Golf ball

A golf ball having excellent rebound characteristics, excellent shot feel and good controllability at approach shot which contains a core and a cover formed on the core, wherein the cover is formed from a base resin mainly containing the mixture of an ionomer and polybutadiene having trans-1,4-structure of at least 60%.

FIELD OF THE INVENTION 
The present invention relates to a golf ball. More particularly, it relates 
to a golf ball having excellent rebound characteristics, excellent shot 
feel and good controllability at approach shot. 
BACKGROUND OF THE INVENTION 
Recently, ionomer resins have been widely used as of cover material for 
golf balls (e.g. Japanese Patent Kokai Publication No. 49727/1974, etc.). 
This is because the ionomer resin is superior in rebound characteristics, 
durability, etc. and it is easily processed. However, since the ionomer 
resin has high rigidity and hardness, the resulting golf ball is poor in 
shot feel and controllability in comparison with balata 
(transpolyisoprene)-covered thread wound golf ball. 
In order to improve shot feel and controllability of the ionomer-covered 
golf balls, it has been attempted to soften the ionomer resin by various 
means, but golf balls having sufficient performance have not been 
obtained. For example, Japanese Patent Kokai Publications Nos. 
188270/1982, 54928/1994, 79017/1994, and the like propose that the ionomer 
resin is mixed with a rubber to soften it, but it is difficult to have 
stable physical properties depending on the variability of the state of 
crosslinking of the rubber. 
OBJECTS OF THE INVENTION 
A main object of the present invention is to provide a golf ball having 
excellent shot feel and good controllability, while maintaining excellent 
rebound characteristics inherent to ionomer resins. 
According to the present invention, the object described above has been 
accomplished by employing a cover which is formed from a base resin mainly 
containing the mixture of an ionomer and polybutadiene having 
trans-1,4-structure of at least 60%. 
SUMMARY OF THE INVENTION 
The present invention provides a golf ball comprising a core and a cover 
formed on the core, wherein the cover is formed from a base resin mainly 
containing the mixture of an ionomer and polybutadiene having 
trans-1,4-structure of at least 60%. 
DETAILED DESCRIPTION OF THE INVENTION 
A cover is covered on a core to obtain the golf ball of the present 
invention. The core used may be either a core for thread wound golf ball 
(thread wound core) or a core for solid golf ball (solid core). The thread 
wound core is composed of a center and a thread rubber layer formed by 
winding the thread rubber in a stretched state around the center. The 
center may be a solid center of a vulcanized molded article of a rubber 
composition, or a liquid center wherein a liquid such as water, etc. is 
encapsulated in a center cover made of a vulcanized rubber. The solid core 
may be not only a core having a single-layer structure, that is uniform 
and integral, but also may be a core with a multi-layer structure having 
two or more layers. These are well known in the art. In order to simply 
explain the present invention, a two-piece golf ball using a uniform and 
integral solid core will be use hereinafter for explanation. The solid 
core may be obtained by vulcanizing or press-molding a rubber composition 
into a core of a typical two-piece golf ball. The rubber composition 
typically comprises a base rubber, a metal salt of an unsaturated 
carboxylic acid, an organic peroxide, a filler, and the like. 
The base rubber can be natural rubber or synthetic rubber which has been 
conventionally used for solid golf balls. Preferred is high-cis 
polybutadiene rubber containing not less than 40 molar %, preferably not 
less than 80 molar % of a cis-1,4 bond. The base rubber can be mixed with 
natural rubber, polyisoprene rubber, styrene-butadiene rubber, EPDM, and 
the like. 
The metal salt of the unsaturated carboxylic acid, which acts as a 
co-crosslinking agent, includes mono or divalent metal salts, such as zinc 
and magnesium salts of .alpha.,.beta.-unsaturated carboxylic acids having 
3 to 8 carbon atoms (e.g. acrylic acid, methacrylic acid, etc.). Preferred 
co-crosslinking agent is zinc acrylate because it imparts high rebound 
characteristics to the resulting golf ball. An amount of the metal salt of 
the unsaturated carboxylic acid in the rubber composition is preferably 
from 10 to 50 parts by weight, based on 100 parts by weight of the base 
rubber. When the amount of the metal salt of the unsaturated carboxylic 
acid is larger than 50 parts by weight, the core is too hard. Therefore, 
the shot feel is poor. On the other hand, when the amount of the metal 
salt of the unsaturated carboxylic acid is smaller than 10 parts by 
weight, the core is soft. Therefore, the rebound characteristics are 
degraded to reduce flight distance. 
The organic peroxide, which acts as the crosslinking agent or curing agent, 
includes for example dicumyl peroxide, 1,1-bis 
(t-butylperoxy)-3,3,5-trimethylcyclohexane, 
2,5-dimethyl-2,5-di(t-butylperoxy)-hexane, di-t-butyl peroxide and the 
like. Preferred organic peroxide is dicumyl peroxide. An amount of the 
organic peroxide is not limited, but is preferably from 0.5 to 5 parts by 
weight based on 100 parts by weight of the base rubber. When the amount of 
the organic peroxide is smaller than 0.5 parts by weight, the core is too 
soft. Therefore the rebound characteristics is degraded to reduce flight 
distance. On the other hand, when the amount of the organic peroxide is 
larger than 5 parts by weight, the core is too hard. Therefore the shot 
feel is poor. 
The filler, which can be one used for a core of golf ball, includes for 
example, inorganic filler (such as zinc oxide, barium sulfate, calcium 
carbonate and the like), high specific gravity filler. (such as powdered 
tungsten, powdered molybdenum, and the like), and the mixture thereof. An 
amount of the filler is not limited and can vary depending on the specific 
gravity and size of the core and cover, but is preferably from 5 to 50 
parts by weight based on 100 parts by weight of the base rubber. When the 
amount of the filler is smaller than 5 parts by weight, the core is too 
light. Therefore, the resulting golf ball is too light. On the other hand, 
when the amount of the filler is larger than 50 parts by weight, the core 
is too heavy. Therefore, the resulting golf ball is too heavy. 
The rubber composition can contain other components which have been 
conventionally used for preparing the core of solid golf balls, such as 
antioxidants or peptizing agents. 
In the present invention, the core obtained above is covered with a cover. 
The cover is formed from a base resin mainly containing a mixture of an 
ionomer and polybutadiene having trans-1,4-structure of at least 60%. The 
ionomer resin used in the present invention is not limited, but includes a 
copolymer of an .alpha.-olefin and an .alpha.,.beta.-unsaturated 
carboxylic acid, of which a portion of the carboxylic acid groups is 
neutralized with a metal ion. As the .alpha.-olefins, ethylene and 
propylene are preferable. As the .alpha.,.beta.-unsaturated carboxylic 
acid described above, acrylic acid, methacrylic acid, and the like are 
preferable. As the metal ion which neutralizes a portion of the carboxylic 
acid groups of a copolymer of an .alpha.-olefin and an 
.alpha.,.beta.-unsaturated carboxylic acid, alkaline metal ions, such as 
sodium ions, potassium ions, lithium ions and the like; divalent metal 
ions, such as zinc ions, calcium ions, magnesium ions, and the like; 
trivalent metal ions, such as aluminum ions, neodymium ions, and the like; 
and the mixture thereof can be used. Sodium ions, zinc ions, lithium ions 
and the like are typically used, because of the rebound characteristics, 
durability and the like. The ionomer resin is not limited, but examples 
thereof will be shown by a trade name thereof. Examples of the ionomer 
resin, which is commercially available from Mitsui Du Pont Polychemical 
Co., include Hi-milan 1557, Hi-milan 1605, Hi-milan 1705, Hi-milan 1706, 
Hi-milan 1707, Hi-milan 1855 and Hi-milan 1856. Examples of the ionomer 
resin, which is commercially available from Exxon Chemical Co., include 
Iotek 7010 and Iotek 8000. These ionomer resins are used alone or in 
combination thereof. 
The polybutadiene described above is a polymer which is formed by the 
polymerization of a butadiene as a monomer. Both 1,4-addition and 
1,2-addition are produced by the addition polymerization of butadiene, and 
both the cis-1,4-structure and the trans-1,4-structure having the 
following formulae: 
##STR1## 
are formed among the 1,4-addition. It is not very necessary to consider 
the 1,2-addition, since the 1,4-addition occurs more preferentially than 
the 1,2-addition. However, various products are obtained depending on the 
type of catalyst selected for he polymerization, the method of 
polymerization utilized, and the like, between cis-1,4-structure and 
trans-1,4-structure. In the present invention, particularly the 
polybutadiene having a trans-1,4-structure of at least 60%, preferably 75 
to 95% is used. The polybutadiene which has been conventionally used for 
preparing solid golf balls has the cis-1,4-structure much more than the 
trans-1,4-structure, and has the trans-1,4-structure in an amount of less 
than 2%. It is unexpected that the polybutadiene which has the 
trans-1,4-structure in a much higher amount than the trans-1,4-structure 
can be used for the cover of golf balls. When polybutadiene having a 
trans-1,4-structure is used in an amount of less than 60%, the hardness is 
too soft. Therefore, rebound characteristics is degraded. Polybutadiene 
having a trans-1,4-structure of at least 60% according to the present 
invention has a molecular weight of preferably 30,000 to 300,000, more 
preferably 50,000 to 200,000. When the molecular weight is less than 
30,000, the physical properties, particularly the rebound characteristics 
is degraded. When the molecular weight is more than 300,000, the 
durability is degraded. Therefore, the processability, moldability and 
workability of the golf balls are degraded. Suitable examples of the 
polybutadiene, which are commercially available, for example, include 
"TP-049" (trade name) which are commercially available from Asahi Chemical 
Industries Co., Ltd. 
In the cover formulation, a weight ratio (A:B) of ionomer resin (A) to 
polybutadiene having a trans-1,4-structure of at least 60% (B) is within 
the range from 99:1 to 50:50, preferably 98:2 to 70:30. When a weight 
ratio of (A) is more than the range, the effect of (B) is not sufficient. 
On the other hand, when a weight ratio (A) is less than this range, the 
rebound characteristics and durability are degraded from the level of (A). 
Further, inorganic fillers and pigments for coloring can be optionally 
added to the cover composition, in addition to the resin component such as 
ionomer resin, etc. 
The cover used in the present invention may optionally contain fillers 
(such as barium sulfate, etc.), pigments (such as titanium dioxide, etc.), 
and the other additives such as a UV absorber, a photostabilizer and a 
fluorescent agent or a fluorescent brightener, etc., in addition to the 
resin component, as long as the addition of the additives does not 
deteriorate the desired performance of the golf ball cover, but an amount 
of the colorant is preferably from 0.1 to 0.5 parts by weight. 
The cover layer of the present invention is formed by a conventional method 
for forming a golf ball cover well known in the art, such as injection 
molding, pressure molding and the like. A thickness of the cover is 
preferably 1.0 to 2.5 mm. When the thickness is less than 1.0 mm, the 
cover is too thin to exhibit a properties of the cover. When the thickness 
is more than 2.5 mm, the hardness of the resulting golf ball is too large. 
Therefore, controllability and shot feel are degraded. When forming cover, 
many depressions called "dimples" are generally formed on the cover. The 
golf ball having a dimpled cover is then coated with paint to finish for 
serving commercial sell.

EXAMPLES 
The following Examples and Comparative Examples further illustrate the 
present invention in detail but are not to be construed to limit the scope 
thereof. 
Production of solid cores 
A rubber composition for core shown in the formulation of Table 1 was 
kneaded and then was vulcanized under pressure at 150.degree. C. for 30 
minutes to obtain spherical solid cores having the diameter of 39.0 mm. 
TABLE 1 
______________________________________ 
Kind Parts by weight 
______________________________________ 
BR11 *1 100 
Zinc acrylate 30 
Zinc oxide 20 
Dicumyl peroxide 
1.0 
Antioxidant *2 0.5 
______________________________________ 
*1: Polybutadiene (trade name "BR11") from Japan Synthetic Rubber Co., 
Ltd. 
*2: Trade name "Yoshinox 425" from Yoshitomi 
Pharmaceutical Inds., Ltd. 
Preparation of cover composition 
The materials for cover shown in Table 2 were extruded using a kneading 
type twin-screw extruder to obtain pelletized cover compositions. 
TABLE 2 
______________________________________ 
Comparative 
Example Example 
Kind 1 2 3 1 2 
______________________________________ 
Hi-milan 1605 *3 
45 35 25 50 20 
Hi-milan 1706 *4 
45 35 25 50 20 
Hi-milan 1855 *5 
-- 25 30 -- 60 
TP-049 *6 10 5 20 -- -- 
Titanium dioxide 
2 2 2 2 2 
______________________________________ 
*3: Himilan 1605 (trade name), ethylenemethacrylic acid copolymer ionomer 
resin obtained by neutralizing with sodium ion, manufactured by Mitsui Du 
Pont Polychemical Co., Ltd. 
*4: Himilan 1706 (trade name), ethylenemethacrylic acid copolymer ionomer 
resin obtained by neutralizing with zinc ion, manufactured by Mitsui Du 
Pont Polychemical Co., Ltd. 
*5: Himilan 1855 (trade name), ethylenebutyl acrylatemethacrylic acid 
terpolymer ionomer resin obtained by neutralizing with zinc ion, 
manufactured by Mitsui Du Pont Polychemical Co., Ltd. 
*6: TP049 (trade name), polybutadiene, manufactured by Asahi Chemical 
Industries Co., Ltd., average molecular weight = 80,000 to 130,000, 
content of trans1,4-structure = 87% 
Examples 1 to 3 and Comparative Examples 1 to 2 
The core obtained above was directly covered by injection molding the cover 
composition to obtain golf balls having a weight of 45.4 g. 
With respect to the resulting golf balls, ball compression (compression), 
coefficient of restitution, shot feel and controllability at approach shot 
were determined, and the results were shown in Table 3. The test method is 
as follows. 
Test method 
(1) Ball compression 
The ball compression of golf balls was measured by the ATTI measuring 
equipment. 
(2) Coefficient of restitution 
A metal cylinder having weight of 198.4 g struck against the golf ball, and 
the velocity of the cylinder and golf ball before and after strike were 
measured. The coefficient of restitution was calculated from the velocity 
and the weight of the cylinder and golf ball. 
(3) Shot feel and controllability at approach shot 
The resulting golf ball were evaluated by 10 top professional and top 
amateur golfers according to practical hitting test. The evaluation 
criteria are as follows. 
Evaluation criteria 
o: Not less than 8 out of 10 golfers felt that the golf ball is good shot 
feel and spin is readily applied to stop ball easily. 
.DELTA.: 7 to 4 out of 10 golfers felt that the golf ball is good shot feel 
and spin is readily applied to stop ball easily. 
x: Not more than 3 out of 10 golfers felt that the golf ball is good shot 
feel and spin is readily applied to stop ball easily. 
TABLE 3 
______________________________________ 
Comparative 
Example example 
Item 1 2 3 1 2 
______________________________________ 
Compression (PGA) 
97 97 96 99 97 
Coefficient of 
99 98 98 100 95 
restitution 
Shot feel .largecircle. 
.largecircle. 
.largecircle. 
X .DELTA. 
Controllability 
.largecircle. 
.largecircle. 
.largecircle. 
X .DELTA. 
at approach shot 
______________________________________ 
As is apparent from the physical properties of the golf balls of Examples 1 
to 3 are comparative with those of the golf balls of Comparative Examples 
1 to 2 shown in Table 3, the golf balls of the present invention have 
excellent shot feel and excellent controllability at approach shot without 
degrading coefficient of restitution. 
In a golf ball comprising a core and a cover formed on the core, the golf 
ball, which the cover is formed from a base resin mainly containing the 
mixture of an ionomer and polybutadiene having trans-1,4-structure of at 
least 60%, has excellent rebound characteristics, excellent shot feel and 
good controllability at approach shot.