Silica sol dispersed in nitrogen-containing organic solvent containing organic acid, and insulating resin composition

Abstract

Provided are: a silica sol dispersed in a nitrogen-containing organic solvent that contains an organic acid; and an insulating resin composition containing the silica sol and a nitrogen-containing polymer.　This silica sol is obtained by dispersing silica particles having an average primary particle size of 5-100 nm in a nitrogen-containing organic solvent. In the silica sol, a carboxylic acid having 1-3 carbon atoms is contained at a proportion of 80-1500 ppm. In the silica sol, the nitrogen-containing organic solvent is an amide-based solvent. In the silica sol, the nitrogen-containing organic solvent is dimethylacetamide, dimethylformamide, or dimethylpropionamide. In the silica sol, the carboxylic acid having 1-3 carbon atoms is formic acid, acetic acid, or propionic acid. This insulating resin composition contains the silica sol and a nitrogen-containing polymer.

Description

Silica sol dispersed in nitrogen-containing organic solvent containing organic acid and insulating resin composition

[0001] The present invention relates to a silica sol dispersed in a nitrogen-containing organic solvent containing an organic acid such as acetic acid, an insulating resin composition using the same, and methods for producing the same.

[0002] Silica sols in which surface-modified silica particles are dispersed in a solvent are known. For example, a method has been disclosed in which hydroxyl groups on the surface of inorganic oxide particles such as silica react with alcohol to introduce alkoxyl groups, thereby organifying the particles and obtaining an inorganic oxide sol dispersed in an organic solvent such as toluene (see Patent Document 1). This method discloses reacting phenyltrimethoxysilane with a methanol-dispersed silica sol and dispersing the resulting silica sol in a toluene solvent.

[0003] Also disclosed is a silica sol obtained by solvent-substituting a methanol-dispersed silica sol with acetonitrile to obtain a silica sol dispersed in an acetonitrile-methanol mixed solvent, and then reacting the resulting silica sol with phenyltrimethoxysilane (see Patent Document 2).

[0004] Furthermore, a silica sol in which the surfaces of silica particles are modified with an aluminum compound has been disclosed (see Patent Document 3).

[0005] Also disclosed is an aluminum-containing silica sol dispersed in a nitrogen-containing solvent, and an insulating resin composition using the same (see Patent Document 4).

[0006] JP 2005-200294 A, WO 2009 / 008509 A, JP 2011-026183 A, WO 2022 / 097694 A

[0007] The present invention aims to provide a silica sol in which silica particles are dispersed in a nitrogen-containing organic solvent that allows for good compatibility with polar resins such as polyimide and polyamide resins, and also to provide a resin composition containing the silica sol and a resin, which, when used as an insulating resin composition, can provide an insulating coated conductor that can maintain a long insulation life.

[0008] The present invention provides, as a first aspect, a silica sol in which silica particles having an average primary particle size of 5 to 100 nm are dispersed in a nitrogen-containing organic solvent, the silica sol containing a carboxylic acid having 1 to 3 carbon atoms in an amount of 80 to 1500 ppm; as a second aspect, the silica sol according to the first aspect, in which the nitrogen-containing organic solvent is an amide solvent; as a third aspect, the silica sol according to the first or second aspect, in which the nitrogen-containing organic solvent is dimethylacetamide, dimethylformamide, or dimethylpropionamide; as a fourth aspect, the silica sol according to any one of the first to third aspects, in which the carboxylic acid having 1 to 3 carbon atoms is formic acid, acetic acid, or propionic acid; as a fifth aspect, the silica sol according to any one of the first to fourth aspects, in which the amount of water in the silica sol is 0.1 to 10.0 mass %; and as a sixth aspect, SiO ２ The silica sol according to any one of the first to fifth aspects, which has a viscosity of 3 to 500 mPa s when measured at 25°C in a concentration of 30% by mass; the silica sol according to any one of the first to sixth aspects, which contains alkali metal ions (wherein the alkali metal ions are alkali metal ions consisting of lithium, sodium, and potassium) in a proportion of 300 ppm or less; and the silica sol according to an eighth aspect, which is a silica sol represented by any one of formulas (1) to (3):

[0009]

[0010] (In formula (1), R １ are each an alkyl group, a halogenated alkyl group, an alkenyl group, an aryl group, or an organic group having a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a carboxyl group, an acid anhydride group, a carboxylic acid ester group, an epoxy group, a hydroxyl group, or a cyano group, and are bonded to a silicon atom by a Si—C bond, and R ２ represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group, and a represents an integer of 1 to 3. In formulas (2) and (3), R ３ and R ５are each an alkyl group having 1 to 3 carbon atoms or an aryl group having 6 to 30 carbon atoms and bonded to a silicon atom by a Si—C bond, and R ４ and R ６ each represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; Y represents an alkylene group, an NH group, or an oxygen atom; b is an integer of 1 to 3; c is an integer of 0 or 1; and d is an integer of 1 to 3. the silica sol according to any one of the first to seventh aspects, which is coated with at least one silane compound selected from the group consisting of: or a hydrolyzate thereof; as a ninth aspect, an insulating resin composition comprising the silica sol according to any one of the first to eighth aspects and a nitrogen-containing polymer; as a tenth aspect, the insulating resin composition according to the ninth aspect, in which the ratio of the nitrogen-containing polymer to 1 part by mass of silica contained in the silica sol is 1 to 100; as an eleventh aspect, the insulating resin composition according to the ninth or tenth aspect, in which the nitrogen-containing polymer is polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, or polyesterimide; as a twelfth aspect, an insulating resin composition comprising the silica sol according to any one of the first to eighth aspects and a polyamic acid comprising 4,4'-diaminodiphenyl ether (DDE) and pyromellitic anhydride (PMDA) as a resin, and a resin / SiO ２ a silica-blended polyamic acid adjusted to a mass ratio of 80 / 20, the viscosity (mPa·s) of which after storage at 50°C for 7 days is 1.20 times or less of the viscosity before storage; and as a thirteenth aspect, an insulating resin composition comprising the silica sol according to any one of the first to eighth aspects, 4,4′-diaminodiphenyl ether (DDE), and polyamic acid composed of pyromellitic anhydride (PMDA) as a resin, and a resin / SiO ２an insulating resin composition in which a silica-blended polyamic acid adjusted to a mass ratio of 80 / 20 is heated at 290°C on a Cu plate to obtain a silica-blended polyimide (coating thickness: 29 to 32 µm), and a dielectric breakdown life is 50 minutes or longer at a test temperature of 155°C (in air), an applied voltage of 3.0 kV, and a frequency of 50 Hz; as a fourteenth aspect, an insulating coated conductor that is insulated and coated with the insulating resin composition according to any one of the ninth to thirteenth aspects; as a fifteenth aspect, the present invention provides an insulating resin composition comprising the following steps (A) and (B): step (A): preparing a silica sol in which silica particles having an average primary particle size of 5 to 100 nm are dispersed in an aqueous medium; a method for producing an insulating resin composition according to any one of the first to eighth aspects, the method including: a step (B) of substituting the silica sol obtained in the step (A) with a nitrogen-containing organic solvent while adjusting the silica sol to contain a carboxylic acid having 1 to 3 carbon atoms at a ratio of 80 to 1500 ppm; a method for producing an insulating resin composition according to the fifteenth aspect, the method including, as a sixteenth aspect, a step (C) of adding at least one silane compound represented by any one of formulas (1) to (3) of claim 7 during the step (B) or after completion of the step (B); a method for producing an insulating resin composition according to any one of the ninth to fourteenth aspects, the method including, as a seventeenth aspect, a step (D) of mixing the silica sol obtained in the fifteenth or sixteenth aspect with a nitrogen-containing polymer; and an eighteenth aspect, the method for producing an insulating resin composition according to the seventeenth aspect, the method including, in addition to the step (D), a step (E) of removing part or all of the nitrogen-containing organic solvent from the insulating resin composition.

[0011] The insulating resin obtained by coating and curing an insulating resin composition can contain silica particles in the insulating resin composition to improve the insulation resistance of the substrate. The silica particles form a tight, strong coating layer with the insulating resin, thereby protecting the substrate from dielectric breakdown due to discharge. Nitrogen-containing polymers with high insulating properties are often used as insulating resins. These nitrogen-containing polymers include, for example, polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, and polyesterimide. These nitrogen-containing polymers are synthesized from diamines and acid anhydrides, and have both polar moieties, such as an imide skeleton, a carboxyl group, or an amide bond, and hydrophobic moieties contained in the diamine molecule or the acid anhydride molecule.

[0012] The insulating resin composition is produced by mixing a silica sol dispersed in a nitrogen-containing organic solvent that is highly compatible with the nitrogen-containing polymer with the nitrogen-containing polymer. When the insulating resin composition is used as an enameled wire coating material, proper viscosity control is important in the field to coat the enameled wire with a uniform film thickness.

[0013] According to the present invention, at the stage of silica sol dispersed in a nitrogen-containing organic solvent, it is possible to suppress an increase in viscosity of the silica sol by adding a specific amount of organic acid to the nitrogen-containing organic solvent.Furthermore, according to the present invention, when the silica sol dispersed in the nitrogen-containing organic solvent is mixed with a nitrogen-containing polymer to form an insulating resin composition (varnish), it is possible to suppress an increase in viscosity of the insulating resin composition by adding a specific amount of organic acid.

[0014] The present invention also relates to a silica sol dispersed in a nitrogen-containing organic solvent and a method for producing an insulating resin composition by adding the sol to a nitrogen-containing polymer, and includes cases where an organic acid is originally contained in the nitrogen-containing organic solvent or the nitrogen-containing polymer, and cases where an organic acid is newly added. By measuring these and adjusting the content within the range set by the present invention, it is possible to achieve stabilization of the viscosity of the silica sol or the insulating resin composition.

[0015] Preferred embodiments of the present invention will be described below. However, the following embodiments are merely examples for explaining the present invention, and the present invention is not limited to these embodiments. One embodiment of the present invention is a silica sol in which silica particles having an average primary particle size of 5 to 100 nm are dispersed in a nitrogen-containing organic solvent, and the silica sol contains 80 to 1500 ppm of a carboxylic acid having 1 to 3 carbon atoms.

[0016] In one embodiment of the present invention, the silica particles contained in the silica sol have an average primary particle size of 5 to 100 nm, and the average primary particle size of the silica particles can be a particle size (nm) measured by a nitrogen gas adsorption method (BET method).

[0017] The nitrogen-containing organic solvent used in the present invention has a functional group containing at least a nitrogen atom. Examples of functional groups containing a nitrogen atom include an amino group, a nitro group, and a cyano group. Among these, amide-based solvents in which a nitrogen-containing functional group and a carbonyl group exist in one solvent molecule are preferred, and examples of such solvents include those having a chain structure or a cyclic structure. Examples of functional groups containing a nitrogen atom include an amino group, a nitro group, and a cyano group, with the amino group being preferred. The amino group and the carbonyl group can be adjacent to each other or can exist via a carbon atom, but can be used, for example, as an amide bond, and amide-based solvents are preferred.

[0018] Specific examples of nitrogen-containing organic solvents include dimethylformamide, diethylformamide, dimethylacetamide, diethylacetamide, dimethylpropionamide, N-methylpyrrolidone, N-ethylpyrrolidone, tetramethylurea, hexamethylphosphoric triamide, dimethylacrylamide, acryloylmorpholine, hydroxyethylacrylamide, isopropylacrylamide, diethylacrylamide, dimethylaminopropylacrylamide, 3-methoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide, dimethylaminopropylacrylamide methyl chloride quaternary salt, and dimethylaminoethyl acrylate benzyl chloride quaternary salt.

[0019] Among these, dimethylacetamide, dimethylformamide, or dimethylpropionamide can be preferably used as the nitrogen-containing organic solvent. The nitrogen-containing organic solvent may contain other solvents as long as the effects of the present invention are not impaired.

[0020] That is, the nitrogen-containing organic solvent may be contained in an amount of 50 to 100% by volume, 90 to 100% by volume, 98 to 100% by volume, or 99 to 100% by volume based on the total solvent, and other solvents may be contained in an amount of 0 to less than 50% by volume, 0 to less than 10% by volume, 0 to less than 2% by volume, or 0 to less than 1% by volume. Examples of other solvents include water, ketone-based solvents, ester-based solvents, alcohol-based solvents, glycol ether-based solvents, hydrocarbon-based solvents, halogen-based solvents, ether-based solvents, glycol-based solvents, and amine-based solvents.

[0021] Specific examples of such solvents include ketone-based solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, and cyclohexanone; ester-based solvents such as methyl acetate, ethyl acetate, and butyl acetate; alcohol-based solvents such as methanol, ethanol, isopropanol, and benzyl alcohol; glycol ether-based solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoethyl ether acetate, ethylene glycol monopropyl ether, and diethylene glycol monobutyl ether; hydrocarbon-based solvents such as benzene, toluene, xylene, n-hexane, and cyclohexane; halogen-based solvents such as dichloromethane, trichloroethylene, and perchloroethylene; ether-based solvents such as dioxane, diethyl ether, and tetrahydrofuran; glycol-based solvents such as ethylene glycol, diethylene glycol, propylene glycol, and polyethylene glycol; and amine-based solvents such as monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, N-methylethanolamine, and 2-amino-2-methyl-1-propanol.

[0022] In one embodiment of the present invention, the organic acid is preferably a carboxylic acid having 1 to 3 carbon atoms, and more preferably formic acid, acetic acid, or propionic acid, because these organic acids are highly compatible with nitrogen-containing organic solvents and nitrogen-containing polymers.

[0023] In one embodiment of the present invention, the water content in the silica sol is preferably 0.1 to 10.0% by mass. ２ When the concentration is 30% by mass, the viscosity measured at 25° C. is preferably 3 to 500 mPa·s.

[0024] The silica sol may contain alkali metal ions (which include lithium, sodium, and potassium) in an amount of 300 ppm or less, 30 to 300 ppm, or 30 to 200 ppm. For example, by controlling the amount of sodium ions in the alkali metal ions to fall within the above range, an increase in viscosity of the silica sol dispersed in the nitrogen-containing organic solvent can be suppressed.

[0025] In one embodiment of the present invention, the silica sol may contain alkali metal ions (wherein alkali metals refer to alkali metal ions consisting of lithium, sodium, and potassium) at a concentration of 300 ppm or less, or 30 to 300 ppm, or 30 to 200 ppm, and may contain 0.03 mass % or less, or 0.003 mass % to 0.03 mass %, or 0.003 mass % to 0.02 mass % in the silica sol. For example, the SiO contained in a silica sol having a silica concentration of 30 mass % may be ２ M of 1000 ppm or less, or 100 ppm to 1000 ppm or less, or 100 ppm to 6.70 ppm or less, relative to the mass of ２ In addition, when the insulating resin composition is produced by mixing the silica sol dispersed in the nitrogen-containing organic solvent with the nitrogen-containing polymer, the insulating resin composition may also contain an alkali metal converted to SiO. ２ M of 1000 ppm or less, or 100 ppm to 1000 ppm or less, or 100 ppm to 670 ppm or less ２It is preferable that an alkali metal converted into O is contained.

[0026] The silica particles in the silica sol of the present invention can be coated by adding at least one silane compound selected from the group consisting of formulas (1) to (3). １ are each an alkyl group, a halogenated alkyl group, an alkenyl group, an aryl group, or an organic group having a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a carboxyl group, an acid anhydride group, a carboxylic acid ester group, an epoxy group, a hydroxyl group, or a cyano group, and are bonded to a silicon atom by a Si—C bond, and R ２ represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group, and a represents an integer of 1 to 3. In formulas (2) and (3), R ３ and R ５ are each an alkyl group having 1 to 3 carbon atoms or an aryl group having 6 to 30 carbon atoms and bonded to a silicon atom by a Si—C bond, and R ４ and R ６ each represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; Y represents an alkylene group, an NH group, or an oxygen atom; b is an integer of 1 to 3; c is an integer of 0 or 1; and d is an integer of 1 to 3.

[0027] The alkyl group is an alkyl group having 1 to 18 carbon atoms, and examples thereof include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, a cyclopropyl group, an n-butyl group, an i-butyl group, an s-butyl group, a t-butyl group, a cyclobutyl group, a 1-methyl-cyclopropyl group, a 2-methyl-cyclopropyl group, an n-pentyl group, a 1-methyl-n-butyl group, a 2-methyl-n-butyl group, a 3-methyl-n-butyl group, a 1,1-dimethyl-n-propyl group, a 1,2-dimethyl-n-propyl group, a 2,2-dimethyl-n-propyl group, a 1-ethyl -n-propyl group, cyclopentyl group, 1-methyl-cyclobutyl group, 2-methyl-cyclobutyl group, 3-methyl-cyclobutyl group, 1,2-dimethyl-cyclopropyl group, 2,3-dimethyl-cyclopropyl group, 1-ethyl-cyclopropyl group, 2-ethyl-cyclopropyl group, n-hexyl group, 1-methyl-n-pentyl group, 2-methyl-n-pentyl group, 3-methyl-n-pentyl group, 4-methyl-n-pentyl group, 1,1-dimethyl-n-butyl group, 1,2-dimethyl-n-butyl group, 1,3-dimethyl-n-butyl group group, 2,2-dimethyl-n-butyl group, 2,3-dimethyl-n-butyl group, 3,3-dimethyl-n-butyl group, 1-ethyl-n-butyl group, 2-ethyl-n-butyl group, 1,1,2-trimethyl-n-propyl group, 1,2,2-trimethyl-n-propyl group, 1-ethyl-1-methyl-n-propyl group, 1-ethyl-2-methyl-n-propyl group, cyclohexyl group, 1-methyl-cyclopentyl group, 2-methyl-cyclopentyl group, 3-methyl-cyclopentyl group, 1-ethyl-cyclobutyl group, 2-ethyl-cyclobutyl group group, 3-ethyl-cyclobutyl group, 1,2-dimethyl-cyclobutyl group, 1,3-dimethyl-cyclobutyl group, 2,2-dimethyl-cyclobutyl group, 2,3-dimethyl-cyclobutyl group, 2,4-dimethyl-cyclobutyl group, 3,3-dimethyl-cyclobutyl group, 1-n-propyl-cyclopropyl group, 2-n-propyl-cyclopropyl group, 1-i-propyl-cyclopropyl group, 2-i-propyl-cyclopropyl group, 1,2,2-trimethyl-cyclopropyl group, 1,2,3-trimethyl-cyclopropyl group, 2,2,Examples of alkylene groups include, but are not limited to, 3-trimethylcyclopropyl, 1-ethyl-2-methylcyclopropyl, 2-ethyl-1-methylcyclopropyl, 2-ethyl-2-methylcyclopropyl, and 2-ethyl-3-methylcyclopropyl groups, as well as hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl groups. Examples of alkylene groups include those derived from the alkyl groups described above.

[0028] The aryl group is an aryl group having 6 to 30 carbon atoms, and examples thereof include a phenyl group, a naphthyl group, an anthracene group, and a pyrene group. Examples of the alkenyl group include alkenyl groups having 2 to 10 carbon atoms, such as ethenyl, 1-propenyl, 2-propenyl, 1-methyl-1-ethenyl, 1-butenyl, 2-butenyl, 3-butenyl, 2-methyl-1-propenyl, 2-methyl-2-propenyl, 1-ethylethenyl, 1-methyl-1-propenyl, 1-methyl-2-propenyl, 1-pentenyl, 2-pentenyl, 3-pentenyl, 4-pentenyl, 1-n-propylethenyl, 1-methyl-1-butenyl, 1-methyl-2-butenyl, 1-methyl-3-butenyl, 2-ethyl-2-propenyl, 2-methyl-1-butenyl, 2-methyl-2-butenyl, and 2-methyl-3-butenyl. Examples of alkyl groups include, but are not limited to, a 3-methyl-1-butenyl group, a 3-methyl-2-butenyl group, a 3-methyl-3-butenyl group, a 1,1-dimethyl-2-propenyl group, a 1-i-propylethenyl group, a 1,2-dimethyl-1-propenyl group, a 1,2-dimethyl-2-propenyl group, a 1-cyclopentenyl group, a 2-cyclopentenyl group, a 3-cyclopentenyl group, a 1-hexenyl group, a 2-hexenyl group, a 3-hexenyl group, a 4-hexenyl group, a 5-hexenyl group, a 1-methyl-1-pentenyl group, a 1-methyl-2-pentenyl group, a 1-methyl-3-pentenyl group, a 1-methyl-4-pentenyl group, a 1-n-butylethenyl group, a 2-methyl-1-pentenyl group, and a 2-methyl-2-pentenyl group.

[0029] Examples of the alkoxy group include alkoxy groups having 1 to 10 carbon atoms, such as methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy, t-butoxy, n-pentyloxy, 1-methyl-n-butoxy, 2-methyl-n-butoxy, 3-methyl-n-butoxy, 1,1-dimethyl-n-propoxy, 1,2-dimethyl-n-propoxy, 2,2-dimethyl-n-propoxy, 1-ethyl-n-propoxy, and n-hexyloxy groups, but are not limited to these.

[0030] The acyloxy group includes acyloxy groups having 2 to 10 carbon atoms, such as a methylcarbonyloxy group, an ethylcarbonyloxy group, an n-propylcarbonyloxy group, an i-propylcarbonyloxy group, an n-butylcarbonyloxy group, an i-butylcarbonyloxy group, an s-butylcarbonyloxy group, a t-butylcarbonyloxy group, an n-pentylcarbonyloxy group, a 1-methyl-n-butylcarbonyloxy group, a 2-methyl-n-butylcarbonyloxy group, a 3-methyl-n-butylcarbonyloxy group, a 1,1-dimethyl-n-propylcarbonyloxy group, a 1,2-dimethyl-n-propylcarbonyloxy group, a 2,2-dimethyl-n-propylcarbonyloxy group, a 1-ethyl-n-propylcarbonyloxy group, an n-hexylcarbonyloxy group, a 1-methyl-n-pentylcarbonyloxy group, and a 2-methyl-n-pentylcarbonyloxy group. Examples of the halogen group include fluorine, chlorine, bromine, and iodine.

[0031] The (meth)acryloyl group refers to both an acryloyl group and a methacryloyl group. Examples of organic groups having a (meth)acryloyl group include a 3-methacryloxypropyl group and a 3-acryloxypropyl group. Examples of organic groups having a mercapto group include a 3-mercaptopropyl group. Examples of organic groups having an amino group include a 2-aminoethyl group, a 3-aminopropyl group, an N-2-(aminoethyl)-3-aminopropyl group, an N-(1,3-dimethyl-butylidene)aminopropyl group, an N-phenyl-3-aminopropyl group, and an N-(vinylbenzyl)-2-aminoethyl-3-aminopropyl group.

[0032] An example of an organic group having a ureido group is a 3-ureidopropyl group. An example of an organic group having an epoxy group is a glycidyl group or a 3,4-epoxycyclohexyl group. This may include a form in which these epoxy groups are ring-opened to generate a hydroxyl group. An example of an organic group having a cyano group is a 3-cyanopropyl group. As the compounds of the above formulas (2) and (3), compounds capable of forming trimethylsilyl groups on the surface of silica particles are preferred. Examples of such compounds include the following.

[0033]

[0034] In the above formula, R １２ is an alkoxy group, for example, a methoxy group or an ethoxy group. As the silane compound, a silane compound manufactured by Shin-Etsu Chemical Co., Ltd. can be used. A step can be carried out in which the silane compound reacts with hydroxyl groups on the surface of silica particles, for example, silanol groups in the case of silica particles, to coat the surfaces of the silica particles with the silane compound through siloxane bonds. The reaction temperature can be from 20°C to the boiling point of the dispersion medium, for example, in the range of 20°C to 100°C. The reaction time can be about 0.1 to 6 hours.

[0035] The silane coupling agent is used in such a manner that the number of silicon atoms in the silane compound is 0.1 / nm as the coating amount on the surface of the silica particles.２ ~5.0 pieces / nm ２ The silica particle surfaces can be coated by adding a silane compound corresponding to the coating amount to the silica sol. Water is required for the hydrolysis of the silane compound, and if the sol is an aqueous solvent, that aqueous solvent can be used. When the aqueous medium is replaced with an organic solvent such as methanol or ethanol, the water remaining in the solvent can be used. For example, water present in an amount of 0.01 to 10.0 mass % or 0.1 to 7.0 mass % can be used. Furthermore, the hydrolysis can be carried out with or without a catalyst.

[0036] When the hydrolysis is performed without a catalyst, the silica particle surface is on the acidic side. When a catalyst is used, examples of the hydrolysis catalyst include metal chelate compounds, organic acids, inorganic acids, organic bases, and inorganic bases. Examples of metal chelate compounds used as hydrolysis catalysts include triethoxy mono(acetylacetonato)titanium and triethoxy mono(acetylacetonato)zirconium. Examples of organic acids used as hydrolysis catalysts include acetic acid and oxalic acid. Examples of inorganic acids used as hydrolysis catalysts include hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and phosphoric acid. Examples of organic bases used as hydrolysis catalysts include pyridine, pyrrole, piperazine, and quaternary ammonium salts. Examples of inorganic bases used as hydrolysis catalysts include ammonia, sodium hydroxide, and potassium hydroxide.

[0037] One embodiment of the present invention is an insulating resin composition containing the above silica sol and a nitrogen-containing polymer, wherein the nitrogen-containing polymer may be contained in an amount of 1 to 100 parts by mass per part by mass of silica contained in the silica sol.

[0038] In one embodiment of the present invention, the silica sol contains a carboxylic acid having 1 to 3 carbon atoms in an amount of 80 to 1500 ppm, which allows the viscosity of the silica sol measured with a Brookfield viscometer to be set in the range of 3 to 500 mPa s. If the carboxylic acid is not contained, the silica sol will have a viscosity of 500 mPa s or more, for example, 790 mPa s to 3600 mPa s, which is not preferred because it tends to thicken.

[0039] In one embodiment of the present invention, the organic acid having 1 to 3 carbon atoms may be contained in the silica sol at a ratio of 80 to 1500 ppm, or may be contained in the silica sol at a ratio of 0.008 mass % to 0.15 mass %. For example, this is the SiO contained in a silica sol having a silica concentration of 30 mass %. ２ The amount of SiO contained in the silica sol is equivalent to 0.00027 g to 0.005 g relative to the mass of the silica sol. ２ When the insulating resin composition is produced by mixing the silica sol dispersed in the nitrogen-containing organic solvent with the nitrogen-containing polymer, the insulating resin composition may also contain 270 ppm to 5000 ppm of the organic acid. ２ It is preferable that the organic acid is contained in a proportion of 2.70 ppm to 5000 ppm relative to the total weight of the material.

[0040] The nitrogen-containing polymer may be polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, or polyesterimide. The stability of the insulating resin composition is improved by using a polyamic acid resin composed of silica sol, 4,4'-diaminodiphenyl ether (DDE), and pyromellitic anhydride (PMDA) as the resin, and a resin / SiO ２ The viscosity (mPa s) of a silica-blended polyamic acid adjusted to a mass ratio of 80 / 20 after storage at 50°C for 7 days can be set to 1.20 times or less, or in the range of 0.80 to 1.20 times, or in the range of 1.00 to 1.20 times, or in the range of 1.05 to 1.20 times, the viscosity before storage.

[0041] The insulating resin composition has insulating properties, and is made of silica sol, 4,4'-diaminodiphenyl ether (DDE), and polyamic acid made of pyromellitic anhydride (PMDA) as the resin, and a resin / SiO ２A silica-blended polyamic acid adjusted to a mass ratio of 80 / 20 is heated at 290°C on a Cu plate to obtain a Cu plate (coating thickness: 29 to 32 μm) on which a silica-blended polyimide is baked, and an insulating resin composition is obtained having a dielectric breakdown life of 50 minutes or more, or 50 to 1000 minutes, or 60 to 500 minutes, or 60 to 200 minutes at a test temperature of 155°C (in air), an applied voltage of 3.0 kV, and a frequency of 50 Hz. These insulating resin compositions can be used to provide insulating coatings on enameled wires or the like to obtain insulating coated conductors.

[0042] The silica sol of the present invention can be produced by a method including the following steps (A) and (B): step (A): preparing a silica sol in which silica particles having an average primary particle size of 5 to 100 nm are dispersed in an aqueous medium; and step (B): substituting the silica sol obtained in step (A) with a nitrogen-containing organic solvent while adjusting the content of a carboxylic acid having 1 to 3 carbon atoms to 80 to 1500 ppm. In step (B), the carboxylic acid can be added during the solvent substitution from the aqueous medium to the nitrogen-containing organic solvent. Alternatively, the carboxylic acid can be added before the solvent substitution with the nitrogen-containing organic solvent. However, since some of the carboxylic acid may be removed during the solvent substitution process, the carboxylic acid can be added after the solvent substitution so that the content falls within the predetermined range.

[0043] During or after step (B), step (C) of adding at least one silane compound represented by formula (1) to formula (3) can be added. By adding the silane compound, the surface of the silica particles can be coated with the silane compound. The silica sol dispersed in the nitrogen-containing organic solvent of the present invention can be combined with a nitrogen-containing polymer to obtain an insulating resin composition (resin varnish).

[0044] The insulating resin composition (resin varnish) can be produced by further adding steps (D) and (E) to steps (A) and (B) or after steps (A) and (C): step (D): mixing the silica sol dispersed in the nitrogen-containing organic solvent with a nitrogen-containing polymer; and step (E): removing part or all of the nitrogen-containing organic solvent from the silica sol obtained in step (D).

[0045] The nitrogen-containing polymer can be blended in a ratio of 1 part by mass of silica contained in the silica sol to 1 part by mass of the nitrogen-containing polymer in a range of 1 to 100. Examples of the nitrogen-containing polymer include polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, and polyesterimide.

[0046] The insulating resin composition can be applied to a conductor that requires insulation and then heated and cured at a temperature at which the solvent evaporates, forming an insulating film on the surface of the conductor. The heating temperature for removing the solvent is determined by the temperature and pressure, but is about 150°C to 300°C at normal pressure, or about 150°C to 400°C for imidization of the resin.

[0047] The conductor is a metal wire, and in particular, a copper wire. Copper wire is coated with an enamel film to form an electric wire, which is used in industrial and household motors, transformers, coils, etc. The insulating resin composition of the present invention can be used to produce an insulating coated conductor wire by coating an enamel-coated copper wire or by directly coating a copper wire with the insulating resin composition instead of enamel.

[0048] The insulating resin composition is obtained by mixing 1 part by mass of silica contained in silica sol with 1 to 100, 1 to 50, or 1 to 10 parts by mass of nitrogen-containing polymer.

[0049] The insulating resin composition can be obtained by mixing or stirring the silica sol and the polymer using a mixer or disperser. Additives can be added to the mixture as desired. The conductor coated with the insulating resin composition of the present invention has insulating properties and flexibility.

[0050] Flexibility is measured in accordance with JIS C 3216-3, Section 5. For example, an insulating coated conductor having a 35 μm-thick insulating coating layer obtained from an insulating resin composition containing 1 part by mass of silica and 4 parts by mass of a nitrogen-containing polymer preferably has a flexibility of 1d to 2d. However, the flexibility is measured by determining the minimum winding diameter d at which cracks do not occur in the insulating coating of an insulating coated conductor stretched 20% compared to an insulating coated conductor stretched at no stretch, and is measured in the range from the original diameter (1d) to n times the original diameter (nd).

[0051] [SiO ２ Measurement of Concentration] The silica sol was placed in a crucible and dried at 150°C, and the resulting gel was fired at 1000°C, and the firing residue was weighed and calculated.

[0052] [Measurement of average primary particle diameter (particle diameter by nitrogen adsorption method)] The specific surface area of ​​the powder obtained by drying the acidic silica sol at 300°C was measured using a specific surface area measuring device Monosorb (registered trademark) MS-16 (manufactured by Yuasa Ionics Co., Ltd.).

[0053] [Measurement of Water Content] The water content was determined by Karl Fischer titration.

[0054] [Measurement of Viscosity] The viscosity of the silica sol was measured using a B-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd.).

[0055] [Measurement of Cationic Components in Organosol] Pure water was added to the sol to adjust the silica concentration to 3% by mass. 200 μL of 1N aqueous nitric acid solution was added to 8 g of the diluted sol, and the mixture was left overnight. The resulting solution was centrifuged (5,000 rpm x 30 minutes) using a Merck Amicon Ultra-15 10k (molecular weight cutoff: 10,000). The resulting filtrate was diluted 10-fold with pure water and measured by cation chromatography.

[0056] [Measurement of Anionic Components in Organosol] The pretreatment conditions for the silica sol are described below. 50 μL of the organosilica sol sample was dissolved in 950 μL of electrophoresis buffer (containing 40 mM quinolinic acid, 90 mM 2-amino-2-hydroxymethyl-1,3-propanediol (Tris), and 0.7 mM hexadecyltrimethylammonium hydroxide (HDTMA), pH 7.4), and the solution was centrifuged using a centrifuge (Model 6200, Kubota Shoji Co., Ltd.) (centrifugation conditions: 10,000 rpm, 10 minutes, 15°C). 475 μL of the resulting supernatant was placed in an electrophoresis vial. 25 μL of a solution (sodium nitrate concentration: 100 ppm) prepared by dissolving 1 mg of sodium nitrate in 10 mL of electrophoresis buffer was added to the sample collected in the electrophoresis vial to prepare a sample for capillary electrophoresis.

[0057] The pretreatment conditions for the capillary column are described below. Before measurement, the electrophoresis solution was passed through the capillary at a pressure of 915 mbar for 20 minutes to perform preconditioning. Subsequently, before and after each sample measurement, the capillary was washed by passing ethanol (Junsei Chemical Co., Ltd., special-grade reagent) for 180 seconds, 0.1 M aqueous sodium hydroxide solution (Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric analysis) for 360 seconds, ultrapure water (trade name Milli-Q) for 300 seconds, and the electrophoresis solution for 300 seconds, all at a pressure of 915 mbar.

[0058] The conditions for measuring capillary electrophoresis are as follows: Apparatus: A capillary electrophoresis system (trade name: Agilent 7100, manufactured by Agilent Technologies, Inc.) was used.・Capillary: Agilent Model No. G1600-64311 (inner diameter 75 μm, total length 112.5 cm, effective length 104 cm, fused silica capillary) ・Detector: PDA detector (Sig.=400 nm±10 nm, Ref.=265 nm±5 nm) ・Voltage: −25 kV ・Electrophoresis temperature: 25°C ・Electrophoresis buffer: containing 40 mM quinolinic acid, 90 mM 2-amino-2-hydroxymethyl-1,3-propanediol (Tris), and 0.7 mM hexadecyltrimethylammonium hydroxide (HDTMA), pH 7.4 Sample injection: pressure 50 mbar, injection time 6 seconds (pressure injection method) For analysis, quantification was performed by normalizing with the peak area of ​​nitrate ions (the peak area of ​​the organic acid ions divided by the peak area of ​​sodium nitrate).

[0059] [Solid content of polyamic acid] The polyamic acid was placed in an aluminum cup and baked at 200°C, and the baking residue was weighed and calculated.

[0060] Example 1 412 g of water-dispersed silica sol (trade name PL-3) (average primary particle size 35 nm, silica concentration 20% by mass, manufactured by Fuso Chemical Co., Ltd.) was placed in a 1 L recovery flask, and the solvent was evaporated and distilled off using a rotary evaporator at a reduced pressure of 150 to 70 Torr and a bath temperature of 80 to 90°C while DMAC (dimethylacetamide) was supplied to replace the dispersing medium of the sol with DMAC, thereby obtaining a DMAC-dispersed silica sol (R1) (silica concentration 30.0% by mass, water content 6.7% by mass, viscosity 790 mPa s). 172.3 g of the obtained sol was placed in a 500 mL recovery flask, and 0.058 g of acetic acid was added while stirring the sol with a magnetic stirrer. The mixture was then kept at room temperature for 2 hours to obtain a DMAC-dispersed silica sol (1) (silica concentration 30.0 mass%, water 6.7 mass%, viscosity 257 mPa s, acetic acid content in the sol 370 ppm, alkali metal ion content in the sol below the lower detection limit (less than 10 ppm)).

[0061] (Example 2) 169 g of the DMAC-dispersed silica sol (1) obtained in Example 1 was charged into a 500 mL eggplant flask, and while stirring the sol with a magnetic stirrer, 0.1514 g of 4N aqueous sodium hydroxide solution was added. Then, by supplying DMAC while evaporating the solvent at a reduced pressure of 70 Torr and a bath temperature of 90 ° C. in a rotary evaporator, a DMAC-dispersed silica sol was obtained (silica concentration 30.0 mass%, water 3.4 mass%, viscosity 23 mPa s). Then, additional DMAC replacement was performed at a reduced pressure of 70 Torr and a bath temperature of 90 ° C. in a rotary evaporator to obtain a DMAC-dispersed silica sol (2) (silica concentration 30.0 mass%, water 3.4 mass%, viscosity 23 mPa s, alkali metal ions in the sol Na ion content 82 ppm, acetic acid content in the sol 370 ppm).

[0062] Example 3 616 g of water-dispersed silica sol (trade name PL-2L, average primary particle size 17 nm, silica concentration 19% by mass, manufactured by Fuso Chemical Co., Ltd.) was placed in a 2 L recovery flask, and the solvent was evaporated and distilled off using a rotary evaporator at a reduced pressure of 150 to 70 Torr and a bath temperature of 80 to 90°C while DMAC (dimethylacetamide) was added. The sol's dispersion medium was replaced with DMAC, yielding 557 g of a DMAC-water mixed solvent-dispersed silica sol (silica concentration 21.0% by mass, water content 17.3% by mass). While stirring the sol with a magnetic stirrer, 3.1 g of phenyltrimethoxysilane (trade name KBM-103, manufactured by Shin-Etsu Chemical Co., Ltd.) was added, and the liquid temperature was maintained at 90°C for 2 hours. 282 g of the resulting sol was placed in a 1 L recovery flask, and 0.075 g of acetic acid was added, followed by 0.26 g of a 4N aqueous sodium hydroxide solution, followed by stirring for 30 minutes. Thereafter, DMAC was supplied while evaporating and distilling off the solvent using a rotary evaporator at a reduced pressure of 100 to 70 Torr and a bath temperature of 110°C, thereby obtaining a DMAC-dispersed silica sol (3) (silica concentration: 30.3 mass%, water content: 0.9 mass%, viscosity: 8 mPa s, alkali metal ion content in the sol: Na ion: 120 ppm, acetic acid content in the sol: 400 ppm).

[0063] Example 4 351 g of water-dispersed silica sol (trade name Snowtex O-33) (average primary particle size 12 nm, silica concentration 33% by mass, manufactured by Nissan Chemical Industries, Ltd.) was placed in a 1 L eggplant flask, and while stirring the sol with a magnetic stirrer, 0.25 g of a 4N NaOH aqueous solution was added, and stirring was continued for 30 minutes. Thereafter, DMAC was supplied while evaporating and distilling off the solvent in a rotary evaporator at a reduced pressure of 150 to 110 Torr and a bath temperature of 90 ° C., and the dispersion medium of the sol was replaced with DMAC, thereby obtaining 352 g of a silica sol dispersed in a DMAC-water mixed solvent (silica concentration 33.0% by mass, water content 11.7% by mass). 4.3 g of phenyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KBM-103) was added, and the liquid temperature was maintained at 90 ° C. for 2 hours. Thereafter, DMAC was added while evaporating and distilling off the solvent in a rotary evaporator at a reduced pressure of 100 to 70 Torr and a bath temperature of 110°C, thereby obtaining a DMAC-dispersed silica sol (4) (silica concentration 30.2% by mass, water 0.3% by mass, viscosity 8 mPa s, alkali metal ion content in the sol: Na ion content 200 ppm, acetic acid content in the sol 111 ppm, formic acid content 36 ppm). At this time, the amount of acetic acid generated by hydrolysis of DMAC was controlled within the range, thereby obtaining a DMAC-dispersed silica sol.

[0064] Example 5 140 g of methanol-dispersed silica sol (trade name: methanol silica sol) (average primary particle size: 12 nm, silica concentration: 30% by mass, manufactured by Nissan Chemical Industries, Ltd.) was placed in a 0.5 L recovery flask, and 3.3 g of phenyltrimethoxysilane (trade name: KBM-103, manufactured by Shin-Etsu Chemical Co., Ltd.) was added, followed by maintaining the liquid temperature at 60° C. for 5 hours. Thereafter, 6.8 g of dimethylpolysiloxane (trade name: KF-96L 0.65cs, manufactured by Shin-Etsu Chemical Co., Ltd.) was added, followed by adding 14 g of DMAC, and maintaining the liquid temperature at 60° C. for 3 hours. Thereafter, the solvent was evaporated and distilled off in a rotary evaporator at a reduced pressure of 450 to 110 Torr and a bath temperature of 85 to 125°C while DMAC was supplied, and the dispersion medium of the sol was replaced with DMAC to obtain a DMAC-dispersed silica sol (5) (silica concentration 30.2% by mass, water 0.1% by mass, viscosity 7 mPa s, alkali metal ion content in the sol: Na ion content 150 ppm, acetic acid content in the sol 65 ppm, formic acid content 87 ppm). At this time, the amount of carboxylic acid generated by hydrolysis of DMAC was controlled within the range to obtain a DMAC-dispersed silica sol.

[0065] (Example 6) 200 g of water-dispersed silica sol, trade name Snowtex OXS (average primary particle size according to Sears: 5 nm, silica concentration: 10.5% by mass, pH 2.8, manufactured by Nissan Chemical Co., Ltd.) was placed in a 1 L eggplant flask, and while stirring the sol with a magnetic stirrer, 4.3 g of 3-glycidoxypropyltrimethoxysilane (trade name: KBM-403, manufactured by Shin-Etsu Chemical Co., Ltd.) was added, and the liquid temperature was maintained at 80 ° C. for 4 hours. Thereafter, the solvent was evaporated and distilled off in a rotary evaporator at a pressure of 150 to 70 Torr and a bath temperature of 90 ° C. while DMF (N,N-dimethylformamide) was supplied, and the dispersion medium of the sol was replaced with DMF to obtain a DMF-dispersed silica sol (6) (silica concentration: 15.9% by mass, water content: 1.0% by mass, viscosity: 4 mPa s, alkali metal ion in the sol: Na ion content: 100 ppm, formic acid content in the sol: 1375 ppm). At this time, the amount of carboxylic acid generated by hydrolysis of DMF was controlled within a certain range to obtain a DMF-dispersed silica sol.

[0066] Comparative Example 1 412 g of water-dispersed silica sol (trade name PL-3) (average primary particle size 35 nm, silica concentration 20 mass%, manufactured by Fuso Chemical Co., Ltd.) was placed in a 1 L recovery flask, and the solvent was evaporated and distilled off using a rotary evaporator at a reduced pressure of 150 to 70 Torr and a bath temperature of 80 to 90°C while DMAC (dimethylacetamide) was supplied to replace the dispersing medium of the sol with DMAC, thereby obtaining a DMAC-dispersed silica sol (R1) (silica concentration 30.0 mass%, water content 6.7 mass%, viscosity 790 mPa s). 172.3 g of the obtained sol was placed in a 500 mL recovery flask, and the sol was stirred with a magnetic stirrer while being kept at room temperature for 2 hours, thereby obtaining a DMAC-dispersed silica sol (R1) (silica concentration 30.0 mass%, water 6.7 mass%, viscosity 790 mPa s, alkali metal ions in the sol below the lower detection limit (less than 10 ppm), acetic acid content in the sol 30 ppm).

[0067] (Comparative Example 2) 15.5 g of the DMAC-dispersed silica sol (R1) obtained in Comparative Example 1 was collected in a 20 ml glass bottle, and 0.039 g of an 8% aqueous sulfuric acid solution was added to the sol and shaken, whereupon the sol lost its fluidity and gelled.

[0068] Comparative Example 3 616 g of water-dispersed silica sol (trade name PL-2L, average primary particle size 17 nm, silica concentration 19 mass%, manufactured by Fuso Chemical Co., Ltd.) was placed in a 2 L recovery flask, and the solvent was evaporated and distilled off using a rotary evaporator at a reduced pressure of 150 to 70 Torr and a bath temperature of 80 to 90°C while DMAC (dimethylacetamide) was supplied to replace the dispersing medium of the sol with DMAC, thereby obtaining 557 g of a DMAC-water mixed solvent-dispersed silica sol (silica concentration 21.0 mass%, water content 17.3 mass%). While stirring the sol with a magnetic stirrer, 3.1 g of phenyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-103) was added, and the liquid temperature was maintained at 90°C for 2 hours to obtain a high-viscosity DMAC-dispersed silica sol (R3) (silica concentration 27.6 mass%, water 5.2 mass%, viscosity 3600 mPa s, alkali metal ions in the sol below the lower detection limit (less than 10 ppm), acetic acid content in the sol 31 ppm).

[0069] (Comparative Example 4) 130 g of the DMAC-dispersed silica sol (4) obtained in Example 4 was placed in a 500 mL recovery flask, and 0.26 g of acetic acid was added while stirring with a magnetic stirrer. The mixture was then kept at room temperature for 2 hours to obtain a DMAC-dispersed silica sol (R4) (silica concentration 30.2 mass%, water 0.3 mass%, viscosity 16 mPa s, alkali metal ion in the sol Na ion content 200 ppm, acetic acid content in the sol 1926 ppm, formic acid content 39 ppm).

[0070] Synthesis Example 1 Preparation of Polyamic Acid 4,4'-Diaminodiphenyl ether (DDE), pyromellitic dianhydride (PMDA), and NMP (N-methylpyrrolidone) and DMAC (dimethylacetamide) as solvents were polymerized at a temperature of 50°C with stirring to obtain a polyamic acid (solid content 17% by mass, viscosity at 25°C measured with an E-type viscometer of 13,640 mPa·s) corresponding to formula (4). The polyamic acid was polymerized using an equimolar ratio of 1:1 between the DDE and PMDA. The weight-average molecular weight of the obtained polyamic acid was 63,000. n in formula (4) is the number of repeating units.

[0071]

[0072] (Thermal Stability Test of Insulating Resin Composition) The DMAC-dispersed silica sol obtained in Example 4 and Comparative Example 4 was mixed with the polyamic acid obtained in Synthesis Example 1 in a mass ratio of resin / SiO ２ The components were mixed in a glass bottle so that the viscosity was 80 / 20, and the mixture was degassed and stirred for 20 minutes using a vacuum degasser (manufactured by EME, product name V-mini300) to obtain a silica-blended polyamic acid. The initial viscosity (mPa s) at 25°C and the viscosity (mPa s) measured after storing at 50°C for 7 days and then cooling to 25°C are shown in the table below.

[0073]

[0074] The silica-blended polyamic acid obtained in Example 4 and Comparative Example 4 was applied to a Cu plate (manufactured by AS ONE Corporation, product name HC0536, 300 mm x 300 mm, 0.5 mm thick) using an applicator (manufactured by BEVS, product name: Film applicator with film thickness adjustment function B / M150 mm), and the solvent was removed and the plate was thermally cured at 70°C for 30 minutes, 100°C for 30 minutes, 150°C for 30 minutes, and 290°C for 60 minutes to obtain a Cu plate (coating thickness: 29 to 32 μm) with a baked silica-blended polyimide. This was cut into a 5 cm square to serve as an insulation test sample.

[0075] (Measurement of dielectric breakdown life) A plate-shaped sample measuring 50 mm x 50 mm and 0.5 mm thick was used to measure the dielectric breakdown life of the above-mentioned insulation test sample using a dielectric breakdown tester, model YST-243WS, manufactured by Yamayo Testing Instruments Co., Ltd., at a test temperature of 155°C (in air), an applied voltage of 3.0 kV, and a frequency of 50 Hz. The electrodes used were a flat electrode (φ = 25 mm) at the bottom and a spherical electrode (φ = 20 mm) at the top, with both electrodes being installed so as to be in contact with the sample. Three to four measurements were performed at an applied voltage of 3.0 kV, and the average value was recorded. As a blank, a sample containing only silica-free polyimide resin was used and similarly measured.

[0076]

[0077] A silica sol dispersed in a nitrogen-containing organic solvent containing a predetermined amount of an organic acid (such as acetic acid) does not have a higher viscosity than a DMAC-dispersed silica sol that does not contain an organic acid (such as acetic acid) when compared at the same solid content, and therefore has good workability even when blended with a nitrogen-containing polymer to form an insulating resin composition.

[0078] An insulating resin composition prepared by blending a silica sol dispersed in a nitrogen-containing organic solvent containing an organic acid (such as acetic acid) in excess of a predetermined amount with a nitrogen-containing polymer showed an increase in viscosity in a thermal stability test after storage at 50°C for 7 days, compared to an insulating resin composition prepared by blending a DMAC-dispersed silica sol containing a predetermined amount of organic acid (such as acetic acid) with a nitrogen-containing polymer.

[0079] It was also found that silica sol dispersed in a nitrogen-containing organic solvent containing a predetermined amount of organic acid (acetic acid, etc.) has a longer insulation life than polyimide resin that does not contain silica.

[0080] In the present invention, the silica sol dispersed in a nitrogen-containing organic solvent containing a predetermined amount of organic acid (e.g., acetic acid) has low viscosity, so when the silica sol is mixed with a nitrogen-containing polymer to form an insulating resin composition and applied to a substrate, a coating film that can coat the substrate while maintaining the solid content required to maintain insulating properties can be obtained, and the resulting insulating substrate has a long insulating life.

[0081] Silica sol dispersed in a nitrogen-containing organic solvent containing a predetermined amount of organic acid (such as acetic acid) has low viscosity, so when it is mixed with a nitrogen-containing polymer to form an insulating resin composition and coated onto a substrate, an insulating coating can be obtained that retains the solid content necessary to maintain insulating properties and can be applied to the substrate.

Claims

1. A silica sol in which silica particles having an average primary particle diameter of 5 to 100 nm are dispersed in a nitrogen-containing organic solvent, and the silica sol contains a carboxylic acid having 1 to 3 carbon atoms in a proportion of 80 to 1,500 ppm.

2. The silica sol according to claim 1, wherein the nitrogen-containing organic solvent is an amide solvent.

3. The silica sol according to claim 1 or 2, wherein the nitrogen-containing organic solvent is dimethylacetamide, dimethylformamide, or dimethylpropionamide.

4. The silica sol according to claim 1 or 2, wherein the carboxylic acid having 1 to 3 carbon atoms is formic acid, acetic acid or propionic acid.

5. The silica sol according to claim 1 or 2, wherein the water content in the silica sol is 0.1 to 10.0% by mass.

6. SiO ２ 3. The silica sol according to claim 1 or 2, which has a viscosity of 3 to 500 mPa·s measured at 25° C. when the concentration is 30% by mass.

7. The silica sol according to claim 1 or 2, containing alkali metal ions (wherein the alkali metal ions are alkali metal ions consisting of lithium, sodium and potassium) in a ratio of 300 ppm or less.

8. Silica particles represented by formula (1) to formula (3): (In formula (1), R １ each represents an alkyl group, a halogenated alkyl group, an alkenyl group, an aryl group, or an organic group having a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a carboxyl group, an acid anhydride group, a carboxylate group, an epoxy group, a hydroxyl group, or a cyano group, and is bonded to a silicon atom by a Si-C bond; R ２ each represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group, a represents an integer of 1 to 3, and in formula (2) and formula (3), R ３ and R ５ are each an alkyl group having 1 to 3 carbon atoms or an aryl group having 6 to 30 carbon atoms and bonded to a silicon atom by a Si—C bond, R ４ and R ６ each represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group, Y represents an alkylene group, an NH group, or an oxygen atom, b is an integer of 1 to 3, c is an integer of 0 or 1, and d is an integer of 1 to 3. The silica sol according to claim 1 or 2, wherein the silica sol is coated with at least one silane compound or a hydrolyzate thereof selected from the group consisting of:

9. An insulating resin composition comprising the silica sol according to claim 1 and a nitrogen-containing polymer.

10. The insulating resin composition according to claim 9, wherein the ratio of parts by mass of the nitrogen-containing polymer to 1 part by mass of silica contained in the silica sol is 1 to 100.

11. The insulating resin composition according to claim 9, wherein the nitrogen-containing polymer is a polyimide, a polyamide, a polyamic acid, a polyamideimide, a polyetherimide, or a polyesterimide.

12. A resin / SiO2 composite comprising the silica sol according to claim 1, 4,4'-diaminodiphenyl ether (DDE), and polyamic acid made of pyromellitic anhydride (PMDA) as a resin. ２ The insulating resin composition is an insulating resin composition in which a viscosity (mPa·s) of a silica-blended polyamic acid, which has been adjusted to a mass ratio of 80 / 20, after storage at 50°C for 7 days is 1.20 times or less than the viscosity before storage.

13. A resin / SiO2 composite comprising the silica sol according to claim 1, 4,4'-diaminodiphenyl ether (DDE), and polyamic acid made of pyromellitic anhydride (PMDA) as a resin. ２ A silica-blended polyamic acid adjusted to a mass ratio of 80 / 20 is heated at 290°C on a Cu plate to obtain a silica-blended polyimide baked onto the Cu plate (coating thickness: 29 to 32 µm), and the insulating resin composition has a dielectric breakdown life of 50 minutes or more at a test temperature of 155°C (in air), an applied voltage of 3.0 kV, and a frequency of 50 Hz.

14. An insulating coated conductor, which is coated with the insulating resin composition according to any one of claims 9 to 13.

15. A method for producing a silica sol according to claim 1 or 2, comprising the following steps (A) and (B): step (A): preparing a silica sol in which silica particles having an average primary particle diameter of 5 to 100 nm are dispersed in an aqueous medium; and step (B): replacing the silica sol obtained in step (A) with a nitrogen-containing organic solvent while adjusting the content of a carboxylic acid having 1 to 3 carbon atoms in the silica sol to 80 to 1,500 ppm.

16. A method for producing a silica sol according to claim 15, further comprising the step (C) of adding at least one silane compound represented by formula (1) to formula (3) according to claim 7 during or after the completion of step (B).

17. A method for producing the insulating resin composition according to any one of claims 9 to 13, comprising a step (D) of mixing the silica sol according to claim 1 with a nitrogen-containing polymer.

18. The method for producing an insulating resin composition according to claim 17, further comprising the step (E) of removing a part or all of the nitrogen-containing organic solvent from the insulating resin composition, in addition to the step (D).

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