Rubber composition for tires

Claim 1

A rubber composition for a pneumatic tire tread, characterized by comprising 10 to 120 parts by weight of carbon black and 0.5 to 5 parts by weight of a vulcanizer based on 100 parts by weight of a styrene- -butadiene copolymer rubber alone or a blend rubber of not less than 30 parts by weight of the said styrene- -butadiene copolymer rubber and not more than 70 parts by weight of other diene rubber, the said styrene- -butadiene copolymer rubber being obtained by random copolymerization of styrene with 1,3-butadiene and satisfying the following requirements : (1) the content of bound styrene is 10 to 30% by weight; (2) the content of 1,2-bonds in the butadiene is 42 to 70% by weight; (3) the content of trans-1,4 bonds in the butadiene is not less than 25% by weight; (4) the value obtained by subtracting the content of cis-1,4 bonds in the butadiene from the said content of trans-1,4 bonds is not less than 8% by weight; (5) the relation between the said content of bound styrene and the said content of 1,2-bonds is 65 ≦ 1.7 x the content of bound styrene (% by weight) + the content of 1,2-bonds (% by weight) ≦ 100; - (6) the weight mean molecular weight (M ) is 35 x 10 4 to 65 x 10 4 ; (7) the ratio of the weight mean molecular weight ( M w ) to the number mean molecular weight ( M n ) is not more than 2.3; (8) the number of peaks in a molecular weight distribution curve is not less than 2; and (9) the relation between the said weight mean molecular weight and the said content of trans-1,4 bonds is 75 < M w x 10 -4 + 1.3 x the content of trans-1,4 bonds (%by weight) < 1200