Golf ball

1. A golf ball comprising a core composed of a spherical center and an envelope layer and at least one cover layer disposed outside the core,

wherein

the spherical center has a hardness difference between a maximum hardness and a minimum hardness within the spherical center of less than 5 in JIS-C hardness,

the envelope layer is formed from an envelope layer rubber composition containing

(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 and

(d) a carboxylic acid and/or a salt thereof excluding the (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof,

the outermost cover layer has a hardness that is highest from among the spherical center, envelope layer and any other cover layer of the golf ball, and

the envelope layer rubber composition contains (d) the carboxylic acid and/or the salt thereof in an amount of 1 part by mass or more and less than 40 parts by mass with respect to 100 parts by mass of (a) the base rubber, and

the spherical center is formed from a center resin composition comprising

at least one selected from the group consisting of an ionomer resin, a thermoplastic polyamide elastomer, a thermoplastic polyester elastomer, a thermoplastic polyurethane elastomer, and a thermoplastic styrene elastomer as a resin component.

2. The golf ball according to claim 1 , wherein the envelope layer rubber composition further contains (e) an organic sulfur compound.

3. The golf ball according to claim 2 , wherein (e) the organic sulfur compound includes at least one compound selected from the group consisting of thiophenols, polysulfides having 2 to 4 sulfur atoms, thionaphthols, thiurams, or metal salts thereof.

4. The golf ball according to claim 2 , wherein the envelope layer rubber composition contains (e) the organic sulfur compound in an amount ranging from 0.05 part by mass to 5 parts by mass with respect to 100 parts by mass of (a) the base rubber.

5. The golf ball according to claim 1 , wherein the envelope layer rubber composition contains (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof in an amount ranging from 15 parts by mass to 50 parts by mass with respect to 100 parts by mass of (a) the base rubber.

6. The golf ball according to claim 1 , wherein the envelope layer rubber composition further contains (f) a metal compound in case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

7. The golf ball according to claim 1 , wherein the envelope layer rubber composition contains (c) the crosslinking initiator in an amount of 0.2 part by mass to 5 parts by mass with respect 100 parts by mass of (a) the base rubber.

8. The golf ball according to claim 1 , wherein a total thickness of at least one cover layer is 2.5 mm or less.

9. The golf ball according to claim 1 , wherein the center resin composition further includes at least one selected from the group consisting of an anionic surfactant having an anionic portion and containing sulfur, an amphoteric surfactant having a cationic and anionic portions, and a basic metal salt of a fatty acid.

10. The golf ball according to claim 1 , wherein the center resin composition further includes at least one selected from the group consisting of an anionic surfactant having an anionic portion and containing sulfur, an amphoteric surfactant having a cationic and anionic portions.