GB2278364A - Golf balls - Google Patents

Abstract

A golf ball has a core enclosed in a cover which is mainly formed from a polymer blend of 60 - 95% by weight of at least one of zinc and sodium salts of an ethylene- (meth)acrylic acid copolymer having a flexural modulus of 20,000 to 56,000 psi and 40 - 5% by weight of an ethylene-vinyl acetate copolymer having a flexural modulus of 1,000 to 10,000 psi. The ball has excellent hitting and spin characteristics, high initial speed, and durability and experiences minimized damage by iron shots.

Description

GOLF BALLS This specification relates to golf balls having a core enclosed in a cover, and more particularly, to the nature of the cover.

Because of their impact durability and cut resistance, ionomer resins in the form of ethylene (meth)acrylic acid copolymers have been widely used and accepted as the cover material for most two-piece golf balls and some wound golf balls. It is, however, said that golf balls with ionomer resin covers are difficult to give a spin as intended when hit by an iron and thus relatively poor in ball control probably because the ionomer resins offer a harder feel on hitting and have higher hardness than balata rubber which is a conventional cover material.

It was thus proposed to use a blend of soft and hard ionomer resins as the golf ball cover as disclosed in USP 4,884,814 and Japanese Patent Application Kokai (JP-A) No. 308577/ 1989. More particularly, an ionomer resin in the form of an ethylene (meth)acrylic acid copolymer having a certain spectrum of physical properties is blended with a relatively soft ionomer resin in the form of an ethylene (meth)acrylic acid - (meth)acrylate terpolymer.

While the conventional golf balls using an ionomer resin in the form of an ethylene (meth)acrylic acid copolymer as the cover suffered from hard hitting feel and difficult ball control, the use of such a blend was effective for achieving significant improvement in these properties.

The golf ball cover of a soft and hard ionomer blend by this proposal, however, has the following problems. The fact that the cover is softer and more receptive to spin on an iron shot indicates an increased frictional force between the club face and the cover. Then balls using a hard core like two-piece golf balls tend to be burred or finely split on the surface since the cover surface can be ablated or scraped by the iron club grooves on an iron shot.

In addition, the ionomer cover itself is low in repulsion due to low hardness, resulting in a substantial loss of ball repulsion.

There is thus a desire to have a golf ball which is satisfactory in feel, spin receptivity, ball control, and burring resistance, and repulsion while maintaining the flying distance inherent to solid golf balls such as two-piece golf balls.

Working to produce a golf ball which has a soft feel, good spin receptivity, and acceptable repulsion, and is improved in resistance against burring or fine splitting on an iron shot, our inventors have found that a cover formed of a blend of a hard ionomer resin and/or mid-hardness ionomer resin having an intermediate hardness between hard and soft ionomer resins and a relatively low hardness resin having flexibility and elasticity, more particularly a polymer blend of a zinc and/or sodium salt of an ethylene (meth)acrylic acid copolymer having a flexural modulus of 20,000 to 56,000 pounds per square inch gauge (psi) and an ethylene vinyl acetate copolymer having a flexural modulus of 1,000 to 10,000 psi has high repulsion when it has equivalent -hardness to the above-mentioned soft and hard ionomer resin blend cover. A ball using this cover is satisfactory in feel and spin properties and experiences significantly less surface damage on an iron shot.

In one aspect, therefore, we propose a golf ball having a core enclosed in a cover which is formed with a polymer blend consisting essentially of 60 to 95% by weight of a zinc or sodium salt of an ethylene (meth)acrylic acid copolymer having a flexural modulus of 20,000 to 56,000 psi or a mixture of zinc and sodium salts of ethylene(meth)acrylic acid copolymers having a flexural modulus of 20,000 to 56,000 psi and 40 to 5% by weight of an ethylene vinyl acetate copolymer having a flexural modulus of 1,000 to 10,000 psi.

FURTHER EXPLANATIONS: OPTIONAL FEATURES The cover of a golf ball as proposed herein is mainly formed of a polymer blend consisting essentially of at least one of zinc and sodium salts of an ethylene (meth)acrylic acid copolymer having a flexural modulus of 20,000 to 56,000 psi and an ethylene vinyl acetate copolymer having a flexural modulus of 1,000 to 10,000 psi. The flexural modulus is measured according to ASTM D-790.

One component is a zinc or sodium salt of an ethylene (meth)acrylic acid copolymer or a mixture thereof. Any of zinc and sodium salts of ethylene (meth)acrylic acid copolymers may be used as long as they have a flexural modulus of 20,000 to 56,000 psi. Where a mixture of zinc and sodium salts of ethylene (meth)acrylic acid copolymers is used, the individual zinc and sodium salts may have a flexural modulus outside the range insofar as the mixture has a flexural modulus within the range. The mixture includes a mixture of zinc salts, a mixture of sodium salts, and a mixture of zinc and sodium salts.

These zinc and sodium salt of ethylene (meth)acrylic acid copolymers are commercially available, for example, under the trade name of HimilanXfrom Mitsui duPont Poly-Chemical K.K., corresponding brands of E.I. duPont in U.S.A., and ethylene - acrylic acid copolymer ionomer "Iotek#"from Exon. Useful examples are listed in Table 1.

Table 1

Noutreli@ing Fl@@@r@l modulue metal (psi) Himilan 1707 Na 55,100 Himilan 1605 Na 53,600 Himilan 1706 Zn 47,900 Himilan 1601 Na 37,700 Himilan 1557 Zn 36,300 Himilan 1705 Zn 33,400 Himilan 1650 Zn 24,700 Himilan 1702 Zn 21,700 The other component is an ethylene-vinyl acetate copolymer having a flexural modulus of 1,000 to 10,000 psi. Often ethylene vinyl acetate copolymers containing 10 to 32% by weight of vinyl acetate are used.Such are commercially available under the trade name of Evaflexofrom DU PONT - MITSUI POLYCHEMICALS CO., LTD. For example, Evaflex 150 is an ethylene vinyl acetate copolymer having a vinyl acetate content of 33% by weight and a flexural modulus of 1,200 psi, and Evaflex 150 is an ethylene vinyl acetate copolymer having a vinyl acetate content 28% by weight and a flexural modulus of 2,500 psi.

These components are mixed to provide a polymer blend. The polymer blend consists essentially of the zinc and/or sodium salt of an ethylene (meth)acrylic acid copolymer and the ethylene vinyl acetate copolymer in a weight ratio of from 60:40 to 95:5, preferably from 70:30 to 90:10. Impact durability lowers at ethylene-vinyl acetate copolymer contents of more than 40% by weight, whereas scant improvement;. in hitting feel and spin is available with contents of less than 5% by weight.

In addition to the polymer blend as a major component, the resin composition for the cover may contain optional additives, for example, dyestuffs, pigments such as titanium dioxide, zinc oxide, and barium sulfate, UV absorbers, anti-oxidants, and dispersing aids such as metal soaps. The resins and optional additives are mixed in a conventional mixer such as a closed kneader (e.g., Banbury mixer and kneader) and an extruder and then molded in a conventional manner.

Covers as proposed herein are suitable for solid golf balls, typically two-piece and three-piece golf balls.

Cores of these may be conventional, but are preferably of a composition comprising a base rubber, a metal salt of an unsaturated carboxylic acid, and a peroxide, to which pentachlorothiophenol or a metal salt thereof is added as an essential component. The base rubber is preferably cis 1,4-polybutadiene rubber containing at least 90% of cis-configuration because of its high repulsion. Although another rubber component such as natural rubber and polyisoprene rubber may be blended in the cis - 1,4 - polybutadiene rubber, it is preferred that the base rubber contains at least 80% by weight of the high cis - 1,4 - polybutadiene rubber.With less than 80% by weight of high cis - 1,4 - polybutadiene rubber, the base rubber would not take full advantage of the repulsion thereof. The metal salt of an unsaturated carboxylic acid is blended as a co-crosslinking agent, preferably in an amount of about 25 to 40 parts by weight per 100 parts by weight of the base rubber.

Zinc acrylate is a preferred salt. The peroxide includes dicumyl peroxide, t -- butylperoxy benzoate, di - t - butylperoxide, and 1,1, bis(t - butylperoxy) - 3,3,5 - trimethyl cyclohexane. Preferably the peroxide is blended in an amount of about 0.5 to 3 parts, more preferably 0.8 to 2 parts by weight per 100 parts by weight of the base rubber. Penta- chlorothiophenol or its salt is blended in an amount of about 0.2 to 1.5 parts by weight per 100 parts by weight of the base rubber in order that this blend system be enhanced in repulsion.

Beyond this range, the rubber composition for the core can be retarded in crosslinking reaction, If desired, the rubber composition for the core may contain any additive commonly used in the core of two-piece solid golf balls, for example, zinc oxide, anti-oxidant, and barium sulfate. The core rubber composition is mixed in a conventional mixer such as a Banbury mixer and kneader and then molded and cured into a spherical core by hot pressure vulcanization in a conventional manner. The vulcanized composition or core should preferably have a hardness in a specific range. That is, the distortion of the core under a load of 100 kg should preferably be 2.3 to 3.3 mm. With a distortion of less than 2.3 mm, a ball after enclosure in a cover as defined above would be hard and give an unpleasant hitting feel.With a distortion of more than 3.3 mm, the resulting ball would be less repulsive.

The golf ball may be prepared by conventional processes, for example, by molding a solid core from a core material, forming leaf cups from a cover material by a conventional molding technique, and enclosing the core with the leaf cups followed by heat compression molding. Alternatively the solid core is directly covered with a cover material by injection molding.

After the completion of molding, the ball is worked up in a conventional manner as by buffing, lacquering, and stamping.

The golf ball typically has a size and weight defined in golf ball regulations and, in general, a diameter of 42.67 mm or more and a weight of 45.93 grams or less.

The thickness of the cover is preferably 1.3 to 2.4 mm, more preferably 1.5 to 2.1 mm.

EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation.

Examples 1 - 5 & Comparative Examples 1 - 2 Large-size two-piece golf balls with a diameter of 38.5 mm were prepared. A solid core was formed from a rubber composition of the following components in a conventional manner.

Component Parts by weight cis - 1,4 - polybutadiene rubber 100 (JSR BR01) zinc acrylate 33 zinc oxide 9.7 barium sulfate 9.7 anti-oxidant (Nolak NS-6) 0.2 dicumyl peroxide 1.0 The solid core was directly covered by injection molding with a cover material of the composition shown in Table 2. The cover had a radial thickness of 2.1 mm.

The resulting two-piece golf balls were measured for weight, hardness, initial speed, hitting feel, back spin, stop on the green, and burr. The results are shown in Table 2. It is to be noted that each reported value is an average of 12 ball samples.

Ball hardness Hardness is expressed by a distortion (mm) of a ball under a load of 100 kg between flat plates.

The greater the value, the softer:is the ball.

Ball initial speed Initial speed (m/sec.) was measured using an initial speed meter as prescribed by golf ball authorized organization R & (USGA).

Feel test In a sensory test, three experienced players hit the ball with W#1 and I#7 clubs.

The hitting feel was rated "#" for soft feel as comfortable as balata-would balls, "#" for soft fee, and "#" for hard feel corresponding to conventional two-piece balls.

Back spin Using a swing robot manufactured by True Temper Co., the ball was hit by a pitching wedge at a head speed of 37 m/s. The spin quantity was determined by taking a photograph of the ball immediately after impact followed by image analysis.

Stop on the green Using a swing robot manufactured by True Temper Co., the ball was hit under the same conditions as the back spin test so as to fly directly on the green. The distance between the landing and stop positions was measured. A negative value is the distance the ball covers due to back spin. A positive value is a run in a flying direction.

Iron burring test A burring test was carried out by hitting the ball at three different points, once at each point, under the same conditions as the back spin test. The three hit points on the ball were visually observed and evaluated in accordance with the following criteria.

0: slight, substantially unnoticeable club face mark L: distinct club face mark, but not feathered on cover surface X : surface burred and noticeably feathered Table 2

Example Comparative 1 2 3 4 5 1 2 Cover composition (pbw) Himilan 1706 (Zn) 38 35 50 28 Himilan 1605 (Na) 38 35 50 28 Himilan 1601 (Na) 41 38 Himilan 1557 (Zn) 41 38 Himilan 1650 (Zn) 88 Evaflex 150 24 18 12 Evaflex 260 30 24 44 Titanium oxide 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Mgnesium stearate 0.9 0.9 0.9 0.9 0.9 0.9 0.9 Test Cover hardness (Shore D) 56 56 56 56 57 63 52 Ball weight (g) 45.2 45.2 45.2 45.2 45.2 45.2 45.2 Ball distortion (mm) 2.23 2.21 2.23 2.22 2.24 1.97 2.23 Ball initial speed (m/s) 77.51 77.50 77.40 77.39 77.38 77.96 77.10 Hitting feel # # # # # # # Back spin (rpm) 9320 9300 9360 9270 9300 8700 9400 Stop on the green (m) 0 0.5 0 0 0 4.0 - 0.5 Iron burring # # # # # # or # # Note: Himilan is the trade name of ionomer resin commercially available from DU PONT - MITSUI POLYCHEMICALS CO., LTD.

Evaflex is the trade name of ethylene vinyl acetate copolymer commercially available from DU PONT - MITSUI POLYCHEMICALS CO., LTD.

The golf balls embodying the new concepts were excellent in hitting and spin characteristics, initial speed, and durability and experience minimized damage by iron shots.

Japanese Patent Application No. 5-145566 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Ranges of 135 to 390 MNm and 6 to 70 MNm may be taken as respective alternatives to the ranges 20000 to 56000 psi and 1000 to 10000 psi stated herein.

Claims (6)

CLAIMS:

1. A golf ball comprising a core and a cover enclosing the core, the cover being mainly formed of a polymer blend consisting essentially of 60 to 95% by weight of at least one of zinc and sodium salts of an ethylene (meth)acrylic acid copolymer having a flexural modulus of 20,000 to 56,000 psi and 40 to 5% by weight of an ethylene-vinyl acetate copolymer having a flexural modulus of 1,000 to 10,000 psi.

2. The golf ball of claim 1 wherein said core is a solid core.

3. The golf ball of claim 2 wherein said solid core is made of a composition comprising 100 parts by weight of a base rubber, 25 to 40 parts by weight of a metal salt of unsaturated carboxylic acid, 0.5 to 3 parts by weight of a peroxide, and 0.2 to 1.5 parts by weight of pentachlorothiophenol or a metal salt thereof.

4. The golf ball of claim 3 wherein said base rubber contains at least 80% by weight of cis - 1,4 - polybutadiene rubber containing at least 90% of cis-configuration and said metal salt of an unsaturated carboxylic acid is zinc acrylate.

5. A golf ball according to any one of the preceding claims in which the ethylene-vinyl acetate copolymer has a vinyl acetate content of from 10 to 32wt%.

6. A golf ball substantially as described herein with reference to any one of Examples 1 to 5.