JP7271932B2 - Rubber composition for tennis balls - Google Patents

Description

The present invention relates to a rubber composition for tennis balls. In particular, the present invention relates to rubber compositions for use in tennis ball cores.

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

In a pressurized tennis ball, the internal pressure of the core, which is higher than the atmospheric pressure, imparts excellent resilience performance and a good feel at impact. On the other hand, since the internal pressure of the core is higher than the atmospheric pressure, the filled compressed gas gradually leaks out of the core. Gas leakage may reduce the internal pressure of the core to near atmospheric pressure. A tennis ball with a reduced internal core pressure is inferior in resilience performance and feel at impact. There is a demand for a tennis ball whose resilience performance and feel at impact are less likely to change over time.

Also, during play, the tennis ball is hit with great force and bounces on the ground. When hitting and bouncing, the tennis ball expands and contracts at high speed. The physical properties of the core rubber deteriorate due to repeated expansion and contraction at high speed. A tennis ball whose core has deteriorated rubber physical properties is inferior in resilience performance and feel at impact. Tennis balls are also required to have improved durability against impact.

Japanese Patent Application Laid-Open No. 106471/1985 discloses a core made of a rubber composition in which 1,2-bond-containing polybutadiene, natural rubber and/or 1,4-bond-containing high-cis polybutadiene are blended in a predetermined proportion. Japanese Patent Application Laid-Open No. 10-323408 proposes a tennis ball using a core obtained from a rubber composition containing short polyamide fibers having an average fiber diameter of 1 μm or less. Japanese Patent Application Laid-Open No. 2004-16532 discloses a tennis ball having a core formed by cross-linking a rubber composition containing a composite containing nylon short fibers, which is derived from a rubber-polyolefin-nylon terpolymer. is disclosed.

JP-A-60-106471 JP-A-10-323408 Japanese Patent Application Laid-Open No. 2004-16532

In the rubber composition disclosed in JP-A-60-106471, no measures are taken to prevent gas leakage, and the improvement in durability against impact by changing the base rubber is not sufficiently satisfactory. The rubber compositions disclosed in JP-A-10-323408 and JP-A-2004-16532 do not provide sufficient impact durability. Also, depending on the amount added, the hardness of the rubber composition may increase and the feel at impact may deteriorate. Furthermore, in some cases, the desired effect of preventing gas leakage cannot be obtained despite the inclusion of flat fillers.

SUMMARY OF THE INVENTION An object of the present invention is to provide a rubber composition for obtaining a tennis ball which is excellent in durability to impact and can maintain an appropriate feel at impact and resilience performance for a long period of time.

The present inventors have made intensive investigations into why polyamide fibers do not provide the desired durability to impact. That is, under the kneading conditions usually used in the field of rubber, the polyamide fiber having a high melting point cannot be melt-mixed sufficiently uniformly with the base rubber and the like, so that the rigidity of the core made of this rubber composition increases. , it was not possible to follow the large deformation at the time of impact and bounce, and sufficient impact durability was not obtained. Furthermore, it was found that the polyamide fibers dispersed in the matrix of the rubber component without being melted interfered with the orientation of other fillers and inhibited their gas leak prevention effect.

A rubber composition according to the present invention includes a base rubber and a modified polyamide resin. The melting point of this modified polyamide resin is 150° C. or lower. The amount of the modified polyamide resin with respect to 100 parts by mass of the base rubber is 1 part by mass or more and 100 parts by mass or less.

Preferably, this modified polyamide resin is a polyamide resin in which some or all of the hydrogen atoms of the amide groups contained in its main skeleton are substituted with alkoxyalkyl groups. Preferably, the alkoxyalkyl group is an alkoxymethyl group represented by the general formula ( _--CH.sub.2 --O--R), where R is a linear, branched or cyclic alkyl having 1 to 8 carbon atoms. is the base.

Preferably, the rubber composition further contains a curing catalyst for the modified polyamide resin. The amount of the curing catalyst for 100 parts by mass of the base rubber is 0.01 parts by mass or more and 1.0 parts by mass or less. The curing catalyst is selected from the group consisting of C1-C10 organic sulfonic acids and C1-C10 carboxylic acids.

Preferably, the modified polyamide resin has a modification rate of 10 mol % or more and 50 mol % or less.

Preferably, the modified polyamide resin is a modified polyamide resin selected from the group consisting of nylon 6, nylon 66, nylon 610, nylon 6/66 copolymer and nylon 6/12 copolymer.

Preferably, the modified polyamide resin has a number average molecular weight Mn of 10,000 or more and 100,000 or less before modification.

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

Preferably, the sulfur content of this rubber composition is 0.01 mass % or more and 10 mass % or less.

Preferably, the rubber composition further contains an inorganic filler. The inorganic filler is selected from the group consisting of silica, clay, talc, mica, diatomaceous earth, calcium carbonate, magnesium carbonate, magnesium hydroxide, titanium oxide, zinc oxide and bismuth oxide.

Preferably, the amount of the inorganic filler is 1 part by mass or more and 150 parts by mass or less with respect to 100 parts by mass of the base rubber. Preferably, the average particle size _D50 of this inorganic filler is 0.01 μm or more and 50 μm or less.

Preferably, the rubber composition further contains a carbon-based filler. The carbon-based filler is selected from the group consisting of carbon black, activated carbon, graphite, graphene, fullerenes and carbon nanotubes.

Preferably, the amount of the carbon-based filler is 50 parts by mass or less with respect to 100 parts by mass of the base rubber.

Preferably, the Shore A hardness Ha of this rubber composition is 20 or more and 88 or less.

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

A predetermined amount of a modified polyamide resin having a melting point of 150° C. or less is compounded in the rubber composition according to the present invention. This modified polyamide resin is sufficiently uniformly melt-kneaded with the base rubber and the like under normal kneading conditions. A core obtained from this rubber composition can withstand high-speed expansion and contraction deformation when hit or the like. In this core, deterioration of rubber physical properties due to impact or the like is suppressed. A tennis ball with this core has excellent durability against repeated hits. Further, according to this rubber composition, the gas leakage prevention effect of the core and the feel at impact of the tennis ball are not impaired. This tennis ball maintains proper resilience performance and good feel at impact for a long period of time.

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

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

FIG. 1 is a partially cutaway cross-sectional view showing a tennis ball 2 according to one embodiment of the present invention. This tennis ball 2 has a hollow core 4 , two felt portions 6 covering the core 4 , and a seam portion 8 positioned 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 from the rubber composition according to one embodiment of the invention. This rubber composition contains 1 part by mass or more and 100 parts by mass or less of the modified polyamide resin with respect to 100 parts by mass of the base rubber. The melting point of this modified polyamide resin is 150° C. or lower.

A rubber composition is usually obtained by melt-kneading a base rubber and various additives. From the viewpoint of the heat resistance of the base rubber and additives, the kneading temperature is set to 200° C. or lower. A modified polyamide resin having a melting point of 150° C. or less melts at a temperature of 200° C. or less and is easily kneaded with a base rubber or the like.

Modified polyamide resins are characterized by impact resistance and gas barrier properties. The melt-mixing of this modified polyamide resin improves the impact resistance and gas barrier properties of the rubber composition. Furthermore, excessive increases in hardness and rigidity are suppressed in this rubber composition. Further, the melt-mixed modified polyamide resin does not hinder the orientation of the filler compounded in the rubber composition. In the core 4 obtained from this rubber composition, the reinforcing effect of the modified polyamide resin is exerted without impairing the feel at impact and the effect of preventing gas leakage, and deterioration of physical properties of the rubber due to impact or the like is suppressed. The tennis ball 2 provided with this core 4 has excellent impact durability. With this tennis ball 2, a good feel at impact and appropriate resilience performance can be maintained for a long period of time.

From the viewpoint that the effect of the modified polyamide resin is significantly obtained, the amount of the modified polyamide resin is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, relative to 100 parts by mass of the base rubber. From the viewpoints of ease of melt-kneading and feel at impact, the amount of the modified polyamide resin is preferably 70 parts by mass or less, more preferably 50 parts by mass or less.

From the viewpoint of facilitating melt-kneading, the melting point of the modified polyamide resin is preferably 145° C. or lower, more preferably 140° C. or lower. The melting point of the modified polyamide resin is preferably 100° C. or higher, more preferably 110° C. or higher, from the viewpoints of ease of production and reinforcing effect. The melting point of the modified polyamide resin is measured by DSC (differential scanning calorimetry) in accordance with JIS K7121 "Method for measuring transition temperature of plastics". Specifically, it is measured using a differential scanning calorimeter (manufactured by Shimadzu Corporation under the trade name of "DSC-60") in a nitrogen atmosphere at a heating rate of 5°C/min.

The melting point of the modified polyamide resin decreases with increasing degree of modification. From the viewpoint of ease of melt-kneading, the modification rate of the modified polyamide resin is preferably 10 mol % or more, more preferably 15 mol % or more, and particularly preferably 20 mol % or more. From the viewpoint of gas barrier properties and reinforcing effects, the modification rate is preferably 50 mol % or less, more preferably 45 mol % or less, and particularly preferably 40 mol % or less. The modification rate of the modified polyamide resin (ratio of amide groups in which hydrogen atoms are substituted with alkoxyalkyl groups to all amide groups) is measured by ¹ H-NMR method (solvent: heavy chloroform).

From the viewpoint of ease of production, a modified polyamide resin in which some or all of the hydrogen atoms of the amide group (-NHCO-) contained in the main skeleton is substituted with an alkoxyalkyl group (hereinafter referred to as N-alkoxyalkylated polyamide resin ) is preferred. Examples of this alkoxyalkyl group include an alkoxymethyl group, an alkoxyethyl group, an alkoxypropyl group, an alkoxybutyl group and the like. An alkoxymethyl group represented by the general formula ( _--CH.sub.2 --OR) (here, R is a linear, branched or cyclic alkyl group having 1 to 8 carbon atoms) is more preferred. Specific examples of such alkoxymethyl groups include methoxymethyl, ethoxymethyl, propoxymethyl, i-propoxymethyl, n-butoxymethyl, t-butoxymethyl, n-hexyloxymethyl, cyclohexyloxy A methyl group etc. are mentioned. A methoxymethyl group is particularly preferred.

In embodiments in which the modified polyamide resin is an N-alkoxyalkylated polyamide resin, it is preferred that the rubber composition further comprises a curing catalyst for this modified polyamide resin. In this rubber composition, a dealcoholization reaction occurs by heating the N-alkoxyalkylated polyamide resin in the presence of a curing catalyst. This dealcoholization reaction forms crosslinks between the nitrogen atoms of the amide groups in the N-alkoxyalkylated polyamide resin.

Formation of the crosslinked structure further improves the gas barrier properties of the N-alkoxyalkylated polyamide resin. However, the crosslinked N-alkoxyalkylated polyamide resin has an increased melting point and an increased crystallinity, which may result in poor melt-kneadability with the base rubber. From this point of view, it is preferable to melt-knead the N-alkoxyalkylated polyamide resin and the base rubber in advance, and then add the curing catalyst and heat. As a result, excellent gas barrier properties due to the formation of a crosslinked structure can be imparted without impairing compatibility with the base rubber. Furthermore, the crosslinked structure formed by this method does not significantly affect the hardness and stiffness of the resulting rubber composition. According to this rubber composition, it is possible to further improve the effect of preventing gas leakage while maintaining the shot feel and impact durability.

In addition, when a sufficient gas leakage prevention effect is obtained by blending a predetermined amount of the modified amide resin, it is not always necessary to add a curing catalyst for the modified amide resin to the rubber composition.

The type of curing catalyst is not particularly limited as long as the effects of the present invention are not impaired, and organic acids such as carboxylic acids and organic sulfonic acids are appropriately selected and used. From the viewpoint of reactivity with the modified polyamide resin, a curing catalyst selected from the group consisting of carboxylic acids having 1 to 10 carbon atoms and organic sulfonic acids having 1 to 10 carbon atoms is preferred. Examples of carboxylic acids having 1 to 10 carbon atoms include acetic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, succinic acid, glutaric acid, glycolic acid, adipic acid, citric acid, benzoic acid, and phthalic acid. . Examples of organic sulfonic acids having 1 to 10 carbon atoms include benzenesulfonic acid, benzene-1,2-disulfonic acid, benzene-1,3-disulfonic acid, benzene-1,4-disulfonic acid, p-toluenesulfonic acid, o-toluenesulfonic acid, m-toluenesulfonic acid, xylenesulfonic acid, 2,4,6-trimethylbenzenesulfonic acid and the like. Citric acid, p-toluenesulfonic acid and benzenesulfonic acid are more preferred. You may use together 2 or more types of curing catalysts.

The amount of the curing catalyst added to the rubber composition is appropriately adjusted depending on the type. From the viewpoint that cross-linking can effectively occur, the amount of the curing catalyst is preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, relative to 100 parts by mass of the base rubber. The amount of the curing catalyst is preferably 1.0 parts by mass or less from the viewpoint of not impairing the feel at impact. Moreover, the amount of the curing catalyst added to the modified polyamide resin in the rubber composition is preferably 0.1% by mass or more and 10% by mass or less. When two or more curing catalysts are used in combination, the total amount is adjusted within the above range.

From the viewpoint of forming a crosslinked structure, the heating temperature after addition of the curing catalyst is preferably 100° C. or higher, more preferably 110° C. or higher. A preferable heating temperature is 150° C. or less from the viewpoint of preventing thermal deterioration. The heating time is appropriately set according to the type of curing catalyst, the amount added, and the heating temperature. From the viewpoint of improving gas barrier properties, the heating time is preferably 5 minutes or longer. From the viewpoint of preventing thermal deterioration, the preferable heating time is 60 minutes or less.

As long as the effect of the present invention can be obtained, the method for obtaining the modified polyamide resin is not particularly limited. For example, a polyamide resin modified with an alkoxymethyl group can be obtained by reacting a polyamide resin with formaldehyde in an acid solvent such as formic acid in the presence of a lower alcohol. Polyamide resins before modification (unmodified polyamide resins) include polylactams such as nylon 6, nylon 11 and nylon 12; polycondensates of dicarboxylic acids and diamines such as nylon 46, nylon 66, nylon 610 and nylon 612; Copolymers such as 6/66, nylon 6/12, nylon 6/610, and nylon 6/612 are exemplified. Modified polyamide resin obtained using a polyamide resin selected from the group consisting of nylon 6, nylon 66, nylon 610, nylon 6/66 copolymer and nylon 6/12 copolymer is more preferable, and nylon 6 is used. The resulting modified polyamide resin (modified 6-nylon) is particularly preferred.

From the viewpoint of reinforcing effect, the number average molecular weight Mn of the unmodified polyamide resin is preferably 10,000 or more, more preferably 15,000 or more. For ease of melt-kneading, the number average molecular weight Mn of the unmodified polyamide resin is preferably 100,000 or less, more preferably 80,000 or less. The number average molecular weight of the unmodified polyamide resin is measured by gel permeation chromatography (GPC method) under the following conditions.
Measuring device: Shodex GPC SYSTEM-11 (manufactured by Showa Denko)
Eluent: 0.1 mol% sodium trifluoroacetate/hexafluoroisopropanol solution Eluent flow rate: 1.0 ml/min Column: HFIP-806M (manufactured by Showa Denko Co., Ltd., 2 columns), HFIP-800 (manufactured by Showa Denko Co., Ltd., two)
Column temperature: 40°C
Reference material: Polymethyl methacrylate

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

When natural rubber and polybutadiene rubber are used together, the mass ratio B/N of the blending amount B of polybutadiene rubber to the blending amount N of natural rubber is preferably 1.4 or less, and 1.0 or less, from the viewpoint of feel at impact. is more preferable, and 0.4 or less is particularly preferable. The total amount of the base rubber may be natural rubber.

Preferably, the rubber composition further contains an inorganic filler. Inorganic fillers consist of a large number of particles. In this rubber composition, a large number of particles forming an inorganic filler are dispersed in a matrix containing a base rubber as a main component. In the specification of the present application, a matrix containing base rubber as a main component may be referred to as a "rubber component".

In the rubber composition according to the present invention, the type of inorganic filler is not particularly limited. For example, clay, diatomaceous earth, mica, talc, bentonite, hallosite, montmorillonite, beidellite, saponite, hectorite, nontronite, vermiculite, illite, allophane, silica, magnesium carbonate, calcium carbonate, magnesium hydroxide, zinc oxide, oxide Examples include titanium, bismuth oxide, barium sulfate, and the like. Preferably, one or more selected from the group consisting of silica, clay, talc, mica, diatomaceous earth, calcium carbonate, magnesium carbonate, magnesium hydroxide, titanium oxide, zinc oxide and bismuth oxide. More preferably, one or more selected from the group consisting of silica, clay, mica, calcium carbonate and zinc oxide.

The average particle diameter _D50 of the inorganic filler is appropriately set according to the type of inorganic filler selected. From the viewpoint of gas leakage prevention, the average particle diameter _D50 of the inorganic 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 feel at impact, the average particle diameter _D50 of the inorganic filler is preferably 50 μm or less, more preferably 30 μm or less, still 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) refers to the cumulative particle size distribution from the small diameter side in the particle size distribution measured by a laser diffraction particle size distribution analyzer (for example, LMS-3000 manufactured by Seishin Enterprise Co., Ltd.). means the average particle size of 50% by volume.

As long as the effects of the present invention are not hindered, the amount of the inorganic filler to be blended is not particularly limited. From the viewpoint of gas leakage prevention, the amount of the inorganic filler compounded is preferably 1 part by mass or more, more preferably 10 parts by mass or more, and particularly preferably 20 parts by mass or more with respect to 100 parts by mass of the base rubber. From the viewpoint of feel at impact and durability, the amount of the inorganic filler to be blended is preferably 150 parts by mass or less, more preferably 100 parts by mass or less, and particularly preferably 70 parts by mass or less. When two or more inorganic fillers are used in combination, the total amount is adjusted within the above range.

Preferably, the rubber composition further contains a carbon-based filler. In the present specification, the carbon-based filler means a filler composed of a large number of particles whose main component is carbon atoms. Preferably, 90% by weight or more, more preferably 95% by weight or more, particularly preferably 98% by weight or more of the constituents are carbon atoms. In the specification of the present application, carbon-based fillers are not included in the concept of inorganic fillers.

A large number of particles forming the carbon-based filler compounded in this rubber composition are dispersed in a matrix containing the base rubber as a main component. From the viewpoint of improving dispersibility, the carbon-based filler is preferably added to the rubber composition in the form of a masterbatch that is premixed with the base rubber. By adding the carbon-based filler as a masterbatch, the feel at impact and resilience performance of the resulting tennis ball are improved.

The type of carbon-based filler is not particularly limited as long as the effects of the present invention are not impaired. For example, carbon black, activated carbon, carbon fiber, graphite, graphene, fullerene, carbon nanotubes, and the like can be mentioned. Two or more carbon-based fillers may be included.

From the viewpoint of gas leakage prevention and resilience performance, preferred carbon-based fillers are selected from the group consisting of carbon black, graphite, graphene, fullerene and carbon nanotubes. More preferred carbon-based fillers are graphite and graphene.

Graphene usually has a monolayer structure in which a large number of carbon atoms are bonded in a plane, and is also called a graphene sheet. The graphene may include stacks or aggregates of graphene sheets. The number of laminated graphene sheets is preferably 100 or less, more preferably 50 or less, and particularly preferably 20 or less.

In the rubber composition according to the present invention, the method for producing graphene is not particularly limited. For example, it may be obtained from graphite, graphite oxide, etc. by a known method such as a peeling method, an ultrasonic treatment method, a chemical vapor deposition method, an epitaxial growth method, or the like.

Graphite is a carbon material in which a plurality of graphene sheets are laminated with each layer being slightly displaced, and is also called graphite. The type and production method of graphite are not particularly limited. Examples include artificial graphite obtained by heat-treating carbon.

Examples of carbon black include ketjen black, acetylene black, furnace black, oil furnace black, channel black, and thermal black. Examples of activated carbon include powdered activated carbon, granular activated carbon, crushed carbon, granular carbon, and the like. Examples of carbon fibers include polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, vegetable carbon fibers, and the like. Carbon fibers may be of the long fiber type or of the short fiber type.

The carbon nanotube may be a single-walled carbon nanotube (SWNT), a multi-walled carbon nanotube (MWNT), or a mixture thereof. . Also, the carbon nanotube may have an armchair structure, a zigzag structure, a spiral structure, or a mixture thereof.

As used herein, fullerene means a carbon molecule with a spherical structure, and is not limited to C60 fullerene. A fullerene polymer in which a plurality of C60 fullerenes are bonded may be used as long as the effects of the present invention are not impaired.

From the viewpoint of reinforcing effect and reduction of gas leakage, the average particle diameter _D50 of the carbon-based filler is preferably 0.01 μm or more, more preferably 0.02 μm or more, and particularly preferably 0.05 μm or more. From the viewpoint of miscibility with the rubber component, the average particle diameter _D50 of the carbon-based filler is preferably 100 µm or less, more preferably 80 µm or less, and particularly preferably 50 µm or less. The average particle diameter D ₅₀ (μm) of the carbon-based filler is measured in the same manner as the inorganic filler. When the carbon-based filler is graphene or graphite, the average particle diameter _D50 is preferably 1 μm or more, more preferably 5 μm or more. The average particle size _D50 of graphene and graphite is preferably 50 μm or less.

The amount of the carbon-based filler to be blended is not particularly limited as long as the effects of the present invention are not impaired. From the viewpoint of reinforcing effect and reduction of gas leakage, the amount of the carbon-based filler compounded is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and particularly 5 parts by mass or more based on 100 parts by mass of the base rubber. preferable. From the viewpoint of durability to impact, the amount of the carbon-based filler compounded is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and particularly preferably 30 parts by mass or less.

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

Examples of suitable vulcanization accelerators include guanidine-based compounds, sulfenamide-based compounds, thiazole-based compounds, thiuram-based compounds, thiourea-based compounds, dithiocarbamic acid-based compounds, aldehyde-amine-based compounds, aldehyde-ammonia-based compounds, and imidazolines. and xanthate-based compounds. From the viewpoint of resilience performance, the amount of the vulcanization accelerator compounded is preferably 1.0 parts by mass or more, more preferably 2.0 parts by mass or more, relative to 100 parts by mass of the base rubber. The content of the vulcanization accelerator is preferably 6.0 parts by mass or less.

Examples of vulcanizing aids include fatty acids such as stearic acid, metal oxides such as zinc oxide, and fatty acid metal salts such as zinc stearate. The rubber composition may further contain additives such as anti-aging agents, antioxidants, light stabilizers, softeners, processing aids, colorants, etc., as long as they do not impair the effects of the present invention.

Preferably, the rubber composition contains sulfur. Sulfur contained in the rubber composition can contribute to the formation of crosslinked structures. The crosslink density of the rubber composition affects the shot feeling and resilience performance of the tennis ball 2 obtained from this rubber composition. From the viewpoint of resilience performance, the sulfur content of the 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 Japanese Pharmacopoeia 17th Edition, General Test Methods. The sulfur contained in the rubber composition may be sulfur as an element, or may be a sulfur atom contained in a sulfur compound. This sulfur may come from vulcanizing agents or vulcanization accelerators.

From the viewpoint of durability and resilience performance, 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 feel at impact, 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 tester (trade name "Digitest II" by H. Burleith). A slab having a thickness of about 2 mm and formed by hot pressing is used for the measurement. A slab that has been stored for two weeks at a temperature of 23° C. is used for the measurements. Three slabs are superimposed during the measurement.

From the viewpoint of impact durability, the elongation at break EB of this rubber composition is preferably 50% or more, more preferably 100% or more, and particularly preferably 300% or more. From the viewpoint of feel at impact, the elongation at break EB is preferably 500% or less. The elongation at break EB of the rubber composition is measured according to the description of JIS K6251 "Vulcanized rubber and thermoplastic rubber-Determination of tensile properties".

From the standpoint of preventing leakage of gas from the core 4 and maintaining a suitable shot feel and resilience performance, 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, and more preferably 8.0×10 ⁻¹⁰ (cm ³ ·cm/cm ² /sec/cmHg) or less. 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 this rubber composition is not particularly limited as long as the object of the present invention is achieved. For example, after melt-kneading the base rubber and the modified polyamide resin using a known kneader such as a Banbury mixer, a kneader, or a roll, an inorganic filler and/or a carbon-based filler and other additives appropriately selected are added. The rubber composition may be produced by adding a vulcanizing agent or the like to the obtained kneaded product and applying pressure and heat to the kneaded product. When adding a curing catalyst for the modified polyamide resin, after melt-kneading the base rubber and the modified polyamide resin and before adding other additives, etc., the curing catalyst is added and heated at a predetermined temperature. This rubber composition may be obtained. When a carbon-based filler is blended, a masterbatch containing the base rubber and the carbon-based filler is prepared in advance, and the masterbatch is fed into the kneader described above to produce the rubber composition. good. The kneading conditions and vulcanization conditions are selected according to the compounding of the rubber composition. A preferred kneading temperature is 50°C or higher and 180°C or lower. A preferable 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 also not particularly limited. For example, by vulcanizing and molding this rubber composition in a predetermined mold, two hemispherical half shells are formed. The core 4, which is a hollow spherical body, is formed by bonding the two half shells together in a state in which the ammonium salt and the nitrite are contained therein, and then compression molding them. Inside the core 4, nitrogen gas is generated by the chemical reaction of ammonium salts and nitrites. This nitrogen gas increases the internal pressure of the core 4 . Next, the tennis ball 2 is obtained by bonding the felt portion 6, which has been previously cut into a dumbbell shape and has seam glue applied to its cross section, to the surface of the core 4. As shown in FIG.

The effects of the present invention will be clarified by examples below, but the present invention should not be construed in a limited manner based on the description of these examples.

[Example 1]
80 parts by mass of natural rubber (trade name “SMR CV60”), 20 parts by mass of polybutadiene rubber (trade name “BR01” manufactured by JSR Corporation) and 50 parts by mass of modified polyamide resin 1 (modified 6 nylon manufactured by Namuichi Co., Ltd.) , trade name “FR-101”) was put into a Banbury mixer and kneaded at 130° C. for 5 minutes. After visually confirming that the modified polyamide resin had melted, 0.5 parts by mass of p-toluenesulfonic acid (manufactured by Tokyo Chemical Industry Co., Ltd.) was added, and kneading was continued for an additional 10 minutes. Subsequently, 5 parts by mass of silica (trade name "Nipsil VN3" manufactured by Tosoh Silica Co., Ltd.), 56 parts by mass of clay (trade name "ECKALITE 120" manufactured by Imerys), 5 parts by mass of zinc oxide (correct Chemical Co., Ltd. trade name “zinc oxide type 2”) and 1 part by mass of stearic acid (NOF Corporation trade name “Tsubaki”) were added and further kneaded for 3 minutes.

After cooling the obtained kneaded product to 40 ° C. or less, 3.6 parts by mass of sulfur (manufactured by Sanshin Chemical Co., Ltd., trade name "Sunfer EX", containing 20% oil), 1 part by mass of vulcanization accelerator 1 ( Sanshin Chemical Co., Ltd. trade name "Sancellar D"), 1 part by mass of vulcanization accelerator 2 (Ouchi Shinko Kagaku Co., Ltd. trade name "Noccellar CZ") and 1.9 parts by mass of vulcanization accelerator 3 (manufactured by Ouchi Shinko Kagaku Co., Ltd. under the trade name of "Noccellar DM") and kneaded at 50°C for 3 minutes using an open roll to obtain a rubber composition of Example 1.

[Example 2-17 and Comparative Example 1-11]
The types and amounts of the base rubber, modified polyamide resin, curing catalyst, inorganic filler and carbon-based filler were changed to those shown in Table 1-5 below, and in the same manner as in Example 1, Example 2-17 and Comparative Examples 1-11 were produced. In addition, when the modified polyamide resin is not blended, materials other than sulfur and vulcanization accelerator are put into a Banbury mixer, kneaded at 90 ° C. for 5 minutes, and sulfur and vulcanization accelerator are added to the resulting kneaded product. and kneaded at 50° C. for 3 minutes on an open roll. When blending a 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. , and this masterbatch was added so as to have the formulation shown in Tables 1 to 5 below to obtain each rubber composition.

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"
Curing catalyst: p-toluenesulfonic acid modified polyamide resin manufactured by Tokyo Kasei Co., Ltd. Modified polyamide resin 1: Modified 6 nylon resin manufactured by Namiichi Co., Ltd., trade name “FR101”, modification rate 30 mol%, number average molecular weight 20,000, melting point 130 ° C. ～140℃
Modified polyamide resin 2: Modified 6 nylon resin manufactured by Namarishi Co., Ltd., trade name "FR104", modification rate 30 mol%, number average molecular weight 40,000, melting point 130°C to 140°C
Modified polyamide resin 3: Modified 12 nylon copolymer manufactured by Namiichi Co., Ltd., trade name "FR301", modification rate 20 mol%, number average molecular weight 17,000, melting point 130°C to 140°C
Polyamide fiber: 6 nylon fiber manufactured by Toray Amtex Co., Ltd., trade name "Tough Binder", fiber length 5 mm, melting point 225 ° C.
Carbon black: trade name “Show Black N330” manufactured by Cabot Japan, average particle size (D ₅₀ ) 0.03 μm, BET specific surface area 79 m ² /g
Graphite: trade name “SFG44” manufactured by Imerys, average particle size (D ₅₀ ) 49 μm, BET specific surface area 5 m ² /g
Silica: trade name “Nipsil VN3” manufactured by Tosoh Silica Corporation, average particle size (D ₅₀ ) 20 μm, BET specific surface area 200 m ² /g
Clay: kaolin clay manufactured by Imerys, trade name "ECKALITE120", average particle size ( _D50 ) 4 μm, BET specific surface area 18 m ² /g
Calcium carbonate: trade name “BF300” manufactured by Shiraishi Calcium Co., Ltd., average particle size (D ₅₀ ) 8 μm, BET specific surface area 0.27 m ² /g
Mica: trade name “SYA-21” manufactured by Yamaguchi Mica Co., Ltd., average particle size (D ₅₀ ) 27 μm, BET specific surface area 2 m ² /g
Zinc oxide: Trade name “zinc oxide type 2” manufactured by Seido Chemical Co., Ltd., average particle size (D ₅₀ ) 0.6 μm, BET specific surface area 4 m ² /g
Stearic acid: trade name "Tsubaki" manufactured by NOF Corporation
Sulfur: insoluble sulfur manufactured by Sanshin Chemical Co., Ltd., trade name "Sanfer EX", containing 20% oil Vulcanization accelerator 1: 1,3-diphenylguanidine (DPG) manufactured by Sanshin Chemical Co., Ltd., trade name "Suncellar D"
Vulcanization accelerator 2: N-cyclohexyl-2-benzothiazolylsulfenamide (CZ) manufactured by Ouchi Shinko Kagaku Co., Ltd., trade name "Noccellar CZ"
Vulcanization accelerator 3: di-2-benzothiazolyl disulfide (MBTS) manufactured by Ouchi Shinko Kagaku Co., Ltd., trade name "Noxeller DM"

[Sulfur content measurement]
The sulfur content of the rubber compositions of Examples 1-17 and Comparative Examples 1-11 was measured according to the oxygen flask combustion method described in the Japanese Pharmacopoeia 17th Edition, General Test Methods. After burning 10 mg of each rubber composition, a total of 55 mL of methanol was added, followed by exactly 20 mL of 0.005 mol/L perchloric acid solution. The sulfur content was quantified by measuring the solution obtained by standing for 10 minutes using ion chromatography (HIC-SP manufactured by Shimadzu Corporation). The sulfur content (wt.%) obtained is shown in Tables 6-10 below.

[hardness]
The rubber compositions of Examples 1-17 and Comparative Examples 1-11 were each put into a mold and press-vulcanized at 160° C. for 2 minutes to obtain a test piece 3 having a thickness of 2 mm, a width of 4 mm and a length of 30 mm. A sheet was prepared and each test piece was stored at a temperature of 23° C. for 2 weeks. After that, in accordance with the provisions of "ASTM-D 2240-68", the hardness was measured by pressing an automatic hardness tester equipped with a type A durometer (the above-mentioned "Digitest II") to each of the three superimposed test pieces. was measured. The Shore A hardness obtained is shown in Table 6-10 below as Ha.

[Elongation at break]
The rubber compositions of Examples 1-17 and Comparative Examples 1-11 were put into molds and press-vulcanized at 160° C. for 2 minutes to prepare No. 3 dumbbell-shaped test pieces with a thickness of 2 mm. According to JIS K6251 "Vulcanized rubber and thermoplastic rubber - Determination of tensile properties", a tensile test was performed at room temperature to measure the elongation at break of each test piece. The measured values obtained are shown in Table 6-10 below as EB (%).

[Measurement of nitrogen gas permeability coefficient]
Using the rubber compositions of Examples 1-17 and Comparative Examples 1-11, test pieces each having a thickness of 2 mm, a width of 4 mm, and a length of 30 mm were prepared. According to JIS K7126-1 "Plastics - Film and sheet - Gas permeability test method - Part 1: Differential pressure method", the nitrogen gas permeability coefficient in the thickness direction of each test piece (cm ³ · cm / cm ² /sec /cmHg) was measured. For the measurement, a gas permeation tester "GTR-30ANI" manufactured by GTR Tech was used. 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 performed indoors at 23±0.5°C. A numerical value obtained by multiplying the obtained nitrogen gas permeability coefficient by 10 ¹⁰ is shown in Table 6-10 below as G (×10 ¹⁰ ).

[Manufacture 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: 3.2±0.4 mm). After adding ammonium chloride, sodium nitrite and water to one half shell, it was adhered to another half shell and heated at 150° C. for 4 minutes to form a spherical core. A tennis ball was obtained by laminating two sheets of felt having seam paste on the cross section of the core on the surface of the core. The internal pressure of the core was 180 kPa. Similarly, tennis balls having cores made of the rubber compositions of Examples 2-17 and Comparative Examples 1-11 were produced.

[feeling at impact]
After production, the tennis balls were stored for 24 hours in an environment of atmospheric pressure, temperature of 20±2° C., and relative humidity of 60%. The following ratings were given based on the number of players who responded that "the feel at impact is good." The results are shown in Tables 6-10 below.
A: 40 or more B: 30-39 C: 20-29 D: 19 or less

[Durability evaluation 1]
After production, the tennis balls were stored for 24 hours under atmospheric pressure at a temperature of 20±2° C. and a relative humidity of 60%, and the DF values (forward deformation values) of the tennis balls were measured according to the regulations of the International Tennis Federation. Specifically, the tennis ball is conditioned by compressing it by 1 inch three times in each of three directions (X-axis direction, Y-axis direction, and Z-axis direction) perpendicular to each other through the center of the tennis ball. removed. After that, the amount of deformation (mm) when a load of 80.07 N was applied to the tennis ball was measured as the DF value. The temperature at the time of measurement was 25°C. Deformation amounts were measured when loads were applied from three directions of the X-axis, Y-axis, and Z-axis, and the average value was calculated. Then, after hitting the same tennis ball 30 times with a hammering tester, the DF value was measured again.

The rate of change (%) is calculated as the ratio of the amount of deformation DF2 after the hammering test to the amount of deformation DF1 before the hammering test. evaluated the sex. This evaluation result is shown in Table 6-10 below as durability 1.
A: Less than 5.0% B: 5.0% or more and less than 15.0% C: 15.0% or more and less than 25.0% D: 25.0% or more

[Durability evaluation 2]
After manufacturing, tennis balls stored under atmospheric pressure, temperature 20 ± 2 ° C., relative humidity 60% for 24 hours, and tennis balls stored under the same conditions for 3 months, the internal pressure (atmospheric pressure difference) was measured. The ratio of the internal pressure of the tennis ball after storage for 3 months to the internal pressure of the tennis ball after storage for 24 hours (internal pressure decrease rate (%)) was calculated and evaluated according to the following criteria. This evaluation result is shown as durability 2 in Table 6-10 below.
A: Less than 15% B: 15% or more and less than 20% C: 20% or more and less than 25% D: 25% or more

(summary)
(1) From a comparison between Comparative Example 1 and Examples 1-8, it was found that the break elongation EB (%) and the nitrogen gas permeability coefficient G of the rubber composition of the core decreased due to the blending of the modified polyamide resin. It can be seen that durability 1 against impact and durability 2 against a decrease in internal pressure over time have improved. On the other hand, in Comparative Example 2 in which the polyamide fiber was blended, although the elongation at break EB (%) decreased, the hardness Ha increased, resulting in poor feel at impact and durability against impact, and no gas leakage prevention effect.
(2) From the comparison between Example 6 and Examples 2, 4 and 5, the addition of the curing catalyst further decreased the nitrogen gas permeability coefficient G and allowed gas leakage without impairing other performances of the tennis ball. It can be seen that the prevention effect has improved. On the other hand, addition of only a curing catalyst as in Comparative Example 11 did not significantly affect the physical properties of the rubber composition.
(3) From the comparison of Examples 9-11 and Comparative Examples 3-5, even when used in combination with a carbon-based filler, the addition of the modified polyamide resin does not hinder the gas leakage prevention effect, and the impact durability improved. Specifically, from the comparison between Example 9 and Comparative Example 3, in which the same amount of carbon-based filler was blended, it can be seen that durability 1 and durability 2 were improved by blending the modified polyamide resin and the curing catalyst. Similarly, from the comparison between Example 10 and Comparative Example 4 and between Example 11 and Comparative Example 5, durability 1 was improved while maintaining durability 2 by blending the modified polyamide resin and the curing catalyst. I understand. Moreover, in Examples 10 and 11, the gas permeability coefficient G is lower than in Comparative Examples 4 and 5, respectively, and further improvement in the gas leakage prevention effect is expected.
(4) From the comparison of Examples 12-17 and Comparative Examples 6-10, it can be seen that even when the type and blending amount of the inorganic filler were changed, the performance improvement effect of the modified polyamide resin was obtained. Specifically, from the comparison between Example 14 and Comparative Example 8 in which the same amount of the same type of inorganic filler was blended, it can be seen that both durability 1 and durability 2 were improved by blending the modified polyamide resin and the curing catalyst. . From the comparison between Example 12 and Comparative Example 6, it can be seen that durability 1 was improved while durability 2 was maintained. Moreover, in Example 12, the gas permeability coefficient G is lower than in Comparative Example 6, and further improvement in the gas leakage prevention effect is expected. In a comparison between Example 13 and Comparative Example 7, the evaluation result of durability 1 in the 30-times impact test was the same as B, but the elongation at break EB (%) decreased. Improved durability is expected.
(5) From the comparison between Examples 15 and 16 and Comparative Example 9, even when the type of inorganic filler is changed, the effect is low due to the addition of the modified polyamide resin and the curing catalyst, but durability against impact It can be seen that the property 1 and the durability 2 against the decrease in internal pressure over time were improved. Also, from the comparison between Example 17 and Comparative Example 10, it can be seen that durability 1 was improved while durability 2 was maintained. In Example 17, the gas permeability coefficient G is lower than in Comparative Example 10, and further improvement in the gas leakage prevention effect is expected.
(6) The evaluation result of Example 16 was lower than that of Example 2 and Example 17 containing the same modified polyamide resin and curing catalyst as in Example 16, but Example 15 and Example 16 It can be seen from the comparison that the feel at impact and durability 2 were improved by increasing the blending amount of the modified polyamide resin. This indicates that it is necessary to select the optimum blending amount of the modified polyamide resin depending on the type of inorganic filler used in combination.

As described above, as shown in Tables 6 to 10, the rubber compositions of Examples are evaluated higher than the rubber compositions of Comparative Examples. From this evaluation result, the superiority of the present invention is clear.

The rubber composition described above can also be applied to manufacture various hollow balls filled with compressed gas.

2... tennis ball 4... core 6... felt part 8... seam part

Claims (16)

including a base rubber and a modified polyamide resin,
The modified polyamide resin has a melting point of 150° C. or less,
A rubber composition for a tennis ball, wherein the amount of the modified polyamide resin is 1 part by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the base rubber. 2. The rubber composition according to claim 1, wherein said modified polyamide resin is a polyamide resin in which some or all of hydrogen atoms of amide groups contained in its main skeleton are substituted with alkoxyalkyl groups. The alkoxyalkyl group is an alkoxymethyl group represented by the general formula ( _--CH.sub.2 --O--R), and R is a linear, branched or cyclic alkyl group having 1 to 8 carbon atoms. The rubber composition according to claim 2. It further contains a curing catalyst for the modified polyamide resin,
The amount of the curing catalyst with respect to 100 parts by mass of the base rubber is 0.01 parts by mass or more and 1.0 parts by mass or less,
The rubber composition according to claim 2 or 3, wherein said curing catalyst is p-toluenesulfonic acid . The rubber composition according to any one of claims 1 to 4, wherein the modified polyamide resin has a modification rate of 20 mol% or more and 50 mol% or less. 6. The method of claims 1 to 5, wherein the modified polyamide resin is a modified polyamide resin selected from the group consisting of nylon 6, nylon 66, nylon 610, nylon 6/66 copolymer and nylon 6/12 copolymer. A rubber composition according to any one of the above. The rubber composition according to any one of claims 1 to 6, wherein the modified polyamide resin has a number average molecular weight Mn of 10,000 or more and 100,000 or less before modification. The base rubber contains polybutadiene rubber and natural rubber, and the mass ratio B/N of the butadiene rubber content B to the natural rubber content N in the base rubber is 1.4 or less. Item 8. The rubber composition according to any one of Items 1 to 7. The rubber composition according to any one of claims 1 to 8, wherein the sulfur content is 0.01% by mass or more and 10% by mass or less. further comprising an inorganic filler,
10. Any one of claims 1 to 9, wherein the inorganic filler is selected from the group consisting of silica, clay, talc, mica, diatomaceous earth, calcium carbonate, magnesium carbonate, magnesium hydroxide, titanium oxide, zinc oxide and bismuth oxide. The described rubber composition. 11. The rubber composition according to claim 10, wherein the amount of the inorganic filler is 1 part by mass or more and 150 parts by mass or less with respect to 100 parts by mass of the base rubber. 12. The rubber composition according to claim 10, wherein the inorganic filler has an average particle size D50 of 0.01 [mu]m or more and 50 [mu]m or less. further comprising a carbon-based filler,
13. The rubber composition according to any one of claims 1 to 12, wherein the carbon-based filler is selected from the group consisting of carbon black, activated carbon, graphite, graphene, fullerene and carbon nanotubes. 14. The rubber composition according to claim 13, wherein the amount of the carbon-based filler is 50 parts by mass or less with respect to 100 parts by mass of the base rubber. The rubber composition according to any one of claims 1 to 14, which has a Shore A hardness Ha of 20 or more and 88 or less. A tennis ball comprising a core obtained using the rubber composition according to any one of claims 1-15.

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