GOLF BALL

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims priority under 35 U.S.C. § 119 (a) on Patent Application No. 2024-223681 filed in Japan on Dec. 19, 2024, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball including a core, an intermediate layer, and a cover, in which a film layer is formed between the intermediate layer and the cover.

BACKGROUND ART

Required characteristics of a golf ball are mainly an increase in distance, but in addition, a characteristic that the ball stops well on approach shots and a characteristic of scuff resistance are also required. That is, until now, many golf balls have been developed with good flight on shots with a driver and with which backspin is suitably applied on approach shots. Recently, polyurethane has been adopted as an alternative to ionomer resin materials in many cases for professionals and advanced players.

In addition, as a structure of a golf ball, multi-piece solid golf balls such as a three-piece or a four-piece golf ball in which an intermediate layer is interposed between a core and a cover are mainly used. Therefore, if polyurethane is used as a chief material for the cover (outermost layer), it is desirable to improve adhesion at an interface with an intermediate layer made of an ionomer resin or the like. As a method for improving adhesion between a polyurethane cover and an inner layer (intermediate layer) thereof, various methods have been conventionally studied. For example, Patent Document 1 proposes a technique of providing an adhesive agent layer between an intermediate layer and a cover. Patent Document 2 proposes a technique for improving durability by acid-treating an inner layer of a cover. Patent Document 3 proposes a technique for improving adhesion to a urethane cover by treating a core surface with a halogenated isocyanuric acid solution. In addition, as a technique for improving adhesion between an intermediate layer and a cover, techniques described in Patent Document 4 and Patent Document 5 have been proposed.

However, effects of these proposed prior art techniques are only an improvement in durability by improving adhesion between a core and a polyurethane cover and between an intermediate layer and the polyurethane cover, and no superiority in other performance characteristics, particularly flight performance, has been found. In addition, in the polyurethane cover, if a harder urethane material is used for the cover, there has been a problem in that a shear stress is reduced and a spin rate on shots with a middle iron is reduced, but a spin performance on approach shots is also reduced at the same time.

CITATION LIST

• • Patent Document 1: JP-A H10-179795
  • Patent Document 2: JP-A 2003-339912
  • Patent Document 3: JP-A 2009-131631
  • Patent Document 4: JP-A 2016-127911
  • Patent Document 5: JP-A 2019-107229

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a golf ball that sufficiently exhibits low spin performance when using a middle iron, while maintaining good spin performance on approach shots, and has high adhesion to a polyurethane cover.

As a result of intensive studies to achieve the above object, the present inventors have found that the above problems may be solved by forming a film layer between an intermediate layer and a polyurethane cover in a golf ball including a core, the intermediate layer, and the polyurethane cover, in which the film layer is formed of a resin composition containing an aziridine group-containing compound in a two-liquid curable polyurethane composition containing a polyol component and a polyisocyanate component, and thus have completed the present invention.

That is, conventionally, if a harder urethane material is used for the cover as a cover material of a urethane ball, a shear stress is reduced and a spin rate on shots with a middle iron is reduced, but a spin performance on approach shots is also reduced at the same time. In the present invention, by forming a film with a composition containing a mixture of a two-liquid curable urethane coating material and polyfunctional aziridine inside the polyurethane cover, the spin performance on approach shots is not deteriorated, and it is possible for the spin rate on shots with a middle iron to be reduced. The present inventors have found a golf ball that forms a network structure by an action of polyfunctional aziridine on urethane bonds in the urethane coating material, that may adjust the shear stress of the film layer, that eliminates spin performance dependent on a club number, and that may exhibit such striking performance that flight is reliable on shots with a middle iron and the golf ball reliably stops on shots with a short iron.

Accordingly, the present invention provides a multi-piece solid golf ball including

• • a core, an intermediate layer, and a cover, wherein a film layer is formed between the intermediate layer and the cover, the film layer is formed of a resin composition containing an aziridine group-containing compound in a two-liquid curable polyurethane composition containing a polyol component and a polyisocyanate component, and the cover is formed of a resin composition containing polyurethane as a principal component.

In a preferred embodiment of the golf ball according to the invention, a compounding amount of the aziridine group-containing compound is from 0.05 to 20 parts by weight per 100 parts by weight of a solid content of the two-liquid curable polyurethane composition.

In another preferred embodiment of the inventive golf ball, an aziridine content of the aziridine group-containing compound is from 0.01 to 10 mmol/g.

In yet another preferred embodiment, the film layer has a thickness of from 10 to 200 μm.

In still another preferred embodiment, a polyurethane resin obtained by curing the two-liquid curable polyurethane composition has a storage elastic modulus (E′) at 23° C. of from 500 to 2,500 MPa.

In a further preferred embodiment, the intermediate layer has a hardness of from 60 to 80 on the Shore D hardness scale.

Advantageous Effects of the Invention

With the golf ball of the present invention, adhesion between the polyurethane cover and the intermediate layer, which is an inner layer of the polyurethane cover, is improved, and while the spin performance on approach shots is favorably maintained, low spin performance on shots with a middle iron is sufficiently exhibited, and a desired flight performance may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

At FIG. 1 is a schematic cross-sectional view of a golf ball according to one embodiment of the present invention.

FIG. 2 is an explanatory view of a test piece for measurement used when adhesive strength between a cover (outermost layer) and an intermediate layer is measured.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is described in more detail.

The golf ball of the present invention includes a core, an intermediate layer, and a cover, and further has a ball structure in which a film layer is formed between the intermediate layer and the cover. For example, an example thereof is shown in FIG. 1. A golf ball G shown in FIG. 1 has a single-layer core 1 and a single-layer cover 3 with a single-layer intermediate layer 2 between the core and the cover. Furthermore, a thin film layer 4 is formed inside the cover. The cover 3 is positioned at an outermost layer in the layer structure of the golf ball except for a coating layer. A surrounding layer made of a single layer or a plurality of layers of resin may be included between the core and the intermediate layer. A large number of dimples D are usually formed on a surface of the cover (outermost layer) 3 in order to improve aerodynamic properties. Although not particularly illustrated, the coating layer is usually formed on the surface of the cover 3. Hereinafter, each of the above layers is described in detail.

The core may be formed in a single layer, a two-layer core including an inner layer and an outer layer, or a plurality of other layers.

The core is formed of a rubber composition containing a base rubber and an α,β-unsaturated carboxylic acid metal salt. As a suitable rubber composition, a rubber composition having the following formulation may be exemplified.

The base rubber is not particularly limited, but polybutadiene is particularly preferably used.

It is preferable that the polybutadiene has a cis-1,4-bond of at least 60%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95% in a polymer chain thereof. If the amount of the cis-1,4-bond in a bond in a polybutadiene molecule is too small, a rebound may be reduced.

A content of a 1,2-vinyl bond contained in the polybutadiene is usually not more than 2%, preferably not more than 1.7%, and still more preferably not more than 1.5% in the polymer chain. If the content of the 1,2-vinyl bond is too large, the rebound may be reduced.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably at least 20, and more preferably at least 30, and an upper limit thereof is preferably not more than 120, more preferably not more than 100, and still more preferably not more than 80.

The above-mentioned Mooney viscosity is an index of industrial viscosity (JIS K 6300) measured by a Mooney viscometer, which is one type of rotational plastometer, and ML₁₊₄ (100° C.) is used as a unit symbol. M represents the Mooney viscosity, L represents a large rotor (L type), and 1+4 represents a preheating time of one minute and a rotation time of the rotor of four minutes, and measurement performed under conditions of 100° C. is indicated.

As the polybutadiene, a polybutadiene synthesized using a rare earth element-based catalyst or a Group VIII metal compound catalyst may be used.

In the base rubber, a polybutadiene rubber synthesized with a catalyst different from a lanthanum-series rare earth element compound may be blended. Styrene-butadiene rubber (SBR), natural rubber, polyisoprene rubber, ethylene propylene diene rubber (EPDM), and the like may be blended, and these may be used singly, or two or more may be used in combination.

A proportion of the polybutadiene in an entire base rubber of the rubber composition is preferably at least 60 wt %, more preferably at least 70 wt %, and most preferably at least 90 wt %. Further, 100 wt % of the base rubber, that is, all of the base rubber, may be the polybutadiene.

The α,β-unsaturated carboxylic acid metal salt is usually used as a co-crosslinking agent. The number of carbon atoms of the unsaturated carboxylic acid is preferably 3 to 8, and specific examples thereof include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Specific examples of the metal of the unsaturated carboxylic acid include zinc, sodium, magnesium, calcium, and aluminum, and zinc is particularly preferable. Therefore, zinc acrylate is most preferable as the co-crosslinking agent.

The α,β-unsaturated carboxylic acid metal salt may be preferably at least 5 parts by weight, more preferably at least 10 parts by weight, and still more preferably at least 15 parts by weight per 100 parts by weight of the base rubber, and the upper limit of the compounding amount may be preferably not more than 60 parts by weight, more preferably not more than 50 parts by weight, and still more preferably not more than 45 parts by weight. If the compounding amount is too large, there is a case where the ball becomes too hard and gives an intolerable feel at impact, and if the compounding amount is too small, there is a case where the rebound is reduced.

The rubber composition may contain, in addition to the components described above, which are essential, a co-crosslinking agent other than those described above, an organic peroxide, an inert filler, sulfur, an antioxidant, an organosulfur compound, and the like.

If the core is a single layer, a single-layer core may be produced from the rubber composition described above. If the core has a plurality of layers, as materials of a center core (inner core) and an outer layer core thereof, in addition to the rubber materials described above, a rubber composition or a known resin material in which types and compounding amounts of blending components are different from each other may be adopted.

A vulcanized molded product (core) may be produced by vulcanizing and hardening the rubber composition. This vulcanized molded product may be particularly used for all or a part of a single-layer core or a multi-layer core. For example, a molded body is kneaded using a mixing apparatus such as a Banbury mixer or a roll mill, subjected to compression molding or injection molding using a core mold, and appropriately heated at a temperature of about 100 to 200° C. for 10 to 40 minutes as a temperature sufficient for the organic peroxide or the co-crosslinking agent to act, whereby the molded body may be cured to manufacture a core that is the vulcanized molded product.

A diameter of the core is not particularly limited and depends on a layer structure of the golf ball to be manufactured, although the diameter is preferably at least 30 mm, and more preferably at least 35 mm, and as an upper limit, the diameter is preferably not more than 41 mm, and more preferably not more than 40 mm. If the diameter of the core deviates from this range, an initial velocity of the ball may be low or appropriate spin characteristics may not be obtained.

In the present invention, at least one intermediate layer is provided between the core and the cover (outermost layer).

A resin material of the intermediate layer is not particularly limited, but known thermoplastic resin materials such as various ionomer resins used for golf balls may be used.

As the ionomer resin, specifically, products of the brands “Himilan” and “Nucrel” may be adopted. Specifically, commercially available products such as Himilan 1554, Himilan 1557, Himilan 1601, Himilan 1605, Himilan 1706, Himilan AM7311, Himilan AM7316, and Himilan AM7339 (all manufactured by Dow-Mitsui Polychemicals Co., Ltd.) may be used.

In addition, it is particularly preferable to use a highly neutralized ionomer material as the intermediate layer material in order to further realize a low spin rate of the ball. Specifically, it is preferable to use a material in which the following components (i) to (iv) are blended.

A base resin (i) is obtained by mixing an olefin-unsaturated carboxylic acid binary random copolymer and/or a metal ion neutralized product of an olefin-unsaturated carboxylic acid binary random copolymer (i-1) with an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer and/or a metal ion neutralized product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer (i-2) at a weight ratio of 100:0 to 0:100.

• • 100 parts by weight of a resin component obtained by mixing a non-ionomer thermoplastic elastomer (ii) with the base resin (i) at a weight ratio of 100:0 to 50:50;
  • 5 to 80 parts by weight of a fatty acid having a molecular weight of 228 to 1,500 and/or a derivative thereof (iii); and
  • 0.1 to 17 parts by weight of a basic inorganic metal compound (iv) capable of neutralizing un-neutralized acid groups in the component (i) and the component (iii)
  • are blended to produce a mixed material. In particular, if the mixed material of the above components (i) to (iv) is used, it is preferable to employ a material in which at least 70% of the acid groups are neutralized.

A material hardness of the intermediate layer is not particularly limited, and is preferably at least 60, and more preferably at least 65 on the Shore D hardness scale. The upper limit thereof may be preferably not more than 80, and more preferably not more than 70.

A thickness of the intermediate layer is set to not more than 2.0 mm, preferably not more than 1.8 mm, and more preferably not more than 1.5 mm. Although not particularly limited, the lower limit is preferably at least 0.8 mm, more preferably at least 1.0 mm, and still more preferably at least 1.2 mm. If the thickness of the intermediate layer deviates from the above numerical range, a spin rate-lowering effect by a driver (W #1) becomes insufficient, and a distance may not be increased.

A method for forming the intermediate layer may be performed by a customary method such as a known injection molding method. For example, the intermediate layer material may be injected around the core with an injection mold to obtain a layer-encased sphere, or the core may be enclosed with two half-cups pre-molded into hemispherical shapes as an intermediate layer material and molded under applied heat and pressure to produce an intermediate layer-encased sphere surrounding the core.

Next, the cover (outermost layer) is described.

A resin material of the cover is formed of a resin composition containing polyurethane as a principal component. A proportion of the polyurethane in an entire cover resin composition is not particularly limited, although the proportion may be at least 50 wt %, and preferably at least 80 wt %. The polyurethane is described below.

A structure of the polyurethane is composed of a soft segment composed of a polymer polyol (polymeric glycol), which is a long-chain polyol, and a chain extender and a polyisocyanate constituting a hard segment. Here, as the polymer polyol as a raw material, any polymer polyol that is conventionally used in a technique related to a polyurethane material may be used, and is not particularly limited. Examples thereof include polyester-based polyols, polyether-based 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. Specific examples of the polyester-based polyol include adipate-based polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol, and polyhexamethylene adipate glycol, and lactone-based polyols such as polycaprolactone polyol. Examples of a polyether polyol include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol), and poly(methyltetramethylene glycol). These may be used singly, or two or more may be used in combination.

A numerical average molecular weight of the long-chain polyol is preferably within a range of 1,000 to 5,000. By using a long-chain polyol having such a numerical average molecular weight, it is possible to reliably obtain a golf ball made of a polyurethane composition excellent in various properties such as productivity and the above-mentioned rebound. The numerical average molecular weight of the long-chain polyol is more preferably within a range of 1,500 to 4,000, and still more preferably within a range of 1,700 to 3,500.

The numerical average molecular weight is a numerical average molecular weight calculated based on a hydroxyl value measured in accordance with JIS K 1557 (the same applies hereinafter.).

As the chain extender, those used in conventional techniques related to polyurethanes may be suitably used, and the chain extender is not particularly limited. In the present invention, a low molecular weight compound having at least two active hydrogen atoms capable of reacting with an isocyanate group in a molecule and having a molecular weight of not more than 2,000 may be used, and among these compounds, an aliphatic diol having 2 to 12 carbon atoms may be suitably used. Specific examples thereof include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, and 2,2-dimethyl-1,3-propanediol, and among these examples, 1,4-butylene glycol may be particularly suitably used.

As the polyisocyanate, those used in conventional techniques related to polyurethanes may be suitably used, and there is no particular limitation. Specifically, 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, naphthylene 1,5-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 may be used. However, it is difficult to control a crosslinking reaction during injection molding depending on isocyanate species.

In addition, a compounding ratio of active hydrogen atoms:isocyanate group in the polyurethane forming reaction described above may be appropriately adjusted within a preferable range. Specifically, when the above-described long-chain polyol, a polyisocyanate compound, and the chain extender are reacted to manufacture the polyurethane, it is preferable to use each component at such a ratio that the isocyanate group contained in the polyisocyanate compound is from 0.95 to 1.05 mol, based on 1 mol of active hydrogen atoms of the long-chain polyol and the chain extender.

A method for manufacturing the polyurethane is not particularly limited, and the polyurethane may be manufactured by either a prepolymer method or a one-shot method using the long-chain polyol, the chain extender, and the polyisocyanate compound by utilizing a known urethanization reaction. Among these methods, it is preferable to perform melt polymerization substantially in the absence of a solvent, and it is particularly preferable to manufacture the polyurethane by continuous melt polymerization using a multi-screw extruder.

As the polyurethane described above, a thermoplastic polyurethane material is preferably used. As the thermoplastic polyurethane material, a commercially available product may be suitably used, and examples thereof include “Pandex” (trade name) manufactured by DIC Covestro Polymer, Ltd., and “Resamine” (trade name) manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

A material hardness of the cover is not more than 60, more preferably not more than 50, and still more preferably not more than 45 on the Shore D hardness scale. Although not particularly limited, the lower limit is preferably at least 20, and more preferably at least 30 on the Shore D hardness scale.

A thickness of the cover is set to not more than 2.0 mm, preferably not more than 1.5 mm, and more preferably not more than 1.2 mm. Although not particularly limited, the lower limit is preferably at least 0.6 mm, more preferably at least 0.7 mm, and still more preferably at least 0.8 mm. If the thickness of the cover deviates from the above numerical range, the spin rate-lowering effect by a driver (W #1) or a middle iron becomes insufficient, and the distance may not be increased. If the thickness of the cover is too small, durability may deteriorate.

In the present invention, the film layer is formed between the intermediate layer and the cover. This film layer contains an aziridine group-containing compound in a two-liquid curable polyurethane composition including a polyol component and a polyisocyanate component. In general, physical properties of polyurethanes are determined by a balance between hard and soft segments and a formation of hydrogen bonds, and adjusting these factors is important in designing golf balls. A polyfunctional polyaziridine-containing polyurethane used in the present invention brings about the balance between hard and soft segments of polyurethane as a substrate and the formation of hydrogen bonds by adding the polyfunctional polyaziridine to the polyurethane. As a result, while maintaining a response to a shear force applied on shots with a short iron or a wedge, the response to the shear force applied on shots with a middle iron may be suppressed. That is, by mixing and applying the polyfunctional polyaziridine with a two-liquid curable urethane coating material, film molding may be facilitated, and a preferable shear stress may be exhibited.

Examples of the polyol component in the two-liquid curable polyurethane composition include an acrylic polyol and a polyester-based polyol. These polyols include modified products of polyols, and other polyols may also be added in order to further improve workability.

On the other hand, the polyisocyanate is not particularly limited, and is a generally used aromatic, aliphatic, alicyclic polyisocyanate, or the like, and specific examples thereof include hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), diphenylmethane diisocyanate, xylylene diisocyanate, tetramethylene diisocyanate, lysine diisocyanate, isophorone diisocyanate, 1,4-cyclohexylene diisocyanate, naphthalene diisocyanate, trimethylhexamethylene diisocyanate, dicyclohexylmethane diisocyanate, 1-isocyanato-3,3,5-trimethyl-4-isocyanatomethylcyclohexane, and the like. These may be used singly or in combination.

As a curing catalyst (organometallic compound), an amine-based catalyst or an organometallic catalyst may be used, and as the organometallic compound, a compound conventionally blended as a curing agent for a two-liquid curable urethane coating material, such as metal soap of aluminum, nickel, zinc, tin, or the like, may be suitably used.

Various organic solvents may be mixed with the two-liquid curable polyurethane composition depending on coating conditions. Examples of such organic solvents include aromatic solvents such as toluene, xylene, and ethylbenzene; ester solvents such as ethyl acetate, butyl acetate, propylene glycol methyl ether acetate, and propylene glycol methyl ether propionate; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; ether solvents such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, and dipropylene glycol dimethyl ether; alicyclic hydrocarbon solvents such as cyclohexane, methyl cyclohexane, and ethyl cyclohexane; and petroleum hydrocarbon solvents such as mineral spirits.

In order to reliably obtain desired effects of the present invention, a storage elastic modulus (E′) at 23° C. of a polyurethane resin obtained by curing the two-liquid curable polyurethane composition is preferably from 500 to 2,500 MPa. This storage elastic modulus (E′) may be measured with a commercially available measuring apparatus, for example, a device for measuring dynamic viscoelasticity (DMA Q800) manufactured by TA Instruments.

The two-liquid curable polyurethane composition includes the aziridine group-containing compound. That is, it is presumed that a polyfunctional compound having an aziridine group reacts with a reactive functional group such as isocyanate remaining in the polyurethane cover to improve adhesion between the film layer and the cover. By reacting with a functional group such as a carboxyl group contained in the intermediate layer material by a ring-opening reaction of an aziridine group, adhesion between the intermediate layer and the film layer is also improved, and a function as an adhesive agent on both surfaces of the intermediate layer-cover layer is exhibited.

Examples of the aziridine group-containing compound include N,N′-toluene-2,4-bis(1-aziridine carboxide), N,N′-diphenylmethane-4,4′-bis(1-aziridine carboxide), N,N′-hexamethylene-1,6-bis(1-aziridine carboxamide), triethylenemelamine, bisisoprotaloyl-1-(2-methylaziridine), tri-1-aziridinylphosphine oxide, tetramethylolmethane-tri-β-aziridinylpropionate, and trimethylolpropane-tri-β-aziridinylpropionate.

As the aziridine group-containing compound, a commercially available product may be used. Examples of commercially available products of the aziridine group-containing compound include “CHEMITITE PZ-33” and “CHEMITITE DZ-22E” manufactured by Nippon Shokubai Co., Ltd., and “NeoAdd PAX-523” manufactured by Covestro AG, but the present invention is not limited thereto.

A compounding amount of the aziridine group-containing compound is preferably at least 0.05 parts by weight, more preferably at least 0.1 parts by weight, and still more preferably at least 0.2 parts by weight, and the upper limit thereof is preferably not more than 20 parts by weight, more preferably not more than 10 parts by weight, and still more preferably not more than 5 parts by weight per 100 parts by weight of a solid content of the two-liquid curable polyurethane composition. If the compounding amount is too large, it is difficult to form a film layer by coating, whereas if the compounding amount is too small, the function as an adhesive agent on both surfaces of the intermediate layer-cover layer is deteriorated, and an effect on flight performance is reduced.

A content of aziridine contained in the aziridine group-containing compound is preferably at least 0.01 mmol/g, more preferably at least 0.1 mmol/g, and still more preferably at least 1.0 mmol/g, and the upper limit thereof is preferably not more than 10 mmol/g, and more preferably not more than 8 mmol/g. If the content is too large, viscosity of the aziridine group-containing compound increases, and dispersibility of the two-liquid curable polyurethane composition deteriorates. On the other hand, if the content is too small, it is necessary to increase an addition amount of the aziridine group-containing compound in the two-liquid curable polyurethane composition, and it becomes difficult to form the film layer by coating as described above.

As a method for forming the film layer, the two-liquid curable polyurethane composition containing the aziridine group-containing compound is applied to a surface of the intermediate layer-encased sphere, and the film layer may be formed on the surface of the intermediate layer-encased sphere through a drying step. In this case, as a coating method, a spray coating method, an electrostatic coating method, a dipping method, or the like may be suitably adopted, and there is no particular limitation.

A thickness of the film layer formed on the surface of the intermediate layer-encased sphere as described above is preferably at least 10 μm, more preferably at least 20 μm, and still more preferably at least 30 μm, and the upper limit thereof is preferably not more than 200 μm, more preferably not more than 150 μm, and still more preferably not more than 120 μm. That is, the gist of the present invention is to replace a part (inner side) of a urethane cover layer, which is the outermost layer, with the film layer, and it is necessary to balance the thicknesses of the cover and the film layer in order to encase the intermediate layer with a different polyurethane material (film layer) and exhibit performance as a double cover while making use of an original ball performance of a urethane cover material. In order to achieve this balance, the thickness of the film layer is preferably in the above range.

A method for manufacturing a golf ball formed by laminating the above-described core, intermediate layer, film layer, and cover (outermost layer) may be performed by a customary method such as a known injection molding method. For example, the intermediate layer material is injected around the core with an injection mold to obtain the intermediate layer-encased sphere, and then a resin composition is spray-coated on the intermediate layer-encased sphere to form the film layer having a predetermined thickness. Furthermore, this sphere encased with the film layer is set in another injection mold, and the material of the cover, which is the outermost layer, is injection-molded to obtain a golf ball with a structure of four layers or more. A golf ball may also be produced by enclosing the layer-encased sphere with two half-cups pre-molded into hemispherical shapes as each of the encasing layers and molding the layer-encased sphere under applied heat and pressure.

A large number of dimples are formed on the surface of the outermost layer of the cover, and various treatments such as base treatment, stamping, and coating may be performed on the cover. In particular, if such a surface treatment is applied to the cover formed of the cover material of the present invention, since moldability of the cover surface is good, the treatment may be performed with good workability.

The golf ball of the present invention is preferably formed to have a diameter and a weight according to the Rules of Golf, and is usually formed to have a diameter of at least 42.67 mm and a weight of not more than 45.93 g, although the golf ball preferably has a diameter of from 42.67 to 42.9 mm.

EXAMPLES

Hereinafter, the present invention is specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

Examples 1 to 5, Comparative Examples 1 to 4

[Formation of Core]

A core having a diameter of 38.7 mm was produced. Compounding of the core was common to all Examples and Comparative Examples. As a base rubber, two types of polybutadienes were used, and 20 parts by weight of polybutadiene (trade name “BR 51” manufactured by ENEOS Materials Corporation) and 80 parts by weight of polybutadiene (trade name “BR 730” manufactured by ENEOS Materials Corporation) were mixed and used. In addition, 29.5 parts by weight of zinc acrylate (“ZN-DA85S” manufactured by Nippon Shokubai Co., Ltd.), 0.6 parts by weight of dicumyl peroxide (trade name “Percumyl D” manufactured by NOF Corporation) as an organic peroxide, 0.1 parts by weight of 2,2-methylenebis(4-methyl-6-butylphenol) (trade name “Nocrac NS-6” manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.) as an antioxidant, 19.3 parts by weight of zinc oxide (trade name “Grade 3 Zinc Oxide” manufactured by Sakai Chemical Industry Co., Ltd.), and 0.3 parts by weight of pentachlorothiophenol zinc salt (manufactured by Wako Pure Chemical Industries, Ltd.) as an organosulfur compound are blended into the base rubber. The rubber composition is vulcanized at a temperature of 155° C. for a time of 15 minutes. A specific gravity of the blend is 1.138.

[Formation of Intermediate Layer]

Next, a resin material for an intermediate layer was injection-molded around the core having a diameter of 38.7 mm to produce an intermediate layer-encased sphere having the intermediate layer having a thickness of 1.2 mm. The resin material of the intermediate layer was common to all Examples and Comparative Examples, and trade names “Himilan AM7318” and “Himilan 1706” (ionomer resins manufactured by Dow-Mitsui Polychemicals Co., Ltd.) were blended at 85:15 (weight ratio), respectively. A material hardness is 68 on the Shore D hardness scale.

[Formation of Film Layer]

Next, in each of the Examples and Comparative Examples excluding Examples 2 and 5, a resin material for a film layer was applied to a periphery of the intermediate layer-encased sphere with a spray gun to produce a film layer-encased sphere having a thickness of from 10 to 100 μm. The resin material of the film layer is as shown in Table 1 below.

Example 2 is an example in which the thickness of the film layer of Example 3 was changed from 100 μm to 50 μm, and Example 5 is an example in which a polyol 1 as a main component of Example 3 was changed to a polyol 3. For these examples, similarly to the above, the resin material for the film layer is applied to the periphery of the intermediate layer-encased sphere with a spray gun to produce the film layer-encased sphere having a predetermined thickness.

TABLE 1
Blend (pbw)	A	B	C	D	E	F	G	H

Main	Polyol 1	30		—
component	Polyol 2		30	—	30	30	30	30
	Polyol 3			—					30
	Polycarbonate			—
	Solvent (ethyl acetate)	70	70	—	70	70	70	70	70
Curing	HDI	21.0	21.0	—	21.0	20.6	20.6	18.9	20.6
agent	Solvent (butyl acetate)	29.0	29.0	—	28.9	28.4	28.4	26.1	28.4
Others	Aziridine group-containing compound				0.1	1.0	1.0	5.0	1.0
	Polyurethane dispersion			100

Solid content	51.0	51.0	—	51.0	50.6	50.6	48.9	50.6
of two-liquid curable polyurethane composition
Compounding amount			—	0.2	2.0	2.0	10.2	2.0
of aziridine group-containing compound
Storage elastic modulus (E′) [MPa]	1803	1332	—	1332	1332	1332	1332	2389
of two-liquid curable polyurethane composition
Thickness [μm]	100	100	10	100	50	100	100	100

Components of the main component to be blended in the resin composition are as follows.

• • “Polyol 1, Polyol 2”: Polyester polyols manufactured by Asia Industry Co., Ltd., wherein polyol 1 is a polyester polyol in which a two-liquid curable polyurethane resin has a storage elastic modulus of 1,803 MPa at 23° C., and polyol 2 is a polyester polyol in which the two-liquid curable polyurethane resin has a storage elastic modulus of 1,332 MPa at 23° C.
  • “Polyol 3”: Polyester polyol manufactured by Cashew Co., Ltd., wherein the two-liquid curable polyurethane resin has a storage elastic modulus of 2,389 MPa at 23° C. A method for measuring the storage elastic modulus of each of these polyurethane resin compositions is described below.
  • “Aziridine group-containing compound”: Trade name “CHEMITITE PZ-33” (liquid having an aziridine content of at least 6 mmol/g) manufactured by Nippon Shokubai Co., Ltd.
  • “Polyurethane dispersion”: Product obtained by coating, with a spray gun, a composition obtained by blending water with product name “Resamine D-6031” (polycarbonate polyurethane dispersion) and product name “Resamine D-52” (crosslinking agent), both manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., at a weight ratio of 100:3:3.

[Storage Elastic Modulus E′ of Two-Liquid Curable Polyurethane Resin Composition]

The storage elastic modulus E′ (MPa) of the film was measured under the following conditions.

• • Apparatus: Dynamic viscoelasticity measuring apparatus “Q800” manufactured by TA Instruments.
  • Measurement sample: A resin composition containing a main component and a curing agent (excluding formulation of the aziridine group-containing compound) was dried and cured at 40° C. for four hours to produce a film (coating film) having a thickness of from 0.11 to 0.14 mm. A sample piece was cut out from the film so as to have a width of 4 mm and a distance between clamps of 20 mm.
  • Measurement mode: Tensile
  • Measurement temperature: −100° C. to 150° C.
  • Heating rate: 4° C./min
  • Measurement data fetching interval: 4° C.
  • Vibration frequency: 10 Hz
  • Measurement strain: 0.10%

[Formation of Cover]

Next, in Examples and Comparative Examples excluding Examples 2 and 5, an ether-type thermoplastic polyurethane (trade name “Pandex” manufactured by DIC Covestro Polymer Ltd., Shore D hardness 47) was used as a resin material of a cover (outermost layer). Using another injection mold, the resin material was injection-molded around the film layer-encased sphere to produce a golf ball having a four-layer structure with a diameter of 42.7 mm and an outermost layer with a thickness of 0.8 mm. In this process, a large number of dimples were formed on a cover surface.

In Examples 2 and 5, similarly to the above, the resin material is injection-molded around the film layer-encased sphere to produce a golf ball having the four-layer structure. In this process, dimples similar to the above are formed on the cover surface.

[Formation of Coating Layer (Coating Film)]

Next, in each of the Examples and Comparative Examples excluding Examples 2 and 5, a coating composition containing a polyester polyol (main component) and an isocyanate curing agent was applied to the surface of the cover having a large number of dimples with an air spray gun to produce a golf ball of each example in which a coating layer (coating film) having a thickness of 15 μm was formed. For Examples 2 and 5, in the same manner as described above, the coating composition is applied to the surface of the cover to produce a golf ball of each example in which a coating layer is formed.

For the obtained golf balls of each example, a spin rate and adhesion of golf balls struck with a number six iron and a sand wedge are evaluated by the following methods, and the results are shown in Table 2.

[Spin Rate with Number Six Iron (I #6, HS 42 m/s)]

A number six iron (I #6) is mounted on a swing robot machine, and a spin rate when a ball is struck at a head speed (HS) of 42 m/s is measured. The spin rate immediately after the ball is struck is measured by a device for measuring initial conditions. The club used is a JGR Forged I #6 (2016 model) manufactured by Bridgestone Sports Co., Ltd. and is evaluated according to the following rating criteria.

[Rating Criteria]

• • ⊚: A difference in spin rate is smaller than that in Comparative Example 1 by at least 150 rpm.
  • ◯: The difference in spin rate is 50 to 149 rpm smaller than that in Comparative Example 1.
  • Δ: The spin rate is equal to or larger than that in Comparative Example 1.

[Evaluation of Spin Rate on Approach Shots]

A determination is made based on the spin rate when a sand wedge is mounted on a golf swing robot and a ball is struck at a head speed (HS) of 15 m/s. The spin rate immediately after the ball is struck is measured by a device for measuring initial conditions. The sand wedge used is a TOURSTAGE TW-03 (loft angle 57°) 2002 model manufactured by Bridgestone Sports Co., Ltd.

[Rating Criteria]

• • ◯: The spin rate is equal to or slightly larger than that of Comparative Example 1.
  • Δ: The difference in spin rate is smaller than that in Comparative Example 1 by at least 50 rpm.

[Adhesion]

As shown in FIG. 2, when a flat plane having a distance of 2 mm from a center of a golf ball 10 was p1, and a flat plane that was point symmetrical to p1 with respect to the ball center was p2, adhesion strength between an outermost layer 30 and an intermediate layer 20 at a ball site s1 between p1 and p2 was measured. In the outermost layer 30 of the ball, a cut T was made in a portion where the outermost layer 30 and p1 overlap and a portion where the outermost layer 30 and p2 overlap, and the outermost layer 30 in a portion other than s1 was peeled off. Next, a cut T was made in the outermost layer 30 so as to be perpendicular to p1 and p2, the outermost layer 30 about 20 mm from the cut was peeled off from the intermediate layer 20, and a gripping margin was provided to obtain a test piece. With reference to JIS K 6256 “Adhesion Testing Methods For Rubber, Vulcanized Or Thermoplastic”, a moving speed of a gripping jig was set to 50 mm/min, and a tensile strength was measured every 0.1 mm. The tensile strength for about 100 mm was measured for one test piece, and these values were taken as an adhesive force (unit: N). The test piece obtained as described above was used, and the gripping margin provided in the outermost layer 30 was held with the gripping jig. A fixing jig for the test piece allows the test piece to rotate while maintaining its center position, and may peel off the outermost layer 30 wound around the intermediate layer 20 without sagging along with the movement of the gripping jig. The adhesive force (peel value) measured in each example is evaluated according to the following criteria.

[Rating Criteria]

• • ⊚: Peel value may not be measured because adhesion is appreciably strong. That is, the intermediate layer and the cover are not peeled off, but the test piece of the intermediate layer or the cover is torn.
  • ◯: Peel value (N) is at least 0.5.
  • Δ: Peel value (N) is at least 0.2 and less than 0.5.
  • X: Peel value (N) is less than 0.2.

	TABLE 2
	Comparative Example	Example

	1	2	3	4	1	2	3	4	5

Core	Common	Common	Common	Common	Common	Common	Common	Common	Common
Intermediate layer	Common	Common	Common	Common	Common	Common	Common	Common	Common

Film layer	Presence	Absent	Present	Present	Present	Present	Present	Present	Present	Present
	or absence
	Material	—	A	B	C	D	E	F	G	H
	Thickness (μm)	—	100	100	10	100	50	100	100	100

Cover

Common

Common

Common

Common

Common

Common

Common

Common

Common

Spin rate	Initial velocity	57.5	57.4	57.4	57.6	57.3	57.2	57.3	57.4	57.4
on shots	(m/s)
with a	Spin rate (rpm)	5,074	5,025	4,981	5,087	4,945	4,997	4,910	4,973	4,918
middle
iron	Evaluation	(Reference)	Δ	◯	Δ	◯	◯	⊚	◯	⊚
(I#6)
Spin rate	Initial velocity	12.8	12.8	12.8	12.9	12.8	12.8	12.8	12.8	12.8
on	(m/s)
approach	Spin rate (rpm)	5,247	5,258	5,175	5,261	5,210	5,243	5,246	5,300	5,288
shots	Evaluation	(Reference)	◯	Δ	◯	◯	◯	◯	◯	◯
(SW)
Adhesion	Peel value (N)	0.27	0.43	0.09	Un-	2.22	Un-	Un-	1.11	Un-
					measurable		measurable	measurable		measurable
	Evaluation	(Reference)	Δ	X	⊚	◯	⊚	⊚	◯	⊚

As shown in Table 2, considering the performance of the golf balls of Examples 1 to 5 with the golf ball of Comparative Example 1 as a reference, the golf balls are excellent in adhesion between the cover and the intermediate layer, have a small spin rate and good flight performance on shots with a middle iron (I #6), and have a high spin rate on approach shots.

On the other hand, in Comparative Example 2, since the film layer does not contain the aziridine group-containing compound, adhesion is not sufficient, the spin rate on shots with a middle iron (I #6) is equivalent, and the spin rate is not sufficiently reduced.

In Comparative Example 3, since the film layer does not contain the aziridine group-containing compound, adhesion is not sufficient, and the spin rate on approach shots is reduced.

In Comparative Example 4, while polyurethane dispersion is used for the film layer, and adhesion is excellent, the spin rate on shots with a middle iron (I #6) is not reduced as compared with Comparative Example 1 (reference).

Japanese Patent Application No. 2024-223681 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.