Liquid curable polyorganosiloxane compositions

Claim 1

A curable composition yielding an optically transparent polyorganosiloxane elastomer, said composition comprising the product obtained by blending the following ingredients to homogeneity: A. at least one liquid polydiorganosiloxane exhibiting a viscosity of from 0.1 to 40 Pa.s at 25°C. and containing a vinyl radical at each of the two terminal positions; B. from 5 to about 40 percent, based on the total weight of said composition, of a benzene soluble resinous copolymer consisting essentially of triorganosiloxy units of the general formula R′′₃ SiO 1/2 , diorganovinylsiloxy units of the general formula CH₂=CH(R′′′)₂SiO 1/2 , and SiO₂ units, where R′′ and R′′′ are individually monovalent hydrocarbon or halohydrocarbon radicals containing from 1 to about 20 carbon atoms and free of ethylenic unsaturation, the molar ratio of combined triorganosiloxy units and diorganovinylsiloxy units to SiO₂ units is from 0.7 to 1.2 and said copolymer contains from 0.1 to 8 weight percent of silicon-bonded vinyl radicals; C. an organohydrogensiloxane curing agent containing at least three silicon-bonded hydrogen atoms per molecule, D. a diorganohydrogensiloxy terminated polydiorganosiloxane containing two silicon-­bonded hydrogen atoms and an average of up to 50 diorganosiloxane units per molecule in an amount sufficient to provide from l0 to 85 percent of the silicon-bonded hydrogen atoms present in said composition, and E. a platinum-containing hydrosilation catalyst in an amount sufficient to promote curing of said composition;