Golf ball

1. A golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition consisting essentially of:

(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,

(d) a carboxylic acid having a benzene ring and/or a salt thereof,

(e) a metal compound and,

(f) an organic sulfur compound

provided that the rubber composition further contains (e) a metal compound if only (b) the αβ-unsaturated carboxylic acid having 3 to 8 carbon atoms is contained as the co-crosslinking agent,

wherein (a) the base rubber consists of a polybutadiene having a molecular weight distribution of 2.2 or more and 3.4 or less,

the spherical core is such that R 2 of a linear approximation curve obtained from a least square method is 0.90 or higher, when JIS-C hardness, which is measured at nine points obtained by dividing a radius of the spherical core into equal parts having 12.5% intervals therebetween, is plotted against distance (%) from a core center, and

the spherical core has a hardness difference in a range from 30 to 70 in JIS-C hardness between a surface hardness and a center hardness thereof, the center hardness of from 30 to 70 in JIS-C hardness, and the surface hardness of from 76 to 100 in JIS-C hardness.

2. A golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition consisting essentially of:

(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,

(d) a carboxylic acid having a benzene ring substituted with a halogen atom and/or a salt thereof,

(e) a metal compound and,

(f) an organic sulfur compound

provided that the rubber composition further contains (e) a metal compound if only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is contained as the co-crosslinking agent,

wherein (a) the base rubber consists of a polybutadiene having a molecular weight distribution of 2.2 or more and 3.4 or less,

the spherical core is such that R 2 of a linear approximation curve obtained from a least square method is 0.90 or higher, when JIS-C hardness, which is measured at nine points obtained by dividing a radius of the spherical core into equal parts having 12.5% intervals therebetween, is plotted against distance (%) from a core center, and

the spherical core has a hardness difference in a range from 30 to 70 in JIS-C hardness between a surface hardness and a center hardness thereof, the center hardness of from 30 to 70 in JIS-C hardness, and the surface hardness of from 76 to 100 in JIS-C hardness.

3. The golf ball according to claim 2 , wherein (d) the carboxylic acid having the benzene ring substituted with a halogen atom and/or the salt thereof is at least one compound selected from the group consisting of a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a fluoro group, a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a chloro group, a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a bromo group, and a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with an iodo group.

4. The golf ball according to claim 2 , wherein (d) the carboxylic acid having the benzene ring substituted with a halogen atom and/or the salt thereof includes at least one compound selected from the group consisting of chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, and/or a salt thereof.

5. The golf ball according to claim 2 , wherein (f) the organic sulfur compound includes at least one compound selected from thionaphthols and metal salts thereof.

6. The golf ball according to claim 2 , wherein the rubber composition contains (f) the organic sulfur compound in a content ranging from 0.05 part to 5 parts by mass with respect to 100 parts by mass of (a) the base rubber.

7. The golf ball according to claim 2 , the rubber composition contains (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof in a content ranging from 15 parts to 50 parts by mass with respect to 100 parts by mass of (a) the base rubber.

8. The golf ball according to claim 2 , wherein the rubber composition contains (b) the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

9. The golf ball according to claim 2 , wherein 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 2.0 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.

10. The golf ball according to claim 2 , wherein the rubber composition contains (d) the carboxylic acid having the benzene ring substituted with a halogen atom and/or the salt thereof in a content ranging from 2 parts by mass to 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.

11. The golf ball according to claim 2 , wherein the spherical core has the center hardness of from 30 to 48.6 in JIS-C hardness.

12. The golf ball according to claim 2 , wherein the spherical core has the center hardness of from 30 to 47.3 in JIS-C hardness.

13. The golf ball according to claim 2 , wherein (f) the organic sulfur compound consists of at least one of thionaphthols.

14. The golf ball according to claim 2 ,

wherein (d) the carboxylic acid having the benzene ring substituted with a halogen atom and/or the salt thereof is at least one compound selected from the group consisting of a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a fluoro group, a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a chloro group, a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a bromo group, and a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with an iodine group, and

(f) the organic sulfur compound consists of at least one of thionaphthols.

15. The golf ball according to claim 2 ,

wherein (d) the carboxylic acid having the benzene ring substituted with a halogen atom and/or the salt thereof is at least one compound selected from the group consisting of a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a fluoro group, a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a chloro group, a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with a bromo group, and a carboxylic acid where at least one of hydrogen atoms of benzoic acid is substituted with an iodine group,

(f) the organic sulfur compound consists of at least one of thionaphthols and

the spherical core has the center hardness of from 30 to 48.6 in JIS-C hardness.