Polyorganosiloxane compositions

Claim 1

A curable thixotropic, liquid polyorganosiloxane composition comprising the product obtained by mixing (A) a polyorganosiloxane containing at least two vinyl radicals per molecule and represented by the average unit formula R.sub.a SiO.sub.(4-a)/2, where at least 50 mole % of the radicals represented by R are methyl, the remainder being selected from the group consisting of unsubstituted monovalent hydrocarbon radicals other than methyl,alkoxy and substituted monovalent hydrocarbon radicals where the substituent is selected from the group consisting of acryloxy, methacryloxy, chlorine and fluorine, and the value of a is from 1.8 to 2.2, inclusive; (B) a polyorganosiloxane containing at least two silicon-bonded nitrogen atoms per molecule and represented by the average unit formula R.sub.b.sup.1 SiO.sub.(4-b)/2, where R 1 is selected from the group consisting of unsubstituted monovalent hydrocarbon radicals, hydrogen, alkoxy, and substituted monovalent hydrocarbon radicals wherein the substituent is selected from acryloxy, methacryloxy, chlorine, and fluorine, the value of b is from 1.5 to 2.5, inclusive, and the molar ratio of silicon-bonded hydrogen atoms in (B) to vinyl radicals in (A) is from 0.2 to 5, inclusive; (C) an amount of a platinum-containing catalyst equivalent to from 0.1 to 200 parts by weight of platinum per million parts of combined weight of (A) and (B); (D) from 0.5 to 50 weight %, based on the combined weight of (A) and (B), of fine silica powder; and (E) from 0.05 to 10 weight %, based on the combined weight of ingredients (A) through (D), of a polyorganosiloxane containing at least 0.5 weight percent of hydroxyl groups per molecule and represented by the average unit formula R.sub.c.sup.2 SiO.sub.(4-c)/2, where the value of c is from 1.8 to 3.0, R 2 is selected from the group consisting of monovalent hydrocarbon radicals, methoxy, ethoxy, substituted alkyl radicals wherein the substituent is, in turn, selected from the group consisting of chlorine, fluorine, and hydroxyl, and radicals of the formula --R.sup.3 (OC.sub.2 H.sub.4).sub.x (OC.sub.3 H.sub.6).sub.y OH, where R 3 is alkylene, and the values of x and y are from 0 to 30, inclusive, with the proviso that the sum of x and y is at least 1, and that (E) contains a number of silicon-bonded hydroxyl groups sufficient to provide a total of at least 0.5 percent by weight of hydroxyl groups per molecule of (E) in combination with any hydroxyl groups present in R 2 ; wherein said composition cures to form a gel exhibiting a penetration of from 8 to 50 mm.