Golf ball

Abstract: Disclosed is a golf ball which comprises a core and a cover covering the said core, characterized by said cover comprising a resin component of (a) 10 to 80 wt % of a saponified polymer obtained by saponifying at least a portion of the ester groups of a copolymer of an olefin having 2 to 8 carbon atoms and an unsaturated acrylate having 3 to 8 carbon atoms with an alkali metal which is Li, Na, K or a mixture thereof, and (b) the remaining balance of an ionomer resin.

Description: The present invention relates to a golf ball, particularly to an ionomer covered golf ball. As a cover material for golf balls, ionomer resins have been widely used (Japanese Kokai Publication 49727/1974). In particular, for two-piece golf balls which contain solid cores, almost all balls use an ionomer resin as the cover material. This is because ionomer resins have an excellent durability, anti-cutting property, preferred rebound property and easy processing property and are cheaper than other cover materials usable for golf balls. However, ionomer resins are inferior to a Balata cover (a trans-polyisoprene cover) which is used as the cover material of thread-wound golf balls, with regard to shot feel and controllability (ease of imparting spin to the ball), because of high hardness and high stiffness. Many trials have been made to improve the shot feel (hit feeling) and controllability of the golf ball by making the ionomer resin flexible. Japanese Kokai Publication 308577/1989 discloses a flexible ionomer resin which is a sodium salt or zinc salt of a terpolymer which is prepared from an olefin having 2 to 8 carbon atoms, an unsaturated monocarboxylic acid having 3 to 8 carbon atoms and an acrylate monomer having 2 to 22 carbon atoms. The flexible ionomer resins do not have sufficiently good properties for golf balls. We have now developed an improved ionomer resin, which provides a golf ball with a good shot feel, excellent controllability and flying properties. Accordingly, the present invention provides a golf ball comprising a core and a cover covering the core, wherein the cover comprises a resin component of (a) from 10 to 80 wt % of a saponified polymer obtained by saponifying at least a portion of the ester groups of a copolymer of an olefin having from 2 to 8 carbon atoms and an unsaturated acrylate or a methyl, ethyl, propyl, n-butyl or isobutyl ester of methacrylic acid, maleic acid or fumaric acid in an amount of from 15 to 40 wt % based on the amount of the copolymer and having from 4 to 8 carbon atoms with an alkali ion which is Li, Na, K or a mixture thereof, wherein the amount of the ester groups saponified is from 10 to 40 wt % of the total of ester groups and wherein the copolymer has a stiffness of less than 1000 kgf/cm 2 , and (b) the balance of an ionbmer resin obtained by neutralizing a portion of the carboxyl groups of a copolymer of from 10 to 20 wt % of an α,β-unsaturated carboxylic acid having from 3 to 8 carbon atoms and from 80 to 90 wt % of an α-olefin, with a metal ion. According to the present invention, by mixing the ionomer resin with the copolymer with a specified degree of saponification, the ionomer is made flexible and thereby the shot feel and controllability of the ball are improved and yet severe deterioration of flying performance and anti-cutting property can be avoided. In the present invention, the ionomer resin used for the cover material is a resin which is obtained by neutralizing a portion of the carboxyl groups of a copolymer of 10 to 20 wt % of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and 80 to 90 wt % of an α-olefin, with a metal ion (ion of Na, Li, Zn and Mg). The ionomer resin is generally commercially available and there are many types, typically under the trade name of Surlyn from E.I. DuPont de Nemours Co. In the present invention, the ionomer resin is mixed with the copolymer of an olefin having 2 to 8 carbon atoms and an unsaturated acrylate having 3 to 8 carbon atoms, of which a portion of the ester groups is saponified with an alkali metal selected from the group consisting of Li, Na, K or a mixture thereof. The olefins include ethylene, propylene, 1-butene, isobutylene, 1-pentene and 1-octene, and ethylene or propylene is preferred. The acrylates include methyl, ethyl, propyl, n-butyl or isobutyl ester of acrylic acid. Preferred are an acrylate and a methacrylate. Saponification is conducted in the normal way, for instance, by adding caustic alkali to the copolymer and heating it. The amount of the saponified copolymer in the resin is 10 to 80 wt %, preferably 20 to 60 wt %, based on the total weight of the resin made mainly of ionomer resin. If it is less than 10%, the resin is not sufficiently flexible and the shot feel and controllability of the ball are not improved. If it is more than 80%, the anti-cutting property is inferior and a satisfactory ball is not obtained. The amount of the unsaturated acrylate in the copolymer of the olefin and unsaturated acrylate is 15 to 40 wt %. If it is less than 15 wt %, the shot feel and controllability of the ball are not improved while if it is over 40 wt %, the anti-cutting property is inferior. The amount of the ester groups saponified may be 10 to 40 wt % of the total of ester groups. When it is less than 10 wt %, the anti-cutting property is inferior and if it is over 40 wt %, the melt viscosity is too high and consequently, the MI (melt index) is too small to conduct mixing. The stiffness of the copolymer is preferably less than 1,000 kgf/cm 2 , more preferably 500 to 30 kgf/cm 2 . If it is above 1,000 kgf/cm 2 , a sufficient improvement of the shot feel and controllability of ball are not realized. The cover of the present invention mainly comprises a resin component of (a) 10 to 80 wt % of the above mentioned saponified copolymer and (b) the balance of the ionomer resin. The cover may further comprise various additives, for example, pigments, dispersants, antioxidants, UV absorbents, or optical stabilizers, if necessary. The cover of the present invention can cover cores of golf balls. The cores include not only solid cores for solid golf balls, but also thread-wound cores for thread-wound golf balls. The solid golf balls may include two or more-piece solid golf balls, for example, two-piece golf balls and three-piece golf balls. For example, the core of a two-piece golf ball may be obtained by press-vulcanizing a rubber composition obtained by mixing 100 wt parts of polybutadiene with 10 to 60 wt parts of a co-crosslinking agent, 10 to 30 wt parts of a filler (such as zinc oxide and barium sulfate), 0.5 to 5.0 wt parts of a peroxide (such as dicumyl peroxide) and 0.1 to 1.0 wt parts of an antioxidant. The co-cross-linking agent includes an α,β-monoethylenic unsaturated carboxylic acid (e.g acrylic acid and methacrylic acid), a metal salt thereof, and a functional monomer (e.g trimethylolpropane trimethacrylate). Press-vulcanization may be conducted by compression heating at 140 to 170°C for 19 to 40 minutes to form the composition into spherical article. The thread-wound core is composed of a center and the rubber thread wound around the center. The center may be either liquid base or rubber base. The rubber case center may be obtained by vulcanizing a rubber composition of the same type as that used for the solid core. The rubber thread may be one having been conventionally used, for example, one obtained by vulcanizing a rubber composition composed of natural rubber or a combination of natural rubber and synthetic polyisoprene, an antioxidant, a vulcanization accelerator and sulfur. The method to cover the core with the cover is not limited to any specific method but it may be the normal methods used in the art. For example, the cover composition of the present invention may be formed into semispherical shapes, the core is covered with two such half-shells and formed under pressure at 130 to 170°C for 1 to 5 min. or, alternatively, the core may be covered by direct injection-forming of the composition for the cover around the core. The thickness of the cover is normally about 1.0 to 3.0 mm. On forming the cover, dimples are formed at the surface of the ball as required and finishing by painting or stamping is conducted as required. Examples The present invention is described in further detail in reference to the examples and comparative examples. Examples 1 to 4 and Comparative Examples 1 to 3 30 wt parts of zinc acrylate (manufactured by Nippon Shokubai Kagaku Kogyo K.K.), 20 wt parts of zinc oxide (manufactured by Toho Aen Co.) and 1 wt part of dicumyl peroxide (manufactured by Nippon Oil & Fat Co.) were added to 100 wt parts of cis-1,4-butadiene (JSR BRO1® of Japan Synthetic Rubber CO.) and after kneading, it was formed under pressure at 150°C for 30 minutes in a mold for core to obtain a solid core with diameter of 38.5 mm. Separately, an ionomer resin composition as shown in Table 1 was blended, of which 100 wt parts was mixed with 2 wt parts of titanium oxide (TiO 2 ), and then extruded in an extruder to obtain a cover composition. The amount of composition of ionomer resin shown in Table 1 is by weight parts and it is so arranged that the entire amount makes 100 wt parts. Therefore the numbers indicating the compounding amount simultaneously indicate weight percent in the total. Thus obtained cover composition was covered directly on the solid core by injection molding to obtain a two-piece golf-ball. Thus obtained ball was coated by paint to prepare the golf-ball with diameter of 42.8 mm. Weight, compression, initial velocity, flight distance (carry) of the golf ball and stiffness of the cover were measured. The results are shown in Table 1. The methods of measurement of these characteristics are as follows. (1) Ball compression According to PGA method (2) initial velocity (according to R & A measuring method): A ball was hit by a No. 1 Wood Club at the head speed of 45 m/sec using a swing robot manufactured by True Temper Corp. and an initial velocity of the ball was measured. Measuring was made on 10 balls of each kind and the results were shown by their average value. (3) Flight distance (carry) A ball was hit by a No. 1 wood club at the head speed of 45 m/sec using a swing robot manufactured by True Temper Corp. and a flight distance (carry) was measured. Measurement was made on 10 balls of each kind and the results were shown by their average value. (4) Stiffness of the cover: Measurement was made by a stiffness tester manufactured by Toyo Seiki K.K according to ASTM D-747. The sample for measurement was a flat plate prepared by press-forming. After press-forming, the sample plate had been left standing for two weeks at 23 °C in the humidity of 50% and then measurement was made on such plate. Hit feeling of ball and controllability were evaluated by actual hitting test made by 10 top professional golfers. As it is evident from Table 1, the golf balls of Example 1 and 2 indicate the carry flight distance of 225 to 226 yard (207-208 m) which was roughly equal to that of the golf ball of Comparative Example 1, while the golf balls of Comparative Examples 2 and 3 were inferior as for the carry by 5 to 7 yard (4,6 - 6,4 m) when compared to the balls of Comparative Example 1 and Examples 1 and 2. Hit feeling of ball, controllability and flight distance of golf balls of Examples 1 and 2 and comparative examples 1 to 2 were evaluated by actual hitting test made by top professional golfers. As the result, it was found that the golf balls of Examples 1 and 2 have the hit feeling of ball and controllability roughly equal to the thread wound golf balls covered with the transpolyisoprene (Balata) and the flight distance of carry was also at satisfactory level. Whereas the golf ball of Comparative Example 1 exhibited a rigid feel when hit and poor controllability owing to the difficulty in applying spin, the golf balls of comparative Examples 2 and 3 exhibited a good feel when hit and possessed good controllability, but had inferior carry. Anti-cutting property of golf balls of Examples 1 and 2 and Comparative Examples 1 to 3 was investigated by fixing a No. 12 pitching wedge to the swing robot manufactured by True Temper Corp. and the ball was hit at the top at the speed of 30 m/sec and the production of cut injury was investigated. As the result, it was found that cutting injury was not produced for the golf balls of Example 1 and 2 and comparative example 1 but small cut injury was produced for the golf balls of comparative examples 2 and 3. Anti-cutting property was investigated under the same conditions also with the thread wound golf balls covered with the cover made mainly of transpolyisoprene (Balata) and it was found that large cut injuries which make the ball unusable have been produced. According to the present invention, a golf ball with excellent feeling of hitting of ball, controllability and flying performance is obtained.

Claims: A golf ball comprising a core and a cover covering the core, characterized by the cover comprising a resin component of (a) from 10 to 80 wt % of a saponified polymer obtained by saponifying at least a portion of the ester groups of a copolymer of an olefin having from 2 to 8 carbon atoms and an unsaturated acrylate or a methyl, ethyl, propyl, n-butyl or isobutyl ester of methacrylic acid, maleic acid or fumaric acid in an amount of from 15 to 40 wt % based on the amount of the copolymer and having from 4 to 8 carbon atoms with an alkali ion which is Li, Na, K or a mixture thereof, wherein the amount of the ester groups saponified is from 10 to 40 wt % of the total of ester groups and wherein the copolymer has a stiffness of less than 1000 kgf/cm 2 , and (b) the balance of an ionomer resin obtained by neutralizing a portion of the carboxyl groups of a copolymer of from 10 to 20 wt of an α,β-unsaturated carboxylic acid having from 3 to 8 carbon atoms and from 80 to 90 wt % of an α-olefin, with a metal ion. A golf ball as claimed in claim 1, wherein the olefin is ethylene, propylene, 1-butene, isobutylene, 1-pentene or 1-octene. A golf ball as claimed in claim 1 or claim 2, wherein the acrylate comprises the methyl, ethyl, propyl, n-butyl or isobutyl ester of acrylic acid.

Inventors: HAMADA AKIHIKO, HORIUCHI KUNIYASU, HAMADA, AKIHIKO, HORIUCHI, KUNIYASU
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, CO. LTD
Application year: 1993
Main classification: C08L23
IPC codes: A63B37, C08L23, C08F210