WO2025089213A1 - Silica particles having improved hydrophobicity, and composition for coating enamel wire - Google Patents

Abstract

The present invention addresses the problem of providing a silica sol in which silica particles are dispersed in a nitrogen-containing solvent and which is for mixing with a polyimide-based or polyamide-based polar resin with which the silica particles have good compatibility, and of providing an insulating resin composition in which the silica sol and the resin are combined, and an insulated and covered conductive wire which maintains high insulation life over a long period of time. Said problem is solved by means of a silica sol in which silica particles are dispersed in a nitrogen-containing solvent, wherein the silica particles have an average primary particle size of 5-100 nm, the surfaces of at least a portion of the silica particles are coated with a hydrolysate of a carboxylic acid-based silane coupling agent (a), and the degree of hydrophobization as determined by methanol titration is 0.8 vol% or higher.

Description

Silica particles with improved hydrophobicity and composition for enameled wire coating

The present invention relates to a hydrophobic silica sol dispersed in a nitrogen-containing solvent, an insulating resin composition using the same, and a method for producing the same.

A method has been disclosed in which hydroxyl groups on the surface of inorganic oxide particles such as silica react with alcohol, introducing alkoxysilyl groups to make them organic, and obtaining an inorganic oxide sol dispersed in an organic solvent such as toluene. In this method, phenyltrimethoxysilane is reacted with a methanol-dispersed silica sol, and a silica sol dispersed in a toluene solvent is disclosed (see Patent Document 1).

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 Brochure

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

（式（１）及び（２）中、Ｒ^１及びＲ^３はそれぞれカルボキシル基、酸無水物基、カルボン酸エステル基、又はそれらを含む有機基で且つＳｉ－Ｃ結合によりケイ素原子と結合しているものであり、Ｒ^２及びＲ^４はそれぞれアルコキシ基、アシルオキシ基、ヒドロキシル基、又はハロゲン基を示し、Ｘはアルキレン基、ＮＨ基、又は酸素原子を示し、ａは１～３の整数を示し、ｂは１～２の整数であり、ｃは０又は１の整数である。）からなる群より選ばれる少なくとも１種のシラン化合物を含むものである第１観点又は第２観点に記載のシリカゾル、
第４観点として、更にシリカ粒子がその粒子表面に非カルボン酸系シランカップリング剤（ｂ）の加水分解物が被覆されたシリカ粒子である第１観点乃至第３観点の何れか一つに記載のシリカゾル、
第５観点として、非カルボン酸系シランカップリング剤（ｂ）が、少なくともアルキル基、（メタ）アクリロイル基、又はアリール基を含む有機基を含むシランカップリング剤である第４観点に記載のシリカゾル、
第６観点として、非カルボン酸系シランカップリング剤（ｂ）が式（３）、式（４）、及び式（５）：
［化２］

（式（３）中、Ｒ^５はそれぞれアルキル基、ハロゲン化アルキル基、アルケニル基、アリール基、又はエポキシ基、（メタ）アクリロイル基、メルカプト基、アミノ基、ウレイド基、もしくはシアノ基を有する有機基で且つＳｉ－Ｃ結合によりケイ素原子と結合しているものであって、Ｒ^６はそれぞれアルコキシ基、アシルオキシ基、ヒドロキシル基、又はハロゲン基を示し、ｄは１～３の整数を示し、
式（４）及び式（５）中、Ｒ^７及びＲ^９はそれぞれ炭素原子数１～３のアルキル基、又は炭素原子数６～３０のアリール基で且つＳｉ－Ｃ結合によりケイ素原子と結合しているものであり、Ｒ^８及びＲ^１０はそれぞれアルコキシ基、アシルオキシ基、ヒドロキシル基、又はハロゲン基を示し、Ｙはアルキレン基、ＮＨ基、又は酸素原子を示し、ｅは１～３の整数であり、ｆは０又は１の整数であり、ｇは１～３の整数である。）
からなる群より選ばれる少なくとも１種のシラン化合物を含むものである第４観点又は第５観点に記載の有機溶媒シリカゾル、
第７観点として、カルボン酸系シランカップリング剤（ａ）：非カルボン酸系シランカップリング剤（ｂ）が質量比で１：０．１～１０の割合でその加水分解物が被覆されたシリカ粒子を含む第４観点又は第５観点に記載のシリカゾル、
第８観点として、窒素含有溶媒がアミド系溶媒である第１観点乃至第７観点のいずれか一つに記載のシリカゾル、
第９観点として、前記窒素含有溶媒がジメチルアセトアミド、ジメチルホルムアミド、Ｎ－メチルピロリドン、又はＮ－エチルピロリドンである第１観点乃至第８観点の何れか一つに記載のシリカゾル、
第１０観点として、第１観点乃至第９観点の何れか一つに記載のシリカゾルと窒素含有ポリマーとを含む絶縁性樹脂組成物、
第１１観点として、前記シリカゾルに含まれるシリカの１質量部に対し前記窒素含有ポリマーの質量部が１～１００である第１０観点に記載の上記組成物、
第１２観点として、前記窒素含有ポリマーが、ポリイミド、ポリアミド、ポリアミック酸、ポリアミドイミド、ポリエーテルイミド、又はポリエステルイミドのいずれかである第１０観点又は第１１観点に記載の上記組成物、
第１３観点として、第１０観点乃至第１２観点の何れか一つに記載の絶縁性樹脂組成物により絶縁被覆した絶縁性被覆導線、
第１４観点として、４，４’－ジアミノジフェニルエーテル(ＤＤＥ)、及び無水ピロメリット酸無水物（ＰＭＤＡ）からなるポリアミック酸を樹脂として、質量比で樹脂／ＳｉＯ_２＝８５／１５になるように調整したシリカ配合ポリアミック酸をＣｕ板に２９０℃で加熱してシリカ配合ポリイミドを焼き付けたＣｕ板（皮膜厚：２９～３２μｍ）を得て、試験温度１５５℃（空気中）、印加電圧３．０ｋＶ、周波数５０Ｈｚでの絶縁破壊寿命が、５０分以上である第１０観点乃至第１２観点の何れか一つに記載の絶縁性樹脂組成物、
第１５観点として、下記（Ａ）工程乃至（Ｄ）工程：
（Ａ）工程：５～１００ｎｍの平均一次粒子径を有するシリカ粒子が水性媒体に分散したシリカゾルを準備する（Ａ）工程、
（Ｂ）工程：（Ａ）工程で得られたシリカゾルに式（１）乃至式（２）からなる群より選ばれる少なくとも１種のカルボン酸系シランカップリング剤（ａ）を添加する（Ｂ）工程、
（Ｃ）工程：（Ｂ）工程で得られたシリカゾルの分散媒を窒素含有溶媒に溶媒置換する（Ｃ）工程、
（Ｄ）工程：（Ｃ）工程で得られたシリカゾルに、式（３）乃至式（５）からなる群から選ばれる少なくとも１種の非カルボン酸系シランカップリング剤（ｂ）を添加する（Ｄ）工程、を含む、第１観点乃至第９観点の何れか一つに記載のシリカゾルの製造方法、及び
第１６観点として、第１５観点に記載の（Ａ）工程乃至（Ｄ）工程に、更に（Ｅ）乃至（Ｆ）工程：
（Ｅ）工程：（Ｄ）工程で得られたシリカ粒子が窒素含有溶媒に分散したシリカゾルと、窒素含有ポリマーを混合する（Ｅ）工程、
（Ｆ）工程：（Ｅ）工程で得られたシリカゾルから窒素含有溶媒の一部又は全部を除去する工程、を追加する第１０観点乃至第１２観点の何れか一つに記載の絶縁性樹脂組成物の製造方法である。 In a first aspect, the present invention provides a silica sol in which silica particles having an average primary particle size of 5 to 100 nm are coated on the particle surface with a hydrolysate of a carboxylic acid-based silane coupling agent (a), the silica particles have a hydrophobicity of 0.8 volume % or more as determined by a methanol titration method, and the silica particles are dispersed in a nitrogen-containing solvent;
As a second aspect, the silica sol according to the first aspect, in which the carboxylic acid-based silane coupling agent (a) is a silane coupling agent containing a carboxyl group, an acid anhydride group, a carboxylic acid ester group, or an organic group containing any of them.
As a third aspect, the carboxylic acid-based silane coupling agent (a) is represented by the formula (1) and the formula (2):
[Chemical formula 1]

(in formulas (1) and (2), R ¹ and R ³ each represent a carboxyl group, an acid anhydride group, a carboxylate group, or an organic group containing any of them and are bonded to a silicon atom via a Si-C bond; R ² and R ⁴ each represent an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; X represents an alkylene group, an NH group, or an oxygen atom; a represents an integer of 1 to 3; b represents an integer of 1 to 2; and c represents an integer of 0 or 1).
As a fourth aspect, the silica sol according to any one of the first to third aspects, wherein the silica particles are silica particles whose particle surfaces are coated with a hydrolysate of the non-carboxylic acid silane coupling agent (b).
As a fifth aspect, the silica sol according to the fourth aspect, in which the non-carboxylic acid silane coupling agent (b) is a silane coupling agent containing an organic group containing at least an alkyl group, a (meth)acryloyl group, or an aryl group.
As a sixth aspect, the non-carboxylic acid-based silane coupling agent (b) is represented by formula (3), formula (4), or formula (5):
[Chemical formula 2]

(In formula (3), R ⁵ is an alkyl group, a halogenated alkyl group, an alkenyl group, an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, or a cyano group, and is bonded to a silicon atom via a Si-C bond; R ⁶ is an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; d is an integer of 1 to 3;
In formula (4) and formula (5), R ⁷ and R ⁹ 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 via a Si-C bond; R ⁸ and R ¹⁰ 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; e is an integer of 1 to 3; f is an integer of 0 or 1; and g is an integer of 1 to 3.
The organic solvent silica sol according to the fourth or fifth aspect, which contains at least one silane compound selected from the group consisting of:
As a seventh aspect, the silica sol according to the fourth or fifth aspect, which contains silica particles coated with a hydrolysate of a carboxylic acid silane coupling agent (a) and a non-carboxylic acid silane coupling agent (b) in a mass ratio of 1:0.1 to 10;
As an eighth aspect, the silica sol according to any one of the first to seventh aspects, in which the nitrogen-containing solvent is an amide solvent.
As a ninth aspect, the silica sol according to any one of the first to eighth aspects, in which the nitrogen-containing solvent is dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone.
According to a tenth aspect, there is provided an insulating resin composition comprising the silica sol according to any one of the first to ninth aspects and a nitrogen-containing polymer.
According to an eleventh aspect, the composition according to the tenth 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 a twelfth aspect, the composition according to the tenth or eleventh aspect, wherein the nitrogen-containing polymer is any one of polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, and polyesterimide;
According to a thirteenth aspect, there is provided an insulating coated conductor that is insulatingly coated with the insulating resin composition according to any one of the tenth to twelfth aspects.
As a fourteenth aspect, an insulating resin composition according to any one of the tenth to twelfth aspects, in which a polyamic acid composed of 4,4'-diaminodiphenyl ether (DDE) and pyromellitic anhydride (PMDA) is used as a resin, and a silica-blended polyamic acid adjusted to a mass ratio of resin/SiO ₂ = 85/15 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, 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;
As a fifteenth aspect, the present invention provides a method for producing a method comprising the following steps (A) to (D):
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): adding at least one carboxylic acid-based silane coupling agent (a) selected from the group consisting of formulas (1) and (2) to the silica sol obtained in step (A);
Step (C): A step (C) of replacing the dispersion medium of the silica sol obtained in step (B) with a nitrogen-containing solvent;
A method for producing a silica sol according to any one of the first to ninth aspects, comprising: a step (D) of adding at least one non-carboxylic acid silane coupling agent (b) selected from the group consisting of formulas (3) to (5) to the silica sol obtained in the step (C). As a sixteenth aspect, the method for producing a silica sol according to the fifteenth aspect, further comprising steps (E) to (F) in addition to the steps (A) to (D) according to the fifteenth aspect:
Step (E): A step (E) of mixing the silica sol obtained in step (D), in which the silica particles are dispersed in a nitrogen-containing solvent, with a nitrogen-containing polymer;
The method for producing an insulating resin composition according to any one of the tenth to twelfth aspects, further comprising the step of: (F) removing a part or all of the nitrogen-containing solvent from the silica sol obtained in the step (E).

（式（１）及び（２）中、Ｒ^１及びＲ^３はそれぞれカルボキシル基、酸無水物基、カルボン酸エステル基、又はそれらを含む有機基で且つＳｉ－Ｃ結合によりケイ素原子と結合しているものであり、Ｒ^２及びＲ^４はそれぞれアルコキシ基、アシルオキシ基、ヒドロキシル基、又はハロゲン基を示し、Ｘはアルキレン基、ＮＨ基、又は酸素原子を示し、ａは１～３の整数を示し、ｂは１～２の整数であり、ｃは０又は１の整数である。）からなる群より選ばれる少なくとも１種のシラン化合物を含むものである、［１］又は［２］に記載のシリカゾル。
［４］
　シリカ粒子の少なくとも一部が、その粒子表面を非カルボン酸系シランカップリング剤（ｂ）の加水分解物でさらに被覆された、［１］乃至［３］のいずれかに記載のシリカゾル。
［５］
　非カルボン酸系シランカップリング剤（ｂ）が、少なくともアルキル基、（メタ）アクリロイル基、又はアリール基を含む有機基を含むシランカップリング剤である、［４］に記載のシリカゾル。
［６］
　非カルボン酸系シランカップリング剤（ｂ）が式（３）、式（４）、及び式（５）：
［化２］

（式（３）中、Ｒ^５はそれぞれアルキル基、ハロゲン化アルキル基、アルケニル基、アリール基、又はエポキシ基、（メタ）アクリロイル基、メルカプト基、アミノ基、ウレイド基、もしくはシアノ基を有する有機基で且つＳｉ－Ｃ結合によりケイ素原子と結合しているものであって、Ｒ^６はそれぞれアルコキシ基、アシルオキシ基、ヒドロキシル基、又はハロゲン基を示し、ｄは１～３の整数を示し、
式（４）及び式（５）中、Ｒ^７及びＲ^９はそれぞれ炭素原子数１～３のアルキル基、又は炭素原子数６～３０のアリール基で且つＳｉ－Ｃ結合によりケイ素原子と結合しているものであり、Ｒ^８及びＲ^１０はそれぞれアルコキシ基、アシルオキシ基、ヒドロキシル基、又はハロゲン基を示し、Ｙはアルキレン基、ＮＨ基、又は酸素原子を示し、ｅは１～３の整数であり、ｆは０又は１の整数であり、ｇは１～３の整数である。）
からなる群より選ばれる少なくとも１種のシラン化合物を含むものである、［４］又は［５］に記載のシリカゾル。
［７］
　カルボン酸系シランカップリング剤（ａ）：非カルボン酸系シランカップリング剤（ｂ）が質量比で１：０．１～１０の割合で、その加水分解物で被覆されたシリカ粒子を含む、［４］又は［５］に記載のシリカゾル。
［８］
　前記窒素含有溶媒がアミド系溶媒である、［１］乃至［７］のいずれかに記載のシリカゾル。
［９］
　前記窒素含有溶媒がジメチルアセトアミド、ジメチルホルムアミド、Ｎ－メチルピロリドン、又はＮ－エチルピロリドンである、［１］乃至［８］のいずれかに記載のシリカゾル。
［１０］
　［１］乃至［９］のいずれかに記載のシリカゾルと窒素含有ポリマーとを含む絶縁性樹脂組成物。
［１１］
　前記シリカゾルに含まれるシリカの１質量部に対し前記窒素含有ポリマーの質量部が１～１００である、［１０］に記載の絶縁性樹脂組成物。
［１２］
　前記窒素含有ポリマーが、ポリイミド、ポリアミド、ポリアミック酸、ポリアミドイミド、ポリエーテルイミド、又はポリエステルイミドのいずれかである、［１０］又は［１１］に記載の絶縁性樹脂組成物。
［１３］
　［１０］乃至［１２］のいずれかに記載の絶縁性樹脂組成物により絶縁被覆した絶縁性被覆導線。
［１４］
　４，４’－ジアミノジフェニルエーテル(ＤＤＥ)、及び無水ピロメリット酸無水物（ＰＭＤＡ）からなるポリアミック酸を樹脂として、質量比で樹脂／ＳｉＯ_２＝８５／１５になるように調整したシリカ配合ポリアミック酸をＣｕ板に２９０℃で加熱してシリカ配合ポリイミドを焼き付けたＣｕ板（皮膜厚：２９～３２μｍ）を得て、試験温度１５５℃（空気中）、印加電圧３．０ｋＶ、周波数５０Ｈｚでの絶縁破壊寿命が、５０分以上である、［１０］乃至［１２］のいずれかに記載の絶縁性樹脂組成物。
［１５］
　下記（Ａ）工程乃至（Ｄ）工程：
　（Ａ）工程：５～１００ｎｍの平均一次粒子径を有するシリカ粒子が水性媒体に分散したシリカゾルを準備する（Ａ）工程、
　（Ｂ）工程：（Ａ）工程で得られたシリカゾルに式（１）乃至式（２）からなる群より選ばれる少なくとも１種のカルボン酸系シランカップリング剤（ａ）を添加する（Ｂ）工程、
　（Ｃ）工程：（Ｂ）工程で得られたシリカゾルの分散媒を窒素含有溶媒に溶媒置換する（Ｃ）工程、
　（Ｄ）工程：（Ｃ）工程で得られたシリカゾルに、式（３）乃至式（５）からなる群から選ばれる少なくとも１種の非カルボン酸系シランカップリング剤（ｂ）を添加する（Ｄ）工程、
　を含む、［１］乃至［９］のいずれかに記載のシリカゾルの製造方法。
［１６］
　請求項１５に記載の（Ａ）工程乃至（Ｄ）工程に、更に（Ｅ）乃至（Ｆ）工程：
　（Ｅ）工程：（Ｄ）工程で得られたシリカ粒子が窒素含有溶媒に分散したシリカゾルと、窒素含有ポリマーを混合する（Ｅ）工程、
　（Ｆ）工程：（Ｅ）工程で得られたシリカゾルから窒素含有溶媒の一部又は全部を除去する工程、
　を追加する、［１０］乃至［１２］のいずれかに記載の絶縁性樹脂組成物の製造方法。 Although overlapping with other descriptions, various aspects of the present invention are as follows. However, the present invention is not limited to the following.
[1]
A silica sol in which silica particles are dispersed in a nitrogen-containing solvent,
The silica particles have an average primary particle size of 5 to 100 nm,
At least a part of the silica particles has a particle surface coated with a hydrolyzate of a carboxylic acid-based silane coupling agent (a), and has a hydrophobicity of 0.8 volume % or more as measured by a methanol titration method.
Silica sol.
[2]
The silica sol according to [1], wherein the carboxylic acid-based silane coupling agent (a) is a silane coupling agent containing a carboxyl group, an acid anhydride group, a carboxylic acid ester group, or an organic group containing them.
[3]
The carboxylic acid-based silane coupling agent (a) is represented by the formula (1) and the formula (2):
[Chemical formula 1]

(in formulas (1) and (2), R ¹ and R ³ each represent a carboxyl group, an acid anhydride group, a carboxylate group, or an organic group containing any of them and are bonded to a silicon atom via a Si-C bond; R ² and R ⁴ each represent an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; X represents an alkylene group, an NH group, or an oxygen atom; a represents an integer of 1 to 3; b represents an integer of 1 to 2; and c represents an integer of 0 or 1). The silica sol according to [1] or [2],
[4]
The silica sol according to any one of [1] to [3], wherein at least a part of the silica particles is further coated with a hydrolyzate of the non-carboxylic acid silane coupling agent (b) on the particle surface.
[5]
The silica sol according to [4], wherein the non-carboxylic acid silane coupling agent (b) is a silane coupling agent containing an organic group containing at least an alkyl group, a (meth)acryloyl group, or an aryl group.
[6]
The non-carboxylic acid-based silane coupling agent (b) is represented by the formula (3), the formula (4), and the formula (5):
[Chemical formula 2]

(In formula (3), R ⁵ is an alkyl group, a halogenated alkyl group, an alkenyl group, an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, or a cyano group, and is bonded to a silicon atom via a Si-C bond; R ⁶ is an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; d is an integer of 1 to 3;
In formula (4) and formula (5), R ⁷ and R ⁹ 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 via a Si-C bond; R ⁸ and R ¹⁰ 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; e is an integer of 1 to 3; f is an integer of 0 or 1; and g is an integer of 1 to 3.
The silica sol according to [4] or [5], which contains at least one silane compound selected from the group consisting of:
[7]
The silica sol according to [4] or [5], comprising silica particles coated with a hydrolysate of a carboxylic acid silane coupling agent (a) and a non-carboxylic acid silane coupling agent (b) in a mass ratio of 1:0.1 to 10.
[8]
The silica sol according to any one of [1] to [7], wherein the nitrogen-containing solvent is an amide solvent.
[9]
The silica sol according to any one of [1] to [8], wherein the nitrogen-containing solvent is dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone.
[10]
[1] An insulating resin composition comprising the silica sol according to any one of [1] to [9] and a nitrogen-containing polymer.
[11]
The insulating resin composition according to [10], 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.
[12]
The insulating resin composition according to [10] or [11], wherein the nitrogen-containing polymer is any one of polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, and polyesterimide.
[13]
An insulating coated conductor coated with the insulating resin composition according to any one of [10] to [12].
[14]
The insulating resin composition according to any _{one of [10] to [12], wherein a polyamic acid composed of 4,4'-diaminodiphenyl ether (DDE) and pyromellitic anhydride (PMDA) is used as a resin, and a silica-blended polyamic acid adjusted to a mass ratio of resin/SiO 2} = 85/15 is heated at 290°C on a Cu plate to obtain a silica-blended polyimide baked onto the Cu plate (film thickness: 29 to 32 μm), and the insulating breakdown life is 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.
[15]
The following steps (A) to (D):
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): adding at least one carboxylic acid-based silane coupling agent (a) selected from the group consisting of formulas (1) and (2) to the silica sol obtained in step (A);
Step (C): A step (C) of replacing the dispersion medium of the silica sol obtained in step (B) with a nitrogen-containing solvent;
Step (D): adding at least one non-carboxylic acid silane coupling agent (b) selected from the group consisting of formulas (3) to (5) to the silica sol obtained in step (C);
The method for producing a silica sol according to any one of [1] to [9], comprising:
[16]
The process according to claim 15 further comprises the steps (A) to (D) and further steps (E) to (F):
Step (E): A step (E) of mixing the silica sol obtained in step (D), in which the silica particles are dispersed in a nitrogen-containing solvent, with a nitrogen-containing polymer;
Step (F): A step of removing a part or all of the nitrogen-containing solvent from the silica sol obtained in step (E);
The method for producing an insulating resin composition according to any one of [10] to [12], further comprising:

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 can protect the substrate from insulation breakdown due to discharge by forming a dense and strong coating layer with the insulating resin. Insulating resins are often made of nitrogen-containing polymers with high insulation properties. 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 diamine molecules or acid anhydride molecules. In order for a resin having such a structure and silica particles to form an insulating coating layer with good compatibility, it is necessary to impart similar properties to the silica particles. According to the present invention, a silica sol can be provided that can increase the affinity and dispersibility of silica particles to insulating resins in order to form a coating layer with high insulation properties.

In order to impart such properties, silica particles are coated with two types of silane compounds. One is to coat silica particles with a silane compound (silane coupling agent) having a highly polar carboxyl group, and the other is to coat silica particles with a silane compound (silane coupling agent) having a functional group other than a carboxyl group, particularly a highly hydrophobic silane compound (silane coupling agent). The partial structure due to the carboxyl group formed on the surface of the silica particles forms a close interaction with the polar part of the nitrogen-containing polymer used as the insulating resin by hydrogen bonding or a covalent bond accompanied by a reaction. And, the hydrophobic group as a functional group other than the carboxyl group formed on the surface of the silica particles forms a close interaction with the hydrophobic part contained in the diamine molecule or acid anhydride molecule in the nitrogen-containing polymer used as the insulating resin by affinity. Since both functional groups on the silica particles can interact with the polar part and hydrophobic part of the insulating resin, it is thought that the insulating resin and the silica particles have improved insulation resistance because the surface modification part of each silica particle and the functional group part of the resin were able to improve adhesion to each other in the nano region.

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 solvent, the silica particles having a surface coated with a hydrolysate of a carboxylic acid-based silane coupling agent (a) and a hydrophobicity of 0.8% by volume or more as measured by a methanol titration method. Alternatively, the present invention is a silica sol in which silica particles are dispersed in a nitrogen-containing solvent, the silica particles having an average primary particle size of 5 to 100 nm, at least a portion of the silica particles having a surface coated with a hydrolysate of a carboxylic acid-based silane coupling agent (a) and a hydrophobicity of 0.8% by volume or more as measured by a methanol titration method.

The silica particles contained in the silica sol of the present invention have an average primary particle diameter of 5 to 100 nm, 10 to 70 nm, or 10 to 50 nm. The average primary particle diameter of the silica particles can be a particle diameter (nm) measured by a nitrogen gas adsorption method (BET method). The particle diameter (average primary particle diameter D (nm)) measured by the nitrogen gas adsorption method (BET method) is given by the formula D (nm) = 6000/ρ x S from the specific surface area S ( ^m2 /g) and density ρ (g/ ^cm3 ) measured by the nitrogen gas adsorption method. In the present invention, the density ρ of the silica particles used is 2.2 (g/ ^cm3 ).

In addition, the silica particles contained in the silica sol of the present invention have good dispersibility in nitrogen-containing solvents, and the particle size measured by dynamic light scattering (DLS) in a nitrogen-containing solvent is in the range of 5 to 100 nm or 10 to 70 nm.

The nitrogen-containing solvent used in the present invention has a functional group that contains at least a nitrogen atom (nitrogen-containing functional group). Examples of functional groups that have a nitrogen atom include amino groups, nitro groups, and cyano groups. The nitrogen-containing solvent used in the present invention is preferably an amide-based solvent in which a nitrogen-containing functional group and a carbonyl group exist in one solvent molecule, and examples of the nitrogen-containing solvent include a chain structure and a ring structure. Examples of the nitrogen-containing functional group include amino groups, nitro groups, and cyano groups, but 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, but can be used, for example, as an amide bond, and amide-based solvents are preferably used.

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.

Preferred examples of nitrogen-containing solvents include dimethylacetamide, dimethylformamide, N-methylpyrrolidone, and N-ethylpyrrolidone.

In the present invention, the nitrogen-containing solvent can contain other solvents as long as the effect is not impaired.

In other words, the total solvent may contain nitrogen-containing solvents at a ratio of 50-100 volume%, 90-100 volume%, 98-100 volume%, or 99-100 volume%, and other solvents may be contained at 0-50 volume%, 0-10 volume%, 0-2 volume%, or 0-1 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.

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

In the present invention, as the carboxylic acid-based silane coupling agent (a), a silane coupling agent containing a carboxyl group, an acid anhydride group, a carboxylate group, or an organic group containing them can be used. These functional groups can be present in a ratio of 1 to 3 per molecule of the silane coupling agent. The organic group connected to the functional group of the carboxyl group, the acid anhydride group, or the carboxylate group can be an aliphatic structure, an aromatic structure, or a combination thereof. The aliphatic structure can be a saturated or unsaturated structure, and an alkyl group or an alkenyl group can be used.

As the carboxylic acid-based silane coupling agent (a), at least one silane compound selected from the group consisting of formulas (1) and (2) can be used.

In formulas (1) and (2), ^R1 and ^R3 each represent a carboxyl group, an acid anhydride group, a carboxylate group, or an organic group containing any of these and are bonded to a silicon atom via a Si-C bond; ^R2 and ^R4 each represent an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; X represents an alkylene group, an NH group, or an oxygen atom; a represents an integer of 1 to 3; b represents an integer of 1 to 2; and c represents an integer of 0 or 1.

Examples of silane coupling agents containing an organic group that contains a carboxyl group include a compound of formula (1-1) in which two carboxyl group structures are bonded to a tetrahydroxydisiloxane skeleton, and the product name X-12-1135 manufactured by Shin-Etsu Chemical Co., Ltd. can be used.

Examples of silane coupling agents containing an organic group containing an acid anhydride group include those having an alkylene chain with 1 to 10 or 3 to 5 carbon atoms between the acid anhydride structure and the alkoxysilyl group, and silane coupling agents having a succinic anhydride structure can be used. For example, 3-trimethoxysilylpropyl succinic anhydride can be used, and the product name X-12-967C, manufactured by Shin-Etsu Chemical Co., Ltd., represented by the formula (1-2), can be used.

Silane coupling agents containing an organic group containing a carboxylate group include alkyl esters, aryl esters, and arylalkyl esters in which the ester portion is an alkyl ester. For example, the alkyl group may be a linear or branched alkyl group having 1 to 10 carbon atoms, or an aryl group having 6 to 40 carbon atoms. The arylalkyl group may be a structure in which an alkylene group having 1 to 10 carbon atoms is bonded to an aryl group having 6 to 40 carbon atoms. Examples of the structure of the ester portion include a methyl carboxylate structure, a t-butyl carboxylate structure, and a benzyl carboxylate structure. In addition, an alkylene group having 1 to 10 carbon atoms, which may contain a heteroatom, is connected between the carboxylate portion and the alkoxysilyl group by an alkylene group. Examples of the heteroatom include a nitrogen atom and an oxygen atom, and an NH group and an O group. The carboxylate portion is hydrolyzed to become a carboxylic acid, and when it contains a nitrogen atom as a heteroatom, it becomes an amino acid, and the organic group containing a carboxylate group can be used as an amino acid generator. For example, the product name X-88-475 manufactured by Shin-Etsu Chemical Co., Ltd., represented by the formula (1-3), can be used.

In the present invention, it is preferable that the hydrophobicity of the silica particles is 0.8% by volume or more as measured by methanol titration. There is no particular problem with the upper limit of the hydrophobicity as long as it is 0.8% by volume or more, but in practice it can be set in the range of 0.8% by volume to 40.0% by volume, 0.8% by volume to 20.0% by volume, preferably 0.8% by volume to 10.0% by volume, preferably 0.8% by volume to 8.0% by volume, preferably 0.8% by volume to 5.0% by volume, preferably 1.0% by volume to 10.0% by volume, preferably 1.0% by volume to 5.0% by volume.

The degree of hydrophobicity of hydrophobically treated silica particles can be evaluated by methanol wettability. Specifically, the degree of hydrophobicity is determined by adding pure water to a sample in a container, stirring the mixture, dripping methanol, and reading the drip volume when the entire amount of the sample is suspended in pure water. The value is expressed as a percentage (volume of methanol dripped) / (volume of methanol dripped + volume of pure water).

For example, 5 mL of organic solvent-dispersed silica sol is evaporated and removed in a rotary evaporator at a reduced pressure of 50 Torr and a bath temperature of 80 to 130°C to obtain silica powder. The resulting powder is pulverized in a mortar and dried again in a rotary evaporator at a reduced pressure of 50 Torr and a bath temperature of 130°C to obtain a sample (silica powder) for measuring hydrophobicity.

Put 50 mL of pure water into a 100 mL beaker, add 0.2 g of the above silica powder, and stir using a magnetic stirrer. Then, add methanol dropwise. The hydrophobicity can be calculated from the amount of methanol added (X mL) required until the silica powder floating on the liquid surface is completely submerged in the liquid, using the following formula.
Hydrophobicity (volume%)={(X)/(50+X)}×100

In the present invention, the silica particles can further be silica particles whose particle surfaces are coated with a hydrolyzate of a non-carboxylic acid silane coupling agent (b).

The non-carboxylic acid silane coupling agent (b) refers to a silane coupling agent having a structure other than a carboxyl group. As the non-carboxylic acid silane coupling agent (b), a silane coupling agent containing an organic group including at least an alkyl group, a (meth)acryloyl group, or an aryl group can be used. As long as an alkyl group, a (meth)acryloyl group, or an aryl group is contained, it may also contain functional groups other than a carboxyl group. Examples of such functional groups include an alkenyl group, an epoxy group, a mercapto group, an amino group, a ureido group, and a cyano group.

In the present invention, the non-carboxylic acid silane coupling agent (b) contains at least one silane compound selected from the group consisting of formulas (3), (4) and (5).

In formula (3), R ⁵ is 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, or a cyano group, and is bonded to a silicon atom via a Si-C bond; R ⁶ is an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; d is an integer of 1 to 3;
In formula (4) and formula (5), R ⁷ and R ⁹ 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 via a Si—C bond; R ⁸ and R ¹⁰ 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; e is an integer of 1 to 3; f is an integer of 0 or 1; and g is an integer of 1 to 3.

The alkyl group is an alkyl group having 1 to 18 carbon atoms, such as 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-cyclo ...,1-dimethyl-n-propyl group, a 1,2-dimethyl-n-propyl group, a 2,2-dimethyl-n-propyl group, a 1,2-dimethyl-n-propyl group, a 1,2-dimethyl-n-propyl group, a 1,2-dimethyl 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 -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, 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, etc., but are not limited to these.

Alkylene groups can also be alkylene groups derived from the alkyl groups mentioned above.

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 is an alkenyl group having 2 to 10 carbon atoms, and includes 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, 2-methyl-3-butenyl, 3 -methyl-1-butenyl group, 3-methyl-2-butenyl group, 3-methyl-3-butenyl group, 1,1-dimethyl-2-propenyl group, 1-i-propylethenyl group, 1,2-dimethyl-1-propenyl group, 1,2-dimethyl-2-propenyl group, 1-cyclopentenyl group, 2-cyclopentenyl group, 3-cyclopentenyl group, 1-hexenyl group, 2-hexenyl group, 3-hexenyl group, 4-hexenyl group, 5-hexenyl group, 1-methyl-1-pentenyl group, 1-methyl-2-pentenyl group, 1-methyl-3-pentenyl group, 1-methyl-4-pentenyl group, 1-n-butylethenyl group, 2-methyl-1-pentenyl group, 2-methyl-2-pentenyl group, etc., but are not limited to these.

The alkoxy group includes alkoxy groups having 1 to 10 carbon atoms, such as, for example, a methoxy group, an ethoxy group, an n-propoxy group, an i-propoxy group, an n-butoxy group, an i-butoxy group, an s-butoxy group, a t-butoxy group, an n-pentyloxy group, a 1-methyl-n-butoxy group, a 2-methyl-n-butoxy group, a 3-methyl-n-butoxy group, a 1,1-dimethyl-n-propoxy group, a 1,2-dimethyl-n-propoxy group, a 2,2-dimethyl-n-propoxy group, a 1-ethyl-n-propoxy group, and an n-hexyloxy group, but are not limited to these.
The acyloxy group has 2 to 10 carbon atoms, and examples of the acyloxy group include 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 are not limited thereto.

The above halogen groups include fluorine, chlorine, bromine, iodine, etc.

The above (meth)acryloyl group refers to both acryloyl and methacryloyl groups. Examples of organic groups having a (meth)acryloyl group include a 3-methacryloxypropyl group and a 3-acryloxypropyl group.

An example of an organic group having a mercapto group is the 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 an organic group having a ureido group is the 3-ureidopropyl group.

An example of an organic group having a cyano group is the 3-cyanopropyl group.

The above formulas (4) and (5) are preferably compounds capable of forming trimethylsilyl groups on the surface of silica particles.

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.

The above silane compound reacts with hydroxyl groups such as silanol groups present on the surface of the silica particles to form siloxane bonds, thereby coating the surface of the silica particles with the silane compound. In the reaction of the silane compound with the hydroxyl groups, 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.

In the present invention, the total amount of the silane compound consisting of the carboxylic acid-based silane coupling agent (a) and the non-carboxylic acid-based silane coupling agent (b) for coating the silica particle surface can be an amount equivalent to a coating amount of 0.4 ^to 5.0 silicon atoms/ ^nm2 in the silane compound as the coating amount on the silica particle surface.

The amount of the carboxylic acid-based silane coupling agent (a) added can be an amount equivalent to a coating amount in which the number of silicon atoms in the silane compound of the carboxylic acid-based silane coupling agent (a) is 0.1 atoms/nm ² to 3.0 atoms/nm ² .

The amount of the non-carboxylic acid silane coupling agent (b) added can be an amount equivalent to a coating amount in which the number of silicon atoms in the silane compound of the non-carboxylic acid silane coupling agent (b) is 0.3 atoms/nm ² to 4.0 atoms/nm ² .

Silica particles coated with the hydrolyzate of a carboxylic acid silane coupling agent (a) and a non-carboxylic acid silane coupling agent (b) in a mass ratio of 1:0.1 to 10 can be obtained.

Water is required for the hydrolysis of the above silane compounds, but if the sol is an aqueous solvent, that aqueous solvent can be used. When the aqueous medium is replaced with an organic solvent such as methanol or ethanol, the water remaining in the solvent can be used. For example, water present at 0.01 to 4 mass % can be used. Furthermore, the hydrolysis can be carried out 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.

The silica sol of the present invention can be prepared by the following steps (A) to (D):
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): adding at least one carboxylic acid-based silane coupling agent (a) selected from the group consisting of formulas (1) and (2) to the silica sol obtained in step (A);
Step (C): A step (C) of replacing the dispersion medium of the silica sol obtained in step (B) with a nitrogen-containing solvent;
Step (D): The silica sol obtained in step (C) is added with at least one non-carboxylic acid silane coupling agent (b) selected from the group consisting of formulas (3) to (5).

The silica sol used in step (A) is preferably an acidic aqueous silica sol having a pH of 1.0 to 7.0, preferably 2.0 to 5.0. The _SiO2 concentration is preferably in the range of 0.1 to 50% by mass, or 10 to 40% by mass.

In step (B), a carboxylic acid-based silane coupling agent (a) represented by formula (1) is added to the silica sol obtained in step (A) to coat the surface of the silica particles with a hydrolysate of the silane coupling agent of formula (1). In step (B), the reaction can be carried out at 50 to 100°C or 60 to 90°C for 0.1 to 10 hours. This reaction causes the hydrolysate of the silane coupling agent of formula (1) to react with the surface of the silica particles to form a siloxane bond. This reaction is preferably carried out at a pH in the acidic range described above; if it is carried out in the alkaline range, the carboxylic acid-based silane coupling agent of formula (1) may oligomerize, resulting in a reduced amount of coating on the silica particles.

If the carboxylic acid-based silane coupling agent of formula (1) has an acid anhydride group, two carboxyl groups can be generated by hydrolysis of part or all of the acid anhydride group. Also, if it is a carboxylic acid ester group, a carboxyl group is generated by hydrolysis.

In step (C), the dispersion medium of the silica sol obtained in step (B) is replaced with a nitrogen-containing solvent. The nitrogen-containing solvent is preferably an amide solvent, such as dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone. In replacing the solvent with a nitrogen-containing solvent, the water content can be adjusted to a range of 0.1 to 4.0% by mass, 1.0 to 2.5% by mass, or 1.5 to 2.0% by mass in the total solvent.

In step (D), at least one non-carboxylic acid silane coupling agent (b) selected from the group consisting of formulas (3) to (5) can be added to the silica sol obtained in step (C). In step (D), the non-carboxylic acid silane coupling agent (b) contains a hydrophobic silane coupling agent, so that the hydrolyzate of the non-carboxylic acid silane coupling agent (b) reacts to form siloxane bonds by solvent replacement from the aqueous silica sol to the nitrogen-containing solvent silica sol. In step (D), the reaction can be carried out at 50 to 100°C or 60 to 90°C for 0.1 to 10 hours. This reaction causes the hydrolyzate of the silane coupling agent of formula (1) to react on the surface of the silica particles to form siloxane bonds.

The silica sol of the present invention, in which silica particles are dispersed in a nitrogen-containing solvent, can be combined with a nitrogen-containing polymer to obtain an insulating resin composition (resin varnish).

The insulating resin composition (resin varnish) is produced by the steps (A) to (D) and further steps (E) and (F):
Step (E): A step (E) of mixing the silica sol obtained in step (D), in which the silica particles are dispersed in a nitrogen-containing solvent, with a nitrogen-containing polymer;
It can be produced by a method further comprising the step (F): of removing a part or all of the nitrogen-containing solvent from the silica sol obtained in the step (E).

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, and copper wire in particular is used. Copper wire is coated with an enamel film and used as electrical wire in industrial and domestic motors, transformers, coils, etc.

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 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 insulating resin composition can be obtained by mixing and stirring the silica sol and 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. The insulated coated conductor of the present invention has an insulating coating layer adjusted to a thickness of 35 μm and a silica concentration of 20 mass% using an insulating resin composition, and the insulating coating layer preferably has a flexibility of 1d to 2d. However, the above flexibility is determined by determining the minimum winding diameter d at which cracks are not observed in the insulating coating of an insulated coated conductor stretched 20% compared to an insulated coated conductor that is not stretched, and is measured in the range from the own diameter (1d) to n times the own diameter (nd).

A polyamic acid composed of 4,4'-diaminodiphenyl ether (DDE) and pyromellitic anhydride (PMDA) is used as a resin, and the silica-blended polyamic acid is adjusted to a mass ratio of resin/ _SiO2 = 85/15. The polyamic acid is heated at 290°C on a Cu plate to obtain a Cu plate (film thickness: 29 to 32 μm) on which a silica-blended polyimide is baked. An insulating resin composition is obtained whose dielectric breakdown life is 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.

The present invention will be specifically explained below with reference to examples. Note that the present invention is not limited in any way by the following examples.

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

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

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

[pH Measurement]
The pH was measured at 20° C. using a pH meter (manufactured by DKK Toa Corporation).

[Measurement of Viscosity]
The viscosity of the silica sol was measured at 20° C. using an Ostwald viscometer.
In the examples, the converted viscosity measured at 20° C. using a B-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd.) is shown in parentheses.

[Measurement of dynamic light scattering particle size (DLS particle size)]
The particle size was measured using a dynamic light scattering particle size measuring device (manufactured by Malvern Instruments, trade name: Zetasizer Nano).

[Hydrophobicity measurement]
A silica powder was obtained by evaporating and distilling off the solvent from 5 mL of the organic solvent-dispersed silica sol in a rotary evaporator at a reduced pressure of 50 Torr and a bath temperature of 80 to 130° C. (120° C. for the DMAC-dispersed silica sol). The obtained powder was pulverized in a mortar and dried again in a rotary evaporator at a reduced pressure of 50 Torr and a bath temperature of 130° C. to obtain a sample for measuring the hydrophobicity.

50 mL of pure water was placed in a 100 mL beaker, 0.2 g of the silica powder was added, and the mixture was stirred using a magnetic stirrer. Methanol was then added dropwise, and the hydrophobicity was calculated from the amount of methanol (X mL) required for the silica powder floating on the liquid surface to be completely submerged in the liquid, using the following formula.
Hydrophobicity (volume%)={(X)/(50+X)}×100

[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
Step (A): Water-dispersed silica sol ST-O-33 (average primary particle size 12 nm, pH 3, silica concentration 33 mass%, manufactured by Nissan Chemical Industries, Ltd.) was prepared.
Step (B): 1,000 g of water-dispersed silica sol ST-O-33 (average primary particle size 12 nm, pH 3, silica concentration 33 mass%, manufactured by Nissan Chemical Industries, Ltd.) was charged into a 2 L eggplant flask, and 33.1 g of 3-trimethoxypropylsuccinic anhydride (manufactured by Shin-Etsu Chemical Co., Ltd., product name X-12-967C) was added while stirring the sol with a magnetic stirrer, and then the liquid temperature was maintained at 80° C. for 2 hours.
Step (C): The above water-dispersed silica sol was evaporated and distilled off at a reduced pressure of 170 to 110 Torr and a bath temperature of 105 to 125° C. in a rotary evaporator while supplying DMAC (dimethylacetamide) to replace the dispersion medium of the sol with DMAC, thereby obtaining a DMAC-dispersed silica sol (silica concentration 29.8% by mass, water content 2.0% by mass) treated with 3-trimethoxypropylsuccinic anhydride.
Step (D): Next, 25.0 g of phenyltrimethoxysilane was added while stirring the above DMAC-dispersed silica sol with a magnetic stirrer, and the liquid temperature was then maintained at 80° C. for 2 hours.
Thereafter, the solvent was evaporated off using a rotary evaporator at a reduced pressure of 150 to 110 Torr and a bath temperature of 105 to 125° C., thereby obtaining a DMAC-dispersed silica sol (silica concentration 30.7% by mass, pH 4.2, viscosity (20° C.) 5 mPa s (7 mPa s), water content 0.1% by mass, dynamic light scattering particle size 13 nm, average primary particle size 12 nm, hydrophobicity 1.6% by volume).

Example 2
The same operation as in Example 1 was performed except that in the step (B) of Example 1, the amount of 3-trimethoxypropylsuccinic anhydride added as the carboxylic acid-based silane coupling agent (a) was changed to 16.5 g, and a DMAC-dispersed silica sol (silica concentration 30.8 mass%, pH 4.4, viscosity (20°C) 4 mPa s (6 mPa s), moisture 0.1 mass%, dynamic light scattering particle size 13 nm, average primary particle size 12 nm, hydrophobicity 3.7 volume%) was obtained.

Example 3
The same operation as in Example 1 was performed except that in the step (B) of Example 1, the amount of 3-trimethoxypropylsuccinic anhydride added as the carboxylic acid-based silane coupling agent (a) was changed to 8.3 g, and a DMAC-dispersed silica sol (silica concentration 30.8 mass%, pH 4.6, viscosity (20°C) 4 mPa s (6 mPa s), moisture 0.1 mass%, dynamic light scattering particle size 14 nm, average primary particle size 12 nm, hydrophobicity degree 4.6 volume%) was obtained.

Example 4
In the step (D) of Example 1, except that the non-carboxylic acid-based silane coupling agent (b) was changed to 17.2 g of methyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-13), the same operation as in Example 1 was performed to obtain a DMAC-dispersed silica sol (silica concentration 30.8 mass%, pH 4.2, viscosity (20°C) 4 mPa·s (6 mPa·s), moisture 0.2 mass%, dynamic light scattering particle size 13 nm, average primary particle size 12 nm, hydrophobicity 1.0 volume%).

Example 5
In step (D) of Example 1, the same operation as in Example 1 was performed except that the non-carboxylic acid-based silane coupling agent (b) was changed to 31.3 g of 3-methacryloxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-503), to obtain a DMAC-dispersed silica sol (silica concentration 30.6 mass%, pH 4.3, viscosity (20°C) 4 mPa·s (6 mPa·s), moisture 0.3 mass%, dynamic light scattering particle size 13 nm, average primary particle size 12 nm, hydrophobicity 1.0 volume%).

Example 6
In the step (B) of Example 1, except that 3-trimethoxypropylsuccinic anhydride was used as the carboxylic acid-based silane coupling agent (a) with 21.6 g of a carboxy group-containing organosiloxane (manufactured by Shin-Etsu Chemical Co., Ltd., product name X-12-1135), the same operation as in Example 1 was performed to obtain a DMAC-dispersed silica sol (silica concentration 30.8 mass%, pH 4.1, viscosity (20°C) 6 mPa s (7 mPa s), moisture 0.3 mass%, dynamic light scattering particle size 15 nm, average primary particle size 12 nm, hydrophobicity 1.0 volume%).

(Comparative Example 1)
Step (A): Water-dispersed silica sol ST-O-33 (average primary particle size 12 nm, pH 3, silica concentration 33 mass%, manufactured by Nissan Chemical Industries, Ltd.) was prepared.
Step (B): 1000 g of water-dispersed silica sol ST-O-33 (average primary particle size 12 nm, pH 3, silica concentration 33 mass%, manufactured by Nissan Chemical Industries, Ltd.) was charged into a 2 L eggplant flask, and 33.1 g of 3-trimethoxypropylsuccinic anhydride was added as a carboxylic acid-based silane coupling agent (a) while stirring the sol with a magnetic stirrer, and the liquid temperature was maintained at 80° C. for 2 hours.
Step (C): The above water-dispersed silica sol was evaporated and distilled off at a reduced pressure of 170 to 110 Torr and a bath temperature of 105 to 125° C. in a rotary evaporator while DMAC (dimethylacetamide) was supplied to replace the dispersion medium of the sol with DMAC, thereby obtaining a DMAC-dispersed silica sol (silica concentration 30.8% by mass, pH 4.4, viscosity (20° C.) 8 mPa·s (10 mPa·s), moisture 0.2% by mass, particle size by dynamic light scattering method 24 nm, average primary particle size 12 nm, hydrophobicity degree 0.6% by volume).

(Comparative Example 2)
In step (D) of Example 1, except that the non-carboxylic acid-based silane coupling agent (b) was changed to 29.8 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-403), the same operation as in Example 1 was carried out to obtain a DMAC-dispersed silica sol (silica concentration 30.5 mass%, pH 4.2, viscosity (20°C) 5 mPa s (7 mPa s), moisture 0.4 mass%, dynamic light scattering particle size 12 nm, average primary particle size 12 nm, hydrophobicity degree 0.3 volume%).

(Comparative Example 3)
Step (A): Water-dispersed silica sol ST-O-33 (average primary particle size 12 nm, pH 3, silica concentration 33 mass%, manufactured by Nissan Chemical Industries, Ltd.) was prepared.
Step (C): 1000 g of the above silica sol was placed in a 2 L recovery flask, 500 g of DMAC (dimethylacetamide) was added, and DMAC was supplied while evaporating and distilling off the solvent in a rotary evaporator at a reduced pressure of 200 to 100 Torr and a bath temperature of 85 to 125° C., thereby replacing the dispersion medium of the sol with DMAC, thereby obtaining a colorless and transparent DMAC-dispersed silica sol (silica concentration 20.5 mass%, pH 4.5, viscosity (20° C.) 3 mPa s (6 mPa s), moisture 0.9 mass%, particle size by dynamic light scattering method 18 nm, average primary particle size 12 nm, hydrophobicity 0.0 volume%).

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%, viscosity at 25°C measured with an E-type viscometer of 13640 mPa·s) corresponding to formula (6). 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 (6) is the number of repeating units.

The DMAC-dispersed silica sols obtained in Examples 1 to 7 and Comparative Examples 1 to 4 were added to and mixed with the polyamic acid obtained in Synthesis Example 1 in a glass bottle so as to give a mass ratio of resin/SiO ₂ = 85/15, and the mixture was degassed and stirred for 20 minutes using a vacuum degasser (EME, product name V-mini300) to obtain a silica-blended polyamic acid.

Then, the obtained silica-blended polyamic acid 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 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 (film thickness: 29-32 μm) with a silica-blended polyimide baked on. This was then cut into 5 cm squares to serve as a sample for insulation tests.

(Paint film condition test/paint film appearance and cross section)
The coating appearance and cross section of the insulation test sample obtained by the above method were observed visually and by SEM.

For SEM observation, a focused ion beam scanning electron microscope (Helicos G3, Thermo Fisher Scientific, magnification 100,000 times) was used to view the coating cross section.

In the examples and comparative examples, if the coating film was not cloudy or foamed when visually inspected, and no silica particle aggregates were found when cross-sectionally observed with a SEM, the silica particles were well dispersed in the resin and the result was deemed "OK."

In addition, in the examples and comparative examples, when the coating film was found to be cloudy or foaming when visually inspected, and when aggregates of silica particles were observed in cross-sectional SEM observation, the silica particles were not dispersed in the resin, and the result was rated "NG."

Then, for the blank (a composition containing only polyimide resin without silica particles), the appearance of the coating was visually checked for the presence or absence of turbidity or bubbles in the coating, and a state in which these were not present was rated as "OK" and a state in which these were present was rated as "NG."

(Measurement of dielectric breakdown life)
Plate-shaped samples with dimensions of 50 mm x 50 mm and thickness of 0.5 mm were used to measure the dielectric breakdown life 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 electrodes used were 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, a polyimide resin containing no silica was used as a sample and similarly measured.

Table 1 shows the hydrophobicity of Examples 1 to 6 and Comparative Examples 1 to 3, as well as the results of the coating film condition test and the dielectric breakdown life measurement.

The DMAC-dispersed silica sol obtained in Examples 1 to 6 was able to improve the insulation life of the cured film formed by baking silica-blended polyimide, compared to Comparative Examples 1 to 3.

The present invention provides a silica sol in which silica particles are dispersed in a nitrogen-containing solvent to be mixed with a polyimide or polyamide polar resin with good compatibility. It also provides a resin composition in which the silica sol is mixed with a resin. Furthermore, it provides an insulating coated conductor that can maintain a long insulation life when used as an insulating resin composition.

Claims (16)

A silica sol in which silica particles are dispersed in a nitrogen-containing solvent,
The silica particles have an average primary particle size of 5 to 100 nm,
At least a part of the silica particles has a particle surface coated with a hydrolyzate of a carboxylic acid-based silane coupling agent (a), and has a hydrophobicity of 0.8 volume % or more as measured by a methanol titration method.
Silica sol. The silica sol according to claim 1, wherein the carboxylic acid-based silane coupling agent (a) is a silane coupling agent containing a carboxyl group, an acid anhydride group, a carboxylic acid ester group, or an organic group containing any of them. The carboxylic acid-based silane coupling agent (a) is represented by the formula (1) and the formula (2):

(In the formulas (1) and (2), ^R1 and ^R3 each represent a carboxyl group, an acid anhydride group, a carboxylate group, or an organic group containing any of them and are bonded to a silicon atom via a Si-C bond; ^R2 and ^R4 each represent an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; X represents an alkylene group, an NH group, or an oxygen atom; a represents an integer of 1 to 3; b represents an integer of 1 to 2; and c represents an integer of 0 or 1. The silica sol according to claim 1 or 2, The silica sol according to any one of claims 1 to 3, wherein at least a portion of the silica particles is further coated on its particle surface with a hydrolysate of a non-carboxylic acid silane coupling agent (b). The silica sol according to claim 4, wherein the non-carboxylic acid silane coupling agent (b) is a silane coupling agent containing an organic group containing at least an alkyl group, a (meth)acryloyl group, or an aryl group. The non-carboxylic acid-based silane coupling agent (b) is represented by the formula (3), the formula (4), and the formula (5):

(In formula (3), R ⁵ is an alkyl group, a halogenated alkyl group, an alkenyl group, an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, or a cyano group, and is bonded to a silicon atom via a Si-C bond; R ⁶ is an alkoxy group, an acyloxy group, a hydroxyl group, or a halogen group; d is an integer of 1 to 3;
In formula (4) and formula (5), R ⁷ and R ⁹ 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 via a Si-C bond; R ⁸ and R ¹⁰ 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; e is an integer of 1 to 3; f is an integer of 0 or 1; and g is an integer of 1 to 3.
The silica sol according to claim 4 or 5, which contains at least one silane compound selected from the group consisting of: The silica sol according to claim 4 or 5, which contains silica particles coated with a hydrolyzate of a carboxylic acid silane coupling agent (a) and a non-carboxylic acid silane coupling agent (b) in a mass ratio of 1:0.1 to 10. The silica sol according to any one of claims 1 to 7, wherein the nitrogen-containing solvent is an amide solvent. The silica sol according to any one of claims 1 to 8, wherein the nitrogen-containing solvent is dimethylacetamide, dimethylformamide, N-methylpyrrolidone, or N-ethylpyrrolidone. An insulating resin composition comprising the silica sol according to any one of claims 1 to 9 and a nitrogen-containing polymer. The insulating resin composition according to claim 10, 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 10 or 11, wherein the nitrogen-containing polymer is any one of polyimide, polyamide, polyamic acid, polyamideimide, polyetherimide, and polyesterimide. An insulating coated conductor coated with the insulating resin composition according to any one of claims 10 to 12. The insulating resin composition according to any one of claims 10 to 12, wherein a silica-blended polyamic acid, which is made of polyamic acid consisting of 4,4'-diaminodiphenyl ether (DDE) and pyromellitic anhydride (PMDA) as a resin and is adjusted to a mass ratio of resin/SiO ₂ = 85/15, is heated at 290°C on a Cu plate to obtain a silica-blended polyimide baked onto the Cu plate (film thickness: 29 to 32 μm), and the insulating breakdown life is 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. The following steps (A) to (D):
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): adding at least one carboxylic acid-based silane coupling agent (a) selected from the group consisting of formulas (1) and (2) to the silica sol obtained in step (A);
Step (C): A step (C) of replacing the dispersion medium of the silica sol obtained in step (B) with a nitrogen-containing solvent;
Step (D): adding at least one non-carboxylic acid silane coupling agent (b) selected from the group consisting of formulas (3) to (5) to the silica sol obtained in step (C);
The method for producing the silica sol according to any one of claims 1 to 9, comprising: The process according to claim 15 further comprises the steps (A) to (D) and further steps (E) to (F):
Step (E): A step (E) of mixing the silica sol obtained in step (D), in which the silica particles are dispersed in a nitrogen-containing solvent, with a nitrogen-containing polymer;
Step (F): A step of removing a part or all of the nitrogen-containing solvent from the silica sol obtained in step (E);
The method for producing an insulating resin composition according to any one of claims 10 to 12, further comprising: