HIGH FLOW IONOMER RESIN COMPOSITIONS

USEFUL FOR GOLF BALL COVERS

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to ionomer resin compositions based on ethylene copolymers which have high melt flow. They are ideally suited for use particularly as blending resins with other ionomers, to provide ionomer blends which also have high flow, yet have good resilience. The compositions are readily processed to produce covers for golf balls. The resulting balls also have good resilience, and also have good durability. More particularly, the compositions contain a certain amount of ionomer prepared from ethylene carboxylic acid copolymers with a very high melt index, neutralized to a fairly high level. Description of Related Art

Ethylene (meth)acrylic acid copolymers neutralized with metal ions are known as ionic copolymers or ionomers or ionomer resins. They are well known for use as covers for golf balls. The properties of the ionomer are reflected to a considerable extent in the behavior of golf balls when the ionomer is used as cover material. The resilience of the golf ball is, for instance, dependent on the resilience of the cover material, and the durability of a golf ball on repeated impact reflects the toughness of the cover material.

Many variations in the compositions of ionomers is known, and these variations have been used to optimize particular characteristics of golf balls when the ionomer material is used as its cover. For instance, the amount of acid and which acid, and the particular ion used to neutralize, as well as the amount of neutralization, affects the nature of the ionomer. Blending ionomers also has become common in production of such cover materials. Blends may be of different ionomers having different metal ions, different ionomers having different flexural modulus, or different ionomers having different acid level and/or type. Such blending has been found to provide a certain degree of synergism with respect to key properties important for golf ball covers, particularly resilience. Use of blending is now the norm for golf ball covers, and any new cover

composition may well employ blending, together with utilization of any new technical development or discovery.

Ethylene copolymer ionomers are made by neutralizing the corresponding ethylene/acid copolymer with materials which supply metal ions. The process of making ionomers is disclosed in U.S. Patent

No. 3,264,272 (Rees). Neutralization causes ionic crosslinking to occur and at use temperatures (for golf balls this means ambient temperatures), this ionic crosslinking has a major beneficial effect on certain properties. Fortunately, at higher temperatures, for instance above 160 deg. C, the crosslinks become labile and the material can become fluid enough to process as a thermoplastic melt rather than remain intractable, as covalent crosslinked materials are. The crosslinking effect caused by the ions does not completely disappear at processing temperatures however, and the melt viscosity of these ion neutralized resins is still significantly higher than the parent un-neutralized resin. As the level of neutralization increases, viscosity increases many fold, and at levels above 90%, the resin can indeed become almost intractable. For this reason, neutralization is typically in the range of 20 to 70%.

The melt viscosity, is normally quantified by Melt Index, (MI) which is a melt flow measurement, and therefore has an inverse relation to viscosity. Rees discloses that the 'base resin', i.e., the acid copolymer, may have a melt index of from 0.1 to 1000, (using ASTM D-1238 at 190 deg.C), but preferably from 1.0 to 100. The reference further discloses that optimum solid state properties for ionomers are actually achieved using a base copolymer with an MI of 1 to 5, an acid concentration of 5 to 10 weight percent, and a degree of neutralization of the acid groups of from 50 to 80 percent. The final melt index of the ionomers is typically from 0.1 to about 25. Rees disclosed that the lower the MI of the derived ionomer, the better the properties in general. Generally, before the present invention, high MI base resins, particularly those having an MI of greater than 100, were believed to result in low impact strength ionomers.

For some packaging applications, ethylene methacrylic acid copolymer ionomers are used which are made from a base resin with an MI as high as 100. Resins of this type have in some instances been

blended with other ionomers in an attempt to improve processability. Such resins are sold by E. I. du Pont de Nemours and sold under the trade name of SURLYN. Ionomers made from acid copolymers having a very high melt index of about 400 have also been used for certain adhesives and binder applications, where mechanical property requirements are quite different from those required in materials used in golf ball covers. Ionomers made from ethylene/methacrylic acid copolymers having an MI between these values, i.e. between 100 and 400, have not been used.

For golf ball cover applications which utilize ionomers made from ethylene/methacrylic acid copolymers, for instance those manufactured and sold by E. I. du Pont de Nemours, also sold under the trade name of SURLYN, normal practice is to use a much lower starting MI, typically about 60 or less, and neutralize sufficiently to reach an MI of from about 0.4 to about 2.8. Using these starting and finishing Ms of acid copolymer and resulting ionomer respectively, the ionomers were ideally suited, particularly in blends, to provide golf ball cover materials which allowed the balls to have premium properties - the desired high resilience, the durability, and the other characteristics sought after in golf balls. Ionomers with Mis much above 3.0 generally gave less than desirable properties due to their low ion content, which results from the low degree of neutralization which corresponds to such Mis.

Unfortunately, resins with low Mis, particularly about 3 or less do not lend themselves to as efficient and rapid processing to form golf ball covers as would be liked. This imposes a cost penalty both because of slower processing, and because of inconsistency in parts when molding a resin of insufficient fluidity. Typically the result is a high number of rejects. Thus, conventional wisdom imposes a window on the ionomer composition. The limits of this window are a starting acid copolymer MI below about 60, and derived ionomer MI below 3.0 for properties. It is clearly possible, when starting with an acid copolymer having an MI of 60 to make an ionomer with high MI, simply by neutralizing only to a relatively low level. However, high neutralization levels are considered critical in obtaining the beneficial properties of ionomer. Thus one is forced into the best compromise between optimum flow and optimum properties.

There is a need for ionomer resins for use in golf ball cover materials, to be used alone, but preferably as part of ionomer blends, which have more desirable processibility, or impart better processability to a blend, without substantial loss of properties in the resin or blend, and which lead to acceptable characteristics in golf balls when such materials are used as the cover, particularly durability and resilience characteristics. SUMMARY OF THE INVENTION The key to the invention is the discovery that those properties of ionomers which translate into durability and resilience in golf balls, when used as cover material, are not substantially reduced when the ionomers have an MI above 3.0 provided they are made from base copolymers with an MI above about 110, and in the 110 to 350 range, because it is possible to achieve high levels of neutralization before flow becomes too low, when the base MI is high. These high levels of neutralization, suφrisingly, adequately compensate for high MI, both in the starting acid copolymer, and in the resulting ionomer. Thus there is a small window of base resin MI, specifically between 110 to 350, which has not heretofore been recognized as useful for producing precisely the combination of flow and properties in the resulting ionomer, and in the performance characteristics of golf balls using that ionomer (or blends including that ionomer), that have been sought after.

More specifically, there is provided a composition comprising an ethylene copolymer ionomer with a melt index of from 3.0 to 8.0, which is prepared from a precursor ethylene methacrylic acid or acrylic acid copolymer having a melt index of from about 110 to 350, having greater than about 40 percent of the acid groups present neutralized with metal ions.

Further provided are ionomer blends which contain above 25 percent of the above ionomer. DETAILED DESCRIPTION OF THE INVENTION

In this disclosure, the word 'copolymer' means a polymer polymerized from two or more monomers, and includes teφolymers. The more specific description 'ethylene carboxylic acid copolymer', 'ethylene methacrylic acid copolymer' and the like, is meant to include copolymers which may also have a third monomer present.

As noted above, ionomers derived from ethylene/carboxylic acid copolymers by partial neutralization with metal ions are materials which behave substantially as crosslinked polymers at ambient temperatures, yet whose crosslinks are sufficiently labile as temperature is increased that they can be processed as thermoplastics. The preparation of such ionomers is described in U.S. Patent No. 3,264,272 (Rees), which is hereby incoφorated by reference. The precursor or 'base' resins used to prepare the ionomers, ethylene/carboxylic acid copolymers are direct copolymers, that is to say not graft copolymers, and may be prepared as described in U.S. Patent No. 4,351,931 (Armitage), which is also hereby incoφorated by reference. That reference describes polymers with up to about 10 weight percent acid. Higher levels of acid are common in the base polymers used to prepare ionomers now commonly used in golf ball cover materials. Acid levels as high as 30 weight percent have been considered, and are of importance in the present invention. Acid copolymers with high levels of acid are difficult to prepare in continuous polymerizers, because of monomer/polymer phase separation. This difficulty can be avoided, however, by use of 'cosolvent technology' as described in U.S. Patent No. 5,028,674 which is also hereby incoφorated by reference.

Like the properties of any crosslinked polymers, ionomer properties depend fundamentally on the average molecular weight of the underlying un-crosslinked polymer, and on the degree of effective crosslinking. These may be considered the 'architectural' features of a polymer, and are strongly dominant, whatever the chemical nature of the polymer. Increasing crosslinking affects a characteristic of molecular weight which affects flow, but it has virtually no effect on the underlying length of the linear precursor molecules, and this is a key factor for many mechanical properties. The resins of this invention, prepared from high MI base resins, above about 110, will be referred to, for descriptive convenience, as non-conventional architectural type ionomers resins or, for short, non-conventional ionomers. Ionomer resins made from an MI base resin of below 110 will be referred to as conventional architectural type ionomer resins or, for short, conventional ionomers. These types of architectural features in a covalently crosslinked polymer are sometimes

characterized in terms of the underlying linear polymer molecular weight and its distribution, and the crosslink density. However, these ionomers may be more readily characterized empirically, simply in terms of base resin MI, derived ionomer resin MI, and degree of neutralization or percent of acid groups neutralized.

The other major factor which affects properties is the chemical nature of the chain and of the crosslinks. In the case of ionomers the chemical nature of the chain depends on the amount of acid comonomer and type, the amount of any other comonomer (such as 'softening' comonomers which lower modulus) and type, and the particular metal ions which form the crosslinks.

The major thrust in varying and optimizing ionomer properties, particularly for use in golf ball cover materials, has been concerned with varying the chemical nature, namely the amount and type of acid, use of additional softening comonomers, particularly alkyl acrylates, and the nature and amount of ions in the composition. Most particularly, various synergisms or pseudo-synergisms have been found or are puφorted to occur, when mixes of ionomers of these various chemical types are utilized. By contrast, although the amount of crosslinking, corresponding to different amounts of neutralization, has been varied, the most sacrosanct of the architectural features of polymers, the molecular weight of the underlying equivalent linear chains, has not been varied or examined to any degree. As a result, the MI of the underlying precursor acid copolymer, which is a measure of molecular weight, and therefore the length of the polymer chains, has always been kept below about 60 when the derived ionomer is to be used in applications where premium properties are necessary. In no application are premium properties more essential than that of golf ball covers. Even when the chemical nature of the underlying polymer chain varies, i.e. the amount and type of acid and other comonomers, the rule has been to use precursor polymers with an MI of 60 or less. It has always been recognized that ionomers can be made from higher MI precursor polymers, even as high as 1000. However, conventional wisdom, unfortunately based on limited experimentation, indicated that such polymers would have limited impact

durability - the resins might be suitable for adhesives perhaps, but not much else.

Suφrisingly, it has now been found that high MI precursor polymer, within limits, can be used to obtain higher flow ionomers (or ionomer blends) without sacrificing those properties essential to allow use of the material for golf ball covers. In effect this means that, suφrisingly, one can reduce underlying polymer chain length provided there is an increase in crosslinking. While some properties may be adversely affected by this change, no deficiencies have been noted which are critical for translation into golf ball covers having good characteristics.

The ionomers of this invention may be prepared from ethylene methacrylic acid or ethylene acrylic acid copolymers containing from 10 to 30 weight percent acid. They base copolymer and resulant ionomer may also contain a 'softening' monomer which may be an alkyl acylate or methacrylate having an alkyl group with from 1 to 8, preferably 4 to 8 carbon atoms. The amount of softening monomer in such teφolymers may be up to about 40 weight percent in the base resin. Generally, the ionomers of the invention will not be 100 percent teφolymer. They may contain no teφolymer, or they may be a blend containing up to about 90 percent teφolymer.

The resins of this invention, non-conventional ionomers, may advantageously be used in blends. Melt blending of ionomers is used for various puφoses. If the synergism known to exist in the properties of compositions with mixed metal ion types is to be utilized, the obvious way to do this is to mix two ionomers which have been neutralized with different ions. Another blend used is that of a 'hard' stiff ionomer, and a 'soft' more flexible ionomer which typically contains a softening comonomer such as an alkyl acrylate, in addition to the carboxylic acid comonomer. Another type of blend may be that of different acid ionomers, such as ionomers derived from acrylic and methacrylic acid. Alternately, a blend of a high acid comonomer content ionomer and a lower acid comonomer content ionomer may be advantageous. Another reason for blending is to obtain a given MI resin. It is usually easier to blend resins of known Mis in calculated amounts to obtain a required MI than to obtain that MI by adjusting neutralization level.

Ions in an ionomer tend to be labile, and ionomer blends may have the ions uniformly dispersed throughout the polymer. Blends of ionomers having different amounts and types of comonomers however are somewhat different, since unlike ions which move from position to position, comonomers are locked into the original polymer chain in which they were polymerized.

The blends of this invention may comprise blends of varying chemical type as described above. The other resin or resins in a blend may be another non-conventional ionomer or a conventional ionomer. At least 25 percent of the blend however must be non-conventional ionomer. It is clear that there exists an almost endless number of variations achievable by blending. The blends of this invention however will always contain some non-conventional ionomer, whatever chemical- nature type of blending is also employed. In this way, all the advantages in properties resulting from chemical-nature type blending are combined with the flow advantage achieved by using at least 25 percent non- conventional ionomer in the composition. While one non-conventional ionomer could be used alone in golf ball covers, generally, the advantage achieved by chemical nature type ionomer blending will be utilized, even if this is achieved by using two non-conventional ionomers blended together - in which case the blend would be 100 percent non-conventional ionomer. However, while it is necessary in a blend to have least 25 percent non-conventional ionomer, it is preferred also to have less than 70 percent. Resins which are 100 percent non- conventional ionomer, even in blends which are optimum as far as mixed ions are concerned, while adequate, can show slightly poorer durability properties. In single ion compositions, conventional as well as non-conventional ionomers vary in low temperature impact durability. Sodium ionomers particularly, tend to be poorer in durability than zinc ionomers whether conventional or non-conventional.

Very generally, the percent of acid groups present which have been neutralized determines the final MI for a given starting MI. That is to say, the change in flow depends on the percent of acid groups neutralized. If there are more acid groups to start with, say for a 15 weight percent acid comonomer instead of 10 percent, more metal ions will be required to

achieve the same percent neutralization of the total acid groups present. Thus a higher acid copolymer will have a higher weight percent of metal (as ions) in it than a lower acid copolymer. At first sight, it might be assumed that the amount of effective crosslinking, and hence reduction in melt flow which occurs on neutralization, would depend on the total amount of ions present irrespective of the amount of acid groups in the copolymer. However, it appears this is not so, and that the percent of acid groups present neutralized is the more important factor in determining flow. In addition, it has been found that the change in flow, for a given percent neutralization, is not particularly dependent on the particular metal ion for many metal ions, unless the ion produces a more covalent bond, such as aluminum does. Thus, the same starting resin, with the same percent of original acid groups neutralized with either sodium or zinc, produces, very nearly, the same MI ionomer. This understanding will enable one skilled in the art to achieve a particular MI from a particular base resin MI with minimal experimentation.

For golf ball covers, when hard, stiff ionomers are used, either alone or as they often are, in blends with soft ionomers, the copolymers of choice contain 15 weight percent or more acid comonomer. Earlier, ionomers with only 10 percent acid were commonly used. Even higher levels of 20 percent acid and even up to 30 percent are now of interest in obtaining premium properties. The high acid materials will contain more metal ions for a given percent of acid groups neutralized. While ionomers with lower levels of acid in the base polymer are part of the present invention, copolymers with 15 percent or more acid of preferred.

While not committing to any particular theory, it may be that the higher metal content possible in ionomers whose base resin has a high MI, particularly together with a high acid comonomer level, is partly an offsetting factor against the relatively low underlying molecular weight of the parent acid copolymer. It should be noted that the high flow ionomers, including blends, of this invention do not usually give superior properties to materials with poor flow unless the flow of the latter is so low that the materials are difficult to process, and produce flaws in molded parts.. Rather, the properties are unexpectedly adequate, and better than would generally be obtained from high flow materials whose

high flow was merely a result of a low level of neutralization. Use of high flow materials which have good properties also has a subtle additional advantage. When a material flows well, the inherent properties are more likely to be realized compared with a low flow, poor processing material. A low flow material which has inherently good properties may not always reflect those properties unless excessive care is taken during processing. Thus, not only do the high flow materials of this invention have adequate properties, they are likely to produce more consistent properties. Consistency as a quality can be of extreme importance. The compositions of this invention are especially useful in molding golf ball covers. However, other applications where high flow combined with premium properties is required, can take advantage of the materials of this invention. Such applications include bowling-pin covers, certain footwear parts, and various other applications such as perfume bottle caps, extruded cord and co-extruded films.

TESTING CRITERIA AND SAMPLE PREPARATION

Melt Index, measured on the acid copolymers and on the ionomers is measured using ASTM D-1238, condition E, at 190 deg. C, using a weight of 2160 grams. The degree of neutralization (percent of acid groups neutralized) may be measured by several techniques. Thus, infrared analysis may be employed, and the degree of neutralization calculated from the changes resulting in the absoφtion bands. Another method comprises the titration of a solution of the ionic copolymer with a strong base. Tests used to determine the utility of the materials were carried out on two-piece golf balls. While some properties can be carried out on spheres made of the ionomer material itself as a guide to the inherent properties of the material, the preferred testing is carried out on balls, since for golf ball applications, the final measure is, of course, in the behavior of the ball. Two-piece ball were made, using a thermoset poly(butadiene) compound core, and molding on covers having a thickness of 60 - 90 mils, usually about 75 mils, using an Arburg vertical injection molding machine. Melt temperature was about 215 deg. C. Resilience, as reflected in a measurement of the coefficient of restitution or COR, was measured by firing the ball from an air cannon at an initial velocity of 180 ft./sec. or 125 ft./sec„ as measured using a speed monitoring device over a distance of 3 to 6 feet from the cannon. The ball strikes a steel plate, positioned 9 feet away from the cannon, and rebounds through the speed-monitoring device. The return velocity divided by the initial velocity is the COR. The COR values are strongly affected by the particular ion or ion blend, so that compositions can only be compared when the ion or ion mix is comparable. In preparing a ball material, the ion mix will be optimized, in addition to utilization of the high flow materials of the present invention. The desired result is that the COR of the ball, utilizing the high flow materials of the present invention are adequate, and generally comparable to those which could be obtained with comparable conventional ionomer having made from the same ion or ion mix.

Durability was also measured using an air cannon. The ball has an initial velocity of 175 ft./sec, and it hits a steel plate about 2 feet away.

Typically five balls were tested. The average number of hits to crack the ball is measured. The higher the number of hits to crack, the greater the durability. Any value of hits to crack above 10 is considered adequate, significant deficiency being registered by lower numbers. Values higher than 10 are considered very good increasing to excellent. Measurements were made at ambient temperatures and also at -20 deg. F. Durability, particularly low temperature durability, while somewhat dependent on ion mix and other factors, is strongly affected by MI. Thus durability can decrease dramatically with increasing MI. It is of great concern therefore, that durability be adequate when the present invention is utilized, since the invention is intimately concerned with MI manipulation. This property thus represents the most important determinant of suitability of materials of the invention.

Various other properties such as PGA compression, and hardness were also measured to ascertain that there was no deficiency in these properties. For ionomers with comparable ion mix these properties showed little variation, and did not appear to be significantly dependent on whether conventional or non-conventional ionomer was used, and very little dependent on MI at all. EXAMPLES

In the following examples, the ionomers used are listed in Table 1, which lists the base resin MI, the final MI of the ionomer, and the degree of neutralization.

Table 2 shows examples of the compositions of the invention and some comparative examples. Examples falling within the bounds of the invention are shown with an example number without a 'c' suffix. Comparative examples are shown with a 'c' suffix. All examples have an MI above 3.0, as required, and have good processibility. Comparative example 6c also has an MI above 3.0, but has no non-conventional resin component. Of the examples, 1, 2, 3, 4, 5, 8 and 9 are one hundred percent non-conventional resin, examples 1, 2 and 3 being single resins, the remainder blends of non-conventional resins. Examples 6 and 7 have fifty percent non-conventional resin, and thus fall within the bounds of the invention.

COR values of 2-piece balls with non-blended ionomer resin covers, either at 125 or 180 ft sec are slightly lower than for blend covers, which contain two different-ion ionomers. This is to be expected, since blending ionomers with different metal ions is well known to improve resilience. The difference is much greater when measured on spheres of the ionomer itself, but in covered balls, the difference is relatively small. The important thing to note is that for a given ion or ion blend, golf balls with high flow, non-conventional ionomer, or non-conventional ionomer blend covers, have resilience comparable with that of balls using conventional ionomer covers. Thus, there is no loss in behavior with respect to resilience when high flowing superior processing ionomers materials of the present invention are used as cover materials.

Durability tests at ambient temperatures demonstrate that values for balls using cover materials of the invention are generally adequate. All balls show values of 10 or more hits to break, which is regarded as adequate. Durability appears to show a much wider variation than COR. The best values are generally for sodium/zinc blends, and the poorer ones for lithium ionomer or lithium ionomer blend covers, both with sodium and zinc (examples 3, 8 and 9). With the exception of lithium and lithium blend ionomer covers, values are at least as good for covers using the materials of the invention to those using conventional material.

Durability at -20 deg. F is also generally adequate. Again, for sodium zinc ionomer blend covered balls, values tend to be highest. Comparative example 6c, which uses a high flow material made from conventional ionomers only, is severely deficient, as is comparative example 8c which uses lithium in its blend. All blend ionomer covers provide adequate low temperature durability for the ball. For non blended ionomer covers, high flow non-conventional lithium ionomer actually shows a much higher value that conventional material (compare example 3 and comparative example 3 c). High flow sodium ionomer cover (example 1) does however show some deficiency, giving a value of 3. However, as noted previously, and as can be seen from exanple lc, sodium ionomers are generally poorer in low temperature durability than zinc ionomers. Overall however, materials of the invention containing

non-conventional ionomer, provide at least as good balls as materials with lower flow conventional ionomers do.

TABLE : I

IONOMER RESIN COMPOSITIONS

MI

Base

Cpde Resin Ionomer Metal % Acid Groups Neutralized

Cl 25 2.5(2.7) Na -29

C2 60 4.2(4.0) Na -33

C3 60 1.2(0.9) Na -59

C4 60 5.7(4.5) Zn -35

C5 60 0.9(0.6) Zn -58

C6 60 2.8 Li -47

NCI 213 4.4 Na -51

NC2 213 3.4 Zn -56

NC3 175 4.3(4.6) Zn -51

NC4 175 3.3(3.6) Zn -58

NC5 237 4.5(4.7) Zn -58

NC6 213 3.8 Li -47

All compositions have a base resin of ethylene/methacrylic acid with 15 weight percent methacrylic acid. C code refers to conventional ionomers. NC code refers to non-conventional ionomers.

MI values in parenthesis were measured on the same resin in a different laboratory.

TABLE 2

BEHAVIOR OF 2-PIECE GOLF BALLS WITH IONOMER COVERS

Composition (of Dur¬ Dur- ionomer cover) MI ability ability-

Ex # COR180 COR125 (ionomer) RT 20 (hits/ (hits/ break) break) lc C3 (Na) .705 .774 1.2 30 16

1 NCI (Na) .711 .776 4.4 30 3

2c C5 (Zn) .703 .772 0.9 34 23

2 NC2 (Zn) .702 .770 3.4 36 22

3c C6 (Li) .717 .781 2.8 54 8

3 NC6 (Li) .709 .777 3.8 27 33

4c C3/C5 (Na/Zn) .713 .779 0.9 20 22

5c C1/C5 (Na/Zn) .715 .780 1.6 52 41

6c C1/C4 (Na/Zn) .711 .780 4.7 59 2

4 NC1/NC2 (Na/Zn) .712 .781 3.7 39 33

5 NC1/NC2 (Na/Zn) .708 .777 3.8e 37 29

6 C1/NC4 (Na/Zn) .719 .782 3.8 73 43

7 C1/NC5 (Na/Zn) .723 .784 4.2 69 42

7c C6/C5 (Li/Zn) .714 .779 1.5 30 28

8 NC6/NC2 (Li/Zn) .714 .780 3.4 14 17

8c C6/C1 (Li/Na) .716 .781 2.9 47 1

9 NC6/NC1 (Li Na) .711 .777 3.7 10 13

e = estimate

COR 180 and COR 125 are Coefficient of Restitution at 180 and

125 feet/second.

Durability RT measured at Room Temperature and Durability/-20 measured at -20 deg. F. (-29 deg, C),

All blends are 50/50 except example 5 which is a 70/30 blend.