An object of the present invention is to provide a golf ball traveling a greater distance by improving the properties of the spherical core. The present invention provides a golf ball comprising a spherical core, and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing a base rubber, a co-crosslinking agent, a crosslinking initiator, a filler and a polynuclear aromatic organic sulfur compound or a derivative thereof, and the spherical core has a specific hardness distribution without decreasing hardness from a central point toward a surface of the spherical core in JIS-C hardness.
1. A golf ball comprising:
a spherical core, and
at least one cover layer covering the spherical core,
wherein the spherical core is formed from a rubber composition containing a base rubber, a co-crosslinking agent, a crosslinking initiator, a filler and 2-naphthalene thiol or a metal salt thereof, the spherical core has a hardness distribution that satisfies B<A≦C, and JIS-C hardness does not decrease in a direction from the spherical core central point toward the spherical core surface, wherein
A=(Hardness H5 at a distance of 5 mm from the central point of the spherical core)−(Central hardness HO at the central point of the spherical core);
B=(Hardness H12.5 at a distance of 12.5 mm from the central point of the spherical core)−(Hardness H5 at a distance of 5 mm from the central point of the spherical core);
C=(Surface hardness Hs at the surface of the spherical core)−(Hardness H12.5 at a distance of 12.5 mm from the central point of the spherical core); and
B ranges from 0 to 4 in JIS-C hardness.
2. The golf ball according to claim 1 , wherein the rubber composition contains 2-naphthalene thiol or a metal salt thereof in an amount of 0.03 part to 3.5 parts by mass with respect to 100 parts by mass of the base rubber.
3. The golf ball according to claim 1 , wherein a hardness difference D between the surface hardness Hs and the central hardness HO at the central point of the spherical core is 22 or more in JIS-C hardness.
4. The golf ball according to claim 1 , wherein A ranges from 6 to 15 in JIS-C hardness.
5. The golf ball according to claim 1 , wherein C ranges from 10 to 20 in JIS-C hardness.
6. The golf ball according to claim 1 , wherein the central hardness HO ranges from 40 to 70 in JIS-C hardness.
7. The golf ball according to claim 1 , wherein the hardness H5 ranges from 63 to 73 in JIS-C hardness.
8. The golf ball according to claim 1 , wherein the hardness 1112.5 ranges from 64 to 76 in JIS-C hardness.
9. The golf ball according to claim 1 , wherein the surface hardness Hs ranges from 78 to 100 in JIS-C hardness.
10. A golf ball comprising:
a spherical core, and
at least one cover layer covering the spherical core,
wherein the spherical core is formed from a rubber composition containing a base rubber, a co-crosslinking agent, a crosslinking initiator, a filler and 2-naphthalene thiol or a metal salt thereof, and the spherical core has a hardness distribution that satisfies B<A≦C, and JIS-C hardness does not decrease in a direction from the spherical core central point toward the spherical core surface,
wherein
A=(Hardness H5 at a distance of 5 mm from the central point of the spherical core)−(Central hardness HO at the central point of the spherical core);
B=(Hardness H12.5 at a distance of 12.5 mm from the central point of the spherical core)−(Hardness H5 at a distance of 5 mm from the central point of the spherical core);
C=(Surface hardness Hs at the surface of the spherical core)−(Hardness H12.5 at a distance of 12.5 mm from the central point of the spherical core); and
B ranges from 0 to 4 in JIS-C hardness.
11. The golf ball according to claim 10 , wherein a hardness difference D between the surface hardness Hs and the central hardness HO at the central point of the spherical core ranges from 22 to 35 in JIS-C hardness.
12. The golf ball according to claim 1 , wherein a hardness difference D between the surface hardness Hs and the central hardness HO at the central point of the spherical core ranges from 22 to 35 in JIS-C hardness.
13. The golf ball according to claim 1 , wherein B ranges from 0.5 to 3 in JIS-C hardness.
14. The golf ball according to claim 10 , wherein B ranges from 0.5 to 3 in JIS-C hardness.