Advancements in Solventless Technology for Silicone PSAs By Alexander Knott, Dow Corning Corporation Paper Industry Specialist Dow Corning Corporation



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Advancements in Solventless Technology for Silicone PSAs By Alexander Knott, Dow Corning Corporation Paper Industry Specialist

Introduction Most commercial-use PSAs, including acrylics, rubber-based and epoxies, are based on organic chemistry. However, for applications where excellent performance at high temperatures and resistance to chemicals, moisture, weathering and UV are required, silicone-based PSAs are preferred. Silicone PSAs have good conformability, can be cleanly removed and show excellent adhesion to lowenergy surfaces. Due to their basic chemistry, they exhibit exceptional electrical insulating properties and provide some degree of sound and vibration dampening. Because they can withstand high temperatures and chemical attack, silicone PSAs are useful in applications such as plasma, flame spray and electronic circuit board masking tape, where conventional, organic-based PSAs are unable to perform satisfactorily. Their high-temperature performance also makes silicone PSAs suitable for use in powder coating applications, and their electrical properties make them suitable for tapes used in circuit assembly and EMI shielding. Further, their ability to adhere to low-energy surfaces makes silicone PSAs suitable for use as splicing tapes for siliconecoated release liners.

Figure 1. Silicone gum polymer

The MQ resin contains a level of silanol functionality on the surface. The MQ name derives from its structure, which consists of a core of three-dimensional Q units (SiO4/2) surrounded by a shell of M units (Me3SiO). (See Figure 2.) The resin is supplied as a dispersion in solvent. Silicone PSAs are produced by blending a specified ratio of the resin and polymer in a hydrocarbon solvent. The mixture is heated to promote a condensation reaction between the silanol functionality on the resin and the polymer, further enhancing the properties of the final adhesive. The ratio of resin to polymer is the most important formulation detail when optimizing the balance of performance properties for a given adhesive. Even slight variations in the ratio dramatically affect the properties of the PSA. Figure 2. MQ resin molecular model

The Chemistry of Silicone PSAs The composition of silicone PSAs parallels that of many common organicbased PSAs. The two main components of a silicone PSA are a high-molecularweight linear siloxane polymer and a highly condensed silicate tackifying (MQ) resin. Current silicone PSAs use either polydimethylsiloxane (PDMS) or polydimethyldiphenylsiloxane (PDMDPS) polymers, which contain silanol functionality at the polymer chain ends. (See Figure 1.) 

Adhesive Cure Chemistry Most silicone PSAs exhibit pressuresensitive behavior immediately following solvent removal. However, further crosslinking is generally required to sufficiently reinforce the adhesive network. Two basic cure systems for silicone PSAs are commercially available. The first is a peroxide-catalyzed free-radical cure system. Most commercially available silicone PSAs require a peroxide-catalyzed (benzoyl peroxide or 2,4-dichlorobenzoyl peroxide) freeradical reaction to achieve additional crosslink density. This is a two-stage process in which solvent is removed at lower temperatures (60 to 90°C) to ensure it is not inadvertently cured in the PSA matrix. This

stage is followed by catalyst decomposition at elevated temperatures (130 to 200°C) to form free radicals that attack the organic substituents along the polymer chains, extracting protons and generating free radicals. The free radicals then combine to form crosslinks, as shown at the right.

1) C6H5C(=O)OO(O=)CC6H5

2 C6H5C(=O)O•

Δ

2) C6H5C(=O)O• + ≡SiCH3

C6H5C(=O)OH + ≡SiCH2•

3) ≡SiCH2• + ≡SiCH2•

≡SiCH2CH2Si≡

Disadvantages of this type of silicone PSA system include the need to handle volatile solvents and peroxides, the impact of generated peroxide byproducts on stability, and the need to prime certain substrates to improve adhesive anchorage in self-wound tape.

Curing this type of silicone PSA can be accomplished at lower overall temperatures (100 to 150°C). This system’s lower-temperature cure offers benefits that include lower temperature variation sensitivity, the use of substrates (i.e., polyethylene and polypropylene) with lower thermal stability and the generation of no catalyst by-products. Further, unlike peroxide-catalyzed systems, which require solvent for viscosity but also to keep the peroxide dissolved within the adhesive bath, the platinumcatalyzed system requires solvent for viscosity control only, reducing the amount of solvent removal required.

The second basic cure system is a platinum-catalyzed silicone addition system. Silicone PSAs have been introduced that use a platinum-catalyzed reaction of silicon hydride with vinyl. (See Figure 3.)

In the early 1990s, Dow Corning commercialized a first-generation platinum-catalyzed “VOC-Compliant Silicone PSA.” The system demonstrated significant performance advantages over solvent-based silicone PSAs, including

The main benefit of the peroxidecatalyzed system is its ability to control properties by peroxide addition level (in the range from 0 to 4%). Additional crosslinking increases cohesive strength and slightly decreases adhesion and tack.

high peel adhesion, high tack and primerless adhesion. The only disadvantage was inferior high-temperature shear performance. Recent developments in silicone materials have led to the formation of a new solventless silicone PSA with the tack, adhesion and high-temperature shear of a common solvent-based silicone PSA.

Comparative Performance When comparing the performance of various silicone PSAs, it is possible to see how the different silicone PSA chemistries perform. In this study, four silicone PSAs were evaluated: two peroxide-cured and two platinum-cured. 1) Solvent-based, peroxide-catalyzed dimethyl silicone PSA, “Commercial Dimethyl” 2) Solvent-based, peroxide-catalyzed diphenyl silicone PSA, “Commercial Diphenyl”

Figure 3. Platinum-catalyzed reaction

3) A previously commercialized solventless silicone PSA, “FirstGeneration Solventless” 4) The new solventless silicone PSA, “New Solventless” The peroxide-catalyzed adhesives were prepared at 50 wt% solids in solvent using 2 wt% benzoyl peroxide. The platinum-catalyzed adhesives were prepared as 100% solids coatings using 0.5 wt% platinum catalysts. Using 180° peel measurements, the adhesion performance of the silicone PSA Commercial Dimethyl and Diphenyl adhesives were very similar. The First-Generation Solventless PSA



Figure 4. 180° Peel Adhesion – 38-50 g/m2 adhesive

had a slightly superior adhesion performance when compared to the peroxide PSAs, and the New Solventless PSA was similar to that of the typical peroxidecured adhesives. (See Figure 4.) In terms of the cohesive strength performance of silicone PSAs, standard lap-shear tests using a 25-mm x 25-mm overlap of tape on stainless steel surfaces illustrate that the performance of Commercial Dimethyl and Diphenyl adhesives is very similar, with good performance shown up to a temperature of 260°C in a 5-day test. However, the First-Generation Solventless PSA had a drawback: It was not as good in terms of high-temperature shear. This key area was addressed and rectified during the development of the New Solventless PSAs. (See Figure 5.)

Figure 5. High-temperature shear – 25 x 25 mm overlap, 1 kg weight, 5 days

Polyken Probe tack measurements of the silicone PSAs show that the Commercial Diphenyl adhesives generally develop greater tack than the Commercial Dimethyl adhesives. The First-Generation Solventless PSA had an equivalent tack to the Diphenyl, whereas the New Solventless PSA has an intermediate tack performance. (See Figure 6.) Polyken Probe tack is only one measure of tack and does not always agree with the results of other methods. There was a considerable level of variation in the results of Polyken Probe tack measurements taken at different times. This technique yields only a single measurement point. Comparing the Polyken Probe tack results to assessments made on the finger tack of the adhesives, the First-Generation Solventless PSA appears tackier than the Diphenyl PSA.

Figure 6. Polyken Probe Tack – 38-50 g/m2 adhesive

The problem with the assessment of tack by finger is its intrinsically subjective nature. One solution is to use a texture analyzer to measure tack behavior. When tack behavior was measured for these four PSAs using a TA-XT2i instrument and a 0.5-inch diameter spherical probe, failure energy trends, measured as the area under the peak, closely mirrored the finger tack assessments. Results indicated that the First-Generation Solventless PSA was the best, with Diphenyl as a very close second, followed by the New Solventless PSA and the Dimethyl last. (See Table 1.)

Adhesive Rheology In terms of the rheology, the main differences between these silicone PSAs

Table 1. Texture analyzer results 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

1st Gen. Solventless

114.80

66.72

1.25

New Solventless

109.84

45.68

0.70

Adhesive



is the temperature of the peak in tan delta, with the higher tack PSAs (namely the Diphenyl and First-Generation Solventless PSA) showing a tan delta peak at lower temperature. The New Solventless adhesive is similar, but still slightly higher in temperature. An interesting observation from all of the rheology measurements for silicone PSAs is how the elastic modulus remains high even at an elevated temperature, evidence of the high-temperature performance of silicone PSAs in general. Unfortunately, the test temperature was not high enough to determine the differences between the First-Generation Solventless PSA and other silicone PSAs. In summary, these results show that the new generation of solventless PSAs is quite comparable in its performance to

the existing, solvent-based silicone PSAs and resolves many of the concerns with the First-Generation Solventless PSAs.

Changing Trends, Future Developments A number of changing trends in the use of tapes are likely to affect silicone PSA applications. These include the continuing trend away from using mechanical fastening to using chemical fastening, the increased use of masking tapes to reduce process steps, and the increasing demand in Asia for all kinds of tapes. The increasing general market for tapes impacts the market for silicone PSAcoated tapes. Some trends specifically impact the use of silicone PSAs, such as the drive toward smaller and smaller electronics, which give off more heat. This creates a greater need for adhesives with superior heat stability. Another trend is the increasing use of substrates with low surface energy (polypropylene and polyethylene, for instance), where adhesion of conventional adhesives may be a challenge. A number of developments in silicone PSA technology are already underway to support some of these market trends.



These developments include: • Development of silicone PSAs with ultra-high temperature stability (in excess of 280°C) to meet the challenges posed by the new solders being used in the electronics industry, which require higher processing temperatures. • Development of even lowertemperature-curing silicone PSAs for use on temperature-sensitive substrates. • Continued development of alternative delivery systems for silicone PSAs such as hot melt and emulsion for easier and faster application. • Development of organic/silicone PSA hybrids to combine some of the advantages of both types of adhesives.

Acknowledgements The author would like to thank Luc Dusart, Tim Mitchell and Norm Kanar of the Dow Corning Paper Industry Group for their help in gathering the data for this paper.

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