GB2315071A

GB2315071A - Golf ball cover compositions

Info

Publication number: GB2315071A
Application number: GB9718606A
Authority: GB
Inventors: Yoshinori Egashira; Hisashi Yamagishi; Hideo Watanabe; Yasushi Ichikawa
Original assignee: Bridgestone Sports Co Ltd
Current assignee: Bridgestone Sports Co Ltd
Priority date: 1994-02-10
Filing date: 1995-02-06
Publication date: 1998-01-21
Anticipated expiration: 2015-02-06
Also published as: GB2315071B; GB9718606D0

Abstract

A golf ball cover is formed of a composition comprising (a) 70 - 95% by weight of a sodium or zinc salt of an ethylene-unsaturated carboxylic acid - unsaturated carboxylic ester terpolymer or a mixture thereof and (b) 30 - 5% by weight of a sodium or zinc salt of an ethylene - unsaturated carboxylic acid copolymer, wherein said copolymer has an M.F.R. of at least 4, the composition curing into a product having a hardness of 55 - 80 on JIS C scale and a tensile stress at 10% distortion of 40 - 140 kg/cm 2 . The composition is easy to mold and allows for manufacture of a golf ball having improved spin properties and resistant to impact by iron and other clubs.

Description

GOLF BALL COVER COMPOSITIONS The specification relates to golf ball cover compositions, their preparation and use.

Prior Art As the cover material of two-piece golf balls and some wound golf balls, ionomer resins in the form of ethylene- (meth)acrylic acid copolymer are widely used and accepted because of their long - lasting impact resistance and cut resistance. Since the ionomer resins provide hard hitting feel and high hardness as compared with the balata rubber conventionally used as the cover material, golf balls with an ionomer resin cover are said difficult to impart a desired quantity of spin and inferior to control on iron shots.

For improvements in these respects, USP 4,884,814 or JP-A 308677/1989 proposes to blend an ionomer resin in the form of an ethylene (meth)acrylic acid copolymer having a certain spectrum of physical properties with a specific amount of an ethylene- (meth)acrylic acid (meth)acrylate terpolymer ionomer which is a relatively soft ionomer resin. The soft/hard ionomer blend is used as a golf ball cover.

This is a quite effective technique for improving the hitting feel and control of golf balls using a conventional ionomer resin in the form of an ethylene- (meth)acrylic acid copolymer as the cover.

Nevertheless, the golf ball cover of the above-mentioned soft/hard ionomer blend has several problems. The cover is softer and allows the ball to receive more spin on iron shots, which means the increased frictional force between the club face and the cover. Then balls using a hard core such as two-piece solid golf balls have the likelihood that the ball surface be peeled as a consequence of iron shots because the cover surface can be peeled by grooves across the iron club face.

An improvement in this regard is proposed in JP-A 277208/1993. A mixture of two or more metal salts of an ethylene - unsaturated carboxylic acid - unsaturated carboxylate terpolymer having a low flexural modulus is used as the cover for the purpose of protecting the ionomer cover against fretting by iron club shots. This technique leads however to problems in manufacturing or molding because of poor flow of the material used therein and the resulting balls are inferior in performance. For example, when a conventional cover-enclosing process (involving interposition of a core between half cups of the cover material followed by heat pressure molding or injection molding of the cover material around a core) is used, the low flowing material has the problems that as compared with smooth flowing resins, dimples on the inner surface of a mold are poorly transferred thereto, the mold configuration is not faithfully reflected so detracting from ball sphericity, and weld lines appear to reduce impact durability.

The aim here is to provide new and useful compositions for golf ball covers, as well as methods for preparation and use thereof.

In the light of the discussion above, preferred aims are to provide good feel, ease of spin or control, and resistance to peeling.

Making efforts to develop a golf ball cover composition which is flexible and readily moldable and which offers a soft feel, good spin property, and improved resistance to peeling by iron shots when used as the golf ball cover, we have found that when a composition comprises (a) 70 to 95% by weight of a sodium or zinc salt of an ethylene - unsaturated carboxylic acid - unsaturated carboxylate terpolymer o; a mixture thereof and (b) 30 to 5% by weight of a sodium or zinc salt of an ethylene-unsaturated carboxylic acid copolymer, an ethyleneunsaturated carboxylic acid - unsaturated carboxylate terpolymer or a mixture thereof, having a melt flow rate (M.F.R., defined below) of at least 4, the composition can be cured to a product having a hardness of 55 to 80 on JIS C scale and a tensile stress at 10% distortion of 40 to 140 kg/cm2, and we can then obtain a golf ball cover which meets at least some of the problems mentioned above and so offers good flexibility, soft hitting feel, susceptibility to spin, and ease of control, and is resistant to scuff and damages by iron clubs.

According to a first aspect herein, there is provided a composition for forming a golf ball cover, comprising (a) 70 to 95% by weight of a sodium or zinc salt of an ethylene-unsaturated carboxylic acid - unsaturated carboxylate terpolymer or a mixture thereof and (b) 30 to 5% by weight of a sodium or zinc salt of an ethylene-unsaturated carboxylic acid copolymer, an ethylene - unsaturated carboxylic acid-unsaturated carboxylate terpolymer or a mixture thereof, having an M.F.R. of at least 4, the composition curing into a product having a hardness of 55 to 80 on JIS C hardness scale and a tensile stress at 10% distortion of 40 to 140 kg/cm2.

Other aspects are methods comprising preparation of such compositions, their use to form golf ball covers, and golf balls having such covers.

According to the present invention, the cover of the golf ball is formed of a composition comprising (a) a sodium or zinc salt of an ethylene - unsaturated carboxylic acid unsaturated carboxylate terpolymer or a mixture thereof and (b) a sodium or zinc salt of an ethylene - unsaturated carboxylic acid copolymer, an ethylene - unsaturated carboxylic acid unsaturated carboxylate terpolymer or a mixture thereof.

Component (a) This is a sodium or zinc salt of an ethylene - unsaturated carboxylic acid unsaturated carboxylate terpolymer. The unsaturated carboxylic acid is preferably selected from aliphatic monocarboxylic acids having 3 to 8 carbon atoms, especially acrylic acid and methacrylic acid. The unsaturated carboxylate, qenerally an ester, is preferably of unsaturated carboxylic acid having 3 to 8 carbon atoms (e.g., acrylic and methacrylic acid) with alcohol having 1 to 9 carbon atoms, the ester desirably having 4 to 12 carbon atoms in total.

Exemplary esters include methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, and isobutyl methacrylate.

Examples of the sodium and zinc salts of terpolymer are shown in Table 1.

Table 1

Neutral - Flexural M.F.R.

Trade name Manufacturer izing modulus (g/10 metal (psi) min.) Mitsui - DuPont Himilan 1855 Polychemical Zn 13,000 1.0 K.K.

Mitsui - DuPont Himilan 1856 Polychemical Na 10,100 1.0 K.K.

E.I. DuPont de Surlyn 8120 Na 7,100 0.9 Nemours Co.

E.I. DuPont de Surlyn 8320 Na 2,800 0.9 Nemours Co.

Iotak 7510 Exxon Chemical Zn 5,200 0.8 Corp.

Exxon Chemical Iotak 7520 Zn 3,800 2.0 Corp.

(Note: Himilan, Surlyn and Iotak are registered trade marks) The sodium and zinc salts of terpolymers may be used alone or in admixture of two or more. A blend of sodium and zinc salts is preferred from the standpoints of repulsion and durability. In such a blend, the sodium and zinc salts are desirably mixed in a weight ratio of from 20:80 to 80:20, especially from 40:60 to 70:30.

It is seen from Table 1 that these sodium and zinc salts of terpolymers are relatively soft among ionomer resins since they have a flexural modulus of about 2,500 to 15,000 psi, indicating that they are suitable as soft feeling covers. However, their M.F.R. which is an index of flow is not more than 2.0, mostly not more than 1.0, indicating that they are low flowing and difficult to mold.

It is to be noted that the melt flow rate (M.F.R.) as used herein is measured according to JIS K7210 (1976) Method A.

Component (b) Therefore, it is necessary to improve the flow of the sodium or zinc salt of terpolymer or mixture thereof. According to the present invention, to the sodium or zinc salt of terpolymer or mixture thereof is added component (b) which is smooth flowing and compatible with the metal salt of terpolymer. Component (b) is a sodium or zinc salt of an ethylene unsaturated carboxylic acid copolymer, an ethylene - unsaturated carboxylic acid unsaturated carboxylate terpolymer or a mixture Component (b) should preferably have an M.F.R. not more than 30 because above this limit, the resulting balls may be deteriorated in durability (scuff resistance) and performance (repulsion).

In the sodium and zinc salts of ethylene unsaturated carboxylic acid copolymer, the unsaturated carboxylic acid is preferably selected from aliphatic monocarboxylic acids having 3 to 8 carbon atoms, especially acrylic acid and methacrylic acid. These salts should have an M.F.R. of at least 4. Salts with MFR substantially less than 4 are less effective for improving moldability. Illustrative examples of the sodium and zinc salts of ethylene unsaturated carboxylic acid copolymer are shown below.

Table 1

Flexural M.F.R.

Trade name Manufacturer modulus (g/10 (psi) min.) Mitsui - DuPont Himilan 1577 Polychemical 36,000 5.0 K.K.

Mitsui - DuPont Himilan 1652 Polychemical 24,000 5.0 K.K.

Mitsui - DuPont Himilan 1705 Polychemical 34,000 5.0 K.K.

Mitsui - DuPont Himilan 1702 Polychemical 22,000 14.0 K.K.

E.I. DuPont de Surlyn 9650 Nemours Co. 32,000 5.0 E.I. DuPont de Surlyn 9450 Nemours Co. 19,000 5.5 E.I. DuPont de Surlyn 9950 37,000 5.5 Nemours Co.

E.I. DuPont de Surlyn 9970 28,000 14.0 Nemours Co.

Another useful component (b) is an ethylene - unsaturated carboxylic acid unsaturated carboxylate terpolymer. Options for unsaturated carboxylic acid and unsaturated carboxylate moieties may be the same as in the sodium or zinc salt of an ethylene - unsaturated carboxylic acid - unsaturated carboxylate terpolymer used as main component (a) although the terpolymer as component (b) should have an M.F.R. of at least 4. Exemplary terpolymers are shown below.

Table 3

Flexural M.F.R.

Trade name Manufacturer modulus (g/10 (psi) min.) Mitsui - DuPont Nucrel AN 4307 Polychemical 3,100 15 K.K.

Mitsui - DuPont Nucrel AN 4212C Polychemical 4,100 12 K.K.

Mitsui - DuPont Nucrel N 0825J Polychemical 2,900 25 K.K.

(Note: Nucrel is a registered trade mark) Also included are mixtures of sodium and zinc salts of ethylene-unsaturated carboxylic acid copolymer, mixtures of ethylene unsaturated carboxylic acid - unsaturated carboxylate terpolymers, and mixtures of a sodium or zinc salt of ethylene-unsaturated carboxylic acid copolymer with an ethylene unsaturated carboxylic acid - unsaturated carboxylate terpolymer.

According to the present invention, components (a) and (b) are blended. The (total) amount of component (a) blended is 70 to 95% by weight of the resin components combined while the (total) amount of component (b) blended is 5 to 30% by weight of the resin components combined. If the amount of component (b) substantially exceeds 30% by weight, peeling resistance becomes low. If the amount of (b) is substantially less than 5%, the effect of improving ~ moldability is not fully achieved. The preferred amount of component (b) blended is 10 to 20% by weight of the resin components combined.

In the cover composition of the invention, dyes, pigments (e.g., titanium dioxide, zinc oxide and barium sulfate), UV absorbers, anti- oxidants, and dispersing aids (e.g., metal soaps) may be added to the resin component consisting of components (a) and (b), if desired. These components are mixed in a conventional mixing means, for example, a closed kneading machine (Banbury mixer and kneader) and extruder.

The cover composition of the invention is molded and cured into a golf ball cover e.g. by any conventional technique. The cover as cured should have a hardness of 55 to 80, preferably 65 to 75 on JIS C hardness scale and a tensile stress at 10% distortion of 40 to 140 kg/cm2, preferably 70 to 110 kg/cm2. If the cover's hardness is substantially over 80, hitting feel and spin property become poor. If it is substantially less than 55, the cover is too soft to provide the ball with sufficient repulsion so that the ball does not fly well. If the cover's tensile stress at 10% distortion is less than 40 kg/cm2, the cover is less repulsive and susceptible to damages by club hitting. If the tensile stress at 10% distortion is more than 140 kg/cm2, the cover is likely to be peeled by iron shots.

It is to be noted that the tensile stress at 10% distortion is defined in JIS K-7113 as the tensile stress at which a specimen having a length of 100 is longitudinally stretched at a pulling rate of 500 mm/min. to a length of 110, the tensile stress being divided by the transverse cross - sectional area of the specimen before the tensile test.

The cover composition may be used as covers for solid golf balls such as two and three-piece golf balls and thread- wound golf balls. The cover composition exerts its function to the maximum when used as solid golf ball covers.

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 - 4 For the manufacture of large size two piece golf balls, a core having a diameter of 38.5 mm was prepared by furnishing a rubber composition of the following formulation and effecting vulcanization at 155"C for 18 minutes.

Rubber composition (Core) Parts by weight Polybutadiene rubber 100 Barium sulfate 14.0 Zinc oxide 5.0 Anti - oxidant 0.2 Zinc acrylate 36.0 Peptizer 0.5 Dicumyl peroxide 1.0 Next, a cover composition was prepared by adding additives to the resin composition shown in Table 4 so as to give the following formulation. The cover composition was injection molded over the core to form a cover of 2.0 mm thick thereon.

Parts by weight Resin composition 100 Titanium oxide 5 Magnesium stearate 1 Bluing agent 0.03 The resulting two-piece golf balls having a diameter of 42.7 mm and a weight of 45.2 grams were examined by the following tests.

(1) Dimple edge When the cover composition was injection molded, it was examined how a dimple pattern on the mold was transferred to the molded ball.

The edge definition of dimples was visually observed.

(2) Sphericity The outer diameter (mm) of a ball was measured at 15 points. Reported is a variation range R (mm) of the measured diameter. Higher values indicate less spherical balls.

(3) Hardness Hardness is expressed by a distortion (mm) of a ball under a load of 100 kg. Higher values indicate softer balls.

(4) Initial speed An initial speed (m/s) was measured ~by an initial speed meter of the same type as prescribed by USGA.

(5) Spin With a &num;7 iron mounted on a swing robot, a ball was hit at a head speed of 36 m/s. The motion of the ball immediately after impact was analyzed using high-speed photographs, determining a back spin quantity (rpm).

(6) Scuff test A ball was kept at a temperature of 230C.

Three commercial pitching wedges were mounted on a robot machine. The ball was hit at three positions by the three pitching wedges, once at each position, at a head speed of 37 m/s. The three hit areas were visually observed.

Evaluation was made according to the following criterion. The result is an average of four balls tested.

Point Remarks 10: no change on the ball surface 9: intermediate points 10 and 8 8: very slight, substantially unperceivable club face dent 7: intermediate points 8 and 6 6: substantial club face dent, but no peeling from the cover surface 5: intermediate points 6 and 4 4: perceivably peeled surface 3: intermediate points 4 and 2 2: peeled surface with some cracks 1: outstanding cracks The results are shown in Table 4.

Table 4

Example Comparative Example 1 2 3 4 5 1 2 3 4 Resin composition (pbw) H1706 20 H1557 20 35 H1855 30 40 70 50 50 10 15 30 H1856 90 25 S8120 50 50 20 50 50 50 50 N0825J 10 10 10 25 40 Cover physical property N.F.R. 1.7 1.6 1.6 1.6 2.5 0.9 5.3 2.4 1.0 JIS C hardness 74 72 71 74 66 68 60 79 79 10% Tensile stress 100 80 77 81 65 62 52 120 124 Golf ball configuration somewhat Dimple edge sharp sharp sharp sharp sharp blunt sharp sharp blunt Sphericity 0.12 0.13 0.13 0.12 0.10 0.31 0.13 0.12 0.22 Golf ball performance Hardness 2.66 2.75 2.72 2.77 2.87 2.85 2.95 2.58 2.58 Initial speed 76.9 76.7 76.6 76.4 76.3 '76.5 75.9 77.0 77.0 Spin 8300 8600 8500 8200 8800 8600 9100 7700 7600 Scuff 8 8 8 7 6 8 4 5 4 The cover composition of the invention is easily moldable and offers golf balls having improved spin properties and resistant to impact by iron and other clubs.

Japanese Patent Application No. 6 - 37958 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.

Claims

CLAIMS 1. A composition for forming a golf ball cover comprising a resin consisting essentially of (a) 70 to 95% by weight of a first resin constituent selected from sodium and zinc salts of ethyleneunsaturated carboxylic acid-unsaturated carboxylate terpolymers; and (b) 30 to 5% by weight of a second resin constituent selected from sodium and zinc salts of ethyleneunsaturated carboxylic acid copolymers, and having a melt flow rate of at least 4, the composition being curable to a product having a hardness of from 55 to 80 on the JIS C hardness scale and a tensile stress at 10% distortion of from 40 to 140 kg/cm2.
2. A composition of claim 1 wherein unsaturated carboxylic acid of components (a) and (b) has from 3 to 8 carbon atoms and the unsaturated carboxylate is selected from esters of unsaturated carboxylic acids having from 3 to 8 carbon atoms with alcohols having from 1 to 9 carbon atoms.
3. A golf ball having a cover formed of a cured composition of claim 1 or 2.
4. A method comprising the preparation of a composition according to claim 1 or claim 2, comprising blending the ingredients thereof.
5. A method of making a golf ball, comprising forming the cover thereof from a composition according to claim 1 or claim 2.
6. A golf ball cover composition, composition preparation method, golf ball or method of making a golf ball substantially as any described herein with reference to the Examples (but not the Comparative Examples).