Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-01-21
2003-02-25
Buttner, David J. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S09200D, C525S208000, C525S217000, C525S221000, C525S223000, C473S378000, C473S385000
Reexamination Certificate
active
06525139
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a golf ball which is improved in durability and restitution while providing a soft hitting feel.
2. Prior Art
Ionomer resins have been widely used as the golf ball cover stock. Ionomer resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid and maleic acid wherein the acidic group is partially neutralized with a metal ion such as sodium and zinc. Because of many superior properties including durability and resiliency, they are suitable as a base resin of golf ball cover stocks.
When used as a golf ball cover stock, however, the ionomer resins have the drawback that the cover gives a hard hitting feel and lacks the flexibility required to impart a necessary spin rate to control the ball in flight. More particularly, since the ionomer resins are relatively hard, the ball is not compressed into full contact with the club face upon impact and hence, less spin is imparted. A golf ball using a harder ionomer resin as the cover stock becomes very hard in hitting feel.
It was desired to develop a golf ball cover stock which can provide spin susceptibility and feeling fit for skilled low-handicap golfers and professional golfers and has improved durability and restitution.
To solve the above-mentioned problems, extensive investigations were made on many two- and three-piece solid golf balls. With respect to the core, it is known in the art to use a soft core in order to soften a two-piece solid golf ball for achieving a soft hitting feel. The soft core, however, has the problem that the ball becomes less durable because of a larger amount of ball deformation upon impact.
With respect to the golf ball cover, a cover composition comprising an ionomer resin in admixture with a core-shell polymer consisting of a core of rubbery polymer and a shell of vitreous polymer was proposed as providing durability (see JP-A 319832/1994). More particularly, the cover composition is described as comprising an ionic ethylene copolymer based on an ethylene-unsaturated carboxylic acid copolymer in admixture with a core-shell polymer. The core-shell polymer consists of a core of a rubbery polymer having an epoxy, carboxyl or acid anhydride group on the surface and a shell of a vitreous polymer free of an epoxy, carboxyl and acid anhydride group and a functional group reactive therewith and serves to improve impact resistance. Using the ionomer resin having the core-shell polymer blended therein as a cover stock, the golf ball is improved in durability.
However, since the core-shell polymer contains as the shell the vitreous polymer which is not effective for improving durability, a large amount of the core-shell polymer must be blended in order to provide practical durability. If the amount of the core-shell polymer blended is increased to a sufficient level to achieve a significant improvement in durability, the golf ball suffers from a substantial loss of restitution.
Also a golf ball cover composition comprising an ionomer resin in admixture with a diene rubber was proposed in JP-A 79017/1994 as achieving an improvement in hitting feel. This cover composition contains the ionomer resin and the diene rubber in a weight ratio of 95:5 to 60:40. The diene rubber which is non-functional rubber is crosslinked with a crosslinking agent upon mixing it under heat with the ionomer resin. The cover composition comprising an ionomer resin in admixture with a conventional non-functional rubber is improved in hitting feel, but suffers from substantial losses of mechanical strength, impact resistance, oil resistance, resilience, and moldability. This cover composition is not practically acceptable as a golf ball cover stock. Furthermore, the cover composition using the diene rubber which is not previously crosslinked but is crosslinked upon heat mixing with the ionomer resin by a crosslinking agent would give a considerable variability of particle sizes in the rubber. This may cause a low compatibility and low durability. In addition, since the crosslinking reaction depends on the molding conditions of the rubber composition, it is very difficult to stabilize the physical properties of the cover such as melt viscosity and hardness. Thus, the use of the cover composition of JP-A 79017/1994 would result in a remarkably low productivity.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a novel and improved golf ball offering a soft hitting feel and having improved durability and restitution.
We have found that when a core is enclosed with a cover having one or more layers at least one of which is composed mainly of a rubber-modified thermoplastic resin composition consisting essentially of a base resin component in the form of an ionomer resin, a non-ionomer thermoplastic elastomer or a mixture thereof and a previously crosslinked functional rubbery copolymer preferably having a gel content of at least 20% by weight, the resulting golf ball offers a soft hitting feel and is improved in durability and restitution.
More particularly, the base resin component in the form of an ionomer resin, a non-ionomer thermoplastic elastomer or a mixture thereof is blended with a crosslinked functional rubbery copolymer of (a) a monomer having at least one functional group selected from the group consisting of carboxyl, epoxy, hydroxyl and amino groups, (b) at least one monomer selected from the group consisting of acrylonitrile, butadiene, isoprene, and acrylates, and (c) a crosslinker in the form of a polyfunctional monomer, instead of the conventional non-functional rubber or core-shell polymer containing a vitreous polymer. Then, reaction or interaction occurs between a reactive group in the functional rubbery copolymer and a reactive group in the base resin to form a high bond strength interface between the components, achieving a drastic improvement in durability at no sacrifice of resilience. The functional rubbery copolymer has the effect of softening the resin, contributing to a soft hitting feel. As opposed to the blending of a core-shell polymer containing a vitreous polymer, only a small amount of the functional rubbery copolymer can achieve a drastic improvement in durability without adversely affecting resilience. This leads to molding and economic advantages. Drastic improvements in mechanical strength, impact resistance, and oil resistance are achieved as compared with the base resin having non-functional rubber blended therein. The cover stock containing the functional rubbery copolymer is also improved in feeling, modulus of elasticity, and molding or dimensional stability, overcoming the problems of the prior art cover stocks.
The present invention provides a golf ball comprising a core and a cover enclosing the core, said cover having one or more layers, at least one cover layer being composed mainly of a rubber-modified thermoplastic resin composition consisting essentially of a base resin in the form of an ionomer resin, a non-ionomer thermoplastic elastomer or a mixture thereof and a crosslinked functional rubbery copolymer preferably having a gel content of at least 20% by weight.
Preferably, the functional rubbery copolymer is a crosslinked copolymer of (a) a monomer having at least one functional group selected from the group consisting of carboxyl, epoxy, hydroxyl and amino groups, (b) at least one monomer selected from the group consisting of acrylonitrile, butadiene, isoprene, and acrylates, and (c) a crosslinker in the form of a polyfunctional monomer. The preferred functional rubbery copolymer is in particulate form having a mean particle size of up to 500 nm.
The ionomer resin is preferably comprised of an olefin-unsaturated carboxylic acid copolymer having an acid content of 10 to 25% by weight. The non-ionomer thermoplastic elastomer is preferably a polyurethane elastomer, polyester elastomer, polyamide elastomer or a mixture thereof.
The cover stock according to the present invention is obtained by mixing the previously
Ichikawa Yasushi
Kashiwagi Shunichi
Takesue Rinya
Bridgestone Sport Co., Ltd.
Buttner David J.
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