GOLF BALL

Abstract

In a golf ball having a rubber core of at least one layer and a cover of at least one layer that encases the core, at least one layer of the cover is formed of a resin composition which includes (A) a polyurethane or a polyurea, (B) an alkyl methacrylate/alkyl acrylate copolymer, and (C) a fatty acid amide. The golf ball has an excellent mold releasability during molding of the cover while retaining a good spin rate on approach shots, scuff resistance and paint film durability.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2022-089434 filed in Japan on Jun. 1, 2022, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a golf ball having a core of at least one layer and a cover of at least one layer, and more particularly to a golf ball in which the cover is made using a polyurethane material of excellent mold releasability during molding of the cover.

BACKGROUND ART

Golf ball covers made of conventional thermoplastic polyurethane materials have a poor mold releasability when molded. To address this problem, a metal salt of stearic acid or a polyethylene wax is sometimes used as a lubricant ingredient or a dispersant. However, although metal salts of stearic acid do have the salutary effect of increasing mold releasability, it has been confirmed that—perhaps because metal salts act as decomposition catalysts on thermoplastic polyurethane materials—metal salts of stearic acid, when used as a dispersant, lower the heat resistance of thermoplastic polyurethane materials. As for polyethylene waxes, owing to their poor compatibility with polyurethane materials, when a polyethylene wax is used as a dispersant, the pigment dispersibility tends to worsen.

Regarding the formulation of cover resin compositions in which the base resin is a polyurethane resin, JP-A 2002-336382 discloses a golf ball formed using a cover stock which is composed primarily of a thermoplastic polyurethane material and includes also, as a dispersant, a fatty acid amide and/or montan wax. JP-A 2014-171895 describes a golf ball formed using a cover stock which is composed primarily of a thermoplastic polyurethane material and includes a montanic acid ester-based lubricant. Yet, although these golf balls do contain in the cover resin material a fatty acid amide or the like as a lubricant or a mold release agent, the releasability remains inadequate, especially when the cover is molded of a urethane resin material having a low degree of hardness.

When using a mold release agent, there is a risk of worsening the features of the cover resin composition and so care is required in selecting the type and content of release agent to be included in the composition. The norm in recent years has been to use, as the cover stock in golf balls for professional golfers and skilled amateurs, a soft urethane resin material instead of an ionomer resin material. This makes it possible to achieve not only a good flight performance, but also a high spin rate on approach shots and an excellent scuff resistance. There exists a desire for the production of golf balls which, while retaining these properties, also have an increased mold releasability and thus a good productivity.

In other related art, JP-A 2019-107401 describes a cover-forming resin composition which contains a polyurethane and a methacrylic resin. However, no mention is made therein of including a mold release agent such as a fatty acid amide.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball having a polyurethane resin cover, which golf ball has an excellent mold releasability during molding of the cover yet retains such properties as a good spin rate on approach shots, scuff resistance and paint film durability.

As a result of intensive investigations, we have found that by preparing, as a resin composition for golf ball covers, a resin composition which includes an alkyl methacrylate/alkyl acrylate copolymer and a fatty acid amide in a polyurethane resin having a low degree of hardness, when a golf ball in which a molding of this resin composition serves as the cover is produced, the golf ball has an excellent mold releasability during molding of the cover and also retains a good spin rate on approach shots, scuff resistance and paint film (or “coating”) durability.

Accordingly, the present invention provides a golf ball having a rubber core of at least one layer and a cover of at least one layer which encases the core, wherein at least one layer of the cover is formed of a resin composition which includes (A) a polyurethane or a polyurea, (B) an alkyl methacrylate/alkyl acrylate copolymer, and (C) a fatty acid amide.

In a preferred embodiment of the golf ball of the invention, the amount of the alkyl methacrylate/alkyl acrylate copolymer included as component B per 100 parts by weight of component A is 1.5 parts by weight or less.

In another preferred embodiment of the inventive golf ball, the amount of the fatty acid amide included as component C per 100 parts by weight of component A is 1.0 part by weight or less.

In yet another preferred embodiment, the resin composition making up the cover has a material hardness on the Shore D hardness scale of 65 or less.

In still another preferred embodiment, the resin composition making up the cover has a rebound resilience, as measured according to JIS-K 6255, which is 65 or less.

In a further preferred embodiment, the value obtained by dividing the rebound resilience of the resin composition making up the cover by the material hardness of the resin composition on the Shore D hardness scale is 0.5 or more.

Advantageous Effects of the Invention

The golf ball of the invention, which has a cover made of a polyurethane resin, possesses an excellent mold releasability during molding of the cover yet retains a good spin rate on approach shots, scuff resistance and paint film durability.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic cross-sectional view of the golf ball according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagram.

The golf ball of this invention is a golf ball having a core of at least one layer which is encased by a cover of at least one layer, i.e., a single-layer or multilayer cover.

The core may be formed using a known rubber material as the base material. A known base rubber such as a natural rubber or a synthetic rubber may be used as the base rubber of the core. More specifically, it is recommended that polybutadiene, especially cis-1,4-polybutadiene having a cis structure content of at least 40%, be chiefly used. If desired, natural rubber, polyisoprene rubber, styrene-butadiene rubber or the like may be used together with the foregoing polybutadiene in the base rubber.

The polybutadiene may be synthesized with a metal catalyst, such as a neodymium or other rare-earth catalyst, a cobalt catalyst or a nickel catalyst.

Co-crosslinking agents such as unsaturated carboxylic acids and metal salts thereof, inorganic fillers such as zinc oxide, barium sulfate and calcium carbonate, and organic peroxides such as dicumyl peroxide and 1,1-bis(t-butylperoxy)cyclohexane may be included in the base rubber. If necessary, commercial antioxidants and the like may be suitably added.

The core may be produced by vulcanizing/curing the rubber composition containing the above ingredients. For example, production may be carried out by kneading the composition using a mixer such as a Banbury mixer or a roll mill, compression molding or injection molding the kneaded composition using a core mold, and curing the molded body by suitably heating it at a temperature sufficient for the organic peroxide and the co-crosslinking agent to act, i.e., between 100° C. and 200° C., and preferably between 140 and 180° C., for a period of 10 to 40 minutes.

In the golf ball of the invention, the core is encased by a single-layer or multilayer cover. Such a golf ball may take the form of, for example, a golf ball having a single-layer cover over a core, or a golf ball having a core, an intermediate layer encasing the core, and an outermost layer encasing the intermediate layer. For example, the FIGURE shows a golf ball G having a three-layer construction that consists of a core 1, an intermediate layer 2 encasing the core 1, and an outermost layer 3 encasing the intermediate layer 2. The outermost layer 3 is the layer positioned outermost, aside from a coating layer, in the layered structure of the golf ball. Numerous dimples D are typically formed on the surface of the outermost layer 3 in order to enhance the aerodynamic properties of the ball. Although not shown in the FIGURE, a coating layer is generally formed on the surface of the outermost layer 3.

In this invention, at least one layer of the cover is formed of a resin composition containing components A to C below:

• • (A) a polyurethane or a polyurea,
  • (B) an alkyl methacrylate/alkyl acrylate block copolymer, and
  • (C) a fatty acid amide.

(A) Polyurethane or Polyurea

The polyurethane or polyurea is a substance that is capable of serving as the base resin of the above cover material (resin composition). The polyurethane (A-1) or polyurea (A-2) used as this component is described in detail below.

(A-1) Polyurethane

The polyurethane has a structure which includes soft segments composed of a polymeric polyol that is a long-chain polyol, and hard segments composed of a chain extender and a polyisocyanate. Here, the polymeric polyol serving as a starting material may be any that has hitherto been used in the art relating to polyurethane materials, and is not particularly limited. It is exemplified by polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. Specific examples of polyester polyols that may be used include adipate-type polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol and polyhexamethylene adipate glycol; and lactone-type polyols such as polycaprolactone polyol. Examples of polyether polyols include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol). These polyols may be used singly, or two or more may be used in combination.

It is preferable to use a polyether polyol as the above polymeric polyol.

The long-chain polyol has a number-average molecular weight that is preferably in the range of 1,000 to 5,000. By using a long-chain polyol having a number-average molecular weight in this range, golf balls made with a polyurethane composition that have excellent properties, including a good rebound and good productivity, can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in the range of 1,500 to 4,000, and even more preferably in the range of 1,700 to 3,500.

Here and below, “number-average molecular weight” refers to the number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS-K1557.

The chain extender is not particularly limited; any chain extender that has hitherto been employed in the art relating to polyurethanes may be suitably used. In this invention, low-molecular-weight compounds with a molecular weight of 2,000 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups may be used. Of these, preferred use can be made of aliphatic diols having from 2 to 12 carbon atoms. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3 butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The use of 1,4 butylene glycol is especially preferred.

Any polyisocyanate hitherto employed in the art relating to polyurethanes may be suitably used without particular limitation as the polyisocyanate. For example, use can be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane and dimer acid diisocyanate. However, depending on the type of isocyanate, crosslinking reactions during injection molding may be difficult to control.

The ratio of active hydrogen atoms to isocyanate groups in the polyurethane-forming reaction may be suitably adjusted within a preferred range. Specifically, in preparing a polyurethane by reacting the above long-chain polyol, polyisocyanate and chain extender, it is preferable to use the respective components in proportions such that the amount of isocyanate groups included in the polyisocyanate per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

The method of preparing the polyurethane is not particularly limited. Preparation using the long-chain polyol, chain extender and polyisocyanate may be carried out by either a prepolymer process or a one-shot process via a known urethane-forming reaction. Of these, melt polymerization in the substantial absence of solvent is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

It is preferable to use a thermoplastic polyurethane material as the polyurethane, with an ether-based thermoplastic polyurethane material being especially preferred. The thermoplastic polyurethane material used may be a commercial product, illustrative examples of which include those available under the registered trademark PANDEX from DIC Covestro Polymer, Ltd., and those available under the trade name RESAMINE from Dainichiseika Color & Chemicals Mfg. Co., Ltd.

(A-2) Polyurea

The polyurea is a resin composition made up primarily of urea linkages formed by reacting (i) an isocyanate with (ii) an amine-terminated compound. This resin composition is described in detail below.

(i) Isocyanate

The isocyanate is not particularly limited. Any isocyanate used in the prior art relating to polyurethanes may be suitably used here. Use may be made of isocyanates similar to those mentioned above in connection with the polyurethane material.

(ii) Amine-Terminated Compound

An amine-terminated compound is a compound having an amino group at the end of the molecular chain. In this invention, the long-chain polyamines and/or amine curing agents shown below may be used.

A long-chain polyamine is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number average molecular weight of from 1,000 to 5,000. In this invention, the number average molecular weight is more preferably from 1,500 to 4,000, and even more preferably from 1,900 to 3,000. Examples of such long-chain polyamines include, but are not limited to, amine-terminated hydrocarbons, amine-terminated polyethers, amine-terminated polyesters, amine-terminated polycarbonates, amine-terminated polycaprolactones, and mixtures thereof. These long-chain polyamines may be used singly, or two or more may be used in combination.

An amine curing agent is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number average molecular weight of less than 1,000. In this invention, the number average molecular weight is more preferably less than 800, and even more preferably less than 600. Specific examples of such amine curing agents include, but are not limited to, ethylenediamine, hexamethylenediamine, 1-methyl-2,6-cyclohexyldiamine, tetrahydroxypropylene ethylenediamine, 2,2,4- and 2,4,4-trimethyl-1,6-hexanediamine, 4,4′-bis(sec-butylamino)dicyclohexylmethane, 1,4-bis(sec-butylamino)cyclohexane, 1,2 bis(sec-butylamino)cyclohexane, derivatives of 4,4′ bis(sec butylamino)dicyclohexylmethane, 4,4′-dicyclohexylmethanediamine, 1,4 cyclohexane bis(methylamine), 1,3-cyclohexane bis(methylamine), diethylene glycol di(aminopropyl) ether, 2-methylpentamethylenediamine, diaminocyclohexane, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, propylenediamine, 1,3 diaminopropane, dimethylaminopropylamine, diethylaminopropylamine, dipropylenetriamine, imidobis(propylamine), monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, isophoronediamine, 4,4′ methylenebis(2-chloroaniline), 3,5-dimethylthio-2,4-toluenediamine, 3,5 dimethylthio-2,6-toluenediamine, 3,5-diethylthio-2,4-toluenediamine, 3,5 diethylthio 2,6-toluenediamine, 4,4′-bis(sec-butylamino)diphenylmethane and derivatives thereof, 1,4-bis(sec-butylamino)benzene, 1,2-bis(sec-butylamino)benzene, N,N′-dialkylaminodiphenylmethane, N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine, trimethylene glycol di-p-aminobenzoate, polytetramethylene oxide di-p-aminobenzoate, 4,4′-methylenebis(3-chloro-2,6-diethyleneaniline), 4,4′-methylenebis(2,6-diethylaniline), m-phenylenediamine, p-phenylenediamine and mixtures thereof. These amine curing agents may be used singly or two or more may be used in combination.

(iii) Polyol

Although not an essential ingredient, in addition to above components (i) and (ii), a polyol may also be included in the polyurea. The polyol is not particularly limited, but is preferably one that has hitherto been used in the art relating to polyurethanes. Specific examples include the long-chain polyols and/or polyol curing agents mentioned below.

The long-chain polyol may be any that has hitherto been used in the art relating to polyurethanes. Examples include, but are not limited to, polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin-based polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or two or more may be used in combination.

The long-chain polyol has a number-average molecular weight of preferably from 1,000 to 5,000, and more preferably from 1,700 to 3,500. In this average molecular weight range, an even better rebound and productivity are obtained.

The polyol curing agent is preferably one that has hitherto been used in the art relating to polyurethanes, but is not subject to any particular limitation. In this invention, use may be made of a low-molecular-weight compound having on the molecule at least two active hydrogen atoms capable of reacting with isocyanate groups and having a molecular weight of less than 1,000. Of these, the use of aliphatic diols having from 2 to 12 carbon atoms is preferred. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The use of 1,4-butylene glycol is especially preferred. The polyol curing agent has a number-average molecular weight of preferably less than 800, and more preferably less than 600.

A known method may be used to produce the polyurea. A prepolymer process, a one-shot process or some other known method may be suitably selected for this purpose.

Component A has a material hardness on the Shore D hardness scale which, from the standpoint of the spin properties and scuff resistance that can be obtained in the golf ball, is preferably 65 or less, more preferably 60 or less, even more preferably 55 or less, still more preferably 50 or less, and most preferably 45 or less. From the standpoint of the moldability, the lower limit in the material hardness on the Shore D scale is preferably at least 35, more preferably at least 38, and even more preferably at least 40.

Component A has a rebound resilience which, from the standpoint of golf ball properties such as the initial velocity of the ball when struck, the scuff resistance and the spin rate on approach shots, is preferably at least 40%, more preferably at least 45%, and even more preferably at least 50%. The upper limit in the rebound resilience is preferably 70% or less, more preferably 68% or less, and even more preferably 65% or less. The rebound resilience is measured based on JIS-K 6255:2013.

Component A serves as the base material of the resin composition making up the cover. To fully impart the scuff resistance of the urethane resin, it accounts for at least 50 wt %, preferably at least 60 wt %, more preferably at least 70 wt %, even more preferably at least 80 wt %, and most preferably at least 90 wt %, of the resin composition.

In this invention, an alkyl methacrylate/alkyl acrylate copolymer is included as component B in the resin composition making up the cover. Component B is used in particular for the purpose of enhancing the dispersibility of the subsequently described component C in the urethane resin or the like serving as component A. Component B is described in detail below.

The alkyl acrylate/alkyl methacrylate copolymer serving as component B is an acrylic processing aid obtained by copolymerizing an alkyl acrylate with an alkyl methacrylate. By adding this copolymer to the thermoplastic resin, it is possible to impart melt elasticity and increase the melt tension. This advantageous effect reportedly arises due to the creation of a pseudo-crosslinked state on account of entanglement by the molecular chain of this copolymer with molecules of the matrix resin. Examples of the alkyl methacrylate include, but are not particularly limited to, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate and 2-ethylhexyl methacrylate. One of these may be used alone or two or more may be used in combination. Of these, alkyl methacrylates in which the ester group has from 1 to 4 carbon atoms, such as methyl methacrylate and n-butyl methacrylate, are preferred. Methyl methacrylate is especially preferred. Examples of the alkyl acrylate include, but are not particularly limited to, ethyl acrylate, propyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. One of these may be used alone or two or more may be used in combination. Of these, an alkyl acrylate in which the ester group has from 2 to 4 carbon atoms is preferred, with n-butyl acrylate being especially preferred because the compatibility of the resulting copolymer composition with the thermoplastic resin is controlled, resulting in a resin processing aid that can better impart lubricity. The weight-average molecular weight of the alkyl acrylate/alkyl methacrylate copolymer is preferably from 200,000 to 5,000,000, more preferably from 350,000 to 3,500,000, and even more preferably from 500,000 to 2,000,000. Examples of commercial products include the Kane Ace™ PA Series processing aids from Kaneka Corporation and the Metablen™ P Type acrylic processing aids from Mitsubishi Chemical Corporation.

The content of component B per 100 parts by weight of component A is preferably 1.5 parts by weight or less, and more preferably 1.4 parts by weight or less. Above this value, the scuff resistance may decrease or the spin performance of the ball when struck may worsen. The lower limit of this content per 100 parts by weight of component A is preferably 0.05 part by weight or more, and more preferably 0.1 part by weight or more. At a content lower than this value, an excellent mold releasability may not be attainable.

In this invention, to improve the mold releasability during molding of the resin composition that forms the cover, a fatty acid amide is included as component C in the composition. The fatty acid amides used in this invention can be divided into bisamides and monoamides. Fatty acid amides are compounds that are chemically neutral and stable, but they have within the molecule a long-chain alkyl group and a highly polar amide group. Hydrogen bonds are created between hydrogens on the amide group and oxygens on other molecules, forming a chain-like or network polymer that exhibits singular properties. Exemplary monoamides include saturated fatty acid amides, unsaturated fatty acid amides, hydroxy fatty acid amides and N-methylol fatty acid amides of 8 to 22 carbons. Exemplary bisamides include methylene bisamides and ethylene bisamides which use a saturated or unsaturated fatty acid of 8 to 22 carbons. In the practice of this invention, although not particularly limited, the fatty acid amide is suitably selected from among ones that are commonly sold as fatty acid amides; from the standpoint of compatibility with urethane, an unsaturated fatty acid bisamide may be advantageously used, with ethylenebis(oleamide) or the like being preferred. Alternatively, ethylenebis(stearamide) is preferred in terms of the balance between performance and price. Commercial ethylenebis(oleamides) include Slipax O from Mitsubishi Chemical Corporation. Commercial ethylenebis(stearamides) include Kao Wax EB-P, EB-FF and EB-G from Kao Corporation, and Slipax E from Mitsubishi Chemical Corporation.

The component C content per 100 parts by weight of component A is preferably 1.0 part by weight or less, more preferably 0.9 part by weight or less, and even more preferably 0.8 part by weight or less. Above this value, the paint film durability may decrease or the spin performance of the ball on shots may worsen. The lower limit of this content is preferably at least 0.05 part by weight, and more preferably at least 0.08 part by weight, per 100 parts by weight of component A.

The resin composition of components A to C may be prepared by using a masterbatch. That is, the resin composition can be prepared via steps (i) and (ii) below:

• • (i) producing a masterbatch containing components B and C, and
  • (ii) mixing the masterbatch into component A and thereby obtaining a resin composition containing components A, B and C.

Rather than charging components A, B and C all at once, by instead first preparing the above lubricant masterbatch and then blending these ingredients with the urethane resin serving as the base resin, it is possible to increase even further the dispersibility of component C and thus enhance the mold releasability.

Aside from the above resin ingredients, the resin composition containing components A, B and C may also include other resin materials for such purposes as to further enhance the flow properties of the golf ball resin composition and improve various properties of the golf ball, such as the rebound, durability to cracking, scuff resistance, spin performance and controllability.

Such other resin materials may be selected from, but are not limited to, polyester elastomers, polyamide elastomers, ionomer resins, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, polyacetals, polyethylenes, nylon resins, styrene-ethylene/butylene-styrene block copolymers (SEBS), styrene-isobutylene-styrene block copolymers (SIBS), styrene-isoprene-styrene block copolymers (SIS), styrene isobutylene block copolymers (SIB), styrene-ethylene/propylene-styrene block copolymers (SEPS), styrene-ethylene/ethylene/propylene-styrene block copolymers (SEEPS), styrene-butadiene/butylene-styrene block copolymers (SBBS), styrene ethylene-propylene block copolymers (SEP), polyvinyl chlorides, polycarbonates, polyphenylene ethers, polyarylates, polysulfones, polyethersulfones, polyetherimides and polyamideimides. One, two or more of these may be used.

In addition, an active isocyanate compound may be included in the above resin composition. This active isocyanate compound reacts with the polyurethane or polyurea serving as the base resin, enabling the scuff resistance of the overall resin composition to be further enhanced. Moreover, the isocyanate has a plasticizing effect which increases the flow properties of the resin composition, enabling the moldability to be improved.

Any isocyanate compound employed in ordinary polyurethanes may be used without particular limitation as the above isocyanate compound. For example, aromatic isocyanate compounds that may be used include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and mixtures of both, 4,4-diphenylmethane diisocyanate, m-phenylene diisocyanate and 4,4′-biphenyl diisocyanate. Use can also be made of the hydrogenated forms of these aromatic isocyanate compounds, such as dicyclohexylmethane diisocyanate. Other isocyanate compounds that may be used include aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (HDI) and octamethylene diisocyanate; and alicyclic diisocyanates such as xylene diisocyanate. Further examples of isocyanate compounds that may be used include blocked isocyanate compounds obtained by reacting the isocyanate groups on a compound having two or more isocyanate groups on the ends with a compound having active hydrogens, and uretdiones obtained by isocyanate dimerization.

The amount of the above isocyanate compounds included per 100 parts by weight of the polyurethane or polyurea serving as component (A) is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 30 parts by weight, and more preferably not more than 20 parts by weight. When too little is included, sufficient crosslinking reactions may not be obtained and an improvement in the properties may not be observable. On the other hand, when too much is included, discoloration over time due to heat and ultraviolet light may increase, or problems such as a loss of thermoplasticity or a decline in resilience may arise.

Depending on the intended use of the above resin composition, optional additives may be suitably added to the composition. For example, in cases where this golf ball material is to be used as the cover stock, various additives such as fillers (inorganic fillers), organic staple fibers, reinforcing agents, crosslinking agents, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be suitably added to the above ingredients. When including such additives, the amount thereof per 100 parts by weight of the base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight; the upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

In order to increase the initial velocity performance of the golf ball when struck, the scuff resistance and the spin rate on approach shots, the above resin composition has a rebound resilience, as measured according to JIS-K 6255:2013, which is preferably at least 40%, more preferably at least 45%, and even more preferably at least 50%. The upper limit is preferably not more than 70%, more preferably not more than 68%, and even more preferably not more than 65%.

The resin composition has a material hardness on the Shore D hardness scale which, from the standpoint of the scuff resistance and imparting a suitable spin rate on approach shots, is preferably not more than 65, more preferably not more than 60, even more preferably not more than 55, still more preferably not more than 50, and most preferably not more than 45. From the standpoint of moldability, the lower limit in the material hardness on the Shore D hardness scale is preferably at least 35, more preferably at least 38, and even more preferably at least 40.

The value obtained by dividing the rebound resilience of the cover by the Shore D hardness of the cover material is, for example, 1.4 in the case of a thermoplastic polyurethane having a Shore D hardness of 43 and a rebound resilience of 62% that is available under the trade name Pandex from DIC Covestro Polymer, Ltd., 1.1 in the case of a thermoplastic polyurethane having a Shore D hardness of 47 and a rebound resilience of 54% that is available under the trade name Pandex from DIC Covestro Polymer, Ltd., 0.9 in the case of a thermoplastic polyurethane having a Shore D hardness of 56 and a rebound resilience of 60% that is available under the trade name Pandex from DIC Covestro Polymer, Ltd, and 0.7 in the case of a thermoplastic polyurethane having a Shore D hardness of 64 and a rebound resilience of 43% that is available under the trade name Pandex from DIC Covestro Polymer, Ltd. The value is higher for low-hardness urethanes, which provide good golf ball properties such as the spin rate on approach shots. That is, from the standpoint of golf ball properties such as the initial velocity performance on golf ball shots and the spin rate on approach shots, the value obtained by dividing the rebound resilience of the cover by the Shore D hardness of the cover material is preferably at least 0.5, more preferably at least 0.6, even more preferably at least 0.8, still more preferably at least 0.9, and most preferably at least 1.0. From the standpoint of properties such as the feel at impact, the upper limit is preferably 2.5 or less, and more preferably 2.0 or less.

The resin composition may be prepared by mixing together the ingredients using any of various types of mixers, such as a kneading-type single-screw or twin-screw extruder, a Banbury mixer, a kneader or a Labo Plastomill. Alternatively, the ingredients may be mixed together by dry blending when the resin composition is to be injection-molded. In addition, when an active isocyanate compound is used, it may be incorporated at the time of resin mixture using various types of mixers, or a resin masterbatch already containing the active isocyanate compound and other ingredients may be separately prepared and the various components mixed together by dry blending when the resin composition is to be injection molded.

The method of molding the cover from the above resin composition may involve, for example, feeding the resin composition into an injection molding machine and molding the cover by injecting the molten resin composition over the core. In this case, the molding temperature differs according to the type of polyurethane or polyurea (A) serving as the chief ingredient, but is typically in the range of 150 to 270° C.

The cover has a thickness which is at least preferably 0.4 mm, more preferably at least 0.5 mm, and even more preferably at least 0.6 mm. The upper limit is preferably not more than 3.0 mm, more preferably not more than 2.0 mm, and most preferably not more than 1.0 mm.

In the golf ball of the invention, numerous dimples are provided on the surface of the outermost layer for reasons having to do with the aerodynamic performance. The number of dimples formed on the surface of the outermost layer is not particularly limited. However, to enhance the aerodynamic performance and increase the distance traveled by the ball, this number is preferably at least 250, more preferably at least 270, even more preferably at least 290, and most preferably at least 300. The upper limit is preferably not more than 400, more preferably not more than 380, and even more preferably not more than 360.

In this invention, a coating layer is formed on the cover surface. A two-part curable urethane coating may be suitably used as the coating that forms this coating layer. Specifically, in this case, the two-part curable urethane coating is one that includes a base resin composed primarily of a polyol resin and a curing agent composed primarily of a polyisocyanate.

A known method may be used without particular limitation as the method for applying this coating onto the cover surface and forming a coating layer. Use can be made of a desired method such as air gun painting or electrostatic painting.

The thickness of the coating layer, although not particularly limited, is typically from 8 to 22 μm, and preferably from 10 to 20 μm.

The golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and is not more than 42.80 mm, and to a weight which is preferably between 45.0 and 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 3, Comparative Examples 1 to 3

Common Core

A core-forming rubber composition formulated as shown in Table 1 and common to all of the examples was prepared and then molded and vulcanized to produce a 38.6 mm diameter core.

TABLE 1
Rubber composition    parts by weight
cis-1,4-Polybutadiene 100
Zinc acrylate         27
Zinc oxide            4.0
Barium sulfate        16.5
Antioxidant           0.2
Organic peroxide (1)  0.6
Organic peroxide (2)  1.2
Zinc salt of          0.3
pentachlorothiophenol
Zinc stearate         1.0

Details on the above core material are given below.

cis-1,4-Polybutadiene: Available under the trade name BR01 from JSR Corporation
Zinc acrylate: Available from Nippon Shokubai Co., Ltd.
Zinc oxide: Available from Sakai Chemical Co., Ltd.
Barium sulfate: Available from Sakai Chemical Co., Ltd.
Antioxidant: Available under the trade name “Nocrac NS6” from Ouchi Shinko Chemical Industry Co., Ltd.
Organic peroxide (1): Dicumyl peroxide, available under the trade name “Percumyl D” from NOF Corporation
Organic peroxide (2): A mixture of 1,1-di(tert-butylperoxy)cyclohexane and silica, available under the trade name “Perhexa C-40” from NOF Corporation
Zinc stearate: Available from NOF Corporation

Common Intermediate Layer

An intermediate layer-forming resin material was injection-molded over the 38.6 mm diameter core, thereby producing an intermediate layer-encased sphere having a 1.25 mm thick intermediate layer. This intermediate layer-forming resin material, which was a resin blend common to all of the examples, consisted of 50 parts by weight of the sodium neutralization product of an ethylene-unsaturated carboxylic acid copolymer having an acid content of 18 wt % and 50 parts by weight of the zinc neutralization product of an ethylene-unsaturated carboxylic acid copolymer having an acid content of 15 wt %, for a total of 100 parts by weight.

Cover (Outermost Layer)

Next, the outermost layer-forming cover resin composition shown in Table 2 below was injection-molded over the intermediate layer-encased sphere, thereby producing a 42.7 mm-diameter three-piece golf ball having a 0.8 mm thick outermost layer. Dimples common to all of the examples were formed at this time on the surface of the cover in each Example and Comparative Example. The resin compositions for the cover were designed so as to include the respective ingredients in the amounts shown in Table 2 below and, using a mold having a poor releasability that was furnished for evaluating the mold releasability, were injection molded at a molding temperature of between 200 and 250° C.

Details on the ingredients included in the compositions shown in Table 2 are given below.

TPU: A thermoplastic polyurethane available from DIC Covestro Polymer, Ltd. as Pandex® (Shore D hardness, 43; rebound resilience, 61%)
Alkyl acrylate/alkyl methacrylate copolymer: Available as Metablen™ P-501A from Mitsubishi Chemical Corporation; weight average molecular weight, 700,000

Properties of Resin Composition

(1) Shore D Hardness

The resin material was molded into 2 mm thick sheets and left to stand for two weeks at a temperature of 23±2° C. At the time of measurement, three sheets were stacked together. The material hardness of the resin was measured using a Shore D durometer in accordance with ASTM D2240. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) equipped with a Shore D durometer was used for measuring the hardness.

(2) Rebound Resilience

The rebound resiliences of the resin compositions measured based on JIS-K 6255:2013 are shown in Table 2.

The spin rate on approach shots, scuff resistance, paint film durability and moldability (mold releasability) for each golf ball were evaluated by the following methods. The results are shown in Table 2.

Spin Rate on Approach Shots

A sand wedge (SW) was mounted onto a golf swing robot, and the initial velocity and backspin rate of the ball immediately after being struck at a head speed (HS) of 20 m/s were measured with a launch monitor. In Comparative Examples 1 to 3, the moldability (mold releasability) was poor and so only a limited number of samples suitable for testing were obtained. For this reason, the spin rate was not measured in Comparative Examples 2 and 3.

Evaluation of Scuff Resistance

The golf balls were held isothermally at 23° C. and five balls of each type were hit at a head speed of 33 m/s using as the club a pitching wedge (PW) mounted on a golf swing robot. The damage to the ball from the impact was visually rated according to the following criteria.

Good: No scuffing whatsoever, or slight scuffing that is substantially inconspicuous.
NG: Substantial scuffing is apparent.

Paint Film Durability

The molded golf ball was painted by an ordinary process and then left to stand for one week, following which the ball was placed together with a sand/water slurry in a sealed container and a two-hour abrasion test by rotation was carried out. The degree of peeling by the surface paint was checked and was rated according to the following criteria.

Rating Criteria:

Good: No peeling whatsoever of paint, or substantially no peeling.
NG: Peeling of paint was observed in several places.

Moldability (Mold Releasability)

In each example, the releasability of the golf ball from the mold following injection molding of the cover was rated according to the following criteria.

Exc: Molded body does not stick to mold during demolding, and external defects such as runner stubs and ejector pin marks do not arise.
Good: Molded body sticks slightly to mold during demolding, but external defects such as runner stubs and ejector pin marks do not arise.
NG: Molded body sticks to mold during demolding, and external defects such as runner stubs and ejector pin marks arise.

	TABLE 2
	Example	Comparative Example
	1	2	3	1	2	3
Cover	(A)	TPU	100	100	100	100	100	100
composition	(B)	Alkyl methacrylate/alkyl	0.3	0.8	1.3			1.0
(pbw)		acrylate copolymer
	(C)	Ethylenebis(oleamide)	0.1	0.4	0.7		0.4
Cover	Shore D hardness (I)	43	43	43	43	43	43
material	Rebound resilience (%) (II)	62	62	62	62	62	62
properties	(II)/(I)	1.4	1.4	1.4	1.4	1.4	1.4
Evaluation	Backspin rate (rpm)	6,382	6,403	6,403	6,382	—	—
results	Scuff resistance	Good	Good	Good	Good	Good	Good
	Paint film durability	Good	Good	Good	Good	Good	Good
	Moldability (mold releasability)	Good	Exc	Exc	NG	NG	NG

As demonstrated by the results in Table 2, the golf balls in Examples 1 to 3 have an excellent mold releasability. Moreover, it is apparent that the golf balls in Examples 1 to 3 retain a good spin rate on approach shots, scuff resistance and paint film durability.

By contrast, in the golf ball of Comparative Example 1, which does not include Components B and C in the cover material, the mold releasability during molding of the cover is clearly inadequate. Likewise, in the golf balls of Comparative Examples 2 and 3, which lack either component B or component C in the cover material, the mold releasability during molding of the cover is clearly inadequate.

Japanese Patent Application No. 2022-089434 is incorporated herein by reference. Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

Claims

1. A golf ball comprising a rubber core of at least one layer and a cover of at least one layer that encases the core, wherein at least one layer of the cover is formed of a resin composition comprising:

(A) a polyurethane or a polyurea,

(B) an alkyl methacrylate/alkyl acrylate copolymer, and

(C) a fatty acid amide.

2. The golf ball of claim 1, wherein the amount of the alkyl methacrylate/alkyl acrylate copolymer included as component B per 100 parts by weight of component A is 1.5 parts by weight or less.

3. The golf ball of claim 1, wherein the amount of the fatty acid amide included as component C per 100 parts by weight of component A is 1.0 part by weight or less.

4. The golf ball of claim 1, wherein the resin composition making up the cover has a material hardness on the Shore D hardness scale of 65 or less.

5. The golf ball of claim 1, wherein the resin composition making up the cover has a rebound resilience, as measured according to JIS-K 6255, which is 65 or less.

6. The golf ball of claim 1, wherein the value obtained by dividing the rebound resilience of the resin composition making up the cover by the material hardness of the resin composition on the Shore D hardness scale is 0.5 or more.