WO2025158750A1 - Varnish for insulated flat electric wire, insulated flat electric wire, and method for manufacturing insulated flat electric wire - Google Patents

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

This disclosure relates to a varnish for rectangular insulated wire, rectangular insulated wire, and a method for manufacturing 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.

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 has a viscosity of 10 Pa·s or more at 30°C.

International Publication No. 2013/073397

A varnish for rectangular insulated electric wire according to one embodiment of the present disclosure contains a polyimide precursor, which 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 higher and 25 Pa·s or lower.

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

[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.

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

Description of the embodiments of the present disclosure
First, embodiments of the present disclosure will be listed and described.
Item 1.
A varnish for a rectangular insulated electric wire 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 mass % or more of an amide solvent having a boiling point of 150°C or more and 190°C or less; and the varnish has a viscosity at 30°C of 5 Pa s or more and 25 Pa s or less.
Item 2.
2. The varnish for a rectangular insulated electric wire according to Item 1, wherein the boiling point of the amide-based solvent is 150° C. or higher and 170° C. or lower.
Item 3.
3. The varnish for a rectangular insulated electric wire according to Item 1 or 2, wherein the amide solvent is N,N-dimethylacetamide, N,N-dimethylformamide, or a combination thereof.
Item 4.
4. The varnish for a rectangular insulated electric wire according to any one of items 1 to 3, wherein the content of the amide solvent in the organic solvent is 70 mass % or more.
Item 5.
5. The varnish for a rectangular insulated electric wire according to any one of items 1 to 4, wherein the content of the amide solvent in the organic solvent is 95% by mass or less.
Item 6.
Item 6. The varnish for a rectangular insulated electric wire 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.
Section 7.
7. The varnish for a rectangular insulated electric wire according to any one of items 1 to 6, having a viscosity at 30° C. of 10 Pa·s or more and 20 Pa·s or less.
Section 8.
8. The varnish for a rectangular insulated electric wire according to any one of items 1 to 7, which does not contain a filler.
Item 9.
Item 10. The varnish for a rectangular insulated electric wire according to any one of items 1 to 8, wherein when an insulating coating is formed on a rectangular conductor using the varnish for a rectangular insulated electric wire according to any one of items 1 to 8, the variation rate, expressed by the following formula, is 12.0% or less with respect to the average film thickness at 16 points on 30 cross sections at 50 cm intervals in the longitudinal direction of the insulating coating (average film thickness at 16 points x 30 surfaces = 480 points):
Variation rate (%) = (4σ/average film thickness) × 100
(wherein σ represents the standard deviation.)
Item 10.
Item 10. A rectangular insulated electric wire comprising a rectangular conductor and an insulating coating covering the rectangular conductor, the insulating coating being formed from the varnish for rectangular insulated electric wire according to any one of items 1 to 9.
Item 11.
Item 11. A method for producing a rectangular insulated wire according to item 10, comprising the steps of: applying the varnish for a rectangular insulated 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 wire applied in the applying step.

[Details of the embodiment 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 embodiment of the present disclosure will be described.

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

This varnish for rectangular insulated electric wires contains a specific amount of the specific amide solvent as an organic solvent and has a specific range of viscosity at 30°C, thereby achieving the effect of forming an insulating coating with a highly uniform thickness on rectangular insulated electric wires.

While not wishing to be limiting, when a varnish with a low viscosity (viscosity of 25 Pa·s or less at 30°C) is applied to the surface of a conductor and passed through a die, it is subject to the influence of surface tension before drying, and is unable to maintain the shape it had when passed through the die, which could result in the shape changing. Therefore, by including a specific amount of the above-mentioned specific amide-based solvent as the organic solvent, it is possible to maintain the shape it had when passed through the die. As a result, it is believed that an insulating coating with a highly uniform thickness can be formed on rectangular insulated electric wires.

In this specification, "an insulating coating with a high uniformity in thickness can be formed on a rectangular insulated wire" means that when the insulating coating is formed on a rectangular conductor using the varnish for rectangular insulated wire, the variation rate, expressed by the following formula, is 12.0% or less relative to the average thickness of 16 points on 30 cross sections spaced 50 cm apart in the longitudinal direction of the insulating coating (average thickness at 16 points x 30 surfaces = 480 points).
Variation rate (%) = (4σ/average film thickness) × 100
(wherein σ represents the standard deviation.)

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

The viscosity of the varnish for rectangular insulated electric wires at 30°C is 5 Pa·s or more and 25 Pa·s or less. The viscosity is a value measured using a B-type viscometer. The lower limit of the viscosity is 5 Pa·s, and may be 7 Pa·s or 10 Pa·s. The upper limit of the viscosity is 25 Pa·s, and may be 22 Pa·s or 20 Pa·s. When the viscosity of the varnish for rectangular insulated electric wires 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 electric wire can be further improved.

The viscosity of this 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, it is possible to prevent a decrease in the elongation of the insulating coating.

This varnish for rectangular insulated electric wires can be suitably used as a varnish for forming an insulating coating on rectangular insulated electric wires.

"Rectangular insulated wire" refers to an insulated wire with a rectangular cross-sectional shape. "Rectangle" generally refers to a quadrilateral with all corners being right angles, but in this specification, "rectangular" is a broader concept that includes any shape that is roughly recognizable as a rectangle.

The following explains each component contained in this varnish for rectangular insulated electric wire.

(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 known as a polyamic acid (polyamic acid). The polyimide precursor undergoes a dehydration cyclization reaction (imidization reaction) to form a cyclic imide, resulting in a polyimide.

If the aromatic tetracarboxylic dianhydride contains pyromellitic dianhydride (PMDA), the heat resistance of the insulating coating can be improved. This is because PMDA has a rigid and linear molecular structure. The aromatic tetracarboxylic dianhydride may also contain aromatic tetracarboxylic dianhydrides other than PMDA (hereinafter also referred to as "other aromatic tetracarboxylic dianhydrides").

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, Examples of other aromatic tetracarboxylic acid 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 acid dianhydride, and 2,3,6,7-naphthalenetetracarboxylic acid dianhydride. These other aromatic tetracarboxylic acid dianhydrides may be used alone or in combination of two or more.

Using biphenyltetracarboxylic dianhydride (BPDA) as another aromatic tetracarboxylic dianhydride can favorably improve the hydrolysis resistance of the polyimide precursor.

The lower limit of the PMDA content relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 10 mol%, 20 mol%, or 30 mol%. The upper limit of the PMDA content 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 achieved by using 10 mol% or more of PMDA.

The content of the other aromatic tetracarboxylic dianhydride relative to 100 mol% of the aromatic tetracarboxylic dianhydride can be determined appropriately 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%.

The aromatic diamine containing diaminodiphenyl ether (ODA) can improve the heat resistance of the insulating coating. This is because ODA has a rigid, 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). Of these, using 4,4'-diaminodiphenyl ether (4,4'-ODA) can favorably improve the elongation of the insulating coating.

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

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 other aromatic diamines include 4,4'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 2,4'-diaminodiphenyl sulfide, 2,2'-diaminodiphenyl sulfide, paraphenylenediamine, metaphenylenediamine, p-xylylenediamine, m-xylylenediamine, 2,2'-dimethyl-4,4'-diaminobiphenyl (mTBHG), 1,5-diaminonaphthalene, 4,4'-benzophenonediamine, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, and 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane. These other aromatic diamines may be used alone or in combination of two or more.

Other aromatic diamines, such as 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) or 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), can lower the dielectric constant of the insulating film.

The content of the other aromatic diamine relative to 100 mol% of the aromatic diamine can be determined appropriately 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%.

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, and the number of coating and heating steps can be reduced. 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.

From the perspective of ease of synthesis of the polyimide precursor, the molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine (aromatic tetracarboxylic dianhydride:aromatic diamine) used as raw materials for the polyimide precursor 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. 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. "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.

(Method for synthesizing polyimide precursor)
The polyimide precursor can be obtained by a condensation polymerization reaction between the aromatic tetracarboxylic dianhydride and the aromatic diamine described above. The condensation polymerization reaction can be carried out by a method similar to conventional methods for synthesizing polyimide precursors. A specific example of the condensation polymerization reaction is a method in which the aromatic tetracarboxylic dianhydride and the aromatic diamine are mixed in an organic solvent. This method allows the aromatic tetracarboxylic dianhydride and the aromatic diamine to polymerize, thereby obtaining a solution in which the polyimide precursor is dissolved in the organic solvent. For example, the degree of polymerization (weight-average molecular weight) can be controlled by carrying out the condensation polymerization reaction in the presence of a reaction inhibitor.

Examples of reaction inhibitors include water (H ₂ O) and alcohols having 1 to 15 carbon atoms. Examples of alcohols having 1 to 15 carbon atoms include monohydric alcohols such as ethanol, methanol, propanol, butanol, and pentanol; and polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin.

The reaction conditions for the condensation polymerization can be set appropriately 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.

The organic solvent used in the condensation polymerization reaction may be similar to the organic solvents described below.

(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 solvents may be used alone or in combination of two or more.

The lower limit of the boiling point of the amide solvent is 150°C, and may be 153°C. The upper limit of the boiling point of the amide solvent is 190°C, and 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 on the rectangular insulated electric wire can be further improved.

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).

If 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.

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, and 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.

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. An "aprotic solvent" refers to an organic solvent that does not have a group that releases a proton.

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 gamma-butyrolactone (boiling point: 204°C).

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, it is possible to further improve the uniformity of the thickness of the insulating coating on the rectangular insulated electric wire, and to form an insulating coating with strong interlayer adhesion.

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 will need to be volatilized when forming the insulating coating, which may take 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.

(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.

The pore-forming agent can be any known additive used to form insulating coatings with pores, and is not particularly limited. Examples of pore-forming agents include chemical foaming agents, thermally expandable microcapsules, particles containing thermally decomposable resins, and high-boiling-point solvents.

When the pore-forming agent is thermally decomposable resin-containing particles, an insulating coating with a good appearance can be formed even when the porosity is high. The thermally decomposable resin-containing particles are gasified by thermal decomposition, and pores are formed in the areas of the insulating coating where the thermally decomposable resin-containing particles were present. In this case, the thermally decomposable resin-containing particles can be evenly distributed as islands of fine particles in the resin portion that makes up the insulating coating (the sea phase of the resin matrix), forming independent pores in the insulating coating.

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 makes up the insulating coating. The baking temperature of the polyimide is set appropriately depending on the type of material that makes up the polyimide precursor, but is usually approximately 200°C or higher and 600°C or lower. "Thermal decomposition temperature" refers to the temperature at which a mass loss rate of 50% occurs 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" from SII Nanotechnology, Inc.).

Examples of the thermally decomposable resin contained in the thermally decomposable resin-containing particles include compounds such as polyethylene glycol and polypropylene glycol in which one, both, or a portion of the terminals 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. An example of the (meth)acrylic acid ester polymer is polymethyl methacrylate (PMMA). The term "(meth)acrylic acid" is a general term for "acrylic acid" and "methacrylic acid," and refers to either or both of them. Additionally, the term "(meth)acrylate" is a general term for "acrylate" and "methacrylate," and refers to either or both of them.

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 whose main component is the thermally decomposable resin and a shell whose main component is a resin whose thermal decomposition temperature is 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 interconnected pores and reduce variation in pore size.

The main component of the shell is not particularly limited as long as it has a higher thermal decomposition temperature than the core, and may be a synthetic resin with a low dielectric constant and high heat resistance. Examples include polystyrene, silicone, fluororesin, and polyimide. In particular, when the main component of the shell is silicone, it is easy to increase elasticity, which 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.

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 and the target porosity of the insulating coating.

(Other ingredients)
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.

Curing agents include, for example, 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, it can further improve the uniformity of the film thickness on the rectangular insulated electric wire.

<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.

The cross-sectional shape of this rectangular insulated wire is rectangular (flat wire). Because it is a flat wire, it can be wound at high density during coil processing.

This rectangular insulated wire can be suitably used as coil winding wire (magnet wire).

(Rectangular conductor)
The cross section of the rectangular conductor is rectangular.

The material of the rectangular conductor may be a metal with high conductivity and mechanical strength. Examples of such metals include copper, copper alloys, aluminum, nickel, silver, mild steel, steel, and stainless steel. The rectangular conductor can be made of a wire-shaped material made from the above metal, 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.

The lower limit of the average cross-sectional area of the rectangular conductor may be 0.01 ^mm2 or 0.1 ^mm2 . In this case, the volume of the insulating coating relative to the rectangular conductor in the rectangular insulated electric wire can be made appropriate, thereby improving the volumetric efficiency of coils and the like formed using the rectangular insulated electric wire. The upper limit of the average cross-sectional area of the rectangular conductor may be 20 ^mm2 or 10 ^mm2 . In this case, the need to form a thick insulating coating to sufficiently reduce the dielectric constant can be reduced, and unnecessary increases in the diameter of the rectangular insulated electric wire can be avoided.

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

The insulating coating contains a resin matrix. The insulating coating is formed from the above-mentioned varnish for rectangular insulated electric wire. Therefore, the resin matrix is primarily composed of polyimide.

There are no particular restrictions on the average thickness of the insulating coating, and it can usually be between 2 μm and 200 μm.

The insulating coating may contain multiple voids. In this case, the dielectric constant of the insulating coating can be reduced. When the insulating coating contains multiple voids, the multiple voids are dispersed throughout the resin matrix in the insulating coating.

When the insulating coating contains multiple pores, the porosity of the insulating coating may be 20% by volume or more and 60% by volume or less. A porosity of 20% by volume or more can further reduce the dielectric constant of the insulating coating. 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 resin matrix and the insulating coating containing pores. Specifically, the porosity is measured as follows: The porosity is calculated using the formula (W1 - W2) x 100/W1, where W1 is the mass when there are no pores, 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, and W2 is the actual mass of the insulating coating.

(vacancy)
The voids may be derived from thermally decomposable resin-containing particles. The thermally decomposable resin-containing particles are gasified by thermal decomposition, and voids 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, and independent voids are formed in the insulating coating.

If the lower limit of the average diameter of the plurality of pores is 0.1 μm, the mechanical properties of the insulating coating can be improved. If the upper limit of the average diameter is 10 μm, the insulating properties of the insulating coating can be improved. 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 (for example, Porous Materials' "Automatic Pore Diameter Distribution Measuring System for Porous Materials").

The rectangular insulated wire may have a configuration other than that 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 adhesion improvers include mercaptans such as 2-mercaptoimidazole and 5-amino-1,3,4-thiadiazole-2-thiol.

The rectangular insulated electric wire may have a surface friction adjusting layer as its outermost layer. Examples of surface friction adjusting layers 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 that makes up the rectangular insulated electric wire, with the conductor side facing inward.

The rectangular insulated wire may have an outermost adhesive layer containing an additive such as a foaming agent. Examples of foaming agents 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.

The rectangular insulated wire may have, as its outermost layer, a 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.

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

In the coating process, the rectangular insulated wire varnish described above is applied to the outer surface of the rectangular conductor. One method for applying the rectangular insulated wire varnish to the outer surface of the rectangular conductor is to use a coating device equipped with a liquid composition tank that stores 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 surface of the conductor. The rectangular conductor then passes through the coating die, coating the rectangular insulated wire varnish to a uniform thickness.

In the heating step, the varnish for rectangular insulated electric wires that was applied to the conductor in the application step is heated. This heating volatilizes the organic solvent in the varnish for rectangular insulated electric wires and hardens the polyimide precursor, forming polyimide.

The equipment used in the heating step is not particularly limited, and for example, a cylindrical baking furnace that is long in the direction in which the conductor travels can be used. The heating method is not particularly limited, and can be any conventionally known method such as hot air heating, infrared heating, or high-frequency heating.

The heating temperature can be, for example, 300°C or higher and 800°C or lower. The heating time can be, for example, 5 seconds or higher and 1 minute or lower.

The coating and heating processes are typically repeated multiple times. By repeating the process 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.

The present invention will be explained in more detail below using experimental examples. The present invention is not limited to these experimental examples.

<Preparation of Varnish for Rectangular Insulated Wire and Production of Rectangular Insulated Wire>
The various components used in preparing the varnish for rectangular insulated electric wire are shown below.
(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)

[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 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. Varnish No. 1 (solids concentration: 28% by mass) was obtained as a polyimide precursor solution containing NMP as a solvent.

(Production 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 similar in shape to 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, producing rectangular insulated electric wire No. 1.

[No. 2 to No. 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 produced.

[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.

<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.

[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.

[Film thickness uniformity]
The average thickness of the insulating coating was determined for the rectangular insulated wires No. 1 to No. 21 produced 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 indicate a large variation.

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

In Table 1 below, "acid anhydride" means "aromatic tetracarboxylic dianhydride." "Diamine" means "aromatic diamine." In the "organic solvent" column, "specific amide solvent" 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.

Table 1 shows that No. 2 to No. 21 have superior film thickness uniformity compared to No. 1.

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 configuration of the above-described embodiments, but is indicated by the claims, and is intended to include all modifications within the meaning and scope of the claims.

1. Flat insulated wire 2. Flat conductor 3. Insulating coating

Claims (11)

a polyimide precursor which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine;
an organic solvent and
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,
A varnish for rectangular insulated electric wires having a viscosity at 30°C of 5 Pa·s or more and 25 Pa·s or less. The varnish for rectangular insulated electric wires according to claim 1, wherein the boiling point of the amide-based solvent is 150°C or higher and 170°C or lower. The varnish for rectangular insulated electric wires according to claim 1 or 2, wherein the amide solvent is N,N-dimethylacetamide, N,N-dimethylformamide, or a combination thereof. The varnish for rectangular insulated electric wire according to any one of claims 1 to 3, wherein the content of the amide solvent in the organic solvent is 70 mass% or more. The varnish for rectangular insulated electric wire according to any one of claims 1 to 4, wherein the content of the amide solvent in the organic solvent is 95 mass% or less. The varnish for rectangular insulated electric 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. The varnish for rectangular insulated electric wires according to any one of claims 1 to 6, having a viscosity at 30°C of 10 Pa·s or more and 20 Pa·s or less. The varnish for rectangular insulated electric wires according to any one of claims 1 to 7, which does not contain a filler. 2. The varnish for rectangular insulated wire according to claim 1, wherein when an insulating coating is formed on a rectangular conductor using the varnish for rectangular insulated wire according to claim 1, the variation rate represented by the following formula is 12.0% or less.
Variation rate (%) = (4σ/average thickness of insulating film) x 100
(where σ represents the standard deviation of the thickness of the insulating coating.) A rectangular conductor;
and an insulating coating that covers the rectangular conductor.
A rectangular insulated electric wire, wherein the insulating coating is formed from the varnish for a rectangular insulated electric wire according to any one of claims 1 to 9. The method for producing a rectangular insulated electric wire according to claim 10,
A step of applying the varnish for a rectangular insulated wire according to any one of claims 1 to 9 to an outer peripheral surface of the rectangular conductor;
a step of heating the varnish for a rectangular insulated electric wire applied in the application step.