GB697057A - Improvements in or relating to heat-hardenable siloxane compositions - Google Patents

Abstract

A heat-hardenable composition comprises a polysiloxane having a viscosity of at least 10,000 cs. at 25 DEG C. and containing 1.99-2.0 organic radicals per silicon atom, the radicals being methyl, ethyl, siliconeopentyl or phenyl radicals or any combination thereof provided that at least 50 per cent of the radicals are alkyl radicals, and a silica filler having a pore volume of 3 to 10 c.c. per gram, in an amount of 5 to 150 parts per 100 parts by weight of the siloxane, the silica being combined with from 0.008 to less than 0.08 mols. per mol. of SiO2 of organosilyl radicals of the general formula R2R1Si- or RR1Si=, where R is an alkyl radical containing less than 5 carbon atoms and R1 is an alkyl radical containing less than 5 carbon atoms or a phenyl radical. The treated silicas may be made according to Specification 682,574, and may be used alone or together with, for example, 0.5 to 50 parts by weight per 100 parts of siloxane of untreated silicas having pour volumes of at least 3 c.c. per gram. The compositions may be formed into elastomers by heating to 250 DEG C., or to at least 110 DEG C. by using one or more organic peroxide catalysts selected from benzoyl peroxide, t.-butyl perbenzoate and t.-butyl per acetate. Examples are given. Specifications 671,140, 673,149 and 682,540 also are referred to.