WO2025115547A1 - Silica sol dispersed in nitrogen-containing organic solvent containing organic acid, and insulating resin composition - Google Patents

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

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 a method for producing the same.

Silica sols in which surface-modified silica particles are dispersed in a solvent are known. For example, a method has been disclosed in which the hydroxyl groups on the surface of inorganic oxide particles such as silica react with alcohol to introduce alkoxyl groups and organize the particles to obtain an inorganic oxide sol dispersed in an organic solvent such as toluene (see Patent Document 1). In this method, phenyltrimethoxysilane is reacted with a methanol-dispersed silica sol to produce a silica sol dispersed in a toluene solvent.

Also, a silica sol dispersed in methanol is solvent-substituted with acetonitrile to obtain a silica sol dispersed in an acetonitrile-methanol mixed solvent, and then reacted with phenyltrimethoxysilane (see Patent Document 2).

Also, a silica sol in which the surface of silica particles is modified with an aluminum compound has been disclosed (see Patent Document 3).

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).

JP 2005-200294 A International Publication No. 2009/008509 JP 2011-026183 A International Publication No. 2022/097694

The present invention aims to provide a silica sol in which silica particles are dispersed in a nitrogen-containing organic solvent to achieve good compatibility with polyimide or polyamide polar resins. It also aims to provide a resin composition in which the silica sol is blended with a resin, and when used as an insulating resin composition, provides an insulated conductor that can maintain a high insulation life for a long period of time.

According to a first aspect of the present invention, there is provided 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 a proportion of 80 to 1,500 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 a water content in the silica sol is 0.1 to 10.0 mass%.
As a sixth aspect, the silica sol according to any one of the first to fifth aspects, in which the viscosity measured at 25° C. is 3 to 500 mPa s when the SiO ₂ concentration is 30% by mass;
As a seventh aspect, the silica sol according to any one of the first to sixth aspects contains an alkali metal ion (wherein the alkali metal ion refers to an alkali metal ion consisting of lithium, sodium, and potassium) at a ratio of 300 ppm or less.
As an eighth aspect, the silica particles are represented by formula (1) to formula (3):

(In formula (1), R ¹ 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 via a Si-C bond; R ² represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; a represents an integer of 1 to 3;
In formula (2) and formula (3), ^R3 and ^R5 are each an alkyl group having 1 to 3 carbon atoms or an aryl group having 6 to 30 carbon atoms, and are bonded to a silicon atom by a Si-C bond; ^R4 and ^R6 are each an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; Y is 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 or a hydrolyzate thereof selected from the group consisting of:
According to a ninth aspect, there is provided an insulating resin composition comprising the silica sol according to any one of the first to eighth aspects and a nitrogen-containing polymer.
According to a tenth aspect, the insulating resin composition according to the ninth aspect, 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.
As an eleventh aspect, the insulating resin composition according to the ninth or tenth aspect, wherein the nitrogen-containing polymer is polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, or polyesterimide;
According to a twelfth aspect, there is provided 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 made of pyromellitic anhydride (PMDA) as a resin, the polyamic acid being adjusted to a mass ratio of resin/SiO ₂ = 80/20, the viscosity (mPa·s) of the polyamic acid being 1.20 times or less after storage at 50°C for 7 days compared to the viscosity before storage;
According to a thirteenth aspect, there is provided 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 made of pyromellitic anhydride (PMDA) as a resin, the polyamic acid being adjusted to a mass ratio of resin/SiO ₂ = 80/20, the polyamic acid being 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, the insulating resin composition having a dielectric breakdown life of 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;
According to a fourteenth aspect, there is provided an insulating coated conductor that is insulatingly coated with the insulating resin composition according to any one of the ninth to thirteenth aspects.
As a fifteenth aspect, the present invention provides a method for producing a method comprising the steps (A) and (B) below:
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;
(B) step: replacing the silica sol obtained in the (A) step with a nitrogen-containing organic solvent while adjusting the content of the carboxylic acid having 1 to 3 carbon atoms in the silica sol at a ratio of 80 to 1500 ppm;
As a sixteenth aspect, the method for producing a silica sol according to the fifteenth aspect, further comprising a step (C) of adding at least one silane compound represented by any one of formulae (1) to (3) according to claim 7 during the step (B) or after completion of the step (B);
As a seventeenth aspect, there is provided a method for producing an insulating resin composition according to any one of the ninth to fourteenth aspects, the method including a step (D) of mixing the silica sol obtained according to the fifteenth or sixteenth aspect with a nitrogen-containing polymer; and as an eighteenth aspect, there is provided a method for producing an insulating resin composition according to the seventeenth aspect, the method further including a 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).

The insulating resin obtained by coating and curing the 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 and strong coating layer with the insulating resin, thereby protecting the substrate from insulation breakdown due to discharge. Nitrogen-containing polymers with high insulating properties are often used as insulating resins. These nitrogen-containing polymers are, for example, polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, or polyesterimide, and are synthesized from diamines and acid anhydrides, but have both partial structures of polar parts such as imide skeletons, carboxyl groups, and amide bonds, and hydrophobic parts contained in the diamine molecules or acid anhydride molecules.

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 using the insulating resin composition as an enameled wire coating material, proper viscosity management is important on-site to coat the enameled wire with a uniform film thickness.

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 the 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 the viscosity of the insulating resin composition by adding a specific amount of organic acid.

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, including cases in which an organic acid is contained in the nitrogen-containing organic solvent or the nitrogen-containing polymer from the beginning, and cases in which an organic acid is newly added. By measuring these and setting the content within the range set by the present invention, it is possible to achieve stabilization of the viscosity of the silica sol or insulating resin composition.

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 the following 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 a carboxylic acid having 1 to 3 carbon atoms in a proportion of 80 to 1,500 ppm.

In one embodiment of the present invention, the silica particles contained in the silica sol have an average primary particle diameter of 5 to 100 nm. The average primary particle diameter of the silica particles can be measured using the particle diameter (nm) measured by the nitrogen gas adsorption method (BET method).

The nitrogen-containing organic solvent used in the present invention has a functional group that contains at least a nitrogen atom. Examples of functional groups that contain a nitrogen atom include amino groups, nitro groups, and cyano groups. 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 that have a chain structure or a ring structure. Examples of functional groups that contain a nitrogen atom include amino groups, nitro groups, and cyano groups, and it is preferable to use an amino group. The amino group and the carbonyl group can be adjacent to each other or can exist via a carbon atom, and can be used, for example, as an amide bond, and amide-based solvents are preferably used.

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, dimethylaminoethyl acrylate benzyl chloride quaternary salt, etc.

Among them, dimethylacetamide, dimethylformamide, or dimethylpropionamide can be preferably used as the nitrogen-containing organic solvent. Note that the nitrogen-containing organic solvent can contain other solvents as long as the effect of the present invention is not impaired.

That is, the total solvent may contain the nitrogen-containing organic solvent in an amount of 50 to 100 volume %, 90 to 100 volume %, 98 to 100 volume %, or 99 to 100 volume %, and other solvents may be contained in an amount of 0 to less than 50 volume %, 0 to less than 10 volume %, 0 to less than 2 volume %, or 0 to less than 1 volume %.
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.

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.

In one embodiment of the present invention, a carboxylic acid having 1 to 3 carbon atoms is preferably used as the organic acid, and for example, formic acid, acetic acid, or propionic acid is more preferably used. These organic acids are preferred because they have high compatibility with nitrogen-containing organic solvents and nitrogen-containing polymers.

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

Alkali metal ions (wherein alkali metal ions refer to alkali metal ions consisting of lithium, sodium, and potassium) can be contained at a ratio of 300 ppm or less, or 30 to 300 ppm, or 30 to 200 ppm. For example, by keeping the alkali metal ions, sodium ions, within the above range, it is possible to suppress an increase in the viscosity of silica sol dispersed in a nitrogen-containing organic solvent.

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 ratio of 300 ppm or less, or 30 to 300 ppm, or 30 to 200 ppm, and may contain 0.03% by mass or less, or 0.003% to 0.03% by mass, or 0.003% to 0.02% by mass in the silica sol. For example, a silica sol having a silica concentration of 30% by mass may contain 1000 ppm or less, or 100 ppm to 1000 ppm or less, or 100 ppm to 6.70 ppm or less of alkali metal converted to M ₂ O relative to the mass of SiO ₂ contained therein. 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 also preferably contains an alkali metal, calculated as M ₂ O, of 1000 ppm or less, or 100 ppm to 1000 ppm or less, or 100 ppm to 670 ppm or less relative to SiO ₂ .

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).
In formula (1), R ¹ is an organic group having an alkyl group, a halogenated alkyl group, an alkenyl group, an aryl group, or 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 via a Si-C bond; R ² is an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; a is an integer of 1 to 3;
In formula (2) and formula (3), ^R3 and ^R5 are each an alkyl group having 1 to 3 carbon atoms or an aryl group having 6 to 30 carbon atoms, which are bonded to a silicon atom via a Si-C bond; ^R4 and ^R6 are each an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; Y is 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 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, a cyclopentyl group, a 1-methyl-cyclobutyl group, a 2-methyl-cyclobutyl group, a 3-methyl-cyclobutyl 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, a cyclopentyl group, a 1-methyl-cyclobutyl group, a 2-methyl-cyclobutyl group, a 3-methyl-cyclobutyl group, a 2 ... 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, 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, ethyl-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, 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 Examples of the cyclopropyl cyclopropyl group include, but are not limited to, 1-propyl-cyclopropyl group, 2-i-propyl-cyclopropyl group, 1,2,2-trimethyl-cyclopropyl group, 1,2,3-trimethyl-cyclopropyl group, 2,2,3-trimethyl-cyclopropyl group, 1-ethyl-2-methyl-cyclopropyl group, 2-ethyl-1-methyl-cyclopropyl group, 2-ethyl-2-methyl-cyclopropyl group and 2-ethyl-3-methyl-cyclopropyl group, hexyl group, heptyl group, octyl group, nonyl group, decyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group and the like.
Examples of the alkylene group include those derived from the above-mentioned alkyl groups.

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.
The alkenyl group includes 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 the aryl group 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.

The above alkoxy groups include alkoxy groups having 1 to 10 carbon atoms, such as, but not limited to, 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.

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, but is not limited thereto.
The halogen group includes fluorine, chlorine, bromine, iodine, and the like.

The above-mentioned (meth)acryloyl group refers to both an acryloyl group and a methacryloyl group. Examples of the organic group having a (meth)acryloyl group include a 3-methacryloxypropyl group and a 3-acryloxypropyl group.
An example of the organic group having a mercapto group is 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.

An example of the organic group having a ureido group is a 3-ureidopropyl group.
Examples of organic groups having an epoxy group include a glycidyl group and a 3,4-epoxycyclohexyl group. These epoxy groups may be ring-opened to generate hydroxyl groups.
An example of the organic group having a cyano group is a 3-cyanopropyl group.
As the compounds of the above formula (2) and formula (3), compounds capable of forming trimethylsilyl groups on the surface of silica particles are preferred.
Examples of such compounds include the following.

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 process 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 surface 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.

The silane coupling agent can be added to the silica sol in an amount equivalent to a coating amount of ^0.1 to 5.0 silicon atoms/nm ² on the silica particle surface.
Water is necessary for the hydrolysis of the silane compound, and if the sol is an aqueous solvent, the aqueous solvent is 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 at 0.01 to 10.0 mass %, or 0.1 to 7.0 mass % can be used. The hydrolysis can be performed with or without a catalyst.

　When the reaction is carried out 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 as hydrolysis catalysts include triethoxy mono(acetylacetonato)titanium and triethoxy mono(acetylacetonato)zirconium. Examples of organic acids as hydrolysis catalysts include acetic acid and oxalic acid. Examples of inorganic acids as hydrolysis catalysts include hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and phosphoric acid. Examples of organic bases as hydrolysis catalysts include pyridine, pyrrole, piperazine, and quaternary ammonium salts. Examples of inorganic bases as hydrolysis catalysts include ammonia, sodium hydroxide, and potassium hydroxide.

One embodiment of the present invention is an insulating resin composition containing the above-mentioned silica sol and a nitrogen-containing polymer. 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.

In one embodiment of the present invention, the silica sol contains 80 to 1500 ppm of a carboxylic acid having 1 to 3 carbon atoms, which allows the viscosity of the silica sol to be set in the range of 3 to 500 mPa·s when measured with a Brookfield viscometer. If the carboxylic acid is not contained, the viscosity will be 500 mPa·s or more, for example 790 mPa·s to 3600 mPa·s, and there will be a tendency for the viscosity to increase, which is not preferred.

In one embodiment of the present invention, the silica sol may contain an organic acid having 1 to 3 carbon atoms at a ratio of 80 to 1500 ppm, and may contain an organic acid having 1 to 3 carbon atoms at a ratio of 0.008% by mass to 0.15% by mass. This corresponds to 0.00027 g to 0.005 g of SiO ₂ mass contained in a silica sol having a silica concentration of 30% by mass, and the organic acid may be contained at 270 ppm to 5000 ppm of SiO ₂ contained in the silica _{sol. When the insulating resin composition is produced by mixing the silica sol dispersed in the nitrogen-containing organic solvent and the nitrogen-containing polymer, it is preferable that the insulating resin composition also contains the organic acid at a ratio of 2.70 ppm to 5000 ppm of SiO 2} .

The nitrogen-containing polymer may be a polyimide, a polyamide, a polyamic acid, a polyamideimide, a polyetherimide, or a polyesterimide.
Regarding the stability of the insulating resin composition, a silica-blended polyamic acid composed of silica sol, 4,4'-diaminodiphenyl ether (DDE), and pyromellitic anhydride (PMDA) is used as the resin, and the resin is adjusted to a mass ratio of resin/ _SiO2 = 80/20. After storing the polyamic acid at 50°C for 7 days, the viscosity (mPa·s) 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, compared to the viscosity before storage.

The insulating properties of the insulating resin composition are such that the resin is a polyamic acid composed of silica sol, 4,4'-diaminodiphenyl ether (DDE), and pyromellitic anhydride (PMDA), and the silica-blended polyamic acid is adjusted to a mass ratio of resin/ _SiO2 = 80/20, and the silica-blended polyamic acid 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 form insulating coatings on enameled wires or the like to obtain insulating coated conductors.

The silica sol of the present invention can be produced by 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;
Step (B): The silica sol obtained in step (A) is substituted with a nitrogen-containing organic solvent while adjusting the content of a carboxylic acid having 1 to 3 carbon atoms in the sol to 80 to 1500 ppm.
In the step (B), the carboxylic acid can be added during the process of replacing the aqueous medium with the nitrogen-containing organic solvent. The carboxylic acid can also be added before the solvent replacement with the nitrogen-containing organic solvent, but since a part of the carboxylic acid may be removed during the solvent replacement process, the carboxylic acid can be added after the solvent replacement so as to fall within a predetermined range.

During or after the step (B), a step (C) of adding at least one silane compound represented by any one of formulas (1) to (3) can be added. The surface of the silica particles can be coated with the silane compound by adding 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).

The insulating resin composition (resin varnish) may be subjected to steps (A) and (B) or steps (A) and (C) and then further subjected to steps (D) and (E):
Step (D): mixing the silica sol dispersed in the nitrogen-containing organic solvent with a nitrogen-containing polymer;
Step (E): The silica sol obtained in the step (D) may be subjected to an additional step (E) of removing a part or all of the nitrogen-containing organic solvent from the silica sol obtained in the step (D).

The nitrogen-containing polymer can be mixed in a ratio of 1 to 100 parts by mass per part by mass of silica contained in the silica sol.
The nitrogen-containing polymer may be a polyimide, a polyamide, a polyamic acid, a polyamideimide, a polyetherimide, or a polyesterimide.

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 to remove the solvent is determined by the temperature and pressure, but is around 150°C to 300°C at normal pressure, or 150°C to 400°C to imidize the resin.

The conductor is a metal wire, particularly a copper wire, which is coated with enamel to form electric wires for use in industrial and domestic motors, transformers, coils, and the like.
The insulating resin composition of the present invention can be used to produce an insulating coated conductor by coating an enamel coated copper wire or by directly coating a copper wire with the insulating resin composition instead of enamel.

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

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

Flexibility is measured in accordance with JIS C 3216-3, Section 5. For example, an insulated conductor having an insulating coating layer 35 μm thick obtained from an insulating resin composition containing 1 part by mass of silica and 4 parts by mass of nitrogen-containing polymer preferably has a flexibility of 1d to 2d. However, the above flexibility is determined by determining the minimum winding diameter d at which cracks do not occur in the insulating coating of an insulated conductor stretched 20% compared to an insulated conductor that is not stretched, and is measured in the range from the own diameter (1d) to n times the own diameter (nd).

[Measurement of _SiO2 concentration]
The silica sol was placed in a crucible and dried at 150° C., and the resulting gel was then fired at 1000° C., and the firing residue was weighed and calculated.

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

[Moisture measurement]
The value was determined by Karl Fischer titration.

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

[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 nitric acid aqueous 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), and the resulting filtrate was diluted 10 times with pure water and measured by cation chromatography.

[Measurement of anion 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 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 collected in an electrophoresis measurement vial. 25 μL of a solution (sodium nitrate concentration 100 ppm) in which 1 mg of sodium nitrate was dissolved in 10 mL of electrophoresis buffer was added to the sample collected in the electrophoresis measurement vial, and used as a sample for capillary electrophoresis measurement.

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

The measurement conditions for capillary electrophoresis are described below.
Apparatus: A capillary electrophoresis system (product name: Agilent 7100, manufactured by Agilent Technologies, Inc.) was used.
Capillary: Agilent model number 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: -25kV
Electrophoresis temperature: 25°C
Running solution: 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)
The analysis was performed by standardizing the peak area of the nitrate ion (dividing the peak area of the organic acid ion by the peak area of sodium nitrate) and quantifying the organic acid ion.

[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.

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 eggplant 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%, moisture 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)).

Example 2
169 g of the DMAC-dispersed silica sol (1) obtained in Example 1 was charged into a 500 mL eggplant flask, and 0.1514 g of 4N aqueous sodium hydroxide solution was added while stirring the sol with a magnetic stirrer. Then, DMAC was supplied while evaporating and distilling the solvent with a rotary evaporator at a reduced pressure of 70 Torr and a bath temperature of 90 ° C. to obtain a DMAC-dispersed silica sol (silica concentration 30.0 mass%, moisture 3.4 mass%, viscosity 23 mPa · s,). Then, additional DMAC replacement was performed with a rotary evaporator at a reduced pressure of 70 Torr and a bath temperature of 90 ° C. to obtain a DMAC-dispersed silica sol (2) (silica concentration 30.0 mass%, moisture 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).

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 charged into a 2 L eggplant 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, and the dispersion medium of the sol was replaced with DMAC to obtain 557 g of 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., trade name KBM-103) was added, and the liquid temperature was maintained at 90 ° C. for 2 hours. 282 g of the obtained sol was charged into a 1 L eggplant flask, and 0.075 g of acetic acid was added, followed by adding 0.26 g of a 4N aqueous sodium hydroxide solution, and stirring was continued for 30 minutes. Thereafter, the solvent was evaporated and distilled off in a rotary evaporator at a reduced pressure of 100 to 70 Torr and a bath temperature of 110° C. while DMAC was being supplied, thereby obtaining a DMAC-dispersed silica sol (3) (silica concentration: 30.3 mass %, moisture: 0.9 mass %, viscosity: 8 mPa s, alkali metal ion content in the sol: Na ion content: 120 ppm, acetic acid content in the sol: 400 ppm).

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 Co., Ltd.) was charged into a 1 L eggplant flask, and 0.25 g of 4N NaOH aqueous solution was added while stirring the sol with a magnetic stirrer, and stirring was continued for 30 minutes. Thereafter, DMAC was supplied while evaporating and distilling the solvent at a reduced pressure of 150 to 110 Torr and a bath temperature of 90° C. in a rotary evaporator, and the dispersion medium of the sol was replaced with DMAC, thereby obtaining 352 g of a DMAC-water mixed solvent-dispersed silica sol (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 supplied 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., to obtain a DMAC-dispersed silica sol (4) (silica concentration 30.2 mass%, moisture 0.3 mass%, viscosity 8 mPa·s, alkali metal ion in the sol Na ion content 200 ppm, acetic acid content in the sol 111 ppm, formic acid content 36 ppm). At that time, the amount of acetic acid generated by hydrolysis of DMAC was controlled within the range to obtain a DMAC-dispersed silica sol.

Example 5
Methanol-dispersed silica sol: 140 g of 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 eggplant flask, and 3.3 g of phenyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-103) was added, after which the liquid temperature was maintained at 60° C. for 5 hours. Thereafter, 6.8 g of dimethylpolysiloxane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KF-96L 0.65cs) was added, followed by the addition of 14 g of DMAC, and the liquid temperature was maintained at 60° C. for 3 hours. Thereafter, DMAC was supplied while evaporating and distilling the solvent in a rotary evaporator at a reduced pressure of 450 to 110 Torr and a bath temperature of 85 to 125° C., and the dispersion medium of the sol was replaced with DMAC to obtain a DMAC-dispersed silica sol (5) (silica concentration 30.2 mass%, moisture 0.1 mass%, viscosity 7 mPa·s, alkali metal ion in the sol Na ion content 150 ppm, acetic acid content in the sol 65 ppm, formic acid content 87 ppm). At that time, the amount of carboxylic acid generated by hydrolysis of DMAC was controlled within the range to obtain a DMAC-dispersed silica sol.

Example 6
200 g of a water-dispersed silica sol, trade name Snowtex OXS (average primary particle size according to Sears: 5 nm, silica concentration: 10.5 mass%, pH: 2.8, manufactured by Nissan Chemical Industries, Ltd.) was placed in a 1 L recovery flask, and 4.3 g of 3-glycidoxypropyltrimethoxysilane (trade name: KBM-403, manufactured by Shin-Etsu Chemical Co., Ltd.) was added while stirring the sol with a magnetic stirrer, and the liquid temperature was then 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 mass%, moisture 1.0 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 that time, the amount of carboxylic acid generated by hydrolysis of DMF was controlled within the range to obtain a DMF-dispersed silica sol.

(Comparative 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 charged into a 1 L eggplant 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, and the dispersion medium of the sol was replaced with DMAC to obtain a DMAC-dispersed silica sol (R1) (silica concentration 30.0% by mass, water 6.7% by mass, viscosity 790 mPa s). 172.3 g of the obtained sol was charged into a 500 mL eggplant flask, and the sol was stirred with a magnetic stirrer while being held at room temperature for 2 hours to obtain a DMAC-dispersed silica sol (R1) (silica concentration 30.0% by mass, water 6.7% by mass, viscosity 790 mPa s, alkali metal ions in the sol were below the detection limit (less than 10 ppm), acetic acid content in the sol was 30 ppm).

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

(Comparative 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 eggplant 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% by mass, water content: 17.3% by 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 %, moisture 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).

(Comparative Example 4)
130 g of the DMAC-dispersed silica sol (4) obtained in Example 4 was placed in a 500 mL recovery flask, 0.26 g of acetic acid was added while stirring with a magnetic stirrer, and the mixture was kept at room temperature for 2 hours to obtain a DMAC-dispersed silica sol (R4) (silica concentration 30.2 mass%, moisture 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).

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 under stirring to obtain a polyamic acid (solid content 17% by mass, viscosity at 25°C measured with an E-type viscometer of 13640 mPa·s) corresponding to formula (4). The polyamic acid was polymerized with the above DDE and PMDA in an equimolar ratio of 1:1. The weight average molecular weight of the obtained polyamic acid was 63,000. n in formula (4) is the number of repeating units.

(Thermal Stability Test of Insulating Resin Composition)
The DMAC-dispersed silica sol obtained in Example 4 and Comparative Example 4 was added to and mixed with the polyamic acid obtained in Synthesis Example 1 in a glass bottle so that the mass ratio was resin/SiO ₂ = 80/20, and the mixture was degassed and stirred for 20 minutes using a vacuum degassing machine (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.

The silica-blended polyamic acid obtained in Example 4 and Comparative Example 4 was applied to a Cu plate (manufactured by AS ONE, 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/M 150 mm), and then 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: 29-32 μm) with a silica-blended polyimide baked on. This was then cut into a 5 cm square to prepare a sample for insulation testing.

(Measurement of dielectric breakdown life)
A plate-shaped sample with a size of 50 mm x 50 mm and a thickness of 0.5 mm was used to measure the dielectric breakdown life of the above-mentioned dielectric test sample at a test temperature of 155°C (in air), an applied voltage of 3.0 kV, and a frequency of 50 Hz using a dielectric breakdown tester manufactured by Yamayo Testing Instruments Co., Ltd., model: YST-243WS. The electrode shape used was a flat electrode (φ = 25 mm) at the bottom and a spherical electrode (φ = 20 mm) at the top, and both electrodes were installed so as to be in contact with the sample. Three to four measurements were made at an applied voltage of 3.0 kV, and the average value was recorded. As a blank, only a polyimide resin containing no silica was used as a sample and similarly measured.

Silica sol dispersed in a nitrogen-containing organic solvent containing a specified amount of organic acid (such as acetic acid) does not have a high viscosity when compared with silica sol dispersed in DMAC that does not contain organic acid (such as acetic acid) at the same solid content, and therefore has good workability when mixed with a nitrogen-containing polymer to form an insulating resin composition.

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

It was also found that silica sol dispersed in a nitrogen-containing organic solvent containing a specified amount of organic acid (such as acetic acid) has a longer insulation life than polyimide resin that does not contain silica.

In the present invention, the silica sol dispersed in a nitrogen-containing organic solvent containing a specified amount of organic acid (acetic acid, etc.) has a low viscosity, so when it is mixed with a nitrogen-containing polymer to form an insulating resin composition and applied to a substrate, a coating is obtained that retains the solid content required to maintain insulating properties and can be applied to the substrate. As a result, the insulating substrate obtained has a long insulating life.

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

Claims (18)

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 1,500 ppm of a carboxylic acid having 1 to 3 carbon atoms. The silica sol according to claim 1, wherein the nitrogen-containing organic solvent is an amide solvent. The silica sol according to claim 1 or 2, wherein the nitrogen-containing organic solvent is dimethylacetamide, dimethylformamide, or dimethylpropionamide. 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. 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. The silica sol according to claim 1 or 2, having a viscosity of 3 to 500 mPa·s measured at 25°C when the _SiO2 concentration is 30% by mass. The silica sol according to claim 1 or 2, containing alkali metal ions (wherein alkali metal ions refer to alkali metal ions consisting of lithium, sodium, and potassium) at a ratio of 300 ppm or less. The silica particles are represented by formula (1) to formula (3):

(In formula (1), R ¹ 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 via a Si-C bond; R ² represents an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; a represents an integer of 1 to 3;
In formula (2) and formula (3), ^R3 and ^R5 are each an alkyl group having 1 to 3 carbon atoms or an aryl group having 6 to 30 carbon atoms, and are bonded to a silicon atom by a Si-C bond; ^R4 and ^R6 are each an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; Y is 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.
3. The silica sol according to claim 1 or 2, which is coated with at least one silane compound or a hydrolyzate thereof selected from the group consisting of: An insulating resin composition comprising the silica sol of claim 1 and a nitrogen-containing polymer. The insulating resin composition according to claim 9, wherein the nitrogen-containing polymer is present in an amount of 1 to 100 parts by mass per part by mass of silica contained in the silica sol. 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. An insulating resin composition comprising the silica sol according to claim 1, 4,4'-diaminodiphenyl ether (DDE), and polyamic acid composed of pyromellitic anhydride (PMDA) as a resin, and a silica-blended polyamic acid adjusted to a mass ratio of resin/SiO ₂ = 80/20, the viscosity (mPa·s) of which after storage at 50°C for 7 days is 1.20 times or less compared to the viscosity before storage. An insulating resin composition comprising the silica sol according to claim 1, 4,4'-diaminodiphenyl ether (DDE), and polyamic acid composed of pyromellitic anhydride (PMDA) as a resin, and adjusted to a mass ratio of resin/SiO ₂ = 80/20, and heated at 290°C to a Cu plate to obtain a silica-blended polyimide-baked Cu plate (coating thickness: 29 to 32 μm), the insulating resin composition having 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. An insulating coated conductor coated with the insulating resin composition according to any one of claims 9 to 13. 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;
The method for producing a silica sol according to claim 1 or 2, further comprising: (B) a step of substituting the silica sol obtained in the step (A) with a nitrogen-containing organic solvent while adjusting the content of the carboxylic acid having 1 to 3 carbon atoms in the organic solvent to 80 to 1500 ppm. The 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). A method for producing the insulating resin composition according to any one of claims 9 to 13, comprising step (D) of mixing the silica sol according to claim 1 with a nitrogen-containing polymer. 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).
?