ADVANCEMENTS IN SLVENTLESS TECHNLGY FR SILICNE PSAs Tim Mitchell, Senior Development Chemist, Dow Corning Corporation, Midland, MI Abstract licone Pressure Sensitive Adhesives (PSAs) have been used for many years in areas where typical organic PSAs have failed. ne of the most important uses is in applications where large temperature extremes occur. A wide variety of silicone PSAs are available on the market today and in all cases these materials are offered at 50-60% solids in an organic carrier solvent. While many organic PSAs have found alternative delivery systems (emulsion, hotmelt, etc.), industrial silicone PSAs have been slow to evolve with the times. Attempts have been made in the past to make solventless versions of an industrial silicone PSA without much commercial success. The PSA would have the tack and adhesion performance that was typical of a silicone PSA, but the high temperature shear performance was lacking. Recently, there have been advancements in silicone-based raw materials that have opened up the ability to formulate solventless silicone PSAs that have the tack, adhesion and high temperature shear performance of common solvent-based silicone PSAs. This paper will discuss the performance of past solventless attempts, current solvent-based products and a novel solventless silicone PSA (patent pending) using 180 degree peel, Polyken probe tack, Texture Analyzer, High Temperature Shear testing and Rheology testing procedures. Introduction The composition of silicone PSAs parallels that of many common organic based 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 utilize either a polydimethylsiloxane (PDMS) or polydimethyldiphenylsiloxane (PDMDPS) polymers that contain silanol functionality at the polymer chain ends. R1 R2 R2 R2 R1 = (H, vinyl, alkyl, phenyl, etc.) R2 = (methyl, phenyl, alkyl, vinyl, etc.) Figure 1: licone Gum Polymer n R1 The silicate resin, often referred to as a 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 (4/2) surrounded by a shell of M-units (Me 3 ). 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.
Figure 2: MQ Resin Molecular Model licone PSAs are produced by blending a specified ratio of the 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 for 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. Figure 3: Effect of composition on silicone PSA properties Tack Hold/Shear Peel Adhesion Inc rea sin g Val ue s -- --> 45 50 55 60 65 70 Increasing R/P ---->
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. There are two basic cure systems 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 to 90 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 to 200 C) the catalyst decomposes to form free radicals, which primarily attack the organic substituents along the polymer chains, extracting protons and generating free radicals 1. The free radicals then combine to form crosslinks as shown below: 1) C 6 H 5 C(=)(=)CC 6 H 5 Δ 2 C 6 H 5 C(=) 2) C 6 H 5 C(=) + C 6 H 5 C(=)H + CH 2 3) CH 2 + CH 2 CH 2 CH 2 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 from 0 to 4% 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 2. Some of the disadvantages of this type of silicone PSA system include the handling of volatile solvents, generation of peroxide by-products, 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 which utilize a different type of curing mechanism 3. These adhesives are cured by a platinum-catalyzed reaction of silicon hydride to vinyl. This chemistry is analogous to the typical solvent-based 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 to 150 C), even though these systems are supplied in hydrocarbon solvents. As the solvent evaporates, the platinum-catalyzed reaction occurs without any generation of by-products as shown in figure 4.
Figure 4: Platinum-catalyzed reaction C H 2 C H H CH 2 x C + C H 3 H y Pt C H 2 C H H 2 x C CH 2 H 3 C H y -1 H 3 C H 3 C 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 by-products. 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. In the early 1990 s, Dow Corning commercialized a VC Compliant licone PSA 4 that was meant to address the market need for the reduction of solvents. This product was based on the platinum-catalyzed chemistry and showed significant performance advantages such as high peel adhesion, high tack, primer-less adhesion to multiple substrates and a lower volatile siloxane content. The one disadvantage that it had in comparison to other traditional silicone PSAs was its inferior high temperature shear performance. To be commercially viable, a silicone PSA ultimately needs to be able to perform in temperature extremes where an organic PSA fails. This product was eventually removed from the commercial market. 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, bsolete Solventless and 4) a new prototype solventless silicone PSA, Prototype Solventless. The peroxide catalyzed adhesives were prepared at 50 wt. % solids in solvent using 2 wt. % benzoyl peroxide based on the silicone solids. Figures 5 and 6 show the peel adhesion (PSTC-1) and Polyken probe tack performance (ASTM D-2979) for each PSA on 2 different substrates, respectively. Figure 5: 180 Degree Peel Adhesion 70 180 Degree Peel Adhesion: 1.5 to 2.0 mils adhesive 2-mil Polyester 1-mil Polyimide 60 60 50 43.7 46.2 Peel Adhesion (oz/in) 40 30 26.3 32 36.5 20 21.7 20.5 10 0 Commercial Dimethyl Commercial Diphenyl bsolete Solventless Prototype Solventless
Figure 6: Polyken Probe Tack Polyken Probe Tack: 1.5 to 2.0 mils adhesive 1600 1400 1500 1417 2-mil Polyester 1-mil Polyimide 1229 Tack (grams) 1200 1000 800 1060 795 1120 996 890 600 400 200 0 Commercial Dimethyl Commercial Diphenyl bsolete Solventless Prototype Solventless Comparison of these four adhesives on the chart illustrates an important point. The Commercial Dimethyl PSA is known in the marketplace to be an all-purpose adhesive, while the Commercial Diphenyl PSA has the reputation of having excellent tack properties for a silicone. When the bsolete 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 bsolete 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.
Figure 7: High Temperature Shear on 1-mil Polyimide High Temperature Shear: 1" x 1" on SS, 1000g weight, 5 days 550 500 450 Temperature (F) 400 350 300 250 200 Commercial Dimethyl Commercial Diphenyl bsolete Solventless Prototype Solventless To be considered for most high temperature applications, a silicone must be able to pass at 500 F (260 C) for 5 days. Both the Commercial Dimethyl and Commercial Diphenyl routinely pass the shear test under these conditions. The maximum temperature for the bsolete Solventless was 400 F before failing the shear test. This limited the widespread utility of this adhesive. By comparison, the Prototype Solventless has been improved to withstand up to 500 F without failure. Each material was also analyzed on the TA-XT2i Texture Analyzer 5 as a supplement to traditional Polyken probe tack and peel adhesion data. Figure 8 shows the resulting output.
Figure 8: Texture Analyzer Curves Texture Analyzer: 1.5 to 2.0 mils of adhesive 130 120 110 100 90 Commercial Dimethyl Commercial Diphenyl bsolete Solventless Prototype Solventless 80 70 60 Force (grams) 50 40 30 20 10 0 0-10 0.5 1 1.5 2 2.5-20 -30-40 -50 Time (seconds) In the last few years, the Texture Analyzer and other probe type tests that generate stress-strain curves have been gaining acceptance in 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 6, 7, 8. The calculations done on the curves above are in Table 1. These include peak force, total area under the curve and the area ratio (area after peak force/area before peak force). Table 1: Texture Analyzer Results Adhesive Peak Force (g) Total Area (g.s) Area Ratio Commercial Dimethyl 82.86 37.49 1.72 Commercial Diphenyl 86.22 63.35 2.95 bsolete Solventless 114.80 66.72 1.25 Prototype Solventless 109.84 45.68 0.70 These four adhesives were also evaluated with a Rheometrics Dynamic Spectrometer (RDS II) using a temperature sweep from 150 C to 150 C, 3 C/min at 1 rad/s, 1% strain in dynamic mode. The G curves for each adhesive is in Figures 9.
Figure 9: G versus Temperature 1.00E+09 1.00E+08 Commercial Dimethyl Commercial Diphenyl bsolete Solventless Prototype Solventless G' (dynes/cm2) 1.00E+07 1.00E+06 1.00E+05 1.00E+04-50.00-30.00-10.00 10.00 30.00 50.00 70.00 90.00 110.00 130.00 150.00 Temperature ( C) Comparing the G for each adhesive at RT versus the Total Area under the curve of the Texture Analyzer, as in Figure 10, shows that a reasonable correlation exists. This correlation is not surprising, as the ability of the material to wet out the probe under short contact times and low contact forces should correlate to the materials modulus at a given temperature.
Figure 10: G versus Total Area of TA 100000000 y = -104773x + 7E+06 R 2 = 0.9629 10000000 G' (dynes/cm2) at 25C 1000000 100000 10000 20 30 40 50 60 70 80 Total TA Area (g.s) 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 11 displays the release profiles of each PSA.
Figure 11: Performance of licone PSAs on Fluorosilicone Release Liner Release Profile from Fluorosilicone Release System: Direct Cast at 1.5 to 2.0 mils adhesive 300 250 Commercial Dimethyl Commercial Diphenyl bsolete Solventless Prototype Solventless 200 Release Force (g/in) 150 100 50 0 10 100 1000 10000 100000 Delamination Speed (ipm) The Commercial Dimethyl, bsolete 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 (patent pending) using the standard test methods of peel adhesion, Polyken probe tack and high temperature shear. The Texture Analyzer and DMA (dynamic mechanically analysis) were 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. These benefits are summarized in Table 2.
Table 2: Benefits of the Prototype licone PSA 1. Lower volatile organic content 2. Lower volatile siloxane content 3. No cure by-products 4. ngle-zone oven cure 5. Fast cure 6. Lower temperature cure 7. Curable over a wide range of temperatures and times 8. Reduced worker exposure to hazardous solvents 9. Primer-less adhesion to substrates 10. Wider range of backing materials possible 11. Compatible with standard silicone PSA release system (Syl-ff Q2-7785) References 1. Tangney, T.; licone Pressure Sensitive Adhesives for High Performance Applications. Presented at Adhesives 86 Conference, Baltimore Md., September, 1986. 2. Sobieski, L.; Formulating licone Pressure Sensitive Adhesives for Application Performance. In Technical Seminar Proceedings, Pressure Sensitive Tape Council, Itasca, Ill., May 7-9, 1986. 3. Tangney, T.; Adhesive Development ffers Low Temps, Savings. Paper, Film & Foil CNVERTER, November, 1990. 4. Schmidt, Randall, Vincent, Gary and Brady, William; VC Compliant licone PSAs. In Technical Seminar Proceedings, Pressure Sensitive Tape Council, Northbrook, Ill., May 5-7, 1993. 5. Stable Micro Systems, Godalming, Surrey, U.K. 6. Zosel, A.; The effect of bond formation on the tack of polymers. Journal of Adhesion Science and Technology, 11(11):1447-1457, 1997. 7. Zosel, A.; The effect of fibrillation on the tack of pressure sensitive adhesives. International Journal of Adhesion & Adhesives, 18(4):265-272, 1998. 8. Zosel, A.; Molecular Structure, Mechanical Behaviour and Adhesion Performance of Pressure Sensitive Adhesives. Technical Seminar Proceedings, Pressure Sensitive Tape Council, Northbrook, Ill., May 3-5, 2000. 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.