When used with appropriate crosslinkers and tackifiers, acrylic polymer systems can be designed to adhere to many different surfaces. One area, however, where acrylic-based systems have not performed so well is in adhesion to low energy plastic surfaces. This article examines one company's efforts to fill such a void in the pressure-sensitive adhesive industry.
Figure 1. Schematic Representation of Acrylic Hybrid Polymer
There are many excellent acrylic polymer systems in use today that exhibit good environmental stability and degradation resistance. In the hands of a skilled chemist, these polymers - together with appropriate crosslinkers and tackifiers - can be designed and tailored to have a range of adhesive properties and adhere to many different surfaces. One area, however, where acrylic-based systems typically do not perform as well is in adhesion to low-energy plastic surfaces - an area of increasing importance. While certain tackifying resins will aid this adhesion, such gains are made at the detriment of cohesive strength and often aging and stability properties.
Rubber-based systems have always been able to perform well on low surface energy materials, but they often exhibit poor stability over time and limited cohesive strength - part of the reason that led to the increased use of acrylic systems. We introduced our developments with rubber-acrylic hybrid adhesives1 at the 2001 PSTC meeting. These new systems were designed to fill a void in the pressure-sensitive adhesive industry. Our hybrid polymer systems combine the best performance attributes of both acrylic and rubber systems in a single material. This article discusses our most recent developments - an increased understanding of how these systems behave and perform, together with improvements to our original material.
Figure 2. Representation of Acrylic Hybrid Polymer Structure and Formulated Structure
The Rubber-Acrylic Hybrid Polymer
To create a polymer with all of the benefits of an acrylic and a rubber, we chose to create a graft copolymer using an acrylic backbone and a saturated hydrocarbon side chain - an approach first described by Mallya and Smith2
and shown schematically in Figure 1.
The acrylic backbone contains the typical elements of a pressure-sensitive acrylic: low-Tg monomers (such as 2-ethylhexyl acrylate, butyl acrylate or iso-octyl acrylate) to give tack, higher Tg monomers (such as methyl acrylate or methyl methacrylate) to give cohesive strength, and functional monomers (such as acrylic acid or 2-hydroxyethyl acrylate) to give specific adhesion as well as the possibility of sites for metal crosslinkers to bind. The hydrocarbon side chains are a low-molecular-weight poly(ethylene-butylene) polymer with a methacrylate end group to allow it to be used as a comonomer in the polymerization reaction. The ethylene-butylene polymer has a molecular weight of 4000 and a glass-transition temperature (Tg) of -63 degrees C.
Figure 3. DMA Curve of Acrylic Hybrid Polymer
The combination of rubber and acrylic phases within the material offers broad formulating latitude. The polymer is compatible with tackifiers typically associated with acrylic polymers, such as rosin esters and terpene phenolics, and with many hydrocarbon tackifiers not normally compatible with a regular acrylic polymer. In our initial developments, we chose to tackify the rubber phase with a hydrocarbon tackifier; this approach gives the broadest range of performance. A hydrocarbon tackifier of this type will interact solely with the rubber domains of the polymer, due to its incompatibility with the acrylic portion. This will result in the formation of hydrocarbon domains (see Figure 2).
Prior to tackification, the polymer has two distinct Tgs, with the rubber being lower than the acrylic phase. This is shown in the DMA curve in Figure 3.
When the polymer is tackified with a hydrocarbon tackifier, the Tg of the acrylic phase remains unchanged. This tackifier is incompatible with the acrylic phase and so does not interact with it. The Tg of the rubber phase is shifted to a higher temperature, now becoming a shoulder on the high temperature side of the acrylic peak. This can be seen in the DMA curve in Figure 4. The adhesive has also been crosslinked, giving rise to a relatively flat storage modulus (G') at elevated temperatures.
Figure 4. DMA Curve of Formulated, Cured Acrylic Hybrid Adhesive
Understanding the System
Over the past year, we have spent significant time looking at the performance and behavior of our acrylic hybrid system and attempting to understand some of the causes of the interesting performance we see. Our results are as follows.
Figure 5. Peel Build on Film Aging
When the polymer was tackified with a C5-based hydrocarbon resin, we noticed some interesting changes in performance properties over time. Peel adhesion tests, using a 20-minute dwell time, showed differing results, depending upon the age of the film. A recently coated film would give a lower peel value than a one- to two-week-old film. Through systematic testing, we discovered that the peel value does increase as the film is stored on the liner, reaching a maximum value after approximately two weeks (see Figure 5). As the peel increases, the failure mode begins to change from adhesive to cohesive. Once the peel from steel reaches a maximum value of approximately 120 ozf/in, with a film around two weeks old, the failure is entirely cohesive.
We had hypothesized that this was due to changes in the phase morphology of the system. The two distinct phases would most likely rearrange slowly within the film. The lower energy tackified rubber phase would orient itself toward the low-energy surface of the release liner. Until recently, we had been unable to prove this theory. Several attempts with microscopic techniques, including atomic force microscopy, had failed. This can, in part, be attributed to the fact that the glass-transition temperatures of the acrylic phase and the tackified rubber phase are close together, making distinction difficult. The use of sum frequency generation IR spectroscopy, however, has yielded some interesting results.
Figure 6. SFG-IR Spectrum of Polymer-Sapphire Interface
Sum frequency generation (SFG) IR spectroscopy examines material interfaces by the use of a visible laser and a tunable IR laser, and is detailed in recent literature.3
Through collaboration with Ali Dhinojwala and Gary Harp at the University of Akron, we have been able to see the surfaces of the hybrid system for the first time. Although our work is very much in its infancy, the initial findings were startling. The acrylic hybrid polymer (without crosslinker or tackifier) was coated and annealed on a sapphire crystal. The SFG-IR technique was then used to probe the interface between the polymer and the sapphire and the polymer and air. This placed both a high- and a low-energy material in contact with the polymer. The polymer surface against the sapphire showed the IR bands typically associated with methylene groups next to oxygen (see Figure 6), provisionally assigned as those found around an acrylate ester. Conversely, the interface with the air showed a predominance of methyl and methylene resonances (see Figure 7) as would be found in a hydrocarbon chain - such as the poly(ethylene-butylene) domains in our polymer. There is more work to be done to establish how the spectral effects change as the level of hydrocarbon in the polymer is changed, or when the system is tackified. These results, however, represent the first piece of evidence to support our theory that the acrylic hybrid can exist in distinct phase domains at an interface, which differ from the bulk material.
Figure 7. SFG-IR Spectrum of Polymer-Air Interface
Kinetics of Chelation
We had chosen a titanium-based crosslinker system for our adhesive. In order to provide sufficient stability to the adhesive before coating and curing, a moderately high level of pentane-2,4-dione was added as a stabilizer. While this pentanedione provides excellent storage stability for solventborne adhesives, it presents an additional problem - it can affect the rate of cure. Pentanedione is not particularly volatile and so it evaporates only slowly in the drying oven. Longer residence times and higher temperatures will result in greater removal and therefore greater degree of crosslinking. Obviously, this can start to present economic challenges to the coater and therefore could easily become undesirable. An excellent way to look at the affect of curing conditions on adhesive performance is by measuring the shear adhesion failure temperature (SAFT). An example of this is shown in Figure 8.
Figure 8. Affect of Drying Time on SAFT
This study was conducted with 5-mil-thick adhesive films to maximize the effect. Films were dried in a laboratory oven at 120 degrees C for the times indicated; the test was performed using a 1" x 1" test area and a 1 kg weight. The oven ramp rate was 10 degrees F every 10 min., starting at 120 degrees F. Standard laboratory drying times of a few minutes will produce SAFT values around 200 degrees F. Extending the drying for as long as 10 min. can enhance the SAFT performance, reaching values of 230 degrees F. Such time and temperature combinations are obviously not practical on a commercial coater, but our experience has shown that a similar range of properties can be found with 2 mil adhesive films on commercial coaters as the oven profile is changed. A web temperature of at least 100 degrees C in the final zone of the oven will produce SAFT values of 200 degrees F and above.
This critical dependence of film performance on curing conditions was one of the motivations that we had to create an improved system that would be easier to use.
Figure 9. Affect of Crosslinker Type on Peel Adhesion
New Improvements to the Acrylic Hybrid System
Although we were very pleased with our initial system, improvements could be made. The system performs extremely well in many different environments and is a significant improvement over a rosin ester tackified acrylic for adhesion to low-surface-energy material. The aging characteristics of the material are also better than many simple tackified acrylics and will not give any concern in many applications. Users who subject the adhesive to extremely challenging environments, such as high heat or extended exposure to ultraviolet light, may begin to see problems with stability. The hydrocarbon tackifier we used contains unsaturation, which leads easily to oxidative degradation. In addition, our titanium crosslinking system requires substantial stabilization in the unformulated material. The lack of volatility of the pentane-2,4-dione used for stability has already been discussed. While it is fairly straightforward to achieve sufficient cure to see the desired performance properties, residual pentanedione is not desirable. It will slowly evaporate if the tape is exposed to high-heat environments and therefore result in additional crosslinking and stiffening of the adhesive and a reduction in peel adhesion. If the tape has already been used to form a secure bond before such heat exposure, there will be little noticeable effect. If, however, the tape is exposed to heat before being used to form a bond, the resulting bond strength will be compromised as the stiffer adhesive has lower peel and tack values.
A number of alternative crosslinker systems could have been used. We could have moved away from a transition metal system completely to materials such as isocyanates. These would react with hydroxyl groups in the polymer side chains and give strong, stable covalent crosslinks. Such materials have to be formulated immediately prior to use, resulting in more complex handling for the tape producer. We therefore chose to focus on crosslinking systems that could be pre-formulated into the adhesive and supplied as a one-part system. Isocyanates are still a possibility in such as system. A number of blocked isocyanate materials are available. Such materials have protecting groups to prevent them from crosslinking the adhesive during formulation and storage in solution. The protecting groups are heat labile and therefore are removed in the drying oven and can then crosslink the adhesive film. We found this to be a difficult concept to execute successfully. The temperatures required to activate the blocked isocyanates were generally greater than the temperatures achieved during the normal drying conditions of pressure-sensitive adhesive films. The films, therefore, were normally under-cured and did not exhibit the desired performance characteristics.
Figure 10a. Affect of Crosslinker Type on Shear and SAFT
We therefore returned to transition metal crosslinking agents. We wanted to select a more reactive titanate than our original choice, but one that could be stabilized sufficiently to allow it to be formulated into the adhesive during manufacture. Obviously, stabilization would still be called for, but it was important that we found a stabilizer that was sufficiently volatile to be easily removed under typical coating conditions. We chose an organic titanate, which was known for high reactivity. As with most such materials, it contained alkoxy groups as ligands on the metal center. The analogous alcohol could be used as a stabilizer. This was significantly more volatile than the pentanedione we had originally been using. When we substituted this new crosslinker into our formulation, we achieved very satisfying results, the peel performance of which is shown in Figure 9. These peel measurements were all conducted with 20-min. dwell times. The peel adhesion remained very similar to the levels previously obtained, across a range of substrates.
Our shear strength was improved, as was our SAFT performance, which was particularly beneficial (see Figure 10).
Figure 10b. Affect of Crosslinker Type on Shear and SAFT
One problem remained: our system still contained an unsaturated aliphatic tackifier; while our cure speed and crosslink stability had been enhanced, the product would still exhibit poor oxidative aging characteristics. Substitution of the tackifier for a saturated resin was the obvious approach. However, this is not completely straightforward. There are many readily available, hydrogenated tackifiers, but not all of these are readily compatible with our polymer. In fact, many we tried showed only borderline compatibility and resulted in decreased adhesive performance. Luckily, we were able to find a resin that was chemically very similar to our original choice, but fully saturated. When this resin was substituted into our system, we saw all of the same benefits that the crosslinker change had given us, but now with enhanced environmental stability. The oxidative degradation and darkening of the product was greatly reduced. After seven days of heat aging at 300 degrees F, the original material had darkened significantly, becoming a deep brown color with almost no tack left. In contrast, the improved formulation still retained tack after seven days of aging at 300 degrees F. While there was some darkening of the new material, it was at a distinctly lower level than with the original material and was comparable or superior to many tackified acrylics. This enhancement, coupled with the fact that there was little significant additional crosslinking taking place, meant that we also had much superior retention of adhesive properties after heat exposure.
Work is continuing on the acrylic hybrid platform and we are investigating a range of opportunities. These include enhanced foam bonding, with particular attention to polyurethane foams, as well as continued work in low-color, environmentally stable hybrid adhesives. We would like to produce an adhesive with a SAFT above 300 degrees F that will still have outstanding adhesion to many substrates and be straightforward to coat and cure.
I would like to thank the Pressure Sensitive Adhesive Research and Development group at National Starch, but particularly JoAnn Hansen and Brent Sellers, who performed most of the formulation and testing work, as well as Jim Miller for polymer synthesis. Also, Dr. Paul Foreman, Dr. Jennifer Jensen and Dr. Clay Kellam for their technical insight and discussions, and Dr. Rama Chandran for technical advice and management support. Finally, I would like to thank Ali Dhinojwala and Gary Harp at the University of Akron for permission to reproduce their results.
This article is based on a paper presented at PSTC's TECH XXVI Technical Seminar, May 2003 in Washington, D.C.
1. Foreman, P.; Eaton, P.; Shah, S.
Pressure Sensitive Tape Council TECH XXIV, May 2001.
2. Mallya, P.; Smith,
C.C. U. S. Patent 5,625,005; April 29, 1997.
3. For examples of the technique, see:
Harp, G.R.; Gautam, K.S.; Dhinojwala, A.
J. Am. Chem. Soc., 2002, 124, 7908.
Gautam, K.S.; Dhinojwala, A.
Macromolecules, 2001, 34, 1137.
Further references can be found in these papers.
For more information:
For more information on acrylic hybrid technology, contact the National Starch & Chemical Information Center, One Matrix Drive, Monroe, NJ 08831; phone 800-797-4992; fax 609-409-5699; e-mail email@example.com
; or visit www.nationalstarch.com