Platinum-catalyzed addition cure

The formulation ofmoisture-ujriiig silicones includes a silicone polymer, filler, a moisture-reactive cross-linker, and sometimes a catalyst. A newer class of silicone sealants are known as the silicone latex sealants. These sealants are silicone-in-water emulsions that cure by evaporation of the emulsifying water. Addition-curing silicones in general are two-part systems that cure by the platinum-catalyzed reaction of a silicone hydride with typically a vinyl group attached to silicon. Because no by-products are generated by the cure, there are few volatiles and no shrink in thick sections. 

The additive approach also was shown to be effective in making a silicone foam. Kittle and Ronk (9 ) demonstrated that addition of ferric oxide or alcohols to platinum catalyzed foam formulation would allow the material to foam when heated by micro-wave, whereas the same composition without ferric oxide or alcohol did not foam or cure. 

The reaction is also catalyzed by platinum. The addition of some vinyl-containing polysiloxane can thus improve properties such as density, tensile strength, cure rate, and so forth. 

There are numerous applications where cure reaction by-products are not acceptable as they may contaminate other sensitive areas of the devices, and where fast deep section cure is required. These constraints have directed the choice to the platinum-catalyzed hydrosilylation addition cure system. The reaction involves the addition of a hydrido silane (SiH) to an alkenyl organic group (-CH=CH2), typically the vinyl or hexenyl groups. The product of the reaction is the ethylenic bridge (-CH2-CH2-). The alkenyl groups are usually placed at the end of the polymer base chains in structures such as The SiH groups are usually placed in a combed structure within the polymeric or copolymeric chains called cross-linker M-D -M or M-D -Dy-M. (The M, D, etc. nomenclature is explained in Silicones structures.) Typical values of n cover a wide range from 100 to 1000, while the values of x and y vary from 3 to 100. 

Alternatively, addition curing of a two-component silicone based on the platinum-catalyzed reaction between a hydride functional siloxane polymer (component A) and a vinyl functional siloxane (component B) may lead to sensor films. The result is an ethyl bridge between the two polymers according to Scheme 22.2. 

These examples illustrate that augmenting the thermal crosslinking reaction of vinyl-substituted precursors with platinum catalysts provides a means to obtain crosslinked precursors more rapidly or with enhanced ceramic yield. Still, the processing times described for both the thermal and the platinum-catalyzed cure methods are too long for rapid ceramic processing. In addition, hydrosilylation of nitrogen-containing silicon compounds is difficult at best. 

Another two-pack RTV formulation cures by hydrosilation, which involves the addition reaction between a polysiloxane containing vinyl groups (obtained by including methylvinyldichlorosilane in the original reaction mixture for synthesis of polysiloxane) and a siloxane cross-linking agent that contains Si-H functional groups, such as Si[OSi(CH3)2H]4. The reaction is catalyzed by chloroplatinic acid or other soluble platinum compounds. 

Uses Encapsulant, molding and coating compd. tor electronic devices, optical devices, supported membranes Features Filler-tree high optical transmission relatively low vise. extended pot life flexible RTV vinyl-addition (platinum) curing Properties Vise. 1750-2500 cSt (1 1 catalyzed) Cured props. ret. index 1.407 tens. str. > 300 psi elong. 140-200% tear str. 5-15 pli hardness (Shore A) 15-30 Gelest OE 42 [Gelest]... 

---