JPH0350779B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a rubber composition for the core of a tennis ball. In general, rubber compositions require softeners to improve the mixability and dispersibility of compounding ingredients, improve rubber flow, and facilitate molding operations such as extraction and rolling. It contains higher fatty acids such as cotton oil, higher fatty acid esters such as castor oil, vegetable oil softeners such as pine tar and tall oil, and petroleum softeners such as spindle oil and process oil. In the rubber composition for the core, the above-mentioned softening agent has the disadvantage of increasing compression (that is, softening the core), which is an important characteristic of the core. Therefore, the amount of vulcanizing agent is increased to increase the degree of vulcanization and achieve proper compression. In this case, the durability and feel (hitting feeling) are reduced. In view of such circumstances, the present invention uses low molecular weight liquid polyisoprene rubber in place of conventional softeners to improve the processability of unvulcanized rubber (that is, to improve the ease of mixing of compounding agents and Improves dispersibility, improves rubber flow, improves molding workability such as extraction and rolling), and allows appropriate compression to be obtained without increasing the amount of vulcanizing agent. It is. That is, the present invention uses 100 parts of natural rubber (parts by weight,
1 to 7 parts of liquid polyisoprene rubber with an average molecular weight of 20,000 to 70,000, 3 to 20 parts of zinc oxide
A rubber composition for a tennis ball core, characterized in that it contains 20 to 60 parts of a filler, 2 to 5 parts of sulfur, and 1 to 5 parts of an accelerator. In the present invention, liquid polyisoprene rubber is used with the expectation that it will improve the processability of the rubber when unvulcanized, but this liquid polyisoprene rubber is compatible with conventional softeners that do not participate in crosslinking. Unlike softeners, they are vulcanized by a vulcanizing agent, and after vulcanization they themselves have some rubber-like elasticity, so unlike conventional softeners, they do not cause a large deterioration in rubber properties. It also has a good effect on improving the processability of unvulcanized rubber, and can be used satisfactorily as a softening agent. In the present invention, the liquid polyisoprene rubber used has an average molecular weight in the range of 20,000 to 70,000.
This is because those with an average molecular weight of less than 20,000 deteriorate the physical properties of the vulcanized rubber, while those with an average molecular weight of more than 70,000 have little effect on improving molding workability such as ease of mixing, dispersibility, and extraction. The amount of liquid polyisoprene rubber mixed with natural rubber is in the range of 1 to 7 parts of liquid polyisoprene rubber per 100 parts of natural rubber, as described above.
This is because if the amount of liquid polyisoprene rubber blended is less than the above range, it will have little effect on improving molding workability such as ease of mixing, dispersibility, and extraction, and conversely, if it exceeds the above range, the physical properties of the vulcanized rubber will deteriorate. This is because it makes you The composition for the core of a tennis ball of the present invention contains 1 to 7 parts of liquid polyisoprene rubber to 100 parts of natural rubber as described above, and further contains 3 parts of zinc oxide.
~20 parts, 20 to 60 parts of filler such as calcium carbonate, magnesium carbonate, silicate compound, clay, 2 to 5 parts of sulfur, 1 to 5 parts of accelerator (vulcanization accelerator), and other ingredients if necessary. It can be obtained by appropriately blending the following rubber compounding agents. In the tennis ball core composition of the present invention, natural rubber is used as the base rubber because natural rubber has excellent processability and vulcanization properties, and is also highly compatible with liquid polyisoprene rubber. be. Further, a part of the natural rubber may be replaced with butadiene rubber, ethylene propylene diene rubber, styrene butadiene rubber, etc., within a range that does not impair the properties of the natural rubber. Furthermore, the reason why zinc oxide, filler, sulfur, accelerator, etc. are blended in the above proportions is that blending them in such proportions is suitable for obtaining proper compression. In order to obtain a core for a tennis ball from the rubber composition of the present invention, the rubber composition is usually formed into a hemispherical shell shape, the hollow part is filled with a gas generating agent, and two pieces are bonded together to form a ball shape. Vulcanize. Next, the present invention will be explained with reference to Examples. Examples 1-2 and Comparative Examples 1-3 Rubber compositions having the compositions shown in Table 1 were prepared, and their Mooney viscosity and physical properties after vulcanization were measured. The results are shown in Table 2. For kneading, the rubber and compounding ingredients other than sulfur and the accelerator were kneaded using a Banbury mixer, and the sulfur and accelerator were added to the above-mentioned kneaded material using a roll and kneaded. Vulcanization is
The test was carried out under pressure at 141°C for 30 minutes, and the physical properties of the vulcanized rubber were measured according to the test method specified in JIS K 6301.

【table】

[Table] *2: Dibenzothiazyl disulfide

[Table] As shown in Table 2, the compositions of Examples 1 and 2 of the present invention had a lower Mooney viscosity than the composition of Comparative Example 3, which did not contain a softener, and thereby It has been revealed that the compositions of Examples 1 and 2 have good molding workability such as ease of mixing, dispersibility, and extraction characteristics. Further, Examples 1 to 1 of the present invention
Compositions No. 2 are Comparative Examples 1 to 2, in which conventional softeners were blended.
It is clear that the modulus is higher than that of composition No. 2 and that it is suitable as a core rubber composition. Next, cores for tennis balls were manufactured using the rubber compositions of Examples 1 and 2 and Comparative Examples 1 and 2, and their properties were measured. The results are shown in Table 3. The core is manufactured by molding the rubber composition into a hemispherical shell shape, filling the hollow space with a gas generating agent made of a mixture of ammonium salt and nitrite, bonding them together to form a spherical shape, and heating at 150°C. This was done by vulcanization under pressure for 10 minutes. The outer diameter of the core (core ball) after vulcanization is 60.5 mm.

[Table] A smaller compression value indicates a firmer core ball, but a range of 6.8±0.2 is considered to be good for a core for a tennis ball. Therefore, the target value is set at 6.8, and the compositions of the present invention fall within the above range in both Examples 1 and 2, and can be said to be preferable as compositions for tennis ball cores from the compression point of view. In other words, when the compression is within the range of 6.8±0.2, a good shot feel is obtained; when the compression is smaller than the above range, the ball feels hard and has a large impact; when the compression is larger than the above range, The composition of Example 1 had a compression of 6.80, and the composition of Example 2 had a compression of 6.88.
It falls within the range of 6.8±0.2, and can be said to be preferable as a composition for the core of a tennis ball. In addition, the repulsion height is 174±3cm, which is considered to be good as a core for tennis balls, and the target value is
174cm, but the composition of the present invention is Example 1,
Both values are close to the target values, and from this point of view, it can be said that the composition is preferable as a composition for the core of a tennis ball.

Claims (1)

[Claims] 1 Average molecular weight 20,000 per 100 parts by weight of natural rubber
~70000 liquid polyisoprene rubber 1 to 7 parts by weight,
A rubber composition for a tennis ball core, comprising 3 to 20 parts by weight of zinc oxide, 20 to 60 parts by weight of a filler, 2 to 5 parts by weight of sulfur, and 1 to 5 parts by weight of an accelerator.