Rubber composition for steel cord coating and pneumatic tire

Abstract

The present invention provides a rubber composition for covering steel cords that can improve sustainability and adhesion to steel cords without impairing processability. [Solution] The rubber composition for covering steel cord according to the embodiment comprises a diene rubber, a vegetable oil, a melamine derivative, and a resorcinol compound.

Description

Technical Field

[0001] The present invention relates to a rubber composition for coating steel cords and a pneumatic tire using the same.

Background Art

[0002] In pneumatic tires, particularly radial tires, steel cords are widely used as reinforcing materials such as the belt layer of passenger car tires, the belts of large tires such as truck and bus tires, carcass plies, and chafers. These days, the service life of tires has been extended, and it is important to enhance the reinforcing effect of steel cords and maintain durability over a long period. Therefore, the rubber composition for coating steel cords is required to have excellent adhesion to steel cords.

[0003] In order to improve the adhesion to steel cords, it is known to incorporate resorcin-based compounds such as resorcin or resorcin derivatives into the rubber composition for coating steel cords. For example, Patent Document 1 describes a rubber-steel cord composite having excellent initial adhesiveness and wet heat adhesiveness, in which a rubber composition containing a resorcin-based compound and a melamine derivative is vulcanization-bonded to a steel cord.

[0004] Patent Document 2 discloses a resorcin derivative that can be substituted for resorcinol during rubber production, and a rubber composition containing the resorcin derivative as a methylene acceptor and a methylene donor is disclosed.

[0005] Patent Document 3 discloses a belted ply in which a cord array is coated with a topping rubber mainly composed of raw materials derived from non-petroleum resources, and it is described that vegetable oil is incorporated as an extender oil and a resorcin derivative is incorporated as an adhesive into the topping rubber.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-195413 [Patent Document 2] International Publication No. 2002 / 050206 [Patent Document 3] Japanese Patent Publication No. 2009-40347 [Overview of the project] [Problems that the invention aims to solve]

[0007] As a measure to address global environmental problems, there is a growing desire to use sustainable raw materials, such as those derived from biomass resources, instead of petroleum-derived raw materials. Currently, petroleum-based oils (e.g., aromatic oils) are mainly used as oil components in the production of tire rubber compositions from a processability standpoint. Therefore, the development of rubber compositions that replace petroleum-based oils with sustainable raw materials is desirable from a sustainability perspective.

[0008] In view of the above, embodiments of the present invention aim to provide a rubber composition for covering steel cords that can improve sustainability and adhesion to steel cords without impairing processability. [Means for solving the problem]

[0009] The present invention includes embodiments shown below. [1] A rubber composition for coating steel cords, comprising a diene rubber, a vegetable oil, a melamine derivative, and a resorcinol compound. [2] The rubber composition for covering steel cords according to [1], wherein the content of the vegetable oil is 1 to 10 parts by mass per 100 parts by mass of the diene rubber. [3] The rubber composition for covering steel cords according to [1] or [2], wherein the content of the melamine derivative is 0.1 to 5 parts by mass and the content of the resorcinol compound is 0.1 to 5 parts by mass per 100 parts by mass of the diene rubber. [4] The rubber composition for covering steel cords according to any one of [1] to [3], wherein the iodine value of the vegetable oil is 60 to 140. A pneumatic tire comprising a rubber-steel cord composite in which a steel cord is vulcanized and bonded with a steel cord and a rubber composition for covering steel cords as described in any one of items [1] to [4]. [Effects of the Invention]

[0010] According to embodiments of the present invention, sustainability and adhesion to steel cords can be improved without impairing processability. [Modes for carrying out the invention]

[0011] The rubber composition for covering steel cords according to this embodiment (hereinafter also referred to as the rubber composition) comprises (A) a diene rubber, (B) vegetable oil, (C) a melamine derivative, and (D) a resorcinol compound. By blending the vegetable oil together with the melamine derivative and the resorcinol compound in this way, the adhesion to the steel cord can be improved without impairing the processability. The reason for this is presumed to be as follows, although this is not intended to be the sole reason.

[0012] When a melamine derivative (methylene donor) and a resorcinol-based compound (methylene acceptor) are added as additives to a rubber composition, the methylene group of the melamine derivative reacts with the hydroxyl group of the resorcinol-based compound during vulcanization. This reaction forms a network-like, highly stable resin layer inside the vulcanized rubber. In particular, this reaction preferentially proceeds at the adhesive interface between rubber and metal, resulting in the formation of a stronger resin layer at the interface and, consequently, improved moist heat adhesion. Incidentally, rubber compositions for steel cord coatings generally contain petroleum-derived aromatic oils to improve processability. Since such aromatic oils have a chemical structure similar to that of resorcinol-based compounds, they are thought to compete with the reaction between the resorcinol-based compound and the melamine derivative. In contrast, vegetable oils have a linear chemical structure and are significantly different in chemical structure from resorcinol-based compounds. Therefore, competition does not occur, and as a result, a stronger resin layer is formed at the adhesive interface between rubber and metal, improving adhesion. Furthermore, by using vegetable oil instead of petroleum-derived aromatic oil as the oil component, a desirable rubber composition can be obtained not only from the standpoint of adhesion between rubber and metal, but also from the standpoint of sustainability.

[0013] [(A) Diene-based rubber] In this embodiment, diene rubber refers to rubber having repeating units corresponding to a diene monomer having a conjugated double bond, and the polymer's main chain contains a carbon-carbon double bond. Specific examples of diene rubber include natural rubber (NR), synthetic isoprene rubber (IR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), styrene-isoprene copolymer rubber, butadiene-isoprene copolymer rubber, styrene-isoprene-butadiene copolymer rubber, and various other diene rubbers commonly used in rubber compositions. These diene rubbers also include those with modified ends or main chains as needed (e.g., end-modified SBR) or those modified to impart desired properties (e.g., modified NR), as well as those modified to impart desired properties. These diene rubbers may be used individually or in combination of two or more. Among these, at least one selected from the group consisting of natural rubber, synthetic isoprene rubber, butadiene rubber, and styrene-butadiene rubber is preferred as the diene rubber.

[0014] In one embodiment, the diene rubber preferably contains natural rubber. The amount of natural rubber in 100 parts by mass of diene rubber is not particularly limited, but may be 50 parts by mass or more, 60 parts by mass or more, 70 parts by mass or more, 80 parts by mass or more, 90 parts by mass or more, or 100 parts by mass.

[0015] [(B) Vegetable oil] The rubber composition according to this embodiment contains vegetable oil as an oil component. Vegetable oil refers to oils and fats extracted from plant seeds, fruits, kernels, etc. The vegetable oil may be solid or liquid at room temperature (25°C). "Solid" refers to a state that does not have fluidity, and "liquid" refers to a state that has fluidity. From the viewpoint of sustainability, it is preferable that the oil component does not contain petroleum-based oils.

[0016] Specific examples of vegetable oils include linseed oil, soybean oil, sunflower oil, palm oil, palm olein, palm kernel oil, olive oil, rapeseed oil, sesame oil, cottonseed oil, safflower oil, rice bran oil, corn oil, coconut oil, peanut oil, grape seed oil, etc. These may be used alone or in combination of two or more of them.

[0017] In one embodiment, the iodine value of the vegetable oil is preferably 60 to 140, more preferably 65 to 130. Here, the iodine value is the value obtained by converting the amount of halogen bound when reacting 100 g of the sample with halogen into the amount of iodine (g), and is measured in accordance with JIS K0070:1992. The iodine value correlates with the amount of unsaturated bonds in the fatty acids contained in the vegetable oil, and indicates that the greater the iodine value, the greater the amount of unsaturated bonds in the fatty acids contained in the vegetable oil.

[0018] Since the unsaturated bond portion of the fatty acids contained in the vegetable oil has high electron reactivity, it reacts with sulfur. When the iodine value of the vegetable oil is 140 or less, excessive consumption of sulfur due to the reaction between the unsaturated bond portion of the fatty acids contained in the vegetable oil and sulfur can be suppressed. Therefore, a decrease in the crosslink density of the vulcanized rubber is suppressed, and it is easier to make the bond between the resin layer and the rubber layer stronger. Also, when the iodine value of the vegetable oil is 60 or more, sulfur is moderately consumed by the reaction between the unsaturated bond portion of the fatty acids contained in the vegetable oil and sulfur, and formation of a large amount of thermally unstable polysulfides in the vulcanized rubber can be suppressed, and the effects of the present embodiment can be enhanced.

[0019] In one embodiment, the content of the vegetable oil is preferably 1 to 10 parts by mass, more preferably 1 to 8 parts by mass, and still more preferably 1 to 6 parts by mass with respect to 100 parts by mass of the diene rubber. By the content of the vegetable oil being 1 part by mass or more, sufficient processability can be obtained. Also, by the content of the vegetable oil being 10 parts by mass or less, the reinforcing property of the vulcanized rubber is maintained, so breakage is suppressed and adhesiveness can be improved.

[0020] [(C) Melamine derivative] The rubber composition according to this embodiment contains a melamine derivative. Specific examples of the melamine derivative include hexamethylol melamine, hexamethoxymethyl melamine, hexaethoxymethyl melamine, hexakis-(methoxymethyl) melamine, pentamethoxymethylol melamine, N,N’,N’’-trimethyl-N,N’,N’’-trimethylol melamine, N,N’,N’’-trimethylol melamine, N-methylol melamine, N,N’-(methoxymethyl) melamine, N,N’,N’’-tributyl-N,N’,N’’-trimethylol melamine, and the like. Any one of these or a combination of two or more thereof may be used. As the melamine derivative, hexamethoxymethyl melamine is particularly preferably used.

[0021] In one embodiment, the content of the melamine derivative is preferably 0.1 to 5 parts by mass, more preferably 0.5 to 4 parts by mass, and still more preferably 1 to 3 parts by mass with respect to 100 parts by mass of the diene rubber.

[0022] [(D) Resorcin-based compound] The rubber composition according to this embodiment contains a resorcin-based compound. The resorcin-based compound refers to resorcin and resorcin derivatives. As the resorcin derivative, a resorcin-formalin resin obtained by condensing resorcin and formaldehyde, a resorcin-alkylphenol-formalin resin in which a part of the repeating unit of the resorcin-formalin resin is alkylated, and further, a phenol-formalin resin obtained by condensing phenols (such as phenol and cresol) and aldehyde, and phenolic resins such as cresol-formalin resin can also be used. Any one of these or a combination of two or more thereof may be used. As the resorcin-based compound, resorcin-alkylphenol-formalin resin and resorcin are particularly preferably used.

[0023] In one embodiment, the content of the resorcin-based compound is preferably 0.1 to 5 parts by mass, more preferably 0.5 to 4 parts by mass, and still more preferably 1 to 3 parts by mass with respect to 100 parts by mass of the diene rubber.

[0024] The mass ratio of the melamine derivative to the resorcinol compound (melamine derivative / resorcinol compound) is not particularly limited, but is preferably 0.25 to 4, and more preferably 0.5 to 2.

[0025] [Other ingredients] In addition to the components mentioned above, the rubber composition according to this embodiment may contain various additives commonly used in rubber compositions, such as fillers, zinc oxide, stearic acid, cobalt organic acid salts, vulcanizing agents, and vulcanization accelerators.

[0026] Examples of fillers include carbon black and silica, which may be used individually or in combination.

[0027] The carbon black is not particularly limited, and various known grades can be used. Specifically, examples include SAF grade (N100 series), ISAF grade (N200 series), HAF grade (N300 series), FEF grade (N500 series), GPF grade (N600 series), and SRF grade (N700 series) (all ASTM grades). Any one or more of these carbon blacks can be used in combination. The carbon black content is not particularly limited; for example, it may be 20 to 100 parts by mass, 40 to 80 parts by mass, or 50 to 70 parts by mass per 100 parts by mass of diene rubber.

[0028] The silica is not particularly limited and includes wet silica, dry silica, etc. Preferably, wet silica such as wet sedimentation silica or wet gelation silica is used. The silica content is not particularly limited and may be 1 to 40 parts by mass, 3 to 30 parts by mass, or 5 to 20 parts by mass per 100 parts by mass of diene rubber.

[0029] The zinc oxide content is not particularly limited; for example, it may be 0 to 20 parts by mass, 3 to 15 parts by mass, or 5 to 10 parts by mass per 100 parts by mass of diene rubber.

[0030] The stearic acid content is not particularly limited; for example, it may be 0 to 10 parts by mass, 0.3 to 5 parts by mass, or 0.5 to 3 parts by mass per 100 parts by mass of diene rubber.

[0031] Organic cobalt acid salts are preferably added as adhesion enhancers to steel cords. Examples of organic cobalt acid salts include cobalt oleate, cobalt stearate, cobalt naphthenate, cobalt neodecanoate, cobalt borate, cobalt maleate, and cobalt rosinate, with cobalt stearate being particularly preferred. The content of the organic cobalt acid salt is not particularly limited; for example, it may be 0.03 to 0.50 parts by mass in terms of metal content per 100 parts by mass of diene rubber.

[0032] Examples of vulcanizing agents include sulfur such as powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, and highly dispersible sulfur. The content of the vulcanizing agent is not particularly limited and may be 1 to 10 parts by mass, 2 to 9 parts by mass, or 4 to 8 parts by mass per 100 parts by mass of diene rubber.

[0033] Examples of vulcanization accelerators include sulfenamide-based, thiuram-based, thiazole-based, and guanidine-based vulcanization accelerators, which can be used individually or in combination of two or more. The content of the vulcanization accelerator is not particularly limited; for example, it may be 0 to 5 parts by mass, 0.3 to 3 parts by mass, or 0.5 to 3 parts by mass per 100 parts by mass of diene rubber.

[0034] The rubber composition according to this embodiment can be prepared by kneading in accordance with conventional methods using a commonly used mixer such as a Banbury mixer, kneader, or roll. That is, in the first mixing stage, other additives excluding the vulcanizing agent and vulcanization accelerator are added and mixed to the diene rubber, and then, in the final mixing stage, the vulcanizing agent and vulcanization accelerator are added and mixed to the resulting mixture to prepare the rubber composition.

[0035] The rubber composition can be used as a rubber composition for coating various steel cords, and a rubber-steel cord composite is obtained in which the rubber composition and the steel cord are vulcanized and bonded. Such a rubber-steel cord composite can be manufactured by vulcanizing the rubber composition in contact with the steel cord. The heating temperature during vulcanization is not particularly limited and may be, for example, 140 to 180°C. As the steel cord, one which has been plated on its surface with brass plating, bronze plating, zinc plating, etc., is preferably used, and more preferably a brass-plated steel cord is used.

[0036] The rubber-steel cord composite can be used as a reinforcing material for tires, such as belts, carcass plies, and chafers in pneumatic tires, or as a reinforcing material for industrial belt components, etc. Preferably, it is used as a reinforcing material for pneumatic tires, and therefore, the pneumatic tire according to this embodiment is equipped with the above-mentioned rubber-steel cord composite.

[0037] When used as a reinforcing material for belts, carcass plies, chafers, etc., in pneumatic tires, a steel cord topping is produced by topping steel cords with a rubber composition using a topping device such as a steel calender, in accordance with conventional methods. This topping is then used as a belt, carcass ply, chafer, etc., to produce an unvulcanized tire, which can then be vulcanized and molded at, for example, 140 to 180°C to produce a pneumatic tire.

[0038] The pneumatic tire can be a passenger car tire or a heavy-duty tire for trucks, buses, etc., and is not particularly limited. The structure of the pneumatic tire itself is well known and is not particularly limited. Generally, a pneumatic tire comprises a pair of left and right bead portions and sidewalls, a tread provided between the two sidewalls so as to connect the radially outer ends of the left and right sidewalls, and at least one layer of carcass ply extending across the pair of left and right bead portions. The carcass ply extends from the tread through the sidewalls and both ends are locked at the bead portions, reinforcing the above-mentioned parts. In addition, a belt is usually provided in two or more layers between the tread rubber on the outer circumference side of the carcass ply on the tread, reinforcing the tread on the outer circumference of the carcass ply. A chafer is embedded in the bead portion to reinforce the bead portion. When the above-mentioned rubber-steel cord composite is used as a tire reinforcing material, it may be applied to one of the belt, carcass, and chafer, or to two or more. [Examples]

[0039] The following are examples of the present invention, but the present invention is not limited to these examples.

[0040] The raw materials used in the examples and comparative examples are as follows: • Natural rubber: RSS#3 • Carbon Black: HAF-LS, manufactured by Tokai Carbon Co., Ltd. "N326 Seast 300" • Silica: "Nip Seal AQ" manufactured by Tosoh Silica Co., Ltd. • Petroleum-based oil: ENEOS Corporation's "Process NC-140" • Vegetable oil-1: Sunflower oil, "High-Ole Sunflower Oil NS" manufactured by J-Oil Mills Co., Ltd. (iodine value = 84) • Vegetable oil-2: Soybean oil, manufactured by Nisshin Oillio Group Ltd., "Soybean Refined Oil" (iodine value = 128) • Vegetable oil-3: Palm olein, manufactured by Nisshin Oillio Group Ltd., "PL65" (iodine value = 65) • Melamine derivative: Hexamethoxymethylmelamine, manufactured by Ornex Japan Co., Ltd. as "Sailets 963L" • Resorcinol derivative: Resorcinol alkylphenol formalin resin, manufactured by Sumitomo Chemical Co., Ltd. as "Sumikanol 620" • Zinc oxide: "3 types of zinc oxide" manufactured by Mitsui Mining & Smelting Co., Ltd. • Stearic acid: "Lunaq S-20" manufactured by Kao Corporation • Cobalt stearate: "Corebond CS-9.5" manufactured by Taiko Precision Chemicals Co., Ltd. (Co content 9.5% by mass) • Sulfur: Powdered sulfur manufactured by Tsurumi Chemical Industries, Ltd. • Vulcanization accelerator: "Noxellar DZ-G" manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0041] The evaluation methods for each example and comparative example are as follows.

[0042] [Workability] In accordance with JIS K6300-1:2013, a rotaryless Mooney viscosity measuring instrument manufactured by Toyo Seiki Seisakusho Co., Ltd. was used to preheat the unvulcanized rubber composition at 100°C for 1 minute, and then measure the torque value in Mooney units after 4 minutes. The reciprocal of the measured torque value is set to 100 in Table 1 for Comparative Example 1 and 100 in Table 2 for Comparative Example 3, and the values ​​for each example and comparative example are shown as indices. A larger index indicates lower Mooney viscosity and superior processability. The target value for processability is 95 or higher.

[0043] [Wet heat adhesion] Unvulcanized samples of rubber-steel cord composites were prepared using each of the obtained rubber compositions. Specifically, 12 steel cords for belts (3×0.20 + 6×0.35 mm structure, copper / zinc = 64 / 36 (mass ratio), brass plating with an adhesion amount of 5 g / kg) were arranged in parallel at a density of 25 mm. Both sides of these were covered with a 1 mm thick rubber sheet made of the above rubber compositions, and these two sheets were laminated so that the cords were parallel to each other to prepare an unvulcanized sample. The prepared unvulcanized sample was left at room temperature for 24 hours, then vulcanized at 150°C for 30 minutes. The vulcanized test piece was left in saturated steam at 105°C for 96 hours, and then a peel test was performed between the two layers of steel cords using a Shimadzu Autograph "DCS500" to determine the peeling force. Regarding the measured peeling force, Table 1 sets the value for Comparative Example 1 to 100, and Table 2 sets the value for Comparative Example 3 to 100, with the values ​​for each example and comparative example shown as indices. A higher index indicates superior moist heat adhesion.

[0044] [First Experimental Example] Rubber compositions for Comparative Examples 1-2 and Examples 1-5 were prepared using a Banbury mixer according to the formulations (parts by mass) listed in Table 1 below. Specifically, in the first mixing stage, all additives except sulfur and vulcanization accelerator were added and kneaded (discharge temperature = 160°C). In the final mixing stage, sulfur and vulcanization accelerator were added to the resulting mixture and kneaded (discharge temperature = 90°C) to prepare the rubber composition. The processability and wet heat adhesion of each obtained rubber composition were evaluated.

[0045] The results are shown in Table 1 below. In Table 1, only carbon black was used as a filler. Comparative Example 1 is a conventional formulation using petroleum-based oil as the oil component. In Comparative Example 2, no oil component was used compared to Comparative Example 1, resulting in significantly reduced processability, falling below the target value, and slightly inferior moist heat adhesion.

[0046] In contrast, Examples 1 to 5, which used vegetable oils with various iodine values ​​as the oil component, exhibited improved moist heat adhesion while maintaining processability equivalent to or better than that of Comparative Example 1.

[0047] [Table 1]

[0048] [Second Experimental Example] Comparative Example 3 and Example 6 rubber compositions were prepared according to the formulations (parts by mass) listed in Table 2 below, with the rest of the preparation being the same as in Experimental Example 1. The processability and wet heat adhesion of each obtained rubber composition were evaluated.

[0049] The results are shown in Table 2 below. In Table 2, carbon black and silica were used in combination as fillers. Comparative Example 3 is a conventional formulation using petroleum-based oil as the oil component. In contrast, Example 6, which used vegetable oil (sunflower oil) as the oil component, showed significantly improved moist heat adhesion compared to Comparative Example 3 while maintaining comparable processability.

[0050] [Table 2]

[0051] Furthermore, the various numerical ranges described in the specification can be any combination of their upper and lower limits, and all such combinations are described herein as preferred numerical ranges. Also, the description of a numerical range as "X~Y" means X or greater and Y or less.

[0052] Although several embodiments of the present invention have been described above, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, and modifications are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

Claims

1. A rubber composition for coating steel cords, comprising diene rubber, vegetable oil, melamine derivative, and resorcinol compound.

2. The rubber composition for covering steel cords according to claim 1, wherein the content of the vegetable oil is 1 to 10 parts by mass per 100 parts by mass of the diene rubber.

3. The rubber composition for covering steel cords according to claim 1, wherein the content of the melamine derivative is 0.1 to 5 parts by mass and the content of the resorcinol compound is 0.1 to 5 parts by mass per 100 parts by mass of the diene rubber.

4. The rubber composition for covering steel cords according to claim 1, wherein the iodine value of the vegetable oil is 60 to 140.

5. A pneumatic tire comprising a rubber-steel cord composite in which a steel cord and a steel cord are vulcanized and bonded together using the steel cord coating rubber composition described in any one of claims 1 to 4.

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