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 the spherical core has a difference between absorbance of the absorption peak appeared at 1560±30 cm −1 at a surface thereof and absorbance of the absorption peak appeared at 1560±30 cm −1 at a center thereof of 0.024 or more, when measuring the spherical core with a Fourier transform infrared spectrophotometer.
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 comprising:
(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 1 to 30 carbon atoms and/or a salt thereof in a content ranging from 0.1 part by mass to 40.0 parts by mass with respect to 100 parts by mass of the base rubber, said carboxylic acid excluding an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms or a metal salt thereof, and
(f) an organic sulfur compound in a content ranging from 0.05 parts to 5 parts by mass with respect to 100 parts by mass of the base rubber,
provided that the rubber composition further contains (e) a metal compound if only an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is present as the co-crosslinking agent,
wherein the spherical core exhibits a difference in infrared absorbance measured at a surface thereof and measured at a center thereof of 0.024 or more, with respect to an absorption peak appearing at 1560±30 cm −1 when measuring the spherical core with a Fourier transform infrared spectrophotometer,
the spherical core has a hardness difference ranging from 18 to 60 in JIS-C hardness between a surface hardness and a center hardness thereof, and
if JIS-C hardness values are measured at nine points obtained by dividing a radius of the spherical core into equal parts having 12.5% intervals therebetween so as to include the center and the surface, and the hardness values are plotted on a graph against distance (%) including the core center and core surface, then R 2 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 spherical core is a single-layered core.
3. The golf ball according to claim 1 , wherein (d) the carboxylic acid and/or the salt thereof is a fatty acid and/or a salt thereof.
4. The golf ball according to claim 1 , wherein (d) the carboxylic acid and/or the salt thereof is a carboxylic acid having 1 to 14 carbon atoms, and the rubber composition contains (d) the carboxylic acid in a content ranging from 0.1 part by mass to 13.0 parts by mass with respect to 100 parts by mass of (a) the base rubber.
5. The golf ball according to claim 1 , wherein (d) the carboxylic acid and/or the salt thereof is a salt of a carboxylic acid having 1 to 14 carbon atoms, and the rubber composition contains (d) the salt of the carboxylic acid in a content ranging from 0.1 part by mass to 30.0 parts by mass with respect to 100 parts by mass of (a) the base rubber.
6. The golf ball according to claim 1 , wherein (f) the organic sulfur compound includes thiophenol or a derivative thereof, diphenyl disulfide or a derivative thereof, thionaphthol or a derivative thereof, thiuram disulfide or a derivative thereof, or a metal salt of these compounds.
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 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 4 , wherein (d) the carboxylic acid and/or the salt thereof is a carboxylic acid having 1 to 9 carbon atoms.
9. The golf ball according to claim 5 , wherein (d) the carboxylic acid and/or the salt thereof is a salt of a carboxylic acid having 1 to 9 carbon atoms.
10. The golf ball according to claim 1 , wherein the carboxylic acid (d) has 5 to 30 carbon atoms and the α,β-unsaturated carboxylic acid or the metal salt thereof (b) has 3 to 4 carbon atoms.
11. The golf ball according to claim 1 , wherein the carboxylic acid (d) is an aliphatic carboxylic acid.
12. The golf ball according to claim 10 , wherein the carboxylic acid (d) is an aliphatic carboxylic acid.
13. The golf ball according to claim 1 , wherein the spherical core has a center hardness of 54.4 or less in JIS-C hardness.
14. The golf ball according to claim 1 , wherein the spherical core has a center hardness of 51.6 or less in JIS-C hardness.