Liquid curable polyorganosiloxane compositions

Claim 1

A curable composition yielding an optically transparent polyorganosiloxane elastomer, said cmposition 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 CH 2 ═CH(R'") 2 SiO 1/2 , and SiO 2 units, where R' and R'" are individually monovalent hydrocabon 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 SiO 2 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 10 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; with the proviso that the composition contains at least 1.5 vinyl radicals for each silicon-bonded hydrogen atom, and the combined concentration of said curing agent and said diorganohydrogensiloxy terminated polydiorganosiloxane is sufficient to cure said composition.