GB788964A - Improvements in or relating to siloxane compositions and elastomers - Google Patents

Abstract

A heat-hardenable polysiloxane composition comprises by weight (1) 100 parts of a diorganopolysiloxane gum; (2) from 10 to 35 parts of a liquid copolymer having a viscosity of less than 1,000,000 cs. at 25 DEG C. and being composed of from 20 to 60 mol. per cent of dimethylsiloxane, from 30 to 60 mol. per cent of phenylmethylsiloxane, from 5 to 15 mol. per cent of siloxane units of the general formula RnViSiO <FORM:0788964/IV (a)/1> in which R is an alkyl or phenyl radical, Vi is a vinyl radical, and n is 0 or 1, and from 0.0003 to 35 mol. per cent of siloxane units of the general formula RMe2SiO0.5, in which R has the above meaning, and (3) an organic peroxide as vulcanizing agent. Copolymer (2) is either a copolymer of all four of the siloxanes defined above or a mixture of copolymers, such mixture containing all four of the defined siloxane units. The siloxane gum (1) may have silicon bonded aliphatic, aromatic or halogenated hydrocarbon radicals. Vulcanizing agents specified are benzoyl, dichlorobenzoyl and monochlorobenzoyl peroxides and t-butylperbenzoate. The compositions may also contain fillers such as titania, zinc oxide, ferric oxide, diatomaceous earth, crushed quartz, silica aerogels, fume silicas and silica xerogels (if desired, the fillers may be treated with hydrolysable silanes) and other known additives. The compositions may be compounded in conventional manner, but advantageously by compounding (1) and (2), and, if desired, the filler, and heating the mixture for a few hours at 150 DEG to 250 DEG C. prior to adding (3) and curing. Examples are given. The elastomers are useful for sealing purposes and for electrical insulation.