The present invention relates to a thread wound golf ball. More particularly, it relates to a thread wound golf ball which keeps the good shot feel inherently present in thread wound golf balls and attains a long flight distance.

A thread wound golf ball is obtained by winding a thread rubber on a solid or liquid rubber centre to form a thread rubber layer and covering on the thread rubber layer with a cover material (e.g. ionomer, balata, etc.).

The thread wound golf ball is superior in shot feel and control properties to a two-piece solid golf ball using a solid core. However, it can not attain a long flight distance because of high spin and a low launch angle in comparison with the two-piece solid golf ball. Therefore, general amateur golfers tend to prefer the two-piece solid golf ball which attains a long flight distance in comparison with the thread wound golf ball, and the two-piece solid golf ball have recently been put on the market, exclusively.

The present invention has been accomplished in order to solve a problem that the conventional thread wound golf ball merely attains a relatively short flight distance. The main object of the present invention is to provide a thread wound golf ball which keeps the good shot feel inherently present in thread wound golf balls and attains a long flight distance which is equal to that attained by the two-piece solid golf ball.

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 with reference to the accompanying drawing in which:

Fig. 1 is a schematic cross section illustrating one embodiment of a thread wound golf ball of the present invention.

According to the present invention, the above object has been accomplished by increasing the diameter of the centre of the thread wound golf ball and making the centre hard to control the amount of spin applied and to increase the launch angle to increase the flight distance.

That is, the present invention provides a thread wound golf ball comprising a centre comprising a vulcanized moulded article of a rubber composition, a thread rubber layer formed on said centre and a cover covering on said thread rubber layer, wherein said centre has a diameter of 30 to 35 mm and wherein the strain amount formed between an initial loading of 10 kg and a final loading of 30 kg is 1.2 to 2.5 mm.

The centre can be obtained, for example, by formulating 5 to 80 (preferably 15 to 50) parts by weight of a thermoplastic resin as a filler for adjusting hardness, 2 to 12 (preferably 8 to 10) parts by weight of sulfur,1 to 4 (preferably 1 to 2) parts by weight of a vulcanization accelerator and 10 to 150 (preferably 50 to 120) parts by weight of a weight adjustor and, if necessary, 3 to 10 (preferably 5 to 9) parts by weight of a vulcanization auxiliary, based on 100 parts by weight of a rubber, to give a rubber composition and subjecting the rubber composition to a vulcanization moulding.

In the present invention, the reason why large flight distance of the thread wound golf ball can be attained while maintaining good shot feel by adjusting the diameter of the centre at 30 to 35 mm and the strain amount formed between initial loading (10 kg) and final loading (30 kg) at 1.2 to 2.5 mm is not clear at present, but is considered as follows.

It is considered that, by adjusting the diameter of the centre at 30 to 35 mm and the strain amount formed between initial loading (10 kg) and final loading (30 kg) at 1.2 to 2.5 mm, the deformation behaviour of the golf ball at the time of hitting becomes similar to that of the two-piece solid golf ball. As a result, the amount of spin formed at the time of hitting is controlled and the launch angle becomes large, thereby increasing the flight distance.

Further, the reason why the thread wound golf ball can keep a good shot feel which is an inherent characteristic in thread wound golf balls regardless of improvement of the flight distance as described above is considered to be because the tension of the thread rubber layer is small in comparison with a conventional thread wound golf ball.

When the diameter of the centre is smaller than 30 mm, the thread rubber layer becomes thick and, as a result, the launch angle becomes small and the spin amount becomes large. On the other hand, when the diameter of the centre is larger than 35 mm, the thread rubber layer becomes thin and the thread rubber has already been wound before the tension is formed so that a suitable hardness of the golf ball can not be obtained.

Further, when the strain amount of the centre is larger than 2.5 mm under the above condition, the thread rubber must be wound tightly so as to obtain a proper ball hardness. As a result, the tension of the thread rubber layer becomes large and the deformation at the time of hitting does not easily arise, thereby obtaining no desired technical effects. On the other hand, when the strain amount of the centre is smaller than 1.2 mm under the above condition, the shot feel becomes inferior.

Further, it is preferred that the height of rebound of the centre is 120 cm or more, particularly 140 to 240 cm, when dropping it on a concrete board from the height of 254 cm. That is, the fact that the centre has the large height of rebound shows that the impact resilience of the centre is large, and when the centre has the large impact resilience, the ball initial velocity at the time of hitting becomes large and, therefore, good flying performances can be obtained. On the other hand, when the height of rebound of the centre is smaller than the above range, the ball initial velocity becomes small and, therefore, long flight distances can not be obtained easily.

As described above, the centre may be composed of the vulcanized moulded article of the rubber composition obtained by formulating 5 to 80 (preferably 15 to 50) parts by weight of a thermoplastic resin as a filler for adjusting hardness, 2 to 12 (preferably 8 to 10) parts by weight of sulfur, 1 to 4 (preferably 1 to 2) parts by weight of a vulcanization accelerator and 10 to 150 (preferably 50 to 120) parts by weight of a weight adjustor and, if necessary, 3 to 10 (preferably 5 to 9) parts by weight of a vulcanization auxiliary, based on 100 parts by weight of a rubber. The vulcanization is normally conducted by heating at 140 to 170°C (preferably 150 to 160 °C) under pressure for 5 to 30 minutes (preferably 10 to 20 minutes).

The rubber as the centre is not specifically limited, and a polybutadiene having high resilient performances (particularly high-cis polybutadiene) is preferred. It is preferred that high-cis polybutadiene or a rubber containing high-cis polybutadiene as a main component is used in the preparation of the rubber composition for the centre.

The thermoplastic resin is not specifically limited, and there can be preferably used high-molecular weight polyolefins such as highstyrene resin, high-molecular weight polyethylene, high-molecular weight polypropylene, etc., or a mixture thereof.

The above rubber composition for a centre differs from the conventional rubber composition for a centre in formulating the thermoplastic resin as the filler for adjusting hardness.

The thermoplastic resin is used for adjusting the hardness of the centre and the amount is, as described above, 5 to 80 parts by weight, preferably 15 to 50 parts by weight, based on 100 parts by weight of the rubber. When the amount of the thermoplastic resin is smaller than the above range, the hardness of the centre can not be sufficiently increased. Therefore, the hardness becomes the same as that of a conventional centre, thereby affording no desired effect. On the other hand, when the amount of the thermoplastic resin is larger than the above range, the hardness becomes too high and, therefore, the shot feel becomes inferior and the workability at the time of kneading of rubber also becomes inferior.

The amount of sulfur is, as described above, 2 to 12 parts by weight, preferably 8 to 10 parts by weight, based on 100 parts by weight of the rubber. When the amount of sulfur is smaller than the above range, the vulcanization degree becomes low and, therefore, the desired hardness of the centre can not be obtained easily. On the other hand, when the amount of sulfur is larger than the above range, the hardness of the centre becomes too high and, therefore, the desired effect can not be obtained.

Examples of the vulcanization auxiliary include metal oxides (e.g. zinc oxide, magnesium oxide, etc.) and higher fatty acids (e.g. stearic acid, palmitic acid, oleic acid, lauric acid, etc.). This vulcanization auxiliary is preferably used for conducting the vulcanization smoothly, but is not necessarily required.

The vulcanization accelerator may be any one which can be used as the normal vulcanization accelerator of the sulfur vulcanization, and typical examples thereof include thiazole vulcanization accelerators such as 2-mercaptobenzothiazole, dibenzothiazyl disulfide, etc.; thiuram vulcanization accelerators such as tetramethylthiuram monosulfide, tetramethylthiuram disulfide, etc.; sulfenamide vulcanization accelerators such as N-cyclohexyl-2-benzothiazyl sulfenamide, etc., but it is not limited to those described above.

The vulcanization auxiliary and vulcanization accelerator may be formulated according to the amount of sulfur to be formulated. The amount of the vulcanization auxiliary is preferably 3 to 10 parts by weight, particularly 5 to 9 parts by weight, based on 100 parts by weight of the rubber, and the amount of the vulcanization accelerator is preferably 1 to 4 parts by weight, particularly 1 to 2 parts by weight, based on 100 parts by weight of the rubber. When the amount of them is smaller than the above range, the vulcanization due to sulfur can not be sufficiently conducted. On the other hand, when the amount of them is larger than the above range, the hardness of the centre becomes too high, which results in deterioration of shot feel.

Examples of the weight adjustor include barium sulfate, clay,
   calcium carbonate, silica filler and the like, and the amount is, as described above, preferably 10 to 150 parts by weight, particularly 50 to 120 parts by weight, based on 100 parts by weight of the rubber.

When the amount of the weight adjustor is smaller than the above range, the weight of the centre becomes small and, the proper weight as the golf ball can not be obtained. On the other hand, when the amount of the weight adjustor is larger than the above range, the weight of the centre becomes large and, therefore, the ball weight exceeds the standard value.

To the rubber composition for centre, antioxidants, etc. may be added, in addition to the above components.

The construction of the golf ball of the present invention will be explained with reference to the accompanying drawing. Fig. 1 is a schematic cross section illustrating one embodiment of the thread wound golf ball of the present invention. In Fig. 1, 1 is a centre, 2 is a thread rubber layer and 3 is a cover.

The centre 1 is composed of a vulcanized moulded article of a rubber composition and the diameter of the centre is 30 to 35 mm. Further, the strain amount of the centre, which is formed between initial loading (10 kg) and final loading (30 kg), is within a range of 1.2 to 2.5 mm.

The thread rubber layer 2 is formed by winding a thread rubber around the centre 1, and a so-called thread wound core is composed of the centre 1 and thread rubber layer 2 constitute.

As the thread rubber used for forming the thread rubber layer 2, there can be used the same thread rubber which has hitherto been used. For example, there can be used those obtained by vulcanizing a rubber composition wherein sulfur, a vulcanization auxiliary, a vulcanization accelerator, an antioxidant, etc. are formulated in a natural rubber, or a natural rubber and a synthetic polyisoprene.

The thread rubber layer 2 is covered with the cover 3 and 3a is a dimple provided on the cover 3 and, if necessary, a suitable number of dimples 3a may be provided according to the desired characteristics.

As the cover 3, there can be used both ionomer cover containing ionomer as a main material and balata cover, but the ionomer cover is preferred in view of increase in flight distance.

A method of covering the core with cover is not specifically limited, and the covering is conducted by a normal method. For example, there can be used a method comprising moulding two half-shells having a semispherical shape in advance, covering a core using them and subjecting to a pressure moulding at 130 to 1 70GC for 1 to 15 minutes, or a method comprising injection-moulding a composition for cover directly on a core to cover the core. The thickness of the cover is normally about 1 to 4 mm. Then, a dimple may be optionally formed on the surface of the golf ball at the time of cover moulding. Further, paint finishing, stamping, etc. may be optionally provided after cover moulding.

According to the present invention, there could be provided a thread wound golf ball which attains a long flight distance while maintaining a good shot feel as a characteristic of the thread wound golf ball, by increasing a diameter of the centre and making the centre hard in comparison with a conventional thread wound golf ball.

The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof.

Examples 1 to 5 and Comparative Examples 1 to 7.

A rubber composition for centre having a formulation shown in Tables 1 and 2, and the resulting rubber composition was charged in a mould for centre and subjected to compression moulding/vulcanization at 155°C for 20 minutes to give a centre, respectively.

The diameter, the JIS-A hardness (hardness measured by a JIS-A hardness tester), the strain amount, the height of rebound and the weight of the resulting centre are shown in Tables 1 and 2.

The amount of the respective formulation amount to be formulated shown in Tables 1 and 2 is represented by parts by weight. The diameter, the JIS-A hardness (hardness measured by a JIS-A hardnesstester), the strain amount, the height of rebound and the weight of the resulting centre are shown in Tables 1, in addition to the composition of Examples 1 to 5. Those as to Comparative Examples 1 to 7 are shown in Table 2.

Further, the measuring method of the strain amount and height of rebound is as follows. The explanation of the formulation component will be described the back of Table 2.

The strain amount formed between initial loading (10 kg) and final loading (30 kg) is measured.

The height of rebound of the centre is measured when dropping it on a concrete board from the height of 254 cm.
disulfide, manufactured by Ohuchi Shinko Kagaku Kogyo Co., Ltd.) and 1.25 parts by weight of Noxxelar CZ-G (trade name, Ncyclohexyl-2-benzothiazyl sulfenamide, manufactured by Ohuchi Shinko Kagaku Kogyo Co., Ltd.)
6:  Barium sulfate (manufactured by Sakai Kagaku Kogyo Co., Ltd.

Next, a thread rubber comprising a natural rubber/low-cis synthetic polyisoprene (50:50, weight ratio) [Shell IR-309 (trade name), manufactured by Shell Kagaku Co., Ltd.] as the base rubber was wound around each centre thus obtained as described above to form a thread rubber layer, thereby preparing a thread wound core of 39.5 mm in outer diameter.

The above core was covered with a pair of semispherical halfshells moulded from the composition for cover, followed by subjecting to a press moulding in a mould for ball at 150°C for 3 minutes to give a thread wound golf ball. The resulting golf ball was coated with a paint to finish a golf ball of 42.7 mm in outer diameter. The composition for cover is obtained by formulating 2 parts by weight of titanium dioxide in an ionomer mixture of Hi-milane 1605 (trade name) and Hi-milane 1706 (trade name) (50:50, weight ratio), which are ionomer neutralized with a sodium ion manufactured by Mitsui Du Pont Polychemical Co., Ltd. and ionomer neutralized with a zinc ion manufactured by Mitsui Du Pont Polychemical Co., Ltd., respectively.

The weight, the compression, the flight performances and the shot feel of the resulting golf ball were examined. The flight performances were examined as to the case when hitting with a No. 1 wood club (flight performances 1) and the case when hitting with a No. 5 iron club (flight performances 2).

The evaluation results of the weight, the compression, the flight performances 1 ) and 2) and the shot feel of the golf balls of Examples 1 to 5 are shown in Table 3. Those of the golf balls of Comparative Examples 1 to 4 were shown in Table 4 and those of the golf balls of Comparative Examples 5 to 7 are shown in Table 5.

Further, the measuring method of the compression, flight performances 1 and flight performances 2 as well as the evaluation method of the shot feel and the evaluation criteria are as follows.

An initial load (10 kg) is applied on the golf ball, and then the load is gradually increased to the final load (130 kg). The amount of strain formed between initial loading and final loading of the golf ball is measured according to PGA system.

A Swing robot manufactured by True Temper Co. is equipped with a No. 1 wood club and the golf ball is hit at a head speed of about 45 m/second to measure the flight performances 1. The spin amount is determined by taking a photograph of the golf ball to be hit. The carry is the distance of the golf ball from the point where it was dropped. The total is the total of the carry and a distance of the golf ball running from the point where it was dropped.

A Swing robot manufactured by True Temper Co. is equipped with a No. 5 iron club and the golf ball is hit at a head speed of about 38 m/second to measure the flight performances 2.

The golf ball is practically hit with No. 1 wood club by 10 top professional golfers to evaluate the shot feel. The evaluation criteria are as follows. The results shown in Tables 3 to 5 are based on the fact that not less than 8 out of 10 golfers evaluated with the same criterion.

O:

Good shot feel which is similar to that of a standard thread wound golf ball using a balata cover

XH:

Heavy and inferior

XS:

Soft and heavy feel, inferior

As is apparent from comparing the results shown in Tables 3, 4 and 5, regarding the golf balls of Examples 1 to 5, the spin amount was small and the launch angle was large and, further the flying distance was large in comparison with the golf balls of Comparative Examples 1 to 7. The flying distance (carry) due to the No. 1 wood club of a standard two-piece solid golf ball using a solid core is normally 225.0 to 235.0 yards and, therefore, it is understood that the flight distance of the golf balls of Examples 1 to 5 is large, which is equal to that of a two-piece solid golf ball. Further, in the evaluation of Examples 1 to 4 using the same thermoplastic resin, as the diameter of the centre becomes larger and the centre becomes harder, the spin tends to become small and the launch angle tends to become large, which results in large flight distance.

To the contrary, regarding the golf balls of Comparative Examples 1 to 3 and 6, the centre was soft and its strain amount was large and, therefore, a large flight distance could not be attained. Also, regarding the golf ball of Comparative Example 4, the strain amount of the centre was large and, therefore, a large flight distance could not be attained. Regarding the golf ball of Comparative Example 5, the centre was too hard and its strain amount was small and, therefore, a large flight distance could not be attained. Regarding the golf ball of Comparative Example 7, the height of rebound was small and the impact resilient is insufficient and, therefore, the initial velocity became small, thereby attaining a small flight distance.

Further, the golf balls of Comparative Examples 1 to 5 maintained a good shot feel which is similar to that of a standard thread wound golf ball using a balata cover, but some golf balls among the golf balls of Comparative Examples 1 to 6 were too hard or soft and, therefore, they were inferior.