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

Abstract

An insulated wire according to one embodiment of the present disclosure comprises a conductor and an insulating film that covers the conductor. The insulating film has a resin matrix and a plurality of holes. The main component of the resin matrix is a polyimide derived from a polyimide precursor which is a reaction product of an aromatic tetracarboxylic acid dianhydride and an aromatic diamine. The resin matrix contains an aliphatic polyvalent carboxylic acid ester.

Description

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

[0001] This disclosure relates to a resin composition, an insulated wire, and a method for producing an insulated wire. This application claims priority to Japanese Application No. 2024-054791, filed March 28, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] Patent Document 1 describes an insulated wire having a heat-cured insulating varnish film. The insulating varnish contains a coating resin and a heat-decomposable resin that decomposes at a temperature lower than the baking temperature of the coating resin. 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 embodiment of the present disclosure includes a conductor and an insulating coating covering the conductor. The insulating coating has a resin matrix and a plurality of pores. The resin matrix is ​​primarily composed of a polyimide derived from a polyimide precursor, which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine. The resin matrix contains an aliphatic polycarboxylic acid ester.

[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 with high operating voltages, such as motors operated at high voltages, high voltages are applied to the insulated wires included 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 lifespan of the insulated wires 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] The problem to be solved by the present disclosure is to provide an insulated wire with a low relative dielectric constant.

[0008] The insulated wire according to one aspect of the present disclosure has a low dielectric constant.

[0009] First, embodiments of the present disclosure will be listed and described. (1) An insulated electric wire according to one embodiment of the present disclosure includes a conductor and an insulating coating covering the conductor. The insulating coating has a resin matrix and a plurality of pores. The resin matrix is ​​mainly composed of a polyimide derived from a polyimide precursor, which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine. The resin matrix contains an aliphatic polycarboxylic acid ester. (2) In the insulated electric wire described in (1) above, the content of the aliphatic polycarboxylic acid ester in the resin matrix may be 800 ppm or less. (3) In the insulated electric wire described in (1) or (2) above, the aliphatic polycarboxylic acid ester may be an aliphatic tricarboxylic acid ester. (4) In the insulated electric wire described in (3) above, the aliphatic tricarboxylic acid ester may be a citric acid ester. (5) In the insulated electric wire described in (4) above, the citric acid ester may be tributyl citrate. (6) In the insulated wire according to any one of (1) to (5), the flattening ratio of the plurality of pores may be less than 2.00. (7) In the insulated wire according to any one of (1) to (6), the average diameter of the plurality of pores may be less than 2.45 μm. (8) In the insulated wire according to any one of (1) to (7), the porosity of the insulating coating may be 20% by volume or more.

[0010] Hereinafter, an insulated wire according to one embodiment of the present disclosure will be described.

[0011] 1 includes a conductor 2 and an insulating coating 3 that covers the conductor 2. The insulating coating 3 contains a resin matrix 4 and a plurality of pores 5.

[0012] The cross-sectional shape of the insulated electric wire 1 perpendicular to the longitudinal axis 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 shape.

[0013] The insulated wire 1 can be suitably used as a winding wire for a coil (magnet wire).

[0014] [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.

[0015] The conductor 2 may be made of a metal having high electrical conductivity and high mechanical strength. Examples of such metals include copper, copper alloy, 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 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.

[0016] 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 If the average cross-sectional area of ​​the conductor 2 is equal to or greater than 20 mm, the volume of the insulating coating 3 relative to the conductor 2 in the insulated wire 1 can be adjusted appropriately, and the volume efficiency of the coil formed using the insulated wire 1 can be improved. ２ 10 mm ２ The average cross-sectional area of ​​the conductor 2 may be 20 mm ２ If the dielectric constant is equal to or less than this, it is not necessary to form the insulating coating 3 thick in order to sufficiently reduce the relative dielectric constant, and the diameter of the insulated wire 1 does not become unnecessarily large.

[0017] [Insulating Coating] The insulating coating 3 is laminated on the outer peripheral surface of the conductor 2 so as to cover the conductor 2. The insulating coating 3 is composed of one or more layers. For example, when the insulating coating 3 is formed by the method described below (a method in which a resin varnish is applied and baked multiple times), the insulating coating 3 has a laminated structure composed of multiple layers formed using the resin varnish.

[0018] The insulating coating 3 contains a resin matrix 4 and a plurality of pores 5 .

[0019] The average thickness of the insulating coating 3 is not particularly limited, and is, for example, 2 μm or more and 300 μm or less.

[0020] The porosity of the insulating coating 3 may be 20% by volume or more and 60% by volume or less. When the porosity of the insulating coating 3 is 20% by volume or more, the relative dielectric constant of the insulating coating can be further reduced. The lower limit of the porosity of the insulating coating 3 may be 25% by volume, 30% by volume, or 40% by volume. The upper limit of the porosity of the insulating coating 3 may be 60% by volume, 50% by volume, or 45% by volume. "Porosity" refers to the percentage (unit: vol%) of the volume of pores relative to the volume of the insulating coating 3. The porosity is specifically measured by the following method. 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 4.

[0021] (Resin Matrix) The main component of the resin matrix 4 is a polyimide derived from a polyimide precursor, which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine. 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 4.

[0022] Polyimides are derived from polyimide precursors, which are reaction products of aromatic tetracarboxylic dianhydrides and aromatic diamines. The polyimide precursors are reaction products obtained by condensation polymerization of aromatic tetracarboxylic dianhydrides and aromatic diamines. The polyimide precursors are also called polyamic acids (polyamic acids). The polyimide precursors undergo cyclodehydration (imidization) to form cyclic imides, resulting in polyimides.

[0023] When the aromatic tetracarboxylic dianhydride contains pyromellitic dianhydride (PMDA), the heat resistance of the insulating coating 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").

[0024] 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(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, or 2,3,6,7-naphthalenetetracarboxylic acid dianhydride. The other aromatic tetracarboxylic acid dianhydrides may be used alone or in combination of two or more.

[0025] When the other aromatic tetracarboxylic dianhydride is biphenyltetracarboxylic dianhydride (BPDA), the hydrolysis resistance of the polyimide precursor can be improved.

[0026] 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%.

[0027] The content of the other aromatic tetracarboxylic dianhydride relative to 100 mol% of the aromatic tetracarboxylic dianhydride can be set appropriately as long as the effects of the present disclosure are not impaired. The upper limit of the content may be 30 mol% or 20 mol%. The lower limit of the content may be 0 mol% or 10 mol%.

[0028] When the aromatic diamine contains diaminodiphenyl ether (ODA), the heat resistance of the insulating coating 3 is improved. This is because ODA has a rigid and linear molecular structure. Examples of diaminodiphenyl ether 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 insulating coating 3 can be improved.

[0029] 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%.

[0030] 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'-diaminodiphenylmethane ... The other aromatic diamines may be used alone or in combination of two or more thereof.

[0031] If the other aromatic diamine is 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) or 4,4′-bis(4-aminophenoxy)biphenyl (BAPB), the relative dielectric constant of the insulating coating 3 can be further reduced.

[0032] The content of the other aromatic diamine relative to 100 mol% of the aromatic diamine can be set appropriately 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%.

[0033] From the viewpoint of ease of synthesis of the polyimide precursor, the molar ratio of aromatic tetracarboxylic dianhydride to aromatic diamine (aromatic tetracarboxylic dianhydride:aromatic diamine) used as raw materials for the polyimide precursor may be 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. In particular, a ratio of 99:101 or more and 101:99 or less allows for easy increase in the molecular weight of the polyimide precursor.

[0034] 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 50,000. 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.

[0035] The polyimide precursor can be obtained by a condensation polymerization reaction between an aromatic tetracarboxylic dianhydride and an 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 polymerizes the aromatic tetracarboxylic dianhydride and the aromatic diamine, thereby obtaining a solution in which the polyimide precursor is dissolved in the organic solvent. For example, the degree of polymerization (weight average molecular weight) can be controlled by adding a reaction inhibitor to perform the condensation polymerization reaction.

[0036] The reaction control agent is, for example, water (H ２ 0), alcohols having 1 to 15 carbon atoms, or acid anhydrides. 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.

[0037] The condensation polymerization reaction conditions 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.

[0038] The organic solvent used in the condensation polymerization reaction is, for example, the organic solvent described below.

[0039] The resin matrix 4 contains an aliphatic polycarboxylic acid ester. When the insulating coating 3 is formed using a resin varnish containing a polyimide precursor, an aliphatic polycarboxylic acid ester, and an organic solvent, it is believed that the polyimide precursor and the aliphatic polycarboxylic acid ester undergo phase separation to form the plurality of voids 5.

[0040] The term "aliphatic polycarboxylic acid ester" refers to an ester derived from an aliphatic polycarboxylic acid. The term "aliphatic polycarboxylic acid" refers to an aliphatic carboxylic acid having two or more carboxy groups. The term "carboxylic acid" refers to a compound having a carboxy group, and includes not only carboxylic acids in the narrow sense but also carboxylic acids in the broad sense such as hydroxycarboxylic acids.

[0041] Aliphatic polycarboxylic acid esters are classified by the number of carboxy groups. Examples of aliphatic polycarboxylic acid esters include 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, it has a smaller molecular weight, a lower boiling point, and a lower thermal decomposition temperature than aliphatic polycarboxylic acids having four or more carboxy groups (e.g., aliphatic tetracarboxylic acid esters). Therefore, the amount of residual aliphatic polycarboxylic acid ester in the insulating coating 3 can be reduced. When the aliphatic polycarboxylic acid ester is an aliphatic tricarboxylic acid ester, the amount of residual aliphatic polycarboxylic acid ester in the insulating coating 3 can be further reduced. By reducing the amount of residual aliphatic polycarboxylic acid ester in the insulating coating 3, the dielectric constant of the insulating coating 3 can be reduced.

[0042] Examples of the aliphatic dicarboxylic acid ester include dibutyl fumarate, dibutyl succinate, diethyl sebacate, diisobutyl adipate, dibutyl adipate (DBA), dibutyl sebacate, and di(2-butoxyethyl) adipate. Examples of the aliphatic tricarboxylic acid ester include citric acid esters such as triethyl citrate, O-acetyl triethyl citrate, tributyl citrate (TBC), and O-acetyl tributyl citrate. When the total number of carbon atoms in the aliphatic dicarboxylic acid ester and the total number of carbon atoms in the citric acid ester are the same, an insulating coating 3 with a high porosity is more likely to be formed when the resin matrix 4 contains a citric acid ester than when the resin matrix 4 contains an aliphatic dicarboxylic acid ester. When the citric acid ester is tributyl citrate (TBC), the relative dielectric constant of the insulating coating 3 can be further reduced.

[0043] The upper limit of the content of the aliphatic polycarboxylic acid ester in the resin matrix 4 may be 8000 ppm, 7000 ppm, 6000 ppm, 5000 ppm, 3000 ppm, 1000 ppm, 800 ppm, 500 ppm, 200 ppm, 100 ppm, 50 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, or 1 ppm. The lower limit of the content is not particularly limited as long as it is an amount that can confirm the presence of the aliphatic polycarboxylic acid ester, and may be, for example, the detection limit or 0.1 ppm. "ppm" means ppm by mass.

[0044] When the content of the aliphatic polycarboxylic acid ester in the resin matrix 4 is 800 ppm or less, the adhesion between the conductor 2 and the insulating coating 3 can be improved.

[0045] The content of the aliphatic polycarboxylic acid ester in the resin matrix 4 can be adjusted by adjusting, for example, the type of aliphatic polycarboxylic acid ester, the heating temperature or heating time in the method for producing the insulated electric wire 1 described later, the content of the aliphatic polycarboxylic acid ester in the resin varnish, or two or more of these conditions.

[0046] The resin matrix 4 may contain other components in addition to the above components. The other components are, for example, additives to be blended into the insulating coating 3 of the insulated wire 1. Specifically, the other components are, for example, fillers, antioxidants, leveling agents, curing agents, or adhesion aids.

[0047] (Pores) As described above, the plurality of pores 5 are considered to be formed by phase separation between the polyimide precursor and the aliphatic polycarboxylic acid ester. The plurality of pores 5 are uniformly distributed within the resin matrix 4.

[0048] The shape of the plurality of pores 5 may be spherical or flat. If the pores 5 have a flat shape, the plurality of pores 5 are less likely to come into contact with each other, and the pores 5 can more easily remain independent pores.

[0049] The lower limit of the flattening ratio of the plurality of voids 5 is not particularly limited and may be 1.50, 1.70, or 1.80. The upper limit of the flattening ratio may be 2.00, 1.90, or 1.86. The flattening ratio of the plurality of voids may be less than 2.00. If the flattening ratio is less than 2.00, the breakdown voltage (BDV) during bending is less likely to decrease. "Flattening ratio" refers to the ratio of the major axis of the voids 5 to the minor axis of the voids 5. The major axis (length of the major axis) of the voids 5 is the maximum diagonal length passing through the center of gravity of the cross-sectional shape of the voids 5. The minor axis (length of the minor axis) of the voids 5 is the minimum diagonal length passing through the center of gravity of the cross-sectional shape of the voids 5. Since the shape and size of the voids 5 are often not uniform, the major axis and minor axis of the voids 5 are determined as follows. First, a cross section of the insulating coating 3 is observed with a scanning electron microscope (SEM). Next, the open-source image processing software "ImageJ" is used to measure the major and minor axes of 100 or more pores 5 in the observed image. The average of these major axes is the "major axis of pores 5," and the average of these minor axes is the "minor axis of pores 5."

[0050] The lower limit of the major axis of the pores 5 is not particularly limited, but may be 1.40 μm, 1.50 μm, or 1.90 μm. The upper limit of the major axis of the pores 5 may be 5.00 μm, 3.50 μm, or 3.00 μm.

[0051] The lower limit of the minor axis of the pores 5 is not particularly limited, but may be 0.80 μm or 1.00 μm. The upper limit of the minor axis of the pores 5 may be 2.00 μm, 1.80 μm, or 1.50 μm.

[0052] When the major axis of the pores 5 is 5.00 μm or less and the minor axis is 2.00 μm or less, the breakdown voltage (BDV) during bending is less likely to decrease.

[0053] The upper limit of the average diameter of the plurality of pores 5 may be 3.00 μm, 2.50 μm, 2.45 μm, 2.00 μm, or 1.50 μm. The average diameter of the plurality of pores 5 may be less than 2.45 μm. When the average diameter is less than 2.45 μm, the breakdown voltage (BDV) during bending is less likely to decrease, and the adhesion between the conductor 2 and the insulating coating 3 is improved. The lower limit of the average diameter is not particularly limited and is, for example, 0.10 μm. The lower limit of the average diameter may be 1.10 μm or 1.40 μm. The "average diameter" is (major diameter + minor diameter) / 2.

[0054] The insulated wire 1 may include components other than those described above. For example, the insulated wire 1 may have an adhesion layer containing an additive such as an adhesion improver between the conductor 2 and the insulating coating 3. Examples of the adhesion improver include mercaptans such as 2-mercaptoimidazole and 5-amino-1,3,4-thiadiazole-2-thiol.

[0055] The insulated wire 1 may have a surface friction adjusting layer as its outermost layer. The surface friction adjusting layer may be, for example, a polyamide-imide layer, a self-lubricating amide-imide layer, a polyimide layer, or a self-lubricating polyimide layer. The "outermost layer" refers to the layer located outermost in the laminate structure constituting the insulated wire 1, i.e., the layer farthest from the conductor 2.

[0056] The insulated wire 1 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.

[0057] The insulated wire 1 may have an anti-surge layer containing an inorganic filler as its outermost layer. The anti-surge layer may be, for example, a silica layer, an alumina layer, a magnesia layer, a beryllium oxide layer, a silicon carbide layer, a titanium carbide layer, a boron carbide layer, a tungsten carbide layer, a boron nitride layer, or a silicon nitride layer. The inorganic filler may be surface-treated. The surface treatment agent may be, for example, a silane coupling agent.

[0058] [Method for manufacturing insulated wire] The insulated wire 1 can be manufactured by a method including a step of applying a resin varnish to the outer peripheral surface of the conductor 2 (coating step) and a step of heating the resin varnish applied in the coating step (heating step).

[0059] In the coating process, 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 this coating device is used, the conductor 2 passes through the liquid composition tank, so that the resin varnish adheres to the outer peripheral surface of the conductor 2. The conductor 2 then passes through the coating die, so that the applied resin varnish has a uniform thickness.

[0060] The resin varnish is, for example, a resin varnish containing a polyimide precursor, an aliphatic polycarboxylic acid ester, and an organic solvent.

[0061] The organic solvent may be any known organic solvent conventionally used in resin varnishes for forming insulating films on insulated 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.

[0062] 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.

[0063] The lower limit of the solid content concentration of the resin varnish may be 15% by mass, 20% by mass, or 30% by mass. When forming the insulating coating 3 using the resin varnish, it is necessary to achieve the desired thickness of the insulating coating 2. If the solid content concentration of the resin varnish is 15% by mass or more, the amount of resin varnish required in the entire manufacturing process can be reduced. Furthermore, 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 50% by mass or 40% by mass. If the solid content concentration of the resin varnish is 50% by mass or less, the viscosity of the resin varnish can be appropriately adjusted, and the storage stability and coatability of the resin varnish can be improved.

[0064] 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.

[0065] 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 resin varnish can be heated by a conventionally known method such as hot air heating, infrared heating, or high-frequency heating.

[0066] The heating temperature can be, for example, 300° C. to 800° C. The heating time can be, for example, 5 seconds to 1 minute.

[0067] 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.

[0068] 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.

[0069] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0070] The abbreviations of the components used in the examples are as follows: PMDA: pyromellitic dianhydride ODA: diaminodiphenyl ether NMP: N-methyl-2-pyrrolidone DBA: dibutyl adipate TBC: tributyl citrate

[0071] <Preparation of Insulated Wire> [No. 1] (Preparation of Resin Varnish) 4,4'-ODA as an aromatic diamine was dissolved in NMP. The boiling point of NMP is 202°C. PMDA as an aromatic tetracarboxylic dianhydride was added to NMP so that the mixing ratio (molar ratio) of the aromatic tetracarboxylic dianhydride to the aromatic diamine was 100:100. This solution was reacted at 30°C for 3 hours while stirring under a nitrogen atmosphere to synthesize a polyimide precursor. In this way, a polyimide precursor solution (solids concentration 28% by mass) containing NMP as a solvent was obtained. DBA was added to the polyimide precursor solution to prepare Resin Varnish No. 1.

[0072] (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 wide and 2 mm long. Resin varnish No. 1 was applied to the surface of the conductor, and the process of heating the conductor coated with the resin varnish was repeated 30 times to prepare insulated wire No. 1 having an insulating coating with an average thickness of 100 μm.

[0073] [No. 2 and No. 3] Resin varnishes No. 2 and No. 3 were prepared in the same manner as insulated wire No. 1, except that TBC was used as the aliphatic polycarboxylic acid ester, and insulated wires No. 2 and No. 3 were fabricated.

[0074] [Reference Example 1] Resin varnish Reference Example 1 was prepared in the same manner as insulated wire No. 1, except that an aliphatic polycarboxylic acid ester was not used and core-shell structured particles with an average particle diameter of 3 μm were used, and insulated wire Reference Example 1 was produced. The core-shell structured particles were a pore-forming agent, the cores of the core-shell structured particles were polymethyl methacrylate particles, and the shell material was silicone. Insulated wire Reference Example 1 is an example of an insulated wire provided with an insulating coating having a plurality of pores.

[0075] <Evaluation> For insulated wires No. 1 to No. 3, the dielectric constant, aliphatic polycarboxylic acid ester content, porosity pore diameter, dielectric breakdown voltage, and coating lift length at 25% elongation were measured according to the following methods. For insulated wire Reference Example 1, the dielectric constant and porosity were measured according to the following methods. The results are shown in Table 1.

[0076] [Measurement of Relative Dielectric Constant] The relative dielectric constants of the insulating coatings of insulated wires No. 1 to No. 3 and Reference Example 1 were measured. 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 to prepare a measurement sample. The silver paste was applied along the longitudinal axis of the insulated wire at 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. After heating the three samples at 105°C for 1 hour, the relative dielectric constants of the three samples were measured and the average value was calculated.

[0077] [Measurement of Aliphatic Polycarboxylic Acid Ester Content] The aliphatic polycarboxylic acid ester content in the insulating coating of insulated wires No. 1 to No. 3 was measured. Using a Pyrolyzer PY-3030 manufactured by Frontier Labs, the samples were heated at 500°C for 1 minute, and the components of the gas evolved upon heating were measured using an Agilent 5977 gas chromatograph mass spectrometer manufactured by Agilent Technologies. The measurement was performed with the insulating coating still attached to the conductor. The mass of the insulating coating was calculated by subtracting the mass of the conductor from the mass of the insulated wire, and the content of the aliphatic polycarboxylic acid ester in the insulating coating was determined.

[0078] [Measurement of Porosity] The insulating coatings of insulated wires No. 1 to No. 3 and Reference Example 1 were peeled from the conductors in the form of tubes. The peeled insulating coatings were used as coating test pieces. The porosity of these coating test pieces was measured by the method described above.

[0079] [Measurement of pore diameter] The major and minor diameters of the pores of insulated wires No. 1 to No. 3 were measured by the method described above.

[0080] [Measurement of Breakdown Voltage] The breakdown voltage BDV1 of insulated wires No. 1 to No. 3 was measured as follows. The insulated wires were immersed in a mixed solution of glycerin and saturated saline (volume ratio of glycerin to saturated saline = 85:15). A 60 Hz AC voltage with a waveform close to a sine wave was applied between the conductor of the insulated wire and the mixed solution, and the breakdown voltage BDV1 was measured. The AC voltage was increased at a rate of 500 V / sec. The detection current for breakdown was 15 mA. The breakdown voltage BDV2 of insulated wires No. 1 to No. 3 was measured as follows. A metal pin with a radius of 1.5 mm was placed at the center of the insulated wire so that it was perpendicular to the insulated wire, and the insulated wire was bent at an angle of 180°. The bent portion was immersed in the mixed solution of glycerin and saturated saline, and the breakdown voltage BDV2 was measured in the same manner as for measuring the breakdown voltage BDV1. The breakdown voltages BDV1 and BDV2 of the five wire test pieces were measured, and the average values ​​were calculated. The BDV reduction rate during bending was calculated using the following formula 3. Formula 3: BDV reduction rate during bending (%) = (BDV1 - BDV2) x 100 / BDV1

[0081] [Measurement of the Length of Lifted Coating at 25% Elongation] Insulated wires No. 1 to No. 3 were elongated by 25% along their longitudinal axes. The length of lifted coating between the conductor and the insulating coating that occurred during elongation was measured visually using a common tape measure with a minimum scale of 0.1 mm.

[0082]

[0083] As shown in Table 1, the dielectric constants of insulated wires No. 1 to No. 3 were approximately the same as that of Reference Example 1. Therefore, the dielectric constants of insulated wires No. 1 to No. 3 were sufficiently low.

[0084] REFERENCE SIGNS LIST 1 insulated wire 2 conductor 3 insulating coating 4 resin matrix 5 voids

Claims

1. An insulated electric wire comprising: a conductor; and an insulating coating covering the conductor, wherein the insulating coating has a resin matrix and a plurality of pores, the resin matrix is ​​primarily composed of a polyimide derived from a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and the resin matrix contains an aliphatic polycarboxylic acid ester.

2. The insulated wire according to claim 1, wherein the content of the aliphatic polycarboxylic acid ester in the resin matrix is ​​800 ppm or less.

3. An insulated wire according to claim 1 or 2, wherein the aliphatic polycarboxylic acid ester is an aliphatic tricarboxylic acid ester.

4. The insulated wire according to claim 3, wherein the aliphatic tricarboxylic acid ester is a citric acid ester.

5. The insulated wire of claim 4, wherein said citrate ester is tributyl citrate.

6. An insulated wire according to any one of claims 1 to 5, wherein the flattening ratio of the plurality of pores is less than 2.

00.

7. An insulated wire according to any one of claims 1 to 6, wherein the average diameter of the plurality of pores is less than 2.45 μm.

8. An insulated wire according to any one of claims 1 to 7, wherein the insulating coating has a porosity of 20% by volume or more.

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