Liquid curable polyorganosiloxane compositions

Claim 1

A curable liquid polyorganosiloxane composition consisting essentially of the product obtained by mixing (A) 100 parts by weight of at least one liquid triorganosiloxy endblocked polydimethylsiloxane, said triorganosiloxy radicals being selected from dimethylvinylsiloxy and methylphenylvinylsiloxy, where said polydimethylsiloxane exhibits a viscosity of from 1.0 to 500 Pa's at 25°C; (B) from 5 to 25 parts of a benzene-soluble copolymer consisting essentially of units of the formulae CH 2 =CH(CH 3 ) 2 SiO 1/2 , (CH 3 ) 3 SiO 1/2 and SiO 4/2 , where the molar ratio of the combination of CH 2 =CH(CH 3 ) 2 SiO 1/2 and (CH 3 ) 3 SiO 1/2 units to SiO 4/2 units is from 0.7:1 to 1.2:1, inclusive, and CH 2 =CH(CH 3 ) 2 SiO 2 units constitute from 2 to 8 percent by weight of said copolymer; (C) an organosiloxane of the formula H(CH 3 ) 2 SiO(Si(CH 3 ) 2 O] x Si(CH 3 ) 2 H, where x is an integer from 0 to 50, in an amount sufficient to provide at least 1.6 silicon-bonded hydrogen atoms per vinyl radical present in said composition; (D) a polyorganosiloxane containing at least three silicon-bonded hydrogen atoms per molecule in an amount sufficient to provide from 5 to 15 percent of the silicon-bonded hydrogen atoms present in (C) and (D); and (E) an amount of platinum catalyst sufficient to promote curing of said composition, characterized in that the total number of silicon-bonded hydrogen atoms present in (C) and (D) is from 1.8 to - 2.9 times the number of vinyl radicals present in said composition.