Golf ball and method manufacturing the same

Abstract

To provide a golf ball having better controllability on approach shots and better scratch resistance as compared with conventional golf balls with urethane covers, and a method of manufacturing the golf ball.SOLUTION: A golf ball includes a core and a cover, where the cover is formed from a resin material primarily composed of polyurethane. The crosslink density of the polyurethane is 1.4×102 mol / m3 or more and 35×102 mol / m3 or less. Preferably, the polyol component of the polyurethane is polytetramethylene glycol (PTMG). The value of (T / D)×(Mn) is 1000 or less, where the crosslink density is (T)×102 mol / m3, the Shore D hardness of the cover is D, and the number average molecular weight of polytetramethylene glycol (PTMG) is Mn. A method of manufacturing the golf ball is also provided.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a golf ball comprising a core and a cover mainly made of a polyurethane resin having a relatively low crosslink density, and a method for manufacturing the same.

Background Art

[0002] The required characteristics of a golf ball are mainly an increase in flight distance. In addition, there are performance such as the ball stopping well during an approach shot and scratch resistance. Conventionally, many golf balls that fly well when struck with a driver and have suitable backspin during an approach shot have been developed. Recently, for professionals and advanced players, as the cover (outermost layer) of a golf ball, many are adopting a polyurethane resin material as an alternative to an ionomer resin material.

[0003] The polyurethane resin is composed of a soft segment made of a high molecular polyol which is a long-chain polyol, and a chain extender and a polyisocyanate that constitute a hard segment. As the polyurethane resin used for a golf ball, thermoplastic polyurethane elastomer (TPU) is often used, and as the polyol component, PTMG (polytetramethylene glycol) is often used. Some cover materials of polymer blends in which other resin materials are mixed with a polyurethane resin as a base resin have been proposed, but there are few documents focusing on the chemical structure and physical properties of the urethane resin itself.

[0004] As a prior art document focusing on the crosslink density of the polyurethane resin in order to improve the physical properties of a golf ball, Japanese Patent Application Laid-Open No. 2012-75911 can be cited. However, this document aims at and has the effect of increasing the crosslink density of the polyurethane resin by electron beam irradiation to improve durability, and does not disclose improving controllability and scratch resistance during an approach shot.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention has been made in view of the above circumstances, and an object thereof is to provide a golf ball having excellent controllability during an approach shot and excellent scratch resistance, as compared with a conventional golf ball having a cover made of a polyurethane resin, and a method for manufacturing the same.

Means for Solving the Problems

[0007] As a result of intensive studies to achieve the above object, the present inventor has found that in a golf ball having a core and a cover, when polyurethane is used as a material for the cover, by adjusting the crosslink density thereof to a specific range, the amount of spin during approach can be increased, and a golf ball excellent in scratch resistance can be obtained, and thus the present invention has been completed. Further, paying attention to the above crosslink density, the Shore D hardness of the cover, and the number average molecular weight of the polyol component of the polyurethane, it has been found that the above effects of the present invention can be sufficiently exhibited by specifying these relationships.

[0008] Therefore, the present invention provides the following golf ball and a method for manufacturing the same. 1. In a golf ball having a core and a cover, the cover is formed of a resin composition mainly composed of a polyurethane resin, and the crosslink density of the polyurethane resin is 1.4×10 2 mol / m 3 or more and 35×10 2 mol / m 3 or less. A golf ball characterized by the above. 2. The polyol component of the polyurethane resin is polytetramethylene glycol (PTMG), and the crosslink density is T×10 2 mol / m 3When the Shore D hardness of the cover is D and the number average molecular weight of polytetramethylene glycol (PTMG) is Mn, the golf ball according to Item 1 above, wherein the value of (T / D) × Mn is 1000 or less. 3. The golf ball according to Item 2 above, wherein the value of T / D is 0.6 or less. 4. The golf ball according to Item 2 above, wherein the value of Mn is 1000 or more. 5. The golf ball according to Item 2 above, wherein the value of Mn is 2000 or more. 6. The golf ball according to Item 2 above, wherein the value of Mn is 3000 or more. 7. The golf ball according to Item 1 or 2 above, wherein the thickness of the cover is 1.5 mm or less. 8. The golf ball according to Item 1 or 2 above, wherein the Shore D hardness of the polyurethane resin is 30 to 60. 9. A method for manufacturing a golf ball having a core and a cover, in the step of forming a cover with a resin composition mainly composed of a polyurethane resin, measuring the crosslink density of the polyurethane resin based on a toluene swelling test, and the crosslink density is 1.4 × 10 2 mol / m or more and 35 × 10 2 mol / m 3 The method for manufacturing a golf ball, characterized in that a polyurethane resin that is less than or equal to is selected as the main material of the resin composition of the cover.

Advantages of the Invention

[0009] According to the golf ball and the method for manufacturing the same of the present invention, by adjusting the crosslink density of the polyurethane cover to a specific range, the amount of spin during approach can be increased, and a golf ball excellent in scratch resistance can be obtained.

Brief Description of the Drawings

[0010]

Figure 1

Embodiments for Carrying Out the Invention

[0011] The present invention will be described in more detail below. The golf ball of the present invention has a ball structure in which at least one layer of a cover, that is, a single-layer or multi-layer cover, is coated on a core composed of at least one layer.

[0012] The above core can be formed using a known rubber material as a base material. As the base rubber, a known base rubber of natural rubber or synthetic rubber can be used. More specifically, it is recommended to mainly use polybutadiene, particularly cis-1,4-polybutadiene having at least 40% or more of a cis structure. Further, in the base rubber, natural rubber, polyisoprene rubber, styrene-butadiene rubber, etc. can be used in combination with the above-mentioned polybutadiene as desired.

[0013] Also, polybutadiene can be synthesized by a metal catalyst such as a rare earth element-based catalyst of an Nd catalyst, a cobalt catalyst, and a nickel catalyst.

[0014] To the above base rubber, a co-crosslinking agent such as an unsaturated carboxylic acid and its metal salt, an inorganic filler such as zinc oxide, barium sulfate, and calcium carbonate, and an organic peroxide such as dicumyl peroxide and 1,1-bis(t-butylperoxy)cyclohexane can be blended. Further, if necessary, a commercially available antioxidant or the like can be appropriately added.

[0015] The above core can be manufactured by vulcanizing and curing a rubber composition containing the above components. For example, kneading is performed using a kneader such as a Banbury mixer or a roll, and compression molding or injection molding is performed using a core mold. The molded body is appropriately heated under conditions of 100 to 200 °C, preferably 140 to 180 °C, and 10 to 40 minutes, which is a temperature sufficient for the action of the organic peroxide and the co-crosslinking agent, to cure the molded body and manufacture it.

[0016] The golf ball of the present invention has a core covered with a single-layer or multiple-layer cover. Examples of such golf ball embodiments include, for example, a golf ball having a single-layer cover on the core, and a golf ball having a core, an intermediate layer covering the core, and an outermost layer covering the intermediate layer.

[0017] The present invention is formed by a cover resin composition containing a polyurethane resin as at least one layer of the resin material of the cover. The polyurethane resin can be the main material or the base resin of the cover resin composition. The details of this component, polyurethane resin (hereinafter referred to as "polyurethane"), are as follows.

[0018] The structure of the polyurethane consists of a soft segment composed of a high molecular polyol which is a long-chain polyol, and a chain extender and a polyisocyanate constituting the hard segment. Here, as the high molecular polyol used as a raw material, any of those conventionally used in the technology related to polyurethane materials can be used, and there is no particular limitation. For example, polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymerization polyols, castor oil polyols, silicone polyols, vinyl polymerization polyols, etc. can be mentioned. As the polyester polyol, specifically, adipate polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol, polyhexamethylene adipate glycol, and lactone polyols such as polycaprolactone polyol can be adopted. Examples of the polyether polyol include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and polymethyltetramethylene glycol. These may be used alone or in combination of two or more.

[0019] As the above-mentioned polymer polyol, it is preferable to use a polyether-based polyol, particularly polytetramethylene glycol (PTMG). As for the polyol used in polyurethane, a larger number-average molecular weight of PTMG is preferable in terms of the approach spin performance and scratch resistance performance of the golf ball. However, although the ball performance is higher when the cover material has a softer hardness, pinning during ball molding and injuries during polishing are likely to occur, and the moldability tends to deteriorate.

[0020] The number-average molecular weight of the above-mentioned polymer polyol is preferably in the range of 1,000 to 5,000. By using a polymer polyol having such a number-average molecular weight, a golf ball made of a polyurethane composition excellent in various properties such as the above-mentioned resilience and productivity can be surely obtained. The number-average molecular weight of the polymer is more preferably in the range of 1,500 to 4,000, and still more preferably in the range of 1,700 to 3,500.

[0021] Note that the above-mentioned number-average molecular weight is the number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS-K1557 (hereinafter the same).

[0022] As the chain extender, those used in the conventional technology related to polyurethane can be preferably used, and there is no particular limitation. In the present invention, a low-molecular compound having two or more active hydrogen atoms capable of reacting with an isocyanate group in the molecule and having a molecular weight of 2,000 or less can be used, and among them, an aliphatic diol having 2 to 12 carbon atoms can be preferably used. Specifically, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, 2,2-dimethyl-1,3-propanediol, etc. can be mentioned, and among them, 1,4-butylene glycol can be particularly preferably used.

[0023] As the polyisocyanate, those used in the prior art related to conventional polyurethanes can be preferably used, and there is no particular limitation. Specifically, 4,4'-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, naphthylene 1,5-diisocyanate, tetramethylxylylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane, dimer acid diisocyanate, one or more selected from the group consisting thereof can be used. However, depending on the isocyanate species, it may be difficult to control the crosslinking reaction during injection molding.

[0024] In addition, the mixing ratio of the active hydrogen atom to the isocyanate group in the polyurethane formation reaction can be adjusted within an appropriate preferable range as appropriate. Specifically, when producing a polyurethane by reacting the above-mentioned long-chain polyol, polyisocyanate compound and chain extender, with respect to 1 mol of the active hydrogen atoms possessed by the long-chain polyol and the chain extender, it is preferable to use each component at a ratio such that the isocyanate groups contained in the polyisocyanate compound are 0.95 to 1.05 mol.

[0025] The method for producing the polyurethane is not particularly limited, and it may be produced by any of the prepolymer method and the one-shot method using a known urethanization reaction using a long-chain polyol, a chain extender and a polyisocyanate compound. Among them, it is preferable to carry out melt polymerization substantially in the absence of a solvent, and particularly preferably to produce by continuous melt polymerization using a multi-screw type extruder.

[0026] As the above-mentioned polyurethane, it is preferable to use a thermoplastic polyurethane material, and particularly preferably an ether-based thermoplastic polyurethane material. As the thermoplastic polyurethane material, commercially available products can be preferably used. For example, products named "Pandex" manufactured by DIC Covestro Polymer Co., Ltd., and products named "Resamin" manufactured by Dainichi Seika Kogyo Co., Ltd. can be mentioned.

[0027] The above-mentioned polyurethane is the main material of the resin composition of the cover. From the viewpoint of sufficiently imparting scratch resistance to the golf ball, it is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, and most preferably 90% by mass or more of the resin composition.

[0028] In the present invention, a polyurethane resin having a crosslink density of 1.4×10 2 mol / m 3 or more and 35×10 2 mol / m 3 or less is used as the main material of the resin composition of the cover. Thereby, it is possible to provide a golf ball having excellent controllability during an approach shot and excellent scratch resistance. The crosslink density and network chain concentration that effectively act on the elasticity that controls the mechanical properties of the polyurethane elastomer are contributed by chemical crosslinking and various physical crosslinkings, and it is very difficult to separate and quantify them due to side reactions during the crosslinking reaction, the complexity of the crosslinking form and properties. However, the crosslink density of the polyurethane elastomer can be measured by the swelling method. An example of this swelling method is as follows.

[0029] The polyurethane was molded into a sheet with a thickness of 2 mm, cut into test pieces measuring 3 mm in length × 3 mm in width × 2 mm in thickness, and used as samples. The sample weights were measured using an electronic balance capable of measuring in units of four decimal places (mg). A 10-ml vial was prepared, 8 ml of toluene and the samples were added, stoppered and sealed, and left standing for 72 hours or more. Then, the internal solution was discarded, the toluene adhering to the sample surface was wiped off, and the sample weight after immersion was measured. The crosslink density of the polyurethane was calculated using the Flory-Rehner equation from the sample weights before and after swelling. ν = -(ln(1 - v r ) + v r + χv r 2 ) / V S (v r 1 / 3 - v r / 2) [ν: Crosslink density, v r : Volume fraction of polyurethane during swelling, χ: Interaction constant, V S : Molar volume of toluene] v r = V TPU / (V TPU + V T ) V TPU = (w f - w f v f ) / ρ V T = (w s - w f ) / ρ T [V TPU : Volume of polyurethane, V T : Volume of swollen toluene, v f : Weight fraction of filler in polyurethane, ρ: Density of polyurethane, w f : Sample weight before immersion, w s : Sample weight after immersion, ρ T : Density of toluene] Note that Vs is 0.1063 × 10 -3 m 3 / mol, ρ Tis 0.8669, and χ is calculated at 0.47 based on the literature (Macromolecules 2007, 40, 3669 - 3675).

[0030] Also, when the polyol component of the polyurethane is polytetramethylene glycol (PTMG), the crosslink density is T×10 2 mol / m 3 If the Shore D hardness of the cover material is D, the value of T / D indicating the crosslink density in the hardness of the polyurethane is preferably 0.6 or less, preferably 0.5 or less, more preferably 0.4 or less, still more preferably 0.35 or less, and most preferably 0.2 or less. If this value exceeds 0.6, the scratch resistance may be inferior.

[0031] Furthermore, when the number average molecular weight of polytetramethylene glycol (PTMG) is Mn, the value of (T / D)×Mn is preferably 1000 or less, preferably 900 or less, more preferably 800 or less, still more preferably 700 or less, and most preferably 600 or less. This value is the product of the crosslink density in the hardness of the polyurethane and the number average molecular weight of PTMG. If this value exceeds 1000, the scratch resistance may be inferior.

[0032] Also, increasing the number average molecular weight (Mn) of PTMG can improve the performance of the entire golf ball without changing the softness and hardness. That is, when the number average molecular weight (Mn) of PTMG is greater than 1000 and less than 2000, or greater than 2000 and less than 3000, at the same hardness level, the approach spin performance is excellent, and the resilience and scratch resistance are also excellent.

[0033] In the above cover resin composition, a resin material other than the above-mentioned polyurethane may be blended. The purpose is to further improve the fluidity of the resin composition for golf balls and enhance various physical properties such as resilience and crack durability. Also, as a resin component other than polyurethane, a resin component with low resilience may be blended to improve the control performance.

[0034] Examples of resin materials other than polyurethane include, specifically, polyamide elastomers, ionomer resins, ethylene-ethylene / butylene-ethylene block copolymers or modified products thereof, polyacetals, polyethylene, nylon resins, styrene resins, polyvinyl chloride, polycarbonates, polyphenylene ethers, polyarylates, polysulfones, polyethersulfones, polyetherimides, and polyamideimides. One or more of these can be used.

[0035] Regarding the hardness of polyurethane itself, a softer hardness is preferable in terms of approach spin performance and scratch resistance performance. The preferred hardness of polyurethane is in the range of 30 to 60 on the Shore D hardness scale.

[0036] Furthermore, the above cover resin composition can further contain an active isocyanate compound. This active isocyanate compound can react with the main components, polyurethane or polyurea, to further improve the scratch resistance of the entire resin composition. Additionally, due to the plasticizing effect of isocyanate, the fluidity can be improved, thereby enhancing the moldability.

[0037] As the above isocyanate compound, any isocyanate compound commonly used in ordinary polyurethanes can be used without particular limitation. For example, as aromatic isocyanate compounds, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate or a mixture of both, 4,4-diphenylmethane diisocyanate, m-phenylene diisocyanate, 4,4'-biphenyl diisocyanate, etc. can be mentioned. Hydrogenated products of these aromatic isocyanate compounds, such as dicyclohexylmethane diisocyanate, etc. can also be used. In addition, aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (HDI), octamethylene diisocyanate, and alicyclic diisocyanates such as xylene diisocyanate can be mentioned. Furthermore, blocked isocyanate compounds obtained by reacting the isocyanate groups of a compound having two or more isocyanate groups at the terminal with a compound having active hydrogen, uretdione bodies formed by dimerization of isocyanates, etc. can be mentioned.

[0038] The compounding amount of the above isocyanate compound is preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, based on 100 parts by mass of the polyurethane. Also, as the upper limit value, it is preferably 30 parts by mass or less, more preferably 20 parts by mass or less. If this compounding amount is too small, a sufficient cross-linking reaction may not be obtained, and improvement in physical properties may not be recognized in some cases. On the other hand, if this compounding amount is too large, discoloration over time, due to heat or ultraviolet rays, may increase, or problems such as loss of thermoplasticity or reduction of resilience may occur in some cases.

[0039] Furthermore, optional additives can be appropriately incorporated into the above cover resin composition according to the intended use. For example, when using the golf ball material of the present invention as a cover material, various additives such as fillers (inorganic fillers), organic short fibers, reinforcing agents, crosslinking agents, pigments, dispersants, antioxidants, ultraviolet absorbers, and light stabilizers can be added to the above components. When incorporating these additives, the blending amount is preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, and the upper limit is preferably 10 parts by mass or less, more preferably 4 parts by mass or less, based on 100 parts by mass of the base resin.

[0040] Regarding the resilience modulus of the above cover resin composition, it is required to be 48% or more, preferably 50% or more, more preferably 52% or more, as measured according to JIS-K 6255:2013, for the purpose of achieving low resilience and improving the approach spin amount of the golf ball. The upper limit value is 72% or less, preferably 70% or less, more preferably 68% or less.

[0041] Also, regarding the material hardness of the above cover resin composition, from the viewpoints of scratch resistance of the golf ball and imparting an appropriate approach spin amount, it is preferably 50 or less, more preferably 48 or less, and even more preferably 45 or less in Shore D hardness. From the viewpoint of moldability, the lower limit is preferably 30 or more, more preferably 35 or more, and even more preferably 37 or more in Shore D hardness.

[0042] Regarding the preparation method of each component of the above cover resin composition, for example, it can be mixed using various kneaders such as kneading type (single-screw or) twin-screw extruders, Banbury mixers, kneaders, and lab plast mills, or the components can be mixed by dry blending during the injection molding of the resin composition. Furthermore, when using the above active isocyanate compound, it may be incorporated during resin mixing using various kneaders, or a masterbatch containing the active isocyanate compound and other components may be separately prepared in advance and dry blended during the injection molding of the resin composition to mix the components.

[0043] For example, as a method of molding a cover with the above cover resin composition, for example, the above resin composition can be supplied to an injection molding machine, and the cover can be molded by injecting the molten resin composition around the core. In this case, the molding temperature varies depending on the type of polyurethane, but is usually in the range of 150 to 270°C.

[0044] The thickness of the cover is preferably 0.4 mm or more, more preferably 0.5 mm or more, still more preferably 0.6 mm or more, and as the upper limit, preferably 2.0 mm or less, more preferably 1.5 mm or less.

[0045] When at least one intermediate layer is interposed between the above core and the above cover, as the material of the intermediate layer, it is preferable to employ various thermoplastic resins used for the cover material of a golf ball, particularly ionomer resin, and a commercially available product can be used as the ionomer resin. In this case, the thickness of the intermediate layer can be set within the same range as the thickness of the above cover.

[0046] In the golf ball of the present invention, from the viewpoint of aerodynamic performance, a large number of dimples are provided on the surface of the outermost layer. There is no particular limitation on the number of dimples formed on the surface of the outermost layer, but from the point of enhancing aerodynamic performance and increasing the flying distance, it is preferably 250 or more, more preferably 270 or more, still more preferably 290 or more, and most preferably 300 or more, and as the upper limit value, preferably 400 or less, more preferably 380 or less, still more preferably 360 or less.

[0047] In the present invention, a paint layer is formed on the cover surface. As the paint for forming this paint layer, it is preferable to employ a two-component curable urethane paint. Specifically, in this case, the two-component curable urethane paint contains a main agent mainly composed of a polyol resin and a curing agent mainly composed of a polyisocyanate.

[0048] As a method of forming a paint layer by applying the above paint to the cover surface, there are no particular limitations, and known methods can be used. Desired methods such as an air gun coating method or an electrostatic coating method can be used.

[0049] Regarding the thickness of the paint layer, there are no particular limitations, but it is usually 8 to 22 μm, preferably 10 to 20 μm.

[0050] In addition, the golf ball of the present invention can conform to the golf rules for competitive use, and the ball outer diameter can be formed to be 42.80 mm or less, which does not pass through a ring with an inner diameter of 42.672 mm, and the mass is preferably 45.0 to 45.93 g.

Examples

[0051] Hereinafter, examples and comparative examples will be shown to specifically explain the present invention, but the present invention is not limited to the following examples.

[0052] 〔Examples 1 to 34, Comparative Examples 1, 2〕 Common core A core having a diameter of 38.6 mm is produced by preparing and vulcanizing and molding a rubber composition for the core common to all examples according to the formulation shown in Table 1.

[0053]

Table 1

[0054] The details of the above core material are as follows. · "cis-1,4-polybutadiene" manufactured by ENEOS MATERIALS Co., Ltd., trade name "BR01" · "Zinc acrylate" manufactured by Nippon Catalyst Co., Ltd. · "Zinc oxide" manufactured by Sakai Chemical Industry Co., Ltd. · "Barium sulfate" manufactured by Sakai Chemical Industry Co., Ltd. · "Antioxidant" trade name "No Crack NS6" (manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.) · "Organic peroxide (1)" dicumyl peroxide, trade name "Perk Mill D" (manufactured by NOF Corporation) · "Organic peroxide (2)", a mixture of 1,1-di(tert-butylperoxy)cyclohexane and silica, trade name "Perhexa C-40" (manufactured by NOF Corporation) · "Zinc stearate", manufactured by NOF Corporation

[0055] Common intermediate layer An intermediate layer-coated sphere having an intermediate layer with a thickness of 1.25 mm is produced by injection molding a resin material for the intermediate layer around a core with a diameter of 38.6 mm. The resin material for the intermediate layer has a resin formulation common to all examples, which is a blend of 50 parts by mass of the sodium neutralized product of an ethylene-unsaturated carboxylic acid copolymer with an acid content of 18% by mass and 50 parts by mass of the zinc neutralized product of an ethylene-unsaturated carboxylic acid copolymer with an acid content of 15% by mass, totaling 100 parts by mass.

[0056] Resin composition of the cover (outermost layer) For the resin compositions of the covers in Examples 1 to 7 and 14 to 20, 4,4'-diphenylmethane diisocyanate as the isocyanate, 1,4-butylene glycol as the chain extender, and PTMG1000 or PTMG2000 as the long-chain polyol were adjusted to the predetermined mixing ratios so as to obtain the Shore D hardness shown in Tables 2 to 3, and polyurethane was synthesized by the one-shot method. Also, for Examples 8 to 13, Examples 21 to 34, and Comparative Examples 1 and 2, 4,4'-diphenylmethane diisocyanate as the isocyanate, 1,4-butylene glycol as the chain extender, and PTMG1000, PTMG2000, or PTMG3000 as the long-chain polyol were adjusted to the predetermined mixing ratios so as to obtain the Shore D hardness shown in Tables 2 to 4, and polyurethane was synthesized by the one-shot method.

[0057] Cover (outermost layer) and paint layer Next, the above cover resin composition is injection molded around the intermediate layer-coated sphere to produce a three-piece golf ball with a diameter of 42.7 mm having an outermost layer with a thickness of 0.8 mm. At this time, although not particularly illustrated, common dimples are formed on the cover surfaces of each of the examples and comparative examples. Further, a paint layer of about 15 μm is formed on the surface of the golf ball of each example with the urethane paint composition.

[0058] Shore D hardness Each of the above polyurethane resins is molded into a sheet with a thickness of 2 mm and left to stand at a temperature of 23 ± 2°C for 2 weeks. Three sheets are overlapped during measurement. The material hardness of the resin is measured with a Shore D durometer conforming to ASTM D2240 standard. For the hardness measurement, an automatic rubber hardness meter "P2" manufactured by Polymer Instruments Co., Ltd. equipped with a Shore D type durometer is used.

[0059] Crosslink density (toluene swelling test) Each of the above polyurethane resins is molded into a sheet with a thickness of 2 mm, cut into test pieces with a length of 3 mm × width of 3 mm × thickness of 2 mm, used as samples, and the sample weights are measured with an electronic balance capable of measuring in units of four decimal places (mg). A 10 ml vial is prepared, 8 ml of toluene and the sample are added, stoppered and sealed. After standing for 72 hours, the internal solution is discarded, the toluene adhering to the sample surface is wiped off, and the sample weight after immersion is measured. The crosslink density of the polyurethane resin is calculated from the sample weights before and after swelling using the Flory-Rehner equation. The Flory-Rehner equation is based on the description in paragraph

[0029] .

[0060] The coefficient of restitution, the amount of spin during an approach shot, and the scratch resistance of the golf balls produced in each example are evaluated by the following methods. Further, from the relationship between the Shore D hardness of each example and the respective physical property values obtained, Examples 8 to 13, 21 to 34, and Comparative Examples 1 and 2 can be evaluated. The results are shown in Tables 2, 3, and 4.

[0061] Amount of spin during approach shot Attach a sand wedge (SW) to a golf hitting robot, and measure the amount of backspin immediately after hitting at a head speed (HS) of 20 m / s using an initial condition measuring device.

[0062] Evaluation of scratch resistance Keep the ball at 23°C, and using a swing robot machine, hit each ball five times each with a pitching wedge (PW) at a head speed of 33 m / s, and visually evaluate the impact damage according to the following criteria. ◎ ··· No damage. ○ ··· Slightly damaged, but the damage is not very noticeable.

[0063] [Table 2]

[0064] [Table 3]

[0065] [Table 4]

[0066] As shown in Tables 2 to 4, the golf balls of each example have a crosslink density of the polyurethane resin of the cover material of 1.4×10 2 mol / m 3 or more and 35×10 2 mol / m 3 or less. As a result, a high spin amount at approach is obtained, and the abrasion resistance is excellent. In particular, when comparing Comparative Example 1, Example 27, and Example 34 with the same Shore D hardness (Shore D hardness 50), Example 27 and Example 34 are superior in abrasion resistance to Comparative Example 1, and the difference in the spin amount at approach is large. Also, Example 34 using a PTMG average molecular weight of 3000 has a higher spin amount at approach than Example 27 using a PTMG average molecular weight of 2000.

[0067] In addition, from the data shown in Tables 2 to 4, a graph showing the relationship between the crosslink density of polyurethane and the Shore D hardness is shown in FIG. 1. When viewed at the same hardness, the crosslink density is higher for the PTMG with an average molecular weight of 1000. That is, it is presumed that the hardness is obtained by having more crosslinking points and more network chains. When viewed at the same Shore D hardness, the crosslink density is lower for the PTMG with an average molecular weight of 2000. That is, because the number average molecular weight of the polyol is high, it is presumed that a predetermined Shore D hardness can be obtained even if the number of network chains is not so large, flexibility can be ensured, and scratch resistance is good. A smaller value of the crosslink density with respect to the Shore D hardness is excellent in golf ball physical properties.

Claims

Claim 1 In a golf ball having a core and a cover, the cover is formed of a resin composition mainly composed of a polyurethane resin, and the crosslinking density of the polyurethane resin is 1.4 × 10 2 mol / m 3 or more and 35 × 10 2 mol / m 3 or less. A golf ball characterized by this. Claim 2 The polyol component of the polyurethane resin is polytetramethylene glycol (PTMG), and the crosslinking density is T × 10 2 mol / m 3 . When the Shore D hardness of the cover is D and the number average molecular weight of polytetramethylene glycol (PTMG) is Mn, the golf ball according to claim 1, wherein the value of (T / D) × Mn is 1000 or less. Claim 3 The golf ball according to claim 2, wherein the value of T / D is 0.6 or less. Claim 4 The golf ball according to claim 2, wherein the value of Mn is 1000 or more. Claim 5 The golf ball according to claim 2, wherein the value of Mn is 2000 or more. Claim 6 The golf ball according to claim 2, wherein the value of Mn is 3000 or more. Claim 7 The golf ball according to claim 1 or 2, wherein the thickness of the cover is 1.5 mm or less. Claim 8 The golf ball according to claim 1 or 2, wherein the Shore D hardness of the polyurethane resin is 30 to 60. Claim 9 A method for manufacturing a golf ball having a core and a cover, in the step of forming the cover with a resin composition mainly composed of a polyurethane resin, measuring the crosslink density of the polyurethane resin based on a toluene swelling test, and the crosslink density is 1.4 × 10 2 mol / m or more and 35 × 10 2 mol / m 3 or less, and selecting the polyurethane resin as the main material of the resin composition of the cover. A method for manufacturing a golf ball is characterized by this.

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