Thread wound golf ball

The present invention provides a thread wound golf ball having excellent durability, heat resistance and impact resilience, which comprises a core, a thread rubber layer and an outer layer. A thread rubber constituting the thread rubber layer comprising a solid rubber containing a high-cis polyisoprene rubber as a main rubber material and a liquid polybutadiene rubber and/or liquid butadiene-isoprene copolymer rubber as a rubber component.

FIELD OF THE INVENTION 
The present invention relates to a thread wound golf ball. More 
particularly, it relates to a thread wound golf ball comprising a thread 
rubber layer having high strength and excellent impact resilience. The 
resulting golf ball has excellent durability, heat resistance and impact 
resilience. 
BACKGROUND OF THE INVENTION 
A thread wound golf ball is a golf ball comprising a core (liquid core or 
solid core), a thread rubber layer formed by winding a thread rubber 
around the core in a stretched state and a cover covering the thread 
rubber layer. 
Generally, it is necessary that the golf ball has suitable impact 
resilience so as to obtain excellent flight performances. Particularly, 
the thread wound golf ball requires a thread rubber having excellent 
impact resilience, because impact resilience and flight performances of 
the ball is extremely influenced by the properties of the thread rubber 
layer. 
A thread rubber having excellent impact resilience can be obtained by 
vulcanizing a rubber blend comprising a low-cis polyisoprene rubber as a 
base rubber, however, the low-cis polyisoprene rubber is inferior in 
mechanical strength and heat resistance. The thread rubber formed from the 
low-cis polyisoprene rubber imparts disadvantage to the resulting golf 
ball. That is, the thread rubber is liable to be damaged on forming the 
thread rubber layer by winding around the core at the stretched state, 
which is the cause of deterioration of durability of the ball. Further, 
heat upon forming the cover damages the thread wound layer, thus 
deteriorating ball compression. 
Therefore, in order to improve the strength of thread rubber, it has been 
proposed to blend natural rubber or synthesized high-cis polyisoprene 
rubber in with the low-cis polyisoprene rubber. However, impact resilience 
of the resulting golf ball adversely declines, although its strength is 
improved. It is therefore difficult to obtain both impact resilience and 
high durability. 
On the other hand, in the case of using a method of preparing the thread 
rubber from a coagulated sheet of a latex blend, a kneading process 
wherein breakage of molecular chain occurs is unnecessary. Therefore, the 
above disadvantage in strength is slightly improved. However strength and 
impact resilience thereof are still insufficient. 
As described above, in the thread wound golf ball, impact resilience of the 
ball is extremely influenced by the properties of the thread rubber 
constituting the thread rubber layer, whereby flight performances of the 
ball are extremely influenced. According to the prior art, when impact 
resilience is increased, strength is lowered and, therefore, the thread 
rubber having both high strength and excellent impact resilience were not 
obtained. 
OBJECTS OF THE INVENTION 
Under these circumstances, the present inventors have intensively studied. 
As a result, it has been found that a thread wound golf ball having 
excellent durability, heat resistance and impact resilience can be 
obtained, by making a thread rubber from a rubber compound comprising a 
solid rubber containing a high-cis polyisoprene rubber and a liquid 
polybutadiene rubber and/or liquid butadiene-isoprene copolymer rubber and 
forming a thread rubber layer with the resulting thread rubber having high 
strength and excellent impact resilience. 
Main object of the present invention is to provide a thread wound golf ball 
comprising a thread rubber layer having high strength and excellent impact 
resilience, which has excellent durability, heat resistance and impact 
resilience. 
This object as well as other objects and advantages of the present 
invention will become apparent to those skilled in the art from the 
following description. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a thread wound golf 
ball comprising a core, a thread rubber layer formed on said core and a 
cover covering said thread rubber layer. The thread rubber constituting 
said thread rubber layer is obtained by vulcanizing a rubber compound 
comprising a solid rubber containing a high-cis polyisoprene rubber and a 
liquid polybutadiene rubber and/or liquid butadiene-isoprene copolymer 
rubber. 
DETAILED DESCRIPTION OF THE INVENTION 
The term "high-cis polyisoprene rubber" used for the thread rubber in the 
present invention means natural rubber or synthesized high-cis 
polyisoprene rubber. The high-cis polyisoprene rubber may be solid in the 
dry state, and it may be used in the state of a solid rubber or in the 
state of a latex. 
The natural rubber may be any one which is normally used in the rubber 
industry, and non-limited examples thereof include RSS#1, RSS#2, Pale 
Crape, Brown Crape and the like. 
Among the natural rubbers, some kinds of natural rubbers are commercially 
available in the state of a latex. The natural rubber latex is not also 
limited to a specific one, but high ammonia type latex containing a solid 
content of about 60% by weight is preferred because its stability on 
formulation of latex is excellent. 
The synthesized high-cis polyisoprene rubber is a rubber having a cis-1,4 
bond of at least 98% which has a fine structure similar to the natural 
rubber, and it can be used in place of the natural rubber. Examples of the 
synthesized high-cis polyisoprene rubber in a solid state include JSR IR 
2200 manufactured by Nippon Gosei Rubber Co., IR-10 manufactured by 
Kuraray Co., Nippol IR-2200 manufactured by Nippon Zeon Co. and the like. 
As the solid rubber component, a low-cis polyisoprene rubber can be added 
in a predetermined amount so as to conduct auxiliary improvement of impact 
resilience, in addition to the high-cis polyisoprene rubber. When the 
level of low-cis polyisoprene rubber added is too great, the strength is 
extremely deteriorated and the improvement of the strength of the thread 
rubber is hindered. Therefore, the amount is preferably not more than 50% 
by weight, particularly not more than 30% by weight, based on the weight 
of the solid rubber. As is apparent from this description, the solid 
rubber comprising a high-cis polyisoprene rubber as a main rubber material 
in the present invention means that the solid rubber component is 
constituted only with the high-cis polyisoprene rubber or that the solid 
rubber component is constituted with the high-cis polyisoprene rubber and 
low-cis polyisoprene rubber. 
The low-cis polyisoprene rubber contains a cis-1,4 bond of about 92%. 
Examples of those which are commercially available in the state of the 
solid rubber include Califlex IR-305 and Califlex IR-309 manufactured by 
Shell Chemical Co. Further, examples of the low-cis polyisoprene rubber 
which is commercially available in the state of the latex include Maxplene 
IR latex manufactured by Sumitomo Seika Co. 
In the present invention, a liquid polybutadiene rubber and/or liquid 
butadiene-isoprene copolymer rubber as the liquid rubber is blended in 
(mixed with) the solid rubber. By blending the liquid rubber, strength and 
balance are obtained in high level, and impact resilience can be improved 
without deterioration of strength. Further, workability on winding of the 
thread rubber is improved by blending the liquid rubber. 
Regarding the liquid polybutadiene rubber or liquid butadiene-isoprene 
rubber, a viscosity-average molecular weight (Mv) is preferably in the 
range of about 10,000 to 80,000, wherein the rubber shows viscous fluidity 
or semisolid state at room temperature (25.degree. C.). When the 
viscosity-average molecular weight exceeds 80,000, it's property becomes 
similar to that of the solid rubber and the effect of improving impact 
resilience is deteriorated. When the viscosity-average molecular weight of 
the liquid rubber is less than 10,000, the rubber is difficult to 
vulcanize and the strength of the thread rubber is deteriorated. 
It is suitable for improving impact resilience of the thread rubber that 
the liquid polybutadiene rubber or liquid butadiene-isoprene rubber 
contains at least 50%, particularly at least 80% of a cis-1,4 bond. 
The weight ratio of butadiene to isoprene in the liquid butadiene-isoprene 
copolymer rubber is preferably 100:0 to 30:70. 
As the commercially available article of the liquid polybutadiene rubber, 
for example, there are Kuraray LIR-300 manufactured by Kuraray Co. 
[viscosity-average molecular weight: 40,000; cis-1,4 bond in butadiene 
component: about 85%] and the like. As the commercially available article 
of the liquid butadiene-isoprene copolymer rubber, for example, there are 
Kuraray LIR-390 [weight ratio of butadiene to isoprene: 90:10; 
viscosity-average molecular weight: 48,000; cis-1,4 bond in butadiene 
component: about 85%], Kuraray LIR-340 [weight ratio of butadiene to 
isoprene: 40:60; viscosity-average molecular weight: 34,000; cis-1,4 bond 
in butadiene component: about 85%] (manufactured by Kuraray Co.) and the 
like. These are suitably used in the present invention. 
These liquid rubbers are not commercially available in the latex state at 
present and, therefore, they must be converted to a latex in the case of 
using the solid rubber in the latex state. The liquid rubber is easily 
converted to the latex by dissolving it in a volatile solvent (e.g. 
n-pentane, n-hexane, n-heptane, toluene, etc.) followed by emulsification 
with an emulsifier, removal of the solvent by flash distillation and a 
further concentration step of centrifugation or ultrafiltration. 
The weight ratio of the solid rubber to liquid rubber is preferably 97:3 to 
70:30. When the amount of the liquid rubber is smaller than the above 
lower limit, the effect for improving winding workability of the thread 
rubber and impact resilience is not sufficiently obtained. When the amount 
of the liquid rubber is larger than the above upper limit, stretch becomes 
too large and winding workability is rather deteriorated. 
The thread rubber can be prepared as follows. Vulcanization additives (e.g. 
vulcanizing agents, vulcanization accelerators, vulcanization auxiliaries, 
etc.), antioxidants, if necessary, small amount of fillers and oils are 
added to the rubber component to form a dry rubber compound or latex 
compound, which is formed into a thin sheet of about 0.15 mm in thickness 
and vulcanized. Then, the vulcanized rubber sheet is cut into thread 
pieces of about 1 to 2 mm in width. 
Thereafter, the thread rubber thus obtained is wound around a core in the 
stretched state to form a thread wound layer which is then covered with a 
cover which generally comprises an ionomer resin or a trans-polyisoprene 
as a base material and then finished it with paint to obtain the thread 
wound golf ball of the present invention. 
As the core, a conventional solid or liquid core can be used. The thickness 
of the thread rubber layer varies depending on a material of the cover and 
diameter of the core, but it is normally 4 to 8 mm.

EXAMPLE 
The following Examples further illustrate the present invention in detail 
but are not to be construed to limit the scope thereof. 
EXAMPLES 1 TO 4 AND COMATIVE EXAMPLES 1 AND 2 
In order to prepare a thread rubber, six kinds of rubber latex blends shown 
in Tables 1 and 2 were firstly prepared. Among these six kinds of rubber 
latex blends, blends 1 to 4 shown in Table 1 are used for preparing thread 
rubbers of the golf balls of Examples 1 to 4, and blends 5 and 6 shown in 
Table 2 are used for preparing thread rubbers of the golf balls of 
Comparative Examples 1 and 2. 
In Tables 1 and 2, the unit of a numerical value of each component is dry 
parts by weight. Regarding latex, a rubber content is shown. Regarding 
accelerator (vulcanization accelerator), sulfur and antioxidant, an active 
component content is shown. The details as to each component are explained 
at the notes attached to Table 2. 
TABLE 1 
______________________________________ 
Blend Blend Blend Blend 
1 2 3 4 
______________________________________ 
Dunlop C-60 (natural rubber 
80 80 70 70 
latex, rubber component: 
60%) *1 
Kuraray LIR-300 (emulsion, 
20 -- 20 -- 
rubber component: 45%) *2 
Kuraray LIR-390 (emulsion, 
-- 20 -- 20 
rubber component: 45%) *3 
Maxplene IR (latex, 
-- -- 10 10 
rubber component: 65%) *4 
Accelerator (emulsion, active 
1 1 1 1 
component: 20%) *5 
Sulfur (dispersion, active 
2 2 2 2 
component: 50%) 
Antioxidant (dispersion, active 
1 1 1 1 
component: 50%) *6 
______________________________________ 
TABLE 2 
______________________________________ 
Blend 5 
Blend 6 
______________________________________ 
Dunlop C-60 (natural rubber latex, 
100 50 
rubber component: 60%) *1 
Kuraray LIR-300 (emulsion, 
-- -- 
rubber component: 45%) *2 
Kuraray LIR-390 (emulsion, 
-- -- 
rubber component: 45%) *3 
Maxplene IR (latex, -- 50 
rubber component: 65%) *4 
Accelerator (emulsion, active 
1 1 
component: 20%) *5 
Sulfur (dispersion, active 
2 2 
component: 50%) 
Antioxidant (dispersion, active 
1 1 
component: 50%) *6 
______________________________________ 
(Note) 
*1: Dunlop C-60 (trade name), natural rubber latex stored by 
adding a large amount of ammonia, manufactured by Malaysia 
Dunlop Estate Co. 
*2: Kuraray LIR-300 (trade name), liquid polybutadiene rubber 
(viscosity-average molecular weight: 40,000; cis-1,4 bond of 
butadiene: about 85%) manufactured by Kuraray Co. 
*3: Kuraray LIR-390 (trade name), liquid butadiene-isoprene 
copolymer rubber (butadiene:isoprene = 90:10 (weight ratio); 
viscosity-average molecular weight: 48,000; cis-1,4 bond of 
butadiene component: about 85%) manufactured by Kuraray Co. 
*4: Maxplene IR (trade name), low-cis isoprene rubber latex, 
manufactured by Sumitomo Seida Co. 
*5: Accelerator, Noxelar 8 (trade name), butylaldehyde-aniliine 
condensate, manufactured by Ohuchi Shinko Kagaku Kogyo Co. 
*6: Antioxidant, Yoshinox 425 (trade name), 2,2'-methylene-bis- 
(4-ethyl-6-t-butylphenol), manufactured by Yoshitomi Seiyaku Co. 
The above latex blends 1 to 6 were placed on an endless belt on which a 
coagulant solution (calcium chloride solution) was applied to coagulate on 
the belt, respectively. The coagulated sheet thus obtained was rinsed with 
water and dried. Then, it was rolled round a drum and placed in a 
vulcanizer to vulcanize at 140.degree. C. for 2 hours. 
Thus, a vulcanized rubber sheet of 350 mm in width, 0.54 mm in thickness 
and about 30 m in length was prepared and the resulting vulcanized rubber 
sheet was cut in pieces of 1.6 mm in width to prepare a thread rubber. 
The thread rubber thus obtained was wound around a solid core of vulcanized 
polybutadiene rubber [outer diameter: 28.5 mm; hardness: 80 (JIS-A); 
weight: 18.2 g ] at the stretched state to form a thread wound core of 
about 40 mm in outer diameter, which was covered with an outer layer 
material of a mixture comprising 100 parts by weight of ionomer resin and 
2 parts by weight of titanium oxide to form an outer layer. After 
pretreatment, paint mark was provided to produce a thread wound golf ball 
of 42.7 mm in outer diameter. 
The ionomer resin used on the formation of the outer layer is a mixture of 
HI-MILAN 1706, HI-MILAN 1605, HI-MILAN 1557 and HI-MILAN 1555 manufactured 
by Mitsui Du Pont Polychemicals Co. (weight ratio=45:40:5:10). The weight 
of the resulting thread wound golf ball was in the range of 45.4 to 45.6 
g. 
Ball properties (compression, initial velocity, durability and heat 
resistance) of the thread wound golf ball thus obtained are shown in 
Tables 3 and 4, together with a kind of the blend of the thread rubber. 
The measuring method of the ball properties are explained in detail at the 
notes attached to Table 4. 
TABLE 3 
______________________________________ 
Blend of Ex. 1 Ex. 2 Ex. 3 Ex. 4 
thread rubber 
Blend 1 Blend 2 Blend 3 
Blend 4 
______________________________________ 
Compression (PGA 
85 85 85 85 
system) 
Initial velocity 
252.3 252.0 252.8 252.5 
(feet/second) *7 
Durability (index) *8 
140 141 134 136 
Heat resistance 
-9 -9 -11 -11 
(compression deteri- 
oration point) *9 
______________________________________ 
TABLE 4 
______________________________________ 
Comp. Comp. 
Ex. 1 Ex. 2 
Blend of thread rubber 
Blend 5 Blend 6 
______________________________________ 
Compression (PGA system) 
85 85 
Initial velocity (feet/second) *7 
248.0 252.5 
Durability (index) *8 
140 100 
Heat resistance (compression 
-9 -18 
deterioration point) *9 
______________________________________ 
(Note) 
*7: It is measured by R & A method. 
*8: A golf ball is allowed to bump at a speed of 45 m/second, 
repeatedly. The number of times at which the ball is broken is 
determined. The number is expressed as an index when the 
number of Comparative Example 2 is made 100. 
*9: A golf ball is aged in an oven at 70.degree. C. for 72 hours. 
Difference between compression after aging and initial 
compression is expressed as a value of PGA system. The minus 
value indicates that compression is deteriorated. 
As is shown in Table 3, regarding the golf balls of Examples 1 to 4, the 
index which indicates durability was large and the initial velocity was 
also large. The golf balls had excellent durability and impact resilience. 
To the contrary, regarding the golf ball of Comparative Example 1 
comprising only a natural rubber as the base component, the index which 
indicates durability was large and durability was excellent, but the 
initial velocity was small and impact resilience was extremely 
deteriorated, as shown in Table 4. Regarding the golf ball of Comparative 
Example 2 comprising a mixture of natural rubber and low-cis polyisoprene 
rubber as the rubber component, the initial velocity was large and impact 
resilience was excellent, but the index which indicates durability was 
small and durability was inferior in comparison with others, as shown in 
Table 4. 
The golf balls of Examples 1 to 4 showed small compression deterioration 
point in comparison with that of Comparative Example 2 and heat resistance 
was excellent. 
As described above, a thread wound golf ball having excellent durability, 
heat resistance and impact resilience can be obtained, by making a thread 
rubber layer from a thread rubber of a blend comprising a solid rubber 
containing a high-cis poyisoprene rubber as a main rubber material and a 
liquid polybutadiene rubber and/or liquid butadiene-isoprene copolymer 
rubber as a rubber component.