JP2019005549A - Golf ball - Google Patents

Abstract

To provide a golf ball having extremely high durability, having low reduction of resilience over time, capable of reducing a load to a production facility and securing preferable productivity rate.SOLUTION: There is provided a golf ball comprising a one-piece golf ball or core and a cover formed of one or plural layers, in which the one-piece golf ball or the core is formed by a heating molded product of a rubber composition in which (A)-(E) components are blended, in which (A) is a base material rubber, (B) is an organic peroxide, (C) is water, (D) is methacrylate metal salt, (E) is zinc oxide, and a blended amount of (D) component is 30-45 pts.mass to 100 pts.mass of the (A) components, a molar ratio of the (C) and (D) components satisfies the following formula 0.9<(C)/(D)<1.1, and initial speed Vof the core is 72.0 m/s or more and 75.0 m/s or less.SELECTED DRAWING: None

Description

  The present invention relates to a golf ball suitably used for a one-piece golf ball or a golf ball having a core and a single-layer or multi-layer cover, particularly a practice golf ball.

  In JP 2012-228252 A, when zinc methacrylate is used for the rubber composition of the core of the golf ball for practice, since sufficient durability cannot be ensured, a blend using methacrylic acid is proposed instead. . However, since methacrylic acid has the ability to corrode metals as an acid, it requires expensive capital investment, such as having to make it resistant to acid when designing production equipment. Also, safety must be taken into consideration in the process, and the burden on production is very large.

Japanese Patent Application Laid-Open No. 10-85367 proposes a technique for suppressing a decrease in core resilience by reducing the water present in a rubber composition for a golf ball as much as possible.
Japanese Patent Laid-Open No. 2002-102388 also attempts to add a powder rubber that has absorbed moisture to a rubber composition as one of means for reducing the rebound of a golf ball for practice at a low price (low price). ing.

  The above prior art refers to a method for reducing repulsion and a method for maintaining high durability. However, the above prior art does not mention that good productivity can be ensured by reducing the burden on the production facility.

JP 2012-228452 A Japanese Patent Laid-Open No. 10-85367 JP 2002-102388 A

  The present invention has been made in view of the above circumstances, has extremely high durability, has little decrease in resilience with time, and further reduces the burden on production facilities to ensure good productivity. An object of the present invention is to provide a golf ball that can be played.

  As a result of intensive studies to achieve the above object, the present inventors have determined that the core is made of (A) base rubber, (B) organic peroxide, (C) water, (D) metal methacrylate, And (E) By adopting a heat-molded product of a rubber composition containing the components (A) to (E) of zinc oxide, a core material that is hard to break and has little reduction in repulsion over time can be used. I found a way to get it safely without needing it. Then, when these core materials are applied to the whole one-piece golf ball or the core of the golf ball, the golf ball is found to be excellent in durability and hardly change in physical properties after long-term use. That led to

The present invention provides the following golf balls.
1. In a one-piece golf ball or a golf ball having a core and a cover of one or more layers, the one-piece golf ball or core includes the following components (A) to (E) (A) Base rubber (B) Organic peroxide ( C) Water (D) Metal salt of methacrylic acid (E) It is formed from a heat-molded product of a rubber composition containing zinc oxide, and the amount of component (D) is 100 mass of component (A). The molar ratio (C) / (D) of the components (C) and (D) is from the following formula: 0.9 <(C) / (D) <1.1.
And the initial velocity V _{0 of the} core is 72.0 m / s or more and 75.0 m / s or less.
2. In the golf ball after molding, when the core initial velocity was measured after peeling off the cover and V _0, the core initial velocity was measured 60 days after the measurement of the V ₀ and _{V 60, (V 0 -V 60} ) ≦ 0 2. The golf ball according to 1 above, wherein 8 is established.
3. 3. The golf ball according to 1 or 2, wherein a value of a JIS-C hardness difference obtained by subtracting the center hardness from the surface hardness of the core is 4 or more and 20 or less.
4. A golf ball according to any one of 1 to 3, wherein the golf ball has a cover of one layer or a plurality of layers, wherein at least one cover layer is formed using urethane resin as a main material.
5. 5. The golf ball according to any one of 1 to 4, wherein methacrylic acid is not contained in the volatile gas when the core is heated at 300 ° C.

  The golf ball of the present invention has little decrease in resilience over time, maintains very high durability, and further reduces the burden on equipment in production, and can ensure good productivity.

Hereinafter, the present invention will be described in more detail.
The present invention relates to a one-piece golf ball, or a golf ball having a core and a single-layer or multi-layer cover, wherein the one-piece golf ball or core comprises the following components (A) to (E): (A) base rubber (B) organic Peroxide (C) Water (D) Metal methacrylate (E) It is formed by hot molding of a rubber composition containing zinc oxide.

  The base rubber of the component (A) is not particularly limited, but it is particularly preferable to use polybutadiene.

  The polybutadiene preferably has 60% or more, preferably 80% or more, more preferably 90% or more, and most preferably 95% or more of cis-1,4-bonds in the polymer chain. If there are too few cis-1,4-bonds in the bonds in the polybutadiene molecule, the resilience may be lowered.

  The content of 1,2-vinyl bond contained in the polybutadiene is usually 2% or less, preferably 1.7% or less, more preferably 1.5% or less in the polymer chain. If the content of 1,2-vinyl bond is too large, the resilience may be lowered.

The polybutadiene has (ML _{1 + 4} (100 ° C.)) of preferably 20 or more, more preferably 30 or more, and the upper limit is preferably 120 or less, more preferably 100 or less, and still more preferably 80 or less. is there.

The Mooney viscosity referred to in the present invention is an industrial viscosity index (JIS K6300) measured with a Mooney viscometer, which is a kind of rotational plasticity meter, and ML _{1 + 4} (100 ° C.) as a unit symbol. ) Is used. Further, M is Mooney viscosity, L is a large rotor (L type), 1 + 4 is a preheating time of 1 minute, and a rotor rotation time is 4 minutes, which is measured at 100 ° C.

  As the polybutadiene, those synthesized using a rare earth element-based catalyst or a Group VIII metal compound catalyst can be used. In the present invention, those synthesized using a rare earth element-based catalyst can be suitably used. Moreover, it is also optional to use these catalysts in combination with organoaluminum compounds, alumoxanes, halogen-containing compounds, Lewis bases, and the like as necessary. In the present invention, as the various compounds exemplified above, those described in JP-A No. 11-35633 can be preferably used.

  Among the rare earth element-based catalysts, use of a neodymium-based catalyst using a neodymium compound that is a lanthanum series rare earth element compound is particularly recommended. In this case, a high content of 1,4-cis bonds and a 1,2-vinyl bond are present. A low content of polybutadiene rubber can be obtained with excellent polymerization activity.

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

  Next, the organic peroxide (B) used in the present invention is not particularly limited, but it is preferable to use an organic peroxide having a 1-minute half-life temperature of 110 to 185 ° C. One or more organic peroxides can be used. The compounding amount of the organic peroxide is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more with respect to 100 parts by mass of the base rubber, and the upper limit is preferably 5 parts by mass. Part or less, more preferably 4 parts by weight or less, and still more preferably 3 parts by weight or less. Commercially available products can be used as the above-mentioned organic peroxides. Specifically, trade names “Park Mill D”, “Perhexa C-40”, “Niper BW”, “Parroyl L”, etc. For example, Luperco 231XL (manufactured by Atchem).

  In the present invention, as the component (C), water (including a material containing water) is directly blended into the core material, so that a part of zinc methacrylate in the core is ionized and added from the state. By starting the vulcanization reaction, a tougher vulcanizate can be obtained. Water also has the property of promoting the decomposition of organic peroxides, and if vulcanized at a high temperature, the amount of decomposed radicals will increase, causing recombination and inactivation between radicals. It will be. As a result, radicals that effectively work for crosslinking are reduced. The temperature near the core surface is almost the same as the temperature of the vulcanization mold, but the heat of decomposition of the organic peroxide decomposed from the outside accumulates near the core center, so it is much higher than the mold temperature. Therefore, the decomposition of the organic peroxide by water is promoted, and the hardness at the core center tends to be softer than that of the normal core composition. In addition, since the core which has more than necessary internal and external hardness difference is disadvantageous for durability, it is necessary to control vulcanization at an appropriate temperature.

  There is no restriction | limiting in particular about the water of (C) component, Although distilled water or tap water may be sufficient, Especially using distilled water which does not contain an impurity is employ | adopted suitably. The amount of water is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and the upper limit is preferably 5 parts by mass or less, based on 100 parts by mass of the base rubber. Preferably it is 3.5 mass parts or less. If the amount of water added to the rubber composition is too small, the initial speed cannot be kept low, and if it is too large, the core becomes too soft, and the core initial speed decreases more than necessary, which adversely affects durability. There is.

Although it is possible to add water directly to the rubber composition, the following methods (i) to (iii) can be employed.
(I) Method of applying mist-like water to all or part of rubber composition (compounding material) by steam or ultrasonic wave (ii) Method of immersing all or part of rubber composition in water (iii) Rubber composition A method in which all or part of the object is left in a high-humidity environment for a certain period of time in a humidity-controllable place such as a humidity chamber. Note that a high-humidity environment is an environment that can wet a rubber composition, etc. Although not limited, it is preferable that the humidity is 40 to 100%.

  Moreover, water can be processed into a jelly form and blended with the rubber composition. Alternatively, a material in which water is previously supported on a filler, unvulcanized rubber, rubber powder or the like can be used and blended with the rubber composition. Since such an aspect is more workable than directly adding water, the production efficiency of the golf ball can be improved. The type of material containing a predetermined amount of water is not particularly limited, but examples include sufficiently filled water, unvulcanized rubber, rubber powder, etc., particularly impairing durability and resilience. It is preferred to use materials without The water content of the above material is preferably 3% by mass or more, more preferably 5% by mass or more, further preferably 10% by mass or more, and the upper limit is preferably 99% by mass or less, more preferably 95% by mass. % Or less.

  In the rubber composition, in addition to the components (A) to (C) described above, (D) a metal salt of methacrylic acid and (E) zinc oxide are blended. If necessary, other compounding agents such as an anti-aging agent can be blended. These components are described in detail below.

(D) Metal salt of methacrylic acid metal salt Although the metal salt of unsaturated carboxylic acid is used as a kind of co-crosslinking agent, in the present invention, methacrylic acid is used as the α, β-unsaturated carboxylic acid, and the metal of the metal salt is Zinc is used.

  The zinc methacrylate as the component (D) preferably has an average particle size of 3 to 30 μm, more preferably 5 to 25 μm, and still more preferably 8 to 15 μm. If the average particle size of zinc methacrylate is less than 3 μm, the rubber composition tends to aggregate, and if the average particle size exceeds 30 μm, the co-crosslinking agent particles become too large, resulting in large variations in the characteristics of the resulting golf ball. . About the compounding quantity of the said component (D) zinc methacrylate, it is 30-45 mass parts with respect to 100 mass parts of base rubber. When the blending amount is less than the above range, the durability is deteriorated or becomes too soft, and when it is more than the above range, the durability is deteriorated or becomes too hard and the shot feeling is deteriorated.

About the molar ratio (C) / (D) of the said (C) component and (D) component, following formula 0.9 <(C) / (D) <1.1
Is required. When this molar ratio deviates from the above range, the durability of the golf ball is deteriorated.

(E) In the present invention, zinc oxide is used as a kind of zinc oxide inert filler. In the present invention, zinc oxide is essential, and other inert fillers such as inorganic fillers such as barium sulfate and calcium carbonate can be used in combination. (E) As a compounding quantity of the zinc oxide which is a component, Preferably it is 1 mass part or more with respect to 100 mass parts of base rubber, More preferably, it is 3 mass parts or more, More preferably, it is 5 mass parts or more, Is preferably 60 parts by mass or less, more preferably 40 parts by mass or less, still more preferably 30 parts by mass or less, and most preferably 15 parts by mass or less.

  In the present invention, an anti-aging agent can be blended in the rubber composition as necessary. For example, commercially available products such as Nocrack NS-6, NS-30, and 200 (manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.) Can be adopted. These may be used individually by 1 type and may use 2 or more types together. Although there is no restriction | limiting in particular about the compounding quantity of anti-aging agent, Preferably it is 0.1 mass part or more with respect to 100 mass parts of base rubbers, More preferably, it can be 0.15 mass part or more. Moreover, the upper limit becomes like this. Preferably it is 5.0 mass parts or less, More preferably, it is 4.0 mass parts or less, More preferably, it can be 3.0 mass parts or less. If the blending amount of the anti-aging agent is too much or too little, a suitable core hardness gradient may not be obtained, and suitable resilience, durability, and low spin effect during full shot may not be obtained.

  The core in the present invention can be obtained by vulcanizing and curing the above-described rubber composition in the same manner as a known golf ball rubber composition. Examples of the vulcanization conditions include a vulcanization temperature of 140 to 200 ° C., preferably 150 to 180 ° C., and a vulcanization time of 5 to 40 minutes.

  The diameter of the core is preferably 30 mm or more, more preferably 33 mm or more, and further preferably 35 mm or more. The upper limit is preferably 41 mm or less, more preferably 40.5 mm or less, and even more preferably 40 mm or less. Recommended. If the diameter is too small, sufficient resilience may not be obtained. On the other hand, if the value is larger than the above value, the effect imparted by the cover may not be obtained, for example, a sufficient low spin effect cannot be obtained. In the case of a one-piece golf ball, it is 42.67 mm or more according to the rules of the golf ball, and the upper limit is about 45 mm, but it is not particularly limited.

  In addition, the deflection amount when the final load 1275N (130 kgf) is applied from the state where the initial load 98N (10 kgf) is applied to the core (heated product) is preferably 2.3 mm or more, more preferably 2.4 mm. The upper limit is preferably 3.5 mm or less, and more preferably 3.4 mm or less. If the value is larger than the above value, the core becomes too soft, so the resilience is reduced and the core feels hard to hit. Will be burdensome.

Next, the hardness of the core will be described.
The central hardness of the core is not particularly limited, but is preferably 45 or more, more preferably 50 or more according to JIS-C standard, and the upper limit is preferably 70 or less, more preferably 65 or less. When the center hardness of the core is out of the above range, the hit feeling may be deteriorated or the durability may be lowered.

  Although there is no restriction | limiting in particular about the surface hardness of a core, Preferably it is 65 or more by a JIS-C specification, More preferably, it is 70 or more, As an upper limit, Preferably it is 90 or less, More preferably, it is 85 or less. If the surface hardness of the core is too higher than the above range, the durability to cracking due to repeated impacts will deteriorate.

  Since the value of JIS-C hardness difference [(core surface hardness) − (core center hardness)] obtained by subtracting the core center hardness from the core surface hardness has an effect on crack durability, it is preferably 4 or more, more preferably Is 6 or more, and the upper limit is preferably 25 or less, more preferably 20 or less. If the value of the hardness difference is too large, the durability to cracking when the golf ball is repeatedly hit is deteriorated. Here, the above-mentioned center hardness means the hardness measured at the center of a cross section obtained by cutting the core in half (through the center), and the surface hardness is measured on the surface (spherical surface) of the core. Means hardness. Moreover, JIS-C hardness means the hardness measured with the spring-type hardness meter (JIS-C type) prescribed | regulated to JISK6301-1975.

When the initial velocity of the core was measured after removing the cover from the molded ball cover material to the core and V _0, the core initial velocity was measured 60 days after the measurement date of the V ₀ and V _60, V ₀ is 72 The upper limit is 75.0 m / s or less, preferably 74.0 m / s. Also, V ₆₀ is preferably not significantly lower than V ₀ . As a practice golf ball, it is preferable to adjust the core initial speed within a range of 72.0 to 75.0 m / s. The same applies to one-piece golf balls.

The value of V ₀ -V _60, the following formula V ₀ -V ₆₀ <0.9
Is preferably established, and more preferably,
V ₀ −V ₆₀ <0.8
Is to satisfy.
In the present invention, when moisture is blended in the core in a well-balanced manner, even if the core is in direct contact with the atmosphere, it is not easily affected by humidity, and changes in the core initial speed can be suppressed.

  The initial speed of the core can be measured using an initial speed measuring device of the same type as the USGA drum rotation type initial speed meter approved by R & A. In this case, the core can be conditioned at a temperature of 23 ± 1 ° C. for 3 hours or more and tested in a room at a room temperature of 23 ± 2 ° C.

  When the core is heated at 300 ° C., it is preferable that the volatile gas does not contain methacrylic acid. That is, at the time of core vulcanization, the temperature near the center of the core may be about 250 ° C., and it is preferable that methacrylic acid near this temperature is not detected. Since methacrylic acid has the ability to corrode metals as an acid, if methacrylic acid is not detected, there is no need to provide an acid-resistant specification facility, which is industrially advantageous.

Next, the cover of one layer or a plurality of layers covering the core will be described.
The cover material is not particularly limited, but materials such as various ionomer resins used for golf balls and polyurethane resins such as thermoplastic urethane elastomers can be used. The material of the outermost layer of the cover is preferably a polyurethane resin, particularly a thermoplastic urethane elastomer, from the viewpoint of durability.

  In order to obtain a cover according to the present invention, for example, a single layer or a multilayer core of two or more layers prepared in advance according to the type of ball is placed in a mold, the mixture is heated, mixed, melted, and injection molded. A method of covering a desired cover around the core can be employed. In this case, the cover can be manufactured with excellent thermal stability, fluidity, and moldability, so that the golf ball finally obtained has high resilience and hit feeling. It has good durability and scratch resistance. In addition to the above, the cover is formed by, for example, previously forming a pair of hemispherical half cups with the cover material of the present invention, and wrapping the core with the half cups at 120 to 170 ° C. for 1 to 5 minutes. Alternatively, a pressure molding method or the like can be employed.

  When the cover has one layer, the thickness can be 0.3 to 3 mm. When the cover has two layers, the thickness of the outer cover can be in the range of 0.3 to 2.0 mm, and the thickness of the inner cover can be in the range of 0.3 to 2.0 mm. Further, the Shore D hardness of each layer (cover layer) constituting the cover is not particularly limited, but is preferably 40 or more, more preferably 42 or more, and the upper limit is preferably 67 or less. Preferably it is 65 or less.

  In addition, a large number of dimples are formed on the surface of the outermost layer of the cover, and various treatments such as base treatment, stamping, and painting can be performed on the cover. In particular, when such a surface treatment is applied to a cover formed of the cover material of the present invention, the workability can be improved because the cover surface has good moldability.

  The present invention is a golf ball in which the rubber composition is used as a golf ball body (that is, a one-piece golf ball) or at least one layer of a core material. It can be used for a solid golf ball such as a multi-piece golf ball having a three-layer structure or more.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not restrict | limited to the following Example.

[Examples 1-4, Comparative Examples 1-5]
After preparing a rubber composition for a core mainly composed of polybutadiene shown in Table 1 below, a core having a diameter of 39.9 mm was produced by vulcanization molding at 170 ° C. for 25 minutes.

The details of the above blending are as follows.
・ Polybutadiene rubber: Trade name “BR01” (manufactured by JSR)
Polymerized with Ni-based catalyst / Organic peroxide: Dicumyl peroxide Product name “Park Mill D” (manufactured by NOF Corporation)
・ Water: Distilled water (Wako Pure Chemical Industries, Ltd.)
・ Zinc methacrylate: Asada Chemical Co., Ltd. ・ Methacrylic acid: EVONIK Co., Ltd. ・ Zinc oxide: Trade name “Three kinds of zinc oxide” (manufactured by Sakai Chemical Co., Ltd.)
・ Anti-aging agent: Trade name “NOCRACK NS-6” (manufactured by Ouchi Shinsei Chemical Co., Ltd.)

  About each core produced above, in addition to predetermined load deformation, hardness distribution, and initial initial speed, initial speed after leaving the core was evaluated.

Predetermined load deformation amount of the core This is a value obtained by measuring the deformation amount from when the initial load 98N (10 kgf) is applied to the core to when the final load 1275N (130 kgf) is applied. The average value of 30 is shown in Table 2.

Core hardness distribution The surface of the core is spherical, but a hardness meter needle was set to be substantially perpendicular to the spherical surface, and the core surface hardness was measured with JIS-C hardness according to JIS K6301-1975 standard.
About the center hardness of the core, the core was cut in half so as to pass through the center with a fine cutter, and the center of the cross section was measured by JIS-C hardness.

Core initial speed V ₀ and initial speed V ₆₀ after 60 days
The golf ball cover was peeled off to prepare a core. The initial core speed measured on the day the cover was peeled off was defined as day 0, and the initial core speed after 60 days was measured therefrom. The core was left in a room controlled at a room temperature of 24 ° C. and a humidity of 40%. The initial speed was measured using an initial speed measuring device of the same type as the USGA drum rotation type initial speed meter approved by R & A. The core was conditioned at a temperature of 23 ± 1 ° C. for more than 3 hours and tested in a room at room temperature of 23 ± 2 ° C. Each of the 20 cores was hit twice, the time required to pass between 6.28 ft (1.91 m) was measured, and the initial speed was calculated. This cycle was performed in about 15 minutes.

Using GC / MS equipped with a core acid volatile component detection pyrolyzer (pyrolyzer), 0.1 mg of the sample piece cut out from the core was put into the pyrolyzer and heated at 300 ° C to volatilize the gas. Analysis was performed through GC / MS. The presence or absence of detection of the acid volatile component is shown in Table 2.

  Next, for each of the cores produced above, a polyurethane elastomer (trade name “Pandex T8195” manufactured by DIC Covestro Polymer Co., Ltd.) is used as the resin material for the cover (outer layer), and the thickness is 1.4 mm. By the method, a cover was covered around the core to obtain a two-piece solid golf ball having a two-layer structure. At this time, common dimples having a predetermined pattern were formed on the cover surfaces of the balls of all the examples and the comparative examples, although not particularly illustrated.

  Various characteristics of the golf balls of these examples and comparative examples were evaluated as follows. The results are listed in Table 2.

Ball load deformation (mm)
The amount of deformation (mm) from when the initial load of 98 N (10 kgf) was applied to the golf ball to when the final load of 1275 N (130 kgf) was applied was measured.

Initial speed of the ball (m / s)
The ball initial speed was measured with a measuring instrument and conditions in the same manner as the core initial speed.

Feeling of the ball
A golf club (W # 1) was used, and sensory evaluation was performed according to the following criteria by 10 amateur golfers having a head speed of 35 to 40 m / s.
○: 7 or more out of 10 good hit feelings Δ: 5 or 6 good hit feelings ×: 4 or less out of 10 people felt good hit feeling

Ball durability Durability of golf balls was evaluated by ADC Ball COR Durability Tester manufactured by Automated Design Corporation. This test machine has a function of causing a golf ball to be blown with air pressure and then continuously colliding with two metal plates installed in parallel. The incident speed on the metal plate was 43 m / s. The number of firings required to break the golf ball was measured, and the average value of five golf balls was measured.
○: 1000 times or more Δ: 500 times or more and less than 1000 times ×: less than 500 times

Based on the above results, the golf balls of Examples 1 to 4 have a good hit feeling, have very high durability, have little decrease in resilience over time, and no acid volatile component is detected. There is no need to design production equipment with specifications, and the burden on production equipment is small.
On the other hand, Comparative Example 1 is an example in which the core blend contains a large amount of zinc methacrylate, which is the component (D) of the present invention, and as a result, the golf ball feels hard.
In Comparative Example 2, in the core formulation, the molar ratio of the component (C) / component (D) of the present invention is lower than 0.9 (the lower limit of the present invention), and as a result, the durability of the golf ball is poor.
Comparative Example 3 is an example in which methacrylic acid is blended in the core blend, and unlike the blend in which water and zinc methacrylate as the components (C) and (D) of the present invention are blended, the gas of acid is in the process. Oxidation-resistant equipment is necessary because it occurs.
In Comparative Example 4, since zinc methacrylate, which is the component (D) of the present invention, is not blended, the core initial speed becomes large, too much flight, and is not suitable as a golf ball for practice.
Comparative Example 5 is an example in which the zinc blend, which is the component (D) of the present invention, is small in the core formulation, and as a result, the golf ball has a soft feel and is slightly inferior in ball durability.

Claims (5)

In a one-piece golf ball or a golf ball having a core and a cover of one or more layers, the one-piece golf ball or core includes the following components (A) to (E) (A) Base rubber (B) Organic peroxide ( C) Water (D) Metal salt of methacrylic acid (E) It is formed from a heat-molded product of a rubber composition containing zinc oxide, and the amount of component (D) is 100 mass of component (A). The molar ratio (C) / (D) of the components (C) and (D) is from the following formula: 0.9 <(C) / (D) <1.1.
And the initial velocity V _{0 of the} core is 72.0 m / s or more and 75.0 m / s or less. In the golf ball after molding, when the core initial velocity was measured after peeling off the cover and V _0, the core initial velocity was measured 60 days after the measurement of the V ₀ and _{V 60, (V 0 -V 60} ) ≦ 0 The golf ball according to claim 1, wherein.   The golf ball according to claim 1 or 2, wherein a value of a JIS-C hardness difference obtained by subtracting the center hardness from the surface hardness of the core is 4 or more and 20 or less.   4. The golf ball according to claim 1, wherein the golf ball has a single-layer or multiple-layer cover, and at least one cover layer is formed mainly of a polyurethane resin. 5.   The golf ball according to claim 1, wherein methacrylic acid is not contained in the volatile gas when the core is heated at 300 ° C. 6.