A golf ball 2 includes a core 4 , amid layer 6 , a reinforcing layer 8 , and a cover 10 . The core 4 includes a center 12 and an envelope layer 14 . The envelope layer 14 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, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein
the core comprises a center and an envelope layer positioned outside the center,
the envelope layer is formed from a crosslinked rubber composition,
the crosslinked rubber composition includes:
(a) a base rubber;
(b) a co-crosslinking agent;
(c) a crosslinking initiator; and
(d) an acid and/or a salt thereof,
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,
wherein the acid and/or the salt (d) is present in an amount 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),
wherein the acid or salt (d) excludes the co-crosslinking agent (b),
wherein the rubber composition includes 32 parts by weight or greater but 38 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a),
wherein the acid and/or the salt (d) is an aromatic carboxylic acid with a carbon number equal to or greater than 5 but equal to or less than 30 and/or the salt thereof,
wherein if the distance from the core center to the core surface is divided into intervals of 12.5% to include nine distance points that include the core central point and the core surface; JIS-C hardness values are measured at each of the nine distance points; and the outer six distance points are plotted on a graph against the JIS-C hardness values; then R 2 of a linear approximation curve of the graph obtained by a least-square method is equal to or greater than 0.95, and
wherein the base rubber comprises at least one of polybutadiene, polyisoprene, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubber.
2. The golf ball according to claim 1 , wherein the aromatic carboxylic acid with a carbon number equal to or greater than 5 but equal to or less than 30 and/or the salt thereof is a zinc salt of a carboxylic acid.
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 3 , 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.
5. The golf ball according to claim 4 , wherein the organic sulfur compound (e) is 2-thionaphthol, bis(pentabromophenyl)disulfide, or 2,6-dichlorothiophenol.
6. 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).
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 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).
9. The golf ball according to claim 1 , 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).
10. The golf ball according to claim 1 , wherein
a core central point JIS-C hardness H(0) is equal to or greater than 40 but equal to or less than 70, and
a core surface JIS-C hardness H(100) is equal to or greater than 80 but equal to or less than 96.
11. The golf ball according to claim 10 , wherein the core surface JIS-C hardness H(100) is greater than the core central point JIS-C hardness H(0) by an amount that is equal to or greater than 15.
12. The golf ball according to claim 1 , wherein
the mid layer Shore D hardness Hm is greater than the cover Shore D hardness Hc, and
the mid layer Shore D hardness Hm is greater than the cover Shore D hardness He by an amount that is equal to or greater than 18.
13. The golf ball according to claim 1 , wherein
a thickness of the mid layer is equal to or greater than 0.5 mm but equal to or less than 1.6 mm, and
a thickness of the cover is equal to or less than 0.8 mm.
14. The golf ball according to claim 1 , wherein
the mid layer is formed from a resin composition including a base resin,
the cover is formed from a resin composition having a base resin that is different from the base resin of the mid layer, and
the golf ball further comprises a reinforcing layer between the mid layer and the cover.
15. The golf ball according to claim 1 , wherein a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm.
16. The golf ball according to claim 1 , wherein if the distance from the core center to the core surface is divided into intervals of 12.5% to include nine distance points that include the core central point and the core surface; JIS-C hardness values are measured at each of the nine distance points; and the outer six distance points are plotted on a graph against the JIS-C hardness values; then the gradient a of the linear approximation curve obtained by a least-square method is equal to or greater than 0.30.
17. The golf ball according to claim 10 , wherein the core central point JIS-C hardness H(0) is equal to or greater than 62 but equal to or less than 70.
18. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein
the core comprises a center and an envelope layer positioned outside the center,
the envelope layer is formed from a crosslinked rubber composition,
the crosslinked rubber composition includes:
(a) a base rubber;
(b) a co-crosslinking agent;
(c) a crosslinking initiator; and
(d) an acid and/or a salt thereof,
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,
wherein the acid and/or the salt (d) is present in an amount 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),
wherein the acid or salt (d) excludes the co-crosslinking agent (b),
wherein the rubber composition includes 32 parts by weight or greater but 38 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a),
wherein the acid and/or the salt (d) is a carboxylic acid with a carbon number equal to or greater than 5 but equal to or less than 30 and/or the salt thereof,
wherein the acid and/or salt (d) is a fatty acid and/or salt thereof which excludes stearic acid and stearate salts,
wherein if the distance from the core center to the core surface is divided into intervals of 12.5% to include nine distance points that include the core central point and the core surface; JIS-C hardness values are measured at each of the nine distance points; and the outer six distance points are plotted on a graph against the JIS-C hardness values; then R 2 of a linear approximation curve of the graph obtained by a least-square method is equal to or greater than 0.95, and
wherein the base rubber comprises at least one of polybutadiene, polyisoprene, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubber.
19. The golf ball according to claim 18 , wherein the acid and/or salt (d) is a fatty acid and/or a salt thereof, in which a carbon number of the fatty acid component is equal to or greater than 5 but equal to or less than 8.
20. The golf ball according to claim 18 , wherein the acid and/or the salt (d) is an octanoic acid and/or a salt thereof.