Rubber composition and pneumatic tire

1. A rubber composition, comprising:

5 to 150 parts by mass of carbon black;

10 to 150 parts by mass of silica, wherein the silica comprises silica (1) that has a nitrogen adsorption specific surface area in the range 50 m 2 /g to less than 120 m 2 /g and silica (2) that has a nitrogen adsorption specific surface area of 120 m 2 /g or more;

100 parts by mass of a rubber component comprising styrene butadiene rubber; and

5 to 50 parts by mass of a modified polymer obtained by modifying a styrene-butadiene copolymer, wherein the modified polymer contains a moiety -(A)-C(O)R 1 in which R 1 is —OR 4 or —B—(CO)OR 5 , A and B being, independently, a divalent saturated or unsaturated hydrocarbon group and R 4 and R 5 being, independently, hydrogen or a divalent saturated or unsaturated hydrocarbon group, and wherein the modified polymer has a weight average molecular weight of from 1000 to 100,000,

wherein the rubber composition has a tanδ peak temperature of −20° C. or higher, a rubber hardness of from 63 to 75, and a tanδ at 70° C. of from 0.15 to 0.45.

2. The rubber composition according to claim 1 , wherein A is represented by the following formula (3):

wherein m represents an integer of from 0 to 6; and R 2 and R 3 are the same as or different from each other and each represent a hydrogen atom, a C 1-2 hydrocarbon group, or an aryl group, and B is represented by any one of the following formulas (4) to (7):

wherein n represents an integer of 2 or 3; R 6 and R 7 are the same as or different from each other and each represent a hydrogen atom or a C 1-18 hydrocarbon group; R 8 represents a hydrogen atom or a methyl group; and R 9 represents a hydrogen atom or a C 1-4 hydrocarbon group.

3. The rubber composition according to claim 1 , wherein the modified group is present in a proportion of at least 0.1 groups on average per molecule of the polymer in the polymer mixture.

4. The rubber composition according to claim 1 , wherein the modified polymer has a viscosity at 25° C. of from 1.0×10 4 to 8.0×10 5 .

5. The rubber composition according to claim 1 , wherein the modified polymer has a styrene content of from 5 to 45% by mass.

6. The rubber composition according to claim 1 , wherein the modified polymer comprises a backbone-modified polymer.

7. The rubber composition according to claim 1 , wherein the nitrogen adsorption specific surface areas and amounts of the silicas (1) and (2) satisfy the following formulas:

(Nitrogen adsorption specific surface area of silica (2))/(Nitrogen adsorption specific surface area of silica (1))≥1.4; and

(Amount of silica (1))×0.06≤(Amount of silica (2))≤(Amount of silica (1))×15.

8. The rubber composition according to claim 1 , wherein the rubber composition satisfies at least one of the following conditions (1) to (7):

(1) a styrene butadiene rubber content is 70% by mass or more based on 100% by mass of a rubber component in the rubber composition;

(2) a polybutadiene rubber content is 30% by mass or less based on 100% by mass of the rubber component;

(3) an amount of silica is 70 parts by mass or more per 100 parts by mass of the rubber component;

(4) an amount of oil is 5 parts by mass or more per 100 parts by mass of the rubber component;

(5) the rubber composition comprises styrene butadiene rubber having a styrene content of 30% by mass or more;

(6) an acetone extractable content (an amount of acetone-extractable matter in the vulcanized rubber composition) is 17 to 25% by mass as determined in accordance with the method A in JIS K 6229 (1998); and

(7) a mass ratio of carbon black to silica (carbon black/silica) is less than 0.65.

9. The rubber composition according to claim 8 , wherein the rubber composition satisfies all the conditions (1) to (7).

10. A rubber composition for tires, comprising the rubber composition according to claim 1 .

11. A pneumatic tire, formed from the rubber composition according to claim 10 .

12. A pneumatic tire, formed from the rubber composition according to claim 2 .