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1. A pneumatic rubber truck tire having a circumferential stratified dual cap rubber tread comprised of an outer tread cap and an underlying tread sub cap wherein, based on parts by weight per 100 parts by weight rubber (phr),
(A) said tread cap rubber composition is comprised of:
(1) conjugated diene based elastomers comprised of about 20 to about 80 phr of natural cis 1,4-polyisoprene rubber (NR) and about 80 to about 20 phr of cis 1,4-polybutadiene rubber (BR),wherein said polybutadiene rubber (BR) is comprised of a combination of designated (BR1) and specialized (BR2) cis 1,4-polybutadiene rubbers, wherein, in terms of phr:
(a) if BR is less than or equal to 40 phr, BR=BR1,(b) if BR is greater than 40 phr,
BR1=25+((BR−25)/2)×Y where Y=is a value in a range of from 0.2 to 1.6, and(2) about 40 to about 90 phr of reinforcing filler comprised of about 50 to about 90 weight percent of a first rubber reinforcing carbon black and from about 10 to about 50 weight percent precipitated silica together with silica coupler for said precipitated silica containing a moiety reactive with hydroxyl groups on said precipitated silica and another different moiety interactive with said diene-based elastomers, (B) said tread sub-cap rubber composition is comprised of:
(1) conjugated diene based elastomers comprised of about 40 to about 60 phr of natural cis 1,4-polyisoprene rubber (NR) and about 60 to about 40 phr of cis 1,4-polybutadiene rubber (BR),wherein said polybutadiene rubber (BR) is comprised of a combination of designated (BR1) and specialized (BR2) cis 1,4-polybutadiene rubbers, wherein, in terms of phr:
(a) if BR is less than or equal to 40 phr, BR=BR1,(b) if BR is greater than 40 phr,
BR1=16+((BR−16)/2)×Y where Y=is a value in a range of from 0.2 to 1.6, and(2) about 40 to about 90 phr of reinforcing filler comprised of about 50 to about 90 weight percent of a second rubber reinforcing carbon black and from about 10 to about 50 weight percent precipitated silica together with silica coupler for said precipitated silica containing a moiety reactive with hydroxyl groups on said precipitated silica and another different moiety interactive with said diene-based elastomers,wherein for said tread cap and said tread sub-cap rubber compositions, said specialized cis 1,4-polybutadiene rubber has a microstructure comprised of about 90 to about 99 percent cis 1,4-isomeric units, a number average molecular weight (Mn) in a range of from about 90,000 to about 220,000 and a heterogeneity index (Mw/Mn) in a range of from about 2.5/1 to about 5/1, andsaid designated cis 1,4-polybutadiene rubber has a microstructure comprised of about 96 to about 99 percent cis 1,4-isomeric units, a number average molecular weight (Mn) in a range of from about 150,000 to about 300,000 and a heterogeneity index (Mw/Mn) in a range of from about 1.5/1 to about 2/1,wherein said first rubber reinforcing carbon black for said tread cap rubber composition has an iodine adsorption value (ASTM D1510) in a range of from about 100 to about 150 g/kg together with a dibutyphthalate (DBP) adsorption number (ASTM D2414) of from about 110 to about 150 cc/100 g, andwherein said second rubber reinforcing carbon black for said tread sub cap rubber composition has an iodine adsorption value (ASTM D1510) in a range of from about 70 to about 130 g/kg together with a dibutyphthalate (DBP) adsorption number (ASTM D2414) of from about 80 to about 130 cc/100 g. |
