JP2020097674A - Rubber composition for tennis ball - Google Patents

Abstract

To provide a rubber composition for a tennis ball excellent in a speed feeling and a striking feeling.SOLUTION: The rubber composition for a tennis ball at least includes a base material rubber and a flat filler. The product M×E is 2.0(MPa)-20(MPa) with regard to a tensile stress M at a tensile strain 10% and a tensile elastic modulus E in a region of a tensile strain 5%-20% measured at a temperature 23±1°C in accordance with JIS K6251, of a vulcanizate obtained through vulcanizing the rubber composition. A tennis ball 2 includes: a core 4 formed with the rubber composition and being hollow; two felt parts 6 covering the core 4; and a seam part 8 positioned in a gap between the two felt parts 6.SELECTED DRAWING: Figure 1

Description

The present invention relates to a rubber composition for tennis balls. More particularly, the present invention relates to rubber compositions used in tennis ball cores.

The tennis ball has a core made of a rubber composition and a felt (melton) covering the core. This core is a hollow sphere. In a tennis ball used in hard tennis, a compressed gas having a pressure higher than the atmospheric pressure by 40 kPa to 120 kPa is filled inside the core. This tennis ball is also called a pressure tennis ball (pressure ball).

In a pressurized tennis ball, an excellent rebound performance and a good shot feeling are imparted by the inner pressure of the core higher than the atmospheric pressure. When a tennis ball with excellent resilience is hit on a tennis court, it bounces at a high speed. A tennis ball with a high ball speed after bounce makes players and spectators feel a sense of speed. Tennis balls that give you a sense of speed make the game interesting. On the other hand, the filled compressed gas gradually leaks from the core due to the internal pressure of the core being higher than the atmospheric pressure. Gas leakage may reduce the internal pressure of the core to near atmospheric pressure. A tennis ball with reduced internal pressure in the core is inferior in speed and shot feeling.

In recent years, with the improvement of rackets and the like, there has been a demand for tennis balls with a faster feeling. In Japanese Patent Laid-Open No. 61-143455, a rubber material containing a scale-like or flat-plate-like filler is studied to suppress a decrease in ball internal pressure with time and maintain resilience performance. JP-A-2011-188877, JP-A-2011-188878, and JP-A-2011-177369 disclose a core during play by providing a core with increased internal pressure and a felt formed of a nonwoven fabric. A tennis ball is disclosed in which the flexure is reduced to prevent the bounce from deteriorating over time.

Japanese Patent Laid-Open No. 61-143455 JP, 2011-188877, A JP, 2011-188878, A JP, 2011-177369, A

In the disclosure of JP-A-61-143455, JP-A-2011-188877, JP-A-2011-188878 and JP-A-2011-177369, a technique for reducing deterioration with time of resilience performance and the like is proposed. However, it does not disclose a rubber composition having a high ball speed at the time of bouncing. Further, according to the rubber material proposed in JP-A-61-143455, the rubber material becomes excessively hard due to the addition of the scale-like or flat-plate-like filler, and the shot feeling of the tennis ball may be deteriorated.

There is still room for improvement in improving the speed of tennis balls. An object of the present invention is to provide a rubber composition for obtaining a tennis ball having an enhanced sense of speed without impairing the feel at impact.

The rubber composition according to the present invention contains at least a base rubber and a flat filler. The vulcanized rubber obtained by vulcanizing this rubber composition has a tensile stress M at a tensile strain of 10% and a tensile strain of 5% to 20% measured at a temperature of 23±1° C. according to JIS K6251. The product M×E with the tensile elastic modulus E in the region is 2.0 (MPa ² ) or more and 20 (MPa ² ) or less.

Preferably, the flat filler has an average particle diameter D ₅₀ of 0.01 μm or more and 50 μm or less. Preferably, the flatness DL obtained by dividing the average particle diameter D ₅₀ by the average thickness T is 20 or more and 200 or less.

Preferably, the flatness DL is 20 or more and 200 or less, and the amount of the flat filler is 5 parts by mass or more and 150 parts by mass or less with respect to 100 parts by mass of the base rubber.

Preferably, the flat filler is from the group consisting of talc, kaolin clay, graphite, graphene, bentonite, halloysite, montmorillonite, mica, magnesium carbonate, beidellite, saponite, hectorite, nontronite, vermiculite, illite and allophane. Selected.

Preferably, the rubber composition further contains a coupling agent. The coupling agent has a functional group X capable of interacting with the flat filler and a functional group Y capable of interacting with the base rubber, and is represented by the general formula XY. It is a compound.

Preferably, the functional group X, an alkoxysilyl group _{^{(-Si-R 1 3-n}} (OR 2) n), 4 quaternary ammonium group ^{(-N + R 3 R 4 R} 5 Z) and the group consisting of an epoxy group R ¹ , R ² , R ³ , R ⁴ and R ⁵ are each independently a linear, branched or cyclic alkyl group, alkenyl group or alkylphenyl group having 1 to 30 carbon atoms. , An alkenylphenyl group, a hydroxyalkyl group or a hydroxyalkenyl group, n is a natural number of 1 to 3, and Z is an anion. The functional group Y is a linear, branched or cyclic alkyl group or alkenyl group having 1 to 30 carbon atoms.

Preferably, the flat filler is surface-treated with the coupling agent. Preferably, the amount of the coupling agent with respect to the flat filler is 1% by mass or more and 50% by mass or less.

Preferably, the rubber composition further contains other filler which does not correspond to the flat filler. This other filler is selected from the group consisting of silica, carbon black, calcium carbonate, magnesium hydroxide, diatomaceous earth, titanium oxide, zinc oxide, bismuth oxide and barium sulfate.

Preferably, the base rubber contains butadiene rubber and natural rubber. The mass ratio B/N of the compounding amount B of butadiene rubber to the compounding amount N of natural rubber in this base rubber is 1.4 or less.

The sulfur content of this rubber composition is preferably 0.01% by mass or more and 10% by mass or less. Preferably, the rubber composition has a loss tangent tan δ at 0° C. of 0.10. The Shore A hardness Ha of this rubber composition is preferably 20 or more and 88 or less.

This tennis ball has a core obtained by using any one of the rubber compositions described above.

The rubber composition according to the present invention contains a flat filler having excellent adhesion to the rubber component. In the core obtained by vulcanizing this rubber composition, the stress and elastic modulus in the low strain region are proper. In a tennis ball provided with this core, deformation when hit and touching the court is suppressed. The ground resistance that this tennis ball receives when bouncing is small. According to this rubber composition, a tennis ball having an improved sense of speed can be obtained without impairing the feel at impact. Further, in this tennis ball, a suitable speed feeling and a good shot feeling can be maintained for a long period of time due to the gas leakage preventing action of the flat filler.

FIG. 1 is a partially cutaway sectional view of a tennis ball according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the drawings as appropriate.

FIG. 1 is a partially cutaway sectional view showing a tennis ball 2 according to an embodiment of the present invention. The tennis ball 2 has a hollow core 4, two felt portions 6 covering the core 4, and a seam portion 8 located in a gap between the two felt portions 6. The thickness of the core 4 is usually about 3 mm to 4 mm. The inside of the core 4 is filled with compressed gas. Two felt portions 6 are attached to the surface of the core 4 with an adhesive.

The core 4 is formed of the rubber composition according to one embodiment of the present invention. This rubber composition contains at least a base rubber and a flat filler.

The flat filler is composed of a large number of flat particles. In this rubber composition, a large number of flat particles forming the flat filler are dispersed in a matrix containing a base rubber as a main component. In this specification, a matrix containing a base rubber as a main component may be referred to as a “rubber component”. This flat filler adheres well to the rubber component. The flat filler having high adhesion to the rubber component has excellent reinforcing properties.

In the vulcanized rubber obtained by vulcanizing this rubber composition, by containing a flat filler having excellent reinforcing properties, proper tensile properties can be obtained particularly in a low strain region that contributes to deformation at the time of ground contact. To be Specifically, in the vulcanized rubber obtained by vulcanizing the rubber composition containing this flat filler, the tensile stress at a tensile strain of 10% measured at a temperature of 23±1° C. is measured in accordance with JIS K6251. The product M×E of M (MPa) and the tensile elastic modulus E (MPa) in the region of tensile strain of 5% to 20% is 2.0 (MPa ² ) or more and 20 (MPa ² ) or less.

The product M×E is an index showing the adhesion between the flat filler and the rubber component. In the tennis ball 2 including the core 4 obtained by using this rubber composition, the product M×E is 2.0 (MPa ² ) or more, so that the reinforcing effect by the flat filler is exerted, and at the time of bouncing. Deformation when contacting the tennis court is suppressed. In this tennis ball 2, the ground resistance due to the deformation is reduced, so that the ball speed after bouncing does not decrease significantly. The product M×E is preferably 2.5 (MPa ² ) or more, and more preferably 3.0 (MPa ² ) or more, from the viewpoint of suppressing deformation during bounding.

Further, in this tennis ball 2, since the product M×E is 20 (MPa ² ) or less, the vulcanized rubber is prevented from becoming excessively hard. The shot feeling of this tennis ball 2 is good. According to this rubber composition, the tennis ball 2 having improved speed feeling can be obtained without impairing the shot feeling. From the viewpoint of shot feeling, the product M×E is preferably 15.0 (MPa ² ) or less, and more preferably 12.0 (MPa ² ) or less.

Further, in the core 4 obtained from this rubber composition, a large number of flat particles dispersed in the matrix of the rubber component impede the movement of gas molecules inside. In the tennis ball 2 including the core 4, gas leakage is prevented by the large number of flat particles forming the flat filler. With this tennis ball 2, a suitable speed feeling and a good shot feeling can be maintained for a long time.

In this rubber composition, the tensile stress M is not particularly limited as long as the product M×E is 2.0 (MPa ² ) or more and 20 (MPa ² ) or less. From the viewpoint of improving the feeling of speed, the tensile stress M at a tensile strain of 10% is preferably 0.3 MPa or more, and more preferably 0.4 MPa or more. From the viewpoint of feel at impact, the preferable tensile stress M is 3.5 MPa or less. A method for measuring the tensile stress M at a tensile strain of 10% will be described later in Examples.

In this rubber composition, the tensile modulus E is not particularly limited as long as the product M×E is 2.0 (MPa ² ) or more and 20 (MPa ² ) or less. From the viewpoint of improving the feeling of speed, the tensile elastic modulus E in the region of tensile strain of 5% to 20% is preferably 3.0 MPa or more, more preferably 3.5 MPa or more. From the viewpoint of feel at impact, the preferred tensile elastic modulus E is 14.0 MPa or less. The tensile elastic modulus E in the region of tensile strain of 5% to 20% is measured by the method described later in Examples and calculated according to the following equation.
_{E = (M 20 -M 5)} / (20-5)
Here, M ₂₀ is the tensile stress at 20% tensile strain, and M ₅ is the tensile stress at 5% tensile strain.

In the rubber composition according to the present invention, the kind of the flat filler is not particularly limited, and can be appropriately selected so that the product M×E is 2.0 (MPa ² ) or more and 20 (MPa ² ) or less. A flat filler selected from the group consisting of talc, kaolin clay, graphite, graphene, bentonite, halloysite, montmorillonite, mica, magnesium carbonate, beidellite, saponite, hectorite, nontronite, vermiculite, illite and allophane is preferred. Talc, kaolin clay, graphite and bentonite are more preferred. Two or more flat fillers may be used in combination.

The average particle diameter D ₅₀ of the flat filler is appropriately selected depending on its type. From the viewpoint of preventing gas leakage, the average particle diameter D ₅₀ of the flat filler is preferably 0.01 μm or more, more preferably 0.05 μm or more, and particularly preferably 0.1 μm or more. From the viewpoint of mixability with the base rubber, the average particle diameter D ₅₀ is preferably ₅₀ μm or less, more preferably 20 μm or less, and particularly preferably 10 μm or less. In the specification of the present application, the average particle diameter D ₅₀ (μm) means a cumulative particle diameter of _{50 from the} smaller diameter side in the particle diameter distribution measured by a laser diffraction particle size distribution measuring device (for example, LMS-3000 manufactured by Seishin Enterprise Co., Ltd.). It means the average particle diameter which becomes volume %.

The average thickness T of the flat filler is appropriately selected depending on its type. From the viewpoint of preventing gas leakage, the average thickness T of the flat filler is preferably 1.00 μm or less, more preferably 0.50 μm or less, and particularly preferably 0.20 μm or less. From the viewpoint of the compatibility with the base rubber, the average thickness T is preferably 0.001 μm or more, more preferably 0.002 μm or more, and particularly preferably 0.003 μm or more. In the present specification, the average thickness T (μm) of the flat filler is measured by observation with a microscope such as a transmission electron microscope. Specifically, from the image obtained by observing a plurality of particles collected from the flat filler with a transmission electron microscope (for example, H-9500 manufactured by Hitachi High-Technologies Corporation), A particle having a size equivalent to the average particle diameter D ₅₀ is selected and its thickness is measured. The average value of the measured values obtained for 12 particles is taken as the average thickness T of this flat filler.

From the viewpoint of preventing gas leakage, the flatness DL obtained by dividing the average particle diameter D ₅₀ (μm) of the flat filler by the average thickness T (μm) is preferably 20 or more, more preferably 40 or more. , 50 or more are more preferable, and 100 or more are particularly preferable. The preferable flatness DL is 200 or less from the viewpoint of the compatibility with the rubber component. When a plurality of flat particles forming the flat filler are aggregated or multi-layered to form an aggregate, the flatness DL is an average particle diameter D ₅₀ obtained by measuring the aggregate including the aggregate. And the average thickness T. This rubber composition may contain a flat filler having a flatness of less than 20 as long as the effects of the present invention are not impaired.

As long as the product M×E satisfies the above range, the compounding amount of the flat filler is not particularly limited, and is appropriately adjusted depending on its type and particle shape. From the viewpoint that a predetermined product M×E is easily obtained, the amount of the flat filler having a flatness DL of 20 or more and 200 or less is preferably 5 parts by mass or more based on 100 parts by mass of the base rubber. It is 150 parts by mass or less. From the viewpoint of improving the feeling of speed, the amount of the flat filler having a flatness DL of 20 or more and 200 or less is more preferably 30 parts by mass or more, and particularly preferably 40 parts by mass or more. From the viewpoint of shot feeling, this amount is preferably 120 parts by mass or less, and particularly preferably 100 parts by mass or less.

As long as the product M×E satisfies the above range, the rubber composition may include other fillers that are not flat fillers. Here, the other filler that does not correspond to the flat filler means a filler having a flatness DL of about 1. Other fillers include silica, carbon black, calcium carbonate, magnesium hydroxide, diatomaceous earth, titanium oxide, zinc oxide, bismuth oxide, barium sulfate and the like. Fillers selected from the group consisting of silica, carbon black and zinc oxide are preferred. From the viewpoint of adhesion to the rubber component, carbon-based fillers are preferable, and carbon black is particularly preferable. You may use together 2 or more types as another filler. In the present specification, the carbon-based filler means a filler composed of a large number of particles containing carbon atoms as a main constituent. It preferably means a filler in which 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 98% by mass or more of its constituents are carbon atoms.

The average particle diameter D ₅₀ of the other filler used in combination with the flat filler is appropriately set according to the kind of the selected filler. From the viewpoint of preventing gas leakage, the average particle diameter D ₅₀ of the other filler is preferably 0.01 μm or more, more preferably 0.05 μm or more, and particularly preferably 0.10 μm or more. From the viewpoint of shot feeling, the average particle diameter D ₅₀ of the other filler is preferably 50 μm or less, more preferably 30 μm or less, further preferably 20 μm or less, and particularly preferably 10 μm or less. The average particle diameter D ₅₀ (μm) of the other fillers is measured by the same method as for the flat filler.

The amount of the other filler compounded is not particularly limited as long as the effects of the present invention are not impaired. From the viewpoint of reinforcing properties, the amount of the other filler compounded is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and particularly preferably 5 parts by mass or more, relative to 100 parts by mass of the base rubber. From the viewpoint of feel at impact, the amount of the other filler compounded is preferably 50 parts by mass or less, more preferably 30 parts by mass or less, and particularly preferably 20 parts by mass or less. When two or more types are used in combination, the total amount is adjusted to be within the above range. The rubber composition of the present invention may not include other fillers that do not correspond to the flat filler.

When the flat filler is used in combination with another filler, the total amount thereof is preferably 5 parts by mass or more with respect to 100 parts by mass of the base rubber, from the viewpoints of reinforcement and gas leakage prevention. The amount is more preferably at least 20 parts by mass, particularly preferably at least 20 parts by mass. From the viewpoint of shot feeling, the total content is preferably 200 parts by mass or less, and more preferably 150 parts by mass or less.

When the flat filler and other fillers are used in combination, the ratio of the amount of the flat filler to the total blending amount of the flat filler and the other filler is 70 from the viewpoint of improving the feeling of speed. The content is preferably at least mass%, more preferably at least 80 mass%, particularly preferably at least 90 mass%, and the upper limit value is 100 mass%.

Preferably, the rubber composition further contains a coupling agent. This coupling agent has a functional group X capable of interacting with the flat filler and a functional group Y capable of interacting with the base rubber, and is a compound represented by the general formula XY. is there. In the rubber composition containing this coupling agent, the functional group X interacts with the flat filler, and the functional group Y interacts with the base rubber, whereby the adhesion between the flat filler and the base rubber is improved. Is improved. In the core 4 obtained from this rubber composition, the tensile stress is particularly increased in the low strain region. In the tennis ball 2 including the core 4, the deformation at the time of bouncing is suppressed, and the sense of speed is improved.

The type of the functional group X is not particularly limited as long as it has a function of interacting with a hydroxyl group, a carbonyl group, a carboxyl group and the like existing on the surface of the flat filler. Preferably, the functional group X is from the group consisting of alkoxysilyl group _{^{(-Si-R 1 3-n}} (OR 2) n), 4 quaternary ammonium group ^{(-N + R 3 R 4 R} 5 Z) and an epoxy group Selected. Here, R ¹ , R ² , R ³ , R ⁴ and R ⁵ are each independently a linear, branched or cyclic alkyl group, alkenyl group, alkylphenyl group, alkenylphenyl group having 1 to 30 carbon atoms. , A hydroxyalkyl group or a hydroxyalkenyl group, n is a natural number of 1 to 3, and Z is an anion such as a hydroxide ion or a halide ion. R ¹ , R ² , R ³ , R ⁴ and R ⁵ may further have a substituent unless the effects of the present invention are impaired.

The functional group Y interacts with a functional group contained in the molecular chain of the base rubber (for example, carbon-carbon double bond) or a radical generated by the molecular chain of the base rubber being cut during vulcanization molding described later. The type is not particularly limited as long as it has a function to act, but the functional group Y is preferably a linear, branched or cyclic alkyl group or alkenyl group having 1 to 30 carbon atoms.

From affinity aspects of the flat filler, preferred functional groups X is a quaternary ammonium group ^{(-N + R 3 R 4 R} 5 Z). From the viewpoint of reducing steric hindrance, a quaternary ammonium salt in which at least one of R ³ , R ⁴ and R ⁵ is a linear alkyl group having 1 to 30 carbon atoms is preferable. 1-25 are more preferable, as for carbon number of a linear alkyl group, 1-20 are more preferable, and 1-10 are especially preferable. A quaternary ammonium group in which at least one of R ³ , R ⁴ and R ⁵ is a methyl group is particularly preferable. As the anion Z, a hydroxide ion and a chloride ion are preferable.

The coupling agent in which the functional group X is a quaternary ammonium group is also called a quaternary ammonium salt. Specific examples of such a quaternary ammonium salt include trimethylstearylammonium, dimethylstearylbenzylammonium, dimethyldioctadecylammonium, oleylbis(2-hydroxyethyl)methylammonium, lauryltrimethylammonium, trioctylammonium, distearyldimethylammonium, Examples of hydroxides or halides of behenyltrimethylammonium, tetraethylammonium, tetrabutylammonium, cetyltrimethylammonium, n-dodecyltrimethylammonium, cetyldimethylbenzylammonium, methylcetyldibenzylammonium, cetyldimethylethylammonium, octadecyltrimethylammonium and the like. To be Preferred are hydroxides or halides of trimethylstearyl ammonium, dimethylstearylbenzyl ammonium, dimethyldioctadecyl ammonium, oleylbis(2-hydroxyethyl)methylammonium. More preferred quaternary ammonium salts are halides of trimethylstearylammonium, dimethylstearylbenzylammonium, dimethyldioctadecylammonium and oleylbis(2-hydroxyethyl)methylammonium, with chloride being particularly preferred. Two or more kinds may be used in combination.

From the viewpoint that the effect of the present invention is easily obtained, it is preferable that the flat filler is surface-treated with a coupling agent in advance. This rubber composition contains a coupling agent bonded to the surface of the flat filler. From the viewpoint of adhesion with the base rubber, the amount of the coupling agent contained in the surface-treated flat filler (hereinafter, also referred to as modification rate) is preferably 1% by mass or more, and more preferably 10% by mass or more. 20 mass% or more is especially preferable. From the viewpoint of shot feeling, the modification rate is preferably 50% by mass or less, more preferably 45% by mass or less, and particularly preferably 40% by mass or less.

The method of surface-treating the flat filler with a coupling agent is not particularly limited as long as the effects of the present invention can be obtained. For example, surface treatment by adding a predetermined amount of coupling agent to a flat filler dispersed in a solvent such as water, reacting under reflux while heating if necessary, and then drying. A flattened filler is obtained. It is also possible to use a commercially available product as it is.

The modification rate of the surface-treated flat filler can be adjusted by the amount of the coupling agent added to the flat filler. The amount of coupling added to the flat filler is preferably 1% by mass or more, more preferably 10% by mass or more, and particularly preferably 25% by mass or more. From the viewpoint of shot feeling, the amount of the coupling agent added is preferably 100% by mass or less, more preferably 80% by mass or less, and particularly preferably 65% by mass or less.

In the rubber composition according to the present invention, examples of suitable base rubbers include natural rubber, polybutadiene, polyisoprene, styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, polychloroprene, ethylene-propylene copolymer, An ethylene-propylene-diene copolymer, an isobutylene-isoprene copolymer and an acrylic rubber. More preferred base rubbers are natural rubber and polybutadiene. Two or more of these rubbers may be used in combination.

When natural rubber and polybutadiene are used in combination, the mass ratio B/N of the compounding amount B of the polybutadiene rubber to the compounding amount N of the natural rubber is preferably 1.4 or less, and 1.0 or less from the viewpoint of shot feeling. More preferably, 0.4 or less is particularly preferable. The total amount of base rubber may be natural rubber.

Preferably, the rubber composition further contains a vulcanizing agent, a vulcanization accelerator and a vulcanization aid. Examples of the vulcanizing agent include sulfur such as powdered sulfur, insoluble sulfur, precipitated sulfur, and colloidal sulfur; and sulfur compounds such as morpholine disulfide and alkylphenol disulfide. The compounding amount of the vulcanizing agent is adjusted according to the type, but from the viewpoint of resilience performance, it is preferably 0.5 part by mass or more, and more preferably 1.0 part by mass or more with respect to 100 parts by mass of the base rubber. preferable. The compounding amount of the vulcanizing agent is preferably 5.0 parts by mass or less.

Suitable vulcanization accelerators include, for example, guanidine compounds, sulfenamide compounds, thiazole compounds, thiuram compounds, thiourea compounds, dithiocarbamic acid compounds, aldehyde-amine compounds, aldehyde-ammonia compounds, imidazolines. System compounds, xanthate compounds and the like. From the viewpoint of resilience performance, the compounding amount of the vulcanization accelerator is preferably 1.0 part by mass or more, and more preferably 2.0 parts by mass or more with respect to 100 parts by mass of the base rubber. The compounding amount of the vulcanization accelerator is preferably 6.0 parts by mass or less.

Examples of the vulcanization aid include fatty acids such as stearic acid, metal oxides such as zinc white, and fatty acid metal salts such as zinc stearate. The rubber composition may further contain additives such as an antioxidant, an antioxidant, a light stabilizer, a softening agent, a processing aid, and a coloring agent as long as the effects of the present invention are not impaired.

Preferably, the rubber composition contains sulfur. Sulfur contained in the rubber composition can contribute to the formation of a crosslinked structure. The crosslink density of the rubber composition affects the feel at impact and speed of the tennis ball 2 obtained from this rubber composition. From the viewpoint of a sense of speed, the sulfur content of this rubber composition is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and particularly preferably 1.0% by mass or more. From the viewpoint of feel at impact, the sulfur content is preferably 10% by mass or less, more preferably 8% by mass or less, and particularly preferably 7% by mass or less. In the present specification, the sulfur content of the rubber composition is measured according to the oxygen flask combustion method described in the 17th Revised Japanese Pharmacopoeia, General Test Method. The sulfur contained in the rubber composition may be sulfur as a simple substance or may be a sulfur atom contained in the sulfur compound. This sulfur may be derived from a vulcanizing agent or a vulcanization accelerator.

The loss tangent tan δ at 0° C. of the rubber composition affects the feel at impact of the tennis ball 2 including the core 4 made of this rubber composition. In the core 4 having a large tan δ at 0° C., the resilience from deformation at the time of hitting the ball is lowered, so that the shot feeling tends to be heavy. From this viewpoint, the loss tangent tan δ at 0° C. of this rubber material is preferably 0.10 or less, more preferably 0.085 or less, particularly preferably 0.075 or less, and its lower limit value is 0. In the present specification, the loss tangent tan δ at 0°C is the value of tan δ at 0°C of the temperature dispersion curve obtained by a viscoelastic spectrometer. Details of the measuring method will be described later.

From the viewpoint of speed, the Shore A hardness Ha of this rubber composition is preferably 20 or more, more preferably 40 or more, and particularly preferably 50 or more. From the viewpoint of shot feeling, the hardness Ha is preferably 88 or less, more preferably 85 or less, and particularly preferably 80 or less. The hardness Ha is measured by a type A durometer attached to an automatic hardness meter (trade name "Digitest II" manufactured by H. Barreith Co., Ltd.). For the measurement, a slab formed by hot pressing and having a thickness of about 2 mm is used. Slabs stored at a temperature of 23° C. for 2 weeks are used for the measurement. At the time of measurement, three slabs are overlaid.

From the viewpoint that the leakage of gas from the core 4 is prevented and a suitable shot feeling and repulsion performance are maintained, the nitrogen gas permeability coefficient G of this rubber material at 40° C. is 9.0×10 ⁻¹⁰ ( cm ³ ·cm/cm ² /sec/cmHg) or less is preferable, and 8.0×10 ⁻¹⁰ (cm ³ ·cm/cm ² /sec/cmHg) or less is more preferable. In the present specification, the nitrogen gas permeability coefficient G is measured according to the differential pressure method described in JIS K7126-1.

The method for producing the rubber composition is not particularly limited as long as the object of the present invention is achieved. For example, a known kneader such as a Banbury mixer, a kneader, or a roll, the base rubber, the flat filler, and other additives appropriately selected are charged and melt-kneaded. This rubber composition may be produced by adding a vulcanizing agent and the like and applying pressure and heating. When the carbon-based filler is added, it is preferable to add it to the rubber composition in the state of a masterbatch that has been mixed with the base rubber in advance from the viewpoint of dispersibility.

The kneading conditions and vulcanization conditions are selected depending on the composition of the rubber composition. The preferable kneading temperature is 50° C. or higher and 180° C. or lower. The preferred vulcanization temperature is 140° C. or higher and 180° C. or lower. The vulcanization time is preferably 2 minutes or more and 60 minutes or less.

The method of manufacturing the tennis ball 2 using this rubber composition is not particularly limited. For example, two hemispherical half shells are formed by vulcanizing and molding this rubber composition in a predetermined mold. The two half shells are bonded together with the ammonium salt and the nitrite contained therein, and then compression molded to form the core 4, which is a hollow spherical body. Inside the core 4, nitrogen gas is generated by the chemical reaction of ammonium salt and nitrite. The internal pressure of the core 4 is increased by this nitrogen gas. Next, a dumbbell-shaped piece is cut in advance, and the felt portion 6 having a seam glue adhered to the cross section thereof is attached to the surface of the core 4 to obtain the tennis ball 2.

Hereinafter, the effects of the present invention will be clarified by examples, but the present invention should not be limitedly interpreted based on the description of the examples.

[Example 1]
80 parts by mass of natural rubber (product name "SMR CV60"), 20 parts by mass of polybutadiene rubber (product name "BR01" manufactured by JSR), 66 parts by mass of talc A (product name "Mistron HAR" manufactured by Imerys) ) 5 parts by mass of zinc oxide (trade name "Zinc Oxide 2" manufactured by Shodo Kagaku Co., Ltd.) and 0.5 parts by mass of stearic acid (trade name "Tsubaki" manufactured by NOF CORPORATION) are put into a Banbury mixer. Then, the mixture was kneaded at 90° C. for 5 minutes. After cooling the obtained kneaded product to 40° C. or lower, 3.6 parts by mass of sulfur (trade name “SAN FEL EX” manufactured by Sanshin Chemical Co., containing 20% oil), 1 part by mass of vulcanization accelerator 1( Sanshin Chemical Co., Ltd. product name "Suncellar D"), 1 part by mass of vulcanization accelerator 2 (Ouchi Shinko Chemical Co., Ltd. product name "NOXCELLER CZ") and 1.9 parts by mass of vulcanization accelerator 3. (Trade name “NOXCELLER DM” manufactured by Ouchi Shinko Chemical Co., Ltd.) was added and kneaded at 50° C. for 3 minutes using an open roll to obtain a rubber composition of Example 1.

[Example 2-15 and Comparative Example 1-7]
The amounts of the base rubber, the flat filler and the other fillers were changed to those shown in Table 1-5 below, and in the same manner as in Example 1, Example 2-15 and Comparative Example 1- A rubber composition of No. 7 was produced. When blending the carbon-based filler, 100 parts by mass of the base rubber and 50 parts by mass of the carbon-based filler are kneaded in advance at 90° C. for 5 minutes using a Banbury mixer to prepare a masterbatch. After that, this masterbatch was added so as to have the composition shown in Table 1-5 below to obtain respective rubber compositions.

Details of the compounds listed in Tables 1-5 are as follows.
NR: Natural rubber, trade name "SMR CV60"
BR: Polybutadiene rubber manufactured by JSR, trade name "BR01"
Talc A: trade name "Mistron HAR" manufactured by Imerys Co., average particle diameter (D ₅₀ ) 6.7 μm, flatness (DL) 100.
Surface-treated clay A: Quaternary ammonium cation-modified bentonite clay manufactured by Hojun Co., Ltd., trade name “ESBEN NX80”, average particle diameter (D ₅₀ ) 2 μm, flatness (DL) 50, modification rate about 45% by mass.
Surface-treated clay B: Quaternary ammonium cation-modified bentonite clay manufactured by Hojun Co., Ltd., trade name “Lebonite 400”, average particle diameter (D ₅₀ ) 5 μm, flatness (DL) 40, modification rate about 35% by mass
Graphite: trade name "SFG44" manufactured by Imerys Co., average particle diameter (D ₅₀ ) 49 μm, flatness (DL) 120, BET specific surface area 5 m ² /g
Graphene: trade name “xGn-M-5” manufactured by XG Sciences, average particle diameter (D ₅₀ ) 5 μm, flatness (DL) 700
Talc B: trade name “SSS” manufactured by Nippon Talc Co., average particle diameter (D ₅₀ ) 13 μm, flatness (DL) 20
Clay: Kaolin clay manufactured by Imerys Co., Ltd., trade name “ECKALITE 120”, average particle diameter (D ₅₀ ) 4 μm, flatness (DL) 20, BET specific surface area 18 m ² /g
Mica: Yamaguchi Mica product name “SYA-21”, average particle diameter (D ₅₀ ) 27 μm, flatness (DL) 90, BET specific surface area 2 m ² /g
Carbon black: trade name “SHOW BLACK N330” manufactured by Cabot Japan, average particle diameter (D ₅₀ ) 0.03 μm, flatness (DL) 1, BET specific surface area 79 m ² /g
Silica: trade name “Nipseal VN3” manufactured by Tosoh Silica, average particle diameter (D ₅₀ ) 20 μm, flatness (DL) 1, BET specific surface area 200 m ² /g
Diatomaceous earth: trade name "Radiolite 500" manufactured by Showa Chemical Industry Co., Ltd., average particle size (D ₅₀ ) 35 μm, flatness (DL) 1
Magnesium carbonate: trade name “Venus” manufactured by Kamijima Chemical Industry Co., Ltd., average particle diameter (D ₅₀ ) 6 μm, flatness (DL) 10
Calcium carbonate: trade name “BF300” manufactured by Shiraishi Calcium Co., average particle diameter (D ₅₀ ) 8 μm, flatness (DL) 1, BET specific surface area 0.27 m ² /g.
Zinc oxide: trade name “Zinc oxide two types” manufactured by Shodo Kagaku Co., average particle diameter (D ₅₀ ) 0.6 μm, flatness (DL) 1, BET specific surface area 4 m ² /g
Stearic acid: Trade name "Tsubaki" manufactured by NOF CORPORATION
Sulfur: Sanshin Chemical Co., Ltd. insoluble sulfur, trade name “San Fel EX”, containing 20% oil Vulcanization accelerator 1: Sanshin Chemical Co., 1,3-diphenylguanidine (DPG), trade name “Sunceller D”
Vulcanization accelerator 2: N-cyclohexyl-2-benzothiazolylsulfenamide (CZ) manufactured by Ouchi Shinko Chemical Co., Ltd., trade name "NOXCELLER CZ"
Vulcanization accelerator 3: Di-2-benzothiazolyl disulfide (MBTS) manufactured by Ouchi Shinko Kagaku Co., Ltd., trade name "NOXCELLER DM"

[Sulfur content measurement]
The sulfur contents of the rubber compositions of Examples 1-15 and Comparative Examples 1-7 were measured according to the oxygen flask combustion method described in General Tests, 17th Revised Japanese Pharmacopoeia. After burning 10 mg of each rubber composition, a total of 55 mL of methanol was added, and then 20 mL of 0.005 mol/L perchloric acid solution was accurately added. The sulfur content was quantified by measuring the solution obtained by leaving it for 10 minutes using ion chromatography (HIC-SP manufactured by Shimadzu Corporation). The resulting sulfur content (wt.%) is shown in Table 6-10 below.

[Tensile test]
The rubber compositions of Example 1-15 and Comparative Example 1-7 were each put into a mold and press-vulcanized at 160° C. for 2 minutes to prepare a No. 3 dumbbell-shaped test piece having a thickness of 2 mm. According to JIS K6251 "Vulcanized Rubber and Thermoplastic Rubber-Determination of Tensile Properties", a tensile test was performed at 23 ± 1°C to measure a tensile stress M (MPa) at a tensile strain of 10%. Further, the tensile modulus E (MPa) in the region of 5% to 20% of tensile strain was obtained according to the following equation.
_{E = (M 20 -M 5)} / (20-5)
Here, M ₂₀ is the tensile stress at 20% tensile strain, and M ₅ is the tensile stress at 5% tensile strain. The resulting tensile stress M, tensile modulus E and their product M×E are shown in Tables 6-10 below.

[Viscoelasticity measurement]
The rubber compositions of Example 1-15 and Comparative Example 1-7 were put into a mold and press-vulcanized at 160° C. for 2 minutes to prepare vulcanized rubber sheets each having a thickness of 2 mm. This vulcanized rubber sheet was cut to prepare test pieces each having a width of 4 mm and a length of 30 mm. The loss tangent of each test piece at 0° C. was measured using a viscoelasticity spectrometer (E4000 manufactured by UBM) in a tensile mode, an initial strain of 10%, a frequency of 10 Hz, and an amplitude of 0.05%. The obtained loss tangent is shown as tan δ (0° C.) in Table 6-10 below.

[hardness]
Test pieces 3 having a thickness of 2 mm, a width of 4 mm, and a length of 30 mm were put into the molds by press-vulcanizing the rubber compositions of Examples 1-15 and Comparative Examples 1-7, respectively, at 160° C. for 2 minutes. Sheets were prepared and each test piece was stored at a temperature of 23° C. for 2 weeks. Then, according to the regulations of "ASTM-D 2240-68", the hardness is obtained by pressing the automatic hardness tester (the above-mentioned "Digitest II") equipped with the type A durometer onto each of the three test pieces. Was measured. The resulting Shore A hardness is shown as Ha in Table 6-10 below.

[Nitrogen gas permeability coefficient measurement]
Using the rubber compositions of Example 1-15 and Comparative Example 1-7, test pieces having a thickness of 2 mm, a width of 4 mm and a length of 30 mm were prepared. According to JIS K7126-1 "Plastic-Film and Sheet-Gas Permeability Test Method-Part 1: Differential Pressure Method", the nitrogen gas permeability coefficient (cm ³ ·cm/cm ² /sec in the thickness direction of each test piece /cmHg) was measured. A gas permeation tester “GTR-30ANI” manufactured by GTR Tech Co. was used for the measurement. The measurement conditions were a sample temperature of 40° C., a transmission cross-sectional area of the measurement cell of 15.2 cm ² , and a differential pressure of 0.2 MPa. All measurements were made in a room at 23±0.5°C. Numerical values obtained by multiplying the obtained nitrogen gas permeation coefficient by 10 ¹⁰ are shown in Table 6-10 below as G(×10 ¹⁰ ).

[Manufacturing of tennis balls]
The rubber composition of Example 1 was put into a mold and heated at 150° C. for 4 minutes to form two half shells (thickness 3.6±0.4 mm). After adding ammonium chloride, sodium nitrite and water to one half shell, the half shell was bonded to another half shell and heated at 150° C. for 4 minutes to form a spherical core. A tennis ball was obtained by bonding two felt portions (made by Milliken Co., Ltd., thickness 3.1 mm) having seam glue adhered to the cross section to the surface of this core. Similarly, a tennis ball having cores made of the rubber compositions of Example 2-15 and Comparative Example 1-7 was manufactured. The diameter of each tennis ball was 65±1 mm, and the internal pressure thereof was 0.08±0.02 MPa.

[Sense of speed]
According to the rules of the International Tennis Standards (ITF CS 01/01), the speed (incident speed) Vi (m/s) immediately before the tennis ball collides with the tennis court and the speed (reflection speed) Vf (m) immediately after the collision. /S) and the ratio (Vf/Vi) were determined. Specifically, a tennis ball is shot toward a tennis court (trade name "Omni Court LT20" manufactured by Sumitomo Rubber Industries, Ltd.) using a ball launching device (trade name "Teniser PM100" manufactured by Silver Seiko Co., Ltd.). The incident velocity Vi (m/s) and the reflection velocity Vf (m/s) were measured by using a speed gun (trade name “Starter Sports 2” manufactured by Applied Concepts), and a ratio (Vf/Vi) was calculated. .. The measurement was performed under the conditions of a temperature of 19° C. and a relative humidity of 57%. The speed of the tennis ball at the time of launch was 30±2 (m/s), the number of revolutions was less than 3 revolutions per second, and the angle of incidence on the tennis court was 16±2° (degree). The average values of each tennis ball measured three times are shown in Table 6-10 below as Vi (m/s), Vf (m·s) and ratio (Vf/Vi). The higher the ratio (Vf/Vi), the higher the evaluation.

The following rating was performed based on the obtained ratio (Vf/Vi). The results are shown in Table 6-10 below as "speed feeling".
A: Ratio (Vf/Vi) is 0.72 or more B: Ratio (Vf/Vi) is 0.70 or more and less than 0.72 C: Ratio (Vf/Vi) is less than 0.70

[Shooting feel]
After manufacture, 50 players were hit with a tennis racket to hit a tennis ball stored for 24 hours in an environment of an atmospheric pressure, an air temperature of 20±2° C., and a relative humidity of 60%, and the feeling of hitting was heard. The following ratings were given based on the number of players who responded that "the shot feel was good". The results are shown in Tables 6-10 below.
A: 40 or more B: 30-39 C: 20-29 D: 19 or less

As shown in Tables 6-10, the rubber compositions of the examples improved the feeling of speed without significantly impairing the shot feeling, as compared with the rubber compositions of the comparative examples. From this evaluation result, the superiority of the present invention is clear.

The rubber composition described above can also be applied to the production of various balls required to have a feeling of speed and a feel at impact.

2... Tennis ball 4... Core 6... Felt part 8... Seam part

Claims (14)

A rubber composition comprising at least a base rubber and a flat filler,
The vulcanized rubber obtained by vulcanizing the rubber composition has a tensile stress M at a tensile strain of 10% and a tensile strain of 5% to 20% measured at a temperature of 23±1° C. according to JIS K6251. A rubber composition for a tennis ball, wherein a product M×E with a tensile elastic modulus E in a region is 2.0 (MPa ² ) or more and 20 (MPa ² ) or less. The average particle diameter D ₅₀ of the flat filler is 0.01 μm or more and 50 μm or less, and the flatness DL obtained by dividing the average particle diameter D ₅₀ by the average thickness T is 20 or more and 200 or less. The rubber composition according to claim 1. The rubber composition according to claim 2, wherein the amount of the flat filler having the flatness DL of 20 or more and 200 or less is 5 parts by mass or more and 150 parts by mass or less with respect to 100 parts by mass of the base rubber. The flat filler is selected from the group consisting of talc, kaolin clay, graphite, graphene, bentonite, halloysite, montmorillonite, mica, magnesium carbonate, beidellite, saponite, hectorite, nontronite, vermiculite, illite and allophane. The rubber composition according to any one of claims 1 to 3. Further including a coupling agent,
The coupling agent has a functional group X capable of interacting with the flat filler and a functional group Y capable of interacting with the base rubber, and is represented by the general formula XY. The rubber composition according to any one of claims 1 to 4, which is a compound. The functional group X is selected from the group consisting of an alkoxysilyl group (—Si—R ¹ _3-n (OR ² ) _n ), a quaternary ammonium group (—N ⁺ R ³ R ⁴ R ⁵ Z) and an epoxy group. Here, R ¹ , R ² , R ³ , R ⁴ and R ⁵ are each independently a linear, branched or cyclic alkyl group, alkenyl group, alkylphenyl group, alkenylphenyl having 1 to 30 carbon atoms. A group, a hydroxyalkyl group or a hydroxyalkenyl group, n is a natural number of 1 to 3, Z is an anion,
The rubber composition according to claim 5, wherein the functional group Y is a linear, branched or cyclic alkyl group or alkenyl group having 1 to 30 carbon atoms. The rubber composition according to claim 5, wherein the flat filler is surface-treated with the coupling agent. The rubber composition according to claim 7, wherein the amount of the coupling agent contained in the surface-treated flat filler is 1% by mass or more and 50% by mass or less. Further including other fillers that do not correspond to the flat filler,
9. The filler according to claim 1, wherein the other filler is selected from the group consisting of silica, carbon black, calcium carbonate, magnesium hydroxide, diatomaceous earth, titanium oxide, zinc oxide, bismuth oxide and barium sulfate. Rubber composition. The base rubber contains butadiene rubber and natural rubber, and the mass ratio B/N of the blending amount B of the butadiene rubber to the blending amount N of the natural rubber in the base rubber is 1.4 or less. Item 10. A rubber composition according to any one of items 1 to 9. The rubber composition according to claim 1, having a sulfur content of 0.01% by mass or more and 10% by mass or less. The rubber composition according to any one of claims 1 to 11, wherein the loss tangent tan δ at 0°C is 0.10 or less. The rubber composition according to any one of claims 1 to 12, having a Shore A hardness Ha of 20 or more and 88 or less. A tennis ball provided with a core obtained by using the rubber composition according to claim 1.

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