An object of the present invention is to provide a golf ball excellent in flying performance, in particular, to provide a golf ball traveling a great flight distance. 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 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 consisting essentially of:
(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) an aliphatic saturated carboxylic acid and/or an aromatic carboxylic acid,
(f) an organic sulfur compound which is a compound or metal salt of thiophenol or a derivative thereof, diphenyl disulfide or a derivative thereof, or thionaphthol or a derivative thereof, and
at least one inorganic filler selected from the group consisting of zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder,
wherein the rubber composition contains (d) the aliphatic saturated carboxylic acid and/or the aromatic carboxylic acid 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 if the JIS-C hardness is measured in the spherical core at nine points, including the core center and core surface, 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, and
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.
2. The golf ball according to claim 1 , wherein the spherical core has a hardness difference of 18 or more in JIS-C hardness between a surface hardness and a center hardness thereof.
3. 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.
4. The golf ball according to claim 1 , wherein (d) the aliphatic saturated carboxylic acid is a fatty acid.
5. The golf ball according to claim 1 , wherein (d) the aliphatic saturated carboxylic acid and/or the aromatic carboxylic acid is a carboxylic acid having 4 to 30 carbon atoms.
6. The golf ball according to claim 1 , wherein the rubber composition contains (f) the organic sulfur compound 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.
7. 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.
8. A two-piece golf ball having a spherical core and a single layered cover covering the spherical core, wherein the spherical core is formed from a rubber composition consisting essentially of:
(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) an aliphatic saturated carboxylic acid and/or an aromatic carboxylic acid having 4 to 30 carbon atoms,
(f) an organic sulfur compound which is a compound or metal salt of thiophenol or a derivative thereof, diphenyl disulfide or a derivative thereof, or thionaphthol or a derivative thereof, and
at least one inorganic filler selected from the group consisting of zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder,
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, and
wherein if the JIS-C hardness is measured in the spherical core at nine points, including the core center and core surface, 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 two-piece golf ball according to claim 8 , wherein the rubber composition contains (d) the aliphatic saturated carboxylic acid and/or the aromatic carboxylic acid in an amount ranging from 5 parts to 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.
10. The two-piece 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.
11. The two-piece golf ball according to claim 8 , wherein (d) the aliphatic saturated carboxylic acid is a fatty acid.
12. The two-piece golf ball according to claim 8 , wherein the rubber composition contains (f) the organic sulfur compound 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.
13. The two-piece 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 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.