Multi-piece solid golf ball

Abstract

The present invention provides a multi-piece solid golf ball, of which flight performance and shot feel are improved. The present invention relates to a multi-piece solid golf ball comprising a core composed of a center and an intermediate layer formed on the center, and at least one layer of a cover formed on the core, wherein

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a multi-piece solid golf ball. More particularly, it relates to a multi-piece solid golf ball, which has good shot feel by accomplishing very soft hardness of a center, and of which flight distance is improved, when compared with a conventional golf ball.

BACKGROUND OF THE INVENTION

[0002] Amateur golfers generally regard flight distance as most important factor for golf balls and use a solid golf ball having good rebound characteristics and little spin amount, such as a two-piece golf ball, by choice. On the other hand, professional golfers and high level-amateur golfers generally regard controllability as most important factor for golf balls, and regard soft and good shot feel and flight performance as the next important factor for golf balls. Therefore they have mainly used thread wound golf balls, which have good controllability and soft and good shot feel. However, since the thread wound golf ball has a structure easily putting spin thereon, there has been a problem that the spin amount is large when hit by any type of golf club, which reduces the flight distance. In order to solve the problem, many solid golf balls having good shot feel and excellent flight performance while maintaining good controllability have been proposed in Japanese Patent Kokai Publication Nos. 332247/1996, 313643/1997 and the like.

[0003] The golf balls have a three-piece structure, which are formed by placing an intermediate layer between the core and the cover of the two-piece solid golf ball, in order to accomplish both excellent flight and good shot feel, because of accomplishing various hardness distributions as compared with the two-piece golf ball. Therefore the solid golf balls having a three-piece structure occupy the greater part of the golf ball market.

[0004] Japanese Patent Kokai Publication No. 332247/1996 suggests a three-piece solid golf ball comprising a core having a two-layered structure of an inner core and outer core, and a cover. The inner core has a diameter of 25 to 37 mm, a center hardness in JIS-C hardness of 60 to 85 and a hardness difference in JIS-C hardness between the center point and surface of the inner core of not more than 4, the outer core has a surface hardness in JIS-C hardness of 75 to 90, and the cover has a flexural modulus of 1,200 to 3,600 kg/cm2. However, in the golf ball, there has been a problem that it is difficult to obtain sufficient rebound characteristics and high launch angle when hit by a middle iron club to a short iron club, because the intermediate layer of the golf ball has large thickness.

[0005] Japanese Patent Kokai Publication No. 313643/1997 suggests a three-piece solid golf ball that an intermediate layer is placed between a core and a cover. The core has a center hardness in JIS-C hardness of not more than 75, a surface hardness in JIS-C hardness of not more than 85, the surface hardness is higher than the center hardness by 5 to 25, a hardness of the intermediate layer is higher than the surface hardness of the core by less than 10, and a hardness of the cover is higher than that of the intermediate layer. However, in the golf ball, there has been a problem that rebound characteristics are poor and shot feel is hard and poor, because the intermediate layer is formed from thermoplastic resin.

[0006] It is required for all golfers regardless of amateur golfers or professional golfers to provide a multi-piece solid golf ball having better flight performance and better shot feel by solving the problem.

OBJECTS OF THE INVENTION

[0007] A main object of the present invention is to provide a multi-piece solid golf ball, of which flight performance and shot feel are improved.

[0008] According to the present invention, the object described above has been accomplished by providing a multi-piece solid golf ball comprising a core composed of a center and an intermediate layer and at least one layer of a cover formed on the core, and adjusting a diameter and center point hardness of the center, and a thickness and surface hardness of the intermediate layer to specified ranges, thereby providing a multi-piece solid golf ball, of which flight performance and shot feel are improved.

[0009] This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.

BRIEF EXPLANATION OF DRAWINGS

[0010] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

[0011] FIG. 1 is a schematic cross section illustrating one embodiment of the golf ball of the present invention.

[0012] FIG. 2 is a schematic cross section illustrating one embodiment of a mold for molding a semi-spherical half-shell for the intermediate layer of the golf ball of the present invention.

[0013] FIG. 3 is a schematic cross section illustrating one embodiment of a mold for molding the core of the golf ball of the present invention.

SUMMARY OF THE INVENTION

[0014] The present invention provides a multi-piece solid golf ball comprising a core composed of a center and an intermediate layer formed on the center, and at least one layer of a cover formed on the core, wherein

[0015] the center has a diameter of 20 to 35 mm and a center point hardness in Shore A hardness of 10 to 70,

[0016] the intermediate layer has a thickness of 2.5 to 10.0 mm and a surface hardness in Shore D hardness of 40 to 65.

[0017] In the golf ball of the present invention, when compared with the conventional golf ball described above, shot feel is soft and good; and it is possible to control spin amount, which improves flight distance; particularly by adjusting a hardness of the center to 10 to 70 in Shore A hardness (not more than 45 in Shore D hardness), which is very low.

[0018] In order to put the present invention into a more suitable practical application, it is desired for the cover to have a Shore D hardness of 45 to 60.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The multi-piece solid golf ball of the present invention will be explained with reference to the accompanying drawing in detail. FIG. 1 is a schematic cross section illustrating one embodiment of the golf ball of the present invention. As shown in FIG. 1, the golf ball of the present invention comprises a core 4 composed of a center 1 and an intermediate layer 2 formed on the center, and at least one layer of cover 3 covering the core. In order to explain the golf ball of the present invention simply, a golf ball having one layer of cover 3 will be used hereinafter for explanation. However, the golf ball of the present invention may be applied for the golf ball having two or more layers of cover.

[0020] In the golf ball of the present invention, the center 1 is suitably formed from vulcanized molded article mainly comprising silicone rubber composition, which is easy to obtain soft rubber molded article, or vulcanized molded article comprising rubber composition containing oily substance. Preferred is silicone rubber composition because a rubber-molded article, which is soft and has high rebound characteristics, is easy to obtained.

[0021] The silicone rubber composition is obtained by dispersing filler, vulcanization agent and the like in raw silicone rubber. The raw silicone rubber, which is colorless elastic body slightly flowable at room temperature, contains dimethyl siloxane unit [(CH3)2SiO] as a main component, contains 103 diorganosiloxane units per a molecular on an average, and has an average molecular weight of about 105 to 106. The raw silicone rubber is formed by ring opening polymerization of cyclic dimethyl siloxane, which is formed by hydrolyzing dimethyl dichloro silane, with acid or alkali under applied heat. Examples of the raw silicone rubber includes dimethyl silicone, methyl vinyl silicone, methyl phenyl vinyl silicone and the like.

[0022] The raw silicone rubber is not limited, but examples thereof will be shown by a trade name thereof. Examples of the dimethyl silicone rubber include “KE-76 (trade name)”, which is commercially available from Shin-Etsu Chemical Co., Ltd.; “TS-959 (trade name)”, which is commercially available from Toshiba Silicone Co., Ltd.; “Silastic 400 (trade name)” and “Silastic 401 (trade name)”, which are commercially available from Dow Corning Co. (D.C.); “SE-76 (trade name)”, which is commercially available from General Electric Co. (G.E.); “W-95 (trade name)”, which is commercially available from Union Carbide Co. (U.C.C.); and the like. Examples of methyl vinyl silicone include “KE-77 (trade name)”, which is commercially available from Shin-Etsu Chemical Co., Ltd.; “TS-959B (trade name)”, which is commercially available from Toshiba Silicone Co., Ltd.; “SH-430 (trade name)”, which is commercially available from Toray Silicone Co., Ltd.; “Silastic 410 (trade name)” and “Silastic 430 (trade name)”, which are commercially available from Dow Corning Co. (D.C.); “SE-31 (trade name)”, which is commercially available from General Electric Co. (G.E.); “W-96 (trade name)”, which is commercially available from Union Carbide Co. (U.C.C.); and the like. Examples of methyl phenyl vinyl silicone include “KE-79 (trade name)”, which is commercially available from Shin-Etsu Chemical Co., Ltd.; “Silastic 440 (trade name)”, which is commercially available from Dow Corning Co. (D.C.); “W-97 (trade name)”, which is commercially available from Union Carbide Co. (U.C.C.); and the like.

[0023] When a vulcanized molded article of only raw silicone rubber is used, tensile strength thereof tends to be low. Therefore the silicone rubber may comprise a filler in order to improve the strength of the vulcanized molded article of silicone rubber. As the filler, there are a reinforcable filler for reinforcing the silicone rubber and a non-reinforcable filler for extending the silicone rubber. It is required in all cases to select the filler, which has excellent heat resistance and does not deteriorate the performance of the silicone rubber at high temperature. Examples of the reinforcable fillers include a surface treated silica, high purity fumed silica, silica aerogel, precipitated silica and the like.

[0024] Examples of the non-reinforcable fillers include a calcined diatomaceous silica, precipitated calcium carbonate, ground silica, ground quartz, titanium oxide, zinc oxide and the like.

[0025] A method for vulcanizing the silicone rubber includes a method of using organic peroxide, fatty azo compound, radiation and the like, but organic peroxide is generally used as a vulcanizing agent. Examples of the organic peroxide include benzoyl peroxide, bis(2,4-dichlorobenzoyl)peroxide, dicumyl peroxide, d-t-butyl peroxide and the like. The amount of the vulcanizing agent is preferably from 0.2 to 8.0 parts by weight, based on 100 parts by weight of the base silicone rubber. When the amount of the vulcanizing agent is smaller than 0.2 parts by weight, the vulcanization is not perfectly conducted, and the hardness is low, which degrades the rebound characteristics. On the other hand, when the amount of the vulcanizing agent is larger than 8.0 parts by weight, the center is hard, and the spin amount is high, which reduces the flight distance.

[0026] The vulcanization of the silicone rubber may be heat vulcanizing type, which is conducted by heating, or room temperature vulcanizing type, which is conducted by leaving it at room temperature. The room temperature vulcanizing type silicone rubber is classified into one-component type and two-component type. The one-component type silicone rubber is filled in a cartridge or tube in a state of compounding the vulcanizing agent with the raw silicone rubber, and is vulcanized by condensation reaction when it is contacted with air. The two-component type silicone rubber is vulcanized by mixing the vulcanizing agent at the time of using.

[0027] The silicone rubber may be dimethyl silicone rubber, methyl vinyl silicone rubber and methyl phenyl vinyl silicone rubber as described above, but copolymer thereof may be used. Preferred is heat vulcanizing type silicone rubber, which is copolymer of dimethyl siloxane rubber as a main component and methyl vinyl siloxane as small component, in view of rebound characteristics. Content of vinyl group in the silicone rubber is preferably 0.1 to 10 molar %, more preferably 0.15 to 0.8 molar %.

[0028] In the vulcanized molded article, the amount of the silicone rubber is not less than 70 parts by weight, preferably not less than 80 parts by weight, based on 100 parts by weight of base rubber of the center. When the amount of the silicone rubber is smaller than 70 parts by weight, good retentivity of spin is not obtained. The silicone rubber may be mixed with polybutadiene rubber (BR), ethylene-propylene-diene monomer terpolymer rubber (EPDM), acrylonitrile butadiene rubber (NBR), acrylonitrile rubber, polynorbornene rubber and the like.

[0029] When the center 1 is prepared by using the silicone rubber composition, the center is obtained by mixing 0.5 to 5 parts by weight of vulcanizing agent and a desired amount of weight adjuster, based on 100 parts by weight of the silicone rubber using a Banbury mixer or a mixing roll and by press molding and vulcanizing at 150 to 170° C. for 10 to 20 minutes. The vulcanizing agent and weight adjuster may be one which has been conventionally used. However, as the weight adjuster, high specific gravity filler is used in order to accomplish high weight ratio of the rubber component in the center and low hardness of the center. Example of the weight adjuster includes barium sulfate, calcium carbonate, clay, silica and the like.

[0030] When the center 1 is formed from the rubber composition containing oily substance, the center is obtained by mixing 100 to 500 parts by weight of the oily substance, 0.5 to 5 parts by weight of sulfur, 3 to 10 parts by weight of vulcanization aid, 1 to 5 parts by weight of vulcanization accelerator and a desired amount of weight adjuster, based on 100 parts by weight of the base rubber using a Banbury mixer or a mixing roll and by press molding and vulcanizing at 150 to 170° C. for 10 to 20 minutes.

[0031] The base rubber is not limited, but includes polybutadiene rubber (BR), natural rubber (NR), ethylene-propylene-diene monomer terpolymer rubber (EPDM), polynorbornene rubber and the like. Preferred is polynorbornene rubber. Polynorbornene rubber can contain an oily substance in high amount to obtain a soft rubber, which imparts suitable rebound characteristics to the resulting center by selecting the oily substance.

[0032] The oily substance, which is used for the rubber composition, is not limited, but includes alkylbenzene oil; naphthenic oil; paraffinic oil; aromatic oil; ester plasticizer such as dioctyl adipate (DOA), dioctyl sebacate (DOS) and the like. Preferred are naphthenic oil and alkylbenzene oil, because it is difficult to occur bleeding in case of containing in high amount and they impart high rebound characteristics to the resulting center.

[0033] The vulcanization aid, vulcanization accelerator and weight adjuster may be one which has been conventionally used for a rubber composition of solid golf ball. However, the weight adjuster may be the same as described in the silicone rubber composition.

[0034] The intermediate layer 2 is then formed on the center 1 to obtain the core 4. In the golf ball of the present invention, the intermediate layer 2 is obtained by press-molding a rubber composition under applied heat. The rubber composition contains a base rubber, a co-crosslinking agent, an organic peroxide, a filler, an antioxidant and the like.

[0035] The base rubber may be natural rubber and/or synthetic rubber, which has been conventionally used for solid golf balls. Preferred is high-cis polybutadiene rubber containing a cis-1, 4 bond of not less than 40%, preferably not less than 80%. The high-cis polybutadiene rubber may be optionally mixed with natural rubber, polyisoprene rubber, styrene-butadiene rubber, ethylene-propylene-diene rubber (EPDM) and the like.

[0036] The co-crosslinking agent can be a metal salt of &agr;,&bgr;-unsaturated carboxylic acid, including mono or divalent metal salts, such as zinc or magnesium salts of &agr;,&bgr;-unsaturated carboxylic acids having 3 to 8 carbon atoms (e.g. acrylic acid, methacrylic acid, etc.), or a blend of the metal salt of &agr;,&bgr;-unsaturated carboxylic acid and acrylic ester or methacrylic ester and the like. The preferred co-crosslinking agent is zinc acrylate because it imparts high rebound characteristics to the resulting golf ball. The amount of the co-crosslinking in the rubber composition is from 25 to 40 parts by weight, preferably from 27 to 35 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the co-crosslinking is larger than 40 parts by weight, the core is too hard, and the shot feel is poor. On the other hand, when the amount of the co-crosslinking is smaller than 25 parts by weight, the rebound characteristics are degraded, which reduces the flight distance. In addition, the resulting golf ball is too soft, and it has poor shot feel such that the rebound characteristics are poor, because soft rubber is used for the center.

[0037] The organic peroxide includes, for example, dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butyl peroxide and the like. The preferred organic peroxide is dicumyl peroxide. The amount of the organic peroxide is from 0.5 to 2.0 parts by weight, preferably 0.8 to 1.5 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the organic peroxide is smaller than 0.5 parts by weight, the core is too soft, and the rebound characteristics are degraded, which reduces the flight distance. On the other hand, when the amount of the organic peroxide is larger than 2.0 parts by weight, the core is too hard, and the shot feel is poor.

[0038] The filler, which can be typically used for the core of solid golf ball, includes for example, inorganic filler (such as zinc oxide, barium sulfate, calcium carbonate and the like), high specific gravity metal powder filler (such as tungsten powder, molybdenum powder and the like), and mixtures thereof. The amount of the filler is 5 to 50 parts by weight, preferably 8 to 40 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the filler is smaller than 5 parts by weight, it is required to use a large amount of filler in the center in order to obtain a desired weight of the resulting golf ball, and the rebound characteristics of the center are degraded. On the other hand, when the amount of the filler is larger than 50 parts by weight, the weight of the core is too large, it is difficult to adjust the weight of the resulting golf ball.

[0039] The rubber composition for the center 1 and intermediate layer 2 of the golf ball of the present invention can contain other components, which have been conventionally used for preparing the core of solid golf balls, such as antioxidant or peptizing agent. If used, the amount of the antioxidant is preferably 0.2 to 0.5 parts by weight, and that of the peptizing agent is preferably 0.2 to 5.0 parts by weight, based on 100 parts by weight of the base rubber.

[0040] The process of producing the core of the golf ball of the present invention will be explained with reference to FIG. 2 and FIG. 3. FIG. 2 is a schematic cross section illustrating one embodiment of a mold for molding a semi-spherical half-shell used for the intermediate layer of the golf ball of the present invention. FIG. 3 is a schematic cross section illustrating one embodiment of a mold for molding a core of the golf ball of the present invention. The rubber composition for the center is mixed, and press-molded in a mold at 150 to 170° C. for 10 to 20 minutes to prepare a vulcanized spherical molded article for the center. The rubber composition for the intermediate layer then is mixed, and press-molded using a mold having a semi-spherical cavity 5 and a male plug mold 6 having a semi-spherical convex having the same diameter as the vulcanized spherical molded article for the center as described in FIG. 2 to obtain an unvulcanized semi-spherical half-shell 7 for the intermediate layer. The vulcanized molded article for the center 9 is covered with the two semi-spherical half-shells 7 for the intermediate layer, and then press-molded at 140 to 180° C. for 10 to 60 minutes in a mold 8, which is composed of an upper mold and a lower mold having a semi-spherical cavity, as described in FIG. 3 to prepare a two-layer structured core 4 comprising the center 1 and the intermediate layer 2 formed on the center.

[0041] In the golf ball of the present invention, it is required for the center 1 to have a diameter of 20 to 35 mm, preferably 22 to 33 mm, more preferably 25 to 32 mm. When the diameter is smaller than 20 mm, the spin amount at the time of hitting is high, which reduces the flight distance. On the other hand, when the diameter is larger than 35 mm, ball compression (deformation amount) is large, and the golf ball is too soft. Therefore the golf ball has poor shot feel such that the rebound characteristics are poor.

[0042] In the golf ball of the present invention, it is required for the center 1 to have a center point hardness in Shore A hardness of 10 to 70, preferably 15 to 65, more preferably 30 to 60. When the center point hardness is lower than 10, the rebound characteristics of the resulting golf ball are degraded, which reduces the flight distance. On the other hand, when the hardness is higher than 70, the center is too hard, and the shot feel is poor. In addition, the spin amount when hit by a driver is high, which reduces the flight distance. The term “center point hardness of the center” as used herein means the hardness determined by cutting the center into two equal parts and then measuring a hardness in Shore A hardness at the center point of the section.

[0043] In the golf ball of the present invention, it is required for the intermediate layer 2 to have a thickness of 2.5 to 10.0 mm, preferably 3.0 to 9.5 mm, more preferably 3.5 to 7.5 mm. When the thickness is smaller than 2.5 mm, ball compression (deformation amount) is large, and the golf ball is too soft. Therefore the golf ball has poor shot feel such that the rebound characteristics are poor. On the other hand, when the thickness is larger than 10.0 mm, the spin amount at the time of hitting is high, which reduces the flight distance. In addition, the shot feel is poor.

[0044] In the golf ball of the present invention, it is required for the intermediate layer to have a surface hardness in Shore D hardness of 40 to 65, preferably 42 to 60, more preferably 44 to 60. When the surface hardness is lower than 40, the resulting golf ball is too soft, and the rebound characteristics are degraded, which reduces the flight distance. On the other hand, when the hardness is higher than 65, the resulting golf ball is too hard, and the shot feel is poor. In addition, the spin amount when hit by a driver is high, which reduces the flight distance. The term “a surface hardness of the intermediate layer” as used herein means the surface hardness in Shore D hardness of the core 4 having a two-layered structure, which is formed by integrally press-molding the intermediate layer 2 on the center 1.

[0045] At least one layer of the cover 3 is covered on the core 4. In the golf ball of the present invention, it is desired for the cover 3 to have a thickness of 1.0 to 4.0 mm, preferably 1.5 to 3.8 mm, more preferably 2.0 to 3.5 mm. When the thickness of the cover is smaller than 1.0 mm, sufficient controllability is obtained. On the other hand, when the thickness is larger than 4.0 mm, sufficient flight distance is obtained. The cover of the golf ball of the present invention may have single-layered structure or multi-layered structure, which has two or more layers. If the cover has a multi-layered structure, the thickness of each layer is not limited as long as the total thickness of the cover layers is within the above range.

[0046] In the golf ball of the present invention, it is desired for the cover 3 to have a Shore D hardness of 45 to 60, preferably 47 to 58, more preferably 48 to 57, most preferably 50 to 55. When the hardness is lower than 45, the spin amount when hit by a driver is high, which reduces the flight distance. On the other hand, when the hardness is higher than 60, the cover is too hard, and the shot feel is poor. In addition, the spin amount at approach shot is low, and the controllability is poor. If the cover has a multi-layered structure, the hardness of each layer is not limited as long as that of the outmost layer of the cover is within the above range. The term “a hardness of the cover” as used herein means the surface hardness in Shore D hardness of the golf ball, which is formed by covering the core 4 with the cover 3.

[0047] The cover 3 of the present invention contains thermoplastic resin, particularly ionomer resin, which has been conventionally used for the cover of golf balls, as a base resin. The ionomer resin may be a copolymer of ethylene and &agr;,&bgr;-unsaturated carboxylic acid, of which a portion of carboxylic acid groups is neutralized with metal ion, or a terpolymer of ethylene, &agr;,&bgr;-unsaturated carboxylic acid and &agr;,&bgr;-unsaturated carboxylic acid ester, of which a portion of carboxylic acid groups is neutralized with metal ion. Examples of the &agr;,&bgr;-unsaturated carboxylic acid in the ionomer include acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like, preferred are acrylic acid and methacrylic acid. Examples of the &agr;,&bgr;-unsaturated carboxylic acid ester in the ionomer include methyl ester, ethyl ester, propyl ester, n-butyl ester and isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like. Preferred are acrylic acid esters and methacrylic acid esters. The metal ion which neutralizes a portion of carboxylic acid groups of the copolymer or terpolymer includes a sodium ion, a potassium ion, a lithium ion, a magnesium ion, a calcium ion, a zinc ion, a barium ion, an aluminum, a tin ion, a zirconium ion, cadmium ion, and the like. Preferred are sodium ions, zinc ions, magnesium ions and the like, in view of rebound characteristics, durability and the like.

[0048] The ionomer resin is not limited, but examples thereof will be shown by a trade name thereof. Examples of the ionomer resins, which are commercially available from Mitsui Du Pont Polychemical Co., Ltd. include Hi-milan 1555, Hi-milan 1557, Hi-milan 1605, Hi-milan 1652, Hi-milan 1702, Hi-milan 1705, Hi-milan 1706, Hi-milan 1707, Hi-milan 1855, Hi-milan 1856 and the like. Examples of the ionomer resins, which are commercially available from Du Pont Co., include Surlyn 8945, Surlyn 9945, Surlyn 6320 and the like. Examples of the ionomer resins, which are commercially available from Exxon Chemical Co., include Iotek 7010 Iotek 8000 and the like. These ionomer resins may be used alone or in combination.

[0049] As the materials suitably used in the cover 3 of the present invention, the above ionomer resin may be used alone, but the ionomer resin may be used in combination with at least one of thermoplastic elastomer, diene block copolymer and the like. Examples of the thermoplastic elastomers include polyamide-based thermoplastic elastomer, which is commercially available from Toray Co., Ltd. under the trade name of “Pebax” (such as “Pebax 2533”); polyester-based thermoplastic elastomer, which is commercially available from Toray-Do Pont Co., Ltd. under the trade name of “Hytrel” (such as “Hytrel 3548”, “Hytrel 4047”); polyurethane-based thermoplastic elastomer, which is commercially available from Takeda Bardishe Co., Ltd. under the trade name of “Elastollan” (such as “Elastollan ET880”); and the like.

[0050] The diene-based block copolymer is a block copolymer or partially hydrogenated block copolymer having double bond derived from conjugated diene compound. The base bock copolymer is block copolymer composed of block polymer block A mainly comprising at least one aromatic vinyl compound and polymer block B mainly comprising at least one conjugated diene compound. The partially hydrogenated block copolymer is obtained by hydrogenating the block copolymer. Examples of the aromatic vinyl compounds comprising the block copolymer include styrene, &agr;-methyl styrene, vinyl toluene, p-t-butyl styrene, 1,1-diphenyl styrene and the like, or mixtures thereof. Preferred is styrene. Examples of the conjugated diene compounds include butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene and the like, or mixtures thereof. Preferred are butadiene, isoprene and combinations thereof. Examples of the diene-based block copolymers which is commercially available include the diene-based block copolymers, which are commercially available from Daicel Chemical Industries, Ltd. under the trade name of “Epofriend” (such as “Epofriend A1010”), the diene-based block copolymers, which are commercially available from Kuraray Co., Ltd. under the trade name of “Septon” (such as “Septon HG-252”) and the like.

[0051] The amount of the thermoplastic elastomer or diene-based block copolymer is 1 to 60 parts by weight, preferably 1 to 35 parts by weight, based on 100 parts by weight of the base resin for the cover. When the amount is smaller than 1 parts by weight, the technical effect of absorbing the impact force at the time of hitting accomplishing by using them is not sufficiently obtained. On the other hand, when the amount is larger than 60 parts by weight, the cover is too soft and the rebound characteristics are degraded, or the compatibility with the ionomer resin is degraded and the durability is degraded.

[0052] The composition for the cover 3 used in the present invention may optionally contain pigments (such as titanium dioxide, etc.) and the other additives such as a dispersant, an antioxidant, a UV absorber, a photostabilizer and a fluorescent agent or a fluorescent brightener, etc., in addition to the resin component, as long as the addition of the additives does not deteriorate the desired performance of the golf ball cover.

[0053] A method of covering on the core 4 with the cover 3 is not specifically limited, but may be a conventional method. For example, there can be used a method comprising molding the cover composition into a semi-spherical half-shelled in advance, covering the core, which is covered with the outer core, with the two half-shells, followed by pressure molding at 130 to 170° C. for 1 to 5 minutes, or a method comprising injection molding the cover composition directly on the core, which is covered with the core, to cover it. At the time of molding the cover, many depressions called “dimples” may be optionally formed on the surface of the golf ball. Furthermore, paint finishing or marking with a stamp may be optionally provided after the cover is molded for commercial purposes.

EXAMPLES

[0054] The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope of the present invention.

[0055] (i) Production of Vulcanized Spherical Center

[0056] The rubber compositions for the center having the formulation shown in Table 1 were mixed, and then vulcanized by press-molding in the mold at 160° C. for 20 minutes to obtain vulcanized spherical molded articles for the centers. The diameter, center point hardness and specific gravity of the resulting centers were measured. The results are shown in Table 3.

[0057] (ii) Production of Unvulcanized Semi-spherical Half-shell for the Intermediate Layer

[0058] The rubber compositions for the intermediate layer having the formulation shown in Table 1 were mixed, and then press-molded in the mold (5, 6) having a semi-spherical convex having the same diameter as the vulcanized spherical molded article for the center produced in the step (i) as described in FIG. 2 to obtain unvulcanized semi-spherical half-shells 7 for the intermediate layer.

[0059] (iii) Production of Core

[0060] The vulcanized spherical molded articles for the center 9 produced in the step (i) were covered with the two unvulcanized semi-spherical half-shells 7 for the intermediate layer produced in the step (ii), and then vulcanized by press-molding at 165° C. for 20 minutes in the mold 8 as described in FIG. 3 to obtain two-layer structured cores 4 having a diameter of 39.0 mm. The surface hardness, thickness and specific gravity of the resulting cores 4 were measured. The results are shown in Table 3. The surface hardness of the core is shown in the same Table as the surface hardness in Shore D hardness of the intermediate layer. 1 TABLE 1 (parts by weight) Comparative Example No. Example No. 1 2 3 4 1 2 3 (Center composition) BR-11 *1 — — — — — — 100 Zinc acrylate — — — — — — 15 Zinc oxide — — — — — — 15 Antioxidant *2 — — — — — — 0.5 Dicumyl peroxide — — — — — — 1.2 Barium sulfate — — — — — — 14 KE530-U *3 100 — — — 100 — — KE540-U *4 — 100 — 100 — — — KE1551-U *5 — — 100 — — 100 — C-4 *6 4 4 4 4 4 4 (Intermediate layer composition) BR-11 *1 100 100 100 100 100 100 100 Zinc acrylate 28 34 38 34 28 38 22 Zinc oxide 15 15 15 15 15 15 15 Antioxidant *2 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Dicumyl peroxide 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Barium sulfate 10.5 8.5 5.0 8.5 10.0 5.0 11.0 *1: BR-11 (trade name), high-cis polybutadiene commercially available from JSR Co., Ltd. (Content of 1,4-cis-polybutadiene: 96.0) *2: Nocrac NS-6 (trade name), 2,5-t-butylhydroquinone commercially available from Ouchishinko Chemical Industrial Co., Ltd. *3: KE530-U (trade name), silicone compound commercially available from Shin-Etsu Chemical Co., Ltd. *4: KE540-U (trade name), silicone compound commercially available from Shin-Etsu Chemical Co., Ltd. *5: KE1551-U (trade name), silicone compound commercially available from Shin-Etsu Chemical Co., Ltd. *6: C-4 (trade name), vulcanization accelerator commercially available from Shin-Etsu Chemical Co., Ltd.

[0061] (iv) Preparation of Cover Compositions

[0062] The formulation materials shown in Table 2 were mixed using a kneading type twin-screw extruder to obtain pelletized cover compositions. The extrusion condition was,

[0063] a screw diameter of 45 mm,

[0064] a screw speed of 200 rpm, and

[0065] a screw L/D of 35.

[0066] The formulation materials were heated at 150 to 260° C. at the die position of the extruder. 2 TABLE 2 (parts by weight) Comparative Example No. Example No. Cover composition 1 2 3 4 1 2 3 Hi-milan 1605 *7 — — — 50 — — — Hi-milan 1706 *8 — — — 50 — — — Hi-milan 1855 *9 — 20 — — 20 20 20 Surlyn 9945 *10 30 20 — — 20 20 20 Surlyn 8945 *11 30 20 20 — 20 20 20 Surlyn 6320 *12 — — 80 — — — — Taftek Z514 *13 — 25 — — 25 25 25 Bondine AX8390 *14 — 15 — — 15 15 15 Epofriend A1010 *15 10 — — — — — — Septon HG-252 *16 30 — — — — — — Titanium dioxide 2 2 2 2 2 2 2 Barium sulfate 2 2 2 2 2 2 2 *7: Hi-milan 1605 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with sodium ion, manufactured by Mitsui Du Pont Polychemical Co., Ltd., Shore D hardness = 61, Flexural modulus = about 310 MPa *8: Hi-milan 1706 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with zinc ion, manufactured by Mitsui Du Pont Polychemical Co., Ltd., Shore D hardness = 60, Flexural modulus = about 260 MPa *9: Hi-milan 1855 (trade name), ethylene-methacrylic acid-acrylic acid ester terpolymer ionomer resin obtained by neutralizing with zinc ion, manufactured by Mitsui Du Pont Polychemical Co., Ltd., Shore D hardness = 54, Flexural modulus = about 90 MPa *10: Surlyn 9945 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with zinc ion, manufactured by Du Pont Co., Shore D hardness = 59, Flexural modulus = about 220 MPa *11: Surlyn 8945 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with sodium ion, manufactured by Du Pont Co., Shore D hardness = 61, Flexural modulus = about 270 MPa *12: Surlyn 6320 (trade name), ethylene-methacrylic acid-acrylic acid ester terpolymer ionomer resin obtained by neutralizing with magnesium ion, manufactured by Du Pont Co., Shore D hardness: 44, Flexural modulus = about 35 MPa *13: Taftek Z514 (trade name), glycidyl methacrylate adduct of hydrogenated styrene-butadiene-styrene block copolymer, manufactured by Asahi Kasei Co., Ltd., JIS-A hardness = 65, content of styrene = about 20% by weight, content of hydrogenated butadiene = about 80% by weight, content of glycidyl methacrylate = about 1% by weight *14: Bondine AX8390 (trade name), ethylene-ethyl acrylate-maleic anhydride terpolymer resin, manufactured by Sumitomo Chemical Industries Co., Ltd., MI = 7.0, Shore D hardness = 14, content of ethyl acrylate + maleic anhydride = 32% (content of maleic anhydride: 1 to 4%) *15: Epofriend A1010 (trade name), styrene-butadiene-styrene (SBS) block copolymer with epoxy groups, manufactured by Daicel Chemical Industries, Ltd., JIS-A hardness = 67, styrene/butadiene (weight ratio) = 40/60, content of epoxy: about 1.5 to 1.7% by weight *16: Septon HG-252 (trade name), hydrogenated styrene-isoprene-styrene (SIS) block copolymer having a terminal OH group, manufactured by Kuraray Co. Ltd., JIS-A hardness = 80, content of styrene = about 40% by weight

Examples 1 to 4 and Comparative Examples 1 to 3

[0067] The cover composition was covered on the resulting core 4 having two-layered structure by injection molding to form a cover layer 3 having the Shore D hardness shown in Table 3. Then, paint was applied on the surface to produce golf ball having a diameter of 42.8 mm and a weight of 45.4 g. With respect to the resulting golf balls, the initial velocity, spin amount, flight distance (total)) and shot feel were measured or evaluated. The results are shown in Table 3. The test methods are as follows.

[0068] (Test method)

[0069] (1) Hardness

[0070] (i) Center Point Hardness of the Center

[0071] The center point hardness of the center, which is formed by press-molding, is determined by measuring a hardness in Shore A hardness at the center point of the inner core in section, after the resulting center is cut into two equal parts. The Shore A hardness (which is substantially the same as JIS-A hardness according to JIS K 6253A) was measured with a Shore hardness meter.

[0072] (ii) Surface Hardness of the Intermediate Layer

[0073] The surface hardness of the intermediate layer is determined by measuring a hardness in Shore D hardness at the surface of the core having two-layered structure, which is formed by integrally press-molding the intermediate layer on the center. The Shore D hardness was measured using a Shore D hardness meter according to ASTM D 2240-68.

[0074] (iii) Hardness of Cover

[0075] After the golf ball is obtained by covering the core 4 with the cover, a Shore D hardness of the cover is determined by measuring a hardness at the surface of the golf ball at 23° C. using a Shore D hardness meter according to ASTM D 2240-68.

[0076] (2) Flight Performance

[0077] (2-1) Flight Performance 1

[0078] After a No. 1 wood club (W#1, a driver) was mounted to a swing robot manufactured by Golf Laboratory Co. and the golf ball was hit at a head speed of 45 m/sec, the initial velocity, spin amount and flight distance was measured. As the flight distance, total that is a distance to the stop point of the hit golf ball were measured. The measurement was conducted by using 12 golf balls for every sample (n=12), and the average is shown as the result of the golf ball.

[0079] (2-2) Flight Performance 2

[0080] After a sand wedge (SW) was mounted to a swing robot manufactured by Golf Laboratory Co. and the golf ball was hit at a head speed of 21 m/sec, the spin amount was measured. The spin amount was measured by continuously taking a photograph of a mark provided on the hit golf ball using a high-speed camera. The measurement was conducted 12 golf balls for each sample (n=12), and the average is shown as the result of the golf ball.

[0081] (3) Shot Feel

[0082] The shot feel of the golf ball is evaluated by 10 golfers according to a practical hitting test using a No. 1 wood club (W#1, a driver) having a metal head. The evaluation criteria are as follows. The results shown in the Tables below are based on the fact that the most golfers evaluated with the same criterion about shot feel.

[0083] Evaluation Criteria I (Impact Force)

[0084] ∘: The golfers felt that the golf ball has small impact force, and good shot feel.

[0085] &Dgr;: The golfers felt that the golf ball has fairly good shot feel.

[0086] x: The golfers felt that the golf ball has large impact force, and poor shot feel.

[0087] Evaluation Criteria II (Rebound Characteristics)

[0088] ∘: The golfers felt that the golf ball has rebound characteristics, and good shot feel.

[0089] &Dgr;: The golfers felt that the golf ball has fairly good shot feel.

[0090] x: The golfers felt that the golf ball has poor rebound characteristics, and heavy and poor shot feel. 3 TABLE 3 Comparative Example No. Example No. Test item 1 2 3 4 1 2 3 (Center) Diameter (mm) 24 28 32 28 18 36 28 Center point hard- 32 42 58 42 32 58 75 ness (Shore A) Specific gravity 1.13 1.14 1.16 1.14 1.13 1.16 1.16 (Intermediate layer composition) Surface hardness 44 55 60 55 44 60 40 (Shore D) Thickness (mm) 7.5 5.5 3.5 5.5 10.5 1.5 5.5 Specific gravity 1.17 1.16 1.16 1.16 1.16 1.16 1.16 (Cover) Shore D hardness 58 54 52 70 54 54 54 (Properties of golf ball) Flight performance 1 (W#1; 45 m/sec) Initial velocity 65.1 65.2 65.0 65.4 65.2 64.9 65.0 (m/sec) Spin amount (rpm) 2750 2700 2670 2600 3180 2620 3040 Total (yard) 233.8 234.6 235.0 236.5 221.5 230.5 224.2 Flight performance 2 (SW; 21 m/sec) Spin amount (rpm) 6980 7020 7100 5600 7180 6850 7150 Shot Impact ∘ ∘ ∘ &Dgr; x ∘ x feel Rebound ∘ ∘ ∘ ∘ ∘ x ∘

[0091] As is apparent from Table 3, the golf balls of Examples 1 to 4 of the present invention, when compared with the golf balls of Comparative Examples 1 to 3, had long flight distance when hit by a driver, and good shot feel such that the impact force is small and rebound characteristics are good.

[0092] In the golf ball of Example 4, which is within the scope of the present invention, both the flight distance and shot feel are superior to the golf balls of Comparative Examples, but the cover hardness is high, and the spin amount when hit by a sand wedge is low. When hit by a short iron club, such as a sand wedge, the controllability is generally regarded as most important factor for golf balls. Therefore it is desired for the spin amount when hit by a sand wedge to be not less than 5500 rpm, preferably not less than 6950 rpm. In the golf ball of Example 4, when compared with the golf balls of the other Examples, the cover hardness is high, and the spin amount when hit by a sand wedge is low, which degrades the controllability.

[0093] On the other hand, in the golf ball of Comparative Example 1, since the diameter of the center is small, the spin amount at the time of hitting is high, which reduces the flight distance. In addition, since the thickness of the intermediate layer is large, the spin amount is high, which reduces the flight distance, or the impact force at the time of hitting is large, and the shot feel is poor.

[0094] In the golf ball of Comparative Example 2, since the diameter of the center is large, the resulting golf ball is too soft, and has a poor shot feel such that the rebound characteristics are poor. In addition, since the thickness of the intermediate layer is small, the resulting golf ball is too soft, and has a poor shot feel such that the rebound characteristics are poor.

[0095] In the golf ball of Comparative Example 3, since the center point hardness of the center is high, the resulting golf ball is too hard, and the shot feel is poor. In addition, the spin amount when hit by a driver is high, which reduces the flight distance.

Claims

1. A multi-piece solid golf ball comprising a core composed of a center and an intermediate layer formed on the center, and at least one layer of a cover formed on the core, wherein

the center has a diameter of 20 to 35 mm and a center point hardness in Shore A hardness of 10 to 70,

the intermediate layer has a thickness of 2.5 to 10.0 mm and a surface hardness in Shore D hardness of 40 to 65.

2. The multi-piece solid golf ball according to claim 1, wherein the cover has a Shore D hardness of 45 to 60.

3. The multi-piece solid golf ball according to claim 1, wherein the center is formed from a vulcanized molded article mainly comprising silicone rubber composition.

4. The multi-piece solid golf ball according to claim 1, wherein the cover has a single-layered structure.