A golf ball 2 includes a spherical core 4 , a mid layer 6 positioned outside the core 4 , and a cover 8 positioned outside the mid layer 6 . The core 4 includes a spherical center 10 and an envelope layer 12 positioned outside the center 10 . The center 10 has a diameter of 1 mm or greater and 15 mm or less. The center 10 has a JIS-C hardness H1, at its central point, of 20 or greater and 50 or less. The difference (H4−H3) between the JIS-C hardness H4 of the core 4 at its surface and the JIS-C hardness H3 of the envelope layer 12 at its innermost portion is equal to or greater than 10. The cover 8 has a flexural modulus of 300 MPa or greater and 1000 MPa or less, and has a tensile modulus of 400 MPa or greater and 1500 MPa or less.
1. A golf ball comprising a core and a cover positioned outside the core, wherein
the core includes a center and an envelope layer positioned outside the center,
the center has a spherical space and an outer layer,
the center has an inner diameter of 2 mm or greater and 13 mm or less,
the center has an outer diameter of 5 mm or greater and 15 mm or less,
the center has a JIS-C hardness H2, at its surface, of 25 or greater and 55 or less,
a difference (H4−H3) between a JIS-C hardness H4 of the core at its surface and a JIS-C hardness H3 of the envelope layer at its innermost portion is equal to or greater than 10,
the cover has a flexural modulus of 300 MPa or greater and 1000 MPa or less, and
the cover has a tensile modulus of 400 MPa or greater and 1500 MPa or less.
2. The golf ball according to claim 1 , wherein the cover has a Shore D hardness H6 of 65 or greater and 75 or less.
3. The golf ball according to claim 1 , wherein
the cover is formed from a resin composition that includes
(A) a highly elastic polyamide resin with a flexural modulus of 700 MPa or greater and 5000 MPa or less,
(B) an ethylene-(meth)acrylic acid copolymer that is neutralized with a metal ion, and
(C) a resin having a polar functional group,
a weight ratio of the polyamide resin (A) with respect to the copolymer (B) is equal to or greater than 5/95 and equal to or less than 80/20, and
the amount of the resin (C) is equal to or greater than 0.1 part by weight and equal to or less than 20 parts by weight, per total 100 parts by weight of the polyamide resin (A) and the copolymer (B).
4. The golf ball according to claim 3 , wherein the polyamide resin (A) includes one or more members selected from the group consisting of polyamide 6, polyamide 11, polyamide 12, polyamide 66, polyamide 610, polyamide 6T, polyamide 61, polyamide 9T, polyamide MST, polyamide 612, and a polyether block amide copolymer.
5. The golf ball according to claim 3 , wherein the copolymer (B) includes an ethylene-(meth)acrylic acid binary copolymer and/or an ethylene-(meth) acrylic acid-(meth)acrylic acid ester ternary copolymer.
6. The golf ball according to claim 3 , wherein the resin (C) includes one or more members selected from the group consisting of an ethylene-glycidyl methacrylate copolymer, an ethylene-acrylic acid-glycidyl methacrylate copolymer, and a methyl methacrylate-glycidyl methacrylate copolymer.
7. The golf ball according to claim 1 , further comprising a mid layer positioned between the core and the cover, wherein
a shore D hardness H6 of the cover is greater than a Shore D hardness H5 of the mid layer.
8. The golf ball according to claim 7 , wherein the mid layer is formed from a thermoplastic resin composition.
9. The golf ball according to claim 1 , wherein the outer layer has a JIS-C hardness H1, at its innermost portion, of 35 or less.
10. The golf ball according to claim 1 , wherein a difference (H3−H2) between the hardness H3 and the hardness H2 is equal to or less than 35.
11. The golf ball according to claim 1 , wherein a difference (H4−H2) between the hardness H4 and the hardness H2 is equal to or greater than 40.
12. The golf ball according to claim 1 , wherein
the outer layer is formed by crosslinking a rubber composition, and
the rubber composition includes sulfur as a crosslinking agent.
13. The golf ball according to claim 1 , wherein a rubber composition for the outer layer includes a natural rubber.
14. The golf ball according to claim 1 , wherein
the envelope layer is formed by crosslinking a rubber composition, and
the principal component of the base rubber of the rubber composition is a polybutadiene.