Rubber composition for golf balls and golf balls

Abstract

To provide a rubber composition for a golf ball capable of improving flight performance with low-spin characteristic when striking a golf ball by setting hardness difference large in hardness distribution in the core while keeping desired core hardness, and to provide a golf ball made of the composition.SOLUTION: A rubber composition for a golf ball contains following each component (a)-(d): (a) base rubber, (b) α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-cross-linking agent, (c) a cross-linking initiator, and (d) an alcohol having a value of 70 or less obtained by dividing the molecular weight by the number (valence) of the hydroxy groups. A golf ball has a core and a single layer or a plurality of layers covering the core, where the core is made of the rubber composition.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a rubber composition for golf balls and a golf ball using the same, and more particularly to a rubber composition for golf balls that is suitable for use as a core material for golf balls consisting of one or more core layers and one or more cover layers, and a golf ball using the same. [Background technology]

[0002] Recently, two-piece and three-piece solid golf balls have become mainstream. These golf balls typically have a rubber core structure surrounded by a single-layer or multi-layer cover made of various resin materials. The core occupies a large portion of the golf ball's volume and significantly affects various physical properties, such as resilience, feel, and durability. Recently, various technologies have been proposed to achieve a unique core hardness gradient by appropriately adjusting the cross-sectional hardness of the core, thereby optimizing spin characteristics during full shots with a driver or iron, thereby improving flight distance. It has been shown that increasing the hardness difference between the surface and center of the core reduces spin during full shots with a driver, and conventional knowledge has shown that reducing spin during full shots leads to improved flight distance. Therefore, a technology to further increase the hardness difference within the core is needed to improve the flight distance of golf balls. One method to achieve this technology has been proposed, which uses a core structure made of two rubber layers. However, because the labor required to produce a core is greater than that required for a single-layer rubber core, a technology to increase the hardness difference within the single-layer core remains desirable.

[0003] Methods for adjusting the cross-sectional hardness of the core include appropriately adjusting the compounding components of the rubber composition of the core, and the vulcanization temperature and time. Regarding the compounding components of the rubber composition of the core, examples include selecting the type and amount of co-crosslinking agent and organic peroxide. Regarding co-crosslinking agents, in the field of golf balls, it is known to use methacrylic acid, acrylic acid, and metal salts thereof. However, the adjustment of the compounding of the co-crosslinking agent is primarily aimed at adjusting the feel of the ball by adjusting the hardness of the core, and does not result in satisfactory spin characteristics.

[0004] Japanese Patent Application Laid-Open No. 11-169485 proposes a technique of compounding a specific amount of polyethylene glycol into a core rubber composition. However, this technique aims to improve the mold releasability of the rubber molded product (core) by compounding polyethylene glycol as an internal mold release agent, and does not propose a technique for further improving the internal hardness of the rubber molded product or reducing the spin rate of the ball by selecting the types of compounding components in the core rubber composition. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-169485 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above circumstances, and has as its object to provide a rubber composition for a golf ball, which can achieve low spin characteristics when hitting the golf ball and improve flight performance by setting a large hardness difference in the hardness distribution within the core while maintaining a desired core hardness, and a golf ball using the same. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above-mentioned object, the present inventors have discovered that by incorporating the essential components (a) to (d) of the rubber composition of a golf ball core, namely (a) base rubber, (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and (d) an alcohol whose molecular weight divided by the number of hydroxyl groups (valence) is 70 or less, it is possible to set a large hardness difference in the hardness distribution within the core while maintaining the desired core hardness, thereby fully demonstrating low spin characteristics when hitting the golf ball, and have thus completed the present invention. The reason for this is not clear, but is presumed to be as follows.

[0008] That is, by blending the above-mentioned specific alcohol into the core material, the decomposition of the organic peroxide in the core compound is promoted, resulting in a unique crosslinked structure of the butadiene rubber. It is known that the decomposition efficiency of organic peroxides in the core rubber composition varies with temperature, with the decomposition efficiency increasing as the temperature rises. If the temperature is too high, the amount of decomposed radicals increases, causing the radicals to recombine with each other or become inactivated. As a result, the number of radicals that effectively contribute to crosslinking decreases. When the organic peroxide decomposes during core vulcanization and generates heat, the temperature near the core surface remains approximately the same as the vulcanization mold temperature, but the temperature near the core center becomes significantly higher than the mold temperature due to the accumulation of heat from the decomposition of the organic peroxide decomposed from the outside. It is speculated that blending a specific alcohol into the core allows the hydroxyl groups of the specific alcohol to promote the decomposition of the organic peroxide, thereby altering the radical reaction described above between the core center and the core surface. In other words, it is thought that the decomposition of the organic peroxide is further promoted near the core center, and the deactivation of radicals is further promoted, further reducing the amount of available radicals, resulting in a core with a large difference in crosslink density between the core center and the core surface.

[0009] Accordingly, the present invention provides the following rubber composition for a golf ball and golf ball. 1. Each of the following components (a) to (d): (a) base rubber; (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent; (c) organic peroxides, and (d) An alcohol having a molecular weight divided by the number of hydroxyl groups (valence) of 70 or less and a valence selected from the group consisting of tetrahydric, pentahydric, and hexahydric alcohols. and the compounding amount of component (d) is 0.5 to 3 parts by mass per 100 parts by mass of the base rubber (a). 2. The rubber composition for golf balls according to 1 above, wherein the value obtained by dividing the molecular weight of the alcohol of component (d) by the number of hydroxyl groups (valence) is 40.1 or greater. 3. The above component (d) , Ji (trimethylolpropane), pentaerythritol and sorbitol. 1 or 2 The rubber composition for golf balls described above. 4 The blending amount of the component (b) is 15 to 65 parts by mass per 100 parts by mass of the base rubber (a). 3 2. The rubber composition for a golf ball according to claim 1, wherein the rubber composition for a golf ball is a rubber composition for a golf ball. 5 Furthermore, the above-mentioned 1 to 3 containing an organic sulfur compound as component (e) 4 2. The rubber composition for a golf ball according to claim 1, wherein the rubber composition for a golf ball is a rubber composition for a golf ball. 6 The above-mentioned 1 to 3 wherein the vulcanized molded product of the rubber composition is a golf ball core. 5 2. The rubber composition for a golf ball according to claim 1, wherein the rubber composition for a golf ball is a rubber composition for a golf ball. 7 The above-mentioned rubber composition has a hardness difference between the surface and the center of a vulcanized molded product of the above-mentioned rubber composition of 20 or more in JIS-C hardness. 6 The rubber composition for golf balls described above. 8 A golf ball having a core and a single-layer or multi-layer cover covering the core, wherein the core is 5 2. A golf ball formed from the rubber composition according to claim 1. 9 In the core hardness distribution, the difference in hardness between the surface and the center of the core is 20 or more in JIS-C hardness.8 The golf ball described herein. [Effects of the Invention]

[0010] According to the rubber composition for golf balls of the present invention, when the rubber composition is used as each component of a golf ball, particularly as the core, the golf ball exhibits low spin characteristics upon impact, thereby improving flight performance.

[0011] The present invention will be described in more detail below. The rubber composition for golf balls of the present invention is characterized by containing the following components (a) to (d): (a) base rubber; (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent; (c) a crosslinking initiator, and (d) Alcohols whose molecular weight divided by the number of hydroxyl groups (valence) is 70 or less

[0012] The base rubber of component (a) is not particularly limited, but it is particularly preferred to use polybutadiene.

[0013] The polybutadiene preferably has cis-1,4-bonds in its polymer chain at 60% or more, preferably 80% or more, more preferably 90% or more, and most preferably 95% or more. If the proportion of cis-1,4-bonds in the polybutadiene molecule is too low, the resilience may decrease.

[0014] The content of 1,2-vinyl bonds contained in the polybutadiene is usually 2% or less, preferably 1.7% or less, and more preferably 1.5% or less in the polymer chain. If the content of 1,2-vinyl bonds is too high, the resilience may decrease.

[0015] The polybutadiene is (ML 1+4(100°C)) is preferably 20 or more, more preferably 30 or more, and the upper limit is preferably 120 or less, more preferably 100 or less, and even more preferably 80 or less.

[0016] The Mooney viscosity referred to in the present invention is an industrial viscosity index (JIS K 6300) measured by a Mooney viscometer, which is a type of rotational plasticity meter, and is expressed in units of ML. 1+4 (100°C). M indicates Mooney viscosity, L indicates a large rotor (L-type), 1+4 indicates a preheating time of 1 minute, a rotor rotation time of 4 minutes, and the measurements were taken under conditions of 100°C.

[0017] The polybutadiene that can be used is one synthesized using a rare earth element catalyst or a Group VIII metal compound catalyst.

[0018] The base rubber may contain polybutadiene rubber synthesized using a catalyst other than the lanthanum series rare earth compound. Also, styrene butadiene rubber (SBR), natural rubber, polyisoprene rubber, ethylene propylene diene rubber (EPDM), etc. may be compounded, either alone or in combination.

[0019] The proportion of the polybutadiene in the entire rubber is preferably at least 60% by mass, more preferably at least 70% by mass, and most preferably at least 90% by mass. Alternatively, 100% by mass of the base rubber, i.e., the entire base rubber, may be the polybutadiene.

[0020] Next, component (b) is a co-crosslinking agent, which is an α,β-unsaturated carboxylic acid and / or a metal salt thereof. The unsaturated carboxylic acid preferably has 3 to 8 carbon atoms, and specific examples thereof include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Specific examples of metals in the unsaturated carboxylic acids include zinc, sodium, magnesium, calcium, and aluminum, with zinc being particularly preferred. Therefore, zinc acrylate is the most preferred co-crosslinking agent.

[0021] The amount of component (b) blended per 100 parts by weight of the base rubber of component (a) is preferably at least 10 parts by weight, more preferably at least 15 parts by weight, and even more preferably at least 20 parts by weight, with the upper limit being preferably no more than 65 parts by weight, more preferably no more than 60 parts by weight, and even more preferably no more than 55 parts by weight. If the blending amount is less than the above range, the golf ball will be too soft and have poor resilience, while if the blending amount is more than the above range, the golf ball will be too hard, resulting in a poor shot feel and being brittle and having poor durability.

[0022] The co-crosslinking agent (b) preferably has an average particle size of 3 to 30 μm, more preferably 5 to 25 μm, and even more preferably 8 to 15 μm. If the average particle size of the co-crosslinking agent is less than 3 μm, it is likely to aggregate in the rubber composition, which increases the reactivity between acrylic acids and decreases the reactivity between base rubbers, which can result in an insufficient golf ball resilience. If the average particle size of the co-crosslinking agent exceeds 30 μm, the co-crosslinking agent particles become too large, resulting in greater variation in the properties of the resulting golf balls.

[0023] Component (c) is a crosslinking initiator. It is preferable to use an organic peroxide as the crosslinking initiator, particularly an organic peroxide with a 1-minute half-life temperature of 110 to 185°C. Examples of such organic 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. Other commercially available products include "Perhexa C-40," "Nyper BW," and "Perroil L" (all manufactured by NOF Corp.), as well as Luperco 231XL (manufactured by Atochem). These may be used alone or in combination.

[0024] The amount of component (c) added, per 100 parts by mass of the base rubber, is preferably at least 0.1 part by mass, and more preferably at least 0.3 part by mass, and the upper limit is preferably at most 5 parts by mass, more preferably at most 4 parts by mass, and even more preferably at most 3 parts by mass.

[0025] Next, component (d) is an alcohol component, i.e., an alcohol with a molecular weight divided by the number of hydroxyl groups (valence) of 70 or less. By using such a specific alcohol component as component (d), a cured rubber (core) with the desired core hardness distribution can be obtained during vulcanization (curing) of the rubber composition, thereby sufficiently achieving low spin on impact and achieving excellent flight performance. The term "alcohol" as used herein refers to a substance having one or more alcoholic hydroxy groups, and includes the condensation polymerization of polyhydric alcohols having two or more hydroxy groups. The above alcohols also include sugar alcohols such as alditol.

[0026] The alcohol is preferably a hexahydric or lower alcohol (an alcohol having six or fewer alcoholic hydroxy groups). Specific examples include, but are not limited to, methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, glycerin, butanetriol, trimethylolethane, trimethylolpropane, di(trimethylolpropane), pentaerythritol, and sorbitol. There are no particular restrictions on the molecular weight of these alcohols, but they are preferably 300 or less, more preferably 250 or less, and even more preferably 200 or less. If the molecular weight is too large, i.e., if the number of carbon atoms is too large, the desired core hardness distribution cannot be achieved, and the ball's spin rate upon impact cannot be adequately reduced.

[0027] The amount of component (d) is preferably at least 0.1 part by weight, more preferably at least 0.5 part by weight, per 100 parts by weight of the base rubber, with the upper limit being preferably no more than 10 parts by weight, more preferably no more than 6 parts by weight, and even more preferably no more than 3 parts by weight. If the amount of component (d) is too high, the hardness will be too soft and the desired feel, durability, and resilience will not be achieved, while if the amount is too low, the desired core hardness distribution will not be achieved, and the ball may not be able to fully achieve low spin when hit.

[0028] In addition to the components (a) to (d) described above, various additives such as fillers, antioxidants, and organic sulfur compounds may be added as long as they do not impair the effects of the present invention.

[0029] Suitable fillers include, for example, zinc oxide, barium sulfate, and calcium carbonate. These may be used alone or in combination. The amount of filler added is preferably at least 1 part by weight, more preferably at least 3 parts by weight, and even more preferably at least 5 parts by weight per 100 parts by weight of the base rubber. The upper limit of the amount added is preferably no more than 100 parts by weight, more preferably no more than 60 parts by weight, and even more preferably no more than 40 parts by weight per 100 parts by weight of the base rubber. If the amount added is too high or too low, it may be difficult to achieve the appropriate weight and appropriate resilience.

[0030] The antioxidant is not particularly limited, but examples include phenolic antioxidants such as 2,2-methylenebis(4-methyl-6-tert-butylphenol), 4,4-butylidenebis(3-methyl-6-tert-butylphenol), and 2,2-methylenebis(4-ethyl-6-tert-butylphenol). Commercially available products include Nocrac NS-6, Nocrac NS-30, and Nocrac NS-5 (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.). These antioxidants may be used alone or in combination of two or more. The amount of antioxidant is not particularly limited, but is preferably at least 0.05 parts by weight, more preferably at least 0.1 parts by weight, and preferably up to 1.0 parts by weight, more preferably up to 0.7 parts by weight, and even more preferably up to 0.4 parts by weight, per 100 parts by weight of base rubber. If the amount is too high or too low, the appropriate core hardness gradient may not be achieved, resulting in the failure to achieve favorable resilience, durability, and low spin effect on full shots.

[0031] The organic sulfur compound is not particularly limited, and examples thereof include thiophenols, thionaphthols, diphenyl polysulfides, halogenated thiophenols, and metal salts thereof. Specific examples include zinc salts of pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, parachlorothiophenol, and the like; diphenyl polysulfide, dibenzyl polysulfide, dibenzoyl polysulfide, dibenzothiazoyl polysulfide, dithiobenzoyl polysulfide, and the like, each having 2 to 4 sulfur atoms. These compounds may be used alone or in combination of two or more. Among these, the zinc salt of pentachlorothiophenol and / or diphenyl disulfide are preferred.

[0032] The amount of the organic sulfur compound is preferably at least 0.05 parts by mass, more preferably at least 0.1 parts by mass, and even more preferably at least 0.2 parts by mass, per 100 parts by mass of the base rubber, and it is recommended that the upper limit be preferably at most 3 parts by mass, more preferably at most 2 parts by mass, and even more preferably at most 1 part by mass. If the amount of the organic sulfur compound is too high, the hardness of the hot-molded product of the rubber composition may become too soft, while if the amount is too low, improvement in resilience may not be expected.

[0033] The core can be produced by vulcanizing and curing a rubber composition containing the above components. For example, the components are kneaded using a kneader such as a Banbury mixer or a roll, compression molded or injection molded using a core mold, and then appropriately heated to a temperature sufficient for the organic peroxide and co-crosslinking agent to act, at about 100 to 200°C for 10 to 40 minutes, thereby curing the molded product.

[0034] The above-described compounding allows the molded rubber for a golf ball after vulcanization and curing to have a hardness gradient with a large difference in hardness between the surface and the center. By using the above-described molded rubber for a golf ball as a core for a golf ball, the durability of the golf ball can be improved while maintaining good spin characteristics.

[0035] There are no particular restrictions on the center hardness of the core, but it is preferably at least 40, more preferably at least 45, and even more preferably at least 50, in JIS-C standard, with the upper limit being preferably no more than 75, more preferably no more than 70, and even more preferably no more than 65. If the center hardness of the core deviates from the above range, the feel at impact may be poor or durability may be reduced, and the low spin effect may not be achieved.

[0036] The surface hardness of the core is not particularly limited, but is preferably at least 65, more preferably at least 70, and even more preferably at least 72, according to the JIS-C standard, with the upper limit being preferably no more than 95, more preferably no more than 90, and even more preferably no more than 88. If the surface hardness of the core is lower than the above range, the resilience may be reduced and sufficient distance may not be achieved. On the other hand, if the surface hardness of the core is higher than the above range, the feel on impact may be too hard and the durability to cracking due to repeated impacts may be reduced.

[0037] The core's hardness distribution should be such that the hardness difference between the surface and center is sufficiently large. Specifically, the difference in hardness between the core's surface A and center B is preferably 20 or more, more preferably 25 or more, and even more preferably 30 or more, on the JIS-C hardness scale. The upper limit is preferably 50 or less, more preferably 45 or less, and even more preferably 40 or less. If the hardness difference is too small, the low-spin effect upon W#1 shots may be insufficient, resulting in a loss of distance. On the other hand, if the hardness difference is too large, the initial velocity of the golf ball may be low when actually struck, resulting in a loss of distance or reduced durability to cracking due to repeated impacts. Here, the "center hardness" refers to the hardness measured at the center of a cross section obtained by cutting the core in half (through the center), and the "surface hardness" refers to the hardness measured at the surface (spherical surface) of the core. The "JIS-C hardness" refers to the hardness measured using a spring-type hardness tester (JIS-C type) specified in JIS K 6301-1975.

[0038] The hardness gradient of the core used in the present invention is preferably such that the hardness remains constant or increases, rather than decreases, from the center of the core toward the surface.

[0039] Furthermore, the compression hardness (deformation amount) of the core (heat-molded product) when an initial load of 98 N (10 kgf) is applied followed by a final load of 1275 N (130 kgf) is not particularly limited, but is preferably 2.0 mm or more, more preferably 2.3 mm or more, and even more preferably 2.5 mm or more. The upper limit is preferably 6.0 mm or less, more preferably 5.5 mm or less, and even more preferably 5.0 mm or less. If the core is too soft, the spin reduction effect may not be sufficient and the resilience may be reduced. If the core is too small, the spin reduction effect may not be obtained and the feel may be hard.

[0040] The diameter of the core is not particularly limited and depends on the layer structure of the golf ball to be manufactured, but is preferably at least 30 mm, more preferably at least 35 mm, with the upper limit being preferably not more than 41 mm, more preferably not more than 40 mm. If the core diameter is outside this range, the initial velocity of the ball may be low or the appropriate spin characteristics may not be obtained.

[0041] The rubber composition is preferably used as a core for a golf ball, as described above. The golf ball of the present invention preferably has a structure including a core and a cover having one or more layers.

[0042] Next, the cover layer or layers that encase the core will be described. There are no particular restrictions on the cover material, but any of the various known materials used in golf balls, such as ionomer resins and urethane elastomers, can be used.

[0043] To further reduce the spin rate of the ball, it is particularly preferable to use a highly neutralized ionomer material in the layer adjacent to the core. Specifically, it is preferable to use a material containing the following components (i) to (iv): (i-1) an olefin-unsaturated carboxylic acid binary random copolymer and / or a metal ion-neutralized product of an olefin-unsaturated carboxylic acid binary random copolymer; (ii-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer and / or a metal ion-neutralized product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer in a mass ratio of 100:0 to 0:100 (i) a base resin, and (ii) a non-ionomer thermoplastic elastomer in a mass ratio of 100:0 to 50:50 (ii-3), per 100 parts by mass of a resin component in which (iii) 5 to 80 parts by mass of a fatty acid and / or a derivative thereof having a molecular weight of 228 to 1500; (ix) 0.1 to 17 parts by mass of a basic inorganic metal compound capable of neutralizing unneutralized acid groups in the above components (i) and (iii). In particular, when using a mixed material of the above components (i) to (ix), it is preferable to use one in which 70% or more of the acid groups have been neutralized.

[0044] The outermost layer of the cover is preferably made primarily of a urethane material, particularly a thermoplastic urethane elastomer.

[0045] Furthermore, one or more cover layers (intermediate layers) may be molded between the layer adjacent to the core and the outermost cover layer, and in this case, the intermediate layer is preferably made of a thermoplastic resin such as an ionomer.

[0046] The cover of the present invention can be obtained, for example, by placing a single-layer or multi-layer core (depending on the type of ball) pre-prepared in a mold, heating, mixing, and melting the mixture, and injection-molding the mixture to form the desired cover around the core. In this case, the cover can be produced under conditions that ensure excellent thermal stability, fluidity, and moldability, resulting in a golf ball with high resilience, a good feel, and excellent abrasion resistance. In addition to the above, the cover can also be formed by molding a pair of hemispherical half cups from the cover material of the present invention in advance, encasing the core in these half cups, and then pressure-molding the core at 120 to 170°C for 1 to 5 minutes.

[0047] When the cover has one layer, its thickness can be 0.3 to 3 mm. When the cover has two layers, the outer cover layer can be 0.3 to 2.0 mm thick, and the inner cover layer can be 0.3 to 2.0 mm thick. There are no particular restrictions on the Shore D hardness of each layer (cover layer) constituting the cover, but it is preferably 40 or greater, more preferably 45 or greater, and the upper limit is preferably 70 or less, more preferably 65 or less.

[0048] Numerous dimples are formed on the surface of the outermost layer of the cover, and the cover may be further subjected to various treatments such as priming, stamping, painting, etc. In particular, when such surface treatments are applied to a cover formed from the cover material of the present invention, the good moldability of the cover surface allows for easy workability.

[0049] The present invention relates to a golf ball in which the above rubber composition is used as a core material for at least one layer. The type of golf ball is not particularly limited, so long as it has a core and at least one cover layer. For example, the rubber composition can be used in solid golf balls such as two-piece and three-piece solid golf balls in which a solid core is covered with a cover, and multi-piece golf balls with a three or more layer structure, as well as in the core of a thread-wound golf ball in which a thread-wound core is covered with a single-layer or two or more multi-layer cover. [Example]

[0050] 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.

[0051] [Examples 1 to 10, Comparative Examples 1 to 3] Using the core material whose main component is polybutadiene shown in Table 1 below, core compositions were prepared using the rubber blends of Examples 1 to 10 and Comparative Examples 1 to 3. These were then vulcanized at 155°C for 20 minutes, and the core surface was polished to produce cores with a diameter of 38.6 mm.

[0052] [Table 1]

[0053] Details of the above formulation are as follows: Polybutadiene rubber: Product name "BR01" (manufactured by JSR Corporation) Zinc oxide: Product name "Triple Zinc Oxide" (manufactured by Sakai Chemical Industry Co., Ltd.) Antioxidant: Product name "Nocrac NS-6" (phenolic antioxidant: manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Zinc acrylate: Product name "ZN-DA85S" (85% zinc acrylate / 15% zinc stearate), manufactured by Nippon Shokubai Co., Ltd. Pentachlorothiophenol zinc salt: Wako Pure Chemical Industries, Ltd. Organic peroxide (dicumyl peroxide): Trade name "Percumyl D" (manufactured by NOF Corporation) Propylene glycol (dihydric alcohol): molecular weight 76.1 (Hayashi Pure Chemical Industries, Ltd.) Glycerin (trihydric alcohol): molecular weight 92.1 (Hayashi Pure Chemical Industries, Ltd.) 1,2,4-butanetriol (trihydric alcohol): molecular weight 106.1 (Tokyo Chemical Industry Co., Ltd.) Trimethylolpropane (trihydric alcohol): molecular weight 134.8 (Tokyo Chemical Industry Co., Ltd.) Di(trimethylolpropane) (tetrahydric alcohol): molecular weight 250.3 (Tokyo Chemical Industry Co., Ltd.) Trimethylolethane (trihydric alcohol): molecular weight 120.2 (Tokyo Chemical Industry Co., Ltd.) Pentaerythritol (tetrahydric alcohol): molecular weight 136.2 (Fujifilm Wako Pure Chemical Industries, Ltd.) Sorbitol (hexahydric alcohol): molecular weight 182.2 (Fujifilm Wako Pure Chemical Industries, Ltd.) Stearyl alcohol (monohydric alcohol): molecular weight 270.5 (trade name "NAA-45" / NOF Corporation) Polyethylene glycol (dihydric alcohol): average molecular weight 400.0 (product name "Polyethylene Glycol #400" / NOF Corporation)

[0054] Core cross-sectional hardness For each of the cores having a diameter of 38.6 mm in each of the above examples and comparative examples, the cross-sectional hardness was measured at various positions including the surface and center by the following method. (1) Core surface hardness At a temperature of 23±1°C, the needle of a hardness tester was set perpendicular to the surface of the spherical core, and the JIS-C hardness was measured at four random points on the surface of the core. The average of these measurements was used as the measurement value for one ball, and the average of three cores was calculated. The measurements are shown in Table 3. (2) Core cross-sectional hardness The core was cut into a flat surface so that the cross section passed through the center of the core. At a temperature of 23±1°C, a hardness tester needle was placed perpendicular to the flat cross section, and the hardness was measured using a JIS-C hardness tester at the center of the hemispherical core and at 2mm intervals from the center toward the surface. The hardness was measured as a single ball, and the average of three cores was calculated. The measured values ​​are shown in Table 3.

[0055] Compression hardness of core and ball The core and ball were compressed at a rate of 10 mm / s at a temperature of 23±1°C, and the compression hardness (deformation) (mm) of the core and ball was measured from an initial load of 98 N (10 kgf) to a final load of 1275 N (130 kgf). The average value of 10 measurements was calculated.

[0056] Formation of the cover (intermediate and outermost layers) Next, using an injection molding die, the intermediate layer material (ionomer resin material) shown in Table 2 was injection molded around the surface of the core to form an intermediate layer with a thickness of 1.3 mm and a Shore D hardness of 64. Next, using another injection molding die, the outermost layer material (urethane resin material) shown in Table 2 was injection molded around the intermediate layer-coated sphere to form an outermost layer with a thickness of 0.8 mm and a Shore D hardness of 40.

[0057] [Table 2]

[0058] The details of the ingredients in the above table are as follows: "Himilan 1706," "Himilan 1557," and "Himilan 1605": Ionomer resins manufactured by Mitsui Dow Polychemicals "TPU": "Pandex" manufactured by DIC Covestro Polymer, an ether-type thermoplastic polyurethane with a Shore D hardness of 40. Polyethylene wax: Sanwax 161P (manufactured by Sanyo Chemical Industries, Ltd.) Isocyanate compound: 4,4'-diphenylmethane diisocyanate

[0059] The driver spin rate of the resulting golf balls was evaluated by the following method, and the results are shown in Table 3.

[0060] Driver spin rate A driver (W#1) was attached to the golf hitting robot, and the spin rate of the ball was measured immediately after hitting it at a head speed of 45 m / s using an initial condition measurement device. The club used was the Bridgestone Sports TourB XD-3 Driver (2016 model) (loft angle 9.5°).

[0061] [Table 3]

[0062] As shown in Table 3, Examples 1 to 10, which used as the core material a rubber composition containing an alcohol whose molecular weight divided by the number of hydroxyl groups (valence) was 70 or less, had a sufficient hardness difference of 20 or more in JIS-C scale between the center and surface of the core compared to Comparative Examples 1 to 3, and the spin rate of the golf ball when hit with a driver was approximately 100 to 500 revolutions lower than in Comparative Examples 1 to 3, which did not have a sufficient hardness difference of less than 20. This shows that the distance traveled was improved.

Claims

1. Each of the following components (a) to (d): (a) a base rubber; (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent; (c) organic peroxides, and (d) an alcohol having a molecular weight divided by the number of hydroxyl groups (valence) of 70 or less and a valence selected from the group consisting of tetrahydric, pentahydric, and hexahydric alcohols; wherein the amount of component (d) blended is 0.5 to 3 parts by weight per 100 parts by weight of the base rubber (a).

2. 2. The rubber composition for golf balls according to claim 1, wherein the alcohol of component (d) has a molecular weight divided by the number of hydroxyl groups (valence) of 40.1 or more.

3. 3. The rubber composition for golf balls according to claim 1, wherein component (d) is one or more alcohols selected from the group consisting of di(trimethylolpropane), pentaerythritol, and sorbitol.

4. 4. The rubber composition for golf balls according to claim 1, wherein the blending amount of component (b) is 15 to 65 parts by weight per 100 parts by weight of the base rubber (a).

5. 5. The rubber composition for golf balls according to claim 1, further comprising an organic sulfur compound as component (e).

6. 6. The rubber composition for golf balls according to claim 1, wherein the vulcanized molded product of said rubber composition is a core for a golf ball.

7. 7. The rubber composition for golf balls according to claim 6, wherein the difference in hardness between the surface and the center of a vulcanized product of said rubber composition is 20 or more in JIS-C hardness.

8. A golf ball having a core and a single-layer or multi-layer cover covering the core, wherein the core is formed from the rubber composition according to any one of claims 1 to 5.

9. 9. The golf ball of claim 8, wherein the core has a hardness distribution in which the difference in hardness between the surface and the center of the core is 20 or more in JIS-C hardness.

Images