GB2168059A - Golf balls and covers - Google Patents

Abstract

A composition suitable for molding a cover for a golf ball, said cover having a Shore C hardness less than substantially 85 (but optionally in the range 70 to 85) comprises: (1) a thermoplastic urethane having a Shore A hardness less than substantially 95; and (2) an ionomer comprising olefinic groups having 2 to 4 carbon atoms copolymerized with a monocarboxylic acid having 3 or 4 carbon atoms, said acid groups being at least partly neutralized with metal ion(s), said ionomer having a Shore D hardness at least substantially 55. The cover improves characteristics of a golf ball, especially of golf balls having molded spherical cores, e.g. spin rate, short iron and wood playability, puttability, and durability.

Description

SPECIFICATION Golf balls and covers This invention relates to a composition for preparing a golf ball cover; a golf ball cover corresponding to the ingredients of a said composition; and a golf ball comprising a said cover and a core. The golf balls have superior short iron and other playability characteristics.

Balata and blends of balata with elastomeric or plastics materials were for many years the materials of choice in the manufacture of covers for top grade golf balls. These materials have good molding properties and accordingly could be readily compression molded about a spherical wound core to produce a high quality golf ball. An experienced player can apply spin to a balata covered wound ball such that it will fade or draw in flight or have the backspin necessary to stop abruptly on the green.

These playability properties are most important in short iron play and can be exploited significantly only by relatively skilled players.

Balata and its synthetic substitutes, trans polybutadiene and trans polyisoprene, have today essentially been replaced by new materials. With the exception of a few lines of golf balls distributed through pro shops to professional golfers and those who would emulate them, newer synthetic polymers are the cover materials of choice. Of the new synthetics, by far the most commonly used are a line of ionomers sold by E.I. Dupont de Nemours & BR< Company under the trademark SURLYN. These materials comprise copolymers of olefins, typically ethylene, with an alpha, beta ethylenically unsatu rated carboxylic acid such as methacrylic acid. Metal ions such as sodium or zinc ions are used to neutralize some portion of the acidic groups in the copolymer, resulting in a thermoplastic elastomer which has several advantages including a cost advantage over balata.The ionomers may be manufactured with a wide variety of properties by altering the identity of the comonomers, the fraction of the polymer comprising the carboxylic acid, the molecu lar weight of the polymer, the degree of neutralization of the polymer, and the identity of the metal ions used. Control of these parameters results in ionomer resins of different melt index, hardness, resilience and other mechanical properties which, in a golf ball cover, affect cut resistance, shear resistance, general durability, and resilience.

U.S. Patent 3,819,768 issued to R.P. Molitor dis closes that blends of sodium neutralized ionomer resins with zinc neutralized ionomer resins, as a class, have certain advantages which have not been achievable in any other way. Among these is the production of an unexpectedly high coefficient of restitution of golf balls having the blended ionomer cover. Such covers also resist cold cracking, have excellent aging properties, and are unexpectedly durable. The development of the SURLYN blended cover has been a major factor in the production of two-piece balls having covers which for all practical purposes cannot be cut in play, and which travel further when hit than any other USGA regulation ball as measured by controlled tests when hit by golfers or testing machines.

While the balata-covered, thread-wound balls are easily cut and very expensive, they nevertheless have excellent short iron playability. it is much more difficult to impart spin to an ionomer covered two-piece ball. Frequently, experienced players note that the ionomer covered two-piece balls have an unsatisfactory "feel".

The patent literature is replete with proposed cover formulations seeking to improve upon the balata and ionomer covers which have been commercially successful. Polyurethanes, thermoplastic rubbers, various block copolymers, polyesters, and polyamides, as well as various blends including such materials, have been proposed. Examples of patent specifications are: U.S. 3,359,231; U.S. 4,398,000; U.S. 4,234,184; U.S. 4,295,652; U.S. 4,248,432; U.S.

3,989,568; U.S. 3,310,102; U.S. 4,337,947; U.S.

4,123,061; and U.S. 3,490,146.

The manufacture of two-piece balls (i.e. balls comprising a solid, molded, resilient core and a cover) has many significant advantages over the more expensive wound balls. There is accordingly a need for two-piece balls having short iron playability characteristics comparable to wound, balatacovered balls.

A first aspect of the present invention provides a composition suitable for molding a cover for a golf ball, said cover having a Shore C hardness less than substantially 85 (but optionally in the range 70 to 85, e.g. 75 to 80 or substantially 78), said composition comprising: a thermoplastic urethane having a Shore A hard- ness less than substantially 95; and an ionomer comprising olefinic groups having 2 to 4 carbon atoms copolymerized with a monocarboxylic acid having 3 or 4 carbon atoms, said acid groups being at least partly neutralized with metal ion(s), said ionomer having a Shore D hardness at least substantially 55 (e.g. greater than substantially 55).

A second aspect of the present invention provides a cover for a golf ball, wherein said cover comprises a cover according to the first aspect of the present invention, e.g. having a Shore C hardness of 70 to 85 (e.g. 75 to 80 or substantially 78). Preferably, the cover has a thickness no greater than substantially 0.06 in (0.15 cm). Thinner covers appearto maximize the short iron playability characteristics of said golf ball.

Athird aspect of the present invention provides a golf ball, comprising a cover according to the second aspect of the invention; and a core. Preferably, the golf ball is adapted to have a spin rate greater than substantially 9000 rpm, when hit with a nine iron.

A cover composition of the present invention may be used in manufacture of thread-wound balls, but is preferably used in manufacture of balls having molded cores. Two-piece balls made with a cover of the present invention have short iron playability properties as good as or better than balata-covered wound balls but are significantly more durable. They also have better wood playability properties than conventional two-piece balls, and permit experienced golfers to apply spin so as to fade or draw a shot. Puttability is also improved.

Accordingly, the present invention can provide a cover composition which can be injection molded or compression molded about a solid molded core to produce golf ball covers which are more durable and cut resistent than balata rubber-covered balls. Furthermore, such a cover composition can provide a two-piece ball having short iron and other playability characteristics equal to or exceeding thread-wound balata-covered balls. The cover compositions can be compatible with urethane-based paints that resist chipping and peeling.

Tests have shown that the spin rate off a nine iron of two-piece balls comprising a cover material of the present invention is about 9,550 rpm vs. about 5,400 rpm for state of the art, two-piece balls having ionomer covers. This compares favourably to balatacovered wound balls which under the same hitting conditions have a spin rate of about 9,700 rpm. The phrase "two-piece ball" as used herein refers primarilyto balls consisting of a molded core and a cover, but also includes e.g. balls having a separate solid layer beneath the cover as disclosed e.g. in U.S.

Patent No. 4,431,193 issued to Nesbitt, and other balls having non-wound cores.

The increased spin that can be applied during a proper hitto balls having a cover of the present invention is believed to be due primarily to the softness of the blended cover material. On impact, the cover deforms significantly more than balls having ionomer covers, and tests have shown that there is a greater area of contact between the ball and the club face. This permits the better player to impart fade, draw, or backspin to the ball as the situation dictates, and permits the player to execute better chips and other shots to the green. On shots of short range (e.g. less than 50 yards (45.7 metres)), the two-piece balls of the present invention often outperform balata-covered wound balls.Regarding puttability, two-piece balls including a cover of the present invention have a lower drop rebound relative to conventional two-piece balls, and have increased contact time on a putter face. These characteristics combine to enable the player better to control the putting line and distance.

As some examples of use of cover compositions of the present invention, the cover compositions may be used to formulate golf ball covers by injection or compression molding about wound cores, solid molded cores of known composition, or high coefficient molded cores such as those disclosed in our U.K. patent application No.8530055 (P.2303). Various combinations of the solid core technologies may also be used.

The thermoplastic urethanes preferably comprise (e.g. consist essentially of) linear primary polymer chains having a preponderence of relatively long, flexible chain segments joined end to end by rigid chain segments through covalent chemical bonds.

The flexible segments may be diisocyanate coupled, low melting polyester or polyether chains. The rigid segments may comprise single diurethane bridges resulting when a diisocyanate couples to polyester or polyether molecules. The rigid segments may comprise longer high melting urethane chain segments formed by the reaction of a diisocyanate with a low molecular weight glycol chain extendercom- ponent. The polar nature of recurring rigid, urethane chain segments results in their strong mutual attraction, aggregation, and ordering into crystalline and polycrystalline domains in a mobile polymer matrix.

Urethane hydrogen atoms and carbonyl and ether oxygen partners are believed to permit extensive hydrogen bonding among the polymer chains. This apparently restricts the mobility of the urethane chain segments in the domains. These attractions and chain entanglements produce thermo-labile psuedo cross-links, and produce resins which can display the superficial properties of strong rubbery vulcanizates. Preferred thermoplastic urethanes have a tensile strength of at least 2500 b/in2 (446.45 Kg/cm2) and at break an elongation at least substantially 250 per cent. Most preferably, tensile strength and elongation at break are 4000 Ib/in2 (0.71 Kg/cm2) and 400 per cent, respectively. Such materials are available commercially from Mobay Chemical Company under the trademark TEXIN, from B.F.Goodrich underthetrademark ESTANE,from Upjohn Company under the trademark PELLETHANES, or from K. J. Quinn Company under the trademark Q THANES. Mobay Chemical Company's TEXIN 480AR is an especially preferred thermoplastic urethane.

TEXIN 591A may also be used.

The preferred thermoplastic polyurethane consists essentially of linear primary polymer chains. The structure of such chains preferably comprises long, flexible chain segments joined end to end by rigid chain segments through covalent chemical bonds.

The flexible segments typically are diisocyanatecoupled, low melting polyester or polyether chains.

The rigid segments include single diurethane bridges and high melting urethane chain segments formed by the reaction of diisocyanate with small glycols. Hydrogen bonding and other attractive forces among the rigid chain segments are believed to produce domains which are mutually attractive, behave somewhat like cross-links, and are reversible with heat and with solvation.

Monocarboxylic acid in the ionomers is preferably acrylic or methacrylic acid, at least partially neutralized (or cross-linked) with metal ion(s). The metal ions are preferably sodium and/or zinc ions. Examples of the ionomers are those sold under the trademark SURLYN by E.l. Dupont de Nemours Co.

They come in a wide variety of grades which differ in average molecular weight, percent carboxylic acid monomer, degree of neutralization, identity of neutralizing ion, and other factors. Any of the SURLYN ionomers or other ionomers of the type described having a shore D hardness of at least 55 may be used. An especially preferred material is SURLYN 1702. SURLYN 1702 is a copolymer of ethylene and methacrylic acid, and is believed to comprise about 96.5% by weight free acid and 1.2% by weight zinc.

The copolymer's average melt index is about 14.

The preferred ionomer comprises olefinic groups having two or four carbon atoms copolymerized with acrylic or methacrylic acid groups, which are at least partially neutralized with a sodium or zinc ion, e.g. a SURLYN ionomer. Inclusion of the ionomer imparts to the cover a degree of cut resistence far in excess of balata, and helps to control the melt index of the blend to facilitate molding. Thus, the presence of the ionomer in the blend can serve to set the melt flow index to optimal levels for injection or compression molding, and imparts cut resistence and general durability to covers made with the blend.

The urethane and ionomer resin components thus coact in use to provide a unique combination of durability, playability, and ease of manufacture. The nature of the cover material is such that better players can fade, draw, or apply backspin to the ball even though it is of two-piece construction, yet the cover is more cut resistant and has generally better durability than balata-covered wound balls. It is believed that the primary reason why more spin can be given to the ball during a hit is that the cover material deforms more easily than do ionomer covers. Thus, irons, particularly short irons, tend to "bite" into the ball without permanently deforming it.

The nature of the molded cores which may be used with the cover form no part of this invention, although molded cores are preferred compound with wound cores because of lower cost and superior performance. Thus the covers of the invention may be used with cores of the types described in U.S. patent numbers 4,264,075; 4,169,599; or 4,141,559.

Conventional solid cores are typically compression molded from a slug of uncured or lightly cured elastomer composition comprising a high cis content polybutadiene and a metal salt of an alpha, beta, ethylenically unsaturated carboxylic acid such as zinc mono or di acrylate or methacrylate. To achieve high coefficient of restitution in the core, manufacturers include a small amount of a metal oxide such as zinc oxide. Larger amounts of metal oxide than are needed for that coefficient may be included to increase core weight so that the finished ball more closely approaches the USGA upper weight limit of 1.62 ounce (45.93 gram). Other materials may be used in the core composition, e.g. compatible rubbers or ionomers, and low molecular weight fatty acids such as stearic acid.Free radical initiator catalysts such as peroxides are admixed with the core composition so that, on the application of heat and pressure, a complex curing or cross-linking reaction takes place.

Copending U.K. patent application No. 8530055 (P.2303) (the disclosure of which is incorporated herein by reference) discloses a golf ball core that is cured with a peroxide, and comprising a polyfunctional isocyanate having 3 to 30 carbon atoms. Balls having cores made using a small amount of such as an isocyanate (preferably a diisocyanate, e.g., 1.0% by weight of 4,4' diphenyl methane diisocyanate) have an improved, higher coefficient of restitution.

Coefficient of restitution is defined as the ratio of outgoing to incoming velocity of the test core or ball projected against a hard plate at a selected initial velocity, e.g. 125 ft./sec. (38.1 metres/sec). The composition may otherwise be identical to prior art polybutadiene-metal acrylate core compositions.

These cores may be used with the cover composition of the invention to produce a ball of high coefficient and high initial velocity, closely approaching the USGA limit, and which has excellent short iron playability properties.

Copending U.S. patent application Serial No.

(attorney docket No. SPH-007) (filed on the same day as the U.S. application corresponding to the present application) discloses a core consisting of a hard, inner central portion and a softer outer layer. The core may be manufactured by formulating a conventional core composition of the type described above, extruding slugs of the proper size, applying a material which alters cross-linking to surface layers of the slug (e.g. sulfur or sulfur bearing materials), and then compression molding. Inner portions of the core cure in the normal way to produce a hard, resilient centre. An outer layer of the core (at least about 0.2 in (0.51 cm) thick) is cured differently because of the presence of e.g. sulfur or sulfur bearing material to produce a soft outer region, e.g.

having a Shore A hardness of about 35. That type of core may also be used with the cover of this invention to produce balls of excellent short iron playability characteristics.

The invention will be further illustrated from the following non-limiting Examples.

Examples A cover composition according to the present invention was made by blending 80 parts by weight TEXIN 480AR thermoplastic urethane (Mobay); 20 parts by weight SURLYN 1702 (Dupont); and 0.4 parts by weight TiO2. After thorough blending, the composition had a melt flow index of 0.89. The composition was injection molded about conventional cores of the type described above at approximately 450"F (232.2"C) to produce covers 0.06 in (0.15 cm) thick. The balls were painted and their properties were tested versus a golf ball of similar construction, except that the cover material was a blend of a sodium ionomer with a zinc ionomer, and versus a wound ball having a balata cover.

Off a nine iron, balls having the above cover of the present invention had a spin rate of 9550 rpm, whereas ionomer covered balls had a spin rate of 5400 rpm, and balata covered balls a spin rate of 9700 rpm. When "bladed" with a three iron, the balls of the present invention are very slightly bruised, the ionomer covered balls are unmarked, and the balata covered ball is cut through to the threads. When tested by professional golfers, the balls of the invention were perceived as being equal or superior to balata covered balls for short iron playability.

The present invention includes equivalents and modifications within the scope ofthe above disclosures and/or of the appended claims. A preferred core is a molded, resilient, solid sphere.

Claims (20)

1. A composition suitable for molding a cover for a golf ball, said cover having a Shore C hardness less than substantially 85 (but optionally in the range 70 to 85), said composition comprising: a thermoplastic urethane having a Shore A hard ness less than substantially 95; and an ionomer comprising olefinic groups having 2 to 4 carbon atoms copolymerized with a monocarboxylic acid having 3 or 4 carbon atoms, said acid groups being at least partly neutralized with metal ion(s), said ionomer having a Shore D hardness at least substantially 55.

2. A composition as claimed in claim 1, wherein the relative weight ratios of said urethane and said ionomer are predetermined for enabling a said Shore D hardness.

3. A composition as claimed in claim 1 or 2, wherein there are substantially 8 parts by weight of said urethane to 1 to 4 parts by weight of said onomen

4. A composition as claimed in claim 3, wherein there are substantially 2 parts by weight of said ionomer.

5. A composition as claimed in any one of claims 1 to 4, wherein said urethane has a tensile strength of at least substantially 2500 Ib in2 (446.45 Kg;cm2), and at break an elongation greater than substantially 250 per cent.

6. A composition as claimed in any one of claims 1 to 5, wherein said monocarboxylic acid is acrylic acid or methacrylic acid.

7. A composition as claimed in any one of claims 1 to 6, wherein said metal ion is a sodium or zinc ion.

8. A composition as claimed in any one of claims 1 to 7, comprising titanium dioxide.

9. A composition as claimed in claim 1, substantially as described in the Examples.

10. A cover for a golf ball, wherein said cover comprises a cover corresponding to the ingredients of a composition as claimed in any one of claims 1 to 9, said cover having a Shore C hardness less than substantially 85 (but optionally in the range 70 to 85).

11. A cover as claimed in claim 10, wherein said cover has a Shore C hardness of 75 to 80.

12. A cover as claimed in claim 10 or 11, wherein said cover has a Shore C hardness of substantially 78.

13. A cover as claimed in any one of claims 10 to 12, wherein said cover has a thickness no greater than substantially 0.06 in (0.15 cm).

14. A cover as claimed in claim 10, substantially as described in the Examples.

15. A golf ball, comprising a cover as claimed in any one of claims 10 to 14; and a core.

16. A golf ball as claimed in claim 15, when adapted to have a spin rate greater than substantially 9000 rpm, when hit with a nine iron.

17. A golf ball as claimed in claim 15 or 16, wherein said core comprises a molded spherical core.

18. A golf ball as claimed in claim 15 or 16, wherein said core comprises a wound core.

19. A golf ball as claimed in any one of claims 15 to 18, comprising a coat of polyurethane paint.

20. A golf ball as claimed in claim 15, substantially as described in the Examples.