Rubber composition having excellent gripping power and rolling resistance, and production thereof

A process for producing a rubber composition which comprises:

• blending, by using a rubber processing machine, (A) 100 parts by weight of a rubber component comprising at least 50% by weight of a styrene/butadiene copolymer rubber at a rubber temperature of 50.degree. to 190.degree. C. with (B) 1 to 6 parts by weight of an azodicarboxylic ester compound represented by the formula (I):
• ##STR6## wherein X is an alkyl of 3 to 8 carbon atoms, an unsubstituted or substituted phenyl, or an unsubstituted or substituted benzyl, and (C) 60 to 250 parts by weight of carbon black having a nitrogen absorption specific surface area of 100 to 250 m.sup.2 /g and a dibutyl phthalate absorption number of 110 to 170 ml/100 g;
• blending the resulting mixture at a rubber temperature of 10.degree. to 120.degree. C. with (D) 0.5 to 4 parts by weight of sulfur, and (E) 0.3 to 3 parts by weight of a benzothiazole vulcanization accelerator;
• and
• vulcanizing the resulting mixture.

A method for increasing a loss factor of a vulcanized rubber in a low temperature range of -20.degree. to +30.degree. C. as well as lowering the loss factor of the vulcanized rubber in a high temperature range of 50.degree. to 80.degree. C., said loss factor being determined by a dynamic viscoelasticity test with a frequency of 10 to 100 Hz, which method comprises:

• blending, in a rubber processing step, (A) 100 parts by weight of a rubber component comprising at least 50% by weight of a styrene/butadiene copolymer rubber at a rubber temperature of 50.degree. to 190.degree. C. with (B) 1 to 6 parts by weight of an azodicarboxylic ester compound represented by the formula (I):
• ##STR7## wherein X is an alkyl of 3 to 8 carbon atoms, an unsubstituted or substituted phenyl, or an unsubstituted or substituted benzyl, and (C) 60 to 250 parts by weight of carbon black having a nitrogen absorption specific surface area of 100 to 250 m.sup.2 /g and a dibutyl phthalate absorption number of 110 to 170 ml/100 g;
• blending the resulting mixture at a rubber temperature of 10.degree. to 120.degree. C. with (D) 0.5 to 4 parts by weight of sulfur, and (E) 0.3 to 3 parts by weight of a benzotriazole vulcanization accelerator;
• and
• vulcanizing the resulting mixture.