Patent ID: 8242194

Claim:
A method of preparing a silica reinforcement containing rubber composition with a delayed addition of fatty acid salt processing aid which comprises the sequential steps of, based upon parts by weight per 100 parts by weight rubber (phr): (A) preparing a rubber mixture by mixing, in an internal rubber mixer at a temperature in a range of from about 140° C. to about 190° C.: (1) 100 parts by weight of at least one sulfur vulcanizable elastomer selected from conjugated diene homopolymers and copolymers of at least one of isoprene and 1,3-butadiene and copolymers of styrene and at least one of isoprene and 1,3-butadiene; (2) about 15 to about 120 phr of particulate reinforcing filler comprised of precipitated silica and rubber reinforcing carbon black, wherein said reinforcing filler contains from 75 to about 90 weight percent of said precipitated silica; (3) a coupling agent comprised of a bis(3-trialkoxysilylalkyl)polysulfide having an average of from 2 to about 4 connecting sulfur atoms in its polysulfidic bridge exclusive of an alkoxyorganomercaptosilane, and (4) fatty acid salt processing aid as an in situ formed product of zinc oxide and fatty acid within the rubber composition in an amount of about 55 to about 80 percent of fatty acid; and thereafter: (B) mixing about 45 to about 20 percent of said fatty acid with said rubber mixture in a subsequent mixing step in a separate internal rubber mixer, thereby forming a resultant rubber composition; wherein said fatty acid is comprised of stearic acid and at least one of palmitic acid and oleic acid; and (C) thereafter blending, at a temperature in a range of from about 100° C. to about 120° C., elemental sulfur and at least one sulfur vulcanization accelerator with the resultant rubber composition in an internal rubber mixer.