An object of the present invention is to provide a golf ball having a low spin rate on driver shots and excellent in flight distance performance. The present invention provides 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, and (d) a salt of a carboxylic acid, provided that the rubber composition further contains (e) 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.
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 consisting of a polybutadiene having a Mooney viscosity ML 1+4 (100° C.) of 55 or less,
(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 salt of a carboxylic acid having 4 to 10 carbon atoms, or a sole salt selected from the group consisting of zinc laurate, zinc myristate, zinc palmitate, zinc stearate, zinc behenate, and zinc 12-hydroxystearate, and
(f) an organic sulfur compound that is diphenyl disulfide or a derivative thereof, thionaphthol or a derivative thereof, thiuram disulfide or a derivative thereof, a metal salt of thionaphthol, or a metal salt of the derivative of thionaphthol,
provided that the rubber composition further contains (e) a metal compound if the co-crosslinking agent (b) is only an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and
wherein if the JIS-C hardness is measured in the spherical core including the core center at nine points obtained by dividing the radius of the spherical core into equal parts having 12.5% intervals therebetween, and is plotted 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 salt of the carboxylic acid in an amount of 10 parts by mass or more and less than 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.
3. The golf ball according to claim 1 , wherein (d) the salt of the carboxylic acid is a salt of a fatty acid.
4. The golf ball according to claim 1 , wherein the rubber composition contains (f) the organic sulfur compound in a content ranging from 0.05 parts by mass 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 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 by mass to 50 parts by mass with respect to 100 parts by mass of (a) the base rubber.
6. 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.
7. The golf ball according to claim 1 , wherein the spherical core has a hardness difference of 15 or more in JIS-C hardness between the surface hardness and the center hardness thereof.
8. 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 of:
(a) a base rubber consisting of a polybutadiene having a Mooney (ML 1+4 (100° C.)) of 55 or less,
(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 salt of a carboxylic acid having 4 to 10 carbon atoms, or a sole salt selected from the group consisting of zinc laurate, zinc myristate, zinc palmitate, zinc stearate, zinc behenate, and zinc 12-hydroxystearate, in an amount of 10 parts by mass or more and less than 40 parts by mass with respect to 100 parts by mass of (a) the base rubber,
(f) an organic sulfur compound that is diphenyl disulfide or a derivative thereof, thionaphthol or a derivative thereof, thiuram disulfide or a derivative thereof, a metal salt of thionaphthol, or a metal salt of the derivative of thionaphthol, and
(e) a metal compound,
wherein the spherical core has a hardness difference of 20 or more in JIS-C hardness between the surface hardness and the center hardness thereof,
and
wherein if the JIS-C hardness is measured in the spherical core including the core center at nine points obtained by dividing the radius of the spherical core into equal parts having 12.5% intervals therebetween, and is plotted 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.
9. The golf ball according to claim 8 , wherein the rubber composition contains (f) the organic sulfur compound in a content ranging from 0.05 parts by mass to 5 parts by mass with respect to 100 parts by mass of (a) the base rubber.
10. The golf ball according to claim 8 , 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 a content ranging from 15 parts by mass to 50 parts by mass with respect to 100 parts by mass of (a) the base rubber.
11. The golf ball according to claim 8 , 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.
12. The golf ball according to claim 1 , wherein the spherical core has a diameter ranging from 38.8 mm to 42.2 mm.
13. The golf ball according to claim 8 , wherein the spherical core has a diameter ranging from 38.8 mm to 42.2 mm.