GB793594A - Vulcanisable compositions - Google Patents

Abstract

Vulcanizable compositions consist essentially of (a) one or more polysiloxanes having a viscosity of at least 10,000 cs. at 25 DEG C. and having an average of from 1.9 to 2 saturated aliphatic, saturated cycloaliphatic or aryl hydrocarbon radicals per silicon atom; (b) from 20 to 60 parts by weight per 100 parts of siloxane of a silica filler, the surface of which is saturated with <FORM:0793594/IV (a)/1> units in which n has an average value from 2 to 3 inclusive, the units being attached to silicon through Si.O.Si linkages; and (c) from 0.5 to 10 parts (by weight) per 100 parts of siloxane of a vulcanizing agent which is a thiuram disulphide or a polyvalent metal salt of a dialkyldithiocarbamic acid. Vulcanization is effected by heating preferably at temperatures from 110 DEG to 250 DEG C. Ingredients specified are (a) siloxanes containing methyl, ethyl, propyl, octadecyl, cyclohexyl, cyclopentyl, phenyl, xylyl or naphthyl radicals; (b) fillers made treating a finely-divided silica aerogel or fume silica with a chlorosilane or by the method of Specification 783,868; (c) vulcanizing agents:-tetramethyl-, tetraoctadecyl-and di-n-pentamethylene-thiuram disulphides and dithiocarbamates of the general formula <FORM:0793594/IV (a)/2> where Y is a monovalent hydrocarbon radical, e.g. methyl, ethyl, butyl, benzyl and phenyl, M is a polyvalent metal, e.g. zinc, lead, copper, bismuth, selenium and tellurium and x is at least 2 and equal to the valency of M. Pigments, compression set additives, &c., may also be added. The vulcanized compositions are useful for electrical insulation and gaskets. Specification 781,278 also is referred to.