GB773324A - Improvements in silicone rubbers - Google Patents

Abstract

In making a heat-convertible organopolysiloxane containing an average of 1.98 to 2.01 hydrocarbon or halogenated hydrocarbon radicals per Si atom and which in the cured, solid, elastic state has low shrink characteristics, the volatile siloxanes boiling below 250 DEG C. when measured at 760 mm. present in the convertible polymer are removed therefrom to an extent that less 2 per cent by weight of the convertible siloxane consists of the volatile siloxanes. The volatiles, which are low molecular weight cyclic siloxane polymers, may be removed in the presence or absence of fillers or other additives other than the curing catalysts, by (1) heating the siloxane at 100 DEG to 250 DEG C., preferably under reduced pressure and desirably after neutralization of the polymerization catalyst; (2) selectively extracting the volatiles with suitable solvents, e.g. ethanol, butanol, isopropanol, ketones or mixtures thereof; or (3) by subjecting the siloxane to a kneading and air-stripping action as described and claimed in Specification 773,325. Specified are silica, aerogel, calcium carbonate, diatomaceous earth, other finely-divided silicas, iron oxide, titania and lithopone as fillers, and benzoyl peroxide and t-butyl perbenzoate as curing catalysts. In examples, a convertible dimethylpolysiloxane was (1) mixed with silica aerogel and compounded on a mill to form thin sheets which were then heated at 250 DEG C. in an oven until the volatiles had been removed; and (2) neutralized by addition of triphenyl phosphate, as described and claimed in Specification 772,088 and then subjected to a kneading and airstripping action. The filled siloxanes may be compounded with compression set additives, as described and claimed in Specification 768,286, and curing catalysts and cured in known manner. The products find use in making gaskets and electrical insulation. Specifications 594,481, [Group IV], 618,453 and 640,067 also are referred to.