WO1996006133A1

WO1996006133A1 - Golf balls with co-neutralized ethylene copolymer ionomer covers

Info

Publication number: WO1996006133A1
Application number: PCT/US1995/008954
Authority: WO
Inventors: Will Devoy Rogers; Craig Howard Evans
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1994-08-18
Filing date: 1995-08-01
Publication date: 1996-02-29
Also published as: AU3130495A

Abstract

Golf balls comprising a core and cover are made having a cover which consists of a co-neutralized ethylene copolymer ionomer. The co-neutralized ionomer is an ionomer which has been neutralized with two or more metal ion sources during its preparation, and may also be prepared using two or more underlying acid copolymers. Use of co-neutralized ionomer provides greater security in composition uniformity, and also allows for greater versatility in chemical composition, in that metal mixtures and underlying acid copolymers can be independently varied.

Description

133 PCMJS95/08954

TITLE

GOLF BALLS WITH CO-NEUTRALIZED

ETHYLENE COPOLYMER IONOMER COVERS

BACKGROUND OF THE TNVENTTON Field of the Invention

This invention relates to golf balls which have ethylene copolymer ionomer covers made from ionomers having a special form. More particularly, the covers are made of ionomers prepared from ethylene/acid copolymer resins which have been neutralized with more than one metal ion. The use of such mixed metal ionomers avoids the need to blend standard single metal ionomers, and provides increased composition versatility. Blending errors and blending inconsistencies are also avoided. Description of Related Art Ethylene copolymer ionomers are well known in the art. They were first described in U.S. Patent No. 3,264,272 (Rees). ionomers of this sort are based on copolymers of ethylene and a carboxylic acid such as acrylic or methacrylic acid, optionally with other monomers such as alkyl acrylates, these acid copolymers having been neutralized with a single metal ion such as sodium or zinc. The possibility of neutralizing with more than one metal ion was however disclosed in the above patent, where it is stated that 'It is not essential that only one metal ion be employed... and more than one metal ion may be preferred in certain applications.' In practice, no mixed metal ionomers made by neutralizing acid copolymers with two (or more) metal ion sources have ever been commercialized, and no particular utility or specific application for such a composition has ever been identified.

Golf balls have employed covers made of ionomers for may years. The mechanical properties of the ionomer are a critical factor in the behavior and play characteristics of golf balls. As a result, rigorous investigations into the differences between different ionomers, and how these ionomers affect play characteristics of balls which utilize covers of them, have been carried out over the years. One of the key findings, as 133 PCMJS95/08954

disclosed in several U.S. Patents, is that blends of different ionomers may be better than single ionomers.

U.S. Patent No. 3,819,768 (Molitor) discloses that a blend of sodium ionomer and a zinc ionomer produces a particularly attractive golf ball cover. Such blends have been used for about the twenty years since the issuance of the patent. Very recently, U.S. Patent No. 5,298,571 (Statz et al.) described a sophisticated investigation into mixtures of metal ionomers, where blends of up to four different metal ionomers were studied, and optimum compositions for certain types of golf ball covers found.

Other patents such as U.S. Patent No. 4,884,814 (Sullivan) and 5,120,791 (also Sullivan) disclose specific blends of ionomers where, while different metal ionomers could be employed in the blends, the key disclosure related to the underlying acid copolymer nature. Thus, the first of these patents discloses blends of hard and soft ionomers, where hard ionomers are ionomers of any metal where the acid copolymer on which it is based is a bipolymer of ethylene and acrylic or methacrylic acid, and soft ionomers are ionomers of any metal where the acid copolymer is a terpolymer containing, in addition, a 'softening' monomer such as an alkyl acrylate. The second patent discloses blends where the specific acid, acrylic acid, must be the acid in one of the ethylene copolymer ionomers. In both types of blends, blends of different metal ionomers and blends of ionomers which are based on different underlying acid copolymers, two methods have been employed in preparing the blends for making golf ball covers. The first methods is to extrusion blend two ionomers together. Pellets of this extrusion blend may then be injection molded as a cover on a pre-formed core. The second method is simply to make a pellet blend of the two ionomers. The mixed pellets may then be directly injection molded as a cover on the core, hoping the injection molding process provides sufficient mixing of pellets to produce a uniform and intimate blend. Clearly, the second is more economical, since no separate extrusion step is necessary.

Notably absent in any of these disclosures is any mention of the possibility of ionomers which are neutralized with more than one metal. Nor does there appear to have been any disclosure of a reason for making or utilizing such a mixed metal ionomer, despite the long period during which ionomer blends have been used for golf ball covers.

SUMMARY OF THE INVENTION

The invention is directed to the discovery that the advantageous properties of blends of different metal ionomers, when utilized as golf ball covers, can also be obtained without blending different metal ionomers. This is accomplished by use of mixed metal ionomers. Use of such a single mixed metal ionomer provides a number of distinct advantages over use of blends of ionomers. Specifically, the invention is to a golf ball comprising a core and a cover, the cover comprising an ionomer neutralized with more than one metal.

The mixed metal ionomer may also be based on more than one underlying acid copolymer. DETAILED DESCRTPTTON OF THE INVENTION

In this disclosure the term acid copolymer is synonymous with ethylene/acid copolymer, and is the 'underlying' or 'precursor' copolymer of the ionomers.

Also in this disclosure, the word 'co-neutralized' is used to describe an ionomer where more than one metal ion source is used in the process of neutralizing the underlying acid copolymer or copolymers. The phrase 'direct copolymer' is used to describe polymers prepared 'directly' from mixed monomers, resulting in polymers where the comonomer derived units are close to randomly distributed along the polymer chains, as distinct from graft copolymers where pendent units contain units derived from comonomer which has been grafted and polymrized onto an existing polymer chain.

Different metal ionomers, based on direct ethylene/acid copolymers, are essentially miscible with one another, unless the underlying acid copolymer on which each metal ionomer is based is radically different. To date, all commercially available ethylene copolymer based metal ionomers can be considered miscible. Nevertheless, to obtain uniform properties in a blend it is necessary to provide sufficient mixing. Melt extruders with adequate mixing screws, and even molding machines with good mixing before injection can be used to prepare such blends. Clearly molding machines with adequate blending are economically preferable, since when an object such as a golf ball cover is to be molded, it can be molded at the same time pellets of the two ionomers are blended, in a single machine. With golf ball covers, perhaps more than with other molded objects, good mixing is particularly critical. One could not, for instance, accept a golf ball with one surface side being predominantly a relatively high resilient material, and the other a much lower resilient material.

In this disclosure, ionomers blends which are mixed without pre-extrusion will be referred to as 'pellet blends'. Ionomer blends which are first blended in an extruder will be referred to as 'extrusion blends'.

Pellet blends of ionomers have been found to be acceptable, in general, for use in preparing golf ball covers. However, they can have certain disadvantages. Any operator error in weighing the ionomer components will result in the wrong blend composition on the golf ball covers. Extrusion blends of ionomers and the co-neutralized ionomers of this invention can provide a check on the composition before the molding process, thus providing a greater measure of control at an earlier juncture. Another disadvantage is that in mixing pellets of different ionomers, if the pellets are of different size and shape, (a situation which is likely if the ionomers are from quite different sources), a degree of 'sedimentation' can occur, making it difficult to obtain uniform blends. The factors become more significant when the blend has three or more components.

The co-neutralized ionomers used in the present invention, however, have another advantage, both over pellet blends and over extrusion blends. New, optimum property blend compositions may be more compositionally complex than earlier compositions. If one is limited to blending commercially available ionomers, it will not always be possible to obtain an optimum blend. Suppose, for instance, a 50% sodium neutralized acrylic acid ionomer and a 50% zinc neutralized methacrylic acid ionomer are available. Any blend of the two, will be proportioned in the level of underlying acid copolymer at the same level as the proportion of metal. Thus a 75/25 sodium ionomer/zinc ionomer blend will have a 75/25 ratio of sodium to zinc but will also contain 75% underlying acrylic acid copolymer and 25% underlying methacrylic acid 133 PCMJS95/08954

copolymer. In other words, the metal proportion is locked in with the underlying acid copolymer proportion. This is not the case with co- neutralized ionomers, since in their preparation, described below, the proportions of underlying acid copolymers may be varied independently of the proportions of metal ions, one may have a 50/50 blend of ethylene/acrylic acid and ethylene/methacrylic acid polymer chains, yet a 90/10 sodium/zinc ion blend, the versatility provided by this independence, when three or more metals and three or more underlying acid copolymers are to be considered for the composition, becomes even more significant.

On the debit side, a point of control of the composition no longer exists at the golf ball producer level. Thus, with pellet blends, a producer could readily decide that a little more lithium ionomer, for instance, would more readily produce a ball with a desired play characteristic. It would be a simple matter to increase the level of lithium ionomer in the pellet blend, rather than going back to the producer to provide the changed composition. Of course, one could always pellet blen. a small amount of single metal lithium ionomer with a co- neuiralized ionomer, but this defeats the advantage of co-neutralization. Wiih extrusion blends, the point of control of the composition, of course, depends on whether the golf ball producer of the resin producer prepares the extrusion blend.

In summary, there are hitherto unrecognized advantages which make co-neutralized ionomers a valuable new resource. Co-neutralized ionomers, as noted, have long ago been disclosed as being possible. However, no such materials have ever been made available, presumably because no advantage to doing so had been recognized.

Co-neutralized ionomers may be prepared in an extruder in a similar way to single metal ionomers. Other melt mixing devices are also suitable. Preparation of ionomers using extruders and other mixing devices is described in U.S. Patent No. 3,264,272 (Rees), which is hereby incorporated by reference. The reference describes typical metal ion sources. Typically, the underlying acid copolymer, or mix of acid copolymers, is fed into an extruder provided with sufficient mixing ability. A mixture of the neutralizing metal ion sources is fed into the extruder feed zone or a port along the barrel, in the same way as for preparing a single metal ionomer. These sources may be, for instance, metal hydroxides or acetates, possibly dissolved in water, or oxides such as zinc oxide, either as a polymer concentrate or together with some acetic acid. The output of the reaction/extrusion process will be co-neutralized ionomer. The extrudate will be pelletized for molding. There is no limit to the number of metal ion sources which can be used in producing a co- neutralized ionomer. The requirements for the ion source will be similar to that for single metal ionomers, namely that they be sufficiently reactive to neutralize the acid groups in the acid copolymer or copolymer mixture. It is possible, however, that a particularly reactive metal ion source may activate a less active different metal source, though no investigations in this regard have been made. Co-neutralization is particularly advantageous when three metals are to be present. Three metal ionomer compositions may be increasingly important. An example is U.S. Patent No. 5,298,571 which describes optimum composition containing sodium, lithium and zinc ionomers. The potential for error and poor mixing increases as one increases the number of ionomers in the blend. More than one acid copolymer also may be used in preparation of a co-neutralized ionomer. Ethylene/acid copolymers used to prepare ionomers may differ in the acid. The acid may be acrylic acid or methacrylic acid. OR they may differ in the level of acid, which may be from about 7 to 25 weight percent in the copolymer. Or they may have an additional monomer acting as a softening monomer. The softening monomer may be present in an amount between 10 and 40 weight percent and may be an alkyl acrylate or methacrylate wherein the alkyl group has 1 to 8 carbon atoms. A preferred softening monomer is n-alkyl acrylate. Any mix of precursor ethylene/acid copolymers is suitable for the preparation of co-neutralized ionomers.

The level of mixing of ions throughout the polymer matrix should be intimate in co-neutralized ionomers, even without superior extruder mixing, since the mixing of metal ions which are input into the extruders does not require mixing of different polymers associated with each ion, as does extrusion blending single metal ionomers, particularly if the ion sources are premixed. If the co-neutralized ionomer is based on a mix of acid copolymer precurors however, the extruder must not only blend and react the ions with the polymer, but must blend the different acid copolymer chains. However, it may be easier to mix acid copolymers themselves than already neutralized acid copolymers (i.e. ionomers) since the latter will have much higher viscosity as a result of the neutralization. With co-neutralization, mixing of the polymers can occur before ion source input, avoiding mixing high viscosity materials. However, in co-neutralized ionomers based on more than one acid copolymer, and in extrusion blend ionomers the final composition should, in principle, be similar if melt mixing is very good. Both types of ionomer will consist of a blend of the differing acid copolymer chains, together with a blend of the ions. The ions are believed to be relatively labile, and not associated with any given chain acid group as would a covalently bonded moiety. For this reason, provided mixing is optimum, properties of co-neutralized ionomers should be similar to those of extrusion blended ionomers having comparable levels of each metal and each underlying acid copolymer. It is, of course, as can be inferred from the above discussion, possible to prepare co-neutralized ionomers having a composition which could only be duplicated by blending a complex mix of single metal ionomers if the metal ion ratio is radically different from the ratio of underlying acid copolymer precursors.

While co-neutralization can readily be used to product far more varied compositions that pellet or extrusion blending, it is important to ascertain that a co-neutralized ionomer has properties (or more specifically produces golf balls having covers of the ionomer, with properties or characteristic) which are at least as good as standard pellet blends or extrusion blends when the final composition is identical with respect to metal mix, and underlying acid copolymer mix. For viability, co-neutralized ionomers do not need to have better properties than pellet or extrusion blends, only equivalent properties. The advantages of composition precision, and the advantage of ability to vary composition more readily with co-neutralization, provide incentive to produce co-neutralized ionomers only provided there is no property deficiency relative to extrusion and pellet blend ionomers. In that regard, properties of simple zinc/sodium, ethylene methacrylic acid blends were determined on co-neutralized ionomers, and on the equivalent composition pellet and extrusion blends. No attempt was made to compare pellet blends with co- neutralized ionomer where a mixture of more than one underlying acid copolymer was used. Any difference between co-neutralized ionomers and pellet blends would presumably only be comparable with the difference between pellet blends and extrusion blends of different acid copolymer ionomers, since co-neutralized ionomers require a comparable extrusion process to extrusion blend ionomers. It will be understood that the golf ball cover material may also have standard additives such as whiteners, as is common practice.

The ionomers used as golf ball covers in this invention will also find utility in other applications where the composition versatility inherent in the co-neutralization process, and/or avoidance of the inconsistencies inherent in pellet blending and the molding of pellet blends is important. Test Procedures and Criteria

In the examples below, several key properties used in determining the behavior of golf balls were measured. Measurements were made on molded spheres made from the ionomer itself, and on golf balls using the ionomers as a cover. Spheres of size similar to standard golf ball cores were molded and properties typically measured on golf balls were measured on them. Similar properties were also measured on covered balls. Durability is only meaningful when measured on balls themselves. Properties on neat spheres of the ionomer, however, can be a useful guide to the material's behavior as a cover as well as being more sensitive to variations of the material itself. Obviously, properties of a ball with a cover of the ionomer are far more significant than 'neat-sphere' properties, but neat-sphere properties can be measured without the necessity of actually preparing golf balls covered with the ionomer.

Coefficient of Restitution (COR) is a measure of resilience, and is a guide to the 'length' or distance a ball will travel when struck. Higher values correspond to higher distances. Hardness generally increases with increasing COR. PGA compression is a measure of resistance to deformation, and generally increases with increasing COR. PGA compression, however, is often used as a guide to payability, where lower PGA values correspond to better playability, high softness and higher spin. Durability, measured by a repeated impact test on golf balls themselves tends to show less correlation with COR or PGA compression. The important determination with regard to the present invention, however, is not that any of these properties should be superior, but that these properties for co-neutralized ionomers should be at least comparable to those for pellet blend materials and extrusion blend materials.

COR is measured by firing either a sphere of the ionomer itself or a golf ball covered with the ionomer, by firing from an air cannon at an initial speed of 180 or 125 ft./seα, as measured by a speed monitoring device, over a distance of 3 to 6 ft. from the cannon. The ball strikes a steel plate position 9 feet away from the cannon, and rebounds through the speed monitoring device. The ratio of the speeds is the COR. When this and other properties were measured on a ball itself, a standard core, made from cis 1,4 polybutadiene, with zinc acrylate crosslinking agent, peroxide and other additive such as antioxidants and fillers was used.

PGA compression, the resistance to compression, is measured using a standard industry ATTI machine. Shore hardness (D scale), measured on both molded ionomer spheres and covered balls is measured using ASTM, D-2240.

Melt Index (MI) measured on the polymer itself was measured using ASTM D-1238, condition E, at 190°C, using a 2160 gram weight. Values are in units of grams/10 minutes. Durability is measured using a repeated impact test on covered balls, using the same machine as for COR, but with an initial velocity of 175 ft./sec. Durability measures the number of hits to break. Measurements at ambient and low temperatures (-20°C) were measured. Low temperature durability can be important in cold climates. EXAMPLES

The ionomers used in the comparison testing are as follows. The co-neutralized ionomer is designated CO-1. A comparable pellet blend is designated P-l, and a melt extrusion blend as E-l. The pellet blend, P-l was a blend of two commercially available ionomers. One was a sodium ionomer having a melt index of 2.8, derived from an underlying acid copolymer which was an ethylene/methacrylic acid copolymer containing 15 weight percent methacrylic acid, with a melt index of 25, neutralized to about 29%. This equivalent to 1.58 weight percent calculated as sodium hydroxide or 0.91 weight percent calculated as sodium. The other was a zinc ionomer having a melt index of 0.7 prepared by neutralizing an ethylene/methacrylic acid copolymer containing 15 weight percent methacrylic acid and a melt index of 60 to about 57% neutralization. This calculates as 1.08 weight percent calculated as zinc oxide, or 0.86 weight percent calculated as zinc. Final melt index of the pellet blend can not be readily measured because of the pellet nature, but calculates out as about 2.3.

Extrusion blend E-l was an identical mixture as the above pellet blend, but was extrusion blended in a Davis standard extruder. Melt temperature was about 240°C. Measured melt index was about 1.5.

Co-neutralized ionomer CO-1 was prepared by reacting in an SLM

a 75/25 ratio of two ethylene/methacrylic acid copolymer both containing 15 weight percent methacrylic acid, the firs with a melt index of 25, the second with a melt index of 60 (this simulates the underlying acid copolymer mix in the pellet and extrusion blends above), with 1.08 weight percent zinc oxide, and 1058 percent sodium hydroxide. The zinc oxide was incorporated using a 45 weight percent concentrate in an ethylene/methacrylic acid copolymer containing 10 weight percent acid and having a melt index of 500 and was fed into the extruder with the precursor acid copolymer mix. The sodium hydroxide was incorporated using a 50 weight percent aqueous solution through a port in the extruder (providing the same amounts of metal ions as in the pellet and extrusion blends above). Melt temperature ranges from 100°C at the front end of the extruder to about 250°C. Measured melt index was 2.0. This ionomer thus provides a direct comparison to the pellet and extrusion blends. Property comparisons are shown in Table 1. An examination of the properties show that co-neutralized ionomer has virtually identical properties to the pellet blend and the extrusion blend. In one instance, ambient temperature durability measured on a ball, the co-neutralized ionomer seems significantly superior, but the difference is not regarded as really significant. Nor are slightly lower COR and PGA compression values when measured on cores made from the ionomer. Any possible difference is not reflected in the COR and PGA compression of balls having covers of the materials. Overall, therefore, it is clear that a co-neutralized blend shows equivalent properties to a pellet blend or an extrusion blend. It should be recognized that the pellet blend was carefully and well made, and so does not reflect the real potential for poor pellet blending, due to factors such as pellet sedimentation, poor mixing in the molding machine or, of course, operator weighing errors. Co-neutralized ionomers, therefore, represent a viable, and potentially more versatile alternative to either pellet blended or extrusion blended ionomers.

33

TABLE 1 COMPARISON OF CO-NEUTRALIZED IONOMER WTTH PELLET AND EXTRUSION BLENDED IONOMERS

Property P-l E-l CO-1

Neat Sphere Properties: (Molded sphere, 100% of ionomer)

Hardness 68 68 68

PGA Compression 168 169 163

COR( 180 ft/sec) .689 .690 .686

COR( 125 ft/sec) .740 .739 .736

Golf Ball Properties (Standard Core, Cover 100% of ionomer)

Hardness 67 68 69

PGA Compression 96 96 98

COR (180 fVsec) .719 .718 .718

COR (125 ft/sec) .784 .781 .782

Durability

(Ambient Temp.) 63 69 84

Durability (-20°C) 46 49 50

Claims

WHAT IS CLAIMED IS:

1. A golf ball comprising a core and a cover, wherein the cover consists essentially of a co-neutralized ethylene/acid copolymer ionomer, prepared by neutralizing one or more than one direct ethylene/acid copolymer with a mixture of at least two metal ion sources, in a melt mixing device.

2. The golf ball of claim 1, wherein the mixing device is a melt extruder.

3. The golf ball of claim 1, wherein at least two ethylene/acid copolymers are used in preparing the co-neutralized ionomer.

4. The golf ball of claim 1 , where three metal ion sources are used in preparing the co-neutralized ionomer.

5. The golf ball of claim 3, where three metal ion sources are used in preparing the co-neutralized ionomer.

6. A method of preparing injection molded objects which comprises the step of injection molding pellets of a co-neutralized ethylene/acid copolymer ionomer.