JP7779190B2 - golf balls - Google Patents

Description

The present invention relates to a golf ball with a multi-layer structure having a single-layer or multi-layer core, an intermediate layer, and a two-layer cover consisting of an inner cover layer and an outer cover layer.

The primary characteristic required of a golf ball is increased flight distance, but another required characteristic is the ability to stop the ball well on approach shots. To achieve these characteristics, urethane resin materials have recently been increasingly used as golf ball cover materials. In particular, thermoplastic urethane elastomers are increasingly being used as urethane resin materials, and their physical properties are being improved year by year. Several technologies have been proposed to further improve the physical properties of cover resin compositions by blending other resins and additives while using thermoplastic urethane elastomers as the base resin.

For example, Patent Document 1 proposes using a blend of thermoplastic polyurethane and a styrene-based block copolymer as the primary cover material in order to improve the cover's abrasion resistance. However, this blend cover is insufficient in terms of impact resilience and abrasion resistance. Furthermore, Patent Document 2 discloses a golf ball in which a plasticizer is blended into a resin composition based on a urethane resin to soften the resin composition. However, this merely softens the resin composition, and is insufficient to improve spin rate on approach shots.

In addition, when blending urethane resin materials with other resin materials to achieve lower hardness, it is also desirable to avoid changes in impact resilience and deterioration of moldability as much as possible.

Japanese Patent Application Publication No. 11-9721 JP 2017-12737 A

The present invention was developed in consideration of the above circumstances, and aims to provide a golf ball that increases spin rate on approach shots and provides a good feel without compromising distance performance on driver (W#1) shots.

As a result of extensive research to achieve the above-mentioned objectives, the inventors have discovered that, rather than using the conventional method of blending various other polymers and additives into a urethane resin material to improve controllability on approach shots in golf balls having a core, intermediate layer, and cover (outermost layer), by forming a relatively thin, film-like layer on the inside of the cover (outermost layer) of the golf ball and selecting a material for this thin layer with a specific range of rebound resilience and hardness, or by specifying the relationship between the rebound resilience of this thin layer and the cover (outermost layer), golf balls having this outermost layer and thin layer will achieve a high spin rate on approach shots, provide a good feel on approach shots, and not lose distance when hit with a driver (W#1), resulting in a golf ball that is generally advantageous to golf players, and this has led to the creation of the present invention.

Accordingly, the present invention provides the following golf balls.
1. A golf ball having a single-layer or multi-layer core, an intermediate layer, and a cover, wherein the cover is composed of two layers: an inner cover layer and an outer cover layer, the inner cover layer having a material hardness of 20 to 40 Shore D hardness and the outer cover layer having a material hardness of 35 to 55 Shore D hardness, the total thickness of these two layers being 0.6 to 1.2 mm, and wherein, where A is the rebound resilience (%) of the inner cover layer material according to JIS-K 6255, and B is the rebound resilience (%) of the outer cover layer material according to JIS-K 6255, the relationship 0<(A) - (B)<18 is satisfied.
2. The golf ball according to claim 1, wherein the resin component of the material for the outer cover layer is a thermoplastic polyurethane elastomer.
3. The golf ball according to 1 or 2 above, wherein the resin component of the material for the inner cover layer is a thermoplastic elastomer.
4. The golf ball according to claim 1 or 2, wherein the resin component of the material of the inner cover layer is a multicomponent copolymer having conjugated diene units, non-conjugated olefin units, and aromatic vinyl units, the conjugated diene units include butadiene units, the non-conjugated olefin units include units selected from the group consisting of ethylene units, propylene units, and 1-butene units, the aromatic vinyl units include styrene units, and the content of the conjugated diene units in the multicomponent copolymer is 5% by mass or greater.
5. The golf ball according to 4 above, wherein the content of the conjugated diene units in the multicomponent copolymer is 10% by mass or more.
6. The golf ball according to 4 above, wherein the content of the non-conjugated olefin units in the multicomponent copolymer is 90% by mass or less.
7. The golf ball according to 4 above, wherein the content of the aromatic vinyl units in the multi-component copolymer is 30% by mass or less.
8. The golf ball of claim 4, wherein the non-conjugated olefin units are ethylene units.
9. The golf ball according to claim 4, wherein the multi-component copolymer is a copolymer polymerized with a gadolinium metallocene complex catalyst.
10. The golf ball according to 1 or 2 above, wherein the thickness of the inner cover layer is 0.1 to 0.5 mm.
11. The golf ball according to 1 or 2 above, wherein the thicknesses of the intermediate layer, inner cover layer, and outer cover layer satisfy the following relationship: intermediate layer thickness > inner cover thickness < outer cover thickness.
12. The golf ball according to 1 or 2 above, wherein the ratio of the thickness (a) of the inner cover layer to the thickness (b) of the outer cover layer is (a)/(b)=0.10 to 0.60.

The golf ball of the present invention increases the amount of spin on approach shots and provides a good feel without compromising distance performance on driver (W#1) shots.

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

The present invention will be described in more detail below.
As shown in Figure 1, the golf ball of the present invention is a golf ball G having a core 1, an intermediate layer 2 enclosing the core, an outer cover layer 4 enclosing the intermediate layer, and a thin inner cover layer 3 inside the outer cover layer 4. The core 1 and the intermediate layer 2 can each be formed as a single layer or multiple layers. The surface of the outer cover layer 4 usually has numerous dimples D formed thereon, and is usually coated with a paint layer 5.

The core can be formed using a known rubber material as the base material. Known natural or synthetic rubbers can be used as the base rubber. More specifically, it is recommended to primarily use polybutadiene, especially cis-1,4-polybutadiene, which has at least 40% or more of a cis structure. Furthermore, if desired, natural rubber, polyisoprene rubber, styrene-butadiene rubber, etc. can be used in combination with the polybutadiene in the base rubber.

Polybutadiene can also be synthesized using metal catalysts such as rare earth element catalysts like neodymium catalysts, cobalt catalysts, and nickel catalysts.

The above base rubber can be compounded with co-crosslinking agents such as unsaturated carboxylic acids and their metal salts, 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. If necessary, commercially available antioxidants can also be added as needed.

There are no particular restrictions on the diameter of the core, but it is preferably 20 mm or more, more preferably 25 mm or more, and even more preferably 30 mm or more, with the upper limit being preferably 41 mm or less, more preferably 40 mm or less.

The material hardness of the intermediate layer is not particularly limited, but can be set to 55 or more, preferably 60 or more, and more preferably 65 or more, in Shore D hardness. There is also no particular upper limit, but it is preferably set to 70 or less.

There are no particular restrictions on the thickness of the intermediate layer, but it is preferably 0.5 mm or more, more preferably 0.6 mm or more, and even more preferably 0.7 mm or more, with an upper limit of preferably 1.8 mm or less, more preferably 1.7 mm or less, and even more preferably 1.6 mm or less.

The material for the intermediate layer is not particularly limited, but known thermoplastic resins and thermoplastic elastomers can be used, and it is particularly preferable to use an ionomer material. Specifically, it is preferable to use a commercially available ionomer resin or a resin mixture containing the following components (i) to (iv):
(i-1) an olefin-unsaturated carboxylic acid binary random copolymer and/or a metal ion-neutralized product of an olefin-unsaturated carboxylic acid binary random copolymer;
(ii-2) 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 in a mass ratio of 100:0 to 0:100 (i) a base resin and (ii) a non-ionomer thermoplastic elastomer in a mass ratio of 100:0 to 50:50, per 100 parts by mass of a resin component,
(iii) 5 to 80 parts by mass of a fatty acid and/or a derivative thereof having a molecular weight of 228 to 1500;
(ix) a mixed material containing 0.1 to 17 parts by mass of a basic inorganic metal compound capable of neutralizing unneutralized acid groups in the components (i) and (iii). In particular, when using a mixed material of the components (i) to (ix), it is preferable to use one in which 70% or more of the acid groups have been neutralized.

In the present invention, an inner cover layer is formed adjacent to the inside of the outer cover layer described below. This layer is as thin as a film and has appropriate resilience performance.

The thickness of the inner cover layer is preferably 0.1 mm or more, more preferably 0.2 mm or more, with an upper limit of preferably 0.5 mm or less, more preferably 0.4 mm or less. If the thickness is less than this range, the spin rate may not be high when hitting with a club such as a sand wedge (SW) on an approach shot. On the other hand, if the thickness is greater than this range, the spin rate may increase when hitting with a driver (W#1), resulting in a limited distance and a poor feel.

The material hardness of the inner cover layer is 20 or greater, preferably 24 or greater, and more preferably 28 or greater, on the Shore D scale. The upper limit is 40 or less, preferably 38 or less, and more preferably 36 or less. If the material hardness of this inner cover layer is lower than the above range, the spin rate upon impact with a driver (W#1) increases, making it impossible to increase the distance. On the other hand, if the material hardness of this inner cover layer is higher than the above range, the spin rate upon approach shots with a club such as a sand wedge (SW) is not high, and the feel upon impact is poor.

The rebound resilience of the inner cover layer material, as measured according to JIS-K 6255, is preferably 50% or higher, more preferably 55% or higher, with an upper limit of 75% or lower, more preferably 70% or lower. If the rebound resilience is too low, the spin rate on approach shots will be high, but the distance on driver shots may decrease. Furthermore, if the rebound resilience is too high, the desired high spin rate on approach shots may not be achieved. Furthermore, the rebound resilience of the inner cover layer material is always set higher than the rebound resilience of the outer cover layer material, which will be described later.

The material for the inner cover layer preferably uses, as its resin component, various thermoplastic elastomers, as well as a multicomponent copolymer having conjugated diene units, non-conjugated olefin units, and aromatic vinyl units. This multicomponent copolymer is the one described in Japanese Patent No. 6780827, and has optimal impact resilience and other properties for the present invention. This multicomponent copolymer is described as follows:

<Conjugated diene unit>
The multicomponent copolymer contains a conjugated diene unit. The conjugated diene unit is a structural unit derived from a conjugated diene compound as a monomer. Because the multicomponent copolymer can be polymerized using a conjugated diene compound as a monomer, it has superior crosslinking properties compared to copolymers polymerized using a non-conjugated diene compound such as the well-known EPDM. The conjugated diene compound contains a butadiene unit. The butadiene unit is a structural unit derived from a butadiene compound, and specific examples of butadiene compounds include 1,3-butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Furthermore, the conjugated diene unit in the multicomponent copolymer preferably contains a 1,3-butadiene unit, and more preferably consists solely of a 1,3-butadiene unit.

Furthermore, the multicomponent copolymer preferably has a cis-1,4 bond content of 50% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 90% or more, in all of the conjugated diene units. Such a multicomponent copolymer having a high cis-1,4 bond content in all of the conjugated diene units can be obtained by using a conjugated diene compound, a non-conjugated olefin compound, and an aromatic vinyl compound as monomers. On the other hand, the vinyl bond content (1,2 vinyl bond, 3,4 vinyl bond, etc.) in all of the conjugated diene units is preferably 30% or less, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 6% or less. Furthermore, the trans-1,4 bond content in all of the conjugated diene units is preferably 30% or less, more preferably 15% or less, and even more preferably 10% or less. The respective contents of cis-1,4 bonds, trans-1,4 bonds, and vinyl bonds can be determined from the integral ratios of the 1H-NMR and 13C-NMR measurement results.

The conjugated diene compound may be used alone or in combination of two or more types. That is, the multicomponent copolymer may contain one type of conjugated diene unit alone or two or more types. The content of conjugated diene units is preferably 10% by mass or more of the entire multicomponent copolymer, and more preferably 15% by mass or more. Furthermore, the content of conjugated diene units is preferably 80% by mass or less of the entire multicomponent copolymer, more preferably 60% by mass or less, and even more preferably 50% by mass or less.

<Non-conjugated olefin unit>
The multicomponent copolymer contains non-conjugated olefin units. The non-conjugated olefin units are structural units derived from a non-conjugated olefin compound as a monomer. The non-conjugated olefin compound is selected from the group consisting of ethylene, propylene, and 1-butene. In particular, in order to impart sufficient impact resilience and softness to the golf ball material, it is preferable that the non-conjugated olefin units be ethylene units.

The non-conjugated olefin compound may be used alone or in combination of two or more types. That is, the above-mentioned multicomponent copolymer may contain one type of non-conjugated olefin unit alone or two or more types. The content of non-conjugated olefin units is preferably more than 20% by mass and less than 90% by mass of the entire multicomponent copolymer, more preferably 30 to 85% by mass, even more preferably 40 to 80% by mass, and most preferably 45 to 75% by mass.

<Aromatic vinyl unit>
The above-mentioned multicomponent copolymer contains an aromatic vinyl unit. The aromatic vinyl unit is a structural unit derived from an aromatic vinyl compound as a monomer. Examples of aromatic vinyl compounds include styrene compounds, specifically styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o,p-dimethylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, etc. The aromatic vinyl unit in the above-mentioned multicomponent copolymer contains a styrene unit, and more preferably consists of only a styrene unit. Note that the aromatic ring in the aromatic vinyl unit is not included in the main chain of the copolymer unless it is bonded to an adjacent unit.

Aromatic vinyl compounds may be used alone or in combination of two or more. That is, the above-mentioned multicomponent copolymer may contain one type of aromatic vinyl unit alone or two or more types. The content of aromatic vinyl units is 3 to 30% by mass of the entire multicomponent copolymer. If the content of aromatic vinyl units is less than 3% by mass or more than 30% by mass, the length of the non-conjugated olefin portion of the copolymer cannot be suppressed, and the improvement in durability derived from non-conjugated olefin crystals is not achieved. The content of aromatic vinyl units is preferably 3 to 30% by mass of the entire multicomponent copolymer, more preferably 5 to 25% by mass, and even more preferably 10 to 20% by mass.

There are no particular restrictions on the number of types of monomers in the above-mentioned multicomponent copolymer, so long as the multicomponent copolymer contains conjugated diene units, non-conjugated olefin units, and aromatic vinyl units. The multicomponent copolymer may contain other structural units in addition to conjugated diene units, non-conjugated olefin units, and aromatic vinyl units. From the perspective of achieving the desired effects, the content of other structural units is preferably 30% by mass or less of the entire multicomponent copolymer, more preferably 20% by mass or less, and even more preferably 10% by mass or less. It is particularly preferable that no other structural units are contained, i.e., the content is 0% by mass.

The multicomponent copolymer is a multicomponent copolymer containing at least one conjugated diene unit, one non-conjugated olefin unit, and one aromatic vinyl unit. From the viewpoint of obtaining favorable breaking properties, the multicomponent copolymer is preferably a polymer obtained by polymerizing at least one conjugated diene compound, one non-conjugated olefin compound, and one aromatic vinyl compound as monomers.
The multicomponent copolymer is more preferably a terpolymer consisting of only one type of conjugated diene unit, one type of non-conjugated olefin unit, and one type of aromatic vinyl unit, and even more preferably a terpolymer consisting of only 1,3-butadiene units, ethylene units, and styrene units. Here, "one type of conjugated diene unit" encompasses conjugated diene units with different bonding modes.

Multicomponent copolymers contain conjugated diene units, non-conjugated olefin units, and aromatic vinyl units, and one of their major characteristics is that their main chains are composed exclusively of acyclic structures. If the main chain contains a cyclic structure, the fracture properties (especially elongation at break) decrease. NMR is the primary measurement method used to confirm whether the main chain of a multicomponent copolymer contains a cyclic structure. Specifically, if peaks attributable to cyclic structures present in the main chain (e.g., peaks appearing at 10 to 24 ppm for three- to five-membered rings) are not observed, this indicates that the main chain of the multicomponent copolymer is composed exclusively of acyclic structures. Furthermore, as described below, multicomponent copolymers can be synthesized in a single reaction vessel, i.e., one-pot synthesis, allowing for simplified production.

The multicomponent copolymer preferably has a polystyrene-equivalent weight-average molecular weight (Mw) of 10,000 to 10,000,000, more preferably 100,000 to 9,000,000, and even more preferably 150,000 to 8,000,000. A multicomponent copolymer with an Mw of 10,000 or more ensures sufficient strength as a golf ball material, while an Mw of 10,000,000 or less ensures high workability. The weight-average molecular weight and molecular weight distribution described above are determined by gel permeation chromatography (GPC) using polystyrene as the standard substance.

The chain structure of the multicomponent copolymer is not particularly limited and can be selected appropriately depending on the purpose. For example, where A is a conjugated diene unit, B is a non-conjugated olefin unit, and C is an aromatic vinyl unit, the multicomponent copolymer can be a block copolymer having a structure such as Ax-By-Cz (x, y, and z are integers of 1 or greater), a random copolymer having a structure in which A, B, and C are randomly arranged, a tapered copolymer comprising a mixture of the above random copolymer and a block copolymer, or an alternating copolymer having a structure such as (A-B-C)w (w is an integer of 1 or greater). Furthermore, the multicomponent copolymer may have a structure in which the conjugated diene units, non-conjugated olefin units, and aromatic vinyl units are linearly linked (linear structure), or a structure in which at least one of the conjugated diene units, non-conjugated olefin units, and aromatic vinyl units are linked to form branched chains (branched structure). Furthermore, when a multicomponent copolymer has a branched structure, the branched chains can also be bicomponent or multicomponent (i.e., the branched chains can contain at least two of conjugated diene units, non-conjugated olefin units, and aromatic vinyl units). Therefore, among multicomponent copolymers, multicomponent copolymers with a branched structure having bicomponent or multicomponent branched chains can be clearly distinguished from conventional graft copolymers in which the backbone chain and side chains are formed from a single type of unit that is different from each other.

The manufacturing method, polymerization process, and manufacturing conditions for the multicomponent copolymer, including the polymerization catalyst used, can be those described in Japanese Patent No. 6780827. It is preferable that the multicomponent copolymer be polymerized using a gadolinium metallocene complex catalyst.

The material hardness of the outer cover layer is 35 or greater, preferably 40 or greater, and more preferably 42 or greater, in Shore D hardness. The upper limit is 55 or less, preferably 52 or less, and more preferably 50 or less. If the material hardness of the outer cover layer deviates from the above range, it may not be possible to sufficiently achieve low spin on full shots with a driver (W#1), or it may not be possible to achieve high spin rate and high controllability on approaches.

There are no particular restrictions on the thickness of the outer cover layer, but it is preferably at least 0.4 mm, more preferably at least 0.5 mm, with an upper limit of preferably not more than 1.0 mm, more preferably not more than 0.9 mm.

In the present invention, the thickness relationship between the intermediate layer, inner cover layer, and outer cover layer satisfies the following formula: intermediate layer thickness > inner cover thickness < outer cover thickness. Furthermore, the ratio of the inner cover layer thickness (a) to the outer cover layer thickness (b), expressed as a value of (a)/(b), is preferably 0.10 or greater, more preferably 0.20 or greater, with the upper limit being preferably 0.60 or less, more preferably 0.50 or less. If the above range is exceeded, the spin rate upon impact with a driver (W#1) may increase, resulting in a poorer flight distance or a worsening feel.

The sum of the thickness (a) of the inner cover layer and the thickness (b) of the outer cover layer, (a) + (b), is 0.6 mm or greater, and preferably 0.7 mm or greater. The upper limit is 1.2 mm or less, and preferably 1.0 mm or less. If the sum of the above thicknesses deviates from the specified range, the spin rate may not be high when approaching with a club such as a sand wedge (SW), or the spin rate may increase when hitting with a driver (W#1), resulting in a decrease in flight distance.

The rebound resilience of the outer cover layer material, as measured in accordance with JIS-K 6255, is preferably 40% or greater, more preferably 45% or greater, with an upper limit of preferably 60% or less, more preferably 55% or less. If the rebound resilience is too low, the spin rate on driver shots may increase, resulting in reduced distance, and the spin rate on approach shots may decrease. Furthermore, if the rebound resilience is too high, the desired high spin rate on approach shots may not be achieved.

The present invention is also characterized in that, where the rebound resilience (%) of the inner cover layer material based on JIS-K 6255 is (A) and the rebound resilience (%) of the outer cover layer material based on JIS-K 6255 is (B), the relationship 0<(A)-(B)<18 is satisfied. By consistently setting the rebound resilience of the inner cover layer material higher than that of the outer cover layer material and setting the difference in rebound resilience between the two within a specific range, the desired effects of the present invention can be achieved, such as increased spin rate and improved feel on approach shots without compromising distance performance on driver (W#1) shots. The lower limit of the difference in rebound resilience between the two, i.e., (A)-(B), is preferably 3% or more, more preferably 5% or more, and even more preferably 7% or more, with the upper limit preferably 17% or less. If the value of (A)-(B) deviates from this range, insufficient spin will be applied on approach shots, preventing the desired high spin rate from being achieved.

There are no particular restrictions on the resin material for the outer cover layer, but it is preferable to use a polyurethane resin material. Details of the polyurethane material are as follows:

Polyurethanes are structured with a soft segment made of a long-chain polyol (polymeric glycol), and a hard segment made of a chain extender and polyisocyanate. The polymeric polyol used as the raw material can be any of those conventionally used in polyurethane material technology, and is not particularly limited. 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. 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 may be used alone or in combination of two or more.

It is preferable to use a polyether-based polyol as the polymer polyol.

The number-average molecular weight of the long-chain polyol is preferably within the range of 1,000 to 5,000. By using a long-chain polyol with such a number-average molecular weight, it is possible to reliably obtain a golf ball made from a polyurethane composition that has various excellent properties, such as the resilience and productivity described above. The number-average molecular weight of the long-chain polyol is more preferably within the range of 1,500 to 4,000, and even more preferably within the range of 1,700 to 3,500.

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

Chain extenders used in conventional polyurethane technology can be used suitably, and there are no particular limitations. In the present invention, low-molecular-weight compounds having two or more active hydrogen atoms in the molecule that can react with isocyanate groups and a molecular weight of 2,000 or less can be used. Among these, aliphatic diols having 2 to 12 carbon atoms are 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. Of these, 1,4-butylene glycol is particularly preferred.

The polyisocyanate may be any of those used in conventional polyurethane technology, and there are no particular restrictions. Specifically, one or more of the following may be used: 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. However, with some isocyanate types, it may be difficult to control the crosslinking reaction during injection molding.

The ratio of active hydrogen atoms to isocyanate groups in the polyurethane-forming reaction can be adjusted within 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 a ratio such that 0.95 to 1.05 moles of isocyanate groups are contained in the polyisocyanate compound per mole of active hydrogen atoms contained in the long-chain polyol and chain extender.

There are no particular limitations on the method for producing polyurethane; it can be produced using either the prepolymer method or the one-shot method, utilizing a known urethane reaction using a long-chain polyol, a chain extender, and a polyisocyanate compound. Of these, melt polymerization in the substantial absence of solvent is preferred, and continuous melt polymerization using a multi-screw extruder is particularly preferred.

The above-mentioned polyurethane is preferably a thermoplastic polyurethane material, and an ether-based thermoplastic polyurethane material is particularly suitable. Suitable thermoplastic polyurethane materials include commercially available products such as "Pandex" manufactured by DIC Covestropolymer and "Rezamin" manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

The resin materials for the intermediate layer, inner cover layer, and outer cover layer described above can be blended with various additives as needed, such as pigments, dispersants, antioxidants, light stabilizers, UV absorbers, and release agents. Furthermore, each of the resin materials described above can be obtained by mixing the components described above using various kneading machines, such as a kneading-type (single-screw or) twin-screw extruder, a Banbury mixer, or a kneader.

To manufacture the sphere (unpainted golf ball) having the above-described core, intermediate layer, inner cover layer, and outer cover layer of the present invention, conventional methods such as injection molding and hot-press molding can be used. For example, the materials for the intermediate layer and inner cover layer can be sequentially injected around the core using respective injection molds to obtain each coated sphere, and finally, the material for the outer cover layer, which is the outermost layer, can be injection molded to obtain a multi-layered golf ball. Alternatively, a golf ball can be produced by encasing the coated sphere in two half-cups previously molded into a spherical half-shell and then hot-press molding each coated layer.

The golf ball of the present invention has a large number of dimples on the surface of the outermost layer from the viewpoint of aerodynamic performance. There are no particular restrictions on the number of dimples formed on the surface of the outermost layer, but from the viewpoint of improving aerodynamic performance and increasing flight distance, the number is preferably 250 or more, more preferably 270 or more, even more preferably 290 or more, and most preferably 300 or more, with the upper limit being preferably 400 or less, more preferably 380 or less, and even more preferably 360 or less.

In the present invention, a paint layer is formed on the cover surface. It is preferable to use a two-component curing urethane paint as the paint that forms this paint layer. Specifically, in this case, the two-component curing urethane paint contains a base agent whose main component is polyol resin and a curing agent whose main component is polyisocyanate.

There are no particular restrictions on the method for applying the above-mentioned paint to the cover surface to form a paint layer, and any known method can be used, such as air gun painting or electrostatic painting.

There are no particular restrictions on the thickness of the paint layer, but it is usually 8 to 22 μm, preferably 10 to 20 μm.

The golf ball of the present invention can be made to conform to the Rules of Golf for competitive use, with an outer diameter of 42.80 mm or less so that it does not pass through a ring with an inner diameter of 42.672 mm, and a mass of preferably 45.0 to 45.93 g.

The present invention will be explained in detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[Examples 1 to 5, Comparative Examples 1 to 11]
A rubber composition for the core common to all examples is prepared according to the formulation shown in Table 1 and vulcanized and molded to produce a core having a diameter of 38.7 mm.

Details of the core material are as follows:
- "cis-1,4-polybutadiene" manufactured by JSR Corporation, product name "BR01"
・"Zinc acrylate" manufactured by Nippon Shokubai Co., Ltd. ・"Zinc oxide" manufactured by Sakai Chemical Industry Co., Ltd. ・"Barium sulfate" manufactured by Sakai Chemical Industry Co., Ltd. ・"Anti-aging agent" product name "Nocrac NS6" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)
"Organic peroxide (1)" Dicumyl peroxide, trade name "Percumyl D" (manufactured by NOF Corporation)
"Organic peroxide (2)": a mixture of 1,1-di(tert-butylperoxy)cyclohexane and silica, trade name "Perhexa C-40" (manufactured by NOF Corporation)
・"Zinc stearate" manufactured by NOF Corporation

The resin material for the intermediate layer was injection molded around a core having an intermediate layer diameter of 38.7 mm to produce an intermediate layer-coated sphere having an intermediate layer thickness of 1.0 mm, 1.2 mm, or 1.4 mm. The resin material for the intermediate layer was a resin formulation common to all examples, a blend of 50 parts by weight of a sodium-neutralized ethylene-unsaturated carboxylic acid copolymer having an acid content of 18% by weight and 50 parts by weight of a zinc-neutralized ethylene-unsaturated carboxylic acid copolymer having an acid content of 15% by weight, totaling 100 parts by weight.

Next, an inner cover layer was formed around the mid layer-covered ball of each example using resin materials No. 1 to No. 8 shown in Table 2 (Examples 1 to 5 and Comparative Examples 1 to 3) and Table 3 (Comparative Examples 4 to 11). This formation method involves first molding a pair of hemispherical half cups from the inner cover layer material, and then encasing the mid layer in these half cups and molding them under heat and pressure.

Details of the components contained in the compositions shown in Tables 2 and 3 below are as follows.
No. 1: "Multicomponent copolymer A" (details below) No. 2: "Multicomponent copolymer B" (details below) No. 3: "Hytrel 3001" Thermoplastic polyether ester elastomer manufactured by DuPont-Toray Co., Ltd. No. 4: "Hytrel 2401" Thermoplastic polyether ester elastomer manufactured by DuPont-Toray Co., Ltd. No. 5: "HPF2000" manufactured by The Dow Chemical Company No. 6: "AN4319""Nucrel" manufactured by Dow Mitsui Polychemicals
No. 7: "AN42012""Nucrel" manufactured by Mitsui Dow Polychemicals No. 8: "S.O.E.S1611" Styrene-ethylene-butylene-styrene block copolymer (SEBS) manufactured by Asahi Kasei Corporation

Multi-component copolymer A
80 g of styrene and 600 mL of toluene are placed in a thoroughly dried 1,000 mL pressure-resistant stainless steel reactor.
In a glove box under a nitrogen atmosphere, 0.25 mmol of mono(bis(1,3-tert-butyldimethylsilyl)indenyl)bis(bis(dimethylsilyl)amide) gadolinium complex (1,3-[(t-Bu) _{Me2Si ] 2C9H5Gd} [N( _SiHMe2 ) ₂ ] ₂ ), 0.275 mmol of dimethylanilinium tetrakis(pentafluorophenyl)borate [ _Me2NHPhB ( _C6F5 ) _{4 ]} , and 1.1 mmol _of diisobutylaluminum hydride were placed in a glass vessel, and 40 mL of toluene was added to prepare a catalyst solution.
The catalyst solution is added to the pressure-resistant stainless steel reactor and heated to 70°C.
Next, ethylene is charged into the pressure-resistant stainless steel reactor at a pressure of 1.5 MPa, and then 80 mL of a toluene solution containing 20 g of 1,3-butadiene is charged into the pressure-resistant stainless steel reactor over 8 hours, and copolymerization is carried out at 70° C. for a total of 8.5 hours.
Next, 1 mL of a 5% by mass solution of 2,2'-methylene-bis(4-ethyl-6-t-butylphenol) (NS-5) in isopropanol is added to the pressure-resistant stainless steel reactor to terminate the reaction.
Next, the copolymer is separated using a large amount of methanol and dried in vacuum at 50° C. to obtain copolymer A.

Multi-component copolymer B
65 g of styrene and 600 mL of toluene are placed in a thoroughly dried 1,000 mL pressure-resistant stainless steel reactor.
In a glove box under a nitrogen atmosphere, 0.26 mmol of mono(bis(1,3-tert-butyldimethylsilyl)indenyl)bis(bis(dimethylsilyl)amide)gadolinium complex (1,3-[(t-Bu) _Me2Si ] _2C9H5Gd [N(SiHMe2) ₂ ] _{2 )} , 0.283 _mmol of _{dimethylanilinium} tetrakis(pentafluorophenyl)borate [ _Me2NHPhB ( _{C6F5 )4 ]} , and 0.83 mmol of diisobutylaluminum hydride were placed in a glass vessel, and 40 mL of toluene was added to form a catalyst solution. This catalyst solution was then added to the pressure-resistant stainless steel reactor and heated to 70°C.
Next, ethylene is charged into the pressure-resistant stainless steel reactor at a pressure of 1.5 MPa, and then 120 mL of a toluene solution containing 24 g of 1,3-butadiene is charged into the pressure-resistant stainless steel reactor over 8 hours, and copolymerization is carried out at 70° C. for a total of 8.5 hours.
Next, 1 mL of a 5% by mass solution of 2,2'-methylene-bis(4-ethyl-6-t-butylphenol) (NS-5) in isopropanol is added to the pressure-resistant stainless steel reactor to terminate the reaction.
Next, the copolymer is separated using a large amount of methanol and dried in vacuum at 50° C. to obtain copolymer B.

For each of the "multicomponent copolymer A" and "multicomponent copolymer B," the contents (mass %) of butadiene, ethylene, and styrene are measured and evaluated by the following method.
<Butadiene, ethylene, and styrene content>
The contents of butadiene, ethylene, and styrene in each copolymer were determined by ¹ H-NMR, and the results are as follows:
"Multicomponent copolymer A": butadiene/ethylene/styrene = 14/70/16 (mass%), weight average molecular weight (Mw) 273 x 10 ³
"Multicomponent copolymer B": butadiene/ethylene/styrene = 16/72/12 (mass%), weight average molecular weight (Mw) 338 x 10 ³

Next, a urethane resin material common to all examples (trade name "Pandex" manufactured by DIC Covestro Polymer, an ether-type thermoplastic polyurethane with a Shore D hardness of 47) was injection molded around the sphere formed by covering the intermediate layer-covered sphere with the inner cover layer, to produce golf balls of a predetermined thickness so as to achieve a diameter of 42.7 mm in all examples. In this process, common dimples were formed on the cover surface of each example and comparative example, although not specifically shown. In addition, the cover surface of each example and comparative example was coated with a common two-component curing polyurethane resin composition to a thickness of 15 μm.

The material hardness and rebound resilience of the cover of the above golf ball were measured as follows, and the values shown in Table 2 (Examples 1-5 and Comparative Examples 1-3) and Table 3 (Comparative Examples 4-11) were obtained.

Material hardness (Shore D hardness) of the inner cover layer and the outer cover layer
The resin material for each cover layer is formed into a 2 mm thick sheet and left to stand for at least two weeks, after which the Shore D hardness is measured in accordance with ASTM D2240-95 standard.

The resin material is formed into a sheet having a thickness of 2 mm using a press, and the sheet is stacked to a thickness of 4 mm. The temperature is adjusted to 23±1°C, and the rebound resilience is measured using a Tripson type rebound resilience device in accordance with JIS-K 6255 (2013). (Note that the drop angle specified in JIS-K 6255 is adjusted to 40 degrees to measure the rebound resilience.)

The spin performance of each golf ball on driver (W#1) shots and approach shots, as well as the feel on approach shots, were evaluated using the methods described below. The results are shown in Table 2 (Examples 1-5 and Comparative Examples 1-3) and Table 3 (Comparative Examples 4-11).

Spin performance with driver A driver (W#1) was attached to a golf hitting robot, and the amount of backspin immediately after hitting with a head speed (HS) of 45 m/s was measured using an initial condition measuring device and evaluated according to the following criteria. The driver (W#1) used was a "Tour B XD-3 (loft angle 9.5°)" manufactured by Bridgestone Sports Co., Ltd.
[Judgment evaluation]
◎: Spin rate less than 3000 rpm ○: Spin rate 3000 rpm or more but less than 3100 rpm △: Spin rate 3100 rpm or more but less than 3200 rpm ×: Spin rate 3200 rpm or more

Spin performance during approach: A sand wedge (SW) was attached to a golf hitting robot, and the amount of backspin immediately after hitting the ball at a head speed (HS) of 10 m/s was measured using an initial condition measuring device, and the ball was evaluated according to the following criteria. The sand wedge (SW) used was a "Tour B XW-B (loft angle 56°)" manufactured by Bridgestone Sports Co., Ltd.
[Judgment evaluation]
◎: Spin rate is 3200 rpm or more. ○: Spin rate is 3150 rpm or more but less than 3200 rpm. △: Spin rate is 3050 rpm or more but less than 3150 rpm. ×: Spin rate is less than 3050 rpm.

Feeling Evaluation: A sensory evaluation of the feel of the ball when hitting an approach shot is carried out using the following method. The club used is a sand wedge (SW) manufactured by Bridgestone Sports Co., Ltd., product name "TOUR B XW-B" (loft angle 56°), and amateur golfers evaluate the feel of the ball when actually hitting it using the following criteria.
[Judgment evaluation]
◎: Very good feel ○: Good feel ×: Poor feel The feel is judged based on the impact (hardness) of the ball hitting the club face, the sound of the ball hitting the club, and how the ball is launched.

As shown in Tables 2 and 3, the golf balls of Comparative Examples 1 to 11 are inferior to the products of the present invention (Examples) in the following respects.
Comparative Example 1, which does not have an inner cover layer, has a poor feel on impact and slightly less spin on approach shots.
In Comparative Example 2, no inner cover layer was formed, and the spin rate on approach shots was slightly lower.
In Comparative Example 3, no inner cover layer is formed, and the spin rate on approach shots is low.
In Comparative Example 4, the difference in resilience between the inner cover layer and the outer cover layer exceeds 18%, resulting in a low spin rate on approach shots.
In Comparative Example 5, the difference in the resilience modulus between the inner cover layer and the outer cover layer exceeds 18%, and the spin rate on approach shots is low.
In Comparative Example 6, the inner cover layer has a material hardness of more than 40 in Shore D hardness, resulting in a poor feel on approach shots.
In Comparative Example 7, the material hardness of the inner cover layer exceeds 40 in Shore D hardness, and the spin rate on approach shots is slightly low.
In Comparative Example 8, the material hardness of the inner cover layer exceeded 40 in Shore D hardness, and the difference in resilience between the inner cover layer and the outer cover layer exceeded 18%, resulting in a slightly increased spin rate when hit with a driver (W#1) and a poor feel.
In Comparative Example 9, the outer cover layer had a higher rebound resilience than the inner cover layer, resulting in a slightly higher spin rate on hits with a driver (W#1) and a slightly lower spin rate on approach shots.
In Comparative Example 10, the inner cover layer had a material hardness of over 40 Shore D hardness, and the outer cover layer had a higher rebound resilience than the inner cover layer, resulting in low spin rate and poor feel on approach shots.
In Comparative Example 11, the outer cover layer had a higher rebound resilience than the inner cover layer, resulting in a slightly lower spin rate on approach shots.

G Golf ball 1 Core 2 Mid layer 3 Inner cover layer 4 Outer cover layer 5 Paint layer D Dimples

Claims (12)

A golf ball having a single-layer or multi-layer core, an intermediate layer, and a cover, wherein the cover is composed of two layers, an inner cover layer and an outer cover layer, the inner cover layer having a material hardness of 20 to 40 Shore D hardness and the outer cover layer having a material hardness of 35 to 55 Shore D hardness, the total thickness of these two layers being 0.6 to 1.2 mm, and wherein the rebound resilience (%) of the inner cover layer material based on JIS-K 6255 is (A), and the rebound resilience (%) of the outer cover layer material based on JIS-K 6255 is (B), the golf ball satisfies the relationship 0 < (A) - (B) < 18. The golf ball of claim 1, wherein the resin component of the material for the outer cover layer is a thermoplastic polyurethane elastomer. The golf ball according to claim 1 or 2, wherein the resin component of the material for the inner cover layer is a thermoplastic elastomer. The golf ball of claim 1 or 2, wherein the resin component of the inner cover layer is a multicomponent copolymer having conjugated diene units, non-conjugated olefin units, and aromatic vinyl units, the conjugated diene units include butadiene units, the non-conjugated olefin units include units selected from the group consisting of ethylene units, propylene units, and 1-butene units, the aromatic vinyl units include styrene units, and the content of the conjugated diene units in the multicomponent copolymer is 5% by mass or greater. The golf ball of claim 4, wherein the content of the conjugated diene units in the multicomponent copolymer is 10% by mass or more. The golf ball of claim 4, wherein the content of the non-conjugated olefin units in the multicomponent copolymer is 90% by mass or less. The golf ball of claim 4, wherein the content of the aromatic vinyl units in the multicomponent copolymer is 30% by mass or less. The golf ball of claim 4, wherein the non-conjugated olefin units are ethylene units. The golf ball according to claim 4, wherein the multicomponent copolymer is a copolymer polymerized using a gadolinium metallocene complex catalyst. The golf ball according to claim 1 or 2, wherein the thickness of the inner cover layer is 0.1 to 0.5 mm. 3. The golf ball of claim 1, wherein the thicknesses of the intermediate layer, inner cover layer, and outer cover layer satisfy the following relationship: intermediate layer thickness > inner cover thickness < outer cover thickness. The golf ball according to claim 1 or 2, wherein the ratio of the thickness (a) of the inner cover layer to the thickness (b) of the outer cover layer is (a)/(b) = 0.10 to 0.60.