Golf ball

Abstract: The present invention provides a golf ball having excellent hit feeling and control property as well as satisfactory flying performance and cut resistance, which comprises a core and a cover for covering the core. The cover is prepared from a heated mixture of the following three components: (A) an ionomer resin; (B) a polymer containing at least maleic anhydride as one constituent component; and (C) a metal salt selected from the group consisting of metal hydroxides, metal carbonates and metal acetates.

Description: The present invention relates to a golf ball with a cover which provides the ball with improved shot feel and control properties. As a base resin for the cover material of a golf ball, an ionomer resin has previously widely been used (e.g. Japanese Patent Kokai No. 49-49727, etc.). Particularly, in almost all two-piece golf balls using a solid core, the cover material is formed from an ionomer resin. The reason is that the ionomer resin is superior in durability, cut resistance and impact resilience and is easily processed and, further, it is cheap in comparison with other cover materials. However, since the ionomer resin has a considerably high hardness and stiffness, it is inferior in shot feel and control properties (easy spinning) in comparison with balata (transpolyisoprene), which is used as the cover base of thread wound golf balls. Therefore, attempts have been made to improve the shot feel and control properties by softening the ionomer resin by various means e.g. US-A-4 986 545, but, satisfactory results have not been obtained at present. In order to improve both the shot feel and flying performance, it has been found that a golf ball having excellent shot feel and control properties, as well as a satisfactory flying performance and cut resistance, can be obtained, by using a cover material comprising a heated mixture of the following components: (A) an ionomer resin; and (B) a polymer containing at least maleic anhydride as one constituent component; characterized in that the heated mixture also includes (C) a metal salt selected from the group consisting of metal hydroxides, metal carbonates and metal acetates. According to the present invention, there is provided a golf ball comprising a core and a cover for covering the core, said cover comprising a heated mixture of the following components: (A) an ionomer resin; and (B) a polymer containing at least maleic anhydride as one constituent component; characterized in that the heated mixture also includes (C) a metal salt selected from the group consisting of metal hydroxides, metal carbonates and metal acetates. That is, by mixing the polymer containing maleic anhydride as one constituent component with the ionomer resin with heating, the ionomer resin is softened, whereby, the shot feel and control properties are improved. Furthermore, the strength of the heated mixture is increased by a crosslinking effect due to the addition of the above metal salt and a deterioration of the excellent properties of the ionomer resin is prevented, whereby, the shot feel and cut resistance are satisfactorily maintained. The ionomer resin is a copolymer of from 10 to 20% by weight of an α, β-unsaturated carboxylic acid containing from 3 to 8 carbon atoms with 80 to 90% by weight of α-olefin, a portion of the carboxyl groups in the copolymer being neutralized with a metal ion. In the present invention, the ionomer resin may be any such resin which is used for the cover of the golf ball, and one or more types may be used in combination. Representative examples of the ionomer resins are the ionic ethylene-methacrylic acid copolymer based ionomer resin include HI-MILAN 1605 (of the type neutralized by sodium ion), HI-MILAN 1706 (of the type neutralized by zinc ion), HI-MILAN 1707 (of the type neutralized by sodium ion), HI-MILAN 1705 (of the type neutralized by zinc ion) which are commercially available from Mitsui Du Pont Polychemicals Co.; SURLYN 7930 ( of the type neutralized by lithium ion), SURLYN 7940 (of the type neutralized by lithium ion) which are commercially available from Du Pont de Nemours & Co. and the like. The ionic ethylene-acrylic acid copolymer based ionomer resins include one that is commercially available from EXXON Chemicals Co. under the trade name of Iotek. As the polymer containing at least maleic anhydride as one constituent component, there can be used various forms of polymers, for example, modified types wherein maleic acid is added to a polymer main chain, copolymer types containing maleic anhydride as a copolymeric component, and the like. It is preferred that the polymer backbone is formed from an α-olefin as the main monomer since it has excellent compatibility with the ionomer resin and, therefore, it is suitably used in the present invention. Examples of such polymers are maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene monomer terpolymer, maleic anhydride-modified polyethylene, maleic anhydride-modified EPDM rubber, maleic anhydride-modified polypropylene, ethylene-ethyl acrylate-maleic anhydride terpolymer and the like. The polymer containing maleic anhydride as one constituent component has elastomeric properties and is superior in processability, thermostability and compatibility with the ionomer resin, and it softens the ionomer resin to impart flexibility to the mixture. Since the polymer containing maleic anhydride as one constituent component is used for the purpose of imparting flexibility, as described above, those having a low stiffness modulus in comparison with a normal ionomer resin are preferred. Particularly, those having a stiffness modulus of not more than 2000 kgf/cm², preferably about 50 to 1000 kgf/cm² are preferred. When the stiffness modulus exceeds 2000 kgf/cm², it becomes equal to that of the ionomer resin (generally, 2500 to 4000 kgf/cm²) and it becomes impossible to soften the ionomer resin. In the present invention, by adding the component (C) (i.e. metal hydroxides, metal carbonates or metal acetates) to the resin component of two sorts of resins and mixing them with heating, a residual carboxyl group in the ionomer resin and carboxylic anhydride in the polymer containing maleic anhydride as one constituent component reacts with the metal salt, whereby, cocrosslinking due to the metal ion occurs between the ionomer resin and the polymer containing maleic anhydride as one constituent component, which results in an improvement of strength. Examples of the metal in the above component (C) include alkali metals such as lithium, sodium, potassium, etc.; alkaline earth metals such as magnesium, calcium, barium, etc.; transition metals such as zinc, copper, iron, nickel, etc. Representative examples of the metal hydroxides, metal carbonates and metal acetates are magnesium hydroxide, basic magnesium carbonate, magnesium acetate, basic zinc carbonate, zinc acetate, sodium carbonate, lithium carbonate, copper hydroxide, calcium hydroxide and the like. In the three-component mixture constituting the cover base, a weight ratio of the ionomer resin (A) to the polymer containing maleic anhydride as one constituent component (B) is preferably 95:5 to 30:70. When the amount of the ionomer resin (A) is more than the above range, the amount of the polymer containing maleic anhydride as one constituent component (B) is small and, therefore, a sufficient effect of softening the heated mixture is not obtained, whereby, a sufficient effect of improving the shot feel and control properties of the golf ball is not obtained. Further, when the amount of the ionomer resin (A) is less than the above range, the impact resilience and cut resistance of the ionomer resin deteriorate, whereby, the flying performance and cut resistance of the golf ball are lowered. The amount of the metal salt (C) is preferably 0.2 to 3 parts by weight based on 100 parts by weight of the resin component comprising the ionomer resin (A) and the polymer containing maleic anhydride as one constitutent component (B). When the amount of the metal salt (C) is less than 0.2 parts by weight based on 100 parts of weight of the resin component (A) + (B), the amount of cocrosslinkage formed due to the metal ion becomes small and, therefore, an insufficient modifying effect is obtained. When the amount of the metal salt (C) is more than 3 parts by weight based on 100 parts by weight of the resin component (A) + (B), the amount of cocrosslinkage formed due to the metal ion becomes excessive and, therefore, the fluidity of the mixture decreases, which results in a deterioration of processability. In the present invention, a heated mixture comprising an ionomer resin (A), a polymer containing maleic anhydride as one constituent component (B) and a metal salt (C) is used as the main component of the base for the cover. To the heated mixture, if necessary, various additives such as pigments, dispersants, antioxidants, UV absorbers, UV stabilizers and the like can be added. Further, in the present invention, the fact that a heated mixture of three components, an ionomer resin (A), a polymer containing maleic anhydride as one consituent component (B) and a metal salt (C) is used as a main component of the base for the cover means either that the base for the cover is only composed of the above heated mixture, or that the base for the cover is composed of the heated mixture, to which other resins are added in an amount which does not affect the properties of the heated mixture (normally, in the range of not more than 20% by weight based on the total weight of the heated mixture). The above ionomer resin, polymer containing maleic anhydride as one constituent component and metal salt are mixed each other with heating at a resin temperature of from 150 to 280°C, using an extruder for plastic materials or an internal mixer such as a plastmill, Banbury mixer or the like. In order to conduct the above mixing uniformly and efficiently, it is most suitable to use a biaxial kneading type extruder among the above mixers. The cover composition containing the heated mixture as the main component of the cover material is coated onto the core in order to obtain a golf ball. In that case, any solid core or thread wound cores can be used. The solid core is obtained by vulcanizing (crosslinking) a rubber composition mainly composed of a rubber followed by integral molding. The thread wound core is obtained by winding a thread rubber on a core material. The present invention thus provides a golf ball having an excellent shot feel and control properties, as well as a satisfactory flying performance and cut resistance. The following Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Examples 1 to 3 and Comparative Examples 1 to 4 (1) Preparation of core A rubber composition obtained by adding 35 parts by weight of zinc acrylate, 20 parts by weight of zinc oxide, 1 part by weight of dicumyl peroxide and 0.05 parts by weight of antioxidant [Yoshinox 425 (trade name), manufactured by Yoshitomi Seiyaku K.K.] to 100 parts by weight of polybutadiene rubber [BR-11 (trade name), manufactured by Nippon Gosei Gomu K.K.) was vulcanized and molded at 165°C for 20 minutes to obtain a solid core. The average diameter of the resulting solid core was 38.4 mm. (2) Preparation of cover composition Formulation components shown in Tables 1 and 2 were mixed and extruded by a twin-screw kneading type extruder to obtain a pellet-like cover composition. Further, formulations of Preparation Examples 1 to 3 are shown in Table 1 and those of Preparation Comparative Examples 1 to 4 are shown in Table 2, respectively. The extrusion was conducted under the condition of a screw diameter of 45 mm, a screw revolution speed of 200 rpm and a screw L/D of 35. A composition temperature was 220 to 260°C. Further, formulation components shown in Table 1 are the same as those shown in Table 2 and, therefore, they will be explained only in Table 1. Preparation Example No. 1 2 3 HI-MILAN 1605 80 80 80 Exxelor VA1803 20 0 0 Bondine AX8390 0 20 0 Bondine TX8030 0 0 20 Magnesium hydroxide 0.7 0.7 0.7 Titanium oxide 1.0 1.0 1.0 Preparation Comparative Example No. 1 2 3 4 HI-MILAN 1605 100 80 80 80 Exxelor VA1803 0 20 0 0 Bondine AX8390 0 0 20 0 Bondine TX8030 0 0 0 20 Magnesium hydroxide 0 0 0 0 Titanium oxide 1.0 1.0 1.0 1.0 (3) Preparation of golf ball The core prepared in the above item (1) was coated with the cover composition prepared in the above item (2) by an injection molding method to obtain a golf ball. A ball weight, a ball compression, a ball initial velocity and a flying distance of the resulting golf ball were measured. The ball compression was measured by PGA system and the ball initial velocity gas measured by R & A initial velocity measuring method. Further, the flying distance was measured by hitting a golf ball with a golf club of No. 1 wood at a club-head speed of 45 m/second, using a swing-robot manufactured by True Temper Co. In Table 3, ball weight, ball compression, ball initial velocity, flying distance and kinds of cover composition used in the production of the golf balls of Examples 1 to 3 are shown. In Table 4, ball weight ball compression, ball initial velocity, flying distance and kinds of cover composition used in the production of the golf balls of Comparative Examples 1 to 4 are shown. The cover compositions are respectively shown by Preparation Example No. and Comparative Examples No. The golf ball of Comparative Example 1 is a golf ball wherein the ionomer resin is used alone as the resin component for the cover. Example No. 1 2 3 Cover composition Ball weight (g) 45.2 45.4 45.4 Ball compression 91 90 94 Ball initial velocity (feet/second) 252.5 252.3 252.6 Flying distance (yard) 228 227 229 Comparative Example No. 1 2 3 4 Cover composition Ball weight (g) 45.4 45.2 45.4 45.4 Ball compression 102 90 88 92 Ball initial velocity (feet/second) 252.5 251.2 250.5 251.5 Flying distance (yard) 228 223 222 224 As is shown in Table 3, regarding the golf balls of Examples 1 to 3, flying distance is 227 to 229 yard and is the same as that of the golf ball of Comparative Example 1 shown in Table 4. Substantial deterioration of flying distance was not observed. On the contrary, regarding the golf balls of Comparative Examples 2 to 4, flying distance was about 4 to 6 yard smaller than that of the golf ball of Comparative Example 1, as shown in Table 4. Further, regarding the golf balls of Examples 1 to 3 and Comparative Examples 1 to 4, ten professional golfers were asked to hit the golf balls and their evaluation about hit feeling, control property and flying distance were asked. As a result, hit feeling and control property of the golf balls of Examples 1 to 3 were similar to those of a thread wound golf ball coated with a cover mainly composed of transpolyisoprene (balata) and flying distance was also satisfactory. On the contrary, regarding the golf ball of Comparative Example 1, hit feeling was hard and spinning was hardly obtained and control property was inferior. Regarding the golf balls of Comparative Examples 2 to 4, hit feeling and control property was good, but flying distance was inferior. Further, in order to evaluate cut resistance of the golf balls of Examples 1 to 3 and Comparative Examples 1 to 4, the top part of the golf ball was hit at a club-head speed of 30 m/second using a swing-robot provided with a pitching wedge manufactured by True Temper Co. to examine whether cut flaw is arisen or not. As a result, no cut flaw was arisen in the golf balls of Examples 1 to 3 and Comparative Example 1. However, small cut flaw was arisen in the golf balls of Comparative Examples 2 to 4. Further, regarding the thread wound golf ball coated with the cover mainly composed of transpolyisoprene (balata), considerably large cut flaw was arisen after test was conducted under the same conditions.

Claims: A golf ball comprising a core and a cover for covering the core, the cover comprising a heated mixture of the following components: (A) an ionomer resin; and (B) a polymer containing at least maleic anhydride as one constituent component; characterized in that the heated mixture also includes (C) a metal salt selected from the group consisting of metal hydroxides, metal carbonates and metal acetates. A golf ball as claimed in claim 1 wherein the ionomer resin (A) is an ionic ethylene-methacrylic acid copolymer based ionomer resin or an ionic ethylene-acrylic acid copolymer based ionomer resin. A golf ball as claimed in claim 1 or claim 2 wherein the polymer (B) is maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene monomer terpolymer, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, ethylene-ethyl acrylate-maleic anhydride terpolymer or a mixture thereof. A golf ball as claimed in any one of the preceding claims wherein the polymer (B) has a stiffness modulus of not more than 2,000 kgf/cm². A golf ball as claimed in any one of the preceding claims wherein the weight ratio of the ionomer resin (A) to the polymer (B) is in the range of from 95:5 to 30:70. A golf ball as claimed in any one of the preceding claims wherein the metal salt (C) is present in the mixture in an amount of from 0.2 to 3 parts by weight based on 100 parts by weight of the resin component comprising the ionomer resin (A) and the polymer (B).

Inventors: HAMADA AKIHIKO, HORIUCHI KUNIYASU, KATO AKIRA, HAMADA, AKIHIKO, HORIUCHI, KUNIYASU, KATO, AKIRA
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1992
Main classification: A63B37
IPC codes: A63B37, C08K3, C08K5, C08L23, C08L51