A golf ball 2 includes a spherical core 4 and a cover 6 covering the core 4 . The cover 6 includes an inner cover 8 and an outer cover 10 positioned outside the inner cover 8 . The core 4 is obtained by a rubber composition being crosslinked. The rubber composition includes a base rubber (a), a co-crosslinking agent (b), a crosslinking initiator (c), and a carboxylate (d). The co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. The amount of the carboxylate (d) is equal to or greater than 1 parts by weight but less than 40 parts by weight per 100 parts by weight of the base rubber (a). A hardness of an innermost layer of the cover is greater than a surface hardness of the core.
1. A golf ball comprising a spherical core and a cover which covers the core and has two or more layers, wherein the core is formed from a crosslinked rubber composition that includes:
(a) base rubber;
(b) a co-crosslinking agent;
(c) a crosslinking initiator; and
(d) a carboxylate having a carboxylic acid component that includes 4 to 14 carbon atoms, wherein the co-crosslinking agent (b) is:
(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and/or
(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,
the carboxylate (d) is present in an amount that is equal to or greater than 1 parts by weight but less than 10 parts by weight per 100 parts by weight of the base rubber (a),
the carboxylate carboxylic acid component excludes the α,β-unsaturated carboxylic acid,
a JIS-C hardness Hi of an innermost layer of the cover is greater than a JIS-C hardness Hs at a surface of the core,
the core has a hardness distribution such that if JIS-C hardness values are measured at eight points in the core, including the core central point, at intervals of 2.5 mm, as well as the core surface, and the hardness values are plotted versus distance in a graph, then R 2 of a linear approximation curve obtained by the least-square method is equal to or greater than 0.95;
the JIS-C hardness Hs at the surface of the core is equal to or greater than 78.0 but equal to or less than 95.0;
the core central point JIS-C hardness H(0) is equal to or greater than 40.0 but equal to or less than 70.0; and
the JIS-C hardness Hs at the surface of the core is greater than the core central point JIS-C hardness H(0) by a difference of equal to or greater than 15.
2. The golf ball according to claim 1 , wherein the JIS-C hardness Hi of the innermost layer of the cover is equal to or greater than 80.0 but equal to or less than 95.0.
3. The golf ball according to claim 1 , wherein the rubber composition further includes an organic sulfur compound (e).
4. The golf ball according to claim 1 , wherein the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (f).
5. The golf ball according to claim 1 , wherein the carboxylate (d) is a fatty acid salt.
6. The golf ball according to claim 5 , wherein the carboxylate (d) is a salt of a saturated fatty acid.
7. The golf ball according to claim 1 , wherein the rubber composition includes the metal salt (b2) of the α,β-unsaturated carboxylic acid.
8. The golf ball according to claim 3 , wherein the organic sulfur compound (e) is a thiophenol, a polysulfide having 2 to 4 sulfur atoms, a thionaphthol, a thiuram, or a metal salt thereof.
9. The golf ball according to claim 1 , wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a).
10. The golf ball according to claim 1 , wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).
11. The golf ball according to claim 3 , wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).
12. The golf ball according to claim 1 , wherein a difference (Hi−Hs) between the JIS-C hardness Hi of the innermost layer of the cover and the hardness Hs at the surface of the core is equal to or greater than 1 but equal to or less than 5.
13. The golf ball according to claim 1 , wherein a difference (Ho−Hi) between a JIS-C hardness Ho of an outermost layer of the cover and the JIS-C hardness Hi of the innermost layer of the cover is equal to or greater than 2 but equal to or less than 10.
14. The golf ball according to claim 1 , wherein, in a hardness distribution curve from a central point of the core to an outermost layer of the cover, a hardness of the outermost layer is the greatest.
15. The golf ball according to claim 1 , wherein a total thickness of the cover is equal to or less than 2.5 mm.