Rubber composition for golf ball

Abstract

To provide a rubber composition for a golf ball excellent in crack durability by setting a hardness difference in a core internal hardness distribution large while maintaining a desired core hardness.SOLUTION: The present invention provides a rubber composition for golf balls comprising (a) a base rubber, (b) an α, β - unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent, (c) an organic peroxide, (d) water or a water donor, and (e) a hindered phenol-based antioxidant having a substituent with a thioether structure, wherein the amount of component (e) is at least 0.2 part by weight per 100 parts by weight of component (a).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a rubber composition for golf balls, and more particularly to a rubber composition for golf balls that is suitable for use as a core material for golf balls that consist of one or more core layers and one or more cover 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 that increases the hardness difference within the core is needed to improve the flight distance of golf balls. One method for achieving this technology has been proposed, which involves 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 that increases the hardness difference within the single-layer core remains desirable.

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

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

[0005] Furthermore, Japanese Patent Application Laid-Open Nos. 2013-108079 and 2013-108080 propose a technique for improving the resilience of a rubber vulcanization molded product and providing it with a suitable hardness by compounding a specific benzimidazole such as 2-mercaptobenzimidazole as a result of examining various additives to be compounded in a rubber composition for a golf ball. However, the above rubber compositions do not propose a technique for further improving the internal hardness of a rubber molded product or for further reducing the spin rate of a ball.

[0006] Furthermore, JP 2015-47502 A proposes a technology in which water and / or a monocarboxylic acid metal salt is blended into the base rubber of a core rubber composition, thereby maintaining good resilience of the golf ball, reducing spin rate to increase flight distance, and also improving durability. However, even with this technology, the effect of reducing spin rate of the golf ball is insufficient, and there is still room for improvement in reducing spin rate. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 11-169485 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-108079 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-108080 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-47502 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a rubber composition for golf balls that has excellent crack resistance by setting a large hardness difference in the hardness distribution within the core while maintaining a desired core hardness. [Means for solving the problem]

[0009] As a result of extensive research into achieving the above-mentioned object, the present inventors discovered that by incorporating the following essential components into the rubber composition of a golf ball core: (a) base rubber; (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent; (c) an organic peroxide; (d) water or a moisture-providing agent; and (e) a hindered phenol-based antioxidant having a substituent with a thioether structure, it is possible to achieve a large difference in hardness within the core hardness distribution while maintaining the desired core hardness, thereby achieving high crack resistance, and thereby completing the present invention. The reason for this is not clear, but is presumed to be as follows.

[0010] That is, by compounding a hindered phenol-based antioxidant having a thioether substituent with water or a moisture provider into the core material, a significant difference in the promotion of decomposition of organic peroxides in the core compound occurs between the core surface and the core center, resulting in a difference in the crosslink structure of the butadiene rubber. It is known that the decomposition efficiency of organic peroxides in the core rubber composition varies with temperature, and the higher the temperature, the greater the decomposition efficiency. If the temperature is too high, the amount of decomposed radicals increases, causing them to recombine and become inactivated. As a result, the number of radicals effectively contributing to crosslinking decreases. When the organic peroxide decomposes and generates heat during core vulcanization, the temperature near the core surface remains approximately the same as the vulcanization mold temperature, but the temperature near the core center becomes significantly higher than the mold temperature due to the accumulation of heat from the decomposition of the organic peroxide decomposed from the outside. It is presumed that when water or the like is blended into the core, the hydroxyl groups of the water or the like promote the decomposition of the organic peroxide, thereby changing the radical reaction as described above between the core center and the core surface. That is, it is presumed that the decomposition of the organic peroxide is further promoted near the core center, further promoting the inactivation of radicals and further reducing the amount of effective radicals, thereby enabling the production of a core with a low crosslink density near the core center. On the other hand, it is presumed that the blending of a hindered phenol-based antioxidant having a substituent with a thioether structure has the effect of improving the heat resistance of polymer components that deteriorate due to the thermal history during core molding compared to conventional hindered phenol-based antioxidants, thereby improving the antioxidant effect of the blend system of the present invention.

[0011] Accordingly, the present invention provides the following rubber composition for golf balls. 1. Each of the following components (a) to (e): (a) base rubber; (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent; (c) organic peroxide; (d) water or a moisture-providing agent; (e) Hindered phenol antioxidants having a thioether structure as a substituent wherein the moisture provider is a substance that contains a water component other than free water in its structure and releases water when heated, or a substance that releases water when thermally decomposed by heating, and the blending amount of component (e) is 0.2 parts by mass or more per 100 parts by mass of component (a). 2. The rubber composition for golf balls according to 1 above, wherein the hindered phenol-based antioxidant, component (e), has a chemical structure having at least one methyl group at the ortho position. 3. The rubber composition for golf balls according to 1 above, wherein the hindered phenol antioxidant (component (e)) has two or more substituents with a thioether structure. 4. The rubber composition for golf balls according to 1 above, wherein the hindered phenol-based antioxidant (e) is represented by the following general formula (I): [ka] (wherein, x is an integer of 1 or more). 5. The rubber composition for golf balls according to the above item 1, wherein x in the formula is an integer of 8 or greater. 6. The rubber composition for golf balls according to 1 above, further comprising, as component (f), a benzimidazole and / or a metal salt thereof represented by the following general formula (II): [ka] (In the formula, R is a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, m is an integer of 1 to 4, and when m is 2 or more, they may be the same or different.) 7. The rubber composition for golf balls according to 6 above, wherein the component (f) is selected from the group consisting of 2-mercaptobenzimidazole, 2-mercaptomethylbenzimidazole, and metal salts thereof. 8. The rubber composition for golf balls according to 1 above, further comprising, as component (g), sulfur or an organic sulfur compound which is an alkylphenol disulfide polymer represented by the following chemical structural formula (III): [ka] (In the formula, R represents an alkyl group, and n represents a degree of polymerization ranging from 2 to 20.) A rubber composition for a golf ball, comprising: 9. The rubber composition for golf balls according to the above item 8, wherein in formula (III), R is a lower alkyl group having 1 to 6 carbon atoms selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-amyl (pentyl), isoamyl (pentyl), tert-amyl (pentyl), sec-isoamyl, neopentyl, n-hexyl, isohexyl, and tert-hexyl. 10. The rubber composition for golf balls according to the above item 8, wherein the (g) organic sulfur compound is an amylphenol disulfide polymer. 11. The rubber composition for golf balls according to the above item 8, further comprising as component (h) an organosulfur compound different from component (g). 12. The rubber composition for golf balls according to the above item 1, wherein the vulcanized molded product of the rubber composition is used as the core of a golf ball. 13. A rubber composition for golf balls according to the above item 1, wherein the difference in hardness between the surface and the center of a vulcanized product of the rubber composition is 15 or more in JIS-C hardness. 14. A rubber composition for golf balls according to 1 above, wherein the hardness of a region of 25 to 30% from the surface toward the center of a vulcanized molded product of the rubber composition is at least 5 JIS-C hardness lower than the surface hardness. [Effects of the Invention]

[0012] According to the rubber composition for golf balls of the present invention, when the rubber composition is used as each component of a golf ball, particularly as the core, excellent crack resistance can be achieved. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in more detail below. The rubber composition for golf balls of the present invention is characterized by containing the following components (a) to (e): (a) base rubber; (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent; (c) organic peroxide; (d) Water or a hydration agent (e) Hindered phenol antioxidants having a thioether structure as a substituent

[0014] The base rubber of component (a) is not particularly limited, but it is particularly preferred to use polybutadiene.

[0015] The polybutadiene preferably has cis-1,4-bonds in its polymer chain at 60% or more, preferably 80% or more, more preferably 90% or more, and most preferably 95% or more. If the proportion of cis-1,4-bonds in the polybutadiene molecule is too low, the resilience may decrease.

[0016] 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.

[0017] The polybutadiene has an (ML1+4(100°C)) of 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.

[0018] The Mooney viscosity referred to in the present invention is an industrial viscosity index (JIS K 6300) measured with a Mooney viscometer, which is a type of rotational plasticity meter, and uses the unit symbol ML1+4 (100°C). M indicates Mooney viscosity, L indicates a large rotor (L-type), and 1+4 indicates a preheating time of 1 minute, a rotor rotation time of 4 minutes, and the measurement was performed under the condition of 100°C.

[0019] The polybutadiene that can be used is one synthesized using a rare earth element catalyst or a Group VIII metal compound catalyst.

[0020] The base rubber may contain polybutadiene rubber synthesized using a catalyst other than the lanthanum series rare earth compound. Also, styrene butadiene rubber (SBR), natural rubber, polyisoprene rubber, ethylene propylene diene rubber (EPDM), etc. may be compounded, either alone or in combination.

[0021] The proportion of the polybutadiene in the entire rubber is preferably at least 60% by mass, more preferably at least 70% by mass, and most preferably at least 90% by mass. Alternatively, 100% by mass of the base rubber, i.e., the entire base rubber, may be the polybutadiene.

[0022] Next, component (b) is a co-crosslinking agent, which is an α,β-unsaturated carboxylic acid and / or a metal salt thereof. The unsaturated carboxylic acid preferably has 3 to 8 carbon atoms, and specific examples thereof include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Specific examples of metals in the unsaturated carboxylic acids include zinc, sodium, magnesium, calcium, and aluminum, with zinc being particularly preferred. Therefore, zinc acrylate is the most preferred co-crosslinking agent.

[0023] The amount of component (b) blended per 100 parts by weight of the base rubber of component (a) is preferably at least 10 parts by weight, more preferably at least 15 parts by weight, and even more preferably at least 20 parts by weight, with the upper limit being preferably no more than 65 parts by weight, more preferably no more than 60 parts by weight, and even more preferably no more than 55 parts by weight. If the blending amount is less than the above range, the golf ball will be too soft and have poor resilience, while if the blending amount is more than the above range, the golf ball will be too hard, resulting in a poor shot feel and being brittle and having poor durability.

[0024] The co-crosslinking agent (b) preferably has an average particle size of 3 to 30 μm, more preferably 5 to 25 μm, and even more preferably 8 to 15 μm. If the average particle size of the co-crosslinking agent is less than 3 μm, it is likely to aggregate in the rubber composition, which increases the reactivity between acrylic acids and decreases the reactivity between base rubbers, which can result in an insufficient golf ball resilience. If the average particle size of the co-crosslinking agent exceeds 30 μm, the co-crosslinking agent particles become too large, resulting in greater variation in the properties of the resulting golf balls.

[0025] Component (c) is an organic peroxide, and it is particularly preferable to use an organic peroxide with a 1-minute half-life temperature of 110 to 185°C. Examples of such organic peroxides include dicumyl peroxide (NOF Corp.'s "Percumyl D"), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (NOF Corp.'s "Perhexa 25B"), and di(2-t-butylperoxyisopropyl)benzene (NOF Corp.'s "Perbutyl P"), with dicumyl peroxide being preferred. Other commercially available products include "Perhexa C-40," "Nyper BW," and "Perroil L" (all manufactured by NOF Corp.), as well as Luperco 231XL (manufactured by Atochem). These may be used alone or in combination.

[0026] The amount of component (c) added, per 100 parts by mass of the base rubber, is preferably at least 0.1 part by mass, and more preferably at least 0.3 part by mass, and the upper limit is preferably at most 5 parts by mass, more preferably at most 4 parts by mass, and even more preferably at most 3 parts by mass.

[0027] Component (d) is water or a moisture-providing agent. There are no particular limitations on the water in component (d), and it may be distilled water or tap water, but it is particularly preferable to use distilled water that does not contain impurities.

[0028] Furthermore, when component (d) is a moisture-providing agent, this moisture-providing agent is defined as a substance that contains water components other than free water in its structure and desorbs water when heated, or a substance that releases water components through thermal decomposition caused by heating. Common types of water include free water, adsorbed water, interlayer water, zeolite water, and bound water. It is said that clay minerals contain adsorbed water, interlayer water, and free water, and clay minerals containing such interlayer water can be used as component (d).

[0029] Examples of the clay minerals include layered double hydroxides such as hydrotalcite. Layered double hydroxides (hereinafter also referred to as "LDH") are minerals with a multi-layer structure, in which water (interlayer water) exists between layers, chemically bonded to the layers. For example, in the case of Mg-Al-based LDHs, the interlayer water is almost completely desorbed in the range of 180 to 300°C. In addition, in the case of Zn-Al-based LDHs, the interlayer water is desorbed at a lower temperature of 170 to 200°C.

[0030] Further, examples of component (d) include substances containing bound water. Specific examples include substances containing water (coordinated water) that acts as a ligand to form complex ions, such as hydrates of inorganic compounds. Examples of the inorganic compounds include calcium sulfate 0.5-hydrate, calcium sulfate dihydrate, aluminum sulfate 14-18 hydrate, magnesium sulfate heptahydrate, beryllium sulfate tetrahydrate, zirconium sulfate tetrahydrate, manganese sulfate pentahydrate, iron sulfate heptahydrate, cobalt sulfate heptahydrate, nickel sulfate hexahydrate, cupric sulfate pentahydrate, zinc sulfate heptahydrate, cadmium sulfate octahydrate, indium sulfate nonahydrate, and zinc sulfate dihydrate. These inorganic compounds can be used alone or in combination of two or more selected from the group consisting of calcium sulfate 0.5-hydrate, calcium sulfate dihydrate, aluminum sulfate 14-18 hydrate, magnesium sulfate heptahydrate, beryllium sulfate tetrahydrate, zirconium sulfate tetrahydrate, manganese sulfate pentahydrate, iron sulfate heptahydrate, cobalt sulfate heptahydrate, nickel sulfate hexahydrate, cupric sulfate pentahydrate, zinc sulfate heptahydrate, cadmium sulfate octahydrate, indium sulfate nonahydrate, and zinc sulfate dihydrate.

[0031] Furthermore, examples of component (d) include substances that release water by thermal decomposition due to heating. For example, these substances exist as hydroxide ions, but when heated, they are released as water (HO), such as aluminum hydroxide and magnesium hydroxide.

[0032] The moisture provider preferably has a moisture dissociation rate of 60% or more by mass when the rubber composition is heated to the vulcanization temperature or when the core reaches its maximum temperature due to the heat of self-reaction during vulcanization. In addition, from the viewpoint of increasing the moisture supply efficiency, it is preferable to use a moisture provider with a high moisture content by mass.

[0033] Specifically, for example, the water content in the molecular formula of the moisture-providing agent is preferably 6% or more by mass, more preferably 15% or more. The higher the water content in the molecular formula of the moisture-providing agent, the more preferable it is. Although there is no particular upper limit, from the viewpoint of ease of availability, it can be, for example, 90% or less by mass.

[0034] It is preferable that the moisture provider be capable of releasing as much moisture as possible when vulcanizing the rubber composition. However, vulcanization conditions such as vulcanization temperature and vulcanization time may vary depending on the base rubber and components contained in the rubber composition such as organic peroxides. Therefore, it is preferable to select a moisture provider that can release an amount of moisture appropriate for the purpose under vulcanization conditions such as vulcanization temperature, in accordance with the vulcanization conditions of the rubber composition to which the moisture provider is added.

[0035] Furthermore, since it is preferable that the moisture provider has a low water dissociation rate when the rubber composition is kneaded, it is preferable that the water dissociation rate when heated to, for example, 90°C, i.e., the cumulative water dissociation rate when heated to 90°C, is 60% or less by mass.

[0036] The amount of component (d) 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 15 parts by weight, more preferably no more than 10 parts by weight, even more preferably no more than 5 parts by weight, and even more preferably no more than 3 parts by weight. If the amount of component (d) is too high, the hardness will be too soft and the desired feel, durability, and resilience will not be achieved, while if the amount is too low, the desired core hardness distribution will not be achieved, and the ball may not be able to fully achieve low spin when hit.

[0037] Component (e) is a hindered phenol-based antioxidant having a substituent with a thioether structure. This hindered phenol-based antioxidant preferably has a chemical structure with at least one methyl group at the ortho position. Furthermore, the hindered phenol-based antioxidant preferably has two or more substituents with a thioether structure.

[0038] In the present invention, by compounding the above component (e) into the rubber composition, it is possible to improve normal impact durability performance, and even if foreign matter is mixed into the rubber material, it is possible to maintain a certain level or more of reduction in impact durability.

[0039] Specifically, the hindered phenol-based antioxidant, component (e), is preferably represented by the following general formula (I). [ka]

[0040] In the above formula, x is an integer of 1 or more, and preferably, x is an integer of 8 or more.

[0041] Specific examples of component (e) that can be used include trade names "ANTAGE HP500" and "ANTAGE HP400" (both manufactured by Kawaguchi Chemical Industry Co., Ltd.) and trade name "Irganox 1520L" manufactured by BASF.

[0042] The blend amount of component (e) is at least 0.2 parts by weight, preferably at least 0.3 parts by weight, and more preferably at least 0.5 parts by weight, per 100 parts by weight of the base rubber. The upper limit is preferably at most 3.0 parts by weight, more preferably at most 2.0 parts by weight, and even more preferably at most 1.5 parts by weight. If the blend amount of component (e) is too high, the hardness will be too low, and the desired feel, durability, and resilience will not be achieved. If the blend amount is too low, the desired impact durability effect will not be achieved.

[0043] The component (e) may be added to the component (a) during the production of the rubber composition, or may be added in advance during the production of the component (a), or these addition methods may be used in combination.

[0044] In addition to the components (a) to (e) described above, various additives such as the components (f), (g), and (h) described below, fillers, and processing aids may be blended, as long as they do not impede the effects of the present invention.

[0045] Component (f) is benzimidazole and / or its metal salt represented by the following general formula (II), and is used as an antioxidant. [ka]

[0046] In the above formula (II), R is a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, and m is an integer of 1 to 4, and when m is 2 or more, these may be the same or different. Specific examples of benzimidazoles having the above formula (II) include 2-mercaptobenzimidazole, 2-mercaptomethylbenzimidazole, and metal salts thereof, and the metal salt is preferably a zinc salt.

[0047] The amount of benzimidazole and / or its metal salt represented by the above specific formula (f) compounded per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, with the upper limit being preferably at most 5 parts by weight, more preferably at most 3 parts by weight. If the amount of component (f) is too small, the crosslinking reaction near the core surface will not be efficiently promoted, resulting in an insufficient crosslink density, an insufficient layer of hardness, an insufficient difference in hardness between the core surface and the core center, and insufficient impact durability. On the other hand, if the amount of component (f) is excessively large, the effect obtained will not change beyond the above-mentioned preferred amount.

[0048] Component (g) is sulfur or an alkylphenol disulfide polymer having the following chemical structure: [ka]

[0049] In the formula (III), R represents an alkyl group, and n represents a degree of polymerization ranging from 2 to 20. The alkyl group represented by R is preferably a lower alkyl group having 1 to 6 carbon atoms, specifically, methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-amyl (pentyl), isoamyl (pentyl), tert-amyl (pentyl), sec-isoamyl, neopentyl, n-hexyl, isohexyl, and tert-hexyl. More preferably, the organic sulfur compound of component (g-1) is an amylphenol disulfide polymer, specifically, commercially available products such as "Sanceler AP" (manufactured by Sanshin Chemical Industry Co., Ltd.) and "Vultac5" (manufactured by Arkema Japan) can be used.

[0050] The amount of the alkylphenol disulfide polymer (component (g)) is not particularly limited, but is preferably at least 0.05 parts by mass, more preferably at least 0.1 parts by mass, and most preferably at least 0.3 parts by mass per 100 parts by mass of the rubber component. The upper limit is preferably at most 5.0 parts by mass, more preferably at most 3.0 parts by mass, and most preferably at most 2.0 parts by mass. If the amount is too large, the crosslinking reaction by the organic peroxide is inhibited by the influence of sulfur, and the overall hardness of the molded product tends to be significantly softened.

[0051] On the other hand, when the component (g) is sulfur, commercially available sulfur products can be used, such as "Sulfax 5" manufactured by Tsurumi Chemical Industry Co., Ltd., "Sunmix S-80N" and "Sunmix IS-60N" manufactured by Sanshin Chemical Industry Co., Ltd., and "AKROFORM S-80 / EPR / P" manufactured by Akrochem.

[0052] The amount of sulfur is not particularly limited, but is preferably at least 0.01 part by mass, more preferably at least 0.03 part by mass, and most preferably at least 0.05 part by mass per 100 parts by mass of the rubber component. The upper limit is preferably at most 5.0 parts by mass, more preferably at most 2.0 parts by mass, and most preferably at most 1.0 part by mass. If the amount is too high, the crosslinking reaction by the organic peroxide is inhibited by the influence of sulfur, and the overall hardness of the molded product tends to be significantly softened. On the other hand, if the amount is too low, it may be impossible to increase the difference in hardness between the surface and center of the core interior.

[0053] In addition, sulfur is preferably used in the form of a masterbatch to enhance the dispersibility of a small amount of sulfur. Examples of such sulfur masterbatches include the above-mentioned trade names "Sunmix S-80N," "Sunmix IS-60N," and "AKROFORM S-80 / EPR / P."

[0054] 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 obtain the appropriate weight and appropriate resilience.

[0055] Component (h) is an organic sulfur compound different from component (g). This organic sulfur compound is not particularly limited, but 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 polysulfides having 2 to 4 sulfur atoms, dibenzyl polysulfide, dibenzoyl polysulfide, dibenzothiazoyl polysulfide, dithiobenzoyl polysulfide, and 2-thionaphthol. 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.

[0056] 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 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.

[0057] Suitable processing aids include higher fatty acids and their metal salts. Examples of higher fatty acids include stearic acid, palmitic acid, oleic acid, linoleic acid, linolenic acid, and myristic acid, with stearic acid being particularly preferred. Examples of metal salts of higher fatty acids include lithium salts, sodium salts, potassium salts, copper salts, magnesium salts, calcium salts, strontium salts, barium salts, tin salts, cobalt salts, nickel salts, zinc salts, and aluminum salts, with zinc stearate being particularly preferred. The amount of processing aid 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 this amount is preferably no more than 20 parts by weight, more preferably no more than 15 parts by weight, and even more preferably no more than 10 parts by weight per 100 parts by weight of the base rubber. If the amount is too high, sufficient hardness and resilience may not be achieved, while if the amount is too low, the additive chemicals may not be sufficiently dispersed, resulting in the failure to achieve the desired physical properties. Methods for adding the processing aid include, but are not limited to, adding it to a mixer simultaneously with other chemicals, pre-mixing it with other chemicals such as component (b) above and adding it, coating it on the surface of other chemicals such as component (b) above and adding it, or preparing a masterbatch together with component (a) above and adding it.

[0058] In the present invention, an antioxidant different from component (e) may be contained. Specific examples include hindered phenol-based antioxidants such as 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 1,3,5-tris(3',5'-di-tert-butyl-4-hydroxybenzyl)isocyanuric acid, 2,2'-methylenebis(4-methyl-6-tert-butylphenol), and 4,4',4''-(1-methylpropanyl-3-ylidene)tris(6-tert-butyl-m-cresol). Commercially available antioxidants include Nocrac 200, Nocrac M-17, and Nocrac NS-6 (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), Irganox 1010 (manufactured by BASF), and Adeka STAB. AO-20, AO-30 (manufactured by ADEKA Corporation), etc. can be used. These may be used alone or in combination of two or more. There are no particular restrictions on the amount of this antioxidant blended, but it is preferably 1.0 part by mass or less, more preferably 0.7 parts by mass or less, and even more preferably 0.5 parts by mass or less, per 100 parts by mass of base rubber. If the blended amount is too high, the durability improving effect of component (e) may not be obtained.

[0059] A vulcanized molded product can be produced by vulcanizing and curing the rubber composition for golf balls of the present invention. This vulcanized molded product can be used, in particular, for all or part of a single-layer or multi-layer core. For example, the composition can be kneaded using a kneading machine such as a Banbury mixer or roll, compression molded or injection molded using a core mold, and then appropriately heated at 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 and producing a vulcanized molded core.

[0060] The above-described compounding allows the molded rubber for a golf ball after vulcanization and curing to have a hardness gradient with a large difference in hardness between the surface and the center. By using the above-described molded rubber for a golf ball as a core for a golf ball, the durability of the golf ball can be improved while maintaining good spin characteristics.

[0061] 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.

[0062] 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.

[0063] Regarding the core's hardness distribution, the hardness difference between the surface and center should be sufficiently large. Specifically, the hardness difference (A)-(B) between the surface (A) and center (B) of the core is preferably 20 or more, more preferably 25 or more, and even more preferably 30 or more, on the JIS-C hardness scale. The upper limit is preferably 50 or less, more preferably 45 or less, and even more preferably 40 or less. If the hardness difference is too small, the low-spin effect upon W#1 shots may be insufficient, resulting in a loss of distance. On the other hand, if the hardness difference is too large, the initial velocity of the ball when actually struck may be low, resulting in a loss of distance, or the durability to cracking due to repeated impacts may be reduced. 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. Furthermore, JIS-C hardness refers to the hardness measured using a spring hardness tester (JIS-C type) specified in JIS K 6301-1975.

[0064] 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.

[0065] The core, which is a vulcanized molded product of the rubber composition, preferably includes a region in which the hardness of a region extending from the surface toward the center by 25 to 30% is at least 5 JIS-C hardness lower than the surface hardness.

[0066] Furthermore, the compression hardness (deformation amount) of the core (heat-molded product) when an initial load of 98 N (10 kgf) is applied followed by a final load of 1275 N (130 kgf) is not particularly limited, but is preferably 2.0 mm or more, more preferably 2.3 mm or more, and even more preferably 2.5 mm or more. The upper limit is preferably 6.0 mm or less, more preferably 5.5 mm or less, and even more preferably 5.0 mm or less. If the core is too soft, the spin reduction effect may not be sufficient and the resilience may be reduced. If the core is too small, the spin reduction effect may not be obtained and the feel may be hard.

[0067] 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.

[0068] The rubber composition is preferably used as a core for a golf ball, as described above. The golf ball of the present invention preferably has a structure including a core and a cover having one or more layers.

[0069] 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 thermoplastic polyurethane elastomers, can be used.

[0070] To further reduce the spin rate of the ball, it is particularly preferable to use a highly neutralized ionomer material in the layer adjacent to the core. Specifically, it is preferable to use a material containing the following components (i) to (iv): (i-1) an olefin-unsaturated carboxylic acid binary random copolymer and / or a metal ion-neutralized product of an olefin-unsaturated carboxylic acid binary random copolymer; (ii-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer and / or a metal ion-neutralized product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer in a mass ratio of 100:0 to 0:100 (i) a base resin and (ii) a non-ionomer thermoplastic elastomer in a mass ratio of 100:0 to 50:50, (iii) 5 to 80 parts by mass of a fatty acid and / or a derivative thereof having a molecular weight of 228 to 1500; (iv) 0.1 to 17 parts by mass of a basic inorganic metal compound capable of neutralizing unneutralized acid groups in the above components (i) and (iii). In particular, when using a mixed material of the above components (i) to (iv), it is preferable to use one in which 70% or more of the acid groups have been neutralized.

[0071] The outermost layer of the cover is preferably made primarily of a urethane material, particularly a thermoplastic polyurethane elastomer.

[0072] 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.

[0073] The cover of the present invention can be obtained, for example, by placing a single-layer or multi-layer core (depending on the type of ball) pre-prepared in a mold, heating, mixing, and melting the mixture, and injection-molding the mixture to form the desired cover around the core. In this case, the cover can be produced under conditions that ensure excellent thermal stability, fluidity, and moldability, resulting in a golf ball with high resilience, a good feel, and excellent abrasion resistance. In addition to the above, the cover can also be formed by molding a pair of hemispherical half cups from the cover material of the present invention in advance, encasing the core in these half cups, and then pressure-molding the core at 120 to 170°C for 1 to 5 minutes.

[0074] When the cover has one layer, its thickness can be 0.3 to 3 mm. When the cover has two layers, its outermost layer can be 0.3 to 2.0 mm thick, and its inner cover (intermediate 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.

[0075] Numerous dimples are formed on the surface of the outermost layer of the cover, and the cover may be further subjected to various treatments such as priming, stamping, painting, etc. In particular, when such surface treatments are applied to a cover formed from the cover material of the present invention, the good moldability of the cover surface allows for easy workability.

[0076] The present invention relates to a golf ball in which the above rubber composition is used as a core material for at least one layer. The type of golf ball is not particularly limited, so long as it has a core and at least one cover layer. For example, the rubber composition can be used in solid golf balls such as two-piece and three-piece solid golf balls in which a solid core is covered with a cover, and multi-piece golf balls with a three or more layer structure, as well as in the core of a thread-wound golf ball in which a thread-wound core is covered with a single-layer or two or more multi-layer cover. [Example]

[0077] 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.

[0078] [Examples 1 to 5, Comparative Examples 1 to 6] Using the core material whose main component is polybutadiene shown in Table 1 below, core compositions were prepared using the rubber blends of Examples 1 to 5 and Comparative Examples 1 to 6. These were then vulcanized at 155°C for 20 minutes, and the core surface was polished to produce cores with a diameter of 38.6 mm.

[0079] [Table 1]

[0080] Details of the above formulation are as follows: Polybutadiene rubber: Product name "BR01" (High-cis polybutadiene rubber manufactured by ENEOS Materials) 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) Water: Pure water (Seiki Pharmaceutical Co., Ltd.) Antioxidant (1): Product name "ANTAGE HP500" (Kawaguchi Chemical Industry Co., Ltd.) Antioxidant (2): Product name "ANTAGE HP400" (Kawaguchi Chemical Industry Co., Ltd.) Antioxidant (3): Product name "Nocrac NS-6" (hindered phenol-based antioxidant, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Antioxidant (4): Trade name "ADEKA STAB AO-30" (hindered phenolic antioxidant): ADEKA Antioxidant (5): Sumilizer TP-D (sulfur-based antioxidant, manufactured by Sumitomo Chemical Co., Ltd.) Antioxidant (6): Product name "Nocrac 400" (sulfur-based antioxidant: manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Antioxidant (7): Trade name "Nocrac MB" (benzimidazole-based antioxidant: manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Sulfur: "Sulfax 5" (95% finely divided sulfur) manufactured by Tsurumi Chemical Industry Co., Ltd. Amylphenol disulfide polymer: Trade name "SANCELER AP" (manufactured by Sanshin Chemical Industry Co., Ltd.) Pentachlorothiophenol zinc salt: Wako Pure Chemical Industries, Ltd. Zinc oxide: Product name "Triple Zinc Oxide" (manufactured by Sakai Chemical Industry Co., Ltd.) Zinc stearate: Product name "Zinc Stearate G" (NOF Corporation)

[0081] [Core cross-sectional hardness] For each of the cores having a diameter of 38.6 mm in each of the above examples and comparative examples, the cross-sectional hardness was measured at various positions including the surface and center by the following method. (1) Core surface hardness At a temperature of 23±1°C, the needle of a hardness tester was set perpendicular to the surface of the spherical core, and the JIS-C hardness was measured at four random points on the surface of the core. The average of these measurements was used as the measurement value for one ball, and the average of three cores was calculated. The measurements are shown in Table 3. (2) Core cross-sectional hardness The core was cut into a flat surface so that the cross section passed through the center of the core. At a temperature of 23±1°C, a hardness tester needle was placed perpendicular to the flat cross section, and the hardness was measured using a JIS-C hardness tester at the center of the hemispherical core and at 2mm intervals from the center toward the surface. The hardness was measured as a single ball, and the average of three cores was calculated. The measured values ​​are shown in Table 3.

[0082] [Compression hardness of core and ball] The core and ball were compressed at a rate of 10 mm / s at a temperature of 23±1°C, and the compression hardness (amount of deformation) (mm) of the core and ball was measured from an initial load of 98 N (10 kgf) to a final load of 1275 N (130 kgf). The average value of 10 measurements was calculated.

[0083] [Cover (mid layer and outermost layer) formation] Next, using an injection molding die, the intermediate layer material (ionomer resin material) shown in Table 2 was injection molded around the surface of the core to form an intermediate layer with a thickness of 1.25 mm and a Shore D hardness of 64. Next, using another injection molding die, the outermost layer material (urethane resin material) shown in Table 2 was injection molded around the intermediate layer-coated sphere to form an outermost layer with a thickness of 0.8 mm and a Shore D hardness of 41.

[0084] [Table 2]

[0085] The details of the ingredients in the above table are as follows: "Himilan 1706," "Himilan 1557," and "Himilan 1605": Ionomer resins manufactured by Mitsui Dow Polychemicals "TPU": "Pandex" manufactured by DIC Covestro Polymer, an ether-type thermoplastic polyurethane with a Shore D hardness of 41. Polyethylene wax: Sanwax 161P (manufactured by Sanyo Chemical Industries, Ltd.) "Isocyanate compound": 4,4'-diphenylmethane diisocyanate

[0086] The durability of the resulting golf balls against impact was evaluated by the following method, and the results are shown in Table 3.

[0087] [Impact 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 43 m / s. The number of shots required for the golf ball to break was measured, and the average value of the measurements for 10 golf balls was calculated. The index was calculated by setting the average number of times the ball in Comparative Example 1 (without foreign matter) broke as 1.00 (reference value), and the index was calculated and is shown in Table 3. In addition, artificial foreign matter was mixed into the rubber composition of each example, and the above-mentioned breakage impact durability was evaluated. In this case, as in the above, an index was calculated, with the average number of times that the ball of Comparative Example 1 (without foreign matter) broke being set as 1.00 (reference value), and the results are shown in Table 3.

[0088] [Method for producing artificial foreign bodies in Table 1] Artificial foreign matter was mixed into the rubber composition of each example to make the core more susceptible to cracking, and the impact durability was evaluated. The artificial impurities were prepared by first mixing zinc oxide and finely powdered zinc stearate in a 2:1 mass ratio, heating the mixture in an oven at 150°C for 30 minutes, cooling to room temperature, and then roughly pulverizing the solidified mixture. The pulverized mixture was then sieved through 1 mm and 0.5 mm sieves to obtain agglomerates with diameters of 0.5 to 1.0 mm. These agglomerates were then added as artificial impurities to a kneaded rubber having the rubber composition shown in Table 1 using a roll mill in the amount (1 phr) shown in Table 1.

[0089] [Table 3]

[0090] As shown in Table 3, the golf balls of Examples 1 to 5 have improved durability against normal impact compared to Comparative Examples 1 to 6, and it can be seen that even if foreign matter is mixed in, the decrease in durability against impact can be maintained at a certain level or above.

Claims

1. Each of the following components (a) to (e): (a) a base rubber; (b) an α,β-unsaturated carboxylic acid and / or a metal salt thereof as a co-crosslinking agent; (c) organic peroxide; (d) water or a moisture providing agent; (e) Hindered phenol-based antioxidants having a substituent with a thioether structure wherein the moisture provider is a substance that contains a water component other than free water in its structure and releases water when heated, or a substance that releases water when thermally decomposed by heating, and the blending amount of component (e) is 0.2 parts by mass or more per 100 parts by mass of component (a).

2. 2. The rubber composition for golf balls according to claim 1, wherein the hindered phenol-based antioxidant (e) has a chemical structure having at least one methyl group at the ortho position.

3. 2. The rubber composition for golf balls according to claim 1, wherein the hindered phenol antioxidant (component (e)) has two or more substituents with a thioether structure.

4. 2. The rubber composition for golf balls according to claim 1, wherein the hindered phenol-based antioxidant (component (e)) is represented by the following general formula (I): 【Chemistry 1】 (wherein, x is an integer of 1 or more.)

5. 2. The rubber composition for golf balls according to claim 1, wherein x is an integer of 8 or greater.

6. 2. The rubber composition for golf balls according to claim 1, further comprising, as component (f), a benzimidazole and / or a metal salt thereof represented by the following general formula (II): 【Chemistry 2】 (In the formula, R is a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, m is an integer of 1 to 4, and when m is 2 or more, they may be the same or different.)

7. 7. The rubber composition for golf balls according to claim 6, wherein component (f) is selected from the group consisting of 2-mercaptobenzimidazole, 2-mercaptomethylbenzimidazole, and metal salts thereof.

8. 2. The rubber composition for golf balls according to claim 1, further comprising, as component (g), sulfur or an organic sulfur compound which is an alkylphenol disulfide polymer represented by the following chemical structural formula (III): 【Transformation 3】 (wherein R represents an alkyl group, and n represents a degree of polymerization ranging from 2 to 20.)

9. 9. The rubber composition for golf balls according to claim 8, wherein in formula (III), R is a lower alkyl group having 1 to 6 carbon atoms selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-amyl (pentyl), isoamyl (pentyl), tert-amyl (pentyl), sec-isoamyl, neopentyl, n-hexyl, isohexyl, and tert-hexyl.

10. 9. The rubber composition for golf balls according to claim 8, wherein said organic sulfur compound (g) is an amylphenol disulfide polymer.

11. 9. The rubber composition for golf balls according to claim 8, further comprising an organosulfur compound as component (h) that is different from component (g).

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

13. 2. The rubber composition for golf balls according to claim 1, wherein the difference in hardness between the surface and the center of a vulcanized product of said rubber composition is 15 or more in JIS-C hardness.

14. 2. The rubber composition for golf balls according to claim 1, wherein a vulcanized molded product of the rubber composition includes a region having a hardness of 25 to 30% from the surface toward the center that is at least 5 JIS-C hardness lower than the surface hardness.

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