Golf Ball

Abstract

The invention provides a golf ball having a core, at least one intermediate layer encasing the core, and a cover. The intermediate layer is formed primarily of a specific ionomer resin composition that is highly neutralized. The cover is formed by injection molding a single resin blend composed primarily of a thermoplastic polyurethane and (g) a polyisocyanate compound in at least some portion of which all the isocyanate groups on the molecule are present in an unreacted state. The golf ball of the invention has a low spin rate on shots with a driver, enabling it to travel a good distance, and achieves a sufficient spin rate on shots with a short iron such as a wedge. The ball also has an excellent scuff resistance.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a golf ball which achieves a good distance on shots with a driver and has good spin properties on shots with an iron, and which moreover has an excellent scuff resistance and a good durability.

[0002] Most golf balls currently in use are generally manufactured by using a process such as injection molding or compression molding to encase the periphery of a solid core made primarily of a rubber such as diene rubber with a material composed primarily of, for example, urethane resin or ionomer resin.

[0003] The major performance attributes required of golf balls include distance, controllability, durability and feel on impact; balls having the highest levels of such attributes are constantly being sought. In this context, there has emerged among recent golf balls a succession of balls with multilayer structures--typically three-piece balls. Providing a golf ball with a multilayer structure makes it possible to combine many materials of differing properties; by assigning various functions to the respective layers, a wide diversity of ball designs can be achieved.

[0004] Generally, when the distance traveled by a golf ball is regarded as important, the core or cover is formed so as to be rather hard, thereby increasing the resilience of the ball when struck. Although the distance in such a case can be increased, when the ball is hit with a short iron such as a wedge, sufficient spin is not achieved, resulting in a poor controllability and a diminished feel. When this concern is addressed by forming the ball so as to be rather soft, sufficient spin is obtained on shots with a short iron, enabling controllability to be improved, in addition to which the feel is also better. Yet, at the same time, making the ball softer lowers the rebound and also increases the spin rate on shots with a driver, making it difficult to achieve an increased distance. A cover made of an ionomer resin that does not readily crack and is somewhat soft is commonly used in such soft (low hardness) balls, but the resulting balls tend to have a poor scuff resistance. Sometimes the cover is made instead of a thermoplastic polyurethane resin, although prior-art covers made of thermoplastic polyurethane resins also have a poor scuff resistance.

[0005] Although forming an intermediate layer of a highly neutralized ionomer resin composition in which the degree of neutralization of the ionomer resin has been increased through the addition of a basic inorganic metal compound is effective for enhancing rebound and lowering the spin rate, the resulting ball often has a poor durability.

[0006] Hence, there exists a need for golf balls which satisfy the conflicting demands for improved distance, controllability, durability and feel. In particular, there exists a desire for the development of a golf ball which increases the distance by keeping the spin rate low on shots with a driver; provides a suitable spin rate and good controllability on shots with an iron; and moreover has both an excellent scuff resistance and an excellent durability.

[0007] Prior art relating to the present invention includes the three-piece solid golf ball disclosed in JP-A 2001-79116 which has an outermost layer composed of various types of thermoplastic elastomers to which an inorganic granular filler has been added. In addition, JP-A 2003-761 discloses a golf ball in which an inorganic filler has been included in a cover material composed primarily of an ionomer resin, JP-A 2003-126298 discloses a golf ball wherein an inorganic filler has been included in a high-hardness resin, and JP-A 2008-49153 discloses a golf ball having a cover composed of a polyurethane resin of improved scuff resistance. However, further improvement is desired.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to provide a golf ball which achieves a good distance by suitably reducing the spin rate on shots with a driver, which achieves a suitable spin rate on shots with a short iron, and which has an excellent scuff resistance and a good durability.

[0009] The inventors have conducted extensive investigations in order to achieve the above object. As a result, they have discovered that, in a golf ball having a core, at least one intermediate layer and a cover, by injection-molding the cover from a single resin blend composed primarily of (f) a thermoplastic polyurethane and (g) a polyisocyanate compound in at least some portion of which all the isocyanate groups on the molecule are present in an unreacted state, and by combining such a cover with an intermediate layer made of a highly neutralized ionomer resin composition obtained by adding a basic inorganic metal compound or the like to a conventional ionomer resin so as to increase the degree of neutralization, there can be obtained a golf ball which has a reduced spin rate on shots with a driver and a suitable degree of spin on shots with a short iron, thus enabling both distance and controllability to be achieved, and which is also endowed with an excellent scuff resistance and a good durability.

[0010] Accordingly, the invention provides the following golf balls.

[1] A golf ball comprising a core, at least one intermediate layer and a cover, wherein the intermediate layer has a Shore D hardness of from 40 to 60 and is formed primarily of a resin mixture having a degree of neutralization of at least 70% and including:

[0011] 100 parts by weight of a resin component composed of, in admixture, [0012] a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio of from 100:0 to 0:100, and

[0013] (e) a non-ionomeric thermoplastic elastomer

in a weight ratio of from 100:0 to 50:50;

[0014] (c) about 15 to about 150 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and

[0015] (d) about 0.1 to about 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c), and the cover has a Shore D hardness of from 58 to 65 and is formed by injection molding a single resin blend composed primarily of (f) a thermoplastic polyurethane and (g) a polyisocyanate compound in at least some portion of which all the isocyanate groups on the molecule are present in an unreacted state.

[2] The golf ball of claim 1, wherein the hardness of the cover is equal to or greater than the hardness of the intermediate layer. [3] The golf ball of claim 1, wherein the resin blend which forms the cover includes (h) a thermoplastic elastomer other than a thermoplastic polyurethane. [4] The golf ball of claim 4, wherein some portion of the isocyanate groups in the polyisocyanate compound (g) included in the resin blend prior to injection molding form bonds with active hydrogens in component (f) and/or component (h), and the remaining isocyanate groups are present within the resin blend in an unreacted state. [5] The golf ball of claim 4, wherein the components in the resin blend which forms the cover have a compositional ratio, expressed as a weight ratio, of (f):(g):(h)=100:2 to 50:0 to 50. [6] The golf ball of claim 4, wherein the components in the resin blend which form the cover have a compositional ratio, expressed as a weight ratio, of (f):(g):(h)=100:2 to 30:8 to 50. [7] The golf ball of claim 1, wherein the resin blend which forms the cover has a melt mass flow rate (MFR) value at 210.degree. C. of at least 5 g/10 min. [8] The golf ball of claim 1, wherein the polyisocyanate compound of component (g) is one or more polyisocyanate compound selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4- (or) 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. [9] The golf ball of claim 1, wherein the polyisocyanate compound of component (g) is one or more polyisocyanate compound selected from the group consisting of 4,4'-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate. [10] The golf ball of claim 4, wherein the thermoplastic elastomer of component (h) is one or more thermoplastic elastomer selected from the group consisting of polyester elastomers, polyamide elastomers, ionomer resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins. [11] The golf ball of claim 4, wherein the thermoplastic elastomer of component (h) is one or more selected from the group consisting of polyester elastomers, polyamide elastomers and polyacetals. [12] The golf ball of claim 1, wherein the ball has a plurality of dimples formed on a surface thereof, which dimples number in all from 250 to 392 and have a total volume of from 400 to 750 mm.sup.3.

BRIEF DESCRIPTION OF THE DIAGRAMS

[0016] FIG. 1 is a top view of a golf ball showing an Arrangement (I) of dimples used in the examples of the invention and in the comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The invention is described in greater detail below.

[0018] The golf ball of the present invention has a solid core, at least one intermediate layer, and a cover.

[0019] In the invention, the solid core may be formed using a known rubber composition. Although not subject to any particular limitation, suitable rubber compositions include those formulated as shown below.

[0020] A rubber core which has been molded and vulcanized from a rubber composition made up primarily of a commonly used rubber base may be employed as the core in the present invention. Specifically, the core is formed using a molded and vulcanized rubber composition obtained by compounding the following with a base rubber: a crosslinking agent, a vulcanizing agent and, optionally, additives such as organosulfur compounds, antioxidants and fillers.

[0021] Polybutadiene is preferably used as the base rubber of the rubber composition that forms the core. It is preferable to use cis-1,4-polybutadiene having a cis structure of at least 40% as the polybutadiene. If desired, natural rubber, polyisoprene rubber, styrene-butadiene rubber or ethylene-propylene-diene rubber, for example, may be suitably compounded together with the polybutadiene in the base rubber.

[0022] An .alpha.,.beta.-unsaturated carboxylic acid such as zinc methacrylate or zinc acrylate may be included as a co-crosslinking agent in the rubber composition. The use of zinc acrylate is especially preferred. The amount in which these unsaturated carboxylic acids are included per 100 parts by weight of the base rubber may be set to at least 10 parts by weight, and especially at least 15 parts by weight. The upper limit is set to preferably not more than 40 parts by weight, and most preferably not more than 35 parts by weight.

[0023] A vulcanizing agent is included in the above rubber composition. An organic peroxide is preferably used as the vulcanizing agent. The organic peroxide, an illustrative example of which is dicumyl peroxide, may be a single type used alone or may be a mixture of two or more types. The organic peroxide may be a commercially available product, specific examples of which include Perhexa 3M (produced by NOF Corporation), Percumyl D (NOF Corporation), and Luperco 231XL and Luperco 101XL (both produced by Atochem Co.). The amount of vulcanizing agent included may be set to more than 0, and may be set to preferably at least 0.1 part by weight per 100 parts by weight of the base rubber. The upper limit in the amount of the vulcanizing agent, although not subject to any particular limitation, may be set to preferably not more than 2 parts by weight, and more preferably not more than 1.5 parts by weight, per 100 parts by weight of the base rubber.

[0024] In the invention, an organosulfur compound may be included so as to further improve the core resilience. Specifically, it is recommended that thiophenols, thionaphthols, halogenated thiophenols or metal salts thereof be included. Illustrative examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having from 2 to 4 sulfurs. The use of diphenyldisulfide or the zinc salt of pentachlorothiophenol is especially preferred.

[0025] In addition, an antioxidant may be included. Examples of commercial products include Nocrac NS-6, Nocrac NS-30 and Nocrac SP-N (Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (Yoshitomo Pharmaceutical Industries, Ltd.). These may be used singly or as combinations of two or more thereof.

[0026] The filler is not subject to any particular limitation. For example, zinc oxide, barium sulfate and calcium carbonate may be suitably included.

[0027] The core-forming rubber composition which includes the above ingredients is prepared by mastication using a conventional mixer, such as a Banbury mixer or a roll mill. In cases where the core is molded using such a rubber composition, molding may be carried out by compression molding or injection molding using a specific core-forming mold. The resulting molded body is then heated and cured under temperature conditions sufficient for the crosslinking agent and co-crosslinking agent included in the rubber composition to act, thereby giving a core having a specific hardness profile. The vulcanization conditions are not subject to any particular limitation. For example, when dicumyl peroxide is used as the crosslinking agent and zinc acrylate is used as the co-crosslinking agent, the conditions are generally set to about 100 to 200.degree. C., and especially 130 to 180.degree. C., for 10 to 40 minutes, and especially 12 to 20 minutes.

[0028] The diameter of the core obtained by the above manufacturing method, although not subject to any particular limitation, is preferably at least 36.5 mm, more preferably at least 36.8 mm, and even more preferably at least 37.2 mm. The upper limit in the diameter, although not subject to any particular limitation, is preferably not more than 40 mm, more preferably not more than 39.7 mm, and even more preferably not more than 39.5 mm.

[0029] In the present invention, although not subject to any particular limitation, the core has a deformation, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), of preferably at least 3.5 mm but not more than 6.0 mm. The lower limit in this deformation is more preferably at least 4.0 mm, and even more preferably at least 4.3 mm. The upper limit is more preferably not more than 5.5 mm, and even more preferably not more than 5.0 mm. If the core is softer than the above value (large deformation), the core resilience will diminish. Conversely, if the core is harder than the above value (small deformation), the feel of the ball may worsen.

[0030] The structure of the core is not limited to one layer, and may be a multilayer structure of two or more layers. By giving the core a multilayer structure, it is possible to reduce the spin rate on shots with a driver, and it is possible to further increase the distance traveled by the ball. In addition, the spin properties and feel of the ball at the time of impact can be further improved. In such cases, the core will have at least an inner core layer (center sphere) and an outer core layer.

[0031] The golf ball of the invention has at least one intermediate layer which encases the core, and a cover which encases the intermediate layer. The materials of the intermediate layer and cover are described in detail below.

[0032] In the present invention, the intermediate layer is formed primarily of a resin composition which includes: 100 parts by weight of a resin component composed of, in admixture,

[0033] a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio of from 100:0 to 0:100, and

[0034] (e) a non-ionomeric thermoplastic elastomer

in a weight ratio of from 100:0 to 50:50; (c) about 15 to about 150 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and (d) about 0.1 to about 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c)

[0035] Above components (a) to (e) are described below.

[0036] Component (a) and component (b) serve as the base resin of the resin composition which forms the intermediate layer. Component (a) is an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer, and component (b) is an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer. In the present invention, either of above components (a) and (b) may be used singly or both may used in combination.

[0037] Here, the olefin in above component (a) and component (b) generally has at least two carbons but not more than 8 carbons, and most preferably not more than 6 carbons. Illustrative examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

[0038] Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

[0039] In addition, the unsaturated carboxylic acid ester included in above component (b) is preferably a lower alkyl ester of the above unsaturated carboxylic acid, illustrative examples of which include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) is especially preferred.

[0040] The random copolymer of component (a) and component (b) may be obtained by the random copolymerization of the above components by a known method. Here, the content (acid content) of the unsaturated carboxylic acid included in the random copolymer, although not subject to any particular limitation, may be set to preferably at least 2 wt %, more preferably at least 6 wt %, and even more preferably at least 8 wt %. It is recommended that the upper limit in the unsaturated carboxylic acid content (acid content), although not subject to any particular limitation, be preferably not more than 25 wt %, more preferably not more than 20 wt %, and even more preferably not more than 15 wt %.

[0041] The random copolymer neutralization products of components (a) and (b) may be obtained by neutralizing some of the acid groups in the above random copolymer with metal ions. Here, illustrative examples of metal ions which neutralize the acid groups include Na.sup.+, K.sup.+, Li.sup.+, Zn.sup.++, Cu.sup.++, Mg.sup.++, Ca.sup.++, Co.sup.++, Ni.sup.++ and Pb.sup.++. Of these, preferred use may be made of Ne.sup.+, Li.sup.+, Zn.sup.++ and Mg.sup.++; Mg.sup.++ and Zn.sup.++ are especially recommended. The degree of neutralization of the random copolymer by these metal ions is not subject to any particular limitation. Such neutralization products may be obtained by a known method. For example, these metal ions may be introduced into the random copolymer by using compounds such as formates, acetates, nitrates, carbonates, bicarbonates, oxides, hydroxides or alkoxides thereof.

[0042] A commercial product may be used as above component (a). Illustrative examples include Nucrel AN4311, Nucrel AN4318, Nucrel AN4319, Nucrel 1560 and Nucrel AN4213C (all produced by DuPont-Mitsui Polychemicals Co., Ltd.).

[0043] Likewise, a commercial product may be used as above component (b). Illustrative examples include Himilan 1554, Himilan 1557, Himilan 1601, Himilan 1605, Himilan 1706, Himilan 1855, Himilan 1856 and Himilan AM7316 (all produced by DuPont-Mitsui Polychemicals Co., Ltd.); and Surlyn 6320, Surlyn 7930 and Surlyn 8120 (all produced by E.I. DuPont de Nemours & Co.). The use of a zinc-neutralized ionomer resin (e.g., Himilan AM7316) is especially preferred.

[0044] Above component (a) and component (b) may be used individually or both may be used in combination as the base resin of the above intermediate layer-forming resin composition. The mixing ratio of these two components, expressed by weight as (a):(b), is from 100:0 to 0:100. Although not subject to any particular limitation, the mixing ratio is preferably from 80:20 to 50:50, and more preferably from 70:30 to 60:40.

[0045] Component (c) is a fatty acid or fatty acid derivative having a molecular weight of at least 228. This component contributes to improving the flow properties of the resin composition; it has a very small molecular weight compared with the thermoplastic resin of above component (a), and thus contributes to a marked decrease in the melt viscosity of the mixture. Because the fatty acid (or fatty acid derivative) in the present invention has a molecular weight of 228 or more and contains a high content of acid groups (or derivative moieties thereof), its addition results in little loss in resilience.

[0046] The fatty acid or fatty acid derivative serving as component (c) has a molecular weight which is at least 228, preferably at least 256, more preferably at least 280, and even more preferably at least 300, with an upper limit of not more than 1,500, preferably not more than 1,000, more preferably not more than 600, and even more preferably not more than 500. Here, if the molecular weight is too low, it will be impossible to achieve an improvement in the heat resistance, in addition to which the content of acid groups will be so high that interactions with acid groups present in the base resin may lower the flow-improving effects. On the other hand, if the molecular weight is too high, a distinct flow-improving effect may not appear.

[0047] Specific examples of the fatty acid serving as component (c) include stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Of these, preferred use may be made of stearic acid, arachidic acid, behenic acid and lignoceric acid.

[0048] The fatty acid derivative is exemplified by derivatives in which the proton on the acid group of the fatty acid has been substituted. Exemplary fatty acid derivatives of this type include metallic soaps in which the proton has been substituted with a metal ion. Metal ions that may be used in such metallic soaps include Li.sup.+, Ca.sup.++, Mg.sup.++, Zn.sup.++, Mn.sup.++, Al.sup.+++, Ni.sup.++, Fe.sup.++, Fe.sup.+++, Cu.sup.++, Sn.sup.++, Pb.sup.++ and Co.sup.++. Of these, Ca.sup.++, Mg.sup.++ and Zn.sup.++ are especially preferred.

[0049] Specific examples of fatty acid derivatives that may be used as component (c) include magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. Of these, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate are preferred.

[0050] The amount of component (c) used per 100 parts by weight of above component (a) and/or component (b) (referred to below as the "base resin") is from about 50 to about 150 parts by weight. The lower limit is preferably at least about 81 parts by weight, and the upper limit is preferably not more than about 120 parts by weight.

[0051] Use may also be made of known metallic soap-modified ionomers (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Disclosure WO 98/46671) when using above component (a) and/or component (b), and component (c).

[0052] The basic inorganic filler of component (d) is included to neutralize the acid groups in above component (a) and/or component (b), and in component (c). When above component (d) is not included, and in particular when a metal-modified ionomer resin alone (e.g., a metal soap-modified ionomer resin of the type mentioned in the foregoing patent publications, alone) is mixed under applied heat, as mentioned below, the metallic soap and unneutralized acid groups present on the ionomer undergo exchange reactions, generating a fatty acid. Because the fatty acid has a low thermal stability and readily vaporizes during molding, it causes molding defects. Moreover, if the fatty acid thus generated deposits on the surface of the molded material, it substantially lowers paint film adhesion.

##STR00001##

(1) unneutralized acid group present on the ionomer resin (2) metallic soap (3) fatty acid X: metal atom

[0053] To solve such problems, it is essential to include as component (d) a basic inorganic metal compound which neutralizes acid groups present in above component (a) and/or component (b), and in component (c). The inclusion of component (d) confers excellent properties. Namely, the acid groups in above component (a) and/or component (b) and in component (c) are neutralized, and synergistic effects from the inclusion of each of these components increase the thermal stability of the resin composition while at the same time imparting a good moldability, and also enhance the resilience as a golf ball-forming material.

[0054] It is recommended that above component (d) be a basic inorganic metal compound--preferably a monoxide--which is capable of neutralizing acid groups in above component (a) and/or component (b), and in component (c). Because such compounds have a high reactivity with the ionomer resin and the reaction by-products contain no organic matter, the degree of neutralization of the resin composition can be increased without a loss of thermal stability.

[0055] The metal ions used here in the basic inorganic metal compound are exemplified by Li.sup.+, Na.sup.+, K.sup.+, Ca.sup.++, Mg.sup.++, Zn.sup.++, Al.sup.+++, Ni.sup.+, Fe.sup.++, Fe.sup.+++, Cu.sup.++, Mn.sup.++, Sn.sup.++, Pb.sup.++ and Co.sup.++. Illustrative examples of the inorganic metal compound include basic inorganic fillers containing these metal ions, such as magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. Of these, as noted above, a monoxide is preferred. The use of magnesium oxide, which has a high reactivity with ionomer resins, is especially preferred in the present invention.

[0056] Component (d) is included in an amount, per 100 parts by weight of the above resin component, of from about 0.1 to about 17 parts by weight. Here, the lower limit is preferably at least 0.5 part by weight, and the upper limit is preferably not more than about 15 parts by weight. If the amount of above component (d) included is too low, improvements in thermal stability and resilience will not be observed. On the other hand, if the amount is too high, the thermal resistance of the composition may instead decline due to excessive basic inorganic metal compound.

[0057] The non-ionomeric thermoplastic elastomer serving as component (e) is included to further improve the feel of the ball on impact and the rebound. Illustrative examples include thermoplastic elastomers such as thermoplastic polyester elastomers, thermoplastic block copolymers and thermoplastic urethanes.

[0058] The above component (e) is included in an amount, expressed as a weight ratio of the above-described base resin to component (e), of from 100:0 to 50:50.

[0059] From the standpoint of processability, it is desirable for the intermediate layer-forming resin composition which includes above components (a) to (e) to have a melt index (measured in accordance with JIS-K6760 at a test temperature of 190.degree. C. and a test load of 21 N (2.16 kgf)) of at least 0.5 g/10 min, preferably at least 0.8 g/10 min, and more preferably at least 1.0 g/min. The upper limit in this melt index is not more than 20 g/10 min, and more preferably not more than 15 g/10 min. If the melt index of the resin composition is too low, the processability may markedly decrease.

[0060] Although the specific gravity of this resin composition itself is not subject to any particular limitation, it may be set to preferably at least 0.9. The upper limit of this specific gravity, although not subject to any particular limitation, is preferably set to not more than 1.3, and more preferably set to no more than 1.2.

[0061] The above resin composition is obtained by mixing under applied heat the above-described component (a) and/or component (b), component (c), component (d) and component (e), and has an optimized melt index. It is recommended that at least 70 mol %, preferably at least 80 mol %, and more preferably at least 90 mol %, of the acid groups in the resin composition be neutralized. A high degree of neutralization more reliably suppresses the exchange reactions that pose a problem in the above-described cases where the base resin and the fatty acid (or fatty acid derivative) alone are used, thus making it possible to prevent the generation of fatty acids. As a result, a material can be obtained which has a markedly increased thermal stability, a good moldability, and a substantially higher resilience than conventional ionomer resins.

[0062] An inorganic granular filler may be optionally included in the resin composition so as to further improve durability. This inorganic granular filler may be a known inorganic granular filler and is not subject to any particular limitation, although the use of titanium dioxide and barium sulfate is preferred in the present invention. The amount per 100 parts by weight of the above base resin is set to preferably at least 5 parts by weight, and more preferably at least 9 parts by weight. The upper limit is preferably not more than 36 parts by weight, and more preferably not more than 26 parts by weight.

[0063] Various additives may be optionally added to the resin composition containing components (a) to (e). Additives which may be used include pigments, antioxidants, ultraviolet absorbers and light stabilizers.

[0064] The method used to form the intermediate layer may be a known method and is not subject to any particular limitation. For example, a method may be employed which involves placing a prefabricated core within a mold, then melting under applied heat, or mixing and melting under applied heat, and subsequently injection-molding the intermediate layer-forming resin composition.

[0065] The Shore D hardness of the intermediate layer is preferably at least 40, and more preferably at least 43. The upper limit is preferably not more than 65, and more preferably not more than 60. At a Shore D hardness below 40, the rebound may decrease, whereas at more than 65, the ball may crack more easily, resulting in a poor durability.

[0066] The thickness of the intermediate layer is not subject to any particular limitation, although it is recommended that the thickness be at least 0.8 mm, and especially 1 mm. It is recommended that the upper limit in the intermediate layer thickness be not more than 4.0 mm, and especially not more than 3.0 mm.

[0067] The specific gravity of the intermediate layer, although not subject to any particular limitation, is preferably at least 0.9, and more preferably at least 0.95. The upper limit in the specific gravity also is not subject to any particular limitation, although a value of not more than 1.3, and especially not more than 1.15, is recommended. If the specific gravity is too large, thorough dispersion and mixing may become difficult to carry out. On the other hand, if the specific gravity is too small, there may be cases where.

[0068] Moreover, the hardness of the intermediate layer is set to a Shore D value of preferably from 40 to 60, and more preferably from 43 to 60. At an intermediate layer hardness having a Shore D value of less than 40, the resilience may be diminish; on the other hand, at a Shore D value greater than 60, the ball may have a diminished feel on impact and may crack more easily.

[0069] The construction of the intermediate layer is not limited to a single layer. If necessary, two or more intermediate layers having different properties may be formed within the above-described range. By forming a plurality of intermediate layers, the spin rate on shots with a driver can be reduced, enabling an even greater increase in distance to be achieved. Also, the spin properties and feel at the time of impact can be further improved.

[0070] The golf ball of the invention is arrived at by forming a cover over, and thereby encasing, the outside of the intermediate layer. The cover is composed primarily of a thermoplastic polyurethane, and is formed from a single resin blend composed primarily of (f) a thermoplastic polyurethane and (g) a polyisocyanate compound. Golf balls which use such a cover made of a thermoplastic polyurethane have a high rebound and an excellent spin performance and scuff resistance. Moreover, the cover-forming material has high flow properties and an excellent productivity.

[0071] As used herein, a "single" resin blend means that the cover is molded, not by feeding as the resin blend a plurality of pellet types, but by feeding to injection molding one type of pellet obtained by formulating a plurality of ingredients into single pellets.

[0072] This cover is composed primarily of (f) a thermoplastic polyurethane and (g) a polyisocyanate compound. Specifically, it is recommended that the combined weight of components (f) and (g) be at least 60%, and preferably at least 70%, of the overall weight of the cover layer.

[0073] The thermoplastic polyurethane (f) is described. This thermoplastic polyurethane has a structure which includes soft segments made of a polymeric polyol that is a long-chain polyol (polymeric glycol), and hard segments made of a chain extender and a polyisocyanate compound. Here, the long-chain polyol used as a starting material is not subject to any particular limitation, and may be any that is used in the prior art relating to thermoplastic polyurethanes. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof. Of the long-chain polyols mentioned here, polyether polyols are preferred because they enable the synthesis of thermoplastic polyurethanes having a high rebound resilience and excellent low-temperature properties.

[0074] Illustrative examples of the above polyether polyol include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol) obtained by the ring-opening polymerization of a cyclic ether. The polyether polyol may be used singly or as a combination of two or more polyether polyols. Of these, poly(tetramethylene glycol) and/or poly(methyltetramethylene glycol) are preferred.

[0075] It is preferable for these long-chain polyols to have a number-average molecular weight in a range of 1,500 to 5,000. By using a long-chain polyol having a number-average molecular weight within this range, golf balls made of a thermoplastic polyurethane composition having various excellent properties such as resilience and manufacturability can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in a range of 1,700 to 4,000, and even more preferably in a range of 1,900 to 3,000.

[0076] As used herein, "number-average molecular weight of the long-chain polyol" refers to the number-average molecular weight computed based on the hydroxyl number measured in accordance with JIS K-1557.

[0077] Suitable chain extenders include those used in the prior art relating to thermoplastic polyurethanes. For example, low-molecular-weight compounds which have a molecular weight of 400 or less and bear on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these chain extenders, aliphatic diols having 2 to 12 carbons are preferred, and 1,4-butylene glycol is especially preferred.

[0078] The polyisocyanate compound is not subject to any particular limitation; preferred use may be made of one employed in the prior art relating to thermoplastic polyurethanes. Specific examples include one or more selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4- or 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Depending on the type of isocyanate used, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4'-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

[0079] It is most preferable for the thermoplastic polyurethane serving as component (f) to be a thermoplastic polyurethane synthesized using a polyether polyol as the long-chain polyol, using an aliphatic diol as the chain extender, and using an aromatic diisocyanate as the polyisocyanate compound. It is desirable, though not essential, for the polyether polyol to be a polytetramethylene glycol having a number-average molecular weight of at least 1,900, for the chain extender to be 1,4-butylene glycol, and for the aromatic diisocyanate to be 4,4'-diphenylmethane diisocyanate.

[0080] The mixing ratio of active hydrogen atoms to isocyanate groups in the above polyurethane-forming reaction can be controlled within a desirable range so as to make it possible to obtain a golf ball which is composed in part of a thermoplastic polyurethane composition and has various improved properties, such as rebound, spin performance, scuff resistance and manufacturability. Specifically, in preparing a thermoplastic polyurethane by reacting the above long-chain polyol, polyisocyanate compound and chain extender, it is desirable to use the respective components in proportions such that the amount of isocyanate groups on the polyisocyanate compound per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

[0081] No particular limitation is imposed on the method of preparing the thermoplastic polyurethane used as component (f). Production may be carried out by either a prepolymer process or a one-shot process in which the long-chain polyol, chain extender and polyisocyanate compound are used and a known urethane-forming reaction is effected. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

[0082] Illustrative examples of the thermoplastic polyurethane serving as component (f) include commercial products such as Pandex T8295, Pandex T8290, Pandex T8260, Pandex T8295 and Pandex T8290 (all available from DIC Bayer Polymer, Ltd.).

[0083] Next, concerning the polyisocyanate compound used as component (g), it is essential that, in at least some portion thereof prior to injection molding, all the isocyanate groups on the molecule remain in an unreacted state. That is, polyisocyanate compound in which all the isocyanate groups on the molecule remain in a completely free state must be present in the single resin blend prior to injection molding. Such a polyisocyanate compound may be present together with a polyisocyanate compound in which only some of the isocyanate groups on the molecule are in a free state.

[0084] Various isocyanates may be employed without particular limitation as this polyisocyanate compound. Illustrative examples include one or more selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4- or 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Of the above group of isocyanates, the use of 4,4'-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate is preferable in terms of the balance between the influence on processability of such effects as the rise in viscosity that accompanies reaction with the thermoplastic polyurethane serving as component (f) and the physical properties of the resulting golf ball cover material.

[0085] In the cover of the inventive golf ball, although not an essential constituent, a thermoplastic elastomer other than the above-described thermoplastic polyurethane may additionally be included as component (h) together with components (f) and (g). Including this component (h) in the above resin composition enables the flow properties of the resin blend to be further improved and enables increases to be made in various properties required of golf ball cover materials, such as resilience and scuff resistance.

[0086] Component (h), which is a thermoplastic elastomer other than the above thermoplastic polyurethane, is exemplified by one or more thermoplastic elastomer selected from among polyester elastomers, polyamide elastomers, ionomer resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins. The use of polyester elastomers, polyamide elastomers and polyacetals is especially preferred because the resilience and scuff resistance are enhanced, owing to reactions with isocyanate groups, while a good manufacturability is retained.

[0087] The relative proportions of above components (f), (g) and (h) are not subject to any particular limitation. However, to fully and effectively achieve the objects of the invention, it is preferable for the weight ratio (f):(g):(h) of the respective components to be 100:2 to 50:0 to 50, and more preferably 100:2 to 30:8 to 50.

[0088] In the present invention, the cover-forming resin blend is prepared by mixing together component (f), component (g), and also optional component (h). It is necessary to select the mixing conditions at this time such that at least some polyisocyanate compound in which all the isocyanate groups on the molecule remain in an unreacted state is present in the polyisocyanate compound. For example, treatment such as mixture in an inert gas (e.g., nitrogen) or in a vacuum state must be furnished. The resin blend is then injection-molded around a core which has been placed in a mold. For easy, trouble-free handling, it is preferable that the resin blend be formed into pellets having a length of 1 to 10 mm and a diameter of 0.5 to 5 mm. A sufficient number of isocyanate groups in an unreacted state remain within these resin pellets; while the resin blend is being injection-molded about the core, or due to post-treatment such as annealing thereafter, the unreacted isocyanate groups react with component (f) and component (h) to form a crosslinked material.

[0089] In addition, various additives may also be optionally included in this cover-forming resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers, and parting agents may be suitably included.

[0090] The melt mass flow rate (MFR) of this resin blend at 210.degree. C. is not subject to any particular limitation. However, to increase the flow properties and manufacturability, the MFR is preferably at least 5 g/10 min, and more preferably at least 6 g/10 min. If the melt mass flow rate of the resin blend is low, the flow properties will decrease, which may cause eccentricity during injection molding and may also lower the degree of freedom in the thickness of the cover that can be molded. The measured value of the melt mass flow rate is obtained in accordance with JIS K-7210 (1999 edition).

[0091] The method of molding the cover layer may involve feeding the above-described resin blend to an injection-molding machine and injecting the molten resin blend around the core. Although the molding temperature in this case will vary depending on the type of thermoplastic polyurethane, the molding temperature is generally in a range of from 150 to 250.degree. C.

[0092] When injection molding is carried out, it is desirable though not essential to carry out molding in a low-humidity environment such as by purging with an inert gas (e.g., nitrogen) or a low-humidity gas (e.g., low dew-point dry air), or by vacuum treating, some or all places on the resin paths from the resin feed area to the mold interior. Illustrative, non-limiting, examples of the medium used for transporting the resin include low-humidity gases such as low dew-point dry air or nitrogen. By carrying out molding in such a low-humidity environment, reaction by the isocyanate groups is kept from proceeding before the resin has been charged into the mold interior. As a result, polyisocyanate in which the isocyanate groups are to some degree in an unreacted state can be included within the resin blend, thus making it possible to reduce variable factors such as an unwanted rise in viscosity and enabling the actual crosslinking efficiency to be enhanced.

[0093] Techniques that may be used to confirm the presence of polyisocyanate compound in an unreacted state within the resin blend prior to injection molding about the core include those which involve extraction with a suitable solvent that selectively dissolves out only the polyisocyanate compound. An example of a simple and convenient method is one in which confirmation is carried out by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) measurement in an inert atmosphere. For example, when the resin blend (cover material) used in the invention is heated in a nitrogen atmosphere at a temperature ramp-up rate of 10.degree. C./min, a gradual drop in the weight of diphenylmethane diisocyanate can be observed from about 150.degree. C. On the other hand, in a resin sample in which the reaction between the thermoplastic polyurethane material and the isocyanate mixture has been carried out to completion, a weight drop is not observable from about 150.degree. C., but a weight drop is observable from about 230 to 240.degree. C.

[0094] After the resin blend has been molded as described above to form a cover, its properties as a golf ball cover can be further improved by carrying out annealing so as to induce the crosslinking reaction to proceed further. "Annealing," as used herein, refers to aging the cover in a fixed environment for a fixed length of time.

[0095] This cover layer is set to a surface hardness, expressed as the Shore D hardness, of preferably from 58 to 65, and more preferably from 59 to 62. If the surface hardness of the cover layer is too low, the spin rate on shots with a driver may increase, resulting in a shorter distance. On the other hand, if the surface hardness of the cover layer is too high, the ball may have a poor feel and the resilience and durability attributes of the urethane material may diminish.

[0096] The hardness of this cover is preferably set so as to be equal to or greater than the hardness of the intermediate layer; i.e., cover hardness z intermediate layer hardness. If the cover hardness is lower than the intermediate layer hardness, the ball may have a hard and poor feel on impact, which may make it impossible to achieve the objects of the invention.

[0097] The cover layer has a rebound resilience of generally at least 35%, preferably at least 40%, more preferably at least 45%, and even more preferably at least 47%. Because thermoplastic polyurethanes basically do not have such an outstanding resilience, strict selection of the above rebound resilience is preferred. If the rebound resilience of the cover layer is too low, the golf ball may undergo a large decline in distance. On the other hand, if the cover layer has an excessively high rebound resilience, the initial velocity on puts and on shots of less than 100 yards requiring control may be too high, and may not result in a good feel on impact for the golfer. In the present invention, "rebound resilience" refers to the rebound resilience obtained in accordance with JIS K7311.

[0098] The thickness of this cover, although not subject to any particular limitation, is set to preferably at least 0.4 mm, and more preferably at least 0.7 mm. The upper limit, although not subject to any particular limitation, is set to preferably not more than 1.5 mm, and more preferably not more than 1.3 mm. If the cover thickness is too large, the rebound may decrease. On the other hand, if the cover thickness is too small, the durability may decrease.

[0099] The structure of the cover is not limited to one layer; if necessary, a cover of two or more layers may be formed with materials having different properties. In this case, it is recommended that the cover have at least one layer which is formed of the above resin blend composed primarily of components (f) and (g), and that the hardness and thickness be adjusted so that these values for the overall cover fall within the above-indicated ranges.

[0100] In the golf ball of the present invention, to further enhance the aerodynamic properties and improve the distance, as in ordinary golf balls, it is preferable for numerous dimples to be formed on the surface of the cover. By optimizing such parameters as the number of dimple types and the total number of dimples, through synergistic effects with the above-described ball construction, there can be obtained a golf ball having a more stable trajectory and an improved distance performance. Moreover, to improve the design and durability of the golf ball, various treatments such as surface treatment, stamping and painting may be carried on the cover.

[0101] Here, it is recommended that the number of dimple types, which refers to the number of dimple types of mutually differing diameter and/or depth, be preferably at least two types, and more preferably at least three types. It is recommended that the upper limit be not more than eight types, and in particular not more than six types.

[0102] Because the golf ball of the present invention, owing to the above-described ball construction, tends to have a decreased spin rate at the time of impact, and thus a lower trajectory, it is preferable to carry out dimple design in such a way as to enable a large lift to be obtained.

[0103] First, the total number of dimples is set to preferably at least 250, and more preferably at least 270. The upper limit is preferably set to not more than 392, and more preferably not more than 360. If the total number of dimples is too low or too high, an optimal lift may not be achieved and the ball may travel a less than desirable distance.

[0104] Nor is any particular limitation imposed on the geometrical arrangement of the dimples; use may be made of a known arrangement, such as an octahedral or an icosahedral arrangement. At this time, from the standpoint of reducing variability in the flight of the ball, preferred use may be made of a dimple arrangement such that the surface of the ball has thereon not even a single great circle which intersects no dimples. The dimple shapes are not limited to circular shapes, and may also be suitably selected from among polygonal, teardrop, oval and other shapes. The dimple diameter (in polygonal shapes, the diagonal length) be set to preferably at least 1 mm, and more preferably at least 2 mm. The upper limit is set to preferably not more than 8 mm, and more preferably not more than 7 mm.

[0105] It is recommended that the dimple surface coverage, from the standpoint of reducing air resistance, be at least 75%, and especially at least 79%. This surface coverage can be increased by raising the number of dimples formed, interspersing a plurality of dimples types of differing diameter, and using dimple shapes in which the distance between neighboring dimples (land width) becomes substantially 0.

[0106] The total volume of the dimples refers to the sum of the volumes of those portions circumscribed by dimple walls and the curved surfaces of land areas on the ball surface. This total volume is preferably set to from 400 to 750 mm.sup.3, and especially from 450 to 700 mm.sup.3.

[0107] The golf ball of the invention may be made to conform with the Rules of Golf for competitive play, and may be formed to a diameter of not less than 42.67 mm. It is generally suitable to set the weight to not less than 45.0 g, and preferably not less than 45.2 g, but not more than 45.93 g.

[0108] Although not subject to any particular limitation, the golf ball of the present invention has an overall ball deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of preferably at least 2.5 mm, and more preferably at least 2.7 mm. The upper limit is preferably not more than 4.0 mm, and more preferably not more than 3.7 mm. If the deflection is too small, the feel on impact may worsen and, on long shots such as with a driver in which the ball incurs a large deformation, may subject the ball to an excessive rise in the spin rate, shortening the distance traveled by the ball. On the other hand, if the deflection is too large, the ball may have a dead feel and a less than adequate rebound, shortening the distance traveled, in addition to which the ball may have a poor durability to cracking on repeated impact.

[0109] Although not subject to any particular limitation, it is desirable for the golf ball of the invention to exhibit a decline in initial velocity one year after manufacture relative to the initial velocity one week after manufacture (after molding) of preferably not more than 0.5 m/s, and more preferably not more than 0.4 m/s. Such an initial velocity may be achieved by adjusting the formulation of the intermediate layer-forming material in accordance with the core and cover within the range set forth in the present invention. As used herein, "initial velocity" refers to the value measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The specific measurement conditions are given in the examples described below.

EXAMPLES

[0110] The following Examples and Comparative Examples are provided by way of illustration and not by way of limitation.

Examples 1 to 3, Comparative Examples 1 to 4

Formation of Core

[0111] Solid cores were fabricated by preparing the rubber compositions shown in Table 1 below, then molding and vulcanizing at 155.degree. C. for 15 minutes.

TABLE-US-00001 TABLE 1 Formulation No. 1 No. 2 Formulation Polybutadiene 100 100 (pbw) Dicumyl peroxide 1.2 1.2 Zinc oxide 18.02 19.14 2,2'-Methylenebis(4-methyl-6-t- 0.1 0.1 butylphenol) Zinc diacrylate 32.71 29.99 Zinc salt of pentachlorothiophenol 0.10 0.10

[0112] Details on the materials in Table 1 are given below. [0113] Polybutadiene: Available under the trade name "BR 730" from JSR Corporation. [0114] Dicumyl peroxide: Available under the trade name "Percumyl D" from NOF Corporation. [0115] Zinc oxide: Available from Sakai Chemical Industry Co., Ltd. [0116] 2,2'-Methylenebis(4-methyl-6-t-butylphenol): Available under the trade name "Nocrac NS-6" from Ouchi Shinko Chemical Industry Co., Ltd. [0117] Zinc diacrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. [0118] Zinc salt of pentachlorothiophenol: [0119] Available from Tokyo Kasei Kogyo Co., Ltd.

Formation of Intermediate Layer and Cover

[0120] Next, an intermediate layer and a cover of the formulations shown below were successively injection-molded around the core obtained as described above, thereby producing three-piece solid golf balls (of three types in the examples according to the invention and of four types in the comparative examples) having the core, intermediate layer and cover combinations shown in Table 3. At this time, the dimples shown in FIG. 1 were formed on the cover surface. Details of these dimples are shown in Table 2.

[0121] Formation of the covers in Working Examples 1 to 4 and Comparative Examples 1 to 4 was carried out as follows.

[0122] First, each of the following cover materials was mixed in a nitrogen environment with a twin-screw extruder to give a cover resin blend in pellet form (length, 3 mm; diameter, 1 to 2 mm). The cover material was then injection-molded around a core to form a cover.

Intermediate Layer Formulation A

TABLE-US-00002 [0123] Nucrel AN4319 (acid content, 8.0 wt %; 100 parts by weight ester content, 17.0 wt %) Magnesium stearate 100 parts by weight Magnesium oxide 2.8 parts by weight Polytail 4.0 parts by weight Degree of neutralization: 110.4 mol % Shore D hardness: 50

Intermediate Layer Formulation B

TABLE-US-00003 [0124] Nucrel AN4319 (acid content, 8.0 wt %; 30 parts by weight ester content, 17.0 wt %) Nucrel AN4221C (ester content, 0.9 wt %) 60 parts by weight Dynaron 6100P 10 parts by weight Magnesium stearate 60 parts by weight Magnesium oxide 1.3 parts by weight Degree of neutralization: 80.4 mol % Shore D hardness: 56

Intermediate Layer Formulation C

TABLE-US-00004 [0125] Himilan H1605 68.8 parts by weight Himilan AM7331 31.3 parts by weight Behenic acid 18 parts by weight Calcium hydroxide 2.3 parts by weight Polytail 2.0 parts by weight Degree of neutralization: 56.0 mol % Shore D hardness: 56

Cover Layer Formulation A

TABLE-US-00005 [0126] Pandex T8260 50 parts by weight Pandex T8295 50 parts by weight Polyisocyanate compound 7.5 parts by weight Elastomer 11 parts by weight Polyethylene wax 1 part by weight Titanium oxide 3 parts by weight Shore D hardness: 59

Cover Layer Formulation B

TABLE-US-00006 [0127] Pandex T8260 100 parts by weight Polyisocyanate compound 7.5 parts by weight Elastomer 11 parts by weight Polyethylene wax 1 part by weight Titanium oxide 3 parts by weight Shore D hardness: 61

Cover Layer Formulation C

TABLE-US-00007 [0128] Himilan H1557 75 parts by weight Himilan H1855 25 parts by weight Polyethylene wax 1 parts by weight Magnesium stearate 1.8 parts by weight Titanium oxide 3.8 parts by weight Shore D hardness: 58

Cover Layer Formulation D

TABLE-US-00008 [0129] Pandex T8295 100 parts by weight Polyisocyanate compound 7.5 parts by weight Elastomer 11 parts by weight Polyethylene wax 1 part by weight Titanium oxide 3 parts by weight Shore D hardness: 57

Cover Layer Formulation E

TABLE-US-00009 [0130] Pandex T8295 75 parts by weight Pandex T8290 25 parts by weight Polyisocyanate compound 7.5 parts by weight Elastomer 11 parts by weight Polyethylene wax 1 part by weight Titanium oxide 3 parts by weight Shore D hardness: 54

[0131] Details on the materials used in above Intermediate Layer Formulations A to C and Cover Layer Formulations A to E are given below. [0132] Nucrel AN4319: An olefin-unsaturated carboxylic acid-carboxylic acid ester terpolymer available from DuPont-Mitsui Polychemicals Co., Ltd. [0133] Nucrel AN4221C: An olefin-unsaturated carboxylic acid copolymer available from DuPont-Mitsui Polychemicals Co., Ltd. [0134] Himilan AM7331: An ionomer resin in the form of a sodium ion-neutralized ethylene-methacrylic acid-acrylic acid ester copolymer available from DuPont-Mitsui Polychemicals Co., Ltd. [0135] Himilan 1605: An ionomer resin in the form of a sodium ion-neutralized ethylene-methacrylic acid copolymer available from DuPont-Mitsui Polychemicals Co., Ltd. [0136] Himilan 1557: An ionomer resin in the form of a zinc ion-neutralized ethylene-methacrylic acid copolymer available from DuPont-Mitsui Polychemicals Co., Ltd. [0137] Himilan 1855: An ionomer resin in the form of a zinc ion-neutralized ethylene-methacrylic acid copolymer available from DuPont-Mitsui Polychemicals Co., Ltd. [0138] Dynaron 6100P: A CEBC-type olefinic thermoplastic elastomer available from JSR Corporation. [0139] Pandex T8260: A MDI-PTMG type thermoplastic polyurethane material available from DIC Bayer Polymer. Durometer D resin hardness, 56. Rebound resilience, 45%. [0140] Pandex T8295: A MDI-PTMG type thermoplastic polyurethane material available from DIC Bayer Polymer. JIS-A resin hardness, 97. Rebound resilience, 44%. [0141] Pandex T8260: A MDI-PTMG type thermoplastic polyurethane material available from DIC Bayer Polymer. JIS-A resin hardness, 93. Rebound resilience, 52%. [0142] Elastomer: Hytrel 4001, available from DuPont Toray Co., Ltd. [0143] Behenic acid: Available as "NAA-222S (powder)" from NOF Corporation. [0144] Magnesium stearate: Available as "Nissan Magnesium Stearate" from NOF Corporation. [0145] Polyisocyanate compound: 4,4'-Diphenylmethane diisocyanate. [0146] Polyethylene wax: Available as "Sanwax 161P" from Sanyo Chemical Industries, Ltd. [0147] Polytail: A low-molecular-weight polyolefin polyol available from Mitsubishi Chemical Corporation. [0148] Magnesium oxide: A high-activity type magnesium oxide available as "Micromag 3-150" from Kyowa 1.0 Chemical Industry. [0149] Calcium hydroxide: Available as "CLS-B" from Shiraishi Calcium Kaisha, Ltd. [0150] Titanium oxide: Available under the trade name "Tipaque R550" from Ishihara Sangyo Kaisha, Ltd.

TABLE-US-00010 [0150] TABLE 2 Total Diameter Depth volume No. Number (mm) (mm) V.sub.0 (mm.sup.3) SR VR 1 12 4.60 0.15 0.47 568 0.81 0.784 2 234 4.40 0.15 0.47 3 60 3.80 0.14 0.47 4 12 3.50 0.13 0.47 5 12 2.50 0.10 0.47 Total 330

Dimple Definitions

[0151] Diameter: Diameter of flat plane circumscribed by edge of dimple. [0152] Depth: Maximum depth of dimple from flat plane circumscribed by edge of dimple. [0153] V.sub.0: Spatial volume of dimple below flat plane circumscribed by dimple edge, divided by volume of cylinder whose base is the flat plane and whose height is the maximum depth of dimple from the base. [0154] Total volume: Sum of volume of portions circumscribed by dimple walls and curved surfaces of land areas on ball surface. [0155] SR: Sum of individual dimple surface areas, each defined by the surface area of the flat plane circumscribed by the edge of a dimple, as a percentage of surface area of ball sphere were it to have no dimples thereon. [0156] VR: Sum of volumes of individual dimples formed below flat plane circumscribed by the edge of the dimple, as a percentage of volume of ball sphere were it to have no dimples thereon.

[0157] For each of the golf balls obtained, various properties, including the thickness, hardness and deflection of the respective layers, and the flight performance and scuff resistance were measured by the following methods. The results are shown in Table 3.

Measuring the Ball Properties

Deflection (mm) of Core and Finished Ball

[0158] The core and the finished ball were placed on a hard plate, and the deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

Deflection (mm) of Intermediate Layer

[0159] A sphere composed of the core encased by the intermediate layer was placed on a hard plate, and the deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

Cover Hardness and Intermediate Layer Hardness

[0160] The Shore D hardness of the cover layer alone and the intermediate layer alone, as measured in accordance with ASTM D-2240.

Flight Performance

[0161] Distance and Spin Rate on Shots with a W#1: The distance traveled by the ball when hit at a head speed of 50 m/s with a W#1 mounted on a golf swing robot was measured. A Tour Stage X-Drive 701 driver (2009 model; loft, 8.5.degree.) manufactured by Bridgestone Sports Co., Ltd. was used as the club. The spin rate was obtained by using an apparatus for measuring initial conditions to measure the ball immediately after being hit in the same way. [0162] Spin Rate on Shots with a Wedge: The spin rate was obtained by using an apparatus for measuring initial conditions to measure the ball immediately after being hit at a head speed of 24 m/s with a pitching wedge mounted on a golf swing robot. A J's Classical Edition pitching wedge manufactured by Bridgestone Sports Co., Ltd. was used as the club.

Scuff Resistance

[0163] Golf balls were held at a temperature of 23.degree. C., 13.degree. C. or 0.degree. C. and the respective balls were hit at a head speed of 33 m/s using a non-plated pitching wedge mounted on a swing robot machine, following which damage from the impact was visually rated. The ball was assigned a scuff resistance rating of "Good" when six or more out of ten judges thought the ball could be used again, and was assigned a scuff resistance rating of "NG" when five or fewer judges thought the ball was no longer fit for use.

Initial Velocity

[0164] The initial velocity one week after production (molding) of the resulting ball (Initial Velocity A), and the initial velocity one year after production (Initial Velocity B) were measured by the following method, and the decrease in initial velocity when one year had elapsed following production was determined.

Method of Measuring Initial Velocity:

[0165] The initial velocity was measured under the following conditions using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The ball was hit using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s). Ten balls were each hit four times. The time taken for the balls to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity. In conducting this test, the balls were held isothermally at a temperature of 23.+-.1.degree. C. for at least 3 hours, then tested in a room temperature (23.+-.2.degree. C.) chamber.

TABLE-US-00011 TABLE 3 Example Comparative Example 1 2 3 4 1 2 3 4 Core Formulation No. 1 No. 2 No. 1 No. 2 No. 1 No. 2 No. 1 No. 1 Diameter (mm) 38.1 38.1 38.1 38.1 38.1 38.1 38.1 38.1 Deflection (mm) 3.4 3.7 3.4 3.7 3.4 3.7 3.4 3.4 Intermediate Formulation A B A B A C A B layer Thickness (mm) 1.31 1.31 1.31 1.31 1.31 1.31 1.31 1.31 Shore D hardness 50 56 50 56 50 56 50 56 Deflection (mm) 3.2 3.2 3.2 3.2 3.2 3.1 3.1 3.2 Cover Formulation A A B B C A D E Thickness (mm) 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 Shore D hardness 59 59 61 61 58 59 57 54 Ball Diameter 42.70 42.70 42.70 42.70 42.70 42.70 42.70 42.70 Deflection (mm) 2.9 2.9 2.9 2.9 2.9 2.9 2.9 3 Dimples FIG. 1, FIG. 1, FIG. 1, FIG. 1, FIG. 1, FIG. 1, FIG. 1, FIG. 1, Table 2 Table 2 Table 2 Table 2 Table 2 Table 2 Table 2 Table 2 Performance Spin rate with 2550 2530 2520 2500 2650 2680 2700 2900 W#1 (rpm) Spin rate with 5700 5700 5500 5500 5700 5700 5900 6100 wedge (rpm) Distance with 235 234 240 241 230 230 230 226 W#1 (m) Scuff resistance Good Good Good Good NG Good Good Good Initial velocity A 77.30 77.30 77.30 77.30 77.30 77.20 77.30 77.30 Initial velocity B 76.50 77.00 76.50 77.00 76.50 46.90 76.50 77.00 Initial velocity -0.80 -0.30 -0.80 -0.30 -0.80 -0.30 -0.80 -0.30 difference (A - B)

[0166] As shown in Table 3, the golf balls of the present invention had relatively low spin rates on shots with a driver, enabling the distance traveled by the ball to be increased. The same balls also had good spin rates on shots with a wedge and thus an excellent controllability. Moreover, the inventive golf balls had an excellent scuff resistance and a good durability.

Claims

1. A golf ball comprising a core, at least one intermediate layer and a cover, wherein the intermediate layer has a Shore D hardness of from 40 to 60 and is formed primarily of a resin mixture having a degree of neutralization of at least 70% and including: 100 parts by weight of a resin component composed of, in admixture, a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio of from 100:0 to 0:100, and (e) a non-ionomeric thermoplastic elastomer in a weight ratio of from 100:0 to 50:50; (c) about 50 to about 150 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and (d) about 0.1 to about 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c), and the cover has a Shore D hardness of from 58 to 65 and is formed by injection molding a single resin blend composed primarily of (f) a thermoplastic polyurethane and (g) a polyisocyanate compound in at least some portion of which all the isocyanate groups on the molecule are present in an unreacted state.

2. The golf ball of claim 1, wherein the hardness of the cover is equal to or greater than the hardness of the intermediate layer.

3. The golf ball of claim 1, wherein the resin blend which forms the cover includes (h) a thermoplastic elastomer other than a thermoplastic polyurethane.

4. The golf ball of claim 3, wherein some portion of the isocyanate groups in the polyisocyanate compound (g) included in the resin blend prior to injection molding form bonds with active hydrogens in component (f) and/or component (h), and the remaining isocyanate groups are present within the resin blend in an unreacted state.

5. The golf ball of claim 3, wherein the components in the resin blend which forms the cover have a compositional ratio, expressed as a weight ratio, of (f):(g):(h)=100:2 to 50:0 to 50.

6. The golf ball of claim 3, wherein the components in the resin blend which forms the cover have a compositional ratio, expressed as a weight ratio, of (f):(g):(h)=100:2 to 30:8 to 50.

7. The golf ball of claim 1, wherein the resin blend which forms the cover has a melt mass flow rate (MFR) value at 210.degree. C. of at least 5 g/10 min.

8. The golf ball of claim 1, wherein the polyisocyanate compound of component (g) is one or more polyisocyanate compound selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4- (or) 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate.

9. The golf ball of claim 1, wherein the polyisocyanate compound of component (g) is one or more polyisocyanate compound selected from the group consisting of 4,4'-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate.

10. The golf ball of claim 3, wherein the thermoplastic elastomer of component (h) is one or more thermoplastic elastomer selected from the group consisting of polyester elastomers, polyamide elastomers, ionomer resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins.

11. The golf ball of claim 3, wherein the thermoplastic elastomer of component (h) is one or more selected from the group consisting of polyester elastomers, polyamide elastomers and polyacetals.

12. The golf ball of claim 1, wherein the ball has a plurality of dimples formed on a surface thereof, which dimples number in all from 250 to 392 and have a total volume of from 400 to 750 mm.sup.3.