Rubber composition for tire sidewall and pneumatic tire

Abstract

To provide a tire sidewall rubber composition used as a raw material of a tire sidewall improved in low fuel consumption and steering stability in good balance when used as the tire sidewall of a pneumatic tire.SOLUTION: (In the formula, at least one of R1 to R5 is an - OH group or an - OCH3 group, and the others are - H groups or hydrocarbon groups having 1 to 20 carbon atoms. A is an alkylene group having 1 to 20 carbon atoms which may have unsaturated bonds or -H groups, - CH3 groups, - NH2 groups, - O - groups, or - OH groups, and n is an integer of 0 to 10. B represents a -COOH group, an -OH group, or a = O group, and may be bonded to an adjacent R1 or R5 to form a ring structure. A rubber composition for a tire sidewall, comprising 0.2 to 7 parts by mass of a compound according to [1]: SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, from the viewpoint of energy conservation, pneumatic tires are required to have low fuel consumption. Another required performance of pneumatic tires is steering stability. One method for improving the former is to improve the low heat buildup of vulcanized rubber, and one method for improving the latter is to increase the rigidity of vulcanized rubber even at high temperatures (increasing hardness).

[0003] Incidentally, Patent Document 1 listed below describes a rubber composition in which a specific compound is blended in a predetermined amount when the total amount of rubber components is taken as 100 parts by mass, with the aim of improving the heat aging resistance of vulcanized rubber. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-089553 Summary of the Invention [Problem to be solved by the invention]

[0005] When aiming to achieve both low fuel consumption and handling stability in pneumatic tires, it is important to optimize the loss tangent (tan δ) and rubber hardness of the raw rubber composition. Generally, fuel economy is highly dependent on the tan δ at 60°C (hereinafter also referred to as "tan δ(60°C)") of the raw rubber composition, and the smaller the tan δ(60°C), the better the fuel economy of the pneumatic tire. Furthermore, the higher the rubber hardness of the vulcanized rubber, the better the handling stability of the pneumatic tire.

[0006] The vulcanized rubber of the rubber composition described in Patent Document 1 has excellent heat aging resistance. However, as a result of extensive research by the present inventors, it has been found that there is room for further improvement in terms of improving fuel economy and handling stability in a balanced manner in order to use the vulcanized rubber as the rubber portion of a pneumatic tire.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a tire sidewall rubber composition that serves as a raw material for tire sidewalls that, when used as tire sidewalls of pneumatic tires, have a well-balanced improvement in fuel economy and handling stability.

[0008] Another object of the present invention is to provide a pneumatic tire having a tire sidewall that exhibits a good balance between fuel economy and driving stability. [Means for solving the problem]

[0009] 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, 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.)

[0010] The rubber composition (1) for a tire sidewall is preferably a rubber composition (2) for a tire sidewall, which further contains carbon black, and the content of the carbon black is less than 50 parts by mass when the total amount of the rubber components is 100 parts by mass.

[0011] In the above rubber composition for tire sidewall (1) or (2), when the total amount of the rubber components is taken as 100 parts by mass, a rubber composition for tire sidewall (3) containing 30 to 70 parts by mass of natural rubber and 30 to 70 parts by mass of butadiene rubber is preferred.

[0012] In any one of the rubber compositions for tire sidewalls (1) to (3), a rubber composition for tire sidewalls (4) is preferred in which the compound represented by the general formula (1) is a naturally occurring compound.

[0013] In any one of the rubber compositions (1) to (4) for tire sidewalls, a rubber composition (5) for tire sidewalls is preferred in which the compound represented by the general formula (1) is at least one of 3,4-dihydroxycinnamic acid and 3,4-dimethoxycinnamic acid.

[0014] The present invention also relates to a pneumatic tire (6) having a vulcanized rubber of any one of the rubber compositions (1) to (5) for tire sidewalls in the tire sidewall portion. [Effects of the Invention]

[0015] The rubber composition for tire sidewalls according to the present invention contains 0.2 to 7 parts by mass of the compound represented by the general formula (1) above per 100 parts by mass of a rubber component containing at least a diene rubber. This allows the final vulcanized rubber to be improved in a balanced manner in fuel economy and handling stability. The reasons for this effect are believed to be as follows:

[0016] Because the compound represented by the general formula (1) is highly hydrophilic, it tends to aggregate in the rubber component, thereby increasing the viscosity of the rubber composition for tire sidewalls. As a result, during rubber mixing, the rubber composition can be maintained in a high shear state while suppressing excessive temperature rise. This allows the reaction between the compound represented by the general formula (1) and the rubber component to proceed at a high level. This increases the rubber hardness of the final vulcanized rubber and reduces tan δ(60°C) at 60°C. As a result, the vulcanized rubber achieves a balanced improvement in fuel economy and handling stability. DETAILED DESCRIPTION OF THE INVENTION

[0017] The rubber composition for a tire sidewall according to the present invention comprises a compound 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 an -OH group, and n is an integer of 0 to 10. B is a -COOH group, -OH group or ═O group and may bond with 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.

[0018] From the viewpoint 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 3,4-dihydroxycinnamic acid (caffeic acid), 3,4-dimethoxycinnamic acid, curcumin, sesamol, coumaric acid, ferulic acid, sinapic acid, chlorogenic acid, rosmarinic acid, 4-(4-hydroxy-3-methoxyphenyl)-2-butanone, naringin, hesperidin, quercetin, and tocopherol. Among these compounds, in the present invention, it is particularly preferable to use at least one of 3,4-dihydroxycinnamic acid (caffeic acid) and 3,4-dimethoxycinnamic acid from the viewpoint of improving the fuel economy and handling stability of the vulcanized rubber. The reason why the use of at least one of 3,4-dihydroxycinnamic acid and 3,4-dimethoxycinnamic acid improves the fuel economy and handling stability of the vulcanized rubber is not clear, but the following reasons (1) and (2) are thought to be possible.

[0019] (1) When compounded with zinc compounds such as zinc oxide in a rubber composition for tire sidewalls, 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) When two or more molecules of 3,4-dihydroxycinnamic acid or 3,4-dimethoxycinnamic acid form a complex, the tendency of aggregation in the rubber composition for tire sidewalls is enhanced, and therefore the viscosity of the rubber composition for tire sidewalls is more effectively increased. As a result of the more effective increase in viscosity of the rubber composition for tire sidewalls, the reaction between the compound represented by general formula (1) and the rubber component proceeds more effectively, and the rubber hardness of the finally obtained vulcanized rubber increases and the tan δ(60°C) at 60°C decreases, which is thought to result in a balanced improvement in the fuel economy and handling stability of the vulcanized rubber.

[0020] It should be noted that compounds of non-natural origin can also be used as the compound represented by the general formula (1). Examples of non-naturally occurring compounds include 2,3-dimethoxycinnamic acid, 2,4-dimethoxycinnamic acid, 2,5-dimethoxycinnamic acid, 2,3,4-tritoxycinnamic acid, 3,4,5-tritoxycinnamic acid, protocatechuic acid, 3-(3,4-dihydroxyphenyl)-L-alanine, 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobiindane, carvacrol, 3,4-dimethoxyhydrocinnamic acid, 5,6-dimethoxy-1-indanone, and 3,4-dihydroxyhydrocinnamic acid.

[0021] As the rubber component, for example, a diene rubber can be suitably used. In the present invention, among the diene rubbers, natural rubber (NR) and butadiene rubber (BR) are preferably contained, and the blending amounts thereof are preferably 30 to 70 parts by mass of natural rubber and 30 to 70 parts by mass of butadiene rubber, when the total amount of the rubber components is 100 parts by mass. In the rubber composition according to the present invention, examples of diene rubbers other than natural rubber and butadiene rubber as the rubber component include isoprene rubber (IR), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), chloroprene rubber (CR), styrene-isoprene copolymer rubber, butadiene-isoprene copolymer, and styrene-isoprene-butadiene copolymer rubber.

[0022] The rubber composition for tire sidewalls according to the present invention may contain carbon black as a filler. Examples of carbon black that can be used 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. The amount of carbon black in the rubber composition for tire sidewalls is preferably less than 50 parts by mass when the total amount of the rubber components is taken as 100 parts by mass. There is no particular lower limit for the carbon black content, but it is preferably 25 parts by mass per 100 parts by mass of the rubber components.

[0023] The rubber composition for a tire sidewall according to the present invention may contain, in addition to the rubber component and the compound represented by the general formula (1), carbon black, silica, a silane coupling agent, a vulcanizing agent, a vulcanization accelerator, an antioxidant, stearic acid, petroleum resin, a softener such as wax or oil, a processing aid, and the like.

[0024] As the vulcanizing agent, sulfur can be suitably used. The sulfur may be any ordinary sulfur for rubber, such as powdered sulfur, precipitated sulfur, insoluble sulfur, or highly dispersible sulfur. The amount of sulfur in the rubber composition according to the present invention is preferably 0.5 to 5 parts by mass per 100 parts by mass of the rubber component.

[0025] As the vulcanization accelerator, vulcanization accelerators commonly used for rubber vulcanization, such as sulfenamide-based vulcanization accelerators, thiuram-based vulcanization accelerators, thiazole-based vulcanization accelerators, thiourea-based vulcanization accelerators, guanidine-based vulcanization accelerators and dithiocarbamate-based vulcanization accelerators, may be used alone or in appropriate mixtures.

[0026] The rubber composition for a tire sidewall according to the present invention may use, as the antioxidant, 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, either alone or in appropriate mixtures.

[0027] The rubber composition for a tire sidewall according to the present invention can be obtained by kneading together, in addition to the rubber component and the compound represented by the general formula (1), carbon black, silica, a silane coupling agent, a vulcanizing agent, a vulcanization accelerator, an antioxidant, stearic acid, petroleum resin, 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.

[0028] 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.

[0029] The vulcanized rubber of the rubber composition for tire sidewalls according to the present invention is excellent in fuel economy and handling stability, and therefore the rubber composition for tire sidewalls according to the present invention is useful as a raw material for tire sidewalls for pneumatic tires. [Example]

[0030] Examples that specifically illustrate the configuration and effects of the present invention will be described below.

[0031] (Preparation of Rubber Composition) Rubber compositions were prepared by compounding the rubber compositions for tire sidewalls of Examples 1 to 6 and Comparative Examples 1 to 4 according to the compounding recipes in Tables 1 and 2 and kneading them using a conventional Banbury mixer. The compounding ingredients listed in Tables 1 and 2 are shown below (in Tables 1 and 2, the compounding amount of each compounding ingredient is shown in parts by mass per 100 parts by mass of the rubber component). Natural rubber; RSS#3 Butadiene rubber: Ube Industries, Ltd., "UBEPOL BR150B" Carbon black: Tokai Carbon Co., Ltd., product name "Seast 3" Aroma oil: JX Nippon Oil & Energy Corporation, product name "Process NC140" Zinc oxide: manufactured by Mitsui Mining & Smelting Co., Ltd., product name "Zinc Oxide No. 3" Stearic acid: Kao Corporation, product name "Lunac S-20" Anti-aging agent: Ouchi Shinko Chemical Industry Co., Ltd., product name "Nocrac 6C" 3,4-Dimethoxycinnamic acid 3,4-Dihydroxycinnamic acid (caffeic acid) Acetamidocinnamic acid (a compound not corresponding to the compound represented by general formula (1)) Sulfur: Powdered sulfur, manufactured by Tsurumi Chemical Industry Co., Ltd. Vulcanization accelerator: Ouchi Shinko Chemical Industry Co., Ltd., product name "Noccela CZ"

[0032] The vulcanized rubbers of Examples 1 to 6 and Comparative Examples 1 to 3 were evaluated for rubber hardness (Hs.) and tan δ (60° C.) by the following methods.

[0033] <Rubber hardness of vulcanized rubber (Hs.)> The rubber hardness was evaluated by measuring the hardness of sample rubber test pieces obtained by heating and vulcanizing the rubber compositions for tire sidewalls of Examples 1 to 6 and Comparative Examples 1 to 3 in a predetermined mold at 160°C for 30 minutes using a durometer type A in accordance with JIS K6253 at a temperature of 23°C, and expressing the hardness as an index with the value of Comparative Example 1 set to 100. The larger the index, the higher the rubber hardness at room temperature, indicating excellent steering stability when used in the tread portion of a pneumatic tire.

[0034] <Vulcanized rubber tanδ (60℃)> The rubber compositions for tire sidewalls of Examples 1 to 6 and Comparative Examples 1 to 3 were heated and vulcanized at 160°C for 30 minutes using a predetermined mold to obtain sample rubbers, which were used as measurement specimens. For each measurement specimen, the storage modulus (E') and loss modulus (E") were measured using a dynamic viscoelasticity measuring device (product name: "Fully Automatic Viscoelasticity Analyzer VR-7110," manufactured by Ueshima Seisakusho Co., Ltd.), and tan δ (60°C) was measured. In Tables 1 and 2, the values ​​are expressed as an index, with the value of tan δ (60°C) of Comparative Example 1 set to 100. For tan δ (60°C), the smaller the index, the better the fuel economy when used in the tread of a pneumatic tire. The measurement conditions were as follows: Measurement sample size: length 40 mm, width 3 mm, thickness 2 mm Measurement mode: Tensile mode Measurement temperature: 60℃ Frequency: 100Hz Dynamic distortion: 0.15%

[0035] [Table 1]

[0036] The results in Table 1 show that the rubber composition for tire sidewalls of Comparative Example 2 contains an excessive amount of 3,4-dimethoxycinnamic acid, which corresponds to the compound represented by general formula (1), and therefore the 3,4-dimethoxycinnamic acid, which corresponds to the compound represented by general formula (1), aggregates excessively, resulting in a deterioration in tan δ (60°C).The vulcanized rubber of the rubber composition for tire sidewalls of Comparative Example 3 contains an acetamidocinnamic acid that does not correspond to the compound represented by general formula (1), and therefore the tan δ (60°C) and rubber hardness are deteriorated.On the other hand, the vulcanized rubbers of the rubber compositions for tire sidewalls of Examples 1 to 3, when used in the tire sidewall portion of a pneumatic tire, show improvements in both tan δ (60°C) and rubber hardness, resulting in a balanced improvement in both fuel economy and handling stability.

[0037] [Table 2]

[0038] The results in Table 2 show that the vulcanized rubber of the rubber composition of Example 4, which contains 3,4-dihydroxycinnamic acid corresponding to compound (2) in general formula (1), improves both tan δ (60°C) and rubber hardness when used in the sidewall of a pneumatic tire, resulting in a balanced improvement in both fuel economy and handling stability.The vulcanized rubber of the rubber compositions for tire sidewalls of Examples 5 and 6, which contain 3,4-dihydroxycinnamic acid corresponding to compound (2) in general formula (1), improves both tan δ (60°C) and rubber hardness when used in the sidewall of a pneumatic tire, resulting in a balanced improvement in both fuel economy and handling stability.

Claims

1. With respect to 100 parts by mass of a rubber component containing at least a diene rubber, a compound represented by the following general formula (1): 【Chemistry 1】 (In the above general formula (1), R １ ~R ５ At least one of the groups is an —OH group or an —OCH ３ A is an unsaturated bond, a —H group, a —CH ３ group, -NH ２ is an alkylene group having 1 to 20 carbon atoms which may have a —COOH group, an —O— group, or an —OH group, and n is an integer of 0 to 10. B is a —COOH group, an —OH group, or an ═O group, and the adjacent R １ or R ５ and a cyclic structure may be formed by bonding to the compound described in (1).

2. 2. The rubber composition for a tire sidewall according to claim 1, further comprising carbon black, wherein the content of the carbon black is less than 50 parts by mass when the total amount of the rubber components is taken as 100 parts by mass.

3. 2. The rubber composition for tire sidewalls according to claim 1, which contains 30 to 70 parts by mass of natural rubber and 30 to 70 parts by mass of butadiene rubber, when the total amount of the rubber components is 100 parts by mass.

4. 2. The rubber composition for a tire sidewall according to claim 1, wherein the compound represented by the general formula (1) is a naturally occurring compound.

5. 2. The rubber composition for tire sidewalls according to claim 1, wherein the compound represented by the general formula (1) is at least one of 3,4-dihydroxycinnamic acid and 3,4-dimethoxycinnamic acid.

6. A pneumatic tire having a vulcanized rubber of the rubber composition for tire sidewalls according to claim 1 in the tire sidewall portion.

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