Insulated wire, coil, motor, and thermoplastic polyimide resin composition

Abstract

This insulated wire comprises a conductor and a coating film that covers the conductor. The coating film contains a thermoplastic polyimide containing a structural unit represented by formula (1). In formula (1), each of the benzene rings contained in the structural unit represented by formula (1) are optionally substituted.

Description

Insulated wire, coil, motor, and thermoplastic polyimide resin composition

[0001] The present disclosure relates to an insulated wire, a coil, a motor, and a thermoplastic polyimide resin composition.

[0002] Magnet wire (hereinafter sometimes referred to as insulated wire) is used for coils in industrial motors and motors for home appliances, and in recent years, insulated wire has also been used in drive motors for electric vehicles (EVs). In these fields, from the viewpoint of improving the power performance, fuel economy, and power cost of electric vehicles, there is a demand for higher output and smaller drive motors, and there is a trend toward improving the output density of drive motors.

[0003] On the other hand, with the increase in power density, higher voltages than ever before are being applied to insulated wires. This can cause partial discharges on the surface of the insulated wire, which can lead to dielectric breakdown. Insulated wires are now required to have higher heat resistance and insulating properties to suppress partial discharges than ever before.

[0004] Thermoplastic polyimides are widely used as materials for various parts in industrial materials, automobiles, electrical and electronic equipment, and other industrial applications due to their excellent moldability, mechanical properties, and chemical resistance.

[0005] Patent Document 1: International Publication No. 2014 / 084063 discloses an insulated wire in which an insulating film is formed by extrusion coating a thermoplastic polyimide on the outer periphery of a conductor.

[0006] However, when the insulating film is formed using the thermoplastic polyimide described in Patent Document 1, there is a problem that partial discharge may occur in the insulating film, leading to dielectric breakdown.

[0007] The present disclosure has been made in view of the above-described conventional circumstances. One problem to be solved by one embodiment of the present disclosure is to provide an insulated wire having excellent insulation properties, and a coil and a motor using the insulated wire. Another problem to be solved by another embodiment of the present disclosure is to provide a thermoplastic polyimide resin composition having excellent insulation properties.

[0008] Specific means for achieving the above object are as follows: <1> An insulated wire comprising a conductor and a coating covering the conductor, wherein the coating contains a thermoplastic polyimide containing a structural unit represented by the following formula (1):

[0009]

[0010] In the formula (1), the benzene rings contained in the structural unit represented by the formula (1) may each independently have a substituent.

[0011] <2> The insulated wire according to <1>, wherein the coating has a crystallinity of 10% or less after being heated at 300°C for 4 hours in an inert gas atmosphere.

[0012] <3> The insulated wire according to <1> or <2>, wherein the structural unit represented by the formula (1) accounts for 20 mol % to 80 mol % of the entire thermoplastic polyimide.

[0013] <4> The insulated wire according to any one of <1> to <3>, wherein the thermoplastic polyimide further contains a structural unit represented by the following formula (2):

[0014]

[0015] In the formula (2), the benzene rings contained in the structural unit represented by the formula (2) may each independently have a substituent.

[0016] <5> The insulated wire according to <4>, wherein the structural unit represented by formula (1) accounts for 20 mol % to 80 mol % of the total of the structural unit represented by formula (1) and the structural unit represented by formula (2) in the thermoplastic polyimide.

[0017] <6> The insulated wire according to any one of <1> to <5>, wherein the imide group concentration of the thermoplastic polyimide is 4% to 35%.

[0018] <7> The insulated wire according to any one of <1> to <6>, wherein the coating further contains an acid anhydride, and when a total amount of the thermoplastic polyimide and the acid anhydride contained in the coating is taken as 100% by mass, the content of the acid anhydride is more than 0% by mass and 0.002% by mass or less.

[0019] <8> A coil comprising the insulated wire according to any one of <1> to <7>.

[0020] <9> A motor including the coil according to <8>.

[0021] <10> A thermoplastic polyimide resin composition comprising a thermoplastic polyimide containing a structural unit represented by the following formula (1) and an acid anhydride, wherein the acid anhydride content is greater than 0% by mass and 0.002% by mass or less when the total amount of the thermoplastic polyimide and the acid anhydride is taken as 100% by mass:

[0022]

[0023] In the formula (1), the benzene rings contained in the structural unit represented by the formula (1) may each independently have a substituent.

[0024] <11> The thermoplastic polyimide resin composition according to <10>, which is used for forming an insulating coating by extrusion coating.

[0025] According to one embodiment of the present disclosure, there is provided an insulated wire having excellent insulation properties, and a coil and a motor using the insulated wire. According to another embodiment of the present disclosure, there is provided a thermoplastic polyimide resin composition having excellent insulation properties.

[0026] FIG. 1 is a cross-sectional view of an example of an insulated wire according to the present disclosure.

[0027] Hereinafter, embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure.

[0028] In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. Furthermore, in numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in an example.

[0029] In the present disclosure, each component may contain multiple substances corresponding to the component. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified.

[0030] In the present disclosure, the thickness of a coating is defined as the arithmetic mean value of thicknesses measured at five points on the coating, measured using a micrometer or the like.

[0031] In the present disclosure, "thermoplastic polyimide" refers to a polyimide resin having thermoplastic properties. The thermoplastic polyimide is preferably a polyimide resin having a deflection temperature under load of 200°C or higher. The deflection temperature under load is measured in accordance with ASTM D648, and the load is 1.82 MPa. The higher the measured deflection temperature under load, the better the deflection strength.

[0032] <Insulated wire> The insulated wire according to the present disclosure includes a conductor and a coating that coats the conductor, and the coating contains a thermoplastic polyimide (hereinafter, may be referred to as a specific polyimide) that includes a structural unit represented by the following formula (1):

[0033]

[0034] In the formula (1), the hydrogen atom bonded to the benzene ring contained in the formula (1) may be substituted.

[0035] The insulated wire of the present disclosure has excellent insulating properties. The reason for this is not clear, but is presumed to be as follows.

[0036] When the specific polyimide contained in the coating that coats the conductor contains a structural unit represented by formula (1), steric hindrance is introduced into the specific polyimide. The introduction of steric hindrance into the specific polyimide can reduce the crystallinity of the coating containing the specific polyimide. When the crystallinity of the coating is reduced, free electrons become less likely to move in the coating, and the voltage required to generate a partial discharge tends to increase. As a result, the partial discharge inception voltage of the coating increases, and it is presumed that the insulating properties of the insulated wire of the present disclosure, which has a conductor coated with a coating containing the specific polyimide, are improved.

[0037] The conductor, the specific polyimide, and the coating containing the specific polyimide that constitute the insulated wire of the present disclosure, as well as the method for producing the insulated wire, will be described below.

[0038] Specific examples of the insulated wire of the present disclosure will be described below with reference to the drawings, but the present disclosure is not limited thereto. Furthermore, the sizes of the components in Fig. 1 are conceptual, and the relative size relationships between the components are not limited thereto.

[0039] A cross-sectional view of an example of an insulated wire according to the present disclosure is shown in Fig. 1. As shown in Fig. 1, the insulated wire 10 includes a conductor 11 and a coating 12. The coating 12 directly covers the conductor 11. In other words, the coating 12 and the conductor 11 are in direct contact with each other.

[0040] (Conductor) The insulated wire of the present disclosure includes a conductor. The conductor has a function of allowing current to flow.

[0041] The cross-sectional shape of the conductor when cut along a plane perpendicular to the axial direction of the insulated wire (hereinafter simply referred to as "cross-sectional shape") is not particularly limited, and examples include a rectangle (e.g., a square, a rectangle, etc.), a polygon (excluding rectangles), a circle, an ellipse, etc. Among these, a rectangular cross-sectional shape is preferable. Hereinafter, a conductor having a rectangular cross-sectional shape will also be referred to as a "rectangular conductor."

[0042] From the viewpoint of suppressing the occurrence of partial discharge, the cross-sectional shape of the rectangular conductor is preferably a shape in which the four corners of the rectangular conductor are chamfered (with a radius of curvature) as shown in Fig. 1. The radius of curvature is not particularly limited and may be appropriately selected depending on the application of the insulated wire, and is preferably 0.6 mm or less, more preferably 0.2 mm to 0.4 mm.

[0043] The size of the conductor is not particularly limited.

[0044] In the cross section of the rectangular conductor when cut along a plane perpendicular to the axial direction of the insulated wire, the lengths of the long and short sides of the rectangular conductor are not particularly limited and are selected appropriately depending on the application of the insulated wire, and are preferably within the following ranges. The length of the long side of the rectangular conductor is preferably 1.0 mm to 5.0 mm, more preferably 1.4 mm to 4.0 mm. The length of the short side of the rectangular conductor is preferably 0.4 mm to 3.0 mm, more preferably 0.5 mm to 2.5 mm. The ratio of the length of the long side of the rectangular conductor to the length of the short side of the rectangular conductor (length of the long side of the rectangular conductor / length of the short side of the rectangular conductor) is preferably 1 to 4.

[0045] When the cross section of the conductor is circular, the diameter of the conductor is preferably 0.3 mm to 3.0 mm, more preferably 0.4 mm to 2.7 mm.

[0046] The material of the conductor may be a metal. The metal is not particularly limited, and examples thereof include copper, iron, nickel, gold, silver, platinum, cobalt, zinc, lead, tin, titanium, chromium, aluminum, magnesium, manganese, and alloys thereof (stainless steel, brass, phosphor bronze, etc.). The copper may be low-oxygen copper or oxygen-free copper. The oxygen content of low-oxygen copper is 30 ppm or less, preferably 20 ppm or less.

[0047] The conductor may be subjected to a surface modification treatment in order to improve adhesion to the coating. Examples of the surface modification treatment include mechanical treatments such as blasting, polishing, and embossing, physical treatments such as laser treatment, corona treatment, plasma treatment, and dry etching, electrochemical treatments such as plating and anodizing, and chemical treatments such as chemical etching (wet etching) and surface coating.

[0048] In blasting, particulate or finely powdered media is sprayed onto a conductor and collided with it, creating irregularities on the conductor surface. The irregularities increase the surface roughness of the conductor surface, making it easier for the coating to adhere due to frictional resistance (anchor effect). This can improve adhesion between the conductor and the coating. Blasting is also advantageous because it can remove oxide coatings formed on the conductor surface, further improving adhesion with the coating. Examples of media that can be used include metal particles, carbon particles, oxide particles, carbide particles, and nitride particles. The conductor surface may be roughened by machining it to the shape of a die during conductor manufacturing.

[0049] Corona treatment involves modifying (hydrophilizing) the conductor surface by corona discharge treatment, which can improve adhesion between the conductor and the coating through chemical interaction.

[0050] In the plating process, the conductor is coated with a plating material. From the viewpoints of cost and productivity, wet plating is preferred. Electrolytic plating is more preferred. Examples of plating materials include nickel plating and silver plating. The plating thickness is preferably 0.1 μm to 10 μm, more preferably 0.5 μm to 5 μm.

[0051] In surface coating, a coating agent is applied or sprayed onto the conductor. Coating prevents the surface of the conductor from oxidizing, and chemical interactions improve adhesion between the conductor and the coating. Known silane coupling agents can be used as the coating agent.

[0052] (Coating) The insulated wire of the present disclosure includes a coating containing a specific polyimide that covers the conductor. The coating has a function of insulating the conductor.

[0053] The thickness of the coating is not particularly limited and may be appropriately selected depending on the intended use of the insulated wire, and is preferably 30 μm to 300 μm, more preferably 40 μm to 250 μm.

[0054] The insulated wire may be provided with a layer other than the coating containing the specific polyimide, depending on the application, etc. Examples of the other layer include an electrical insulating layer and an adhesive layer.

[0055] When an insulated wire is provided with a layer other than the coating containing the specific polyimide, the other layer may or may not be interposed between the coating containing the specific polyimide and the conductor, but in order to further improve the insulating properties of the insulated wire, it is preferable that the coating containing the specific polyimide directly coats the conductor. "Coating the conductor directly" means that no other resin layer other than the coating containing the specific polyimide is interposed between the conductor and the coating containing the specific polyimide.

[0056] -Specific Polyimide- The specific polyimide in the present disclosure contains a structural unit represented by formula (1).

[0057] In formula (1), the benzene rings contained in the structural unit represented by formula (1) may or may not each independently have a substituent.

[0058] When the benzene ring has a substituent, examples of the substituent include alkyl groups such as a methyl group and an ethyl group; alkoxy groups such as a methoxy group and an ethoxy group; halogenated alkyl groups such as a fluoromethyl group and a trifluoromethyl group; halogenated alkoxy groups such as a fluoromethoxy group; and halogen atoms such as a chlorine atom and a fluorine atom. The alkyl group, alkoxy group, halogenated alkyl group, and halogenated alkoxy group may have a substituent.

[0059] When the benzene ring contained in the structural unit represented by formula (1) has a substituent, it is preferable that a specific benzene ring among the multiple benzene rings contained in the structural unit represented by formula (1) has a substituent. More specifically, formula (1) is preferably the following formula (1)'.

[0060]

[0061] In formula (1)', R1 to R8 each independently represent a hydrogen atom, an alkyl group which may have a substituent, an alkoxy group which may have a substituent, a halogenated alkyl group which may have a substituent, a halogenated alkoxy group which may have a substituent, or a halogen atom.

[0062] The proportion of the structural unit represented by formula (1) in the entire specific polyimide is preferably 20 mol% or more, more preferably 30 mol% or more, even more preferably 40 mol% or more, and particularly preferably 45 mol% or more, from the viewpoint of increasing the effect of steric hindrance introduced into the specific polyimide. Furthermore, the proportion of the structural unit represented by formula (1) in the entire specific polyimide is preferably 80 mol% or less, more preferably 70 mol% or less, even more preferably 60 mol% or less, and particularly preferably 55 mol% or less. From the viewpoint of reducing the crystallinity, the proportion of the structural unit represented by formula (1) in the entire specific polyimide is preferably 20 mol% to 80 mol%, more preferably 30 mol% to 70 mol%, even more preferably 40 mol% to 60 mol%, and particularly preferably 45 mol% to 55 mol%.

[0063] The specific polyimide preferably contains a structural unit represented by formula (2) in addition to the structural unit represented by formula (1).

[0064]

[0065] In formula (2), the benzene rings contained in the structural unit represented by formula (2) may or may not each independently have a substituent. Specific examples of the substituent are the same as those in the structural unit represented by formula (1).

[0066] When the benzene ring contained in the structural unit represented by formula (2) has a substituent, a specific benzene ring among the multiple benzene rings contained in the structural unit represented by formula (2) may have a substituent. More specifically, formula (2) is preferably the following formula (2)'.

[0067]

[0068] In formula (2)', R9 to R14 each independently represent a hydrogen atom, an alkyl group which may have a substituent, an alkoxy group which may have a substituent, a halogenated alkyl group which may have a substituent, a halogenated alkoxy group which may have a substituent, or a halogen atom.

[0069] When the specific polyimide contains a structural unit represented by formula (1) and a structural unit represented by formula (2), the proportion of the structural unit represented by formula (1) in the total of the structural unit represented by formula (1) and the structural unit represented by formula (2) is preferably 20 mol % to 80 mol %, more preferably 30 mol % to 70 mol %, and even more preferably 40 mol % to 60 mol %.

[0070] The specific polyimide may contain structural units other than the structural units represented by formula (1) and the structural units represented by formula (2). The other structural units are not particularly limited. When the specific polyimide contains other structural units, the total proportion of the structural units represented by formula (1) and the structural units represented by formula (2) contained as needed in the entire specific polyimide is preferably 80 mol% or more, more preferably 90 mol% or more, and even more preferably 99 mol% or more.

[0071] The total proportion of the structural unit represented by formula (1) and the structural unit represented by formula (2) contained as needed in the entire specific polyimide may be 100 mol %.

[0072] The coating preferably has a crystallinity of 10% or less, more preferably 8% or less, and even more preferably 5% or less, after heating the coating at 300°C for 4 hours in an inert gas atmosphere. When the thermoplastic polyimide contained in the coating contains a structural unit represented by formula (1) in its molecular structure, the crystallinity of the coating after heating the coating at 300°C for 4 hours in an inert gas atmosphere is easily controlled to 10% or less.

[0073] The crystallinity of a coating included in an insulated electric wire can be measured after heating the coating at 300°C for 4 hours in an inert gas atmosphere using the following method. First, the insulated electric wire is placed in an inert oven (e.g., Model FJ-30 manufactured by Advantec Toyo Co., Ltd.) in a nitrogen atmosphere, heated from room temperature to 300°C over 1 hour, held at 300°C for 4 hours, and cooled to 200°C or below over 1 hour. The coating is stripped from the insulated electric wire that has been subjected to the above-described heat treatment to obtain a measurement sample. Alternatively, the coating may be stripped from the insulated electric wire before the heat treatment, and the stripped coating may be subjected to the above-described heat treatment to obtain a measurement sample. Wide-angle X-ray diffraction measurement is performed on the obtained measurement sample to obtain an X-ray diffraction chart (X-ray diffraction pattern) in the measurement range 2θ = 5° to 35°. The obtained X-ray diffraction chart is separated into peaks derived from crystalline portions and peaks derived from amorphous portions using a multiple peak separation program, and the crystallinity (%) is calculated from the peak intensities of the peaks derived from crystalline portions and peaks derived from amorphous portions according to the following formula. The separation of peaks derived from crystalline portions and peaks derived from amorphous portions can be achieved by classifying each peak separated by the multiple peak separation program into those derived from crystalline portions and those derived from amorphous portions according to the half-width of each peak. For example, the half-width may be set at 2θ = 2° as the threshold, and a peak derived from 2° or more may be considered to be derived from a crystalline portion, and a peak derived from an amorphous portion may be considered to be derived from an amorphous portion if it is less than 2°.

[0074] Crystallinity (%)=(Σ(peak area of ​​peaks derived from crystalline portions) / (Σ(peak area of ​​peaks derived from crystalline portions)+Σ(peak area of ​​peaks derived from amorphous portions)))×100

[0075] The MFR (melt flow rate) of the specific polyimide measured in accordance with JIS K7210-1:2014 at 400°C under a load of 1.05 kg is not particularly limited, but is preferably 2 g / 10 min to 30 g / 10 min, more preferably 4 g / 10 min to 20 g / 10 min. The method for measuring the MFR of the resin is the same as that described in the examples.

[0076] The logarithmic viscosity of the specific polyimide is not particularly limited, but is preferably 0.1 dl / g to 3.0 dl / g, more preferably 0.2 dl / g to 2.0 dl / g, even more preferably 0.3 dl / g to 1.5 dl / g, and particularly preferably 0.4 dl / g to 1.0 dl / g. If the logarithmic viscosity is 0.1 dl / g to 3.0 dl / g, the molecular weight of the specific polyimide is appropriate. Therefore, the mechanical strength of the coating is excellent, and the specific polyimide has excellent fluidity for producing the coating by injection molding or extrusion molding.

[0077] The inherent viscosity is measured by heating a solution in a mixed solvent of parachlorophenol / phenol (90 / 10 mass ratio) at a concentration of 0.5 g / 100 ml to 200° C. and then cooling it to 35° C. The definition of inherent viscosity is given in the 1995 first edition of "Polymer Handbook," published by Asakura Shoten and published by the Japan Society for Analytical Chemistry, p. 58.

[0078] The glass transition temperature Tg of the specific polyimide is not particularly limited, and from the viewpoint of heat resistance, it is preferably 200° C. or higher, more preferably 210° C. or higher, and even more preferably 220° C. or higher. The glass transition temperature Tg of the specific polyimide is not particularly limited, and is preferably 300° C. or lower, more preferably 290° C. or lower, and even more preferably 280° C. or lower. The method for measuring the glass transition temperature Tg is the same as that described in the examples.

[0079] The imide group concentration of the specific polyimide is preferably 4% or more, more preferably 7% or more, and even more preferably 10% or more, from the viewpoint of adhesion between the conductor and the coating. The imide group concentration of the specific polyimide is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less, from the viewpoint of improving insulating properties. The imide group concentration of the specific polyimide is preferably 4% to 35%.

[0080] The imide group concentration of the specific polyimide is a value calculated by (molecular weight of imide group portion) / (total molecular weight)×100(%) in the molecular structure of the specific polyimide. For example, if the specific polyimide is a polyimide containing 100% of the structural unit represented by formula (1) (provided that each benzene ring in formula (1) has no substituent), the imide group concentration is ((molecular weight of imide group portion) / (total molecular weight))×100(%)=(70.03×2) / (626.64)=22.35%.

[0081] The coating may contain a thermoplastic polyimide other than the specific polyimide. When the coating contains another thermoplastic polyimide, the proportion of the specific polyimide in the thermoplastic polyimide contained in the coating is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 99% by mass or more. The proportion of the specific polyimide in the thermoplastic polyimide contained in the coating may be 100% by mass.

[0082] The specific polyimide may be a commercially available product, such as AURUM (registered trademark, manufactured by Mitsui Chemicals, Inc.).

[0083] The specific polyimide and other thermoplastic polyimides used as needed may be synthesized using a tetracarboxylic acid anhydride and a diamine. The method for synthesizing the thermoplastic polyimide is not particularly limited. The thermoplastic polyimide can be obtained by dehydration co-condensation of a tetracarboxylic acid anhydride and a diamine.

[0084] The amount of raw material compounds used when synthesizing a thermoplastic polyimide is usually 0.90 to 0.99 equivalents of tetracarboxylic dianhydride per equivalent of diamine. The amount of tetracarboxylic dianhydride used is preferably 0.93 to 0.985 equivalents, more preferably 0.95 to 0.983 equivalents. When the amount of tetracarboxylic dianhydride used is 0.90 to 0.99 equivalents, the molecular weight of the thermoplastic polyimide resin is sufficiently high, resulting in excellent mechanical properties of the coating and excellent flowability of the thermoplastic polyimide resin.

[0085] In the synthesis of thermoplastic polyimides, it is preferable to add an acid anhydride as an end-capping agent and cap the reactive ends of the molecules with the acid anhydride (e.g., phthalic anhydride). Capturing the reactive ends improves the thermal stability of the thermoplastic polyimide resin.

[0086] The reaction is preferably carried out in an organic solvent. Examples of the organic solvent include N,N-dimethylformamide, N,N-diethylacetamide, N,N-dimethoxyacetamide, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, N-methylcaprolactam, 1,2-dimethoxyethane, bis(2-methoxyethyl)ether, 1,2-bis(2-methoxyethoxy)ethane, bis[2-(2-methoxyethoxy)ethyl]ether, tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, pyrroline, picoline, dimethyl sulfoxide, dimethyl sulfone, tetramethylurea, hexamethylphosphoramide, phenol, o-cresol, m-cresol, p-cresol, p-chlorophenol, anisole, benzene, toluene, and xylene. The organic solvent may be used alone or in combination.

[0087] The reaction temperature is usually room temperature to 250°C, preferably 140°C to 200°C. The reaction pressure is not particularly limited and can be carried out at normal pressure. The reaction time varies depending on the type of solvent and the reaction temperature, but is usually 4 to 24 hours. As for the imidization method, the precursor polyamic acid is heated to 100°C to 300°C to be imidized, or chemically imidized using an imidizing agent such as acetic anhydride, to obtain the desired thermoplastic polyimide.

[0088] Acid Anhydride: To improve the thermal stability of the thermoplastic polyimide, the coating preferably further contains an acid anhydride. The acid anhydride refers to an acid anhydride other than the acid anhydride used as a raw material for forming the skeleton of the thermoplastic polyimide, and is preferably a dicarboxylic acid anhydride, and more preferably phthalic anhydride.

[0089] The acid anhydride content in the coating is preferably greater than 0% by mass but not greater than 0.002% by mass, more preferably greater than 0% by mass but not greater than 0.001% by mass, even more preferably greater than 0% by mass but not greater than 0.0005% by mass, and particularly preferably greater than 0% by mass but not greater than 0.0003% by mass, relative to the total amount of thermoplastic polyimide and acid anhydride (100% by mass). An acid anhydride content greater than 0% by mass in the coating means that the coating contains acid anhydride, thereby improving the thermal stability of the thermoplastic polyimide. Furthermore, by ensuring that the acid anhydride content in the coating is 0.002% by mass or less (20 ppm or less) relative to the total amount of thermoplastic polyimide and acid anhydride, gelation of the thermoplastic polyimide during extrusion coating can be suppressed. As a result, deterioration of the thermoplastic polyimide and an increase in surface roughness of the coating layer are suppressed, and degradation of insulating properties can be suppressed.

[0090] The content of acid anhydride in the coating is a value measured using a gas chromatograph by the GC-FID method. Details of the measurement are shown in the Examples section.

[0091] -Other Additives- The coating may contain additives other than the specific polyimide, other thermoplastic polyimides used as needed, and acid anhydrides. The coating may contain at least one fiber component selected from the group consisting of carbon fiber, glass fiber, potassium titanate fiber, aluminum borate fiber, metal fiber, ceramic fiber, boron fiber, silicon carbide fiber, asbestos fiber, rock wool fiber, and aramid fiber, as long as the properties required of the coating are not impaired.

[0092] The coating may contain, as needed, at least one of the following fillers such as mica, synthetic mica, wollastonite, talc, silicone oil, fluorine-based oil, glass beads, molybdenum disulfide, clay, silica, alumina, diatomaceous earth, hydrated alumina, shirasu balloons, carbon nanotubes, calcium carbonate, hydrotalcite, graphite (e.g., artificial graphite, natural graphite (e.g., flake graphite, scaly graphite, earthy graphite)), lubricants, release agents, stabilizers, colorants, crystal nucleating agents, etc., within the range that does not impair the properties required of the coating.

[0093] The coating may contain, as needed, at least one of various liquid crystal polymers, thermoplastic resins (e.g., fluororesin, polyetherimide, polyethernitrile, polyetherketone, polyetheretherketone, polyetherketoneketone, polyetherketoneetherketoneketone, polyamideimide, polyethersulfone, polysulfone, polyarylate and / or polyphenylene sulfide), and thermosetting resins (e.g., epoxy resin, polybenzimidazole resin, etc.), within the range that does not impair the properties required of the coating.

[0094] When the coating contains an additive, the content of the additive is not particularly limited and is selected appropriately depending on the type of additive, etc., and is preferably 40 mass % or less relative to the total amount of the coating.

[0095] (Coating Method) In the insulated wire of the present disclosure, the method for coating the conductor with a coating is not particularly limited. A coating can be produced by extrusion coating a conductor with a thermoplastic polyimide resin composition containing the specific polyimide and the various components described above that are used as needed. This produces an insulated wire.

[0096] The composition of the thermoplastic polyimide resin composition is not particularly limited as long as it contains the specific polyimide. Among these, a preferred composition is the thermoplastic polyimide resin composition of the present disclosure described below. The proportion of the specific polyimide in the thermoplastic polyimide resin composition is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 99% by mass or more. The proportion of the specific polyimide in the thermoplastic polyimide resin composition may be 100% by mass. The content of the specific polyimide in the coating and the content of the specific polyimide in the thermoplastic polyimide resin composition may be the same or different.

[0097] A method for coating a conductor with a coating can be, for example, a method using an extruder equipped with a die. The extruder may be any known extruder. The shape of the die hole is similar to the cross-sectional shape of the conductor. The thickness of the coating can be adjusted by adjusting the size of the shape of the die hole.

[0098] The heating temperature of the extruder is not particularly limited, but is preferably 380°C to 420°C.

[0099] When the thermoplastic polyimide resin composition is extrusion coated onto a conductor, the conductor is preferably preheated. Preheating the conductor can improve adhesion between the thermoplastic polyimide resin composition and the conductor. The preheating temperature of the conductor is not particularly limited, but is preferably 100°C to 400°C, more preferably 250°C to 350°C.

[0100] By preheating the conductor within the above temperature range, the temperature difference between the conductor and the molten thermoplastic polyimide resin composition is reduced, and stress-strain is alleviated, thereby improving adhesion between the conductor and the coating. Furthermore, this small temperature difference allows the molten thermoplastic polyimide resin composition to remain in contact with the surface of the conductor for a longer period of time. As a result, the thermoplastic polyimide resin composition can be impregnated into the fine irregularities formed on the surface of the conductor, improving adhesion between the conductor and the coating.

[0101] The preheating method is not particularly limited, and may be a heating method in which a heat source or flame heated to a high temperature is brought into direct contact with the conductor, or a heating method using microwaves, high frequency waves, etc. In the case of a heating method using microwaves, high frequency waves, etc., heating may be a non-contact method in which the conductor is not brought into contact. When preheating using a heating method using microwaves, high frequency waves, etc., induction heating using high frequency waves is preferred from the viewpoint of preheating at a high temperature. The preheating unit may be provided integrally with the unwinding unit. Alternatively, the winding core (bobbin) of the unwinding unit around which the conductor is wound may be heated.

[0102] When the conductor is a copper wire and a high-frequency induction heating device is used, the preheating time is preferably 0.1 seconds to 5 minutes, more preferably 0.1 seconds to 1 minute.

[0103] Moreover, the preheating is preferably carried out in an inert gas atmosphere such as nitrogen or argon in order to prevent the conductor surface from being oxidized by high temperatures and forming an oxide film on the conductor surface.

[0104] After extrusion coating, the coated conductor (i.e., the insulated wire) may be cooled by air or water.

[0105] After extrusion coating and before cooling the insulated wire, the insulated wire may be kept at a temperature of from the heating temperature of the extrusion coating part (e.g., the die part) −10° C. to the heating temperature of the extrusion coating part +10° C. Holding the temperature under these conditions improves compatibility between the conductor and the coating, thereby improving adhesion between the conductor and the coating.

[0106] After extrusion coating, the insulated wire may be further post-heated before cooling. By further heating after extrusion coating, compatibility between the conductor and the coating improves, and adhesion between the conductor and the coating improves.

[0107] Alternatively, after extrusion coating, the insulated wire may be rolled before cooling. By rolling the manufactured insulated wire at a predetermined pressure, the adhesion between the conductor and the coating is improved. Rolling can be performed, for example, by roll pressing, cold isostatic pressing (CIP), or the like.

[0108] <Thermoplastic Polyimide Resin Composition> The thermoplastic polyimide resin composition of the present disclosure includes a thermoplastic polyimide containing a structural unit represented by the following formula (1) and an acid anhydride, and when the total amount of the thermoplastic polyimide and the acid anhydride is taken as 100 mass%, the content of phthalic anhydride is more than 0 mass% and 0.002 mass% or less.

[0109]

[0110] In formula (1), the benzene rings contained in the structural unit represented by formula (1) may each independently have a substituent.

[0111] In the thermoplastic polyimide resin composition of the present disclosure, the details and preferred embodiments of the thermoplastic polyimide containing the structural unit represented by formula (1), the acid anhydride, and other additives have been described above, and therefore further description thereof will be omitted here.

[0112] The thermoplastic polyimide resin composition of the present disclosure has excellent insulating properties and can therefore be suitably used in applications such as insulating treatment of conductors, and producing insulating materials and insulators, and is particularly suitably used in applications for forming insulating coatings by extrusion coating around conductors.

[0113] (Uses of Insulated Wire) The insulated wire of the present disclosure has excellent insulating properties and is therefore suitable for use in forming coils used in motors. Because the insulated wire of the present disclosure can exhibit good insulating properties even when the thickness of the insulating coating is reduced, use of the insulated wire of the present disclosure enables motors to be made more powerful and more compact. Therefore, the insulated wire of the present disclosure is particularly suitable for use in forming coils used in drive motors for electric vehicles.

[0114] <Coil and Motor> The coil of the present disclosure includes the insulated wire of the present disclosure. For example, a coil is formed by winding the insulated wire of the present disclosure around a core made of a magnetic material. A member made of the core and the coil is used as a rotor or stator of a motor.

[0115] The motor of the present disclosure includes the coil of the present disclosure. The configuration of the motor is not particularly limited as long as it includes the coil of the present disclosure. For example, the above-described stator can be used as a split stator, and multiple split stators can be combined and arranged in a ring shape to form a stator that can be used as a component of the motor.

[0116] Hereinafter, the present disclosure will be specifically explained using examples, but the present disclosure is not limited to the following examples as long as it does not deviate from the gist of the disclosure.

[0117] Example 1 A thermoplastic polyimide (TPI(a1)) represented by the following formula (3) was synthesized according to the following procedure: In formula (3), the ratio of m to n was 1:1.

[0118]

[0119] A vessel equipped with a stirrer, reflux condenser, water separator, and nitrogen inlet tube was prepared. 3.684 kg (10.0 mol) of 4,4'-bis(3-aminophenoxy)biphenyl, 1.081 kg (4.96 mol) of pyromellitic dianhydride, 1.459 kg (4.86 mol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 0.148 kg (1.0 mol) of phthalic anhydride, and 21.53 kg of m-cresol were added to the vessel. The mixture was then heated to 200°C with stirring under a nitrogen atmosphere. During this time, approximately 350 ml of water was confirmed to be distilled off. The reaction was continued for another 6 hours at 200°C. The mixture was then cooled to room temperature, and 10.8 kg of methanol was added. This was then filtered to obtain a yellow polyimide powder. This polyimide powder was washed with methanol and then dried at 200°C for 24 hours to obtain 5.46 kg (yield 98.5%) of polyimide powder (powder of thermoplastic polyimide (TPI (a1))). Using a Takayasu 25 mm diameter extruder, this polyimide powder was extruded at 400°C to obtain pellets (1). The glass transition temperature of the thermoplastic polyimide (TPI (a1)) was 235°C, and the imide group concentration was 23.9%. Furthermore, when the acid anhydride in the pellets was confirmed by the GC-FID analysis described below, pellets (1) contained no acid anhydrides other than phthalic anhydride. The content of phthalic anhydride (acid anhydride) in pellets (1) was measured and found to be 0.0006% by mass (6 ppm) relative to the total amount of TPI (a1) and phthalic anhydride. The method for measuring the content will be described later.

[0120] A rectangular conductor (material: copper with an oxygen content of 15 ppm) was prepared as the conductor. The length of the long side of the rectangular cross section of the rectangular conductor was 3.2 mm. The length of the short side of the rectangular cross section of the rectangular conductor was 2.4 mm. The curvature radius of the chamfer at the four corners of the rectangular cross section of the rectangular conductor was 0.3 mm.

[0121] An extruder equipped with a die (screw: 20 mm diameter, full flight, L / D = 24, compression ratio: 3.0) was prepared. The heating temperature in the cylinder section of the extruder was controlled by dividing it into three zones, C1, C2, and C3, in order from the material input side. The heating temperatures were set to 380°C in the C1 zone, 410°C in the C2 and C3 zones, and 420°C in the head section behind the cylinder of the extruder and the die at the end of the head section. The shape of the die hole was similar to the cross-sectional shape of the flat rectangular conductor. Pellets (1) were introduced into the extruder, and the molten pellets (1) were directly coated on the outer periphery of a conductor preheated to 350°C, followed by cooling. This produced an insulated electric wire of Example 1. The insulated electric wire included a conductor and a coating directly covering the conductor. The coating was a film of TPI (a1), and when the acid anhydrides in the coating were confirmed by the GC-FID analysis described below, it was found that the coating contained no acid anhydrides other than phthalic anhydride. The content of phthalic anhydride (acid anhydride) in the coating (the content ratio relative to the total amount of TPI (a1) and phthalic anhydride) was greater than 0% by mass (0 ppm) and 0.0006% by mass or less (6 ppm or less). The outer shape of the cross section of the insulated wire was similar to the shape of the conductor. The thickness of the coating was 100 μm. The method for measuring the content will be described later.

[0122] [Example 2] An insulated wire of Example 2 was obtained in the same manner as Example 1, except that the die was changed. The thickness of the coating was 80 μm. The coating was a TPI (a1) film, and did not contain any acid anhydrides other than phthalic anhydride. The content of phthalic anhydride (acid anhydride) in the coating (the content ratio relative to the total amount of TPI (a1) and phthalic anhydride) was greater than 0% by mass (0 ppm) and 0.0006% by mass or less (6 ppm or less). The acid anhydride content was confirmed and the content was measured in the same manner as Example 1.

[0123] [Comparative Example 1] An insulated wire of Comparative Example 1 was obtained in the same manner as in Example 1, except that pellet (2) was used instead of pellet (1). Pellet (2) was made of a thermoplastic polyimide (TPI (a2-1)) having a structural unit represented by the following formula (4). The thermoplastic polyimide (TPI (a2-1)) was synthesized by the following procedure.

[0124]

[0125] A vessel equipped with a stirrer, reflux condenser, water separator, and nitrogen inlet tube was prepared. 3.680 kg (10 mol) of 4,4'-bis(3-aminophenoxy)biphenyl, 2.071 kg (9.5 mol) of pyromellitic dianhydride, 0.148 kg (1.0 mol) of phthalic anhydride, and 21.53 kg of m-cresol were added to the vessel. The mixture was then heated to 200°C with stirring under a nitrogen atmosphere. During this time, approximately 350 ml of water was confirmed to be distilled off. The reaction was continued for another 6 hours at 200°C. The mixture was then cooled to room temperature, and 10.8 kg of toluene was added. The mixture was then filtered to obtain a yellow polyimide powder. This polyimide powder was washed with toluene and dried at 180°C for 24 hours to obtain 5.46 kg (yield 98.5%) of polyimide powder (powder of thermoplastic polyimide (TPI (a2-1))). This polyimide powder was extruded at 400°C using a Takayasu type 25 mm diameter extruder to obtain pellets (2). The glass transition temperature of the thermoplastic polyimide (TPI (a2-1)) was 245°C, and the imide group concentration was 25.5%.

[0126] Comparative Example 2 Except for changing the die, an insulated wire of Comparative Example 2 was obtained in the same manner as in Comparative Example 1. The thickness of the coating was 80 μm.

[0127] [Comparative Example 3] An insulated wire of Comparative Example 3 was obtained in the same manner as in Example 1, except that pellet (3) was used instead of pellet (1). Pellet (3) was made of a thermoplastic polyimide (TPI (a2-2)) having a structural unit represented by the above formula (4). The thermoplastic polyimide (TPI (a2-2)) was synthesized according to the following procedure.

[0128] A vessel equipped with a stirrer, reflux condenser, water separator, and nitrogen inlet tube was prepared. 2.873 kg (7.8 mol) of 4,4'-bis(3-aminophenoxy)biphenyl, 1.570 kg (7.25 mol) of pyromellitic dianhydride, 0.148 kg (1.0 mol) of phthalic anhydride, and 21.53 kg of m-cresol were added to the vessel. The mixture was then heated to 200°C with stirring under a nitrogen atmosphere. During this time, approximately 350 ml of water was confirmed to be distilled off. The reaction was continued for another 6 hours at 200°C. The mixture was then cooled to room temperature, and 10.8 kg of toluene was added. The mixture was then filtered to obtain a yellow polyimide powder. This polyimide powder was washed with toluene and dried at 180°C for 24 hours to obtain 5.46 kg (yield 98.5%) of polyimide powder (powder of thermoplastic polyimide (TPI (a2-2))). This polyimide powder was extruded at 400°C using a Takayasu type 25 mm diameter extruder to obtain pellets (3). The thermoplastic polyimide (TPI (a2-2)) had a glass transition temperature of 240°C and an imide group concentration of 25.5%.

[0129] Comparative Example 4 Except for changing the die, an insulated wire of Comparative Example 4 was obtained in the same manner as in Comparative Example 3. The thickness of the coating was 80 μm.

[0130] Comparative Example 5 An insulated wire of Comparative Example 5 was obtained in the same manner as in Example 1, except that pellet (4) was used instead of pellet (1). Pellet (4) was made of a thermoplastic polyimide (TPI (a3)) having a structural unit represented by the following formula (5). The thermoplastic polyimide (TPI (a3)) represented by the following formula (5) was synthesized according to the following procedure. In formula (5), the ratio of m to n was 1:9.

[0131]

[0132] A vessel equipped with a stirrer, reflux condenser, water separator, and nitrogen inlet tube was charged with 3.312 kg (9.0 mol) of 4,4'-bis(3-aminophenoxy)biphenyl, 0.2 kg (1.0 mol) of 4,4'-diaminodiphenyl ether, 2.071 kg (9.5 mol) of pyromellitic dianhydride, 0.148 kg (1.0 mol) of phthalic anhydride, and 21.53 kg of m-cresol. The mixture was then heated to 200°C with stirring under a nitrogen atmosphere. During this time, approximately 350 ml of water was confirmed to be distilled off. The reaction was continued for another 6 hours at 200°C. The mixture was then cooled to room temperature, and 10.8 kg of methanol was added. The mixture was then filtered to obtain a yellow polyimide powder. This polyimide powder was washed with methanol and dried at 180°C for 24 hours to obtain 5.26 kg (yield 98%) of polyimide powder (powder of thermoplastic polyimide (TPI (a3))). This polyimide powder was extruded at 400°C using a Takayasu type 25 mm diameter extruder to obtain pellets (4). The thermoplastic polyimide (TPI (a3)) had a glass transition temperature of 253°C and an imide group concentration of 26.3%.

[0133] Comparative Example 6 Except for changing the die, an insulated wire of Comparative Example 6 was obtained in the same manner as in Comparative Example 5. The thickness of the coating was 80 μm.

[0134] [Comparative Example 7] An insulated wire of Comparative Example 7 was obtained in the same manner as in Example 1, except that pellet (5) was used instead of pellet (1) and the extrusion temperature and the conductor preheating temperature were changed as follows.

[0135] As the pellets (5), polyether ether ketone (PEEK) (manufactured by Solvay Specialty Polymers, trade name: "KetaSpire KT-851NL", glass transition temperature: 150°C) was used.

[0136] The extrusion temperatures were changed to 300°C in the C1 zone, 380°C in the C2 and C3 zones, 390°C in the head behind the cylinder of the extruder, and 400°C in the die at the end of the head. The conductor preheating temperature was changed to 180°C.

[0137] Comparative Example 8 Except for changing the die, an insulated wire of Comparative Example 8 was obtained in the same manner as in Comparative Example 7. The thickness of the coating was 200 μm.

[0138] <Evaluation> The partial discharge inception voltage (25° C.) and other properties of Examples 1 and 2 and Comparative Examples 1 to 8 were measured as follows. The measurement results are shown in Table 1.

[0139] (Partial Discharge Inception Voltage (25°C)) A partial discharge tester (manufactured by Kikusui Electronics Co., Ltd., product name: "KPD 2050") was used to measure the partial discharge inception voltage of an insulated wire. A sample was prepared by closely bonding two insulated wires. In the sample, the flat surfaces of the outer circumferential surfaces of the two insulated wires were closely bonded to each other with no gap over a length of 150 mm. The "flat surface" refers to the surface formed by the long side of the longitudinal cross section of a rectangular insulated wire, which is continuous in the axial direction. Electrodes were connected between the two conductors of this sample, and a 50 Hz AC voltage was applied at a measurement temperature of 25°C, and the voltage at which a 10 pC partial discharge occurred was read as the peak voltage (Vp). The read peak voltage (Vp) was defined as the partial discharge inception voltage (25°C) (hereinafter also referred to as "Vp (25°C)"). An acceptable Vp (25°C) is 1000 Vp or higher.

[0140] (Partial Discharge Inception Voltage (200°C)) In the same manner as in the partial discharge inception voltage (25°C), except that the measurement temperature was changed to 200°C, the voltage at the time when a partial discharge of 10 pC occurred was read as the peak voltage (Vp). The peak voltage (Vp) read out was taken as the partial discharge inception voltage (200°C) (hereinafter also referred to as "Vp (200°C)"). The allowable Vp (200°C) is 1000 Vp or more.

[0141] (Heat Resistance) The heat resistance of the insulated wire was quantitatively evaluated using the reduction rate of the partial discharge inception voltage. The reduction rate of the partial discharge inception voltage is expressed by the following formula (A). An acceptable reduction rate of the partial discharge inception voltage is 3% or less.

[0142] Formula (A): Reduction rate of partial discharge inception voltage = ((Vp (25°C) - Vp (200°C)) / Vp (25°C)) × 100

[0143] (Adhesion) The adhesion of the insulated wire was evaluated by the bending workability test described below. The insulated wire was cut to obtain a straight test piece with a length of 300 mm. A scratch (notch) approximately 5 μm deep and 2 μm long was made in the center of the coating on the edge surface of the straight test piece using a special tool in both the longitudinal and perpendicular directions. At this time, the coating and the conductor were in close contact with each other and did not peel off. The "edge surface" refers to the surface formed by the short side of the rectangular insulated wire in the cross section, which continues in the axial direction. The straight test piece was bent 180° (in a U-shape) with the scratch as the apex and the rectangular conductor as the axis, and maintained this state for 5 minutes. The progression of peeling between the conductor and the coating, which occurred near the apex of the straight test piece, was visually observed. The adhesion of the insulated wire was evaluated according to the following evaluation criteria. An acceptable adhesion rating for an insulated wire was "A."

[0144] -Evaluation criteria- A: None of the scratches formed in the coating expanded, and the coating did not peel off from the conductor. B: At least one of the scratches formed in the coating expanded, and the entire coating peeled off from the conductor.

[0145] (Melt Flow Rate (MFR)) The melt flow rate (MFR) was measured using a melt indexer (manufactured by Toyo Seiki Seisakusho, Ltd., model number: A-371401705). The measurement was performed in accordance with JIS K7210-1:2014, with a cylinder temperature of 400°C and a load of 1.05 kg. The unit is g / 10 min.

[0146] (Glass Transition Temperature) The glass transition temperature was measured using a differential scanning calorimeter (DSC220C, manufactured by Seiko Instruments Inc.). Specifically, approximately 5 mg of material was sealed in a measurement aluminum pan and set in the differential scanning calorimeter. Then, the material was heated from room temperature to 450°C at 10°C / min. To completely melt the resin, the material was held at 450°C for 5 minutes and then cooled to 30°C at 10°C / min. After leaving the material at 30°C for 5 minutes, the material was heated a second time to 450°C at 10°C / min. The inflection point corresponding to the glass transition was taken as the glass transition temperature (Tg). The acceptable glass transition temperature is 200°C.

[0147] (Acid Anhydride Content) The acid anhydride content in the pellets or coating was measured by GC-FID analysis using a gas chromatograph (GC-20214, manufactured by Shimadzu Corporation). Specifically, a calibration curve was prepared in advance from a solvent containing varying concentrations of acid anhydride (phthalic anhydride in this example), and the peak area value of the obtained component was determined from the calibration curve to calculate the acid anhydride concentration. <Measurement Conditions> Column: DB-5 (30 m x 0.53 mm, film thickness 5 μm) Column temperature: 80°C Inlet temperature: 250°C Detector temperature: 250°C Carrier gas: Helium

[0148] (Crystallinity) The crystallinity was determined by the method described above. The results are shown in Table 1. The measurement device and measurement conditions used are as follows: Device: Rigaku RINT2550 Accessory: Rotating sample stage X-ray source: CuKα Output: 40 kV, 370 mA Detector: Scintillation counter Scanning range (measurement range): 2θ = 5° to 35°

[0149]

[0150] The evaluation results in Table 1 show that the insulated wire of Example 1 has a higher partial discharge inception voltage than the insulated wires of Comparative Examples 1, 3, 5, and 7. The insulated wire of Example 2 also has a higher partial discharge inception voltage than the insulated wires of Comparative Examples 2, 4, and 6. These results show that the insulated wires of the Examples have excellent insulating properties.

[0151] The disclosure of Japanese Patent Application No. 2024-044135, filed on March 19, 2024, is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual publication, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. An insulated wire comprising a conductor and a coating covering the conductor, wherein the coating comprises a thermoplastic polyimide containing a structural unit represented by the following formula (1): (In the formula (1), the benzene rings contained in the structural unit represented by the formula (1) may each independently have a substituent.) 2. The insulated wire according to claim 1, wherein the coating has a crystallinity of 10% or less after being heated at 300°C for 4 hours in an inert gas atmosphere.

3. The insulated wire according to claim 1, wherein the structural unit represented by formula (1) accounts for 20 mol % to 80 mol % of the entire thermoplastic polyimide.

4. The insulated wire according to claim 1, wherein the thermoplastic polyimide further contains a structural unit represented by the following formula (2): (In the formula (2), the benzene rings contained in the structural unit represented by the formula (2) may each independently have a substituent.) 5. The insulated wire according to claim 4, wherein the proportion of the structural unit represented by formula (1) to the total of the structural unit represented by formula (1) and the structural unit represented by formula (2) in the thermoplastic polyimide is 20 mol % to 80 mol %.

6. The insulated wire according to claim 1, wherein the imide group concentration of the thermoplastic polyimide is 4% to 35%.

7. The insulated wire according to claim 1, wherein the coating further contains an acid anhydride, and when the total amount of the thermoplastic polyimide and the acid anhydride contained in the coating is taken as 100% by mass, the acid anhydride content is greater than 0% by mass and 0.002% by mass or less.

8. A coil comprising an insulated wire according to any one of claims 1 to 7.

9. A motor comprising the coil according to claim 8.

10. A thermoplastic polyimide resin composition comprising a thermoplastic polyimide containing a structural unit represented by the following formula (1) and an acid anhydride, wherein the acid anhydride content is greater than 0% by mass and not more than 0.002% by mass when the total amount of the thermoplastic polyimide and the acid anhydride is taken as 100% by mass. (In the formula (1), the benzene rings contained in the structural unit represented by the formula (1) may each independently have a substituent.) 11. The thermoplastic polyimide resin composition according to claim 10, which is used to form an insulating coating by extrusion coating.

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