Silicone rubber composition and cured object obtained therefrom

Abstract

Provided is a silicone rubber composition comprising: 100 parts by mass of (A) an organopolysiloxane having a degree of polymerization of 100 or greater and having two or more silicon-atom-bonded alkenyl groups in the molecule; 25-70 parts by mass of (B) reinforcing silica having a specific surface area, as measured by a BET method, of 50 m2 / g or larger; 5-50 parts by mass of (C) flat boron nitride having an average particle diameter of 3-50 μm and an aspect ratio of 5 or higher; and (D) a curing agent in an amount effective in curing the component (A).

Description

Silicone rubber composition and cured product thereof

[0001] The present invention relates to a silicone rubber composition that provides a cured product with excellent dielectric breakdown strength, and to the cured product thereof.

[0002] Silicone rubber has excellent properties such as weather resistance, electrical properties, low compression set, heat resistance, and cold resistance, and is therefore widely used in a variety of fields, including electrical equipment, automobiles, construction, medicine, and food products. For example, it is used in rubber contacts used as rubber contacts in remote controllers, musical instruments, etc., construction gaskets, office equipment rolls such as fixing rolls, developing rolls, transfer rolls, charging rolls, and paper feed rolls, vibration-proof rubber for audio equipment, etc., packing for compact discs, and wire coating materials for various high-voltage cables.

[0003] In recent years, with the increasing performance of in-vehicle cables for electric vehicles, hybrid vehicles, etc., and various electronic devices, there has been a noticeable trend toward higher voltages and currents, and higher voltage resistance characteristics are required. In order to improve the dielectric breakdown characteristics of silicone rubber, Patent Document 1 proposes blending boron nitride powder with an average particle size of 20 μm or less.

[0004] However, the demand for higher dielectric breakdown strength of silicone rubber is increasing year by year, and these conventional techniques are unable to achieve higher dielectric breakdown strength characteristics in the dielectric breakdown test specified in JIS K-6249.

[0005] Japanese Patent Application Publication No. 1-221454

[0006] The present invention has been made in view of the above circumstances, and has as its object to provide a silicone rubber composition that gives a cured product of silicone rubber that exhibits high dielectric breakdown strength, and a cured product of the silicone rubber composition.

[0007] As a result of extensive research to achieve the above object, the present inventors have discovered a method for producing an organopolysiloxane having (A) a degree of polymerization of 100 or more and having two or more alkenyl groups bonded to silicon atoms in each molecule, and (B) a polymerizable organopolysiloxane having a specific surface area of ​​50 m2 or less as measured by the BET method. 2 / g or more, (C) flat boron nitride particles having an average particle size of 3 to 50 μm and an aspect ratio of 5 or more, and (D) a curing catalyst. The present inventors have found that a silicone rubber composition containing such particles gives a cured product with excellent dielectric breakdown strength, which has led to the completion of the present invention.

[0008] That is, the present invention provides the following silicone rubber composition and its cured product: [1] (A) 100 parts by mass of an organopolysiloxane having a degree of polymerization of 100 or more and having two or more silicon-bonded alkenyl groups per molecule; (B) a silicone rubber composition having a specific surface area of ​​50 m2 measured by the BET method; 2 [2] A silicone rubber composition comprising: (A) a specific surface area of ​​50 m2 or more of component (B); (B) a specific surface area of ​​50 m2 or more of component (B) being 50 to 50 parts by mass of a reinforcing silica having a specific surface area of ​​50 m2 or more of component (B); (C) a flat boron nitride having an average particle size of 3 to 50 μm and an aspect ratio of 5 or more of component (C); and (D) a curing agent in an amount effective to cure component (A). 2 / g or more 400m 2 / g or less. [3] The silicone rubber composition according to [1] or [2], wherein the silicone rubber compound containing the components (A) and (B) has a plasticity of 200 or more. [4] The silicone rubber composition according to any one of [1] to [3], wherein the curing agent of the component (D) is an organic peroxide. [5] The silicone rubber composition according to any one of [1] to [3], further comprising (E) a silicone rubber compound having the following general formula (2): (In the formula, R 1 are hydrogen atoms or the same or different unsubstituted or substituted alkyl groups, and R 2 are the same or different unsubstituted or substituted monovalent hydrocarbon groups, and m is a positive number from 1 to 50. The silicone rubber composition according to any one of [1] to [4], containing 0.1 to 50 parts by mass of an organosilane or organosiloxane compound represented by the following formula (1):

[0009] According to the present invention, it is possible to obtain a silicone rubber composition that gives a cured silicone rubber product with excellent dielectric breakdown strength.

[0010] The present invention will be described in detail below, but the present invention is not limited to the following description. [Silicone Rubber Composition] The silicone rubber composition of the present invention contains the following components (A) to (D) and provides a cured silicone rubber product with excellent dielectric breakdown strength. (A) An organopolysiloxane having a degree of polymerization of 100 or more and having two or more alkenyl groups bonded to silicon atoms per molecule; (B) A silicone rubber composition having a specific surface area of ​​50 m2 or less as measured by the BET method; 2 / g or more; (C) flat boron nitride having an average particle size of 3 to 50 μm and an aspect ratio of 5 or more; (D) a hardener;

[0011] [(A) Alkenyl Group-Containing Organopolysiloxane] Component (A) in the silicone rubber composition of the present invention is an organopolysiloxane having a degree of polymerization of at least 100 and having two or more silicon-bonded alkenyl groups per molecule. This alkenyl group-containing organopolysiloxane is not particularly limited as long as it has the above degree of polymerization, but examples include those represented by the following average composition formula (1):

[0012] R a SiO (4-a) / 2 (1) (In the formula, R is the same or different, unsubstituted or substituted monovalent hydrocarbon group, and a is a positive number between 1.95 and 2.05. However, two or more of the R in one molecule are alkenyl groups.)

[0013] In the above average composition formula (1), R represents the same or different unsubstituted or substituted monovalent hydrocarbon groups, which may be linear, branched, or cyclic, preferably having 1 to 12 carbon atoms, more preferably having 1 to 8 carbon atoms, and even more preferably having 1 to 6 carbon atoms. Specific examples thereof 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. Further examples include groups in which some or all of the hydrogen atoms bonded to the carbon atoms of these groups have been substituted with halogen atoms such as chlorine, fluorine, or bromine, or with cyano groups (e.g., chloromethyl, trifluoropropyl, and cyanoethyl). Among these, R is preferably a methyl group, a vinyl group, a phenyl group, or a trifluoropropyl group.

[0014] Suitable examples of component (A) include organopolysiloxanes represented by the average composition formula (1) above, in which the main chain of the organopolysiloxane is composed of dimethylsiloxane units, and dimethylpolysiloxanes in which diphenylsiloxane units having phenyl groups, vinyl groups, 3,3,3-trifluoropropyl groups, or the like, methylvinylsiloxane units, methyl-3,3,3-trifluoropropylsiloxane units, or the like have been introduced into part of the main chain of the dimethylpolysiloxane.

[0015] The organopolysiloxane of component (A) has two or more alkenyl groups (preferably vinyl groups) bonded to silicon atoms per molecule, and of the groups bonded to silicon atoms in component (A) (i.e., R in the average composition formula (1) above), preferably 0.01 to 10 mol %, and more preferably 0.02 to 5 mol %, are alkenyl groups.

[0016] Although this alkenyl group may be bonded to a silicon atom at a molecular chain terminal, to a silicon atom in a side chain, or both, it is preferable that at least one be bonded to a silicon atom at a molecular chain terminal, and it is even more preferable that at least one be bonded to silicon atoms at both molecular chain terminals. In other words, the organopolysiloxane of component (A) is preferably one in which both molecular chain terminals are blocked with dimethylvinylsilyl groups, methyldivinylsilyl groups, trivinylsilyl groups, or the like.

[0017] 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. There are no particular restrictions on the shape of the organopolysiloxane of component (A), and it is generally a linear chain in which both molecular chain terminals are capped with triorganosiloxy groups and the main chain is composed of repeating diorganosiloxane units, although it may be branched as long as it does not impair rubber elasticity.

[0018] The degree of polymerization of the organopolysiloxane of component (A) is at least 100, preferably 3,000 to 100,000, and more preferably 4,000 to 20,000. If the degree of polymerization is less than 100, sufficient rubber strength cannot be obtained.

[0019] In this specification, the term "degree of polymerization" refers to the average degree of polymerization, which is determined from the weight-average molecular weight measured by gel permeation chromatography (GPC) under the following conditions using polystyrene as a standard (the same applies hereinafter): [Measurement conditions] Developing solvent: toluene Flow rate: 1 mL / min Detector: differential refractive index detector (RI) Column: TSKgel Super H-RC x 2 (manufactured by Tosoh Corporation) Column temperature: 25°C Sample injection volume: 20 μL (toluene solution with a concentration of 0.1% by mass)

[0020] Specific examples of component (A) include dimethylpolysiloxanes capped at both molecular chain terminals with dimethylvinylsiloxy groups, dimethylpolysiloxanes capped at both molecular chain terminals with methyldivinylsiloxy groups, dimethylpolysiloxanes capped at both molecular chain terminals with trivinylsiloxy groups, dimethylsiloxane-methylvinylsiloxane copolymers capped at both molecular chain terminals with trimethylsiloxy groups, dimethylsiloxane-methylvinylsiloxane copolymers capped at both molecular chain terminals with dimethylvinylsiloxy groups, and dimethylsiloxane-methylvinylsiloxane copolymers in which one molecular chain terminal is capped with dimethylvinylsiloxy groups and the other terminal is capped with trimethylsiloxy groups, etc. Among these, preferred are dimethylpolysiloxanes capped at both molecular chain terminals with dimethylvinylsiloxy groups, dimethylsiloxane-methylvinylsiloxane copolymers capped at both molecular chain terminals with trimethylsiloxy groups, and dimethylsiloxane-methylvinylsiloxane copolymers capped at both molecular chain terminals with dimethylvinylsiloxy groups.

[0021] The organopolysiloxane of component (A) may be used alone or in combination of two or more types with different molecular structures or degrees of polymerization.

[0022] Such organopolysiloxanes of component (A) can be obtained by known methods, such as (co)hydrolytic condensation of one or more organohalogenosilanes, or by ring-opening polymerization of a cyclic polysiloxane using an alkaline or acidic catalyst.

[0023] [(B) Reinforcing Silica] The component (B) in the silicone rubber composition of the present invention is a reinforcing silica having a specific surface area of ​​50 m2 as measured by the BET method. 2 / g or more.

[0024] The reinforcing silica of component (B) is a powdered reinforcing filler, and its type is not particularly limited, but examples include fumed silica (dry silica or fumed silica), calcined silica, and precipitated silica (wet silica), and of these, fumed silica is preferred from the viewpoint of dielectric breakdown strength.

[0025] The specific surface area of ​​the reinforcing silica of component (B) measured by the BET method is 50 m 2 / g or more, preferably 100m 2 The upper limit is not particularly limited, but is preferably 400 m 2 / g or less, more preferably 300m 2 / g or less. The specific surface area measured by the BET method is 50 m 2 If the viscosity is less than 1 / g, the resulting cured silicone rubber will have insufficient mechanical strength.

[0026] The reinforcing silica may be surface-untreated or surface-treated, and is not particularly limited, but if necessary, it is preferable to use silica whose surface has been hydrophobized, in terms of dispersibility in component (A) and reinforcing properties. In this case, silica that has been surface-hydrophobized directly with a silica surface treatment agent in a powder state may be used, or the silica fine powder may be mixed with component (A) by adding a silica surface treatment agent and heating and mixing, so that the surface is hydrophobized.

[0027] The surface treatment method is not particularly limited and can be selected appropriately from conventionally known methods. For example, a method can be used in which untreated silica fine powder and a silica surface treatment agent are placed in a mechanical kneading device or fluidized bed sealed at atmospheric pressure, and mixed at room temperature or by heat treatment, if necessary in the presence of an inert gas. In some cases, a catalyst can be used to promote the surface treatment. After kneading, drying can produce a reinforcing silica fine powder with a hydrophobic surface.

[0028] Specific examples of silica surface treatment agents include known treatment agents such as chlorosilanes such as trimethylchlorosilane, dimethyldichlorosilane, and methyltrichlorosilane, and silazanes such as hexamethyldisilazane and 1,3-divinyl-1,1,3,3-tetramethyldisilazane. The amount of silica surface treatment agent used is not particularly limited, but is preferably 5 to 75 parts by mass, and more preferably 5 to 60 parts by mass, per 100 parts by mass of the surface-untreated silica fine powder.

[0029] Commercially available products can be used as component (B), and examples of surface-untreated fumed silica include Aerosil 130, Aerosil 200, and Aerosil 300 manufactured by Nippon Aerosil Co., Ltd., and Reolosil QS-10, Reolosil QS-102, and Reolosil QS-30 manufactured by Tokuyama Corporation. Examples of surface-hydrophobized fumed silica include Aerosil R-812, Aerosil R-974, and Aerosil R976S manufactured by Nippon Aerosil Co., Ltd., and Reolosil DM-20 and DM-30 manufactured by Tokuyama Corporation. These may be used alone or in combination of two or more.

[0030] The amount of reinforcing silica (component (B)) added is 30 to 70 parts by mass, preferably 30 to 60 parts by mass, per 100 parts by mass of the organopolysiloxane (component (A)). If the amount of component (B) added is less than 30 parts by mass, the amount is too small, and the strength and processability of the resulting silicone rubber will be reduced. If it exceeds 70 parts by mass, the processability of the silicone rubber composition will be poor and the mechanical strength of the cured silicone rubber will be reduced.

[0031] [(C) Flake-like Boron Nitride] Component (C) in the silicone rubber composition of the present invention is flat boron nitride. The flat boron nitride of component (C) functions as a barrier layer against electrical breakdown in the cured silicone rubber.

[0032] The crystal structure of the flat boron nitride of component (C) is not particularly limited, but from the viewpoint of forming a barrier layer, hexagonal boron nitride is preferred. Furthermore, component (C) is particulate, and its shape is flat, but this also includes scale-like, plate-like, and flake-like shapes. By using such hexagonal boron nitride, the hexagonal boron nitride is oriented in its plane direction in the silicone rubber composition, and the function as a barrier layer against electrical breakdown after curing is strengthened.

[0033] The aspect ratio of the flat boron nitride of component (C) is 5 or more, preferably 7 or more. If the aspect ratio is less than 5, the orientation of the boron nitride in the plane direction is suppressed, making it impossible to obtain sufficient dielectric breakdown strength. The upper limit is not particularly limited, but is preferably 40 or less, more preferably 35 or less, and even more preferably 30 or less. This aspect ratio is the value obtained by dividing the major axis of the particle by the thickness of the particle, i.e., major axis / thickness. When the particle is spherical, the aspect ratio is 1, and as the degree of flatness increases, the aspect ratio increases. In the present invention, the above aspect ratio can be obtained by measuring the major axis and thickness of the particle using a scanning electron microscope and calculating the major axis / thickness.

[0034] The average particle size of the flat boron nitride particles of component (C) is 3 to 50 μm, preferably 5 to 50 μm, and more preferably 8 to 50 μm. If the particle size is less than 3 μm, the gaps between the boron nitride particles oriented in the silicone rubber composition will become large, and the function of the barrier layer against electrical breakdown after curing will be reduced, preventing the expression of sufficient dielectric breakdown strength. If the particle size is greater than 50 μm, processability will be significantly reduced.

[0035] The flat boron nitride of component (C) may have a single average particle size, or a mixture of multiple types having different average particle sizes. In the present invention, the average particle size is the cumulative 50% particle size (D50) on a volume basis measured using a particle size distribution analyzer MT3000II manufactured by Microtrac Bell.

[0036] The amount of component (C) added is 5 to 50 parts by mass, preferably 10 to 50 parts by mass, and more preferably 10 to 40 parts by mass, per 100 parts by mass of component (A).

[0037] [(D) Curing Agent] The component (D) in the silicone rubber composition of the present invention is a curing agent. There are no particular limitations on the curing agent for component (D), 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 that combine an organohydrogenpolysiloxane (a hydrosilyl group-containing organopolysiloxane) with a platinum group metal catalyst (a hydrosilylation catalyst), etc., can be used.

[0038] (D-1) Organic Peroxide Curing Agent Examples of organic peroxide curing agents include benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-methylbenzoyl peroxide, o-methylbenzoyl peroxide, 2,4-dicumyl peroxide, 2,5-dimethyl-bis(2,5-t-butylperoxy)hexane, di-t-butyl peroxide, t-butyl perbenzoate, 1,6-hexanediol-bis-t-butylperoxycarbonate, etc. These may be used alone or in combination of two or more.

[0039] The amount of organic peroxide curing agent added should be an effective amount for curing component (A), and is preferably 0.1 to 10 parts by mass, and more preferably 0.2 to 5 parts by mass, per 100 parts by mass of the organopolysiloxane of component (A). If this amount is too small, the silicone rubber composition may not cure sufficiently, while if the amount added is too large, the decomposition residue of the organic peroxide may cause discoloration of the cured silicone rubber.

[0040] (D-2) Addition Reaction Curing Agent When curing with an addition reaction curing agent, an organohydrogenpolysiloxane and a platinum group metal catalyst are used. The organohydrogenpolysiloxane may be linear, branched, or cyclic, as long as it contains two or more, preferably three or more, more preferably 3 to 200, and even more preferably about 4 to 100 hydrogen atoms bonded to silicon atoms (SiH groups) per molecule, but preferably has a degree of polymerization of 300 or less.

[0041] Specific examples of such organohydrogenpolysiloxanes include dimethylpolysiloxanes blocked at both ends with dimethylhydrogensiloxy groups, dimethylsiloxane-methylhydrogensiloxane copolymers blocked at both ends with trimethylsiloxy groups, dimethylsiloxane-methylhydrogensiloxane copolymers blocked at both ends with dimethylhydrogensiloxy groups, and copolymers containing dimethylhydrogensiloxane units (H(CH3)2SiO 0.5 units) and SiO units, 1,3,5,7-tetrahydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, 1-propyl-3,5,7-trihydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, 1,5-dihydrogen-3,7-dihexyl-1,3,5,7-tetramethylcyclotetrasiloxane, and the like, as well as compounds in which some or all of the methyl groups in the above exemplary compounds have been substituted with other alkyl groups or aryl groups such as phenyl groups.

[0042] The amount of organohydrogenpolysiloxane added as a curing agent is preferably such that hydrogen atoms directly bonded to silicon atoms (SiH groups) account for 50 to 500 mol % of the aliphatic unsaturated groups (alkenyl groups) in the organopolysiloxane of component (A) ((SiH groups / aliphatic unsaturated groups) x 100). If this ratio is less than 50 mol %, crosslinking will be insufficient, which may have adverse effects on the physical and electrical properties of the cured silicone product, while if it exceeds 500 mol %, the physical properties after curing may be reduced.

[0043] On the other hand, platinum group metal catalysts include platinum group metals (platinum, palladium, rhodium, etc.) and compounds thereof, and specific examples thereof include elemental platinum, platinum compounds, platinum complexes, chloroplatinic acid, alcohol compounds of chloroplatinic acid, aldehyde compounds, ether compounds, and complexes with various olefins.

[0044] The amount of platinum group metal catalyst added is preferably 1 to 2,000 ppm (by mass) of platinum group metal atoms relative to the organopolysiloxane of component (A). If this amount is less than the above lower limit, the addition reaction may not be sufficiently promoted, resulting in insufficient curing. On the other hand, if the amount exceeds the above upper limit, the effect on reactivity may be reduced, which may be uneconomical.

[0045] [(E) Organosilane / Organosiloxane Compound] In addition to the above components, the silicone rubber composition of the present invention preferably contains, as an optional component, (E) an organosilane or organosiloxane compound represented by the following general formula (2): Component (E) acts as a dispersant (wetter) to improve the dispersibility of the reinforcing silica of component (B) in component (A), and by incorporating component (E), the workability, extrusion characteristics, etc. of the silicone rubber composition of the present invention are improved.

[0046] (In the formula, R 1 are hydrogen atoms or the same or different unsubstituted or substituted alkyl groups, and R 2 are the same or different unsubstituted or substituted monovalent hydrocarbon groups, and m is a positive number from 1 to 50.

[0047] Here, R 1 are hydrogen atoms or the same or different alkyl groups, and the organosilane or organosiloxane represented by the general formula (2) has an alkoxy group or a hydroxyl group at the molecular chain terminal.

[0048] In formula (2), R 1 The alkyl group in R preferably has 1 to 4 carbon atoms, and specific examples thereof include a methyl group, an ethyl group, a propyl group, and a butyl group. Also included are groups in which some or all of the hydrogen atoms in these groups have been substituted with halogen atoms such as chlorine, fluorine, or bromine, or with a cyano group (for example, a trifluoropropyl group, a cyanoethyl group, etc.). Among these, R 1 is preferably a hydrogen atom, a methyl group, or an ethyl group.

[0049] R 2The monovalent hydrocarbon group may be linear, branched, or cyclic, and preferably has 1 to 12 carbon atoms, more preferably 1 to 8 carbon atoms, and even more preferably 1 to 6 carbon atoms. Specific examples thereof include the same groups as those exemplified for R, and among these, a methyl group, a vinyl group, a phenyl group, and a trifluoropropyl group are preferred, and a methyl group, a vinyl group, and a trifluoropropyl group are more preferred.

[0050] m is a positive number of 1 to 50, preferably a positive number of 1 to 30, and more preferably a positive number of 1 to 20. When m is 50 or less, a sufficient effect of addition can be obtained without compounding a large amount, and therefore there is no risk of deterioration in the rubber properties due to compounding a large amount.

[0051] The kinematic viscosity of component (E) is 1 to 500 mm 2 / s is preferred, and 1 to 100 mm 2 The kinematic viscosity is a value measured at 25°C using an Ostwald viscometer.

[0052] Specific examples of component (E) include dimethylsilanediol, diethylsilanediol, diphenylsilanediol; dimethylpolysiloxane having silanol groups at both ends; and methylphenylpolysiloxane having silanol groups at both ends.

[0053] When component (E) is compounded, the amount of organosilane / organosiloxane of component (E) added is preferably 0.1 to 50 parts by mass, more preferably 0.5 to 30 parts by mass, and even more preferably 0.5 to 20 parts by mass, per 100 parts by mass of the organopolysiloxane of component (A). When the amount is 0.1 part by mass or more, the workability, extrusion characteristics, etc. of the silicone rubber composition of the present invention are sufficient, while when the amount is 50 parts by mass or less, there is no risk of the resulting silicone rubber composition becoming tacky, and therefore there is no risk of reduced processability or reduced physical properties of the resulting rubber.

[0054] [Other Components] In addition to the above components, the silicone rubber composition of the present invention may contain optional components, as needed, within the range that does not impair the effects of the present invention. These optional components include platinum compounds other than the platinum group metal catalysts described above, iron oxides, flame retardants such as halogen compounds, heat resistance improvers such as cerium oxide, antioxidants, ultraviolet absorbers, colorants, and mold release agents, and are known additives for silicone rubber compositions.

[0055] The silicone rubber composition of the present invention can be produced by kneading the predetermined amounts of the above-mentioned components using a known kneading machine, such as a two-roll mill, kneader, or Banbury mixer. Heat treatment (kneading under heat) may be performed as needed. Specifically, components (A) and (B) are kneaded together (when component (E) is added, components (A), (B), and (E) are kneaded together), and then component (D) is added at room temperature. While component (C) may be added before or after heat treatment, it is preferable to add component (D) after heat treatment. Furthermore, component (D) is preferably added after component (C) has been added. While the heat treatment temperature and time are not particularly limited, they are preferably 100 to 250°C, more preferably 140 to 180°C, and are preferably about 30 minutes to 5 hours.

[0056] In the present invention, the mixture obtained by mixing components (A) and (B) (or components (A), (B), and (E) when component (E) is blended) is referred to as a silicone rubber compound. In the present invention, the plasticity of this silicone rubber compound is preferably 200 or more, more preferably 220 or more. If the plasticity is less than 200, the orientation of boron nitride (C) in the silicone rubber composition may be insufficient when blended, and the desired dielectric breakdown strength may not be obtained after curing. The above-mentioned plasticity value refers to the Williams plasticity value measured in accordance with JIS-K6249:2003 for the silicone rubber compound obtained by blending components (A) and (B) (or components (A), (B), and (E) when component (E) is blended).

[0057] [Cured Product] When molding the silicone rubber composition of the present invention, the molding method can be appropriately selected depending on the required application (molded product). Specific molding methods include compression molding, injection molding, transfer molding, atmospheric hot air vulcanization, and steam vulcanization. Curing conditions are not particularly limited and can be appropriately selected depending on the curing method and molded product. Generally, primary vulcanization is preferably carried out at 80 to 600°C, more preferably 100 to 450°C, for a period of preferably several seconds to several days, more preferably about 5 seconds to 1 hour. Secondary vulcanization may also be performed if necessary. Secondary vulcanization can be carried out, for example, at 180 to 250°C for about 1 to 10 hours.

[0058] As described above, the silicone rubber composition of the present invention can provide a cured silicone rubber product with excellent dielectric breakdown properties. That is, the cured product of the silicone rubber composition of the present invention preferably has a dielectric breakdown strength of 30 kV / mm or more, more preferably 35 kV / mm. The dielectric breakdown strength value is measured in accordance with JIS K 6249:2003.

[0059] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. The measurement methods used in the following examples are as follows. [Average Degree of Polymerization (GPC Measurement Conditions)] Measurement apparatus: HLC-8420GPC manufactured by Tosoh Corporation Developing solvent: toluene Flow rate: 1 mL / min Detector: Differential refractive index detector (RI) Column: TSKgel Super H-RC x 2 (manufactured by Tosoh Corporation) Column temperature: 25°C Sample injection amount: 20 μL (toluene solution with a concentration of 0.1% by mass) [Aspect Ratio] The major axis and thickness of the particles were measured using a scanning electron microscope (model number JSM-7900F, manufactured by JEOL Ltd.), and the major axis / thickness ratio was calculated. [Average Particle Size] The volume-based cumulative 50% particle size (D50) was measured using a particle size distribution analyzer MT3000II manufactured by Microtrac Bell. [Kinematic viscosity] Measured using an Ostwald viscometer at 25°C. [Plasticity] The silicone rubber compound obtained by kneading components (A), (B), and (E) was kneaded 15 times using a triple roll mill, and 10 minutes later, the Williams plasticity was measured using the method described in JIS-K6249:2003.

[0060] Example 1: 100 parts by mass of methylvinylpolysiloxane crude rubber (A) consisting of 99.85 mol % dimethylsiloxane units and 0.15 mol % methyl-vinylsiloxane units, with both molecular chain terminals blocked with dimethylvinylsiloxane units and an average degree of polymerization of approximately 7,000; 2 35 parts by mass of fumed silica (Aerosil 200 (manufactured by Nippon Aerosil Co., Ltd.)) (B) of 1 / g, a dispersant having silanol groups at both ends, an average degree of polymerization of 13, and a kinematic viscosity at 25°C of 15 mm 2 A silicone rubber compound was obtained by kneading 5 parts by mass of dimethylpolysiloxane (E) having a viscosity of 1 / s in a kneader. The obtained silicone rubber compound was heat-treated at 180°C for 3 hours, and after cooling, 30 parts by mass of flat boron nitride (C-1) having an aspect ratio of 7 and an average particle size of 3 μm was added and kneaded to obtain a millable silicone rubber composition. 0.8 parts by mass of organic peroxide p-methylbenzoyl peroxide (D-1) was added to this, and the mixture was uniformly mixed using a two-roll mill to obtain a silicone rubber composition.

[0061] The resulting silicone rubber composition was press-cured for 10 minutes at 120°C and 686 Pa to prepare a 1 mm thick sheet for testing dielectric breakdown strength. This test sheet was then post-cured for 1 hour in an oven at 150°C. The dielectric breakdown strength of the resulting cured product was measured according to the method described in JIS K 6249:2003 using Trans Oil A oil manufactured by Showa Shell Sekiyu K.K. at a voltage rise rate of 2 kV / sec, and the results are shown in Table 1.

[0062] [Example 2] In Example 1, the specific surface area measured by the BET method was 200 m 2 The dielectric breakdown strength was measured using the same millable silicone rubber composition, except that the amount of fumed silica (B) having a molecular weight of 1 / g was changed to 47 parts by mass and the amount of dimethylpolysiloxane (E) having silanol groups at both ends as a dispersant was changed to 10 parts by mass.

[0063] Example 3 The dielectric breakdown strength was measured using the same millable silicone rubber composition as in Example 2, except that the component (C-1) was changed to 30 parts by mass of flat boron nitride (C-2) having an aspect ratio of 10 and an average particle size of 10 μm.

[0064] Example 4 The dielectric breakdown strength was measured using the same millable silicone rubber composition as in Example 2, except that the component (C-1) was changed to 30 parts by mass of flat boron nitride (C-3) having an aspect ratio of 25 and an average particle size of 45 μm.

[0065] Example 5 The dielectric breakdown strength was measured using the same millable silicone rubber composition as in Example 2, except that the component (C-1) was changed to 30 parts by mass of flat boron nitride (C-4) having an aspect ratio of 28 and an average particle size of 50 μm.

[0066] Example 6 The dielectric breakdown strength was measured using the same millable silicone rubber composition as in Example 4, except that the amount of flat boron nitride (C-3) having an aspect ratio of 25 and an average particle size of 45 μm was changed to 5 parts by mass.

[0067] Comparative Example 1 The dielectric breakdown strength was measured using a millable type silicone rubber composition that was the same as in Example 2, except that the flat boron nitride (C) was not added.

[0068] [Comparative Example 2] In Example 1, the specific surface area measured by the BET method was 200 m 2 The dielectric breakdown strength was measured using the same millable silicone rubber composition except that the amount of fumed silica (B) of 1 / g was changed to 25 parts by mass and the amount of dimethylpolysiloxane (E) having silanol groups at both ends as a dispersant was changed to 4 parts by mass, and the flat boron nitride (C) was not added.

[0069] Comparative Example 3 The dielectric breakdown strength was measured using the same millable silicone rubber composition as in Comparative Example 2, except that 30 parts by mass of flat boron nitride (C-1) having an aspect ratio of 7 and an average particle size of 3 μm was added.

[0070] Comparative Example 4 The dielectric breakdown strength was measured using the same millable silicone rubber composition as in Example 2, except that the component (C-1) was changed to 30 parts by mass of flat boron nitride (C-5) having an aspect ratio of 3 and an average particle size of 3 μm.

[0071] Comparative Example 5 The dielectric breakdown strength was measured using the same millable silicone rubber composition as in Example 2, except that the component (C-1) was changed to 30 parts by mass of flat boron nitride (C-6) having an aspect ratio of 4 and an average particle size of 20 μm.

[0072]

[0073] From the results in Table 1 above, as shown in Comparative Example 4, when the aspect ratio of the added boron nitride is as low as 3, a cured product having a dielectric breakdown characteristic exceeding 30 kV / mm cannot be obtained, even with the same amount of boron nitride added as in Examples 1 to 5. Furthermore, as shown in Comparative Example 3, even when the aspect ratio of the added boron nitride is 7, a cured product having a dielectric breakdown characteristic exceeding 30 kV / mm cannot be obtained if the amount of silica is small and the plasticity is low at 140. On the other hand, as shown in Examples 1 and 2, when the components specified in the present invention are contained and the plasticity is 200 or more, high dielectric breakdown characteristics exceeding 30 kV / mm can be achieved after curing. Furthermore, as shown in Examples 4 and 5, even if the particle size of the boron nitride is large, a cured product having good dielectric breakdown characteristics can be obtained by adding boron nitride with a high aspect ratio.

Claims

1. (A) 100 parts by mass of an organopolysiloxane having a degree of polymerization of 100 or more and having two or more alkenyl groups bonded to silicon atoms per molecule; (B) a polymer having a specific surface area of ​​50 m2 measured by the BET method; 2 / g or more: 30 to 70 parts by mass of reinforcing silica; (C) flaky boron nitride having an average particle size 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 component (A).

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

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

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

5. Furthermore, (E) the following general formula (2): (In the formula, R 1 are hydrogen atoms or the same or different unsubstituted or substituted alkyl groups, and R 2 are the same or different unsubstituted or substituted monovalent hydrocarbon groups, and m is a positive number from 1 to 50.

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

7. The cured product according to claim 6, which has a dielectric breakdown strength of 30 kV / mm or more as measured in accordance with JIS K 6249:2003.

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