Insulated wire and resin composition for insulating layer

Abstract

An insulated wire according to the present disclosure comprises: a conductor; and a first insulating layer that is laminated on the surface of the conductor and has a plurality of pores derived from an aliphatic polyvalent carboxylic acid ester. The first insulating layer contains a polyimide that is the main component, and a hydrophobic nanosilica.

Description

Insulated wire and resin composition for insulating layer

[0001] The present disclosure relates to an insulated wire and a resin composition for an insulating layer.

[0002] In electric equipment with a high applied voltage, such as a motor used under high voltage, high voltage is applied to the insulated wire used for wiring, and partial discharge (corona discharge) tends to occur on the surface of the insulating coating. When corona discharge causes local temperature rise, ozone generation, ion generation and the like, dielectric breakdown occurs at an early stage, which shortens the service life of the insulated wire and consequently the electric equipment. Therefore, in addition to excellent insulation properties, mechanical strength and the like, insulated wires used in electric equipment with high applied voltage are also required to have an increased corona discharge inception voltage.

[0003] As a contrivance for increasing the corona discharge inception voltage, reducing the dielectric constant of the insulating coating is effective. In order to reduce the dielectric constant of the insulating coating, it has been proposed to form a heat-cured film (insulating coating) using a resin composition for an insulating layer that includes a coating-forming resin and a thermally decomposable resin that decomposes at a temperature lower than the baking temperature of the coating-forming resin (see Patent Document 1). In Patent Document 1, voids are formed in the heat-cured film by utilizing the fact that the thermally decomposable resin thermally decomposes during baking of the coating-forming resin and the decomposed parts become voids, and the formation of these voids achieves a lower dielectric constant of the insulating coating.

[0004] Japanese Unexamined Patent Publication No. 2012-224714

[0005] The insulated wire of the present disclosure includes a conductor, and a first insulating layer laminated on a surface of the conductor and having a plurality of voids derived from an aliphatic polycarboxylic acid ester, wherein the first insulating layer contains polyimide as a main component and hydrophobic nano-silica.

[0006] FIG. 1 is a schematic cross-sectional view showing an insulated wire according to an embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view showing an insulated wire according to another embodiment.

[0007] [Problems this disclosure aims to solve] When forming voids in the insulating layer of an insulated wire, using a high-boiling-point solvent as a void-forming agent can reduce the viscosity of the insulating layer resin composition, thereby obtaining an insulating layer resin composition with better workability than the above-mentioned pyrolytic resin. However, while the formation of voids in the insulating layer promotes a lower dielectric constant of the insulated wire, when a high-boiling-point solvent is used as a void-forming agent, the porosity increases, the distance between adjacent voids shortens, and adjacent voids merge together, resulting in a larger void diameter. Thus, the presence of large voids presents the problem of reduced resistance to compression processing used when forming coils, etc.

[0008] This disclosure aims to provide an insulated wire that exhibits excellent resistance to compression processing, even when the dielectric constant is low and the porosity of the insulating layer is high.

[0009] [Effects of this disclosure] According to this disclosure, it is possible to provide an insulated wire that has excellent resistance to compression processing even when the dielectric constant is low and the porosity of the insulating layer is high.

[0010] [Description of Embodiments of the Disclosure] First, embodiments of the Disclosure will be listed and described.

[0011] (1) The insulated wire of the present disclosure comprises a conductor and a first insulating layer laminated on the surface of the conductor and having a plurality of pores derived from an aliphatic polycarboxylic acid ester, wherein the first insulating layer comprises polyimide as the main component and hydrophobic nanosilica.

[0012] In this insulated wire, the inclusion of polyimide improves the toughness, insulation, and heat resistance of the insulating layer. Furthermore, the presence of multiple pores in the insulating layer allows for a lower dielectric constant. Moreover, using an aliphatic polycarboxylic acid ester, which is a high-boiling-point solvent, as a pore-forming agent reduces variations in pore size. Additionally, while the insulated layer's dielectric constant is reduced by including hydrophobic nanosilica, the expansion of pore diameter is suppressed even when the porosity of the insulating layer is high. The reason for these effects is not entirely clear, but the following reason is speculated: When the first insulating layer is manufactured, it is thought that multiple pores are formed by phase separation between the polyimide precursor, which is the main component, and the aliphatic polycarboxylic acid ester. In this insulated wire, the insulating layer contains hydrophobic nanosilica, which allows the hydrophobic groups of the hydrophobic nanosilica to bond with the hydrophobic groups of the aliphatic polycarboxylic acid ester through interaction. This suppresses the fusion of adjacent pores, thereby enabling a low dielectric constant and miniaturization of pore size even when the porosity of the insulating layer is high. Consequently, this insulated wire exhibits excellent resistance to compression processing, even when the porosity of the insulating layer is high, while maintaining a low dielectric constant. The "main component" above refers to the component with the largest mass content, for example, a component with a content of 70% by mass or more. "Nanosilica" refers to nano-sized silicon dioxide fine particles with an average particle diameter of 1 μm or less, and its shape is not particularly limited. The above "average particle size" refers to the value at which the volume-based integrated distribution calculated in accordance with JIS-Z-8819-2 (2001) becomes 50%, based on the particle size distribution measured by laser diffraction / scattering on a dilution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013). "Hydrophobic nanosilica" refers to silica in which some or all of the silanol groups on the surface of nanosilica particles have chemically reacted with hydrophobic groups.

[0013] (2) In (1) above, the content of the hydrophobic nanosilica in the first insulating layer may be 0.50% by mass or more and 3.10% by mass or less. By having the hydrophobic nanosilica content in the first insulating layer within the above range, it is possible to reduce the dielectric constant while also reducing the pore size in the insulating layer and improving the elongation of the insulating layer, even when the porosity of the insulating layer is high. By having a hydrophobic nanosilica content of 0.50% by mass or more, sufficient interaction is obtained between the hydrophobic groups of the hydrophobic nanosilica and the hydrophobic groups of the aliphatic polycarboxylic acid ester, making it easier to reduce the pore size. By having a hydrophobic nanosilica content of 3.10% by mass or less, it is possible to prevent a decrease in mechanical properties such as elongation due to the hydrophobic nanosilica acting as a fracture initiation point.

[0014] (3) In (1) or (2) above, the average pore diameter of the plurality of pores may be 0.40 μm or more and 0.70 μm or less. Having the average pore diameter of the plurality of pores within the above range improves the dielectric constant and resistance to compression processing of the insulated wire. The average pore diameter of the plurality of pores can be determined by measuring the diameters of 100 or more arbitrary pores on an image of the cross-section of the insulated wire using a scanning electron microscope (SEM) with the open-source image processing software "ImageJ", and calculating the average value of the 100 or more pore diameters.

[0015] (4) The insulating layer resin composition of the present disclosure is an insulating layer resin composition used to form the first insulating layer of an insulated electric wire described in any of (1) to (3) above, and contains a polyimide precursor, an organic solvent, an aliphatic polycarboxylic acid ester, and hydrophobic nanosilica. The insulating layer resin composition contains an aliphatic polycarboxylic acid ester, which is a pore-forming agent, and hydrophobic nanosilica, so that the hydrophobic groups of the hydrophobic nanosilica and the hydrophobic groups of the aliphatic polycarboxylic acid ester bond through interaction, and the integration of adjacent pores is suppressed, thereby enabling a low dielectric constant and miniaturization of pore size even when the porosity of the formed insulating layer is high. Therefore, the insulating layer resin composition can form an insulating layer that is highly resistant to compression processing, even when the porosity of the insulating layer is high, while achieving a low dielectric constant.

[0016] [Details of Embodiments of the Disclosure] The insulated wire and resin composition for the insulating layer according to the embodiments of the Disclosure will be described below with reference to the drawings.

[0017] <Insulated Wire> An insulated wire according to one embodiment of the present disclosure comprises a conductor and a first insulating layer laminated on the surface of the conductor and having a plurality of pores derived from an aliphatic polycarboxylic acid ester. The insulated wire 1 shown in Figure 1 comprises a conductor 2 and a first insulating layer 3 laminated on the surface of the conductor 2. In the insulated wire 1, the first insulating layer 3 has a plurality of pores 4. By having a plurality of pores 4 in the first insulating layer 3, the dielectric constant of the insulated wire 1 can be reduced.

[0018] The cross-sectional shape of the insulated wire 1 is not particularly limited; for example, in addition to the circular shape (round wire) shown in Figure 1, other shapes include elliptical, square (square wire), and rectangular (flat wire). Furthermore, it is preferable that the cross-sectional shape of the insulated wire 1 and the cross-sectional shape of the conductor 2 described later be of the same type.

[0019] [Conductor] The cross-sectional shape of the conductor 2 can be, for example, circular, elliptical, square, or rectangular. If the insulated wire 1 is a flat rectangular wire, the cross-sectional shape of the conductor 2 is preferably rectangular.

[0020] The material of conductor 2 should be a metal with high conductivity and high mechanical strength. Examples of such metals include copper, copper alloys, aluminum, nickel, silver, soft iron, steel, and stainless steel. Conductor 2 can be a metal wire formed from the above metal, or a multilayer structure in which another metal is coated on a wire-like material, such as nickel-coated copper wire, silver-coated copper wire, copper-coated aluminum wire, or copper-coated steel wire.

[0021] The lower limit of the average cross-sectional area of ​​conductor 2 is 0.01 mm². ２ It may be 0.1 mm ２ This may also be the case. In this case, the volume of the first insulating layer 3 relative to the conductor 2 in the insulated wire 1 can be made appropriate, and the volume efficiency of coils, etc., formed using the insulated wire 1 can be improved. The upper limit of the average cross-sectional area of ​​the conductor 2 is 40 mm². ２ It may also be 30 mm ２ This is also acceptable. In this case, it is possible to avoid a decrease in the output efficiency of coils, etc., due to increased copper loss caused by eddy currents.

[0022] [Insulating Layer] The first insulating layer 3 is laminated on the surface of the conductor 2. The first insulating layer 3 is formed from an insulating layer resin composition described later. The first insulating layer 3 has a plurality of vacancies 4 derived from aliphatic polycarboxylic acid esters and contains polyimide, which is the main component forming the matrix, and hydrophobic nanosilica.

[0023] The average thickness of the first insulating layer 3 is not particularly limited and can usually be between 5 μm and 200 μm.

[0024] The insulated wire 1 can be suitably used as a coil winding (magnet wire).

[0025] (Polyimide) By using polyimide as the main component of the first insulating layer 3, toughness, insulating properties, and heat resistance can be improved.

[0026] The polyimide described above is derived from a polyimide precursor, which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine. The polyimide precursor is a reaction product obtained by the polycondensation reaction of an aromatic tetracarboxylic dianhydride and an aromatic diamine. The polyimide precursor is also called a polyamic acid (polyamic acid). The polyimide precursor forms a cyclic imide through a dehydration cyclization reaction (imidization reaction), becoming a polyimide.

[0027] The lower limit of the polyimide content in the first insulating layer 3 may be 70% by mass, 80% by mass, 85% by mass, or 90% by mass. A polyimide content of 70% by mass or more allows the first insulating layer 3 to achieve sufficient elongation.

[0028] (Aliphatic polycarboxylic acid ester) The first insulating layer 3 contains an aliphatic polycarboxylic acid ester, which is a high-boiling point solvent, as a vacancy-forming agent. By containing an aliphatic polycarboxylic acid ester as a vacancy-forming agent in the first insulating layer 3, the variation in the size of the vacancies 4 can be reduced. "Aliphatic polycarboxylic acid ester" means an ester derived from an aliphatic polycarboxylic acid. "Aliphatic polycarboxylic acid" means an aliphatic carboxylic acid having two or more carboxyl groups. "Carboxylic acid" means a compound having a carboxyl group, and is not limited to carboxylic acids in the narrow sense, but also includes carboxylic acids in the broad sense such as hydroxycarboxylic acids.

[0029] The above-mentioned aliphatic polycarboxylic acid esters can be classified by the number of carboxyl groups, for example, into aliphatic dicarboxylic acid esters, aliphatic tricarboxylic acid esters, and aliphatic tetracarboxylic acid esters. Because the above-mentioned aliphatic polycarboxylic acid ester is an aliphatic dicarboxylic acid ester or an aliphatic tricarboxylic acid ester, it has a smaller molecular weight and lower boiling point and thermal decomposition temperature compared to aliphatic polycarboxylic acids with four or more carboxyl groups, such as aliphatic tetracarboxylic acid esters, thus reducing the amount of residue in the first insulating layer 3. Furthermore, because it is an aliphatic tricarboxylic acid ester, the amount of residue in the first insulating layer 3 can be reduced even further. By reducing the amount of residue in the first insulating layer 3, the relative permittivity of the first insulating layer 3 can be reduced.

[0030] Examples of the above aliphatic dicarboxylic acid esters include dibutyl fumarate, dibutyl succinate, diethyl sebacate, diisobutyl adipate, dibutyl adipate (DBA), dibutyl sebacate, and di(2-butoxyethyl) adipate. Examples of the above aliphatic tricarboxylic acid esters include citrate esters such as triethyl citrate, triethyl O-acetylcitrate, tributyl citrate (TBC), and tributyl O-acetylcitrate. When comparing the above aliphatic dicarboxylic acid esters with the above citrate esters, if the total number of carbon atoms is the same, citrate esters tend to form insulating layers with higher porosity. By using tributyl citrate (TBC) as the citrate ester, the relative permittivity of the first insulating layer 3 can be further reduced.

[0031] The upper limit of the aliphatic polycarboxylic acid ester content in the first insulating layer 3 may be 8,000 ppm, 7,000 ppm, 6,000 ppm, 5,000 ppm, 3,000 ppm, 1,000 ppm, 800 ppm, 500 ppm, 200 ppm, 100 ppm, 50 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, or 1 ppm. The lower limit of the above content is not particularly limited as long as the amount in which the aliphatic polycarboxylic acid ester can be confirmed to be present may be, for example, above the detection limit or 0.1 ppm. "ppm" refers to mass ppm.

[0032] The content of the aliphatic polycarboxylic acid ester in the first insulating layer 3 can be adjusted, for example, by adjusting the type of aliphatic polycarboxylic acid ester, the heating temperature and heating time in the manufacturing method of the insulated wire 1 described later, and the content of the aliphatic polycarboxylic acid ester in the resin composition for the insulating layer.

[0033] (Hydrophobic Nanosilica) Generally, silica is hydrophilic because it has silanol groups on its surface, but hydrophobic nanosilica becomes hydrophobic when the surface of the nanosilica is modified with a surface treatment agent such as a silane coupling agent. The surface treatment state of hydrophobic nanosilica can be determined by performing diffuse reflectance FT-IR measurement on the first insulating layer 3.

[0034] As a surface treatment agent for hydrophobic nanosilica, known agents such as silane coupling agents can be used. Examples of silane coupling agents include silane compounds having functional groups such as vinyl, methyl, methacrylic, acrylic, amino, and phenyl groups. The method for producing hydrophobic nanosilica is not particularly limited, but it can be obtained, for example, by reacting the hydroxyl groups on the surface of silica particles with hexamethyldisilazane or dimethyldichlorosilane to introduce trimethylsilyl or dimethylsilyl groups. The surface treatment method is not particularly limited, but it can be carried out by adding a silane coupling agent to a dispersion (organosilica sol) and stirring at 20°C to 80°C for about 1 to 10 hours. At this time, a catalyst to promote the reaction may be added. In this way, for example, trimethylsilane hydrophobic nanosilica can be obtained by reacting the hydroxyl groups on the surface of silica particles with hexamethyldisilazane to introduce trimethylsilyl groups.

[0035] The lower limit of the average particle diameter of hydrophobic nanosilica may be 7 nm or 12 nm. The upper limit of the average particle diameter of hydrophobic nanosilica may be 40 nm or 30 nm. By having an average particle diameter of hydrophobic nanosilica above the lower limit, aggregation of hydrophobic nanosilica particles can be reduced. On the other hand, by having an average particle diameter of hydrophobic nanosilica below the upper limit, good uniformity of thickness in the insulating layer can be maintained.

[0036] The lower limit of the hydrophobic nanosilica content in the first insulating layer 3 may be 0.50% by mass, 0.60% by mass, or 0.70% by mass. By having a hydrophobic nanosilica content above the above lower limit, the miniaturization of pore size can be further improved even when the porosity of the first insulating layer 3 is high. On the other hand, the upper limit of the hydrophobic nanosilica content may be 3.10% by mass or 3.05% by mass. By having a hydrophobic nanosilica content below the above upper limit, the elongation of the insulating layer can be further improved.

[0037] (Other components) The first insulating layer 3 may contain other components besides those listed above. The other components are not particularly limited and include, for example, antioxidants, radical scavengers, UV inhibitors, surface lubricants, leveling agents, curing agents, and adhesion aids.

[0038] [Vacancies] The multiple vacancies 4 originate from an aliphatic polycarboxylic acid ester, which is a vacancy-forming agent. As described above, the multiple vacancies 4 are thought to be formed by the phase separation of the polyimide precursor and the aliphatic polycarboxylic acid ester. The multiple vacancies 4 are uniformly distributed as island phases of fine particles in the ocean phase of the first insulating layer 3 that forms the first insulating layer 3. The shape of the multiple vacancies 4 may be spherical or flattened.

[0039] The porosity of the first insulating layer 3 is preferably 20% by volume or more and 50% by volume or less. If the porosity of the first insulating layer 3 is 20% by volume or more, the dielectric constant of the insulating layer can be further reduced. The lower limit of the porosity of the first insulating layer 3 may be 22% by volume, 25% by volume, or 30% by volume. The upper limit of the porosity of the first insulating layer 3 may be 45% by volume or 40% by volume. "Porosity" means the percentage (unit: volume %) of the volume of pores 4 relative to the volume of the first insulating layer 3 having multiple pores 4. Specifically, the porosity is measured by the following method. The mass of the coated test piece in air (W1) and in hexane (W2) is measured using an electronic balance specific gravity measurement kit. First, the specific gravity (H) of the coated test piece is calculated using formula 1: W1 / (W1 - W2) × 0.66. In formula 1 above, 0.66 is the specific gravity of hexane. Then, the porosity can be calculated using equation 2: (1 - (H / ρ)) × 100. In equation 2 above, ρ is the density of the insulating layer material.

[0040] The lower limit of the average diameter of the plurality of voids 4 may be 0.40 µm or 0.50 µm. When the average diameter of the plurality of voids 4 is 0.40 µm or more, the dielectric constant reduction of the insulated wire 1 and the resistance to compression processing can be improved. The upper limit of the average diameter of the plurality of voids 4 may be 0.70 µm or 0.68 µm. When the average diameter of the plurality of voids 4 is 0.70 µm or less, the dielectric constant reduction of the insulated wire 1 and the resistance to compression processing can be improved.

[0041] The insulated wire 1 includes an insulating layer that achieves low dielectric constant and has excellent resistance to compression processing even when the porosity of the insulating layer is high. The insulated wire 1 can be suitably used as a coil winding (magnet wire).

[0042] <Resin Composition for Insulating Layer> The resin composition for an insulating layer is used for forming the first insulating layer of the insulated wire described above. The resin composition for an insulating layer contains a polyimide precursor, an organic solvent, an aliphatic polycarboxylic acid ester, and hydrophobic nanosilica.

[0043] [Polyimide Precursor] The polyimide precursor is a reaction product obtained by a polymerization condensation reaction of an aromatic tetracarboxylic dianhydride and an aromatic diamine. The polyimide precursor is a compound also referred to as polyamic acid. The polyimide precursor forms a cyclic imide through a dehydration cyclization reaction to become polyimide.

[0044] The lower limit of the weight average molecular weight of the polyimide precursor may be 15,000 or 16,000. The upper limit of the weight average molecular weight may be 100,000 or 50,000. When the weight average molecular weight is not less than the above lower limit, good coating properties can be obtained. When the weight average molecular weight is not more than the above upper limit, good coatability of the resin composition for an insulating layer can be obtained.

[0045] The lower limit of the content of the polyimide precursor in the resin composition for an insulating layer may be 10% by mass, or may be 20% by mass. The upper limit of the content may be 50% by mass, or may be 40% by mass. By setting the content to be equal to or higher than the above lower limit, when forming a first insulating layer using the resin composition for an insulating layer, the amount of the resin composition for an insulating layer required in the entire manufacturing process to obtain the first insulating layer having a desired thickness can be reduced, and the number of steps of applying the resin composition for an insulating layer and the step of heating the same can be reduced. By setting the content to be equal to or lower than the above upper limit, the viscosity of the resin composition for an insulating layer can be appropriately adjusted while maintaining good film properties, and the coatability can be improved.

[0046] The aromatic tetracarboxylic dianhydride may include pyromellitic dianhydride (PMDA). Since PMDA has a rigid and linear molecular structure, when the aromatic tetracarboxylic dianhydride includes PMDA, the heat resistance of the first insulating layer can be improved.

[0047] The above aromatic tetracarboxylic dianhydride may further contain aromatic tetracarboxylic dianhydrides other than PMDA (hereinafter also referred to as "other aromatic tetracarboxylic dianhydrides"). Examples of the above 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 acid dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bi Examples include s(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. The above other aromatic tetracarboxylic acid dianhydrides may be used individually or in combination of two or more.

[0048] The use of biphenyltetracarboxylic dianhydride (BPDA) as another aromatic tetracarboxylic dianhydride can improve the hydrolysis resistance of the polyimide.

[0049] The lower limit of the PMDA content in 100 mol% of the above aromatic tetracarboxylic dianhydride may be 60 mol% or 70 mol%. The upper limit of the PMDA content in 100 mol% of the above aromatic tetracarboxylic dianhydride may be 100 mol%, 90 mol%, or 80 mol%.

[0050] The content of the other aromatic tetracarboxylic dianhydride in 100 mol% of the above aromatic tetracarboxylic dianhydride can be appropriately determined within a range that does not impair the effects of this disclosure. The upper limit of the above content may be 40 mol% or 30 mol%. The lower limit of the above content may be 0 mol%, 10 mol%, or 20 mol%.

[0051] The above aromatic diamine can have its heat resistance improved if it contains diaminodiphenyl ether (ODA). This is because ODA has a rigid and linear molecular structure. Examples of diaminodiphenyl ethers 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). By using 4,4'-diaminodiphenyl ether (4,4'-ODA), the elongation of the first insulating layer can be improved.

[0052] The above aromatic diamine may further contain aromatic diamines other than ODA (hereinafter also referred to as "other aromatic diamines"). Examples of the above other aromatic diamines 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 include -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. The above other aromatic diamines may be used individually or in combination of two or more.

[0053] The dielectric constant of the first insulating layer can be reduced by using 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) or 4,4'-bis(4-aminophenoxy)biphenyl (BAPB) as the other aromatic diamine.

[0054] The lower limit of the ODA content in 100 mol% of the above aromatic diamine may be 50 mol%, 60 mol%, or 70 mol%. The upper limit of the ODA content in 100 mol% of the above aromatic diamine may be 100 mol%, or 90 mol%.

[0055] The content of the other aromatic diamines in 100 mol% of the above aromatic diamine can be appropriately determined within a range that does not impair the effects of this disclosure. The upper limit of the above content may be 50 mol%, 40 mol%, or 30 mol%. The lower limit of the above content may be 0 mol%, or 10 mol%.

[0056] The molar ratio (aromatic tetracarboxylic dianhydride:aromatic diamine) of aromatic tetracarboxylic dianhydride to aromatic diamine used as a raw material for the above polyimide precursor may be, for example, 95:105 to 105:95, 97:103 to 103:97, or 99:101 to 101:99, from the viewpoint of ease of synthesis of the polyimide precursor. The aromatic tetracarboxylic dianhydride and aromatic diamine may be substantially equimolar. In this case, the molecular weight of the polyimide precursor can be easily increased. "Substantially equimolar" means that the molar ratio (aromatic tetracarboxylic dianhydride:aromatic diamine) of aromatic tetracarboxylic dianhydride to aromatic diamine is in the range of 99:101 to 101:99.

[0057] (Method for synthesizing polyimide precursors) The above polyimide precursor can be obtained by a polymerization condensation reaction between the above aromatic tetracarboxylic dianhydride and an aromatic diamine. The polymerization condensation reaction can be carried out in the same manner as the conventional synthesis of polyimide precursors. A specific method for the polymerization condensation reaction is to mix the aromatic tetracarboxylic dianhydride and the aromatic diamine in an organic solvent. By this method, the aromatic tetracarboxylic dianhydride and the aromatic diamine polymerize, and a solution in which the polyimide precursor is dissolved in the organic solvent can be obtained. The degree of polymerization (weight-average molecular weight) of the above polyimide precursor can be controlled, for example, by carrying out the polymerization condensation reaction using a reaction control agent.

[0058] Examples of reaction control agents include water (H ２O), Examples include 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.

[0059] The reaction conditions for the polymerization described above can be appropriately set depending on the raw materials used. For example, the reaction temperature can be set to 10°C or higher and 100°C or lower, and the reaction time to 0.5 hours or higher and 24 hours or lower.

[0060] As the organic solvent used in the above polymerization condensation reaction, aprotic polar organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide, dimethyl sulfoxide, and γ-butyrolactone can be used. These organic solvents may be used individually or in combination of two or more. An "aprotic polar organic solvent" refers to a polar organic solvent that does not have a proton-releasing group.

[0061] Examples of aprotic solvents include amide solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF); sulfur-containing solvents such as dimethyl sulfoxide; and lactone solvents such as γ-butyrolactone.

[0062] The amount of organic solvent used in the synthesis of the polyimide precursor is not particularly limited, as long as it is sufficient to uniformly dissolve and disperse the aromatic tetracarboxylic dianhydride and aromatic diamine. If the amount is too large, it will be necessary to volatilize a large amount of organic solvent when forming the first insulating layer, which may prolong the formation of the first insulating layer. Therefore, for example, the amount of organic solvent used can be between 100 parts by mass and 1,000 parts by mass per 100 parts by mass of the total of the aromatic tetracarboxylic dianhydride and aromatic diamine.

[0063] [Organic solvent] The organic solvent used in the resin composition for the insulating layer is the same as the organic solvent used in the polymerization condensation reaction described above.

[0064] [Aliphatic Polycarboxylic Acid Esters] Details of aliphatic polycarboxylic acid esters are as described above. The content of the aliphatic polycarboxylic acid ester in the insulating layer resin composition can be appropriately set within the range in which the effects of this disclosure are exhibited. The upper limit of the content of the aliphatic polycarboxylic acid ester in the insulating layer resin composition may be 20 parts by mass, 18 parts by mass, 16 parts by mass, 14 parts by mass, or 13 parts by mass per 100 parts by mass of the total of the polyimide precursor and the organic solvent. When the content is 20 parts by mass or less, the formed first insulating layer will have a good shape without foaming in appearance. Although we do not want a restrictive interpretation, it is presumed that when the content is 20 parts by mass or less, the balance between the island parts and sea parts of the sea-island structure generated by the phase separation of the polyimide precursor and the aliphatic polycarboxylic acid ester is appropriately adjusted, thereby reducing the occurrence of appearance defects such as foaming, blistering, and peeling of the insulating layer. The lower limit of the above aliphatic polycarboxylic acid ester may be 2 parts by mass, 4 parts by mass, or 6 parts by mass, based on 100 parts by mass of the total of the polyimide precursor and the organic solvent.

[0065] [Hydrophobic Nanosilica] Details of hydrophobic nanosilica are as described above. The lower limit of the hydrophobic nanosilica content in the insulating layer resin composition may be 0.10 parts by mass, 0.12 parts by mass, or 0.14 parts by mass per 100 parts by mass of polyimide precursor. By having a hydrophobic nanosilica content above the lower limit, the miniaturization of pore size can be further improved even when the porosity of the first insulating layer 3 is high. On the other hand, the upper limit of the hydrophobic nanosilica content may be 0.95 parts by mass, 0.90 parts by mass, or 0.85 parts by mass per 100 parts by mass of polyimide precursor. By having a hydrophobic nanosilica content below the upper limit, the elongation in the insulating layer can be further improved.

[0066] (Other components) The insulating layer resin composition may contain other components besides those listed above. Other components are not particularly limited as long as they are additives blended into the insulating layer resin composition for forming the insulating layer of an insulated wire, and include, for example, antioxidants, radical scavengers, UV inhibitors, surface lubricants, leveling agents, curing agents, and adhesion aids.

[0067] [Method for Manufacturing Insulated Wires] The method for manufacturing insulated wires comprises a conductor and a first insulating layer covering the conductor. The method for manufacturing insulated wires comprises a step of coating the outer surface of the conductor with the above-mentioned insulating layer resin composition (coating step) and a step of heating the insulating layer resin composition coated in the coating step (heating step).

[0068] (Coating process) In the above coating process, the aforementioned insulating layer resin composition is applied to the outer surface of the conductor.

[0069] The method for producing the insulating layer resin composition comprises, for example, the steps of synthesizing a polyimide precursor and adding hydrophobic nanosilica and other optional components to the polyimide precursor solution obtained after the step of synthesizing the polyimide precursor.

[0070] One method for coating the insulating layer resin composition onto the outer surface of a conductor is to use a coating apparatus that includes a liquid composition tank containing the insulating layer resin composition and a coating die. With this coating apparatus, the insulating layer resin composition adheres to the outer surface of the conductor as it passes through the liquid composition tank, and then the insulating layer resin composition is coated to a uniform thickness as it passes through the coating die.

[0071] (Heating step) In the heating step described above, the insulating layer resin composition coated onto the conductor in the coating step described above is heated. This heating causes the organic solvent in the insulating layer resin composition to volatilize, and the polyimide precursor hardens, forming a polyimide.

[0072] The apparatus used in the above heating process is not particularly limited; for example, a long cylindrical baking furnace arranged along the direction of travel of the conductor can be used. The heating method is not particularly limited and can be carried out by conventionally known methods such as hot air heating, infrared heating, or high-frequency heating.

[0073] The heating temperature can be, for example, 300°C to 800°C. The heating time can be, for example, 5 seconds to 1 minute.

[0074] The above coating process and heating process are usually repeated multiple times. By repeating the process multiple times, the thickness of the insulating layer can be increased. The hole diameter of the coating die can be adjusted as appropriate according to the number of repetitions.

[0075] According to this insulating layer resin composition, it is possible to form an insulating layer that has excellent resistance to compression processing, even when the dielectric constant of the insulating layer is high, while also achieving a low dielectric constant.

[0076] [Other Embodiments] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is not limited to the configurations of the embodiments described herein, but is indicated by the claims, and all modifications within the meaning and scope of the claims are intended to be included.

[0077] The insulated wire may further include a second insulating layer laminated directly or indirectly on the surface of the first insulating layer as a surge-resistant layer. Figure 2 is a schematic cross-sectional view showing an insulated wire 10 according to another embodiment. The insulated wire 10 further includes a second insulating layer 5 laminated directly on the surface of the first insulating layer 3 described above. In the insulated wire 10, the second insulating layer 5 contains polyimide as the main component and inorganic particles 6, and the inorganic particles 6 may be silica, alumina, aluminum hydroxide, boehmite, or a combination thereof. By further including the second insulating layer 5 containing polyimide as the main component and inorganic particles 6, the insulated wire 10 has excellent properties for reducing dielectric breakdown caused by surge voltage (surge resistance). In electrical equipment with high applicable voltages, such as motors used at high voltages, if a voltage is suddenly applied to the insulated wires contained in the electrical equipment, small discharges (surges) occur between the insulated wires, and this surge can cause premature dielectric breakdown, potentially shortening the lifespan of the insulated wires and, consequently, the electrical equipment. The insulated wire 10 can further include a second insulating layer 5 to prevent premature dielectric breakdown.

[0078] The inorganic particles 6 may be surface-treated. Examples of surface treatment agents include silylation agents and silane coupling agents. Using surface-treated silica as the inorganic particles 6 can further improve surge resistance.

[0079] The average thickness of the second insulating layer 5 is not particularly limited and can usually be between 5 μm and 200 μm.

[0080] The insulated wire may have configurations other than those described above. For example, the insulated wire may have an adhesion layer between the conductor and the insulating layer containing an adhesion improver or other additive. Examples of adhesion improvers include mercaptans such as 2-mercaptoimidazole and 5-amino-1,3,4-thiadiazole-2-thiol, and melamine compounds.

[0081] The present disclosure will be further described below with reference to examples. This disclosure is not limited to these examples.

[0082] <Insulated wires No. 3 to No. 5 and No. 7 to No. 9> [Preparation of resin composition for insulating layer] The various components used in the examples are shown below. (Aromatic tetracarboxylic dianhydride) Pyromellitic dianhydride (Aromatic diamine) 4,4'-diaminodiphenyl ether (Organic solvent) N-methyl-2-pyrrolidone (boiling point: 202°C) (Aliphatic polycarboxylic acid ester) Tributyl citrate (Hydrophilic nanosilica) Nissan Chemical Corporation's "DMAC-ST": Colloidal hydrophilic nanosilica dispersion (dimethylacetamide dispersion containing 20% ​​by mass of surface-untreated nanosilica with a particle size of 10 nm to 15 nm and OH groups on the surface) (Hydrophobic nanosilica) Nissan Chemical Corporation's "DMAC-ST" was formulated with a methylsilane surface treatment agent as a silane coupling agent to create a colloidal hydrophobic nanosilica dispersion.

[0083] First, 4,4'-diaminodiphenyl ether (ODA), as an aromatic diamine, was dissolved in N-methyl-2-pyrrolidone (NMP). Pyromellitic dianhydride (PMDA), as an aromatic tetracarboxylic dianhydride, was added in a mixing ratio (molar ratio) of 100:100 between the aromatic tetracarboxylic dianhydride and the aromatic diamine. The mixture was reacted at 30°C for 3 hours with stirring under a nitrogen atmosphere to synthesize a polyimide precursor, and a polyimide precursor solution was obtained using NMP as the solvent (solid content concentration: 28% by mass). To 100 parts by mass of the above polyimide precursor, tributyl citrate, as an aliphatic polycarboxylic acid ester, in the amounts listed in Table 1, and a hydrophobic nanosilica dispersion in the amounts listed in Table 1 were added to prepare insulating layer resin compositions No. 3 to No. 5 and No. 7 to No. 9. In Table 1 below, "-" indicates that the corresponding component is not present.

[0084] [Fabrication of Insulated Wires] Round copper wire with an average diameter of 1 mm was used as the conductor. Insulating layer resin compositions No. 3 to No. 5 and No. 7 to No. 9 were applied to the surface of the conductor, respectively. The conductor coated with the insulating layer resin compositions was heated in a heating furnace at an inlet temperature of 400°C, an outlet temperature of 450°C, and a wire speed of 3 m / min. This process was repeated 10 times to form a first insulating layer with an average thickness of 35 μm, thereby producing insulated wires.

[0085] <Insulated Wire No. 1> Resin composition No. 1 for the insulating layer was prepared in the same manner as No. 3, except that a hydrophobic nanosilica dispersion was not added, and an insulated wire was manufactured.

[0086] <Insulated Wire No. 6> Resin composition No. 6 for the insulating layer was prepared in the same manner as No. 7, except that a hydrophobic nanosilica dispersion was not added, and an insulated wire was manufactured.

[0087] <Insulated Wire No. 2> Resin composition No. 2 for the insulating layer was prepared in the same manner as No. 3, except that 0.28 parts by mass of hydrophilic nanosilica dispersion was added instead of hydrophobic nanosilica dispersion, and an insulated wire was manufactured.

[0088] [Evaluation] (Measurement of porosity) For the insulated wire prepared above, the first insulating layer was peeled off from the conductor in a tubular shape, and the mass W2 of the cylindrical first insulating layer was measured. The apparent volume V1 was determined from the outer shape of the cylindrical first insulating layer, and the mass W1 in the case of no voids was calculated by multiplying the volume V1 by the density ρ1 of the material of the first insulating layer. From these values ​​of W1 and W2, the porosity (unit: volume %) was calculated using the following formula 1. Formula 1: Porosity = (W1 - W2) × 100 / W1

[0089] (Measurement of average void diameter of multiple voids) For the insulated wires No. 1 to No. 9 that were fabricated, the average void diameter of multiple voids in the cross-section of the insulated wire was calculated. The above average void diameter was measured by performing SEM observation of the cross-section of the first insulating layer and using the open-source image processing software "ImageJ". The above average void diameter was the average value when measuring 100 or more voids.

[0090] (2% proof stress) For the insulated wire prepared above, a round conductor was dissolved in nitric acid to peel off the first insulating layer from the conductor, and a sample consisting of the first insulating layer with a diameter of approximately 1 mm and a length of 6 mm was prepared. Next, a tensile test was performed using a tensile testing machine (Shimadzu Corporation's "Autograph AGS-X series") and the 0.2% proof stress [MPa] was measured according to the following procedure. The higher the 0.2% proof stress, the better the mechanical properties and the less deformation and film loss can be reduced during compression processing.

[0091]

[0092] As shown in Table 1, when comparing insulated wires No. 3 to No. 5, which contain polyimide as the main component and hydrophobic nanosilica in the first insulating layer having multiple pores derived from aliphatic polycarboxylic acid esters, with insulated wire No. 1, which does not contain hydrophobic nanosilica under the same conditions, it can be seen that the average pore diameter in the first insulating layer is reduced by the inclusion of hydrophobic nanosilica. Furthermore, as the hydrophobic nanosilica content increases, there was a tendency for the average pore diameter in the first insulating layer to decrease even further. In addition, insulated wires No. 3 to No. 5 showed improved 0.2% yield strength due to the reduction in the average pore diameter in the first insulating layer.

[0093] Comparing insulated wires No. 7 to No. 9, which have increased aliphatic polycarboxylic acid ester content, with No. 3 (No. 7 and hydrophobic nanosilica content are the same), No. 4 (No. 8 and hydrophobic nanosilica content are the same), and No. 5 (No. 9 and hydrophobic nanosilica content are the same), it is observed that as porosity increases, the average pore diameter in the first insulating layer tends to increase as well. Furthermore, from the results for insulated wire No. 6, which has increased aliphatic polycarboxylic acid ester content similar to insulated wires No. 7 to No. 9 but without hydrophobic nanosilica, it can be seen that even when the porosity of the first insulating layer is high, such as 35.0 volume%, the inclusion of hydrophobic nanosilica reduces the average pore diameter in the first insulating layer and improves the 0.2% yield strength.

[0094] On the other hand, insulated wire No. 2 containing hydrophilic nanosilica showed a slightly smaller average pore size in the first insulating layer compared to insulated wire No. 1 without hydrophobic nanosilica, but its 0.2% yield strength did not improve.

[0095] From the above, it can be seen that the insulated wire exhibits excellent resistance to compression processing, even when the dielectric constant is low and the porosity of the insulating layer is high.

[0096] 1. Insulated wire 2. Conductor 3. First insulating layer 4. Void 5. Second insulating layer 6. Inorganic particles

Claims

1. An insulated wire comprising a conductor and a first insulating layer laminated on the surface of the conductor and having a plurality of pores derived from an aliphatic polycarboxylic acid ester, wherein the first insulating layer contains polyimide as the main component and hydrophobic nanosilica.

2. The insulated wire according to claim 1, wherein the content of the hydrophobic nanosilica in the first insulating layer is 0.50% by mass or more and 3.10% by mass or less.

3. The insulated wire according to claim 1 or claim 2, wherein the average pore diameter of the plurality of pores is 0.40 μm or more and 0.70 μm or less.

4. An insulating layer resin composition used to form the first insulating layer of an insulated electric wire according to any one of claims 1 to 3, comprising a polyimide precursor, an organic solvent, an aliphatic polycarboxylic acid ester, and hydrophobic nanosilica.

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