An object of the present invention is to provide a golf ball excellent in hitting durability. The present invention provides a golf ball comprising a spherical core and at least one cover layer covering the spherical core, wherein at least a part of the spherical core is formed from a rubber composition containing (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, and (x) an olefin-alkyl (meth)acrylate copolymer, and a content of (x) the olefin-alkyl (meth)acrylate copolymer in the rubber composition ranges from 0.5 part by mass to 7.0 parts by mass with respect to 100 parts by mass of (a) the base rubber.
1. A golf ball comprising a spherical core and at least one cover layer covering the spherical core,
wherein all of layers of the spherical core are formed from a rubber composition containing (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, and (x) an olefin-alkyl (meth)acrylate copolymer,
a content of (b) the co-crosslinking agent in the rubber composition ranges from 25 parts by mass to 50 parts by mass with respect to 100 parts by mass of (a) the base rubber, and
a content of (x) the olefin-alkyl (meth)acrylate copolymer in the rubber composition ranges from 0.5 parts by mass to 7.0 parts by mass with respect to 100 parts by mass of (a) the base rubber.
2. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer has a melt flow rate (2.16 kg, 190° C.) of 1.0 g/10 min or more and 40 g/10 min or less.
3. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer has a tensile elastic modulus (−25° C.) of 20 MPa or more and 500 MPa or less.
4. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer has a Vicat softening temperature of 40° C. or more and 70° C. or less.
5. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer has a melting point of 80° C. or more and 100° C. or less.
6. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer has a brittleness temperature of −50° C. or less.
7. The golf ball according to claim 6 , wherein (x) the olefin-alkyl (meth)acrylate copolymer has the brittleness temperature of −60° C. or less.
8. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer is at least one copolymer selected from the group consisting of an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, and an ethylene-butyl acrylate copolymer.
9. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer contains the alkyl (meth)acrylate in an amount ranging from 5.0 mass % to 35 mass %.
10. The golf ball according to claim 1 , wherein (x) the olefin-alkyl (meth)acrylate copolymer has a solubility parameter ranging from 7.5 to 11.5.
11. The golf ball according to claim 1 , wherein the content of (x) the olefin-alkyl (meth)acrylate copolymer in the rubber composition ranges from 1 part by mass to 5 parts by mass with respect to 100 parts by mass of (a) the base rubber.
12. The golf ball according to claim 1 , wherein the spherical core has a surface hardness (Hs) ranging from 65 to 95 in JIS-C hardness.
13. The golf ball according to claim 1 , wherein the spherical core has a center hardness (Ho) ranging from 35 to 70 in JIS-C hardness.
14. The golf ball according to claim 1 , wherein the spherical core has a hardness difference (Hs−Ho) ranging from 5 to 50 in JIS-C hardness between a surface hardness (Hs) and a center hardness (Ho).
15. The golf ball according to claim 1 , wherein (a) the base rubber consists of polybutadiene rubber having a 1,2-vinyl bond in a content of 2 mass % or less.
16. The golf ball according to claim 1 , wherein the rubber composition further contains (e) a carboxylic acid and/or a salt thereof selected from the group consisting of caprylic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, benzoic acid, butylbenzoic acid, anisic acid, dimethoxybenzoic acid, trimethoxybenzoic acid, dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, acetoxybenzoic acid, biphenylcarboxylic acid, naphthalene carboxylic acid, anthracene carboxylic acid, furan carboxylic acid, thenoic acid, and a salt thereof.
17. The golf ball according to claim 1 , wherein the rubber composition consists essentially of (a) the base rubber, (b) the co-crosslinking agent, (c) the crosslinking initiator, (x) the olefin-alkyl (meth)acrylate copolymer, and at least one inorganic filler selected from the group consisting of barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder.
18. The golf ball according to claim 1 , wherein
(x) the olefin-alkyl (meth)acrylate copolymer is an ethylene-ethyl acrylate copolymer, and
the spherical core has a surface hardness (Hs) ranging from 65 to 95 in JIS-C hardness, a center hardness (Ho) ranging from 35 to 70 in JIS-C hardness, and a hardness difference (Hs−Ho) ranging from 5 to 50 in JIS-C hardness between the surface hardness (Hs) and the center hardness (Ho).
19. The golf ball according to claim 1 , wherein the rubber composition consists essentially of (a) the base rubber, (b) the co-crosslinking agent, (c) the crosslinking initiator, (x) the olefin-alkyl (meth)acrylate copolymer, and at least one inorganic filler selected from the group consisting of barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder,
(a) the base rubber consists of polybutadiene rubber having a cis-1,4 bond in a proportion of 80% or more, a 1,2-vinyl bond in a content of 2 mass % or less, and a Mooney viscosity (ML 1+4 (100° C.)) of 55 or more and 140 or less,
(x) the olefin-alkyl (meth)acrylate copolymer is at least one copolymer selected from the group consisting of an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, and an ethylene-butyl acrylate copolymer,
(x) the olefin-alkyl (meth)acrylate copolymer contains the alkyl (meth)acrylate in an amount ranging from 5.0 mass % to 35 mass %,
(x) the olefin-alkyl (meth)acrylate copolymer has a melt flow rate (2.16 kg, 190° C.) of 1.0 g/10 min or more and 40 g/10 min or less, a tensile elastic modulus (−25° C.) of 20 MPa or more and 500 MPa or less, a Vicat softening temperature of 40° C. or more and 70° C. or less, a melting point of 80° C. or more and 100° C. or less, and a brittleness temperature of −50° C. or less, and
the spherical core has a surface hardness (Hs) ranging from 65 to 95 in JIS-C hardness, a center hardness (Ho) ranging from 35 to 70 in JIS-C hardness, and a hardness difference (Hs−Ho) ranging from 5 to 50 in JIS-C hardness between the surface hardness (Hs) and the center hardness (Ho).
20. The golf ball according to claim 1 , wherein (a) the base rubber consists of polybutadiene rubber having a cis-1,4 bond in a proportion of 80% or more and a Mooney viscosity (ML 1+4 (100° C.)) of 55 or more and 140 or less.