Golf Ball Cover

This invention relates in general to golf balls and, more particularly, to a golf ball having an improved cover material.

The assembly of a golf ball generally involves molding a cover around a solid or wound core. Golf balls with wound cores are typically referred to as "three piece" balls because they consist of three basic components: (1) a solid or liquid-filled center; (2) rubber winding around the center, and (3) the cover. Similarly, solid cores are referred to as having a "two piece" construction, since they consist solely of a solid core and a cover. A third type of ball, known as a "one-piece" ball is also known in the art. As the name suggests, one piece balls are solid balls of homogeneous construction made by any conventional molding method suitable for the purpose.

The composition of the golf ball cover has proven to be a factor in overall golf ball performance. Historically, three piece balls had covers made of Balata, a natural resin obtained from trees in South and Central America and is essentially made up of transpolyisoprene.

Balata is still used today as a cover material because of the "click" and "feel" provided by the balata cover to the golfer. "Click" is the sound made by a golf club head when it hits the ball. "Feel" is the overall sensation transmitted to the golfer after hitting the ball. Balata covered golf balls are also used because of the excellent playability and good flight performance qualities. These balls give good flight distance and skilled golfers can impart desirable spin to the ball, enabling it to draw or fade in flight. Also, especially with high-lofted clubs, the backspin allows the ball to stop abruptly on approach shots.

However, balata type materials are expensive and the manufacturing procedures required are time-consuming, labor-intensive and therefore expensive.

The majority of modern golf balls use a two piece construction with covers made of a durable synthetic thermoplastic resin such as Surlyn, a product of E. I. DuPont de Nemours Company, Incorporated. Synthetic thermoplastic cover materials have been used with limited success. They are durable and produce satisfactory flight distance. Additionally, they are less costly to manufacture than balata covers. However, these materials are hard and therefore lack the "click" and "feel" of a balata ball. Moreover, it is not possible to impart to such balls the desirable spin characteristics required by skilled golfers. It is desirable, however, to utilize a synthetic cover material for improved process manufacturing and to increase desirability and resilience performance.

In an attempt to overcome the negative factors of the hard ionomer covers, DuPont introduced low modulus SURLYN ionomers in the early 1980's. These SURLYN ionomers have a flexural modulus of from about 3000 to about 7000 PSI and a hardness of from 25 to about 40 as measured on the Shore D scale - ASTM 2240. The low modulus ionomers are terpolymers of ethylene, methacrylic acid and n- or iso-butylacrylate, neutralized with sodium and zinc cations. E. I. DuPont De Nemours & Company has disclosed that the low modulus ionomers can be blended with other grades of previously commercialized ionomers of high flexural modulus of from about 30,000 to 55,000 PSI to produce balata-like properties.

The low modulus ionomers when used without blends produced covers with very similar physical properties to those of balata, including poor cut resistance. Worse, such covers tend to go "out-of-round" quicker than balata covers. Blending with hard SURLYN ionomers was found to improve these properties.

However, the relatively high cost of the soft SURLYN ionomers, together with the need to blend several grades of hard and soft ionomers to achieve the desired properties, militates against the use of soft/hard ionomer blends in the commercial manufacture of a golf ball.

Accordingly, it is an object of the present invention to provide a golf ball having a synthetic cover material.

Another object of the present invention is to provide an improved synthetic cover material for a golf ball.

It is another object of the present invention to provide a golf ball having a synthetic cover material that achieves the click, feel, playability and flight performance qualities of Balata covered golf balls.

A further object of the present invention is to provide a golf ball having a cover material that has improved process manufacturing as well as durability and resilience.

The present invention is a cover material for a golf ball, especially a three piece liquid center golf ball. The cover material includes ethylene-methyl acrylate (E-MA) blended with an ionomer resin, such as SURLYN and a compatabilizer. The result is a golf ball cover with cover hardness and feel comparable to balata, but yielding improved resilience, durability and cut resistance over balata.

These and other objects and features of the present invention will be apparent from a reading of the following detailed description of the invention.

The cover composition of the present invention includes blends of Ethylene-Methyl Acrylate (E-MA) with an ionomer resin and a compatabilizer.

For the purpose of the present invention, E-MA is defined as a copolymer of 65-94% by weight of ethylene and 35-6% by weight methyl-acrylate. Suitable for use in the present invention are Optema.RTM. copolymers, a trademark of Exxon Chemical Company of Houston, Tex. for ethylene methyl acrylate (EMA) copolymers. Three versions of Optema.RTM. are useful in the present invention and have the following physical properties:

The term "ionomer resin" as used herein means a copolymer of ethylene with a metal salt of methacrylic acid, a metal salt of acrylic acid or a mixture thereof. Preferred metal salts are those of lithium, sodium, zinc and magnesium. Specifically, the ionomer resin will be a copolymer of 75-95% of an olefin, preferably ethylene, and 5-25% of an alpha, beta, ethnically unsaturated carboxylic acid, such as acrylic or methacrylic acid, with 10%-90% of the carboxylic acid groups being neutralized with sodium, zinc, lithium or magnesium ions. For example, Surlyn, available from E. I. DuPont deNemours Company, Incorporated of Wilmington, Del. is acceptable for use in the present invention as the ionomer resin. Also suitable is Iotek resin, available from Exxon Chemical Company of Houston, Tex.

Certain ionomer resins useful in the present invention have the following physical properties:

In a preferred embodiment, the cover material of the present invention includes 30-90% ethylene-methyl acrylate copolymer, consisting of 94-65% by weight of ethylene and 6-35% by weight of methyl-acrylate, having a flexural modulus of 2,800-19,000 psi, Shore "D" hardness of 17-45 and melt flow index of 0.7-20.0 g/10 min; and 70-10% of an ionomer resin containing 95-75% by weight of ethylene and 5-25% by weight of a carboxylic acid, 10-90% of which are neutralized with one of the metal ions Na, Zn, Li, or Mg, having a flexural modulus of 10,000-90,000 psi, and a melt index of 0.5-5.0 g/10 min.

Preferably, the blend for the cover material of the present invention includes a compatabilizer to improve the miscibility of the E-MA/Ionomer blend. The compatabilizer shall consist of a copolymer containing 80-95% by weight of an olefin, preferably ethylene, 20-5% by weight of an alpha, beta ethnically unsaturated carboxylic acid, preferably acrylic or methacrylic acid, and possibly containing a small amount of an n-alkyl acrylate, such as n-butyl acrylate. A suitable compatabilizer is Escor.RTM., a trademark of Exxon Chemical Company of Houston, Tex. for a terpolymer of ethylene, acrylic acid and n-butyl acrylate having the following physical properties:

For blends of the cover material that include a compatabilizer, a preferred embodiment is:

30 to 90 php of EMA

10 to 70 php of ionomer resin

5 to 15 php of compatabilizer--copolymer/terpolymer.

It may be desirable for the cover material of the present invention to include a colorant. The colorant may be present in the composition in an amount up to 5 parts per hundred polymer (p.h.p.) of the composition. Typically, the colorant will consist of titanium dioxide, ultramarine blue, ultramarine violet and an ionomeric carrier. A suitable colorant is available from AMERICHEM Inc. of Cuyahuga, Ohio.

The blends are made and produced into a golf ball cover by processes known in the art for producing golf ball covers, processes such as extrusion molding or injection molding. It is within the skill of those in this field to produce a blend from the components of the present invention.

The present invention will be described by way of the following examples, which are provided for illustration but are not to be considered as limiting. A control example was included, ID No. 12 consisting of a Maxfli HT-90 balata covered golf ball, for comparison purposes.

The following physical properties of compositions ID Nos. 1-11 were also measured:

Balls having cover compositions of Examples 1 to 11 above were also tested for torsional modulus at various temperatures and the results are below:

Three piece wound golf balls having covers made from the compositions of Examples 1 to 12 above were driven by a TRUE TEMPER machine and measured for carry, total distance and trajectory. The results are given below.

Balls having covers of the compositions of Examples 1 to 12 above were tested for spin-rate at various launch angles using a Driver, No. 5 iron and pitching wedge. Spin rate was measured by observing the ball in flight using stop action strobe photography and the results are given below.

Balls having cover compositions of Examples 1 to 11 above and having the torsional modulus values shown in Table III were tested for loss of properties over temperature changes and the results are below. Values calculated represent the loss in torsional modulus against previous temperature value.

Each of the compositions ID No. 1-11 and the Maxfli HT Balata (12) were then the subject of an impact deflection study, testing procedures completed in accordance with Dunlop golf standards. The results are below:

Blends I.D. Nos. 1-11 shown in Table I all contain 30-90% ethylene-methyl acrylate with 70-10% ionomer. These blends show significantly improved durability and COR versus the Example ID No. 12, the prior art Balata control. Table IV illustrates the carry distance and initial velocity improvements during flight testing vs. Maxfli HT control. Table V illustrates spin comparativeness of E-MA/ionomer blends with respect to Balata control.

Other blends were prepared and tested for various physical properties as follow:

Example blends I.D. Nos. 13-16 (Table VIII) demonstrate the effective need of a compatabilizer in E-MA/ionomer blends. Blends I.D. Nos. 13-15 contain 5-15 phr Escor compatabilizing copolymer, while blend 16 contains only the E-MA/ionomer blend. Physical property testing results show blend No. 16 yields significant decrease in elongation break vs. blends No. 13-15. This indicates a separation of materials if compatabilizer is not present.

The E-MA/ionomer resin blends of the present invention yield comparable or improved coefficient of restitution properties compared to Balata covered golf balls. Moreover, these properties did not change as cover hardness changed.

Cover blends in the hardness range of 47-62 Shore "D" yield superior durability properties compared to Balata, in the range of 145-182 hits. This is comparable to a standard Surlyn covered golf ball. Cover blends with Shore "D" hardness in range of 35-47 yielded lower durability than a harder blend, but still yielded significantly improved durability in comparison with Balata covered balls.

Example ID No. 17 is a cover consisting of 100% ethylene-methyl acrylate. Ball properties are acceptable, however, cut resistance durability is significantly lower than blends containing the ionomer resin. Thus, the blends of present invention are shown to be superior to a cover of EMA alone.

Example blends ID Nos. 18-21 support observations regarding ID Nos. 1-11, covering ratio of ionomer from 20-50% by weight.

Example blends ID Nos. 22-26 cover usage of ionomers of methacrylic acid neutralized with Na, Zn, Li or Mg metal ions. All examples in this group yield desirable properties, comparable to Example ID No. 12 (prior art Balata). The present invention is therefore shown to be valid for all types of metal ion ionomers mentioned.

Example ID Nos. 27-28 further support the conclusions mentioned in Examples 13-16, regarding effectiveness of a compatabilizer. A significant decrease in both COR and ball durability are observed on Example ID No. 27, without compatabilizer.

Example ID Nos. 28-29 indicate that the present invention yields comparable properties to Example 12 (prior art Balata) when ionomer used contains acrylic acid as the carboxylic acid.

In short, the compositions of the present invention produce a cover for a golf ball that is cost effective to produce, and yet yields the playability and flight performance qualities of a Balata covered ball.

It will be appreciated that the instant specification and claims are set forth by way of illustration and not limitation and that various changes and modifications may be made without departing from the spirit and scope of the present invention.