Thread wound golf ball

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

Description: The present invention relates to a thread wound golf ball. More particularly, it relates to a thread wound golf ball comprising thread rubber layer having a high strength and an excellent impact resilience. The resulting golf ball has an excellent durability, heat resistance and impact resilience. 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 a suitable impact resilience so as to obtain an excellent flight performance. Particularly, the thread wound golf ball requires a thread rubber having an excellent impact resilience, because the impact resilience of the ball is extremely influenced by the thread rubber layer and the flight performance of the ball is extremely influenced by the properties of the thread rubber layer. A thread rubber layer having an excellent impact resilience can be obtained by vulcanizing a rubber blend comprising a low-cis polyisoprene rubber as the base rubber, however, the low-cis polyiscoprene 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 in a stretched state, which is the cause of the deterioration of the durability of the ball. Furthermore, heat upon forming the cover damages the thread wound layer, thus deteriorating the ball compression. Therefore, in order to improve the strength of the thread rubber, it has been proposed to blend a natural rubber or a synthetic high-cis polyisoprene rubber with the low-cispolyisoprene rubber. However, the impact resilience of the resulting soft ball declines adversely, although its strength is improved. It is therefore difficult to obtain both a high impact resilience and high durability. GB-A-2164260 discloses a thread rubber formed by vulcanizing a rubber composition comprising solid rubber comprising high cis polyisoprene rubber and butadiene-isoprene copolymer (not in liquid form). On the other hand, when a method of preparing the thread rubber from a coagulated sheet of a latex blend is used, a kneading process in which breakage of the 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 a thread wound golf ball, the impact resilience on the ball is extremely influenced by the properties of the thread rubber constituting the thread rubber layer, whereby the flight performance of the ball is extremely influenced. According to the prior art, when the impact resilience is increased, the strength is lowered and, therefore, a thread rubber having both a high strength and excellent impact resilience was not obtained. We have found that a thread wound golf ball having an 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 liquid polybutadiene rubber and/or a liquid butatadiene-isoprene copolymer rubber having a high strength and excellent impact resilience. The main object of the present invention is to provide a thread wound golf ball comprising a thread rubber layer having a high strength and an excellent impact resilience, which has excellent durability, heat resistance and impact resilience. According to the present invention, there is provided a process for the production of a thread wound golf ball comprising the steps of: (i) vulcanizing a rubber composition comprising (a) a solid rubber comprising a high-cis polyisoprene rubber and (b) a liquid polybutadiene rubber and/or liquid butadiene-isoprene copolymer rubber to obtain a thread rubber; (ii) winding the thread rubber obtained in step (i) around a core; and (iii) covering the thread rubber wound core of step (ii) with a cover. The present invention further provides for the use of a rubber composition comprising (a) a solid rubber comprising a high-cis polyisoprene rubber and (b) a liquid polybutadiene rubber and/or liquid butadiene-isoprene copolymer rubber in a process for the production of a thread wound golf ball having a core, a thread rubber layer formed around the core and a cover, wherein the said rubber composition is vulcanized to obtain the thread rubber layer. By the term "high-cis polyisoprene rubber" as used herein is meant natural rubber or synthetic 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 anyone 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 also is not limited to a specific one, but a high ammonia type latex containing a solids content of about 60% by weight is preferred because of its excellent stability on the formulation of a latex. 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 synthetic high-cis polyisoprene rubber in the cold 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 effect an auxiliary improvement of the impact resilience, in addition to the high-cis polyisoprene rubber. When low-cis polyisoprene rubber is added the strength deteriorates and the improvement of the strength of the thread rubber is hindered. Therefore, the amount is preferably not more than 50% by weight, more preferably not more than 30% by weight, based on the weight of the solid rubber. As is apparent from the description, the solid rubber comprising a high-cis polyisoprene rubber as the main rubber material in the present invention comprises the solid rubber component comprising only the high-cis polyisoprene rubber or comprising only 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 a solid rubber include Califlex IR-305 and Califlex IR-309 manufactured by Shell Chemical Co. Further, examples of the low-cis polyisoprene rubber which are commercially available in the state of a latex include Maxplene IR latex manufactured by Sumitomo Seika Co. In the present invention, a liquid polybutadiene bubber 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 the impact resilience can be improved without deterioration of strength. Furthermore, the workability on winding of the thread rubber is improved by blending with 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 from 10,000 to 80,000, wherein the rubber shows viscosity fluidity or a semisolid state at room temperature (25°C). When, the viscosity-average molecular weight is above 80,000, it's property becomes similar to those of the solid rubber and the effect of improving impact resilience deteriorates. When the viscosity-average molecular weight of the liquid rubber is below 10,000, the rubber is hardly vulcanized and the strength of the thread rubber deteriorates. It is suitable for improving the impact resilience of the thread rubber that the liquid polybutadiene rubber or liquid butadiene-isoprene rubber contains at least 50%, preferably 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. Commercially available examples of the liquid polybutadiene rubber, include 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. Commercially available examples of the liquid butadiene-isoprene copolymer rubber include 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 Kuraray Co.) and the like. These may suitably be used in the present invention. These liquid rubber 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 as the latex state. The liquid rubber is easily converted to the latex form by dissolving it in a volatile solvent (e.g. n-pentane, n-hexane, n-heptane or toluene.) followed by emulsification with an emulsifier, removal of the solvent by flash distillation and further concentration by 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 less than the above lower limit, the improvement in the winding worakability of the thread rubber and the impact resilience are not sufficiently obtained. When the amount of the liquid rubber is above the above upper limit, the stretch becomes too large and the winding workability deteriorates. The thread rubber may be prepared as follows. Vulcanization additives (e.g. vulcanizing agents, vulcanization accelerators, vulcanization auxiliaries, etc.), antioxidants, if necessary, small amount of fillers and oils may be 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 a 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 the base material and then coated with paint to obtain the thread wound golf ball of the present invention. As the core, a conventional solid or liquid core may be used. The thickness of the thread rubber layer varies depending on the material of the cover and the diameter of the core, but it is normally in the range of from 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 Comparative 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. Blend 1 Blend 2 Blend 3 Blend 4 Dunlop C-60 (natural rubber latex, rubber component: 60%) 80 80 70 70 Kuraray LIR-300 (emulsion, rubber component: 45 %) 20 - 20 - Kuraray LIR-390 (emulsion, rubber component: 45 %) - 20 - 20 Maxplene IR (latex, rubber component: 65 %) - - 10 10 Accelerator (emulsion, active component: 20 %) 1 1 1 1 Sulfur (dispersion, active component: 50 %) 2 2 2 2 Antioxidant (dispersion, active component: 50 %) 1 1 1 1 Blend 5 Blend 6 Dunlop C-60 (natural rubber latex, rubber component: 60 %) 100 50 Kuraray LIR-300 (emulsion, rubber component: 45 %) - - Kuraray LIR-390 (emulsion, rubber component: 45 %) - - Maxplene IR (latex, rubber component: 65 %) - 50 Accelerator (emulsion, active component: 20 %) 1 1 Sulfur (dispersion, active component: 50 %) 2 2 Antioxidant (dispersion, active component: 50 %) 1 1 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°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. Ex.1 Ex.2 Ex.3 Ex.4 Blend of thread rubber Blend 1 Blend 2 Blend 3 Blend 4 Compression (PGA system) 85 85 85 85 Initial velocity (feet/second) 252.3 252.0 252.8 252.5 Durability (index) 140 141 134 136 Heat resistance (compression deterioration point) -9 -9 -11 -11 Comp. Ex.1 Comp. Ex.2 Blend of thread rubber Blend 5 Blend 6 Compression (PGA system) 85 85 Initial velocity (feet/second) 248.0 252.5 Durability (index) 140 100 Heat resistance (compression deterioration point) -9 -18 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.

Claims: A process for the production of a thread wound golf ball comprising the steps of: (i) vulcanizing a rubber composition comprising (a) a solid rubber comprising a high-cis polyisoprene rubber and (b) a liquid polybutadiene rubber and/or liquid butadiene-isoprene copolymer rubber to obtain a thread rubber; (ii) winding the thread rubber obtained in step (i) around a core; and (iii) covering the thread rubber wound core of step (ii) with a cover. A process as claimed in claim 1, wherein the solid rubber further comprises a low-cis polyisoprene rubber. A process as claimed in claim 1 or claim 2, wherein the liquid polybutadiene rubber and/or liquid butadiene isoprene copolymer rubber has a viscosity average molecular weight (Mv) in the range of from 10,000 to 80,000. A process as claimed in any one of the preceding claims, wherein the polybutadiene portion of the liquid rubber component (b) has a cis-1, 4-structure of at least 50% A process as claimed in any one of the preceding claims, wherein the solid rubber (a) and liquid rubber (b) are used in a weight ratio (a:b) of 97:3 to 70:30. A process as claimed in any one of the preceding claims, wherein the core is rubber or liquid. A process as claimed in any one of the preceding claims, wherein the cover is an ionomer resin or a trans-polyisoprene. A process as claimed in any one of the preceding claims, wherein the thread rubber layer has a thickness in the range of from 4 to 8 mm. A process as claimed in any one of the preceding claims, wherein the rubber composition comprising (a) and (b) is a latex blend. Use of a rubber composition comprising (a) a solid rubber comprising a high-cis polyisoprene rubber and (b) a liquid polybutadiene rubber and/or liquid butadiene-isoprene copolymer rubber in a process for the production of a thread wound golf ball having a core, a thread rubber layer formed around the core and a cover, wherein the said rubber composition is vulcanized to obtain the thread rubber layer.

Inventors: EBISUNO TADAHIRO, HAMADA AKIHIKO, EBISUNO, TADAHIRO, HAMADA, AKIHIKO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1993
Main classification: A63B37
IPC codes: C08L9, A63B37, C08L21