Rubber composition and pneumatic tire

Abstract

To provide a rubber composition having excellent thermal aging resistance and to provide a pneumatic tire equipped with a vulcanized rubber of the rubber composition in the rubber part.SOLUTION: There is provided a rubber composition which comprises, when the total amount of the rubber component is defined as 100 pts.mass, 0.1 to 10 pts.mass of a compound represented by the following general formula (1): (wherein, at least one of R1 to R5 is -OH group or -OCH3 group and the others are -H group or a hydrocarbon group having 1 to 20 carbon atoms; A is an unsaturated bond or an alkylene group having 1 to 20 carbon atoms which have -H group, -CH3 group, -NH2 group, -O- group or -OH group; n is an integer of 0 to 10; B is -COOH group, -OH group or =O group, which may be bonded with adjacent R1 or R5 to form a ring structure.)SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a rubber composition and a pneumatic tire having a rubber portion made of a vulcanized rubber of the rubber composition. [Background technology]

[0002] Rubber products with rubber parts, such as pneumatic tires and anti-vibration rubber, generally deteriorate due to oxygen and ozone in the atmosphere over long periods of use, causing cracks and resulting in reduced durability. For this reason, various antioxidants are blended into the rubber composition used as the raw material to prevent oxidation and ozone degradation of the rubber parts.

[0003] Patent Document 1 listed below describes a rubber composition obtained by compounding 0.5 parts by weight of an amine-based antioxidant and 0.5 to 8 parts by weight of a polyphenol-based compound with 100 parts by weight of a rubber component made of a diene-based rubber. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-59327 Summary of the Invention [Problem to be solved by the invention]

[0005] However, after extensive investigations, the present inventors have found that the rubber compositions described in the above patent documents still have room for improvement in terms of the heat aging resistance of the vulcanized rubbers that are finally obtained.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a rubber composition having excellent heat aging resistance, and a pneumatic tire having a rubber portion made of vulcanized rubber of the rubber composition. [Means for solving the problem]

[0007] The above object can be achieved by the present invention as described below. That is, the present invention provides a rubber composition comprising a compound represented by the following general formula (1): [ka] (In the above general formula (1), at least one of R1 to R5 is a -OH group or a -OCH3 group, and the others are a -H group or a hydrocarbon group having 1 to 20 carbon atoms. A is an alkylene group having 1 to 20 carbon atoms which may have an unsaturated bond, or a -H group, a -CH3 group, a -NH2 group, a -O- group or a -OH group, and n is an integer of 0 to 10. B is a -COOH group, a -OH group or a ═O group, and may be bonded to an adjacent R1 or R5 to form a ring structure.)

[0008] In the rubber composition, the compound represented by the general formula (1) is preferably a naturally occurring compound.

[0009] In the rubber composition, the compound represented by the general formula (1) is preferably at least one of 3,4-dihydroxycinnamic acid and 3,4-dimethoxycinnamic acid.

[0010] The present invention also relates to a pneumatic tire having a rubber portion made of a vulcanized rubber of any one of the rubber compositions described above. [Effects of the Invention]

[0011] The rubber composition according to the present invention contains a predetermined amount of the compound represented by formula (1), and therefore has excellent heat aging resistance when vulcanized into rubber. In particular, when the compound represented by formula (1) is a naturally occurring compound, this is more preferable from the viewpoint of environmental protection. Furthermore, when the compound represented by formula (1) is 3,4-dihydroxycinnamic acid or 3,4-dimethoxycinnamic acid, this is particularly preferable from the viewpoint of both heat aging resistance and environmental protection. Note that 3,4-dihydroxycinnamic acid and 3,4-dimethoxycinnamic acid may be used in combination. DETAILED DESCRIPTION OF THE INVENTION

[0012] The rubber composition according to the present invention comprises a rubber component represented by the following general formula (1): [ka] (In the above general formula (1), at least one of R1 to R5 is an -OH group or an -OCH3 group, and the others are an -H group or a hydrocarbon group having 1 to 20 carbon atoms. A is an alkylene group having 1 to 20 carbon atoms which may have an unsaturated bond or an -H group, -CH3 group, -NH2 group, -O- group or -OH group, and n is an integer of 0 to 10. B is a -COOH group, -OH group or =O group and may be bonded to an adjacent R1 or R5 to form a ring structure.) The compounding amount of the compound described in the above general formula (1) is more preferably 0.5 to 5 parts by mass when the total amount of the rubber component is 100 parts by mass.

[0013] In terms of environmental protection, it is more preferable that the compound represented by the general formula (1) is a naturally occurring compound. Examples of naturally occurring compounds include the compounds shown below.

[0014] [ka] Among the above compounds, in the present invention, from the viewpoint of improving the heat aging resistance of the vulcanized rubber, it is particularly preferable to use at least one of 3,4-dihydroxycinnamic acid (caffeic acid) and 3,4-dimethoxycinnamic acid. The reason why the heat aging resistance of the vulcanized rubber improves when at least one of 3,4-dihydroxycinnamic acid and 3,4-dimethoxycinnamic acid is used as an antioxidant is not clear, but the following reasons (1) to (3), for example, are thought to be the cause. (1) When compounded with zinc compounds such as zinc oxide in a rubber composition as a raw material, two or more molecules of 3,4-dihydroxycinnamic acid or 3,4-dimethoxycinnamic acid coordinate to zinc through the hydroxyl or methoxy groups at the R2 and R3 positions in the rubber composition, forming a complex that increases the molecular weight. (2) Two or more molecules of 3,4-dihydroxycinnamic acid or 3,4-dimethoxycinnamic acid form a complex, so that the 3,4-dihydroxycinnamic acid or 3,4-dimethoxycinnamic acid remains in the rubber composition and vulcanized rubber without blooming. (3) By suppressing the blooming of 3,4-dihydroxycinnamic acid or 3,4-dimethoxycinnamic acid from the rubber composition and vulcanized rubber, the antioxidant effect of 3,4-dihydroxycinnamic acid or 3,4-dimethoxycinnamic acid is effectively exerted, resulting in improved heat aging resistance of the vulcanized rubber.

[0015] It should be noted that compounds that are not naturally occurring can also be used as the compounds represented by the general formula (1). Examples of non-naturally occurring compounds include the compounds described below.

[0016] [ka]

[0017] As the rubber component, for example, a diene rubber can be suitably used. The diene rubber is not particularly limited, and examples thereof include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), chloroprene rubber (CR), styrene-isoprene copolymer rubber, butadiene-isoprene copolymer, and styrene-isoprene-butadiene copolymer rubber. These can be used alone or in combination of two or more. Preferred diene rubbers are natural rubber, isoprene rubber, butadiene rubber, styrene-butadiene rubber, or a blend of two or more of these.

[0018] The rubber composition according to the present invention preferably contains carbon black as a filler. Examples of the carbon black include carbon blacks commonly used in the rubber industry, such as SAF, ISAF, HAF, FEF, and GPF, as well as conductive carbon blacks such as acetylene black and ketjen black.

[0019] It is also preferable to contain silica as a filler. As the silica, wet silica, dry silica, sol-gel silica, surface-treated silica, etc., which are commonly used for rubber reinforcement, are used. Of these, wet silica is preferred.

[0020] When silica is contained as a filler, it is also preferable to contain a silane coupling agent. The silane coupling agent is not particularly limited as long as it contains sulfur in the molecule, and various silane coupling agents that are compounded together with silica in rubber compositions can be used. Examples of the silane include sulfide silanes such as bis(3-triethoxysilylpropyl)tetrasulfide (e.g., "Si69" manufactured by Degussa), bis(3-triethoxysilylpropyl)disulfide (e.g., "Si75" manufactured by Degussa), bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)disulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, and bis(2-trimethoxysilylethyl)disulfide; mercaptosilanes such as γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptopropylmethyldimethoxysilane, mercaptopropyldimethylmethoxysilane, and mercaptoethyltriethoxysilane; and protected mercaptosilanes such as 3-octanoylthio-1-propyltriethoxysilane and 3-propionylthiopropyltrimethoxysilane.

[0021] As the vulcanizing agent, sulfur can be preferably used. The sulfur may be any ordinary sulfur for rubber, such as powdered sulfur, precipitated sulfur, insoluble sulfur, or highly dispersible sulfur.

[0022] As the vulcanization accelerator, sulfenamide-based vulcanization accelerators, thiuram-based vulcanization accelerators, thiazole-based vulcanization accelerators, thiourea-based vulcanization accelerators, guanidine-based vulcanization accelerators, dithiocarbamate-based vulcanization accelerators, and other vulcanization accelerators commonly used for rubber vulcanization may be used alone or in appropriate mixtures. However, in the present invention, it is preferable to use sulfenamide-based vulcanization accelerators because they provide excellent reinforcing properties for the vulcanized rubber and excellent peel resistance between the coating rubber and the steel cord.

[0023] The rubber composition according to the present invention may contain, in addition to the rubber component, the compound represented by formula (1), a filler, a vulcanizing agent, and a vulcanization accelerator, an antioxidant other than the compound represented by formula (1), stearic acid, a softener such as wax or oil, a processing aid, and the like.

[0024] The rubber composition according to the present invention is characterized in that it contains the compound represented by formula (1) as an antioxidant, but it may also use an antioxidant other than the compound represented by formula (1). As the antioxidant other than the compound represented by formula (1), antioxidants commonly used for rubber, such as aromatic amine-based antioxidants, amine-ketone-based antioxidants, monophenol-based antioxidants, bisphenol-based antioxidants, polyphenol-based antioxidants, dithiocarbamate-based antioxidants, and thiourea-based antioxidants, may be used alone or in appropriate mixtures.

[0025] The rubber composition according to the present invention can be obtained by kneading the rubber component, the compound represented by the formula (1), a filler, a vulcanizing agent, a vulcanization accelerator, an antioxidant, stearic acid, a softener such as wax or oil, a processing aid, and the like, using a kneading machine typically used in the rubber industry, such as a Banbury mixer, a kneader, or a roll.

[0026] The method for compounding the above-mentioned components is not particularly limited, and any of the following may be used: a method in which the compounding components other than the vulcanization-based compounding agents, such as the vulcanizing agent and vulcanization accelerator, are pre-mixed to form a master batch, and the remaining components are then added and further kneaded; a method in which the components are added in any order and kneaded; or a method in which all the components are added simultaneously and kneaded. [Example]

[0027] Examples that specifically illustrate the configuration and effects of the present invention will be described below.

[0028] (Preparation of Rubber Composition) Rubber compositions were prepared by compounding the rubber compositions of Examples 1 to 11 and Comparative Example 1 according to the compounding recipes in Table 1 and kneading them using a conventional Banbury mixer. The compounding ingredients listed in Table 1 are shown below (in Table 1, the compounding amount of each compounding ingredient is shown in parts by mass per 100 parts by mass of the rubber component). a) Natural rubber (RSS#3) b) Carbon black; trade name "N339 Seast KH" manufactured by Tokai Carbon Co., Ltd. c) Stearic acid; product name "Lunac S-20" manufactured by Kao Corporation d) Zinc oxide; trade name "Zinc Oxide No. 1", manufactured by Mitsui Kinzoku Co., Ltd. e) Compounds represented by general formula (1) (naturally occurring compounds) (i) 3,4-dihydroxycinnamic acid; trade name “Caffeic Acid”, manufactured by BLD Pharmatech Ltd. (ii) Curcumin; trade name "Curcumin (Natural)", manufactured by Tokyo Chemical Industry Co., Ltd. (iii) Sesamol; trade name "Sesamol, 98%", manufactured by Sigma-Aldrich Japan Co., Ltd. (iv) Coumaric acid; trade name "trans-p-Coumaric Acid", manufactured by Tokyo Chemical Industry Co., Ltd. (v) 3,4-Dimethoxycinnamic acid; trade name "3,4-Dimethoxycinnamic Acid", manufactured by Tokyo Chemical Industry Co., Ltd. f) Compounds represented by general formula (1) (non-naturally occurring compounds) (vi) 3-(3,4-Dihydroxyphenyl)-L-alanine; trade name "3-(3,4-Dihydroxyphenyl)-L-alanine", manufactured by Tokyo Chemical Industry Co., Ltd. (vii) Protocatechuic acid; trade name "3,4-Dihydroxybenzoic Acid", manufactured by Tokyo Chemical Industry Co., Ltd. (viii) 5,5',6,6'-Tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobiindane; trade name "5,5',6,6'-Tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobiindane", manufactured by Tokyo Chemical Industry Co., Ltd. g) Sulfur; product name "Powdered sulfur", manufactured by Tsurumi Chemical Co., Ltd. h) Vulcanization accelerator; trade name "Suncerer NS-G", manufactured by Sanshin Chemical Industry Co., Ltd.

[0029] The rubber compositions of Examples 1 to 11 and Comparative Example 1 were heated and vulcanized at 160° C. for 10 or 20 minutes using a predetermined mold, and the durability of the obtained sample rubbers was evaluated by the following method.

[0030] (Heat aging resistance of vulcanized rubber) According to JIS K6253, the retention of elongation at break (the retention of elongation at break after aging relative to the initial elongation at break) of sample rubbers aged by leaving them in a Geer oven for a predetermined time was measured. In Table 1, the retention of Comparative Example 1 is shown as an index, with the retention being set at 100. The larger the value, the better the heat aging resistance. The results are shown in Table 1.

[0031] [Table 1]

[0032] The results in Table 1 show that the vulcanized rubbers of the rubber compositions according to Examples 1 to 11 are superior to the vulcanized rubber of the rubber composition according to Comparative Example 1 in heat aging resistance.

Claims

[Claim 1] A pneumatic tire having a rubber portion made of a vulcanized rubber of a rubber composition, The rubber composition contains at least one of 3,4-dihydroxycinnamic acid and 3,4-dimethoxycinnamic acid, When the rubber composition contains the 3,4-dihydroxycinnamic acid, the rubber composition contains 2.4 to 10 parts by mass of the 3,4-dihydroxycinnamic acid when the total amount of the rubber component is taken as 100 parts by mass, When the rubber composition contains the 3,4-dimethoxycinnamic acid, the pneumatic tire contains 0.1 to 10 parts by mass of the 3,4-dimethoxycinnamic acid when the total amount of the rubber component is 100 parts by mass.

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