Multipiece solid golf ball

Abstract

To provide a golf ball excellent in carry performance and controllability at the time of approach, and further excellent in hitting durability.SOLUTION: The present invention relates to a multipiece solid golf ball comprising a two-layer overall core having an inner core and an outer core, an intermediate layer, and a cover, the outer surface of the cover being formed with a large number of dimples, the inner core and the outer core being formed from a rubber composition, the intermediate layer and the cover being formed from a resinous material, and the hardness difference (JIS-C hardness) A between the surface and center of the overall core being 31 or more between the surfaces, wherein when the JIS-C hardness at a point 0.5 mm radially outward from the boundary between the inner core and outer core is defined as X and the JIS-C hardness at a point 0.5 mm radially inward from the boundary between the inner core and outer core is defined as Y, the following two equations are satisfied. 3.0<X-Y<7.9, and A / (X-Y)≥4.0.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a multi-piece solid golf ball including a two-layer core (overall core) composed of an inner layer and an outer layer, an intermediate layer, and a cover, and having a large number of dimples formed on the outer surface of the cover.

Background Art

[0002] Conventionally, in order to increase the flight distance and improve the hitting feeling and controllability during approach, efforts have been made to design the ball structure into a multi-layer structure, and various multi-layer golf balls have been proposed with a two-layer structure not only for the cover but also for the core structure. Examples of golf balls having a two-layer core composed of an inner layer and an outer layer include the following Patent Documents 1 to 7.

[0003] Patent Document 1 describes a golf ball in which the hardness distribution and outer diameter of a two-layer core are specified from the viewpoint of achieving various performances. That is, in the golf ball described in Patent Document 1, an elastic rubber having an inner layer and an outer layer is used as a core material, and the hardness of the core material decreases from the surface of the outer layer of the core material toward the center of the inner layer of the core material, and the hardness difference at the interface between the inner layer and the outer layer is within 7 in Shore D hardness. Further, Patent Documents 2 to 7 describe that the hardness difference at the interface of the two-layer core is 0 or more in Shore C hardness.

[0004] However, the golf balls of Patent Documents 1 to 7 do not sufficiently specify the relationship between the hardness relationship near the boundary of the two-layer core and the hardness gradient of the entire two-layer core in terms of their specific configurations. Further, the golf balls described in these documents are not golf balls that satisfy all performances such as controllability during approach, flight distance when hit with an iron such as a 6-iron (I♯6), and crack durability.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention has been made in view of the above circumstances, and an object thereof is to provide a golf ball having good flight distance performance during iron hitting, maintaining good controllability during approach, and excellent crack durability.

Means for Solving the Problems

[0007] As a result of intensive studies to achieve the above object, the inventors of the present invention focused on optimizing the hardness gradient of the entire two-layer core and the vicinity of the boundary of a golf ball having a two-layer overall core including an inner core and an outer core. Specifically, the hardness difference between the surface and the center of the entire core is 31 or more in JIS-C hardness, and this hardness difference is A, the JIS-C hardness at a point 0.5 mm outside in the radial direction from the boundary between the inner core and the outer core is X, and the JIS-C hardness at a point 0.5 mm inside is Y. When, the following two formulas 3.0 < X - Y < 7.9 A / (X - Y) ≧ 4.0 By satisfying the above, while maintaining good controllability during approach, the flight distance performance during iron hitting such as with a 6-iron is high, and it has been found that the crack durability is excellent, and the present invention has been completed.

[0008] Accordingly, the present invention provides the following multi-piece solid golf ball. 1. A multi-piece solid golf ball comprising a two-layer overall core having an inner core and an outer core, an intermediate layer, and a cover, wherein a large number of dimples are formed on the outer surface of the cover. The inner core and the outer core are formed of a rubber composition, the intermediate layer and the cover are formed of a resin material, the hardness difference (JIS-C hardness) A between the surface and the center of the overall core is 31 or more, and when the JIS-C hardness at a point 0.5 mm outside in the radial direction from the boundary between the inner core and the outer core is X and the JIS-C hardness at a point 0.5 mm inside is Y, the following two formulas 3.0 < X - Y < 7.9 A / (X - Y) ≧ 4.0 A multi-piece solid golf ball characterized by satisfying the above. 2. The inner core is formed of a rubber composition containing (a) a base rubber and (b) an organic peroxide, and when the blending amount of the (b) component with respect to 100 parts by mass of the (a) component is M and the half-life temperature of the (b) component for 1 minute is T, (M / T) × A ≧ 0.25 The golf ball according to Item 1 above, which satisfies the above. 3. The multi-piece solid golf ball according to Item 2 above, wherein the (b) organic peroxide is an organic peroxide having a half-life temperature of less than 160°C for 1 minute, and the blending amount thereof is 1.0 part by mass or more with respect to 100 parts by mass of the (a) component. 4. The multi-piece solid golf ball according to Item 1 or 2 above, wherein the diameter of the inner core is 20 to 25 mm. 5. The multi-piece solid golf ball according to Item 1 or 2 above, wherein the Shore D hardness of the resin material of the cover is 43 or less. 6. When the volume occupancy rate (%) of the dimples is VR and the Shore D hardness of the resin material of the cover is H, VR × H ≧ 27 The multi-piece solid golf ball according to Item 1 or 2 above, which satisfies the above. 7. The multi-piece solid golf ball according to claim 1 or 2, wherein when a load is applied to the golf ball from an initial load of 98 N (10 kgf) to a final load of 1,275 N (130 kgf), the amount of deflection (mm) is 2.6 mm or less. 8. When the volume occupancy rate (%) of the dimples is VR and the Shore D hardness of the resin material of the cover is H, {A / (X - Y)} × VR × H ≧ 150 The multi-piece solid golf ball according to claim 1 or 2, which satisfies the above condition.

Advantages of the Invention

[0009] According to the golf ball of the present invention, while maintaining good controllability during approach, it has high flight distance performance during iron shots such as with a 6-iron, and is excellent in crack durability.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0011] Hereinafter, the present invention will be described in more detail. As shown in FIG. 1, the multi-piece solid golf ball of the present invention has an overall core 1 having an inner core 10 and an outer core 20, an intermediate layer 2 covering the core, and a cover 3 covering the intermediate layer. This cover 3 is located in the outermost layer in the layer structure of the golf ball except for the paint layer. The intermediate layer can be formed not only as a single layer as shown in FIG. 1 but also as a plurality of layers. A large number of dimples D are formed on the surface of the cover (outermost layer) 3 in order to make the aerodynamic characteristics the target characteristics of the present invention. In addition, although not particularly shown, a paint layer is usually formed on the surface of the cover 3. Each of the above layers will be described in detail below.

[0012] In the present invention, the overall core is formed of two layers, an inner core and an outer core. In the following description, the overall core composed of the inner core and the outer core may be simply referred to as the "core".

[0013] Both the inner core and the outer core are obtained by vulcanizing a rubber composition mainly composed of a rubber material. The inner core preferably has a high resilience and a formulation of a rubber composition suitable for achieving the target hardness distribution of the present invention.

[0014] The rubber material of the outer core surrounding the inner core may be of the same type or different type as the material of the inner rubber. Both the inner core and the outer core are mainly composed of a base rubber, and a rubber composition can be prepared by blending a co-crosslinking agent, an organic peroxide, an inert filler, an organic sulfur compound, etc. with this.

[0015] The base rubber can include a diene rubber. Examples of this diene rubber include polybutadiene, natural rubber, isoprene rubber, ethylene propylene diene rubber, etc. It is preferable to use polybutadiene as the base rubber.

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

[0017] The co-crosslinking agent is α,β-unsaturated carboxylic acid and / or its metal salt. Specific examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid, fumaric acid, etc., and acrylic acid and methacrylic acid are particularly preferably used. The metal salt of the unsaturated carboxylic acid is not particularly limited, but examples include those obtained by neutralizing the above unsaturated carboxylic acid with a desired metal ion. Specific examples include zinc salts and magnesium salts of methacrylic acid, acrylic acid, etc., and zinc acrylate is particularly preferably used.

[0018] Regarding the above unsaturated carboxylic acid and / or its metal salt, in the case of the inner layer core, usually 7 parts by mass or more, preferably 10 parts by mass or more, more preferably 13 parts by mass or more, and the upper limit is usually 25 parts by mass or less, preferably 20 parts by mass or less, more preferably 16 parts by mass or less are blended based on 100 parts by mass of the base rubber. If this blending amount is too large, it may become too hard and the spin will increase during iron hitting, and the desired flying distance may not be obtained. On the other hand, if this blending amount is too small, the resilience may decrease or the crack durability due to repeated hitting may deteriorate. In the case of the outer layer core, usually 30 parts by mass or more, preferably 35 parts by mass or more, more preferably 38 parts by mass or more, and the upper limit is usually 50 parts by mass or less, preferably 45 parts by mass or less, more preferably 42 parts by mass or less are blended based on 100 parts by mass of the base rubber. If this blending amount is too large, it may become too hard and the hitting feeling may deteriorate or the crack durability due to repeated hitting may deteriorate. On the other hand, if this blending amount is too small, the spin may increase during iron hitting and the desired flying distance may not be obtained.

[0019] The organic peroxide is a crosslinking initiator. Examples of the organic peroxide include dicumyl peroxide ("Perkyl D" manufactured by NOF Corporation), 1,1-di(t-butylperoxy)cyclohexane ("Perhexa C" manufactured by NOF Corporation), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane ("Perhexa 25B" manufactured by NOF Corporation), di(2-t-butylperoxyisopropyl)benzene ("Perbutyl P" manufactured by NOF Corporation), etc. Dicumyl peroxide can be preferably used. Other commercially available products include "Perhexa C-40", "Niper BW", "Peroyl L", etc. (all manufactured by NOF Corporation), or Luperco 231XL (manufactured by Atochem), etc. These may be used alone or in combination of two or more.

[0020] Regarding the rubber composition of the inner layer core, it is particularly preferable to use an organic peroxide having a half-life temperature of less than 160°C for 1 minute. Specifically, 1,1-di(t-butylperoxy)cyclohexane ("Perhexa C" manufactured by NOF Corporation) having a half-life temperature of 154°C for 1 minute and 1,1-di(t-hexylperoxy)cyclohexane ("Perhexa HC" manufactured by NOF Corporation) having a half-life temperature of 149°C for 1 minute can be mentioned. If the half-life temperature of the organic peroxide is 160°C or higher, a desired hardness gradient in the inner layer core cannot be obtained, and the durability may deteriorate. When using two or more organic peroxides, it is not necessary that the half-life temperature of all the organic peroxides is less than 160°C for 1 minute, and at least one type of organic peroxide only needs to satisfy the above-mentioned predetermined half-life.

[0021] The above-mentioned organic peroxide is preferably 1.0 part by mass or more, more preferably 1.2 parts by mass or more, and still more preferably 1.5 parts by mass or more, based on 100 parts by mass of the above-mentioned base rubber. The upper limit is preferably 5 parts by mass or less, more preferably 4 parts by mass or less, and still more preferably 3 parts by mass or less. If the blending amount is small, a desired hardness gradient in the inner layer core cannot be obtained, and the durability may deteriorate.

[0022] Regarding the rubber composition of the inner core, when the compounding amount of the organic peroxide with respect to 100 parts by mass of the base rubber is M, the one-minute half-life temperature of the organic peroxide, and the value obtained by subtracting the core surface hardness from the core center hardness is A, (M / T)×A≧0.25 it is preferable to satisfy this condition. The larger the value of the formula (M / T)×A, the lower the spin of the ball during a full shot, and the better the flight distance performance. Conversely, the smaller the value, the higher the spin and the worse the flight distance performance.

[0023] As the inert filler, for example, zinc oxide, barium sulfate, calcium carbonate, etc. can be preferably used. These may be used alone or in combination of two or more. The compounding amount of this filler can be preferably 5 parts by mass or more with respect to 100 parts by mass of the base rubber. Also, the upper limit value of this compounding amount can be preferably 50 parts by mass or less, more preferably 40 parts by mass or less, still more preferably 35 parts by mass or less with respect to 100 parts by mass of the base rubber. If the compounding amount is too large or too small, it may not be possible to obtain an appropriate mass and suitable resilience.

[0024] Furthermore, an antioxidant can be compounded as needed. For example, commercially available products such as Nocrack NS-6, NS-30, 200, MB (manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.), Yoshinox 425 (manufactured by Yoshitomi Pharmaceutical Industries, Ltd.) can be adopted. These may be used alone or in combination of two or more.

[0025] Regarding the compounding amount of the antioxidant, there is no particular limitation. However, with respect to 100 parts by mass of the base rubber, it is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more. The upper limit value is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, still more preferably 1 part by mass or less, and most preferably 0.5 parts by mass or less. If the compounding amount is too large or too small, an appropriate core hardness gradient may not be obtained, and suitable resilience and durability may not be achieved.

[0026] The organic sulfur compound can be compounded in either or both of the inner core and the outer core to impart good resilience. Specifically, it is recommended to compound thiophenol, thionaphthol, halogenated thiophenol or their metal salts. More specifically, zinc salts such as pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, parachlorothiophenol, pentachlorothiophenol, etc., diphenyl polysulfide, dibenzyl polysulfide, dibenzoyl polysulfide, dibenzothiazoyl polysulfide, ditthiobenzoyl polysulfide, etc. with 2 to 4 sulfur atoms can be mentioned. In particular, zinc salts of pentachlorothiophenol and diphenyldisulfide can be preferably used.

[0027] The compounding amount of the organic sulfur compound is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, based on 100 parts by mass of the above base rubber. The upper limit value is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 2.5 parts by mass or less. If this compounding amount is too large, the improvement effect of resilience (especially impact by the driver (W#1)) cannot be expected further, and the core may become too soft or the hitting feeling may deteriorate. On the other hand, if this compounding amount is too small, the improvement effect of resilience cannot be expected.

[0028] The diameter of the inner core is usually 20 mm or more, preferably 21 mm or more, and more preferably 22 mm or more. The upper limit value of this diameter is preferably 30 mm or less, and more preferably 25 mm or less. If the diameter of the inner core is too small, the actual hitting initial velocity may be low during a full shot, or the low spin effect may be insufficient and the aimed flight distance may not be obtained during iron hitting. On the other hand, if the diameter of the inner core is too large, the crack durability during repeated hitting may deteriorate, or the low spin effect during a full shot may be insufficient and the aimed flight distance may not be obtained.

[0029] With respect to the inner core, the amount of deflection (mm) when a load is applied from an initial load of 98 N (10 kgf) to a final load of 1,275 N (130 kgf) is not particularly limited, but is preferably 5.0 mm or more, more preferably 5.5 mm or more, still more preferably 6.0 mm or more. As the upper limit value, it is preferably 7.2 mm or less, more preferably 6.8 mm or less, and still more preferably 6.3 mm or less. If the amount of deflection of the inner core is too small, that is, too hard, the spin amount at full shot may increase too much and the flying distance at iron strike may not be sufficient, or the hitting feeling may become too hard. On the other hand, if the amount of deflection of the inner core is too large, that is, too soft, the hitting feeling at full shot may become too soft, or the crack durability at repeated strikes may deteriorate.

[0030] The component of the rubber material of the outer core covering the inner core may be the same type or different type from the material of the inner core. However, since the target hardness of the inner core and the outer core is different, the component ratios of the two are different.

[0031] The thickness of the outer layer covering the inner core is preferably 4.2 mm or more, more preferably 6.7 mm or more, in order to obtain the hardness distribution aimed at in the present invention. Also, the upper limit value is preferably 8.8 mm or less, more preferably 8.5 mm or less, and still more preferably 8.0 mm or less.

[0032] The method for manufacturing the inner core is not particularly limited, but the inner core can be formed by a method such as heating and compressing at 140 to 180 °C for 8 to 30 minutes according to a conventional method to form a spherical shape. As a method for forming the outer core on the surface of the inner core, a method can be adopted in which a pair of half cups are formed using a sheet-like unvulcanized rubber, the inner core is placed in this cup and further encapsulated, and then pressure heating and molding are performed. For example, after primary vulcanization (semi-vulcanization) to produce a pair of hemispherical cup bodies, then, the inner core with the previously fabricated outer core coated thereon is placed on one hemispherical cup body, and secondary vulcanization (full vulcanization) is performed with the other hemispherical cup body covering it. Or, the rubber composition is made into a sheet in an unvulcanized state to create a pair of sheets for the outer core, the sheets are molded by a half mold provided with hemispherical protrusions to produce unvulcanized hemispherical cup bodies, and then these pair of hemispherical cup bodies are covered on the previously fabricated inner core and heated and compressed at 140 to 180 °C for 8 to 30 minutes to form a spherical shape, so that a method in which the vulcanization process is divided into two steps can be preferably adopted.

[0033] The diameter of the core (entire core) is usually 37.5 mm or more, preferably 38.0 mm or more, more preferably 38.4 mm or more. The upper limit value of this diameter is preferably 40.0 mm or less, more preferably 39.4 mm or less, and even more preferably 38.8 mm or less. If the diameter of the core is too small, the initial ball speed may be too low, or the hardness of the entire ball may become hard (the amount of deflection becomes small), and the desired flight distance may not be obtained during iron hitting. On the other hand, if the diameter of the core is too large, the amount of spin during a full shot may increase and the desired flight distance during iron hitting may not be obtained, or the crack durability during repeated hitting may deteriorate.

[0034] Next, the hardness distribution of the core (entire core) will be described. The hardness of the core described below means JIS-C hardness, and it is the hardness value measured according to the JIS K6301-1975 standard. Regarding the cross-sectional hardness at the center of the core and at predetermined positions of each core, the outer core including the inner core is cut into a hemispherical shape to make the cross-section flat, and the needle of the hardness tester is vertically pressed against the measurement part for measurement.

[0035] The core center hardness is preferably 49 or more, more preferably 51 or more, still more preferably 53 or more, and the upper limit value is preferably 61 or less, more preferably 59 or less, still more preferably 57 or less. If this value is too large, the spin amount at full shot will increase, and the desired flight distance at iron hitting cannot be obtained, or the hitting feeling may become too hard. On the other hand, if the above value is too small, the resilience will be low and the aimed flight distance at iron hitting cannot be obtained, or the crack durability when hitting repeatedly may deteriorate, or the slice will be large at iron hitting, and the needs of professionals and advanced players may not be satisfied.

[0036] The surface hardness of the core is preferably 80 or more, more preferably 82 or more, still more preferably 85 or more, and as the upper limit value, it is preferably 95 or less, more preferably 93 or less, still more preferably 90 or less.

[0037] The value obtained by subtracting the core center hardness from the surface hardness of the core is preferably set to 31 or more, preferably 28 or more, more preferably 25 or more. On the other hand, the upper limit value is preferably 42 or less, more preferably 40 or less. In order to increase the flight distance when hitting a full shot with a driver (W#1) or a 6-iron, it is necessary to increase the core hardness gradient, but if it is too large, the durability at the time of hitting will decrease. From these viewpoints, it is desirable to set it within the above numerical range.

[0038] In the present invention, when the JIS-C hardness at a point 0.5 mm outside in the radial direction from the boundary between the inner core and the outer core is X and the JIS-C hardness at a point 0.5 mm inside is Y, the following two equations 3.0 < X - Y < 7.9 ···(1) A / (X - Y) ≧ 4.0 ···(2) It is characterized by satisfying the following. Here, the boundary and hardness of the two-layer core will be described with reference to FIG. 2. In FIG. 2, reference numeral 1 represents the entire core, 10 represents the inner core, and 20 represents the outer core. A point 0.5 mm outside the boundary line 30 is 20a, and the JIS-C hardness at that point is X. Also, a point 0.5 mm inside the boundary line 30 is 10a, and the JIS-C hardness at that point is Y.

[0039] In the above formula (1), the value of X - Y represents the hardness difference near the boundary of the two-layer core, and shows a value greater than 3.0 and less than 7.9. Increasing the difference in the hardness gradient at the interface between the inner core and the outer core makes it more likely to crack, while decreasing it results in insufficient ball performance. Therefore, it is desirable to set it within the above numerical range from these viewpoints. The preferable lower limit value of the value of X - Y is 3.2 or more, more preferably 3.5 or more, and the upper limit value is preferably 7.5 or less, more preferably 7.0 or less.

[0040] Also, in the above formula (2), the value of A / (X - Y) represents the value obtained by dividing the hardness difference from the surface to the center of the two-layer core (i.e., the value contributing to the spin performance in a full shot) by the hardness difference near the boundary of the two-layer core (i.e., the value contributing to the durability of the core), and it is required to be 4.0 or more. When this value is large, the spin amount in a full shot tends to be small and the flight distance performance tends to be good, and the durability also tends to be good. On the other hand, when this value is small, the spin amount in a full shot becomes excessive, and the durability also deteriorates. The preferable lower limit value of the value of A / (X - Y) is 5.0 or more, more preferably 6.0 or more, and the upper limit value is preferably 15.0 or less, more preferably 12.0 or less.

[0041] Next, the intermediate layer will be described. The hardness of the material of the intermediate layer is not particularly limited, but in terms of Shore D hardness, it is preferably 61 or more, more preferably 63 or more, still more preferably 65 or more, and as the upper limit value, it is preferably 72 or less, more preferably 70 or less, still more preferably 67 or less. If the hardness of the material of the intermediate layer is too soft compared to the above range, when full shot is performed, too much spin amount may be applied or the actual hitting initial velocity may become low, and the flying distance during iron hitting may not be achieved. On the other hand, if the hardness of the material of the intermediate layer is too hard compared to the above range, the crack durability during repeated hitting may deteriorate, or the hitting feeling during putter or short approach execution may become hard, or it may be difficult to apply spin when approaching.

[0042] The thickness of the intermediate layer is preferably 0.9 mm or more, more preferably 1.0 mm or more, still more preferably 1.1 mm or more. On the other hand, as the upper limit value of the thickness of the intermediate layer, it is preferably 1.6 mm or less, more preferably 1.4 mm or less, still more preferably 1.3 mm or less. Also, it is preferable that the thickness of the intermediate layer is thicker than the cover described later. If the thickness of the intermediate layer is out of the above range or thinner than the cover, the low spin effect of the ball during full shot may be insufficient and the aimed flying distance during iron hitting may not be achieved. Also, if the intermediate layer is too thin, the crack durability during repeated hitting may deteriorate. On the other hand, if the thickness of the intermediate layer is too thick compared to the above range, the hitting feeling may deteriorate.

[0043] Regarding the material of the intermediate layer, it is preferable to adopt an ionomer resin as the main material. When an ionomer resin is adopted as the main material, it is desirable to use a mixture of a zinc-neutralized ionomer resin and a sodium-neutralized ionomer resin as the main material. The blending ratio is 5 / 95 to 95 / 5, preferably 10 / 90 to 90 / 10, more preferably 15 / 85 to 85 / 15 in terms of zinc-neutralized / sodium-neutralized (mass ratio). If the Zn-neutralized ionomer and the Na-neutralized ionomer are not included within this ratio, the repulsion may become too low, resulting in no flying distance during the driver (W#1) strike and iron strike of the average hitter. Furthermore, the crack durability during repeated strikes at normal temperature may deteriorate, or the crack durability at low temperature (sub-zero) may deteriorate.

[0044] Also, as the ionomer resin material, it is preferable to include a high acid content ionomer resin with an unsaturated carboxylic acid content (also referred to as "acid content") of 16% by mass or more.

[0045] Also, the content rate of the high acid content ionomer resin is preferably 25% by mass or more, more preferably 50% by mass or more, still more preferably 75% by mass or more with respect to 100% by mass of the resin material. As the upper limit, it is preferably 100% by mass or less, more preferably 90% by mass or less, still more preferably 85% by mass or less. If the blending amount of the above high acid content ionomer resin is too small, the spin amount of the ball during a full shot may increase, and the flying distance may not be achieved. On the other hand, if the blending amount of the above high acid content ionomer resin is too large, the repeated strike durability may deteriorate.

[0046] Any additive can be appropriately blended into the intermediate layer material according to the application. For example, various additives such as pigments, dispersants, anti-aging agents, ultraviolet absorbers, and light stabilizers can be added. When these additives are blended, the blending amount is preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more with respect to 100 parts by mass of the base resin, and as the upper limit, it is preferably 10 parts by mass or less, more preferably 4 parts by mass or less.

[0047] Regarding the intermediate layer material, it is preferable to polish the surface of the intermediate layer in order to enhance the adhesion with the polyurethane that is preferably used as the cover material described later. Further, it is preferable to apply a primer (adhesive) to the surface of the intermediate layer after the polishing treatment, or to add an adhesion enhancer to the material.

[0048] Next, the cover will be described. The hardness of the cover material is not particularly limited, but in terms of Shore D hardness, it is preferably 30 or more, more preferably 35 or more, and as the upper limit value, it is preferably 48 or less, more preferably 45 or less, and even more preferably 43 or less. If the hardness of the cover material is too soft compared to the above range, the spin amount may increase during a full shot, and the flying distance during iron hitting may not be achieved. On the other hand, if the hardness of the cover material is too hard compared to the above range, the spin performance during approach may deteriorate.

[0049] The thickness of the cover is preferably 0.3 mm or more, more preferably 0.5 mm or more, and even more preferably 0.6 mm or more. On the other hand, as the upper limit value of the thickness of the cover, it is preferably 1.2 mm or less, more preferably 0.9 mm or less, and even more preferably 0.8 mm or less. If the above cover is too thick, the rebound property of the ball may be insufficient during a full shot, or the spin amount may increase, resulting in the flying distance during iron hitting not being achieved. On the other hand, if the above cover is too thin, the scratch resistance may deteriorate, or the spin during approach may not be applied and the controllability may be insufficient.

[0050] As the above cover material, from the viewpoints of spin controllability and scratch resistance in short games, various urethane resins used for the cover material of golf balls can be used. Further, from the viewpoint of mass productivity, it is preferable to use a resin material mainly composed of thermoplastic polyurethane. Furthermore, it is preferably formed by a resin composition mainly composed of (I) thermoplastic polyurethane and (II) polyisocyanate compound.

[0051] It is recommended that the total mass of the above components (I) and (II) combined be 60% or more, more preferably 70% or more, based on the total amount of the resin composition of the cover. The above components (I) and (II) will be described in detail below.

[0052] Regarding the above (I) thermoplastic polyurethane, its structure includes a soft segment composed of a high molecular polyol (polymeric glycol) which is a long-chain polyol, and a hard segment composed of a chain extender and a polyisocyanate compound. Here, as the raw long-chain polyol, any of those conventionally used in technologies related to thermoplastic polyurethane can be used and is not particularly limited. For example, polyester polyol, polyether polyol, polycarbonate polyol, polyester polycarbonate polyol, polyolefin-based polyol, conjugated diene polymerization-based polyol, castor oil-based polyol, silicone-based polyol, vinyl polymerization-based polyol, etc. can be mentioned. These long-chain polyols may be used alone or in combination of two or more. Among these, polyether polyol is preferable in that a thermoplastic polyurethane with a high resilience modulus and excellent low-temperature properties can be synthesized.

[0053] As the chain extender, those used in conventional technologies related to thermoplastic polyurethane can be preferably used. For example, it is preferably a low molecular compound with a molecular weight of 400 or less having two or more active hydrogen atoms capable of reacting with isocyanate groups in the molecule. Examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, 2,2-dimethyl-1,3-propanediol, etc., but are not limited thereto. Among these, aliphatic diols having 2 to 12 carbon atoms are preferable as the chain extender, and 1,4-butylene glycol is more preferable.

[0054] As the polyisocyanate compound, those used in the technologies related to conventional thermoplastic polyurethanes can be preferably used, and there is no particular limitation. Specifically, one or more selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4-(or)2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, naphthylene 1,5-diisocyanate, tetramethylxylylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, 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. In the present invention, from the viewpoint of the balance between the stability during production and the physical properties exhibited, 4,4'-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferred.

[0055] As the specific thermoplastic polyurethane of component (I), commercially available products can also be used. For example, Pandex T8295, T8290, T8260 (all manufactured by DIC Covestro Polymer Co., Ltd.) and the like can be mentioned.

[0056] Although not an essential component, a thermoplastic elastomer other than the above-mentioned thermoplastic polyurethane can be blended as component (III), which is another component, with the above-mentioned components (I) and (II). By blending this component (III) into the resin blend, various physical properties required for a golf ball cover material, such as further improvement in the fluidity, resilience, and scratch resistance of the resin blend, can be enhanced.

[0057] Although there are no particular restrictions on the composition ratios of the above components (I), (II), and (III), in order to sufficiently and effectively exhibit the effects of the present invention, it is preferably (I):(II):(III) = 100:2 to 50:0 to 50 by mass ratio, and more preferably (I):(II):(III) = 100:2 to 30:8 to 50 (mass ratio).

[0058] Furthermore, various additives other than the components constituting the above thermoplastic polyurethane can be blended into the above resin composition as necessary. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers, mold release agents, etc. can be appropriately blended.

[0059] Regarding the manufacturing method of the multi-piece solid golf ball formed by laminating each of the above-described core, intermediate layer, and cover (outermost layer), it can be carried out by a conventional method such as a known injection molding method. For example, an intermediate layer material can be injected with an injection molding die around the core to obtain an intermediate layer-coated spherical body, and finally, a multi-piece golf ball can be obtained by injecting the material of the cover, which is the outermost layer. Also, two half cups molded into a semi-shell shape can be prepared in advance, and a golf ball can be produced by wrapping the core and the intermediate layer-coated spherical body with these and performing heat and pressure molding.

[0060] For the golf ball, the deflection amount (mm) when a load is applied from an initial load of 98 N (10 kgf) to a final load of 1,275 N (130 kgf) is preferably 2.0 mm or more, more preferably 2.1 mm or more, and even more preferably 2.2 mm or more. On the other hand, as the upper limit value of the above deflection amount, it is preferably 2.7 mm or less, more preferably 2.6 mm or less, and even more preferably 2.5 mm or less. If the deflection amount of the golf ball is too small, that is, too hard, the spin amount may increase too much and the flying distance during iron hitting may not be achieved, or the hitting feeling may become too hard. On the other hand, if the above deflection amount is too large, that is, the above spherical body is too soft, the spin amount will decrease during iron hitting and it will be difficult to control the desired distance, the hitting feeling may become too soft, or the crack durability during repeated hitting may deteriorate.

[0061] A large number of dimples can be formed on the outer surface of the cover. There is no particular limitation on the dimples arranged on the cover surface, but preferably there are 280 or more, preferably 300 or more, more preferably 310 or more, and as the upper limit, it can preferably have 450 or less, more preferably 400 or less, and even more preferably 350 or less. If the number of dimples deviates from the above range, the flying distance during iron hitting may decrease.

[0062] Regarding the shape of the dimples, one type or a combination of two or more types such as circular, various polygonal, teardrop-shaped, and other elliptical shapes can be appropriately used. For example, when using circular dimples, the diameter can be about 2.5 mm or more and 6.5 mm or less, and the depth can be 0.08 mm or more and 0.30 mm or less.

[0063] The dimple occupancy rate of the dimple on the spherical surface of the golf ball, specifically, the ratio (SR value) of the total dimple area defined by the perimeter of the plane surrounded by the edge of the dimple to the spherical surface area of the ball assuming no dimples is preferably 75% or more, more preferably 80% or more, still more preferably 84% or more, and the upper limit is 90% or less, more preferably 88% or less, still more preferably 86% or less. If this SR value deviates from the above range, the carry distance during iron shots may be reduced.

[0064] The total dimple volume formed downward from the plane surrounded by the edge of the dimple, and the VR value of the total dimple volume occupied by the ball volume assuming no dimples is 0.78% or more, preferably 0.79% or more, more preferably 0.81% or more, and the upper limit is 0.89% or less, more preferably 0.88% or less, more preferably 0.87% or less. If this VR value is greater than the above range, the carry distance during driver (W#1) or iron shots may be too short. Also, in this case, the trajectory may be low, making it difficult to achieve carry and difficult to cross valleys or ponds. On the other hand, if the above value is too small, the ballooning during a full shot may be large, and conversely, there is a risk of losing carry distance.

[0065] Also, the value (VR×H) obtained by multiplying the above dimple volume occupancy rate VR (%) by the Shore D hardness H of the cover resin material is preferably 27 or more, more preferably 28 or more, still more preferably 29 or more. The technical meaning of this is that when using a soft cover with a low cover hardness, the spin amount of the ball during a full shot increases and the ballooning becomes large. Therefore, using dimples with a high VR value is more likely to obtain the desired effects of the present invention.

[0066] Furthermore, from the viewpoint of increasing the spin amount during approach and enhancing durability, it is desirable that the value obtained by multiplying the above VR×H by A / (X - Y) described above is 150 or more. That is, the following formula {A / (X - Y)}×VR×H≧150 is preferably satisfied.

[0067] The value V0 obtained by dividing the spatial volume of the under-plane dimples surrounded by the edge of each dimple by the cylindrical volume having the said plane as the bottom surface and the maximum depth of the dimple from this bottom surface as the height is preferably 0.35 or more, more preferably 0.38 or more, still more preferably 0.40 or more, and as the upper limit value, it is 0.80 or less, more preferably 0.70 or less, still more preferably 0.60 or less. When this V0 value deviates from the above range, the flight distance at the time of driver (W#1) hitting and iron hitting may become lower than the target.

[0068] In addition, the multi-piece solid golf ball of the present invention can conform to golf rules for competitive use, the outer diameter of the ball is of a size that does not pass through a ring with an inner diameter of 42.672 mm, and the mass can preferably be formed to be 45.0 to 45.93 g.

Examples

[0069] Hereinafter, examples and comparative examples will be shown to specifically explain the present invention, but the present invention is not limited to the following examples.

[0070] 〔Examples 1 to 11, Comparative Examples 1 to 10〕 Formation of the core Regarding Examples 4, 6, 7, 9 to 11 and Comparative Examples 1 to 11, after preparing the rubber compositions of each example shown in Table 1, inner layer cores were produced by vulcanization molding under the vulcanization conditions of each example shown in Table 1.

[0071] Regarding Examples 1 to 3, 5, 8, inner layer cores were produced based on the formulations in Table 1 in the same manner as above.

[0072]

Table 1

[0073]

Table 2

[0074] Details of the above formulation are as follows. · Polybutadiene: Trade name "BR T700" (manufactured by ENEOS MATERIALS Co., Ltd.) · Zinc acrylate: Trade name "ZN-DA85S" (manufactured by Nippon Shokubai Co., Ltd.) · Organic peroxide A: 1,1-di(t-butylperoxy)cyclohexane, trade name "Perhexa C" (manufactured by NOF Corporation), half-life temperature of 153.8 °C in 1 minute · Organic peroxide B: Dicumyl peroxide, trade name "Perkyl D" (manufactured by NOF Corporation), half-life temperature of 175.2 °C in 1 minute · Antioxidant (I): 2-mercaptobenzimidazole, trade name "No Crack MB" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) · Antioxidant (II): 2,2-methylenebis(4-methyl-6-butylphenol), trade name "No Crack NS-6" (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) · Zinc oxide: Trade name "Three Kinds of Zinc Oxide" (manufactured by Sakai Chemical Industry Co., Ltd.) · Zinc pentachlorothiophenolate: Manufactured by Fujifilm Wako Pure Chemical Corporation

[0075] 〔Method for forming outer core〕 For Examples 4, 6, 7, 9 to 11 and Comparative Examples 1 to 11, after preparing the above inner core, the rubber material for the outer core was placed in the outer core mold, sandwiched between a convex mold with the same radius as the inner core, heated under the conditions of 145 °C for 3 minutes for primary vulcanization, then removed from the mold to produce a pair of half-cup-shaped outer cores. These were placed on the previously vulcanized and molded inner core and vulcanized and molded under the conditions of 145 °C for 10 minutes to produce the overall core (inner core + outer core) for each example. After removing the core from the mold, the surface of the core was polished.

[0076] For Examples 1 to 3, 5, and 8, in the same manner as above, the overall core (inner core + outer core) was produced. After removing the core from the mold, the surface of the core was polished.

[0077] Formation of the intermediate layer and the cover (outermost layer) Next, for Examples 4, 6, 7, 9 to 11 and Comparative Examples 1 to 11, using an injection mold, around the core surface, injection molding was performed with Resin Material No.1 of the intermediate layer shown in Table 3 to form the intermediate layer. Next, using another injection mold, around the above intermediate layer-coated sphere, injection molding was performed with Resin Material No.3 or No.4 of the cover (outermost layer) shown in Table 3 to form the cover. At this time, a predetermined number of dimples described below are formed on the cover surface.

[0078] For Examples 1 to 3, 5, and 8, in the same manner as above, injection molding was performed with Resin Materials No.1 to No.4 shown in Table 3 to form the intermediate layer and the cover.

[0079]

Table 3

[0080] The details of the compounding components in Table 3 are as follows. "Himilan 1706", "AM7318", ionomers manufactured by Mitsui Dow Chemical Co., Ltd. "Trimethylolpropane" (TMP), manufactured by Tokyo Chemical Industry Co., Ltd. "TPU1", trade name "Pandex" manufactured by DIC Covestro Polymer Co., Ltd., ether-type thermoplastic polyurethane, material hardness (Shore D) "47" "TPU2", trade name "Pandex" manufactured by DIC Covestro Polymer Co., Ltd., ether-type thermoplastic polyurethane, material hardness (Shore D) "43" "TPU3", trade name "Pandex" manufactured by DIC Covestro Polymer Co., Ltd., ether-type thermoplastic polyurethane, material hardness (Shore D) "40"

[0081] Regarding the dimples of each example and comparative example, the following dimples (1) to (3) were used respectively. Each dimple mode includes 8 types of circular dimples numbered No. 1 to No. 8 with different diameters and depths. The details are shown in Table 4 below. Also, the arrangement modes (patterns) of dimples (1) to (3) are shown in Fig. 3. Fig. 3(A) shows the plan view of the dimples, and Fig. 3(B) is the side view thereof.

[0082]

Table 4

[0083] Definition of dimples Edge: The highest point in the cross-section passing through the center of the dimple Diameter: The diameter of the plane surrounded by the edge of the dimple Depth: The maximum depth of the dimple from the plane surrounded by the edge of the dimple SR: The ratio of the total dimple area defined by the plane surrounded by the edge of the dimple to the spherical area of the ball assuming no dimples Dimple volume: The dimple volume under the plane surrounded by the edge of the dimple Cylinder volume ratio: The ratio of the dimple volume to the volume of a cylinder with the same diameter and depth as the dimple VR: The ratio of the total dimple volume formed downward from the plane surrounded by the edge of the dimple to the ball volume of the ball assuming no dimples

[0084] For each obtained golf ball, various physical properties such as the internal hardness at each position of the core, the outer diameters of the core and each coated sphere, the thickness and material hardness of each layer, and the deflection amounts of the core and the ball were evaluated by the following methods and are shown in Tables 5 and 6.

[0085] Core hardness distribution The surface of the core is spherical. Set the needle of the hardness tester to be almost perpendicular to the spherical surface, and measure the surface hardness in JIS-C hardness according to JIS K6301-1975 standard. For the center and predetermined positions of the core, cut the core into a hemispherical shape to make the cross-section flat, press the needle of the hardness tester vertically against the center part and the predetermined positions shown in Table 5 and Table 6, and show the hardness of the center and each position as the value of JIS-C hardness. For the measurement of hardness, an automatic rubber hardness tester "P2" manufactured by Polymer Instruments Co., Ltd. equipped with a JIS-C type hardness tester is used. Read the maximum value of the hardness value. All measurements are made in an environment of 23 ± 2°C. Note that the values in the table are the values of JIS-C hardness.

[0086] Inner core, overall core, and outer diameter of the ball Adjust the temperature in a thermostatic bath at a temperature of 23.9 ± 1°C for at least 3 hours or more, measure 5 arbitrary surface locations in a room at 23.9 ± 2°C, take the average value as the measured value of each sphere, and obtain the average value with 10 measurement samples.

[0087] Amount of deflection of the inner core, overall core, and ball Place each coated sphere on a hard board, and measure the deflection amount when the load is increased from an initial load of 98 N (10 kgf) to a final load of 1275 N (130 kgf). Note that the above deflection amount is the measured value measured in a room at 23.9 ± 2°C after temperature adjustment in a thermostatic bath at a temperature of 23.9 ± 1°C for at least 3 hours or more. A high-load compression tester manufactured by Myu Seiki Co., Ltd. is used as the measuring instrument, and the down speed of the pressure head for compressing each coated sphere is 10 mm / s.

[0088] Material hardness of the intermediate layer and the cover (Shore D hardness) Mold the resin material of each layer into a sheet with a thickness of 2 mm and leave it for 2 weeks at a temperature of 23 ± 2°C. When measuring, 3 sheets are stacked. For the measurement of hardness, an automatic rubber hardness tester "P2" manufactured by Polymer Instruments Co., Ltd. equipped with a Shore D type hardness tester is used. Read the maximum value of the hardness value. The measurement method follows ASTM D2240 standard.

[0089] Evaluate the flight (I#6), controllability during approach, and crack durability of each golf ball using the following methods. The results are shown in Table 7.

[0090] Jump evaluation (I#6) Attach a 6-iron (I#6) to a golf hitting robot and measure the amount of backspin and flight distance (carry and total) when hitting at a head speed (HS) of 42 m / s. Use the "JGR Forged I#6 (2016 model)" manufactured by Bridgestone Sports Co., Ltd. for the club.

[0091] Judge by the amount of spin when attaching a sand wedge (SW) to a golf hitting robot and hitting at a head speed (HS) of 15 m / s. Measure the amount of spin of the ball immediately after hitting in the same way using an initial condition measuring device. Use the "TourStage TW-03 (loft angle 57°) 2002 model" manufactured by Bridgestone Sports Co., Ltd. for the sand wedge.

[0092] Evaluation of the amount of spin during approach Crack durability Evaluate the durability of the ball using the ADC Ball COR Durability Tester manufactured by Automated Design Corporation of the United States. This tester has a function of continuously colliding a golf ball against two parallel metal plates after firing it with air pressure. Set the incident speed on the metal plate to 43 m / s. Use 10 golf balls, measure the number of firings required until the balls crack, and evaluate according to the following criteria. ○ None of the 10 balls crack even after 150 or more firings. △ One or more cracked balls are included in the number of firings from 100 to 150 times. × One or more cracked balls are included in the number of firings of 100 times or less.

[0093]

Table 5

[0094]

Table 6

[0095] As shown in the results of Tables 5 and 6, the golf balls of Comparative Examples 1 to 10 are inferior to the product of the present invention (Example) in the following points. Comparative Example 1 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and also does not satisfy the formula "A / (X - Y) ≥ 4.0", and the crack durability is not good. Comparative Example 2 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and also does not satisfy the formula "A / (X - Y) ≥ 4.0", and the crack durability is poor. Comparative Example 3 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and the crack durability is not good. Comparative Example 4 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and the crack durability is not good. Comparative Example 5 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and the crack durability is poor. Comparative Example 6 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and also does not satisfy the formula "A / (X - Y) ≥ 4.0", and the crack durability is poor. Comparative Example 7 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and also does not satisfy the formula "A / (X - Y) ≥ 4.0", and the crack durability is poor. Comparative Example 8 does not satisfy the formula "3.0 < X - Y < 7.9" regarding the hardness near the boundary between the inner core and the outer core, and also does not satisfy the formula "A / (X - Y) ≥ 4.0", and the crack durability is poor. In Comparative Example 9, since the hardness difference A between the surface and the center of the entire core is small, the flying distance in the flying evaluation (I#6) is not good. Comparative Example 10 does not satisfy the formulas "3.0 < X - Y < 7.9" and "A / (X - Y) ≥ 4.0" regarding the hardness near the boundary between the inner core and the outer core. Since the hardness difference A between the surface and the center of the entire core is small, the flying distance in the flight evaluation (I#6) is not good.

Explanation of Signs

[0096] G Golf ball 1 Core (entire) 2 Intermediate layer 3 Cover D Dimple 10 Inner core 20 Outer core 30 Boundary line 10a Point 0.5 mm inside from the boundary 20a Point 0.5 mm outside from the boundary

Claims

1. A multi-piece solid golf ball comprising a two-layer overall core having an inner core and an outer core, an intermediate layer, and a cover, wherein a large number of dimples are formed on the outer surface of the cover, the inner core and the outer core are formed of a rubber composition, the intermediate layer and the cover are formed of a resin material, the hardness difference (JIS-C hardness) A between the surface and the center of the overall core is 31 or more, and when the JIS-C hardness at a point 0.5 mm outside in the radial direction from the boundary between the inner core and the outer core is X and the JIS-C hardness at a point 0.5 mm inside is Y, the following two equations 3.0 < X - Y < 7.9 A / (X - Y) ≥ 4.0 are satisfied, characterized in that it is a multi-piece solid golf ball.

2. The inner core is formed of a rubber composition containing (a) a base rubber and (b) an organic peroxide, and when the blending amount of the component (b) with respect to 100 parts by mass of the component (a) is M and the half-life temperature of the component (b) for 1 minute is T, (M / T) × A ≥ 0.25 The golf ball according to claim 1, which satisfies the above.

3. The above-mentioned (b) organic peroxide is an organic peroxide having a half-life temperature of less than 160 ° C for 1 minute, and its blending amount is 1.0 part by mass or more with respect to 100 parts by mass of the above-mentioned (a) component. The multi-piece solid golf ball according to claim 2.

4. The multi-piece solid golf ball according to claim 1 or 2, wherein the inner core has a diameter of 20 to 25 mm.

5. The multi-piece solid golf ball according to claim 1 or 2, wherein the Shore D hardness of the resin material of the cover is 43 or less.

6. When the volume occupancy rate (%) of the above-mentioned dimples is VR and the Shore D hardness of the resin material of the cover is H, VR × H ≥ 27 The multi-piece solid golf ball according to claim 1 or 2, which satisfies the above.

7. The multi-piece solid golf ball according to claim 1 or 2, wherein the amount of deflection (mm) when a load is applied to the golf ball from an initial load of 98 N (10 kgf) to a final load of 1,275 N (130 kgf) is 2.6 mm or less.

8. When the volume occupancy rate (%) of the above-mentioned dimples is VR and the Shore D hardness of the resin material of the cover is H, {A / (X - Y)} × VR × H ≥ 150 The multi-piece solid golf ball according to claim 1 or 2, which satisfies the above.

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