golf ball

Abstract

The objective is to provide a golf ball that offers excellent adhesion between the cover and the intermediate layer, low spin rate and good distance performance when using middle irons (I#6), and high spin rate when approaching the ball. [Solution] The present invention provides a multi-piece solid golf ball comprising a core, an intermediate layer, and a cover, wherein a film layer is formed between the intermediate layer and the cover, and the film layer is formed from a resin composition containing an aziridine group-containing compound in a two-component curable polyurethane composition consisting of a polyol component and a polyisocyanate component, and the cover is formed from a resin composition mainly composed of polyurethane.

Description

Technical Field

[0001] The present invention relates to a multi-piece solid golf ball including a core, an intermediate layer, and a cover, and further having a film layer formed between the intermediate layer and the cover.

Background Art

[0002] The required characteristics of a golf ball mainly include an increase in flight distance. In addition, there are performance such as the ball stopping well during an approach shot and scratch resistance. That is, until now, many golf balls that fly well when struck with a driver and have suitable backspin during an approach shot have been developed. Recently, for professionals and advanced players, many are adopting polyurethane as a substitute for ionomer resin materials.

[0003] Also, as the structure of a golf ball, multi-piece solid golf balls such as three-piece and four-piece types with an intermediate layer interposed between the core and the cover have become mainstream. Therefore, when the cover (outermost layer) is made mainly of polyurethane, it is desirable to improve the adhesion at the interface with the intermediate layer made of an ionomer resin or the like. Various methods have been conventionally studied as techniques for enhancing the adhesion between the polyurethane cover and its inner layer (intermediate layer). For example, Patent Document 1 proposes a technique of providing an adhesive layer between the intermediate layer and the cover. Patent Document 2 proposes a technique of improving durability by acid-treating the inner layer of the cover. Patent Document 3 proposes a technique of improving the adhesion with a urethane cover by treating the core surface with a solution of halogenated isocyanuric acid. In addition, as techniques for improving the adhesion between the intermediate layer and the cover, the techniques described in Patent Document 4 and Patent Document 5 have been proposed.

[0004] However, the effects of these proposed prior arts were limited to improved durability due to enhanced adhesion between the core or intermediate layer and the polyurethane cover, and no advantages in other performance aspects, particularly distance performance, were found. Furthermore, with polyurethane cover materials, using a harder urethane material reduces shear stress, which decreases spin with mid-irons, but simultaneously reduces approach spin performance. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-179795 [Patent Document 2] Japanese Patent Publication No. 2003-339912 [Patent Document 3] Japanese Patent Publication No. 2009-131631 [Patent Document 4] Japanese Patent Publication No. 2016-127911 [Patent Document 5] Japanese Patent Publication No. 2019-107229 [Overview of the project] [Problems that the invention aims to solve]

[0006] This invention has been made in view of the above circumstances, and aims to provide a golf ball that maintains good spin performance during approach shots, fully exhibits low spin performance when using middle irons, and has high adhesion to the polyurethane cover. [Means for solving the problem]

[0007] The present inventors conducted diligent studies to achieve the above objectives and found that the above problems can be solved by configuring a golf ball comprising a core, an intermediate layer, and a polyurethane cover to have a film layer formed between the intermediate layer and the polyurethane cover, and by forming this film layer with a resin composition containing an aziridine group-containing compound in a two-component curable polyurethane composition consisting of a polyol component and a polyisocyanate component, thus leading to the present invention.

[0008] In other words, conventionally, using a harder urethane material as the cover material for a urethane ball reduces shear stress, which decreases the amount of spin with middle irons, but at the same time, the approach spin performance also decreases. In the present invention, by forming a film on the inside of the polyurethane cover with a composition containing a mixture of a two-component curing urethane paint and a polyfunctional aziridine, it was possible to reduce the amount of spin with middle irons without reducing the approach spin performance. This is because the action of the polyfunctional aziridine on the urethane bonds in the urethane paint forms a network structure, making it possible to adjust the shear stress of the film layer, eliminating spin performance that is dependent on the club number, and resulting in a golf ball that exhibits impact performance that ensures reliable distance with middle irons and reliable stopping power with short irons.

[0009] Therefore, the present invention provides the following golf ball. 1. A multi-piece solid golf ball comprising a core, an intermediate layer, and a cover, wherein a film layer is formed between the intermediate layer and the cover, and the film layer is formed from a resin composition containing an aziridine group-containing compound in a two-component curable polyurethane composition consisting of a polyol component and a polyisocyanate component, and the cover is formed from a resin composition mainly composed of polyurethane. 2. The golf ball according to claim 1, wherein the amount of the aziridine group-containing compound is 0.05 to 20 parts by mass per 100 parts by mass of the solid content of the two-component curable polyurethane composition. 3. The golf ball according to claim 1 or 2 above, wherein the aziridine content of the aziridine group-containing compound is 0.01 to 10 mmol / g. 4. The golf ball according to item 1 or 2 above, wherein the thickness of the film layer is 10 to 200 μm. 5. The golf ball according to claim 1 or 2 above, wherein the storage modulus (E') of the two-component curable polyurethane resin at 23°C is 500 to 2500 MPa. 6. A golf ball according to item 1 or 2 above, wherein the Shore D hardness of the intermediate layer is 60 to 80. [Effects of the Invention]

[0010] According to the golf ball of the present invention, the adhesion between the polyurethane cover and the inner intermediate layer is improved, and while maintaining good spin performance during approach shots, it is possible to fully demonstrate low spin performance when struck with a middle iron, thereby obtaining the desired flight performance. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view of a golf ball, which is one embodiment of the present invention. [Figure 2] This is an explanatory diagram of the test specimen used to measure the adhesive strength between the cover (outermost layer) and the intermediate layer. [Modes for carrying out the invention]

[0012] The present invention will be described in more detail below. The golf ball of the present invention comprises 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. The golf ball G shown in FIG. 1 has a single-layer core 1 and a single-layer cover 3, and a single-layer intermediate layer 2 is provided therebetween. Further, a thin film layer 4 is formed inside the cover. The cover 3 is located at the outermost layer in the layer structure of the golf ball except for the paint layer. Note that a surrounding layer made of a single layer or a plurality of layers of resin can be included between the core and the intermediate layer. Note that a large number of dimples D are usually formed on the surface of the cover (outermost layer) 3 in order to improve aerodynamic characteristics. Also, although not particularly shown, a paint layer is usually formed on the surface of the cover 3. Hereinafter, each of the above layers will be described in detail.

[0013] In addition to being a single layer, the core can be formed into a two-layer core having an inner layer and an outer layer or other multiple layers.

[0014] The core is formed of a rubber composition containing a base rubber and a metal salt of α,β-unsaturated carboxylic acid. As a suitable rubber composition, a rubber composition having the following formulation can be exemplified.

[0015] The above base rubber is not particularly limited, but it is particularly preferable to use polybutadiene.

[0016] The above polybutadiene preferably has 60% or more, preferably 80% or more, more preferably 90% or more, and most preferably 95% or more of cis-1,4 bonds in its polymer chain. If the cis-1,4 bond in the polybutadiene molecule occupies too little of the bonds, the resilience may decrease.

[0017] Also, the content of 1,2-vinyl bonds contained in the above polybutadiene is usually 2% or less, preferably 1.7% or less, and more preferably 1.5% or less in its polymer chain. If the content of 1,2-vinyl bonds is too large, the resilience may decrease.

[0018] The above polybutadiene has a Mooney viscosity (ML 1+4 (100 °C)), preferably 20 or more, more preferably 30 or more, and as the upper limit, preferably 120 or less, more preferably 100 or less, still more preferably 80 or less.

[0019] The above Mooney viscosity is an index of industrial viscosity (JIS K 6300) measured by a Mooney viscometer, which is a type of rotational plasticity meter, and the unit symbol ML 1+4 (100 °C) is used. Also, M represents the Mooney viscosity, L represents the large rotor (L type), 1 + 4 represents a preheating time of 1 minute, and the rotation time of the rotor is 4 minutes, indicating that the measurement was carried out under the condition of 100 °C.

[0020] The above polybutadiene can be synthesized using a rare earth element-based catalyst or a Group VIII metal compound catalyst.

[0021] In the base rubber, polybutadiene rubber synthesized with a catalyst different from the above lanthanum series rare earth element compound may be blended. Also, styrene-butadiene rubber (SBR), natural rubber, polyisoprene rubber, ethylene-propylene-diene rubber (EPDM), etc. may be blended, and one of these may be used alone or two or more may be used in combination.

[0022] The proportion of the above polybutadiene in the entire base rubber of the rubber composition is preferably 60% by mass or more, more preferably 70% by mass or more, and most preferably 90% by mass or more. Also, it may be 100% by mass of the base rubber, that is, all of the base rubber may be the above polybutadiene.

[0023] α,β-unsaturated carboxylic acid metal salts are commonly used as co-crosslinking agents. The unsaturated carboxylic acid preferably has 3 to 8 carbon atoms; specific examples include acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Specific examples of metals for the unsaturated carboxylic acid include zinc, sodium, magnesium, calcium, and aluminum, with zinc being particularly preferred. Therefore, zinc acrylate is the most preferred co-crosslinking agent.

[0024] The above α,β-unsaturated carboxylate metal salt can preferably be added in an amount of 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, per 100 parts by mass of the base rubber, and the upper limit of the amount added can preferably be 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 45 parts by mass or less. If the amount added is too high, it may become too hard and the feel of hitting it may become unbearable, and if the amount added is too low, the rebound may decrease.

[0025] In addition to the essential components mentioned above, the above rubber composition may also contain other co-crosslinking agents, organic peroxides, inert fillers, sulfur, antioxidants, organic sulfur compounds, and the like.

[0026] Furthermore, if the core is a single layer, a single-layer core can be manufactured using the rubber composition described above. If the core has multiple layers, in addition to the rubber material described above, rubber compositions or known resin materials with different types and amounts of constituent components can be used for the center core (inner core) and its outer layer core.

[0027] A vulcanized molded product (core) can be produced by vulcanizing and curing the above rubber composition. This vulcanized molded product can be used in whole or in part for single-layer or multi-layer cores. For example, a core, which is a vulcanized molded product, can be produced by kneading using a kneader such as a Banbury mixer or roll, compression molding or injection molding using a core mold, and then appropriately heating the molded body at a temperature of about 100 to 200°C for 10 to 40 minutes, which is sufficient for the organic peroxide and co-crosslinking agent to act, thereby curing the molded body.

[0028] There are no particular restrictions on the core diameter, and it depends on the layer structure of the golf ball being manufactured, but it is preferably 30 mm or more, more preferably 35 mm or more, with an upper limit of preferably 41 mm or less, more preferably 40 mm or less. If the core diameter deviates from this range, the initial velocity of the ball may be low, or the appropriate spin characteristics may not be obtained.

[0029] In this invention, at least one intermediate layer is provided between the core and the cover (outermost layer).

[0030] There are no particular restrictions on the resin material for the intermediate layer, but known thermoplastic resin materials such as various ionomer resins used in golf balls can be used.

[0031] Specifically, the ionomer resins used can include products from the brands "Hymiran" and "Nucrel." Specifically, commercially available products such as Hymiran 1554, 1557, 1601, 1605, 1706, AM7311, AM7316, and AM7339 (all manufactured by Mitsui Dow Polychemicals) can be used.

[0032] Furthermore, to further reduce the spin rate of the ball, it is particularly preferable to use a highly neutralized ionomer material as the intermediate layer material. Specifically, it is preferable to use a material containing the following components (i) to (iv). (i-1) A binary random copolymer of olefin-unsaturated carboxylic acid and / or a metal ion neutralized product of a binary random copolymer of olefin-unsaturated carboxylic acid, (ii-2) A base resin (i) is a blend of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terranandic copolymer and / or a metal ion neutralized product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terranandic copolymer in a mass ratio of 100:0 to 0:100, and (ii) a non-ionomer thermoplastic elastomer is blended in a mass ratio of 100:0 to 50:50. (iii) 5 to 80 parts by mass of fatty acids and / or derivatives thereof having a molecular weight of 228 to 1500, (ix) 0.1 to 17 parts by mass of a basic inorganic metal compound capable of neutralizing the unneutralized acid groups in the above components (i) and (iii). A mixed material comprising the above components (i) to (ix). In particular, when using a mixed material of the above components (i) to (ix), it is preferable to use one in which 70% or more of the acid groups have been neutralized.

[0033] The material hardness of the above-mentioned intermediate layer is not particularly limited, but is preferably 60 or higher on the Shore D hardness scale, and more preferably 65 or higher. Furthermore, its upper limit can be preferably 80 or lower, and more preferably 70 or lower.

[0034] The thickness of the intermediate layer described above is set to 2.0 mm or less, preferably 1.8 mm or less, and more preferably 1.5 mm or less. There is no particular lower limit, but it is preferably 0.8 mm or more, more preferably 1.0 mm or more, and even more preferably 1.2 mm or more. If the thickness of the intermediate layer deviates from the above numerical range, the low-spin effect of the driver (W#1) may be insufficient, and the distance may be reduced.

[0035] The above-mentioned intermediate layer can be formed by conventional methods such as known injection molding. For example, an intermediate layer material can be injected around the core using an injection molding die to obtain a coated sphere, or an intermediate layer coated sphere surrounding the core can be produced by wrapping the core with two pre-formed hemispherical half-cups as the intermediate layer material and then heating and pressurizing the wrapped cups.

[0036] Next, we will explain the cover (outermost layer) mentioned above. The resin material of the cover described above is formed from a resin composition mainly composed of polyurethane. The proportion of polyurethane in the overall cover resin composition is not particularly limited, but it can be 50% by mass or more, preferably 80% by mass or more. The polyurethane described above will be explained below.

[0037] The structure of polyurethane consists of a soft segment made of a long-chain polymer polyol (polymeric glycol) and a hard segment made of a chain extender and polyisocyanate. Here, any polymer polyol conventionally used in polyurethane material technology can be used as the raw material, and there are no particular restrictions. Examples include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer polyols, castor oil polyols, silicone polyols, and vinyl polymer polyols. Specifically, polyester polyols include adipate polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol, and polyhexamethylene adipate glycol, as well as lactone polyols such as polycaprolactone polyols. Polyether polyols include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol), and poly(methyltetramethylene glycol). These can be used individually or in combination of two or more types.

[0038] The number-average molecular weight of the long-chain polyol described above is preferably in the range of 1,000 to 5,000. By using a long-chain polyol having such a number-average molecular weight, it is possible to reliably obtain a golf ball made of a polyurethane composition that is excellent in various properties such as resilience and productivity as described above. 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.

[0039] The number-average molecular weight mentioned above is the number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS-K1557 (the same applies hereinafter).

[0040] As chain extenders, those used in conventional polyurethane technologies can be suitably used and are not particularly limited. In the present invention, low molecular weight compounds having two or more active hydrogen atoms capable of reacting with an isocyanate group and a molecular weight of 2,000 or less can be used, and among these, aliphatic diols having 2 to 12 carbon atoms can be suitably used. Specifically, examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, 2,2-dimethyl-1,3-propanediol, and among these, 1,4-butylene glycol can be suitably used.

[0041] As for the polyisocyanate, those used in conventional polyurethane technologies can be suitably used, and there are no particular restrictions. Specifically, one or more selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylenediisocyanate, xylylene diisocyanate, naphthylene 1,5-diisocyanate, tetramethylxylylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane, and dimer acid diisocyanate can be used. However, depending on the isocyanate species, it may be difficult to control the crosslinking reaction during injection molding.

[0042] Furthermore, the ratio of active hydrogen atoms to isocyanate groups in the polyurethane formation reaction described above can be adjusted to a suitable range. Specifically, when producing polyurethane by reacting the long-chain polyol, polyisocyanate compound, and chain extender, it is preferable to use each component in such a ratio that the isocyanate groups contained in the polyisocyanate compound are 0.95 to 1.05 moles for every 1 mole of active hydrogen atoms contained in the long-chain polyol and chain extender.

[0043] The method for producing polyurethane is not particularly limited, and it may be produced using a long-chain polyol, a chain extender, and a polyisocyanate compound, utilizing known urethane formation reactions, by either a prepolymer method or a one-shot method. Among these, melt polymerization in the substantially absence of solvent is preferred, and production by continuous melt polymerization using a multi-screw extruder is particularly preferred.

[0044] As the polyurethane mentioned above, it is preferable to use a thermoplastic polyurethane material. Commercially available thermoplastic polyurethane materials can be suitably used, such as "Pandex" manufactured by DIC Covestropolymer Co., Ltd. and "Rezamin" manufactured by Dainichi Seika Kogyo Co., Ltd.

[0045] The material hardness of the cover described above is 60 or less on the Shore D hardness scale, more preferably 50 or less, and even more preferably 45 or less. There is no particular lower limit, but it is preferably 20 or more on the Shore D hardness scale, and more preferably 30 or more.

[0046] The thickness of the cover described above should be set to 2.0 mm or less, preferably 1.5 mm or less, and more preferably 1.2 mm or less. There is no particular lower limit, but it is preferably 0.6 mm or more, more preferably 0.7 mm or more, and even more preferably 0.8 mm or more. If the thickness of the cover described above deviates from the above numerical range, the low-spin effect of the driver (W#1) and mid-irons may be insufficient, resulting in a loss of distance. Also, if the thickness of the cover described above is too small, the durability may deteriorate.

[0047] In the present invention, a film layer is formed between the intermediate layer and the cover. This film layer is made of a two-component curable polyurethane composition consisting of a polyol component and a polyisocyanate component, with an aziridine group-containing compound added. Generally, the physical properties of polyurethane are determined by the balance between hard segments and soft segments, and the formation of hydrogen bonds, and adjusting these is important in designing golf balls. The polyfunctional polyaziridine-containing polyurethane used in the present invention achieves a balance between the hard segments and soft segments of the polyurethane base material, as well as the formation of hydrogen bonds, by adding polyfunctional polyaziridine to the polyurethane. As a result, it is possible to maintain the response to shear force received during short iron and wedge shots, while suppressing the response to shear force received during middle iron shots. That is, by mixing polyfunctional polyaziridine with a two-component curable urethane paint and applying it, film molding is facilitated, and a desirable shear stress can be achieved.

[0048] Examples of the polyol component in the above-mentioned two-component curable polyurethane composition include acrylic polyols and polyester polyols. These polyols may include modified polyols, and other polyols may be added to further improve workability.

[0049] On the other hand, there are no particular restrictions on the polyisocyanates used; commonly used aromatic, aliphatic, and alicyclic polyisocyanates are acceptable. Specifically, examples 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, and 1-isocyanato-3,3,5-trimethyl-4-isocyanatomethylcyclohexane. These can be used individually or in combination.

[0050] As the curing catalyst (organometallic compound), amine-based catalysts or organometallic catalysts can be used. Suitable organometallic compounds include metal soaps such as aluminum, nickel, zinc, and tin, which have conventionally been used as curing agents for two-component urethane coatings.

[0051] Various organic solvents can be mixed into the two-component curable polyurethane composition depending on the 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, methylcyclohexane, and ethylcyclohexane; and petroleum hydrocarbon solvents such as mineral spirits.

[0052] Furthermore, in order to reliably obtain the desired effects of the present invention, the storage modulus (E') at 23°C of the polyurethane resin obtained by curing the above two-component curable polyurethane composition is preferably 500 to 2500 MPa. This storage modulus (E') can be measured using a commercially available measuring device, for example, a dynamic viscoelasticity analyzer (DMA Q800) manufactured by T.A. Instruments.

[0053] The above two-component curable polyurethane composition contains an aziridine group-containing compound. That is, it is presumed that the polyfunctional compound having an aziridine group reacts with reactive functional groups such as isocyanates remaining in the polyurethane cover, thereby improving the adhesion between the film layer and the cover. Furthermore, the ring-opening reaction of the aziridine group reacts with functional groups such as carboxyl groups contained in the intermediate layer material, thereby improving the adhesion between the intermediate layer and the film layer, and exhibiting the function of an adhesive on both the intermediate layer and the cover layer.

[0054] Examples of the above-mentioned aziridine group-containing compounds 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), triethylene melamine, bisisoprotaloyl-1-(2-methylaziridine), tri-1-aziridinylphosphine oxide, tetramethylolmethane-tri-β-aziridinylpropionate, and trimethylolpropane-tri-β-aziridinylpropionate.

[0055] The above-mentioned aziridine group-containing compounds can be commercially available products. Examples of commercially available aziridine-containing compounds include "Chemitight PZ-33" and "Chemitight DZ-22E" manufactured by Nippon Shokubai Co., Ltd., and "NeoAdd PAX-523" manufactured by Covestro Corporation, but the present invention is not limited to these.

[0056] The amount of the aziridine group-containing compound is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and even more preferably 0.2 parts by mass or more, per 100 parts by mass of solid content of the two-component curable polyurethane composition, with an upper limit of preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less. If the amount is too high, it becomes difficult to form a film layer by coating, while if it is too low, the function as an adhesive on both sides of the intermediate layer and cover layer deteriorates, and the effect on the flying performance is reduced.

[0057] Furthermore, the aziridine content in the above-mentioned aziridine group-containing compound is preferably 0.01 mmol / g or more, more preferably 0.1 mmol / g or more, and even more preferably 1.0 mmol / g or more, with an upper limit of preferably 10 mmol / g or less, and more preferably 8 mmol / g or less. If the content is too high, the viscosity of the aziridine group-containing compound will increase, and the dispersibility of the two-component curable polyurethane composition will deteriorate. On the other hand, if the content is too low, it will be necessary to increase the amount of aziridine group-containing compound added to the two-component curable polyurethane composition, and as described above, it will become difficult to form a film layer by coating.

[0058] As a method for forming the above-mentioned film layer, a two-component curable polyurethane composition containing the above-mentioned aziridine group-containing compound can be applied to the surface of the intermediate layer-coated sphere, and a film layer can be formed on the surface of the intermediate layer-coated sphere through a drying process. In this case, spray coating, electrostatic coating, dipping, etc., can be suitably used as the coating method, and there are no particular restrictions.

[0059] As described above, the thickness of the film layer formed on the surface of the intermediate layer-coated sphere is preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 30 μm or more, with an upper limit of preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 120 μm or less. In other words, the gist of the present invention is to replace a part (inside) of the outermost urethane cover layer with a film layer, and in order to utilize the ball performance of the original urethane cover material while coating the intermediate layer with a different polyurethane material (film layer) and exhibiting performance as a double cover, it is necessary to balance the thickness of the cover and the film layer. To achieve this balance, it is preferable to set the thickness of the film layer within the above range.

[0060] A golf ball formed by laminating the aforementioned core, intermediate layer, film layer, and cover (outermost layer) can be manufactured by conventional methods such as known injection molding. For example, an intermediate layer material is injected around the core using an injection molding die to obtain an intermediate layer-coated sphere, and then a resin composition is spray-painted onto this intermediate layer-coated sphere to form a film layer of a predetermined thickness. Furthermore, this sphere coated with the film layer is set in another injection molding die, and the material for the outermost cover layer is injection-molded to obtain a golf ball with a four-layer structure or more. Alternatively, a golf ball can be manufactured by wrapping the coated sphere with two pre-formed hemispherical half-cups and then heat-pressure-molding them to form each coating layer.

[0061] Furthermore, numerous dimples are formed on the surface of the outermost layer of the cover, and various treatments such as surface preparation, stamping, and painting can be applied to the cover. In particular, when such surface treatments are applied to a cover formed with the cover material of the present invention, the moldability of the cover surface is good, so the workability is good.

[0062] The golf ball of the present invention is preferably formed to have a diameter and weight in accordance with the rules of golf, and is usually formed to have a diameter of 42.67 mm or more and a weight of 45.93 g or less, but the diameter is preferably 42.67 to 42.9 mm. [Examples]

[0063] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0064] [Examples 1-5, Comparative Examples 1-4] [Core formation] A core with a diameter of 38.7 mm was prepared. The core composition was the same in all examples and comparative examples. Two types of polybutadiene were used as the base rubber: 20 parts by mass of polybutadiene (product name "BR51" manufactured by ENEOS Material Co., Ltd.) and 80 parts by mass of polybutadiene (product name "BR730" manufactured by ENEOS Material Co., Ltd.) were mixed and used. Furthermore, this base rubber is blended with 29.5 parts by mass of zinc acrylate ("ZNDA-85S" manufactured by Nippon Shokubai Co., Ltd.), 0.6 parts by mass of dicumyl peroxide ("Parkmyl D" manufactured by NOF Corporation) as an organic peroxide, 0.1 parts by mass of 2,2-methylenebis(4-methyl-6-butylphenol) ("Nocrac NS-6" manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) as an antioxidant, 19.3 parts by mass of zinc oxide ("Triple Zinc Oxide" manufactured by Sakai Chemical Industry Co., Ltd.), and 0.3 parts by mass of pentachlorothiophenol zinc salt (manufactured by Wako Pure Chemical Industries, Ltd.) as an organic sulfur compound. The rubber composition is vulcanized at a temperature of 155°C for 15 minutes. The specific gravity of the blend is 1.138.

[0065] [Formation of the intermediate layer] Next, a resin material for the intermediate layer was injection molded around a core with a diameter of 38.7 mm to produce an intermediate layer-coated sphere with an intermediate layer thickness of 1.2 mm. The resin material for the intermediate layer was the same in all examples and comparative examples, and consisted of "Hymiran AM7318" and "Hymiran 1706" (both ionomer resins manufactured by Mitsui Dow Polychemicals) blended in a ratio of 85:15 (mass ratio). The material hardness was Shore D hardness "68".

[0066] [Formation of the film layer] Next, for each example and comparative example except for Examples 2 and 5, a resin material for the film layer was spray-coated around the intermediate layer-coated sphere using a spray gun to produce film-coated spheres with a thickness of 10 to 100 μm. The resin materials for the film layer are as shown in Table 1 below.

[0067] Furthermore, Example 2 is an example in which the thickness of the film layer in Example 3 is changed from 100 μm to 50 μm, and Example 5 is an example in which polyol 1, the main component in Example 3, is changed to polyol 3. In these examples, as described above, the resin material for the film layer is sprayed around the intermediate layer-coated sphere using a spray gun to produce a film-coated sphere of a predetermined thickness.

[0068] [Table 1]

[0069] The main components to be incorporated into the above-mentioned tree composition are as follows: • "Polyol 1, 2" ... These are polyester polyols manufactured by Asia Industries Co., Ltd. Polyol 1 is a two-component curable polyurethane resin with a storage modulus of 1803 MPa at 23°C, and Polyol 2 is a two-component curable polyurethane resin with a storage modulus of 1332 MPa at 23°C. • "Polyol 3" ... This is a polyester polyol manufactured by Cashew Co., Ltd., and is a two-component curable polyurethane resin with a storage modulus of 2389 MPa at 23°C. The method for measuring the storage modulus of these polyurethane resin compositions is described below. • "Aziridine group-containing compound" ... Nippon Shokubai Co., Ltd. product name "Chemitite PZ-33" (liquid with an aziridine content of 6 mmol / g or more) • "Polyurethane Dispersion" ... A composition made by spraying a mixture of "Rezamin D-6031" (polycarbonate-type polyurethane dispersion) and "Rezamin D-52" (crosslinking agent), both manufactured by Dainichi Seika Kogyo Co., Ltd., in a mass ratio of 100:3:3, with water.

[0070] [Two-component curable polyurethane resin composition has a storage modulus of E'] The storage modulus E' (MPa) of the film was measured under the following conditions. • Equipment: TA Instruments' "Q800" dynamic viscoelasticity measuring instrument. • Measurement sample: A resin composition containing the main agent and curing agent (excluding the inclusion of aziridine group-containing compounds) was dried and cured at 40°C for 4 hours to produce a film (coating) with a thickness of 0.11 to 0.14 mm. From this film, sample pieces were cut out to a width of 4 mm and a clamping distance of 20 mm. • Measurement mode: Tensile ·Measurement temperature: -100℃~150℃ • Heating rate: 4°C / min • Measurement data acquisition interval: 4℃ ·Excitation frequency: 10Hz • Measurement distortion: 0.10%

[0071] [Formation of the cover] Next, for each example and comparative example except for Examples 2 and 5, an ether-type thermoplastic polyurethane (product name "Pandex" manufactured by DIC Covestropolymer, with a Shore D hardness of "47") was used as the resin material for the cover (outermost layer). Using a different injection molding die, the above resin material was injection molded around the above film-coated sphere to produce a four-layer golf ball with a diameter of 42.7 mm and an outermost layer of 0.8 mm thickness. At this time, numerous dimples were formed on the cover surface.

[0072] In Examples 2 and 5, a four-layer golf ball is produced by injection molding the above-mentioned resin material around a film-coated sphere, similar to the above. In this case, dimples similar to those described above are formed on the cover surface.

[0073] [Formation of paint layer (paint film)] Next, for each example and comparative example, excluding Examples 2 and 5, a paint composition containing polyester polyol (main component) and isocyanate curing agent was applied to the surface of a cover with numerous dimples using an air spray gun to form a paint layer (coating film) with a thickness of 15 μm to produce a golf ball for each example. For Examples 2 and 5, the same procedure was followed as above, applying the above paint composition to the surface of the cover to form a paint layer to produce a golf ball for each example.

[0074] For each of the obtained golf balls, the spin rate and adhesion were evaluated by striking them with a 6-iron and a sand wedge using the method described below, and the results are shown in Table 2.

[0075] Spin rate of a 6-iron (I#6, HS 42m / s) Using a swing robot machine, the spin rate will be measured when hitting a 6-iron (I#6) at a head speed (HS) of 42 m / s. The spin rate will be measured immediately after impact using an initial condition measurement device. The club used will be a Bridgestone Sports "JGR Forged I#6 (2016 model)" and will be evaluated according to the following criteria. 〔Judgment criteria〕 ◎...The difference in spin rate compared to Comparative Example 1 is 150 rpm or less. ○ ··· The difference in spin rate compared to Comparative Example 1 is 50-149 rpm less. △ ··· The spin amount is equivalent to or greater than that of Comparative Example 1.

[0076] Evaluating spin rate during approach shots The golf swing robot will be fitted with a sand wedge and the amount of spin will be measured when it hits the ball at a head speed (HS) of 15 m / s. The amount of spin will be measured immediately after impact using an initial condition measurement device. The sand wedge used will be a Bridgestone Sports "TourStage TW-03 (loft angle 57°) 2002 model". 〔Judgment criteria〕 ○ ··· The spin amount is equivalent to or slightly higher than that of Comparative Example 1. △ ··· The difference in spin rate compared to Comparative Example 1 is 50 rpm or less.

[0077] Adhesion As shown in Figure 2, when p1 is a plane 2 mm from the center of the golf ball 10, and p2 is a plane point-symmetric to p1 with respect to the ball center, the adhesion strength between the outermost layer 30 and the intermediate layer 20 was measured at ball portion s1 between p1 and p2. Cuts T were made in the outermost layer 30 of the ball where the outermost layer 30 overlaps with p1, and where the outermost layer 30 overlaps with p2, and the outermost layer 30 was peeled off in parts other than s1. Next, cuts T were made in the outermost layer 30 perpendicular to p1 and p2, and the outermost layer 30 was peeled off from the intermediate layer 20 by approximately 20 mm from the cut to create a gripping surface and obtain a test specimen. Referring to JIS K6256 "Adhesion Test Method for Vulcanized Rubber and Thermoplastic Rubber," the movement speed of the gripping jig was set to 50 mm / min, and the tensile strength was measured every 0.1 mm. The tensile strength of approximately 100 mm was measured for one test specimen, and these values ​​were defined as the adhesion force (unit: N). The test specimens used were those obtained as described above, and the gripping tab provided on the outermost layer 30 was held by a gripping jig. The fixing jig for the test specimen was such that the test specimen could rotate while maintaining its center position, and the outermost layer 30, which was wrapped around the intermediate layer 20, could be peeled off without slack as the gripping jig moved. The adhesion force (peel value) measured in each example was evaluated according to the following criteria. 〔Judgment criteria〕 ◎...The adhesion is so strong that the peel value cannot be measured. In other words, the test piece of the intermediate layer or cover does not separate, but rather tears. ○ ··· Peel value (N) is 0.5 or higher △ ··· Peel value (N) is 0.2 or higher, but less than 0.5. × ··· Peel value (N) is less than 0.2

[0078] [Table 2]

[0079] As shown in Table 2, when the performance of the golf balls of Examples 1 to 5 is considered in comparison to the golf ball of Comparative Example 1, they exhibit excellent adhesion between the cover and the intermediate layer, low spin rate and good distance performance when using a middle iron (I#6), and high spin rate when approaching the ball. In contrast, Comparative Example 2 does not contain an aziridine group-containing compound in the film layer, resulting in insufficient adhesion, and the spin rate for mid-irons (I#6) is the same, indicating that sufficient spin reduction has not been achieved. Comparative Example 3 does not contain an aziridine group-containing compound in the film layer, resulting in insufficient adhesion and a reduced spin rate during approach. Comparative Example 4 uses polyurethane dispersion in the film layer, and although it has excellent adhesion, the amount of spin when using a middle iron (I#6) is not reduced compared to Comparative Example 1 (the baseline).

Claims

1. A multi-piece solid golf ball comprising 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 from a resin composition containing an aziridine group-containing compound in a two-component curable polyurethane composition consisting of a polyol component and a polyisocyanate component, and the cover is formed from a resin composition mainly composed of polyurethane.

2. The golf ball according to claim 1, wherein the amount of the aziridine group-containing compound is 0.05 to 20 parts by mass per 100 parts by mass of the solid content of the two-component curable polyurethane composition.

3. The golf ball according to claim 1 or 2, wherein the aziridine content of the above aziridine group-containing compound is 0.01 to 10 mmol / g.

4. The golf ball according to claim 1 or 2, wherein the thickness of the above-mentioned film layer is 10 to 200 μm.

5. The golf ball according to claim 1 or 2, wherein the storage modulus (E') of the above two-component curable polyurethane resin at 23°C is 500 to 2500 MPa.

6. The golf ball according to claim 1 or 2, wherein the Shore D hardness of the intermediate layer is 60 to 80.

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