Rubber compositions

Claim 1

A rubber composition curable by heating into a rubbery elastomer comprising: (a) 100 parts by weight of a rubber blend composed of (a-1) from 50 to 95% by weight of an organic rubber which is one of natural rubber and an organic synthetic rubber, chosen from the group consisting of styrene-butadiene rubbers, butadiene rubbers, ethylene-propylene rubbers, ethylene-propylenediene terpolymer rubbers, butadiene-acrylonitrile rubbers, ethylene-vinyl acetate copolymer rubbers, and isobutylene-isoprene rubbers, and (a-2) from 50 to 5% by weight of an organopolysiloxane expressed by the average unit formula R.sub.a Si O.sub.(4-a/2) where R is a substituted or unsubstituted monovalent hydrocarbon group and a is a number from 1.98 to 2.05 both inclusive, and having at least two functional units selected from the class consisting of mercapto groups and aliphatically unsaturated linkages in the molecule, (b) from 0.1 to 20 parts by weight of an organosilicon compound chosen from the group consisting of bis (trimethoxysilylpropyl) tetrasulfide, bis (methyldiethoxysilypropyl) tetrasulfide, (EtO).sub.3 SiCH.sub.2 CH.sub.2 CH.sub.2 S.sub.4 CH.sub.2 CH.sub.2 CH.sub.2 Si(OEt).sub.3 ##STR5## wherein Me, Et and Bu denote methyl, ethyl, and butyl groups, respectively, (c) from 50 to 200 parts by weight of a reinforcing siliceous filler having a specific surface area of at least 50 m.sup.2 g, and (d) from 0.1 to 10 parts by weight of a curing agent chosen from the group consisting of sulfur, sulfur-containing organic compounds, organic peroxides, a combination of an organic peroxide and sulfur, and a combination of an organic peroxide and a sulfur-containing organic compound.