Advancements in Solventless Technology for Silicone PSAs

Silicone pressure-sensitive adhesives (PSAs) have been used for many years in areas where typical organic PSAs have failed. One of the most important uses is applications where large temperature extremes occur.

The composition of silicone PSAs parallels that of many common organic PSAs. The two main components that dictate the performance of the silicone PSA are a high molecular weight, linear siloxane polymer and a highly condensed, silicate tackifying resin (MQ resin). Figure 1 shows the structure of a typical silicone polymer. Commercially available silicone PSAs use either polydimethylsiloxane (PDMS) or polydimethyldiphenylsiloxane (PDMDPS) polymers that contain silanol functionality at the polymer chain ends.

The silicate resin, often referred to as an MQ resin, is a solid particle supplied in a hydrocarbon solvent. The MQ name derives from the fact that its structure consists of a core of three-dimensional Q units (SiO₄/₂) surrounded by a shell of M units (Me₃SiO). The resin also contains a level of silanol functionality on the surface. The ratio of M:Q is typically in the range of 0.6-1.2:1. Figure 2 shows a computer-generated molecular model of a silicate resin.

Silicone PSAs are produced by blending a specified ratio of resin and polymer together in a hydrocarbon solvent. Heating the mixture to promote a condensation reaction between the available silanol functionality on the resin and polymer can further enhance the initial cohesive strength of the adhesive. The ratio of resin to polymer is the most important formulation detail when trying to optimize the balance of performance properties for a given adhesive. Figure 3 shows an example of how the balance of resin and polymer can affect the tack, peel adhesion, and shear performance of a silicone PSA. The exact positioning of these curves with respect to the x and y axes and each other is determined primarily by the resin properties.

Adhesive Cure Chemistry

Although most silicone PSAs will exhibit pressure-sensitive behavior immediately after solvent removal, further crosslinking is done to reinforce the adhesive network. Two basic cure systems are commercially available for silicone PSAs: peroxide-catalyzed free-radical cure and platinum-catalyzed silicon hydride to vinyl addition cure. The majority of silicone PSAs available employ the use of a peroxide-catalyzed (benzoyl peroxide or 2,4-dichlorobenzoyl peroxide) free-radical reaction to achieve additional crosslink density. Curing of these types of adhesives is done in multi-zoned ovens due to the use of non-specific peroxides. Solvent removal is first required at lower temperatures (60-90 degrees C) to ensure the peroxide does not inadvertently cure solvent in the PSA matrix, which would result in reduced performance and poor temperature stability. At elevated temperatures (130-200 degrees C) the catalyst decomposes to form free radicals, which primarily attack the organic substituents along the polymer chains, extracting protons and generating free radicals.₁The free radicals then combine to form crosslinks as shown in the graphic.

The main benefit of the peroxide-catalyzed system is the ability to control properties by addition level of peroxide used. The tape producer has the flexibility to use a range of 0-4 percent peroxide. The additional curing with the peroxide results in a more tightly cured PSA. An increase in cohesive strength, as evidenced by performance in shear tests, is observed. The increase in cohesive strength is accompanied by a slight decrease in adhesion and tack.₂Some disadvantages of this type of silicone PSA system include the handling of volatile solvents, generation of peroxide byproducts, more sophisticated curing ovens and the need for priming of certain substrates to improve adhesive anchorage in the construction of self-wound tapes.2 Some disadvantages of this type of silicone PSA system include the handling of volatile solvents, generation of peroxide byproducts, more sophisticated curing ovens and the need for priming of certain substrates to improve adhesive anchorage in the construction of self-wound tapes.

As an alternative to the peroxide-catalyzed system, silicone PSAs have been introduced that use a different type of curing mechanism.₃These adhesives are cured by a platinum-catalyzed reaction of silicon hydride to vinyl. This chemistry is analogous to the typical solventborne and solventless platinum-catalyzed silicone release coating systems used for release liners of organic PSAs. The curing of this type of silicone PSA can be accomplished in a single-zone oven at lower overall temperatures (100-150 degrees C), even though these systems are supplied in hydrocarbon solvents. As the solvent evaporates, the platinum-catalyzed reaction occurs without any generation of byproducts (see Figure 4).

The ability of this type of system to be cured at a single, lower temperature offers benefits that are not seen with a peroxide-catalyzed system. These benefits include faster line speeds (or cure time), lower sensitivity to temperature variation, the ability to use substrates with lower thermal stability (PE, PP, etc.) and no generation of volatile byproducts.

Another benefit of the platinum-catalyzed silicone system is the fact that it does not inherently need the hydrocarbon solvent for anything other than viscosity control. The peroxide-catalyzed system not only needs the solvent for viscosity control, the solvent keeps the peroxide dissolved within the adhesive bath prior to coating on the web. This advantage for the platinum-catalyzed chemistry has led to the successful commercialization of many solventless silicone systems over the last couple of decades, including silicone release coatings. Unfortunately, not much success has been made in producing an industrial solventless silicone PSA for tape applications. But as time has moved forward, silicone raw materials have continued to evolve much like any other chemistry. This evolution with time has ultimately expanded the toolbox for the development chemist. In recent years, work in the lab has led to the development of a prototype, solventless silicone PSA that has the tack, adhesion and high temperature shear of a common solvent-based silicone PSA.

Discussion

In this study, four adhesives were evaluated: 1) a solvent-based, peroxide-catalyzed dimethyl silicone PSA, “commercial dimethyl”, 2) a solvent-based, peroxide-catalyzed diphenyl silicone PSA, “commercial diphenyl”, 3) a previously commercial solventless silicone PSA, “obsolete solventless” and 4) a new prototype solventless silicone PSA, “prototype solventless”. The peroxide catalyzed adhesives were prepared at 50 wt. percent solids in solvent using 2 wt. percent benzoyl peroxide based on the silicone solids. Figures 5-6 show the peel adhesion (PSTC-1) and probe tack performance (ASTM D 2979) for each PSA on two different substrates, respectively.

Comparison of these four adhesives on these figures illustrates an important point. The commercial dimethyl PSA is known in the marketplace to be an all-purpose adhesive, while the commercial diphenyl PSA is reputed to have excellent tack properties for a silicone. When the obsolete solventless was developed, it too had excellent tack and adhesion properties. The focus of making the current prototype was to de-emphasize the need for the best tack and adhesion performance but rather improve the high temperature performance. Although the tack and peel adhesion performance is slightly lower than the commercial diphenyl and that of the obsolete solventless, the performance is well within the acceptable range for a silicone PSA. The biggest step change for the prototype solventless exists in the high temperature area. Figure 7 shows the high temperature shear performance (PSTC-7) for each PSA.

To be considered for most high-temperature applications, a silicone must be able to pass at 500 degrees F (260 degrees C) for five days. Both the commercial dimethyl and commercial diphenyl routinely pass the shear test under these conditions. The maximum temperature for the obsolete solventless was 400 degrees F before failing the shear test. This limited the widespread use of this adhesive. By comparison, the prototype solventless has been improved to withstand up to 500 degrees F without failure.

Each material was also analyzed on the TA-XT2i Texture Analyzer₄ as a supplement to traditional probe tack and peel adhesion data. Figure 8 shows the resulting output.

In the last few years, the texture analyzer and other probe-type tests that generate stress-strain curves have been gaining acceptance in the testing of pressure-sensitive adhesives due to the simplicity and relative ease of testing. This has also been applied to the development and evaluation of silicone PSAs. Studies have shown that the failure energy, as calculated by integration of the area under the curve, relates to the behavior of the adhesive during bonding and debonding processes._5-7The calculations done on the curves in Figure 8 are shown in the table. These include peak force, total area under the curve and the area ratio (area after peak force/area before peak force).

Additional evaluations of the prototype solventless have shown that this new PSA is also compatible with the industry standard, fluorosilicone-coated release liner system for silicone PSAs. Figure 9 displays the release profiles of each PSA. The commercial dimethyl, obsolete solventless and prototype solventless all show flat and consistent release across many delamination speeds.

Summary

Recent development work at Dow Corning has produced a new prototype solventless silicone PSA using the standard test methods of peel adhesion, probe tack and high-temperature shear. The texture analyzer has been further used to enhance the understanding of the structure/property relationships between the resin and polymer. The prototype solventless silicone PSA exhibits tack and peel adhesion properties that are typical of silicone PSAs, and most importantly, it also exhibits the high temperature shear performance that is required in many silicone PSA applications. The use of a platinum-cured solventless system also offers many advantages over that of a traditional peroxide-catalyzed system.

Acknowledgements

The author wishes to acknowledge Glenn Gordon for discussions on the use of rheology for silicone adhesives, and Lacy Brondstetter and Patricia Moore for their formulation and testing work.

References

_{1 Tangney, T. Silicone Pressure Sensitive Adhesives for High Performance Applications. Presented at Adhesives ’86 Conference, Baltimore, Sept. 1986.
2 Sobieski, L. Formulating Silicone Pressure Sensitive Adhesives for Application Performance. In Technical Seminar Proceedings, Pressure Sensitive Tape Council, Itasca, IL, May 7-9, 1986.

3 Tangney, T. Adhesive Development Offers Low Temps, Savings. Paper, Film & Foil CONVERTER, Nov. 1990.

4 Stable Micro Systems, Godalming, Surrey, U.K.

5 Zosel, A. The Effect of Bond Formation on the Tack of Polymers. J. Adhesion Science and Technol., 11(11):1447-1457, 1997.

6 Zosel, A. The Effect of Fibrillation on the Tack of Pressure Sensitive Adhesives. International J. Adhesion & Adhesives, 18(4):265-272, 1998.

7 Zosel, A.; Molecular Structure, Mechanical Behaviour and Adhesion Performance of Pressure Sensitive Adhesives. Technical Seminar Proceedings, Pressure Sensitive Tape Council, Northbrook, IL, May 3-5, 2000.}