Patent ID: 7884154

Claim:
A silica compounded rubber composition for a tire, the composition comprising 100 parts by weight of a rubber component including from 20 to 80% by weight of (a) and 80 to 20% by weight of (b), wherein (a) comprises a vinyl-cis-polybutadiene rubber composition containing 1,2-polybutadiene having a melting point of 170° C. or higher, a cis-polybutadiene rubber, and a first substance having at least one unsaturated double bond per repeating unit and comprising at least one member selected from the group consisting of polyisoprene, crystalline polybutadiene having a melting point of not higher than 150° C., liquid polybutadiene, and derivatives thereof, and wherein (b) comprises a diene-based rubber other than (a); and wherein the rubber composition further includes 40 to 100 parts by weight of (c), which comprises a rubber reinforcing agent containing 40% or more of silica, wherein the vinyl-cis-polybutadiene rubber composition (a) is produced by a process for producing a vinyl-cis-polybutadiene rubber composition by subjecting 1,3-butadiene to cis-1,4-polymerization in a hydrocarbon-based solvent by using a cis-1,4-polymerization catalyst to provide a resulting polymerization reaction mixture, subsequently making a 1,2-polymerization catalyst copresent in the resulting polymerization reaction mixture to subject 1,3-butadiene to 1,2-polymerization, thereby forming 1,2-polybutadiene having a melting point of 170° C. or higher, and then separating, recovering and obtaining the vinyl-cis-polybutadiene rubber composition as formed, wherein the process for producing the vinyl-cis-polybutadiene rubber comprises a step of adding the first substance during the production of the vinyl-cis-polybutadiene rubber, and wherein the 1,2-polybutadiene is in a short crystalline fiber form in which a minor axis average length of monodispersed fiber crystals is not more than 0.2 μm, an aspect ratio is not more than 10, and a number of average monodispersed fiber crystals is 10 or more per 400 μm 2 .