Multi-piece solid golf ball

Abstract

The present invention provides a multi-piece solid golf ball having good shot feel, excellent flight performance and excellent controllability. The present invention relates to a multi-piece solid golf ball comprising a core composed of a center and at least one intermediate layer formed on the center, and a cover covering the core, wherein assuming that a central point hardness of the center, a surface hardness of the center, a hardness of the intermediate layer and a hardness of the cover in Shore D hardness are represented by HM, HS, HI and HC, respectively, the HM, HS, HI and HC satisfy a correlation represented by the following formula:

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 having good shot feel, excellent flight performance and excellent controllability.

BACKGROUND OF THE INVENTION

[0002] In golf balls commercially selling, there are solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. Recently, the two-piece golf ball and three-piece golf ball, of which flight distance can be improved while maintaining soft and good shot feel at the time of hitting as good as the conventional thread wound golf ball, generally occupy the greater part of the golf ball market. Multi-piece solid golf balls represented by three-piece golf ball have good shot feel while maintaining excellent flight performance, because they can vary hardness distribution and design of golf balls, when compared with the two-piece golf ball.

[0003] The three-piece solid golf balls are obtained by inserting an intermediate layer between the core and the cover layer constituting the two-piece solid golf ball and have been described in Japanese Patent Kokai Publication Nos. 313643/1997, 305114/1998, 151226/1998, 360740/2002 and the like. In the golf balls, it has been attempted to compromise the balance of flight performance and shot feel at the time of hitting by using thermoplastic resin, such as polyurethane-based thermoplastic elastomer, ionomer resin, or mixtures thereof, for the intermediate layer, to adjust a hardness, hardness distribution, deformation amount, specific gravity, elastic modulus and the like of the core, intermediate layer and cover to proper ranges.

[0004] In Japanese Patent Kokai Publication No. 313643/1997, a three-piece solid golf ball, of which an intermediate layer is placed between a core and a cover, is disclosed. The core has a center hardness in JIS-C hardness of not more than 75 and has 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 the hardness of the intermediate layer. In the golf ball, since the cover is formed from ionomer resin as a base resin, the spin performance of the resulting golf ball is not sufficiently obtained, and the controllability is poor. In addition, the scuff resistance is not sufficiently obtained.

[0005] In Japanese Patent Kokai Publication No. 305114/1998, a golf ball comprising a solid core, an intermediate layer and a cover, of which the surface of the cover has many dimples, is disclosed. The core has a surface hardness in Shore D hardness of not more than 48, the intermediate layer has a hardness in Shore D hardness of 53 to 60 and the hardness of the intermediate layer is higher than the surface hardness of the core by not less than 8, the cover has a hardness in Shore D hardness of 55 to 65 and the hardness of the cover is higher than that of the intermediate layer, the dimples are consisted of two types having different diameter and/or depth from each other, the total number of the dimples is within the range of 370 to 450, the dimples cover at least 63% of the ball surface, and the index Dst of the overall dimple surface area is at least 4. In the golf ball, since the cover is formed from ionomer resin as a base resin, the hardness of the cover is high, and the shot feel and controllability are poor.

[0006] In Japanese Patent Publication No. 151226/1998, a multi-piece golf ball, of which the center has a distortion of at least 2.5 mm under a load of 100 kg, the hardness in Shore D hardness of the intermediate layer is at least 13 degrees higher than that of the cover, and the ball as a whole has an inertia moment of at least 83 g-cm2, is described. However, since the cover has low hardness and large thickness, the rebound characteristics of the resulting golf ball are poor and the spin amount is large, and the flight distance when hit by a driver is not sufficiently obtained.

[0007] In Japanese Patent Publication No. 360740/2002, a three-piece solid golf ball comprising a core composed of a center and an intermediate layer formed on the center, and a cover formed on the intermediate layer, of which the cover is formed from a mixture of polyurethane-based thermoplastic elastomer and polyamide-based thermoplastic elastomer as a base resin, is disclosed. Sine the cover has low hardness, the spin amount of the resulting golf ball is large, and the flight distance when hit by a driver is not sufficiently obtained.

[0008] In the conventional golf balls, sufficient performances have not been obtained in view of the balance of the flight performance and shot feel, and durability at a level of practical use, as described above. Therefore, a golf ball, of which the shot feel, flight performance, and durability are further improved, has been required.

OBJECTS OF THE INVENTION

[0009] A main object of the present invention is to provide a multi-piece solid golf ball having good shot feel, excellent flight performance and excellent controllability.

[0010] According to the present invention, the object described above has been accomplished by providing a multi-piece solid golf ball, of which an intermediate layer is placed between a center and a cover, by forming the cover from polyurethane material, and by adjusting a hardness distribution of the central point and surface of the center, the intermediate layer and the cover, and a hardness of the cover to specified ranges, thereby providing a multi-piece solid golf ball having good shot feel, excellent flight performance and excellent controllability.

[0011] 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

[0012] The present invention will become more fully understood from the detailed description given hereinbelow and the accomplishing drawings which are given by way of illustrating only, and thus are not limitative of the present invention, and wherein:

[0013] FIG. 1 is a schematic cross section illustrating one embodiment 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 center, at least one intermediate layer formed on the center and a cover covering the intermediate layer, wherein

[0015] assuming that a central point hardness of the center, a surface hardness of the center, a hardness of the intermediate layer and a hardness of the cover in Shore D hardness are represented by HM, HS, HI and HC, respectively, the HM, HS, HI and HC satisfy a correlation represented by the following formula:

HM<HS<HI<HC

[0016] and

[0017] the cover is formed from polyurethane material as a main component, and has a hardness in Shore D hardness of 45 to 60.

[0018] Generally, urethane cover has been widely used for golf balls, because the urethane material imparts the resulting golf ball to excellent controllability. Particularly, soft urethane cover has been widely used in order to improve the controllability. However, since the soft urethane cover had poor rebound characteristics, it was required to use an intermediate layer having high stiffness (Japanese Patent Publication No. 151226/1998). Therefore, it was problem that the resulting golf ball had also large spin amount when hit by a driver, and hit golf ball created blown-up trajectory, which reduced the flight distance.

[0019] Moreover, golf balls satisfying the above correlation represented by the following formula:

(Central point hardness of the core)<(Surface hardness of the core)<(Intermediate layer hardness)<(Cover hardness)

[0020] have been proposed (Japanese Patent Kokai Publication Nos. 313643/1997 and 305114/1998), but it was required to use a cover comprising ionomer resin as a main component in order to have high stiffness cover. Therefore, it was problem that the cover hardness was too high or the spin amount at approach shot of the resulting golf ball was small, and the controllability was not sufficiently obtained.

[0021] In the conventional golf balls, since the soft urethane cover has been used as an urethane cover, there has no golf ball using urethane cover and satisfying the correlation represented by the above formula. In the golf ball of the present invention, it was accomplished to decrease the spin amount when hit by a driver, which improved the flight distance, by forming the cover from polyurethane material as a main component; and to improve the controllability at approach shot by satisfying the above correlation represented by the following formula:

(Central point hardness of the core)<(Surface hardness of the core)<(Intermediate layer hardness)<(Cover hardness)

[0022] Moreover, in the present invention, it was accomplished to impart the golf ball to excellent scuff resistance by using a urethane cover having higher hardness than the conventional urethane cover. Therefore, even if the cover is thin, the durability of the resulting golf ball can be sufficiently obtained.

[0023] In order to put the present invention into a more suitable practical application, it is preferable that

[0024] the golf ball have a deformation amount of 3.0 to 4.0 mm, when applying from an initial load of 98 N to a final load of 1274 N; and

[0025] the polyurethane material comprise polyurethane-based thermoplastic elastomer as a main component.

DETAILED DESCRIPTION OF THE INVENTION

[0026] 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 multi-piece solid 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 at least one intermediate layer 2 formed on the center, and a cover 3 covering the core. The intermediate layer 2 may have single-layer structure or multi-layer structure, which has two or more layers. In FIG. 1, in order to explain the golf ball of the present invention simply, a golf ball having one layer of intermediate layer 2, that is, a three-piece solid golf ball will be used hereinafter for explanation. The center 1 of the golf ball of the present invention is obtained by press-molding a rubber composition under applied heat using a method and condition, which has been conventionally used for preparing cores of solid golf balls. The rubber composition comprises a base rubber, a co-crosslinking agent, an organic peroxide, a filler and the like.

[0027] The base rubber used in the present invention may be also the same one that has been conventionally used for solid golf balls, but preferred is polybutadiene rubber, particularly so-called 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.

[0028] Examples of the co-crosslinking agents are not limited, but include &agr;,&bgr;-unsaturated carboxylic acids having 3 to 8 carbon atoms (such as acrylic acid, methacrylic acid, etc.) or mono or divalent metal salts (such as zinc or magnesium salts) thereof, or a combination thereof. Preferred is zinc acrylate or zinc methacrylate, because they impart high rebound characteristics to the resulting golf ball. The amount of the co-crosslinking agent is preferably from 10 to 40 parts by weight, more preferably from 10 to 35 parts, most preferably from 15 to 30 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the co-crosslinking agent is smaller than 10 parts by weight, the center is not sufficiently crosslinked, and the rebound characteristics and the durability are degraded. On the other hand, when the amount of the co-crosslinking agent is larger than 40 parts by weight, the center is too hard, and the shot feel of the resulting golf ball is hard and poor.

[0029] Examples of the organic peroxides include 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 preferably from 0.1 to 3.0 parts by weight, more preferably from 0.3 to 2.5 parts by weight, most preferably from 0.5 to 2.0 parts by weight based on 100 parts by weight of the base rubber. When the amount of the organic peroxide is smaller than 0.1 parts by weight, the center is not sufficiently vulcanized. On the other hand, when the amount of the organic peroxide is larger than 3.0 parts by weight, the center is hard, but the rebound characteristics are not sufficiently improved. In addition, the shot feel is poor.

[0030] 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, magnesium oxide and the like), high specific gravity metal powder filler (such as tungsten powder, molybdenum powder and the like), and the mixture thereof. The amount of the filler is preferably from 1 to 30 parts by weight, more preferably from 3 to 20 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the filler is smaller than 1 part by weight, it is difficult to adjust the weight of the resulting golf ball. On the other hand, when the amount of the filler is larger than 30 parts by weight, the weight ratio of the rubber component in the center is small, and the rebound characteristics of the resulting golf ball are degraded too much.

[0031] Where appropriate, it is possible to compound a component which is typically used in the manufacture of solid golf ball cores together with the rubber composition; e.g., other additives such as organic sulfides, antioxidants and the like. If used, preferably the amount of the additives is preferably 0.5 to 5.0 parts by weight, more preferably 0.7 to 4.0 parts by weight, based on 100 parts by weight of the base rubber.

[0032] The center 1 used for the golf ball of the present invention can be obtained by mixing, and then vulcanizing and press-molding the above rubber composition under applied heat in a mold. The vulcanization condition is not limited, but the vulcanization may be conducted at 140 to 180° C. and 2.8 to 11.8 MPa for 10 to 60 minutes.

[0033] In the golf ball of the present invention, it is suitable for the center 1 to have a diameter of 25 to 41 mm, preferably 27 to 40 mm. When the diameter of the center 1 is smaller than 25 mm, it is required to increase the thickness of the intermediate layer or the cover, and the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the diameter of the center is larger than 41 mm, the thickness of the intermediate layer is too small, and it is difficult to mold the intermediate layer.

[0034] In the golf ball of the present invention, it is desired for the center 1 to have a deformation amount when applying from an initial load of 98 N to a final load of 1274 N of 3.5 to 5.5 mm, preferably 3.5 to 5.0 mm, more preferably 3.8 to 4.8 mm, most preferably 4.0 to 4.5 mm. When the deformation amount of the center is smaller than 3.5 mm, the center is too hard, and the shot feel of the resulting golf ball is hard and poor. On the other hand, when the deformation amount is larger than 5.5 mm, the center is too soft, and the shot feel of the resulting golf ball is heavy and poor.

[0035] In the present invention, assuming that a central point hardness of the center, a surface hardness of the center, a hardness of the intermediate layer and a hardness of the cover in Shore D hardness are represented by HM, Hs, HI and HC, respectively, it is required for the golf ball to satisfy a correlation represented by the following formula: HM<HS<HI<HC Therefore, in the center 1 of the golf ball of the present invention, it is required that the surface hardness (HS) is higher than the central point hardness (HM), and the hardness difference thereof (HS−HM) is preferably 5 to 30, more preferably 8 to 30, most preferably 10 to 25. When the surface hardness is not more than the central point hardness, the hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not obtained, and the spin amount is large, which reduces the flight distance. In addition, when the hardness difference is smaller than 5, the technical effects accomplished by having hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not sufficiently obtained, and the spin amount is large, which reduces the flight distance. On the other hand, when the hardness difference is larger than 30, the durability is poor.

[0036] In the golf ball of the present invention, it is desired for the center 1 to have a central point hardness (HM) in Shore D hardness of 20 to 40, preferably 22 to 38, more preferably 25 to 35. When the central point hardness is lower than 20, the center is too soft, and the rebound characteristics of the resulting golf ball are degraded, which reduces the flight distance. On the other hand, the central point hardness is higher than 40, it is difficult to satisfy the correlation represented by the above formula, and the hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not obtained. Therefore, the spin amount is large, which reduces the flight distance. The term “a central point hardness of the center” as used herein refers to the hardness, which is determined by cutting the resulting center into two equal parts and then measuring a hardness at its central point in section.

[0037] In the golf ball of the present invention, it is desired for the center 1 to have the surface hardness (HS) in Shore D hardness of 35 to 55, preferably 38 to 52, more preferably 40 to 50. When the surface hardness is lower than 35, the center is too soft, and the rebound characteristics of the resulting golf ball are degraded, which reduces the flight distance. On the other hand, when the surface hardness is higher than 55, the intermediate layer and cover are too hard in order to satisfy the correlation represented by the above formula, and the shot feel of the resulting golf ball is poor. The term “a surface hardness of the center” as used herein refers to the hardness, which is determined by measuring a hardness at the surface of the resulting center. The intermediate layer 2 is then formed on the center 1.

[0038] The intermediate layer 2 of the golf ball of the present invention may be formed from the rubber composition as used for the center 1, or from thermoplastic resin as a base resin. Examples of the thermoplastic resins include one or combination of two or more selected from the group consisting of ionomer resin and thermoplastic elastomer, which can be typically used for the cover of golf balls. Examples of the thermoplastic elastomers include polyester-based thermoplastic elastomer, polyamide-based thermoplastic elastomer, polyurethane-based thermoplastic elastomer, polyolefin-based thermoplastic elastomer, styrene-based thermoplastic elastomer and the like. The thermoplastic elastomers may have functional groups, such as carboxyl group, glycydyl group, sulfone group, epoxy group and the like.

[0039] When using the rubber composition for the intermediate layer 2, the amount of the co-crosslinking agent and organic peroxide is slightly different from the rubber composition used for the center 1 in order to satisfy the correlation represented by the above formula. That is, the amount of the co-crosslinking agent is preferably from 15 to 50 parts by weight, more preferably from 20 to 45 parts by weight, based on 100 parts by weight of the base rubber. Preferred are zinc acrylate, zinc methacrylate, magnesium acrylate or magnesium methacrylate. The amount of the organic peroxide is preferably from 0.1 to 6.0 parts by weight, more preferably from 0.3 to 5.0 parts by weight, most preferably from 0.5 to 4.0 parts by weight based on 100 parts by weight of the base rubber.

[0040] The ionomer resin may be a copolymer of &agr;-olefin and &agr;,&bgr;-unsaturated carboxylic acid having 3 to 8 carbon atoms, of which a portion of carboxylic acid groups is neutralized with metal ion, a terpolymer of &agr;-olefin, &agr;,&bgr;-unsaturated carboxylic acid having 3 to 8 carbon atoms and &agr;,&bgr;-unsaturated carboxylic acid ester, of which a portion of carboxylic acid groups is neutralized with metal ion or mixture thereof. Examples of the &agr;-olefins in the ionomer preferably include ethylene, propylene and the like. 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 an alkali metal ion, such as a sodium ion, a potassium ion, a lithium ion and the like; a divalent metal ion, such as a zinc ion, a calcium ion, a magnesium ion and the like; a trivalent metal ion, such as an aluminum, a neodymium ion and the like; and mixture thereof. Preferred are sodium ions, zinc ions, lithium ions and the like, in view of rebound characteristics, durability and the like.

[0041] 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 Du Pont-Mitsui Polychemicals Co., Ltd. include Hi-milan 1555, Hi-milan 1557, Hi-milan 1601, Hi-milan 1605, Hi-milan 1652, Hi-milan 1702, Hi-milan 1705, Hi-milan 1706, Hi-milan 1707, Hi-milan 1855, Hi-milan 1856, Hi-milan AM7316 and the like. Examples of the ionomer resins, which are commercially available from Du Pont Co., include Surlyn 8945, Surlyn 9945, Surlyn 6320, Surlyn 8320, Surlyn AD8511, Surlyn AD8512 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.

[0042] Examples of the thermoplastic elastomers, which are commercially available, include 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”); polyamide-based thermoplastic elastomer, which is commercially available from Atofina Japan Co., Ltd. under the trade name of “Pebax” (such as “Pebax 2533”); polyurethane-based thermoplastic elastomer, which is commercially available from BASF Japan Co., Ltd. under the trade name of “Elastollan” (such as “Elastollan ET880”); olefin-based thermoplastic elastomer available from Mitsubishi Chemical Co., Ltd. under the trade name “Thermoran” (such as “Thermoran 3981N”); polyolefin-based thermoplastic elastomer, which is commercially available from Sumitomo Chemical Co., Ltd. under the trade name of “Sumitomo TPE” (such as “Sumitomo TPE3682” and “Sumitomo TPE9455”); styrene-based thermoplastic elastomer, which are commercially available from Mitsubishi Chemical Co., Ltd. under the trade name of “Rabalon” (such as “Rabalon SR04”); styrene-based thermoplastic elastomer available from Asahi Kasei corporation under the trade name “Tuftec” (such as “Tuftec H1051”); and the like.

[0043] The composition for the intermediate layer 2 used in the present invention may optionally contain fillers, pigments and the other additives such as an antioxidant in addition to the thermoplastic resin as a base resin.

[0044] Examples of the fillers include 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 the mixture thereof.

[0045] The intermediate layer 2 of the present invention may be formed by conventional methods, which have been known in the art and used for forming the cover of the golf balls. When the intermediate layer 2 is formed from the rubber composition, the rubber composition for the intermediate layer is mixed, and coated on the center 1 into a concentric sphere, and then vulcanized by press-molding at 160 to 180° C. for 10 to 20 minutes in the mold to obtain a core 4, which is formed by covering the intermediate layer 2 on the center 1. When the intermediate layer 2 is formed from thermoplastic resin, the resin composition for the intermediate layer is molded into a semi-spherical half-shell, and the center is covered with the two half-shells, followed by pressure molding; or the resin composition for the intermediate layer is injection molded directly on the center 1; to obtain the core 4. It is preferable for the surface of the resulting core to be buffed to improve the adhesion to the cover formed on the core.

[0046] In the present invention, assuming that a central point hardness of the center, a surface hardness of the center, a hardness of the intermediate layer and a hardness of the cover in Shore D hardness are represented by HM, HS, HI and HC, respectively, it is required for the golf ball to satisfy a correlation represented by the following formula: HM<HS<HI<HC as described above. Therefore, in the golf ball of the present invention, it is required that the hardness (HI) of the intermediate layer 2 is higher than the surface hardness (HS) of the center 1, and the hardness difference thereof (HI−HS) is preferably 1 to 15, more preferably 2 to 10, most preferably 3 to 8. When the hardness (HI) of the intermediate layer 2 is not more than the surface hardness (HS) of the center 1, the hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not obtained, and the spin amount is large, which reduces the flight distance. In addition, when the hardness difference is smaller than 1, the technical effects accomplished by having hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not sufficiently obtained, and the spin amount is large, which reduces the flight distance. On the other hand, when the hardness difference is larger than 15, the intermediate layer is too hard, and the controllability and shot feel of the resulting golf ball are degraded.

[0047] In the golf ball of the present invention, it is desired for the intermediate layer 2 to have a hardness (HI) in Shore D hardness of 38 to 58, preferably 40 to 55, more preferably 42 to 52. When the hardness of the intermediate layer is lower than 38, the core is too soft, and the rebound characteristics and durability of the resulting golf ball are degraded. On the other hand, the hardness of the intermediate layer is higher than 58, the shot feel of the resulting golf ball is poor. In addition, the cover is too hard in order to satisfy the correlation represented by the above formula, and the controllability at approach shot of the resulting golf ball is degraded.

[0048] It is desired for the intermediate layer 2 to have a thickness, which is determined by the diameter of the center 1 and core 4, of 0.5 to 4.0 mm, preferably 0.5 to 3.0 mm, more preferably 0.7 to 2.0 mm. When the thickness of the intermediate layer is smaller than 0.5 mm, the technical effects accomplished by the presence of the intermediate layer are not sufficiently obtained, and the rebound characteristics are degraded and the spin amount is increased, which reduces the flight distance. On the other hand, when the thickness is larger than 4.0 mm, the shot feel of the resulting golf ball is hard and poor, because the intermediate layer is relatively hard. The cover 3 is then formed on the intermediate layer 2.

[0049] In the golf ball of the present invention, it is suitable for the core 4 to have a diameter of 39.0 to 42.2 mm, preferably 40.0 to 42.0 mm, more preferably 40.5 to 42.0 mm. When the diameter of the core is smaller than 39.0 mm, the cover is thick, and the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the diameter of the core is larger than 42.2 mm, the cover is too thin, and the technical effects accomplished by the presence of the cover are not sufficiently obtained. In addition, it is difficult to mold the cover.

[0050] In the golf ball of the present invention, it is desired for the core 4 to have a deformation amount when applying from an initial load of 98 N to a final load of 1274 N of 3.2 to 5.0 mm, preferably 3.2 to 4.8 mm, more preferably 3.4 to 4.5 mm, most preferably 3.2 to 3.8 mm. When the deformation amount of the core is smaller than 3.2 mm, the shot feel of the resulting golf ball is hard and poor. On the other hand, when the deformation amount is larger than 5.0 mm, the core is too soft, and the shot feel of the resulting golf ball is heavy and poor.

[0051] The cover 3 is then covered on the core 4. In the golf ball of the present invention, it is required for the cover 3 to be formed from polyurethane material as a main component. In the polyurethane materials, there are thermosetting type and thermoplastic type polyurethane materials, but preferred are thermoplastic type polyurethane materials, such as polyurethane-based thermoplastic elastomer, in view of processability and cost.

[0052] Polyurethane-based thermoplastic elastomer generally contains polyurethane structure as hard segment and polyester or polyether as soft segment. The polyurethane structure generally contains diisocyanate and curing agent, such as amine-based curing agent. The polyurethane-based thermoplastic elastomer includes polyurethane-based thermoplastic elastomer that the diisocyanate is aromatic diisocyanate, cycloaliphatic diisocyanate or aliphatic diisocyanate.

[0053] Examples of the aromatic diisocyanate include tolylene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), tolidine diisocyanate (TODI), xylylene diisocyanate (XDI) and the like. Preferred is MDI. Concrete examples of the polyurethane-based thermoplastic elastomer formed by using the MDI include polyurethane-based thermoplastic elastomer, which is commercially available from BASF Japan Co., Ltd. under the trade name of “Elastollan ET890”, and the like.

[0054] Examples of the cycloaliphatic diisocyanates include 4,4′-dicyclohexylmethane diisocyanate (H12MDI), which is hydrogenated compound of MDI; 1,3-bis(isocyanatomethyl)cyclohexane (H6XDI), which is hydrogenated compound of XDI; isophorone diisocyanate (IPDI); and trans-1,4-cyclohexane diisocyanate (CHDI). Preferred is the H12MDI in view of general-purpose properties and processability. Concrete examples of the polyurethane-based thermoplastic elastomer formed by using the H12MDI include polyurethane-based thermoplastic elastomers, which are commercially available from BASF Japan Co., Ltd. under the trade name of “Elastollan XNY90A”, “Elastollan XNY97A”, “Elastollan XNY585”, “Elastollan XKP016”, and the like.

[0055] Examples of the aliphatic diisocyanates include hexamethylene diisocyanate (HDI), lysine diisocyanate (LDI), and the like. Preferred is HDI. Concrete examples of the polyurethane-based thermoplastic elastomer formed by using the HDI include polyurethane-based thermoplastic elastomer, which is commercially available from Dainippon Ink & Chemicals Inc. under the trade name of “Pandex T-7890”(trade name), and the like.

[0056] Preferred are polyurethane-based thermoplastic elastomers formed by using diisocyanate having no double bond in backbone structure in molecule, that is, aliphatic diisocyanate and cycloaliphatic diisocyanate in view of yellowing resistance. Preferred are polyurethane-based thermoplastic elastomers formed by using cycloaliphatic diisocyanate and aromatic diisocyanate, which have high mechanical strength, in view of scuff resistance. Therefore, in the present invention, preferred is polyurethane-based thermoplastic elastomer formed by using cycloaliphatic diisocyanate in view of both the yellowing resistance and scuff resistance.

[0057] For the cover 3 of the golf ball of the present invention, the above polyurethane-based thermoplastic elastomer may be used alone, or the polyurethane-based thermoplastic elastomer may be used in combination with at least one of the ionomer resin and the thermoplastic elastomer as used for the intermediate layer 2. When using the combination, polyamide-based thermoplastic elastomer is most preferable in view of the compatibility with the polyurethane-based thermoplastic elastomer and the rebound characteristics, and a weight ratio (a/b) of the polyurethane-based thermoplastic elastomer (a) to the polyamide-based thermoplastic elastomer (b) is preferably 95/5 to 70/30.

[0058] In the golf ball of the present invention, the cover composition may optionally contain fillers such as barium sulfate, pigments such as titanium dioxide, and 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 base resin as a main component, as long as the addition of the additive does not deteriorate the desired performance of the golf ball cover. If used, the amount of the pigment is preferably 0.1 to 5.0 parts by weight, based on the 100 parts by weight of the base resin of the cover.

[0059] In the golf ball of the present invention, it is desired for the cover 3 to have a thickness of 0.3 to 2.0 mm, preferably 0.3 to 1.5 mm, more preferably 0.5 to 1.0 mm. When the thickness of the cover is smaller than 0.3 mm, the technical effects accomplished by the presence of the cover are not sufficiently obtained, and the controllability and durability are degraded. On the other hand, when the thickness is larger than 2.0 mm, the technical effects accomplished by the presence of the core and intermediate layer are not sufficiently obtained, and the rebound characteristics of the resulting golf ball are degraded, which reduces the flight distance.

[0060] In the present invention, assuming that a central point hardness of the center, a surface hardness of the center, a hardness of the intermediate layer and a hardness of the cover in Shore D hardness are represented by HM, HS, HI and HC, respectively, it is required for the golf ball to satisfy a correlation represented by the following formula: HM<HS<HI<HC as described above. Therefore, in the golf ball of the present invention, it is required that the hardness (HC) of the cover is higher than the hardness (HI) of the intermediate layer 2, and the hardness difference thereof (HC−HI) is preferably 1 to 15, more preferably 3 to 12, most preferably 5 to 10. When the hardness (HC) of the cover 3 is not more than the hardness (HI) of the intermediate layer 2, the hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not obtained, and the spin amount is large, which reduces the flight distance. In addition, when the hardness difference is smaller than 1, the technical effects accomplished by having hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not sufficiently obtained, and the spin amount is large, which reduces the flight distance. On the other hand, when the hardness difference is larger than 15, the cover is too hard, and the controllability and shot feel of the resulting golf ball are degraded.

[0061] In the golf ball of the present invention, it is desired for the cover 3 to have a hardness (HC) in Shore D hardness of 45 to 60, preferably 47 to 60, more preferably 50 to 58. When the hardness of the cover is lower than 45, the deformation amount of the surface of the resulting golf ball at the time of hitting is large even if adjusting the hardness of the core, and the rebound characteristics are degraded, which reduces the flight distance. In addition, the durability is poor. The term “a hardness of the intermediate layer” and “a hardness of the cover” as used herein refer to the hardness, which is determined by measuring a hardness using a sample of a stack of the three or more heat and press molded sheets having a thickness of about 2 mm from the resulting compositions for the intermediate layer and cover, which had been stored at 23° C. for 2 weeks.

[0062] The cover of the present invention may be formed by the same methods as used in the intermediate layer. At the time of molding the cover, many depressions called “dimples” are formed on the surface of the golf ball. The term “an area of the dimple” as used herein refers to the area enclosed in the periphery (edge) of the dimple when observing the central point of the golf ball from infinity, which is the area of plane. When the dimple is spherical, the area of the dimple S is determined by calculating from the following formula:

S=&pgr;(d/2)2

[0063] wherein “d” is a diameter of the dimple. The ratio of the golf ball surface occupied by the dimple to the total surface area of the golf ball is determined by calculating a ratio of (the total of the area “S” of each dimple) to (the-surface area of the phantom-sphere assuming that the golf ball is a true sphere having no dimples on the surface thereof).

[0064] In the golf ball of the present invention, it is desired for the dimple to have a ratio of the golf ball surface occupied by the dimple of 70 to 90%, preferably 72 to 88%, more preferably 74 to 86%, based on the total surface area of the golf ball. When the ratio of the golf ball surface occupied by the dimple is smaller than 70%, the technical effects of improving the flight distance accomplished by the presence of the dimple are not sufficiently obtained, which reduces the flight distance. On the other hand, when the ratio of the golf ball surface occupied by the dimple is larger than 90%, the depressions on the surface of the golf ball are too many, and the shape of the golf ball is not spherical to break symmetry of airflow around the golf ball on the fly. In addition, since there is not sufficient space between dimples adjacent to each other on the surface of the golf ball, it is difficult to design the dimples having the ratio of the golf ball surface occupied by the dimples of larger than 90%. The total volume of the dimples and the ratio of the golf ball surface occupied by the dimples as used herein are determined by measuring at the surface of the resulting golf ball, and if paint is applied on the cover, they are determined by measuring at the surface of the applied golf ball.

[0065] In the golf ball of the present invention, it is desired for the dimple to have total number of 250 to 500, preferably 300 to 480, more preferably 320 to 450. When the total number of the dimples is smaller than 250, the technical effects accomplished by the presence of the dimples are not sufficiently obtained. On the other hand, when the total number of the dimples is larger than 500, the size of each dimple is small, and the technical effects accomplished by the presence of the dimples are not sufficiently obtained. Therefore, in the both cases, the technical effects accomplished by the presence of the dimples are not sufficiently obtained to improve the flight performance.

[0066] Furthermore, paint finishing or marking with a stamp may be optionally provided after the cover is molded for commercial purposes.

[0067] In the golf ball of the present invention, it is desired to have a deformation amount when applying from an initial load of 98 N to a final load of 1274 N of 3.0 to 4.0 mm, preferably 3.0 to 3.8 mm, more preferably 3.2 to 3.8 mm. When the deformation amount is smaller than 3.0 mm, the golf ball is too hard, and the shot feel is hard and poor. On the other hand, when the deformation amount is larger than 4.0 mm, the golf ball is too soft, and the rebound characteristics are degraded, which reduces the flight distance.

[0068] The multi-piece solid golf ball of the present invention is formed to a diameter of at least 42.67 mm (preferably 42.67 to 42.82 mm) and a weight of no more than 45.93 g, in accordance with the regulations for golf balls.

[0069] The diameter of golf balls is limited to not less than 42.67 mm in accordance with the regulations for golf balls as described above. Generally, when the diameter of the golf ball is large, air resistance of the golf ball on the fly is large, which reduces the flight distance. Therefore, most of golf balls commercially available are designed to have a diameter of 42.67 to 42.82 mm. The present invention is applicable to the golf balls having the diameter. There are golf balls having large diameter in order to improve the easiness of hitting. In addition, there are cases where golf balls having a diameter out of the regulations for golf balls are required depending on the demand and object of users. Therefore, it can be considered for golf balls to have a diameter of 42 to 44 mm, more widely 40 to 45 mm. The present invention is also applicable to the golf balls having the diameter.

EXAMPLES

[0070] 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.

[0071] Production of Core

[0072] (i) Production of Center

[0073] The rubber compositions for the center having the formulations shown in Table 1 were mixed, and then vulcanized by press-molding at 160° C. for 20-minutes in a mold to obtain spherical centers. The diameter, deformation amount, the central point hardness and the surface hardness of the resulting center were measured. The results are shown in Table 4 (Examples) and Table 5 (Comparative Examples). Test methods are described later. 1 TABLE 1 (parts by weight) Center composition I II III BR-11 *1 100 100 100 Zinc acrylate 26 24 20 Zinc oxide 5 5 5 Dicumyl peroxide 0.7 0.7 0.7 Diphenyl disulfide 0.5 0.5 0.5 Barium sulfate *2 proper proper proper amount amount amount *1: High-cis Polybutadiene rubber available from JSR Co., Ltd. *2: The amount of barium sulfate was adjusted to a proper amount such that the weight of the resulting golf ball was 45.4 g.

[0074] (ii) Preparation of Intermediate Layer Composition

[0075] The formulation materials shown in Table 2 were mixed using a kneading type twin-screw extruder to obtain pelletized intermediate layer compositions. The extrusion condition was,

[0076] a screw diameter of 45 mm,

[0077] a screw speed of 200 rpm, and

[0078] a screw L/D of 35.

[0079] The formulation materials were heated at 200 to 260° C. at the die position of the extruder. The hardness of the intermediate layer was measured, using a sample of a stack of the three or more heat and press molded sheets having a thickness of about 2 mm from the resulting composition for the intermediate layer, which had been stored at 23° C. for 2 weeks. The results are shown in Table 2, Table 4 (Examples) and Table 5 (Comparative Examples). 2 TABLE 2 (parts by weight) Intermediate layer composition A B C D Hi-milan 1555 *3 — — — 25 Hi-milan 1557 *4 — — — 25 Surlyn 8945 *5 30 25 50 — Surlyn 9945 *6 30 25 50 — Rabalon SR04 *7 40 50 — 50 Shore D hardness 46 43 64 40 *3: Hi-milan 1555 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with sodium ion, manufactured by Du Pont-Mitsui Polychemicals Co., Ltd. *4: Hi-milan 1557 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with zinc ion, manufactured by Du Pont-Mitsui Polychemicals Co., Ltd. *5: Surlyn 8945 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with sodium ion, manufactured by Du Pont Co. *6: Surlyn 9945 (trade name), ethylene-methacrylic acid copolymer ionomer resin obtained by neutralizing with zinc ion, manufactured by Du Pont Co. *7: Rabalon SR04 (trade name), styrene-ethylene-butylene-styrene (SEBS)-based thermoplastic elastomer, manufactured by Mitsubishi Chemical Co., Ltd.

[0080] (iii) Preparation of Two-Layered Core

[0081] The resulting intermediate layer compositions were directly injection molded on the center produced in the (ii) to form a spherical two-layered core. The thickness of the resulting intermediate layer, the diameter and deformation amount of the resulting two-layered core were measured, and the results are shown in Table 4 (Examples) and Table 5 (Comparative Examples).

[0082] Preparation of Cover Composition

[0083] The formulation materials shown in Table 3 were mixed using a kneading type twin-screw extruder to obtain pelletized cover compositions. The extrusion condition was,

[0084] a screw diameter of 45 mm,

[0085] a screw speed of 200 rpm, and

[0086] a screw L/D of 35.

[0087] The formulation materials were heated at 200 to 260° C. at the die position of the extruder. The hardness of the cover was measured, using a sample of a stack of the three or more heat and press molded sheets having a thickness of about 2 mm from the resulting composition for the cover, 5 which had been stored at 23° C. for 2 weeks. The results are shown in Table 3, Table 4 (Examples) and Table 5 (Comparative Examples). 3 TABLE 3 (parts by weight) Cover composition a b c d e Elastollan XNY97A *8 30 60 — — — Elastollan XNY90A *9 — — — — 90 Elastollan XKP-016 *10 60 30 — — — Pebax 5533 *11 10 10 — — 10 Surlyn 8945 *5 — — 50 50 — Surlyn 9945 *6 — — 50 — — Surlyn 6320 *12 — — — 50 — Titanium dioxide  4  4  4  4  4 Shore D hardness 55 52 64 53 42 *8: Elastollan XNY97A (trade name), polyurethane-based thermoplastic elastomer formed by using 4,4′-dicyclohexylmethane diisocyanate (H12MDI), commercially available from BASF Japan Co., Ltd.; Shore A (JIS-A) hardness = 97 *9: Elastollan XNY90A (trade name), polyurethane-based thermoplastic elastomer formed by using 4,4′-dicyclohexylmethane diisocyanate (H12MDI), commercially available from BASF Japan Co., Ltd.; Shore A (JIS-A) hardness = 90 *10: Elastollan XKP-016 (trade name), polyurethane-based thermoplastic elastomer formed by using 4,4′-dicyclohexylmethane diisocyanate (H12MDI), commercially available from BASF Japan Co., Ltd.; Shore D hardness = 64 *11: Pebax 2533 (trade name), polyamide-based thermoplastic elastomer (hard segment: polyamide, commercially available from Atofina Japan Co., Ltd. *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.

Examples 1 to 5 and Comparative Examples 1 to 5

[0088] The resulting cover compositions were directly injection molded on the intermediate layer to form a cover layer having a thickness shown in Table 4 (Examples) and Table 5 (Comparative Examples). Then, clear paint was coated on the surface to obtain a golf ball having a weight of 45.4 g and a diameter of 42.7 mm. The ratio of the golf ball surface occupied by the dimple was 84% and the total number of the dimples was 410. With respect to the resulting golf balls, the deformation amount, flight performance (spin amount and flight distance) were measured, and the shot feel, controllability and scuff resistance were evaluated. The results are shown in Table 6 (Examples) and Table 7 (Comparative Examples). The test methods are as follows.

(Test Method)

[0089] (1) Deformation Amount of Center

[0090] The deformation amount of center was determined by measuring a deformation amount when applying from an initial load of 98 N to a final load of 1274 N on the center.

[0091] (2) Hardness

[0092] (i) Hardness of center

[0093] The surface hardness of the center was determined by measuring a hardness at the surface of the resulting center. The central point hardness was determined by cutting the resulting center into two equal parts and then measuring a hardness at its central point in section. The hardness was measured using a Shore D hardness meter according to ASTM D 2240-68.

[0094] (ii) Hardness of Intermediate Layer and Cover

[0095] The hardness of the intermediate layer and cover were determined by measuring a hardness, using a sample of a stack of the three or more heat and press molded sheets (slab) having a thickness of about 2 mm from the intermediate layer composition and cover composition, which had been stored at 23° C. for 2 weeks, with a Shore D hardness meter according to ASTM D 2240-68.

[0096] (3) Flight Performance

[0097] (i) Flight Performance 1

[0098] After a No. 1 wood club (W#1, a driver) having metal head was mounted to a swing robot manufactured by Golf Laboratory Co. and the resulting golf ball was hit at a head speed of 45 m/sec, the spin amount (backspin amount) immediately after hitting and flight distance were measured. As the flight distance, total that is a distance to the stop point of the hit golf ball was measured. The measurement was conducted 5 times for each golf ball (n=5), and the average is shown as the result of the golf ball.

[0099] (ii) Flight Performance 2

[0100] 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 20 m/sec, the launch angle, spin amount and flight distance were measured. The spin amount (backspin amount) immediately after hitting was measured. The measurement was conducted 5 times for each golf ball (n=5), and the average is shown as the result of the golf ball.

[0101] (4) Shot Feel

[0102] 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 results shown in the Tables below are based on the fact that most of golfers evaluated with the same criterion about shot feel. The evaluation criteria are as follows.

[0103] Evaluation criteria

[0104] ∘: The golfers felt that the golf ball has good shot feel such that impact force at the time of hitting is small and rebound characteristics are good.

[0105] &Dgr;: The golfers felt that the golf ball has fairly good shot feel.

[0106] x: The golfers felt that the golf ball has poor shot feel such that impact force at the time of hitting is large or the golf ball has heavy and poor shot feel.

[0107] (5) Controllability

[0108] The controllability of the golf ball is evaluated by high-level 10 golfers according to a practical hitting test using a pitting wedge (PW). The evaluation criteria are as follows. The results shown in the Tables below are based on the fact that most of golfers evaluated with the same criterion about controllability.

[0109] Evaluation Criteria

[0110] ∘: The golfers felt that it is easy to apply spin on the golf ball, and the golf ball has good controllability.

[0111] &Dgr;: The golfers felt that the golf ball has fairly good controllability.

[0112] x: The golfers felt that it is difficult to apply spin on the golf ball such that the golf ball slips on the face of golf club, and the golf ball has poor controllability.

[0113] (6) Scuff Resistance

[0114] After a pitching wedge (PW) commercially available was mounted to a swing robot manufactured by Golf Laboratory Co., two points on the surface of each golf ball was hit at a head speed of 36 m/sec one time for each point. The two points were evaluated by checking the surface appearance by visual observation. The evaluation criteria are as follows.

[0115] Evaluation Criteria

[0116] ∘: The surface of the golf ball slightly has a cut, but it is not particularly noticeable.

[0117] &Dgr;: The surface of the golf ball clearly has a cut, and the surface becomes fluffy.

[0118] x: The surface of the golf ball is considerably chipped off, and the surface noticeably becomes fluffy.

Test Results

[0119] 4 TABLE 4 Example No. Test item 1 2 3 4 5 (Center) Composition I I I I II Diameter (mm) 37.7 37.7 36.7 38.5 36.7 Deformation 4.1 4.1 4.2 4.0 4.5 amount (mm) Central point 28 28 28 29 28 hardness (Shore D) Surface hardness 43 43 42 44 41 (Shore D) (intermediate layer) Composition A A A A B Thickness (mm) 1.3 1.3 1.6 1.3 1.6 Hardness (Shore D) 46 46 46 46 43 (Core) Diameter (mm) 40.3 40.3 39.9 41.1 40.3 Deformation 3.7 3.7 3.6 3.7 4.1 amount (mm) (Cover) Composition a b b b b Thickness (mm) 1.2 1.2 1.4 0.8 1.2 Hardness (Shore D) 55 52 52 52 52

[0120] 5 TABLE 5 Comparative Example No. Test item 1 2 3 4 5 (Center) Composition I I I II III Diameter (mm) 37.7 37.7 37.7 36.7 37.7 Deformation 4.1 4.1 4.1 4.1 5.5 amount (mm) Central point 29 29 29 29 26 hardness (Shore D) Surface hardness 43 43 43 43 38 (Shore D) (intermediate layer) Composition C A A D D Thickness (mm) 1.3 1.3 1.3 1.3 1.3 Hardness (Shore D) 64 46 46 40 40 (Core) Diameter (mm) 40.3 40.3 40.3 40.3 40.3 Deformation 3.3 3.7 3.7 3.8 4.7 amount (mm) (Cover) Composition a c d b e Thickness (mm) 1.2 1.2 1.2 1.2 1.2 Hardness (Shore D) 55 64 53 52 42

[0121] 6 TABLE 6 Example No. Test item 1 2 3 4 5 (Golf ball) Deformation 3.3 3.4 3.2 3.5 3.8 amount (mm) Flight performance 1 (W#1; 45 m/sec) Spin amount (rpm) 2630 2720 2760 2680 2550 Total (m) 223.5 221.61 222.0 222.7 221.2 Flight performance 2 (SW; 20 m/sec) Spin amount (rpm) 6460 6620 6710 6600 6530 Shot feel ◯ ◯ ◯ ◯ &Dgr; Controllability ◯ ◯ ◯ ◯ ◯ Scuff resistance ◯ ◯ ◯ ◯ ◯

[0122] 7 TABLE 7 Comparative Example No. Test item 1 2 3 4 5 (Golf ball) Deformation 3.0 2.9 3.3 3.4 4.2 amount (mm) Flight performance 1 (W#1; 45 m/sec) Spin amount (rpm) 2820 2650 2750 2900 2850 Total (m) 220.3 224.5 221.9 217.0 211.3 Flight performance 2 (SW; 20 m/sec) Spin amount (rpm) 6490 4830 6200 6770 6830 Shot feel X X ◯ &Dgr; X Controllability ◯ X &Dgr; ◯ ◯ Scuff resistance ◯ ◯ X ◯ ◯

[0123] As is apparent from Tables 6 and 7, the golf balls of Examples 1 to 5 of the present invention, when compared with the golf balls of Comparative Examples 1 to 5, had good shot feel, excellent flight performance and excellent controllability.

[0124] On the other hand, in the golf ball of Comparative Example 1, since the hardness of the intermediate layer is very high, the hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not obtained, and the spin amount is large, which reduces the flight distance. In addition, the impact force at the time of hitting is large, and the shot feel is poor. In the golf ball of Comparative Example 2, since the cover is formed from ionomer resin, the cover hardness is high, and the spin amount is small, which degrades the controllability. In addition, the impact force is large, and the shot feel is very poor.

[0125] In the golf ball of Comparative Example 3, since the cover is formed from the mixture of hard ionomer resin and soft ionomer resin, the shot feel is good, but the scuff resistance is very poor. In the golf ball of Comparative Example 4, since the hardness of the intermediate layer is low, the hardness distribution such that the outer portion is hard and the inner portion is soft in the whole golf ball are not obtained, and the spin amount is large, which reduces the flight distance. In addition, the shot feel is heavy and poor.

[0126] In the golf ball of Comparative Example 5, since the cover hardness is low, the deformation amount is large, and the rebound characteristics are degraded, which reduces the flight distance. In addition, the shot feel is heavy and poor.

Claims

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

assuming that a central point hardness of the center, a surface hardness of the center, a hardness of the intermediate layer and a hardness of the cover in Shore D hardness are represented by HM, HS, HI and HC, respectively, the HM, HS, HI and HC satisfy a correlation represented by the following formula:

HM<HS<HI<HC

and

the cover is formed from polyurethane material as a main component, and has a hardness in Shore D hardness of 45 to 60.

2. The multi-piece solid golf ball according to claim 1, wherein the golf ball has a deformation amount of 3.0 to 4.0 mm, when applying from an initial load of 98 N to a final load of 1274 N.

3. The multi-piece solid golf ball according to claim 1, wherein the polyurethane material comprises polyurethane-based thermoplastic elastomer as a main component.