GOLF BALL

Abstract

An object of the present disclosure is to provide a golf ball having a great change in an initial velocity associated with a change in a swing speed. The present disclosure provides a golf ball comprising a constituent member, wherein at least a part of the constituent member is formed of a cured product of a rubber composition comprising (a) a base rubber containing a natural rubber, (b) a co-crosslinking agent containing methacrylic acid and/or a metal salt thereof and (c) a crosslinking initiator.

Description

FIELD OF THE DISCLOSURE

The present disclosure relates to a golf ball.

DESCRIPTION OF THE RELATED ART

Conventionally, as a golf ball used in a driving range, a golf ball travelling a restricted flight distance has been developed. For example, JP S60-92780 A discloses a practice range golf ball obtained by vulcanizing a composition containing 3 to 35 parts by weight of a rubber with low resilience, 20 to 30 parts by weight of methacrylic acid, and 20 to 50 parts by weight of a metal compound which can form a metal salt with methacrylic acid, with respect to 100 parts by weight of a base rubber.

In addition, JP S61-71069 A discloses a practice range golf ball obtained from a composition containing 3 to 35 parts by weight of an epoxidized natural rubber with an epoxidation degree of 10 to 60 mole %, 20 to 35 parts by weight of methacrylic acid, and 20 to 50 parts by weight of zinc oxide, with respect to 100 parts by weight of a base rubber.

SUMMARY OF THE DISCLOSURE

Conventionally, as a practice range golf ball, a golf ball travelling a restricted flight distance has been proposed. However, such golf ball having low resilience provides a small change in the flight distance or initial velocity even if the swing speed has been changed, and a golfer is hard to feel the improvement in the swing speed.

The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a golf ball having a great change in an initial velocity associated with a change in a swing speed.

The present disclosure that has solved the above problem provides a golf ball comprising a constituent member, wherein at least a part of the constituent member is formed of a cured product of a rubber composition comprising

• • (a) a base rubber containing a natural rubber,
  • (b) a co-crosslinking agent containing methacrylic acid and/or a metal salt thereof, and
  • (c) a crosslinking initiator.

The golf ball according to the present disclosure has a great change in the flight distance or initial velocity associated with the change in the swing speed. Thus, if the golf ball according to the present disclosure is used for practice, a golfer can intuitively feel the improvement in the swing speed by the change in the flight distance or initial velocity of the golf ball.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view showing a one-piece golf ball according to one embodiment of the present disclosure;

FIG. 2 is a partially cutaway cross-sectional view showing a multi-piece golf ball according to one embodiment of the present disclosure;

FIG. 3 is a graph showing a relationship between a compression deformation amount and a coefficient of resilience of a golf ball; and

FIG. 4 is a graph showing a relationship between a compression deformation amount and an initial velocity difference of a golf ball.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[Rubber Composition]

The present disclosure provides a golf ball comprising a constituent member, wherein at least a part of the constituent member is formed of a cured product of a rubber composition comprising

• • (a) a base rubber containing a natural rubber,
  • (b) a co-crosslinking agent containing methacrylic acid and/or a metal salt thereof, and
  • (c) a crosslinking initiator.

Although the reason why the golf ball according to the present disclosure has a great change in the flight distance or initial velocity associated with a change in the swing speed when hitting is still unclear, it is considered as follows. In the case that the swing speed when hitting is relatively slow (the head speed is 40 m/s or less), the energy input of the hitting by the club is consumed for the motion of the polymer molecule of the natural rubber, and thus the energy input is hardly consumed for the resilience energy. Thus, the golf ball tends to travel a shorter flight distance or have a smaller initial velocity than the golf ball that does not contain the natural rubber. In addition, in the case that the swing speed when hitting is relatively fast (the head speed is 50 m/s or more), the motion speed of the polymer molecule of the natural rubber cannot keep up with the energy input of the hitting by the club, and thus the energy input is easily consumed for the resilience energy. Thus, the golf ball tends to travel a greater flight distance or have a faster initial velocity than the golf ball that does not contain the natural rubber.

((a) Base Rubber)

• • (a) The base rubber contains a natural rubber (NR). The natural rubber is, for example, prepared by slicing plants that produce natural rubber latex, collecting the latex, and coagulating the rubber component contained in the latex. The natural rubber may be used solely, or at least two of them may be used in combination.

Examples of the plant that produce the natural rubber latex include Para rubber tree and Ceara rubber tree which belong to the Euphorbiaceae family; Indian rubber tree, Panama rubber tree and Lagos rubber tree which belong to the Moraceae family; Arabia rubber tree and Tragacanth rubber tree which belong to the Fabaceae family; Jelutong tree, Zanzibar rubber tree, Funtumia elastica and Urceola which belong to the Apocynaceae family; Guayule rubber tree and Rubber dandelion which belong to the Composite family; Gutta-percha tree, Balata rubber tree and Sapodilla which belong to the Sapotaceae family; Ipomoea nil which belongs to the Asclepiadaceae family; and Eucommia which belongs to the Eucommiaceae family.

Examples of the natural rubber include a CV grade in which the rubber viscosity is stabilized by adding a viscosity stabilizer or the like to a raw latex, and a non-CV grade in which the rubber viscosity is not stabilized. The natural rubber may be used solely, or two of them may be used in combination. Among them, the CV grade having the stabilized viscosity is particularly preferable. It is noted that the natural rubber may be either SMR (Standard Malaysia Rubber) or SVR (Standard Vietnam Rubber).

The natural rubber is cis-1,4-polyisoprene, and either a sheet rubber or a block rubber can be used. In addition, the natural rubber also includes a modified product of the natural rubber, i.e. a modified natural rubber such as an epoxidized natural rubber, a methacrylic acid modified natural rubber, a halogen modified natural rubber, a deproteinized natural rubber, a maleic acid modified natural rubber, a sulfonic acid modified natural rubber, and a styrene modified natural rubber. Among them, the natural rubber preferably does not contain the epoxidized natural rubber.

As the natural rubber, Technical Specified Rubbers (TSR), or Ribbed Smoked Sheet (RSS) is preferable. In addition, the natural rubber may contain a viscosity stabilizer.

The Mooney viscosity (ML₁₊₄ (100° C.)) of the natural rubber is preferably 30 or more, more preferably 35 or more, and even more preferably 40 or more, and is preferably 80 or less, more preferably 75 or less, and even more preferably 70 or less. It is noted that the Mooney viscosity (ML₁₊₄ (100° C.)) in the present disclosure is a value measured according to JIS K6300 (2013) using an L rotor under the conditions of preheating time: 1 minute, rotor rotation time: 4 minutes, and temperature: 100° C.

(a) The base rubber may consist of the natural rubber or contain the natural rubber and a synthetic rubber.

When (a) the base rubber contains the natural rubber and the synthetic rubber, the amount of the natural rubber is preferably 10 mass % or more, more preferably 20 mass % or more, and even more preferably 30 mass % or more, and is preferably 80 mass % or less, more preferably 75 mass % or less, and even more preferably 70 mass % or less in 100 mass % of (a) the base rubber. If the amount is 10 mass % or more, the change in the initial velocity associated with the change in the swing speed is greater, and if the amount is 80 mass % or less, the golf ball has better hardness.

Examples of the synthetic rubber include a diene based rubber such as a polybutadiene rubber (BR), a polyisoprene rubber (IR), a styrene-polybutadiene rubber (SBR), a chloroprene rubber (CR), a butyl rubber (IIR), and an acrylonitrile-butadiene rubber (NBR); and a non-diene based rubber such as an ethylene-propylene rubber (EPM), an ethylene-propylene-diene rubber (EPDM), a urethane rubber, a silicone rubber, an acrylic rubber, an epichlorohydrin rubber, a polysulfide rubber, a fluorine rubber, and a chlorosulfonated polyethylene rubber. The synthetic rubber may be used solely, or two of them may be used in combination.

(a) The base rubber preferably contains the diene based rubber as the synthetic rubber. In this case, the amount of the diene based rubber is preferably 20 mass % or more, more preferably 25 mass % or more, and even more preferably 30 mass % or more, and is preferably 90 mass % or less, more preferably 80 mass % or less, and even more preferably 70 mass % or less in 100 mass % of (a) the base rubber. If the amount of the diene based rubber is 20 mass % or more, the golf ball has better hardness, and if the amount of the diene based rubber is 90 mass % or less, the change in the initial velocity associated with the change in the swing speed is greater.

(a) The base rubber preferably contains a polybutadiene rubber as the diene based rubber. In particular, (a) the base rubber more preferably contains a high-cis polybutadiene having a cis-1,4 bond in an amount of 40 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more. The amount of the high-cis polybutadiene in the diene based rubber is preferably 50 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more. The diene based rubber also preferably consists of the high-cis polybutadiene rubber.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in an amount of 2 mass % or less, more preferably 1.7 mass % or less, and even more preferably 1.5 mass % or less. The smaller amount of the 1,2-vinyl bond means the more enhanced resilience of the golf ball.

The high-cis polybutadiene is preferably one synthesized using a rare-earth element catalyst. When a neodymium catalyst employing a neodymium compound, which is a lanthanum series rare-earth element compound, is used, a polybutadiene rubber having a high amount of the cis-1,4 bond and a low amount of the 1,2-vinyl bond is obtained with an excellent polymerization activity, and thus such polybutadiene rubber is particularly preferable.

The molecular weight distribution Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) of the high-cis polybutadiene is preferably 2.0 or more, more preferably 2.2 or more, and even more preferably 2.4 or more, and is preferably 6.0 or less, more preferably 5.0 or less, and even more preferably 4.0 or less. If the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is 2.0 or more, the processibility is enhanced, and if the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is 6.0 or less, the golf ball has further enhanced resilience. It is noted that the molecular weight distribution is measured by gel permeation chromatography (“HLC-8120GPC” available from Tosoh Corporation) using a differential refractometer as a detector under the conditions of column: GMHHXL (available from Tosoh Corporation), column temperature: 40° C., and mobile phase: tetrahydrofuran, and calculated by converting based on polystyrene standard.

The Mooney viscosity (ML₁₊₄ (100° C.)) of the high-cis polybutadiene is preferably 30 or more, more preferably 32 or more, and even more preferably 35 or more, and is preferably 140 or less, more preferably 120 or less, and even more preferably 100 or less.

((b) Co-Crosslinking Agent)

(b) The co-crosslinking agent has an action of crosslinking a rubber molecule by graft polymerization to a molecular chain of the base rubber.

The rubber composition contains methacrylic acid and/or a metal salt thereof as (b) the co-crosslinking agent. If methacrylic acid and/or the metal salt thereof is contained as (b) the co-crosslinking agent, the hardness and the resilience performance of the golf ball can be enhanced.

Examples of the metal component constituting the metal salt of methacrylic acid include a monovalent metal ion such as sodium, potassium, and lithium; a divalent metal ion such as magnesium, calcium, zinc, barium, and cadmium; a trivalent metal ion such as aluminum; and other metal ions such as tin, and zirconium. The metal component may be used solely, or at least two of them may be used in combination. Among them, as the metal component, the divalent metal such as magnesium, calcium, zinc, barium, and cadmium is preferable.

(b) The co-crosslinking agent of the rubber composition may consist of methacrylic acid and/or the metal salt thereof, or may further contain other co-crosslinking agents different from methacrylic acid and/or the metal salt thereof. Examples of the other co-crosslinking agents include an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof. The α,β-unsaturated carboxylic acid used as the other co-crosslinking agents preferably has 3 to 8 carbon atoms, more preferably has 3 to 6 carbon atoms, and even more preferably has 3 or 4 carbon atoms. It is noted that the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof may be used solely, or two or more of them may be used in combination. Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms other than methacrylic acid include acrylic acid, fumaric acid, maleic acid, and crotonic acid.

Examples of the metal component constituting the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include a monovalent metal ion such as sodium, potassium, and lithium; a divalent metal ion such as magnesium, calcium, zinc, barium, and cadmium; a trivalent metal ion such as aluminum; and other metal ions such as tin, and zirconium. The metal component may be used solely, or at least two of them may be used in combination. Among them, as the metal component, the divalent metal such as magnesium, calcium, zinc, barium, and cadmium is preferable.

When (b) the co-crosslinking agent further contains the other co-crosslinking agents than methacrylic acid and/or the metal salt thereof, the amount of methacrylic acid and/or the metal salt thereof is preferably 20 mass % or more, more preferably 40 mass % or more, and even more preferably 80 mass % or more in 100 mass % of (b) the co-crosslinking agent.

The amount of (b) the co-crosslinking agent is preferably 15 parts by mass or more, more preferably 18 parts by mass or more, and even more preferably 20 parts by mass or more, and is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, even more preferably 60 parts by mass or less, and most preferably 40 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the amount of (b) the co-crosslinking agent is 15 parts by mass or more, the constituent member formed from the rubber composition has a suitable hardness in a small amount of (c) the crosslinking initiator, and the golf ball has enhanced impact durability. In addition, if the amount of (b) the co-crosslinking agent is 100 parts by mass or less, the constituent member formed from the rubber composition is not excessively hard, and thus the golf ball has enhanced shot feeling.

In the case that (a) the base rubber contains the diene based rubber, the amount of (b) the co-crosslinking agent is preferably 15 parts by mass or more, more preferably 18 parts by mass or more, and even more preferably 20 parts by mass or more, and is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, even more preferably 60 parts by mass or less, and most preferably 45 parts by mass or less, with respect to 100 parts by mass of the diene based rubber. If the amount of (b) the co-crosslinking agent is 15 parts by mass or more, the constituent member formed from the rubber composition has a suitable hardness with a small amount of (c) the crosslinking initiator, and the golf ball has enhanced impact durability. In addition, if the amount of (b) the co-crosslinking agent is 100 parts by mass or less, the constituent member formed from the rubber composition is not excessively hard, and thus the golf ball has enhanced shot feeling.

((c) Crosslinking Initiator)

(c) The crosslinking initiator is blended in order to crosslink (a) the base rubber component. As (c) the crosslinking initiator, an organic peroxide is preferable. Specific examples of the organic peroxide include an organic peroxide such as dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, and di-t-butylperoxide. These organic peroxides may be used solely, or at least two of them may be used in combination. Among them, dicumyl peroxide is preferably used.

The amount of (c) the crosslinking initiator is preferably 0.2 part by mass or more, more preferably 0.5 part by mass or more, and even more preferably 0.7 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 2.5 parts by mass or less, and even more preferably 2.0 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the amount of (c) the crosslinking initiator is 0.2 part by mass or more, the crosslinked rubber molded product formed from the rubber composition is not excessively soft and the shot feeling is better, and if the amount of (c) the crosslinking initiator is 5.0 parts by mass or less, the crosslinked rubber molded product formed from the rubber composition has suitable hardness and thus has better durability.

((d) Organic Sulfur Compound)

The rubber composition preferably further contains (d) an organic sulfur compound. If the rubber composition contains (d) the organic sulfur compound, the resilience performance of the golf ball can be enhanced.

Examples of (d) the organic sulfur compound include at least one compound selected from the group consisting of thiophenols, thionaphthols, polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, thiazoles, and their metal salts. (d) The organic sulfur compound may be used solely, or two of them may be used in combination. As (d) the organic sulfur compound, the organic sulfur compound having the thiol group (—SH) or the metal salt thereof is preferable, the thiophenols, thionaphthols or their metal salts are preferable.

Examples of the thiophenols include thiophenol, thiophenol substituted with a halogen group, and their metal salts. Examples of the thiophenol substituted with the halogen group include thiophenols substituted with a fluoro group, such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, and pentafluorothiophenol; thiophenols substituted with a chloro group, such as 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, and pentachlorothiophenol; thiophenols substituted with a bromo group, such as 4-bromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenols substituted with an iodo group, such as 4-iodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; and their metal salts. As the metal salt, zinc salt is preferable.

Examples of the thionaphthols (naphthalenethiols) include 2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol, 2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and their metal salts. 2-thionaphthol, 1-thionaphthol, or their metal salts are preferable. As the metal salt, a divalent metal salt is preferable, zinc salt is more preferable. Specific examples of the metal salt include zinc salt of 1-thionaphthol, and zinc salt 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond, and examples thereof include disulfides, trisulfides, and tetrasulfides. As the polysulfides, diphenylpolysulfides are preferable.

Examples of the diphenyl polysulfides include diphenyl disulfide; diphenyl disulfides substituted with a halogen group, such as bis(4-fluorophenyl) disulfide, bis(2,5-difluorophenyl) disulfide, bis(2,6-difluorophenyl) disulfide, bis(2,4,5-trifluorophenyl) disulfide, bis(2,4,5,6-tetrafluorophenyl) disulfide, bis(pentafluorophenyl) disulfide, bis(4-chlorophenyl) disulfide, bis(2,5-dichlorophenyl) disulfide, bis(2,6-dichlorophenyl) disulfide, bis(2,4,5-trichlorophenyl) disulfide, bis(2,4,5,6-tetrachlorophenyl) disulfide, bis(pentachlorophenyl) disulfide, bis(4-bromophenyl) disulfide, bis(2,5-dibromophenyl) disulfide, bis(2,6-dibromophenyl) disulfide, bis(2,4,5-tribromophenyl) disulfide, bis(2,4,5,6-tetrabromophenyl) disulfide, bis(pentabromophenyl) disulfide, bis(4-iodophenyl) disulfide, bis(2,5-diiodophenyl) disulfide, bis(2,6-diiodophenyl) disulfide, bis(2,4,5-triiodophenyl) disulfide, bis(2,4,5,6-tetraiodophenyl) disulfide, and bis(pentaiodophenyl) disulfide; and diphenyl disulfides substituted with an alkyl group, such as bis(4-methylphenyl) disulfide, bis(2,4,5-trimethylphenyl) disulfide, bis(pentamethylphenyl) disulfide, bis(4-t-butylphenyl) disulfide, bis(2,4,5-tri-t-butylphenyl) disulfide, and bis(penta-t-butylphenyl) disulfide.

Examples of the thiurams include thiuram monosulfides such as tetramethylthiuram monosulfide; thiuram disulfides such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and tetrabutylthiuram disulfide; and thiuram tetrasulfides such as dipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylic acids include a naphthalene thiocarboxylic acid. Examples of the dithiocarboxylic acids include a naphthalene dithiocarboxylic acid. Examples of the sulfenamides include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide.

Examples of the thiazoles include 2-mercaptobenzothiazole, a metal salt of 2-mercaptobenzothiazole, and 2-(4-morpholinodithio) benzothiazole. Examples of the metal salt of 2-mercaptobenzothiazole include zinc salt of 2-mercaptobenzothiazole and cyclohexylamine salt of 2-mercaptobenzothiazole.

As (d) the organic sulfur compound, the thiophenols and/or the metal salt thereof, the thionaphthols and/or the metal salt thereof, the diphenyl disulfides, the thiuram disulfides, and the thiazoles and/or the metal salt thereof are preferable, 2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol, 2-mercaptobenzothiazole, and their metal salts are more preferable.

As (d) the organic sulfur compound, (d1) a compound of the thiophenols and/or the metal salt thereof and (d2) a compound of the thiazoles and/or the metal salt thereof are preferably contained, and the thiophenols substituted with the halogen group and/or the metal salt thereof and 2-mercaptobenzothiazole and/or the metal salt thereof are particularly preferably contained. In this case, the mass ratio (d1/d2) of (d1) the compound of the thiophenols and/or the metal salt thereof to (d2) the compound of thiazoles and/or the metal salt thereof in (d) the organic sulfur compound is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.3 or more, and is preferably 10.0 or less, more preferably 7.5 or less, even more preferably 5.0 or less, and most preferably 1.0 or less. If the mass ratio (d1/d2) is 0.1 or more, the change in the initial velocity associated with the change in the swing speed when hitting is greater, and if the mass ratio (d1/d2) is 5.0 or less, the golf ball has better hardness.

The amount of (d) the organic sulfur compound in the rubber composition is preferably 0.01 part by mass or more, more preferably 0.1 part by mass or more, and even more preferably 0.2 part by mass or more, and is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, even more preferably 10 parts by mass or less, and most preferably 5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the amount of (d) the organic sulfur compound is 0.01 part by mass or more, the change in the initial velocity associated with the change in the swing speed when hitting is greater, and if the amount of (d) the organic sulfur compound is 20 parts by mass or less, the golf ball has better hardness.

((e) Metal Compound)

The rubber composition may further contain (e) a metal compound. If (e) the metal compound is contained, a metal crosslinking with methacrylic acid blended as (b) the co-crosslinking agent can be formed.

(e) The metal compound is not particularly limited, as long as (e) the metal compound can neutralize methacrylic acid in the rubber composition. Examples of (e) the metal compound include a metal hydroxide such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; a metal oxide (excluding titanium oxide) such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and a metal carbonate such as magnesium carbonate, zinc carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. (e) The metal compound is preferably the divalent metal compound, more preferably the zinc compound. This is because the divalent metal compound reacts with the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, thereby forming a metal crosslinking. In addition, use of the zinc compound provides a golf ball with better hardness. (e) The metal compound may be used solely, or at least two of them may be used in combination.

The amount of (e) the metal compound is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, and is preferably 100 parts by mass or less, more preferably 90 parts by mass or less, even more preferably 80 parts by mass or less, particularly preferably 70 parts by mass or less, and most preferably 30 parts by mass or less, with respect to 100 parts by mass of (b) the co-crosslinking agent. If the amount of (e) the metal compound falls within the above range, the golf ball can keep a suitable weight.

(Other Components)

The rubber composition may further contain additives such as a pigment, a filler for adjusting a weight or the like, an antioxidant, a peptizing agent, a softening agent, and a carboxylic acid, where necessary. In addition, the rubber composition may contain a rubber powder which is obtained by pulverizing a golf ball core or offcuts produced when preparing a core.

Examples of the pigment blended in the rubber composition include a white pigment, a blue pigment, and a purple pigment. As the white pigment, titanium oxide is preferably used. The type of titanium oxide is not particularly limited, but a rutile type is preferably used because of the high opacity. In addition, the amount of titanium oxide is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and is preferably 8 parts by mass or less, more preferably 5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber.

It is also a preferable embodiment that the rubber composition contains a white pigment and a blue pigment. The blue pigment is blended in order to cause white color to be vivid, and examples thereof include ultramarine blue, cobalt blue, and phthalocyanine blue. In addition, examples of the purple pigment include anthraquinone violet, dioxazine violet, and methyl violet.

The filler blended in the rubber composition is used as a weight adjusting agent for adjusting the mass of the obtained crosslinked rubber molded product. The filler may be blended where necessary. Examples of the filler include an inorganic filler such as calcium carbonate, barium sulfate, tungsten powder, and molybdenum powder.

The amount of the antioxidant is preferably 0.1 part by mass or more and 1 part by mass or less with respect to 100 parts by mass of (a) the base rubber. In addition, the amount of the peptizing agent is preferably 0.1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of (a) the base rubber.

(Preparation of Rubber Composition)

The rubber composition can be obtained by mixing and kneading (a) the base rubber, (b) the co-crosslinking agent, (c) the crosslinking initiator, and the other optional additives. The kneading method is not particularly limited, for example, a conventional kneading machine such as a kneading roll, a Banbury mixer, and a kneader can be used in the kneading method.

(Cured Product of Rubber Composition)

The cured product of the rubber composition can be obtained by heat-molding the kneaded rubber composition in a mold. The molding temperature is preferably 120° C. or more, more preferably 150° C. or more, and is preferably 250° C. or less. In addition, the molding pressure preferably ranges from 2.9 MPa to 11.8 MPa. The molding time preferably ranges from 10 minutes to 60 minutes.

[Golf Ball]

The present disclosure includes a golf ball comprising a constituent member, wherein at least a part of the constituent member is formed of a cured product of the rubber composition. Examples of the golf ball include a one-piece golf ball wherein the golf ball body is formed of the cured product of the rubber composition; and a multi-piece golf ball comprising a spherical core and a cover composed of at least one layer and covering the spherical core, wherein at least a part of the spherical core is formed of the cured product of the rubber composition.

(One-Piece Golf Ball)

Examples of the one-piece golf ball include a one-piece golf ball consisting of a golf ball body; and a one-piece golf ball composed of a golf ball body and a paint film covering the golf ball body.

The one-piece golf ball preferably has a diameter in a range from 40 mm to 45 mm. In light of satisfying a regulation of US Golf Association (USGA), the diameter is particularly preferably 42.67 mm or more. In light of prevention of air resistance, the diameter is more preferably 44 mm or less, particularly preferably 42.80 mm or less. In addition, the one-piece golf ball preferably has a mass of 40 g or more and 50 g or less. In light of obtaining greater inertia, the mass is more preferably 44 g or more, particularly preferably 45.00 g or more. In light of satisfying a regulation of USGA, the mass is particularly preferably 45.93 g or less.

When the one-piece golf ball has a diameter in the range from 40 mm to 45 mm, the compression deformation amount of the one-piece golf ball (shrinking amount of the one-piece golf ball along the compression direction) when applying a load from an initial load of 98 N to a final load of 1275 N to the one-piece golf ball is preferably 2.0 mm or more, more preferably 2.2 mm or more, and even more preferably 2.5 mm or more, and is preferably 6.0 mm or less, more preferably 5.5 mm or less, and even more preferably 5.0 mm or less. If the compression deformation amount is 2.0 mm or more, the golf ball has better shot feeling, and if the compression deformation amount is 6.0 mm or less, the golf ball has better shot feeling.

When the one-piece golf ball has a diameter in the range from 40 mm to 45 mm, the coefficient of resilience (e40) thereof is preferably 0.500 or more, more preferably 0.520 or more, and even more preferably 0.550 or more, and is preferably 0.800 or less, more preferably 0.780 or less, and even more preferably 0.750 or less. If the coefficient of resilience (e40) is 0.500 or more, the change in the initial velocity associated with the change in the swing speed when hitting is greater, and if the coefficient of resilience (e40) is 0.800 or less, the golf ball has restricted resilience performance and is more suitable for a narrow practice range. The measurement method of the coefficient of resilience (e40) will be described later.

Concave portions called “dimple” are usually formed on the surface of the golf ball body of the one-piece golf ball. The total number of the dimples is preferably 200 or more and 500 or less. If the total number of the dimples is 200 or more, the dimple effect is greater, and if the total number of the dimples is 500 or less, the dimple effect is greater because the size of the respective dimples is large. The shape (shape in a plan view) of the formed dimples includes, for example, without limitation, a circle, a polygonal shape such as a roughly triangular shape, a roughly quadrangular shape, a roughly pentagonal shape, a roughly hexagonal shape, and other irregular shapes. The shape of dimples is employed solely or at least two of them may be used in combination.

The golf ball body of the one-piece golf ball can be produced by heat-molding the rubber composition in a mold. The molding temperature is preferably 100° C. or more, more preferably 120° C. or more, and even more preferably 150° C. or more, and is preferably 200° C. or less. In addition, the molding pressure preferably ranges from 2.9 MPa to 11.8 MPa. The molding time preferably ranges from 10 minutes to 60 minutes.

The one-piece golf ball may have a paint film or a mark formed on the surface of the golf ball body. The thickness of the paint film is not particularly limited, and is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 7 μm or more, and is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less. If the thickness of the paint film is 5 μm or more, the paint film is hard to wear off due to the continued use of the golf ball, and if the thickness of the paint film is 50 μm or less, the dimple effect is not lowered and thus the flight performance of the golf ball is better.

(Multi-Piece Golf Ball)

The multi-piece golf ball is a golf ball comprising a spherical core and a cover composed of at least one layer and covering the spherical core, wherein at least a part of the spherical core is formed of the cured product of the rubber composition.

The construction of the spherical core may be single-layered or multiple-layered. Examples of the spherical core include a single-layered core formed of the cured product of the rubber composition; and a dual layered core having an inner layer and an outer layer, wherein the inner layer and/or the outer layer is formed of the cured product of the rubber composition.

The diameter of the spherical core is preferably 34.8 mm or more, and is preferably 42.2 mm or less, more preferably 41.8 mm or less, even more preferably 41.2 mm or less, and most preferably 40.8 mm or less. If the diameter of the spherical core is 34.8 mm or more, the cover is not excessively thick, and thus the impact durability is better. On the other hand, if the diameter of the spherical core is 42.2 mm or less, the cover is not excessively thin, and thus the cover functions better.

When the core has a diameter in the range from 34.8 mm to 42.2 mm, the compression deformation amount of the core (shrinking amount of the core along the compression direction) when applying a load from an initial load of 98 N to a final load of 1275 N to the core is preferably 2.0 mm or more, more preferably 2.3 mm or more, and even more preferably 2.5 mm or more, and is preferably 5.0 mm or less, more preferably 4.5 mm or less, and even more preferably 4.3 mm or less. If the compression deformation amount is 2.0 mm or more, the shot feeling is better, and if the compression deformation amount is 5.0 mm or less, the impact durability is better.

The spherical core can be produced by mixing, kneading and molding the rubber composition in a mold. The molding condition is not particularly limited, and the molding is generally carried out at a temperature in a range from 130° C. to 200° C. under a pressure of 2.9 MPa to 11.8 MPa for 10 minutes to 60 minutes.

The cover of the golf ball is formed from a cover composition containing a resin component. Examples of the resin component include an ionomer resin, a thermoplastic polyurethane elastomer having a trade name of “Elastollan (registered trademark)” available from BASF Japan Ltd., a thermoplastic polyamide elastomer having a trade name of “Pebax (registered trademark)” available from Arkema K. K., a thermoplastic polyester elastomer having a trade name of “Hytrel (registered trademark)” available from Du Pont-Toray Co., Ltd., and a thermoplastic styrene elastomer having a trade name of “Tefabloc” available from Mitsubishi Chemical Corporation.

In addition to the resin component described above, the cover composition may further contain a pigment component such as a white pigment (e.g. titanium oxide), a blue pigment and a red pigment, a weight adjusting agent such as zinc oxide, calcium carbonate and barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material or fluorescent brightener, as long as these additives don't impair the function of the cover.

Examples of the method for molding the cover of the golf ball include a method which comprises molding the cover composition into a hollow shell, covering the core with a plurality of the hollow shells and performing compression molding (preferably a method which comprises molding the cover composition into a hollow half shell, covering the core with two of the half shells and performing compression molding); and a method which comprises injection molding the cover composition directly onto the core.

The thickness of the cover is preferably 4.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less. If the thickness of the cover is 4.0 mm or less, the obtained golf ball has better shot feeling. The thickness of the cover is preferably 0.3 mm or more, more preferably 0.4 mm or more, and even more preferably 0.5 mm or more. If the thickness of the cover is 0.3 mm or more, the durability or wear resistance of the cover is better. In the case that the golf ball comprises a plurality of cover layers, the total thickness of a plurality of cover layers preferably falls within the above range.

Concave portions called “dimple” are usually formed on the surface of the cover when molding the cover. The total number of the dimples formed on the cover is preferably 200 or more and 500 or less. If the total number of the dimples is 200 or more, the dimple effect is greater, and if the total number of the dimples is 500 or less, the dimple effect is greater because the size of the respective dimples is large. The shape (shape in a plan view) of the formed dimples includes, for example, without limitation, a circle, a polygonal shape such as a roughly triangular shape, a roughly quadrangular shape, a roughly pentagonal shape, a roughly hexagonal shape, and other irregular shapes. The shape of dimples is employed solely or at least two of them may be used in combination.

The golf ball body having the cover formed thereon is ejected from the mold, and is preferably subjected to surface treatments such as deburring, cleaning and sandblast where necessary. In addition, if desired, a paint film or a mark may be formed. The thickness of the paint film is not particularly limited, and is preferably 5 μm or more, more preferably 7 μm or more, and is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less. If the thickness of the paint film is 5 μm or more, the paint film is hard to wear off due to the continued use of the golf ball, and if the thickness of the paint film is 50 μm or less, the dimple effect is not lowered and thus the flight performance of the golf ball is better.

The multi-piece golf ball preferably has a diameter in a range from 40 mm to 45 mm. In light of satisfying a regulation of US Golf Association (USGA), the diameter is particularly preferably 42.67 mm or more. In light of prevention of air resistance, the diameter is more preferably 44 mm or less, particularly preferably 42.80 mm or less. In addition, the golf ball preferably has a mass of 40 g or more and 50 g or less. In light of obtaining greater inertia, the mass is preferably 44 g or more, particularly preferably 45.00 g or more. In light of satisfying a regulation of USGA, the mass is particularly preferably 45.93 g or less.

Examples of the golf ball according to the present disclosure will be explained with reference to FIGS. 1 and 2. FIG. 1 is a partially cutaway cross-sectional view showing a one-piece golf ball according to one embodiment of the present disclosure. FIG. 2 is a partially cutaway cross-sectional view showing a multi-piece golf ball according to one embodiment of the present disclosure.

The golf ball 1 of FIG. 1 is a one-piece golf ball composed of a golf ball body 2 and a paint film 3 covering the golf ball body 2. A plurality of dimples 21 are formed on the surface of the golf ball body 2. Other portions than the dimples 21 on the surface of the golf ball 1 are lands 22. The golf ball 1 has the paint film 3 formed on an outer side of the golf ball body 2.

The golf ball 1 of FIG. 2 has a golf ball body composed of a spherical core 4 and a cover 5 covering the spherical core 4. A plurality of dimples 51 are formed on the surface of the cover 5. Other portions than the dimples 51 on the surface of the golf ball 1 are lands 52. The golf ball 1 has a paint film 3 formed on an outer side of the cover 5.

EXAMPLES

Next, the present disclosure will be described in detail by way of examples. However, the present disclosure is not limited to the examples described below. Various changes and modifications without departing from the spirit of the present disclosure are included in the scope of the present disclosure.

[Evaluation Method]

(1) Compression Deformation Amount (Mm)

The deformation amount of the golf ball along the compression direction (the shrinking amount of the golf ball along the compression direction), when applying a load from an initial load of 98 N to a final load of 1275 N to the golf ball, was measured.

The compression deformation amount was measured with a YAMADA type compression tester “SCH”. The golf ball was placed on a metal rigid plate of the tester. A metal cylinder slowly fell toward the golf ball. The golf ball sandwiched between the bottom of the cylinder and the rigid plate deformed. The travelling distance of the cylinder was measured when applying a load from an initial load of 98 N to a final load of 1275 N to the golf ball. The travelling speed of the cylinder before applying the initial load was 0.83 mm/s. The travelling speed of the cylinder when applying the load from the initial load to the final load was 1.67 mm/s.

(2) Coefficient of Resilience

A metal cylindrical object having a mass of 198.4 g was allowed to collide with each golf ball at a speed of 40 m/s, and the speeds of the cylindrical object and the golf ball before and after the collision were measured. The coefficient of resilience of each golf ball was calculated based on the respective speeds and mass of the golf ball and the cylindrical object. The measurement was conducted using twelve samples for each golf ball, and the average value thereof was adopted as the coefficient of resilience of the golf ball.

(3) Initial Velocity

A driver (“SRIXON ZX7”, Shaft hardness: S, Loft angle: 10.5°, available from Sumitomo Rubber Industries, Ltd.) was installed on a swing machine available from Golf Laboratories, Inc. The hitting point was set at the face center. The golf ball was hit at head speeds of 40 m/sec and 50 m/sec, and the ball velocity right after hitting the golf ball was measured. The measurement was conducted twelve times for each golf ball, and the average value thereof was adopted as the measurement value for that golf ball.

[Production of One-Piece Golf Ball]

According to the formulations shown in Table 1, the rubber compositions were kneaded with a kneading roll, and heat-pressed at a temperature of 170° C. for 20 minutes in upper and lower molds, each having a hemispherical cavity, to obtain golf ball bodies having a diameter of 40.86 mm.

	TABLE 1
	Golf ball No.
	1	2	3
Formulation	BR730	40	40	40
of	BR01	55.0	55.0	55.0
rubber	IR2200	5	5	5
composition	CV60 (NR)	—	—	—
(parts	Methacrylic acid	22	22	22
by	DCP	0.7	0.9	0.5
mass)	PCTP-Zn	—	—	—
	MBT	—	—	—
	Zinc oxide	23.4	23.4	23.4
	Calcium carbonate	2	2	2
	Titanium oxide	1	1	1
Evaluation	Compression deformation amount (mm)	2.7	2.5	3.1
	Coefficient of resilience (e40)	0.724	0.736	0.705
	Initial velocity (m/s)	v1 (40 m/s)	57.0	57.4	56.2
		v2 (50 m/s)	70.7	71.3	69.4
	Initial velocity difference (v2-v1)	13.8	13.9	13.2
	Golf ball No.
	4	5	6
Formulation	BR730	25.3	25.3	25.3
of	BR01	34.7	34.7	34.7
rubber	IR2200	—	—	—
composition	CV60 (NR)	40	40	40
(parts	Methacrylic acid	22	22	22
by	DCP	0.9	1.1	1.3
mass)	PCTP-Zn	—	—	—
	MBT	—	—	—
	Zinc oxide	23.4	23.4	23.4
	Calcium carbonate	2	2	2
	Titanium oxide	1	1	1
Evaluation	Compression deformation amount (mm)	3.6	3.1	2.8
	Coefficient of resilience (e40)	0.649	0.670	0.683
	Initial velocity (m/s)	v1 (40 m/s)	54.4	55.2	55.7
		v2 (50 m/s)	67.1	68.3	69.2
	Initial velocity difference (v2-v1)	12.8	13.1	13.4
	Golf ball No.
	7	8	9
Formulation	BR730	25.3	25.3	25.3
of	BR01	34.7	34.7	34.7
rubber	IR2200	—	—	—
composition	CV60 (NR)	40	40	40
(parts	Methacrylic acid	22	22	22
by	DCP	1.3	1.5	1.7
mass)	PCTP-Zn	0.5	0.5	0.5
	MBT	1	1	1
	Zinc oxide	23.4	23.4	23.4
	Calcium carbonate	2	2	2
	Titanium oxide	1	1	1
Evaluation	Compression deformation amount (mm)	3.5	3.3	3.1
	Coefficient of resilience (e40)	0.697	0.705	0.710
	Initial velocity (m/s)	v1 (40 m/s)	56.0	56.3	56.5
		v2 (50 m/s)	69.2	69.8	70.1
	Initial velocity difference (v2-v1)	13.2	13.5	13.6

• • BR730: “BR730” (high-cis polybutadiene rubber (amount of cis-1,4 bond=96 mass %, amount of 1,2-vinyl bond=1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.)=55, molecular weight distribution (Mw/Mn)=3)) available from JSR Corporation
  • BR01: “BR01” (high-cis butadiene rubber (amount of cis-1,4 bond=95 mass %, Moony viscosity (ML₁₊₄ (100° C.)=45) available from JSR Corporation
  • IR2200: isoprene rubber (Moony viscosity (ML₁₊₄ (100° C.)=82) available from Zeon Corporation
  • CV60 (NR): natural rubber (Moony viscosity (ML₁₊₄ (100° C.)=60)
  • Methacrylic acid: available from Mitsubishi Chemical Corporation
  • DCP: “Percumyl (registered trademark) D” (dicumyl peroxide, amount of active oxygen: 5.92 mass %) available from NOF Corporation
  • PCTP-Zn: zinc pentachlorothiophenol (containing 28 mass % to 32 mass % of zinc stearate) available from FUJIFILM Wako Chemicals Corporation
  • MBT: “Nocceler M-P” (2-mercaptobenzothiazole) available from Ouchi Shinko Chemical Industrial Co., Ltd.
  • Zinc oxide: “Ginrei R” (zinc oxide) available from Toho Zinc Co., Ltd.
  • Calcium carbonate: “Whiton BF-300” available from Shiraishi Calcium Kaisha Ltd.
  • Titanium oxide: “CR-60” available from Ishihara Sangyo Kaisha, Ltd.

Evaluation results of the obtained golf balls are shown in Table 1. In addition, the relationship between the compression deformation amount and the coefficient of resilience is shown in FIG. 3, and the relationship between the compression deformation amount and the initial velocity difference is shown in FIG. 4.

In general, a golf ball having a smaller compression deformation amount has a greater coefficient of resilience, and a golf ball having a greater compression deformation amount has better shot feeling. In other words, a golf ball having a great compression deformation amount and a great coefficient of resilience has excellent shot feeling and resilience. As shown in Table 1, Golf balls No. 1 to 3, Golf balls No. 4 to 6, and Golf balls No. 7 to 9 differ from each other only in the amount of (c) the crosslinking initiator. In these golf balls, the golf ball having a smaller compression deformation amount has a greater coefficient of resilience, as shown in FIG. 3. Further, for example, if the results where the compression deformation amount is 3.1 mm are compared, Golf balls No. 7 to 9 formed of the cured product of the rubber composition in which (a) the base rubber contains the natural rubber and (b) the co-crosslinking agent contains methacrylic acid have a greatest coefficient of resilience. In other words, the golf ball according to the present disclosure has improved shot feeling while maintaining the coefficient of resilience.

Further, as shown in FIG. 4, for example, if the results where the compression deformation amount is 3.1 mm are compared, Golf balls No. 7 to 9 formed of the cured product of the rubber composition in which (a) the base rubber contains the natural rubber and (b) the co-crosslinking agent contains methacrylic acid have a greatest initial velocity difference. In other words, the golf ball according to the present disclosure has good shot feeling and a great change in the initial velocity associated with a change in the swing speed. Thus, if the golf ball according to the present disclosure is used, the golfer can intuitively feel the improvement in the swing speed by the change in the flight distance or initial velocity of the golf ball which has good shot feeling.

The preferable embodiment (1) of the present disclosure is a golf ball comprising a constituent member, wherein at least a part of the constituent member is formed of a cured product of a rubber composition comprising (a) a base rubber containing a natural rubber, (b) a co-crosslinking agent containing methacrylic acid and/or a metal salt thereof and (c) a crosslinking initiator.

The preferable embodiment (2) of the present disclosure is the golf ball according to the preferable embodiment (1), wherein the rubber composition further contains (d) an organic sulfur compound.

The preferable embodiment (3) of the present disclosure is the golf ball according to the preferable embodiment (2), wherein (d) the organic sulfur compound contains (d1) a compound of thiophenols and/or a metal salt thereof and (d2) a compound of thiazoles and/or a metal salt thereof.

The preferable embodiment (4) of the present disclosure is the golf ball according to the preferable embodiment (2) or (3), wherein the rubber composition contains (d) the organic sulfur compound in an amount ranging from 0.01 part by mass to 20 parts by mass with respect to 100 parts by mass of (a) the base rubber.

The preferable embodiment (5) of the present disclosure is the golf ball according to any one of the preferable embodiments (1) to (4), wherein the golf ball is a one-piece golf ball, and a golf ball body thereof is formed of the cured product of the rubber composition.

This application is based on Japanese Patent application No. 2023-022445 filed on Feb. 16, 2023, the content of which is hereby incorporated by reference.

Claims

1. A golf ball comprising a constituent member, wherein at least a part of the constituent member is formed of a cured product of a rubber composition comprising

(a) a base rubber containing a natural rubber,

(b) a co-crosslinking agent containing methacrylic acid and/or a metal salt thereof, and

(c) a crosslinking initiator.

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

3. The golf ball according to claim 2, wherein the rubber composition contains (d) the organic sulfur compound in an amount ranging from 0.01 part by mass to 20 parts by mass with respect to 100 parts by mass of (a) the base rubber.

4. The golf ball according to claim 3, wherein (d) the organic sulfur compound contains (d1) a compound of thiophenols and/or a metal salt thereof and (d2) a compound of thiazoles and/or a metal salt thereof.

5. The golf ball according to claim 4, wherein a mass ratio (d1/d2) of (d1) the compound of thiophenols and/or the metal salt thereof to (d2) the compound of thiazoles and/or the metal salt thereof ranges from 0.1 to 10.0.

6. The golf ball according to claim 1, wherein the natural rubber has a Mooney viscosity (ML1+4 (100° C.)) ranging from 30 to 80.

7. The golf ball according to claim 1, wherein (a) the base rubber further contains a synthetic rubber, and an amount of the natural rubber ranges from 10 mass % to 80 mass % in 100 mass % of (a) the base rubber.

8. The golf ball according to claim 7, wherein the synthetic rubber is a high-cis polybutadiene rubber.

9. The golf ball according to claim 1, wherein (b) the co-crosslinking agent consists of methacrylic acid and/or the metal salt thereof.

10. The golf ball according to claim 1, wherein the golf ball is a one-piece golf ball, and a golf ball body thereof is formed of the cured product of the rubber composition.

11. The golf ball according to claim 10, wherein the one-piece golf ball has a diameter ranging from 40 mm to 45 mm and a compression deformation amount ranging from 2.0 mm to 6.0 mm when applying a load from an initial load of 98 N to a final load of 1275 N to the one-piece golf ball.

12. A one-piece golf ball having a golf ball body,

wherein the golf ball body is formed from a rubber composition comprising

(a) a base rubber containing a natural rubber,

(b) a co-crosslinking agent containing methacrylic acid and/or a metal salt thereof,

(c) a crosslinking initiator, and

(d) an organic sulfur compound.

13. The one-piece golf ball according to claim 12, wherein the rubber composition contains (d) the organic sulfur compound in an amount ranging from 0.01 part by mass to 20 parts by mass with respect to 100 parts by mass of (a) the base rubber.

14. The one-piece golf ball according to claim 13, wherein (d) the organic sulfur compound contains (d1) a compound of thiophenols and/or a metal salt thereof and (d2) a compound of thiazoles and/or a metal salt thereof.

15. The one-piece golf ball according to claim 14, wherein a mass ratio (d1/d2) of (d1) the compound of thiophenols and/or the metal salt thereof to (d2) the compound of thiazoles and/or the metal salt thereof ranges from 0.1 to 10.0.

16. The one-piece golf ball according to claim 15, wherein the natural rubber has a Mooney viscosity (ML1+4 (100° C.)) ranging from 30 to 80.

17. The golf ball according to claim 16, wherein (a) the base rubber further contains a synthetic rubber, and an amount of the natural rubber ranges from 10 mass % to 80 mass % in 100 mass % of (a) the base rubber.