Rubber composition for tires and pneumatic tire

1. A rubber composition for tires comprising a diene rubber, silica, a silane coupling agent, a fatty acid metal salt, and an alkyltrialkoxysilane represented by Formula (I);

the diene rubber containing 60 mass % or greater of a modified conjugated diene rubber and 30 mass % or greater of a particular conjugated diene rubber which corresponds to a part or all of the modified conjugated diene rubber;

an average glass transition temperature of the diene rubber being from −45 to −20° C.;

a content of the silica being from 60 to 180 parts by mass per 100 parts by mass of the diene rubber;

a content of the silane coupling agent being from 4 to 20 parts by mass relative to the content of the silica;

a content of the fatty acid metal salt being from 2 to 8 mass % relative to the content of the silica;

a content of the alkyltrialkoxysilane being from 0.1 to 20 mass % relative to the content of the silica;

wherein, in the silica, a silica (X) having a CTAB adsorption specific surface area of 144 to 176 m 2 /g and a silica (Y) having a CTAB adsorption specific surface area of 180 to 220 m 2 /g are used in combination, and a mass ratio (X:Y) of the silica (X) to the silica (Y) is from 4:1 to 1:3,

wherein, R 11 represents an alkyl group having from 1 to 20 carbons, and R 12 each independently represents a methyl group or an ethyl group;

the particular conjugated diene rubber being a conjugated diene rubber produced by a method of producing a conjugated diene rubber, the method comprising steps A, B, and C below in this order;

the particular conjugated diene rubber having an aromatic vinyl unit content of 38 to 48 mass %, a vinyl bond content of 20 to 35 mass %, and a weight average molecular weight of 500000 to 800000;

Step A: a step of forming a polymer block A having an active terminal, the polymer block A having an isoprene unit content of 80 to 95 mass %, an aromatic vinyl unit content of 5 to 20 mass %, and a weight average molecular weight of 500 to 15000, by polymerizing a monomer mixture containing isoprene and an aromatic vinyl;

Step B: a step of obtaining a conjugated diene-based polymer chain having an active terminal, the conjugated diene-based polymer chain having the polymer block A and a polymer block B, by forming the polymer block B having an active terminal by mixing the polymer block A with a monomer mixture containing 1,3-butadiene and an aromatic vinyl to continue polymerization reaction to form the polymer block B in series with the polymer block A; and

Step C: a step of reacting a polyorganosiloxane represented by Formula (1) with the active terminal of the conjugated diene-based polymer chain;

wherein, R 1 to R 8 are the same or different and are each an alkyl group having from 1 to 6 carbons or an aryl group having from 6 to 12 carbons; X 1 and X 4 are the same or different and are groups selected from the group consisting of alkyl groups having from 1 to 6 carbons, aryl groups having from 6 to 12 carbons, alkoxy groups having from 1 to 5 carbons, and groups having an epoxy group and from 4 to 12 carbons; X 2 is an alkoxy group having from 1 to 5 carbons or a group having an epoxy group and from 4 to 12 carbons, and a plurality of the X 2 moieties are the same or different; X 3 is a group having from 2 to 20 alkylene glycol repeating units, and when a plurality of the X 3 moieties exists, the X 3 moieties are the same or different; and m is an integer of 3 to 200, n is an integer of 0 to 200, and k is an integer of 0 to 200.

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

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

3. The rubber composition for tires according to claim 2 , wherein a CTAB adsorption specific surface area of the silica is 180 m 2 /g or greater.

4. The rubber composition for tires according to claim 1 , wherein a CTAB adsorption specific surface area of the silica is 180 m 2 /g or greater.

5. A pneumatic tire having the rubber composition for tires described in claim 4 in a cap tread.

6. A pneumatic tire having the rubber composition for tires described in claim 3 in a cap tread.

7. A pneumatic tire having the rubber composition for tires described in claim 2 in a cap tread.

8. A pneumatic tire having the rubber composition for tires described in claim 1 in a cap tread.