JP2012228457A - Practice golf ball - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a practice golf ball endowed with the properties required of practice balls which moreover has a carry that is controlled so as to be appropriate for a practice field.SOLUTION: In the practice golf ball having a core and a cover, the ball deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), is 2.0 to 4.5 mm; a carry C, when a driver (W#1) having a loft angle of 9° is mounted on a golf swing robot and struck at a head speed (HS) of 52 m/s, is preset, a ball deflection BH is adjusted to satisfy formula (1): C=A-33XBH (where 295≤A≤325). The practice golf ball is endowed with the properties required of practice balls and moreover has a carry that is suitably controlled so as to be appropriate for practice fields of variable size.

Description

  The present invention relates to a golf ball for practice that is suitably used in a practice field and the like. More specifically, the required size of the practice ball includes various practice fields of large and small sizes. It is related with the golf ball for practice that the flight distance was controlled according to.

  In general, it is desirable that the performance required for a golf ball for practice is to obtain ball characteristics that are not different from the golf ball used in an actual round. For example, if the hit feeling and the flying characteristics are different from those of the game ball, the technique acquired by practice cannot be fully utilized in the actual round.

On the other hand, since there is a restriction on the size of the practice field, in order to prevent jumping out, a low flying distance ball is desired at present.
Therefore, a ball feel similar to that of a game ball and a low flight distance is desired.

  In addition, since practice golf balls are used repeatedly at a golf driving range, the balls can withstand repeated use for as long a period as possible in order to reduce the cost of operating the driving range. It is desirable to be. That is, since a golf ball used in a driving range is repeatedly used over a long period of time by a large number of practitioners, it is desired that the golf ball has superior durability such as cracking durability than a game ball that is normally used. .

  As a ball structure of the practice golf ball, it is desirable to use a two-piece solid golf ball rather than a one-piece golf ball in order to give flying and feeling close to a game feeling.

  Regarding the golf balls described in JP 2007-61614 A, JP 2007-301357 A, JP 2010-115485 A, and JP 2010-115486 A, acrylic resin is added to cis-1,4-polybutadiene rubber. A predetermined amount of zinc acid is blended. However, these golf balls are golf balls for the purpose of improving flying performance, and there is a danger of jumping over the net of the practice field when this ball is used in the practice field. In other words, since conventional balls are mainly designed for game balls, they do not flexibly correspond to the size of the practice field, and as practice golf balls whose flight distance can be controlled according to the practice field. Was unsuitable.

  As other prior arts related to the present invention, JP 2004-180793 A, JP 2008-149190 A, JP 2009-195761 A, JP 2005-27814 A, and JP 2010 are listed. Although the golf balls described in Japanese Patent No. -269147 are mentioned, there is a risk of jumping out of the practice field if these balls are used as they are.

JP 2007-61614 A JP 2007-301357 A JP 2010-115485 A JP 2010-115486 A JP 2004-180793 A JP 2008-149190 A JP 2009-195761 A JP 2005-27814 A JP 2010-269147 A

  The present invention has been made in view of the above circumstances. As a required characteristic of a practice ball, there is a practice golf ball whose flying distance is controlled in accordance with the size of the practice field in various large and small practice fields. The purpose is to provide.

  As a result of intensive studies to achieve the above object, the present inventors have created a practice golf ball having a core and a cover, and the ball cover hardness, thickness, material, and ball rebound (initial By optimizing the speed and dimples, we found that there is no risk of jumping over the net of the practice field when used in the practice field, and the flight distance can be controlled according to the practice field, and the present invention is made. It is obtained.

  Further, as a preferred embodiment of the present invention, methacrylic acid is blended as a co-crosslinking agent for the core rubber composition, and by optimizing the blending amount of methacrylic acid and the blending amount of the crosslinking initiator. The durability against cracking can be very good as compared with the game ball. In addition, it is preferable to use a cover material mainly composed of polyurethane, and a golf ball for practice excellent in crack durability and wear durability can be obtained. Furthermore, in the present invention, it is preferable to optimize the cross-sectional shape of the dimples formed on the ball surface, and it is possible to obtain a practice golf ball having excellent mark durability in long-term use.

Accordingly, the present invention provides the following golf balls for practice.
Claim 1:
The ball has a deflection amount BH when the ball is loaded from an initial load of 98 N (10 kgf) to a final load of 1,275 N (130 kgf) of 2.0 to 4.5 mm. Carry C when hitting the ball at a head speed (HS) of 52 m / s with a TourStage X-DRIVE 701 (Loft angle: 9 °) manufactured by Bridgestone Sports as a driver (W # 1) attached to the striking robot. Is set in advance, and the following formula (1)
C = A−33 × BH (1)
(However, 295 ≦ A ≦ 325)
A golf ball for practice characterized in that the deflection amount BH of the ball is adjusted so as to satisfy the above.
Claim 2:
The golf ball for practice according to claim 1, wherein the core is formed of a rubber composition containing a base rubber, a co-crosslinking agent, a crosslinking initiator, and a metal oxide, and methacrylic acid is used as the co-crosslinking agent.
Claim 3:
The golf ball for practice according to claim 1 or 2, wherein the initial velocity of the ball is 76 m / s or less.
Claim 4:
The ratio (VR) of the total volume of the dimple space formed below the plane surrounded by the edge of the dimple to the volume of the phantom sphere assuming that there is no dimple on the ball surface is 0.8 to 1.7. The golf ball for practice according to any one of claims 1 to 3.
Claim 5:
The golf ball for practice according to any one of claims 1 to 4, wherein dimples satisfying the following requirements (1) and (2) are formed on a surface.
(1) It has a dimple provided with a radius of curvature (R) of 0.5 to 2.5 mm at the peripheral edge.
(2) The ratio (ER) of the total number of dimples (RA) to the total number of dimples (N) in which the ratio (R / D ratio) of the radius of curvature (R) to the diameter (D) is 20% or more is 15 to 95. %.
Claim 6:
Furthermore, the golf ball for practice of Claim 5 which satisfies the following requirements (3).
(3) A dimple having a plurality of types of dimples having different diameters (D) and having a radius of curvature (R) of a dimple having a diameter (D) larger than its own diameter (D) equal to or less than its own radius of curvature (R) The ratio (DER) of the total number (DE) of the maximum diameter dimples having the maximum diameter (D) to the total number of dimples (N) is 80% or more.
Claim 7:
Furthermore, the golf ball for practice of Claim 6 which satisfies the following requirements (4)-(6).
(4) It has three or more types of dimples having different diameters (D).
(5) The total number of dimples (N) is 380 or less.
(6) The surface occupancy (SR), which is the ratio of the total dimple area defined by the plane surrounded by the dimple edges to the surface area of the phantom sphere assuming that no dimples exist on the ball surface, is 60 to 74 %.
Claim 8:
The resin component of the cover is mainly composed of polyurethane, the material hardness of the cover is 30 to 57 in Shore D hardness, and the thickness of the cover is 0.3 to 1.9 mm. The golf ball for practice according to any one of 1 to 7.
Claim 9:
In the blending of each component of the rubber composition, 10 to 40 parts by weight of methacrylic acid, 15 to 30 parts by weight of metal oxide, and 0.3 to 5. The golf ball for practice according to claim 1, wherein 0 part by mass is blended.

  According to the practice golf ball of the present invention, the required characteristics of the practice ball are optimal as a practice golf ball whose flying distance is controlled in accordance with the size of the practice field, among large and small practice fields. .

1 is a schematic cross-sectional view of a golf ball for practice showing an embodiment of the present invention. It is the schematic which shows an example of the cross section of a dimple. An example of a dimple arrangement | sequence is shown, (A) is a top view, (B) is a front view. It is a top view which shows the mark attached | subjected to the golf ball produced by the Example and the comparative example.

Hereinafter, the present invention will be described in more detail.
The structure of the practice golf ball of the present invention is, for example, a two-piece solid golf ball G comprising a core 1 and a cover 2 covering the core, as shown in FIG. Note that many dimples D are usually formed on the surface of the cover 2. The core 1 and the cover 2 are each formed in a single layer in the figure, but a plurality of cores may be provided, and a plurality of covers may be provided.

  The core is obtained by vulcanizing a rubber composition mainly composed of a rubber material. There is no restriction | limiting in particular as this rubber composition, As preferable embodiment, it forms, for example using the rubber composition containing base rubber, a co-crosslinking agent, a crosslinking initiator, a metal oxide, and an anti-aging agent. Can be mentioned. Moreover, it is preferable to use polybutadiene as the base rubber of this rubber composition.

  The polybutadiene as the rubber component has a cis-1,4-bond of 60% (mass%, the same shall apply hereinafter) or more, preferably 80% or more, more preferably 90% or more, and most preferably 95% or more. is necessary. If there are too few cis-1,4-bonds, the resilience will decrease. Further, the content of 1,2-vinyl bonds is preferably 2% or less, more preferably 1.7% or less, and still more preferably 1.5% or less.

The polybutadiene has a Mooney viscosity (ML _{1 + 4} (100 ° C.)) of preferably 30 or more, preferably 35 or more, more preferably 40 or more, most preferably 45 or more, and the upper limit is preferably 100 or less, more preferably 80 or less, more preferably 70 or less, and most preferably 60 or less.

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.). 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.

  The polybutadiene is preferably synthesized with a rare earth element-based catalyst or a Group VIII metal compound catalyst from the viewpoint of obtaining a vulcanized molded product of a rubber composition having good resilience.

  The rare earth element-based catalyst is not particularly limited, but a catalyst using a lanthanum series rare earth element compound can be preferably used. Further, if necessary, a lanthanum series rare earth element compound can be used in combination with an organoaluminum compound, an alumoxane, a halogen-containing compound, and a Lewis base. As the various compounds exemplified above, those described in JP-A-11-35633, JP-A-11-164912, and JP-A-2002-293996 can be suitably used.

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

  As the polybutadiene, the molecular weight distribution Mw / Mn (Mw: weight average molecular weight, Mn: number average molecular weight) is preferably 2.0 or more, more preferably 2.2 or more, still more preferably 2.4 or more, and most preferably. Is 2.6 or more, and the upper limit is preferably 6.0 or less, more preferably 5.0 or less, and even more preferably 4.5 or less. If Mw / Mn is too small, workability is improved. If it is too low, the resilience may decrease.

  The above-mentioned polybutadiene is used as the base rubber. In this case, the proportion of polybutadiene in the entire rubber is preferably 40% by mass or more, more preferably 60% by mass or more, still more preferably 80% by mass or more, and most preferably. Is 90% by mass or more. Moreover, 100 mass% of base rubber may be the said polybutadiene, 98 mass% or less is preferable, More preferably, it is 95 mass% or less.

  Specifically, as the cis-1,4-polybutadiene rubber, high cis BR01, BR11, BR02, BR02L, BR02LL, BR730, BR51 manufactured by Nippon Synthetic Rubber Co., Ltd. (manufactured by JSR) can be used.

  In addition to the polybutadiene, other rubber components can be blended with the rubber base material as long as the effects of the present invention are not impaired. Examples of the rubber component other than the polybutadiene include polybutadiene other than the polybutadiene, and other diene rubbers such as styrene butadiene rubber, natural rubber, isoprene rubber, and ethylene propylene diene rubber.

The above isoprene rubber contains cis-1,4-bond of 60% or more, preferably 80% or more, more preferably 90% or more, and Mooney viscosity (ML _{1 + 4} (100 ° C.)) of 70 or more. The upper limit is preferably 75 or more, more preferably 80 or more, and the upper limit is 90 or less, preferably 85 or less. Specifically, IR2200 manufactured by Nippon Synthetic Rubber Co. (manufactured by JSR) or the like can be used. Moreover, solution polymerization styrene butadiene rubber and emulsion polymerization styrene butadiene rubber can be used as the styrene butadiene rubber. Solution polymerized styrene butadiene rubber does not contain organic acids and low molecular weight components derived from the production method compared to emulsion polymerized styrene butadiene rubber, so it has poor processability, and when used in a rubber composition for the core, the ball is used in winter and summer. The difference in repulsive force increases. Conversely, when emulsion-polymerized styrene-butadiene rubber is used in the core rubber composition, it effectively prevents the core from becoming hard due to temperature changes due to seasonal differences, compared to when solution-polymerized styrene-butadiene rubber is used. Can do. Therefore, by using two kinds of styrene butadiene rubbers with different properties at a predetermined ratio, it may be possible to suppress the decrease in resilience and change in hardness during winter use while maintaining processability. is there. Specifically, solution polymerization SBR-SL552, SL555, SL563 manufactured by Nippon Synthetic Rubber Co., Ltd. (manufactured by JSR) as solution polymerized styrene butadiene rubber, etc. Emulsion polymerization SBR1500, 1502, 1507, 0202 and the like can be used. The commercially available solution-polymerized styrene butadiene rubber has a styrene bond amount of 5 to 50%, and the emulsion polymerized styrene butadiene rubber has a styrene bond amount of 15 to 50%. The proportion of rubber components other than polybutadiene in the entire rubber is preferably 0% by mass or more, more preferably 2% by mass or more, and most preferably 5% by mass or more. Moreover, it is 60 mass% or less, More preferably, it is 40 mass% or less, More preferably, it is 20 mass% or less, Most preferably, it is 10 mass% or less.

  The co-crosslinking agent is not particularly limited in the present invention, but methacrylic acid is preferably used. In this case, the blending amount of methacrylic acid is preferably 10 parts by weight, preferably 12 parts by weight or more, more preferably 14 parts by weight or more, and further preferably 16 parts by weight with respect to 100 parts by weight of the base rubber. The upper limit is preferably 40 parts by mass or less, more preferably 35 parts by mass or less, still more preferably 30 parts by mass or less, and still more preferably 25 parts by mass or less. If the amount is too large, it may become too hard and unbearable feel may occur, and if the amount is too small, it may become too soft and unbearable feel.

  As the crosslinking initiator, it is preferable to use an organic peroxide. Specifically, Park Mill D (manufactured by NOF Corporation), Perhexa 3M (manufactured by NOF Corporation), Perhexa C40 (manufactured by Nippon Oil Corporation) A commercially available product such as Oil Co., Ltd. can be suitably used. These may be used alone or in combination of two or more.

  The crosslinking initiator is preferably 0.3 parts by mass or more, more preferably 0.5 parts by mass or more, still more preferably 0.7 parts by mass or more with respect to 100 parts by mass of the base rubber. Is blended in an amount of 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, still more preferably 3.0 parts by mass or less, and most preferably 2.0 parts by mass or less. If the total amount is too large, it will become too hard and unbearable feel will be caused, and crack durability will also be greatly reduced. On the other hand, if the blending amount is too small, the feel becomes too soft and unbearable, and the productivity may be greatly reduced.

  In the present invention, it is preferable to use zinc oxide as the metal oxide, but metal oxides other than zinc oxide can be used as long as the effects of the present invention are not impaired. About the compounding quantity of a metal oxide, Preferably it is 15 mass parts or more with respect to 100 mass parts of said base rubber, More preferably, it is 17 mass parts or more, More preferably, it is 19 mass parts or more, Most preferably, it is 21 mass parts or more. The upper limit is preferably 30 parts by mass or less, more preferably 28 parts by mass or less, still more preferably 26 parts by mass or less, and most preferably 24 parts by mass or less. If the amount is too large or too small, it may not be possible to obtain a proper mass and suitable hardness and resilience.

  Moreover, in this invention, anti-aging agent is mix | blended with a rubber composition, For example, commercial items, such as NOCRACK NS-6, NS-30, and 200 (made by Ouchi Shinsei Chemical Co., Ltd.), are used. 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 rubber, More preferably, it is 0.2 mass part or more, Preferably it is 1.0 as an upper limit. It is 0.7 parts by mass or less, more preferably 0.4 parts by mass or less. If the amount is too large or too small, it may not be possible to obtain suitable resilience, durability, and low spin effect during full shot.

  In the present invention, from the viewpoint of resource recycling, pulverized rubber pulverized powder or abrasive powder can be blended in a small amount of 40 parts by mass or less with respect to 100 parts by mass of the base rubber. In this case, the pulverized powder and the abrasive powder are more than 0, preferably 2% by mass or more, most preferably 5% by mass or more, and the upper limit is 40% by mass or less, more preferably 100% by mass of the base rubber. Can be blended in a small amount of 35% by mass or less, more preferably 30% by mass or less, and most preferably 25% by mass or less. Vulcanized rubber pulverized powder and abrasive powder are vulcanized products containing rubber, unsaturated carboxylic acid or metal salt thereof. The particle size of the vulcanized rubber pulverized powder or abrasive powder is 20 μm or more, preferably 25 μm or more, most preferably 30 μm or more, and the upper limit is 1000 μm or less, more preferably 900 μm or less, and most preferably 800 μm or less. preferable. Addition of vulcanized rubber pulverized powder or abrasive powder improves the productivity of the vulcanizate and improves crack durability. However, if the amount is too large, workability and productivity may be greatly reduced.

  The core can be produced by vulcanizing and curing a rubber composition containing the above components by a known method. For example, kneading using a kneader such as a Banbury mixer or roll, compression molding or injection molding using a core mold, and a temperature sufficient for the organic peroxide or co-crosslinking agent to act is about 100 The molded body can be cured and manufactured by appropriately heating the molded body under the conditions of ~ 200 ° C and 10 to 40 minutes.

  Although there is no restriction | limiting in particular as a diameter of a core, Preferably it is 38.9 mm or more, More preferably, it is 39.3 mm or more, As an upper limit, Preferably it is 42.1 mm or less, More preferably, it is 41.1 mm or less. If the diameter of the core is out of this range, the cracking durability of the ball may be significantly reduced or the initial speed of the ball may be lowered.

  The specific gravity of the core is 1.05 or more, preferably 1.08 or more, more preferably 1.1 or more, and the upper limit is 1.2 or less, preferably 1.15 or less, more preferably 1.13 or less. Is recommended

  The amount of deflection when the core is loaded, that is, the amount of deflection (mm) (CH) from when the initial load 98N (10 kgf) to the final load 1,275 N (130 kgf) is applied to the core is 2 0.0 mm or more, preferably 2.3 mm or more, more preferably 2.4 mm or more, and the upper limit is 4.5 mm or less, more preferably 4.3 mm or less, still more preferably 4.1 mm or less, and most preferably 3.9 mm or less. It is. If the deflection amount (CH) of the core is too small, the feel as a golf ball for practice will be too hard and it will be unbearable to use, and conversely if it is too large it will be too soft and unbearable and will be highly productive. May decrease.

  The core resilience (CV) is 65 m / s or more, preferably 68 m / s or more, more preferably 71 m / s or more, most preferably 73 m / s or more, and the upper limit is 76 m / s or less, more preferably 75.7 m / s or less, more preferably 75.4 m / s or less, and most preferably 75 m / s or less. If it deviates from this range, there is a problem that the flight distance is drastically reduced or that it jumps too much in the practice field and exceeds the net.

  In the present invention, the core may be surface-treated with a solution containing a halogenated isocyanuric acid and / or a metal salt thereof.

  Further, before the surface treatment is performed on the core with a solution containing a halogenated isocyanuric acid and / or a metal salt thereof shown below, the core surface is subjected to a polishing treatment, thereby adjoining the core surface. The adhesiveness with the material can be further improved.

  At this time, the polishing treatment removes the skin layer from the core surface after vulcanization, increases the permeability of the halogenated isocyanuric acid and / or its metal salt solution to the core surface, and the contact area with the adjacent cover material. Can be increased. Specific methods include buffing, barrel polishing, centerless polishing, and the like.

（ここで、Ｘは、水素原子、ハロゲン原子又はアルカリ金属原子を表す。Ｘの少なくとも１個はハロゲン原子である。ハロゲン原子としては、Ｆ、Ｃｌ、Ｂｒが好ましく、特にＣｌが好ましい。アルカリ金属原子としては、Ｌｉ、Ｎａ、Ｋが好ましい。） Said halogenated isocyanuric acid and its metal salt are compounds represented by the following formula (I).

(Here, X represents a hydrogen atom, a halogen atom or an alkali metal atom. At least one of X is a halogen atom. F, Cl and Br are preferable, and Cl is particularly preferable. Alkali metal. (As atoms, Li, Na, and K are preferable.)

  Specific examples of the halogenated isocyanuric acid and / or metal salts thereof include chloroisocyanuric acid, sodium chloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuric acid, sodium dichloroisocyanurate, sodium dichloroisocyanurate dihydrate, dichloroisocyanuric Examples thereof include salts of potassium acid, trichloroisocyanuric acid, isocyanuric tribromide, isocyanuric dibromide, isocyanuric bromide, sodium isocyanurate dibromide, hydrates thereof, difluoroisocyanuric acid and the like. Among these, chloroisocyanuric acid, sodium chloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuric acid, sodium dichloroisocyanurate, potassium dichloroisocyanurate, and trichloroisocyanuric acid are preferable. This is because they are easily hydrolyzed by moisture to generate an acid and chlorine and serve as an initiator for an addition reaction to a double bond in a diene rubber molecule. In particular, when trichloroisocyanuric acid is used, the effect of improving adhesiveness is remarkable.

  The halogenated isocyanuric acid and / or metal salt thereof is preferably dissolved in an organic solvent and used as a solution. As said organic solvent, a well-known thing can be used and especially the organic solvent soluble in water can be used conveniently. Specific examples thereof include ethyl acetate, acetone, methyl ethyl ketone and the like. Among these, acetone is particularly preferable from the viewpoint of permeability to the core surface. Solvents that are soluble in water are easy to incorporate moisture into the solvent, and the incorporated moisture is liable to hydrolyze with the halogenated isocyanuric acid and / or its metal salt adhering to the core surface, or to the next step. When washing is used, it is preferably used for the reason that the affinity to the core surface is improved and the hydrolysis reaction between water and a halogenated isocyanuric acid and / or a metal salt thereof easily occurs.

  When dissolved in an organic solvent, the content of the halogenated isocyanuric acid and / or metal salt thereof in the solution is preferably 0.3% by mass or more, more preferably 1% by mass or more, and further preferably 2.5% by mass. If it is above and it is less than 0.3 mass%, the adhesive improvement effect expected after core surface treatment cannot be obtained, and impact durability may be inferior. The upper limit can be up to a saturated solution concentration. However, in consideration of cost effectiveness, for example, when an acetone solution is used, it is preferable that the upper limit is about 10% by mass. Further, the immersion time of the core in the solution is preferably 0.3 seconds or more, more preferably 3 seconds or more, still more preferably 10 seconds or more, and the upper limit is preferably within 5 minutes, more preferably within 1 minute, More preferably, it is within 30 seconds. If it is too short, the treatment effect may not be obtained, and if it is too long, the productivity may be impaired.

  As a method for treating the core surface with a halogenated isocyanuric acid and / or a metal salt thereof, a method of applying a halogenated isocyanuric acid and / or a metal salt solution thereof to the core surface by brushing or spraying, or the core is isocyanuric halide Although the method of immersing in an acid and / or its metal salt solution etc. is mentioned, the immersion method is used especially suitably from a viewpoint of productivity and the high permeability of the solution to the core surface.

  The core surface is preferably washed with water after the surface treatment is performed with a solution containing a halogenated isocyanuric acid and / or a metal salt thereof. Washing the core surface with water can be done by running water, spraying, soaking using a washing layer, etc., but it is not only for washing, but also for the purpose of starting and promoting the desired treatment reaction. A too rapid cleaning method is not suitable. Therefore, pickling and washing provided with a washing basket is preferably used. In this case, it is preferable to put it about 1 to 5 times in a washing bowl containing fresh water.

  By treating the core surface with a halogenated isocyanuric acid and / or a metal salt thereof, adhesion to the cover is greatly improved. The reason is not clear, but the following can be considered.

  First, the halogenated isocyanuric acid and / or its metal salt penetrates into the diene rubber forming the core together with the solvent, and approaches the double bond of the main chain. Thereafter, water enters the core surface, and the halogenated isocyanuric acid and / or its metal salt is hydrolyzed with water to release halogen. The halogen attacks the double bond of the nearby diene rubber main chain, and the addition reaction proceeds. In the course of the addition reaction, liberated isocyanuric acid is added to the diene rubber main chain together with chlorine while maintaining the ring structure. The added isocyanuric acid has three —NHCO— structures in the molecule.

  Accordingly, the core surface treated with the halogenated isocyanuric acid and / or metal salt thereof is given a structure of -NHCO-, so that the adhesion to the cover material is further improved, so that the hitting durability as a golf ball is improved. It is thought that it is done. Further, when a polyurethane elastomer or a polyamide elastomer having the same —NHCO— structure in the polymer molecule is used as the cover material, it is considered that the impact durability is further increased because the affinity is further increased.

In addition, when addition of isocyanuric acid and chlorine to the diene rubber surface occurred, the state change of the bond before and after the addition is an absorption peak of C═O bond (stretching) at 1725 to 1705 cm ⁻¹ in the infrared absorption spectrum, broad absorption peak of N-H bond in 3450~3300cm ^-1 (stretching), it appears to increase the absorption peak of the C-Cl bond in 800~600cm ^-1. Therefore, by measuring the infrared absorption spectrum of the surface-treated core and confirming the increase of these absorption peaks, it was confirmed that the addition of isocyanuric acid and chlorine into the diene rubber molecule on the core surface occurred. Can be qualitatively supported.

  Further, the material of the surface portion of the solid core after the surface treatment does not show any exothermic or endothermic peak from room temperature to 300 ° C. by differential scanning calorimetry (DSC). This means that the introduced functional group is kept stable in this temperature range. In other words, the introduced functional group does not cause thermal decomposition in the molding process of the cover material, and maintains its effectiveness. Also, it does not cause melting like a hot-melt resin and begins to bleed into the parting line. This means that there is no concern about adverse effects on durability and appearance quality. In addition, as described above, the stability of the material of the surface portion of the solid core after the surface treatment is one of the evidence that isocyanuric acid having a melting point of 300 ° C. or higher was added while maintaining its molecular structure. It can be said that there is.

  In the present invention, although not particularly limited, it is preferable that the resin component of the cover is mainly made of polyurethane, and a thermoplastic polyurethane elastomer or a thermosetting polyurethane resin can be used. It is preferable to adopt.

  The structure of the thermoplastic polyurethane elastomer is composed of a soft segment composed of a polymer polyol (polymeric glycol), a chain extender constituting a hard segment, and a diisocyanate. Here, as the raw material polymer polyol, any of those conventionally used in the technology relating to thermoplastic polyurethane materials can be used, and is not particularly limited. The polyether type is preferable to the polyester type in that a thermoplastic polyurethane material having a high elastic modulus and excellent low-temperature characteristics can be synthesized. Examples of the polyether polyol include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is particularly preferable from the viewpoint of the resilience modulus and low temperature characteristics. The average molecular weight of the polymer polyol is preferably 1000 to 5000, and particularly preferably 2000 to 4000 in order to synthesize a thermoplastic polyurethane material having high resilience.

  As the chain extender, those used in the conventional technology relating to thermoplastic polyurethane materials can be suitably used. For example, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1 , 6-hexanediol, 2,2-dimethyl-1,3-propanediol and the like, but are not limited thereto. These chain extenders preferably have an average molecular weight of 20 to 15000.

  As the diisocyanate, those used in the conventional technology relating to thermoplastic polyurethane materials can be suitably used. For example, aromatics such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, and 2,6-toluene diisocyanate can be used. Aliphatic diisocyanates, aliphatic diisocyanates such as hexamethylene diisocyanate, and the like, but are not limited thereto. However, some isocyanate species make it difficult to control the crosslinking reaction during injection molding. In the present invention, 4,4'-diphenylmethane diisocyanate which is an aromatic diisocyanate is most preferable.

  Commercially available products can be suitably used as the thermoplastic polyurethane material composed of the above-mentioned materials. For example, DIC Bayer Polymer Co., Ltd .: Pandex T8180, T8195, T8290, T8295, T8260, Dainichi Seikagaku ( Resamine 2593, 2597 manufactured by Co., Ltd. and the like.

  The cover thickness is 0.3 mm or more, preferably 0.5 mm or more, more preferably 0.7 mm or more, and the upper limit is 1.9 mm or less, preferably 1.8 mm or less, more preferably 1. 7 mm or less. If the thickness of the cover is larger than the above range, the resilience is lowered and the flying performance may be deteriorated. When the thickness of the cover is smaller than the above range, the durability to cracking is lowered. In particular, the cover may tear when it is topped.

  The specific gravity of the cover is preferably 1.13 or more, more preferably 1.14 or more, still more preferably 1.15 or more, and the upper limit is preferably 1.30 or less, more preferably 1.20 or less, and further Preferably it is 1.17 or less.

  The material hardness of the cover is Shore D hardness of 30 or more, preferably 35 or more, more preferably 38 or more, and the upper limit is 57 or less, preferably 55 or less, more preferably 53 or less, and particularly preferably 51 or less. It is. If the Shore D hardness of the cover is higher than the above range, the appearance performance (mark durability) during long-term use may be reduced, and the flying performance may be further reduced. If the Shore D hardness of the cover is softer than the above range, the cracking durability is greatly reduced, and the cover may be torn especially when it is topped. Also, the spin becomes very large, which may lead to a decrease in flight distance.

  The practice golf ball of the present invention is usually provided with a large number of dimples on the surface thereof. In this case, although not particularly limited, the ratio of the total volume of the dimple space formed below the plane surrounded by the edge of the dimple to the volume of the phantom sphere assuming that there is no dimple on the ball surface (VR) is preferably from 0.8 to 1.7, more preferably a lower limit of 0.83, still more preferably 0.85, most preferably 0.86, and a more preferable upper limit is 1.5. More preferably, it is 1.3, and most preferably 1.2.

  Although not particularly limited, it is preferable that the dimples formed on the practice golf ball of the present invention satisfy the following requirements (1) and (2). In addition, it is preferable that the following requirements (1) and (2) are satisfied at the same time, but it is also preferable that each of the following requirements is satisfied.

  First, as the requirement (1), as shown in FIG. 2, it is preferable to have dimples having rounded edges with a radius of curvature (R) of 0.5 to 2.5 mm as shown in FIG. A more preferable lower limit value of the radius of curvature (R) is 0.6 mm, more preferably 0.7 mm, and a more preferable upper limit value is 1.8 mm, still more preferably 1.5 mm.

  Next, as requirement (2), the ratio (ER) of the total number of dimples (RA) to the total number of dimples (N) in which the ratio (R / D ratio) of the radius of curvature (R) to the diameter (D) is 20% or more. ) Is preferably 15 to 95%. In this case, the R / D ratio is represented by (R / D) × 100%, and the larger the ratio, the larger the ratio of the rounded portion to the size of the dimple, and the dimple having a smoother cross-sectional shape. It means that. The ER represents the ratio of the smooth dimples to all the dimples. By setting the ER to 15 to 95% as described above, damage to the coating film at the edge portion of the dimples can be effectively suppressed. It can be done. In addition, there is no restriction | limiting in particular in the upper limit of the said R / D ratio, However, Preferably it is 60% or less, More preferably, it is 40% or less. Further, the more preferable lower limit of the ER is 20%, more preferably 25%, and the more preferable upper limit is 90%, still more preferably 85%, and most preferably 70%.

  Further, although not particularly limited, it is preferable to satisfy the following requirement (3). That is, as a requirement (3), when a plurality of types of dimples having different diameters (D) are provided, the radius of curvature (R) of the dimple having a diameter (D) larger than its own diameter (D) is set to the radius of curvature of its own ( R) The dimples are formed so that the ratio (DER) of the total number (DE) of the dimples with a maximum diameter (D) and the maximum diameter dimple (DE) to the total number of dimples (N) is 80% or more. Is preferred.

  Normally, if the depth of the dimple (see FIG. 2) is constant, the radius of curvature (R) of the roundness provided at the periphery decreases as the diameter decreases, but the requirement (3) adjusts the depth. By setting the radius R of the peripheral edge to be as large as possible even for dimples with a small diameter by means such as making a dimple having a smooth cross-sectional shape and setting the DER to 80% or more, the ratio of such a smooth dimple can be reduced. In many cases, damage to the coating film is more effectively suppressed. A more preferable value of this DER is 85% or more, more preferably 90% or more, and most preferably 93% or more. The upper limit value of DER is 100%.

  Furthermore, the dimple of the present invention is not limited, but preferably satisfies the following requirements (4) to (6). In addition, it is preferable that the following requirements (4) to (6) are all satisfied simultaneously, but it is also preferable that each of the following requirements (4) to (6) is satisfied individually.

  First, as requirement (4), it is preferable to provide three or more types of dimples having different diameters (D), and more preferably, five or more types of dimples are formed. In this case, the diameter (D) of the dimple is not particularly limited, but is preferably 1.5 to 7 mm, a more preferable lower limit is 1.8 mm, and a more preferable upper limit is 6.5 mm. . Further, the depth of the dimple is not particularly limited, but is preferably 0.05 to 0.35 mm, more preferably a lower limit is 0.1 mm, and a more preferable upper limit is 0.3 mm. Is 0.25 mm.

  Further, as the requirement (5), the total number (N) of dimples is preferably 380 or less, more preferably 350 or less, and more specifically 220 to 340.

  Furthermore, as a requirement (6), the total dimple area defined by the plane surrounded by the dimple edge (the chain line in FIG. 2) is the phantom sphere (in FIG. 2) assuming that there is no dimple on the ball surface. It is preferable to form the dimples so that the surface occupancy (SR), which is the ratio of the surface area to the broken line), is 60 to 74%. If the SR is 74% or more, the interval between adjacent dimples may be too narrow, and it may be difficult to provide the radius of curvature of the requirement (1) at the periphery of the dimple, while the SR is 60 If it is less than%, the aerodynamic performance may decrease and the flight distance may decrease. A more preferable lower limit value of SR is 65%, still more preferably 68%, and a more preferable upper limit value is 73%.

  In general, a one-piece golf ball has a slightly yellow rubber color, so a white enamel paint is applied to the first layer, and then a clear paint is applied. From the viewpoint of securing the appearance, it is preferable to apply clear coating to the ball surface of the present invention. The film thickness of the clear coating is 10 μm or more for the dimple bank (Y), preferably 12 μm or more, most preferably 13 μm or more, and the upper limit is 30 μm or less, preferably 25 μm or less, most preferably 20 μm or less. ) Is 8 μm or more, preferably 10 μm or more, most preferably 11 μm or more, and the upper limit is 28 μm or less, preferably 23 μm or less, and most preferably 18 μm or less. Further, the ratio of the bank portion (Y) to the edge portion (Z), Z / Y × 100 is 60% or more, preferably 70% or more, most preferably 80% or more, and the upper limit is 100% or less, preferably 95%. It is as follows. When deviating from the above range, the mark durability at the dimple edge portion may be significantly deteriorated during long-term use.

  The diameter of the ball is 42 mm or more, preferably 42.3 mm or more, more preferably 42.67 mm or more, and the upper limit is 44 mm or less, preferably 43.8 mm or less, more preferably 43.5 mm or less, still more preferably 43 mm or less.

  The weight of the ball is preferably 44.5 g or more, more preferably 44.7 g or more, further preferably 45.1 g or more, and most preferably 45.2 g or more. The upper limit is preferably It is 47.0 g or less, more preferably 46.5 g or less, still more preferably 46.0 g or less.

  The deflection amount (BH) when the initial load of 98N (10 kgf) to the final load of 1,275 N (130 kgf) of the practice golf ball of the present invention is preferably 2.0 mm or more, more preferably 2.3 mm or more. More preferably, it is 2.4 mm or more, and the upper limit is preferably 4.5 mm or less, more preferably 4.3 mm or less, still more preferably 4.1 mm or less, and most preferably 3.9 mm or less.

  When the initial load 98N (10 kgf) and the final load 1,275N (130 kgf) are applied to the core and the ball, when the respective deflection amounts are (CH) and (BH), (CH) / (BH) The value is 0.95 or more, preferably 0.96 or more, more preferably 0.97 or more, and the upper limit is 1.1 or less, preferably 1.08 or less, more preferably 1.07 or less. When the value of (CH) / (BH) is too large, the amount of deflection of the product is very small with respect to the amount of deflection of the core. That is, since the cover hardness is high, it may lead to a decrease in feel and a decrease in appearance after long-term use. On the other hand, if it is too small, the cover becomes very soft, so the cracking durability is greatly reduced, and the cover may break, especially when it is topped. Also, the spin becomes very large, which may lead to a decrease in flight distance.

  On the other hand, the ball rebound (BV) is 65 m / s or more, preferably 68 m / s or more, more preferably 71 m / s or more, most preferably 73 m / s or more, and the upper limit is 76 m / s or less. Preferably it is 75.7 m / s or less, More preferably, it is 75.4 m / s or less, Most preferably, it is 75 m / s or less. If it deviates from this range, there is a problem that the flight distance is drastically reduced or that it jumps too much in the practice field and exceeds the net.

Further, according to the present invention, as a driver (W # 1), a TourStage X-DRIVE 701 (loft angle: 9 °) manufactured by Bridgestone Sports is attached to a striking robot, and the ball is hit at a head speed (HS) of 52 m / s. Carry C is set in advance, and the following formula (1)
C = A−33 × BH (1)
(However, 295 ≦ A ≦ 325)
The amount of deflection BH of the ball is adjusted so that it satisfies the requirements. As a result, there are various practice grounds as the required characteristics of the practice ball. Can be controlled. For this reason, when the golf ball for practice having a core and a cover is used at a practice field by optimizing the cover hardness, thickness, material, and ball rebound (initial speed) and dimple, There is no risk of jumping over, and the flight distance can be controlled according to the practice range.

  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 1-6, Comparative Examples 1-7]
When preparing golf balls for practice in the examples and comparative examples, rubber materials having the composition shown in Table 1 below are prepared. This rubber composition is appropriately kneaded with a kneader or a roll, and then vulcanized according to the temperature and time conditions shown in Table 1 to prepare solid cores for each example. In addition, the number of each material in the following table | surface is represented by a mass part.

The material of the core composition in the above table has the following contents.
(1) BR01: Ni catalyst butadiene rubber manufactured by JSR: Mooney viscosity ML "46"
(2) IR2200: JSR isoprene rubber: Mooney viscosity ML “82”
(3) BR730: JSR Nd-catalyzed butadiene rubber: Mooney viscosity ML “55”
(4) BR51: Nd catalyst butadiene rubber manufactured by JSR: Mooney viscosity ML “36”
(5) Perhexa C-40: Organic peroxide made by NOF (6) Park mill D: Organic peroxide made by NOF (7) Zinc oxide: Sakai Chemical Industry Co., Ltd. (8) Anti-aging agent: Ouchi “NOCRAK NS-6” manufactured by Shinsei Chemical Industry Co., Ltd.
(9) Methacrylic acid: Made by Kuraray (10) Zinc methacrylate: Made by Asada Chemical Industry Co., Ltd. (11) Zinc acrylate: Made by Nippon Distillation Co., Ltd.

  After the rubber composition was vulcanized and molded in accordance with the ingredients shown in Table 1, the core surface was polished to a desired outer diameter, and then the core surface was trichloroisocyanuric acid in acetone (concentration 3 wt%). The core surface treatment is carried out by washing with water after immersion for 30 seconds. Next, this core is set in a cover injection mold, and the cover composition shown in Table 2 below is injection molded around the solid core.

The explanation in the table is as follows.
Product name "High Milan": Made by Mitsui DuPont Polychemical, ionomer resin Product name "Pandex": Dainippon Ink & Chemicals, thermoplastic polyurethane elastomer
Stoma Magnesium Stearate: Nippon Oil & Fats Titanium Dioxide: Trade name “Taipeke R550”, Ishihara Sangyo Co., Ltd. Polyethylene wax: Trade name “Sun Wax 161P”, Sanyo Chemical Industries

  The mold cavity has a number of convex protrusions corresponding to the dimple pattern so that a predetermined dimple pattern is formed on the cover surface. Is also typed. Details of the dimples are shown in Table 3 below. Further, the mark shown in FIG. 4 is printed on the surface of the ball, and further, the base material: polyester resin (acid value 6, OH value 168) (solid content) / butyl acetate / PMA (propylene glycol) Adduct body of hexamethylene diisocyanate as a curing agent: non-yellowing polyisocyanate (Takeda Pharmaceutical Co., Ltd., Takenate D-) with respect to 100 parts by mass of the main agent, with 70/15/15 parts by mass of monomethyl ether acetate) 160N, NCO content of 8.5% by mass, solid content of 50% by mass) is 150 parts by mass and butyl acetate is 150 parts by mass.

Details of each item in Table 3 are as follows.
R: radius of curvature R / D ratio of the roundness provided at the peripheral portion: ratio of radius of curvature (R) to diameter (D) N: total number of dimples RA: total dimples in which (R / D ratio) is 20% or more Number ER: Ratio of (RA) to total number of dimples (N) DE: A dimple whose radius of curvature (R) of a dimple having a diameter (D) larger than its own diameter (D) is equal to or less than its own radius of curvature (R) The total number DER of the largest diameter dimples having the largest diameter (D): the ratio of (DE) to the total number of dimples (N) SR: the sum of the dimple areas defined by the plane surrounded by the edges of the dimples, Surface occupancy rate VR, which is the ratio of the surface area of the phantom sphere assuming that no dimples exist on the surface of the ball: Dimple space volume formed downward from the plane surrounded by the edges of the dimples Is the proportion of the volume of the phantom sphere assuming that no dimples exist on the ball surface.

  The physical properties of the cores and covers of the examples and comparative examples thus obtained and the physical properties, flight distance, durability and feel of the practice balls are evaluated according to the following criteria. The results are shown in Table 4.

Deflection amount of core and ball products (mm)
The core and ball products, which are the target spheres, are compressed at a speed of 10 mm / min using “4204 type” manufactured by Instron Corporation, and an initial load of 98 N (10 kgf) to a final load of 1,275 N (130 kgf) is loaded. Measure the difference (mm) in the amount of deflection up to the time.

Resilience of core and ball products (initial speed)
The initial speed is 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 cores and balls as samples were temperature-controlled at 23 ± 1 ° C. for 3 hours or more and tested in a room at room temperature 23 ± 2 ° C. Each of 10 samples is hit twice, the time passing between 6.28 ft (1.91 m) is measured, and the initial speed is calculated.

Cover material hardness A cover sheet is prepared, and the hardness after temperature adjustment to 23 ± 1 ° C. is measured according to Shore D (ASTM D-2240).

Cracking durability Head speed (HS) = 38 m / s, No. 9 iron (X-BLADE GR manufactured by Bridgestone Sports Co., Ltd.) at the leading edge after hitting the same part of the ball 5 times (top hit), with an incident speed of 43 m / s The average number of N = 3 cracks when repeatedly hitting the wall is obtained.

Put 10 golf balls and 3 liters of sand for bunker in a magnetic ball mill with 8 liters of wear test capacity and mix for 144 hours. Degree of mark breakage, surface damage, and gloss reduction due to sand wear. And the degree of sand adhesion was examined visually. The appearance of the ball is evaluated on the basis of good (◯), normal (Δ), and bad (×).

Film thickness measurement bank (Y): Measures the thickness of clear coating on the bank at the center between the dimples.
Edge part (Z): The thickness of the clear coating of the dimple edge part is measured.
Two balls are measured at three locations, and the average value is calculated.

As a flying distance driver (W # 1), a Bridge Stage Sports Tour Stage X-DRIVE 701 (loft angle: 9 °) is attached to the striking robot, and the carry flying distance is measured at a head speed (HS) of 52 m / s. The hitting condition is the above head speed. When hitting the ball of Example 1, the average number of hits is 5 times, the initial ball speed is about 69 m / s, the launch angle is about 11 degrees, and the initial backspin amount is about 3100 rpm. Set the conditions to
Further, the ball after the wear test is hit under the same conditions as described above, and the carry flight distance is measured.

The golf ball for practice of Comparative Example 1 is obtained by blending zinc methacrylate as a co-crosslinking agent for the core rubber composition, and as a result, the crack durability is poor.
The golf ball for practice of Comparative Example 2 is obtained by blending zinc acrylate as a co-crosslinking agent for the core rubber composition, and as a result, the crack durability is very poor.
The golf ball for practice of Comparative Example 3 is obtained by blending zinc acrylate as a co-crosslinking agent for the core rubber composition, and as a result, the crack durability is very poor. Further, the cover is formed using ionomer as a main material. As a result, the mark durability is very poor, and the flying performance after the wear test (after mark durability evaluation) is greatly reduced.
The golf ball for practice of Comparative Example 4 is obtained by blending zinc acrylate as a co-crosslinking agent for the core rubber composition, and as a result, the crack durability is very poor. Further, since the radius of curvature of the dimple peripheral portion is small and the film thickness ratio between the dimple edge and the bank portion is small, the mark durability is very poor and the flying performance after the wear test (after mark durability evaluation) is also lowered.
The golf ball for practice of Comparative Example 5 has a small radius of curvature at the periphery of the dimple and a small film thickness ratio between the dimple edge and the bank, so that the mark durability is very poor, and after the wear test (after the mark durability evaluation) The flight performance will also be reduced.
In the golf balls for practice of Comparative Examples 6 and 7, the cover is formed using ionomer as the main material, the radius of curvature of the dimple peripheral edge is small, and the film thickness ratio of the dimple edge to the bank is small. As a result, the mark durability is very poor, and the flight performance after the wear test (after mark durability evaluation) is greatly reduced.

1 Core 2 Cover D Dimple G Golf ball

Claims (9)

The ball has a deflection amount BH when the ball is loaded from an initial load of 98 N (10 kgf) to a final load of 1,275 N (130 kgf) of 2.0 to 4.5 mm. Carry C when hitting the ball at a head speed (HS) of 52 m / s with a TourStage X-DRIVE 701 (Loft angle: 9 °) manufactured by Bridgestone Sports as a driver (W # 1) attached to the striking robot. Is set in advance, and the following formula (1)
C = A−33 × BH (1)
(However, 295 ≦ A ≦ 325)
A golf ball for practice characterized in that the deflection amount BH of the ball is adjusted so as to satisfy the above.   The golf ball for practice according to claim 1, wherein the core is formed of a rubber composition containing a base rubber, a co-crosslinking agent, a crosslinking initiator, and a metal oxide, and methacrylic acid is used as the co-crosslinking agent.   The golf ball for practice according to claim 1 or 2, wherein the initial velocity of the ball is 76 m / s or less.   The ratio (VR) of the total volume of the dimple space formed below the plane surrounded by the edge of the dimple to the volume of the phantom sphere assuming that there is no dimple on the ball surface is 0.8 to 1.7. The golf ball for practice according to any one of claims 1 to 3. The golf ball for practice according to any one of claims 1 to 4, wherein dimples satisfying the following requirements (1) and (2) are formed on a surface.
(1) It has a dimple provided with a radius of curvature (R) of 0.5 to 2.5 mm at the peripheral edge.
(2) The ratio (ER) of the total number of dimples (RA) to the total number of dimples (N) in which the ratio (R / D ratio) of the radius of curvature (R) to the diameter (D) is 20% or more is 15 to 95. %. Furthermore, the golf ball for practice of Claim 5 which satisfies the following requirements (3).
(3) A dimple having a plurality of types of dimples having different diameters (D) and having a radius of curvature (R) of a dimple having a diameter (D) larger than its own diameter (D) equal to or less than its own radius of curvature (R) The ratio (DER) of the total number (DE) of the maximum diameter dimples having the maximum diameter (D) to the total number of dimples (N) is 80% or more. Furthermore, the golf ball for practice of Claim 6 which satisfies the following requirements (4)-(6).
(4) It has three or more types of dimples having different diameters (D).
(5) The total number of dimples (N) is 380 or less.
(6) The surface occupancy (SR), which is the ratio of the total dimple area defined by the plane surrounded by the dimple edges to the surface area of the phantom sphere assuming that no dimples exist on the ball surface, is 60 to 74 %.   The resin component of the cover is mainly composed of polyurethane, the material hardness of the cover is 30 to 57 in Shore D hardness, and the thickness of the cover is 0.3 to 1.9 mm. The golf ball for practice according to any one of 1 to 7.   In the blending of each component of the rubber composition, 10 to 40 parts by weight of methacrylic acid, 15 to 30 parts by weight of metal oxide, and 0.3 to 5. The golf ball for practice according to claim 1, wherein 0 part by mass is blended.

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