GOLF BALL

Info

Publication number: 20140323242
Type: Application
Filed: Apr 29, 2014
Publication Date: Oct 30, 2014
Applicant: DUNLOP SPORTS CO. LTD. (Kobe-shi)
Inventors: Hidetaka INOUE (Kobe-shi), Toshiyuki TARAO (Kobe-shi)
Application Number: 14/264,138

Abstract

The present invention provides a golf ball that achieves a low spin rate on driver shots to thereby improve flight distance performance, and also has improved durability, especially at high clubhead speeds. The present invention relates to a golf ball including a core, at least one middle layer covering the core, and a cover covering the middle layer, wherein at least one piece or layer of the middle layer includes a material for a middle layer that includes a thermoplastic polyurethane with a slab hardness of 65 to 80 in Shore D hardness and a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60.

Description

Description

TECHNICAL FIELD

The present invention relates to a golf ball.

BACKGROUND ART

Golf balls that fly longer distances on driver shots have been desired. In order to enhance this flight distance performance, golf balls having a multiple layer structure and improved materials for golf balls are being developed. One of the techniques to increase flight distance is to design a golf ball to have an outer-hard/inner-soft structure. For example, a golf ball is proposed which includes a highly rigid, hard middle layer to enable the golf ball itself to have an outer-hard/inner-soft structure and thereby to achieve a low spin rate on driver shots.

Various kinds of ionomer resins are generally used as materials for providing high rigidity to middle layers of golf balls to achieve low spin rates. Unfortunately, however, the use of such a resin reduces durability due to high rigidity.

Patent Literature 1 discloses a golf ball with excellent flight distance performance and the like, which includes a middle layer containing, as a main component, a thermoplastic resin other than ionomer resins, such as a thermoplastic polyurethane elastomer, and having a certain thickness and a certain Shore D hardness. Patent Literatures 2 and 3 disclose golf balls with longer flight distances which include a middle layer containing a thermoplastic polyurethane elastomer and the like and having a certain bending rigidity.

Unfortunately, the above-mentioned golf balls with a middle layer containing a thermoplastic polyurethane elastomer leave room for improvement as they are not sufficiently durable at high clubhead speeds.

CITATION LIST Patent Literature

Patent Literature 1: JP 2004-8404 A

Patent Literature 2: JP 2004-187991 A

Patent Literature 3: JP 2004-242850 A

SUMMARY OF INVENTION Technical Problem

The present invention aims to solve the above problems and to provide a golf ball that achieves a low spin rate on driver shots to thereby improve flight distance performance, and also has improved durability, especially at high clubhead speeds.

Solution to Problem

The present invention relates to a golf ball, including a core, at least one middle layer covering the core, and a cover covering the middle layer, wherein at least one piece or layer of the middle layer includes a material for a middle layer that includes a thermoplastic polyurethane with a slab hardness of 65 to 80 in Shore D hardness and a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60.

The thermoplastic polyurethane preferably has an upper yield stress of not less than 15 MPa and a lower yield stress of not less than 10 MPa. Moreover, the thermoplastic polyurethane preferably has a breaking stress (MPa) of not less than 25 MPa.

The thermoplastic polyurethane preferably has a ratio of breaking stress (MPa) to slab hardness (Shore D hardness) of not less than 0.70. Moreover, the thermoplastic polyurethane preferably has a bending rigidity of 250 to 4000 MPa.

Preferably, the middle layer has a thickness of 0.5 to 2.0 mm. Preferably, the middle layer has a surface hardness of 65 to 80 in Shore D hardness. Preferably, a difference (Hm−Hs) between a surface hardness (Hm) of the middle layer and a surface hardness (Hs) of the core is 3 to 25.

Preferably, the cover includes a thermoplastic polyurethane with a slab hardness of not more than 50 in Shore D hardness, and has a thickness of 0.3 to 1.5 mm.

Advantageous Effects of Invention

The present invention provides a golf ball including a core, at least one middle layer covering the core, and a cover covering the middle layer, wherein at least one piece or layer of the middle layer includes a material for a middle layer that includes a thermoplastic polyurethane with a slab hardness of 65 to 80 in Shore D hardness and a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60. Thus, the golf ball not only achieves a low spin rate on driver shots to thereby improve flight distance performance, but also has improved durability, especially at high clubhead speeds.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a stress-strain curve 1 of a thermoplastic polyurethane (Elastollan 1174D).

FIG. 2 shows a stress-strain curve 2 of a thermoplastic polyurethane.

DESCRIPTION OF EMBODIMENTS

The golf ball of the present invention includes a core, at least one middle layer covering the core, and a cover covering the middle layer, wherein at least one piece or layer of the middle layer includes a material for a middle layer that includes a thermoplastic polyurethane with a slab hardness of 65 to 80 in Shore D hardness and a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60.

By using the thermoplastic polyurethane having a specific slab hardness and a specific ratio of upper yield stress to lower yield stress as a resin component of a middle layer, it is possible to not only achieve a low spin rate on driver shots to thereby increase flight distance, but also to improve durability, especially at high clubhead speeds. It is also possible to ensure a soft ball with a large compressive deformation to thereby give good shot feeling.

First, the material for a middle layer will be explained.

A middle layer is formed of a material for a middle layer which includes, as a resin component, a thermoplastic polyurethane (thermoplastic polyurethane elastomer) that has a specific slab hardness and a specific ratio of upper yield stress to lower yield stress ([upper yield stress]/[lower yield stress]).

The slab hardness of the thermoplastic polyurethane is not less than 65, preferably not less than 67, and more preferably not less than 69 in Shore D hardness, whereas it is not more than 80, preferably not more than 78, and more preferably not more than 76 in Shore D hardness. The use of such a high hardness thermoplastic polyurethane enables a low spin rate on driver shots, thus improving flight distance performance. It should be noted that the slab hardness (Shore D hardness) can be measured by the method mentioned later.

The thermoplastic polyurethane has a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60, preferably not more than 1.50, and more preferably not more than 1.40. In the case that the difference between the upper and lower yield stresses at a yield point is large, large distortion is likely to occur when an impact exceeding the yield point is applied. The deformation further causes a strain, and as a result durability is likely to deteriorate. The difference between the upper and lower yield stresses at a yield point is preferably small. The closer to 1.00 the ratio of upper yield stress (MPa) to lower yield stress (MPa) is, the less likely the distortion due to impact is to occur, and, in turn, the better the durability tends to be.

The thermoplastic polyurethane preferably has an upper yield stress of not less than 15 MPa, more preferably not less than 18 MPa, and still more preferably not less than 21 MPa. If the upper yield stress is too low, a small stress tends to easily exceed the yield point, which tends to result in more deformation and therefore poor resistance to impact. The upper limit of the upper yield stress is not particularly limited, and the upper yield stress is preferably not more than 60 MPa, more preferably not more than 55 MPa, and still more preferably not more than 50 MPa.

The thermoplastic polyurethane preferably has a lower yield stress of not less than 10 MPa, more preferably not less than 13 MPa, and still more preferably not less than 16 MPa. If the lower yield stress is too low, the deformation at a point exceeding the upper yield point tends to become greater so that the resistance to impact is likely to deteriorate. The upper limit of the lower yield stress is not particularly limited, and the lower yield stress is preferably not more than 50 MPa, more preferably not more than 45 MPa, and still more preferably not more than 40 MPa.

In the present invention, the upper yield stress and the lower yield stress are determined from a stress-strain curve obtained by a tensile test performed in accordance with ISO 527-1, as shown in FIG. 1 or 2, for example. Specifically, they are determined by the method mentioned later. If an upper yield stress and a lower yield stress do not clearly appear in an obtained stress-strain curve as shown in FIG. 2, the upper yield stress is regarded to be equal to the lower yield stress, which indicates that the ratio of upper yield stress to lower yield stress is 1.00.

The thermoplastic polyurethane preferably has a breaking stress (MPa) of not less than 25 MPa, more preferably not less than 28 MPa, and still more preferably not less than 30 MPa. A breaking stress of not less than 25 MPa ensures excellent durability. The upper limit of the breaking stress is not particularly limited, and the breaking stress is preferably not more than 65 MPa, more preferably not more than 60 MPa, and still more preferably not more than 55 MPa.

The thermoplastic polyurethane preferably has a ratio of breaking stress (MPa) to slab hardness (Shore D hardness) of not less than 0.70, more preferably not less than 0.72, and still more preferably not less than 0.74. A ratio of not less than 0.70 ensures excellent durability. The maximum ratio of breaking stress (MPa) to slab hardness (Shore D hardness) is not particularly limited. In view of the need of high hardness for improvement in flight distance performance and of simultaneous achievement of durability and flight distance performance, the ratio is preferably not more than 0.88, more preferably not more than 0.86, and still more preferably not more than 0.84. It should be noted that the breaking stress and the slab hardness (Shore D hardness) can be measured by the methods mentioned later.

The thermoplastic polyurethane preferably has a bending rigidity of not less than 250 MPa, more preferably not less than 280 MPa, and still more preferably not less than 300 MPa. The bending rigidity is preferably not more than 4000 MPa, more, preferably not more than 3600 MPa, and still more preferably not more than 3200 MPa. The use of such a high rigidity thermoplastic polyurethane enables a low spin rate on driver shots and thus improves flight distance performance. It should be noted that the bending rigidity herein refers to a value determined in accordance with ISO 178.

The thermoplastic polyurethane preferably has a number average molecular weight of not less than 20,000, more preferably not less than 30,000, and still more preferably not less than 40,000, whereas it preferably has a number average molecular weight of not more than 200,000, more preferably not more than 150,000, and still more preferably not more than 100,000. With a number average molecular weight of not less than 20,000, the resulting golf ball can have further improved scratch resistance. With a number average molecular weight of not more than 200,000, conversely, the thermoplastic polyurethane has good fluidity and good moldability. It should be noted that the number average molecular weight (Mn) can be determined by gel permeation chromatography (GPC) relative to polystyrene standards.

The thermoplastic polyurethane with the certain slab hardness and the certain ratio of upper yield stress to lower yield stress may further have a plurality of urethane bonds within the molecule and thus exhibit thermoplasticity. Examples of such thermoplastic polyurethanes include products obtained by reacting a polyisocyanate with a polyol to form urethane bonds within the molecule, optionally followed by further causing a chain extension reaction with a low-molecular-weight polyol or polyamine, or the like.

The polyisocyanate component forming the thermoplastic polyurethane may be one that has two or more isocyanate groups. Examples thereof include aromatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, mixtures of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethyl xylylene diisocyanate (TMXDI), and para-phenylene diisocyanate (PPDI); and alicyclic or aliphatic polyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenated xylylene diisocyanate (H₆XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornene diisocyanate (NBDI). These may be used alone, or two or more of these may be used in combination. In view of enhancing flight distance performance and durability, xylylene diisocyanate (XDI) or hexamethylene diisocyanate (HDI) is preferred among these polyisocyanate components, and 4,4′-diphenylmethane diisocyanate (MDI) is more preferred.

The polyol component forming the thermoplastic polyurethane may be one that has a plurality of hydroxy groups, such as, for example, a low-molecular-weight polyol or a high-molecular-weight polyol. Examples of the low molecular-weight polyols include diols such as ethylene glycol, diethylene glycol, triethylene glycol, propanediol (e.g. 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol), dipropylene glycol, butanediol (e.g. 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol), neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, 1,6-cyclohexanedimethylol, aniline diols, and bisphenol A diols; triols such as glycerin, trimethylol propane, and hexanetriol; and tetraols and hexanols, such as pentaerythritol and sorbitol. Examples of the high-molecular-weight polyols include polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), or polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. These may be used alone, or two or more of these may be used in combination.

The high-molecular-weight polyol has any number average molecular weight although the number average molecular weight is preferably not less than 400, and more preferably not less than 1,000. The use of a high-molecular-weight polyol having an excessively low number average molecular weight may produce a hard polyurethane, resulting in golf balls with reduced shot feeling. The upper limit of the number average molecular weight is preferably not more than 10,000 and more preferably not more than 8,000.

The polyamine optionally forming the thermoplastic polyurethane may be one that has two or more amino groups. Examples of the polyamines include aliphatic polyamines such as ethylenediamine, propylenediamine, butylenediamine, and hexamethylenediamine; alicyclic polyamines such as isophoronediamine and piperazine; and aromatic polyamines.

The aromatic polyamine may be one that has two or more amino groups directly or indirectly bonded to an aromatic ring. Herein, the term “indirectly bonded” means that the amino group is bonded to the aromatic ring via, for example, a lower alkylene group. The aromatic polyamine may be, for example, a monocyclic aromatic polyamine having two or more amino groups bonded to one aromatic ring or a polycyclic aromatic polyamine having two or more aminophenyl groups in which at least one amino group is bonded to one aromatic ring.

Examples of the monocyclic aromatic polyamines include a kind of polyamines in which amino groups are directly bonded to an aromatic ring, such as phenylenediamine, toluenediamine, diethyltoluenediamine, and dimethylthiotoluenediamine; and a kind of polyamines in which amino groups are bonded to an aromatic ring via a lower alkylene group, such as xylylenediamine. The polycyclic aromatic polyamine may be a poly(aminobenzene) having two or more aminophenyl groups directly bonded to each other, or may be one having two or more aminophenyl groups bonded via a lower alkylene group or an alkylene oxide group. In view of reactivity, 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline) is preferred among these polyamines, and 4,4′-methylene-bis-(2,6-diethylaniline) is more preferred.

Structural embodiments of the thermoplastic polyurethane include: an embodiment in which the thermoplastic polyurethane is formed from a polyisocyanate component and a high-molecular-weight polyol component; an embodiment in which the thermoplastic polyurethane is formed from a polyisocyanate component, a high-molecular-weight polyol component, and a low-molecular-weight polyol component; an embodiment in which the thermoplastic polyurethane is formed from a polyisocyanate component, a high-molecular-weight polyol component, a low-molecular-weight polyol component, and a polyamine component; and an embodiment in which the thermoplastic polyurethane is formed from a polyisocyanate component, a high-molecular-weight polyol component, and a polyamine component.

Specific examples of the thermoplastic polyurethane include Elastollan (registered trademark) 1164D and 1174D, produced by BASF Japan Ltd., and Miractran E568 and E574 produced by Nippon Miractran Co., Ltd.

The resin component forming the middle layer preferably contains the thermoplastic polyurethane in an amount of not less than 50% by mass, more preferably not less than 65% by mass, and still more preferably not less than 80% by mass. The amount of the thermoplastic polyurethane may be 100% by mass. When the resin component contains the thermoplastic polyurethane in an amount of not less than 50% by mass, i.e., contains the thermoplastic polyurethane as a main component, it is more effective in enhancing low spin and in improving durability.

The material for a middle layer may contain a resin component other than the thermoplastic polyurethane, such as various ionomer resins, as long as it does not impair the effects of the present invention. Moreover, other additives may be added to the extent that they do not impair the effects of the present invention. Examples of the additives include: pigments such as white pigments (e.g. titanium oxide), blue pigments, and red pigments; weighting agents such as calcium carbonate or barium sulfate; dispersants; antioxidants; ultraviolet absorbents; light stabilizers; fluorescent materials and fluorescent brighteners.

In cases where the middle layer contains materials other than the thermoplastic polyurethane, the material for a middle layer, including all the materials forming the middle layer, desirably has a slab hardness (Shore D), a ratio of upper yield stress to lower yield stress, an upper yield stress, a lower yield stress, a breaking stress (MPa), a ratio of breaking stress (MPa) to slab hardness (Shore D hardness), and a bending rigidity, all falling within the ranges as defined for the thermoplastic polyurethane. Specifically, these properties can be adjusted to desired values by using the thermoplastic polyurethane as a main component and further appropriately selecting and using other components to the extent that they do not greatly affect the properties. Each property can be measured in the same manner as mentioned above.

The following will explain the golf ball.

The golf ball of the present invention includes a core, at least one middle layer covering the core, and a cover covering the middle layer. At least one piece or layer of the middle layer is formed of the material for a middle layer including the thermoplastic polyurethane.

Any core may be used in the present invention. The core may be, for example, a core consisting of a center, or a multilayered core (e.g. two-layered core) consisting of a center and at least one surrounding layer that covers the center.

The center may be formed of a conventional rubber composition (hereinafter, also referred to simply as “rubber composition for a center”) or a resin composition. For example, the center may be formed by hot-pressing a rubber composition that includes a base rubber, a crosslinking initiator, a co-crosslinking agent, and a filler. The base resin of the resin composition may be a thermoplastic resin such as ionomer resin, thermoplastic olefin copolymer, thermoplastic polyurethane resin, thermoplastic polyamide resin, thermoplastic styrene resin, thermoplastic polyester resin, or thermoplastic acrylic resin.

The base rubber may be natural rubber and/or a synthetic rubber, and examples thereof include polybutadiene rubber, natural rubber, polyisoprene rubber, styrene polybutadiene rubber, and ethylene-propylene-diene rubber (EPDM). Preferred among them, in particular, is high-cis polybutadiene having cis bonds, which advantageously improve resilience, in an amount of 40% by mass or more, preferably 70% by mass or more, and more preferably 90% by mass or more.

The crosslinking initiator is intended to crosslink the base rubber component. The crosslinking initiator is suitably an organic peroxide. Specific examples of the organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide, preferred among which is dicumyl peroxide. The amount of crosslinking initiator to be added is preferably not less than 0.1 parts by mass, more preferably not less than 0.3 parts by mass, and still more preferably not less than 0.5 parts by mass, whereas it is preferably not more than 3 parts by mass, more preferably not more than 2.8 parts by mass, and still more preferably not more than 2.5 parts by mass, per 100 parts by mass of the base rubber. If the amount is less than 0.1 parts by mass, the core tends to become too soft and thus resilience tends to be lowered. If the amount is more than 3 parts by mass, a larger amount of co-crosslinking agent is necessary for a proper hardness, which leads to insufficient resilience.

Any co-crosslinking agent may be used as long as it functions to crosslink rubber molecules by graft polymerization onto the molecular chains of the base rubber. Examples of the co-crosslinking agents include α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms and metal salts thereof, preferably acrylic acid, methacrylic acid, and metal salts thereof. Examples of metals that can be used to form the metal salts include zinc, magnesium, calcium, aluminum, and sodium, preferred among which is zinc because it provides high resilience.

The amount of co-crosslinking agent to be added is preferably not less than 10 parts by mass, and more preferably not less than 15 parts by mass, whereas it is preferably not more than 50 parts by mass, and more preferably not more than 45 parts by mass, per 100 parts by mass of the base rubber. If the amount of co-crosslinking agent is less than 10 parts by mass, a larger amount of crosslinking initiator is likely to necessary for a proper hardness, which tends to reduce resilience. Conversely, if the amount is more than 50 parts by mass, the center may become so hard that shot feeling can be reduced.

The rubber composition for a center may optionally contain a filler mainly as a weighting agent to adjust the specific gravity of a golf ball obtained as an end product in the range of 1.0 to 1.5. Examples of the fillers include inorganic fillers such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder. The amount of filler to be added is preferably not less than 0.5 part by mass, and more preferably not less than 1 part by mass, whereas it is preferably not more than 30 parts by mass, and more preferably not more than 20 parts by mass, per 100 parts by mass of the base rubber. If the amount is less than 0.5 parts by mass, it is difficult to adjust the weight. If the amount is more than 30 parts by mass, the weight ratio of the rubber component is reduced, which tends to lead to reduced resilience.

The rubber composition for a center may further appropriately contain an organic sulfur compound, an antioxidant, a peptizing agent, and other additives in addition to the base rubber, the crosslinking initiator, the co-crosslinking agent and the filler.

The organic sulfur compound may suitably be a diphenyl disulfide. Examples of the diphenyl disulfides include: diphenyl disulfide; mono-substituted diphenyl disulfides 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; di-substituted diphenyl disulfides 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; tri-substituted diphenyl disulfides such as bis(2,4,6-trichlorophenyl)disulfide and bis(2-cyano-4-chloro-6-bromophenyl)disulfide; tetra-substituted diphenyl disulfides such as bis(2,3,5,6-tetra chlorophenyl)disulfide; and penta-substituted diphenyl disulfides such as bis(2,3,4,5,6-pentachlorophenyl)disulfide and bis(2,3,4,5,6-pentabromophenyl)disulfide. These diphenyl disulfides can enhance resilience by somehow affecting the state of cure of rubber vulcanizates. Preferred among them are diphenyl disulfide and bis(pentabromophenyl)disulfide because the resulting golf balls particularly have high resilience. The amount of organic sulfur compound to be added is preferably not less than 0.1 parts by mass, and more preferably not less than 0.3 parts by mass, whereas it is preferably not more than 5.0 parts by mass, and more preferably not more than 3.0 parts by mass, per 100 parts by mass of the base rubber.

The amount of antioxidant to be added preferably ranges from 0.1 to 1 part by mass per 100 parts by mass of the base rubber. The amount of peptizing agent preferably ranges from 0.1 to 5 parts by mass per 100 parts by mass of the base rubber.

The center may be prepared by mixing and kneading the rubber composition and molding it in a die under any conditions. Preferably, for example, the rubber composition is heated at a temperature of 130 to 200° C. for 10 to 60 minutes, or alternatively, is heated in two steps, i.e., at a temperature of 130 to 150° C. for 20 to 40 minutes, and subsequently at a temperature of 160 to 180° C. for 5 to 15 minutes.

The following will explain the surrounding layer forming the multilayered core, if used.

The surrounding layer is formed of a composition for a surrounding layer, examples of which include: thermoplastic resins such as ionomer resins commercially available under the trade name “HIMILAN (registered trademark) (e.g. HIMILAN 1605, HIMILAN 1706)” from DuPont-Mitsui Polychemical, and under the trade name “SURLYN (registered trademark) (e.g. SURLYN 8140, SURLYN 9120)” from E.I. du Pont de Nemours and Company; and thermoplastic elastomers such as thermoplastic polyamide elastomers commercially available under the trade name “PEBAX (registered trademark) (e.g. PEBAX 2533)” from Arkema, thermoplastic polyester elastomers commercially available under the trade name “HYTREL (registered trademark) (e.g. HYTREL 3548, HYTREL 4047)” from Du Pont-Toray Co., Ltd., thermoplastic polyurethane elastomers commercially available under the trade name “ELASTOLLAN (registered trademark) (e.g. ELASTOLLAN XNY97A)” from BASF Japan Ltd., and thermoplastic polystyrene elastomers commercially available under the trade name “RABALON (registered trademark)” from Mitsubishi Chemical Corporation, as well as rubber compositions as mentioned for the composition for a center. These thermoplastic resins and thermoplastic elastomers may be used alone, or two or more of them may be used in admixture.

The surrounding layer may be formed, for example, by covering the center with the composition for a surrounding layer. The method for forming the surrounding layer is not particularly limited. For example, the surrounding layer may be formed by molding the composition for a surrounding layer into hemispherical half-shells in advance, enclosing the center with two pieces of half-shells, and pressing the assembly at a temperature of 130 to 170° C. for 1 to 5 minutes, or by injection molding the composition for a surrounding layer directly onto the center to enclose the center.

The composition for a surrounding layer preferably has a slab hardness of not less than 40, more preferably not less than 42, and still more preferably not less than 43 in Shore D hardness, whereas the slab hardness is preferably not more than 70, more preferably not more than 66, and still more preferably not more than 64 in Shore D hardness. If the composition has a slab hardness of not less than 40, the resulting golf ball has better resilience. If the composition has a slab hardness of not more than 70, the resulting golf ball gives better shot feeling. The slab hardness of the composition for a surrounding layer can be adjusted by appropriately selecting a combination of the resin components or rubber compositions mentioned above.

The center preferably has a diameter of not less than 5.0 mm, more preferably not less than 10.0 mm, whereas it preferably has a diameter of not more than 41.5 mm, more preferably not more than 41.0 mm, and still more preferably not more than 40.5 mm. If the center has a diameter of not less than 5.0 mm, the function of the relatively soft center is more exerted, thus further reducing the spin rate, especially on W #1 shots. Conversely, if the diameter thereof is not more than 41.5 mm, then the surrounding layer, the middle layer, or the cover layer is not too thin, and thus the function of each layer is more exerted.

In cases where the center has a diameter of 5.0 to 41.5 mm, the amount of compressive deformation (shrinkage of the center in the compression direction) when applying from an initial load of 98 N to a final load of 1275 N is preferably not smaller than 4.0 mm, and more preferably not smaller than 4.5 mm, whereas it is preferably not greater than 10.0 mm, and more preferably not greater than 8.0 mm. When the amount of compressive deformation is not smaller than 4.0 mm, then better shot feeling is provided. When the amount of compressive deformation is not greater than 10.0 mm, then better resilience is achieved.

The surrounding layer preferably has a thickness of not smaller than 3.0 mm, more preferably not smaller than 5.0 mm, and still more preferably not smaller than 7.0 mm. The thickness is also preferably not larger than 17.0 mm, more preferably not larger than 15.0 mm, and still more preferably not larger than 13.0 mm. The surrounding layer having a thickness not smaller than the lower limit mentioned above provides a larger effect, thus further enhancing the effect of reducing spin on, for example, driver shots. When the surrounding layer has a thickness not larger than the upper limit mentioned above, then the core exhibits a greater influence, thus making resilience much better.

In the case of a multilayered core including the center and at least one surrounding layer covering the center, the multilayered core preferably has a diameter of not less than 32.0 mm, more preferably not less than 34.0 mm, and still more preferably not less than 39.0 mm, whereas it preferably has a diameter of not more than 41.5 mm, more preferably not more than 41.0 mm, and still more preferably not more than 40.5 mm. When the core has a diameter within the range mentioned above, the effect of reducing spin on, for example, driver shots is further enhanced.

In cases where the core has a diameter of 32.0 to 41.5 mm, the amount of compressive deformation (shrinkage of the core in the compression direction) when applying from an initial load of 98 N to a final load of 1275 N is preferably not smaller than 2.0 mm, more preferably not smaller than 2.2 mm, and still more preferably not smaller than 2.3 mm, whereas it is preferably not greater than 4.5 mm, more preferably not greater than 4.0 mm, and still more preferably not greater than 3.5 mm. When the amount of compressive deformation is not smaller than 2.0 mm, the effect of reducing spin on, for example, driver shots and shot feeling are further improved. When the amount of compressive deformation is not greater than 4.5 mm, resilience becomes much better.

The difference (Hs−Ho) between the surface hardness (Hs) and the center-point hardness (Ho) of the core is preferably not less than 10, more preferably not less than 15, and still more preferably not less than 20 in Shore D hardness. When the surface of the core is harder than the center point thereof, a larger launch angle and reduced spin rate are achieved, so that flight distance is increased. The difference between the surface hardness and the center-point hardness of the core is preferably not more than 55, more preferably not more than 50, and still more preferably not more than 40 in Shore D hardness. If the difference in hardness is too large, durability may be reduced.

Moreover, the center-point hardness (Ho) of the core is preferably not less than 20, more preferably not less than 27, and still more preferably not less than 32 in Shore D hardness. When the center-point hardness is not less than 20, the core is not too soft and thus can provide good resilience. The center-point hardness (Ho) is preferably not more than 70, more preferably not more than 65, and still more preferably not more than 62 in Shore D hardness. When the center-point hardness is not more than 70, the core is not too hard and thus can give a good shot feeling. The center-point hardness of the core herein refers to a hardness measured as follows: the core is divided into two equal parts, and the hardness is then measured at the center point of the cross section using a spring type Shore D hardness tester.

The core of the golf ball of the present invention preferably has a surface hardness (Hs) of not less than 45, more preferably not less than 47, and still more preferably not less than 48 in Shore D hardness. When the surface hardness is not less than 45, the core is not too soft and thus can provide good resilience. The core preferably has a surface hardness (Hs) of not more than 65, more preferably not more than 63, and still more preferably not more than 60 in Shore D hardness. If the surface hardness is not more than 65, the difference in hardness from the middle layer can be increased, which further increases the effect of reducing spin on driver shots.

The following will explain the at least one middle layer covering the core.

A material for a middle layer containing the above-mentioned thermoplastic polyurethane is used in at least one piece or layer of the middle layer.

The middle layer may be formed, for example, by covering the core with the material for a middle layer. The method for forming the middle layer is not particularly limited. For example, the middle layer may be formed by molding the material for a middle layer into hemispherical half-shells in advance, enclosing the core with two pieces of half-shells, and pressing the assembly at a temperature of 130 to 170° C. for 1 to 5 minutes, or by injection molding the material for a middle layer directly onto the core to enclose the core.

The middle layer formed of the material for a middle layer preferably has a thickness of not smaller than 0.5 mm, more preferably not smaller than 0.6 mm, and still more preferably not smaller than 0.7 mm, whereas it preferably has a thickness of not larger than 2.0 mm, more preferably not larger than 1.8 mm, and still more preferably not larger than 1.6 mm. If the middle layer has a thickness smaller than 0.5 mm, durability may be deteriorated due to the thin middle layer. If the middle layer has a thickness larger than 2.0 mm, then the core has a smaller diameter, which may result in low resilience.

The middle layer formed of the material for a middle layer preferably has a surface hardness (Hm) of not less than 65, more preferably not less than 66, and still more preferably not less than 67, whereas it preferably has a surface hardness (Hm) of not more than 80, more preferably not more than 78, and still more preferably not more than 76, in Shore D hardness. When the surface hardness is not less than 65, the middle layer has high hardness and high rigidity and thus the effect of reducing spin on, for example, driver shots is further enhanced. When the surface hardness is not more than 80, the middle layer is not too hard and thus golf ball durability and shot feeling are further improved.

The difference (Hm−Hs) between the surface hardness (Hm) of the middle layer formed of the material for a middle layer and the surface hardness (Hs) of the core is preferably not less than 3, more preferably not less than 4, and still more preferably not less than 5, whereas it is preferably not more than 25, more preferably not more than 18, and still more preferably not more than 16, in Shore D hardness. When the difference (Hm−Hs) in surface hardness falls within the range mentioned above, the spin rate is further reduced, so that flight distance is increased.

Embodiments of combinations of the core and the at least one middle layer include an embodiment in which the core is covered with one middle layer; and an embodiment in which the core is covered with multiple pieces or layers of middle layer.

The core covered with the middle layer preferably has a spherical shape. This is because, if the shape of the middle layer formed is not spherical, the cover has an uneven thickness and thus partially has reduced covering performance. Meanwhile, the core has a spherical shape, in general. The spherical core may be provided with elongated protrusion(s) to divide the surface of the spherical core, for example, to equally divide the surface of the spherical core. For example, in an embodiment in which the elongated protrusion(s) is provided, the elongated protrusion(s) is integrally formed with the surface of the surrounding layer. In another embodiment, the surface of the spherical center is provided with a surrounding layer in the form of elongated protrusion(s).

If the spherical core is regarded as the earth, for example, the elongated protrusion(s) is preferably provided along the equator and any meridians that equally divide the surface of the spherical core. For example, in the case of dividing the surface of the spherical core into 8 parts, elongated protrusions may be provided along the equator, any meridian (0 degrees longitude) and the meridians at 90 degrees east longitude, at 90 degrees west longitude, and at 180 degrees east (west) longitude based on the meridian at 0 degrees longitude. When the elongated protrusion(s) is provided on the surface of the core, concave portions separated by the elongated protrusion(s) are preferably filled by multiple middle layers, or a single middle layer that covers each concave portion, so that the covered core has a spherical shape. The elongated protrusion may have any cross-sectional shape, and may have, for example, an arc shape, a substantially arc shape (for example, a shape in which a notch portion is formed at a part where the elongated protrusions intersect one another or are at right angles to each other), or the like.

Regarding the middle layer, in the case that the core is covered with a single middle layer or multiple middle layers, at least one layer of the middle layer(s) is formed of the material for a middle layer. In the case that the concave portions separated by elongated protrusions provided on the surface of the core are filled by multiple pieces of the middle layer, at least one piece of the multiple pieces of the middle layer is formed of the material for a middle layer. In cases where the core is covered with multiple pieces or layers of the middle layer, the middle layer may include a middle layer formed of a material for a middle layer different from the earlier mentioned material for a middle layer, as long as it does not impair the effects of the present invention. In this case, the middle layer formed of the earlier mentioned material for a middle layer is preferably placed in contact with the cover. Preferably, all of the multiple pieces or layers of the middle layer are formed of the material for a middle layer.

The other material for a middle layer different from the earlier mentioned material for a middle layer may be, for example, a material as mentioned for the composition for a surrounding layer. The material may additionally contain a weighting agent (e.g. barium sulfate, tungsten), an antioxidant, a pigment and the like.

Examples of the resin component in a cover composition forming the cover include, in addition to polyurethane resins and conventional ionomer resins, thermoplastic polyamide elastomers commercially available under the trade name “PEBAX (registered trademark) (e.g. PEBAX 2533)” from Arkema; thermoplastic polyester elastomers commercially available under the trade name “HYTREL (registered trademark) (e.g. HYTREL 3548, HYTREL 4047)” from Du Pont-Toray Co., Ltd.; and thermoplastic polystyrene elastomers commercially available under the trade name “RABALON (registered trademark)” from Mitsubishi Chemical Corporation. These resins used as the resin component may be used alone, or two or more of these may be used in combination. Preferred among these are polyurethane resins.

The cover composition forming the cover of the golf ball of the present invention preferably contains a resin component that includes 50% by mass or more, more preferably 60% by mass or more, and still more preferably 70% by mass or more of a polyurethane resin. Most preferably, the resin component of the cover composition consists only of a polyurethane resin.

The polyurethane resin may be any one that has a plurality of urethane bonds within the molecule. Examples thereof include products obtained by reacting a polyisocyanate component and a high-molecular-weight polyol component to form urethane bonds within the molecule, optionally followed by further causing a chain extension reaction with a low-molecular-weight polyol or polyamine, or the like.

The polyurethane resin preferably has a slab hardness of not higher than 50, more preferably not higher than 40, and still more preferably not higher than 35 in Shore D hardness. The use of a polyurethane resin having a low hardness improves spin performance on approach shots. The slab hardness is preferably not lower than 10, and more preferably not lower than 15 in Shore D hardness. A polyurethane resin having a slab hardness of lower than 10 may lead to excessively high spin rates on approach shots. Specific examples of the polyurethane resin include ELASTOLLAN (registered trademark) XNY75A, XNY80A, XNY83A, XNY85A, XNY97A, XNY90A, and ET880, produced by BASF Japan Ltd.

In the present invention, the cover may contain, in addition to the above-mentioned resin component, any of the following additives: pigments such as titanium oxide, a blue pigments, and red pigments; weighting agents such as zinc oxide, calcium carbonate, and barium sulfate; dispersants; antioxidants; ultraviolet absorbents; light stabilizers; fluorescent materials and fluorescent brighteners, to the extent that they do not impair the performance of the cover.

The amount of white pigment (titanium oxide) per 100 parts by mass of the resin component in the cover is preferably not less than 0.5 parts by mass, and more preferably not less than 1 part by mass, whereas it is preferably not more than 10 parts by mass, and more preferably not more than 8 parts by mass. When the amount of white pigment is not less than 0.5 parts by mass, hiding properties can be imparted to the cover. If the amount of white pigment exceeds 10 parts by mass, the durability of the resulting cover may be reduced.

The cover preferably has a slab hardness (Hc) of not higher than 50, more preferably not higher than 40, and still more preferably not higher than 35 in Shore D hardness. The cover with a slab hardness of not higher than 50 improves the spin performance on approach shots with a short iron or the like, thus enabling to produce a golf ball excellent in controllability on approach shots. The cover preferably has a slab hardness (Hc) of not lower than 10, more preferably not lower than 15 in Shore D hardness. The cover with a slab hardness lower than 10 may excessively increase the spin rate on approach shots with a short iron or the like. The slab hardness of the cover herein refers to the hardness of the cover composition in a sheet form measured by the method mentioned later.

The embodiment of forming the cover from the cover composition is not particularly limited. Examples thereof include: an embodiment in which the cover composition is injection-molded directly onto a core; and an embodiment in which the cover composition is molded into hollow shells, and a core is covered with a plurality of shells and then compression-molded (preferably, a method in which the cover composition is molded into hollow half-shells, and a core is covered with two pieces of half-shells and then compression-molded). In the case of injection molding the cover composition onto a core to form a cover, upper and lower molds for forming a cover each preferably have a hemispherical cavity with pimples, some of the pimples also serving as retractable hold pins. The cover can be formed by injection molding as follows: the hold pins are protruded; a core is placed in the mold and held by the hold pins; then, the cover composition melted by heating is injected onto the core and then cooled to form a cover. For example, the cover composition melted by heating to 150 to 230° C. is injected in 0.1 to 1 second into molds clamped under a pressure of 980 KPa to 1,500 KPa, the composition is then cooled for 15 to 60 seconds, and the molds are opened to obtain a cover.

In the case that the cover is formed by compression molding, half-shells can be formed by either compression molding or injection molding, suitably by compression molding. The conditions for compression molding the cover composition into half-shells may be, for example, under a pressure of at least 1 MPa but not more than 20 MPa at a molding temperature of at least −20° C. but not higher than +70° C. with respect to the flow beginning temperature of the cover composition. Under these molding conditions, half-shells having a uniform thickness can be formed. In an example of the method for forming a cover from the half-shells, a core is covered with two pieces of half-shells and they are subjected to compression molding. The conditions for compression molding the half-shells to form a cover may be, for example, under a molding pressure of at least 0.5 MPa but not more than 25 MPa at a molding temperature of at least −20° C. but not higher than +70° C. with respect to the flow beginning temperature of the cover composition. Under these molding conditions, a cover for golf balls having a uniform thickness can be formed.

When a golf ball body is formed by covering with the cover, the surface of the cover typically has dents called dimples. The total number of dimples formed on the cover is preferably 200 to 500. If the number of dimples is less than 200, the effect of dimples is less likely to be exhibited. If the number of dimples is more than 500, the individual size of dimples is small and thus the effect of dimples is less likely to be exhibited. The dimples formed each may have any shape (shape in plane view), and may have a round shape; a polygonal shape such as substantially triangle, substantially rectangle, substantially pentagon, or substantially hexagon; or other irregular shapes. These shapes may be employed alone, or two or more of these shapes may be employed in combination, for the shapes of dimples.

Moreover, the golf ball body with the thus formed cover is taken out from the mold and may then preferably be subjected to a surface treatment such as deburring, cleaning, and sandblasting as needed. If desired, a paint layer or a mark may be formed. The thickness of the paint layer is not particularly limited, and is preferably not smaller than 5 μm, and more preferably not smaller than 7 μm, whereas it is preferably not larger than 25 μm, and more preferably not larger than 23 μm. If the thickness of the paint layer is smaller than 5 μm, the paint layer is more likely to wear out and disappear after continuous use, while if the thickness of the paint layer is larger than 25 μm, the effect of dimples tends to decrease and thus the resulting golf ball tends to have reduced flight performance.

The cover of the golf ball of the present invention preferably has a thickness of not smaller than 0.3 mm, and more preferably not smaller than 0.4 mm. A thinner cover enables a higher initial speed. The cover preferably has a thickness of not larger than 1.5 mm, and more preferably not larger than 1.0 mm. A thicker cover improves spin performance but may lower the initial speed. The thickness of the cover herein refers to the thickness of portions of the cover without dimples. In other words, the thickness is determined by measuring the thickness of the cover at at least four points beneath a land portion, and calculating the mean value.

The golf ball of the present invention may have any structure as long as it includes a core, at least one middle layer covering the core, and a cover covering the middle layer. Specific examples of the structure of the golf ball of the present invention include: a three-piece golf ball that includes a core, a middle layer covering the core, and a cover covering the middle layer; a four-piece golf ball that includes a core including a center and a surrounding layer covering the center, a middle layer covering the core, and a cover covering the middle layer; and a multi-piece golf ball that includes a core including a center and a surrounding layer covering the center, multiple pieces or layers of middle layer covering the core, and a cover covering the middle layer.

In cases where the golf ball of the present invention has a diameter of 40 to 45 mm, the amount of compressive deformation (shrinkage of the golf ball in the compression direction) when applying from an initial load of 98 N to a final load of 1275 N is preferably not smaller than 2.0 mm, more preferably not smaller than 2.1 mm, and still more preferably not smaller than 2.2 mm, whereas it is preferably not larger than 3.0 mm, more preferably not larger than 2.9 mm, and still more preferably not larger than 2.8 mm. When the amount of compressive deformation is not smaller than 2.0 mm, better shot feeling can be provided. When the amount of compressive deformation is not greater than 3.0 mm, good resilience can be achieved.

EXAMPLES

The present invention will be described in greater detail referring to, but not limited to, examples.

(1) Surface Hardness of Center, Surface Hardness of Surrounding Layer (Shore D Hardness)

Using a P1-series auto rubber hardness tester (produced by Kobunshi Keiki Co., Ltd.) including a spring type Shore D hardness tester in accordance with ASTM-D 2240, the Shore D hardness was measured at the surface of a center and at the surface of a surrounding layer, and used as the surface hardness of the center and the surface hardness of the surrounding layer, respectively.

(2) Slab Hardness (Shore D Hardness)

A material for a middle layer or a cover composition was formed into a sheet having a thickness of about 2 mm, and then stored at 23° C. for 2 weeks. Three or more pieces of this sheet were stacked on one another so as not to be affected by a measurement substrate and the like. The slab hardness of the stack was measured using a P1-series auto rubber hardness tester (produced by Kobunshi Keiki Co., Ltd.) including a spring type Shore D hardness tester in accordance with ASTM-D 2240. The sheet used in the measurement was formed by injection molding.

(3) Upper Yield Stress, Lower Yield Stress (MPa)

A material for a middle layer was injection-molded into a sheet having a thickness of about 2 mm, and then stored at a temperature of 23° C. for 2 weeks. A dumbbell specimen was prepared from this sheet. The specimen was subjected to a tensile test in accordance with ISO 527-1 to prepare a stress-strain curve. A value at a point on the curve at which the stress first started declining due to the increase in strain was taken as upper yield stress. A value at a point on the curve at which the stress first started rising after passing through the upper yield stress due to the increase in strain was taken as lower yield stress.

(4) Breaking Stress (MPa)

A material for a middle layer was injection-molded into a sheet having a thickness of about 2 mm, and then stored at a temperature of 23° C. for 2 weeks. A dumbbell specimen was prepared from this sheet. The specimen was measured for breaking stress in accordance with ISO 527-1.

(5) Bending Rigidity (MPa)

A specimen (length: 80.0±2 mm, width: 10.0±0.2 mm, thickness: 4.0±0.2 mm) was prepared by injection molding a material for a middle layer, and then stored at a temperature of 23° C. for 2 weeks. Then, the bending rigidity of the specimen sheet was measured in accordance with ISO 178. The measurement was performed at a temperature of 23° C. and a humidity of 50% RH.

(6) Amount of Compressive Deformation (mm)

A golf ball was compressed by applying from an initial load of 98 N to a final load of 1275 N to the golf ball, and the amount of deformation of the golf ball in the compression direction (shrinkage of the golf ball in the compression direction) was measured.

(7) Driver Shots

A golf ball was hit at a clubhead speed of 50 m/sec with a metal head driver (W #1) (XXIO S, loft angle: 11°, produced by Dunlop Sports Co., Ltd.) attached to a swing robot M/C (produced by Golf Laboratories Inc.). The flight distance (the distance from a launching point to a stopping point), the initial speed of the ball, and the spin rate (the rate of spin of the ball) were measured. Each golf ball was measured 12 times, and a mean value was calculated and used as the measured value of the golf ball. It should be noted that the spin rate of a golf ball immediately after being hit was determined from serial photographs of the golf ball hit.

(8) Durability

Each golf ball was hit at a clubhead speed of 50 m/sec with a metal head driver (W #1) attached to a swing robot M/C (produced by Golf Laboratories Inc.) to make the golf ball collide with a collision board. This operation was repeated. The number of hits required to break the golf ball was measured. The number of hits for each golf ball is expressed as an index relative to that of a golf ball No. 12 (=100), to show the durability of the golf ball. A higher index indicates that the golf ball has better durability at high clubhead speeds.

[Preparation of Golf Ball] (1) Preparation of Center

A rubber composition was prepared by mixing the materials according to the formulation shown in Table 1. The rubber composition was hot-pressed in upper and lower molds each having a hemispherical cavity at 170° C. for 30 minutes to form a center.

TABLE 1 Composition for center No. A Formulation Polybutadiene rubber 100 (Parts by Zinc acrylate 31.5 mass) Zinc oxide 5 Barium sulfate Appropriate amount Diphenyl disulfide 0.3 Dicumyl peroxide 0.9 Physical Surface hardness 60 properties (Shore D hardness) Polybutadiene rubber: “BR-730 (high-cis polybutadiene)” produced by JSR Corporation Zinc acrylate: “ZNDA-90S” produced by Ninon Jyoryu Kogyo Co., Ltd. Zinc oxide: “GINREI (registered trademark) R” produced by Toho Zinc Co., Ltd. Barium sulfate: “Barium Sulfate BD” produced by Sakai Chemical Industry Co., Ltd. Diphenyl disulfide: product of Sumitomo Seika Chemicals Co., Ltd. Dicumyl peroxide: “PERCUMYL (registered trademark) D” produced by NOF Corporation

The amount of barium sulfate added was appropriately adjusted so that the resulting golf ball had a mass of 45.4 g.

(2) Preparation of Composition for Surrounding Layer

A composition for a surrounding layer was prepared using the materials according to the formulation shown in Table 2.

TABLE 2 Composition for surrounding layer No. B Formulation Polybutadiene rubber 100 (Parts by Zinc acrylate 38 mass) Zinc oxide 5 Barium sulfate Appropriate amount Diphenyl disulfide 0.5 Dicumyl peroxide 0.8 Physical Surface hardness 62 properties (Shore D hardness) Polybutadiene rubber: “BR-730 (high-cis polybutadiene)” produced by JSR Corporation Zinc acrylate: “ZNDA-90S” produced by Nihon Jyoryu Kogyo Co., Ltd. Zinc oxide: “GINREI (registered trademark) R” produced by Toho Zinc Co., Ltd. Barium sulfate: “Barium Sulfate BD” produced by Sakai Chemical Industry Co., Ltd. Diphenyl disulfide: product of Sumitomo Seika Chemicals Co., Ltd. Dicumyl peroxide: “PERCUMYL (registered trademark) D” produced by NOF Corporation

(3) Preparation of Cover Composition and Material for Middle Layer

A pelletized cover composition and a pelletized material for a middle layer were prepared by mixing the materials according to the formulations shown in Table 3 and Table 4 using a twin-screw mixing extruder. The following extrusion conditions were used: a screw diameter of 45 mm, a screw rotation speed of 200 rpm, and a screw L/D ratio of 35. Here, the mixture was heated to 160 to 230° C. in the die of the extruder.

TABLE 3 Cover composition No. 1 2 3 Formulation Elastollan XNY75A 100 — — (Parts by Elastollan XNY83A — 100 — mass) Elastollan XNY85A — — 100 Titanium oxide 4 4 4 Physical Slab hardness 23 30 32 properties (Shore D hardness) Elastollan XNY75A: thermoplastic polyurethane elastomer (Shore D hardness: 23) produced by BASF Elastollan XNY83A: thermoplastic polyurethane elastomer (Shore D hardness: 30) produced by BASF Elastollan XNY85A: thermoplastic polyurethane elastomer (Shore D hardness: 32) produced by BASF

TABLE 4 Material for middle layer No. a b c d e g f Formulation Primalloy CX300 100 — — — — — — (Parts by (Polyester elastomer) mass) Elastollan 1164D — 50 — — — — — (Polyether polyurethane elastomer) Elastollan 1174D — 50 100 — — — — (Polyether polyurethane elastomer) Elastollan HM76D — — — — — 100 — (Polyether polyurethane elastomer) E568 — — — 100 — — — (Polycaprolactone polyurethane elastomer) E574 — — — — 100 — — (Polycaprolactone polyurethane elastomer) Surlyn 8945 — — — — — — 50 (Zn ionomer, Acid component content: 15 wt %) Himilan AM7329 — — — — — — 50 (Na ionomer, Acid component content: 15 wt %) Physical Slab hardness (Shore D hardness) 72 70 74 68 74 76 66 properties Bending rigidity (MPa) 400 360 660 350 640 700 290 of slab Breaking stress (MPa) 29 55 55 55 56 50 27 Breaking stress/Slab hardness (Shore D hardness) 0.40 0.79 0.74 0.81 0.76 0.66 0.41 Upper yield stress (MPa) 23 25 31 28 32 38 18 Lower yield stress (MPa) 14 24 22 25 24 22 17 Upper yield stress/Lower yield stress 1.64 1.04 1.41 1.12 1.33 1.73 1.06 PRIMALLOY CX300: polyester elastomer (Shore D hardness: 72, bending rigidity: 400 MPa, breaking stress: 29 MPa, upper yield stress: 23 MPa, lower yield stress: 14 MPa) produced by Mitsubishi Chemical Corporation Elastollan 1164D: polyether polyurethane elastomer (Shore D hardness: 64, bending rigidity: 330 MPa, breaking stress: 55 MPa, upper yield stress: 23 MPa, lower yield stress: 23 MPa) produced by BASF Elastollan 1174D: polyether polyurethane elastomer (Shore D hardness: 74, bending rigidity: 660 MPa, breaking stress: 55 MPa, upper yield stress: 31 MPa, lower yield stress: 22 MPa) produced by BASF Elastollan HM76D: polyether polyurethane elastomer (Shore D hardness: 76, bending rigidity: 700 MPa, breaking stress: 50 MPa, upper yield stress: 38 MPa, lower yield stress: 22 MPa) produced by BASF Miractran E568: thermoplastic polycaprolactone polyurethane elastomer (Shore D hardness: 68, bending rigidity: 350 MPa, breaking stress: 55 MPa, upper yield stress: 28 MPa, lower yield stress: 25 MPa) produced by Nippon Miractran Co, Ltd. Miractran E574: thermoplastic polycaprolactone polyurethane elastomer (Shore D hardness: 74, bending rigidity: 640 MPa, breaking stress: 56 MPa, upper yield stress: 32 MPa, lower yield stress: 24 MPa) produced by Nippon Miractran Co, Ltd. Surlyn 8945: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin (acid component content: 15% by mass, Shore D hardness: 61, bending rigidity: 254 MPa, breaking stress: 24 MPa, upper yield stress: 15 MPa, lower yield stress: 14 MPa) produced by E. I. du Pont de Nemours and Company Himilan AM7329: zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin (acid component content: 15% by mass, Shore D hardness: 61, bending rigidity: 254 MPa, breaking stress: 24 MPa, upper yield stress: 16 MPa, lower yield stress: 14 MPa) produced by Du Pont-Mitsui Polychemical

(4) Preparation of Golf Ball Body

The above-obtained composition for a surrounding layer was used to form a surrounding layer on the center obtained as above, if necessary, and a core was thus prepared. In the case of using the composition for a surrounding layer, first the composition for a surrounding layer containing the materials according to the formulation shown in Table 2 was kneaded, and then an upper mold for forming a core, in which the center was housed, and a lower mold for forming a core were clamped in a manner that a necessary amount of the composition for a surrounding layer was in contact with a half of the surface of the center, followed by pressing. Thus, an intermediate core molded product in which a surrounding layer was formed on a half of the surface of the center was prepared. Next, a lower mold for forming a core, in which the surrounding layer of the intermediate core molded product was housed, and an upper mold for forming a core were clamped in a manner that a necessary amount of the composition for a surrounding layer was in contact with the other half of the surface of the center, followed by pressing. Thus, a surrounding layer was formed on the other half of the surface of the center, and the resulting product was hot-pressed at 170° C. for 30 minutes to form a core.

The material for a middle layer obtained as above was injection-molded onto the core obtained as above to form a middle layer covering the core. Then, a cover was formed by injection molding the cover composition onto the middle layer. Or alternatively, a cover was formed by injection molding or compression molding the cover composition into half-shells and applying two pieces of half-shells to the core provided with the middle layer so as to cover the core, followed by hot-pressing. Thus, a golf ball was prepared. The upper and lower molds for molding used each had a hemispherical cavity with pimples, some of the pimples also serving as retractable hold pins. After protruding the hold pins, the core was placed in the mold and held by the hold pins, and the molds were clamped under a pressure of 80 t. Then, the resin composition heated to 210° C. was injected into the molds in 0.3 seconds, and then cooled for 30 seconds. The molds were opened to take out a golf ball.

The surface of the obtained golf ball body was sandblasted and marked. Then, a clear paint was applied and dried by heating in an oven at 40° C. for 4 hours. Thus, a golf ball having a diameter of 42.7 mm and a mass of 45.4 g was obtained.

The golf balls thus obtained were evaluated for amount of compressive deformation and other items. Table 5 shows the results.

TABLE 5 Golf ball No. 1 2 3 4 5 6 7 8 9 10 Structure 3PC 3PC 3PC 3PC 3PC 3PC 3PC 3PC 4PC 4PC Core Composition for A A A A A A A A A A center No. Composition for — — — — — — — — B B surrounding layer No. Middle Composition for b c d e b b b b b c layer middle layer No. Cover Cover 2 2 2 2 1 3 2 2 2 2 composition No. Details of Core diameter 39.7 39.7 39.7 39.7 39.7 39.7 40.1 38.1 39.7 39.7 structure (mm) (Center (Center diameter: diameter: 20.1 mm) 20.1 mm) (Surrounding (Surrounding layer layer thickness: thickness: 9.8 mm) 9.8 mm) Middle layer 1.0 1.0 1.0 1.0 1.0 1.0 0.8 1.8 1.0 1.0 thickness (mm) Cover 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 thickness (mm) Ball diameter 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 (mm) Physical Amount of 2.68 2.66 2.68 2.66 2.69 2.65 2.71 2.62 2.62 2.60 properties compressive of golf deformation ball (mm) Ball initial 72.6 72.6 72.6 72.7 72.6 72.7 72.8 72.7 72.7 72.8 speed (m/s) Ball spin 2375 2360 2395 2335 2400 2360 2380 2340 2350 2340 rate (rpm) Flight 252.8 253.1 252.5 253.8 252.1 253.4 253.2 253.6 253.5 253.7 distance (m) Durability 160 125 140 125 165 155 130 170 145 120 (index) Golf ball No. 11 12 13 14 15 16 17 18 19 Structure 3PC 3PC 3PC 3PC 3PC 3PC 3PC 4PC 4PC Core Composition for A A A A A A A A A center No. Composition for — — — — — — — B B surrounding layer No. Middle Composition for b b a f g a a a f layer middle layer No. Cover Cover 2 2 2 2 2 1 3 2 2 composition No. Details of Core diameter 41.1 36.7 39.7 39.7 39.7 39.7 39.7 39.7 39.7 structure (mm) (Center (Center diameter: diameter: 20.1 mm) 20.1 mm) (Surrounding (Surrounding layer layer thickness: thickness: 9.8 mm) 9.8 mm) Middle layer 0.3 2.5 1.0 1.0 1.0 1.0 1.0 1.0 1.0 thickness (mm) Cover 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 thickness (mm) Ball diameter 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 (mm) Physical Amount of 2.74 2.56 2.70 2.72 2.64 2.73 2.67 2.63 2.65 properties compressive of golf deformation ball (mm) Ball initial 73.0 72.6 72.5 72.6 Not 72.4 72.5 72.6 72.7 speed (m/s) evaluable Ball spin 2430 2320 2475 2450 due to 2500 2445 2450 2430 rate (rpm) poor Flight 253.6 252.8 250.6 251.1 durability 249.9 251.2 251.4 251.6 distance (m) Durability 105 180 25 100 30 20 10 90 (index)

It was demonstrated that each of the golf balls Nos. 1 to 10, which included a middle layer including a thermoplastic polyurethane elastomer having a specific slab hardness and a specific ratio of upper yield stress to lower yield stress, showed a lower spin rate, longer flight distance and better durability than the golf balls Nos. 13, 14, and 16 to 19, which included a polyester elastomer or an ionomer resin. Further, excellent flight distance and excellent durability were also ensured in the case of the golf balls No. 11 and 12 including a middle layer with a thickness of 0.3 mm and 2.5 mm, respectively. The golf ball No. 15 had poor durability and thus was not evaluable.

INDUSTRIAL APPLICABILITY

The golf ball of the present invention is useful as it has improved flight distance performance and durability.

Claims

1. A golf ball, comprising

a core,

at least one middle layer covering the core, and

a cover covering the middle layer,

wherein at least one piece or layer of the middle layer comprises a material for a middle layer that comprises a thermoplastic polyurethane with a slab hardness of 65 to 80 in Shore D hardness and a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60.

2. The golf ball according to claim 1,

wherein the thermoplastic polyurethane has an upper yield stress of not less than 15 MPa and a lower yield stress of not less than 10 MPa.

3. The golf ball according to claim 1,

wherein the thermoplastic polyurethane has a breaking stress (MPa) of not less than 25 MPa.

4. The golf ball according to claim 1,

wherein the thermoplastic polyurethane has a ratio of breaking stress (MPa) to slab hardness (Shore D hardness) of not less than 0.70.

5. The golf ball according to claim 1,

wherein the thermoplastic polyurethane has a bending rigidity of 250 to 4000 MPa.

6. The golf ball according to claim 1,

wherein the middle layer has a thickness of 0.5 to 2.0 mm.

7. The golf ball according to claim 1,

wherein the middle layer has a surface hardness of 65 to 80 in Shore D hardness.

8. The golf ball according to claim 1,

wherein a difference (Hm−Hs) between a surface hardness (Hm) of the middle layer and a surface hardness (Hs) of the core is 3 to 25.

9. The golf ball according to claim 1,

wherein the cover comprises a thermoplastic polyurethane with a slab hardness of not more than 50 in Shore D hardness, and has a thickness of 0.3 to 1.5 mm.