CN112280464B

CN112280464B - Method for improving storage stability of coating

Info

Publication number: CN112280464B
Application number: CN202011255788.2A
Authority: CN
Inventors: 尹勇; 高翔
Original assignee: Zhujing Technology Shenzhen Co ltd
Current assignee: Zhujing Technology Shenzhen Co ltd
Priority date: 2020-11-11
Filing date: 2020-11-11
Publication date: 2022-02-22
Anticipated expiration: 2040-11-11
Also published as: CN112280464A

Abstract

The invention provides a method for improving the storage stability of a coating, which comprises the following steps: mixing a base coating composition comprising a polyimide precursor and a solvent with a filler such that the mixture exhibits L of 5 or more and less than 70, a of-10 or more and less than 10, and b of 15 or more and less than 90 in the CIE L a b system to obtain an insulating coating composition.

Description

Method for improving storage stability of coating

Technical Field

The present invention relates to an insulating coating composition having excellent weather resistance and excellent visibility and workability.

Background

The conventional polyimide coating is a brown color derived from the resin composition (polyimide precursor) contained therein, but when stored as a coating or after formation of an insulating film, ultraviolet rays contained in sunlight or the like are irradiated to partially decompose a main solvent component, thereby causing discoloration or deterioration, and causing a problem of affecting the characteristics of the final product.

Disclosure of Invention

Problems to be solved by the invention

In view of the above problems, an object of the present invention is to provide an insulating coating composition having excellent weather resistance, less deterioration, and excellent visibility and workability, and a method for improving the storage stability of the coating composition.

Means for solving the problems

In order to solve the above problems, the present inventors have made intensive studies to develop a polyimide coating material excellent in weather resistance. As a result, it was found that a polyimide coating material having a light scattering and shielding effect of irradiation light and improved storage stability of the coating material and weather resistance of the coated coating film can be obtained by including a filler such that L is 5 or more and less than 70, a is-10 or more and less than 10, and b is 15 or more and less than 90 in the CIELAB color space, and the present invention was completed. Namely, the present invention is as follows.

A first aspect of the present invention relates to a method of improving the storage stability of a coating, comprising: mixing a base coating composition comprising a polyimide precursor and a solvent with a filler such that the mixture exhibits L of 5 or more and less than 70, a of-10 or more and less than 10, and b of 15 or more and less than 90 in the CIE L a b system to obtain an insulating coating composition. The base coating composition may be a polyimide-based varnish. The filler may comprise an organic filler and/or an inorganic filler.

Preferably, the filler comprises an organic filler and/or an inorganic filler having a particle size of 20nm or more and less than 5 um.

Preferably, the filler contains a thermally decomposable acrylic polymer organic filler having a primary particle diameter of 1um or more and less than 5 um.

Preferably, the filler contains an inorganic filler having a primary particle diameter of 20nm or more and less than 100nm and an inorganic filler having a secondary particle diameter of 100nm or more and less than 1000 nm.

Preferably, the filler comprises an inorganic filler subjected to a hydrophobic treatment.

Preferably, the thixotropic index of the insulating coating composition at 30 ℃ is 1.5 or more and 5 or less.

Preferably, the content of the filler is 5 to 30 wt% with respect to the polyimide precursor in the insulating coating composition.

Preferably, the viscosity of the insulating coating composition at 30 ℃ is 50 poise or more and less than 300 poise.

Preferably, the concentration of the polyimide precursor in the insulating coating composition is 20 wt% or more and less than 40 wt%.

Preferably, the imidization rate of the polyimide precursor is 5% or more and less than 30%.

Preferably, the acid dianhydride as the raw material of the polyimide precursor includes biphenyl tetracarboxylic dianhydride in a content of 30 mol% or more and less than 90 mol% of the acid dianhydride.

Preferably, the diamine as a raw material of the polyimide precursor includes phenylenediamine having a content of 10 mol% or more and less than 50 mol% of the diamine.

A second aspect of the present invention relates to an insulating coating composition comprising a polyimide precursor, a filler and a solvent, said insulating coating composition exhibiting, in the CIE L a b system, L of 5 or more and less than 70, a of-10 or more and less than 10, and b of 15 or more and less than 90. The filler may comprise an organic filler and/or an inorganic filler.

The third aspect of the present invention relates to an insulating coating film formed using the insulating coating composition.

Effects of the invention

The present invention provides an insulating coating composition which is excellent in weather resistance, hardly undergoes deterioration, and is excellent in visibility and workability, and a method for improving the storage stability of the coating composition.

Detailed Description

An example of the preferred embodiment will be specifically described below. The insulating coating composition of the present disclosure includes a polyimide precursor, an organic filler, an inorganic filler, and a solvent (embodiment 1), L in the CIE L a b system (CIELAB color space) is 5 or more and less than 70, a is-10 or more and less than 10, and b is 15 or more and less than 90.

< polyimide precursor >

When the coating material is in a state before application, baking, and dehydration (imidization treatment) (for example, storage), for example, the substance related to the imide resin contained in the coating material is a polyimide precursor (obtained by partially imidizing polyamic acid) in the coating composition in a state of polyamic acid + varnish), and the coating material having the polyimide precursor is applied, baked, and dehydrated (imidization treatment) to form the imide resin. In contrast to the reduction in elongation of the film by the addition of the filler, the present disclosure uses polyimide as the base resin, and the effect thereof can be minimized.

Polyimide precursors include any polyimide precursor material derived from diamine and acid dianhydride monomers and capable of being converted to polyimide, such as polyamic acids and the like. The polyimide precursor can be synthesized by a conventional synthesis method, and can also be a polyimide varnish from the market. In some embodiments, an acid dianhydride and a diamine are reacted in the presence of a solvent to provide a polyimide precursor. The reaction temperature may be 0 to 90 ℃. The reaction time may be 1 to 24 hours.

The diamine is preferably an aromatic diamine, and examples thereof include phenylenediamine (PPD), diaminodiphenyl ether (ODA), diaminodimethylbiphenyl (e.g., the following formulae (1) and (2)), bis (4-aminophenyl) sulfide, 3 '-diaminodiphenyl sulfone, 1, 4-bis (4-aminophenoxy) benzene, 1, 3-bis (3-aminophenoxy) benzene, 2-bis [4- (4-aminophenoxy) ] phenyl ] hexafluoropropane, 2-bis (4-aminophenyl) hexafluoropropane, 9-bis (4-aminophenyl) fluorene, 2-bis [4- (4-aminophenoxy) phenyl ] propane, and 4,4' -bis (4-aminophenoxy) biphenyl (e.g., the following formula (3))), 1, 3-bis (4-aminophenoxy) benzene, 2' -bis (trifluoromethyl) benzidine, and the like;

these diamines may be used alone or in combination of two or more. In one example, ODA is used as the diamine. In another example, ODA and PPD are used as diamines.

More preferably, the diamine includes at least one of PPD, diaminodimethylbiphenyl represented by formula (1), diaminodimethylbiphenyl represented by formula (2), and 4,4' -bis (4-aminophenoxy) biphenyl in an amount of 10 mol% or more and less than 50 mol% based on the total diamine, and thereby a material composition having good film toughness can be obtained even when a filler described later is dispersed, hydrolysis resistance can be imparted, and ATF (automatic transmission fluid) resistance when used in a motor can be improved. PPD, diaminodimethylbiphenyl represented by formula (1), diaminodimethylbiphenyl represented by formula (2), and 4,4' -bis (4-aminophenoxy) biphenyl in an amount of 10 mol% or more and less than 50 mol% can enhance intermolecular stacking (packing) of polyimide or reduce intramolecular tension (strain) of polyimide, and thus contribute to improvement of hydrolysis resistance, which is a weak point of polyimide. Regarding the ATF resistance, hydrolysis due to moisture contained in ATF is a main cause of deterioration of the insulating film.

The acid dianhydride is preferably an aromatic acid dianhydride, and examples thereof include pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA), 3',4,4' -benzophenonetetracarboxylic dianhydride, bicyclo [2.2.2] oct-7-ene-2, 3,5, 6-tetracarboxylic dianhydride, 1,2,3, 4-cyclopentanetetracarboxylic dianhydride, 1,2,3, 4-cyclobutanetetracarboxylic dianhydride, 1,2,4, 5-cyclohexanetetracarboxylic dianhydride, 3',4,4' -diphenylsulfonetetracarboxylic dianhydride, 4,4'- (hexafluoroisopropylidene) diphthalic anhydride, 4,4' - (4,4 '-isopropylidenediphenoxy) diphthalic anhydride, 4,4' -oxydiphthalic anhydride, bis (1, 3-dioxo-1, 3-dihydroisobenzofuran) 5-carboxylic acid) -1, 4-phenylene ester, and the like. These acid dianhydrides may be used singly or in combination of two or more. In one example, PMDA is used as the acid dianhydride. In another example, PMDA and BPDA are used as the acid dianhydride.

In a preferred example, the acid dianhydride includes biphenyltetracarboxylic dianhydride in an amount of 30 mol% or more and less than 90 mol% based on the total acid dianhydride, and thereby a material composition having good film toughness is obtained even when a filler described later is dispersed, hydrolysis resistance is further imparted, and ATF resistance when used in a motor is also improved. Introduction of BPDA enhances stacking (packing) between polyimide molecules or reduces tension (strain) in polyimide molecules, and contributes to improvement of hydrolysis resistance, which is a weak point of polyimide. Further, it is preferable that the total amount of the introduced amount of BPDA and the introduced amount of diamine composed of at least one of PPD, diaminodimethylbiphenyl represented by formula (1), diaminodimethylbiphenyl represented by formula (2), and 4,4' -bis (4-aminophenoxy) biphenyl be 50 mol% or more (for example, when BPDA is a mol% of acid dianhydride and PPD is b mol% of diamine, a mol% + b mol% is 50 mol% or more), and the material composition having good film toughness, the effect of imparting hydrolysis resistance and the effect of improving ATF resistance can be obtained. Wet heat deterioration resistance evaluation (ATF resistance evaluation) was performed on winding wires using the coating materials of, for example, examples 4 to 8 described later in the following manner: the winding wire sample was immersed in ATF oil containing 0.5 mass% of water in an SUS-made hermetic container, heated at 155 ℃ for 1000 hours in a hermetically sealed state, and then taken out to evaluate the presence or absence of film cracking. The test specimen in which the film was not cracked was judged to be acceptable by visual observation, and the test specimen in which the film was cracked was judged to be unacceptable. The winding wire using the coating materials of examples 4 to 8 did not suffer from film cracking after 155 ℃/1000 h.

The amount of the diamine compound is 0.95mol or more and 1.05mol or less based on 1mol of the acid dianhydride. By setting the amount of the diamine compound in this range, a polyimide precursor and/or polyimide having an appropriate molecular weight range can be obtained. The weight average molecular weight of the polyimide polymer (resin) is 10,000-100,000. If the weight average molecular weight is not less than the lower limit, sufficient film toughness can be achieved, and if the weight average molecular weight is not more than the upper limit, usable fluidity can be ensured.

The polyimide precursor also contains a polyimide resin. The polyimide precursor preferably has an imidization ratio of 5% or more and less than 30% from the viewpoint of maintaining a suitable filler dispersion state described later. When the imidization ratio is 5% or more, the possibility of insufficient exchange reaction of the polyamic acid to lower the viscosity and stability can be suppressed. When the imidization ratio is less than 30%, the varnish is prevented from thickening easily during storage or coating, and the life of the varnish is shortened. The "imidization ratio of the polyimide precursor" refers to a ratio of imidization (ratio of imide ring closure) in the polyimide precursor, and is a ratio of imidization occurring in a process of forming the polyimide precursor, or an imidization ratio in a state of varnish of the polyimide precursor. As a method for controlling the imidization rate of the polyimide precursor, a reaction temperature, a reaction time, a solvent, and the like may be controlled. For example, the imidization rate may be controlled within the above range by adding a diamine (the acid dianhydride and the diamine may be in equimolar amounts) to a solution containing an acid dianhydride at a high rate, treating the reaction solution at a high temperature of 70 ℃ to 100 ℃ for 0.5 to 2 hours at a time, immediately cooling the reaction solution, and reacting the reaction solution at a low temperature of 10 ℃ to 40 ℃ for 2 to 10 hours. In addition, in the present disclosure, the imidization rate can be controlled by¹The H-NMR method is used for quantitative determination (see, for example, JP 2003-183392A).

The concentration of the polyimide precursor in the insulating coating composition is preferably 20 wt% or more and less than 40 wt% from the viewpoint of maintaining a suitable filler dispersion state described later. When the concentration is 20 wt% or more, the coating efficiency can be prevented from deteriorating due to the increase in the number of times of coating when a film having a predetermined thickness is formed. When the concentration is less than 40 wt%, the decrease in the molecular weight of the polyamic acid can be suppressed, and sufficient film characteristics cannot be obtained. The weight average molecular weight of the polyimide precursor was determined in accordance with JIS-K7252-1: 2008 "plastic: calculation of the average molecular weight and molecular weight distribution of macromolecules using size exclusion chromatography-section 1: general "is measured by Gel Permeation Chromatography (GPC) using monodisperse polystyrene as a standard.

< Filler >.

The insulating coating compositions of the present disclosure may contain both organic and inorganic fillers such that 5 x L < 70, -10 x a < 10, 15 x b < 90 in the CIE la color space system, organic and inorganic fillers are added to achieve a specified hue. In this way, the effect of scattering and shielding light by the irradiation light generated from the filler particles can reduce damage to the resin by light and heat, and improve the storage stability of the coating material and the weather resistance of the coated film. The content of the filler is preferably 5 to 30 wt% relative to the polyimide precursor contained in the insulating coating composition, and both the reflection scattering property and the toughness and elongation of the coating film can be achieved. More preferably 20 ≦ L ≦ 50, 0 ≦ a ≦ 5, and 15 ≦ b ≦ 60, so that the irradiation light scattering and light blocking effect can be further exhibited to the maximum. In addition, L, a, and b denote chromaticity coordinates L, a, and b in the L, a, b chromaticity system (CIELAB color space), respectively. In the CIELAB color space, L is a luminance axis representing luminance. Black when approaching 0 and white when approaching 100. a represents green to red. Negative is green and positive is red. b represents blue to yellow. Negative is blue and positive is yellow. The larger the a and b mean values are, the stronger the color is (the chroma is high). This CIELAB color space is based on JIS Z8729.

The inorganic filler is preferably at least one of silica and alumina. In a preferred example, the inorganic filler is a silica nanofiller, and the silica nanofiller is added in an appropriate dispersion state, whereby the surge resistance after the formation of the insulating film described later can be further provided. In addition, only inorganic fillers may be used as fillers. The filler can be dispersed in the varnish by adding the filler little by little (a small amount of) while stirring the varnish (100 to 1,000rpm) at room temperature in a reaction vessel, and the filler can be uniformly dispersed in the varnish.

The particle diameter (average particle diameter) of the inorganic filler is 20nm or more and less than 5 um. It is preferable that the insulating coating composition contains an inorganic filler having a primary particle diameter (initial average particle diameter) of 20nm or more and less than 100nm, and a part of the filler is aggregated so that the average particle diameter of the secondary aggregate is 100nm or more and less than 1000nm, that is, the insulating coating composition further contains an inorganic filler having a secondary particle diameter of 100nm or more and less than 1000nm, whereby the inorganic filler is further appropriately aggregated to scatter light, and the insulating coating composition of the above color can be obtained, and both of the imparting of reflection-scattering property and the securing of film toughness can be achieved. From the viewpoint of forming a moderate aggregate, it is preferable to include an inorganic filler subjected to a hydrophobic treatment. The filler particles may be treated to achieve hydrophobization, and silane coupling agents and the like may be used as the treating agent. In the present embodiment, when the varnish using the inorganic filler is used as the insulating film of the winding wire, the surge resistance can be imparted to the winding wire in addition to the effect of improving the storage stability of the varnish.

The organic filler is preferably at least one of a thermally decomposable acrylic polymer (acrylic polymer) such as polymethyl methacrylate (PMMA), polyethylene glycol, and polypropylene glycol. In a preferred example, the organic filler uses a thermally decomposable acrylic polymer. Since the acrylic polymer is decomposed and removed when the insulating coating material is heated (fired) as described later, it does not substantially remain in the insulating film to be finally formed, and does not affect the insulating property. Further, the dielectric constant of the insulating film can be further reduced. In addition, only organic fillers may be used as fillers.

The particle size of the organic filler is 20nm or more and less than 5 um. It is preferable to include a thermally decomposable acrylic polymer having a primary particle diameter of 1um or more and less than 5um, and to achieve both the provision of reflection scattering properties and the securing of film toughness. In the present embodiment, when the varnish using the organic filler is used as an insulating film of a winding wire, the organic filler is not left but thermally decomposed when the insulating film is formed. The filler can be added slowly while stirring the varnish to uniformly disperse the varnish.

The viscosity of the insulating coating composition at 30 ℃ is 50 Poise or more and less than 300 Poise from the viewpoint of workability (formation of appropriate aggregates) at the time of varnish synthesis and coating. The viscosity can be adjusted by the solid content of the polyimide precursor, the weight average molecular weight of the polyimide, the glass transition temperature, and the like. For example, the viscosity tends to be lower as the solid content, particularly the content of polyimide, is smaller. Further, the smaller the weight average molecular weight of the polyimide or the lower the glass transition temperature, the lower the viscosity tends to be.

An indication that fillers interact with each other to form moderate aggregates can be identified by the Thixotropic Index (Thixotropic Index). The thixotropic index of the insulating coating composition of the present disclosure at 30 ℃ is 1.5 or more and 5 or less. When the thixotropic index is 1.5 or more, the possibility that the flow of the varnish may deteriorate the uniformity of the coating film when the varnish is baked after coating can be suppressed. When the thixotropic index is 5 or less, the possibility of the workability being deteriorated due to excessively low fluidity at the time of coating can be suppressed. The viscosity was measured at 30 ℃ and 1rpm using an E-type rotational viscometer. The thixotropic index is defined as the ratio of the value measured at 30 ℃ with a rotational speed of 1rpm to the value measured at a rotational speed of 10rpm using an E-type rotational viscometer (thixotropic index ═ viscosity at 1 rpm)/(viscosity at 10 rpm)).

The total content of the organic filler and the inorganic filler is preferably 5 wt% or more with respect to the polyimide precursor contained in the insulating coating composition, so that a sufficient light-shielding effect can be easily obtained. Further, it is preferable that the content of the polyimide precursor in the insulating coating composition is 30 wt% or less, thereby further suppressing the decrease in toughness and elongation of a film described later. From the viewpoint of ensuring the toughness and elongation of the coating film, the content of the organic filler is preferably 20 wt% or less relative to the polyimide precursor contained in the insulating coating composition. From the viewpoint of ensuring sufficient particle reflection scattering properties, the content of the inorganic filler is preferably 10 to 25 wt% with respect to the polyimide precursor contained in the insulating coating composition. The particle reflection and scattering properties can be effectively exhibited by combining an organic filler having a large particle diameter and an inorganic filler having a small particle diameter.

< solvent >.

The solvent in the insulating coating composition is not particularly limited, and may be an organic solvent, and may be at least one selected from N, N-dimethylacetamide (DMAc), N-Dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and xylene, for example.

The insulating coating composition of the present invention can be used for forming an insulating film (e.g., an insulating film for a winding wire). The insulating coating composition of the present invention achieves predetermined L, a, b chromaticity parameters (L is 5 or more and less than 70, a is-10 or more and less than 10, b is 15 or more and less than 90) by adding a filler, and light entering the coating film is reflected and scattered on the surface of the filler particles, and thus cannot enter the coating film, and deterioration of the resin due to light is less likely to occur. The insulating film can be prepared by a conventional method. In one embodiment, the insulating coating composition is coated and heated to imidize to obtain a film. The thickness of the film is 10 to 200 μm.

Modification examples.

The insulating coating composition of embodiment 2 differs from embodiment 1 in that only an inorganic filler is used as a filler, and the other components are the same as those of embodiment 1, and therefore, redundant description is omitted.

The insulating coating composition according to embodiment 2 contains an inorganic filler, and the inorganic filler is added so that the color hue of the CIE L a b system (CIELAB color space) is 5 or less L < 70, -10 or less a < 10, 15 or less b < 90. In this way, the effect of scattering and shielding light by the irradiation light generated from the filler particles can reduce damage to the resin by light and heat, and improve the storage stability of the coating material and the weather resistance of the coated film. The content of the filler is preferably 5 to 30 wt% relative to the polyimide precursor contained in the insulating coating composition, and both the reflection scattering property and the toughness and elongation of the coating film can be achieved. More preferably 20 ≦ L ≦ 50, 0 ≦ a ≦ 5, and 15 ≦ b ≦ 60, so that the irradiation light scattering and light blocking effect can be further exhibited to the maximum.

The content of the inorganic filler is preferably 5 wt% or more with respect to the polyimide precursor contained in the insulating coating composition, so that a sufficient light-shielding effect by scattering can be easily obtained. Further, it is preferable that the content of the polyimide precursor in the insulating coating composition is 30 wt% or less, thereby further suppressing the decrease in toughness and elongation of a film described later. From the viewpoint of ensuring sufficient particle reflection scattering properties, the content of the inorganic filler is more preferably 10 to 25 wt% with respect to the polyimide precursor contained in the insulating coating composition.

The insulating coating composition of embodiment 3 differs from embodiment 1 in that only an organic filler is used as a filler, and the other components are the same as those of embodiment 1, and therefore, redundant description is omitted.

The insulating coating composition of embodiment 3 comprises an organic filler, and the organic filler is added so as to have a predetermined hue in the CIE L a b system (CIELAB color space) of 5L < 70, -10 a < 10, 15 b < 90. In this way, the effect of scattering and shielding light by the irradiation light generated from the filler particles can reduce damage to the resin by light and heat, and improve the storage stability of the coating material and the weather resistance of the coated film. The content of the filler is preferably 5 to 30 wt% relative to the polyimide precursor contained in the insulating coating composition, and both the reflection scattering property and the toughness and elongation of the coating film can be achieved. More preferably 20 ≦ L ≦ 50, 0 ≦ a ≦ 5, and 15 ≦ b ≦ 60, so that the irradiation light scattering and light blocking effect can be further exhibited to the maximum.

The organic filler is preferably at least one of a thermally decomposable acrylic polymer, polyethylene glycol, and polypropylene glycol. In a preferred example, the organic filler uses a thermally decomposable acrylic polymer. Since the acrylic polymer is decomposed and removed when the insulating coating material is heated (fired) as described later, it does not substantially remain in the insulating film to be finally formed, and does not affect the insulating property. Further, the dielectric constant of the insulating film can be further reduced. In addition, only organic fillers may be used as fillers.

The viscosity of the insulating coating composition at 30 ℃ is 50 Poise or more and less than 300 Poise from the viewpoint of workability (formation of appropriate aggregates) at the time of varnish synthesis and coating. The viscosity can be adjusted by the solid content of the polyimide precursor, the weight average molecular weight of the polyimide, the glass transition temperature, and the like. For example, the viscosity tends to be lower as the solid content, particularly the content of polyimide, is smaller. Further, the smaller the weight average molecular weight of the polyimide or the lower the glass transition temperature, the lower the viscosity tends to be. The viscosity was measured at 30 ℃ and 1rpm using an E-type rotational viscometer.

The content of the organic filler is preferably 5 wt% or more with respect to the polyimide precursor contained in the insulating coating composition, so that a sufficient light-shielding effect by scattering can be easily obtained. Further, it is preferable that the content of the polyimide precursor in the insulating coating composition is 30 wt% or less, thereby further suppressing the decrease in toughness and elongation of a film described later. From the viewpoint of ensuring the toughness and elongation of the coating film, the content of the organic filler is more preferably 20 wt% or less relative to the polyimide precursor contained in the insulating coating composition.

A base coating composition comprising a polyimide precursor and a solvent may be mixed with a filler such that the mixture exhibits L of 5 or more and less than 70, a of-10 or more and less than 10, and b of 15 or more and less than 90 in the CIE L a b system to obtain an insulating coating composition.

The present invention will be described in detail by way of examples. It is also to be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the invention, and that certain insubstantial modifications and adaptations of the invention by those skilled in the art may be made in light of the above teachings. The specific process parameters and the like of the following examples are also only one example of suitable ranges, i.e., those skilled in the art can select the appropriate ranges through the description herein, and are not limited to the specific values of the following examples;

in the following examples, reagents, materials and instruments used are all conventional reagents, conventional materials and conventional instruments, which are commercially available, if not specifically mentioned, and the reagents involved therein can also be synthesized by conventional synthesis methods.

Preparation of insulating coating composition: a filler is added to a polyimide-based varnish as a base coating composition, and the polyimide-based varnish and the filler are mixed to give a mixture having a predetermined hue. The filler and the polyimide precursor solution (polyimide-based varnish) can be uniformly mixed by a usual stirring method. The stirring method may be a simple stirring method commonly used, for example, mechanical stirring. The stirring temperature can be 0-80 ℃, and the stirring time can be 0.5-24 hours. While stirring the varnish at 200rpm using an anchor stirrer, a predetermined amount of a filler was gradually added to prepare a filler-dispersed varnish.

Examples

A diamine compound is dissolved in a solvent, and acid dianhydride is added thereto to perform a reaction, thereby obtaining a base polyimide varnish. The kinds and molar ratios of diamine and acid dianhydride are shown in Table 1.

A predetermined amount of a filler is added to a predetermined amount of a base polyimide varnish under stirring to obtain a filler-containing polyimide varnish. The kind, particle diameter, addition amount, stirring method, final solid content and the like of the filler are shown in table 1. Here, the "final solid content" means the weight% of the resin portion and the filler portion in the filler-mixed varnish, calculated by (the weight of the resin portion contained in the varnish + the weight of the filler)/the total weight of the filler-mixed varnish × 100. In the present disclosure, the final solid content may be 25 to 35 wt%.

Comparative example

The difference compared to the examples is that no filler is added. The kinds and molar ratios of the diamine and the acid dianhydride are shown in Table 1.

Preparation of the skin Membrane

Coating film production Using the coating materials prepared in the above examples and comparative examples

The specific method comprises the following steps: a round wire (a wire having a circular shape in a cross section perpendicular to the longitudinal direction) having an average diameter of 1mm and containing copper as a main component was prepared as a conductor. The varnish prepared as described above was applied to the outer peripheral side surface of the conductor. The conductor coated with the varnish was heated in a heating furnace at a heating temperature of 400 ℃ for 35 seconds to apply a coating of about 3 μm at a time. The coating process and the heating process were repeated 10 times each. Thus, an insulated wire comprising the conductor and an insulating film having an average thickness of 36 μm laminated on the outer peripheral surface of the conductor was obtained.

The obtained coating and film were evaluated for various properties. The test methods for each property are as follows:

imidization rate

According to the method described in JP2003-183392A (¹H-NMR method). Specifically, the imidization ratio was measured by 1H-NMR measurement using a model AVANCE III 600 digital NMR apparatus manufactured by Bruker corporation. The prepared polyimide precursor solution was diluted with deuterated dimethyl sulfoxide (d6 form) containing 0.05% TMS, which was sufficiently dehydrated by molecular sieve, and sufficiently dissolved. The concentration of the sample is appropriately diluted and adjusted according to the resin concentration of the polyimide precursor solution so as to obtain sufficient chemical shift strength. An appropriate amount of the resulting diluted solution was filled in a sample tube made of NMR glass having a diameter of about 5 mm. The imidization ratio was determined based on protons derived from a structure which did not change before and after imidization, and was calculated using the proton integrated value and a proton peak derived from an NH group of the polyamic acid appearing in the vicinity of 9.5 to 10.0 ppm. For example, in PMDA-ODA, the amount of 1H protons derived from the NH group of polyamic acid appearing in the vicinity of 9.5 to 10.0ppm is quantified based on 1H protons derived from the benzene ring of PMDA, and the imidization ratio is { 1- (1H integrated value of NH of polyamic acid ÷ 2) } × 100.

Initial viscosity

The viscosity at 30 ℃ was measured with an E-type viscometer.

Thixotropic index

The thixotropic index at 30 ℃ was measured with an E-type viscometer.

Colour(s)

The device comprises the following steps: spectrophotometer (specro 2guide 7075 manufactured by BYK-Gardner Co., Ltd.)

The observation conditions were as follows: 25 deg.C

And (3) observing a light source: d65

And (3) a chrominance system: l a b color system (CIELAB color space)

Color difference formula: Δ E ab (CIE1976) color difference equation

And (3) measuring the diameter: 8 mm.

Evaluation of weather resistance

The accelerated test using a metal halide lamp was conducted by irradiating ultraviolet light corresponding to an outdoor 1-year period. In the varnish discoloration judgment, "OK" indicates that no clear discoloration was observed in visual observation compared with the state before the start of the test, and "NG" indicates that discoloration was clear in visual observation compared with the state before the start of the test. In the determination of the deterioration of the coating, "OK" indicates that the surface of the coating is not cracked or broken, and "NG" indicates that the surface of the coating is cracked or broken.

Table 1.

With reference to table 1, it is seen from examples 1 to 16 and comparative examples 1 to 2 that when the coating material L is 5 or more and less than 70, a is-10 or more and less than 10, and b is 15 or more and less than 90 by including the filler, the insulating coating composition has excellent storage stability (excellent weather resistance, less deterioration, and excellent visibility and workability). The coating film prepared by using the coating material of the example is hard to be discolored and deteriorated even in long-term storage.

Claims

1. A method of improving the storage stability of a coating, comprising: mixing a base coating composition comprising a polyimide precursor and a solvent with a filler such that the mixture exhibits L of 5 or more and less than 70, a of-10 or more and less than 10, and b of 15 or more and less than 90 in the CIE L a b system to obtain an insulating coating composition; the filler contains an organic filler and/or an inorganic filler having a particle diameter of 20nm or more and less than 5 um; the content of the filler is 5-30 wt% relative to the polyimide precursor in the insulating coating composition; the concentration of the polyimide precursor in the insulating coating composition is more than 20 wt% and less than 40 wt%; the imidization rate of the polyimide precursor is more than 5% and less than 30%; acid dianhydride as a raw material of the polyimide precursor includes biphenyl tetracarboxylic dianhydride in a content of 30 mol% or more and less than 90 mol% of the acid dianhydride; the thixotropic index of the insulating coating composition at 30 ℃ is 1.5-5.

2. The method according to claim 1, wherein the filler comprises a thermally decomposable acrylic polymer organic filler having a primary particle diameter of 1um or more and less than 5 um.

3. The method according to claim 1, characterized in that the filler comprises an inorganic filler having a primary particle diameter of 20nm or more and less than 100nm and an inorganic filler having a secondary particle diameter of 100nm or more and less than 1000 nm.

4. The method of claim 1, wherein the filler comprises a hydrophobized inorganic filler.

5. The method according to claim 1, wherein the diamine used as the raw material of the polyimide precursor includes phenylenediamine in an amount of 10 mol% or more and less than 50 mol% based on the diamine.