Method for improving the visibility of letters or figures on a rubber molded body and rubber composition

Abstract

This invention relates to a method for improving the visibility of embossed letters or figures on a rubber molded body obtained from a rubber composition, or letters or figures printed on the surface of a rubber molded body. [Solution] A method for improving the visibility of letters or figures on a rubber molded article obtained from a rubber composition by adding one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds to the rubber composition. Preferably, the aliphatic amine is a tertiary higher alkylamine, and the maleic acid ester compound is a polyoxyalkylene alkyl maleic acid ester.

Description

Technical Field

[0001] The present invention relates to a method for improving the visibility of characters or figures on a rubber molded body.

Background Art

[0002] In order to improve the visibility or designability of a molded body of a rubber composition typified by a tire, or to display a product name, a manufacturing number, or a model number, printing or embossing characters or figures on the surface of the molded body has been performed. However, for example, if characters or figures are displayed in white on the sidewall of a tire, the visibility is high. However, due to carbon black inevitably contained as a reinforcing filler in the tire and an amine-based anti-aging agent that causes brown discoloration (browning), the whiteness decreases over time and the visibility decreases.

[0003] As a rubber composition for tires, Patent Document 1 discloses a rubber composition in which 15 to 85 parts by weight of silica and a specific tertiary amine compound having a molecular weight of 180 or more are blended in an amount of 1 to 15% by weight with respect to 100 parts by weight of natural rubber and / or diene-based synthetic rubber.

[0004] There are also reported examples of additives to rubber compositions. Patent Document 2 discloses an additive composition for a silica-filled rubber composition comprising a glycerin fatty acid ester, wherein the glycerin fatty acid ester is an ester of glycerin and two or more fatty acids, and among the two or more fatty acids constituting the glycerin fatty acid ester, the most abundant fatty acid component is contained in the total fatty acids in an amount of 10 to 90% by mass, and further the monoester component is contained in the glycerin fatty acid ester in an amount of 50 to 100% by mass.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] According to the technology described in Patent Document 1, a rubber composition is provided in which the dispersibility of silica in rubber is improved, and the heat generation and abrasion resistance are enhanced.

[0007] According to the technology described in Patent Document 2, an additive composition is provided that can improve the low loss, tensile properties (fracture resistance), and processability of silica-containing rubber compositions. Tires obtained from a rubber composition containing the additive composition have excellent fracture resistance.

[0008] However, the aforementioned conventional technologies have not considered ways to improve the visibility of characters and figures on molded articles while maintaining the performance of the rubber composition. The present invention relates to a method for improving the visibility of embossed letters or figures on a rubber molded body, or letters or figures printed on the surface of a rubber molded body. [Means for solving the problem]

[0009] The inventors have found that the problems of the prior art described above can be solved by adding an aliphatic amine, a polyoxyethylene alkylamine, or a maleic acid ester compound to a rubber composition.

[0010] The present invention relates to the following [1] to [3]. [1] A method for improving the visibility of letters or figures on a rubber molded article obtained from a rubber composition, by adding an additive to the rubber composition that contains one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds. A rubber composition for use in a method for improving the visibility of letters or figures on the rubber composition described in [2] [1]. [3] A rubber composition for improving the visibility of letters or figures on a rubber molded article, comprising rubber and an additive, wherein the additive comprises one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds, and the content of the additive per 100 parts by mass of rubber is more than 5 parts by mass and less than 7 parts by mass. [Effects of the Invention]

[0011] According to the present invention, it is possible to improve the visibility of characters or figures with raised or recessed patterns embossed on a rubber molded body obtained from a rubber composition, or characters or figures printed on the surface of a rubber molded body. [Modes for carrying out the invention]

[0012] The present invention provides a method for improving the visibility of letters or figures on a rubber molded article by adding one or more substances selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds (hereinafter also referred to as "additives") to a rubber composition. Although the reason is not entirely clear, it is presumed that aliphatic amines, polyoxyethylene alkylamines, or maleic acid ester compounds migrate to the surface of the rubber molded body, improving reflectivity and increasing gloss, thereby improving the visibility of letters and figures printed on the rubber molded body. This effect is particularly pronounced when the outlines of letters and figures have irregularities. Furthermore, even if the surface of the rubber molded body is damaged by repeated use, the gloss is not reduced because the aliphatic amines, polyoxyethylene alkylamines, or maleic acid ester compounds are exposed from within the rubber molded body.

[0013] [Rubber composition] The rubber composition of the present invention comprises rubber and an additive in one or more embodiments. The rubber composition of the present invention comprises rubber and an additive, and may further comprise other components (e.g., reinforcing additives). The following describes each component (rubber, additives, and other components) in the rubber composition.

[0014] <rubber> Examples of rubber used in the rubber composition of the present invention include diene-based rubber, specifically conjugated diene-based rubber. While the main application of the rubber composition of the present invention is tires, the rubber that can be used in the rubber composition of the present invention is not limited to diene-based rubber. Examples of diene rubbers include at least one selected from the group consisting of natural rubber (NR) and synthetic diene rubbers.

[0015] Examples of synthetic diene rubbers include polybutadiene rubber (BR), synthetic polyisoprene rubber (IR), styrene-butadiene copolymer rubber (SBR), and styrene-isoprene copolymer rubber (SIR).

[0016] Of these, the diene rubber is preferably made of styrene-butadiene copolymer rubber (SBR) because it has a small tanδ and reduces the rolling resistance of the tire. The SBR content in the diene rubber is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, and 100% by mass or less.

[0017] Diene rubber may be used alone or in combination of two or more types. Furthermore, the diene rubber used may be modified or unmodified.

[0018] From the viewpoint of exhibiting rubber-derived physical properties, the rubber content in the rubber composition is preferably 30% by mass or more, more preferably 38% by mass or more, even more preferably 45% by mass or more, and preferably 70% by mass or less, more preferably 63% by mass or less, and even more preferably 55% by mass or less.

[0019] <Additives> The rubber composition of the present invention contains an additive comprising one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds.

[0020] <Aliphatic amine> Examples of aliphatic amines include primary to tertiary aliphatic amines. Further, the primary to tertiary aliphatic amines may be alkanolamines. Specific examples of primary aliphatic amines include methylamine, ethylamine, propylamine, isopropylamine, butylamine, isobutylamine, 2-aminobutane, 2-amino-2-methylpropane, pentylamine, isopentylamine, 2-amino-2-methylbutane, hexylamine, heptylamine, octylamine, 2-ethylhexylamine, nonylamine, decylamine, monoethanolamine, monoisopropanolamine, 2-amino-1-butanol, 2-amino-2-methyl-1-propanol, 2-amino-2-methyl-1,3-propanediol, 2-amino-2-ethyl-1,3-propanediol, ethylenediamine, coconut amine, laurylamine, stearylamine, oleylamine, etc. Specific examples of secondary aliphatic amines include dimethylamine, diethylamine, cipropropylamine, diisopropylamine, N-methylethylamine, N-ethylhexylamine, diethanolamine, ethylmonoethanolamine, dipropanolamine, diisopropanolamine, etc. Specific examples of tertiary aliphatic amines include triethylamine, tributylamine, tripropylamine, trioctylamine, trilaurylamine, methyldiethanolamine, ethyldiethanolamine, diethylethanolamine, dimethylhexylamine, dimethyldodecylamine, dimethylcoconut amine, dimethyloctylamine, dimethyldecylamine, dimethyllaurylamine, dimethylpalmitylamine, dimethylstearylamine, dimethylmyristylamine, dimethylbehenylamine, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2,2'-(n-butylimino)diethanol, triethanolamine, etc.

[0021] Among these, primary or tertiary aliphatic amines are preferred, primary or tertiary higher alkylamines are more preferred, and tertiary higher alkylamines are still more preferred. The higher alkyl group means an alkyl group having 12 to 24 carbon atoms. Examples of the primary higher alkylamine include coconut amine, octylamine, laurylamine, stearylamine, oleylamine, etc., and examples of the tertiary higher alkylamine include dimethyl coconut amine, dimethyl octylamine, dimethyl decylamine, dimethyl laurylamine, dimethyl palmitylamine, dimethyl stearylamine, dimethyl myristylamine, dimethyl behenylamine, etc. Among these, tertiary higher alkylamines are more preferred, dimethyl palmitylamine and dimethyl stearylamine are still more preferred, and dimethyl stearylamine is still more preferred.

[0022] <Polyoxyethylene alkylamine> Examples of the polyoxyethylene alkylamine include higher alkylamine ethylene oxide (EO) adducts such as polyoxyethylene stearylamine, polyoxyethylene laurylamine, and N,N-bis(polyoxyethylene) stearylamine. Among these, from the viewpoint of visibility, polyoxyethylene stearylamine and polyoxyethylene laurylamine are preferred. In one or more embodiments, the polyoxyethylene alkylamine is a nonionic surfactant.

[0023] <Maleic acid ester compound> Examples of the maleic acid ester compound include polyoxyalkylene alkyl maleic acid esters. Polyoxypropylene alkyl maleic acid esters and polyoxyethylene alkyl maleic acid esters are preferred. Among these, polyoxypropylene alkyl maleic acid esters are preferred, and the added molar number of oxypropylene is preferably 2 or more and 8 or less.

[0024] From the viewpoint of visibility, polyoxyethylene alkyl maleate and polyoxypropylene alkyl maleate are more preferred as maleate compounds. Polyoxyethylene (6) alkyl maleate is even more preferred as polyoxyethylene alkyl maleate. Polyoxypropylene alkyl maleate is even more preferred as polyoxypropylene alkyl maleate with 2 to 7 oxypropylene additions, and polyoxypropylene (3) alkyl maleate and polyoxypropylene (6) alkyl maleate are even more preferred.

[0025] Among aliphatic amines, polyoxyethylene alkylamines, or maleic acid ester compounds, aliphatic amines are preferred as additives to the rubber composition of the present invention from the viewpoint of visibility.

[0026] The content of aliphatic amines, polyoxyethylene alkylamines, or maleic acid ester compounds in a rubber composition is, as described later, preferably 1 part by mass or more, more preferably 2 parts by mass or more, even more preferably 4 parts by mass or more, even more preferably more than 5 parts by mass, and preferably 15 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 8 parts by mass or less, from the viewpoint of improving the visibility of letters or figures on the rubber molded article obtained from the rubber composition, per 100 parts by mass of rubber.

[0027] <Other ingredients> In addition to the rubber described above, the rubber composition according to the present invention may also contain, to the extent that it does not impair the objectives of the present invention, reinforcing fillers, and rubber additives such as coupling agents, vulcanizing agents such as sulfur, zinc oxide, vulcanization accelerators, softeners such as oil, stearic acid, and anti-aging agents. Examples of vulcanization accelerators include N-cyclohexyl-2-benzothiazole sulfenamide and 1,3-diphenylguanidine. Examples of anti-aging agents include N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine.

[0028] Examples of reinforcing fillers include organic fillers such as carbon black of GPF, FEF, HAF, ISAF, and SAF grades, and functionalized polyvinyl aromatic fillers, as well as inorganic fillers such as silica, aluminum hydroxide, clay, talc, calcium carbonate, and zeolite. The silica can be wet silica (hydrated silica), dry silica (anhydrous silica), calcium silicate, aluminum silicate, etc. These components may be used individually or in combination of two or more.

[0029] From the viewpoint of the processability of the rubber composition, the content of the reinforcing filler in the rubber composition is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, and even more preferably 70 parts by mass or less, per 100 parts by mass of rubber. Furthermore, the content of the reinforcing filler in the rubber composition is preferably 2 parts by mass or more, more preferably 5 parts by mass or more, even more preferably 10 parts by mass or more, and preferably less than 240 parts by mass, more preferably 120 parts by mass or less, and even more preferably 60 parts by mass or less, relative to 1 part by mass of an additive containing one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds.

[0030] The coupling agent is usually a silane coupling agent, preferably a silane coupling agent having a sulfur atom. Specific examples of silane coupling agents having a sulfur atom 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-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane, 2-mercaptoethyltriethoxysilane, 3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyltetrasulfide, and 3-triethoxysilylpropyl Examples include pyr-N,N-dimethylthiocarbamoyl tetrasulfide, 2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, 3-trimethoxysilylpropylbenzothiazolyl tetrasulfide, 3-triethoxysilylpropylbenzothiazolyl tetrasulfide, 3-triethoxysilylpropyl methacrylate monosulfide, 3-trimethoxysilylpropyl methacrylate monosulfide, bis(3-diethoxymethylsilylpropyl)tetrasulfide, 3-mercaptopropyldimethoxymethylsilane, dimethoxymethylsilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide, and dimethoxymethylsilylpropylbenzothiazolyl tetrasulfide.

[0031] The content of the silane coupling agent in the rubber composition is preferably 2 parts by mass or more, more preferably 4 parts by mass or more, preferably 15 parts by mass or less, more preferably 12 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of silica.

[0032] Of the rubber additives, the content of oil in the rubber composition is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, from the viewpoint of softening the rubber composition and improving its processability. The content of rubber additives other than coupling agents and oils in the rubber composition is preferably 10 parts by mass or more, more preferably 30 parts by mass or more, and preferably 150 parts by mass or less, and more preferably 100 parts by mass or less, per 100 parts by mass of rubber, from the viewpoint of the reinforcing properties of the rubber molded article obtained from the rubber composition.

[0033] In this invention, by adding the aforementioned aliphatic amine, polyoxyethylene alkylamine, or maleic acid ester compound to the rubber composition in addition to rubber additives such as reinforcing fillers, the visibility of characters or figures printed or processed by embossing or printing on the surface of a rubber molded article obtained from the rubber composition is improved. This effect is achieved because the aliphatic amine, polyoxyethylene alkylamine, or maleic acid ester compound migrates onto the surface of the rubber composition or its molded article, increasing its reflectivity and gloss. The gloss, which has a slightly higher reflectivity, provides contrast, and it is believed that the visibility of characters or figures on the surface of the rubber molded article obtained from the rubber composition is improved. The effect of improved visibility is particularly noticeable when the outlines of characters or figures have irregularities. Furthermore, even if the surface of the rubber molded article obtained from the rubber composition is damaged by repeated use, the visibility does not decrease because the gloss from which the aliphatic amine, polyoxyethylene alkylamine, or maleic acid ester compound emerges from within improves visibility.

[0034] <Method for manufacturing rubber composition> The rubber composition according to the present invention can be manufactured, for example, by compounding and mixing each component contained in the rubber composition using a kneader such as a Banbury mixer, roll mixer, or intensive mixer.

[0035] In this case, from the viewpoint of suppressing vulcanization during the manufacture of the rubber composition and improving handling during manufacture, it is preferable to manufacture the rubber composition by first blending and mixing the components of the rubber composition, excluding the vulcanizing agent and vulcanization accelerator (first kneading step), and then blending and mixing the vulcanizing agent and vulcanization accelerator (second kneading step). With this method, vulcanization does not occur even if the first kneading step is carried out under high temperature conditions, and the rubber composition of the present invention can be manufactured with high productivity.

[0036] In the first kneading step, the mixing temperature is preferably 250°C or lower, more preferably 200°C or lower, and even more preferably 170°C or lower, from the viewpoint of suppressing thermal decomposition of each component, and preferably 90°C or higher, more preferably 110°C or higher, and even more preferably 130°C or higher, from the viewpoint of productivity.

[0037] In the second kneading step, the mixing temperature is preferably 130°C or lower, more preferably 120°C or lower, from the viewpoint of suppressing vulcanization during mixing, and preferably 80°C or higher, more preferably 90°C or higher, from the viewpoint of productivity.

[0038] [Rubber molded body] The rubber composition of the present invention is used in a method for improving the visibility of letters or figures on a rubber molded article obtained from the rubber composition. In one or more embodiments, the rubber molded article of the present invention is obtained by vulcanizing an unvulcanized rubber composition (the rubber composition of this disclosure). The unvulcanized rubber composition can be molded by a known method, heated or heated and pressurized, and then vulcanized to form a rubber molded article. The aforementioned rubber molded body is suitably used in rubber products that require the printing or processing of characters or figures by embossing or printing. For example, by manufacturing a tire, i.e., a pneumatic tire, using the rubber composition of the present invention, a highly visible model name, manufacturing number, or year and week of manufacture can be displayed on the sidewall. The applications of the aforementioned rubber molded body are not limited to automobile tires, but are suitably used in products that require the printing or processing of part numbers, model numbers, manufacturing numbers, or dates, such as bicycle tires, pipe joints, pedal rubber pads, tool grips, car wipers, and pump systems. [Examples]

[0039] The present invention will be specifically described below with reference to examples, but the present invention is not limited in any way by these examples. In the table, the numerical values ​​for the composition are the solid content values, and "%" means "mass%" unless otherwise specified.

[0040] [Example 1] [Preparation of rubber composition] Using the formulation shown in Table 1, a rubber composition was prepared by kneading in the following order using a standard Banbury mixer: the first kneading step followed by the second kneading step. In the first mixing step, 100 parts by mass of styrene-butadiene copolymer rubber (SBR) (manufactured by Nippon Zeon Co., Ltd., product name "NS210"), 5 parts by mass of carbon black (manufactured by Tokai Carbon Co., Ltd., product name "Seasto 3 (HAF)"), and silica (manufactured by Tosoh Silica Co., Ltd., product name "Nipsil") are mixed. 60 parts by mass of AQ, 4.8 parts by mass of bis(triethoxysilylpropyl)tetrasulfide (manufactured by Evonik, trade name "Si69") as a silane coupling agent, 2 parts by mass of stearic acid (manufactured by Kao Corporation, trade name "Lunaq® S70-V"), 1 part by mass of N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine (manufactured by Tokyo Chemical Industry Co., Ltd.) as an antioxidant, 15 parts by mass of oil (manufactured by Nippon Sun Oil Co., Ltd., trade name "Sansen 410"), and 6 parts by mass of additive 1, dimethylpalmitylamine (manufactured by Kao Corporation, trade name "Farmin® DM6098") were mixed and kneaded in a Banbury mixer for 4 minutes at a maximum temperature of 150°C.

[0041] Next, in the second kneading step, 3 parts by mass of zinc oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 1.5 parts by mass of sulfur (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 1.8 parts by mass of N-cyclohexyl-2-benzothiazole sulfenamide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as vulcanization accelerator 1, and 1.5 parts by mass of 1,3-diphenylguanidine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as vulcanization accelerator 2 were added to the kneaded product obtained in the first kneading step, and the mixture was kneaded in the Banbury mixer for 2 minutes and 30 seconds at a maximum temperature of 110°C to obtain an unvulcanized rubber composition.

[0042] <Reflectance Test> The unvulcanized rubber composition obtained in Example 1 was heated at 160°C for 15 minutes to obtain a 2 mm thick sheet-like vulcanized rubber molded article. The reflectance of the rubber sheet was measured using a colorimeter (Z-300A, manufactured by Nippon Denshoku Industries Co., Ltd.), and the relative value was determined when the reflectance of the rubber sheet of Comparative Example 1, which did not use the additives described later, was set to 100.

[0043] <Visibility Test> The vulcanized rubber molded sheets described above were left standing at 25°C for two weeks, and then visually observed according to the evaluation criteria below. Visual comparison and evaluation were performed using the sheet of Comparative Example 1, which had no gloss, as the reference. 1: Glossy 2: No gloss 3: It is bleached.

[0044] [Examples 2] to [Examples 8] An unvulcanized rubber composition was obtained in the same manner as in Example 1, except that additive 1 was changed to additives 2 to 8. Specifically, using the formulation shown in Table 1 and a standard Banbury mixer, in the same manner as in Example 1, in the first kneading step, components other than zinc oxide, sulfur, and vulcanization accelerators 1 and 2 were kneaded at a maximum temperature of 150°C, and in the second kneading step, zinc oxide, sulfur, and vulcanization accelerators 1 and 2 were kneaded at a maximum temperature of 110°C to obtain an unvulcanized rubber composition. A vulcanized rubber molded sheet was obtained in the same manner as in Example 1, and then the reflectivity was measured and the visibility was evaluated. The results are shown in Table 1.

[0045] In Table 1, the components are as follows: • Rubber: Styrene-butadiene copolymer rubber, manufactured by Zeon Corporation, SBR, product name "NS210" • Carbon Black: Manufactured by Tokai Carbon Co., Ltd., product name "Seast 3 (HAF)" • Silica: Manufactured by Tosoh Silica Co., Ltd., product name "Nipsil AQ" • Silane coupling agent: Bis(3-triethoxysilylpropyl)tetrasulfide, manufactured by Evonik, trade name "Si69"

[0046] • Stearic acid: Manufactured by Kao Corporation, product name "Lunac (registered trademark) S70-V" • Anti-aging agent: N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine, manufactured by Tokyo Chemical Industry Co., Ltd. • Oil: Manufactured by Nippon Sun Oil Co., Ltd., product name "Sunsen 410" • Zinc oxide: Manufactured by Fujifilm Wako Pure Chemical Corporation • Sulfur: Manufactured by Fujifilm Wako Pure Chemical Corporation

[0047] • Vulcanization accelerator 1: N-cyclohexyl-2-benzothiazole sulfenamide, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. • Vulcanization accelerator 2: 1,3-diphenylguanidine, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. • Additive 1: Dimethylpalmitylamine, manufactured by Kao Corporation, "Farmin (registered trademark) DM6098" • Additive 2: Dimethylstearylamine, manufactured by Kao Corporation, "Farmin (registered trademark) DM8680" • Additive 3: Dimethylstearylamine, manufactured by Kao Corporation, "Farmin (registered trademark) DM8098"

[0048] • Additive 4: Polyoxyethylene stearylamine, manufactured by Kao Corporation as "Amite (registered trademark) 320" • Additive 5: Polyoxyethylene laurylamine, manufactured by Kao Corporation as "Amite (registered trademark) 105" • Additive 6: Polyoxyethylene alkyl maleate, C12-EO6-maleic acid • Additive 7: Polyoxypropylene alkyl maleate, C12-PO6-maleic acid • Additive 8: Polyoxypropylene alkyl maleate, C12-PO3-maleic acid

[0049] [Comparative Example 1] A sheet-like vulcanized rubber molded article was obtained in the same manner as in Example 1, except that additive 1 was not added. A vulcanized rubber molded sheet was obtained in the same manner as in Example 1, and then the reflectivity was measured and the visibility was evaluated. The results are shown in Table 1.

[0050] [Comparative Example 2] In Example 1, the reflectance was measured and visibility was evaluated in the same manner as in Example 1, except that calcium stearate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), which is additive C1, was added instead of additive 1. The results are shown in Table 1.

[0051] Examples 1-8, which used additives 1-8, showed higher reflectivity and superior visibility compared to the reference comparative example 1. Amine compounds yielded good results in terms of visibility, with dimethylpalmitylamine showing the best results. Among polyoxyethylene alkylamines, Ami 105 showed the highest effect. Among polyoxyalkylene alkyl maleate esters (additives 6-8), PO addition was more effective than EO addition, with additive 8 showing the highest effect due to its smaller number of PO additions. On the other hand, the rubber molded article of Comparative Example 1, which did not use the additives according to the present invention, lacked gloss, and the rubber molded article of Comparative Example 2, which had calcium stearate (additive C1) added instead of additives 1-8, showed whitening of the rubber molded article's surface due to the calcium stearate. The whitening of the rubber molded article of Comparative Example 2 was so severe that it was unsuitable for practical use, and therefore, reflectivity measurements were not performed.

[0052] [Table 1] [Industrial applicability]

[0053] The rubber composition of the present invention is suitably used in rubber products that are printed or processed with letters or figures by embossing or printing, such as car tires, pipe joints, pedal rubber pads, tool grips, car wipers, and pump systems.

Claims

1. A method for improving the visibility of letters or figures on a rubber molded article obtained from a rubber composition, by adding an additive to the rubber composition that contains one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds.

2. The method for improving visibility according to claim 1, wherein the aliphatic amine is a tertiary higher alkylamine.

3. The method for improving visibility according to claim 1, wherein the maleic acid ester compound is a polyoxyalkylene alkyl maleic acid ester.

4. A rubber composition for use in a method for improving visibility according to any one of claims 1 to 3.

5. It contains rubber and additives, The aforementioned additive comprises one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds. A rubber composition for improving the visibility of letters or figures on a rubber molded article, wherein the content of the additive per 100 parts by mass of rubber is more than 5 parts by mass and less than 7 parts by mass.

6. Furthermore, it includes a reinforcing filler, The rubber composition according to claim 5, wherein the content of the reinforcing filler is 2 parts by mass or more and less than 240 parts by mass relative to 1 part by mass of the additive.

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