Resin composition, insulated wire, and method for producing insulated wire

Abstract

This resin composition contains: a polyimide precursor, which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine; an organic solvent; a pore-forming agent; and a radical scavenger.

Description

Resin composition, insulated wire, and method for producing insulated wire

[0001] This disclosure relates to a resin composition, an insulated wire, and a method for producing an insulated wire. This application claims priority to Japanese Application No. 2023-071272 filed on April 25, 2023, and incorporates by reference all of the contents of that application.

[0002] Patent Document 1 describes an insulating varnish containing a coating film-forming resin and a heat-decomposable resin that decomposes at a temperature lower than the baking temperature of the coating film-forming resin, and an insulated electric wire having a heat-cured film of the insulating varnish, in which pores are formed in the heat-cured film due to the thermal decomposition of the heat-decomposable resin.

[0003] JP 2012-224714 A

[0004] A resin composition according to one embodiment of the present disclosure contains a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, an organic solvent, a pore-forming agent, and a radical scavenger.

[0005] FIG. 1 is a schematic cross-sectional view showing an insulated wire according to one embodiment of the present disclosure.

[0006] [Problem to be Solved by the Present Disclosure] In electrical equipment with a high applied voltage, such as a motor operated at a high voltage, a high voltage is applied to the insulated wire that constitutes the electrical equipment, making partial discharge (corona discharge) likely to occur on the surface of the insulating coating. The occurrence of partial discharge can cause localized temperature increases, ozone generation, ion generation, and the like, which can lead to early insulation breakdown and shorten the life of the insulated wire and, ultimately, the electrical equipment. Therefore, there is a need to suppress the occurrence of partial discharge, and one method that has been considered is to reduce the dielectric constant of the insulating coating. Patent Document 1, cited above, proposes a technique for forming pores in the insulating coating as one method for reducing the dielectric constant of the insulating coating.

[0007] Increasing the porosity of the insulating coating can lower the dielectric constant of the insulating coating, but this can also reduce the elongation of the insulating coating. Therefore, there is a demand for an insulated wire in which the reduction in elongation of the insulating coating that accompanies an increase in the porosity of the insulating coating is suppressed.

[0008] An object of the present disclosure is to provide a resin composition capable of forming an insulating coating in which a decrease in coating elongation due to an increase in porosity is suppressed.

[0009] Effect of the Present Disclosure A resin composition according to one aspect of the present disclosure inhibits a decrease in film elongation that occurs with an increase in the porosity of the insulating film.

[0010] [Explanation of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. Item 1. A resin composition containing a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, an organic solvent, a pore-forming agent, and a radical scavenger. Item 2. The resin composition according to Item 1, wherein the content of the radical scavenger is 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the polyimide precursor. Item 3. The resin composition according to Item 1 or 2, wherein the radical scavenger is a hindered amine-based light stabilizer. Item 4. The resin composition according to any one of Items 1 to 3, wherein the pore-forming agent is a heat-decomposable resin-containing particle. Item 5. The resin composition according to any one of Items 1 to 4, which is used to form an insulating coating for an insulated electric wire. Item 6. Item 7. An insulated electric wire comprising: a conductor; and an insulating coating covering the conductor, wherein the insulating coating has a resin matrix and a plurality of voids, and the insulating coating is formed from the resin composition according to any one of items 1 to 5. Item 7. An insulated electric wire according to item 6, wherein the insulating coating has a porosity P of 20% by volume or more and 60% by volume or less, and wherein a coating elongation Y of the insulating coating and a value X calculated by the following formula (1) satisfy Y≧X: Formula (1): X=−1.3×P+84 Item 8. A method for producing an insulated electric wire comprising: a conductor; and an insulating coating covering the conductor, the insulating coating having a resin matrix and a plurality of voids, the method comprising the steps of: applying the resin composition according to any one of items 1 to 5 to an outer peripheral surface of the conductor; and heating the resin composition applied in the applying step.

[0011] [Details of Embodiment of the Present Disclosure] Hereinafter, a resin composition, an insulated wire, and a method for producing an insulated wire according to one aspect of the present disclosure will be described.

[0012] <Resin Composition> The resin composition contains a polyimide precursor which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, an organic solvent, a pore-forming agent, and a radical scavenger.

[0013] The resin composition contains a radical scavenger, which can suppress the decrease in film elongation that accompanies an increase in the porosity of the insulating coating. In other words, the resin composition contains a radical scavenger, which can provide excellent film elongation even when the porosity of the insulating coating is high.

[0014] Although not wishing to be limiting, it is believed that radicals are generated when an insulating coating having voids is formed. The generated radicals remove hydrogen atoms from carbon-hydrogen bond (C-H bond) in the polyimide precursor, its cured product, polyimide, or its raw materials, aromatic tetracarboxylic dianhydride and aromatic diamine (hereinafter also referred to as "polyimide precursor, etc."), generating carbon radicals (C.). The generated carbon radicals react with carbon radicals in other polyimide precursors, etc., to form undesired crosslinks between polymer chains of the polyimide precursor, etc., resulting in reduced elongation of the insulating coating. Therefore, it is believed that by using a radical scavenger to trap the radicals generated when forming an insulating coating having voids, it is possible to suppress the reduction in elongation of the insulating coating due to the generation of radicals.

[0015] "Porosity" refers to the percentage (unit: volume %) of the volume of pores relative to the volume of the resin matrix and the insulating coating having pores. Specifically, the porosity is measured as follows. The porosity is calculated using the formula (W1 - W2) x 100 / W1, where W1 is the mass of the insulating coating without pores, calculated by multiplying the apparent volume V1 calculated from the outer diameter of the insulating coating by the density ρ1 of the insulating coating material, and W2 is the actual mass of the insulating coating.

[0016] "Coating elongation" refers to the elongation at break (unit: %). Specifically, the coating elongation is measured by the following method. A tubular insulating coating is obtained by removing the conductor from an insulated electric wire, and this sample is subjected to a tensile test using a tensile tester under the conditions of a chuck distance of 20 mm and a pulling rate of 10 mm / min, thereby measuring the elongation at break.

[0017] The resin composition can be suitably used as a resin composition (resin varnish) for forming an insulating coating for an insulated wire.

[0018] Each component contained in the resin composition will be described below.

[0019] (Polyimide Precursor) A polyimide precursor is a reaction product obtained by a condensation polymerization reaction between an aromatic tetracarboxylic dianhydride and an aromatic diamine. A polyimide precursor is a compound also called a polyamic acid (polyamic acid). The polyimide precursor undergoes a dehydration cyclization reaction to form a cyclic imide, becoming a polyimide. Although not intended to be a restrictive interpretation, the curing reaction from the above-mentioned polyimide precursor to polyimide is thought to be one of the causes of radical generation.

[0020] The aromatic tetracarboxylic dianhydride containing pyromellitic dianhydride (PMDA) can improve the heat resistance of the insulating coating. This is because PMDA has a rigid and linear molecular structure. The aromatic tetracarboxylic dianhydride may contain an aromatic tetracarboxylic dianhydride other than PMDA (hereinafter also referred to as "another aromatic tetracarboxylic dianhydride").

[0021] Examples of the other aromatic tetracarboxylic dianhydrides include 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA), 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA), 2,2',3,3'-biphenyltetracarboxylic dianhydride (i-BPDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, and 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride. Examples of the other aromatic tetracarboxylic dianhydrides include bis(2,3-dicarboxyphenyl)propane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, and 2,3,6,7-naphthalenetetracarboxylic dianhydride. These other aromatic tetracarboxylic dianhydrides may be used alone or in combination of two or more.

[0022] When the other aromatic tetracarboxylic dianhydride is biphenyltetracarboxylic dianhydride (BPDA), the hydrolysis resistance of the polyimide precursor can be improved.

[0023] The lower limit of the content of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 0 mol%, 10 mol%, 20 mol%, or 30 mol%, and the upper limit of the content of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 100 mol%, 90 mol%, 80 mol%, or 70 mol%.

[0024] The content of the other aromatic tetracarboxylic dianhydride relative to 100 mol% of the aromatic tetracarboxylic dianhydride can be appropriately determined within a range that does not impair the effects of the present disclosure. The upper limit of the content may be 30 mol% or 20 mol%, and the lower limit of the content may be 0 mol% or 10 mol%.

[0025] When the aromatic diamine contains diaminodiphenyl ether (ODA), the heat resistance of the insulating coating can be improved. This is because ODA has a rigid and linear molecular structure. Examples of diaminodiphenyl ethers include 4,4'-diaminodiphenyl ether (4,4'-ODA), 3,4'-diaminodiphenyl ether (3,4'-ODA), 3,3'-diaminodiphenyl ether (3,3'-ODA), 2,4'-diaminodiphenyl ether (2,4'-ODA), and 2,2'-diaminodiphenyl ether (2,2'-ODA). 4,4'-diaminodiphenyl ether (4,4'-ODA) can improve the elongation of the insulating coating.

[0026] The lower limit of the ODA content relative to 100 mol% of the aromatic diamine may be 50 mol%, 60 mol%, or 70 mol%, and the upper limit of the ODA content relative to 100 mol% of the aromatic diamine may be 100 mol% or 90 mol%.

[0027] The aromatic diamine may further contain an aromatic diamine other than ODA (hereinafter also referred to as "other aromatic diamine"). Examples of the other aromatic diamine include 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane, 2,2'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone, 3,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, 2,4'-diaminodiphenylsulfone, 2,2'-diaminodiphenylsulfone, 4,4' Examples of the other aromatic diamines include 4,4'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 2,4'-diaminodiphenyl sulfide, 2,2'-diaminodiphenyl sulfide, paraphenylenediamine, metaphenylenediamine, p-xylylenediamine, m-xylylenediamine, 2,2'-dimethyl-4,4'-diaminobiphenyl (mTBHG), 1,5-diaminonaphthalene, 4,4'-benzophenonediamine, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, and 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane. The other aromatic diamines may be used alone or in combination of two or more.

[0028] If the other aromatic diamine is 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) or 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), the relative dielectric constant of the insulating film can be reduced.

[0029] The content of the other aromatic diamine relative to 100 mol% of the aromatic diamine can be appropriately determined within a range that does not impair the effects of the present disclosure. The upper limit of the content may be 40 mol% or 30 mol%. The lower limit of the content may be 0 mol% or 10 mol%.

[0030] The lower limit of the concentration of the polyimide precursor in the resin composition may be 10% by mass or 20% by mass. The upper limit of the concentration may be 50% by mass or 40% by mass. By setting the concentration at or above the lower limit, the amount of resin composition required in the entire manufacturing process to obtain an insulating coating of a desired thickness when forming an insulating coating using the resin composition can be reduced, and the number of coating steps and heating steps can be reduced. By setting the concentration at or below the upper limit, the viscosity of the resin composition can be appropriately adjusted while maintaining good coating properties, thereby improving coatability.

[0031] The molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine used as raw materials for the polyimide precursor (aromatic tetracarboxylic dianhydride:aromatic diamine) may be, for example, 95:105 or more and 105:95 or less, 97:103 or more and 103:97 or less, or 99:101 or more and 101:99 or less, from the viewpoint of ease of synthesis of the polyimide precursor. The aromatic tetracarboxylic dianhydride and the aromatic diamine may be substantially equimolar amounts. In this case, the molecular weight of the polyimide precursor can be easily increased. The term "substantially equimolar amount" refers to a molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine (aromatic tetracarboxylic dianhydride:aromatic diamine) in the range of 99:101 or more and 101:99 or less.

[0032] (Method for synthesizing polyimide precursor) The polyimide precursor can be obtained by a polymerization condensation reaction between the aromatic tetracarboxylic dianhydride and the aromatic diamine described above. The polymerization condensation reaction can be carried out in the same manner as in conventional synthesis of polyimide precursors. Specific methods for the polymerization condensation reaction include, for example, mixing the aromatic tetracarboxylic dianhydride and the aromatic diamine in an organic solvent. This method polymerizes the aromatic tetracarboxylic dianhydride and the aromatic diamine, thereby obtaining a solution in which the polyimide precursor is dissolved in the organic solvent. For example, the degree of polymerization (weight average molecular weight) can be controlled by carrying out the polymerization condensation reaction in the presence of a reaction inhibitor.

[0033] The reaction inhibitor may be, for example, water (H ２ and alcohols having 1 to 15 carbon atoms. Examples of the alcohols having 1 to 15 carbon atoms include monohydric alcohols such as ethanol, methanol, propanol, butanol, and pentanol; and polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin.

[0034] The reaction conditions for the polymerization can be appropriately set depending on the raw materials used, etc. For example, the reaction temperature can be set to 10° C. or higher and 100° C. or lower, and the reaction time can be set to 0.5 hours or higher and 24 hours or lower.

[0035] Examples of the organic solvent used in the polycondensation reaction include the same organic solvents as those described below.

[0036] (Organic Solvent) Examples of the organic solvent that can be used include aprotic polar organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide, dimethyl sulfoxide, and γ-butyrolactone. These organic solvents may be used alone or in combination of two or more. The term "aprotic polar organic solvent" refers to a polar organic solvent that does not have a group that releases a proton.

[0037] The content of the organic solvent in the resin composition is not particularly limited as long as it is an amount that can uniformly dissolve and disperse the aromatic tetracarboxylic dianhydride and aromatic diamine. If the amount is too large, a large amount of organic solvent must be volatilized when forming the insulating coating, which may require a long time to form the insulating coating. Therefore, the content of the organic solvent can be, for example, 100 parts by mass or more and 1,000 parts by mass or less per 100 parts by mass of the aromatic tetracarboxylic dianhydride and aromatic diamine combined.

[0038] (Pore-forming agent) The pore-forming agent is not particularly limited and can be any additive known to be used for forming an insulating coating having pores. Examples of the pore-forming agent include chemical foaming agents, thermally expandable microcapsules, thermally decomposable resin-containing particles, high-boiling point solvents, and hollow fillers. Although not intended to be restrictive, it is believed that some types of pore-forming agents can be a cause of radical generation, and for example, thermally decomposable resin-containing particles are believed to generate radicals.

[0039] When the pore-forming agent is a thermally decomposable resin-containing particle, an insulating coating having a good appearance can be formed even when the porosity is high. The thermally decomposable resin-containing particle is gasified by thermal decomposition, and pores are formed in the insulating coating at the location where the thermally decomposable resin-containing particle was present. In this case, the pores can be uniformly distributed as an island phase of fine particles in the sea phase of the resin matrix that constitutes the insulating coating, thereby forming independent pores.

[0040] The thermally decomposable resin contained in the thermally decomposable resin-containing particles is preferably a resin that thermally decomposes at a temperature lower than the baking temperature of the polyimide that is the main component of the resin matrix that constitutes the insulating coating. The baking temperature is set appropriately depending on the type of polyimide, but is usually about 200°C or higher and 600°C or lower. The "thermal decomposition temperature" refers to the temperature at which the mass loss rate reaches 50% when the temperature is increased from room temperature at a rate of 10°C / min in an air atmosphere. The thermal decomposition temperature can be measured by measuring the thermogravimetry using a thermogravimetry-differential thermal analyzer ("TG / DTA" from SII Nanotechnology, Inc.).

[0041] Examples of thermally decomposable resins include compounds in which one or both ends or a portion of the molecular chain of a resin such as polyethylene glycol or polypropylene glycol are alkylated, (meth)acrylated, or epoxidized; polymers of (meth)acrylic acid esters having an alkyl group of 1 to 6 carbon atoms, such as polymethyl(meth)acrylate, polyethyl(meth)acrylate, polypropyl(meth)acrylate, and polybutyl(meth)acrylate; urethane oligomers, urethane polymers, polymers of modified (meth)acrylates such as urethane(meth)acrylate, epoxy(meth)acrylate, and ε-caprolactone(meth)acrylate; poly(meth)acrylic acid; crosslinked products thereof; polystyrene; and crosslinked polystyrene. Polymers of (meth)acrylic acid esters having an alkyl group of 1 to 6 carbon atoms are prone to thermal decomposition at the baking temperature, easily forming voids in the insulating coating. An example of such a (meth)acrylic acid ester polymer is polymethyl methacrylate (PMMA). The term "(meth)acrylic acid" encompasses both "acrylic acid" and "methacrylic acid."

[0042] The thermally decomposable resin-containing particles may be particles consisting solely of the thermally decomposable resin, or may be particles with a core-shell structure having a core primarily composed of the thermally decomposable resin and a shell primarily composed of a resin having a thermal decomposition temperature higher than that of the thermally decomposable resin. When a resin varnish containing particles with a core-shell structure is heated, only the core is thermally decomposed to form pores, with the shell remaining on the outer periphery of the pores. Particles with a core-shell structure can suppress pore interconnection and reduce pore size variation.

[0043] The main component of the shell is not particularly limited as long as it has a thermal decomposition temperature higher than that of the core, and is preferably a synthetic resin with a low dielectric constant and high heat resistance. Examples include polystyrene, silicone, fluororesin, and polyimide. Silicone tends to increase elasticity, which in turn tends to improve the dispersion of pores in the insulating coating, resulting in excellent insulating properties and heat resistance.

[0044] The content of the pore-forming agent in the resin composition can be determined appropriately depending on, for example, the type of pore-forming agent, the target porosity of the insulating coating, and the like.

[0045] (Radical Scavenger) In the present disclosure, the term "radical scavenger" refers not only to a radical scavenger in the narrow sense (primary antioxidant) that captures free radicals generated in the early stages of oxidation, but also to a peroxide decomposer (secondary antioxidant) that decomposes peroxides generated by the reaction of free radicals into harmless compounds. The radical scavenger can be any radical scavenger used as a radical scavenger in the narrow sense or a peroxide decomposer as described above, without any particular limitation. By including a radical scavenger in the resin composition, it is possible to trap radicals generated during the formation of an insulating coating having voids, and as a result, it is believed that a decrease in the elongation of the insulating coating due to the generation of radicals can be suppressed.

[0046] Examples of radical scavengers include hindered amine light stabilizers (HALS), phenolic antioxidants, aromatic amine antioxidants, sulfur antioxidants, and phosphorus antioxidants. Generally, hindered amine light stabilizers, phenolic antioxidants, and aromatic amine antioxidants are classified as antioxidants in the narrow sense described above, while sulfur antioxidants and phosphorus antioxidants are classified as peroxide decomposers. The resin composition can contain one or more radical scavengers.

[0047] Examples of the hindered amine light stabilizer include bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate, 1,2,2,6,6-pentamethyl-4-piperidyl methacrylate, tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate, (2,2,6,6-tetramethyl-4-piperidyl)dodecyl succinimide, and N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)butanetetracarboxylate.

[0048] Examples of phenolic antioxidants include 2,6-di-tert-butyl-p-cresol (BHT), 2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)mesitylene, 2,2'-methylenebis(6-tert-butyl-4-ethylphenol), pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,5-di-t-butylhydroquinone, N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxyhydrocinnamide), 3,5-di-t-butyl-4-hydroxybenzylphos phonate-diethyl ester, 2,4-bis[(octylthio)methyl]-o-cresol, 2,6-di-t-butyl-4-ethylphenol, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), 4,4'-butylidenebis(3-methyl-6-t-butylphenol), 2,5-di-t-amylhydroquinone, 2-t-butyl-6-(3-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, and 4,4'-butylidenebis(3-methyl-6-t-butylphenol).

[0049] Examples of aromatic amine antioxidants include N,N'-di-2-naphthyl-1,4-phenylenediamine and 4,4'-bis(α,α-dimethylbenzyl)diphenylamine.

[0050] Examples of sulfur-based antioxidants include phenothiazine, pentaerythritol-tetrakis(3-laurylthiopropionate), didodecyl sulfide, dioctadecyl sulfide, didodecyl thiodipropionate, dioctadecyl thiodipropionate, dimyristyl thiodipropionate, dodecyl octadecyl thiodipropionate, and 2-mercaptobenzimidazole.

[0051] Examples of phosphorus-based antioxidants include triisodecyl phosphite, diphenylisodecyl phosphite, triphenyl phosphite, and trinonylphenyl phosphite.

[0052] Although not wishing to be limiting, it is believed that if the radical scavenger is a radical scavenger in the narrow sense (primary antioxidant), it can capture free radicals generated in the upstream stage of oxidation and prevent downstream chain reactions, thereby more effectively suppressing a decrease in the elongation of the insulating coating due to the generation of radicals than a secondary antioxidant. Furthermore, the combined use of a radical scavenger in the narrow sense (primary antioxidant) and a peroxide decomposer (secondary antioxidant) can capture free radicals generated in the upstream stage of oxidation while decomposing the peroxides generated by the reaction of free radicals in the downstream stage of oxidation into harmless compounds, thereby more effectively suppressing a decrease in the elongation of the insulating coating.

[0053] When the radical scavenger is a hindered amine light stabilizer, the film elongation can be further improved even when the porosity is high.

[0054] The lower limit of the content of the radical scavenger in the resin composition may be 0.1 parts by mass or more, 0.5 parts by mass or more, or 1 part by mass or more, per 100 parts by mass of the polyimide precursor. The upper limit of the content of the radical scavenger may be 10 parts by mass or less, or 5 parts by mass or less, per 100 parts by mass of the polyimide precursor. When the content of the radical scavenger is within the above range, the film elongation can be further improved even when the porosity is high. Furthermore, when the content of the radical scavenger is 1 part by mass or more and 5 parts by mass or less, the flexibility of the insulating film can be improved.

[0055] (Other Components) The resin composition may contain other components in addition to the above-described components. The other components are not particularly limited as long as they are additives that can be blended into a resin varnish for forming an insulating coating for an insulated wire, and examples thereof include an antioxidant other than the above-described radical scavenger, a filler, a leveling agent, a curing agent, and an adhesion promoter.

[0056] 1 includes a conductor 2 and an insulating coating 3 that covers the conductor 2. The insulating coating 3 contains a resin matrix 5 and a plurality of pores 4.

[0057] The cross-sectional shape of the insulated wire 1 is not particularly limited, and examples thereof include a circular shape (round wire), an elliptical shape, a square shape (rectangular wire), and a rectangular shape (flat wire). It is preferable that the cross-sectional shape of the insulated wire 1 is a rectangular shape, in other words, a flat wire. In this case, the insulated wire 1 can be wound at a high density during coil processing. It is also preferable that the cross-sectional shape of the insulated wire 1 and the cross-sectional shape of the conductor 2, which will be described later, are the same type of shape.

[0058] The insulated wire 1 can be suitably used as a winding wire for a coil (magnet wire).

[0059] (Conductor) Examples of the cross-sectional shape of the conductor 2 include a circular shape (round wire), an elliptical shape, a square shape, and a rectangular shape. When the insulated wire 1 is a rectangular wire, the cross-sectional shape of the conductor 2 is preferably a rectangular shape.

[0060] The conductor 2 is preferably made of a metal having high electrical conductivity and high mechanical strength. Examples of such metals include copper, copper alloys, aluminum, nickel, silver, mild steel, steel, and stainless steel. The conductor 2 may be made of a wire-shaped material made of such metals, or a multilayer structure in which a wire-shaped material is further coated with another metal, such as a nickel-coated copper wire, a silver-coated copper wire, a copper-coated aluminum wire, or a copper-coated steel wire.

[0061] The lower limit of the average cross-sectional area of ​​the conductor 2 is 0.01 mm ２ 0.1 mm ２ In this case, the volume of the insulating coating 3 relative to the conductor 2 in the insulated wire 1 can be made appropriate, and the volume efficiency of a coil or the like formed using the insulated wire can be improved. ２ 10 mm ２ In this case, it is possible to reduce the need to form the insulating coating 3 thick enough to sufficiently reduce the relative dielectric constant, and it is possible to avoid an unnecessary increase in the diameter of the insulated wire.

[0062] (Insulating Coating) The insulating coating 3 is laminated on the outer peripheral surface of the conductor 2 so as to cover the conductor 2. The insulating coating 3 is composed of one or more layers. For example, when the insulating coating 3 is formed by the method described below (a method in which a resin varnish is applied and baked multiple times), the insulating coating 3 has a laminated structure composed of multiple layers formed using the resin varnish.

[0063] The insulating coating 3 contains a resin matrix 5 and a plurality of pores 4 dispersed in the resin matrix 5. The insulating coating 3 is formed from the resin composition described above. Therefore, the resin matrix 5 contains polyimide as a main component.

[0064] The porosity of the insulating coating 3 is preferably 20% by volume or more and 60% by volume or less. When the porosity of the insulating coating 3 is 20% by volume or more, the dielectric constant of the insulating coating can be reduced. Furthermore, since the insulating coating 3 is formed from the resin composition described above, the insulating coating 3 has excellent film elongation even when it has a high porosity of 20% by volume or more. The porosity of the insulating coating 3 may be 40% by volume or more. The upper limit of the porosity of the insulating coating 3 may be 60% by volume or 50% by volume. When the porosity of the insulating coating 3 is 60% by volume or less, the processability of the insulating coating 3 can be improved while maintaining the toughness of the insulating coating 3.

[0065] The insulating coating 3 has excellent coating elongation even when the porosity is high. Specifically, when the porosity P is 20% by volume or more and 60% by volume or less, the coating elongation Y of the insulating coating 3 and the value X calculated by equation (1): X = -1.3 × P + 84 should satisfy Y ≥ X. When the coating elongation Y is equal to or greater than the value X, it can be said that a decrease in coating elongation is suppressed even when the porosity is high.

[0066] The average thickness of the insulating coating 3 is not particularly limited, and can usually be set to 2 μm or more and 200 μm or less.

[0067] (Pores) The voids 4 may be derived from the thermally decomposable resin-containing particles. The thermally decomposable resin-containing particles are gasified by thermal decomposition, and voids 4 are formed in the portions of the insulating coating 3 where the thermally decomposable resin-containing particles were previously present. In this case, the fine particles can be uniformly distributed as an island phase in the sea phase of the resin matrix 5 that constitutes the insulating coating 3, forming independent voids. The insulating coating 3 contains multiple voids 4, which allows the insulated wire to have a reduced dielectric constant.

[0068] A lower limit of the average diameter of the pores 4 of 0.1 μm can improve the mechanical properties of the insulating coating 3. An upper limit of the average diameter of the pores 4 of 10 μm can improve the insulating properties of the insulating coating 3. The average diameter of the pores 4 is a value obtained by measuring the cross section of the insulated wire 1 with a pore diameter distribution measuring device (for example, the "Porous Material Automated Pore Size Distribution Measuring System" manufactured by Porous Materials).

[0069] The insulated wire may have a configuration other than those described above. For example, the insulated wire may have an adhesion layer containing an additive such as an adhesion improver between the conductor and the insulating coating. Examples of the adhesion improver include mercaptans such as 2-mercaptoimidazole and 5-amino-1,3,4-thiadiazole-2-thiol.

[0070] The insulated wire may have a surface friction adjusting layer as its outermost layer. Examples of the surface friction adjusting layer include a polyamide-imide, self-lubricating amide-imide, polyimide, and self-lubricating polyimide layer. The "outermost layer" refers to the layer located outermost in the laminate structure constituting the insulated wire, with the conductor side positioned inside.

[0071] The insulated wire may have an outermost adhesive layer containing an additive such as a foaming agent. Examples of the foaming agent include azo-based foaming agents such as azodicarbonamide and azobisisobutyronitrile, nitroso-based foaming agents such as dinitrosopentamethylenetetramine and N,N'-dinitroso-N,N'-dimethylterephthalamide, hydrazide-based foaming agents such as p-toluenesulfonylhydrazide, p,p'-oxybisbenzenesulfonylhydrazide and benzenesulfonylhydrazide, and trihydrazinotriazine.

[0072] The insulated wire may have an anti-surge layer containing an inorganic filler as its outermost layer. Examples of the anti-surge layer include silica, alumina, magnesia, beryllium oxide, silicon carbide, titanium carbide, boron carbide, tungsten carbide, boron nitride, and silicon nitride. The inorganic filler may be surface-treated. Examples of the surface treatment agent include a silane coupling agent.

[0073] <Method for manufacturing insulated wire> The method for manufacturing an insulated wire is a method for manufacturing the insulated wire described above, and includes a step of applying the resin composition described above to an outer peripheral surface of a conductor (coating step), and a step of heating the resin composition applied in the coating step (heating step).

[0074] In the coating step, the resin composition is applied to the outer peripheral surface of the conductor. For example, a method for applying the resin composition to the outer peripheral surface of the conductor may use a coating device equipped with a liquid composition tank containing the resin composition and a coating die. With this coating device, the conductor passes through the liquid composition tank, causing the resin composition to adhere to the outer peripheral surface of the conductor. The conductor then passes through the coating die, coating the resin composition to a uniform thickness.

[0075] In the heating step, the resin composition applied to the conductor in the application step is heated, which volatilizes the organic solvent in the resin composition and cures the polyimide precursor to form a polyimide.

[0076] The heating step may be performed using any suitable device, such as a cylindrical baking oven that is long in the direction of travel of the conductor. The heating method may be any suitable method, such as hot air heating, infrared heating, or high-frequency heating.

[0077] The heating temperature can be, for example, 300° C. to 800° C. The heating time can be, for example, 5 seconds to 1 minute.

[0078] The coating step and the heating step are usually repeated multiple times. By repeating the steps multiple times, the thickness of the insulating coating can be increased. The hole diameter of the coating die can be adjusted appropriately depending on the number of repetitions.

[0079] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0080] <Preparation of Resin Composition and Production of Insulated Wire> The abbreviations of the various components used in preparing the resin composition are shown below. (Aromatic tetracarboxylic dianhydrides) PMDA: pyromellitic dianhydride BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride (Aromatic diamines) ODA: 4,4'-diaminodiphenyl ether BAPP: 2,2-bis[4-(4-aminophenoxy)phenyl]propane BAPB: 4,4'-bis(4-aminophenoxy)biphenyl (Organic solvents) NMP: N-methyl-2-pyrrolidone (Radical scavenger) HALS1: bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate (ADEKA Corporation's "ADEKA STAB LA-72") HALS2: 1,2,2,6,6-pentamethyl-4-piperidyl methacrylate (ADEKA Corporation's "ADEKA STAB LA-82") HALS 3: tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate (ADEKA CORPORATION's "ADEKA STAB LA-52") Phenol 1: 2,6-di-tert-butyl-p-cresol (Tokyo Chemical Industry Co., Ltd.'s "D0228") Phenol 2: 2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)mesitylene (Tokyo Chemical Industry Co., Ltd.'s "T0916") Amine 1: N,N'-di-2-naphthyl-1,4-phenylenediamine (Tokyo Chemical Industry Co., Ltd.'s "D0812") Amine 2: 4,4'-bis(α,α-dimethylbenzyl)diphenylamine (Tokyo Chemical Industry Co., Ltd.'s "B1970")

[0081] [No. 1] (Preparation of Resin Composition) ODA as an aromatic diamine was dissolved in NMP. PMDA as an aromatic tetracarboxylic dianhydride was added so that the mixing ratio (molar ratio) of the aromatic tetracarboxylic dianhydride to the aromatic diamine was 100:100. The mixture was allowed to react at 30°C for 3 hours with stirring under a nitrogen atmosphere to synthesize a polyimide precursor, obtaining a polyimide precursor solution with NMP as the solvent (solid content: 28% by mass). 0.5 parts by mass of HALS1 and 40 parts by mass of thermally decomposable resin-containing particles were added to 100 parts by mass of the polyimide precursor to prepare Resin Composition No. 1. The thermally decomposable resin-containing particles used were core-shell structured particles with an average particle diameter of 3 μm, in which the core was a polymethyl methacrylate particle and the shell was silicone.

[0082] (Preparation of Insulated Wire) A round copper wire with an average diameter of 1 mm was used as the conductor. Resin Composition No. 1 was applied to the surface of the conductor. The conductor coated with Resin Composition No. 1 was heated 13 times under the conditions of an inlet temperature of 400°C, an outlet temperature of 500°C, and a wire speed of 13 m / min in a heating furnace. In this way, an insulating coating with an average thickness of 40 μm was formed on the conductor, and Insulated Wire No. 1 was prepared.

[0083] [Nos. 2 to 31] Resin compositions No. 2 to No. 31 were prepared in the same manner as No. 1, except that the types and amounts of each component shown in Table 1 below were used, and insulated wires No. 2 to No. 31 were fabricated.

[0084] <Evaluation> For the insulated wires No. 1 to No. 31 prepared above, the porosity was calculated, the relative dielectric constant was measured, and the coating elongation was measured according to the following methods. The results are shown in Table 1 below.

[0085] [Calculation of Porosity] For each of the insulated wires No. 1 to No. 31 prepared above, the insulating coating was peeled off from the conductor to form a tube, and the mass W2 of the cylindrical insulating coating was measured. The apparent volume V1 was determined from the outer diameter of the cylindrical insulating coating, and the mass W1 without pores was calculated by multiplying the volume V1 by the density ρ1 of the insulating coating material. The porosity (unit: volume %) was calculated from the values ​​of W1 and W2 using the following formula. The porosity was calculated for n = 3 samples, and the average value was calculated. Formula: Porosity = (W1 - W2) × 100 / W1

[0086] [Measurement of Relative Dielectric Constant] The relative dielectric constant of the insulating coating was measured for the insulated wires No. 1 to No. 31 prepared above. Measurement samples were prepared by applying silver paste to three locations on the surface of the insulated wire and peeling off the insulating coating at one end of the insulated wire to expose the conductor. The lengths of the silver paste applied to the three locations on the surface of the insulated wire in the longitudinal direction of the insulated wire were 10 mm, 100 mm, and 10 mm, respectively. The two 10-mm-long silver paste applications were grounded, and the capacitance between the 100-mm-long silver paste applied between these two silver paste applications and the exposed conductor was measured using an LCR meter. The relative dielectric constant of the insulating coating was calculated from the measured capacitance and the average thickness of the insulating coating. The relative dielectric constant was measured three times after heating at 105°C for 1 hour, and the average value was calculated.

[0087] [Measurement of Coating Elongation] For each of the insulated wires No. 1 to No. 31 prepared above, the conductor was removed from the insulated wire to form a tubular insulating coating. The elongation at break (unit: %) was measured when the coating was pulled using a tensile tester (Shimadzu Corporation's "Autograph AGS-X") at a chuck distance of 20 mm and a pulling rate of 10 mm / min in an environment of 25°C. The coating elongation measurement was performed five times, and the average value was calculated.

[0088] [Relationship between Porosity and Film Elongation] Using the calculated porosity and the measured film elongation, an evaluation was made to determine whether the insulating coating had excellent film elongation even when the porosity was high. The evaluation was made by comparing the value X calculated by the following formula (1) where P is the porosity (volume %) and the measured film elongation Y. When the porosity P was 20 vol% or more, those satisfying Y≧X were evaluated as "A" (insulating coatings with excellent film elongation even when the porosity was high), and those not satisfying Y≧X (i.e., Y<X) were evaluated as "B" (insulated wires with poor film elongation when the porosity was high). Formula (1): X=−1.3×P+84(P≧20)

[0089] In Table 1 below, "-" indicates that the corresponding component was not used. In the "Polyimide precursor" row, "Acid anhydride" means "Aromatic tetracarboxylic dianhydride", and "Diamine" means "Aromatic diamine". The "Composition ratio" in the "Acid anhydride" and "Diamine" columns indicates the molar ratio. The numerical values ​​in the "Pore-forming agent" and "Radical scavenger" columns indicate the number of parts by mass per 100 parts by mass of polyimide precursor.

[0090]

[0091] As can be seen from Table 1, when resin compositions No. 1 to No. 27 were used, the insulating coatings had superior film elongation compared to when resin compositions No. 28 to No. 31 were used, even when the porosity was high.

[0092] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is not limited to the configurations and examples of the above-described embodiments, but is defined by the scope of the claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

[0093] REFERENCE SIGNS LIST 1 insulated wire 2 conductor 3 insulating coating 4 voids 5 resin matrix

Claims

1. a polyimide precursor which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine; an organic solvent; a pore-forming agent; Radical scavenger and A resin composition comprising:

2. 2. The resin composition according to claim 1, wherein the content of the radical scavenger is 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the polyimide precursor.

3. 2. The resin composition according to claim 1, wherein the radical scavenger is a hindered amine-based light stabilizer.

4. 2. The resin composition according to claim 1, wherein the pore-forming agent is a thermally decomposable resin-containing particle.

5. The resin composition according to claim 1, which is used to form an insulating coating for an insulated wire.

6. A conductor; an insulating coating that covers the conductor; Equipped with the insulating coating has a resin matrix and a plurality of pores, An insulated wire, wherein the insulating coating is formed from the resin composition according to any one of claims 1 to 5.

7. the porosity P of the insulating coating is 20% by volume or more and 60% by volume or less, 7. The insulated wire according to claim 6, wherein a coating elongation Y of the insulating coating and a value X calculated by the following formula (1) satisfy Y≧X: Formula (1): X=-1.3×P+84

8. A method for manufacturing an insulated wire comprising: a conductor; and an insulating coating covering the conductor, the insulating coating having a resin matrix and a plurality of pores, a step of applying the resin composition according to any one of claims 1 to 5 to an outer peripheral surface of the conductor; a step of heating the resin composition applied in the application step; A method for manufacturing an insulated wire comprising: