JP2024118558A - Aromatic polyimide resin varnish for insulating coating, and insulated wire, coil, rotating electric machine, and electric/electronic device using the same - Google Patents

Abstract

[Problem] To provide an aromatic polyimide resin varnish containing an aromatic polyamic acid for use in forming an insulating layer made of an aromatic polyimide resin, which dries more quickly and can effectively suppress the occurrence of bumps even if the insulating layer formed in a single application and baking process is made thicker; an insulated wire having an excellent appearance and having an insulating coating including an insulating layer formed using the resin varnish; and a coil, rotating electric machine, and electric/electronic device using the insulated wire. [Solution] An aromatic polyimide resin varnish for insulating coating, which contains an aromatic polyamic acid with an imidization rate of 10 to 60%. [Selected Figure] None

Description

The present invention relates to an aromatic polyimide resin varnish for insulating coating, and to an insulated wire, coil, rotating electrical machine, and electrical/electronic device that uses the same.

Insulated electric wires with a resin insulating coating on the outer surface of a linear metal conductor are used as magnet wires for coils in electric and electronic devices such as high-speed switching elements, inverter motors, and transformers. The insulating coating of insulated electric wires is formed by applying and baking a thermosetting resin or a thermoplastic resin, extrusion coating a thermoplastic resin, or a combination of these.

In the coating and baking of the thermosetting resin or thermoplastic resin, a resin varnish in which the resin is dissolved in a solvent is used, and the amount of resin varnish applied is determined so that each insulating layer has a desired thickness. The solvent in the resin varnish is removed by volatilization (evaporation) in the baking process following the application of the resin varnish. Therefore, if the amount of resin varnish applied at one time is not too large (the thickness of the insulating layer formed is not too thick) and sufficient baking temperature and baking time are ensured, the amount of residual solvent in the insulating layer formed can be kept low. On the other hand, if the amount of resin varnish applied is large (the insulating layer is thick), the solvent does not volatilize (evaporate) sufficiently in the baking process, and the amount of residual solvent in the insulating layer formed increases. This excess residual solvent foams so as to push the insulating layer from the inside to the outside during repeated application and baking of the resin varnish, causing bumps (bulges, protrusions, etc. on the insulating layer or insulating coating surface due to foaming due to volatile components, etc.), which causes poor appearance. Furthermore, the resulting bumps can cause a decrease in adhesion between the conductor and the insulating layer, as well as between insulating layers themselves.
From the standpoint of reducing the amount of residual solvent in the insulating layer and preventing the occurrence of poor appearance of the insulating coating, the amount of resin varnish applied each time is usually set small (to make the insulating layer thin). For example, the resin varnish is applied and baked so that the insulating layer has an average thickness of 2 to 3 μm per layer. This process is repeated to form a laminated insulating layer, thereby forming an insulating coating of the desired thickness.

Various thermosetting resins are known as materials for the insulating layer (enamel layer) formed by applying and baking a resin varnish. Some thermoplastic resins are also used as materials for the enamel layer. Among them, thermosetting aromatic polyimide resins (hereinafter simply referred to as "aromatic polyimide resins"), which have a structure in which aromatic rings are linked by imide bonds, have excellent heat resistance, low temperature dependence of electrical properties, high dimensional stability, and excellent chemical resistance, and are widely used as materials for forming insulating layers. An insulating layer of aromatic polyimide resin is formed by applying and baking a varnish containing aromatic polyamic acid, which is a precursor of aromatic polyimide resin.

An insulated electric wire has been proposed that reduces the amount of residual solvent in the insulating film containing the polyimide resin. For example, Patent Document 1 describes an invention for an insulated electric wire, which is characterized by forming an insulating film on a conductor by coating and laminating (1) a first insulating layer substantially consisting of at least one of polyamideimide and polyimide, and (2) a second insulating layer made by blending polyamideimide A with thermoplastic resin B having a glass transition temperature of 140°C or higher in a weight ratio A/B of 70/30 to 30/70, in that order, such that the ratio T1/T2 of the thickness T1 of the first insulating layer to the thickness T2 of the second insulating layer is within the range of T1/T2 = 5/95 to 40/60, and the amount of residual solvent is 0.05% by weight or less of the total amount of the insulating film. According to the technology described in Patent Document 1, by controlling the amount of residual solvent in the entire insulating film as described above, it is possible to more reliably prevent the insulating film near the joint from foaming due to heat during joining or from becoming discolored for an extended length during the process of joining the ends of the insulated electric wire.

JP 2001-155551 A

If the evaporation rate of the solvent during each application and baking process of the resin varnish can be increased, it will be possible to form a thicker insulating layer while suppressing the occurrence of bumps. In other words, it will be possible to form an insulating film of the required thickness using fewer insulating layers, reducing the number of processes and, as a result, reducing manufacturing costs.

The present invention aims to provide an aromatic polyimide resin varnish (hereinafter, simply referred to as "aromatic polyimide resin varnish") containing aromatic polyamic acid, which is used to form an insulating layer made of aromatic polyimide resin, and which has excellent quick-drying properties and can effectively suppress the occurrence of bumps even if the insulating layer formed in a single application and baking process is made thicker. The present invention also aims to provide an insulated electric wire with excellent appearance, which has an insulating coating including an insulating layer formed using the resin varnish, and a coil, a rotating electric machine, and an electric/electronic device using this insulated electric wire.

As a result of studies to solve the above problems, the present inventors have found that in an aromatic polyimide resin varnish, by promoting the dehydration and cyclization reaction of the polyamic acid structure to a certain extent to raise the imidization rate to a certain range before the coating and baking process, the quick-drying property of the aromatic polyimide resin varnish can be effectively improved, and the amount of residual solvent in the insulating film formed by repeatedly coating and baking the aromatic polyimide resin varnish can be reduced, and as a result, the occurrence of bumps can be suppressed and an insulated wire with excellent appearance can be obtained. The present invention was completed after further studies based on these findings.

That is, the above-mentioned object of the present invention has been achieved by the following means.
[1]
An aromatic polyimide resin varnish for insulating coating contains an aromatic polyamic acid having an imidization rate of 10 to 60%.
[2]
The aromatic polyimide resin varnish for insulating coating according to [1] above, wherein when a 10 mg sample of the resin varnish, in which the non-volatile content (solid content) is adjusted to 25 mass %, is heated from 35° C. at a rate of 20° C./min in a nitrogen atmosphere with a gas flow rate of 100 ml/min, the weight loss rate at the time when the boiling point of the solvent is reached is 40 mass % or more.
[3]
The aromatic polyimide resin varnish for insulating coating according to [2] above, wherein the weight reduction rate is 50 mass% or more.
[4]
The aromatic polyimide resin varnish for insulating coating according to any one of [1] to [3] above, wherein the resin varnish has a non-volatile content of 10 mass% or more.
[5]
An insulated wire having a conductor and an insulating coating covering the outer periphery of the conductor,
An insulated wire, wherein the insulating coating comprises a laminated insulating layer formed by repeatedly applying and baking the aromatic polyimide resin varnish for insulating coating according to any one of [1] to [4].
[6]
The average thickness of each of the insulating layers constituting the laminated insulating layer is 4 μm or more.
The insulated wire according to [5] above.
[7]
A coil using the insulated wire according to [6] above.
[8]
An electric/electronic device having the coil described in [7].

In the present invention or this specification, the term "insulating layer" simply means a layer formed by applying and baking a resin varnish once. In the present invention, an insulating layer formed by repeatedly applying and baking the same resin varnish multiple times is regarded as a multi-layer insulating layer. In other words, whether the resin varnish is the same or different, a layer formed by applying and baking once is counted as one insulating layer. In other words, when the application and baking are repeated, a laminated insulating layer is formed in which the same number of insulating layers as the number of repetitions are laminated. Note that this number of layers can be confirmed with an optical microscope or a microscope after edging the cross section of the insulating layer.
In the present invention, the insulating coating of the insulated wire, as described above, has as a specific feature that it includes a laminated insulating layer formed by repeatedly applying and baking a resin varnish. However, this simply indicates the state of the insulating coating (i.e., indicates that the insulating coating includes a specific enamel layer), thereby clarifying the structure or characteristics of the insulating coating.
In the present invention and this specification, the cross-sectional shape perpendicular to the longitudinal direction of the insulated electric wire, including the conductor and the insulating coating, may be simply referred to as the cross-sectional shape. The cross-sectional shape in this invention does not simply mean that only the cut surface has a specific shape, but that this cross-sectional shape is continuous in the longitudinal direction of the entire insulated electric wire, and unless otherwise specified, it means that the cross-sectional shape perpendicular to the longitudinal direction is substantially the same for any part of the insulated electric wire in the longitudinal direction.
In the present invention and this specification, a numerical range expressed using "to" means a range including the numerical values before and after it as the lower limit and upper limit.
In the present invention and this specification, "ppm" described as a unit of concentration is based on mass.

The aromatic polyimide resin varnish for insulating coating of the present invention has excellent quick-drying properties during application and baking, and even if the polyimide insulating layer formed by a single application and baking is made thicker, the solvent is less likely to remain in the polyimide insulating layer, and the occurrence of bumps can be effectively suppressed. Furthermore, an insulated electric wire using this resin varnish to form an insulating coating can keep the amount of residual solvent in the insulating coating low, suppressing the occurrence of bumps and providing an excellent appearance, even if the polyimide insulating layer formed by a single application and baking of the resin varnish is made thicker. Furthermore, according to the present invention, a coil, a rotating electric machine, and an electric/electronic device using the insulated electric wire are provided.

FIG. 1 is a schematic exploded perspective view showing a preferred embodiment of a stator used in an electric/electronic device according to the present invention. FIG. 2 is a schematic perspective view showing a preferred embodiment of a stator used in the electric/electronic device of the present invention.

A preferred embodiment of the present invention will be described below, but the present invention is not limited to the following embodiment except as specified in the present invention.

[Aromatic polyimide resin varnish for insulating coating]
The aromatic polyimide resin varnish for insulating coating (for forming an insulating layer) of the present invention (hereinafter also referred to as "the resin varnish of the present invention") has an aromatic polyamic acid as a resin component, and is a resin varnish for forming an insulating layer (polyimide insulating layer) of aromatic polyimide resin by coating and baking in the manufacture of insulated electric wires. In the resin varnish of the present invention, the imidization rate of the aromatic polyamic acid is increased to a specific range, and the interaction with the solvent is suppressed, resulting in excellent quick-drying properties. Therefore, the amount of residual solvent in the insulating layer, laminated insulating layer, or insulating film formed using the resin varnish of the present invention is kept low, thereby making it possible to suppress the occurrence of abnormalities in the appearance of the insulating film caused by the residual solvent (such as the occurrence of bumps).
In addition, since the resin varnish of the present invention has excellent quick-drying properties, a larger amount of the resin varnish can be applied in one application and baking, which allows a thicker polyimide insulating layer to be formed. This reduces the number of repeated application and baking steps required to reach a desired coating thickness, thereby reducing the manufacturing cost of the insulated wire.
The characteristic features of the resin varnish of the present invention will be described below.

<Aromatic polyamic acid>
The resin varnish of the present invention contains an aromatic polyamic acid (aromatic polyimide precursor) as a resin component, and a part of the structure undergoes a dehydration/cyclization reaction at a specific ratio to form an imide bond, and the imidization rate is controlled to the value specified in the present invention. In the following description, such a polymer is also referred to as the "polyamic acid used in the present invention."

Aromatic polyamic acid can be formed by a ring-opening polyaddition reaction of aromatic carboxylic dianhydride and aromatic diamine. The method for preparing aromatic polyamic acid is not particularly limited, and it can be prepared by a conventional method. For example, it can be obtained by dissolving aromatic diamine in an aprotic amide polar solvent and adding aromatic carboxylic dianhydride with stirring at room temperature.

The aromatic polyamic acid has a unit structure represented by the following general formula (1), for example:

In the general formula (1), R ¹ represents a tetravalent group having an aromatic ring, and R ² represents a divalent group having an aromatic ring. Here, R ¹ is derived from an aromatic carboxylic dianhydride, and R ² is derived from an aromatic diamine. Examples of the aromatic ring include a benzene ring and a naphthalene ring, and a benzene ring is preferred. The number of aromatic rings in R ¹ and R ² is preferably 1 to 4, and more preferably 1 or 2 (condensed aromatic rings such as naphthalene are one aromatic ring in their entirety). The aromatic polyamic acids themselves used to form the enamel layer of the insulated wire are widely known, and the present invention is characterized in that the imidization rate of these aromatic polyamic acids is controlled.

The polyamic acid used in the present invention has a structure in which a part of the structure undergoes a dehydration/cyclization reaction (imidization reaction) at a specific ratio to form an imide bond. This dehydration/cyclization reaction can be caused by a normal method such as a thermal imidization method or a chemical imidization method. For example, the method described in the examples can also be adopted.
When the unit structure of the general formula (1) is imidized, the structure is represented by the following general formula (2).

In formula (2), ^R3 has the same meaning as ^R1 , and ^R4 has the same meaning as ^R2.

-Imidization rate-
The polyamic acid used in the present invention has an imidization rate of 10 to 60%. The imidization rate can be measured by a conventional method using a resin varnish. For example, the FT-IR spectra of a reference sample (polyimide) in which the imidization rate of the resin in the resin varnish is set to 100% and the resin (polyamic acid) in the resin varnish as a measurement sample are measured by an ATR (attenuated total reflection) method using a Fourier transform infrared spectrophotometer (FT-IR, product name: IRAffinity-1S) manufactured by Shimadzu Corporation, and the imidization rate can be calculated by comparing the peak intensity around 1774 cm ⁻¹ of the obtained FT-IR spectrum (waveform data). That is, the imidization rate (%) can be calculated by the following formula after baseline correction.

Imidization rate (%)=100×[measurement sample value (A1774 cm ⁻¹ /A1495 cm ⁻¹ )]/[reference sample value (A1774 cm ⁻¹ /A1495 cm ⁻¹ )]

For example, baseline correction is performed using the absorbance at a wavenumber of 4000 cm ^-1 , and normalization is performed using the peak intensity at a wavenumber of about 1495 cm ^-1 (the peak intensity at about 1774 cm ^-1 is divided by the peak intensity at about 1495 cm ^-1 ), and the imidization rate can be calculated by comparing the obtained numerical value of the measured sample (A1774 cm ^-1 /A1495 cm ^-1 ) with the numerical value of a reference sample in which the imidization rate is set to 100% (A1774 cm ^-1 /A1495 cm ^-1 ).
A reference sample in which the imidization rate of the resin in the resin varnish is 100% can be prepared, for example, by heating the polyimide resin varnish at 300° C. for 1 hour to sufficiently promote the dehydration and cyclization reaction of the polyamic acid.
In this specification, for example, "peak intensity near 1774 cm ^-1 " means the peak intensity when there is a peak at a wavelength of 1774 cm ^-1 , and means the peak intensity at the wavenumber position closest to the wavelength of 1774 cm ^-1 when there is no peak at the wavelength of 1774 cm ^-1 . In other words, it does not necessarily mean the absorbance at the wavenumber position of 1774 cm ^-1 . The same applies to other peak intensities.

As a result of extensive investigations, the inventors of the present invention have found that when the imidization rate of the polyamic acid is lower than the imidization rate specified in the present invention, the volatilization (evaporation) of the solvent is difficult to occur when forming an insulating layer (insulating film) by coating and baking a resin varnish. The reason for this is unclear, but is thought to be as follows.
The solvent contained in the resin varnish is coordinated and stabilized by forming hydrogen bonds or the like with the -NH- and -OH groups of the polyamic acid. The stabilized solvent is less likely to volatilize (evaporate) than a free solvent that does not form such bonds. For example, when the solvent is dimethylacetamide or N-methylpyrrolidone, it coordinates with the -NH- and -OH groups of the polyamic acid via the carbonyl groups of these solvents. When the imidization rate of the polyamic acid is lower than the imidization rate specified in the present invention, the interaction between the solvent and the polyamic acid is strong in the formation of an insulating layer (insulating film) by coating and baking the resin varnish, and it is considered that the solvent is less likely to volatilize (evaporate) and the solvent is more likely to remain in the insulating layer.

On the other hand, it has also been found that when the imidization rate of the polyimide resin is higher than the imidization rate specified in the present invention, the quick-drying property of the solvent tends to be impaired. This is thought to be due to the high viscosity of the resin varnish itself. In other words, it is presumed that the resin acts like a lid on the solvent, trapping part of the solvent within the coating layer.

At least a portion of the solvent remaining inside the insulating layer in this way foams and pushes up the outer insulating layer from below as the resin varnish is repeatedly applied and baked, producing lumps and other defects that cause poor appearance of the insulating film.

In the resin varnish of the present invention, the imidization rate of the polyamic acid is controlled to 10-60%, thereby increasing the proportion of free solvent (solvent in a free state) that is not bound to the polyamic acid, promoting the evaporation of the solvent during application and baking, while also appropriately suppressing the viscosity of the resin varnish, effectively reducing the amount of residual solvent in the insulating layer (insulating coating), resulting in an insulated wire with excellent appearance. In addition, by setting the imidization rate within the above range, the generation of by-product water due to the imidization reaction during baking of the resin varnish of the present invention can be suppressed. In other words, the ability to suppress foaming due to bumping of the by-product water is also thought to be one of the reasons why the resulting insulated wire has excellent appearance.

In the resin varnish of the present invention, from the viewpoint of further increasing the quick-drying property of the resin varnish, the imidization rate is preferably 15% or more, more preferably 20% or more, and even more preferably 25% or more. From the same viewpoint as above, the imidization rate is preferably 55% or less, more preferably 50% or less, even more preferably 45% or less, even more preferably 40% or less, and even more preferably 35% or less. The imidization rate is preferably in the range of 15 to 55%, more preferably 20 to 50%, even more preferably 25 to 45%, and is also preferably 25 to 40%, and is also preferably 25 to 35%.

- Aromatic carboxylic acid dianhydride -
The aromatic carboxylic dianhydride is an aromatic tetracarboxylic dianhydride, such as pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride (ODPA), 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 1,1-bis(3, Examples of the aromatic carboxylic acid dianhydride include 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. In the preparation of the aromatic polyamic acid, the aromatic carboxylic acid dianhydride may be used alone or in combination of two or more kinds. In the resin varnish of the present invention, the aromatic carboxylic acid dianhydride is preferably PMDA, BPDA, and/or ODPA.

- Aromatic diamine -
The aromatic diamine is preferably an aromatic diamine compound, such as 4,4'-oxydianiline (ODA), m-phenylenediamine, 3,3'-diamino-4,4'dihydroxydiphenyl sulfone, 4,4'diamino-3,3'dihydroxybiphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 4,4'-diaminodiphenyl ether (DDE), and 3,4'-diaminodiphenyl ether (m-DDE). , 3,3'-diaminodiphenyl ether, 4,4'-diamino-diphenyl sulfone (p-DDS), 3,4'-diamino-diphenyl sulfone, 3,3'-diamino-diphenyl sulfone, 2,4'-diaminodiphenyl ether, 1,3-bis(4-aminophenoxy)benzene (m-TPE), 1,3-bis(3-aminophenoxy)benzene (APB), 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), 2,2-bis[ 4-(4-aminophenoxy)phenyl]hexafluoropropane (HF-BAPP), bis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS), bis[4-(3-aminophenoxy)phenyl]sulfone (m-BAPS), 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), 1,4-bis(4-aminophenoxy)benzene (p-TPE), 4,4'-diaminodiphenyl sulfide (ASD), 3,4'-diaminodiphenyl Examples of the diamines include aryl sulfide, 3,3'-diaminodiphenyl sulfide, 3,3'diamino-4,4'dihydroxydiphenyl sulfone, 2,4-diaminotoluene (DAT), 2,5-diaminotoluene, 3,5-diaminobenzoic acid (DABz), 2,6-diaminopyridine (DAPy), 4,4'diamino-3,3'dimethoxybiphenyl, 4,4'diamino-3,3'dimethylbiphenyl, and 9,9'-bis(4-aminophenyl)fluorene (FDA). In the preparation of the aromatic polyamic acid, the diamines may be used alone or in combination of two or more. In the resin varnish of the present invention, the diamine is preferably ODA and/or BAPP.

<Solvent>
The resin varnish of the present invention contains a solvent (organic solvent, organic solvent) to turn the resin into a varnish. Examples of the solvent include amide-based solvents such as N,N-dimethylacetamide (DMAc, boiling point: 165° C.), N-methyl-2-pyrrolidone (NMP, boiling point: 202° C.), and N,N-dimethylformamide (DMF, boiling point: 153° C.), urea-based solvents such as N,N-dimethylethyleneurea, N,N-dimethylpropyleneurea, and tetramethylurea, lactone-based solvents such as γ-butyrolactone and γ-caprolactone, carbonate-based solvents such as propylene carbonate, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexane. Examples of the solvent include ketone-based solvents such as diethyl ether, isopropyl ether, and the like; ester-based solvents such as ethyl acetate, n-butyl acetate, butyl cellosolve acetate, butyl carbitol acetate, ethyl cellosolve acetate, ethyl carbitol acetate, etc.; glyme-based solvents such as diglyme, triglyme, tetraglyme, etc.; hydrocarbon-based solvents such as toluene, xylene, cyclohexane, etc.; phenol-based solvents such as cresol, phenol, halogenated phenol, etc.; sulfone-based solvents such as sulfolane, etc.; and dimethyl sulfoxide (DMSO).

Among these, from the viewpoint of not having hydrogen atoms that tend to inhibit the crosslinking reaction by heating, etc., those selected from DMAc, NMP, DMF, N,N-dimethylethyleneurea, N,N-dimethylpropyleneurea, tetramethylurea, and DMSO are more preferable, and DMAc and/or NMP are further preferable.
The above solvents and the like may be used alone or in combination of two or more kinds.

The content of polyamic acid in the resin contained in the resin varnish of the present invention is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more, and it is also preferable that all of the resin is polyamic acid. In addition, when a resin other than polyamic acid is contained, the type of the resin is appropriately selected within a range that does not impair the effects of the present invention. For example, the resin varnish of the present invention may contain a polyimide resin in addition to polyamic acid. Note that even when the resin varnish contains a resin other than polyamic acid, the above imidization rate is determined based on the composition of the entire resin varnish (the entire resin varnish is used as a sample) as described above.

The resin varnish of the present invention may contain various additives such as adhesion aids, antioxidants, antistatic agents, ultraviolet protection agents, light stabilizers, fluorescent brighteners, pigments, dyes, compatibilizers, lubricants, reinforcing agents, flame retardants, crosslinking agents, crosslinking aids, plasticizers, thickeners, viscosity reducers, and elastomers, provided that the additives do not affect the properties.
The resin varnish of the present invention may contain inorganic fine particles, provided that the inorganic fine particles do not affect the properties of the varnish. Examples of such inorganic fine particles include zinc oxide, titanium oxide, silica, alumina, tin oxide, silicon carbide, and strontium titanate.

<Non-volatile content>
In the present invention and this specification, the "non-volatile content of the resin varnish" means the evaporation residue (solid content) excluding volatile components such as water and solvent in the resin varnish, and the "non-volatile content rate" means the solid content (mass%) of the resin varnish. The resin varnish of the present invention preferably has a non-volatile content rate of 10 mass% or more, and can be 15 mass% or more, or can be 20 mass% or more. In addition, the upper limit is not particularly limited, but can be, for example, 50 mass% or less, 40 mass% or less, or 35 mass% or less. The non-volatile content rate of the resin varnish of the present invention is preferably in the range of 10 to 50 mass%, more preferably 15 to 40 mass%, and even more preferably 20 to 35 mass%.
The non-volatile content can be calculated by heating the resin varnish of the present invention to thoroughly evaporate the volatile components, measuring the weight of the residue remaining after heating, and calculating the percentage of the weight of the sample before heating. The heating conditions can be appropriately set taking into consideration the boiling points of the solvents and the like contained in the resin varnish. For example, it is also preferable to use a method in accordance with JIS K 5601-1-2:2008, with a heating temperature of 200°C and a heating time of 2 hours.

<Weight reduction rate>
In the resin varnish of the present invention, the imidization rate of the polyamic acid is controlled within a specific range, so that the solvent contained in the resin varnish is more likely to volatilize (evaporate). Therefore, when the resin varnish of the present invention is heated at a constant temperature increase rate, the weight loss rate at the boiling point temperature of the solvent is high. The weight loss rate of the resin varnish of the present invention can be measured by the following method after adjusting the non-volatile content rate to 25 mass% by adding a solvent to the resin varnish or by drying and concentrating under conditions in which imidization does not substantially proceed.

- Measurement conditions -
Non-volatile content in sample: 25% by mass
Sample weight: 10 mg
Apparatus: Differential thermal and thermogravimetric simultaneous measurement apparatus (product name: DTG-60AH, manufactured by Shimadzu Corporation)
・Heating rate: 20°C/min ・Heating range: 35-400°C
Under nitrogen atmosphere Gas flow rate: 100 ml/min

In the above measurement, it is preferable to prepare the sample immediately before the measurement. Under the above conditions, the weight of the sample is measured when the boiling point of the solvent contained in the resin varnish is reached, and the weight reduction rate (mass%) at the boiling point of the solvent can be determined by calculating the ratio of the weight reduction from the weight of the sample before the measurement. For example, if the weight of the sample before the measurement is 10 mg and the weight of the sample when the boiling point of the solvent is reached is 2 mg, the weight reduction rate (mass%) can be calculated as [1-(2/10)]×100=80 (mass%). The boiling point of the solvent can be a publicly known boiling point of the solvent. In addition, when the resin varnish contains multiple solvents, the weighted average of the boiling points of these solvents can be used as the boiling point of the solvent in the resin varnish. For example, the boiling point of a mixed solvent containing 20% by mass of solvent A, which has a boiling point of 100° C., and 80% by mass of solvent B, which has a boiling point of 200° C., can be calculated as follows: [(100×20/100)+(200×80/100)]=180° C.
The resin varnish of the present invention preferably has a weight loss rate, measured under the above conditions, of 40% by mass or more when the temperature reaches the boiling point of the solvent, more preferably 45% by mass or more, and even more preferably 50% by mass or more.

<Liquidity>
When applying the resin varnish to the conductor, the temperature (wire temperature) of the conductor varies depending on the manufacturing conditions, but is generally about 30 to 70°C. Therefore, in the resin varnish of the present invention, from the viewpoint of sufficiently volatilizing (evaporating) the solvent during application and baking of the resin varnish, the viscosity of the resin varnish at at least one temperature between 30 and 70°C (for example, 30°C) is preferably 15000 mPa·s or less, more preferably 10000 mPa·s or less, and even more preferably 3000 mPa·s or less. In addition, from the viewpoint of applying the resin varnish to the conductor or the insulating layer, the viscosity is usually 500 mPa·s or more, and can be 1000 mPa·s or more, or can be 1500 mPa·s or more.
Moreover, the resin varnish of the present invention more preferably has a viscosity within the above-mentioned viscosity range at the line temperature when the resin varnish is applied to the conductor.

[Insulated wire]
The insulated wire of the present invention has a conductor and an insulating coating covering the outer periphery of the conductor. The insulating coating includes a so-called enamel layer (multi-layer enamel layer) formed by repeatedly applying and baking the resin varnish of the present invention. That is, in the insulated wire of the present invention, the insulating coating includes an insulating layer having a polyimide.
It is preferable that the cross-sectional shape of the insulated wire of the present invention is similar to that of the conductor, and it is particularly preferable that the overall shape of the insulating coating, i.e., the cross-sectional shape of the outermost surface of the insulating coating opposite the conductor, is similar to that of the conductor. Note that the similar shape is not limited to a completely similar shape, and may be an approximately similar shape.

<Conductor>
The conductor used in the present invention may be any of those conventionally used as conductors for insulated wires, such as metal conductors such as copper wires and aluminum wires.

The cross-sectional shape perpendicular to the longitudinal direction of the conductor used in the present invention is not particularly limited. For example, conductors having a circular or rectangular (rectangular) cross-sectional shape can be used. In the present invention, a conductor having a rectangular cross-sectional shape, i.e., a rectangular conductor, is preferred. A conductor having a rectangular cross-sectional shape has a higher space factor in the slots of the stator core when wound, compared to a circular conductor. For this reason, it is preferred for applications in which many insulated wires are incorporated in a certain narrow space.
A conductor having a rectangular cross section is preferably shaped such that the four corners are chamfered (with a radius of curvature r) in order to suppress partial discharge from the corners. The radius of curvature r is preferably 0.6 mm or less, and more preferably 0.2 to 0.4 mm.
The size of the conductor is not particularly limited, but in the case of a rectangular conductor, the width (long side) of the rectangular cross-sectional shape is preferably 1.0 to 10.0 mm, more preferably 1.0 to 5.0 mm, and even more preferably 1.4 to 4.0 mm, and the thickness (short side) is preferably 0.4 to 3.0 mm, and more preferably 0.5 to 2.5 mm. The ratio of the length of the width (long side) to the thickness (short side) (thickness:width) is preferably 1:1 to 1:20, and more preferably 1:1 to 1:4. On the other hand, in the case of a conductor having a circular cross-sectional shape, the diameter is preferably 0.3 to 3.0 mm, and preferably 0.4 to 2.7 mm.

<Insulating film>
In the insulated wire of the present invention, the insulating coating includes a laminated insulating layer (multilayer insulating layer) in which a plurality of insulating layers are laminated.
Since the resin varnish of the present invention has excellent quick-drying properties, even if the resin varnish is applied and baked so that each insulating layer is thick, the solvent in the resin varnish quickly volatilizes and evaporates during baking, so that the amount of residual solvent in the insulating layer (insulating film) can be controlled to be low. Furthermore, by making each insulating layer thick, the number of times that the resin varnish is repeatedly applied and baked can be reduced, and the cost of manufacturing the insulated electric wire can be reduced. From the above viewpoint, the average thickness per layer of each insulating layer constituting the insulating film of the insulated electric wire of the present invention can be 1 μm or more, preferably 2 μm or more, more preferably 3 μm or more, even more preferably 4 μm or more, and even more preferably 5 μm or more. Moreover, the average thickness can be 15 μm or less, may be 10 μm or less, or may be 6 μm or less. The average thickness of each of the insulating layers is preferably in the range of 1 to 15 μm, more preferably 2 to 15 μm, even more preferably 3 to 10 μm, still more preferably 4 to 10 μm, even more preferably 4 to 6 μm, and still more preferably 5 to 6 μm.
In this specification, the average thickness is measured by 16-point measurement. The 16-point measurement is a commonly used measurement method in this field, and a specific measurement method is described in the pamphlet of International Publication No. WO 2013/073397.

The thickness of the insulating coating is preferably 30 μm or more and 200 μm or less, more preferably 40 μm or more and 180 μm or less, and even more preferably 50 μm or more and 160 μm or less.
The number of times of coating and baking to form the insulating film is not particularly limited, and can be appropriately set so as to obtain the thickness of the insulating film when the thickness of each insulating layer is within the above-mentioned preferred range. In the present invention, the "number of times of coating and baking" is synonymous with the "number of insulating layers constituting the insulating film". For example, the number of times of coating and baking can be 10 to 35, 12 to 30, or 15 to 25. The number of insulating layers constituting the insulating film can be 10 to 35, 12 to 30, or 15 to 25.
The average thickness of each insulating layer can be, for example, thinner on the conductor side and thicker on the opposite side to the conductor. The average thickness of each insulating layer constituting the insulating coating can be the same as the above-mentioned "average thickness per insulating layer." The average thickness of each insulating layer can be calculated, for example, by dividing the thickness of the insulating coating by the number of insulating layers constituting the insulating coating.

-Residual solvent amount-
The insulating film is formed by repeatedly applying and baking the resin varnish of the present invention, which has excellent quick-drying properties, and therefore the amount of residual solvent in the insulating film (insulating layer) can be kept low.
The amount of residual solvent in the insulating coating of the insulated wire of the present invention is preferably 10,000 ppm or less, more preferably 8,000 ppm or less, even more preferably 4,000 ppm or less, even more preferably 3,600 ppm or less, and even more preferably 3,200 ppm or less, from the viewpoint of suppressing the occurrence of defective appearance of the insulating coating. Also, from the viewpoint of improving the adhesion between the conductor and the insulating layer and between the insulating layers, the amount is preferably 500 ppm or more, more preferably 1,000 ppm or more, even more preferably 2,000 ppm or more, and even more preferably 2,500 ppm or more.
The amount of the residual solvent in the insulating coating can be measured by a conventional method, for example, by the method described in the Examples.

[Method of manufacturing insulated wire]
The insulated wire of the present invention can be obtained by forming a laminated insulating layer by a coating/baking process in which the resin varnish of the present invention is applied to the outer periphery of a conductor and baked multiple times. As long as the resin varnish used for coating/baking satisfies the requirements of the present invention, the same resin varnish may be used for all insulating layers, or different types of resin varnishes may be used for each insulating layer.

In the method for producing an insulated electric wire of the present invention, it is preferable that the imidization rate and/or the weight loss rate of the resin varnish of the present invention are appropriately adjusted during the production of the insulated electric wire so that they are each within the above-mentioned preferred range. The imidization rate and/or the weight loss rate can be measured each time such adjustment is made.

The resin varnish of the present invention can be applied to a conductor by a conventional method, such as using a varnish application die that is similar in shape to the conductor, or, if the cross-sectional shape of the conductor is rectangular, a die called a "universal die" that is shaped like a grid can be used.

The conductor coated with the resin varnish of the present invention is baked in a baking oven in the usual manner. The specific baking conditions depend on the shape of the oven used, but in the case of a natural convection vertical oven of approximately 8 m, this can be achieved by setting the oven temperature at 400-650°C and the passage time at 10-90 seconds. The amount of resin varnish applied of the present invention can be appropriately set so as to obtain the desired thickness of each insulating layer.

[Coils, rotating electrical machines, and electrical/electronic devices]
The insulated wire of the present invention can be used as a coil in fields requiring electrical properties (voltage resistance) and heat resistance, such as rotating electrical machines and various electrical and electronic devices. For example, the insulated wire of the present invention can be used in motors, transformers, etc. to configure high-performance rotating electrical machines and electrical and electronic devices. In particular, the insulated wire can be suitably used as a winding for the drive motor of a hybrid vehicle (HV) or an electric vehicle (EV).

The coil of the present invention may be formed by coiling the insulated electric wire of the present invention, or may be formed by bending the insulated electric wire of the present invention and then electrically connecting predetermined portions thereof.
The coil formed by coil processing the insulated electric wire of the present invention is not particularly limited, and may be a coil formed by winding a long insulated electric wire in a spiral shape. In such a coil, the number of turns of the insulated electric wire is not particularly limited. Usually, an iron core or the like is used to wind the insulated electric wire.

An example of a coil made by bending the insulated electric wire of the present invention and then electrically connecting predetermined portions is a coil used in a stator of a rotating electric machine or the like. As shown in FIG. 1, for example, such a coil is made by cutting the insulated electric wire of the present invention to a predetermined length, bending it into a U-shape or the like to make a plurality of electric wire segments 34, and alternately connecting the two open ends (terminals) 34a of the U-shape or the like of each electric wire segment 34 to make a coil 33 (see FIG. 1 and FIG. 2).

There is no particular limitation on the electric/electronic device using this coil. A preferred embodiment of such an electric/electronic device is a transformer. Another example is a rotating electric machine (particularly a drive motor for HVs and EVs) equipped with a stator 30 shown in Figs. 1 and 2. This rotating electric machine can have the same configuration as a conventional rotating electric machine, except that it is equipped with the stator 30.
The stator 30 may have the same configuration as a conventional stator, except that the electric wire segment 34 is formed of the insulated electric wire of the present invention. That is, the stator 30 has a stator core 31 and a coil 33 formed by inserting an electric wire segment 34 made of the insulated electric wire of the present invention into a slot 32 of the stator core 31 and electrically connecting the open end 34a, as shown in FIG. 1 for example. The coil 33 is fixed by fixing adjacent fusion layers to each other or to the fusion layer and the slot 32. Here, the electric wire segment 34 may be inserted into the slot 32 by itself, but is preferably inserted into a pair as shown in FIG. 2. In the stator 30, the coil 33 formed by connecting the open end 34a, which is the two ends of the electric wire segment 34 bent as described above, alternately is housed in the slot 32 of the stator core 31. At this time, the open end 34a of the wire segment 34 may be connected before being stored in the slot 32, or the insulating segment 34 may be stored in the slot 32 and then the open end 34a of the wire segment 34 may be bent and connected.

The present invention will be described in more detail below based on examples, but the present invention is not limited to these embodiments. In the following examples, "ppm" is based on mass.

[Preparation of aromatic polyimide resin varnish for insulating coating]
The following PI varnishes A and B were prepared as polyimide (PI) varnishes for use in preparing aromatic polyimide resin varnishes for insulating coating (hereinafter also simply referred to as "resin varnishes"). The resins contained in PI varnishes A and B are aromatic polyamic acids, which are precursors of aromatic polyimide resins. The imidization rates of PI varnishes A and B were both less than 3%.

PI Varnish A (solvent: DMAc, aromatic carboxylic dianhydride component: PMDA, aromatic diamine component: ODA)
PI Varnish B (solvent: NMP, aromatic carboxylic dianhydride component: PMDA, aromatic diamine component: ODA)

The imidization rate of these PI varnishes was adjusted by partially promoting the dehydration/cyclization reaction (imidization reaction) by the heat treatment or chemical treatment described below, to prepare the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3. The resin varnishes of Preparation Examples 1 to 4 and Comparative Preparation Examples 1 and 2 are resin varnishes derived from PI varnish A, and the resin varnishes of Preparation Example 5 and Comparative Preparation Example 3 are resin varnishes derived from PI varnish B.

(Heat treatment: thermal imidization method)
The PI varnishes A and B were heated under the conditions shown in Table 1 below to promote the imidization of the aromatic polyamic acid.

(Chemical treatment: chemical imidization method)
To the PI varnish A, 3% by mass of acetic anhydride was added and stirred under basic conditions to promote imidization of the aromatic polyamic acid.

The properties and characteristics of the resin varnishes obtained in Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 were determined by the following tests. The results are summarized in Table 1.

<Imidization rate>
The FT-IR spectra of the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 were measured by the ATR (attenuated total reflection) method using a Shimadzu Fourier transform infrared spectrophotometer (FT-IR, product name: IRAffinity-1S). As a reference sample, a PI film (imidization rate: 100%) prepared by heating PI varnish A or B at 300°C for 1 hour was used, and the measurements were performed in the same manner as for the above resin varnishes. The obtained FT-IR spectra (waveform data) were subjected to baseline correction using the value at 4000 cm ^-1 , and then normalized using the peak intensity near 1495 cm ^-1 . For the normalized waveform data, the peak intensities (A1774 cm ^-1 /A1495 cm ^-1 ) near 1774 cm ^-1 of the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 were compared with the peak intensity (A1774 cm ^-1 /A1495 cm ^-1 ) near 1774 cm ^-1 of the reference sample, and the imidization rate was calculated as described above.

<Non-volatile content>
The non-volatile content (solid content, mass%) of the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 was measured by a method according to JIS K 5601-1-2: 2008. The heating conditions for measuring the non-volatile content were 200° C. and 2 hours.

<Weight reduction rate>
The weight loss rate (mass%) of the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 was measured using a Shimadzu differential thermal and thermogravimetric simultaneous measurement device (product name: DTG-60AH). As described above, the sample weight of the resin varnish was 10 mg, and the heating conditions were a temperature rise rate of 20°C/min, a nitrogen atmosphere, and a gas flow rate of 100 ml/min. The weight loss rate at the start of the measurement (35°C) was set to 0 mass%, and the weight was measured when the heating temperature reached the boiling point of the solvent contained in the resin varnish (DMAc: boiling point 165°C, NMP: boiling point 202°C), to calculate the weight loss rate (mass%) as described above. The non-volatile content rate of the resin varnish before the measurement was 25 mass% in all cases.

<Liquidity>
The viscosity of the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 at 30°C was measured using an E-type digital viscometer (model number: TVE25L, manufactured by Toki Sangyo Co., Ltd.) and evaluated according to the following evaluation criteria. The non-volatile content of each of the resin varnishes before the viscosity measurement was 25% by mass.

-Evaluation criteria-
◯: Viscosity is 15,000 mPa·s or less ×: Viscosity is over 15,000 mPa·s

As is clear from Table 1, the weight loss rate of the resin varnishes of Comparative Preparation Examples 1 to 3 was 35% by mass or less when the nonvolatile content was 25% by mass. In the case of the resin varnish of Comparative Preparation Example 1, which had a low imidization rate of 3%, it was believed that the proportion of the solvent coordinated to the polyamic acid was high, and the solvent was difficult to volatilize (evaporate). In the case of the resin varnishes of Comparative Preparation Examples 2 and 3, which had high imidization rates of 65% and 70%, respectively, the proportion of the solvent coordinated to the polyamic acid was low, but the viscosity was high, and it was believed that the solvent inside the insulating layer was difficult to volatilize (evaporate).
In contrast, the resin varnishes of Preparation Examples 1 to 5 of the present invention, in which the imidization rate was adjusted to be within a specific range (imidization rate: 15 to 60%), all had weight loss rates of 40% by mass or more when the non-volatile content was 25% by mass. In other words, the resin varnishes of Preparation Examples 1 to 5 of the present invention were shown to have a superior quick-drying property compared to the resin varnishes of Comparative Preparation Examples 1 to 3.

[Examples 1 to 10 and Comparative Examples 1 to 6]
Using the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3, insulated wires each consisting of a conductor and an insulating coating were produced by the following method.
The conductor used was a rectangular conductor (copper with an oxygen content of 15 ppm) having a rectangular cross section (3.5 mm wide x 2.0 mm long, with a chamfered corner radius of curvature r = 0.3 mm).
The resin varnishes used for forming the insulating layer were the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3. After adjusting the non-volatile content of the resin varnish, the resin varnish was applied to the surface of the conductor using a die in which the outer shape of the cross section of the innermost insulating layer in contact with the conductor is similar to the cross-sectional shape of the conductor, so that the average thickness of each insulating layer was the thickness shown in Tables 2 and 3 below. The wire was passed through a baking furnace with a length of 8 m set at 500° C. at a speed that gave a passing time of 20 seconds. This application and baking was performed a total of 20 times to form an insulating coating (thickness: 80 μm) consisting of multiple polyimide insulating layers. In this way, the insulated electric wires of Examples 1 to 10 and Comparative Examples 1 to 6 were obtained.
The non-volatile content when the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 were used for the insulated wires of Examples 6 to 10 and Comparative Preparation Examples 4 to 6 was adjusted by diluting the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3 with the same solvent as the solvent contained in the resin varnishes of Preparation Examples 1 to 5 and Comparative Preparation Examples 1 to 3.

The amount of residual solvent in the insulating film and the appearance of the insulated wire were evaluated for each of the insulated wires obtained as described below. The results are summarized in Tables 2 and 3 below.

<Residual Solvent Amount>
Only the insulating coating was peeled off from the insulated wires of Examples 1 to 10 and Comparative Examples 1 to 6, and the amount of residual solvent in the insulating coating was measured for 2 mg of the peeled insulating coating using a gas chromatograph (model: GC-2010 Plus, manufactured by Shimadzu Corporation, carrier gas: nitrogen). The measurement results were evaluated based on the following evaluation criteria.

-Evaluation criteria-
A: Residual solvent amount is more than 2800 ppm and less than 3200 ppm. B: Residual solvent amount is more than 3200 ppm and less than 3600 ppm. C: Residual solvent amount is more than 3600 ppm and less than 4000 ppm. D: Residual solvent amount is more than 4000 ppm and less than 4400 ppm. E: Residual solvent amount is more than 4400 ppm and less than 4800 ppm. F: Residual solvent amount is more than 4800 ppm.

<Appearance Evaluation>
Two 10 cm pieces were cut from each of the insulated wires of Examples 1 to 10 and Comparative Examples 1 to 6, and the number of bumps (abnormal foaming of the insulating coating) that had occurred on the insulating coating of the insulated wires (number of bumps) was visually counted. The number of bumps per insulated wire (average number of bumps) was calculated and evaluated based on the following evaluation criteria.

-Evaluation criteria-
A: Average number of spots is 0 B: Average number of spots is 1-2 C: Average number of spots is 3-5 D: Average number of spots is 6-9 E: Average number of spots is 10 or more

In the insulated wires of Comparative Examples 1 and 4, which were prepared using the resin varnish of Comparative Preparation Example 1, which has an imidization rate lower than the range specified in the present invention, with a non-volatile content of 25% by mass and 10% by mass, respectively, residual solvents were easily accumulated in the insulating film, and many abnormal bubbles were observed in the external appearance of the insulated wire. In addition, in the insulated wires of Comparative Examples 2, 3, 5, and 6, which were prepared using the resin varnish of Comparative Preparation Examples 2 and 3, which has an imidization rate higher than the range specified in the present invention, with a non-volatile content of 25% by mass and 10% by mass, respectively, the amount of residual solvents in the insulating film was large, and many abnormal bubbles were observed in the external appearance of the insulated wire.

In contrast, the insulated wires (Examples 1 to 10) manufactured using the resin varnishes of Preparation Examples 1 to 5, which are resin varnishes that meet all of the requirements of the present invention, all had a certain level of residual solvent in the insulating film that was suppressed, and the insulated wires also had excellent appearance.

30 Stator 31 Stator core 32 Slot 33 Coil 34 Electric wire segment 34a Open end

Claims (8)

An aromatic polyimide resin varnish for insulating coating, containing aromatic polyamic acid with an imidization rate of 10 to 60%. The aromatic polyimide resin varnish for insulating coating according to claim 1, in which a 10 mg sample of the resin varnish, in which the non-volatile content is adjusted to 25 mass %, is heated from 35°C at a rate of 20°C/min in a nitrogen atmosphere with a gas flow rate of 100 ml/min, the weight loss rate at the time the boiling point of the solvent is reached is 40 mass % or more. The aromatic polyimide resin varnish for insulating coating according to claim 2, wherein the weight loss rate is 50% by mass or more. The aromatic polyimide resin varnish for insulating coating according to claim 3, wherein the non-volatile content of the resin varnish is 10% by mass or more. An insulated wire having a conductor and an insulating coating covering the outer periphery of the conductor,
An insulated wire, wherein the insulating coating comprises a laminated insulating layer formed by repeatedly applying and baking the aromatic polyimide resin varnish for insulating coating according to any one of claims 1 to 4. The insulated wire according to claim 5, wherein the average thickness of each insulating layer constituting the laminated insulating layer is 4 μm or more. A coil using the insulated wire according to claim 6. An electric/electronic device having the coil according to claim 7.

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