Silica reinforced rubber composition and use in tires

A process of preparing a rubber composition which comprises the sequential steps of:

• (A) thermomechanically mixing in at least one preparatory mixing step to a temperature of about 140.degree. C. to about 190.degree. C. for a total mixing time of about 2 to about 20 minutes (i) 100 parts by weight of at least one sulfur vulcanizable elastomer selected from conjugated diene homopolymers and copolymers and copolymers of at least one conjugated diene and aromatic vinyl compound, (ii) about 15 to about 100 phr of particulate filler comprised of at least one of precipitated silica, alumina, aluminosilicate and carbon black, wherein said filler contains from about 5 to about to 85 weight percent carbon black, (iii) about 0.05 to about 20 parts by weight per part by weight of said particulate filler of at least one organosilane disulfide/trisulfide compound having the formula:
• Z--R.sup.1 --S.sub.n --R.sup.1 --Z
• wherein n is an integer of from 2 to 6 provided, however, that about 55 to about 75 percent of n is 2, about 15 to about 35 percent of n is 3 and about 2 to about 10 percent of n is 4, and less than 10 percent of n is greater than 4 provided that the sum of n=2 and n=3 is in a range of about 90 to about 98 percent of n;
• wherein Z is selected from the group consisting of:
• ##STR2## wherein R.sup.2 may be the same or different and is individually selected from the group consisting of alkyl group having 1 to 4 carbons and phenyl;
• R.sup.3 may be the same or different and is individually selected from the group consisting of alkyl groups having 1 to 4 carbon atoms, phenyl, alkoxy groups having 1 to 8 carbon atoms and cycloalkoxy groups with 5 to 8 carbon atoms;
• and R.sup.1 is selected from the group consisting of a substituted or unsubstituted alkylene group having a total of 1 to 18 carbon atoms and a substituted or unsubstituted arylene group having a total of 6 to 12 carbon atoms;
• (iv) at least one additional additive as a compound selected from (a) a free sulfur source selected from at least one of (1) elemental sulfur and (2) at least one sulfur donor as a polysulfidic organic compound containing sulfur and having a property of releasing at least a portion of said sulfur at a temperature in a range of about 140.degree. C. to about 190.degree. C.;
• provided, however, that the total free sulfur from said elemental sulfur addition and available from said sulfur donor addition is in a range of about 0.05 to about 2 phr, and (b) about 0.1 to about 0.5 phr of at least one vulcanization accelerator for sulfur vulcanizable elastomers that is not a such a sulfur donor;
• and
• (B) subsequently blending therewith, in a final thermomechanical mixing step at a temperature to about 100.degree. C. to about 130.degree. C. for a time of about 1 to about 3 minutes, about 0.4 to about 3 phr of elemental sulfur provided, however that the total of elemental and/or free sulfur introduced in said preparatory mixing steps and elemental sulfur added in said final mixing step is in a range of about 0.45 to about 5 phr, and at least one sulfur vulcanization accelerator.