This invention relates generally to golf ball cover and intermediate layers and, more particularly, to golf ball layers for use in making golf ball covers and intermediate layers to optimize ball performance. This invention also relates to methods of manufacture of such golf ball layers.
Golf balls generally include a core and at least one layer surrounding the core. Balls can be classified as two-piece, wound, or multi-layer balls. Two-piece balls include a spherical inner core and an outer cover layer. Wound balls include a core, a rubber thread wound under tension around the core to a desired diameter, and a cover layer. Cover layers for wound ball generally are made of balata material or thermoset polyurethane. Multi-layer balls include a core, a cover layer, and one or more intermediate layers.
Generally, two-piece balls have good durability and distance when hit, but poor “feel”—the overall sensation transmitted to the golfer while hitting the ball—and low spin rate, which results in poor ball control. Wound balls having balata covers generally have high spin rate, leading to good control, and they also have good feel, but they have poor durability and short distance in comparison to two-piece balls. Multi-layer balls generally have performance characteristics between those of two-piece and wound balls; that is, multi-layer balls exhibit durability and distance inferior to two-piece balls but superior to wound balata balls, and they exhibit feel and spin rate inferior to wound balata balls but superior to two-piece balls.
Material characteristics of the compositions used in the core, cover, and any intermediate layers of golf balls are among the important factors that determine the performance of the balls. In particular, the composition of the cover layer is important in determining the ball's durability, shear-cut resistance, speed, spin rate, hitting sound (the sound made by a golf club head when it hits the ball), and feel. Various materials having different physical properties are used to make cover layers to create a ball having the most desirable performance possible. In particular, cover layers of many commercially available balls are made using soft or hard ionomeric resins, elastomeric resins, or blends of these.
Ionomeric resins used generally are ionomeric copolymers of an olefin and a metal salt of a unsaturated carboxylic acid, or ionomeric terpolymers having a co-monomer within its structure. These resins vary in resiliency, flexural modulus, and hardness. Examples of these resins include those marketed under the trademark SURLYN manufactured by E.I. DuPont de Nemours & Company of Wilmington, Del., and IOTEK manufactured by ExxonMobil Corporation of Irving, Tex. Ionomeric copolymers have been have been particularly favored for use in golf ball covers because they produce ball covers having excellent durability and high resilience. Ionomeric terpolymers are used to produce covers having improved spin and feel, though at the cost of ball speed and durability. Elastomeric resins used in golf ball covers include a variety of thermoplastic or thermoset elastomers available, such as polyurethane, polyetherester elastomer, polyamide elastomer, and terpolymeric ionomer.
A particular elastomeric material that provides for good performance when used in making ball covers and intermediate layers is a triblock copolymer having a first polymer block comprising an aromatic vinyl compound, a second polymer block comprising a diene compound, and a hydroxyl group located at a block copolymer, or the hydrogenation product of this triblock copolymer. An example of this block copolymer having a hydroxyl group at the terminal block copolymer is sold under the trademark HG-252 by Kuraray Company of Kurashiki, Japan. Incorporation of this material into ball covers and intermediate layers allow for balls having good feel while retaining good ball speed and shear-cut durability, properties which typically are difficult to maximize at once.
However, it has been observed that covers incorporating these triblock copolymers can suffer from cracks after being hit during play. During endurance testing of balls having covers incorporating these triblock copolymers, crack initiation and propagation was observed in the covers. This cracking leads to substantial deterioration in ball performance and long-term durability. These cracks also can initiate in an intermediate layer and subsequently propagate to a cover. Additionally, shear-cut resistance in the covers needs to be further improved for optimal performance.
In view of the above, it is apparent that golf ball cover and intermediate layers are needed that allow the optimization of golf ball performance properties by incorporating triblock copolymers into the layers, while eliminating or reducing formation of cracks in the covers and intermediate layers. The ball layers also should provide little or no processing and preparation difficulties beyond that provided by present layers. The present invention fulfills this need and other needs, and provides further related advantages.