Rubber composition for golf balls

The present invention provides a rubber composition having high impact residence, preferred processability and workability which may be used for the manufacture of one-piece solid golf ball, solid core of multi-layered solid golf ball, solid center of thread-wound golf ball, etc. The rubber composition is characterized by that a rubber component is made from a mixture of: (1) 60 to 95 wt % of a polybutadiene containing at least more than 40% of cis-1,4 bond and having a Mooney viscosity [ML.sub.1+4 (100.degree. C.)] of 50 to 70, obtained by using lanthanum rare earth-based catalyst, nickel-based catalyst or cobalt-based catalyst, or a mixture thereof with another polybutadiene obtained by using a catalyst other than the above mentioned one, and (2) 5 to 40 wt % of a polyisoprene containing at least more than 90% of cis-1,4 bond and a Mooney viscosity of 70 to 90.

FIELD OF THE INVENTION 
The present invention relates to a rubber composition for golf balls. In 
particular, the present invention relates to a rubber composition used for 
manufacture of one-piece solid golf balls, solid cores of multi-layered 
solid golf balls (such as, two-piece solid golf balls and three-piece 
solid golf balls etc.) and solid centers of thread-wound golf balls. 
BACKGROUND OF THE INVENTION 
Conventionally, as a rubber component for a rubber composition used for 
manufacture of one-piece solid golf balls, solid cores of multi-layered 
solid golf balls (such as, two-piece solid golf balls, three-piece solid 
golf balls) or solid centers of thread-wound golf balls, a polybutadiene 
containing more than 40% of cis-1, 4 bond and having a Mooney viscosity 
[ML.sub.1+4 (100.degree. C.)] of less than 50, obtained by using lanthanum 
rare earth-based catalyst, nickel-based catalyst or cobalt-based catalyst, 
has been employed. This is because the polybutadiene has good workability 
in mixing operation using a roller, a kneader or a banbury mixer (for 
instance, U.S. Pat. No. 4,955,613, Japanese Kokai Publication Hei 2-80068, 
Japanese Kokai Publication Hei 4-109971, Japanese Kokai Publication Hei 
3-151985, Japanese Kokai Publication Hei 4-73072, U.S. Pat. 5,215,308 
etc.). 
However, when the polybutadiene of low viscosity is used, there has been 
such problem that high impact resilience is difficult to obtain and 
consequently the golf ball with higher initial speed or higher flying 
distance can not be obtained. 
Thus, in order to obtain a rubber composition which provides high impact 
resilience, it has been proposed to use a rubber having higher Mooney 
viscosity or higher molecular weight (for instance, Japanese Kokai 
Publication Sho 63-275356, Japanese Kokai Publication Sho 62-89750, 
Japanese Kokai Publication Hei 3-106380, Japanese Kokai Publication Hei 
3-151985, etc.). 
However, the use of the rubber having higher viscosity or higher molecular 
weight adversely gives rise to such a problem as deterioration of 
processability or workability. Consequently, to prevent the deterioration 
of processability or workability, it is also proposed to blend a rubber 
having lower Mooney viscosity with the rubber with higher Mooney viscosity 
or to blend a liquid rubber with the rubber with higher Mooney viscosity 
(for instance, Japanese Kokai Publication Hei 4-73072, U.S. Pat. No. 
5,215,308). 
These rubber compositions for golf balls are mixed by a roller, a kneader, 
a banbury mixer, etc. and then extruded by an extruder, followed by 
cutting to an appropriate size to obtain plugs for compression molding, 
which is used for one-piece solid golf balls, solid cores of two-piece 
golf balls, solid cores of three-piece golf balls, solid centers of 
thread-wound golf balls, etc. 
For instance, in case of a rubber composition used for producing one-piece 
solid golf balls, it is made into cylindrical plugs having a diameter of 
about 32.+-.3 mm and a weight of 48.+-.2 g. In case of a rubber 
composition used for producing solid cores of two-piece solid golf balls, 
it is made into cylindrical plugs of about 29.+-.2 mm in diameter and 
38.+-.2 g. In case of a rubber composition used for producing solid cores 
of three-piece solid golf balls or for producing solid centers of 
thread-wound golf balls, it is made into plugs with elliptic oval shape 
having about 38 mm in length, about 23 mm in width and about 21 mm in 
thickness. 
The plugs thus obtained are usually dipped in a solution of an 
anti-sticking agent so that the plugs do not stick with each other and 
after drying, they are aged for about 8 to 48 hrs. The plugs are then 
charged in the respective metal mold and press-molded under 
heat-compression. 
However, if a polybutadiene with higher Mooney viscosity is used in this 
process, the resulting rubber composition has poor mixing ability and poor 
roll workability, and provides plugs having rough surface when extruded, 
often showing a surface like a pinecone. When press-molded after dipping 
the plugs in the solution of anti-sticking agent, the anti-sticking agent 
enters into a gap formed before vulcanization and remains there. When the 
plugs are press-molded, the gap does not close and results in small 
creases on the surface. In the worst case, it gives cracks which cause 
breaking of golf balls. 
Besides, in order to stabilize vulcanization, it is necessary to conduct 
storage the plugs after kneading and extruding and before press-molding, 
wherein the plugs have to be stored in a dehumidified storage chamber at a 
constant temperature usually for about 8 to 48 hrs, as mentioned above. 
However, the cis-polybutadiene tends to cause cold flow and during 
storage, the plugs may collapse and deform from the shape before storage, 
resulting in deterioration of workability when press-molding. 
SUMMARY OF THE INVENTION 
The objective of the present invention is to solve such problems as above 
which are inherent to the process of prior art and to provide a rubber 
composition for golf balls, having improved processability and workability 
and providing higher impact resilience. Particularly, the rubber 
composition of the present invention also provides good extrusion 
characteristics and anti-cold flow characteristics which have not been 
much attended so far. 
The present invention provides a rubber composition for golf balls 
characterized by that a rubber component of the rubber composition is made 
from a mixture of: 
(1) 60 to 95 wt % of a polybutadiene containing at least more than 40% of 
cis-1,4 bond and having a Mooney viscosity [ML.sub.1+4 (100.degree. C.)] 
of 50 to 70, obtained by using lanthanum rare earth-based catalyst, 
nickel-based catalyst or cobalt-based catalyst, or a mixture thereof with 
another polybutadiene obtained by using a catalyst other than the above 
mentioned one, and 
(2) 5 to 40% wt % of a polyisoprene containing at least more than 90% of 
cis-1,4 bond and a Mooney viscosity [ML.sub.1+4 (100.degree. C.)] of 70 to 
90. 
In other words, in the case of the rubber composition of the present 
invention, a high impact resilience is secured by using the polybutadiene 
(1) which contains more than at least 40% of cis-1,4 bond and has Mooney 
viscosity [ML.sub.1+4 (100.degree. C.)] of 50 to 70 which is higher than 
those of the conventional compositions, at such high ratio as 60 to 95 wt 
%. And the problems of inferior processability and workability at the time 
of kneading, extrusion and press-molding and inferior anti-cold flow 
property during storage due to the use of polybutadiene of high Mooney 
viscosity are solved by co-using the polyisoprene (2) which contains more 
than at least 90% of cis-1,4 bond and has Mooney viscosity [ML.sub.1+4 
(100.degree. C.)] of 70 to 90 at the specific ratio of 5 to 40 wt %. 
A point where the rubber composition of the present invention is 
significantly different from the conventional rubber composition used for 
golf balls is that, with the conventional rubber composition, a 
polybutadiene having lower Mooney viscosity or a liquid rubber having 
lower viscosity is blended into the polybutadiene having higher Mooney 
viscosity, whereas in case of the rubber composition of the present 
invention, the polybutadiene having higher Mooney viscosity is blended 
with the polyisoprene which has Mooney viscosity even higher than the 
polybutadiene, and thereby the processability, workability and anti-cold 
flow property of the rubber composition are significantly improved without 
deteriorating high impact resilience thereof.

DETAILED DESCRIPTION OF THE INVENTION 
The rubber composition of the present invention may be used for manufacture 
of one-piece solid golf balls, solid cores of multi-layered solid golf 
balls (such as, two-piece solid golf ball, three-piece solid golf ball 
etc.) or solid centers of thread-wound golf balls. The composition may 
generally comprises, in addition to the above rubber component, a 
crosslinking agent, a filler, a peroxide initiator and if necessary an 
antioxidant. The crosslinking agent may be a metal salt of an 
.alpha.,.beta.-ethylenically unsaturated carboxylic acid or a combination 
of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid and a metal 
oxide (e.g. zinc oxide). If the composition is vulcanized by way of 
sulfur, the rubber composition may comprises sulfur and a vulcanization 
accelerator, and if necessary an auxiliary vulcanization accelerator, 
instead of the co-crosslinking agent and the peroxide. 
In the present invention, it is necessary that the polybutadiene (1) 
contains more than 40% of cis-1,4 bond, preferably more than 80% of 
cis-1,4 bond and that Mooney viscosity [ML.sub.1+4 (100.degree. C.)] 
thereof is 50 to 70, preferably 55 to 65. 
When the content of cis-1,4 bond is less than 40%, high impact resilience 
is not obtained and also when Mooney viscosity is less than 50, high 
impact resilience is not obtained, while when Mooney viscosity is higher 
than 70, processability and workability deteriorate and such problem can 
not be sufficiently eliminated even when the polyisoprene (2) is co-used. 
The polybutadiene (1) is obtained by using lanthanum rare earth-based 
catalyst, nickel-based catalyst or cobalt-based catalyst. The 
polybutadiene obtained by using nickel-based catalyst or cobalt-based 
catalyst may be those conventionally used in this field and it is not 
required to be a special product. 
The polybutadiene obtained by using lanthanum rare earth-based catalyst is 
usually synthesized by polymerizing butadiene in the presence of a 
catalyst comprising a combination of a lanthanum rare earth-compound, an 
organic aluminum compound, a Lewis base and, upon necessity, a Lewis acid. 
The lanthanum rare earth-compound may be a compound containing a rare 
earth atom (atom number of 57 to 71), but particularly preferred is a 
neodymium compound. 
In the polymerization, solvent may be used or bulk polymerization may be 
conducted without using the solvent. A polymerization temperature may be 
normally -30.degree. to 150.degree. C., preferably 10.degree. to 
80.degree. C. and a polymerization pressure may be changed properly 
according to other conditions. 
The concrete examples of polybutadiene (1) containing at least more than 
40% of cis-1,4 bond and having a Mooney viscosity [ML.sub.1+4 (100.degree. 
C.)] of 50 to 70 may be, for example, "Butene 1207" (tradename) 
manufactured by Goodyear Chemical Corp., "Buna CB22" (tradename) or "Buna 
CB23" (tradename) manufactured by Bayer A.G., prototype product of Japan 
Synthetic Rubber Co. (super-high molecular weight polybutadiene with 
Mooney viscosity [ML.sub.1+4 (100.degree. C.)] of 60 obtained by 
laboratory synthesis using nickel-based catalyst, method of polymerization 
being the same as that for JSR BR11 (tradename Hi-cis polybutadiene), and 
an average molecular weight being 15.times.10.sup.4) etc. 
In the present invention, the cis-1,4-polybutadiene (1) is used with the 
polyisoprene (2). The polyisoprene (2) is so-called cis-polyisoprene and 
it is required to contain at least more than 90% of cis-1,4 bond and to 
have Mooney viscosity [ML.sub.1+4 (100.degree. C.)] of 70 to 90. When a 
Mooney viscosity is lower than 70, impact resilience decreases, roll 
workability deteriorates and anti-cold flow property deteriorates whereas 
when it is higher than 90, blending with cis-1,4-polybutadiene (1) becomes 
difficult and inferior dispersion is resulted. 
The concrete example of the polyisoprene containing more than 90% of 
cis-1,4-bond and having a Mooney viscosity [ML.sub.1+4 (100.degree. C.)] 
of 70 to 90 may be, for example, "IR-2200" (tradename) manufactured by 
Nippon Zeon Co., Ltd., "JSR IR-2200" (tradename) of Japan Synthetic Rubber 
Co., "KURAPRENE IR-10" (tradename) manufactured by Kuraray Co., Ltd. etc. 
As for the mixture of polybutadiene (1) and the polyisoprene (2) of the 
rubber composition of the present invention, the ratio of the two 
components is required to be 60 to 95 wt % for polybutadiene (1) and 5 to 
40 wt % for polyisoprene 2), and particularly it is preferred to be 75 to 
95 wt % for polybutadiene (1) and 5 to 25 wt % for polyisoprene (2). 
When the ratio of the polybutadiene (1) is less than 60%, impact resilience 
is not sufficient and consequently initial velocity, flying distance etc. 
of the ball are insufficient whereas when the ratio of the polybutadiene 
(1) is more than 95 wt %, processability and workability at the time of 
kneading, extrusion, press-molding etc. and anti-cold flow property during 
storage become inferior. 
In the present invention, the mixture of polybutadiene (1) and polyisoprene 
(2) is used as the rubber component as aforesaid, but it is unnecessary to 
blend them beforehand. They may be mixed at the specified blending ratio 
at the time of preparation of the rubber composition. In case where the 
polybutadiene (1) is a mixture of the polybutadiene prepared by using the 
specific catalyst and another polybutadiene, they may be mixed at the 
specific blending ratio at the time of preparation of the rubber 
composition, as mentioned above. 
The rubber composition of the present invention may be used, as mentioned 
above, for manufacture of one piece solid golf balls, for manufacture of 
solid cores of multi-layered solid golf balls (such as, two-piece solid 
golf balls, three-piece solid golf balls) or for manufacture of solid 
centers of thread-wound golf balls. 
In the preparation of the golf ball, vulcanization method to be employed 
may be crosslinked by way of .alpha.,.beta.-ethylenically unsaturated 
carboxylic acid-based crosslinking agent or by way of sulfur. 
Vulcanization by .alpha.,.beta.-ethylenically unsaturated carboxylic 
acid-based crosslinking agent may be applied to the manufacture of 
one-piece solid golf ball, the manufacture of the solid core of 
multi-layered solid golf ball (such as, two piece solid golf ball, 
three-piece solid golf ball etc.) or to the manufacture of the solid 
center of thread-wound golf ball. 
Vulcanization by sulfur is suitable for the manufacture of the solid center 
of thread-wound golf ball or the manufacture of the solid core of 
three-piece solid golf ball. 
When the .alpha.,.beta.-ethylenically unsaturated carboxylic acid-based 
crosslinking agent is used, a metal salt of .alpha.,.beta.-ethylenically 
unsaturated carboxylic acid may be directly formulated into the rubber 
composition. Or an .alpha.,.beta.-ethylenically unsaturated carboxylic 
acid and a metal oxide (e.g. zinc oxide) may be formulated into the rubber 
composition and the metal salt of the .alpha.,.beta.-ethylenically 
unsaturated carboxylic acid may be prepared in the rubber composition by 
the reaction thereof during the preparation of the rubber composition. 
The .alpha.,.beta.-ethylenically unsaturated carboxylic acid includes 
acrylic acid, methacrylic acid etc., and the metal salt thereof includes 
zinc acrylate and zinc methacrylate. For the metal oxide to be used in the 
production of metal salt of said .alpha.,.beta.-ethylenically unsaturated 
carboxylic acid from .alpha.,.beta.-ethylenically unsaturated carboxylic 
acid and metal oxide during the preparation of rubber composition, zinc 
oxide is preferred. 
When the .alpha.,.beta.-ethylenically unsaturated carboxylic acid based 
crosslinking agent is used, a peroxide is used as an initiator. Examples 
of the peroxides are dicumyl peroxide, 
1,1-di-t-butylperoxy-3,3-5-trimethylcyclohexane, 
2,5-dimethyl-2,5-di-(t-buthylperoxy)hexane, 1,3-bis 
(t-butylperoxy-isopropyl)benzene, etc. 
When vulcanization is conducted by way of sulfur, a vulcanization 
accelerator may be formulated into the rubber composition. Upon necessity, 
an auxiliary vulcanization accelerator may also be formulated. The 
vulcanization accelerator may be, for example, CZ 
(N-cyclohexyl-2-benzothiazylsulfeneamide), TT (tetramethylthiuram 
disulfide), TS (tetramethylthiuram monosulfide), MOR 
(N-oxydiethylene-2-benzothiazolylsulfeneamide) etc. and the auxiliary 
vulcanization accelerator may be stearic acid, zinc stearate etc. 
Whether it is crosslinked by .alpha.,.beta.-ethylenically unsaturated 
carboxylic acid-based crosslinking agent or vulcanized by sulfur, filler 
is formulated into the rubber composition. The filler may be zinc oxide, 
barium sulfate, calcium carbonate, silica, etc. 
An amount of these primary components is not particularly restricted but 
the preferred ratio may be for 100 wt parts of the rubber component 
composed of the mixture of the polybutadiene (1) and polyisoprene (2), 5 
to 60 wt parts of metal salt of .alpha.,.beta.-ethylenically unsaturated 
carboxylic acid (or 5 to 60 wt parts of .alpha.,.beta.-ethylenically 
unsaturated carboxylic acid and 5 to 60 wt parts of metal oxide), 2 to 300 
wt parts of filler and 0.1 to 10 wt parts of peroxide. 
When sulfur vulcanization is conducted, it is preferred to formulate 1 to 
30 wt parts of sulfur and 0.1 to 5 wt parts of vulcanization accelerator, 
in place of the metal salt of .alpha.,.beta.-ethylenically unsaturated 
carboxylic acid (or .alpha.,.beta.-ethylenically unsaturated carboxylic 
acid and metal oxide) and peroxide. 
Using the rubber composition of the present invention, a solid core of a 
two-piece solid golf ball or a three- piece solid golf ball is prepared. A 
solid center of a thread-wound golf ball is formed and then rubber thread 
is wound around the solid center to form a thread-wound core. It is 
necessary to cover the core with a cover. The cover generally is formed 
from ionomer resin. 
A thickness of the cover is properly determined and it is not particularly 
restricted but it is preferred to be in the range of 0.5 to 3 mm. When the 
thread-wound core is covered, the material mainly made of balata 
(transpolyisoprene) may be used for the cover material. 
The manufacture of one-piece solid golf ball, solid core of multi-layered 
solid golf ball such as two-piece solid golf ball, three piece solid golf 
ball etc. solid center of thread-wound golf ball using the rubber 
composition of the present invention may be conducted in the same manner 
as in the conventional method. 
To describe the method taking the solid core of two-piece solid golf ball 
as an example, the material with the composition suitable for the solid 
core of two-piece golf ball is mixed by a mixing machine such as a banbury 
mixer, a kneader, a roller, etc. to prepare a rubber composition. The 
resulting composition is extruded in rod form by an extruder, and the 
extruded molding is cut to obtain plugs or alternatively the composition 
is spread into a thick board and then is stamped out to produce plugs. 
The obtained plugs are put in a metal mold for a core and press-molded 
under heat-compression. The temperature used at this time is usually 
135.degree. to 180.degree. C. and duration of compression is preferred to 
be 10 to 50 minutes. 
The pressure is sufficient if the metal mold does not open during 
press-molding. It is also possible to form the mixed rubber composition 
into a thin and narrow ribbon form which can be molded into cores by 
injection molding. 
By press-molding or by injection molding under heat compression, the rubber 
composition is vulcanized to obtain elasticity. When the 
.alpha.,.beta.-ethylenically unsaturated carboxylic acid-based 
crosslinking agent is used, no crosslink by sulfur actually occurs and 
therefore the correct expression is crosslinking but in this 
specification, the case of crosslinking by .alpha.,.beta.-ethylenically 
unsaturated carboxylic acid-based crosslinking agent is sometimes called 
vulcanization according to customary practice. 
The method to cover the core with a cover is not particularly limited but 
it may be the method to envelop the core with a pair of cover material 
each formed into a semi-spherical shell beforehand and the two half shells 
are made into one piece by heat-compression molding or made into one piece 
by directly injection molding the cover material around the core. 
To blend polyisoprene into polybutadiene has not necessarily been totally 
unreported in a literature but the practices of such blending reported 
have been for other purposes or for the purposes undefined or they lack 
sometimes the detailed description of Mooney viscosity. For instance, the 
Examples of Japanese Kokai Publication Hei 4-10997 introduces the blending 
of Butadiene BR-01 (tradename) manufactured by Japan Synthetic Rubber Co. 
with Mooney viscosity [ML.sub.1+4 (100.degree. C.)] of 43 and Polyisoprene 
Natsyn 2200 (tradename) manufactured by Goodyear Tire Co. with Mooney 
viscosity [ML.sub.1+4 (100.degree. C.)] of 82 but this Example is for the 
objective of obtaining the solid golf ball having both a specific feeling 
and longer flying distance which is different from the objective of the 
present invention and the range of viscosity is also different. 
Japanese Kokai Publication Hei 2-297384 introduces an example wherein a 
mixture of polybutadiene with unknown Mooney viscosity and polyisoprene 
with unknown Mooney viscosity is formulated with metal salt of unsaturated 
carboxylic acid, organic sulfur compound and/or organic sulfur compound 
containing metal but this example aims mainly at an improvement of initial 
speed of the ball (i.e., improvement of impact resilience) by formulating 
the sulfur compound and besides no reference is made to the Mooney 
viscosity of the rubber to be used. 
Japanese Kokai Publication Sho 63-212377, Japanese Kokai Publication Sho 
63-2200889 etc. also propose to properly blend polybutadiene with 
polyisoprene, natural rubber (natural polyisoprene) or styrene-butadiene 
etc. However, these proposals do not clearly indicate the purpose of 
blending of natural rubber etc. and besides they lack the description of 
Mooney viscosity. Usually when natural rubber is used, it is used after 
reducing the Mooney viscosity to around 50 by a softening agent as the 
processability of natural rubber itself is inferior. 
As the result as indicated in the Comparative Example 5, the workability is 
adversely affected or anti-cold flow property deteriorates. 
Rubber composition of the present invention is the material wherein, as 
described above, polybutadiene with high Mooney viscosity is blended with 
polyisoprene having even higher Mooney viscosity and thus while 
maintaining a high impact resilience, its workability, processability, 
anti-cold flow property are improved and thus the concept is essentially 
different from the aforesaid prior arts. 
EXAMPLES 
The present invention is described in reference to the Examples. However, 
the present invention is not restricted to these Examples. 
Examples 1 to 2 and Comparative Examples 1 to 5 
Rubber compositions having the formulating ratio shown in Tables 1 and 2 
were prepared and then press-molded at 150.degree. C. for 30 minutes to 
form solid cores used for two-piece solid golf balls having an average 
diameter of 38.4 mm. The solid cores were covered with a cover to obtain 
two-piece solid golf balls having an outer diameter of 42.7 mm. 
The cover was made of the blend of two ionomer resins (i.e. Himilan 1605 
(tradename) and Himilan 1705 (tradename) manufactured by DUPONT-MITSU 
POLYCHEMICALS CO., LTD.) at a weight ratio of 50:50, containing 2 wt % of 
titanium oxide (TiO.sub.2). Table 1 shows the composition of Examples 1 
and 2 and Comparative Examples 1 and 2, and Table 2 shows the composition 
of Comparative Examples 3, 4 and 5. The numbers represent amounts of 
formulated materials in Tables 1 and 2 based on weight parts. 
The Mooney viscosities of the rubbers in Tables 1 and 2 are those at 
ML.sub.1+4 (100.degree. C.) and the polybutadiene and polyisoprene used 
are as follows: 
Polybutadiene (Mooney viscosity 60): 
Prototype polybutadiene manufactured by Japan Synthetic Rubber Co. (a 
product obtained by laboratory synthesis using nickel-based catalyst, the 
method of polymerization employed is the same as that for JSR BR11 and 
average molecular weight is 15.times.10.sup.4). 
Polybutadiene (Mooney viscosity 43): 
JSR BRO1 (tradename) manufactured by Japan Synthetic Rubber Co. 
Polyisoprene (Mooney viscosity 82): 
IR-2200 (tradename) manufactured by Nippon Zeon Co. 
Preparation of rubber composition was executed by kneading the components 
with a kneader and the rubber composition obtained was extruded in rod 
form by an extruder, the extruded rod product was cut into plugs. The 
plugs thus obtained were dipped in a solution of anti-sticking agent, 
dried and storaged at 26.degree. C. for 24 hrs. and placed in a metal mold 
for press-molding. 
A die of the extruder used in the extrusion was an elliptical one having a 
length of 35 mm and a width of 15 mm. Since the rubber composition shrunk 
in an extrusion direction at the point where it left the die, the plugs 
obtained had an elliptical shape of 50 mm in length, 28 mm in width and 27 
mm in thickness. Since the plugs had residual orientation in the extrusion 
direction, the dimension of the core after vulcanization came to be 
different depending on the direction of orientation given to the plugs at 
the time of vulcanization and in the case of the rubber composition of 
inferior extrusion processability, the difference in sphericity of the 
core varied greatly. 
The results of investigation of the roll workability, surface texture of 
extruded piece (conditions of the surface of extruded product) and 
anti-cold flow property during storage of the plug are shown in Table 3 
and 4. The evaluation standard of the roll workability, extruded surface 
texture of extruded product and anti-cold flow property is as follows: 
Evaluation Standard of Roll Workability 
Excellent: The product can be rolled with easiness and cutting to size is 
easy. 
Good: Ordinary roll work is possible. 
Fairly good: Bagging or sticking to the roll occurs and operation is 
difficult. 
Poor: Bagging occurs and the composition does not attach on the roller or 
adversely sticks to the roller, which makes it difficult to cut off and 
feed again between the rollers. 
Evaluation Standard of Surface Texture of Extruded Product 
Excellent: Surface texture of the plugs after extrusion is smooth. 
Good: Surface texture of the plug after extrusion is nearly smooth. 
Fairly good: Surface of the plug after extrusion is slightly rough. 
Poor: The surface of the plug after extrusion indicates severe indentation 
just like a pinecone. 
Anti-Cold Flow Property 
Excellent: No deformation when left for 24 hrs. 
Good: Some deformation when left for 24 hrs. but presenting no problem for 
being set into the metal mold. 
Fairly good: Deformation occurs when left for 24 hrs. and the plug can not 
be set in the metal mold unless pushed into it by force. 
Poor: When left for 24 hrs., the plug sticks to the iron plate on which it 
was placed and can not be removed. 
Tables 3 and 4 indicate the results of evaluation of core sphericity, core 
hardness, ball weight, initial speed of ball, distance of carry and total 
distance. The method of measurement of these factors are as follows: 
Core Sphericity: 
A size of the core is measured both in a direction of an orientation 
direction of the rubber composition and in a direction perpendicular to 
the orientation direction, which are along a parting live of the core (a 
parting live of upper and lower molds). The difference between the both 
directions in millimeter is described in the tables. 
Core Compression: 
A difference of deformation at the time when an initial load of 10 kg is 
applied and at the time when an ultimate load of 130 kg is applied is 
measured. When the deformation amount is in 2 to 4 mm, the core 
compression is appropriate and if it is smaller, the core is too hard and 
if it is larger, the core is too soft. 
Initial velocity of Ball: 
Using a swing robot made by True Temper Corp., a ball is hit by No. 1 Wood 
at a head speed of 45 m/sec and its initial velocity is measured. 
Durability of Ball: 
Using a swing robot of True Temper Corp., a ball is hit repeatedly by No. 1 
Wood at a head speed of 45 m/sec. and the number of repetition of hitting 
until crack generates is counted and durability of ball is expressed by an 
index of such count as against the count of hitting in Example 1, which is 
deemed 100. 
Flying Distance: 
Using a swing robot made by True Temper Corp., a ball is hit by No. 1 Wood 
at a head speed of 45 m/sec. and the distance up to a point where the ball 
falls is deemed carry and total distance until the ball stops is deemed 
total distance. 
TABLE 1 
______________________________________ 
Comp. Comp. 
Ex. 1 
Ex. 2 Ex. 1 Ex. 2. 
______________________________________ 
Polybutadiene 90 78 -- 100 
(Mooney viscosity 
60) 
Polybutadiene -- -- 100 -- 
(Mooney viscosity 
43) 
Polyisoprene 10 22 -- -- 
(Mooney viscosity 
82) 
Zinc acrylate 35 35 35 35 
Zinc oxide 22 22 22 22 
Dicumyl peroxide 
2 2 2 2 
Antioxidant 0.5 0.5 0.5 0.5 
Natural rubber 
-- -- -- -- 
(Mooney viscosity 
50) 
______________________________________ 
TABLE 2 
______________________________________ 
Comp. Comp. Comp. 
Ex. 3 Ex. 4 Ex. 5 
______________________________________ 
Polybutadiene -- 50 90 
(Mooney viscosity 60) 
Polybutadiene 90 -- -- 
(Mooney viscosity 43) 
Polyisoprene (Mooney 
10 50 -- 
viscosity 82) 
Zinc acrylate 35 35 35 
Zinc oxide 22 22 22 
Dicumyl peroxide 
2 2 2 
Antioxidant 0.5 0.5 0.5 
Natural rubber -- -- 10 
(Mooney viscosity 50) 
______________________________________ 
TABLE 3 
______________________________________ 
Example 
Example Comp. Comp. 
1 2 Ex. 1 Ex. 2 
______________________________________ 
Roll workability 
Excellent 
Excellent Excellent 
Good- 
Fairly 
good 
Extruded surface 
Excellent 
Excellent Excellent 
Fairly 
good 
Anti-cold flow 
Excellent 
Excellent Fairly Good- 
property good- Fairly 
Poor good 
Difference in core 
0.095 0.098 0.108 0.107 
sphericity (mm) 
Core compression 
Suitable Suitable Suitable 
Suitable 
Ball weight (g) 
45.5 45.3 45.4 45.5 
Initial velocity 
65.2 65.0 64.6 65.3 
(m/s) 
Durability of ball 
100 102 101 90 
Flying distance 
Carry (m) 202 201 197 203 
Total (m) 225 224 216 225 
______________________________________ 
TABLE 4 
______________________________________ 
Comp. Comp. Comp. 
Ex. 3 Ex. 4 Ex. 5 
______________________________________ 
Roll workability 
Excellent 
Good Fairly 
good 
Extruded surface 
Excellent 
Excellent Good 
Anti-cold flow Fairly Excellent Fairly 
property good- good 
Poor 
Difference in core 
0.097 0.095 0.093 
sphericity (mm) 
Core hardness Suitable Soft Suitable 
Ball weight (g) 45.4 45.3 45.5 
Initial velocity 
64.4 62.7 64.2 
(m/s) 
Durability of ball 
102 73 93 
Flying distance 
Carry (m) 196 190 199 
Total (m) 213 209 217 
______________________________________ 
As is apparent from Table 3, Examples 1 to 3 indicate satisfactory results 
for all factors (i.e. roll workability, surface texture of extruded piece, 
anti-cold flow property) and initial speed and flying distance of ball, 
and flying characteristics are excellent. Durability is comparable to the 
conventional products (Comparative Example 1) and shows no substantial 
decrease of durability. 
Comparative Example 1 corresponds to the conventional product wherein 
polybutadiene of Low Mooney viscosity is used independently for the rubber 
component, but it indicates lower initial speed of ball, shorter flying 
distance, inferior anti-cold flow property and the plug tends to fail to 
maintain the original shape, thus making the workability in press-molding 
inferior. 
In Comparative Example 2, since polybutadiene having higher Mooney 
viscosity is independently used as the rubber component, roll workability 
and surface texture of extrude piece are inferior as shown in Table 3. 
This may cause inclusion of anti-sticking agent during press-molding 
process and provides the deterioration of durability. 
In Comparative Example 3, since polybutadiene having lower Mooney viscosity 
is used as the rubber component, the initial ball velocity and flying 
distance are smaller as shown in Table 4, like Comparative Example 1 and 
anti-cold flow property has neither been improved sufficiently. 
In Comparative Example 4, since a large amount of polyisoprene is used as 
rubber components, compression is smaller and as the result, initial 
velocity and flying distance are smaller. 
Comparative Example 5 was an example where natural rubber (natural 
polyisoprene) was blended, but normally, natural rubber has a Mooney 
viscosity of unmeasurably high and workability is so bad that it can not 
be used as it is and it is hard to be blended with other rubber, thus 
making it impossible to use. It is therefore customary to knead it by 
adding a softening agent etc. to reduce Mooney viscosity [ML.sub.1+4 
(100.degree. C.)] to about 50 to 70 to reduce molecular weight and use. 
Therefore in Comparative Example 5, 0.5 wt part of Noctizer SK (tradename 
of a softening agent manufactured by Ouchi Shinko Chemical Industry Co.) 
was added to 100 wt parts of natural rubber (RSS No.3), and mixed for 10 
minutes by a banbury mixer to obtain a mixture having a Mooney viscosity 
[ML.sub.1+4 (100.degree. C.)] of 50, because of enhancing blending 
properties with polybutadiene. However, natural rubber tends to loose its 
molecular weight as it is blended with other chemicals and therefore it 
produced the same results as the case when polyisoprene having very low 
molecular weight was blended. As it is evident in the results shown in 
Table 4 above, it produced higher sticking properties to the rollers 
during roll work and workability deteriorated. Anti-cold flow property 
also became inferior and both initial speed and flying distance became 
undesirable. 
As described above, in the present invention, by blending the polybutadiene 
having higher Mooney viscosity with polyisoprene having higher Mooney 
viscosity and using such blend as the rubber component, it has been 
possible to significantly improve processability, workability and 
anti-cold flow property while maintaining high impact resilience inherent 
to polybutadiene having higher Mooney viscosity. 
That is, in the present invention by using a combination of the 
polybutadiene (1) and the polyisoprene (2), it is made possible to obtain 
a good mixture of rubber component at the time of preparation of rubber 
composition, improve roll workability accompanied by polybutadiene having 
higher Mooney viscosity and eliminate poor extrusion characteristics 
accompanied by polybutadiene having higher Mooney viscosity, thus 
improving workability at the time of extrusion. The present invention also 
improves smoothness of surface texture of the extruded piece, thereby 
preventing inclusion of foreign matters into the rubber composition at the 
time of press-molding and also improves anti-cold flow property inherent 
to polybutadiene. It is also made possible to prevent deformation of plug 
during storage the plug prior to press-molding, thus improving workability 
at the time of press-molding and improving initial velocity and flying 
distance of the golf ball in view of the high impact resilience of 
polybutadiene having higher Mooney viscosity.