Golf Ball

The present invention relates to a golf ball. More particularly, it relates to a golf ball having excellent impact resilience and flying performance, of which the cover is formed from an ionomer resin which contains an alpha, beta-ethylenic unsaturated carboxylic acid in a larger amount than conventional ionomer resins.

A golf ball is composed of a core and a cover covering the core. The cover generally used is mainly formed from an ionomer resin (see Japanese Kokai Publications 135078/1984 and 82768/1986), because the ionomer resin is commercially available cheaper than the other cover resins and has excellent cut resistance.

The ionomer resin is a resin which is prepared by neutralizing a copolymer of an alpha-olefin, an alpha, beta-ethylenic unsaturated carboxylic acid and optionally an alpha, beta-ethylenic unsaturated carboxylic ester with metal, and many types of them are commercially available. The ionomer resins which are used as the cover of the golf balls, however, contain the alpha, beta-ethylenic unsaturated carboxylic acid in an amount of less than 15 % by weight before neutralizing, and the other carboxyl-rich ionomer resins which contain more than 15 % by weight of it have never been employed.

It has been surprisingly found that a carboxyl-rich ionomer resin which contains 16 to 30 % by weight of an alpha, beta-ethylenic unsaturated carboxylic acid significantly improves the properties of the golf balls, such as impact resilience and flying performance. Thus, the present invention provides a golf ball which comprises a core and a cover covering the core, wherein the cover contains at least 20 % by weight of a carboxyl-rich ionomer resin prepared by neutralizing 15 to 80 mol % of carboxylic acid groups of an olefinic copolymer containing 16 to 30 % by weight of an alpha, beta-ethylenic unsaturated carboxylic acid with monovalent or divalent metal ions.

The olefinic copolymer of the carboxyl-rich ionomer resin is obtained by copolymerizing an alpha-olefin with an alpha, beta-ethylenic unsaturated carboxylic acid. In the present invention, an amount of the alpha, beta-ethylenic unsaturated carboxylic acid is limited to 16 to 30 % by weight, preferably 20 to 30 % by weight, based on the total monomer weight. Thus, the balance of the monomer is the alpha-olefin. If the amount of the alpha, beta-ethylenic unsaturated carboxylic acid is less than 16 % by weight, a stiffness modulus is low and an impact resilience is low, thus resulting in poor flying performance. A cut resistance is also deteriorated. If it is more than 30 % by weight, the stiffness modulus of the ball is so high that a hit feeling is poor, and the durability when hit repeatedly is low. A portion of the alpha-olefin may be replaced with an alpha, beta-ethylenic unsaturated carboxylic ester which, however, does not exceed 20 % by weight based on the total monomer weight. Typical examples of the alpha, beta-ethylenic unsaturated carboxylic acids are acrylic acid, methacrylic acid, maleic acid, fumaric acid and the like. Typical examples of the alpha-olefins are ethylene, propylene and the like.

The carboxyl-rich ionomer resin of the present invention is prepared by neutralizing 15 to 80 mol % of the carboxylic acid groups of the above mentioned olefinic copolymer with monovalent or divalent metal ions. Typical examples of the metal ions are monovalent metal ions, such as lithium ions, sodium ions, potassium ions and the like; divalent metal ions, such as alkaline earth metal ions (e.g. magnesium ions and calcium ions) and other metal ions (e.g. zinc ions and copper ions). If the neutralizing degree is less than 15 mol %, the impact resilience and durability of the ball are poor. If it is more than 80 mol %, the molding ability is poor and it is difficult to cover it on the core.

It is preferred that the carboxyl-rich ionomer resin of the present invention has a stiffness modulus of 3,000 to 6,000 Kgf/cm.sup.2. Values of less than 3,000 Kg/cm.sup.2 reduce impact resilience and those of more than 6,000 Kgf/cm.sup.2 deteriorate hit feeling and durability when hit repeatedly.

The cover of the present invention contain at least 20 % by weight, preferably at least 30 % by weight, of the above obtained carboxyl-rich ionomer resin. If the carboxyl-rich ionomer resin is less than 20 % by weight, the technical effects of the improvement of rebound resilience and flying ability are insufficient. The cover resin composition may comprises the carboxyl-rich ionomer and a small amount of additives (e.g. pigment, filler, dispersant, antioxidant, ultraviolet absorber, photostabilizer and the like). The cover may further contain other resins, such as polyolefin, polyester elastomer, polyamide and the like, but these resins are formulated in a small amount, especially 10 by weight or less.

The core of the golf ball of the present invention can be either a solid core or a thread-wound core. The solid core is generally prepared by vulcanizing a rubber composition, for example at 140 to 170 .degree. C for 10 to 49 minutes under pressure. The rubber composition may be 100 parts by weight of polybutadiene rubber, 10 to 60 parts by weight of a co-crosslinking agent (e.g. an alpha, beta-ethylenic unsaturated carboxylic acid, such as acrylic acid and methacrylic acid, a metal salt thereof, and a monohydric monomer, such as trimethylolpropane trimethacrylate), 10 to 30 parts by weight of a filler (e.g. zinc oxide and barium sulfate), 0.5 to 5.0 parts by weight of a peroxide (e.g. dicumyl peroxide) and 0.1 to 1.0 parts by weight of an antioxidant. The solid core may have a multilayer construction if necessary.

The thread-wound core is generally composed of a center and a thread rubber layer wound on the center. The center may be either a liquid center or a solid center. the solid center can be prepared by vulcanizing the rubber composition as explained for the solid core. Thread rubber is known to the art and may be prepared by vulcanizing a rubber composition comprising a natural rubber or a combination of a natural rubber and a synthetic polyisoprene rubber, an antioxidant, a vulcanization promoter and sulfur.

The above described solid core and thread-wound core scope of the present invention is mere exemplification and are not limited thereto.

A method for covering the cover resin composition on the core is not limited and can be conducted by art-known methods. For example, a cover resin composition is formed into hemispherical half shells and a core is surrounded with two half shells, followed by pressure-molding at 130 to 170 .degree. C. for 1 to 5 minutes. Also, the cover composition may be injection-molded to cover the core. A thickness of the cover layer is generally 1.0 to 3.0 mm. Dimples are formed on the surface of the cover and if necessary stamps and paint layers are formed thereon.

The present invention is illustrated by the following Examples which, however, are not to be construed as limiting the invention to their details.

A rubber composition was prepared by mixing 100 parts by weight of cis-1,4-polybutadiene (available from Japan Synthetic Rubber Co., Ltd. as JSR BR01.RTM.), 30 parts by weight of zinc acrylate (available from Nippon Shokubai Kagaku Kogyo Co., Ltd.), 20 parts by weight of zinc oxide (available from Toho Zinc Co., Ltd.) and one part by weight of dicumyl peroxide (available from Nippon Oil & Fats Co., Ltd.), and compression-molded at 150 .degree. C. for 30 minutes to form a solid core having a diameter of 38.5 mm.

Separately, the ionomer resin having a composition as shown in Table 1 was mixed, and 100 parts by weight of the ionomer resin mixture was mixed with 2 parts by weight of titanium dioxide by an extruder to obtain a cover resin composition. In Table 1, the numbers for the ionomer resins are based on parts by weight and the total of them indicates 100. Thus, the numbers also show % by weight.

The solid core was covered with the cover resin composition by an injection molding to obtain a two piece solid golf ball. The ball was then coated with a paint to obtain a golf ball having a diameter of 42.8 mm.

The obtained golf balls were evaluated by ball weight, compression, initial velocity, durability, low temperature durability, flying distance (carry) and stiffness modulus and the results ar shown in Table 1.

As is shown in Table 1, the gold balls of Examples 1 to 4 have superior initial velocities (i.e. higher impact resilience) and superior flying distance to those of Comparative Examples 1 to 3. The golf balls of Examples are excellent in durability and low temperature durability and have good hit feeling.