Solid Golf Ball

1. Field of the Invention

The present invention relates to a solid golf ball comprising a solid core having a three-layered structure coated with a cover.

2. Description of the Related Art

A two-piece solid golf ball is generally known, in which a solid core having a single-layered structure is coated with a cover. Such a two-piece solid golf ball can provide a long carry. However, the two-piece solid golf ball has a drawback in that a hard feeling is given when the ball is hit. In order to improve the drawback of the two-piece solid golf ball, a three-piece solid golf ball comprising a solid core having a two-layered structure has been developed.

For example, a three-piece solid golf ball is disclosed in Japanese Laid-Open Patent Publication No. 60-241464, in which a solid core is divided into two. Namely, the solid core comprises an inner core and an outer core. The inner core has a JIS-C hardness of 50 to 70, and the outer core has a JIS-C hardness of 70 to 90. Thus it is intended to give a soft hitting feeling by using the soft inner core and the hard outer core.

However, in the case of the golf ball as described above, when a large striking force is exerted on the golf ball and even the inner core undergoes deformation of the ball, it is impossible to put suitable spin on the ball in order to obtain an optimum ballistic curve. As a result, an inconvenience arises in that the carry is lowered. Further, when a shot is performed such that an outer portion of the ball is locally deformed, it is impossible to always obtain a good hitting feeling, due to the influence caused by the hardness of the outer core.

A general object of the present invention is to provide a solid golf ball which makes it possible to obtain a sufficient carry and a soft hitting feeling.

A principal object of the present invention is to provide a solid golf ball which makes it possible to put suitable spin on the ball so that an optimum ballistic curve is obtained.

Another object of the present invention is to provide a solid golf ball which makes it possible to suppress distortion or deformation of the ball upon making a shot.

Still another object of the present invention is to provide a solid golf ball which makes it possible to obtain high resilience.

The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.

FIG. 1 shows a cross-sectional structure of a solid golf ball according to the first invention.

FIG. 2 shows a cross-sectional structure of a solid golf ball according to the second invention.

As shown in FIG. 1, a solid golf ball according to the first invention has its cross-sectional structure comprising a solid core 1 composed of three layers, and a cover 2 for coating the solid core 1.

The solid core 1 comprises an inner layer 1a, an intermediate layer 1b having a JIS-C hardness of 50 to 80, and an outer layer 1c having a hardness which is higher than the hardness of the intermediate layer 1b.

If the hardness of the intermediate layer 1b is lower than a JIS-C hardness of 50, the golf ball becomes excessively soft. As a result, the resilience is deteriorated, and the carry is lowered. On the contrary, if the hardness of the intermediate layer 1b is higher than a JIS-C hardness of 80, the golf ball becomes excessively hard. As a result, the hitting feeling is deteriorated. Even when the intermediate layer 1b has a JIS-C hardness of 50 to 80, if the hardness of the outer layer 1c is lower than the hardness of the intermediate layer 1b, it becomes impossible to obtain a sufficient carry.

The inner layer 1a is formed to be relatively hard. However, in some cases, the inner layer 1a may have a hardness lower than the hardness of the intermediate layer 1b. Preferably, the inner layer 1a has a JIS-C hardness of 60 to 95. Preferably, the outer layer 1c has a JIS-C hardness of not less than 65.

Each of the layers of the solid core 1 is usually formed of a rubber composition comprising a base material composed of a natural rubber or a synthetic rubber. However, it is also possible to use a material containing, for example, an ionomer resin and a thermoplastic elastomer composed of styrene, olefin, urethane, ester, or amide.

In the first invention, it is preferable that the intermediate layer 1b has a thickness of 1 to 13 mm, and the inner layer 1a and the intermediate layer 1b have a combined diameter of 21 to 39 mm. It is allowable to alter these ranges to some extent. Usually, the inner layer 1a has a diameter of about 6 to 30 mm, and the solid core 1 has a diameter of about 37 to 41 mm. However, there is no limitation to these ranges. If the size (diameter or thickness) of each of the layers of the solid core 1 is greatly deviated from the ordinary size, it is impossible to obtain a suitable distribution of hardness in the solid core 1, and it is impossible to obtain the effect of the first invention.

Preferably, the solid golf ball according to the first invention is produced in accordance with a specification for the solid core 1 as shown in Table 1, when the inner layer 1a of the solid core 1 has a diameter of 6 to 17 mm.

In order to suppress excessive distortion or deformation caused by a shot in which the ball is greatly distorted or deformed, it is preferable that the hardness of the inner layer 1a is higher than the hardness of the intermediate layer 1b. Even when the hardness of the inner layer 1a is increased, the hitting feeling is soft, because the inner layer 1a has a small diameter. The solid golf ball, which comprises the solid core 1 produced in accordance with the specification described above, makes it possible to put suitable spin on the ball especially in the case of a shot in which the distortion or deformation of the ball is large. Accordingly, the carry is not lowered.

Further, the solid golf ball according to the first invention is preferably produced in accordance with a specification for the solid core 1 as shown in Table 2, when the inner layer 1a of the solid core 1 has a diameter of 17 to 30 mm.

The solid golf ball according to the specification shown in Table 2 has a large diameter of the inner layer 1a as compared with the solid golf ball according to the specification shown in Table 1. Therefore, the inner layer 1a preferably has a low hardness. Further, in the former solid golf ball, the intermediate layer 1b is arranged at an outer portion as compared with the latter solid golf ball. Therefore, the intermediate layer 1b preferably has a high hardness. The solid golf ball comprising the solid core 1 according to the specification shown in Table 2 gives a good hitting feeling especially in the case of a shot in which the distortion or deformation of the ball is small.

As described above, the two types of solid golf balls having different characteristics are obtained depending on the specification for the solid core 1. However, the high resilience is obtained by using any of them, and there is no deterioration of the ballistic curve, which would be otherwise caused by decrease in number of spinning rotation, owing to the solid core 1 having the three-layered structure comprising the inner layer 1a, the intermediate layer 1b having the JIS-C hardness of 50 to 80, the outer layer 1c having the hardness higher than the hardness of the intermediate layer 1b. Therefore, a sufficient carry is obtained. Moreover, a soft hitting feeling is obtained owing to the soft intermediate layer 1b.

As shown in FIG. 2, a solid golf ball according to the second invention has its cross-sectional structure comprising a solid core 3 composed of three layers, and a cover 4 for coating the solid core 3.

The solid core 3 comprises an inner layer 3a having a JIS-C hardness of 40 to 90, an intermediate layer 3b formed of a thermoplastic resin composition and having a JIS-C hardness of 50 to 80, and an outer layer 3c having a JIS-C hardness of not less than 65.

The intermediate layer 3b is formed from a composition comprising a component of a thermoplastic resin, not from a polybutadiene rubber composition as used for the conventional solid core. Those usable as the thermoplastic resin include, for example, an ionomer resin and a thermoplastic elastomer composed of styrene, olefin, urethane, ester, or amide. However, it is preferable to use those having a characteristic that the lower the hardness is, the higher the resilience is. For example, it is preferable to use a thermoplastic polyamide elastomer. It is preferable to use those having a Shore D hardness of 20 to 50 as the thermoplastic resin. If those having a Shore D hardness higher than 50 are used, the intermediate layer 3b has a high hardness. Therefore, the hitting feeling is deteriorated, and the resilience of the ball is deteriorated in some cases. The blending amount of the thermoplastic resin is not especially limited. However, the thermoplastic resin is blended in an amount of not less than 50% by weight with respect to a total weight of the composition. The thermoplastic resin composition may be blended with, for example, a filler, a coloring agent, an anti-aging agent, and a dispersing agent, if necessary.

When the inner layer 3a is soft, the carry may be lowered in the case of a shot in which the distortion or deformation of the ball is large. However, the shortage of carry is supplemented by the high resilience of the intermediate layer 3b. Accordingly, the hardness of the inner layer 3b can be lowered up to a JIS-C hardness of 40. On the other hand, the solid golf ball is provided with the intermediate layer 3b having the low hardness. Therefore, the hitting feeling is not deteriorated even when the hardness of the inner layer 3a is raised up to a JIS-C hardness of 90.

Even when the hardness of the outer layer 3c is lowered up to a JIS-C hardness of 65, a sufficient carry is obtained, because the solid golf ball is provided with the intermediate layer 3b having the high resilience. If the hardness of the outer layer 3c is excessively high, the hitting feeling is deteriorated. Therefore, it is preferable that the outer layer 3c has a JIS-C hardness of 65 to 97.

Preferably, the inner layer 3a and the outer layer 3c are composed of rubber compositions comprising a base material of cis-1,4-polybutadiene which is used for the conventional solid core. However, the outer layer 3c can be also formed of a thermoplastic resin composition containing, for example, an ionomer resin or a thermoplastic elastomer.

In the second invention, it is preferable that the intermediate layer 3b has a thickness of 1 to 5 mm, and the inner layer 3a and the intermediate layer 3b have a combined diameter of 26 to 39 mm. These ranges can be altered to some extent. However, if these ranges are greatly altered, it is impossible to obtain the effect of the second invention. In the second invention, the inner layer 3a may have a diameter of about 17 to 30 mm, and the solid core 3 may have a diameter of about 37 to 41 mm. However, there is no limitation to these ranges.

In order to produce the solid cores 1, 3 according to the first and second inventions, for example, it is possible to use a method equivalent to the conventional method for producing a solid core having a two-layered structure. Namely, the rubber composition for the inner layer 1a or 3a is heated and molded in a mold under a pressurized condition to form the inner layer 1a or 3a. The obtained inner layer 1a or 3a is coated with hemispherical shells formed from the resin composition for the intermediate layer 1b or 3b, followed by heating and molding in a mold under a pressurized condition to produce the intermediate layer 1b or 3b. Next, the outer layer 1c or 3c is produced in the same manner as the intermediate layer 1b or 3b. Further, the resin composition to be used for the intermediate layer 1b or 3b may be molded by using an injection molding machine for thermoplastic resins. Therefore, the intermediate layer 1b or 3b and the outer layer 1c or 3c can be produced by means of injection molding.

The cover 2 or 4 for coating the solid core 1 or 3 can be formed of a resin composition comprising a base material of an ionomer resin used for the conventional solid golf ball. The cover 2 or 4 can have a thickness approximately equivalent to those used for the conventional golf ball (usually, 1.3 to 2.8 mm).

Rubber compositions for inner layers 1a, intermediate layers 1b, and outer layers 1c blended as shown in Table 3 were used to produce solid cores 1 of the three-layered structure having diameters, weights, and hardnesses as shown for Examples 1 to 5 and Comparative Examples 2 and 3 in Table 4. A resin composition for a cover 2 was prepared by blending 1.5 part by weight of titanium dioxide with respect to 100 parts by weight of an ionomer resin. Each of the solid cores 1 was coated with the prepared resin composition for the cover 2, followed by being subjected to polishing and painting to produce solid golf balls concerning Examples 1 to 5 and Comparative Examples 2 and 3.

In Table 4, symbols are as follows:

*1 (carry), *2 (number of spinning rotation): measured by striking the ball by using a swing robot with a wood No. 1 club (1W) at a club head speed of 40 m/sec;

*3 (hitting feeling): evaluated by hitting the ball by an expert golfer (man) with a wood No. 1 club (1W) and an iron No. 9 club (9I); .circleincircle..fwdarw.extremely good, .smallcircle..fwdarw.good, X.fwdarw.bad.

A conventional two-piece solid golf ball was produced by using a rubber composition blended as shown in Table 3 to prepare a solid core having a single-layered structure, and coating the prepared solid core with the resin composition for a cover as used in Examples 1 to 5 and Comparative Examples 2 and 3, followed by being subjected to polishing and painting.

The carry, the number of spinning rotation, and the hitting feeling were investigated and shown in Table 4 for the solid golf balls concerning Examples 1 to 5 and Comparative Examples 1 to 3. As shown in Table 4, the solid golf balls concerning Examples 1 to 5 gave carries approximately equivalent to a carry given by using the conventional two-piece solid golf ball concerning Comparative Example 1. The solid golf balls concerning Examples 1 to 5 were evaluated such that they gave good hitting feelings.

On the contrary, the solid golf ball concerning Comparative Example 2 gave a bad hitting feeling, because the intermediate layer of the solid core was excessively hard. The solid golf ball concerning Comparative Example 3 failed to give a sufficient carry, because the outer layer of the solid core was softer than the intermediate layer.

Rubber compositions for inner layers 3a blended as shown in Table 5 were heated and molded in a mold under a pressurized condition to form the inner layers 3a. Each of intermediate layers 3b was provided on each of outer surfaces of the obtained inner layers 3a by means of injection molding with a thermoplastic polyamide elastomer. Next, hemispherical shells in semi-vulcanized states were formed from resin compositions for outer layers 3c blended as shown in Table 5. Each of outer surfaces of the intermediate layers 3b was coated with the hemispherical shells, followed by heating and molding in a mold under a pressurized condition to provide the outer layer 3c. Thus solid cores 3 having the three-layered structure were produced. Each of the solid cores 3 was subjected to injection molding on its outer surface with a resin composition for a cover 4 obtained by blending 1.5 part by weight of titanium dioxide with respect to 100 parts by weight of an ionomer resin, followed by being subjected to polishing and painting to produce solid golf balls concerning Examples 6 to 9.

Solid golf balls concerning Examples 10 to 12 and Comparative Example 5 were produced in the same manner as Examples 6 to 9 except that mixtures of a thermoplastic polyamide elastomer and an ionomer blended as shown in Table 5 were used for intermediate layers 3b.

A conventional two-piece solid golf ball was produced such that a solid core having a single-layered structure produced from a rubber composition blended as shown in Table 5 was subjected to injection molding on its outer surface with the resin composition for the cover used in Examples 6 to 9, followed by being subjected to polishing and painting.

A solid golf ball was produced in the same manner as Examples 6 to 9 except that a thermoplastic polyamide elastomer harder than those used in Examples 6 to 9 was used for an intermediate layer 3b.

In Table 5, symbols are as follows:

*4: polyether-polyamide block copolymer, Shore D hardness of 25;

*5: polyether-polyamide block copolymer, Shore D hardness of 63;

*6: trade name, ionomer resin of sodium salt of ethylene-methacrylic acid copolymer produced by Du Pont-Mitsui Polychemical, Shore D hardness of 65.

The initial velocity and the hitting feeling were investigated and shown in Table 6 for the solid golf balls concerning Examples 6 to 12 and Comparative Examples 4 to 6. As shown in Table 6, the solid golf balls concerning Examples 6 to 12 gave initial velocities approximately equivalent to or more than an initial velocity given by using the conventional two-piece solid golf ball concerning Comparative Example 4. Therefore, the solid golf balls concerning Examples 6 to 12 were excellent in resilience. Further, the solid golf balls concerning Examples 6 to 12 were evaluated such that they gave good hitting feelings.

On the contrary, the solid golf ball concerning Comparative Example 5 was poor in resilience, because the outer layer of the solid core was soft. The solid golf ball concerning Comparative Example 6 gave a bad hitting feeling and it was poor in resilience, because the intermediate layer of the solid core was formed of the hard thermoplastic polyamide elastomer having a Shore D hardness of 63.

In Table 6, symbols are as follows:

*7: measured by using a method in accordance with the initial velocity test recommended by U.S.G.A. (United States Golf Association); the larger the numerical value is, the better the resilience is;

*8: evaluated by actually hitting the ball by an expert golfer (man); .circleincircle..fwdarw.extremely good, .smallcircle..fwdarw.good, X.fwdarw.bad.