Rubber composition for high-voltage electrical insulator and rubber molded article

Abstract

The present invention relates to a rubber composition for a high-voltage electrical insulator, the rubber composition containing (A) a rubber material that is selected from the group consisting of a nitrile rubber, a hydrogenated nitrile rubber, an acrylic rubber, an ethylene propylene diene rubber, and a butyl rubber, and (B) a filler that is selected from the group consisting of carbon black and silica. The filler (B) contains at least one type of silica, the content of silica with respect to the total mass of the rubber composition is 4.0 mass% or more, and the content of carbon black with respect to the total mass of the rubber composition is 5.0 mass% or less.

Description

Rubber Composition for High-Voltage Electrical Insulator and Rubber Molded Product

[0001] The present invention relates to a rubber composition for a high-voltage electrical insulator and a rubber molded product.

[0002] In recent years, with the trend of miniaturization of electronic components due to the electrification of automobiles, miniaturization of terminal blocks for connecting high-voltage units has been desired, and such terminal blocks are required to have electrical insulation against high voltages. However, miniaturization of terminal blocks raises concerns about the so-called tracking phenomenon, in which a conductive path is formed due to deterioration of the surface of the insulating material, resulting in energization or ignition. The tracking resistance, which is an index indicating the difficulty of occurrence of the tracking phenomenon, is represented by, for example, the comparative tracking index (CTI). Therefore, by applying a material with a high CTI to the terminal block, not only the safety of the components is enhanced, but also the creepage distance between conductors is shortened, enabling miniaturization of the terminal block.

[0003] In the test standard for tracking resistance (JIS C2134: 2021), the maximum voltage used in the test is specified as 600 V (volts). This is because when a voltage of 600 V or more is applied, an air arc that conducts electricity through the air between the electrodes is generated, making it impossible to apply a voltage to the material. On the other hand, in electric vehicles, system voltages of 200 V to 400 V are widely used, but development aiming for system voltages exceeding 600 V is underway, and there is an increasing interest in materials that exhibit tracking resistance with a CTI exceeding 600 V.

[0004] In addition, in in-vehicle components, in addition to electrical properties (high volume resistivity, high CTI), impact resistance, flex resistance, and vibration resistance are required as durability against impacts during driving and vibrations in the engine / motor section. Furthermore, structural members of terminal blocks connecting motors and inverters may also require sealing properties to prevent intrusion of liquids between the electrodes and the structural members.

[0005] While resin materials such as polyamide and polyisobutylene are commonly used as structural components for terminal blocks, their tracking resistance is insufficient for the high voltages of modern units. Furthermore, resin materials lack rubber elasticity, resulting in poor sealing properties. On the other hand, rubber materials possess rubber elasticity, providing sealing properties, impact resistance, bending resistance, vibration resistance, and vibration damping. However, many rubber materials have low tracking resistance, which is a concern. Therefore, the use of silicone rubber, which exhibits relatively good tracking resistance among rubber materials, is known (Patent Document 1).

[0006] On the other hand, using silicone rubber as a rubber material raises concerns about contact failure due to volatile siloxanes, making it undesirable as a structural component for electronic parts. Therefore, there is a need to develop a material that is useful as a high-voltage electrical insulator that exhibits excellent tracking resistance while suppressing siloxane generation.

[0007] Japanese Patent Application Publication No. 4-209655

[0008] The present invention provides a rubber composition for high-voltage electrical insulators that prevents the generation of siloxanes and has excellent tracking resistance, as well as a rubber molded product made using the same.

[0009] A rubber composition for high-voltage electrical insulation according to an embodiment of the present invention contains (A) a rubber material selected from the group consisting of nitrile rubber, hydrogenated nitrile rubber, acrylic rubber, ethylene propylene diene rubber, and butyl rubber, and (B) a filler selected from the group consisting of carbon black and silica, wherein the filler (B) contains at least one type of silica, the silica content relative to the total mass of the rubber composition is 4.0% by mass or more, and the carbon black content relative to the total mass of the rubber composition is 5.0% by mass or less.

[0010] In a rubber composition for high-voltage electrical insulation according to one embodiment of the present invention, the filler (B) contains both carbon black and silica.

[0011] In a rubber composition for high-voltage electrical insulators according to one embodiment of the present invention, the carbon black content relative to the total mass of the rubber composition is 4.0% by mass or less.

[0012] In a rubber composition for high-voltage electrical insulation according to one embodiment of the present invention, the carbon black content relative to the total mass of the rubber composition is 3.5% by mass or less.

[0013] A rubber composition for high-voltage electrical insulators according to one embodiment of the present invention further comprises (C) a vulcanizing agent.

[0014] The rubber molded article according to the embodiment of the present invention is obtained by vulcanizing the above rubber composition.

[0015] In a rubber molded product according to one embodiment of the present invention, the comparative tracking index (CTI), measured in accordance with the JIS C2134:2021 standard, is 600V or higher.

[0016] In a rubber molded product according to one embodiment of the present invention, the comparative tracking index (CTI), measured by reversing the electrodes, is 700V or higher, according to the JIS C2134:2021 standard.

[0017] According to the present invention, it is possible to provide a rubber composition for high-voltage electrical insulators that prevents the generation of siloxanes and has excellent tracking resistance, as well as a rubber molded product made using the same.

[0018] Figure 1 is a schematic diagram showing a test method for measuring the comparative tracking index (CTI). Figure 2 is a schematic diagram showing a test method for measuring the comparative tracking index (CTI) with the electrodes reversed, as in Figure 1.

[0019] [Rubber Composition] Embodiments of the present invention will be described in detail below. The rubber composition for high-voltage electrical insulation of the present invention contains (A) a rubber material selected from the group consisting of nitrile rubber, hydrogenated nitrile rubber, acrylic rubber, ethylene propylene diene rubber, and butyl rubber, and (B) a filler selected from the group consisting of carbon black and silica, wherein the filler contains at least one type of silica. Furthermore, in the rubber composition for high-voltage electrical insulation of the present invention, the silica content relative to the total mass of the rubber composition is 4.0% by mass or more, and the carbon black content relative to the total mass of the rubber composition is 5.0% by mass or less. In the following, "rubber composition for high-voltage electrical insulation" may be simply referred to as "rubber composition".

[0020] <(A) Rubber Material> As the rubber component, a rubber material selected from the group consisting of nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), acrylic rubber, ethylene propylene diene rubber, and butyl rubber is used. Because the main component is rubber, it is possible to impart superior properties of rubber materials to conventional resin products, such as sealing properties, impact resistance, flexural resistance, vibration resistance, and vibration damping properties. These rubber materials may be used individually or in combination of two or more. Such rubber materials do not produce siloxane volatilization, and furthermore, they exhibit sealing properties and can be imparted with desired physical properties such as impact resistance, flexural resistance, and vibration resistance. In addition, the base rubber material may be appropriately selected according to the properties exhibited by the rubber material, such as oil resistance and water resistance.

[0021] (Nitrile Rubber) Nitrile rubber (NBR) is a copolymer of acrylonitrile and butadiene, and is excellent in heat resistance, chemical resistance, mechanical strength, abrasion resistance, etc., and is particularly excellent in oil resistance. Examples of nitrile rubber include very high nitrile content (bound acrylonitrile content of 44% by mass or more), high nitrile content (bound acrylonitrile content of 36% by mass or more and 43% by mass or less), medium-high nitrile content (bound acrylonitrile content of 31% by mass or more and 35% by mass or less), medium nitrile content (bound acrylonitrile content of 25% by mass or more and 30% by mass or less), and low nitrile content (bound acrylonitrile content of 24% by mass or less). Among these, nitrile rubber containing 30% by mass or more of acrylonitrile is preferred.

[0022] Nitrile rubber Mooney viscosity ML 1+4 (100°C) is not particularly limited, but is preferably 25 to 90, and more preferably 30 to 80. Mooney viscosity ML 1+4 (100°C) is measured in accordance with JIS K 6300-1:2013.

[0023] Examples of commercially available nitrile rubber products include "Nipol® DN3350" (acrylonitrile content 33% by mass: manufactured by Nippon Zeon Co., Ltd.), "NANCAR® 3345" (acrylonitrile content 33% by mass: manufactured by Soryu Sangyo Co., Ltd.), "N230S" (acrylonitrile content 35% by mass: manufactured by JSR Corporation), "N220S" (acrylonitrile content 41.5% by mass: manufactured by JSR Corporation), and "Nipol® DN003" (acrylonitrile content 50% by mass: manufactured by Nippon Zeon Co., Ltd.).

[0024] (Hydrogenated Nitrile Rubber) Hydrogenated nitrile rubber (HNBR) is a type of rubber produced by adding hydrogen to nitrile rubber (NBR) to hydrogenate the unsaturated bonds in the nitrile rubber, thereby improving its heat resistance and weather resistance. HNBR has heat resistance similar to that of acrylic rubber and possesses excellent chemical stability. It also has excellent mechanical strength and abrasion resistance.

[0025] The nitrile rubber used in hydrogenated nitrile rubber is not particularly limited, and includes various nitrile rubbers with very high nitrile content, high nitrile content, medium-high nitrile content, medium nitrile content, and low nitrile content as described above. Among these, nitrile rubber with medium-high nitrile content, high nitrile content, and more preferably medium-high nitrile content is used. Also, the Mooney viscosity ML of the hydrogenated nitrile rubber 1+4 (100°C) is preferably 30 to 100, and more preferably 30 to 80, from the viewpoint of mechanical strength and processability. Here, Mooney viscosity ML 1+4 (100°C) is measured in accordance with JIS K 6300-1:2013.

[0026] Examples of commercially available hydrogenated nitrile rubber include "Zetpol® 2010," "Zetpol® 2020," and "Zetpol® 4310" manufactured by Zeon Corporation of Japan.

[0027] (Acrylic Rubber) Acrylic rubber includes acrylic rubber in the narrow sense (ACM) and ethylene-acrylic rubber (AEM). Acrylic rubber in the narrow sense (ACM) is a polymer obtained by copolymerizing alkyl acrylate or alkoxyalkyl acrylate as the main monomer component with a crosslinkable monomer having a crosslinking group. Ethylene-acrylic rubber (AEM) is a polymer obtained by copolymerizing alkyl acrylate and ethylene as the main monomer components with a crosslinkable monomer having a crosslinking group.

[0028] Examples of alkyl acrylates include alkyl acrylates in which the alkyl group has 1 to 20 carbon atoms. Examples of such alkyl acrylates include methyl acrylate, ethyl acrylate, n-butyl acrylate, iso-butyl acrylate, tert-butyl acrylate, propyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, and the like, with methyl acrylate, ethyl acrylate, and n-butyl acrylate being preferred.

[0029] Examples of alkoxyalkyl acrylates include alkoxyalkyl acrylates in which the alkoxy group has 1 to 4 carbon atoms. Examples of such alkoxyalkyl acrylates include methoxymethyl acrylate, methoxyethyl acrylate, ethoxyethyl acrylate, butoxyethyl acrylate, and methoxyethoxyethyl acrylate, with methoxyethyl acrylate, ethoxyethyl acrylate, and butoxyethyl acrylate being preferred.

[0030] Examples of crosslinkable monomers having crosslinkable groups include crosslinkable monomers having carboxyl groups, epoxy groups, halogen groups, hydroxyl groups, amide groups, etc., or diene monomers. Examples of such crosslinkable monomers include chlorine monomers such as 2-chloroethyl vinyl ether and vinyl chloroacetate, epoxy monomers such as allyl glycidyl ether, and diene monomers such as ethylidene norbornene. Examples of crosslinkable monomers having carboxyl groups include monoalkyl esters such as methyl, ethyl, propyl, isopropyl, n-butyl, and isobutyl of unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, and citraconic acid, and unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid. These crosslinkable monomers having crosslinkable groups are preferably used in acrylic rubber at a copolymerization ratio of about 0.5 to 10% by mass.

[0031] Examples of commercially available acrylic rubber (ACM) include "NOXTITE® PA522", "NOXTITE® PA401", and "NOXTITE® PA404K" manufactured by Unimatec, while examples of commercially available ethylene-acrylic rubber (AEM) include "Vamac G" and "Vamac GLS" manufactured by DuPont Dow Elastomers.

[0032] (Ethylene Propylene Diene Rubber) Ethylene propylene diene rubber (EPDM) is a polymer obtained by copolymerizing ethylene propylene rubber (EPM), which is a copolymer of ethylene and propylene, with a small amount of non-conjugated polyene as a third component. It exhibits excellent heat resistance, cold resistance, durability, and chemical resistance.

[0033] Examples of diene components in EPDM include linear non-conjugated dienes such as 1,4-hexadiene, 3-methyl-1,4-hexadiene, 4-methyl-1,4-hexadiene, 5-methyl-1,4-hexadiene, 4,5-dimethyl-1,4-hexadiene, and 7-methyl-1,6-octadiene; and cyclic non-conjugated dienes such as methyltetrahydroindene, 5-vinyl-2-norbornene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene, 5-isopropylidene-2-norbornene, 5-vinylidene-2-norbornene, 6-chloromethyl-5-isopropenyl-2-norbornene, and dicyclopentadiene.

[0034] EPDM Mooney Viscosity ML 1+4 The temperature (125°C) is preferably between 20 and 80 degrees Celsius, and more preferably between 20 and 70 degrees Celsius. Furthermore, the EPDM may be either non-oil-expanding EPDM or oil-expanding EPDM.

[0035] Examples of commercially available EPDM products include "EPT3045" manufactured by Mitsui Chemicals and "EP33" manufactured by JSR Corporation.

[0036] (Butyl rubber) Butyl rubber (IIR) is a copolymer of isobutylene and isoprene, and is a polymer in which isoprene is copolymerized to a maximum of about 3 mol%.

[0037] <(B) Filler> The rubber composition according to the present invention contains a filler selected from the group consisting of carbon black and silica, wherein the filler contains at least one type of silica. That is, silica is an essential component of the filler, and carbon black may be optionally further included. The filler does not have to contain carbon black, and may contain both carbon black and silica, but it is preferable that it contains both carbon black and silica. The amount of filler blended is preferably 5 parts by mass or more and 90 parts by mass or less, more preferably 10 parts by mass or more and 80 parts by mass or less, and even more preferably 20 parts by mass or more and 70 parts by mass or less, per 100 parts by mass of rubber material.

[0038] (Silica) The rubber composition according to the present invention contains at least one type of silica. As the silica, for example, amorphous silica such as dry silica produced by thermal decomposition of halogenated silica or organosilicon compounds, or by air oxidation of silicon oxide (SiO) vaporized by heating and reducing silica sand, and wet silica produced by thermal decomposition of sodium silicate can be used. These silicas may be used individually or in combination of two or more types. Examples of commercially available silica include "Nipsil (registered trademark) E-74P" and "Nipsil (registered trademark) ER#100" manufactured by Tosoh Silica Co., Ltd.

[0039] The silica content relative to the total mass of the rubber composition is preferably 4.0% by mass or more, and more preferably 5.0% by mass or more. Furthermore, the upper limit of the silica content relative to the total mass of the rubber composition is preferably 40% by mass or less, more preferably 20% by mass or less, and even more preferably 16% by mass or less. Furthermore, the silica content per 100 parts by mass of rubber material is preferably 60 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less.

[0040] (Carbon Black) In one embodiment of the present invention, the rubber composition contains carbon black. The type of carbon black can be appropriately selected according to the application. For example, hard carbons such as super abrasion furnace (SAF) carbon black, intermediate super abrasion furnace (ISAF) carbon black, high abrasion furnace (HAF) carbon black, and easy processing channel (EPC) carbon black, and conductive (XCF: extra conductive furnace) carbon black, fast extruding furnace (FEF) carbon black, general purpose furnace (GPF) carbon black, high modulus furnace (HMF) carbon black, semi-reinforcing furnace (SRF) carbon black, fine thermal (FT) carbon black, and medium thermal (MT) carbon black. These carbon blacks may be used alone or in combination of two or more. Among these, soft carbon is preferred as the carbon black, and among the soft carbons, SRF carbon black and MT carbon black are more preferred. Examples of commercially available carbon blacks include SRF carbon blacks such as "SEAST (registered trademark) G-S" and "SEAST (registered trademark) G-SVH" manufactured by Tokai Carbon Co., Ltd.

[0041] The carbon black content relative to the total mass of the rubber composition is preferably 5.0% by mass or less, more preferably 4.0% by mass or less, and more preferably 3.5% by mass or less. Furthermore, the lower limit of the carbon black content relative to the total mass of the rubber composition is preferably 2.0% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3.0% by mass or more. By reducing the amount of carbon black blended, surface carbonization (formation of conductive paths) when a high voltage is applied to a rubber molded product obtained by vulcanizing the rubber composition can be suppressed, and tracking resistance can be improved. Furthermore, the carbon black content per 100 parts by mass of rubber material is preferably 8 parts by mass or less, more preferably 6 parts by mass or less, and even more preferably 5 parts by mass or less.

[0042] <(C) Vulcanizing agent> The rubber composition according to the present invention preferably further contains a vulcanizing agent in order to provide a crosslinked structure to the rubber material. The vulcanizing agent is preferably an organic peroxide-based vulcanizing agent or an amine-based vulcanizing agent. Examples of organic peroxide-based vulcanizing agents include tertiary butyl peroxide, dicumyl peroxide, tertiary butylcumyl peroxide, 1,1-di(tertiary butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(tertiary butylperoxy)hexane, 2,5-dimethyl-2,5-di(tertiary butylperoxy)hexyn-3, 1,3-di(tertiary butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, tertiary butylperoxybenzoate, tertiary butylperoxyisopropyl carbonate, n-butyl-4,4-di(tertiary butylperoxy)valerate, and the like. Furthermore, examples of amine-based vulcanizing agents include N,N'-disinnamyridene-1,6-hexanediamine.

[0043] Commercially available vulcanizing agents include, for example, organic peroxide-based vulcanizing agents such as "PERKADOX (trademark registered) 14S-FL" from Kayaku Akzo Corporation and "PERCUMYL (trademark registered) D" from Nippon Oil & Fats Co., Ltd., as well as amine-based vulcanizing agents such as "CLP5350" from Unimatec Corporation.

[0044] These vulcanizing agents are preferably 1 to 8 parts by mass, more preferably 2 to 6 parts by mass, per 100 parts by mass of the rubber material.

[0045] <(D) Hydrated metal compound> The rubber composition according to the present invention may contain a hydrated metal compound. The hydrated metal compound plays a role in improving tracking suppression characteristics and heat resistance. Examples of such hydrated metal compounds include aluminum hydroxide, magnesium hydroxide, calcium hydroxide, etc. In particular, the tracking resistance can be improved by containing aluminum hydroxide. Further, the hydrated metal compound may be surface-treated with a polymer such as silane.

[0046] The hydrated metal compound is preferably 5 to 300 parts by mass, more preferably 150 to 300 parts by mass, per 100 parts by mass of the rubber component.

[0047] <(E) Other additives> The rubber composition according to the present embodiment can be appropriately added with various additives generally blended in rubber as needed. Examples of such additives include plasticizers, processing aids, anti-aging agents, co-crosslinking agents, silane coupling agents, etc. The blending amounts of these additives are not particularly limited as long as they do not inhibit the object and effect of the present invention, and appropriate amounts according to the blending purpose can be blended.

[0048] <Method for producing rubber composition> The method for producing the rubber composition according to the present embodiment is not particularly limited. For example, after appropriately blending at a predetermined ratio the above-mentioned rubber component, filler, and vulcanizing agent, and further at least one of the above-mentioned components (D) and (E) blended as needed, the rubber composition can be produced by kneading using a kneader such as a single-screw extruder, double-screw extruder, roll, Banbury mixer, kneader, high-shear mixer, etc. Incidentally, pre-kneading may be performed as needed before kneading.

[0049] <Rubber Molded Products> Rubber molded products can be manufactured by vulcanizing the rubber composition according to this embodiment. Vulcanization of the rubber composition is generally carried out by pressure vulcanization at approximately 150 to 230°C for approximately 0.5 to 30 minutes, for example, 1.5 times the 90% crosslinking time, using an injection molding machine, compression molding machine, etc. Furthermore, after such primary vulcanization (pressure vulcanization), secondary vulcanization may be performed as needed to ensure that the inside of the vulcanized molded product is vulcanized. Secondary vulcanization can generally be carried out by oven heating at approximately 150 to 250°C for approximately 0.5 to 24 hours.

[0050] (Hardness) In the rubber molded product according to this embodiment, the hardness (DuroA, Hs) measured in accordance with JIS K6253:2012 is preferably 50 or higher, more preferably 60 or higher, and even more preferably 65 or higher.

[0051] (Tensile Strength) In the rubber molded product according to this embodiment, the tensile strength (Ts) measured in accordance with JIS K6251:2017 is preferably 5.0 MPa or more, more preferably 7.0 MPa or more, even more preferably 10.0 MPa or more, and even more preferably 15.0 MPa or more.

[0052] (Elongation) In the rubber molded product according to this embodiment, it is preferable that the elongation (Eb) measured in accordance with JIS K6251:2017 be 100% or more, more preferably 200% or more, even more preferably 300% or more, and even more preferably 400% or more.

[0053] (Electrical insulation) In the rubber molded product according to this embodiment, the volume resistivity measured in accordance with JIS K6271-1:2015 is 1 × 10⁻¹⁰ １０ It is preferable that it be Ω·cm or more, and 1 × 10 １１ It is more preferable that it be Ω·cm or greater, and 1 × 10 １２ It is even more preferable that the value be Ω·cm or greater.

[0054] (Tracking Resistance) As an indicator of tracking resistance, the comparative tracking index (CTI), measured in accordance with the JIS C2134:2021 standard, is known. Specifically, as shown in Figure 1, a predetermined electrolyte (aqueous solution) and a platinum electrode are used, and a certain amount of the electrolyte is dropped onto a test piece while a voltage is applied to the platinum electrode, and the voltage (V: volts) at which tracking occurs on the test piece is measured. In the rubber molded product according to this embodiment, it is preferable that the CTI is 600V or higher in the tracking resistance test shown in Figure 1.

[0055] In the tracking resistance test shown in Figure 1, if the CTI is 600V or higher, a similar tracking resistance test may be performed in a measurement system with the electrodes reversed. Specifically, as shown in Figure 2, the platinum electrode is reversed from the measurement system shown in Figure 1, and the CTI is measured in the same manner while the air arc is suppressed. In the rubber molded product according to this embodiment, it is preferable that the CTI is 700V or higher in the tracking resistance test shown in Figure 2.

[0056] The rubber composition according to this embodiment exhibits good physical and electrical properties after crosslinking, and furthermore, it shows excellent tracking resistance while suppressing the generation of siloxanes, making it suitable as a material for high-voltage electrical insulators. In particular, high-voltage electrical insulators made using such a rubber composition are useful as materials for high-voltage electronic equipment components such as terminal block structural components for automotive high-voltage units.

[0057] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and includes all aspects included in the concept and claims of the present invention, and can be modified in various ways within the scope of the present invention.

[0058] Based on the embodiments described above, the present invention relates to the following [1] to [8]. [1] A rubber composition for high-voltage electrical insulators comprising (A) a rubber material selected from the group consisting of nitrile rubber, hydrogenated nitrile rubber, acrylic rubber, ethylene propylene diene rubber, and butyl rubber, and (B) a filler selected from the group consisting of carbon black and silica, wherein the (B) filler contains at least one type of silica, the silica content relative to the total mass of the rubber composition is 4.0% by mass or more, and the carbon black content relative to the total mass of the rubber composition is 5.0% by mass or less. [2] The rubber composition according to [1], wherein the (B) filler contains both carbon black and silica. [3] The rubber composition according to [1] or [2], wherein the carbon black content relative to the total mass of the rubber composition is 4.0% by mass or less. [4] The rubber composition according to [3], wherein the carbon black content relative to the total mass of the rubber composition is 3.5% by mass or less. [5] (C) The rubber composition according to any one of [1] to [4] above, further comprising a vulcanizing agent. [6] A molded rubber article obtained by vulcanizing the rubber composition according to any one of [1] to [5] above. [7] The molded rubber article according to [6] above, wherein the comparative tracking index (CTI) measured in accordance with the JIS C2134:2021 standard is 600V or higher. [8] The molded rubber article according to [7] above, wherein the comparative tracking index (CTI) measured by reversing the electrodes according to the JIS C2134:2021 standard is 700V or higher.

[0059] The following describes examples of the present invention, but the present invention is not limited to these examples unless it exceeds the spirit of the invention.

[0060] The materials used in the following examples and comparative examples are as follows: • EPDM: Ethylene-propylene-diene terpolymer ("JSR EP33", manufactured by JSR Corporation) • HNBR: Hydrogenated nitrile rubber ("Zetpol® 2020", manufactured by Zeon Corporation) • ACM: Acrylic rubber ("NOXTITE® PA522", manufactured by Unimatec Corporation) • VMQ: Silicone rubber ("XE20-A6716", manufactured by Momentive Performance Materials) • Carbon black A: SRF carbon black ("SEAST® G-S", manufactured by Tokai Carbon Co., Ltd.), "SEAST® G-SVH" • Carbon black B: SRF-HS carbon black ("SEAST® G-SVH", manufactured by Tokai Carbon Co., Ltd.) • Silica A: Precipitation-type silica ("Nipsil (registered trademark) E-74P", manufactured by Tosoh Silica Co., Ltd.) • Silica B: Precipitation-type silica ("Nipsil (registered trademark) ER#100", manufactured by Tosoh Silica Co., Ltd.) • Vulcanizing agent A: Organic peroxide-based vulcanizing agent ("PERCUMYL (registered trademark) D", manufactured by Nippon Oil & Fats Co., Ltd.) • Vulcanizing agent B: Amine-based vulcanizing agent ("CLP5350", manufactured by Unimatec Co., Ltd.) • Vulcanizing agent C: Peroxide-based vulcanizing agent ("C-8", manufactured by Shin-Etsu Chemical Co., Ltd.) • Co-crosslinking agent: Triallyl isocyanurate ("TAIC (registered trademark)", manufactured by Nippon Chemical Corporation) • Plasticizer: Paraffin-based process oil ("Diana Process Oil PW380", manufactured by Idemitsu Kosan Co., Ltd.) • Silane coupling agent: Oligomer-like reactive vinylsiloxane (containing methoxy group) ("Dynasylan® 6490", manufactured by Evonik Japan). Note that the values ​​for each of the above materials in Table 1 below represent "parts by mass" unless otherwise specified.

[0061] (Example 1) A rubber composition was prepared by kneading 100 parts by mass of EPDM, 3.7 parts by mass of carbon black B, 20 parts by mass of silica A, 3 parts by mass of vulcanizing agent A, 1 part by mass of cocrosslinking agent, 0.3 parts by mass of plasticizer, and 1 part by mass of silane coupling agent using a kneader and an open roll kneader.

[0062] <Preparation of test pieces for property measurement> The obtained rubber composition was subjected to pressure vulcanization at 180°C for 12 minutes (primary vulcanization) and open vulcanization at 175°C for 15 hours (secondary vulcanization) using a sheet mold to produce sheet-shaped rubber molded products with a thickness of 2 mm as test pieces.

[0063] <Normal Properties> The following normal properties were evaluated for the obtained test specimens. The results are shown in Table 1. Hardness Hs: The peak hardness value was measured using a Type A durometer, referring to JIS K6253:2012. Tensile strength Ts (MPa): Measured in accordance with JIS K6251:2017. Elongation Eb (%): Measured in accordance with JIS K6251:2017.

[0064] <Electrical Insulation> For the obtained test specimens, 500V was applied between the electrodes in accordance with JIS K6271-1:2015, and the volume resistivity (Ω·cm) after 1 minute was measured. A higher volume resistivity indicates better electrical insulation. The results are shown in Table 1.

[0065] <Tracking Resistance> As shown in Figure 1, using approximately 0.1% by mass of ammonium chloride electrolyte and a platinum electrode, a voltage was applied to the platinum electrode while the ammonium chloride electrolyte was dropped onto the test specimen one drop every 30 seconds for a total of 50 drops. The voltage (V) at which tracking occurred on the test specimen was measured. A CTI of 600V or higher was considered to indicate excellent tracking resistance. The results are shown in Table 1.

[0066] For the obtained test specimens, as shown in Figure 2, the platinum electrode was rotated 180° from the measurement system shown in Figure 1 to suppress the air arc, and the CTI was measured using the same procedure. A CTI of 600V or higher was evaluated as having excellent tracking resistance. The results are shown in Table 1.

[0067] <Volatility of Siloxane> Using gas chromatography-mass spectrometry, a small amount of rubber sheet was heated at 140°C for 10 minutes, and the amount of siloxane was quantified from the peak area in the chromatogram of the generated outgass.

[0068] (Example 2) Except for replacing EPDM with 100 parts by mass of HNBR, replacing carbon black B with 3.5 parts by mass of carbon black A, adding 1.5 parts by mass of silica A and 3 parts by mass of silica B, and omitting the co-crosslinking agent, plasticizer, and silane coupling agent, a rubber composition and test pieces were prepared in the same manner as in Example 1, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0069] (Example 3) Except for the amount of carbon black A added being 3.7 parts by mass and the amount of silica A added being 8 parts by mass, a rubber composition and test pieces were prepared in the same manner as in Example 2, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0070] (Example 4) Except for the amount of carbon black A added being 4 parts by mass and silica A added being 20 parts by mass, and silica B being omitted, a rubber composition and test pieces were prepared in the same manner as in Example 2, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0071] (Example 5) Except for the amount of carbon black A added being 4.5 parts by mass and the amount of silica A added being 18 parts by mass, a rubber composition and test pieces were prepared in the same manner as in Example 2, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0072] (Example 6) Except for the amount of carbon black A added being 5.3 parts by mass and the amount of silica A added being 20 parts by mass, a rubber composition and test pieces were prepared in the same manner as in Example 2, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0073] (Example 7) Except for adding 100 parts by mass of ACM instead of EPDM and 6 parts by mass of vulcanizing agent B instead of vulcanizing agent A, and reducing the amount of silica A to 60 parts by mass, and omitting carbon black B, co-crosslinking agent, plasticizer, and silane coupling agent, a rubber composition and test pieces were prepared in the same manner as in Example 1, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0074] (Comparative Example 1) Except for adding 20 parts by mass of carbon black A instead of silica A, and omitting the co-crosslinking agent and silane coupling agent, a rubber composition and test pieces were prepared in the same manner as in Example 1, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0075] (Comparative Example 2) Except for the amount of carbon black A added being 10 parts by mass and the amount of silica A added being 50 parts by mass, the rubber composition and test pieces were prepared in the same manner as in Example 2, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0076] (Comparative Example 3) Except for adding 100 parts by mass of VMQ instead of EPDM, omitting fillers, co-crosslinking agents, plasticizers, and silane coupling agents, and adding 1.5 parts by mass of vulcanizing agent C instead of vulcanizing agent A, a rubber composition and test pieces were prepared in the same manner as in Example 1, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0077]

[0078] As can be seen from Table 1, in Examples 1 to 7, where silicone rubber was not used as the rubber component, and the filler contained at least one type of silica, with a silica content of 4.0% by mass or more relative to the total mass of the rubber composition, and a carbon black content of 5.0% by mass or less relative to the total mass of the rubber composition, the CTI was 600V or higher in all cases, demonstrating excellent tracking resistance. Furthermore, since no volatilization of siloxane occurred in Examples 1 to 7, contact failures caused by volatilized siloxane can be suppressed. In particular, Examples 1 and 7, where hydrogenated nitrile rubber and acrylic rubber were used as the rubber material, and with a low amount of carbon black, showed extremely excellent tracking resistance.

Claims

1. A rubber composition for high-voltage electrical insulation comprising (A) a rubber material selected from the group consisting of nitrile rubber, hydrogenated nitrile rubber, acrylic rubber, ethylene propylene diene rubber, and butyl rubber, and (B) a filler selected from the group consisting of carbon black and silica, wherein the (B) filler contains at least one type of silica, the silica content relative to the total mass of the rubber composition is 4.0% by mass or more, and the carbon black content relative to the total mass of the rubber composition is 5.0% by mass or less.

2. The rubber composition according to claim 1, wherein the filler (B) comprises both carbon black and silica.

3. The rubber composition according to claim 1, wherein the carbon black content relative to the total mass of the rubber composition is 4.0% by mass or less.

4. The rubber composition according to claim 3, wherein the carbon black content relative to the total mass of the rubber composition is 3.5% by mass or less.

5. (C) The rubber composition according to claim 1, further comprising a vulcanizing agent.

6. A rubber molded article obtained by vulcanizing a rubber composition according to any one of claims 1 to 5.

7. The rubber molded article according to claim 6, wherein the comparative tracking index (CTI), measured in accordance with the JIS C2134:2021 standard, is 600V or higher.

8. The rubber molded article according to claim 7, wherein the comparative tracking index (CTI) measured by reversing the electrodes is 700V or higher, according to the JIS C2134:2021 standard.

Images