WO1998036801A1

WO1998036801A1 - Golf ball cover compositions

Info

Publication number: WO1998036801A1
Application number: PCT/US1998/003457
Authority: WO
Inventors: Murali Rajagopalan
Original assignee: Acushnet Co
Current assignee: Acushnet Co
Priority date: 1997-02-25
Filing date: 1998-02-23
Publication date: 1998-08-27
Anticipated expiration: 1999-08-25
Also published as: AU6334898A

Abstract

A golfball having a cover which includes 95-30 of a hard Mg ionomer and 5-70 of a very low modulus Mg ionomer. The hard Mg ionomer is typically a neutralized copolymer of ethylene and acrylic or methacrylic acid having a 10-30 % acid content. The very low modulus Mg ionomer is a neutralized terpolymer of ethylene, acrylic or methacrylic acid and n-butyl or isobutyl acrylate having an acid content of about 8-15 %.

Description

GOLF BALL COVER COMPOSITIONS

FIELD OF THE INVENTION

^■ This invention relates generally to golf balls and, in particular, to golf ball cover compositions comprising blends of hard magnesium ionomers and very low flexural modulus magnesium ionomers. The golf ball cover compositions of the invention provide golf balls having unexpectedly improved durability when compared to prior art hard-soft ionomer blends.

BACKGROUND OF THE INVENTION

A majority of golf balls sold today have covers formed either of balata or a blend comprising one or more ionomers. Balata covered, wound balls are typically preferred by skilled golfers because such balls have a high spin rate, allowing skilled players to draw or fade the ball, and to stop the ball quickly on the green with approach shots. However, balls with balata covers are expensive, and are easily cut or otherwise damaged when miss-hit, e.g., by a less than skilled player.

Therefore, most amateur golfers prefer two-piece golf balls having covers which comprise an ionomer or ionomer blend. Two-piece balls with ionomer covers are less expensive than wound, balata covered balls, and are highly resistant to cutting or other damage. However, these balls have a hard feel and a low spin rate when compared to balata covered balls, so that they are more difficult to control.

Although golf ball manufacturers have attempted to produce golf ball covers that provide the spin rate of balata with the cut resistance of an ionomer, i.e., a copolymer of an α-olefin and an ,β-unsaturated carboxylic acid, by forming blends of ionomers that differ in hardness, acid content, and in the metal atom used to neutralize the carboxylic acid groups on the polymer chain, none of these . attempts have been fully successful.

U.S. Patent No. 3,819,768 to Molitor discloses golf ball covers comprising blends of zinc and sodium ionomers, 5 which overcome cold cracking problems.

U.S. Patent No. 4,526,375 to Nakade discloses covers comprising a single ionomer in which part of the carboxylic acid groups has been neutralized with sodium and another part has been neutralized with magnesium or zinc to

10 overcome a fatigue fracture problem in covers comprising sodium/zinc ionomer blends, as well as a cold cracking problem in covers comprising a single ionomer.

U.S. Patent No. 5,000,459 to Issac discloses golf ball covers made from a lithium ionomer resin, wherein up to

15 50 percent of the lithium ionomer resin is replaced with at least one sodium ionomer resin. The resulting golf balls are found to be comparable in durability and initial velocity to golf balls having a cover with lithium ionomer only.

U.S. Patent No. 4,911,451 to Sullivan et al.

20 discloses a golf ball cover including about 10 to 90 percent by weight of an ethylene-acrylic acid copolymer, containing about 11 percent by weight acrylic acid with about 40 percent of the carboxylic acid groups neutralized with sodium, and about 90 to 10 percent by weight of at least one

25 ethylene-acrylic acid copolymer, containing about 11 to 16 percent acrylic acid with about 10 to 40 percent of the carboxylic acid groups neutralized with zinc. Although not exemplified or even further discussed, ionomers neutralized with metal ions other than zinc and sodium are also

30 disclosed. The exemplified golf balls are said to have the durability of balls having ethylene-methacrylic acid ionomer covers, but with an improved coefficient of restitution.

U.S. Patent No. 5,492,972 to Stefani discloses a golf ball cover composition of a blend comprising 95 to 50

35 parts by weight based on 100 parts by weight resin (phr) of a low flexural modulus sodium ionomer resin and about 5 to 50 phr of a lithium ionomer resin. In addition, a second sodium ionomer, having a flexural modulus and melt flow index that . are different from that of the first sodium ionomer, can be added to the blend. The spin rate of such a ball is improved over typical two-piece balls, and approaches the spin rate of a wound, balata covered ball. However, the initial velocity of the disclosed golf balls is less than that of two-piece balls having covers comprising a lithium ionomer.

U.S. Patent No. 4,884,814 to Sullivan discloses golf ball covers comprising blends of a high flexural modulus ("hard") ionomer and a low flexural modulus ("soft") ionomer. The hard ionomers are copolymers of an olefin and an unsaturated monocarboxylic acid, neutralized with sodium or zinc, and have a flexural modulus of about 30,000 to 55,000 p.s.i. and a Shore D hardness of about 60 to 66. The soft ionomers are terpolymers of an olefin, an unsaturated monocarboxylic acid, and an acrylate ester. These terpolymers are neutralized with zinc or sodium, and have a flexural modulus of about 3,000 to 7,000 p.s.i. and a Shore D hardness of about 25 to 40. The resulting cover is disclosed as being intermediate between balata and hard Surlyn in softness to such a degree that adequate spin can be imparted to the ball by a skilled golfer.

U.S. Patent No. 5,120,791 to Sullivan discloses golf ball cover compositions that provide enhanced carrying distance when compared to the golf balls of U.S. 4,884,814 without sacrificing playability or durability. The disclosed compositions are blends of at least one hard ionomeric resin, preferably an acrylic acid ionomer, and at least one acrylic acid based soft ionomer. Hard ionomers neutralized with sodium, zinc, magnesium, or lithium, having a flexural modulus of 15,000 to 70,000 p.s.i. and a Shore D hardness of greater than 50 are disclosed. However, only zinc and sodium ionomers are exemplified. The disclosed soft ionomers are sodium or zinc neutralized terpolymers of an olefin, acrylic acid, and an unsaturated acrylate ester monomer, and have a flexural modulus of about 2,000 to 10,000 p.s.i. and a Shore D hardness of about 20 to 40. Golf balls having such cover blends reportedly have an improved coefficient of restitution and spin rate when compared to prior art golf balls having covers with hard-soft ionomer blends.

U.S. Patent No. 5,415,937 to Cadorniga et al. discloses a golf ball cover material of a blend of 80-50% of a high stiffness ionomer and 20 to 50% of a very low modulus ionomer ("VLMI") to improve the feel and playability of the ball when compared to a standard ionomer cover, while retaining the distance and resilience of the prior art balls. High stiffness ionomers are defined as a copolymer of 75 to 80% of an olefin and 20 to 25% of an alpha, beta ethylenically unsaturated carboxylic acid, having 10 to 90% of the acid groups neutralized with sodium, lithium, zinc, or magnesium. The high stiffness ionomers preferably have a melt index of 0.5 to 5.0 g/10 min., a Shore D hardness of at least 70, and a flexural modulus of 60,000 to 120,000 psi. The VLMI are defined as terpolymers, consisting of 67 to 70 % by weight of ethylene, 20 to 23% by weight of n- or iso-butyl-acrylate, and 10% by weight of methacrylic acid, where 10 to 90% of the acid groups are neutralized by sodium, zinc, or lithium. The VLMI preferably have a melt index of 0.5 to 10 g/10 min, a Shore D hardness of 20 to 50, and a flexural modulus of 2,000 to 8,000. Golf balls having covers incorporating the disclosed blends have a slightly improved coefficient of restitution and initial velocity with spin rates that range from slightly better than prior art blends to significantly lower, depending upon the particular blend and the club used in the test, i.e., driver, 5-iron, or pitching wedge. U.S. Patent No. 5,324,783 to Sullivan discloses golf ball covers having an intermediate modulus with improved distance. The covers include a blend of a "relatively low amount", i.e., about 10 to 30 weight percent, of soft ionomer resins and about 90 to 70 weight percent of hard ionomer resins. The soft ionomers are terpolymers of an olefin, acrylic or methacrylic acid, and an unsaturated acrylate ester. Although U.S. 5,324,783 discloses hard and soft ionomers neutralized with sodium, zinc, magnesium, and lithium, only blends of sodium and zinc ionomers are exemplified and discussed. Golf balls incorporating the disclosed cover composition have an improved spin rate compared to balls incorporating only hard ionomers.

Therefore, there is no teaching in the prior art that golf ball cover compositions having improved durability, as well as the initial velocity and carry of two-piece balls having a hard ionomer cover and improved spin characteristics, can be obtained by blending a hard ionomer and a VLMI, where both ionomers are neutralized with the same type of metal atom.

SUMMARY OF THE INVENTION

The present invention relates to a golf ball comprising a cover and a core, wherein the cover comprises a blend of from about 95 to about 30 weight percent, preferably from about 80 to about 40 weight percent, most preferably from about 60 to about 40 weight percent, of at least one hard magnesium ionomer and from about 5 to about 70 weight percent, preferably from about 20 to about 60 weight percent, most preferably from about 40 to about 60 weight percent, of at least one magnesium VLMI. Typically, the hard magnesium ionomer is a copolymer of ethylene and acrylic or methacrylic acid, having an acid content of from about 10 to about 30% by weight, wherein from about 30 to about 70% of the acid groups are neutralized with magnesium atoms. Preferably the hard magnesium ionomer has an acid content of from about 15 to about 25% by weight, a flexural modulus of at least about 50,000 psi, and a Shore D hardness of at least about 60. The magnesium VLMI is typically a terpolymer of ethylene acrylic acid or methacrylic acid, and n-butylacrylate or iso-butylacrylate, having an acid content of from about 8 to about 15% by weight, a Shore D hardness of from about 20 to about 40, and a flexural modulus of from about 2,000 to about 20,000 psi.

The invention also relates to a method of making a golf ball, comprising forming a cover composition of a blend of a hard magnesium ionomer and a magnesium VLMI, as described above, and molding the cover composition around a golf ball core to form a golf ball.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms "Very Low Modulus Ionomer" and "VLMI" refer to terpolymers of an α-olefin, such as ethylene, a softening agent, such as n- or iso-butyl-acrylate, and an α,/3-unsaturated carboxylic acid, such as acrylic or methacrylic acid, where at least part of the acid groups are neutralized by a metal atom.

The present invention relates to golf ball cover compositions having unexpectedly improved durability. The golf ball cover compositions of the invention comprise blends of at least one hard magnesium ionomer and at least one magnesium Very Low Modulus Ionomer ("VLMI") . Golf balls having covers incorporating the magnesium ionomer blends of the invention have unexpectedly improved durability and initial velocity, as well as similar or improved playability, when compared to hard-soft ionomer blends of the prior art. It is believed that the unexpectedly improved durability and golf ball properties of the present invention result from improved compatibility of the magnesium ionomers when compared to blends of ionomers neutralized with two or more different metal ions.

The golf ball cover compositions of the invention comprise a blend of from about 95 to about 30 weight percent of at least one hard magnesium ionomer, preferably from about 90 to about 40 weight percent, more preferably from about 80 to about 40 weight percent, most preferably from about 60 to about 40 weight percent, and from about 5 to about 70 weight percent of at least one magnesium VLMI, preferably from about 10 to about 60 weight percent, more preferably from about 20 to about 60 weight percent, most preferably from about 40 to about 60 weight percent.

The hard, i.e., high flexural modulus, magnesium neutralized ionomers useful in the golf ball cover compositions of the invention have a Shore D hardness of at least about 60, as measured with ASTM method D-2240, a flexural modulus, as measured with ASTM method D-790, of at least 50,000 psi, preferably from about 50,000 to about 80,000 psi, and a melt index from about 0.5 to about 5.0 g/10 min.

The hard magnesium ionomers are copolymers of an olefin and an unsaturated carboxylic acid, wherein about 30 to about 70% of the acid groups are neutralized with magnesium. Preferably, the hard magnesium ionomers are copolymers of ethylene and either acrylic or methacrylic acid, most preferably methacrylic acid. The acid content of the hard magnesium ionomers useful in the invention is from about 10 to about 30% by weight, preferably from about 15 to about 25%.

Hard magnesium ionomers useful in the invention are available from the E.I. Du Pont de Nemours Company. In particular, SURLYN® AD8172 has provided good results. SURLYN® AD8172 is a copolymer of ethylene and methacrylic acid, wherein the acid content is about 15% by weight, and about 40 to 60% of the acid groups are neutralized with magnesium atoms.

The magnesium VLMI useful in the cover compositions of the invention are terpolymers of an olefin, such as ethylene, acrylic or methacrylic acid, and n- or iso-butyl acrylate, wherein the acid content ranges from about 8 to about 15% by weight, and from about 30 to about 60% of the acid groups are neutralized with magnesium. Commercially available magnesium VLMI include SURLYN® SEP688A, which has a Shore D hardness of about 45, a flexural modulus of about 4,600 psi. Additional materials conventionally included in golf ball cover compositions may be added to the golf ball cover compositions of the invention. These additional materials include, but are not limited to, dyes, whitening agents, UV absorbers, optical brighteners, processing aids, and other conventional additives. Antioxidants, stabilizers, softening agents, plasticizers, processing aids, and reinforcing materials can also be added.

Golf balls and golf ball covers comprising the cover compositions of the invention can be made in any conventional manner, including injection molding cover stock formed from the magnesium ionomer blends of the invention around a core or by compression molding pre-formed half-shells of the cover stock into a half-shell mold in a conventional manner.

After molding, golf balls comprising the magnesium ionomer cover compositions of the invention can be finished by buffing, painting, and stamping, good results have been obtained by placing a mixture of the ionomer resins in a solid, pelletized form in a hopper used to feed the heated barrel of a conventional injection molding machine, which is used to make preformed half-shells for compression molding or directly forming the cover around a core in an injection mold.

EXAMPLES The following non-limiting examples are intended to illustrate the preferred embodiments of the present invention, and are not to be construed as limiting the invention, the scope of which is defined by the amended claims.

Golf balls having covers according to the invention were prepared, and compared to golf balls having prior art covers. The blends used in the covers of the tested balls are given in Table 1. All amounts are given in parts per hundred ("PHR"). TABLE 1

The improved durability of golf balls having covers incorporating the magnesium ionomer blends of the invention is clearly demonstrated by the results given in Table 2. Golf balls having covers according to the invention have unexpectedly superior durability, suffering no failures in hit tests, where prior art balls suffered failures in as few as 350 hits.

TABLE 2

The golf balls of examples 1 to 4 incorporated 87 compression cores having a diameter of 1.58 inch (4.01 cm).

The results of additional tests, provided in Table 3 , demonstrate that magnesium ionomer blends of the invention provide a combination of improved durability, initial velocity, and spin over a wide range of hard magnesium ionomer and magnesium VLMI concentrations that is not available with prior art golf ball cover compositions. In particular, golf balls having covers according to the present invention spin at a rate that is comparable to that of wound, balata covered balls, and have the durability and initial velocity of prior art two piece balls, having hard ionomer covers.

TABLE 3

ω c

CO in

H C H m en x m m

H

"3 c ι- m t σ>

1. Golf balls 5 to 14 incorporate covers injection molded around 87 compression cores having a diameter of 1.58 in. (4.01 cm).

2. Golf ball 15 is a commercially available two-piece ball having a compression molded, Surlyn 8320 and Li ionomer cover.

3. Mg ionomer.

4. Mg VLMI.

5. Li VLMI.

6. Na VLMI.

While it is apparent that the invention disclosed • herein is well calculated to fulfill the objects stated above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art. Therefore, it is intended that the appended claims cover all such modifications and embodiments as falling within the true spirit and scope of the present invention.

Claims

THE CLAIMSI claim:

1. A golf ball comprising a cover and a core, wherein the cover comprises a blend of from about 95 to about 30 weight percent of at least one hard magnesium ionomer and from about 5 to about 70 weight percent at least one magnesium VLMI.

2. The golf ball according to 1, wherein the cover comprises a blend of from about 80 to about 40 weight percent of at least one hard magnesium ionomer and from about 20 to about 60 weight percent of at least one magnesium VLMI.

3. The golf ball according to 1, wherein the cover comprises a blend of from about 60 to about 40 weight percent of at least one hard magnesium ionomer and from about 40 to about 60 weight percent of at least one magnesium VLMI.

4. The golf ball according to 1, wherein the hard magnesium ionomer has a flexural modulus of at least about 50,000 psi and a Shore D hardness of at least about 60.

5. The golf ball according to claim 1, wherein the hard magnesium ionomer is a copolymer of ethylene and acrylic or methacrylic acid, having an acid content of from about 10 to about 30% by weight, wherein from about 30 to about 70% of the acid groups are neutralized with magnesium atoms.

6. The golf ball according to claim 5, wherein the acid content of the hard magnesium ionomer is from about 15 to about 25% by weight.

7. The golf ball according to claim 1, wherein ΓÇó the magnesium VLMI is a terpolymer of ethylene acrylic acid or methacrylic acid, and n-butylacrylate or iso-butylacrylate, having an acid content of from about 8 to about 15% by weight.

8. A method of making a golf ball, comprising: forming a blend of from about 95 to about 30 weight percent of at least one hard magnesium ionomer and from about 5 to about 70 weight percent at least one magnesium VLMI to form a cover composition; and molding the cover composition around a golf ball core to form a golf ball.

9. The method of claim 8, comprising forming a blend of from about 80 to about 40 weight percent of at least one hard magnesium ionomer and from about 20 to about 60 weight percent at least one magnesium VLMI.

10. The method of claim 8, comprising forming a blend of from about 60 to about 40 weight percent of at least one hard magnesium ionomer and from about 40 to about 60 weight percent at least one magnesium VLMI.

11. The method of claim 8, further comprising selecting a hard magnesium ionomer having a flexural modulus of at least about 50,000 psi and a Shore D hardness of at least about 60.

12. The method of claim 8, further comprising selecting a hard magnesium ionomer of a copolymer of ethylene and acrylic or methacrylic acid, having an acid content of from about 10 to about 30% by weight, wherein from about 30 to about 70% of the acid groups are neutralized with magnesium atoms.

13. The method of claim 12 , further comprising selecting a hard magnesium ionomer having an acid content of from about 15 to 25% by weight.

14. The method of claim 8, further comprising selecting a magnesium VLMI of a terpolymer of ethylene acrylic acid or methacrylic acid, and n-butylacrylate or iso-butylacrylate, having an acid content of from about 8 to about 15% by weight.