Golf ball

Abstract

A golf ball is provided which exhibits good rebound performance, long flight distance and favorable anti-flier characteristics and is excellent in hit feeling and controllability. The golf ball is composed of a core and a cover surrounding the core. The cover is formed by vulcanizing a cover composition including thermoplastic elastomer, trans structure-containing rubber and ionomer resin.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to golf balls superior in spin performance, and more particularly to a golf ball having high impact resilience and unlikely to cause a flier shot even when hit from the rough.

[0003] 2. Description of the Background Art

[0004] Conventionally, two types of golf balls have often been used for the round in the golf course. One is a solid golf ball, which has a solid core formed of a vulcanized and molded rubber composition with polybutadiene as its main material that is surrounded by a cover. The other is a thread-wound golf ball, which has a thread-wound core formed by winding a rubber thread around a rubber type or liquid type center that is enveloped by a cover. The balls often sink into high grass or the rough during the play, regardless of the types of the balls.

[0005] A shot from the rough would cause a flier, i.e., a phenomenon that the grass jammed between the club and the ball causes backspin rate of the ball to decrease and as a result the ball rises too high. This often degrades controllability of the ball especially when hitting an approach shot. This phenomenon occurs whether the solid golf ball or the thread-wound golf ball is used, which is a source of trouble to a player. Particularly in the case of a golf ball having a cover made of ionomer resin, although it exhibits a relatively high modulus of elasticity and favorable rebound performance, it is inferior in controllability as it is difficult to apply spin to the ball, resulting in poor controllability of the approach shot.

[0006] Balata, or trans 1,4-polyisoprene (TPI), is known as a material that is unlikely to pose the above-described problems. A golf ball with the balata cover is excellent in hit feeling and controllability. This is considered because it has both the crystalline portion of polymer and the crosslinked rubber portion, giving appropriate hardness and high spin performance to the golf ball. Such excellent hit feeling and controllability make the golf ball with the balata cover favored by skilled players.

[0007] If the balata is subjected to crosslinking at a temperature higher than the melting point of trans 1,4-polyisoprene (50-60° C. according to differential scanning calorimetry or DSC), the crystalline portion will not be formed when cooled after crosslinking. In such a case, the appropriate hardness cannot be obtained.

[0008] Thus, it would be necessary to let the balata undergo vulcanization under a special ambience over a long period of time. For example, Japanese Patent Laying-Open No. 54-99177 discloses a technique for crosslinking reaction, which includes the following steps. First, a composition having organic amine, zinc oxide and sulfur blended in balata is molded as a spherical cover to envelop an elastic core. The molded spherical body is then immersed, for a prescribed period of time, in a solution prepared by dissolving appropriate amounts of xanthate type vulcanization accelerator and sulfur into an organic solvent, to effect sulfur crosslinking of the spherical cover. This technique poses problems in the crosslinking process, as it requires immersion of the object into the solution, making the post-treatment difficult.

[0009] European Patent Publication No. 0524841, for example, discloses a technique for crosslinking balata using metallic salt of unsaturated fatty acid and crosslinking initiator. This however requires vulcanization at a high temperature for a long period of time, which causes various problems. The thread rubber wound around the core would suffer heat deterioration, and rebound performance of the golf ball would be degraded. Further, the appropriate hardness would not be obtained, as the crystal growth is not expected.

[0010] In addition, the vulcanization may start during the cover molding process, which would increase viscosity of the cover material as the vulcanization proceeds. Due to such increase in viscosity, fluidity of the cover material would be degraded, so that the cover material becomes less likely to penetrate into the thread-wound core. This leads to poor adhesion between the cover and the thread rubber layer, and as a result, durability of the golf ball would be deteriorated.

[0011] Proposed to solve these problems is a technique for vulcanizing balata using a low-temperature vulcanization accelerator that starts to react at a low temperature like room temperature, consisting of dithiocarbamates and/or xanthates.

[0012] This technique however poses various problems. The interior core may be degraded during thermoforming. The residue (flash) obtained after vulcanization of balata as a result of the progress of crosslinking may be relatively difficult to put into reuse. In addition, the time that is allowed from rubber kneading to molding is restricted comparatively.

[0013] Further, when molding the balata material as thermoplastic resin, it was difficult to maintain a perfect sphere of the golf ball because of a change in volume, or shrinkage cavity, due to crystallization at the time of cooling thereof.

[0014] Although a cover formed of a mixture of ionomer resin and balata commonly used as a cover material overcomes the disadvantages of balata described above, it tends to exhibit characteristics of the ionomer resin to a relatively large extent. Therefore, compared to the cover formed solely of balata, the cover formed of the mixture of ionomer resin and balata is inferior in controllability, as it is difficult to apply spin to the ball, and thus poses a problem of poor controllability of an approach shot.

SUMMARY OF THE INVENTION

[0015] The present invention is made to solve the above-described problems, and its object is to provide a golf ball that exhibits high rebound performance, long flight distance and good anti-flier characteristics, and is also superior in hit feeling and controllability.

[0016] The golf ball according to the present invention is composed of a core and a cover surrounding the core. The cover is formed by vulcanizing a cover composition that includes, as its polymer components, thermoplastic elastomer, rubber containing a trans structure, and ionomer resin.

[0017] In the golf ball of the present invention, a ratio of the weight of the trans structure-containing rubber to the total weight of the thermoplastic elastomer and the ionomer resin is preferably in a range from 70:30 to 20:80, and a ratio of the weight of the thermoplastic elastomer to the weight of the ionomer resin is preferably in a range from 85:15 to 50:50.

[0018] Further, in the golf ball of the present invention, the trans structure-containing rubber preferably includes at least one of trans 1,4-polyisoprene rubber, trans 1-4-polybutadiene rubber and trans polyocteneline rubber.

[0019] Further, in the golf ball of the present invention, the thermoplastic elastomer preferably includes at least one of styrene type thermoplastic elastomer and urethane type thermoplastic elastomer.

[0020] Still further, in the golf ball of the present invention, the ionomer resin is preferably included 5-60 parts by weight with respect to 100 parts by weight of the polymer components in the cover composition.

[0021] The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention was made based on a novel knowledge that a golf ball composed of a core and a cover surrounding the core, in which the core is formed by vulcanizing a cover composition including thermoplastic elastomer, rubber containing a trans structure and ionomer resin, exhibits good rebound performance, long flight distance and favorable anti-flier characteristics, and is excellent in hit feeling and controllability.

[0023] Inclusion of the thermoplastic elastomer in addition to the trans structure-containing rubber within the cover composition brings about the following benefits. During the cover molding process, fluidity of the cover composition is maintained. The change in volume, or shrinkage cavity, due to crystallization at the time of cooling can be prevented. Degradation of spin performance of the golf ball can also be prevented. Inclusion of the ionomer resin in the cover composition brings about benefits that good rebound performance and spin performance of the golf ball can be obtained together, and cut resistance can also be improved.

[0024] Although superior in cut resistance, the ionomer resin tends to be inferior in spin performance. The trans structure-containing rubber and in particular the thermoplastic elastomer are superior in spin performance. However, the trans structure-containing rubber and in particular the thermoplastic elastomer tend to be inferior in cut resistance. Therefore, the order of superiority in the spin performance is thermoplastic elastomer>trans structure-containing rubber>ionomer resin. The order of superiority in the cut resistance is ionomer resin>trans structure-containing rubber>thermoplastic elastomer. Using only the trans structure-containing rubber and the ionomer resin, the spin performance of the golf ball may be degraded, since the thermoplastic elastomer is not blended. Using only the thermoplastic elastomer and the ionomer resin, due to the absence of the trans structure-containing rubber, poor spin performance and poor cut resistance as disadvantages of the ionomer resin and the thermoplastic elastomer, respectively, may emerge. The golf ball according to the present invention is composed of a core and a cover surrounding the core, and the cover is formed by vulcanizing the cover composition including the thermoplastic elastomer, the trans structure-containing rubber, and the ionomer resin. Therefore, a golf ball superior both in the spin performance and the cut resistance can be obtained.

[0025] A ratio of the weight of the trans structure-containing rubber to the total weight of the thermoplastic elastomer and the ionomer resin may be set in a range from 70:30 to 20:80. If the content of the trans structure-containing rubber is greater than in the case satisfying the foregoing weight ratio of 70:30, the characteristics of the trans structure-containing rubber tend to emerge to a relatively large extent. In this case, a special ambience is required for the vulcanizing process of the cover composition. Otherwise, the change in volume due to crystallization during the molding process of the cover composition, for example, becomes too large, resulting in shrinking cavity at the time of molding the cover. If the total weight of the thermoplastic elastomer and the ionomer resin becomes greater than in the case satisfying the foregoing weight ratio of 20:80, crystallinity of a level necessary to keep the strength of the cover cannot be ensured, since the content of the trans structure-containing rubber is decreased. It also becomes harder to apply spin to the ball, so that controllability of the golf ball and hence controllability of the approach shot tends to be degraded. The ratio of the weight of the trans structure-containing rubber to the total weight of the thermoplastic elastomer and the ionomer resin may further be set in a range from 65:35 to 20:80.

[0026] A ratio of the weight of the thermoplastic elastomer to the weight of the ionomer resin may be set in a range from 85:15 to 50:50. If the content of the thermoplastic elastomer is greater than in the case satisfying the foregoing weight ratio of 85:15, the content of the ionomer resin is decreased comparatively, which would degrade the carry and the cut resistance of the golf ball. If the content of the ionomer resin is greater than in the case satisfying the foregoing weight ratio of 50:50, it may become more difficult to apply spin to the golf ball. This would degrade controllability of the golf ball and hence controllability of the approach shot. The weight ratio between the thermoplastic elastomer and the ionomer resin may be set in a range from 72:28 to 50:50, or further in a range from 65:35 to 50:50.

[0027] The total weight of the thermoplastic elastomer, the trans structure-containing rubber and the ionomer resin with respect to the gross weight of the cover composition may be set to preferably at least 0.7, more preferably at least 0.8, and still more preferably at least 0.9.

[0028] The trans structure-containing rubber may include at least one of trans 1,4-polyisoprene rubber, trans 1,4-polybutadiene rubber and trans polyocteneline rubber. In particular, the trans 1,4-polyisoprene rubber is suitable for use as the trans structure-containing rubber, since it is superior in solubility with the thermoplastic elastomer, and also superior in durability after being molded as the cover.

[0029] Herein, the trans 1,4-polyisoprene refers to one having a trans content of at least 60% in the polyisoprene molecule. One having a trans content of less than 60% has a low degree of crystallinity and thus the melting point thereof is too low. It cannot satisfy the basic characteristics as a cover.

[0030] Similarly, the trans 1,4-polybutadiene refers to one having a trans content of at least 60% in the polybutadiene molecule. One with a trans content of less than 60% is low in crystallinity and thus tends to be easily cut. It would not exhibit a satisfactory value in cut resistance when used as a cover material of the golf ball.

[0031] The trans polyocteneline, i.e., a polymer having a 1,4-trans structure, can be obtained from ring-opening polymerization of cyclooctene. Herein, the trans polyocteneline refers to one having a trans content of at least 60%. One with a trans content of less than 60% is low in crystallinity, and thus would not show a satisfactory value in cut resistance when used as a cover material of the golf ball.

[0032] As the thermoplastic elastomer, one including at least one of styrene type thermoplastic elastomer and urethane type thermoplastic elastomer may be employed. The styrene type or urethane type thermoplastic elastomer is advantageous in that it can improve fluidity of the cover composition during the cover forming process by virtue of its relatively low melting point, and also can comparatively improve spin performance of the golf ball.

[0033] The styrene type thermoplastic elastomer is a block copolymer having polystyrene as its hard segment and polybutadiene, polyisoprene or polyolefin as its soft segment. One having a polymer chain with polystyrene phase (S) at both ends and polybutadiene phase (B) at the intermediate portion is called an SBS structure, or SBS. One having a polymer chain with polystyrene phase (S) at both ends and polyisoprene phase (I) at the intermediate portion is SIS. One having a polymer chain with polystyrene phase (S) at both ends and polyolefin phase (ethylene/butylene=EB or ethylene/propylene=EP) at the intermediate portion is SEBS or SEPS. SEBS and SEPS above correspond respectively to SBS and SIS each saturated by hydrogenation to eliminate double bonding therefrom. To enumerate some trade names of products suitable for use, SBS includes “TR” series available from Japan Synthetic Rubber Co., Ltd., SIS includes “SIS” series also available from Japan Synthetic Rubber Co., Ltd., SEBS includes “Tuftec” series available from Asahi Kasei Corp., and SEPS includes “Septon” series available from Kuraray Co., Ltd.

[0034] The urethane type thermoplastic elastomer is a polymer with relatively high molecule weight having, as its main chain, urethane bond obtained from aromatic diisocyanate and polyol of polyester structure or of polyether structure. Specific examples of the urethane type thermoplastic elastomer include those available from Dainippon Ink and Chemicals, Inc. under the trade names of PANDEX T-7890N and T-2983N.

[0035] Of the hard segment and soft segment constituting the thermoplastic elastomer, the soft segment serves to emerge rubber-like elasticity specific to the thermoplastic elastomer. During the molding process, the hard segment is dissolved and plasticized to ease the molding. It is hardened after completion of the molding, and functions to restrict plastic deformation.

[0036] As other types of thermoplastic elastomer, any of ester type thermoplastic elastomer, olefin type elastomer, and amid type elastomer may be used alone or by mixing them together. They can also be mixed to the styrene type or urethane type thermoplastic elastomer.

[0037] The ionomer resin may be ionomer of ethylene-(meth)acrylate copolymer, which has a copolymer of ethylene-(meth)acrylate neutralized with metal ion as its bone structure. The metal ion for use in neutralization may be monovalent or bivalent metal ion, such as Na ion, K ion, Li ion, Zn ion, Ca ion or Mg ion. Examples of the ionomer include those available from Du Pont-Mitsui Polychemical Co., Ltd. under the trade names of Hi-milan 1605, 1706 and those available from Exxon Corp. under the trade names of IOTEC 7010, 8000. It is noted that the examples of the urethane type thermoplastic elastomer and the ionomer resin enumerated above are only representative; they are not limited thereto.

[0038] The ionomer resin may be included 5-60 parts by weight with respect to 100 parts by weight of the polymer components in the cover composition. If the content of the ionomer resin is less than 5 parts by weight, high rebound performance and high cut resistance of the golf ball by virtue of high rebound performance and high strength inherent in the ionomer resin would not be expected. If the content of the ionomer resin exceeds 60 parts by weight, however, it becomes harder to apply spin to the ball, so that controllability of the golf ball and hence controllability of the approach shot would be degraded.

[0039] The cover of the golf ball according to the present invention is made to have a thickness of preferably 0.7-3.0 mm, and more preferably 1.0-2.5 mm. If the thickness of the cover is less than 0.7 mm, strength and durability of the cover would be deteriorated. If the thickness exceeds 3.0 mm, however, the volume percent of the cover composition to the entire ball becomes too large, so that rebound performance of the ball would be degraded. If the thickness of the cover exceeds 3.0 mm, shrinkage cavity may also be created at the time of molding the cover, causing outside diameter variation of the golf ball. With the golf ball of the present invention, a material exhibiting a relatively high degree of crystallinity is used as the cover composition. Thus, for the purposes of preventing the shrinkage cavity at the time of cover molding and other disadvantages, it is preferable to make the cover thinner than those commonly used for the golf balls. Accordingly, the golf ball of the present invention is made to have the outside diameter of the core of preferably from 36.8 mm to 41.4 mm, and more preferably from 37.8 mm to 40.8 mm.

[0040] A vulcanizing agent may be blended in the cover composition. As the vulcanizing agent, organic peroxide type agent or sulfur type agent may be used alone or by mixing them together. Examples of the organic peroxide type vulcanizing agent include: benzoyl peroxide, dicumyl peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, methyl ethyl ketone peroxide, cumene hydro peroxide, 2,5-dimethyl-2,5-di (t-butyl peroxy) hexane, 2,5-dimethyl-2,5-di (benzoyl peroxy) hexane, 2,5-dimethyl-2,5-di (t-butyl peroxy) hexane-3 or 1,3-bis (t-butyl peroxy propyl) benzene, di-t-butyl peroxy-diisopropyl benzene, t-butyl peroxy benzene, 2,4-dichloro benzoyl peroxide, 1,1-di-t-butyl peroxy-3,3,5-trimethyl siloxane, n-butyl-4,4-di-t-butyl peroxy valerate, and others. Among them, dicumyl peroxide, t-butyl peroxy benzene and di-t-butyl peroxy-diisopropyl benzene are preferable. Examples of the sulfur type vulcanizing agent are sulfur, morpholine disulfide and others, of which sulfur is preferable.

[0041] A vulcanization accelerator may also be blended in the cover composition, which may be dithiocarbamates, xanthates and others. Representative materials of dithiocarbamates are sodium dimethyl dithiocarbamate, sodium diethyl dithiocarbamate, sodium di-n-butyl carbamate, zinc dimethyl dithiocarbamate, zinc diethyl dithiocarbamate, zinc di-n-butyl carbamate, zinc diamyl dithiocarbamate, zinc dibenzyl dithiocarbamate, zinc N-pentamethylene dithiocarbamate, pipecoline methyl pentamethylene dithiocarbamate, pipecoline pipecolyl dithiocarbamate, and pentamethylene dithiocarbamate. Examples of xanthates are sodium isopropyl xantogenate, zinc isopropyl xantogenate, and zinc butyl xantogenate.

[0042] If the vulcanizing agent and/or the vulcanization accelerator is to be blended, the blended amount of each of the vulcanizing agent and the vulcanization accelerator is preferably not greater than 3 parts by weight with respect to 100 parts by weight of the polymer components in the cover composition. If the blended amounts of these agents are too large, vulcanization will occur when mixing the materials with a roll or kneader. This would cause so-called rubber burning, and the degree of crystallinity of the trans structure-containing rubber would be degraded. Therefore, it is most preferable not to blend either the vulcanizing agent or the vulcanization accelerator in the cover composition. If being blended, they are preferably blended at most 3 parts by weight, more preferably at most 2 parts by weight, and still more preferably at most 1 parts by weight each with respect to 100 parts by weight of the polymer components in the cover composition.

[0043] Further, various kinds of additives may also be added to the cover composition where appropriate. They include pigment, dispersant, antioxidant, UV absorbent, photo-stabilizer, lubricant and others. As the lubricant, barium stearate, calcium stearate, zinc stearate, aluminum stearate, magnesium stearate, complex of zinc stearate and barium stearate, complex of zinc stearate and calcium stearate and others may be employed. As the antioxidant, metallic salt of carbamate of amine type, phenol type and imidazole type, and wax and others may be employed.

[0044] A manufacturing method of the golf ball of the present invention will now be described. First, the cover composition is kneaded using a roll or kneader. The time to be spent for mixing should be as short as possible, specifically between 1 minute and 20 minutes. If it is shorter than 1 minute, dispersion of the ingredients within the cover composition will be insufficient. If it is longer than 20 minutes, rubber burning will occur.

[0045] The cover composition may be used to cover both a solid core and a thread-wound core. To provide the cover composition around the solid core or the thread-wound core, hemispherical half shells are fabricated in advance and press-molded over the core. The half shells are preferably fabricated while the material is fluid at a temperature as low as possible, from 60° C. to 90° C., and rapidly cooled thereafter.

[0046] A dimple pattern is then formed on the surface of the ball with the core enveloped by the half shells. At this stage, vulcanization reaction has not yet occurred, or has occurred only partially to the extent that it would not deteriorate the fluidity of the material.

[0047] The cover composition is then subjected to vulcanization. For the vulcanization, either a method in which crosslinking is completed within the mold, or a method in which the cooled object is removed from the mold after press-molding and the crosslinking is completed within an oven or the like at a similar temperature over a similar period of time, may be employed. The vulcanization of the cover material of the resultant ball has now been completely finished.

[0048] The present invention is applicable to a multi-piece core with one or more layers, or a thread-wound core having rubber thread wound around a solid or liquid center.

EXAMPLES

[0049] 1) Preparation of Core

[0050] A rubber thread was wound in an elongated state around a solid center formed of crosslinked rubber having a diameter of 28 mm, to prepare a thread-wound core having an outside diameter of 40 mm.

[0051] 2) Cover Composition

[0052] Cover compositions Nos. 1-13 different in blended ratio of ingredients from each other were prepared as shown in Table 1 below.

[0053] Specifically, cover composition No. 1 has 35 parts by weight of HG 252, 35 parts by weight of TP-301, 30 parts by weight of ionomer resin, 20 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0054] Cover composition No. 2 has 49 parts by weight of HG 252, 21 parts by weight of TP-301, 30 parts by weight of ionomer resin, 21 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0055] Cover composition No. 3 has 45 parts by weight of HG 252, 45 parts by weight of TP-301, 10 parts by weight of ionomer resin, 20 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0056] Cover composition No. 4 has 27 parts by weight of HG 252, 63 parts by weight of TP-301, 10 parts by weight of ionomer resin, 19 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0057] Cover composition No. 5 has 35 parts by weight of ET 890, 35 parts by weight of TP-301, 30 parts by weight of ionomer resin, 5 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0058] Cover composition No. 6 has 50 parts by weight of HG 252, 50 parts by weight of TP-301, 21 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0059] Cover composition No. 7 has 30 parts by weight of HG 252, 70 parts by weight of TP-301, 20 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0060] Cover composition No. 8 has 50 parts by weight of ET 890, 50 parts by weight of TP-301, 3 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0061] Cover composition No. 9 has 10 parts by weight of HG 252, 90 parts by weight of TP-301, 16 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0062] Cover composition No. 10 has 70 parts by weight of TP-301, 30 parts by weight of ionomer resin, 17 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0063] Cover composition No. 11 has 100 parts by weight of HG 252, 22 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0064] Cover composition No. 12 has 100 parts by weight of TP-301, 16 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0065] Cover composition No. 13 has 100 parts by weight of ionomer resin, 20 parts by weight of BaSO4, and 2 parts by weight of pigment blended therein.

[0066] HG 252 is an SEPS type thermoplastic elastomer available from Kuraray Co., Ltd. ET 890 is an urethane type thermoplastic elastomer available from Takeda Badische Urethane Industries Ltd. TP-301 is trans polyisoprene rubber (with a trans content of at least 95%) available from Kuraray Co., Ltd. The ionomer resin is a mixture of Surlyn 8945 and Surlyn 9945, available from Du Pont Co., Ltd., mixed in equal quantities. Here, Surlyn 8945 is an ethylene/methacrylic acid copolymer ionomer resin neutralized with Na. Surlyn 9945 is an ethylene/methacrylic acid copolymer ionomer resin neutralized with Zn. BaSO4 is barium sulfate BMH available from Sakai Chemical Co., Ltd. The pigment is titanium oxide A 220 available from Ishihara Sangyou Kaisha Ltd.

[0067] Cover Compositions Nos. 1-5 were used to manufacture golf balls of Examples of the present invention. Cover Compositions Nos. 6-13 were used to manufacture golf balls as Comparative examples. 1 TABLE 1 Cover Composition No. 1 2 3 4 5 6 7 8 9 10 11 12 13 HG252 *1) 35 49 45 27 50 30 10 100 ET890 *2) 35 50 TP-301 *3) 35 21 45 63 35 50 70 50 90 70 100 Ionomer resin *4) 30 30 10 10 30 30 100 BaSO4 *5) 20 21 20 19 5 21 20 3 16 17 22 16 20 Pigment *6) 2 2 2 2 2 2 2 2 2 2 2 2 2 *1) SEPS type thermoplastic elastomer, from Kuraray Co., Ltd. *2) Urethane type thermoplastic elastomer, from Takeda Badische Urethane Industries Ltd. *3) Trans polyisoprene rubber (with trans content of at least 95%), from Kuraray Co., Ltd. *4) Mixture of Surlyn 8945 and Surlyn 9945, from Du Pont Co., Ltd., in equal quantities *5) Barium Sulfate BMH, from Sakai Chemical Co., Ltd. *6) Titanium oxide A 220, from Ishihara Sangyou Kaisha Ltd.

[0068] 3) Manufacturing Process of Golf Ball

[0069] Each cover composition having ingredients as shown in Table 1 was extruded with a two-axis extruder at a cylinder temperature of 20° C., and injection-molded in a hemispherical mold to fabricate a half shell. The core was enveloped with two such half shells, and subjected to thermoforming in a dimpled mold at 150° C. for 2 minutes, followed by cooling. A ball with dimples was thus obtained.

[0070] 4) Measurement of Physical Properties of the Golf Ball

[0071] Physical properties of the golf ball were measured in the following manners.

[0072] <Hardness>

[0073] Hardness of the surface of the ball after the cover molding process was measured using a Shore D hardness tester.

[0074] <Flexural Rigidity>

[0075] The cover composition was pelletized and subjected to thermoforming at 170° C. for 5 minutes. After cooled in place for 20 minutes, it was press-molded into a flat plate with a thickness of 2 mm. The flexural rigidity thereof was measured with a method in conformity with JIS K7106.

[0076] <Compression>

[0077] The amount of deformation of the ball when applying a load from 10 kg to 130 kg was measured. It is expressed in units of mm.

[0078] <Cut Resistance>

[0079] The ball was hit using a pitching wedge to intentionally cut the surface. The state of the cut (mainly the length of the cut) was evaluated in five grades. The grade 5 means that there was almost no cut. The grade 1 means that a large cut was created.

[0080] <Spin Performance>

[0081] For each ball, likelihood of a flier shot when hit from the rough was examined. The spin performance is expressed as (spin rate when hit from the rough/spin rate when hit under a normal condition)×100. A larger value indicates a more favorable result with more stable spin.

[0082] Results obtained when using Cover composition No. 1 were hardness: 46, flexural rigidity: 162 MPa, compression: 3.09 mm, cut resistance: 5, and spin performance: 70.

[0083] Results with Cover composition No. 2 were hardness: 43, flexural rigidity: 139 MPa, compression: 3.16 mm, cut resistance: 5, and spin performance: 74.

[0084] Results with Cover composition No. 3 were hardness: 42, flexural rigidity: 131 MPa, compression: 3.05 mm, cut resistance: 4, and spin performance: 76.

[0085] Results with Cover composition No. 4 were hardness: 45, flexural rigidity: 160 MPa, compression: 3.15 mm, cut resistance: 4, and spin performance: 74.

[0086] Results with Cover composition No. 5 were hardness: 53, flexural rigidity: 195 MPa, compression: 3.18 mm, cut resistance: 5, and spin performance: 71.

[0087] Results with Cover composition No. 6 were hardness: 41, flexural rigidity: 124 MPa, compression: 3.34 mm, cut resistance: 2, and spin performance: 79.

[0088] Results with Cover composition No. 7 were hardness: 45, flexural rigidity: 154 MPa, compression: 3.21 mm, cut resistance: 2, and spin performance: 76.

[0089] Results with Cover composition No. 8 were hardness: 44, flexural rigidity: 145 MPa, compression: 3.26 mm, cut resistance: 2, and spin performance: 79.

[0090] Results with Cover composition No. 9 were hardness: 48, flexural rigidity: 218 MPa, compression: 3.11 mm, cut resistance: 2, and spin performance: 72.

[0091] Results with Cover composition No. 10 were hardness: 56, flexural rigidity: 262 MPa, compression: 2.79 mm, cut resistance: 5, and spin performance: 49.

[0092] Results with Cover composition No. 11 were hardness: 41, flexural rigidity: 36 MPa, compression: 3.20 mm, cut resistance: 1, and spin performance: 92.

[0093] Results with Cover composition No. 12 were hardness: 54, flexural rigidity: 259 MPa, compression: 2.93 mm, cut resistance: 2, and spin performance: 65.

[0094] Results with Cover composition No. 13 were hardness: 62, flexural rigidity: 331 MPa, compression: 2.61 mm, cut resistance: 5, and spin performance: 35.

[0095] These results are shown in Table 2. 2 TABLE 2 Examples Comparative Examples Cover Composition No. 1 2 3 4 5 6 7 8 9 10 11 12 13 Hardness (D) 46 43 42 45 53 41 45 44 48 56 41 54 62 Flexural rigidity MPa 162 139 131 160 195 124 154 145 218 262 36 259 331 Compression mm 3.09 3.16 3.05 3.15 3.18 3.34 3.21 3.26 3.11 2.79 3.20 2.93 2.61 Cut resistance Grade 5 5 4 4 5 2 2 2 2 5 1 2 5 Spin performance 70 74 76 74 71 79 76 79 72 49 92 65 35

[0096] When the flexural rigidity is less than 80 MPa, the cut resistance tends to be degraded. If it is greater than 300 MPa, the spin rate decreases, resulting in hard hit feeling. Therefore, the flexural rigidity in the range of 80-300 MPa is preferable. The cover compositions of Examples of the present invention each have the flexural rigidity that falls within this range.

[0097] If the Shore D hardness is less than 41, the cut resistance tends to be degraded. If it is greater than 56, the spin rate tends to decrease, giving hard hit feeling to the player. Thus, the Shore D hardness is preferably in the range between 41 and 56. The cover compositions of the present invention each have the Shore D hardness within this range.

[0098] The cover compositions of Examples of the present invention also have relatively good grades in the cut resistance and relatively high values in the spin performance.

[0099] It is possible to blend the vulcanizing agent and/or vulcanization accelerator in the cover composition as explained above. However, if they are blended therein, during the mixing of the thermoplastic elastomer, trans structure-containing rubber and ionomer resin applying heat, they may deteriorate the crystallinity of the trans structure-containing rubber, leading to degradation of the strength of the cover. Accordingly, it is preferable that the golf ball of the present invention is composed of a core and a cover surrounding the core, and the cover is formed by vulcanizing a cover composition including thermoplastic elastomer, trans structure-containing rubber and ionomer resin, but not including either vulcanizing agent or vulcanization accelerator.

[0100] As described above, according to the present invention, the cover composition of the golf ball includes the thermoplastic elastomer, trans structure-containing rubber and ionomer resin blended therein. Therefore, the cover composition is moldable in a state maintaining a high level of fluidity, thereby preventing defective molding. Further, according to the present invention, shrinkage cavity due to crystallization during the molding process is alleviated, so that dimensional stability is improved. When a thread-wound core is employed, it is possible to cause the cover composition to sufficiently permeate into the thread rubber layer of the thread-wound core. This ensures excellent adhesion between the cover and the thread rubber layer, and as a result, a golf ball improved in rebound performance and durability can be obtained. Still further, the cover composition includes the thermoplastic elastomer and the rubber with high crystallinity, like balata, ensuring stable spin. Accordingly, the golf ball exhibiting good spin performance and preventing a flier shot can be obtained.

[0101] Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims

1. A golf ball composed of a core and a cover surrounding the core, said cover being formed by vulcanizing a cover composition including, as its polymer components, thermoplastic elastomer, rubber containing a trans structure, and ionomer resin.

2. The golf ball according to claim 1, wherein

a ratio of a weight of said trans structure-containing rubber to a total weight of said thermoplastic elastomer and said ionomer resin is in a range from 70:30 to 20:80, and

a ratio of the weight of said thermoplastic elastomer to the weight of said ionomer resin is in a range from 85:15 to 50:50.

3. The golf ball according to claim 1, wherein said trans structure-containing rubber includes at least one of trans 1,4-polyisoprene rubber, trans 1,4-polybutadiene rubber and trans polyocteneline rubber.

4. The golf ball according to claim 1, wherein said thermoplastic elastomer includes at least one of styrene type thermoplastic elastomer and urethane type thermoplastic elastomer.

5. The golf ball according to claim 1, wherein said ionomer resin is included 5-60 parts by weight with respect to 100 parts by weight of the polymer components in said cover composition.