GB2276628A

GB2276628A - Golf balls

Info

Publication number: GB2276628A
Application number: GB9404469A
Authority: GB
Inventors: Yoshinori Egashira; Hisashi Yamagishi; Hideo Watanabe; Jun Shindo
Original assignee: Bridgestone Sports Co Ltd
Current assignee: Bridgestone Sports Co Ltd
Priority date: 1993-03-17
Filing date: 1994-03-08
Publication date: 1994-10-05
Anticipated expiration: 2014-03-08
Also published as: GB2276628B; GB9404469D0

Abstract

A solid golf ball includes a solid core enclosed in a cover. The resin component of the cover consists of 30 to 100% by weight of an ethylene-methacrylic acid-acrylate terpolymer ionomer resin having a flexural modulus of 2,500 - 14,000 psi and a Shore D hardness of 20 - 59 and 70 to 0% by weight of an ethylene(meth)acrylic acid copolymer ionomer resin having a flexural modulus of 20,000 - 30,000 psi and a Shore D hardness of 56 - 64. The core is made of a rubber composition comprising 100 parts by weight of a base rubber and 0.2 - 1.5 parts by weight of pentachlorothiophenol and/or metal salt thereof and has a distortion of 2.3 - 3.3 mm under a load of 100 kg. The ball is excellent in spin receptivity, burring resistance, and repulsion.

Description

GOLF BALLS This specification relates to golf balls. Desirable characteristics of a golf ball are ball control, spin receptivity, durability, repulsion, and flying performance.

Prior Art 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 US-A-4,884,814 and JP-A308577/1989. 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 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 improvements in these properties.

A golf ball cover of a soft and hard ionomer blend as mentioned above nevertheless 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. So, 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.

The general aim herein is the provision of novel and useful golf ball constructions. The preferred aim is to provide a solid golf ball which is satisfactory in feel, spin receptivity, ball control, repulsion, and burring resistance, while maintaining the flying distance inherent to solid golf balls such as two piece golf balls.

The inventors have found that as compared with the conventional blend of hard and soft ionomer resins, a blend of a mid-hardness ionomer resin (having a hardness intermediate between hard and soft ionomer resins) and a soft ionomer resin or a soft ionomer resin alone experiences significantly less surface damage on an iron shot. Balls with a cover which is more resistant against burring on the surface are also satisfactory in feel and spin properties.

However, the cover as such is soft and thus low repulsive. It is then difficult to utilize this cover in golf balls. Quite unexpectedly, the above-mentioned requirements can be effectively met by combining the cover with a special core.

By blending pentachlorothiophenol or a metal salt thereof in base rubber, a more repulsive core is obtained so that the resulting ball provides a satisfactory level of repulsion.

So, one aspect of our proposals is a golf ball comprising a solid core and a cover enclosing the core. The cover is based on a resin component consisting of (1) 30 to 100% by weight of a first ionomer resin in the form of an ethylene - methacrylic acid - acrylate terpolymer of flexural modulus 2,500 to 14,000 psi (17 to 97MPa) and a Shore D hardness of 20 to 59 and (2) 70 to 08 by weight of a second ionomer resin in the form of an ethylene (meth)acrylic acid copolymer having flexural modulus 20,000 to 30,000 psi (135 to 210 MPa) and a Shore D hardness of 56 to 64.

The core is made of a rubber composition comprising 100 parts by weight of a base rubber and 0.2 to 1.5 parts by weight of pentachloro- thiophenol or a metal salt thereof. Typically it has a distortion of 2.3 to 3.3 mm under a load of 100 kg.

Another aspect is a method of making such a golf ball.

The golf ball has a generally spherical solid core enclosed in a cover. The cover is essentially made of a blend of (1) 30 to 100% by weight, preferably 50 to 90% by weight of a first ionomer resin in the form of an ethylene - methacrylic acid - acrylate terpolymer and (2) 70 to 0% by weight, preferably 50 to 10% by weight of a second ionomer resin in the form of an ethylene (meth)acrylic acid copolymer. The first ionomer resin in the form of an ethylene methacrylic acid-acrylate terpolymer has a flexural modulus of 2,500 to 14,000 psi and a Shore D hardness of 20 to 59. Suitable ones are available under the trade names of Himilans from DuPont-Mitsui Polychemical Co., Ltd. and Surlyns from E.I.

DuPont (as shown in Table 1). The second ionomer resin in the form of an ethylene (meth)acrylic acid copolymer has a flexural modulus of 20,000 to 30,000 psi and a Shore D hardness of 56 to 64.

Suitable ones are available under the trade name Himilano as mentioned above, and as shown in Table 2.

Table 1

Flexural Shore D Ion modulus hardness (psi) Himilan 1855 Zn 13,000 56 Himilan 1856 Na 10,100 58 Surlyn 8120 Na 7,100 39 Surlyn 8320 Na 2,800 25 Table 2

Flexural Shore p Ion modulus Shore D (psi) Himilan 1650 Zn 24,700 60 Himilan 1652 Zn 23,000 57 Himilan 1702 Zn 21,700 62 In addition to the resin 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 ionomer 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.

The preferred core composition comprises the base rubber, metal salt of an unsaturated carboxylic acid, and . peroxide, to which pentachlorothiophenol or metal salt thereof is added as an essential component. The base rubber is preferably cis - 1,4 - poly - butadiene 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 may not take full advantage of the repulsion thereof. A metal salt of an unsaturated carboxylic acid such as acrylic acid and methacrylic acid may be used 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. Examples of the metal salt of an unsaturated carboxylic acid include zinc salts, magnesium salts and calcium salts of acrylic acid or methacrylic acid.

Among them, zinc acrylate is most preferred.

Suitable peroxides include dicumyl peroxide, t butylperoxybenzoate, 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 and/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 may be retarded in crosslinking reaction. Examples of the salt include zinc salt of pentachlorothiophenol.

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 may be 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 have a hardness in a specific range. That is, the distortion of the core under a load of 100 kg should 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 is hard and gives an unpleasant hitting feel.

With a distortion of more than 3.3 mm, the resulting ball is less repulsive.

The golf ball may be constructed and assembled using conventional techniques, 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.

Desirable thickness of the cover is in the range of 1.4 to 2.5 mm. Typical diameter of the golf ball is 42. 67 mm or more and typical weight of the golf ball is 45.93 g or less.

Examples are given below, by way of illustration and not by way of limitation.

A solid core was formed from a rubber composition of the following components.

Component Parts by weight cis - 1,4 - polybutadiene rubber (BRO1) 100 zinc acrylate 33.5 zinc oxide 10 barium sulfate 9.6 anti - oxidant 0.2 dicumyl peroxide 0.9 The solid core was directly covered by injection molding with a cover material consisting of ionomer resin, titanium dioxide for coloring, and dispersing aids, and having a specific gravity of 0.99. The ionomer resins used were Himilan and Surlyn resins having physical properties as shown in Table 3. They were mixed as shown in Table 4. The cover was surface treated and coated with clear lacquer. The resulting two piece golf balls were measured for weight (g), surface hardness (Shore C and D scales), distortion (mm under a load of 100 kg) and initial speed (m/sec.).

The thickness of the cover was 2.0 mm.

The diameter of the golf ball was 42.7 mm and the weight of the golf ball was 45.2 g.

Table 3

Flexural Ion Elonga - Shore D Designation MER modulus type tion (%) hardness (psi) Himilan 1706 Zn 0.7 290 47,900 66 Himilan 1577 Zn 5.0 410 36,300 63 Himilan 1650 Zn 1.5 460 24,700 60 Himilan 1652 Zn 5.0 500 23,000 57 Himilan 1855 Zn 1.0 510 13,000 56 Himilan 1856 Na 1.0 530 10,100 58 Surlyn 8120 Na 0.9 j 660 7,100 39 Surlyn 8320 Na 0.9 770 2,800 25 A burring test was carried out on the balls using a robot machine equipped with three pitching wedges, Eagle PW, Rextar HT305 PW and MSX PW commercially available from Bridgestone Sports Co., Ltd. A ball was hit at three different points, once at each point, by each pitching wedge at a head speed of 37 m/s. The three hit points on the ball were visually observed and evaluated in accordance with the following criteria.

slight, substantially unnoticeable club face mark distinct club face mark, but not feathered on cover surface surface burred and noticeably feathered The results are also shown in Table 4.

Table 4

Examples Comparative Examples 1 2 3 4 5 6 7 1 2 3 4 5 Cover resin mix Himilan 1706 (Zn) 50 50 25 Himilan 1557 (Zn) 50 Himilan 1650 (Zn) 50 50 50 25 30 75 75 Himilan 1652 (Zn) 25 50 Himilan 1702 (Zn) Himilan 1855 (Zn) 50 Himilan 1856 (Zn) 50 50 Surlyn 8120 (Na) 50 50 50 70 50 50 25 Surlyn 8320 (Na) 50 50 Table 4 (Contd.)

B d O Comparative X tS d 2 3 4 2 , t Ball XXX Weight (g) 45.2 45.2 45.2 45.2 45.2 45.2 45.2 45.2 45.2 45.2 45.2 45.2 Surface Shore C 82 79 76 CV Shore D 60 57 49 55 53 51 58 59 58 53 61 59 Distortion (mm) 2.39 2.43 2.57 2.47 2.44 2.52 2.40 2.37 2.38 2.40 2.30 2.36 Initial speed o 77.0976.8376.5776.8176.5976.6276.8077.2277.2676.1077.1577.01 o test o or 0 or 0 or 0 or > Cor A x > c A Eagle PW A A A A A 0 or A 0 0 Xor A un a w a za > O O O D23 A 2 N N N 1 N W 0 O O < N ~ W N O h O O O n C Pq ;: < &commat; O U < O k g rJ , ç T) 4 4 h r1 X X r1 eJ d C k as a) uz It is evident from Table 4 that the covers embodying the new proposals are fully resistant against burring by the iron club face.

Next, cores (of diameter 38.7 mm) were molded from the compositions shown in Table 5 and measured for a distortion (mm) under a load of 100 kg and initial speed (m/sec.). The core initial speed was measured using an initial speed meter of the same type as prescribed by golf ball authorized organization R & (USGA).

The results are also shown in Table 5.

Table 5

Con trol Examples Core 1 1 2 3 4 5 Composition BRO1 100 100 100 100 100 100 Zinc acrylate 33.5 33.5 33.5 33.5 33.5 33.5 Barium sulfate 9.6 9.6 9.6 9.6 9.6 9.6 Zinc oxide 10 10 10 10 10 10 Anti-oxidant 0.2 0.2 0.2 0.2 0.2 0.2 Lenacit V* - 0.2 0.4 0.6 1.0 1.5 Dicumyl peroxide 0.9 0.9 0.9 0.9 0.9 0.9 Vulcanised core properties Distortion (mm) 2.61 2.51 2.62 2.64 2.67 2.71 Initial speed 77.75 78.14 78.29 78.35 78.64 78.54 * zinc salt of pentachlorothiophenol commercially available from Bayer Japan Ltd.

It is evident from Table 5 that the cores embodying the present concepts offer a good initial speed which indicates a good repulsion.

Next, two-piece golf balls were prepared by combining the cover and the core as shown in Table 6. The balls were measured for initial speed by the same procedure as above. The balls were also determined for back spin, stop on the green, and driver flying distance by the following tests. The thickness of the cover was 2.0 mm.

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

Stop on the green Using the swing robot, the ball was hit by a pitching wedge so as to fly directly on the green. The distance between the landing and stop positions, which was the distance the ball covered due to back spin, was measured.

Flying distance Using the swing robot, the ball was hit by a driver W&num;1 at a head speed of 45 m/s to measure the flying distance.

The results are shown in Table 6.

Table 6

Examples Com- Commercial parative 2 - piece No. 1 No. 4 No. 4 No. 1 golf ball Himilan Himilan Himilan Himilan Himilan 1650/ 1650/ 1855/ 1650/ Cover resin 1706/ Surlyn Surlyn Surlyn Surlyn mix Surlyn 8120 8120 1856 8120 1650 50/ 50/50 50/50 50/50 50/50 1650 50/50 Initial speed (m/s) 77.08 77.30 77.27 76.85 77.10 Back spin 9350 9280 9310 9290 8750 (rpm) Stop on the 0.0 0.5 0.5 0.5 3.5 green (m) Driver flying 225.0 224.5 225.5 220.5 225.0 distance As is evident from Table 6, the golf balls embodying our concepts were excellent in ball control and flying performance.

These golf balls were also good in spin characteristics, burring resistance, and repulsion.

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 practised otherwise than as specifically described.

Claims

CLAIMS:

1. A golf ball comprising a solid core and a cover enclosing the core, wherein said cover is based on a resin component consisting of 30 to 100% by weight of first ionomer resin in the form of ethylene methacrylic acid-acrylate terpolymer having a flexural modulus of 2,500 to 14,000 psi and a Shore D hardness of 20 to 59, and 70 to 0% by weight of second ionomer resin in the form of ethylene (meth)acrylic acid copolymer having a flexural modulus of 20,000 to 30,000 psi and a Shore D hardness of 56 to 64, and said core is made of a rubber composition comprising 100 parts by weight of a base rubber and 0.2 to 1.5 parts by weight of penta- chlorothiophenol and/or metal salt thereof.

2. A golf ball according to claim 1 in which the cover resin component contains 50 to 90wt% of first ionomer resin and 10 to 50wt% of the second ionomer resin.

3. A golf ball according to claim 1 or claim 2 in which the core rubber composition base rubber is at least 80wt% of high-cis 1,4-polybutadiene rubber.

4. A golf ball according to any one of the preceding claims in which the core rubber composition is cured from a composition of the base rubber, a metal salt of an unsaturated carboxylic acid, peroxide and said pentachlorothiophenol and/or salt thereof.

5. A golf ball substantially as described herein with reference to any one of the Examples (but not the Comparative Examples).