Free silanol group-containing surface-treated silica sol dispersed in nitrogen-containing solvent, and insulating resin composition

Abstract

Provided is a silica sol which is obtained by dispersing silica particles in a nitrogen-containing solvent for mixing the silica particles with a polyimide-based or polyamide-based polar resin in a satisfactorily compatible state, the silica sol being capable of being subjected to an additional silane treatment according to intended applications and the like, having excellent storage stability when contained at a high concentration, and having both customizability and reliability.　Specifically provided is a silica sol containing: silica particles which have an average particle diameter of 5-100 nm as determined by a dynamic light scattering method and to the surfaces of which organic groups each containing a carbon-carbon unsaturated bond are bonded; and a nitrogen-containing solvent. The bonding amount of the organic groups each containing a carbon-carbon unsaturated bond, which are bonded to the surfaces of the silica particles, per unit area of the surfaces of the silica particles is 0.1-2.0 particles / nm2, the concentration of the silica particles is 25-70 mass%, free silanol groups are present on the surfaces of the silica particles, and the amount of the free silanol groups per unit area of the surfaces of the silica particles is 0.4-1.2 particles / nm2.

Description

Free silanol group-containing surface-treated silica sol dispersed in a nitrogen-containing solvent and insulating resin composition

[0001] The present invention relates to a surface-treated silica sol having free silanol groups dispersed in a nitrogen-containing solvent and an insulating resin composition containing the silica sol and a nitrogen-containing polymer.

[0002] A method is known for obtaining an inorganic oxide sol dispersed in an organic solvent such as toluene by reacting an alcohol with a hydroxyl group on the surface of inorganic oxide particles such as silica to introduce an alkoxysilyl group and performing a surface treatment for organifying. For example, Patent Document 1 discloses a silica sol obtained by reacting phenyltrimethoxysilane with a methanol-dispersed silica sol and dispersing it in a toluene solvent.

[0003] Patent Document 2 discloses a silica sol obtained by subjecting a methanol-dispersed silica sol to solvent substitution with acetonitrile to obtain an acetonitrile / methanol mixed solvent-dispersed silica sol and then performing a surface treatment with phenyltrimethoxysilane.

[0004] Patent Document 3 discloses a silica-based fine particle aqueous dispersion sol containing silica fine particles obtained by modifying the surface of silica fine particles with aluminum.

[0005] Patent Document 4 discloses an aluminum-containing silica sol in which silica particles containing aluminum atoms are dispersed in a nitrogen-containing solvent, the silica sol in which aluminum atoms are bonded to the surface of the silica particles at a predetermined ratio, and an insulating resin composition using the same.

[0006] Japanese Patent Application Laid-Open No. 2005-200294 International Publication No. 2009-008509 Japanese Patent Application Laid-Open No. 2011-026183 International Publication No. 2022-097694

[0007] Further investigation by the inventors revealed that, when utilizing silica sols subjected to the above-mentioned surface treatment or surface modification, it is industrially useful to provide a silica sol that can be subsequently subjected to additional silane treatment depending on the application (customizability). Furthermore, it is desirable to provide a high-concentration silica sol with excellent storage stability even before the above-mentioned additional silane treatment (reliability). From the standpoint of achieving both such customizability and reliability, it was found that there is room for further improvement.

[0008] The present invention aims to provide a silica sol in which silica particles are dispersed in a nitrogen-containing solvent for mixing with polyimide-based or polyamide-based polar resins in a state of good compatibility, which allows for additional silane treatment depending on the application, and is highly customizable and reliable, with excellent storage stability at high concentrations.

[0009] The inventors have completed the present invention as a result of repeated experiments and diligent studies aimed at solving the above problems. That is, the gist of the present invention is as follows: [1] A silica sol comprising silica particles with an average particle diameter of 5 to 100 nm as determined by dynamic light scattering, wherein organic groups containing unsaturated bonds between carbon atoms are bonded to the surface, and a nitrogen-containing solvent, wherein the amount of organic groups containing unsaturated bonds between carbon atoms bonded to the surface of the silica particles is 0.1 to 2.0 groups / nm per unit area of ​​the surface of the silica particles ２ The silica particle concentration is 25-70% by mass, and free silanol groups are present on the surface of the silica particles, with the amount of free silanol groups per unit area of ​​the silica particle surface being 0.4-1.2 groups / nm. ２ [2] The amount of alkoxy groups bonded to the surface of the silica particles is 0.001 to 0.03 groups / nm per unit area of ​​the surface of the silica particles. ２[1] The silica sol according to [1]. [3] The silica sol according to [1], wherein the organic group containing the unsaturated bond between carbon atoms is an organic group containing a phenyl group or an organic group containing a (meth)acryloyl group. [4] The silica sol according to [1], wherein the organic group containing the unsaturated bond between carbon atoms is a phenyl group, a phenylaminoalkyl group, a (meth)acryloyl group, or a (meth)acryloylalkyl group. [5] The silica sol according to [1], wherein the nitrogen-containing solvent is an amide solvent. [6] The silica sol according to [1], wherein the nitrogen-containing solvent is dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone. [7] An insulating resin composition comprising the silica sol according to any one of [1] to [6] and a nitrogen-containing polymer. [8] The insulating resin composition according to [7], wherein the amount of the nitrogen-containing polymer is 1 to 100 parts by mass per 1 part by mass of silica contained in the silica sol. [9] The insulating resin composition according to [7], wherein the nitrogen-containing polymer is polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, or polyesterimide.

[0010] According to the present invention, a silica sol can be provided in which silica particles are dispersed in a nitrogen-containing solvent for mixing with polyimide-based or polyamide-based polar resins in a state of good compatibility. Additional silane treatment can be performed depending on the application, and a silica sol can be provided that is highly concentrated, has excellent storage stability, and combines customizability and reliability. Furthermore, the silica sol dispersed in the nitrogen-containing solvent of the present invention has good compatibility with nitrogen-containing polymers such as polyimide and polyamide. By combining the silica sol and the nitrogen-containing polymer to form a composition, an insulating resin composition (resin varnish) in which silica particles are sufficiently dispersed can be obtained. The insulating resin composition of the present invention can be used as a coating material for conductors that require insulation. For example, by coating the surface of a conductor such as copper wire or enamel-coated copper wire, an insulating coated conductor can be obtained that can maintain a high insulation life over a long period of time.

[0011] Hereinafter, preferred embodiments of the present invention will be described. However, the following embodiments are examples for explaining the present invention, and the present invention is not limited to the following embodiments in any way. In this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. In this specification, the above-mentioned plurality of numerical ranges shown as stepwise preferable ranges can also be understood as independent preferable upper limit values or lower limit values by focusing only on each upper limit value or each lower limit value thereof.

[0012] [Silica sol and method for producing the same] The silica sol of the present invention is a silica sol containing silica particles having an average particle diameter of 5 to 100 nm by the dynamic light scattering method in which an unsaturated bond-containing organic group between carbon atoms is bonded to the surface, and a nitrogen-containing solvent, and the bonding amount of the unsaturated bond-containing organic group between carbon atoms bonded to the silica particle surface is 0.1 to 2.0 per unit area of the silica particle surface. ２ per nm, the concentration of the silica particles is 25 to 70% by mass, free silanol groups are present on the silica particle surface, and the amount of free silanol groups present per unit area of the silica particle surface is 0.4 to 1.2 per nm. ２ per nm.

[0013] The average primary particle diameter of the silica particles can be used as the particle diameter (nm) of the specific surface area diameter calculated from the specific surface area (S Ｎ２ ) measured by the nitrogen gas adsorption method (BET method). For example, the specific surface area diameter (average primary particle diameter: D (nm)) is the specific surface area S measured by the nitrogen adsorption method (BET method) Ｎ２ (m ２ / g) and density ρ (g / cm ３ ), D (nm) = 6000 / ρ × S Ｎ２ is the primary particle diameter calculated by the formula, and means the particle diameter converted to spherical silica particles. Further, the particle diameter of the silica particles contained in the silica sol can be the average particle diameter measured by the dynamic light scattering method (DLS method). The silica sol of the present invention has good dispersibility in a nitrogen-containing solvent, and the average particle diameter by the dynamic light scattering method in the nitrogen-containing solvent is preferably 5 to 100 nm, and more preferably 10 to 70 nm.

[0014] The surface of the silica particles of the present invention is bonded to an organic group containing an unsaturated bond between carbon atoms. In one embodiment of the present invention, the organic group containing an unsaturated bond between carbon atoms can be an organic group containing a phenyl group or an organic group containing a (meth)acryloyl group. Specific examples include a phenyl group, a phenylaminoalkyl group, a (meth)acryloyl group, or a (meth)acryloylalkyl group. In the present invention, among these, a phenyl group and a phenylaminoalkyl group are preferred, and a phenyl group is more preferred, from the viewpoint of compatibility with nitrogen-containing polymers.

[0015] Considering the effects of the present invention, the number of organic groups containing unsaturated bonds between carbon atoms bonded to the silica particle surface is 0.1 to 2.0 groups / nm per unit area of ​​the silica particle surface. ２ Preferably, the density is 0.1 to 1.5 particles / nm. ２ It is more preferable that the density be 0.2 to 1.0 particles / nm. ２ It is even more preferable that the following conditions are met. A greater amount of unsaturated bond-containing organic groups than the lower limit is expected to result in superior storage stability of the silica sol, while a smaller amount of unsaturated bond-containing organic groups than the upper limit is expected to result in superior customizability (the ability to further bond organic groups to the silica particle surface as needed).

[0016] In one embodiment of the present invention, alkoxy groups may be bonded to the surface of silica particles. Examples of alkoxy groups include linear, branched, or cyclic alkyl groups having 1 to 10 carbon atoms. Specifically, examples include methoxy groups, ethoxy groups, n-propoxy groups, i-propoxy groups, n-butoxy groups, i-butoxy groups, s-butoxy groups, or t-butoxy groups. In the present invention, among these, methoxy groups, ethoxy groups, and n-propoxy groups are preferred from the viewpoint of compatibility with nitrogen-containing polymers and customizability, with methoxy groups being more preferred.

[0017] Considering the effects achieved by the present invention, the amount of alkoxy groups bonded to the silica particle surface is 0.001 to 0.03 groups / nm per unit area of ​​the silica particle surface.２ Preferably, the density is 0.002 to 0.03 particles / nm. ２ It is more preferable that the density be 0.004 to 0.03 particles / nm. ２ It is even more preferable that the following conditions are met. A greater number of alkoxy groups than the lower limit is expected to result in excellent compatibility with nitrogen-containing polymers, while a smaller number of alkoxy groups than the upper limit is expected to result in excellent customizability.

[0018] The silica sol of the present invention exhibits excellent dispersibility and storage stability even at relatively high concentrations of silica particles. The concentration of silica particles in the silica sol of the present invention is preferably 25 to 70% by mass, more preferably 30 to 60% by mass, and even more preferably 40 to 50% by mass. A concentration of silica particles higher than the lower limit is expected to have the effect of suppressing dilution of the resin by the dispersion solvent introduced by the silica sol while increasing the amount of silica particles incorporated into the resin, and a concentration of silica particles lower than the upper limit is expected to have the effect of providing excellent storage stability for the silica sol.

[0019] The silica sol of the present invention is a sol (dispersion) in which silica particles, on which organic groups containing unsaturated bonds between carbon atoms are bonded to the surface, are dispersed in a nitrogen-containing solvent.

[0020] A nitrogen-containing solvent is one having a functional group containing at least a nitrogen atom. Examples of functional groups containing a nitrogen atom include amino groups, nitro groups, and cyano groups, with amino groups being preferred. Amide-based solvents can be suitably used as nitrogen-containing solvents. Specific examples of nitrogen-containing solvents include dimethylformamide, diethylformamide, dimethylacetamide, diethylacetamide, 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 these, dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone can be suitably used.

[0021] Other solvents may be included in the nitrogen-containing solvent as long as they do not impair the effects of the present invention. For example, the nitrogen-containing solvent may be included in the total solvent in a proportion of 50 to 100% by volume, 90 to 100% by volume, 98 to 100% by volume, or 99 to 100% by volume, and other solvents may be included in a proportion 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.

[0022] The silica particle surface of the present invention is characterized by the presence of free silanol groups within a specific range. The free silanol groups can be measured using the method described in the section on "Measurement of Free Silanol Group Amount" below. Considering the effects achieved by the present invention, the amount of free silanol present on the silica particle surface is approximately 0.4 to 1.2 groups / nm per unit area of ​​the silica particle surface. ２ Preferably, the number is 0.6 to 1.2 particles / nm. ２ It is more preferable that the number of particles is 0.8 to 1.1 per nm. ２It is even more preferable that the following conditions are met. A greater abundance of free silanol groups than the lower limit is expected to result in superior customizability, while a smaller abundance of free silanol groups than the upper limit is expected to result in superior storage stability of the silica sol.

[0023] Regarding the amount of free silanol groups present on the surface of silica particles, it is effective to adjust the amount of a specific silane compound represented by formula (1), such as phenyltrimethoxysilane, as described later. By optimizing the reaction conditions when coating the surface of silica particles with a specific silane compound, it is possible to appropriately adjust the amount of free silanol present on the surface of silica particles after solvent replacement with a nitrogen-containing solvent so that it falls within a predetermined range, thereby providing a silica sol that combines customizability and reliability.

[0024] The silica sol of the present invention can be produced by coating the silica surface with a silica sol by surface treatment or surface modification. An example of a method for producing the silica sol of the present invention will be described below.

[0025] In this invention, silica particles are given by formula (1): (In formula (1), R １ R represents an organic group containing unsaturated bonds between carbon atoms and bonded to a silicon atom by a Si-C bond. ２ represents an alkoxy group, and a is an integer from 1 to 3. Examples of organic groups containing unsaturated bonds between carbon atoms include organic groups containing a phenyl group or organic groups containing a (meth)acryloyl group. Specific examples of silane compounds represented by formula (1) include phenyltrimethoxysilane, phenyltriethoxysilane, phenylmethyldimethoxysilane, phenylaminopropyltrimethoxysilane, phenylaminopropyltriethoxysilane, as well as methacryloyloxypropyltrimethoxysilane, acryloyloxypropyltrimethoxysilane, and the like.

[0026] In the present invention, surface modification of silica particles may also be performed using a silane coupling agent of formula (1) and a hydrolyzable silane compound of formula (2) and / or formula (3). (In equations (2) and (3), R ３ and R ５ Each of these is an alkyl group having 1 to 3 carbon atoms and is bonded to a silicon atom by a Si-C bond, R ４ and R ６ Each of the following represents an alkoxy group, Y represents an alkylene group, an NH group, or an oxygen atom, b is an integer from 1 to 3, c is an integer of 0 or 1, and d is an integer from 1 to 3. It is possible to coat with a silane compound represented by ( ) or its hydrolysate.

[0027] Examples of the above-mentioned alkoxy groups include alkoxy groups having a linear, branched, or cyclic alkyl moiety with 1 to 10 carbon atoms, such as methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy, and t-butoxy groups.

[0028] The silane compounds represented by formulas (2) and (3) above are preferably compounds that can form a trimethylsilyl group on the surface of silica particles. Examples of structures that these compounds may have are as follows. In the above equation (2-3), R １２ This represents an alkoxy group, such as a methoxy group or an ethoxy group.

[0029] These silane compounds can include silane coupling agents manufactured by Shin-Etsu Chemical Co., Ltd., Dow-Toray Industries, Inc., and JNC Corporation.

[0030] In this invention, silica sol can be produced using methods such as the water glass method using water glass as a raw material, the alkoxide method using metal alkoxides as a raw material, or the gas phase method using silicon chloride compounds as a raw material. Silica sol obtained by any of these methods may be used, but the water glass method is preferred.

[0031] The silica sol of the present invention can be obtained, as an example, by a manufacturing method comprising the following steps (A), (B), (C), (D), and (E): (A) Step: 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 (A); (B) Step: Adding a nitrogen-containing solvent and an alkaline catalyst to the silica sol obtained in step (A); (C) Step: Solvent-replacing the silica sol obtained in step (B) with a nitrogen-containing solvent (C); (D) Step: Adding a silane compound represented by formula (1) and optionally a silane compound represented by formula (2) or formula (3) (D); (E) Step: Solvent-replacing the dispersion medium of the silica sol obtained in step (D) with a nitrogen-containing solvent (E).

[0032] (A) As the silica sol used in step (A), silica sol dispersed in an aqueous medium with an average particle size in the range of 5 to 100 nm or 10 to 70 nm (aqueous silica sol) can be used. Aqueous silica sol can be produced by heating a silica solution with a pH of 1 to 6, obtained by cation exchange treatment of an aqueous alkali silicate aqueous solution with a solid content concentration of 1 to 10% by mass, at 50 to 110°C in the presence of alkali. The cation exchange treatment can be performed by passing the aqueous alkali silicate aqueous solution through a strongly acidic cation exchange resin packed in a column.

[0033] As the alkali silicate, sodium silicate, potassium silicate, lithium silicate, etc., can be used, and commercially available sodium silicate sold under names such as No. 1 sodium water glass, No. 2 sodium water glass, No. 3 sodium water glass, etc., can be used. Alternatively, alkali silicate obtained by adding sodium hydroxide, potassium hydroxide, lithium hydroxide, and quaternary ammonium hydroxide to a silicic acid solution obtained by hydrolyzing alkoxysilanes such as tetraethoxysilane and tetramethoxysilane can also be used.

[0034] (A) Instead of the aqueous silica sol used in step (A), silica sol obtained by replacing the aqueous medium with an alcohol having 1 to 3 carbon atoms by vacuum distillation, ultrafiltration, etc., can be used.

[0035] (A) As the silica sol used in the process, an aqueous silica sol that has undergone hydrothermal treatment for 0.1 to 10 hours can be used.

[0036] (A) The silica sol obtained in step (A) can be subjected to cation exchange or anion exchange as needed.

[0037] Step (B) is the step of adding a nitrogen-containing solvent and an alkaline catalyst to the silica sol obtained in step (A). To suppress thickening during solvent substitution in step (C), it is preferable to add the nitrogen-containing solvent and alkaline catalyst to the aqueous silica sol beforehand. It is preferable to use an amide-based solvent as the nitrogen-containing solvent, for example, dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone. Examples of alkaline catalysts include alkali metals such as sodium hydroxide and potassium hydroxide, and amines such as ethylenediamine and diethylenetriamine.

[0038] Step (C) is a step in which the dispersion medium of the silica sol obtained in step (B) is replaced with a nitrogen-containing solvent. Solvent replacement can be carried out using an evaporator or UF concentration. When replacing with a nitrogen-containing solvent, the water content can be adjusted to a range of 3.0 to 15.0% by weight or 5.0 to 10.0% by weight in the total solvent.

[0039] Step (D) is a step of adding a silane compound represented by formula (1) (tentatively called silane compound A) and optionally a silane compound represented by formula (2) or formula (3) (tentatively called silane compound B) to the silica sol obtained in step (C). The order in which silane compound A and silane compound B are added is not limited to any order, but it is preferable, for example, to coat the silica particles with the silane compound represented by formula (1) (silane compound A) or its hydrolysis condensate, and then add the silane compound represented by formula (2) or formula (3) (silane compound B).

[0040] Step (E) is a step in which the dispersion medium of the silica sol obtained in step (D) is replaced with a nitrogen-containing solvent. During or after the solvent replacement step, the silica concentration in the silica sol can be adjusted to 5 to 75% by mass, 20 to 70% by mass, or 25 to 70% by mass.

[0041] In step (E), the total solvent may contain nitrogen-containing solvent in a proportion of 50-100% by volume, 90-100% by volume, 98-100% by volume, or 99-100% by volume, and other solvents may be contained in amounts of 0-50% by volume, 0-10% by volume, 0-2% by volume, or 0-1% by volume. Multiple types of nitrogen-containing solvents may also be included. As other solvents, water contained in the silica sol of step (A) and alcohol dispersions with 1-3 carbon atoms are acceptable as long as they do not impair the effect, even if they fall within the above ranges.

[0042] (C) Specific examples of nitrogen-containing solvents used for solvent substitution in step (C) are as described above.

[0043] [Insulating resin composition and method for producing the same] An insulating resin composition (resin varnish) can be obtained by combining a silica sol in which the nitrogen-containing solvent of the present invention is the dispersion medium with a nitrogen-containing polymer.

[0044] An insulating resin composition (resin varnish) can be obtained by a method that further includes steps (F) and (G) in addition to steps (A) to (E): (F) step: mixing a silica sol in which silica particles obtained in step (E) are dispersed in a nitrogen-containing solvent with a nitrogen-containing polymer; (G) step: removing part or all of the nitrogen-containing solvent from the silica sol obtained in step (F).

[0045] By the above method, for example, a silica sol can be obtained in which silica particles coated with phenyl groups from phenyltrimethoxysilane as a silane compound and trimethylsilyl groups from hexamethyldisiloxane as a silane compound are dispersed in dimethylacetamide (DMAC) as a nitrogen-containing solvent. Furthermore, an insulating resin composition can be produced by combining this with a nitrogen-containing polymer.

[0046] A resin varnish such as an insulating resin composition can be obtained, wherein the amount of parts by mass of the nitrogen-containing polymer is 1 to 100, 1 to 50, or 1 to 10 parts by mass of silica particles contained in the silica sol. Examples of nitrogen-containing polymers include polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, and polyesterimide.

[0047] An insulating resin composition can be applied to a conductor requiring insulation, and by heating and curing it at a temperature at which the solvent evaporates, an insulating film (insulating coating layer) can be formed on the surface of the conductor. The heating temperature to remove the solvent is determined by the temperature and pressure, but at atmospheric pressure it is approximately 150°C to 300°C, or approximately 150°C to 400°C for the imidization of the resin.

[0048] Examples of conductors include metal wires such as copper wire and aluminum wire, with copper wire being particularly commonly used. Copper wire, coated with an enamel film, is used in industrial and household motors, transformers, coils, and other applications.

[0049] An insulated conductor can be manufactured by using the insulating resin composition of the present invention as an insulating coating layer and coating a copper wire with the insulating coating layer.

[0050] The above insulating resin composition is obtained by mixing a nitrogen-containing polymer in a ratio of 1 to 100, 1 to 50, or 1 to 10 parts by mass per 1 part by mass of silica particles contained in a silica sol. The insulating resin composition is obtained by mixing or stirring the silica sol and polymer with a mixer or disperser. Additives can be added to these formulations as desired.

[0051] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0052] (Analysis method) [SiO ２ [Measurement of Concentration] Silica sol was placed in a crucible, dried at 150°C, and the resulting gel was calcined at 1000°C under air for 30 minutes. The calcination residue was then weighed and the concentration was calculated.

[0053] [Measurement of average primary particle diameter (particle diameter by nitrogen adsorption method)] Specific surface area value (S) of silica particles by nitrogen adsorption method in water-dispersed silica sol Ｎ２ ) After removing water-soluble cations from the aqueous dispersion silica sol with an H-type cation exchange resin (manufactured by Dow Chemical, trade name: Amberlite IR-120B), the silica sol is dried at 290°C to obtain a measurement sample, which is then measured using a nitrogen adsorption specific surface area analyzer, trade name Monosorb (manufactured by Quantachrome Instruments Japan LLC), with N ２ A mixed gas of 30% nitrogen and 70% helium (He) was used as the carrier gas, and measurements were taken using the B.E.T. single-point method. The specific surface area (S) of silica particles in an organic solvent-dispersed silica sol by nitrogen adsorption was measured. Ｎ２ The specific surface area value S was measured using the B.E.T. one-point method described above, with the measurement sample (silica particles (powder) heated at 150°C) obtained in the (measurement of carbon content) described later. Ｎ２ (m ２ ( / g) and density ρ (g / cm³) ３ From this, the average primary particle diameter (nm) = 6000 / ρ × S Ｎ２ The density of silica particles was calculated using the following method. In this invention, the density of silica particles is ρ (g / cm³). ３ ) is 2.2 (g / cm³) ３ ) was calculated as follows.

[0054] [Moisture content measurement] Moisture content was determined by Karl Fischer titration using a Karl Fischer moisture meter (manufactured by Kyoto Electronics Manufacturing Co., Ltd., product name: MKA-610).

[0055] [pH Measurement] The pH of the water-dispersed silica sol was measured at 20°C using a pH meter (manufactured by Toa DKK Co., Ltd., product name: MM-43X). The pH of the organic solvent-dispersed silica sol was measured at 20°C using a solution prepared by mixing silica sol and pure water in a 1:1 mass ratio when the organic solvent used as the dispersion medium was methanol (MeOH), or by mixing silica sol, MeOH, and pure water in a 1:1:1 mass ratio when the organic solvent was a nitrogen-containing solvent, such as dimethylacetamide (DMAC).

[0056] [Viscosity Measurement] The viscosity of silica sol was measured at 20°C using an Ostwald viscometer.

[0057] [Measurement of average particle diameter by dynamic light scattering (DLS method)] The average particle diameter by the DLS method (average secondary particle diameter by dynamic light scattering: also called Z-average particle diameter or DLS average particle diameter) was measured using a dynamic light scattering particle diameter analyzer (Malvern Panalogical, trade name: Zetasizer Nano). 0.1 g of the target silica sol was placed in a glass cell with a path length of 10 mm, and the solvent, which is the main component of the dispersion medium of the silica sol, was added to obtain a silica sol in which the silica particle concentration was adjusted so that the count rate at an attenuator of 7 was 200 to 400 kcps. For example, MeOH was added to silica sols in which methanol was the main component, and DMAC was added to silica sols in which DMAC was the main component, so that the silica particle concentration was adjusted so that the count rate at an attenuator of 7 was 200 to 400 kcps. The prepared silica sol was placed in the cell, and the liquid level was adjusted so that it was approximately 1 cm above the bottom of the cell. The DLS average particle size of the silica sol was then measured using the attenuator 7.

[0058] [Measurement of Carbon Content] (1) Place 3 mL of organic solvent-dispersed silica sol into a 30 cc centrifuge tube and add 20 mL of toluene. (2) After centrifuging (centrifugal force 11000 G × 30 min), remove the supernatant. (3) Add 4 mL of acetone to redissolve the gel, then add 4 mL of toluene and 10 mL of hexane, and after centrifuging (centrifugal force 11000 G × 30 min), remove the supernatant. (4) Repeat (3) again. (5) After vacuum drying the obtained gel at 60°C, the obtained powder was ground in a mortar and pestle and dried at 150°C for 2 hours to obtain silica powder. The carbon content (mass%) of the powder obtained above was measured using an elemental analyzer (PerkinElmer, product name 2400II CHNS / O).

[0059] [Measurement of Alkoxy Group Bonding Amount] [Measurement of Carbon Content] 0.2 g of the powder obtained in these measurements was mixed and dissolved in 10 mL of 0.05 N sodium hydroxide aqueous solution, and gas chromatography was performed (Shimadzu Corporation, product name: GC-2014s, column: PorapakQ, heating rate: 5°C / min, temperature range: 20-250°C). Gas chromatography measurement results and specific surface area value S Ｎ２ (m ２ From ( / g), the amount of alkoxy groups (groups / nm) bound to the surface. ２ ) was measured.

[0060] [Measurement of the amount of unsaturated bonded organic groups] By subtracting the carbon content corresponding to the amount of alcohol detached from the alkoxy groups contained in the silica particles obtained from the gas chromatography measurement in [Measurement of the amount of alkoxy groups] and the carbon content corresponding to the solvent adsorbed on the silica particles (e.g., DMAC: dimethylacetamide, etc.) from the carbon content of the powder obtained in [Measurement of carbon content], the amount of unsaturated bonded organic groups bonded according to formula (I) (groups / nm) is obtained. ２ The amount of bonded unsaturated bonded organic groups (groups / nm) was quantified. ２ ) = [(C м ―C Ａ ―C Ｓ ) / M Ｐ ×A ｂ ] / ( S Ｎ２ ×10 １８ ×100) ...Formula (I) C м : Carbon content of the powder (mass%) C Ａ : Carbon content (mass%) equivalent to the amount of alcohol removed from the alkoxy group C Ｓ : Carbon content (mass%) equivalent to the solvent adsorbed on silica particles M Ｐ : Amount of carbon per mol contained in the unsaturated bond group-containing organic group (72 g / mol in the case of phenyltrimethoxysilane) A ｂ Avogadro's number: 6.02 × 10⁻¹⁰ ２３ (pieces / mol) S Ｎ２ : Specific surface area value (m ２ / g)

[0061] [Measurement of Free Silanol Group Amount] The silica sol was subjected to additional silane treatment using the following procedure, and the amount of free silanol groups was calculated. (1) 100 g of silica sol was placed in a 250 mL round-bottom flask, and while stirring the sol with a magnetic stirrer, 2 g of pure water was added, followed by 3 hexamethyldisilazane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name SZ-31) at a concentration of 3 groups / nm ２ (1) Add the amount (for example, 7.4 g when the average primary particle size of the silica particles is 12 nm and the silica concentration is 30% by mass), and then maintain the liquid temperature at 90°C for 3 hours. (2) Using a rotary evaporator, evaporate the solvent at a reduced pressure of 450 to 110 Torr and a bath temperature of 85 to 125°C while supplying the main component of the dispersion medium, for example, DMAC in Example 1 described later, and obtain a silica sol that has been further treated with trimethylsilane produced by hydrolysis of hexamethyldisilazane. (3) Calculate the carbon content of the silica sol before and after the additional silane treatment in (2) according to [Measurement of Carbon Content]. (4) Subtract the carbon content corresponding to the amount of alcohol removed from the alkoxy groups contained in the silica particles obtained from the gas chromatography measurement in [Measurement of Alkoxy Group Bonding Amount] and the carbon content corresponding to the solvent adsorbed on the silica particles (for example, DMAC, etc.: dimethylacetamide, etc.) from the carbon content in (3) to calculate the carbon content of unsaturated bond-containing groups and the carbon content derived from the additional silane treatment. (5) From the change in carbon content (mass%) of the silica sol before and after additional silane treatment, the amount of free silanol groups per unit area of ​​the silica surface (groups / nm) can be calculated using formula (II). ２ The amount of free silanol groups per unit area of ​​silica surface (groups / nm) was calculated. ２ ) = [{C ｂ -C ａ} / M ｃ ×A ｂ ] / ( S Ｎ２ ×10 １８ ×100)] ...Formula (II) C ｂ : Carbon content (mass%) of silica sol after additional silane treatment C ａ : Carbon content (mass%) of silica sol before additional silane treatment M ｃ : Amount of carbon per mol in the trimethylsilane molecule (36 g / mol)ｂ Avogadro's number: 6.02 × 10⁻¹⁰ ２３ (pieces / mol) S Ｎ２ : Specific surface area value (m ２ / g)

[0062] [Storage Stability of Silica Sol] 50 g of a concentrated silica sol, obtained by heating silica sol under reduced pressure in a rotary evaporator to a concentration of 40% by mass, was sealed in a 100 ml glass bottle and stored at 50°C for one month. Subsequently, the viscosity ratio was calculated from the viscosity of the silica sol before and after storage at 50°C using the following formula (III) as an indicator of the change over time at 50°C for one month, and the storage stability of the silica sol was evaluated. However, (X) in formula (N) below is omitted. ０ ) indicates the viscosity before the storage stability test (immediately after sol preparation), (X １ｍ ) indicates the viscosity after the storage stability test (after being held at 50°C for 1 month). Viscosity ratio: (X １ｍ ) / (X ０ ) ... (Formula (III)) Storage stability was evaluated in four stages, A to D, as shown below. The results are shown in Table 1. <Thermal Stability Test Evaluation Criteria> A: Viscosity ratio of 0.80 or more and less than 1.06 B: Viscosity ratio of 1.06 or more and less than 1.11 C: Viscosity ratio of 1.11 or more and less than 2.01 D: Viscosity ratio of 2.01 or more, or less than 0.80, or the silica sol becomes cloudy, phase separation occurs, sediment or floating occurs, or it is not possible to concentrate to a silica concentration of 40% by mass.

[0063] (Measurement of Insulation Life) The insulation life of the insulation test samples prepared in the examples and comparative examples described later was measured using a dielectric breakdown tester, model YST-243WS, manufactured by Yamayo Test Instruments Co., Ltd., at a test temperature of 155°C (in air), an applied voltage of 2.0 kV, and a frequency of 50 Hz. The electrode shape used was a flat electrode (φ=25 mm) at the bottom and a spherical electrode (φ=20 mm) at the top, with both electrodes in contact with the sample during the test. Four measurements were taken at an applied voltage of 2.0 kV, and the average value is recorded. The insulation life was evaluated in four stages, A to D, as described below. The results obtained are shown in Table 1.

[0064] (Insulation Life Evaluation Criteria) A: Insulation life of 700 minutes or more B: Insulation life of 400 minutes or more but less than 700 minutes C: Insulation life of 100 minutes or more but less than 400 minutes D: Insulation life of less than 100 minutes

[0065] [Example 1] (A) Step: Water-dispersed silica sol ST-O-33 (average primary particle size 12 nm, pH 3, silica concentration 33% by mass, density 2.2 g / cm³) ３ (A) Step: A silica sol (manufactured by Nissan Chemical Corporation) was prepared. Step (B): 1000 g of the aqueous silica sol prepared in Step (A) was placed in a 2 L round-bottom flask, and while stirring the sol with a magnetic stirrer, 200 g of DMAC (dimethylacetamide, manufactured by Fujifilm Wako Pure Chemical Industries, purity 99.99% by mass) was added, followed by the addition of 0.8 g of N,N-diisopropylethylamine (manufactured by Fujifilm Wako Pure Chemical Industries, purity 99.9% by mass), and then held at 25°C for 30 minutes. Step (C): The sol obtained in Step (B) was supplied with DMAC while evaporating the solvent at a reduced pressure of 300 to 120 Torr and a bath temperature of 108°C using a rotary evaporator, thereby replacing the dispersion medium of the sol with DMAC to obtain a DMAC-dispersed silica sol (silica concentration 31.9% by mass, water content 4.0% by mass). (D) Step: Next, while stirring the DMAC-dispersed silica sol obtained in step (C) with a magnetic stirrer, 7.5 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 1 hour. Then, 1.8 g of N,N-diisopropylethylamine was added, and the liquid temperature was maintained at 90°C for 1 hour. (E) Step: The sol obtained in step (D) was subjected to solvent replacement in the same manner as in step (C), resulting in a surface-treated DMAC-dispersed silica sol (silica concentration 40.7% by mass, pH 4.7, viscosity 9 mPa·s, moisture content 0.9% by mass, average particle size 13 nm by dynamic light scattering method, specific surface area 234 m²). ２ / g, average primary particle diameter 12 nm, carbon content 2.2 mass%, phenyl group bonding amount to silica particles 0.3 / nm ２ The amount of methoxy groups attached to silica particles is 0.005 / nm. ２ The obtained surface-treated DMAC-dispersed silica sol showed that the change in viscosity from the initial state after storage at 50°C for one month was within 10%, indicating excellent storage stability.

[0066] [Example 2] The procedure was carried out in the same manner as in Example 1, except that the amount of phenyltrimethoxysilane added in step (D) was changed to 10.8 g. Surface treatment DMAC dispersed silica sol (silica concentration 40.6% by mass, pH 4.8, viscosity 8 mPa·s, moisture content 0.8% by mass, average particle size 13 nm by dynamic light scattering method, specific surface area 234 m²) ２ / g, average primary particle diameter 12 nm, carbon content 2.5 mass%, phenyl group bonding amount to silica particles 0.4 / nm ２ The amount of methoxy groups attached to silica particles is 0.005 / nm. ２ The obtained surface-treated DMAC-dispersed silica sol showed that the change in viscosity from the initial state after storage at 50°C for one month was within 10%, indicating excellent storage stability.

[0067] [Example 3] The procedure was carried out in the same manner as in Example 1, except that the amount of phenyltrimethoxysilane added in step (D) was changed to 17.5 g. Surface treatment DMAC dispersed silica sol (silica concentration 40.7% by mass, pH 4.7, viscosity 8 mPa·s, moisture content 0.8% by mass, average particle size 12 nm by dynamic light scattering method, specific surface area 234 m²) ２ / g, average primary particle diameter 12 nm, carbon content 3.0 mass%, phenyl group bonding amount to silica particles 0.6 / nm ２ , methoxy group bonding amount to silica particles: 0.02 groups / nm ２ The obtained surface-treated DMAC-dispersed silica sol showed that the change in viscosity from the initial state after storage at 50°C for one month was within 10%, indicating excellent storage stability.

[0068] [Comparative Example 1] The procedure was carried out in the same manner as in Example 1, except that the amount of phenyltrimethoxysilane added in step (D) of Example 1 was changed to 2.5 g. Surface treatment DMAC dispersed silica sol (silica concentration 34.8% by mass, pH 5.5, viscosity 14 mPa·s, moisture content 0.9% by mass, average particle size 22 nm by dynamic light scattering method, specific surface area 234 m²) ２ / g, average primary particle diameter 12 nm, carbon content 1.7 mass%, phenyl group bonding amount to silica particles 0.1 / nm ２ The amount of methoxy groups attached to silica particles is 0.0002 / nm. ２)(was obtained. The rate of change in viscosity from the initial value after storing the obtained surface-treated DMAC-dispersed silica sol at 50°C for one month was 20% or more, and the storage stability was poor.)

[0069] (Synthesis Example 1) Preparation of polyamic acid 496 g of 4,4'-diaminodiphenyl ether (DDE) and 534 g of pyromellitic dianhydride (PMDA) were polymerized at a temperature of 50°C for 13 hours under stirring using NMP (N-methylpyrrolidone) and DMAC (dimethylacetamide) as solvents to obtain a polyamic acid corresponding to formula (4) (solid content 17% by mass, viscosity at 25°C of 13,640 mPa·s measured with an E-type viscometer). The polymerization of the polyamic acid was carried out with DDE and PMDA in an equimolar ratio of 1:1. The weight-average molecular weight of the obtained polyamic acid was 63,000. In formula (4), n is the number of repeating units.)

[0070] (The DMAC-dispersed silica sols obtained in Examples 1 to 3 and Comparative Example 1 were stored at 50°C for one month and then added and mixed in a glass bottle with the polyamic acid obtained in Synthesis Example 1 in a resin (as polyimide) / SiO mass ratio of) ２ = 80 / 20, and defoamed and stirred for 20 minutes with a vacuum defoamer (manufactured by EME Co., Ltd., trade name V-mini300) to obtain a silica-containing polyamic acid.)

[0071] (Next, the obtained silica-containing polyamic acid was applied onto an alkali-free glass (manufactured by AS ONE Corporation, trade name EAGLEXG, 200 mm × 200 mm, 0.7 mm thick) with an applicator (manufactured by BEVS Co., Ltd., trade name: Film Applicator B / M150 mm with film thickness adjustment function, coating film thickness (wet film thickness) 400 μm, coating width 100 mm), and then the solvent was removed and heat-cured under an air atmosphere at 70°C for 30 minutes, then at 100°C for 30 minutes, and finally at 150°C for 30 minutes to obtain a silica-containing polyimide film on the alkali-free glass.)

[0072] Next, the silica-containing polyimide film was separated from the alkali-free glass by making incisions with a cutter, fixed to a metal frame (made of stainless steel), and then heat-cured at 400°C for 1 hour in an atmospheric environment to obtain a silica-containing polyimide self-supporting film (film thickness 30-35 μm). This was cut into 5 cm squares to be used as a sample for insulation testing.

[0073]

[0074] As shown in the evaluation results in Table 1, the silica sols of Examples 1 to 3, which possess the technical features of the present invention, differ from the silica sol of Comparative Example 1 in that they are high-concentration silica sols containing free silanol groups that allow for additional silane treatment, and also exhibit excellent storage stability. Therefore, it has been demonstrated that both customizability and reliability can be achieved by using the silica sol of the present invention. Furthermore, the silica sols obtained in Examples 1 to 3 showed improved insulation lifetime of silica-containing polyimide self-supporting films prepared after storage at 50°C for one month, compared to the silica sol obtained in Comparative Example 1.

[0075] The silica sol of the present invention is a high-concentration silica sol that can undergo additional silane treatment and also has excellent storage stability, thus providing a high-quality silica sol. When the silica sol is combined with a nitrogen-containing polymer and used as an insulating resin composition, it is possible to provide an insulating coated conductor that can maintain a high insulation life over a long period of time.

Claims

1. A silica sol comprising silica particles with an average particle diameter of 5 to 100 nm, determined by dynamic light scattering, on which organic groups containing unsaturated bonds between carbon atoms are bonded to the surface, and a nitrogen-containing solvent, wherein the amount of organic groups containing unsaturated bonds between carbon atoms bonded to the surface of the silica particles is 0.1 to 2.0 groups / nm per unit area of ​​the silica particle surface. ２ The silica particle concentration is 25-70% by mass, and free silanol groups are present on the surface of the silica particles, with the amount of free silanol groups per unit area of ​​the silica particle surface being 0.4-1.2 groups / nm. ２ That is silica sol.

2. The number of alkoxy groups bonded to the surface of the silica particles is 0.001 to 0.03 groups / nm per unit area of ​​the silica particle surface. ２ The silica sol according to claim 1.

3. The silica sol according to claim 1, wherein the organic group containing an unsaturated bond between carbon atoms is an organic group containing a phenyl group or an organic group containing a (meth)acryloyl group.

4. The silica sol according to claim 1, wherein the organic group containing the unsaturated bond between carbon atoms is a phenyl group, a phenylaminoalkyl group, a (meth)acryloyl group, or a (meth)acryloylalkyl group.

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

6. The silica sol according to claim 1, wherein the nitrogen-containing solvent is dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone.

7. An insulating resin composition comprising a silica sol and a nitrogen-containing polymer according to any one of claims 1 to 6.

8. The insulating resin composition according to claim 7, wherein the amount of the nitrogen-containing polymer is 1 to 100 parts by mass per 1 part by mass of silica contained in the silica sol.

9. The insulating resin composition according to claim 7, wherein the nitrogen-containing polymer is polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, or polyesterimide.

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