UNLOCKING THE PERFORMANCE POTENTIAL OF

UNLOCKING THE PERFORMANCE POTENTIAL OF
WATERBORNE LATEX PSAs
Sekhar Sundaram, Project Team Leader, Rohm & Haas Company, Spring House, PA
Laura Picraux, Senior Scientist, Rohm & Haas Company, Spring House, PA
Rob Slone, Global Research Manager, Rohm & Haas Company, Spring House, PA
Abstract
One of the time-tested challenges for waterborne latex pressure sensitive adhesives
(PSAs) is to match the cohesive strength and resistance properties of a solventborne
system while still providing the environmental impact and handling advantages that come
with waterborne adhesives. In this paper, we will discuss the fundamental factors that
have historically prevented acrylic latex PSAs from achieving the cohesive strength of
solventborne dispersions. We will also discuss new technologies (including traditional
and nanotechnology approaches) that have helped bridge the waterborne-solventborne
cohesive strength gap and have enabled the use of waterborne PSA products in adhesive
applications formerly reserved for solventborne systems.
Replacing Solventborne Technology: A Multi-dimensional Problem
Achieving the balance of adhesive and cohesive properties has been a challenge for
waterborne latex PSA technology; however, the medium to high-end specialty PSA
products have many additional requirements for acceptable performance. These
properties include resistance to water degradation, water-whitening, and heat and
humidity.
T ac k
5
4
Heat /
Humidity
on
si
he
Ad
Tack
3
H /H
Peel
Peel
2
1
0
t
sis
Re
s
ce
an
SAFT
SAF T
Shear
H ot s hear
Hot Shear
RT Shear
he
Co
on
si
Figure 1: Multi-dimensional performance space
The multi-dimensional performance space that will have to be met to successfully
compete against solvent technology is illustrated in Figure 1. For simplicity, the fourth
dimension viz. aesthetics of the final film like color stability and clarity, which is critical
for certain end-use applications will not be discussed.
Over the years, various techniques (Figure 2) have been explored to tackle the adhesioncohesion balance of waterborne PSAs. One approach has been to crosslink a soft (low
Tg) polymer backbone. The resultant system provides adequate cohesive strength but
often at the expense of tack and peel (bold arrow in Figure 2). Tackification of a highcohesive strength polymer has been another commonly used approach. The adhesion
properties are improved but the cohesive strength of the polymer decreased to an
unacceptable level (bold arrow in Figure 2). These examples demonstrate that traditional
techniques are not sufficient to move beyond the ‘classical behavior’ of waterborne
PSAs, noted in Figure 2.
Adhesion Æ Peel
Solvent
Acrylics
X-link
Cl
as
sic
al
B
eh
av
Tackification
ior
Cohesion Æ Shear
Figure 2: Various approaches illustrated
In this paper, a more fundamental approach is described to achieve a waterborne adhesive
system that has high cohesive strength while maintaining tack and peel. This is
accomplished by understanding the basic film formation mechanism and the relationship
between particle morphology and visco-elastic properties. These studies have provided
the basis to produce an aqueous acrylic polymer which is comparable to solventborne
acrylic systems.
Emulsion Polymers & Film Formation
For solventborne systems, high cohesive strength can be readily obtained by the
continuous network morphology formed in the dry film. In contrast, emulsion polymers
produce films which contain discrete colloidal particles and, in general, weaker
adhesives1. Understanding the various aspects which are important from the emulsion
polymerization to the final film formation allows a better approach to improving the final
film morphology and thereby improving the PSA properties of the adhesive.
Polymer Particle
10 - 400 nm
Surfactant
Figure 3: Emulsion Polymer
The emulsion contains discrete polymer particles stabilized by surfactants with the
hydrophobic moiety adsorbed onto the polymer particle and the hydrophilic moiety
extended into the aqueous phase (Figure 3). It has been shown that the surfactant
surrounding each polymer particle contributes to the stability of the overall aqueous
system and to the colloidal nature of the dried film. Thus, the choice of surfactant, viz.
anionic, nonionic or polymeric, influences the storage and handling of the emulsion,
morphology of the film, and the performance of the PSA. The emulsion is then drawn
down to form a film. Figure 4 illustrates the various stages of film-formation for
waterborne latexes2. The film formation process can be broken down into three steps.
Polymer particle
X
Water
Evaporation
Y
Water
Evaporation
+
Polymer
Deformation
Coalescence
Z
+
Film Formation
Figure 4: Steps in film formation
In the first-stage, most of the water evaporates from the film bringing the latex particles
close together.
Next, further evaporation occurs and the particles deform creating a mechanically weak
polyhedral cell-like structure. In the initial drying of the film the cell boundaries are
populated by the surfactants and maintain the discrete particle nature. The rate of drying
and, by extension, drying temperature influences the degree of particle deformation. The
Tg of the latex will also affect the particle shape; a lower Tg system will have greater
chain mobility at temperature typically used for drying.
Latex was cast into a film. Ultra
thin sections were cut from the
films by cryo-microtomy at -90 oC.
Stained with RuO4 (ruthenium
tetroxide) and imaged by
Transmission Electron
Microscopy.
These samples have undergone
compaction and fully formed
films.
The dark regions are
surfactant rich.
500 nm
Figure 5: TEM image of surfactant adsorbed at particle boundaries
In the final stage, the cohesive strength of the film is developed by coalescence of the
latex particles and inter-diffusion of polymer molecules across the cell boundary.
Partitioning of surfactants to the interface can retard particle coalescence, and film
formation (see Figure 5) resulting in weaker films.
The fundamental nature of film-formation for waterborne latexes makes it challenging to
build a system that performs analogously to solventborne PSAs. The presence of the
surfactants in the system affects the final coalescence of the film and, as a result,
influences the final cohesive strength of the system. After film formation, the surfactants
remain in the film. These migratory water-soluble species are one of the main reasons for
poor water and moisture resistance. In addition, the mechanical properties of the film
depend upon the particle morphology which in turn impacts the peel and tack. Therefore,
improving the final polymer network will lead to enhanced PSA properties in the
waterborne system.
Particle Morphology
Particle morphology of the latexes has been used as a means to tailor the physical
properties of films3. With the availability of a wide range of monomers and with a
greater understanding of the emulsion polymerization process, chemists have been
successful in designing composite latex particles to meet the requirements for specific
end-use applications.
Opaque
Polymer
Hard-Soft
SSP
Nano-composite
Core/Shell
Figure 6: Some examples where nanotechnology has been utilized to achieve unique
performance
For example, ‘core-shell’ latex particles are prepared by a series of consecutive emulsion
polymerization steps. The resultant particle will have an inner-core with one composition
and an outer-shell with a different composition. Similarly, there are numerous examples
of nano-sized features deliberately designed within the latex particles resulting in
differentiated performance (Figure 6). Some examples of nano-structures are hard and
soft composites4, polymer shell with hollow cores (opaque polymer) and controlled shells
(soluble shell polymers, SSP)5. Each of these systems has been developed for high value
additives, binders and pressure sensitive adhesives. Composites of polymer and nanoclays6 have been evaluated for PSA applications where increased film toughness was
required. A unique balance of PSA properties has been achieved by these systems.
In this study, results are presented from the latest developments in waterborne technology
by leveraging nanotechnology to provide a unique balance of PSA properties.
Experimental Methods
PSA Tests
All samples were prepared by direct coating the emulsion onto 2 mil polyester, and
covering the sample with a release liner. The dried sample had a coat weight of 53 g/m2
± 2 g/m2. All tests were performed in a controlled environment of 21 °C and 50 %
relative humidity. All data reported are the average of six individual tests.
Peel tests were performed using PSTC-101 Test Method A. Two different dwell times
were used; 30 min (initial) and 24 hr. The PSTC-107 Procedure A was followed for the
creep experiments. Loop tack was measured using the PSTC-16 Test Method B.
Tensile Strength and Elongation.
The tensile strength and elongation of a free film were measured according to PSTC-131
test method. The free film was prepared by casting the wet sample in a petri dish and
allowing it to dry for 7 days at 21 °C and 50 % relative humidity. The dry films were
then transferred to a 50 °C oven for 2 days. The film was removed from the petri dishes
by placing them on dry ice for 30 min and twisting the dish motion to eject the film. It
was then warmed to room temperature. It was found that this procedure allowed easy
removal of the films without stretching or deforming them. The film thickness was
approximately .035 ± 2 inches. These films were then cut into a dumb-bell shape, where
the central dimensions were 0.725 in. long by 0.25 in. wide. The samples were elongated
until break at a rate of 2 in/min.
Water Absorption
The water absorption of each sample was measured. Free films were prepared as in the
tensile strength and elongation experiment. Circular 0.375 inch discs were cut from the
films. The weight of each disc was recorded and then the disc was placed in 20 mL of
H2O in a glass vial. The discs were removed at 24 hr, 4 days, and 7 days, blotted with
cheese cloth, and weighed. After each weighing the disc was returned to the glass vial.
The % weight gain was calculated and reported.
DMA
The dynamic mechanical analysis (DMA) was performed on a Rheometrics RDA-II
rheometer. Free films of all samples were prepared as described above. From these
films, 0.375 inch discs (thickness 0.8 to 1.0 mm) were cut and used for sample analysis.
The experiments were performed in the dynamic temperature mode using a frequency of
6.28 rad/s. Parallel plate geometry (8mm diameter) was used for all testing. The
temperature range used for each sample was -70 °C to 180 °C with a 0.5 °C temperature
step increment. All experiments were performed by increasing the temperature from -70
°C.
Results & Discussions
PSA Testing and Water Absorption.
The primary goal for the current development was to build cohesive strength in the
polymers without compromising the adhesion. In an effort to optimize the peel-shear
balance, acrylic latex polymers were synthesized using traditional metal cross-linking (A)
techniques7 and incorporating nanotechnology (B and C). These systems are compared
to a general purpose solvent acrylic (SA) adhesive. Note that values shown for the
general purpose SA system are based on typical target values, rather than test results of a
specific solvent acrylic product. Within the general purpose category there is a range of
performance. Using broad market input, a set of desired characteristics for a general
purpose solventborne PSA was created to allow comparison to the developmental
waterborne PSAs.
1000
RT Shear 1”X1”X4 lbs (hrs)
SS 180o Peel 24 hrs (oz/in)
80
60
40
100
20
0
10
A
B
C
SA
Metal
X-link
General Purpose
Solvent Acrylic
Figure 7: Adhesion-cohesion balance
The PSA properties were evaluated for the new systems. The classical peel-shear balance
is evident from Figure 7. Polymer A has the highest cohesive strength (shear) and the
lowest peel value. With the development of technology, B, the high cohesive strength
was maintained while improving the adhesion characteristics. With latex C, a better
balance of high cohesive strength and good adhesion was achieved. These results are
favorable to a general purpose SA system.
75
50
40
30
1”X1”X1 kg.
SAFT
SAFT
SAFT
SAFT
20
10
Loop Tack (Oz/in)
Hot Shear (hrs)
SAFT (oC) OR,
150
65oC
225
0
0
A
Metal
X-link
B
C
SA
General Purpose
Solvent Acrylic
Figure 8: SAFT, hot shear and loop tack
The resistances of the water-based films were evaluated. Figure 8 illustrates the heat
resistance of the polymers under static (hot shear at 65 oC) and dynamic shear adhesion
failure temperature (SAFT) conditions. For polymers A, B, and C, the static hot shear
experiments were stopped after 100 hrs. It is evident from Figure 8 that the aqueous
systems developed here had excellent static heat resistance that was comparable to or
better than the general purpose solvent acrylic. Polymers A, B and C have excellent
SAFT results. Comparing the improved resistance of these new systems to the tack, it is
apparent that the general purpose solvent acrylic is able to maintain a higher level of tack
(Figure 8); however the overall characteristics are much improved over the typical
waterborne system and are similar to the sovlentborne system noted here.
100
50
80
40
60
30
40
20
20
10
0
0
A
Metal
X-link
B
C
% Water Absorption (7 days)
% SS Peel Retention (Initial)
The impact of aging the PSA film closed with a release liner under heat (35 oC) and
humidity (90 % RH) for 7 days was also tested. Initial (30 min dwell on stainless steel
panels) 180o peel testing was performed on these adhesive strips after removing them
from the oven and equilibrating them in a controlled atmosphere (22 oC and 50 % RH)
for 2-3 hrs.
SA
General Purpose
Solvent Acrylic
Figure 9: Peel retention under heat (35 oC) and humidity (90% RH) and water absorption
Figure 9 reports the percent peel retention compared to the initial peel as obtained
without heat and humidity aging. The metal cross-linking technology (A), had the best
peel retention but is a low peel and tack system which is not acceptable for many
applications. Samples B and C have higher peel and tack but lower heat and humidity
resistance. Sample C approaches the SA system the closest. Figure 9 also illustrates the
percent water absorption after soaking the polymer (see above for experimental protocol)
for 7 days. The polymer A with metal cross-linking technology fares very poorly under
these conditions. With the incorporation of nanotechnology, polymer B achieves water
absorption of about 20 percent compared to an expected value of 10 percent for a SA
PSA.
Tensile Strength and Elongation.
Measurement of the stress-strain behavior of a polymer allows the deformation and
fracture energy of the system to be understood8. The behavior of the systems studied
here is shown in Figure 10. Polymer A demonstrates the expected behavior for a crosslinked system. At high elongation, there is an up-turn in the stress-strain curve due to the
limited extension that the chains can undergo. For polymer B the tensile strength has
increased, but the maximum elongation has decreased.
300
σ
Strain (psi)
250
B
200
A
Tensile
properties
tailored with
the help of
nanotechnology
150
100
50
C
0
0
200
400
600
ε
800 1000 1200 1400 1600 1800
Stress (%)
Figure 10: Stress-strain curve
40
40
30
30
20
20
10
10
0
0
A
B
Toughness (×104 psi/in3)
Modulus (psi)
On the other hand for latex C, we are able to tailor the tensile properties to enable better
adhesion. The tensile properties for latex B and C are consistent with their basic peelshear balance as presented in Figure 7. This underscores the ability to tailor a range of
performance with the use of nanotechnology.
C
break
Modulus: Obtained from the
slope of the initial linear region
of the stress-strain curve
Toughness =
∫σdε
0
V
Figure 11: Modulus and Toughness
The modulus of the samples is a measure of resistance to stretching. This value was
determined from the elastic (linear) region of the stress-strain curve. The modulus of C
was lower than that of A or B. Figure 11 plots the modulus and toughness (see below) of
the three samples studied here. The trend for both modulus and toughness is consistent
with the trend in peel, tack and creep resistance properties for samples A through C.
The toughness, calculated by the area under the stress (ε) - strain (σ) curve divided by the
volume (V), followed a similar trend as the modulus. During a peel experiment, which
demonstrates a nonlinear response, the legging and flow during the de-bonding process
has been suggested to be related to the stress-strain behavior. It was found that the peel
strength (for similar failure modes) followed the same trend as the tensile strength.
Although this trend is suggestive, a complex range of factors such as the viscous flow,
chain entanglement, and adhesive-surface interactions will all be important during the
peel experiment. These examples underscore the interplay between polymer morphology
and the observed properties of the system under tensile elongation.
DMA
The viscoelastic properties can be examined over a large magnitude of temperatures
(frequencies) using DMA. From this experiment, information regarding the polymer
structure and properties may be determined. The DMA curves are presented in Figures
12-13. Correlating the mechanical properties with the performance of PSAs is of great
interest. It has been discussed in the literature that the high temperature terminal zone
region of the DMA curves are relevant to the long time failure tests such as creep
resistance9. Similarly, the low temperature transition zone has been related to the short
time scale, debonding processes such as peel tests9.
9
10
Sample A
Sample B
Sample C
8
10
ω = 6.28 rad/s
G' (Pa)
7
10
B
6
10
5
10
C
4
10
-100
-50
0
50
100
150
200
o
Temperature ( C)
Figure 12: DMA curve, G’ (storage modulus) as a function of temperature
The plateau region of the G’ curves in Figure 12 are relatively flat for sample A which
employs traditional metal cross-linking chemistry. The G’ curve of B has some slope to
it, and sample C turns the corner. Thus, the developments presented here help to build
cohesive strength and increase the creep resistance without using traditional metal crosslinking technology.
8
Sample A
Sample B
Sample C
10
7
G" (Pa)
10
ω = 6.28 rad/s
6
10
B
5
10
4
10
C
-100
-50
0
50
100
150
200
o
Temperature ( C)
Figure 13: DMA curve, G” (loss modulus) as a function of temperature
The transition region of the G” curves (Figure 13) are consistent with the observed tack
behavior of the adhesives. The lower loss modulus and higher dissipation energy is
observed for all samples, indicating the pressure sensitive adhesive nature of the samples.
G’×10-5 (at Tg + 60oC), Pa
2.50
2.00
1.50
1.00
0.50
0.00
A
B
C
Figure 14: G’ (modulus) at [Tg + 60 oC]
The storage modulus of each system can be compared to obtain additional information
about the viscoelastic properties. A summary of G’ for three samples is noted in Figure
14 at Tg + 60oC.
Sample B has the highest G’ value, followed by A, and then C. This roughly predicts the
modulus of the polymer in the terminal zone. It is also interesting to note that structures
have been generated that behave as if they were cross-linked yet with adequate flow.
Technology Optimization
Further technology enhancement of polymer C resulted in two polymer systems, D and E
that met, or, in some cases exceeded all of the SA target performance.
HDPE Peel
5
40
on
si
he
Ad
180o HDPE Peel
(oz/in)
4
Heat/Humidity Aging
(% Retention) 100
H/H
3
SA
D
E
80
24
S S Peel
180o SS Peel
(oz/in)
2
60
48
20
1
8
16
40
120
10
20
200
SAFT (oC)
SAFT
s
Re
s
ce
an
is t
60
100
1 kg Hot Shear 65oC
(hrs)
0
Hot shear
30
30
50
Loop Tack
Loop Tack
(oz/in)
90
RTS hear
150
4 lbs RT Shear
(hrs)
he
Co
si
on
Figure 15: Summary of performance data with the latest technology optimization
Figure 15 summarizes the data in a spider chart. Most notable for the new polymers D
and E is that the peel values for both low surface energy (HDPE) and high surface energy
(SS) substrates as well as heat resistances are enhanced without compromising the
cohesive strength of the polymers. The hot shear (at 65 oC) and SAFT for polymer D is
greater than SA. This balance of adhesion along with cohesive strength is unique.
The results demonstrate the tremendous progress made in utilizing nanotechnology to
build heat, humidity, and water resistance without overly compromising peel and tack for
waterborne latex PSAs.
Summary & Conclusions
Nanotechnology techniques have been utilized to design waterborne latexes with PSA
properties approaching those of a general purpose solvent acrylic. In particular, these
systems demonstrate that polymers can be synthesized with a wide range of adhesion and
cohesion properties to meet the needs of a specific application. In summary, the next
generation water based acrylic technology presented here is closing the gap with general
purpose solvent acrylic performance while providing environmental benefits.
Reference
1. Blackley, P. C. Emulsion Polymerization, Theory and Practice, Applied Science
Publishers, London, 1975.
2. Steward, P. A. et al., Advances in Colloid & Interface Science, 86, p195-267, 2000.
3. Lovell, P. A. and El-Aasser, M. S., Emulsion Polymerization and Emulsion Polymers,
John Wiley & Sons, p291, 1998.
4. Kowalski et al., US Patent 4,427,836.
5. Lorah, D. P., US Patent 4,876, 313.
6. Lofton, L., Clay/Polymer Nanocomposites for PSAs, PSTC Annual Meeting, 2004.
7. Czech, Z., Polymer International, 52, p347-357, 2003.
8. Sperling, L. H. Introduction to Physical Polymer Science; Wiley-Interscience
Publication, New York, 1986.
9. Chang, E. P. J. Adhesion, 60, p233-248, 1997.
Acknowledgements
The authors would like to thank Dr. A. Nakatani for helpful discussions in relation to
polymer rheology and PSA properties, Dr. J. Reffner for providing TEM images, Dr. W.
Griffith for valuable discussions, and finally D. Pierson for his suggestions regarding the
manuscript.