Field of the Invention
The present invention generally relates to methods for making multi-piece golf balls and more particularly to golf balls having at least one component made of a liquid rubber composition comprising liquid ethylene-propylene diene copolymer (EPDM) rubber. The invention also includes the resulting multi-piece golf ball. The ball includes an inner core, preferably a dual-core containing a center and surrounding outer core layer. The ball further includes a cover, preferably a dual-cover having inner and outer cover layers. The outer cover preferably is made of the EPDM rubber. The resulting golf ball has high resiliency, a soft feel, and good weather-resistance.
Brief Review of the Related Art
Multi-piece solid golf balls having a core and surrounding cover are generally known in the industry. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made commonly of a rubber material such as natural and synthetic rubbers: styrene butadiene, polybutadiene, poly (cis-isoprene), or poly (trans-isoprene). The outer cover is made commonly of a thermoplastic such as ionomer resins, polyamides, and polyesters; and thermoplastic and thermoset polyurethane and polyurea elastomers. As new materials and manufacturing processes have become more economically feasible, three-piece, four-piece, and five-piece solid golf balls have been introduced. Both professional and amateur golfers enjoy these multi-piece golf balls because of their properties and playing performance. Different materials can be used to impart specific properties and playing features to the ball.
Multi-layered covers are used normally in constructing these multi-piece balls. For example, the ball may include an inner cover layer made of an ethylene-based acid copolymer ionomer resin that helps impart hardness to the ball. These acid copolymer ionomers contain inter-chain ionic bonding and are generally made of an α-olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic, acrylic acid, or maleic acid. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available ethylene-based ionomer resins are available in various grades and identified based on type of base resin, molecular weight, and type of metal ion, amount of acid, degree of neutralization, additives, and other properties. The outer cover layer, which is disposed about the inner cover layer, may be made from a variety of materials including ionomers, polyamides, polyesters, and thermoplastic and thermoset polyurethane and polyureas. In recent years, golf balls having thin polyurethane covers have become more popular, because such covers tend to provide the ball with a “soft feel.” In general, these balls provide the player with a more natural feel and sensation when he/she strikes the ball with the club face as opposed to balls having a more plastic and “hard feel.”
Various cover materials are known in the industry. For example, Nesbitt, U.S. Pat. No. 6,303,704 discloses golf balls having covers made of non-ionomeric resins that have been subjected to cross-linking by peroxide cross-linking agents such as dicumyl peroxide or by irradiation such as gamma rays/electron beams. Numerous resin materials are listed including ethylene-ethyl acrylate, ethylene-methyl acrylate, ethylene-vinyl acetate, low density polyethylene, linear low density polyethylene, metallocene catalyzed polyolefins, polyamides, non-ionomeric acid copolymers, ethylene propylene elastomers such as EPR and EPDM, and syndiotactic resins such as syndiotactic 1,2-polybutadiene alone or in combination with other dienes. In one embodiment, a cover composition made from 100 parts ethylene-propylene-diene monomer and additives was cross-linked. The resulting composition was molded over a solid core to form a two-piece ball that was tested for scuff and cut-resistance.
Sullivan and Kaltenbacher, U.S. Pat. No. 5,857,926 discloses a cover layer formed from a composition comprising a blend of: 1) ionomeric copolymer; 2) ethylene-propylene-diene monomer; and 3) a copolymer formed from an α-olefin such as ethylene, acrylate ester such as methylacrylate, and acid such as methacrylic acid. The cover composition may be molded over a golf ball core (solid or wound) by injection molding or compression molding. Cores having one, two, or more layers can be used. According to the '926 Patent, the resulting ball has a high coefficient of restitution, soft cover, and excellent cut-resistance.
The industry continues looking for new cover materials for golf balls. It would be desirable to have a cover material that helps provides the ball with high resiliency. This would help the ball travel longer distances. At the same time, the cover material should provide the ball with a nice feel and playability. The cover material should not be excessively hard. Moreover, it would be desirable to have a cover material with high weather-resistance so the ball can resist cracking and thermal aging. The present invention provides golf balls having covers with such properties as well as other advantageous characteristics and features.