Abstract: By blending 5-40 parts by weight of a polyisoprene having an intrinsic viscosity [n] of 0.2-1.5 dl/g as measured in toluene at 30.degree. C. and a total 1,2- and 3,4-configuration content of 30% or less in its micro-structure into 100 parts by weight of an ethylene-propylene rubber, the adhesive property of the ethylene-propylene rubber to fibers can be improved while maintaining the heat resistance, weather resistance, ozone resistance, low temperature properties, electrical insulating property and chemical resistance which are all characteristic of ethylene-propylene rubber.

Abstract: A novel rubber composition is obtained by blending a rubber mixture consisting of 15 to 50 parts by weight of 1,2-polybutadiene having a 1,2-addition unit content of 70% or more, a crystallinity of 5 to 40% and an intrinsic viscosity [.eta.] of 0.7 dl/g or more (as measured in toluene at 30.degree. C.) and 85 to 50 parts by weight of at least one rubber selected from the group consisting of natural rubber and diene-based rubbers with 3 to 10 parts by weight of sulfur per 100 parts by weight of said rubber mixture. Said rubber composition has a desirable processability and exhibits, on curing, high hardness, high modulus, low temperature dependency of physical properties, low heat generation and excellent blow-out resistance. It is suitable as a rubber material for use in various low profile automobile tires such as radial tires.