JP7793941B2 - Rubber composition and crosslinked rubber molded product - Google Patents

Description

This disclosure relates to rubber compositions, and in particular to rubber compositions that require a short crosslinking time.

Rubber compositions containing base rubber, co-crosslinking agents, and crosslinking initiators are widely used as materials for forming the core of golf balls because of their good resilience.

For example, Patent Document 1 describes a golf ball containing a composition including an unsaturated polymer, a crosslinking agent, a peptizer which is a non-metallic salt of an organic sulfur compound, and an accelerator selected from the group consisting of 2-mercaptobenzothiazole and salts of 2-mercaptobenzothiazole (see Patent Document 1 (claims 1, 7, and 11)).

Patent Document 2 also describes a golf ball containing a composition that includes an unsaturated polymer, a crosslinking agent, a peptizer, and a vulcanization accelerator, where the vulcanization accelerator is selected from the group consisting of 2-mercaptobenzothiazole and salts of 2-mercaptobenzothiazole (see Patent Document 2 (Claims 1 and 25)).

Japanese Patent Application Laid-Open No. 2005-000647 Japanese Patent Application Laid-Open No. 2005-000657

Although various rubber compositions have been proposed, conventional rubber compositions have the problem of requiring a long time for crosslinking. Furthermore, in conventional rubber compositions, crosslinking is not efficiently performed, so a large amount of co-crosslinking agent is required to increase the hardness of the cured product.
The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a rubber composition that requires a short time for crosslinking and that can give a molded product having high hardness.

A rubber composition according to a first aspect of the present disclosure that has been able to solve the above-mentioned problems comprises (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, and (d) a benzothiazole compound, wherein the (d) benzothiazole compound is at least one selected from the group consisting of compounds represented by formula (1), compounds represented by formula (2), and compounds represented by formula (3), and wherein, when the amount (parts by mass) of the (b) co-crosslinking agent added per 100 parts by mass of the (a) base rubber is X, the torque (N·m) 35 seconds after the start of measurement when a crosslinking test is performed on the rubber composition at 170°C is T1, and the torque (N·m) 45 seconds after the start of measurement is T2, these satisfy the relationship {(T2 - T1)/(X - 18)} ≥ 0.46.

A rubber composition according to a second aspect of the present disclosure that has been able to solve the above-mentioned problems comprises (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, (d) a benzothiazole compound, and (f) an organic sulfur compound, wherein the (d) benzothiazole compound is at least one selected from the group consisting of compounds represented by formula (1), compounds represented by formula (2), and compounds represented by formula (3), and wherein, when the amount (parts by mass) of the (b) co-crosslinking agent added per 100 parts by mass of the (a) base rubber is X, the torque (N·m) 10 seconds after the start of measurement when a crosslinking test is performed on the rubber composition at 170°C is T3, and the torque (N·m) 60 seconds after the start of measurement is T4, these satisfy the relationship {(T4 - T3) / (X - 18)} > 0.3.

By using the rubber composition disclosed herein, molded products with high hardness can be obtained in a short crosslinking time.

FIG. 2 is a graph showing the relationship between the crosslinking time and torque of a rubber composition. FIG. 2 is a graph showing the relationship between the crosslinking time and torque of a rubber composition. FIG. 2 is a graph showing the relationship between the crosslinking time and torque of a rubber composition. FIG. 2 is a graph showing the relationship between the crosslinking time and torque of a rubber composition. FIG. 2 is a graph showing the relationship between the crosslinking time and torque of a rubber composition.

[Rubber composition]
The rubber composition of the present disclosure contains (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, and a specific (d) benzothiazole compound. The rubber composition of the present disclosure contains the specific (d) benzothiazole compound, and requires a short time for crosslinking.

The raw materials used in the rubber composition are explained below.

(a) Base Rubber As the (a) base rubber, natural rubber and/or synthetic rubber can be used. Examples of the synthetic rubber include diene rubbers such as polybutadiene rubber (BR), polyisoprene rubber (IR), styrene polybutadiene rubber (SBR), chloroprene rubber (CR), butyl rubber (IIR), and acrylonitrile butadiene rubber (NBR); and non-diene rubbers such as ethylene propylene rubber (EPM), ethylene-propylene-diene rubber (EPDM), urethane rubber, silicone rubber, acrylic rubber, epichlorohydrin rubber, polysulfide rubber, fluororubber, and chlorosulfonated polyethylene rubber. These may be used alone or in combination of two or more.

The (a) base rubber preferably contains natural rubber and/or diene rubber. The total content of natural rubber and/or diene rubber in the (a) base rubber is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more. It is also preferable that the (a) base rubber contains only natural rubber and/or diene rubber.

The base rubber (a) preferably contains polybutadiene rubber. High-cis polybutadiene, which has cis-1,4-bonds advantageous for resilience, is particularly suitable, with a content of 40% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more. The content of high-cis polybutadiene in the base rubber (a) is preferably 50% by mass or more, and more preferably 70% by mass or more.

The high-cis polybutadiene preferably has a 1,2-vinyl bond content of 2.0% by mass or less, more preferably 1.7% by mass or less, and even more preferably 1.5% by mass or less. If the 1,2-vinyl bond content is too high, the resilience may decrease.

The high-cis polybutadiene is preferably synthesized using a rare earth element catalyst. In particular, the use of a neodymium catalyst using a neodymium compound, a lanthanum series rare earth element compound, is preferred, as it produces polybutadiene rubber with a high content of 1,4-cis bonds and a low content of 1,2-vinyl bonds with excellent polymerization activity.

The high-cis polybutadiene preferably has a molecular weight distribution Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight) of 2.0 or more, more preferably 2.2 or more, even more preferably 2.4 or more, and most preferably 2.6 or more, and preferably 6.0 or less, more preferably 5.0 or less, even more preferably 4.0 or less, and most preferably 3.4 or less. If the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is too small, workability may decrease, while if it is too large, resilience may decrease. The molecular weight distribution was measured by gel permeation chromatography (Tosoh Corporation, "HLC-8120GPC") using a differential refractometer as a detector, a GMHHXL column (Tosoh Corporation), a column temperature of 40°C, and a mobile phase of tetrahydrofuran, and calculated as a value converted into standard polystyrene.

The high-cis polybutadiene preferably has a Mooney viscosity (ML ₁₊₄ (100°C)) of 30 or more, more preferably 32 or more, even more preferably 35 or more, and preferably 140 or less, more preferably 120 or less, even more preferably 100 or less, and most preferably 80 or less. The Mooney viscosity (ML ₁₊₄ (100°C)) referred to in the present disclosure is a value measured in accordance with JIS K6300 using an L rotor, with a preheating time of 1 minute, a rotor rotation time of 4 minutes, and at 100°C.

(b) Co-Crosslinking Agent The (b) co-crosslinking agent has the effect of crosslinking rubber molecules by graft polymerizing with the base rubber molecular chain. As the (b) co-crosslinking agent, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof is preferred, and a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is more preferred. The α,β-unsaturated carboxylic acid used as the (b) co-crosslinking agent preferably has 3 to 8 carbon atoms, more preferably 3 to 6 carbon atoms, and even more preferably 3 or 4 carbon atoms. The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof may be used alone or in combination of two or more types.

Examples of α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid. When the rubber composition contains only an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as a co-crosslinking agent, the rubber composition preferably further contains (e) a metal compound. This is because neutralizing the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubber composition with a metal compound provides substantially the same effects as when a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used as a co-crosslinking agent.

Examples of metals constituting the metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include monovalent metal ions such as sodium, potassium, and lithium; divalent metal ions such as magnesium, calcium, zinc, barium, and cadmium; trivalent metal ions such as aluminum; and other ions such as tin and zirconium. The metal components can be used alone or in combination. Among these, divalent metals such as magnesium, calcium, zinc, barium, and cadmium are preferred as the metal component. This is because using a divalent metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms facilitates the formation of metal crosslinks between rubber molecules. In particular, zinc salts of α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms are preferred as the divalent metal salt, and zinc acrylate is more preferred, as this enhances the resilience of the resulting golf ball. When an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and its metal salt are used in combination as a co-crosslinking agent, a metal compound (e) may be used as an optional component.

When the metal is a divalent or trivalent metal, (b) the metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms may contain, as the carboxylic acid component, a carboxylic acid other than the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Examples of such other carboxylic acids include saturated carboxylic acids such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, and behenic acid, and unsaturated carboxylic acids such as palmitoleic acid, oleic acid, linoleic acid, linolenic acid, and arachidonic acid.

The content of (b) co-crosslinking agent is preferably more than 18 parts by mass, more preferably 20 parts by mass or more, even more preferably 25 parts by mass or more, particularly preferably 27 parts by mass or more, and preferably 50 parts by mass or less, more preferably 45 parts by mass or less, and even more preferably 35 parts by mass or less, per 100 parts by mass of (a) base rubber. If the content of (b) co-crosslinking agent is more than 18 parts by mass, a small amount of (c) crosslinking initiator can be used to impart appropriate hardness to a member formed from the rubber composition. On the other hand, if the content of (b) co-crosslinking agent is 50 parts by mass or less, the member formed from the rubber composition will not be too hard.

(c) Crosslinking Initiator The (c) crosslinking initiator is blended to crosslink the (a) base rubber component. An organic peroxide is suitable as the (c) crosslinking initiator. Specific examples of the organic peroxide include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. These organic peroxides may be used alone or in combination of two or more. Of these, dicumyl peroxide is preferably used.

The one-minute half-life temperature of the (c) crosslinking initiator is preferably 120°C or higher, more preferably 140°C or higher, and even more preferably 150°C or higher, and is preferably 200°C or lower, more preferably 190°C or lower, and even more preferably 180°C or lower.

The content of the (c) crosslinking initiator is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, even more preferably 0.7 parts by mass or more, and is preferably 5.0 parts by mass or less, more preferably 2.5 parts by mass or less, even more preferably 2.0 parts by mass or less, and particularly preferably 0.9 parts by mass or less, per 100 parts by mass of the (a) base rubber. If the content is 0.2 parts by mass or more, the crosslinked rubber molded article formed from the rubber composition will not be too soft and will have good resilience. If the content is 5.0 parts by mass or less, the crosslinked rubber molded article formed from the rubber composition will have an appropriate hardness and will have good resilience and durability.

(d) Benzothiazole Compound The (d) benzothiazole compound used in the present disclosure will now be described. Examples of the (d) benzothiazole compound include compounds represented by formula (1), formula (2), and formula (3). By incorporating these (d) benzothiazole compounds, crosslinks can be formed efficiently, and the time required for crosslinking can be shortened. Furthermore, because crosslinks can be formed efficiently, molded products with high hardness can be obtained even with a small amount of (b) co-crosslinking agent incorporated.

[R ¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, or a metal atom.
R ² to R ⁵ are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms.]

[R ¹¹ to R ¹⁴ are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms; M represents a metal atom; and n represents 1 or 2.]

The compound represented by formula (3) is more preferably a compound represented by formula (31).

[R ¹¹ to R ¹⁸ are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms.
X represents a divalent metal atom.

The alkyl group having 1 to 8 carbon atoms represented by ^R1 in formula (1) or (2) includes a linear alkyl group, a branched alkyl group, and a cyclic alkyl group. The alkyl group preferably has 1 or more carbon atoms, preferably 6 or less, and more preferably 4 or less.
Examples of the linear alkyl group include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, and an n-pentyl group.
Examples of the branched alkyl group include an isopropyl group, an isobutyl group, a s-butyl group, a t-butyl group, and an isopentyl group.
Examples of the cyclic alkyl group include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.

The number of carbon atoms in the aryl group having 4 to 14 carbon atoms represented by ^R1 is preferably 12 or less, and more preferably 10 or less. Examples of the aryl group include a phenyl group and a naphthyl group.

Examples of the metal atom represented by R ¹ include sodium, potassium, lithium, magnesium, calcium, zinc, barium, and cadmium.

R ¹ is preferably a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and more preferably a hydrogen atom.

Examples of the halogen groups represented by R ² to R ⁵ and R ¹¹ to R ¹⁸ in formula (1), (2), (3) or (31) include a fluoro group (—F), a chloro group (—Cl), and a bromo group (—Br).

The alkyl group having 1 to 8 carbon atoms represented by ^R to ^R and ^{R to R} includes a linear alkyl group, a branched alkyl group, and a cyclic alkyl group. The alkyl group preferably has 1 or more carbon atoms, and 6 or less, and more preferably 4 or less.
Examples of the linear alkyl group include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, and an n-pentyl group.
Examples of the branched alkyl group include an isopropyl group, an isobutyl group, a s-butyl group, a t-butyl group, and an isopentyl group.
Examples of the cyclic alkyl group include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.

The number of carbon atoms in the aryl group having 4 to 14 carbon atoms represented by R ² to R ⁵ and R ¹¹ to R ¹⁸ is preferably 12 or less, more preferably 10 or less. Examples of the aryl group include a phenyl group and a naphthyl group.

The number of carbon atoms in the alkoxy groups having 1 to 8 carbon atoms represented by ^R to ^R and ^{R to R} is preferably 6 or less, more preferably 4 or less. Examples of the alkoxy group include a methoxy group, an ethoxy group, a propoxy group, and a butoxy group.

The number of carbon atoms in the perfluoroalkyl groups having 1 to 8 carbon atoms represented by R ² to R ⁵ and R ¹¹ to R ¹⁸ is preferably 6 or less, more preferably 4 or less. Examples of the perfluoroalkyl group include a trifluoromethyl group (—CF ₃ ), a pentafluoroethyl group (—C ₂ F ₅ ), and a heptafluoropropyl group (—C ₃ F ₇ ).

R ² to R ⁵ and R ¹¹ to R ¹⁸ are preferably a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and more preferably a hydrogen atom.

Examples of monovalent metal atoms represented by M in formula (3) include sodium, potassium, and lithium, and examples of divalent metal atoms include magnesium, calcium, zinc, barium, and cadmium.
Examples of the divalent metal atom represented by X in formula (31) include magnesium, calcium, zinc, barium, and cadmium.

The content of the (d) benzothiazole compound is preferably 0.01 part by mass or more, more preferably 0.1 part by mass or more, and even more preferably 0.5 part by mass or more, per 100 parts by mass of the (a) base rubber, and is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less. If the content of the (d) benzothiazole compound is 0.01 part by mass or more, the hardness of the obtained crosslinked rubber is further improved, and if it is 20 parts by mass or less, the physical properties of the obtained crosslinked rubber are good.

(e) Metal Compound The rubber composition may further contain a (e) metal compound. The (e) metal compound is preferably one capable of neutralizing the (b) α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubber composition. Examples of the (e) metal compound include metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. Divalent metal compounds are preferred as the (e) metal compound, and zinc compounds are more preferred. This is because divalent metal compounds react with α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms to form metal crosslinks. Furthermore, the use of zinc compounds allows for the production of crosslinked rubber molded articles with high resilience. (e) metal compounds may be used alone or in combination. The content of the (e) metal compound may be adjusted as appropriate.

(f) Organic Sulfur Compound The rubber composition may further contain an organic sulfur compound (f). The organic sulfur compound (f) does not include the benzothiazole compound (d). Examples of the organic sulfur compound (f) include at least one compound selected from the group consisting of thiophenols, thionaphthols, polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, thiazoles, and metal salts thereof. The organic sulfur compound (f) is preferably an organic sulfur compound having a thiol group (—SH) or a metal salt thereof, and more preferably a thiophenol, a thionaphthol, or a metal salt thereof.

Examples of thiols include thiophenols and thionaphthols. Examples of the thiophenols include thiophenol; thiophenols substituted with a fluoro group, such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, and pentafluorothiophenol; 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, and pentachlorothiophenol. Examples of suitable metal salts include thiophenols substituted with a chloro group such as 4-bromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenols substituted with an iodo group such as 4-iodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; and metal salts thereof. Zinc salts are preferred as metal salts.

Examples of the thionaphthols (naphthalenethiols) include 2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol, 2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and metal salts thereof, with 2-thionaphthol, 1-thionaphthol, and metal salts thereof being preferred. The metal salt is preferably a divalent metal salt, more preferably a zinc salt. Specific examples of the metal salt include the zinc salt of 1-thionaphthol and the zinc salt of 2-thionaphthol.

Polysulfides are organic sulfur compounds having polysulfide bonds, such as disulfides, trisulfides, and tetrasulfides. Diphenyl polysulfides are preferred as polysulfides.

Diphenyl polysulfides include diphenyl disulfide, as well as bis(4-fluorophenyl) disulfide, bis(2,5-difluorophenyl) disulfide, bis(2,6-difluorophenyl) disulfide, bis(2,4,5-trifluorophenyl) disulfide, bis(2,4,5,6-tetrafluorophenyl) disulfide, bis(pentafluorophenyl) disulfide, bis(4-chlorophenyl) disulfide, and bis(4-chlorophenyl) disulfide. Bis(2,5-dichlorophenyl) disulfide, bis(2,6-dichlorophenyl) disulfide, bis(2,4,5-trichlorophenyl) disulfide, bis(2,4,5,6-tetrachlorophenyl) disulfide, bis(pentachlorophenyl) disulfide, bis(4-bromophenyl) disulfide, bis(2,5-dibromophenyl) disulfide, bis(2,6-dibromophenyl) disulfide, bis(2,4,5-tribromophenyl) disulfide phenyl) disulfide, bis(2,4,5,6-tetrabromophenyl) disulfide, bis(pentabromophenyl) disulfide, bis(4-iodophenyl) disulfide, bis(2,5-diiodophenyl) disulfide, bis(2,6-diiodophenyl) disulfide, bis(2,4,5-triiodophenyl) disulfide, bis(2,4,5,6-tetraiodophenyl) disulfide, bis(pentaiodophenyl) disulfide and diphenyl disulfides substituted with halogen groups such as bis(4-methylphenyl) disulfide, bis(2,4,5-trimethylphenyl) disulfide, bis(pentamethylphenyl) disulfide, bis(4-t-butylphenyl) disulfide, bis(2,4,5-tri-t-butylphenyl) disulfide, and bis(penta-t-butylphenyl) disulfide; and diphenyl disulfides substituted with alkyl groups such as bis(4-methylphenyl) disulfide, bis(2,4,5-tri-t-butylphenyl) disulfide, and bis(penta-t-butylphenyl) disulfide.

Examples of thiurams include thiuram monosulfides such as tetramethylthiuram monosulfide, thiuram disulfides such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and tetrabutylthiuram disulfide, and thiuram tetrasulfides such as dipentamethylenethiuram tetrasulfide. Examples of thiocarboxylic acids include naphthalene thiocarboxylic acid. Examples of dithiocarboxylic acids include naphthalene dithiocarboxylic acid. Examples of sulfenamides include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide.

(f) The above-mentioned organic sulfur compounds can be used alone or in combination. (f) Preferred organic sulfur compounds are thiophenols and/or metal salts thereof, thionaphthols and/or metal salts thereof, diphenyl disulfides, and thiuram disulfides. More preferred are 2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyl disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, bis(pentabromophenyl)disulfide, and metal salts thereof.

When the rubber composition contains an organic sulfur compound (f) other than the benzothiazole compound (d), crosslinking tends to occur more slowly. Therefore, if a faster crosslinking rate is desired, the content of the organic sulfur compound (f) is preferably 5.0 parts by mass or less, more preferably 1.0 part by mass or less, even more preferably 0.05 part by mass or less, and particularly preferably 0.01 part by mass or less, per 100 parts by mass of the base rubber (a). It is also preferable that the rubber composition does not contain any organic sulfur compounds (f) other than the benzothiazole compound (d).

When the rubber composition contains an organic sulfur compound (f) other than the benzothiazole compound (d), the resilience of the resulting cured product improves, but crosslinking tends to occur more slowly. Therefore, by using the benzothiazole compound (d) in combination with the organic sulfur compound (f), the resilience of the cured product can be improved while suppressing a decrease in the crosslinking rate. Therefore, to improve resilience, the content of the organic sulfur compound (f) is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and preferably 5.0 parts by mass or less, more preferably 2.0 parts by mass or less, per 100 parts by mass of the base rubber (a). If the content of the organic sulfur compound (f) is less than 0.05 parts by mass, the effect of adding the organic sulfur compound (f) cannot be obtained, and the resilience of the crosslinked rubber molded product may not improve. Furthermore, if the content of the organic sulfur compound (f) exceeds 5.0 parts by mass, the compressive deformation of the resulting crosslinked rubber molded product may increase, resulting in reduced resilience.

The rubber composition may contain a (g) carboxylic acid and/or a salt thereof. By containing the (g) carboxylic acid and/or a salt thereof, the hardness distribution of the resulting crosslinked rubber molded article can be controlled. Examples of the (g) carboxylic acid and/or a salt thereof include aliphatic carboxylic acids, aliphatic carboxylic acid salts, aromatic carboxylic acids, and aromatic carboxylic acid salts. The (g) carboxylic acid and/or a salt thereof can be used alone or as a mixture of two or more. Note that the (g) carboxylic acid and/or a salt thereof does not include the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof used as the (b) co-crosslinking agent.

The aliphatic carboxylic acid may be either a saturated aliphatic carboxylic acid (hereinafter sometimes referred to as a "saturated fatty acid") or an unsaturated aliphatic carboxylic acid (hereinafter sometimes referred to as an "unsaturated fatty acid"). Furthermore, the aliphatic carboxylic acid may have a branched structure or a cyclic structure. The saturated fatty acid preferably has 6 or more carbon atoms, and 24 or less, more preferably 18 or less, and even more preferably 13 or less. The unsaturated fatty acid preferably has 6 or more carbon atoms, more preferably 7 or more, and even more preferably 8 or more, and 24 or less, more preferably 18 or less, and even more preferably 13 or less.

The aromatic carboxylic acids include those having a benzene ring in the molecule and those having a heteroaromatic ring in the molecule. The aromatic carboxylic acids may be used alone or in combination of two or more. Examples of carboxylic acids having a benzene ring include aromatic carboxylic acids in which a carboxyl group is directly bonded to the benzene ring, aromatic-aliphatic carboxylic acids in which an aliphatic carboxylic acid is bonded to the benzene ring, polynuclear aromatic carboxylic acids in which a carboxyl group is directly bonded to a condensed benzene ring, and polynuclear aromatic-aliphatic carboxylic acids in which an aliphatic carboxylic acid is bonded to a condensed benzene ring. Examples of carboxylic acids having a heteroaromatic ring include those in which a carboxyl group is directly bonded to a heteroaromatic ring.

The aliphatic carboxylate or aromatic carboxylate salt can be a salt of the aliphatic carboxylic acid or aromatic carboxylic acid described above. Examples of the cationic components of these salts include metal ions, ammonium ions, and organic cations. The cationic components can be used alone or as a mixture of two or more. Examples of metal ions include monovalent metal ions such as sodium, potassium, lithium, and silver; divalent metal ions such as magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel, and manganese; trivalent metal ions such as aluminum and iron; and other ions such as tin, zirconium, and titanium. Among these, divalent metal ions are preferred, with magnesium, zinc, and calcium being more preferred.

The organic cation is a cation having a carbon chain. The organic cation is not particularly limited, and examples thereof include organic ammonium ions. Examples of the organic ammonium ion include primary ammonium ions such as stearyl ammonium ion, hexyl ammonium ion, octyl ammonium ion, and 2-ethylhexyl ammonium ion; secondary ammonium ions such as dodecyl (lauryl) ammonium ion and octadecyl (stearyl) ammonium ion; tertiary ammonium ions such as trioctyl ammonium ion; and quaternary ammonium ions such as dioctyl dimethyl ammonium ion and distearyl dimethyl ammonium ion. These organic cations may be used alone or in combination of two or more.

The aliphatic carboxylic acids and/or salts thereof include saturated fatty acids and/or salts thereof, and unsaturated fatty acids and/or salts thereof. The saturated fatty acids and/or salts thereof are preferred, and caprylic acid (octanoic acid), pelargonic acid (nonanoic acid), capric acid (decanoic acid), lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, or their potassium salts, magnesium salts, calcium salts, aluminum salts, zinc salts, iron salts, copper salts, nickel salts, and cobalt salts are preferred. The unsaturated fatty acids and/or salts thereof are palmitoleic acid, oleic acid, linoleic acid, or arachidonic acid, or their potassium salts, magnesium salts, calcium salts, aluminum salts, zinc salts, iron salts, copper salts, nickel salts, and cobalt salts are preferred.

The aromatic carboxylic acid and/or its salt is particularly preferably benzoic acid, butylbenzoic acid, anisic acid (methoxybenzoic acid), dimethoxybenzoic acid, trimethoxybenzoic acid, dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, acetoxybenzoic acid, biphenylcarboxylic acid, naphthalenecarboxylic acid, anthracenecarboxylic acid, furancarboxylic acid, or thenoic acid, or their potassium salts, magnesium salts, calcium salts, aluminum salts, zinc salts, iron salts, copper salts, nickel salts, or cobalt salts.

The content of the (g) carboxylic acid and/or its salt is, for example, preferably 1 part by weight or more, more preferably 2 parts by weight or more, and even more preferably 3 parts by weight or more, per 100 parts by weight of the (a) base rubber, and is preferably 30 parts by weight or less, more preferably 20 parts by weight or less, and even more preferably 15 parts by weight or less.

(Other ingredients)
The rubber composition may contain additives such as pigments, fillers for adjusting the weight, antioxidants, peptizers, softeners, etc. as needed. The rubber composition may also contain rubber powder obtained by pulverizing the core of a golf ball or scraps generated during the production of the core.

Examples of pigments that can be compounded into the rubber composition include white pigments, blue pigments, and purple pigments. Titanium oxide is preferably used as the white pigment. There are no particular restrictions on the type of titanium oxide, but rutile-type titanium oxide is preferably used because of its excellent hiding power. The titanium oxide content is preferably 0.5 parts by mass or more, more preferably 2 parts by mass or more, and preferably 8 parts by mass or less, more preferably 5 parts by mass or less, per 100 parts by mass of (a) base rubber.

It is also a preferred embodiment that the rubber composition contains a white pigment and a blue pigment. The blue pigment is blended to make the white appear more vivid, and examples of the blue pigment include ultramarine, cobalt blue, and phthalocyanine blue. Furthermore, examples of the purple pigment include anthraquinone violet, dioxazine violet, and methyl violet.

The filler used in the rubber composition is blended as a weight adjuster to adjust the mass of the resulting crosslinked rubber molded product, and may be blended as needed. Examples of such fillers include inorganic fillers such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder.

The content of the antioxidant is preferably 0.1 parts by mass or more and 1 part by mass or less per 100 parts by mass of (a) base rubber. Furthermore, the content of the peptizing agent is preferably 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of (a) base rubber.

The rubber composition may include an embodiment (embodiment 1) in which the (d) benzothiazole compound contains at least one selected from the group consisting of compounds represented by formula (1), compounds represented by formula (2), and compounds represented by formula (3); or an embodiment (embodiment 2) in which the (d) benzothiazole compound contains at least one selected from the group consisting of compounds represented by formula (1), compounds represented by formula (2), and compounds represented by formula (3) and (f) an organic sulfur compound.

The molar ratio ((d)/(c)) of the molar amount of the (c) crosslinking initiator to the molar amount of the (d) benzothiazole compound in the rubber composition is preferably 0.5 or greater, more preferably 1.0 or greater, and even more preferably 1.5 or greater, and is preferably 10 or less, more preferably 7 or less, and even more preferably 5 or less. If the molar ratio ((d)/(c)) is 0.5 or greater, the hardness of the resulting crosslinked rubber is improved, and if it is 10 or less, the physical properties of the resulting crosslinked rubber are good.

In the rubber composition of the second embodiment, the mass ratio ((d)/(f)) of the (d) benzothiazole compound to the (f) organic sulfur compound is preferably 0.5 or greater, more preferably 1 or greater, and even more preferably 1.5 or greater, and is preferably 10 or less, more preferably 8 or less, and even more preferably 5 or less. If the mass ratio ((d)/(f)) is 0.5 or greater, crosslinking proceeds efficiently, and if it is 10 or less, the physical properties of the resulting crosslinked rubber are good.

The rubber composition preferably contains (e) a metal compound, more preferably a zinc compound, and particularly preferably zinc oxide.

When the rubber composition contains a compound represented by formula (1) and/or a compound represented by formula (2) and (e) a metal compound, the ratio of the total number of moles M2 of the compound represented by formula (1) and the compound represented by formula (2) to the product of the number of moles M1 of the (e) metal compound and the valence of the contained metal (M2/(M1 x valence)) is preferably 0.01 or more, more preferably 0.02 or more, even more preferably 0.04 or more, and is preferably 1.0 or less, more preferably 0.5 or less, even more preferably 0.1 or less.

When the rubber composition contains a compound represented by formula (3) and an (e) metal compound, the ratio of the number of moles M21 of the structure derived from the compound represented by formula (1) in the compound represented by formula (3) to the sum of the product of the number of moles M1 of the (e) metal compound and the valence of the contained metal and the product of the number of moles M22 of the metal component in the compound represented by formula (3) and the valence of the metal (M21/{(M1 × valence) + (M22 × valence)}) is preferably 0.01 or more, more preferably 0.02 or more, even more preferably 0.04 or more, and is preferably 1.0 or less, more preferably 0.5 or less, even more preferably 0.1 or less.

The rubber composition used in the present disclosure is obtained by mixing and kneading (a) base rubber, (b) co-crosslinking agent, (c) crosslinking initiator, (d) benzothiazole compound, and, if necessary, other additives, etc. The kneading method is not particularly limited, and may be performed using, for example, a known kneading machine such as a kneading roll, a Banbury mixer, or a kneader.

Physical Properties of Rubber Composition The rubber composition of the first aspect is also characterized in that, when X is the amount (parts by mass) of (b) co-crosslinking agent added relative to 100 parts by mass of (a) base rubber, T1 is the torque (N m) 35 seconds after the start of measurement when a crosslinking test of the rubber composition is conducted at 170°C, and T2 is the torque (N m) 45 seconds after the start of measurement, the ratio {(T2-T1)/(X-18)} is within a predetermined range. If the ratio {(T2-T1)/(X-18)} is within the predetermined range, crosslinking can be efficiently formed.

When the rubber composition of Aspect 1 contains a compound represented by Formula (1) and/or a compound represented by Formula (2), the rubber composition preferably has a ratio {(T2-T1)/(X-18)} of 0.46 or greater, more preferably greater than 0.55, more preferably greater than 0.60, and even more preferably greater than 0.65, and is preferably 3 or less, more preferably 2 or less, and even more preferably 1 or less.

When the rubber composition of Aspect 1 contains a compound represented by formula (1) and/or a compound represented by formula (2), the difference (T2-T1) is preferably 6 or more, more preferably 7 or more, and even more preferably 8 or more, and is preferably 20 or less, more preferably 17 or less, and even more preferably 15 or less.

When the rubber composition of Aspect 1 contains a compound represented by Formula (3), the ratio {(T2-T1)/(X-18)} of the rubber composition is preferably 0.46 or greater, more preferably greater than 0.46, even more preferably greater than 0.5, and particularly preferably greater than 0.6, and is preferably 2 or less, more preferably 1.5 or less, and even more preferably 1 or less.

When the rubber composition of Aspect 1 contains a compound represented by formula (3), the difference (T2 - T1) is preferably 0.3 or more, more preferably 0.4 or more, even more preferably 0.5 or more, and particularly preferably 3 or more, and is preferably 20 or less, more preferably 17 or less, even more preferably 15 or less, and particularly preferably 10 or less.

The rubber composition of Aspect 2 (when containing (f) an organic sulfur compound) is characterized in that, when the amount (parts by mass) of (b) co-crosslinking agent added per 100 parts by mass of (a) base rubber is X, the torque (N·m) 10 seconds after the start of measurement when a crosslinking test is conducted on the rubber composition at 170°C is T3, and the torque (N·m) 60 seconds after the start of measurement is T4, the ratio {(T4-T3)/(X-18)} falls within a predetermined range. Efficient crosslinking can be achieved when the ratio {(T4-T3)/(X-18)} falls within the predetermined range.

In the rubber composition of Aspect 2, the ratio {(T4-T3)/(X-18)} is preferably greater than 0.3, more preferably 0.35 or greater, and even more preferably 0.4 or greater, and is preferably 2 or less, more preferably 1.5 or less, and even more preferably 1 or less.

For the rubber composition of Aspect 2, the difference (T4 - T3) is preferably 1.5 or more, more preferably 2 or more, and even more preferably 3 or more, and is preferably 8 or less, more preferably 7 or less, and even more preferably 6 or less.

[Crosslinked rubber molded body]
The crosslinked rubber molded article of the present disclosure is characterized by being formed from the rubber composition. The crosslinked rubber molded article can be obtained by molding the kneaded rubber composition in a mold. The molding temperature is preferably 120°C or higher, more preferably 150°C or higher, and preferably 250°C or lower. The molding pressure is preferably 2.9 MPa to 11.8 MPa. The molding time is preferably 10 minutes to 60 minutes.

Applications of the crosslinked rubber molded article include sporting goods such as golf balls, tennis balls, and grips; industrial goods such as hoses, belts, and mats; shoe soles, tires, resin additives, vibration-isolating rubber, and fenders. Examples of the golf ball include those having components formed from the rubber composition.

The present disclosure will be described in detail below using examples, but the present disclosure is not limited to the examples below, and all modifications and embodiments that do not deviate from the spirit of the present disclosure are included within the scope of the present disclosure.

[Evaluation method]
(1) Slab hardness (Shore C)
Sheets approximately 2 mm thick were prepared by heat press molding (170°C, 20 minutes), and these test specimens were stored at a temperature of 23±2°C and a relative humidity of 50±5% for 12 hours or more. Three or more of these sheets were stacked to avoid the influence of the measurement substrate, and the hardness was measured using an automatic hardness tester (Digitest II, manufactured by H. Burleith Co.). A Shore C detector was used.

(2) Lubke's rebound resilience (%)
The impact resilience test was performed in accordance with JIS K6255 (2013). A sheet approximately 2 mm thick was prepared by hot press molding (170°C, 20 minutes). Six circular pieces with a diameter of 28 mm were punched out from the sheet and stacked to prepare cylindrical test pieces with a thickness of approximately 12 mm and a diameter of 28 mm. These test pieces were stored at a temperature of 23±2°C and a relative humidity of 50±5% for 12 hours or more. The impact resilience of the prepared test pieces was measured using a Lübke impact resilience test measuring device (manufactured by Ueshima Seisakusho Co., Ltd.). The flat surface of the stacked test pieces was supported by a mechanical fixation method, and the measurement conditions were a temperature of 23°C, a relative humidity of 50%, an impact end diameter of 12.50±0.05 mm, an impact mass of 0.35±0.01 kg, and an impact velocity of 1.4±0.01 m/s.

(3) Crosslinking Test After sampling an appropriate amount of each rubber composition, the vulcanization curve was measured at 170°C using a vulcanization tester (Curastometer Model 7, manufactured by JSR Trading Co., Ltd.). According to "9. Die vulcanization test method A" in "Method for determining vulcanization characteristics using a vibration vulcanization tester" of JIS K6300-2 (2001), a low-amplitude sinusoidal vibration was applied to a rubber test piece from the lower die so as not to break the test piece, and the torque transmitted from the test piece to the upper die was measured from under-vulcanization to over-vulcanization. The measurement conditions were a torsional vibration frequency of 100 times per minute and an amplitude angle of 1°.

[Preparation of Rubber Composition]
Rubber compositions were prepared by kneading the rubber compositions having the formulations shown in Table 1 with a kneading roll. The obtained rubber compositions were evaluated.

The raw materials used in Tables 1 to 4 are as follows:
BR: "BR730" manufactured by JSR Corporation (high cis polybutadiene rubber (cis-1,4-bond content = 96 mass%, 1,2-vinyl bond content = 1.3 mass%, Mooney viscosity (ML ₁₊₄ (100°C)) = 55, molecular weight distribution (Mw/Mn) = 3))
ZN-DA90S: Zinc acrylate (containing 10% by mass of zinc stearate), manufactured by Nisshoku Techno Fine Chemical Co., Ltd.
ZnO: "Ginrei R" (zinc oxide) manufactured by Toho Zinc Co., Ltd.
PBDS: bis(pentabromophenyl) disulfide manufactured by Kawaguchi Chemical Industry Co., Ltd. PCTP-Zn: pentachlorothiophenol zinc salt manufactured by Fujifilm Wako Chemical Co., Ltd. Nocceler (registered trademark) EUR: N,N'-diethylthiourea manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Nocceler MP: 2-mercaptobenzothiazole manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Nocceler CZ-G: N-cyclohexyl-2-benzothiazolylsulfenamide manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Nocceler TBZTB: tetrabenzylthiuram disulfide manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Nocceler EZ: zinc diethyldithiocarbamate manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Thiazole disulfide: 2,2'-dithiobisbenzothiazole manufactured by Tokyo Chemical Industry Co., Ltd. Thiazole Zn: 2-mercaptobenzothiazole zinc salt, manufactured by Fujifilm Wako Chemical Co., Ltd. Thiazole Na: 2-mercaptobenzothiazole sodium salt, manufactured by Tokyo Chemical Industry Co., Ltd. DCP: "Percumyl (registered trademark) D" (dicumyl peroxide), manufactured by NOF Corporation

Tables 1 to 4 show the slab hardness and Lübke rebound modulus of the crosslinked rubber obtained from the rubber composition. Figures 1 to 5 also show the relationship between crosslinking time and torque in the crosslinking test.

Rubber compositions No. 4, 18, and 19 contain a compound represented by formula (1) as the (d) benzothiazole compound, and {(T2 - T1)/(X-18)} is 0.46 or greater. Rubber compositions No. 24 and 25 contain a compound represented by formula (3) as the (d) benzothiazole compound, and {(T2 - T1)/(X-18)} is 0.46 or greater. These rubber compositions No. 4, 18, 19, 24, and 25 crosslink in a shorter time than rubber compositions No. 1, 8 to 11. Furthermore, comparing rubber compositions No. 1 and No. 4, rubber composition No. 4 provides a crosslinked rubber with increased hardness without increasing the amount of co-crosslinking agent used. Furthermore, comparing rubber compositions No. 10 and No. 24, rubber composition No. 24 provides a crosslinked rubber with increased hardness. 24 increases the hardness of the resulting crosslinked rubber without increasing the amount of co-crosslinking agent used.

In addition, (d) Rubber compositions No. 3, 5 to 7, which contained compounds other than the benzothiazole compound, all required a long time for crosslinking, and the hardness of the resulting crosslinked rubber was reduced.

In addition, rubber compositions No. 2, 12-17, 22, and 23, which contain bis(pentabromophenyl) disulfide, pentachlorothiophenol zinc salt, and thiazole disulfide, have improved rebound resilience but require longer times for crosslinking.

Rubber compositions Nos. 28 to 31 contain a compound represented by formula (1) as a (d) benzothiazole compound and an organic sulfur compound, and {(T4 - T3) / (X - 18)} is greater than 0.30. Rubber compositions Nos. 32 and 33 contain a compound represented by formula (3) as a (d) benzothiazole compound and an organic sulfur compound, and {(T4 - T3) / (X - 18)} is greater than 0.30. Rubber compositions Nos. 34 to 37 contain an organic sulfur compound but do not contain a (d) benzothiazole compound.
Rubber compositions Nos. 28 to 33 formed crosslinks in a shorter time than rubber compositions Nos. 34 to 37. Furthermore, when comparing rubber compositions Nos. 30 and 33 with No. 35, which contain the same amount of co-crosslinking agent, rubber compositions Nos. 30 and 33 have higher slab hardness than rubber composition No. 35. Therefore, by using the benzothiazole compound (d) represented by formula (1) in combination with the organic sulfur compound (f), it is possible to improve the rebound resilience while suppressing a decrease in hardness.

The present disclosure (1) relates to a rubber composition comprising (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, and (d) a benzothiazole compound, wherein the (d) benzothiazole compound is at least one selected from the group consisting of compounds represented by formula (1), compounds represented by formula (2), and compounds represented by formula (3), and wherein, when the amount (parts by mass) of the (b) co-crosslinking agent added per 100 parts by mass of the (a) base rubber is X, the torque (N·m) 35 seconds after the start of measurement when a crosslinking test is performed on the rubber composition at 170°C is T1, and the torque (N·m) 45 seconds after the start of measurement is T2, these satisfy the relationship {(T2 - T1)/(X-18)} ≥ 0.46.

[R ¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 1 to 8 carbon atoms, or a metal atom.
^R2 to ^R5 are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms.
R ¹¹ to R ¹⁴ are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms. M represents a metal atom. n represents 1 or 2.]

Disclosure (2) relates to a rubber composition comprising (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, (d) a benzothiazole compound, and (f) an organic sulfur compound, wherein the (d) benzothiazole compound is at least one selected from the group consisting of compounds represented by formula (1), compounds represented by formula (2), and compounds represented by formula (3), and wherein, when the amount (parts by mass) of the (b) co-crosslinking agent added per 100 parts by mass of the (a) base rubber is X, the torque (N·m) 10 seconds after the start of measurement when a crosslinking test is performed on the rubber composition at 170°C is T3, and the torque (N·m) 60 seconds after the start of measurement is T4, these satisfy the relationship {(T4 - T3) / (X - 18)} > 0.3.

[R ¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, or a metal atom.
^R2 to ^R5 are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms.
R ¹¹ to R ¹⁴ are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms. M represents a metal atom. n represents 1 or 2.]

The present disclosure (3) is a rubber composition according to disclosure (1) or (2), in which the content of the (d) benzothiazole compound is 0.01 to 20 parts by mass per 100 parts by mass of the (a) base rubber.

Disclosure (4) is a rubber composition according to any one of disclosures (1) to (3), in which the (b) co-crosslinking agent is an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof.

Disclosure (5) is a rubber composition according to any one of disclosures (1) to (4), in which the content of (b) the co-crosslinking agent is 15 to 50 parts by mass per 100 parts by mass of the (a) base rubber.

The present disclosure (6) is a crosslinked rubber molded article formed from the rubber composition described in any one of the present disclosures (1) to (5).

The present disclosure (7) is a golf ball characterized by having components formed from the rubber composition described in any one of the present disclosures (1) to (5).

Using the rubber composition of the present disclosure, crosslinking can be achieved in a short period of time, and a highly hard crosslinked rubber molded article can be obtained. Therefore, the rubber composition of the present disclosure can be used in sports goods such as golf balls, tennis balls, and grips; industrial goods such as hoses, belts, and mats; shoe soles, tires, resin additives, vibration-proof rubber, and fenders.

Claims (7)

(a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, and (d) a benzothiazole compound,
the total content of the natural rubber and/or diene rubber in the (a) base rubber is 90% by mass or more,
the (b) co-crosslinking agent is an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof,
the one-minute half-life temperature of the (c) crosslinking initiator is 150°C or higher and 180°C or lower;
The (d) benzothiazole compound is at least one selected from the group consisting of a compound represented by formula (1), a compound represented by formula (2), and a compound represented by formula (3), and
A rubber composition characterized in that, when X is the amount (parts by mass) of the co-crosslinking agent (b) added relative to 100 parts by mass of the base rubber (a), T1 is the torque (N m) 35 seconds after the start of measurement when a crosslinking test is performed on the rubber composition at 170°C, and T2 is the torque (N m) 45 seconds after the start of measurement, these satisfy the relationship {(T2-T1)/(X-18)}≧0.46.
[R ¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 14 carbon atoms, or a metal atom.
^R2 to ^R5 are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms.
R ¹¹ to R ¹⁴ are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms. M represents a metal atom. n represents 1 or 2.] (a) a base rubber, (b) a co-crosslinking agent, (c) a crosslinking initiator, (d) a benzothiazole compound, and (f) an organic sulfur compound,
the total content of the natural rubber and/or diene rubber in the (a) base rubber is 90% by mass or more,
the (b) co-crosslinking agent is an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof,
the one-minute half-life temperature of the (c) crosslinking initiator is 150°C or higher and 180°C or lower;
The (d) benzothiazole compound is at least one selected from the group consisting of a compound represented by formula (1), a compound represented by formula (2), and a compound represented by formula (3), and
A rubber composition characterized in that, when X is the amount (parts by mass) of the co-crosslinking agent (b) added relative to 100 parts by mass of the base rubber (a), T3 is the torque (N m) 10 seconds after the start of measurement when a crosslinking test is performed on the rubber composition at 170°C, and T4 is the torque (N m) 60 seconds after the start of measurement, these satisfy the relationship {(T4 - T3)/(X - 18)} > 0.3.

[R ¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 14 carbon atoms, or a metal atom.
^R2 to ^R5 are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms.
R ¹¹ to R ¹⁴ are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 14 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms. M represents a metal atom. n represents 1 or 2.] The rubber composition according to claim 1 or 2, wherein the content of the (d) benzothiazole compound is 0.01 to 20 parts by mass per 100 parts by mass of the (a) base rubber. The rubber composition according to any one of claims 1 to 3, wherein the molar ratio ((d)/(c)) of the molar amount of the (c) crosslinking initiator to the molar amount of the (d) benzothiazole compound in the rubber composition is 0.5 or more and 10 or less . The rubber composition according to any one of claims 1 to 4, wherein the content of the (b) co-crosslinking agent is more than 18 parts by mass and not more than 50 parts by mass per 100 parts by mass of the (a) base rubber. A crosslinked rubber molded article formed from the rubber composition described in any one of claims 1 to 5. A golf ball having components formed from the rubber composition described in any one of claims 1 to 5.