Rubber composition for tire and pneumatic tire

1. A rubber composition for a tire comprising diene rubber, silica, and a silane coupling agent, wherein

the diene rubber comprises not less than 10 mass % of a specific conjugated diene rubber represented by Formula (1), and comprising a polymer block A comprising a structural unit based on a monomer and comprising isoprene and a structural unit based on a monomer and comprising aromatic vinyl, a polymer block B comprising a structural unit based on a monomer and comprising 1,3-butadiene, and a polymer block C comprising a polyorganosiloxane;

the diene rubber comprises butadiene rubber having a linearity index value from 120 to 150;

a mass ratio of the structural unit based on a monomer and comprising isoprene to the structural unit based on a monomer and comprising aromatic vinyl, the structural unit based on a monomer and comprising isoprene/the structural unit based on a monomer and comprising aromatic vinyl, in the polymer block A is from 80/20 to 95/5, and a formula weight of the polymer block A is from 500 to 15,000;

a CTAB adsorption specific surface area of the silica is from 180 to 250 m 2 /g;

the silane coupling agent is a silane coupling agent represented by Formula (2);

the polyorganosiloxane is a polyorganosiloxane represented by Formula (3); and

a vinyl bond content in the specific conjugated diene rubber is from 20 to 35 mass %:

A-B-C  Formula (1)

where A represents the polymer block A, B represents the polymer block B, and C represents the polymer block C;

(C p H 2p+1 ) t (C p H 2p+1 O) 3−t —Si—C q H 2q —S—C(O)—C r H 2r+1   Formula (2)

where p represents an integer from 1 to 3, q represents an integer from 1 to 3, r represents an integer from 1 to 15, and t represents an integer from 0 to 2; and

where R 1 to R 8 are each an alkyl group having from 1 to 6 carbons or an aryl group having from 6 to 12 carbons, and are optionally identical to or different from one another; X 1 and X 4 are each any group selected from the group consisting of an alkyl group having from 1 to 6 carbons, an aryl group having from 6 to 12 carbons, an alkoxy group having from 1 to 5 carbons, and an epoxy group-containing group having from 4 to 12 carbons, and are optionally identical to or different from one another; X 2 is an alkoxy group having from 1 to 5 carbons or an epoxy group-containing group having from 4 to 12 carbons, and when a plurality of the X 2 are present, the plurality of X 2 are optionally identical to or different from one another; X 3 is a group containing from 2 to 20 repeating alkylene glycol units, and when a plurality of the X 3 are present, the plurality of X 3 are optionally identical to or different from one another; and m is an integer from 3 to 200, n is an integer from 0 to 200, and k is an integer from 0 to 200.

2. The rubber composition for a tire according to claim 1 , wherein a weight average molecular weight of the specific conjugated diene rubber is from 400,000 to 750,000 and an average glass transition temperature of the specific conjugated diene rubber is from −40 to −20° C.

3. The rubber composition for a tire according to claim 2 , wherein a content of the silica is from 90 to 150 parts by mass per 100 parts by mass of the diene rubber.

4. The rubber composition for a tire according to claim 2 , further comprising a thiuram-based vulcanization accelerator.

5. The rubber composition for a tire according to claim 1 , wherein a content of the silica is from 90 to 150 parts by mass per 100 parts by mass of the diene rubber.

6. The rubber composition for a tire according to claim 5 , further comprising a thiuram-based vulcanization accelerator.

7. The rubber composition for a tire according to claim 1 , further comprising an aromatic modified terpene resin having a softening point of from 100 to 150° C., wherein

a content of the aromatic modified terpene resin is from 1 to 50 parts by mass per 100 parts by mass of the diene rubber.

8. The rubber composition for a tire according to claim 7 , further comprising a thiuram-based vulcanization accelerator.

9. The rubber composition for a tire according to claim 1 , further comprising a thiuram-based vulcanization accelerator.

10. The rubber composition for a tire according to claim 1 , further comprising at least one metal salt selected from the group consisting of a potassium salt, a sodium salt, a calcium salt, and a zinc salt.

11. A pneumatic tire using the rubber composition for a tire according to claim 1 in a tire tread.