Universidad de Castilla-La Mancha FACULTAD DE

Transcription

Universidad de Castilla-La Mancha
FACULTAD DE CIENCIAS Y TECNOLOGÍAS QUÍMICAS
DEPARTAMENTO DE INGENIERÍA QUÍMICA
TESIS DOCTORAL:
VALORIZATION OF POLYSTYRENE WASTES USING NATURAL
TERPENES AND HIGH PRESSURE CO2
Presentada por Cristina Gutiérrez Muñoz para optar al grado de doctor por
la Universidad de Castilla-La Mancha
Dirigida por:
D. Juan Francisco Rodríguez Romero
Dña. Mª Teresa García González
D. Juan Francisco Rodríguez Romero
Catedrático de Universidad
Departamento de Ingeniería Química
y
Dña. Mª Teresa García González
Profesor titular de Universidad
Departamento de Ingeniería Química
Certifican que:
Cristina Gutiérrez Muñoz ha realizado bajo su dirección el trabajo
titulado “Valorization of Polystyrene wastes using natural terpenes and high
pressure CO2”, en el Departamento de Ingeniería Química de la Facultad de
Ciencias y Tecnologías Químicas de la Universidad de Castilla-La Mancha.
Considerando que dicho trabajo reúne los requisitos para ser presentado
como Tesis Doctoral expresan su conformidad con dicha presentación.
Ciudad Real, a ____ de ____________ de 2014
Juan Francisco Rodríguez Romero
Mª Teresa García González
Resumen
El volumen de plásticos producidos en Europa crece anualmente, debido a su bajo
precio, la versatilidad que presentan y el amplio campo de aplicaciones en los que se
incorpora. Sin embargo, tras su uso, se convierten en residuos que es necesario
reciclar para proteger y conservar el medioambiente, tal y como marcan las políticas
Europeas de gestión de residuos, que se centran en la reducción de los impactos
negativos que producen los plásticos. Afortunadamente, a lo largo de la última
década se está produciendo una tendencia positiva en la gestión de los residuos
plásticos, hacia su recuperación y reciclado. De este modo, el reciclaje de residuos
plásticos se presenta como una alternativa medioambientalmente ventajosa, pero
que debe tener en cuenta posibles beneficios económicos para considerarla como
óptima. La recuperación de residuos plásticos en forma de productos de alto valor
añadido, hace posible la viabilidad de los procesos de reciclaje por las siguientes
razones: reducción del consumo de materias primas, recursos y energía, ahorro de
espacio en vertederos, beneficios medioambientales y sociales.
El presente trabajo de investigación, se centra en el reciclaje de residuos de
Poliestireno (PS) mediante una tecnología medioambientalmente sostenible, que
permita la obtención de un producto de alto valor añadido. Se seleccionó el
Poliestireno por tratarse de un plástico no biodegradable, que consume inmensas
cantidades de energía y recursos durante su producción y que debido a su
versatilidad y su escasa vida útil, termina depositado en vertederos o desintegrado
en pequeños fragmentos en el océano.
Con el fin de favorecer el reciclaje de residuos de Poliestireno, se tienen que diseñar
nuevos procesos que permitan reducir el impacto medioambiental, los costes
implicados y que a su vez, mejore las propiedades de los productos recuperados
respecto a los originales o a los obtenidos mediante las tecnologías tradicionales
(reciclado mecánico, reciclado químico o valorización energética). De acuerdo con
esto, y teniendo en cuenta los principios de la Química Verde, se propone un nuevo
proceso. Éste consiste en la disolución de los residuos de Poliestireno en disolventes
naturales de la familia de los terpenos, con lo que se logra reducir el volumen de los
residuos (disminuyendo los costes de transporte), eliminar impurezas insolubles en
los aceites terpénicos, y disolver selectivamente el polímero a partir de una corriente
residual que contenga mezcla de distintos productos plásticos. Además, el polímero
permanece intacto, ya que el disolvente no modifica su estructura interna, ni
favorece su degradación. A continuación, es necesario separar el disolvente del
polímero, para ello, se adiciona un antisolvente que permite la precipitación del
polímero y la eliminación del disolvente. El antisolvente elegido en este trabajo, es el
dióxido de carbono (CO2) en condiciones de alta presión, ya que los aceites terpénicos
presentan una elevada solubilidad en él, y su adsorción sobre el polímero facilita el
procesado. Además, el CO2 puede utilizarse como agente espumante, por lo que
incorporado al proceso, da lugar a espumas microcelulares, que presentan un alto
valor añadido. Cabe destacar que a lo largo del proceso descrito, no se producen
corrientes residuales, ya que tanto los aceites terpénicos como el CO2, se pueden
recuperar y reutilizar en el mismo, consiguiendo que el impacto ambiental sea
prácticamente nulo.
Con el objetivo de diseñar el proceso global que permita el reciclaje de residuos de
Poliestireno mediante disolución en terpenos y CO2 a alta presión, la investigación
se centró en el estudio de los sistemas binarios que constituyen los diferentes pasos
de éste. Inicialmente, se determinó la máxima cantidad de residuos tratados que se
pueden solubilizar en los aceites terpénicos. Para ello, se estudió la influencia de la
temperatura, el peso molecular y el tipo de Poliestireno (Expandido o Extruido) en la
solubilidad del polímero en los terpenos. De acuerdo con esto, se comprobó que los
aceites terpénicos son disolventes adecuados para llevar a cabo la disolución del
Poliestireno a temperatura ambiente, independientemente del peso molecular y el
tipo de procesado. Por otro lado, la solubilidad de los terpenos en CO 2 es
fundamental para conseguir la separación del Poliestireno, de su disolución en los
aceites terpénicos de manera eficaz. Se observó que los terpenos son totalmente
solubles en CO2 a temperatura ambiente y presiones moderadas. El último sistema
binario es el formado por el Poliestireno y el CO2. A pesar de que el polímero es
insoluble en el gas en condiciones de presión y temperatura moderadas, la adsorción
del CO2 en el Poliestireno, es responsable del hinchamiento y la plastificación del
polímero. Este fenómeno conlleva la modificación de las propiedades (viscosidad,
tensión interfacial y temperatura de transición vítrea) del Poliestireno, lo que afecta
a su procesado en condiciones de alta presión.
Una vez estudiados los sistemas binarios, es importante conocer el comportamiento
de la mezcla ternaria (CO2/terpenos/Poliestireno), ya que el conocimiento aislado de
los anteriores no aporta la información suficiente como para definir las condiciones
de operación del proceso. Sin embargo, en base a ellos, se puede decir que la
eliminación del disolvente de la mezcla es posible, ya que los terpenos son
totalmente solubles en CO2 en condiciones de presión y temperatura moderadas,
mientras que ocurre lo contrario en el caso del Poliestireno. Para profundizar el
estudio, se determinó la influencia de la presión, la temperatura y la concentración
del Poliestireno en el terpeno, en la solubilidad de los aceites seleccionados
(Limoneno y p-Cimeno) en CO2. También se estudió el efecto de las variables
mencionadas en la solubilidad del CO2 en la disolución o fase rica en polímero. A
partir de estos resultados, se comprobó que las mejores condiciones para llevar a
cabo la precipitación del Poliestireno a partir de su disolución en terpenos utilizando
CO2 como antisolvente, eran aquellas en las que la densidad del antisolvente era
alta (alta presión y baja temperatura) y la concentración del polímero en la
disolución inicial de terpeno moderada.
A continuación, se determinaron las propiedades de las mezclas a alta presión ya
que para el diseño completo del proceso de reciclaje, el conocimiento del equilibrio de
fases es insuficiente. Así, se observó cómo el efecto plastificante del CO 2 también
afecta a las propiedades de las disoluciones, produciendo un descenso significativo
de la viscosidad y la tensión superficial. Sin embargo, la temperatura de transición
vítrea de las mezclas CO2/terpeno/Poliestireno que constituyen el sistema ternario
es similar a la que se obtuvo en el sistema Poliestireno/CO2.
Una vez conocido el sistema a partir del estudio del equilibrio y de la caracterización
de las propiedades a alta presión, se llevó a cabo el proceso de reciclaje bajo dos
regímenes: continuo y discontinuo. En el proceso continuo, conocido
tradicionalmente como Supercritical Antisolvent (SAS), la disolución polimérica se
dispersa en un recipiente que contiene CO 2. Este proceso es conocido porque permite
controlar el tamaño de las partículas. Sin embargo, el Poliestireno precipitó de
acuerdo a morfologías muy variadas, por lo que se estudió la hidrodinámica del
sistema para intentar conocer el comportamiento de la disolución en el interior de la
cámara de precipitación. El modo en el que la disolución polimérica se pone en
contacto con el CO2 determina la difusión y la mezcla de fluidos y por lo tanto, la
forma en la que el Poliestireno precipita. Debido a las dificultades técnicas para
llevar a cabo la precipitación del polímero siguiendo una morfología preestablecida,
se planteó el proceso en discontinuo, que se centra en la espumación de la disolución
para producir espumas microcelulares de Poliestireno reciclado. En este caso, el CO 2
actúa no sólo como disolvente del terpeno y antisolvente del polímero, sino también,
como agente espumante, lo que permite diseñar el tamaño de las celdas y su
distribución, así como su densidad. La presencia del aceite terpénico favorece la
transferencia de masa que está regida por la difusión del CO 2 en el polímero, pero la
presencia del terpeno hace que ésta esté favorecida por la relajación de las cadenas
poliméricas. El Poliestireno reciclado a través de la técnica explicada, presenta
estructura microcelular, insignificantes trazas de terpeno y ningún signo de
degradación.
Por último, el análisis económico del proceso se llevó a cabo para determinar la
viabilidad del proceso de reciclado de residuos de Poliestireno mediante la tecnología
estudiada. Los costes que implica el proceso de reciclado de Poliestireno se pueden
compensar si el polímero recuperado presenta mejores características que el residuo,
como es el caso de la producción de espumas microcelulares. Pero también hay que
destacar que la legislación reguladora de la gestión de residuos plásticos ha
establecido como objetivo prioritario la eliminación de los impactos negativos que
estos producen, por lo que el reciclaje debe considerarse la única alternativa. La
técnica propuesta se puede llevar a cabo a temperaturas bajas, lo que disminuye los
costes energéticos y el hecho de la selección selectiva del polímero evita la
clasificación previa del residuo, que generalmente es muy costosa. El beneficio
económico calculado, junto con las ventajas medioambientales que implica el
reciclado de residuos de Poliestireno, lo convierten en un proceso muy interesante
para su futura aplicación industrial.
Summary
Larges and increasing volumes of plastics are produced yearly due to their low price,
versatility and suitability for a large number of applications and uses.
Unfortunately, after its use, polymers end up in waste streams and they must be
recovered in order to prevent the environmental pollution and to preserve natural
sources as European waste policies are focused on. In fact, during the last decade,
some positive trends are being seen concerning the recovery and recycling of
polymers. Recycling of plastic wastes is attractive environmentally, but also it
should present a potential value from an economic point of view in order to be a
feasible process. The use of plastic wastes as source of high-added value products
comes true the recycling alternative and offers a great opportunity by many reasons:
the reduction of raw material, resources and energy consumption, the increase of
free volume in landfills, and obviously, the environmental and social benefits.
This research work is focused on the recycling of Polystyrene (PS) wastes by
environmentally friendly technology for the production of high-added value
products. The mentioned polymer is the target plastic since it is not biodegradable,
massive amounts of energy and resource are consumed during its production, its
versatility makes it commonly used but its low lifespan makes it to finish in
landfills or in small pieces in the oceans.
In order to promote the recycling of Polystyrene wastes, new processes should be
designed to reduce the environmental impacts and costs and to improve the quality
of the recovered products regarding to the traditional recycling techniques
(mechanical recycling, chemical recycling or energy recovery). Thus, a technology
which conforms to the Green Chemistry principle is proposed. The new recycling
process of Polystyrene wastes consists on the dissolution of the polymer in natural
terpene solvents to reduce its volume (decreasing transport costs), remove
contaminants, dissolve specifically the polymer from the mixed plastic waste
streams and avoid its degradation. Next, the separation of the terpenes from the
solutions is performed by the addition of an antisolvent which makes the target
polymer that precipitates while the solvent is removed. The selected antisolvent is
carbon dioxide (CO2) since it is able to solubilise terpenes easily and due to its
sorption in the polymer, the processing is enhanced. Furthermore, CO2 has resulted
to be a very promising blowing agent which encourages its use for the production of
microcellular foams, a high-added value product. Moreover, it should be highlighted
that any waste stream from the process is obtained because the terpenes and the
CO2 are easily recovered and reused in several cycles of the process, minimizing the
environmental impact.
With the aim of designing a methodology for Polystyrene recycling, the research
started up by the definition of the binary systems which makes the different steps of
the process. Thus, in order to determine the maximum amount of treated wastes,
the influence of the temperature, the molecular weight and the source of the
Polystyrene wastes (Expanded or Extruded) on its solubility in terpenes was
studied. It was checked that terpenes are suitable solvents for the dissolution of the
polymer at room temperature independently of its molecular weight and processing.
On the other hand, the solubility of terpenes in CO2 is crucial since the success of
the separation process depends on it. It was observed that terpenes are fully soluble
in CO2 at room temperature and mild pressure. Finally, although Polystyrene is
insoluble in CO2, the gas is absorbed into the polymer causing its swelling and
plasticization. This phenomenon entails a modification of the Polystyrene properties
(viscosity, interfacial tension and glass transition temperature), which affects the
processing of the polymer at high pressure.
Once the binary systems were defined and studied, it is important to know the
behaviour
of
the
ternary
mixtures
involved
in
the
process
(CO2/terpenes/Polystyrene) since the binary knowledge is not enough to select the
operating conditions successfully. According to the binary system, the solvent
removal is possible thanks to supercritical CO2 since it provides high solubility of
terpenes and fully PS insolubility at moderated pressures and temperature. The
influence of pressure, temperature and concentration in the terpenes on the
solubility of the selected terpenes (Limonene and p-Cymene) in CO2 and on the
sorption of CO2 into the solutions (Polystyrene/Limonene and Polystyrene/pCymene) were determined. From these results, the most suitable conditions for the
precipitation of Polystyrene from its solution in terpenes using CO 2 as antisolvent
were those where the antisolvent density was higher (high pressure and low
temperature) and the concentration of Polystyrene in the solutions moderated.
Next, the properties of the mixtures were determined since the knowledge of the
phases equilibrium is too poor to define the global process. The plasticizing effect of
CO2 on the polymer is also observed when it is in terpene solutions and
consequently, a deep understanding of how the properties of materials and the
processing parameters affect the behaviour of the mixtures is required to optimize
the further development of high-quality plastic products. The study of the viscosity,
interfacial tension and glass transition temperature of the mixtures was carried out.
Although the viscosity and the interfacial tension of the ternary systems were
significantly affected by the antisolvent, the glass transition temperature showed
similar results to those obtained for the binary system Polystyrene/CO 2.
Once the system was characterized and the equilibrium and properties of the
ternary mixtures were determined, the recycling process was performed. Two main
alternatives were proposed: continuous or discontinuous regime. In the continuous
process, named Supercritical Antisolvent (SAS) the polymer solution is sprayed to a
vessel previously filled with CO2. This process allows the control over the particle
size. Nevertheless, Polystyrene precipitated according to a wide variety of
morphologies and the knowledge of the hydrodynamic of the system was crucial to
understand the process. Thus, the way in which the polymer solution is contacted
with CO2 affects the diffusion and mixing of the fluids and consequently the way on
the polymer precipitates. Due to the technical difficulties found along the
development of the SAS process, the discontinuous one was proposed as alternative,
which is focused on the foaming of the solution for the production of Polystyrene
microcellular foams. In this case, CO2 behaves not only as solvent of the terpene and
antisolvent of the Polystyrene, but also as blowing agent of the polymer, which
allows tuning the cells size, cells size distribution and cells density of the foamed
Polystyrene. The presence of the terpene promotes the mass transfer that is mainly
governed by the diffusion of the CO2 into the polymeric rich phase due to the
relaxation of the polymer chains. The recycled Polystyrene, obtained following this
methodology, presents microcellular structure, non remarkable traces of terpene
and any evidence of degradation.
Finally, the economic analysis of the process was carried out to determine the
feasibility of the recycling process. The traditional high costs involved in the
recycling of PS wastes are balanced by the high quality recycled polymer, but also
due to the mild operating conditions required to carry out the process and the
selective dissolution technique which eliminates the need to sort previously the
polymer from mixed plastic waste streams. The economical benefits, together with
the environmental advantages show the interest of the process for industrial
application.
Table of contents
Resumen ...................................................................................................................... 7
Summary ................................................................................................................... 11
1. 1. Plastics................................................................................................................. 5
1.1.1. Plastics production and demand........................................................................ 7
1.1.2. Plastics Wastes .................................................................................................. 9
1.1.3. Plastic Waste Management ............................................................................. 10
1.1.4. Plastic Waste Recycling ................................................................................... 13
Mechanical recycling ................................................................................................. 14
Chemical recycling .................................................................................................... 14
Energy recovery ......................................................................................................... 15
1.2. Polystyrene Production and Wastes ................................................................... 15
1.2.1. Polystyrene Recycling ...................................................................................... 17
Motivation ................................................................................................................. 27
Objectives .................................................................................................................. 28
Work plan .................................................................................................................. 29
Thesis Overview ........................................................................................................ 30
3.1. Materials............................................................................................................. 35
3.2. Experimental setups........................................................................................... 36
3.2.1. Solubility determination at atmospheric pressure .......................................... 36
3.2.2. Variable-Volume High Pressure View Cell...................................................... 36
3.2.4. High-Pressure Differential Scanning Calorimetry .......................................... 38
3.2.5. Rheometer ........................................................................................................ 39
3.2.6. Quartz Viscometer ........................................................................................... 40
3.2.3. Pendant droplet High Pressure View Cell ....................................................... 41
3.2.7. Foaming setup ................................................................................................. 41
3.2.8. Supercritical Antisolvent precipitation setup.................................................. 42
3.3. Experimental Procedures ................................................................................... 43
3.3.1. Determination of the solubility of Polystyrene in Terpene oils ....................... 43
3.3.2. High Pressure Vapour-Liquid Equilibria of Binary mixtures Terpenes/CO2 .. 44
3.3.3. High Pressure Phase Behaviour of Ternary mixtures
CO2/Terpenes/Polystyrene ........................................................................................ 44
3.3.4. Glass transition temperature at high pressure ............................................... 45
3.3.5. Determination of the viscosity of Polystyrene ................................................. 46
3.3.6. Determination of the viscosity of Polystyrene solutions .................................. 46
3.3.7. Determination of high-pressure Interfacial Tension ....................................... 46
3.3.8. Foaming of Polystyrene at high pressure ........................................................ 47
3.3.9. Precipitation of Polystyrene by Supercritical Antisolvent process .................. 48
3.4. Characterisation ................................................................................................. 48
3.4.1. Thermogravimetric Analysis (TGA) ................................................................ 48
3.4.2. Thermogravimetric and Mass Spectrometer Analysis (TG-MSA) ................... 49
3.4.2. Differential Scanning Calorimetry Analysis (DSC) ......................................... 50
3.4.3. Optical Microscopy (OM) ................................................................................. 50
3.4.4. Scanning Electron Microscopy (SEM).............................................................. 51
i
Table of contents
3.4.5. Determination of Molecular Weight ................................................................ 52
Graphical abstract ..................................................................................................... 57
4.1. General Background ........................................................................................... 61
4.1.1. Polystyrene/Terpene oils.................................................................................. 61
4.1.2. Terpene oils/CO2 .............................................................................................. 63
4.1.3. Polystyrene/CO2 ............................................................................................... 64
4.2. Polystyrene/Terpene oils Results........................................................................ 66
4.2.2. Determination of the solubility parameters .................................................... 66
4.2.3. Influence of the Molecular Weight on the Solubility of Polystyrene ............... 69
4.2.4. Influence of the Temperature on the Solubility of Polystyrene ....................... 71
4.2.5. Applicability to real Polystyrene wastes.......................................................... 73
4.3. Terpene oils/CO2 Results .................................................................................... 76
4.3.1. Procedure ......................................................................................................... 76
4.3.2. Literature data and correlation of binary systems Terpenoids/CO 2................ 79
4.3.3. Prediction of multicomponent mixtures and essential oils .............................. 86
4.3.4. Correlation and prediction of terpenes/CO2 using Equations of State ............ 89
4.4. Polystyrene/CO2 Results ..................................................................................... 94
4.4.1. Sorption of CO2 in Polystyrene. Plasticization................................................. 94
4.4.2. Glass transition temperature .......................................................................... 98
4.4.3. Interfacial Tension......................................................................................... 100
4.4.4. Viscosity ......................................................................................................... 102
References ............................................................................................................... 106
Graphical abstract ................................................................................................... 117
RESUMEN .............................................................................................................. 118
ABSTRACT.............................................................................................................. 119
5.1. Background of the Phases’ equilibrium ............................................................ 121
5.2. The ternary system CO2/Limonene/PS ............................................................. 122
5.2.1. Effect of pressure ........................................................................................... 125
5.2.2. Effect of temperature ..................................................................................... 126
5.2.3. Effect of concentration ................................................................................... 127
5.2.4. Sorption of CO2 into PS/Limonene solutions ................................................. 128
5.2.5. Distribution coefficient .................................................................................. 129
5.3. The ternary system CO2/p-Cymene/PS............................................................. 131
5.3.1. Ternary Diagrams ......................................................................................... 131
5.3.2. Influence of density and concentration on p-Cymene solubility in the vapour
phase ....................................................................................................................... 133
5.3.3. Influence of density and concentration on CO 2 solubility in the liquid phase
................................................................................................................................. 138
5.3.4. Selection of the operating conditions ............................................................. 143
References ............................................................................................................... 147
RESUMEN .............................................................................................................. 152
ABSTRACT.............................................................................................................. 153
ii
Table of contents
6.1. Background of viscosity and interfacial tension of the ternary mixtures
CO2/Limonene/PS and CO2/p-Cymene/PS ............................................................... 155
6.2. Viscosity of mixtures CO2/Limonene/Polystyrene ............................................ 157
6.2.1. Selection of the parameters for the study of the viscosity ............................. 159
6.2.2. Concentration effect on viscosity ................................................................... 160
6.2.3. Pressure effect on viscosity ............................................................................ 162
6.2.4. Temperature effects on viscosity ................................................................... 166
6.2.5. Pressure and temperature combined effects on viscosity .............................. 169
6.2.6. Demixing determination from viscosity measurements ................................ 172
6.3. Interfacial tension of CO2/Limonene/PS and CO2/p-Cymene/PS ...................... 175
6.4. Glass transition temperature of CO2/Limonene/PS and CO2/p-Cymene/PS .... 180
Based on: ................................................................................................................. 189
RESUMEN .............................................................................................................. 190
ABSTRACT.............................................................................................................. 191
7.1. Supercritical Antisolvent Background ............................................................. 193
7.2. Supercritical Antisolvent Process ..................................................................... 194
7.2.1. Solubility Parameters .................................................................................... 194
7.2.2. Extraction process and recovery of solvent.................................................... 195
7.2.3. Supercritical Antisolvent Process .................................................................. 197
7.3. Foaming background ........................................................................................ 214
7.4. Feasibility of the Foaming of Polystyrene/cosolvent system with CO2 ............ 217
7.5. Statistical study of the foaming process of PS/p-Cymene solutions ................. 222
7.5.1. Characterisation ............................................................................................ 224
7.5.2. Optimisation .................................................................................................. 226
7.6. Rigorous Statistical study of the foaming process of Polystyrene/Limonene
solutions .................................................................................................................. 229
7.6.1. Foaming process ............................................................................................ 230
7.6.2. Nucleation mechanisms ................................................................................. 240
7.6.3. Characterisation ............................................................................................ 244
RESUMEN .............................................................................................................. 256
ABSTRACT.............................................................................................................. 257
8.1. Life Cycle Assessment Background .................................................................. 259
8.2. Economical evaluation ...................................................................................... 266
iii
Table of contents
iv
Chapter 1
INTRODUCTION
Introduction
2
Chapter 1
Based on the Book Chapter:
 C. Gutiérrez, M.T. García, I. Gracia, A. De Lucas, J.F. Rodríguez, Polystyrene
Wastes, threat or opportunity?. Environment, Energy and Climate Change I and II
3
Introduction
4
Chapter 1
1. 1. Plastics
History has been frequently classified according to the materials that the man used
for making his implements and other basic necessities. The most well known of
these periods are the Stone Age, the Iron Age and the Bronze Age. During the last
century, a new class of material has been introduced which have not only challenged
the older material for their uses but has also made possible new products. Without
plastics is difficult to conceive our everyday life [1]. The term plastic is derived from
the Greek word plastikos meaning fit for moulding. An adequate definition of plastic
is difficult to show, but generally plastic materials are synthetic of semi-synthetic
organic solid widely used in the manufacture of industrial products. Attending to
their molecular structure, they are mainly polymer of high molecular weight. The
word polymer literally means many [2] units (mer), in which a chemical unit
(monomer) is repeated itself a very large number of times to create the polymer [3].
Nowadays, the majority of commodity plastics are derived from gas or from crude oil
which, after processing and refining, generate monomers that are then used in the
manufacture of polymers. There are as many processing routes as kind of polymers,
and several routes could be used to obtain one polymer (Figure 1.1).
The huge variety of plastics makes necessary its classification in order to group by
properties and applications. The most commonly used classification is based on
thermoplastics (80-90 %) and thermosets (12-20%) since they constitute the majority
of the market accounting for around 75% of the total polymer consumption.
Thermoplastics are capable of changing shape on application of force and retaining
this shape on removal of force (stress produces a nonreversible strain). They are
softened when are heated above the glass transition temperature (Tg) and can be
reshaped and will harden in this form upon cooling. Multiple cycles of heating and
cooling can be repeated without severe damage, allowing reprocessing and recycling
because they are generally linear or branched polymers which present few chemical
interactions between chains. On the other hand, thermosets do not soften on heating
but elevated temperatures produce chemical reactions that could harden the
material into an infusible solid. Generally, they are more brittle and insoluble in
organic solvents than thermoplastics because thermosets present a threedimensional structure obtained by chemical crosslinking produced after or during
the processing. By this reason, thermosets can not tolerate repeated heating cycles
as thermoplastics can [4].
According to their demand, the most representative thermoplastics are:
Polypropylene (PP), Low Density Polyethylene (LDPE), High Density Polyethylene
(HDPE), PolyVinyl Chloride (PVC), Polyethylene Terephthalate (PET) and
Polystyrene (PS). The thermosets are largely represented by amino resins,
polyurethanes, phenolics, polyesters, alkyd resins and epoxy resins (Figure 1.2).
5
6
Crude oil
Nafta
Cyclohexane
Distillation
Benzene
or Phenol
Cyclohexanone
Reformer
Cracking Unit
CH4
CH2
CH3
Gasoline
Extraction
Butenes
H2 C
Propylene
H2C
CH3
Ethylene
H2C
Methane
Styrene
Ethylene oxide
Ethylene glycol
Butadiene
CH2
Polyglycols
Polyglycols
p-Xylene;
m-Xylene
Ethyl
Benzene
Cumene
Phenol
Styrene
Isopropyl alcohol
Acetone
Propylene oxide
Propylene glycol
Caprolactam
Phthalic
anhydride
CH
Ethylene
dichloride
o-Xylene
Mixed
Xylenes
Toluene
Benzene
Benzene
HC
Acetylene
Alkyl resins
Polyesteres
Polyamides
Synthetic
rubbers
Polymethyl
methacrylate
Polypropylene
Polystyrene
Polyester fibres
and films
Polyurethanes
Polyethylene
Polyvinyl chloride
Introduction
Figure 1.1. Conventional production routes for polymers raw materials to commodity
plastics.
Chapter 1
9000
8000
Production (kTonnes)
7000
6000
5000
4000
3000
2000
1000
0
P
LD
P
E
LD
DP
/M
E/L
PE
D
H
PP
PS
C
PV
T
PE
AN
S/S
AB
MA
PM
PC
R
PU
s
her
Ot
Type of plastics
Figure 1.2. Demand of most representative thermoplastics in Europe 2012.
1.1.1. Plastics production and demand
In the second half of the 20th century, plastics became one of the most-universallyused and multipurpose materials in the global economy due to their low price and
versatility. Today, plastics are used in more and more applications and they have
become essential to our modern economy [5]. Among the most important properties
are: versatility, low price, lightweight, durability and strength. Due to their
excellent characteristics, the plastics industry has benefited from 50 years of growth
with a year on year expansion of 8.7% from 1950 to 2012. The production and
consumption of plastic materials have been growing steadily. For instance, the
production of plastics rose to 288 million tonnes from 2011 to 2012 which means an
increase of 2.8 %. Nevertheless in Europe and according to the general economic
situation, plastics production decreased by 3% from 2011 to 2012 (Figure 1.3) [6].
7
Introduction
World Plastic Production
Latin America
4.9%
NAFTA
19.9%
300
World Plastic Production
European Plastic Production
Biopolymers
Plastic Production (Mtonne)
250
200
Concers for wildlife
100
Accumulation in natural habitats
France
9.37%
in
95
60
rt s
)
o
e ( f en
19
t an
70
s ) g l em
19
co
po
Germany
Spain
UK
Italy
France
Benelux
Other EU
Benelux
4.48%
1990
2000
2010
Other EU
34.42%
UK
7.54%
Spain
7.13%
Germany
23.63%
in
by
wi
ld
re
rs t
Fi
lif
s(
i rd
ab
eb
se
of
sd
ts
1980
Year
ed
(1
rd
ed
er
re
ov
ri s
isc
ed
tic
gu
yl
en
as
Pl
1970
en
EU
ts
P
t ro ack
ag
du
in
ce
g
d
b y d ir
M ect i
un
v
i ci e l e
Pl
g
pa
as
tic
l S isla
t
ol
de
id io n
br
is
W
re
as
po
t es
rt e
(1
d
99
in
0s
de
)
ep
se
a(
20
00
)
1960
4)
1950
op
China
23.9%
Italy
13.44%
0
pr
Japan
4.9%
Europe Plastic Production
Product user diversification
ly
Middle East, Africa
7.2%
China
Rest of Asia
NAFTA
Latin America
Europe
CIS
Middle East, Africa
Japan
50
Po
CIS
3%
Rest of Asia
15.8%
Concerns for human life
150
Europe
20.4%
Figure 1.3. Plastics production from 1950 to 2012. The production includes thermoplastics,
polyurethanes, thermosets, elastomers, adhesives, coating, sealants and fibres. But PET, PA
and polyacryl-fibres are not included [6].
The first plastic producer country around the world is China, followed by Europe
and the United States of America, Canada and Mexico (NAFTA). European
countries produced 50 Mtonnes in 2012 and the main producers were Germany,
Italy and France. UK and Spain got the fourth and fifth position, respectively.
In Europe, packaging purposes are the largest application sector for the plastic
industry and represent 39.4% of the total plastics demand. In the second position,
building and construction sector reaches 20.3% followed by the automotive sector,
electrical and electronic applications. Other application sectors such as appliances,
household and consumer products, furniture and medical products comprise a total
of 22.4% of the European plastics demand (Figure 1.4).
8
Chapter 1
Automotive
Electrical&Electronics
3.76 Mtonnes 2.52 Mtonnes
Agriculture
Building & Construction
1.93 Mtonnes
9.32 Mtonnes
Others
10.28 Mtonnes
Packaging
Building & Construction
Automotive
Electrical & Electronics
Agriculture
Others
Packaging
18.08 Mtonnes
Figure 1.4. Main sectors of application of the plastics industry in Europe 2012.
1.1.2. Plastics Wastes
It is obvious that plastics are used in our daily lives in a great number of
applications, but at the end of their useful life, polymers enter in waste streams as
either post-consumer waste or industrial scrap.
Households, distribution and industry sectors are the main sources of the wastes.
But also, the automotive sector, the electrical and electronic, as well as building and
construction provides important amounts of plastic wastes (Figure 1.5).
6000
PE
PP
PVC
ABS
PS
PU
PC
Others
Total Plastic Waste (kTonnes)
5000
4000
3000
2000
1000
0
Municipal solid wastes Automotive
Agriculture
Construction Electrical&Electronic
Origin and Type of Post-consumer Plastic Wastes
Figure 1.5. Types of plastic originated depending on the origin of wastes.
9
Introduction
Plastic wastes have been involved in the municipal solid waste considerably,
concentrated in packaging, films, covers, bags, containers and so on with widely
used in our daily life. According to Figure 1.5, municipal wastes cover the largest
volume of plastics wastes. Packaging materials make up the largest contribution to
polymer waste in the stream and they are mainly constituted by LDPE/LLDPE, PP,
HDPE, EPS/PS, PVC and PET. The polyolefins used for packaging materials are
often discarded after a single use, which results in a large amount of polymer
wastes [7].
In the automotive sectors, plastics have increasingly been used to replace metals for
many components because of their greater processability, lighter weight and
corrosion resistance. The percentage by weight of polymers in the average European
car has risen in the last years, in contrast with packaging applications. Polymers
found in automotive wastes are PP, ABS, HDPE, PP and PU, PVC.
The plastic wastes coming from the construction are mainly PVC and thermosetting
resins together with acrylics or PP. The nature of the plastics generated in the large
industry is mainly PE from sheet used exclusively for all secondary packaging.
LDPE is the predominant plastic from the agricultural plastic wastes. And
eventually, electrical and electronic wastes are a significant contributor of ABS
poly(acrylonitrile-co-butadiene-co-styrene), polycarbonate and modified PP.
From the comparison of Figure 1.2 and 1.4, significant differences between the
plastic demand and plastic waste generation can be observed. The most demanded
plastic in Europe in 2012 was Polypropylene, while the Polyethylene was the most
common plastic presents in waste streams. Although the vast majority of the
industrial wastes are recycled with the processing, the rests are usually sent for
reprocessing. On the other hand, the majority of post-consumer plastics waste
reaches the environment and hence the emphasis in polymer waste management is
on this type of waste stream.
1.1.3. Plastic Waste Management
European waste policy has regulated the management of the municipal solid wastes
through “environmental actions plans and a framework of Legislation” during the
last 30 years (European Commission 2010). The target of the environmental policy
is the reduction of waste negative impacts. In 2005, a long-term strategy on waste
management which started a new era of EU waste policy. The new strategy stated
from the EU’s Sixth Environment Action Programme has been reflected in the
Waste Framework Directive (WFD-2008/98/EC). The new Directive (WFD2008/98/EC) is focused on the promotion of a recycling society with the targets for
“EU Member States to recycle 50% of their municipal wastes and 70% of
construction wastes by 2020” [8]. Wastes management legislation in Europe differs
from country to country; however, basic principles and restrictions are in accordance
10
Chapter 1
with the European Landfill Directive and other important EU legislation in the field
of wastes management.
In this regard, since the last years a positive trend is growing in the recovery and
recycling of plastics in Europe. In 2012, post-consumer plastic wastes reached 25.2
Mtonnes, of which 62% of plastics were recovered and only 38% were ended up in
landfills. It is important to highlight that almost 70% of plastic packaging wastes
were recovered, 34.5% went through energy recovery (incineration or additive as
refuse derived fuel), 34.2% was mechanically recycled and only 0.7% went to
feedstock recycling [6].
It should be outlined that the recycling of plastic wastes is attractive not only
environmentally, but also from an economic point of view due to their high potential
value [9]. European Commission marked a shift away from thinking about wastes as
an unwanted burden to seeing it as a valued resource (European Commission 2010).
But the recycling will be only economically advantageous if the design of the process
consider the amount of energy necessary to produce the virgin polymer plus the
energy to dispose equal to the energy to recover the plastic wastes plus the energy
during the reprocessing. The high cost would be balanced by the reduction cost of
landfilling and of course by the social benefit [10] since the recycling of plastics
greatly contributes to the preservation of the natural resources.
According to the data, recycling methods are being established as priority
techniques for the recovery of polymers from plastic wastes. But the recycling of
materials requires the separation of the different polymers in order to recover
plastics of acceptable quality. Plastic recycling streams are made of dirty mixed
polymers including dyes, fillers, flame retardant and other additives [9]. The use of
polymers in different market sectors complicates the identification and separation of
waste streams because their use in one sector does not guarantee that they will
appear as waste in the same sector [4]. By this reasons, the separation of different
polymers by type is crucial. But also, the differences among the melting
temperature and some chemical incompatibilities hinder the mixture of plastics
previously to its classification, especially if the target recycled product should
present high value [11].
Adequate and cost-effective techniques are demanded for the identification of
plastics since the recycling processes are priority as it was shown in Figure 1.6 [12].
Separation is further complicated by the current design of products in which
polymers are commingled with other materials and polymers are currently sorted
and separated manually or mechanically which means its high cost and inefficiency
[13]. Considerable work has been undertaken to develop automated technologies
that can separate mixed waste streams according to polymer type or to remove
foreign matter (contaminants) from the waste stream. Nevertheless, plastic exhibit
similar properties which make the automatic classification a challenge.
11
12
Washing
Plasticisers
Stabilisers
Thermosets
Plastic wastes
Flame retardant
Pigments
Thermoplastics
Classification
Infrared
Raman
Thermal
Analysis
Fluidization
Spectroscopy
Laser
Manual
SELECTIVE
DISSOLUTION
Energy
Recovery
Chemical
Recycling
Mechanical
Recycling
Precipitation
Co-Combustion
Incineration
Chemolysis
Pyrolysis
Hydrogenation
Gasification
Thermoplastics
Thermosets
Hydrolysis
Glycolysis
Methanolysis
400-900 ºC
Fluidised bed, rotatory kiln,
tubular crackers
Bubble column
20 MPa, 500 ºC
Fluidised-bed, fixed-bed,
pneumatic transport reactor
15-30 MPa, 800-600 ºC
Melting
Extrusion
Grinding
Milling
RECYCLED
POLYMER
MONOMERS
WAX, OIL,
GAS, ENERGY
SYNCRUDE
SYNTHESIS
GAS
RECYCLED
POLYMER
ADDITIVES
Introduction
Figure 1.6. Routes for the management of the plastic wastes: from the sorting to the
recycling.
Chapter 1
Figure 1.6 shows some of the developed techniques for the identification and sorting
of different types of plastics; the substantial development occurred in the last
decade has achieved a so good classification of plastic wastes that only contain small
amounts of other types of polymers [14]. Although, gravity separation method based
on air or centrifugal force was proposed by Choi et al. [15], the most employed
techniques are related with spectroscopy: Laser [16], Raman [17], Infrared [17] or
fluorescence [9]. FTRaman has been proved as the most rapid and selective
technique to recognize the most usual polymers. But generally, the combination of
different analytical techniques is required to sort perfectly the plastic wastes
streams. This fact makes these technologies very expensive and its efficiency is
always limited by the feed characteristics. By these reasons, the choice of the most
appropriate will depend on the complexity of the polymer mixture, its physical form,
the quality or the additives and/or contaminants [4].
The proposal of a general methodology for the classification of plastic wastes is still
nowadays a challenge. There is no clear relationship between quantities of plastics
produced and quantities for plastic wastes, because of the lag between production
and disposal depends mainly on the lifespan of each product. Post-consumer plastics
wastes may arise from a host of products or applications each with differing life
cycles and therefore they should be treated individually. While containers and
packaging wastes generally have lifespans under a year; durable and non-durable
plastics can reach lifespans estimated at five years for transportation applications;
10 years for furniture, housewares, electric and electronic; and 50 years or more for
building and construction materials [18]. Generally and according to this data,
packaging dominates the wastes generated from plastics, covering 62.2 % of the
total. Other applications like building and construction, electrical and electronic
products and agriculture count for 5 till 6% each. It should be more appropriate to
divide in new categories the waste streams in which plastics may be found (Figure
1.5).
Years of research, study and testing have resulted in a numbered of treatment,
recycling and recovery methods that could be feasible and economically
advantageous. Important efforts are making to manage efficiently the plastic
products at the end of their service life, new routes are improving constantly and
fewer of them are ending up in landfills.
1.1.4. Plastic Waste Recycling
According to Figure 1.6, plastic wastes management can be performed mainly by
three different ways: mechanical recycling, chemical recycling and energy recovery
[19]. A fourth approach was defined by Scott [20] as the biological recycling but due
to the high costs, complex procedures and environmental damages involve during its
development, this alternative is not generally considered [10]. Each method provides
13
Introduction
a unique set of advantages that make it particularly beneficial for specific
applications [21]. Next, a brief summary of the different alternatives is shown.
Mechanical recycling
Separation of different polymers is particularly important for mechanical recycling
because processing mixed materials would produce recycled products of low quality,
which could only be used in a limited number of applications. Hence, mechanical
recycling is really best suited to clean plastic wastes such as packaging materials.
Mechanical recycling of mixed wastes necessarily starts with the manual sorting
process because it can only be performed on single-polymer plastics and the mixed
or contaminated wastes can not be recycled following this method. This fact implies
high costs and low quality products when the separation is not as efficient as is
required.
Depending on the type of polymer, the mechanical recycling follows different
processes. Thermosets are grinded and particulated for its re-used as filler in new
materials since they can not be re-melted. They are generally used as additives to
improve the properties of other polymers providing better values of modulus,
elongation at break or impact strength. On the other hand, thermoplastics are remelted, extruded and pelletized to be sold as a raw material for further processing.
Although thermoplastics are soften above their glass transition temperature,
degradation and heterogeneity of wastes should be considered. Polymerization
reactions are irreversible but the heat provided during the mechanical recycling
could cause photo-oxidation. Consequently, length or branching of polymers could
occur and the polymer would present worse properties as a result of the ageing.
Despite this, mechanical recycling has been the oldest practice by common plastics
manufacturers [22].
Chemical recycling
Chemical recycling, is based on the conversion of polymers into monomers
(monomer recycling) or useful petrochemicals (feed-stock recycling). Generally,
polyolefin plastics with a massive production have been subjected to recycled
chemically to produce a mixture of hydrocarbons that could be useful as fuel [21]
[23]. Several processes are included in the chemical recycling and the most relevant
are explained below (Figure 1.6).
Gasification is the partial oxidation of hydrocarbons in the presence of low oxygen
levels. The process is typically carried out at temperatures from 800 ºC up to 1600
ºC, pressures of 15-30 MPa and in presence of a gasifying agent (air, oxygen, steam,
fuel gas). The main products of gasification are syngas [24].
14
Chapter 1
Once plastic wastes are depolymerised, they can be hydrogenated to produce
bitumen and a synthetic crude oil, known as syncrude which present a very high
added value. The syncrude is refined for its use in the petrochemical industry.
Pyrolysis consists on the thermal decomposition of the plastic wastes at temperature
from 350-700 ºC in the absence of oxygen or other gasifying gases. The polymers
decompose to their monomers, oligomers and other organic substances that can be
collected separately and used as a feedstock or for energy generation [25].
Finally, chemolysis is a treatment which uses solvolytic processes (hydrolysis,
glycolysis or alcoholysis) to recycle and convert plastics into their basic monomers
for repolymerisation into new plastics. This method allows up to 90% of the plastic
(by weight) to be re-used but it works best with homogeneous plastic types.
Energy recovery
When the recycling of plastic wastes is not feasible, they could be used in energy
recovery due to its high calorific values. The high calorific value of plastics
underscores the need for an alternative energy recovery technology that is
affordable, efficient, safe and user-friendly. The two ways to obtain energy from
plastic wastes are the incineration and the co-combustion. Incineration without
energy recovery would be also possible but is not acceptable from the sustainability
point of view because it only reduces the amount of wastes, but waste valuable nonrenewable resources and it can not be defined as a recycling option. Furthermore,
the incineration of polymer provokes important social opposition [4].
Despite plastics recycling is increasing and many different alternatives are
proposed, important efforts are still required to recycle efficiently the plastic wastes.
Mechanical recycling is only economically feasible if the plastic wastes are classified
in a single polymer stream and energy recovery must also separate PVC from the
rest of the polymers due to the formation of dioxins. The fact that most of the
recycling options require a severe previous classification of polymers by type is a
challenge that nowadays constrains the recycling. Although the recycling has been
established as a priority option, the mentioned high costs related with the sorting
process limits its application.
1.2. Polystyrene Production and Wastes
In this work, we are focused on the recycling of Polystyrene wastes because in the
last years it has been considered a plague on the environment taking more space in
landfills than paper. The first commercial production of Polystyrene began in the
early 1930s by the Farben Company in Germany. Polystyrene is the polymerization
product of styrene monomer, which is a colourless liquid with a strong odour derived
from petroleum and natural gas by-products (Figure 1.1) [26].
15
Introduction
n
Styrene
Polystyrene
Figure 1.7. Styrene polymerization
Generally, Polystyrene is a clear, hard, glassy material with a bulk density between
0.94-1.05 g/cm3. It founds application in packaging, food and beverage containers,
building and insulation. It is widely used since it is very cheap, light, and versatile
also it presents low dielectric constant and low thermal conductivity.
Unfortunately, due to its resistance to biodegradation, PS plastic wastes have
become one of the major problems to the environment. PS is made from petroleum, a
non-renewable resource, through a very complicated process. From oil to monomer,
monomer to polymer, and polymer to final product (Figure 1.7), not only are massive
amounts of energy and resource consumed, but also serious environmental impact is
generated by the productive processes [27] [28]. The Environmental Protection
Agency (EPA) established PS as the fifth largest source of hazardous wastes.
According to Kyoto protocol, PS foams were banned to the ozone-depleting CFC
gases used as blowing agents. Nowadays, several governments are considering the
extension of the ban on PS foam containers because its impact on marine pollution.
Scientists are concerned that plastics debris in the ocean can transport toxic
substances which may end up in the food chain causing potential harm to ecosystem
and human health (UNEP 2011).
Regarding the cited restrictions, the traditional disposal of plastic wastes in landfills
is not an option due to the high costs, legislative pressures and public concern on
resource conservation. PS should be considered a raw material with high potential
value and its recycling a priority, not only for the environmental issues but also
from an economical point of view. Nevertheless, its excellent properties entail some
environmental inconveniences. It is extremely lightweight and due to the high
volume/weight ratio, it does not seem economically viable to store or transport nor is
an attractive material for collection taking up huge volume in landfills. But if PS is
landfilled there is an important loss of the potentially valuable materials. Its
versatility means a wide variety of applications which makes often the PS wastes a
mixture of different grades and types of polymer. This fact could result in variable
and low quality recycled products, which limits the further uses of the recovered
material.
On the other hand, PS wastes (especially EPS) are frequently contaminated by food
and they are generally not recycled because clean wastes are required in order to
recover high quality plastics. It should be emphasise that recycled plastics should
16
Chapter 1
not present a lower grade than the "virgin" polymer because it could prevent its use
in some applications, such as that for food packaging. Eventually, it should be
underlined that PS generally is a one-time use material and its cost per unit is
around cents. The world’s oversupply of this virgin polymer, due to the excess of
production capacity, has driven its price down. So it is a challenge the recycling of
PS wastes to achieve a high market value product from an easy and cheap process
since the cost for collecting, sorting, cleaning and reprocessing often become
excessive [29]. The main idea to overcome the economical boundaries is the recovery
of the material with enhanced properties and at same time with new uses [10].
Mechanical recycling has been selected as the most adequate way to recover styrenic
plastic wastes from packaging applications. But during the re-melting and
compression of PS, their chemical structure, long-term stability or mechanical
properties could be altered [30]. Although the recycling of the cited polymer has
been highly promoted, nowadays the rate of PS recycling is very low in comparison
to the recycling rate of all other plastic wastes.
1.2.1. Polystyrene Recycling
According to the different aspects about the recovery of PS wastes, all the scientific
efforts should be focused on the recycling since it allows saving energy and raw
materials. This fact is crucial due to the existing situation that Europe relies upon
imports of scarce resources (European Commission, 2010).
New processing schemes should be explored in order to reduce the cost of the
traditional and expensive recycling processes mentioned before. Dissolution and
shrinking has been proposed as one of the cheapest and most efficient methods for
the recycling of PS. The selection of the most suitable solvent is the crucial step for
the selective recycling of polymers by dissolution [31]. The main advantages of the
dissolution of polymeric wastes are shown below:

the dissolution of PS in a suitable solvent will cause considerable volume
reduction (it could be achieved it a volume reduction of more than 100
times).

any insoluble contaminants will be dissolved and they could be removed by
filtration, obtaining a cleaner polymer that could be reprocessed.

the selection dissolution allows the separation of plastics from other types of
waste and polymers non-soluble depending on their chemical nature. For
instance, a single solvent can be used to dissolve several unsorted polymers
at different temperatures or even it can dissolve selectively only one
polymer.

the polymer does not suffer further degradation, unless heat is used for the
dissolution of the plastic wastes.
17
Introduction
In view of these numerous advantages, the design of a process for the recycling of
PS wastes by dissolution can be performed. The dissolution of PS wastes has been
studied but focused on the thermal recycling (pyrolysis) [32]. With this aim, it was
proposed the use of several solvents: biodiesel [33], solvents derived from the
pyrolysis of the PS wastes [34], aliphatic, cyclic, and aromatic solvents [35] or even
automobile lubricating oil wastes [36]. In all cases the addition of solvent provided
solutions with a high specific heat. The heat of combustion of PS is approximately
41 MJ/kg, comparable to the traditional value for oil. In a proper designed
combustion device complete combustion could be achieved resulting in water, carbon
dioxide and trace levels of ash.
The use of the aforementioned solvents has been focused on the dissolution of PS
wastes in low cost solvents but with high specific heat in order to be pyrolysed.
Nevertheless, after the pyrolytic treatment only the energy is recovered but not the
polymer. However, it can be checked that the concept of dissolution could be useful
to propose new methodologies for the recycling of PS wastes.
Polymer dissolution plays a key role in many industrial applications among them,
recycling of polymers by dissolution is becoming a green alternative to perform the
process. A single solvent can be used to dissolve several unsorted polymers at
different temperatures or even it can dissolve selectively only one [37-41]. But the
dissolution of polymeric materials is different from non-polymeric since in polymers,
the dissolution process is generally controlled by the external mass transfer
resistance through a liquid layer and they are not dissolved immediately [42].
Generally, polymer dissolution processes show two steps: the disentanglement of
polymer chains when the solvent penetrates and the polymer swelling. Thus, the
bulk transport process implies the movement of disentangled chains from the
polymer to solvent rich phase [43]. Figure 1.8 shows a scheme about the structure of
the surface layers of glassy polymers during their dissolution in a pure solvent.
Polymer
Infiltration
Layer
Solid Swollen
Layer
Gel Layer
Liquid
Layer
Solvent
Figure 1.8. Schematic diagram of the composition of the surface layer.
As can be observed in Figure 1.8, a polymer contains free volume (channels and
holes of molecular dimensions) where solvent goes in firstly. From those empty
spaces the diffusion of the solvent starts and creates a solid swollen layer. Next, the
gel layer is ensued from the swollen polymer, which contains material in a rubberlike state and a liquid layer surrounding.
18
Chapter 1
Next, solvent removal produces amorphous or crystalline porous materials
depending on the experimental conditions, such as polymer concentration, molecular
mass or temperature [44]. The simplest way to eliminate and recovery the solvent is
evaporation or distillation (vacuum or steam) [45, 46], but high temperature can
promote the degradation of the polymer chains [47]. As an alternative, the addition
of a proper antisolvent makes the polymer precipitate while the solvent is
solubilised in the antisolvent and both can be recovered independently [37, 48]. The
ratio solvent/antisolvent, the concentration of the polymer in the initial solution, the
final solvent content in the recovered polymer or the form of the precipitated are
some of the variables to consider during the design of the recycling of polymers by
dissolution and antisolvent process.
Recycling processes only are valuable if they are also economically beneficial, it
means, the recycled material, prepared from post-consumer waste plastic, should
present high-quality. In general, polymers have no competing materials in terms of
weight, ease of processing, economy and versatility [6]. Particularly, microcellular
foams have enjoyed a continuously growing market due to their high impact
strength, toughness and thermal stability, as well as low dielectric constant and
thermal conductivity; therefore they are a high-quality product. Microcellular foams
are defined as foams characterized by cell sizes of less than 10 microns and cell
densities greater than 109 cells/cm3 [49].
Plastic foams have been produced by a variety of processes such as batch foaming,
extrusion foaming, and injection foam molding. Table 1.1. summarizes the general
technologies used to make polymer foams and the target applications as a function
of their .
Table 1.1. Polymeric Foams methodology.
Technology
Reactive foaming
Extrusion
Injection Mold
Polymers
Applications
Thermosets: Polyurethanes and
phenolics
Thermoplastics: Polystyrene,
Polyvinyl Chloride, Polyethylene,
Polypropyelene
Thermoplastics: Polystyrene,
Polypropylene, Polyethylene
Construction, automotive,
furniture, packaging
Food, construction,
packaging, medical
Automotive
In general, thermosets are foamed by means of reactive methodology and
thermoplastics by extrusion or injection [50]. Table 1.1 shows that the cellular
structure in plastics may be produced mechanically, chemically or physically [51].
Thus, blowing agents are generally classified in chemical or physical blowing
agents. Polystyrene is a thermoplastic generally foamed by physical methodologies
using physical blowing agents.
19
Introduction
An ideal physical blowing agent should be environmentally acceptable, nonflammable, adequately soluble, stable in the process, low toxicity, low volatility, low
vapour thermal conductivity, low molecular weight and low cost [52].
Supercritical carbon dioxide (scCO2) has resulted to be a very promising blowing
agent for the replacement of volatile organic compounds especially used in the
production of foams from glassy polymers [53-56]. The main advantage of CO2 is
that it is relatively inert, non flammable, cheap and presents tuneable
physicochemical properties near its critical point (Tc: 31.1 ºC, Pc: 73.8 bar) [57].
Furthermore, CO2 can be used as antisolvent for the polymer and solubilise the
solvent achieving the precipitation of the Polystyrene from its solution on
microcellular foams like [58].
20
Chapter 1
References
[1] J.A. Brydson, Plastics Materials, Elsevier Science, 1999.
[2] P.P. Council, Polystyrene Packaging Council, in: A. Spangenberg (Ed.),
Polystyrene Packaging Council, n.d.
[3] P. Ghosh, Polymer Science and Technology: Plastics, Rubbers, Blends and
Composites, Tata McGraw-Hill, 2001.
[4] A. Azapagic, A. Emsley, I. Hamerton, Polymers: The Environment and
Sustainable Development, Wiley, Guildford, UK, 2003.
[5] D.W. van Krevelen, K. te Nijenhuis, Properties of Polymers: Their Correlation
with Chemical Structure; their Numerical Estimation and Prediction from Additive
Group Contributions, Elsevier Science, 2009.
[6] PlasticsEurope, PlasticsEurope, in: T.P. Portal (Ed.).
[7] C. Borsoi, L.C. Scienza, A.J. Zattera, Characterization of composites based on
recycled expanded polystyrene reinforced with curaua fibers, Journal of Applied
Polymer Science, 128 (2013) 653-659.
[8] T. Zelenović Vasiljević, Z. Srdjević, R. Bajčetić, M. Vojinović Miloradov, GIS and
the analytic hierarchy process for regional landfill site selection in transitional
countries: A case study from Serbia, Environmental Management, 49 (2012) 445458.
[9] S. Brunner, P. Fomin, D. Zhelondz, C. Kargel, Investigation of algorithms for the
reliable classification of fluorescently labeled plastics, in, Graz, 2012, pp. 1659-1664.
[10] A.F. Ávila, M.V. Duarte, A mechanical analysis on recycled PET/HDPE
composites, Polymer Degradation and Stability, 80 (2003) 373-382.
[11] T. Carvalho, F. Durão, C. Ferreira, Separation of packaging plastics by froth
flotation in a continuous pilot plant, Waste Management, 30 (2010) 2209-2215.
[12] K. Inada, R. Matsuda, C. Fujiwara, M. Nomura, T. Tamon, I. Nishihara, T.
Takao, T. Fujita, Identification of plastics by infrared absorption using InGaAsP
laser diode, Resources, Conservation and Recycling, 33 (2001) 131-146.
[13] S.S. Martínez, J.M.L. Paniza, M.C. Ramírez, J.G. Ortega, J.G. García, A sensor
fusion-based classification system for thermoplastic recycling, in, Loughborough,
Leicestershire, 2012, pp. 290-295.
[14] B. Luijsterburg, H. Goossens, Assessment of plastic packaging waste: Material
origin, methods, properties, Resources, Conservation and Recycling, (2013).
[15] W.Z. Choi, J.M. Yoo, E.K. Park, Separation of individual plastics from mixtures
by gravity separation processes, in, San Diego, CA, 2006, pp. 459-468.
[16] V.K. Unnikrishnan, K.S. Choudhari, S.D. Kulkarni, R. Nayak, V.B. Kartha, C.
Santhosh, Analytical predictive capabilities of Laser Induced Breakdown
Spectroscopy (LIBS) with Principal Component Analysis (PCA) for plastic
classification, RSC Advances, 3 (2013) 25872-25880.
[17] J. Anzano, M.E. Casanova, M.S. Bermúdez, R.J. Lasheras, Rapid
characterization of plastics using laser-induced plasma spectroscopy (LIPS), Polymer
Testing, 25 (2006) 623-627.
[18] H. Alter, The recovery of plastics from waste with reference to froth flotation,
Resources, Conservation and Recycling, 43 (2005) 119-132.
[19] K. Hamad, M. Kaseem, F. Deri, Recycling of waste from polymer materials: An
overview of the recent works, Polymer Degradation and Stability, 98 (2013) 28012812.
[20] G. Scott, "Green" polymers, Polymer Degradation and Stability, 68 (2000) 1-7.
21
Introduction
[21] S.M. Al-Salem, P. Lettieri, J. Baeyens, The valorization of plastic solid waste
(PSW) by primary to quaternary routes: From re-use to energy and chemicals,
Progress in Energy and Combustion Science, 36 (2010) 103-129.
[22] M. Al Shrah, I. Janajreh, Mechanical recycling of cross-link polyethylene:
Assessment of static and viscoplastic properties, in, Ouarzazate, 2013, pp. 456-460.
[23] G. De La Puente, U. Sedran, Recycling polystyrene into fuels by means of FCC:
Performance of various acidic catalysts, Applied Catalysis B: Environmental, 19
(1998) 305-311.
[24] V. Wilk, H. Hofbauer, Conversion of mixed plastic wastes in a dual fluidized bed
steam gasifier, Fuel, 107 (2013) 787-799.
[25] W. Kaminsky, M. Predel, A. Sadiki, Feedstock recycling of polymers by pyrolysis
in a fluidised bed, Polymer Degradation and Stability, 85 (2004) 1045-1050.
[26] W.C. Teach, G.C. Kiessling, Polystyrene, Reinhold Publishing Corporation,
1960.
[27] A.L. Andrady, Plastics and the Environment, Wiley, 2003.
[28] J. Brandrup, Recycling and recovery of plastics, Hanser Publishers, 1996.
[29] C.A. Ambrose, R. Hooper, A.K. Potter, M.M. Singh, Diversion from landfill:
Quality products from valuable plastics, Resources, Conservation and Recycling, 36
(2002) 309-318.
[30] F. Vilaplana, A. Ribes-Greus, S. Karlsson, Degradation of recycled high-impact
polystyrene. Simulation by reprocessing and thermo-oxidation, Polymer Degradation
and Stability, 91 (2006) 2163-2170.
[31] C. Gutiérrez, M.T. García, I. Gracia, A. De Lucas, J.F. Rodríguez, A practical
approximation to design a process for polymers recycling by dissolution, Una
aproximación práctica para el diseño de un proceso de reciclado de polímeros
mediante disolución, 68 (2011) 181-188.
[32] J.M. Arandes, J. Ereña, M.J. Azkoiti, M. Olazar, J. Bilbao, Thermal recycling of
polystyrene and polystyrene-butadiene dissolved in a light cycle oil, Journal of
Analytical and Applied Pyrolysis, 70 (2003) 747-760.
[33] Y. Zhang, S.K. Mallapragada, B. Narasimhan, Dissolution of waste plastics in
biodiesel, Polymer Engineering and Science, 50 (2010) 863-870.
[34] Y. Kodera, Y. Ishihara, T. Kuroki, S. Ozaki, Feedstock Recycling of Plastics, in:
M. Müller-Hagedorn, H. Bockhorn (Eds.) Solvo-Cycle Process: AIST's New recycling
process for used plastic foam using plastics-derived solvent, Karlshrue, 2005, pp.
217-222.
[35] A. Karaduman, E.H. Imşek, B. Çiçek, A.Y. Bilgesü, Thermal degradation of
polystyrene wastes in various solvents, Journal of Analytical and Applied Pyrolysis,
62 (2002) 273-280.
[36] S.S. Kim, J. Kim, J.K. Jeon, Y.K. Park, C.J. Park, Non-isothermal pyrolysis of
the mixtures of waste automobile lubricating oil and polystyrene in a stirred batch
reactor, Renewable Energy, 54 (2013) 241-247.
[37] G. Pappa, C. Boukouvalas, C. Giannaris, N. Ntaras, V. Zografos, K. Magoulas,
A. Lygeros, D. Tassios, The selective dissolution/precipitation technique for polymer
recycling: A pilot unit application, Resources, Conservation and Recycling, 34 (2001)
33-44.
[38] D.S. Achilias, A. Giannoulis, G.Z. Papageorgiou, Recycling of polymers from
plastic packaging materials using the dissolution-reprecipitation technique, Polymer
Bulletin, 63 (2009) 449-465.
22
Chapter 1
[39] A.J. Hadi, G.F. Najmuldeen, K.B. Yusoh, Dissolution/reprecipitation technique
for waste polyolefin recycling using new pure and blend organic solvents, Journal of
Polymer Engineering, 33 (2013) 471-481.
[40] E.M. Kampouris, D.C. Diakoulaki, C.D. Papaspyrides, SOLVENT RECYCLING
OF RIGID POLY(VINYL CHLORIDE) BOTTLES, Journal of Vinyl and Additive
Technology, 8 (1986) 79-82.
[41] E.M. Kampouris, C.D. Papspyrides, C.N. Lekakou, A model recovery process for
scrap polystyrene foam by means of solvent systems, Conservation and Recycling, 10
(1987) 315-319.
[42] B.A. Miller-Chou, J.L. Koenig, A review of polymer dissolution, Progress in
Polymer Science (Oxford), 28 (2003) 1223-1270.
[43] B.S. Kong, Y.S. Kwon, D. Kim, Theoretical and experimental analysis of polymer
molecular weight and temperature effects on the dissolution process of polystyrene in
ethylbenzene, Polymer Journal, 29 (1997) 722-732.
[44] J. De Rudder, H. Berghmans, J. Arnauts, Phase behaviour and structure
formation in the system syndiotactic polystyrene/cyclohexanol, Polymer, 40 (1999)
5919-5928.
[45] K. Hattori, S. Shikata, R. Maekawa, M. Aoyama, Dissolution of polystyrene into
p-cymene and related substances in tree leaf oils, Journal of Wood Science, 56 (2010)
169-171.
[46] S. Shikata, T. Watanabe, K. Hattori, M. Aoyama, T. Miyakoshi, Dissolution of
polystyrene into cyclic monoterpenes present in tree essential oils, Journal of Material
Cycles and Waste Management, 13 (2011) 127-130.
[47] M.T. García, G. Duque, I. Gracia, A. De Lucas, J.F. Rodríguez, Recycling
extruded polystyrene by dissolution with suitable solvents, Journal of Material
[48] J.G. Poulakis, C.D. Papaspyrides, Recycling of polypropylene by the
dissolution/reprecipitation technique: I. A model study, Resources, Conservation and
Recycling, 20 (1997) 31-41.
[49] J.B. Bao, T. Liu, L. Zhao, G.H. Hu, X. Miao, X. Li, Oriented foaming of
polystyrene with supercritical carbon dioxide for toughening, Polymer (United
Kingdom), (2012).
[50] S.T. Lee, N.S. Ramesh, Polymeric Foams: Mechanisms and Materials, Taylor &
Francis, 2004.
[51] R. Gendron, Thermoplastic Foam Processing: Principles and Development,
Taylor & Francis, 2004.
[52] D. Klempner, V. Sendijareviʹc, R.M. Aseeva, Handbook of Polymeric Foams and
Foam Technology, Hanser Publishers, 2004.
[53] S.G. Kazarian, Polymer processing with supercritical fluids, Polymer Science Series C, 42 (2000) 78-101.
[54] A.I. Cooper, Porous materials and supercritical fluids, Advanced Materials, 15
(2003) 1049-1059.
[55] D.L. Tomasko, A. Burley, L. Feng, S.K. Yeh, K. Miyazono, S. Nirmal-Kumar, I.
Kusaka, K. Koelling, Development of CO2 for polymer foam applications, Journal of
Supercritical Fluids, 47 (2009) 493-499.
[56] D.L. Tomasko, H. Li, D. Liu, X. Han, M.J. Wingert, L.J. Lee, K.W. Koelling, A
Review of CO2 Applications in the Processing of Polymers, Industrial and
Engineering Chemistry Research, 42 (2003) 6431-6456.
[57] K. Wu, J. Li, Precipitation of a biodegradable polymer using compressed carbon
dioxide as antisolvent, Journal of Supercritical Fluids, 46 (2008) 211-216.
23
Introduction
[58] D.J. Dixon, K.P. Johnston, Formation of microporous polymer fibers and
oriented fibrils by precipitation with a compressed fluid antisolvent, Journal of
Applied Polymer Science, 50 (1993) 1929-1942.
24
Chapter 2
MOTIVATION AND
OBJECTIVES
Motivation and objectives
26
Chapter 2
Motivation
The wide range of applications that Polystyrene (PS) plastics varieties find in the
daily life, makes that its consumption grows continuously. Nevertheless, due to its
resistance to biodegradation, PS wastes have become one of the most important
environmental issues. By this reason, polymer wastes needs to be dealt with to
preserve the natural resources, decrease the hazardous impacts, reduce the amount
of solid waste ending up in landfills, and enhance the sustainability for future
generations.
However, hitherto recycling of the mentioned polymer wastes is not economically
feasible since high costs are implied. The oversupply of virgin PS is the cause of its
low price, moreover, the low density of PS means high volume of wastes and
consequently, major efforts are needed to offset the costs of transport. Also, the low
quality of the recycled product obtained from the traditional methodologies
promotes the lack of a market for the recovered PS.
In view of the aforementioned environmental and economical issues related to the
production of Polystyrene wastes, new processes should be designed with the aim of
reducing the amount of PS wastes, achieving a high-quality recovered product and
succeeding an economically feasible process. Figure 2.1 shows the scheme of the
suggested process.
Terpene Oil/CO 2
PS/Terpene Oil/CO2
Polystyrene
Terpene
Solvents
CO2
PS/Terpene Oil
Dissolution
Figure 2.1. Schematic diagram for the recycling of Polystyrene wastes to produce high-added
value final product by an economically feasible process.
With regard to the Figure 2.1, the general process for the recycling of Polystyrene
wastes can be divided in two main steps: the dissolution of Polystyrene wastes in
27
terpene oils and the separation of the polymer from the solution by CO2 at high
pressure.
The dissolution of wastes in terpenes allows the effectively decrease of the PS
volume, since the density of the oils is higher than those of PS wastes and volume
could be reduced between 5 and 15 times depending on the concentration. Next, the
CO2 solubilises the terpene while the polymer remains changeless except that CO2 is
absorbed among the polymeric chains and is able to tune the polymer structure.
Finally, Polystyrene is recovered as microcellular foam, a very high-added value
product.
Objectives
According to the environmental issues concerning the Polystyrene wastes, the aim
of this Thesis is their recycling by means of an environmentally friendly technology
based in terpene oils and supercritical carbon dioxide. The traditional recycling
methods entail ageing and degradation of the polymer (mechanical recycling),
environmental contaminations (pyrolysis) or even polluting praxis. In this work, a
new technology has been proposed for the recycling process of Polystyrene wastes
with the aim of achieving microcellular foams according to the Green Chemistry
principles.
In order to achieve this general aim, the research work has been structured in the
following particular objectives:
28

Selection of the most suitable terpene solvents to perform the dissolution of
Polystyrene wastes. Study of the influence of temperature, molecular weight
and source on the solubility results.

Determination of the Vapour-Liquid Equilibrium data of the systems
terpene oils/CO2.

Study of the effect of pressure and temperature on the sorption of CO 2 in PS.

Knowledge of the phases equilibrium of the ternary mixtures
CO2/Limonene/PS and CO2/p-Cymene/PS. Limitation of the regions where
the separation could be achieved.

Analysis of the modification of properties under high pressure of the
mentioned ternary systems. Study of viscosity, glass transition temperature
and interfacial tension.

Establishment of the most suitable conditions to perform the precipitation
and foaming processes for the recycling of Polystyrene wastes. Preparation,
characterization and optimization of the foaming process.

Life Cycle and economic assessment of the global process for the recycling of
PS wastes by dissolution in terpene oils and CO2.
Chapter 2
Work plan
According to the objectives and the main goal of this research vocation, the next work
plan was developed.
Recycling of Polystyrene
wastes by dissolution in
terpene oils and scCO2
Binary
Systems
Polystyrene/
Terpene oils
Terpene oils/
scCO2
Ternary
System
Influence of
Pressure
Influence of
Temperature
Polystyrene/
scCO2
Equilibrium of
phases
Glass transition
temperature
Properties
Influence of
Concentration
Foaming
process
Study of Pressure,
temperature, concentration,
contact time and
depressurization time
Interfacial
Tension
Viscosity
Characterization: pore size
distribution, Tdeg, Tg, molecular
weight distribution, amount of
terpene
Optimization
Figure 2.2. Work plan developed along the thesis.
29
Thesis Overview
Chapter 1, shows a general review about the plastic wastes management and
recycling options. Nowadays, many different techniques are being studied since the
optimum process for the recovery of high-added value products from plastic waste
streams is still a challenge. In Chapter 2, the motivation and objectives of the
research study are shown in order to provide the backbone of the thesis. Next,
Chapter 3 details the most relevant experimental setups and procedures to perform
the experiments whose results are shown in the subsequent sections.
The results have been divided in five main chapters: the binary and the ternary
systems, the study of their properties, the foaming process for the recycling of PS
wastes and the Life Cycle and economic Assessment. Chapter 4 is focused on the
study of the binary systems: Polystyrene/Terpene oils, Terpene oils/CO 2 and
Polystyrene/CO2. Also, several properties of the mentioned systems were studied;
particularly, glass transition temperature (Tg) and viscosity (). This issue has been
part of the next papers: A practical approximation to design a process for polymers
recycling by dissolution, in the journal Afinidad; The selective dissolution technique
as initial step for polystyrene recycling, in the journal Waste and Biomass
Valorization; Modeling the phase behaviour of essential oils in supercritical CO 2, in
the journal Industrial & Engineering Chemistry Research and Modification of
Polystyrene properties by CO2: experimental study and correlation in the journal
Chemical Engineering Science. Although it could be thought that once binary
systems are defined, the design of the process would be carried out, the
determination of the mixtures and the evaluation of their properties is crucial to
succeed the global process.
In Chapter 5, two ternary systems (CO2/Limonene/PS and CO2/p-Cymene/PS) were
selected to study their phases equilibrium. Part of the research concerning the
ternary systems and their properties has published in the following papers: Highpressure phase equilibria of Polystyrene dissolutions in Limonene in presence of CO 2
and Determination of the high-pressure phase equilibria of Polystyrene/p-Cymene in
presence of CO2 both in the Journal of Supercritical Fluids.
Chapter 6: and their properties under high pressure. In this case, glass transition
temperature (Tg), viscosity () and interfacial tension (IFT) at high pressure were
studied. The most relevant results have been published in the papers: The effect of
CO2 on the viscosity of polystyrene/limonene solutions and Development of a strategy
for the foaming of polystyrene dissolutions in scCO2, all of them in the Journal of
Supercritical Fluids.
Chapter 7 addresses the precipitation and foaming process of the ternary systems
previously studied. Initially, the feasibility of the separation process was achieved
and next, the study of the variables that influenced on the process was carried out.
The optimization of the foaming process was performed in order to achieve
microcellular foams, it means, minimize the average cells size and its standard
30
Chapter 2
deviation, while the cells density is maximized. The first results were shown in the
paper entitled Development of a strategy for the foaming of polystyrene dissolutions
in scCO2, in the Journal of Supercritical Fluids and Foaming process from
Polystyrene/p-Cymene solutions using CO2 in Chemical & Engineering Technology.
The further study of the foaming process was presented under the title Preparation
and characterization of polystyrene foams from limonene solutions, in the Journal of
Supercritical Fluids.
Finally, in Chapter 8, the Life Cycle and Economic Assessment of the recycling of
PS wastes using terpene oils and CO2 as antisolvent was performed.
31
32
Chapter 3
MATERIALS AND
EXPERIMENTAL
METHODS
Experimental Chapter has been divided in four different sections: materials,
experimental setups, procedures and characterisation. A brief summary of the most
relevant equipments used in this research work are shown below.
Materials and Experimental Methods
34
Chapter 3
3.1. Materials
Polymers:

Polystyrene, Molecular weight 280000 g/mol. CAS # 9003-53-6. Supplied by
Sigma-Aldrich.

Sigma-Aldrich.

Sigma-Aldrich.

Expanded Polystyrene (EPS), Molecular weight: 79309, Polydispersity inde:
1.791. Waste from packaging.

Extruded Polystyrene (XPS), Molecular weight: 89068, IP: 1.845. Supplied
by Tecnove Fiberglass. The composition of the XPS wastes is detailed in
Table 3.1.
Table 3.1. Extruded polystyrene (XPS) waste composition.
Composition
% w/w
XPS
Flame retardant (HBCD)
Nucleating agent (Talc)
Ethyl chloride
Pigments
90-93
2-3
0-1
0-2
0.2-0.3
Solvents:

Carbon Dioxide, purity 99.8 %. CAS # 124-38-9. Supplied by Praxair.

Anisole, purity 99%. CAS # 100-66-3. Supplied by Sigma-Aldrich.

Cinnamaldehyde, purity 98%. CAS # 104-55-2. Supplied by Sigma-Aldrich.

p-Cymene, purity 99%. CAS # 99-87-6. Supplied by Sigma-Aldrich.

Eucalyptol, purity 98%. CAS # 470-82-6. Supplied by Sigma-Aldrich.

Geraniol, purity 97%. CAS # 106-24-1. Supplied by Sigma-Aldrich.

Limonene, purity 97%. CAS # 138-86-3. Supplied by Sigma-Aldrich.

Linalool, purity 95%. CAS # 78-70-6. Supplied by Sigma-Aldrich.

-Phellandrene, purity 95%. CAS # 99-83-2. Supplied by Sigma-Aldrich.

-Pinene, purity ≥ 95%. CAS # 80-56-8. Supplied by MERCK.

-Terpinene, purity 95%. CAS # 99-85-4. Supplied by Panreac.

-Terpineol, purity 98%. CAS # 98-55-5. Supplied by Sigma-Aldrich.
35
Analytical reagants Gel Permeation Chromatography:

Tetrahydrofuran, (THF), CHROMASOLV Plus, purity ≥ 99.9%. CAS # 10999-9. Supplied by Sigma-Aldrich.

Polystyrene Standard for calibration with molecular weight ranged from 370
and 177000, supplied by Waters.
3.2. Experimental setups
3.2.1. Solubility determination at atmospheric pressure
The solubility of Polystyrene in terpene oils was determined by saturation of the
solvents at atmospheric pressure, room temperature (25 ºC) and stirring at 1000
rpm to promote dissolution and homogenization process. The experimental setup is
shown in Figure 3.1.
Figure 3.1. Experimental setup used for the determination of the solubility of Polystyrene in
terpene oils.
5 g of PS pellets or wastes were placed in glass test tubes with 5 ml of terpene
solvents. The tubes were sealed with Parafilm ® to prevent evaporation of solvents.
3.2.2. Variable-Volume High Pressure View Cell
Experimental measurements of the high-pressure phase equilibria were carried out
using a variable-volume cell model ProVis 500 (from Eurotechnica, Hamburg,
Germany), as it is depicted in Figure 3.2 and Figure 3.3.
Figure 3.2 shows the experimental high-pressure view cell used for the
determination of vapour-liquid equilibrium behaviour of binary mixtures
CO2/terpene oils.
36
Chapter 3
T
A
Pressure
Generator
PI
C
B
G-1
TIC
G-2
F
D
F-1
Gas sample
outlet
Liquid sample
outlet
F-2
CO2 inlet
Figure 3.2. Experimental setup employed for high-pressure phase equilibrium in the binary
systems. PG: pressure generator; PI-1: manometer; PI-2: pressure digital indicator; T: liquid
supply tank; TI: temperature digital controller.
The equipment shown in Figure 3.2. consists of a variable-volume cell, supplied with
a front and upper sapphire window and light for visual observation of phase
separation. The cell has a maximum capacity of 50 cm3 and contains a piston
system, consisting in a manual pressure generator, a cylinder and a movable piston
made of Teflon to avoid pressure drops (Valves A and C) only when samples are
taken. The movable piston separates the equilibrium chamber from the pressurizing
circuit. To allow a smooth displacement of the piston inside the cylinder, the piston
was driven by a manual pressure generator and water was used as the pressurizing
fluid. All the system is heated externally by an air bath made of Poly(methyl
methacrylate) capable to resist temperatures up to 80ºC being the temperature
inside the equilibrium chamber measured by a thermocouple (PT100) coupled to a
digital led model testo 925 (Lenzkirch, Germany).
Figure 3.3 shows a schematic diagram of the high-pressure view cell modified for
the measurement of phase behaviour in ternary mixtures containing CO2/terpene
oils/ Polystyrene.
37
T
A
Pressure
Generator
PI
C
B
TIC
D
G-1
G-2
Gas sample
outlet
CO2 inlet
Figure 3.3. Experimental setup employed for high-pressure phase equilibrium in the ternary
systems. PG: pressure generator; PI-1: manometer; PI-2: pressure digital indicator; T: liquid
supply tank; TI: temperature digital controller.
According to Figure 3.3, a loop of 20 cm3 was introduced into the high-pressure view
cell to promote the homogenisation of the ternary mixtures. The gear pump
(Micropump® SerieGAH) provides appropriate stirring inside the cell by the
recirculation of the gas. The rest of the setup was not modified, although in the
determination of the equilibrium of ternary mixtures only gas samples were
withdrawn.
3.2.4. High-Pressure Differential Scanning Calorimetry
Differential Scanning Calorimetry (DSC) was used to determine the influence of
CO2 on the glass transition temperature (Tg) of Polystyrene and its solution in
terpenes at high pressure. A schematic diagram of the experimental setup is shown
in Figure 3.4.
38
Chapter 3
CO2 inlet
V-3
PI
C-1
V-2
V-1
E-1
V-4
V-5
F-1
C-2
P-1
Water
P-2
CO2 out
V-6
I-1
Figure 3.4. Schematic diagram of the experimental setup for the measurement of the glass
transition temperature. V-1, V-2: check valves; E-1: cooler; V-3: purge valve; F-1: filter; P-1:
membrane pump; P-2: syringe pump; V-4, V-6: regulator valves; V-5: micrometric valve; PI:
digital pressure gauge; C-1, C-2: crucibles for sample and reference, respectively; I-1: turbine
flow meter.
The experiments were performed in a SENSYS evo DSC (Setaram, Madrid, Spain)
equipped with two high pressure Inconel crucibles (C-1 and C-2) enabled
measurements up to 400 bar.
3.2.5. Rheometer
A Physica MCR 301 rheometer (Anton Paar, Hertfor, UK) was used to determine
the effect of pressure and temperature on the viscosity of Polystyrene.
7 8 9
. 4 5 6
0 1 2 3
Figure 3.5. Schematic diagram of the experimental setup for the measurement of the
viscosity of Polystyrene at high pressure.
39
The rheometer is equipped with a pressure cell which allows to work at high
pressure, a synchronous motor with air bearing system to stabilize the
measurement and in our case parallel-plate geometry was used to decrease the error
in gap size. A peltier temperature control allows to get an uniform temperature in
the sample up to 200°C. Data were analyzed by RheoPlus software.
3.2.6. Quartz Viscometer
The effect of pressure, temperature and PS concentration on the solution viscosity
was determined using a Quartz Viscometer (Flucon Fluid Control GmbH, Germany).
A schematic diagram is shown in Figure 3.6. The most important element of the
quartz viscometer is a piezoelectric torsion oscillator of SiO 2, which is fed with
alternating voltage of high-frequency and is placed inside the high pressure vessel.
The quartz sensor is located inside a thermo-stated pressure cell and includes an
electric thermostat using aluminium alloy blocks as the medium of heat transfer.
The temperature is controlled by means of an electrically heated thermostat jacket
with a temperature range between 20 and 200 ºC. A thermocouple inside the high
pressure cell allows monitoring and recording of the temperature. The viscometer is
connected to a controlling computer in which the software (QVis) is installed to
allow automation of measurements and the subsequent evaluation of experimental
data.
TI
PI
(a)
CO2 inlet
CO2 outlet
(b)
VI
TIC
Figure 3.6. Schematic diagram of the high-pressure quartz viscometer experimental setup
(a) high pressure stainless steel vessel; (b) quartz sensor. The implementation connected to
the viscometer is PI: pressure transducer; TI: temperature digital sensor; TIC: temperature
digital controller; VI: viscosity indicator.
40
Chapter 3
3.2.3. Pendant droplet High Pressure View Cell
The pendant droplet method, introduced by Andreas and Hauser [1], was the
selected to determine interfacial tension between PS dissolution and CO 2, because it
has proved to be one of the most practicable methods for systems under high
pressure. The experiments were also carried out in a high-pressure view-cell model
PD-E1700 (Eurotechnica GmbH, Hamburg, Germany) containing sapphire windows
to observe the pendant drop, schematic diagram is shown in Figure 3.7. and it has
been previously referenced [2-4]. The equipment can be applied at pressures up to
500 bar and up to 200 ºC.
Solution
Tank
Pressure
Generator
PI
PI
CO2 inlet
Figure 3.7. Experimental setup with a view chamber where is placed the pendant droplet.
The shape of the drop depends on the interfacial tension and on the density
difference of the surrounding fluid phases. Drop shapes were recorded by a
commercial CCD video technique, and later, the images were analyzed by image
processing software called drop shape analysis (DSA) provided by Krüss GmbH.
Density difference between the coexisting fluids (Polystyrene solution and the
surrounding CO2) were calculated to determine interfacial tension according to
Laplace equation of curved fluid surfaces [3].
3.2.7. Foaming setup
Foams were prepared in a home-made batch-type device consisted on a 316
stainless steel high-pressure vessel (350 ml) as it is depicted in Figure 3.8.
41
BPR
CO2 inlet
V-2
PI
V-3
V-4
V-5
V-6
I-1
V-1
F-1
TIC
Figure 3.8. Schematic diagram of the experimental foaming rig. V-1, V-3: check valves; V-2:
purge valve; E-1: cooling system; F-1: filter; BPR: back-pressure regulator; P-1: pump; V-4, V5, V-6: regulator valves; PI: manometer; C-1: foaming vessel; I-1: turbine flow meter; TIC:
temperature digital controller. Green lines represent the refrigerated pipes.
As Figure 3.8 shows, the foaming rig is divided in three modules:
 Pressurization module: where CO2 from the tank is cooled and pumped up to the
operating pressure by a dosing pump (Milton Roy-Milroyal D, France). A purge
valve allows the checking of the CO2 phase before the pumping.
 Main module: the high-pressure stainless steel vessel was heated by an electric
coat digitally controlled.
 Venting module: consists of several regulator valves which allows the control of
the depressurization flow.
3.2.8. Supercritical Antisolvent precipitation setup
The antisolvent precipitation of Polystyrene solutions by CO 2 was performed in a
setup similar to the described in the previous section. Figure 3.9 shows the
experimental setup.
42
Chapter 3
V-2
PI
BPR
V-3
V-4
PI
V-6
V-5
V-1
TIC
V-7
V-8
Figure 3.9. Schematic diagram of the experimental setup for the precipitation of Polystyrene
from its solution. V-1, V-3: check valves; V-2: purge valve; V-4, V-5, V-6, V-7, V-8: regulator
valves; PI: manometer; TIC: temperature digital controller. Green lines represent the
refrigerated pipes and red lines show the heated pipes to balance Joule-Thomson effect.
The general scheme is similar to the described in the section 3.2.7, where three
modules are distinguished. But in this case, the pressurization module is divided in
two parts: CO2 and solution fed. Polystyrene solution is compressed and fed from a
burette by mean of a dosing pump (Milton Roy-Milroyal D, France) through a
stainless steel nozzle of variable diameter (1/16 inch or 500 nm) located at the top of
the high-pressure vessel. Also, the depressurization module is modified, the CO 2 and
the soluble terpene are removed from the vessel through and outlet point at the
bottom and expanded into the liquid solvent recovery vessel. This expansion and the
flow were regulated by a micrometering valve and a bubble flowmeter.
3.3. Experimental Procedures
3.3.1. Determination of the solubility of Polystyrene in Terpene oils
Solubility determination was carried out by thermogravimetric analysis (TA
Instruments-SDT 2960) at a heating rate of 10ºC/min from room temperature to
600ºC under a nitrogen atmosphere. Once the samples were saturated (around 48
43
hours) two samples were withdrawn from the clear saturated solution to minimize
experimental errors. Thermogram analysis obtained, showed two weight losses, the
first one belongs to the solvent boiling (175ºC) and the second one to Polystyrene
decomposition (421ºC).
3.3.2. High Pressure Vapour-Liquid Equilibria of Binary mixtures
Terpenes/CO2
The experimental setup used to determine the Vapour-Liquid Equilibria of binary
mixtures consisting of terpene and CO 2 was shown in Figure 3.2. Terpenes (pCymene, Limonene and -Terpinene) were introduced into the cell that was purged
with CO2 at low pressure to remove the residual air. Then CO 2 was allowed to flow
into the cell (Valve D) and operating pressure and temperature were reached. Once
the temperature was stabilized and the desired pressure was achieved, the magnetic
stirrer was turned on.
To confirm that equilibrium conditions were reached, previous experiments were
carried out taking samples small enough not to affect the global composition of
system at different times. When the solvent concentration in CO 2 remained constant
between two samples in the less favourable conditions (low temperature and
pressure), the time was selected to be sure that the equilibrium was reached and
withdraws the following samples. Once the equilibrium conditions were reached,
stirring was stopped, and the mixture was allowed to repose in order to favour the
separation of the phases.
Samples from the top (Valves F and F1) and the bottom (Valves G1 and G2) of the
equilibrium cell were withdrawn using capillary lines and needle valves and
decompressed to atmospheric pressure. Valves were thermostatized at the same
temperature of the equilibrium cell to avoid condensation. The manual pressure
generator was employed to keep pressure constant during sampling (± 0.01 MPa).
For the determination of the amount of CO2 and terpene in each phase, two samples
were withdrawn from the bottom and the top of the equilibrium cell and expanded
into a glass vial to check the reproducibility. The vials were weighted in a precision
analytical balance with 0.0001 g accuracy to determine the amount of terpene oil
collected. The amount of CO2 was measured during the sampling through a gas
meter.
3.3.3. High Pressure Phase
CO2/Terpenes/Polystyrene
Behaviour
of
Ternary
mixtures
The experimental equipment used to determine the high pressure phase equilibrium
of ternary mixtures was shown in Figure 3.3. Polystyrene and Terpene oil solutions
44
Chapter 3
were prepared in glass vials and introduced into the cell which was purged with CO2
at low pressure to remove the residual air. Then CO 2 was allowed to flow into the
cell (by valve D). Once the temperature was stabilized, the desired pressure was
achieved. After that, the gear pump was stopped, and the mixture was allowed to
repose in order that phase segregation occurred. As it was explained in the
preceding experimental procedure, the time to reach equilibrium conditions was
previously determined.
Samples from the top of the equilibrium cell were withdrawn isobarically through a
six port valve connected to a 20 cm3 loop using capillary lines and needle valves and
decompressed to atmospheric pressure (by Valves F and F1). Valves were
thermostatized at the same temperature of the equilibrium cell. The manual
pressure generator was employed to keep pressure constant during sampling (± 0.1
MPa). For the determination of the amount of solvent solubilised in CO 2, two
samples were withdrawn from the equilibrium cell and expanded into a glass vial,
which was weighted before and after sampling in a precision analytical balance with
0.0001 g accuracy. Limonene was collected in glass vials and separated from the
CO2 by using a trap. The amount of CO2 was measured through a gas meter. The
CO2 density was calculated as a function of pressure and temperature with the
equation of Bender [5].
3.3.4. Glass transition temperature at high pressure
The influence of CO2 on the glass transition (Tg) of Polystyrene and its solution in
terpenes was determined using the experimental setup shown in Figure 3.4.
Initially, the samples (the polymer or its solutions) were placed in the crucibles,
weighted and filled with CO2. Next, CO2 was cooled (in E-1) and pressurized to the
desired level by means of a positive-displacement pump (P-1) and a syringe pump
(P-2) which controlled the feed amount of gas. After that, samples were annealed at
the desired pressure for 24 hours to ensure the total sorption of CO2. Differential
Scanning Calorimetry scans were made using an initial heating at 25 ºC/min up to
120 ºC to release thermal and absorption history of Polystyrene and to provide a
better fit of the polymer in the crucible. The samples were then cooled at the same
rate and annealing for 30 minutes. Tg measurements were carried out during the
second heating, and it was identified from the change in heat flow resulting from a
change in heat capacity at the transition temperature during each scan. Once the
measurements were performed, the vessel was vented by opening a discharge valve
(V-6) and the amount of CO2 was measured by a turbine flow meter (I-1).
45
3.3.5. Determination of the viscosity of Polystyrene
The experimental measurement of the viscosity of Polystyrene at high pressure was
carried out using the rheometer described in Figure 3.5.
The Polystyrene was milled to fill the surface of the cell that was sealed and charged
with CO2 using a syringe pump. The samples where left for 15 min to allow the
diffusion of CO2 into the samples. The shear rate was ranged from 0.001 to 1000
min-1 over 5 min and data points were recorded every 3 seconds. At least three
analyses were run for the different conditions.
3.3.6. Determination of the viscosity of Polystyrene solutions
The study of the influence of pressure, temperature and concentration on the
viscosity of Polystyrene/Terpene oil solutions in the presence of CO 2 was performed
using the quartz viscometer depicted in Figure 3.6. The experimental procedure
requires the Polystyrene/terpene solution to be placed inside the autoclave which
was then filled with CO2. The pressure was measured by a pressure sensor and was
kept constant during each measurement. The major requirement for obtaining
correct results when measuring was to ensure the complete immersion of the quartz
into the solution and to ensure the absence of bubbles in the fluid mixture. This was
achieved by observing the solutions under the same pressures in a view cell. In all
cases, the samples were left for 5 minutes to allow the CO 2 to diffuse and
homogenise into the polymer solution.
A series of standard oils covering a wide range of viscosities from 0.11 to 10000 cP
were used as comparison fluids with known Newtonian behaviour. The calibration of
the quartz viscometer was carried out in a range of temperature from 20 to 100 ºC,
using a mineral oil with low viscosity (D90) and with high viscosity (D7500); the
average standard deviation obtained among the measurements was ± 1.36 mPa·s
and 174.56 mPa·s, respectively.
3.3.7. Determination of high-pressure Interfacial Tension
The measurement of the Interfacial Tension (IFT) of Polystyrene/terpene oil
solutions at high pressure were carried out using the experimental setup shown in
Figure 3.7. The experimental procedure consisted of bringing to the working
temperature the viewing chamber, and then introducing CO2 to the desired
pressure. Next, the drop liquid is generated at the capillary tip using a hydraulic
piston system. When a stable droplet is obtained, it is recorded by the CCD-unit.
Once the droplet shape is digitalized, the software finds the solution that best fits
the experimental drop profile by minimizing the deviation between experimental
46
Chapter 3
and calculated. The result is the value of the IFT at the desired temperature and
pressure.
Drop age was considered to assure that the equilibrium value of the interfacial
tension was kept constant along time (Figure 3.10).
40
35
30
 (mN/m)
25
20
15
10
PS/Cymene C0=0.05 g/ml
5
0 bar
50 bar
0
0
100
200
300
400
500
600
Time (s)
Figure 3.10. Influence of time on the Interfacial tension of Polystyrene/p-Cymene solution at
30 ºC.
As it is observed in Figure 3.10, interfacial tension is reached relatively fast as a
result of a rapid achievement of the equilibrium composition between the drop and
the inter-phase. Since the real mixture densities that resulted from mutual
solubility are not known, the experimental density values for the pure components
as a function of temperature and pressure were used. In case the density must be
corrected after the experiments, the old IFT directly obtained must be divided by the
old density difference. The resulting density-related value is then multiplied by the
new density difference.
3.3.8. Foaming of Polystyrene at high pressure
The foaming of Polystyrene from its solution at high pressure was performed using
the experimental setup described in Figure 3.8. Initially, the polymer solution was
placed in the vessel and it was slightly heated. Next, liquid CO 2 from a stainlesssteel cylinder was cooled (E-1), filtered (F-1), and compressed by a positivedisplacement pump (P-1). The lines were cooled to avoid cavitation of the pump. The
pressure was regulated by a back-pressure regulator (BPR) and checked by a
manometer (PI) before passed to a 350 mL stainless-steel cylinder (C-1) through a
regulation valve (V-4). Maximum operating conditions of the equipment were 120 ºC
and 330 bar. The CO2 stayed inside the vessel during the selected contact time in
47
order to assure that the saturation conditions were reached. The temperature was
kept constant through a digital controller (TIC) which regulated the electric current
by means of a resistance placed around the vessel (± 0.1 ºC). Temperature and
pressure were kept constant during the time of the experiment. At the end of this
period, the vessel was depressurized opening a discharge and a regulator valve (V-5
and V-6, respectively) that was controlled manually by the measurement of the flow
in a turbine flow meter (I-1) in order to induce and control the cell nucleation. Once
the pressure was completely released, the resistance was got away and the highpressure vessel was allowed to cool, and quickly opened up to take out the foamed
samples for subsequent analyses.
3.3.9. Precipitation of Polystyrene by Supercritical Antisolvent
process
The precipitation of Polystyrene from its solution using CO 2 as antisolvent was
performed using the experimental setup described in Figure 3.9. Initially, the highpressure vessel was heated or cooled to the operating temperature. The temperature
was kept constant through a digital controller (TIC) which regulated the electric
current by means of a resistance placed around the vessel (± 0.1 ºC). Next, the CO2
was cooled, pressurized and fed to the vessel at the working pressure. The pressure
was regulated by a back-pressure regulator (BPR) and checked by a manometer (PI).
On the other hand, polymer solution was fed to the pump to be pressurized up to the
operating pressure controlling the flow. Once the conditions (temperature, pressure
and flow) were reached, the CO2 together with the soluble terpene were removed
from the vessel by opening the heated valves V-7 and V-8. The outflow was
controlled manually by mean of a bubble flowmeter and terpenes were condensed in
the liquid solvent recovery glass vessel. Temperature, pressure and flows were kept
constant during the time of the experiment. When the solution was completely fed to
the reactor, valves V-5 and V-6 were closed and CO2 was flushed to the reactor
through the bottom pipe.
3.4. Characterisation
3.4.1. Thermogravimetric Analysis (TGA)
Thermogravimetric analysis (TA Instruments-SDT 2960) was used to:
48

determine the composition of solution (Polystyrene and terpene) after
equilibrium conditions at high pressure were reached.

quantify the amount of residual solvent absorbed on the particles, fibres or
foams.
Chapter 3

calculate the degradation temperature of the Polystyrene.
Figure 3.11. Thermogravimetric balance.
The standard procedure consisted of heat the samples (3-10 mg) at aheating rate of
10ºC/min from room temperature to 600ºC under a nitrogen atmosphere.
3.4.2. Thermogravimetric and Mass Spectrometer Analysis (TGMSA)
The analysis of the gas products from the pyrolysis of Polystyrene was carried out in
a thermogravimetric analyser (TGA-DSC 1; METTLER TOLEDO) coupled to a mass
spectrometer (Thermostar-GSD 320/quadrupole mass analyser; PFEIFFER
VACUUM) with an electron ionization voltage at 70 eV (Figure 3.12).
Figure 3.12. Thermogravimetric and Mass Spectrometer Analysis
The samples were heated from room temperature up to 600 ºC at a heating rate of
10 °C/min. The experiment was performed under argon atmosphere. In order to
identify ions with m/z in the range 0–175, a preliminary broad scan was performed
at a heating rate of 10 ºC/min. Each sample was analyzed at least three times, and
the average value was recorded. The experimental error of these measurements was
calculated, obtaining an error for all studied samples of ±0.5% in weight loss
measurement and ±2 ºC in temperature measurement.
49
3.4.2. Differential Scanning Calorimetry Analysis (DSC)
The differential scanning calorimetry (DSC) was used to determine the glass
transition temperature of the recovered Polystyrene. The apparatus used in the
analysis is presented in Figure 3.12.
Figure 3.12. Differential scanning calorimeter.
It consisted of a calorimeter DSC Q1000 (TA Instruments) equipped with
refrigerated cooling system (RCS) and autosampler. Typical sample weights were 410 mg. All the measurements were run in aluminium hermetic pans under a
nitrogen flow of 50 mL/min. The software used in the analysis was TA Universal
Analysis 2000.
The experimental run consisted of a first heating up to 120 ºC, a cooling up to 0 ºC
and a second heating where the glass transition temperature was determined.
During the three stages, the heating or cooling rate was 10 ºC/min.
3.4.3. Optical Microscopy (OM)
Polymer foams, microparticles and fibres were analyzed and imaged using optical
microscopy in order to determine their pore size and morphology. It is a Carl Zeiss,
model Axio Imager A1 10x and 40x (Figure 3.13), equipped with a camera Nikon
Digital Sight model DS-2Mv which did the microphotographs. The software used to
process the data was Nis Elements BR.
50
Chapter 3
Figure 3.13. Optical microscopy.
A Nikon Eclipse E 200 microscope (Kingston, England) equipped with Nikon
Application Suite Interactive Measurement software was used to analyze the
polymer foams. The optical microscopy technique provides images of the foams with
good resolution and this allows study of the morphology of the recovered polymer.
3.4.4. Scanning Electron Microscopy (SEM)
Scanning electron microscope is an extremely useful tool for the study of the surface
of the samples because it offers a better resolution than the optical microscope. The
surface features of the Polystyrene foams were observed by using a Quanta 250 (FEI
Company) for scan electron microscopy (SEM) (Figure 3.14).
Figure 3.14. Scanning Electron Microscopy.
51
3.4.5. Determination of Molecular Weight
Polymer average molecular weight and its distribution were determined using gel
permeation chromatography (GPC). This technique is included in the size exclusion
chromatography methods and it is the most powerful and widely used in this field.
The average molecular weights resulted from this analysis are in weight (M w), in
number (Mn) and the polydispersity index (PDI).
The apparatus used for this characterization, shown in Figure 3.15, was a
chromatograph (Waters) equipped with two different columns, Styragel HR1 and
Styragel HR4 whose molecular weight interval covered from 100 to 500 000 g/mol. It
is controlled by a computer system supplied with the Breeze software which controls
the entire process but also stores and processes the results.
Figure 3.15. Gel permeation chromatograph.
The analysis conditions are shown in Table 3.15. The polymer solutions were also
filtered before the measurements using a syringe with an attached filter of a pore
size of 0.45 µm. The chromatograph was calibrated using the kit of PS standards
described in Table 3.2.
Table 3.2. Analysis conditions in molecular weight measurements.
Parameter
Solvent
Temperature
Injection volume
Sample concentration
Flow rate
52
Values
THF
35 ºC
100 µL
1.5 mg/mL
1 mL/min
Chapter 3
References
[1] J.M. Andreas, E.A. Hauser, W.B. Tucker, Boundary tension by pendant drops,
Journal of Physical Chemistry, 42 (1938) 1001-1019.
[2] P.T. Jaeger, M.B. Alotaibi, H.A. Nasr-El-Din, Influence of compressed carbon
dioxide on the capillarity of the gas-crude oil-reservoir water system, Journal of
Chemical and Engineering Data, 55 (2010) 5246-5251.
[3] P.T. Jaeger, R. Eggers, H. Baumgartl, Interfacial properties of high viscous
liquids in a supercritical carbon dioxide atmosphere, Journal of Supercritical Fluids,
24 (2002) 203-217.
[4] P.T. Jaeger, J.V. Schnitzler, R. Eggers, Interfacial tension of fluid systems
considering the nonstationary case with respect to mass transfer, Chemical
Engineering and Technology, 19 (1996) 197-202.
[5] E. Bender, Equations of state for ethylene and propylene, Cryogenics, 15 (1975)
667-673.
53
54
Chapter 4
THE BINARY
SYSTEMS
In this Chapter the binary systems involved in the recycling of Polystyrene were
studied individually. Since the recycling process is planned, the main idea is to
maximize the amount of Polystyrene in the solutions. By this reason, a first
screening among the most common terpene oils was studied. Next, the selected
solvents must be soluble in CO2 to recover the terpene and precipitate the polymer.
The Vapour-Liquid Equilibrium (VLE) of the binary mixture CO 2/terpene oils was
determined at mild temperatures. Finally, the influence of CO2 on the properties of
Polystyrene was investigated as initial approach of the global process.
Binary Systems
56
Chapter 4
Based on the papers:
 C. Gutiérrez, M.T. García, I. Gracia, A. De Lucas, J.F. Rodríguez, A practical
mediante disolución, Afinidad, 68 (2011) 181-188.
 C. Gutiérrez, M.T. García, I. Gracia, A. De Lucas, J.F. Rodríguez, The selective
dissolution technique as initial step for polystyrene recycling, Waste and Biomass
Valorization, 4 (2013) 29-36.
 C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. de Lucas, M.T. García, Modeling the
phase behavior of essential oils in supercritical CO2, sent to Industrial &
Engineering Chemistry Research.
 C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. de Lucas, M.T. García, Modification of
Polystyrene properties by CO2: experimental study and correlation, sent to Chemical
Engineering Science.
Graphical abstract
140
T: 25ºC
T: 30ºC
T: 40ºC
Experimental data at 30ºC
Experimental data at 40ºC
120
Pressure (bar)
100
80
60
40
20
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 0.98 0.99 1.00
wCO Liq/Vap
2
Terpene
oils/CO2
Polystyrene/
Terpene oils
CO2/Polystyrene
0.20
0.15
0.10
This work at T: 30 ºC
Density of CO2 at T: 30 ºC
1200
1000
0.15
800
0.10
600
400
0.05
3
0.05
200
Anisole
Linalool
g-Terpinene
Geraniol
Limonene
Eucalyptol
a-Pinene
a-Phellandrene
Cynammaldehyde
p-Cymene
0.00
a-Terpineol
Solubility (g/ml)
0.25
0.20
Sorption (gCO 2/gPS)
0.30
Density of CO 2 (kg/m )
Recycling
of
Polystyrene
wastes
0.35
0.00
0
57
Binary Systems
RESUMEN
El Poliestireno (PS) es uno de los polímeros más comúnmente utilizados debido a su
gran versatilidad, lo que genera una gran cantidad de residuos que es necesario
gestionar de manera adecuada. En este trabajo, se ha propuesto un nuevo proceso
para su reciclado que consiste en la disolución de los residuos en aceites terpénicos y
su posterior separación mediante CO2, ya que éste es capaz de solubilizar a los
componentes terpénicos, pero no al polímero.
En este Capítulo, se muestra el estudio pormenorizado de cada uno de los sistemas
binarios implicados en el proceso. En primer lugar, se estudió la solubilidad del PS
en los aceites terpénicos seleccionados. En este primer apartado, se determinó tanto
teórica, como experimentalmente, la influencia del peso molecular del polímero, la
temperatura a la que se lleva a cabo la disolución y el origen de los residuos en la
solubilidad del polímero.
En segundo lugar, se estudió el equilibrio líquido-vapor entre los aceites terpénicos y
el CO2. Para ello, se llevó a cabo una recopilación bibliográfica que permitiera
generalizar el comportamiento de éstos, para a continuación aplicar modelos
semiempíricos (Chrastil) o teóricos (Ecuación de estado de Peng-Robinson).
Por último, se analizó el efecto del CO2 sobre el Poliestireno, ya que su adsorción
entre las cadenas poliméricas, es la responsable del hinchamiento y la plastificación
del polímero. Debido a este fenómeno, se producen cambios en las propiedades
térmicas y físicas del Poliestireno que repercuten en una disminución de su
temperatura de transición vítrea, su tensión superficial o su viscosidad.
De acuerdo con esto, en este primer Capítulo de resultados, se pretenden sentar las
bases que permitirán conocer mejor el proceso global de reciclado de residuos de
Poliestireno.
58
Chapter 4
ABSTRACT
Polystyrene (PS) is widely used in different applications such as packaging or
insulation materials and consequently enormous volume of wastes are generated. In
this work, a new recycling process has been proposed, consisting of the dissolution of
Polystyrene wastes in terpene oils and its separation using CO2 as solvent of the
terpenes and antisolvent of the polymer.
In this Chapter, a detailed study of each binary system is shown. Firstly, the
solubility of PS in some selected terpenes was studied. In this section, the influence
of the molecular weight of the polymer, the temperature of the solutions and the
waste sources was analyzed theoretically and experimentally over the solubility of
the polymer.
Secondly, the vapour-liquid equilibrium (VLE) of terpene oils in CO2 was
determined. A wide literature review was accomplished with the aim to generalize a
procedure which estimates general trends of terpenes in CO 2. Semi-empirical
(Chrastil’s equation) and theoretical models (Peng-Robinson Equation of State) were
applied to correlate and predict VLE data.
Finally, the interactions between the CO 2 and Polystyrene were analyzed. The
sorption of CO2 into the polymer causes its swelling and plasticisation which
modifies the physical and thermal properties. The decrease of glass transition
temperature, interfacial tension and viscosity was determined experimentally and
correlated with the sorption of the gas.
This Chapter provides the basis of the process for a better understanding of the
different steps during the recycling of PS wastes.
59
Binary Systems
60
Chapter 4
4.1. General Background
4.1.1. Polystyrene/Terpene oils
According to the literature, aromatic solvents are efficient for the solubilisation of
Polystyrene [1-4], but they are not environmentally friendly and would make
difficult the further application of the recycled polymer. The development of a
recycling technology must consider the global benefit, sense and coherence of the
process; by this reason the screening of the solvents should be carried out according
to the principles of the green chemistry. One of the 12 principles of Green Chemistry
is concerned about the “use of safer solvents and auxiliaries”. The devastating effect
of traditional solvents makes not surprisingly the promotion of alternative solvents
[5]. These have low toxicity, are easy to recycle, inert and do not contaminate the
final product.
Nevertheless, there is no perfect green solvent applicable to all processes. But in
this work, we decided to replace the aromatic solvents and/or lubricating oils by
terpene oils. The viability of terpenes to solubilise PS was shown in the literature [611], which turns the dissolution of plastic wastes in an environmentally friendly
technology.
Terpenes are natural compounds originated mainly from plants and in its molecule
at least a part is similar to the isoprene subunit, by this reason they are also
denoted as isoprenoids. They are present in numerous plants and flowers that smell
pleasantly, taste spicy or exhibit pharmacological activities. By these reasons, they
have been used throughout history for a broad variety of purposes including
perfume, medicine, and flavoring [10]. There are more than 30000 terpenes
described in literature, but all of them exhibit the general structure shown below
(Figure 4.1).
H2C
CH2
CH3
Figure 4.1. 2-Methyl-1,3-Butadiene or Isoprene.
In nature, terpenes are found mainly as hydrocarbons, alcohols, aldehydes or
ketones [12]. In accordance with the wide variety of structures presented by
terpenes, the most suitable solvents to carry out the dissolution of PS should
present similar structures to the traditional aromatic solvents. By this reason, the
monoterpenes were selected as the most adequate.
Monoterpenes contain between 10 and 16 atoms of Carbon and are derived from 2,6dimethyloctane. Some of the most well known monoterpenes are /-myrcene,
geraniol, o-/m-/p-cymene, linalool, /-phellandrene, terpinolene, -/-terpynene,
61
Binary Systems
limonene, cymene, etc. They are present in bosil, bay, hops, roses, lemons, oranges,
lavender, mandarins, eucalyptus or orchids, among others. Therefore, it implies that
can be obtained from renewable sources.
The feasibility of the recycling process could be achieved if the terpenes solvent
solubilise selectively the PS, while the rest of the polymers in the waste stream
remain insoluble. In general, the rule “like dissolves like” allows the qualitatively
comprehension of solubility and miscibility. In general, amorphous polymers can be
dissolved in non-polar solvents by mean of ionic and Van der Waals interactions
between them. From a thermodynamic point of view, the formation of solutions from
pure substances is always favoured by entropy since the spontaneous dissolution
process is governed by the free energy of Gibbs [13]. Thus, when two substances are
mixed, the disorder of the binary system increases and the enthalpy should be
minimized in order to achieve an instantaneously dissolution. The energy of mixing
could be represented by the following expression:
[4.1]
where the left term represents the increment in the cohesive energy per unit of
volume; i represents the volume fraction of the substances; 0 the solubility
parameter of the solvent and 1’ is the solubility parameter of the polymer (similar
to the solubility parameter of the monomer which makes it up) [14]. Thus, solubility
parameters () represent the total cohesive energy densities which are the result of
several contributions: non polar, dipole-dipole and hydrogen bonds. They represent
the numerical values for the application of the rule “like dissolves like” [15]. Thus,
Polystyrene will be soluble in those terpenes whose solubility parameters present
similar values to itself. But a first screening could be performed according to the
chemical structure of the polymer and the solvents. In the case of PS, solvents which
exhibit similar solubility parameters will present low tendency to form hydrogen
bonds and relatively low polarity.
The calculation of  could be carried out by approximated [16] or groups contribution
methods [13] although most of the solubility parameters of typical solvents are
summarized in the literature [15]. The estimation of the solubility parameters of PS
and terpenes and the confirmation of its use as predictive tool for the selection of the
most suitable solvents is shown below. Also, the effect of temperature, molecular
weight and source of the PS wastes was studied experimentally and the results are
shown in this section.
62
Chapter 4
4.1.2. Terpene oils/CO2
Compressed supercritical fluids (SCFs) have been proposed as an alternative for
several extraction and purification processes since the separation of essential oils
can be performed at low temperature which makes them suitable to preserve the
thermo labile compounds of the natural oils [17, 18]. The term supercritical fluid is
defined as any fluid which is above both its critical temperature and pressure. A
fluid in this pseudostate combines properties of gases and liquids. Thus, it can
present liquid-like density and no surface tension while interacting with solids
surfaces; but also, it has gas-like low viscosity and high diffusivity. Furthermore, its
solvent power can be easily tuneable by shifting temperature and pressure [19].
Table 4.1 shows the order of typical properties exhibited by gases, liquids, and
supercritical fluids.
Table 4.1. Comparison of the physical properties of gases, liquids and supercritical fluids
[20].
Gases
Supercritical Fluids
Liquids
Density (kg/m3)
1
100-1000
1000
Viscosity (Pa·s)
10
50-100
500-1000
Diffusivity (mm2/s)
1-10
0.01-0.1
0.001
Supercritical carbon dioxide (scCO2) is by far the most widely SCF use and it is
presented as a very interesting substance because its low critical constants (Tc: 31.1
ºC, Pc:73.8 bar) are easily achieved, it is non-reactive, non-toxic, non-flammable, and
available at low cost [21].
Although scCO2 has been shown as a promising and suitable technique for the
fractionation of natural oils due to its excellent properties, there is an important
lack of information concerning the vapour-liquid equilibrium (VLE) data. The
solubility dependence of the terpene oils on pressure and temperature forms the
basis for the design of the conditions in the separator unit [22], but the complexity
of thermodynamic models make difficult the obtaining of generic equilibrium
predictions for multicomponent mixtures [23-25]. Because the predictive capabilities
of solubility models are rather limited, several authors concluded that it is mostly
necessary to determine the solubility experimentally [26].
Although VLE should be obtained experimentally, general trends are observed.
Thus, all pure terpenes behaved similarly on all pressures when subjected to the
different temperatures [24, 27-30]. By this reason, the solubility of terpenoids in
CO2 could be fitted as a function of pressure and temperature in order to generalize
and predict the behaviour of essential oils in a wide range of conditions.
The aim of this section is to provide a simple semi-empirical model to predict the
solubility of essential oils at high pressure on basis of the physico-chemical
properties of their compounds. Vapour-Liquid Equilibrium data of binary systems
63
Binary Systems
(terpenoids/CO2) from the literature or determined in our research group were used
to correlate the semi-empirical model proposed in this work.
4.1.3. Polystyrene/CO2
CO2 has been used in many different applications, such as extraction (as it was
shown in the previous section) but mainly in the field of polymers [31-35] since it is
non-toxic, non-flammable, inexpensive, presents a solvent strength adjustable and
moderate critical temperature and pressure, which makes it an excellent fluid for
polymer processing [32].
But the behaviour of amorphous polymers with gas atmospheres is still nowadays a
source of scientific interest and a search for new industrial applications [36].
Solubility, diffusivity, density and permeability data are very important for
understanding the systems containing a polymer and supercritical CO2 (scCO2) [34].
The gas is diffused between the polymer chains more easily than a large molecule
and its sorption increases the free volume and the polymer segment mobility. The
absorbed CO2 acts as “lubricant”, making easier for chain molecules to slip over one
another and causing polymer softening; by this reason CO2 is known as plasticizer
[37]. There are several factors determining the interaction between the plasticizer
and the polymers: the chemical nature of the polymer and its physical state, its
crystallinity and degree of crosslinking, the intermolecular forces between the
polymer and the gas and the molecular size of the diluent (CO 2) [35, 38].
An increase of CO2 concentration promotes swelling and changes in the mechanical
and thermal properties of the polymer as a consequence of the swelling and
plasticization [36, 39]. As it was mentioned, plasticization causes shifts in the
properties of the polymer that results in the decrease of the rigidity at room
temperature, depression of the glass transition temperature (Tg), increase of
elongation, flexibility and toughness [40, 41].
According to the explained phenomena, for the rational design of processes and
applications, the study of the behaviour of the system polymer/CO 2 and the
knowledge of the mechanical and physical property changes is crucial. For instance,
nucleation of foams using CO2 takes place at temperature close to the Tg [42]. The
selection of the most suitable conditions to perform the purification of polymeric
matrix is determined by the influence of pressure and temperature on the behaviour
of the binary system [37]. The control over the properties and design of functional
materials is influenced by the gas sorption into the polymer [36, 43]. Also, reliable
high pressure rheological data are essential for the application of CO 2 to induce
plasticization in the industrial plastics processes [40, 44].
The gas solubility in the polymer can be described by Henry’s law:
c=H·P
64
[4.1]
Chapter 4
where c is the concentration of the gas in the polymer, H is Henry’s law constant
and P is the equilibrium gas pressure. Henry’s law is only valid on the ideal solution
state and in the case of diluted solutions because it does not consider the
interactions between the gas and the polymer. Although, it is not recommended its
use at high pressure, it has been widely used due to its simplicity. As an alternative,
CO2 sorption in amorphous polymers is described using the Dual-Mode model [36,
45]. Dual Mode Sorption Model is composed of Henry’s law dissolution in the
equilibrium region (low pressures) and Langmuir-type sorption in a non-equilibrium
region [46]. The non-equilibrium region is related to the excess free volume or
unrelaxed free volume in a glassy polymer resulting from the presence of
microcavities capable of retaining solute molecules[47]. The Dual Mode sorption
model describes the solubility in the isotherm according to the expression shown
below:
[4.2]
where S is the sorption of CO2 in the polymer, kH is analogous to Henry’s law
constant, P is the pressure, C’H is the saturation of the cavities and b represents the
affinity between the solute molecules and the Langmuir sites present in the
polymeric matrix.
On the other hand, the Sánchez-Lacombe Equation of State (EoS) has been also
used to correlate the sorption of CO 2 in PS [48] since it is the most widely used
thermodynamic model to describe the behaviour of polymer/gas solutions. It is based
on the classical lattice-fluid theory extended with vacancies in the lattice in order to
account for compressibility. The equation of state for pure fluids is shown below:
[4.3]
where ,
, and represents the reduced density, pressure and temperature,
respectively. The reduced parameters are defined from characteristic fluid
parameters:
[4.4]
[4.5]
[4.6]
[4.7]
[4.8]
[4.9]
[4.10]
[4.11]
[4.12]
[4.13]
where P*, T*,* and v* are the characteristic pressure, temperature, close-packed
mass density and molar volume, respectively and they are typically fit to vapour
pressure or liquid density data;  is the closest distance allowed between two mers, 
is the mer interaction energy, v0 is the hole volume, R is the gas ideal constant, MW
is the molecular weight, v* is the hard-core molecular volume and r is a size
parameter. The lattice fluid model for mixtures is obtained from the application of
the mixing rules.
65
Binary Systems
[4.14]
[4.15]
[4.16]
where i is the close-packed volume fraction of component i in the mixture. It is
related to mass fraction (wi) by:
[4.17]
The parameter Xij is calculated from a geometric mean:
[4.18]
where kij is an adjustable binary interaction parameter essential in accurately
modelling the mixture behaviour, and on understanding the affinity of gas
molecules to the polymer. It is obtained from the correlation of experimental data
and its aim is the correction for the deviation of the mixture.
Results have been divided in two main blocks. Initially, the sorption of CO 2 was
determined experimentally and correlated using Sánchez-Lacombe and Dual-Mode
model. Next, the influence of the CO 2 sorption on the modification of the PS
properties was studied. The sorption of CO 2 in Polystyrene produces polymer
swelling [49-51], large depression in glass transition temperatures [42, 52, 53],
interfacial tension [54, 55] and viscosity [56].
4.2. Polystyrene/Terpene oils Results
4.2.2. Determination of the solubility parameters
Many methods have been developed for the determination of the solubility
parameter, from theoretical calculation, to empirical correlations [16]. In this work,
the groups contribution method was selected because it allows the calculation of the
partial contribution to the solubility parameters, which helps to a better
understanding than the global solubility parameter. Thus, the global solubility
parameter is composed by:
[4.19]
where d is the energy from dispersion forces, p is the energy from dipolar
intermolecular forces and h is the energy due to the hydrogen bonds between
molecules. There are several authors who estimated the different groups
contribution to the solubility parameters. Among the best well known, are Small
[57], Hoy, van Krevelen and van Krevelen and Hoftyzer [58, 59]. In this work,
66
Chapter 4
values shown by Hoy were selected to calculate the individual solubility parameters
because they fitted pretty well the estimation and the experimental  shown in the
literature for a few terpenes. Furthermore, it allows the calculation of the partial
solubility parameters of the Polystyrene since his method included a corrective term
for non-ideality, to be applied in the case of polymers [60]. Table 4.2 shows the
predicted solubility parameters for the selected terpene oils according to Hoy’s
method.
Table 4.2. Solubility partial parameters of the selected terpene oils.
Solvent
d(MPa1/2) p(MPa1/2) h(MPa1/2)
Anisole
16.82±0.98 10.15±6.05 9.14±2.44
p-Cymene
17.95
6.88
4.59
Cinnamaldehyde
15.80±3.60 12.10±0.30 10.26±4.06
Eucalyptol
15.01±0.89 7.41±3.51 9.08±5.68
-Phellandrene
15.93
5.68
7.14
Geraniol
14.76
8.38
12.11
Limonene
15.93
4.85
6.59
Linalool
14.02±2.28 7.72±3.32 11.28±0.08
-Pinene
16.48
6.87
7.33
-Terpinene
16.45
5.88
6
14.08±0.08 7.47±0.53 11.72±1.52
-Terpineol
Experimental values
1 -Terpineol
2 Cinnamaldehyde
3 Eucalyptol
4 Linalool
d(MPa1/2)
14.00
19.40
15.90
16.30
p(MPa1/2)
8.00
12.40
3.90
4.40
h(MPa1/2)
10.20
6.20
3.40
11.20
67
Binary Systems
According to Table 4.2 the predicted and experimental vales of d and h are very
similar; however the most important deviations are due to the energy due to the
hydrogen bonds. In order to establish a relationship between the solubility
parameters of the terpene and the solubility of PS, Hansen proposed that a polymer
will be soluble in a solvent if  of the solvent is within its solubility sphere [15].
Thus, the knowledge of the radius and the solubility parameters of the Polystyrene
are necessary in order to fix the central point of the sphere.
The establishment of the solubility parameter of PS is not so evident since a wide
variety of characteristics are involved in its structure (molecular weight,
polidispersity index, crystallinity, tacticity or crosslinking) and consequently, they
will influence on its solubility. According to the literature, the solubility parameters
shown by Szydlowski et al. [61] and the interaction ratio calculated by Bernardo and
Veseley [62] presented similarities to the PS used in this work (Table 4.3).
Table 4.3. Solubility partial parameters of Polystyrene from the literature [61].
Polystyrene
d(MPa1/2)
p(MPa1/2)
h(MPa1/2)
17.90
4.20
5.00
Figure 4.2 shows the graphical methodology established by Hansen [15] to predict
the ability of terpenes to dissolve PS, where the markers represent the intersection
of the partial solubility parameters and the sphere the interaction ratio of the
polymer.
p-Cymene
15
-Terpineol
Menthol
Cinnamaldehyde
-Pinene
-Phellandrene
Eucalyptol
Geraniol
Limonene
Linalool
-Terpinene
Anisole
Polystyrene
12
p
9
6
3
0
0
5
10
15
d
20
25
3
30
0
6
9
12
15

h
Figure 4.2. Prediction of PS solubility in terpene oils from the solubility parameters shown
in Table 4.2 for terpenes.
As it was mentioned, the ability of solvents to dissolve a polymer is indicated by the
proximity between the polymer and the studied solvent. In Figure 4.2, the solubility
parameter of PS is placed in the centre of the sphere of radius h. If the solubility
68
Chapter 4
parameters of the terpene solvents remain inside the sphere, the terpene is
considered as good solvent. Whereas, if the solubility parameters of the solvents are
placed outside, it is may be assumed that the studied terpene will not dissolve the
polymer [63]. According to Figure 4.2, the most suitable solvents to perform the
recycling of PS by dissolution are p-Cymene, Limonene, -Terpinene, Phellandrene and -Pinene.
The solubility of PS in the selected monoterpenes was determined experimentally to
check the feasibility of the theoretical methods for the prediction of the polymer
dissolution. Table 4.4. shows the values of experimental solubility of PS in the set of
terpene oils at 25 ºC.
Table 4.4. Experimental solubility of PS in terpene oils at 25 ºC.
Solvent
Solubility (g/ml)
Solvent
Solubility (g/ml)
p-Cymene
0.3529
-Terpineol
0.0063
Anisole
0.2578
-Pinene
0.2901
Cinnamaldehyde
0.1957
-Phellandrene
0.2879
Linalool
0.0059
Geraniol
0.0029
-Terpinene
0.2427
D-Limonene
0.2473
Eucalyptol
0.2977
According to Table 4.4, the most suitable solvent to carry out the dissolution process
agrees with the predicted theoretically (Figure 4.2). p-Cymene is the most suitable
solvent to carry out the dissolution process, because presents the greatest solubility
values. Moreover to dissolve easily the PS, a good solvent to carry out its recycling
should show high volatility, which allows its removal from the polymer. Also it is
highly appreciated that the terpenes present low cost, low toxicity and easy
availability [64]. The strong interactions between PS and some of the cited solvents
(those who exhibited high solubility values) could prevent its complete removal from
the polymer-rich solution [65, 66]. According to the described reasons, the most
suitable terpene oils for the recycling of PS wastes should be: p-Cymene, Terpinene, Limonene, -Pinene and -Phellandrene. The subsequent sections will
be focused on the dissolution of PS in the mentioned terpene oils.
4.2.3. Influence of the Molecular Weight on the Solubility of
Polystyrene
The molecular weight of a polymer is an intrinsic property with a great influence on
the properties of their solutions. The activity coefficient at infinite dilution (Ω ∞1)
could be useful to predict the phase equilibrium over the whole concentration range
of a polymer solution. The influence of molecular weight on the activity coefficient at
infinite dilution is predicted using Equation 4.20 [67].
69
Binary Systems
ln 1  1 
MW1
 MW1·12
MW2
[4.20]
where MW1 is the molecular weight of the solvent, MW2 is the molecular weight of
the polymer and 12 is the binary interaction parameter, which shows the physical
interaction between polymer and solvent, it was optimized from experimental
solubility data, obtaining a value of 0.3220. If experimental data are not available,
12 could be approximated to 1 [15], particularly for systems where dispersion forces
dominate over polar and hydrogen-bonding ones, but it systematically overestimates
the infinite dilution solvent activity coefficients [68]. This indicates that better
results may be obtained with lower 12 values as we present.
From a theoretically point of view, the relationship between PS molecular weight
and the activity coefficient at infinite dilution should be approximately constant in
the range of molecular weight studied. Figure 4.3 shows the theoretical relationship
between activity coefficients at infinite dilution versus MW, the interval includes
from styrene monomer (104.1 Da) to 108 Da molecular weight.
4.0
3.5
3.0


2.5
2.0
p-Cymene
-Terpinene
D-Limonene
-Pinene
-Phellandrene
1.5
1.0
0.5
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
10
9
Molecular Weight (g/mol)
Figure 4.3. Evolution of the activity coefficient at infinite dilution as a function of Molecular
Weight.
Figure 4.3 can be divided in two regions. the first one belongs to low molecular
weight (100-5000 g/mol) where the increase of Ω∞1 is directly proportional to the
polymer molecular weight [69, 70], and the second corresponding to higher
molecular weights (> 10000 g/mol), where Ω∞1 remains practically constant.
According to the observed trend, solubility does not decrease when polymer
molecular weight increases in the range of study [71, 72].
70
Chapter 4
In order to determine the real effect of PS molecular weight on the solubility in
terpenes, three commercial samples were used. Generally, the average molecular
weight of the common PS available in plastics is ranged between 150000 and 400000
g/mol [73]; in this work the samples had molecular weight values of 192000, 280000
and 350000 g/mol (Figure 4.4).
0.45
Solubility (g/ml)
Mw: 192000 g/mol
0.40
Mw: 280000 g/mol
0.35
Mw: 350000 g/mol
0.30
0.25
0.20
0.15
0.10
0.05
0.00
p-Cymene
Terpinene
Limonene
Pinene
Phellandrene
Terpene solvents
Figure 4.4. Experimental solubility determined as a function of the Molecular Weight of PS
at 25 ºC.
Figure 4.4 shows the effect of molecular weight on the solubility of PS in the
previously selected terpenes. It is observed that solubility remains practically
constant over the whole studied range since the theoretical prediction showed that
molecular weight does not influence significantly from 105 g/mol. By this reason,
several authors have established that molecular weight is not the most relevant
variable during the polymer dissolution process.
4.2.4. Influence of the Temperature on the Solubility of Polystyrene
It is generally known that an increase on temperature produces an increase in the
solubility. This fact is explained according to the thermodynamic rule which says
that a substance is soluble in a solvent if the Gibbs free energy is negative. Flory–
Huggins theory [74, 75] describes the thermodynamic equilibrium for polymer and
solvent binary systems at constant pressure, as it is shown in Equation 4.21.

2
 Gm   1
 R  T    V ln 1  V ln 2  121 2 

  1
2

[4.21]
71
Binary Systems
where T is the temperature, R is the ideal gas constant,  is the volume fraction of
each component in the mixture, V is the molar volume of the two components, 12 is
the Flory–Huggins interaction parameter expressing the interaction enthalpy
between two different molecules, the subscript 1 represents the solvent and 2
represents the polymer. The first two terms on the right side of this equation denote
the entropy of mixing. which quantitatively is negative[76], since in solution the
molecules display a more chaotic arrangement than in the solid state. Enthalpy of
mixing (ΔHm) at constant pressure for polymer–solvent mixtures can be calculated
as:
H m  1212
[4.22]
where the 12 parameter expresses the interaction enthalpy between two different
molecules. This parameter is always positive, opposing the negative entropy effect of
mixing [77], thus the lower values of 12 show greater results in the solubility as it
can be checked in Figure 4.5, where the influence of temperature over 12 is
depicted.
0.8
0.14
0.12
p-Cymene
-Terpinene
D-Limonene
-Pinene
-Phellandrene
0.7
0.6
Solubility (g/ml)
0.10
X12
p-Cymene
-Terpinene
D-Limonene
-Pinene
-Phellandrene
0.08
0.06
0.04
0.5
0.4
0.3
0.2
0.02
0.1
0.00
0.0020
0.0025
0.0030
1/T (K-1)
0.0035
20
30
40
50
60
70
Temperature (ºC)
Figure 4.5. (Left) Prediction of the influence of the temperature on 12. (Right) Effect of
temperature on PS pellets solubility obtained experimentally.
As it is observed in Figure 4.5 (left). there is a linear relationship between 12 and
the inverse of temperature [78]. Accordingly, when the temperature increases, the
value of 12 decreases, and therefore, as the temperature increase, dissolution
process is favoured.
72
Chapter 4
Furthermore, the effect of temperature on the solubility was investigated
experimentally in the temperature range from 25 to 60 ºC (Figure 4.5 right), higher
temperatures were not used to avoid polymer degradation. PS sample selected was
280000 g/mol because as it was explained it is not the most important characteristic
to carry out the dissolution process, so it was decided to select the average molecular
weight among the previously tested samples. As it is observed, an increase on
temperature produces an increase in the solubility in all the solvents studied.
Nevertheless, the enhancement of solubility due to the temperature increase should
be compared with the energetic costs in order to decide the most economically
valuable way to dissolve the PS. In this work, room temperature or slightly higher
were selected as the most convenient to manage PS wastes by dissolution.
4.2.5. Applicability to real Polystyrene wastes
Polystyrene is processed on different ways to adapt the market needs. Generally, PS
wastes can be found as expanded (EPS) or extruded (XPS) Polystyrene, which
provide a wide applicability field. The processing of polymers could affect to their
dissolution in terpene oils since the dissolution process can be affected by the chain
chemistry, composition and stereochemistry [50]. The dissolution of a polymer into a
solvent is not immediate and involves two transport processes, namely solvent
diffusion and chain disentanglement. Initially, the solvent begins its aggression by
pushing the swollen polymer substance into the solvent, and, as time progresses, a
more dilute upper layer is pushed in the direction of the solvent stream. Further
penetration of the solvent into the solid polymer increases the swollen surface layer
until, at the end of the swelling time, a quasistationary state is reached where the
transport of the macromolecules from the surface into the solution prevents a
further increase of the layer [51]. The way in which the wastes are processed could
affect the dissolution process.
Since the previous parts of this study were performed using pellets of PS, in this
section, the influence of different sources on the solubility of PS in terpene oils was
evaluated (Figure 4.6.)
73
Binary Systems
0.40
XPS
PS Pellets
EPS
0.35
Solubility (g/ml)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
p-Cymene
Terpinene
Limonene
Pinene
Phellandrene
Terpene solvents
Figure 4.6. Influence of the origin on the solubility of PS and PS wastes in terpene oils at 25
ºC.
According to Figure 4.6, the solubility of PS from different sources remains almost
constant independently of their origin. As it was observed in the previous section
entitled “Influence of Molecular Weight”, when the molecular weight of the polymer
exceeds 105 g/mol, does not affect on the solubility. In the case of commercial
polymers, particularly PS, molecular weight is always higher than the mentioned
value and consequently, solubility remains almost constant, as it was expected.
Nevertheless, differences regarding the dissolution process were observed and by
this reason, the minimum time required to dissolve PS pellets was studied. Figure
4.7 shows the thermograms which represent the amount of polymer solubilised in
Limonene in the samples withdrawn at different times from 10 seconds to 24 hours.
According to Figure 4.7, the amount of polymer dissolved in the case of PS (left)
increases slightly during the first 60 minutes, but 24 hours are necessary to
solubilised fully the pellets. Similar behaviours were observed with the others
terpene oils, which required almost 24 hours to dissolve completely PS pellets due to
its high compression grade. On the other hand, Figure 4.7 right represents the
amount of XPS dissolved in Limonene.
74
Chapter 4
100
80
70
60
XPS 10 sec
XPS 30 sec
XPS 60 sec
XPS 5 min
XPS 15 min
XPS 60 min
XPS 24 h
90
80
70
Weight (%)
90
Weight (%)
100
PS 10 sec
PS 30 sec
PS 60 sec
PS 5 min
PS 15 min
PS 60 min
PS 24 h
50
40
60
50
40
30
30
20
20
10
10
0
0
0
100
200
300
400
500
0
100
Temp (ºC)
200
300
400
500
Temp (ºC)
Figure 4.7. Evolution of PS and XPS solubilised in Limonene by thermogravimetric analysis.
Temperature: 25 ºC; stirring: 500 rpm.
The dissolution of XPS wastes increases gradually when time increases, although 24
hours are required to achieve a fully solubilisation. Billmeyer [14] explained that
the dissolution process of the polymers as a slow process that occurs in two stages.
Figure 4.8 summarizes the evolution of the solubility of PS pellets as a function of
time in the five terpene oils. The tendency shows that during the initial 100 seconds
PS in pellets is practically insoluble in the solvent, but since that time, the
dissolution process starts following an exponential behaviour.
0.5
0.5
PS pellets
p-Cymene
-Terpinene
D-Limonene
-Pinene
-Phellandrene
0.3
0.4
Solubility (g/ml)
Solubility (g/ml)
0.4
XPS wastes
p-Cymene
-Terpinene
D-Limonene
-Pinene
-Phellandrene
0.2
0.1
0.3
0.2
0.1
0.0
1
10
100
1000
Time (s)
10000
100000
1
10
100
1000
10000
100000
Time (s)
Figure 4.8. (Left) Time influence in PS pellets solubility and XPS wastes solubility (right).
The higher porosity and surface area of the foam make easier the sorption of the
solvent in the polymer, and hence the dissolution process is faster. From the
gathering of Figures 4.6, 4.7 and 4.8 is possible to assert that results obtained for PS
75
Binary Systems
pellets are applicable to XPS and EPS. Firstly, solvent molecules slowly diffuse into
the polymer to produce a swollen gel. This may be all that happens if, for example,
the polymer-polymer intermolecular forces are high because of crosslinking,
crystallinity, or strong hydrogen bonding. But if these forces can be overcome by the
introduction of strong polymer-solvent interactions and the second stage of solution
can take place. Here the gel gradually disintegrates into a true solution and only
this stage can be materially speeded by agitation [79].
To sum up, terpene solvents present excellent characteristics for PS recycling due to
their low toxicity, cost and high solubility, that has been checked experimental and
theoretically. Moreover, in the range of molecular weight studied, the solubility of
the polymer remains constant independently of it and as it was expected,
temperature favours the solubility of the polymer. Finally, theoretical results can be
applied to real wastes which confirm the feasibility of the dissolution of PS wastes
in terpenes as initial step for its recycling.
4.3. Terpene oils/CO2 Results
4.3.1. Procedure
The vapour phase was correlated following the traditional Chrastil’s equation [80]
which is based on the hypothesis that each molecule of solute (terpene) associates
with k molecules of supercritical solvent (CO2) to form a solvate-complex, which is in
equilibrium with the system.
[4.23]
where S is the solubility in [kg/m3], C1 is a constant dependent on the molecular
weights of the solute and solvent and on the association constant, C2 is a constant
dependent on the total heat of vaporization, T is the temperature in [K], k is the
association number of molecules in the solvate-complex and  is the CO2 density in
[kg/m3].
The new empirical relationship proposed in this work for the correlation of the
liquid phase was:
[4.24]
where xCO2 is the mole fraction of CO2 in the liquid phase, Ai are the correlated
variables, P is the pressure in [bar] and T is the temperature in [K].
Initially, the calibration of Ai, Ci and k was carried out from experimental data
obtained from the literature or in our research group (Table 4.4). They were
calculated by performing a multiple linear regression and minimising the sum of the
square differences between experimental and calculated solubility. Although there
are some binary systems well described in the literature, there is also an important
76
Chapter 4
deficiency on VLE data of minority or non common compounds of the essential oils.
The presence of the cited components could modify significantly the solubility of the
terpenoids in CO2. By this reason, the fitted constants (Ai, Ci and k) were related
with the physical properties of the terpenes in order to get an empirical relationship
which allows the prediction of new components whose VLE data in CO2 were not
previously described in the literature.
Table 4.5 shows a good sample of the available data from the literature about the
equilibrium of the selected terpenoids in CO2 as a function of pressure and
temperature. VLE measurements of terpenoids were carried out in a pressure range
from 7.8 to 133.4 bar and in a temperature range from 303 to 343 K. Different
methods could be employed to determine the vapour-liquid equilibrium for the
binary systems containing CO2/terpenoids, but according to the literature the static
method was the most commonly used [24, 27-29].
Table 4.5. Chemical structure, range of pressure, temperature, experimental method and
source for the binary system terpenoids/scCO 2.
Temperature
(K) range
Pressure
Number
(bar) range of Data
Carvone
Method
Ref
303.55-324.05
18-96
30
Dynamic VLE
Cell with
stirring
[27]
303.15-343.15
1.9-110.7
27
Static VLE
Cell with
stirring
This
work
318.2-323.2
77.5-98.0
15
Static VLE
Cell
[28]
313.18-333
7.8-110.6
29
Static VLE
Cell with
recirculation
[29]
313.78-333.25
8.3-110.0
29
Static VLE
Cell with
recirculation
[29]
313-333
62.9-109.3
24
Static VLE
Cell
[24]
p- Cymene
Eucalyptol
Linalool
Limonene
-Pinene
77
Binary Systems
Table 4.5 (Continued). Chemical structure, range of pressure, temperature, experimental
method and source for the binary system terpenoids/scCO 2.
Temperature
(K) range
Pressure
Number
(bar) range of Data
-Terpinene
313.15-343.15
4.9-110.5
26
Method
Ref
Static VLE
Cell with
stirring
This
work
This semi-empirical relationship, proposed in this work, provides an important tool
for the generalization and prediction of minority species which will enhance the
estimation of the global phase behaviour of the essential oils. The equation which
allows the calculation of Ai, Ci or k constants from the physical properties of the
terpenoids could be generalized for the three constants:
[4.25]
where Zi means Ai, Ci or k; zi,j means ai,j, ci,j or ki (respectively) and the subscript i
means 1 or 2 and the subscript j varies from 1 to 5. The properties of the terpenes
are: MW is the molecular weight in [g/mol], Tb is the boiling temperature in [K],  is
the density in [kg/m3] and  is the solubility parameter calculated following the
method previously described in the literature and in the previous section of this
Chapter [13].
Once the composition of the essential oils is defined, the prediction of the vapourliquid equilibrium data for the essential oils in carbon dioxide could be achieved
with the simply knowledge of the described properties. The essential oil solubility
will be the weighted average of the individual solubility of each of the terpene in
CO2. The global procedure is described in Figure 4.9.
78
Chapter 4
Figure 4.9. Process scheme to correlate and predict essential oil equilibrium data.
Validity of the predictions was checked by comparison with literature data
concerning ternary systems (Limonene + Linalool + CO 2) and real mixtures (Lemon
and orange oils + CO2) which contain compounds whose VLE data are not described
in the literature.
4.3.2. Literature
Terpenoids/CO2
data
and
correlation
of
binary
systems
Essential oils are a complex mixture of terpenes or their derived compounds but
data for natural and well characterized oils mixtures with supercritical CO 2 are
scarce in the literature. Our approach considered that essential oils consisted
mainly of Limonene, Linalool, -Terpinene, Carvone, -pinene, p-Cymene and
Eucalyptol. It is necessary to emphasise that large database for CO 2 and the cited
terpenoids are available in the literature which could enhance the accuracy of the
calibration of the constants, but also the dispersion could be responsible of
deviations in the estimation of parameters
Figures 4.10-4.16 show the experimental data (markers) and the correlated values
(lines) of solubility of binary systems CO2/terpenoids at different temperatures.
From the correlation of the experimental data following eq. [4.6] and [4.7], the
79
Binary Systems
constants Ai, Ci and k were obtained. Table 4.6 shows the values for the fitting
constants employed in the solubility modelling of terpenoids using the semiempirical equation. In all cases, the deviations were below the constants values.
120
20
T: 303.5 K
T: 313.5 K
T: 323.5 K
16
Solubility (kg Carv/m CO2)
100
T: 313.5 K
18
14
Pressure (bar)
3
80
60
40
20
12
10
8
6
4
2
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0
xCO2
50
100
150
200
250
300
3
Density of CO2 (kg/m )
Figure 4.10. Solubility of Carvone in CO2. Markers represent experimental data at 303.5 K
(■), 313.5 K (●) and 323. 5 K (▲) obtained from [27] and lines the correlation at 303.5 K (—),
313.5 K (—) and 323.5 K (—).
120
20
T: 313.15 K
T: 323.15 K
T: 343.15 K
Solubility (kg p-Cym/m CO2)
100
16
14
3
80
Pressure (bar)
T: 313.15 K
T: 323.15 K
T: 343.15 K
18
60
40
20
12
10
8
6
4
2
0
0
0.0
0.2
0.4
0.6
xCO2
0.8
1.0
0
50 100 150 200 250 300 350 400 450
3
Figure 4.11. Solubility of p-Cymene in CO2. Markers represent experimental data at 313.5 K
(■), 323.5 K (●) and 343. 5 K (▲) and lines the correlation at 313.5 K (—), 323.5 K (—) and
343.5 K (—).
80
Chapter 4
120
35
T: 318.2 K
T: 323.2 K
30
Solubility (kg Euc/m CO2)
110
100
25
3
Pressure (bar)
T: 318.5 K
T: 323.5 K
90
80
70
15
10
5
0
20
15
10
5
0
0.0
0.2
0.4
0.6
0.8
1.0
0
xCO2
50 100 150 200 250 300 350 400
3
Figure 4.12. Solubility of Eucalytpol in CO2. Markers represent experimental data at 318.2
K (■), and 323.2 K (●) from [28] and lines the correlation at 318.2 K (—), 323.5 K (—).
20
T: 313.8 K
T: 323.2 K
T: 333.2 K
120
16
Solubility (kg Lim/m CO2)
100
3
80
Pressure (bar)
T: 313.8 K
T: 323.2 K
T: 333.2 K
18
60
40
20
14
12
10
8
6
4
2
0
0
0.0
0.2
0.4
0.6
xCO2
0.8
1.0
0
50 100 150 200 250 300 350 400 450
3
Figure 4.13. Solubility of Limonene in CO2. Markers represent experimental data at 313.8 K
(■), 323.2 K (●) and 333.2 K (▲) and lines the correlation at 313.8 K (—), 323.2 K (—) and
333.2 K (—).
81
Binary Systems
20
T: 313.8 K
T: 323.2 K
T: 333.2 K
120
16
Solubility (kg Lin/m CO2)
100
3
80
Pressure (bar)
T: 313.8 K
T: 323.2 K
T: 333.2 K
18
60
40
14
12
10
8
6
4
20
2
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0
xCO2
50 100 150 200 250 300 350 400 450
3
Figure 4.14. Solubility of Linalool in CO2. Markers represent experimental data at 313.8 K
(■), 323.2 K (●) and 333.2 K (▲) from [29] and lines the correlation at 313.8 K (—), 323.2 K
(—) and 333.2 K (—).
120
20
T: 313 K
T: 323 K
T: 333 K
T: 313 K
T: 323 K
T: 333 K
18
16
14
Pressure (bar)
3
Solubility (kg -Pin/m CO2)
100
80
60
15
10
5
0
12
10
8
6
4
2
0
0.0
0.2
0.4
0.6
xCO2
0.8
1.0
0
50 100 150 200 250 300 350 400 450
3
Figure 4.15. Solubility of -Pinene in CO2. Markers represent experimental data at 313 K
(■), 323 K (●) and 333 K (▲) obtained from [24] and lines the correlation at 313 K (—), 323 K
(—) and 333 K (—).
82
Chapter 4
120
20
T: 313.15 K
T: 323.15 K
T: 343.15 K
Solubility (kg -Terp/m CO2)
100
16
14
3
80
Pressure (bar)
T: 313.15 K
T: 323.15 K
T: 343.15 K
18
60
40
20
12
10
8
6
4
2
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0
50 100 150 200 250 300 350 400 450
3
xCO2
Figure 4.16. Solubility of -Terpinene in CO2. Markers represent experimental data at
313.15 K (■), 323.15 K (●) and 333 K (▲) and lines the correlation at 313.5 K (—), 323.5 K
() and 343.15 K (…).
According to Figures 4.10-4.16, it is observed that the semi-empirical model tested
for the liquid and vapour phase fit well the experimental results, but the model of
Chrastil for the vapour phase presented less standard deviation than the semiempirical equation which describes the liquid phase.
Table 4.6. Fitted constants and associated error from the correlation of the semi-empirical
model for the selected terpenoids.
Terpenoid
Carvone
p-Cymene
Eucalyptol
Linalool
Limonene
-Pinene
-Terpinene
A1
43.33 ± 1.46
38.51 ± 1.03
32.84 ± 0.71
29.07 ± 0.60
29.80 ± 0.80
29.80 ± 1.02
37.53 ± 1.02
A2
-2.23·10-5 ±
8.87·10-6
-1.41·10-5 ± 3.52·10-6
-2.94·10-6 ± 2.03·10-6
-2.18·10-6 ± 6.88·10-7
-2.38·10-6 ± 1.08·10-6
-2.97·10-6 ± 1.50·10-6
-2.97·10-6 ± 6.14·10-7
83
Binary Systems
Table 4.6 (Continued). Fitted constants and associated error from the correlation of the
semi-empirical model for the selected terpenoids.
Terpenoid
C1
C2
k
Carvone
p-Cymene
Eucalyptol
Linalool
Limonene
-Pinene
-Terpinene
-32.33
-4.82 ± 6.22
-34.86 ± 8.80
-22.01 ± 2.73
-6.68 ± 1.65
-14.63 ± 4.38
-4.82 ± 6.22
2549.93
-3462.09 ± 1534.80
-10442.96 ± 3765.92
3024.05 ± 742.38
-1902.82 ± 456.42
-3127.34 ± 860.00
-3462.09 ± 1534.88
4.78
3.06 ± 0.62
11.97 ± 1.71
2.56 ± 0.21
2.57 ± 0.20
4.58 ± 0.40
3.06 ± 0.62
The values of Ai, Ci and k constants were in an enclosed range since they belong to
the same compounds family which implies similar structures and properties.
Nevertheless, some discrepancies were observed among the constants values of the
different terpenoids, which suggested the classification of data into two groups.
Attending to the chemical structures it could be observed that the presence of an
oxygen atom entails higher molecular weight and boiling point which leads lower
solubility of the terpene in CO2 due to the increase in polarity. Following this
argument, we propose the classification of terpenoids in oxygenated (Linalool,
Carvone and Eucalyptol) and non oxygenated compounds (Limonene, -Terpinene,
-Pinene and p-Cymene) in order to get the fitting parameter which correlate the
physico-chemical properties of the compounds with Ai, Ci or k. The physico-chemical
properties required for the correlation of eq. [4.25] are shown in Table 4.7.
Table 4.7. Physico-chemical properties of the terpenoids. MW: molecular weight; T b: boiling
temperature;  density;  solubility parameter.
MW (g/mol)
Carvone
p-Cymene
Eucalyptol
Linalool
Limonene
-Pinene
-Terpinene
150.22
134.22
154.25
154.25
136.24
136.23
152.23
Tb (K) (kg/m3) (MPa)1/2
504.15
450.15
449.15
471.65
449.65
428.65
458.15
960.00
857.00
922.50
863.00
841.10
858.00
850.00
19.70
19.76
19.04
19.58
17.91
19.30
18.47
Table 4.8 shows the values obtained from the fitting of the Zi (Ai, Ci and k) constants
using eq. [4.25] attending the classification of oxygenated and non oxygenated
terpenes.
84
Chapter 4
Table 4.8. Correlation constants obtained from the Equation [4.25] for oxygenated and non
oxygenated terpenes. The Average Standard Deviation (ASD) of the correlated and predicted
values for each constant was shown.
Non Oxygenated Terpenes
A1
a1,1
a1,2
a1,3
a1,4
a1,5
ASD
0.275
0.228
-0.285
7.036
2.921
2.342·10-4
C1
c1,1
c1,2
c1,3
c1,4
c1,5
ASD
A2
a2,1
a2,2
a2,3
a2,4
a2,5
ASD
1.360
-0.851
-3.630
30.796
2696.852
1.618·10-2
C2
-0.095
0.396
-0.289
3.774
3.614
8.112·10-4
c2,1
c2,2
c2,3
c2,4
c2,5
ASD
k
-142.010
3.709
49.009
-1421.616
21.551
0.330
k1
k2
k3
k4
k5
ASD
0.088
-0.074
0.023
0.054
3.660
2.767·10-4
Oxygenated Terpenes
A1
a1,1
a1,2
a1,3
a1,4
a1,5
ASD
-0.645
0.071
0.096
0.632
3.613
2.126·10-3
A2
a2,1
a2,2
a2,3
a2,4
a2,5
ASD
17.469
1.626
-0.747
-144.885
2702.536
8.806·10-3
Oxygenated Terpenes
C1
C2
k
c1,1
c1,2
c1,3
c1,4
c1,5
0.129
0.150
-0.152
0.767
3.655
c2,1
c2,2
c2,3
c2,4
c2,5
-245.203
296.389
-114.363
-12.908
2.506
k1
k2
k3
k4
k5
0.009
-0.200
0.089
0.751
3.654
ASD
2.779·10-3
ASD
1.505
ASD
1.960·10-3
The fit of the constants as a function of the molecular weight, the boiling
temperature, the density and the solubility parameter of the terpenoids is so
accurate that the values of solubility obtained overlay the correlation data showed
85
Binary Systems
in Figures 4.10-4.16. The low values of the standard deviation show in Table 4.8
confirmed that the fit of Ai, Ci and k following eq. [4.25] is highly accurate.
4.3.3. Prediction of multicomponent mixtures and essential oils
The feasibility of the prediction of the VLE data of terpenoids in CO 2 was checked
using the mixture of Limonene and Linalool. The ternary system
Limonene/Linalool/CO2 has been studied since they are considered the key
compounds of the citrus oil, and they are the most difficult compounds to separate
[81, 82]. The literature data was obtained from Cháfer et al. [81] and the
comparison with the prediction is shown in Table 4.9 at 318.15 and 328.15 K in a
pressure range between 70 and 110 bar.
Table 4.9. Comparison between experimental and predicted solubility of the ternary system
Limonene/Linalool/CO2 at 318.15 K and 328.15 K.
P (bar)
70
74
78
82
86
90
70
74
78
82
86
90
95
100
105
110
SLim (kg/m3) SLim (kg/m3) Std. SLin (kg/m3) SLin (kg/m3) Std.
Exp.
Pred.
Dev.
Exp.
Pred.
Dev.
Temperature: 318.15 K
2.070
2.163
0.066
0.649
2.401
1.239
2.748
3.012
0.187
1.108
3.339
1.578
3.522
4.039
0.365
1.574
4.473
2.050
8.716
6.047
1.887
6.524
6.687
0.115
25.387
9.630
11.142
17.888
10.630
5.132
83.621
14.279
49.032
57.308
15.738
29.394
4.089
1.896
1.551
2.305
1.315
0.701
4.619
2.432
1.546
2.540
1.685
0.604
5.107
3.053
1.452
2.844
2.113
0.517
6.016
3.882
1.509
3.505
2.685
0.580
7.453
4.976
1.751
4.451
3.438
0.716
10.612
6.240
3.091
5.691
4.308
0.978
19.550
8.900
7.530
7.807
6.135
1.182
32.958
12.158
14.708
18.245
8.371
6.982
54.791
15.294
27.928
33.411
10.521
16.186
133.848
18.882
81.293
63.947
12.978
36.040
Table 4.9 shows that the proposed semi-empirical model predicts pretty well the
solubilities of Limonene and Linalool in CO2, although higher deviations were
observed at higher pressure. However in the range of pressure between 7 and 9
MPa the prediction was accurate. These pressure are particularly relevant for the
natural oils rich in terpenoids since they represent potential working conditions for
86
Chapter 4
the efficient application of the supercritical processes [83, 84]. The predicted values
are underestimated, but according to the VLE data used for correlation [29], the
maximum solubility of the terpenoids at 323.15 K was 20 kg Limonene/m3 CO2 and
15 kg Linalool/m3 CO2. Experimentally the mixture reached solubility values of 133
kg Limonene/m3 CO2 and 63 kg Linalool/m3 CO2, the increase of solubility in the
ternary mixture was attributed by some authors to the interactions between the
compounds [30].
With the aim of validating the semi-empirical model, the prediction of the solubility
of natural oils in CO2 was carried out [84, 85]. The Lemon and Orange oils contain
terpenoids whose VLE data have not been described in the literature and the
prediction of Ai, Ci and k constants based on their properties will be very useful to
predict their behaviour. In the case of Lemon oil, Limonene and Citral were
assumed as the majority compounds and in the Orange oil, the prediction of
solubility was based on the Limonene, Linalool and Geranial.
The solubility of Lemon oil in CO2 was initially predicted. Gironi and Maschietti
determined the composition of the Lemon oil by means of Gas Chromatographic
analysis and concluded that the main classes of compounds were: monoterpenes
(93.7 wt.%), monoterpene oxygenated derivatives (4.3 wt.%) and sesquiterpenes (2
wt.%) [84]. In this work, we considered that the lemon oil could be modelled using
Limonene as monoterpene and Citral as monoterpene oxygenated derivative
because they are the most relevant compounds of each class, while the
sesquiterpenes are minority components and we did not predict their behaviour. The
prediction of the Ai, Ci and k constants of Citral was carried out on the basis of their
physico-chemical properties shown in Table 4.10.
Table 4.10. Physico-chemical properties of Citral and Geranial. MW: molecular weight; Tb:
boiling temperature;  density;  solubility parameter.
MW (g/mol)
Citral
Geranial
152.24
152.24
Tb (K) (kg/m3) (MPa)1/2
502.20
502.20
893.00
893.00
19.58
19.47
Once the constants were obtained, the prediction of the VLE of the Lemon oil was
achieved employing the equations [4.23] and [4.24] and considering the composition
of the mixture. The experimental measurements were performed at 323.15 and
343.15 K in a pressure range between 81 and 130 bar. Table 4.11 shows the
experimental and predicted values of solubility of the Lemon oil.
The predicted data fitted accurately the experimental solubility, although as it was
observed previously, deviation increased at high pressure. In this case, the average
standard deviation was 4.6 kg Lemon oil/ m3 CO2. It is important to emphasise that
the solubility of the Lemon oil could vary in comparison with its pure components
(Limonene and Citral) because when a component takes part in a complex real
mixture, the accompanying compounds affect the global solubility. This could be the
reason of the underestimation of the predicted data.
87
Binary Systems
Table 4.11. Comparison between the experimental and predicted values of solubility of
Lemon oil in supercritical CO2 at 323.15 K and 343.15 K.
P (MPa)
8.1
8.6
8.9
9.2
9.5
9.8
9.95
10.1
8.6
9.7
10.8
12
12.6
13
SLemonOil (kg/m3) Exp. SLemon Oil (kg/m3) Pred.
3.039
3.944
4.048
5.513
4.335
6.624
5.201
8.322
7.069
10.537
11.435
13.089
33.591
14.497
51.417
15.621
3.151
3.832
3.850
6.461
6.833
10.859
14.133
17.695
22.789
21.904
36.494
25.021
Std. Dev.
0.640
1.036
1.618
2.206
2.453
1.170
13.502
25.312
0.481
1.846
2.847
2.518
0.626
8.113
Next, the prediction of the behaviour of Orange oil in CO 2 was studied since this is a
complex multicomponent mixture of terpenoids. Monoterpenes content of orange
peel oil depends on the origin of the oranges and in this work three different
compositions were studied (named as A, B and C). In general, they presented from
98.26 to 98.77 wt % of monoterpenes, where the most common were Limonene
(95.35 wt %) and Linalool (0.39 wt %) obtained from Budich and Brunner [85].
Decanal and Geranial were also presented in the composition of the orange oil, but
in this work only Geranial was considered in the mixture since Decanal is not a
terpenoid. As far as we know, VLE data of Geranial have not been studied yet
although it is present in several plants and fruits [86-91]. According to the described
procedure, the Ai, Ci and k constants for Geranial were predicted based on its
physico-chemical properties shown in Table 4.9 and considering the composition of
the mixture [85], the prediction of the VLE of three different types of orange oil was
achieved.
Figure 4.17. shows the composition of the mixtures CO 2 + orange peel oil (three
types: A, B, C) determined experimentally (markers) and by means of the proposed
prediction (lines) at 313 and 323 K and in a pressure range from 70 to 130 bar. In
this case, the solubility of the ternary mixture (Limonene, Linalool, Geranial) was
comparatively lower than that of the single components.
88
Chapter 4
30
120
25
Pressure (bar)
3
Solubility (kg /m )
140
100
80
60
Type A
T: 313.15 K
T: 323.15 K
40
20
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
140
15
10
0
20
40
60
80
100
120
Pressure (bar)
15
Type B
3
Solubility (kg/m )
100
80
60
Type B
T: 313.15 K
T: 323.15 K
40
20
12
T: 313.15 K
T: 323.15 K
9
6
3
0
0
xCO2 (wt. %)
20
Type C
25
3
100
80
Type C
T: 313.15 K
T: 323.15 K
40
20
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
xCO2 (wt. %)
60
80
100
120
100
120
30
120
60
40
Pressure (bar)
Solubility (kg/m )
Pressure (bar)
5
0
xCO2 (wt. %)
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Pressure (bar)
T: 313.15 K
T: 323.15 K
20
120
140
Type A
20
T: 313.15 K
T: 323.15 K
15
10
5
0
0
20
40
60
80
Pressure (bar)
Figure 4.17. Comparison between experimental solubility data [85] for three different types
of orange oils (A,B and C) at 313.15 K (■) and 323.15 K (▲), and the prediction at 313.15 K
(—) and 323.15 K (—).
According to the prediction of VLE for multicomponent mixtures shown in Figure
4.17, which contain compounds described or not in the literature, the semi-empirical
model proposed is validated and the procedure can be used to estimate the solubility
of natural oils in CO2 in a range of temperature between 313 and 333 K and at from
atmospheric pressure up to 12 MPa.
4.3.4. Correlation and prediction of terpenes/CO2 using Equations of
State
Although the explained methodology is very easy, simple and versatile and allows
the prediction of VLE data from the knowledge of the composition of natural oils,
cubic Equations of State (EoS) have been generally used in the prediction of
terpenes behaviour. Particularly, Peng-Robinson equation of state (PR-EoS) has
been widely applied for the prediction of the phase equilibrium of nonpolar
component mixtures at high pressures [24, 27, 82, 85, 92]. The general expression to
describe PR-EoS is shown below:
[4.26]
89
Binary Systems
where P is the pressure, R is the ideal gas constant, T is the temperature, V is the
molar volume, a is a measure of the attractive forces between the molecules and b is
related to the size of the molecules. These two parameters are defined as:
[4.27]
[4.28]
[4.29]
[4.30]
[4.31]
where TR is the reduced temperature and  is the acentric factor. The application of
PR-EoS to mixtures (CO2/terpene oils, in this case) requires the knowledge of the
critical constants (Table 4.11) and the use of mixing rules which relate the
properties of the pure components to the properties of the mixtures [93]. The Van
der Waals mixing rules are the most widely used:
[4.32]
[4.33]
In equations [4.32] and [4.33], xi or xj represents the mole fraction of the component
i or j in the mixture, respectively; aij and bij are parameteres corresponding to pure
component when i=j, but when i≠j, are called the unlike-interaction parameter, and
can be calculated following the next expressions:
[4.34]
[4.35]
where kij is known as the coupling or binary parameter and is obtained from the
correlation of experimental data CO2/terpene. Generally, the values of k12 for the
most known binary systems CO2/terpenoids are collected in the literature, but the
interactions between the different compounds involved in a complex mixture at high
pressure are difficult to achieve. In this section, the correlation of experimental VLE
data using PR-EoS was studied in order to obtain the binary parameters of the
mixture. The k12 between the mentioned terpenoids and CO2 was obtained from
fitting the referenced data via Newton-Raphson nonlinear regression procedure
using as objective function the maximum likelihood with a convergence tolerance of
0.0001. Their values together with their standard deviation have been shown in the
Table 4.12.
90
Chapter 4
Table 4.12. Critical constants, acentric factor and binary interaction parameter obtained
from the vapour-liquid equilibrium correlation of terpenoids in CO 2 in the specific range of
pressure and temperature.
Terpenoids
Tc (K)
Pc (bar)

Carvone
724.90
36.14
0.502
p- Cymene
652.00
28.00
0.376
Eucalyptol
643.16
27.00
0.419
Linalool
630.50
24.20
0.748
Limonene
653.00
28.20
0.381
-Pinene
644.00
27.60
0.221
-Terpinene
661.00
28.00
0.376
Terpenoids
Temperature (K) Pressure (bar)
k12
Std. Dev.
Ref.
Carvone
303.55-324.05
18-96
0.099
0.028
[27]
p- Cymene
303.15-343.15
1.9-110.7
0.074
0.054
This work
Eucalyptol
318.2-323.2
77.5-98.0
0.083
[28]
Linalool
313.18-333
7.8-110.6
0.058
[29]
Limonene
313.78-333.25
8.3-110.0
0.083
0.003
This work
-Pinene
313-333
62.9-109.3
0.072
0.011
[24]
-Terpinene
313.15-343.15
4.9-110.5
0.085
This work
It should be outlined that VLE data of some terpenoids has been widely studied and
there are many different references in the literature. Generally, when there is an
important amount of equilibrium data, dispersion of the binary interaction
parameters could be observed. Nevertheless, on a regular basis, k 12 ranges between
0.058 and 0.099 when the working conditions of pressure are between 2 and 120 bar
and temperature from 30 to 70 ºC, which fits pretty well with the typical values
showed by the hydrocarbons/CO2 binary systems [94]. Figures 4.18-4.20 show the
experimental (markers) and the PR-EoS correlated (line) data obtained in this work
for p-Cymene, Limonene and -Terpinene.
91
Binary Systems
140
120
Pressure (bar)
100
T:30 ºC
T:30 ºC Correlation
T:50 ºC
T:70 ºC
80
60
40
20
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.990
0.995
1.000
x/y CO2
Figure 4.18. Equilibrium phase composition experimental data (markers) from this work and
predicted for the CO2/p-Cymene binary system at 30, 50 and 70ºC. k 12:0.074.
140
120
T:30 ºC
T:40 ºC
Pressure (bar)
100
80
60
40
20
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.990
0.995
1.000
x/y CO2
predicted for the CO2/Limonene binary system at 30 and 40ºC. k 12:0.083.
92
Chapter 4
140
120
Pressure (bar)
100
T:40ºC
T:40ºC Correlation
T:50ºC
T:50ºC Correlation
T:70ºC
T:70ºC Correlation
80
60
40
20
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.990
0.995
1.000
x/y CO2
predicted for the CO2/-Terpinene binary system at 40, 50 and 70ºC. k 12:0.085.
In general, good agreement between the experimental and correlated vapour-liquid
equilibrium data was achieved. The behaviour of the experimental data is well
represented using the PR-EoS model with the optimal values of the binary
interaction parameter shown. It is observed that at constant temperature, raising
the pressure higher amounts of Terpene are transferred from the liquid to the
vapour phase because the density of the supercritical fluid increases as well as its
solvating power becomes greater.
Thus, it can be concluded that the knowledge of the phase behaviour by semiempirical or theoretical methods would allow the determination of the most suitable
working conditions to solubilise the terpenes in CO 2. But on a regular basis, at mild
temperatures, pressures above 100 bar are enough to ensure that the selected
terpene oil is in the vapour phase.
93
Binary Systems
4.4. Polystyrene/CO2 Results
4.4.1. Sorption of CO2 in Polystyrene. Plasticization
The characteristic parameters for carbon dioxide and Polystyrene are tabulated in
the literature [95, 96], but in the case of CO2, the mentioned parameters vary widely
depending on the reference. Possibly it is due to the fact that different
thermodynamic properties were used to obtain the parameters and different regions
of pressure and temperature were studied in each case. By this reason, an initial
screening of the most suitable characteristic parameters for CO2 was carried out to
establish a reliable prediction of sorption. Table 4.13 gathers a set of different
characteristic parameters shown in the literature.
Table 4.13. Sanchez-Lacombe EOS characteristic parameters for carbon dioxide
P* (bar)
T*(K)
*(kg/m3)
Ref.
7203
f(t)
1580
[97]
4181
316
1369
[98]
6596
283
1620
[99]
5745
305
1510
[100]
7195
280
1618
[101]
5745
305
1510
[102]
5737
309
1504
[103]
4643
328
1426
[104]
4126
316
1369
[98]
6510
283
1620
[99]
5670
305
1510
[100]
7101
280
1618
[101]
6590
283
1620
[105]
According to the characteristic parameters compile in Table 4.13, density of CO2 was
calculated following SL-EoS and the values were compared with those obtained from
the equation of Bender [106] and Span & Wagner [107] (Figure 4.21).
94
Chapter 4
1000
900
800
*
Density (kg/m3)
*
*
3
P (bar) T (K)  (kg/m ) Ref.
4181
316
1369
[99]
6596
283
1620
[100]
5745
305
1510
[101]
7195
280
1618
[102]
5745
305
1510
[103]
5737
309
1504
[104]
4643
328
1426
[105]
4126
316
1369
[99]
6510
283
1620
[100]
5670
305
1510
[101]
7101
280
1618
[102]
6590
283
1620
[106]
Eq. of Span and Wagner [108]
Eq. of Bender [107]
700
600
500
400
300
200
100
0
0
25
50
75
100
125
150
175
200
225
Pressure (bar)
Figure 4.21. Influence of the characteristic parameters of CO 2 on its density.
Regarding Figure 4.21, the values reported by Panayiotou and Sanchez [103]
(P*=5737 bar; T*= 309 K; *= 1510 kg/m3) achieved the best fit among the studied
parameters, although the supercritical phase transition is not pretty well fitted in
any case. The mentioned characteristics parameters together with P*=3870 bar; T*=
7399 K; *= 1108 kg/m3 from Sato et al. [97] in the case of PS, were selected to apply
SL-EoS.
CO2 sorption in PS data vary mainly due to the different methods and experimental
conditions used. Aubert [108] and Zhang et al. [51] measured the solubility using
gravimetric methods by means of a quartz crystal microbalance while Sato et al.
determined the sorption by the pressure-decay method [96] or by the magnetic
suspension balance [97]. Also, some authors employed combination of two methods
to be more accurate due to changes in the volume of the polymer. Thus, Hilic et al.
[47] combined the magnetic suspension balance with the pressure-decay method. On
the other hand, high temperature were generally used to melt the polymer (Tg ≈ 373
K), but the plasticizing effect of CO2 decreases dramatically the Tg and low
operating temperatures can be used to perform the sorption experiments.
The experimental data of CO2 sorption in Polystyrene at 30 ºC are depicted in
Figure 4.22. The sorption of CO2 increases almost linearly with pressure up to 100
bar where it reaches a constant value close to 0.16 g CO2/ g PS.
95
Binary Systems
1000
0.15
800
0.10
600
400
0.05
0
Experimental data at T: 30 ºC
Shieh and Liu [109] at T: 32 ºC
Zhang et al. [89] at T: 35 ºC
Aubert [108] at T: 40 ºC
Pantoula and Panayiotou [72] at T: 51 ºC
Hilic et al. [85] at T: 65 ºC
0.20
0.15
3
200
0.00
Sorption (gCO2/gPS)
1200
This work at T: 30 ºC
Density of CO2 at T: 30 ºC
Density of CO 2 (kg/m )
Sorption (gCO2/gPS)
0.20
0.10
0.05
0.00
0
50
100
150
200
250
300
350
Pressure (bar)
Figure 4.22. (Upper) CO2 sorption in PS as a function of pressure at 30 ºC. The relationship
between the sorption of CO2 and density can be observed in the right axis. (Below)
Comparison between the experimental data (■) obtained in this work and those from the
literature as it is indicated by the markers: (○) Shieh and Liu [109]; () Zhang et al. [51]; ()
Aubert [108]; (◊) Pantoula and Panayitou [34] and ( ) Hilic et al. [47].
A comparison between our experimental data and those reported in the literature at
temperature between 32 and 65 ºC are shown in Figure 4.21. Data could be divided
in two groups attending to the maximum sorption values. Shieh and Liu [109],
Zhang et al. [51] and Pantoula and Panayiotou [34] data reached values of sorption
around 0.06 g CO2/g PS at 32, 35 and 51 ºC, respectively. On the other hand, Aubert
[108] and Hilic et al. [47] showed an increase on sorption reaching 0.16 g CO 2/g PS
at 40 and 65 ºC, respectively. The last cited references present experimental data
similar to those shown in Figure 4.22.
According to the literature, when temperature increases, solubility of CO 2 decreases
due to the density decrease [110]. This trend has been generally observed in many
gas/polymer systems, where the highest values of sorption could be reached at high
pressure and low temperature. By this reason, the sorption of CO2 is commonly
96
Chapter 4
related with its density, thus, when density increases higher amount of CO 2 are
absorbed into the polymer.
The sorption of CO2 in Polystyrene was correlated using Henry, Dual-Mode model
equations and Sánchez-Lacombe EoS (Figure 4.23). The easiest model to correlate
the sorption of CO2 into the PS follows the law of Henry (equation 4.1), where the
sorption of CO2 is proportional to the pressure. It is especially suitable to fit the
sorption of CO2 at low pressures.
0.20
0.18
Sorption (gCO2/gPS)
0.16
0.14
0.12
0.10
0.08
0.06
Experimental data
Henry equation
Dual-mode model
Sánchez-Lacombe prediction
0.04
0.02
0.00
0
50
100
150
200
250
Pressure (bar)
Figure 4.23. Correlation of sorption experimental data (■) using different models: Henry’s
law (), Dual-mode model () and Sánchez-Lacombe Equation of State (). The good
agreement between the experimental data and Sánchez-Lacombe EoS can be checked.
Figure 4.23 compares the model prediction and correlation and the experimental
data for the systems. It is observed that SL-EoS correlates the experimental data
with reasonable accuracy and improves the results shown by the Dual-Mode model.
At low pressure ( 75 bar) Henry’s law correlates pretty well the experimental
results, while the dual-mode model and the SL-EoS overestimates the values of
sorption. Nevertheless, when pressure increases dual-mode model and the SL-EoS
enhance the correlation of the sorption of CO2 in PS. In contrast to Henry’s law that
predicts a linear relation between the solubility and the saturation pressure, SLEoS can correlate the thermodynamic data with enhanced quality in a wider
pressure range. It is observed that at pressure between 75 and 100 bar, the
experimental values are higher than the correlated. Glassy polymers have nonequilibrium excess free volume and the CO2 could be absorbed into these
microcavities achieving an increase in sorption [46].
97
Binary Systems
4.4.2. Glass transition temperature
The CO2 sorption into the PS is the responsible of the subsequent plasticization of
the polymer. These molecular level phenomena are observed macroscopically by the
decrease of the glass transition temperature (Tg) of the polymer [37]. Polymers in
glassy state present a fixed free volume and any polymeric chain, but when
temperature increases or a plasticizer is added, the free volume is expanded and the
energy of molecular thermodynamic movement is enlarged promoting the movement
of the polymeric chains [111].
Sorption of CO2 in PS lowers its Tg significantly below that observed at atmospheric
pressure (Tg ≈ 373 K). In this case, the glass transition temperature of PS decrease
is related with the increase of CO2 sorption. The influence of pressure on the CO 2
sorption and on the glass transition temperature of PS is shown in Figure 4.24.
120
0.18
110
0.16
100
0.14
80
0.12
Tg (ºC)
70
0.10
60
0.08
50
40
0.06
30
0.04
20
Glass transition Temperature (ºC)
Sorption of CO2 in PS (g CO2 / g PS)
10
0
0
25
50
75
100
125
150
175
200
Sorption (gCO2/gPS)
90
0.02
0.00
225
Pressure (bar)
Figure 4.24. Influence of pressure on the glass transition temperature of PS (●-left axis) and
the sorption of CO2 (-■- right axis). The decrease of Tg is related with the increase of CO 2
sorption.
Figure 4.24 shows an initial almost linear decrease of Tg at low pressures following
the opposite trend to the sorption of CO2. Thus, the Tg of PS decreases with
increasing the mass of CO2. When the critical pressure of CO 2 is reached, Tg
remains approximately constant around 30ºC due to the saturation of PS. The
plasticization of PS with compressed CO2 depends on many factors, such as the free
volume of the polymer under pressure, the polymeric chains flexibility, the critical
temperature and in particular, the sorption of CO2 in PS [41].
98
Chapter 4
It is confirmed that the solubilisation of CO 2 into the polymer induces the
plasticization of the plastic which is noticeable in a reduction of Tg [112, 113]. This
effect could be predicted according to Chow’s equation [114]:
[4.36]
[4.37]
[4.38]
where Tg is the glass transition temperature of the polymer under pressure; Tg,0 is
the glass transition temperature of the pure polymer at atmospheric pressure;  is a
nondimensional parameter defined in expression [4.39]; Mm is the molar mass of
the monomeric unit which makes up the polymer (104.15 g/mol for PS) ; Md is the
molar mass of the dissolved gas (44.01 g/mol for CO2); R is the gas constant; Cpp is
the excess transition isobaric specific heat of the pure polymer (0.2593 J/g·K) and z
is the lattice coordination number which can be either 1 or 2. Generally for polymer
with small repeat units, as the PS, z is equal to 1 [100] but Chow proposed z=2 for
the PS [114]. The use of Chow’s model becomes complicated due to the uncertainty
of the estimation of z values. When z is two or greater, retrograde vitrification of PS
is predicted, nevertheless, in the range of working conditions it was not observed
experimentally. Furthermore, Wingert et al. [115] concluded that optimization of z
is also dependant of the excess transition isobaric specific heat of the polymer. Due
to the inexistence of retrograde vitrification in our data, in this work z=1 was
selected according to Cao et al. [100]. To predict the Chow’s curve of Tg vs pressure,
the calculation of concentration is required. Sánchez-Lacombe EoS (Figure 4.22) was
used and the results of the model were compared with the values of Tg obtained
experimentally (Figure 4.25).
110
100
90
80
Tg (ºC)
70
60
50
40
30
20
10
0
0
20
40
60
80
100
120
140
160
180
200
Pressure (bar)
Figure 4.25. Prediction of the decrease of Tg of PS (●) using the Chow model. The perfect
agreement between the experimental data and the prediction is observed.
99
Binary Systems
The predicted data (solid curve) shown in Figure 4.25 fitted accurately the
experimental values of the glass transition temperature of PS under CO 2 pressure,
and small deviation was observed in the range of pressure studied.
4.4.3. Interfacial Tension
The influence of CO2 on the Interfacial Tension (IFT) of PS was studied theoretically
and results were compared with the literature [54, 55, 116]. It should be outlined
that the molecular weight and crystallinity of PS plays a crucial role on the IFT and
by this reason, important dispersion of data is shown in the literature. On a regular
basis, IFT of PS decreases at rising pressure in CO 2 atmosphere and the slope of the
IFT-curve decreases at elevated pressure. The decrease of IFT is attributed to two
phenomena: when pressure increases, the free energy density of CO 2 becomes closer
to that of the polymer phase and the second effect is related with the amount of CO 2
absorbed in the PS, which plasticizes the polymer [117]. The relationship between
the sorption of CO2 and the IFT was described by Pastore Carbone et al. [117] using
the empirical expression shown below:
[4.39]
where sol is the interfacial tension of the mixture PS/CO2, wCO2 is the weight
fraction of the dissolved CO2, pol is the interfacial tension of the polymer and r is a
fitting parameter. The estimation of wCO2 in the range where the IFT was measured
experimentally was predicted according to S-L EoS. Initially, the agreement
between the predicted wCO2 and the values shown in the literature [96, 97] in a
wider range of temperature are depicted in Figure 4.26.
0.10
Experimental data from Sato et al. [96,97] at 423.15 K
Sánchez-Lacombe prediction at 423.15 K
Sorption (gCO2/gPS)
0.08
0.06
0.04
0.02
0.00
0
25
50
75
100
125
150
175
200
Pressure (bar)
Figure 4.26. Prediction and comparison of the sorption of CO 2 in PS at 423.15 K to validate
the SL-EoS at higher temperature. Experimental data were obtained from Sato et al. [96, 97].
100
Chapter 4
The average standard deviation between experimental and predicted data was 0.006
gCO2/ gPS which means that sorption of CO2 in PS could be successfully predicted
using S-L EoS in all the range of temperature and pressure. Thus, the predicted
values of sorption were used to correlate the IFT of the mixture according to eq.
[4.40] The IFT of PS (pol) was calculated following the Macleod-Sugden equation
[118] and the effect of temperature and pressure on its volume was estimated
according to the Tait-relation [59]. The fitted parameter r ranged between 0.34 and
0.37, nevertheless, the correlated values and the experimental data reported in the
literature did not fit well, although the predicted values of sorption were reliable.
This fact highlights the importance of the experimental measurement of the IFT of
polymers at high pressure. This property is affected by the molecular weight or the
crystallinity of the polymer which implies huge estimation uncertainty.
40
Temperature: 90 ºC from Jaeger et al. [92]
Temperature: 160 ºC from Jaeger et al. [92]
Temperature: 200 ºC from Li et al. [93]
Temperature: 210 ºC from Li et al. [93]
35
30
 (mN/m)
25
20
15
10
5
0
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
wCO2
40
Temp: 90 ºC from Jaeger et al. [92]
Temp: 160 ºC from Jaeger et al. [92]
Temp: 200 ºC from Li et al. [93]
Temp: 210 ºC from Li et al. [93]
35
30
 (mN/m)
25
20
15
10
5
0
0
20 40 60 80 100 120 140 160 180 200
Pressure (bar)
Figure 4.27. Influence of sorption (upper figure) and pressure (figure below) on the decrease
of the interfacial tension of PS. Experimental data are from Jaeger et al.[54] and Li et al. [55].
The influence of molecular weight distribution and crystallinity makes difficult the accurate
prediction of the decrease of IFT.
101
Binary Systems
The relationship between the sorption of CO 2 in PS and the decrease of the IFT is
observed in Figure 4.27 (upper). Although a general trend is observed, the
references studied different types of PS, which complicates the design of a general
fitting independently of the kind of polymer used. The best agreement between
experimental and predicted data was achieved using the general Macleod-Sugden
[119] correlation for mixtures. The exponential factor (n) was fitted using the cited
references and the optimum value was between 3.4 and 3.7 (Figure 4.27 down).
4.4.4. Viscosity
The determination of viscosity at high pressure is still nowadays a challenge since
the diluents must be retained in solution under working conditions of pressure and
temperature [120]. CO2 has lower density and higher compressibility than pure
polymer, so its dissolution into the PS results in swelling of the polymer.
Consequently, an increase in free volume is produced and the transport properties
such as viscosity can be enhanced [121]. Figure 4.28 shows a typical curve obtained
from the rheometer measurements at 120 ºC and atmospheric pressure.
10
7
10
6
10
5
10
4
10
3
10
2
10
1
10
0
Viscosity (Pa·s)
120 ºC P atm
10
-1
1E-3
0.01
0.1
1
10
100
1000
Speed [1/min]
Figure 4.28. Determination of the linear viscoelastic region of PS at 120 ºC and atmospheric
pressure. The polymer shows Newtonian behaviour between 0.001 and 0.01 min-1. The error
bars represent the standard deviation of the measurements from the mean.
There are three different regions: a linear viscoelastic region (shear rate  0.01 min1), a depression of viscosity (above the critical shear rate) and finally the
stabilization of the viscosity. During the initial stage, the viscosity is independent of
shear rate and PS behaves as a Newtonian fluid. In the second stage the viscosity
decrease suggests that the polymeric chains are disentangled in the direction of the
shear. Finally, when the viscosity is kept constant at high shear rate implies the
102
Chapter 4
alienation of the chains [122]. PS exhibited a viscosity of 313.38 kPa·s at 120 ºC and
0.1 MPa, similar to the values showed in the literature by Wingert et al. [120] at 140
ºC.
The effect of pressure and temperature on the viscosity of PS at a constant shear
rate (0.01 min-1) was studied (Figure 4.29). The presence of CO2 (right axis) is the
responsible of the viscosity reduction in the PS due to its plasticizing effect.
Viscosity decreases significantly when CO 2 is absorbed in the polymer and an
increase in the sorption entails a higher decrease of viscosity.
10
6
0.18
0.16
Viscosity (Pa·s)
0.14
5
0.12
10
4
10
3
10
0.10
0.08
2
0
25
50
75
CO2 Sorption at T: 30 ºC
0.06
Viscosity at T: 100 ºC
0.04
100
125
0.02
Sorption (g CO2/g PS)
10
0.00
150
Pressure (bar)
Figure 4.29. Effect of pressure and temperature on the decrease of viscosity of PS in CO 2.
The relationship between the drop of viscosity (left axis) and the sorption of CO 2 (right axis)
can be observed.
In general, in the standard systems, when pressure increases, viscosity raises; but
in this case, the viscosity is decreased by the presence of CO 2. This small molecule is
absorbed among the chains of the PS increasing the free volume and their mobility
which causes the reduction in the viscosity [123].
To obtain reliable results, experiments were carried out when the sample was
saturated with CO2.
The changes in the viscosity along time allow the
determination of the diffusion of CO2 in the PS. The sorption of CO2 in the polymer
causes a decrease in viscosity, till the PS is saturated (Figure 4.30).
103
1.2x10
6
1.1x10
6
1.0x10
6
9.0x10
5
8.0x10
5
7.0x10
5
6.0x10
5
5.0x10
5
4.0x10
5
3.0x10
5
2.0x10
5
1.0x10
5
P: 100 bar; T: 60 ºC
Viscosity (Pa·s)
 (R)
1.0
0.8
0.6
0.4
 (R)
Viscosity (Pa·s)
Binary Systems
0.2
0.0
0
500
1000
1500
2000
2500
3000
3500
4000
0.0
4500
Time (s)
Figure 4.30. Evolution of viscosity of PS as a function of time till saturation of polymer is
reached, at 60 ºC and 100 bar.
The initial point (left axis) is the viscosity of the polymer without CO 2 at 100 bar
and 60 ºC and the points shown in the right axis represent the relative viscosity
every 5 min. Relative viscosity R (t) is defined as:



 

[4.40]
where (t) is the viscosity at time t, (0) is the viscosity at atmospheric pressure, it
means, when the concentration of CO 2 is zero and () is the viscosity at saturation.
From experimental data shown in Figure 4.30, the diffusion coefficient of CO2 in PS
was determined assuming that the gas is diffused into the sample through the
upper side of the polymer disk, the diffusion coefficient is constant at each value of
pressure and temperature and Fickian diffusion is accomplished [124]. The effect of
pressure and temperature on the diffusion coefficient of CO 2 in PS is shown in
Figure 4.31.
104
1E-9
1E-10
1E-10
2
1E-9
1E-11
1E-12
20
40
60
80
100
60
90
120
150
1E-12
180
1E-13
210
2
1E-13
1E-11
Pressure: 100 bar
Pressure: 80 bar
Pressure: 83.53 bar from Sato et al. [97]
Temperature: 60 ºC
Temperature: 80 ºC
Temperature: 100 ºC
Temperature: 100 ºC from Sato et al. [97]
Mutual Diffusion Coefficient (m /s)
Mutual Diffusion Coefficient (m /s)
Chapter 4
Pressure (bar)
Temperature (ºC)
Figure 4.31. Influence of pressure (left figure) and temperature (right figure) on the mutual
diffusion coefficients of CO2 in PS in a range of pressure from 20 to 100 bar and temperature
from 40 to 100 ºC. Our experimental data were compared with the literature [97] and a good
agreement between them was observed.
The diffusion coefficient of CO2 in rubbery PS (above its Tg) is around 10-10 m2/s,
typical value exhibited by high molecular weight polymers [52, 125]. From the
comparison of the diffusion coefficient obtained in this work and the data from Sato
et al. [97], the feasibility of the viscosity measurements to determine the diffusivity
is confirmed. According to Figure 4.31, when pressure and temperature increase,
the mutual diffusion coefficient increases, meaning that CO2 is diffuses faster at
those conditions. The effect of temperature on the diffusion coefficients has been
generally described attending Arrhenius or Williams-Landel-Ferry model [126], but
due to the lack of experimental data, this fact could not be checked.
To sum up, the sorption of CO2 in Polystyrene is the responsible of the swelling and
plasticization of the polymer which changes its glass transition temperature, its
interfacial tension and its viscosity. The inclusion of a small molecule, as CO 2 is,
among the polymeric chains causes an increase of the free volume and enlarges the
movement of the polymer chains. On the other hand, the insolubility of the polymer
is checked and it is used for confirming that precipitation of the PS from a terpene
solution should be easily performed.
105
Binary Systems
References
[1] J.M. Arandes, J. Ereña, M.J. Azkoiti, M. Olazar, J. Bilbao, Thermal recycling of
polystyrene and polystyrene-butadiene dissolved in a light cycle oil, Journal of
Analytical and Applied Pyrolysis, 70 (2003) 747-760.
[2] Y. Zhang, S.K. Mallapragada, B. Narasimhan, Dissolution of waste plastics in
biodiesel, Polymer Engineering and Science, 50 (2010) 863-870.
[3] Y. Kodera, Y. Ishihara, T. Kuroki, S. Ozaki, Feedstock Recycling of Plastics, in:
M. Müller-Hagedorn, H. Bockhorn (Eds.) Solvo-Cycle Process: AIST's New recycling
process for used plastic foam using plastics-derived solvent, Karlshrue, 2005, pp.
217-222.
[4] S.S. Kim, J. Kim, J.K. Jeon, Y.K. Park, C.J. Park, Non-isothermal pyrolysis of the
mixtures of waste automobile lubricating oil and polystyrene in a stirred batch
reactor, Renewable Energy, 54 (2013) 241-247.
[5] F. Kerton, R. Marriott, Alternative Solvents for Green Chemistry, 2nd ed., 2013.
[6] S. Shikata, T. Watanabe, K. Hattori, M. Aoyama, T. Miyakoshi, Dissolution of
polystyrene into cyclic monoterpenes present in tree essential oils, Journal of Material
[7] K. Hattori, S. Shikata, R. Maekawa, M. Aoyama, Dissolution of polystyrene into
p-cymene and related substances in tree leaf oils, Journal of Wood Science, 56 (2010)
169-171.
[8] C. Gutiérrez, M.T. García, I. Gracia, A. De Lucas, J.F. Rodríguez, The selective
[9] K. Hattori, S. Naito, K. Yamauchi, H. Nakatani, T. Yoshida, S. Saito, M.
Aoyama, T. Miyakoshi, Solubilization of polystyrene into monoterpenes, Advances in
Polymer Technology, 27 (2008) 35-39.
[10] M.T. García, G. Duque, I. Gracia, A. De Lucas, J.F. Rodríguez, Recycling
extruded polystyrene by dissolution with suitable solvents, Journal of Material
[11] T. Noguchi, M. Miyashita, Y. Lnagaki, H. Watanabe, A new recycling system for
expanded polystyrene using a natural solvent. Part 1. A new recycling technique,
Packaging Technology and Science, 11 (1998) 19-27.
[12] E. Breitmaier, Terpenes: Flavors, Fragrances, Pharmaca, Pheromones, Wiley,
2006.
[13] C. Gutiérrez, M.T. García, I. Gracia, A. De Lucas, J.F. Rodríguez, A practical
mediante disolución, 68 (2011) 181-188.
[14] F.W. Billmeyer, Textbook of polymer science, Wiley, New York, 1984.
[15] C.M. Hansen, Hansen solubility parameters: a user's handbook, CRC Press,
2000.
[16] C.D. Vaughan, Using solubility parameters in cosmetics formulation, Journal of
the Society of Cosmetic Chemists of Japan, 36 (1985) 319-333.
[17] S. Raeissi, S. Diaz, S. Espinosa, C.J. Peters, E.A. Brignole, Ethane as an
alternative solvent for supercritical extraction of orange peel oils, Journal of
[18] M. Sato, M. Goto, T. Hirose, Fractional extraction with supercritical carbon
dioxide for the removal of terpenes from citrus oil, Industrial and Engineering
Chemistry Research, 34 (1995) 3941-3946.
106
Chapter 4
[19] A.I. Cooper, Polymer synthesis and processing using supercritical carbon
dioxide, Journal of Materials Chemistry, 10 (2000) 207-234.
[20] P.T. Anastas, W. Leitner, P.G. Jessop, Handbook of Green Chemistry, Green
Solvents, Supercritical Solvents, Wiley, 2013.
[22] M. Drescher, O. Seidel, D. Geanǎ, High pressure vapor-liquid equilibria in the
ternary system orange peel oil (limonene) + ethanol + carbon dioxide, Journal of
[23] I. Gracia, M.T. García, J.F. Rodríguez, M.P. Fernández, A. de Lucas, Modelling
of the phase behaviour for vegetable oils at supercritical conditions, Journal of
[24] M. Akgün, N.A. Akgün, S. Dinçer, Phase behaviour of essential oil components
in supercritical carbon dioxide, Journal of Supercritical Fluids, 15 (1999) 117-125.
[25] Y.J. Sheng, P.C. Chen, Y.P. Chen, D.S.H. Wong, Calculations of solubilities of
aromatic compounds in supercritical carbon dioxide, Industrial and Engineering
[26] F.E. Wubbolts, O.S.L. Bruinsma, G.M. Van Rosmalen, Measurement and
modelling of the solubility of solids in mixtures of common solvents and compressed
gases, Journal of Supercritical Fluids, 32 (2004) 79-87.
[27] T. Gamse, R. Marr, High-pressure phase equilibria of the binary systems
carvone-carbon dioxide and limonene-carbon dioxide at 30, 40 and 50ºC, Fluid
Phase Equilibria, 171 (2000) 165-174.
[28] H.A. Matos, E.G. De Azevedo, P.C. Simoes, M.T. Carrondo, M.N. Da Ponte,
Phase equilibria of natural flavours and supercritical solvents, Fluid Phase
Equilibria, 52 (1989) 357-364.
[29] C.-M.J. Chang, C.-C. Chen, High-pressure densities and P-T-x-y diagrams for
carbon dioxide+linalool and carbon dioxide+limonene, Fluid Phase Equilibria, 163
(1999) 119-126.
[30] J.d.C. Francisco, B. Sivik, Solubility of three monoterpenes, their mixtures and
eucalyptus leaf oils in dense carbon dioxide, The Journal of Supercritical Fluids, 23
(2002) 11-19.
[31] S.D. Yeo, E. Kiran, High-pressure density and viscosity of polystyrene solutions
in methylcyclohexane, Journal of Supercritical Fluids, 15 (1999) 261-272.
[32] H.M. Woods, M.M.C.G. Silva, C. Nouvel, K.M. Shakesheff, S.M. Howdle,
Materials processing in supercritical carbon dioxide: Surfactants, polymers and
biomaterials, Journal of Materials Chemistry, 14 (2004) 1663-1678.
[33] O.R. Davies, A.L. Lewis, M.J. Whitaker, H. Tai, K.M. Shakesheff, S.M. Howdle,
Applications of supercritical CO2 in the fabrication of polymer systems for drug
delivery and tissue engineering, Advanced Drug Delivery Reviews, 60 (2008) 373387.
[34] M. Pantoula, C. Panayiotou, Sorption and swelling in glassy polymer/carbon
dioxide systems: Part I. Sorption, Journal of Supercritical Fluids, 37 (2006) 254-262.
[35] I. Kikic, Polymer-supercritical fluid interactions, Journal of Supercritical
Fluids, 47 (2009) 458-465.
[36] O. Hölck, M. Böhning, M. Heuchel, M.R. Siegert, D. Hofmann, Gas sorption
isotherms in swelling glassy polymers-Detailed atomistic simulations, Journal of
Membrane Science, 428 (2013) 523-532.
107
Binary Systems
[37] I. Kikic, F. Vecchione, P. Alessi, A. Cortesi, F. Eva, N. Elvassore, Polymer
plasticization using supercritical carbon dioxide: Experiment and modeling,
Industrial and Engineering Chemistry Research, 42 (2003) 3022-3029.
[38] P. Alessi, A. Cortesi, I. Kikic, F. Vecchione, Plasticization of polymers with
supercritical carbon dioxide: Experimental determination of glass-transition
temperatures, Journal of Applied Polymer Science, 88 (2003) 2189-2193.
[39] A. Bos, I.G.M. Pünt, M. Wessling, H. Strathmann, CO2-induced plasticization
phenomena in glassy polymers, Journal of Membrane Science, 155 (1999) 67-78.
[40] J.R. Royer, Y.J. Gay, J.M. Desimone, S.A. Khan, High-pressure rheology of
polystyrene melts plasticized with CO 2: Experimental measurement and predictive
scaling relationships, Journal of Polymer Science, Part B: Polymer Physics, 38
(2000) 3168-3180.
[41] L. Yu, H. Liu, L. Chen, Thermal behaviors of polystyrene plasticized with
compressed carbon dioxide in a sealed system, Polymer Engineering & Science, 49
(2009) 1800-1805.
[42] C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. De Lucas, M.T. García, Development
of a strategy for the foaming of polystyrene dissolutions in scCO2, Journal of
[43] C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. de Lucas, M.T. García, Preparation
and characterization of polystyrene foams from limonene solutions, The Journal of
[44] H.E. Park, J.M. Dealy, Effects of supercritical fluids, pressure, temperature,
and molecular structure on the rheological properties of molten polymers, in,
Monterey, CA, 2008, pp. 366-368.
[45] T. Spyriouni, G.C. Boulougouris, D.N. Theodorou, Prediction of sorption of CO2
in glassy atactic polystyrene at elevated pressures through a new computational
scheme, Macromolecules, 42 (2009) 1759-1769.
[46] S. Kanehashi, K. Nagai, Analysis of dual-mode model parameters for gas
sorption in glassy polymers, Journal of Membrane Science, 253 (2005) 117-138.
[47] S. Hilic, S.A.E. Boyer, A.A.H. Pádua, J.P.E. Grolier, Simultaneous measurement
of the solubility of nitrogen and carbon dioxide in polystyrene and of the associated
polymer swelling, Journal of Polymer Science, Part B: Polymer Physics, 39 (2001)
2063-2070.
[48] I.C. Sanchez, R.H. Lacombe, Statistical thermodynamics of polymer solutions,
Macromolecules, 11 (1978) 1145-1156.
[49] M. Pantoula, J. von Schnitzler, R. Eggers, C. Panayiotou, Sorption and swelling
in glassy polymer/carbon dioxide systems. Part II-Swelling, Journal of Supercritical
Fluids, 39 (2007) 426-434.
[50] L.N. Nikitin, M.O. Gallyamov, R.A. Vinokur, A.Y. Nikolaec, E.E. Said-Galiyev,
A.R. Khokhlov, H.T. Jespersen, K. Schaumburg, Swelling and impregnation of
polystyrene using supercritical carbon dioxide, Journal of Supercritical Fluids, 26
(2003) 263-273.
[51] Y. Zhang, K.K. Gangwani, R.M. Lemert, Sorption and swelling of block
copolymers in the presence of supercritical fluid carbon dioxide, Journal of
[52] C. Tsioptsias, C. Panayiotou, Simultaneous determination of sorption, heat of
sorption, diffusion coefficient and glass transition depression in polymer-CO2
systems, Thermochimica Acta, 521 (2011) 98-106.
108
Chapter 4
[53] A. Kasturirangan, C.A. Koh, A.S. Teja, Glass-transition temperatures in CO 2 +
polymer systems: Modeling and experiment, Industrial and Engineering Chemistry
Research, 50 (2011) 158-162.
[54] P.T. Jaeger, R. Eggers, H. Baumgartl, Interfacial properties of high viscous
liquids in a supercritical carbon dioxide atmosphere, Journal of Supercritical Fluids,
24 (2002) 203-217.
[55] H. Li, L.J. Lee, D.L. Tomasko, Effect of Carbon Dioxide on the Interfacial
Tension of Polymer Melts, Industrial and Engineering Chemistry Research, 43
(2004) 509-514.
[56] S.P. Nalawade, V.H.J. Nieborg, F. Picchioni, L.P.B.M. Janssen, Prediction of the
viscosity reduction due to dissolved CO 2 of and an elementary approach in the
supercritical CO2 assisted continuous particle production of a polyester resin, Powder
Technology, 170 (2006) 143-152.
[57] P.A. Small, Some factors affecting the solubility of polymers, Journal of Applied
Chemistry, 3 (1953) 71-80.
[58] A.F.M. Barton, CRC handbook of solubility parameters and other cohesion
parameters, CRC Press, 1991.
[60] K.L. Hoy, Solubility parameter as a design parameter for water-borne polymers
and coatings, Journal of Coated Fabrics, (1989) 53-67.
[61] J. Szydlowski, L.P. Rebelo, H. Wilczura, W.A. Van Hook, Y. Melnichenko, G.D.
Wignall, Liquid-liquid demixing from polystyrene solutions. Studies on temperature
and pressure dependences using dynamic light scattering and neutron scattering,
Fluid Phase Equilibria, 150 (1998) 687-694.
[62] G. Bernardo, D. Vesely, Equilibrium solubility of alcohols in polystyrene
attained by controlled diffusion, European Polymer Journal, 43 (2007) 938-948.
[63] A. Güner, The algorithmic calculations of solubility parameter for the
determination of interactions in dextran/certain polar solvent systems, European
Polymer Journal, 40 (2004) 1587-1594.
[64] M.T. García, I. Gracia, G. Duque, A.d. Lucas, J.F. Rodríguez, Study of the
solubility and stability of polystyrene wastes in a dissolution recycling process, Waste
Management, 29 (2009) 1814-1818.
[65] P. Subra, P. Jestin, Screening design of experiment (DOE) applied to
supercritical antisolvent process, Industrial and Engineering Chemistry Research,
39 (2000) 4178-4184.
[66] I.H. Lin, P.F. Liang, C.S. Tan, Preparation of polystyrene/poly(methyl
methacrylate) blends by compressed fluid antisolvent technique, Journal of
[67] J. Holten-Andersen, K. Eng, Activity coefficients in polymer solutions, Progress
in Organic Coatings, 16 (1988) 77-97.
[68] T. Lindvig, M.L. Michelsen, G.M. Kontogeorgis, A Flory-Huggins model based
on the Hansen solubility parameters, Fluid Phase Equilibria, 203 (2002) 247-260.
[69] W.J. Cooper, P.D. Krasicky, F. Rodriguez, Effects of molecular weight and
plasticization on dissolution rates of thin polymer films, Polymer, 26 (1985) 10691072.
[70] K. Ueberreiter, The Solution Process, (1968) 219-257.
[71] B.E. Poling, J.M. Prausnitz, J.P. O'Connell, Properties of Gases and Liquids
(5th Edition), in, McGraw-Hill.
109
Binary Systems
[72] J.S. Papanu, D.W. Hess, D.S. Soane, A.T. Bell, Dissolution of thin poly(methyl
methacrylate) films in ketones, binary ketone/alcohol mixtures, and hydroxy ketones,
Journal of the Electrochemical Society, 136 (1989) 3077-3083.
[73] M. Okubo, H. Ahmad, Synthesis of temperature-sensitive submicron-size
composite polymer particles, Colloid & Polymer Science, 273 (1995) 817-821.
[74] P.J. Flory, Thermodynamics of high polymer solutions, The Journal of Chemical
Physics, 9 (1941) 660-661.
[75] M.L. Huggins, Solutions of long chain compounds, The Journal of Chemical
Physics, 9 (1941) 440.
[76] E. Eastwood, S. Viswanathan, C.P. O'Brien, D. Kumar, M.D. Dadmun, Methods
to improve the properties of polymer mixtures: Optimizing intermolecular
interactions and compatibilization, Polymer, 46 (2005) 3957-3970.
[77] D.Z. Icoz, J.L. Kokini, Examination of the validity of the Flory-Huggins solution
theory in terms of miscibility in dextran systems, Carbohydrate Polymers, 68 (2007)
59-67.
[78] G. Ovejero, P. Pérez, M.D. Romero, I. Guzmán, E. Díez, Solubility and Flory
Huggins parameters of SBES, poly(styrene-b-butene/ethylene-b-styrene) triblock
copolymer, determined by intrinsic viscosity, European Polymer Journal, 43 (2007)
1444-1449.
[79] A.R. Imre, W.A. Van Hook, The effect of branching of alkanes on the liquidliquid equilibrium of oligostyrene/alkane systems, Fluid Phase Equilibria, 187-188
(2001) 363-372.
[80] J. Chrastil, Solubility of solids and liquids in supercritical gases, Journal of
Physical Chemistry, 86 (1982) 3016-3021.
[81] A. Cháfer, A. Berna, J.B. Montón, A. Mulet, High Pressure Solubility Data of
the System Limonene + Linalool + CO 2, Journal of Chemical & Engineering Data, 46
(2001) 1145-1148.
[82] S.A.B. Vieira de Melo, G.M.N. Costa, A.M.C. Uller, F.L.P. Pessoa, Modeling
high-pressure vapor-liquid equilibrium of limonene, linalool and carbon dioxide
systems, Journal of Supercritical Fluids, 16 (1999) 107-117.
[83] M. Kondo, N. Akgun, M. Goto, A. Kodama, T. Hirose, Semi-batch operation and
countercurrent extraction by supercritical CO2 for the fractionation of lemon oil,
Journal of Supercritical Fluids, 23 (2002) 21-27.
[84] F. Gironi, M. Maschietti, Phase equilibrium of the system supercritical carbon
dioxide-lemon essential oil: New experimental data and thermodynamic modelling,
[85] M. Budich, G. Brunner, Vapor–liquid equilibrium data and flooding point
measurements of the mixture carbon dioxide+orange peel oil, Fluid Phase Equilibria,
158 (1999) 759-773.
[86] M.C. Mesomo, M.L. Corazza, P.M. Ndiaye, O.R. Dalla Santa, L. Cardozo, A.D.P.
Scheer, Supercritical CO2 extracts and essential oil of ginger (Zingiber officinale R.):
Chemical composition and antibacterial activity, Journal of Supercritical Fluids, 80
(2013) 44-49.
[87] B. Marongiu, A. Piras, S. Porcedda, E. Tuveri, Comparative analysis of the oil
and supercritical CO2 extract of Cymbopogon citratus Stapf, Natural Product
Research, 20 (2006) 455-459.
[88] L.H.C. Carlson, R.A.F. Machado, C.B. Spricigo, L.K. Pereira, A. Bolzan,
Extraction of lemongrass essential oil with dense carbon dioxide, Journal of
110
Chapter 4
[89] L.C. Argenta, J.P. Mattheis, X. Fan, F.L. Finger, Production of volatile
compounds by Fuji apples following exposure to high CO2 or low O2, Journal of
Agricultural and Food Chemistry, 52 (2004) 5957-5963.
[90] C. Besada, G. Sanchez, A. Salvador, A. Granell, Volatile compounds associated
to the loss of astringency in persimmon fruit revealed by untargeted GC-MS analysis,
Metabolomics, 9 (2013) 157-172.
[91] Š.I. Kirbalar, A. Gök, F.G. Kirbašlar, S. Tepe, Volatiles in Turkish clementine
(Citrus clementina Hort.) peel, Journal of Essential Oil Research, 24 (2012) 153-157.
[92] A.L. Oliveira, V.L.S. Melo, A.R. Guimaraes, F.A. Cabral, Modelling of highpressure phase equilibrium in systems of interest in the food engineering field using
the peng-robinson equation of state with two different mixing rules, Journal of Food
Process Engineering, 33 (2010) 101-116.
[93] B.E. Poling, J.M. Prausnitz, J.P. O'Connell, The properties of gases and liquids,
McGraw-Hill, New York, 2001.
[94] E.A. Turek, R.S. Metcalfe, L. Yarborough, R.L. Robinson Jr, PHASE
EQUILIBRIA IN CO2 -MULTICOMPONENT HYDROCARBON SYSTEMS:
EXPERIMENTAL DATA AND AN IMPROVED PREDICTION TECHNIQUE,
Society of Petroleum Engineers journal, 24 (1984) 308-324.
[95] E. Kiran, Y. Xiong, W. Zhuang, Modeling polyethylene solutions in near and
supercritical fluids using the Sanchez-Lacombe model, The Journal of Supercritical
Fluids, 6 (1993) 193-203.
[96] Y. Sato, M. Yurugi, K. Fujiwara, S. Takishima, H. Masuoka, Solubilities of
carbon dioxide and nitrogen in polystyrene under high temperature and pressure,
Fluid Phase Equilibria, 125 (1996) 129-138.
[97] Y. Sato, T. Takikawa, S. Takishima, H. Masuoka, Solubilities and diffusion
coefficients of carbon dioxide in poly(vinyl acetate) and polystyrene, The Journal of
[98] R. Hariharan, B.D. Freeman, R.G. Carbonell, G.C. Sarti, Equation of state
predictions of sorption isotherms in polymeric materials, Journal of Applied Polymer
Science, 50 (1993) 1781-1795.
[99] D.S. Pope, I.C. Sanchez, W.J. Koros, G.K. Fleming, Statistical thermodynamic
interpretation of sorption/dilation behavior of gases in silicone rubber,
Macromolecules, 24 (1991) 1779-1783.
[100] G.P. Cao, T. Liu, G.W. Roberts, Predicting the effect of dissolved carbon dioxide
on the glass transition temperature of poly(acrylic acid), Journal of Applied Polymer
Science, 115 (2010) 2136-2143.
[101] P.K. Kilpatrick, S.H. Chang, Saturated phase equilibria and parameter
estimation of pure fluids with two lattice-gas models, Fluid Phase Equilibria, 30
(1986) 49-56.
[102] M.N. Lotfollahi, H. Modarress, B. Khodakarami, VLE Predictions of strongly
non-ideal binary mixtures by modifying Van der Waals and Orbey-Sandler mixing
rules, Iranian Journal of Chemistry and Chemical Engineering, 26 (2007) 73-80.
[103] C. Panayiotou, I.C. Sanchez, Swelling of network structures, Polymer, 33
(1992) 5090-5093.
[104] A. Garg, E. Gulari, C.W. Manke, Thermodynamics of polymer melts swollen
with supercritical gases, Macromolecules, 27 (1994) 5643-5653.
[105] X. Wang, I.C. Sanchez, Welding immiscible polymers with a supercritical fluid,
Langmuir, 23 (2007) 12192-12195.
667-673.
111
Binary Systems
[107] R. Span, W. Wagner, A new equation of state for carbon dioxide covering the
fluid region from the triple-point temperature to 1100 K at pressures up to 800 MPa,
Journal of Physical and Chemical Reference Data, 25 (1996) 1509-1596.
[108] J.H. Aubert, Solubility of carbon dioxide in polymers by the quartz crystal
microbalance technique, Journal of Supercritical Fluids, 11 (1998) 163-172.
[109] Y.T. Shieh, K.H. Liu, The effect of carbonyl group on sorption of CO 2 in glassy
polymers, Journal of Supercritical Fluids, 25 (2003) 261-268.
[110] Y. Sato, K. Fujiwara, T. Takikawa, Sumarno, S. Takishima, H. Masuoka,
Solubilities and diffusion coefficients of carbon dioxide and nitrogen in
polypropylene, high-density polyethylene, and polystyrene under high pressures and
temperatures, Fluid Phase Equilibria, 162 (1999) 261-276.
[111] X.K. Li, G.P. Cao, L.H. Chen, R.H. Zhang, H.L. Liu, Y.H. Shi, Study of the
anomalous sorption behavior of CO2 into poly(methyl methacrylate) films in the
vicinity of the critical pressure and temperature using a quartz crystal microbalance
(QCM), Langmuir, 29 (2013) 14089-14100.
[112] A.G. Wonders, D.R. Paul, Effect of CO2 exposure history on sorption and
transport in polycarbonate, Journal of Membrane Science, 5 (1979) 63-75.
[113] S.A. Stern, A.H. De Meringo, Solubility of Carbon Dioxide in cellulose acetate
at elevated pressures, J Polym Sci Polym Phys Ed, 16 (1978) 735-751.
[114] T.S. Chow, Molecular interpretation of the glass transition temperature of
polymer-diluent systems, Macromolecules, 13 (1980) 362-364.
[115] K. Liu, F. Schuch, E. Kiran, High-pressure viscosity and density of poly(methyl
methacrylate) + acetone and poly(methyl methacrylate) + acetone + CO 2 systems,
[116] H. Park, R.B. Thompson, N. Lanson, C. Tzoganakis, C.B. Park, P. Chen, Effect
of temperature and pressure on surface tension of polystyrene in supercritical carbon
dioxide, Journal of Physical Chemistry B, 111 (2007) 3859-3868.
[117] M.G. Pastore Carbone, E. Di Maio, G. Scherillo, G. Mensitieri, S. Iannace,
Solubility, mutual diffusivity, specific volume and interfacial tension of molten
PCL/CO2 solutions by a fully experimental procedure: Effect of pressure and
temperature, Journal of Supercritical Fluids, 67 (2012) 131-138.
[118] R.C. Reid, J.M. Prausnitz, B.E. Poling, The properties of gases and liquids,
1987.
[119] D.B. Macleod, On a relation between surface tension and density, Transactions
of the Faraday Society, 19 (1923) 38-41.
[120] M.J. Wingert, S. Shukla, K.W. Koelling, D.L. Tomasko, L.J. Lee, Shear
viscosity of CO2-plasticized polystyrene under high static pressures, Industrial and
[121] J.E. Mark, Physical properties of polymers handbook, AIP Press, 1996.
[122] C.A. Kelly, S.M. Howdle, K.M. Shakesheff, M.J. Jenkins, G.A. Leeke, Viscosity
studies of poly(DL -lactic acid) in supercritical CO2, Journal of Polymer Science, Part
B: Polymer Physics, 50 (2012) 1383-1393.
[123] C. Gutiérrez, M.T. Garcia, S. Curia, S.M. Howdle, J.F. Rodriguez, The effect of
CO2 on the viscosity of polystyrene/limonene solutions, Journal of Supercritical
Fluids, 88 (2014) 26-37.
[124] M.H. Klopffer, B. Flaconnèche, Transport properties of gases in polymers:
Bibliographic review, Transport de molécule gazeuses dans les polymères: Revue
bibliograpique, 56 (2001) 223-244.
112
Chapter 4
[125] K. Taki, K. Nitta, S.I. Kihara, M. Ohshima, CO2 foaming of poly(ethylene
glycol)/polystyrene blends: Relationship of the blend morphology, CO 2 mass transfer,
and cellular structure, Journal of Applied Polymer Science, 97 (2005) 1899-1906.
[126] G. Bernardo, R.P. Choudhury, H.W. Beckham, Diffusivity of small molecules
in polymers: Carboxylic acids in polystyrene, Polymer, 53 (2012) 976-983.
113
Binary Systems
114
Chapter 5
TERNARY
SYSTEMS:
EQUILIBRIUM
Chapter 5 deals about the study of the ternary mixtures CO 2/terpene oils/PS.
Although the individual knowledge of the binary solutions provides a good idea
regarding the global process, the determination of the phases’ equilibrium of the
mixture is crucial for the accurate design. But also, the influence of CO 2 on the
properties of the solutions is a key factor since the sorption of CO 2 plasticises the
mixture changing dramatically their physic-chemical and rheological
characteristics. According to this, the Chapter has been divided in three different
sections: the study of CO2/Limonene/PS equilibrium, the determination of CO 2/pCymene/PS equilibrium and the measurement of viscosity and interfacial tension of
both mixtures.
Equilibrium of Ternary Mixtures
116
Chapter 5
Based on:

C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. de Lucas, M.T. García, High-pressure
phase equilibria of Polystyrene dissolutions in Limonene in presence of CO 2, The

C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. de Lucas, M.T. García, Determination
of the High-pressure phase equilibria of Polystyrene/p-Cymene in presence of CO2,
The Journal of Supercritical Fluids, enviado.
Graphical abstract
Polystyrene
Two-phases
Terpene
CO2
117
RESUMEN
La disolución de residuos de Poliestireno en aceites terpénicos es una excelente
alternativa para su reciclaje. En concreto, se seleccionaron el Limoneno y el pCimeno por tratarse de los disolventes en los que el polímero presentaba una
elevada solubilidad, además de ser compuestos naturales, no tóxicos y estar
disponibles comercialmente a un bajo precio. A continuación, mediante CO 2 a alta
presión es posible retirar el disolvente para que precipite el polímero, ya que tanto el
Limoneno como el p-Cimeno son muy solubles en CO2 y por el contrario, el
Poliestireno presenta una total insolubilidad. Para definir las condiciones de
operación más adecuadas a las que realizar la separación de los componentes, es
fundamental el conocimiento del equilibrio de fases de la mezcla ternaria
(CO2/terpeno/PS).
En este Capítulo, se determina la solubilidad del Limoneno y del p-Cimeno en la
fase rica en CO2. Para ello, se llevaron a cabo los experimentos en una celda provista
de visores de volumen variable haciendo uso del método estático. Las condiciones a
las que se realizaron las medidas experimentales fueron en un rango de
temperatura de 25 a 40 ºC, concentración de la disolución inicial entre 0.05 y 0.80 g
PS/ml terpeno y a presiones de hasta 150 bar. A partir de estos datos, se concluyó
que las condiciones más adecuadas para conseguir la precipitación del Poliestireno a
partir de su disolución en Limoneno o p-Cimeno son aquellas en las que se alcancen
altos valores de densidad (alta presión y baja temperatura) y valores moderados de
concentración.
118
Chapter 5
ABSTRACT
Dissolution with terpenic solvents is presented as an alternative and original route
to recycle Polystyrene wastes at room temperature. Limonene and p-Cymene were
the selected solvent to perform the dissolution. The mentioned terpenes present
high compatibility with Polystyrene besides being natural, non toxic and relatively
low cost. The solvent removal is possible thanks to supercritical CO 2 since it
provides high solubility of Limonene and p-Cymene and complete PS insolubility at
moderated pressures and temperature. In order to determine the proper working
conditions to conduct the precipitation of the polymer, accurate knowledge of the
phase equilibrium for the ternary mixtures should be known.
In this section, the solubility of Limonene and p-Cymene in the ternary system was
determined. The phase equilibrium experiments were conducted in a variablevolume view cell employing the static method. These experiments were carried out
in the temperature range of 25-40 ºC, at pressures up to 150 bar and in the
concentration range of 0.05-0.80 g PS/ml Terpene. The optimum conditions to carry
out the precipitation process of Polystyrene were predicted on the basis of the
calculated values. The most suitable conditions to recycle Polystyrene were reached
at high values of density, it means, high pressure, low temperature and moderated
concentration.
119
120
Chapter 5
5.1. Background of the Phases’ equilibrium
The use of carbon dioxide as a non-solvent for polymer phase separation has been
extensively reported in the literature [1-3]. It is based on the use of two liquid
solvents that are completely miscible. The polymer is soluble in the first solvent, but
not soluble in the second solvent. Therefore, the addition of the antisolvent induces
the formation of a solution of the two liquids and the supersaturation and
precipitation of the solute. This process is a powerful tool for solvent recovery in
solution polymerisation processes as well as for separation or precipitation of
polymers [4-6].
The ability of scCO2 to tune physico-chemical properties near its critical point (Tc:
31.1 ºC, Pc: 73.8 bar) provides a high diffusivity, high solvent power and the absence
of surface tension which means higher phase separation velocities [7]. By the
reported reasons, CO2 was selected as antisolvent in the recycling process of
Polystyrene from its solution in terpenes. In this process, CO2 is miscible with the
terpenes, but it acts as antisolvent for the polymer because its solubility in the CO 2
is negligible [3]. The polymer selected in this work presented high molecular weight
and consequently it was insoluble in CO2 [8, 9], but terpene oils present low
molecular weight and non polarity and by these reasons they are fully soluble in
CO2 at mild operating conditions (Chapter 4). Due to the small size of the CO2
molecule, two-way diffusion is much faster than in the case of conventional liquid
antisolvent. Thus, during the antisolvent precipitation process, the accelerated mass
transfer between the mediums can facilitate very rapid phase separation and hence
the production of materials with a well defined structure [1].
The knowledge of phase behaviour of the solute + solvent + antisolvent system plays
an important role to determine suitable operating conditions to conduct
precipitation of polymer and understanding the kinetic nucleation mechanism and
growth of the particles. The control of products morphology is possible since CO2
modifies the properties of polymers as a consequence of its sorption and swelling
and a good control of size and size distribution of the material is expected. The
morphology of the spheres, foams or fibres is related to the location of phase
separation [2]. Although many parameters should be considered during the
optimization of these processes, the most important is the phase behaviour of the
systems. A thermodynamic study of the system solute-solvent-antisolvent is
extremely useful to address the feasibility of the process and to exploit the effects of
pressure and temperature because it governs the mass transport [10].
Although the binary equilibriums have been described and they can be predicted
based on the experimental data compilated in the literature (Chapter 4), they are
not enough to determine the behaviour of the mixtures. The behaviour of the binary
systems solvent-antisolvent could be dramatically modified because of the addition
of a solute, especially when it concerns a polymer. Due to the presence of polymers
in the process, equilibrium data and accurate predictions of the phases behaviour
121
are very difficult to achieve. Thus experimental work is needed because it will
complete and improve the existing models.
The potential applications of polymer in the chemical industries have generated a
corpus of literature available on the scCO2 and polymer processing techniques.
However, most of the studies using polymers and supercritical technology are
carried out at high pressure and temperature under which the polymers are molten
but consequently they can be degraded, changing the polystyrene morphology [1114]. As far as we know, few works provide phase equilibrium data of polymer
solutions at low temperatures but it is important since polymer does not undergo
chain degradation and a lower energetic consumption is required [15].
In this Chapter, we present the phase equilibrium data for the ternary systems
CO2/Limonene/Polystyrene and CO2/p-Cymene/Polystyrene. The limits of the
regions where an antisolvent process could be carried out were determined in order
to select the most suitable working conditions. The influence of the pressure, the
temperature and the initial concentration of the solution was evaluated and
correlated. Also the ternary diagrams were built to highlight the importance of
understanding high-pressure phase behaviour. Finally, in order to drawn some
general trends, the solubility and sorption data were fitted to semi-empirical
equations which allowed a global comprehension of the solubility of p-Cymene and
CO2 in the ternary system.
5.2. The ternary system CO2/Limonene/PS
In this section, phase equilibrium data for the ternary systems CO 2 (1) + Limonene
(2) + PS (3) at different concentrations of PS in the organic solvent and at different
temperatures and pressures are reported.
With the aim of separate the PS from its solution in Limonene, it is necessary to
determine when the homogeneous and/or heterogeneous regions are presented.
Since working conditions of the separation operation are suited to ensure an initial
homogeneous region that turns into a heterogeneous region where the PS
precipitates and it is separated from the initial mixture. To determine the
homogeneous and heterogeneous regions, experiments were performed at mild
working conditions in the temperature and pressure ranges from 25 to 40 ºC and 50
to 150 bar. Higher temperatures were not used to decrease the energetic consume
and not exceed the glass transition temperature of the polymer (dramatically
decrease by the presence of CO2) (Chapter 4) [16-18]. The initial concentration of PS
in Limonene has been considered in the range from 0.05 to 0.80 g/ml since at fixed
temperature and pressure, the composition of the phases depends on the initial
concentration of the mixture. The presence of solute in a mixture solvent/antisolvent
may change the phase behaviour, particularly, in the case of polymers. There are
different considerations about the relevance of the solute presence in the
122
Chapter 5
morphology of the precipitated particles or foams, by this reason we decided to
include the polymer initial concentration as a variable of study. To test the
reproducibility of the experiments, we checked the mass balance of each component,
in the two regions, which was better than 2.08 % in vapour phase.
The experimental data have been illustrated in a two-dimensional diagram by using
an isothermal pressure-composition prism with a triangle as a base [19] (Figures
5.1, 5.2 and 5.3). The binary equilibrium data previously obtained are also included
as the limits of the different axis.
On analysis of the experimental ternary diagrams (Figure 5.1, 5.2 and 5.3) a twophase region is observed. According to experimental data depicted, it is observed
that Polystyrene is not collected in the vapour phase. Samples from vapour phase
were analyzed by GPC and any peak belonging to the polymer was detected.
At low carbon dioxide concentrations the system is homogeneous when PS
concentration in Limonene is lower than around 30% and at higher carbon dioxide
concentrations the phase boundary is reached. When PS concentration increases,
two phases are made, liquid-liquid phase separation is produced and a polymer rich
(and Limonene lean) or lean (and Limonene rich) phases are stood out. When
pressure increased, the two one-phase regions slightly shrink; it means that the
mutual miscibility of the three components increases as pressure increased, as it
was studied with the binary equilibriums (Chapter 4). This fact is better observed
when pressure reaches 84.4 bar at 40ºC, after the cited point Limonene is fully
miscible with CO2.
When temperature increases, two effects are noticeable, by one side, higher
temperature improves the volatility of Limonene [20, 21], but on the other side, PS
solubility in Limonene is also enhanced [22]. Furthermore, the two-phase region of
the Limonene/carbon dioxide system increases, while the solubility of carbon dioxide
in PS decreases with increasing temperature [23].
Nevertheless, it was observed that the position of the binodal was not significantly
affected by changes in pressure or temperature, in the studied range 50-150 bar and
25-40 ºC.
In ternary diagrams the tie lines are also represented, in the two-phases region.
They connect the compositions of bottom phase (rich in polymer) and top phase (rich
in CO2). As it is observed, these dashed lines present negative slope probably due to
the higher interaction between Limonene and PS than with CO2, especially at high
initial polymer concentration into the solutions.
123
Figure 5.3. Phase behaviour of the
system CO2/Limonene/PS at 40ºC at 50,
75, 100 and 150 bar. Concentrations are
given in mass fraction.
Figure 5.2 Phase behaviour of the system
CO2/Limonene/PS at 30ºC at 50, 75, 100
and 150 bar. Concentrations are given in
mass fraction.
Figure 5.1. Phase behaviour of the
system CO2/Limonene/PS at 25ºC and at
50, 75, 100 and 150 bar. Concentrations
are expressed as mass fraction.
124
Chapter 5
In the ternary representation it is difficult to observe the influence of pressure,
temperature and gas content on the phase behaviour at the same time. Next, we
have analyzed particularly each variable to better understand their influence in the
Limonene solubility in CO2 rich phase. Limonene solubility was subject of study to
obtain a polymer similar to the virgin and completely free of solvent. Thus,
maximum Limonene solubility in CO2 is the target of this section.
5.2.1. Effect of pressure
Figure 5.4 shows the limonene solubility in the vapour phase obtained
experimentally in the ternary system CO 2/Limonene/PS. Also the theoretical
solubility (represented as lines) is shown from 25 to 40ºC.
120
T: 25ºC
0.05 g/ml
0.10 g/ml
80
0.20 g/ml
S (mg/g)
100
60
40
20
0
T: 30ºC
0.05 g/ml
0.10 g/ml
80
0.20 g/ml
S (mg/g)
100
60
40
20
0
T: 40ºC
0.05 g/ml
0.10 g/ml
80
0.20 g/ml
S (mg/g)
100
60
40
20
0
20
40
60
80
100
120
140
160
180
200
Pressure (bar)
Figure 5.4. Effect of pressure on the experimental solubility of Limonene in top phase
(markers) Theoretical solubility predicted at 25ºC (a), 30ºC (b) and 40ºC (c).
125
With increasing pressure of CO2, the solvent power of the molecule to dissolve the
terpene increases, and therefore, the concentration of Limonene in the vapour phase
becomes larger. Nevertheless, it is observed that solubility increases with pressure,
describing an asymptotic tendency that is kept constant at a pressure value around
100 bar. This fact is due to the increase of CO2 density that is directly related to the
dissolving power [24], thus, the density of the saturated CO2 phase increases rapidly
with pressure, while the density of the liquid phase, saturated with CO 2, although
slightly increasing at low and medium pressure, decreases at higher pressure
[25].
5.2.2. Effect of temperature
The effect of temperature, at constant pressure, on the experimental (markers) and
theoretical (lines) solubility of Limonene in the top phase is shown in Figure 5.5.
60
(a) P: 50 bar
0.05 g PS/ml Lim
20
0.10 g PS/ml Lim
0.20 g PS/ml Lim
15
10
5
0
30
35
20
10
P: 75 bar
0.05 g PS/ml Lim
0.10 g PS/ml Lim
0.20 g PS/ml Lim
25
30
35
40
Temp (ºC)
120
(c)
(d)
100
S (mgLim/g CO2)
80
S (mgLim/g CO2)
30
40
Temp (ºC)
100
40
0
25
(b)
50
S (mgLim/g CO2)
S (mgLim/g CO2)
25
60
40
P: 100 bar
0.05 g PS/ml Lim
20
0.10 g PS/ml Lim
0.20 g PS/ml Lim
0
25
30
35
Temp (ºC)
40
80
60
40
P: 150 bar
0.05 g PS/ml Lim
20
0.10 g PS/ml Lim
0.20 g PS/ml Lim
0
25
30
35
40
Temp (ºC)
Figure 5.5. Effect of temperature on the experimental solubility of Limonene in top phase
(markers). Theoretical solubility predicted at 50 (a), 75 (b), 100 (c) and 150 bar (d).
According to Figure 5.5, at low pressures, when increasing temperature Limonene
solubility increases as it was observed also by Chang and Chen in the binary system
[25]. When pressure increases, the critical pressure of the mixture is reached
(Limonene/CO2): at 30 ºC, 80 bar and at 40ºC, 85 bar [20] and Limonene is fully
miscible with CO2 above this pressure. After reaching the mixture critical point, at
126
Chapter 5
concentration higher than 0.10 g PS/ml Limonene, when temperature increases
solubility decreases. It is important to stand out that polymer solubility in
Limonene increases when temperature increases as it was shown in previous work
[26], which means a decrease in the solubility of Limonene in the CO2 rich phase.
This behaviour will be studied in the subsequent section because as it can be
observed, the effect of temperature on solubility is quite hazy because it is
influenced by several effects.
5.2.3. Effect of concentration
Figure 5.6 shows the influence of concentration increase from 0.05 to 0.80 g/ml on
the solubility of Limonene in CO2 rich phase.
25
25 ºC
30 ºC
40 ºC
50
S (mgLim/g CO2)
20
S (mgLim/g CO2)
60
(a) P: 50 bar
15
10
5
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Conc (g PS/ml Lim)
100
40
30
20 (b) P: 75 bar
25 ºC
30 ºC
40 ºC
10
0
0.00
0.05
0.10
0.15
60
40
20
0
0.0
25 ºC
30 ºC
40 ºC
100
S (mgLim/g CO2)
S (mgLim/g CO2)
80
0.25
(d) P: 150bar
(c) P: 100 bar
25 ºC
30 ºC
40 ºC
0.20
Conc (g PS/ml Lim)
120
80
60
40
20
0.1
0.2
0.3
0.4
0.5
Conc (g PS/ml Lim)
0.6
0.7
0.8
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Conc (g PS/ml Lim)
Figure 5.6. Effect of initial concentration of PS on the experimental solubility of Limonene
(markers) at (a) 50 bar; (b) 75 bar; (c) 100 bar; (d) 150 bar.
When the concentration of Polystyrene in the solution increases Limonene is
embedded in the polymeric matrix making it more difficult to solve in CO 2,
independently of the working pressure. The most pronounced negative slope is
observed at 100 bar and at high temperature (40ºC) because at those conditions, CO 2
is saturated of Limonene [20], but when initial PS concentration increases,
Limonene is more akin to the polymer [26]. In general, Limonene solubility in
vapour phase decreases when the initial concentration increases.
127
Based on these results, the initial concentration of polymer in Limonene is crucial to
carry out the separation process by high pressure CO 2. Because contrary to
preliminary belief, high concentrations of PS in Limonene do not assist the global
recycling process, since values over 0.40 g PS/ml Limonene decreases dramatically
the solvent solubility in CO2, at the studied working pressure and temperature.
5.2.4. Sorption of CO2 into PS/Limonene solutions
Phase separation produced during CO2 sorption in solutions of PS is crucial in
generation of PS porous matrices or foams. While in the unswollen state the
diffusion coefficient for large molecules is negligible, it increases by orders of
magnitude in the swollen state. The CO2 sorption into PS/Limonene solutions was
determined in a range of pressure from 50 to 150 bar and concentration from 0.05 to
0.20 g PS/ml Limonene at two different temperatures, 30 and 40ºC. Figure 5.7
shows data of CO2 sorption obtained experimentally in solutions of PS/Limonene
together with the obtained Shieh and Liu in the binary system CO2/PS [27].
According to Figure 5.7, CO2 sorption in PS/Limonene solutions decreases when
pressure increases reaching PS sorption typical values over 80 bar approximately,
which can be explained if the critical pressure of Limonene solutions is considered
[28]. Above this pressure, the terpene oil is fully miscible with CO 2 and the gas
molecule could only be retained in the polymer rich phase which is mainly composed
by PS and the solvent not solubilised into CO2. As Limonene solubility does not
increase once critical mixture conditions are reached, the amount of CO 2 soaked are
kept constant.
0.45
T:30ºC; C0: 0.05 gPS/ml Lim
0.40
Sorption (g CO 2/g PS)
0.35
0.30
0.25
T:32ºC. Shieh and Liu, 2003 [27]
0.20
0.15
0.10
0.05
0.00
0
25
50
75
100
125
150
175
200
Pressure (bar)
Figure 5.7. Sorption data for Polystyrene/Limonene solutions exposed to carbon dioxide.
Experimental data from this work at 30ºC and 40ºC are shown as markers (Average Standard
Deviation 0.42 mg) and data from Shieh and Liu [27] at 32ºC are shown as a solid line.
128
Chapter 5
Experimental sorption data of CO2 into PS obtained from Shieh and Liu in PS [34]
are shown in Figure 5.7. According to the experimental data shown in the literature,
when pressure increases the sorption of CO2 increases, reaching a constant value
close to its critical point [35]. Comparing our experimental data and the
summarized from the literature, it is observed that higher values of sorption are
obtained in the case of PS/Limonene solutions, which suggests that the sorption of
CO2 is not exclusively stayed into the polymer but also into the Limonene.
Nevertheless, when the critical pressure of the mixture is reached, Limonene is fully
solubilised in CO2 and it goes into the vapour phase, obtaining values of sorption
similar to the described in the literature because the polymeric rich phase was
mainly composed of pure PS.
5.2.5. Distribution coefficient
Considering the interest of a separation process design for a ternary mixture and
taking into account that the CO2 used as supercritical solvent is removed from the
system after the separation, a global process can be designed. With respect to a
separation process, the partition coefficients (Ki) are more relevant than the
solubility. Ki is a key thermodynamic parameter, which is defined as the ratio of the
mole fraction of the component in the top phase (yi) to that in the bottom phase (xi)
and it is expressed by the following equation.
The distribution coefficient of Limonene as a function of pressure, temperature and
concentration is depicted in Figure 5.8.
Limonene distribution coefficient increases when temperature and pressure
increase, having similar shapes to the solubility curves (Figures 5.4 and 5.5.). The
effect of concentration can also be observed, when concentration increases from 0.05
to 0.20 g PS/ml Limonene, KLim decreases. According to the separation process a
maximum KLim is desired. It can be achieved at pressure close to 100 bar (the
plateau is reached), higher temperature and low polymer concentration.
129
0
80
0.015
0.012
25ºC; 0.05 g PS/ml Lim
KLim
0.009
0.006
0.003
90
100
110
120
130
140
150
160
0.000
Pressure (bar)
0.005
KLim
0.004
0.003
0.002
0.001
90
100
110
120
130
140
150
0.000
0.015
160
Pressure (bar)
0.012
KLim
80
0.009
0.006
0.003
0.000
40
60
80
100
120
140
160
Pressure (bar)
90
100
110
120
130
Pressure (bar)
140
150
160
Figure 5.8. Distribution coefficient of Limonene at 25, 30 and 40ºC.
According to the results described along this section, it can be concluded that it is
possible to separate polymer and Limonene rich phases from an initial solution
PS/Limonene, using CO2 as antisolvent. The most suitable operational conditions to
achieve the Limonene recovery, and subsequently dried Polystyrene recovery, were
determined according to experimental results of distribution coefficients. As it was
showed, it is possible to solubilise the maximum amount of Limonene at higher
pressure and temperature and lower concentrations.
130
Chapter 5
5.3. The ternary system CO2/p-Cymene/PS
In the antisolvent techniques the supercritical fluid is not able to dissolve the solid
(PS in this case) but it is completely miscible with the liquid. The precipitation
process will take place at those conditions where solvent and antisolvent are
miscible over the whole composition range while the polymer and the antisolvent
are totally immiscible. The development of this section has been divided in four
parts: the definition of the ternary diagrams to limit the regions, the study of the
solubility of the terpene in the CO2 rich phase, the solubility of the CO2 into the
polymeric solution and finally the selection of the most suitable conditions to
perform the precipitation of PS from its solution in p-Cymene using CO2.
Figure 5.9 shows the vapour-liquid equilibrium composition of the binary system
CO2/p-Cymene. According to the Figure, temperatures between 30 and 40 ºC and
pressures up to 120 bar would be suitable to solubilise fully the p-Cymene and
therefore to precipitate the Polystyrene.
According to the VLE data of p-Cymene/CO2 (Figure 5.9), all the experiments were
carried out at mild working conditions in the temperature and pressure ranges from
25 to 40 ºC and 50 to 150 bar. The working conditions were selected attending to the
experimental solubility of p-Cymene in CO2 in order to achieve the fully
solubilisation of p-Cymene in the vapour phase, while the energy consumption was
moderate and the glass transition temperature of the polymer was not exceeded
(dramatically decrease by the presence of CO 2) [16-18]. The concentration was
ranged from 0.05 to 0.80 g PS/ml p-Cymene because it would allow tuning the
morphology of the PS during the precipitation hence its importance on the selection
of working conditions.
At the equilibrium conditions of this work there were two phases: vapour phase
enriched in CO2 and liquid phase enriched in PS. It was assumed and checked
experimentally that the polymer kept in the liquid phase [9, 29]. To test the
reproducibility of the experiments, we checked the mass balance of each component,
in the two regions, which was better than 1.14 % in vapour phase and 3.86 % in the
liquid phase.
5.3.1. Ternary Diagrams
From the binary systems described in the literature (CO 2/p-Cymene [30], pCymene/PS [22] and PS/CO2 [31]), only the limits of the ternary phase diagram
could be generated. But the overall knowledge of the system CO 2/p-Cymene/PS must
be experimentally obtained since the presence of a solute in a mixture
solvent/antisolvent may change the phase behaviour, particularly if the solute is a
polymer.
131
120
(a)
110
T: 30ºC
T: 50ºC
T: 70ºC
100
90
Pressure (bar)
80
70
60
50
40
30
20
10
0
0.0
0.1
0.2
0.3
0.4
0.5
Solubility of CO 2 (g CO 2/g Total)
3
0.7
2
(b)
Solubility of p-Cymene (kg/m )
0.6
0.8
0.9
0.99
1.00
x/y CO
120
100
80
60
40
20
0
200
400
600
800
3
1000
3
(c)
2
1
0
0
200
400
600
800
1000
0.8
0.6
0.4
0.2
25
50
75
100
125
150
175
3
Figure 5.9. (a) Equilibrium phase composition diagram for the CO 2/p-Cymene binary system
at 30, 50 and 70 ºC. (b) Solubility of p-Cymene in the vapour rich phase as a function of
density. (c) Effect of density on the solubility of CO 2 into the liquid phase.
Homogeneous and heterogeneous regions should be delimited as initial step for the
design of a process to recover a polymer from a solution using CO 2 as antisolvent.
Next, the selection of the operating conditions should be performed according to the
initial homogeneity of the solution which is turned into a heterogeneous mixture in
order to precipitate the polymer and separate the solvent.
The experimental data have been depicted in a diagram by using an isothermal
pressure-composition prism with a triangle as a base [19]. From the analysis of the
experimental ternary diagrams (Figure 5.10) the two-phase region was observed
(shaded area).
At atmospheric pressure, the system is homogeneous till reaches the critical
concentration of the polymer ( 29% of PS in weight) [22]. But when CO2 is added to
the system, the concentration of PS necessary to induce the phase separation
decreases, due to the antisolvent effect of CO 2; the phase boundary is reached and
the phase separation occurs. When pressure increases, the solubility of p-Cymene
increases and the heterogeneous area was expected to shrink because the mutual
miscibility of the three compounds was enhanced. An increase of temperature
132
Chapter 5
entails two noticeable effects, the improvement of p-Cymene solubility in CO2 [30]
and the enhancement of PS solubility in p-Cymene [22].
Also, the two-phase region of the p-Cymene/carbon dioxide system increased, while
the solubility of carbon dioxide in PS decreased when temperature increased [23].
The increase of PS concentration does not show any crucial shift on the position of
the phase boundary.
The tie lines shown in Figure 5.10 represent the composition in the bottom phase
(polymer rich phase) and the top phase (CO 2 rich phase). The most relevant effect
over the position of the tie lines was caused by the PS concentration in the initial
solution. The increase in the polymer concentration is the responsible of the increase
in the negative slope of the cited lines, probably due to the higher interaction
between p-Cymene and PS than with CO2. When pressure rises, the solubility of pCymene in the vapour rich phase increases and the slope of the tie-lines become
more pronounced.
The position of the binodal line was not significantly influence by shifts in pressure
or temperature in the range of the studied conditions. According to Fig. 5.8, the
effect of pressure and temperature was not evident from the ternary diagrams since
the position of the binodal was not significantly affected by changes in the cited
variables. By these reasons, the analysis of the pressure, temperature and
concentration on the solubility of p-Cymene in the vapour rich phase and the CO 2 in
the polymer rich phase was studied. The pressure and temperature effects was
gathered and expressed through the density of CO 2 because it compiles the solvent
power of the mixture.
5.3.2. Influence of density and concentration on p-Cymene solubility
in the vapour phase
The influence of the CO2 density on the solubility of p-Cymene in the CO2 rich phase
was studied in the range of pressure from 50 to 150 bar and temperature from 25 to
40 ºC. Figure 5.11 shows the effect of density on the solubility of p-Cymene in the
vapour phase at three different concentration of polymer.
According to Figure 5.11, when the density of CO 2 increases an increase on pCymene solubility is observed. The most pronounced effect is noticed from 700 kg/m3
belonging to the critical point of the CO 2. This fact demonstrates the improved
solvent power of the CO2 above its critical point, which enhances the dissolution of
the terpene in the vapour phase. Nevertheless, when temperature increases a
density decrease is produced and consequently a decrease on p-Cymene solubility.
133
Figure 5.10. Phase behaviour of the system CO2/p-Cymene/PS at (a) 25 ºC, (b) 30 ºC and (c)
40 ºC at constant pressure: 50, 100 and 150 bar. Concentrations are given in mass fraction.
134
Chapter 5
150
(a)
Binary system CO2/p-Cymene
C0:0.05 g PS/ml Cym
120
90
60
150
(b)
3
Solubility of p-Cymene (kg/m )
30
C0:0.10 g PS/ml Cym
120
90
60
30
150
(c)
C0:0.20 g PS/ml Cym
120
90
60
30
0
0
150
300
450
600
750
900
3
Figure 5.11. Influence of CO2 density on the solubility of p-Cymene (kg p-Cymene/m3 CO2) in
the vapour rich phase at three different concentration (a) ■ 0.05 gPS/ml Cymene; (b) ● 0.10
gPS/ml Cymene; (c) ▲ 0.20 gPS/ml Cymene. Dashed lines represent the solubility of pCymene in the binary system CO 2/p-Cymene.
Also, it is showed that the presence of PS prevents the maximum solubility of pCymene in the vapour phase from being reached. The PS effect is observed by
comparison between the solubility of p-Cymene in the binary system CO2/p-Cymene
(dashed lines) and in the ternary system CO2/p-Cymene/PS (markers and
continuous lines) in Figure 5.11. When there is not any PS in the solution, the
highest values of solubility in the binary system are around 150 kg p-Cymene/m3
CO2 (: 900 kg/m3). Nevertheless, when the concentration of polymer is 0.05 g PS/ml
p-Cymene, the solubility of p-Cymene in the CO2 rich phase at the same working
conditions is around 90 kg p-Cymene/m3 CO2.
With the aim of thoroughly study the effect of PS concentration of the initial
solution on the solubility of p-Cymene a wider range of concentration was
investigated (Figure 5.12).
135
100
45
3
0.5
0.0
0.00
2.0
0.05
0.10
0.15
0.20
0.25
Concentration (g PS/ml Cym)
30
15
0
0.00
60
0.05
0.20
0.25
0.5
0.0
0.00
0.05
0.10
0.15
0.20
0.25
30
15
0
0.00
20
0.5
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
0.00
80
0.05
0.10
0.15
0.20
0.25
0.05
0.10
0.15
0.20
(h) P: 150 bar; T: 30 ºC
60
40
20
0
0.00
30
25
0.05
0.10
0.15
0.20
0.25
(i) P: 150 bar; T: 40 ºC
10
20
5
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
15
10
5
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Figure 5.12. Effect of concentration of PS (abscissa axis) on the solubility of p-Cymene (kg pCymene/m3 CO2) in the vapour rich phase at (a) P: 50 bar; T: 25 ºC; (b) P: 50 bar; T: 30 ºC; (c)
P: 50 bar; T: 40 ºC; (d) P: 100 bar; T:25 ºC; (e) P: 100 bar; T: 30 ºC; (f) P: 100 bar; T: 40 ºC; (g)
P: 150 bar; T: 25 ºC; (h) P: 150 bar; T: 30 ºC; (i) P: 150 bar; T: 40 ºC.
As it is observed, an increase in the PS concentration entailed a decrease in the
solubility of p-Cymene in the CO2 rich phase. When the concentration of PS in the
solution increases, p-Cymene is swelling the polymeric solution and the dissolution
of the terpene in CO2 is not enhanced. In all cases, at low PS concentration ( 0.1 g
PS/ml p-Cymene) there is a decrease of solubility regarding the concentration, but
at higher concentration the solubility decrease is softer. Thus, the decrease of pCymene solubility in CO2 as a consequence of the polymer in the initial solution has
been confirmed. Polystyrene precipitation process would be preferably achieved at
low concentration in order to recover a dried polymer since at those conditions the
solvent could be fully soluble in CO2.
The nonlinear dependence of density and concentration on the solubility of pCymene was studied. Chrastil’s equation [23] was selected since it has been widely
used to describe the phase behaviour of carbon dioxide and terpenes. But according
136
0.25
3
15
3
3
1.0
25
(f) P: 100 bar; T: 40 ºC
S (kg Cym/m CO2)
(c) P: 50 bar; T: 40 ºC
1.5
50
3
45
3
1.0
2.0
S (kg Cym/m CO2)
0.15
(e) P: 100 bar; T: 30 ºC
S (kg Cym/m CO2)
3
S (kg Cym/m CO2)
(b) P: 50 bar; T: 30 ºC
1.5
0.10
S (kg Cym/m CO2)
1.0
(g) P: 150 bar; T: 25 ºC
75
3
S (kg Cym/m CO2)
1.5
3
S (kg Cym/m CO2)
(d) P: 100 bar; T: 25 ºC
S (kg Cym/m CO2)
60
(a) P: 50 bar; T: 25 ºC
S (kg Cym/m CO2)
2.0
Chapter 5
to the results shown in Figure 5.11 and 5.12, a modification of the cited equation
was proposed to include the effect of polymer concentration in the initial solution.
[5.1]
where S is the solubility of p-Cymene in CO2 rich phase in [kg/m3], C1 is a constant
dependent on the molecular weights of the solute and solvent and on the association
constant, C2 is a constant dependent on the total heat of vaporization, T is the
temperature in [K], k is the association number of molecules in the solvate-complex,
 is the CO2 density in [kg/m3], C3 is the new fitting constant and CPS is the
concentration of PS in the initial solution in [g PS/ml p-Cymene]. The results of the
fitting and deviations are shown in Table 5.1.
Table 5.1. Estimated parameters obtained from the correlation of eq. [5.1] for the solubility of
p-Cymene in vapour phase as a function of CO 2 density, temperature and concentration of PS
in the initial solution.
C1
C2
C3
k
R2
-43.26 ± 2.06
1460.15±614.02
-0.51±0.04
6.09±0.33
0.93
The correlation fits the experimental data very closely and presents an acceptable
value of correlation in a range of pressure from 50 to 150 bar, temperature from 25
to 40 ºC and concentration from 0.05 to 0.80 g PS/ml p-Cymene. The opposed effects
of density and concentration on the solubility of p-Cymene in CO2 from the ternary
system are observed through the opposite sign of k and C3, where the positive
influence of density prevails over the negative effect of PS concentration.
In Figure 5.13 the comparison between solubility of p-Cymene correlated and
experimental values is observed.
80
3
Solubilitycorrel (kg p-Cymene/m CO2)
100
60
40
20
0
0
10
20
30
40
50
60
70
80
90
100
3
Solubilityexp (kg p-Cymene/m CO2)
Figure 5.13. Comparison between experimental and correlated solubility with the modified
Chrastil’s equation of p-Cymene in CO2.
137
The modification of the Chrastil’s semi-empirical equation including the effect of the
polymer concentration fits suitably the experimental results for the ternary system
CO2/p-Cymene/PS.
5.3.3. Influence of density and concentration on CO2 solubility in the
liquid phase
The CO2 absorbed into the polymeric solution would determine the feasibility to
carry out the antisolvent process and the possible modification of the PS structure.
The effect of density on the CO2 sorption in the liquid phase in a range of pressure
from 50 to 150 bar, temperature from 25 to 40 ºC and at three different values of
concentration (0.05-0.10-0.20 g PS/ml Cymene) is shown in Figure 5.14.
0.20
(a)
Binary system CO2/PS
C0:0.05 g PS/ml Cym
0.15
Sorption of CO 2 (g CO 2/g Solution)
0.10
0.05
0.00
(b)
0.12
C0:0.10 g PS/ml Cym
0.09
0.06
0.03
0.00
(c)
0.08
C0:0.20 g PS/ml Cym
0.06
0.04
0.02
0.00
0
150 300 450 600 750 900
3
Figure 5.14. Influence of CO2 density on the solubility of CO2 (g CO2/ g Solution) in the
polymeric rich phase in a range of concentration from (a) ■ 0.05 gPS/ml Cymene; (b) ● 0.10
gPS/ml Cymene; (c) ▲ 0.20 gPS/ml Cymene.
138
Chapter 5
The influence of density on CO2 sorption inside the solution shows that when
density increases, the sorption of CO2 in the polymeric rich phase (PS/p-Cymene)
decreases. Independently of the PS concentration in the initial solution, the CO 2
sorption reaches a constant value at density above 600 kg/m3. Figure 5.14 shows
that the sorption of CO2 decreased up to 300-400 kg/m3 where it is kept practically
constant around 0.02 g CO2/g of solution. According to the literature, the sorption of
CO2 in PS at 100 bar and 32 ºC is close to 0.05 g CO2/ g PS which agrees with the
data obtained in this work [24].
Nevertheless, the highest value of CO2 sorption in the ternary system is reached at
C: 0.05 g PS/ ml p-Cymene and : 100 kg/m3 ( 0.2 g CO2/g Solution). Similar values
are observed in the binary system CO2/p-Cymene shown in Figure 5.9 c, which could
be explained by the low concentration of polymer into the solution. The comparison
among the sorption of CO2 in the binary systems (CO2/p-Cymene and CO2/PS) with
the values of sorption obtained in this work in the case of the ternary system (CO2/pCymene/PS) at concentration 0.05 g PS/ ml p-Cymene are shown in Figure 5.15.
1.0
Ternary system CO2/p-Cymene/PS
Sorption of CO 2 (g CO2/g Total)
0.8
0.6
0.4
0.2
0.0
0
150
300
450
600
3
750
900
Figure 5.15. Comparison between the solubility of CO2 into p-Cymene (■), PS (●) and in the
solution PS/p-Cymene (*) as a function of density.
At low values of density, the sorption of CO 2 in the polymeric solution is similar to
the values of solubility in the binary system CO2/p-Cymene; but when density
increases, the sorption of CO2 decreases reaching similar values to the solubility of
CO2 in Polystyrene. According to Figure 5.15, the behaviour of the solubility of CO2
in the polymeric solutions could be explained attending to the presence of p-Cymene
in the mixture which could modify the sorption of CO 2. It can be inferred that CO2
sorption could be distributed between the solvent and the polymer following the
next expression.
139
[5.2]
where wCO2sol is the total amount of CO2 retained in the solution, wCO2PS and
wCO2Cym represents the CO2 retained in PS and p-Cymene, respectively, and  is a
fitting parameter. wCO2sol was obtained experimentally; the CO2 sorption in PS
(wCO2PS) was calculated following Sanchez-Lacombe equation of state and data were
compared with the literature [25-28]. The CO2 absorbed in p-Cymene (wCO2Cym) was
obtained according to Peng-Robinson equation of state and they were also checked
with the literature [19] and our own data.  is a fitting variable depending on the
pressure, temperature and initial concentration of the solution that was obtained
from eq. [5.2].
The influence of pressure and temperature (expressed as density) on  is shown in
Figure 5.16, where  decreased when CO2 density increased and reached a constant
value near zero when p-Cymene is fully soluble in CO2 and it is only retained inside
the PS.
0.30
(a)
C0:0.05 g PS/ml Cym
0.25

0.20
0.15
0.10
0.05
0.00
(b)
C0:0.1 g PS/ml Cym
(c)
C0:0.20 g PS/ml Cym

0.12
0.09
0.06
0.03
0.00
0.08

0.06
0.04
0.02
0.00
0
150
300
450
600
750
900
3
Figure 5.16. Influence of density and initial concentration of PS/p-Cymene solutions on the
parameter at (a) ■ 0.05 gPS/ml Cymene; (b) ● 0.10 gPS/ml Cymene; (c) ▲ 0.20 gPS/ml
Cymene.
140
Chapter 5
An increase on CO2 density promotes an increase of the CO2 absorbed in p-Cymene
that reaches the saturation around 400 kg/m3 corresponding with the critical point
of the mixture ( 84 bar and 40ºC) [9]. This behaviour is expected since the amount
of available p-Cymene for the CO2 to be absorbed in is lower when the PS
concentration increases. The solvent is the responsible to increase the sorption of
CO2 inside the solution but when the critical point of the mixture is reached, the
sorption is exclusively due to the polymer.
Thus, the decrease of CO2 sorption in the PS/p-Cymene mixtures at low pressures is
due to the solubilisation of p-Cymene in the vapour rich phase which reduced the
capability of sorption of the solutions since the solvent is volatilised. The effect of
temperature on the CO2 sorption shows that an increase of temperature entails a
decrease of density and consequently an increase on CO 2 solubility inside the PS/pCymene solutions.
The influence of the concentration of PS in the p-Cymene solution on the sorption of
CO2 into the polymeric rich phase was studied from 25 to 40º C, at three different
pressures (50-100-150 bar) and in a wider range of concentration from 0.05-0.80 g
PS/ml p-Cymene (Figure 5.17).
According to Figure 5.17, the sorption of CO2 into the polymeric solutions decreases
when pressure increases and temperature decreases, it means, when the density of
CO2 increases. At constant temperature, when pressure increases, the solvent power
of CO2 is enhanced and the amount of p-Cymene in the solution decreases since it is
in vapour phase and consequently, the concentration of CO 2 into the polymer liquid
phase decreases.
The effect of density and concentration on the sorption of CO 2 in the polymer rich
phase could be fitted following a modification of the Dual-Mode sorption model [29,
30] (Chapter 4). It is typically used to describe the sorption of gases in polymers but
in this work the innovation proposes the replacement of pressure by density and the
addition of a third term to represent the polymer concentration:
[5.3]
where S is the solubility of CO2 in the polymeric rich phase in [g CO2/g solution], kD
is the Henry’s law coefficient, b represents the hole affinity parameter, which is a
measure of the affinity between the solute molecules and the Langmuir sites, C’ H is
the capacity parameter,  is the density replacing the traditional value of pressure
shown in the model to include the temperature effect in [kg/m3], C’P and a are the
new variables introduced to include the concentration of polymer and C is the
concentration of PS in the initial solution in [g PS/ml p-Cymene].
141
S (g CO 2/g Solution)
0.06
0.03
0.04
0.02
0.02
0.00
0.00
0.05
0.10
0.15
0.20
0.01
0.00
0.25
0.00
(b) P: 50 bar; T: 30 ºC
0.05
0.10
0.15
0.20
0.25
0.10 Concentration (g
PS/ml Cym)
(e) P: 100 bar; T: 30 ºC
0.08
0.15
0.20
0.05
0.10
0.15
0.20
0.25
(c) P: 50 bar; T: 40 ºC
0.05
0.10
0.15
0.20
0.25
(f) P: 100 bar; T: 40 ºC
0.08
0.15
0.05
0.05
0.10
0.15
0.20
0.25
(i) P: 150 bar; T: 40 ºC
0.01
0.00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.00
0.00
0.02
0.02
0.00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.25
0.03
0.04
0.05
0.20
0.04
0.06
0.10
0.15
0.01
0.00
0.00
0.10
0.10
0.02
0.02
0.00
0.00
0.05
(h) P: 150 bar; T: 30 ºC
0.03
0.04
0.05
0.05
0.04
0.06
0.10
0.00
0.00
0.05
0.10
(g) P: 150 bar; T: 25 ºC
0.04
0.05
(d) P: 100 bar; T: 25 ºC
0.08
0.15
0.20
0.10
(a) P: 50 bar; T: 25 ºC
0.20
0.00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Figure 5.17. Influence of CO2 density on the solubility of CO2 (g CO2/ g Solution) in the
polymeric rich phase from an initial PS/p-Cymene solution at constant pressure and
temperature (a) P: 50 bar; T: 25 ºC; (b) P: 50 bar; T: 30 ºC; (c) P: 50 bar; T: 40 ºC; (d) P: 100
bar; T:25 ºC; (e) P: 100 bar; T: 30 ºC; (f) P: 100 bar; T: 40 ºC; (g) P: 150 bar; T: 25 ºC; (h) P: 150
bar; T: 30 ºC; (i)c P: 150 bar; T: 40 ºC.
The results of the fitting are shown in Table 5.2. The value of correlation was
acceptable especially at high density since the model is recommended for glassy
polymers which exhibit lower values of CO2 sorption.
Table 5.2. Estimated parameters obtained from the correlation of eq. [5.3] for the sorption of
CO2 in the polymeric rich phase as a function of CO2 density, and concentration of PS in the
initial solution.
kD (m3/kg)
C’H
b (m3/kg)
C’P
a (ml/g)
R2
-9.02·10-5 ± 2.38·10-5
0.19±0.07
-0.19±0.04
-0.14±0.06
22.46±21.10
0.78
Figure 5.18 shows the comparison between the experimental sorption of CO 2 into
the polymeric rich phase and the correlation according to equation [5.3].
142
Chapter 5
Sorptioncorrel (g CO2/g Total)
0.100
0.075
0.050
0.025
0.000
0.000
0.025
0.050
0.075
0.100
Sorptionexp (g CO2/g Total)
Figure 5.18. Comparison between experimental and correlated solubility with the modified
dual-modal equation of CO2 in the polymeric solution PS/p-Cymene.
The modification of the Dual-Mode sorption including the influence of the
concentration in the initial solution fits pretty well the experimental results.
5.3.4. Selection of the operating conditions
Considering the interest of a precipitation process for the ternary mixture CO2/pCymene/PS and taking into account that the CO 2 used as supercritical solvent is
removed from the system after the separation, the optimum conditions could be
selected. With respect to a separation process, the partition coefficients (Ki) are
more relevant than the solubility. Ki is a key thermodynamic parameter, which is
defined as the ratio of the mole fraction of the component i (CO 2 or p-Cymene) in the
top phase (yi) to that in the bottom phase (xi) and it is expressed by the equation.
[5.4]
The distribution coefficients of CO2 and p-Cymene as a function of pressure,
temperature and concentration were depicted in Figure 5.19.
CO2 and p-Cymene distribution coefficient increases when pressure increases, but
decreases when temperature increases. The effect of concentration shows a negative
influence in the case of p-Cymene distribution coefficients, especially observed at 40
ºC but a positive effect on the partition coefficient of CO 2. According to the
experimental data, when PS concentration increases the solubility of p-Cymene and
CO2 in the vapour or liquid phase, respectively, decreases. The maximum solubility
of p-Cymene in the vapour phase is reached at the highest values of density, while
143
the highest values of CO2 sorption in the polymeric solutions are achieved when the
density of CO2 is low, showing opposite trends. Thus, the most suitable conditions to
perform the antisolvent process would be selected on the basis of the maximum
solubility of the solvent in the vapour phase since the solubility of CO 2 in the
solution could be considered constant and equal to the sorption in PS.
20
0.04
25 ºC
25 ºC
C0: 0.05 g PS/ml p-Cymene
16
0.03
kp-Cymene
2
kCO
12
8
0.02
0.01
4
20
0.030
30 ºC
30 ºC
16
0.020
2
kp-Cymene
12
kCO
0.025
8
4
0.015
0.010
0.005
15
0.015
40 ºC
40 ºC
12
0.012
kp-Cymene
kCO
0.009
2
9
6
3
0.006
0.003
0
0.000
0
25
50
75
100
125
Pressure (bar)
150
175
0
25
50
75
100
125
150
175
Pressure (bar)
Figure 5.18. Distribution coefficient of CO2 (left) and p-Cymene (right) as a function of
temperature (25, 30 and 40ºC) and concentration (■) 0.05 g PS/ ml p-Cymene; (○) 0.10 g PS/
ml p-Cymene; (▲) 0.20 g PS/ ml p-Cymene.
New operating conditions could be tried theoretically following the methodology
described in this work for the recovery of PS. Employing the correlation described in
equations [5.1] and [5.3] it should be possible to predict the solubility of p-Cymene
in CO2 and the sorption of CO2 into the solution in order to optimize the
precipitation. The aim of this work is the recycling of PS wastes. By this reason, the
maximum amount of polymer should be recovered without traces of solvent.
Furthermore, the process should be economically interesting which implies the
minimization of pressure and temperature. In accordance with these premises, a set
144
Chapter 5
of pressure, temperature and concentration values were studied theoretically to
define the optimum conditions where the precipitation could be performed. The
selected pressure to study the solubility of the system was set at 75 bar while the
temperature and concentration was varied from 25 to 40 ºC and from 0.01 to 0.60 g
PS/ml p-Cymene, respectively. The ternary diagrams could be predicted and are
depicted in Figure 5.20 where the heterogeneous region shows the composition in
the polymer and CO2 rich phases.
Figure 5.20. Phase diagrams of the ternary mixture CO 2/p-Cymene/PS at 75 bar and 25, 30,
35 and 40 ºC.
The partition coefficients were calculated in order to select the most suitable
temperature and concentration. As it was explained, the target is the minimization
of KCO2 and the maximization of Kp-Cymene while the maximum value of Kp-Cymene
should prevail over the minimum of K CO2. Figure 5.21 shows the influence of
temperature and concentration on the partition coefficients at 75 bar.
145
120
0.040
Pressure: 75 bar
Temperature: 25 ºC
Temperature: 30 ºC
Temperature: 35 ºC
Temperature: 40 ºC
0.035
0.030
0.025
kp-Cymene
kCO
2
Pressure: 75 bar
Temperature: 25 ºC
100
Temperature: 30 ºC
Temperature: 35 ºC
80
Temperature: 40 ºC
60
40
0.020
0.015
0.010
20
0
0.0
0.005
0.1
0.2
0.3
0.4
0.5
0.6
Concentration (g PS/ml p-Cymene)
0.7
0.000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Concentration (g PS/ml p-Cymene)
Figure 5.21. Distribution coefficient of CO2 (left) and p-Cymene (right) as a function of
Polystyrene concentration and temperature: () 25 ºC, (○) 30 ºC, (▲) 35 ºC and () 40ºC.
According to Figure 5.21, the optimum conditions to perform the precipitation of PS
at 75 bar would be 25 ºC (although lower temperature would show better results)
and 0.1 g PS/ml p-Cymene to maximize the amount of recovered PS. The selected
operating conditions assure the both premises, the maximum solubility of p-Cymene
in the vapour phase and of CO2 into the polymeric rich phase.
In the light of the results obtained, the separation of Polystyrene from its solutions
in terpenes by CO2 at high pressure is feasible due to the differences between the
solubility. The most suitable operating conditions to perform the separation process
are high pressure, low temperature and moderate concentration since the aim of
this work is the recycling of PS wastes into high quality products.
146
Chapter 5
References
[1] G. Luna-Bárcenas, S.K. Kanakia, I.C. Sanchez, K.P. Johnston, Semicrystalline
microfibrils and hollow fibres by precipitation with a compressed-fluid antisolvent,
Polymer, 36 (1995) 3173-3182.
[2] S. Mawson, S. Kanakia, K.P. Johnston, Metastable polymer blends by
precipitation with a compressed fluid antisolvent, Polymer, 38 (1997) 2957-2967.
[3] D.J. Dixon, K.P. Johnston, Formation of microporous polymer fibers and oriented
fibrils by precipitation with a compressed fluid antisolvent, Journal of Applied
Polymer Science, 50 (1993) 1929-1942.
[4] Y. Pérez De Diego, F.E. Wubbolts, G.J. Witkamp, T.W. De Loos, P.J. Jansens,
Measurements of the phase behaviour of the system dextran/DMSO/CO2 at high
pressures, Journal of Supercritical Fluids, 35 (2005) 1-9.
[5] J.L. Owens, K.S. Anseth, T.W. Randolph, Mechanism of microparticle formation
in the compressed antisolvent precipitation and photopolymerization (CAPP) process,
Langmuir, 19 (2003) 3926-3934.
[6] E. Elizondo, A. Córdoba, S. Sala, N. Ventosa, J. Veciana, Preparation of
biodegradable poly (methyl vinyl ether-co-maleic anhydride) nanostructured
microparticles by precipitation with a compressed antisolvent, Journal of
[7] I. De Marco, E. Reverchon, Nanostructured cellulose acetate filaments produced
by supercritical antisolvent precipitation, The Journal of Supercritical Fluids, 55
(2011) 1095-1103.
[8] A.I. Cooper, Polymer synthesis and processing using supercritical carbon dioxide,
Journal of Materials Chemistry, 10 (2000) 207-234.
[9] F. Rindfleisch, T.P. DiNoia, M.A. McHugh, Solubility of polymers and copolymers
in supercritical CO2, Journal of Physical Chemistry, 100 (1996) 15581-15587.
[10] I. Kikic, M. Lora, A. Bertucco, A Thermodynamic Analysis of Three-Phase
Equilibria in Binary and Ternary Systems for Applications in Rapid Expansion of a
Supercritical Solution (RESS), Particles from Gas-Saturated Solutions (PGSS), and
Supercritical Antisolvent (SAS), Industrial and Engineering Chemistry Research, 36
(1997) 5507-5515.
[11] D. Browarzik, M. Kowalewski, Calculation of the stability and of the phase
equilibrium in the system polystyrene + cyclohexane + carbon dioxide based on
equations of state, Fluid Phase Equilibria, 163 (1999) 43-60.
[12] B. Bungert, G. Sadowski, W. Arlt, Supercritical antisolvent fractionation:
Measurements in the systems monodisperse and bidisperse polystyrene-cyclohexanecarbon dioxide, Fluid Phase Equilibria, 139 (1997) 349-359.
[13] R.A. Krenz, R.A. Heidemann, Modelling the fluid phase behaviour of
polydisperse polyethylene blends in hydrocarbons using the modified SanchezLacombe equation of state, Fluid Phase Equilibria, 262 (2007) 217-226.
[14] T. Tassaing, P. Lalanne, S. Rey, F. Cansell, M. Besnard, Spectroscopic study of
the polystyrene/CO2/ethanol system, Industrial and Engineering Chemistry
Research, 39 (2000) 4470-4475.
[15] A.P. Mathew, S. Packirisamy, S. Thomas, Studies on the thermal stability of
natural rubber/polystyrene interpenetrating polymer networks: Thermogravimetric
analysis, Polymer Degradation and Stability, 72 (2001) 423-439.
[16] Z. Zhang, Y.P. Handa, An in situ study of plasticization of polymers by highpressure gases, Journal of Polymer Science, Part B: Polymer Physics, 36 (1998) 977982.
147
[17] C. Tsioptsias, C. Panayiotou, Simultaneous determination of sorption, heat of
sorption, diffusion coefficient and glass transition depression in polymer-CO2
systems, Thermochimica Acta, 521 (2011) 98-106.
[18] M. Pantoula, C. Panayiotou, Sorption and swelling in glassy polymer/carbon
dioxide systems: Part I. Sorption, Journal of Supercritical Fluids, 37 (2006) 254-262.
[19] J.M. Prausnitz, R.N. Lichtenthaler, E.G. de Azevedo, Molecular
Thermodynamics of Fluid-Phase Equilibria, Pearson Education, 1998.
[20] H. Sovová, R.P. Stateva, A.A. Galushko, Essential oils from seeds: Solubility of
limonene in supercritical CO2 and how it is affected by fatty oil, Journal of
[21] S.A.B. Vieira de Melo, G.M.N. Costa, A.M.C. Uller, F.L.P. Pessoa, Modeling
high-pressure vapor-liquid equilibrium of limonene, linalool and carbon dioxide
systems, Journal of Supercritical Fluids, 16 (1999) 107-117.
[22] C. Gutiérrez, M.T. García, I. Gracia, A. De Lucas, J.F. Rodríguez, The selective
[23] Y. Sato, T. Takikawa, S. Takishima, H. Masuoka, Solubilities and diffusion
coefficients of carbon dioxide in poly(vinyl acetate) and polystyrene, The Journal of
[24] M.E. Paulaitis, V.J. Krukonis, R.T. Kurnik, R.C. Reid, Supercritical Fluid
Extraction, Reviews in Chemical Engineering, 1 (1983) 179-250.
[25] C.-M.J. Chang, C.-C. Chen, High-pressure densities and P-T-x-y diagrams for
carbon dioxide+linalool and carbon dioxide+limonene, Fluid Phase Equilibria, 163
(1999) 119-126.
[26] C. Gutiérrez, M.T. García, I. Gracia, A. de Lucas, J.F. Rodríguez, Recycling of
extruded polystyrene wastes by dissolution and supercritical CO 2 technology, Journal
of Material Cycles and Waste Management, 14 (2012) 308-316.
[29] M.L. O'Neill, Q. Cao, M. Fang, K.P. Johnston, S.P. Wilkinson, C.D. Smith, J.L.
Kerschner, S.H. Jureller, Solubility of homopolymers and copolymers in carbon
dioxide, Industrial and Engineering Chemistry Research, 37 (1998) 3067-3079.
[30] Z. Wagner, J. Pavlíček, Vapour-liquid equilibrium in the carbon dioxide- pcymene system at high pressure, Fluid Phase Equilibria, 90 (1993) 135-141.
[31] Y. Sato, K. Fujiwara, T. Takikawa, Sumarno, S. Takishima, H. Masuoka,
Solubilities and diffusion coefficients of carbon dioxide and nitrogen in
polypropylene, high-density polyethylene, and polystyrene under high pressures and
temperatures, Fluid Phase Equilibria, 162 (1999) 261-276.
148
Chapter 6
TERNARY
SYSTEMS:
PROPERTIES
Chapter 6 concerns the influence of CO2 on the properties of the solutions is a key
factor since the sorption of CO2 plasticises the mixture changing dramatically their
physic-chemical and rheological characteristics. According to this, the Chapter has
been divided in three different sections: the study of the glass transition
temperature, the viscosity and the interfacial tension of the mixtures
CO2/Limonene/PS and CO2/p-Cymene/PS.
Ternary Systems: Properties
150
Chapter 6
Based on the papers:
 C. Gutiérrez, M.T. Garcia, S. Curia, S.M. Howdle, J.F. Rodriguez, The effect of
CO2 on the viscosity of polystyrene/limonene solutions, Journal of Supercritical
Fluids, 88 (2014) 26-37.
 C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. De Lucas, M.T. García, Development of
a strategy for the foaming of polystyrene dissolutions in scCO 2, Journal of
Graphical abstract
151
RESUMEN
Con el fin de diseñar un proceso para el reciclaje de residuos de Poliestireno
mediante su disolución en terpenos y posterior separación mediante CO 2 a alta
presión, es necesario estudiar los cambios que éste produce en las propiedades de las
disoluciones. El estudio del equilibrio del sistema ternario formado es fundamental
para determinar la viabilidad el proceso, sin embargo, también es esencial el estudio
de los cambios producidos en la viscosidad, la tensión superficial y la temperatura
de transición vítrea de las disoluciones como consecuencia de la plastificación.
La viscosidad de las disoluciones de PS/Limoneno en presencia de CO 2 se midió en
un viscosímetro de cuarzo. Las condiciones de presión se variaron desde presión
atmosférica hasta 50 bar, en un rango de temperatura de 20 a 40 ºC y en un
intervalo de concentración de entre 0.025 y 0.40 g PS/ml Limoneno. Se observó que
en todos los casos, al aumentar la presión de CO2, la viscosidad de las disoluciones
disminuía por el efecto plastificante que ejerce el gas.
La determinación de la tensión interfacial se realizó a través del método de la gota
colgante en una celda de visión de alta presión bajo condiciones de operación
moderadas (presión: 0-90 bar; temperatura: 30-40 ºC). Por último, se estudió el
efecto del CO2 sobre la temperatura de transición vítrea del Poliestireno en
disolución. A partir de la extrapolación de los datos de viscosidad y tensión
interfacial se determinaron los puntos críticos de las disoluciones en presencia de
CO2.
152
Chapter 6
ABSTRACT
With the aim of designing accurately the global process, the modification of
solutions properties in CO2 should be considered. Although the knowledge of the
equilibrium is crucial to determine the feasibility of the recycling process, the shifts
in the viscosity, the interfacial tension and the glass transition temperature of
Polystyrene/Terpene oils solutions should be also studied.
The viscosity measurements were performed in a quartz crystal viscometer to
determine the viscosity of solutions of Polystyrene in Limonene in the presence of
high pressure carbon dioxide. These measurements were determined up to 50 bar in
the range of temperature from 20 to 40ºC and in the range of concentration from
0.025 to 0.4 g PS/ml Limonene. The viscosities of the solutions at all loadings were
found to decrease with increasing temperature and pressure while the plastising
effect of CO2 prevails over the hydrostatic pressure applied by the gas.
On the other hand, interfacial tension measurements were carried out by the
pendant droplet method at mild working conditions (pressure: 0-90 bar,
temperature: 30-40 ºC). Also, the effect of CO2 on the glass transition Polystyrene
solutions was studied. Finally, the viscosity and interfacial tension data were used
to determine the solvent quality and to quantify the demixing points of the
mixtures.
153
154
Chapter 6
6.1. Background of viscosity and interfacial tension of the
ternary mixtures CO2/Limonene/PS and CO2/p-Cymene/PS
As it has been mentioned, the use of high-pressure fluids for polymer processing and
polymer synthesis has grown significantly in recent years [1-3]. Several research
groups are focused on the use of supercritical CO 2 as a reaction medium for
polymerisation [4-6], as a solvent for fractionation [7, 8], as an antisolvent for
encapsulation [9, 10] or as a blowing agent for non-reactive processes [11-13]. For
polymer solutions, high-pressure viscosity data are especially important in a variety
of applications such as high-pressure synthesis, melt polymer processing and
polymer blending [1]. Thus, the viscosity of a polymer coupled with a solvent or
plasticizer is a subject of considerable interest for the manufacturing of polymers
[14]. A fundamental understanding of how the properties of materials and the
processing parameters affect the behaviour of the mixtures is required to optimize
the future development of high-quality plastic products.
Viscosity also influences non-equilibrium processes, such as those met in the
transient stages during phase separation from polymer solutions [15]. Phase
separation studies are important in a wide variety of applications, such as spray
coating, foaming, spinning (i.e. fibre formation), manufacture of composite
materials, membranes and other porous materials. In all these cases, viscosity data
of polymer solutions are used to gain fundamental insights into polymer–solvent
interactions and to determine if the solvent is a “poor” or a “good” solvent [1]
according to their spatial conformation and dissolution ability. In the case of “poor
solvents” the polymeric chains stay packed and could form a hard sphere because
the solvent can not penetrate easily and dissolve the polymer; nevertheless in the
“good solvents” the chains are expanded and the contact between the polymer and
the surrounding fluid is maximized.
The viscosity of a polymer coil is a property related to the chain dimensions [16].
The degree of polymer-chain expansion depends on several factors but mainly the
quality of the solvent which can be altered by changing the temperature, pressure,
or even its composition [15]. The pressure dependency of the viscosity of polymer
solutions is fundamental to understand the relationship between the polymersolvent interactions in near or supercritical conditions, and the relation between the
chain conformation and viscosity. This is because, in addition to temperature,
pressure can provide an interesting and useful route to controlling the properties of
solutions [1].
The viscosity asymmetry of the components molecules (long chain polymers and
smaller solvent) and the interfacial tension of the polymer solutions are mainly
responsible for the different morphologies obtained during phase separation: well
defined droplets, fibres, membranes, foams etc [15]. Rheological measurements
could be used to relate the composition of solutions and the final structure [17].
155
Although polymer solutions in scCO2 are involved in several polymer processing
operations, such as fractionation, dyeing, foaming, encapsulation, impregnation, etc.
[18-21] there is an important lack of information on their rheological properties.
This has been exacerbated by the difficulty in performing experiments at uniform
shear rate, at high pressure and under homogeneous conditions.
According to the previous section of this Chapter, the knowledge of the equilibrium
is very important for the design of the antisolvent processes, but the determination
of the thermophysical properties (density, viscosity and interfacial tension) are
among the most influential parameters on fluid characteristics. Thus, when a
solution is subjected to a quench at the critical polymer concentration, the system
enters in the unstable region and phase separation proceeds spontaneously by a
mechanism known as spinodal decomposition. For the successful and large scale use
of the described recycling process, an intensive investigation of the physico-chemical
properties is necessary, especially because of the scarce data shown in the
literature. The difference between the viscosity of the solutions and the surrounding
medium is one of key contributors to the phase separation since the CO 2 acts as
antisolvent of the polymer inducing its precipitation [22].
On the other hand, Interfacial Tension (IFT) is also one of the most important
physicochemical parameters used in many polymer engineering processes like
foaming, since this property exerts a notable influence on the morphology of the
final polymer products. Generally low interfacial tension values are desired to
increase the nucleation rate and produce small and uniform cells during the
foaming process [23-25]. The nucleation cell is expressed following the Classical
homogeneous Nucleation Theory (CNT):
Nhom = C· f· exp(−ΔGhom / kB·T)
[6.1]
where Nhom is the rate of homogeneous nucleation, C is the concentration of
nucleation sites, f is the frequency factor of the gas molecules, ΔGhom is the Gibbs
free energy for homogeneous nucleation, and kB is Boltzmann’s constant [26, 27].
The energy barrier of cell nucleation can be lowered by decreasing the IFT according
the equation below:
ΔG = 16 π ϭ3 /3Δp2
[6.2]
where ϭ is the surface tension between the polymer phase and nucleating bubble
phase, and Δp is the pressure difference across the polymer-gas interface. In
summary the classical nucleation theory predicts that the Gibbs free energy will be
reduced by the cubic power of the surface tension, and hence the nucleation rate will
increase exponentially.
As it was shown in Chapter 4, the sorption of supercritical CO2 in polymers results
in their swelling and plasticization. Under this condition, carbon dioxide swells the
entangled melt polymer, causing an increase in free volume, and consequently, a
decrease in chain entanglement. The increase in free volume also increases the
156
Chapter 6
chain mobility, these facts cause changes in their mechanical and physical
properties [28, 29], which ultimately can be used to reduce interfacial tension [30,
31]. Furthermore, foaming processes of polymers using CO 2 are generally performed
at high temperatures to promote the diffusivity and solubility of the foaming agent
in the polymer [32]. In fact, polymer foaming is usually carried out at temperature
that exceeds the polymer melting point, to increase mobility in polymer chains and
to enhance its deformation [59].
In this work, low temperatures are required for the recycling of PS since the
polymer is solubilised in terpenes and they are separated by CO2. The presence of a
solvent decreases the operating temperature and consequently, the energy
consumption and possible polymer chain degradation. In this case, the solvent
expands the coil and increases the chain mobility behaving like a polymer melt
where individual chains are sufficiently separated. However, at higher
concentrations the solution becomes semidilute and eventually entangled depending
on the degree of overlap of adjacent coils and their molar mass [33].
To sum up, the sorption of CO2 in PS/solutions is the responsible of the
plasticisation which entails physico-chemical changes of their properties. The study
of the properties shifts as a function of pressure, temperature and concentration will
determine the morphology of the recovered Polystyrene and they are very useful for
the design of the process.
According to this issue, the first aim of the section is to obtain viscosity data for
PS/Limonene solutions in the presence of CO 2 using a high pressure quartz crystal
viscometer. These measurements concern essentially solvent-rich one-phase
systems, which mean that they are focused on the influence of a known amount of
CO2 on the homogeneous polymeric solution. The pressure and temperature
dependence of the viscosity was analysed using the Arrhenius equation and through
this theory the activation energy for viscous flow was investigated. Finally the
demixing points of the solutions were determined using the experimental viscosity
data. On the other hand, the second objective of this Chapter is to analyze the
interfacial tension of Polystyrene, in dilute solution with carbon dioxide at high
pressure. The solvent allows to work at lower temperature than the used
traditionally, where temperature exceeds the polymer melting point. IFT can affect
the dispersion, fluid dynamics and mass-transfer rates during processes, and its
knowledge it is necessary for the design and modelling of multiphase systems with
supercritical fluids.
6.2. Viscosity of mixtures CO2/Limonene/Polystyrene
The viscosity of polymer solutions in the presence of CO 2 depends on multiple
factors. In this work, we varied: pressure, temperature and PS concentration, while
157
the solvent (Limonene) and the molecular weight distribution of the polymer were
kept constant.
Viscosity data were obtained for solutions of PS in Limonene at polymer
concentrations of 0.025 to 0.400 g PS/ml. The viscosity measurements were carried
out in the temperature range from 20 to 40ºC and at pressures between atmospheric
and 50 bar. Higher pressures were not performed because there could be phase
separation [34]. The concentration of CO2 in the mixtures was in the range 0–35 wt
%. Figure 6.1 shows the viscosity of PS solutions in Limonene at various
temperatures, pressures and polymer concentrations.
0.6
C0: 0.05 g PS/ml Lim
Viscosity (mPa·s)
0.5
Patm
0.4
0.3
P: 6.89 bar
0.2
P: 13.79 bar
0.1
P: 20.68 bar
0.0
Viscosity (mPa·s)
6
5
P: 6.89 bar
4
3
P: 27.58 bar
2
1
0
Viscosity (mPa·s)
Patm
P: 34.47 bar
P: 41.37 bar
P: 48.26 bar
Patm
20
15
P: 6.89 bar
10
P: 13.79 bar
P: 27.58 bar
P: 48.26 bar
5
0
20
25
30
Temperature (ºC)
158
35
40
Chapter 6
Figure 6.1. Viscosity data showing the effect of variables: pressure, temperature and PS
concentration in the initial solutions. The effect of pressure (from atmospheric pressure to
48.26 bar) is observed at constant concentration in each graph as it is shown with the
different lines pointing to the arrows and labels. The effect of temperature at constant
concentration is shown in the abscissa axis and the PS concentration in Limonene is noticed
along the different graphs: (a) C0: 0.05 gPS/ml Limonene; (b) C0: 0.10 gPS/ml Limonene; (c)
C0: 0.30 gPS/ml Limonene.
The reproducibiliy of the experimental viscosity measurements was determined by
comparing the results from two independent runs carried out under identical
conditions. In these experiments the viscosity was similar with an average deviation
of 0.508 mPa·s, indicating good reproducibility.
6.2.1. Selection of the parameters for the study of the viscosity
Initially, a design of experiments (DOE) was carried out to determine if the selected
variables showed a significant influence on the viscosity. These data (Figure 6.1.)
were used to determine the most relevant and/or significant factors influencing
viscosity.
There are different ways to design a test, but the most common is a full factorial
experiment. In this case, we selected the simplest factorial designs which involve
the three factors at two levels and three central points [35]. Next, statistical
analysis of variance (ANOVA) was applied to decide whether the effect of the input
factors was significant. The range of the input factors was selected according to
those used in recycling of PS wastes by dissolution and supercritical CO 2 [36]. Also
the working range was selected considering the phases stability which was
previously observed in a high pressure view cell [37].
The statistical analysis of the experimental results was made using commercial
software package (Statgraphics Plus 5.1 Manugistics, Inc. Rockville, MD, USA). The
test of statistical significance (p-value) was determined accordingly to the total error
criteria giving a confidence level of 95%.
Figure 6.2 illustrates the Pareto chart which shows the influence of the studied
variables on the viscosity. For a significance level of 5%, the vertical line indicates
the minimum statistically-significant effect. The horizontal bar lengths are
proportional to the degree of significance for each effect, and every effect that
exceeds the vertical line is judged significant.
According to Figure 6.2, all the selected variables have a statistically significant
influence over the response. Also the effects of combinations of
pressure/temperature, pressure/concentration or temperature/ concentration are
shown to be statistically significant over the viscosity. The filled black bars show a
negative influence over the response which means that an increase in the variable
159
acts to decrease the viscosity of the mixture. By contrast, the striped bars show a
positive influence over the viscosity which means that variable increases the
viscosity (this effect was only apparent in the case of concentration). The selected
variables will be investigated in the subsequent section according to their
statistically significant magnitude.
C:Concentration
CC
A:Pressure
AC
B:Temperature
BC
BB
AB
AA
+
-
0
10
20
30
40
Standarized Effects
Figure 6.2. Pareto chart of the standardized effects on Viscosity. The vertical line defines the
statistically significant variables, in this case all the studied. A: pressure, B: temperature, C:
concentration, AB: combined effect between pressure and temperature, AC: combined effect
between pressure and concentration, BC: combined effect between temperature and
concentration.
6.2.2. Concentration effect on viscosity
Polystyrene is a solid at room temperature, whose melt viscosity is in the order 10 5
Pa·s at 175 ºC [38, 39] so the presence of the polymer certainly increases the
viscosity of the solvent.
Most theories about viscosity of polymer solutions are formulated around dilute
solutions but, for real polymer solutions, the study of viscosity is often more complex
than simple theory predicts. In this work, all the polymer solutions lie on the
semidilute entangled and concentrated entangled regimes following Graessley’s
classification [40].
The presence of a small amount of polymer increases significantly the viscosity of
the Limonene solutions entailing an important asymmetry between the components
responsible of thermodynamic and dynamics changes that distance from the ideal
behaviour and makes very difficult its accurate prediction. Viscosity data are
typically presented in dimensionless form, relating the ratio between the solution
and solvent viscosities through the relative viscosity (rel).


160

[6.1]
Chapter 6
where sol in [mPa·s] is the viscosity of the PS/Limonene/CO 2 solution and solvent in
[mPa·s] is the viscosity of the solvent under identical working conditions. The
parameter rel is a function of the pressure, temperature and initial PS
concentration; it should be always above 1 because of the presence of the polymer
which will increase the viscosity of the solution.
The influence of pressure, temperature and concentration on the relative viscosity
have been studied (Figure 6.3), but in this section only the concentration effect will
be discussed. The data show an almost linear increase of rel when concentration
increases, especially at low pressure. However, at higher pressure, the effect of
concentration on viscosity becomes less important and it does not contribute
linearly. This effect is only significant from 0.10 g/ml because the presence of CO 2
leads to a dilution effect, as well as the plasticisation of the polymer which will be
explained below. The fact that the concentration effect prevails over the
temperature is also shown as in the Pareto chart (Figure 6.3).
The values of viscosity of the PS/Limonene solutions are lower than the values
reported in the literature for methylcyclohexane and toluene [1, 41], which suggests
a lower chain extension of the polymer in Limonene due to the thermodynamic
qualities of the solvent [42] that will influence on the mobility of the entangled
chains and consequently in the values of viscosity.
(a) T: 25 ºC
Relative viscosity
20
15
10
5
0
0.4
0.3
40
0.2
30
0.1
Concentration (g/ml)
20
0
10
Pressure (bar)
Figure 6.3. Three-dimensional plot of the relative viscosity (rel) in a range of pressure
between 10 and 50 bar, concentration from 0.05 g PS/ ml Limonene to 0.40 g PS/ ml
Limonene and at three different temperatures: (a) T: 25ºC, (b) T: 30ºC and (c) T: 40ºC.
161
(b) T: 30 ºC
Relative Viscosity
20
15
10
5
0
0.4
0.3
40
0.2
30
0.1
20
0
10
Pressure (bar)
(c) T: 40 ºC
Relative viscosity
20
15
10
5
0
0.4
0.3
40
0.2
30
0.1
20
0
10
Pressure (bar)
Figure 6.3 (Continued). Three-dimensional plot of the relative viscosity (rel) in a range of
pressure between 10 and 50 bar, concentration from 0.05 g PS/ ml Limonene to 0.40 g PS/ ml
Limonene and at three different temperatures: (a) T: 25ºC, (b) T: 30ºC and (c) T: 40ºC.
6.2.3. Pressure effect on viscosity
The influence of pressure on the viscosity at constant temperature and PS
concentration (Figure 6.4) shows that when pressure increases, the viscosity
decreases following a logarithmic behaviour that becomes less pronounced at higher
values of temperature and concentration.
Since the viscosity decreases when pressure increases, a good correlation can be
obtained using an Arrhenius-type equation [43]:

[6.2]
where  is the viscosity in [mPa·s], A in [mPa·s], B in [mPa·s] and C in [bar -1] are
three constants depending on the temperature, and p is the CO 2 pressure in [bar].
However there is not a perfect fit between the Arrhenius-type equation and the
experimental data (Figure 6.4). In this case, ln does not decrease linearly when
162
Chapter 6
pressure increases and other effects should be considered to describe the non linear
behaviour.
When pressure increases two opposite influences occur: the plasticising effect due to
the dissolution of CO2 inside the PS/Limonene solutions and the compression of the
solutions as a consequence of the applied pressure.
50
(a) Temperature: 25ºC
Viscosity (mPa·s)
45
2
40
C0: 0.2 g PS/ml Lim. R : 0.88
35
C0: 0.3 g PS/ml Lim. R : 0.87
30
C0: 0.4 g PS/ml Lim. R : 0.86
2
2
25
20
15
10
5
40
0
(b) Temperature: 30ºC
Viscosity (mPa·s)
35
2
C0: 0.2 g PS/ml Lim. R : 0.97
30
C0: 0.3 g PS/ml Lim. R : 0.90
25
C0: 0.4 g PS/ml Lim. R : 0.89
2
2
20
15
10
5
0
(c) Temperature: 40ºC
Viscosity (mPa·s)
30
2
C0: 0.2 g PS/ml Lim. R : 0.97
2
25
C0: 0.3 g PS/ml Lim. R : 0.93
20
C0: 0.4 g PS/ml Lim. R : 0.88
2
15
10
5
0
0
10
20
30
40
50
60
70
Pressure (bar)
Figure 6.4. Viscosity data showing the variation of the viscosity as a function of pressure at
three different temperatures (a) T: 25ºC; (b) T: 30ºC; (c) T: 40ºC and at different concentration
as it is indicated by the markers: (▲) C0: 0.2 g PS/ml Limonene; () C0: 0.3 g PS/ml Limonene;
() C0: 0.4 g PS/ml Limonene. Error bars represent standard deviation based on two
measurements.
The first effect is the increase of the CO 2 density along with the CO2 sorption into
the polymer and/or the polymeric solution causing the plasticisation effect [44]. The
smaller CO2 molecules placed among the entangled polymeric chains decrease their
interactions and contribute to a decrease in viscosity by increasing the spatial
163
distance among them (i.e. the free volume fraction of the polymer). This effect
becomes more important at higher pressure and lower temperature, because the
solubility of CO2 in the liquid phase increases as the pressure increases, whilst it
decreases as temperature increases [45, 46]. An accurate prediction of CO2 sorption
inside the solution is complex to obtain so experimental data are necessary to check
the estimation. The amount of CO2 absorbed in the PS/Limonene solution causes the
plasticisation and consequently the decrease of viscosity. Based on the study of
other properties in previous work [37], the viscosity depression was connected with
the amount of CO2 inside the solution (Figure 6.5).
Pressure (bar)
0
5
10
15
20
Viscosity (mPa·s)
35
25
30
35
(a) Temperature:25ºC
C0: 0.05 g/ml
30
C0: 0.10 g/ml
25
C0: 0.20 g/ml
C0: 0.30 g/ml
20
C0: 0.40 g/ml
15
10
5
0
0
5
10
15
20
25
30
35
25
30
35
% wt CO2
Pressure (bar)
0
5
10
15
20
Viscosity (mPa·s)
35
(b) Temperature:30 ºC
C0: 0.05 g/ml
30
C0: 0.10 g/ml
25
C0: 0.20 g/ml
C0: 0.30 g/ml
20
C0: 0.40 g/ml
15
10
5
0
0
5
10
15
20
25
30
% wt CO2
Pressure (bar)
0
5
10
15
Viscosity (mPa·s)
30
20
25
30
35
(c) Temperature:40 ºC
C0: 0.05 g/ml
25
C0: 0.10 g/ml
20
C0: 0.20 g/ml
15
C0: 0.40 g/ml
C0: 0.30 g/ml
10
5
0
0
5
10
15
20
25
% wt CO2
Figure 6.5. Influence of CO2 sorption on the viscosity of solutions at 25ºC (a); 30ºC (b) and
40ºC (c) as a function of concentration (■) C0: 0.05 gPS/ml Limonene; (○) C0: 0.10 gPS/ml
Limonene; (▲) C0: 0.20 gPS/ml Limonene; () C0: 0.30 gPS/ml Limonene; ()C0: 0.40 gPS/ml
Limonene.
As shown in Figure 6.5, the CO2 solubility in PS/Limonene solutions increases with
increasing pressure and decreases with increasing temperature. The effect of
164
Chapter 6
temperature on CO2 sorption is observed through the length of the lines obtained as
a function of temperature and PS concentration. At 25 ºC the CO2 sorption inside
the solution reaches around 30% while at 40 ºC it decreases up to 20%.The effect of
CO2 pressure is diminished at higher temperatures because less CO 2 is dissolved in
the solution [47]. Also it should be pointed out that the reduction in the viscosity is
smaller at higher CO2 concentration because the pressure effect starts to dominate
at higher pressures [48]. CO2 sorption is also affected by the polymer initial
concentration, when PS concentration increases CO2 solubility in solutions
decreases and subsequently viscosity depression was less pronounced [49].
The second effect of pressure over viscosity is that the density of the CO2 and the
bulk increases, which decreases both the molecular mobility of the polymer chains
and the free volume between the chains [50]. This increases the internal frictional
forces and, as a consequence, the flow resistance. Lucas suggested that changes in
the pressure resulted in an increase in viscosity (when no CO 2 is added) and those
modifications can be estimated from a theoretical point of view [51, 52]. The
estimation method is based on the correlation of polar and nonpolar liquids, where
the ratio between the viscosity of the liquid at the studied pressure and the viscosity
of the saturated liquid at the vapour pressure was obtained as a function of the Pr
that is defined as:
[6.3]
where P is the studied pressure expressed in [bar], Pv is the vapour pressure of
Limonene [53] and Pc is the critical pressure of Limonene [51]. Also the ratio
between viscosities is influenced by the acentric factor and the working
temperature. Following the method described by Lucas [52] the effect of pressure on
the solvent was calculated theoretically (Figure 6.6 solid lines).
The effect of CO2 pressure on the viscosity of Limonene (Figure 6.6 dashed lines)
was also predicted theoretically according to Grunberg-Nissan method [54] because
it is widely recommended for liquid mixture viscosity at reduced temperatures below
0.7. The interaction parameter was obtained according to the procedure established
by Isdale [55] instead of regression because this technique yields quite acceptable
estimates of low-temperature liquid mixture viscosities.
The effect of hydrostatic pressure and CO 2 pressure on the viscosity of Limonene
clearly shows the plasticisation effect as viscosity decreases dramatically as the CO 2
pressure increases almost linearly. This behaviour has been discussed by several
authors [47, 48]. Nevertheless, in the absence of CO2, the viscosity of Limonene
increases slightly while the pressure increases.
165
Viscosity (mPa·s)
1.0
0.8
0.6
0.4
Viscosity (mPa·s)
0.2 (a) Temperature: 25 ºC
Limonene
Limonene/CO2
0.0
0.8
0.6
0.4
Viscosity (mPa·s)
0.2 (b) Temperature: 30 ºC
Limonene
Limonene/CO2
0.0
0.8
0.6
0.4
0.2 (c) Temperature: 40 ºC
Limonene
Limonene/CO2
0.0
0
5 10 15 20 25
30
35 40
45
Pressure (bar)
Figure 6.6. Predicted theoretically viscosity of Limonene as a function of hydrostatic
pressure and temperature (solid lines) and predicted theoretically viscosity of Limonene when
the pressure is provided by CO 2 (dashed lines).
Clearly, the main effect of CO2 pressure on viscosity is a depression as a
consequence of the plasticisation effect but also it is clear that the hydrostatic
pressure is important when the working pressure is high enough, but in general,
this effect is outweigh by the plasticisation caused by CO2 [46].
6.2.4. Temperature effects on viscosity
The influence of temperature on the viscosity at constant pressure (each graph) and
PS concentration (different markers) is shown in Figure 6.7. As the temperature of
the solution increases (top axis), the viscosity decreases. When the solutions are
heated the cohesive and attraction forces between the molecules are reduced while
the kinetic energy of the molecules increases which allows the free movement and
consequently reduces the viscosity of the mixture.
166
Chapter 6
Temperature (ºC)
4.0
40
35
30
25
20
3.6
3.2
3.2
ln (Viscosity)
ln (Viscosity)
3.6
2.8
2.4
2.0 (a) Patm
C0=0.1 g/ml
1.6
C =0.2 g/ml
0
1.2
C0=0.3 g/ml
0.8
C0=0.4 g/ml
0.0031
Temperature
(ºC)25
30
40
35
C0=0.2 g/ml
C0=0.3 g/ml
2.8
C0=0.4 g/ml
2.4
2.0
1.6
1.2
0.0032
0.0033
0.0034
0.0031
-1
20
(c) P:13.79 bar
C0=0.1 g/ml
3.2
C0=0.2 g/ml
2.8
C0=0.3 g/ml
2.4
0.0034
Temperature (ºC)
25
3.6
ln (Viscosity)
ln (Viscosity)
3.6
30
0.0033
1/Temperature (K )
Temperature (ºC)
35
0.0032
-1
1/Temperature (K )
40
20
(b) P:6.89 bar
C0=0.1 g/ml
C0=0.4 g/ml
2.0
1.6
40
35
30
25
20
3.2 (d) P:27.58 bar
C0=0.1 g/ml
2.8
C0=0.2 g/ml
2.4
C =0.3 g/ml
0
2.0
C0=0.4 g/ml
1.6
1.2
0.8
1.2
0.4
0.8
0.0
0.0031
0.0032
0.0033
0.0034
-1
1/Temperature (K )
0.0031
0.0032
0.0033
0.0034
-1
1/Temperature (K )
Figure 6.7. Influence of temperature on the logarithm of viscosity at different concentrations:
0.1 g PS/ml Lim (○), 0.2 g PS/ml Lim (▲), 0.3 g PS/ml Lim (), 0.4 g PS/ml Lim (). The
influence of pressure is observed in each graph: (a) atmospheric pressure; (b) p: 6.89 bar; (c) p:
13.79 bar and (d) p: 27.58 bar.
The viscosity dependence of the temperature can also be described using an
exponential Arrhenius-type equation.

[6.6]
where B is a constant in [mPa·s], Ea is the flow activation energy in [J/mol] , R is the
gas constant [8.3145 J/K·mol] and T is the temperature in [K].
The activation energies of the PS solutions were evaluated from the slope of ln
versus 1/T plots at each pressure and concentration. The calculated activation
energy ranged from 1.5 to 43 kJ/mol (Figure 6.9).
167
50
C0: 0.1 g/ml
45
C0: 0.2 g/ml
40
C0: 0.3 g/ml
Ea (kJ/mol)
35
30
25
20
15
10
5
0
0
10
20
30
40
50
60
Pressure (bar)
Figure 6.8. Effect of pressure on the flow activation energy (Ea). The markers represent the
influence of PS concentration on the flow activation Energy: (○) 0.1 g PS/ml Lim; (▲) 0.2 g
PS/ml Lim; () 0.3 g PS/ml Lim.
The energy of activation is expected to be related to the latent heat of vaporization
of the solvent, since the removal of a molecule from the surroundings of its
neighbours forms a part of both processes. In this case, the Limonene heat of
vaporization is around 40 kJ/mol [56]. Nevertheless the obtained data showed lower
values than those reported in the literature. The CO 2 is less viscous than the
Limonene and obviously less viscous than the Limonene/PS solutions. When the
CO2 is absorbed into the solutions, they become homogeneous expanded solutions
whose viscosity is decreased and therefore the Ea is lower than the heat of
vaporization of Limonene. The fact that in some particular case (C0: 0.3 g/ml – P: 50
bar) Ea reaches higher values is due to the presence of PS.
The Ea data obtained were in the same order of magnitude as other PS solutions
using different solvents [1, 41, 57]. In the case of PS/Toluene or
PS/Methylcyclohexane solutions, Ea was in the range of 7-10 kJ/mol and the authors
considered the energy of activation essentially unchanged with the CO 2 addition in
the investigated range. Nevertheless, in the case of PS/Limonene solutions the
activation energy was higher because our measurements were conducted with
samples of higher molecular weight and at higher concentration. Generally, more
energy is required to enhance the polymeric chain mobility when they present
higher molecular weight, indicating an increasing effective size for the flow unit
[58].
Flow activation energy increases when pressure increases because the polymeric
chains are restrained to move freely. The increase of CO 2 density is the responsible
168
Chapter 6
to press the solution which will make necessary a higher energy contribution to shift
the polystyrene chains. Also the flow activation energy increases when
concentration is higher because the polymer chains are not so accessible as in the
case of dilute solutions. Higher energy should be applied to enhance the movement
of the chains which are more strongly interacting. The cohesive forces in solids are
at a maximum because the molecules are closer than in the case of the liquids, so
the presence of the polymer increases significantly the activation energy.
6.2.5. Pressure and temperature combined effects on viscosity
Although all the combined effects were significant in influencing the viscosity, only
Pressure-Temperature was studied because it can be easily related through the
density. If viscosity is dependent upon pressure and temperature, it should be easier
to consider both effects by considering CO 2 density. Density will comprise the effect
of temperature and pressure, so it will be possible to check the most relevant effect
upon the viscosity as described previously by Pareto chart (Figure 6.2).
The effect of density on the viscosity (Figure 6.9) shows that viscosity decreases
when the density of CO2 increases. Higher values of density correspond to higher
values of pressure and lower values of temperature. At higher density the solubility
of CO2 into the PS/Limonene solutions increases which causes a dispersion and
dilution of the large polymeric molecules.
50
C0:0.2 g/ml
45
C0:0.3 g/ml
Viscosity (mPa·s)
40
C0:0.4 g/ml
35
30
25
20
15
10
5
0
0
25
50
75
100
125
150
175
200
3
Density (kg/m )
Figure 6.9. Influence of CO2 density on viscosity. Experimental data (markers) represent the
viscosity at C0: 0.2 g PS/ml Lim (▲); C0: 0.3 g PS/ml Lim () ; C0: 0.4 g PS/ml Lim ().Lines
show the correlation of viscosity data vs density at C0: 0.2 g PS/ml Lim (—); C0: 0.3 g PS/ml
169
Lim () ; C0: 0.4 g PS/ml Lim (…).
An empirical model was developed by Seifried and Temelli [59] to correlate the
viscosity of fish oils triglycerides to temperature and pressure. In this work, some
modifications were proposed to obtain more accurate data in the case of the
PS/Limonene solutions in presence of CO 2. The following equations were used to
correlate the experimental data obtained.




[6.5]

[6.6]

[6.7]
[6.8]
where  (P,T) is the experimental viscosity as a function of pressure and
temperature in [mPa·s]. The term  (P,T) explains the asymptotic behaviour of the
solution at high pressure and temperature expressed in [mPa·s]. It was amended to
include the effect of pressure previously not described by Seifried and Temelli [59].
The term 0 (T) represents the viscosity at atmospheric pressure in [mPa·s], for this
reason only the temperature affects the term since the pressure is constant (1 atm).
A4 and A5 can be obtained from the correlation of experimental data shown by Clará
et al. [56]. The reported data were obtained at atmospheric pressure as a function of
temperature. Nevertheless, it is important to emphasise that the presence of the PS
will modify the cited values. Finally, k (P,T) shows the exponential decline of
viscosity as a function of pressure and temperature as explained previously. The
described model is totally empirical with the calculated parameters (Table 6.1)
describing the experimental data accurately over the range of the studied
conditions.
Table 6.1. Model parameters for correlation of viscosity of PS/Limonene solutions in presence
of CO2.
C0
A1
A2
A3
A4
(g/ml) [mPa·s] [mPa·s·K-1] [mPa·s·bar-1] [mPa·s]
A5
[K-1]
875.770
A6
A7
[bar-1] [bar·K-1]
0.05
-0.285
0.002
-0.115
1.680E-02
0.651
-0.090
0.10
9.204
-0.015
-0.848
9.900E-03 3500.000 0.440
-0.090
0.20
20.236
-0.049
-1.231
6.406E-05 3544.232 0.197
-0.096
0.30
61.788
-0.173
-1.638
3.174E-05 3925.024 0.120
-0.024
0.40
54.295
-0.092
-15.878
2.470E-02 2096.700 0.058
-0.005
Figure 6.10 shows the experimental (markers) and correlated (lines) viscosity data
as a function of concentration (each graph), pressure and temperature. In all cases
there was a good fit between the experimental and the correlated data which allows
to interpolate new data in the experimental range studied and to extrapolate to
170
Chapter 6
moderately higher pressures and temperatures. The increase of the concentration of
PS would not imply a worsening of the data because the parameters were obtained
from the fitting of five different concentrations covering a wide range of working
conditions. The modification of the semi-empirical equations allows the
improvement of the correlation overcoming the non ideality that could provide the
presence of the polymer.
5.0
10
(a) C0: 0.1 gPS/ml Lim
T: 25 ºC
T: 30 ºC
T: 40 ºC
Viscosity (mPa·s)
4.0
3.5
8
3.0
2.5
2.0
1.5
1.0
0.5
7
6
5
4
3
2
1
0.0
0
0
10
20
30
40
50
60
70
80
0
Pressure (bar)
20
14
20
30
40
10
8
6
4
2
60
70
80
(d) C0: 0.4 gPS/ml Lim
T: 25 ºC
T: 30 ºC
T: 40 ºC
24
12
50
Pressure (bar)
27
Viscosity (mPa·s)
16
10
30
(c) C0: 0.3 gPS/ml Lim
T: 25 ºC
T: 30 ºC
T: 40 ºC
18
Viscosity (mPa·s)
(b) C0: 0.2 gPS/ml Lim
T: 25 ºC
T: 30 ºC
T: 40 ºC
9
Viscosity (mPa·s)
4.5
21
18
15
12
9
6
3
0
0
0
5
10
15
20
25
30
35
40
Pressure (bar)
45
50
55
60
0
5
10
15
20
25
30
35
40
45
50
55
60
Pressure (bar)
Figure 6.10. Viscosity of PS/Limonene solutions as a function of pressure, temperature at 25
ºC (■), 30 ºC (●) and 40 ºC (▲) and concentration (a) C0: 0.10 g PS/ml Lim; (b) C0: 0.20 g PS/ml
Lim; (c) C0: 0.30 g PS/ml Lim; (d) C0: 0.40 g PS/ml Lim. The correlated data calculated
according to eq. (6.5) are shown at 25 ºC (—), 30 ºC () and 40 ºC (…).
Clearly, the pressure and temperature or density effect on viscosity is an indirect
measurement of the CO2 sorption which has been shown as the real cause of
changes in viscosity of the polymeric solutions. CO2 is a small molecule that can
easily penetrate between the polymer chains causing a dilution effect and increasing
their free volume [39, 46]. As gas content increases, viscosity decreases because the
CO2 sorption inside the solution increases while pressure increases and temperature
decreases.
171
6.2.6. Demixing determination from viscosity measurements
Pressure-induced phase separation is a relatively new technique that provides an
alternative path to enter the unstable regions in polymer solutions [22]. The
viscosity can be used to predict the pressure of vanishing, it means, the free energy
density of CO2 becomes closer to that of the polymer-solvent phase and the viscosity
decreases. When gas pressure increases the concentration of CO 2 in the polymersolvent phase increases promoting a decrease of viscosity, since the two phases in
contact become more similar.
In the late 1930s and 1940s Mark, Houwink and Sakurada (MHS) arrived at an
empirical relationship between the molecular weight and the intrinsic viscosity

[6.9]
Where k is a constant in [cm3·g-1], MW is the molecular weight of the polymer and a
is called Mark-Houwink-Sakurada exponent and it provides a measure of the chain
rigidity. According to the experimental viscosity data obtained and knowing the
concentration of the solutions as well as the molecular weight of the polymer, the
MHS parameter can be calculated from the previous equation.
The value of a is thought to be indicative of the solvent power. As a polymer chain is
a good solvent of itself, the polymer chains are much more stretched and sometimes
the chain form molecular entanglements as in the case of this work.
As explained, the viscosity of PS/Limonene solution decreases in the presence of CO 2
as a consequence of the plasticisation effect. In previous work it was noticed that
polymer-solvent phase separation could be accomplished if CO 2 is added as
antisolvent [34]. In compressible fluids, without changing the fluid itself, solvent
quality is altered by increasing the pressure inducing phase separation in polymer
solutions [60]. The parameter a could reflect the change in the solvent power of
Limonene when CO2 is added. Generally, a is 0.5 in the case of theta solvents, while
in good solvents a is 0.8 [61]. When PS is dissolved in Limonene at atmospheric
pressure and 25ºC, a is around 0.35 which is indicative of the poorer solvent power
of the terpene. The parameter a was calculated in the working range of pressure,
temperature and concentration (Figure 6.11).
172
Chapter 6
0.36
0.34
0.32
0.30
a
0.28
0.26
T: 25 ºC
0.24
T: 30 ºC
T: 40 ºC
0.22
0.20
0
5
10
15
20
25
Pressure (bar)
Figure 6.11. Influence of pressure and temperature on parameter a. Markers represent the
different temperatures: (■) 25 ºC, (○) 30 ºC and (▲) 40 ºC.
The viscosity exponent decreases with increasing pressure (Figure 6.11), that is
with increasing the solubility of CO2 in the solution, revealing that the solvent
goodness lowers as the CO2 concentration in the mixture increases. When Limonene
is fully miscible in CO2, its solvent power with regard to the polymer will become
poorer and the parameter a will be close to 0. On the basis of an extrapolation of a,
it should be possible to determine the critical pressure at which phase separation
will appear (Table 6.2).
Table 6.2. Critical pressure of the PS/Limonene solutions as a function of temperature based
on the determination of viscosity.
Temperature (ºC)
Pressure (bar)
24
63.29
29
71.29
34
78.39
The viscosity depends on the CO2 pressure to a greater degree in higher
concentration solutions, presumably because CO 2 is an antisolvent for PS that
reduces the favourability of interactions between the polymer and solvent. It was
proposed that the solvent power of Limonene/CO 2 mixtures decreased with
increasing CO2 pressure, and this increased the importance of polymer–polymer
interactions and thereby reduced the entanglement density [47].
Finally, the influence of pressure, temperature and concentration on the viscosity of
PS/p-Cymene solutions was also studied. Results have been depicted in Figure 6.12.
173
6
(a) C0: 0.3 g PS/ml p-Cymene
P: 6.89 bar
P: 20.68 bar
P: 27.58 bar
P: 34.47 bar
P: 48.26 bar
Viscosity (mPa·s)
5
4
3
2
1
0
(b) C0: 0.4 g PS/ml p-Cymene
Patm
P: 6.89 bar
P: 13.79 bar
P: 20.68 bar
P: 27.58 bar
Viscosity (mPa·s)
40
30
20
10
0
20
25
30
35
40
Temperature (ºC)
Figure 6.12. Viscosity data showing the effect of variables: pressure, temperature and
concentration of PS in the initial solutions of PS/p-Cymene at (a) C0: 0.3 g PS/ml p-Cymene
and (b) C0: 0.4 g PS/ml p-Cymene.
According to Figure 6.12, the viscosity decrease due to the increase of temperature
and pressure is confirmed, as well as the viscosity increase as a consequence of the
polymer concentration increase.
In conclusion, the viscosity of PS/Terpene solutions was studied with the aid of a
quartz viscometer in the presence of high pressure CO 2. All the studied variables:
pressure, temperature and concentration presented statistical significant over the
viscosity. It was checked that the viscosity of the solutions decreases on increasing
pressure, independently of the concentration and temperature. Despite the
reduction of the viscosity because of the plasticisation effect of CO2 the increase of
density of the bulk due to the application of pressure to an incompressible fluid
should be also considered. When temperature increases, the viscosity of
PS/Terpenes solutions decreases and the flow activation energy (Ea) could be
calculated. The values of Ea showed that higher energy is required when the
174
Chapter 6
pressure of CO2 and the concentration of PS increase. The changes of viscosity could
be used to determine the solvent-solute interaction in order to determine the
stability of each phase by means of the Mark-Houwink-Sakurada equation. The
results determined the critical pressure which the phase separation between the
polymer and the solvent by CO2 (antisolvent) could be appeared.
6.3. Interfacial tension of CO2/Limonene/PS and CO2/p-Cymene/PS
The effect of pressure, temperature, concentration and type of solvent on the
interfacial tension (IFT) of PS solutions in the presence of CO 2 was studied. IFT
data were obtained for solutions of 0.05 and 0.20 g PS/ ml terpene. The IFT
measurements were carried out in the temperature range from 30 to 40 ºC and at
pressures from atmospheric up to the vanishing point.
The accuracy of the experimental IFT measurements was determined by comparing
the results from four independent runs carried out under identical conditions. In
these experiments the IFT were similar with a deviation of 0.0373 mN/m, indicating
that reproducibility of the data was also good. The influence of pressure,
temperature and the initial concentration the solvent over the IFT of CO 2/pCymene/PS and CO2/Limonene/PS systems is presented in Figure 6.13.
35
35
(a) C0=0.05 g PS/ml p-Cymene
25
20
15
30 ºC
40ºC
25
20
15
10
10
5
5
0
0
0
10
20
30
40
50
60
70
80
90
0
Pressure (bar)
35
20
 (mN/m)
20
15
30
40
50
60
70
80
90
Pressure (bar)
(d) C0=0.20 g PS/ml Limonene
30
30 ºC
40ºC
25
10
35
(c) C0=0.20 g PS/ml p-Cymene
30
 (mN/m)
(b) C0=0.05 g PS/ml Limonene
30
30 ºC
40ºC
 (mN/m)
 (mN/m)
30
30 ºC
40ºC
25
20
15
10
10
5
5
0
0
0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
80
90
Pressure (bar)
Pressure (bar)
Figure 6.13. Influence of pressure, temperature and initial concentration on IFT of PS
dissolved in Cymene (a, c) and Limonene (b, d).
175
In these Figures it can be seen that the interfacial tension decreases linearly as the
pressure of CO2 increases. This decrease is generally attributed to two phenomena
[62-64]: as pressure increases, the free energy density of CO 2 becomes closer to that
of the polymer-solvent phase and the interfacial tension decreases; and as gas
pressure increases, the concentration of CO 2 in the polymer-solvent phase increases
thus further promoting a decrease of interfacial tension since the two phases in
contact become more similar.
On the other hand, in both systems the pressure and the interfacial tension value at
which the interfacial tension vanishes (both reported in Table 6.3) slightly increase
with the temperature, especially at higher concentration.
Table 6.3. Effect of solvent, temperature and initial concentration of PS dissolution on the
critical pressure obtained from experimental IFT extrapolation.
Solvent
C0 (g PS /ml)
Temperature (ºC)
Slope (mN/m·bar)
Intercept (mN/m)
Critical Pressure (bar)
Solvent
p- Cymene
0.05
30
0.3766
28.7824
76.42
0.20
40
0.3320
27.9334
84.31
30
0.3756
27.0665
72.06
40
0.3337
27.7747
83.24
Limonene
C0 (g PS/ml)
0.05
Temperature (ºC)
30
40
Slope (mN/m·bar)
0.3233
0.3299
Intercept (mN/m)
26.1692
28.1363
Critical Pressure (bar) 80.94
85.29
0.20
30
40
0.4268 0.3289
30.9898 27.4318
72.61
83.40
The effect of temperature on the interfacial tension requires a more complex
explanation, since the temperature produces two opposite effects: i) as temperature
increases the average kinetic energy of the molecules also do. So, an increase of the
temperature produces a decrease on the interfacial tension, since, the interfacial
tension accounts the energy required to create a new surface at the interface
between two phases [65]; ii) as the temperature increases the CO2 density decreases,
and therefore the solubility in CO2 of the polymer/cosolvent system decreases. In
Figure 6.12 and Table 6.3 it can be seen that the second effect is dominant in the
experimental range since when temperature increases, it is observed an increase of
both: the interfacial tension and the pressure of vanishing. Nevertheless, in the case
of 0.05 gPS/ml Limonene, where IFT slope is kept constant, both effect (increase of
kinetic energy and CO2 density) are similar and the effect should be study in a
wider range to explain the exception.
Quantitatively IFT decrease is confirmed by the negative slope of the straight lines
in Figure 6.13 (values are reported in Table 6.3). While IFT is not noticeably
176
Chapter 6
affected by the PS initial concentration temperature causes a significant slope
reduction. The average correlation obtained was 0.9957.
These results are in good agreement with those reported by Park et al. [66] which
studied the surface tension of molten PS in supercritical CO 2 at high temperature
(170-210 ºC). They found that surface tension varies linearly with temperature and
pressure, concluding that the effect of temperature on it becomes lower with
increasing pressure. This fact implies that a temperature increase is only effective if
mild pressure is used during PS processing.
The possibility of correlate the interfacial tension of polymer is important for the
design of further processes. There are several techniques to estimate the surface
tension of pure liquids, but the most useful are empirical. Macleod-Sugden [67]
proposed a correlation to calculate the surface tension of liquid mixture. That is not
a simple equation relating the surface tensions of the pure components of the
mixture because the composition near the surface is not the same than that of the
bulk.
σ 1/4= [P] (ρL- ρV)
[6.10]
where P is the Parachor number, a parameter temperature independent that might
be estimated from the molecular structure and LV are the liquid and vapour
densities, respectively. Following the previous equations the IFT of solvents was
calculated, without considering the polymer concentration at 30 and 40ºC at
atmospheric pressure. The results are shown in Table 6.4.
Table 6.4. Predicted values and Average Absolute Deviation (AAD) of p-Cymene and
Limonene IFT according to semiempirical methods.
p-Cymene
Harrison
et al., 1998
[67]
Brock and
Bird,
1955[68]
Miller,
1963[69]
Pitzer,
1995 [70]
Sastri and
Rao, 1995
[71]
AAD
30ºC
26.019
27.113
27.050
28.208
27.485
0.537
40ºC
25.158
26.166
26.105
27.223
26.525
0.511
Limonene
Harrison
et al., 1998
[67]
Brock and
Bird,
1955[68]
Miller,
1963[69]
Pitzer,
1995 [70]
Sastri and
Rao, 1995
[71]
AAD
30ºC
25.225
26.760
26.697
26.871
27.539
0.558
40ºC
24.523
25.826
25.765
25.940
26.585
0.482
According to Table 6.4, the calculated IFT values present an average absolute
deviation of 0.522 mN/m (maximum), sufficiently small to consider that the
referenced methods are able to predict the interfacial tension with accuracy enough
177
for most practical purposes. Also, IFT predicted values at 0 bar are in good
agreement with the cited by Simões et al. [72] who presented the IFT measurements
of lemon oil, whose main constituents are monoterpenes and largely Limonene (≈
67%) [73].
Among the described equations to predict the IFT, only the developed by MacleodSugden includes the effect of pressure by means of the density of the vapour and
liquid phase. This equation was used to predict Limonene/CO2 and p-Cymene/CO2
surface tension to calculate the global parameter (IFT).
Park et al. [66], on the basis of their statistical investigations, proposed an equation
to estimate the surface tension of the molten Polystyrene in CO2. Based on Park
treatment of IFT, PS in solution can be considered as a molten polymer because of
the high mobility of its chains. With this hypothesis the effect of the polymer
solubilised on the IFT of the solution can be estimated.
Based on the calculated IFT for the solvent/CO2 and for the PS/CO2 and considering
the initial concentration of the solutions, IFT of solution value was calculated as a
function of pressure, temperature and concentration, following next equation [51].
σmr=Σinxi σir
[6.11]
where mr is the surface tension of mixture, xi is the mole fraction of component i in
the bulk phase andir is the surface tension of pure component i. The exponent r is
recommended to be 1 for most hydrocarbon mixtures which would predict linear
behaviour in surface tension vs. composition. Initially the mixing rule applied was
the sum of the corresponding individual IFT multiplied by their mass fraction (dash
line) and secondly a quadratic mixing rule was applied (continuous line), making
use of an adjustable parameter depending on the initial concentration of the
solution. In mixtures containing CO2 an interaction parameter is required to fit
experimental data [51]. In this work, mass fraction of the solvent and the polymer
was considered instead of mole fraction because PS mole fraction is difficult to
calculate attending to the molecular weight distribution. The results are showed in
Figure 6.14 and 6.15.
As it observed in Figures 6.14 and 6.15, empirical methods to estimate IFT are more
accurate at 0.05 g/ml (initial concentration) and 40º C because the error usually
increases as the concentration of the component with the largest pure component
surface tension increases (Polystyrene in this case). Using quadratic mixing rule at
40ºC, the IFT prediction improves slightly as it is particularly observed in Limonene
at 0.20 g PS/ml.
IFT prediction is useful to establish initial values or ranges, but accuracy has been
demonstrated to be only acceptable in the case of low polymer concentrations
because the error increases when higher concentrations are used. IFT experimental
data cannot be replaced by predictions since the global aim of the research line is
the development of a recycling process of PS foams by dissolution with terpenic
178
Chapter 6
solvents and during the process high polymer concentrations are preferable to work.
Consequently, IFT must be checked experimentally in the case of large PS
concentrations.
35
35
(a) C0:0.05 g PS/ml p-Cymene
30
T: 30ºC
T: 30ºC
25
 (mN/m)
 (mN/m)
25
(b) C0:0.20 g PS/ml p-Cymene
30
20
15
10
5
20
15
10
5
0
0
0
10
20
30
40
50
60
70
80
90
0
10
20
Pressure (bar)
40
50
60
70
80
90
Pressure (bar)
35
35
(c) C0:0.05 g PS/ml p-Cymene
30
T: 40ºC
25
(d) C0:0.20 g PS/ml p-Cymene
30
 (mN/m)
 (mN/m)
30
20
15
10
5
T: 40ºC
25
20
15
10
5
0
0
0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
80
90
Pressure (bar)
Pressure (bar)
Figure 6.14. IFT of PS/p-Cymene solutions versus the CO2 pressure. Experimental points
and correlated curve. () Dash line: linear mixing rule; (—) Solid line: quadratic mixing rule.
35
35
(a) C0:0.05 g PS/ml Limonene
30
T: 30 ºC
T: 30 ºC
25
 (mN/m)
 (mN/m)
25
(b) C0:0.20 g PS/ml Limonene
30
20
15
10
5
20
15
10
5
0
0
0
10
20
30
40
50
60
70
80
90
0
10
20
Pressure (bar)
40
50
60
70
80
90
Pressure (bar)
35
35
(c) C0:0.05 g PS/ml Limonene
30
(d) C0:0.20 g PS/ml Limonene
30
T: 40 ºC
T: 40 ºC
25
 (mN/m)
25
 (mN/m)
30
20
15
10
5
20
15
10
5
0
0
0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
80
90
Pressure (bar)
Pressure (bar)
Figure 6.15. IFT of PS/Limonene solutions versus the CO2 pressure. Experimental points
and correlated curve. () Dash line: linear mixing rule; (—) Solid line: quadratic mixing rule.
179
To conclude, Interfacial tension of Polystyrene solutions decreases linearly with
pressure and temperature while the polymer concentration has not a significant
influence in the studied range. The behaviour of the PS in solution in the presence
of CO2 is similar to molten polymers and by this reason, it is possible to predict
easily the interfacial tension of the mixture.
6.4. Glass transition temperature of CO2/Limonene/PS and CO2/pCymene/PS
A final set of experiments was carried out in order to study the influence of a
cosolvent on the glass transition temperature of the polymer-solvent mixture. The
influence of CO2 on the Tg of PS/Limonene solutions was assayed (Figure 6.16).
According to Figure 6.16, the presence of the terpene does not produce a significant
decrease regarding the Tg of the Polystyrene/CO2 binary system (Chapter 4)
independently of the initial concentration of the polymer in the solution. During
plasticization, polymers are swelled with a consequent increase in the free volume of
the polymer, small molecules, as CO2, soak easily in through the matrix than the
bigger ones. The main influence of CO2 over the PS glass transition temperature
makes the Limonene plays only a secondary role on the system. By this reason, Tg
reaches very close values to the obtained without solvent.
110
PS/CO2
100
Solutions C0=0.9 g PS/ml Limonene
90
Solutions C0=0.2 g PS/ml Limonene
80
Tg (ºC)
70
60
50
40
30
20
10
0
0
20
40
60
80
100
120
Pressure (bar)
140
160
180
200
Figure 6.16. Influence of CO2 pressure on the glass transition temperature of PS in
Limonene solutions.(*) This work; () Initial concentration: 0.90 g PS/ml Limonene; (▲)
Initial concentration: 0.20 g PS/ml Limonene.
180
Chapter 6
These results agree with the previously obtained by Stafford et al. [74] that
concluded that the presence of low molecular weight substances in a polymer/CO 2
system does not affect greatly the value of the Tg observed.
Also, the influence of CO2 on the Tg of PS/p-Cymene solutions was studied. Results
and errors are shown in Table 6.5.
Table 6.5. Influence of the pressure of CO 2 on the glass transition temperature of PS /pCymene solutions at 0.2 g PS/ ml p-Cymene.
P (bar)
Tg (ºC)
0
101.4±0.99
20
76.8±1.42
40.4
51.2±0.74
60.4
39.5±2.40
79.6
33.6±1.15
99.6
33.1±0.06
According to Table 6.5 is observed that Tg decreases when pressure of CO2 increases
from 0 to 100 bar. From the comparison of results shown in Tables 6.5 and 4.23 (Tg
of the binary system PS/CO2) is observed that the presence of p-Cymene does not
decrease dramatically the Tg of the solutions. The plastising effect of CO 2 is the
main responsible of the Tg drop since it is a very small molecule which penetrates
better inside the polymeric chains. Results agree pretty well with the previously
shown in Figure 6.16, it confirms that terpene solvents are not the main cause of the
glass transition temperature decrease when there are CO 2 in the system.
In conclusion, the sorption of CO2 in the Polystyrene/terpene oils solutions decreases
the viscosity, interfacial tension and glass transition temperature because of the
plasticisation effect of CO2. Temperature and concentration also affects the
mentioned physico-chemical properties of the polymeric solutions. The vanishing of
viscosity and interfacial tension can be used to determine the solvent-solute
interaction in order to determine the stability of each phase and the demixing
points.
181
References
[1] S.D. Yeo, E. Kiran, High-pressure density and viscosity of polystyrene solutions in
methylcyclohexane, Journal of Supercritical Fluids, 15 (1999) 261-272.
[2] H.M. Woods, M.M.C.G. Silva, C. Nouvel, K.M. Shakesheff, S.M. Howdle,
Materials processing in supercritical carbon dioxide: Surfactants, polymers and
biomaterials, Journal of Materials Chemistry, 14 (2004) 1663-1678.
[3] O.R. Davies, A.L. Lewis, M.J. Whitaker, H. Tai, K.M. Shakesheff, S.M. Howdle,
Applications of supercritical CO2 in the fabrication of polymer systems for drug
delivery and tissue engineering, Advanced Drug Delivery Reviews, 60 (2008) 373387.
[4] A.I. Cooper, Polymer synthesis and processing using supercritical carbon dioxide,
Journal of Materials Chemistry, 10 (2000) 207-234.
[5] S. Villarroya, K.J. Thurecht, A. Heise, S.M. Howdle, Supercritical CO2: An
effective medium for the chemo-enzymatic synthesis of block copolymers?, Chemical
Communications, (2007) 3805-3813.
[6] J. Jennings, M. Beija, A.P. Richez, S.D. Cooper, P.E. Mignot, K.J. Thurecht, K.S.
Jack, S.M. Howdle, One-pot synthesis of block copolymers in supercritical carbon
dioxide: A simple versatile route to nanostructured microparticles, Journal of the
American Chemical Society, 134 (2012) 4772-4781.
[7] L. An, B.A. Wolf, Combined effects of pressure and shear on the phase separation
of polymer solutions, Macromolecules, 31 (1998) 4621-4625.
[8] B. Bungert, G. Sadowski, W. Arlt, Supercritical antisolvent fractionation:
Measurements in the systems monodisperse and bidisperse polystyrene-cyclohexanecarbon dioxide, Fluid Phase Equilibria, 139 (1997) 349-359.
[9] I.H. Lin, P.F. Liang, C.S. Tan, Precipitation of submicrometer-sized poly(methyl
methacrylate) particles with a compressed fluid antisolvent, Journal of Applied
Polymer Science, 117 (2010) 1197-1207.
[10] I. Asghari, F. Esmaeilzadeh, Investigation of key influence parameters for
synthesis of submicron carboxymethylcellulose particles via rapid expansion of
supercritical CO2 solution by Taguchi method, Journal of Supercritical Fluids, 69
(2012) 34-44.
[11] E. Reverchon, S. Cardea, Production of controlled polymeric foams by
supercritical CO2, Journal of Supercritical Fluids, 40 (2007) 144-152.
[12] X. Han, K.W. Koelling, D.L. Tomasko, L.J. Lee, Continuous microcellular
polystyrene foam extrusion with supercritical CO 2, Polymer Engineering and
Science, 42 (2002) 2094-2106.
[13] S. Cotugno, E. Di Maio, G. Mensitieri, S. Iannace, G.W. Roberts, R.G.
Carbonell, H.B. Hopfenberg, Characterization of microcellular biodegradable
polymeric foams produced from supercritical carbon dioxide solutions, Industrial
and Engineering Chemistry Research, 44 (2005) 1795-1803.
[14] Y.C. Bae, Gas plasticization effect of carbon dioxide for polymeric liquid,
Polymer, 37 (1996) 3011-3017.
[15] L.J. Gerhardt, A. Garg, C.W. Manke, E. Gulari, Concentration-dependent
viscoelastic scaling models for polydimethysiloxane melts with dissolved carbon
dioxide, Journal of Polymer Science, Part B: Polymer Physics, 36 (1998) 1911-1918.
[16] M. Onishi, Y. Nakamura, T. Norisuye, Intrinsic viscosity of polystyrene in
toluene-supercritical carbon dioxide mixtures, Polymer Journal, 41 (2009) 477-481.
[17] R. Liao, W. Yu, C. Zhou, Rheological control in foaming polymeric materials: I.
Amorphous polymers, Polymer, 51 (2010) 568-580.
182
Chapter 6
[18] Y.W. Kho, D.S. Kalika, B.L. Knutson, Precipitation of Nylon 6 membranes using
compressed carbon dioxide, Polymer, 42 (2001) 6119-6127.
[19] G.H. Chong, R. Yunus, N. Abdullah, T.S.Y. Choong, S. Spotar, Coating and
encapsulation of nanoparticles using supercritical antisolvent, American Journal of
Applied Sciences, 6 (2009) 1352-1358.
[20] M. Karimi, M. Heuchel, T. Weigel, M. Schossig, D. Hofmann, A. Lendlein,
Formation and size distribution of pores in poly(ε-caprolactone) foams prepared by
pressure quenching using supercritical CO 2, Journal of Supercritical Fluids, 61
(2012) 175-190.
[21] L.N. Nikitin, M.O. Gallyamov, R.A. Vinokur, A.Y. Nikolaec, E.E. Said-Galiyev,
A.R. Khokhlov, H.T. Jespersen, K. Schaumburg, Swelling and impregnation of
polystyrene using supercritical carbon dioxide, Journal of Supercritical Fluids, 26
(2003) 263-273.
[22] C. Dindar, E. Kiran, High-pressure viscosity and density of polymer solutions at
the critical polymer concentration in near-critical and supercritical fluids, Industrial
[23] K. Wasai, G. Kaptay, K. Mukai, N. Shinozaki, Modified classical homogeneous
nucleation theory and a new minimum in free energy change. 1. A new minimum and
Kelvin equation, Fluid Phase Equilibria, 254 (2007) 67-74.
nucleation theory and a new minimum in free energy change. 2. Behavior of free
energy change with a minimum calculated for various systems, Fluid Phase
Equilibria, 255 (2007) 55-61.
[25] K. Nishioka, I. Kusaka, Thermodynamic formulas of liquid phase nucleation
from vapor in multicomponent systems, The Journal of Chemical Physics, 96 (1992)
5370-5376.
[26] J.W. Cahn, J.E. Hilliard, Free energy of a nonuniform system. III. Nucleation in
a two-component incompressible fluid, The Journal of Chemical Physics, 31 (1959)
688-699.
[27] S.K. Goel, E.J. Beckman, Generation of microcellular polymeric foams using
supercritical carbon dioxide. I: effect of pressure and temperature on nucleation,
Polymer Engineering and Science, 34 (1994) 1137-1147.
[29] M. Sauceau, C. Nikitine, E. Rodier, J. Fages, Effect of supercritical carbon
dioxide on polystyrene extrusion, Journal of Supercritical Fluids, 43 (2007) 367-373.
[30] D. Liu, H. Li, M.S. Noon, D.L. Tomasko, CO2-induced PMMA swelling and
multiple thermodynamic property analysis using Sanchez-Lacombe EOS,
Macromolecules, 38 (2005) 4416-4424.
[31] B.A. Miller-Chou, J.L. Koenig, A review of polymer dissolution, Progress in
Polymer Science (Oxford), 28 (2003) 1223-1270.
[32] H. Park, C.B. Park, C. Tzoganakis, K.H. Tan, P. Chen, Surface tension
measurement of polystyrene melts in supercritical carbon dioxide, Industrial and
[33] I. Teraoka, Polymer Solutions: An Introduction to Physical Properties, Wiley,
New York, 2002.
183
[35] P. A. Gauglitz, F. Friedmann, S. I. Kam, W. R. Rossen, Foam generation in
homogeneous porous media, Chemical Engineering Science, 57 (2002) 4037-4052.
[36] C. Gutiérrez, M.T. García, I. Gracia, A. de Lucas, J.F. Rodríguez, Recycling of
extruded polystyrene wastes by dissolution and supercritical CO 2 technology, Journal
of Material Cycles and Waste Management, 14 (2012) 308-316.
[37] C. Kwag, C.W. Manke, E. Gulari, Effects of dissolved gas on viscoelastic scaling
and glass transition temperature of polystyrene melts, Industrial and Engineering
[38] C. Kwag, C.W. Manke, E. Gulari, Rheology of molten polystyrene with dissolved
supercritical and near-critical gases, Journal of Polymer Science, Part B: Polymer
Physics, 37 (1999) 2771-2781.
[39] J.R. Royer, Y.J. Gay, J.M. Desimone, S.A. Khan, High-pressure rheology of
polystyrene melts plasticized with CO 2: Experimental measurement and predictive
scaling relationships, Journal of Polymer Science, Part B: Polymer Physics, 38
(2000) 3168-3180.
[40] W.W. Graessley, Polymer chain dimensions and the dependence of viscoelastic
properties on concentration, molecular weight and solvent power, Polymer, 21 (1980)
258-262.
[41] S.D. Yeo, E. Kiran, High-pressure viscosity of polystyrene solutions in
toluene+carbon dioxide binary mixtures, Journal of Applied Polymer Science, 75
(2000) 306-315.
[42] B.A. Wolf, R. Jend, Pressure and temperature dependence of the viscosity of
polymer solutions in the region of phase separation, Macromolecules, 12 (1979) 732737.
[43] D. Gourgouillon, H.M.N.T. Avelino, J.M.N.A. Fareleira, M. Nunes da Ponte,
Simultaneous viscosity and density measurement of supercritical CO 2-saturated PEG
400, Journal of Supercritical Fluids, 13 (1998) 177-185.
[44] Y. Zhang, K.K. Gangwani, R.M. Lemert, Sorption and swelling of block
copolymers in the presence of supercritical fluid carbon dioxide, Journal of
[45] S.A.E. Boyer, J.P.E. Grolier, Modification of the glass transitions of polymers by
high-pressure gas solubility, Pure and Applied Chemistry, 77 (2005) 593-603.
[46] M. Lee, C.B. Park, C. Tzoganakis, Measurements and modeling of
PS/supercritical CO2 solution viscosities, Polymer Engineering and Science, 39
(1999) 99-109.
[47] R.E. Whittier, D. Xu, J.H. Van Zanten, D.J. Kiserow, G.W. Roberts, Viscosity of
polystyrene solutions expanded with carbon dioxide, Journal of Applied Polymer
Science, 99 (2006) 540-549.
Technology, 170 (2006) 143-152.
[49] M. Lee, C. Tzoganakis, C.B. Park, Effects of supercritical CO2 on the viscosity
and morphology of polymer blends, Advances in Polymer Technology, 19 (2000) 300311.
[50] W. Strauss, N.A. D'Souza, Supercritical CO2 processed polystyrene
nanocomposite foams, Journal of Cellular Plastics, 40 (2004) 229-241.
184
Chapter 6
[52] K. Lucas, Die Druckabhängigkeit der Viskosität von Flüssigkeiten – eine
einfache Abschätzung, Chemie Ingenieur Technik, 53 (1981) 959-960.
[53] Y.P. Sun, Supercritical fluid technology in materials science and engineering:
syntheses, properties, and applications, Marcel Dekker, 2002.
[54] H.J. Kim, H.C. Moon, H. Kim, K. Kim, J.K. Kim, J. Cho, Pressure effect of
various inert gases on the phase behavior of polystyrene- block -poly(n-pentyl
methacrylate) copolymer, Macromolecules, 46 (2013) 493-499.
[55] J.D. Isdale, G. Cartwright, J.C. MacGillivray, Prediction of Viscosity of Organic
Liquid Mixtures by a Group Contribution Method, 1981.
[56] R.A. Clará, A.C.G. Marigliano, H.N. Sólimo, Density, viscosity, and refractive
index in the range (283.15 to 353.15) K and vapor pressure of α-pinene, d-limonene,
(±)-linalool, and citral over the pressure range 1.0 kPa atmospheric pressure, Journal
of Chemical and Engineering Data, 54 (2009) 1087-1090.
[57] K. Liu, F. Schuch, E. Kiran, High-pressure viscosity and density of poly(methyl
methacrylate) + acetone and poly(methyl methacrylate) + acetone + CO 2 systems,
[58] E. Kiran, Z. Gokmenoglu, High-pressure viscosity and density of polyethylene
solutions in n-pentane, Journal of Applied Polymer Science, 58 (1995) 2307-2324.
[59] B. Seifried, F. Temelli, Viscosity and rheological behaviour of carbon dioxideexpanded fish oil triglycerides: Measurement and modeling, Journal of Supercritical
Fluids, 59 (2011) 27-35.
[60] E. Kiran, Polymer miscibility, phase separation, morphological modifications
and polymorphic transformations in dense fluids, Journal of Supercritical Fluids, 47
(2009) 466-483.
[62] H. Li, L.J. Lee, D.L. Tomasko, Effect of Carbon Dioxide on the Interfacial
Tension of Polymer Melts, Industrial and Engineering Chemistry Research, 43
(2004) 509-514.
[63] K.L. Harrison, S.R.P. Da Rocha, M.Z. Yates, K.P. Johnston, D. Canelas, J.M.
DeSimone, Interfacial activity of polymeric surfactants at the polystyrene-carbon
dioxide interface, Langmuir, 14 (1998) 6855-6863.
[64] M.G. Pastore Carbone, E. Di Maio, G. Scherillo, G. Mensitieri, S. Iannace,
Solubility, mutual diffusivity, specific volume and interfacial tension of molten
PCL/CO2 solutions by a fully experimental procedure: Effect of pressure and
temperature, Journal of Supercritical Fluids, 67 (2012) 131-138.
[65] X. Liao, Y.G. Li, C.B. Park, P. Chen, Interfacial tension of linear and branched
PP in supercritical carbon dioxide, Journal of Supercritical Fluids, 55 (2010) 386394.
[66] H. Park, R.B. Thompson, N. Lanson, C. Tzoganakis, C.B. Park, P. Chen, Effect
of temperature and pressure on surface tension of polystyrene in supercritical carbon
dioxide, Journal of Physical Chemistry B, 111 (2007) 3859-3868.
[67] D.B. Macleod, On a relation between surface tension and density, Transactions
of the Faraday Society, 19 (1923) 38-41.
[68] J.R. Brock, R.B. Bird, Surface tension and the principle of corresponding states,
AIChE Journal, 1 (1955) 174-177.
[69] D.G. Miller, Graphical methods for determining a nonlinear constant in vapor
pressure equations, Industrial and Engineering Chemistry Fundamentals, 2 (1963)
68-73.
185
[70] K.S. Pitzer, Thermodynamics, 3rd ed. ed., McGraw-Hill, New York :, 1995.
[71] S.R.S. Sastri, K.K. Rao, A simple method to predict surface tension of organic
liquids, The Chemical Engineering Journal and The Biochemical Engineering
Journal, 59 (1995) 181-186.
[72] P.C. Simões, R. Eggers, P.T. Jaeger, Interfacial tension of edible oils in
supercritical carbon dioxide, European Journal of Lipid Science and Technology, 102
(2000) 263-265.
[73] J. Rao, D.J. McClements, Impact of lemon oil composition on formation and
stability of model food and beverage emulsions, Food Chemistry, 134 (2012) 749-757.
[74] C.M. Stafford, T.P. Russell, T.J. McCarthy, Expansion of polystyrene using
supercritical carbon dioxide: effects of molecular weight, polydispersity, and low
molecular weight components, Macromolecules, 32 (1999) 7610-7616.
186
Chapter 7
APPLICATIONS
Chapter 7 shows the precipitation and foaming processes for the recycling of
Polystyrene from its homogeneous solutions in terpene oils using CO 2 as antisolvent
and blowing agent. Once the phases’ equilibrium was studied and the effect of CO 2
on the shift of physical properties was measured, the design of a foaming process
was carried out. Initially, the feasibility of the processes was determined on the
selection of the solubility parameters and the vanishing points of the solutions in
CO2.
Supercritical Antisolvent and Foaming Processes
188
Chapter 7
Based on:
 C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. De Lucas, M.T. García, Development of
a strategy for the foaming of polystyrene dissolutions in scCO 2, Journal of
 C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. De Lucas, M.T. García, Foaming
process from Polystyrene/p-Cymene solutions using CO2, Chemical Engineering &
Technology, in press.
 C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. de Lucas, M.T. García, Preparation
and characterization of polystyrene foams from limonene solutions, The Journal of
Graphical abstract
189
RESUMEN
Las técnicas que emplean un antisolvente en estado supercrítico (SAS) se han
convertido en una de las tecnologías más apropiadas para conseguir la precipitación
de un polímero dando lugar a una morfología bien definida. El estudio del equilibrio
termodinámico y las propiedades de las fases que forman parte del proceso es
fundamental para seleccionar las condiciones de operación a las que llevar a cabo el
proceso de precipitación.
En primer lugar, se determinó la viabilidad del proceso de separación en base a la
predicción de los parámetros de solubilidad a alta presión y la extracción del aceite
terpénico, que contiene Poliestireno, con CO2. Sin embargo, cuando se plantea el
proceso SAS en continuo, el Poliestireno no precipita de acuerdo a una morfología
previamente seleccionada. Por este motivo, es necesario estudiar otros factores como
la hidrodinámica del sistema, ya que es fundamental para lograr precipitar el
polímero adecuadamente. Con el objetivo de preseleccionar el tamaño de las
partículas de Poliestireno, se calcularon los coeficientes de difusión de las mezclas
binarias para aplicarlos de acuerdo con la segunda ley de Fick. Así, se consiguieron
las condiciones para precipitar el Poliestireno, a partir de su disolución en terpeno
usando CO2 como antisolvente, dando lugar a partículas secas y con una estructura
bien definida.
Sin embargo, debido a la complejidad del proceso SAS, se considera más adecuada la
espumación de residuos de Poliestireno para dar lugar a espumas microcelulares
que presentan un alto valor añadido. En este apartado, inicialmente se determinó la
viabilidad del proceso de espumación y a continuación, se llevó a cabo un diseño
completo de experimentos.
En un proceso de espumación, hay numerosos parámetros que permiten modificar
las propiedades de las espumas. Por este motivo, se estudió el efecto de la presión, la
temperatura, la concentración de la disolución, el tiempo de contacto y el tiempo de
despresurización en el diámetro medio de las celdas y su distribución, así como en su
densidad. Gracias a la presencia del disolvente terpénico, el proceso de espumación
se puede desarrollar a condiciones de presión y temperatura moderadas. De acuerdo
con el diseño factorial, se obtuvo que las condiciones más adecuadas para obtener
espumas microcelulares de Poliestireno a partir de su disolución en Limoneno,
fueron 90 bar, 30 ºC, 0.1 g PS/ml Limoneno, 240 minutos de tiempo de contacto y 30
minutos de despresurización. Por último, se analizaron las muestras del polímero
recuperado para determinar la cantidad residual de disolvente en la espuma, la
temperatura de transición vítrea y la temperatura de degradación, demostrando que
el Poliestireno espumado contenía cantidades inferiores a un 5% de Limoneno, y en
ningún caso hubo signos de degradación.
190
Chapter 7
ABSTRACT
The Supercritical Antisolvent (SAS) techniques are shown as a very promising
technology to achieve precipitation of polymer according to a well-defined
morphology. The study of the thermodynamic properties of the phases involved in
the process is crucial to select the most suitable conditions to perform the process.
Initially, the feasibility of the separation process was studied according to the
prediction of solubility parameters at high pressure and the extraction of the
terpene from a polymer solution using CO2. Nevertheless, during continuous SAS
process, Polystyrene did not precipitate according to previously selected morphology.
By this reason, other factors such as the hydrodynamic of the system are crucial for
that to succeed. Diffusion coefficients were calculated and Fick’s second law was
applied with the aim of fixing the size particles. Finally, well defined and dried
structures were obtained from the precipitation of Polystyrene solutions using CO 2.
Nevertheless, because of the complexity of the process, the foaming of Polystyrene
wastes was considered as a very valuable alternative to precipitate the polymer into
a final product with high-added value. In this section, the feasibility of the foaming
process was determined and next, a complete design of experiments was performed.
During the foaming process, many parameters can be tuned to customize the foams.
In this section, a design of experiment was proposed to determine the effect of
pressure, temperature, concentration of the solution, contact time and vent time
over the diameter of cells, its standard deviation and the cells density. The proposed
foaming process can be simply performed at mild pressure and temperature thanks
to the presence of the solvent. The results showed that the most suitable conditions
to foam Polystyrene from Limonene solutions are 90 bar, 30 ºC, 0.1 g PS/ml
Limonene, 240 minutes contacting and 30 minutes venting. Finally, the samples
were characterised to determine the amount of residual solvent, their glass
transition and degradation temperature checking that the foams presented around
5% of solvent traces but did not show any evidence of degradation.
191
192
Chapter 7
7.1. Supercritical Antisolvent Background
Precipitation processes using a supercritical antisolvent allow solid microparticles
with controlled size and morphology to be obtained from a raw product [1]. CO2 has
been widely used as antisolvent for the preparation of polymeric microparticles due
to its excellent properties, its mild supercritical conditions and the immiscibility of
polymers into the gas [2-6]. An increasing number of processes in which CO2 can be
used as antisolvent have been developed during the last decades. These processes
have been also defined as precipitation with a compressed antisolvent (PCA) since
the solute to be precipitated is dissolved in the organic solvent, and the solution is
sprayed across a capillary into a compressed or supercritical fluid, which is miscible
with the solvent, but behaves as an antisolvent for the solute [7, 8]. The general
process is basically named Supercritical Antisolvent (SAS) although there are
specific acronyms to denote the different techniques: GAS (gas antisolvent), PCA
(precipitation by compressed antisolvent), ASES (aerosol solvent extraction system),
SEDS (solution enhanced dispersion by supercritical fluids) or RESS (rapid
expansion of supercritical solutions) are among the most common [9, 10].
The use of compressed or supercritical antisolvent for the precipitation of polymer
presents several advantages over the conventional techniques (spray drying,
mechanical comminution, recrystallization…): reduce the use of organic solvents,
prevent thermal and chemical degradation of solute, decrease residual solvent
concentration and control particle size and particle size distribution [11]. Depending
on the process conditions, nozzle configuration and species involved, particles with
different characteristics can be obtained [12-14]. Under appropriate operating
conditions, high mass transfer occurs [6], the high diffusion rates and intense
atomization can lead to rapid phase separation producing submicrometre particles
[8]. The simultaneous diffusion of the antisolvent into the organic phase and the
solubilisation of the organic solvent into the bulk antisolvent are enhanced due to
the gas-like diffusivities of the CO2 and the density reduction of the expanded
organic phase [15].
The way in which the liquid solution is dispersed in the CO2 at high pressure
depends mainly on the pressure, when it is above the mixture critical point, mass
transfer takes place by turbulent diffusion and mixing of the fluids is faster than
droplet formation. Above the mixture critical pressure formation of droplets is not
expected, due to the inexistence of surface tension between the solvent and the CO 2.
On the other hand, when pressure is below the mixture critical points, the jet is
disintegrated and mass transfer occurs by molecular diffusion through the solvent
droplet interface [16, 17]. In this case, the precipitated particles usually exhibit
different morphology and higher size than those precipitated above the mixture
critical point, in which high supersaturation levels arise, affording smaller particles
[11]. Either below or above the critical pressure of the mixture, carbon dioxide
induces a liquid–liquid phase split in the polymer solution [16].
193
The phase equilibrium in the SAS process is well known from a theoretical point of
view, but there is a serious lack of experimental data for the multicomponent
systems of interest for this process [13]. However, the mechanism of particle
formation is not very clear yet and in several cases, unsuccessful precipitations were
reported, even when conditions were correctly selected on the basis of binary and
ternary systems behaviour [18]. The reasons for this are that some authors
considered only binary diagrams CO2/organic solvent due to the difficulties in the
construction of ternary diagrams [19], the complexity of the process, in which phase
behaviour, mass transfer, precipitation kinetics and hydrodynamics strongly
interact with each other [16] needs also to be included.
In this section, the precipitation of Polystyrene from its solution in terpenes using
CO2 as antisolvent was studied. Initially, the technical feasibility of the separation
process was studied theoretically from the solubility parameters point of view and
experimentally from an extraction procedure. Next, the continuous process, where a
solution of PS/terpene is fed to a vessel previously filled with high pressure CO 2,
was performed. The mass transfer and the hydrodynamic of the system was studied
in order to achieve some trends which relate the effect of pressure, temperature,
concentration, flow or nozzle diameter on the morphology of the recovered polymer.
7.2. Supercritical Antisolvent Process
7.2.1. Solubility Parameters
It has been demonstrated the power of solubility parameters as a tool for the
prediction of solubility (Chapter 4). The methodology established by Hansen [20]
was extended to be applied at high pressure by means of Equations of State. On a
general basis, when the solubility parameters of a solute is within 4 MPa1/2 of that of
the solvent, the solute will be pretty soluble in it. This rule can be applied to
gaseous, liquid, crystalline and polymeric solutes [21].
In the antisolvent processes, the organic solvent should be soluble in CO 2, while the
solid (PS) should not be. According to this fact, the solubility parameters () of
CO2/PS and CO2/Limonene were calculated in vapour, liquid and solid phase in
order to select the most suitable conditions to perform the antisolvent process. When
the solubility parameters of liquid-vapour in the case of binary system
CO2/Limonene are lower than 4 MPa1/2, while the solubility parameters of solidvapour in the binary system CO2/PS are higher, the antisolvent process could be
accomplished.
Figure 7.1 shows the solubility parameters of the defined systems as a function of
pressure and temperature. The solubility parameters of the vapour phase are
depicted with reddish tones, and the solubility parameters of the liquid or solid
phases are shown as a bluish shade.
194
Chapter 7
5
50
4
40
3

30

2
20
1
10
0
25
30
35
Temperature (ºC)
40
0
10
20
30
40
50
Pressure (bar)
60
70
80
0
25
30
35
Temperature (ºC)
40
0
10
20
30
40
50
60
Pressure (bar)
Figure 7.1 (Left) Effect of pressure and temperature on the solubility parameters of the
vapour and liquid phase of the system CO 2/Limonene. (Right) Influence of pressure and
temperature on the solubility parameters of the vapour and solid phase of the binary system
CO2/PS.
According to Figure 7.1 (left) is observed that CO 2 and Limonene are soluble in all
the studied range of pressure (0-80 bar) and temperature (25-30 ºC) since the
difference between the solubility parameters is lower than 4 MPa1/2. Regarding the
solubility parameter of the vapour and liquid phase, the highest solubility of
Limonene in CO2 is achieved in the range of pressure between 60 and 70 bar in the
interval of temperature between 25 and 30 ºC because the solubility parameter in
vapour and liquid phase are identical. On the other hand, Figure 7.1 (right) shows
that independently of the pressure and temperature, the differences between the
solubility parameters of solid and vapour phases is always higher than 4 MPa1/2 and
the precipitation of PS should be occurred instantaneously.
7.2.2. Extraction process and recovery of solvent
According to the explained previously, a first set of experiments was performed to
remove the Limonene from the PS/Limonene solutions and obtain a solvent-free
solid material. The first experiment was carried out to ensure that no
Polystyrene/Limonene mixture dragging occurred during the extraction runs. In
these experiments, the extractor was loaded with 10 g of mixture and CO 2 was fed
at 77 bar, 40 ºC (= 250 Kg/m3) and Q = 2 L/min. The flow was selected previously,
as the maximum value which avoids PS dragging according to the previous
experience on oil extraction [22-24].
Figure 7.2 shows the Limonene concentration in CO 2 measured in the ternary
mixture CO2/Limonene/PS. Limonene solubility was determined from the slope of
the linear parts of extraction curves, drawn as grams limonene obtained versus
grams CO2 used at the beginning of each extraction. The dash horizontal line
195
70
80
represents the solubility of pure limonene in a binary system CO 2/Limonene
(yLimonene = 0.00216), obtained from the literature [25].
0.0024
Limonene solubility in CO2
0.0022
yLimonene
0.0020
0.0018
0.0016
0.0014
0.0012
0
1
2
3
4
5
6
7
8
Limonene in the mixture (g)
Figure 7.2. Limonene concentration (in mole fraction) in the CO 2 exit stream on the amount
of limonene in the PS/Limonene solution at 40 ºC and 77 bar.
When the amount of Limonene in the solution increases the solubility of Limonene
in CO2 rich phase increases due to the negative effect of the polymer concentration
on the solubility of terpene in the vapour phase. Also it is observed that Limonene
solubility in the ternary system is lower than in the binary one (CO 2/Limonene,
dashed line), because the distribution coefficient decreases as a consequence of the
presence of Polystyrene in the system [26]. These effects were explained in Chapter
5.
The second experiment was performed to determine the extraction yield of the
recovered Limonene. The selected conditions to run the experiment were shown
above and it was carried out until Limonene extinction (Figure 7.3).
100
Yield (%)
80
60
40
20
0
0
1
2
3
4
5
6
7
8
Time (h)
Figure 7.3. Yield evolution of the extraction of limonene. Conditions: P=77 bar; T=40 ºC; L=
10 g ; Q=2 L/min.
196
Chapter 7
Figure 7.3 shows that after 7 hours, the yield of the extraction of Limonene in the
CO2 rich phase does not increase significantly, reaching a value of 95%. The
remaining Limonene is not collected because the residence time of the solvent in the
collector is not enough for the Limonene to condensate.
These easy experiments were used to check that PS can be recycled from its solution
in Limonene, using CO2 as antisolvent. The amount of Limonene in the polymer, its
molecular weight distribution and the degradation temperature were studied in
order to determine the quality of PS recovered after the recycling process. Table 7.1
shows the results from the thermal and calorimetric analysis and Figure 7.1.
Table 7.1. Comparison between the properties of PS wastes and recycled.
PS wastes
PS recovered
Glass transition temperature (ºC)
100.00
96.36
Degradation temperature (ºC)
Polydispersity index
421.90
1.70
421.01
1.74
According to Table 7.1, the degradation of PS does not occur during the extraction
since the characteristic of the original polymer were kept constant after the process.
From the thermogravimetric analysis it is observed that the recycled PS is almost
free of solvent. Therefore, considering the results obtained, it is possible to confirm
that polymer degradation does not occur during the solvent removal using
supercritical CO2.
7.2.3. Supercritical Antisolvent Process
Regarding the promising results obtained for the recovery of PS wastes and
Limonene by extraction with CO2, and the feasibility of the process according the
solubility differences among the components, the Supercritical Antisolvent process
was considered. The mentioned process was selected due to the growing interest
that has arisen during the last decade for the development of micro- and
nanoparticles.
Once the feasibility of the separation was studied and with the aim of widen the
study, the next plan of experiments was performed. For the design of a SAS process,
the studied variables were pressure, temperature and concentration of PS in the
solution, while the volume of solution feed, the flow and nozzle diameter were kept
constant. Table 7.2 shows the range of the mentioned variables and the morphology
of the recycled PS at 20 ºC.
The results shown in Table 7.2 could be subdivided according to the working
pressure (100, 150 and 200 bar). Regarding the pressure, any significance change in
the morphology is observed in the range of study while the concentration is kept
197
constant. Some authors have observed that the process conditions (mainly the
pressure) have little influence on the particle size and morphology [27, 28].
Table 7.2. Summary of the operational conditions and results for the experiments performed.
For all experiments: Temperature: 20 ºC; volume of solution: 25 ml; Qsolution: 0.5 mL/min;
nozzle diameter: 0.0016 m.
Run
P (bar)
CPS (g/ml)
Morphology
1
100
0.004
Wet Microparticles
2
100
0.009
Wet agglomerated microparticles
3
100
0.050
Wet microparticles+ PS wet paste
4
100
0.100
Wet fibres
5
100
0.200
PS wet paste
6
100
0.250
PS wet membranes-like
7
100
0.500
Wet Fibres
8
150
0.045
Foamed PS
9
150
0.500
Foamed PS
10
200
0.050
Wet microparticles+ PS wet paste
11
200
0.200
PS wet paste
On the other hand, the effect of concentration on the morphology of the Polystyrene
shows that at constant pressure, when concentration of PS increases, the recovered
polymer presents higher amounts of p-Cymene and consequently, the structure is
agglomerated, wet and shows an amorphous appearance. Nevertheless, it is worth
mentioning that when the concentration of PS in p-Cymene is higher than the
maximum solubility (0.30 g PS/ml p-Cymene at 30 ºC, see references in Chapter 4),
runs 7 and 9, a foamed PS is recovered. Dixon et al.[8] observed that spraying a
PS/Toluene solutions into subcritical CO2, microspheres and microballoons were
formed. However, when the concentration of polymer increased from 6 to 20%,
microspheres were no hollow, but fully porous, with a continuous pore structure
inside. This fact agrees with the results shown in this work.
When temperature increased up to 40 ºC, while the volume of solution, its flow and
the nozzle of the solution feed were kept constant, any precipitated polymer was
recovered. Table 7.3 summarizes the most representative experiments performed at
40 ºC.
198
Chapter 7
Table 7.3. Summary of the operational conditions and results for the experiments performed.
For all experiments: Temperature: 40 ºC; volume of solution: 25 ml; Qsolution: 0.5 mL/min;
nozzle diameter: 0.0016 m.
Run
P (bar)
C (g/ml)
Morphology
Final
Concentration
(g/ml)
12
100
0.045
Higher concentration solution
1.720
13
100
0.009
1.890
14
100
0.200
2.457
15
100
0.250
2.886
16
150
0.045
1.825
17
150
0.200
2.474
18
200
0.050
2.467
19
200
0.200
2.346
According to Table 7.3, independently of the working pressure and concentration, at
40 ºC any solid PS was recovered. Nevertheless, an increase in the concentration of
the solution (~ 2.25 g/ml) was observed as a consequence of the partial solubilisation
of p-Cymene in CO2. The fact that any discrete particles are formed may be due to
the agglomeration of droplets. The wet droplets collide with others from the wall of
the vessel which makes more difficult their drying. Moreover, the flying time (from
the nozzle to the bottom of the vessel) for agglomerated particles is too short for the
drops to dry and they become sticky forming agglomerated droplets which finally
make higher concentration solutions [28]. Also, Obrzut et al. [29] observed that at
higher temperature, the jet breaks up as a gas-like plume without the formation of
liquid droplets due to the dramatically decrease in the interfacial tension.
In view of these results and considering the ternary equilibriums shown in Chapter
5, the ternary diagrams of the experiments shown above are depicted in Figures 7.4
to 7.6 in order to try to draw some general conclusions.
199
e
1.0
0.2
0.8
0.4
0.6
0.6
e
p-C
en
tyr
ym
lys
en
0.0
Po
Pressure: 100 bar; T: 20 ºC
Run Solution SAS
1
2
3
4
5
6
7
0.4
0.8
0.2
1.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
CO2
Figure 7.4. Ternary diagram comparing mass-transfer pathways for precipitation of PS/pCymene solutions (filled markers) with compressed CO 2 as antisolvent at 100 bar and 20 ºC.
The composition of the ternary mixtures inside the vessel has been depicted as empty
symbols.
Run Solution SAS
8
9
0.0
1.0
0.2
en
0.4
0.6
lys
en
p-C
tyr
ym
Po
e
0.8
e
0.6
0.4
0.8
0.2
1.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
CO2
symbols.
200
Chapter 7
Run Solution SAS
10
11
0.0
1.0
0.2
0.8
ym
p-C
en
tyr
0.6
lys
en
e
Po
0.4
e
0.6
0.4
0.8
0.2
1.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
CO2
symbols.
From Figures 7.4-6, it is observed that an increase in the initial concentration of PS
in the solutions promotes an increase in the slope of the tie lines, as it was
previously described in Chapter 5. However, the increase of pressure does not
produce any significantly shift on the position of the markers in the ternary
diagrams. From the comparison of Figures 7.4-6 with the Figure 5.9, it is checked
that during the SAS process, the composition of all the ternary mixtures inside the
vessel is located in the two-phase regions. By this reason, two phases should be
recovered, a polymer rich phase and a vapour rich phase which contains CO 2 and pCymene. The amount of solvent of the recovered polymer depends on the position of
the tie line.
Next, the ternary diagram of the experiments performed at 100 bar and 40 ºC is
shown (Figure 7.7).
201
Run Solution SAS
1
2
3
4
0.0
1.0
0.2
en
0.4
0.6
lys
en
p-C
ty r
ym
Po
e
0.8
e
0.6
0.4
0.8
0.2
1.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
CO2
symbols.
From the comparison of Figure 7.7 obtained at 40 ºC and Figure 7.4 at 20 ºC, both at
100 bar, any remarkable difference is observed, although at higher temperature any
solid is recovered as it was also shown in Table 7.3.
Considering these results it could be thought that the feasibility of the SAS process
requires not only knowledge of the phase equilibrium ternary systems, but also the
hydrodynamics of the system [17], the nozzle design and the ways of mass transfer
[30]. When the solution is fed to the high pressure vessel different regimes could be
produced: the Rayleigh breakup, the sine wave breakup and the atomization. On a
regular basis, for the precipitation of homogeneous microparticles, atomization is
preferred, but when the working pressure is located above the mixture critical point,
it is not possible to distinguish droplets since there is not interfaces between the
solution and the dense CO2 [17, 31]. Taking into account the importance of the
hydrodynamics of the system, a relation between the experimental morphology of
the PS recovered and the regimes was attempted.
Czerwonatis and Eggers defined two new dimensionless numbers, Z* and Z**:

[7.1]


202
[7.2]
Chapter 7
where l is the viscosity of the liquid, is the velocity of the liquid jet,  is the
interfacial tension and g and l represent the density of the gas and liquid phase,
respectively. By representing the Z* and Z** numbers vs Reynolds (Re) and Morton
(Mo) dimensionless numbers of the liquid phase it would be possible to determine
the limits of the working conditions. The mentioned dimensionless numbers are
defined as:
[7.3]
Re expresses the ratio of fluid inertial and viscous forces, where v is the mean
velocity of the fluid, D is the hydraulic diameter,  is the density of the fluid, is the
dynamic viscosity.
[7.4]
Mo represents the characteristics of the shape of bubbles and/or drops moving in a
surrounding fluid; where g is the acceleration of gravity, c is the viscosity of the
surrounding fluid, c is the density of the continuous phase,  is the difference in
density of the phases and  is the surface tension. The density of the solutions
(was calculated according to the equations developed by Ref. for the compressed
liquid mixture [32]. The dynamic viscosity ( was obtained from the experimental
values of viscosity shown in Chapter 6 and the velocity of the fluid. The viscosity of
CO2 (c) at high pressure was calculated following the Lucas method for compressed
gases at high pressure [33], the density of the surrounding fluid (c) was obtained
from the Bender equation [34] and finally, the surface tension ( was obtained
experimentally and data were shown in Chapter 6. The physical properties obtained
for the calculations at the different temperature are listed in Table 7.4.
Table 7.4. Physical properties for the system CO 2/p-Cymene/PS at 20, 30 and 40 ºC. Volume
of solution: 25 ml; Qsolution: 0.5 mL/min; nozzle diameter: 0.0016 m.
Run
v
l
P
T
g
G·106
(bar) (ºC) (m/s) (kg/m3) (kg/m3) (kg/m·s)
G·10-6
(m2/s)
1
100
20
0.004 857.159 846.121
84.420
0.099
20
100
30
0.004 771.500 846.489
66.160
0.086
12
100
40
0.004 622.641 846.833
47.840
0.076
Run
v
P
T
L
L·106
(bar) (ºC) (m/s) (kg/m·s) (m2/s)
t=0
(N/m)
vcrit
(m/s)
1
100
20
0.004
0.005
5.909
0.003
1.301
20
100
30
0.004
0.003
3.544
0.003
1.371
12
100
40
0.004
0.002
2.362
0.001
0.922
203
For the study of the hydrodynamic regimes a wide set of experiments was
considered, in which the solution flow was varied from 0.2 to 3 mL/min. The
morphology of the recovered PS was classified in three different categories: foams,
wet microparticles or higher concentration solutions as it was shown in Tables 7.2
and 7.3. The study of Z* and Z** numbers vs Re and Mo at different operating
conditions are shown in Figure 7.8.
0.010
0.100
b) Z** vs Re
Foams
Wet microparticles
0.075
Solutions
0.008
Z**
Z*
0.006
0.004
0.050
*
a) Z vs Re
Foams
Wet microparticles
Solutions
0.002
0.000
0
1
2
3
0.025
0.000
4
5
6
0
Re
0.020
0.15
c) Z* vs Mo
Foams
Wet microparticles
0.015
Solutions
1
2
3
4
5
6
Re
d) Z** vs Mo
Foams
Wet microparticles
Solutions
Z**
Z*
0.10
0.010
0.05
0.005
0.0
2.0x10
-5
4.0x10
Mo
-5
6.0x10
-5
0.00
0.0
2.0x10
-5
4.0x10
-5
6.0x10
-5
Mo
Figure 7.8. Relation between the hydrodynamic conditions and the morphology of the
recovered PS from a PS/p-Cymene solution in pressurized CO2.
According to Figure 7.8 several trends could be observed but in general, the low
values of Re and Mo dimensionless numbers should be highlighted. Since the
Reynolds number describes the inertial force to friction force ratio, the droplet size
should decrease with increasing Re number. But some authors observed that the
smallest particles were produced in the subcritical region, where the Re number was
either very small [28]. After considering the results shown, Rayleigh breakup was
the only regime achieved due to the low values of Re and Mo, although the critical
points of the mixtures were reached (see Chapter 6). From Figures 7.8 a and b (Z* vs
Re and Z** vs Re, respectively), at higher values of Re and Z* or Z**, any dried
structure was observed. On the other hand, from Figures 7.8 c and d (Z* vs Mo and
Z** vs Mo, respectively) three zones are identified. Higher concentration solutions
are observed when Mo is around 510-5, Z*=0.075 and Z**=0.05, while when Z* and
Z** increases, PS/p-Cymene solutions precipitate in the form of wet microparticles
204
Chapter 7
and when Z* and Z** decreases, PS/p-Cymene are foamed. Agglomerated particles
could lead into foams; this fact could be explained by the increase of CO2 sorption in
the polymeric solution (see Chapter 5) and the existence of significant mechanisms
that stabilize the liquid jet. It means that although equilibrium interfacial tension is
zero (Table 7.4.), a certain interface exist between the solution jet and the
surrounding gas [17]. Attending to the results, changes in the fed flow (velocity)
does not have any significant effect on the morphology of the precipitated PS [28],
although faster solution flow rates should produce spherical particles with
increasing diameters while fibres are generally attributed to the precipitation before
the jet break up [1].
The morphology of the precipitate obtained at pressure above the mixture critical
point should have microparticle shape, since the prerequisites for successful
antisolvent process are the complete miscibility between the organic solvent and the
antisolvent and the insolubility of the polymer in the antisolvent [35]. However,
dried microparticles or fibres were not precipitated from the PS/p-Cymene solutions.
With the aim of study the effect of solvent into the precipitated PS in order to
achieve microparticles, the system CO 2/Limonene/PS was studied following the
procedure described by the system CO2/p-Cymene/PS.
According to Figure 7.9 similar trends to those observed in the ternary system
CO2/p-Cymene/PS are shown. In the case of PS/Limonene solutions, Re
dimensionless number increases, but Mo dimensionless numbers decreases one
order. The decrease of Mo implies little deformation of the droplets that acquire a
nearly spherical shape instead of ellipsoidal [36] which is result of a higher relative
strength of interfacial forces versus viscosity. Viscosity values of PS/Limonene and
PS/p-Cymene were similar, but when the concentration of PS in Limonene increases
the interfacial tension of the mixture increases reaching values higher than in the
case of PS/p-Cymene solutions (Figure 6.13). By this reason, the interfacial forces
prevail over the viscosity which promotes the formation of spherical droplets in
PS/Limonene solutions. However, due to the low value of Re and Mo, Rayleigh
breakup was also the regime achieved.
205
0.3
a) Z* vs Re
Foams
Wet microparticles
Solutions
0.10
0.08
b) Z** vs Re
Foams
Wet microparticles
Solutions
0.2
Z**
Z*
0.06
0.04
0.1
0.02
0.00
0.0
0
2
4
6
12
14
16
18
20
0
0.15
c) Z* vs Mo
Foams
Wet microparticles
Solutions
2
4
6
8
10
12
14
16
18
20
Re
d) Z** vs Mo
Foams
Wet microparticles
Solutions
0.10
0.015
Z**
Z*
0.020
10
Re
0.030
0.025
8
0.010
0.05
0.005
0.000
0.0
0.00
5.0x10
-7
1.0x10
Mo
-6
1.5x10
-6
2.0x10
-6
1.0x10
-6
2.0x10
-6
3.0x10
-6
4.0x10
-6
Mo
Figure 7.9. Relation between the hydrodynamic conditions and the morphology of the
recovered PS from a PS/Limonene solution in pressurized CO 2.
Figures 7.9 a and b show the trend line which separate the precipitation of PS as a
higher concentration solution or as foams. Wet microparticles can not be
summarized according to any specific trend line. Nevertheless, in Figures 7.9 c and
d clear trends are shown for the precipitation of wet microparticles or foams,
although the formation of higher concentration solutions is performed at low values
of Mo (10-6) and Z* around 0.005 or Z** 0.025.
According to the literature, decreasing the initial affinity of the solute for a solvent
(PS presents lower solubility in Limonene than in p-Cymene, Chapter 4) should
precipitate larger particles because they stay suspended in the solution longer and
continue to grow [37]. Nevertheless, from the comparison of Figure 7.9 and 7. 8 this
fact was not observed maybe due to the similarities between both solvents.
Considering the difficulties to the precipitation of PS into homogeneous
microparticles shape, several authors has determined that the fast disappearance of
the surface tension of the injected solution is the cause of microparticles nucleation
and growth. When the liquid and the supercritical CO 2 are contacted, mutual
diffusion leads to a progressive decrease of the dynamic surface tension [38] and
consequently, the organic solvent is solubilised into the CO 2 while the polymer is
precipitated. By this reason and taking into account that mass transfer is driven by
206
Chapter 7
diffusion, the diffusivity coefficients of the binary systems CO2/p-Cymene, pCymene/CO2 and CO2/Polystyrene were calculated as a function of pressure and
temperature. It should be outlined that the diffusion coefficients of CO 2/polymeric
solutions were not calculated because there was any experimental value to check
the prediction.
For the calculation of the diffusion coefficients, several correlation were found in the
literature: Hayduck-Minhas [32], He-Yu [39], Wilke-Chang [32], Reddy-Doraiswamy
[40], Catchpole-King [40], Scheibel [41] and Lusis-Ractliff [40]. All of them were
tested initially against some experimental diffusion coefficients reported in the
literature for organic solvents [40, 42] and Hayduck-Minhas correlation was the
selected to calculate the diffusion coefficients of CO2/p-Cymene as follows:

[7.5]
[7.6]
where Dab is the diffusion coefficient of substance a in b, T is the temperature, a is
the viscosity of substance a and Vb is the molar volume of substance b. The viscosity
of CO2 was calculated following the Lucas method for compressed gases at high
pressure [33] the viscosity of p-Cymene solutions was determined experimentally
and results were shown in Chapter 6 and the viscosity of PS was obtained
experimentally by the use of a rheometer (Chapter 4) . On the other hand, the molar
volume of CO2 and PS was calculated from the Sánchez-Lacombe EoS but the molar
volume of p-Cymene was obtained from the Peng-Robinson EoS (see Chapter 4).
This analysis does not consider the effect of the polymer on the increase in viscosity
of p-Cymene since the binary systems were studied individually. Figure 7.10 shows
the diffusion coefficients of the binary system CO 2/p-Cymene and p-Cymene/CO2.
x 10
-5
CO2/p-Cymene
5
Diffusivity Coefficient (m2/s)
4.5
4
3.5
3
2.5
2
1.5
1
0.5
20
25
200
150
30
100
35
Temperature (ºC)
p-Cymene/CO2
50
40
0
Pressure (bar)
Figure 7.10. Diffusion coefficients of the binary systems CO 2/p-Cymene and p-Cymene/CO2.
207
Figure 7.10 shows that diffusion of CO2 inside the droplet and diffusion of p-Cymene
from the droplet to the surrounding CO 2 is balanced at the lowest pressure, while
when pressure increases the diffusion coefficient of CO 2 inside p-Cymene increases
and its sorption is enhanced but the solubilisation of p-Cymene is not. According to
Figure 7.10, the antisolvent process should be performed at atmospheric pressure,
nevertheless, the solubility of p-Cymene in CO2 at low pressure is negligible (see
Chapters 4 and 5).
The effect of pressure and temperature on the diffusion coefficients of CO 2 into
Polystyrene was predicted according to He and Yu [43] correlation for supercritical
fluids. The calculated data were checked experimentally (Chapter 4) and results are
shown in Figure 7.11.
Figure 7.11 shows that when pressure increases, the diffusion coefficient of CO 2 into
Polystyrene increases, as well as when temperature increases. Although at low
pressure, the effect of temperature is not significantly. In this case, the diffusion
coefficient of Polystyrene into CO2 was not calculated since its solubility is negligible
[44, 45].
x 10
-10
Diffusivity Coefficient (m2/s)
1.8
1.6
1.4
1.2
20
25
200
150
30
100
35
Temperature (ºC)
50
40
Pressure (bar)
Figure 7.11. Diffusion coefficients of CO2 into Polystyrene.
From the comparison of Figures 7.10 and 7.11, it is observed that solubility of CO2 is
enhanced in p-Cymene solutions, while the presence of the polymer would decrease
it because the DCO2/PS presents 5 orders of magnitude lower. This fact could explain
the results obtained in Chapter 5 (Figure 5.11), when the concentration of
Polystyrene increased, the solubility of p-Cymene in CO2 decreased. Finally, the
suitability of the selected ternary system to carry out the antisolvent process was
checked by the several differences between the diffusion coefficients.
208
Chapter 7
Once the diffusion coefficients are obtained, the concentration of each compound as
a function of time and position could be determined using the Fick’s Second Law of
diffusion [46].
[7.7]
where C is the concentration, t is the time and x is the studied position. The
solutions to Fick’s second law depend on the boundary conditions imposed by the
particular case; in this case, a spherical droplet of p-Cymene with radius r was
supposed. The radius of the droplet can be calculated from the nozzle diameter
because when a liquid is forced through a nozzle, at the nozzle exit it forms a
continuous cylindrical jet with a diameter equal to that of the nozzle [47].
Attending to the spherical geometry of the droplet, expression [7.7] becomes:
[7.8]
If the substitution u=Cr is made and the boundary conditions shown below [7.97.12] are applied, the Fick’s second law equation is expressed as equation [7.12].
u=0,
r=0,
t>0
[7.9]
u=aC0,
r=a,
t>0
[7.10]
u=rf(r),
0< r < a,
t=0
[7.11]
[7.12]
where C is the concentration of the surface of the sphere, C 0 is the initial bulk
concentration and erf refers to the Gaussian error function. According to the
expression [7.12] and considering the diffusion coefficients shown in Figure 7.10, the
concentration of p-Cymene and CO2 in relative terms along time in a droplet of
radius 7.94·10-4 m (experimental nozzle diameter) is shown in Figure 7.12.
209
0
10
10
20
Time (s)
0
CO2 inside the droplet
Temperature: 20 ºC
30
P: 1 bar
P: 100 bar
P: 200 bar
0
p-Cymene outside the droplet
Temperature: 20 ºC
P: 1 bar
30
P: 100 bar
P: 200 bar
40
0
10
10
20
Temperature: 30 ºC
30
P: 1 bar
P: 100 bar
P: 200 bar
0
40
Temperature: 30 ºC
P: 1 bar
30
P: 100 bar
P: 200 bar
10
Time (s)
20
20
0
40
10
Time (s)
20
40
Time (s)
Time (s)
Time (s)
Temperature: 40 ºC
30
P: 1 bar
P: 100 bar
P: 200 bar
40
0.0
0.2
0.4
0.6
Concentration
0.8
1.0
20
Temperature: 40 ºC
P: 1 bar
30
P: 100 bar
P: 200 bar
40
0.0
0.2
0.4
0.6
Concentration
0.8
1.0
Figure 7.12. Influence of pressure and temperature on the relative concentration of CO2
inside the droplet (left figures) and p-Cymene outside the droplet (right figures) along the
time p-Cymene/CO2 system.
According to Figure 7.12, the solubility of CO2 inside the droplet increases
dramatically up to 5 seconds, independently of working pressure and temperature.
But as time goes over 5 seconds, the droplet saturation is achieved since the
concentration of CO2 inside the droplet remains almost constant in a range of
concentration between 0.91 and 0.96 (in relative terms of concentration), depending
on the operating pressure and temperature. On the other hand, the effect of
pressure and temperature affects on the solubility of p-Cymene in the CO2 rich
phase (right figures) significantly. When pressure is above the mixture critical point
(> 70 bar), the solubility of the solvent outside the droplet remains almost constant
independently of the temperature. Nevertheless, higher temperatures promote a
faster diffusion of p-Cymene from the droplet to the surrounding media. Regarding
Figure 7.12, dried PS could not be recovered at the studied working conditions since
the concentration of p-Cymene outside the droplet does not reach 1. This fact was
observed experimentally in Table 7.2 and 7.4.
210
Chapter 7
0
0
10
10
20
Time (s)
Time (s)
The initial size of the droplet depends upon the nozzle diameter and initial velocity
of the droplet in addition to the physical properties of the organic solvent, such as,
density, surface tension, and viscosity [48]. Considering the results shown a
decrease in the diameter of the droplet could improve the diffusivity of p-Cymene
outside the droplet, and consequently, dried PS could be achieved. By this reason,
new simulations were performed to study the influence of the nozzle diameter on the
concentration of p-Cymene in the vapour rich phase along the time. Figure 7.13
represents the evolution of CO2 and p-Cymene concentration in liquid and vapour
phase, respectively through a 500 nm nozzle.
Temperature: 20 ºC
P: 1 bar
P: 100 bar
P: 200 bar
0
40
30
Time (s)
Time (s)
10
Temperature: 30 ºC
30
P: 1 bar
P: 100 bar
P: 200 bar
0
40
Temperature: 30 ºC
P: 1 bar
30
P: 100 bar
P: 200 bar
10
Time (s)
Time (s)
20
0
40
10
20
Temperature: 20 ºC
P: 1 bar
30
P: 100 bar
P: 200 bar
40
0
10
20
20
Temperature: 40 ºC
30
P: 1 bar
P: 100 bar
P: 200 bar
40
0.0 0.5 0.9990
0.9995
Concentration
1.0000
20
Temperature: 40 ºC
P: 1 bar
30
P: 100 bar
P: 200 bar
40
0.0 0.5 0.9990
0.9995
1.0000
Concentration
Figure 7.13. Influence of pressure and temperature on the relative concentration of CO2
inside the droplet (left figures) and p-Cymene outside the droplet (right figures) along the
time p-Cymene/CO2 system through a 500 nm nozzle.
Figure 7.13 shows that a decrease in the nozzle size produces smaller drops that are
dried faster since the concentration of p-Cymene in the surrounding media reaches
saturation at times shorter than 1 second when the pressure is above the mixture
critical point. As it was observed in Figure 7.12, the effect of pressure affects
211
significantly on the diffusion of p-Cymene outside the droplet but reducing the
nozzle diameter up to 500 nm, saturation of CO2 can be easily achieved from 100
bar.
Thanks to these encouraging results and with the aim of generating small,
homogeneous and fine droplets with a high surface area, the nozzle was replaced by
a smaller one (500 nm). Rantakylä determined the efficiency of an antisolvent
process by the droplets size produced in the nozzle and by the way in which the
gaseous phase mixes with the droplets [28]. In this cases, when the nozzle was
smaller, dried fibres or microparticles were produced. Figure 7.14 and 7.15 show
some of the most representative microphotographs of the recovered Polystyrene
from its precipitation in CO2 through a nozzle of 500 nm.
(a)
(b)
Figure 7.14. Microphotographs of the precipitated PS using CO 2 as antisolvent at 20 ºC;
Volume of solution: 10 mL. a) P: 100 bar, C: 0.05 gPS/ml p-Cymene, Qsolution: 0.5 mL/min; b)
P: 200 bar, C: 0.05 gPS/ml p-Cymene, Qsolution: 0.5 mL/min.
(a)
(b)
Figure 7.15. a) P: 100 bar, C: 0.05 gPS/ml Limonene, Qsolution: 1.8 mL/min; b) P: 200 bar, C:
0.05 gPS/ml Limonene, Qsolution: 1.9 mL/min.
212
Chapter 7
From Figures 7.14 and 7.15, it is observed that well defined and dried structures
were obtained independently of the operating conditions and the solvent. The
average ratio of microparticles was 250 m and the average size of the fibres
diameter was 20-60 m. An increase in pressure, from 100 to 200 bar does not
produced any significant change in the morphology of the precipitated polymer.
However, an increase in the liquid flow promotes the coalescence of droplets which
leads to fibres (Figure 7.15). This fact could be the issue that limits the volume of
the solution and the concentration of polymer due to the clogging of the nozzle [28].
According to the results, the precipitation of PS from its solution in Limonene or pCymene using CO2 as antisolvent is feasibly. Nevertheless, important efforts should
be carried out to achieve well defined polymeric structures due to the complexity of
the process and the numerous variables involved.
213
7.3. Foaming background
Microcellular foams are defined as foams with average cell sizes of less than 10
microns and cell densities greater than 109 cells/cm3.They are the target products
since they typically exhibit high impact strength, toughness and thermal stability,
as well as low dielectric constant and thermal conductivity [49]. The use of
Supercritical Fluids (SCFs) as physical blowing agents has being developed for the
production of polymer foams because they can create highly functional materials
with new features and dramatically improve conventional manufacturing methods,
especially from an environmental point of view. Currently, the focus is on carbon
dioxide due to its relative ease of handling and more favourable interaction with
polymers compared to other inert gas candidates like nitrogen [50].
The key advantages related with the usage of scCO2 to foam are due to the easily
removal from the final product, its tunable solvent power and its capability to
modify the polymer properties as a result of its sorption [51]. Having diffusion
coefficients higher than that of liquids, viscosities close to that of gases and low
surface tension, scCO2 provides better penetration and complete wetting of the
substrates which is advantageous for impregnation or extraction applications as
compared to conventional solvents [52]. Also, scCO2 has received an increasing
attention with respect to traditional blowing agents, i.e. chlorofluorocarbons (CFCs),
hydrochlorofluorocarbons (HCFCs) or hydrocarbons, due to its low cost and mild
impact on the environment.
Several authors have demonstrated that CO 2 can be used to foam amorphous
materials such as poly(methylmethacrylate), polystyrene, polycarbonate, and
poly(ethyleneterephthalate). Table 7.5 shows a good sample of the available
literature data [49, 51, 53-69] for the foaming of polymers or polymer blends using
supercritical CO2 and the working conditions used to carry out the process: pressure,
temperature, contact time and depressurization rate ranges.
Generally high pressure and temperature are required to foam the polymers but
also there are other parameters playing an important role during the process. By
this reason, polymeric foams can be customized and many polymers are screened for
their foamability and resulting foam structure, which determines to a great extent
the properties of the foam [68].
214
90-160
300
69-210.7
100-200
150-200
86-140
180-380
80-230
140-610
100-250
103-276
230
70-280
78-200
250-450
120-350
150-190
Poly(ethylene terephthalate)
isotactic Polypropylene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene and Poly(D,L-lactic acid)
Polystyrene and cellulose acetate
Poly(l-lactic acid)
Poly(lactic acid-co-glycolic acid)
Poly(D-lactide) and Poly(D-lactide-co-glycolide)
Poly(D,L-lactide) and Poly (D,L-lactide-co-glycolide)
Poly(e-caprolactone-co-lactide)
Poly(e-caprolactone)
Poly(p-dioxanone)
Poly(heteroarylenes)
Polypropylene, Polystyrene and Poly(styrene-butadiene-styrene)
Polystyrene, Polymethylmethacrylate, Poly (styrene-co-butadiene-comethylmethacrylate) and Poly (methylmethacrylate-co-butylacrylate-comethylmethacrylate)
300
80-160
25-80
160-190
40-100
80-100
40-50
60-80
90
35-100
35-85
60-180
55-125
35-55
60-110
100
50-110
60-180
200
180-260
280
0.03-1 MPa/s
120 min
600 min
0.01 MPa/s
3-120 sec
60-120 min
[62]
[61]
[60]
[51]
[59]
[69]
[68]
[67]
[66]
[65]
[64]
0.006-0.013 MPa/s [63]
0.3-0.4 MPa/s
0.05 -0.20 MPa/s
20 seconds
120 min
1-40 min
20 min
300 min
60-240 min
0.1-1.6 MPa/s
1 MPa/s
240 min
240 min
[57]
80-400 MPa/s
0-60min
[58]
[49]
100-400 MPa/s
240 min
[56]
[55]
[54]
[53]
0.91-1.17 MPa/s
180-180 MPa/s
24 h
20-25 min
~ 60 min
30-50 min
Pressure Temperature Contact time Depressurization
Refs
range (bar) range (ºC)
range
rate or time
Poly(ethylene terephtalate)
Polymer
Table 7.5. Literature review of foamed polymers. Range of pressure, temperature, contact time and depressurization used.
Chapter 7
215
The process to prepare foams using CO 2 as foaming agent can be divided into three
steps: (1) sorption of CO2 until saturation in the plastic; (2) nucleation of foam
bubbles, it is need induce a phase separation by a thermodynamic instability (either
a temperature increase or a pressure decrease); and (3) growth of foam’s cells. The
main steps are schematically depicted in Figure 7.16.
Figure 7.16. Schematic diagram of the steps during a typical foaming process: sorption,
saturation, foaming and cell growth. The sorption of CO 2 into the polymer (continuous line,
left axis) is the responsible of the decrease of the glass transition temperature (dashed line,
right axis).
As shown in Figure 7.16, the foaming process starts by melting the polymer and
adding CO2. The type of polymer, together with the applied pressure and
temperature determine to a large extent the amount of CO 2 that can be dissolved
into. During the sorption step, the gas is dissolved inside the polymer inducing
swelling, increasing its free volume and segment mobility, causing its plasticization
which involves the modification of the mechanical and physical properties of the
polymers. The absorbed gas behaves like a “lubricant” of the polymeric chains which
are easier slid and consequently, the glass transition temperature (Tg), melting
temperature (Tm), viscosity () and surface tension () are decreased. The nucleation
stage occurs instantaneously at lower Tg when the polymer matrix becomes less
rigid. Cell growth will stop once the polymer matrix returns to the glassy state,
either due to a decrease in temperature or a decrease in the CO 2 concentration in
the polymer and the polymer is no longer plasticized [70].
216
Chapter 7
During the depressurization stage, run at constant temperature, the pressure is
reduced, the CO2 phase changes from supercritical to gas and nucleation and growth
of gas bubbles occurs in the rubbery polymer generating pores. However not only
depressurization rate should be control, but also the cooling rate, because it affects
the polymer viscosity (Figure 7.16) [63].
Important efforts have been carried out to determine the real influence of the
experimental parameters on the final foam structure and as a consequence on their
properties. In general, increasing pressure, the pore diameter and the bulk density
decreases, while the cell density increases. At higher pressure, more fluid is
dissolved into the polymer matrix causing a more pronounced plasticization and
viscosity reduction. Nevertheless, the pore cell increases while the bulk foam
density and cell population decreases when temperature is increased [59].
In this section, p-Cymene and Limonene were used as solvents for the PS since in
Chapter 4 and Chapter 5, the high solubility of PS in terpenes and of terpenes in
CO2 was checked. Furthermore, they allow to run the foaming process feasibly at
mild working temperature. Moreover, the use of binary fluid mixtures of carbon
dioxide and an organic solvent provide flexibilities towards control of pore sizes and
connectivity in forming porous matrices [60]. CO2 is behaved as nonsolvent of the
polymer and the mass transfer in the systems occurs in two ways, the Limonene is
solubilised from the bulk polymeric phase at the same time as CO 2 is absorbed in
the liquid phase. The mass transfer during the foaming is mainly governed by the
diffusion of the CO2 into the polymeric rich phase and it has been checked that is
enhanced because of the presence of the solvent which promotes the relaxation of
the polymeric chains.
According to these facts, the first aim consisted on the verification of the feasibility
of the foaming process in the surroundings of the vanishing point, where the
interfacial tension is vanished. Next, the study of the foaming process of a PS/pCymene solution using CO2 as antisolvent was carried out. Finally, the third
purpose was the preparation, characterization and optimization of the foams to
obtain high cells density and low pore cells by mean of a process which required low
energy consumption and does not affect the polymeric chain structure because of the
low operation temperature.
7.4. Feasibility of the Foaming of Polystyrene/cosolvent system
with CO2
In this section, the feasibility of the foaming process of PS/terpene oils solution was
studied. The selection of the operating limits was performed according to the glass
transition temperature, viscosity and interfacial tension of the mixtures (Chapter 6).
Generally low interfacial tension (IFT) is desired to increase the nucleation rate and
produce small and uniform cells [71-73]. In Chapter 6 was shown that IFT of PS/p-
217
Cymene and PS/Limonene solutions in presence of CO2, vanishes around 70-80 bar,
depending on the temperature and initial concentration. These results indicate that
Polystyrene should be able to foam at temperatures only slightly above room
temperature, because IFT at mild working conditions practically vanishes and
presents a pronounced decreasing rate.
Also, the effect of CO2 on the glass transition temperature of PS and its solutions is
crucial to limit the working range of the temperature during the foaming process.
This parameter is key since the saturation of CO2 into polymer can occur either in a
glassy or rubbery state, but nuclei growth can mainly occur in the rubbery zone or
near the Tg of an amorphous polymer. Considering that the growth of nucleis in
foams are achieved around Tg and low IFT are recommended to get homogeneous
foams, the selected working conditions were 70-90 bar and 30-40 ºC. Taking under
consideration the mentioned facts, a set of PS foams were produced at different
temperatures, pressures and initial concentrations, while contact and
depressurization time were kept constant (at 240 and 1.5 minutes, respectively).
The working conditions were selected based on Reverchon’s studies [51]. On this
way, the temperature values assayed was 30 and 40ºC, pressures 70 and 90 bar and
contact time was set at 240 minutes in all cases to assure homogeneous
microcellular structured.
The structure and cell size of the processed PS foams analyzed by optical microscopy
are shown in Figure 7.17 (from PS/p-Cymene solutions) and Figure 7.18 (from
PS/Limonene solutions).
Figure 7.17. Micrographs with 50x magnification of the foams obtained as a function of the
initial concentration, temperature, pressure and solvent used.
Cymene
p: 70 bar
p: 90 bar
218
T:30ºC
C0=0.05 g/ml
C0=0.20 g/ml
T:40ºC
C0=0.05 g/ml
C0=0.20 g/ml
Chapter 7
Figure 7.18. Micrographs with 50x magnification of the foams obtained as a function of the
initial concentration, temperature, pressure and solvent used.
Limonene
T:30ºC
C0=0.05 g/ml
C0=0.20 g/ml
T:40ºC
C0=0.05 g/ml
C0=0.20 g/ml
p: 70 bar
p: 90 bar
As it is observed in Figure 7.17 and 7.18, foams were not produced at 70 bar and 40
ºC, independently of the concentration and solvent used. According to Tables 7.6 and
7.7 (runs 5 and 6) it is observed that at 70 bar and 40 ºC, the CO 2 mass fraction in
the system is the lowest, and probably it is not sufficient to thoroughly solubilise the
p-Cymene nor the Limonene and PS can not be foamed.
Table 7.17. Mass fraction composition of the system adopted for PS foam synthesis from pCymene solutions. Subscripts: 1: CO2; 2: p-Cymene; 3: PS.
Run
P (bar)
T (ºC)
C (g/ml)
w1
w2
w3
1
70
30
0.05
0.9114
0.0837
0.0049
2
70
30
0.20
0.8982
0.0825
0.0193
3
90
30
0.05
0.9664
0.0318
0.0019
4
90
30
0.20
0.9610
0.0316
0.0074
5
70
40
0.05
0.8843
0.1094
0.0064
6
70
40
0.20
0.8677
0.1073
0.0250
7
90
40
0.05
0.9493
0.0479
0.0028
8
90
40
0.20
0.9415
0.0475
0.0111
219
Table 7.18. Mass fraction composition of the system adopted for PS foam synthesis from
Limonene solutions. Subscripts: 1: CO 2; 2: Limonene; 3: PS.
Run P (bar)
T (ºC)
C (g/ml)
w1
w2
w3
1
70
30
0.05
0.9128
0.0823
0.0049
2
70
30
0.20
0.8996
0.0811
0.0193
3
90
30
0.05
0.9669
0.0312
0.0019
4
90
30
0.20
0.9616
0.0310
0.0074
5
70
40
0.05
0.8861
0.1075
0.0064
6
70
40
0.20
0.8694
0.1055
0.0251
7
90
40
0.05
0.9502
0.0470
0.0028
8
90
40
0.20
0.9423
0.0466
0.0111
In the case of using Limonene as solvent with the lowest concentration, thick,
colourless and nonporous layers can be appreciated (Figure 7.18) combined with
areas where foams apparently present higher density and nucleation bubbles
appear to be not distributed homogeneously. The formation of a non-porous skin is
due to the presence of gas molecules near the surface of the sample diffusing out of
the sample faster than they can form nuclei. Rapid diffusion out of the sample
creates a reduction of the layer near the surface in which the gas concentration is
too low to contribute to nucleate and growth [51].
When Polystyrene initial concentration increased, the structures obtained are more
rigid, compact and less rubbery that in the case of lower concentration. Similar
trends were observed when cymene was used as solvent. Nevertheless when the
initial concentration was 0.20 g PS/ml p-Cymene, the foam structure is more
uniform and cells are more and smaller. This fact is mainly determined by the
competition between bubble nucleation and growth rates [74].
At higher temperature, foams produced present larger cells because when the CO 2
concentration in the dissolution is low because the effect of pressure on the foam
structure is negligible. In future works the study of the influence of the variables
affecting the foaming process will be thoroughly studied and it will be checked the
foam structures by means of scanning electronic microscopy (Figure 7.19).
220
Chapter 7
Figure 7.19. SEM images with 400x magnification of PS foam obtained at 90 bar, 40ºC and
0.20 g PS/ml Limonene.
Finally, it is important to stand out that limonene and/or cymene are effectively
removed from the PS and the foams obtained are completely free of solvent as was
confirmed by TGA measurements. Figure 7.20 shows the weight loss profile of one of
the foamed samples. The first of the weight losses at around 176 ºC, corresponds to
the boiling point of the terpene oil. The second weight loss at 420 ºC corresponds to
PS degradation. In this way, it was confirmed that the final solvent concentration in
the foam is always lower than 5 % at the studied working conditions.
100
3.0
90
Weight (%)
70
2.5
2.0
60
50
1.5
40
1.0
30
20
0.5
Deriv. Weight (%/ºC)
Weight
Deriv. Weight
80
10
0
0
50
0.0
100 150 200 250 300 350 400 450 500 550 600
Temperature (ºC)
Figure 7.20. Thermogravimetric analysis (TGA) and differential Termogravimetric Analysis
(DTGA) of PS foam obtained at 90 bar, 30 ºC and 0.20 g/ml Cymene as initial concentration.
221
7.5. Statistical study of the foaming process of PS/p-Cymene
solutions
In this section, a full factorial two level design with 3 factors (pressure, temperature
and concentration) and a center point (23+1) was proposed and nine runs were
carried out to determine the influence of the mentioned variables on the foams cells
size in order to optimize this parameter. The levels of each factor are indicated in
Table 7.19.
Table 7.19. Coded and uncoded matrix of the experiments. Contact time: 240 min,
depressurization time: 1.5 min.
Run
p
T
C
p (bar)
T (ºC)
C (g/ml)
1
-1
-1
-1
70
30
0.05
2
-1
-1
1
70
30
0.20
3
1
-1
-1
90
30
0.05
4
1
-1
1
90
30
0.20
5
0
0
0
80
35
0.125
6
-1
1
-1
70
40
0.05
7
-1
1
1
70
40
0.20
8
1
1
-1
90
40
0.05
9
1
1
1
90
40
0.20
The coding scheme used to describe the factor levels is based on the “1” and “−1”
signs, where denote the high and low level of each factor and 0 corresponds to the
mean between the two levels of all factors. The limits of the operating conditions
were selected following the principles explained in the previous section: vanishing of
the interfacial tension and temperature above glass transition temperature of the
ternary mixture.
As it was explained, among the most important solid-state properties defined by the
foaming process are the dimensional properties of the final product (cells diameter
and morphology). The variables studied in this work were the diameter of the pore
cells, its standard deviation and the cells density to determine the foams structure
and homogeneity. Results have been divided in three sections. Initially, the results
of the foaming process are shown, next the characterisation of samples was
performed to optimise the results during the last section.
All experiments in the design matrix shown in Table 7.19 were carried out according
to the same experimental procedure, with the operating conditions indicated by the
coding scheme (high and low levels for each factor).
Figure 7.21 shows the microphotographs analyzed by scanning electron microscopy
where the structure and cell size of the foamed PS can be observed. The most
222
Chapter 7
homogeneous foams which presented higher cells density, smaller and well
distributed pores were obtained at high level of pressure and concentration and low
values of temperature.
Figure 7.21. SEM images with 2000x magnification of PS foam obtained as a function of the
initial concentration of PS, temperature and pressure.
T:30ºC
T:35ºC
T:40ºC
C0=0.05 g/ml C0=0.20 g/ml C0=0.125 g/ml C0=0.05 g/ml C0=0.20 g/ml
p: 70 bar
p: 85 bar
No foam
No foam
No foam
p: 90 bar
From the microphotographs, data on the average diameter of cells data were fitted
to a normal distribution curve. As it was shown n the previous section, the foaming
process can not be performed at 70 bar and 40ºC nor at 85 bar and 35ºC, because it
is not feasible and higher PS concentration solutions were obtained instead of foams
[31]. The foaming process was not achieved under the mentioned conditions because
the CO2 concentration was too low to solubilise fully p-Cymene. The solubility of
CO2 in polymers increases under higher pressure and lower temperature [75] (see
Chapter 5). Also, the CO2 density increases at high pressure and low temperature
like its solvent power [76] so p-Cymene should be preferably removed at high
densities. Furthermore, the critical points and the Tg of the mixtures was studied
and results are shown in Figure 7.22.
According to Figure 7.22, at 30 ºC foams were obtained independently of the
working pressure since they are below the glass transition temperature of the
mixture. Nevertheless, at higher temperature, the foaming process is achieved when
the working pressure is above the critical pressure of the mixture, which
demonstrates that p-Cymene is fully soluble in CO2 and the separation process can
be performed. According to these reasons, runs 5, 6 and 7 provided the less
favourable conditions to perform the foaming process.
223
110
100
Pres (bar)
90
80
70
60
Tg PS/p-Cymene solutions
Critical mixing point of 0.05 g PS/ml p-Cymene solutions
Critical mixing point of 0.20 g PS/ml p-Cymene solutions
Foam
No foam
50
40
25
30
35
40
45
Temp (ºC)
Figure 7.22. Influence of glass transition temperature and phase equilibrium boundary on
the feasibility of the foaming process: (▲) successful and/or () unsuccessful. (—) Decrease of
Tg of the PS/p-Cymene solutions as a function of CO2 pressure; (…) critical point of the
mixture at 0.05 g PS/ml p-Cymene and () 0.20 g PS/ml p-Cymene.
7.5.1. Characterisation
All samples were characterized to determine the amount of residual solvent, their
glass transition and the degradation temperature (Table 7.20).
Table 7.20. Characterisation of the Polystyrene foams. Concentration of p-Cymene in the
final products (% wt Cym), glass transition temperature (Tglass) and degradation
temperature (Tdeg).
Run p
T
C p (bar) T (ºC) C (g/ml) % wt Cym
Tglass (ºC)
Tdeg (ºC)
1
-1
-1
-1
70
30
0.05
2.70
104.29
415.54
2
-1
-1
1
70
30
0.2
5.00
104.04
416.85
3
1
-1
-1
90
30
0.05
4.00
101.81
415.82
4
1
-1
1
90
30
0.2
5.49
101.85
417.56
5
0
0
0
80
35
0.125
No foam
6
-1
1
-1
70
40
0.05
No foam
7
-1
1
1
70
40
0.2
No foam
8
1
1
-1
90
40
0.05
8.22
98.88
421.18
9
1
1
1
90
40
0.2
7.61
101.30
418.76
224
Chapter 7
The average concentration of p-Cymene present in PS foams is around 5% but the
foams are not structurally affected because of the solvent traces. Lower
temperatures enhance the solubility of p-Cymene in CO2 and consequently, pCymene traces in the final foam are reduced. Also, terpene oil traces would be easily
removed from the foams by adding an extra stream of CO 2 once depressurization is
performed.
The degradation temperature and the glass transition temperature were kept
almost constant in all samples compared with the original PS pellets (105.81 ºC and
408.92 ºC, respectively). An average deviation of 3.58% in the Tg and 2.13% in the
Tdeg was obtained which confirmed that PS maintained its original calorimetric
properties. In addition, the molecular weight distribution by means of GPC to
confirm that fractionation of polymer do not occur during the experiments.
Since the cell size of the final product is defined as one of the most important
properties while the calorimetric and structural properties remain constant, the
pore cells, its standard deviation and the cells density were studied (Table 7.21).
Table 7.21. Average diameter of the pore cells (m), standard deviation and cells density
(cells/cm3) of the PS foams.
Run p
T
C p (bar) T (ºC) C (g/ml)
Diameter
Cells Density
Std. Dev.
(cells/cm3)
m)
1
-1
-1
-1
70
30
0.05
4.472
2.264
2.40·109
2
-1
-1
1
70
30
0.20
10.899
4.549
5.52·108
3
1
-1
-1
90
30
0.05
7.228
2.560
2.04·1010
4
1
-1
1
90
30
0.20
2.822
1.275
2.47·1011
5
0
0
0
80
35
0.125
No foam
6
-1
1
-1
70
40
0.05
No foam
7
-1
1
1
70
40
0.20
No foam
8
1
1
-1
90
40
0.05
5.270
1.217
1.51·1010
9
1
1
1
90
40
0.20
20.345
5.838
6.45·108
Generally higher cells densities are achieved when the pore size are smaller, but in
the cases in which the foams are heterogeneous, small cell diameters could lead to
low values of cells density. According to Table 7.21, the smallest and narrowest
distributed pores are obtained in run 4 which also shows the highest value of cells
density. By contrast, the foams obtained in run 9 exhibit big pores, non
homogeneously distributed and low values of cells density.
225
7.5.2. Optimisation
Analysis of the experimental data is focused on the production of homogeneous
foams with high cells density which means minimization of the cells size and its
standard deviation and maximization of the cell density. With three samples not
foaming it is difficult to draw any accurate conclusions using the traditional
statistical analysis of variance to determine the input factors statistically significant
and finding the optimum levels. Therefore, the influence of each variable on the cell
structure is studied individually.
The effect of pressure on the cell size and its distribution was studied at constant
temperature (30 ºC) and concentration (0.20 g PS/ml p-Cymene). PS foams were
produced at 70 and 90 bar (runs 2 and 4). The effect of pressure on the average pore
diameter and its distribution is illustrated in Figure 7.23 where an increase in CO2
pressure tended to produce smaller and narrower distributed pores.
80
Runs
Average Std. Dev.
2; P: 70 bar
10.90
4.549
4; P: 90 bar
2.822
1.275
70
Probability Density
60
T: 30 ºC; C0: 0.20 g PS/ml p-Cymene
50
40
30
20
10
0
0
5
10
15
Average Pore Cells
20
Figure 7.23. Effect of pressure on the cells size through runs 2 and 4 performed at 30 ºC,
0.20 g PS/ml p-Cymene, 70 and 90 bar, respectively.
As the pressure increases, more CO2 was dissolved into the polymer solution and
caused higher plasticization and viscosity reduction which facilitated the growth in
the number of pores and reduced their size leading to higher rate of nucleation when
pressure was reduced [8, 24, 25]. This fact was observed through the increase of
cells density from 5.52·108 to 2.47·1011 cells/cm3.
The influence of temperature on the porous structure was studied at constant
pressure (90 bar) and concentration (0.20 g PS/ml p-Cymene) while temperature
ranged between 30 and 40 ºC (runs 4 and 9) and is observed in Figure 7.24.
226
Chapter 7
140
Average Std. Dev.
Runs
2.822
1.275
4; T: 30 ºC
20.34
5.838
9; T: 40 ºC
120
P: 90 bar; C0: 0.20 g PS/ml p-Cymene
Probability Density
100
80
60
40
20
0
0
8
16
Average Pore Cells
24
32
Figure 7.24. Effect of temperature on the cells size through runs 4 and 9 performed at 90
bar, 0.20 g PS/ml p-Cymene, 30 and 40 ºC, respectively.
It should be emphasized that the foaming process is hampered when the working
temperature was above the glass transition temperature of the mixture and foams
are only obtained at higher pressure. With higher temperatures (Figure 7.24), the
cell size increases from 2 to 20 m in the studied range. The increase of the pore
diameter is due to a reduction in the viscosity of the solution and a decrease of the
CO2 solubility into the PS/p-Cymene solution which produces fewer nuclei but
consequently larger cells [51, 58, 59, 77], i.e., a lower cell density.
The effect of Polystyrene concentration on the cell size was studied depending on the
pressure and temperature used because different trends were obtained (Figure
7.25). Runs 1-2 (70 bar and 30 ºC) and runs 8-9 (90 bar and 40 ºC) indicate an
increase in the cell size when PS concentration increases from 4-5 m at 0.05 g
PS/ml p-Cymene to 10-20 m, at 0.20 g PS/ml p-Cymene. An increase of the pore
size could be mainly attributed to the lower CO2 solubility which is produced at low
pressure and high temperature [75]. With higher initial concentration of the
solution, the CO2 sorption decreases and the nucleation rate is lower as the amount
of cells formed, but they are bigger and inhomogeneously distributed.
Comparing runs 3 and 4, it can be stated that with higher PS concentration, the cell
size decreases. At 90 bar and 30 ºC the solubility of CO 2 in the solution was
improved and consequently nucleation is enhanced which causes the formation of
many homogenously distributed nuclei.
227
a)
50
Runs
Average Std. Dev.
1; C: 0.05 g/ ml 4.472
2.264
2; C: 0.20 g/ ml 10.90
4.549
Probability Density
40
P: 70 bar; T: 30 ºC
30
20
10
0
0
5
10
15
20
Average Pore Cells
(b)
140
Runs
Average Std. Dev.
8; C: 0.05 g/ml
5.270
1.217
9; C: 0.20 g/ml
20.34
5.838
Probability Density
120
P: 90 bar; T: 40 ºC
100
80
60
40
20
0
4
8
12
16
20
24
28
32
Average Pore Cells
Figure 7.25. (a) Effect of concentration on the cells size through runs 1 and 2 performed at
70 bar, 30 ºC, 0.05 and 0.20 g PS/ml p-Cymene, respectively. (b) Effect of concentration on the
cells size through runs 8 and 9 performed at 90 bar, 40 ºC, 0.05 and 0.20 g PS/ml p-Cymene,
respectively.
In order to obtain some general conclusions about the effect of the working
conditions over the cells size, the global effect of pressure, temperature and
concentration was studied. The main purpose of the section is the foaming of PS/pCymene solutions in order to get small pores and homogeneous cells foams.
According to Figures 7.23 - 7.25, the PS foams with the smallest and narrowest pore
diameter together with the highest cell density are obtained at 90 bar, 30 ºC and
0.20 g PS/ml p-Cymene because a decrease of pressure produces an increase of the
cell size while the pore diameter distribution remains practically constant. The
optimum results are displayed in Figure 7.26 where the foams presented small and
homogeneously distributed pores.
228
Chapter 7
(a)
(b)
Figure 7.26. Microphotographs of the optimum PS foams obtained at 90 bar, 30 ºC and 0.20 g
PS/ml p-Cymene (a) 3000x magnification and (b) 6000x magnification
To sum up, a proper foaming of Polystyrene under mild working conditions could be
achieved and the structure of the foams produced can be tailored by altering the
pressure and the temperature. But a rigorous study is shown in the subsequent
section with the aim of study all the variables involved in the foaming process.
7.6. Rigorous Statistical study of the foaming process of
Polystyrene/Limonene solutions
As it has been explained in the previous section, during the foaming process, several
parameters should be studied since they have influence on the final structure or
characteristics of the foams. Among the most important solid-state properties
defined by the foaming process are the dimensional properties of the final product
(diameter and morphology). The responses studied in this work were: the diameter
of the pore cells, its standard deviation (indicative of the foam homogeneity) and the
cells density.
In this research section, the pressure, temperature, initial concentration of the PS
solution, contact time and depressurization time were considered as variable.
Although the most frequently design of tests is a full factorial in order to minimize
the number of tests required, fractional factorial experiments (FFEs) design of
experiments was developed. In particular, a design called Taguchi method was
applied by using an orthogonal array of L27, with only twenty seven experiments.
The three levels for each factor are shown in Table 7.11, and the values were chosen
according to previous studies on PS foaming from solutions [31].
229
Table 7.11. Factors studied in the design of experiments (DOE) of the foaming process and
their levels.
Pressure (bar)
Temperature (ºC)
Contact time (min)
Depressurization time (min)
Level1
Level2
Level3
70
30
0.10
60
1.5
80
35
0.15
150
15.45
90
40
0.20
240
30
Furthermore, statistical analysis of variance (ANOVA) was applied to determine the
effect of the input factors that was statistically significant and to find the optimum
levels. The statistical analysis performed with the experimental results was made
using commercial software package Statgraphics Plus 5.1 (Manugistics, Inc.
Rockville, MD, USA). The test of statistical significance, p-value, was determined
accordingly to the total error criteria considering a confidence level of 95%.
Results are divided in three sections. Initially, experimental data together with the
results obtained from the design of experiments is analyzed. Next, a brief
description of the Classical Nucleation Theory is applied and finally, the
characterization of the foams is assessed.
7.6.1. Foaming process
All experiments in the design matrix shown in Table 7.11 were carried out according
to the same experimental procedure, with the operating conditions indicated by the
coding and the runs were established under a randomness criterion. Table 7.12
shows the structure of Taguchi’s orthogonal array design and some representative
microphotographs. The average cells diameter, their standard deviation and the cell
density were calculated from the analysis of several microphotographs in each
experiment.
230
70
70
70
70
70
70
70
70
70
2
3
4
5
6
7
8
9
40
40
40
35
35
35
30
30
30
0.20
0.20
0.20
0.15
0.15
0.15
0.10
0.10
0.10
240
150
60
240
150
60
240
150
60
30
30
30
15.45
15.45
15.45
1.5
1.5
1.5
p
C
tcontact tdep
T (ºC)
(bar)
(g/ml) (min) (min)
1
Run
Scanning Electron
Microphotography
No foams were obtained
42.473
3.531
Highly porous skin
Heterogeneous distribution
4.18
Very well defined pore surface
2.932
4.259
Very well defined pore surface
Noncontinuous foam,
nonporous homogeneous skin
Average
(m)
Observation
11.667
0.913
0.834
2.323
1.202
Std.
Dev
3.134·106
2.540·109
4.599·109
3.930·109
7.051·109
Cells
Density
(cells/cm3)
Table 7.12. L27 orthogonal array. Runs, experimental working conditions, microphotographs, diameter cells and their standard
deviations and cells density.
Chapter 7
231
232
p
(bar)
80
80
80
80
80
80
Run
10
11
12
13
14
15
35
35
35
30
30
30
0.10
0.10
0.10
0.20
0.20
0.20
240
150
60
240
150
60
30
30
30
15.45
15.45
15.45
T
C
tcontact tdep
(ºC) (g/ml) (min) (min)
Scanning Electron
Microphotography
8.82
8.051
3.821
30.057
43.71
7.5
Very homogeneous pore
distribution
Very homogeneous
distribution and small size
pores
Nonporous skin, very few big
pores
Nonporous and rough skin,
very few big pores
Heterogeneous, rough and few
pores skin
3.236
8.862
0.908
5.019
2.561
Average Std.
Dev
(m)
Fine, brittle, medium size
pores foam
Observation
3.199·108
7.932·108
1.266·109
1.756·109
4.433·108
2.738·108
Cells
Density
(cells/cm3)
Table 7.12 (continued) L27 orthogonal array. Runs, experimental working conditions, microphotographs, diameter
cells and their standard deviations and cells density
p
(bar)
80
80
80
90
90
90
Run
16
17
18
19
20
21
30
30
30
40
40
40
T
(ºC)
0.15
0.15
0.15
0.15
0.15
0.15
240
150
60
240
150
60
30
30
30
1.5
1.5
1.5
C
tcontact tdep
(g/ml) (min) (min)
Scanning Electron
Microphotography
Average
(m)
57.116
5.747
25.695
3.939
9.918
6.982
Observation
Non very porous skin, big size
and heterogeneous pores
Pores inside cavities, non
homogeneous pores
Low porosity, big size pores
Very homogeneous and small
pores
Very well defined and big
cells, high cells density
Heteregeneous and non very
porous skin
3.243
2.362
1.122
10.769
2.207
28.85
Std.
Dev
7.363·1010
2.539·1010
4.867·1010
4.868·107
2.223·109
2.801·107
Cells
Density
(cells/cm3)
Table 7.12 (continued) L27 orthogonal array. Runs, experimental working conditions, microphotographs, diameter cells and
their standard deviations and cells density
Chapter 7
233
234
p
(bar)
90
90
90
90
90
90
Run
22
23
24
25
26
27
40
40
40
35
35
35
T
(ºC)
0.10
0.10
0.10
0.20
0.20
0.20
240
150
60
240
150
60
15.45
15.45
15.45
1.5
1.5
1.5
C
tcontact tdep
(g/ml) (min) (min)
Scanning Electron
Microphotography
Average
(m)
8.82
8.051
3.821
30.057
43.71
7.5
Observation
Fine, brittle, medium size
pores foam
Very homogeneous pore
distribution
Very homogeneous
distribution and small size
pores
Nonporous skin, very few big
pores
Nonporous and rough skin,
very few big pores
Heterogeneous, rough and few
pores skin
3.236
8.862
0.908
5.019
2.561
Std.
Dev
3.199·108
7.932·108
1.266·109
1.756·109
4.433·108
2.738·108
Cells
Density
(cells/cm3)
Table 7.12 (continued) L27 orthogonal array. Runs, experimental working conditions, microphotographs, diameter cells and
their standard deviations and cells density
Chapter 7
According to the microphotographs shown in Table 7.12 the most homogeneous
foams which presented higher cells density, smaller size and are well distributed
are obtained at low temperature and high concentration. From the analysis of
microphotographs, the mean diameter of cells data were fitted to a normal
distribution curve and the selected runs are shown in Figure 7.27. The smallest
pores with the narrowest distribution were obtained from runs 3, 5 and 19.
0.5
Run
3
13
19
21
Probability Density
0.4
Average Std. Dev.
3.531
0.8344
30.06
8.862
3.939
1.122
6.982
3.243
0.3
0.2
0.1
0.0
0
8
16
24
Mean Cell Size
32
40
48
Figure 7.27. Probability density function of average cell pore diameter (m) for the runs ( )
3, ( ) 13, ( ) 19 and ( ) 21.
It is important to highlight that at 70 bar and 40ºC, no foams were obtained as it
was previously observed [31]. To try to explain the lack of foams during the runs 79, the evolution of the working conditions as well as the Tg modification during the
process were studied (Figure 7.28). The left axis represents the variation of the
temperature (operating and glass transition temperature) along the experiments
while the right axis shows the evolution of pressure. In the cited runs, the Tg is
lower than the working temperature because the plasticising effect of CO 2, so the
crystallization of polymer chains is practically not expected, unless the state of the
solution is supercooled and the rate of crystallization high enough. The rest of the
experiments presented values of Tg higher than different the operating temperature,
which allows the nucleation of bubbles and foaming of the polymer.
235
100
Temp (ºC)
75
80
60
50
40
25
20
0
0
0
10
20
30
40
50
60
70
80
90
100
100
80
80
60
60
40
40
20
20
0
0
0
25
50
75
100
125
150
175
100
100 Run 9
60
60
40
40
20
20
0
0
25
50
75
100
125
150
175
200
225
250
Pressure (bar)
80
80
0
Pressure (bar)
Temp (ºC)
100 Run 8
Temp (ºC)
Pressure (bar)
100 Run 7
275
Time (min)
Figure 7.28. Evolution of the operating conditions during the foaming process during the
runs 7, 8 and 9. Evolution of temperature (dashed lines, left axis): glass transition
temperature (■), and working temperature (○) and pressure (solid line, right axis).
According to Figure 7.28, it could be concluded that a gap between the working
temperature and the Tg of PS is required to promote the polymer crystallization and
foaming.
Taguchi approach involves two steps: the proper selection of the orthogonal arrays
and its analysis of variance (ANOVA). Thus, ANOVA determines the contribution of
each parameter on the responses factors (Table 7.13). The influence factor will be
significant if the value of critical level (p) is lower than 0.05. As can be observed
from p-value, the effects statistically significant over the mean pore diameter are
the temperature, the concentration and the pressure while the contact time and the
depressurization time are not statistically significant with a p-value higher than
0.05. The standard distribution of the pore cells was also affected by the previously
mentioned variables. Nevertheless, the pressure is the only effect which affects
significantly the cells density.
236
Chapter 7
Table 7.13. Main effects and interactions from L27 orthogonal array on the mean pore cell
diameter, the standard deviation and the cells density.
Factor
Cells
Diameter (m)
Std. Dev.
Cells Density
(cells/cm3)
p effect (±error std.)
-27.55 (±10.92)
-30.40 (±12.69)
7.53·1010
(±3.54·1010)
p-value
0.0197
0.0281
0.0456
T effect (±error std.)
56.80 (±10.92)
50.89 (±12.69)
-1.75·1010
(±3.54·1010)
p-value
0.0001
0.0007
0.6258
C effect (±error std.)
32.12 (±10.92)
33.66(±12.69)
-4.56·1010
(±3.54·1010)
p-value
0.0078
0.0182
0.2120
Contact time effect
(±error std.)
7.09 (±10.92)
13.99(±12.69)
5.80·1010
(±3.54·1010)
p-value
0.5233
0.2979
0.1168
Dep time effect
(±error std.)
-12.28 (±10.92)
-12.83(±12.69)
4.09·1010
(±3.54·1010)
p-value
0.2731
0.3238
0.2609
According to the results shown in Table 7.13, the morphology of the foams is mainly
affected by the pressure and the temperature. The standardized effects of the
independent variables on the cells diameter, its standard distribution and the cells
density were investigated by mean of a Pareto chart (Figure 7.29). The length of
each bar indicates the standardized effect of the selected factor on the different
responses and its colour represents if the contribution was positive or negative. The
positive effects (grey colour) presented a favourable effect on the response while the
negative effects (black bars) shown an antagonistic effect on it.
237
(a)
(b)
(c)
Figure 7.29. Pareto chart showing the standardized effect of independent variables (A:
pressure; B: temperature; C: concentration; D: contact time; E: venting time) on the average
cells diameter (a), its standard deviation (b) and cells density (c).
According to Figure 7.29 (a) in the case of the average cell pore diameter, the
temperature and depressurization time promote the increase of the pore sizes,
whereas the pressure favours the formation of smaller pore cells. In Figure 7.29 (b)
the significantly influence of pressure, temperature, concentration and
depressurization time on the standard distribution of the pore cells was shown. Only
an increase of pressure facilitates the homogeneity of the foam while an increase of
the mentioned variables tends to increase the standard deviation of the pore cells
diameter. Finally, Figure 7.29 (c) shows that only the pressure significantly affects
the cells density. This fact supports the non homogeneity of the foams, since the cell
density should be significantly influenced by the same variables than the cell pore
diameter, but they should indicate opposite effects on the studied response. Next,
the effect of each factor in the foaming characteristics is discussed.
238
Chapter 7
The effect of pressure on the average diameter of the cells was studied at 70, 80 and
90 bar. With increasing pressure, the mean pore diameter decreases and it can be
found that if the cell size decreases, the amount of cells increases with pressure.
They became narrower distributed with increasing pressure, which means that
more uniform foam structure could be obtained at higher pressure [51, 58-60, 77]. At
higher pressure, more CO2 is dissolved into the polymer solution and caused higher
plasticization and viscosity reduction (see Chapters 5 and 6) [78]. These facts imply
higher amount of CO2 used to nucleation which facilitates the growth of nucleis and
results in more and smaller cells (higher cells density and lower pores diameter).
The influence of the working temperature was investigated between 30 and 40ºC.
An increase in the temperature of the sample produces an increase in the mean pore
diameter as well as an increase in its heterogeneity and a decrease in the cells
density [51, 77]. With increasing temperature, the CO2 solubility in the polymer
solution decreases. Since at higher temperatures there is less dissolved fluid into
the polymer matrix available for the nucleation and growth of pores, fewer nuclei
(that should share this fluid) are formed [59, 77]. Higher temperature lower the
viscosity of the matrix and increase the diffusion rate of CO 2, rendering the cell
growth faster, thus a narrow cell size distribution could be achieved at lower
processing temperature.
The effect of concentration on the average diameter of the cells was examined in the
range of 0.1 to 0.2 gPS/ml Limonene. According to Table 7.13 and Figure 7.29,
increasing the initial concentration of PS in the solution, the mean pore diameter
and the heterogeneity increase. Concentration is not statistically significant over
the cells density, nevertheless a decrease in the cells density was observed when
concentration of Polystyrene increases. With increasing the initial concentration of
the solution, the CO2 sorption decreases as it was observed experimentally in Figure
5.7, so the nucleation rate is lower as the amount of cells formed, but they are bigger
and non homogeneously distributed.
The contact time of CO2 in PS/Limonene solutions was studied in a range between
60 and 240 min according previous literature references [51] An increase in the
contact time produces more uniform structures and smaller pore cells. Although
Reverchon’s studies concluded that 150 min was enough to promote homogeneous
microcellular structure, we observe that when Limonene is presented in the
mixture, the minimum contact time increases up to 240 minutes.
Finally, the effect of depressurization time was considered in the study of
PS/Limonene solution foaming. Fast (1.5 min) or slow (30 min) depressurization
time was tuned to determine its influence on the mean pore cells. The mean pore
diameter decreases when the depressurization time increased. At high
depressurization time (low depressurization rates) the CO 2 entrapped in the
swelling solution is slowly removed, decreasing the cells size and increasing the cell
population. Nevertheless, high depressurization rate produces the coalescence of
neighbour cells as a consequence of the abrupt elimination of CO 2 [59, 79].
239
In order to obtain some general conclusions about the effects over the studied
responses, Figure 7.30 summarized the response surfaces of the average mean pore
of the cells and their standard deviation. In this case the cells density is not
considered because only one factor (the
pressure)
was statistically
significant.
Superficie
de Respuesta
Estimada
Concentration=0.15,Contact time=150.0,Dep time=15.75
Mean Cells Diameter
(a)
100
80
60
40
20
40
0
70
Mean Cells Diameter
0.0
12.0
24.0
36.0
48.0
60.0
72.0
84.0
96.0
108.0
120.0
35
80
Pressure (bar)
90
30
Temperature (ºC)
Superficie de Respuesta Estimada
Concentration=0.15,Contact time=150.0,Dep time=15.75
Std. Dev.
(b)
90
75
60
45
30
15
0
40
Standard Deviation
-20.0
-8.0
4.0
16.0
28.0
40.0
52.0
64.0
76.0
88.0
100.0
112.0
35
30
70
80
Temperature (ºC)
90
Pressure (bar)
Figure 7.30. Response surfaces showing the relation between the pore cells (a) and its
standard deviation (b) as a function of pressure and temperature.
According to Figure 7.30 (a), an increase in pressure and a decrease in temperature
at constant concentration, contact time and depressurization time acted increasing
the sorption of the supercritical solvent into the polymeric matrix which decreases
the pore size while increases its homogeneity. Whereas, as it is observed in Figure
7.30 (b) the heterogeneity (higher standard deviation) of the pores sizes was
obtained at higher temperatures and lower pressures.
7.6.2. Nucleation mechanisms
The main objective of any foaming process is to obtain foams with high cells density
and low cell size which implies high nucleation rate. Nevertheless, the formation of
nuclei in a viscous liquid is a complex mechanism influenced by several parameters
240
Chapter 7
[70]. During this section the main factors which affected the formation of
microcellular foams are determined.
Various mechanisms of nucleation have been proposed in the literature for
nucleation of gas bubbles in thermoplastics, including homogeneous, heterogeneous,
mixed-mode, shear-induced and void nucleation theories. However, none of these
theories give quantitative predictions for even the simplest cases of bubble
nucleation in polymeric liquids.
One of the key processes to control the cellular structure is the nucleation and
growth of gas cells dispersed throughout the polymer. The resulting structure
depends on a balance between several mechanisms, such as diffusion of CO2 in the
polymer, the solubility and the plasticization effect, which depends intrinsically of
the polymer and the saturation conditions [80]. The nucleation theory can be used to
describe the thermodynamic and kinetic effects on the foam generation. The
Classical Nucleation Theory (CNT) has been selected to explain some of the
experimental facts obtained because it is still regarded as the most successful
approach for prediction of nucleation phenomena [81]. The steady state nucleation
rate, defined as the number of critical nuclei formed per unit time in a unit volume
of the parent phase is given by:
[7.13]
where Jss is the number of critical nuclei formed per unit time in a unit volume, W
is the reversible work of critical nucleus formation, k B is the Boltzmann factor, and
T is the absolute temperature. J0, according to the kinetic consideration of the
bubble formation process, is defined as:
[7.14]
where N is the number of gas molecules per unit volume, it means the density of the
blowing agent molecule in polymer-blowing agent solutions, m is the mass of the gas
molecule, and  is the surface tension between the metastable parent phase and the
nucleating phase. B is a dimensionless factor that varies between 2/3 and 1,
depending on the viscosity of the material; in the case of polymer is considered to
have a value of 1.
W presents a larger impact on the value of the nucleation rate and its accurate
calculation is considered more essential that the proper calculation of J0. In the
cases where p is large, W becomes close to zero and the exponential term of the
equation which represents the steady state nucleation rate (Eq. [7.13]) is mostly
affected by the pre-exponential factor J0. For a homogeneous nucleation, W is
evaluated to be:
[7.15]
241
where P is the difference between Pnucleus, the pressure of the metastable phase
(inside the gas nucleus) and Psystem, the pressure of the nucleating phase
(polymer/gas solution) [56]. P is the real driven force of the foaming process and
due to the importance of its accurate determination it is calculated from the
experimental data obtained because it is difficult to obtain experimentally. Next, the
calculated drop pressure could be used to predict the behaviour of the foam using
the Classical Nucleation Theory. According to equations 7.13, 7.14 and 7.15 and
considering the experimental data shown, the value of P required to obtain the
critical nuclei (JSS) is calculated. N can be obtained from the equilibrium data shown
in Chapter 5, was determined experimentally and it was shown in our previous
work [31] and in Chapter 6 and B was fixed at 2/3 because the solutions were not
very viscous. In those cases where foams are not obtained, an arbitrary value of 109
cells/cm3 was selected as cell densities because of the microcellular foams definition.
Results are shown in Table 7.14.
According to Table 7.14 P decreases when pressure increases and temperature
decreases. The main variables that present influence over the P are the interfacial
tension between phases and the temperature. The interfacial tension of Polystyrene
in Limonene solution decreases when the pressure of CO2 increases, by this reason,
P required to foam also decreases. The small range of concentration does not
influence significantly on the interfacial tension and consequently it is not affected
by far the minimum pressure required to foam. The contact time promoted the CO 2
sorption into the polymeric solution. Nevertheless, this parameter does not affect
the interfacial tension, because it is checked that a rapid achievement of the
equilibrium composition is got between the drop and the interphase [31]. In general,
minimum values of P are achieved when interfacial tension is close to the
vanishing point. In these cases, the concentration of CO 2 in the polymer-solvent
phase increase promoting a decrease of interfacial tension since the two phases in
contact become more similar. The most suitable conditions to carry out the foaming
process are those in which P is lower, because a very low driven force is necessary
to ease nucleation and cell growth. In Table 7.14, the lowest values of P are
observed at runs 10-12, where low and homogeneous cell sizes are obtained.
242
Chapter 7
Table 7.14. P required to carry out the foaming process.
Run
p (bar)
T (ºC)
C (g/ml)
tcontact
(min)
tdep
(min)
P (bar)
1
70
30
0.10
60
1.5
17.23
2
70
30
0.10
150
1.5
17.05
3
70
30
0.10
240
1.5
17.10
4
70
35
0.15
60
15.45
22.23
5
70
35
0.15
150
15.45
22.59
6
70
35
0.15
240
15.45
20.32
7
70
40
0.20
60
30
35.46
8
70
40
0.20
150
30
35.46
9
70
40
0.20
240
30
35.46
10
80
30
0.20
60
15.45
0.00381
11
80
30
0.20
150
15.45
0.00385
12
80
30
0.20
240
15.45
0.00395
13
80
35
0.10
60
30
2.72
14
80
35
0.10
150
30
2.69
15
80
35
0.10
240
30
2.65
16
80
40
0.15
60
1.5
6.15
17
80
40
0.15
150
1.5
6.61
18
80
40
0.15
240
1.5
6.20
19
90
30
0.15
60
30
0.00426
20
90
30
0.15
150
30
0.00420
21
90
30
0.15
240
30
0.00430
22
90
35
0.20
60
1.5
0.00426
23
90
35
0.20
150
1.5
0.00420
24
90
35
0.20
240
1.5
0.00444
25
90
40
0.10
60
15.45
0.00397
26
90
40
0.10
150
15.45
0.00385
27
90
40
0.10
240
15.45
0.00376
The difference between the initial interfacial tension of the solutions and the
interfacial tension of the polymer (without Limonene, only CO 2) at the same
working conditions are represented in Figure 7.31. As it was previously observed,
the lowest values of interfacial tension promote the polymer solutions foaming.
However, the highest values of interfacial tension of solutions give the lowest gap
243
between the initial interfacial tension of solutions and the interfacial tension of the
polymer at the same pressure. The impossibility to process the foams from the
solution at those conditions (P: 70 bar, T:40ºC) could also be attributed to the high
driven force required.
0.0400
Initial Surface Tension (Solution)
Final Surface Tension (Polystyrene)
0.0375
0.0350
 (N/m)
0.0325
0.0300
0.0050
0.0025
0.0000
0
3
6
9
12
15
18
21
24
27
Run
Figure 7.31. Interfacial tension of Polystyrene solutions (-■-) and Polystyrene (-○-) in the
different runs.
On the other hand, the influence of temperature on P was given by the inverse of
the square root, thus, when temperature increased, P should decrease linearly
which would help the foaming of the solutions at higher temperature. Nevertheless,
experimental data do not show the expected trend. It is important to consider that
when temperature increases CO2 sorption in PS or its solution decreases and the
minimum driven force required to foam is increased because lower amounts of the
gas are imbedded into the polymeric matrix. This fact confirms that lower
temperature values are desired to foam the Polystyrene solutions.
7.6.3. Characterisation
All samples (except those on which foaming were not produced) were characterized
to determine the amount of residual solvent, their glass transition (Tg) and
degradation temperature (Tdeg). Virgin Polystyrene pellets were also characterized
in order to compare the results obtained from the analysis of the processed
polymeric foams (Table 7.15).
244
Chapter 7
Table 7.15. Characterization of the Polystyrene foams. Concentration of Limonene in the
final products (% wt Lim), glass transition temperature (Tglass) and degradation
temperature (Tdeg).
Run
p (bar)
T (ºC)
C (g/ml)
% wt Lim
Tglass
(ºC)
Tdeg
(ºC)
1
70
30
0.10
2.43
104.43
414.35
2
70
30
0.10
2.97
104.15
416.72
3
70
30
0.10
7.03
103.92
416.97
4
70
35
0.15
5
70
35
0.15
7.61
96.25
420.06
6
70
35
0.15
6.95
101.52
420.57
7
70
40
0.20
No observable foaming
8
70
40
0.20
9
70
40
0.20
10
80
30
0.20
2.78
104.08
415.81
11
80
30
0.20
2.12
102.28
414.77
12
80
30
0.20
3.54
95.15
414.55
13
80
35
0.10
6.58
103.06
421.97
14
80
35
0.10
6.38
94.57
415.97
15
80
35
0.10
6.74
104.46
416.90
16
80
40
0.15
6.34
101.65
421.14
17
80
40
0.15
4.37
100.78
416.46
18
80
40
0.15
7.84
95.91
420.07
19
90
30
0.15
3.56
101.00
415.83
20
90
30
0.15
4.44
102.62
415.81
21
90
30
0.15
6.53
101.07
419.30
22
90
35
0.20
4.59
23
90
35
0.20
5.36
24
90
35
0.20
2.66
104.14
417.33
25
90
40
0.10
8.52
76.62
423.40
26
90
40
0.10
7.91
101.13
418.95
27
90
40
0.10
7.30
101.46
418.57
105.81
408.92
Virgin Polystyrene pellets
The amount of Limonene embedded in the foams os studied and results are shown
in Table 7.15. The average concentration of solvent present in PS foams is around
245
5%, although there are some runs where Limonene amount is higher, the foams are
not structurally affected because of the traces of solvent. Limonene would be easily
removed from the foams adding an extra stream of CO 2 once depressurization is
carried out because it has been assessed that Limonene is fully miscible in CO 2 at
the working conditions (Chapter 6) [25].
According to Table 7.15, the degradation temperature and the glass transition
temperature are kept almost constant in all samples. Low values of standard
deviation confirm that polymer degradation or structural modification do not occur
during the foaming process with CO 2 at high pressure. The decrease of Tg values in
some cases can be attributed to the presence of Limonene which acts as plasticizer
and consequently promotes the mobility of the polymeric chains decreasing its glass
transition temperature.
Finally, a comparison between commercial Polystyrene wastes (particularly, XPS
which is used as insulation material) and the foams produced in our research group
was accomplished (Figure 7.32).
Average Pore (m)
Cells Density (cells/cm3)
Density (kg/m3)
Recycled Polystyrene
Commercial Polystyrene
3-20
1·109-1011
304.02  18.34
400-600
3-5 ·104
35  10
Figure 7.32. Comparison between the recycled Polystyrene (left) and the original wastes
(right).
As it is observed, in the recycled Polystyrene obtained in this research work, the
cells are smaller, more homogeneous and consequently it presents higher cells
density as well as a significant increase on bulk density. According to the values
obtained it can be assessed that the production of microcellular foams from
Polystyrene solutions is a feasible process. On that way, a very high added value
product is obtained because of the improvement of the structural properties which is
responsible of higher impact strength, toughness and better thermal stability.
The foaming of Polystyrene at mild working conditions can be achieved and the
structure of the foams produced can be tailored by altering mainly the pressure and
the temperature. The use of a terpene solvent to perform an initial solution provided
246
Chapter 7
an increase in CO2 sorption which was the responsible of the plasticization of the
Polystyrene. The polymer properties modification (decrease of glass transition
temperature, interfacial tension, viscosity…), as a consequence of gas sorption,
increases the CO2 diffusion coefficient which also benefited the nucleation and cell
growth. According to the design of experiments analysis, the most suitable
conditions to carry out the process of foaming Polystyrene/Limonene solutions were
higher pressures (90 bar), low temperature (30ºC), low concentration (0.1 gPS/ml
Limonene), high contact time (240 min) and depressurization time (30 min). At
these conditions, the cell size decreased while the cell density increases which
provided foams with high quality properties. The characterization of the foams
showed that all the properties of the initial polymer were kept constant.
247
References
[1] E. Carretier, E. Badens, P. Guichardon, O. Boutin, G. Charbit, Hydrodynamics of
supercritical antisolvent precipitation: Characterization and influence on particle
morphology, Industrial and Engineering Chemistry Research, 42 (2003) 331-338.
[2] C. Chen, S. Zhan, M. Zhang, Z. Liu, Z. Li, Preparation of poly(L-lactide)
microparticles by a supercritical antisolvent process with a mixed solvent, Journal of
[3] Y. Park, C.W. Curtis, C.B. Roberts, Formation of nylon particles and fibers using
precipitation with a compressed antisolvent, Industrial and Engineering Chemistry
Research, 41 (2002) 1504-1510.
[4] Y. Pérez De Diego, F.E. Wubbolts, G.J. Witkamp, T.W. De Loos, P.J. Jansens,
Measurements of the phase behaviour of the system dextran/DMSO/CO 2 at high
pressures, Journal of Supercritical Fluids, 35 (2005) 1-9.
[5] K. Wu, J. Li, Precipitation of a biodegradable polymer using compressed carbon
dioxide as antisolvent, Journal of Supercritical Fluids, 46 (2008) 211-216.
[6] J.O. Werling, P.G. Debenedetti, Numerical modeling of mass transfer in the
supercritical antisolvent process: Miscible conditions, Journal of Supercritical Fluids,
18 (2000) 11-24.
[7] J.O. Werling, P.G. Debenedetti, Numerical modeling of mass transfer in the
supercritical antisolvent process, Journal of Supercritical Fluids, 16 (1999) 167-181.
[8] D.J. Dixon, G. Luna-Bárcenas, K.P. Johnston, Microcellular microspheres and
microballoons by precipitation with a vapour-liquid compressed fluid antisolvent,
Polymer, 35 (1994) 3998-4005.
[9] E. Reverchon, Supercritical antisolvent precipitation of micro- and nanoparticles, Journal of Supercritical Fluids, 15 (1999) 1-21.
[10] C. Vemavarapu, M.J. Mollan, M. Lodaya, T.E. Needham, Design and process
aspects of laboratory scale SCF particle formation systems, International Journal of
Pharmaceutics, 292 (2005) 1-16.
[11] E. Franceschi, M.H. Kunita, M.V. Tres, A.F. Rubira, E.C. Muniz, M.L. Corazza,
C. Dariva, S.R.S. Ferreira, J.V. Oliveira, Phase behavior and process parameters
effects on the characteristics of precipitated theophylline using carbon dioxide as
antisolvent, Journal of Supercritical Fluids, 44 (2008) 8-20.
[12] E. Reverchon, I. De Marco, Supercritical antisolvent precipitation of
Cephalosporins, Powder Technology, 164 (2006) 139-146.
[13] A. Martín, M.J. Cocero, Numerical modeling of jet hydrodynamics, mass
transfer, and crystallization kinetics in the supercritical antisolvent (SAS) process,
[14] S.D. Yeo, E. Kiran, Formation of polymer particles with supercritical fluids: A
review, Journal of Supercritical Fluids, 34 (2005) 287-308.
[15] Y.W. Kho, D.S. Kalika, B.L. Knutson, Precipitation of Nylon 6 membranes using
compressed carbon dioxide, Polymer, 42 (2001) 6119-6127.
[16] Y.P. De Diego, H.C. Pellikaan, F.E. Wubbolts, G.J. Witkamp, P.J. Jansens,
Operating regimes and mechanism of particle formation during the precipitation of
polymers using the PCA process, Journal of Supercritical Fluids, 35 (2005) 147-156.
[17] A. Tenorio, P. Jaeger, M.D. Gordillo, C.M. Pereyra, E.J.M. De La Ossa, On the
selection of limiting hydrodynamic conditions for the Supercritical AntiSolvent
(SAS) process, Industrial and Engineering Chemistry Research, 48 (2009) 92249232.
248
Chapter 7
[18] P. Subra, C.G. Laudani, A. Vega-González, E. Reverchon, Precipitation and
phase behavior of theophylline in solvent-supercritical CO2 mixtures, Journal of
[19] R. Adami, L.S. Osséo, R. Huopalahti, E. Reverchon, Supercritical AntiSolvent
micronization of PVA by semi-continuous and batch processing, Journal of
[20] C.M. Hansen, Hansen solubility parameters: a user's handbook, CRC Press,
2000.
[21] Y. Marcus, Are solubility parameters relevant to supercritical fluids?, Journal of
[22] I. Gracia, J.F. Rodríguez, A. De Lucas, M.P. Fernandez-Ronco, M.T. García,
Optimization of supercritical CO2 process for the concentration of tocopherol,
carotenoids and chlorophylls from residual olive husk, Journal of Supercritical
Fluids, 59 (2011) 72-77.
[23] M.P. Fernández-Ronco, C. Ortega-Noblejas, I. Gracia, A. De Lucas, M.T. García,
J.F. Rodríguez, Supercritical fluid fractionation of liquid oleoresin capsicum:
Statistical analysis and solubility parameters, Journal of Supercritical Fluids, 54
(2010) 22-29.
[24] J. Rincón, P. Cañizares, M.T. García, I. Gracia, Regeneration of used lubricant
oil by propane extraction, Industrial and Engineering Chemistry Research, 42 (2003)
4867-4873.
[25] H. Sovová, R.P. Stateva, A.A. Galushko, Essential oils from seeds: Solubility of
limonene in supercritical CO2 and how it is affected by fatty oil, Journal of
[26] C. Gutiérrez, J.F. Rodríguez, I. Gracia, A. de Lucas, M.T. García, High-pressure
phase equilibria of Polystyrene dissolutions in Limonene in presence of CO 2, The
[27] A.T. Griffith, Y. Park, C.B. Roberts, Separation and recovery of nylon from
carpet waste using a supercritical fluid antisolvent technique, Polymer-Plastics
Technology and Engineering, 38 (1999) 411-431.
[28] M. Rantakylä, M. Jäntti, O. Aaltonen, M. Hurme, The effect of initial drop size
on particle size in the supercritical antisolvent precipitation (SAS) technique, Journal
of Supercritical Fluids, 24 (2002) 251-263.
[29] D.L. Obrzut, P.W. Bell, C.B. Roberts, S.R. Duke, Effect of process conditions on
the spray characteristics of a PLA + methylene chloride solution in the supercritical
antisolvent precipitation process, Journal of Supercritical Fluids, 42 (2007) 299-309.
[30] A. Martín, M.J. Cocero, Micronization processes with supercritical fluids:
Fundamentals and mechanisms, Advanced Drug Delivery Reviews, 60 (2008) 339350.
of a strategy for the foaming of polystyrene dissolutions in scCO 2, Journal of
[33] K. Lucas, Die Druckabhängigkeit der Viskosität von Flüssigkeiten – eine
einfache Abschätzung, Chemie Ingenieur Technik, 53 (1981) 959-960.
667-673.
249
[35] E. Reverchon, I. De Marco, E. Torino, Nanoparticles production by supercritical
antisolvent precipitation: A general interpretation, Journal of Supercritical Fluids,
43 (2007) 126-138.
[36] K.B. Deshpande, W.B. Zimmerman, Simulation of interfacial mass transfer by
droplet dynamics using the level set method, Chemical Engineering Science, 61
(2006) 6486-6498.
[37] A.D. Randolph, M.A. Larson, Theory of particulate processes: analysis and
techniques of continuous crystallization, Academic Press, 1988.
[38] E. Reverchon, R. Adami, G. Caputo, I. De Marco, Spherical microparticles
production by supercritical antisolvent precipitation: Interpretation of results,
[39] H. Chao-Hong, Y. Yong-Sheng, Estimation and infinite-dilution diffusion
coefficients in supercritical fluids, Industrial and Engineering Chemistry Research,
36 (1997) 4430-4433.
[40] J.L. Bueno, J.J. Suárez, I. Medina, Experimental binary diffusion coefficients of
benzene and derivatives is supercritical carbon dioxide and their comparison with
the values from the classic correlations, Chemical Engineering Science, 56 (2001)
4309-4319.
[41] C. Mantell, M. Rodríguez, E. Martínez De La Ossa, Estimation of the diffusion
coefficient of a model food dye (malvidin 3,5-diglucoside) in a high pressure CO2 +
methanol system, Journal of Supercritical Fluids, 29 (2004) 165-173.
[42] T. Fadli, A. Erriguible, S. Laugier, P. Subra-Paternault, Simulation of heat and
mass transfer of CO2-solvent mixtures in miscible conditions: Isothermal and nonisothermal mixing, Journal of Supercritical Fluids, 52 (2010) 193-202.
[43] C.H. He, Y.S. Yu, New equation for infinite-dilution diffusion coefficients in
supercritical and high-temperature liquid solvents, Industrial and Engineering
[44] F. Rindfleisch, T.P. DiNoia, M.A. McHugh, Solubility of polymers and
copolymers in supercritical CO2, Journal of Physical Chemistry, 100 (1996) 1558115587.
[45] M.L. O'Neill, Q. Cao, M. Fang, K.P. Johnston, S.P. Wilkinson, C.D. Smith, J.L.
Kerschner, S.H. Jureller, Solubility of homopolymers and copolymers in carbon
dioxide, Industrial and Engineering Chemistry Research, 37 (1998) 3067-3079.
[46] J. Crank, The Mathematics of Diffusion, Clarendon Press, 1979.
[47] M. Lora, A. Bertucco, I. Kikic, Simulation of the semicontinuous supercritical
antisolvent recrystallization process, Industrial and Engineering Chemistry
Research, 39 (2000) 1487-1496.
[48] M. Mukhopadhyay, S.V. Dalvi, Mass and heat transfer analysis of SAS: Effects
of thermodynamic states and flow rates on droplet size, Journal of Supercritical
Fluids, 30 (2004) 333-348.
[49] J.B. Bao, T. Liu, L. Zhao, G.H. Hu, X. Miao, X. Li, Oriented foaming of
polystyrene with supercritical carbon dioxide for toughening, Polymer (United
Kingdom), (2012).
[50] D.L. Tomasko, A. Burley, L. Feng, S.K. Yeh, K. Miyazono, S. Nirmal-Kumar, I.
Kusaka, K. Koelling, Development of CO2 for polymer foam applications, Journal of
[51] E. Reverchon, S. Cardea, Production of controlled polymeric foams by
250
Chapter 7
[52] D. Sanli, S.E. Bozbag, C. Erkey, Synthesis of nanostructured materials using
supercritical CO2: Part I. Physical transformations, Journal of Materials Science, 47
(2012) 2995-3025.
[53] H. Zhong, Z. Xi, T. Liu, Z. Xu, L. Zhao, Integrated process of supercritical CO 2assisted melt polycondensation modification and foaming of poly(ethylene
terephthalate), Journal of Supercritical Fluids, 74 (2013) 70-79.
[54] L. Sorrentino, E. Di Maio, S. Iannace, Poly(ethylene terephthalate) foams:
Correlation between the polymer properties and the foaming process, Journal of
[55] D.C. Li, T. Liu, L. Zhao, W.K. Yuan, Foaming of linear isotactic polypropylene
based on its non-isothermal crystallization behaviors under compressed CO 2,
[56] Z. Guo, A.C. Burley, K.W. Koelling, I. Kusaka, L.J. Lee, D.L. Tomasko, CO2
bubble nucleation in polystyrene: Experimental and modeling studies, Journal of
[57] J.-B. Bao, T. Liu, L. Zhao, G.-H. Hu, A two-step depressurization batch process
for the formation of bi-modal cell structure polystyrene foams using scCO2, The
[58] R. Liao, W. Yu, C. Zhou, Rheological control in foaming polymeric materials: I.
Amorphous polymers, Polymer, 51 (2010) 568-580.
[59] I. Tsivintzelis, A.G. Angelopoulou, C. Panayiotou, Foaming of polymers with
supercritical CO2: An experimental and theoretical study, Polymer, 48 (2007) 59285939.
[60] E. Kiran, Foaming strategies for bioabsorbable polymers in supercritical fluid
mixtures. Part I. Miscibility and foaming of poly(l-lactic acid) in carbon dioxide +
acetone binary fluid mixtures, Journal of Supercritical Fluids, 54 (2010) 308-319.
[61] M. Xanthos, Functional Fillers for Plastics, Wiley, 2010.
[62] S.A.E. Boyer, J.P.E. Grolier, Modification of the glass transitions of polymers by
high-pressure gas solubility, Pure and Applied Chemistry, 77 (2005) 593-603.
[63] C. Gualandi, L.J. White, L. Chen, R.A. Gross, K.M. Shakesheff, S.M. Howdle,
M. Scandola, Scaffold for tissue engineering fabricated by non-isothermal
supercritical carbon dioxide foaming of a highly crystalline polyester, Acta
Biomaterialia, 6 (2010) 130-136.
[64] E. Kiran, Foaming strategies for bioabsorbable polymers in supercritical fluid
mixtures. Part II. Foaming of poly(ε-caprolactone-co-lactide) in carbon dioxide and
carbon dioxide + acetone fluid mixtures and formation of tubular foams via solution
extrusion, Journal of Supercritical Fluids, 54 (2010) 296-307.
[65] M. Karimi, M. Heuchel, T. Weigel, M. Schossig, D. Hofmann, A. Lendlein,
Formation and size distribution of pores in poly(ε-caprolactone) foams prepared by
pressure quenching using supercritical CO 2, Journal of Supercritical Fluids, 61
(2012) 175-190.
[66] E. Kiran, Modification of biomedical polymers in dense fluids. Miscibility and
foaming of poly(p-dioxanone) in carbon dioxide + acetone fluid mixtures, Journal of
[67] I.A. Ronova, O.V. Sinitsyna, S.S. Abramchuk, A.Y. Nikolaev, S. Chisca, I. Sava,
M. Bruma, Study of porous structure of polyimide films resulting by using various
methods, Journal of Supercritical Fluids, 70 (2012) 146-155.
[68] H.X. Huang, H.F. Xu, Preparation of microcellular polypropylene/polystyrene
blend foams with tunable cell structure, Polymers for Advanced Technologies, 22
(2011) 822-829.
251
[69] H. Zweifel, R.D. Maier, M. Schiller, Plastics Additives Handbook, Carl Hanser
GmbH, 2009.
[70] L.J.M. Jacobs, M.F. Kemmere, J.T.F. Keurentjes, Sustainable polymer foaming
using high pressure carbon dioxide: A review on fundamentals, processes and
applications, Green Chemistry, 10 (2008) 731-738.
nucleation theory and a new minimum in free energy change. 1. A new minimum and
Kelvin equation, Fluid Phase Equilibria, 254 (2007) 67-74.
nucleation theory and a new minimum in free energy change. 2. Behavior of free
energy change with a minimum calculated for various systems, Fluid Phase
Equilibria, 255 (2007) 55-61.
[73] K. Nishioka, I. Kusaka, Thermodynamic formulas of liquid phase nucleation
from vapor in multicomponent systems, The Journal of Chemical Physics, 96 (1992)
5370-5376.
[74] S. Cotugno, E. Di Maio, G. Mensitieri, S. Iannace, G.W. Roberts, R.G.
Carbonell, H.B. Hopfenberg, Characterization of microcellular biodegradable
polymeric foams produced from supercritical carbon dioxide solutions, Industrial
[76] J. Rincón, P. Cañizares, M.T. García, Improvement of the waste-oil vacuumdistillation recycling by continuous extraction with dense propane, Industrial and
[77] J. Wang, X. Cheng, X. Zheng, M. Yuan, J. He, Preparation and characterization
of microcellular polystyrene/polystyrene ionomer blends with supercritical carbon
dioxide, Journal of Polymer Science, Part B: Polymer Physics, 41 (2003) 368-377.
Technology, 170 (2006) 143-152.
[79] L.I. Cabezas, V. Fernández, R. Mazarro, I. Gracia, A. De Lucas, J.F. Rodríguez,
Production of biodegradable porous scaffolds impregnated with indomethacin in
[80] A.R. Imre, G. Melnichenko, W.A. Van Hook, Liquid-liquid equilibria in
polystyrene solutions: The general pressure dependence, Physical Chemistry
Chemical Physics, 1 (1999) 4287-4292.
[81] T. Kalwarczyk, N. Ziebacz, S.A. Wieczorek, R. Holyst, Kinetics and dynamics of
dissolution/mixing of a high-viscosity liquid phase in a low-viscosity solvent phase,
Journal of Physical Chemistry B, 111 (2007) 11907-11914.
252
Chapter 8
Life Cycle and
Economic
Assessment
Chapter 7 shows the foaming process of PS from its homogeneous solutions in
terpene oils using CO2 as antisolvent and blowing agent. Once the phases’
equilibrium was studied and the effect of CO2 on the shift of physical properties was
measured, the design of a foaming process was carried out. Initially, the feasibility
of the foaming was determined on the selection of the vanishing points of the
solutions in CO2. Next, a further study concerning the optimization of the process in
order to achieve microcellular foams from PS/Limonene solutions.
Life Cycle and Economic Assessment
254
Chapter 8
Graphical abstract
255
RESUMEN
Normalmente, se cree que el reciclado de residuos plásticos es una de las
alternativas de gestión que presenta mayores ventajas medioambientales. Sin
embargo, también puede tener algunas implicaciones negativas, relacionadas con
emisiones peligrosas, contaminación de suelos, o incluso riesgos para la salud. La
identificación y evaluación de los impactos medioambientales sobre el ciclo de vida
del proceso explicado en este trabajo, es el objetivo de este Capítulo. Así, se
comprobó que la tecnología propuesta presenta numerosas ventajas respecto a las
técnicas comúnmente usadas.
Debido a los resultados tan prometedores que mostró el reciclado de residuos de
Poliestireno, se estimó su viabilidad económica. En primer lugar, se determinó el
coste de la inversión y se realizó un análisis financiero en dos escenarios distintos,
en función del volumen de residuos tratados. A continuación se calculó el Valor
Actual Neto (VAN) y la Tasa Interna de Rentabilidad (TIR), ya que estos valores se
usan normalmente para determinar la inversión y la rentabilidad de un proceso. Por
último, se obtuvo la matriz DAFO, que es una metodología de estudio de la situación
de un proyecto, analizando sus características internas (Debilidades y Fortalezas) y
su situación externa (Amenazas y Oportunidades) en una matriz cuadrada.
256
Chapter 8
ABSTRACT
Generally, recycling of plastic wastes could be thought as an environmentally
beneficial alternative respect to other management ways. Unfortunately, there
could be negative implications due to hazardous emissions, soil contamination or
health risks. The identification and evaluation of the environmental impacts over
the life cycle of the process is the aim of the first section of this Chapter. Thus, it
was checked that the proposed technology presents several environmental
advantages versus the traditional techniques used.
Due to the promising results offer by this process for the recycling of Polystyrene
wastes, its economic feasibility was investigated. Initially, the investment and
financial analysis were determined in two different scenarios depending on the
amount of treated wastes. Next, the Net Price Value (NPV) and the Internal Rate of
Return (IRR) were calculated since the mentioned parameters are generally used for
determining investments and projects profitability. Finally, the StrengthsWeaknesses-Opportunities- Threats (SWOT) matrix of the process was determined,
revealing the interest of the process for industrial application.
257
258
Chapter 8
8.1. Life Cycle Assessment Background
Life Cycle Assessment (LCA) is an environmental management tool which identifies
all resources used and wastes generated to all environmental media (air, water and
soil) over the whole life cycle of a specific good or service [1]. LCA was introduced in
the early 1970s, by simple calculations of energy usage during the entire life span of
the product [2]. The main asset of LCA is its comprehensive character which make it
very popular in analyzing plastic wastes management as it is shown by the
numerous published studies of the life cycle emissions of these systems [1]. The
increasing number of research papers has been focused on the LCA, particularly
concerning the recycling of plastics (Figure 8.1).
3000
Life Cycle Assessment
Research Paper
2500
2000
1500
1000
500
0
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015
Year
18
16
Life Cycle Assessment
Plastics Recycling
Research Paper
14
12
10
8
6
4
2
0
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014
Year
Figure 8.1. Evolution of research papers concerning Life Cycle Assesment (a) and Life Cycle
Assesment of plastics recycling (b) along the last decades.
Recycling is shown as environmentally beneficial because it reduces resources,
decreases demand for landfill space and promotes savings energy, in principle.
Nevertheless, the collection of wastes also shows environmental impacts as a
259
consequence of collection, sorting and recovery. By this reason, it is necessary to
evaluate the costs and benefits from the recycling as well as the emissions
generated. One methodology for undertaking this evaluation is LCA, which
quantifies the environmental impacts of a product or material over its entire lifecycle [3].
An LCA comprises four major stages: goal definition, inventory, impact assessment
and improvement assessment. According to this classification, the section below has
been divided according to the mentioned aspects.
The aim of this research is the recycling of Polystyrene wastes according to an
environmentally friendly technology. The new recycling process proposed in this
work is shown in Figure 8.2.
Customer
Shrinking and
Transport
Recycling Plant
High Quality
Polystyrene
Volume reduction: 1/5-1/35
Figure 8.2. Schematic diagram of the recycling process of Polystyrene wastes for the
production of microcellular foams.
Figure 8.2 shows a schematic diagram of the recycling methodology where the PS
wastes from customer are dissolved in Limonene and transported to the recycling
plant by the trucks. Recycled microcellular foam PS can be reused as raw material
for high quality applications and recovered Limonene can be reused to shrink new
PS wastes.
Next, the inventory is defined, mass and energy balances required to produce the
functional unit are established and the environmental burdens are quantified [4].
The different studied scenarios were:
1.
2.
3.
4.
5.
6.
Virgin Polystyrene production
Landfilling
Pyrolysis of Polystyrene wastes
Thermal treatment of Polystyrene wastes
Dissolution of Polystyrene wastes in Limonene
Dissolution of Polystyrene wastes in Limonene, precipitation and foaming by
scCO2.
They are summarized in Figure 8.3.
260
Chapter 8
Low quality Recycled Polystyrene
Emissions
Emissions to
to
air
air
Emissions
Emissions to
to
air
air
Emissions
Emissions to
to
air
air
Landfill
Landfill
Transport
Transport
Emissions
Emissions to
to
air
air and
and
water
water
Polymerization
Styrene
Styrene
Pyrolysys
Pyrolysys
Sorting
Sorting and
and
Classification
Classification
Virgin
Virgin
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Wastes
Wastes
Thermal
Thermal
treatment
treatment
Emissions
Emissions to
to
air
air
Dissolution
Dissolution in
in
terpene
terpene oils
oils
Terpene
Terpene oils
oils
Transport
Transport
Additives
Additives for
for
Cyanoacrylates
Cyanoacrylates
CO
CO22
Microcellular
Microcellular
Foams
Foams
Terpene
Terpene oils/
oils/ CO
CO22
High quality recycled Polystyrene
Figure 8.3. Process flow of the different scenarios for the management of Polystyrene wastes.
The different scenarios are shown below together with a brief description.
1. Virgin Polystyrene production
Polystyrene is a commodity thermoplastic with many special properties which make
it very useful in a wide range of applications. Styrene monomers are polymerized
via chain-growth by free-radicals initiation with the aim to produce atactic
Polystyrene (aPS). Once the radical initiator initiates (generally, benzoyl peroxide)
the polymerization of styrene, propagation occurs which “builds up” the polymer
chain. Due to the reactivity of vinyl monomers, it is often necessary to add an
inhibitor to stabilize the monomer and prevent premature radical formation and
polymerization. Once the polymer chain has “grown” and at a desirable length or
molecular weight, the polymerization is terminated [5]. The polymer is then
isolated, possibly purified, characterized, and used for material use. On the other
hand, syndiotactic Polystyrene (sPS) is polymerized by coordination using
metallocenes catalysts [6]. sPS, in contrast to aPS is semi-crystalline, and exhibits
enhanced chemical resistance and thermal stability. Although the improved
properties of sPS, aPS is more widely ended up in waste streams and by this reason,
the LCA of aPS polymerization was studied.
According to Figure 8.3, during the polymerization of styrene, emissions to the air
and water are produced.
261
2. Landfilling
The amount of space used in landfills by all plastics is between 25-30 %. Although
Polystyrene wastes account for less than 1% of the total weight of landfill materials,
but the fraction of landfill space it takes up is much higher due to their low density.
Furthermore, it is essentially non-biodegradable, taking hundreds or thousands of
years to decompose. Even after disposal in landfill, EPS can easily be carried by the
wind to litter streets or pollute water bodies or could be eaten by animals. Moreover,
toxic chemicals could leach out of the wastes into the rainwater and be moved into
the soils. From Figure 8.2 it is observed that the main emissions of PS wastes in
landfills are to the air.
3. Pyrolysis of Polystyrene wastes
The pyrolysis process for plastics takes the long chain polymer molecules and breaks
or cracks them into shorter chains through heat producing energy. Energy recovery
is an effective way to reduce the volume of organic materials by incineration but this
method has been widely accused of being ecologically unacceptable, owing to the
health risk from air born toxic substances [7]. Depending on the components of the
waste plastics used as feedstock for energy recovery, the resulting emissions may
contain contaminants such as amines, alcohols, waxy hydrocarbons and some
inorganic substances [8]. Due to the wide variety of pollutants emitted during the
energy recovery, the main substances produced during the pyrolysis of real PS
wastes were analyzed and results are shown in Figure 8.4.
100
3.0
90
2.5
70
2.0
60
50
1.5
40
1.0
30
20
(a)
Weight
Deriv. Weight
10
Deriv. Weight (%/ºC)
Weight (%)
80
C2H2
-HCN
CH3Cl
0.5
0
0.0
C2H2
Intensity (a.u.)
(b)
Intensity (a.u.)
-HCN
CH3Cl
C4H3
+
C4H4
+
C4H3
+
C4H4
+
74
C6H6
C8H8
74
C6H6
C8H8\
50
100
150
200
250
300
350
400
Temperature (ºC)
450
500
550
350
400
450
500
Temperature (ºC)
Figure 8.4. Thermogravimetric and differential thermogravimetric analysis for the pyrolysis
of PS wastes (a) and main components produced during its pyrolysis (b).
262
Chapter 8
According to Figure 8.4 during the pyrolysis of PS wastes highly toxic pollutants are
produced due to the degradation of the polymer. The main free radical generated is
C4H3+, but also cyclohexane and benzene are yielded.
4. Thermal treatment of Polystyrene wastes
PS wastes become in densified PS by compacting at temperature above its Tg ( > 100
ºC). Typically, an EPS cushion package consists of 2% Polystyrene and 98% air.
Removal of the air by mechanical or thermal densification results in a weight-tovolume ratio 30 to 50 times greater than the raw EPS package.
The PS pellets cannot be recycled as high quality polymers due to the thermal
degradation and ageing that suffer along the process [9, 10]. Densification will tend
to lock in contaminates, lowering or negating the value of the densified materials.
Moreover, the melting of the polymer requires high energy consumption and high
temperature could promote the release of harmful vapours.
5. Dissolution of Polystyrene wastes in Limonene
The dissolution of PS wastes has been shown not only as a volume reduction way,
but also as a potentially cheap source for further applications. In this work, its
application as fillers in cyanoacrylates based adhesives is studied. The addition of
PS/Limonene solutions in different cyanoacrylate monomers was studied. Limonene
was added as carrier in a variable concentration in order to homogenize the
dissolution of PS into cyanoacrylates and avoid their instantaneous polymerization.
PS was added as filler at maximum concentration of 9% (from a solution at 19% in
Limonene) while the adhesive properties of cyanoacrylates remained constant or
still enhanced.
From the application of the explained management of PS wastes only emissions to
air due to the transport of solutions should be considered for the study of the life
cycle assessment.
6. Dissolution of Polystyrene wastes in Limonene, precipitation and
foaming by scCO2.
The process described in Chapter 7 allows the recovery of a high quality PS from
wastes by an environmentally friendly recycling process. According to Figure 8.3,
during the process any emissions are produced to air or water since all streams are
recirculated. By this reason, the environmental impact should only consider the
transport of PS wastes and results will be comparable to the previous section.
There are many references in the literature concerning the Life Cycle Assessment of
the synthesis of polymers, landfilling, recycling process and energy recovery [11-13].
In this section, the impact assessment was limited to the main resources consumed,
the CO2, SO2 and NOx emissions and other environmentally relevant factors, such as
acidification or greenhouse effect. Figure 8.5 summarizes the mentioned factors.
263
CO2 emissions
SO2 emissions
Landfill of PS
Virgin PS
NOx emissions
CO2 emissions
SO2 emissions
NOx emissions
Greenhouse effect
Acidification
Energy
0
10
20
30
40
50
60
70
Emissions to air
80
0
10
20
30
40
50
60
70
40
50
60
70
Emissions to air
Thermal treatment
CO2 emissions
SO2 emissions
Pyrolysis
NOx emissions
CO2 emissions
SO2 emissions
NOx emissions
Greenhouse effect
Acidification
Energy
10
15
20
25
30
0
10
20
30
Emissions to air
Foaming
5
Emissions to air
Dissolution
0
CO2 emissions
SO2 emissions
NOx emissions
CO2 emissions
SO2 emissions
NOx emissions
Greenhouse effect
Acidification
Energy
0
3
6
9
12
Emissions to air
Greenhouse effect
Acidification
Energy
15
0
3
6
9
12
Emissions to air
15
Figure 8.5. Amount of emissions to air from 1 kg of PS.
The amount of CO2 emissions to air from 1 kg of PS in the different scenario is
shown in Figure 8.5. The most important CO 2 emissions to the atmosphere are due
to the landfill and pyrolysis of PS wastes. Otherwise, the lowest CO 2 emissions are
achieved by the thermal treatment, dissolution and foaming of PS which produces
between three and twelve times lower than that from landfill or pyrolysis.
Furthermore it is important to highlight that during PS wastes collection and
dissolution, the volume reduction enables a more efficient transportation due to the
volume reduction which reduces significantly the emissions during the transport.
Regarding SO2 emissions, the pyrolysis and landfill of PS are the main sources of
emissions, followed by the thermal treatment while the dissolution of wastes in
terpenes emits the lowest value of SO2. On the other hand, NOx emissions show the
264
Chapter 8
same trend as that of SO2. This fact is due to the emission related to the efficient in
transport and by this reason the production of virgin PS exhibits low SO 2 and NOx
values of emissions because transport is not included, moreover the higher density
of PS pellets means lower volume of transport than the processed wastes. Finally,
other emissions such as CH4, dioxins or CO should be considered, but due to the lack
of data makes it quite difficult.
Other parameters to evaluate the environment impact of the management of PS
wastes were normalized and included. Thus, the greenhouse effect, the acidification
potential and the equivalent energy consumption were quantified. According to data
shown in Figure 8.5 the most negative impacts are due the production of virgin
polymer, while the rest of alternatives are more environmentally friendly,
especially, those related with recycling and recovery of PS wastes.
In view of these facts, the results of this LCA indicate that recycling scenarios are
generally the environment preferable options versus all impact categories.
Unfortunately, the different references show an important disparity and due to the
high sensitivity of the impacts, results could vary considerably. Nevertheless, if high
quality recycled PS (as additive fillers of microcellular foams) is produced and it can
substitute virgin plastic, these are the most favourable alternatives for the waste
management of PS wastes.
According to the explained facts, the economical assessment of the recycling of PS
wastes for the production of high quality products, microcellular foams, is shown in
the subsequent section.
265
8.2. Economical evaluation
One of the main objectives of a plant feasibility study is to assess the commercial
viability of the proposed plant. The essential parameters of this calculation are the
capital cost and cost of debt, the operating costs, the revenue on operation, and the
plant life [14]. On a regular basis, supercritical technology presents high investment
requirements due to the high pressure operation [15] but the products obtained
after the supercritical process present improved characteristics which make them
high added value products [16]. Among the most promising purposes of supercritical
fluid processes are materials and pollution abatement both areas are covered in this
work. Furthermore, the recycling of Polystyrene wastes entails huge environmental
benefits which would be sufficient to convince consumers to change their habits and
customs on polymer applications.
In this section, the economical assessment includes the calculation of the equipment
costs required for the process and then the total capital investment of the plant.
According to the promising results shown in Chapter 7, the recycling of Polystyrene
to produce microcellular foams would be the most beneficial alternative. Two
different scenarios have been fixed to accomplish the calculations and to compare
the feasibility of the process based on market demand.
1.- Building and operating a new plant located in Alcázar de San Juan due to the
potential management capacities which offers the village. The scope is the recycling
of PS wastes from Castilla-La Mancha region. In this scenario, the considered
amount of PS wastes was 7000 kg/day according to the population of the region and
the rates of the mentioned polymer wastes produced per habitant in Spain [17].
Along the following sections, the amount of treated wastes will be discussed as a
function of the price.
2.- Construction and operating trucks containing the facilities for the recycling of
daily PS wastes. The considered area is also Castilla-La Mancha region. In the case
of scenario number 2, due to the smaller capacities of the plant (restricted to the
volume of the truck), the treated PS wastes were 100 kg/day since it is the estimated
value from household yielding.
In the case of the industrial plant (scenario 1), five vessels for the foaming of PS
wastes were designed because they allow to avoid dead times, reduce the energy
requirements and fulfil the recycling of 7000 kg/day of Polystyrene wastes [18].
However, when the recycling process is carried out in the trucks (scenario 2), only
one vessel was designed due to volumetric restrictions.
The flow diagram of the plant (scenario 1) is shown in Figure 8.6, where the
operating pressure and temperature of the main equipments are reported. PS
wastes are accumulated in different collection points and dissolved in tank trucks
containing Limonene. The solution and the CO 2 are fed in liquid and gas phase by a
membrane pump (P-1) and a compressor (K-1), respectively to their corresponding
266
Chapter 8
foaming tank (C-1). In the foaming tanks (C-1 to C-5), Polystyrene is precipitated
adopting microcellular structures and once CO 2/Limonene vapour phase is removed
by means of the corresponding valves, it is discharged by means of a conveyor belt
(C-8). The saturated CO2/Limonene solution is expanded by a turbine (K-2) and in
the separator tank (C-6) both Limonene and CO2 can be recovered in liquid and gas
phase, respectively. CO2 is returned to the feed tank (C-7) by means of a compressor
(K-3) while collection trucks are fed by Limonene in order to dissolve again new PS
wastes. The study of the economical feasibility of CO2 recovery (using compressor K3) was performed and results are shown below.
A set of heat exchangers were designed and optimized with the aim of recovery a
99.99% of Limonene during the depressurization of the mixture CO 2/Limonene. The
selection of the most suitable temperature to achieve the condensation of the
terpene was performed according to the equilibrium results shown in Chapter 4.
Finally to improve energy efficiency of the process and take advantage from the
thermal shift, the heating or cooling water forms part of a closed circuit, where it is
recirculated.
On the other hand, for the recycling of PS wastes in recovery trucks, the flow
diagram was modified to be fitted into them. The modified flow diagram of the
recycling plant for the scenario 2 is represented in Figure 8.7. In this case, only one
foaming vessel was designed and plant requirements were also modified. In this
case, PS wastes are directly fed to the foaming vessel (C-1) that is previously filled
with the Limonene where dissolution and foaming are carried out in two steps, but
in the same vessel. CO2 is pressurized by the compressor (K-1) and fed to the
foaming vessel. The process is performed following the previously described
procedure.
Due to the lower amount of PS wastes treated, the economical feasibility of the
recirculation compressor (K-3) was not considered in scenario 2. On a regular basis,
recovery of CO2 is only valuable when high volume of gas is required. Thus, an
easier diagram is obtained, which will affect mainly to the investment costs.
267
-5
-57
E-2
5
5
30
1
25
30
Pressure (bar)
Temperature (ºC)
P
T
25
C-0
C-7
1
E-1
P-1
84
K-1
90
90
5
I-1
C-1
I-2
C-2
I-3
C-3
I-4
C-4
I-5
C-5
20
K-2
C-8
E-3
50
217
50
K-3
C-6
PIC
Figure 8.6. Schematic diagram of the industrial plant for the recycling of PS wastes to
produce microcellular foams.
268
Chapter 8
I-24
C-3
C-2
I-20
K-1
K-2
C-1
C-4
Figure 8.7. Schematic diagram of the truck containing the recycling plant for the
management of PS wastes to produce microcellular foams.
3.2. Investment analysis
Next, equipments defined in the previous section were designed and their costs were
determined to accomplish the first step of the investment analysis. Methods to
estimate equipment cost are usually based on correlations which link a specific
characteristic of the equipment with its final price. Generally, these correlations are
referred to previous years and cost must be updated to the current year [19]. The
accuracy of these methods depend on the specific type of equipment and the working
conditions, therefore some authors think that the most accurate method for
determine process equipment costs is directly from suppliers [20]. In this work, the
correlations studied by Peters et al.[19] were used and prices were updated
according to the overall Consumer Price Index (CPI) [21]. Equipments described in
Figures 8.6 and 8.7 have been grouped in vessels, impulsion devices and heat
exchangers. Tables 8.1-8.3 summarize the most relevant characteristics of the
different equipments and their updated costs scenario 1.
269
Table 8.1. Estimated tanks and vessels cost (€2014) for the recycling of Polystyrene wastes
plant in scenario 1.
Equipment
C-0
C-1 to C-5
C-6
C-7
Volume
(m3)
2.1
15.83
0.42
Cost
(€2014)
28844.3
347094.6
15428.9
100000
The impulsion devices of scenario 1 are compressors (K-1 and K-3), pump (P-1) and
turbine (K-2).
Table 8.3. Estimated impulsion equipments cost (€2014) for the recycling of Polystyrene
wastes plant in scenario 1.
Equipment
K-1
K-2
K-3
P-1
W (kW)
2020
-1969
2714
17.2
Cost (€2014)
774887.5
739175.3
7860550.9
34935.0
Next, the optimization of the compressor (K-3) will be studied because it implies
high investment costs.
Heat exchangers are necessary to keep constant the temperature not only in the
vessels but also in the pipes and valves in order to optimize the global process.
Nevertheless, during compression important heat is produced that can be useful to
balance the temperature decrease during expansions. The integration of energy is
especially highlighted in scenario 1 because of the high values of flow. Furthermore,
the design of the heat exchanger was similar to simplify the construction and
building.
Table 8.3. Estimated heat exchangers cost (€2014) for the recycling of Polystyrene wastes
Equipment
E-1
E-2
E-3
A (m2)
60.32
60.32
60.32
Cost (€2014)
18374.1
18374.1
18374.1
Table 8.4 summarizes the cost of the equipments. All of them were designed in
stainless steel 316 and the thickness of the vessels was calculated according to the
most restrictive criteria.
270
Chapter 8
Table 8.4.Estimated equipment cost (€ ) for PS wastes recycling plant in scenario 1.
2014
Equipment
C-0
C-1 to C-5
C-6
C-7
K-1
K-2
K-3
P-1
E-1
E-2
E-3
Cost (€2014)
28844.3
347094.6
15428.9
100000
774887.5
739175.3
7860550.9
34935.0
18374.1
18374.1
18374.1
TOTAL EQUIPMENT COSTS
11,344,417.20
Capital investment and operating costs
The capital investment represents both the amount of money which must be
supplied for the manufacturing and plant facilities, namely fixed capital investment,
and that required for the operation of the plant, known as working capital.
Percentage of delivered equipment cost is the method used for estimating the fixed
or total capital investment. The other items included in the total direct plant cost
are then estimated as percentage of the delivered equipment. The addition
components of the capital investment are based on average percentage of total direct
plant cost total direct and indirect plant costs or total capital investment.
Estimating by percentage of delivered equipment cost is commonly used for
preliminary and study estimates [19, 22]. Using the fixed-percentage method the
capital investment was calculated and results are shown in Table 8.5.
Table 8.5. Capital investment (€2014) by the percentage of delivered-equipment cost in
scenario 1.
Equipments installation
53,31,876.1
Instrumentation
2,041,995.1
Piping
7,487,315.4
Electrical services
1,247,885.9
Buildings
2,041,995.1
Land improvement
1,134,441.7
Utilities services
7,941,092.0
Land
DIRECT COSTS
680,665.0
39,251,683.5
271
Table 8.5. (Continued) Capital investment (€2014) by the percentage of delivered-equipment
cost in scenario 1.
Engineering
3,743,657.68
Construction
4,651,211.05
Taxes
453,776.69
Contractor Fees
2,495,771.78
Contingency
4,991,543.57
INDIRECT COSTS
16,335,960.77
Operating costs include the manufacturing operation, the raw materials, and
energetic requirements, utilities and plant maintenance. The cost of raw materials
was not directly taken into account because PS wastes are provided freely by the
consumers, but the transportation costs from the collection point to the recycling
plant were considered as 0.7 €/km [23]. In both scenarios, a working radius of 200
km/truck was defined to cover the area of influence. Energetic requirements were
calculated from the electrical power used for pumping and compression; the price of
electricity was established at 0.117 €/kWh as it was shown in Eurostat [24]. With
the aim of decrease energy consumption, the energy produced during the expansion
of the CO2/Limonene stream in K-2 was used to feed K-1 and K-3. Maintenance and
repairs were established as 7% of the total capital investment attending to the
complexity of the process due to the high values of operating conditions, especially of
pressure, and taxes were calculated as 0.5% of the fixed capital investment. The
labour costs were established according to the number of workers in each category:
truck drivers, operators and engineer. The summary of the operating costs are
shown in Table 8.6.
Table 8.6. Operating costs for the recycling of PS wastes process in scenario 1.
Raw materials
Electricity
Direct operating labour
Maintenance and repairs
Directive management
Taxes
260,470.6
1,253,743.0
210,000.0
1,667,629.3
52,500.0
277,938.2
Total
425,8048.0
Income statement
Finally, the Net Price Value (NPV) is generally used for judging investments and
projects, it means, whether the company made or lost money during the period
being reported. NPV is the difference between the present value of cash inflows and
the present value of cash outflows. NPV analysis is sensitive to the reliability of
272
Chapter 8
future cash inflows that an investment or project will yield and is used in capital
budgeting to assess the profitability of an investment or project. If the NPV of a
prospective project is positive, the project should be accepted. However, if NPV is
negative, the project should probably be rejected because cash flows will also be
negative.
On the other hand, Internal Rate of Return (IRR) is the discount rate often used in
capital budgeting that makes the NPV of all cash flows from a particular project
equal to zero. Generally speaking, the higher a project's internal rate of return, the
more desirable it is to undertake the project. As such, IRR can be used to rank
several prospective projects a firm is considering. Assuming all other factors are
equal among the various projects, the project with the highest IRR would probably
be considered the best and undertaken first.
To determine the optimum selling price and to consider the trend market, the
Polystyrene price curve for the recycling process was determined in the range of
production between 700 and 9000 kg/day as shown in Figure 8.8.
60
55
50
45
Price (€/kg)
40
35
30
25
20
15
10
5
0
0
1500
3000
4500
6000
7500
9000
Sales of recycled Polystyrene (kg/day)
Figure 8.8. Price curve for the recycling of Polystyrene wastes into microcellular foams by
dissolution and supercritical CO2 in scenario 1.
Each marker of the curve shown in Figure 8.8 represents for a specific sales, the
price to meet the Break-even-Point, it means, the point at which a business begins
to make profits. According to Figure 8.8., when the sales of the recycled Polystyrene
increase, its price decreases. Considering the amount of wastes in Castilla-La
Mancha region and assuming that all the wastes are recycled (7000 kg/day), a sale
price of 4.02 €/kg assures the economic feasibility of the explained process.
Nevertheless, a higher price was considered (6 €/kg) to achieve economical benefits
from the recycling process.
273
The estimation of the income statement for the recycling of Polystyrene wastes
considered a linear amortization in 15 years (6.67%). Considerations for the
accounting period 2014–2030 are reported in Table 8.7.
Table 8.7. Income statement over the accounting period 2014-2030. Recycling of PS wastes in
scenario 1.
Amortisation along 15 years
Investment capital
Inflation
Working capital
0.045
Investment curve
55,587,644.3
82,298.1
Sales (€)
12,600,000.0
3,722,281.2
t año=0
0.60
Costs (€)
t año=1
0.40
Amortisation
0.067
Interest rate
10%
Taxes
0.35
Figure 8.9 represents the cash flow and the Net Price Value along the temporary
horizon of the recycling plant in scenario 1.
Cash Flow (€)
Net Cash Flow (€)
10000000
0
Cash Flow (€)
2014
2016
2018
2020
2022
2024
2026
2028
2030
Temporary horizon
-10000000
-20000000
-30000000
-40000000
Figure 8.9. Evolution of Cash Flow and Net Cash Flow along the operating time of the
recycling plant in scenario 1.
Next, the most relevant results for the recycling of Polystyrene wastes in scenario 1
are shown in Table 8.8.
274
Chapter 8
Table 8.8. Results from the cash flow analysis for the recycling of PS wastes.
Temporary horizon: 2 years of proyect+15 years of operation
Price of recycled PS (€/kg)
6
Price of recycled PS (€/dm3)
1.82
NPV > 0
6,584,829.0 €
IRR (%)
11.73
From these studies, it could be concluded that proposed process seemed to be an
interesting process to recycled Polystyrene wastes into high added-value foam since
the final product is industrially interesting due to the wide range of applications
that it can cover at a low price.
Next, the study of feasibility of recovering CO2 was performed. In this case, CO2
would not be recycled to the storage tank, and consequently, the compressor K-3
would not be necessary; however, CO2 should be supplied to the recycling plant in
each batch. The price curve increases significantly as it is observed in Figure 8.10.
800
700
Price (€/kg)
600
500
400
300
200
100
0
0
1500
3000
4500
6000
7500
9000
dissolution and supercritical CO2 in scenario 1 without CO2 recovery.
From the comparison of Figure 8.8 and 8.9 is observed that recovery of CO 2
decreases more than order of magnitude the cost of the recycled Polystyrene.
Considering 7000 kg/day as the treated amount of wastes, a sale price of 75.42 €/kg
assures the economic feasibility of the explained process. Nevertheless, a higher
price was considered (76 €/kg) to achieve economical benefits from the recycling
process. Thus, the main results concerning the feasibility of the explained option are
shown in Table 8.9
275
Table 8.9. Results from the cash flow analysis for the recycling of PS wastes without CO 2
recovery.
76
23.11
NPV > 0
1,463,869.9 €
IRR (%)
11.22
According to Table 8.9, the CO2 recovery is more advantageous than the continuous
supply of the gas and by this reason, this option was discarded for the scenario 2.
Following the explained procedure, the economical feasibility for the recycling of PS
wastes in scenario 2 are shown below, where the amount of treated polymer is 100
kg/day. The cost of the vessels depicted in Figure 8.7 is shown in Table 8.10.
Table 8.10. Estimated tanks and vessels cost (€2014) for the recycling of Polystyrene wastes
Equipment
C-1
C-2
C-3
Volume (m3)
5.65
0.16
Cost (€2014)
71,349.1
10,514.1
100,000
The cost of the equipments in scenario 2 is very much affected by the smaller size of
the installation. In the case of scenario 2, there were only one compressor (K-1) and
a turbine (K-2) since the Limonene is recovered directly to the vessel C-1 while CO2
recovery was not economically valuable. The costs of impulsion equipments are
shown in Table 8.11.
Table 8.11. Estimated impulsion equipments cost (€2014) for the recycling of Polystyrene
wastes plant in scenario 2.
Equipment
K-1
K-2
W (kW)
485.6
-1112.9
Cost (€2014)
733306.5
308308.4
It should be remarked that turbine (K-2) releases higher amount of energy than the
required in the compression of CO2 and consequently it can be used in further
application.
Next, the Table 8.12 summarizes the cost of the main equipments in scenario 2. All
of them were designed in stainless steel 316 and the thickness of the vessels was
calculated according to the most restrictive criteria.
276
Chapter 8
Table 8.12.Estimated equipment cost (€ ) for PS wastes recycling plant in scenario 2.
2014
Equipment
C-1
C-2
C-3
K-1
K-2
TOTAL EQUIPMENT COSTS
Cost (€2014)
71,349.1
10,514.1
100,000
733,306.5
308,308.4
1,223,478.1
The capital investment was calculated according to the percentage method [19, 22],
but some budget items were missed since they are not necessary (for instance, land
or buildings). The capital investment results are shown in Table 8.13.
Table 8.13. Capital investment (€2014) by the percentage of delivered-equipment
cost in scenario 2.
Equipments installation
575,034.7
Instrumentation
220,226.1
Piping
807,495.5
Electrical services
134,582.6
Utilities services
856,434.7
DIRECT COSTS
3,817,251.67
Engineering
403,747.8
Taxes
48,939.1
Contractor Fees
269,165.2
Contingency
538,330.4
INDIRECT COSTS
1,260,182.44
As it was explained previously, operating costs include the manufacturing
operation, the raw materials, and energetic requirements, utilities and plant
maintenance and they are shown in Table 8.14.
Table 8.14. Operating costs for the recycling of PS wastes process in scenario 2.
Raw materials
Direct operating labour
Maintenance and repairs
Directive management
Taxes
253,994.1
40,000.0
152,323.02
10,000
25,387.17
Total
481,704.29
277
Before examining the feasibility of recycling Polystyrene wastes in trucks, the price
curve as a function of the amount of wastes was studied and it is shown in Figure
8.10.
350
300
Price (€/kg)
250
200
150
100
50
0
0
20
40
60
80
100 120 140 160 180 200 220
dissolution and supercritical CO2 in scenario 2.
According to Figure 8.10, the price of the recycled Polystyrene decreases when their
sales increase. Setting the values of the treated polymer in 100 kg/day, a minimum
sale price of 30.76 €/kg should be economically feasible. Nevertheless, it is important
to highlight that if the amount of wastes increases up to 200 kg/day, the sale price
decreases up to 15.36 €/kg, that presents the same order of magnitude than that
obtained in the scenario 1.In order to achieve economical benefits from the recycling
process of 100 kg of wastes per day, a higher sale price was set (35 €/kg).
The income statement was determined in the same way that in the previous section.
The most relevant parameters are shown in Table 8.15.
Table 8.15. Income statement over the accounting period 2014-2030. Recycling of PS wastes
in scenario 1.
Amortisation along 15 years
Investment capital
Inflation
Working capital
0.045
Investment curve
278
5,077,434.1
5500.0
Sales (€)
600,000.0
217,358.5
t año=0
0.60
Costs (€)
t año=1
0.40
Amortisation
0.067
Interest rate
10%
Taxes
0.35
Chapter 8
Figure 8.11 represents the cash flow and the Net Price Value along the temporary
horizon of the recycling plant in scenario 2.
1500000
Cash Flow (€)
Net Cash Flow (€)
1000000
500000
0
Cash Flow (€)
2014
2016
-500000
2018
2020
2022
2024
2026
2028
2030
Temporary horizon
-1000000
-1500000
-2000000
-2500000
-3000000
-3500000
Figure 8.11. Evolution of Cash Flow and Net Cash Flow along the operating time of the
recycling plant in scenario 2.
Next, the most relevant results for the recycling of Polystyrene wastes in scenario 2
are shown in Table 8.16.
Table 8.16. Results from the cash flow analysis for the recycling of PS wastes in scenario 2.
30
9.12
NPV > 0
733,383.8 €
IRR (%)
12.09
Table 8.16 shows that the design process could be also applicable to smaller volume
of production according to the modified scheme, but considering a slightly higher
sale price.
Although the recycling process of Polystyrene by means of an industrial plant or by
recycling trucks might be a challenge, but the subsequent considerations should be
taken into account:

There could be government subsidies and aid to promote the recycling of the
polymer.
279

Any management costs were considered, which could decrease the costs of
the process.

Future bans concerning the use of Polystyrene are expected.
According to the mentioned points, the recycling of Polystyrene wastes by
dissolution in terpenes and CO2 at high pressure for the production of microcellular
foams is stated as a promising technology. Thus, the studied recycling process is
presented as environmentally and economically advantageous since it is valuable
even though the aims and management profits were not considered in the
economical analysis. In summary, the recycling of Polystyrene wastes for the
production of microcellular foams could be positioned as very valuable in an
immediate future.
Finally, the analysis of SWOT matrix (Table 8.17) will be a helpful tool to determine
and clarify the real interest of a process, discarding non interesting ideas for
industrial applications.
Table 8.17. SWOT matrix for the recycling of Polystyrene by dissolution and supercritical
CO2 to produce microcellular foams.
Strengths
Environment friendly
Clean facilities
Recycling that reduces overall volume as
well as generates profits
Low resource consumption
Cheap raw material
Previous experience on scCO2
Easy scale-up and transport of the plant
Favourable access to distribution
network
Established organizational structure
Opportunities
Employment opportunities
New technology
No competence on recycling XPS/EPS
with high quality
New regulation about waste
management
Social conscience about responsible
consumption
Unfulfilled customer need
Strategic positioning
Trade Monopoly
Weaknesses
Finite storage facilities
Poor participation by resident in
materials collection
High cost plant
High cost process
No previous experience on trade
Only one scale-up plant
Weak brand name
Trained and qualified staff
Threats
High capital requirements
New competitors
Emergence of similar products or
technology
Social and economical instability
Low level of financial resources
As can be seen in Table 8.17, the analysis of opportunities and strengths confirmed
the interest of the microcellular foams process. Nowadays, the environmental
280
Chapter 8
awareness is increasing but the consumption of Polystyrene does not decrease,
which assure a potential market of the mentioned polymer products. Furthermore,
the wide knowledge of the system provides an important versatility respect to the
target products, which could change attending the necessity of the markets. It
means, not only microcellular foams can be produced, but also, higher concentration
solutions without PS ageing, or in a future, microparticles and foams.
From the analysis of the threats and weaknesses of Table 8.17, the high cost of the
new technology could be overcome if customers are able to differentiate the new
product from regular Polystyrene, which should be focused on highlighting the
environmentally benefits of the new products.
281
References
[1] J. Cleary, Life cycle assessments of municipal solid waste management systems: A
comparative analysis of selected peer-reviewed literature, Environment
International, 35 (2009) 1256-1266.
[2] B. Hannon, SYSTEM ENERGY AND RECYLING: A STUDY OF THE
CONTAINER INDUSTRY, ASME Pap, (1972).
[3] A.L. Craighill, J.C. Powell, Lifecycle assessment and economic evaluation of
recycling: A case study, Resources, Conservation and Recycling, 17 (1996) 75-96.
[4] A. Azapagic, A. Emsley, I. Hamerton, Polymers: The Environment and
Sustainable Development, Wiley, Guildford, UK, 2003.
[5] P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, 1953.
[6] J. Schellenberg, Syndiotactic Polystyrene: Synthesis, Characterization,
Processing, and Applications, Wiley, 2009.
[7] D.S. Achilias, I. Kanellopoulou, P. Megalokonomos, E. Antonakou, A.A. Lappas,
Chemical recycling of polystyrene by pyrolysis: Potential use of the liquid product for
the reproduction of polymer, Macromolecular Materials and Engineering, 292 (2007)
923-934.
[8] F. Pinto, P. Costa, I. Gulyurtlu, I. Cabrita, Pyrolysis of plastic wastes. 1. Effect of
plastic waste composition on product yield, Journal of Analytical and Applied
Pyrolysis, 51 (1999) 39-55.
[9] M. Guaita, THERMAL DEGRADATION OF POLYSTYRENE, British Polymer
Journal, 18 (1985) 226-230.
[10] A. Karaduman, E.H. Imşek, B. Çiçek, A.Y. Bilgesü, Thermal degradation of
polystyrene wastes in various solvents, Journal of Analytical and Applied Pyrolysis,
62 (2002) 273-280.
[11] T. Noguchi, H. Tomita, K. Satake, H. Watanabe, A new recycling system for
expanded polystyrene using a natural solvent. Part 3. Life cycle assessment,
Packaging Technology and Science, 11 (1998) 39-44.
[12] R.B.H. Tan, H.H. Khoo, Life cycle assessment of EPS and CPB inserts: Design
considerations and end of life scenarios, Journal of Environmental Management, 74
(2005) 195-205.
[13] S. Madival, R. Auras, S.P. Singh, R. Narayan, Assessment of the environmental
profile of PLA, PET and PS clamshell containers using LCA methodology, Journal of
Cleaner Production, 17 (2009) 1183-1194.
[14] P. Watermeyer, Handbook for Process Plant Project Engineers, John Wiley &
Sons, 2002.
[15] M. Perrut, Supercritical fluid applications: Industrial developments and
economic issues, Industrial and Engineering Chemistry Research, 39 (2000) 45314535.
[16] M.P. Fernández-Ronco, A. De Lucas, J.F. Rodríguez, M.T. García, I. Gracia,
New considerations in the economic evaluation of supercritical processes: Separation
of bioactive compounds from multicomponent mixtures, Journal of Supercritical
Fluids, 79 (2013) 345-355.
[17] ANAPE, http://www.anape.es/ in, Asociación Nacional del Poliestireno
Expandido, 2014.
[18] G.A. Núñez, C.A. Gelmi, J.M. del Valle, Simulation of a supercritical carbon
dioxide extraction plant with three extraction vessels, Computers and Chemical
Engineering, 35 (2011) 2687-2695.
282
Chapter 8
[19] M. Peters, K. Timmerhaus, R. West, Plant Design and Economics for Chemical
Engineers, McGraw-Hill Education, 2003.
[20] K. Hillstrom, L.C. Hillstrom, Encyclopedia of Small Business: A-I, Gale Group,
2002.
[21] INE, http://www.ine.es/calcula/index.do?L=1, in, Instituto Nacional de
Estadística, 2014, pp. Update a personal income or spending with the overall CPI.
[22] R.H. Perry, D.O.N.W.A. GREEN, J.O.H. Maloney, Perry's Chemical Engineers'
Handbook, McGraw-Hill Professional Publishing, 1997.
[23] M.d. Fomento, www.fomento.gob.es, in, Madrid, 2014, pp. Observatorio de
mercado del transporte de mercancías por carretera.
[24]
Eurstat,
Electricity
prices
for
industrial
consumers,
in,
http://epp.eurostat.ec.europa.eu/tgm/table.do, 2013.
283
284
Chapter 9
Conclusions
and Future
work
Chapter 7 shows the foaming process of PS from its homogeneous solutions in
terpene oils using CO2 as antisolvent and blowing agent. Once the phases’
equilibrium was studied and the effect of CO2 on the shift of physical properties was
measured, the design of a foaming process was carried out. Initially, the feasibility
of the foaming was determined on the selection of the vanishing points of the
solutions in CO2. Next, a further study concerning the optimization of the process in
order to achieve microcellular foams from PS/Limonene solutions.
Conclusions and Future Work
286
Chapter 9
Fruto de los resultados obtenidos en esta investigación, se pueden concluir que:

La obtención de productos de alto valor añadido, a partir de residuos de
Poliestireno, mediante disolución en aceites naturales y separación con CO 2
a alta presión es viable.

El proceso de disolución de residuos de Poliestireno en aceites terpénicos
como paso inicial del proceso de reciclado es ventajoso, ya que el residuo
presenta una elevada solubilidad en los disolventes mencionados, que se
incrementa con la temperatura, sin presentar signos de degradación. Los
resultados son extrapolables a residuos reales de Poliestireno Extruido y
Expandido.

La introducción de CO2 a alta presión en el sistema Poliestireno/aceite
terpénico, provoca dos efectos principales. Por un lado, los terpenos
presentan una elevada solubilidad en el CO2, en condiciones moderadas de
presión y temperatura, por lo que son eliminados con relativa facilidad. Por
otro lado, el CO2 se adsorbe en el polímero, provocando su plastificación y
por lo tanto, facilitando su procesado.

El proceso de separación de la disolución Poliestireno/aceite terpénico
utilizando CO2 como antisolvente a alta presión, es técnicamente viable.

La precipitación del polímero a partir de su disolución en terpenos,
adicionando CO2 como antisolvente, de acuerdo con una estructura
predefinida (micropartículas, fibras, espumas…) resulta muy compleja. Es
necesario conocer el equilibrio de las mezclas ternarias, sus propiedades y la
hidrodinámica del sistema.

El proceso de espumación de disoluciones Poliestireno/aceite terpénico
permite la separación del polímero adoptando una estructura microcelular.
Las condiciones más adecuadas para llevar a cabo el proceso y conseguir
espumas con un menor tamaño de poro y una distribución homogénea de los
mismos, fueron alta presión (90 bar), baja temperatura (30 ºC), baja
concentración (0.05 g PS/ml terpeno), elevado tiempo de contacto y
despresurización (4 horas y 30 minutos, respectivamente).

A partir del análisis económico y del ciclo de vida, se confirma la viabilidad
del proceso de reciclaje de residuos de Poliestireno mediante disolución y
CO2 a alta presión.
Además de las conclusiones generales, los resultados más reseñables por Capítulos
son:
Capítulo 4:
i) La estimación de la solubilidad del Poliestireno (PS) en disolventes de la familia
de los terpenos a partir de métodos teóricos es una buena alternativa frente a las
287
tradicionales determinaciones experimentales. Así, se puede estudiar el efecto de la
temperatura y el peso molecular sobre la solubilidad del polímero.
ii) Los aceites terpénicos son totalmente solubles en CO 2 en condiciones
moderadas de temperatura (25- 40 ºC) a presiones cercanas al punto crítico del gas,
lo que permitiría su separación de las mezclas Poliestireno/aceite terpénico.
iii) El CO2 se absorbe entre las cadenas poliméricas del Poliestireno causando el
hinchamiento y la plastificación del polímero. Este fenómeno se observa a través de
la modificación de sus propiedades a nivel macromolecular: descenso de la
temperatura de transición vítrea (Tg), viscosidad () y tensión interfacial ().
Capítulo 5:
iv) El equilibrio de las mezclas ternarias formadas por CO 2/aceite
terpénico/Poliestireno está principalmente determinado por la presión, la
temperatura y la concentración de polímero en la disolución inicial. Un aumento de
presión, favorece tanto la solubilidad del terpeno en la fase rica en CO2, como la
adsorción del CO2 en la disolución. El incremento de la temperatura promueve un
aumento de la solubilidad de los aceites terpénicos en CO 2, y también favorece la
solubilidad del Poliestireno en el disolvente, por lo que es necesario estudiar cuál de
los efectos es más relevante. Por último, cuando aumenta la concentración del
polímero en la disolución, la solubilidad del aceite terpénico y la absorción del CO 2
disminuyen.
Capítulo 6:
v)
El efecto de la presión sobre la viscosidad de las mezclas CO2/aceite
terpénico/Poliestireno mostró que no sólo hay que tener en cuenta el efecto
plastificante causado por el CO2, sino también el efecto de compresión que ejerce
éste contra la disolución polimérica. El descenso de la viscosidad de las mezclas
ternarias como resultado del incremento de la temperatura, sigue la ley de
Arrhenius, a partir de cuyos resultados se puede calcular la energía de activación.
Por último, el efecto más relevante que causa el aumento de la viscosidad es la
adición de polímero a la disolución inicial.
vi) La tensión interfacial de las disoluciones Poliestireno/terpeno disminuye
linealmente cuando aumenta la presión de CO2 sobre la mezcla ternaria. Sin
embargo, el efecto de la temperatura y la concentración no fueron muy significativos
en el rango de estudio.
vii) La temperatura de transición vítrea de las mezclas ternarias se ve
principalmente afectada por el efecto plastificante del CO2 más que por la presencia
del disolvente (terpeno).
Capítulo 7:
viii) La morfología del Poliestireno precipitado a partir de su disolución en aceites
terpénicos y utilizando CO2 como antisolvente, depende principalmente de la
288
Chapter 9
transferencia de materia. A partir del cálculo de los coeficientes de difusividad y de
la aplicación de la segunda ley de Fick, se puede conocer el perfil de concentraciones
del CO2 y del terpeno en el interior de una gota de disolución.
ix) La modificación de las propiedades del polímero en disolución favorecen la
nucleación y el crecimiento de celdas, cuando se ponen en contacto CO2 y una
disolución PS/aceite terpénico. Por este motivo, el proceso de espumación de
disoluciones poliméricas permite la producción de espumas microcelulares de una
manera relativamente sencilla.
Capítulo 8:
x) Las distintas alternativas de reciclaje de residuos de Poliestireno presentan
ventajas medioambientales respecto al resto de técnicas de gestión de los mismos.
En concreto, el reciclaje de residuos de Poliestireno mediante disolución en aceites
terpénicos y espumación utilizando CO2 a alta presión, se posiciona como una de las
mejores opciones para el tratamiento de residuos poliméricos, por ser
medioambientalmente sostenible.
xi) La evaluación económica del proceso estudiado, confirma que el reciclaje de
residuos de Poliestireno para la producción de espumas microcelulares presenta
mayores ventajas si el CO2 y el disolvente terpénico son recirculados en el proceso y
el volumen de la planta de tratamiento es superior a 7000 kg/día.
289
From the present research about the following main conclusions can be concluded:

The production of high-added value products from the recycling of
Polystyrene wastes using natural terpene oils and high pressure CO 2 is
feasible.

The dissolution of Polystyrene wastes in terpene oils is the first step for the
recycling since the polymer is highly soluble in terpenoids and its solubility
is enhanced by the increase of temperature. During the dissolution process
at mild temperatures, any degradation was observed. The results concerning
solubility can be applied to real wastes, i.e. Extruded or Expanded
Polystyrene.

The addition of high pressure CO2 in the dissolution Polystyrene/terpene oils
cause two effects. On one hand, the easy removal of terpenes from the
mixture due to their high solubility in CO2 and could be easily removed. On
the other hand, the plasticization of the polymer due to the CO 2 sorption
which enhance the processing.

The separation process of the Polystyrene/terpene oils using high-pressure
CO2 as antisolvent is technically feasible.

The precipitation of the polymer according to a previously selected
morphology (microparticles, fibres, foams…) is not so evident. Equilibrium
and hydrodynamics considerations are necessary to be studied.

The foaming of Polystyrene/terpene oil solutions using CO2 is an easy
process which allows the recovery of the polymer according to microcellular
structures. The most suitable conditions to achieve foams with smaller, well
defined and homogeneous cells are high pressure (90 bar), low temperature
(30 ºC), low concentration (0.05 gPS/ml terpene), high contact time (4 hours)
and high depressurization time (30 min).

The life cycle and economic assessment showed excellent results concerning
the feasibility of the explained recycling process for the Polystyrene wastes.
In addition to the aforementioned conclusions, the most relevant results from the
different chapters are:
Chapter 4:
i)
The solubility of Polystyrene in terpene oils prediction is an acceptable
alternative versus the traditional experimental measurements. The influence of the
temperature and the polymer molecular weight on the solubility are studied.
ii) Terpene oils are fully soluble in CO2 at mild temperatures (25- 40 ºC) and
pressure close to the critical point of the gas, which allows the separation of binary
mixtures composed by Polystyrene and terpene oils.
290
Chapter 9
iii) CO2 is absorbed among the polymer chains of Polystyrene causing its swelling
and plasticisation. This fact is observed through the decrease of glass transition
temperature (Tg), viscosity () and interfacial tension ().
Chapter 5:
iv) The equilibrium of ternary mixtures containing CO2/terpene oil/Polystyrene is
mainly affected by pressure, temperature and concentration of polymer in the
solution. When pressure increases, the solubility of terpenes in CO2 and the sorption
of CO2 in the dissolution are enhanced. An increase of temperature enhances the
solubility of terpenes in CO2, but also, promotes the solubility of Polystyrene in
terpenes. Finally, when the concentration of polymer in the initial solution
increases, the solubility of the terpenoids and the sorption of CO 2 decreases.
Chapter 6:
v) The effect of pressure on the viscosity of CO2/terpene oil/Polystyrene showed
that not only plasticisation of solutions should be considered, but also the
compression effect of the gas on the solutions. The decrease of viscosity due to the
increase of temperature follows an Arrhenius-type equation and activation energy
can be calculated. The most relevant effect on the increase of viscosity of the ternary
mixtures is caused by the increase of polymer concentration.
vi) Interfacial tension of Polystyrene/terpene oil solutions decreases linearly when
pressure of CO2 in the ternary mixtures CO2/terpene oil/Polystyrene increases. The
effect of temperature and concentration were not very significant in the studied
range.
vii) The glass transition temperature of the ternary mixture is mainly affected by
the plasticising effect of CO2 instead of the presence of a solvent (terpene oil).
Chapter 7:
viii) The morphology of the recovered Polystyrene, from its solution in terpenes and
using CO2 as antisolvent, is mainly dependant on the mass transfer. From the
knowledge of the mutual diffusion coefficients and the application of Fick’s second
law, the concentration profile was calculated.
ix) The modification of the polymer properties enhances the nucleation and the cell
growth during the foaming process. Thus, microcellular foams are easily produced.
Chapter 8:
x) Among the different alternatives for the plastic wastes management, the
recycling was risen as the most environmentally friendly. Particularly, the recycling
of Polystyrene wastes by dissolution in natural terpene oils and foaming using CO2
291
as blowing agent is arranged as one of the best options for the polymer wastes
treatment.
xi) The economic evaluation of the explained process confirm that the recycling of
Polystyrene wastes for the production of microcellular foams takes more advantages
if the CO2 and the terpenes are recirculated when the treated wastes are higher
than 7000 kg/day.
292
Chapter 9
A partir de las conclusiones obtenidas en este trabajo de investigación, se
recomienda:
i)
Desarrollar un modelo teórico completo y fiable que permita la predicción del
comportamiento de mezclas ternarias a alta presión con el fin de ampliar el proceso
desarrollado y aplicarlo a otros polímeros y disolventes.
ii) Determinar las isopletas del sistema ternario y la influencia de la concentración
de CO2 en la separación de las mezclas, lo que permitiría hacer una selección inicial
de las condiciones más adecuadas para precipitar el polímero.
iii) Llevar a cabo un estudio pormenorizado de la influencia de las condiciones
hidrodinámicas en la precipitación de disoluciones formadas por Poliestireno y
aceites terpénicos.
iv) Diseñar y construir una instalación experimental para conseguir la
precipitación de micropartículas de Poliestireno a partir de su disolución en
terpenos y utilizando CO2 como antisolvente.
v) Incorporar sustancias de alto valor añadido a las espumas microcelulares para
aumentar la utilidad del producto final.
vi) Estudiar el proceso de reciclaje de residuos plásticos mediante disolución y CO2
a alta presión.
vii) Mezclar los residuos de Poliestireno con otros con un importante volumen de
producción, como la celulosa.
viii) Optimizar las condiciones de operación de la planta piloto con el fin de
adaptarse a criterios económicos y a las demandas del mercado.
293
After the conclusions obtained in this research work, several guidelines for future
work can be proffered:
i)
To develop a complete and reliable theoretical model for the prediction of the
behaviour of ternary mixtures at high pressure in order to enlarge the application of
the process to new polymers and solvents.
ii) To determine the isoplets and the influence of CO2 concentration on the
separation of ternary mixtures for a first screening of the most suitable conditions to
precipitate the polymer.
iii) To carry out a detailed study about the influence of hydrodynamic on the
precipitation of Polystyrene/Terpene oils solutions.
iv) To design and build an experimental setup for the efficient precipitation of
Polystyrene microparticles from its solution in terpenes using CO2 as antisolvent.
v) To load the microcellular foams with high-added value to increase its value.
vi) To study the recycling of new plastic wastes by dissolution and high-pressure
CO2.
vii) To blend the Polystyrene with traditional wastes (cellulose).
viii) To optimize the pilot plant operational conditions on the basis of economic
criteria and market trends.
294
NOMENCLATURE
Abbreviations:
AAD: Average Absolute Deviation
ABS: Poly (Acrylonitrile-co-butadiene-co-styrene)
ANOVA: Analysis of Variance
aPS: atactic Polystyrene
ASES: Aerosol Solvent Extraction System
ASD: Average Standard Deviation
BPR: Back-Pressure Regulator
CCD: Charged Coupled Device
CFCs: Chlorofluorocarbons
CNT: Classical Nucleation Theory
CPI: Consumer Price Index
DOE: Design of Experiments
DSA: Drop Shape Analysis
DSC: Differential Scanning Calorimetry
DTGA: Differential Thermogravimetric Analysis
EoS: Equation of State
EPA: Environmental Protection Association
EPS: Expanded Polystyrene
FFEs: Fractional Factorial Experiments
GAS: Gas Antisolvent
GPC: Gel Permeation Chromatography
HCFC: Hydrochlorofluorocarbon
HDPE: High Density Polyethylene
IFT: Interfacial Tension (mN/m)
IRR: Internal Rate of Return
LCA: Life Cycle Assessment
LDPE: Low Density Polyethylene
MHS: Mark Houwink and Sakurada empirical relationship
295
MW: molecular weight (g/mol)
NPV: Net Price Value
PC: Polycarbonate
PCA: Precipitation with a Compressed Antisolvent
PDI: Polydispersity Index
PET: Polyethylene Terphthalate
PI: Pressure indicator (manometer)
PR-EoS: Peng-Robinson Equation of State
PP: Polypropylene
PS: Polystyrene
PU: Polyurethane
PVC: PolyVinyl Chloride
RCS: Refrigerated Cooling Systems
RESS: Rapid Expansion of Supercritical Solution
SAS: Supercritical Antisolvent
scCO2: supercritical CO2
SCF: Supercritical Fluid
SEDS: Solution Enhanced Dispersion by Supercritical Fluids
SEM: Scan Electron Microscope
SL-EoS: Sánchez-Lacombe Equation of State
sPS: syndiotactic Polystyrene
Std. Dev.: Standard Deviation
SWOT: Strenghts, Weakness, Opportunities and Threats Matrix
TGA: Thermogravimetric Analysis
THF: Tetrahydrofuran
TIC: Temperature Indicator Controller
VLE: Vapour-Liquid Equilibrium
XPS: Extruded Polystyrene
296
Symbols:
a: attractive forces between the molecules in Peng-Robinson Equation of State
a: polymer chain rigidity applied to Mark-Houwink-Sakurada
A: area of the micrograph (cm2)
b: molecules size
B: dimensionless factor depending on the viscosity of the material used in the
Classical Nucleation Theory
C’H: saturation of the cavities (g CO2/ g PS)
C: concentration (g Polystyrene/ml terpene)
C0: initial concentration (g/ml)
Cpp: excess transition isobaric specific heat of the pure polymer (J/g·K)
D: hydraulic diameter (m)
Dab: diffusion coefficient of substance a in b (m2/s)
erf: Gaussian error function
Ea: flow activation energy (J/mol)
g: acceleration of gravity (m2/s)
Gm: Gibbs free energy (J/mol)
H: Henry's law constant (g CO2/ bar·g PS)
Hm: enthalpy of mixing (J/mol)
Jss: number of critical nuclei formed during the Classical Nucleation Theory
kB: Boltzmann’s constant (J/K)
kH: analogous to Henry’s law used in Dual-Mode sorption model
kij: coupling or binary interaction parameter of components i and j
m: mass of the gas molecule in the Classical Nucleation Theory (g/mol)
M: magnification factor
Md: molar mass of the dissolved gas (g/mol)
Mm: molar mass of the monomeric unit (g/mol)
Mo: Morton dimensionless number
n: number of cells
N: number of gas molecules per unit volume (number of molecules/cm3)
Nhom: rate of homogeneous nucleation (cells/s)
297
p: critical level of significance in the Analysis of Variance
P: pressure (bar)
reduced pressure
P*: characteristic pressure (bar)
Pc: critical pressure (bar)
PR: reduced pressure
Pv: vapour pressure (bar)
P: difference between the pressure of the metastable and nucleating phase used in
the Classical Nucleation Theory (bar)
Q: flow (m3/s)
r: size parameter used in Peng-Robinson Equation of State
r: radius of the droplet used in Fick’s second law (m)
R: ideal gas constant (J/K·mol)
Re: Reynolds dimensionless number
S: solubility (kg/m3)
tcontact: contact time during foaming process (min)
tdep: depressurization time during foaming process (min)
T: temperature (ºC)
: reduced temperature
T*: characteristic temperature (ºC)
Tc: critical temperature (ºC)
Tb: boiling temperature (ºC)
Tdeg: degradation temperature (ºC)
Tg: glass transition temperature (ºC)
Tg,0: glass transition temperature of the pure polymer at atmospheric pressure (ºC)
Tm: melting temperature (ºC)
: velocity of the liquid jet (m/s)
V: molar volume (cm3/mol)
xi: mole fraction of the component i in the liquid rich phase
W: reversible work of critical nucleus formation (J)
wi: mass fraction of the component i
298
wCO2Cym: total amount of CO2 retained in p-Cymene (g)
wCO2PS: total amount of CO2 retained in the Polystyrene (g)
wCO2sol: total amount of CO2 retained in the solution (g)
yi: mole fraction of the component i in the vapour rich phase
z: lattice coordination number

12: binary interaction parameter of terpene/polymer systems
 surface tension (N/m)
d : solubility parameter from dispersion forces (MPa1/2)
h: solubility parameters due to the hydrogen bonds between molecules (MPa1/2)
i: global solubility parameter of the substance i (MPa1/2)
p: solubility parameter from dipolar intermolecular forces (MPa1/2)
 mer interaction energy
: viscosity (Pa·s)
R (t): relative viscosity time dependant (Pa·s)
(0): viscosity at atmospheric pressure(Pa·s)
(t): viscosity at time t (Pa·s)
(): viscosity at saturation (Pa·s)
c: viscosity of the surrounding fluid (Pa·s)
l: viscosity of the liquid (Pa·s)
sol: viscosity of the PS/Limonene/CO2 solution (Pa·s)
solvent: viscosity of the solvent (Pa·s)
rel: relative viscosity between the solution and the solvent
Ki: partition or distribution coefficients
i: volume fraction of the substance i
: density (kg/m3)
c: density of the continuous phase (kg/m3)
g: density of the gas phase (kg/m3)
l: density of the liquid phase (kg/m3)
: reduced density
299
: characteristic density (kg/m3)
 closest distance allowed between two mers (m)
m: interfacial tension of the mixture PS/Terpene and CO 2 (mN/m)
pol: interfacial tension of the polymer (mN/m)
sol: interfacial tension of the solution of CO2 in PS (mN/m)
v*: characteristic molar volume(cm3/mol)
v0: hole volume (cm3/mol)
12: interaction enthalpy between terpenes and polymers (J/mol)
: acentric factor
Ω∞1: activity coefficient at infinite dilution of component i
300

Universidad de Castilla-La Mancha FACULTAD DE

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