golf ball

Abstract

It comprises a core and a cover, and the core comprises the following components (a) to (d). (a) Base rubber (b) Water and / or lower alcohols with a molecular weight of less than 200 (c) Unsaturated carboxylic acids and / or their metal salts (d) Organic peroxide A heat-molded rubber composition containing the following: (a) is a rubber obtained by mixing (a-1) polybutadiene and (a-2) an unneutralized ethylene-unsaturated carboxylic acid copolymer, followed by neutralization with (a-3) a metal oxide, wherein (a-2) is acrylic acid or methacrylic acid, the acid content of (a-2) is 5% by mass or more, the amount of (a-2) blended is 10 parts by mass or less per 100 parts by mass of the total of (a-1) and (a-2), and the difference between the crosslink density of the core surface and the crosslink density of the core center is 5.0 × 10 2 mol / m 3 That's all for golf balls. [Effect] According to the present invention, it is possible to achieve both low spin and excellent durability while maintaining high rebound properties.

Description

[Technical Field]

[0001] The present invention relates to a golf ball having a core and one or more layers of cover. [Background technology]

[0002] Recently, two-piece and three-piece solid golf balls have become the mainstream. These golf balls typically have a rubber core covered with a single or multiple layers of various resin materials. The core occupies the majority of the golf ball's volume and greatly influences various physical properties such as rebound, 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 improving distance by optimizing spin characteristics during full shots with drivers and irons. It is known that increasing the hardness difference between the surface and the center of the core reduces spin during full shots with drivers, and conventional knowledge has shown that reducing spin during full shots leads to increased distance. Therefore, technologies that further increase the hardness difference inside the core are needed to improve the distance of golf balls. One way to realize this technology is to propose a structure in which the core is made of two layers of rubber. However, since the number of man-hours required to produce the core is greater compared to a single-layer rubber core, technologies that increase the hardness difference within a single-layer core are still highly anticipated.

[0003] Furthermore, methods for adjusting the cross-sectional hardness of the core include appropriately adjusting the compounding components of the core's rubber composition, as well as the vulcanization temperature and time. Regarding the compounding components of the core's rubber composition, this includes selecting the type and amount of co-crosslinking agents and organic peroxides used. In the golf ball field, methacrylic acid, acrylic acid, and their metal salts are known to be used as co-crosslinking agents. However, the adjustment of the above-mentioned co-crosslinking agent formulations mainly focuses on adjusting the feel of the ball by adjusting the hardness of the core, and does not satisfy the spin characteristics.

[0004] As a new technology to achieve low spin on the ball during a full shot by increasing the hardness difference between the surface and the center of the core, a golf ball described in Japanese Patent Publication No. 2015-47502 (corresponding to U.S. Patent Application Publication No. 2015-0065268) is an example. In this publication, the core is obtained by vulcanizing a rubber composition for the core with water. Another example is a golf ball described in Japanese Patent Publication No. 2019-213606 (corresponding to U.S. Patent Application Publication No. 2019-0375917). In this publication, the core is obtained by vulcanizing a rubber composition for the core with a lower alcohol. However, these technologies have the drawback that increasing the hardness difference between the surface and the center of the core reduces the ball's impact durability.

[0005] Furthermore, the following technologies have been proposed as specific techniques to improve the ball's impact durability while maintaining the core's resilience. One technique involves compounding an ionomer resin with polybutadiene rubber in a core rubber composition, as described in U.S. Patent Publication No. 2002-0086745. Another technique involves mixing polybutadiene rubber with an unneutralized ethylene-unsaturated carboxylic acid copolymer and then neutralizing it with a metal cation source, as described in Japanese Patent Publication No. 2007-209472 (corresponding U.S. Patent Publication No. 2007-0184916). In addition, rubber compositions containing polybutadiene rubber, an olefin-containing polymer having a specific amount of acid content, and an inorganic metal compound are described in Japanese Patent Publication No. 2007-061605 and Japanese Patent Publication No. 2012-254304 (corresponding U.S. Patent Publication No. 2007-0049419). However, all of these technologies involve incorporating resin components into the rubber composition, and a drawback is that increasing the amount of resin components significantly reduces the rebound properties. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-47502

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Patent Document 9

Patent Document 10

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention has been made in view of the above circumstances, and an object thereof is to provide a golf ball that achieves both low spin and excellent durability while maintaining high resilience.

Means for Solving the Problems

[0008] As a result of intensive studies to achieve the above object, the inventors of the present invention have found that for a golf ball having a core and one or more layers of covers, the core is composed of the following components (a) to (d): (a) Base rubber (b) Water and / or lower alcohol having a molecular weight of less than 200 (c) α,β-unsaturated carboxylic acid and / or its metal salt (d) Organic peroxide The rubber composition containing the above is formed by heat molding, and in this process, the base rubber of component (a) is specified to be a rubber obtained by mixing (a-1) polybutadiene and (a-2) an unneutralized ethylene-unsaturated carboxylic acid copolymer, and then neutralizing it with (a-3) a metal oxide, the unsaturated carboxylic acid of component (a-2) is specified to be acrylic acid or methacrylic acid, the acid content of component (a-2) is set to 5% by mass or more, 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 component (a-1) and component (a-2), and the difference between the crosslink density at the core surface and the crosslink density at the core center, as measured based on the toluene swelling test, is set to 5.0 × 10 2 mol / m 3 By preparing the core in the manner described above, we discovered that it is possible to maintain the desired core hardness while setting a large difference in hardness in the internal cross-sectional hardness of the core, thereby fully exhibiting low-spin characteristics when hitting a golf ball, and also achieving excellent impact durability. This led to the present invention.

[0009] Therefore, the present invention provides the following golf ball. 1. In a golf ball having a core and one or more layers of cover, the core is composed of the following components (a) to (d). (a) Base rubber (b) Water and / or lower alcohols with a molecular weight of less than 200 (c) α,β-unsaturated carboxylic acids and / or their metal salts (d) Organic peroxide The rubber composition is formed by a heat-molded product of a rubber composition containing the following: (a) The base rubber of component (a) is obtained by mixing (a-1) polybutadiene and (a-2) an unneutralized ethylene-unsaturated carboxylic acid copolymer, and then neutralizing with (a-3) a metal oxide, wherein the unsaturated carboxylic acid of component (a-2) is acrylic acid or methacrylic acid, the acid content of component (a-2) is 5% by mass or more, the amount of component (a-2) is 10 parts by mass or less per 100 parts by mass of the total amount of component (a-1) and component (a-2), and the crosslink density measured based on a toluene swelling test is such that the difference between the crosslink density of the core surface and the crosslink density of the core center is 5.0 × 10 2mol / m 3 A golf ball characterized by the above. 2. The golf ball according to item 1 above, wherein (a-2) the unneutralized ethylene-unsaturated carboxylic acid copolymer is completely neutralized by (a-3) a metal oxide. 3. The golf ball according to item 1 above, wherein the amount of component (b) is 0.1 to 10 parts by mass per 100 parts by mass of component (a). 4. The golf ball according to item 1 above, wherein the lower alcohol of component (d) is one or more alcohols selected from the group consisting of butanol, glycerin, ethylene glycol, propylene glycol, butantriol, trimethylolethane, trimethylolpropane, di(trimethylolpropane), pentaerythritol, and sorbitol. [Effects of the Invention]

[0010] The golf ball of this invention can achieve both low spin and excellent durability while maintaining high rebound properties. [Modes for carrying out the invention]

[0011] The present invention will be described in more detail below. The golf ball of the present invention has a core and one or more layers of cover, and the core may be made up of not only one layer but also two or more layers as needed. The core is composed of the following components (a) to (d): (a) Base rubber (b) Water and / or lower alcohols with a molecular weight of less than 200 (c) α,β-unsaturated carboxylic acids and / or their metal salts (d) Organic peroxide It is formed by a heat-molded product of a rubber composition that contains as an essential component.

[0012] 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 it with (a-3) a metal oxide.

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

[0014] Furthermore, the 1,2-vinyl bond content in the polybutadiene is typically 2% or less, preferably 1.7% or less, and more preferably 1.5% or less in the polymer chain. If the 1,2-vinyl bond content is too high, the resilience may decrease.

[0015] The above polybutadiene has a Mooney viscosity (ML). 1+4 The temperature (at 100°C) is preferably 20 or higher, more preferably 30 or higher, and the upper limit is preferably 120 or lower, more preferably 100 or lower, and even more preferably 80 or lower.

[0016] The Mooney viscosity mentioned above is an industrial viscosity index (JIS K 6300) measured with a Mooney viscometer, a type of rotational plasticity meter, and its unit symbol is ML. 1+4 (100°C) is used. Also, M is Mooney viscosity, L is large rotor (L type), 1+4 indicates a preheating time of 1 minute and a rotor rotation time of 4 minutes, and indicates that the measurement was taken under conditions of 100°C.

[0017] The polybutadiene described above can be synthesized using rare earth element catalysts or group VIII metal compound catalysts.

[0018] The unneutralized ethylene-unsaturated carboxylic acid copolymer of component (a-2) is particularly effective in reducing the spin performance of the ball after hitting it with a driver (W#1). The unsaturated carboxylic acid has 3 to 8 carbon atoms, and specifically includes acrylic acid, methacrylic acid, maleic acid, fumaric acid, etc., with acrylic acid or methacrylic acid being preferred because they impart high rebound properties.

[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 too low, a sufficiently high initial velocity and 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 amount of component (a-2) should be 10 parts by mass or less per 100 parts by mass of the total amount of component (a-1) and component (a-2). The higher the proportion of component (a-2), the better the durability, but if it exceeds 10 parts by mass, the core rebound performance will decrease, which may lead to a decrease in ball initial velocity.

[0021] (a-2) The melt flow rate (MFR) of the unneutralized ethylene-unsaturated carboxylic acid copolymer of component (a-2) is preferably 10 g / 10 min or more, more 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, from the viewpoint of improving dispersibility in the matrix of the base rubber. 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, more 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 this 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] (a-2) As the unneutralized ethylene-unsaturated carboxylic acid copolymer of component (a-2), commercially available products can be used, for example, the trade names "Nucrel N1110H" and "Nucrel N1560" (both manufactured by Mitsui Dow Polychemicals), and "NUCREL30707" (manufactured by The Dow Chemical Company). These may be used individually or in combination of two or more.

[0024] (a-3) 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. One of these can be used alone, or two or more can be used.

[0025] The amount of component (a-3) is preferably 1 to 50 parts by mass, and particularly preferably 2 to 30 parts by mass, per 100 parts by mass of the total amount of components (a-1) and (a-2). If the amount is too small, the neutralization reaction may not be sufficient, and the rebound performance and durability may not be sufficient. On the other hand, if the amount is too large, the mass of the core will become too heavy, exceeding the weight range suitable for a golf ball, which may be undesirable.

[0026] When the metal oxide of component (a-3) is blended in the above-mentioned proportions, it is preferable that 80 mol% or more of the unneutralized unsaturated carboxylic acid of component (a-2) is neutralized. More preferably, the degree of neutralization of this unsaturated carboxylic acid is 90 mol% or more, and most preferably 100 mol%. That is, it is most preferable that the unneutralized ethylene-unsaturated carboxylic acid copolymer of (a-2) is completely neutralized by the metal oxide of (a-3) in order to fully exhibit the desired effects of the present invention.

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

[0028] Next, there are no particular restrictions on the water of component (b) used in the present invention; it 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 to be blended is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, with an upper limit of preferably 5 parts by mass or less, and more preferably 4 parts by mass or less, per 100 parts by mass of the base rubber.

[0029] Furthermore, by adding an appropriate amount of the above-mentioned water, the moisture content of the rubber composition before vulcanization is preferably 1000 ppm or more, and more preferably 1500 ppm or more. The upper limit is preferably 8500 ppm or less, and more preferably 8000 ppm or less. If the moisture content of the above-mentioned rubber composition is too low, it becomes difficult to obtain an appropriate crosslinking density, and it may become difficult to mold a golf ball that has low energy loss and low spin. If the moisture content of the above-mentioned rubber composition is too high, the core becomes too soft, and it may become difficult to obtain an appropriate core initial velocity.

[0030] Although it is possible to directly add water to the above rubber composition, the following methods (i) to (iii) can be used. (i) A method of applying mist-like water to all or part of the rubber composition (compounding material) using steam or ultrasound. (ii) A method of immersing all or part of a rubber composition in water. (iii) A method of leaving all or part of a rubber composition in a high-humidity environment for a certain period of time in a humidity-controlled place such as a constant-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 40-100% is preferred.

[0031] Furthermore, water can be processed into a jelly-like substance and incorporated into the above rubber composition. Alternatively, a material in which water is pre-supported on a filler, unvulcanized rubber, rubber powder, etc., can be used and incorporated into the above rubber composition. This method offers superior workability compared to directly incorporating water, and can improve the production efficiency of golf balls. There are no particular restrictions on the type of material containing a predetermined amount of water, but examples include fillers, unvulcanized rubber, rubber powder, etc., that contain a sufficient amount of water, and it is particularly preferable to use a material that does not impair durability or rebound properties. The water content of the above material is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more, with an upper limit of preferably 99% by mass or less, and more preferably 95% by mass or less.

[0032] Furthermore, in this invention, a lower alcohol with a molecular weight of less than 200 can be used instead of water. Here, "alcohol" refers to a substance having one or more alcoholic hydroxyl groups, and includes polyhydric alcohols with two or more hydroxyl groups that have undergone condensation polymerization. Lower alcohol refers to an alcohol with a small number of carbon atoms, i.e., a small molecular weight. By incorporating this lower alcohol into the rubber composition, a rubber cured product (core) having a desired core hardness gradient can be obtained during the vulcanization (curing) of the rubber composition, thereby sufficiently reducing the spin of the ball during impact and improving its flight performance.

[0033] The lower alcohols mentioned above are particularly preferably hexavalent or lower alcohols (alcohols having 6 or fewer alcoholic hydroxyl groups), and specifically 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, their molecular weight is less than 200, preferably less than 150, and more preferably less than 100. If the molecular weight is too large, i.e., if the number of carbon atoms is too high, the desired core hardness gradient cannot be obtained, and it becomes impossible to sufficiently achieve low spin on the ball during impact.

[0034] The amount of the lower alcohol blended is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 0.5 parts by mass or more, per 100 parts by mass of the base rubber, with an upper limit of preferably 10 parts by mass or less, more preferably 6 parts by mass or less, and even more preferably 3 parts by mass or less. If the blending amount is too high, the hardness will soften and the desired feel, durability, and rebound properties cannot be obtained, and if the blending amount is too low, the desired core hardness gradient cannot be obtained, and it may not be possible to sufficiently achieve low spin on the ball when hitting it.

[0035] Next, component (c) is an α,β-unsaturated carboxylic acid and / or its metal salt. The number of carbon atoms in this unsaturated carboxylic acid is preferably 3 to 8, and specific examples include acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Specific examples of metals for the above unsaturated carboxylic acid include zinc, sodium, magnesium, calcium, and aluminum, with zinc being particularly preferred. Therefore, zinc acrylate is the most preferred cocrosslinking agent.

[0036] The amount of component (c) blended is preferably 10 parts by mass or more, more preferably 15 parts by mass or more, and even more preferably 20 parts by mass or more, per 100 parts by mass of the base rubber of component (a), with an upper limit of preferably 65 parts by mass or less, more preferably 60 parts by mass or less, and even more preferably 55 parts by mass or less. If the blending amount is less than the above range, it will become too soft and have poor rebound properties, and if it is more than the above range, it will become too hard and have a poor feel when hitting the ball, as well as be brittle and have poor durability.

[0037] (c) The co-crosslinking agent of component (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 tends to aggregate in the rubber composition, increasing the reactivity between the acrylic acids and decreasing the reactivity between the base rubbers, which may result in insufficient rebound performance 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, leading to greater variation in the properties of the resulting golf ball.

[0038] (d) Component is an organic peroxide. Preferably, this organic peroxide has a half-life temperature of 110 to 185°C. Examples of such organic peroxides include dicumyl peroxide (NOF Corporation's "Permil D"), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (NOF Corporation's "Perhexa 25B"), and di(2-t-butylperoxyisopropyl)benzene (NOF Corporation's "Perbutyl P"), with dicumyl peroxide being a suitable choice. Other commercially available products include "Perhexa C-40", "Niper BW", "Perloyl L" (all manufactured by NOF Corporation), or Luperco 231XL (Atochem Corporation). These may be used individually or in combination of two or more.

[0039] (d) The amount of component blended is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, per 100 parts by mass of base rubber, with an upper limit of preferably 5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3 parts by mass or less.

[0040] In addition to the components (a) to (d) described above, various additives such as fillers, antioxidants, and organic sulfur compounds may be incorporated, as long as they do not interfere with the effects of the present invention.

[0041] Suitable fillers include, for example, zinc oxide, barium sulfate, and calcium carbonate. These may be used individually or in combination of two or more. The amount of filler added is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, per 100 parts by mass of the base rubber. The upper limit of this addition is preferably 100 parts by mass or less, more preferably 60 parts by mass or less, and even more preferably 40 parts by mass or less, per 100 parts by mass of the base rubber. If the amount added is too much or too little, it may not be possible to obtain the appropriate mass and suitable rebound properties.

[0042] There are no particular restrictions on the anti-aging agent, but examples include phenolic anti-aging agents such as 2,2-methylenebis(4-methyl-6-tertbutylphenol), 4,4-butylidenebis(3-methyl-6-tertbutylphenol), and 2,2-methylenebis(4-ethyl-6-tertbutylphenol). Commercially available products include Nocrack NS-6, NS-30, and NS-5 (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.). These may be used individually or in combination of two or more. There are no particular restrictions on the amount of anti-aging agent to be blended, but preferably it is 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, with an upper limit of preferably 1.0 parts by mass or less, more preferably 0.7 parts by mass or less, and even more preferably 0.4 parts by mass or less, per 100 parts by mass of base rubber. If the amount is too high or too low, it may not be possible to obtain an appropriate core hardness gradient, and suitable rebound properties, durability, and low spin effect during full shots may not be obtained.

[0043] There are no particular restrictions on the organic sulfur compounds, but examples include thiophenols, thionaphthols, diphenyl polysulfides, halogenated thiophenols, or their metal salts. Specifically, examples include zinc salts of pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, parachlorothiophenol, etc., diphenyl polysulfides with 2 to 4 sulfur atoms, dibenzyl polysulfides, dibenzoyl polysulfides, dibenzothiazoyl polysulfides, dithiobenzoyl polysulfides, etc. These may be used individually or in combination of two or more. Among these, zinc salts of pentachlorothiophenol and / or diphenyl disulfides can be preferably used.

[0044] The amount of organic sulfur compound added is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and even more preferably 0.2 parts by mass or more, per 100 parts by mass of the base rubber, with an upper limit of preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less. If the amount of organic sulfur compound added is too high, the hardness of the heat-molded product of the rubber composition may become too soft, while if it is too low, improvement in rebound properties may not be expected.

[0045] The above-mentioned core can be manufactured by vulcanizing and curing a rubber composition containing the above-mentioned components. For example, it can be manufactured by kneading using a kneader such as a Banbury mixer or roll, compression molding or injection molding using a core mold, and then curing the molded body by appropriately heating it at a temperature of approximately 100 to 200°C for 10 to 40 minutes, which is sufficient for the organic peroxide and co-crosslinking agent to act.

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

[0047] There are no particular restrictions on the core's central hardness, but according to the JIS-C standard, it is preferably 40 or higher, more preferably 45 or higher, and even more preferably 50 or higher. The upper limit is preferably 75 or lower, more preferably 70 or lower, and even more preferably 65 or lower. If the core's central hardness deviates from the above range, the feel of the ball may deteriorate, or its durability may decrease, and it may not be possible to obtain a low-spin effect.

[0048] Regarding the surface hardness of the core, there are no particular restrictions, but in accordance with the JIS-C standard, it is preferably 65 or higher, more preferably 70 or higher, and even more preferably 72 or higher. As the upper limit value, it is preferably 95 or lower, more preferably 90 or lower, and even more preferably 88 or lower. If the surface hardness of the core is too low compared to the above range, the resilience may be low and the flying distance may not be sufficiently obtained. Also, if the surface hardness of the core is too high compared to the above range, the hitting feeling may become too hard, and the crack durability due to repeated hitting may deteriorate.

[0049] Regarding the hardness difference between the surface and the center of the above core, there are no particular restrictions, but it is preferably 15 or higher in JIS-C hardness, more preferably 20 or higher, even more preferably 24 or higher, and most preferably 30 or higher. As the upper limit, it is preferably 50 or lower, more preferably 45 or lower, and even more preferably 40 or lower. If the value of the above hardness difference is too small, the low spin effect during W#1 hitting may not be sufficient and the flying distance may not be obtained. On the other hand, if the value of the above hardness difference is too large, the initial ball speed when actually hitting the golf ball may be low and the flying distance may not be obtained, or the crack durability due to repeated hitting may deteriorate. Here, the above center hardness means the hardness measured at the center of the cross-section obtained by cutting the core in half (so as to pass through the center), and the surface hardness means the hardness measured on the surface (spherical surface) of the above core. Also, the JIS-C hardness means the hardness measured with a spring-type hardness meter (JIS-C type) specified in JIS K 6301-1975.

[0050] Also, it is preferable that the hardness gradient of the core used in the present invention is such that the hardness is equal to or increases from the center of the core toward the surface and does not decrease.

[0051] Next, the crosslink density of the core will be described. In the present invention, the difference in crosslink density between the core center and the core surface [(the value of (crosslink density of the core surface)-(crosslink density of the core center))] is 5.0×10 2 mol / m 3 or more, and as the upper limit value, it is preferably 30.0×10 2mol / m 3 The following applies: If the crosslinking density at the core center or core surface deviates from the above range, the water in the rubber composition may not contribute sufficiently to the decomposition of organic peroxides during vulcanization, and as a result, the ball may not achieve a sufficient low-spin effect.

[0052] The crosslinking density mentioned above can be measured specifically by the following procedure. The core is cut into a 2mm thick circular plate passing through its geometric center. Then, on the circular plate, a φ3mm sample is punched out using a punch at measurement points within 4mm inward from the core center and core surface, and the weight of the sample is measured using an electronic balance capable of measuring to two decimal places (mg). The sample and 8ml of toluene are added to a 10ml vial, the vial is sealed and left to stand for 72 hours or more, after which the solution is discarded and the weight of the sample after immersion is measured. The crosslinking density of the rubber composition is calculated using the Flory-Rehner formula from the sample weights before and after swelling. ν = -(ln(1-v) r ) + v r + χv r 2 ) / V S (v r 1 / 3 -v r / 2) [ν: crosslink density, v r : Volume fraction of rubber during swelling, χ: Interaction constant, V S [Molar volume of toluene] v r = V BR / (V BR + V T ) V BR = ( w f - w f v f ) / ρ V T = ( w s - w f ) / ρ T [V BR : Volume of BR in the rubber composition, V T: Volume of swollen toluene, v f : Weight fraction of filler in the rubber composition, ρ: Density of the rubber composition, w f : Sample weight before immersion, w s : Sample weight after immersion, ρ T [Density of toluene] Note that Vs is 0.1063 × 10 -3 m 3 / mol, ρ T The calculations were performed using 0.8669 for x and 0.47 for x, based on the literature (Macromolecules 2007, 40, 3669-3675).

[0053] Furthermore, regarding the compressive hardness (amount of deformation) of the core (heat-molded product) when an initial load of 98N (10kgf) is applied and a final load of 1275N (130kgf) is applied, there are no particular restrictions, but it is preferably 2.0mm or more, more preferably 2.3mm or more, even more preferably 2.5mm or more, and most preferably 2.8mm or more. As an upper limit, it is recommended that it be preferably 6.0mm or less, more preferably 5.5mm or less, and even more preferably 5.0mm or less. If the value is too large compared to the above, the core will become too soft, and a sufficient low-spin effect may not be obtained, and the rebound may decrease. Also, if the value is too small compared to the above, a low-spin effect may not be obtained, and the feel of the ball may become hard.

[0054] There are no particular restrictions on the core diameter, and it depends on the layer structure of the golf ball being manufactured, but it is preferably 30 mm or more, more preferably 35 mm or more, with an upper limit of preferably 41 mm or less, more preferably 40 mm or less. If the core diameter deviates from this range, the initial velocity of the ball may be low, or the appropriate spin characteristics may not be obtained.

[0055] Next, we will describe the one or more layers of cover that surround the core. There are no particular restrictions on the cover material, but known materials such as various ionomer resins and urethane elastomers used in golf balls can be used.

[0056] Furthermore, in order 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) A binary random copolymer of olefin-unsaturated carboxylic acid and / or a metal ion neutralized product of a binary random copolymer of olefin-unsaturated carboxylic acid, (A-2) A base resin is prepared by blending (A) a olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terranandic copolymer and / or a metal ion neutralized product of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terranandic copolymer in a mass ratio of 100:0 to 0:100, and (B) a non-ionomer thermoplastic elastomer is prepared by blending (A) a resin component in a mass ratio of 100:0 to 50:50. (C) A mixed material comprising 5 to 80 parts by mass of a fatty acid and / or its derivative having a molecular weight of 228 to 1500, and (D) 0.1 to 17 parts by mass of a basic inorganic metal compound capable of neutralizing the unneutralized acid groups in components (A) and (B). In particular, when using a mixed material of components (A) to (D) above, it is preferable to use one in which 70% or more of the acid groups have been neutralized.

[0057] Furthermore, it is preferable to use a urethane material, particularly a thermoplastic urethane elastomer, as the main material for the outermost layer of the cover.

[0058] Furthermore, one or more layers of cover (intermediate layers) may be molded between the layer adjacent to the core and the outermost cover. In this case, it is preferable to use a thermoplastic resin such as an ionomer as the intermediate layer material.

[0059] To obtain the above-mentioned cover, for example, a method can be employed in which a single-layer or multi-layer core of two or more layers, which is manufactured in advance according to the type of ball, is placed in a mold, the above-mentioned mixture is heated, mixed and melted, and then injection molded to cover the core with the desired cover. In this case, the manufacturing of the cover can be carried out while ensuring excellent thermal stability, fluidity, and moldability, and as a result, the resulting golf ball has high rebound, good feel, and excellent abrasion resistance. In addition to the above, another method for forming the cover can be employed, for example, in which a pair of hemispherical half-cups are molded in advance from the cover material, the core is wrapped in these half-cups, and pressure molded at 120-170°C for 1-5 minutes.

[0060] If the above cover is a single layer, its thickness can be 0.3 to 3.0 mm. If the above cover is a two-layer cover, the thickness of the outer layer cover can be in the range of 0.3 to 2.0 mm, and the thickness of the inner layer cover can be in the range of 0.3 to 2.0 mm. Furthermore, there are no particular restrictions on the Shore D hardness of each layer (cover layer) constituting the above cover, but it is preferably 40 or higher, more preferably 45 or higher, and the upper limit is preferably 70 or lower, more preferably 65 or lower.

[0061] Furthermore, numerous dimples are formed on the outermost surface of the cover, and various treatments such as surface preparation, stamping, and painting can be applied to the cover. In particular, when applying such surface treatments to the cover, the good moldability of the cover surface allows for easy work.

[0062] The types of golf balls of the present invention are not particularly limited, as long as they have a core and at least one cover layer. Examples include two-piece or three-piece solid golf balls in which a solid core is covered with a cover, multi-piece golf balls with a three-layer or more structure, and wound golf balls in which a wound core is covered with a single-layer or two-layer or more multi-layer structure cover. [Examples]

[0063] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0064] [Examples 1-14, Comparative Examples 1-5] Core compositions were prepared using the polybutadiene-based rubber compositions shown in Tables 1 and 2 below, according to the rubber formulations (each component and its proportion) of Examples 1-14 and Comparative Examples 1-5. The mixing and molding methods for these rubber compositions are as follows.

[0065] Method for mixing and molding rubber compositions The core composition is prepared using the core materials shown in Table 1 below. First, (a-1) polybutadiene rubber and (a-2) or (a-2') ethylene-methacrylic acid copolymer or ethylene-acrylic acid copolymer are mixed using a Plastmill manufactured by Toyo Seiki Co., Ltd. at a set temperature of 120°C, 30 rpm, and for 10 minutes. Then, (a-3) zinc oxide is added and mixed at 30 rpm for 5 minutes. After that, the mixture is discharged and cooled. In the second step, (c) zinc acrylate, an antioxidant, and pentachlorothiophenol zinc salt are added to the above mixture and mixed at a set temperature of 100°C, 30 rpm, and for 5 minutes using a Plastmill. After that, the mixture is discharged and cooled. In the third step, (b) water or alcohol and (d) organic peroxide are added to the above mixture and mixed at a set temperature of 40°C, 30 rpm, and for 5 minutes using a Plastmill. After that, the mixture is discharged. Each of the resulting mixtures is vulcanized at 155°C for 20 minutes, and after a core surface polishing process, a core with a diameter of 38.5 mm is produced.

[0066] [Table 1]

[0067] [Table 2]

[0068] Details regarding the rubber compounds in Tables 1 and 2 are as follows. • Polybutadiene rubber: Product name "BR01" (manufactured by JSR Corporation) • "AN4214C", "N1110H", and "N1560": All are brands of "Nucrel" (ethylene-unsaturated carboxylic acid copolymer) manufactured by Mitsui Dow Polychemicals. • "NUCREL30707": A brand name "Nucrel" (ethylene-acrylic acid copolymer) manufactured by The Dow Chemical Company. • Zinc oxide: Product name "Triple Zinc Oxide" (manufactured by Sakai Chemical Co., Ltd.) • Water: Distilled water • Propylene glycol (lower dihydric alcohol): Molecular weight 76.1 (manufactured by 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): Product name "Perkmyl 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 "Nocrack MBN" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) • Pentachlorothiophenol zinc salt: Manufactured by Wako Pure Chemical Industries, Ltd.

[0069] Details of the above brand "Nucrel" (ethylene-unsaturated carboxylic acid copolymer) are shown in Table 3 below.

[0070] [Table 3]

[0071] Core cross-sectional hardness For each of the above examples and comparative examples, the 38.5 mm diameter core was measured at the surface and center using the following method, and the difference in hardness between them is shown in Tables 5 and 6 below. (1) Core surface hardness At a temperature of 23±1℃, the needle of a hardness tester is set perpendicular to the surface of a spherical core, and four points on the surface of the core are randomly measured according to the JIS-C hardness scale. The average value of these measurements is taken as the measurement value for one ball, and the average value of three cores is calculated. (2) Core hardness The core is cut into a flat shape so that the cross-section passes through the center of the core. At a temperature of 23±1℃, the needle of a hardness tester is set 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 measurement is taken as the value for one ball, and the average value of three cores is calculated.

[0072] Formation of the cover (intermediate and outermost layers) Next, using an injection molding die, the intermediate layer material (ionomer resin material) shown in Table 4 is injection molded around the core surface to form an intermediate layer with a thickness of 1.3 mm and a Shore D hardness of 64. Then, using another injection molding die, the outermost layer material (urethane resin material) shown in Table 4 is injection molded around the intermediate layer-covered sphere to form an outermost layer with a thickness of 0.8 mm and a Shore D hardness of 40.

[0073] [Table 4]

[0074] The details of the ingredients listed in Table 4 above are as follows. • "Hymilan 1706", "Hymilan 1557", and "Hymilan 1605": Ionomer resins manufactured by Mitsui Dow Polychemicals. • "TPU": "Pandex," a product name manufactured by DIC Covestropolymer, Inc., is an ether-type thermoplastic polyurethane with a Shore D hardness of 40. • "Polyethylene Wax": Product name "Sunwax 161P" (manufactured by Sanyo Chemical Industries, Ltd.) • Isocyanate compound: 4,4'-diphenylmethane diisocyanate

[0075] The resulting golf balls were evaluated for their compression deformation, initial velocity, spin rate, and durability using the methods described below. The results are shown in Tables 5 and 6.

[0076] Amount of compression deformation of the ball The balls were compressed at a temperature of 23±1℃ at a speed of 10 mm / s. The amount of compression deformation (mm) of the balls was measured from an initial load of 98 N (10 kgf) to a final load of 1275 N (130 kgf), and the average value of 10 measurements was calculated.

[0077] Initial velocity and backspin amount when hitting with a driver (W#1) A golf swing robot is fitted with a driver (W#1), and the initial velocity and backspin of the ball immediately after impact at a head speed of 45 m / s are measured using an initial condition measurement device. The club used is a Bridgestone Sports "TourB XD-3 driver (2016 model)" (loft angle 9.5°).

[0078] durability The durability of the golf balls will be evaluated using the ADC Ball COR Durability Tester manufactured by Automated Design Corporation, USA. This test machine launches a golf ball using air pressure and then continuously impacts it against two parallel metal plates. The incident velocity on the metal plates is set to 43 m / s. The number of shots required for the golf ball to break is measured, and the average value of the measurements for 10 golf balls is calculated. Additionally, an index is calculated using the average number of shots required for the balls in Comparative Example 2 to break as a baseline of 100, and these results are recorded in Tables 5 and 6.

[0079] [Table 5]

[0080] [Table 6]

[0081] Tables 5 and 6 show that Examples 1 to 14 all improve durability without reducing rebound while maintaining spin performance. Furthermore, it is expected that the higher the acid content of component (a-2) used, the more ionic crosslinking between the unsaturated carboxylic acid and the (a-3) metal oxide occurs in the (a-1) polybutadiene, resulting in improved durability. In contrast, Comparative Examples 1 to 5 are inferior to the present invention (examples) in the following respects. Comparative Example 1 does not contain component (b) in the core rubber composition, and because the difference in hardness between the center and surface of the core is small, the amount of backspin when hitting with a driver (W#1) is high. Comparative Example 2 does not contain component (a-2) in the core rubber composition, and therefore its durability is not improved. Comparative Example 3 does not sufficiently improve initial velocity and durability because the acid content of component (a-2) incorporated into the core rubber composition is low. Comparative Example 4 has a low initial velocity because the acid content of component (a-2) blended into the core rubber composition is low. Comparative Example 5 has a low initial velocity because it contains a large amount of component (a-2) in the core rubber composition.

Claims

1. A golf ball having a core and one or more layers of cover, wherein the core is composed of the following components (a) to (d). (a) Base rubber (b) Water and / or lower alcohols with a molecular weight of less than 200 (c) α,β-unsaturated carboxylic acids and / or their metal salts (d) Organic peroxide The rubber composition is formed by a heat-molded product of a rubber composition containing the following: The base rubber of component (a) is obtained by mixing (a-1) polybutadiene and (a-2) an unneutralized ethylene-unsaturated carboxylic acid copolymer, and then neutralizing it with (a-3) a metal oxide, wherein the unsaturated carboxylic acid of component (a-2) is acrylic acid or methacrylic acid, the acid content of component (a-2) is 5% by mass or more, the amount of component (a-2) is 10 parts by mass or less per 100 parts by mass of the total amount of component (a-1) and component (a-2), and the crosslink density measured based on a toluene swelling test is such that the difference between the crosslink density of the core surface and the crosslink density of the core center is 5.0 × 10 2 mol / m 3 A golf ball characterized by the above.

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

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

4. (d) The golf ball according to claim 1, wherein the lower alcohol of component is one or more alcohols selected from the group consisting of butanol, glycerin, ethylene glycol, propylene glycol, butantriol, trimethylolethane, trimethylolpropane, di(trimethylolpropane), pentaerythritol, and sorbitol.

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