golf balls

Abstract

To provide a golf ball that has a dominant flight distance performance in the iron full shot, has a high spin performance in the short game, and gives a very soft hitting feeling and is superior in breakage durability in the repetitive hitting.SOLUTION: A golf ball is made by interposing a single-layered rubber core 1 and an intermediate layer 2 of 1-layer between a single-layered resin cover 3 and itself. In the golf ball, a surface hardness relationship between a sphere obtained by covering the core with the intermediate layer (intermediate layer-covering sphere) and the ball satisfies the following expression (surface hardness of the intermediate layer-covering sphere)>(surface hardness of the ball), and satisfies the following 2-expressions (Atti compression of the core) / (core surface shore C hardness - core center shore C hardness)≤1.1 (surface shore C hardness of the intermediate layer-covering sphere) / {(core surface shore C hardness - core center shore C hardness)×thickness (mm) of the intermediate layer}≤2.7.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a golf ball having a three-layer structure with a single-layer core, a single-layer cover, and one intermediate layer sandwiched between them, and particularly to a golf ball that satisfies the needs of advanced amateur golfers. [Background technology]

[0002] Among amateur golfers, there are many advanced-level amateur golfers who are skilled at iron shots. Achieving advantageous distance with full shots using not only a driver (W#1) but also long and middle irons is an important element in playing golf. In addition to advantageous distance on full iron shots, a golf ball must also provide high spin performance during the short game, an extremely soft feel, and excellent durability during repeated shots to fully satisfy the needs of advanced amateur golfers.

[0003] The following Patent Documents 1 to 5 are three-piece golf balls for so-called spin golf balls in which the intermediate layer is harder than the cover, and the cover layer is formed mainly of polyurethane. However, even the golf balls described in the above Patent Documents were not sufficient to satisfy all of the requirements for distance, feel, and durability against repeated shots on full iron shots. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-120898 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-112308 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-000183 [Patent Document 4] Japanese Patent Application Laid-Open No. 2017-000470 [Patent Document 5] Japanese Patent Publication No. 2020-175021 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made in consideration of the above circumstances, and provides a golf ball that has excellent distance performance on full iron shots, high spin performance in short games, gives a very soft hitting feel, and is excellent in crack resistance when hit repeatedly. [Means for solving the problem]

[0006] As a result of extensive research into achieving the above object, the inventors of the present invention have found that, for a golf ball having one intermediate layer interposed between a single-layer rubber core and a single-layer resin cover, the relationship between the surface hardness of a sphere in which the core is covered with an intermediate layer (an intermediate layer-covered sphere) and the surface hardness of the ball satisfies the relationship (surface hardness of the intermediate layer-covered sphere) > (surface hardness of the ball), and satisfies the following two formulas: (Core Atti Compression) / (Core Surface Shore C Hardness - Core Center Shore C Hardness)≦1.1, and (Surface hardness of intermediate layer coated sphere) / {(Core surface Shore C hardness - Core center Shore C hardness) × Intermediate layer thickness (mm)} ≦ 2.7 The inventors have found that by constructing a golf ball so as to satisfy the above requirements, advanced amateur golfers who are skilled mainly in iron shots can obtain superior distance performance on full iron shots, high spin performance in short games, a very soft feel on impact, and excellent crack resistance when hit repeatedly, which has led to the completion of the present invention.

[0007] In other words, the golf ball of the present invention is a golf ball that satisfies the needs of advanced amateur golfers, combining a relatively soft cover that allows for high-level spin control in short games, a hard intermediate layer that suppresses excessive spin on full iron shots, and a core with a hardness distribution that provides an extremely soft feel and excellent resistance to cracking during repeated hits.

[0008] In the present specification, the term "advanced amateur golfer" refers to a target range of golfers who hit a 6-iron with a head speed range of approximately 35 to 45 m / s and have a handicap of approximately 12 or less.

[0009] Accordingly, the present invention provides the following golf balls. 1. In a three-piece golf ball having a single rubber core, a single resin cover, and one intermediate layer sandwiched between them, the surface hardness relationship between a sphere in which the core is covered with an intermediate layer (an intermediate layer-covered sphere) and the ball satisfies the following formula: (Shore C hardness of the surface of the ball coated with the intermediate layer) > (Shore C hardness of the surface of the ball) and the following two equations are satisfied: 0.5≦ (Core Atti Compression) / (Core Surface Shore C Hardness - Core Center Shore C Hardness)≦1.1 (Shore C hardness of the surface of the intermediate layer-coated sphere) / {(Shore C hardness of the core surface - Shore C hardness of the core center) × thickness of the intermediate layer (mm)} ≦ 2.7 A golf ball characterized by satisfying the above. 2. In terms of core hardness distribution, if the Shore C hardness of the core surface is H100, the Shore C hardness at a position 87.5% of the core radius from the core center is H87.5, the Shore C hardness at a position 75% of the core radius from the core center is H75, the Shore C hardness at a position 50% of the core radius from the core center is H50, and the Shore C hardness at the core center is H0, then the following formula can be used: (H100-H0)≧24 1. A golf ball that satisfies the above requirement. 3.The following formula: (H100-H0) / (H50-H0)≧3.0 2. A golf ball as defined in claim 2 above that satisfies the above requirements. 4.The following formula: (H87.5-H75)-(H100-H87.5)≧-0.5 4. The golf ball according to claim 2 or 3 above, which satisfies the above condition. 5. The core comprises the following components (A) to (D): (A) Base rubber (B) Organic peroxide (C) Water and / or a metal salt of a carboxylic acid (D) Sulfur 5. The golf ball according to any one of 1 to 4 above, which is a hot-molded product of a rubber composition comprising: 6. The golf ball according to the above item 5, wherein the mass ratio of component (C) to component (D) is (D) / (C)=0.010 to 0.200. 7. When the lift coefficient measured under the conditions of a Reynolds number of 80,000 and a spin rate of 2,000 rpm is CL1, and the lift coefficient measured under the conditions of a Reynolds number of 70,000 and a spin rate of 1,900 rpm is CL2, CL1 and CL2 are calculated using the following formula: 0.950 ≦ CL2 / CL1 7. The golf ball according to any one of 1 to 6 above, which satisfies the above conditions. 8. When the lift coefficient measured under the conditions of a Reynolds number of 200,000 and a spin rate of 2,500 rpm is CL3, and the lift coefficient measured under the conditions of a Reynolds number of 120,000 and a spin rate of 2,250 rpm is CL4, CL3 and CL4 can be calculated by the following formula: 1.250 ≦ CL4 / CL3 ≦ 1.300 8. The golf ball according to any one of the above items 1 to 7, which satisfies the above conditions. [Effects of the Invention]

[0010] The golf ball of the present invention has excellent distance performance on full iron shots, high spin performance during short games, a very soft feel on impact, and excellent crack resistance on repeated impacts, and is a ball that particularly satisfies the needs of advanced amateur golfers who are skilled in iron shots. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic cross-sectional view of a golf ball according to one embodiment of the present invention. [Figure 2] FIG. 2 is a plan view showing the dimple arrangement (Type-A) used in the examples and comparative examples. [Figure 3] FIG. 2 is a plan view showing the dimple arrangement (Type-B) used in the examples and comparative examples. [Figure 4] 1 is a graph showing the core hardness distribution of Examples 1 to 4. [Figure 5] 1 is a graph showing the core hardness distribution of Comparative Examples 1 to 6. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in more detail below. The golf ball of the present invention has a single-layer core, an intermediate layer, and a single-layer cover, an example of which is shown in FIG. 1. The golf ball G shown in FIG. 1 has a single-layer core 1, a single-layer intermediate layer 2 encasing the core 1, and a single-layer cover 3 encasing the intermediate layer. This cover 3 is the outermost layer in the layer structure of the golf ball, excluding the paint layer. As shown in FIG. 1, the intermediate layer is formed as a single layer. A large number of dimples D are usually formed on the surface of the cover (outermost layer) 3 to improve aerodynamic characteristics. Although not specifically shown, a paint layer is usually formed on the surface of the cover 3. Each of the above layers will be described in detail below.

[0013] The core is primarily made of rubber, and the rubber composition can be prepared by blending a base rubber with a co-crosslinking agent, an organic peroxide, an inert filler, an organic sulfur compound, etc.

[0014] The core used in the present invention comprises the following components (A) to (D): (A) Base rubber (B) Organic peroxide (C) Water and / or a metal salt of a carboxylic acid (D) Sulfur It is preferable that the rubber composition is a hot-molded product of a rubber composition containing the above.

[0015] Polybutadiene is preferably used as the base rubber (A). Commercially available polybutadiene products can be used, such as BR01, BR51, BR730, and T0700 (manufactured by JSR Corporation). The proportion of polybutadiene in the base rubber is preferably 60% by mass or more, and more preferably 80% by mass or more. Rubber components other than the polybutadiene may be blended into the base rubber within a range that does not impair the effects of the present invention. Examples of rubber components other than polybutadiene include polybutadienes other than the polybutadienes listed above, and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber, and ethylene-propylene-diene rubber.

[0016] As the (B) organic peroxide, it is preferable to use an organic peroxide with a relatively high thermal decomposition temperature. Specifically, an organic peroxide with a 1-minute half-life temperature of approximately 165 to 185°C is used, and examples thereof include dialkyl peroxides. Examples of dialkyl peroxides include dicumyl peroxide (NOF Corp.'s "Percumyl D"), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (NOF Corp.'s "Perhexa 25B"), and di(2-t-butylperoxyisopropyl)benzene (NOF Corp.'s "Perbutyl P"), with dicumyl peroxide being preferred. These may be used alone or in combination. The half-life is an index of the decomposition rate of an organic peroxide and is expressed as the time required for the original organic peroxide to decompose and reduce its active oxygen content by half. The vulcanization temperature for the core rubber composition is usually within the range of 120 to 190°C, and within this range, organic peroxides, which have a high one-minute half-life temperature of approximately 165 to 185°C, thermally decompose relatively slowly. With the rubber composition used in the present invention, it is possible to obtain a core, which is a cross-linked rubber product having a specific internal hardness profile, as described below, by adjusting the amount of free radicals produced, which increases with the passage of vulcanization time.

[0017] The water (C) is not particularly limited and may be distilled water or tap water, but it is particularly preferred to use distilled water that does not contain impurities. The amount of water blended is preferably 0.1 part by weight or more, more preferably 0.2 part by weight or more, per 100 parts by weight of the base rubber, and the upper limit is preferably 2 parts by weight or less, more preferably 1 part by weight or less.

[0018] By directly blending water or a water-containing material as component (C) into the core material, the decomposition of organic peroxides during core compounding can be accelerated. It is known that the decomposition efficiency of organic peroxides in core rubber compositions varies with temperature, with the decomposition efficiency increasing as the temperature rises. If the temperature is too high, too many radicals are decomposed, leading to recombination and inactivation of the radicals. As a result, the number of radicals available for crosslinking is reduced. When the organic peroxide decomposes during core vulcanization and generates heat, the temperature near the core surface remains roughly the same as the vulcanization mold temperature. However, the temperature near the core center rises significantly higher than the mold temperature due to the accumulation of heat from the decomposition of the organic peroxide decomposed from the outside. When water or a water-containing material is directly blended into the core, the water promotes the decomposition of the organic peroxide, thereby altering the radical reaction described above between the core center and the core surface. That is, near the core center, the decomposition of the organic peroxide is further promoted, and the deactivation of radicals is further promoted, further reducing the amount of effective radicals. This makes it possible to obtain cores with significantly different crosslink densities between the core center and the core surface, and cores with different dynamic viscoelastic properties in the core center.

[0019] Alternatively, a monocarboxylate metal salt can be used instead of the water described above. It is believed that a carboxylic acid is coordinately bonded to the metal salt in a monocarboxylate metal salt, and it is distinct from dicarboxylate metal salts such as zinc diacrylate, which has the chemical formula [CH₂=CHCOO]₂Zn. A monocarboxylate metal salt introduces water into the rubber composition through a dehydration condensation reaction, thereby achieving the same effects as water. Furthermore, a monocarboxylate metal salt can be incorporated into the rubber composition as a powder, simplifying the processing steps and facilitating uniform dispersion throughout the rubber composition. To effectively carry out the reaction, a monosalt is required. The amount of the monocarboxylate metal salt is preferably 1 part by weight or more, more preferably 3 parts by weight or more, per 100 parts by weight of the base rubber. The upper limit of the amount of the monocarboxylate metal salt is preferably 60 parts by weight or less, more preferably 50 parts by weight or less. If the amount of the monocarboxylate metal salt is too small, it may be difficult to achieve an appropriate crosslink density, which may result in an insufficient golf ball spin reduction effect. On the other hand, if the blending amount is too high, the core becomes too hard, which may make it difficult to maintain an appropriate feel on impact.

[0020] Examples of the carboxylic acid that can be used include acrylic acid, methacrylic acid, maleic acid, fumaric acid, and stearic acid. Examples of the substituting metal include Na, K, Li, Zn, Cu, Mg, Ca, Co, Ni, and Pb, with Zn being preferred. Specific examples include zinc monoacrylate and zinc monomethacrylate, with zinc monoacrylate being particularly preferred.

[0021] Specific examples of (D) sulfur include products such as "Sunmix S-80N" (manufactured by Sanshin Chemical Industry Co., Ltd.) and "Sulfax-5" (manufactured by Tsurumi Chemical Industry Co., Ltd.). The amount of sulfur can be greater than 0, preferably at least 0.005 parts by weight, and more preferably at least 0.01 parts by weight, per 100 parts by weight of the base rubber. There is no upper limit to the amount of sulfur added, but it is preferably no more than 0.1 parts by weight, more preferably no more than 0.05 parts by weight, and even more preferably no more than 0.03 parts by weight. The addition of sulfur can increase the hardness difference in the core. However, if the amount of sulfur added is too high, the resilience may be significantly reduced and durability against repeated impacts may be reduced.

[0022] Regarding the blending ratio of the above-mentioned components (C) and (D), the mass ratio (D) / (C) is preferably 0.010 or more, more preferably 0.013 or more, and even more preferably 0.016 or more, with the upper limit being preferably 0.200 or less, more preferably 0.100 or less, and even more preferably 0.060 or less. Outside the above numerical range, it becomes difficult to achieve the desired core hardness distribution, and it may become impossible to achieve both advantageous distance due to low spin on full shots and good durability against repeated impacts. Note that the above-mentioned component (D) refers to the mass of the sulfur component contained in the product, not the mass of the sulfur product itself.

[0023] In addition to the above-mentioned components (A) to (D), the rubber composition may contain a co-crosslinking agent (E) and an inert filler (F), and may also contain an antioxidant or an organic sulfur compound as needed. These components are described in detail below.

[0024] Examples of the (E) co-crosslinking agent include unsaturated carboxylic acids and metal salts of unsaturated carboxylic acids. Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, and fumaric acid, with acrylic acid and methacrylic acid being particularly preferred. Metal salts of unsaturated carboxylic acids are not particularly limited, but include, for example, those obtained by neutralizing the above-mentioned unsaturated carboxylic acids with desired metal ions. Specific examples include zinc salts and magnesium salts of methacrylic acid, acrylic acid, and the like, with zinc acrylate being particularly preferred.

[0025] The unsaturated carboxylic acid and / or metal salt thereof is blended in an amount of typically 5 parts by weight or more, preferably 9 parts by weight or more, and more preferably 13 parts by weight or more, per 100 parts by weight of the base rubber, with the upper limit typically being 60 parts by weight or less, preferably 50 parts by weight or less, and more preferably 40 parts by weight or less. If the blended amount is too high, the ball may become too hard, resulting in an unbearable feel at impact, while if the blended amount is too low, the resilience may decrease.

[0026] Suitable examples of (F) inert fillers include zinc oxide, barium sulfate, and calcium carbonate. These may be used alone or in combination of two or more. The amount of inert filler blended is preferably at least 1 part by weight, more preferably at least 5 parts by weight, per 100 parts by weight of the base rubber, with the upper limit being preferably 50 parts by weight or less, more preferably 40 parts by weight or less, and even more preferably 36 parts by weight or less. If the blended amount is too high or too low, it may not be possible to obtain the appropriate weight and suitable resilience.

[0027] Furthermore, an antioxidant can be added as needed, and examples of commercially available products include Nocrac MB, Nocrac NS-6, Nocrac NS-30 (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), Yoshinox 425 (manufactured by Yoshitomi Pharmaceutical Co., Ltd.), etc. These may be used alone or in combination of two or more.

[0028] The amount of antioxidant blended, per 100 parts by weight of the base rubber, is preferably at least 0 part by weight, more preferably at least 0.05 part by weight, and particularly preferably at least 0.1 part by weight, with the upper limit being preferably at most 3 parts by weight, more preferably at most 2 parts by weight, particularly preferably at most 1 part by weight, and most preferably at most 0.5 parts by weight. If the blended amount is too high or too low, it may be impossible to obtain suitable rebound properties and durability.

[0029] Furthermore, an organic sulfur compound can be blended into the core to impart good resilience. The organic sulfur compound is not particularly limited as long as it can improve the resilience of the golf ball, and examples thereof include thiophenols, thionaphthols, halogenated thiophenols, and metal salts thereof. More specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, parachlorothiophenol, zinc salt of pentachlorothiophenol, zinc salt of pentafluorothiophenol, zinc salt of pentabromothiophenol, zinc salt of parachlorothiophenol, diphenyl polysulfide having 2 to 4 sulfur atoms, dibenzyl polysulfide, dibenzoyl polysulfide, dibenzothiazoyl polysulfide, and dithiobenzoyl polysulfide. The zinc salt of pentachlorothiophenol is particularly preferred. The amount of the organic sulfur compound is preferably at least 0 parts by weight, more preferably at least 0.05 parts by weight, and even more preferably at least 0.1 parts by weight, per 100 parts by weight of the base rubber, and is preferably no more than 5 parts by weight, more preferably no more than 3 parts by weight, and even more preferably no more than 2.5 parts by weight. If the amount is too high, the resilience improvement effect (especially on shots with a W#1 club) cannot be expected, the core may become too soft, or the feel may be poor. On the other hand, if the amount is too low, the resilience improvement effect cannot be expected.

[0030] The core can be produced by vulcanizing and curing a rubber composition containing the above components. For example, the core can be produced by kneading the rubber composition using a kneader such as a Banbury mixer or a roll, compression molding or injection molding using a core mold, and curing the molded product by appropriately heating the molded product at a temperature sufficient for the organic peroxide and co-crosslinking agent to act, 100 to 200°C, preferably 140 to 180°C, for 10 to 40 minutes.

[0031] In the present invention, the core is formed as a single layer. In the case of a rubber core made of multiple layers, peeling may occur at the interface when repeatedly hit, resulting in poor durability.

[0032] The core diameter is not particularly limited, but is preferably at least 37.1 mm, more preferably at least 37.5 mm, and even more preferably at least 37.9 mm, with the upper limit being preferably no greater than 39.7 mm, more preferably no greater than 39.1 mm, and even more preferably no greater than 38.7 mm. If the core diameter is too small, the initial velocity on full shots may be low, resulting in a failure to achieve the desired distance, or the feel may be too hard. On the other hand, if the core diameter is too large, the durability to repeated shots may be poor, or the spin rate may be too high on full shots, resulting in a failure to achieve the desired distance.

[0033] The core's Atti compression is not particularly limited, but is preferably 7 or greater, more preferably 12 or greater, and even more preferably 17 or greater, with an upper limit of preferably 42 or less, more preferably 36 or less, and even more preferably 30 or less. If the core's compression value is too high, the ball may spin too much, especially on full iron shots, resulting in a loss of distance and an excessively hard feel. On the other hand, if the core's Atti compression value is too low, the ball may feel too soft or may have poor crack resistance after repeated impacts. Atti compression has been widely used in the golf ball industry since the 1940s, and the same measuring device and method are also known as PGA compression. Atti compression is a numerical value, expressed in kilograms, of the load required to deflect the ball (or core, etc.) 0.1 inches (2.54 mm), with the minimum value being 0. Most golf balls on the market fall within a value of 140 or less. The above Atti compression is measured using an Atti Engineering ATTI compression tester, which is designed to measure spherical objects with a diameter of 42.7 mm (1.68 inches). Therefore, when measuring the compression of the core, since the core has a small diameter, a spacer is inserted between the pressure head and the core so that the (core diameter + spacer thickness) is 42.7 mm.

[0034] The core used in the present invention must satisfy the following formula: (Core Atti Compression) / (Core Surface Shore C Hardness - Core Center Shore C Hardness)≦1.1 In other words, a small value of this index means that the core compression (Atti) is small and the direction in which the difference in hardness between the surface and center of the core is large is suppressed. Also, a large value of this index means that the core compression (Atti) is large and the direction in which the difference in hardness between the surface and center of the core is small is suppressed. Specifically, the value of (core Atti compression) / (core surface Shore C hardness - core center Shore C hardness) is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more, with the upper limit being 1.1 or less, preferably 1.0 or less, and more preferably 0.9 or less. If this value is too large, spin may increase on full shots, resulting in a loss of distance, or the feel may be too hard. On the other hand, if this value is too small, crack resistance may be poor when hit repeatedly, or the feel may be too soft.

[0035] Next, the hardness distribution of the core will be described. Note that the hardness of the core described below refers to Shore C hardness. This Shore C hardness is a hardness value measured using a Shore C hardness tester conforming to the ASTM D2240 standard.

[0036] In the following description of the core hardness distribution, the Shore C hardness of the core surface is defined as H100, the Shore C hardness at a position 87.5% of the core radius from the core center as H87.5, the Shore C hardness at a position 75% of the core radius from the core center as H75, the Shore C hardness at a position 62.5% of the core radius from the core center as H62.5, the Shore C hardness at a position 50% of the core radius from the core center as H50, the Shore C hardness at a position 37.5% of the core radius from the core center as H37.5, the Shore C hardness at a position 25% of the core radius from the core center as H25, the Shore C hardness at a position 12.5% ​​of the core radius from the core center as H12.5, and the Shore C hardness at the core center as H0.

[0037] The surface hardness (H100) of the core is not particularly limited, but is preferably 75 or higher, more preferably 77 or higher, and even more preferably 79 or higher. The upper limit is also not particularly limited, but is preferably 91 or lower, more preferably 89 or lower, and even more preferably 87 or lower. If this value is too low, the resilience will be low, resulting in poor flight performance and poor durability to cracking after repeated impacts. On the other hand, if this value is too high, the feel will be hard, or spin will increase on full shots, making it difficult to achieve the desired flight distance.

[0038] The hardness (H87.5) at a position 87.5% outward from the center of the core radius is not particularly limited, but is preferably at least 71, more preferably at least 73, and even more preferably at least 75. There is no upper limit to this hardness, but it is preferably at most 84, more preferably at most 82, and even more preferably at most 80. Any deviation from these hardness limits may result in the same adverse effects as those described for the surface hardness (H100) of the core.

[0039] The hardness (H75) at a position 75% outward from the center of the core radius is not particularly limited, but is preferably at least 65, more preferably at least 67, and even more preferably at least 69. There is no upper limit to this hardness, but it is preferably at most 76, more preferably at most 74, and even more preferably at most 72. Any deviation from these hardness limits may result in the same adverse effects as those described for the surface hardness (H100) of the core.

[0040] The hardness (H50) at a position 50% outward from the center of the core radius is not particularly limited, but is preferably at least 55, more preferably at least 57, and even more preferably at least 59. There is no upper limit to this hardness, but it is preferably at most 68, more preferably at most 66, and even more preferably at most 64. Any deviation from these hardness limits may result in the same adverse effects as those described for the surface hardness (H100) of the core.

[0041] The center hardness (H0) of the core is not particularly limited, but is preferably at least 49, more preferably at least 51, and even more preferably at least 53. There is no upper limit to the center hardness, but it is preferably at most 61, more preferably at most 59, and even more preferably at most 57. If the hardness deviates from these limits, the same adverse effects as those described for the surface hardness (H100) of the core may occur.

[0042] The hardness difference between the surface and center of the core, i.e., the value of (H100-H0), is preferably 24 or greater, more preferably 26 or greater, and even more preferably 28 or greater, with the upper limit being preferably 35 or less, more preferably 33 or less, and even more preferably 31 or less. If this value is too large, the initial velocity on full shots may be low, resulting in a loss of distance, and durability to cracking during repeated hits may be poor. On the other hand, if this value is too small, the low spin effect on full shots may be insufficient, resulting in a loss of distance.

[0043] The core used in the present invention preferably satisfies the following formula: (H100-H0) / (H50-H0)≧3.0 That is, the above formula means that the hardness gradient from the midpoint of the core radius to the core surface is greater than the hardness gradient from the center to the midpoint. Specifically, the value of (H100-H0) / (H50-H0) is preferably 3.0 or greater, more preferably 3.2 or greater, and even more preferably 3.4 or greater, with the upper limit being preferably 7.0 or less, more preferably 6.0 or less, and even more preferably 5.6 or less. If this value is too small, the low spin effect on full shots may be insufficient, resulting in a loss of distance. On the other hand, if this value is too large, the initial velocity on actual impact may be low, resulting in a loss of distance, or the durability to cracking may be poor when repeatedly hit.

[0044] The core used in the present invention preferably satisfies the following formula: (H87.5-H75)-(H100-H87.5)≧-0.5 In other words, the above formula means that the hardness gradient from the position 87.5% radially outward from the center of the core to the core surface should not be too steep compared to the hardness gradient from the position 75% radially outward to the position 87.5% radially outward from the center of the core. Specifically, the value of (H87.5-H75)-(H100-H87.5) is preferably at least -0.5, more preferably at least 0.5, and even more preferably at least 0.8, with the upper limit being preferably at most 4.5, more preferably at most 4.0, and even more preferably at most 3.6.

[0045] Next, the intermediate layer will be described. The intermediate layer is formed as a single layer, and each layer is preferably formed from a resin material, as will be described later.

[0046] The material hardness of the intermediate layer is not particularly limited, but is preferably 60 or more, more preferably 62 or more, and even more preferably 64 or more, in Shore D hardness, with an upper limit of preferably 72 or less, more preferably 70 or less, and even more preferably 68 or less. The surface hardness of a sphere (an intermediate layer-coated sphere) in which the core is coated with the intermediate layer is preferably 66 or more, more preferably 68 or more, and even more preferably 70 or more, with an upper limit of preferably 78 or less, more preferably 76 or less, and even more preferably 74 or less. If the material hardness and surface hardness of the intermediate layer are softer than the above ranges, the spin rate on full shots may increase too much, resulting in a loss of distance, or the initial velocity of the ball may decrease, resulting in a loss of distance on full shots. On the other hand, if the material hardness and surface hardness are too hard, the durability to cracking due to repeated impacts may be poor, and the feel may be poor.

[0047] The material hardness of the intermediate layer, expressed in Shore C hardness, is preferably 88 or more, more preferably 89 or more, and even more preferably 92 or more, with an upper limit of preferably 98 or less, more preferably 96 or less, and even more preferably 94 or less. The surface hardness of the intermediate layer-coated sphere, expressed in Shore C hardness, is preferably 92 or more, more preferably 94 or more, and even more preferably 96 or more, with an upper limit of preferably 100 or less, more preferably 99 or less, and even more preferably 98 or less.

[0048] The thickness of the intermediate layer is preferably 0.9 mm or more, more preferably 1.2 mm or more, and even more preferably 1.4 mm or more. The upper limit of the thickness of the intermediate layer is preferably 2.0 mm or less, more preferably 1.8 mm or less, and even more preferably 1.6 mm or less. If the intermediate layer is too thin, the durability against cracking due to repeated impacts may be poor, or the spin rate may increase on full shots with an iron, resulting in a loss of distance. On the other hand, if the intermediate layer is too thick, the initial velocity may be low, resulting in a loss of the desired distance and a poor feel.

[0049] The intermediate layer can be made of any of the thermoplastic resins used in golf balls, particularly ionomer resin-based resins. By using ionomer resin as the intermediate layer material, low spin and high resilience can be achieved on full shots with a driver (W#1) by golfers with slow head speeds, ensuring the desired distance and crack resistance when hit repeatedly.

[0050] Any additives can be added to the intermediate layer material depending on the application. For example, various additives such as pigments, dispersants, antioxidants, UV absorbers, and light stabilizers can be added. When these additives are added, the amount of the additives added is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and preferably 10 parts by mass or less, more preferably 4 parts by mass or less, per 100 parts by mass of the base resin.

[0051] It is preferable to polish the surface of the intermediate layer material to improve adhesion with the polyurethane that is preferably used in the cover material described below. After the polishing process, it is also preferable to apply a primer (adhesive) to the surface of the intermediate layer or to add an adhesion enhancer to the material.

[0052] The specific gravity of the intermediate layer material is usually less than 1.1, preferably 0.90 to 1.05, and more preferably 0.93 to 0.99. If the specific gravity is outside this range, the ball's overall resilience will decrease, resulting in a shorter flight distance and reduced durability against cracking due to repeated impacts.

[0053] Next, the cover (outermost layer) will be described. The cover material hardness is not particularly limited, but is preferably 35 or higher, more preferably 40 or higher, and even more preferably 43 or higher, in Shore D hardness, with an upper limit of preferably 60 or lower, more preferably 55 or lower, and even more preferably 50 or lower. The surface hardness of the ball (ball surface hardness) obtained by covering the intermediate layer-covered ball with the cover is preferably 50 or higher, more preferably 53 or higher, and even more preferably 56 or higher, with an upper limit of preferably 70 or lower, more preferably 67 or lower, and even more preferably 64 or lower, in Shore D hardness. If the cover material hardness and ball surface hardness are too softer than the above ranges, spin may increase on full iron shots, resulting in a loss of distance under any hitting conditions. On the other hand, if the above material hardness and surface hardness are too hard, the desired spin rate may not be achieved on approaches, or abrasion resistance may be poor.

[0054] The cover material hardness, expressed in Shore C hardness, is preferably 57 or greater, more preferably 63 or greater, and even more preferably 67 or greater, with an upper limit of preferably 89 or less, more preferably 83 or less, and even more preferably 76 or less. The ball surface hardness, expressed in Shore C hardness, is preferably 75 or greater, more preferably 80 or greater, and even more preferably 84 or greater, with an upper limit of preferably 95 or less, more preferably 92 or less, and even more preferably 90 or less.

[0055] The cover thickness is preferably 0.3 mm or more, more preferably 0.45 mm or more, and even more preferably 0.6 mm or more. The upper limit of the cover thickness is preferably 1.2 mm or less, more preferably 1.15 mm or less, and even more preferably 1.0 mm or less. If the cover is too thick, full shots with an iron may result in insufficient repulsion or excessive spin, resulting in a failure to achieve the desired distance. On the other hand, if the cover is too thin, the scuff resistance may be poor, or insufficient spin may be applied on approaches, resulting in a lack of controllability.

[0056] The total thickness of the cover and intermediate layer is preferably 1.5 mm or more, more preferably 1.8 mm or more, and even more preferably 2.0 mm or more. The upper limit of this total thickness is preferably 2.8 mm or less, more preferably 2.6 mm or less, and even more preferably 2.4 mm or less. If this total thickness is too thin, durability to cracking due to repeated impacts may be poor, and the feel may be poor. On the other hand, if this total thickness is too thick, the initial velocity on full shots may be low, resulting in a loss of distance.

[0057] The cover may be made of any of the thermoplastic resins used in golf ball cover materials, but from the standpoint of spin controllability and abrasion resistance in the short game, it is preferable to use a resin material primarily composed of thermoplastic polyurethane. That is, it is preferable to form the cover from a resin blend primarily composed of (I) thermoplastic polyurethane and (II) a polyisocyanate compound.

[0058] The total mass of the components (I) and (II) is preferably 60% or more, and more preferably 70% or more, of the total mass of the resin composition of the cover. Components (I) and (II) are described in detail below.

[0059] Regarding the above-mentioned (I) thermoplastic polyurethane, the structure of the thermoplastic polyurethane includes a soft segment composed of a long-chain polyol (polymeric glycol) and a hard segment composed of a chain extender and a polyisocyanate compound. The long-chain polyol used as the raw material can be any of those conventionally used in thermoplastic polyurethane-related technologies, and is not particularly limited. Examples of such long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer polyols, castor oil polyols, silicone polyols, and vinyl polymer polyols. These long-chain polyols may be used alone or in combination. Among these, polyether polyols are preferred because they can synthesize thermoplastic polyurethanes with high rebound resilience and excellent low-temperature properties.

[0060] The chain extender may be any of those used in conventional thermoplastic polyurethane technologies, and is preferably a low-molecular-weight compound having two or more active hydrogen atoms in the molecule that can react with isocyanate groups and a molecular weight of 400 or less. Examples of the chain extender include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, and 2,2-dimethyl-1,3-propanediol. Of these, aliphatic diols having 2 to 12 carbon atoms are preferred, and 1,4-butylene glycol is more preferred.

[0061] The polyisocyanate compound may be any of those used in conventional thermoplastic polyurethane technologies, and is not particularly limited. Specifically, one or more compounds selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4-(or) 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, naphthylene 1,5-diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate may be used. However, some isocyanate species may make it difficult to control the crosslinking reaction during injection molding. In the present invention, from the viewpoint of the balance between stability during production and the physical properties that are exhibited, 4,4'-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferred.

[0062] Specific thermoplastic polyurethanes of component (I) may be commercially available products, such as Pandex T8295, T8290, and T8260 (all manufactured by DIC Covestro Polymers).

[0063] Although not a required component, a thermoplastic elastomer other than the thermoplastic polyurethane may be blended as component (III) in addition to components (I) and (II). By blending component (III) in the resin blend, it is possible to further improve the flowability of the resin blend and enhance the various physical properties required of a golf ball cover material, such as resilience and abrasion resistance.

[0064] There are no particular limitations on the composition ratio of the above components (I), (II), and (III). However, in order to fully and effectively exert the effects of the present invention, the mass ratio of (I):(II):(III) is preferably 100:2-50:0-50, and more preferably (I):(II):(III) is 100:2-30:8-50 (mass ratio).

[0065] Furthermore, various additives other than the components constituting the thermoplastic polyurethane may be blended into the resin blend as needed. For example, pigments, dispersants, antioxidants, light resistance stabilizers, ultraviolet absorbers, mold release agents, etc. may be blended as appropriate.

[0066] The golf ball formed by laminating the above-mentioned core, intermediate layer, and cover (outermost layer) layers can be manufactured by a conventional method such as a known injection molding method. For example, a golf ball can be obtained by injecting the intermediate layer material around the core using an injection mold to obtain each covered sphere, and then injection molding the cover material, which is the outermost layer. Alternatively, each covered layer can be formed by encasing the covered sphere in two half-cups, which have been previously molded into a spherical half-shell shape, and then molding the covered sphere under heat and pressure.

[0067] [ Hardness relationship of each layer 〕 In the golf ball of the present invention, the surface hardness of the intermediate layer-covered sphere is set to be higher than the surface hardness of the ball. That is, the value obtained by subtracting the Shore C hardness of the ball surface from the Shore C hardness of the intermediate layer-covered sphere surface is greater than 0, preferably 5 or greater, more preferably 9 or greater, with the upper limit being preferably 20 or less, more preferably 17 or less, and even more preferably 15 or less. When this value is small and is due to the material hardness of the intermediate layer, spin may increase on full shots, resulting in a failure to achieve the desired distance. When this value is due to the material hardness of the cover, spin controllability and abrasion resistance in shot games may be poor. On the other hand, when this value is large and is due to the material hardness of the intermediate layer, crack resistance due to repeated impacts may be poor and the feel may be too hard. When this value is due to the material hardness of the cover, spin may increase on full shots, resulting in a failure to achieve the desired distance.

[0068] The surface hardness of the intermediate layer-covered sphere is preferably higher than that of the core. The value obtained by subtracting the surface hardness of the core from the surface hardness of the intermediate layer-covered sphere is preferably 1 or more, more preferably 5 or more, and even more preferably 10 or more, in Shore C hardness, with the upper limit being preferably 30 or less, more preferably 24 or less, and even more preferably 18 or less. If this value is too small, spin may increase on full shots, resulting in a loss of distance. On the other hand, if this value is too large, crack resistance may be reduced when hit repeatedly.

[0069] The golf ball of the present invention must also satisfy the following formula: (Shore C hardness of the surface of the intermediate layer-coated sphere) / {(Shore C hardness of the core surface - Shore C hardness of the core center) × thickness of the intermediate layer (mm)} ≦ 2.7 In other words, although the golf ball of the present invention is a golf ball with a two-layer cover, with a hard inner layer and a soft outer layer, the above formula means that the difference in hardness between the surface and center of the core is relatively large, and the intermediate layer is relatively hard but not too thin. The value of (surface hardness of intermediate layer-coated ball) / {(Shore C hardness of core surface - Shore C hardness of core center) × thickness of intermediate layer (mm)} is 2.7 or less, preferably 2.6 or less, and more preferably 2.5 or less. The lower limit is preferably 1.5 or more, more preferably 1.8 or more, and even more preferably 2.0 or more. If this value is too large, full shots may result in excessive spin, making it difficult to achieve the desired distance. On the other hand, if this value is too small, the durability to repeated impacts and the feel may be poor.

[0070] [ Compression relationship between the core and the ball 〕 The Atti compression of the ball is not particularly limited, but is preferably 48 or greater, more preferably 53 or greater, and even more preferably 58 or greater, with the upper limit being preferably 80 or less, more preferably 75 or less, and even more preferably 70 or less. If this compression value is too high, the ball may have too much spin, particularly on full iron shots, resulting in a loss of distance or an excessively hard feel. On the other hand, if the compression value is too low, the ball may have too soft a feel or may have poor crack resistance during repeated impacts. The Atti compression is measured using an Atti Engineering ATTI compression tester, similar to the core compression measurement described above.

[0071] The ratio of the Atti compression of the ball to the core, i.e., the Atti compression of the core and ball, C1 and C2, respectively, should preferably be 1.7 or greater, more preferably 2.0 or greater, and even more preferably 2.3 or greater, with the upper limit being preferably 3.6 or less, more preferably 3.3 or less, and even more preferably 3.0 or less. If this ratio is too high, durability to cracking after repeated impacts may be impaired. On the other hand, if this ratio is too low, spin may increase on full shots, resulting in a loss of the desired distance.

[0072] The difference in Atti compression between the ball and the core, i.e., the value of C2-C1, is preferably 28 or greater, more preferably 33 or greater, and even more preferably 38 or greater, with the upper limit being preferably 50 or less, more preferably 46 or less, and even more preferably 42 or less. If this value is too small, full shots may result in increased spin, which may hinder the desired distance. On the other hand, if this value is too large, crack resistance may be impaired when hit repeatedly.

[0073] A large number of dimples can be formed on the outer surface of the cover. There are no particular restrictions on the number of dimples to be arranged on the cover surface, but the number is preferably at least 323, more preferably at least 326, and more preferably at least 330, with the upper limit being preferably at most 380, more preferably at most 360, and even more preferably at most 350. If the number of dimples exceeds the above range, the ball's trajectory may be lowered, resulting in a shorter flight distance. Conversely, if the number of dimples is lower, the ball's trajectory may be higher, resulting in a shorter flight distance.

[0074] The dimples may be of one or a combination of two or more shapes, such as circular, polygonal, dewdrop, or elliptical. For example, if circular dimples are used, the diameter may be approximately 2.5 mm to 6.5 mm, and the depth may be 0.08 mm to 0.30 mm.

[0075] The dimple coverage (SR value), which is the ratio of the total dimple area defined by the planes surrounded by the dimple edges to the ball's area assuming no dimples, is preferably 70% to 90% in order to fully demonstrate aerodynamic characteristics. Furthermore, V0, the value obtained by dividing the spatial volume of each dimple below the plane surrounded by the dimple edges by the volume of a cylinder whose base is the plane and whose height is the maximum depth of the dimple from the bottom, is preferably 0.35 to 0.80 in order to optimize the ball's trajectory. Furthermore, the VR value, which is the ratio of the total dimple volume below the planes surrounded by the dimple edges to the ball's area assuming no dimples, is preferably 0.6% to 1.0%. Any deviation from the above-mentioned ranges may result in a poor trajectory and an insufficient flight distance.

[0076] In the golf ball of the present invention, when the lift coefficient measured under conditions of a Reynolds number of 80,000 and a spin rate of 2,000 rpm is CL1, the lift coefficient measured under conditions of a Reynolds number of 70,000 and a spin rate of 1,900 rpm is CL2, the lift coefficient measured under conditions of a Reynolds number of 200,000 and a spin rate of 2,500 rpm is CL3, and the lift coefficient measured under conditions of a Reynolds number of 120,000 and a spin rate of 2,250 rpm is CL4, it is desirable to optimize CL2 / CL1 and CL4 / CL3.

[0077] In this specification, "lift coefficients (CL1, CL2, CL3, CL4)" are measured in accordance with the ITR (Indoor Test Range) established by the USGA (United States Golf Association). The lift coefficient can be adjusted by adjusting the configuration of the dimples on the golf ball (arrangement, diameter, depth, volume, number, shape, etc.). The lift coefficient is independent of the internal configuration of the golf ball. The Reynolds number (Re) is a dimensionless number used in the field of fluid dynamics. The Reynolds number (Re) is calculated using the following formula (I): Re = ρvL / μ (I) In the above equation (I), ρ represents the density of the fluid, v represents the average velocity of the object relative to the fluid flow, L represents the characteristic length, and μ represents the viscosity coefficient of the fluid.

[0078] The conditions for measuring the lift coefficient CL1, a Reynolds number of 80,000 and a spin rate of 2,000 rpm, generally correspond to the state when the lift coefficient begins to decrease (and thus the golf ball begins to fall) after the golf ball has reached its highest point since being hit. The conditions for measuring the lift coefficient CL2, a Reynolds number of 70,000 and a spin rate of 1,900 rpm, generally correspond to the state when the golf ball reaches its highest point since being hit and is about to fall to the ground. These conditions are particularly true when the golf ball is launched at high speed (e.g., initial velocity of 66 m / s, spin rate of 2,600 rpm, launch angle of 11°). These high-speed conditions correspond to the conditions used by an average amateur golfer using a driver.

[0079] The value of CL2 / CL1 is preferably 0.950 or greater, more preferably 0.960 or greater, and even more preferably 0.970 or greater. By satisfying this range, it is possible to suppress a decrease in lift force during the flight of the golf ball, which in turn facilitates an increase in flight distance (and therefore an increase in carry) and run during flight. This results in an improvement in flight distance (total). If CL2 / CL1 is too low, the golf ball is likely to drop precipitously, making it difficult to achieve a sufficient increase in carry and run. From the perspective of improving flight distance, a higher CL2 / CL1 is preferable. However, if it is too high, the carry will increase but the run will decrease, resulting in a risk of the total flight distance falling short of the optimum value. Therefore, the upper limit of CL2 / CL1 is 1.100 or less, preferably 1.050 or less, more preferably 1.044 or less, and even more preferably 1.022 or less.

[0080] The conditions for measuring the lift coefficient CL3, i.e., a Reynolds number of 200,000 and a spin rate of 2,500 rpm, generally correspond to the state of a golf ball immediately after it is launched at high speed (e.g., initial speed of 72 m / s, spin rate of 2,500 rpm, launch angle of 10°).The conditions for measuring the lift coefficient CL4, i.e., a Reynolds number of 120,000 and a spin rate of 2,250 rpm, generally correspond to the state of a golf ball rising after it is launched at high speed (e.g., initial speed of 72 m / s, spin rate of 2,500 rpm, launch angle of 10°), approximately two seconds have passed.

[0081] The CL4 / CL3 value is preferably 1.250 or greater, more preferably 1.252 or greater, and even more preferably 1.255 or greater, with the upper limit being preferably 1.300 or less, more preferably 1.295 or less, and even more preferably 1.290 or less. By setting the CL4 / CL3 value within the above range, when the golf ball is hit under high-speed conditions (for example, when hit with a W#1), excessive lift of the golf ball can be suppressed (and thus lift can be suppressed), improving wind resistance and carry. Furthermore, run can be improved. Consequently, the total distance can be improved.

[0082] From the viewpoint of improving flight distance, the lift coefficient CL1 is preferably 0.230 or more. Furthermore, the lift coefficient CL1 is preferably 0.240 or less. From the same viewpoint, the lift coefficient CL2 is preferably 0.230 or more. Furthermore, the lift coefficient CL2 is preferably 0.240 or less. From the same viewpoint, the lift coefficient CL3 is preferably 0.145 or more. Furthermore, the lift coefficient CL3 is preferably 0.155 or less. From the same viewpoint, the lift coefficient CL4 is preferably 0.185 or more. Furthermore, the lift coefficient CL4 is preferably 0.195 or less.

[0083] A coating layer is formed on the surface of the cover. This coating layer can be applied using various types of paint, and as the paint, a coating composition containing a urethane paint composed of a polyol and a polyisocyanate as its main component is preferred because it must be able to withstand the harsh conditions of use of the golf ball.

[0084] Examples of the polyol component include acrylic polyols, polyester polyols, etc. These polyols include modified polyols, and other polyols may be added to further improve workability.

[0085] Examples of acrylic polyols include homopolymers or copolymers of monomers having a functional group that reacts with isocyanate, and examples of such monomers include (meth)acrylic acid alkyl esters, specifically methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, etc. These may be used alone or in combination of two or more.

[0086] In addition, examples of modified acrylic polyols that can be used include polyester-modified acrylic polyols. Other polyols include polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG); condensation polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PH2A); lactone polyester polyols such as poly-ε-caprolactone (PCL); and polycarbonate polyols such as polyhexamethylene carbonate. These polyols can be used alone or in combination. The proportion of these polyols relative to the total amount of acrylic polyols is preferably 50% by mass or less, more preferably 40% by mass or less.

[0087] Polyester polyols are obtained by polycondensation of polyols and polybasic acids. Examples of polyols include diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, hexylene glycol, dimethylolheptane, polyethylene glycol, and polypropylene glycol, as well as triols, tetraols, and polyols having an alicyclic structure. Examples of polybasic acids include aliphatic dicarboxylic acids such as succinic acid, adipic acid, sebacic acid, azelaic acid, and dimer acid; aliphatic unsaturated dicarboxylic acids such as fumaric acid, maleic acid, itaconic acid, and citraconic acid; aromatic polycarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, and pyromellitic acid; dicarboxylic acids having an alicyclic structure such as tetrahydrophthalic acid, hexahydrophthalic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and endomethylenetetrahydrophthalic acid; and tris-2-carboxyethyl isocyanurate.

[0088] As the polyol component, it is preferable to use two types of polyester polyols in combination. In this case, if the two types of polyester polyols are components (A) and (B), the polyester polyol of component (A) can be a polyester polyol having a cyclic structure introduced into the resin skeleton, such as a polyester polyol obtained by polycondensation of a polyol having an alicyclic structure such as cyclohexanedimethanol with a polybasic acid, or a polyol having an alicyclic structure with a diol or triol with a polybasic acid. On the other hand, the polyester polyol of component (B) can be a polyester polyol having a multi-branched structure, such as a polyester polyol having a branched structure such as "NIPPOLAN 800" manufactured by Tosoh Corporation.

[0089] The overall weight-average molecular weight (Mw) of the base material comprising the two polyester polyols is preferably 13,000 to 23,000, more preferably 15,000 to 22,000. The overall number-average molecular weight (Mw) of the base material comprising the two polyester polyols is preferably 1,100 to 2,000, more preferably 1,300 to 1,850. If these average molecular weights (Mw and Mn) fall outside the above ranges, the abrasion resistance of the coating layer may be reduced. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) are values ​​(polystyrene equivalent) measured by gel permeation chromatography (hereinafter abbreviated as GPC) using a differential refractometer.

[0090] The blending amounts of the two polyester polyols (A) and (B) are not particularly limited, but it is preferable that the blending amount of component (A) is 20 to 30 mass % of the total amount of the base material, and the blending amount of component (B) is 2 to 18 mass % of the total amount of the base material.

[0091] On the other hand, the polyisocyanate is not particularly limited and may be a commonly used aromatic, aliphatic, alicyclic, or other polyisocyanate, specifically, tolylene diisocyanate, diphenylmethane diisocyanate, xylylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, isophorone diisocyanate, 1,4-cyclohexylene diisocyanate, naphthalene diisocyanate, trimethylhexamethylene diisocyanate, dicyclohexylmethane diisocyanate, 1-isocyanato-3,3,5-trimethyl-4-isocyanatomethylcyclohexane, etc. These may be used alone or in combination.

[0092] Examples of the modified hexamethylene diisocyanate include polyester modified hexamethylene diisocyanate and urethane modified hexamethylene diisocyanate. Examples of the derivatives of hexamethylene diisocyanate include nurate (isocyanurate), biuret, and adduct of hexamethylene diisocyanate.

[0093] The molar ratio (NCO / OH) of the hydroxyl groups (OH) of the polyol to the isocyanate groups (NCO) of the polyisocyanate must be in the range of 0.5 to 1.5, preferably 0.8 to 1.2, and more preferably 1.0 to 1.2. If the molar ratio is less than 0.5, unreacted hydroxyl groups may remain, which may deteriorate the performance and water resistance of the coating layer. On the other hand, if the molar ratio exceeds 1.5, the isocyanate groups will be in excess, which will react with water to produce urea groups (which are brittle), which may result in a deterioration in the performance of the coating layer.

[0094] As the curing catalyst (organometallic compound), an amine catalyst or an organometallic catalyst can be used, and as this organometallic compound, metal soaps such as aluminum, nickel, zinc, tin, etc., which have traditionally been compounded as curing agents in two-component curing urethane paints, can be suitably used.

[0095] Various organic solvents can be mixed into the coating composition depending on the coating conditions. Examples of such organic solvents include aromatic solvents such as toluene, xylene, and ethylbenzene, ester solvents such as ethyl acetate, butyl acetate, propylene glycol methyl ether acetate, and propylene glycol methyl ether propionate, ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone, ether solvents such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, and dipropylene glycol dimethyl ether, alicyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, and ethylcyclohexane, and petroleum hydrocarbon solvents such as mineral spirits.

[0096] If necessary, known paint compounding components may be added to the paint composition. Specifically, appropriate amounts of thickeners, ultraviolet absorbers, fluorescent brighteners, slipping agents, pigments, etc. may be added.

[0097] The thickness of the coating layer made from the above coating composition is not particularly limited, but is usually 5 to 40 μm, preferably 10 to 20 μm. Note that the thickness of the coating layer referred to here does not refer to the coating layer formed within the dimples, but to the thickness of the coating formed on the surface of the ball other than the dimples (i.e., the land areas or banks).

[0098] In the present invention, the elastic work recovery rate of the coating layer made of the above-mentioned paint composition must be 60% or more, preferably 70% or more, and more preferably 80% or more. If the elastic work recovery rate of this coating layer is within the above range, the coating layer will have high elasticity, therefore a high self-repair function and excellent abrasion resistance. Furthermore, the performance of golf balls coated with the above-mentioned paint composition can be improved. The above-mentioned elastic work recovery rate can be measured as follows.

[0099] The elastic work recovery is a parameter of the nanoindentation method, which evaluates the physical properties of a coating layer. Conventional methods can only measure the size of the deformation mark (plastic deformation mark) corresponding to the maximum load. However, the nanoindentation method automatically and continuously measures the relationship between the indentation load and the indentation depth. This allows for highly accurate evaluation of the physical properties of a coating layer without the individual variability that occurs when visually measuring deformation marks with an optical microscope. Because the coating layer on a golf ball's surface is significantly affected by impacts from drivers and various clubs, and the coating layer's influence on the physical properties of a golf ball is significant, measuring the coating layer with the ultramicrohardness test method, which can be performed with higher precision than conventional methods, is a highly effective evaluation method.

[0100] The hardness of the paint layer is preferably 40 or more, more preferably 60 or more, with an upper limit of 95 or less, more preferably 85 or less, in Shore M hardness. This Shore M hardness conforms to ASTM D2240. The hardness of the paint layer is preferably 40 or more, with an upper limit of 80 or less, in Shore C hardness. This Shore C hardness conforms to ASTM D2240. If the paint layer has a hardness that is too high above the above range, the paint may become brittle upon repeated impacts, potentially making it unable to protect the cover layer. If the paint layer has a hardness that is too low below the above range, the ball surface may be easily scratched or soiled when hit against hard objects, which is undesirable.

[0101] When using the above coating composition, the coating composition of the present invention can be prepared at the time of coating a golf ball manufactured by a known method, applied to the surface using a conventional coating process, and then dried to form a coating layer on the surface of the ball. In this case, the coating method can be suitably spray coating, electrostatic coating, dipping, or the like, and is not particularly limited. [Example]

[0102] EXAMPLES The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0103] [Examples 1 to 4, Comparative Examples 1 to 6] Core formation The rubber compositions of Examples 1 to 4 and Comparative Examples 1 to 3 shown in Table 1 were prepared, and then vulcanized and molded at the temperature and for the time shown in Table 1 to prepare solid cores. For Comparative Examples 4 to 6, solid cores are prepared using the rubber compositions and vulcanization conditions shown in Table 1 in the same manner as above.

[0104] [Table 1]

[0105] The details of each component listed in Table 1 are as follows: Polybutadiene A: JSR Corporation, product name "BR01" Polybutadiene B: JSR Corporation, product name "T0700" Zinc acrylate: "ZN-DA85S" (Nippon Shokubai Co., Ltd.) Organic peroxide: Dicumyl peroxide, trade name "Percumyl D" (NOF Corporation), 1-minute half-life temperature 175.2°C Zinc stearate: Product name "Zinc Stearate G" (NOF Corporation) Sulfur: Product name "Sunmix S-80N" (Sanshin Chemical Industry), sulfur masterbatch containing 80% by mass of powdered sulfur for rubber Water: Pure water (Seiki Pharmaceutical Co., Ltd.) Antioxidant A: 2,2-methylenebis(4-methyl-6-butylphenol), product name "Nocrac NS-6" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Antioxidant B: 2-mercaptobenzimidazole, product name "Nocrac MB" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Zinc oxide: Product name "Triple Zinc Oxide" (manufactured by Sakai Chemical Industry Co., Ltd.) Pentachlorothiophenol zinc salt: Wako Pure Chemical Industries, Ltd.

[0106] Formation of the intermediate layer and cover (outermost layer) Next, for Examples 1 to 4 and Comparative Examples 1 to 3, an intermediate layer was formed around the core obtained above by injection molding using intermediate layer material No. 1 or No. 2 having the composition shown in Table 2, to produce an intermediate layer-covered sphere. Next, a cover (outermost layer) was formed around the intermediate layer-covered sphere obtained above by injection molding using cover material No. 4 or No. 5 having the composition shown in the same table, to produce a golf ball. At this time, Type-A or Type-B dimples described below were formed on the surface of the cover. For Comparative Examples 4 to 6, golf balls were produced by injection molding using intermediate layer material No. 1 and cover material No. 2, as described above, with the formulations shown in Table 2. In addition, the following Type-A or Type-B dimples were formed on the surface of the covers of Comparative Examples 4 to 6.

[0107] [Table 2]

[0108] The trade names of the main materials listed in the table are as follows: "Himilan 1605", "Himilan 1557", and "Himilan 1706" are ionomers manufactured by Mitsui Dow Polychemicals "AM7318" Ionomer manufactured by Mitsui Dow Polychemicals "Trimethylolpropane" (TMP) manufactured by Tokyo Chemical Industry Co., Ltd. "TPU 1" is a product name "Pandex" manufactured by DIC Covestro Polymer, an ether-type thermoplastic polyurethane with a material hardness (Shore D) of 50. "TPU 2" is a product name "Pandex" manufactured by DIC Covestro Polymer, an ether-type thermoplastic polyurethane with a material hardness (Shore D) of 43.

[0109] Dimple Type-A uses six types of circular dimples, the details of which are shown in Table 3 below, and their arrangement is as shown in Figure 2. Figure 2(A) shows a plan view of the dimples, and Figure 2(B) shows a side view.

[0110] [Table 3]

[0111] Dimple Type-B uses eight types of circular dimples, the details of which are shown in Table 4 below, and their arrangement is as shown in Figure 3. Figure 3(A) shows a plan view of the dimples, and Figure 3(B) shows a side view.

[0112] [Table 4]

[0113] Dimple definition Edge: The highest point on the cross section passing through the center of the dimple Diameter: The diameter of the plane enclosed by the dimple edge Depth: The maximum depth of the dimple from the plane bounded by the dimple edge SR: The ratio of the total dimple area, defined as the plane surrounded by the edges of the dimples, to the area of ​​the ball as if no dimples existed. Dimple volume: The volume of the dimple below the plane surrounded by the dimple edge Cylinder volume ratio: The ratio of the dimple volume to the volume of a cylinder with the same diameter and depth as the dimple. VR: The total volume of the dimples formed below the plane surrounded by the edges of the dimples is the volume of the ball as if no dimples existed.

[0114] For balls with the above Type-A and Type-B dimples formed on the cover surface, the lift coefficient CL1 measured under conditions of a Reynolds number of 80,000 and a spin rate of 2,000 rpm, the lift coefficient CL2 measured under conditions of a Reynolds number of 70,000 and a spin rate of 1,900 rpm, the lift coefficient CL3 measured under conditions of a Reynolds number of 200,000 and a spin rate of 2,500 rpm, the lift coefficient CL4 measured under conditions of a Reynolds number of 120,000 and a spin rate of 2,250 rpm, and the values ​​of CL2 / CL1 and CL4 / CL3 are shown in Table 5. These lift coefficients were measured in accordance with the ITR (Indoor Test Range) established by the USGA.

[0115] [Table 5]

[0116] Formation of paint layer (coating layer) Next, for Examples 1 to 4 and Comparative Examples 1 to 3, the coating composition shown in Table 6 below was used as the coating composition common to all Examples and Comparative Examples, and the coating was applied to the surface of a large number of covers (outermost layers) using an air spray gun to produce golf balls with a coating layer 15 μm thick. Similarly, for Comparative Examples 4 to 6, the above paint was applied to prepare golf balls having a paint layer with a thickness of 15 μm.

[0117] [Table 6]

[0118] [Synthesis example of polyester polyol (A)] A reaction apparatus equipped with a reflux condenser, a dropping funnel, a gas inlet tube, and a thermometer was charged with 140 parts by mass of trimethylolpropane, 95 parts by mass of ethylene glycol, 157 parts by mass of adipic acid, and 58 parts by mass of 1,4-cyclohexanedimethanol, and the mixture was heated to 200-240°C with stirring and heated (reacted) for 5 hours. After that, a "polyester polyol (A)" with an acid value of 4, a hydroxyl value of 170, and a weight average molecular weight (Mw) of 28,000 was obtained. Next, the polyester polyol (A) synthesized above was dissolved in butyl acetate to prepare a varnish having a nonvolatile content of 70% by mass.

[0119] The coating composition in Table 5 was prepared by mixing 23 parts by mass of the polyester polyol solution with 15 parts by mass of "polyester polyol (B)" (saturated aliphatic polyester polyol "NIPPOLAN 800" manufactured by Tosoh Corporation, weight average molecular weight (Mw) 1,000, solid content 100%) and an organic solvent to form the base. This mixture had a non-volatile content of 38.0% by mass.

[0120] Elastic work recovery rate The elastic work recovery rate of the paint is measured using a 50 μm thick paint sheet. The measuring device used is the Elionix ultra-micro hardness tester "ENT-2100," and the measurement conditions are as follows: Indenter: Berkovich indenter (material: diamond, angle α: 65.03°) Load F: 0.2mN Loading time: 10 seconds ·Holding time: 1 second ·Unloading time: 10 seconds The elastic work recovery rate is calculated using the following formula based on the indentation work load Welast (Nm) due to the return deformation of the paint and the mechanical indentation work load Wtotal (Nm). Elastic work recovery rate = Welast / Wtotal × 100 (%)

[0121] Shore C hardness and Shore M hardness The Shore C hardness and Shore M hardness in Table 6 above are measured by preparing 2 mm thick sheets, stacking three of them as test pieces, and using a Shore C hardness tester and a Shore M hardness tester in accordance with ASTM D2240 standard.

[0122] For each of the resulting golf balls, various physical properties such as the internal hardness at each position of the core, the outer diameter of the core and each coated sphere, the thickness and material hardness of each layer, and the surface hardness of each coated sphere were evaluated using the methods described below, and the results are shown in Table 7.

[0123] The outer diameter of each core and intermediate layer coated sphere The sphere to be measured is placed in a thermostatic chamber adjusted to 23.9±1°C for at least three hours, and then measured in a room at 23.9±2°C. Five randomly selected points on the surface are measured, and the average value is taken as the measurement value for each sphere, and the average value for all 10 measurements is calculated.

[0124] Ball diameter The ball to be measured is placed in a thermostatic chamber adjusted to 23.9±1°C for at least three hours, and then measured in a room at 23.9±2°C. Measurements are taken at 15 randomly selected non-dimpled locations, and the average value is used as the measurement value for one ball. The average value for 10 balls is then calculated.

[0125] Atti compression of core and ball The core or ball is measured using an ATTI compression tester manufactured by Atti Engineering. The tester is designed to measure spherical objects with a diameter of 42.7 mm (1.68 inches). When measuring the compression of the core, a spacer is inserted between the compression head and the core, so that the core diameter plus the spacer thickness equals 42.7 mm.

[0126] Core hardness distribution The core's surface is spherical, and the needle of a hardness tester is set nearly perpendicular to the spherical surface, and the surface hardness is measured in Shore C hardness in accordance with ASTM D2240. For the center and designated locations of the core, the core is cut into a hemisphere, the cross section is flattened, and the needle of the hardness tester is pressed perpendicularly against the center and the designated locations listed in Table 7. The hardness at the center and each location is shown in Shore C hardness. Hardness measurements were performed using an automatic rubber hardness tester "P2" manufactured by Kobunshi Keiki Co., Ltd., equipped with a Shore C hardness tester. The maximum hardness value was read. All measurements were performed in an environment of 23±2°C. The values ​​in Table 7 are Shore C hardness values.

[0127] Graphs of core hardness distribution for Examples 1 to 4 and Comparative Examples 1 to 6 are shown in FIGS. 4 and 5, respectively.

[0128] Hardness of the mid layer and cover materials The resin material for each layer is molded into a 2mm thick sheet and left to stand for two weeks. The Shore D hardness and Shore C hardness are then measured in accordance with the ASTM D2240 standard. The hardness is measured using an automatic rubber hardness tester "P2" manufactured by Kobunshi Keiki Co., Ltd. Attachments for Shore D hardness and Shore C hardness are attached and the respective hardness values ​​are measured. The maximum hardness value is read. All measurements are performed in an environment of 23±2°C.

[0129] Surface hardness of each sphere of the intermediate layer coated sphere and the ball Measurements are taken by pressing a needle perpendicularly against the surface of each sphere. The surface hardness of the ball (cover) is measured on the land area on the ball surface where no dimples are formed. Shore D hardness and Shore C hardness are measured in accordance with the ASTM D2240 standard. Hardness is measured using an automatic rubber hardness tester "P2" manufactured by Kobunshi Keiki Co., Ltd. Attachments for Shore D hardness and Shore C hardness are attached and the respective hardnesses are measured. The maximum hardness value is read. All measurements are taken in an environment of 23±2°C.

[0130] [Table 7]

[0131] The distance (I#6), spin rate on approach, feel on impact, and durability against repeated impact of each golf ball were evaluated by the following methods. The results are shown in Table 8.

[0132] Distance (Iron (W#6)) A 6-iron was attached to a golf hitting robot and hit at a head speed of 43.5 m / s, and the distance was measured and judged according to the following criteria. The club used was the Bridgestone Sports JGR Forged (2016 model). The spin rate was also measured immediately after hitting using an initial condition measuring device. <Judgment criteria> ◎ Total flying distance of 181.0m or more 〇 ··· Total distance: 178.0m or more and 180.9m or less × Total flying distance less than 178.0m

[0133] Evaluating spin rate during approach The sand wedge was attached to a golf hitting robot and hit at a head speed (HS) of 15 m / s, and the amount of spin was measured. The amount of spin was measured using an initial condition measuring device on the ball immediately after hitting. The sand wedge used was the "TourStage TW-03 (loft angle 57°) 2002 model" manufactured by Bridgestone Sports. 〔Judgment criteria〕 〇 ··· Spin rate of 4500 rpm or more × Spin rate less than 4500 rpm

[0134] Hitting feel Amateur golfers with a handicap of 12 or less hit the ball and the feel is evaluated based on the number of people who rated the club as "very soft and has a good feel." <Judgment criteria> ◎ ··· 18 or more out of 20 people 〇 15 or more and 17 or less out of 20 people △ ··· 10 or more and 14 or less out of 20 people × ··· 9 or less out of 20 people

[0135] Repeated impact durability A test was conducted in which a ball was repeatedly struck against a steel plate at a launch speed of 43 m / s to see how many times it took for the plate to break. N=30 balls were struck repeatedly, and the minimum number of strikes at which the ball began to break was used for evaluation. The number of breaks in Example 2 was assigned an index of 100. <Judgment criteria> 〇 ··· Index 90 or more × Index less than 90

[0136] [Table 8]

[0137] As shown in the results in Table 8, the golf balls of Comparative Examples 1 to 6 are inferior to the products of the present invention (Examples) in the following respects. In Comparative Example 1, the value of (core Atti compression) / (core surface Shore C hardness - core center Shore C hardness) was greater than 1.1, and the value of (surface Shore C hardness of intermediate layer-covered sphere) / {(core surface Shore C hardness - core center Shore C hardness) x intermediate layer thickness (mm)} was greater than 2.7. As a result, the flight distance on full shots with an iron was short, and a soft feel was not achieved. In Comparative Example 2, the value of (core Atti compression) / (core surface Shore C hardness - core center Shore C hardness) was greater than 1.1, and the value of (surface Shore C hardness of intermediate layer-covered sphere) / {(core surface Shore C hardness - core center Shore C hardness) × intermediate layer thickness (mm)} was greater than 2.7. As a result, the flight distance on full shots with an iron was short and a soft feel was not achieved. In Comparative Example 3, the value of (core Atti compression) / (core surface Shore C hardness - core center Shore C hardness) was greater than 1.1, and the value of (surface Shore C hardness of intermediate layer-covered sphere) / {(core surface Shore C hardness - core center Shore C hardness) × intermediate layer thickness (mm)} was greater than 2.7. As a result, the flight distance on full shots with an iron was short and a soft feel was not achieved. In Comparative Example 4, the value of (core Atti compression) / (core surface Shore C hardness - core center Shore C hardness) was greater than 1.1, and the value of (surface Shore C hardness of intermediate layer-coated sphere) / {(core surface Shore C hardness - core center Shore C hardness) x intermediate layer thickness (mm)} was greater than 2.7. As a result, the flight distance on full shots with an iron was short and a soft feel was not achieved. In Comparative Example 5, the value of (Shore C hardness of surface of intermediate layer-coated sphere) / {(Shore C hardness of core surface - Shore C hardness of core center) × intermediate layer thickness (mm)} was greater than 2.7, resulting in poor crack resistance when hit repeatedly. In Comparative Example 6, the value of (Atti compression of core) / (Shore C hardness of core surface-Shore C hardness of core center) is greater than 1.1, resulting in a short flight distance on full shots with an iron and an unsoft feel on impact.

Claims

1. In a three-piece golf ball having a single rubber core, a single resin cover, and one intermediate layer sandwiched between them, the surface hardness relationship between a sphere in which the core is covered with an intermediate layer (an intermediate layer-covered sphere) and the ball satisfies the following formula: (Surface Shore C hardness of intermediate layer-coated ball)>(Surface Shore C hardness of ball) and the following two equations are satisfied: 0.5≦(Core Atti Compression) / (Core Surface Shore C Hardness−Core Center Shore C Hardness)≦1.1 (Shore C hardness of surface of intermediate layer-coated sphere) / {(Shore C hardness of core surface−Shore C hardness of core center)×thickness of intermediate layer (mm)}≦2.7 A golf ball characterized by satisfying the above.

2. In the core hardness distribution, if the Shore C hardness of the core surface is H100, the Shore C hardness at a position 87.5% of the core radius from the core center is H87.5, the Shore C hardness at a position 75% of the core radius from the core center is H75, the Shore C hardness at a position 50% of the core radius from the core center is H50, and the Shore C hardness at the core center is H0, then the following formula can be used: (H100-H0)≧24 2. The golf ball of claim 1, wherein the above formula satisfies the above formula.

3. The following formula: (H100-H0) / (H50-H0)≧3.0 3. The golf ball of claim 2, which satisfies the following:

4. The following formula: (H87.5-H75)-(H100-H87.5)≧-0.5 4. The golf ball according to claim 2, wherein the above formula satisfies the above formula.

5. The core comprises the following components (A) to (D): (A) Base rubber (B) Organic peroxide (C) Water and / or a metal salt of a carboxylic acid (D) sulfur 5. The golf ball of claim 1, which is a hot-molded product of a rubber composition comprising:

6. 6. The golf ball of claim 5, wherein the mass ratio of component (C) to component (D) is (D) / (C)=0.010 to 0.

200.

7. When the lift coefficient measured under the conditions of a Reynolds number of 80,000 and a spin rate of 2,000 rpm is defined as CL1, and the lift coefficient measured under the conditions of a Reynolds number of 70,000 and a spin rate of 1,900 rpm is defined as CL2, CL1 and CL2 are expressed by the following formula: 0.950≦CL2 / CL1 7. The golf ball of claim 1, which satisfies the following:

8. When the lift coefficient measured under the conditions of a Reynolds number of 200,000 and a spin rate of 2,500 rpm is defined as CL3, and the lift coefficient measured under the conditions of a Reynolds number of 120,000 and a spin rate of 2,250 rpm is defined as CL4, CL3 and CL4 are expressed by the following formula: 1.250≦CL4 / CL3≦1.300 8. The golf ball of claim 1, which satisfies the following:

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