Patent Publication Number: US-8974319-B2

Title: Golf ball material and golf ball using the same

Description:
FIELD OF THE INVENTION 
     The present invention relates to a novel golf ball material and a golf ball using the same. 
     DESCRIPTION OF THE RELATED ART 
     As a resin component constituting a cover of a golf ball, an ionomer resin or polyurethane is used. Covers containing an ionomer resin are widely used for their excellent repulsion, durability and processability. However, the problems have been pointed out that the shot feeling is poor because of the high rigidity and hardness and that the controllability is also poor because of the insufficient spin performance. On the other hand, if polyurethane is used as the resin component constituting the cover, it is known that the shot feeling and spin performance are improved compared with an ionomer resin. 
     It is an ultimate goal for those who develop golf balls to provide a golf ball traveling a great distance on driver shots, and stopping quickly on the green on approach shots. In order to make a golf ball travel a long distance on driver shots, from the view point of the material, employing a core material having a high resilience has been studied in Japanese patent publication No. 2003-154035. From the view point of the golf ball construction, employing a core having a larger diameter has been disclosed in Japanese patent publication No. 2006-034740. On the other hand, in order to make a golf ball stop quickly on the green on the approach shots, soft cover materials are used to increase a spin rate on the approach shots. Further, the inventors of the present invention has filed a Japanese patent application (published as Japanese Patent publication No. 2009-131508) that the spin rate is increased by regulating the steric structure of the polyurethane which is a resin component of the cover. 
     SUMMARY OF THE INVENTION 
     An attempt to increase the spin rate on the approach shot by employing conventional soft cover materials also increases the spin rate on the driver shots. If the spin rate on the driver shots increases, the initial velocity of the golf ball reduces, resulting in the short flight distance. Thus, it is difficult to strike a balance between stopping quickly on the green on the approach shots and traveling a great distance on the driver shots. The present invention has been achieved in view of the above circumstances. An object of the present invention is to provide a golf ball which has a high spin rate on the approach shots and a low spin rate on the driver shots. 
     With respect to the deformation of the cover when hitting the golf ball, it is considered that the compressive deformation is dominant on the driver shots and the shear deformation is dominant on the approach shots. Based on this hypothesis, the inventors of the present invention have studied characteristics of the golf ball material, and found that the spin rate on the driver shots correlates with the tensile loss modulus E″ measured in a tensile mode, and the spin rate on the approach shots correlates with the shear loss modulus G″ measured in a shear mode at the conditions of a temperature of 0° C. and oscillation frequency of 10 Hz using a dynamic viscoelasticity measuring apparatus. The inventors of the present invention have made the present invention based on the findings that the material having a shear loss modulus G″ of 2.11×10 7  Pa or less, and a ratio (E″/G″) of a tensile loss modulus E″ to the shear loss modulus G″ of 1.78 or more, when measuring the shear loss modulus G″ in a shear mode and the tensile loss modulus E″ in a tensile mode at conditions of a temperature of 0° C. and oscillation frequency of 10 Hz using a dynamic viscoelasticity measuring apparatus, produces a high spin rate on the approach shots and a low spin rate on the driver shots. The present invention includes a golf ball having a constituting member formed from the golf ball material of the present invention. 
     According to the present invention, it is possible to provide a golf ball with a high spin rate on the approach shots and a low spin rate on the driver shots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing a correlation between the spin rate on approach shots and the shear loss modulus G″; and 
         FIG. 2  is a graph showing a correlation between the spin rate on driver shots and the tensile loss modulus E″. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a golf ball material having a shear loss modulus G″ of 2.11×10 7  Pa or less, and a ratio (E″/G″) of a tensile loss modulus E″ to the shear loss modulus G″ of 1.78 or more, when measuring the shear loss modulus G″ in a shear mode and the tensile loss modulus E″ in a tensile mode at conditions of a temperature of 0° C. and oscillation frequency of 10 Hz using a dynamic viscoelasticity measuring apparatus. In the present invention, the reason why the viscoelasticity is measured at the conditions of the temperature of 0° C. and oscillation frequency of 10 Hz is as follows. The contact time between the golf ball and the golf club when hitting the golf ball is several hundreds micro seconds. If this impact is considered as one deformation, this deformation corresponds to the deformation at the frequency of several thousands Hertz. Based on the time-temperature superposition principle of the general polyurethane elastomer, the viscoelasticity measured at the conditions of temperature: room temperature and oscillation frequency: several thousands Hertz correspond to the viscoelasticity measured at the conditions of temperature: 0° C. and oscillation frequency: 10 Hz. 
     If the shear loss modulus G″ when measured in a shear mode at conditions of a temperature of 0° C., oscillation frequency of 10 Hz using a dynamic viscoelasticity measuring apparatus is small, the spin rate on the approach shots becomes high. Thus, the golf ball material of the present invention has the shear loss modulus G″ of 2.11×10 7  Pa or less, preferably 1.95×10 7  Pa or less, more preferably 1.83×10 7  Pa or less. The golf ball material of the present invention has no limitation on the lower limit of the shear loss modulus G″, but preferably has the shear loss modulus G″ of 1.00×10 6  Pa or more, more preferably 1.10×10 6  Pa or more. If the shear loss modulus G″ is 1.00×10 6  Pa or more, the handling of the golf ball material becomes better in a production process. 
     If the tensile loss modulus E″ when measured in a tensile mode at conditions of a temperature of 0° C., oscillation frequency of 10 Hz using a dynamic viscoelasticity measuring apparatus becomes large, the spin rate on the driver shots becomes low. Thus, regulating a ratio of the tensile loss modulus E′ to the shear loss modulus G″ at a certain level or more, it is possible to provide a golf ball with a high spin rate on the approach shots and a low spin rate on the driver shots. The ratio (E″/G″) of the tensile loss modulus E″ to the shear loss modulus G″ is 1.78 or more, preferably 1.86 or more, more preferably 1.90 or more. The ratio (E″/G″) of the tensile loss modulus E″ to the shear loss modulus G″ is, but not limited to, preferably 6 or less, more preferably 5.5 or less. If the ratio (E″/G″) of the loss moduli is 6 or less, the handling of the golf ball material becomes better in a production process. The tensile loss modulus E″ is preferably 2.00×10 7  Pa or more, more preferably 2.20×10 7  Pa or more, even more preferably 2.40×10 7  Pa or more. The shear loss modulus G″ and tensile loss modulus E″ are adjusted by, for example, composition ratio of the components constituting a polyurethane contained in the golf ball material, molecular weight, or the like. 
     The golf ball material of the present invention preferably contains polyurethane as a resin component. The polyurethane is a polymer having plurality of urethane bonds in a molecular chain thereof and is obtained by, for example, a reaction between a polyol and a polyisocyanate. 
     As a polyol component constituting the preferable polyurethane, preferably used is a polyol having a number average molecular weight ranging from 200 to 3,000. The polyol having a number average molecular weight ranging from 200 to 3,000 forms a soft segment and imparts the softness to the polyurethane. The number average molecular weight of the polyol is preferably 250 or more, more preferably 300 or more, and even more preferably 1,500 or more. If the number average molecular weight of the polyol is too small, the obtained polyurethane may become hard. The number average molecular weight of the polyol is preferably 6,000 or less, more preferably 4,000 or less, even more preferably 3,000 or less. If the number average molecular weight is 6,000 or less, it is possible to provide a golf ball with a less spin rate on the driver shots. 
     The number average molecular weight of the polyol component can be measured by Gel permeation Chromatography using two columns of TSK-GEL SUPREH 2500 (TOSOH Corporation) as a column, polystyrene as a standard material, and tetrahydrofuran as an eluate. 
     The polyol component having a number average molecular weight from 200 to 3,000 is preferably a polymer polyol. The polymer polyol is a polymer obtained by polymerizing a low molecular compound, and has plurality of hydroxyl groups. Among them, a polymer diol is more preferable. Use of the polymer diol provides a linear thermoplastic polyurethane and facilitates the molding of the obtained polyurethane into the constituting member of the golf ball. 
     Examples of the polyol having a number average molecular weight from 200 to 3,000 include a polyether polyol such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polytetramethylene ether glycol (PTMG); a condensed polyester polyol such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); a lactone polyester polyol such as poly-ε-caprolactone (PCL); a polycarbonate polyol such as polyhexamethylene carbonate; and an acrylic polyol. The above polyols may be used alone or as a mixture of at least two of them. Among them, as the polyol component, polytetramethylene ether glycol is preferably used. Use of the polytetramethylene ether glycol makes it possible to control the spin rates on the driver shots and the approach shots at the higher level. 
     The polymer polyol constituting the polyurethane used in the present invention preferably has a hydroxyl value of 561 mgKOH/g or less, more preferably 173 mgKOH/g or less and preferably has a hydroxyl value of 94 mgKOH/g or more, more preferably 112 mgKOH/g or more, even more preferably 132 mgKOH/g or more. The hydroxyl value of the polyol component can be measured, for example, by an acetylation method according to JIS K1557-1. 
     The polyurethane used in the present invention may further have a chain extender as a constituent, unless the effect of the preset invention does not deteriorate. The chain extender includes a low-molecular weight polyol or a low-molecular weight polyamine. Examples of the low-molecular weight polyol may include a diol such as ethylene glycol, diethylene glycol, triethylene glycol, propanediol (e.g., 1,2-propanediol, 1,3-propanediol, and 2-methyl-1,3-propanediol), dipropylene glycol, butanediol (e.g., 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, and 2,3-dimethyl-2,3-butanediol), neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, and 1,4-cyclohexane dimethylol; a triol such as glycerin, trimethylol propane, and hexanetriol; a tetraol or a hexanol such as pentaerythritol and sorbitol. 
     The low-molecular weight polyamine that can be used as a chain extender may include any polyamine, as long as it has at least two amino groups. The polyamine includes an aliphatic polyamine such as ethylenediamine, propylenediamine, butylenediamine, and hexamethylenediamine, an alicyclic polyamine such as isophoronediamine, piperazine, and an aromatic polyamine. 
     The aromatic polyamine has no limitation as long as it has at least two amino groups directly or indirectly bonded to an aromatic ring. Herein, the “indirectly bonded to the aromatic ring”, for example, means that the amino group is bonded to the aromatic ring via a lower alkylene bond. Further, the aromatic polyamine includes, for example, a monocyclic aromatic polyamine having at least two amino groups bonded to one aromatic ring or a polycyclic aromatic polyamine having at least two aminophenyl groups each having at least one amino group bonded to one aromatic ring. 
     Examples of the monocyclic aromatic polyamine include a type such as phenylenediamine, tolylenediamine, diethyltoluenediamine, and dimethylthiotoluenediamine wherein amino groups are directly bonded to an aromatic ring; and a type such as xylylenediamine wherein amino groups are bonded to an aromatic ring via a lower alkylene group. Further, the polycyclic aromatic polyamine may include a poly(aminobenzene) having at least two aminophenyl groups directly bonded to each other or a compound having at least two aminophenyl groups bonded via a lower alkylene group or an alkylene oxide group. Among them, a diaminodiphenylalkane having two aminophenyl groups bonded to each other via a lower alkylene group is preferable. Typically preferred are 4,4′-diaminodiphenylmethane or the derivatives thereof. 
     The chain extender preferably has a molecular weight of 400 or less, more preferably 350 or less, even more preferably less than 200 and preferably has a molecular weight of 30 or more, more preferably 40 or more, even more preferably 45 or more. If the molecular weight is too large, it is difficult to distinguish the chain extender from the high-molecular weight polyol (polymer polyol) constituting a soft segment of the polyurethane. “Low molecular weight polyol” and “Low molecular weight polyamine” are low molecular compounds which are not obtained by polymerization, and are distinguished from the polymer polyol having a number average molecular weight from 200 to 3,000 obtained by polymerization of the low molecular weight compound. 
     The polyisocyanate component constituting the polyurethane used in the present invention is not limited, as long as it has at least two isocyanate groups. Examples of the polyisocyanate include an aromatic polyisocyanate such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylenediisocyanate (TMXDI), para-phenylene diisocyanate (PPDI); an alicyclic polyisocyanate or aliphatic polyisocyanate such as 4,4′-dicyclohexylmethane diisocyanate (H 12 MDI), hydrogenated xylylenediisocyanate (H 6 XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornene diisocyanate (NBDI). These may be used either alone or as a mixture of at least two of them. 
     In view of improving the abrasion-resistance, the aromatic polyisocyanate is preferably used as the polyisocyanate component of the polyurethane. Use of the aromatic polyisocyanate improves the mechanical property of the obtained polyurethane and provides the cover with the excellent abrasion-resistance. In addition, in view of improving the weather resistance, as the polyisocyanate component of the polyurethane, a non-yellowing type polyisocyanate such as TMXDI, XDI, HDI, H 6 XDI, IPDI, H 12 MDI and NBDI is preferably used. More preferably, 4,4′-dicyclohexylmethane diisocyanate (H 12 MDI) is used. Since 4,4′-dicyclohexylmethane diisocyanate (H 12 MDI) has a rigid structure, the mechanical property of the resulting polyurethane is improved, and thus the cover which is excellent in abrasion-resistance can be obtained. 
     The polyurethane used in the present invention has no limitation on the constitutional embodiments thereof. Examples of the constitutional embodiments are the embodiment where the polyurethane consists of the polyisocyanate component, the polyol component having a number average molecular weight from 200 to 3,000, and the embodiment where the polyurethane consists of the polyisocyanate component, the polyol component having a number average molecular weight from 200 to 3,000, and the chain extender component. 
     The polyurethane used in the present invention preferably has dicyclohexylmethane diisocyanate as the polyisocyanate component, polytetramethylene ether glycol having a number average molecular weight from 200 to 3,000 as the polyol component, more preferably further has 1,4-butane diol as the chain extender component. 
     The polyurethane used in the present invention preferably has a slab hardness of 5 or more, more preferably 10 or more, even more preferably 15 or more, and preferably has a slab hardness of 80 or less, more preferably 75 or less, even more preferably 70 or less, even more preferably 55 or less in Shore D hardness. If the slab hardness is too low, the spin rate on the driver shots may increase, while if the slab hardness is too high, the spin rate on the approach shots may decrease. 
     The polyurethane used in the present invention may be either thermoplastic polyurethane or thermosetting polyurethane (two-component curing type polyurethane). The thermoplastic polyurethane is polyurethane exhibiting plasticity by heating and generally means a polyurethane having a linear chain structure of a high molecular weight to a certain extent. On the other hand, the thermosetting polyurethane (two-component curing type polyurethane) is a polyurethane obtained by polymerization through a reaction between a relatively low-molecular weight urethane prepolymer and a chain extender (curing agent). The thermosetting polyurethane includes a polyurethane having a linear chain structure or polyurethane having a three-dimensional crosslinked structure depending on a number of a functional group of the prepolymer or the chain extender (curing agent) to be used. In the present invention, the thermoplastic polyurethane is preferable. 
     Examples of a method for synthesizing the polyurethane include a one-shot method and a prepolymer method. The one-shot method is a method of reacting a polyisocyanate component, a polyol component or the like at once. The prepolymer method is a method of reacting a polyisocyanate component and a polyol component or the like in multiple steps. For example, a relatively low-molecular weight urethane prepolymer is synthesized, followed by further polymerization to have a higher-molecular weight. The polyurethane used in the present invention is preferably produced by the prepolymer method. 
     As an example of producing the polyurethane by the prepolymer method, the following case will be described in detail, wherein an isocyanate group terminated urethane prepolymer is synthesized and then polymerized with the chain extender. 
     First, a polyisocyanate component is subjected to a urethane reaction with a polymer polyol component to synthesize an isocyanate group terminated urethane prepolymer. In this case, the charging ratio of the polyisocyanate component to the polymer polyol component is, preferably 1 or larger, more preferably 1.2 or larger, and even more preferably 1.5 or larger, and is preferably 10 or smaller, more preferably 9 or smaller, and even more preferably 8 or smaller in a molar ratio (NCO/OH) of the isocyanate group (NCO) contained in the polyisocyanate component to the hydroxyl group (OH) contained in the polyol component. 
     The temperature at which the prepolymer reaction is performed is preferably 10° C. or higher, more preferably 30° C. or higher, and even more preferably 50° C. or higher, and is preferably 200° C. or lower, more preferably 150° C. or lower, and even more preferably 100° C. or lower. The reaction time for the prepolymer reaction is preferably 10 minutes or longer, more preferably 1 hour or longer, and even more preferably 3 hours or longer, and is preferably 32 hours or shorter, more preferably 16 hours or shorter, and even more preferably 8 hours or shorter. 
     Next, the obtained isocyanate group terminated urethane prepolymer is subjected to a chain extension reaction with the chain extender component to obtain the polyurethane having a high molecular weight. In this case, the charging ratio of the isocyanate group terminated urethane prepolymer to the chain extender component is preferably 0.9 or larger, more preferably 0.92 or larger, and even more preferably 0.95 or larger, and is preferably 1.1 or smaller, more preferably 1.08 or smaller, and even more preferably 1.05 or smaller in a molar ratio (NCO/OH) of the isocyanate group (NCO) contained in the isocyanate group terminated urethane prepolymer to the hydroxyl group (OH) contained in the chain extender component. 
     The temperature at which the chain extension reaction is performed is preferably 10° C. or higher, more preferably 30° C. or higher, and even more preferably 50° C. or higher, and is preferably 220° C. or lower, more preferably 170° C. or lower, and even more preferably 120° C. or lower. The reaction time for the chain extension reaction is preferably 10 minutes or longer, more preferably 30 minutes or longer, and even more preferably 1 hour or longer, and is preferably 20 days or shorter, more preferably 10 days or shorter, and even more preferably 5 days or shorter. 
     Both of the prepolymer reaction and the chain extension reaction are preferably conducted in an atmosphere of dry nitrogen. 
     In synthesizing the polyurethane, a publicly known catalyst may be used as long as it does not impair the effect of the present invention. Examples of the catalyst include a monoamine such as triethylamine, and N,N-dimethylcyclohexylamine; a polyamine such as N,N,N′,N′-tetramethylethylenediamine, and N,N,N′,N″,N″-pentamethyldiethylenetriamine; a cyclic diamine such as 1,8-diazabicyclo-[5.4.0]-7-undecene (DBU), triethylenediamine; a tin-based catalyst such as dibutyl tin dilaurylate, and dibutyl tin diacetate. Each of these catalysts may be used solely, or two or more of these catalysts may be used in combination. Among these catalysts, a tin-based catalyst such as dibutyl tin dilaurylate, and dibutyl tin diacetate are preferable, and in particular, dibutyl tin dilaurylate is preferably used. 
     The golf ball material of the present invention preferably contains only the polyurethane as the resin composition, but may further contain ionomer resins or thermoplastic elastomers, as long as they do not impair the effect of the present invention. In this case, the content of the polyurethane is preferably 50 mass % or more, more preferably 60 mass % or more, even more preferably 70 mass or more in the resin component. 
     Examples of the ionomer resin include one prepared by neutralizing at least a part of carboxyl groups in a copolymer, composed of ethylene and α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with a metal ion; one prepared by neutralizing at least a part of carboxyl groups in a terpolymer composed of ethylene, α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and α,β-unsaturated carboxylic acid ester with a metal ion; or a mixture of these two. Examples of the α,β-unsaturated carboxylic acid include acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid, or the like. In particular, acrylic acid and methacrylic acid are preferable. Examples of the α,β-unsaturated carboxylic acid ester include methyl ester, ethyl ester, propyl ester, n-butyl ester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, and maleic acid. In particular, acrylic acid ester and methacrylic acid ester are preferable. Examples of the metal ion for neutralizing at least a part of the carboxyl groups in the copolymer composed of ethylene and the α,β-unsaturated carboxylic acid or in the terpolymer composed of ethylene, the α,β-unsaturated carboxylic acid, and the α,β-unsaturated carboxylic acid ester are; monovalent metal ions such as sodium, potassium, and lithium; divalent metal ions such as magnesium, calcium, zinc, barium, and cadmium; trivalent metal ions such as aluminum, or other metal ions such as tin and zirconium. In particular, sodium ion, zinc ion, and magnesium ion are preferably used in view of the resilience and durability of the golf ball. 
     Specific examples of the ionomer resin include “Himilan (registered trade mark)” available from MITSUI-DUPONT POLYCHEMICAL CO., LTD, “Surlyn (registered trade mark)” available from DUPONT CO, and “Iotek (registered trade mark)” available from Exxon Co. 
     Specific examples of the thermoplastic elastomers are a thermoplastic polyamide elastomer having a commercial name of “Pebax (registered trademark) (e.g. “Pebax 2533”)” commercially available from Arkema K. K.; a thermoplastic polyester elastomer having a commercial name of “Hytrel (registered trademark) (e.g. “Hytrel 3548”, “Hytrel 4047”)” commercially available from Du Pont-Toray Co., Ltd.; a thermoplastic polystyrene elastomer having a commercial name of “Rabalon (registered trademark) (e.g. “Rabalon T3221C”)” commercially available from Mitsubishi Chemical Corporation. The ionomer resins and the thermoplastic elastomers can be used solely or as a mixture of at least two of them. 
     The golf ball material of the present invention may contain a pigment component such as a white pigment (for example, titanium oxide) and a blue pigment, a gravity adjusting agent such as calcium carbonate and barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material or a fluorescent brightener. 
     The content of the white pigment (for example, titanium oxide) is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and is preferably 10 parts by mass or less, more preferably 8 parts by mass or less based on 100 parts by mass of the resin component. The white pigment in an amount of 0.5 part by mass or more can impart opacity to the golf ball material, while the white pigment in an amount of more than 10 parts by mass may lower the durability of the golf ball material. 
     The golf ball of the present invention is not limited, as long as it comprises a constituent member formed from the golf ball material of the present invention. For example, in a two-piece golf ball comprising a single-layered core and a cover disposed around the core, in a three-piece golf ball comprising a core having a center and a single-layered intermediate layer disposed around the center, and a cover disposed around the core, and in a multi-piece golf ball comprising a core having a center and at least two intermediate layer disposed around the center, and a cover disposed around the core, anyone of constituent members may be formed from the above golf ball material. Among them, it is preferable that the cover is formed from the above golf ball material. If the golf ball material of the present invention is used for the cover, the golf ball with a high spin rate on the approach shots and a low spin rate on the driver shots is obtained. 
     In the followings, the present invention will be explained based on the preferable golf ball that comprises a core and a cover, wherein the cover is formed from the above golf ball material. However, the present invention is not limited to this embodiment. 
     The cover of the golf ball of the present invention is formed from the golf ball material of the present invention (hereinafter, sometimes merely referred to as “cover composition”). A method for molding a cover is not particularly limited, and includes an embodiment which comprises injection molding the cover composition directly onto the core, or an embodiment which comprises molding the cover composition into a hollow-shell, covering the core with a plurality of the hollow-shells and subjecting the core with a plurality of the hollow shells to the compression-molding (preferably an embodiment which comprises molding the cover composition into a half hollow-shell, covering the core with the two half hollow-shells, and subjecting the core with the two half hollow-shells to the compression-molding). 
     Molding of the half shell can be performed by either compression molding method or injection molding method, and the compression molding method is preferred. The compression-molding of the cover composition into half shell can be carried out, for example, under a pressure of 1 MPa or more and 20 MPa or less at a temperature of −20° C. or more and 70° C. or less relative to the flow beginning temperature of the cover composition. By performing the molding under the above conditions, a half shell having a uniform thickness can be formed. Examples of a method for molding the cover using half shells include compression molding by covering the core with two half shells. The compression molding of half shells into the cover can be carried out, for example, under a pressure of 0.5 MPa or more and 25 MPa or less at a temperature of −20° C. or more and 70° C. or less relative to the flow beginning temperature of the cover composition. By performing the molding under the above conditions, a golf ball cover having a uniform thickness can be formed. 
     In the present invention, molding the cover by injection molding the cover composition directly on the core is also preferable. In this case, it is preferred to use upper and lower molds for forming a cover having a spherical cavity and pimples, wherein a part of the pimple also serves as a retractable hold pin. When forming the cover by injection molding, the hold pin is protruded to hold the core, and the cover composition which has been heated and melted is charged and then cooled to obtain a cover. For example, the cover composition heated and melted at the temperature of 150° C. to 250° C. is charged into a mold held under the pressure of 9 MPa to 15 MPa for 0.5 second to 5 seconds. After cooling for 10 to 60 seconds, the mold is opened and the golf ball with the cover molded is taken out from the mold. 
     When molding a cover, the concave portions called “dimple” are usually formed on the surface. After the cover is molded, the mold is opened and the golf ball body is taken out from the mold, and as necessary, the golf ball body is preferably subjected to surface treatments such as deburring, cleaning, and sandblast. If desired, a paint film or a mark may be formed. The paint film preferably has a thickness of, but not limited to, 5 μm or larger, and more preferably 7 μm or larger, and preferably has a thickness of 25 μm or smaller, and more preferably 18 μm or smaller. If the thickness is smaller than 5 μm, the paint film is easy to wear off due to continued use of the golf ball, and if the thickness is larger than 25 μm, the effect of the dimples is reduced, resulting in lowering flying performance of the golf ball. 
     In the present invention, the thickness of the cover of the golf ball is preferably 2.0 mm or less, more preferably 1.5 mm or less, even more preferably 1.0 mm or less. If the thickness of the cover is 2.0 mm or less, since it is possible to increase the diameter of the core, the resilience of the obtained golf ball is improved. The thickness of the cover is not limited, but is preferably 0.3 mm or more, more preferably 0.4 mm or more, and even more preferably 0.5 mm or more. If the thickness of the cover is less than 0.3 mm, it may become difficult to mold the cover. 
     The cover composition preferably has a slab hardness of 5 or more, more preferably 10 or more, and preferably has a slab hardness of 80 or less, more preferably 75 or less, even more preferably 70 or less, even more preferably 60 or less in Shore D hardness. If the slab hardness of the cover is less than 5, the repulsion property of the golf ball may be lowered, resulting in shortening a flight distance, while if the cover hardness is more than 80, the durability of the obtained golf ball may be lowered. Herein, the slab hardness of the cover is a measured hardness of the cover composition that is molded into a sheet form by a measuring method described later. 
     Next, a preferred embodiment of the core of the golf ball of the present invention will be explained. The core of the golf ball of the present invention includes, for example, a single-layered core, and a core consisting of a center and at least one intermediate layer covering the center. The core consisting of a center and at least one intermediate layer covering the center includes, for example, a core consisting of a center and a single-layered intermediate layer covering the center; and a core consisting of a center and multi-piece or multi-layer of intermediate layers covering the center. The core preferably has a spherical shape. If the core does not have a spherical shape, the cover does not have a uniform thickness. As a result, there exist some portions where the performance of the cover is lowered. On the other hand, the center generally has the spherical shape, but the center may be provided with a rib on the surface thereof so that the surface of the spherical center is divided by the ribs, preferably the surface of the spherical center is evenly divided by the ribs. In one embodiment, the ribs are preferably formed as a part of the center in an integrated manner on the surface of the center, and in another embodiment, the ribs are formed as an intermediate layer on the surface of the spherical center. 
     The ribs are preferably formed along an equatorial line and meridians that evenly divide the surface of the spherical center, if the spherical center is assumed as the earth. For example, if the surface of the spherical center is evenly divided into 8, the ribs are formed along the equatorial line, any meridian as a standard, and meridians at the longitude 90 degrees east, longitude 90 degrees west, and the longitude 180 degrees east(west), assuming that the meridian as the standard is at longitude 0 degree. If the ribs are formed, the depressed portion divided by the ribs are preferably filled with a plurality of intermediate layers or with a single-layered intermediate layer that fills each of the depressed portions to make a core in the spherical shape. The shape of the ribs, without limitation, includes an arc or an almost arc (for example, a part of the arc is removed to obtain a flat surface at the cross or orthogonal portions thereof). 
     The core or the center of the golf ball of the present invention, is preferably molded by, for example, heat-pressing a rubber composition (hereinafter, sometimes simply referred to as “core rubber composition”) containing a base rubber, a crosslinking initiator, a co-crosslinking agent, and where necessary a filler. 
     As the base rubber, a natural rubber or a synthetic rubber can be used. Such examples include a polybutadiene rubber, a natural rubber, a polyisoprene rubber, a styrene polybutadiene rubber, and ethylene-propylene-diene terpolymer (EPDM). Among them, typically preferred is the high cis-polybutadiene having cis-1,4 bond in a proportion of 40% or more, more preferably 70% or more, even more preferably 90% or more in view of its superior repulsion property. 
     The crosslinking initiator is blended to crosslink the base rubber component. As the crosslinking initiator, an organic peroxide is preferably used. Examples of the organic peroxide for use in the present invention are dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Among them, dicumyl peroxide is preferable. An amount of the crosslinking initiator to be blended in the rubber composition is preferably 0.2 part by mass or more, more preferably 0.3 part by mass or more, and is preferably 3 parts by mass or less, more preferably 2 parts by mass or less based on 100 parts by mass of the base rubber. If the amount is less than 0.2 part by mass, the core becomes too soft, and the resilience tends to be lowered, and if the amount is more than 3 parts by mass, the amount of the co-crosslinking agent needs to be increased in order to obtain an appropriate hardness, which may cause the insufficient resilience. 
     The co-crosslinking agent is not particularly limited, as long as it has the effect of crosslinking a rubber molecule by graft polymerization to a base rubber molecular chain; for example, α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms or a metal salt thereof, more preferably acrylic acid, methacrylic acid or a metal salt thereof may be used. As the metal constituting the metal salt, for example, zinc, magnesium, calcium, aluminum and sodium may be used, and among them, zinc is preferred because it provides high resilience. 
     The amount of the co-crosslinking agent to be used is preferably 10 parts or more, more preferably 20 parts or more, and is preferably 50 parts or less, more preferably 40 parts or less based on 100 parts of the base rubber by mass. If the amount of the co-crosslinking agent to be used is less than 10 parts by mass, the amount of the organic peroxide must be increased to obtain an appropriate hardness which tends to lower the resilience. On the other hand, if the amount of the co-crosslinking agent to be used is more than 50 parts by mass, the core becomes too hard, so that the shot feeling may be lowered. 
     The filler contained in the core rubber composition is mainly blended as a gravity adjusting agent in order to adjust the specific gravity of the golf ball obtained as the final product in the range of 1.0 to 1.5, and may be blended as required. Examples of the filler include an inorganic filler such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder. The amount of the filler to be blended in the rubber composition is preferably 2 parts or more, more preferably 3 parts or more, and preferably 50 parts or less, more preferably 35 parts or less based on 100 parts of the base rubber by mass. If the amount of the filler to be blended is less than 2 parts by mass, it becomes difficult to adjust the weight, while if it is more than 50 parts by mass, the weight ratio of the rubber component becomes small and the resilience tends to be lowered. 
     As the core rubber composition, an organic sulfur compound, an antioxidant or a peptizing agent may be blended appropriately in addition to the base rubber, the crosslinking initiator, the co-crosslinking agent and the filler. 
     As the organic sulfur compound, a diphenyl disulfide or a derivative thereof may be preferably used. Examples of the diphenyl disulfide or the derivative thereof include diphenyl disulfide; a mono-substituted diphenyl disulfide such as bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide and bis(4-cyanophenyl)disulfide; a di-substituted diphenyl disulfide such as bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, and bis(2-cyano-5-bromophenyl)disulfide; a tri-substituted diphenyl disulfide such as bis(2,4,6-trichlorophenyl)disulfide, and bis(2-cyano-4-chloro-6-bromophenyl)disulfide; a tetra-substituted diphenyl disulfide such as bis(2,3,5,6-tetra chlorophenyl)disulfide; a penta-substituted diphenyl disulfide such as bis(2,3,4,5,6-pentachlorophenyl)disulfide and bis(2,3,4,5,6-pentabromophenyl)disulfide. These diphenyl disulfides or the derivative thereof can enhance resilience by having some influence on the state of vulcanization of vulcanized rubber. Among them, diphenyl disulfide and bis(pentabromophenyl)disulfide are preferably used, since a golf ball having particularly high resilience can be obtained. The amount of the organic sulfur compound to be blended is preferably 0.1 part by mass or more, more preferably 0.3 part by mass or more, and preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less relative to 100 parts by mass of the base rubber. 
     The amount of the antioxidant to be blended is preferably 0.1 part or more and is preferably 1 part or less based on 100 parts of the base rubber by mass. Further, the amount of the peptizing agent is preferably 0.1 part or more and is preferably 5 parts or less based on 100 parts of the base rubber by mass. 
     The conditions for press-molding the core rubber composition may be determined appropriately depending on the rubber composition. The press-molding is preferably carried out for 10 to 60 minutes at the temperature of 130 to 200° C. Alternatively, the press-molding is preferably carried out in a two-step heating, for example, for 20 to 40 minutes at the temperature of 130 to 150° C., and continuously for 5 to 15 minutes at the temperature of 160 to 180° C. 
     The core used in the golf ball of the present invention preferably has a diameter of 38 mm or larger, more preferably 39.0 mm or larger, and even more preferably 40.8 mm or larger, and preferably has a diameter of 42.2 mm or smaller, more preferably 42 mm or smaller, and even more preferably 41.8 mm or smaller. If the diameter of the core is smaller than the above lower limit, the cover becomes so thick that the resulting golf ball would have reduced resilience. On the other hand, if the diameter of the core is larger than the above upper limit, the cover becomes so thin that it is difficult to mold a cover. 
     In the case that the core has a diameter ranging from 38 mm to 42.2 mm, the compression deformation amount (shrinking amount of the core in a compressive direction) of the core when applying a load from 98 N as an initial load to 1275 N as a final load is preferably 2.40 mm or more, more preferably 2.50 mm or more, even more preferably 2.60 mm or more, and is preferably 3.20 mm or less, and more preferably 3.10 mm or less. If the above deformation amount is less than 2.40 mm, the shot feeling becomes poor, while if the above deformation amount is larger than 3.20 mm, the repulsion property may be lowered. 
     In a preferable embodiment, the core has a hardness difference between the center and the surface. The difference between the surface hardness and the center hardness is preferably 10 or more, more preferably 12 or more, and is preferably 40 or less, more preferably 35 or less, and even more preferably 30 or less in JIS-C hardness. If the hardness difference is more than 40, the durability may be lowered, while if the hardness difference is less than 10, the shot feeling may be hard because of a large impact. The surface hardness of the core is preferably 65 or more, more preferably 70 or more, even more preferably 72 or more, and is preferably 100 or less in JIS-C hardness. If the surface hardness of the core is less than 65 in JIS-C hardness, the core is so soft and the repulsion property may be lowered, resulting in the short flight distance. On the other hand, if the surface hardness of the core is more than 100, the core is so hard and the shot feeling may deteriorate. The center hardness of the core is preferably 45 or more, more preferably 50 or more, and is preferably 70 or less, and more preferably 65 or less in JIS-C hardness. If the center hardness of the core is less than 45, the core is so soft and the durability may be lowered, while if the center hardness of the core is more than 70, the core is so hard and the shot feeling may be worsened. The hardness difference of the core can be provided by forming an intermediate layer having a higher hardness than that of the center or by properly selecting the heat molding conditions of the core. The center hardness of the core means a JIS-C hardness obtained by cutting a spherical core into halves and measuring at the central point of the cut surface using a JIS-C type spring hardness tester. The surface hardness means a hardness measured at a surface part of the core using a JIS-C type spring hardness tester. In the case that the core has a multi-layered structure, the surface hardness of the core means the hardness measured at the surface of the outermost layer of the core. 
     In the case that the core consists of a center and at least one intermediate layer covering the center, the center can be formed from the core rubber composition described above. The diameter of the center is preferably 30 mm or more, more preferably 32 mm or more, and is preferably 41 mm or less, more preferably 40.5 mm or less. If the diameter of the center is less than 30 mm, the intermediate layer or the cover layer must be made thicker than the desired thickness, resulting in the lowered resilience. On the other hand, if the diameter of the center is more than 41 mm, the intermediate layer or the cover must be made thinner than the desired thickness, and hence the intermediate layer or the cover does not function well. 
     Examples of the material for the intermediate layer are a thermoplastic polyamide elastomer having a commercial name of “Pebax (registered trademark) (e.g. Pebax 2533)” available from Arkema; a thermoplastic polyester elastomer having a commercial name of “Hytrel (registered trademark) (e.g. Hytrel 3548, Hytrel 4047)” available from Du Pont-Toray Co., Ltd.; a thermoplastic polyurethane elastomer having a commercial name of “Elastollan (registered trademark) (e.g. Elastollan XNY97A)” available from BASF Japan Co., a thermoplastic polystyrene elastomer having a commercial name of “Rabalon (registered trademark) (e.g. Rabalon SR04, Rabalon T3339C, Rabalon T3221C)” available from Mitsubishi Chemical Corporation, in addition to the cured product of the rubber composition or the conventional ionomer resin. The above materials for the intermediate layer can be used solely or as a mixture of at least two of them. 
     Examples of the ionomer resin include one prepared by neutralizing at least a part of carboxyl groups in a copolymer, composed of ethylene and α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with a metal ion; one prepared by neutralizing at least a part of carboxyl groups in a terpolymer composed of ethylene, α,β-unsaturated carboxylic acid having 3 to 8 carbons atoms, and α,β-unsaturated carboxylic acid ester with a metal ion; or a mixture of these two. 
     Specific examples of the ionomer resins include trade name “Himilan (registered trademark) (e.g. the binary copolymerized ionomer such as Himilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706 (Zn), Himilan 1707 (Na), Himilan AM7311 (Mg), Himilan AM7329 (Zn); and the ternary copolymerized ionomer such as Himilan 1856 (Na), Himilan 1855 (Zn))” commercially available from Du Pont-Mitsui Polychemicals Co., Ltd. 
     Further, examples include “Surlyn (registered trademark) (e.g. the binary copolymerized ionomer such as Surlyn 8945 (Na), Surlyn 9945 (Zn), Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn), Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li), Surlyn 7940 (Li), Surlyn AD8546 (Li); and the ternary copolymerized ionomer such as Surlyn 6320 (Mg), Surlyn 8120 (Na), Surlyn 8320 (Na), Surlyn 9320 (Zn))” and the ternary copolymerized ionomer such as “HPF 1000 (Mg), HPF 2000 (Mg)” commercially available from E.I. du Pont de Nemours and Company. 
     Further, examples include “Iotek (registered trademark) (e.g. the binary copolymerized ionomer such as Iotek 8000 (Na), Iotek 8030 (Na), Iotek 7010 (Zn), Iotek 7030 (Zn); and the ternary copolymerized ionomer such as Iotek 7510 (Zn), Iotek 7520 (Zn))” commercially available from ExxonMobil Chemical Corporation. 
     It is noted that Na, Zn, Li, and Mg described in the parentheses after the trade names indicate metal types of neutralizing metal ions for the ionomer resins. The intermediate layer may further contain a specific gravity adjusting agent such as barium sulfate or tungsten or the like; an antioxidant; or a pigment component. 
     In the case of using the intermediate layer composition containing a rubber composition as a main component (50 mass % or more), the intermediate layer preferably has a thickness of 1.2 mm or more, more preferably 1.8 mm or more, even more preferably 2.4 mm or more, and preferably has a thickness of 6.0 mm or less, more preferably 5.2 mm or less, even more preferably 4.4 mm or less. 
     In the case of using the intermediate layer composition containing the resin composition as a main component (50 mass % or more), the intermediate layer preferably has a thickness of 0.3 mm or more, more preferably 0.4 mm or more, even more preferably 0.5 mm or more, and preferably has a thickness of 2.5 mm or less, more preferably 2.4 mm or less, even more preferably 2.3 mm or less. If the thickness of the intermediate layer is more than 2.5 mm, the resilience performance of the obtained golf ball may be lowered, while if the thickness of the intermediate layer is less than 0.3 mm, it may be difficult to suppress the excessive spin rate on the driver shot. 
     A method for molding the intermediate layer is not particularly limited, and includes an embodiment which comprises injection molding the intermediate layer composition directly onto the center, or an embodiment which comprises molding the intermediate layer composition into a half hollow-shell, covering the center with the two hollow-shells and subjecting the center with the two hollow-shells to the compression-molding. 
     The intermediate layer of the golf ball of the present invention preferably has a slab hardness of 40 or larger, more preferably 45 or larger, and even more preferably 50 or larger, and preferably has a slab hardness of 80 or smaller, more preferably 70 or smaller, and even more preferably 65 or smaller in Shore D hardness. The intermediate layer having the slab hardness of 40 or more in shore D hardness makes the core have the higher degree of “hard outer and soft inner” structure, thereby providing a high launch angle and a less amount of spin and hence achieving a great flight distance of the gold ball. On the other hand, the intermediate layer having the slab hardness of 80 or less in shore D hardness provides an excellent shot feeling as well as improves the spin performance of the golf ball, thereby improving controllability of the golf ball. Herein, the slab hardness of the intermediate layer is the measured hardness of the intermediate layer composition in the form of a sheet, and is measured by a later-described measuring method. The slab hardness of the intermediate layer can be adjusted, for example, by appropriately selecting a combination of the above resin component and the rubber material and the amount of additives. 
     When preparing a wound golf ball in the present invention, a wound core may be used as the core. In that case, for example, a wound core comprising a center formed by curing the above rubber composition for the core and a rubber thread layer which is formed by winding a rubber thread around the center in an elongated state can be used. In the present invention, the rubber thread, which is conventionally used for winding around the center, can be adopted for winding around the center. The rubber thread, for example, is obtained by vulcanizing a rubber composition including a natural rubber, or a mixture of a natural rubber and a synthetic polyisoprene, a sulfur, a vulcanization auxiliary agent, a vulcanization accelerator, and an antioxidant. The rubber thread is wound around the center in elongation of about 10 times length to form the wound core. 
     According to the present invention, it is possible to provide a golf ball which has a high spin rate on the approach shots and a low spin rate on the driver shots. The spin rate on the approach shots is preferably 6,500 rpm or more, more preferably 6,550 rpm or more. If the spin rate on the approach shots is 6,500 rpm or more, the golf ball stops quickly on the green on the approach shots. The upper limit of the spin rate on the approach shots is not limited, but if the spin rate on the approach shots is too high, the spin rate on the driver shots may also become high. From this aspect, the spin rate on the approach shots is preferably 8,000 rpm or less, more preferably 7,800 rpm or less. On the other hand, the spin rate on the driver shots is preferably 2,600 rpm or less, more preferably 2,580 rpm or less. If the spin rate on the driver shots is 2,600 rpm or less, since the spin rate becomes low, the golf ball traveling a great distance is obtained. In order to increase the flight distance on the driver shots, the certain degree of the spin rate is necessary. Thus, the spin rate on the driver shots is preferably 2,000 rpm or more, more preferably 2,100 rpm or more, even more preferably 2,200 rpm or more. The spin rates on the approach shots and the driver shots are determined by the method described later. 
     EXAMPLES 
     The following examples illustrate the present invention, however these examples are intended to illustrate the invention and are not to be construed to limit the scope of the present invention. Many variations and modifications of such examples will exist without departing from the scope of the inventions. Such variations and modifications are intended to be within the scope of the invention. 
     [Evaluation Methods] 
     (1) Shear Loss Modulus G″ 
     The shear loss modulus G″ of the polyurethane was measured at the following conditions. 
     Apparatus: Rheometer ARES available from TA instruments 
     Test piece: A polyurethane sheet having a thickness of 2 mm was produced by a press molding and a test piece was cut out to have a width 10 mm and a length between the clamps of 10 mm. 
     Measuring mode: shear mode 
     Measuring temp.: 0° C. 
     Oscillation frequency: 10 Hz 
     Measuring strain: 0.1% 
     (2) Tensile Loss Modulus E″ 
     The tensile loss modulus E″ of the polyurethane was measured at the following conditions. 
     Apparatus: Viscoelasticity measuring apparatus Rheogel-E4000 available from UBM CO., Ltd. 
     Test piece: A polyurethane sheet having a thickness of 2 mm was produced by a press molding and a test piece was cut out to have a width 4 mm and a length between the clamps of 20 mm. 
     Measuring mode: tensile mode 
     Measuring temp.: 0° C. 
     Oscillation frequency: 10 Hz 
     Measuring strain: 0.1% 
     (3) Spin Rate on the Approach Shots (Dry Spin Rate, Wet Spin Rate, Spin Retention) 
     An approach wedge (SRIXON I-302, Shaft S available from SRI Sports Limited) was installed on a swing robot available from Golf Laboratories, Inc. A golf ball was hit at a head speed of 21 m/sec., and a sequence of photographs of the hit golf ball were taken for measuring the spin rate (rpm). The measurement was performed ten times for each golf ball, and the average value is regarded as the spin rate(rpm). “Dry spin rate” means a spin rate when the test was conducted under the condition that the club face and the golf ball were dry, and “Wet spin rate” means a spin rate when the test was conducted under the condition that the club face and the golf ball were wet with water. Spin retention can be calculated by the following mathematical expression.
 
Spin retention(%)=100×Wet spin rate/Dry spin rate
 
(4) Spin Rate on the Driver Shots
 
     A driver (XXIO, shaft S, Loft angle: 11° available from SRI Sports Limited) was installed on a swing robot available from Golf Laboratories, Inc. A golf ball was hit at a head speed of 50 m/sec., and a sequence of photographs of the hit golf ball were taken for measuring the spin rate (rpm). The measurement was performed ten times for each golf ball, and the average value is regarded as the spin rate(rpm). 
     (5) Slab Hardness (Shore D Hardness) 
     Sheets having a thickness of about 2 mm were prepared from, polyurethane, the cover composition or the intermediate layer composition by hot press molding and preserved at the temperature of 23° C. for two weeks. Three or more of the sheets were stacked on one another to avoid being affected by the measuring substrate on which the sheets were placed, and the stack was subjected to the measurement using a P1 type auto hardness tester provided with the Shore D type spring hardness tester prescribed by ASTM-D2240, available from KOUBUNSHI KEIKI CO., LTD to obtain the respective slab hardness of the polyurethane, the cover composition or the intermediate layer composition. 
     (6) Core Hardness (JIS-C) 
     The hardness measured at a surface part of a spherical core using a P1 type auto hardness tester provided with the JIS-C type spring hardness tester available from KOUBUNSHI KEIKI CO., LTD, was determined as the surface hardness of the spherical core, and the JIS-C hardness obtained by cutting a spherical core into halves and measuring at the central point of the cut surface was determined as the center hardness of the spherical core. 
     (7) Number Average Molecular Weight of Polyol component 
     Gel permeation chromatography was conducted to determine the number average molecular weight of the polyol component under the following conditions. 
     Measuring Conditions: 
     Apparatus: HLC-8120GPC manufactured by Tosoh Corporation 
     Eluent: THF 
     Temperature: 40° C. 
     Column: TSK gel Super HM-M (manufactured by Tosho Corporation) 
     Polyol concentration: 0.2 mass % (Polyol/(polyol+THF)) 
     Sample injection volume: 5 ul 
     Flow rate: 0.5 ml/min 
     Molecular weight standard: polystyrene (PSt Quick Kit-H, manufactured by Tosoh Corporation). 
     [Production of the Golf Ball] 
     (1) Preparation of the Center 
     The center rubber compositions having formulation shown in Table 1 were kneaded and pressed in upper and lower molds, each having a hemispherical cavity, at a temperature of 170° C. for 15 minutes to obtain the center in a spherical shape (diameter 38.5 mm). 
                                 TABLE 1                       Center rubber composition   A                                                    Polybutadiene rubber   100           Zinc acrylate   35           Zinc oxide   5           Diphenyl disulfide   0.5           Dicumyl peroxide   1                       Notes on table 1:           Parts by mass           Polybutadiene rubber: “BR730 (high cis-polybutadiene)” manufactured by JSR Corporation           Zinc acrylate: “ZNDA-90S” manufactured by NIHON JYORYU KOGYO Co,. LTD.           Zinc oxide: “Ginrei R” manufactured by Toho-Zinc Co.           Diphenyl disulfide: manufactured by Sumitomo Seika Chemicals Company Limited           Dicumyl peroxide: “Percumyl D” manufactured by NOF Corporation            
(2) Preparation of Core
 
     Next, the materials for the intermediate layer shown in Table 2 were extruded by a twin-screw kneading extruder to prepare an intermediate layer composition in the form of pellet. Extrusion was performed in the following conditions: screw diameter=45 mm; screw revolutions=200 rpm; and screw L/D=35. The mixtures were heated to a temperature ranging from 150° C. to 230° C. at a die position of the extruder. The obtained intermediate layer composition was injection molded on the center which had been obtained as described above, to prepare a core (diameter 41.7 mm) consisting of the center and the intermediate layer covering the center. 
                             TABLE 2                       Core No.           1                                                Center   Center composition   A           Center diameter (mm)   38.5       Intermediate layer   Intermediate layer composition   a           Himilan 1605   50           Himilan AM7329   50           Slab hardness (Shore D)   64           Thickness (mm)   1.6       Core Property   Diameter (mm)   41.7           Surface hardness (JIS-C)   98           Center hardness (JIS-C)   65           Hardness difference (JIS-C)   33           Compression deformation amount (mm)   2.55               Formulation: parts by mass       Notes on table 2:       Himilan 1605: sodium ion neutralized ethylene-methacrylic acid copolymerized ionomer resin manufactured by MITSUI-DUPONT POLYCHEMICAL CO., LTD.       Himilan AM7329: zinc ion neutralized ethylene-methacrylic acid copolymerized ionomer resin manufactured by MITSUI-DUPONT POLYCHEMICAL CO., LTD.            
(3) Synthesis of Polyurethane
 
     Polyurethanes having the compositions shown in Tables 3 to 4 were synthesized as follows. First, polytetramethylene ether glycol (PTMG) heated at the temperature of 80° C. was added to dicyclohexylmethane diisocyanate (H 12 MDI) heated at the temperature of 80° C. Then, dibutyl tin dilaurate (dibutyl tin dilaurate available from Aldrich, Inc.) of 0.005 mass % of the total amount of the raw materials (H 12 MDI, PTMG, and BD) was added thereto. Then, the mixture was stirred at the temperature of 80° C. for 2 hours under a nitrogen gas flow. Under a nitrogen gas flow, butane diol (BD) heated at the temperature of 80° C. was added as a chain extender to the mixture, and the mixture was stirred at the temperature of 80° C. for 1 minute. Then, the reaction liquid was cooled, and degassed under the reduced pressure for 1 minute at the room temperature. After the degassing, the reaction liquid was spread in a container, kept at the temperature of 110° C. for 6 hours under a nitrogen gas atmosphere to carry out a chain extending reaction, thereby obtaining polyurethanes. 
     (4) Molding of Half Shells 
     The polyurethane thus obtained were dry blended with titanium oxide, and mixed by a twin-screw kneading extruder to prepare cover compositions in the form of pellet. Extrusion was performed in the following conditions: screw diameter=45 mm; screw revolutions=200 rpm; and screw L/D=35. The mixtures were heated to a temperature ranging from 150° C. to 230° C. at a die position of the extruder. Compression molding of half shells were performed by, charging one pellet of the cover composition obtained as described above into each of depressed parts of lower molds for molding half shells, and applying pressure to mold half shells. Compression molding was performed at a temperature of 170° C. for 5 minutes under a molding pressure of 2.94 MPa. 
     (5) Molding of the Cover 
     The core obtained in (2) was covered with the two half shells obtained in (4) in a concentric manner, and the cover was molded by compression molding. Compression molding was performed at a temperature of 145° C. for 2 minutes under a molding pressure of 9.8 MPa. 
     The surface of the obtained golf ball body was subjected to a sandblast treatment, and marking, and then clear paint was applied thereto and dried in an oven at a temperature of 40° C. to obtain a golf ball having a diameter of 42.7 mm and a weight of 45.3 g. The spin performance of the obtained golf ball was evaluated, and results thereof are also shown in Tables 3 and 4. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 Golf ball No. 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                   
               
               
                 Core 
                 Core No. 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
                 Intermediate layer Slab hardness (Shore D) 
                 64 
                 64 
                 64 
                 64 
                 64 
                 64 
                 64 
               
               
                   
                 Intermediate layer thickness (mm) 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
               
               
                   
                 Core Diameter (mm) 
                 41.7 
                 41.7 
                 41.7 
                 41.7 
                 41.7 
                 41.7 
                 41.7 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Cover 
                 Polyurethane 
                 PTMG MW = 650 
                 1 
                 1 
                 1 
                 1 
                 — 
                 — 
                 — 
               
               
                 composition 
                 comp. 
                 PTMG MW = 850 
                 — 
                 — 
                 — 
                 — 
                 1 
                 1 
                 1 
               
               
                   
                 (molar ratio) 
                 PTMG MW = 1000 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
                 H 12 MDI 
                 2.82 
                 2.46 
                 2.21 
                 1.78 
                 3.8 
                 3.2 
                 2.6 
               
               
                   
                   
                 BD (Butane diol) 
                 1.82 
                 1.46 
                 1.21 
                 0.78 
                 2.8 
                 2.2 
                 1.6 
               
               
                   
                   
                 Slab hardness of Polyurethane 
                 53 
                 38 
                 34 
                 18 
                 42 
                 35 
                 27 
               
               
                   
                   
                 (Shore D) 
               
               
                   
                   
                 Shear loss modulus G″ (×10 7  Pa) 
                 7.86 
                 4.41 
                 4.09 
                 1.43 
                 5.03 
                 4.12 
                 3.02 
               
               
                   
                   
                 Tensile loss modulus E″ 
                 9.98 
                 9.22 
                 7.95 
                 6.48 
                 9.22 
                 8.22 
                 8.22 
               
               
                   
                   
                 (×10 7  Pa) 
               
               
                   
                   
                 E″/G″ 
                 1.27 
                 2.09 
                 1.94 
                 4.53 
                 1.83 
                 2.00 
                 2.72 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Slab hardness of Cover composition (Shore D) 
                 54 
                 39 
                 35 
                 19 
                 43 
                 36 
                 28 
               
               
                 Ball 
                 Cover thickness (mm) 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
               
               
                   
                 Dry spin rate on Approach shots (rpm) 
                 5861 
                 6208 
                 6288 
                 6752 
                 6202 
                 6301 
                 6506 
               
               
                   
                 Wet spin rate on Approach shots (rpm) 
                 3800 
                 4200 
                 4700 
                 5000 
                 4800 
                 5400 
                 5600 
               
               
                   
                 Spin retention (wet/dry) % 
                 65 
                 68 
                 75 
                 74 
                 77 
                 86 
                 86 
               
               
                   
                 Spin rate on Driver shots (rpm) 
                 2375 
                 2368 
                 2421 
                 2450 
                 2383 
                 2402 
                 2423 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Golf ball No. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 8 
                 9 
                 10 
                 11 
                 12 
               
               
                   
                   
               
               
                   
                 Core 
                 Core No. 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
                   
                 Intermediate layer Slab hardness (Shore D) 
                 64 
                 64 
                 64 
                 64 
                 64 
               
               
                   
                   
                 Intermediate layer thickness (mm) 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
               
               
                   
                   
                 Core Diameter (mm) 
                 41.7 
                 41.7 
                 41.7 
                 41.7 
                 41.7 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Cover 
                 Polyurethane 
                 PTMG MW = 650 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 composition 
                 comp. 
                 PTMG MW = 850 
                 1 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
                 (molar ratio) 
                 PTMG MW = 1000 
                 — 
                 1 
                 1 
                 1 
                 1 
               
               
                   
                   
                   
                 H 12 MDI 
                 1.73 
                 3.82 
                 3.22 
                 2.63 
                 1.7 
               
               
                   
                   
                   
                 BD (Butane diol) 
                 0.73 
                 2.82 
                 2.22 
                 1.63 
                 0.7 
               
               
                   
                   
                   
                 Slab hardness of Polyurethane 
                 17 
                 44 
                 36 
                 29 
                 18 
               
               
                   
                   
                   
                 (Shore D) 
               
               
                   
                   
                   
                 Shear loss modulus G″ (×10 7  Pa) 
                 1.79 
                 4.66 
                 4.34 
                 3.00 
                 1.38 
               
               
                   
                   
                   
                 Tensile loss modulus E″ 
                 5.52 
                 9.26 
                 7.01 
                 6.26 
                 3.71 
               
               
                   
                   
                   
                 (×10 7  Pa) 
               
               
                   
                   
                   
                 E″/G″ 
                 3.08 
                 1.99 
                 1.62 
                 2.09 
                 2.69 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Slab hardness of Cover composition (Shore D) 
                 18 
                 45 
                 37 
                 30 
                 19 
               
               
                   
                 Ball 
                 Cover thickness (mm) 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
               
               
                   
                   
                 Dry spin rate on Approach shots (rpm) 
                 6804 
                 6229 
                 6308 
                 6462 
                 6788 
               
               
                   
                   
                 Wet spin rate on Approach shots (rpm) 
                 4200 
                 5400 
                 5700 
                 5700 
                 3800 
               
               
                   
                   
                 Spin retention (wet/dry) % 
                 62 
                 87 
                 90 
                 88 
                 56 
               
               
                   
                   
                 Spin rate on Driver shots (rpm) 
                 2460 
                 2385 
                 2446 
                 2434 
                 2502 
               
               
                   
                   
               
               
                   
                 Cover composition: polyurethane 100 parts, titanium oxide 4 parts 
               
            
           
         
       
     
                             TABLE 4                          Golf ball No.                                                     13   14   15   16   17   18               Core   Core No.   1   1   1   1   1   1           Intermediate layer Slab hardness (Shore D)   64   64   64   64   64   64           Intermediate layer thickness (mm)   1.6   1.6   1.6   1.6   1.6   1.6           Core Diameter (mm)   41.7   41.7   41.7   41.7   41.7   41.7                                                 Cover   Polyurethane   PTMG MW = 1500   1   1   1   1   —   —       composition   comp.   PTMG MW = 2000   —   —   —   —   1   1           (molar ratio)   PTMG MW = 3000   —   —   —   —   —   —               H 12 MDI   4.89   4.12   3.85   3.39   6.03   5.12               BD (Butane diol)   3.89   3.12   2.85   2.39   5.03   4.12               Slab hardness of Polyurethane   47   37   34   30   50   38               (Shore D)               Shear loss modulus G″ (×10 7  Pa)   3.59   1.95   1.83   1.02   3.02   2.11               Tensile loss modulus E″   1.05   6.08   5.15   1.80   6.00   3.75               (×10 7  Pa)               E″/G″   0.29   3.12   2.81   1.76   1.99   1.78                                                 Slab hardness of Cover composition (Shore D)   48   38   35   31   51   39       Ball   Cover thickness (mm)   0.5   0.5   0.5   0.5   0.5   0.5           Dry spin rate on Approach shots (rpm)   6250   6561   6651   6850   6475   6609           Wet spin rate on Approach shots (rpm)   5700   5800   5700   5000   5400   5400           Spin retention (wet/dry) %   91   88   86   73   83   82           Spin rate on Driver shots (rpm)   2338   2463   2510   2607   2466   2550                                 Golf ball No.                                                     19   20   21   22                       Core   Core No.   1   1   1   1               Intermediate layer Slab hardness (Shore D)   64   64   64   64               Intermediate layer thickness (mm)   1.6   1.6   1.6   1.6               Core Diameter (mm)   41.7   41.7   41.7   41.7                                                 Cover   Polyurethane   PTMG MW = 1500   —   —   —   —           composition   comp.   PTMG MW = 2000   1   —   —   —               (molar ratio)   PTMG MW = 3000   —   1   1   1                   H 12 MDI   4   6   5.1   4                   BD (Butane diol)   3   5   4.1   3                   Slab hardness of Polyurethane   29   53   40   30                   (Shore D)                   Shear loss modulus G″ (×10 7  Pa)   1.70   3.02   2.11   1.70                   Tensile loss modulus E″   1.75   7.82   3.75   1.02                   (×10 7  Pa)                   E″/G″   1.03   2.59   1.78   0.60                                                 Slab hardness of Cover composition (Shore D)   30   54   41   31           Ball   Cover thickness (mm)   0.5   0.5   0.5   0.5               Dry spin rate on Approach shots (rpm)   6663   6471   6601   6669               Wet spin rate on Approach shots (rpm)   4300   5400   5200   3800               Spin retention (wet/dry) %   65   83   79   57               Spin rate on Driver shots (rpm)   2595   2421   2571   2656                       Cover composition: polyurethane 100 parts, titanium oxide 4 parts            
Materials in tables 3 to 4:
 
H 12 MDI: Desmodur available from Sumika Bayer Urethane Co., Ltd.
 
PTMG650: Polytetramethylene ether glycol, PTMG-650 (Number average molecular weight 650) available from DIA CHEMICAL Co., Ltd.
 
PTMG850: Polytetramethylene ether glycol, PTMG-850SN (Number average molecular weight 850) available from HODOGAYA CHEMICAL Co., Ltd.
 
PTMG1000: Polytetramethylene ether glycol, PTMG-1000SN (Number average molecular weight: 1000) available from HODOGAYA CHEMICAL Co., Ltd.
 
PTMG1500: Polytetramethylene ether glycol, PTMG-1500SN (Number average molecular weight 1500) available from HODOGAYA CHEMICAL Co., Ltd.
 
PTMG2000: Polytetramethylene ether glycol, PTMG-2000SN (Number average molecular weight 2000) available from HODOGAYA CHEMICAL Co., Ltd.
 
PTMG3000: Polytetramethylene ether glycol, PTMG-3000SN (Number average molecular weight 3000) available from HODOGAYA CHEMICAL Co., Ltd.
 
BD: 1,4-butanediol available from WAKO Pure Chemicals, Industries, Ltd.
 
Dibutyl tin dilaurate: dibutyl tin dilaurate available from Aldrich.
 
       FIG. 1  is a graph showing a correlation between the spin rate on the approach shots and the shear loss modulus G″ shown in Tables 3 and 4. As apparent from  FIG. 1 , the spin rate on the approach shots and the shear loss modulus G″ showed a good correlation. It was found that the spin rate on the approach shots increases as the shear loss modulus G″ gets small. 
       FIG. 2  is a graph showing a correlation between the spin rate on the driver shots and the tensile loss modulus E″ shown in Tables 3 and 4. As apparent from  FIG. 2 , the spin rate on the driver shots and the tensile loss modulus E″ showed a good correlation. It was found that the spin rate on the driver shots decreases as the tensile loss modulus E″ gets large. 
     In tables 3 and 4, it is apparent that the golf balls satisfying a shear loss modulus G″ of 2.11×10 7  Pa or less, and a ratio (E″/G″) of a tensile loss modulus E″ to the shear loss modulus G″ of 1.78 or more, when measuring the shear loss modulus G″ in a shear mode and the tensile loss modulus E′ in a tensile mode at conditions of a temperature of 0° C., oscillation frequency of 10 Hz using a dynamic viscoelasticity measuring apparatus have a low spin rate on the driver shots and a high spin rate on the approach shots. Especially, in the case that the polyol component constituting the polyurethane having a number average molecular weight raging from 1,500 to 3,000, the spin retention was high, and thus the golf balls would stop quickly on the green even in the rainy weather. 
     According to the present invention, it is possible to provide a golf ball with a high spin rate on the approach shots and a low spin rate on the driver shots. This application is based on Japanese Patent application No. 2009-285367 filed on Dec. 16, 2009, the contents of which are hereby incorporated by reference.