Two-piece golf ball

Abstract: A two-piece golf ball comprising of a core formed of a rubber composition containing a base rubber, a co-crosslinking agent, and an organic peroxide, and a cover covering the core, characterized in that: the core has the following hardness distribution according to JIS - C type hardness meter readings: (1) hardness at centre : 58 - 73 (2) hardness 5 to 10 mm from centre : 65 - 75 (3) hardness 15 mm from centre : 74 - 82 (4) surface hardness : 76 - 84 (wherein hardness (2) is generally constant within the range shown, with a variation range of ±3, and satisfies the relation (1)<(2)<(3)≦(4), and the core has a compression deflection of 3.5 to 4.3 mm when a load range of initial 10 kg to final 130 kg is applied, the cover having a thickness of 2.1 to 2.8 mm.

Description: The present invention relates to a two-piece golf ball and, more particularly, to a two-piece golf ball having a good shot feel when the ball is hit. Two-piece golf balls are widely accepted for use because of their good flight characteristics. However, with such a ball, the problem is that it has a harder shot feel as compared to a thread wound golf ball; especially when the ball is mishit, such feeling is far greater. As such, there exists a demand for a two-piece golf ball having a good shot feel. Recently, various efforts have been made to provide a ball with a shot feel closer to that of a thread wound golf ball. For example, an attempt has been made to arrange the core of a two-piece golf ball to be made softer to moderate the hardness of the ball as a whole, thereby to provide an improved shot feel. However, any golf ball produced on the basis of such an idea lacks the concept of core hardness distribution and feels heavy although it has some soft feel. Indeed, such a ball can hardly be said as having a feel close to that of a thread-wound golf ball. The present invention is directed to controlling the core characteristics and cover thickness with respect to a two-piece golf ball thereby to provide a two-piece golf ball having a shot feel closer to that of a thread-wound golf ball. The present inventors conducted research on two-piece golf balls, with particular attention directed to such factors as core hardness distribution, compressive strength, and cover thickness. As a result, the inventors found that by designing the golf ball as set out in claims 1 and 2, it would be possible to provide a two-piece golf ball having a cover free of hard feel and a core feeling light and moderately soft, so that the ball feels soft and light, has a good shot feel and can travel a greater flight distance than conventional two-piece golf balls. GB-A-2127303 relates to a two-piece solid golf ball. EP-A-0589647 also relates to a two-piece golf ball and is comprised in the state of the art by virtue of Article 54(3) EPC. Thus, according to the invention, there is provided a two-piece golf ball comprising a core formed of a rubber composition containing a base rubber, a co-crosslinking agent, and an organic peroxide, and a cover covering the core wherein: the core has the following hardness distribution according to JIS - C type hardness meter readings: (1) hardness at centre : 58 - 73 (2) hardness 5 to 10 mm from centre : 65 - 75 (3) hardness 15 mm from centre : 74 - 82 (4) surface hardness : 76 - 84 wherein hardness (2) is generally constant within the range shown, with a variation range of ±3, and satisfies the relation (1)<(2)<(3)≦(4), and the core has a compression deflection of 3.5 to 4.3 mm when a load range of initial 10 kg to final 130 kg is applied, the cover having a thickness of from 2.1 to 2.8 mm, excluding a thickness of 2.1 mm. The base rubber used in the invention may be any natural rubber and/or synthetic rubber which is coventionally used in two-piece golf ball cores. In particular, a cis-1,4-polybutadiene rubber having at least 40% of a cis structure is preferred and, if necessary, the polybutadiene rubber may be suitably mixed with natural rubber, polyisoprene rubber, styrene-butadiene rubber or EPDM. In another aspect, the present invention provides a two-piece golf ball comprising a core formed of a rubber composition containing a base rubber, a co-crosslinking agent, and an organic peroxide, and a cover covering the core, the core has the following hardness distribution according to JIS - C type hardness meter readings: (1) hardness at centre : 58 - 73 (2) hardness 5 to 10 mm from centre : 65 - 75 (3) hardness 15 mm from centre : 74 - 82 (4) surface hardness : 76 - 84 wherein hardness (2) is generally constant within the range shown, with a variation range of ±3, and satisfies the relation (1)<(2)<(3)≦(4), and the core has a compression deflection of 3.5 to 4.3 mm when a load range of initial 10 kg to final 130 kg is applied, the cover having a thickness of 2.1 to 2.8 mm, and wherein the rubber composition for making the core comprises cis-1,4-polybutadiene, zinc (meth)acrylate and dicumyl peroxide, zinc(meth)acrylate being present in an amount of 20 to 35 parts by weight and dicumyl peroxide being present in an amount of 0.5 to 5.0 parts by weight, based on 100 parts by weight of cis-1,4-polybutadiene. The co-crosslinking agent is not particularly limited, and may be, for example, a metal salt of an unsaturated carboxylic acid, more particularly, a monovalent or bivalent metal salt of such an acid having 3 to 8 carbon atoms (e.g. acrylic acid or methacrylic acid). Zinc acrylate is especially preferred. The proportion of the co-crosslinking agent is 20 to 35 parts by weight, preferably 20 to 30 parts by weight, based on 100 parts by weight of the base rubber. If the proportion is less than 20 parts by weight, the hardness of the ball is insufficient, which results in a heavy and unsatisfactory feel and poor durability. If the proportion exceeds 35 parts by weight, the ball is exceedingly hard and produces a poor feel. The organic peroxide may be any peroxide that is used in this field, including dicumyl peroxide and di-t-butyl peroxide. Dicumyl peroxide is especially preferred. The proportion of the organic peroxide is 0.5 to 5.0 parts by weight, preferably 1.0 to 3.0 parts by weight, relative to 100 parts by weight of the base rubber. If the proportion is less than 0.5 parts by weight, the hardness of the ball is insufficient, which results in a heavy and unsatisfactory feel. If the proportion is greater than 5.0 parts by weight, the ball is excessively hard and produces a poor feel. The rubber composition used in the golf ball of the invention may be admixed with additives such as fillers and anti-oxidizing agents, if necessary. Examples of the fillers generally include zinc oxide and barium sulfate. The proportion of such additives is dependent on the specific gravity and size of the cover and core and not particularly limited, but it is generally 10 to 40 parts by weight relative to 100 parts by weight of the base rubber. The rubber composition is thoroughly kneaded, and vulcanized and molded in a mold, whereby a core of a two-piece golf ball is formed. Conditions for kneading and vulcanizing are well known to persons skilled in the art. For example, the vulcanizing operation is generally carried out at temperatures of 140 to 180°C. for a period of 15 to 55 minutes. The core of the golf ball of the invention has a hardness distribution which meets the following conditions: (1) hardness at core centre is 58 to 73, (2) hardness 5 to 10 mm from centre is 65 to 75, (3) hardness 15 mm from centre is 74 to 82, and (4) surface hardness is 76 to 84, according to JIS-C type hardness meter readings. The hardness distribution also satisfies the relation of (1)<(2)<(3)≦(4). In particular, hardness (2) involves no hardness dispersion and is generally constant, with a variation range of not more than ±3. An approach which specifies a hardness distribution in such a way is described in Japanese Patent Application Laid-Open No. 60-90575. Where the hardness profile is lower than the above noted range, the ball has a poor durability and feels excessively soft and heavy. Where the hardness profile is higher than the above noted range, the ball is subject to a strong impact force when hit and feels poor. Where the hardness distribution is higher towards the core surface, the ball is subjected to a stronger impact force and lacks a soft feel. Where the hardness profile includes a flat region as in the present invention, the ball is subjected to a smaller impact force and has a good shot feel. When the core is subjected to a load range of initial 10 kg to final 130 kg, it is necessary that the ball should have a compression deflection of 3.5 to 4.3 mm. Such a compression deflection may be controlled mainly by adjusting the proportion of the metal salt of the unsaturated carboxylic acid. It is also possible to control the compression deflection by adjusting the proportion of other chemicals and the vulcanizing conditions. Whichever method may be employed for such control, if the deflection of the core is greater than 4.3 mm, the ball has a lower rebound coefficient and an unsatisfactory flight performance. Basically, in such a case, the ball feels excessively soft, heavy and rather dull, and is less durable. Conversely, where the core deflection is less than 3.5 mm, the ball feels hard and unsatisfactory. The golf-ball core thus obtained is covered with a 2.1 to 2.8 mm thick cover to provide a two-piece golf ball. Usually, the cover is formed from a member comprising mainly an ionomer resin, with a filler (e.g. titanium dioxide or barium sulfate) contained therein for coloration purposes, as required. Where the cover is less than 2.1 mm in thickness, the ball is subjected to excessive spin so that the ball is unsatisfactory in its flight performance; further, the ball feels too soft and rather dull, which means that the ball has a feel close to that of a one-piece ball. Where the thickness of the cover is more than 8 mm, the ball feels hard and heavy and feels unsatisfactory. According to the invention, the flexural rigidity of the cover is preferably in the range of 3000 to 4500 kg/cm 2 . Where the flexural rigidity is less than 3000 kg/cm 2 , the cover is relatively soft and, therefore, the ball feels soft, heavy and rather dull. Conversely, when the flexural rigidity is greater than 4500 kg/cm 2 , the ball feels hard and heavy. The method of forming a cover from an ionomer resin is known in the art, which method is usually carried out by using an injection molding technique. EXAMPLES The following Examples are given to illustrate the invention in further detail. It is to be understood that the invention is not limited by these Examples. Examples 1 - 6 and Comparative Examples 1 - 4 . Core ingredients shown in Table 1 were mixed and kneaded to obtain rubber compositions. Each of the rubber compositions was vulcanized and molded in a mold under the conditions shown in Table 1. The hardness distribution and compressive strength of each core obtained is shown in Table 1. Covers formed from ingredients shown in Table 1 and according to the normal procedure were used to cover the respective cores. The flexural rigidity and thickness of each cover is shown in Table 1. Various features of the two-piece golf balls obtained, including hardness (PAG number), durability index, rebound coefficient, flying characteristics, such as hitting angle, spin, and carry, and feel evaluation, are shown in Table 2. As is apparent from Table 1 and Table 2 measurement results, balls of Examples 1 to 6 are light and soft and have such good feel as is close to a thread-wrapped ball. They are also found satisfactory in durability and flying characteristics. The ball of Comparison Example 1 is inferior in core compressive strength (4.6 mm), is low in ball hardness, and is too soft and heavy, thus giving a dull and unfavorable feel. The ball is also inferior in durability. The ball of Comparison Example 2 has a low core compressive strength (3.35 mm), higher ball hardness, and has a hard and unfavorable feel. The ball of Comparison Example 3 has a thin cover giving a feel close to that of a one-piece ball; it feels soft and heavy. The ball is more subject to spin and is inferior in flight and durability characteristics. The ball of Comparison Example 4 is rather hard and heavy because of its thick cover, and has an unfavorable feel. The two-piece golf ball of the invention has a feel of hitting which is close to that of a thread-wrapped golf ball. In addition, the ball has good flight characteristics inherent to a two-piece golf ball. Thus, the golf ball of the invention has a good advantage in both flying characteristics and feel of hitting.

Claims: A two-piece golf ball comprising a core formed of a rubber composition containing a base rubber, a co-crosslinking agent, and an organic peroxide, and a cover covering the core, the core having the following hardness distribution according to JIS - C type hardness meter readings: (1) hardness at centre : 58 - 73 (2) hardness 5 to 10 mm from centre : 65 - 75 (3) hardness 15 mm from centre : 74 - 82 (4) surface hardness : 76 - 84 wherein hardness (2) is generally constant within the range shown, with a variation range of ±3, and satisfies the relation (1)<(2)<(3)≦(4), characterized in that: the core has a compression deflection of 3.5 to 4.3 mm when a load range of initial 10 kg to final 130 kg is applied, the cover having a thickness of from 2.1 to 2.8 mm, excluding a thickness of 2.1 mm. A two-piece golf ball comprising a core formed of a rubber composition containing a base rubber, a co-crosslinking agent, and an organic peroxide, and a cover covering the core, the core having the following hardness distribution according to JIS - C type hardness meter readings: (1) hardness at centre : 58 - 73 (2) hardness 5 to 10 mm from centre : 65 - 75 (3) hardness 15 mm from centre : 74 - 82 (4) surface hardness : 76 - 84 wherein hardness (2) is generally constant within the range shown, with a variation range of ±3, and satisfies the relation (1)<(2)<(3)≦(4), characterized in that: the core has a compression deflection of 3.5 to 4.3 mm when a load range of initial 10 kg to final 130 kg is applied, the cover having a thickness of 2.1 to 2.8 mm, and wherein the rubber composition for making the core comprises cis-1,4-polybutadiene, zinc (meth)acrylate and dicumyl peroxide, zinc(meth)acrylate being present in an amount of 20 to 35 parts by weight and dicumyl peroxide being present in an amount of 0.5 to 5.0 parts by weight, based on 100 parts by weight of cis-1,4-polybutadiene. A two-piece golf ball as claimed in claim 1 wherein the rubber composition for making the core comprises cis-1,4-polybutadiene, zinc (meth)acrylate and dicumyl peroxide, zinc(meth)acrylate being present in an amount of 20 to 35 parts by weight and dicumyl peroxide being present in an amount of 0.5 to 5.0 parts by weight, based on 100 parts by weight of cis-1,4-polybutadiene. A two-piece golf ball as claimed in claim 2 or claim 3 wherein the rubber composition further comprises a filler and an anti-oxidizing agent. A two-piece golf ball as claimed in any one of the preceding claims wherein the cover is made of an ionomer resin. A two-piece golf ball as claimed in any one of the preceding claims wherein the cover has a flexural rigidity of from 3000 to 4500 Kg/cm 2 .

Inventors: ENDO SEIICHIRO, YABUKI YOSHIKAZU, ENDO, SEIICHIRO, YABUKI, YOSHIKAZU
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1993
Main classification: A63B37
IPC codes: A63B37, C08K5