1. Field of the Invention
The present invention relates to golf ball covers comprising terpolymers consisting of at least three different monomeric units, one of which is olefinic, one of which is an ester and one of which is carbon monoxide, an anhydride monomer or a monomer containing an epoxy, i.e., glycidyl group.
2. Description of the Prior Art
Throughout its history, the golf ball has undergone an extensive evolution in an effort to improve its play-related characteristics, e.g., durability, distance, and control. The first golf balls appeared around the 14th century and consisted of a leather sack which had been soaked in water and subsequently filled with wet feathers. As the sack dried, the leather would contract and the feathers would expand, resulting in a very hard mass which was then hammered into a round ball. These "feathery" golf balls were in use until about the mid-19th century, when a single piece golf ball made from "gutta percha" was introduced. Gutta percha is a rubber-like material which is made from tree sap. Gutta percha balls were found to be more durable than the feathery balls, but did not offer the same distance as these older balls.
Following the introduction of gutta percha balls, it was discovered that a ball's flight was notably longer and more accurate when its surface had been nicked and/or scratched during use. Accordingly, golf ball manufacturers thereafter developed various methods for providing balls with a gutta percha cover that had a variety of patterns of nicks or markings on the surface. This eventually lead to the complex dimple patterns which are in use on golf balls today.
In the early 1900's, the wound rubber ball, which was considered a major breakthrough in the design of golf balls, was introduced. It consisted of a ball in which a rubber thread was tightly wound around a solid rubber core, which was then encased in a gutta percha cover. The early wound rubber balls lacked uniformity in size and weight, but the ball had a livelier core and the golfer was better able to control the spin of the ball.
Modern day golf balls can be classified as one-piece, two-piece, and three-piece (also known as "wound"). One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded therein. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid core. These are the most popular types of balls in use today. Two-piece balls typically have a hard "cutproof" cover which gives a longer distance ball, but which has lower spin rates, resulting in a decreased ability to control the ball.
Three-piece or wound balls are made by molding a cover about a wound core. The core is typically made of rubber and can be solid, semi-solid or have a liquid center. A wound core is prepared by winding a thin thread of elastic material about the center core. The wound core is then covered with a durable cover material. Wound balls are generally softer and provide more spin, resulting in increased control over the ball, but such balls typically travel shorter distances than a two piece ball. As a result of their more complex construction, wound balls generally require a longer time to manufacture and are more expensive to produce than two-piece balls.
The covers of today's golf balls are made from a variety of materials, such as balata, Surlyn.TM. and Iotek.TM.. Balata is a type of rubber and is the softest of all other cover materials. For many years, balata was the standard cover stock material for most golf balls. Balata covered balls are favored among professionals and more advanced amateur players because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter approach shots.
However, balata covered balls are expensive and less durable as compared to the other covering materials. In particular, balata covered balls are subject to nicks or cuts as a result of a mis-swung golf club, which is not uncommon with the average recreational golfer. Such nicks or cuts detract from the flight characteristics of such balls, rendering them of little use. Accordingly, cover compositions have been developed in an attempt to provide spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance.
In the mid-1960's, E.I. DuPont de Nemours and Co. discovered a new species of resins known as ionomer resins. These resins are sold under the trademark SURLYN.TM. and, to a large extent, have replaced balata as a cover stock material. Chemically, these ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic acid groups neutralized by a metal ion. See U.S. Pat. No. 3,264,272. Today, the only commercially available ionomer resins are copolymers of ethylene and methacrylic or acrylic acid. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization.
Dunlop Rubber Company obtained the first patent on the use of SURLYN.TM. for the cover of a golf ball, i.e., U.S. Pat. No. 3,454,280 issued Jul. 8, 1969. Since then, there have been a number of disclosures on the use of these ionomer resins in the cover composition of a golf ball. See, for example, U.S. Pat. Nos. 3,819,768 issued Jun. 25, 1974; 4,323,247 issued Apr. 6, 1982; 4,526,375 issued Jul 2, 1985; 4,884,814 issued Dec. 3, 1989; and 4,911,451 issued Mar. 27, 1990. However, while Surlyn.TM. covered golf balls as described in the preceding patents possess virtually cutproof covers, they have inferior spin and feel properties as compared to balata covered balls.
In November, 1986, DuPont introduced a sodium and zinc ionomer resin having a low flexural modulus and suggested using and blending the same with other ionomer resins for making a golf ball cover. Golf ball covers made from these low flexural modulus ionomer resins have improved spin and feel characteristics but relatively low velocity.
In December, 1986, DuPont introduced a lithium ionomer resin which was a copolymer of ethylene and methacrylic acid. These lithium ionomer resins have a very high flexural modulus, typically about 60,000 psi (415 MPa). DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e., more durable) than a golf ball made from other known ionomer resins such as sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with other ionomer resins where they can impart better cut resistance to those other resins.
The United States Golf Association (USGA) has promulgated a rule that no golf ball shall have an initial velocity that exceeds 255 feet (78 m) per second, i.e., 250 feet (76 m) per second with a 2% tolerance. Golf balls with covers made from ionomer resins with a low flexural modulus are woefully below this maximum and, as should be appreciated, all golf ball manufacturers strive to come as close as possible to this limit.
In order to approximate the characteristics of balata covered balls at lower cost, the art has developed balls having a variety of cover compositions. As noted above, these prior art compositions have a considerably higher cut resistance and durability as compared to balata covered balls. However, despite numerous attempts to replicate the performance of balata covered balls, the golf ball cover compositions of the prior art generally suffer from low spin rates which makes them difficult to control near the greens. Further, such balls tend to have relatively poor "click" and feel as compared to the balata covered balls. Additionally, many of the prior art golf ball cover compositions are made with low flexural modulus ionomer resins which have improved spin and feel characteristics, but relatively low velocity, which results in shorter overall distance.
Consequently, a need exists for a golf ball cover composition which provides spin rates and a feel more closely approximating those of balata covered balls, while also providing as high or a higher degree of durability than that provided by the balls presently available or disclosed in the prior art.