Golf Ball

Abstract

The invention provides a golf ball which includes a core and a cover encasing the core, wherein the cover has a thickness of at least 0.1 mm but less than 1.3 mm, the core has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 4.1 mm, the ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 3.8 mm, and the ratio of the core deflection to the ball deflection (core/ball) is at least 0.8 but not more than 1.2. The ball of the invention can be sufficiently deformed on shots with a driver, even when played by a low head speed golfer, allowing a good ball rebound to be obtained and thus resulting in an increased distance. The ball also has a good feel when played and an excellent controllability on approach shots.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a golf ball for women golfers and junior golfers which has a soft feel on impact and is able to achieve an excellent flight performance even when struck at a low head speed.

[0002] To address the needs of professional golfers and skilled amateurs, many golf balls have hitherto been developed which, when hit at a high head speed, have an excellent flight performance and spin properties and also provide a good feel on impact. However, because such balls are generally designed to achieve an optimal deformation when struck at a high head speed, a sufficient deformation often cannot be imparted to the ball when played by women golfers and junior golfers having a low head speed. Hence, low head-speed golfers have been unable to make the most of the performance inherent to such balls, failing to achieve a sufficient distance, and have also experienced a poor feel on impact.

[0003] Consequently, developing a golf ball which allows even a low head-speed golfer to impart sufficient deformation to the ball and which enables an excellent flight performance and a good feel to be obtained is important for expanding the golfer base.

[0004] Prior-art relating to this invention includes JP 2822926 (U.S. Pat. No. 5,695,413), U.S. Pat. No. 6,142,886, JP 2576787 (U.S. Pat. No. 5,452,898), U.S. Pat. No. 5,743,817 and U.S. Pat. No. 5,813,924.

SUMMARY OF THE INVENTION

[0005] It is therefore an object of the present invention to provide a golf ball which has a soft feel and enables an excellent flight performance to be obtained even when hit by a low head-speed golfer.

[0006] As a result of extensive investigations, the inventor has discovered that, by forming a soft, very thin cover around a soft core which has been set to a large deflection, it is possible to confer the ball with a good feel and excellent controllability on approach shots while at the same time enabling sufficient deformation to be imparted to the ball on shots with a driver, even when struck under low head speed conditions. This allows a good ball rebound to be obtained and also suppresses an excessive spin rate, resulting in an increased distance.

[0007] Accordingly, the invention provides the following golf balls.

[1] A golf ball comprising a core and a cover encasing the core, wherein the cover has a thickness of at least 0.1 mm but less than 1.3 mm, the core has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 4.1 mm, the ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 3.8 mm, and the ratio of the core deflection to the ball deflection (core/ball) is at least 0.8 but not more than 1.2. [2] The golf ball of claim 1, wherein the cover has a material hardness, expressed as the Shore D hardness, of not more than 58. [3] The golf ball of claim 1, wherein the cover is formed of a resin composition which is composed primarily of an ionomer and which has a melt flow rate of at least 4. [4] The golf ball of claim 3, wherein an ethylene-methacrylic acid-methacrylic acid ester copolymer resin accounts for between 10 and 80 wt % of the polymer components included in the ionomer composition. [5] The golf ball of claim 1, wherein the cover is formed of a resin composition which is composed primarily of polyurethane and which has a melt flow rate of at least 16.

BRIEF DESCRIPTION OF THE DIAGRAM

[0008] FIG. 1 is a schematic cross-sectional view of a golf ball according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The golf ball is described more fully below.

[0010] Referring to FIG. 1, the golf ball of the invention has a construction which is exemplified by a two-piece solid golf ball G composed of a core 1 and a cover 2 encasing the core. The cover 2 typically has a surface on which numerous dimples D are formed. Here, the core 1 and the cover 2 may each be composed of a single layer, although it is also possible for the core to be given a multilayer construction of from 2 to 6 layers, and for the cover to be composed of a plurality of two or more layers.

[0011] The core may be obtained by vulcanizing a rubber composition composed primarily of a rubber material. Specifically, use may be made of a rubber composition containing, for example, a base rubber, a crosslinking initiator, a co-crosslinking agent, an antioxidant and a filler.

[0012] The base rubber of the rubber composition is not subject to any particular limitation, although it is preferable to use polybutadiene. Preferred use may be made of cis-1,4-polybutadiene having a cis structure of at least 40% as the polybutadiene. In addition, natural rubber, polyisoprene rubber, styrene butadiene rubber and the like may be suitably included, where desired, in this base rubber. The rebound of the golf ball can be enhanced by increasing the amount of rubber components.

[0013] In the invention, preferred use may be made of an organic peroxide as the crosslinking initiator. Preferred examples of organic peroxides that may be used include dicumyl peroxide and 1,1-bis(tert-butylperoxy)cyclohexane. A commercial product may be used as the organic peroxide. Illustrative examples include Perhexa 3M, Perhexa C40, Percumyl D (NOF Corporation), Luperco 231XL and Luperco 101XL (Atochem Co.). These may be used singly or as mixtures of two or more thereof.

[0014] The amount of crosslinking initiator included, although not subject to any particular limitation, is preferably at least 0.2 part by weight, more preferably at least 0.4 part by weight, and even more preferably at least 0.6 part by weight, per 100 parts by weight of the base rubber. The upper limit in the amount included, although not subject to any particular limitation, may be set to preferably not more than 2.0 parts by weight, more preferably not more than 1.5 parts by weight, even more preferably not more than 1.2 parts by weight, and most preferably not more than 0.9 part by weight. If the amount included is too high, this may cause scorching during rubber compounding and hot molding, in which case a golf ball core and golf ball of sufficient durability will not be obtained. If too little is included, the workability when hot molding the rubber composition may decrease, as a result of which a hardness sufficient for the golf ball core may not be achieved. Although not subject to any particular limitation, in this invention, sulfur may also be included in the above rubber composition. The sulfur is exemplified by powdered sulfur, illustrative examples of which include the product available under the trade name Sulfur Z (dispersible sulfur) from Tsurumi Chemical Industry Co., Ltd. The amount of sulfur included per 100 parts by weight of the above base rubber is typically from 0.01 to 0.5 part by weight, preferably from 0.01 to 0.4 part by weight, and more preferably from 0.01 to 0.1 part by weight. If the amount of sulfur included is too low, the hardness distribution of the solid core may be limited in the degree to which it can be increased, as a result of which the rebound resilience may become lower, resulting in a shorter distance. On the other hand, if the amount of sulfur included is too high, undesirable effects such as explosion of the rubber composition may arise during hot molding.

[0015] .alpha.,.beta.-Unsaturated carboxylic acids such as zinc methacrylate and zinc acrylate may be included as the co-crosslinking agent in the present invention. The use of zinc acrylate is especially preferred. The amount of co-crosslinking agent included is not subject to any particular limitation, but may be set to at least 10 parts by weight, and preferably at least 15 parts by weight, per 100 parts by weight of the base rubber. The upper limit in the amount of co-crosslinking agent is not subject to any particular limitation, although it is recommended that it be set to not more than 50 parts by weight, and preferably not more than 39 parts by weight. If too much co-crosslinking agent is included, the core will become too hard, as a result of which the ball too will be hard. For this reason, low head speed golfers may be unable to impart sufficient deformation to the ball at the time of impact, and so the ball may not achieve a good distance. On the other hand, if too little co-crosslinking agent is included, sufficient durability may not be obtained.

[0016] In the invention, an antioxidant may be included in the rubber composition. For example, a commercial product such as Nocrac NS-6, Nocrac NS-30 or Nocrac SP-N (all available from Ouchi Shinko Chemical Industry Co., Ltd.) may be used. These may be used singly or as combinations or two or more thereof.

[0017] The amount of antioxidant included per 100 parts by weight of the base rubber, although not subject to any particular limitation, is preferably at least 0.02 part by weight, and more preferably at least 0.05 part by weight. The upper limit is preferably not more than 1 part by weight, more preferably not more than 0.8 part by weight, and even more preferably not more than 0.6 part by weight. If the amount included is too high, the golf ball may not be able to achieve a sufficient initial velocity. On the other hand, if the amount is too low, this may cause scorching during rubber compounding and hot molding, in which case a golf ball core and golf ball having sufficient durability may not be attainable.

[0018] The filler is not subject to any particular limitation. Illustrative examples of fillers that may be included are zinc oxide, barium sulfate, titanium dioxide and calcium carbonate. The amount-in which these are included is not subject to any particular limitation, although the amount may be set to at least 3 parts by weight, and preferably at least 5 parts by weight, per 100 parts by weight of the base rubber. The upper limit in the amount of filler included is not subject to any particular limitation, although it is recommended that this be set to not more than 60 parts by weight, and preferably not more than 50 parts by weight.

[0019] The core can be produced by using a known method to vulcanize the rubber composition containing the various above ingredients. For example, the various ingredients may be kneaded together using a mixing apparatus such as a Banbury mixer or a roll mill so as give a rubber composition, and the composition may then be compression-molded or injection-molded using a core mold to give a spherical molding. The molding is then suitably heated under conditions sufficient for the crosslinking initiator and co-crosslinking agent to act and is thereby cured, giving a core having a given hardness. In cases where, for example, dicumyl peroxide is used as the crosslinking initiator and zinc acrylate is used as the co-crosslinking agent, the heating conditions for the molding may be set to generally about 130 to 170.degree. C., and especially 150 to 160.degree. C., and to 10 to 40 minutes, and especially 12 to 20 minutes.

[0020] The core diameter, although not subject to any particular limitation, is preferably at least 24 mm, more preferably at least 26 mm, and even more preferably at least 28 mm. The upper limit is preferably not more than 42 mm, and more preferably not more than 41.5 mm. At a core diameter outside of this range, a sufficient rebound performance may not be achieved, excess spin may arise, or a good feel may not be obtained.

[0021] It is critical for the core to have a deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of at least 4.1 mm, preferably at least 4.2 mm, and more preferably at least 4.3 mm. The upper limit in the deflection, although not subject to any particular limitation, is preferably not more than 8.0 mm, more preferably not more than 7.0 mm, even more preferably not more than 6.0 mm, and most preferably not more than 5.5 mm. If the core is softer than the above value, the core rebound may be poor. On the other hand, if the core is harder than the above value, the ball may have a poor feel.

[0022] In the invention, the cover formed over the core may be made of a known material. Although not subject to any particular limitation, exemplary cover materials include thermoplastic resins such as ionomers, and various types of thermoplastic elastomers. Specific examples of thermoplastic elastomers include polyester-type, thermoplastic elastomers, polyamide-type thermoplastic elastomers, polyurethane-type thermoplastic elastomers, olefin-type thermoplastic elastomers, and styrene-type thermoplastic elastomers.

[0023] In the invention, such a cover material is not subject to any particular limitation, although preferred use may be made of the following compositions composed primarily of an ionomer or a polyurethane. Each of these materials is described in turn below.

[0024] First, the resin composition composed primarily of an ionomer is, in this invention, a resin composition composed primarily of a metal salt of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymer and/or a metal salt of an olefin-unsaturated carboxylic acid random copolymer.

[0025] The olefin is exemplified by olefins in which the number of carbons is at least 2, but not more than 8, and preferably not more than 6. Illustrative examples of such olefins include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

[0026] Illustrative examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. In the invention, of these, acrylic acid and methacrylic acid are preferred, and methacrylic acid is especially preferred.

[0027] The unsaturated carboxylic acid ester included is preferably a lower alkyl ester of the above-mentioned unsaturated carboxylic acid. Illustrative examples 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.

[0028] The random copolymer may be obtained by random copolymerization of the above ingredients in accordance with a known method. Here, the content of unsaturated carboxylic acid (acid content) included in the random copolymer, although not subject to any particular limitation, is generally at least 2 wt %, 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 generally not more than 25 wt %, preferably not more than 20 wt %, and more preferably not more than 15 wt %. At a low acid content, the rebound may decrease, whereas at a high acid content, the processability of the material may decrease.

[0029] The acid groups in the above random copolymers are partially or completely neutralized with metal ions. In this case, although the degree of neutralization is not subject to any particular limitation, it is preferable for at least 20 mol % of the acid groups to be neutralized; this proportion may be set to more preferably at least 30 mol %, and even more preferably at least 40 mol %. The upper limit also is not subject to any particular limitation, although it may be set to not more than 100 mol %, preferably not more than 90 mol %, and even more preferably not more than 80 mol %. At a degree of neutralization lower than 20 mol %, the rebound may become too low. Here, the metal ions which neutralize the acid groups are exemplified by Na.sup.+, K.sup.+, Li.sup.+, Zn.sup.++, Cu.sup.++, Mg.sup.++, Ca.sup.++, Co.sup.++, Ni.sup.++ and Pb.sup.++. In the invention, of these, Na.sup.+, Li.sup.+, Zn.sup.++, Mg.sup.++ and Ca.sup.++ are especially preferred.

[0030] The content of the above metal salt of a random copolymer (ionomer resin) is not subject to any particular limitation, although it is preferable for between 100 and 20 wt %, based on the overall resin composition, to be included. In this case, the lower limit is more preferably at least 40 wt %, and even more preferably at least 50 wt %. The upper limit is more preferably not more than 90 wt %, and even more preferably not more than 85 wt %.

[0031] Although not subject to any particular limitation, from the standpoint of providing good flow properties and processability during injection molding, it is preferable for the proportion of ethylene-methacrylic acid-methacrylic acid ester copolymer included in the ionomer composition as a polymer component other than the ionomer to be from 10 to 80 wt %. An illustrative example is that available under the trade name Nucrel AN4319 (DuPont-Mitsui Polychemicals Co., Ltd.). The lower limit value of this ratio is preferably at least 15 wt %, and more preferably at least 20 wt %. The upper limit value is preferably not more than 70 wt %, more preferably not more than 60 wt %, and even more preferably not more than 50 wt %. If the flow properties are too low, forming a thin cover over the core by an injection-molding process may be difficult, as a result of which production may be inefficient.

[0032] In the invention, a known product may be used as the above ionomer. Illustrative examples include the products available under the trade names Himilan 1557 and Himilan 1601 (DuPont-Mitsui Polychemicals Co., Ltd.), the products available under the trade names HPF1000 and HPF2000 (available from E.I. DuPont de Nemours and Co.), and the resin compositions mentioned in U.S. patent application Ser. No. 12/340,790 (or U.S. patent application Ser. No. 12/706,175). These may be used singly or as mixtures of two or more thereof.

[0033] Various additives may be optionally included in the above resin composition. For example, additives such as pigments, dispersants, antioxidants, UV absorbers and optical stabilizers may be suitably included.

[0034] The above resin composition composed primarily of an ionomer has a melt flow rate (MFR) which is not subject to any particular limitation. However, to provide good flow properties and processability during injection molding, the melt flow rate, as measured at a test temperature of 190.degree. C. and under a test load of 21.18 N (2.16 kgf) in general accordance with JIS K-7210, is preferably at least 4 g/10 min, more preferably at least 5 g/10 min, and even more preferably at least 6 g/10 min. The upper limit also is not subject to any particular limitation, but is recommended to be not more than 14 g/10 min, and more preferably not more than 13 g/10 min. If the flow properties are too low, forming a thin cover over the core by an injection-molding process may be difficult, as a result of which production may be inefficient.

[0035] Next, in cases where the cover is formed using a resin composition composed primarily of a polyurethane, a thermoplastic polyurethane elastomer or a thermoset polyurethane resin may be used in the invention. The use of a thermoplastic polyurethane elastomer is especially preferred.

[0036] The thermoplastic polyurethane elastomer has a structure composed of soft segments made of a polymeric polyol (polymeric glycol) and hard segments made of a chain extender and a diisocyanate. Here, the polymeric polyol serving as a starting material may be any which has hitherto been used in the art relating to thermoplastic polyurethane materials, and is not subject to any particular limitation. Exemplary polymeric polyols include polyester polyols and polyether polyols. Polyether polyols are more preferable than polyester polyols because thermoplastic polyurethane materials having a high rebound resilience and excellent low-temperature properties can be synthesized. Illustrative examples of polyether polyols include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is especially preferred from the standpoint of the rebound resilience and the low-temperature properties. The polymeric polyol has an average molecular weight of preferably from 1,000 to 5,000. To synthesize a thermoplastic polyurethane material having a high rebound resilience, an average molecular weight of from 2,000 to 4,000 is especially preferred.

[0037] The chain extender employed is preferably one which has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative examples include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. These chain extenders have an average molecular weight of preferably from 20 to 15,000.

[0038] The diisocyanate employed is preferably one which has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative examples include, but are not limited to, aromatic diisocyanates such as 4,4'-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, and aliphatic diisocyanates such as hexamethylene diisocyanate. Depending on the type of isocyanate, control of the crosslinking reaction during injection molding may be difficult. In this invention, the use of 4,4'-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferred.

[0039] A commercial product may be advantageously used as the thermoplastic polyurethane material composed of the above materials. Illustrative examples include those available under the trade names Pandex T8180, Pandex 18195, Pandex T8290, Pandex T8295 and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.), and those available under the trade names Resamine 2593 and Resamine 2597 (available from Dainichiseika Color & Chemicals Mfg. Co., Ltd.).

[0040] Various additives may be optionally included in the above thermoplastic polyurethane material. For example, additives such as pigments, dispersants, antioxidants, UV absorbers and optical stabilizers may be suitably included.

[0041] In the invention, the resin composition composed primarily of a polyurethane has a melt flow rate which is not subject to any particular limitation. However, to provide good flow properties and processability during injection molding, the melt flow rate, as measured at a test temperature of 210.degree. C. and under a test load of 21.18 N (2.16 kgf) in general accordance with JIS K-7210, is preferably at least 16 g/10 min, more preferably at least 17 g/10 min, and even more preferably at least 18 g/10 min. The upper limit also is not subject to any particular limitation, but is recommended to be not more than 40 g/10 min, and more preferably not more than 35 g/10 min. If the flow properties are too low, forming a thin cover over the core by an injection-molding process may be difficult, as a result of which production may be inefficient.

[0042] It is critical for the cover thickness to be set to at least 0.1 mm, preferably at least 0.3 mm, and more preferably at least 0.6 mm. It is also critical for the upper limit to be set to less than 1.3 mm, and preferably not more than 1.1 mm. At a cover thickness outside of the above range, a good feel on impact may not be obtained. Also, a sufficient distance on shots with a driver and a good controllability on approach shots may not be obtained.

[0043] The cover has a material hardness which, although not subject to any particular limitation, when expressed as the Shore D hardness, may be set to preferably not more than 58, more preferably not more than 57, and even more preferably not more than 56. The lower limit is not subject to any particular limitation, but may be set to a JIS-A hardness of preferably at least 30, more preferably at least 35, even more preferably at least 40, yet more preferably at least 45, and most preferably a Shore D hardness of at least 50. If the material hardness of the cover is too high, the cover will have difficulty following the core deformation when the ball is struck, which may lead to a worsening in the durability of the ball to repeated impact. On the other hand, in cases where the material hardness of the core is too low, the rebound will often be inadequate. Here, with regard to the above material hardnesses, the Shore D hardness is the hardness measured using a Type D durometer in general accordance with ASTM D2240 for a sheet of the cover-forming material that has been molded under pressure to a thickness of about 2 mm; and the JIS-A hardness is the hardness measured in general accordance with JIS K-6301 for a similar sheet.

[0044] The cover may be formed by a known method, such as injection-molding or compression-molding. For example, when the cover is formed by injection molding, a solid core fabricated beforehand using the above-described rubber composition may be set inside a core-forming mold, and the above-described cover material may be injected into the mold by an ordinary method. Alternatively, another method that may be used involves using the above-described cover material to pre-mold a pair of half-cups, enclosing the above core in the half-cups, and compression-molding at, for example, between 120 and 170.degree. C. for a period of 1 to 5 minutes.

[0045] It is critical for the golf ball of the invention to have a deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of at least 3.8 mm. The deflection, is preferably at least 3.9 mm, and more preferably at least 4.0 mm. The upper limit in this deflection, although not subject to any particular limitation, is preferably not more than 8.0 mm, more preferably not more than 7.0 mm, even more preferably not more than 6.0 mm, yet more preferably not more than 5.0 mm, and most preferably not more than 4.7 mm. If the ball is too soft, a suitable launch angle may not be obtained. On the other hand, if the ball is too hard, the ball may not fly well when struck by a low head-speed golfer, and the feel on impact may be poor.

[0046] The ratio of the core deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to the ball deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) (core/ball) is at least 0.8, preferably at least 0.82, more preferably at least 0.84, and even more preferably at least 0.86. The upper limit is not more than 1.2, preferably not more than 1.19, and more preferably not more than 1.18. If this ratio is outside the above range, the cover thickness and hardness will be inappropriate for the core deflection, and a good feel will not be obtained at the time of impact.

[0047] In the golf ball of the invention, although not subject to any particular limitation, to further improve the aerodynamic properties and increase the distance of the ball, as in conventional golf balls, a plurality of dimples may be formed on the surface of the cover. By optimizing the types and total number of such dimples, the trajectory can be made more stable, enabling a ball having an excellent flight performance to be obtained. In addition, to enhance the design and durability of the golf ball, the cover surface may be painted, at which time various treatments such as surface preparation and stamping may be optionally carried out.

[0048] The preferred form of the dimples in the golf ball of the invention is described in detail below.

[0049] The number of dimples on the entire surface of the golf ball is preferably at least about 200, more preferably at least about 220, even more preferably at least about 260, and yet more preferably at least about 300. The upper limit in the number of dimples is preferably not more than about 450, more preferably not more than about 430, and even more preferably not more than about 410. By setting the number of dimples within this range, the golf ball readily incurs lift, enabling the distance to be increased, particularly on shots with a driver.

[0050] A plurality of types of dimples of differing diameter and/or depth may be formed. The number of dimple types is preferably at least 4, more preferably at least 5, and even more preferably at least 6. The upper limit in the number of dimple types is preferably not more than about 20, more preferably not more than about 15, and even more preferably not more than about 12. By setting the number of dimple types in this range, the dimple surface coverage can be easily increased and the distance traveled by the ball enhanced.

[0051] The shape of the dimples is preferably circular as seen from above. The average diameter is preferably at least about 2.8 mm, more preferably at least about 3.5 mm, and even more preferably at least about 3.8 mm. The upper limit in the average diameter is preferably not more than about 5.0 mm, more preferably not more than about 4.6 mm, and even more preferably not more than about 4.3 mm. The average dimple depth, from the standpoint of obtaining a suitable trajectory, is preferably at least about 0.120 mm, more preferably at least about 0.130 mm, and even more preferably at least about 0.140 mm. The upper limit in the average depth is preferably not more than about 0.185 mm, more preferably not more than about 0.180 mm, and even more preferably not more than about 0.174 mm.

[0052] As used herein, "average diameter" refers to the average of the diameters of all the dimples, and "average depth" refers to the average of the depths of all the dimples. In most cases, the golf ball is painted, with measurement of the dimple diameters and depths being carried out on the dimples after the coat of paint has been applied.

Measurement of a dimple diameter is carried out by measuring the span across the dimple between points where land areas (non-dimple forming portions) of the golf ball surface are tangent with the recessed surface of the dimple. Measurement of the dimple depth is carried out by connecting points where the dimple is tangent with land areas so as to trace an imaginary circle in a flat plane, and measuring the vertical distance from the center of the circle to the bottom of the dimple.

[0053] To fully manifest the aerodynamic properties, the dimple coverage or, specifically, the ratio (SR value) of the sum of the individual dimple surface areas, each defined by the flat plane circumscribed by the edge of the dimple, to the surface area of an imaginary sphere were the ball surface assumed to have no dimples thereon, is preferably at least about 60%, more preferably at least about 65%, and even more preferably at least about 68%. In the invention, the upper limit in the dimple coverage, although not subject to any particular limitation, is preferably not more than about 90%, more preferably not more than about 85%, and even more preferably not more than about 80%. From the standpoint of optimizing the ball trajectory, the value V.sub.0 obtained by dividing the spatial volume of the individual dimples below the flat plane circumscribed by the edge of each particular dimple by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from that base is preferably at least about 0.35. The upper limit of V.sub.0, although not subject to any particular limitation, is preferably not more than about 0.80. In addition, the VR value, defined as the sum of the volumes of the dimples formed below the flat plane circumscribed by the edge of each dimple as a proportion of the volume of the imaginary sphere were the ball to have no dimples thereon, is preferably at least about 0.6%, more preferably at least about 0.7%, and even more preferably at least about 0.8%. The upper limit in the VR value is preferably not more than about 1.0%, and more preferably not more than about 0.9%.

[0054] The golf ball of the invention may be manufactured so as to conform with the Rules of Golf for competitive play, with the ball preferably being set to a diameter of not less than 42.67 mm and a weight of not more than 45.93 g.

[0055] As described above, the golf ball of the invention can be sufficiently deformed on shots with a driver, even when played by a woman golfer or junior golfer having a low head speed, allowing a good ball rebound to be obtained, and thus enabling an increased distance to be achieved. The ball also has a good feel when played and an excellent controllability on approach shots.

EXAMPLES

[0056] Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 10, Comparative Examples 1 to 7

[0057] Rubber compositions were formulated as shown in Table 1 (Examples of the invention) and Table 2 (Comparative Examples) below, then molded and vulcanized at 155.degree. C. for 10 minutes to produce solid cores. Next, covers formulated as shown in Table 1 or Table 2 were injection-molded over the cores obtained above, thereby giving two-piece solid golf balls for each Example of the invention and each Comparative Example.

TABLE-US-00001 TABLE 1 Example 1 2 3 4 5 6 7 8 9 10 Core Formulation Zinc oxide 14.5 16.8 18.6 18.0 15.9 15.9 15.9 15.2 16.1 18.3 (pbw) cis-1,4-Polybutadiene 100 100 100 100 100 100 100 100 100 100 Dicumyl peroxide 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 1,1-Bis(tert-butylperoxy)- 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 cyclohexane 2,2'-Methylenebis(4-methyl- 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 6-t-butylphenol ) Zinc acrylate 26 23.5 20 19 25 23 25.5 26 24 18 Zinc stearate 5 5 5 5 5 5 5 5 Titanium dioxide 2 2 2 2 2 2 2 2 2 2 Zinc salt of 1.00 0.60 0.20 0.15 1.00 0.80 0.60 1.00 1.00 0.40 pentachlorothiophenol Diameter (mm) 41.1 40.7 40.7 41.1 40.7 41.1 40.7 40.7 40.7 41.1 Weight (g) 40.5 39.6 39.6 40.5 39.6 40.5 39.6 39.4 39.4 40.5 Deflection (mm) 4.1 4.1 4.2 4.3 4.3 4.5 4.5 4.1 4.2 5.1 Cover Formulation Himilan 1557 27.5 27.5 32.5 37.5 37.5 42.5 42.5 37.5 (pbw) Himilan 1601 27.5 27.5 32.5 37.5 37.5 42.5 42.5 37.5 Nucrel AN4319 45 45 35 25 25 15 15 25 Pandex T8260 55 70 Pandex T8195 45 30 Titanium dioxide 3.5 3.5 Polyethylene wax 1.5 1.5 MFR (g/10 min), 190.degree. C. 9.6 9.6 8.1 6.7 6.7 5.2 5.2 6.7 MFR (g/10 min), 210.degree. C. 18.7 18.2 Thickness (mm) 0.8 1.0 1.0 0.8 1.0 0.8 1.0 1.0 1.0 0.8 Material hardness (Shore D) 52 52 54 56 56 58 58 52 54 56 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 45.3 45.3 45.3 45.3 45.3 45.3 Deflection (mm) 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.6 Deflection ratio (core/ball) 1.03 1.03 1.04 1.08 1.08 1.11 1.11 1.03 1.05 1.11 Distance on shots with driver 1.5 1.5 1.5 2.5 2.0 3.3 3.0 1.5 1.5 2.9 (difference in m) Spin rate on shots with driver -30 -30 -30 -50 -40 -65 -60 -30 -30 -60 (difference in rpm) Spin rate on approach shots 115 100 60 55 45 20 0 120 100 45 (difference in rpm) Feel on shots with a driver Exc Exc Exc Exc Exc Exc Exc Exc Exc Exc

TABLE-US-00002 TABLE 2 Comparative Example 1 2 3 4 5 6 7 Core Formulation Zinc oxide 19.3 20.8 19.9 22.8 19.3 20.5 22.6 (pbw) cis-1,4-Polybutadiene 100 100 100 100 100 100 100 Dicumyl peroxide 0.6 0.6 0.6 0.6 0.6 0.6 0.6 1,1-Bis(tert-butylperoxy)- 0.6 0.6 0.6 0.6 0.6 0.6 0.6 cyclohexane 2,2'-Methylenebis(4-methyl- 0.1 0.1 0.1 0.1 0.1 0.1 0.1 6-t-butylphenol) Zinc acrylate 23.5 24 22 19 23 24.5 19.5 Zinc stearate 5 5 5 5 5 5 5 Titanium dioxide 2 2 2 2 2 2 2 Zinc salt of 0.15 0.05 0.10 0.10 0.40 0.25 0.20 pentachlorothiophenol Diameter (mm) 40.0 39.3 40.0 39.3 40.0 39.3 39.3 Weight (g) 38.7 37.0 38.7 37.0 38.7 37.0 37.0 Deflection (mm) 3.9 4.0 4.2 4.2 4.5 4.1 4.4 Cover Formulation Himilan 1557 27.5 27.5 37.5 37.5 50 50 50 (pbw) Himilan 1601 27.5 27.5 37.5 37.5 50 50 50 Nucrel AN4319 45 45 25 25 Pandex T8260 Pandex T8195 Titanium dioxide Polyethylene wax MFR (g/10 min), 190.degree. C. 9.6 9.6 6.7 6.7 3.7 3.7 3.7 MFR (g/10 min), 210.degree. C. Thickness (mm) 1.4 1.7 1.4 1.7 1.4 1.7 1.7 Material hardness (Shore D) 52 52 56 56 60 60 60 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 45.3 45.3 45.3 Deflection (mm) 3.8 3.8 3.8 3.8 3.8 3.8 3.6 Deflection ratio (core/ball) 1.03 1.05 1.11 1.09 1.18 1.08 1.21 Distance on shots with driver 0.0 1.0 1.3 1.3 2.5 2.0 -1.0 (difference in m) Spin rate on shots with driver 0 -20 -25 -25 -50 -40 20 (difference in rpm) Spin rate on approach shots 35 -45 -5 -35 -80 -105 45 (difference in rpm) Feel on shots with a driver good good good good good good NG

[0058] Details of the above core formulations are provided below. [0059] Zinc oxide: Available from Sakai Chemical Co., Ltd. [0060] Polybutadiene: Available under the trade name "BR730" from JSR Corporation [0061] Dicumyl peroxide: Available under the trade name "Percumyl D" from NOF Corporation [0062] 1,1-Bis(tert-butylperoxy)cyclohexane, 40% dilution: Available under the trade name "Perhexa C40" from NOF Corporation [0063] 2,2'-Methylenebis(4-methyl-6-t-butylphenol): [0064] Available under the trade name "Nocrac NS-6" from Ouchi Shinko Chemical Industry Co., Ltd. [0065] Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. [0066] Zinc stearate: Available from NOF Corporation [0067] Titanium dioxide: Available under the trade name "Tipaque R550" from Ishihara Sangyo Kaisha, Ltd. [0068] Zinc salt of pentachlorothiophenol: [0069] Available from Tokyo Chemical Industry Co., Ltd.

[0070] Details of the above cover formulations are provided below. [0071] Himilan 1557: A zinc ionomer of a binary copolymer, available from DuPont-Mitsui Polychemicals Co., Ltd. [0072] Himilan 1601: A sodium ionomer of a binary copolymer, available from DuPont-Mitsui Polychemicals Co., Ltd. [0073] Nucrel AN4319: A terpolymer available from DuPont-Mitsui Polychemicals Co., Ltd. [0074] Titanium dioxide: Available under the trade name "Tipaque R550" from Ishihara Sangyo Kaisha, Ltd. [0075] Pandex: An MDI-PTMG type thermoplastic polyurethane available from DIC Bayer Polymer [0076] Polyethylene wax: Available under the trade name "Sanwax 161P" from Sanyo Chemical Industries, Ltd.

[0077] The following physical properties of the resulting golf balls were investigated. In addition, flight tests were carried out by the method described above, and the controllability (spin rate on approach shots) and feel on impact were also evaluated. Those results are shown in Tables 1 and 2.

Material Hardness of Cover

[0078] The Shore D hardness, measured in general accordance with ASTM D-2240, of a test specimen obtained by melting the cover material (resin composition) under applied heat and molding the material into a 2 mm-thick sheet.

Core and Ball Deflections (mm)

[0079] The deflection (mm) when the test object was compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

Flight Performance

[0080] The distance traveled by the ball when struck at a head speed of 35 m/s with a driver mounted on a golf swing robot was measured. The club was a TourStage ViQ HT 460 driver (loft, 9.5.degree.) manufactured by Bridgestone Sports Co., Ltd. The differences in the measured values relative to a reference value of "0" for Comparative Example 1 are shown in the tables.

Spin Rate on Shots with a Driver (rpm)

[0081] The spin rate was measured under the same conditions as mentioned above. The differences in the measured values relative to a reference value of "0" for Comparative Example 1 are shown in the tables.

Spin Rate on Approach Shots (rpm)

[0082] The distance traveled by the ball when struck at a head speed of 24 m/s with an iron mounted on a golf swing robot was measured. The club used was a TourStage X-WEDGE TW-03 driver (loft, 57.degree.) manufactured by Bridgestone Sports Co., Ltd. The differences in the measured values relative to a reference value of "0" for Example 7 are shown in the tables.

Feel on Shots with a Driver

[0083] Five golfers having a head speed of about 35 m/s hit the test ball and rated the feel at that time according to the following criteria. The club used was a TourStage ViQ HT 460 driver (loft, 9.5.degree.) manufactured by Bridgestone Sports Co., Ltd.

[0084] Exc: excellent feel

[0085] Good: good feel

[0086] NG: poor feel

[0087] The balls in Comparative Examples 1 and 7 had a distance on shots with a driver which was poor compared with the other examples. The balls in Comparative Examples 2, 3, 4, 5 and 6 had a low spin rate on approach shots, and thus a poor controllability, compared with the other examples. By contrast, the balls in Examples 1 to 10 of the invention had both an excellent distance on shots with a driver and an excellent performance on approach shots.

Claims

1. A golf ball comprising a core and a cover encasing the core, wherein the cover has a thickness of at least 0.1 mm but less than 1.3 mm, the core has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 4.1 mm, the ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 3.8 mm, and the ratio of the core deflection to the ball deflection (core/ball) is at least 0.8 but not more than 1.2.

2. The golf ball of claim 1, wherein the cover has a material hardness, expressed as the Shore D hardness, of not more than 58.

3. The golf ball of claim 1, wherein the cover is formed of a resin composition which is composed primarily of an ionomer and which has a melt flow rate of at least 4.

4. The golf ball of claim 3, wherein an ethylene-methacrylic acid-methacrylic acid ester copolymer resin accounts for between 10 and 80 wt % of the polymer components included in the ionomer composition.

5. The golf ball of claim 1, wherein the cover is formed of a resin composition which is composed primarily of polyurethane and which has a melt flow rate of at least 16.