Insulated electric wire

Abstract

An insulated electric wire includes a conductor in a wire shape and an insulating layer that covers an outer peripheral surface of the conductor, wherein the insulating layer contains a first solvent having a relative dielectric constant of 15 or more and a second solvent having a relative dielectric constant lower than 15 as residual solvents, a ratio of a content of the second solvent contained in the insulating layer with respect to a total content of the first solvent and the second solvent contained in the insulating layer, that is, a first ratio is 50% by mass or more, and a ratio of a content of the second solvent contained in the insulating layer after a heating treatment in which the insulated electric wire is heated at 350°C for 1 minute with respect to a total content of the first solvent and the second solvent contained in the insulating layer after the heating treatment, that is, a second ratio is higher than the first ratio.

Description

Technical Field

[0001] This disclosure relates to insulated electrical wires. This application claims priority based on Japanese Patent Application No. 2021-136332, filed on August 24, 2021. The entire contents of that Japanese patent application are incorporated herein by reference. Background Technology

[0002] Conventionally, insulated wires having a linear conductor and an insulating layer covering the outer periphery of the conductor are known. These insulated wires are suitable for use as coils in, for example, motors and transformers. Japanese Patent Application Publication No. 2004-269770 (Patent Document 1) discloses a polyurethane-based coating for insulated wires that enables the manufacture of insulated wires with excellent insulation properties by minimizing the amount of residual solvent in the insulating film.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2004-269770 Summary of the Invention

[0006] The insulated wire disclosed herein includes a linear conductor and an insulating layer covering the outer peripheral surface of the conductor. The insulating layer contains a first solvent with a relative permittivity of 15 or higher and a second solvent with a relative permittivity of less than 15 as residual solvents. The ratio of the content of the second solvent in the insulating layer to the total content of the first and second solvents in the insulating layer, i.e., a first ratio, is 50% by mass or higher. The ratio of the content of the second solvent in the insulating layer after heat treatment of the insulated wire at 350°C for 1 minute to the total content of the first and second solvents in the heat-treated insulating layer, i.e., a second ratio, is higher than the first ratio. Detailed Implementation

[0007] [The problem this disclosure aims to solve]

[0008] From the perspective of reducing manufacturing costs, it is desirable to cover the outer periphery of the conductor with an insulating layer by drying the insulating varnish applied and baked on the outer periphery of the conductor in a short time and at a low temperature. In this case, polar solvents from the insulating varnish tend to remain in the insulating layer, causing the dielectric constant of the insulating layer to increase, thereby potentially hindering the insulation properties. On the other hand, from the perspective of resin storage stability, it is necessary to contain a specified amount of polar solvent in the insulating varnish. Therefore, in insulated wires where the insulating layer is formed by drying the insulating varnish in a short time and at a low temperature, there is an urgent need to develop an insulated wire that, by containing a specified amount of polar solvent, can maintain the storage stability of the insulating varnish and reduce the dielectric constant of the insulating layer, thereby improving the insulation properties.

[0009] In view of the above-mentioned actual situation, the purpose of this disclosure is to provide an insulated wire with a reduced dielectric constant of the insulation layer.

[0010] [The Effects of This Disclosure]

[0011] According to this disclosure, it is possible to provide insulated wires with reduced dielectric constant of the insulation layer.

[0012] [Description of embodiments of this disclosure]

[0013] The inventors conducted repeated and in-depth research to solve the aforementioned problems, resulting in this disclosure. Specifically, they focused on reducing the residual amount of high-dielectric-constant solvent in the insulating layer by adding a low-dielectric-constant solvent and a polar solvent (hereinafter also referred to as "high-dielectric-constant solvent") to the insulating varnish used to form the insulating layer. As a result, it was surprisingly found that when an insulating layer is formed on the outer peripheral surface of a conductor using an insulating varnish containing a low-dielectric-constant solvent with a boiling point higher than that of the high-dielectric-constant solvent, the dielectric constant of the insulating layer can be reduced, thereby improving the insulation properties.

[0014] First, embodiments of this disclosure will be described.

[0015] [1] One aspect of the insulated wire disclosed herein includes a linear conductor and an insulating layer covering the outer peripheral surface of the conductor. The insulating layer contains a first solvent with a relative permittivity of 15 or higher and a second solvent with a relative permittivity of less than 15 as residual solvents. The ratio of the content of the second solvent in the insulating layer to the total content of the first and second solvents in the insulating layer, i.e., a first ratio, is 50% by mass or higher. After heat treatment of the insulated wire at 350°C for 1 minute, the ratio of the content of the second solvent in the insulating layer to the total content of the first and second solvents in the heat-treated insulating layer, i.e., a second ratio, is higher than the first ratio. An insulated wire with these characteristics can reduce the dielectric constant of the insulating layer, thereby improving insulation properties.

[0016] [2] Preferably, the insulating layer comprises one or more resins selected from the group consisting of polyimide resin, polyamide-imide resin, polyester-imide resin, and polyether-imide resin. This reduces the dielectric constant in the insulating layer comprising a resin commonly used in insulated wires.

[0017] [3] Preferably, the insulating layer comprises the first solvent and the second solvent in a total amount of 0.2% by mass or more and 10% by mass or less. This further reduces the dielectric constant of the insulating layer.

[0018] [4] Preferably, the insulating layer contains 5% by mass or less of the first solvent and 0.1% by mass or more and 5% by mass or less of the second solvent. This further reduces the dielectric constant of the insulating layer.

[0019] [5] Preferably, the insulating layer contains a curing agent, which comprises one or more selected from the group consisting of alicyclic anhydrides, aliphatic anhydrides, aromatic anhydrides, imidazole, triethylamine, titanium compounds, isocyanate compounds, terminated isocyanates, urea, melamine, melamine compounds, and acetylene derivatives. This promotes the formation of an insulating layer with the above-described characteristics from the insulating varnish.

[0020] [Details of the embodiments of this disclosure]

[0021] The embodiments of this disclosure (hereinafter also referred to as "this embodiment") will be described in further detail below. In this specification, expressions such as "A to B" refer to the upper and lower limits of a range (i.e., above A and below B). Where no unit is specified in A but only in B, the unit of A is the same as the unit of B. Furthermore, in this specification, the terms "dielectric constant" and "relative dielectric constant" are used in the same context as indicators of the conductivity of a medium. In this specification, "high dielectric constant" means a relative dielectric constant of 15 or higher, and "low dielectric constant" means a relative dielectric constant lower than 15. Additionally, in this specification, "solvent" refers to a substance used to dissolve other substances. Therefore, in this specification, within the scope of "solvent," any material used to dissolve other substances naturally includes materials that are liquid at room temperature and materials that are solid at room temperature.

[0022] [Insulated wire]

[0023] The insulated wire of this embodiment comprises a linear conductor and an insulating layer covering the outer periphery of the conductor. The insulating layer contains a first solvent with a relative permittivity of 15 or higher and a second solvent with a relative permittivity of less than 15 as residual solvents. The ratio of the content of the second solvent in the insulating layer to the total content of the first and second solvents in the insulating layer, i.e., a first ratio, is 50% by mass or higher. Furthermore, after heat treatment of the insulated wire at 350°C for 1 minute, the ratio of the content of the second solvent in the insulating layer to the total content of the first and second solvents in the heat-treated insulating layer, i.e., a second ratio, is higher than the first ratio. Insulated wires with these characteristics can reduce the dielectric constant of the insulating layer, thereby improving insulation properties.

[0024] Regarding the reasons why the aforementioned insulated wires can reduce the dielectric constant of the insulation layer and thus improve insulation properties, although the details are unclear, the following speculations are made.

[0025] That is, the insulated wire of this embodiment contains not only a first solvent with a relative permittivity of 15 or higher as a residual solvent in the insulation layer, but also a second solvent with a relative permittivity of less than 15 as a residual solvent. Furthermore, the ratio of the content of the second solvent in the insulation layer to the total content of the first and second solvents in the insulation layer, i.e., the first ratio, is 50% by mass or higher. It can be considered that since the first solvent has a relative permittivity of 15 or higher and is a high-dielectric-constant solvent, and since the second solvent has a relative permittivity of less than 15 and is a low-dielectric-constant solvent, the dielectric constant of the insulation layer, in which more than half of the residual solvent is the second solvent, is suppressed to a low level.

[0026] Furthermore, in this embodiment, when the insulated wire is heated at 350°C for 1 minute, the ratio (second ratio) of the content of the second solvent in the insulation layer to the total content of the first solvent and the second solvent in the insulation layer is higher than the first ratio. The second ratio being higher than the first ratio means that the first solvent evaporates from the insulation layer more actively than the second solvent during the heat treatment. Here, the heat treatment simulates a part of the drying process performed in the manufacturing process of the insulated wire, and therefore can be understood as follows: in the manufacturing process of the insulated wire, the second solvent preferentially remains in the insulation layer compared to the first solvent. Thus, it can be understood that by slightly reducing the content of the second solvent in the insulating varnish and ensuring a sufficient content of the first solvent, the preservation stability of the resin can be maintained, and more than half of the residual solvent in the insulation layer in the insulated wire can be the second solvent, thereby reducing the dielectric constant of the insulation layer as described above. In summary, it is speculated that the insulated wire of this embodiment can maintain the preservation stability of the insulating varnish and reduce the dielectric constant of the insulation layer, thereby improving insulation properties.

[0027] <Conductor>

[0028] As described above, the insulated wire of this embodiment includes a linear conductor. This conductor is a conductor. Preferably, the conductor material is a metal with high conductivity and high mechanical strength. Specifically, examples include copper, copper alloys, aluminum, aluminum alloys, nickel, silver, soft iron, steel, and stainless steel. The conductor can be a wire formed from these metals, a coated wire formed by coating the surface of the wire with another metal, or a stranded wire formed by twisting multiple wires together. Examples of coated wires include nickel-coated copper wire, silver-coated copper wire, silver-coated aluminum wire, and copper-coated steel wire, but the method is not limited to these.

[0029] The shape of the conductor is not particularly limited, and round or square wires can be appropriately selected based on the intended use and electrical characteristics of the insulated wire. That is, in the cross-section obtained by cutting the insulated wire with a plane perpendicular to its length, the conductor's cross-sectional shape can be circular or flat. The conductor's diameter or outer circumference length is also not particularly limited and can be appropriately selected based on the intended use and electrical characteristics of the insulated wire.

[0030] In this specification, "flat" as one of the cross-sectional shapes of a conductor includes rectangles and squares, and also includes shapes in which the four corners of these rectangles and squares are chamfered, as well as shapes with a radius (R-shape), etc.

[0031] The lower limit of the conductor's cross-sectional area is preferably 0.01 mm². 2More preferably 0.1mm 2 The upper limit is preferably 20mm. 2 More preferably 10mm 2 When the cross-sectional area of ​​the conductor is less than 0.01 mm² 2 In cases where the volume ratio of the insulation layer to the conductor increases, the volumetric efficiency of coils formed using insulated wires may decrease. This is especially true when the conductor's cross-sectional area exceeds 20 mm². 2 In order to fully improve the insulation of the insulated wire, the insulation layer needs to be thickened. As a result, the insulated wire will be larger in diameter and there is a tendency for it to be difficult to wind it densely around the iron core.

[0032] <Insulation layer>

[0033] As described above, the insulated wire of this embodiment includes an insulating layer covering the outer peripheral surface of the conductor. This insulating layer can be any of the conventionally known resins used to form the insulating layer in this insulated wire.

[0034] Specifically, examples of resins contained in the insulating layer include: polyvinyl alcohol formaldehyde resin, polyurethane resin, alkyl resin, epoxy resin, phenoxy resin, polyester resin, polyesterimide resin, polyesteramideimide resin, polyamideimide resin, polyimide resin, and other thermosetting resins; as well as thermoplastic resins such as polyetherimide resin, polyetheretherketone resin, polyethersulfone resin, and polyimide resin. These resins may be contained individually or in combination of two or more.

[0035] Particularly preferred is that the above-mentioned insulating layer comprises one or more resins selected from the group consisting of polyimide resin, polyamide-imide resin, polyester-imide resin, and polyether-imide resin. This allows for a reduction in the dielectric constant of the insulating layer comprising a resin commonly used in insulated wires. From the viewpoint of improving strength and heat resistance, the above-mentioned insulating layer is more preferably composed of a thermosetting polyimide resin.

[0036] Furthermore, when the insulating layer contains a polyimide resin, the polyimide resin may be a monomer containing a small amount of polyimide or a polyimide resin with introduced functional groups.

[0037] The minimum thickness of the insulation layer is preferably 5 μm, and the maximum thickness is preferably 200 μm. If the insulation layer thickness is less than 5 μm, the insulation layer is prone to damage, and the insulation of the conductor may become insufficient. If the insulation layer thickness exceeds 200 μm, the volumetric efficiency of coils formed using insulated wires tends to decrease.

[0038] When the conductor has a flat cross-sectional shape, the insulation thickness refers to the average thickness of the insulation layer on two pairs of opposing surfaces (upper surface, lower surface, left surface, and right surface) of the outer periphery of the conductor, obtained by cutting the insulated wire with a plane perpendicular to its long dimension. Specifically, the cross-section obtained by cutting the insulated wire with a plane perpendicular to its long dimension is ground to create the measurement surface. Then, the measurement surface is photographed using a digital microscope VHX-7000 (manufactured by KEYENCE Co., Ltd.) to obtain an image. Finally, the insulation layer thickness on the two pairs of opposing surfaces of the outer periphery of the conductor in the image can be calculated by selecting one point each from the upper surface, lower surface, left surface, and right surface of the conductor, and the average value is calculated based on the values ​​obtained by measuring the insulation layer thickness at these four points. This average value is taken as the insulation layer thickness. When the cross-sectional shape of the conductor is circular, the thickness of the insulation layer can be determined as follows: Four measurement sites are selected at equal intervals in the annular insulation layer in the image of the measured object surface taken by the digital microscope, and the thickness of the insulation layer at these four measurement sites is measured and determined by the average value of the thickness.

[0039] (Residual solvent)

[0040] The aforementioned insulating layer contains a first solvent with a relative permittivity of 15 or higher and a second solvent with a relative permittivity of less than 15 as residual solvents. In this specification, "residual solvent" refers to the solvent component contained in the insulating varnish applied to the outer periphery of the conductor during the manufacturing process of the insulated wire, which remains in the insulating layer even after the insulating layer is formed by baking the insulating varnish onto the conductor. The aforementioned insulating varnish is sometimes prepared by diluting the aforementioned resin or its resin precursor with an organic solvent containing at least the first and second solvents. The aforementioned organic solvent may consist of the first and second solvents.

[0041] 1) First solvent

[0042] The first solvent is a solvent with a high dielectric constant of 15 or higher. As a first solvent, any solvent with a high dielectric constant of 15 or higher can be a conventionally known organic solvent or a material that is solid at room temperature. Specifically, examples of first solvents include polar organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide, dimethyl sulfoxide, tetramethylurea, hexaethyl phosphate triamide, and γ-butyrolactone; and ketone-based organic solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. These organic solvents can be used individually or in combination of two or more.

[0043] 2) Second solvent

[0044] The second solvent is a low dielectric constant solvent with a relative permittivity of less than 15. As the second solvent, conventionally known organic solvents with a relative permittivity of less than 15 and whose relationship with the first solvent satisfies the conditions described later, as well as materials that are solid at room temperature, can be used. Specifically, examples of second solvents include: ester-based organic solvents such as methyl acetate, ethyl acetate, butyl acetate, and diethyl oxalate; ether-based organic solvents such as diethyl ether, ethylene glycol dimethyl ether, diethylene glycol monomethyl ether, ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol dimethyl ether, and tetrahydrofuran; hydrocarbon-based organic solvents such as hexane, heptane, benzene, toluene, xylene, and naphtha; halogen-based organic solvents such as dichloromethane and chlorobenzene; phenol-based organic solvents such as cresol and chlorophenol; and amine-based organic solvents such as pyridine. Examples of materials that are solid at room temperature and serve as the second solvent include paraffin wax. These organic solvents and materials that are solid at room temperature can be used individually or in combination of two or more.

[0045] In this specification, when the second solvent is naphtha or paraffin, the "boiling point" of the second solvent refers to the boiling point (the so-called "dry point") of the compound contained in the second solvent that has the highest boiling point.

[0046] Here, the low-dielectric-constant solvent that can be used as the second solvent satisfies the following two conditions in relation to the first solvent (high-dielectric-constant solvent). Specifically, the first condition is that the ratio of the content of the second solvent contained in the insulation layer of the aforementioned insulated wire to the total content of the first solvent and the second solvent contained in the insulation layer of the aforementioned insulated wire, i.e., the first ratio, is 50% by mass or more. Therefore, it has a relationship where, as a residual solvent contained in the aforementioned insulation layer, the second solvent is equal in mass to the first solvent, or the second solvent is equal in mass to or greater than the first solvent.

[0047] Furthermore, the second condition is that the ratio of the content of the second solvent contained in the insulation layer after heat treatment (heating the insulated wire at 350°C for 1 minute) to the total content of the first solvent and the second solvent contained in the insulation layer after heat treatment, i.e., the second ratio, is higher than the first ratio. In this case, in the insulation layer of the insulated wire of this embodiment, the first solvent evaporates more actively than the second solvent during the heat treatment, so it can be understood that the boiling point of the second solvent is higher than that of the first solvent. That is, there is a relationship that the boiling point of the second solvent, as a residual solvent contained in the insulation layer, is preferably higher than that of the first solvent. By satisfying the first and second conditions described above, the insulated wire of this embodiment has a slightly lower content of the second solvent in the insulating varnish and a sufficient content of the first solvent, thereby maintaining the storage stability of the resin and suppressing the dielectric constant of the insulation layer to a low level, as more than half of the residual solvent contained in the insulation layer of the insulated wire is the second solvent.

[0048] Regarding the aforementioned first ratio, from the viewpoint of further reducing the dielectric constant of the insulating layer, its value is preferably 70% by mass or more, more preferably 80% by mass or more. If the aforementioned second ratio can be 100% by mass, then the upper limit of the aforementioned first ratio is 99.99% by mass. There is no particular limitation as long as the value of the aforementioned second ratio is higher than the value of the aforementioned first ratio. From the viewpoint of further reducing the dielectric constant of the insulating layer, the difference between the value of the aforementioned second ratio and the value of the first ratio (second ratio - first ratio) is preferably 5 or more, more preferably 10 or more. It should be noted that the upper limit of the aforementioned second ratio is also 100% by mass, which is considered an ideal value.

[0049] 3) The content and total content of the first and second solvents in the insulation layer of insulated wires, etc.

[0050] The insulating layer preferably comprises a total of 0.2% by mass and 10% by mass of the first solvent and the second solvent. This further reduces the dielectric constant of the insulating layer.

[0051] If the total content of the first solvent and the second solvent in the insulating layer is less than 0.2% by mass, the effect of reducing the dielectric constant caused by the presence of the second solvent may not be sufficiently confirmed. If the total content of the first solvent and the second solvent in the insulating layer exceeds 10% by mass, there may be concerns about an increase in the dielectric constant based on the first solvent and a decrease in the strength of the insulating layer. More preferably, the insulating layer contains a total of 0.5% by mass or more and 5% by mass or less, and more preferably, a total of 1% by mass or more and 4% by mass or less of the first solvent and the second solvent.

[0052] Furthermore, preferably, the insulating layer contains 5% by mass or less of the first solvent, and the insulating layer contains 0.1% by mass or more and 5% by mass or less of the second solvent. In this case, the dielectric constant of the insulating layer can be further reduced.

[0053] When the content of the first solvent in the aforementioned insulating layer exceeds 5% by mass, there may be concerns about an increase in the dielectric constant based on the first solvent. On the other hand, the lower limit of the content of the first solvent in the aforementioned insulating layer can be set to 0.02% by mass. When the content of the second solvent in the aforementioned insulating layer before the aforementioned heat treatment is less than 0.1% by mass, the effect of reducing the dielectric constant based on the second solvent may be insufficient. When the content of the second solvent in the aforementioned insulating layer before the aforementioned heat treatment exceeds 5% by mass, the strength of the insulating layer may be impaired. More preferably, the aforementioned insulating layer before the aforementioned heat treatment contains 3% or less of the first solvent by mass, and more preferably, it contains 1.5% or less of the first solvent by mass. More preferably, the aforementioned insulating layer contains 0.5% or more and 4.5% or less of the second solvent by mass, and more preferably, it contains 1.5% or more and 4% or less of the second solvent by mass.

[0054] (Determination Method)

[0055] The contents of the first and second solvents in the insulation layer of the insulated wire can be determined, for example, using a pyrolysis gas chromatography-mass spectrometry (Py-GC / MS, trade name: "6890N / 5973Network", manufactured by Agilent Technologies). In this case, the atmosphere is He gas, and the flow rate is 1 mL / min. Furthermore, the pyrolysis temperature is set to 500°C × 1 min, which is sufficient to completely evaporate the residual solvent.

[0056] Details of the determination conditions for the contents of the first solvent and the second solvent in the insulating layer using the above-described Py-GC / MS are as follows.

[0057] Pyrolysis apparatus: Double-Shot Pyrolyzer (trade name: "PY-2020iD", manufactured by FRONTIER LAB) and MicroJet Cryo-Trap (trade name: MJT-1030E, manufactured by FRONTIER LAB).

[0058] Column: UA-5 (inner diameter 0.25 mm × length 30 m, film thickness 0.25 μm, manufactured by FRONTIER LAB).

[0059] Pyrolysis (temperature × time): 500℃ × 1 min.

[0060] Inlet temperature: 300℃, split ratio: 100:1.

[0061] Trap: -150℃.

[0062] Oven: 50℃ → (25℃ / min.) → 320℃ (5min.).

[0063] Ionization method: Electron ionization (EI).

[0064] MS temperature: 230℃ (ion source), 150℃ (quadrupole).

[0065] Quality range: 33~550a.mu.

[0066] Furthermore, the amounts of the first and second solvents remaining in the insulation layer after heat treatment (heating the insulated wire at 350°C for 1 minute) can be determined using the same method as for determining the amounts of the first and second solvents in the insulation layer of the insulated wire, by employing the aforementioned pyrolysis gas chromatography-mass spectrometry with the atmosphere set to He gas (flow rate 1 mL / min). That is, the amounts of solvent produced when the pyrolysis temperature in the aforementioned measurement conditions is changed to 350°C × 1 min can be subtracted from the amounts of the first and second solvents remaining in the insulation layer to determine the amounts of each solvent remaining after heat treatment.

[0067] (Curing agent)

[0068] The aforementioned insulating layer preferably contains a curing agent. In this case, the curing agent preferably comprises one or more selected from the group consisting of alicyclic anhydrides, aliphatic anhydrides, aromatic anhydrides, imidazole, triethylamine, titanium compounds, isocyanate compounds, terminal isocyanates, urea, melamine, melamine compounds, and acetylene derivatives. This promotes the formation of an insulating layer with a reduced dielectric constant from the insulating varnish. These curing agents are appropriately selected based on the type of resin or resin precursor in the insulating varnish; for example, imidazole, melamine, and melamine compounds are preferred.

[0069] [Manufacturing method of insulated wires]

[0070] Regarding the insulated wire of this embodiment, it can be obtained by applying conventionally known manufacturing methods related to such insulated wires, except for preparing a prescribed insulating varnish containing at least a first solvent and a second solvent. For example, from the viewpoint of manufacturing with good yield, the insulated wire of this embodiment is preferably obtained using the following insulated wire manufacturing method.

[0071] That is, the manufacturing method of the insulated wire according to this embodiment preferably includes: a step of preparing a conductor and an insulating varnish (first step); and a step of coating the outer peripheral surface with an insulating layer (second step). Furthermore, the coating step (second step) preferably includes: a step of applying an insulating varnish to the outer peripheral surface (step A); and a step of baking the insulating varnish onto the conductor (step B). Hereinafter, each step included in the manufacturing method of the insulated wire according to this embodiment will be described.

[0072] <First Process>

[0073] The first step is to prepare the conductor and the insulating varnish. The conductor can be prepared, for example, by obtaining commercially available products. Alternatively, it can be prepared by casting, stretching, drawing into wire, and further softening the aforementioned metal, which serves as the conductor material.

[0074] Insulating varnish can be prepared by diluting the aforementioned resin or resin precursor, which serves as the insulating layer material, with an organic solvent containing at least the aforementioned first solvent and second solvent. The resin solids concentration in the insulating varnish is preferably 15% by mass (lower limit), more preferably 20% by mass (upper limit), and preferably 50% by mass (upper limit), more preferably 30% by mass (upper limit). When the insulating varnish contains a resin precursor, the aforementioned resin solids concentration refers to the concentration of the resin precursor. Furthermore, the insulating varnish may contain a curing agent, fillers, various additives, etc., in addition to the first solvent, the second solvent, and the resin or resin precursor. The insulating varnish may also contain a solvent different from the aforementioned first solvent and second solvent.

[0075] Here, regarding the first and second solvents contained in the insulating varnish, the conditions (relative permittivity, boiling point, and residual amount before and after heat treatment, etc.) required for the solvents used as the first or second solvent, as well as the specific names of the solvents, will not be repeated. Furthermore, the specific names of the resins or resin precursors contained in the insulating varnish will also not be repeated. The mass ratio of the first solvent to the second solvent contained in the insulating varnish is preferably set to first solvent : second solvent = 99.8 : 0.2 to 70 : 30. The content of the first solvent in the insulating varnish can be set to 55 to 97% by mass, and the content of the second solvent in the insulating varnish can be set to 0.1 to 30% by mass.

[0076] As a curing agent that can be included in insulating varnishes, curing agents that have the function of curing resins or promoting the polymerization of resin precursors can be used. Specifically, examples include alicyclic anhydrides such as methyltetrahydrophthalic anhydride, aliphatic anhydrides, aromatic anhydrides, imidazoles, triethylamine, titanium compounds, isocyanate compounds, terminal isocyanates, urea, melamine compounds, and acetylene derivatives. These curing agents are appropriately selected according to the type of resin or resin precursor in the insulating varnish. For example, if the insulating varnish contains a thermosetting polyimide precursor, imidazoles are preferably used as the curing agent.

[0077] Examples of the aforementioned titanium-based compounds include tetrapropyl titanate, tetraisopropyl titanate, tetramethyl titanate, tetrabutyl titanate, and tetrahexyl titanate. Examples of the aforementioned isocyanate compounds include: aromatic diisocyanates such as toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), terephthalic diisocyanate, and naphthalene diisocyanate; aliphatic diisocyanates with 3 to 12 carbon atoms such as hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexane diisocyanate, and lysine diisocyanate; and 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), and 4,4'-dicyclohexylmethane diisocyanate (hydrogenated MDI). Alicyclic isocyanates with 5 to 18 carbon atoms, such as cyclohexane diisocyanate, isopropylidene dicyclohexyl-4,4'-diisocyanate, 1,3-diisocyanate-methylcyclohexane (hydrogenated XDI), hydrogenated TDI, 2,5-bis(isocyanate-methyl)-bicyclo[2,2,1]heptane, and 2,6-bis(isocyanate-methyl)-bicyclo[2,2,1]heptane; aliphatic diisocyanates with aromatic rings, such as phenylenediamine diisocyanate (XDI) and tetramethylphenylenediamine diisocyanate (TMXDI); and their modified forms.

[0078] Examples of the aforementioned end-capped isocyanates include diphenylmethane-4,4'-diisocyanate (MDI), diphenylmethane-3,3'-diisocyanate, diphenylmethane-3,4'-diisocyanate, diphenyl ether-4,4'-diisocyanate, benzophenone-4,4'-diisocyanate, diphenyl sulfone-4,4'-diisocyanate, toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, naphthalene-1,5-diisocyanate, isophthalimide diisocyanate, and terephthalimide diisocyanate. Examples of the aforementioned melamine compounds include melamine, methylated melamine, butylated melamine, hydroxymethylated melamine, and hydroxybutylated melamine. Examples of the aforementioned acetylene derivatives include acetylenyl aniline and acetylenyl phthalic anhydride.

[0079] <Second Process>

[0080] The second step is to coat the outer periphery of the conductor with an insulating layer. The second step may include: applying an insulating varnish to the outer periphery of the conductor (step A); and baking the insulating varnish onto the conductor (step B).

[0081] (Process A)

[0082] Step A is a process of applying the insulating varnish prepared in the first step to the outer peripheral surface of the conductor. Specifically, Step A can be performed by passing the conductor coated with the aforementioned insulating varnish through the opening of a mold. In Step A, it is preferable to apply the insulating varnish to the outer peripheral surface of the conductor with a uniform thickness using a mold having an opening.

[0083] (Process B)

[0084] Step B is the process of baking the aforementioned insulating varnish onto the conductor. Specifically, Step B can be performed by placing the conductor coated with insulating varnish, which has undergone Step A, in a baking oven and baking the insulating varnish onto the conductor. As a result, the resin solidifies as the solvent in the insulating varnish vaporizes, thereby forming an insulating layer on the outer periphery of the conductor.

[0085] The baking temperature and time of the insulating varnish in the aforementioned baking oven can be appropriately selected from known temperature and time conditions in the manufacture of such insulated wires, based on the types of resin and organic solvents in the insulating varnish. Specifically, firstly, the amount of heat sufficient to form an insulating layer on the outer periphery of the conductor and to reduce residual solvent to the desired level is calculated based on the type of resin, and then the baking temperature and time are determined based on this amount of heat. This amount of heat corresponds to the product of the baking temperature and time. If the baking temperature is high, the amount of heat can be reduced by shortening the time; if the baking temperature is low, the amount of heat can be reduced by extending the time.

[0086] Furthermore, in the second step, it is preferable to repeatedly apply and bake the aforementioned insulating varnish to create one or more insulating layers, achieving a predetermined thickness. The application and baking methods for the insulating varnish can be any conventionally known methods. Afterward, the insulating layer can be dried using conventionally known methods to obtain an insulated wire.

[0087] In this specification, an insulating layer obtained by applying and baking an insulating varnish once is referred to as a "single" insulating layer, and an insulating layer obtained by applying and baking an insulating varnish multiple times is referred to as a "multi-layer" insulating layer.

[0088] <Effects>

[0089] In summary, the insulated wire of this embodiment can be manufactured. Regarding the insulated wire manufactured by the above-described method, the dielectric constant of the insulation layer is reduced, thus improving insulation properties.

[0090] [Postscript]

[0091] The above description includes the implementation methods noted below.

[0092] <Postscript 1>

[0093] An insulated wire includes a linear conductor and an insulating layer covering the outer periphery of the conductor, wherein the insulating layer contains a first solvent with a relative permittivity of 15 or more and a second solvent with a relative permittivity of less than 15 as residual solvents, the ratio of the content of the second solvent in the insulating layer to the total content of the first solvent and the second solvent in the insulating layer, i.e., a first ratio, is 50% by mass or more, and the boiling point of the second solvent is higher than that of the first solvent.

[0094] <Appendix 2>

[0095] According to Appendix 1, the insulated wire comprises one or more resins selected from the group consisting of polyimide resin, polyamide-imide resin, polyester-imide resin, and polyether-imide resin.

[0096] <Appendix 3>

[0097] According to Appendix 1 or 2, the insulated wire contains, in which the insulation layer comprises, in aggregate, more than 0.2% by mass and less than 10% by mass of the first solvent and the second solvent.

[0098] <Appendix 4>

[0099] The insulated wire according to any one of Appendices 1 to 3, wherein the insulation layer contains less than 5% by mass of the first solvent, and the insulation layer contains more than 0.1% by mass and less than 5% by mass of the second solvent.

[0100] [Example]

[0101] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited thereto. The insulated wires of samples 2, 4, 6, 8, 10 and 12 described below are examples, and the insulated wires of samples 1, 3, 5, 7, 9, 11, 13 and 14 are comparative examples.

[0102] [Measurement and Evaluation Methods]

[0103] First, the evaluation items and their measurement methods in this embodiment will be explained.

[0104] <Thickness of insulation layer>

[0105] For each insulated wire in the examples and comparative examples, the thickness of the insulation layer (in μm) was determined using a digital microscope VHX-7000 (manufactured by KEYENCE Co., Ltd.) based on the above measurement method.

[0106] <The content and ratio of the first and second solvents in the insulation layer of insulated wires>

[0107] For each insulated wire of the Examples and Comparative Examples, based on the above-described measurement method, a pyrolysis gas chromatography-mass spectrometry (Py-GC / MS, trade name: "6890N / 5973Network", manufactured by Agilent Technologies) was used to determine the content (in ppm) of the first and second solvents in the insulation layer. The ratio of the second solvent content (first ratio) was also calculated based on the sum of these contents.

[0108] <The content and ratio of the first and second solvents remaining in the insulation layer after heat treatment>

[0109] For each insulated wire of the examples and comparative examples, based on the above-described measurement method, a pyrolysis gas chromatography-mass spectrometry (Py-GC / MS, trade name: "6890N / 5973Network", manufactured by Agilent Technologies) was used to perform heat treatment, and the contents of the first and second solvents remaining in the insulation layer after heat treatment (each residual amount, in ppm) were determined. Furthermore, based on the sum of these contents, the ratio of the content of the second solvent remaining in the insulation layer after heat treatment (second ratio) was also calculated. Here, the conditions for the above-described heat treatment are set as follows: heating temperature 350°C, heating time 1 minute, pressure 1 atm, and He atmosphere (flow rate: 1 mL / min).

[0110] <Determination of Dielectric Constant>

[0111] For each insulated wire in the examples and comparative examples, the dielectric constant of the insulation layer was determined using an impedance analyzer (trade name (model): "ZA5405", manufactured by NF Corporation). Furthermore, in the tests described later, the difference in dielectric constant of the insulation layer between the samples was also calculated.

[0112] [First Test]

[0113] <Sample Preparation>

[0114] (Sample 1)

[0115] 1) First process

[0116] Prepared to be formed of copper alloy with a flat cross-section (cross-sectional area: 5mm²). 2 A commercially available linear conductor. Then, after dissolving tetracarboxylic acid dianhydride and diamine in an equimolar ratio in a first solvent composed of N-methyl-2-pyrrolidone (NMP, relative permittivity: 32.2, boiling point: 204°C), a condensation reaction is promoted to prepare polyamic acid (a polyamide precursor) as an insulating varnish. The tetracarboxylic acid dianhydride used is pyromellitic dianhydride. The diamine used is 4,4'-diaminodiphenyl ether. This insulating varnish does not contain a second solvent.

[0117] 2) Second process

[0118] The conductor prepared in the first step is immersed in the insulating varnish prepared in the first step, thereby coating the outer peripheral surface of the conductor with the insulating varnish. Next, the conductor coated with the insulating varnish is passed through the opening of a coating mold having a shape similar to the cross-sectional shape of the conductor (Step A). ​​Then, the conductor, which has been uniformly coated with insulating varnish through Step A, is subjected to a baking process in a baking oven, thereby covering the outer peripheral surface of the conductor with an insulating layer (Step B). In the baking process, a predetermined amount of heat (+++) sufficient to form the insulating layer is calculated, and the baking temperature and time to impart this heat (+++) are determined.

[0119] Next, the insulating layer formed in the second process is repeatedly coated with the above-mentioned insulating varnish and baked on the insulating layer to obtain the insulated wire of sample 1 with multiple insulating layers.

[0120] (Sample 2)

[0121] In the first step, naphtha (relative permittivity: 1.8, boiling point: 247°C) is added to the first solvent as the second solvent, such that the mass ratio of the first solvent to the second solvent is 9:1, thereby preparing an insulating varnish. Otherwise, the insulating wire of sample 2 is obtained by the same method as sample 1.

[0122] <Evaluation of Insulated Wires>

[0123] For each insulated wire of Sample 1 and Sample 2, the material name and thickness of the insulation layer, the dielectric constant of the insulation layer, the ratio of the content of the second solvent in the insulation layer of the insulated wire based on the total content of the first solvent and the second solvent (first ratio), the residual amount of the first solvent and the second solvent in the insulation layer after heat treatment, and the ratio of the residual amount of the second solvent based on the above residual amounts (second ratio) were determined. The results are shown in Table 1. Table 1 also shows the dielectric constant and boiling point of NMP, which is used as the first solvent, and the dielectric constant and boiling point of naphtha, which is used as the second solvent.

[0124] [Second Test]

[0125] <Sample Preparation>

[0126] (Sample 3)

[0127] In step B of the second process, the heat used to perform the baking treatment is set to two-thirds of the heat of sample 1 (++). Otherwise, the insulated wire of sample 3 is obtained by the same method as sample 1.

[0128] (Sample 4)

[0129] In the first step, naphtha (relative permittivity: 1.8, boiling point: 247°C) is added to the first solvent as the second solvent, such that the mass ratio of the first solvent to the second solvent is 9:1, thereby preparing an insulating varnish. Otherwise, the insulating wire of sample 4 is obtained by the same method as sample 3.

[0130] <Evaluation of Insulated Wires>

[0131] For each insulated wire of samples 3 and 4, various measurements were performed on the same evaluation items as in the first test described above. The results are shown in Table 1.

[0132] [Third Test]

[0133] <Sample Preparation>

[0134] (Sample 5)

[0135] In step B of the second process, the heat used to perform the baking treatment is set to one-third of the heat of sample 1 (+). Otherwise, the insulated wire of sample 5 is obtained by the same method as sample 1.

[0136] (Sample 6)

[0137] In the first step, naphtha (relative permittivity: 1.8, boiling point: 247°C) is added to the first solvent as the second solvent, such that the mass ratio of the first solvent to the second solvent is 9:1, thereby preparing an insulating varnish. Otherwise, the insulating wire of sample 6 is obtained by the same method as sample 5.

[0138] <Evaluation of Insulated Wires>

[0139] For each insulated wire of samples 5 and 6, various measurements were performed on the same evaluation items as in the first test described above. The results are shown in Table 1.

[0140] [Fourth Test]

[0141] <Sample Preparation>

[0142] (Sample 7)

[0143] In the first step, an insulating varnish was prepared using a first solvent consisting of N,N-dimethylacetamide (DMAc, relative permittivity: 37.8, boiling point: 165°C) instead of NMP. Otherwise, the insulated wire of sample 7 was obtained by the same method as sample 5. This insulating varnish did not contain a second solvent.

[0144] (Sample 8)

[0145] In the first step, naphtha (relative permittivity: 1.8, boiling point: 247°C) is added to the first solvent as the second solvent, such that the mass ratio of the first solvent to the second solvent is 9:1, thereby preparing an insulating varnish. Otherwise, the insulating wire of sample 8 is obtained by the same method as sample 7.

[0146] <Evaluation of Insulated Wires>

[0147] For the insulated wires of samples 7 and 8, various measurements were performed on the same evaluation items as in the first test described above. The results are shown in Table 2. Table 2 also shows the dielectric constants and boiling points of DMAc and NMP, which were used as the first solvent, and the dielectric constant and boiling point of naphtha, which was used as the second solvent.

[0148] [Fifth Test]

[0149] <Sample Preparation>

[0150] (Sample 9)

[0151] The insulated wire of sample 9 was obtained by preparing the same insulated wire as sample 5.

[0152] (Sample 10)

[0153] Naphtha (relative permittivity: 1.8, boiling point: 247°C) was added to the first solvent as the second solvent, such that the mass ratio of the first solvent to the second solvent was 8.5:1.5, thereby preparing an insulating varnish. Otherwise, the insulating wire of sample 10 was obtained by the same method as sample 9.

[0154] <Evaluation of Insulated Wires>

[0155] For each insulated wire of samples 9 and 10, various measurements were performed on the same evaluation items as in the first test described above. The results are shown in Table 2.

[0156] [Sixth Test]

[0157] <Sample Preparation>

[0158] (Sample 11)

[0159] In the first step, pyromellitic dianhydride and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride were used in a molar ratio of 3:7 as tetracarboxylic acid dianhydride as a raw material for polyamic acid, thereby preparing an insulating varnish. Otherwise, the insulated wire of sample 11 was obtained by the same method as sample 3. This insulating varnish does not contain a second solvent.

[0160] (Sample 12)

[0161] In the first step, naphtha (relative permittivity: 1.8, boiling point: 247°C) is added to the first solvent as the second solvent, such that the mass ratio of the first solvent to the second solvent is 9:1, thereby preparing an insulating varnish. Otherwise, the insulating wire of sample 12 is obtained by the same method as sample 11.

[0162] <Evaluation of Insulated Wires>

[0163] For each insulated wire of specimens 11 and 12, various measurements were performed on the same evaluation items as in the first test described above. The results are shown in Table 2.

[0164] [Seventh Test]

[0165] <Sample Preparation>

[0166] (Sample 13)

[0167] The insulated wire of sample 13 was obtained by preparing the same insulated wire as sample 5.

[0168] (Sample 14)

[0169] In the first step, naphtha (relative permittivity: 1.8, boiling point: 175°C) is added to the first solvent as the second solvent, such that the mass ratio of the first solvent to the second solvent is 8:2, thereby preparing an insulating varnish. Otherwise, the insulating wire of sample 14 is obtained by the same method as sample 13.

[0170] <Evaluation of Insulated Wires>

[0171] For each insulated wire of samples 13 and 14, various measurements were performed on the same evaluation items as in the first test described above. The results are shown in Table 2.

[0172] [Table 1]

[0173]

[0174] [Table 2]

[0175]

[0176] <Inspection>

[0177] According to Table 1, when the insulation layer contains a first solvent and a second solvent as residual solvents, and the ratio of the content of the second solvent in the insulation layer to the total content of the first solvent and the second solvent in the insulation layer (i.e., the first ratio) is 50% by mass or more, and the residual amount of the second solvent in the insulation layer after heat treatment by heating the insulated wire at 350°C for 1 minute is higher than the ratio of the residual amount of the first solvent and the second solvent in the insulation layer after heat treatment (i.e., the second ratio) is higher than the first ratio, the effect of reducing the dielectric constant is confirmed. That is, in the first test, the dielectric constant of the insulation layer of sample 2 is reduced compared with sample 1; in the second test, the dielectric constant of the insulation layer of sample 4 is reduced compared with sample 3; and in the third test, the dielectric constant of the insulation layer of sample 6 is reduced compared with sample 5. Similarly, in the fourth test, the dielectric constant of the insulating layer of sample 8 was lower than that of sample 7; in the fifth test, the dielectric constant of the insulating layer of sample 10 was lower than that of sample 9; and in the sixth test, the dielectric constant of the insulating layer of sample 12 was lower than that of sample 11.

[0178] On the other hand, in the seventh test, the boiling point of naphtha, the second solvent in sample 14, was 175°C, which was lower than the boiling point of NMP, the first solvent (204°C). In the second step described above, naphtha (the second solvent) evaporated in greater quantities compared to NMP (the first solvent). Therefore, the first ratio of the insulated wire in sample 14 was less than 50% by mass, and the second ratio was lower than the first ratio, resulting in no decrease in dielectric constant compared to sample 13. From the above, it can be understood that the dielectric constant of the insulation layer of the insulated wires in samples 2, 4, 6, 8, 10, and 12 was reduced, thereby improving insulation properties.

[0179] As described above, embodiments and examples of this disclosure have been explained, but appropriate combinations of the above-described embodiments and examples are contemplated from the outset.

[0180] The embodiments and examples disclosed herein should be considered exemplary rather than limiting in all respects. The scope of the invention is set forth not by the described embodiments and examples, but by the claims, and is intended to include all modifications in the sense and scope equivalent to the claims.

Claims

1. An insulated wire comprising a linear conductor and an insulating layer covering the outer peripheral surface of said conductor, wherein, The insulating layer contains a first solvent with a relative permittivity of 15 or higher and a second solvent with a relative permittivity of less than 15 as residual solvents. The ratio of the content of the second solvent in the insulating layer to the total content of the first solvent and the second solvent in the insulating layer, i.e., the first ratio, is 50% by mass or more. The ratio of the content of the second solvent in the insulation layer after heat treatment of the insulated wire at 350°C for 1 minute to the total content of the first solvent and the second solvent in the heat-treated insulation layer, i.e., the second ratio, is higher than the first ratio.

2. The insulated wire according to claim 1, wherein, The insulating layer comprises one or more resins selected from the group consisting of polyimide resin, polyamide-imide resin, polyester-imide resin, and polyether-imide resin.

3. The insulated wire according to claim 1 or 2, wherein, The insulating layer comprises a total of more than 0.2% by mass and less than 10% by mass of the first solvent and the second solvent.

4. The insulated wire according to claim 1 or 2, wherein, The insulating layer contains less than 5% by mass of the first solvent. The insulating layer contains more than 0.1% by mass and less than 5% by mass of the second solvent.

5. The insulated wire according to claim 1 or 2, wherein, The insulating layer contains a curing agent. The curing agent comprises one or more selected from the group consisting of alicyclic anhydrides, aliphatic anhydrides, aromatic anhydrides, imidazole, triethylamine, titanium compounds, isocyanate compounds, urea, melamine compounds, and acetylene derivatives.

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