Rubber composition, crosslinked rubber, and tire

Abstract

To improve the strength of a diene rubber crosslinked by a re-bindable bond.SOLUTION: A crosslinked rubber contains a diene rubber having epoxy groups crosslinked with a dicarboxylic acid represented by the general formula (1) defined by HOOC-R1-COOH and an imidazole represented by the general formula (2) in the figure. In the formula (1), R1 represents a divalent hydrocarbon group having 3 to 50 carbon atoms which is optionally interrupted by one or more heteroatoms. In the formula (2), R2 represents a hydrogen atom or a methyl group.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a rubber composition, a cross-linked rubber product, and a tire. [Background technology]

[0002] Generally, rubber is cross-linked with sulfur. In recent years, the importance of recycling rubber materials has increased, but recycling sulfur-crosslinked rubber is difficult because the cross-linking points of the sulfur cross-links cannot be restored.

[0003] There are two main methods that can be considered to realize rubber material recycling. One is desulfurization treatment. However, in desulfurization treatment, it is generally difficult to selectively sever sulfur bonds, and the main chain is also severed, resulting in a decrease in physical properties. The other is an alternative to sulfur crosslinking, which realizes material recycling by changing sulfur crosslinking to a reversible crosslinking.

[0004] As an alternative technique to sulfur crosslinking, Patent Document 1 discloses a rubber composition containing an epoxide elastomer crosslinked with polycarboxylic acid. In Patent Document 1, a diene elastomer containing an epoxide functional group is mixed with a crosslinking system containing two specific polycarboxylic acids and a specific imidazole compound to prepare a diene elastomer crosslinked with polycarboxylic acid. Specifically, Patent Document 1 discloses a rubber composition in which dodecanedioic acid, poly(acrylonitrile-co-butadiene) dicarboxy terminal, and 1-benzyl-2-methylimidazole are blended with epoxidized natural rubber, but does not disclose the use of imidazole that can be crosslinked by ionic bonds.

[0005] On the other hand, Non-Patent Document 1 discloses that an ionomer is formed by grafting 1H-imidazole to epoxidized natural rubber, thereby introducing crosslinks by ionic bonds (i.e., ionic crosslinks) into epoxidized natural rubber. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2016-501941 [Non-patent literature]

[0007] [Non-Patent Document 1] S. Mandal et al., “Transformation of Epoxidized Natural Rubber into Ionomers by Grafting of 1H-Imidazolium Ion and Development of a Dynamic ReversibleNetwork”, Applied Polymer Materials, 2022, 4, 9, 6612-6622 Summary of the Invention [Problem to be solved by the invention]

[0008] By crosslinking diene rubber with epoxy groups with dicarboxylic acid, the epoxy groups react with the carboxyl groups to form ester bonds. Since ester bonds can be rearranged by transesterification, they are recombinable bonds and exhibit self-repairing and reworkability. However, crosslinking with dicarboxylic acid may not always provide sufficient strength, and improvements in strength are required.

[0009] In view of the above, an object of an embodiment of the present invention is to improve the strength of a cross-linked rubber product cross-linked with a dicarboxylic acid. [Means for solving the problem]

[0010] The present inventors, while earnestly studying alternative technologies to sulfur crosslinking, discovered that the strength of cross-linked rubber products can be improved by introducing ion crosslinking with imidazole in addition to crosslinking of ester bonds with dicarboxylic acid, and thus completed the present invention.

[0011] The present invention includes the embodiments set forth below. [1] A rubber composition comprising a diene rubber having an epoxy group, a dicarboxylic acid represented by the following general formula (1), and an imidazole represented by the following general formula (2). HOOC-R 1 -COOH (1) (In formula (1), R 1 represents a divalent hydrocarbon group having 3 to 50 carbon atoms, which may be interrupted by one or more heteroatoms. [ka] (In formula (2), R 2 represents a hydrogen atom or a methyl group. [2] The rubber composition according to [1], wherein the amount of the dicarboxylic acid per 100 parts by mass of the diene rubber is 0.2 to 20 parts by mass. [3] The rubber composition according to [1] or [2], wherein the amount of the imidazole per 100 parts by mass of the diene rubber is 0.5 to 15 parts by mass. [4] A cross-linked rubber comprising a diene rubber having an epoxy group cross-linked with a dicarboxylic acid represented by the above general formula (1) and an imidazole represented by the above general formula (2). [5] A tire comprising the cross-linked rubber according to [4]. Effect of the Invention

[0012] According to an embodiment of the present invention, the strength of a cross-linked rubber cross-linked with a dicarboxylic acid can be improved. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The rubber composition according to the present embodiment contains a diene rubber having an epoxy group, a dicarboxylic acid represented by general formula (1), and an imidazole represented by general formula (2). These may be contained as a mixture in which the components are mixed together without reacting with each other, or may be contained as a product obtained by at least partial reaction, or a mixture of unreacted and reacted components may be present.

[0014] Diene rubber having epoxy (hereinafter referred to as epoxidized diene rubber) is a diene rubber in which some of the carbon-carbon double bonds are epoxidized. Diene rubber refers to a rubber having a repeating unit corresponding to a diene monomer having a conjugated double bond, and contains a carbon-carbon double bond in the main chain of the polymer. Examples of diene rubbers that serve as the base for epoxidized diene rubber include natural rubber (NR), synthetic isoprene rubber (IR), polybutadiene rubber (BR), styrene butadiene rubber (SBR), styrene-isoprene copolymer rubber, butadiene-isoprene copolymer rubber, and styrene-isoprene-butadiene copolymer rubber.

[0015] The epoxidized diene rubber is preferably at least one selected from the group consisting of epoxidized natural rubber (ENR), epoxidized synthetic isoprene rubber, epoxidized polybutadiene rubber, and epoxidized styrene butadiene rubber, more preferably epoxidized natural rubber.

[0016] Epoxidized natural rubber is obtained by epoxidizing natural rubber. For example, natural rubber epoxidized by the chlorohydrin method, direct oxidation method, hydrogen peroxide method, alkyl hydroperoxide method, peracid method, or the like can be used.

[0017] The epoxidation rate of the epoxidized diene rubber is not particularly limited, and may be, for example, 1 to 80 mol%, 5 to 70 mol%, 10 to 60 mol%, or 20 to 55 mol%. The epoxidation rate is the ratio of the number of epoxidized double bonds to the total number of double bonds in the diene rubber before epoxidation, and can be measured by Fourier transform nuclear magnetic resonance spectroscopy.

[0018] The molecular weight of the epoxidized diene rubber is not particularly limited, and those that are solid at room temperature (25° C.) can be preferably used.

[0019] In the rubber composition according to the embodiment, the rubber component may be only an epoxidized diene rubber, or may contain a non-epoxidized rubber having no epoxy group together with the epoxidized diene rubber. The non-epoxidized rubber is not particularly limited, and examples thereof include natural rubber, synthetic isoprene rubber, butadiene rubber, and styrene-butadiene rubber, and these may be used alone or in combination of two or more.

[0020] The rubber component preferably contains an epoxidized diene rubber as a main component, and the amount of the epoxidized diene rubber in 100% by mass of the rubber component is preferably 50 to 100% by mass, more preferably 70 to 100% by mass, even more preferably 80 to 100% by mass, and particularly preferably 100% by mass.

[0021] The dicarboxylic acid is a compound represented by the following general formula (1), and is a component that crosslinks the epoxidized diene rubber. As the dicarboxylic acid, any one of the compounds represented by formula (1) may be used, or two or more of them may be used in combination. The two carboxy groups of the dicarboxylic acid react with the epoxy groups of the epoxidized diene rubber, respectively, to form ester bonds. Therefore, the polymer chains of the epoxidized diene rubber are crosslinked via the dicarboxylic acid. The ester bonds can be rearranged by an ester exchange reaction. Since the bonds have recombination properties, they have self-repair properties and reprocessability, and material recycling can be realized.

[0022] HOOC-R 1 -COOH (1) In formula (1), R 1 represents a divalent hydrocarbon group having 3 to 50 carbon atoms which may be interrupted by one or more heteroatoms.

[0023] The divalent hydrocarbon group may be an aliphatic hydrocarbon group or an aromatic hydrocarbon group. The aliphatic hydrocarbon group may be saturated or unsaturated, may have a straight chain or a branched chain, or may contain an alicyclic structure (i.e., an alicyclic hydrocarbon group). Preferably, it is a saturated or unsaturated divalent aliphatic hydrocarbon group (preferably an alkanediyl group or an alkenediyl group) that may have a branched chain and / or an alicyclic structure.

[0024] R 1 The number of carbon atoms in the dicarboxylic acid is preferably 6 to 45, more preferably 8 to 40, and even more preferably 15 to 40. The number of carbon atoms Nc in the carbon chain linearly connecting two carboxy groups of the dicarboxylic acid is preferably 3 to 30, more preferably 6 to 25, and even more preferably 10 to 20. Here, the number of carbon atoms Nc is an index of the length of the crosslinked chain, and in the case of the dimer acid DFA in the examples described later, it is 16, and in the case of sebacic acid, it is 8.

[0025] R 1 The hydrocarbon group may be interrupted by at least one heteroatom selected from, for example, oxygen, nitrogen and sulfur, preferably by an oxygen atom, i.e., it may contain an ether bond.

[0026] Specific examples of the dicarboxylic acid include glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, terephthalic acid, cyclohexanedicarboxylic acid, dimer acid, and hydrogenated dimer acid, and these can be used alone or in combination of two or more.

[0027] The amount of the dicarboxylic acid is preferably 0.2 to 20 parts by mass, more preferably 0.5 to 15 parts by mass, even more preferably 0.8 to 10 parts by mass, and still more preferably 1 to 5 parts by mass, based on 100 parts by mass of the epoxidized diene rubber.

[0028] The imidazole is a compound represented by the following general formula (2), and is a component that crosslinks epoxidized diene rubber. In detail, the imidazole has a hydrogen atom at the nitrogen atom at position 1, and reacts with the epoxy group of the epoxidized diene rubber to bond to the epoxidized diene rubber. When the epoxy group of another epoxidized diene rubber reacts with the bonded imidazole ring, the polymer chains of the epoxidized diene rubber are bonded to each other via the imidazole, and the positively charged imidazole ring and the negative ions of oxygen atoms (O - ) occurs. As a result, the imidazolium ion becomes a neutral repeating unit, and the epoxidized diene rubber forms an ionomer, and crosslinks (ionic crosslinks) due to ionic bonds are introduced into the epoxidized diene rubber (see Non-Patent Document 1). By additionally introducing ionic crosslinks by imidazole in this way, the strength of the crosslinked rubber can be improved. The reason for this is not particularly limited, but it is presumed that the ionic crosslinks function as sacrificial bonds in the system. In other words, it is believed that the strength of the entire system is increased by the ionic crosslinks being preferentially broken while dispersing the breaking energy as sacrificial bonds.

[0029] [ka] In formula (2), R 2 represents a hydrogen atom or a methyl group. Therefore, the imidazole is 1H-imidazole or 2-methyl-1H-imidazole, which may be used alone or in combination.

[0030] The amount of the imidazole is preferably 0.5 to 15 parts by mass, more preferably 1 to 10 parts by mass, even more preferably 1.5 to 8 parts by mass, and still more preferably 2 to 5 parts by mass, based on 100 parts by mass of the epoxidized diene rubber.

[0031] The rubber composition according to the present embodiment may contain a metal salt and / or an N-substituted imidazole in order to promote the ester reaction of the dicarboxylic acid with the epoxidized diene rubber. Since these are components that promote the ester reaction, they do not bond to the epoxidized diene rubber and do not become a crosslinking component.

[0032] The metal salt is not particularly limited, and examples thereof include heavy metal salts such as zinc salts (e.g., zinc acetate, zinc chloride) and manganese salts (e.g., manganese acetate), and alkaline earth metal salts such as calcium salts (e.g., calcium acetate) and magnesium salts (e.g., magnesium acetate), and the like, and these can be used alone or in combination of two or more.

[0033] The amount of the metal salt is not particularly limited, and may be 1 to 20 parts by mass, or may be 2 to 15 parts by mass, based on 100 parts by mass of the epoxidized diene rubber.

[0034] As the N-substituted imidazole, there can be used an imidazole in which the hydrogen atom at the 1-position of the imidazole ring is substituted with an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group, etc. Specific examples of the N-substituted imidazole include 1-methylimidazole, 1,2-dimethylimidazole, 1-butylimidazole, 1-propylimidazole, etc., and any one of these can be used alone or in combination of two or more.

[0035] The amount of the N-substituted imidazole is not particularly limited, and may be 1 to 20 parts by mass, or may be 3 to 10 parts by mass, based on 100 parts by mass of the epoxidized diene rubber.

[0036] The rubber composition according to the present embodiment may contain a filler. For example, carbon black and / or silica is preferable as the filler. The carbon black is not particularly limited, and various known types can be used. The silica is also not particularly limited, and examples thereof include wet silica and dry silica. It is preferable to use wet silica such as wet precipitation silica and wet gelation silica.

[0037] The amount of the filler is not particularly limited and can be appropriately set depending on the application. For example, it may be 10 to 200 parts by mass or 30 to 100 parts by mass per 100 parts by mass of the rubber component.

[0038] In addition to the above components, the rubber composition according to this embodiment may contain various additives that are generally used in rubber compositions, such as zinc oxide, stearic acid, antioxidants, wax, and oil.

[0039] The rubber composition according to the present embodiment preferably does not substantially contain a vulcanizing agent such as sulfur and a vulcanization accelerator. That is, the rubber composition does not contain a vulcanizing agent such as sulfur and a vulcanization accelerator, or even if it does contain one, the total amount of the vulcanizing agent and the vulcanization accelerator is preferably less than 1 part by mass, more preferably less than 0.5 parts by mass, and even more preferably less than 0.2 parts by mass, per 100 parts by mass of the rubber component.

[0040] The rubber composition according to the present embodiment can be prepared by mixing using a commonly used mixer such as a Banbury mixer, kneader, or roll. The mixing procedure is not particularly limited, and for example, the dicarboxylic acid and the imidazole may be added to the epoxidized diene rubber (A) at once and mixed, the dicarboxylic acid may be added to the epoxidized diene rubber (B) and mixed, and then the imidazole may be added to the obtained mixture and mixed, or the imidazole may be added to the epoxidized diene rubber (C) and mixed, and then the dicarboxylic acid may be added to the obtained mixture and mixed. In this case, additives such as a filler may be added to the epoxidized diene rubber in advance and mixed prior to mixing the dicarboxylic acid and the imidazole, or one or both of the dicarboxylic acid and the imidazole may be added to the epoxidized diene rubber and mixed, and then added to the mixture obtained and mixed.

[0041] The rubber composition thus obtained can be further heated to complete the crosslinking, thereby obtaining a crosslinked rubber product.

[0042] The cross-linked rubber according to the embodiment contains an epoxidized diene rubber cross-linked with a dicarboxylic acid represented by the above formula (1) and an imidazole represented by the above formula (2), and is obtained by cross-linking the above rubber composition. In the cross-linked rubber, the epoxidized diene rubber is cross-linked by the above imidazole in addition to the cross-linking of ester bonds by the above dicarboxylic acid, and has improved strength. These cross-links are recombinable bonds, so they have self-repairing properties and reprocessability, and therefore the strength can be improved while realizing material recycling.

[0043] The rubber composition or cross-linked rubber according to the embodiment can be used for various rubber members such as tires, anti-vibration rubber, and conveyor belts. It is preferably for tires. That is, a tire according to a preferred embodiment contains the above-mentioned cross-linked rubber. Examples of tires include pneumatic tires of various sizes for various uses such as tires for passenger cars and large tires for trucks and buses, and the rubber composition or cross-linked rubber according to the embodiment can be applied to each portion thereof such as the tread, sidewall, and bead portion. EXAMPLES

[0044] Examples will be shown below, but the present invention is not limited to these examples.

[0045] The ENR and DFA used in the examples and comparative examples are as follows. ENR: Epoxidized natural rubber, "Epoxyrene 50" (epoxidation rate: 50 mol%) manufactured by Muang Mai Guthrie Public Company Limited DFA: Dimer acid represented by the following formula, manufactured by Sigma-Aldrich "Dimer acid, hydrogenated" [ka]

[0046] Preparation methods X, Y, and Z in the examples and comparative examples are as follows. Preparation method X: Using a Banbury mixer (Toyo Seiki Seisakusho's "Labo Plastomill 10S100"), zinc acetate, 1,2-dimethylimidazole and dicarboxylic acid were added to ENR and mixed for 10 minutes at 60°C and 100 rpm. Next, using the Banbury mixer, 1H-imidazole was added to the resulting mixture and mixed for 3 minutes at 60°C and 100 rpm. Next, the resulting mixture was formed into a sheet with a thickness of 1 mm and heated at 170°C for 60 minutes to obtain a sheet-shaped cross-linked rubber product.

[0047] Preparation method Y: Using a Banbury mixer (Toyo Seiki Seisakusho's "Labo Plastomill 10S100"), 1H-imidazole was added to ENR and mixed for 3 minutes at 60°C and 100 rpm. Next, using the Banbury mixer, zinc acetate, 1,2-dimethylimidazole and dicarboxylic acid were added to the resulting mixture and mixed for 10 minutes at 60°C and 100 rpm. Next, the resulting mixture was formed into a sheet with a thickness of 1 mm and heated at 170°C for 60 minutes to obtain a sheet-shaped cross-linked rubber product.

[0048] Preparation method Z: Using a Banbury mixer (Toyo Seiki Seisakusho's "Labo Plastomill 10S100"), zinc acetate, 1,2-dimethylimidazole and dicarboxylic acid were added to ENR and mixed for 10 minutes at 60°C and 100 rpm. The resulting mixture was then molded into a sheet with a thickness of 1 mm and heated at 170°C for 60 minutes to obtain a sheet-shaped cross-linked rubber product.

[0049] The evaluation methods for the cross-linked rubber products in the examples and comparative examples are as follows. Stress at 100% elongation: A tensile test (dumbbell-shaped No. 7, thickness 1 mm) was conducted in accordance with JIS K6251:2017 to measure the tensile stress (MPa) at 100% elongation at 25°C. The obtained tensile stress is shown as an index with the value for Comparative Example 1 in Table 1, the value for Comparative Example 3 in Table 2, and the value for Comparative Example 4 in Table 3 each set at 100. A larger index indicates a larger tensile stress at 100% elongation and better fracture properties (mechanical strength).

[0050] [First Experimental Example] Cross-linked rubber products of Comparative Examples 1 and 2 and Examples 1 to 6 were prepared according to the formulations (parts by mass) and preparation methods shown in the following Table 1. The cross-linked rubber products obtained were evaluated for stress at 100% elongation.

[0051] [Table 1]

[0052] The results are shown in Table 1. Comparative Examples 1 and 2 are examples in which a crosslinked structure by an ester bond using DFA as a dicarboxylic acid is introduced. Examples 1 to 6 are examples in which an ionic crosslink by 1H-imidazole is introduced in addition to an ester bond by DFA, and the introduction of the ionic crosslinks significantly improved the strength compared to Comparative Examples 1 and 2. Comparing Example 3 with Example 4, in terms of the preparation method, Example 3, in which an ionic crosslink was introduced first, was more excellent in improving the strength than Example 4, in which a crosslinked structure by an ester bond was introduced first. The same results were obtained when comparing Example 5 with Example 6, in which the amount of 1H-imidazole was increased.

[0053] [Second Experimental Example] The cross-linked rubber products of Comparative Example 3 and Examples 7 to 10 were prepared according to the formulation (parts by mass) and preparation method shown in the following Table 2. The cross-linked rubber products obtained were evaluated for stress at 100% elongation.

[0054] [Table 2]

[0055] The results are shown in Table 2. Even when the dicarboxylic acid was changed from DFA to sebacic acid, in Examples 7 to 10 in which ionic crosslinking by 1H-imidazole was introduced in addition to ester bonds, the strength was improved compared to Comparative Example 3 in which only ester bonds were used. However, the degree of improvement was smaller than that of DFA. The reason for this is unclear, but it is presumed that sebacic acid, because the crosslinking chain length is shorter than that of DFA, penetrates into the ionic association formed by ionic crosslinking and destroys part of the ionic association. Alternatively, the lower reactivity of sebacic acid compared to DFA may also be a factor.

[0056] [Third Experimental Example] Cross-linked rubber products of Comparative Examples 4 to 8 were prepared according to the formulations (parts by mass) and preparation methods shown in the following Table 3. The cross-linked rubber products obtained were evaluated for stress at 100% elongation.

[0057] [Table 3]

[0058] The results are shown in Table 3. When the dicarboxylic acid was changed to malonic acid, even if ionic crosslinking with 1H-imidazole was introduced in addition to the ester bond, no improvement in strength was observed, and the strength actually decreased, as shown in Comparative Examples 5 to 8. This is thought to be due to the fact that malonic acid has an even shorter crosslinking chain length and lower reactivity than sebacic acid.

[0059] In addition, the various numerical ranges described in the specification can be arbitrarily combined with their upper and lower limits, and all such combinations are considered to be preferred numerical ranges described in the specification. In addition, a numerical range described as "X to Y" means from X to Y.

Claims

1. A rubber composition comprising a diene rubber having an epoxy group, a dicarboxylic acid represented by the following general formula (1), and an imidazole represented by the following general formula (2), P.S. １ -COOH (1) In formula (1), R １ represents a divalent hydrocarbon group having 3 to 50 carbon atoms, which may be interrupted by one or more heteroatoms; 【Chemistry 1】 In formula (2), R ２ represents a hydrogen atom or a methyl group; Rubber composition.

2. The rubber composition according to claim 1, wherein the amount of the dicarboxylic acid per 100 parts by mass of the diene rubber is 0.2 to 20 parts by mass.

3. 3. The rubber composition according to claim 1, wherein the amount of the imidazole is 0.5 to 15 parts by mass based on 100 parts by mass of the diene rubber.

4. A cross-linked rubber product comprising a diene rubber having an epoxy group cross-linked with a dicarboxylic acid represented by the following general formula (1) and an imidazole represented by the following general formula (2), P.S. １ -COOH (1) In formula (1), R １ represents a divalent hydrocarbon group having 3 to 50 carbon atoms, which may be interrupted by one or more heteroatoms; 【Chemistry 2】 In formula (2), R ２ represents a hydrogen atom or a methyl group; Cross-linked rubber.

5. A tire comprising the cross-linked rubber according to claim 4.

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