golf balls

Abstract

To provide a golf ball which is excellent in a carry by a driver shot and is good in a spin amount by a middle iron shot.SOLUTION: A golf ball has a spherical core and a cover. Provided that a radius of the spherical core is equally divided into eight segments and points between the segments have a shore C hardness, labelled from the center as C0 (center), C1, C2, C3, C4, C5, C6, C7, and C8 (surface), a hardness difference (C1-C0), a hardness difference (C2-C1), a hardness difference (C3-C2), and a hardness difference (C4-C3) are more than 0 and equal to or less than 6.0; a hardness difference (C5-C4) is 5.0 or more; a hardness difference (C6-C5), a hardness difference (C7-C6), and a hardness difference (C8-C7) is more than 0 and equal to or less than 3.5; and a difference {(C5-C4)-(C4-C3)} between the hardness difference (C5-C4) and the hardness difference (C4-C3) is 1.0 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a golf ball, and more particularly to the hardness distribution of a spherical core. [Background technology]

[0002] Golf balls are required to have excellent distance performance on driver shots. One way to improve the distance on driver shots is to appropriately select the hardness distribution of the spherical core. Specifically, it is known that increasing the difference between the surface hardness and the center hardness of the spherical core can reduce the amount of spin on driver shots and improve the distance.

[0003] For example, Patent Document 1 describes a golf ball having a core, an intermediate layer, and a cover, in which the core is formed primarily from a base rubber and has a diameter set within a specific range, and the intermediate layer and the cover are each formed from a resin material, and the internal hardness of the core is designed to be within a predetermined range for the hardness at the core center and at every 2 mm interval up to 16 mm from the core center, as well as the core surface hardness, and the difference between these hardnesses is set within a predetermined range, and the surface hardness of the ball is set lower than the surface hardness of a sphere covered with the intermediate layer (Patent Document 1).

[0004] Patent Document 2 also describes a multi-piece solid golf ball with an intermediate layer interposed between a core and a cover, in which the surface hardnesses of the core, intermediate layer-covered sphere, and ball satisfy a specified relationship, the thickness of the intermediate layer and the thickness of the cover satisfy a specified relationship, and the core hardness distribution has a specified relationship among the core surface hardness (Cs), core center C hardness (Cc), hardness at a position 5 mm from the core center (C5), and hardness at a position midway between the core surface and center (Cm) (Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-062036 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-112308 Summary of the Invention [Problem to be solved by the invention]

[0006] However, professional golfers and advanced golfers desire not only improved distance on driver shots but also increased spin on middle iron shots. However, when the spin on driver shots is reduced by controlling the difference between the surface hardness and the center hardness of the spherical core, the spin on middle iron shots also tends to decrease.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a golf ball that has an excellent flight distance on driver shots and a good spin rate on middle iron shots. [Means for solving the problem]

[0008] The golf ball of the present invention, which has solved the above-mentioned problems, is a golf ball having a spherical core and a cover enclosing the spherical core, and is characterized in that, when a straight line from the center of the spherical core to the surface is divided into eight equal parts, the center hardness (C0), the hardness (C1) at a point 12.5% ​​from the center, the hardness (C2) at a point 25.0% from the center, the hardness (C3) at a point 37.5% from the center, the hardness (C4) at a point 50.0% from the center, the hardness (C5) at a point 62.5% from the center, the hardness (C6) at a point 75.0% from the center, the hardness (C7) at a point 87.5% from the center, and the surface hardness (C8) of the spherical core satisfy, in Shore C hardness, formulas (1) to (9). 0<(C1-C0)≦6.0 (1) 0<(C2-C1)≦6.0 (2) 0<(C3-C2)≦6.0 (3) 0<(C4-C3)≦6.0 (4) 5.0≦(C5-C4) (5) 0<(C6-C5)≦3.5 (6) 0<(C7-C6)≦3.5 (7) 0<(C8-C7)≦3.5 (8) 1.0≦{(C5-C4)-(C4-C3)} ···(9)

[0009] During driver shots, the entire spherical core deforms significantly. Therefore, by setting the lower limit values ​​of formulas (1) to (4) and (6) to (8) to greater than 0 and the lower limit value of formula (5) to 5.0, the hardness distribution throughout the spherical core becomes an outer-hard, inner-soft structure, resulting in a recoil effect during driver shots and reduced spin rate. Furthermore, by setting the upper limit values ​​of formulas (1) to (4) to 6.0 and the upper limit values ​​of formulas (6) to (8) to 3.5, the deformation balance throughout the spherical core during driver shots is improved, improving resilience performance. This results in improved distance performance during driver shots.

[0010] During middle iron shots, the amount of deformation of the spherical core is smaller than during driver shots, making the hardness of the spherical core near its surface important. Therefore, by setting the upper limit values ​​of formulas (6) to (8) to 3.5, the amount of deformation near the spherical core surface during middle iron shots can be suppressed, improving spin. Furthermore, by satisfying formulas (5) and (9), the amount of deformation at a point 50% to 62.5% from the center of the spherical core during middle iron shots is increased, further reducing deformation near the center. This reduces recoil and further improves spin during middle iron shots. [Effects of the Invention]

[0011] According to the present invention, a golf ball is obtained which has an excellent flight distance on driver shots and a good spin rate on middle iron shots. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a partially cutaway cross-sectional view showing a golf ball according to an embodiment of the present invention; [Figure 2]Graph showing the hardness distribution of a spherical core. [Figure 3] Graph showing the hardness distribution of a spherical core. [Figure 4] Graph showing the hardness distribution of a spherical core. [Figure 5] Graph showing the hardness distribution of a spherical core. [Figure 6] Graph showing the hardness distribution of a spherical core. [Figure 7] Graph showing the hardness distribution of a spherical core. [Figure 8] Graph showing the hardness distribution of a spherical core. [Figure 9] Graph showing the hardness distribution of a spherical core. [Figure 10] Graph showing the hardness distribution of a spherical core. DETAILED DESCRIPTION OF THE INVENTION

[0013] The golf ball of the present invention has a spherical core and a cover enclosing the spherical core, and is characterized in that when the length of a straight line extending from the center of the spherical core in a radial direction toward the surface is divided into eight equal parts, the center hardness (hardness at 0%) (C0), the hardness at 12.5% ​​from the center (C1), the hardness at 25.0% from the center (C2), the hardness at 37.5% from the center (C3), the hardness at 50.0% from the center (C4), the hardness at 62.5% from the center (C5), the hardness at 75.0% from the center (C6), the hardness at 87.5% from the center (C7), and the surface hardness (hardness at 100% from the center) (C8) of the spherical core satisfy, in Shore C hardness, formulas (1) to (9). 0<(C1-C0)≦6.0 (1) 0<(C2-C1)≦6.0 (2) 0<(C3-C2)≦6.0 (3) 0<(C4-C3)≦6.0 (4) 5.0≦(C5-C4) (5) 0<(C6-C5)≦3.5 (6) 0<(C7-C6)≦3.5 (7) 0<(C8-C7)≦3.5 (8) 1.0≦{(C5-C4)-(C4-C3)} ···(9)

[0014] The above formulas (1) to (4) define the hardness distribution near the center of the spherical core. If there is a location near the center where the hardness gradient is too large, the amount of deformation at that location will be large during driver shots. This will disrupt the overall balance of deformation of the golf ball, reducing resilience and resulting in reduced flight distance. Therefore, satisfying formulas (1) to (4) will improve resilience and improve driver flight distance. Furthermore, by providing a hardness gradient near the center of the spherical core, the amount of recoil during driver shots can be increased. This will reduce the amount of spin during driver shots, further improving flight distance.

[0015] The spherical core has a hardness difference (C1-C0) between the center hardness (C0) and the hardness (C1) at a point 12.5% ​​from the center, in Shore C hardness, of greater than 0, preferably 0.5 or more, more preferably 1.0 or more, and is 6.0 or less, preferably 5.5 or less, more preferably 5.0 or less.

[0016] The difference in hardness (C2-C1) between the hardness (C1) at a point 12.5% ​​from the center of the spherical core and the hardness (C2) at a point 25.0% from the center, in Shore C hardness, is greater than 0, preferably 0.5 or more, more preferably 1.0 or more, and is 6.0 or less, preferably 5.5 or less, more preferably 5.0 or less.

[0017] The difference in hardness (C3-C2) between the hardness (C2) at a point 25.0% from the center of the spherical core and the hardness (C3) at a point 37.5% from the center, in Shore C hardness, is greater than 0, preferably 0.5 or more, more preferably 1.0 or more, and is 6.0 or less, preferably 5.5 or less, more preferably 5.0 or less.

[0018] The difference in hardness (C4-C3) between the hardness (C3) at a point 37.5% from the center of the spherical core and the hardness (C4) at a point 50.0% from the center, in Shore C hardness, is greater than 0, preferably 0.5 or more, more preferably 1.0 or more, and is 6.0 or less, preferably 5.5 or less, more preferably 5.0 or less.

[0019] The formula (5) defines the hardness difference (C5-C4) between the hardness (C4) at a point 50.0% from the center of the spherical core and the hardness (C5) at a point 62.5% from the center. The hardness difference (C5-C4) is, in Shore C hardness, 5.0 or more, preferably 5.5 or more, and more preferably 6.0 or more, and is preferably 12.0 or less, more preferably 11.0 or less, and even more preferably 10.0 or less. If the hardness difference (C5-C4) is 5.0 or more, the spin rate on middle iron shots can be improved while achieving a high initial velocity on driver shots.

[0020] The formulas (6) to (8) define the hardness distribution near the surface of the spherical core. If there is an area near the surface where the hardness gradient is too large, the amount of deformation at that area will be large on driver shots, preventing the entire golf ball from deforming and resulting in reduced resilience. Furthermore, if there is an area near the surface where the hardness gradient is too large, the deformation near the surface will be large on middle iron shots, resulting in increased energy loss and reduced spin rate. Therefore, by satisfying formulas (6) to (8), it is possible to achieve both improved driver shot distance and improved spin rate on middle iron shots.

[0021] The difference in hardness (C6-C5) between the hardness (C5) at a point 62.5% from the center of the spherical core and the hardness (C6) at a point 75.0% from the center, in Shore C hardness, is greater than 0, preferably 0.5 or more, more preferably 1.0 or more, and is 3.5 or less, preferably 3.0 or less, more preferably 2.5 or less.

[0022] The difference in hardness (C7-C6) between the hardness (C6) at a point 75.0% from the center of the spherical core and the hardness (C7) at a point 82.5% from the center, in Shore C hardness, is greater than 0, preferably 0.5 or more, more preferably 1.0 or more, and is 3.5 or less, preferably 3.0 or less, more preferably 2.5 or less.

[0023] The difference in hardness (C8-C7) between the hardness (C7) at a point 82.5% from the center of the spherical core and the surface hardness (C8) is, in Shore C hardness, greater than 0, preferably 0.5 or more, more preferably 1.0 or more, and is 3.5 or less, preferably 3.0 or less, more preferably 2.5 or less.

[0024] The formula (9) defines the difference {(C5-C4)-(C4-C3)} between the hardness difference (C5-C4) and the hardness difference (C4-C3). Satisfying formula (9) improves the spin rate on middle iron shots and reduces the spin rate on driver shots. The difference {(C5-C4)-(C4-C3)} is preferably 1.0 or more, more preferably 1.5 or more, and even more preferably 2.0 or more, in Shore C hardness, and is preferably 10.0 or less, more preferably 9.0 or less, and even more preferably 8.0 or less.

[0025] The spherical core preferably has hardnesses (C4), (C5), and (C6) that satisfy the following relationship in Shore C hardness: By satisfying formula (10), the spin rate on middle iron shots is improved and the initial velocity on driver shots is increased. 1.0≦{(C5-C4)-(C6-C5)} ···(10)

[0026] The difference {(C5-C4)-(C6-C5)} is preferably 1.0 or more, more preferably 1.5 or more, and even more preferably 2.0 or more, in Shore C hardness, and is preferably 10.0 or less, more preferably 9.0 or less, and even more preferably 8.0 or less.

[0027] The spherical core preferably has hardnesses (C0), (C4), (C5), and (C8) that satisfy formula (11) in Shore C. By satisfying formula (11), the spin rate on middle iron shots is improved and the spin rate on driver shots is reduced. (C4-C0)>(C5-C4)>(C8-C5)...(11)

[0028] The hardness difference (C2-C0) between the center hardness (C0) of the spherical core and the hardness (C2) at a point 25.0% from the center is preferably 5.5 or more, more preferably 6.0 or more, and even more preferably 6.5 or more, in Shore C hardness, and is preferably 12.0 or less, more preferably 11.0 or less, and even more preferably 10.0 or less. If the hardness difference (C2-C0) is within the above range, the amount of deformation near the center of the spherical core increases, further increasing the recoil effect on driver shots and further reducing the spin rate.

[0029] The difference in hardness (C4-C0) between the center hardness (C0) of the spherical core and the hardness (C4) at a point 50.0% from the center, in Shore C hardness, is preferably greater than 0, more preferably 3.0 or more, and even more preferably 6.0 or more, and is preferably 24.0 or less, more preferably 22.0 or less, and even more preferably 20.0 or less.

[0030] The difference in hardness (C8-C5) between the hardness (C5) at a point 62.5% from the center of the spherical core and the surface hardness (C8) is preferably 0 or more, more preferably 1.0 or more, and even more preferably 2.0 or more, in Shore C hardness, and is preferably 10.5 or less, more preferably 10.0 or less, and even more preferably 9.5 or less.

[0031] The hardness difference (C8-C0) between the center hardness (C0) and surface hardness (C8) of the spherical core is preferably 18.0 or more, more preferably 19.0 or more, and even more preferably 20.0 or more, in Shore C hardness, and is preferably 32.0 or less, more preferably 30.0 or less, and even more preferably 28.0 or less. If the hardness difference (C8-C0) is within the above range, the degree of outer hardness / inner softness in the hardness distribution of the entire spherical core becomes greater, and the recoil effect becomes greater, further reducing the spin rate on driver shots and improving the flight distance.

[0032] The spherical core has a ratio of the hardness difference (C2-C0) to the hardness difference (C5-C4) {(C2-C0) / (C5-C4)}, in Shore C hardness, of preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more, and preferably 3.5 or less, more preferably 3.3 or less, and even more preferably 3.0 or less. If the ratio {(C2-C0) / (C5-C4)} is within the above range, the spin rate on middle iron shots is further improved and the spin rate on driver shots is further reduced.

[0033] The spherical core preferably has a ratio of the hardness difference (C2-C0) to the hardness difference (C4-C2) {(C2-C0) / (C4-C2)}, in Shore C hardness, of 1.0 or more, more preferably 1.2 or more, and even more preferably 1.4 or more, and preferably 6.5 or less, more preferably 6.0 or less, and even more preferably 5.5 or less. If the ratio {(C2-C0) / (C4-C2)} is within the above range, the spin rate on middle iron shots is improved and the spin rate on driver shots is reduced.

[0034] The spherical core preferably has hardnesses (C0), (C2), (C4), and (C8) that satisfy formula (12) in Shore C. By satisfying formula (12), the spin rate on middle iron shots is improved and the spin rate on driver shots is reduced. {(C8-C4) / (C2-C0)}≦3.0 ···(12)

[0035] The ratio {(C8-C4) / (C2-C0)} is preferably 0 or more, more preferably 0.5 or more, and even more preferably 1.0 or more, and is preferably 3.0 or less, more preferably 2.5 or less, and even more preferably 2.0 or less.

[0036] The spherical core has, in Shore C hardness, a hardness difference between hardness (C1) and hardness (C0) of (C1-C0), a hardness difference between hardness (C2) and hardness (C1) of (C2-C1), a hardness difference between hardness (C3) and hardness (C2) of (C3-C2), a hardness difference between hardness (C4) and hardness (C3) of (C4-C3), a hardness difference between hardness (C5) and hardness (C4) of (C5-C4), When the largest value of the hardness difference (C6-C5) between hardness (C6) and hardness (C5), the smallest value of the hardness difference (C7-C6) between hardness (C7) and hardness (C6), and the smallest value of the hardness difference (C8-C7) between hardness (C8) and hardness (C7) are Cbmax and Cbmin, respectively, the ratio (Cbmax / Cbmin) is preferably 4.0 or greater. Having areas with large and small hardness differences throughout the spherical core creates areas where the golf ball bends upon impact and areas where the movement of the golf ball is suppressed, allowing the deformed areas to be efficiently converted into resilience. It is preferable that the hardness difference (C5-C4) is Cbmax.

[0037] The spherical core preferably has a center hardness (C0) of 50.0 or more, more preferably 52.0 or more, and even more preferably 54.0 or more, in Shore C hardness, and preferably 70.0 or less, more preferably 68.0 or less, and even more preferably 66.0 or less. If the center hardness (C0) is 50.0 or more, the ball will not collapse too much when deformed, providing good resilience performance, while if it is 70.0 or less, deformation will extend to the inside of the ball, resulting in a good feel.

[0038] The spherical core preferably has a surface hardness (C8) of 70.0 or more, more preferably 72.0 or more, and even more preferably 74.0 or more, in Shore C hardness, and preferably 90.0 or less, more preferably 88.0 or less, and even more preferably 86.0 or less. A surface hardness (C8) of 70.0 or more results in a ball that is not crushed too much and exhibits good resilience, while a surface hardness of 90.0 or less results in a ball with good durability.

[0039] The hardness (C4) of the spherical core at a point 50.0% from the center is preferably 60.0 or more, more preferably 62.0 or more, and even more preferably 64.0 or more, on the Shore C scale, and is preferably 80.0 or less, more preferably 78.0 or less, and even more preferably 76.0 or less. If the hardness (C4) is 60.0 or more, the ball will not be crushed too much and will exhibit good resilience, while if it is 80.0 or less, it will promote recoil and result in low spin when used with a driver.

[0040] The diameter of the spherical core is preferably 34.8 mm or more, more preferably 36.3 mm or more, even more preferably 37.8 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 resilience is improved. On the other hand, if the diameter of the spherical core is 42.2 mm or less, the function of the cover is fully exhibited.

[0041] 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 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 amount of compressive deformation is 2.0 mm or more, the shot feel will be better, and if it is 5.0 mm or less, the resilience will be better.

[0042] The spherical core may have either a single-layer structure or a multi-layer structure of two or more layers, but a single-layer structure is preferred. A single-layer spherical core eliminates energy loss at the interface of the multi-layer structure upon impact, improving resilience.

[0043] [Rubber composition] The spherical core is preferably 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 spherical core can be obtained by molding the core rubber composition in a mold. The molding conditions are not particularly limited, but are typically 130°C to 200°C, 2.9 MPa to 11.8 MPa, and 10 to 60 minutes.

[0044] The spherical core is preferably 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, (c) a crosslinking initiator, and (d) a monophenol compound having a substituent only at the p-position. By using a rubber composition containing specific raw materials, the hardness distribution of the resulting spherical core can be easily controlled.

[0045] (a) Base rubber (a) The base rubber can be natural rubber and / or synthetic rubber. (a) Examples of the base rubber that can be used include polybutadiene rubber, natural rubber, polyisoprene rubber, styrene-butadiene rubber, and ethylene-propylene-diene rubber (EPDM). These may be used alone or in combination of two or more. Among these, high-cis polybutadiene, which has cis-1,4-bonds that are advantageous for resilience, is particularly suitable, having 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.

[0046] 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.

[0047] 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 2.0% by mass or less, the resilience is further improved.

[0048] 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.

[0049] 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.

[0050] 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. When the molecular weight distribution (Mw / Mn) of the high-cis polybutadiene is within the above range, the workability of core molding is improved, and the resilience of the resulting spherical core is improved. The molecular weight distribution was measured by gel permeation chromatography (manufactured by Tosoh Corporation, "HLC-8120GPC") using a differential refractometer as a detector, a GMHHXL column (manufactured by Tosoh Corporation), a column temperature of 40°C, and a mobile phase of tetrahydrofuran, and calculated as a value converted into a standard polystyrene.

[0051] (b) Co-crosslinking agent (b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or its metal salt is blended into the rubber composition as a co-crosslinking agent, and has the effect of crosslinking rubber molecules by graft polymerizing to the base rubber molecular chain.

[0052] 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.

[0053] 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.

[0054] The content of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or its metal salt is preferably at least 15 parts by weight, more preferably at least 20 parts by weight, even more preferably at least 25 parts by weight, and preferably at most 50 parts by weight, more preferably at most 45 parts by weight, and even more preferably at most 35 parts by weight, per 100 parts by weight of the (a) base rubber. When the content of component (b) is 15 parts by weight or more, the amount of (c) crosslinking initiator required to impart an appropriate hardness to the core formed from the core rubber composition is reduced, thereby improving the resilience of the resulting golf ball. Furthermore, when the content of component (b) is 50 parts by weight or less, the resulting golf ball has a good shot feel.

[0055] (c) Crosslinking initiator (c) The crosslinking initiator is blended to crosslink the (a) base rubber component. (c) The crosslinking initiator is preferably an organic peroxide. 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.

[0056] The content of (c) the crosslinking initiator, per 100 parts by weight of (a) base rubber, 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 is 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. If the content of component (c) is at least 0.2 parts by weight, the core formed from the core rubber composition will not be too soft, and the resilience of the resulting golf ball will be improved, while if it is at most 5.0 parts by weight, the resilience and durability of the resulting golf ball will be good.

[0057] (d) Monophenolic compounds with substituents only at the p-position (d) A monophenol compound having a substituent only at the p-position is a compound having a substituent only at the p-position of a monophenol. The monophenol compound having a substituent only at the p-position is a compound in which a substituent is directly bonded to the p-position relative to one hydroxy group of a phenol, and has no substituents at the o- and m-positions of the hydroxy group. Examples of the substituent at the p-position include an alkoxy group, a halogen group, a hydrocarbon group, a nitro group, a cyano group, an amino group, and a hydroxy group, with an alkoxy group being preferred.

[0058] The (d) monophenol compound having a substituent only at the p-position is preferably one represented by the following general formula (1).

[0059] [ka] [In general formula (1), R represents an alkoxy group, a halogen group, a hydrocarbon group, a nitro group, a cyano group, an amino group, or a hydroxy group.]

[0060] Examples of the alkoxy group include a group in which an alkyl group having one or more carbon atoms is bonded to an oxygen atom. The number of carbon atoms in the alkoxy group is not particularly limited as long as it is one or more, but is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 8. The structure of the alkyl portion of the alkoxy group may be linear, branched, or cyclic. Specific examples of the alkyl moiety of the alkoxy group include linear or branched alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl, tert-pentyl, neopentyl, n-hexyl, isohexyl, sec-hexyl, tert-hexyl, n-heptyl, isoheptyl, sec-heptyl, tert-heptyl, n-octyl, isooctyl, sec-octyl, and tert-octyl; and cyclic alkyl groups such as cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. Specific examples of the alkoxy group include a methoxy group, an ethoxy group, a propoxy group (including n- and iso-structures), a butoxy group (including n-, iso-, sec-, tert-, and cyclo-structures), a pentyloxy group (including n-, iso-, sec-, tert-, and cyclo-structures), a hexyloxy group (including n-, iso-, sec-, tert-, and cyclo-structures), a heptyloxy group (including n-, iso-, sec-, tert-, and cyclo-structures), and an octyloxy group (including n-, iso-, sec-, tert-, and cyclo-structures). The alkoxy group may have a substituent (e.g., a halogen group, a hydroxy group, an amino group, a nitro group, a cyano group, etc.).

[0061] Examples of the halogen group include a fluoro group, a chloro group, a bromo group, and an iodo group.

[0062] Examples of the hydrocarbon group include an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, and an aryl group.

[0063] The number of carbon atoms in the alkyl group is not particularly limited as long as it is 1 or more, but is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 8. The alkyl group may have a linear, branched, or cyclic structure, and specific examples thereof include linear or branched alkyl groups such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a neopentyl group, an n-hexyl group, an isohexyl group, a sec-hexyl group, a tert-hexyl group, an n-heptyl group, an isoheptyl group, a sec-heptyl group, a tert-heptyl group, an n-octyl group, an isooctyl group, a sec-octyl group, and a tert-octyl group, and cyclic alkyl groups such as a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, and a cyclooctyl group. The alkyl group may have a substituent (for example, a halogen group, a hydroxy group, an amino group, a nitro group, a cyano group, etc.).

[0064] The number of carbon atoms in the alkenyl group is not particularly limited as long as it is 2 or more, but is preferably 2 to 20, more preferably 2 to 10, and even more preferably 2 to 8. Specific examples of the alkenyl group include a vinyl group, an allyl group, a propenyl group, an isopropenyl group, a butenyl group, an isobutenyl group, and a hexenyl group. The alkenyl group may have a substituent (for example, an alkyl group, a halogen group, a hydroxy group, an amino group, a nitro group, or a cyano group).

[0065] The number of carbon atoms in the alkynyl group is not particularly limited as long as it is 2 or more, but is preferably 2 to 20, more preferably 2 to 10, and even more preferably 2 to 8. Specific examples of the alkynyl group include an ethynyl group, a 1-propynyl group, a 2-propynyl group (propargyl group), and a butynyl group. The alkynyl group may have a substituent (for example, an alkyl group, a halogen group, a hydroxy group, an amino group, a nitro group, or a cyano group).

[0066] The number of carbon atoms in the aralkyl group is not particularly limited as long as it is 7 or more, but is preferably 7 to 20, more preferably 7 to 10, and even more preferably 7 to 8. Specific examples of the aralkyl group include a benzyl group, a phenylethyl group, a phenylbutyl group, and an α-cumyl group. The aralkyl group may have a substituent (for example, an alkyl group, a halogen group, an amino group, a nitro group, a cyano group, etc.).

[0067] The number of carbon atoms in the aryl group is not particularly limited as long as it is 6 or more, but is preferably 6 to 20, more preferably 6 to 15, and even more preferably 6 to 10. Specific examples of the aryl group include a phenyl group and a naphthyl group. The aryl group may have a substituent (for example, an alkyl group, a halogen group, an amino group, a nitro group, a cyano group, etc.).

[0068] In the general formula (1), the substituent represented by R is preferably an alkoxy group, more preferably an alkoxy group having 1 to 8 carbon atoms, and particularly preferably a methoxy group, an ethoxy group, a propoxy group, or a butoxy group.

[0069] The (d) monophenol compound having a substituent only at the p-position may be used alone or in combination of two or more kinds.

[0070] The amount of the (d) monophenol compound having a substituent only at the p-position is preferably at least 0.05 parts by weight, more preferably at least 0.07 parts by weight, even more preferably at least 0.10 parts by weight, and is preferably at most 2.0 parts by weight, more preferably at most 1.8 parts by weight, and even more preferably at most 1.6 parts by weight, per 100 parts by weight of the (a) base rubber. When the amount of the (d) monophenol compound having a substituent only at the p-position is 0.05 parts by weight or more, the effect of adding the monophenol compound having a substituent only at the p-position is greater, and when it is 2.0 parts by weight or less, the flight distance on driver shots is further improved.

[0071] The mass ratio of component (b) to component (d) (component (b) / component (d)) is preferably 25 or more, more preferably 30 or more, even more preferably 35 or more, still more preferably 100 or more, particularly preferably 200 or more, and is preferably 500 or less, more preferably 450 or less, and even more preferably 400 or less. When the mass ratio (component (b) / component (d)) is within the above range, the shot feel is good and the flight distance on driver shots is further improved.

[0072] The mass ratio of component (c) to component (d) (component (c) / component (d)) is preferably 1.0 or greater, more preferably 1.5 or greater, even more preferably 2.0 or greater, and is preferably 150 or less, more preferably 130 or less, even more preferably 110 or less, even more preferably 50 or less, and particularly preferably 20 or less. When the mass ratio (component (c) / component (d)) is within the above range, the shot feel is good and the flight distance on driver shots is further improved.

[0073] (e)Organic sulfur compounds The core rubber composition preferably further contains (e) an organic sulfur compound, which increases the resilience of the resulting core.

[0074] The (e) organic sulfur compound may be at least one compound selected from the group consisting of thiols (thiophenols, thionaphthols), polysulfides, thiazoles, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, and dithiocarbamates.

[0075] 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. The metal salt is preferably a divalent metal salt, more preferably a zinc salt.

[0076] 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. The metal salts are preferably divalent metal salts, more preferably zinc salts.

[0077] Polysulfides are organic sulfur compounds having polysulfide bonds, such as disulfides, trisulfides, and tetrasulfides. Diphenyl polysulfides are preferred as the polysulfides.

[0078] 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.

[0079] Examples of thiazoles include 2-mercaptobenzothiazole, di-2-benzothiazolyl disulfide, 2-(N,N-diethylthiocarbamoylthio)benzothiazole, 2-(4'-morpholinodithio)benzothiazole, 4-methyl-2-mercaptobenzothiazole, di-(4-methyl-2-benzothiazolyl)disulfide, 5-chloro-2-mercaptobenzothiazole, 2-mercapto-6-nitrobenzothiazole, 2-mercapto-naphtho[1,2-d]thiazole, 2-mercapto-5-methoxybenzothiazole, 6-amino-2-mercaptobenzothiazole, or metal salts thereof.

[0080] 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.

[0081] The (e) organic sulfur compound is preferably at least one compound selected from the group consisting of halogen-substituted thiophenols, metal salts of halogen-substituted thiophenols, halogen-substituted diphenyl disulfides, thiazoles, and metal salts of thiazoles.

[0082] The (e) organic sulfur compounds can be used alone or in combination of two or more.

[0083] The content of the (e) organic sulfur compound, per 100 parts by weight of the (a) base rubber, 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, 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 component (e) is within the above range, the resilience of the resulting golf ball will be better.

[0084] The mass ratio of component (e) to component (d) (component (e) / component (d)) is preferably 1.0 or greater, more preferably 1.5 or greater, even more preferably 2.0 or greater, and is preferably 100 or less, more preferably 90 or less, even more preferably 80 or less, even more preferably 40 or less, and particularly preferably 20 or less. When the mass ratio (component (e) / component (d)) is within the above range, the recoil effect on driver shots is increased, further improving the flight distance.

[0085] (f) Metal compounds When the core rubber composition contains only an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as a co-crosslinking agent, it preferably further contains (f) a metal compound. This is because, by neutralizing the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the core rubber composition with a metal compound, substantially the same effects as when a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used as a co-crosslinking agent. Note that, when an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and its metal salt are used in combination as a co-crosslinking agent, (f) a metal compound may be used as an optional component.

[0086] The (f) metal compound is not particularly limited as long as it can neutralize the (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the core rubber composition. 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 (f) 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 crosslinks. Furthermore, the use of zinc compounds can result in golf balls with high resilience.

[0087] 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.

[0088] (g) Carboxylic acid and / or its salt The core rubber composition may contain (g) a carboxylic acid and / or a salt thereof. By containing the (g) carboxylic acid and / or a salt thereof, the outer hardness / inner softness of the resulting spherical core can be increased. Examples of the (g) carboxylic acid and / or a salt thereof include aliphatic carboxylic acids, aliphatic carboxylic acid salts, aromatic carboxylic acids, and aromatic carboxylic acid salts. The (g) carboxylic acid and / or a salt thereof can be used alone or as a mixture of two or more kinds.

[0089] The number of carbon atoms in the carboxylic acid is preferably 1 or more and 30 or less, more preferably 18 or less, and even more preferably 13 or less. (g) Carboxylic acid and / or salt thereof does not include (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and / or metal salt thereof used as a co-crosslinking agent.

[0090] Examples of the carboxylic acid and / or its salt include saturated aliphatic carboxylic acids such as caprylic acid (octanoic acid), pelargonic acid (nonanoic acid), capric acid (decanoic acid), lauric acid, myristic acid, palmitic acid, stearic acid, and behenic acid; unsaturated aliphatic carboxylic acids such as palmitoleic acid, oleic acid, linoleic acid, and arachidonic acid; aromatic carboxylic acids such as benzoic acid, butylbenzoic acid, anisic acid (methoxybenzoic acid), dimethoxybenzoic acid, trimethoxybenzoic acid, dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, acetoxybenzoic acid, biphenylcarboxylic acid, naphthalenecarboxylic acid, anthracenecarboxylic acid, furancarboxylic acid, and thenoic acid; and potassium, magnesium, calcium, aluminum, zinc, iron, copper, nickel, and cobalt salts of these carboxylic acids. Among these, aromatic carboxylic acids and / or their salts are preferred, and carboxylic acids and / or their salts having a benzene ring are more preferred.

[0091] The content of the (g) carboxylic acid and / or its salt is, for example, preferably at least 0.5 part by weight, more preferably at least 1.0 part by weight, and even more preferably at least 1.5 parts by weight, per 100 parts by weight of the (a) base rubber, and is preferably at most 40 parts by weight, more preferably at most 35 parts by weight, and even more preferably at most 30 parts by weight. If the content of component (g) is 0.5 part by weight or more, the spherical core's outer hardness / inner softness ratio will be increased, and if it is 40 parts by weight or less, a decrease in core hardness will be suppressed, resulting in good resilience.

[0092] The core rubber composition may contain additives such as pigments, fillers for adjusting the weight, peptizers, softeners, etc., as required.

[0093] 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 (a) base rubber. A filler content of 30 parts by weight or less provides good resilience.

[0094] 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.

[0095] The core rubber composition can be prepared by mixing and kneading the raw materials. The kneading method is not particularly limited, and may be carried out using a known kneading machine such as a kneading roll, a Banbury mixer, or a kneader.

[0096] [Cover, mid-layer] The golf ball has a cover that encases a spherical core, and the cover is the outermost layer of the golf ball body excluding the paint film.

[0097] The material hardness of the cover composition constituting the cover is preferably set appropriately depending on the desired performance of the golf ball. For example, in the case of a distance-type golf ball that emphasizes distance, the material hardness of the cover composition is preferably 50 or more, more preferably 55 or more, even more preferably 60 or more, and preferably 80 or less, more preferably 70 or less, and even more preferably 68 or less, in Shore D hardness. By making the material hardness of the cover composition 50 or more, a golf ball with a high launch angle and low spin rate can be obtained on driver shots and iron shots, further improving the distance. Furthermore, by making the material hardness of the cover composition 80 or less, a golf ball with excellent durability can be obtained. Furthermore, in the case of a spin-type golf ball that emphasizes controllability, the material hardness of the cover composition is preferably less than 50, more preferably 48 or less, even more preferably 45 or less, preferably 20 or more, more preferably 25 or more, and even more preferably 30 or more, in Shore D hardness. If the cover composition has a Shore D hardness of less than 50, the spin rate on approach shots will be increased, resulting in a golf ball that stops easily on the green. Furthermore, by setting the material hardness to 20 or more, the abrasion resistance will be improved. The cover material hardness is a slab hardness measured by molding the cover composition into a sheet.

[0098] 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 cover thickness is 4.0 mm or less, the resilience and shot feel of the resulting golf ball will be better. 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 cover thickness is 0.3 mm or more, the impact durability and abrasion resistance of the cover will be improved.

[0099] The golf ball may have an intermediate layer between the spherical core and the cover, which may be a single layer or two or more layers, but is preferably a single layer.

[0100] The material hardness of the intermediate layer composition constituting the intermediate layer is preferably 55 or more, more preferably 57 or more, even more preferably 59 or more, and preferably 74 or less, more preferably 72 or less, even more preferably 70 or less, on the Shore D hardness scale. An intermediate layer having a material hardness of 55 or more further reduces the spin rate on driver shots and improves distance, while a material hardness of 74 or less provides good durability. When the golf club has two or more intermediate layers, it is preferable that the material hardness of the composition constituting the outermost intermediate layer be within the above-mentioned range. The material hardness of the intermediate layer is a slab hardness measured by molding the intermediate layer composition into a sheet. When multiple intermediate layers are used, the material hardness of each layer may be the same or different, but it is preferable that the hardness of all intermediate layers be within the above-mentioned range.

[0101] The thickness of the intermediate layer is preferably 0.8 mm or more, more preferably 0.9 mm or more, and even more preferably 1.0 mm or more, and is preferably 4.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less. A thickness of 0.8 mm or more of the intermediate layer provides good impact durability, and a thickness of 4.0 mm or less provides a good feel. When the golf club has two or more intermediate layers, it is preferable that the thickness of the outermost intermediate layer be within the above range.

[0102] It is preferable that the thickness (mm) and material hardness (Shore D) of the intermediate layer and the thickness (mm) and material hardness (Shore D) of the cover satisfy formula (21). By satisfying formula (21), spin performance on approach shots is improved, and the recoil effect on driver shots is increased, further reducing the amount of spin. {(intermediate layer thickness × material hardness) / (cover thickness × material hardness)} ≥ 4.0 (21)

[0103] The ratio ((intermediate layer thickness × material hardness) / (cover thickness × material hardness)) is preferably 4.0 or greater, more preferably 4.5 or greater, and even more preferably 5.0 or greater, and is preferably 10.0 or less, more preferably 9.0 or less, and even more preferably 8.0 or less.

[0104] The cover and intermediate layer are preferably formed from a cover composition and intermediate layer 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 (registered trademark)," a thermoplastic polyamide elastomer commercially available from Arkema K.K. under the trade name "Pebax (registered trademark)," a thermoplastic polyester elastomer commercially available from DuPont-Toray Co., Ltd. under the trade name "Hytrel (registered trademark)," or a thermoplastic styrene elastomer commercially available from Mitsubishi Chemical Corporation under the trade name "TEFABLOC (registered trademark)."

[0105] 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.

[0106] The cover composition 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.

[0107] The intermediate layer composition preferably contains 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 ionomer resin in the resin component of the intermediate layer composition is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more.

[0108] In addition to the resin components described above, the cover composition and intermediate layer 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.

[0109] 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 is 10 parts by mass or less, the durability of the resulting cover is good.

[0110] The method for forming the intermediate layer is not particularly limited, but examples include a method in which the intermediate layer composition is first molded into a hemispherical half shell, two of which are used to encase a spherical core and then pressure-molded, or a method in which the intermediate layer composition is directly injection-molded onto the spherical core to encase the sphere.

[0111] Examples of methods for molding the cover include a method in which a hollow shell is molded from a cover composition, and a sphere (a sphere with a spherical core or intermediate layer formed thereon) is coated with multiple shells and compression molded (preferably a method in which a hollow half shell is molded from a cover composition, and the sphere is coated with two half shells and compression molded), or a method in which the cover composition is directly injection molded onto a sphere.

[0112] 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 200 to 500, the size of each dimple can be increased, thereby enhancing 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.

[0113] The golf ball body with the molded cover is removed from the mold, and is preferably subjected to surface treatment such as deburring, cleaning, and sandblasting, as required.

[0114] If desired, a coating film or markings can also be formed. The thickness of the coating film is not particularly limited, but is preferably 5 μm or more, more preferably 6 μm or more, even 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. If the thickness is 5 μm or more, the coating film is less likely to wear away even with continuous use, and if the thickness is 50 μm or less, the effect of the dimples is fully obtained, improving the flight performance of the golf ball.

[0115] [Golf balls] Examples of the golf ball of the present invention include a two-piece golf ball consisting of a spherical core and a single-layer cover enclosing the spherical core, a three-piece golf ball having a spherical core, a single-layer intermediate layer enclosing the spherical core, and a single-layer cover enclosing the intermediate layer, and a multi-piece golf ball having a spherical core, two or more intermediate layers enclosing the spherical core, and a single-layer cover enclosing the intermediate layer. The present invention can be suitably used with golf balls of any of the above structures.

[0116] The diameter of the golf ball is preferably 40 mm to 45 mm. From the viewpoint of meeting the United States Golf Association (USGA) standards, 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. The mass of the golf ball is preferably 40 g or more and 50 g or less. From the viewpoint of obtaining high inertia, a mass of 44 g or more is more preferred, and 45.00 g or more is particularly preferred. From the viewpoint of meeting the USGA standards, a mass of 45.93 g or less is particularly preferred.

[0117] When the golf ball has 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 2.0 mm or more, more preferably 2.1 mm or more, and even more preferably 2.2 mm or more, and is preferably 3.0 mm or less, more preferably 2.9 mm or less, and even more preferably 2.8 mm or less. Golf balls with a compressive deformation of 2.0 mm or more have a good feel at impact. On the other hand, by setting the compressive deformation to 3.0 mm or less, the resilience is improved.

[0118] When the golf ball has a mid layer, it is preferable that the surface hardness (C8) of the spherical core, the surface hardness of the mid layer, and the surface hardness of the ball satisfy formula (20) in Shore C hardness. Core surface hardness <Mid layer surface hardness> Ball surface hardness (20)

[0119] An example of a golf ball of the present invention is shown in Figure 1. Figure 1 is a partially cutaway cross-sectional view showing golf ball 1 according to one embodiment of the present invention. Golf ball 1 has a core 2, a mid layer 3 encasing core 2, and a cover 4 encasing mid layer 3. A large number of dimples 41 are formed on the surface of cover 4. The portion of the surface of this golf ball other than dimples 41 is land 42. Golf ball 1 has a paint layer and a mark layer on the outside of cover 4, but these layers are not shown in the figure. [Example]

[0120] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the following examples, and all modifications and embodiments that do not deviate from the spirit of the present invention are included within the scope of the present invention.

[0121] [Evaluation method] (1) Compression deformation (mm) The amount of deformation in the compression direction (the amount by which the spherical core or golf ball shrinks in the compression direction) was measured when an initial load of 98 N was applied to the spherical core or golf ball and a final load of 1275 N was applied.

[0122] (2) Core hardness (Shore C hardness) The hardness measured at the surface of the core was taken as the core surface hardness. The core was cut into a hemisphere, and the hardness was measured at the center of the cut surface and at a specified distance in the radial direction from the center. The center of the core was taken as 0% and the surface as 100%. The core hardness was calculated by measuring the hardness at four points at specified distances from the center of the core cross section and averaging these values. The hardness was measured using an automatic hardness tester (DigiTest II, manufactured by H. Burleith Co.). The detector used was "Shore C."

[0123] (3) Golf ball surface hardness, intermediate layer surface hardness The hardness measured on the land portion of the surface of the golf ball was defined as the ball surface hardness. The hardness measured on the surface of an intermediate layer-coated sphere, which had an intermediate layer formed on the surface of a spherical core, was defined as the intermediate layer surface hardness. The hardness was measured using an automatic hardness tester (DigiTest II, manufactured by H. Burleith Co.). A "Shore C" detector was used.

[0124] (4) Material hardness (Shore D hardness) The intermediate layer composition and cover composition were injection molded into sheets approximately 2 mm thick and stored at 23°C for two weeks. Three or more of these sheets were stacked to avoid the influence of the measurement substrate, and their hardness was measured using an automatic hardness tester (DigiTest II, manufactured by H. Burleith Co.). A Shore D detector was used.

[0125] (5) Driver shot test A driver (Sumitomo Rubber Industries, Ltd., "SRIXON ZX7," shaft hardness: S, loft angle: 10.5°) was attached to a Golf Laboratory swing machine. The impact point was set at the face center. Golf balls were hit at a head speed of 50 m / sec, and the ball velocity immediately after impact, spin rate, and flight distance (distance from the launch point to the landing point) were measured. Measurements were performed 12 times for each golf ball, and the average values ​​were used as the measurement value for that golf ball. The initial velocity, spin rate, and flight distance of each golf ball in Tables 5 and 6 are shown as the difference from golf ball No. 6.

[0126] (6) Middle iron test An iron (Sumitomo Rubber Industries, "SRIXON ZX7", club number: #7, loft angle: 32°) was attached to a Golf Laboratory swing machine. The impact point was set at the face center. The golf balls were hit at a head speed of 39 m / sec, and the spin rate immediately after impact was measured. Measurements were made 12 times for each golf ball, and the average value was used as the measured value for that golf ball. The spin rate of each golf ball in Tables 5 and 6 is shown as the difference from golf ball No. 6.

[0127] [Manufacturing golf balls] (1) Preparation of rubber composition The raw materials were kneaded with a kneading roll so as to obtain the composition shown in Table 1, thereby obtaining a rubber composition.

[0128] [Table 1]

[0129] The materials used in Table 1 are as follows: BR730: High cis polybutadiene rubber (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) ZN-DA90S: Zinc acrylate (containing 10% zinc stearate), manufactured by Nisshoku Techno Fine Chemical Co., Ltd. Zinc oxide: "Ginrei R" manufactured by Toho Zinc Co., Ltd. Barium sulfate: Sakai Chemical Industry Co., Ltd., "Barium Sulfate BD" Benzoic acid: Emerald Kalama Chemical 4-Methoxyphenol: Tokyo Chemical Industry Co., Ltd. PBDS: Bis(pentabromophenyl) disulfide manufactured by Kawaguchi Chemical Industry Co., Ltd. DPDS: Diphenyl disulfide, manufactured by Sumitomo Seika Chemicals Dicumyl peroxide: manufactured by Tokyo Chemical Industry Co., Ltd.

[0130] (2) Preparation of intermediate layer composition The raw materials were extruded using a twin-screw kneading extruder so as to have the composition shown in Table 2, to prepare pellets of a composition for an intermediate layer.

[0131] [Table 2] Surlyn® 8150: Sodium ion-neutralized ethylene-methacrylic acid copolymer ionomer resin, manufactured by DuPont Himilan (registered trademark) AM7329: Sodium ion-neutralized ethylene-methacrylic acid copolymer ionomer resin, manufactured by DuPont-Mitsui Polychemicals Co., Ltd. Titanium dioxide: Ishihara Sangyo Kaisha, Ltd., A-220

[0132] (3) Preparation of Cover Composition The raw materials were extruded using a twin-screw kneading extruder so as to have the composition shown in Table 3, to prepare pellets of a cover composition.

[0133] [Table 3] Elastollan (registered trademark) NY84A: BASF Japan, thermoplastic polyurethane elastomer Tinuvin (registered trademark) 770: BASF Japan, hindered amine light stabilizer Titanium dioxide: Ishihara Sangyo Kaisha, Ltd., A-220

[0134] (4) Core fabrication Golf balls No. 1-6, 9 The rubber compositions shown in Table 4 were hot-pressed in upper and lower molds having hemispherical cavities to obtain spherical cores. An appropriate amount of barium sulfate was added so that the mass of the resulting golf ball would be 45.6 g.

[0135] Golf balls No. 7 and 8 The rubber composition (inner layer formulation) shown in Table 4 was hot-pressed in upper and lower molds having hemispherical cavities to obtain an inner core layer. Next, half shells were molded using the rubber composition (outer layer formulation) shown in Table 4. The inner core layer was covered with these two half shells. The inner core layer and half shells were hot-pressed in upper and lower molds both having hemispherical cavities to obtain a spherical core.

[0136] (5) Formation of the mid-layer and cover The intermediate layer composition was injection molded onto a spherical core to obtain an intermediate layer-coated sphere. The obtained intermediate layer-coated sphere was placed in a final mold with numerous dimples on the cavity surface. Half shells were obtained from the cover composition by compression molding. Two half shells were placed on the intermediate layer-coated sphere placed in the final mold to coat it, resulting in a golf ball with a cover having numerous dimples with shapes that were the inverse of the dimples on the cavity surface. The evaluation results of the obtained golf balls are shown in Tables 5 and 6.

[0137] [Table 4]

[0138] [Table 5]

[0139] [Table 6]

[0140] Golf balls Nos. 1 to 4 have a spherical core with a hardness distribution in which the difference in hardness (C1-C0), (C2-C1), (C3-C2), and (C4-C3) is greater than 0 and not greater than 6.0, the difference in hardness (C5-C4) is 5.0 or greater, the difference in hardness (C6-C5), (C7-C6), and (C8-C7) is greater than 0 and not greater than 3.5, and the difference {(C5-C4) - (C4-C3)} is 1.0 or greater. Golf ball No. 6 has a spherical core with a hard outer core and a soft inner core, with a hardness gradient that is substantially linear from the center to the surface (the difference in hardness (C5-C4) is less than 5). Compared to golf ball No. 6, golf balls Nos. 1 to 4 have improved driver shot distances and improved spin rates on middle iron shots.

[0141] The present invention (1) is a golf ball having a spherical core and a cover enclosing the spherical core, characterized in that when a line extending from the center of the spherical core to the surface is divided into eight equal parts, the center hardness (C0), the hardness (C1) at a point 12.5% ​​from the center, the hardness (C2) at a point 25.0% from the center, the hardness (C3) at a point 37.5% from the center, the hardness (C4) at a point 50.0% from the center, the hardness (C5) at a point 62.5% from the center, the hardness (C6) at a point 75.0% from the center, the hardness (C7) at a point 87.5% from the center, and the surface hardness (C8) of the spherical core satisfy the following requirements in Shore C hardness: 0<(C1-C0)≦6.0, 0<(C2-C1)≦6.0, 0<(C3-C2)≦6.0, 0<(C4-C3)≦6.0, 5.0≦(C5-C4), 0<(C6-C5)≦3.5, 0<(C7-C6)≦3.5, 0<(C8-C7)≦3.5, 1.0≦{(C5-C4)-(C4-C3)}

[0142] Invention (2) is the golf ball according to invention (1), wherein the spherical core has hardnesses (C4), (C5) and (C6) that satisfy the following relationship in Shore C hardness: 1.0≦{(C5-C4)-(C6-C5)}

[0143] The present invention (3) is a golf ball according to the present invention (1) or (2), wherein the spherical core has hardnesses (C0), (C4), (C5), and (C8) that satisfy the following relationship in Shore C hardness: (C4-C0) > (C5-C4) > (C8-C5)

[0144] The present invention (4) is characterized in that the spherical core has, in Shore C hardness, the hardness difference between hardness (C1) and hardness (C0) (C1-C0), the hardness difference between hardness (C2) and hardness (C1) (C2-C1), the hardness difference between hardness (C3) and hardness (C2) (C3-C2), the hardness difference between hardness (C4) and hardness (C3) (C4-C3), the hardness difference between hardness (C5) and hardness (C4) (C5-C4), the hardness difference between hardness (C6) and hardness (C7) (C7-C8), the hardness difference between hardness (C7) and hardness (C8) (C8-C9), the hardness difference between hardness (C8) and hardness (C9) (C9-C10), the hardness difference between hardness (C9) and hardness (C10) (C1-C11), the hardness difference between hardness (C10) and hardness (C11) (C1-C12), the hardness difference between hardness (C11) and hardness (C12) (C1-C13), the hardness difference between hardness (C12) and hardness (C13) (C1-C14), the hardness difference between hardness (C12) and hardness (C13) (C1-C15), the hardness difference between hardness (C12) and hardness (C14) (C1-C16), the hardness difference between hardness (C13) and hardness (C14) (C1-C17), the hardness difference between hardness (C14) and hardness (C15) (C1-C18), the hardness difference between hardness (C14) and hardness (C15) (C1-C19), the hardness difference between hardness (C15) and hardness (C16) (C1-C19), the hardness difference between hardness (C The golf ball according to any one of present inventions (1) to (3) has a ratio (Cbmax / Cbmin) of 4.0 or greater, where Cbmax is the largest value among the hardness difference (C6-C5) between the hardness (C5), the hardness difference (C7-C6) between the hardness (C7) and the hardness (C6), and the hardness difference (C8-C7) between the hardness (C8) and the hardness (C7).

[0145] The present invention (5) is the golf ball according to any one of the present inventions (1) to (4), wherein the spherical core has hardnesses (C0) and (C8) that satisfy the following relationship in Shore C hardness: 18.0≦(C8-C0)

[0146] The present invention (6) is the golf ball according to any one of the present inventions (1) to (5), which has an intermediate layer between the spherical core and the cover, and in which the surface hardness (C0) of the spherical core, the surface hardness of the intermediate layer, and the surface hardness of the ball satisfy the following relationship in Shore C hardness: Core surface hardness <Mid layer surface hardness> Ball surface hardness

[0147] The present invention (7) is the golf ball according to any one of the present inventions (1) to (6), which has an intermediate layer between the spherical core and the cover, and the thickness (mm) and material hardness (Shore D) of the intermediate layer, and the thickness (mm) and material hardness (Shore D) of the cover satisfy the following relationship: (Intermediate layer thickness x Material hardness) / (Cover thickness x Material hardness) ≥ 4.0

[0148] The present invention (8) is the golf ball according to any one of the present inventions (1) to (7), wherein the amount of compressive deformation when a final load of 1275 N is applied from an initial load of 98 N is 2.8 mm or less.

[0149] The present invention (9) is a golf ball according to any one of the present inventions (1) to (8), 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, (c) a crosslinking initiator, and (d) a monophenol compound having a substituent only at the p-position.

[0150] The present invention (10) is the golf ball according to the present invention (9), wherein the core rubber composition contains (d) 0.05 to 2.0 parts by mass of the monophenol compound having a substituent only at the p-position per 100 parts by mass of the (a) base rubber.

[0151] Invention (11) is the golf ball according to invention (9) or (10), wherein the (d) monophenol compound having a substituent only at the p-position is represented by general formula (1).

[0152] [ka] [In general formula (1), R represents an alkoxy group, a halogen group, a hydrocarbon group, a nitro group, a cyano group, an amino group, or a hydroxy group.]

[0153] The present invention (12) is the golf ball according to any one of the present inventions (9) to (11), wherein the core rubber composition further contains (g) an aromatic carboxylic acid and / or a salt thereof. [Explanation of symbols]

[0154] 1: Golf ball, 2: Spherical core, 3: Mid layer, 4: Cover, 41: Dimples, 42: Land

Claims

1. A golf ball having a spherical core and a cover enclosing the spherical core, the spherical core has a single-layer structure, When a line extending from the center of the spherical core to the surface is divided into eight equal parts, the center hardness (C0), the hardness (C1) at a point 12.5% ​​from the center, the hardness (C2) at a point 25.0% from the center, the hardness (C3) at a point 37.5% from the center, the hardness (C4) at a point 50.0% from the center, the hardness (C5) at a point 62.5% from the center, the hardness (C6) at a point 75.0% from the center, the hardness (C7) at a point 87.5% from the center, and the surface hardness (C8) of the spherical core satisfy the following relationships in Shore C hardness: A golf ball characterized in that, when Cbmax is the largest value among the hardness difference (C1-C0) between hardness (C1) and hardness (C0), the hardness difference (C2-C1) between hardness (C2) and hardness (C1), the hardness difference (C3-C2) between hardness (C3) and hardness (C2), the hardness difference (C4-C3) between hardness (C4) and hardness (C3), the hardness difference (C5-C4) between hardness (C5) and hardness (C4), the hardness difference (C6-C5) between hardness (C6) and hardness (C5), the hardness difference (C7-C6) between hardness (C7) and hardness (C6), and the hardness difference (C8-C7) between hardness (C8) and hardness (C7), the hardness difference (C5-C4) is Cbmax. 0<(C1-C0)≦6.0, 0<(C2-C1)≦6.0, 0<(C3-C2)≦6.0, 0<(C4-C3)≦6.0, 5.0≦(C5-C4)≦12.0, 0<(C6-C5)≦3.0, 0<(C7-C6)≦3.0, 0<(C8-C7)≦3.0, 1.0≦{(C5-C4)-(C4-C3)}≦10.0 6.0≦(C4-C0)≦20.0 18.0≦(C8-C0)≦32.0 50.0≦C0≦70.0 70.0≦C8≦90.0

2. 2. The golf ball according to claim 1, wherein the spherical core has hardnesses (C4), (C5) and (C6) that satisfy the following relationship in Shore C hardness: 1.0≦{(C5-C4)-(C6-C5)}

3. 3. The golf ball according to claim 1, wherein the spherical core has hardnesses (C0), (C4), (C5) and (C8) that satisfy the following relationship in Shore C hardness: (C4-C0)>(C5-C4)>(C8-C5)

4. 2. The golf ball according to claim 1, wherein the spherical core has a ratio (Cbmax / Cbmin) of 4.0 or greater, where Cbmax is the largest value among the Shore C hardness difference between the hardness (C1) and the hardness (C0), (C1-C0), the hardness difference between the hardness (C2) and the hardness (C1), (C2-C1), the hardness difference between the hardness (C3) and the hardness (C2), (C3-C2), the hardness difference between the hardness (C4) and the hardness (C3), (C4-C3), the hardness difference between the hardness (C5) and the hardness (C4), (C5-C4), the hardness difference between the hardness (C6) and the hardness (C5), (C6-C5), the hardness difference between the hardness (C7) and the hardness (C6), and (C8-C7) is the smallest value.

5. 2. The golf ball according to claim 1, wherein the spherical core has hardnesses (C0), (C2), (C4) and (C8) that satisfy the following relationship in Shore C hardness: {(C8-C4) / (C2-C0)}≦3.0

6. 2. The golf ball according to claim 1, further comprising an intermediate layer between the spherical core and the cover, wherein the surface hardness (C8) of the spherical core, the surface hardness of the intermediate layer, and the surface hardness of the ball satisfy the following relationship in Shore C hardness: Core surface hardness <Mid layer surface hardness> Ball surface hardness

7. 2. The golf ball according to claim 1, further comprising an intermediate layer between the spherical core and the cover, wherein the thickness (mm) and material hardness (Shore D) of the intermediate layer, and the thickness (mm) and material hardness (Shore D) of the cover satisfy the following relationship: {(intermediate layer thickness×material hardness) / (cover thickness×material hardness)}≧4.0

8. 2. The golf ball according to claim 1, wherein the golf ball undergoes a compressive deformation of 2.8 mm or less when an initial load of 98 N is applied and a final load of 1275 N is applied.

9. 2. The golf ball according to claim 1, 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; (c) a crosslinking initiator; and (d) a monophenol compound having a substituent only at the p-position.

10. 10. The golf ball according to claim 9, wherein the core rubber composition contains, per 100 parts by weight of the base rubber, (d) 0.05 to 2.0 parts by weight of the monophenol compound having a substituent only at the p-position.

11. 11. The golf ball according to claim 9, wherein the (d) monophenol compound having a substituent only at the p-position is represented by general formula (1). 【Chemistry 1】 [In general formula (1), R represents an alkoxy group, a halogen group, a hydrocarbon group, a nitro group, a cyano group, an amino group, or a hydroxy group.]

12. 10. The golf ball according to claim 9, wherein the core rubber composition further contains (g) an aromatic carboxylic acid and / or a salt thereof.

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