Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-04-17
2002-07-09
Woodward, Ana (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C524S487000, C524S490000
Reexamination Certificate
active
06417282
ABSTRACT:
This invention relates to golf balls, and more particularly, cover stocks and golf balls using the same.
BACKGROUND OF THE INVENTION
Ionomer resins are now in widespread use as the golf ball cover material. Ionomer resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, wherein some acidic groups are neutralized with metal ions such as sodium or zinc ions. Because of the excellent properties of resilience and scratch resistance, the ionomer resins are best suited as the base resin of the golf ball cover stock.
Since golf balls using ionomer cover stocks were marketed, cover stocks surpassing the properties of the ionomer cover stock have not been commercialized. The ionomer covers predominate in the current golf ball covers.
An attempt to develop a new type of cover stock surpassing the properties of the ionomer cover stock requires to find another resin that can be blended with an ionomer resin as the base so as to improve the properties of the blend without detracting from the properties of the ionomer resin itself.
JP-B 63-58856 discloses a cover stock in which a medium to low density polyethylene is blended with an ionomer resin as the base. More specifically, 1 to 9 parts by weight of a medium to low density polyethylene is blended with 100 parts by weight of an ionomer resin to formulate a cover stock. This is effective for improving the durability of a golf ball.
Of the medium and low density polyethylene resins used therein, the low density polyethylene polymerized by the high pressure process is characterized by a low hardness and low cost. It is thus expected that a low cost cover stock providing a soft feel when hit is obtained by blending the low density polyethylene with the ionomer resin. It has been desired to develop a cover stock by blending with the ionomer resin a more proportion of the high-pressure-produced low density polyethylene.
However, the high-pressure-produced low density polyethylene is less dispersible in the ionomer resin because it contains many branches owing to the polymerization process. For this reason, the high-pressure-produced low density polyethylene is not recommended in JP-B 63-58856.
Allegedly, if more than 9 parts by weight of a medium to low density polyethylene is blended with 100 parts by weight of an ionomer resin, the blend experiences a substantial loss of resilience to below the practical level because of the poor dispersion of the medium to low density polyethylene. Also allegedly, if a high density polyethylene is blended with an ionomer resin, despite the reduced content of polyethylene, the resulting cover stock loses durability and becomes impractical.
SUMMARY OF THE INVENTION
An object of the invention is to provide a golf ball cover stock that has solved the above-mentioned problems of conventional golf ball cover stocks based on an ionomer resin in admixture with polyethylene, that is, a cover stock having improved durability, wear resistance, resilience and economy. Another object of the invention is to provide a golf ball using the cover stock.
The inventors are interested in a golf ball cover stock based on an ionomer resin in admixture with polyethylene. The inventors have found that by blending in this cover stock an epoxy group-modified polyolefin or low molecular weight polyethylene wax as a third component, the dispersion of polyethylene in the ionomer resin is significantly improved, allowing a larger amount of polyethylene to be blended. The resulting cover stock is highly durable and wear resistant. Additionally, the cover stock is improved in resilience and cost as compared with the conventional golf ball cover stock based on an ionomer resin in admixture with polyethylene.
The invention provides a golf ball cover stock primarily comprising a heated mixture of (a) an ionomer resin, (b) polyethylene, and (c) an epoxy group-modified polyolefin or (d) a low molecular weight polyethylene wax or both (c) and (d). A golf ball comprising a cover made of the cover stock is also provided.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The golf ball cover stock of the invention contains as a main component a heated mixture of
(a) an ionomer resin,
(b) polyethylene, and
(c) an epoxy group-modified polyolefin and/or (d) a low molecular weight polyethylene wax.
The ionomer resin (a) is preferably selected from metal ion-neutralized copolymers of an olefin with an unsaturated carboxylic acid. The olefins used herein include those of 2 to 8 carbon atoms, with ethylene being preferred. The unsaturated carboxylic acids used herein include those of 3 to 6 carbon atoms, for example, acrylic acid, methacrylic acid, maleic acid and fumaric acid, with methacrylic acid and acrylic acid being preferred.
In the copolymers, the content of unsaturated carboxylic acid is generally 5 to 25% by weight, and preferably 10 to 20% by weight. Too less contents of unsaturated carboxylic acid would lead to low rigidity and poor resilience whereas too more contents would lead to brittle resins having a too high rigidity so that the durability of a ball against strikes lowers. That is, contents outside the range would raise problems on practical use.
In the copolymers, 20 to 80 mol % and more preferably 25 to 70 mol % of the carboxyl groups of the unsaturated carboxylic acid are preferably neutralized with metal ions. Copolymers with a degree of neutralization of less than 20 mol % would be short in rigidity and resilience whereas a degree of neutralization in excess of 80 mol % would detract from flow and workability and achieve no improvement in physical properties. Exemplary ions used for neutralization are Li
+
, Na
+
, K
+
, Zn
++
, Ni
++
, Cu
++
, Pb
++
, and Mg
++
. Of these, Li
+
, Na
+
, Zn
++
and Mg
++
are especially preferred. These metal ions are supplied as suitable compounds such as formates, acetates, nitrates, carbonates, hydrogen carbonates, oxides, hydroxides, and alkoxides.
The ionomer resins may be used alone. Also useful is a mixture of two or more ionomer resins neutralized with different metal cations.
If desired, the copolymer may contain an additional comonomer such as an acrylate because a softer terpolymer can be obtained.
The ionomer resin used herein is commercially available from Mitsui-Dupont Polychemical K.K. as Himilan 1557, Himilan 1601, Himilan 1605, and Himilan AM7318 which are sodium ion-neutralized ethylene-methacrylic acid copolymers; Himilan 1650 and Himilan 1706 which are zinc ion-neutralized ethylene-methacrylic acid copolymers; and Himilan 1855 and Himilan 1856 which are terpolymers with an unsaturated monomer such as acrylate.
Any polyethylene may be used as component (b). A low density polyethylene polymerized by the high pressure process is preferred. A high density polyethylene polymerized by the medium or low pressure process may also be advantageously used when the low molecular weight polyethylene wax (d) is used. The low density polyethylene usually has a specific gravity of 0.90 to 0.94. The high density polyethylene usually has a specific gravity of 0.95 to 0.97.
The high pressure process is by polymerizing purified ethylene under a pressure of about 600 to 1,000 atm., with a minor amount of air added, while heating at about 200° C. The medium pressure process is by polymerizing ethylene under a pressure of about 20 to 30 atm. in the presence of a chromium or similar catalyst while heating at about 150° C. The low pressure process is by polymerizing ethylene in the presence of titanium tetrachloride and triethylaluminum catalysts at a pressure of atmospheric pressure to about 5 atm. and a temperature of room temperature to about 70° C.
The high-pressure-produced polyethylene has branched side chains in places because ethylene monomers are forcedly joined under the impetus of high temperature and pressure during polymerization. These side chains prevent crystallization of polyethylene, which exhibits a low hardn
Ichikawa Yasushi
Kashiwagi Shunichi
Takesue Rinya
Bridgestone Sports Co. Ltd.
Woodward Ana
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