golf balls

Abstract

To provide a golf ball having excellent durability and flight distance and also having a good shot feeling for an average golfer who hits a golf ball at a slow head speed.SOLUTION: The present disclosure provides a golf ball comprising a spherical core and at least two cover layers covering the spherical core. A difference between a core surface crosslinking density and a core center crosslinking density is more than 1.0×102 mol / m3 and less than 9.0×102 mol / m3. A hardness difference between a core surface hardness Cs (Shore C hardness) and a core center hardness Co (Shore C hardness) is 13.0 or more and 30.0 or less. A compression deformation amount of the core when applying a load from an initial load of 98 N to a final load of 1275 N to the core is 3.8 mm or more. The at least two cover layers include a first cover layer and a second cover layer positioned closer to the core than the first cover layer, wherein an average hardness Dave=(Ti×Hi+To×Ho) / (Ti+To) of the first cover layer and the second cover layer is 55 or more, where To (mm) is a thickness of the first cover layer, Ho (Shore D) is a slab hardness of the first cover layer, Ti (mm) is a thickness of the second cover layer, and Hi (Shore D) is a slab hardness of the second cover layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to golf balls, and more particularly to improvements in golf ball cores and covers. [Background technology]

[0002] Golf is a sport that can be played by golfers of all ages, regardless of gender. Beginners, women, and seniors tend to have slow head speeds and use relatively soft golf balls. This is because a slow head speed makes it difficult to crush a hard golf ball when hitting it, resulting in a poor feel. Furthermore, a golfer with a slow head speed is unable to crush a hard golf ball when hitting it, resulting in an increased amount of spin and a reduced distance.

[0003] A golf ball generally comprises a spherical core and a cover that encases the spherical core. A rubber composition containing a base rubber, a co-crosslinking agent, and a crosslinking initiator is widely used as the material for forming the golf ball core because of its good resilience.

[0004] For example, Patent Document 1 discloses a golf ball having a core and at least one cover layer, in which the cross-sectional hardness of the core is expressed as follows: R (mm) is the radius of the core, A is the JIS-C hardness at the center of the core, B is the JIS-C hardness at a position R / 3 mm away from the center of the core, C is the JIS-C hardness at a position R / 1.8 mm away from the center of the core, D is the JIS-C hardness at a position R / 1.3 mm away from the center of the core, and E is the JIS-C hardness of the core surface. (1) DC ≥ 7 (2)CB≦7 (3)(DC)-(CB)≧7 (4) EA≧16 A golf ball characterized by satisfying the following is disclosed.

[0005] Patent Document 2 also describes a golf ball having a core and a cover of one or more layers, in which the core comprises the following components (A) to (C): (A) Base rubber (B) Organic peroxide (C) Water and / or a metal monocarboxylate The rubber composition is formed by hot molding the rubber composition containing the following: The difference in crosslink density between the core surface and the core center, P (mol / m 3 ) and the deflection E (mm) of the core from the initial load of 98N (10kgf) to the final load of 1275N (130kgf) (P × E) is 28 × 10 2 mol / m 3 The present invention discloses a golf ball having a thickness of 1 / 2 mm or more.

[0006] Patent Document 3 discloses a one-piece golf ball comprising a ball body made of a white rubber composition containing a base rubber, an α,β-unsaturated carboxylic acid and / or a metal salt thereof, and an organic peroxide, and a clear coat applied to the surface of the ball body, wherein the white rubber composition is a rubber composition containing 0.1 to 5.0 parts by mass of an antioxidant and 0.05 to 3.0 parts by mass of a light stabilizer per 100 parts by mass of the base rubber, and the clear coat contains 0.05 to 5.0 parts by mass of an ultraviolet absorber per 100 parts by mass of the resin component.

[0007] Patent Document 4 describes a polymer containing 60% by weight or more of cis-1,4-bonds and having a Mooney viscosity (ML 1+4 The present invention discloses a golf ball characterized by comprising, as a constituent element, a heat-molded product of a rubber composition containing polybutadiene having a modulus of elasticity (at 100°C) of 40 or more, an unsaturated carboxylic acid and / or a metal salt thereof, an organic peroxide, and a monophenol-based antioxidant.

[0008] Patent Document 5 discloses a solid golf ball having at least a portion of an elastic portion formed from a rubber composition containing a base rubber, a metal salt of an α,β-ethylenically unsaturated carboxylic acid, an ethylenically unsaturated carboxylic acid ester having a hindered phenol group, and a peroxide. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-077405 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-47502 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-149504 [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-121815 [Patent Document 5] Japanese Patent Application Publication No. 63-212377 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present disclosure is to provide a golf ball that is excellent in durability and distance and that provides a good shot feel for an average golfer with a slow head speed. [Means for solving the problem]

[0011] The golf ball of the present disclosure is a golf ball having a spherical core and at least two layers of a cover enclosing the spherical core, wherein the spherical core is formed from a core 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, and (c) a crosslinking initiator, and wherein the difference between the surface crosslink density of the spherical core and the center crosslink density of the spherical core (core surface crosslink density - core center crosslink density) is 1.0 × 10 2 mol / m 3 Super, 9.0×10 2 mol / m3 the difference in hardness (Cs-Co) between the surface hardness Cs (Shore C hardness) and the center hardness Co (Shore C hardness) of the spherical core is 13.0 or greater and 30.0 or less; the amount of compressive deformation when an initial load of 98 N is applied to the spherical core and a final load of 1275 N is applied to the spherical core is 3.8 mm or greater; and the average hardness Dave of the first cover and the second cover, which are at least two-layered, is 55 or greater, where To (mm) is the thickness of the first cover, Ho (Shore D) is the slab hardness of the first cover, Ti (mm) is the thickness of the second cover, and Hi (Shore D) is the slab hardness of the second cover.

[0012] The golf ball of the present disclosure, having the above-described configuration, provides excellent durability, distance, and a good shot feel for average golfers with slow head speeds. [Effects of the Invention]

[0013] According to the present disclosure, a golf ball that is excellent in durability and distance and provides a good shot feel can be obtained for average golfers with slow head speeds. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a partially cutaway cross-sectional view showing a golf ball according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0015] The golf ball of the present disclosure is a golf ball having a spherical core and at least two layers of a cover enclosing the spherical core, wherein the spherical core is formed from a core 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, and (c) a crosslinking initiator, and wherein the difference between the surface crosslink density of the spherical core and the center crosslink density of the spherical core (core surface crosslink density - core center crosslink density) is 1.0 × 10 2 mol / m 3 Super, 9.0×10 2 mol / m 3 the difference in hardness (Cs-Co) between the surface hardness Cs (Shore C hardness) and the center hardness Co (Shore C hardness) of the spherical core is 13.0 or greater and 30.0 or less; the amount of compressive deformation when an initial load of 98 N is applied to the spherical core and a final load of 1275 N is applied to the spherical core is 3.8 mm or greater; and the average hardness Dave of the first cover and the second cover, which are at least two-layered, is 55 or greater, where To (mm) is the thickness of the first cover, Ho (Shore D) is the slab hardness of the first cover, Ti (mm) is the thickness of the second cover, and Hi (Shore D) is the slab hardness of the second cover.

[0016] The spherical core of the golf ball of the present disclosure is formed from a core 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, and (c) a crosslinking initiator. First, the materials used in the spherical core of the golf ball of the present disclosure will be described.

[0017] [(a) Base rubber] The base rubber (a) can be natural rubber and / or synthetic rubber, such as polybutadiene rubber, natural rubber, polyisoprene rubber, styrene polybutadiene rubber, or ethylene-propylene-diene rubber (EPDM). These can be used alone or in combination of two or more. Among these, high-cis polybutadiene, which has cis-1,4 bonds in an amount of 40% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, is particularly suitable, as it provides excellent resilience.

[0018] In order to obtain a core with higher resilience, the content of high-cis polybutadiene in the base rubber is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. (a) It is also preferable that the base rubber consists solely of high-cis polybutadiene.

[0019] The high-cis polybutadiene preferably has a 1,2-vinyl bond content of 2.0% by mass or less, more preferably 1.7% by mass or less, and even more preferably 1.5% by mass or less. If the 1,2-vinyl bond content is too high, the resilience may decrease.

[0020] The high-cis polybutadiene is preferably synthesized using a rare earth element catalyst, and in particular, the use of a neodymium-based catalyst using a neodymium compound, which is a lanthanum series rare earth element compound, is preferred because it can produce polybutadiene rubber having a high content of 1,4-cis bonds and a low content of 1,2-vinyl bonds with excellent polymerization activity.

[0021] The high-cis polybutadiene has a Mooney viscosity (ML 1+4 (100°C)) is preferably 30 or more, more preferably 32 or more, even more preferably 35 or more, and is preferably 140 or less, more preferably 120 or less, even more preferably 100 or less, and most preferably 55 or less. 1+4(100°C)) is a value measured in accordance with JIS K6300 using an L rotor, with a preheating time of 1 minute, rotor rotation time of 4 minutes, and at 100°C.

[0022] The high-cis polybutadiene preferably has a molecular weight distribution Mw / Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight) of 2.0 or more, more preferably 2.2 or more, even more preferably 2.4 or more, and most preferably 2.6 or more, and preferably 6.0 or less, more preferably 5.0 or less, even more preferably 4.0 or less, and most preferably 3.0 or less. If the molecular weight distribution (Mw / Mn) of the high-cis polybutadiene is too small, workability may decrease, while if it is too large, resilience may decrease. The molecular weight distribution was measured by gel permeation chromatography (manufactured by Tosoh Corporation, "HLC-8120GPC") using a differential refractometer as a detector, a column: GMHHXL (manufactured by Tosoh Corporation), a column temperature: 40°C, and a mobile phase: tetrahydrofuran, and calculated as a value converted into a standard polystyrene.

[0023] [(b) Co-crosslinking agent] The (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or its metal salt used in the rubber composition is blended into the rubber composition as a co-crosslinking agent, and has the effect of crosslinking rubber molecules by graft polymerization with the base rubber molecular chains.

[0024] Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid.

[0025] Examples of metals constituting the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include monovalent metal ions such as sodium, potassium, and lithium; divalent metal ions such as magnesium, calcium, zinc, barium, and cadmium; trivalent metal ions such as aluminum; and other ions such as tin and zirconium. The metal components can be used alone or in combination of two or more. Among these, divalent metals such as magnesium, calcium, zinc, barium, and cadmium are preferred as the metal component. This is because the use of a divalent metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms facilitates the formation of metal crosslinks between rubber molecules. Zinc acrylate is particularly preferred as the divalent metal salt, as it enhances the resilience of the resulting golf ball. The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or its metal salt may be used alone or in combination of two or more.

[0026] The amount of the (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or its metal salt is preferably at least 20 parts by weight, more preferably at least 25 parts by weight, even more preferably at least 30 parts by weight, and preferably at most 55 parts by weight, more preferably at most 50 parts by weight, and even more preferably at most 45 parts by weight, per 100 parts by weight of the (a) base rubber. If the amount of the (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or its metal salt is less than 20 parts by weight, the amount of the (c) cross-linking initiator (described below) must be increased to achieve an appropriate hardness in the cured product (e.g., core) formed from the rubber composition, which tends to result in a decrease in the resilience of the resulting golf ball. On the other hand, if the amount of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or its metal salt exceeds 55 parts by weight, the cured product (e.g., core) formed from the rubber composition may become too hard, potentially resulting in a decrease in the feel at impact of the resulting golf ball.

[0027] [(c) Crosslinking initiator] The (c) crosslinking initiator used in the rubber composition is compounded to crosslink the (a) base rubber component. An organic peroxide is suitable as the (c) crosslinking initiator. Specific examples of the organic peroxide include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. These organic peroxides may be used alone or in combination of two or more. Among these, dicumyl peroxide is preferably used.

[0028] When the rubber composition contains at least one of the following (d) components, the content of the (c) cross-linking initiator is preferably at least 0.2 parts by weight, more preferably at least 0.5 parts by weight, even more preferably at least 0.7 parts by weight, and preferably at most 5.0 parts by weight, more preferably at most 2.5 parts by weight, and even more preferably at most 2.0 parts by weight, per 100 parts by weight of the (a) base rubber. If the content of the cross-linking initiator is less than 0.2 parts by weight, the cured product (e.g., core) formed from the rubber composition will tend to be too soft, resulting in reduced resilience of the resulting golf ball. If the content of the cross-linking initiator is more than 5.0 parts by weight, the amount of the (b) co-cross-linking agent must be reduced to achieve an appropriate hardness in the cured product (e.g., core) formed from the rubber composition, which could result in insufficient resilience or poor durability of the resulting golf ball.

[0029] When the rubber composition does not contain the following component (d), it is also preferable to set the content of the (c) cross-linking initiator to 0.1 parts by mass or less per 100 parts by mass of the base rubber. By setting the content of the (c) cross-linking initiator to 0.1 parts by mass or less per 100 parts by mass of the base rubber, the cross-linking density relative to the amount of co-cross-linking agent added decreases, the breaking strain increases, and durability improves.

[0030] The core rubber composition preferably contains at least one additive (d) selected from the group consisting of (d1) hindered phenol compounds and (d2) hindered amine compounds. By including the additive (d) in the core rubber composition, the core hardness difference can be maintained at a certain level or more while reducing the crosslink density difference in the core.

[0031] [(d1) Hindered phenolic compounds] A hindered phenol compound is a compound having a hydroxyphenyl structure in which a hydroxy group is sterically protected by a bulky functional group. The bulky functional group is preferably located adjacent to the hydroxy group. Examples of the bulky functional group include a t-butyl group and a long-chain alkyl group in which some of the carbon atoms may be substituted with sulfur. As the hindered phenol compound, a compound having a tert-butylhydroxyphenyl structure having at least one tert-butyl group is preferred, and a compound having a di-tert-butylhydroxyphenyl structure having two tert-butyl groups is more preferred.

[0032] Examples of the compound having a tert-butylhydroxyphenyl structure having at least one tert-butyl group include compounds having a structure such as 3-tert-butyl-4-hydroxyphenyl or 3,5-di-tert-butyl-4-hydroxyphenyl, etc. Among these, compounds having a 3,5-di-tert-butyl-4-hydroxyphenyl structure are preferred.

[0033] Specific examples of the hindered phenol compounds include compounds having one hydroxyphenyl structure, such as dibutylhydroxytoluene (BHT), 4,6-bis(octylthiomethyl)-o-cresol, 4,6-bis[(dodecylthio)methyl]-o-cresol, 2,4-dimethyl-6-(1-methylpentadecyl)phenol (e.g., Irganox 1141 manufactured by BASF Japan Ltd.), and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate (e.g., Adekastab AO-50 manufactured by Adeka Corporation).

[0034] Other specific examples of the hindered phenol compound include 2,2'-methylenebis(4-ethyl-6-tert-butylphenol) (e.g., Yoshinox 425 manufactured by Mitsubishi Chemical Corporation), 2,2'-methylenebis(4-methyl-6-tert-butylphenol) (e.g., Sandant 2246 manufactured by Sanshin Chemical Industry Co., Ltd.), 4,4'-butylidenebis(3-methyl-6-tert-butylphenol) (e.g., Yoshinox BB manufactured by Mitsubishi Chemical Corporation), 4,4'-thiobis(3-methyl-6-tert-butylphenol) (e.g., Knocklac 300 manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), 4,4 Examples of compounds having two hydroxyphenyl structures include 2,6-methylenebis(2,6-di-tert-butylphenol), 2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propan-2-yl}sulfanyl)phenol (probucol), and 3,9-bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (e.g., Adeka Stab AO-80 manufactured by Adeka Corporation).

[0035] Other specific examples of the hindered phenol compound include compounds having three hydroxyphenyl structures, such as 1,3,5-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3-5-triazine-2,4,6(1H,3H,5H-)-trione (e.g., Adeka STAB AO-20 manufactured by Adeka Corporation) and 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., Adeka STAB AO330 manufactured by Adeka Corporation).

[0036] Another specific example of the hindered phenol compound is a compound having four hydroxyphenyl structures, such as pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (e.g., Adeka Stab AO-60 manufactured by Adeka Corporation).

[0037] The hindered phenol compounds may be used alone or in combination of two or more.

[0038] The hindered phenol compound is preferably at least one compound selected from the group consisting of dibutylhydroxytoluene, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 4,4'-methylenebis(2,6-di-tert-butylphenol), 2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propan-2-yl}sulfanyl)phenol, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0039] (d2) As the hindered amine compound, a compound having a 2,2,6,6-tetramethyl-4-piperidyl group, such as that represented by the following chemical formula (1), is preferred. [ka] In formula (1), R 11 is a hydrogen atom, a hydroxy group, an alkyl group having 1 to 30 carbon atoms, a hydroxyalkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a hydroxyalkoxy group having 1 to 30 carbon atoms, or an oxy radical.

[0040] The hindered amine compound represented by chemical formula (1) includes hindered amine compounds represented by chemical formula (2) or chemical formula (3).

[0041] The hindered amine compounds represented by the following chemical formula (2) are so-called N-alkyl hindered amine compounds and NH-type hindered amine compounds. [ka] In formula (2), R 12 is a hydrogen atom, a hydroxy group, an alkyl group having 1 to 30 carbon atoms, a hydroxyalkyl group having 1 to 30 carbon atoms, or an oxy radical.

[0042] The hindered amine compound represented by the following chemical formula (3) is what is called an N-alkoxy hindered amine compound. [ka] In chemical formula (3), R 12 is an alkyl group having 1 to 30 carbon atoms or a hydroxyalkyl group having 1 to 30 carbon atoms.

[0043] Specific examples of the (d2) hindered amine compound include compounds represented by chemical formulas (4) and (5). [ka] In chemical formula (4), R 14 , R 15are each independently a hydrogen atom, a hydroxy group, an alkyl group having 1 to 30 carbon atoms, a hydroxyalkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a hydroxyalkoxy group having 1 to 30 carbon atoms, or an oxy radical. 13 is an alkylene group having 1 to 20 carbon atoms.

[0044] [ka] In chemical formula (5), R 16 R is a hydrogen atom, a hydroxy group, an alkyl group having 1 to 30 carbon atoms, a hydroxyalkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a hydroxyalkoxy group having 1 to 30 carbon atoms, or an oxy radical. 17 is an alkyl group having 1 to 30 carbon atoms or an alkenyl group having 2 to 30 carbon atoms.

[0045] [ka] In chemical formula (6), R 18 , R 19 are each independently a hydrogen atom, a hydroxy group, an alkyl group having 1 to 30 carbon atoms, a hydroxyalkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a hydroxyalkoxy group having 1 to 30 carbon atoms, or an oxy radical.

[0046] (d2) Specific examples of the hindered amine compound include ADK STAB LA-52 (tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate), ADK STAB LA-57 (tetrakis(2,2,6,6-tetramethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate), ADK STAB LA-63P, ADK STAB LA-68, ADK STAB LA-72 (bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate), ADK STAB LA-77Y (bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate), and ADK STAB LA-81 (bis(1-undecanoxy-2,2,6,6-tetramethylpiperidin-4-yl)carbonate), all manufactured by Adeka Corporation.

[0047] (d2) Specific examples of the hindered amine compound include the following, which are commercially available from BASF Japan Ltd. 1) Chimassorb 2020FDL 1,6-Hexanediamine, N,N'―bis(2,2,6,6-tetramethyl-4-piperidinyl)-polymer with 2,4,6-trichloro-1,3,5-triazine, reaction products with N-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine 2) Chimassorb 944FDL Poly[[6-[(1,1,3,3-tetramethylbutyl)amino]-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-1,6-hexanediyl[(2,2,6,6-tetramethyl-4-piperidinyl)imino]]) 3) Chinuvin 622SF Butanedioic acid, dimethylester, polymer with 4-hydroxy-2,2,6,6- tetramethyl-1-piperidine ethanol), 4) Tinuvin PA144 Bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-2-butyl-2-(4-hydroxy-3,5-di-tert.-butylbenzyl)propanedioate

[0048] The hindered amine compounds (d2) may be used alone or in combination of two or more. Also, the hindered phenol compound (d1) and the hindered amine compound (d2) may be used in combination.

[0049] When the core rubber composition contains a (d1) hindered phenol compound and / or a (d2) hindered amine compound as the (d) additive, the amount of the (d1) hindered phenol compound and / or the (d2) hindered amine compound is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, even more preferably 1.5 parts by mass or more, and preferably 15 parts by mass or less, more preferably 13 parts by mass or less, and even more preferably 11 parts by mass or less, per 100 parts by mass of the (a) base rubber. This is because, when the amount of the (d1) hindered phenol compound and / or the (d2) hindered amine compound is within the above range, the crosslink density is low relative to the amount of co-crosslinking agent added, the breaking strain is increased, and durability is improved.

[0050] [(e) Organic sulfur compounds] The core rubber composition preferably further contains (e) an organic sulfur compound, which improves the resilience of the resulting core.

[0051] The (e) organic sulfur compound is preferably at least one compound selected from the group consisting of thiols (thiophenols, thionaphthols), polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, and thiazoles.

[0052] Examples of thiols include thiophenols and thionaphthols. Examples of the thiophenols include thiophenol; fluoro-substituted thiophenols such as 4-fluorothiophenol, 2,4-difluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, and pentafluorothiophenol; and chloro-substituted thiophenols 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 bromo groups such as 4-bromothiophenol, 2,4-dibromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenols substituted with iodo groups such as 4-iodothiophenol, 2,4-diiodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; or metal salts thereof. Zinc salts are preferred as the metal salt.

[0053] Examples of the thionaphthols (naphthalene thiols) 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 metal salts thereof, with 2-thionaphthol and 1-thionaphthol being preferred. The metal salt is preferably a divalent metal salt, more preferably a zinc salt. Specific examples of the metal salt include the zinc salt of 1-thionaphthol and the zinc salt of 2-thionaphthol.

[0054] Polysulfides are organic sulfur compounds having polysulfide bonds, such as disulfides, trisulfides, and tetrasulfides. Diphenyl polysulfides are preferred as the polysulfides.

[0055] Diphenyl polysulfides include, in addition to diphenyl disulfide, 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-chloro ... 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 phenyl) 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, bis(pentaiodophenyl) disulfide diphenyl disulfides substituted with a halogen 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; and diphenyl disulfides substituted with an alkyl group, such as bis(4-methylphenyl) disulfide, bis(2,4,5-tri-t-butylphenyl) disulfide, and bis(penta-t-butylphenyl) disulfide.

[0056] Examples of 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 thiocarboxylic acids include naphthalene thiocarboxylic acid. Examples of dithiocarboxylic acids include naphthalene dithiocarboxylic acid. Examples of sulfenamides include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and Nt-butyl-2-benzothiazole sulfenamide.

[0057] The (e) organic sulfur compound is preferably a thiophenol and / or a metal salt thereof, a thionaphthol and / or a metal salt thereof, a diphenyl disulfide, or a thiuram disulfide, more preferably 2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyl disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, or bis(pentabromophenyl)disulfide.

[0058] The (e) organic sulfur compounds can be used alone or in combination of two or more.

[0059] The content of the (e) organic sulfur compound is preferably at least 0.05 parts by weight, more preferably at least 0.1 parts by weight, and even more preferably at least 0.2 parts by weight, per 100 parts by weight of the (a) base rubber, and is preferably at most 5.0 parts by weight, more preferably at most 3.0 parts by weight, and even more preferably at most 2.0 parts by weight. If the content of the (e) organic sulfur compound is less than 0.05 parts by weight, the effect of adding the (e) organic sulfur compound may not be obtained, and the resilience of the golf ball may not be improved. On the other hand, if the content of the (e) organic sulfur compound exceeds 5.0 parts by weight, the amount of compression deformation of the resulting golf ball may be large, which may reduce resilience.

[0060] [(f) Metal compounds] The core rubber composition preferably further contains (f) a metal compound, which can be used, for example, as a weight adjuster for the rubber composition or as a neutralizer for an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

[0061] Examples of the (f) metal compound include metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. Divalent metal compounds are preferred as the (g) metal compound, and zinc compounds are more preferred. This is because divalent metal compounds react with α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms to form metal bridges. Furthermore, the use of zinc compounds allows for the production of golf balls with high resilience.

[0062] The (f) metal compound may be used alone or in combination of two or more thereof. The content of the (f) metal compound may be appropriately adjusted depending on the desired degree of neutralization of the (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

[0063] The core rubber composition may contain additives such as pigments, fillers for adjusting the weight, antioxidants, peptizers, and softeners, as required.

[0064] The filler used in the core rubber composition is primarily blended as a weight adjuster to adjust the weight of the final golf ball product, and may be blended as needed. Examples of such fillers include inorganic fillers such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder. Zinc oxide is particularly preferred as the filler. Zinc oxide is thought to function as a vulcanization aid, increasing the overall hardness of the core. The content of the filler is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, and preferably 30 parts by weight or less, more preferably 25 parts by weight or less, and even more preferably 20 parts by weight or less, per 100 parts by weight of the (a) base rubber. If the filler content is less than 0.5 parts by weight, weight adjustment becomes difficult, while if it exceeds 30 parts by weight, the weight fraction of the rubber component becomes small, tending to reduce resilience.

[0065] The content of the peptizing agent is preferably 0.1 part by mass or more and 5 parts by mass or less per 100 parts by mass of the (a) base rubber.

[0066] The golf ball of the present disclosure is a golf ball having a spherical core and a cover with at least two layers that encases the spherical core, and the spherical core is formed from the core rubber composition.

[0067] The spherical core of the golf ball of the present disclosure has a difference between the surface crosslink density of the spherical core and the center crosslink density of the spherical core (core surface crosslink density - core center crosslink density) of 1.0 × 10 2 mol / m 3 Super, 9.0×10 2 mol / m 3 The spherical core has a surface hardness Cs (Shore C hardness) and a center hardness Co (Shore C hardness) of 13.0 or greater and 30.0 or less in hardness difference (Cs-Co) between the surface hardness Cs and the center hardness Co (Shore C hardness) of the spherical core.

[0068] The golf ball of the present disclosure is characterized by a core having a core surface with a hardness difference of at least a certain level and a core center with a crosslink density difference of at most a certain level. Golf balls having such cores have excellent durability and a good shot feel.

[0069] The core of the golf ball of the present disclosure has a difference between the surface crosslink density of the spherical core and the center crosslink density of the spherical core (core surface crosslink density - core center crosslink density) of 1.0 × 10 2 mol / m 3 More than 1.5 x 10 is preferred 2 mol / m 3 More preferably, 2.0 x 10 2 mol / m 3 More preferably, 9.0 x 10 2 mol / m 3 Preferably, it is less than 8.9 x 10 2 mol / m 3 Preferably, it is 8.8 x 10 or less. 2 mol / m 3 It is more preferable that the difference between the surface crosslink density of the spherical core and the center crosslink density of the spherical core (core surface crosslink density - core center crosslink density) is within the above range, because a good feel can be achieved while maintaining durability.

[0070] The surface crosslinking density of the spherical core is 9.0×10 2 mol / m 3 It is preferable that the value is 9.5×10 or more. 2 mol / m 3 More preferably, it is 10.0×10 2 mol / m 3 More preferably, it is 22.0×10 or more. 2 mol / m 3 It is preferable that the value is 21.0×10 or less. 2 mol / m 3 More preferably, it is 20.0×10 2 mol / m 3It is more preferable that the surface crosslinking density of the spherical core is within the above range because the rubber composition does not become too hard and provides a good feel.

[0071] The central crosslink density of the spherical core is 4.0×10 2 mol / m 3 It is preferable that the value is equal to or greater than 4.5×10 2 mol / m 3 More preferably, it is 5.0×10 or more. 2 mol / m 3 More preferably, it is 13.0×10 or more. 2 mol / m 3 It is preferable that the value is less than 12.0 × 10 2 mol / m 3 More preferably, it is 11.0×10 2 mol / m 3 It is more preferable that the center crosslink density of the spherical core is within the above range because the rubber composition does not become too soft and the resilience can be maintained.

[0072] The difference in hardness (Cs-Co) between the surface hardness (Cs) and the center hardness (Co) of the spherical core is preferably 13.0 or more, more preferably 14.0 or more, and even more preferably 15.0 or more, and is preferably 30.0 or less, more preferably 28.0 or less, and even more preferably 26.0 or less, in Shore C hardness. Durability can be maintained (improved) if the difference in hardness (Cs-Co) between the surface hardness (Cs) and the center hardness (Co) of the core is 13.0 or more, in Shore C hardness. Furthermore, a golf ball with an improved feel on driver shots can be obtained if the difference in hardness (Cs-Co) between the surface hardness (Cs) and the center hardness (Co) of the core is 30.0 or less, in Shore C hardness.

[0073] The surface hardness (Cs) of the spherical core is preferably 60.0 or more, more preferably 65.0 or more, and even more preferably 70.0 or more, and is preferably 90.0 or less, more preferably 88.0 or less, and even more preferably 85.0 or less, in Shore C hardness. If the surface hardness (Cs) of the spherical core is 60.0 or more, the resilience of the core will be improved. Furthermore, if the surface hardness (Hs) of the core is 90.0 or less, the feel at impact upon driver shots will be improved.

[0074] The spherical core preferably has a center hardness (Co) of 30.0 or more, more preferably 35.0 or more, and even more preferably 40.0 or more, in Shore C hardness. If the core has a center hardness (Co) of 30.0 or more, in Shore C hardness, it will not be too soft and will have good resilience. Furthermore, the core preferably has a center hardness (Co) of 70.0 or less, more preferably 68.0 or less, and even more preferably 67.0 or less, in Shore C hardness. If the center hardness (Co) is 70.0 or less, it will not be too hard and will have a good shot feel.

[0075] The diameter of the spherical core of the golf ball of the present disclosure is preferably 34.8 mm or more, more preferably 35.8 mm or more, even more preferably 36.8 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 will not be too thick, resulting in better resilience. On the other hand, if the diameter of the core is 42.2 mm or less, the cover will not be too thin, allowing the cover to perform its functions to the fullest.

[0076] When the spherical core has a diameter of 34.8 mm to 42.2 mm, the amount of compressive deformation (the amount the core shrinks in the compressive direction) from an initial load of 98 N to a final load of 1275 N is preferably 3.8 mm or more, more preferably 3.9 mm or more, and even more preferably 4.0 mm or more, and preferably 6.0 mm or less, more preferably 5.5 mm or less, and even more preferably 5.0 mm or less. If the amount of compressive deformation is 3.8 mm or more, the shot feel will be better, and if it is 6.0 mm or less, the resilience will be better.

[0077] The core of the golf ball of the present disclosure can be obtained by mixing and kneading the core rubber composition described above and molding it in a mold. The conditions for this process are not particularly limited, but the process is typically carried out at 130°C to 200°C and a pressure of 2.9 MPa to 11.8 MPa for 10 to 60 minutes. For example, the core rubber composition is preferably heated at 130°C to 200°C for 10 to 60 minutes, or alternatively, in two stages: first at 130°C to 150°C for 20 to 40 minutes, and then at 160°C to 180°C for 5 to 15 minutes.

[0078] [cover] The golf ball of the present disclosure has a spherical core and at least two-layer covers enclosing the spherical core. For a first cover of the at least two-layer covers and a second cover positioned closer to the core than the first cover, the average hardness Dave of the first cover and the second cover, Dave = (Ti × Hi + To × Ho) / (Ti + To), is 55 or greater, where To (mm) is the thickness of the first cover, Ho (Shore D) is the slab hardness of the first cover, Ti (mm) is the thickness of the second cover, and Hi (Shore D) is the slab hardness of the second cover.

[0079] The average hardness Dave of the first cover and the second cover is preferably 56 or greater, and more preferably 57 or greater. If the average hardness Dave is 55 or greater, both a high initial velocity and low spin rate can be achieved on driver shots. The average hardness Dave of the first cover and the second cover is preferably 71 or less, more preferably 70 or less, and even more preferably 69 or less. If the average hardness Dave is 71 or less, good shot feel and durability can be achieved.

[0080] The second cover is not particularly limited as long as it is located closer to the core than the first cover. For example, it is preferable that the first cover is the outermost cover layer and the second cover is the inner cover layer adjacent to the outermost cover layer.

[0081] It is preferable that the slab hardness Hi of the second cover is greater than the slab hardness Ho of the first cover. The difference in slab hardness (Hi - Ho) between the second cover and the first cover is preferably 2 or more, more preferably 4 or more, and even more preferably 6 or more, and is preferably 30 or less, more preferably 28 or less, and even more preferably 26 or less.

[0082] The slab hardness Ho (Shore D) of the first cover is preferably 32 or more, more preferably 33 or more, and even more preferably 34 or more, and is preferably 72 or less, more preferably 71 or less, and even more preferably 70 or less.

[0083] The thickness To of the first cover is preferably 0.3 mm or more, more preferably 0.4 mm or more, and even more preferably 0.5 mm or more, and is preferably 2.7 mm or less, more preferably 2.6 mm or less, and even more preferably 2.5 mm or less.

[0084] The second cover preferably has a slab hardness Hi (Shore D) of 32 or more, more preferably 33 or more, and even more preferably 34 or more, and preferably has a slab hardness Hi of 72 or less, more preferably 71 or less, and even more preferably 70 or less.

[0085] The thickness Ti of the second cover is preferably 0.3 mm or more, more preferably 0.4 mm or more, and even more preferably 0.5 mm or more, and is preferably 2.7 mm or less, more preferably 2.6 mm or less, and even more preferably 2.5 mm or less.

[0086] The total thickness of the cover is preferably 3.0 mm or less, more preferably 2.9 mm or less, and even more preferably 2.8 mm or less. If the total thickness of the cover is 3.0 mm or less, the resilience and shot feel of the resulting golf ball will be better. The total thickness of the cover is preferably 1 mm or more, more preferably 1.1 mm or more, and even more preferably 1.2 mm or more. If the total thickness of the cover is less than 1 mm, the durability and abrasion resistance of the cover may be reduced.

[0087] The cover of the golf ball of the present disclosure is formed from a cover composition containing a resin component, such as an ionomer resin, a thermoplastic polyurethane elastomer commercially available from BASF Japan Ltd. under the trade name "Elastollan®," a thermoplastic polyamide elastomer commercially available from Arkema K.K. under the trade name "Pebax®," a thermoplastic polyester elastomer commercially available from DuPont-Toray Co., Ltd. under the trade name "Hytrel®," or a thermoplastic styrene elastomer commercially available from Mitsubishi Chemical Corporation under the trade name "TEFABLOC®."

[0088] Examples of the ionomer resin include a binary copolymer of an olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, in which at least a portion of the carboxyl groups are neutralized with metal ions; a terpolymer of an olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylic acid ester, in which at least a portion of the carboxyl groups are neutralized with metal ions; or a mixture thereof. The olefin is preferably an olefin having 2 to 8 carbon atoms, such as ethylene, propylene, butene, pentene, hexene, heptene, and octene, with ethylene being particularly preferred. Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid, with acrylic acid or methacrylic acid being particularly preferred. Examples of the α,β-unsaturated carboxylic acid ester include methyl, ethyl, propyl, n-butyl, and isobutyl esters of acrylic acid, methacrylic acid, fumaric acid, and maleic acid, with acrylic acid esters and methacrylic acid esters being particularly preferred. Among these, metal ion-neutralized products of ethylene-(meth)acrylic acid binary copolymers and metal ion-neutralized products of ethylene-(meth)acrylic acid-(meth)acrylic acid ester terpolymers are preferred as the ionomer resins.

[0089] The cover composition constituting the cover of the golf ball of the present disclosure preferably contains a thermoplastic polyurethane elastomer or an ionomer resin as a resin component. When an ionomer resin is used, it is also preferable to use a thermoplastic styrene elastomer in combination. The content of the polyurethane or ionomer resin in the resin component of the cover composition is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more.

[0090] In addition to the resin components described above, the cover composition may contain pigment components such as white pigments (e.g., titanium oxide), blue pigments, and red pigments, weight adjusters such as zinc oxide, calcium carbonate, and barium sulfate, dispersants, antioxidants, ultraviolet absorbers, light stabilizers, fluorescent materials, or fluorescent brighteners, to the extent that the performance of the cover is not impaired.

[0091] The content of the white pigment (e.g., titanium oxide) is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, and preferably 10 parts by mass or less, more preferably 8 parts by mass or less, per 100 parts by mass of the resin components constituting the cover. By making the content of the white pigment 0.5 parts by mass or more, it is possible to impart hiding power to the cover. Furthermore, if the content of the white pigment exceeds 10 parts by mass, the durability of the resulting cover may decrease.

[0092] Methods for molding the cover of the golf ball of the present disclosure include, for example, a method in which a hollow shell is molded from a cover composition, the core is coated with the multiple shells, and the resulting mixture is compression molded (preferably, a method in which a hollow half shell is molded from the cover composition, the core is coated with two half shells, and the resulting mixture is compression molded), or a method in which the cover composition is directly injection molded onto the core.

[0093] When a cover is molded by compression molding, the half shells can be formed by either compression molding or injection molding, but compression molding is preferred. Conditions for compression molding the cover composition into the half shells include, for example, a pressure of 1 MPa to 20 MPa and a molding temperature of -20°C to 70°C relative to the flow initiation temperature of the cover composition. Using these molding conditions allows for the formation of half shells with uniform thickness. A method for molding a cover using half shells includes, for example, a method in which the core is covered with two half shells and compression molded. Conditions for compression molding the half shells into the cover include, for example, a molding pressure of 0.5 MPa to 25 MPa and a molding temperature of -20°C to 70°C relative to the flow initiation temperature of the cover composition. Using these molding conditions allows for the formation of golf ball covers with uniform cover thickness.

[0094] When a cover is formed by injection molding a cover composition, the cover composition may be extruded and then pelletized, or alternatively, cover materials such as base resin components and pigments may be dry-blended and directly injection-molded. The upper and lower molds for molding the cover preferably have hemispherical cavities with pimples, some of which double as retractable hold pins. The cover can be formed by injection molding by pushing out the hold pins, inserting and holding the core, injecting the cover composition, and cooling. For example, the cover composition heated to 200°C to 250°C is injected over 0.5 to 5 seconds into a mold clamped at a pressure of 9 MPa to 15 MPa, allowing it to cool for 10 to 60 seconds, and then opening the mold.

[0095] When molding a cover, depressions called dimples are usually formed on the surface. The total number of dimples formed on the cover is preferably 200 to 500. If the total number of dimples is less than 200, the effect of the dimples is difficult to obtain. Furthermore, if the total number of dimples exceeds 500, the size of each dimple becomes small, making it difficult to obtain the effect of the dimples. The shape (shape in plan view) of the formed dimples is not particularly limited, and the following may be used alone or in combination: circular; polygonal such as approximately triangular, approximately rectangular, approximately pentagonal, or approximately hexagonal; or other irregular shapes.

[0096] The golf ball body with the molded cover is preferably removed from the mold and, if necessary, subjected to surface treatments such as deburring, cleaning, and sandblasting. A coating film or markings can also be formed as desired. The thickness of the coating film is not particularly limited, but is preferably 5 μm or more, more preferably 7 μm or more, and preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less. A coating film thickness of less than 5 μm is prone to wear and wear with continued use, while a coating film thickness of more than 50 μm reduces the effect of the dimples, resulting in a decrease in the flight performance of the golf ball.

[0097] [Golf balls] The structure of the golf ball of the present disclosure is not particularly limited as long as it has a spherical core and two or more cover layers enclosing the spherical core. FIG. 1 is a partially cutaway cross-sectional view showing a golf ball 1 according to one embodiment of the present disclosure. The golf ball 1 has a spherical core 2 and a second cover 3 and a first cover 4 enclosing the spherical core 2. The second cover 3 is located closer to the core than the first cover. The first cover 4 is the outermost cover layer. The second cover 3 is an inner cover layer adjacent to the outermost cover layer. A large number of dimples 31 are formed on the surface of this cover. The portions of the surface of the golf ball 1 other than the dimples 31 are lands 32. The golf ball 1 has a paint layer and a mark layer on the outside of the first cover 4, but these layers are not shown in the figure.

[0098] The spherical core may have either a single-layer structure or a multi-layer structure, but a single-layer structure is preferred. A single-layer core eliminates energy loss at the interface of the multi-layer structure upon impact, improving resilience. The cover may have a multi-layer structure of two or more layers. Examples of golf balls of the present disclosure include three-piece golf balls having a core, an inner cover layer disposed to encase the core, and an outermost cover layer disposed to encase the inner cover, and multi-piece golf balls having a core and three or more covers disposed to encase the core. The present disclosure can be suitably used with golf balls of any of the above structures.

[0099] The diameter of the golf ball of the present disclosure is preferably 40 mm to 45 mm. From the viewpoint of meeting the standards of the United States Golf Association (USGA), a diameter of 42.67 mm or more is particularly preferred. From the viewpoint of reducing air resistance, a diameter of 44 mm or less is more preferred, and 42.80 mm or less is particularly preferred. Furthermore, the weight of the golf ball of the present disclosure is preferably 40 g or more and 50 g or less. From the viewpoint of obtaining high inertia, a weight of 44 g or more is more preferred, and 45.00 g or more is particularly preferred. From the viewpoint of meeting the standards of the USGA, a weight of 45.93 g or less is particularly preferred.

[0100] For golf balls of the present disclosure with a diameter of 40 mm to 45 mm, the compressive deformation (amount of shrinkage in the compressive direction) when an initial load of 98 N is applied and a final load of 1275 N is applied is preferably 2.8 mm or more, more preferably 3.0 mm or more, even more preferably 3.2 mm or more, and preferably 4.5 mm or less, more preferably 4.3 mm or less, and even more preferably 4.1 mm or less. Golf balls with a compressive deformation of 2.8 mm or more are not too hard and have a good feel at impact. On the other hand, by setting the compressive deformation to 4.5 mm or less, the resilience is improved. [Example]

[0101] The present disclosure will be described in detail below using examples, but the present disclosure is not limited to the following examples, and all modifications and embodiments that do not deviate from the spirit of the present disclosure are included within the scope of the present disclosure.

[0102] [Evaluation method] (1) Crosslink density (mol / m 3 ) The core is cut into a 2 mm thick circular plate passing through its geometric center. Then, a 3 mm diameter sample is punched out of the circular plate at measurement locations within 4 mm inward from the core center and each location corresponding to the core surface using a punching tool. The sample weight is measured using an electronic balance capable of measuring in units of two decimal points (mg). The sample and 8 ml of toluene are added to a 10 ml vial, the vial is sealed with a stopper, and the vial is left to stand for at least 72 hours. The solution is then discarded, and the weight of the sample after immersion is measured. The crosslink density of the rubber composition is calculated using the Flory-Rehner equation from the sample weights before and after swelling.

[0103] ν=-(ln(1-v r )+v r +χv r 2 ) / V S (v r 1 / 3 -v r / 2) [ν: crosslink density, v r : rubber volume fraction during swelling, χ: interaction constant, V S :molar volume of toluene] v r =V BR / (V BR +V T ) V BR =(w f -w f v f ) / ρ V T =(w s -w f ) / ρ T [V BR : Volume of BR (butadiene rubber) in the rubber composition, V T: volume of swollen toluene, v f : weight fraction of filler in rubber composition, ρ: density of rubber composition, w f : sample weight before immersion, w s : sample weight after immersion, ρ T :density of toluene] Vs is 0.1063 x 10 -3 m 3 / mol, ρ T is calculated as 0.8669, and χ is calculated as 0.47 based on the literature (Macromolecules 2007, 40, 3669-3675).

[0104] (2) Compression deformation (mm) The deformation in the compression direction (the amount by which the core or golf ball shrinks in the compression direction) was measured when an initial load of 98 N was applied to the core or golf ball and a final load of 1275 N was applied.

[0105] (3) Core hardness (Shore C hardness) The surface hardness of the core was measured using a DigiTest II automatic hardness tester manufactured by H. Burleith Co. The Shore C hardness of the surface of the core was recorded. The core was also cut into two hemispheres, and the center hardness of the cut surface was measured.

[0106] (4) Slab hardness The cover composition was injection molded into a sheet approximately 2 mm thick and stored at 23°C for two weeks. The sheets were stacked in a stack of three or more sheets to avoid the influence of the measurement substrate, and the hardness was measured using an automatic hardness tester (DigiTest II, manufactured by H. Burleith Co.). A Shore D detector was used.

[0107] (5)Durability A metal head W#1 driver (Sumitomo Rubber Industries, Ltd., 2021 XXIO PRIME shaft, hardness R, loft 10.5°) was attached to a swing robot M / C manufactured by Golf Laboratory, and golf balls were repeatedly struck, measuring the number of strikes until cracks occurred. The head speed during strike was set to 35 m / s, assuming an average golfer. Measurements were taken for 12 golf balls, and the average was used as the number of strikes for that golf ball. The number of strikes for each golf ball was indexed, with the number of strikes for golf ball No. 16 set at 100, and the values ​​were evaluated according to the following criteria. ◎: 115 or more 〇: 105 or more and less than 115 △: 95 or more and less than 105 ×: Less than 95

[0108] (6) Flight distance A metal head W#1 driver (Sumitomo Rubber Industries, Ltd., 2021 XXIO PRIME shaft, hardness R, loft 10.5°) was attached to a swing robot M / C manufactured by Golf Laboratory, and a golf ball was hit to measure the total distance. The head speed at the time of impact was set to 35 m / s, assuming an average golfer. ◎:171.5m or more 〇: 170m or more and less than 171.5m △: 168m or more and less than 170m ×: Less than 168m

[0109] (7) Feel at impact A test was conducted using a driver by 20 average golfers. The evaluation was based on the number of people who answered that the shot felt soft. Evaluation criteria ◎:16 or more people 〇: More than 10 people, less than 15 people △: 3 or more people, 9 or less people ×: 2 people or less

[0110] (8) Overall rating Durability, shot feel, and flight distance were evaluated comprehensively based on the following criteria, with an overall rating of B being within the acceptable range. A: The evaluation results for durability, shot feel, and flight distance are only ◯ or ◎. B: Either one of the evaluation results of durability, shot feel, or flight distance is △. C: The evaluation results for durability, hitting feel, and flight distance have an × or two or more △.

[0111] [Manufacturing golf balls] (1) Core preparation A rubber composition having the formulation shown in Table 1 was kneaded with a kneading roll and then hot-pressed at a predetermined temperature for 30 minutes in upper and lower molds having hemispherical cavities to obtain a spherical core having a diameter of 38.6 mm.

[0112] [Table 1]

[0113] The materials used in Table 1 are as follows: BR730: High-cis polybutadiene rubber manufactured by JSR Corporation (cis-1,4-bond content = 95 mass%, 1,2-vinyl bond content = 1.3 mass%, Mooney viscosity (ML 1+4 (100℃)=55, molecular weight distribution (Mw / Mn)=3) ZDA: Zinc acrylate (10% surface treatment) manufactured by Nisshoku Techno Fine Chemical Co., Ltd. Dicumyl peroxide: Tokyo Chemical Industry Co., Ltd. (1-minute half-life temperature: 175.2°C) Zinc oxide: White Seal by INDOLYSAGHT BHT: Dibutylhydroxytoluene manufactured by Tokyo Materials Benzoic acid: Tokyo Chemical Industry Co., Ltd. Barium sulfate: Sakai Chemical Industry Co., Ltd., "Barium Sulfate BD"

[0114] (2) Making the cover and making the golf ball A cover material having the formulation shown in Table 2 was extruded using a twin-screw kneading extruder to prepare a pelletized cover composition. The extrusion conditions for the cover composition were a screw diameter of 45 mm, a screw rotation speed of 200 rpm, and a screw L / D of 35. The compound was heated to 160 to 230°C at the die of the extruder. The resulting cover composition was injection molded onto the spherical core obtained as described above to produce a golf ball (diameter 42.70 mm, weight 45.50 g) having a spherical core and a two-layer cover encasing the core. The first cover was the outermost cover, and the second cover was the inner layer cover adjacent to the outermost cover. The evaluation results of the resulting golf balls are shown in Tables 3 and 4.

[0115] [Table 2]

[0116] The materials used in Table 2 are as follows: Himilan AM7337: Sodium neutralized ionomer manufactured by Mitsui DuPont Polychemicals Himilan AM7329: Zn-neutralized ionomer manufactured by Mitsui DuPont Polychemicals Himilan 1555: Sodium-neutralized ionomer manufactured by Mitsui DuPont Polychemicals Tefablock T3221C: Styrene elastomer manufactured by Mitsubishi Chemical Corporation Titanium dioxide: Ishihara Sangyo Kaisha JF-90: Light stabilizer manufactured by Johoku Chemical Co., Ltd.

[0117] [Table 3]

[0118] [Table 4]

[0119] As shown in Tables 3 and 4, the golf balls of the present disclosure are excellent in durability and distance, and have a good feel when hit, even for average golfers with slow head speeds. [Industrial Applicability]

[0120] The present disclosure can be suitably used as a golf ball.

[0121] The golf ball of the present disclosure (1) is a golf ball having a spherical core and at least two layers of a cover enclosing the spherical core, wherein the spherical core is formed from a core 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, and (c) a crosslinking initiator, and wherein the difference between the surface crosslink density of the spherical core and the center crosslink density of the spherical core (core surface crosslink density - core center crosslink density) is 1.0 × 10 2 mol / m 3 Super, 9.0×10 2 mol / m 3 the difference in hardness (Cs-Co) between the surface hardness Cs (Shore C hardness) and the center hardness Co (Shore C hardness) of the spherical core is 13.0 or greater and 30.0 or less; the amount of compressive deformation when an initial load of 98 N is applied to the spherical core and a final load of 1275 N is applied to the spherical core is 3.8 mm or greater; and the average hardness Dave of the first cover and the second cover, which are at least two-layered, is 55 or greater, where To (mm) is the thickness of the first cover, Ho (Shore D) is the slab hardness of the first cover, Ti (mm) is the thickness of the second cover, and Hi (Shore D) is the slab hardness of the second cover.

[0122] The golf ball of the present disclosure (2) is the golf ball according to the present disclosure (1), in which the slab hardness Hi of the second cover is greater than the slab hardness Ho of the first cover.

[0123] The golf ball of the present disclosure (3) is the golf ball according to the present disclosure (1) or (2), in which the total thickness of the cover is 1.0 mm to 3.0 mm.

[0124] In the golf ball of the present disclosure (4), the surface crosslink density of the spherical core is 9.0×10 2 mol / m 3 That's it, 22.0 x 10 2 mol / m 3 The golf ball is described in any one of the following disclosures (1) to (3).

[0125] In the golf ball of the present disclosure (5), the central crosslink density of the spherical core is 4.0×10 2 mol / m 3 That's it, 13.0 x 10 2 mol / m 3 The golf ball is described in any one of the following aspects (1) to (4) of the present disclosure.

[0126] The golf ball of the present disclosure (6) is the golf ball according to any one of the present disclosures (1) to (5), wherein the spherical core has a surface hardness Cs of 60.0 or more and 90.0 or less in Shore C hardness.

[0127] The golf ball of the present disclosure (7) is the golf ball according to any one of the present disclosures (1) to (6), wherein the spherical core has a center hardness Co, in Shore C hardness, of 30.0 or more and 70.0 or less.

[0128] The golf ball of the present disclosure (8) is the golf ball according to any one of the present disclosures (1) to (7), wherein the core rubber composition contains at least one additive (d) selected from the group consisting of (d1) hindered phenol compounds and (d2) hindered amine compounds.

[0129] The golf ball of the present disclosure (9) is the golf ball according to any one of the present disclosures (1) to (8), wherein the core rubber composition contains 0.5 parts by mass to 15 parts by mass of (d1) a hindered phenol compound and / or (d2) a hindered amine compound per 100 parts by mass of the (a) base rubber.

[0130] The golf ball of present disclosure (10) is the golf ball of present disclosure (8) or (9), in which (d1) the hindered phenol compound is at least one compound selected from the group consisting of dibutylhydroxytoluene, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 4,4'-methylenebis(2,6-di-tert-butylphenol), 2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propan-2-yl}sulfanyl)phenol, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0131] The golf ball of the present disclosure (11) is the golf ball according to any one of the present disclosures (1) to (10), in which the first cover is the outermost cover layer and the second cover is the inner cover layer adjacent to the outermost cover layer.

Claims

1. A golf ball having a spherical core and a cover having at least two layers surrounding the spherical core, the spherical core is formed from a core 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, and (c) a crosslinking initiator; The difference between the surface crosslink density of the spherical core and the center crosslink density of the spherical core (core surface crosslink density - core center crosslink density) is 1.0 × 10 ２ mol / m ３ Super, 9.0×10 ２ mol / m ３ is less than the difference in hardness (Cs-Co) between the surface hardness Cs (Shore C hardness) and the center hardness Co (Shore C hardness) of the spherical core is 13.0 or greater and 30.0 or less; The amount of compressive deformation when an initial load of 98 N is applied to the spherical core and a final load of 1275 N is applied is 3.8 mm or more, Regarding a first cover and a second cover located closer to the core than the first cover, among the at least two cover layers, The thickness To (mm) of the first cover, the slab hardness Ho (Shore D) of the first cover, When the thickness of the second cover is Ti (mm) and the slab hardness of the second cover is Hi (Shore D), an average hardness Dave of the first cover and the second cover (Dave=(Ti×Hi+To×Ho) / (Ti+To)) is 55 or greater; The golf ball is characterized in that the second cover has a slab hardness Hi greater than the slab hardness Ho of the first cover.

2. A golf ball as described in claim 1, wherein the difference (Hi-Ho) between the slab hardness Hi of the second cover and the slab hardness Ho of the first cover is 2 or more and 30 or less in Shore D hardness.

3. 3. The golf ball according to claim 1, wherein the cover has a total thickness of 1.0 mm to 3.0 mm.

4. The surface crosslinking density of the spherical core is 9.0 × 10 ２ mol / m ３ That's it, 22.0 x 10 ２ mol / m ３ 4. The golf ball according to claim 1, wherein:

5. The central crosslink density of the spherical core is 4.0 × 10 ２ mol / m ３ That's it, 13.0 x 10 ２ mol / m ３ 5. The golf ball according to claim 1, wherein:

6. 6. The golf ball according to claim 1, wherein the spherical core has a surface hardness Cs of 60.0 or greater and 90.0 or less in Shore C hardness.

7. 7. The golf ball according to claim 1, wherein the spherical core has a center hardness Co of 30.0 or greater and 70.0 or less in Shore C hardness.

8. 8. The golf ball according to claim 1, wherein the core rubber composition contains at least one additive (d) selected from the group consisting of (d1) a hindered phenol compound and (d2) a hindered amine compound.

9. 9. The golf ball according to claim 1, wherein the core rubber composition contains 0.5 to 15 parts by mass of (d1) a hindered phenol-based compound and / or (d2) a hindered amine-based compound per 100 parts by mass of the base rubber (a).

10. (d1) The golf ball according to claim 8 or 9, wherein the hindered phenol compound is at least one compound selected from the group consisting of dibutylhydroxytoluene, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 4,4'-methylenebis(2,6-di-tert-butylphenol), 2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propan-2-yl}sulfanyl)phenol, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

11. 11. The golf ball according to claim 1, wherein the first cover is an outermost cover layer, and the second cover is an inner cover layer adjacent to the outermost cover layer.

12. The golf ball according to claim 1, wherein the spherical core has a surface hardness Cs of 60.0 or greater and 76.0 or less in Shore C hardness.

13. The golf ball according to claim 1, wherein the spherical core has a center hardness Co of 30.0 or greater and 55.0 or less in Shore C hardness.

14. A golf ball described in any one of claims 1 to 13, wherein the amount of compressive deformation when the spherical core is subjected to an initial load of 98 N to a final load of 1275 N is 4.0 mm or more.

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