golf balls

Abstract

SOLUTION: To provide a golf ball comprising a core and one or more covers, the core is formed of a heated molded object of a rubber composition which has (a)-(d) components: (a) a substrate rubber; (b) water or / and a lower alcohol whose molecular weight is less than 200; (c)α,β-unsaturated carboxylic acid and / or a metallic salt of the same; (d) an organic peroxide, the substrate rubber of the (a) component is configured so that, the same is obtained by mixing, (a-1) polybutadiene, (a-2) non-neutralized ethylene-unsaturated carboxylic acid copolymer, then, neutralizing the mixture by (a-3) a metal oxide, where an acid content of the (a-2) component is 5 mass% or greater, a blending amount of (a-2) component is 10 pts.mass or lower to 100 pts.mass of a total amount of the (a-1) and (a-2) components, and a hardness difference of a surface and a center of the core is 15 or greater in JIS-C hardness.EFFECT: According to the invention, a golf ball can hold high resilience and achieve both of, low spin and excellent durability.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a golf ball having a core and a cover with one or more layers. [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] An example of a new technology for increasing the hardness difference between the surface and center of a core to reduce the spin rate of the ball during full shots is the golf ball described in Japanese Patent Application Publication No. 2015-47502 (corresponding U.S. Patent Application Publication No. 2015-0065268). In this publication, a core is obtained by blending water into a rubber composition for the core and vulcanizing the resulting composition. Another example is the golf ball described in Japanese Patent Application Publication No. 2019-213606 (corresponding U.S. Patent Application Publication No. 2019-0375917). In this publication, a lower alcohol is blended into a rubber composition for the core and vulcanizing the resulting composition to obtain the core. However, these technologies have the drawback of reducing the impact durability of the ball by increasing the hardness difference between the surface and center of the core.

[0005] The following specific techniques have been proposed to improve the impact durability of a ball while maintaining the core's resilience. One technique for compounding an ionomer resin with polybutadiene rubber in a core rubber composition is described in U.S. Patent Application Publication No. 2002-0086745. Japanese Patent Application Publication No. 2007-209472 (corresponding to U.S. Patent Application Publication No. 2007-0184916) describes a technique for mixing polybutadiene rubber with an unneutralized ethylene-unsaturated carboxylic acid copolymer and then neutralizing the mixture with a metal cation source. Japanese Patent Application Publication Nos. 2007-061605 and 2012-254304 (corresponding to U.S. Patent Application Publication No. 2007-0049419) also describe rubber compositions containing polybutadiene rubber, an olefin-containing polymer having a specific acid content, and an inorganic metal compound. However, all of these techniques involve incorporating a resin component into the rubber composition, and have the drawback that if the amount of resin component added is increased, the resilience is significantly reduced. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-47502 [Patent Document 2] U.S. Patent Application Publication No. 2015-0065268 [Patent Document 3] Japanese Patent Application Publication No. 2019-213606 [Patent Document 4] U.S. Patent Application Publication No. 2019-0375917 [Patent Document 5] U.S. Patent Application Publication No. 2002-0086745 [Patent Document 6] Japanese Patent Application Laid-Open No. 2007-209472 [Patent Document 7] U.S. Patent Application Publication No. 2007-0184916 [Patent Document 8] Japanese Patent Application Laid-Open No. 2007-061605 [Patent Document 9] Japanese Patent Application Laid-Open No. 2012-254304 [Patent Document 10] U.S. Patent Application Publication No. 2007-0049419 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in view of the above circumstances, and has as its object to provide a golf ball that maintains high resilience while achieving low spin and excellent durability. [Means for solving the problem]

[0008] As a result of extensive research into achieving the above-mentioned object, the present inventors have found that the rubber composition of a golf ball core essentially comprises the above-mentioned components (a) to (d), namely, (a) base rubber, (b) water and / or a lower alcohol having a molecular weight of less than 200, (c) an α,β-unsaturated carboxylic acid and / or a metal salt thereof, and (d) an organic peroxide, and that the base rubber of component (a) is a rubber obtained by mixing (a-1) polybutadiene with (a-2) an unneutralized ethylene-unsaturated carboxylic acid copolymer and then neutralizing the mixture with (a-3) a metal oxide, and that component (a-2) has an acid content of 5% by mass or more and the amount of component (a-2) is adjusted to 10 parts by mass or less per 100 parts by mass of the total amount of components (a-1) and (a-2), thereby achieving a large difference in hardness in the cross-sectional hardness within the core while maintaining the desired core hardness, thereby sufficiently exhibiting low spin characteristics when hitting the golf ball and providing excellent durability upon impact. This finding led to the completion of the present invention.

[0009] Accordingly, the present invention provides the following golf balls. 1. Core and A three-piece solid golf ball comprising a two-layer cover, i.e., an intermediate layer and an outermost layer, wherein the intermediate layer is formed primarily from an ionomer resin, and the outermost layer is formed primarily from a urethane resin, and The core comprises the following components (a) to (d): (a) Base rubber (b) water and / or a lower alcohol with a molecular weight of less than 200 (c) α,β-unsaturated carboxylic acid and / or its metal salt (d) Organic peroxide The base rubber of component (a) is a rubber obtained by mixing (a-1) polybutadiene and (a-2) unneutralized ethylene-unsaturated carboxylic acid copolymer, followed by neutralization with (a-3) metal oxide, and the acid content of component (a-2) is 5~15% by mass wherein the blending amount of component (a-2) is 10 parts by mass or less per 100 parts by mass of the combined amount of components (a-1) and (a-2), and the difference in hardness between the surface and center of the core is 15 or more in JIS-C hardness. 2. The golf ball according to 1 above, wherein the unsaturated carboxylic acid in component (a-2) is acrylic acid or methacrylic acid. 3. The golf ball according to 1 or 2 above, wherein (a-2) the unneutralized ethylene-unsaturated carboxylic acid copolymer is completely neutralized with (a-3) a metal oxide. 4. The golf ball according to any one of 1 to 3 above, wherein the blending amount of component (b) is 0.1 to 10 parts by mass per 100 parts by mass of component (a). 5. (b) 5. The golf ball according to any one of 1 to 4 above, wherein the lower alcohol component is one or more alcohols selected from the group consisting of butanol, glycerin, ethylene glycol, propylene glycol, butanetriol, trimethylolethane, trimethylolpropane, di(trimethylolpropane), pentaerythritol, and sorbitol. [Effects of the Invention]

[0010] The golf ball of the present invention can achieve both low spin and excellent durability while maintaining high resilience. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described in more detail below. The golf ball of the present invention has a core and a cover of one or more layers, and the core may be composed of one layer or, if necessary, two or more layers. The core is composed of the following components (a) to (d): (a) Base rubber (b) water and / or a lower alcohol with a molecular weight of less than 200 (c) α,β-unsaturated carboxylic acid and / or its metal salt (d) Organic peroxide The rubber composition is formed by hot molding a rubber composition containing the above as an essential component.

[0012] The base rubber of component (a) is a rubber obtained by mixing (a-1) polybutadiene and (a-2) unneutralized ethylene-unsaturated carboxylic acid copolymer, followed by neutralization with (a-3) metal oxide.

[0013] The polybutadiene of component (a-1) 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 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 has a Mooney viscosity (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 is an industrial viscosity index (JIS K 6300) measured with 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 unneutralized ethylene-unsaturated carboxylic acid copolymer of component (a-2) is particularly effective in reducing the spin rate of the ball after hitting with a driver (W#1). The unsaturated carboxylic acid has 3 to 8 carbon atoms, and specific examples include acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Acrylic acid and methacrylic acid are preferred because they provide high resilience.

[0019] The acid content of component (a-2) is 5% by mass or more, preferably 7% by mass or more, more preferably 8% by mass or more, even more preferably 9% by mass or more, and most preferably 10% by mass or more. If this acid content is low, a sufficiently high initial velocity or good durability may not be obtained. The upper limit of this acid content is preferably 26% by mass or less, more preferably 23% by mass or less, and even more preferably 20% by mass or less.

[0020] The blend amount of component (a-2) is 10 parts by weight or less per 100 parts by weight of the combined total of components (a-1) and (a-2). The greater the proportion of component (a-2), the better the durability, but if it exceeds 10 parts by weight, the core resilience may decrease, resulting in a decrease in the initial velocity of the ball.

[0021] The melt flow rate (MFR) of the unneutralized ethylene-unsaturated carboxylic acid copolymer of component (a-2) is desirably 10 g / 10 min or more to enhance dispersibility in the matrix of the base rubber, and is preferably 20 g / 10 min or more, more preferably 30 g / 10 min or more, even more preferably 40 g / 10 min or more, and most preferably 50 g / 10 min or more. This melt flow rate (MFR) value is measured in accordance with JIS-K7210-1 under conditions of a test temperature of 190°C and a test load of 21.18 N (2.16 kgf).

[0022] The melting point of the unneutralized ethylene-unsaturated carboxylic acid copolymer of component (a-2) is preferably 120°C or lower, preferably 115°C or lower, more preferably 110°C or lower, even more preferably 105°C or lower, and most preferably 100°C or lower. The lower the melting point, the easier it is for component (a-2) to disperse in the matrix of the base rubber. The lower limit of this melting point is preferably 60°C or higher, more preferably 70°C or higher, and most preferably 80°C or higher.

[0023] The unneutralized ethylene-unsaturated carboxylic acid copolymer of component (a-2) can be a commercially available product, such as a product name "Nucrel N1110H" or "Nucrel N1560." These may be used alone or in combination of two or more.

[0024] The metal oxide of component (a-3) is not particularly limited, but specific examples include magnesium carbonate, magnesium acetate, magnesium oxide, zinc oxide, zinc acetate, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide, lithium carbonate, potassium hydroxide, potassium carbonate, etc. These may be used alone or in combination of two or more.

[0025] The amount of component (a-3) is preferably 1 to 50 parts by weight, and particularly preferably 2 to 30 parts by weight, per 100 parts by weight of the total amount of components (a-1) and (a-2). If this amount is too small, the neutralization reaction may be insufficient, resulting in insufficient resilience performance and durability. On the other hand, if this amount is too large, the weight of the core may become too heavy, exceeding the weight range suitable for a golf ball, which may be undesirable.

[0026] By blending the metal oxide (a-3) in the above-mentioned amount, it is preferable that 80 mol % or more of the unneutralized unsaturated carboxylic acid (a-2) is neutralized. The degree of neutralization of this unsaturated carboxylic acid is more preferably 90 mol % or more, and most preferably 100 mol %. That is, it is most preferable that the unneutralized ethylene-unsaturated carboxylic acid copolymer (a-2) is completely neutralized with the metal oxide (a-3), in order to fully exert the desired effects of the present invention.

[0027] There are no particular limitations on the method for mixing (a-1) polybutadiene and (a-2) unneutralized ethylene-unsaturated carboxylic acid copolymer and then neutralizing with (a-3) metal oxide, but one example is the following preparation method. For example, the (a-1) and (a-2) components are thoroughly mixed in a rubber pressure kneader, and then the (a-3) component is added at a rubber temperature of 100°C or higher, preferably 120°C or higher, and the mixture is mixed at a rotor rotation speed of 20 to 40 rpm for 3 to 20 minutes, followed by cooling. This preparation method can produce a rubber composition having a chemical structure in which all or part of the carboxyl groups in the (a-2) component have been neutralized with the (a-3) metal oxide in the presence of the (a-1) component.

[0028] The water (b) used in the present invention is not particularly limited, and may be distilled water or tap water, but it is particularly preferable to use distilled water that does not contain impurities. The amount of water blended is preferably 0.1 parts by weight or more, more preferably 0.3 parts by weight or more, per 100 parts by weight of the base rubber, and the upper limit is preferably 5 parts by weight or less, more preferably 4 parts by weight or less.

[0029] Furthermore, by blending an appropriate amount of the water, the water content of the rubber composition before vulcanization is preferably 1000 ppm or more, more preferably 1500 ppm or more. The upper limit is preferably 8500 ppm or less, more preferably 8000 ppm or less. If the water content of the rubber composition is too low, it may be difficult to obtain an appropriate crosslink density, making it difficult to mold a golf ball with low energy loss and low spin. If the water content of the rubber composition is too high, the core may become too soft, making it difficult to obtain an appropriate core initial velocity.

[0030] Although it is possible to directly compound water into the rubber composition, the following methods (i) to (iii) can be employed. (i) A method in which mist-like water is applied to all or part of the rubber composition (compounded material) using steam or ultrasonic waves. (ii) A method in which the rubber composition is wholly or partially immersed in water (iii) A method in which all or a part of the rubber composition is left in a high humidity environment for a certain period of time in a place where humidity can be controlled, such as a humidity chamber. The high humidity environment is not particularly limited as long as it is an environment that can moisten the rubber composition, etc., but a humidity of 40 to 100% is preferable.

[0031] Water can also be processed into a jelly-like form and blended into the rubber composition. Alternatively, water can be preliminarily supported on fillers, unvulcanized rubber, rubber crumb, or the like, and then blended into the rubber composition. This embodiment offers superior workability compared to directly blending water, thereby improving golf ball production efficiency. There are no particular restrictions on the type of material containing a predetermined amount of water. Examples include fillers, unvulcanized rubber, rubber crumb, and the like that contain sufficient water. It is particularly preferable to use a material that does not impair durability or resilience. The moisture content of the above-mentioned material is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more. The upper limit is preferably 99% by mass or less, more preferably 95% by mass or less.

[0032] In addition, in the present invention, a lower alcohol having a molecular weight of less than 200 can be used instead of the water. The term "alcohol" as used herein refers to a substance having one or more alcoholic hydroxy groups, and includes alcohols obtained by condensation polymerization of polyhydric alcohols having two or more hydroxy groups. Furthermore, the term "lower alcohol" refers to an alcohol having a small number of carbon atoms, i.e., a small molecular weight. By incorporating this lower alcohol into the rubber composition, a cured rubber (core) having the desired core hardness gradient can be obtained during vulcanization (curing) of the rubber composition, thereby sufficiently realizing low spin on impact and achieving excellent flight performance.

[0033] The lower alcohol is particularly 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. Furthermore, the molecular weight of these alcohols is less than 200, preferably less than 150, and more preferably less than 100. If the molecular weight is too high, i.e., if the number of carbon atoms is too large, the desired core hardness gradient cannot be achieved, and the ball's spin rate upon impact cannot be adequately reduced.

[0034] The amount of the lower alcohol is preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, and even 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 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 gradient will not be achieved, and the ball may not be able to achieve a sufficient low spin rate upon impact.

[0035] Next, component (c) 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 acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Specific examples of the metal of the unsaturated carboxylic acid include zinc, sodium, magnesium, calcium, and aluminum, with zinc being particularly preferred. Therefore, zinc acrylate is the most preferred co-crosslinking agent.

[0036] The amount of component (c) 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, brittleness, and poor durability.

[0037] The co-crosslinking agent (c) 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 the base rubbers, which can result in an insufficient resilience of the golf ball. 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.

[0038] Component (d) is an organic peroxide. It is preferable to use an organic peroxide with a one-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.

[0039] The amount of component (d) 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] The above-described compounding allows the core, which is a molded rubber product after vulcanization and hardening, to have a hardness gradient with a large difference in hardness between the surface and the center. By using the above-described molded rubber product for a golf ball as a core for a golf ball, it is possible to improve the durability of the golf ball while maintaining its good spin characteristics.

[0047] 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.

[0048] 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.

[0049] The difference in hardness between the surface and center of the core is preferably 15 or more, more preferably 20 or more, even more preferably 24 or more, and most 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.

[0050] 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.

[0051] Furthermore, the compression hardness (deformation amount) of the core (heat-molded product) when an initial load of 98 N (10 kgf) is applied and a final load of 1275 N (130 kgf) is applied is not particularly limited, but is preferably 2.0 mm or more, more preferably 2.3 mm or more, even more preferably 2.5 mm or more, and most preferably 2.8 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 low spin effect may not be achieved and the resilience may be reduced. If the core is too soft, the low spin effect may not be achieved and the feel may be hard.

[0052] 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.

[0053] 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.

[0054] 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 (A) to (D): (A-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; (A-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 are blended in a mass ratio of 100:0 to 0:100 (A) a base resin, and (B) a non-ionomer thermoplastic elastomer are blended in a mass ratio of 100:0 to 50:50 (B), A mixed material containing 5 to 80 parts by mass of (C) a fatty acid and / or a derivative thereof having a molecular weight of 228 to 1500, and 0.1 to 17 parts by mass of (D) a basic inorganic metal compound capable of neutralizing the unneutralized acid groups in the components (A) and (B). In particular, when using a mixed material of the components (A) to (D), it is preferable to use one in which 70% or more of the acid groups have been neutralized.

[0055] The outermost layer of the cover is preferably made primarily of a urethane material, particularly a thermoplastic urethane elastomer.

[0056] 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.

[0057] To obtain the cover, for example, a single-layer or multi-layer core (depending on the type of ball) previously prepared may be placed in a mold, the mixture heated, mixed, and melted, and injection-molded to form the desired cover around the core. In this case, the cover can be manufactured 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, encasing the core in these half cups, and then pressure-molding the half cups at 120 to 170°C for 1 to 5 minutes.

[0058] When the cover has one layer, its thickness can be 0.3 to 3.0 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.

[0059] 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 the cover, the good formability of the cover surface allows for easy workability.

[0060] The type of golf ball of the present invention is not particularly limited as long as it has a core and at least one cover layer, including, for example, two-piece and three-piece solid golf balls in which a solid core is covered with a cover, multi-piece golf balls with three or more layers, and thread-wound golf balls in which a thread-wound core is covered with a single-layer or two or more multi-layer cover. [Example]

[0061] 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.

[0062] [Examples 1 to 6, Comparative Examples 1 to 5] Using the rubber compositions containing polybutadiene as the main component shown in Table 1 below, core compositions are prepared using the rubber blends (components and blending amounts) of Examples 1 to 6 and Comparative Examples 1 to 5. The mixing and molding methods for these rubber compositions are as follows.

[0063] Rubber composition mixing and molding methods The core composition is prepared using the core materials listed in Table 1 below. First, (a-1) polybutadiene rubber and (a-2) or (a-2') ethylene-methacrylic acid copolymer are mixed in a Plastomill manufactured by Toyo Seiki Seisakusho Co., Ltd. at a set temperature of 120°C and 30 rpm for 10 minutes, then (a-3) zinc oxide is added and mixed at 30 rpm for 5 minutes. The mixture is then discharged and cooled. In the second step, (c) zinc acrylate, antioxidant, and pentachlorothiophenol zinc salt are added to the mixture, and the mixture is mixed in the Plastomill at a set temperature of 100°C and 30 rpm for 5 minutes. The mixture is then discharged and cooled. In the third step, (b) water or alcohol and (d) organic peroxide are added to the mixture, and the mixture is mixed in the Plastomill at a set temperature of 40°C and 30 rpm for 5 minutes. The mixture is then discharged. The resulting mixtures are vulcanized at 155°C for 20 minutes, and the core surface is polished to produce cores with a diameter of 38.5 mm.

[0064] [Table 1]

[0065] Details of the rubber compounding in Table 1 below are as follows: Polybutadiene rubber: Product name "BR01" (manufactured by JSR Corporation) "AN4214C", "N1110H", and "N1560" are all "Nucrel" (ethylene-unsaturated carboxylic acid copolymer) manufactured by Mitsui Dow Polychemicals. Zinc oxide: Product name "Triple Zinc Oxide" (manufactured by Sakai Chemical Industry Co., Ltd.) Water: Distilled water Propylene glycol (lower dihydric alcohol): molecular weight 76.1 (Hayashi Pure Chemical Industries, Ltd.) Zinc acrylate: Product name "ZN-DA85S" (85% zinc acrylate / 15% zinc stearate), manufactured by Nippon Shokubai Co., Ltd. Organic peroxide (dicumyl peroxide): Trade name "Percumyl D" (manufactured by NOF Corporation) Anti-aging agent (1): Product name "Nocrac NS-6" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Anti-aging agent (2): Product name "Nocrac MBN" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Pentachlorothiophenol zinc salt: Wako Pure Chemical Industries, Ltd.

[0066] Details of the above brand "Nucrel" (ethylene-unsaturated carboxylic acid copolymer) are as shown in Table 2 below.

[0067] [Table 2]

[0068] Core cross-sectional hardness For each of the cores having a diameter of 38.5 mm in each of the above Examples and Comparative Examples, the cross-sectional hardness of the surface and center was measured by the following method. The difference in hardness between these values ​​is shown in Table 1. (1) Core surface hardness At a temperature of 23±1°C, the needle of a hardness tester is set perpendicular to the surface of the spherical core, and four points on the surface of the core are measured randomly using the JIS-C hardness scale. The average of these measurements is used as the measurement value for one ball, and the average value for three cores measured is calculated. (2) Core center hardness The core is cut into a flat surface so that the cross section passes through the center of the core. At a temperature of 23±1°C, the needle of a hardness tester is placed perpendicular to the flat cross section, and the hardness of the center of the hemispherical core is measured using a JIS-C hardness tester. This is the measurement value for one ball, and the average value of three cores is calculated.

[0069] Formation of the cover (intermediate and outermost layers) Next, using an injection molding die, the intermediate layer material (ionomer resin material) shown in Table 3 is injection molded around the surface of the core to form an intermediate layer 1.3 mm thick and with a Shore D hardness of 64. Next, using another injection molding die, the outermost layer material (urethane resin material) shown in Table 3 is injection molded around the intermediate layer-coated sphere to form an outermost layer 0.8 mm thick and with a Shore D hardness of 40.

[0070] [Table 3]

[0071] The details of the ingredients in Table 3 above 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

[0072] The resulting golf balls were evaluated for the amount of compression deformation, initial velocity, spin rate, and durability by the methods described below. The results are shown in Table 4.

[0073] Amount of compression deformation of the ball The ball is compressed at a rate of 10 mm / s at a temperature of 23±1°C, and the amount of compression deformation (mm) of the ball is measured from the initial load of 98 N (10 kgf) to the final load of 1275 N (130 kgf), and the average value of 10 measurements is calculated.

[0074] Initial velocity and backspin amount when hitting with a driver (W#1) A driver (W#1) was attached to the golf hitting robot, and the initial velocity and backspin of the ball were measured immediately after hitting it at a head speed of 45 m / s using an initial condition measuring device. The club used was the Bridgestone Sports TourB XD-3 Driver (2016 model) (loft angle 9.5°).

[0075] durability The durability of the balls was evaluated using an ADC Ball COR Durability Tester manufactured by Automated Design Corporation in the United States. This tester has the function of launching a golf ball using air pressure and then impacting it continuously against two parallel metal plates. The speed of the ball entering the metal plates was set to 43 m / s. The number of shots required for the golf ball to break was measured, and the average of the measurements for 10 golf balls was calculated. An index was also calculated, with the average number of times the balls of Comparative Example 2 broke being set to 100 (reference value), and the index is listed in Table 4.

[0076] [Table 4]

[0077] Table 4 shows that in all of Examples 1 to 6, durability was improved without reducing resilience while maintaining spin performance. It is also expected that the higher the acid content of component (a-2) used, the more ionic crosslinking between the unsaturated carboxylic acid and the metal oxide (a-3) occurs in the polybutadiene (a-1), resulting in improved durability. In contrast, Comparative Examples 1 to 5 are inferior to the present invention (Examples) in the following respects. In Comparative Example 1, the core rubber composition does not contain component (b), and the hardness difference between the center and surface of the core is small, resulting in a large amount of backspin when hit with a driver (W#1). In Comparative Example 2, the component (a-2) was not blended into the core rubber composition, and durability was not improved. In Comparative Example 3, the acid content of the component (a-2) blended in the core rubber composition was low, so the initial velocity and durability were not sufficiently improved. Comparative Example 4 has a low initial velocity due to the low acid content of the component (a-2) blended in the core rubber composition. In Comparative Example 5, the initial velocity is low because the amount of component (a-2) compounded in the core rubber composition is large.

Claims

1. A three-piece solid golf ball comprising a core and a two-layer cover including an intermediate layer and an outermost layer, wherein the intermediate layer is formed primarily from an ionomer resin, the outermost layer is formed primarily from a urethane resin, and the core comprises the following components (a) to (d): (a) Base rubber (b) water and / or a lower alcohol having a molecular weight of less than 200 (c) α,β-unsaturated carboxylic acid and / or metal salt thereof (d) Organic peroxide and wherein the base rubber of component (a) is a rubber obtained by mixing (a-1) polybutadiene and (a-2) an unneutralized ethylene-unsaturated carboxylic acid copolymer and then neutralizing the mixture with (a-3) a metal oxide, wherein the acid content of component (a-2) is 5 to 15 mass %, the amount of component (a-2) is 10 parts by mass or less per 100 parts by mass of the total amount of components (a-1) and (a-2), and the difference in hardness between the surface and center of the core is 15 or more in JIS-C hardness.

2. 2. The golf ball of claim 1, wherein the unsaturated carboxylic acid in component (a-2) is acrylic acid or methacrylic acid.

3. 3. The golf ball according to claim 1, wherein (a-2) the unneutralized ethylene-unsaturated carboxylic acid copolymer is completely neutralized with (a-3) a metal oxide.

4. 4. The golf ball of claim 1, wherein the blending amount of component (b) is 0.1 to 10 parts by weight per 100 parts by weight of component (a).

5. A golf ball according to any one of claims 1 to 4, wherein the lower alcohol of component (b) is one or more alcohols selected from the group consisting of butanol, glycerin, ethylene glycol, propylene glycol, butanetriol, trimethylolethane, trimethylolpropane, di(trimethylolpropane), pentaerythritol, and sorbitol.

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