Varnish for insulated flat electric wire, insulated flat electric wire, and method for manufacturing insulated flat electric wire

Abstract

This varnish for an insulated flat electric wire contains an organic solvent and a polyimide precursor which is a reaction product between an aromatic tetracarboxylic acid dianhydride and an aromatic diamine. The organic solvent contains at least 50 mass% of an amide-based solvent having a boiling point of 150-190°C, and has a viscosity of 5-25 Pa·s at 30°C.

Description

Varnish for rectangular insulated electric wire, rectangular insulated electric wire, and method for manufacturing rectangular insulated electric wire

[0001] This disclosure relates to a varnish for rectangular insulated wires, a rectangular insulated wire, and a method for manufacturing a rectangular insulated wire. This application claims priority to Japanese Application No. 2024-007622, ​​filed January 22, 2024, and incorporates by reference all of the contents of that Japanese application.

[0002] Patent Document 1 describes an insulating varnish that is applied to the surface of a conductor, then passes through a die to remove excess applied insulating varnish, and is then dried and baked to form an insulating coating on the surface of the conductor, and that has a viscosity of 10 Pa·s or more at 30°C.

[0003] International Publication No. 2013 / 073397

[0004] A varnish for a rectangular insulated electric wire according to one embodiment of the present disclosure contains a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and an organic solvent, wherein the organic solvent contains 50 mass % or more of an amide-based solvent having a boiling point of 150°C or more and 190°C or less, and has a viscosity at 30°C of 5 Pa s or more and 25 Pa s or less.

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

[0006] [Problem to be Solved by the Present Disclosure] The problem to be solved by the present disclosure is to provide a varnish for rectangular insulated electric wires that can form an insulating coating with a highly uniform thickness on rectangular insulated electric wires.

[0007] Effect of the Present Disclosure The varnish for a rectangular insulated electric wire according to one aspect of the present disclosure can form an insulating coating with a highly uniform thickness on a rectangular insulated electric wire.

[0008] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. Item 1. A varnish for rectangular insulated electric wires, comprising a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and an organic solvent, wherein the organic solvent contains 50% by mass or more of an amide-based solvent having a boiling point of 150°C or higher and 190°C or lower, and has a viscosity of 5 Pa·s or higher and 25 Pa·s or lower at 30°C. Item 2. The varnish for rectangular insulated electric wires according to Item 1, wherein the amide-based solvent has a boiling point of 150°C or higher and 170°C or lower. Item 3. The varnish for rectangular insulated electric wires according to Item 1 or 2, wherein the amide-based solvent is N,N-dimethylacetamide, N,N-dimethylformamide, or a combination thereof. Item 4. The varnish for rectangular insulated electric wires according to any one of Items 1 to 3, wherein the content of the amide-based solvent in the organic solvent is 70% by mass or higher. Item 5. Item 6. The rectangular insulated wire varnish according to any one of Items 1 to 4, wherein the organic solvent contains 95% by mass or less of the amide solvent. Item 7. The rectangular insulated wire varnish according to any one of Items 1 to 5, wherein the organic solvent contains 85% by mass or more and 95% by mass or less of N,N-dimethylacetamide and 5% by mass or more and 15% by mass or less of N-methyl-2-pyrrolidone. Item 8. The rectangular insulated wire varnish according to any one of Items 1 to 6, wherein the viscosity at 30°C is 10 Pa s or more and 20 Pa s or less. Item 9. The rectangular insulated wire varnish according to any one of Items 1 to 7, wherein the content of the amide solvent in the organic solvent is 95% by mass or less. Item 10. The rectangular insulated wire varnish according to any one of Items 1 to 6, wherein the viscosity at 30°C is 10 Pa s or more and 20 Pa s or less. Item 11. The rectangular insulated wire varnish according to any one of Items 1 to 7, wherein the content of the amide solvent in the organic solvent is 95% by mass or less. Item 10. A rectangular insulated electric wire varnish according to any one of Items 1 to 8, wherein, when an insulating coating is formed on a rectangular conductor using the varnish for rectangular insulated electric wire according to any one of Items 1 to 8, the variation in thickness, expressed by the following formula, is 12.0% or less relative to the average of film thicknesses at 16 points on 30 cross sections spaced 50 cm apart in the longitudinal direction of the insulating coating (16 points x 30 surfaces = 480 average film thicknesses): Variation (%) = (4σ / average film thickness) x 100 (where σ represents standard deviation). Item 11. A rectangular insulated electric wire comprising a rectangular conductor and an insulating coating covering the rectangular conductor, wherein the insulating coating is formed using the varnish for rectangular insulated electric wire according to any one of Items 1 to 9.Item 11. A method for producing a rectangular insulated electric wire according to Item 10, comprising the steps of: applying the varnish for a rectangular insulated electric wire according to any one of Items 1 to 9 to the outer peripheral surface of the rectangular conductor; and heating the varnish for a rectangular insulated electric wire applied in the applying step.

[0009] [Details of Embodiments of the Present Disclosure] Hereinafter, a varnish for a rectangular insulated electric wire, a rectangular insulated electric wire, and a method for producing a rectangular insulated electric wire according to one aspect of the present disclosure will be described.

[0010] <Varnish for rectangular insulated electric wire> The varnish for rectangular insulated electric wire contains a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and an organic solvent, wherein the organic solvent contains 50 mass % or more of an amide solvent having a boiling point of 150°C or more and 190°C or less, and has a viscosity at 30°C of 5 Pa s or more and 25 Pa s or less.

[0011] The varnish for rectangular insulated electric wire contains a specific amount of the specific amide solvent as an organic solvent and has a viscosity at 30°C within a specific range, thereby achieving the effect of being able to form an insulating coating with a highly uniform thickness on rectangular insulated electric wires.

[0012] Although not intended to be a restrictive interpretation, a varnish with a low viscosity (25 Pa·s or less at 30°C) may be applied to the surface of a conductor, pass through a die, and then, due to the influence of surface tension, may not be able to maintain the shape it had when passed through the die before drying, resulting in potential deformation. Therefore, by including a specific amount of the specific amide-based solvent as the organic solvent, the shape of the wire passed through the die can be maintained. As a result, it is believed that an insulating coating with a highly uniform thickness can be formed on a rectangular insulated electric wire.

[0013] As used herein, "being able to form an insulating coating with high uniformity of thickness on a rectangular insulated electric wire" means that when an insulating coating is formed on a rectangular conductor using the varnish for rectangular insulated electric wire, the variation in thickness, expressed by the following formula, is 12.0% or less, relative to the average thickness measured at 16 points on 30 cross sections spaced 50 cm apart along the longitudinal direction of the insulating coating (16 points x 30 surfaces = 480 average thicknesses): Variation (%) = (4σ / average thickness) x 100 (where σ represents standard deviation).

[0014] The upper limit of the variation rate may be 11.5%, 11.0%, 10.5%, 10.0%, or 9.5%, and the lower limit of the variation rate is not particularly limited and may be 0%, 0.5%, or 1.0%.

[0015] The viscosity of the rectangular insulated wire varnish at 30°C is 5 Pa·s or more and 25 Pa·s or less. The viscosity is a value measured using a Brookfield viscometer. The lower limit of the viscosity is 5 Pa·s, or may be 7 Pa·s or 10 Pa·s. The upper limit of the viscosity is 25 Pa·s, or may be 22 Pa·s or 20 Pa·s. When the viscosity of the rectangular insulated wire varnish at 30°C is 10 Pa·s or more and 20 Pa·s or less, the uniformity of the film thickness of the rectangular insulated wire can be further improved.

[0016] The viscosity of the rectangular insulated wire varnish at 30°C can be adjusted by changing the degree of polymerization (weight-average molecular weight) of the polyimide precursor, which will be described later. A common method for adjusting the viscosity of a varnish is to add a solid material such as a filler. However, adjusting the viscosity by adding a filler or the like can result in a decrease in the elongation of the insulating coating. Therefore, if the rectangular insulated wire varnish does not contain a filler, the elongation of the insulating coating can be prevented from decreasing.

[0017] The varnish for rectangular insulated electric wires can be suitably used as a varnish for forming an insulating coating on rectangular insulated electric wires.

[0018] The term "rectangular insulated wire" refers to an insulated wire having a rectangular cross section. A "rectangle" generally refers to a quadrilateral with right angles at all corners, but in this specification, the term "rectangular" is a broad concept that includes any shape that can be roughly recognized as a rectangle.

[0019] Hereinafter, each component contained in the varnish for rectangular insulated electric wire will be described.

[0020] (Polyimide Precursor) A polyimide precursor is a reaction product obtained by a condensation polymerization reaction between an aromatic tetracarboxylic dianhydride and an aromatic diamine. The polyimide precursor is a compound also called a polyamic acid (polyamic acid). The polyimide precursor undergoes a dehydration cyclization reaction (imidization reaction) to form a cyclic imide, thereby becoming a polyimide.

[0021] The aromatic tetracarboxylic dianhydride containing pyromellitic dianhydride (PMDA) can improve the heat resistance of the insulating coating. This is because PMDA has a rigid and linear molecular structure. The aromatic tetracarboxylic dianhydride may contain an aromatic tetracarboxylic dianhydride other than PMDA (hereinafter also referred to as "another aromatic tetracarboxylic dianhydride").

[0022] Examples of the other aromatic tetracarboxylic dianhydrides include 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA), 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA), 2,2',3,3'-biphenyltetracarboxylic dianhydride (i-BPDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, and 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride. Examples of other aromatic tetracarboxylic dianhydrides include bis(2,3-dicarboxyphenyl)propane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, and 2,3,6,7-naphthalenetetracarboxylic dianhydride. These other aromatic tetracarboxylic dianhydrides may be used alone or in combination of two or more.

[0023] When biphenyltetracarboxylic dianhydride (BPDA) is used as another aromatic tetracarboxylic dianhydride, the hydrolysis resistance of the polyimide precursor can be suitably improved.

[0024] The lower limit of the amount of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 10 mol%, 20 mol%, or 30 mol%. The upper limit of the amount of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 100 mol%, 90 mol%, 80 mol%, or 70 mol%. Good film elongation can be obtained by using 10 mol% or more of PMDA.

[0025] The content of the other aromatic tetracarboxylic dianhydride relative to 100 mol% of the aromatic tetracarboxylic dianhydride can be appropriately determined within a range that does not impair the effects of the present disclosure. The upper limit of the content may be 30 mol% or 20 mol%. The lower limit of the content of the other aromatic tetracarboxylic dianhydride may be 0 mol% or 10 mol%.

[0026] The aromatic diamine containing diaminodiphenyl ether (ODA) can improve the heat resistance of the insulating coating. This is because ODA has a rigid and linear molecular structure. Examples of ODA include 4,4'-diaminodiphenyl ether (4,4'-ODA), 3,4'-diaminodiphenyl ether (3,4'-ODA), 3,3'-diaminodiphenyl ether (3,3'-ODA), 2,4'-diaminodiphenyl ether (2,4'-ODA), and 2,2'-diaminodiphenyl ether (2,2'-ODA). Among these, the use of 4,4'-diaminodiphenyl ether (4,4'-ODA) can favorably improve the elongation of the insulating coating.

[0027] The lower limit of the ODA content relative to 100 mol% of the aromatic diamine may be 50 mol%, 60 mol%, or 70 mol%, and the upper limit of the ODA content relative to 100 mol% of the aromatic diamine may be 100 mol% or 90 mol%.

[0028] The aromatic diamine may further contain an aromatic diamine other than ODA (hereinafter also referred to as "other aromatic diamine"). Examples of the other aromatic diamine include 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane, 2,2'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone, 3,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, 2,4'-diaminodiphenylsulfone, 2,2'-diaminodiphenylsulfone, 4,4' Examples of the other aromatic diamines include 4,4'-dimethyl-4,4'-diaminodiphenylmethane, 4,4'-benzophenonediamine, 2,2'-dimethyl-4,4'-diaminodiphenylmethane ...

[0029] When 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) or 4,4'-bis(4-aminophenoxy)biphenyl (BAPB) is used as another aromatic diamine, the relative dielectric constant of the insulating film can be lowered.

[0030] The content of the other aromatic diamine relative to 100 mol% of the aromatic diamine can be appropriately determined within a range that does not impair the effects of the present disclosure. The upper limit of the content may be 40 mol% or 30 mol%. The lower limit of the content may be 0 mol% or 10 mol%.

[0031] The lower limit of the concentration of the polyimide precursor in the rectangular insulated wire varnish may be 10% by mass or 20% by mass. The upper limit of the concentration of the polyimide precursor in the rectangular insulated wire varnish may be 50% by mass or 40% by mass. By setting the concentration of the polyimide precursor in the rectangular insulated wire varnish at or above the lower limit, the amount of varnish required throughout the entire manufacturing process to obtain an insulating coating of the desired thickness when forming an insulating coating using the rectangular insulated wire varnish can be reduced, thereby reducing the number of coating and heating steps. By setting the concentration at or below the upper limit, the viscosity of the rectangular insulated wire varnish can be appropriately adjusted while maintaining good coating properties, thereby improving coatability.

[0032] The molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine used as raw materials for the polyimide precursor (aromatic tetracarboxylic dianhydride:aromatic diamine) may be, for example, 95:105 or more and 105:95 or less, 97:103 or more and 103:97 or less, or 99:101 or more and 101:99 or less, from the viewpoint of ease of synthesis of the polyimide precursor. The aromatic tetracarboxylic dianhydride and the aromatic diamine may be substantially equimolar amounts. In this case, it is easy to increase the molecular weight of the polyimide precursor. The term "substantially equimolar amounts" refers to a molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine (aromatic tetracarboxylic dianhydride:aromatic diamine) in the range of 99:101 or more and 101:99 or less.

[0033] (Method for Synthesizing Polyimide Precursor) The polyimide precursor can be obtained by a condensation polymerization reaction between the aromatic tetracarboxylic dianhydride and aromatic diamine described above. The condensation polymerization reaction can be performed by a method similar to conventional methods for synthesizing polyimide precursors. Specific examples of the condensation polymerization reaction include a method in which an aromatic tetracarboxylic dianhydride and an aromatic diamine are mixed in an organic solvent. This method allows the aromatic tetracarboxylic dianhydride and the aromatic diamine to polymerize, resulting in a solution in which the polyimide precursor is dissolved in the organic solvent. For example, the degree of polymerization (weight average molecular weight) can be controlled by performing the condensation polymerization reaction in the presence of a reaction inhibitor.

[0034] The reaction inhibitor may be, for example, water (H ２ and alcohols having 1 to 15 carbon atoms. Examples of the alcohols having 1 to 15 carbon atoms include monohydric alcohols such as ethanol, methanol, propanol, butanol, and pentanol; and polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin.

[0035] The reaction conditions for the condensation polymerization can be appropriately set depending on the raw materials used, etc. For example, the reaction temperature can be set to 10° C. or higher and 100° C. or lower, and the reaction time can be set to 0.5 hours or higher and 24 hours or lower.

[0036] Examples of the organic solvent used in the condensation polymerization reaction include those similar to the organic solvents described below.

[0037] (Organic Solvent) The organic solvent contains 50% by mass or more of an amide-based solvent having a boiling point of 150° C. to 190° C. The organic solvent may be used alone or in combination of two or more.

[0038] The lower limit of the boiling point of the amide solvent is 150° C. or may be 153° C. The upper limit of the boiling point of the amide solvent is 190° C., or may be 180° C. or 170° C. When the boiling point of the amide solvent is 150° C. or higher and 170° C. or lower, the uniformity of the thickness of the insulating coating of the rectangular insulated electric wire can be further improved.

[0039] Examples of the amide solvent include N,N-dimethylacetamide (DMAc, boiling point: 165° C.), N,N-dimethylformamide (DMF, boiling point: 153° C.), and N,N-diethylformamide (DEF, boiling point: 177° C.).

[0040] When the amide solvent is N,N-dimethylacetamide, N,N-dimethylformamide, or a combination thereof, the uniformity of the film thickness of the rectangular insulated electric wire can be further improved.

[0041] The content of the amide solvent in the organic solvent is 50% by mass or more. The lower limit of the content of the amide solvent is 50% by mass, or may be 55% by mass, 60% by mass, 65% by mass, 70% by mass, 75% by mass, 80% by mass, or 85% by mass. The upper limit of the content of the amide solvent is not particularly limited, and may be 100% by mass, 99% by mass, 98% by mass, 97% by mass, 96% by mass, or 95% by mass.

[0042] The organic solvent may contain a solvent other than the amide-based solvent. The solvent other than the amide-based solvent is not particularly limited as long as it is a solvent that can be used in the synthesis of a polyimide precursor, and examples thereof include aprotic solvents. The term "aprotic solvent" refers to an organic solvent that does not have a group that releases a proton.

[0043] Examples of the aprotic solvent include amide solvents such as N-methyl-2-pyrrolidone (NMP, boiling point: 202°C); sulfur-containing solvents such as dimethyl sulfoxide (boiling point: 189°C); and lactone solvents such as γ-butyrolactone (boiling point: 204°C).

[0044] When the organic solvent contains 85% by mass or more and 95% by mass or less of N,N-dimethylacetamide and 5% by mass or more and 15% by mass or less of N-methyl-2-pyrrolidone, the uniformity of the thickness of the insulating coating in the rectangular insulated electric wire can be further improved, and an insulating coating with high interlayer adhesion can be formed.

[0045] The content of the organic solvent in the varnish for rectangular insulated electric wire is not particularly limited as long as it is an amount that can uniformly dissolve and disperse the aromatic tetracarboxylic dianhydride and aromatic diamine. However, if the amount is too large, a large amount of organic solvent must be volatilized when forming the insulating coating, which may require a long time to form the insulating coating. Therefore, the content of the organic solvent can be, for example, 100 parts by mass or more and 1,000 parts by mass or less per 100 parts by mass of the aromatic tetracarboxylic dianhydride and aromatic diamine combined.

[0046] (Pore-forming agent) The varnish for a rectangular insulated electric wire may contain a pore-forming agent. When the varnish for a rectangular insulated electric wire contains a pore-forming agent, an insulating coating having a plurality of pores can be formed.

[0047] The pore-forming agent can be any known additive used to form an insulating coating having pores, and examples of the pore-forming agent include chemical foaming agents, thermally expandable microcapsules, particles containing thermally decomposable resins, and high-boiling-point solvents.

[0048] When the pore-forming agent is a thermally decomposable resin-containing particle, an insulating coating having a good appearance can be formed even when the porosity is high. The thermally decomposable resin-containing particle is gasified by thermal decomposition, and pores are formed in the insulating coating in the area where the thermally decomposable resin-containing particle was present. In this case, the thermally decomposable resin-containing particle can be uniformly distributed as an island phase of fine particles in the resin portion (the sea phase of the resin matrix) that constitutes the insulating coating, and independent pores can be formed in the insulating coating.

[0049] The thermally decomposable resin contained in the thermally decomposable resin-containing particles may be a resin that thermally decomposes at a temperature lower than the baking temperature of the polyimide that constitutes the insulating coating. The baking temperature of the polyimide is set appropriately depending on the type of material that constitutes the polyimide precursor, but is usually about 200°C or higher and 600°C or lower. The "thermal decomposition temperature" refers to the temperature at which the mass loss rate reaches 50% when the temperature is increased from room temperature at a rate of 10°C / min in an air atmosphere. The thermal decomposition temperature can be measured by measuring the thermogravimetry using a thermogravimetry-differential thermal analyzer ("TG / DTA" manufactured by SII NanoTechnology, Inc.).

[0050] Examples of the thermally decomposable resin contained in the thermally decomposable resin-containing particles include compounds in which one, both, or a portion of one or both ends of polyethylene glycol, polypropylene glycol, etc. are alkylated, (meth)acrylated, or epoxidized; polymers of (meth)acrylic acid esters having an alkyl group of 1 to 6 carbon atoms, such as polymethyl(meth)acrylate, polyethyl(meth)acrylate, polypropyl(meth)acrylate, and polybutyl(meth)acrylate; urethane oligomers, urethane polymers, polymers of modified (meth)acrylates, such as urethane(meth)acrylate, epoxy(meth)acrylate, and ε-caprolactone(meth)acrylate; poly(meth)acrylic acid; crosslinked products thereof; polystyrene; and crosslinked polystyrene. Polymers of (meth)acrylic acid esters having an alkyl group of 1 to 6 carbon atoms are prone to thermal decomposition at the baking temperature of the polyimide, easily forming voids in the insulating coating. Examples of the (meth)acrylic acid ester polymers include polymethyl methacrylate (PMMA). The term "(meth)acrylic acid" is a general term for "acrylic acid" and "methacrylic acid," and refers to either one or both of them. Furthermore, the term "(meth)acrylate" is a general term for "acrylate" and "methacrylate," and refers to either one or both of them.

[0051] The thermally decomposable resin-containing particles may be particles consisting solely of the thermally decomposable resin, or may be particles with a core-shell structure having a core primarily composed of the thermally decomposable resin and a shell primarily composed of a resin having a thermal decomposition temperature higher than that of the thermally decomposable resin. When a varnish containing particles with a core-shell structure is heated, only the core is thermally decomposed to form pores, with the shell remaining on the outer periphery of these pores. Particles with a core-shell structure can reduce the interconnection of pores and reduce the variation in pore size.

[0052] The main component of the shell is not particularly limited as long as it has a thermal decomposition temperature higher than that of the core, and may be a synthetic resin with a low dielectric constant and high heat resistance. Examples include polystyrene, silicone, fluororesin, and polyimide. In particular, when the main component of the shell is silicone, it is easy to increase elasticity, which results in good dispersion of pores in the insulating coating. This reduces interconnection of pores and reduces variation in pore size. An insulating coating with this configuration has excellent conductivity and heat resistance.

[0053] The content of the pore-forming agent in the varnish for rectangular insulated electric wires can be determined appropriately depending on, for example, the type of pore-forming agent, the target porosity of the insulating coating, and the like.

[0054] (Other Components) The varnish for rectangular insulated electric wire may contain other components in addition to the above-mentioned components. The other components are not particularly limited as long as they are blended into varnishes for forming insulating coatings on general insulated electric wires. Examples of the other components include antioxidants, leveling agents, curing agents, and adhesion promoters.

[0055] Examples of curing agents include imidazole, titanium-based curing agents, isocyanate compounds, blocked isocyanates, urea or melamine compounds, amino resins, acetylene derivatives, and methyltetrahydrophthalic anhydride. When the varnish for rectangular insulated electric wire contains a curing agent, the uniformity of the film thickness of the rectangular insulated electric wire can be further improved.

[0056] <Rectangular insulated wire> The rectangular insulated wire includes a rectangular conductor and an insulating coating covering the rectangular conductor. The rectangular insulated wire 1 shown in Fig. 1 includes a rectangular conductor 2 and an insulating coating 3 covering the rectangular conductor 2.

[0057] The rectangular insulated wire has a rectangular cross-sectional shape (flat wire). The flat wire allows the rectangular insulated wire to be wound at a high density during coil processing.

[0058] The rectangular insulated wire can be suitably used as a coil winding wire (magnet wire).

[0059] (Rectangular Conductor) The cross-sectional shape of the rectangular conductor is rectangular.

[0060] The rectangular conductor may be made of a metal with high electrical conductivity and mechanical strength. Examples of such metals include copper, copper alloys, aluminum, nickel, silver, mild steel, steel, and stainless steel. The rectangular conductor may be made of a wire-shaped material or a multilayer structure in which a wire-shaped material is coated with another metal, such as nickel-coated copper, silver-coated copper, copper-coated aluminum, or copper-coated steel.

[0061] The lower limit of the average cross-sectional area of ​​the rectangular conductor is 0.01 mm ２ 0.1 mm ２ In this case, the volume of the insulating coating relative to the rectangular conductor in the rectangular insulated wire can be made appropriate, and the volume efficiency of a coil or the like formed using the rectangular insulated wire can be improved. ２ 10 mm ２ In this case, it is possible to reduce the need to form a thick insulating coating in order to sufficiently reduce the relative dielectric constant, and it is possible to avoid an unnecessary increase in the diameter of the rectangular insulated electric wire.

[0062] (Insulating coating) The insulating coating is laminated on the outer surface of the rectangular conductor so as to cover the rectangular conductor. The insulating coating is composed of one or more layers. For example, when the insulating coating is formed by the method described below (a method in which varnish is applied and baked multiple times), the insulating coating has a laminated structure composed of multiple layers formed using varnish.

[0063] The insulating coating contains a resin matrix, which is formed from the above-mentioned varnish for rectangular insulated electric wire, and the resin matrix is ​​therefore primarily composed of polyimide.

[0064] The average thickness of the insulating coating is not particularly limited, and can usually be set to 2 μm or more and 200 μm or less.

[0065] The insulating coating may contain a plurality of pores. In this case, the dielectric constant of the insulating coating can be reduced. When the insulating coating contains a plurality of pores, the plurality of pores are dispersed in the resin matrix of the insulating coating.

[0066] When an insulating coating contains multiple pores, the porosity of the insulating coating may be 20% by volume or more and 60% by volume or less. When the porosity of the insulating coating is 20% by volume or more, the dielectric constant of the insulating coating can be further reduced. The porosity of the insulating coating may be 40% by volume or more. The upper limit of the porosity of the insulating coating may be 60% by volume or 50% by volume. "Porosity" refers to the percentage (unit: volume %) of the volume of pores relative to the volume of the insulating coating containing the resin matrix and pores. Specifically, the porosity is measured as follows. The porosity is calculated using the formula (W1 - W2) × 100 / W1, where W1 is the mass of the insulating coating without pores, which is calculated by multiplying the apparent volume V1 calculated from the outer diameter of the insulating coating by the density ρ1 of the insulating coating material.

[0067] (Pores) The pores may be derived from thermally decomposable resin-containing particles. The thermally decomposable resin-containing particles are gasified by thermal decomposition, and pores are formed in the insulating coating at the locations where the thermally decomposable resin-containing particles were present. In this case, the thermally decomposable resin-containing particles can be uniformly distributed as islands of fine particles in the sea phase of the resin matrix that constitutes the insulating coating, forming independent pores in the insulating coating.

[0068] The mechanical properties of the insulating coating can be improved by setting the lower limit of the average diameter of the plurality of pores to 0.1 μm. The insulating properties of the insulating coating can be improved by setting the upper limit of the average diameter to 10 μm. The average diameter is a value obtained by measuring the cross section of the rectangular insulated electric wire using a pore diameter distribution measuring device (e.g., the "Porous Materials Automated Pore Size Distribution Measuring System" manufactured by Porous Materials).

[0069] The rectangular insulated wire may have a configuration other than those described above. For example, the rectangular insulated wire may have an adhesion layer containing an additive such as an adhesion improver between the rectangular conductor and the insulating coating. Examples of the adhesion improver include mercaptans such as 2-mercaptoimidazole and 5-amino-1,3,4-thiadiazole-2-thiol.

[0070] The rectangular insulated wire may have a surface friction adjusting layer as its outermost layer. Examples of the surface friction adjusting layer include polyamide-imide, self-lubricating amide-imide, polyimide, and self-lubricating polyimide layers. The "outermost layer" refers to the layer located outermost in the laminate structure constituting the rectangular insulated wire, with the conductor side facing inward.

[0071] The rectangular insulated electric wire may have an outermost adhesive layer containing an additive such as a foaming agent. Examples of the foaming agent include azo-based foaming agents such as azodicarbonamide and azobisisobutyronitrile, nitroso-based foaming agents such as dinitrosopentamethylenetetramine and N,N'-dinitroso-N,N'-dimethylterephthalamide, hydrazide-based foaming agents such as p-toluenesulfonylhydrazide, p,p'-oxybisbenzenesulfonylhydrazide and benzenesulfonylhydrazide, and trihydrazinotriazine.

[0072] The rectangular insulated wire may have an outermost surge-resistant layer containing an inorganic filler. Examples of inorganic fillers include silica, alumina, magnesia, beryllium oxide, silicon carbide, titanium carbide, boron carbide, tungsten carbide, boron nitride, and silicon nitride. The inorganic filler may be surface-treated. Examples of surface treatment agents include silane coupling agents.

[0073] <Method for manufacturing a rectangular insulated wire> The method for manufacturing a rectangular insulated wire includes a step of applying the above-mentioned rectangular insulated wire varnish to the outer peripheral surface of a rectangular conductor (coating step), and a step of heating the rectangular insulated wire varnish applied in the coating step (heating step).

[0074] In the coating step, the rectangular insulated wire varnish is applied to the outer peripheral surface of the rectangular conductor. For example, a method for applying the rectangular insulated wire varnish to the outer peripheral surface of the rectangular conductor uses a coating device equipped with a liquid composition tank containing the rectangular insulated wire varnish and a coating die. With the coating device, the rectangular conductor passes through the liquid composition tank, causing the rectangular insulated wire varnish to adhere to the outer peripheral surface of the conductor. The rectangular conductor then passes through the coating die, coating the rectangular insulated wire varnish to a uniform thickness.

[0075] In the heating step, the varnish for a rectangular insulated electric wire that has been applied to the conductor in the application step is heated to volatilize the organic solvent in the varnish for a rectangular insulated electric wire and cure the polyimide precursor to form a polyimide.

[0076] The heating step may be carried out by any known method, such as hot air heating, infrared heating, or high-frequency heating.

[0077] The heating temperature can be, for example, 300° C. to 800° C. The heating time can be, for example, 5 seconds to 1 minute.

[0078] The coating step and the heating step are usually repeated multiple times. By repeating the steps multiple times, the thickness of the insulating coating can be increased. The hole diameter of the coating die can be adjusted appropriately depending on the number of repetitions.

[0079] The present invention will be described in more detail below with reference to experimental examples, but the present invention is not limited to these experimental examples.

[0080] <Preparation of Varnish for Rectangular Insulated Wire and Fabrication of Rectangular Insulated Wire> The various components used in preparing the varnish for rectangular insulated wire are as follows: (aromatic tetracarboxylic dianhydride) PMDA: pyromellitic dianhydride (aromatic diamine) ODA: 4,4'-diaminodiphenyl ether (organic solvent) DMAc: N,N-dimethylacetamide (boiling point: 166°C) DMF: N,N-dimethylformamide (boiling point: 153°C) NMP: N-methyl-2-pyrrolidone (boiling point: 202°C)

[0081] [No. 1] (Preparation of Varnish for Rectangular Insulated Wire) ODA as an aromatic diamine was dissolved in NMP. PMDA as an aromatic tetracarboxylic dianhydride was added so that the mixing ratio (molar ratio) of the aromatic tetracarboxylic dianhydride to the aromatic diamine was 100:100. The mixture was allowed to react at 30°C for 3 hours with stirring under a nitrogen atmosphere to synthesize a polyimide precursor, and Varnish No. 1 (solids concentration: 28% by mass) was obtained as a polyimide precursor solution containing NMP as a solvent.

[0082] (Preparation of Rectangular Insulated Wire) A rectangular copper wire with a cross section of 3 mm × 2 mm was used as the rectangular conductor. Varnish No. 1 was applied to the surface of the rectangular conductor, and the rectangular conductor coated with Varnish No. 1 was passed through a die having a shape similar to that of the conductor. The conductor was then heated in a heating furnace at an inlet temperature of 400°C, an outlet temperature of 500°C, and a wire speed of 6.0 m / min. This process was repeated 13 times to form an insulating coating with an average thickness of 40 μm, thereby producing rectangular insulated wire No. 1.

[0083] [Nos. 2 to 20] Varnishes No. 2 to No. 20 were prepared in the same manner as No. 1, except that the types and amounts of each component shown in Table 1 below were used, and rectangular insulated wires No. 2 to No. 20 were fabricated.

[0084] [No. 21] Varnish No. 21 was prepared in the same manner as No. 1, except that the types and amounts of each component shown in Table 1 below were used and imidazole was added so as to give 1 mass %, and rectangular insulated wire No. 21 was produced.

[0085] <Evaluation> The viscosity of each of the prepared varnishes No. 1 to No. 21 was measured according to the following method. Furthermore, the uniformity of the film thickness of each of the prepared rectangular insulated electric wires No. 1 to No. 21 was evaluated according to the following method.

[0086] [Viscosity] The viscosity of each of the prepared varnishes No. 1 to No. 21 at 30°C at the time of preparation was measured using a Brookfield viscometer (RB-80L manufactured by Toki Sangyo Co., Ltd.). The results are shown in the "Viscosity" column in Table 1 below.

[0087] [Thickness Uniformity] The average thickness of the insulating coating was determined for the rectangular insulated wires No. 1 to No. 21 prepared above. The average thickness of the insulating coating was determined as follows: The thickness was measured at 16 points on each of 30 cross sections of the insulating coating. The 30 cross sections of the insulating coating were arranged at 50 cm intervals along the longitudinal axis of the rectangular insulated wire. The average thickness of the insulating coating was determined by averaging the thicknesses at 480 points (16 points x 30 surfaces). The deviation 4σ of the measured values ​​(σ is the standard deviation) was also determined. Even if the average thicknesses were similar, a large deviation 4σ could be considered to have a large variation.

[0088] Furthermore, the percentage of variation was calculated from the average film thickness and the deviation 4σ, as expressed by the following formula: Percentage of variation (%) = (4σ / average film thickness) × 100 The uniformity of the film thickness was evaluated as "good" when the percentage of variation was 12.0% or less, and as "poor" when the percentage of variation was more than 12.0%. This is because when the percentage of variation is more than 12.0%, it can be said that a so-called dog-bone shaped insulating film is likely to be formed.

[0089] In Table 1 below, "acid anhydride" means "aromatic tetracarboxylic dianhydride." "Diamine" means "aromatic diamine." "Specific amide solvent" in the "organic solvent" column means a solvent that falls under the category of amide solvents with a boiling point of 150°C or higher and 190°C or lower. "Other solvent" means a solvent that does not fall under the category of the specific amide solvents. "-" indicates that the corresponding component is not used.

[0090]

[0091] From Table 1, it can be seen that No. 2 to No. 21 are superior in film thickness uniformity compared to No. 1.

[0092] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is not limited to the configurations of the above-described embodiments, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0093] 1. Flat insulated wire 2. Flat conductor 3. Insulating coating

Claims

1. A varnish for a flat insulating wire, which contains a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and an organic solvent, wherein the organic solvent contains 50% by mass or more of an amide-based solvent having a boiling point of 150°C or higher and 190°C or lower, and has a viscosity at 30°C of 5 Pa·s or more and 25 Pa·s or less.

2. The varnish for a flat insulating wire according to claim 1, wherein the boiling point of the amide-based solvent is 150°C or higher and 170°C or lower.

3. The varnish for a flat insulating wire according to claim 1 or claim 2, wherein the amide-based solvent is N,N-dimethylacetamide, N,N-dimethylformamide, or a combination thereof.

4. The varnish for a flat insulating wire according to any one of claims 1 to 3, wherein the content of the amide-based solvent in the organic solvent is 70% by mass or more.

5. The varnish for a flat insulating wire according to any one of claims 1 to 4, wherein the content of the amide-based solvent in the organic solvent is 95% by mass or less.

6. The varnish for a flat insulating wire according to any one of claims 1 to 5, wherein the organic solvent contains 85% by mass or more and 95% by mass or less of N,N-dimethylacetamide, and 5% by mass or more and 15% by mass or less of N-methyl-2-pyrrolidone.

7. The varnish for a flat insulating wire according to any one of claims 1 to 6, wherein the viscosity at 30°C is 10 Pa·s or more and 20 Pa·s or less.

8. The varnish for a flat insulating wire according to any one of claims 1 to 7, which does not contain a filler.

9. The varnish for a flat insulating wire according to claim 1, wherein when an insulating film is formed on a flat conductor using the varnish for a flat insulating wire according to claim 1, the variation ratio represented by the following formula is 12.0% or less. Variation ratio (%) = (4σ / average film thickness of the insulating film) × 100 (In the formula, σ represents the standard deviation of the film thickness of the insulating film.) 10. A flat insulating wire comprising a flat conductor and an insulating film covering the flat conductor, wherein the insulating film is formed from the varnish for a flat insulating wire according to any one of claims 1 to 9.

11. A method for manufacturing a flat insulated wire according to claim 10, comprising the steps of: applying a varnish for a flat insulated wire according to any one of claims 1 to 9 to the outer peripheral surface of the flat conductor; and heating the varnish for a flat insulated wire applied in the applying step.

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