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 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,

(d) a carboxylic acid and/or a carboxylic acid salt, and

(e) an organic sulfur compound having a naphthalene ring,

provided that the rubber composition further contains (f) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent;

wherein the rubber composition contains (d) the carboxylic acid and/or the carboxylic acid salt in an amount ranging from 7.5 parts to 40 parts by mass with respect to 100 parts by mass of (a) the base rubber;

wherein the carboxylic acid salt includes zinc laurate, zinc myristate, zinc palmitate, zinc stearate, zinc behenate, zinc montanate, or zinc 12-hydroxystearate, and

wherein if the JIS-C hardness is measured in the spherical core at nine points obtained by dividing the radius of the spherical core into equal parts having 12.5% interval therebetween, and is plotted on a graph against distance (%) from the spherical core center, the R 2 value of a linear approximation curve obtained from the least square method is 0.95 or higher.

2. The golf ball according to claim 1 , wherein the rubber composition contains (d) the carboxylic acid in an amount ranging from 7.5 parts by mass to 20 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 contains (d) the carboxylic acid salt in an amount ranging from 10 parts by mass to 25 parts by mass with respect to 100 parts by mass of (a) the base rubber.

4. The golf ball according to claim 1 , wherein the rubber composition contains (e) the organic sulfur compound having the naphthalene ring in an amount ranging from 0.05 part to 5 parts by mass with respect to 100 parts by mass of (a) the base rubber.

5. The golf ball according to claim 1 , wherein (e) the organic sulfur compound having the naphthalene ring includes thionaphthol or a derivative thereof.

6. The golf ball according to claim 5 , wherein the rubber composition contains thionaphthol or a derivative thereof in an amount ranging from 0.05 part to 2 parts by mass with respect to 100 parts by mass of (a) the base rubber.

7. 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 the co-crosslinking agent.

8. The golf ball according to claim 1 , wherein (d) the carboxylic acid is a fatty acid.

9. The golf ball according to claim 1 , wherein (d) the carboxylic acid is a carboxylic acid having 4 to 30 carbon atoms.

10. The golf ball according to claim 1 , wherein (d) the carboxylic acid is a carboxylic acid having 9 to 30 carbon atoms.

11. The golf ball according to claim 1 , wherein the rubber composition contains (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof in an amount ranging from 15 parts by mass to 50 parts by mass with respect to 100 parts by mass of (a) the base rubber.

12. 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 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,

(d) a carboxylic acid having 4 to 10 carbon atoms and/or a salt thereof, and

(e) thionaphthol or a derivative thereof,

provided that the rubber composition further contains (f) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, and

wherein if the JIS-C hardness is measured in the spherical core at nine points obtained by dividing the radius of the spherical core into equal parts having 12.5% interval therebetween, and is plotted on a graph against distance (%) from the spherical core center, the R 2 value of a linear approximation curve obtained from the least square method is 0.95 or higher.

13. The golf ball according to claim 12 , wherein the rubber composition contains thionaphthol or the derivative thereof in an amount ranging from 0.05 part to 2 parts by mass with respect to 100 parts by mass of (a) the base rubber.

14. The golf ball according to claim 12 , wherein the rubber composition contains (d) the carboxylic acid having 4 to 10 carbon atoms and/or the salt thereof in an amount ranging from 5 parts to 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.

15. The golf ball according to claim 12 , wherein the rubber composition contains (d) the carboxylic acid having 4 to 10 carbon atoms in an amount ranging from 5 parts by mass to 20 parts by mass with respect to 100 parts by mass of (a) the base rubber.

16. The golf ball according to claim 12 , wherein the rubber composition contains (d) the salt of the carboxylic acid having 4 to 10 carbon atoms in an amount ranging from 10 parts by mass to 25 parts by mass with respect to 100 parts by mass of (a) the base rubber.

17. The golf ball according to claim 12 , wherein the spherical core has a hardness difference of 15 or more in JIS-C hardness between a surface hardness and a center hardness thereof.

18. The golf ball according to claim 12 , 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.

19. The golf ball according to claim 12 , wherein (d) the carboxylic acid and or the salt thereof is a fatty acid and/or a fatty acid salt.