Insulated wire

Abstract

This insulated wire comprises: a conductor; a first insulating layer covering the conductor; and a second insulating layer covering the first insulating layer, wherein the first insulating layer has a resin matrix containing polyimide as a main component and a plurality of voids, and the second insulating layer has a resin matrix containing polyimide as a main component, the plurality of voids having an outer shell containing silicone or silica as a main component.

Description

insulated wire

[0001] This application claims priority to Japanese Patent Application No. 2024-134141, filed on August 9, 2024, and incorporates by reference all of the contents of said Japanese 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] An insulated wire according to one aspect of the present disclosure includes a conductor, a first insulating layer covering the conductor, and a second insulating layer covering the first insulating layer, wherein the first insulating layer has a resin matrix primarily composed of polyimide and a plurality of voids, the second insulating layer has a resin matrix primarily composed of polyimide, and the plurality of voids have an outer shell primarily composed of silicone or silica.

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

[0006] In electrical equipment operating at high voltages, such as motors, high surge voltages are applied to the insulated wires contained in the electrical equipment, which can easily cause partial discharges (corona discharges) on the surface of the insulating coating of the insulated wires. Partial discharges can cause, for example, localized temperature increases, ozone generation, or ion generation, which can lead to early insulation breakdown and shorten the life of the insulated wire and ultimately the electrical equipment. Therefore, lowering the dielectric constant of the insulating coating has been considered to reduce the occurrence of partial discharges. Patent Document 1 proposes a technique for forming pores in the insulating coating to lower the dielectric constant of the insulating coating.

[0007] Furthermore, there is a demand for excellent properties to reduce dielectric breakdown due to surge voltages (hereinafter also referred to as "surge resistance").

[0008] The problem to be solved by the present disclosure is to provide an insulated wire that has a low relative dielectric constant and excellent surge resistance.

[0009] An insulated wire according to one aspect of the present disclosure has a low dielectric constant and excellent surge resistance.

[0010] First, embodiments of the present disclosure will be listed and described. (1) An insulated electric wire comprising a conductor, a first insulating layer covering the conductor, and a second insulating layer covering the first insulating layer, wherein the first insulating layer has a resin matrix mainly composed of polyimide and a plurality of voids, the second insulating layer has a resin matrix mainly composed of polyimide, and the plurality of voids have an outer shell mainly composed of silicone or silica. (2) The insulated electric wire according to (1), wherein a ratio of the average thickness of the first insulating layer to the sum of the average thickness of the first insulating layer and the average thickness of the second insulating layer is 35% or more and 95% or less. (3) The insulated electric wire according to (1) or (2), wherein the porosity of the first insulating layer is 25% by volume or more and 60% by volume or less. (4) The insulated electric wire according to any one of (1) to (3), wherein the average diameter of the plurality of voids is 0.5 μm or more and 10 μm or less. (5) The insulated electric wire according to any one of (1) to (4), further comprising an adhesive layer between the conductor and the first insulating layer. (6) The insulated electric wire according to any one of (1) to (5), wherein the second insulating layer further comprises inorganic fine particles. (7) The insulated electric wire according to (6), wherein the inorganic fine particles are surface-treated silica, alumina, aluminum hydroxide, or boehmite. (8) The insulated electric wire according to any one of (1) to (7), which is a rectangular wire.

[0011] Hereinafter, an insulated wire according to one embodiment of the present disclosure will be described.

[0012] 1 includes a conductor 2, a first insulating layer 3 covering the conductor 2, and a second insulating layer 4 covering the first insulating layer 3. The first insulating layer 3 has a resin matrix 3 a and a plurality of pores 3 b.

[0013] The cross-sectional shape perpendicular to the longitudinal axis of the insulated electric wire 1 is not particularly limited and may be, for example, circular, elliptical, square, or rectangular. An insulated electric wire 1 with a circular cross-sectional shape is a round wire, an insulated electric wire 1 with a square cross-sectional shape is a rectangular wire, and an insulated electric wire 1 with a rectangular cross-sectional shape is a rectangular wire. The cross-sectional shape of the insulated electric wire 1 may also be rectangular. In other words, the insulated electric wire 1 may be a rectangular wire. If the insulated electric wire 1 is a rectangular wire, the insulated electric wire 1 can be wound at a high density during coil processing. The cross-sectional shape of the insulated electric wire 1 and the cross-sectional shape of the conductor 2, which will be described later, may be the same type of shape.

[0014] The insulated wire 1 can be suitably used as a winding wire for a coil (magnet wire).

[0015] [Conductor] The cross-sectional shape of the conductor 2 is, for example, circular, elliptical, square, or rectangular. When the insulated wire 1 is a rectangular wire, the cross-sectional shape of the conductor 2 may be rectangular.

[0016] The conductor 2 may be 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, for example, a metal wire or a multilayer wire in which a metal wire is further coated with another metal. Specifically, the conductor 2 may be, for example, a nickel-coated copper wire, a silver-coated copper wire, a copper-coated aluminum wire, or a copper-coated steel wire.

[0017] The lower limit of the average cross-sectional area of ​​the conductor 2 is 0.01 mm ２ 0.1 mm ２ The average cross-sectional area of ​​the conductor 2 may be 0.01 mm ２ When the average cross-sectional area of ​​the conductor 2 is equal to or greater than 40 mm, the volume of the first insulating layer 3 relative to the conductor 2 in the insulated wire 1 can be adjusted appropriately, thereby improving the volume efficiency of a coil formed using the insulated wire 1. ２ 30 mm ２ The average cross-sectional area of ​​the conductor 2 may be 40 mm ２ If it is equal to or less than this, it is possible to avoid a decrease in the output efficiency of the coil or the like due to an increase in copper loss caused by eddy current.

[0018] [First insulating layer] The first insulating layer 3 is laminated on the outer peripheral surface of the conductor 2 so as to cover the conductor 2. The first insulating layer 3 is composed of one or more layers. For example, when the first insulating layer 3 is formed by the method described below (a method in which the application and baking of a resin varnish are repeated multiple times), the first insulating layer 3 has a laminate structure composed of multiple layers formed using the resin varnish.

[0019] The first insulating layer 3 contains a resin matrix 3 a and a plurality of voids 3 b. The presence of the voids 3 b reduces the relative dielectric constant of the insulating layer. Furthermore, the electric field strength in the gaps between adjacent insulated wires is alleviated, reducing the frequency of discharge, thereby lengthening the time until breakdown.

[0020] The average thickness of the first insulating layer 3 is not particularly limited, and is, for example, 2 μm or more and 300 μm or less.

[0021] The lower limit of the ratio of the average thickness of the first insulating layer 3 to the sum of the average thickness of the first insulating layer 3 and the average thickness of the second insulating layer 4 (described later) may be 30%, 35%, 40%, 45%, 50%, 55%, or 60%. A ratio of 35% or more can further reduce the dielectric constant. The upper limit of this ratio can be 99%, 95%, 90%, 85%, 80%, 75%, or 70%. A ratio of 95% or less can further improve surge resistance.

[0022] The lower limit of the porosity of the first insulating layer 3 may be 20 vol%, 25 vol%, or 30 vol%. A porosity of 25 vol% or more of the first insulating layer 3 further reduces the dielectric constant. The upper limit of the porosity of the first insulating layer 3 may be 65 vol%, 60 vol%, or 55 vol%. A porosity of 60 vol% or less of the first insulating layer 3 further improves the film elongation. "Porosity" refers to the percentage (unit: vol%) of the volume of pores relative to the volume of the first insulating layer 3. The porosity is specifically measured as follows. Using an electronic balance specific gravity measurement kit, the weight (W1) of the film test piece in air and the weight (W2) in hexane are measured. The specific gravity (H) of the film test piece is calculated using Equation 1: W1 / (W1-W2) x 0.66. In Equation 1, 0.66 is the specific gravity of hexane. The porosity can be calculated by the formula 2: (1-(H / ρ))×100, where ρ is the density of the resin matrix 3a.

[0023] (Resin Matrix) The main component of the resin matrix 3a is polyimide. The "main component" refers to the component that is contained in the largest amount in mass terms. In other words, the "main component of the resin matrix" refers to the component that is contained in the largest amount in mass terms among the components that make up the resin matrix 3a. Polyimide has excellent toughness and a low dielectric constant.

[0024] The polyimide may be derived from a polyimide precursor, which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine. The polyimide precursor is a reaction product obtained by a condensation polymerization reaction between the aromatic tetracarboxylic dianhydride and the aromatic diamine. The polyimide precursor is a compound also known as a polyamic acid (polyamic acid). The polyimide precursor undergoes a dehydration cyclization reaction (imidization reaction) to form a cyclic imide, thereby becoming a polyimide.

[0025] When the aromatic tetracarboxylic dianhydride contains pyromellitic dianhydride (PMDA), the heat resistance of the first insulating layer 3 is improved. 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").

[0026] 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. 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 acid dianhydride, and 2,3,6,7-naphthalenetetracarboxylic acid dianhydride. The other aromatic tetracarboxylic acid dianhydrides may be used alone or in combination of two or more.

[0027] When the other aromatic tetracarboxylic dianhydride is biphenyltetracarboxylic dianhydride (BPDA), the hydrolysis resistance of the polyimide can be improved.

[0028] The lower limit of the amount of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 10 mol%, 20 mol%, or 30 mol%, and the upper limit of the amount of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 100 mol%, 90 mol%, 80 mol%, or 70 mol%.

[0029] The content of the other aromatic tetracarboxylic dianhydride relative to 100 mol% of the aromatic tetracarboxylic dianhydride can be appropriately set as long as the effects of the present disclosure are not impaired. The upper limit of the content may be 90 mol% or 80 mol%. The lower limit of the content may be 0 mol% or 10 mol%.

[0030] When the aromatic diamine contains diaminodiphenyl ether (ODA), the heat resistance of the first insulating layer 3 is 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). When the aromatic diamine is 4,4'-diaminodiphenyl ether (4,4'-ODA), the film elongation of the first insulating layer 3 is improved.

[0031] 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%.

[0032] 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' 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, 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane. The other aromatic diamines may be used alone or in combination of two or more.

[0033] 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 first insulating layer 3 is further reduced.

[0034] The content of the other aromatic diamine relative to 100 mol% of the aromatic diamine can be appropriately set as long as it 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%.

[0035] The components and contents of the aromatic tetracarboxylic dianhydride and the aromatic diamine are determined by subjecting the insulating coating to alkaline hydrolysis. １ It can be determined by analysis using H-NMR (Proton Nuclear Magnetic Resonance).

[0036] From the viewpoint of ease of synthesis of the polyimide precursor, the molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine (aromatic tetracarboxylic dianhydride:aromatic diamine) used as raw materials for the polyimide precursor 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. When the molar ratio is 99:101 or more and 101:99 or less, the molecular weight of the polyimide precursor can be easily increased.

[0037] The lower limit of the weight average molecular weight of the polyimide precursor may be 15,000 or 16,000. The upper limit of the weight average molecular weight may be 100,000 or 80,000. When the weight average molecular weight is equal to or greater than the lower limit, the film properties are improved. When the weight average molecular weight is equal to or less than the upper limit, the coatability of the resin varnish is improved. The weight average molecular weight indicates the weight average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

[0038] The polyimide precursor can be obtained by a condensation polymerization reaction between the aromatic tetracarboxylic dianhydride and the aromatic diamine. The condensation polymerization reaction may be performed by a conventional method for synthesizing a polyimide precursor. A specific method for the condensation polymerization reaction is, for example, a method in which an aromatic tetracarboxylic dianhydride and an aromatic diamine are mixed in an organic solvent. This method allows the aromatic tetracarboxylic dianhydride and the aromatic diamine to polymerize, 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 performing the polymerization condensation reaction in the presence of a reaction inhibitor.

[0039] The reaction control agent is, for example, water (H２ O), alcohols and acid anhydrides having 1 to 15 carbon atoms. Examples of 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. Examples of acid anhydrides include aliphatic acid anhydrides such as phthalic anhydride, maleic anhydride, and succinic anhydride; and aromatic acid anhydrides such as trimellitic acid.

[0040] The reaction conditions for the condensation 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.

[0041] The organic solvent used in the condensation polymerization reaction is, for example, the organic solvent described below.

[0042] The resin matrix 3 a may contain other components in addition to the above components. The other components are, for example, additives that are blended into the insulating coating of an insulated electric wire. Specifically, the other components are, for example, fillers, antioxidants, radical scavengers, ultraviolet inhibitors, surface lubricants, plasticizers, leveling agents, curing agents, and adhesion promoters.

[0043] (Pores) The plurality of pores 3b have an outer shell mainly composed of silicone or silica. Silicone is denatured to silica by heat dissipation when a surge voltage is applied. By providing an outer shell mainly composed of silicone or silica, it is possible to achieve both a low dielectric constant and improved surge resistance. "Silicone" refers to a polymer containing a repeating structure of siloxane bonds in which silicon atoms and oxygen atoms are bonded. The plurality of pores 3b may be derived from a pore-forming agent having a core-shell structure having a core mainly composed of a thermally decomposable resin and a shell mainly composed of silicone or silica.

[0044] Whether the main component of the outer shells of the plurality of voids 3b is silicone or silica can be determined by observing the cross section of the insulated wire using a scanning electron microscope and performing elemental analysis on the outer shell portions of the plurality of voids 3b using an energy dispersive X-ray analyzer.

[0045] When the plurality of pores 3b originates from the pore-forming agent, the plurality of pores 3b are formed by gasification of the core of the pore-forming agent. The outer shell is formed by removing the core of the pore-forming agent to form a hollow shell. In other words, the plurality of pores 3b originates from the core of the pore-forming agent having a core-shell structure, and the outer shell originates from the shell of this pore-forming agent.

[0046] The thermally decomposable resin may be 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 of the first insulating layer 3. The baking temperature is set appropriately depending on the type of polyimide, but is typically about 200°C or higher and 600°C or lower. The lower limit of the thermal decomposition temperature of the thermally decomposable resin may be 200°C, and the upper limit may be 400°C. 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" by SII NanoTechnology, Inc.).

[0047] Examples of the thermally decomposable resin include compounds such as polyethylene glycol and polypropylene glycol in which one or both ends or a portion thereof 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 more likely to form multiple voids 3b. An example of a polymer of (meth)acrylic acid esters is polymethyl methacrylate (PMMA). The term "(meth)acrylic acid" encompasses both "acrylic acid" and "methacrylic acid."

[0048] The lower limit of the average diameter of the plurality of pores 3b may be 0.1 μm, 0.5 μm, or 1 μm, and the upper limit of the average diameter may be 20 μm, 10 μm, or 8 μm.

[0049] The average diameter of the plurality of pores 3b can be determined by observing the cross section of the insulated wire using a scanning electron microscope, measuring the diameters of any 30 pores on the SEM image using "ImageJ," an open-source image processing software, and calculating the average value of the 30 pore diameters.

[0050] [Second Insulating Layer] The second insulating layer 4 is laminated on the outer peripheral surface of the first insulating layer 3 so as to cover the first insulating layer 3. This lamination order reduces the elongation of the first insulating layer 3, which is prone to causing stress concentration between multiple pores and cracks when the insulated wire is bent, thereby obtaining an insulated wire with excellent processability. The second insulating layer 4 is composed of one or more layers. For example, when the second insulating layer 4 is formed by the method described below (a method in which resin varnish is applied and baked multiple times), the second insulating layer 4 has a laminate structure composed of multiple layers formed using the resin varnish.

[0051] The second insulating layer 4 has a resin matrix mainly composed of polyimide, which is the same as that described in the section on the first insulating layer.

[0052] The average thickness of the second insulating layer 4 is not particularly limited, and is, for example, 2 μm or more and 300 μm or less.

[0053] The second insulating layer 4 may further contain inorganic fine particles. When the second insulating layer 4 contains inorganic fine particles, deterioration due to partial discharge that progresses from the surface of the insulated wire is effectively suppressed, and surge resistance is further improved.

[0054] Examples of inorganic fine particles include silica, alumina, titania, aluminum hydroxide, magnesia, boehmite, beryllium oxide, silicon carbide, titanium carbide, boron carbide, tungsten carbide, boron nitride, and silicon nitride. Surface-treated inorganic fine particles may be used. Examples of surface treatment agents include silylating agents and silane coupling agents. The use of surface-treated silica, alumina, aluminum hydroxide, or boehmite as inorganic fine particles further improves surge resistance. Surface-treated silica refers to silica in which some or all of the silanol groups on the surface of silica particles have been chemically reacted.

[0055] Examples of silylating agents used for surface-treated silica include dimethylchlorosilane, trimethylchlorosilane, dimethylchloropropylsilane, dimethylisopropylchlorosilane, dimethyldichlorosilane, diethyldichlorosilane, tetramethyldisilazane, hexamethyldisilazane, diphenyltetramethyldisilazane, and dimethyltetraphenyldisilazane. When the silylating agent is dimethyldichlorosilane, hexamethyldisilazane, or a combination thereof, the silica is uniformly dispersed in the resin matrix, thereby further improving surge resistance.

[0056] Examples of silane coupling agents used for the surface-treated silica include vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3 3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, tris-(trimethoxysilylpropyl)isocyanurate, 3-ureidopropyltrialkoxysilane, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-isocyanatopropyltriethoxysilane, 3-trimethoxysilylpropylsuccinic anhydride, phenyltrimethoxysilane, phenyltriethoxysilane. When the silane coupling agent is 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, or a combination thereof, silica is uniformly dispersed in the resin matrix, thereby further improving surge resistance.

[0057] The surface treatment agent for silica can be identified by, for example, solid-state Si-NMR, pyrolysis gas chromatography mass spectrometry, Fourier transform infrared spectrometry, or the like.

[0058] [Method for Manufacturing Insulated Wire] The insulated wire 1 can be manufactured by a method including a step of forming a first insulating layer and a step of forming a second insulating layer.

[0059] The process of forming the first insulating layer includes a process of applying a resin varnish for forming the first insulating layer to the outer surface of the conductor 2 (coating process), and a process of heating the resin varnish applied in the coating process (heating process).

[0060] In the coating step, the resin varnish is applied to the outer peripheral surface of the conductor 2. For example, the resin varnish can be applied to the outer peripheral surface of the conductor 2 using a coating device equipped with a liquid composition tank that stores the resin varnish and a coating die. When the coating device is used, the resin varnish adheres to the outer peripheral surface of the conductor 2 as the conductor 2 passes through the liquid composition tank. Thereafter, the conductor 2 passes through the coating die, whereby the resin varnish is applied to a uniform thickness.

[0061] The resin varnish is, for example, a resin varnish containing the polyimide precursor, a pore-forming agent, and an organic solvent.

[0062] The organic solvent may be any known organic solvent conventionally used in resin varnishes for forming insulating layers of insulated electric wires. The organic solvent may be, for example, an aprotic solvent. An "aprotic solvent" is an organic solvent that does not have a group that releases a proton.

[0063] Examples of aprotic solvents include amide solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF); sulfur-containing solvents such as dimethyl sulfoxide; and lactone solvents such as γ-butyrolactone.

[0064] The lower limit of the solid content concentration of the resin varnish may be 15% by mass, 18% by mass, or 20% by mass. In this case, the amount of resin varnish required throughout the entire manufacturing process to achieve the desired thickness when forming the first insulating layer 3 using the resin varnish can be reduced. In addition, the number of coating steps and heating steps can be reduced. The upper limit of the solid content concentration of the resin varnish may be 40% by mass or 35% by mass. In this case, the viscosity of the resin varnish can be appropriately adjusted, thereby improving storage stability and coatability.

[0065] In the heating step, the resin varnish applied to the conductor 2 in the application step is heated. This heating volatilizes the organic solvent in the resin varnish and hardens the polyimide precursor, forming polyimide.

[0066] The device used in the heating step is not particularly limited, and for example, a cylindrical baking furnace that is long along the running direction of the conductor 2 can be used. The heating method is not particularly limited, and the heating can be performed by a conventionally known method such as hot air heating, infrared heating, or high-frequency heating.

[0067] The heating temperature can be, for example, 300° C. to 800° C. The heating time can be, for example, 5 seconds to 1 minute.

[0068] 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.

[0069] The process of forming the second insulating layer includes a process of applying a resin varnish for forming the second insulating layer to the outer surface of the first insulating layer (coating process), and a process of heating the resin varnish applied in the coating process (heating process).

[0070] The process for forming the second insulating layer can be carried out in the same manner as the process for forming the first insulating layer, except that a resin varnish for forming the second insulating layer is used instead of the resin varnish for forming the first insulating layer.

[0071] [Other Embodiments] 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 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 of the claims.

[0072] The insulated wire may have a configuration other than the 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, and melamine compounds.

[0073] 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 layer, a self-lubricating amide-imide layer, a polyimide layer, and a self-lubricating polyimide layer. The "outermost layer" refers to the layer located outermost in the laminate structure constituting the insulated wire, i.e., the layer farthest from the conductor.

[0074] The insulated wire may have, as its outermost layer, a bonding 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.

[0075] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0076] The abbreviations of the various components used in the examples are shown below: (aromatic tetracarboxylic dianhydride) PMDA: pyromellitic dianhydride (aromatic diamine) ODA: 4,4'-diaminodiphenyl ether (organic solvent) NMP: N-methyl-2-pyrrolidone DMAc: N,N-dimethylacetamide

[0077] <Preparation of Insulated Wire> [No. 1] (Preparation of Porous Insulating Layer-Forming Resin Varnish) 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, and a polyimide precursor solution containing NMP as the solvent was obtained (solid content: 24% by mass). Next, a pore-forming agent was added to the polyimide precursor solution in an amount such that the calculated porosity of the first insulating layer was 40% by volume, thereby preparing Porous Insulating Layer-Forming Resin Varnish No. 1. Core-shell particles with an average particle diameter of 3 μm (shell content: 15% by mass, shell thickness: 50 nm) having a PMMA particle core and a silicone shell were used as the pore-forming agent.

[0078] (Preparation of Resin Varnish for Filler Insulation Layer Formation) ODA as an aromatic diamine was dissolved in DMAc. 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 using DMAc as the solvent (solid content concentration: 24% by mass). Next, 20 parts by mass of surface-treated silica as inorganic fine particles was added to 100 parts by mass of the polyimide precursor to prepare Resin Varnish for Filler Insulation Layer Formation No. 1. Silica surface-treated with dimethyldichlorosilane was used as the surface-treated silica.

[0079] (Preparation of insulated wire) A rectangular copper wire was used as the conductor. The cross-sectional shape of the rectangular copper wire was a rectangle measuring 3 mm x 2 mm. The porous insulating layer-forming resin varnish No. 1 was applied to the surface of the conductor, and the resin varnish-coated conductor was heated repeatedly to form a first insulating layer with an average thickness of 21 μm. Furthermore, the filled insulating layer-forming resin varnish No. 1 was applied to the surface of each of the first insulating layers, and the heating process was repeated to form a second insulating layer with an average thickness of 49 μm, thereby producing insulated wire No. 1.

[0080] [No. 2 to No. 21] Insulated wires No. 2 to No. 21 were produced in the same manner as No. 1, except that the various components shown in Tables 1 to 3 below were used. In No. 5, hollow silica with an average particle diameter of 3 μm (shell film thickness: 50 nm) was used as the pore-forming agent. In No. 15, a condensation polymer of diphenylmethane-4,4'-diisocyanate and trimellitic anhydride was used as polyamideimide (PAI) instead of polyimide (PI). In Nos. 16 and 17, 20 parts by mass of alumina was used as inorganic fine particles per 100 parts by mass of polyimide precursor. In Nos. 18 and 19, 20 parts by mass of aluminum hydroxide was used as inorganic fine particles per 100 parts by mass of polyimide precursor. In Nos. 20 and 21, 20 parts by mass of aluminum hydroxide was used as inorganic fine particles per 100 parts by mass of polyimide precursor. In No. 21, 20 parts by mass of boehmite was used as inorganic fine particles relative to 100 parts by mass of the polyimide precursor.

[0081] <Evaluation> The dielectric constant, dielectric breakdown time, and coating elongation were measured for the prepared insulated wires No. 1 to No. 21 according to the following methods. The results are shown in Tables 1 and 2 below.

[0082] [Measurement of Relative Dielectric Constant] The relative dielectric constant of the insulating coating was measured for the insulated wires No. 1 to No. 21 prepared above. Measurement samples were prepared by applying silver paste to three locations on the surface of the insulated wire, and then peeling one end of the insulated wire to expose the conductor. The silver paste was applied along the longitudinal axis of the insulated wire in lengths of 10 mm, 100 mm, and 10 mm, respectively. Two 10 mm long strips of silver paste were grounded, and the capacitance between the 100 mm long strip of silver paste and the conductor was measured using an LCR meter. The relative dielectric constant of the insulating coating was calculated from the capacitance and the average thickness of the insulating coating. Three samples were heated at 105°C for 1 hour, and the relative dielectric constants of the three samples were measured and averaged.

[0083] [Measurement of dielectric breakdown time] The dielectric breakdown time was measured using the following procedure. A sample was prepared by bringing two insulated wires into close contact with each other over a length of 100 mm, with no gap between them, and then evaluated. The test conditions were as follows: (1) Waveform: Sine wave (2) Frequency: 50 kHz (3) Voltage: 2,000 V (4) Ambient temperature: 155°C In the durability test under the above test conditions, the elapsed time until dielectric breakdown occurred between the two insulated wires and a short circuit occurred was recorded.

[0084] [Measurement of Coating Elongation] For each of the insulated wires No. 1 to No. 21 prepared above, the conductor was removed from the insulated wire to form a tubular insulating layer, and a tensile test was performed using a tensile tester (Shimadzu Corporation's "Autograph AGS-X") at 25°C under conditions of a chuck distance of 20 mm and a tensile speed of 10 mm / min to measure the coating elongation (elongation at break) (unit: %).

[0085] In Tables 1 to 3 below, "porous insulating layer" means an insulating layer having pores, and "filled insulating layer" means an insulating layer having no pores.

[0086]

[0087]

[0088]

[0089] The results in Tables 1 to 3 show that insulated wires No. 1 to No. 10 and No. 16 to No. 21 have low relative dielectric constants and excellent surge resistance.

[0090] REFERENCE SIGNS LIST 1 insulated wire 2 conductor 3 first insulating layer 3a resin matrix 3b voids 4 second insulating layer

Claims

1. An insulated electric wire comprising: a conductor; a first insulating layer covering the conductor; and a second insulating layer covering the first insulating layer, wherein the first insulating layer has a resin matrix primarily composed of polyimide and a plurality of voids; the second insulating layer has a resin matrix primarily composed of polyimide; and the plurality of voids have an outer shell primarily composed of silicone or silica.

2. An insulated wire according to claim 1, wherein the ratio of the average thickness of the first insulating layer to the sum of the average thickness of the first insulating layer and the average thickness of the second insulating layer is 35% or more and 95% or less.

3. The insulated wire according to claim 1, wherein the porosity of the first insulating layer is 25% by volume or more and 60% by volume or less.

4. The insulated wire according to claim 1, wherein the average diameter of the plurality of pores is 0.5 μm or more and 10 μm or less.

5. The insulated wire according to claim 1, further comprising an adhesion layer between said conductor and said first insulating layer.

6. The insulated wire according to claim 1, wherein said second insulating layer further comprises inorganic fine particles.

7. The insulated wire according to claim 6, wherein the inorganic fine particles are surface-treated silica, alumina, aluminum hydroxide, or boehmite.

8. The insulated wire according to claim 1, which is a rectangular wire.

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