Method for improving visibility of character or figure on rubber molded body and rubber composition

Abstract

The present invention relates to a method for improving visibility of an uneven character or figure embossed on a rubber molded body formed from a rubber composition, or improving visibility of a character or figure printed on a surface of the rubber molded body.　In one aspect, the present invention relates to a method for improving visibility of a character or figure on a rubber molded body formed 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 amines are tertiary higher alkylamines, and the maleic acid ester compounds are polyoxyalkylene alkyl maleic acid esters.

Description

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

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

[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, characters or figures are printed on the surface of the molded body or embossed. 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 No. 4157608 (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 based on 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 No. 6608602 (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.

[0005] The present inventors have found that by adding an aliphatic amine, a polyoxyethylene alkylamine or a maleic acid ester compound to the rubber composition, the problems of the above-mentioned prior art can be solved.

[0006] 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. [2] A rubber composition for use in the method for improving the visibility of letters or figures on a rubber composition described in [1]. [3] A rubber composition for improving the visibility of letters or figures on a rubber molded article obtained, comprising rubber and an additive, wherein the additive contains 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.

[0007] 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.

[0008] 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 rubber compositions containing the additive composition have excellent fracture resistance.

[0009] However, the aforementioned prior art has not considered how to improve the visibility of characters or figures on a molded body while maintaining the performance of the rubber composition. The present invention relates to a method for improving the visibility of characters or figures with raised or recessed areas embossed on a rubber molded body, or characters or figures printed on the surface of a rubber molded body.

[0010] 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.

[0011] The present invention provides a method for improving the visibility of characters or figures on a rubber molded body 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 the 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 characters and figures printed on the rubber molded body. This effect is particularly pronounced when the outlines of the characters or figures have irregularities. Furthermore, even if the surface of the rubber molded body is damaged by repeated use, the visibility does not decrease because the gloss is derived from the aliphatic amines, polyoxyethylene alkylamines, or maleic acid ester compounds emerging from within the rubber molded body.

[0012] [Rubber Composition] In one or more embodiments, 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 contain other components (for example, reinforcing additives). The components of the rubber composition (rubber, additive, and other components) will be described below.

[0013] <Rubber> Examples of rubber used in the rubber composition of the present invention include diene rubber, specifically conjugated diene 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 rubber. Examples of diene rubber include at least one selected from the group consisting of natural rubber (NR) and synthetic diene rubber.

[0014] Examples of synthetic diene rubbers include polybutadiene rubber (BR), synthetic polyisoprene rubber (IR), styrene-butadiene copolymer rubber (SBR), and styrene-isoprene copolymer rubber (SIR).

[0015] 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.

[0016] Diene rubber may be used alone or in combination of two or more types. Furthermore, the diene rubber used may be modified or unmodified.

[0017] 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.

[0018] <Additives> The rubber composition of the present invention contains one or more additives selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds.

[0019] <Aliphatic Amines> Examples of aliphatic amines include primary to tertiary aliphatic amines. Furthermore, primary to tertiary aliphatic amines may also 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, coconutamine, laurylamine, stearylamine, and oleylamine. Specific examples of secondary aliphatic amines include dimethylamine, diethylamine, cypropylamine, and diisopropylamine. Examples of tertiary aliphatic amines include N-methylethylamine, N-ethylhexylamine, diethanolamine, ethylmonoethanolamine, dipropanolamine, and diisopropanolamine. Specific examples of tertiary aliphatic amines include triethylamine, tributylamine, tripropylamine, trioctylamine, trilaurylamine, methyldiethanolamine, ethyldiethanolamine, diethylethanolamine, dimethylhexylamine, dimethyldodecylamine, dimethylcoconutamine, dimethyloctylamine, dimethyldecylamine, dimethyllaurylamine, dimethylpalmitylamine, dimethylstearylamine, dimethylmyristylamine, dimethylbehenylamine, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2,2'-(n-butylimino)diethanol, and triethanolamine.

[0020] Of these, primary or tertiary aliphatic amines are preferred, primary or tertiary higher alkylamines are more preferred, and tertiary higher alkylamines are even more preferred. A higher alkyl group refers to an alkyl group having 12 to 24 carbon atoms. Primary higher alkylamines include coconutamine, octylamine, laurylamine, stearylamine, and oleylamine, while tertiary higher alkylamines include dimethylcoconutamine, dimethyloctylamine, dimethyldecylamine, dimethyllaurylamine, dimethylpalmitylamine, dimethylstearylamine, dimethylmyristylamine, and dimethylbehenylamine. Of these, tertiary higher alkylamines are more preferred, dimethylpalmitylamine and dimethylstearylamine are even more preferred, and dimethylstearylamine is even more preferred.

[0021] <Polyoxyethylene alkylamines> Examples of polyoxyethylene alkylamines include polyoxyethylene stearylamine, polyoxyethylene laurylamine, and higher alkylamine ethylene oxide (EO) adducts such as N,N-bis(polyoxyethylene)stearylamine. Of these, polyoxyethylene stearylamine and polyoxyethylene laurylamine are preferred from the viewpoint of visibility. In one or more embodiments, the polyoxyethylene alkylamine is a nonionic surfactant.

[0022] <Maleic acid ester compounds> Examples of the maleic acid ester compounds include polyoxyalkylene alkyl maleic acid esters. Polyoxypropylene alkyl maleic acid esters and polyoxyethylene alkyl maleic acid esters are preferred. Of these, polyoxypropylene alkyl maleic acid esters are preferred, and the number of moles of oxypropylene added is preferably 2 or more and 8 or less.

[0023] As maleic acid ester compounds, polyoxyethylene alkyl maleic acid esters and polyoxypropylene alkyl maleic acid esters are more preferred from the viewpoint of visibility. As polyoxyethylene alkyl maleic acid esters, polyoxyethylene (6) alkyl maleic acid esters are even more preferred. As polyoxypropylene alkyl maleic acid esters, polyoxypropylene alkyl maleic acid esters with 2 to 7 oxypropylene additions are more preferred, and polyoxypropylene (3) alkyl maleic acid esters and polyoxypropylene (6) alkyl maleic acid esters are even more preferred.

[0024] 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.

[0025] 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 characters or figures on the rubber molded article obtained from the rubber composition, per 100 parts by mass of rubber.

[0026] <Other Components> In addition to the rubber described above, aliphatic amines, polyoxyethylene alkylamines, or maleic acid ester compounds, 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 as appropriate. 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.

[0027] 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.

[0028] 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, and even more preferably 10 parts by mass or more, per 1 part by mass of additive containing one or more selected from the group consisting of aliphatic amines, polyoxyethylene alkylamines, and maleic acid ester compounds, 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.

[0029] 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 These 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, among others.

[0030] 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.

[0031] 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, per 100 parts by mass of rubber, from the viewpoint of softening the rubber composition and improving its processability. The content of coupling agents and rubber additives other than oil 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 reinforcing the rubber molded article obtained from the rubber composition.

[0032] 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.

[0033] <Method for producing the rubber composition> The rubber composition according to the present invention can be produced by compounding and mixing each component contained in the rubber composition using a kneader such as a Banbury mixer, roll mixer, or intensive mixer.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] [Rubber Molded Article] 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 to obtain a vulcanized rubber molded article. The rubber molded article is suitably used in rubber products that need to have letters or figures printed or processed by embossing or printing. For example, by manufacturing a tire, i.e., a pneumatic tire, using the rubber composition of the present invention, it is possible to display a highly visible model name, manufacturing number, or year and week of manufacture on the sidewall. The use of the rubber molded article is not limited to automobile tires, but is suitably used in products that have part numbers, model numbers, manufacturing numbers or dates printed or processed on them, such as bicycle tires, pipe joints, pedal rubber pads, tool grips, car wipers, and pump systems.

[0038] 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 "%" refers to "mass%" unless otherwise specified.

[0039] [Example 1] [Preparation of Rubber Composition] Using the formulation shown in Table 1, a rubber composition was prepared by kneading in the following order: first kneading step, then second kneading step, using a standard Banbury mixer. In the first kneading step, 100 parts by mass of styrene-butadiene copolymer rubber (SBR) (manufactured by Nippon Zeon Co., Ltd., trade name "NS210"), 5 parts by mass of carbon black (manufactured by Tokai Carbon Co., Ltd., trade name "Seasto 3 (HAF)"), and silica (manufactured by Tosoh Silica Co., Ltd., trade name "NipsiL 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.

[0040] 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.

[0041] <Reflectivity Test>The unvulcanized rubber composition obtained in Example 1 was heated at 160 °C for 15 minutes to obtain a vulcanized rubber molded body in the form of a sheet with a thickness of 2 mm. Using a colorimetric color difference meter (Z-300A manufactured by Nippon Denshoku Industries Co., Ltd.), the reflectivity of the rubber sheet was measured, and the relative value was determined when the reflectivity of the rubber sheet of Comparative Example 1 that did not use the additives described below was set to 100.

[0042] <Visibility Test>After leaving the sheet of the vulcanized rubber molded body standing at 25 °C for 2 weeks, visual observation was carried out according to the following evaluation criteria. Visual comparative evaluation was performed based on the sheet of Comparative Example 1 having no gloss. 1: Glossy 2: No gloss 3: Whitened

[0043] [Example 2] to [Example 8] In Example 1, 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. That is, according to the formulation shown in Table 1, using an ordinary Banbury mixer, in the first kneading step, the 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. In the same manner as in Example 1, after obtaining a vulcanized rubber molded body sheet, measurement of reflectivity and visibility evaluation were carried out. The results are shown in Table 1.

[0044] In Table 1, each component is as follows. - Rubber: Styrene-butadiene copolymer rubber, manufactured by Nippon Zeon Co., Ltd., SBR, trade name "NS210" - Carbon black: manufactured by Tokai Carbon Co., Ltd., trade name "Seast 3 (HAF)" - Silica: manufactured by Tosoh Silica Corporation, trade name "Nipsil AQ" - Silane coupling agent: bis(3-triethoxysilylpropyl)tetrasulfide, manufactured by Evonik, trade name "Si69"

[0045] - Stearic acid: Manufactured by Kao Corporation, trade name "Lunac (registered trademark) S70-V" - Antioxidant: Manufactured by Tokyo Chemical Industry Co., Ltd., N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine - Oil: Manufactured by Nippon Sun Oil Co., Ltd., trade name "Sun Sen 410" - Zinc oxide: Manufactured by Fujifilm Wako Pure Chemical Corporation - Sulfur: Manufactured by Fujifilm Wako Pure Chemical Corporation

[0046] - Vulcanization accelerator 1: Manufactured by Fujifilm Wako Pure Chemical Corporation, N-cyclohexyl-2-benzothiazolesulfenamide - Vulcanization accelerator 2: Manufactured by Fujifilm Wako Pure Chemical Corporation, 1,3-diphenylguanidine - Additive 1: Dimethylpalmitylamine, "Farmine (registered trademark) DM6098" manufactured by Kao Corporation - Additive 2: Dimethylstearylamine, "Farmine (registered trademark) DM8680" manufactured by Kao Corporation - Additive 3: Dimethylstearylamine, "Farmine (registered trademark) DM8098" manufactured by Kao Corporation

[0047] - Additive 4: Polyoxyethylene stearylamine, "Amito (registered trademark) 320" manufactured by Kao Corporation - Additive 5: Polyoxyethylene laurylamine, "Amito (registered trademark) 105" manufactured by Kao Corporation - 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

[0048] 〔Comparative Example 1〕 In Example 1, a sheet-shaped vulcanized rubber molded body was obtained in the same manner as in Example 1, except that Additive 1 was not added. After obtaining the vulcanized rubber molded body sheet in the same manner as in Example 1, the reflectance was measured and the visibility was evaluated. The results are shown in Table 1.

[0049] 〔Comparative Example 2〕 In Example 1, calcium stearate (manufactured by Fujifilm Wako Pure Chemical Corporation), which is Additive C1, was added instead of Additive 1, and the reflectance was measured and the visibility was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0050] In Examples 1 to 8, which used additives 1 to 8, the reflectivity was higher and visibility was superior 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 to 8), PO addition was more effective than EO addition, and additive 8, which has fewer PO additions, showed the highest effect. 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 to 8, had a whitened 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 actual use, and therefore the reflectivity was not measured.

[0051]

[0052] 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 or 2, 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. 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.

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

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