Storage stable, foamable two-part polyorganosiloxane compositions

Claim 1

A two-part polyorganosiloxane composition which when combined to form a homogeneous, flowable mixture yields a cured foam under atmospheric pressure and at a temperature above the boiling point of the blowing agent contained therein, said composition comprising (A) a hydroxyl endblocked polydimethylsiloxane exhibiting a viscosity of from 0.1 to 100 Pa.s at 25° C.; (B) a liquid alkyl orthosilicate or partial hydrolyzate thereof in an amount sufficient to cure said polydimethylsiloxane with the proviso that ingredients (A) and (B) are maintained in separate parts of said composition; (C) an amount of a compatible tin compound sufficient to catalyze the reaction of (A) and (B); (D) a blowing agent maintained under superatmospheric pressure and having a boiling point of no higher than 25° C. under atmospheric pressure, the amount of (D) being sufficient to convert said homogeneous flowable mixture to a foam under atmospheric pressure; and (E) a foam stabilizer in an amount sufficient to stabilize said foam during the curing process, where (E) consists essentially of a resinous, benzene-soluble organosiloxane copolymer consisting essentially of SiO 4/2 units, silicon-bonded hydroxyl groups, (CH 3 ) 3 SiO 1/2 units and fluorine-containing units selected from the group consisting of R a R' b SiO.sub.(4-a-b)/2, R"[Si(R') b O.sub.(3-b)/2 ] 2 and mixtures thereof, where R is a monovalent organic radical containing at least four perfluorinated carbon atoms and is bonded to the silicon atom of said fluorine-containing units by a sequence of at least two methylene units or by a silicon-bonded oxygen atom that is, in turn, bonded to a sequence of at least two methylene units, R' is an alkyl radical containing from 1 to 3 carbon atoms, R" is an divalent organic radical containing at least four perfluorinated carbon atoms and is bonded to the silicon atoms of said fluorine-containing units by a sequence of at least two methylene units or by a silicon-bonded oxygen atom that is, in turn, bonded to a sequence of at least two methylene units, a is 1 or 2, b is 0, 1 or 2, and the sum of a and b is 3 or less with the proviso that when R and R" represent fluorine-containing units bonded to silicon via an oxygen atom the orgnosiloxane copolymer optionally contains repeating units of the formula GSiO 3/2 where G represents the residue obtained by removal of the hydrogen atom from a hydroxyl group of a linear organic polymer containing an average of at least one terminal hydroxyl group per molecule and selected from the group consisting of polyethers, polyoxyalkylene glycols, homopolymers of ethylenically unsaturated alcohols and copolymers of ethylenically unsaturated alcohols with ethylenically unsaturated hydrocarbons; the molar ratio of all units other than SiO 4/2 and hydroxyl groups to SiO 4/2 units is from 0.7:1 to 1.1:1, inclusive, and the concentration of said fluorine-containing units and any GSiO 3/2 units is sufficient to (a) impart a surface tension of less than 2.2×10 -4 newtons per centimeter at 25° C. to a 10 percent by weight solution of (E) in a trimethylsiloxy endblocked polydimethylsiloxane exhibiting a viscosity of 0.01 Pa.s at 25° C. and (b) require the addition of from 0 to 100 percent by weight of xylene to said 10 percent by weight solution to achieve optical clarity at 25° C.