Insulated electric wire

Abstract

This insulated electric wire comprises: a conductor; a first insulating layer covering the conductor; and a second insulating layer covering the first insulating layer. The first insulating layer has: a resin matrix primarily comprising polyimide; and a plurality of holes. The second insulating layer has: a resin matrix primarily comprising polyimide; and silica particles. The ratio of the average thickness of the first insulating layer to the total of the average thickness of the first insulating layer and the average thickness of the second insulating layer is between 50% and 95%, inclusive. The porosity of the first insulating layer is between 25% and 60% by volume, inclusive. The silica particles are surface-treated with dimethyldichlorosilane or the like. The silica particle content in the second insulating layer is between 15 parts and 30 parts by mass, inclusive, with respect to 100 parts by mass of the resin matrix. The average primary particle size of the silica particles is between 0.008 μｍ and 0.04 μｍ, inclusive.

Description

insulated wire

[0001] This application claims priority to Japanese Patent Application No. 2024-134144, 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 mainly composed of polyimide and a plurality of pores, and the second insulating layer has a resin matrix mainly composed of polyimide and silica particles, 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 being 50% or more and 95% or less, the porosity of the first insulating layer being 25% by volume or more and 60% by volume or less, and the silica particles being silica particles. The surface is treated with methyldichlorosilane, hexamethyldisilazane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, or a combination of at least two of these, the content of silica particles in the second insulating layer is 15 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the resin matrix, and the average particle size of the primary particle diameter of the silica particles is 0.008 μm or more and 0.04 μm or less.

[0005] 1 is a schematic cross-sectional view showing an insulated wire according to one embodiment of the present disclosure, and FIG. 2 is a schematic cross-sectional view showing an insulated wire according to another embodiment of the present disclosure.

[0006] [Problem to be Solved by the Present Disclosure] In electrical equipment operating at high voltages, such as motors operated at high voltages, high surge voltages are applied to the insulated wires that make up the electrical equipment. This high surge voltage makes partial discharges (corona discharges) more likely to occur on the surface of the insulating coating of the insulated wire. Partial discharges can cause localized temperature increases, ozone generation, ion generation, and other problems, which can lead to early insulation breakdown and shorten the lifespan of the insulated wire and ultimately the electrical equipment. Therefore, there is a need to reduce the occurrence of partial discharges. One method for reducing partial discharges 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 of an insulated wire as one method for reducing the dielectric constant of the insulating coating.

[0007] The insulating coating of an insulated wire is also required to have excellent properties for reducing dielectric breakdown caused by 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 dielectric constant and excellent surge resistance.

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

[0010] [Description of the embodiments of the present disclosure] First, the embodiments of the present disclosure will be listed and described. Item 1. an insulating layer covering the 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 pores; the second insulating layer has a resin matrix mainly composed of polyimide and silica particles; 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 50% or more and 95% or less; the porosity of the first insulating layer is 25% by volume or more and 60% by volume or less; the silica particles have been surface-treated with dimethyldichlorosilane, hexamethyldisilazane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, or a combination of at least two of these; the content of the silica particles in the second insulating layer is 15 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the resin matrix; and the average primary particle diameter of the silica particles is 0.008 μm or more and 0.04 μm or less.

[0011] Item 2. The insulated wire according to Item 1, wherein the average diameter of the plurality of pores is 0.5 μm or more and 15 μm or less.

[0012] Item 3. The insulated wire according to Item 1 or 2, further comprising an adhesive layer between the conductor and the first insulating layer.

[0013] Item 4. The insulated wire according to any one of Items 1 to 3, which is a rectangular wire.

[0014] Item 5. The insulated wire according to any one of Items 1 to 4, wherein the polyimide contained in the second insulating layer is derived from a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and the weight-average molecular weight of the polyimide precursor is 25,000 or more and 80,000 or less.

[0015] Item 6. The insulated wire according to any one of Items 1 to 5, wherein the second insulating layer is formed from a resin varnish having a solid content concentration of 25% by mass or less.

[0016] Item 7. The insulated wire according to any one of Items 1 to 6, wherein the second insulating layer is formed from a resin varnish containing N,N-dimethylacetamide as an organic solvent, and the content of N,N-dimethylacetamide in the organic solvent is 70 mass% or more.

[0017] [Details of the embodiment of the present disclosure] Hereinafter, an insulated wire according to one aspect of the present disclosure will be described.

[0018] 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 3a and a plurality of pores 3b. The second insulating layer 4 has a resin matrix 4a and silica particles 4b.

[0019] The cross-sectional shape of the insulated wire 1 is not particularly limited, and examples thereof include a circular shape (round wire), an oval 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 rectangular, in other words, a flat wire. In this case, the insulated wire 1 can be wound at a high density during coil processing. The cross-sectional shapes of the insulated wire 1 and the conductor 2 (described later) are not particularly limited, but if they have the same type of shape, it is preferable because this makes it easier to adjust the thickness of the insulating layer. The cross-sectional shape of the insulated wire 1 is a cross-sectional shape that intersects (is perpendicular to) the longitudinal direction of the insulated wire 1.

[0020] The insulated wire 1 can be suitably used as a winding wire for a coil (magnet wire).

[0021] [Conductor] The cross-sectional shape of the conductor 2 may be, for example, a circle (round wire), an ellipse, a square, or a rectangle. When the insulated wire 1 is a rectangular wire, the cross-sectional shape of the conductor 2 may be a rectangle. The cross-sectional shape of the conductor 2 is a cross-sectional shape that intersects (is perpendicular to) the longitudinal direction of the conductor 2.

[0022] The material of the conductor 2 may be a metal with high conductivity and high mechanical strength. Examples of materials for the conductor 2 include copper, copper alloy, aluminum, nickel, silver, mild steel, steel, and stainless steel. The conductor 2 may be a wire-shaped metal material 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. High mechanical strength means, for example, high yield strength, tensile strength, and fatigue strength.

[0023] 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 first insulating layer 3 relative to the conductor 2 in the insulated wire 1 can be made appropriate, and the volume efficiency of a coil formed using the insulated wire 1 can be improved. The upper limit of the average cross-sectional area of ​​the conductor 2 is 40 mm ２ 30 mm ２ In this case, it is possible to avoid a decrease in the output efficiency of the coil due to an increase in copper loss caused by eddy currents. The average cross-sectional area of ​​the conductor 2 is the average value obtained by measuring the area of ​​a cross section intersecting (orthogonal to) the longitudinal direction of the conductor 2 three times over a 50 cm length along the longitudinal direction of the conductor 2.

[0024] [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.

[0025] The first insulating layer 3 contains a resin matrix 3 a and a plurality of voids 3 b. The presence of the plurality of voids 3 b can lower the dielectric constant of the insulating layer. This reduces the electric field strength in the gaps between adjacent insulated wires 1 and reduces the frequency of discharge, thereby lengthening the time until breakdown occurs (hereinafter referred to as breakdown time).

[0026] The average thickness of the first insulating layer 3 is not particularly limited and can usually be 2 μm or more and 300 μm or less. In this specification, the average thickness refers to the average thickness at 16 points on each cross section of three cross sections taken at 50 cm intervals in the longitudinal direction of the insulating layer (16 points x 3 surfaces = 48 points).

[0027] 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) is 50% or more and 95% or less. The lower limit of this ratio may be 55% or 60%. When this ratio is 55% or more, the dielectric constant of the insulated wire 1 can be further reduced. The upper limit of this ratio may be 90%, 85%, 80%, 75%, or 70%. When this ratio is 90% or less, the surge resistance of the insulated wire 1 can be further improved.

[0028] The porosity of the first insulating layer 3 is 25% by volume or more and 60% by volume or less. The lower limit of the porosity of the first insulating layer 3 may be 30% by volume. A porosity of 30% by volume or more of the first insulating layer 3 can further reduce the dielectric constant. The upper limit of the porosity of the first insulating layer 3 may be 55% by volume. A porosity of 55% by volume or less of the first insulating layer 3 can further improve the film elongation. "Porosity" refers to the percentage (unit: volume %) of the volume of the multiple pores 3b relative to the volume of the first insulating layer 3 having the resin matrix 3a and the multiple pores 3b. The porosity is specifically measured as follows. Using an electronic balance specific gravity measurement kit, the weight (W1) of the coating test piece in air and the weight (W2) in hexane are measured. The specific gravity (H) of the coating 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.

[0029] (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 by mass. In other words, the "main component of the resin matrix" refers to the component that is contained in the largest amount by mass among the components that make up the resin matrix 3a. Polyimide has excellent toughness and a low dielectric constant.

[0030] 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, resulting in a polyimide.

[0031] When the aromatic tetracarboxylic dianhydride contains pyromellitic dianhydride (PMDA), the heat resistance of the first insulating layer 3 can be 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 "other aromatic tetracarboxylic dianhydride").

[0032] Other aromatic tetracarboxylic dianhydrides include, for example, 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 (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.

[0033] When the other aromatic tetracarboxylic dianhydride is biphenyltetracarboxylic dianhydride (BPDA), the hydrolysis resistance of the polyimide can be improved.

[0034] The lower limit of the content 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 content of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 100 mol%, 90 mol%, 80 mol%, or 70 mol%.

[0035] 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 90 mol% or 80 mol%. The lower limit of the content may be 0 mol% or 10 mol%.

[0036] When the aromatic diamine contains diaminodiphenyl ether (ODA), the heat resistance of the first insulating layer 3 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 film elongation of the first insulating layer 3.

[0037] 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%.

[0038] The aromatic diamine may further contain an aromatic diamine other than ODA (hereinafter also referred to as "other aromatic diamine"). Examples of other aromatic diamines 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 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. One type of the other aromatic diamines may be used alone, or two or more types may be used in combination.

[0039] 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 first insulating layer 3 can be further reduced.

[0040] 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%.

[0041] The components and contents of aromatic tetracarboxylic dianhydride and aromatic diamine are determined by alkaline hydrolysis of the insulating film. １ It can be determined by analyzing with H-NMR (Proton Nuclear Magnetic Resonance). １ In H-NMR, the insulated wire can be analyzed.

[0042] 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. In this case, the molecular weight of the polyimide precursor can be easily increased. "Substantially equimolar amounts" 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.

[0043] 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 of the polyimide precursor may be 100,000 or 50,000. When the weight-average molecular weight of the polyimide precursor is equal to or greater than the lower limit, the film properties are improved. When the weight-average molecular weight of the polyimide precursor is equal to or less than the upper limit, the coatability of the resin varnish is improved. The weight-average molecular weight of the polyimide precursor indicates the weight-average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

[0044] A polyimide precursor can be obtained by a condensation polymerization reaction between an aromatic tetracarboxylic dianhydride and an aromatic diamine. The condensation polymerization reaction can be performed by a method similar to conventional methods for synthesizing polyimide precursors. A specific example of the condensation polymerization reaction is 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, resulting in 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 condensation polymerization reaction in the presence of a reaction inhibitor.

[0045] The reaction inhibitor may be, for example, water (H ２ Examples of the alcohol having from 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 the acid anhydride include aliphatic acid anhydrides such as phthalic anhydride, maleic anhydride, and succinic anhydride; and aromatic acid anhydrides such as trimellitic acid.

[0046] The reaction conditions for the polycondensation can be appropriately set depending on the raw materials used. 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.

[0047] Examples of the organic solvent used in the polycondensation reaction include the same organic solvents as those described below.

[0048] The resin matrix 3 a may contain components other than those described above. Examples of the other components include additives that are blended into the insulating coating of the insulated wire 1. Specific examples of the other components include fillers, antioxidants, radical scavengers, ultraviolet inhibitors, surface lubricants, plasticizers, leveling agents, curing agents, and adhesion promoters.

[0049] (Poles) The plurality of pores 3b are formed by using a pore-forming agent.

[0050] The pore-forming agent can be any known additive used to form an insulating coating having pores, and examples of the pore-forming agent include chemical foaming agents, thermally expandable microcapsules, particles containing thermally decomposable resins, and high-boiling-point solvents.

[0051] When the pore-forming agent is a thermally decomposable resin-containing particle, an insulating coating having a smooth outer surface and a good appearance can be formed on the insulated electric wire 1 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 locations where the thermally decomposable resin-containing particle was present. In this case, the thermally decomposable resin-containing particle can be uniformly distributed as an island phase of fine particles in the sea phase of the resin matrix that constitutes the insulating coating, and unique pores, i.e., independent pores, are formed in the insulating coating without connecting with each other.

[0052] The thermally decomposable resin contained in the thermally decomposable resin-containing particles 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 that constitutes the insulating coating. The baking temperature of the polyimide 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" manufactured by SII NanoTechnology, Inc.).

[0053] Examples of thermally decomposable resins include compounds such as polyethylene glycol and polypropylene glycol in which one, both, or a portion of one end is 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; polymers of modified (meth)acrylates, such as urethane oligomers, urethane polymers, urethane(meth)acrylates, epoxy(meth)acrylates, 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 of polyimide, easily forming voids in the insulating coating. Examples of (meth)acrylic acid ester polymers include polymethyl methacrylate (PMMA). The term "(meth)acrylic acid" is a general term for "acrylic acid" and "methacrylic acid" and includes either one or both of them. The term "(meth)acrylate" is a general term for "acrylate" and "methacrylate" and includes either one or both of them.

[0054] The thermally decomposable resin-containing particles may be particles consisting only of a thermally decomposable resin, or may be particles with a core-shell structure having a core mainly composed of a thermally decomposable resin and a shell mainly 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, and the shell remains on the outer periphery of these pores. Particles with a core-shell structure can reduce the interconnection of pores and reduce the variation in pore size.

[0055] The main component of the shell is not particularly limited as long as it has a higher thermal decomposition temperature than the core, and may be a synthetic resin with a low dielectric constant and high heat resistance. Examples include polystyrene, silicone, fluororesin, and polyimide. In particular, when the main component of the shell is silicone, it is easy to increase elasticity, which in turn improves the dispersion of pores in the insulating coating, resulting in excellent insulating properties and heat resistance.

[0056] When the pore-forming agent is a high-boiling-point solvent, the viscosity of the resin varnish can be reduced, resulting in a resin varnish with excellent workability. When the high-boiling-point solvent evaporates after phase separation between the polyimide precursor and the high-boiling-point solvent, pores are formed in the areas of the insulating coating where the high-boiling-point solvent was previously present. In this case, the islands of fine particles can be uniformly distributed in the sea phase of the resin matrix that constitutes the insulating coating, forming independent pores.

[0057] When the high-boiling point solvent is an aliphatic polycarboxylic acid ester, the interconnection of pores can be reduced, and the variation in pore size can be reduced. "Aliphatic polycarboxylic acid ester" means an ester derived from an aliphatic polycarboxylic acid. "Aliphatic polycarboxylic acid" means an aliphatic carboxylic acid having two or more carboxy groups. "Carboxylic acid" means a compound having a carboxy group, and is not limited to carboxylic acids in the narrow sense, but also includes carboxylic acids in the broad sense, such as hydroxycarboxylic acids.

[0058] Aliphatic polycarboxylic acid esters can be classified according to the number of carboxy groups, and include, for example, aliphatic dicarboxylic acid esters, aliphatic tricarboxylic acid esters, and aliphatic tetracarboxylic acid esters. When the aliphatic polycarboxylic acid ester is an aliphatic dicarboxylic acid ester or an aliphatic tricarboxylic acid ester, the molecular weight is smaller and the boiling point and thermal decomposition temperature are lower than those of aliphatic polycarboxylic acids having four or more carboxy groups (e.g., aliphatic tetracarboxylic acid esters), making it possible to reduce the amount of residue in the insulating layer. When the aliphatic tricarboxylic acid ester is used, the amount of residue in the insulating layer can be further reduced. By reducing the amount of residue in the insulating layer, the dielectric constant of the insulating layer can be reduced.

[0059] Examples of aliphatic dicarboxylic acid esters include dibutyl fumarate, dibutyl succinate, diethyl sebacate, diisobutyl adipate, dibutyl adipate (DBA), dibutyl sebacate, and di(2-butoxyethyl) adipate. Examples of aliphatic tricarboxylic acid esters include citric acid esters such as triethyl citrate, O-acetyl triethyl citrate, tributyl citrate (TBC), and O-acetyl tributyl citrate. When comparing aliphatic dicarboxylic acid esters and citric acid esters, if the total number of carbon atoms is the same, citric acid esters tend to be able to form insulating layers with higher porosity. When the citric acid ester is tributyl citrate (TBC), the dielectric constant of the insulating layer can be further reduced.

[0060] The lower limit of the average diameter of the plurality of pores 3b may be 0.1 μm, 0.5 μm, 0.8 μm, or 1 μm, and the upper limit of the average diameter of the plurality of pores 3b may be 20 μm, 15 μm, 10 μm, or 8 μm.

[0061] The average diameter of the plurality of pores 3b can be determined by observing the cross section of the insulated wire 1 using a scanning electron microscope (SEM) manufactured by ZEISS under conditions of an acceleration voltage of 1.5 kV and a magnification of 40,000 times, 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 diameters of the 30 pores.

[0062] [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. 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 the application and baking of a resin varnish are repeated multiple times), the second insulating layer 4 has a laminate structure composed of multiple layers formed using the resin varnish.

[0063] The second insulating layer 4 includes a resin matrix 4a mainly composed of polyimide and silica particles 4b. Polyimide has excellent toughness and a low dielectric constant. The silica particles 4b have high insulating properties and a low specific gravity, so they can be stably dispersed in the polyimide precursor solution without settling.

[0064] The second insulating layer 4 containing the silica particles 4b can effectively prevent deterioration due to partial discharge that progresses from the surface of the insulated wire 1, thereby improving surge resistance.

[0065] The resin matrix 4a is the same as that described in the section on the first insulating layer. The polyimide, which is the main component of the resin matrix 4a of the second insulating layer 4, is derived from a polyimide precursor, which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine. The weight-average molecular weight of the polyimide precursor may be 25,000 or more and 80,000 or less. When the weight-average molecular weight of the polyimide precursor is 25,000 or more, the film properties are improved. Resistance to molecular chain scission caused by partial discharge is increased, thereby extending the time to dielectric breakdown. When the weight-average molecular weight of the polyimide precursor is 80,000 or less, the coatability of the resin varnish is improved. The weight-average molecular weight of the polyimide precursor is the weight-average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

[0066] The silica particles 4b are surface-treated silica particles that have been surface-treated with dimethyldichlorosilane, hexamethyldisilazane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, or a combination of at least two of these. This allows the silica particles to be uniformly dispersed in the resin matrix, further improving surge resistance. The surface-treated silica particles refer to silica particles in which some or all of the silanol groups on the silica particle surface have been chemically reacted.

[0067] The surface treatment agent of the silica particles 4b can be identified by solid-state Si-NMR, pyrolysis gas chromatography mass spectrometry, or Fourier transform infrared spectrometry.

[0068] The content of silica particles 4b in the second insulating layer 4 is 15 parts by mass or more and 30 parts by mass or less relative to 100 parts by mass of the resin matrix. The lower limit of the content of silica particles 4b may be 16 parts by mass, 17 parts by mass, or 18 parts by mass. The upper limit of the content of silica particles 4b may be 29 parts by mass, 28 parts by mass, 27 parts by mass, or 26 parts by mass. When the content of silica particles 4b is within the above range, the surge resistance and elongation of the insulating layer can be improved. The content of silica particles 4b in the second insulating layer 4 can be determined by thermally decomposing polyimide in the second insulating layer 4 at 750°C and measuring the weight of the silica particle residue.

[0069] The average particle size of the primary particles of the silica particles 4b is 0.008 μm or more and 0.04 μm or less. If the average particle size of the primary particles of the silica particles 4b is less than 0.008 μm, aggregation of the surface-treated silica particles is likely to occur, which may result in a decrease in the surge resistance and elongation of the insulating layer. When the average particle size of the primary particles of the silica particles 4b is 0.008 μm or more, the surge resistance and elongation of the insulating layer can be improved. When the average particle size of the primary particles of the silica particles 4b is 0.04 μm or less, the surge resistance of the insulating layer can be maintained at a good level. The shape of the silica particles 4b is not particularly limited and may be, for example, granular, spherical, scaly, acicular, or plate-like. The average particle diameter of the primary particles of the silica particles 4b can be determined by observing a cross section of the insulated electric wire 1 using a scanning electron microscope (SEM) manufactured by ZEISS under conditions of an acceleration voltage of 1.5 kV and a magnification of 40,000 times, measuring the particle diameters of any 50 primary particles of the silica particles 4b on the SEM image using "ImageJ," an open-source image processing software, and calculating the average value (average particle diameter) of the particle diameters of the measured 50 primary particles.

[0070] The average thickness of the second insulating layer 4 is not particularly limited, and can usually be set to 2 μm or more and 300 μm or less.

[0071] The second insulating layer 4 may further contain inorganic fine particles other than the silica particles 4b. Examples of the inorganic fine particles include alumina, titania, aluminum hydroxide, magnesia, boehmite, beryllium oxide, silicon carbide, titanium carbide, boron carbide, tungsten carbide, boron nitride, and silicon nitride.

[0072] The second insulating layer 4 is preferably formed from a resin varnish having a solid content concentration of 25% by mass or less. By setting the solid content concentration to 25% by mass or less, aggregation of silica particles in the resin varnish is reduced and prevented, improving storage stability.

[0073] The second insulating layer 4 is formed from a resin varnish containing N,N-dimethylacetamide as an organic solvent, and the content of N,N-dimethylacetamide in the organic solvent is preferably 70 mass % or more. In this case, the organic solvent is likely to be completely dried before the shape of the resin varnish applied to the conductor 2 changes, thereby improving the uniformity of the thickness.

[0074] [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.

[0075] 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).

[0076] In the coating process, the resin varnish is applied to the outer peripheral surface of the conductor 2. For example, a method of applying the resin varnish to the outer peripheral surface of the conductor 2 may use a coating device equipped with a liquid composition tank that stores the resin varnish and a coating die. With this coating device, the conductor 2 passes through the liquid composition tank, causing the resin varnish to adhere to the outer peripheral surface of the conductor 2, and then passes through the coating die, thereby applying the resin varnish to a uniform thickness.

[0077] The resin varnish may be, for example, a resin varnish containing a polyimide precursor, a pore-forming agent, and an organic solvent.

[0078] The organic solvent may be any known organic solvent that has been conventionally used in resin varnishes for forming insulating layers of insulated electric wires. For example, an aprotic solvent may be used. The term "aprotic solvent" refers to an organic solvent that does not have a group that releases a proton.

[0079] 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.

[0080] 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. 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.

[0081] 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, hardens the polyimide precursor, and forms polyimide.

[0082] The device used in the heating step is not particularly limited, and for example, a cylindrical baking furnace that is long in 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.

[0083] The heating temperature can be, for example, 300° C. to 800° C. The heating time can be, for example, 5 seconds to 1 minute.

[0084] The coating and heating steps 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.

[0085] 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 3 (coating process), and a process of heating the resin varnish applied in the coating process (heating process).

[0086] 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.

[0087] <Insulated Wire According to Other Embodiments> The insulated wire may have a different configuration. The insulated wire 10 shown in FIG. 2 includes a conductor 20, a first insulating layer 30 covering the conductor 20, a second insulating layer 40 covering the first insulating layer 30, and an adhesion layer 50 between the conductor 20 and the first insulating layer 30. The first insulating layer 30 has a resin matrix 30a and a plurality of pores 30b. The second insulating layer 40 has a resin matrix 40a and silica particles 40b. For example, the insulated wire 10 may include an adhesion layer 50 between the conductor 20 and the first insulating layer 30, the adhesion layer 50 containing an additive such as an adhesion improver. Examples of the adhesion improver include mercaptans such as 2-mercaptoimidazole and 5-amino-1,3,4-thiadiazole-2-thiol, and melamine compounds.

[0088] The insulated wires 1 and 10 may have a surface friction adjusting layer as the 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 wires 1 and 10 when the conductor is positioned inside.

[0089] The insulated wires 1 and 10 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.

[0090] The present invention will be described in more detail below with reference to experimental examples, but the present invention is not limited to these experimental examples.

[0091] 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

[0092] <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, yielding a polyimide precursor solution (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.

[0093] (Preparation of Resin Varnish for Forming Filler Insulation Layer) ODA as an aromatic diamine was dissolved in an organic solvent (NMP:DMAc = 10:90). 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, yielding a polyimide precursor solution (solid content concentration: 24% by mass). Next, 20 parts by mass of surface-treated silica particles were added to 100 parts by mass of the polyimide precursor to prepare Resin Varnish No. 1 for forming a filler insulation layer. As the surface-treated silica particles, silica particles surface-treated with dimethyldichlorosilane (average primary particle diameter: 0.016 μm) were used.

[0094] (Preparation of insulated wire) A rectangular copper wire with a cross section of 3 mm x 2 mm was used as the rectangular conductor. The conductor surface was coated with resin varnish No. 1 for forming a porous insulating layer, and the conductor coated with the resin varnish was repeatedly heated to form a first insulating layer with an average thickness of 46 μm. The filled insulating layer-forming resin varnish No. 1 was then coated on the surface of each first insulating layer, and the heating process was repeatedly performed to form a second insulating layer with an average thickness of 24 μm, thereby producing insulated wire No. 1.

[0095] [No. 2 to No. 26] Insulated wires No. 2 to No. 26 were produced in the same manner as No. 1, except that the various components shown in Tables 1 to 6 below were used.

[0096] <Evaluation> The dielectric constant, eccentricity, time to dielectric breakdown, and coating elongation were measured for the prepared insulated wires No. 1 to No. 26 according to the following methods. The results are shown in Tables 1 to 6 below.

[0097] [Measurement of Relative Dielectric Constant] The relative dielectric constant of the insulating coating was measured for the prepared insulated wires No. 1 to No. 26. First, silver paste was applied to three locations on the surface of each insulated wire, and one end of the insulated wire was peeled off to expose the conductor. Measurement samples were prepared. 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. 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.

[0098] [Measurement of Eccentricity] The thickness uniformity of the insulating layer was evaluated by the eccentricity of the insulating layer. The eccentricity of the insulating layer was measured for the insulated wires No. 1 to No. 26 produced above. The eccentricity of the insulating layer is expressed as the ratio of the maximum thickness to the minimum thickness of the insulating layer of each insulated wire, as shown in the following formula 3. The smaller the eccentricity of the insulating layer, the better the thickness uniformity of the insulating layer. Formula 3: Eccentricity of insulating layer = Maximum thickness of insulating layer / Minimum thickness of insulating layer

[0099] [Measurement of dielectric breakdown time] The dielectric breakdown time was measured using the following procedure. A sample was prepared by closely contacting the surfaces of two insulated wires over a length of 100 mm, leaving no gap between them, and evaluation was performed. 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.

[0100] [Measurement of Coating Elongation] For the prepared insulated wires No. 1 to No. 26, the conductors were removed from the insulated wires to form tubular insulating layers, which were then subjected to a tensile test 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: %).

[0101] In Tables 1 to 6 below, "porous insulating layer" means an insulating layer having pores, and "filled insulating layer" means an insulating layer having no pores.

[0102]

[0103]

[0104]

[0105]

[0106]

[0107]

[0108] The results in Tables 1 to 6 show that insulated wires No. 1 to No. 11 and No. 26 have low dielectric constants and excellent surge resistance.

[0109] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. 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 equivalent to the claims.

[0110] REFERENCE SIGNS LIST 1, 10 Insulated wire 2, 20 Conductor 3, 30 First insulating layer 3a, 30a Resin matrix 3b, 30b Hole 4, 40 Second insulating layer 4a, 40a Resin matrix 4b, 40b Silica particles 50 Adhesion layer

Claims

1. A conductive material 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 pores; the second insulating layer has a resin matrix mainly composed of polyimide and silica particles; 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 50% or more and 95% or less; the porosity of the first insulating layer is 25% by volume or more and 60% by volume or less; the silica particles are surface-treated with dimethyldichlorosilane, hexamethyldisilazane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, or a combination of at least two of these; and the content of the silica particles in the second insulating layer is 15 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the resin matrix. The insulated wire has an average primary particle size of the silica particles of 0.008 μm or more and 0.04 μm or less.

2. The insulated wire according to claim 1, wherein the average diameter of the plurality of pores is 0.5 μm or more and 15 μm or less.

3. The insulated wire according to claim 1 or 2, further comprising an adhesive layer between the conductor and the first insulating layer.

4. The insulated wire according to any one of claims 1 to 3, which is a rectangular wire.

5. An insulated wire according to any one of claims 1 to 4, wherein the polyimide contained in the second insulating layer is derived from a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and the weight-average molecular weight of the polyimide precursor is 25,000 or more and 80,000 or less.

6. An insulated wire according to any one of claims 1 to 5, wherein the second insulating layer is formed from a resin varnish having a solid content concentration of 25% by mass or less.

7. An insulated wire according to any one of claims 1 to 6, wherein the second insulating layer is formed from a resin varnish containing N,N-dimethylacetamide as an organic solvent, and the content of N,N-dimethylacetamide in the organic solvent is 70 mass% or more.

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