Golf ball

Abstract: The present invention provides a golf ball having excellent durability as well as excellent impact resilience and suitable hardness, which comprises a core and a cover for covering the core, wherein a heated mixture of an ionomer resin and a thermoplastic resin containing an oxazoline group is used as the main component of the base resin for the cover.

Description: The present invention relates to a golf ball. As a base resin for the cover of a golf ball, an ionomer resin has previously widely been used (e.g. Japanese Patent Kokai No. 49-49727). The golf ball coated with the ionomer resin cover is hardly damaged and has a superior hitting durability and, furthermore, has an excellent impact resilience and flying performance. The ionomer resin has therefore been highly regarded as having has well-balanced properties as the base resin for the cover. However, the demands of golfers for the golf ball have become higher and higher, and it has been strongly requested that the shot feel of the golf ball is further improved, without affecting other properties. In order to obtain a golf ball having good shot feel, it is necessary to use a softer core. However, as the core becomes softer, the deformation of the ball on hitting becomes larger and the durability of the ball deteriorates remarkably. It is therefore difficult to use the golf ball. Thus, a stronger cover which inhibits deterioration in durability is required. Heretofore, the selection of the ionomer resin and the blending of the ionomer resin with other polymers have been studied, but satisfactory results have not been obtained. It has now been found that a golf ball having excellent durability as well as an excellent impact resilience and suitable hardness can be obtained, by using a mixture produced by mixing, with heating, an ionomer resin and a thermoplastic resin containing an oxazoline group as the main component of the base resin of the cover. The main object of the present invention is to provide a golf ball having an excellent durability, as well as an excellent impact resilience and suitable hardness. According to the present invention, there is provided a golf ball comprising a core and a cover for covering the core, the base resin of the cover comprising a heated mixture of an ionomer resin which is a resin of a copolymer comprising 10 to 20% by weight of an αβ- unsaturated carboxylic acid having from 3 to 8 carbon atoms and from 80 to 90% by weight of an α-olefin, with some carboxyl groups in the copolymer being neutralized with metal ions and a thermoplastic resin containing an oxazoline group as the main component in an amount of from 1,000 to 15,000 (equivalent molecular weight of an oxazoline group) obtainable by heating to a resin temperature of 150 to 250°C wherein the thermoplastic resin containing the oxazoline group is microdispersed in the ionomer resin. When the thermoplastic resin containing the oxazoline group is mixed with the ionomer resin with heating, the oxazoline group of the thermoplastic resin reacts with a carboxyl group of the ionomer resin, and the thermoplastic resin containing the oxazoline group is grafted onto the surface of the ionomer resin to form a compatible mixed system wherein the thermoplastic resin containing the oxazoline group is micro-dispersed in the ionomer resin, i.e a "polymer alloy". This "polymer alloy" synergistically improves the physical properties of the ionomer resin and, therefore, the toughness of the ionomer resin is further improved. As a result, the durability of the golf ball is improved and, furthermore, the impact resilience and hardness are also improved. Accordingly, even if a soft core is used, a remarkable deterioration of durability does not arise and the impact resilience and hardness are suitably maintained, whereby a golf ball having an excellent shot feel can be obtained without affecting other properties. The ionomer resin is a resin of a copolymer comprising 10 to 20% by weight of an α β-unsaturated carboxylic acid having from 3 to 8 carbon atoms and from 80 to 90% by weight of an α-olefin, with some carboxyl groups in the copolymer being neutralized with metal ions. 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 such resins may be used in combination. Examples of the ionic ethylene-methacrylic acid copolymer based ionomer resins include HI-MILAN (Registered Trade Mark) 1605 (of the type neutralized by sodium ion), HI-MILAN (Registered Trade Mark) 1706 (of the type neutralized by zinc ion), HI-MILAN (Registered Trade Mark) 1707 (of the type neutralized by sodium ion), HI-MILAN (Registered Trade Mark) 1705 (of the type neutralized by zinc ion) which are commercially available from Mitsui Du Pont Polychemicals Co.; SURLYN (Registered Trade Mark) 7930 (of the type neutralized by lithium ion), SURLYN (Registered Trade Mark) 7940 (of the type neutralized by lithium ion) which are commercially available from Du Pont de Nemours & Co. Examples of the ionic ethylene-acrylic acid copolymer based ionomer resin include one that is commercially available from EXXON Co. under the trade name of Iotek. As the thermoplastic resin containing oxazoline groups, there are two types of resins, for example, those wherein the oxazoline group is added to the main chain (skeleton) of a polymer and those containing the oxazoline groups at the surface of polymer particles. The main chain part in those resins where the oxazoline group is added to the polymer main chain may be selected from high molecular weight polymers obtained from monomers having radical polymerizability, depending upon physical properties to be required. Examples of the main chain polymer include polystyrene, acrylonitrile-styrene copolymer, acrylic acid-styrene copolymer, polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate and polybutyl acrylate. They may be used alone or in combination. The polymers used in those resins containing the oxazoline groups at the surface of polymer particles are the same as the polymers in which the oxazoline group is attached to the polymer main chain. Those containing the oxazoline group at the surface of polymer particles are generally used as a dispersion in water of fine particles having a particle size in the range of from 0.05 to 0.1 µm. The weight-average molecular weight of the polymer which constitutes the main chain or particle in the thermoplastic resin containing the oxazoline group is preferably in the range of from 100,000 to 500,000. The amount of the oxazoline group is from 1,000 to 15,000 (equivalent molecular weight of a functional group, i.e. oxazoline group). When the thermoplastic resin containing the oxazoline group is mixed with the ionomer resin with heating, the oxazoline group of the thermoplastic resin reacts with the carboxyl group of the ionomer resin to further improve the toughness of the ionomer resin, as described above. As a result, the durability of the golf ball is further improved and the impact resilience and hardness are also improved. Therefore, even if a soft core is used, a remarkable deterioration of durability does not arise and the impact resilience and hardness are suitably maintained, whereby a golf ball having an excellent shot feel can be obtained without affecting other properties. The ionomer resin is mixed with the thermoplastic resin containing oxazoline groups with heating, preferably at a resin temperature of 150 to 250°C, using an extruder for plastic or an internal mixer such as plastmill or Banbury mixer. By mixing with heating, bonding is formed by an aminoesterification reaction of the carboxyl group of the ionomer resin with the oxazoline group of the termoplastic resin containing the oxazoline group, and the thermoplastic resin containing the oxazoline group is micro-dispersed in the ionomer resin. In order to conduct the above mixing and reaction uniformly and efficiently, it is most suitable to use a biaxial kneading type extruder among the above mixers. The mixing ratio (weight ratio) of the ionomer resin (A) and thermoplastic resin containing the oxazoline group (B) is preferably in the range of 95:5 to 60:40. When the mixing ratio of the ionomer resin (A) is larger than the above range, since the amount of the thermoplastic resin containing the oxazoline group (B) is small, sufficient improvement of durability as well as impact resilience and hardness due to the reaction of the thermoplastic resin containing the oxazoline group with the ionomer resin can not be obtained. When the mixing ratio of the ionomer resin (A) is smaller than the above range, the thermoplastic resin containing the oxazoline group (B) constitutes a continuous phase of the mixed system and, therefore, properties such as strength and impact resilience of the ionomer resin per se can not be obtained, sufficiently. In the present invention, a heated mixture of the ionomer resin and thermoplastic resin containing the oxazoline group is used as the main component of the base resin for the cover. To the heated mixture of the ionomer resin and thermoplastic resin containing the oxazoline group, if necessary, various additives such as pigments, dispersants, antioxidants, UV absorbers or UV stabilizers can be added. Further, in the present invention, the fact that the heated mixture of the ionomer resin and thermoplastic resin containing the oxazoline group is used as the main component of the base resin for the cover means the case that the base resin for the cover is only composed of the heated mixture of the ionomer resin and thermoplastic resin containing the oxazoline group, or the case that the base resin for the cover is composed of the heated mixture of the ionomer resin and thermoplastic resin containing the oxazoline group, to which other resins are added in an amount within the range which does not affect 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). Then, a cover composition which is composed of the heated mixture of the ionomer and thermoplastic resin containing the oxazoline group as the main component is coated on the core to obtain a golf ball. In that case, any of solid core and thread wound core 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. According to the present invention, a golf ball having excellent durability as well as excellent impact resilience and suitable hardness can be obtained. Further, according to the present invention, a golf ball having practical durability, suitable impact resilience and hardness as well as excellent hit feeling can be obtained, because remarkable deterioration of durability is not arisen even if a soft core is used and impact resilience and hardness are suitably maintained. The following Examples further illustrate the present invention in detail. Examples 1 to 6 and Comparative Examples 1 and 2 (1) Preparation of cover composition Formulation components shown in Table 1 were mixed and extruded by a twin-screw kneading type extruder to obtain a pellet-like cover composition. 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 cylinder temperature is as follows. Barrel Temperature (°C) 1 200 2 200 3 200 4 230 5 230 6 230 7 250 8 250 9 250 die 250 (2) Preparation of golf ball Two kinds of core compositions shown in Table 2 were prepared. The core composition was placed in a mold for core and vulcanized at 165°C for 20 minutes to prepare a solid core of 38.4 mm in diameter. The core was coated with a cover composition of a formulation shown in Table 1 by a injection molding method and finished through painting and marking processes by a conventional method to prepare a golf ball of 42.8 mm in diameter. Core composition A B Polybutadiene 100 100 Zinc acrylate 35 25 Zinc oxide 20 23 Dicumyl peroxide 1.0 1.0 Antioxidant 0.5 0.5 A ball weight, a ball compression, a ball initial velocity and a ball durability of the resulting golf ball were measured. The results are shown in Tables 3 to 5. Further, the ball compression was measured by PGA system and the ball initial velocity and the ball durability were measured as follows. Ball initial velocity : By using a swing-robot manufactured by True Temper Co., a golf ball was hit at a club-head speed of 45 m/second with a golf club of No. 1 wood. A ball initial velocity at the moment was measured and the average of 10 balls was determined. Ball durability : A golf ball was allowed to bump against a bumping plate at a speed of 45 m/second, repeatedly. The number of times at which the ball was broken was determined. The number is represented by the index which is calculated on the basis of the golf ball of Comparative Example 1 being 100. In Table 3, properties of the golf ball of Example 1 wherein a core composition by which general ball compression can be obtained (i.e. core composition A in Table 2) is used as the core and the golf ball of Comparative Example 1 are shown. Further, the golf ball of Comparative Example 1 is most general golf ball having a conventional constitution wherein an ionomer resin is used as the base resin for cover. Example 1 Comp. Example 1 Core composition A A Cover composition No. 4 6 Ball weight (g) 45.3 45.3 Ball compression 102 100 Ball initial velocity (m/second) 252.8 252.5 Ball durability 140 100 As described above, the golf ball of Comparative Example 1 is most general golf ball having a conventional constitution. As shown in Table 3, regarding the golf ball of Example 1 wherein a core composition which is the same as that of Comparative Example 1 is used, the index indicating ball durability is high as 140 and ball durability is excellent in comparison with the golf ball of Comparative Example 1. That is, the results shown in Table 3 means the fact that durability of the golf ball wherein the cover composition in the present invention is used is remarkably improved in comparison with the golf ball wherein the cover composition having a conventional constitution is used. Then, properties of the golf balls of Examples 2 to 6 wherein a core composition by which a soft core, i.e. a core having low compression can be obtained (core composition B in Table 2) and the golf ball of Comparative Example 2 are shown in Tables 4 and 5. Example 2 Example 3 Example 4 Core composition B B B Cover composition No. 1 2 3 Ball weight (g) 45.3 45.3 45.4 Ball compression 79 80 82 Ball initial velocity (m/second) 251.3 251.5 251.5 Ball durability 95 100 100 Example 5 Example 6 Comp. Example 2 Core composition B B B Cover composition No. 4 5 6 Ball weight (g) 45.3 45.3 45.3 Ball compression 81 83 80 Ball initial velocity (m/second) 251.6 251.6 251.0 Ball durability 105 108 60 As shown in Table 5, regarding the golf ball of Comparative Example 2 wherein a soft core having low compression, i.e. a soft core is used and a cover composition having a conventional constitution (cover composition No. 6) is used, the index indicating ball durability is low as 60 and remarkable deterioration of durability is observed in comparison with the golf ball of Comparative Example 1 which is the most general golf ball having a conventional constitution. Thus, it is considered to be of no practical use. To the contrary, regarding the golf balls of Examples 2 to 6 of the present invention, a soft core which is the same as that of the golf ball of Comparative Example 2 is used, but the index indicating ball durability is about 100 and remarkable deterioration of durability is not observed. Thus, it has practical durability. Regarding the golf balls of Examples 2 to 6 and the golf ball of Comparative Example 1, professional golfers were asked to hit the golf balls and their evaluations were asked. As a result, hit feeling of the golf balls of Examples 2 to 6 is good in comparison with the golf ball of Comparative Example 1. By using a swing-robot manufactured by True Temper Co., the golf balls of Examples 2 to 6 and the golf ball of Comparative Example 1 were hit with a golf club of No. 1 wood to compare an impact force, respectively. As a result, the impact force of the golf balls of Examples 2 to 6 is about 80 % of that of the golf ball of Comparative Example 1.

Claims: A golf ball comprising a core and a cover for covering the core, the base resin of the cover comprising a heated mixture of an ionomer resin which is a resin of a copolymer comprising 10 to 20% by weight of an αβ- unsaturated carboxylic acid having from 3 to 8 carbon atoms and from 80 to 90% by weight of an α-olefin, with some carboxyl groups in the copolymer being neutralized with metal ions, and a thermoplastic resin containing an oxazoline group as the main component in an amount of from 1,000 to 15,000 (equivalent molecular weight of an oxazoline group) obtainable by heating to a resin temperature of 150 to 250°C wherein the thermoplastic resin containing the oxazoline group is microdispersed in the ionomer resin. A golf ball as claimed in claim 1 wherein the thermoplastic resin containing an oxazoline group is a resin in which the oxazoline group is added to a main chain (skeleton) of a polymer or one containing the oxazoline group at the surface of polymer particles. A golf ball as claimed in claim 2 wherein the main chain or polymer particles are selected from polystyrene, acrylonitrile-styrene copolymer, acrylic acid-styrene copolymer, polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate or polybutylacrylate. A golf ball as claimed in claim 1 or claim 2 wherein the polymer particles are present in an aqueous dispersion and have a particle size in the range of from 0.05 to 0.1 µm. A golf ball as claimed in any one of claims 2 to 4 wherein the weight-average molecular weight of the polymer which constitutes the main chain or particle is in the range of from 100,000 to 500,000. A golf ball as claimed in any one of the preceding claims wherein the weight ratio of the ionomer resin (A) and thermoplastic resin containing the oxazoline group (B) is within the range of 95:5 to 60:40. A golf ball as claimed in any one of the preceding claims wherein the cover further contains pigments, dispersants, antioxidants, UV absorbers or UV stabilizers. A golf ball as claimed in any one of the preceding claims wherein the core is a solid core or a thread wound core.

Inventors: HAMADA AKIHIKO, HORIUCHI KUNIYASU, KATO AKIRA, HAMADA, AKIHIKO, HORIUCHI, KUNIYASU, KATO, AKIRA
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, CO. LTD
Application year: 1992
Main classification: C08L57
IPC codes: C08L23, C08L101, C08L33, A63B37, C08L25