Thread Wound Golf Ball

1. Field of the Invention

The present invention relates to a thread wound golf ball.

2. Description of the Related Art

A thread wound golf ball is obtained by winding a thread rubber around a solid or liquid rubber center to form a thread rubber layer and covering the thread rubber layer with a cover material (e.g. ionomer, balata. etc.). As the solid center, a vulcanized rubber of butadiene is conventionally used, which has high hardness and low compression deformation. In a golf ball using such a conventional center, the formation of spin is large and launch angle is small and, therefore, the golf ball is disadvantageous in view of flying distance. Further, the curving of the flight of the golf ball is too large and it is sometimes hard to control.

On the other hand, a liquid center has high compression deformation. In a golf ball using the liquid center, the formation of spin is small and launch angle is large and, therefore, the golf ball is advantageous in view of flying distance in comparison with a conventional solid center. In golf balls having liquid center, however, the production process is complicated. Also, there is a problem of safety, for example, when a ball is cut by a knife or a cutter, a liquid is sprayed out, which may result in loss of eyesight.

In order to solve the above problem, the present inventors have intensively studied. As a result, it has been found that, by using a flexible vulcanized rubber as a solid center of a thread wound golf ball, the formation of spin is suppressed and launch angle is increased, whereby, exceptionally long flying distance can be obtained, and the present invention has been completed.

The main object of the present invention is to provide a thread wound golf ball which is superior in flying distance and hit feeling.

This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description.

According to the present invention, there is provided a thread wound golf ball obtained by winding a thread rubber around a solid rubber center to form a thread rubber layer and covering the thread rubber layer with a cover layer, wherein the solid rubber center has a surface hardness (JIS-A) of not more than 60 and a deformation on 500 g loading of not less than 0.5 mm.

FIG. 1 is a cross-sectional view of a golf ball having a cover layer, a thread-wound layer, and a center.

FIG. 1 shows the cross-sectional structure of a thread wound golf ball, according to the present invention. The rubber center 1 has a surface hardness (JIS-A) of not more than 60, and a deformation on 500 g loading of not less than 0.5 mm, as discussed below. Thread rubber layer 2 comprises a thread rubber wound around the solid rubber center. Cover layer 3 covers the thread rubber layer.

The rubber center used in the present invention has a surface hardness (JIS-A) of not more than 60, preferably 10 to 45, and a deformation (500 g loading) of not less than 0.5 mm. When the surface hardness (JIS-A) of the rubber center is larger than upper limit of the above range, the formation of spin on hitting becomes large and impact strength is large, which results in inferior hit feeling. Further, when the deformation (500 g loading) is smaller than the lower limit of the above range, the formation of spin also becomes large and hit feeling becomes inferior.

The outer diameter of the rubber center is preferably 23 to 34 mm, more preferably 26 to 32 mm. When the outer diameter is smaller than 23 mm, the formation of spin becomes large and launch angle becomes small. On the other hand, when the outer diameter is larger than 34 mm, the thread rubber layer becomes thin and a golf ball having a predetermined hardness can not be obtained.

When the rubber center does not have a suitable impact resilience, the initial velocity on hitting becomes small, which results in bad influence ion the rubber center. The impact resilience is represented by the rebound height of the rubber center which is measured by dropping it on a rigid surface such as concrete block from a height of 254 cm (100 inches) at 23.degree. C. The impact resilience is preferably 90 cm or more. When it is smaller than 90 cm, the initial velocity of the golf ball becomes too small and the effect of the center is hardly obtained.

The rubber for the rubber center is not specifically limited, and examples thereof include butadiene rubber (BR), natural rubber (NR), ethylene-propylene-diene copolymer (EPDM), polynorbornene rubber, silicone rubber and the like. Polynorbornene rubber and silicone rubber are particularly preferred. Polynorbornene rubber can contain a large amount of oil and easily obtain flexibility. The polynorbornene rubber can also be controlled in a suitable impact resilience by selecting the oil.

In order to prevent deformation of the center during winding the thread rubber around the center, it is necessary to freeze the center in advance of winding. It is therefore preferred that the rubber will be solidified at a low temperature. (e.g. -30.degree. to -50.degree. C.). Polynorbornene rubber will solidify at the above range even if it contains a large amount of oil.

In order To reduce hardness and to obtain a suitable impact resilience, oil is added. Non-limited examples thereof include alkylbenzene oil, naphthenic oil, paraffinic oil, aromatic oil, ester plasticizer (e.g. Dioctyl adipate (DOA), Dioctyl phthalate (DOP), etc.) and the like. Among them, naphthenic oil or alkyl benzene oil, which hardly cause blooming even if a large amount of it is added and provide high impact resilience, are particularly preferred.

The rubber center is normally obtained by kneading 100 parts by weight of the rubber, 100 to 500 parts by weight of oil, 0.5 to 5 parts by weight of sulfur, 3 to 10 parts by weight of a vulcanization auxiliary, 1 to 5 parts by weight of a vulcanization accelerator and an optional amount of a modifier by a Banbury mixer or a roll and vulcanizing a mixture at 150.degree. to 170.degree. C. for 10 to 20 minutes.

As the silicone rubber, for example, there are heating vulcanization type and room temperature vulcanization type silicone rubbers. In general, heating vulcanization type silicone rubber obtained by copolymerizing dimethylsiloxane as a main component with a small amount of methyl vinyl siloxane is preferred because of its good impact resilience.

As the vulcanization method of the silicone rubber, for example, there are vulcanization methods using an organic peroxide, an aliphatic azo compound, radiation, and the like.

In general, an organic peroxide is used. 0.5 to 5 Parts by weight of vulcanizing agent and an optional amount of weight modifier (based on 100 parts by weight of silicone rubber) are kneaded by a Banbury mixer or a roll and the resulting mixture is vulcanized at 150.degree. to 170.degree. C. for 10 to 20 minutes to obtain a vulcanized silicone rubber.

By using a flexible vulcanized silicone rubber as a solid ball center for a thread wound core, the formation of spin is suppressed and launch angle is increased. Further, exceptionally long flying distance can be obtained due to the high impact resilience of silicon.

The above vulcanization auxiliary, vulcanization accelerator and weight modifier may be any which are normally used. As the weight modifier, those having large specific gravity are preferred in order to obtain low hardness by increasing a rubber fraction of the rubber center. Examples thereof include barium sulfate, calcium carbonate, clay filler, silica filler, and the like.

A thread rubber is then wound around the rubber center while applying a stretching force to the thread rubber. The thread rubber may be any which is normally used as a thread rubber for a golf ball.

The method for covering the rubber center with a cover (ionomer or balata) is not specifically limited. Normally, the rubber center is covered with two pieces of hemispherical shape covers which have been molded in advance, followed by compression molding. Further, the rubber center may be covered by subjecting a cover composition to injection molding.

As described above, according to the present invention, there is provided a thread wound golf ball superior in flying distance and hit feeling, which has no safety problems. Further, the production process for making this golf ball is simplified.

The following Examples further illustrate the present invention in detail but are not to be construed as limiting the scope thereof.

The formulations shown in Table 1 were mixed and then vulcanized at 155.degree. C. for 20 minutes to make rubber centers having a diameter of 30 mm. A thread rubber which was stretched about 10 times was wound around each rubber center. Then, the thread-wound rubber centers were covered with an ionomer resin by injection molding to make thread-wound golf balls with ionomer covers. The flight performances of the resulting golf balls were evaluated by a conventional method. The results are shown in Table 2.

As is apparent from Table 2, regarding the golf balls of Examples 1 to 7, the flying distance was 4 to 6 yards longer than that of Comparative Examples 1 and 2. Further, as the hardness of the rubber center becomes small and the outer diameter of the rubber center becomes larger, spin was reduced and launch angle was enhanced. Regarding Comparative Example 3, the hardness of the center is small but the elasticity is extremely low and, therefore, long flying distance can not be obtained.

Professional golfers were asked to hit the golf balls and provide their evaluations. As a result, the golf balls of Examples 1 to 7 using a flexible rubber as the center had excellent hit feeling in comparison with Comparative Examples 1 to 3. The flying distance of the golf balls of Examples 1 to 7 was almost the same as that of a two piece golf ball.

According to the same manner as that described in Examples 1 to 7, a thread wound golf ball with a balata cover was made, respectively, using the formulations shown in Table 3. The flight performances of the resulting golf balls were evaluated by a conventional method. The results are shown in Table 4.

According to the same manner as that described in Examples 1 to 7, thread-wound golf balls with an ionomer cover were made, respectively, using the formulations shown in Table 5. The flight performances of the resulting golf balls were evaluated by a conventional method. The results are shown in Table 6.

According to the same manner as that described in Examples 8 to 10, thread-wound golf balls with a balata cover were made, respectively, using the formulations shown in Table 7. The flight performances of the resulting golf balls were evaluated by a conventional method. The results are shown in Table 8.

As is apparent from Table 6, regarding the golf balls of Examples 11 to 14, the flying distance was 3 to 5 yards longer than that of Comparative Examples 5 and 6. Further, as the hardness of the rubber center becomes smaller than that of Examples 11 to 13 and Comparative Example 5 and the outer diameter of the rubber center becomes larger than that of Examples 11 and 14 and Comparative Examples 5 and 6, spin was reduced and launch angle was enhanced. In a golf ball with a balata cover as shown in Table 8, regarding the balls of Examples 15 to 17, the flying distance was 4 to 6 yards longer than that of Comparative Example 7.

Professional golfers were asked to hit the golf balls and provide their evaluations. As a result, the golf balls of Examples 11 to 14 using a vulcanized silicone rubber as the center had excellent hit feeling in comparison with Comparative Examples 5 to 7. The flying distance of the golf balls of Examples 11 to 14 was almost the same as that of a two piece golf ball.