Crosslinkable rubber composition, crosslinked rubber object, and sealing material

Abstract

Provided is a crosslinkable rubber composition which comprises a nitrile-group-containing copolymer rubber having a content of units of an α,β-ethylenically unsaturated nitrile monomer of 35 wt% or less and an iodine value of 120 or less, silica, clay, and an organic-peroxide crosslinking agent, wherein the amount of the silica is 30 parts by weight or larger per 100 parts by weight of the nitrile-group-containing copolymer rubber.

Description

Crosslinkable rubber composition, crosslinked rubber product, and sealing material

[0001] The present invention relates to a cross-linkable rubber composition that provides a cross-linked rubber product that is excellent in low-temperature properties and polycyclic aromatic compound resistance, and that also has good compression set resistance and oil swelling resistance (small volume change in oil), and to a cross-linked rubber product obtained using the cross-linkable rubber composition.

[0002] Compared with general nitrile group-containing copolymer rubbers such as acrylonitrile-butadiene copolymer rubbers, which have many carbon-carbon unsaturated bonds in their main chain structures, saturated nitrile group-containing copolymer rubbers, typified by hydrogenated acrylonitrile-butadiene copolymer rubber, are excellent in heat resistance, high-temperature and long-term oil resistance, ozone resistance, etc., and are therefore suitable for use in rubber parts such as automotive seals, hoses, tubes, etc. In recent years, the oil resistance required of rubber parts such as automotive seals, hoses, tubes, etc. has required not only oil swelling resistance (small volume change in oil) but also excellent resistance to polycyclic aromatic compounds.

[0003] For example, Patent Document 1 discloses a crosslinkable nitrile rubber composition containing a nitrile group-containing copolymer rubber (A) having an iodine value of 120 or less and containing α,β-ethylenically unsaturated nitrile monomer units, conjugated diene monomer units, α,β-ethylenically unsaturated monocarboxylic acid ester monomer units, and carboxyl group-containing monomer units, wherein the content of the α,β-ethylenically unsaturated nitrile monomer units is 8 to 18 wt %, silica (B1), clay (B2), calcium carbonate (B3), and a polyamine-based crosslinking agent (C) in specific proportions. The crosslinkable nitrile rubber composition disclosed in Patent Document 1 can provide a crosslinked rubber product that is excellent in resistance to swelling in oil and has a certain degree of improved resistance to polycyclic aromatic compounds, but there has been a demand for further improvement in resistance to polycyclic aromatic compounds, in particular, further improvement in resistance to polycyclic aromatic compounds after heat aging.

[0004] International Publication No. 2020 / 004284

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a cross-linkable rubber composition that gives a cross-linked rubber product that is excellent in low-temperature properties and polycyclic aromatic compound resistance, and that is also capable of achieving both compression set resistance and oil swelling resistance (small volume change in oil).

[0006] As a result of intensive research into achieving the above object, the present inventors have found that the above object can be achieved by incorporating silica, clay, and an organic peroxide crosslinking agent into a specific nitrile group-containing copolymer rubber containing α,β-ethylenically unsaturated nitrile monomer units in a content of 35% by weight or less, and by controlling the compounding ratio of silica within a specific range, thereby completing the present invention.

[0007] That is, according to the present invention, the following cross-linkable rubber composition, cross-linked rubber product, and sealing material are provided. [1] A cross-linkable rubber composition containing a nitrile group-containing copolymer rubber containing α,β-ethylenically unsaturated nitrile monomer units in a content ratio of 35% by weight or less and having an iodine value of 120 or less, silica, clay, and an organic peroxide cross-linking agent, wherein the content of the silica per 100 parts by weight of the nitrile group-containing copolymer rubber is 30 parts by weight or more. [2] The cross-linkable rubber composition according to [1], further containing a plasticizer, wherein the content of the plasticizer per 100 parts by weight of the nitrile group-containing copolymer rubber is 22 parts by weight or more. [3] The cross-linkable rubber composition according to [1] or [2], further containing a co-cross-linking agent, wherein the content of the co-cross-linking agent per 100 parts by weight of the nitrile group-containing copolymer rubber is 2.5 parts by weight or more. [4] The cross-linkable rubber composition according to any one of [1] to [3], further comprising a silane coupling agent, the content of the silane coupling agent being 0.5 parts by weight or more relative to 100 parts by weight of the nitrile group-containing copolymer rubber. [5] A cross-linked rubber product obtained by cross-linking the cross-linkable rubber composition according to any one of [1] to [4]. [6] A sealing material comprising the cross-linked rubber product according to [5].

[0008] According to the present invention, it is possible to provide a cross-linkable rubber composition that gives a cross-linked rubber product that is excellent in low-temperature properties and polycyclic aromatic compound resistance, and that is also able to achieve both compression set resistance and resistance to swelling in oil (small volume change in oil), as well as a cross-linked rubber product that is obtained using such a cross-linkable rubber composition, and that is excellent in low-temperature properties and polycyclic aromatic compound resistance, and that is also able to achieve both compression set resistance and resistance to swelling in oil (small volume change in oil).

[0009] The crosslinkable rubber composition of the present invention is a crosslinkable rubber composition containing a nitrile group-containing copolymer rubber containing α,β-ethylenically unsaturated nitrile monomer units in a content ratio of 35% by weight or less and having an iodine value of 120 or less, silica, clay, and an organic peroxide crosslinking agent, wherein the content of the silica is in the range of 30 parts by weight or more per 100 parts by weight of the nitrile group-containing copolymer rubber.

[0010] <Nitrile Group-Containing Copolymer Rubber> The nitrile group-containing copolymer rubber used in the present invention contains α,β-ethylenically unsaturated nitrile monomer units in a content ratio of 35% by weight or less and has an iodine value in the range of 120 or less. The nitrile group-containing copolymer rubber used in the present invention can be obtained, for example, by copolymerizing an α,β-ethylenically unsaturated nitrile monomer, a conjugated diene monomer, and other copolymerizable monomers that are added as needed.

[0011] The α,β-ethylenically unsaturated nitrile monomer is not limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples thereof include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and ethacrylonitrile. Of these, acrylonitrile and methacrylonitrile are preferred, and acrylonitrile is particularly preferred. A combination of these α,β-ethylenically unsaturated nitrile monomers may be used.

[0012] The content of α,β-ethylenically unsaturated nitrile monomer units in the nitrile group-containing copolymer rubber used in the present invention is 35% by weight or less, preferably 10 to 35% by weight, more preferably 12 to 30% by weight, even more preferably 15 to 28% by weight, and particularly preferably 18 to 25% by weight, based on the total monomer units. If the content of α,β-ethylenically unsaturated nitrile monomer units is too high, the cold resistance and polycyclic aromatic compound resistance after heat aging will be reduced. Furthermore, by setting the content of α,β-ethylenically unsaturated nitrile monomer units to 10% by weight or more, the oil swelling resistance can be further improved.

[0013] As the conjugated diene monomer, conjugated diene monomers having 4 to 6 carbon atoms such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, and chloroprene are preferred, 1,3-butadiene and isoprene are more preferred, and 1,3-butadiene is particularly preferred. The conjugated diene monomers may be used alone or in combination.

[0014] The content of conjugated diene monomer units (including hydrogenated portions) in the nitrile group-containing copolymer rubber used in the present invention is preferably 20 to 70% by weight, more preferably 30 to 60% by weight, even more preferably 35 to 55% by weight, and particularly preferably 42 to 52% by weight, based on the total monomer units. By setting the content of conjugated diene monomer units within the above range, the obtained cross-linked rubber product can be made excellent in rubber elasticity while maintaining good heat resistance and chemical resistance stability.

[0015] The nitrile group-containing copolymer rubber used in the present invention preferably contains α,β-ethylenically unsaturated monocarboxylic acid ester monomer units in addition to α,β-ethylenically unsaturated nitrile monomer units and conjugated diene monomer units.

[0016] The α,β-ethylenically unsaturated monocarboxylic acid ester monomer forming the α,β-ethylenically unsaturated monocarboxylic acid ester monomer unit is not particularly limited, and examples thereof include α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomer, α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomer, α,β-ethylenically unsaturated monocarboxylic acid aminoalkyl ester monomer, α,β-ethylenically unsaturated monocarboxylic acid hydroxyalkyl ester monomer, and α,β-ethylenically unsaturated monocarboxylic acid fluoroalkyl ester monomer. Among these, α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomer or α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomer is preferred, and α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomer is more preferred. By incorporating an α,β-ethylenically unsaturated monocarboxylic acid ester monomer unit, the resulting cross-linked rubber product can have better low-temperature properties.

[0017] The α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomer preferably has an alkyl group having 3 to 10 carbon atoms, more preferably an alkyl group having 3 to 8 carbon atoms, and even more preferably an alkyl group having 4 to 6 carbon atoms.

[0018] Specific examples of the α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomer include alkyl acrylate monomers such as methyl acrylate, ethyl acrylate, propyl acrylate, isobutyl acrylate, n-butyl acrylate, n-pentyl acrylate, 2-ethylhexyl acrylate, and n-dodecyl acrylate; cycloalkyl acrylate monomers such as cyclopentyl acrylate and cyclohexyl acrylate; alkyl acrylate cycloalkyl ester monomers such as methylcyclopentyl acrylate, ethylcyclopentyl acrylate, and methylcyclohexyl acrylate; and methacrylates such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, n-pentyl methacrylate, and n-octyl methacrylate. Examples thereof include acrylic acid alkyl ester monomers; methacrylic acid cycloalkyl ester monomers such as cyclopentyl methacrylate, cyclohexyl methacrylate, and cyclopentyl methacrylate; methacrylic acid alkyl cycloalkyl ester monomers such as methylcyclopentyl methacrylate, ethylcyclopentyl methacrylate, and methylcyclohexyl methacrylate; crotonate alkyl ester monomers such as propyl crotonate, n-butyl crotonate, and 2-ethylhexyl crotonate; crotonate cycloalkyl ester monomers such as cyclopentyl crotonate, cyclohexyl crotonate, and cyclooctyl crotonate; and crotonate alkyl cycloalkyl ester monomers such as methylcyclopentyl crotonate and methylcyclohexyl crotonate.

[0019] Furthermore, as the α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomer, those having an alkoxyalkyl group with 2 to 8 carbon atoms are preferred, those having an alkoxyalkyl group with 2 to 6 carbon atoms are more preferred, and those having an alkoxyalkyl group with 2 to 4 carbon atoms are even more preferred.

[0020] Specific examples of the α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomer include acrylates such as methoxymethyl acrylate, methoxyethyl acrylate, methoxybutyl acrylate, ethoxymethyl acrylate, ethoxyethyl acrylate, ethoxypropyl acrylate, ethoxydodecyl acrylate, n-propoxyethyl acrylate, i-propoxyethyl acrylate, n-butoxyethyl acrylate, i-butoxyethyl acrylate, t-butoxyethyl acrylate, methoxypropyl acrylate, and methoxybutyl acrylate. Acid alkoxyalkyl ester monomers; methacrylic acid alkoxyalkyl ester monomers such as methoxymethyl methacrylate, methoxyethyl methacrylate, methoxybutyl methacrylate, ethoxymethyl methacrylate, ethoxyethyl methacrylate, ethoxypentyl methacrylate, n-propoxyethyl methacrylate, i-propoxyethyl methacrylate, n-butoxyethyl methacrylate, i-butoxyethyl methacrylate, t-butoxyethyl methacrylate, methoxypropyl methacrylate, and methoxybutyl methacrylate; and the like.

[0021] Among these α,β-ethylenically unsaturated monocarboxylic acid ester monomers, acrylic acid alkyl ester monomers and acrylic acid alkoxyalkyl ester monomers are preferred, n-butyl acrylate and methoxyethyl acrylate are more preferred, and n-butyl acrylate is particularly preferred, from the viewpoint of making the effects of the present invention more remarkable. Furthermore, these α,β-ethylenically unsaturated monocarboxylic acid ester monomers can also be used in combination of two or more kinds.

[0022] The content of the α,β-ethylenically unsaturated monocarboxylic acid ester monomer units in the nitrile group-containing copolymer rubber used in the present invention is preferably 20 to 45% by weight, more preferably 24 to 40% by weight, and even more preferably 27 to 37% by weight. By setting the content of the α,β-ethylenically unsaturated monocarboxylic acid ester monomer units within the above range, the cold resistance of the obtained cross-linked rubber can be improved, and as a result, the low-temperature properties can be improved.

[0023] Furthermore, the nitrile group-containing copolymer rubber used in the present invention may contain, in addition to the α,β-ethylenically unsaturated nitrile monomer units, conjugated diene monomer units, and α,β-ethylenically unsaturated monocarboxylic acid ester monomer units, units of other monomers copolymerizable with the monomers forming these. Examples of such other monomers include α,β-ethylenically unsaturated monocarboxylic acid ester monomers other than those mentioned above, carboxyl group-containing monomers, α,β-ethylenically unsaturated dicarboxylic acid diester monomers, ethylene, α-olefin monomers, aromatic vinyl monomers, fluorine-containing vinyl monomers, copolymerizable antioxidants, etc.

[0024] Examples of the α,β-ethylenically unsaturated monocarboxylic acid ester monomer other than those mentioned above include (meth)acrylic acid esters having a cyanoalkyl group having 2 to 12 carbon atoms, such as α-cyanoethyl acrylate, α-cyanoethyl methacrylate, and cyanobutyl methacrylate; (meth)acrylic acid esters having a hydroxyalkyl group having 1 to 12 carbon atoms, such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 2-hydroxyethyl methacrylate; and (meth)acrylic acid esters having a fluoroalkyl group having 1 to 12 carbon atoms, such as trifluoroethyl acrylate and tetrafluoropropyl methacrylate.

[0025] Examples of carboxyl group-containing monomers include α,β-ethylenically unsaturated monocarboxylic acid monomers, α,β-ethylenically unsaturated polycarboxylic acid monomers, and α,β-ethylenically unsaturated dicarboxylic acid monoester monomers. Also included as carboxyl group-containing monomers are monomers in which the carboxyl groups of these monomers form carboxylates. Furthermore, anhydrides of α,β-ethylenically unsaturated polycarboxylic acids can also be used as carboxyl group-containing monomers because they form carboxyl groups by cleaving the acid anhydride group after copolymerization.

[0026] Examples of the α,β-ethylenically unsaturated monocarboxylic acid monomer include acrylic acid, methacrylic acid, ethylacrylic acid, crotonic acid, and cinnamic acid.

[0027] Examples of the α,β-ethylenically unsaturated polycarboxylic acid monomer include butenedioic acids such as fumaric acid and maleic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, allylmalonic acid, and teraconic acid. Examples of the anhydrides of α,β-unsaturated polycarboxylic acids include maleic anhydride, itaconic anhydride, and citraconic anhydride.

[0028] Examples of the α,β-ethylenically unsaturated dicarboxylic acid monoester monomer include monoalkyl maleates such as monomethyl maleate, monoethyl maleate, monopropyl maleate, and mono-n-butyl maleate; monocycloalkyl maleates such as monocyclopentyl maleate, monocyclohexyl maleate, and monocycloheptyl maleate; monoalkylcycloalkyl maleates such as monomethylcyclopentyl maleate and monoethylcyclohexyl maleate; monoalkyl fumarate esters such as monomethyl fumarate, monoethyl fumarate, monopropyl fumarate, and mono-n-butyl fumarate; monocycloalkyl fumarate esters such as monocyclopentyl fumarate, monocyclohexyl fumarate, and monocycloheptyl fumarate; monoalkylcycloalkyl fumarate esters such as monomethylcyclopentyl fumarate and monoethylcyclohexyl fumarate; monoalkyl citraconic acid esters such as monomethyl citraconic acid, monoethyl citraconic acid, monopropyl citraconic acid, and mono-n-butyl citraconic acid; monocycloalkyl citraconic acid esters such as monocyclopentyl citraconic acid, monocyclohexyl citraconic acid, and monocycloheptyl citraconic acid; monoalkyl citraconic acid cycloalkyl esters such as monomethylcyclopentyl citraconic acid and monoethylcyclohexyl citraconic acid; monoalkyl itaconic acid esters such as monomethyl itaconic acid, monoethyl itaconic acid, monopropyl itaconic acid, and mono-n-butyl itaconic acid; monocycloalkyl itaconic acid esters such as monocyclopentyl itaconic acid, monocyclohexyl itaconic acid, and monocycloheptyl itaconic acid; and monoalkyl itaconic acid cycloalkyl esters such as monomethylcyclopentyl itaconic acid and monoethylcyclohexyl itaconate.

[0029] Examples of the α,β-ethylenically unsaturated dicarboxylic acid diester monomer include dialkyl maleates such as dimethyl maleate and di-n-butyl maleate, wherein the alkyl group has 1 to 18 carbon atoms; dialkyl fumarate esters such as dimethyl fumarate and di-n-butyl fumarate, wherein the alkyl group has 1 to 18 carbon atoms; dicycloalkyl maleates such as dicyclopentyl maleate and dicyclohexyl maleate, wherein the cycloalkyl group has 4 to 16 carbon atoms; dicycloalkyl fumarate esters such as dicyclopentyl fumarate and dicyclohexyl fumarate, wherein the cycloalkyl group has 4 to 16 carbon atoms; dialkyl itaconate esters such as dimethyl itaconate and di-n-butyl itaconate, wherein the alkyl group has 1 to 18 carbon atoms; and dicycloalkyl itaconate esters such as dicyclohexyl itaconate, wherein the cycloalkyl group has 4 to 16 carbon atoms.

[0030] The α-olefin monomer preferably has 3 to 12 carbon atoms, and examples thereof include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

[0031] Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, and vinylpyridine.

[0032] Examples of the fluorine-containing vinyl monomer include fluoroethyl vinyl ether, fluoropropyl vinyl ether, o-trifluoromethylstyrene, vinyl pentafluorobenzoate, difluoroethylene, and tetrafluoroethylene.

[0033] Examples of the copolymerizable antioxidant include N-(4-anilinophenyl)acrylamide, N-(4-anilinophenyl)methacrylamide, N-(4-anilinophenyl)cinnamamide, N-(4-anilinophenyl)crotonamide, N-phenyl-4-(3-vinylbenzyloxy)aniline, and N-phenyl-4-(4-vinylbenzyloxy)aniline.

[0034] A plurality of types of these copolymerizable other monomers may be used in combination. The content of the units of the other monomers is preferably 50% by weight or less, more preferably 40% by weight or less, and even more preferably 10% by weight or less, based on the total monomer units constituting the nitrile group-containing copolymer rubber. It is particularly preferable that the nitrile group-containing copolymer rubber does not substantially contain units of other monomers other than α,β-ethylenically unsaturated nitrile monomer units, conjugated diene monomer units, and α,β-ethylenically unsaturated monocarboxylic acid ester monomer units.

[0035] The iodine value of the nitrile group-containing copolymer rubber used in the present invention is not more than 120, preferably not more than 80, more preferably not more than 50, and particularly preferably not more than 30. If the iodine value of the nitrile group-containing copolymer rubber is too high, the heat resistance and ozone resistance of the obtained cross-linked rubber may decrease.

[0036] The polymer Mooney viscosity (ML1+4, 100°C) of the nitrile group-containing copolymer rubber used in the present invention is preferably 10 to 200, more preferably 15 to 150, even more preferably 15 to 125, and particularly preferably 30 to 90. By setting the polymer Mooney viscosity of the nitrile group-containing copolymer rubber within the above range, it is possible to further improve the mechanical properties of the obtained cross-linked rubber product while maintaining good processability of the cross-linkable rubber composition.

[0037] The method for producing the nitrile group-containing copolymer rubber used in the present invention is not particularly limited, but it can be produced by copolymerizing the above-mentioned monomers and hydrogenating the carbon-carbon double bonds in the resulting copolymer. The polymerization method is not particularly limited, and known emulsion polymerization methods or solution polymerization methods may be used, but emulsion polymerization methods are preferred from the viewpoint of industrial productivity. During emulsion polymerization, commonly used polymerization auxiliary materials can be used in addition to emulsifiers, polymerization initiators, and molecular weight modifiers.

[0038] The emulsifier is not particularly limited, but examples thereof include nonionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, polyoxyethylene alkyl esters, and polyoxyethylene sorbitan alkyl esters; anionic emulsifiers such as salts of fatty acids such as myristic acid, palmitic acid, oleic acid, and linolenic acid, alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate, higher alcohol sulfates, and alkyl sulfosuccinates; and copolymerizable emulsifiers such as sulfoesters of α,β-unsaturated carboxylic acids, sulfate esters of α,β-unsaturated carboxylic acids, and sulfoalkylaryl ethers. The amount of emulsifier added is preferably 0.1 to 10 parts by weight, and more preferably 0.5 to 5 parts by weight, per 100 parts by weight of the monomers used in the polymerization.

[0039] The polymerization initiator is not particularly limited as long as it is a radical initiator, and examples thereof include inorganic peroxides such as potassium persulfate, sodium persulfate, ammonium persulfate, potassium perphosphate, and hydrogen peroxide; organic peroxides such as t-butyl peroxide, cumene hydroperoxide, p-menthane hydroperoxide, t-butylcumyl peroxide, acetyl peroxide, isobutyryl peroxide, octanoyl peroxide, dibenzoyl peroxide, 3,5,5-trimethylhexanoyl peroxide, and t-butylperoxyisobutyrate; and azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, and methyl azobisisobutyrate. These polymerization initiators can be used alone or in combination of two or more. Inorganic or organic peroxides are preferred as the polymerization initiator. When a peroxide is used as the polymerization initiator, it can also be used in combination with a reducing agent to form a redox polymerization initiator. The reducing agent is not particularly limited, but examples thereof include compounds containing reduced metal ions such as ferrous sulfate and cuprous naphthenate; sulfinates such as sodium hydroxymethanesulfinate; sulfites such as sodium sulfite, potassium sulfite, sodium hydrogen sulfite, aldehyde sodium hydrogen sulfite, and potassium hydrogen sulfite; etc. The amount of the polymerization initiator to be added is preferably 0.01 to 2 parts by weight based on 100 parts by weight of the monomers used in the polymerization.

[0040] The molecular weight modifier is not particularly limited, but examples thereof include mercaptans such as t-dodecyl mercaptan, n-dodecyl mercaptan, and octyl mercaptan; halogenated hydrocarbons such as carbon tetrachloride, methylene chloride, and methylene bromide; α-methylstyrene dimer; and sulfur-containing compounds such as tetraethylthiuram disulfide, dipentamethylenethiuram disulfide, and diisopropylxanthogen disulfide. These may be used alone or in combination of two or more. Of these, mercaptans are preferred, and t-dodecyl mercaptan is more preferred. The amount of molecular weight modifier used is preferably 0.1 to 0.8 parts by weight per 100 parts by weight of the monomers used in the polymerization.

[0041] Water is usually used as the medium for emulsion polymerization, and the amount of water is preferably 80 to 500 parts by weight, more preferably 80 to 300 parts by weight, per 100 parts by weight of the monomers used in the polymerization.

[0042] In emulsion polymerization, if necessary, it is possible to use additional polymerization auxiliary materials such as stabilizers, dispersants, pH adjusters, oxygen scavengers, particle size adjusters, etc. When these are used, the types and amounts used are not particularly limited.

[0043] In the present invention, the copolymer obtained by the polymerization operation is subjected to hydrogenation (hydrogenation reaction) to obtain a nitrile group-containing copolymer rubber. The hydrogenation may be carried out by a known method, such as an oil phase hydrogenation method in which the copolymer latex obtained by emulsion polymerization is coagulated and then hydrogenated in an oil phase, or an aqueous phase hydrogenation method in which the obtained copolymer latex is hydrogenated as is.

[0044] When hydrogenation is carried out by the oil phase hydrogenation method, the copolymer latex prepared by the emulsion polymerization is preferably dissolved in an organic solvent through salting out or coagulation with alcohol, filtration, drying, and then hydrogenation reaction (oil phase hydrogenation method) is carried out, and the resulting hydrogenated product is poured into a large amount of water, coagulated, filtration, and drying to obtain a nitrile group-containing copolymer rubber.

[0045] For coagulation of the latex by salting out, known coagulants such as sodium chloride, calcium chloride, aluminum sulfate, etc. can be used. Furthermore, instead of coagulation by salting out, coagulation may be carried out using an alcohol such as methanol. The solvent for the oil phase hydrogenation method is not particularly limited as long as it is a liquid organic compound that dissolves the copolymer obtained by emulsion polymerization, but preferred examples include benzene, chlorobenzene, toluene, xylene, hexane, cyclohexane, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, cyclohexanone, and acetone.

[0046] As the catalyst for the oil phase hydrogenation method, any known selective hydrogenation catalyst can be used without limitation, with palladium-based catalysts and rhodium-based catalysts being preferred, and palladium-based catalysts (such as palladium acetate, palladium chloride, and palladium hydroxide) being more preferred. Two or more of these may be used in combination, in which case it is preferable to use a palladium-based catalyst as the main active component. These catalysts are usually used by being supported on a carrier. Examples of carriers include silica, silica-alumina, alumina, diatomaceous earth, and activated carbon. The amount of catalyst used is preferably 10 to 5,000 ppm by weight, more preferably 100 to 3,000 ppm by weight, based on the copolymer.

[0047] Alternatively, when hydrogenation is carried out by aqueous phase hydrogenation, the copolymer latex prepared by the emulsion polymerization is preferably diluted with water as necessary and then subjected to hydrogenation reaction. The aqueous phase hydrogenation method includes a direct aqueous phase hydrogenation method in which hydrogen is supplied to a reaction system in the presence of a hydrogenation catalyst to carry out hydrogenation, and an indirect aqueous phase hydrogenation method in which hydrogenation is carried out by reduction in the presence of an oxidizing agent, a reducing agent, and an activator, and among these, the direct aqueous phase hydrogenation method is preferred.

[0048] In the aqueous phase direct hydrogenation method, the concentration of the copolymer in the aqueous phase (concentration in the latex state) is preferably 40% by weight or less to prevent aggregation. The hydrogenation catalyst is not particularly limited as long as it is a compound that is not easily decomposed by water. Specific examples of palladium catalysts include palladium salts of carboxylic acids such as formic acid, propionic acid, lauric acid, succinic acid, oleic acid, and phthalic acid; palladium chlorides such as palladium chloride, dichloro(cyclooctadiene)palladium, dichloro(norbornadiene)palladium, and ammonium hexachloropalladate(IV); iodides such as palladium iodide; and palladium sulfate dihydrate. Among these, palladium salts of carboxylic acids, dichloro(norbornadiene)palladium, and ammonium hexachloropalladate(IV) are particularly preferred. The amount of hydrogenation catalyst used may be determined as appropriate, but is preferably 5 to 6,000 ppm by weight, more preferably 10 to 4,000 ppm by weight, based on the copolymer obtained by polymerization.

[0049] In the aqueous phase direct hydrogenation method, the hydrogenation catalyst in the latex is removed after the hydrogenation reaction is completed. For example, an adsorbent such as activated carbon or ion exchange resin is added to adsorb the hydrogenation catalyst under stirring, and then the latex is filtered or centrifuged. It is also possible to leave the hydrogenation catalyst in the latex without removing it.

[0050] In the aqueous layer direct hydrogenation method, the latex thus obtained after the hydrogenation reaction is subjected to coagulation by salting out, filtration, drying, etc., to obtain a nitrile group-containing copolymer rubber. In this case, the steps of filtration and drying following the coagulation can be carried out by known methods, respectively.

[0051] <Silica> The crosslinkable rubber composition of the present invention contains silica as a filler.

[0052] The silica used in the present invention is not particularly limited, but includes natural silica such as quartz powder and silica stone powder; synthetic silica such as silicic anhydride (silica gel, Aerosil, etc.) and hydrous silicic acid; and the like, among which synthetic silica is preferred. Furthermore, these silicas may be surface-treated with a coupling agent such as a silane coupling agent, an aluminum-based coupling agent, or a titanate-based coupling agent, and are preferably surface-treated with a silane coupling agent. Specific examples of silane coupling agents that can be used as appropriate are described below. By surface-treating with a coupling agent, the dispersibility of silica in the crosslinkable rubber composition can be further improved, thereby further enhancing the compounding effect of silica.

[0053] The specific surface area of ​​the silica used in the present invention as measured by the BET method is not particularly limited, but is preferably 10 to 600 m ２ / g, more preferably 50 to 350 m ２ / g, more preferably 100 to 200m ２／ The volume average particle size of the silica used in the present invention is not particularly limited, but is preferably 0.1 to 50 μm, more preferably 0.3 to 40 μm, and even more preferably 0.5 to 30 μm.

[0054] The cross-linkable rubber composition of the present invention contains silica in a proportion of 30 parts by weight or more relative to 100 parts by weight of the nitrile group-containing copolymer rubber. According to the present invention, the cross-linkable rubber composition contains silica, clay, and an organic peroxide cross-linking agent relative to the nitrile group-containing copolymer rubber, and by controlling the silica content within the above range, the cross-linked rubber obtained using such a cross-linkable rubber composition can be made to have excellent low-temperature properties and polycyclic aromatic compound resistance, and can also have excellent compression set resistance and oil swelling resistance (small volume change in oil).

[0055] The content of silica in the cross-linkable rubber composition of the present invention is 30 parts by weight or more, preferably 30 to 100 parts by weight, more preferably 32 to 80 parts by weight, even more preferably 33 to 70 parts by weight, and particularly preferably 34 to 60 parts by weight, per 100 parts by weight of the nitrile group-containing copolymer rubber. If the silica content is too low, the oil swelling resistance and polycyclic aromatic compound resistance after heat aging of the resulting cross-linked rubber product will decrease. Furthermore, by keeping the silica content at 100 parts by weight or less, the processability of the cross-linkable rubber composition can be further improved.

[0056] <Clay> The crosslinkable rubber composition of the present invention further contains clay as a filler. The clay used in the present invention is not particularly limited as long as it is a compound containing silicate as the main component, but it is preferable that the clay contains a compound represented by the following general formula (1) as the main component. Furthermore, it is preferable that the surface of the clay is treated with a surface treatment agent. (M Ｉ ２ O) ａ (M ＩＩ O) ｂ (M ＩＩＩ O ３ ) ｃ (M ＩＶ ２ O ３ ) ｄ xSiO ２ ・mH ２ O (1) (in the above general formula (1), M Ｉ is an alkali metal, M ＩＩ is alkaline earth metal, M ＩＩＩ is an element in group 4 of the periodic table, M ＩＶis an element of Group 13 of the periodic table, a+b+c+d=1, x is a positive real number of 8 or less, and m is 0 or a positive real number of 12 or less.) It is preferable that the clay contains the compound represented by general formula (1) above as the main component (for example, at a content of 90% by weight or more, preferably 95% by weight or more, and more preferably 99% by weight or more), but it may also include clays that contain metal elements other than alkali metals, alkaline earth metals, elements of Group 4 of the periodic table, and elements of Group 13 of the periodic table in amounts on the order of impurities (for example, about 100 ppm by weight of the total of alkali metals, alkaline earth metals, elements of Group 4 of the periodic table, and elements of Group 13 of the periodic table).

[0057] In the above general formula (1), M Ｉ Examples of the alkali metal constituting M include lithium, sodium, and potassium. ＩＩ Examples of the alkaline earth metal constituting M include magnesium, calcium, strontium, and barium, and among these, magnesium is preferred. ＩＩＩ Examples of the group 4 elements of the periodic table that constitute M include titanium and zirconium, and among these, titanium is preferred. ＩＶ Examples of the elements of Group 13 of the periodic table that constitute the element include boron and aluminum, and among these, aluminum is preferred.

[0058] Specific examples of the compound represented by the general formula (1) include magnesium silicate, magnesium silicate hydrate, calcium silicate, calcium silicate hydrate, boron silicate, boron silicate hydrate, aluminum silicate, and aluminum silicate hydrate. Among these, magnesium silicate and aluminum silicate are more preferred, and aluminum silicate is particularly preferred.

[0059] Furthermore, the clay is preferably surface-treated with a surface treatment agent. By treating the clay with a surface treatment agent, the dispersibility of the clay in the crosslinkable rubber composition can be further improved, thereby further enhancing the compounding effect of the clay. In this case, the surface treatment agent used for the surface treatment is not particularly limited, but a silane coupling agent is preferably used, and a vinyl group-containing silane coupling agent, an amino group-containing silane coupling agent, or an epoxy group-containing silane coupling agent is preferably used. Specific examples of the silane coupling agent that can be used include those described below, and one or more types can be used in combination.

[0060] The volume average particle size of the clay is not particularly limited, but is preferably 0.01 to 100 μm, more preferably 0.05 to 50 μm.

[0061] The content of clay in the cross-linkable rubber composition of the present invention is not particularly limited, but is preferably 5 to 80 parts by weight, more preferably 15 to 60 parts by weight, and even more preferably 20 to 50 parts by weight, relative to 100 parts by weight of the nitrile group-containing copolymer rubber. By setting the clay content within the above range, the obtained cross-linked rubber product can be one that has excellent mechanical strength, and further improved resistance to swelling in oil and resistance to polycyclic aromatic compounds after heat aging.

[0062] In the present invention, the clay content may be within the above range, but the clay content is preferably set to an amount such that the total content together with silica is 35 to 180 parts by weight per 100 parts by weight of the nitrile group-containing copolymer rubber. That is, the total content of silica and clay in the crosslinkable rubber composition of the present invention is preferably 35 to 180 parts by weight, more preferably 37 to 140 parts by weight, even more preferably 53 to 120 parts by weight, and particularly preferably 55 to 100 parts by weight.

[0063] <Organic Peroxide Crosslinking Agent> The crosslinkable rubber composition of the present invention further contains an organic peroxide crosslinking agent as a crosslinking agent. The organic peroxide crosslinking agent is not particularly limited as long as it is used as a crosslinking agent in the rubber industry, and examples thereof include dialkyl peroxides, diacyl peroxides, and peroxy esters, and preferably dialkyl peroxides.

[0064] Examples of dialkyl peroxides include dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and 1,3-bis(t-butylperoxyisopropyl)benzene. Examples of diacyl peroxides include benzoyl peroxide and isobutyryl peroxide. Examples of peroxy esters include 2,5-dimethyl-2,5-bis(benzoylperoxy)hexane and t-butylperoxyisopropyl carbonate. Of these, dialkyl peroxides are preferred, and 1,3-bis(t-butylperoxyisopropyl)benzene is more preferred.

[0065] The content of the organic peroxide crosslinking agent in the crosslinkable rubber composition of the present invention is not particularly limited, but is preferably 1 to 16 parts by weight, more preferably 1.5 to 14 parts by weight, and even more preferably 2 to 12 parts by weight, relative to 100 parts by weight of the nitrile group-containing copolymer rubber. By setting the content of the organic peroxide crosslinking agent within the above range, it is possible to ensure that the obtained crosslinked rubber product has good compression set resistance and rubber elasticity, while also achieving sufficient crosslink density.

[0066] <Co-Crosslinking Agent> The crosslinkable rubber composition of the present invention preferably further contains a co-crosslinking agent.

[0067] The co-crosslinking agent is not particularly limited, but examples thereof include triallyl cyanurate, triallyl isocyanurate, trimethylolpropane trimethacrylate, and N,N'-m-phenylene bismaleimide. These may be used alone or in combination, and may be dispersed in silica, clay, or the like to improve the processability of the crosslinkable rubber composition. Among these, triallyl cyanurate, triallyl isocyanurate, and trimethylolpropane trimethacrylate are more preferred, and triallyl isocyanurate and trimethylolpropane trimethacrylate are even more preferred.

[0068] The content of the co-crosslinking agent in the crosslinkable rubber composition of the present invention is not particularly limited, but is preferably 1 to 20 parts by weight, more preferably 2 to 16 parts by weight, even more preferably 2.5 to 14 parts by weight, and particularly preferably 3 to 12 parts by weight, relative to 100 parts by weight of the nitrile group-containing copolymer rubber. By setting the content of the co-crosslinking agent within the above range, it is possible to improve the elongation of the obtained cross-linked rubber product and also improve the compression set resistance.

[0069] <Silane Coupling Agent> The crosslinkable rubber composition of the present invention preferably further contains a silane coupling agent. The silane coupling agent is not particularly limited, but may be a sulfur-containing silane coupling agent such as γ-mercaptopropyltrimethoxysilane, mercaptomethyltrimethoxysilane, mercaptomethyltriethoxysilane, bis(3-triethoxysilylpropyl)tetrasulfane, or bis(3-triethoxysilylpropyl)disulfane; γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; Epoxy group-containing silane coupling agents such as γ-mercaptopropyltrimethoxysilane and γ-glycidoxypropylmethyldiethoxysilane; N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylidene)propylamine , N-phenyl-3-aminopropyltrimethoxysilane and other amino group-containing silane coupling agents; γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltris(β-methoxyethoxy)silane, γ-methacryloxypropylmethyldimethoxysilane, γ-methacryloxypropylmethyldiethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-acryloxypropyltrimethoxysilane and other (meth)acryloxy group-containing silane coupling agents; vinyltrimethoxysilane, vinyl vinyl group-containing silane coupling agents such as vinyltriethoxysilane, vinyltris(β-methoxyethoxy)silane, vinyltrichlorosilane, and vinyltriacetoxysilane; chloropropyl group-containing silane coupling agents such as 3-chloropropyltrimethoxysilane; isocyanate group-containing silane coupling agents such as 3-isocyanatepropyltriethoxysilane; styryl group-containing silane coupling agents such as p-styryltrimethoxysilane; ureido group-containing silane coupling agents such as 3-ureidopropyltriethoxysilane;Examples of suitable silane coupling agents include allyl group-containing silane coupling agents such as diallyldimethylsilane; alkoxy group-containing silane coupling agents such as tetraethoxysilane; phenyl group-containing silane coupling agents such as diphenyldimethoxysilane; fluoro group-containing silane coupling agents such as trifluoropropyltrimethoxysilane; and alkyl group-containing silane coupling agents such as isobutyltrimethoxysilane and cyclohexylmethyldimethoxysilane. These may be used alone or in combination. Among these, amino group-containing silane coupling agents and vinyl group-containing silane coupling agents are preferred, vinyl group-containing silane coupling agents are more preferred, and vinyltrimethoxysilane is even more preferred.

[0070] The content of the silane coupling agent in the crosslinkable rubber composition of the present invention is not particularly limited, but is preferably 0.1 to 10 parts by weight, more preferably 0.2 to 7 parts by weight, even more preferably 0.3 to 4 parts by weight, and particularly preferably 0.5 to 3 parts by weight, per 100 parts by weight of the nitrile group-containing copolymer rubber. When silica or clay that has been surface-treated with a silane coupling agent is used, it is preferable to add a silane coupling agent separately from the silane coupling agent used for surface treatment of the silica or clay, and the content thereof is also preferably within the above-mentioned range. By incorporating the silane coupling agent in the above-mentioned content, the dispersibility of the silica or clay can be further improved, thereby more appropriately improving the tensile strength and compression set resistance of the resulting crosslinked rubber.

[0071] <Plasticizer> The cross-linkable rubber composition of the present invention preferably further contains a plasticizer. By further blending a plasticizer, the processability of the cross-linkable rubber composition and the polycyclic aromatic compound resistance of the obtained cross-linked rubber product after heat aging can be further improved.

[0072] The plasticizer is not particularly limited, but examples thereof include trimellitic acid-based plasticizers, pyromellitic acid-based plasticizers, ether ester-based plasticizers, polyester-based plasticizers, phthalic acid-based plasticizers, adipate ester-based plasticizers, phosphate ester-based plasticizers, sebacic acid ester-based plasticizers, alkylsulfonic acid ester compound plasticizers, and epoxidized vegetable oil-based plasticizers. Specific examples of plasticizers include tris-2-ethylhexyl trimellitate, isononyl trimellitate ester, mixed linear alkyl trimellitate ester, dipentaerythritol ester, 2-ethylhexyl pyromellitic acid ester, polyether ester (molecular weight approximately 300 to 5000), bis[2-(2-butoxyethoxy)ethyl] adipate, dioctyl adipate, adipic acid-based polyester (molecular weight approximately 300 to 5000), dioctyl phthalate, diisononyl phthalate, dibutyl phthalate, tricresyl phosphate, dibutyl sebacate, alkylsulfonic acid phenyl ester, epoxidized soybean oil, diheptanoate, di-2-ethylhexanoate, didecanoate, etc. These can be used alone or in combination of two or more. Among these, ether ester plasticizers, trimellitic acid plasticizers, and adipate plasticizers are preferred, with tris-2-ethylhexyl trimellitate and bis[2-(2-butoxyethoxy)ethyl] adipate being more preferred.

[0073] The content of the plasticizer in the cross-linkable rubber composition of the present invention is not particularly limited, but is preferably 10 to 55 parts by weight, more preferably 15 to 50 parts by weight, even more preferably 20 to 45 parts by weight, and particularly preferably 22 to 35 parts by weight, per 100 parts by weight of the nitrile group-containing copolymer rubber. By setting the content of the plasticizer within the above range, the processability of the cross-linkable rubber composition can be appropriately improved. Furthermore, the tensile strength, cold resistance, and oil swelling resistance of the resulting cross-linked rubber can be made even better.

[0074] <Other Compounding Agents> In addition to the above, the crosslinkable rubber composition of the present invention may contain compounding agents commonly used in the rubber field, such as fillers other than silica and clay, crosslinking aids, crosslinking retarders, antioxidants, antioxidants, light stabilizers, scorch inhibitors such as primary amines, activators such as diethylene glycol, processing aids such as amine-modified silicone, slip agents, adhesives, lubricants, flame retardants, antifungal agents, acid acceptors, antistatic agents, pigments, foaming agents, etc. The amount of these compounding agents to be added is not particularly limited as long as it does not impair the purpose and effects of the present invention, and they may be added in amounts according to the purpose of addition.

[0075] Fillers other than silica and clay are not particularly limited, but examples thereof include carbon black, magnesium oxide, calcium carbonate, short fibers, and metal salts of α,β-ethylenically unsaturated carboxylic acids such as zinc (meth)acrylate and magnesium (meth)acrylate.

[0076] Furthermore, the crosslinkable rubber composition of the present invention may contain rubbers other than the above-mentioned nitrile group-containing copolymer rubber, as long as the effects of the present invention are not impaired. Examples of rubbers other than the nitrile group-containing copolymer rubber include acrylic rubber, ethylene-acrylic acid copolymer rubber, fluororubber, styrene-butadiene copolymer rubber, polybutadiene rubber, ethylene-propylene copolymer rubber, ethylene-propylene-diene terpolymer rubber, epichlorohydrin rubber, urethane rubber, chloroprene rubber, silicone rubber, fluorosilicone rubber, chlorosulfonated polyethylene rubber, natural rubber, and polyisoprene rubber. When a rubber other than the nitrile group-containing copolymer rubber is contained, the amount of the rubber contained other than the nitrile group-containing copolymer rubber is preferably 30 parts by weight or less, more preferably 20 parts by weight or less, and particularly preferably 10 parts by weight or less, per 100 parts by weight of the nitrile group-containing copolymer rubber.

[0077] The crosslinkable rubber composition of the present invention is prepared by mixing the above-mentioned components preferably in a non-aqueous system. There is no limitation on the method for preparing the crosslinkable rubber composition of the present invention, but it can usually be prepared by first kneading the components excluding the crosslinking agent and heat-unstable components (for example, crosslinking aid) in a mixer such as a Banbury mixer, an intermixer, or a kneader, and then transferring the mixture to a roll or the like, adding the crosslinking agent and heat-unstable components, and then secondly kneading.

[0078] <Rubber Cross-Linked Product> The cross-linked rubber product of the present invention is obtained by cross-linking the cross-linkable rubber composition of the present invention described above. The cross-linked rubber product of the present invention can be produced by using the cross-linkable rubber composition of the present invention, molding it into a desired shape using a molding machine such as an extruder, injection molding machine, compressor, or roll, and heating it to cause a cross-linking reaction, thereby fixing the shape as a cross-linked product. In this case, cross-linking may be carried out after molding in advance, or simultaneously with molding. The molding temperature is usually 10 to 200°C, preferably 25 to 120°C. The cross-linking temperature is usually 100 to 200°C, preferably 130 to 190°C, and the cross-linking time is usually 1 minute to 24 hours, preferably 2 minutes to 1 hour.

[0079] Depending on the shape, size, etc. of the cross-linked product, even if the surface is cross-linked, the inside may not be sufficiently cross-linked, so secondary cross-linking may be carried out by further heating. As the heating method, a general method used for cross-linking rubber, such as press heating, steam heating, oven heating, or hot air heating, may be appropriately selected.

[0080] The cross-linked rubber product of the present invention obtained in this manner is obtained using the cross-linkable rubber composition of the present invention described above, and is excellent in low-temperature properties and polycyclic aromatic compound resistance, and is also capable of achieving both compression set resistance and oil swelling resistance (small volume change in oil). In particular, according to the present invention, with regard to polycyclic aromatic compound resistance, the cross-linked rubber product can be made to have excellent polycyclic aromatic compound resistance even after heat aging.

[0081] Therefore, by making use of these properties, the cross-linked rubber of the present invention can be used in a wide range of applications, including O-rings, packings, diaphragms, oil seals, shaft seals, bearing seals, wellhead seals, shock absorber seals, seals for pneumatic equipment, seals for sealing fluorocarbons or fluorohydrocarbons or carbon dioxide used in the cooling devices of air conditioners and compressors for refrigeration units of air conditioners, seals for sealing supercritical carbon dioxide or subcritical carbon dioxide used as a cleaning medium in precision cleaning, seals for rolling devices (rolling bearings, automotive hub units, automotive water pumps, linear guide devices, ball screws, etc.), valves and valve seats, BOP (Blow Out Pump), etc. various sealing materials such as intake manifold gaskets attached to the joint between the intake manifold and the cylinder head, cylinder head gaskets attached to the joint between the cylinder block and the cylinder head, rocker cover gaskets attached to the joint between the rocker cover and the cylinder head, oil pan gaskets attached to the joint between the oil pan and the cylinder block or the transmission case, gaskets for fuel cell separators attached between a pair of housings sandwiching a unit cell having a positive electrode, an electrolyte plate and a negative electrode, gaskets for the top cover of hard disk drives, and various other gaskets; printing rolls, steelmaking rolls, papermaking rolls, industrial rolls, office rolls, etc. various rolls such as rolls for machines; various belts such as flat belts (film core flat belts, cord flat belts, laminated flat belts, single-piece flat belts, etc.), V-belts (wrapped V-belts, raw edge V-belts, etc.), V-ribbed belts (single V-ribbed belts, double V-ribbed belts, wrapped V-ribbed belts, rubber-backed V-ribbed belts, upper cog V-ribbed belts, etc.), CVT belts, timing belts, toothed belts, conveyor belts; various hoses such as fuel hoses, turbo air hoses, oil hoses, radiator hoses, heater hoses, water hoses, vacuum brake hoses, control hoses, air conditioner hoses, brake hoses, power steering hoses, air hoses, marine hoses, risers, flow lines, etc.;The cross-linked rubber product of the present invention can be used in a wide range of applications, including various boots such as CVJ boots, propeller shaft boots, constant velocity joint boots, and rack and pinion boots; damping rubber parts such as cushioning materials, dynamic dampers, rubber couplings, air springs, vibration-isolating materials, and clutch facing materials; dust covers, automotive interior materials, friction materials, tires, coated cables, shoe soles, electromagnetic wave shields, adhesives such as adhesives for flexible printed circuit boards, fuel cell separators, and the electronics field. In particular, the cross-linked rubber product of the present invention has excellent resistance to swelling in oil and resistance to polycyclic aromatic compounds (particularly resistance to polycyclic aromatic compounds after heat aging), and is therefore particularly suitable for applications in which it will come into contact with oil, particularly applications in which it will come into contact with fuel oils containing polycyclic aromatic compounds. Suitable applications in which it will come into contact with fuel oils containing polycyclic aromatic compounds include sealing materials.

[0082] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following, "parts" are by weight unless otherwise specified. The methods for testing or evaluating physical properties and characteristics are as follows.

[0083] <Rubber Composition> The content ratio of each monomer unit constituting the nitrile group-containing copolymer rubber was measured by the following method. That is, the content ratio of acrylonitrile units was calculated by measuring the nitrogen content in the nitrile group-containing copolymer rubber by the Kjeldahl method in accordance with JIS K6451-2. The content ratio of mono-n-butyl maleate units was calculated by adding 100 mL of 2-butanone to 0.2 g of a 2 mm square nitrile group-containing copolymer rubber and stirring for 16 hours, then adding 20 mL of ethanol and 10 mL of water, and while stirring, titrating with a 0.02 N aqueous ethanol solution of potassium hydroxide at room temperature using thymolphthalein as an indicator to determine the number of moles of carboxyl groups per 100 g of nitrile group-containing copolymer rubber, and converting the determined number of moles into the amount of mono-n-butyl maleate units. The content ratios of 1,3-butadiene units and saturated butadiene units were calculated by measuring the iodine value (according to JIS K6235) of a nitrile group-containing copolymer rubber before and after the hydrogenation reaction. The content ratio of n-butyl acrylate units was calculated from the content ratios of mono-n-butyl maleate units, 1,3-butadiene units, saturated butadiene units, and acrylonitrile units determined above.

[0084] <Iodine Value> The iodine value of the nitrile group-containing copolymer rubber was measured in accordance with JIS K6235.

[0085] <Tensile Strength> The cross-linkable rubber composition was placed in a mold measuring 15 cm in length, 15 cm in width, and 0.2 cm in depth, and press-molded at 170°C for 20 minutes while applying a pressure of 10 MPa to obtain a sheet-shaped cross-linked rubber product. The obtained cross-linked rubber product was then transferred to a gear oven and subjected to secondary cross-linking at 170°C for 4 hours, and the obtained cross-linked rubber product in sheet form was punched out with a JIS No. 3 dumbbell to prepare a test specimen. Then, using the obtained test specimen, the tensile strength of the cross-linked rubber product was measured in accordance with JIS K6251.

[0086] <Low-Temperature Properties> The crosslinkable rubber composition was placed in a mold measuring 15 cm in length, 15 cm in width, and 0.2 cm in depth, and press-molded at 170°C for 20 minutes while applying a pressure of 10 MPa to obtain a sheet-like primary crosslinked product. The obtained primary crosslinked product was then transferred to a gear oven and subjected to secondary crosslinking at 170°C for 4 hours to obtain a sheet-like crosslinked rubber product. Using the obtained sheet-like crosslinked rubber product, a TR test (low-temperature elastic recovery test) was performed in a 50% elongated state in accordance with JIS K6261. Specifically, the crosslinked rubber product elongated to 50% was frozen at -70°C, and the temperature was continuously increased to measure the recovery of the elongated test piece, and the temperature TR10 at which the length of the crosslinked product contracted (recovered) by 10% due to the temperature increase was measured. If TR10 is -35°C or less, it can be determined that the product has excellent sealing properties at low temperatures and sufficient low-temperature properties for practical use.

[0087] <Compression set> The crosslinkable rubber composition was pressed using a mold at a temperature of 170°C for 20 minutes under pressure to effect primary crosslinking, and then transferred to a gear oven to effect secondary crosslinking at 170°C for 4 hours to obtain a cylindrical crosslinked rubber product having a diameter of 29 mm and a height of 12.5 mm. The obtained crosslinked rubber product was then placed in an environment of 150°C for 168 hours in a state where it was compressed by 25%, and the compression set was measured according to JIS K6262. If the compression set is 45% or less, it can be determined that the product has sufficient compression set resistance for practical use.

[0088] <Test for swelling resistance in oil (volume change when immersed in fuel oil)> Using a sheet-like cross-linked product obtained in the same manner as in the evaluation of tensile strength above, a test for swelling resistance in oil was carried out in accordance with JIS K6258 by immersing the obtained sheet-like cross-linked rubber product in Fuel C (a mixture of isooctane:toluene=50:50 (volume ratio)) as a test fuel oil adjusted to 40°C for 72 hours. In the test for swelling resistance in oil, the volumes of the cross-linked rubber before and after immersion in fuel oil were measured, and the volume change rate ΔV (unit: %) after immersion in fuel oil was calculated according to "ΔV=([volume after immersion in fuel oil−volume before immersion in fuel oil] / volume before immersion in fuel oil)×100", and evaluation was carried out based on the calculated volume change rate ΔV. If the volume change rate ΔV is within ±100%, it can be determined that the product has sufficient resistance to swelling in oil for practical use.

[0089] <Test for polycyclic aromatic compound resistance after heat aging> First, a crosslinkable rubber composition was pressed using a mold at a temperature of 170°C for 20 minutes while applying pressure to perform primary crosslinking, and then transferred to a gear oven and performed secondary crosslinking at 170°C for 2 hours to obtain an annular sealing material sample. The shape of the annular sealing material sample was shaped to correspond to the seal mounting groove of the first aluminum jig described below.

[0090] A first aluminum jig and a second aluminum jig were used for the polycyclic aromatic compound resistance test after heat aging. The first aluminum jig was a plate-shaped jig with an elliptical recess (major axis 37 mm, minor axis 25 mm) 15 mm deep formed on one side. This recess functions as a liquid storage section and is covered by an outer edge portion having a height of 15 mm and a predetermined width formed on the recess-forming surface of the plate-shaped jig. A seal mounting groove having a width of 3.5 mm and a depth of 5 mm was formed on this outer edge portion 5 mm from the edge of the liquid storage section. The recess also had an injection port for injecting and extracting test fuel oil containing polycyclic aromatic compounds. The second aluminum jig was a plate-shaped jig having a shape corresponding to that of the first aluminum jig. A seal material sample is then attached to the seal mounting groove of the first aluminum jig, and the first aluminum jig is combined with the second aluminum jig via the seal material sample, thereby forming a liquid storage section having an elliptical shape with a major axis of 37 mm, a minor axis of 25 mm, and a height of 15 mm.

[0091] The annular sealing material sample obtained above was then mounted in a sealing material mounting groove provided in a first aluminum jig, and a second aluminum jig was pressed against the sealing material sample to compress the sealing material sample with a compression allowance of 1.5 mm (a gap of 0.2 mm between the first and second aluminum jigs). The aluminum jig with the compressed sealing material sample was left in an environment at 150 ° C for 168 hours to thermally age the sealing material. After thermal aging of the sealing material, 8 ml of phenanthrene-containing test fuel oil (Fuel C mixed with 20% by mass of phenanthrene) was sealed in the space formed between the first and second aluminum jigs and sealed with the sealing material sample (hereinafter referred to as the "containment space"). The space was then kept at 60 ° C in an explosion-proof thermostatic chamber and left for 72 hours. Thereafter, the phenanthrene-containing test fuel oil was removed from the storage space, and the sealing material was subjected to a heat treatment at 110°C for 24 hours while still set in the aluminum jig. After the heat treatment, the sealing material sample was left in the aluminum jig in an environment of -30°C for 4 hours, and then an airtightness test was carried out. In the airtightness test, the pressure was increased from -90 kPa to 120 kPa in 10 kPa increments, and the pressure at which an air leak occurred was measured. The evaluation criteria were as follows: ◯: 50 kPa or more ×: Less than 50 kPa

[0092] Synthesis Example 1 (Production of Nitrile Group-Containing Copolymer Rubber (A-1)) A reactor was charged with 200 parts of ion-exchanged water and 0.2 parts of sodium carbonate. After dissolving the sodium carbonate, 2.25 parts of fatty acid potassium soap (potassium salt of fatty acid) was added to prepare an aqueous soap solution. Then, 9 parts of acrylonitrile, 39 parts of n-butyl acrylate, and 0.5 parts of t-dodecyl mercaptan (molecular weight modifier) ​​were charged to the resulting aqueous soap solution, in that order. The gas inside was purged with nitrogen three times, and then 32 parts of 1,3-butadiene was charged. Next, the reactor was kept at 5°C, and 0.1 parts of cumene hydroperoxide (polymerization initiator), a reducing agent, and appropriate amounts of a chelating agent were charged to initiate the polymerization reaction. Then, when the polymerization conversion rate reached 60%, 10 parts of acrylonitrile and 10 parts of 1,3-butadiene were added, and when the polymerization conversion rate reached 85%, 0.1 parts of a 10% aqueous solution of hydroquinone (polymerization terminator) was added to terminate the polymerization reaction, and the residual monomer was removed using a rotary evaporator with a water temperature of 60°C to obtain a copolymer rubber latex (solid content concentration: approximately 25% by weight).

[0093] Next, the latex obtained above was added to an aqueous solution of aluminum sulfate in an amount of 3% by weight based on the copolymer rubber content, and the mixture was stirred to coagulate the latex. The latex was then filtered while being washed with water, and then vacuum dried at 60°C for 12 hours to obtain a copolymer rubber.

[0094] The obtained copolymer rubber was then dissolved in acetone to a concentration of 12%, and the solution was placed in an autoclave. 700 ppm by weight of a palladium-silica catalyst was added relative to the copolymer rubber, and a hydrogenation reaction was carried out at a hydrogen pressure of 3.0 MPa. After completion of the hydrogenation reaction, the solution was poured into a large amount of water to coagulate, and then filtered and dried to obtain a nitrile group-containing copolymer rubber (A-1). The composition of the obtained nitrile group-containing copolymer rubber (A-1) was 19% by weight of acrylonitrile units, 34% by weight of n-butyl acrylate units, and 47% by weight of 1,3-butadiene units (including saturated portions), and the iodine value was 15.

[0095] Synthesis Example 2 (Production of Nitrile Group-Containing Copolymer Rubber (A-2)) A nitrile group-containing copolymer rubber (A-2) was obtained in the same manner as in Synthesis Example 1, except that the amount of acrylonitrile used at the start of polymerization was changed from 9 parts to 14 parts and the amount of n-butyl acrylate used was changed from 39 parts to 34 parts. The composition of the monomer units of the obtained nitrile group-containing copolymer rubber (A-2) was 24% by weight of acrylonitrile units, 28% by weight of n-butyl acrylate units, and 48% by weight of 1,3-butadiene units (including hydrogenated portions), and the iodine value was 15.

[0096] Synthesis Example 3 (Production of Nitrile Group-Containing Copolymer Rubber (A-3)) A reactor was charged with 180 parts of ion-exchanged water, 25 parts of a 10% by weight aqueous solution of sodium dodecylbenzenesulfonate, 15 parts of acrylonitrile, 39 parts of n-butyl acrylate, 6 parts of mono-n-butyl maleate, and 0.5 parts of t-dodecyl mercaptan (molecular weight modifier), in that order. The gas inside was replaced with nitrogen three times, and then 40 parts of 1,3-butadiene was charged. The reactor was then kept at 5°C, and 0.1 part of cumene hydroperoxide (polymerization initiator), a reducing agent, and appropriate amounts of a chelating agent were charged to initiate the polymerization reaction. When the polymerization conversion rate reached 85%, the polymerization reaction was terminated by adding 0.1 part of a 10% by weight aqueous solution of hydroquinone (polymerization terminator). Residual monomers were removed using a rotary evaporator with water at 60°C, yielding a copolymer rubber latex (solids concentration: approximately 30% by weight).

[0097] Then, the latex obtained above and a palladium catalyst (a solution obtained by mixing a 1 wt % palladium acetate acetone solution with an equal weight of ion-exchanged water) were added to an autoclave, and hydrogenation reaction was carried out at a hydrogen pressure of 3.0 MPa for 7 hours so that the palladium content relative to the dry weight of the copolymer rubber became 3,000 ppm by weight, thereby obtaining a latex of a nitrile group-containing copolymer rubber (A-3).

[0098] Next, a double volume of methanol was added to the obtained latex to coagulate it, and then vacuum dried at 60° C. for 12 hours to obtain a nitrile group-containing copolymer rubber (A-3). The composition of each monomer unit of the obtained nitrile group-containing copolymer rubber (A-3) was 15% by weight of acrylonitrile units, 35% by weight of n-butyl acrylate units, 5% by weight of mono-n-butyl maleate units, and 45% by weight of 1,3-butadiene units (including hydrogenated portions), and the iodine value was 10.

[0099] <Synthesis Example 4 (Production of Nitrile Group-Containing Copolymer Rubber (A-4)> A reactor was charged with 200 parts of ion-exchanged water and 0.2 parts of sodium carbonate. After dissolving the sodium carbonate, 2.25 parts of fatty acid potassium soap (potassium salt of fatty acid) was added to prepare an aqueous soap solution. Then, 37 parts of acrylonitrile and 0.5 parts of t-dodecyl mercaptan (molecular weight modifier) ​​were charged, in this order, to the resulting aqueous soap solution. The internal gas was purged with nitrogen three times, and then 63 parts of 1,3-butadiene was charged. The reactor was then maintained at 5°C, and 0.1 parts of cumene hydroperoxide (polymerization initiator), a reducing agent, and an appropriate amount of chelating agent were charged to initiate the polymerization reaction. When the polymerization conversion rate reached 80%, the polymerization reaction was terminated by adding 0.1 parts of a 10% aqueous solution of hydroquinone (polymerization terminator). Residual monomer was removed using a rotary evaporator at 60°C to obtain a copolymer rubber latex (solids concentration: approximately 25% by weight).

[0100] Next, the latex obtained above was added to an aqueous solution of aluminum sulfate in an amount of 3% by weight based on the copolymer rubber content, and the mixture was stirred to coagulate the latex. The latex was then filtered while being washed with water, and then vacuum dried at 60°C for 12 hours to obtain a copolymer rubber.

[0101] The obtained copolymer rubber was then dissolved in acetone to a concentration of 12%, and the solution was placed in an autoclave. 350 ppm by weight of a palladium-silica catalyst was added relative to the copolymer rubber, and a hydrogenation reaction was carried out at a hydrogen pressure of 3.0 MPa. After completion of the hydrogenation reaction, the solution was poured into a large amount of water to coagulate, and then filtered and dried to obtain a nitrile group-containing copolymer rubber (A-4). The composition of the obtained nitrile group-containing copolymer rubber (A-4) was 36% by weight of acrylonitrile units and 64% by weight of 1,3-butadiene units (including saturated portions), and the iodine value was 15.

[0102] Example 1: 100 parts of the nitrile group-containing copolymer rubber (A-1) obtained in Synthesis Example 1 was mixed with silica (trade name "Carplex (R) #1120", manufactured by Evonik Industries, BET specific surface area: 120 m ２ 50 parts of a silicate (100% by weight / g), 30 parts of a surface-treated clay (trade name "Burgess KE", manufactured by Burgess Pigment, aluminum silicate represented by the above general formula (1) treated with a silane coupling agent, volume average particle size: 1.5 μm), 2 parts of vinyltrimethoxysilane (trade name "KBM-1003", manufactured by Shin-Etsu Chemical Co., Ltd., vinyl-based silane coupling agent), and 30 parts of tris-2-ethylhexyl trimellitate (trade name "Adeka Cizer C-8", manufactured by ADEKA Corporation, plasticizer) were blended and mixed at 50° C. for 5 minutes. The resulting mixture was then transferred to a 50°C roll, and 10 parts of 1,3-bis(t-butylperoxyisopropyl)benzene (trade name "Vulcup 40KE", manufactured by Hercules Chemical Industries, organic peroxide crosslinking agent, 40% product), 1.5 parts of 4,4'-bis(α,α-dimethylbenzyl)diphenylamine (trade name "Nocrac CD", manufactured by Ouchi Shinko Chemical Industry Co., Ltd., antioxidant), 1.5 parts of a zinc salt of 2-mercaptobenzimidazole (trade name "Nocrac MBZ", manufactured by Ouchi Shinko Chemical Co., Ltd., antioxidant), and 8 parts of triallyl isocyanate (trade name "TAIC", manufactured by Nippon Kasei Chemical Co., Ltd., co-crosslinking agent) were blended and kneaded to obtain a crosslinkable rubber composition.

[0103] The crosslinkable rubber compositions thus obtained were subjected to tests and evaluations for tensile strength, compression set, resistance to swelling in oil, and resistance to polycyclic aromatic compounds. The results are shown in Table 1.

[0104] Example 2 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that the amount of silica (trade name "Carplex (R) #1120") was changed from 50 parts to 35 parts. The results are shown in Table 1.

[0105] Example 3 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 2, except that the amount of surface-treated clay was changed from 30 parts to 45 parts. The results are shown in Table 1.

[0106] Example 4 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 2, except that the amount of surface-treated clay was changed from 30 parts to 20 parts. The results are shown in Table 1.

[0107] Example 5 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that 100 parts of the nitrile group-containing copolymer rubber (A-2) obtained in Synthesis Example 2 was used instead of 100 parts of the nitrile group-containing copolymer rubber (A-1) obtained in Synthesis Example 1. The results are shown in Table 1.

[0108] Example 6 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that 8 parts of trimethylolpropane trimethacrylate (trade name "TMPT", manufactured by Shin-Nakamura Chemical Co., Ltd., co-crosslinking agent) was used instead of 8 parts of triallyl isocyanurate. The results are shown in Table 1.

[0109] Example 7 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that 30 parts of bis[2-(2-butoxyethoxy)ethyl] adipate (trade name "Adekacizer RS-107", manufactured by ADEKA Corporation, plasticizer) was used instead of 30 parts of tris-2-ethylhexyl trimellitate. The results are shown in Table 1.

[0110] Example 8: Instead of 50 parts of silica (product name "Carplex (R) #1120"), silica (product name "Nipsil ER", manufactured by Tosoh Silica Corporation, BET specific surface area: 111 m) was used. ２ A crosslinkable rubber composition was obtained in the same manner as in Example 1, except that 50 parts of the hydroxybenzoate (100%) was used, and the composition was evaluated in the same manner. The results are shown in Table 1.

[0111] Example 9 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that the blending amount of tris-2-ethylhexyl trimellitate was changed from 30 parts to 22 parts. The results are shown in Table 1.

[0112] Example 10 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that the blending amount of triallyl isocyanunate was changed from 8 parts to 2.5 parts. The results are shown in Table 1.

[0113] Example 11 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that the blending amount of vinyltrimethoxysilane was changed from 2 parts to 0.5 parts. The results are shown in Table 1.

[0114] Comparative Example 1 100 parts of the nitrile group-containing copolymer rubber (A-3) obtained in Synthesis Example 3 were blended with 50 parts of silica (trade name "Nipsil ER"), 30 parts of surface-treated clay, 2 parts of aminopropyltriethoxysilane (trade name "KBE-903", manufactured by Shin-Etsu Silicones Co., Ltd., amino group-containing silane coupling agent) as an amine-based silane coupling agent, and 30 parts of tris-2-ethylhexyl trimellitate, and mixed at 50°C for 5 minutes. Next, the obtained mixture was transferred to a roll at 50°C, and 2 parts of hexamethylenediamine carbamate (trade name "Diak#1", manufactured by DuPont, polyamine crosslinking agent), 4 parts of 1,8-diazabicyclo[5,4,0]-undecene-7-ene (DBU) (trade name "RHENOGRAN XLA-60 (GE2014), manufactured by RheinChemie, DBU 60% (including the portion that has become a zinc dialkyl diphosphate salt), basic crosslinking accelerator), and 1.5 parts of 4,4'-bis(α,α-dimethylbenzyl)diphenylamine were blended and kneaded to obtain a crosslinkable rubber composition. The obtained crosslinkable rubber composition was then evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0115] Comparative Example 2 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 2, except that the surface-treated clay was not blended. The results are shown in Table 1.

[0116] Comparative Example 3 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that the amount of silica (product name "Carplex (R) #1120") was changed from 50 parts to 20 parts. The results are shown in Table 1.

[0117] Comparative Example 4 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that 100 parts of the nitrile group-containing copolymer rubber (A-4) obtained in Synthesis Example 4 was used instead of 100 parts of the nitrile group-containing copolymer rubber (A-1) obtained in Synthesis Example 1. The results are shown in Table 1.

[0118] Comparative Example 5 A crosslinkable rubber composition was obtained and evaluated in the same manner as in Example 1, except that silica (trade name "Carplex (R) #1120") and vinyltrimethoxysilane were not compounded and the amount of surface-treated clay was changed from 30 parts to 80 parts. The results are shown in Table 1.

[0119]

[0120] As shown in Table 1, the cross-linked rubber products obtained by cross-linking a cross-linkable rubber composition obtained by blending a nitrile group-containing copolymer rubber containing α,β-ethylenically unsaturated nitrile monomer units at a content ratio of 35% by weight or less and having an iodine value of 120 or less with predetermined ratios of silica, clay, and an organic peroxide cross-linking agent were excellent in low-temperature properties and polycyclic aromatic compound resistance, and also had good resistance to compression set and resistance to swelling in oil (small volume change in oil) (Examples 1 to 11).

[0121] On the other hand, when a polyamine crosslinking agent was used as the crosslinking agent, the obtained cross-linked rubber product had low airtightness when immersed in a fuel oil containing a polycyclic condensed aromatic compound after heat aging, and was therefore poor in polycyclic aromatic compound resistance after heat aging (Comparative Example 1). Furthermore, when the crosslinkable rubber composition did not contain clay, the obtained cross-linked rubber product had low airtightness when immersed in a fuel oil containing a polycyclic condensed aromatic compound after heat aging, and was therefore poor in polycyclic aromatic compound resistance after heat aging (Comparative Example 2). Furthermore, when the cross-linkable rubber composition did not contain silica or when the silica content relative to the nitrile group-containing copolymer rubber was too low, the obtained cross-linked rubber product had low airtightness when immersed in a fuel oil containing a polycyclic condensed aromatic compound after heat aging, and was therefore poor in polycyclic aromatic compound resistance after heat aging (Comparative Examples 3 and 5). Furthermore, when a nitrile group-containing copolymer rubber having an α,β-ethylenically unsaturated nitrile monomer unit content of more than 35% by weight was used, the resulting cross-linked rubber had low airtightness when immersed in a fuel oil containing a polycyclic condensed aromatic compound after heat aging, and was poor in resistance to polycyclic aromatic compounds after heat aging (Comparative Example 4).

Claims

1. A crosslinkable rubber composition comprising: a nitrile group-containing copolymer rubber containing α,β-ethylenically unsaturated nitrile monomer units in a content ratio of 35% by weight or less and having an iodine value of 120 or less; silica; clay; and an organic peroxide crosslinking agent, wherein the content of the silica per 100 parts by weight of the nitrile group-containing copolymer rubber is 30 parts by weight or more.

2. The crosslinkable rubber composition according to claim 1, further comprising a plasticizer, the content of said plasticizer being 22 parts by weight or more per 100 parts by weight of said nitrile group-containing copolymer rubber.

3. A crosslinkable rubber composition according to claim 1 or 2, further comprising a co-crosslinking agent, the content of said co-crosslinking agent being 2.5 parts by weight or more per 100 parts by weight of said nitrile group-containing copolymer rubber.

4. A crosslinkable rubber composition according to any one of claims 1 to 3, further comprising a silane coupling agent, the content of said silane coupling agent being 0.5 parts by weight or more per 100 parts by weight of said nitrile group-containing copolymer rubber.

5. A cross-linked rubber product obtained by cross-linking the cross-linkable rubber composition according to any one of claims 1 to 4.

6. A sealing material comprising the cross-linked rubber according to claim 5.

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