Thread wound golf ball

Abstract: The present invention provides a thread wound golf ball having an excellent durability, heat resistance and impact resilience, which comprises a core, a thread rubber layer and an outer layer. The thread rubber comprising the thread rubber layer is obtained from a rubber latex blend containing a depolymerized high-cis polyisoprene rubber latex.

Description: 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 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 for covering the thread rubber layer. Generally, it is necessary that golf balls have 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 performances 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, but 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 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-cis polyisoprene rubber. However, the impact resilience of the resulting golf ball declines adversely, although its strength is improved. It is therefore difficult to obtain both a high impact resilience and high durability. 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 and therefore the above disadvantage in strength is slightly improved, but the strength and impact resilience thereof are still insufficient. GB-A-2164260 discloses a thread-wound golf ball having a high impact resiliency and increased flying distance, comprising a ball core, a rubber thread layer and an outer skin layer. The rubber material constituting the rubber thread of the rubber thread layer and/or the ball core contains more than 30 parts by weight of an isoprene-butadiene random copolymer based on 100 parts by weight of the total rubber ingredient. As described above, in a thread wound golf ball, the impact resilience of 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 has not been 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 latex blend containing a depolymerized high-cis polyisoprene rubber latex and forming a thread rubber layer with the resulting thread rubber having a good balance between high strength and 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 thread wound golf ball comprising a core, a thread rubber layer formed around the core and a cover layer covering the thread rubber layer, the thread rubber comprising the thread rubber layer being obtained from a rubber latex blend characterized in that the rubber latex blend comprises a depolymerized high-cis polyisoprene rubber latex. By the term "depolymerized high-cis polyisoprene rubber latex" as used in the present invention is meant a depolymerized natural rubber latex, depolymerized synthetic high-cis polyisoprene rubber latex or a mixture thereof. The depolymerized high-cis polyisoprene rubber latex is obtained by depolymerizing a natural rubber latex or synthetic high-cis polyisoprene rubber latex in the state of a latex, i.e. the state wherein the colloidal rubber is dispersed in water. The depolymerization can be conducted, for example, by heating in the presence of peroxides or azo compounds, or radical reaction due to a redox initiator under an oxidizing atmosphere or in the presence of chain transfer agents or polymerization inhibitors, or irradiation (e.g. X ray, γ-ray, etc.) under the same conditions. The degree of depolymerization of the depolymerized latex is not specifically limited. For example, when the degree of depolymerization is represented by an adhesiveness measured according to JIS-Z-1522 (adhesive cellophane tape test) or JIS-Z-1528 (double adhesive tape test) in the case of the natural rubber latex, the adhesion is preferably in the range of from 25 g/2.5 cm to 300 g/2.5 cm. When the adhesiveness of the rubber latex is less than 25 g/2.5 cm, the improvement in impact resilience is not sufficient because of an insufficient degree of depolymerization. When the adhesiveness exceeds 300 g/2.5 cm, the strength of the thread rubber is liable to deteriorate because of an excessive degree of depolymerization. On depolymerization of the latex, the reaction proceeds whilst maintaining the state that the latex has rubber particles having a particle size of about 1µ dispersed therein and, therefore, depolymerization preferentially occurs close to the surface of the dispersed particles. At the same time, the radical isomerization reaction proceeds and, on the contrary, the crosslinking reaction under an oxyyen free atmosphere proceeds preferentially in the inside of the particles to form a nonuniform reaction system. As described above, since the improvement of impact resilience due to depolymerization and that of strength due to the crosslinking reaction arise simultaneously, the impact resilience is improved without any deterioration of strength. On the other hand, if the reaction proceeded whilst maintaining a uniform system the impact resilience would be improved whilst the strength would be lowered. The depolymerization of the rubber latex is substantially different from the above, and can improve impact resilience without any deterioration of strength. The depolymerized high-cis polyisoprene rubber latex is commercially available, for example, Aoitex Softack M-M (medium depolymerization degree), Aoitex Softack H (high depolymerization degree), Aoitex Softack L (low depolymerization degree) manufactured by Aoi Rubber Co. These can be suitably used in the present invention. To the rubber latex blend used in the present invention, there may be added alone or in combination, a natural rubber latex, synthetic high-cis polyisoprene rubber latex, synthetic low-cis polyisoprene rubber latex and the like. On formulating the rubber latex, the amount of the depolymerized high-cis polyisoprene rubber latex is preferably not less than 10 % by weight, preferably not less than 20 % by weight, based on the dry weight of the rubber latex blend. The rubber latex blend used in the preparation of the thread rubber can be obtained by adding vulcanization additives (e.g. vulcanizing agents, vulcanization accelerators, vulcanization auxiliaries, etc.), antioxidants and, if necessary, small amounts of fillers and oils to the rubber latex, followed by mixing. A non water-soluble liquid additive is emulsified in water and a non water-soluble solid powder is formed into a dispersion in water, and then they are added to the rubber latex to form a blend. The additive may be anyone which is normally used in the rubber industry and non-limited examples thereof are as follows. (1) Vulcanizing agent: organic sulfur compounds such as sulfur, dithiomorpholine, etc. (2) Vulcanization accelerator aldehyde-aniline accelerators (e.g. butylaldehyde-aniline condensate, etc.) thiazol accelerators (e.g. M, DM, MZ, etc.) sulfenamide accelerators (e.g. CZ, NZ, etc.) thiourams (e.g. TT, TS, TET, etc.) dithiocarbamates (e.g. tepidon, EZ, BZ, etc.) (3) Vulcanization auxilary: zinc oxide (4) Antioxidant: bisphenols such as 2,2'-methylene-bis-(4-ethyl-6-t-butylphenol) 2,2'-methylene-bis-(4-methyl-6-t-butylphenol), etc. (5) Filler: kaolin clay, calcium carbonate, barium sulfate, etc. (6) Oil: naphthene oil, adipate plasticizer, etc. The thread rubber used in the present invention is made as follows. The rubber latex blend is coagulated on an endless belt with a solution of a coagulant (e.g. calcium chloride, calcium nitrate, etc.) and formed continuously into a thin film which is rinsed with water and dried. Then, the dried film is vulcanized to form a continuous vulcanized rubber sheet which is cut into pieces of the appropriate 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 comprising an ionomer resin or 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. EXAMPLES 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. 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. 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. 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 rubber latex containing a depolymerized high-cis polyisoprene rubber latex which has a good balance between high strength and impact resilience.

Claims: A thread wound golf ball comprising a core, a thread rubber layer formed around the core and a cover layer covering the thread rubber layer, the thread rubber comprising the thread rubber layer being obtained from a rubber latex blend characterized in that the rubber latex blend comprising a depolymerized high-cis polyisoprene rubber latex. A thread wound golf ball as claimed in claim 1 wherein the thread rubber has an adhesiveness in the range of from 25 to 300 g/2.5 cm. A thread wound golf ball as claimed in claim 1 or claim 2 wherein the rubber latex blend comprises not less than 10 % by weight of the depolymerized high-cis polyisoprene rubber latex. A thread wound golf ball as claimed in any one of the preceding claims wherein the rubber latex blend comprises a natural rubber latex, synthesized high-cis polyisoprene rubber latex, synthesized low-cis polyisoprene rubber or a mixture thereof, in addition to the depolymerized high-cis polyisoprene rubber latex. A thread wound golf ball as claimed in any one of the preceding claims wherein the rubber latex blend further comprises vulcanizing agents, vulcanization accelerators, vulcanization auxiliaries, antioxidants, fillers or oils. A thread wound golf ball as claimed in any one of the preceding claims wherein the thread rubber is prepared by coagulating the rubber latex blend with a coagulant to form a thin film and vulcanizing to form a vulcanized rubber sheet which is cut into pieces. A thread wound golf ball as claimed in any one of the preceding claims wherein the core is made from rubber or liquid. A thread wound golf ball as claimed in any one of the preceding claims wherein the cover is made from an ionomer resin or transpolyisoprene. A thread wound golf ball as claimed in any one of the preceding claims wherein the rubber layer has a thickness in the range of from 4 to 8 mm.

Inventors: HAMADA AKIHIKO, KATO AKIRA, HAMADA, AKIHIKO, KATO, AKIRA
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1993
Main classification: A63B37
IPC codes: C08L21, A63B37, C08L9