Rubber composition, rubber product, and pneumatic tire

Abstract

Provided are a rubber composition with which it is possible to obtain a rubber product having low rolling resistance, a rubber product obtained from the rubber composition, and a pneumatic tire serving as the rubber product.　There is used a rubber composition containing, per 100 parts by weight of a diene rubber (A), 5-200 parts by weight of silica (B), 0.5-50 parts by weight of carbon black (C), 0.5-20 parts by weight of a silane coupling agent (D), and 1-20 parts by weight of a (meth)acrylic acid ester polymer (E) having a terminal mercapto group.

Description

Rubber composition, rubber product, and pneumatic tire

[0001] The present invention relates to a rubber composition, a rubber product, and a pneumatic tire.

[0002] BACKGROUND ART In the automobile industry, there has been a demand for improved fuel efficiency of automobiles, and there has been a demand for rubber compositions for treads that can provide tires with low rolling resistance.

[0003] As a rubber composition that reduces the rolling resistance of a tire, for example, a rubber composition containing a diene rubber, which is compounded with a (meth)acrylic polymer having an acryloyl group at one molecular end that can react during vulcanization of the diene rubber, is known (see Patent Document 1).

[0004] JP 2014-084363 A

[0005] According to the inventors' investigations, although the use of the rubber composition disclosed in Patent Document 1 reduces the rolling resistance of tires, further improvement is required.

[0006] The present invention has been made in view of the above-mentioned problems, and aims to provide a rubber composition from which a rubber product having low rolling resistance can be obtained, a rubber product obtained from the rubber composition, and a pneumatic tire as the rubber product.

[0007] The present inventors have found that the above-mentioned problems can be solved by blending a (meth)acrylic acid ester polymer having a mercapto group at the end, instead of the (meth)acrylic polymer having an acryloyl group at one molecular end as disclosed in Patent Document 1, and have completed the present invention.

[0008] Aspects of the present disclosure relate to the following rubber compositions, rubber products, and pneumatic tires.

[0009] [1] A rubber composition containing, per 100 parts by weight of diene rubber (A), 5 to 200 parts by weight of silica (B), 0.5 to 50 parts by weight of carbon black (C), 0.5 to 20 parts by weight of a silane coupling agent (D), and 1 to 20 parts by weight of a (meth)acrylic acid ester polymer (E) having a mercapto group at its terminal. [2] The rubber composition according to [1], wherein the diene rubber (A) is at least one selected from the group consisting of natural rubber, isoprene rubber, styrene-butadiene copolymer rubber, and butadiene rubber. [3] The rubber composition according to [1] or [2], wherein the (meth)acrylic acid ester polymer (E) has a mercapto group at one terminal and a group represented by the following general formula (1) at the other terminal. CHR １ R ２ -CR ３ (COOR ４ )-(1) (wherein, R １ , R ２ , R ４ are the same or different and represent a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, or an aryl group having 6 to 20 carbon atoms. １ , R ２ , R ４ represents an oxygen atom, a halogen atom, CN, and NR ５ R ６ wherein R ５ , R ６ are the same or different and represent a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, or an aryl group having 6 to 11 carbon atoms. ３ represents a hydrogen atom or a methyl group.) [4] The rubber composition according to any one of [1] to [3], wherein the (meth)acrylic acid ester polymer (E) having a terminal mercapto group is a block copolymer. [5] A rubber product obtained from the rubber composition according to any one of [1] to [4]. [6] A pneumatic tire which is the rubber product according to [5].

[0010] According to the present invention, it is possible to provide a rubber composition from which a rubber product having low rolling resistance can be obtained, a rubber product obtained from the rubber composition, and a pneumatic tire as the rubber product.

[0011] <Rubber Composition> The rubber composition of the present embodiment contains, relative to 100 parts by weight of the diene rubber (A), 5 to 200 parts by weight of silica (B), 0.5 to 50 parts by weight of carbon black (C), 0.5 to 20 parts by weight of a silane coupling agent (D), and 1 to 20 parts by weight of a (meth)acrylic acid ester-based polymer (E) having a terminal mercapto group.

[0012] According to the rubber composition of the present embodiment, since the (meth)acrylic acid ester polymer (E) having a mercapto group at its terminal is contained, a rubber product having low rolling resistance can be obtained. Hereinafter, essential and optional components that the rubber composition may contain will be described.

[0013] <Diene Rubber (A)> The diene rubber (A) is not particularly limited as long as it is a rubber obtained from a monomer containing a diene compound. Examples of the diene rubber (A) include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile butadiene rubber, ethylene-propylene-diene copolymer rubber, styrene-isoprene copolymer rubber, isoprene-butadiene copolymer rubber, styrene-isoprene-butadiene copolymer rubber, and chloroprene rubber. The styrene-butadiene copolymer rubber (SBR) may be either an emulsion polymerization SBR or a solution polymerization SBR. The diene rubber (A) may also be a rubber into which a functional group has been introduced, such as a modified butadiene rubber. More preferably, the diene rubber (A) is natural rubber, isoprene rubber, styrene-butadiene copolymer rubber, butadiene rubber, or a blend of two or more thereof.

[0014] These diene rubbers (A) may be used in combination with olefin polymer rubbers, such as butyl rubber, chlorinated butyl rubber, brominated butyl rubber, and ethylene-propylene-diene rubber.

[0015] <Silica (B)> Silica (B) is not particularly limited, and examples thereof include wet silica, dry silica, pulverized silica, etc. Among these, wet silica is preferred because it has excellent abrasion resistance, mechanical properties, and is also economical.

[0016] The content of silica (B) is 5 to 200 parts by weight, preferably 10 to 150 parts by weight, and more preferably 20 to 100 parts by weight, per 100 parts by weight of diene rubber (A).

[0017] <Carbon Black (C)> The carbon black (C) is not particularly limited as long as it can be generally compounded as a filler in a rubber composition. Examples of the carbon black (C) include FEF, SRF, HAF, ISAF, and SAF grades. In terms of further improving mechanical properties and abrasion resistance, HAF, ISAF, and SAF grades are preferred for the carbon black (C). The carbon black (C) may be used alone or in combination of two or more types.

[0018] The content of carbon black (C) is 0.5 to 50 parts by weight, preferably 0.5 to 40 parts by weight, and more preferably 1.0 to 20 parts by weight, per 100 parts by weight of diene rubber (A).

[0019] <Silane Coupling Agent (D)> Examples of the silane coupling agent (D) include bis(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)trisulfide, bis(3-triethoxysilylpropyl)disulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)tetrasulfide, 3-mercaptopropyltriethoxysilane, 2-mercaptoethylsilane, 2 ... methyltrimethoxysilane, 2-mercaptoethyltriethoxysilane, 3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide, 3-triethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide, 2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, 3-trimethoxysilylpropyl benzothiazole tetrasulfide, 3-triethoxysilylpropyl benzothiazole tetrasulfide, 3-triethoxysilylpropyl methacrylate monosulfide, 3-trimethoxysilylpropyl methacrylate monosulfide, bis(3-diethoxymethylsilylpropyl)tetrasulfide, 3-mercaptopropyldimethoxymethylsilane, dimethoxymethylsilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide, dimethoxymethylsilylpropyl benzothiazole tetrasulfide, N-(1,3-dimethylbutylidene)-3-(triethoxysilylpropyl)benzothiazole tetrasulfide (3,4-epoxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltriethoxy ...4-epoxycyclohexyl)ethyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-mercaptopropylmethyldimethoxysilane, styryltrimethoxysilane, styryltriethoxysilane, γ-mercaptopropylmethyldiethoxysilane, etc.

[0020] Among these, from the viewpoint of superior abrasion resistance and improved reinforcement, bis(3-triethoxysilylpropyl)tetrasulfide, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, γ-acryloxypropyltriethoxysilane, γ-methacryloxypropyltriethoxysilane, vinyltriethoxysilane, and styryltriethoxysilane are preferred.

[0021] The silane coupling agents (D) can be used either alone or in combination of two or more.

[0022] The content of the silane coupling agent (D) is 0.5 to 20 parts by weight, preferably 0.5 to 15 parts by weight, and more preferably 1.0 to 10 parts by weight, per 100 parts by weight of the diene rubber (A).

[0023] <(Meth)acrylic acid ester polymer (E)> The (meth)acrylic acid ester polymer (E) has a mercapto group at a terminal of the (meth)acrylic acid ester polymer. When the rubber composition of the present embodiment contains the (meth)acrylic acid ester polymer (E), a rubber product having low rolling resistance and excellent tensile stress can be obtained.

[0024] ((Meth)acrylic acid ester-based polymer) The (meth)acrylic acid ester-based polymer contains 50 mol % or more of structural units derived from (meth)acrylic acid ester-based monomers and less than 50 mol % of structural units derived from monomers other than (meth)acrylic acid ester-based monomers, based on 100 mol % of all structural units contained in the polymer.

[0025] Examples of the (meth)acrylic acid ester monomer include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-heptyl (meth)acrylate, n-octyl (meth)acrylate, and 2-methyl (meth)acrylate. -ethylhexyl, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, phenyl (meth)acrylate, toluyl (meth)acrylate, benzyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, stearyl (meth)acrylate, glycidyl (meth)acrylate, (3-trimethoxysilyl)propyl (meth)acrylate propyl, (3-dimethoxymethylsilyl)propyl (meth)acrylate, (2-trimethoxysilyl)ethyl (meth)acrylate, (2-dimethoxymethylsilyl)ethyl (meth)acrylate, trimethoxysilylmethyl (meth)acrylate, (dimethoxymethylsilyl)methyl (meth)acrylate, ethylene oxide adducts of (meth)acrylic acid, trifluoromethylmethyl (meth)acrylate, 2-trifluoromethylethyl (meth)acrylate, 2-perfluoroethylethyl (meth)acrylate, 2-perfluoroethyl-2-perfluorobutylethyl (meth)acrylate, perfluoroethyl (meth)acrylate, trifluoromethyl (meth)acrylate, bis(trifluoromethyl)methyl (meth)acrylate, 2-trifluoromethyl-2-perfluoroethylethyl (meth)acrylate, 2-perfluorohexylethyl (meth)acrylate, 2-perfluorodecylethyl (meth)acrylate, and 2-perfluorohexadecylethyl (meth)acrylate. These monomers may be used alone or in combination of two or more. Here, "(meth)acrylic" refers to acrylic and / or methacrylic.

[0026] When two or more (meth)acrylic acid ester monomers are used in combination, the (meth)acrylic acid ester polymer may be a random copolymer or a block copolymer. Of these, the (meth)acrylic acid ester polymer is preferably a block copolymer, from the viewpoint of obtaining a rubber product having low rolling resistance and excellent tensile stress.

[0027] Monomers other than (meth)acrylic acid ester-based monomers (hereinafter sometimes referred to as "other monomers") are not particularly limited as long as they are vinyl-based monomers copolymerizable with (meth)acrylic acid ester-based monomers. Examples of other monomers include styrene-based monomers such as styrene, vinyltoluene, α-methylstyrene, chlorostyrene, styrenesulfonic acid and salts thereof; fluorine-containing vinyl-based monomers such as perfluoroethylene, perfluoropropylene, and vinylidene fluoride; silicon-containing vinyl-based monomers such as vinyltrimethoxysilane and vinyltriethoxysilane; maleic anhydride, maleic acid, monoalkyl esters and dialkyl esters of maleic acid; fumaric acid, monoalkyl esters and dialkyl esters of fumaric acid; maleimide, methylmaleimide, ethylmaleimide, propylmaleimide, butylmaleimide, Examples of suitable monomers include maleimide monomers such as xylmaleimide, octylmaleimide, dodecylmaleimide, stearylmaleimide, phenylmaleimide, and cyclohexylmaleimide; nitrile group-containing vinyl monomers such as acrylonitrile and methacrylonitrile; amide group-containing vinyl monomers such as acrylamide and methacrylamide; vinyl ester monomers such as vinyl acetate, vinyl propionate, vinyl pivalate, vinyl benzoate, and vinyl cinnamate; alkenyl monomers such as ethylene and propylene; conjugated diene monomers such as butadiene and isoprene; vinyl chloride, vinylidene chloride, allyl chloride, and allyl alcohol. These other monomers may be used alone or in combination of two or more.

[0028] The (meth)acrylic acid ester polymer (E) preferably has a mercapto group at one end of the main chain of the (meth)acrylic acid ester polymer.

[0029] The (meth)acrylic acid ester polymer (E) preferably has a mercapto group at one end and a group represented by the following general formula (1) at the other end: １ R ２ -CR ３ (COOR ４ )-(1) (wherein, R １ , R ２ , R ４ are the same or different and represent a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, or an aryl group having 6 to 20 carbon atoms. １ , R ２ , R ４ represents an oxygen atom, a halogen atom, CN, and NR ５ R ６ wherein R ５ , R ６ are the same or different and represent a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, or an aryl group having 6 to 11 carbon atoms. ３ represents a hydrogen atom or a methyl group.

[0030] R １ , R ２ , and R ４ Examples of the alkyl group having 1 to 20 carbon atoms represented by the formula (I) include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, a 2,2-dimethylpropyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, an n-decyl group, an n-undecyl group, an n-dodecyl group, an n-tridecyl group, an n-tetradecyl group, and an n-pentadecyl group.

[0031] R １ , R ２ , and R ４ Examples of the aryl group having 6 to 20 carbon atoms represented by the formula (R) include a phenyl group, a 1-naphthyl group, a 2-naphthyl group, a 3-phenanthryl group, and a 2-anthryl group. １ , R ２ , and R ４ is appropriately selected depending on the type of monomer and radical polymerization initiator used.

[0032] R ５ and R ６ Examples of the alkyl group having 1 to 8 carbon atoms represented by the formula (I) include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, a 2,2-dimethylpropyl group, an n-hexyl group, an n-heptyl group, and an n-octyl group.

[0033] R ５ and R ６ Examples of the aryl group having 6 to 11 carbon atoms represented by the formula (I) include a phenyl group, a 1-naphthyl group, and a 2-naphthyl group.

[0034] The number average molecular weight of the (meth)acrylic acid ester polymer (E) is not particularly limited, and is preferably 1,000 to 50,000, more preferably 1,000 to 20,000, and even more preferably 1,000 to 15,000, in terms of polystyrene equivalent molecular weight measured by size exclusion chromatography (SEC).

[0035] The molecular weight distribution (Mw / Mn) of the (meth)acrylic acid ester polymer (E) is not particularly limited, and is, for example, preferably 3.0 or less, more preferably 2.0 or less, even more preferably 1.8 or less, even more preferably 1.6 or less, and particularly preferably 1.4 or less. The lower limit is not particularly limited, but it is sufficient as long as it is 1 or more.

[0036] The content of the (meth)acrylic acid ester polymer (E) is 1 to 20 parts by weight, preferably 1 to 15 parts by weight, and more preferably 1 to 10 parts by weight, per 100 parts by weight of the diene rubber (A).

[0037] (Method for producing (meth)acrylic acid ester polymer (E)) The polymerization method for the (meth)acrylic acid ester polymer (E) is not particularly limited, and known polymerization methods can be used (radical polymerization method, cationic polymerization method, anionic polymerization method, etc.). Among them, living polymerization method is preferred because it allows introduction of functional groups to the ends of polymer molecules. Examples of living polymerization method include living radical polymerization method, living cationic polymerization method, and living anionic polymerization method, and among them, living radical polymerization method is suitable for polymerizing acrylic acid ester monomers. Examples of living radical polymerization method include the following. Atom Transfer Radical Polymerization (ATRP) (see J. Am. Chem. Soc. 1995, 117, 5614; Macromolecules. 1995, 28, 1721) Single Electron Transfer Polymerization (SET-LRP) (see J. Am. Chem. Soc. 2006, 128, 14156; JPSC Chem 2007, 45, 1607) Reversible Chain Transfer Polymerization Catalyzed Polymerization (RTCP) (see "Living Radical Polymerization Controlled by Organic Catalysts", Polymer Research Collection, 68, 223-231 (2011); JP 2014-111798 A)) Reversible addition-fragmentation chain transfer polymerization (RAFT polymerization) Nitroxy radical method (NMP method) Polymerization method using an organotellurium compound (TERP method) Polymerization method using an organoantimony compound (SBRP method) Polymerization method using an organobismuth compound (BIRP method) Iodine transfer polymerization Among living radical polymerizations, RAFT polymerization and atom transfer radical polymerization are preferred, with RAFT polymerization being more preferred, because it is easy to introduce a mercapto group into the terminal of the polymer.

[0038] RAFT polymerization is briefly described below.

[0039] In RAFT polymerization, controlled polymerization proceeds via reversible chain transfer reactions in the presence of a RAFT agent and a common radical polymerization initiator.

[0040] The RAFT agent is not particularly limited, but may be a compound containing a thiocarbonylthio group, such as a dithiobenzoate compound, an aromatic or aliphatic heterocyclic carbodithioate compound, a dithiocarbamate compound, a dithiocarbonate compound, a xanthate compound, or a trithiocarbonate compound.

[0041] Examples of trithiocarbonate compounds include S,S-dibenzyl trithiocarbonate, bis[4-(2,3-dihydroxypropylcarbonyl)benzyl]trithiocarbonate, bis[4-(2-hydroxyethoxycarbonyl)benzyl]trithiocarbonate, 2-cyano-2-propyldodecyltrithiocarbonate, etc. The amount of the RAFT agent used is adjusted appropriately depending on the types of monomer and RAFT agent used, etc.

[0042] The polymerization initiator used in RAFT polymerization is not particularly limited, and known radical polymerization initiators such as azo compounds, organic peroxides, and persulfates can be used. However, azo compounds are preferred because they are safe and easy to handle and are less likely to cause side reactions during radical polymerization. Specific examples of azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis[N-(2-propenyl)-2-methylpropionamide], and 2,2'-azobis(N-butyl-2-methylpropionamide). The radical polymerization initiators may be used alone or in combination of two or more.

[0043] The proportion of the radical polymerization initiator used is not particularly limited, but from the viewpoint of obtaining a polymer with a narrower molecular weight distribution, the amount of radical polymerization initiator used per 1 mol of RAFT agent is preferably 0.5 mol or less, and more preferably 0.3 mol or less. Furthermore, from the viewpoint of stably carrying out the polymerization reaction, the lower limit of the amount of radical polymerization initiator used per 1 mol of RAFT agent is 0.001 mol. Therefore, the amount of radical polymerization initiator used per 1 mol of RAFT agent is preferably 0.001 to 0.5 mol, and more preferably 0.005 to 0.3 mol.

[0044] The reaction temperature for RAFT polymerization is preferably 30 to 120°C, more preferably 40 to 110°C, and even more preferably 50 to 100°C. If the reaction temperature is 30°C or higher, the polymerization can proceed smoothly. On the other hand, if the reaction temperature is 120°C or lower, side reactions can be suppressed and restrictions on the initiators and solvents that can be used can be alleviated.

[0045] RAFT polymerization can be performed without a solvent, but can also be performed in various solvents. The type of solvent is not particularly limited, and known polymerization solvents can be used. Specific examples include aromatic compounds such as benzene, toluene, xylene, and anisole; ester compounds such as methyl acetate, ethyl acetate, propyl acetate, and butyl acetate; ketone compounds such as acetone and methyl ethyl ketone; dimethylformamide, acetonitrile, dimethyl sulfoxide, alcohol, and water. These solvents can be used alone or in combination of two or more.

[0046] The polymer obtained by RAFT polymerization can be reacted with a nucleophilic agent to remove the thiocarbonylthio group and introduce a mercapto group into the polymer terminal.

[0047] A known solvent can be used for the reaction between the polymer obtained by RAFT polymerization and the nucleophilic agent. Specific examples include aromatic compounds such as benzene, toluene, xylene, and anisole; ether compounds such as dibutyl ether and tetrahydrofuran; ester compounds such as methyl acetate, ethyl acetate, propyl acetate, and butyl acetate; ketone compounds such as acetone and methyl ethyl ketone; dimethylformamide, acetonitrile, dimethyl sulfoxide, alcohol, and water. These solvents may be used alone or in combination of two or more.

[0048] The nucleophilic agent is not particularly limited as long as it can convert a thiocarbonylthio group to a mercapto group. Specific examples include nucleophilic reagents such as ammonia, amine compounds, and metal alkoxides. These may be used alone or in combination of two or more.

[0049] The amount of nucleophilic agent used can be appropriately selected depending on the equivalent weight of the thiocarbonyl group. From the viewpoint of reaction efficiency, the amount of nucleophilic agent used per 1 mol of thiocarbonyl group is preferably 1 mol or more, more preferably 2 mol or more, and even more preferably 3 mol or more. Furthermore, from the viewpoint of minimizing the influence of odor due to unreacted nucleophilic agent, the amount is preferably 50 mol or less, more preferably 30 mol or less, and even more preferably 20 mol or less. Therefore, the amount of nucleophilic agent used per 1 mol of thiocarbonyl group is preferably 1 to 50 mol, more preferably 2 to 30 mol, and even more preferably 3 to 20 mol.

[0050] A reducing agent can be added to the reaction between the polymer obtained by RAFT polymerization and the nucleophilic agent. Examples of the reducing agent include tributylphosphine, tris(2-carboxyethyl)phosphine, dimethylphenylphosphine, sodium dithionite, sodium bisulfite, and ethylenediaminetetraacetic acid. These may be used alone or in combination of two or more.

[0051] The reaction temperature between the polymer obtained by RAFT polymerization and the nucleophilic agent is preferably 10° C. or higher, more preferably 15° C. or higher, and even more preferably 20° C. or higher, from the viewpoint of reaction efficiency. Moreover, from the viewpoint of preventing side reactions such as nucleophilic reactions on the polymer main chain from occurring, the reaction temperature is preferably 80° C. or lower, more preferably 70° C. or lower, and even more preferably 60° C. or lower.

[0052] (Other Components) The rubber composition may contain components other than the diene rubber (A), silica (B), carbon black (C), silane coupling agent (D), and (meth)acrylic acid ester polymer (E) (hereinafter also referred to as "other components") as long as the effects of the present invention are not impaired. As other components, compounding agents typically used in the rubber industry, such as fillers, plasticizers, vulcanizing agents, vulcanization accelerators, vulcanization accelerator assistants, and antioxidants, can be appropriately blended depending on the purpose and application.

[0053] Examples of the filler include, in addition to the above-mentioned silica (B) and carbon black (C), aluminum oxide, organic short fibers, (meth)acrylic resin fine particles, epoxy resin fine particles, glass fine particles, glass fibers, and flake graphite.

[0054] Examples of plasticizers include petroleum-based process oils such as paraffinic process oil, naphthenic process oil, and aromatic process oil, dialkyl dibasic acids such as diethyl phthalate, dioctyl phthalate, and dibutyl adipate, low-molecular-weight liquid polymers such as liquid polybutene and liquid polyisoprene, and natural oils such as orange oil. Among these, liquid polybutene, liquid polyisoprene, and aromatic process oil are preferred in terms of compatibility with the rubber component.

[0055] Examples of the vulcanizing agent include sulfur, phenolic resin, metal oxide, and peroxide.

[0056] Examples of the vulcanization accelerator include thiazole-based vulcanization accelerators such as 2-mercaptobenzothiazole (M), 2,2-dithiobisbenzothiazole (DM), mercaptobenzothiazole zinc salt (MZ), mercaptobenzothiazole cyclohexylamine salt (M60), 2,4-dinitrophenylthiobenzothiazole (DBM), and N,N-diethylthiocarbamoylthiobenzothiazole (64); N-cyclohexyl-2-benzothiazoylsulfenamide (CZ), N-tert-butyl-2-benzothiazolesulfenamide (NS), N-oxydiethylene-2-benzothiazoylsulfenamide (NOBS), N,N-dicyclohexyl-2-benzothiazoylsulfenamide (DZ), and morpholinodithiobenzothiazole ( Examples of suitable vulcanization accelerators include sulfenamide-based vulcanization accelerators such as butyraldehyde aniline (B) and butyraldehyde monobutylamine (833), aldehyde ammonia-based vulcanization accelerators such as butyraldehyde aniline (B) and butyraldehyde monobutylamine (833), guanidine-based vulcanization accelerators such as diphenyl guanidine (D), di-o-tolyl guanidine (DT), o-tolyl guanidine (BG), and dicatechol borate-di-o-tolyl guanidine salt (PR), and thiuram-based vulcanization accelerators such as tetramethylthiuram monosulfide (TS), tetramethylthiuram disulfide (TT), tetraethylthiuram disulfide (TET), tetrabutylthiuram disulfide (TBT), dipentamethylenethiuram hexasulfide (TRA), and tetrabenzylthiuram disulfide.

[0057] These other components are usually used in an amount of 0.2 to 40 parts by weight per 100 parts by weight of the diene rubber (A). The vulcanization accelerators may be used alone or in combination of two or more.

[0058] <Method for Producing Rubber Composition> The method for producing the rubber composition is not particularly limited as long as it is a method conventionally used in the rubber industry, and examples thereof include a method in which the above-mentioned components are kneaded using a known method or apparatus (e.g., a Banbury mixer, a kneader, a roll, etc.).

[0059] As for the kneading conditions, when additives other than the vulcanizing agent and the vulcanization accelerator are blended, the kneading temperature is usually 50 to 200°C, preferably 80 to 190°C, and the kneading time is usually 30 seconds to 30 minutes, preferably 1 minute to 30 minutes.

[0060] When a vulcanizing agent and a vulcanization accelerator are blended, the kneading temperature is usually 100°C or lower, preferably room temperature to 80°C. Furthermore, a composition blended with a vulcanizing agent and a vulcanization accelerator is usually used after undergoing a vulcanization treatment such as press vulcanization. The vulcanization temperature is usually 120 to 200°C, preferably 140 to 180°C.

[0061] The (meth)acrylic acid ester polymer (E) may be blended in the vulcanization step, or may be blended in advance with the diene rubber (A), or may be reacted with the diene rubber (A) in advance. Blending in the vulcanization step is preferred because of the ease of operation.

[0062] <Rubber Products> Rubber products can be obtained from the rubber composition described above. That is, the rubber composition, to which various additives are optionally added, is molded into a molded product before vulcanization. This unvulcanized molded product can be heated and pressurized in a vulcanizer to produce a rubber product.

[0063] Examples of rubber products include pneumatic tires, cable coverings, hoses, transmission belts, conveyor belts, roll covers, shoe bodies or shoe soles, sealing rings, and vibration-isolating rubber.

[0064] <Pneumatic Tire> A pneumatic tire is manufactured by a conventional method using the above-described rubber composition. That is, the rubber composition, to which various additives are optionally added, is extruded to match the shape of the tire tread while still unvulcanized, molded by a conventional method on a tire building machine, and then bonded together with other tire components to form an unvulcanized tire. This unvulcanized tire can be heated and pressurized in a vulcanizer to manufacture the pneumatic tire of the present invention.

[0065] The pneumatic tire can be suitably used as a tire for a passenger car, or a tire for a truck or bus (heavy duty tire).

[0066] <Tire Performance> When the rubber composition is used in a pneumatic tire, the performance of the tire is estimated by the following indexes.

[0067] (Rolling Resistance) Rolling resistance is a vibration of about 10 to 100 Hz at around 60° C., and is therefore expressed as tan δ from 40° C. to 70° C. at 10 Hz.

[0068] (Wet Grip Performance) Wet grip performance is believed to depend on deformation near the tire surface. This deformation near the surface is known to be a very high frequency vibration, and when temperature frequency conversion is used, wet grip performance is expressed as tan δ from -20°C to 0°C at 10 Hz.

[0069] (Tensile Stress) Tensile stress is considered to be related to the rigidity, strength, and durability of a tire. The tensile stress can be determined using the stress at 300% elongation (M300) as an index.

[0070] (Hardness) Hardness is considered to be related to handling stability.

[0071] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0072] [Measurement Method] (1) Molecular Weight The number average molecular weight (Mn), weight average molecular weight (Mw), and molecular weight distribution (Mw / Mn) of the (meth)acrylic acid ester polymer were measured by standard polystyrene conversion using size exclusion chromatography (SEC). The SEC specifications are as follows: SEC system: HLC-8320 (Tosoh Corporation) Stationary phase: TSKgel Super HM-N (Tosoh Corporation) Mobile phase: Chloroform (2) Rolling Resistance Test specimens measuring 40 mm in length, 5 mm in width, and 2 mm in thickness were cut from the vulcanized cured rubber, and tan δ at 60°C was measured using a dynamic viscoelasticity measuring device DVA-200 (IT Measurement Control Co., Ltd.). This characteristic is expressed as an index, with the reference sample (Comparative Example 1 described below) set to 100. The smaller the index, the better the rolling resistance. The measurement conditions were as follows. Measurement temperature: -20°C to 80°C Heating rate: 2°C / min Initial strain: 10% Amplitude: ±1% Frequency: 10 Hz (3) Wet grip performance Tan δ at 0°C was measured in the same manner as for rolling resistance. This characteristic was expressed as an index, with the reference sample (Comparative Example 1 described below) being 100. The higher this index, the better the wet grip performance. (4) Stress at 300% elongation (M300) A dumbbell No. 7 was punched out of the vulcanized cured rubber, and a tensile test was performed to measure the stress at 300% elongation. An AGS-J (Shimadzu Corporation) was used for the measurement. The tensile speed during the measurement was 500 mm / min. This characteristic was expressed as an index, with the reference sample (Comparative Example 1 described below) being 100. The higher this index, the better the rigidity, strength, and durability are expected to be. (5) Hardness: The vulcanized cured rubber was stacked to a thickness of 6 mm and measured using a Type A durometer. This property was expressed as an index, with the reference sample (Comparative Example 1 described below) being set at 100. A higher index is expected to provide better handling stability.

[0073] [Materials] The materials used in the examples and comparative examples are as follows. Diene rubber (A): Tufden 2000R (Asahi Kasei Corporation) Silica (B): Nipsil AQ (Tosoh Silica Corporation) Carbon black (C): Diablack N339 (Mitsubishi Chemical Corporation) Silane coupling agent (D): Si69 (Evonik) (meth)acrylic acid ester polymer (E): Polymers 1 to 12 shown below: Polymer 1: Obtained in Production Example 1 Polymer 2: Obtained in Production Example 2 Polymer 3: Obtained in Production Example 3 Polymer 4: Obtained in Production Example 4 Polymer 5: Obtained in Production Example 5 Polymer 6: Obtained in Production Example 6 Polymer 7: Obtained in Production Example 7 Polymer 8: Obtained in Production Example 8 Polymer 9: Obtained in Production Example 9 Polymer 10: Obtained in Production Example 10 Polymer 11: Obtained in Production Example 11 Polymer 12: Obtained in Production Example 12 Polymer 13: Obtained in Production Example 13

[0074] (Other ingredients) Anti-aging agent: Nocrac 6C (Ouchi Shinko Chemical Industry Co., Ltd.) Plasticizer: Aromax 3 (ENEOS Corporation) Vulcanizing agent: sulfur (Fujifilm Wako Pure Chemical Industries Co., Ltd.) Vulcanization accelerator: 1,3-diphenylguanidine (Tokyo Chemical Industry Co., Ltd.), N-tert-butyl-2-benzothiazole sulfenamide (Fujifilm Wako Pure Chemical Industries Co., Ltd.) Vulcanization accelerator aid: stearic acid (Fujifilm Wako Pure Chemical Industries Co., Ltd.), zinc oxide (Fujifilm Wako Pure Chemical Industries Co., Ltd.)

[0075] [Production Example 1: Production of Polymer 1] 20.00 g (0.1560 mol) of butyl acrylate, 12 g of toluene as a polymerization solvent, and 1.6231 g (4.6963 mmol) of 2-cyano-2-propyldodecyltrithiocarbonate (CPDT) as a RAFT agent were charged, and 0.9604 g of a separately prepared V-59 solution (a solution prepared by dissolving 0.1000 g (0.5201 mmol) of 2,2'-azobis(2-methylbutyronitrile) (V-59) in 9.9000 g of toluene) was added thereto. Nitrogen bubbling was performed for 30 minutes, and then the mixture was stirred at 70°C to initiate polymerization. The polymerization solution was sampled at regular intervals during the polymerization, and the consumption rate of butyl acrylate was measured by gas chromatography. Eight hours after the start of polymerization, it was confirmed that 77.1% of the butyl acrylate had been consumed, and heating was stopped. The reaction solution was then concentrated under reduced pressure at 105°C for 1 hour. Absorbance measurement of the resulting polymer confirmed that 0.95 groups derived from the CPDT had been introduced into the molecular terminals per polymer molecule. 15 g of the resulting polymer was dissolved in 45 g of tetrahydrofuran, and nitrogen bubbling was performed for 30 minutes. 0.87 g of tributylphosphine and 2.11 g of 1-hexylamine were added to the solution, and the mixture was stirred for 3 hours. The reaction solution was then reprecipitated in methanol cooled to -70°C. The precipitated polymer was concentrated under reduced pressure at 60°C for 1 hour to obtain Polymer 1 having mercapto groups introduced into the molecular terminals. The introduction of mercapto groups was confirmed by absorbance measurement of the resulting Polymer 1, which showed the loss of the groups derived from the CPDT at the molecular terminals. The number average molecular weight of the resulting Polymer 1 was 3,720, and the molecular weight distribution was 1.12.

[0076] [Production Example 2: Production of Polymer 2] 20.00 g (0.1560 mol) of butyl acrylate, 12 g of toluene as a polymerization solvent, and 0.4055 g (1.173 mmol) of CPDT were charged, and 0.9547 g of a separately prepared V-59 solution (a solution prepared by dissolving 0.0500 g (0.260 mmol) of 2,2'-azobis(2-methylbutyronitrile) (V-59) in 9.9500 g of toluene) was added thereto. Nitrogen bubbling was performed for 30 minutes, and then the mixture was stirred at 70°C to initiate polymerization. The polymerization solution was sampled at regular intervals during the polymerization, and the consumption rate of butyl acrylate was measured by gas chromatography. Six hours after the start of polymerization, it was confirmed that 73.3% of the butyl acrylate had been consumed, and heating was stopped. The reaction solution was then concentrated under reduced pressure at 105°C for 1 hour. Absorbance measurement of the resulting polymer confirmed that 0.95 groups derived from the CPDT had been introduced into the molecular terminals per polymer molecule. 12 g of the resulting polymer was dissolved in 36 g of tetrahydrofuran, and nitrogen bubbling was performed for 30 minutes. 0.18 g of tributylphosphine and 0.45 g of 1-hexylamine were added to the solution, and the mixture was stirred for 3 hours. The reaction solution was then reprecipitated in methanol cooled to -70°C. The precipitated polymer was concentrated under reduced pressure at 60°C for 1 hour to obtain Polymer 2, which had mercapto groups introduced into the molecular terminals. The introduction of mercapto groups was confirmed by absorbance measurement of the resulting Polymer 2, which showed the loss of the groups derived from the CPDT at the molecular terminals. The number average molecular weight of the resulting Polymer 2 was 12,700, and the molecular weight distribution was 1.16.

[0077] [Production Example 3: Production of Polymer 3] Polymer 3 was obtained by production in the same manner as in Production Example 1, except that 20.31 g of 2-methoxyethyl acrylate was used instead of 20.00 g of butyl acrylate. The polymerization time was 6.5 hours. The number average molecular weight of the obtained polymer 3 was 3,800, and the molecular weight distribution was 1.17.

[0078] [Production Example 4: Production of Polymer 4] Polymer 4 was obtained by production in the same manner as in Production Example 2, except that 30.54 g of 2-methoxyethyl acrylate was used instead of 20.00 g of butyl acrylate, and the amounts of toluene, CDPT, tributylphosphine, and 1-hexylamine used as polymerization solvents were 16 g, 0.8111 g, 0.23 g, and 0.56 g, respectively. The polymerization time was 5 hours. The number average molecular weight of the obtained polymer 4 was 10,340, and the molecular weight distribution was 1.13.

[0079] Production Example 5 Production of Polymer 5 Polymer 5 was obtained by production in the same manner as in Production Example 1, except that 20.17 g of 2-(dimethylamino)ethyl acrylate was used instead of 20.00 g of butyl acrylate, 0.0903 g of 2,2'-azobis(2-methylbutyronitrile) (V-59) was used instead of the V-59 solution, and the amounts of tributylphosphine and 1-hexylamine used were 0.99 g and 2.41 g, respectively. The polymerization time was 4 hours. The number average molecular weight of the obtained polymer 5 was 3,570, and the molecular weight distribution was 1.29.

[0080] [Production Example 6: Production of Polymer 6] 9.92 g (0.0774 mol) of butyl acrylate, 10 g of toluene as a polymerization solvent, and 1.6107 g (4.6604 mmol) of CPDT were charged, and 2.2400 g of a separately prepared V-59 solution (a solution prepared by dissolving 0.1000 g (0.5201 mmol) of 2,2'-azobis(2-methylbutyronitrile) (V-59) in 9.9000 g of toluene) was added thereto. Nitrogen bubbling was performed for 30 minutes, and then the mixture was stirred at 70°C to initiate polymerization. The polymerization solution was sampled at regular intervals during the polymerization, and the consumption rate of butyl acrylate was measured by gas chromatography. 360 minutes after the start of polymerization, the temperature was raised to 85°C. 440 minutes after the start of polymerization, it was confirmed that 85.0% of the butyl acrylate had been consumed. To this was added 10.08 g (0.07743 mol) of 2-methoxyethyl acrylate, which had been previously bubbled with nitrogen for 30 minutes. 520 minutes after the start of polymerization, it was confirmed that 97.7% of the butyl acrylate and 86.6% of the 2-methoxyethyl acrylate had been consumed, and heating was stopped. The reaction solution was then concentrated under reduced pressure at 105°C for 1 hour. Absorbance measurement of the resulting polymer confirmed that 1.00 groups derived from the CPDT had been introduced to the molecular terminals per polymer molecule. 18 g of the resulting polymer was dissolved in 55 g of tetrahydrofuran, and nitrogen bubbling was performed for 30 minutes. 0.83 g of tributylphosphine and 2.02 g of 1-hexylamine were added to this solution, and the mixture was stirred for 6 hours. The reaction solution was then reprecipitated in hexane. The precipitated polymer was concentrated under reduced pressure at 60°C for 1 hour, yielding Polymer 6, in which mercapto groups had been introduced to the molecular terminals. The introduction of mercapto groups was confirmed by the loss of the groups derived from the CPDT at the molecular terminals, as determined by absorbance measurement of the obtained polymer 6. The obtained polymer 6 had a number average molecular weight of 4,370 and a molecular weight distribution of 1.15.

[0081] [Production Example 7: Production of Polymer 7] 10.08 g (0.0774 mol) of 2-methoxyethyl acrylate, 10 g of toluene as a polymerization solvent, and 1.6107 g (4.6604 mmol) of CPDT were charged, and 2.2400 g of a separately prepared V-59 solution (a solution prepared by dissolving 0.1000 g (0.5201 mmol) of 2,2'-azobis(2-methylbutyronitrile) (V-59) in 9.9000 g of toluene) was added thereto. Nitrogen bubbling was performed for 30 minutes, and then the mixture was stirred at 85°C to initiate polymerization. The polymerization solution was sampled at regular intervals during the polymerization, and the consumption rate of 2-methoxyethyl acrylate was measured by gas chromatography. 2.5 hours after the start of polymerization, it was confirmed that 89.8% of the 2-methoxyethyl acrylate had been consumed. 9.92 g (0.07743 mol) of butyl acrylate that had been previously bubbled with nitrogen for 30 minutes was then added thereto. After 4.5 hours from the start of polymerization, it was confirmed that 98.5% of the 2-methoxyethyl acrylate and 80.5% of the butyl acrylate had been consumed, and heating was stopped. The reaction solution was then concentrated under reduced pressure at 105°C for 1 hour. Absorbance measurement of the resulting polymer confirmed that 0.95 groups derived from CPDT had been introduced at the molecular terminals per polymer molecule. 18 g of the resulting polymer was dissolved in 54 g of tetrahydrofuran, and nitrogen bubbling was performed for 30 minutes. 0.88 g of tributylphosphine and 2.14 g of 1-hexylamine were added to the solution, and the mixture was stirred for 6 hours. The reaction solution was then reprecipitated in hexane. The precipitated polymer was concentrated under reduced pressure at 60°C for 1 hour to obtain Polymer 7, which had mercapto groups introduced at the molecular terminals. The introduction of mercapto groups was confirmed by absorbance measurement of the resulting Polymer 7, which showed the loss of the groups derived from CPDT at the molecular terminals. The number average molecular weight of the resulting Polymer 7 was 3,980, and the molecular weight distribution was 1.15.

[0082] Production Example 8 Production of Polymer 8 Polymer 8 was obtained by production in the same manner as in Production Example 7, except that 2-ethylhexyl acrylate was used instead of butyl acrylate and the amounts of tributylphosphine and 1-hexylamine used were 0.99 g and 2.41 g, respectively. The number average molecular weight of the obtained polymer 8 was 3,880, and the molecular weight distribution was 1.12.

[0083] Production Example 9 Production of Polymer 9 Polymer 9 was obtained by production in the same manner as in Production Example 7, except that 10.10 g of benzyl acrylate was used instead of 9.92 g of butyl acrylate, and the amounts of 2-methoxyethyl acrylate, tributylphosphine, and 1-hexylamine used were 9.90 g, 0.93 g, and 2.27 g. The number average molecular weight of the obtained polymer 9 was 3,720, and the molecular weight distribution was 1.13.

[0084] [Production Example 10: Production of Polymer 10] 440 g (3.43 mol) of butyl acrylate, 220 g of methanol, 5.92 g (58.5 mmol) of triethylamine, and 76 g (0.39 mol) of ethyl 2-bromoisobutyrate were charged, and a separately prepared copper complex solution (a solution prepared by dissolving 0.2178 g (0.9753 mmol) of copper(II) bromide in 22 g of methanol and mixing with 0.2247 g (0.9753 mmol) of 96% pure tris[2-(dimethylamino)ethyl]amine) was mixed therein. Nitrogen bubbling was performed for 30 minutes, and then the mixture was stirred at 40° C. 0.3435 g (1.951 mmol) of ascorbic acid and 0.3947 g (3.901 mmol) of triethylamine were dissolved in 68.00 g of methanol that had been previously bubbled with nitrogen for 30 minutes, and the ascorbic acid solution was added dropwise to initiate polymerization. The dropping rate of the ascorbic acid solution was set to a rate at which 0.0281 g (0.1597 mmol) of ascorbic acid was added to the polymerization system per hour. The polymerization solution was sampled at regular intervals during the polymerization, and the consumption rate of butyl acrylate was measured by gas chromatography. 120 minutes after the start of the dropwise addition of the ascorbic acid solution, it was confirmed that 52.1% of the butyl acrylate had been consumed. To this solution, 660 g (5.15 mol) of butyl acrylate, which had been previously bubbled with nitrogen for 30 minutes, was added dropwise over 120 minutes. 340 minutes after the start of the dropwise addition of the ascorbic acid solution, it was confirmed that 95.2% of the butyl acrylate had been consumed, and the dropwise addition of the ascorbic acid solution was stopped. At this point, the number-average molecular weight of the butyl acrylate polymer was 3,080, and the molecular weight distribution was 1.09. The reaction solution was then concentrated under reduced pressure at 80°C for 1 hour. To the concentrated reaction solution, 1,100 g of butyl acetate, 11 g of Kyoward 500SH (Kyowa Chemical Industry Co., Ltd.), and 11 g of Kyoward 700SEN-S (Kyowa Chemical Industry Co., Ltd.) were added, and the mixture was stirred at 100°C for 1 hour. Next, the resulting reaction solution was filtered to obtain filtrate A. The obtained filtrate A was concentrated under reduced pressure at 120°C for 3 hours to obtain polymer 10. The number average molecular weight of the obtained polymer 10 was 3,110, and the molecular weight distribution was 1.10.

[0085] [Production Example 11: Production of Polymer 11] 6.608 g (0.06600 mol) of pentenoic acid, 5.068 g (0.03667 mol) of potassium carbonate, 0.44 g of Kyoward 700SEN-S (Kyowa Chemical Industry Co., Ltd.), 0.011 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, and 0.132 g of tetra-n-butylammonium bromide were charged to 220 g of filtrate A and stirred at 120°C for 3 hours. The resulting reaction solution was filtered, and the resulting filtrate was concentrated under reduced pressure at 120°C for 3 hours to obtain Polymer 11. 1H NMR measurement of the resulting Polymer 11 confirmed that 0.96 groups derived from pentenoic acid were introduced per polymer molecule at the molecular terminals. The resulting Polymer 11 had a number average molecular weight of 3,250 and a molecular weight distribution of 1.10.

[0086] [Production Example 12: Production of Polymer 12] 20.00 g (0.1560 mol) of butyl acrylate, 12 g of toluene as a polymerization solvent, and 1.6231 g (4.6963 mmol) of 2-cyano-2-propyldodecyltrithiocarbonate (CPDT) as a RAFT agent were charged, and 0.9604 g of a separately prepared V-59 solution (a solution prepared by dissolving 0.1000 g (0.5201 mmol) of 2,2'-azobis(2-methylbutyronitrile) (V-59) in 9.9000 g of toluene) was added thereto. Nitrogen bubbling was performed for 30 minutes, and then the mixture was stirred at 70°C to initiate polymerization. The polymerization solution was sampled at regular intervals during the polymerization, and the consumption rate of butyl acrylate was measured by gas chromatography. Eight hours after the start of polymerization, it was confirmed that 77.1% of the butyl acrylate had been consumed, and heating was stopped. The reaction solution was then concentrated under reduced pressure at 105°C for 1 hour to obtain polymer 12. Absorbance measurement of the obtained polymer 12 confirmed that 0.95 groups derived from CPDT were introduced at the molecular terminals per polymer molecule. The obtained polymer 12 had a number average molecular weight of 3,950 and a molecular weight distribution of 1.06.

[0087] [Production Example 13: Production of Polymer 13] Polymer 13 was obtained by production in the same manner as in Production Example 11, except that pentenoic acid and potassium carbonate were not used, and 4.847 g (0.04400 mol) of potassium acrylate was used, and the mixture was heated and stirred for 4 hours. 1H NMR measurement of the obtained polymer 13 confirmed that 0.92 acryloyl groups were introduced per polymer molecule at the molecular terminals. The number average molecular weight of the obtained polymer 13 was 3,150, and the molecular weight distribution was 1.11.

[0088] Examples 1-10, Comparative Examples 1-5 The components listed in Table 1 were prepared (unit: parts by weight). All components except the vulcanizing agent and vulcanization accelerator were kneaded for 15 minutes using a Labo Plastomill (Toyo Seiki Seisakusho, Ltd., model number "4C150") set at 100°C, and the mixture was then removed and cooled. The mixture was then recharged into the Labo Plastomill, and the vulcanizing agent and vulcanization accelerator were added, ensuring that the resin temperature did not exceed 90°C. The mixture was then kneaded for 5 minutes to obtain a rubber composition. The rubber composition was press-molded at 170°C for 30 minutes using a press (Kamito Metal Industries, Ltd., model number "NSF-50") to produce a 2 mm thick sheet. Test specimens were cut from the sheet, and rolling resistance, wet grip performance, stress at 300% elongation (M300), and hardness were measured. The results are shown in Table 1.

[0089] As can be seen from Table 1, the sheets obtained from the rubber compositions according to the Examples have almost the same wet grip performance and hardness as the sheets obtained from the rubber compositions according to the Comparative Examples, but have improved rolling resistance and M300. In particular, the use of a (meth)acrylic acid ester-based block copolymer having a mercapto group introduced at the molecular end improved the rolling resistance and M300 (see Examples 7 to 10).

[0090]

Claims

1. A rubber composition containing 5 to 200 parts by weight of silica (B), 0.5 to 50 parts by weight of carbon black (C), 0.5 to 20 parts by weight of a silane coupling agent (D), and 1 to 20 parts by weight of a (meth)acrylic acid ester polymer (E) having a mercapto group at the terminal, based on 100 parts by weight of a diene rubber (A).

2. The rubber composition according to claim 1, wherein the diene rubber (A) is at least one selected from the group consisting of natural rubber, isoprene rubber, styrene-butadiene copolymer rubber, and butadiene rubber.

3. The rubber composition according to claim 1 or 2, wherein the (meth)acrylic acid ester polymer (E) has a mercapto group at one end and a group represented by the following general formula (1) at the other end. CHR １ R ２ -CR ３ (COOR ４ ) - (1) (In the formula, R １ , R ２ , R ４ are the same or different and represent a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, or an aryl group having 6 to 20 carbon atoms. However, R １ , R ２ , R ４ may be partially substituted by one or more selected from the group consisting of an oxygen atom, a halogen atom, CN, and NR ５ R ６ . Here, R ５ , R ６ are the same or different and represent a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, or an aryl group having 6 to 11 carbon atoms. R ３ represents a hydrogen atom or a methyl group.) 4. The rubber composition according to claim 1 or 2, wherein the (meth)acrylic acid ester polymer (E) having a mercapto group at the terminal is a block copolymer.

5. A rubber product obtained from the rubber composition according to claim 1 or 2.

6. A pneumatic tire which is the rubber product according to claim 5.

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