Rubber composition for use in tire treads

1. A rubber composition for use in tire treads comprising:

100 parts by mass of a diene rubber and

from 75 to 95 parts by mass of a silica,

wherein the diene rubber comprises from 70 to 90 mass % of a terminal-modified styrene-butadiene rubber having a hydroxy group on an end of the molecule and wherein the terminal-modified styrene-butadiene rubber comprises 20 mass % to 40 mass % styrene, and from 10 to 25 mass % of a butadiene rubber wherein the vinyl bond content in a butadiene moiety of the terminal-modified styrene-butadiene rubber is from 35 to 75 mass %;

the silica has a BET specific surface area of from 194 to 225 m 2 /g, a CTAB specific surface area of from 180 to 210 m 2 /g, a particle size distribution width L d ((d 84 −d 16 )/d 50 ) of at least 0.91, wherein V (d5-d50) /V (d5-d100) is at least 0.66; and a durometer hardness of the rubber composition when measured at 20° C. is from 70 to 75;

the rubber composition further comprises a carbon black having a nitrogen specific surface area (N 2 SA) of from 70 to 100 m 2 /g, the total amount of the carbon black and the silica being from 75 to 120 parts by mass per 100 parts by mass of the diene rubber; and

the rubber composition further comprises a softener, the amount of the softener being from 25 to 40 parts by mass per 100 parts by mass of the diene rubber.

2. The rubber composition according to claim 1 , wherein the terminal-modified styrene-butadiene rubber comprises 0.1 to 10.0 mass % of isoprene units.

3. The rubber composition according to claim 2 , wherein the terminal-modified styrene-butadiene rubber comprises 0.2 to 5.0 mass % of isoprene units.

4. The rubber composition according to claim 1 , wherein the butadiene rubber comprises a high-cis butadiene rubber.

5. The rubber composition according to claim 4 , wherein the high-cis butadiene rubber comprises a high-cis butadiene rubber having 90% or more cis-1,4 bonds.

6. The rubber composition according to claim 1 , wherein a mass-average molecular weight of the butadiene rubber is from 5.0×10 5 to 1.0×10 6 .

7. The rubber composition according to claim 6 , wherein the mass-average molecular weight of the butadiene rubber is from 5.5×10 5 to 9.0×10 5 .

8. The rubber composition according to claim 1 , wherein the butadiene rubber comprises a combination of high molecular weight butadiene rubber and low molecular weight butadiene rubber.

9. The rubber composition according to claim 8 , wherein the combination of high molecular weight butadiene rubber and low molecular weight butadiene rubber comprises a high molecular weight butadiene rubber with a mass-average molecular weight of from 500,000 to 1,000,000 and a low molecular weight butadiene rubber with a mass-average molecular weight of from 6,000 to 60,000.

10. The rubber composition according to claim 1 , wherein the diene rubber comprises from 10 to 20 mass % of the butadiene rubber.

11. The rubber composition according to claim 1 , wherein:

the terminal-modified styrene-butadiene rubber has a styrene content of 23 mass % and the vinyl bond content of the butadiene moiety of the terminal modified styrene-butadiene rubber is 70 mol %;

the silica has a BET specific surface area of 222 m 2 /g and a CTAB specific surface of 200 m 2 /g; and

the softener comprises 25 parts by mass of the composition.

12. A pneumatic tire comprising a tread manufactured from the rubber composition according to claim 1 .