High solids solution acrylics: Controlled architecture hybrid cross- linking pressure sensitive adhesives Christopher L. Lester, Ph.D. Performance Adhesive Center, Avery Dennison William L. Bottorf, Performance Adhesive Center, Avery Dennison Kyle R. Heimbach, Performance Adhesive Center, Avery Dennison
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High solids solution acrylics: Controlled architecture hybrid cross-
linking pressure sensitive adhesives
Christopher L. Lester, Ph.D. Performance Adhesive Center, Avery Dennison
William L. Bottorf, Performance Adhesive Center, Avery Dennison
Kyle R. Heimbach, Performance Adhesive Center, Avery Dennison
Abstract
As a continuation of work reported at the 2009 PSTC Technical Seminar we report herein
the synthesis of acrylic polymers with controlled molecular weight, architecture and
placement of reactive functional groups. In particular, acrylic polymers useful as
pressure sensitive adhesives are described that utilize hybrid cross-linking technology.
The hybrid cross-linking technology described is acid metal chelate used in conjunction
with alkoxysilane sol-gel reactions. The influence of type, amount, and placement of
alkoxy-silane functionalities on viscoelastic properties and corresponding pressure
sensitive adhesive attributes are discussed. Additionally, hybrid cross-linking, controlled
architecture pressure sensitive adhesives are reported with varying glass transition
temperature and solubility parameter. The novel controlled architecture acrylic polymers
allow for the development of high solids solution adhesives at low viscosities and 100%
solids warm melt compositions with processable rheology. Furthermore, the controlled
architectured polymers display enhancements in adhesive performance relative to random
copolymers of the same composition.
Introduction
Polymer architecture and micro-structure have been shown historically to
dramatically influence material and, in particular, adhesive properties. Control of acrylic
polymer architecture and micro-structure has largely consisted of modulating molecular
weight and branching through polymerization temperature, initiator type, and in-process
monomer concentration. Also, some ability to modulate composition spatially along the
polymer chain could be afforded via selection of monomers with reactivity ratios
different from the primary backbone monomers.
While a considerable span of adhesive performance can be attained with the
aforementioned methods, much finer controls are possible with different polymerization
techniques. Controlling polymer architecture in a finer sense has been a subject of
significant research over the past fifty years. It has been demonstrated widely in the
literature that exerting finer control over the polymer architecture results in different and
often enhanced adhesive performance. In some cases, countercurrent properties can often
be decoupled. Previously reported architectures include block copolymers, telechelic
polymers, and random polymers of controlled molecular weight. While, the
aforementioned architectures all provide unique properties, they also have disadvantages.
Random copolymers either require high molecular weight to attain certain
balances of properties or require high degrees of cross-linking which can yield a poor
balance of properties. Telechelic polymers by definition have reactive functional groups
placed exactly at the end-groups and nowhere else in the backbone. The functional
groups then serve solely to increase linear molecular weight and/or form networks in
which free polymer chain ends are eliminated. Telechelic polymers consequently yield
high strength elastomeric materials but lack the viscous liquid character critical to
pressure sensitive adhesive (PSA) performance and require further formulation for
good PSA characteristics. Phase separated block copolymers when formulated
appropriately are known to yield a wide range of adhesive performances. However, due
to the nature of the physical cross-links in phase separated systems the thermal and
solvent resistance can be poor.
In approximately the last 20 years, a variety of pseudo-living or controlled radical
polymerization techniques have been developed to afford good architectural control of
(meth)acrylic monomers. These techniques are more tolerant to a wider variety of
functional groups when compared to living anionic, cationic, and catalytic techniques. A
substantial amount of fundamental research has been performed to understand these types
of polymerization and a thorough review has been edited by Matyjewski.1 Reversible
Addition Fragmentation chain Transfer (RAFT) polymerization is one such technique
that has been shown to work exceedingly well with a wide variety of (meth)acrylic
monomers yielding excellent control of molecular weight and polydispersity.2 The
RAFT mechanism for controlled polymerization is well understood and reported
extensively.1-3
It was previously reported at the 2009 PSTC Technical Seminar that controlled
placement of cross-linkable functional groups could be readily afforded by controlled
radical polymerization.4 These novel polymers allowed for the ability to synthesize
polymers to be of modest to low molecular weight and correspondingly to display low
solution viscosities at high solids content and to also display low viscosities in the melt.
In addition to the desirable solution and melt properties, it was found that the
performance of the resulting adhesives was comparable to high molecular weight
controls and in some cases the adhesive performance was markedly improved.
This study details the synthesis of controlled architecture acrylic polymers with
controlled placement of reactive alkoxy-silane functionalities. These types of polymers
are described as hybrid cross-linking pressure sensitive adhesives. The influence of the
type and amount of alkoxy-silane monomers is described with regards to visco-elastic
properties and corresponding pressure sensitive adhesive performance. Formulated
systems using the hybrid cross-linked materials are described as well as polymers in
which the glass transition and solubility parameter have been varied to modulate
performance.
Experimental
Base acrylic esters such as 2-Ethylhexyl Acrylate (EHA), Butyl Acrylate (BA),
Acrylic Acid (AA) and Isobornyl Acrylate (IBOA) were obtained from various
commercial suppliers and used as received. Methacryloxypropyl Tri-methoxysilane
(MPtMS) and Methacryloxymethyl Tri-ethoxysilane (MMtES) were all obtained from
Wacker Chemical and used as is. Dibenzyl trithiocarbonate (DBTTC) was obtained from
Arkema France and used as received and is shown in Scheme 1. Also in Scheme 1 is a
depiction of how
monomers are incorporated upon sequential addition. All of the polymerizations were
initiated with Azobis(isobutyronitrile) (AIBN). The polymers were all made in organic
solvents, most typically Ethyl Acetate. Unless otherwise stated all of the polymers were
formulated with aluminum acetoacetonate (AAA) at 0.5% by weight based on polymer
solids. All samples were coated at approximately 2.0 mil adhesive thickness onto 2.0mil
mylar. The coatings were all air dried for 10 minutes and placed in a forced air oven for
Scheme 1. Chemical structure of dibenzyl trithiocarbonate (DBTTC) and polymers after a single monomer addition followed by a subsequent monomer addition.
5 minutes at 130oC and closed with 100% solids platinum cured silicone paper liner.
The laminates were all aged in a controlled climate room for 24 hours prior to testing.
Molecular weights were measured using a Polymer Standards Services GPC
outfitted with a refractive index detector and calibrated using polystyrene standards.
Solution viscosities were measured using a Brookfield RVT viscometer. Spindle and
spindle speeds were selected such that a torque value of 40-80% was achieved for
optimal accuracy. Dynamic mechanical analysis (DMA) was performed on a TA
Instrument AR-1000 rheometer using parallel plate clamps. 1.0mm thick samples were
placed in the clamp and annealed at 75oC for 10 minutes to ensure good adhesion. The
samples were then cooled to -80oC for 10 minutes and ramped at 3
oC per minute up to
150oC. During the temperature ramp the sample was oscillated at a frequency of 10
rad/s. Unless otherwise noted, the following test methods were used for evaluating the
adhesive properties of the acrylic polymers.
PSA PERFORMANCE TEST METHODS
Test Condition 180° Peel a, b, 15 Minute Dwell 72 Hour Dwell Shear Strength c Shear Adhesion Failure Temp.(SAFT) d
(a) Peel, sample applied to a stainless steel panel with a 5 pound roller with 1 pass in
each direction. Samples conditioned and tested at 23°C.
(b) Peel, sample applied to a high-density polyethylene or polypropylene panel with a
5 pound roller with 5 passes in each direction. Samples conditioned and tested at
23°C.
(c) Shear: 1 kg weight with a 1/2 inch by 1/2 inch overlap. Sample applied to a
stainless steel panel with a 10 pound roller with 5 passes in each direction.
Samples conditioned and tested at 23°C.
(d) SAFT: 1000 gram weight, 1 inch by 1 inch overlap (2.2 pounds/square inch).
Sample applied to a stainless steel panel with a 10 pound roller with 5 passes in
each direction. Samples conditioned for 1 hour at 23°C and 15 minutes at 40°C.
Temperature increased by 0.5°C/min. until failure.
Results and Discussion
Scheme 2 depicts polymer architectures that are possible through the use
a.
b.
c.
Scheme 2. Varying RAFT/DBTTC mediated architectures including: a.
Random, b. end functional acid, and c. end functional alkoxy-silane.
of well controlled RAFT polymerizations. These architectures were reported previously,
but briefly it was found that segregating the cross-linkable functionalities such as
carboxylic acids or alkoxysilane moities can afford dramatically different material
properties that can be very desirable for pressure sensitive adhesives.4,5
In particular it
has been shown that low molecular weight architectured polymers are advantageous for
processing in that they can be at high concentrations in organic solvents at a low viscosity
or even melt processable in the absence of solvent. In addition to being advantageous for
processing, the low molecular weight architectured polymers were found to yield
pressure sensitive adhesive performance comparable to high molecular weight low solids
analogues. It was also previously reported that placing cross-linkable sites that react
O OH
O OH
O OHO OH
O OH
O OH
O OH
O OH
S
SS
O OHO OH
O OHO OH
O OH
O OHO OH
O OH
S
SS
S
SS
Alkoxy- Silane
Alkoxy- Silane
Alkoxy- Silane
independently from other cross-linkable functional groups can provide significant
performance enhancements in addition to having enhanced processability through lower
molecular weight. This type of system is shown in Scheme 3 in which alkoxy-silane
groups are positioned in the end regions of a
pressure sensitive adhesive acid containing random copolymer. This polymer can be
cross-linked with AAA which also serves as a Lewis acid catalyst for the sol-gel
condensation reaction of the alkoxy-silane moieties. This type of material was previously
compared to a commercial random copolymer of the same composition. To expand on
this work, additional copolymer controls were made and characterized. In all cases,
identical copolymer compositions consisting of 2-EHA, BA and acrylic acid were used
and the architecture and presence of alkoxy silane monomers was varied. Table 1 details
the various polymers molecular weight, solids, and solution viscosities. All of the RAFT
derived materials exhibit similar measured molecular weights with narrow
polydispersities which is indicative of a well controlled polymerization. As a result of
the molecular weights being fairly low, the solids and viscosities of these polymers are all
Scheme 3. Depiction of Hybrid-crosslinked RAFT derived architectured PSA.
O OH
O OH
O OHO OH
O OH
O OH
O OH
O OH
S
SS
Alkoxy- Silane Alkoxy-
Silane
Si
OCH3
OCH3
OCH3
Water
Lewis acid2 Si
OCH3
OCH3
O Si
OCH3
OCH3
+ CH3O
>67.0% and less than 14,000 cps. The commercial controls are higher in molecular
weight with broad polydisperities and correspondingly display
lower solids contents to be at a reasonable viscosity. Figure 1 is a plot of storage
modulus as a function of temperature for the different polymers described in Table 1. All
of the polymers exhibit identical glass transition temperatures (Tg) because of the
identical base compositions. However, at temperatures above
the Tg there are marked differences between the materials. The RAFT polymer with
MPtMS displays a very flat plateau modulus while the RAFT copolymer
without MPtMS does not to the extent that the material actually displays some flow
Table 1
51 EHA 45 BA 4 AA
Type RAFT Architectured
RAFT Random
Commercial Control
Commercial Control
MPtMS Y N Y N
Mn 80,060 81040 63519 61,531
Mw 127,540 122360 366140 380,961
PDI 1.6 1.51 5.76 6.2
Solids 69.0 69.0 51.5 50.0
Solution Viscosity
14,000cps 11600cps 4700cps 5,000cps
-100.0 -50.0 0 50.0 100. 150.0 200.0 Temperature (°C)
1000
10000
1.000E5
1.000E6
1.000E7
1.000E8
1.000E9
1.000E10
G' (dyne/cm^2)
RAFT with MPtMS RAFT no MPtMS Commerical with MPtMS Commerical no MPtMS
Figure 1. Storage modulus as a function of temperature for EHA/BA/AA copolymers of varying architecture and MPtMS amount.
characteristics at elevated temperature. The commercial control without MPtMS displays
similar behavior to that of the silane-free RAFT polymer but with overall higher modulus
as a function of temperature which results from the materials higher molecular weight.
When adding an equivalent amount of MPtMS, the commercial control displays a flat
plateau modulus but with overall higher values than the RAFT polymer containing
MPtMS. The pressure sensitive adhesive performance is very reflective of the DMA data
as can be seen in Table 2. For example, the RAFT copolymer containing alkoxy-silane
monomer exhibits
what could be characterized as the best overall balance of PSA performance. It displays
high ultimate adhesion to stainless steel, moderate adhesion to polypropylene, with
relatively high shear values and >200oC SAFT. The RAFT copolymer without alkoxy-
silane is a low cohesive strength material that displays significant cohesive failures in
peel adhesion coupled with low shear and SAFT values. The commercial control without
alkoxy-silane was better performing than the RAFT analogue in that it displayed high
adhesion values but it still displayed lower shear and SAFT values. The commercial
control with alkoxy-silane is a high cohesive material in that it has high SAFT values and
shears that did not fail cohesively but did not display the high adhesion values of the
RAFT copolymer containing alkoxy-silane. This is a result of the random incorporation
Table 2 51 EHA 45 BA 4 AA
Type RAFT RAFT Commercial Control
Commercial Control
MPtMS Y N Y N
180o Peel to
Stainless Steel 15 min Dwell (Lbs/in)
3.5 5.19 cohesive
2.72 3.8
180o Peel to
Stainless Steel 72 hr Dwell (Lbs/in)
8.4 cohesive
5.58 cohesive
4.63 7.5
180o Peel to
Polypropylene Lbs/in)
2.35 2.54 cohesive
2.00 1.20
SAFT, 1kg/Sq. In (Failure Temp
oC)
>200 66 >200 90
Shear, 2kg/ Sq. In (Failure Time, Mins)
135.1 adhesive
17.0 41.0 adhesive
41.0
of the MPtMS in the polymer back-bone which would produce lower molecular weight
between cross-links that yields an overall higher modulus. Also it is important to note
that even if there were equivalent performance between the RAFT and commercial
control analogues there would still remain the processing advantage of the RAFT
materials afforded by high solids at coatable viscosities.
It is important to note that the coating and drying conditions for MPtMS
containing polymers were 10 minutes at 130oC in a forced air oven in order to ensure
complete cure. While it is possible to reach these kinds of conditions in some coating
assets it may be difficult in others. Additionally, when coating thermally-sensitive
substrates the high temperatures may present difficulties. Scheme 4 displays various
alkoxy-silane methacrylate monomers and relative
propyl analogues. Shown in Figure 2 is a plot of Williams Plasticity Index (WPI) as a
function of temperature for MPtMS and MMtES containing pressure sensitive adhesives.
The MPtMS containing PSA exhibits substantially lower plasticities at all temperatures
when compared to the MMtES containing PSAs. The MMtES exhibits higher plasticities
and of note is the flatter response to
Increasing Reactivity
a.
b.
Scheme 4. Reactivity of varying alkoxy-silane monomers:
a. Methacryloxypropyltrimethoxysilane (MPtMS), b. Methacryloxymethyl
triethoxysilane (MMtES)
temperature over the MPtMS materials. Figure 3 is a plot of storage modulus as a
function of temperature for the PSAs with varying alkoxy-silane types and one can see
that the materials when fully cured are remarkably similar. The similar rheology is
1.5
2
2.5
3
3.5
4
4.5
5
90 100 110 120 130 140 150
Temp (oC)
Willia
ms
Pla
sti
cit
y In
de
x
MPtMS
MMtMS
Figure 2. Williams Plasticity as a function of drying temperature for varying alkoxy-
silane monomer.
manifested in the PSA performance displayed in Table 3 with the
-100.0 -50.0 0 50.0 100.0 150.0 200.0temperature (° C)
10000
1.000E 5
1.000E 6
1.000E 7
1.000E 8
1.000E 9
G' (
Pa)
DE V -8670ADE V -8670A New S ilane
MPtMS MMtES
Figure 3. Storage modulus as a function of temperature for EHA/BA/AA RAFT
polymers with varying alkoxy-silane monomers.
primary difference observed in slightly lower peel performance of the MMtES containing
PSA which is attributable to a slightly higher modulus resulting in mixed
adhesive/cohesive failure modes. This difference in peel can be modulated in a variety of
ways including varying the MMtES content as well as varying AAA cross-linker level in
the same fashion one would modify a standard solution acrylic.
In order to demonstrate how to tune the performance of these types of PSAs, a
series of RAFT polymers was made of the same composition described previously in
which the statistical number of MMtES was varied from 0.5-2.5 per end region. The
molecular weights and physical characteristics of the wet adhesives are shown in Table 4.
The molecular weights and polydispersities are all approximately the same.
Correspondingly, the solids and solution viscosities are similar in that they are all >68%
and <15000cps. It should be noted that at higher levels of MMtES some increase in
polydispersity occurred which resulted in higher viscosities. The higher polydispersity
and corresponding increase in viscosity is likely due to some reaction of the MMtES
during the polymerization. Figure 4 is a plot of storage modulus as a function of MMtES
monomers per chain end. All of the samples display the same glass transition
temperature that
Table 3 51 EHA 45 BA 4 AA
Type MPtMS MMtES
180o Peel to Stainless
Steel 15 min Dwell (Lbs/in)
3.5 3.4
180o Peel to Stainless
Steel 72 hr Dwell (Lbs/in)
8.4 cohesive
7.05 mixed
180o Peel to
Polypropylene Lbs/in) 2.35 2.53
SAFT, 1kg/Sq. In (Failure Temp
oC)
>200 >200
Shear, 1kg/0.25 Sq. In (Failure Time, Mins)
120 adhesive
135 adhesive
one would expect from polymers of the same composition but differ markedly in the
rubbery plateau. In every case the rubbery plateau is extremely flat with the modulus
increasing as the number of MMtES monomers increases. Interestingly, upon raising the
MMtES level from 2 to 2.5 per end region the modulus stays the same. This means that
there are very few unfunctionalized chain ends. The