Gelation/fusion behavior of PVC plastisol with a cyclodextrin

European Polymer Journal 48 (2012) 885–895
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European Polymer Journal
journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Gelation/fusion behavior of PVC plastisol with a cyclodextrin derivative and
an anti-migration plasticizer in flexible PVC
Byong Yong Yu, Ah Reum Lee, Seung-Yeop Kwak ⇑
Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151–744, Republic of Korea
i n f o
Article history:
Received 27 October 2011
Received in revised form 4 January 2012
Accepted 5 February 2012
Available online 24 February 2012
Keywords:
PVC plastisol
Plasticizer
Cyclodextrin derivative
Migration inhibitor
Gelation and fusion process
a b s t r a c t
We successfully evaluated the effects of 2,3,6-per-O-benzoyl-b-cyclodextrin (Bz-b-CD) on
the rheological properties of PVC plastisols and the migration behavior of plasticizer from
flexible PVC. Two types of plasticizer, di-isononyl phthalate (DINP) and diisononyl cyclohex-4-ene-1,2-dicarboxylate (Neocizer), along with Bz-b-CD as a migration inhibitor were
mechanically mixed into an emulsion grade PVC resin to prepare plastisols. The presence of
Bz-b-CD was expected to facilitate formation of stable complexes with DINP or Neocizer in
the flexible PVC. It was necessary to determine whether changes in the processing conditions of the PVC plastisol were needed for use in this application. To this end, the viscoelastic properties of the plastisols, including the elastic modulus, G0 , and the viscous modulus,
G00 , were continuously monitored as a function of temperature during the gelation and
fusion processes using rheological analysis techniques. The results showed that complete
gelation was slightly delayed and both moduli (G0 and G00 ) decreased upon addition of
Bz-b-CD to the PVC matrix. FE-SEM images yielded insight into the gelation and fusion
processes. The curing conditions and physical properties of the flexible PVCs containing
Bz-b-CD were optimized, and the influence of Bz-b-CD on the migration of the plasticizers
and the stability of the flexible PVC was studied. The results showed that Bz-b-CD reduced
migration of DINP and Neocizer from the flexible PVC by almost 40% and 25%, respectively,
thereby favorably restricting migration within the flexible PVC.
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction
Plasticizers are additives that can improve the flexibility
and workability of poly(vinyl chloride) (PVC) by forming
secondary bonds (dipole–dipole interactions) with the
polymer chains. The high density of the ends of the plasticizer chains introduces additional free volume into the PVC
matrix [1]. PVC can be plasticized with varying amounts of
plasticizers to form flexible PVC. Phthalate plasticizers,
such as di-isononyl phthalate (DINP) and di-(2-ethylhexyl)
phthalate (DEHP), are common plasticizers that are widely
used in industrial applications [2]. Phthalate plasticizers
weakly interact with PVC chains and may migrate out of
⇑ Corresponding author. Tel.: +82 2 880 8365; fax: +82 2 885 1748.
E-mail address: [email protected] (S.-Y. Kwak).
the plasticized PVC products into the environment, for
example, into the materials that contact the PVC. Health
risk assessments over the past few decades have examined
the health effects associated with phthalate leaching from
PVC [3,4]. Plasticizer leaching can render the materials in
contact with the PVC useless for some applications due
to plasticizer effects on these materials’ mechanical properties or appearance, and the mechanical properties of
the plasticized PVC products themselves can degrade due
to plasticizer loss. Industries have begun to exploit new
alternative non-phthalate plasticizers (e.g., di-isononyl
cyclohexane-1,2-dicarboxylate (DINCH) and diisononyl
cyclohex-4-ene-1,2-dicarboxylate (Neocizer)) to alleviate
legal concerns. Unfortunately, several popular alternative
plasticizers are structurally similar to phthalate plasticizers and, consequently, also have the potential to migrate
and introduce toxic compounds into the environment.
0014-3057/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.eurpolymj.2012.02.008
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The use of nanoscale particles as functional additives
has attracted significant attention in several polymer
material research fields in recent years [5,6]. b-Cyclodextrin (b-CD) has an internal cavity shaped like a truncated
cone of about 0.8 nm deep and 0.60–0.64 nm in diameter.
This cavity possesses a relatively low polarity that can
accommodate guest organic molecules inside, resulting in
inclusion complexes. In particular, the high density of hydroxyl groups on the exterior of b-CD can easily be modified with various functional groups to endow the b-CD
with specific interactions with other molecules [7]. Grant
et al. recently described the effect of b-CD and hydroxypropyl b-CD incorporation into plasticized PVC on leaching of
DEHP and biocompatibility [8]. In previous reports, we successfully prepared a plasticizer with reduced DEHP migration by directly incorporating nanoscale b-CD derivatives
into DEHP. The addition of a b-CD derivative decreased
the levels of DEHP migration from the flexible PVC samples
by almost 40%. The reduction in DEHP migration from the
flexible PVC was due to the formation of p–p stabilized
complexes and inclusion of DEHP molecules by suitably
oriented b-CD derivatives [9]. In this context, and motivated by the technological and scientific value of these
studies, we intensively pursued a rheological study of the
influence of b-CD derivatives during the gelation and
fusion of PVC plastisols. PVC plastisols are suspensions
consisting of an emulsion-type PVC resin in a liquid continuous phase formed mainly by a plasticizer and a thermal
stabilizer. PVC plastisols are used today to produce many
commercially important products [10]. All industrial processes for preparing plastisols involve heating the plastisol
to 150–200 °C. During heating the plastisol undergoes two
processes, gelation and upon cooling the plastisol is transformed into a relatively soft, flexible substance [11]. Additional components, such as b-CD derivatives, can be added
to modify the rheological properties of the PVC plastisol.
From a commercial perspective, it is useful to optimize
plastisol processing conditions for various PVC formulations. A useful method for monitoring gelation and fusion
is to characterize the viscoelastic properties. When combined with field-emission scanning electron microscopy
(FE-SEM), viscoelastic characterization provides a better
understanding of the viscoelastic behavior, such as gelation and fusion of PVC plastisol. The parameters most frequently used to quantify rheological properties are the
elastic (or storage) modulus, G0 , and the viscous (or loss)
modulus, G00 . These properties show distinct changes at
the initiation and termination of plastisol gelation, and at
the temperature at which fusion becomes complete [12].
In this study, viscoelastic measurements permitted continuous monitoring of the changes in the G0 and G00 as a
function of temperature during gelation and fusion. The
rheological analysis quantified the influence of Bz-b-CD
on the gelation and fusion behavior of the PVC plastisols.
A phthalate or a non-phthalate plasticizer, DINP or Neocizer, respectively, along with Bz-b-CD as a migration inhibitor, were mechanically mixed with emulsion-grade PVC
resin to prepare a PVC plastisol. It is important to note that
the non-phthalate plasticizer, Neocizer, was structurally
similar to the most widely used DINP (see Fig. 1). The
influence of Bz-b-CD on the physical properties of the flex-
Fig. 1. Chemical structure of (a) DINP and (b) Neocizer.
ible PVC was experimentally investigated. Migration tests
of the flexible PVC were conducted according to the
International Organization for Standardization (ISO)
3826:1993(E) method to measure the plasticizer migration
behavior. Gas chromatography (GC) equipped with a flame
ionization detector (FID) was employed to identify and
quantify which migrated to the surface of the PVC part
and dissolved in the contacting solvent.
2. Experimental
2.1. Materials
b-Cyclodextrin (b-CD) was obtained from Tokyo Chemical Industry Co., Ltd. and dried in a vacuum oven at 60 °C
for 7 days prior to use. Benzoyl chloride and anhydrous
pyridine were purchased from Sigma–Aldrich (stated purity P99%). Di-isononyl phthalate (DINP) and diisononyl
cyclohex-4-ene-1,2-dicarboxylate (Neocizer) were kindly
provided by Aekyung Petrochemical Co., Ltd., Korea. Emulsion grade poly(vinyl chloride) (PVC) resin (LG PB1752, degree of polymerization: 1700 ± 50 and kwert (k) value: 76)
was provided by LG Chem. Ltd., Korea. Epoxidized soybean
oil (ESO) and a thermal stabilizer (methyl tin, trade name
MT-800) were purchased from Yakuri Pure Chemicals Co.,
Ltd., Japan and Songwon Co., Ltd., Korea, respectively, and
used to prepare the PVC plastisol. All chemicals except bCD were used as received without further purification.
2.2. Preparation of cyclodextrin derivative as a migration
inhibitor
b-CD was modified with benzoyl chloride, resulting in
2,3,6-per-O-benzoyl- b-cyclodextrin (Bz-b-CD), in accordance with the procedures described in the literature
[13]. Purified and dried b-CD (11.35 g, 10 mmol) was stirred into 240 mL anhydrous pyridine, and 160 mL benzoyl
chloride (1.44 mol) was added. The solution was stirred
at 50 °C for 72 h. Termination of the reaction was accompanied by a solution color change from bright pink to orange–
brown, with some precipitate. The mixture was evaporated
at 50 °C under reduced pressure until it reached about half
the volume. The thick solution was cooled in an ice bath,
and 500 mL anhydrous methanol was added very slowly
with stirring. The copious white precipitate was filtered
off, and the crude product was resuspended in methanol.
The white powders in methanol were filtered out and
washed several times with, alternately, distilled water or
methanol. Finally, the product was dried in a vacuum oven
and ground to a fine white powder. The modification of bCD was verified using Fourier-transform infrared (FT-IR)
spectroscopy using a Perkin-Elmer GX IR spectrophotometer with a spectral resolution of 4 cm1 over the range
4000–400 cm1. All samples were prepared by compression molding, and potassium bromide (KBr) powder was
used as the sample matrix and reference material. The degree of substitution of Bz-b-CD was confirmed by 1H nuclear magnetic resonance (NMR) spectroscopy using a Bruker
Avance spectrometer 500 with dimethyl sulfoxide-d6 as
the solvent. The resulting Bz-b-CD particles were incorporated with DINP or Neocizer as follows. To allow for a
quantitative comparison, the mass of the primary plasticizer, either DINP or Neocizer, was 70 g in each sample
and the mass of Bz-b-CD, where used, was 10 g. To the
plasticizer was carefully added dried Bz-b-CD with stirring
at room temperature. The mixtures were sonicated in a
335 W sonicator bath (Bransonic Co., Ltd., Model 3510R)
for 30 min at room temperature until clear, resulting in a
transparent colloidal solution. The particle sizes and size
distributions of Bz-b-CD in the prepared samples were
measured using dynamic light scattering (DLS) methods
with a Photal DLS-7000 spectrophotometer equipped with
a Photal GC-1000 digital auto-correlator (Otsuka Electronics Co., Ltd., Osaka, Japan). In this procedure, the wavelength (k) of the argon (Ar) laser was 488 nm, and the
scattering angle was 90° with respect to the incident beam.
The correlation functions were analyzed using the constrained regularized CONTIN method to determine the distribution decay rates. The experiments were conducted at
room temperature, and each experiment was repeated
two or more times.
2.3. Rheological measurements of the PVC plastisols
containing Bz-b-CD
PVC pastes, also known as plastisols, were prepared
using a dried emulsion PVC resin, DINP or Neocizer, Bz-bCD, epoxidized soybean oil (ESO), and a thermal stabilizer
(TS), as listed in Table 1. The PVC plastisols were prepared
by slowly adding Bz-b-CD and the other additives to the
dry emulsion PVC resin. A mechanical stirrer with a twoblade propeller was then used to further homogenize the
paste. After mixing had been completed, air bubbles in
the plastisols were removed by applying a vacuum. The
plastisols were then aged for 2 weeks at room temperature
prior to use. The aging is necessary because viscosity increases initially primarily because of de-agglomeration
but it stabilizes after 2 weeks [14]. The viscoelastic behavior of the PVC plastisols was examined to investigate the
influence of Bz-b-CD on the properties and to determine
the optimal processing conditions. A parallel-plate rotational rheometer (Rheometer AR2000, TA instruments
Inc.) was used in the dynamic oscillatory mode with a controlled heating rate. The 40 mm diameter parallel-plate
disks were used with a gap setting of 1 mm. The frequency
of oscillation and shear strain amplitude were kept at 1 Hz
and 2.5%, respectively. The measurement temperature was
varied from 25 to 200 °C, with a programmed rate of increase of 5 °C min1. Morphological analysis of the PVC
plastisols at various stages of gelation and fusion was conducted using an FE-SEM, JEOL JEM-6700F, which used secondary electrons with an acceleration voltage of 15 kV. The
FE-SEM samples were prepared by loading a plastisol between the plates of the rheometer and heating the apparatus to the desired temperature without application of shear
strain. The sampling temperatures were chosen based on
the characteristic features of the viscoelastic curves.
2.4. Physical properties of the flexible PVC sheets containing
Bz-b-CD
Flexible PVC samples were fabricated through gelation
and fusion of the PVC plastisols upon heating. The samples
were then cooled to room temperature. The flexible PVC
sheets containing Bz-b-CD were denoted PVC/DINP-CD
and PVC/Neo-CD, respectively. For comparison, PVC/DINP
and PVC/Neo without Bz-b-CD (blank sample) was also
prepared according to the same method (see Fig. S1). The
influence of Bz-b-CD on the physical properties of the flexible PVCs was assessed experimentally. The glass transition
temperatures, Tg, of the flexible PVCs were determined relative to the indium standards using a TA Instrument 2920
differential scanning calorimetry (DSC) system at a heating
rate of 5 °C min1 over the temperature range 80 to
150 °C. The temperature programmed procedure was performed under a stream of nitrogen. As a measure of the
flexibility of the PVC samples, we measured the% elongation at break using tensile tests performed on a LLOYD
LR10K universal testing machine (UTM). The tests were
conducted at a strain rate of 50 mm min1 with a 1 kN
static load cell. The test specimens assumed dumbbell
shapes with a width of 9.5 mm and a thickness of 2 mm,
Table 1
Compositions of the PVC plastisols.
Sample codes
PVC/DINP
PVC/DINP-CD
PVC/Neo
PVC/Neo-CD
a
b
c
Components (phr)a
PVC resin
DINP or Neocizer
ESOb
TSc
Bz-b-CD
100
70
3
2
–
10
–
10
Parts per hundred resin of PVC.
Epoxidized soybean oil used as a secondary plasticizer.
Thermal stabilizer.
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in accordance with the American Standard Testing Method
(ASTM) D-638 [15]. The thermal decomposition studies
were performed over the temperature range 25–600 °C
using the TA Instrument Q500 thermogravimetric analysis
(TGA) system under flowing nitrogen at a scan rate of
10 °C min1. The masses of the flexible PVC samples were
approximately 5–10 mg. In addition, the optical properties
of the flexible PVC sheets with a thickness of 0.40 mm were
measured using haze tests (BYK Gardner Co., Ltd., HazeGard Plus) with a Commission Internationale de l’Eclairage
(CIE) standard illuminant C (320 < k < 780 nm) as the light
source.
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2.5. Migration tests
Migration tests were carried out on the prepared flexible PVC sheets according to the International Organization
for Standardization (ISO) test method 3826:1993(E) [16].
Distilled water and ethanol were mixed to prepare the
extraction solution. The ratio of distilled water to ethanol
was set to 124:100 by the volume ratio, with a density of
0.9374 g/mL at 25 °C by pyknometer. The migrated plasticizers in the extraction solutions were quantitatively analyzed using a Hewlett Packard model 6890 Series II Plus
GC system with a flame ionization detector and a DB-5
capillary column (30 m 0.32 mm I.D. with a film thickness of 0.25 lm). The column was maintained at 80 °C
for 3 min, ramped up to 320 °C with a heating rate of
10 °C min1, and finally maintained for 13 min. The gas
chromatograph was operated in the splitless injection
mode at a temperature of 320 °C. Helium was used as the
carrier gas at a flow rate of 1.8 mL min1. A calibration
curve was constructed by plotting the ratio of the peak
area of several DINP or Neocizer standard solutions as a
function of concentration between 1 and 20 mg/100 mL.
A summary of the detailed condition of GC analysis is given
in Table S1. The flexible PVC sheets were fabricated in sizes
of 30 30 2 (L H D mm3). The PVC sheets (i.e., PVC/
DINP, PVC/DINP-CD, PVC/Neo, and PVC/Neo-CD) were
rinsed with distilled water to remove dust and impurities
from the surface, and then immersed in 100 mL of the
extraction solution. The testing temperature was maintained at 37 ± 1 °C and the samples remained in solution
for 1–7 days without shaking during the tests. The flasks
were then removed from the oven, inverted gently 10
times, and the contents were transferred to a sample cell.
Quantitative analysis of each sample was repeated three
times to construct a calibration curve and twice for the
other samples. The concentrations of the plasticizer that
had migrated into the extraction solution from the flexible
PVC samples were expressed in mg/100 mL.
3. Results and discussion
3.1. Bz-b-CD as a migration inhibitor
Fig. 2 shows three FT-IR spectra of neat b-CD, benzoyl
chloride, and Bz-b-CD. The broad absorption band in the
range of 3000–3600 cm1 displayed by neat b-CD, shown
in Fig. 2(a), corresponded to the stretching vibrations of
the hydroxyl (–OH) group. Several intense bands in the
Fig. 2. FT-IR spectra of (a) b-CD, (b) benzoyl chloride, and (c) Bz-b-CD.
range 1029–1157 cm1 were assigned to primary and secondary C–OH stretches, and to the C–O–C antisymmetric
stretches, respectively [17]. Modification of the neat b-CD
with benzoyl chloride, however, introduced significant
changes to the FT-IR spectrum of Bz-b-CD, as can be seen
in Fig. 2(c). The broad peak corresponding to the hydroxyl
group stretch disappeared and was accompanied by the
appearance of peaks corresponding to the sp2 C–H stretch
at 3075 cm1 and the aromatic C@C at 1603 and
1448 cm1. These bands corresponded to stretches of the
aromatic ring of the benzoyl group. In addition, the conjugated C@O stretches at 1729 cm1 and the C–O stretches in
the range 1000–1300 cm1 resulted from formation of an
ester. Therefore, the Bz-b-CD spectrum clearly indicated
that b-CD had been modified. Fig. 3 shows the 1H NMR
spectra of neat b-CD, benzoyl chloride, and Bz-b-CD. 1H
NMR spectroscopy provided additional evidence for the
modification and indicated the degree of substitution.
The Bz-b -CD spectrum showed that the hydroxyl group
peaks of b-CD disappeared as the peaks attributed to the
protons of the benzoyl group appeared (2,3,6-COC6H5 at
6.9–8.1 ppm), indicating the substitution of the three hydroxyl groups of b-CD. Each proton (3-H triplet at
6.2 ppm, 1-H doublet at 5.6 ppm, 2-H doublet of doublets
at 5.1 ppm, 5-, 6a-, 6b-H, at 4.8–5.0 ppm, and 4-H triplet
at 4.5 ppm, respectively) of the Bz-b-CD was detected,
and the peaks were shifted downfield upon modification
(see Fig. 3(c)). The number of benzoyl groups, x, introduced
to neat b-CD (which included 21 hydroxyl groups) was easily calculated from the relative peak integrals in the 1H
NMR spectrum. The value of x was determined to be 20.2
(around 96%), which confirmed the successful modification
of neat b-CD to yield Bz-b-CD. Additional discussion of the
modification and general characterization of Bz-b-CD is
provided in our previous publication [9].
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Fig. 3. 1H NMR spectra of (a) b-CD, (b) benzoyl chloride, and (c) Bz-b-CD.
3.2. Dispersion of Bz-b-CD in plasticizers
Several thermodynamic methods can be used to predict
and explain miscibility in multiple phase systems, including the solubility parameter, the equation-of-state, and
the lattice theory [18]. The solubility parameter method
(d (cal/cm3)1/2) was chosen for this study. The solubility
parameters of Bz-b-CD, DINP, and Neocizer can be calculated based on the following equation, d = qRG/M, where
q represents the density, G is the set of group molar attraction constants, and M is the molecular mass of the repeating unit. For simplicity, room temperature was used as the
standard condition. The group molar attraction constants
(commonly Small’s constants) of the specific functional
groups, which are useful in determining the solubility
parameter, have been calculated by Small and Hoy [19].
Compounds with similar solubility parameters (±1.8 (cal/
cm3)1/2) are likely to be moderately miscible [20]. This is
because the energy of mixing two components is balanced
by the energy released by interactions among the pure
components. PVC has a solubility parameter of 9.66 (cal/
cm3)1/2. The calculated solubility parameters for DINP,
Neocizer, and Bz-b-CD were 8.83, 8.72, and 8.01 (cal/
cm3)1/2, respectively. Thus, it was expected that Bz-b-CD
could be well-dispersed in both plasticizers. As shown in
Fig. 4, the quality of the Bz-b-CD dispersion in DINP or
the Neocizer solution was examined by DLS analysis. The
correlation functions were analyzed by means of the constrained regularization method to determine the distribution decay rate. This method analyzes the intensity of
scattered laser light over time, which depends on the particle size. It was very difficult to disperse b-CD in the plasticizers, as predicted by the solubility parameter and
hydrophilicity of b-CD and microscale (around 0.4 lm)
agglomeration of b-CD was observed in both plasticizers.
The DLS results indicated formation of a nanoscale dispersion of Bz-b-CD in both DINP and Neocizer
(d = 2.1 ± 0.6 nm in DINP and 2.3 ± 0.8 nm in Neocizer).
These results showed that, as expected, Bz-b-CD could be
well-dispersed in both plasticizers.
3.3. Rheological behavior of PVC plastisols containing Bz-b-CD
PVC resins obtained by the emulsion polymerization
process are usually used in the preparation of plastisols.
Generally, flexible PVC products are fabricated through a
plastisol by heating briefly to the fusion temperature and
then cooling [21]. As shown in Figs. 5 and 6, G0 and G00 initially decreased as the plastisol was heated from room
temperature. The system behaved as a suspension of
non-interacting PVC particles in the plasticizer, which
formed a continuous phase with a viscosity that decreased
with increasing temperature [22]. During the later stages
of heating, the PVC particles dissolved into the plasticizer
from their outer surfaces, which glued the particles together. In this stage, G0 reached a maximum corresponding
to the complete absorption of plasticizer by the PVC particles. When a PVC plastisol is heated, PVC particles swell
with the plasticizer as the plasticizer is absorbed, and a
steep increase in G0 and G00 is observed. This process is
called gelation. Further temperature increases produce
additional swelling and dissolution of more polymers,
and the microcrystallites of the PVC melt and both G0 and
G00 begin to drop off. This process is called fusion [23].
Fig. S2 illustrates the concept of gelation as part of the
complete fusion scheme. The temperature at which fusion
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Fig. 4. Particle size distributions of Bz-b-CD (solid black bar) and neat
b-CD (shaded gray bar) in (a) DINP and (b) Neocizer.
became essentially complete is indicated by the intersection of the moduli (G0 and G00 ). The processability requirements for PVC plastisol processing emphasize the
viscoelastic behavior, such gelation and fusion, of the
plastisol [24]. During gelation and fusion, the elastic and
viscous modulus in the rheological measurements underwent important changes. Fig. 5 shows the changes in G0
and G00 for the PVC/DINP and PVC/DINP-CD plastisols,
respectively, which were recorded as a function of temperature. The viscoelastic behavior obtained from PVC/DINPCD was similar to that of PVC/DINP. The maximum of the
modulus corresponded to the complete gelation and onset
of fusion. In this stage, the PVC particles became swollen as
the plasticizer was taken up, and some portion of the PVC
molecules may have dissolved into the plasticizer from the
surface layers of the particles. Further temperature increases resulted in a decrease in the modulus, indicating
the dominance of fusion and melting of the PVC microcrystallites. The decreased modulus was due to two processes:
(i) the normal temperature-dependent changes were primarily attributable to thermal expansion, and (ii) melting
of the PVC microcrystallites [25]. As can be seen Fig. 5,
complete gelation of the PVC/DINP-CD plastisols occurred
at 158 °C, a gelation temperature that was slightly higher
than that of the PVC/DINP plastisol (150 °C). Gelation of
the PVC plastisols was delayed upon addition of Bz-b-CD
particles, possibly because of the decreased interaction of
the PVC with the plasticizer. In the Neocizer series, the
gelation point of the PVC/Neo-CD plastisol was 169 °C,
slightly higher than the gelation point of the PVC/Neo plastisol (162 °C). Gelation of the PVC plastisol was also
delayed by the addition of Bz-b-CD particles, as shown in
Fig. 6. The formation of entanglements among the PVC
chains may have been blocked by the Bz-b-CD nanoparticles (see Fig. 7(b)). During processing, partial melting of
the crystallites in the primary PVC particles occurred,
allowing the macromolecules to diffuse through the
Fig. 5. Viscoelastic profile of (a) PVC/DINP and (b) PVC/DINP-CD plastisol.
891
Fig. 6. Viscoelastic profile of (a) PVC/Neo and (b) PVC/Neo-CD plastisol.
boundaries and entangle among the other macromolecules, as shown in Fig. 7(a). The melted crystalline components then recrystallized during cooling to form newly
created ordered domains of secondary crystallites [26].
The presence of well-dispersed Bz-b-CD nanoparticles in
the PVC matrix may have disrupted the physical crosslinks
between neighboring PVC molecules, thereby partially
interrupting formation of the 3-D macromolecular
network. The fusion process completed, according to the
viscoelastic data, at almost the same temperature (about
187 °C) as was observed for Figs. 5 and 6, suggesting that
the macromolecular network reached an equilibrium state
at around 187 °C. These results showed that the presence
of Bz-b-CD did not alter the fusion point of the PVC plastisol. As shown in Fig. S3, the viscoelastic behavior of the
Neocizer series was similar to that of the DINP series.
Fig. 7. Formation of molecular entanglement; (a) neat PVC and (b) PVC with Bz-b-CD.
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However, the temperatures of complete gelation differed
between the DINP and Neocizer series. This difference
could be explained in terms of the compatibility of the
plasticizers and PVC. The solubility parameter and the
polarity parameter, /, can predict the compatibility. The
polarity parameter can be obtained from the equation /
= (Ap/Po)M/1000, where Ap is the number of carbon atoms
in the molecule, excluding aromatic and carboxylic carbon
atoms, Po is the number of polar groups, and M is the
molecular weight. The factor 1000 is used to conveniently
rescale / [27]. Small polarity parameters for the plasticizer
and similar solubility parameters for the plasticizer and
PVC predict good PVC/plasticizer compatibility [28]. The
calculated solubility parameters and polarity parameter
were 8.83 (cal/cm3)1/2, 3.78 for DINP and 8.72 (cal/cm3)1/2,
5.07 for Neocizer. The relative values predicted that DINP
was more compatible with PVC due to the presence of the
aromatic ring. The aliphatic ring in Neocizer decreased
the compatibility with PVC. Therefore, the absorption of
DINP reached completion faster than that of Neocizer, and
the gelation and fusion processes of the DINP series appeared at lower temperatures (12 °C lower) as a consequence of the higher solvent power of the plasticizer. The
morphologies of the PVC plastisols at various stages of gelation and fusion were examined by FE-SEM to investigate
the structural changes associated with the PVC/DINP and
PVC/Neocizer interactions, and the morphologies provided
a qualitative analysis of the gelation and fusion processes.
These results were compared with and used to interpret
the changes in the viscoelastic behavior. Starting from a
two-phase system comprising solid particles dispersed in
the liquid, the plastisol transitioned into a one-phase rubbery solid though gelation and fusion. Figs. S4 and S5 show
SEM images of the PVC/DINP-CD and PVC/Neo-CD system.
The images show a magnification of 5000. At 70 °C, the
PVC particles were clearly identifiable, and the presence
of agglomerates was apparent. At the complete gelation
temperature, 158 or 169 °C for the PVC/DINP-CD or PVC/
Neo-CD systems, respectively, few particles were identifiable, and interparticle boundaries were obscured by entanglement. At 170 °C, fusion occurred, and the particulate
morphology was almost absent. Finally, at 187 °C, fusion
had completed, and recrystallization followed, upon cooling, to form a 3-D structure held together by crystallites
and elastomeric molecules. The fracture surface was continuous, and no domain boundaries could be identified.
During gelation and fusion, the PVC plastisols changed from
PVC particles in DINP or Neocizer to a uniform mass. Overall, the disappearance of the PVC particulate boundaries increased the homogeneity of the PVC plastisols.
3.4. Physical properties of the flexible PVC
Fig. 8 shows the DSC thermograms of the flexible PVC.
The glass transition temperatures, Tg, determined during
the second runs, corresponded to the mid-points of the
small endothermic rises in the pre- and post-transition
baselines. A single Tg for each of the flexible PVCs studied
supported the miscibility among PVC, the plasticizers,
and Bz-b-CD. The flexible PVC samples containing
Bz-b-CD exhibited slightly higher Tg values in comparison
Fig. 8. DSC curves for non-plasticized PVC and flexible PVC samples.
with those of pure flexible PVCs. The addition of plasticizers to a PVC resin increased the free volume of the PVC,
thereby lowering the PVC Tg. The flexibility of PVC is a
main reason why these materials are used so widely. The
percent plasticization efficiency, EDTg, can be calculated
according to the following equation:
EDT g ¼
DT g;PVC=plasticizerCD
100
DT g;PVC=plasticizer
where DTg is the reduction in Tg. The calculated EDTg values
are listed in Table 2 with the glass transition temperatures
of the flexible PVC samples. The plasticizing efficiency of
PVC/DINP-CD was found to be comparable to that of PVC/
DINP, with the EDTg value of PVC/DINP-CD reaching up to
97.8%. EDTg of the PVC/Neo-CD was also found to be comparable to that of PVC/Neo, differing by only 1.7%. These
materials, therefore, were completely flexible at room temperature. Fig. 9 shows the stress–strain curves of the PVC/
DINP and PVC/DINP-CD samples incorporating Bz-b-CD.
These results are typical of soft tough materials. As shown
in Fig. 9, although all samples behaved similarly, the ultimate strength of the stress and elongation at the break
slightly decreased as the Bz-b-CD content in the flexible
PVC sheets increased. Therefore, Bz-b-CD was well-dispersed in the PVC matrix without forming agglomerates,
which act as significant defects. Bz-b-CD apparently interacted with the polar groups of the PVC chains and impeded
the motions of the PVC chains. The plasticization efficiencies, EDeb, were estimated by comparing EDeb due to
DINP-CD and Neo-CD with those of DINP and Neocizer:
EDeb ¼
Deb;PVC=plasticizerCD
100
Deb;PVC=plasticizer
The eb values of the flexible PVC samples and the calculated EDeb values are also presented in Table 2, and the
trends differ only slightly from those observed for the EDTg
data. The transmittance and haze of the flexible PVC sheets
containing Bz-b-CD were investigated and compared with
their counterparts, PVC/DINP and PVC/Neo. In general,
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B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895
Table 2
Glass transition temperatures, ultimate elongation, and percent plasticization efficiency.
c
d
PVC
PVC/DINP
PVC/DINP-CD
PVC/Neo
PVC/Neo-CD
Tg (oC)a
EDTg (%)b
eb (%)c
EDeb (%)d
85.6
0
163.2
0
47.3
100.0
767.6
100.0
44.4
97.8
710.1
90.5
40.9
100.0
657.3
100.0
38.8
98.3
622.7
92.9
Glass transition temperatures.
Percent plasticization efficiencies estimated from the lowering of the glass transition temperatures.
Ultimate elongation.
Percent plasticization efficiencies estimated from the improving of ultimate elongation.
Fig. 9. Tensile stress–strain curves for the flexible PVC sheets.
agglomerates can seriously deteriorate the clarity of PVC
sheets. As shown in Fig. 10, no deterioration in the transmittance of the PVC sheets was observed upon addition
of Bz-b-CD nanoparticles. Bz-b-CD was endowed with
hydrophobic benzoyl groups that limited formation of
agglomerates among Bz-b-CD particles in PVC/DINP-CD
and PVC/Neo-CD. In other words, Bz-b-CD was welldispersed in the plasticized PVC matrix on the nanoscale.
However, the dispersed Bz-b-CD particles slightly increased the haze of the sheets because the tiny Bz-b-CD
particles scattered light in the plasticized PVC matrix.
The thermal stability of each PVC sheet was analyzed by
TGA, and the results are shown in Fig. S6. All sample curves
showed similar trends, and each curve presented two distinct stability stages corresponding to a first weight loss of
about 75%, between 210 and 320 °C, and a second weight
loss of about 15% between 410 and 470 °C. The analysis results suggest that the first weight loss corresponds to the
elimination of HCl with some benzene traces. The second
stage corresponds to the polyacetylene sequences formed
by elimination of HCl from adjacent carbon atoms during
the first stage [29]. At the temperature above 470 °C, the
weight is almost unchanged. These results confirm that
the thermal stability of the PVC/DINP-CD and PVC/NeoCD is similar to that of the PVC/DINP and PVC/Neo samples.
3.5. Anti-migration of the plasticizer in flexible PVC
Two specific types of plasticizer, DINP and Neocizer, included alkyl chains comprising a mixture of C8–C10 chain
Fig. 10. Transmittance (striped bar) and haze (shaded bar) of the flexible
PVC sheets.
isomers, and the largest component of the mixture
included C9 chains [30]. As seen in Fig. S7, the mixture of
isomeric alkyl chains introduced several peaks into the
GC–FID chromatogram. A calibration curve was constructed by plotting the sum of the GC–FID peak areas corresponding to the DINP and Neocizer plasticizers relative
to the peak area of an internal standard for plasticizer concentrations of 1, 2, 5 and 10 mg/100 mL (shown in Fig. S8).
The plasticizer response was linear, with a correlation coefficient, r, of r > 0.9997 for DINP and r > 0.9995 for Neocizer.
The migration behavior of the plasticizers from the flexible
PVC containing Bz-b-CD was examined by extracting into a
mixed water/ethanol solution at 37 °C. The plasticizers that
migrated during extraction of each PVC sample over 1, 3, 5,
and 7 days were detected by GC–FID, and the sum of the
peak areas was compared with the calibration curve to
determine the plasticizer concentration. The total amount
of the migrated plasticizers as a function of time is illustrated in Fig. 11. The figure shows that plasticizer migration dramatically increased during the initial stages, and
the rate of migrated plasticizer extraction slowed remarkably after 24 h. After a seven day extraction, the concentration of the migrated plasticizer extracted from all
plasticized PVC sheets incorporating Bz-b-CD was lower
than that extracted from the neat plasticized PVC sheet,
as listed in Table 3. The plasticizer on the surface of the
PVC sheet diffused through the interface between the sheet
and the extraction solution, and the vacancies left by the
migrated plasticizers were exchanged with the extraction
solution [31]. For thermodynamic reasons, the penetrated
MACROMOLECULAR NANOTECHNOLOGY
a
b
Data
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B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895
extraction solution facilitated the diffusion of plasticizer
from within the PVC sheet to the surface of the PVC sample. During this process, Bz-b-CD prevented interactions
between the plasticizer and the extraction solution. Therefore, Bz-b-CD in the PVC matrix played a key role in inhibiting migration and reducing plasticizer extraction. The
anti-migration efficiency (%) was calculated according to:
C PVC=plasticizerCD
100
Anti-migration efficiencyð%Þ ¼ 1 C PVC=plasticizer
MACROMOLECULAR NANOTECHNOLOGY
Fig. 11. The cumulative amount of migrated plasticizer as a function of
time at 37 °C.
Table 3
Concentration of the plasticizer and anti-migration efficiencies as a
function of time.
Samples
Concentration of plasticizer in the
migration medium (mg/100 mL)
PVC/DINP
PVC/DINP-CD
Anti-migration efficiencies (%)
PVC/Neo
PVC/Neo-CD
Anti-migration efficiencies (%)
1 day
3 day
5 day
7 day
4.89
2.89
40.86
7.03
5.27
24.98
6.26
3.66
41.64
8.84
6.56
25.83
6.66
4.10
38.46
9.75
7.13
26.88
6.86
4.25
38.11
10.11
7.70
23.86
where CPVC/plasticizer-CD and CPVC/plasticizer are the concentration of the migrated plasticizer during migration in the
PVC/plasticizer-CD and PVC/plasticizer samples, respectively. The anti-migration efficiencies of PVC/DINP-CD
and PVC/Neo-CD were 38.11% and 23.86%, respectively.
This indicated that the presence of Bz-b-CD in the flexible
PVC matrix dramatically reduced plasticizer migration,
which was attributed to the formation of an inclusion complex between the Bz-b-CD cavity and the plasticizer, and
stabilizing p–p associations between the benzoyl groups
of Bz-b-CD and the aromatic ring of the plasticizer molecules. Bz-b-CD, which hindered migration of the plasticizer, also will block the plasticizer molecules by winding
pathway (also called tortuous pathway) effect. In other
words, it would promote the path length for transporting
plasticizers and results in a decrease of plasticizer migration (see Fig. 12). Neocizer did not introduce p–p contacts,
and only van der Waals interaction stabilized the interactions between Bz-b-CD and Neocizer. For this reason, the
reduction in the rate of Neocizer migration (with an aliphatic ring) was smaller than the reduction in the rate of
DINP migration (with an aromatic ring). Differences in
the structure strongly affected the plasticizer/Bz-b-CD
interaction process. In addition, gel permeation chromatography (GPC) results confirmed that Bz-b-CD was not released from the flexible PVC sheets during the migration
Fig. 12. Schematic illustration of the anti-migration mechanism of the plasticizer in the PVC matrix.
process (see Fig. S9). The results showed that Bz-b-CD
nanoparticles reduced the rate of migration of both phthalate and non-phthalate plasticizers, thereby preserving the
composition of flexible PVC over longer periods of time.
[9]
[10]
4. Conclusion
[11]
We present, here, a study of the influence of Bz-b-CD on
the rheological properties of PVC plastisols and the prevention of plasticizer migration from flexible PVC. Rheological
analysis showed that the gelation of the PVC plastisols was
slightly delayed upon addition of Bz-b-CD particles, which
obstructed the absorption of both plasticizers. Entanglement among the PVC chains was blocked by the Bz-b-CD
nanoparticles. However, completion of the fusion process,
as observed in the viscoelastic data, occurred at almost
the same temperature (around 187 °C). No significant
changes in the physical properties of the flexible PVC were
observed upon addition of Bz-b-CD, possibly due to the
good dispersion of Bz-b-CD nanoparticles in the PVC matrix on the nanoscale. The presence of Bz-b-CD in the PVC
matrix played a key role in inhibiting plasticizer migration
which may explain the improved stability of the flexible
PVC.
Acknowledgment
This research was supported by the Eco-Innovation
Project through the Korea Ministry of Environment.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.eurpolymj.
2012.02.008.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
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