Rubber composition, its manufacturing method and pneumatic tire

1. A manufacturing method of rubber composition, which is comprised of adding 40-110 mass parts of silica powders as reinforcing fillers to 100 mass parts of diene rubber component and adding silane coupling agent by 3-15 mass % of the silica powders, comprising:

preparing styrene-butadiene rubber modified with a functional group(s) having hetero atom, as hetero-modified SBR, which is to make 20-95 mass % of said 100 mass parts or total of the diene rubber component, in the rubber composition, wherein said functional group(s) is at least one selected from the group consisting of hydroxyl group, amino group, carboxyl group, alkoxyl group, epoxy group, cyano group, and halogen group;

preparing styrene-butadiene rubber having glass transition temperature in a range of −10° C. to 10° C. and having mass-average molecular mass of no less than 900,000, as high-Tg SBR, which is to make 5-25 mass % of said 100 mass parts or total of the diene rubber component, in the rubber composition;

a first kneader-mixing step, at which 80-95 mass parts of the diene rubber component including at least 20 mass parts of the hetero-modified SBR is mixed with substantially all of the reinforcing fillers;

taking out of obtained mixture from the kneader device to give a first master batch; and

a second kneader-mixing step, in which the first master batch is mixed substantially solely with 5-15 mass parts of the high-Tg SBR,

wherein the high-Tg SBR has a glass transition temperature in a range of −10° C. to 0° C.

2. A manufacturing method of pneumatic tire, wherein the rubber composition obtainable obtained by a method according to claim 1 is used for a tread.

3. A manufacturing method of rubber composition according to claim 1 , wherein no rubber component other than the hetero-modified SBR substantially has said functional group.

4. The method according to claim 2 , wherein the hetero-modified SBR has a glass transition temperature in a range of −20° C. to −120° C.; and

if a diene rubber other than the hetero-modified SBR and the high-Tg SBR is included in the diene rubber component, glass transition temperature (Tg) of such other diene rubber is also in a range of −120° C. to −20° C.

5. The manufacturing method according to claim 1 , wherein the hetero-modified SBR has a glass transition temperature in a range of −20° C. to −120° C.; and

if a diene rubber other than the hetero-modified SBR and the high-Tg SBR is included in the diene rubber component, glass transition temperature (Tg) of such other diene rubber is also in a range of −120° C. to −20° C.

6. The manufacturing method according to claim 1 , wherein the high-Tg SBR has a styrene (St) content in a range of 35-50 mass % and a vinyl (Vi) content in butadiene parts in a range of 55-80 mass %.

7. The manufacturing method according to claim 1 , wherein the first master batch is taken out from the kneader device and cooled, to 50° C. or lower, and then charged into the kneader device;

after a second kneader-mixing step, a second master batch is taken out from the kneader device; and then, sulfur and a vulcanizing accelerator are added in a third kneader-mixing step.

8. The manufacturing method according to claim 3 , wherein amount of charging the hetero-modified SBR in the first kneader-mixing step is 40-70 mass parts among 100 parts of the diene-rubber component.

9. The manufacturing method according to claim 5 , wherein a diene rubber other than the hetero-modified SBR and the high Tg SBR is SBR other than the hetero-modified SBR and the high-T g SBR, polybutadiene rubber (BR), natural rubber (NR) or polyisoprene rubber (IR) and included in the diene rubber component by 20-50 mass %.