An object of the present invention is to provide a golf ball showing a great flight distance on driver shots. The present invention provides a golf ball comprising a spherical core and at least one cover layer covering the spherical core, wherein when JIS-C hardness of the spherical core is measured at points located at distances of 0% (core center), 37.5%, 75.0% and 100% (core surface) from the core center of the spherical core, a hardness difference (Hs−Ho) between a surface hardness (Hs) thereof and a center hardness (Ho) thereof is 20 or more, a ratio ((H 37.5 −Ho)/(H 75.0 −Ho)) of a hardness difference (H 37.5 −Ho) to a hardness difference (H 75.0 −Ho) is 0.6 or less, and a compression deformation amount (D (mm)) of the spherical core and the 37.5% point hardness (H 37.5 ) satisfy a relation of H 37.5 ≦−9×D+95.
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) a base rubber,
(b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent,
(c) a crosslinking initiator including 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, and
(d) a carboxylic acid and/or a salt thereof, excluding an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and a metal salt thereof,
a content of (c) the crosslinking initiator is 0.2 part to 0.8 part by mass with respect to 100 parts by mass of (a) the base rubber,
the spherical core has a diameter in a range from 34.8 mm to 42.2 mm and a compression deformation amount in a range from 2.8 mm to 6.0 mm when applying a load from 98N as an initial load to 1275N as a final load to the spherical core, and
when JIS-C hardness of the spherical core is measured at points located at distances of 0% (core center), 37.5%, 75.0% and 100% (core surface) from the core center of the spherical core,
a hardness difference (Hs−Ho) between a surface hardness (Hs) thereof and a center hardness (Ho) thereof is 28 or more,
a ratio ((H 37.5 −Ho)/(H 75.0 −Ho)) of a hardness difference (H 37.5 −Ho) between a 37.5% point hardness (H 37.5 ) and a center hardness (Ho) to a hardness difference (H 75.0 −Ho) between a 75.0% point hardness (H 75.0 ) and a center hardness (Ho) is 0.39 or less, and
the compression deformation amount (D (mm)) of the spherical core and the 37.5% point hardness (H 37.5 ) satisfy a relation of H 37.5 ≦−9×D+95.
2. The golf ball according to claim 1 , wherein the rubber composition contains (d) the carboxylic acid and/or the salt thereof in an amount ranging from 0.5 part to 30 parts by mass with respect to 100 parts by mass of (a) the base rubber.
3. The golf ball according to claim 1 , wherein the rubber composition further contains (f) an organic sulfur compound.
4. The golf ball according to claim 3 , wherein (f) the organic sulfur compound includes at least one compound selected from the group consisting of thiophenols and/or metal salts thereof, thionaphthols and/or metal salts thereof, diphenylpolysulfides and thiuramdisulfides.
5. The golf ball according to claim 1 , wherein the rubber composition contains (b) the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as (b) the co-crosslinking agent.
6. The golf ball according to claim 1 , wherein the ratio ((H 37.5 −Ho)/(H 75.0 −Ho)) is 0.36 or less in JIS-C hardness.
7. The golf ball according to claim 1 , wherein the hardness difference (Hs−Ho) is 29.3 or more in JIS-C hardness.
8. The golf ball according to claim 6 , wherein the hardness difference (Hs−Ho) is 29.3 or more in JIS-C hardness.
9. The golf ball according to claim 1 , wherein component (d) is an aliphatic carboxylic acid having 1 to 11 carbon atoms and/or a salt thereof and/or an aromatic carboxylic acid and/or a salt thereof.
10. The golf ball according to claim 1 , wherein the compression deformation amount of the spherical core is in a range from 3.08 mm to 6.0 mm.
11. The golf ball according to claim 1 , wherein the compression deformation amount of the spherical core is in a range from 3.08 mm to 6.0 mm, and the hardness difference (Hs−Ho) is 29.3 or more in JIS-C hardness.
12. The golf ball according to claim 1 , wherein (d) the carboxylic acid and/or a salt thereof includes an aromatic carboxylic acid and/or a salt thereof.
13. The golf ball according to claim 1 , wherein (c) the crosslinking initiator includes 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3,
a content of (c) the crosslinking initiator is 0.2 part to 0.8 part by mass with respect to 100 parts by mass of (a) the base rubber,
(d) the carboxylic acid and/or a salt thereof includes an aromatic carboxylic acid and/or a salt thereof,
the compression deformation amount of the spherical core is in a range from 3.08 mm to 6.0 mm, and
the hardness difference (Hs−Ho) is 29.3 or more in JIS-C hardness.