Conventional golf balls can be divided into several general types or groups: (1) one piece balls; (2) two piece balls; (3) wound balls; and (4) other balls with three or more layers. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Balls having a two piece construction are generally most popular with the average recreational golfer because they provide maximum distance. Two piece balls commonly include a single solid core, usually formed of a crosslinked rubber. Solid cores are often made of polybutadiene that is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents and is covered by a tough, cut-resistant blended cover, such as SURLYN.RTM., an ionomer resin produced by E.I. Du Pont de Nemours & Co. of Wilmington, Del. The combination of the core and cover materials imparts a relatively high initial velocity to the ball which results in improved distance. Due to the rigidity of these materials, two piece balls have a hard "feel" when struck with a club.
At the present time, the wound ball remain, the preferred ball of the more advanced players due to its spin and feel characteristics. Wound balls typically have either a solid rubber or fluid filled center around which many yards of a stretched elastic thread or yarn are wound. The wound core is then covered with a durable cover material, such as SURLYN.RTM., or a softer material, such as balata or a castable polyurethane. Wound balls are generally softer and provide more spin than the aforementioned two piece balls. Particularly with approach shots into the green, the high spin rate of soft, wound balls enables the golfer to stop the ball very near its landing position.
Relatively recently, a number of golf ball manufacturers have introduced golf balls having three or more layers in an effort to overcome the undesirable aspects of conventional two-piece balls, such as their hard feel, while retaining their positive attributes, such as increased initial velocity and distance. These balls have multiple core layers, i.e., they include a center with one or more intermediate ("mantle") layers, and one or more cover layers. Examples of such multilayer balls include the EPISODE.TM. and the HP2 DISTANCE.TM.(TITLEIST.RTM.).
Although a variety of factors affect which of these types of balls a player will use, all players desire a ball that is affordable and durable. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to develop low-cost, efficient manufacturing techniques that produce golf balls which are resistant to cutting and cracking, yet which exhibit desirable distance, spin rate, and compression.
The durability of a ball depends not only upon its cover, but upon its core as well. A number of elastomers such as polybutadiene, natural rubber, styrene butadiene rubber, and isoprene rubber are commonly used in fabricating golf ball cores. Polybutadiene is most commonly used to obtain desired golfball properties. Manufacturers have added cross-linking agents, such as metallic salts of an .alpha.,.beta.-unsaturated carboxylic acid, to the elastomeric core composition to achieve a desired resiliency, compression, and durability.
Some manufacturers have instead attempted to provided improved golf balls by surrounding polybutadiene solid centers with thermoplastic mantle layers. For example, U.S. Pat. No. 4,337,946 discloses a golf ball having an intermediate layer of thermoplastic resin between a polybutadiene thread-wound center portion and an outer polyester elastomer cover layer which contributes to the ball's impact and cutting resistance characteristics.
U.S. Pat. No. 4,919,434 discloses a golf ball having a solid core of more than 40% cis-1,4-polybutadiene and a cover having an inner layer of 0.1 to 2 mm thickness and an outer layer of 0.1 to 1.5 mm thickness. The inner layer is a thermoplastic resin, such as an ionomer, polyester elastomer, polyamide elastomer, thermoplastic urethane elastomer, propylene-butadiene copolymer, 1,2-polybutadiene, polybutene-1, and styrene-butadiene block copolymer, either individually or in combination.
U.S. Pat. No. 5,439,227 discloses a three-part golf ball having a rubber inner core, and an outer core formed by injection molding a mixture of 100 to 50 weight percent of a polyether ester type thermoplastic elastomer and 0 to 50 weight percent of an ethylene-(meth)acrylate copolymer ionomer.
While materials incorporating a thermoplastic into a polymer blend are known, the use of such blends in portions of a golf ball core is not known. For example, U.S. Pat. No. 4,972,020 discloses an inner cover layer having a modified block copolymer of a thermoplastic polymer and a modified block copolymer consisting essentially of a base block copolymer of a monovinyl substituted aromatic hydrocarbon polymer block and an olefinic compound polymer block having an ethylenic unsaturation degree not exceeding 20 percent, wherein the base block has a molecular unit having a carboxylic acid group and/or a group derived therefrom grafted thereto.
U.S. Pat. No. 5,093,423 discloses a method of making a thermoplastic elastomer produced by dynamic vulcanization of styrene-butadiene-styrene ("SBR") rubber as a dispersed phase of crosslinked SBR, and a co-continuous, matrix of styrene-ethylene-butylene-styrene ("SEBS") and polypropylene.
U.S. Pat. No. 5,100,947 discloses a dynamically vulcanized composition of a polyolefin thermoplastic resin and an elastomer of a rubber material in which a major portion of fillers or specified additives are present in the resin.
U.S. Pat. No. 5,270,386 discloses a cover blend of vinyl aromatic copolymer and a poly(phenylene ether) concentrate containing poly(phenylene ether), a vinyl aromatic copolymer, polyamide, polycarbonate, polyester, poly(alkyl acrylate), and/or poly(alkyl methacrylate). The blend may optionally contain impact modifiers, thermoplastic molding materials including polyester, polystyrene, polyolefin, polyamide, polyvinyl chloride, polyurethane, polyacetal, and conventional additives, such as dyes and pigments.
While some of the references discussed herein describe the use of thermoplastics in forming a golf ball cover, a golf ball core or portion of a core that contains a blend of both thermoplastic and elastomeric materials is not disclosed. There has thus been a long-felt need, which is now satisfied by the present invention, for a golf ball core, or portion thereof, having a blend of at least one high Vicat softening thermoplastic and at least one elastomer to provide an increased geometric stability without substantially affecting the desired golf ball properties.