Electrically conductive rubber composition, molded rubber article, and process for producing the electrically conductive rubber composition

Abstract

An electrically conductive rubber composition with excellent electrical conductivity and colorability, a molded rubber article made from the composition, and a process for producing the composition are made available. The electrically conductive rubber composition contains rubber, carbon nanotubes, and a coloring agent, with the amount of carbon nanotubes ranging from 0.45 parts by weight to 4.5 parts by weight per 100 parts by weight of the rubber.

Description

Title of the invention: Electrically conductive rubber composition, molded rubber article, and method for producing the electrically conductive rubber composition technical field

[0001] The present invention relates to an electrically conductive rubber composition, a molded rubber article made of the composition, and the like. Background of the art

[0002] An automobile axle is supported by a roller bearing, and a bearing seal is used to prevent grease leakage or the ingress of muddy water for the bearing. There have recently been requests for a bearing seal to not only exhibit electrical conductivity but also be colorable for identification purposes. For a bearing seal, an electrically conductive molded rubber article is used. For example, patent literature 1 discloses, as an electrically conductive rubber composition contained in a molded rubber article, an electrically conductive colored rubber in which carbon fibers are contained as filler and which is colored with a pigment to achieve a desired color. List of quotations [Patent literature]

[0003] [Patent Literature 1]

[0004] Published Japanese patent application, Tokukaisho, No. 61-158611 Summary of the invention Technical problem

[0005] However, the conventional technique above has room for improvement from the point of view of obtaining both electrical conductivity and colourability.

[0006] An object of one aspect of the present invention consists of making available: an electrically conductive rubber composition having excellent electrical conductivity and colorability; a molded rubber article made of the composition; and a process for producing the composition. Solution to the problem

[0007] In order for the object to be reached, an electrically charged rubber composition conductor according to one aspect of the present invention comprises: rubber, carbon nanotubes, and a coloring agent, wherein the carbon nanotubes are contained in an amount of 0.45 parts by weight to 4.5 parts by weight per 100 parts by weight of the rubber.

[0008] Furthermore, in order to achieve the objective, a method for producing an electrically conductive rubber composition according to one aspect of the present invention comprises: a mixing step comprising the mixing of rubber, carbon nanotubes, and a coloring agent, in which, in the mixing stage, the carbon nanotubes are mixed in an amount of 0.45 parts by weight to 4.5 parts by weight per 100 parts by weight of the rubber. Advantageous effects of the invention

[0009] One aspect of the present invention provides: an electrically conductive rubber composition having excellent electrical conductivity and colorability; a molded rubber article made of the composition; and a process for producing the composition. Description of implementation methods

[0010] The following description will discuss one embodiment of the present invention in detail. However, the present invention is not limited to the following embodiment but may be altered in various ways within the scope of the description. An embodiment derived from a combination of technical means disclosed in different embodiments is also encompassed by the technical scope of the present invention.

[0011] In this specification, a numerical range "from A to B" means "not less than A and not greater than B". 1. Electrically conductive rubber composition

[0012] The use of electrically conductive carbon black, such as Ketjen black, and / or carbon fibers as fillers has commonly been proposed to enable rubber to be electrically conductive. However, when electrically conductive carbon black and / or carbon fibers are used as fillers, a large amount of the filler must be present to allow the resulting rubber composition to be electrically conductive. Because electrically conductive carbon black and carbon fibers are black, the resulting rubber composition is black. This makes it difficult to color the rubber composition, even when a coloring agent such as a pigment is added.

[0013] Consequently, as a result of diligent research to achieve this objective, the inventors of the present invention focused on the type of filler used. Specifically, the inventors of the present invention discovered for the first time that by using carbon nanotubes as a filler, it is possible to make rubber electrically conductive with a small charge content, and that coloring a resulting molded rubber article is therefore possible by adding a coloring agent.

[0014] An electrically conductive rubber composition according to an embodiment of the present invention (also referred to simply as "rubber composition" in this document) contains a rubber. Examples of rubber include: fluorinated rubber (FKM); nitrile butadiene rubber (NBR); acrylic rubber (ACM) obtained by copolymerization of an acrylic acid ester and a crosslinking monomer; acrylic rubber (AEM) obtained by copolymerization of an acrylic acid ester, ethylene, and a crosslinking monomer; acrylic rubber (ANM) obtained by copolymerization of an acrylic acid ester, acrylonitrile, and a crosslinking monomer; ethylene propylene rubber (EPDM); hydrogenated nitrile butadiene rubber (HNBR); and silicone rubber (VMQ).Each type of these can be used alone, or several types of these can be used in combination in any proportion. In addition, the rubber preferably contains at least one type selected from fluorinated rubber (EKM), nitrile butadiene rubber (NBR), and acrylic rubber (ACM).

[0015] The electrically conductive rubber composition according to an embodiment of the present invention contains carbon nanotubes. Examples of carbon nanotubes include single-walled and multi-walled carbon nanotubes. Single-walled carbon nanotubes are suitable when electrical conductivity is the primary requirement, and multi-walled carbon nanotubes are suitable when economic viability is the primary requirement. Although the average diameter of the carbon nanotubes is not particularly limited, the average diameter is preferably not less than 1 nm. Furthermore, the average diameter is preferably not greater than 100 nm, even better not greater than 50 nm, and more particularly not greater than 20 nm. Although the aspect ratio (average length / average diameter) is not particularly limited, the aspect ratio is preferably from 100 to 100,000.Examples of single-walled carbon nanotubes include "ZEONANO SG101" manufactured by Zeon Corporation and "TUBALL" manufactured by OSCiAl. Examples of multi-walled carbon nanotubes include "FloTube 7000" and "FloTube 9000" manufactured by CNano and "CN7000" manufactured by Nanocyl SA.

[0016] The lower limit value for the carbon nanotube content is not less than 0.45 parts by weight, preferably not less than 0.5 parts by weight, even better not less than 0.8 parts by weight, more particularly not less than 1.0 parts by weight, and especially not less than 1.6 parts by weight, per 100 parts by weight of the rubber. The upper limit value for the carbon nanotube content is not more than 4.5 parts by weight, preferably not more than 4.0 parts by weight, even better not more than 3.5 parts by weight, and more particularly 3.0 parts by weight, per 100 parts by weight of the rubber. A carbon nanotube content of not less than 0.45 parts by weight can impart sufficient electrical conductivity to a resulting molded rubber article.A carbon nanotube content not exceeding 4.5 parts by weight can impart sufficient colorability to a resulting molded rubber article.

[0017] Furthermore, the electrically conductive rubber composition according to an embodiment of the present invention contains a coloring agent. A coloring agent usable in an embodiment of the present invention is not limited to any particular one, and various coloring agents commonly known may be used. Examples of coloring agents include a pigment and a dye. Among these, a pigment is preferable.

[0018] Examples of pigments include organic pigments, inorganic pigments, and fluorescent pigments. Examples of pigment shades include brown, white, red, blue, yellow, and green. Each type of these pigments can be used alone, or several types of these pigments can be used in combination in any proportion. In the following description, a brown pigment will be discussed as an example. Examples of brown pigments include red iron oxide (ferric oxide).

[0019] Examples of dyes include reactive dyes, direct dyes, and fluorescent dyes. Examples of dye shades include brown, white, red, blue, yellow, and green. Each type of dye can be used alone, or several types of dyes can be used in combination in any proportion.

[0020] From the point of view of sufficient coloring of the resulting molded rubber article, the lower limit value of the coloring agent content is preferably not less than 0.7 parts by weight, better still not less than 1.0 parts by weight, and more particularly not less than 3.0 parts by weight, per 100 parts by weight of the rubber. From the point of view of the physical properties of the resulting molded rubber article, the upper limit value of the coloring agent content is preferably not more than 25 parts by weight, better still not more than 20 parts by weight, and more particularly not more than 15 parts by weight, for 100 parts by weight of the rubber.

[0021] The electrically conductive rubber composition according to an embodiment of the present invention preferably contains a white-based filler, from the point of view of color tint and electrical conductivity. The white-based filler is not limited to any particular type, and fillers commonly used in rubber compositions may be used. Examples of such fillers include: inorganic fillers such as silica, clay, calcium carbonate, diatomaceous earth, wollastonite, barium sulfate, and titanium dioxide; and organic fillers such as cellulose powder, reclaimed rubber, and powdered rubber. Among these, inorganic fillers are preferred, and from the point of view of improving adhesion, silica, clay, calcium carbonate, and diatomaceous earth are particularly preferred.Each type of these white-based fillers can be used alone, or several types of these white-based fillers can be used in combination in any proportion.

[0022] From the standpoint of the color tint and electrical conductivity of the resulting molded rubber article, the lower limit of the white filler content is preferably not more than 3 parts by weight, and even better not more than 5 parts by weight, per 100 parts by weight of the rubber. From the standpoint of the physical properties of the rubber in the resulting molded rubber article, the upper limit of the white filler content is preferably not more than 100 parts by weight, even better not more than 75 parts by weight, and more particularly not more than 50 parts by weight per 100 parts by weight of the rubber.

[0023] The electrically conductive rubber composition according to an embodiment of the present invention may contain components other than rubber, carbon nanotubes, and the coloring agent, provided that they do not interfere with the effects of the present invention. Examples of other components, in addition to the white-based filler described above, include various additives such as a processing aid, a vulcanizing agent, a crosslinking co-agent, a vulcanization accelerator, a vulcanization retarder, an adhesive, an acid acceptor, a plasticizer, an anti-aging agent, a coupling agent, an anti-corrosion agent, and a tackifier. 2. Molded rubber article

[0024] The molded rubber article according to an embodiment of the present invention can be obtained by molding with vulcanization of the electrically conductive rubber composition. Examples of a process for molding the com Rubber manufacturing methods include injection molding, extrusion molding, compression molding, and roller molding. Injection molding and compression molding are particularly suitable. In this case, the electrically conductive rubber compound can be pre-molded before vulcanization, or it can be vulcanized and molded simultaneously. It is also possible to vulcanize and mold the electrically conductive rubber compound simultaneously and then subject it to secondary vulcanization. The preferred vulcanization temperature is typically between 150°C and 230°C. The vulcanization time is usually from 0.1 to 60 minutes.Examples of a heating process for vulcanization include a typical process used in rubber vulcanization, such as heating by means of a heating device, steam heating, oven heating, and hot air heating.

[0025] Depending on the shape, dimensions, and similar characteristics of the molded rubber article, the inner part may not be sufficiently vulcanized even when the surface is vulcanized. It is therefore possible to further heat the molded rubber article to achieve secondary vulcanization.

[0026] The vulcanization (crosslinking) process is not limited to any particular method, and examples of the process include polyol vulcanization, sulfur vulcanization, peroxide vulcanization, amine vulcanization, triazine vulcanization, and epoxy group crosslinking. A bisphenol-based compound is used as the vulcanizing agent in polyol vulcanization. Sulfur or a sulfur compound is used as the vulcanizing agent in sulfur vulcanization. An organic peroxide is used as the vulcanizing agent in peroxide vulcanization. The amount of vulcanizing agent used in this case is usually from 0.1 parts by weight to 10 parts by weight per 100 parts by weight of the rubber. The amount of vulcanizing accelerator is usually from 0.1 parts by weight to 10 parts by weight per 100 parts by weight of rubber.

[0027] The applications of the molded rubber article are not particularly limited. The molded rubber article according to an embodiment of the present invention is preferably used in a product that must be electrically conductive and identifiable by color. Examples of such a product include a bearing seal and a gasket. 3. Physical properties of the molded rubber article (Volume resistivity)

[0028] From the point of view of electrical conductivity, the volume resistivity of the molded rubber article according to an embodiment of the present invention is preferably not greater than 1 x 10⁶ Ω*cm, or even better not greater than 1 x 102 Q*cm, and more specifically not greater than 1 x 101 Q*cm. The volume resistivity as used here is the value obtained by a measurement in accordance with Method 3 of JIS K6271-2. (Color shade)

[0029] The color tint of the molded rubber article according to an embodiment of the present invention can be evaluated using the L* value, the a* value, and the b* value, and these values ​​can be measured using a colorimeter. It should be noted that a higher L* value indicates that the black color of the molded rubber article is suppressed to a greater extent, i.e., that the colorability is greater. From the point of view of colorability, the L* value of the molded rubber article is preferably not less than 18.0 and even better not less than 18.5. (Other physical properties)

[0030] The lower limit value of the hardness of the molded rubber article according to an embodiment of the present invention is preferably not less than 50, better still not less than 60, and more particularly not less than 65. The upper limit value of the hardness of the molded rubber article is preferably not more than 87, and better still not more than 86. In this specification, the hardness is measured using a type A durometer by a method in accordance with JIS K6253.

[0031] The lower limit value of the tensile strength of the molded rubber article according to an embodiment of the present invention is preferably not less than 8.0 MPa, better still not less than 8.2 MPa, and more particularly not less than 8.4 MPa. The upper limit value of the tensile strength of the molded rubber article according to an embodiment of the present invention may, for example, not exceed 15.5 MPa.

[0032] The lower limit value of the elongation of the molded rubber article according to an embodiment of the present invention is preferably not less than 70%, even better not less than 75%, and more particularly not less than 78%. The upper limit value of the elongation of the molded rubber article according to an embodiment of the present invention may, for example, not exceed 450%. 4. A process for producing an electrically conductive rubber composition

[0033] An electrically conductive rubber composition according to an embodiment of the present invention comprises a mixing step including the mixing of rubber, carbon nanotubes, and a coloring agent. In the step For mixing, the process for combining the components is not limited to any particular method, and mixing can be carried out using an open roller mill, a mixer, a Banbury mixer, an internal mixer, an extruder, or similar equipment. Of these, it is preferable to perform the mixing using an open roller mill or a mixer. The temperature during mixing is preferably between 20°C and 160°C. 5. Summary

[0034] One embodiment of the present invention includes the following features. <1> An electrically conductive rubber composition containing rubber, carbon nanotubes, and a coloring agent, wherein the carbon nanotubes are contained in an amount of 0.45 parts by weight to 4.5 parts by weight per 100 parts by weight of the rubber. <2> The composition of electrically conductive rubber described in <1> , in which the rubber is of at least one type selected from fluorinated rubber, nitrile-butadiene rubber, and acrylic rubber. <3> The composition of electrically conductive rubber described in <1> Or <2> , in which the coloring agent is contained in an amount of 0.7 parts by weight to 25 parts by weight per 100 parts by weight of the rubber. <4> The electrically conductive rubber composition described in one of <1> has <3> in which the coloring agent is a pigment. <5> The electrically conductive rubber composition described in one of <1> has <4> , also containing at least one type of white-based filler. <6> The composition of electrically conductive rubber described in <5> , in which the white-based filler is contained in a quantity of 3 parts by weight to 100 parts by weight per 100 parts by weight of rubber. <7> A molded article made of the electrically conductive rubber composition described in one of <1> has <6> . <8> The molded rubber article described in <7> , having a volume resistivity not greater than 1 x 106 Q*cm. <9> The molded rubber article described in <7> Or <8> , in which the L* value, measured using a colorimeter, is not less than 18.0. <10> A process for producing an electrically conductive rubber composition, comprising a mixing step including the mixing of rubber, carbon nanotubes, and a coloring agent, wherein, in the mixing step, the carbon nanotubes are mixed in an amount of 0.45 parts by weight to 4.5 parts by weight per 100 parts by weight of the rubber. Examples

[0035] An embodiment of the present invention will be described in more detail below by means of examples. However, the present invention is not limited to these examples. [Materials used in the examples and comparative examples] • Rubber - Fluorinated rubber (FKM): "Viton A-500" manufactured by Chemours Company - Nitrile-butadiene rubber (NBR): "Krynac 3345 F" manufactured by ARLANXEO - Acrylic rubber (ACM): "Nipol AR31" manufactured by Zeon Corporation • Electrically conductive charge - Carbon nanotubes: "TUBALL 01RW02" manufactured by OCSiAl - Carbon black: "Ketjen black" manufactured by Lion Specialty Chemicals Co., Ltd. - Carbon fibers: "HT M100 40 MU" manufactured by Teijin Limited • White-based filler - Silica: "Nipsil VN3" manufactured by Tosoh Silica Corporation - Titanium oxide: TITON Al 10" manufactured by Sakai Chemical Industries Co., Ltd. - Barium sulfate: "Precipitated Barium Sulfate" manufactured by Sakai Chemical Industries Co., Ltd. - Clay: "DIXIE CLAY" manufactured by RT Vanderbilt Company, Inc. • Coloring agent - Pigment: "Resino Brown" manufactured by Resino Color Industry Co., Ltd. [Example 1]

[0036] A mixture or a mixture partially formed into a masterbatch, any one of which has the composition indicated in Table 1, is kneaded using an open roller machine at a temperature of 40°C to 70°C for 15 to 30 minutes, so as to prepare a sheet of unvulcanized rubber having a thickness of 2.0 mm to 3.0 mm. The unvulcanized rubber sheet is then subjected to press vulcanization at 170°C for 10 minutes, so as to obtain a sheet of vulcanized rubber having a thickness of 2 mm. The vulcanized rubber sheet is then subjected to secondary vulcanization as required. [Measurement and evaluation methods] (Volume resistance value)

[0037] The volumetric resistance value of the vulcanized rubber sheet is measured using a process in accordance with Method 3 of HS K6271-2. (Color shade)

[0038] For the color shade of the vulcanized rubber sheet obtained, the L* value, the a* value, and the b* value are measured using a colorimeter (RM200QC portable colorimeter manufactured by X-Rite, Inc.). In addition, the color is also visually verified. of the vulcanized rubber sheet. (Hardness)

[0039] The hardness of the vulcanized rubber sheet obtained is measured using a process in accordance with JIS K6253. Specifically, three vulcanized rubber sheets are superimposed, and the measurement is carried out at 23°C using a type A durometer. The peak value is read. (Tensile strength and elongation)

[0040] The tensile strength of the vulcanized rubber sheet obtained is measured by means of a tensile test. The tensile test is carried out in accordance with JIS K6251. Specifically, the tensile strength (MPa) and elongation (%) are measured using the vulcanized rubber sheet at 23°C and a tensile speed of 500 mm / min. [Examples 2 to 14 and Comparative Examples 1 to 6]

[0041] The same operations are performed as those described in Example 1, except that the types and quantities of the components are changed as indicated in Tables 1 to 3, so that a sheet of unvulcanized rubber and a sheet of vulcanized rubber are obtained. The vulcanized rubber sheet is then measured and evaluated in the same way as in Example 1. Formulation Rubber UC Exemplar 1 Exemplar 2 Exemplar 3 Exemplar 4 Exemplar 5 Exemplar 6 Exemplar 7 Fluorinated UC rubber phr 100 100 100 100 100 100 100 Nitrile-butadiene UC rubber phr - - - - - - - Acrylic UC rubber phr - - - - - - - Electrically conductive filler Carbon nanotubes phr 0.8 1.6 1.6 1.6 1.6 0.5 4.0 Carbon black phr - - - - - - - Carbon fibers phr - - - - - - - White-based filler Silica phr - - 10 - - - - Titanium oxide phr - - - 10 - 10 10 Barium sulfate phr - - - - 10 - - Clay phr - - - - - - - Coloring agent Brown pigment phr 5 5 5 5 5 5 5 Volume resistivity Q*cm 3.6 x 10² 2.4 x 10¹ 1.3 x 10¹ 1.6 x 10¹ 1.6 x 10¹ 5.4 x 10⁴ 3.0 x 10° Color tint (measured using a colorimeter) L* 21.7 18.6 21.4 19.6 28.9 26.3 22.3 a* 10.4 6.7 7.0 6.0 4.9 12.3 2.8 b* 11.0 7.4 7.0 5.9 3.6 12.5 1.2 Color tint (observed) - Brown Brown Brown Brown Brown Brown Brown (visually) dark nnn whitish n light n dark Physical properties Hardness A / O tl 75 80 84 79 79 76 86 Tensile strength MP a 8.6 10.5 14.4 9.7 9.4 9.2 14.6 Elongation % 240 160 160 270 250 220 80 Formulation Exemple 8 Exemple 9 Exemple 10 Exemple 11 Exemple 12 Exemple 13 Exemple 14 UC Rubber Fluorinated UC Rubber phr 100 100 100 100 100 - - Nitrile-Butadiene UC Rubber phr - - - - - 100 - Acrylic UC Rubber phr - - - - - - 100 Electrically Conductive Filler Carbon Nanotubes phr 1.6 1.6 1.6 1.6 1.6 3.0 1.0 Carbon Black phr - - - - - - - Carbon Fibers phr - - - - - - - White-Based Filler Silica phr - - - - - 50 - Titanium Oxide phr 10 10 5 30 10 - - Barium Sulfate phr - - - - 10 - - Clay phr - - - - - - 90 Coloring Agent Brown pigment phr 1 20 5 5 5 5 5 Volume resistivity Q*cm 2.5 x 10¹ 3.6 x 10¹ 2.0 x 10¹ 1.9 x 10¹ 2.4 x 10¹ 1.2 x 10² 9.3 x 10³ Color tint (measured using a colorimeter) L* 29.9 25.8 19.4 19.8 28.8 19.0 27.1 a* -1.8 10.6 4.9 6.2 5.5 3.9 9.0 b* 0.1 11.3 5.2 6.4 3.7 3.9 7.8 Color tint (observed - Brown Brown Brown Marro Marro Marro Marro (visually) dark nnnn whitish n dark n light Physical properties Hardness A / O tl 80 85 78 85 83 69 72 Tensile strength MP a 8.6 8.9 9.4 10.3 8.7 10.8 11.1 Elongation % 180 170 260 240 240 390 180 Formula UC Rubber Example Comparative Example 1 Example Comparative Example 2 Example Comparative Example 3 Example Comparative Example 4 Example Comparative Example 5 Example Comparative Example 6 Fluorinated rubber phr 100 100 100 100 100 - Nitrile-butadiene rubber phr - - - - - 100 Acrylic rubber phr - - - - - - Electrically conductive filler Carbon nanotubes phr 0.4 5.0 1.6 1.6 - - Carbon black phr - - - - 4.0 - Carbon fibers phr - - - - - 40 White-based filler Silica phr - - - - - 50 Titanium oxide phr - 10 10 10 10 - Barium sulfate phr - - - - - - Phr clay - - - - - - Coloring agent Brown pigment phr 5 5 0.5 30 5 - Volume resistivity Q*cm > 10⁶ 3.0 x 10⁻¹ 2.1 x 10¹ 4.2 x 10¹ 3.1 x 10¹ > 10⁶ Color tint (measured using a colorimeter) L* 26.7 16.7 15.7 26.5 12.0 16.0 a* 14.9 1.6 -1.6 11.3 1.8 -0.4 b* 14.8 0.5 0.3 12.1 1.6 0.5 Color tint (observed visually) - Light brown Black Black Brown Black Black Physical properties Hardness A / O tl 74 88 79 88 82 77 Tensile strength MP a 8.5 15.2 8.8 7.9 10.2 14.0 Elongation % 230 60 200 110 220 590 [Results]

[0045] It is confirmed that the results of Examples 1 to 14 have a higher electrical conductivity and colorability than those of comparative Examples 1 to 6. The results will be discussed in more detail in the following description.

[0046] The results of Example 6 indicate a lower volume resistivity than that of Comparative Example 1. Specifically, it is confirmed that a resulting molded rubber article has excellent electrical conductivity by controlling the carbon nanotube content so that it is not less than 0.5 parts by weight. This suggests that a resulting molded rubber article has excellent electrical conductivity by controlling the carbon nanotube content so that it is not less than approximately 0.45 parts by weight.

[0047] In Comparative Example 2, the L* value measured using a colorimeter is lower than that of Example 7, and it is possible to visually confirm that the color is black. Specifically, it is confirmed that a resulting molded rubber article can be colored by controlling the carbon nanotube content so that it does not exceed 4.0 parts by weight. This suggests that a resulting molded rubber article can be colored by controlling the carbon nanotube content so that it does not exceed approximately 4.5 parts by weight.

[0048] In Comparative Example 3, the L* value measured using a colorimeter is lower than that of Example 8, and it is possible to visually confirm that the color is black. Specifically, from the perspective of the colorability of the molded rubber article, the lower limit value for the colorant content is preferably set between 0.5 parts by weight and 1 part by weight (for example, at least 0.7 parts by weight or at least 1 part by weight).

[0049] The results of Example 9 tend to be superior for hardness and tensile strength compared with Comparative Example 4. Namely, we find that, from the point of view of the hardness and tensile strength of the molded rubber article, the upper limit value of the colouring agent content is preferably set between 20 parts by weight and 30 parts by weight (for example at most 25 parts by weight or at most 20 parts by weight).

[0050] In Comparative Example 5, the L* value measured using a colorimeter is lower than those in Examples 1 to 14, and it is possible to visually confirm that the color is black. This confirms that, unlike the case where carbon nanotubes are used, it is not possible to color a molded rubber article when Ketjen black is used as an electrically conductive filler.

[0051] In Comparative Example 6, the L* value measured using a colorimeter is lower than those of Examples 1 to 14, and it is possible to visually confirm that the color is black. Furthermore, the results of Comparative Example 6 indicate a higher volume resistivity than that of Examples 1 to 14. This confirms that, unlike the case where carbon nanotubes are used, a resulting molded rubber article cannot be colored and has lower electrical conductivity when carbon fibers are used as the electrically conductive filler.

[0052] It should be noted that Comparative Example 6 is a reproduction of one of the examples in Patent Literature 1. Patent Literature 1 discloses an electrically conductive colored rubber consisting of a composition containing acrylonitrile-butadiene rubber and carbon fibers. Patent Literature 1 discloses that the specific volume resistance of the composition is 102 Ω·cm and that the addition of a coloring agent results in a desired color. However, in reproducing the present invention, the inventors found that the conductivity and colorability of the electrically conductive colored rubber were not as good as those described in Patent Literature 1 and were at least inferior to those of the molded rubber article in accordance with the examples in that booklet.

[0053] A comparison between Example 2 and Examples 3 to 5 reveals that, when the carbon nanotube content of the electrically conductive rubber composition is identical, the presence of a white-based filler improves the colorability and electrical conductivity of a resulting molded rubber article. This indicates that the electrically conductive rubber composition preferably contains a white-based filler in order to achieve both colorability and electrical conductivity.

[0054] As can be seen from Examples 13 and 14, an electrically conductive rubber composition that achieves both electrical conductivity and Colorability is achieved in the same way even when the type of rubber changes from fluorinated to nitrile-butadiene or acrylic rubber. These results suggest that an electrically conductive rubber composition that achieves both electrical conductivity and colorability can be obtained in the same way even when a rubber other than fluorinated, nitrile-butadiene, or acrylic rubber is used. Industrial applicability

[0055] The present invention can be used for example for a seal for a bearing or the like.

Claims

Demands

1. Electrically conductive rubber composition containing rubber, carbon nanotubes, and a coloring agent, wherein the carbon nanotubes are contained in an amount of 0.45 parts by weight to 4.5 parts by weight per 100 parts by weight of the rubber.

2. Electrically conductive rubber composition according to claim 1, wherein the rubber is of at least one type selected from a fluorinated rubber, a nitrile-butadiene rubber, and an acrylic rubber.

3. Electrically conductive rubber composition according to claim 1 or 2, wherein the coloring agent is contained in an amount of 0.7 parts by weight to 25 parts by weight per 100 parts by weight of the rubber.

4. Electrically conductive rubber composition according to any one of claims 1 to 3, wherein the coloring agent is a pigment.

5. Electrically conductive rubber composition according to any one of claims 1 to 4, further comprising at least one type of blank-based filler.

6. Electrically conductive rubber composition according to claim 5, wherein the white-based filler is contained in an amount of 3 parts by weight to 100 parts by weight per 100 parts by weight of the rubber.

7. Molded article made of the electrically conductive rubber composition according to any one of claims 1 to 6.

8. Molded rubber article according to claim 7, having a volume resistivity not exceeding 1 x 106 Q*cm.

9. Molded rubber article according to claim 7 or 8, wherein the value L*, measured using a colorimeter, is not less than 18.

0.

10. A method for producing an electrically conductive rubber composition, comprising a mixing step of mixing rubber, carbon nanotubes, and a coloring agent, wherein, in the mixing step, the carbon nanotubes are mixed in an amount from 0.45 parts by weight to 4.5 parts by weight per 100 parts by weight of rubber.