Silicone rubber composition and cured product thereof

Abstract

The present application provides a silicone rubber composition containing: (A) an organopolysiloxane having a polymerization degree of 100 or more and having two or more alkenyl groups bonded to silicon atoms in one molecule: 100 parts by mass; (B) a reinforcing silica having a specific surface area of 50 m 2 / g or more measured by the BET method: 25 to 70 parts by mass; (C) a flat boron nitride having an average particle diameter of 3 to 50 μm and an aspect ratio of 5 or more: 5 to 50 parts by mass; and (D) a curing agent: an effective amount for curing the (A) component.

Description

Technical Field

[0001] This invention relates to silicone rubber compositions that provide cured products with excellent dielectric breakdown strength and the cured products thereof. Background Technology

[0002] Silicone rubber is widely used in various fields such as electrical equipment, automobiles, construction, medical, and food due to its excellent weather resistance, electrical properties, low compression set, heat resistance, and cold resistance. Examples include rubber contact parts for remote controls and musical instruments; gaskets for construction; rollers for office equipment such as fixing rollers, developing rollers, transfer rollers, charging rollers, and paper feed rollers; vibration damping rubber for audio equipment; seals for optical discs; and wire sheathing materials for various high-voltage cables.

[0003] In recent years, with the increasing performance of vehicle cables for electric vehicles and hybrid vehicles, as well as various electronic devices, the trend towards higher voltage and higher current has become significant, leading to a pursuit of higher voltage withstand characteristics.

[0004] To improve the dielectric breakdown properties of silicone rubber, Patent Document 1 proposes to combine boron nitride powder with an average particle size of less than 20 μm.

[0005] However, the requirements for dielectric breakdown strength of silicone rubber have been increasing year by year. Among these existing technologies, it is impossible to achieve higher dielectric breakdown strength characteristics in the dielectric breakdown test specified in JIS K-6249.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 1-221454 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] The present invention was made in view of the above circumstances, and its object is to provide a silicone rubber composition and a cured product thereof, said silicone rubber composition being able to provide a silicone rubber cured product exhibiting high dielectric breakdown strength.

[0011] Methods for solving problems

[0012] The inventors conducted in-depth research to achieve the above objectives and found that an organopolysiloxane containing (A) an alkenyl group with a degree of polymerization of 100 or higher and having more than two alkenyl groups bonded to silicon atoms in one molecule, and (B) a specific surface area of ​​50 m² as determined by the BET method, is suitable. 2The present invention is achieved by providing a cured product with excellent dielectric breakdown strength by comprising a silicone rubber composition of reinforcing silica of 6 g or more, (C) flat boron nitride with an average particle size of 3 to 50 μm and an aspect ratio of 5 or more, and (D) a curing catalyst.

[0013] That is, the present invention provides the following silicone rubber composition and its cured product.

[0014] [1] A silicone rubber composition containing:

[0015] (A) Organopolysiloxanes with a degree of polymerization of 100 or higher and having more than two alkenyl groups bonded to silicon atoms in one molecule: 100 parts by mass.

[0016] (B) The specific surface area determined by the BET method is 50 m². 2 Reinforcing silica of / g or more: 30-70 parts by weight

[0017] (C) Flattened boron nitride with an average particle size of 3–50 μm and an aspect ratio of 5 or greater: 5–50 parts by mass, and

[0018] (D) Curing agent: The effective amount of component (A) to cure. [2]

[0020] According to the silicone rubber composition described in [1], wherein the specific surface area of ​​component (B) is 50 m². 2 / g or more and 400m 2 / g or less. [3]

[0022] According to the silicone rubber composition described in [1] or [2], the plasticity (plasticity) of the silicone rubber compound containing components (A) and (B) is 200 or more. [4]

[0024] The silicone rubber composition according to any one of [1] to [3], wherein the curing agent of component (D) is an organic peroxide. [5]

[0026] The silicone rubber composition according to any one of [1] to [4] further contains (E) an organosilane or organosiloxane compound represented by the following general formula (2): in a quantity of 0.1 to 50 parts by mass relative to 100 parts by mass of component (A).

[0027] [Chemistry 1]

[0028]

[0029] (where R) 1R is a hydrogen atom, or an unsubstituted or substituted alkyl group, whether the same or different. 2 (These are the same or different unsubstituted or substituted monovalent hydrocarbon groups, where m is a positive number from 1 to 50.) [6]

[0031] Cured product of the silicone rubber composition according to any one of [1] to [5]. [7]

[0033] According to [6], the cured material has a dielectric breakdown strength of 30 kV / mm or higher as determined by JIS K 6249:2003.

[0034] Technical effect

[0035] According to the present invention, it is possible to obtain a silicone rubber composition that provides a silicone rubber cured product with excellent dielectric breakdown strength. Detailed Implementation

[0036] The present invention will now be described in detail, but the present invention is not limited to the following description.

[0037] [Silicone rubber composition]

[0038] The silicone rubber composition of the present invention contains the following components (A) to (D) to provide a cured silicone rubber with excellent dielectric breakdown strength.

[0039] (A) Organopolysiloxanes with a degree of polymerization of 100 or higher and having two or more alkenyl groups bonded to silicon atoms in one molecule.

[0040] (B) The specific surface area determined by the BET method is 50 m². 2 / g or more of reinforcing silica

[0041] (C) Flat boron nitride with an average particle size of 3–50 μm and an aspect ratio of 5 or higher

[0042] (D) Curing agent

[0043] [(A) Alkenyl-containing organopolysiloxanes]

[0044] Component (A) in the silicone rubber composition of the present invention is an organopolysiloxane with a degree of polymerization of 100 or more and having two or more alkenyl groups bonded to silicon atoms in one molecule. As for the alkenyl-containing organopolysiloxane, there is no particular limitation as long as it has the above-mentioned degree of polymerization, and for example, compounds represented by the following average composition formula (1) can be cited.

[0045]

[0046] (In the formula, R represents the same or different unsubstituted or substituted monovalent hydrocarbon groups, and a is a positive number from 1.95 to 2.05. However, if two or more of the R groups in one molecule are alkenyl groups.)

[0047] In the above average composition formula (1), R is the same or different unsubstituted or substituted monovalent hydrocarbon group, which can be straight-chain, branched, or cyclic, preferably with 1 to 12 carbon atoms, more preferably with 1 to 8 carbon atoms, and even more preferably with 1 to 6 carbon atoms. Specific examples include: alkyl groups such as methyl, ethyl, propyl, butyl, hexyl, and octyl; cycloalkyl groups such as cyclopentyl and cyclohexyl; alkenyl groups such as vinyl, allyl, propenyl, butenyl, and hexenyl; cycloalkenyl groups such as cyclohexenyl; aryl groups such as phenyl and tolyl; and aralkyl groups such as benzyl, 2-phenylethyl, and β-phenylpropyl. In addition, examples include: groups in which some or all of the hydrogen atoms bonded to the carbon atoms of these groups are replaced by halogen atoms such as chlorine, fluorine, and bromine, or cyano groups (e.g., chloromethyl, trifluoropropyl, cyanoethyl, etc.).

[0048] R is preferably methyl, vinyl, phenyl, or trifluoropropyl.

[0049] As component (A), it is suitable to be: a substance in which the main chain of the organopolysiloxane in the above average composition formula (1) is composed of dimethylsiloxane units, or a substance in which diphenylsiloxane units, methyl vinylsiloxane units, methyl-3,3,3-trifluoropropylsiloxane units, etc., having phenyl, vinyl, 3,3,3-trifluoropropyl, etc., are introduced into a part of the main chain of the dimethylpolysiloxane.

[0050] The organopolysiloxane of component (A) has two or more alkenyl groups (preferably vinyl groups) bonded to silicon atoms in one molecule. Among the silicon-bonded groups of component (A) (i.e., R in the above average composition formula (1)), preferably 0.01 to 10 mol%, more preferably 0.02 to 5 mol%, are alkenyl groups.

[0051] It should be noted that the alkenyl group can be bonded to a silicon atom at the end of the molecular chain, or to a silicon atom in the side chain, or to both, but preferably at least one alkenyl group is bonded to a silicon atom at the end of the molecular chain, more preferably at least to silicon atoms at both ends of the molecular chain. That is, the organopolysiloxane as component (A) is specifically preferably a compound whose molecular chain is capped at both ends by dimethylvinylsilyl, methyldivinylsilyl, trivinylsilyl, etc.

[0052] In the above formula (1), a is a positive number from 1.95 to 2.05, preferably from 1.98 to 2.02, and more preferably from 1.99 to 2.01.

[0053] (A) The shape of the organopolysiloxane in the component is not particularly limited. It is basically a straight chain with the ends of the molecular chain capped by three organosiloxane groups and the main chain composed of repeating two organosiloxane units. However, it may also have branches within the range that does not impair the elasticity of the rubber.

[0054] (A) The degree of polymerization of the organopolysiloxane is 100 or higher, preferably 3,000 to 100,000, and more preferably 4,000 to 20,000. When the degree of polymerization is less than 100, sufficient rubber strength cannot be obtained.

[0055] Please note that in this specification, "degree of polymerization" refers to the average degree of polymerization, which is the value obtained by gel permeation chromatography (GPC) under the following conditions using polystyrene as a standard (hereinafter the same).

[0056] [Measurement Conditions]

[0057] Developing solvent: Toluene

[0058] Flow rate: 1 mL / min

[0059] Detector: Differential Refractive Index Detector (RI)

[0060] Chromatographic column: 2 TSKgel SuperH-RC columns (manufactured by Tosoh Corporation)

[0061] Column temperature: 25℃

[0062] Sample injection volume: 20 μL (0.1% by mass toluene solution)

[0063] Specific examples of component (A) include: dimethyl polysiloxane with both ends of the molecular chain capped by dimethyl vinylsiloxy, dimethyl polysiloxane with both ends of the molecular chain capped by methyl divinylsiloxy, dimethyl polysiloxane with both ends of the molecular chain capped by trivinylsiloxy, dimethyl siloxane-methyl vinyl siloxane copolymer with both ends of the molecular chain capped by trimethyl siloxy, dimethyl siloxane-methyl vinyl siloxane copolymer with both ends of the molecular chain capped by dimethyl vinylsiloxy, and dimethyl siloxane-methyl vinyl siloxane copolymer with one end of the molecular chain capped by dimethyl vinylsiloxy and the other end capped by trimethyl siloxy.

[0064] Among them, preferred are dimethyl polysiloxanes whose molecular chains are capped at both ends by dimethyl vinyl siloxane, dimethyl siloxane-methyl vinyl siloxane copolymers whose molecular chains are capped at both ends by trimethyl siloxane, and dimethyl siloxane-methyl vinyl siloxane copolymers whose molecular chains are capped at both ends by dimethyl vinyl siloxane.

[0065] (A) The organopolysiloxane of component A can be used alone or in combination with two or more substances with different molecular structures or degrees of polymerization.

[0066] Such (A) component organopolysiloxanes can be obtained by known methods, such as by (co)hydrolysis and condensation of one or more organohalosilanes, or by ring-opening polymerization of cyclic polysiloxanes using an alkaline or acidic catalyst.

[0067] (B) Reinforcing silica

[0068] Component (B) in the silicone rubber composition of this invention has a specific surface area of ​​50 m² as determined by the BET method. 2 Reinforcing silica (reinforced silica) of / g or higher.

[0069] (B) The reinforcing silica of the component is a powdered reinforcing filler, and its type is not particularly limited. Examples include: fumed silica (dry silica or pyrolytic silica), calcined silica, precipitated silica (wet silica), etc. Among them, fumed silica is preferred from the viewpoint of dielectric breakdown strength.

[0070] (B) The specific surface area of ​​the reinforcing silica, determined by the BET method, is 50 m². 2 / g or more, preferably 100m 2 / g or more. There is no specific upper limit, but 400mg is preferred. 2 / g or less, more preferably 300m 2 / g or less. Specific surface area determined by BET method is less than 50m². 2 When the yield is / g, the mechanical strength of the resulting cured silicone rubber is insufficient.

[0071] The reinforcing silica can be either untreated or treated, without particular limitation. However, when using silica that has undergone hydrophobic treatment on the surface of untreated silica, it is preferred from the perspective of dispersibility and reinforcing properties in component (A). In this case, silica that has been pre-treated with a silica surface treatment agent in powder form can be used, or the silica surface treatment agent can be added when mixing silica micro powder with component (A), and the surface can be hydrophobically treated by heating and mixing.

[0072] There are no particular limitations on the surface treatment method; any conventionally known method can be appropriately selected. For example, one method involves placing untreated silica micropowder and a silica surface treatment agent in a closed mechanical mixing apparatus or fluidized bed under normal pressure, and mixing them at room temperature or with heating in the presence of an inactive gas, as needed. Sometimes a catalyst can be used to promote the surface treatment. After mixing, drying can produce reinforcing silica micropowder with a hydrophobic surface treatment.

[0073] Specific examples of silica surface treatment agents include: chlorosilanes such as trimethylchlorosilane, dimethyldichlorosilane, and methyltrichlorosilane; and silazanes such as hexamethyldisilazane and 1,3-divinyl-1,1,3,3-tetramethyldisilazane.

[0074] The amount of silica surface treatment agent is not particularly limited, but is preferably 5 to 75 parts by weight, more preferably 5 to 60 parts by weight, relative to 100 parts by weight of untreated silica micro powder.

[0075] (B) Commercially available products can be used as the components. For example, untreated fumed silica can be represented by Aerosil 130, Aerosil 200, and Aerosil 300 manufactured by Aerosil Corporation of Japan, and Reolosil QS-10, Reolosil QS-102, and Reolosil QS-30 manufactured by Tokuyama Corporation. Hydrophobically treated fumed silica can be represented by Aerosil R-812, Aerosil R-974, and Aerosil R976S manufactured by Aerosil Corporation of Japan, and Reolosil DM-20 and DM-30 manufactured by Tokuyama Corporation. One of these can be used alone, or two or more can be used in combination.

[0076] The amount of reinforcing silica added in component (B) is 30 to 70 parts by weight, preferably 30 to 60 parts by weight, relative to 100 parts by weight of the organopolysiloxane in component (A). When the amount of component (B) added is less than 30 parts by weight, the strength and processability of the resulting silicone rubber decrease. When it exceeds 70 parts by weight, the processability of the silicone rubber composition deteriorates, and the mechanical strength of the cured silicone rubber decreases.

[0077] [(C) Flattened boron nitride]

[0078] In the silicone rubber composition of the present invention, component (C) is flat boron nitride. The flat boron nitride of component (C) functions as a barrier layer against electrical breakdown in the cured silicone rubber.

[0079] The crystal structure of the flattened boron nitride in component (C) is not particularly limited, but hexagonal boron nitride is preferred from the viewpoint of forming a barrier layer. Furthermore, component (C) is granular and flattened, but it also includes flake-like, plate-like, and sheet-like forms. By using such hexagonal boron nitride, the hexagonal boron nitride is oriented along its face direction in the silicone rubber composition, and its function as a barrier layer against electrical breakdown is enhanced after curing.

[0080] (C) The aspect ratio of the flat boron nitride component is 5 or more, preferably 7 or more. When the aspect ratio is less than 5, the orientation of the boron nitride in the planar direction is suppressed, and therefore sufficient dielectric breakdown strength cannot be obtained. It should be noted that there is no particular upper limit, but it is preferably 40 or less, more preferably 35 or less, and even more preferably 30 or less.

[0081] The aspect ratio is the value obtained by dividing the particle's length by its thickness, i.e., length / thickness. The aspect ratio is 1 when the particle is spherical, and it increases with increasing flatness. Note that in this invention, the aspect ratio can be obtained by measuring the particle's length and thickness using a scanning electron microscope and calculating the aspect ratio / thickness.

[0082] (C) The average particle size of the flat boron nitride in component (C) is 3–50 μm, preferably 5–50 μm, and more preferably 8–50 μm. When the particle size is less than 3 μm, the gaps between the oriented boron nitrides in the silicone rubber composition become larger, reducing their function as a barrier layer against electrical breakdown after curing, and thus they cannot exhibit sufficient dielectric breakdown strength; when the particle size is greater than 50 μm, the processability deteriorates significantly.

[0083] (C) The flattened boron nitride composition can use boron nitride with a single average particle size, or a mixture of multiple boron nitrides with different average particle sizes. It should be noted that in this invention, the aforementioned average particle size is the value of the cumulative 50% particle size (D50) on a volume basis, measured using a MicrotracBEL MT3000II particle size distribution measuring device.

[0084] The amount of component (C) added is 5 to 50 parts by mass relative to 100 parts by mass of component (A), preferably 10 to 50 parts by mass, and more preferably 10 to 40 parts by mass.

[0085] [(D) Curing agent]

[0086] In the silicone rubber composition of the present invention, component (D) is a curing agent. The curing agent for component (D) is not particularly limited as long as it can cure the silicone rubber composition of the present invention. Therefore, known organic peroxide curing agents for silicone rubber, or addition reaction curing agents combining organohydrogen polysiloxanes (organohydrosilyl-containing organosiloxanes) with platinum group metal catalysts (hydrosilane alkylation catalysts), etc., can be used.

[0087] (D-1) Organic peroxide curing agent

[0088] Examples of organic peroxide curing agents include: benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-methylbenzoyl peroxide, o-methylbenzoyl peroxide, 2,4-diisopropylbenzene peroxide, 2,5-dimethyl-bis(2,5-tert-butylperoxide)hexane, di-tert-butyl peroxide, tert-butyl peroxide, and 1,6-hexanediol bis-tert-butyl percarbonate. These can be used individually or in combination with two or more.

[0089] The amount of organic peroxide curing agent added should be an effective amount to cure component (A), preferably 0.1 to 10 parts by weight, more preferably 0.2 to 5 parts by weight, relative to 100 parts by weight of the organopolysiloxane in component (A). If the amount added is too small, the silicone rubber composition may sometimes be insufficiently cured; conversely, if the amount added is too large, the cured silicone rubber may sometimes discolor due to the decomposition residue of the organic peroxide.

[0090] (D-2) Addition reaction curing agent

[0091] When curing is performed using an addition reaction curing agent, organohydrogen polysiloxanes and platinum group metal catalysts are used.

[0092] As long as an organohydrogen polysiloxane contains 2 or more, preferably 3 or more, more preferably 3 to 200, and even more preferably 4 to 100 hydrogen atoms (SiH groups) bonded to silicon atoms in one molecule, it can be linear, branched, or cyclic, but the degree of polymerization is preferably 300 or less.

[0093] Specific examples of such organohydrosiloxanes include: dimethyl polysiloxanes end-capped with dimethylhydrosiloxy groups, dimethylsiloxane-methylhydrosiloxane copolymers end-capped with trimethylsiloxy groups, dimethylsiloxane-methylhydrosiloxane copolymers end-capped with dimethylhydrosiloxy groups, and dimethylsiloxane-methylhydrosiloxane copolymers composed of dimethylhydrosiloxane units (H(CH3)2SiO). 0.5 Low-viscosity fluids composed of SiO2 units, 1,3,5,7-tetrahydro-1,3,5,7-tetramethylcyclotetrasiloxane, 1-propyl-3,5,7-trihydro-1,3,5,7-tetramethylcyclotetrasiloxane, 1,5-dihydro-3,7-dihexyl-1,3,5,7-tetramethylcyclotetrasiloxane, etc., and organohydrogen polysiloxanes in which part or all of the methyl groups in the above-exemplary compounds are replaced by other alkyl or aryl groups such as phenyl groups, etc.

[0094] The amount of organohydrogen polysiloxane added as the curing agent is preferably 50 to 500 mol% ((SiH group / aliphatic unsaturated group) × 100) relative to the aliphatic unsaturated groups (alkenyl groups) of the organosiloxane in component (A). When this proportion is less than 50 mol%, the crosslinking is insufficient, which may sometimes have an adverse effect on the physical or electrical properties of the cured organosilicon; in addition, when it exceeds 500 mol%, the physical properties after curing may sometimes decrease.

[0095] On the other hand, as platinum group metal catalysts, there are platinum group metal elements (platinum, palladium, rhodium, etc.) and their compounds. Specific examples include: elemental platinum, platinum compounds, platinum complexes, chloroplatinic acid, alcoholic compounds of chloroplatinic acid, aldehyde compounds, ether compounds, and complexes with various olefins.

[0096] The amount of platinum group metal catalyst added relative to the organopolysiloxane of component (A), calculated on a platinum group metal atomic basis, is preferably 1 to 2,000 ppm (mass basis). When this amount is below the lower limit mentioned above, the addition reaction is not sufficiently promoted, sometimes resulting in incomplete curing. On the other hand, when the amount exceeds the upper limit mentioned above, the effect on reactivity becomes smaller, sometimes making it uneconomical.

[0097] [(E) Organosilanes / organosiloxanes]

[0098] In addition to the components described above, the silicone rubber composition of the present invention preferably contains, as an optional component, an organosilane or organosiloxane compound represented by the following general formula (2) (E). Component (E) acts as a dispersant (wetting agent) to improve the dispersibility of reinforcing silica in component (B) in component (A). By incorporating component (E), the workability, extrusion characteristics, etc., of the silicone rubber composition of the present invention are improved.

[0099] [Chemistry 2]

[0100]

[0101] (where R) 1 R is a hydrogen atom, or an unsubstituted or substituted alkyl group, whether the same or different. 2 (These are the same or different unsubstituted or substituted monovalent hydrocarbon groups, where m is a positive number from 1 to 50.)

[0102] Here, R 1 The organosilane or organosiloxane represented by the above general formula (2) has an alkoxy or hydroxyl group at the end of the molecular chain, which is a hydrogen atom or the same or different alkyl group.

[0103] In equation (2), R is used as 1Alkyl groups, preferably alkyl groups having 1 to 4 carbon atoms, include methyl, ethyl, propyl, butyl, etc. Other examples include groups in which some or all of the hydrogen atoms are replaced by halogen atoms such as chlorine, fluorine, bromine, or cyano groups (e.g., trifluoropropyl, cyanoethyl, etc.).

[0104] Among them, as R 1 The preferred atoms are hydrogen atoms, methyl groups, and ethyl groups.

[0105] R 2 The monovalent hydrocarbon group can be straight-chain, branched, or cyclic, preferably with 1 to 12 carbon atoms, more preferably with 1 to 8 carbon atoms, and even more preferably with 1 to 6 carbon atoms. As specific examples, groups identical to those exemplified in R can be given, wherein methyl, vinyl, phenyl, and trifluoropropyl are preferred, and methyl, vinyl, and trifluoropropyl are more preferred.

[0106] m is a positive number from 1 to 50, preferably a positive number from 1 to 30, and more preferably a positive number from 1 to 20. When m is less than 50, sufficient additive effect can be obtained even without large-scale compounding, so there is no risk of rubber property degradation due to large-scale compounding.

[0107] Furthermore, the kinematic viscosity of component (E) is preferably 1 to 500 mm. 2 / s, more preferably 1 to 100 mm 2 / s. It should be noted that the above kinematic viscosity is a value measured at 25°C using an Orthocrites viscometer.

[0108] Specific examples of component (E) include: dimethylsilanediol, diethylsilanediol, diphenylsilanediol; dimethylpolysiloxane with silanol groups at both ends, methylphenylpolysiloxane with silanol groups at both ends, etc.

[0109] When incorporating component (E), the amount of organosilane / organosiloxane added in component (E) is preferably 0.1 to 50 parts by weight, more preferably 0.5 to 30 parts by weight, and even more preferably 0.5 to 20 parts by weight, relative to 100 parts by weight of organopolysiloxane in component (A). If it is 0.1 parts by weight or more, the workability and extrusion characteristics of the silicone rubber composition of the present invention can be sufficiently obtained; if it is 50 parts by weight or less, there is no risk of the resulting silicone rubber composition becoming sticky, and therefore there is no risk of decreased processability or decreased physical properties of the resulting rubber.

[0110] [Other ingredients]

[0111] In the silicone rubber composition of the present invention, in addition to the above-mentioned components, as any component, known additives in silicone rubber compositions, such as platinum compounds other than the above-mentioned platinum group metal catalysts, iron oxide, halogen compounds and other flame retardant agents, heat resistance improvers such as cerium oxide, anti-aging agents, ultraviolet absorbers, colorants, release agents and other additives, may be added as needed.

[0112] The method for manufacturing the silicone rubber composition of the present invention is not particularly limited, and it can be obtained by mixing a specified amount of the above-mentioned components using a known mixing machine such as a two-roll mill, kneader, or Banbury mixer. Alternatively, heat treatment (mixing under heat) can be performed as needed.

[0113] Specifically, it is preferable to mix components (A) and (B) (or, in the case of component (E), mix components (A), (B), and (E)), perform heat treatment as needed, and then add component (D) at room temperature. In this case, component (C) can be mixed before or after heat treatment, but it is preferable to mix it after heat treatment. Furthermore, component (D) is preferably added after mixing component (C).

[0114] There are no particular restrictions on the heat treatment temperature and time, but it is preferred to perform the heat treatment at 100-250°C, more preferably 140-180°C, for about 30 minutes to 5 hours.

[0115] In this invention, the mixture obtained by mixing components (A) and (B) (or (A), (B), and (E) when component (E) is incorporated) is called a silicone rubber compound. In this invention, the plasticity of this silicone rubber compound is preferably 200 or higher, more preferably 220 or higher. When the plasticity is lower than 200, when boron nitride (C) is incorporated, the boron nitride is not sufficiently oriented in the silicone rubber composition, and the desired dielectric breakdown strength may not be obtained after curing.

[0116] To clarify, the above-mentioned plasticity value refers to the Williams plasticity value of the silicone rubber compound obtained by mixing components (A) and (B) (in the case of compounding component (E), components (A), (B) and (E)) according to JIS-K6249:2003.

[0117] [Cured material]

[0118] When molding the silicone rubber composition of the present invention, a molding method can be appropriately selected according to the desired application (molded article). Specific examples of molding methods include: compression molding, injection molding, transfer molding, atmospheric pressure hot air vulcanization, steam vulcanization, etc.

[0119] There are no particular limitations on the curing conditions, which can be appropriately selected according to the curing method and the molded product. Generally, primary vulcanization is preferably carried out at 80–600°C, more preferably at 100–450°C, preferably for several seconds to several days, more preferably for about 5 seconds to 1 hour. Alternatively, secondary vulcanization can be performed as needed. Secondary vulcanization, for example, can be carried out at 180–250°C for about 1–10 hours.

[0120] As described above, if the silicone rubber composition of the present invention is used, a silicone rubber cured product with excellent dielectric breakdown characteristics can be provided.

[0121] That is, the dielectric breakdown strength of the cured silicone rubber composition of the present invention is preferably 30 kV / mm or more, and more preferably 35 kV / mm. It should be noted that the dielectric breakdown strength value is based on the value measured according to JIS K 6249:2003.

[0122] Example

[0123] The present invention will be described in more detail below with examples and comparative examples, but the present invention is not limited to these examples. It will be noted that the measurement methods used in the following examples are as described below.

[0124] [Average degree of polymerization (GPC determination conditions)]

[0125] Measurement apparatus: HLC-8420GPC manufactured by Tosoh Corporation

[0126] Developing solvent: Toluene

[0127] Flow rate: 1 mL / min

[0128] Detector: Differential Refractive Index Detector (RI)

[0129] Chromatographic column: 2 TSKgel SuperH-RC columns (manufactured by Tosoh Corporation)

[0130] Column temperature: 25℃

[0131] Sample injection volume: 20 μL (0.1% by mass toluene solution)

[0132] Aspect Ratio

[0133] The length and thickness of the particles were measured using a scanning electron microscope (model JSM-7900F, manufactured by Nippon Electron Ltd.), and the length / thickness ratio was calculated.

[0134] [Average Particle Size]

[0135] The cumulative 50% particle size (D50) value on a volume basis was determined using a MicrotracBEL MT3000II particle size distribution measuring device.

[0136] [Kinematic viscosity]

[0137] The measurements were taken at 25°C using an Auster viscometer.

[0138] [Plasticity]

[0139] The silicone rubber compound obtained by mixing components (A), (B) and (E) was mixed with a three-roll mill 15 times, and the Williams plasticity was determined after 10 minutes according to the method described in JIS-K6249:2003.

[0140] [Example 1]

[0141] 100 parts by weight of methyl vinyl polysiloxane raw rubber (A), composed of 99.85 mol% dimethylsiloxane units and 0.15 mol% methyl-vinylsiloxane units, with an average degree of polymerization of approximately 7,000 and end-capped by dimethylvinylsiloxane units, were tested using the BET method. The specific surface area was 200 m². 2 / g of fumed silica (Aerosil 200 (manufactured by Aerosil Corporation of Japan)) (B) 35 parts by weight, with silanol groups at both ends as a dispersant, an average degree of polymerization of 13 and a kinematic viscosity of 15 mm at 25°C. 2 Five parts by mass of dimethylpolysiloxane (E) were kneaded in a kneader to obtain a silicone rubber compound.

[0142] The obtained silicone rubber compound was heat-treated at 180°C for 3 hours. After cooling, 30 parts by weight of flat boron nitride (C-1) with an aspect ratio of 7 and an average particle size of 3 μm were added and mixed to obtain a composable silicone rubber composition. 0.8 parts by weight of the organic peroxide p-methylbenzoyl peroxide (D-1) were then added and the mixture was uniformly mixed using two rollers to obtain the silicone rubber composition.

[0143] The obtained silicone rubber composition was cured under pressure at 120°C and 686 Pa for 10 minutes to prepare a 1 mm thick sheet for dielectric breakdown strength testing. The sheet was then post-cured in an oven at 150°C for 1 hour. The dielectric breakdown strength of the cured material was determined using TransOil A oil manufactured by Showa Shell Oil Co., Ltd., at a pressure increase rate of 2 kV / s, according to the method described in JISK 6249:2003. The results are recorded in Table 1.

[0144] [Example 2]

[0145] In Example 1, the specific surface area measured by the BET method was 200 m². 2The fumed silica (B) was changed to 47 parts by mass, and the dimethyl polysiloxane (E) with silanol groups at both ends as a dispersant was changed to 10 parts by mass. Otherwise, the exact same composable silicone rubber composition was used, and the dielectric breakdown strength was determined.

[0146] [Example 3]

[0147] In Example 2, the (C-1) component was replaced with 30 parts by mass of flat boron nitride (C-2) with an aspect ratio of 10 and an average particle size of 10 μm. Otherwise, the exact same composable silicone rubber composition was used, and the dielectric breakdown strength was determined.

[0148] [Example 4]

[0149] In Example 2, the (C-1) component was replaced with 30 parts by mass of flat boron nitride (C-3) with an aspect ratio of 25 and an average particle size of 45 μm. Otherwise, the exact same composable silicone rubber composition was used, and the dielectric breakdown strength was determined.

[0150] [Example 5]

[0151] In Example 2, the (C-1) component was replaced with 30 parts by mass of flat boron nitride (C-4) with an aspect ratio of 28 and an average particle size of 50 μm. Otherwise, the exact same composable silicone rubber composition was used, and the dielectric breakdown strength was determined.

[0152] [Example 6]

[0153] In Example 4, the flat boron nitride (C-3) with an aspect ratio of 25 and an average particle size of 45 μm was replaced with 5 parts by mass, while the exact same composable silicone rubber composition was used, and the dielectric breakdown strength was determined.

[0154] [Comparative Example 1]

[0155] In Example 2, without the addition of flat boron nitride (C), the exact same composable silicone rubber composition was used to determine the dielectric breakdown strength.

[0156] [Comparative Example 2]

[0157] In Example 1, the specific surface area measured by the BET method was 200 m². 2 The fumed silica (B) was replaced with 25 parts by mass, the dimethyl polysiloxane (E) with silanol groups at both ends as a dispersant was replaced with 4 parts by mass, and flat boron nitride (C) was not added. Otherwise, the exact same composable silicone rubber composition was used to determine the dielectric breakdown strength.

[0158] [Comparative Example 3]

[0159] In Comparative Example 2, 30 parts by mass of flat boron nitride (C-1) with an aspect ratio of 7 and an average particle size of 3 μm were added, and the exact same composable silicone rubber composition was used to determine the dielectric breakdown strength.

[0160] [Comparative Example 4]

[0161] In Example 2, the (C-1) component was replaced with 30 parts by mass of flat boron nitride (C-5) with an aspect ratio of 3 and an average particle size of 3 μm. Otherwise, the exact same composable silicone rubber composition was used, and the dielectric breakdown strength was determined.

[0162] [Comparative Example 5]

[0163] In Example 2, the (C-1) component was replaced with 30 parts by mass of flat boron nitride (C-6) with an aspect ratio of 4 and an average particle size of 20 μm. Otherwise, the exact same composable silicone rubber composition was used, and the dielectric breakdown strength was determined.

[0164] [Table 1]

[0165]

[0166] As can be seen from the results in Table 1 above, as shown in Comparative Example 4, when the aspect ratio of the added boron nitride is a low value such as 3, even if the same amount of boron nitride is added as in Examples 1 to 5, it is impossible to obtain a cured product with a dielectric breakdown characteristic exceeding 30 kV / mm.

[0167] Furthermore, as shown in Comparative Example 3, even with an aspect ratio of 7 for the added boron nitride, it was impossible to obtain a cured product with a dielectric breakdown characteristic exceeding 30 kV / mm when the amount of silica was small and the plasticity was as low as 140.

[0168] On the other hand, as shown in Examples 1 and 2, when the product contains the components specified in this invention and has a plasticity of 200 or more, it can exhibit high dielectric breakdown characteristics of more than 30 kV / mm after curing.

[0169] Furthermore, as shown in Examples 4 and 5, even if the boron nitride particle size is large, a cured product with good dielectric breakdown characteristics can be obtained by adding boron nitride with a high aspect ratio.

Claims

1. A silicone rubber composition comprising: (A) Organopolysiloxanes with a degree of polymerization of 100 or higher and having more than two alkenyl groups bonded to silicon atoms in one molecule: 100 parts by mass. (B) The specific surface area determined by the BET method is 50 m². 2 Reinforcing silica of / g or more: 30-70 parts by weight (C) Flattened boron nitride with an average particle size of 3–50 μm and an aspect ratio of 5 or greater: 5–50 parts by mass, and (D) Curing agent: The effective amount of component (A) to cure.

2. The silicone rubber composition according to claim 1, wherein, (B) The specific surface area of ​​component B is 50 m². 2 / g or more and 400m 2 / g or less.

3. The silicone rubber composition according to claim 1 or 2, wherein, The plasticity of the silicone rubber compound containing components (A) and (B) is above 200.

4. The silicone rubber composition according to any one of claims 1 to 3, wherein, (D) The curing agent is an organic peroxide.

5. The silicone rubber composition according to any one of claims 1 to 4, further comprising (E) an organosilane or organosiloxane compound represented by the following general formula (2): in a quantity of 0.1 to 50 parts by weight relative to 100 parts by weight of component (A), [Chemistry 1] In the formula, R 1 R is a hydrogen atom, or an unsubstituted or substituted alkyl group, whether the same or different. 2 The groups are the same or different unsubstituted or substituted monovalent hydrocarbon groups, and m is a positive number from 1 to 50.

6. The cured product of the silicone rubber composition according to any one of claims 1 to 5.

7. The cured product according to claim 6, wherein the dielectric breakdown strength measured according to JIS K 6249:2003 is 30 kV / mm or higher.

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