GOLF BALL
A golf ball 2 includes a core 4 , an inner cover 6 , and an outer cover 8 . The core 4 includes a center 10 , an inner envelope layer 12 , and an outer envelope layer 14 . The inner envelope layer 12 is formed by a rubber composition being crosslinked. The rubber composition includes: a base rubber (a); a co-crosslinking agent (b); a crosslinking initiator (c); and an acid and/or a salt (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.
1 . A golf ball comprising a core and a cover positioned outside the core, wherein
the core comprises a center, an inner envelope layer positioned outside the center, and an outer envelope layer positioned outside the inner envelope layer,
the cover comprises one or more layers,
at least one of the center, the inner envelope layer, and the outer envelope layer is formed from a rubber composition that includes:
(a) a base rubber;
(b) a co-crosslinking agent;
(c) a crosslinking initiator; and
(d) an acid and/or a salt,
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, and
a component having the greatest hardness in a range from a central point of the golf ball to a surface of the golf ball is an outermost layer of the cover.
2 . The golf ball according to claim 1 , wherein the cover comprises an inner cover and an outer cover positioned outside the inner cover.
3 . The golf ball according to claim 2 , wherein a difference (Ho−Hs) between a JIS-C hardness Ho of the outer cover and a JIS-C hardness Hs at a surface of the core is equal to or greater than 5.
4 . The golf ball according to claim 2 , wherein a difference (Ho−Hi) between a JIS-C hardness Ho of the outer cover and a JIS-C hardness Hi of the inner cover is equal to or greater than 3.
5 . The golf ball according to claim 1 , wherein an amount of the acid and/or the salt (d) is equal to or greater than 1.0 parts by weight but less than 40 parts by weight, per 100 parts by weight of the base rubber (a).
6 . The golf ball according to claim 1 , wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1).
7 . The golf ball according to claim 6 , wherein a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than 30.
8 . The golf ball according to claim 6 , wherein the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.
9 . The golf ball according to claim 6 , wherein the carboxylic acid and/or the salt thereof (d1) is a zinc salt of a carboxylic acid.
10 . The golf ball according to claim 9 , wherein the zinc salt of the carboxylic acid is one or more members selected from the group consisting of zinc octoate, zinc laurate, zinc myristate, and zinc stearate.
11 . 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).
12 . 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).
13 . The golf ball according to claim 1 , wherein a difference (Hs−H(0.0)) between a JIS-C hardness Hs at a surface of the core and a JIS-C hardness H(0.0) of a central point of the core is equal to or greater than 15.
14 . The golf ball according to claim 2 , wherein a sum (Ti+To) of a thickness Ti of the inner cover and a thickness To of the outer cover is equal to or less than 2.5 mm.
15 . The golf ball according to claim 1 , wherein the rubber composition further includes an organic sulfur compound (e).
16 . The golf ball according to claim 15 , wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
17 . The golf ball according to claim 15 , 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).
18 . 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).