Golf ball

Details Golf ball Golf ball

2001-11-07

2003-05-27

Sewell, Paul T. (Department: 3711)

Games using tangible projectile

Golf

Ball

C473S351000

Reexamination Certificate

active

06569037

ABSTRACT:

This invention relates to a golf ball having excellent travel distance performance because of improved rebound and reception of non-excessive backspin upon driver shots as well as durability against shots.
BACKGROUND OF THE INVENTION
Ionomer resins have been widespread as the cover stock for golf balls in recent years. Ionomer resins are ionic copolymers of olefins such as ethylene with unsaturated carboxylic acids such as acrylic acid, methacrylic acid and maleic acid in which acidic groups are partially neutralized with metal ions such as sodium and zinc. Because of their durability, resilience and excellent other properties, ionomer resins are suitable as the base resin of golf ball cover stocks.
For example, the use of a relatively hard ionomer resin as the golf ball cover stock reduces the friction between the ball and a club face upon driver shots so that the ball receives relatively small backspin upon driver shots. Then the ball flies unaffected by the wind and rolls a good run, leading to a longer travel distance.
However, the use of the ionomer resin as the cover stock raises the problem that the golf ball gives a very hard feel when hit.
One solution to this problem is, as proposed in Sullivan U.S. Pat No. 4,884,814 or JP-A 1-308577, to use a soft/hard ionomer resin blend as the golf ball cover. It is a blend of a soft ionomer resin in the form of an ethylene-(meth)acrylic acid-(meth)acrylate terpolymer with a specific proportion of a hard ionomer resin in the form of an ethylene-(meth)acrylic acid copolymer having certain physical properties. This proposal is very effective for significantly ameliorating the undesired hard feel of prior art golf balls using ionomer resins in the form of ethylene-(meth)acrylic acid copolymers.
However, problems were found with the soft/hard ionomer resin blend as the golf ball cover of the above proposal. Since the relatively soft ionomer resin in the form of an ethylene-(meth)acrylic acid-(meth)acrylate terpolymer is less resilient and mechanically weak, the golf ball molded from a blend thereof is low in rebound. Since the ionomer resin in the form of an ethylene-(meth)acrylic acid-(meth)acrylate terpolymer has a high viscosity, a resin blend thereof also has an increased viscosity and becomes rather difficult to mold, often resulting in balls with poor sphericity.
Molitor et al. U.S. Pat. No. 4,674,751 discloses a cover composition comprising an ionomer resin having a specific hardness blended with a thermoplastic urethane resin having a specific hardness, the cover composition having a specific hardness. The cover composition contains a high proportion of urethane resin relative to the ionomer resin and intends to improve the performance of flexible urethane resin. Using the cover composition, a golf ball which is very soft and receives a relatively much backspin is obtained. The urethane elastomers used therein are insufficiently resilient to provide a long travel distance on driver shots. Most of the urethane elastomers used herein are likely to discolor upon UV exposure, failing to satisfy the characteristics required to withstand the rigorous environment as encountered with golf balls. The urethane elastomers used therein are less resistant to heat and difficult to mold. The durability of the ball against shots is low because of less compatibility between ionomer and urethane resins. Improvements in these respects are desired.
SUMMARY OF THE INVENTION
An object of the invention is to provide a golf ball having excellent travel distance performance because of improved rebound and reception of non-excessive backspin upon driver shots despite a relatively low hardness, as well as durability against shots. Another object of the invention is to provide a golf ball using a cover stock which avoids any discoloration upon long-term UV exposure and is easy to mold.
According to the invention, there is provided a golf ball comprising a core and a cover. The cover is formed of a cover stock based on a mixture of (a) an ionomer resin and (b) a thermoplastic polyurethane elastomer. The cover stock has a hardness of at least 55 as measured by an ASTM D2240 type D durometer.
In a preferred embodiment, the thermoplastic polyurethane elastomer (b) is one synthesized using an aliphatic or alicyclic diisocyanate as the isocyanate component. The aliphatic or alicyclic diisocyanate is typically HDI or hydrogenated MDI. The thermoplastic polyurethane elastomer (b) preferably has a modulus of resilience of at least 49%.
In another preferred embodiment, the ionomer resin (a) and the thermoplastic polyurethane elastomer (b) are mixed in a weight ratio of from 100:30 to 100:1. The cover stock preferably has a polymer decomposition temperature on TG-DTA measurement of at least 290° C.
In a further preferred embodiment, when the ball surface is exposed to a mercury lamp for 24 hours, the ball surface experiences a discoloration before and after the lamp exposure corresponding to a color difference (&Dgr;YI) of up to 8 as measured by JIS K7103 reflection method.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the golf ball comprising a core and a cover according to the invention, the cover is formed of a cover stock primarily comprising a mixture of (a) an ionomer resin and (b) a thermoplastic polyurethane elastomer.
The ionomer resin (a) used herein may be selected from well-known ones, typically metal ion-neutralized products of olefin-unsaturated carboxylic acid copolymers. The olefins used herein include those of 2 to 8 carbon atoms, for example, ethylene, propylene, butene, pentene, hexene, heptene, and octene, with ethylene being preferred. The unsaturated carboxylic acids used herein include those of 3 to 8 carbon atoms, for example, acrylic acid, methacrylic acid, maleic acid and fumaric acid, with acrylic acid and methacrylic acid being preferred.
It is generally recommended that the content of unsaturated carboxylic acid (simply referred to as acid content) be at least 10%, preferably at least 15% by weight and up to 25%, preferably up to 20%, more preferably up to 16% by weight, based on the olefin-unsaturated carboxylic acid copolymer to constitute the ionomer resin. With too low an acid content, some cover stocks may become less rigid and less resilient. With too high an acid content, some cover stocks may become too rigid and detract from the feel when hit.
The carboxyl group of the unsaturated carboxylic acid is partially neutralized with metal ions. It is recommended that the degree of neutralization be at least 20 mol %, more preferably at least 25 mol %, and up to 80 mol %, more preferably up to 70 mol %. With too low a degree of neutralization, the cover stock may become short of rigidity. With too high a degree of neutralization, the cover stock may become less flowable and less workable and the cover may show little improvement in physical properties.
The metal ions used for neutralization include Li
+
, Na
+
, K
+
, Zn
++
, Co
++
, Ni
++
, Cu
++
, Pb
++
and Mg
++
, with the Li
+
, Na
+
, Zn
++
and Mg
++
being preferred. These metal ions may be given by salts of formic acid, acetic acid, nitric acid, and carbonic acid, hydrogen carbonate salts, oxides, hydroxides, and alkoxides.
As the ionomer resin (a), any of commercially available ionomer resins for the golf ball cover stock may be used. Examples include Himilan AM7315, Himilan AM7317, Himilan AM7318, Himilan 1706, Himilan 1605, Himilan 1601, and Himilan 1557 from Dupont-Mitsui Polychemical Co., Ltd., which may be used alone or in admixture of two or more.
The ionomer resin (a) in the base mixture should preferably have a hardness as measured by an ASTM D2240 type D durometer of at least 55, more preferably at least 56, even more preferably at least 60 and up to 75, more preferably up to 65, even more preferably up to 63. With too low a durometer hardness, the cover may become less resilient. With too high a durometer hardness, the ball may sometimes become hard in feel and les

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