A method for preparing an insulated wire

Abstract

This invention discloses a method for preparing insulated wires. Specifically, it includes annealing a rolled conductor to obtain an annealed conductor; coating the outer periphery of the annealed conductor with an insulating varnish to form a first pre-formed core wire; curing the first pre-formed core wire coated with the insulating varnish at high temperature to form an insulating layer, obtaining a second pre-formed core wire; coating the outer side of the insulating layer of the second pre-formed core wire with a topcoat containing a modified polyamide-imide intermediate to form a third pre-formed core wire; curing the third pre-formed core wire coated with the topcoat at high temperature to form a topcoat layer on the outer side of the insulating layer, and further obtaining an insulated wire. The insulated wire provided by this invention improves the flexibility of the wire, adapts to more compact arrangement of complex slots, thereby improving the slot fill factor of the motor, thus reducing the stator core volume at the same power, indirectly reducing the weight of the motor, and achieving the requirement of lightweighting.

Description

Technical Field

[0001] This invention relates to the field of insulated wire technology, and more specifically, to a method for preparing an insulated wire. Background Technology

[0002] With the development of motor systems, the heat resistance of polyamide-imide is approaching its upper limit, making it difficult to independently meet the long-term reliability requirements under higher temperature rise conditions. Therefore, a composite coating of polyamide-imide and polyimide varnish is currently commonly used to synergistically improve the heat resistance and electrical insulation performance of the insulation layer. Furthermore, the high rigidity of the polyamide-imide molecular structure and the high hardness of the varnish film result in insufficient flexibility during winding, easily leading to stress concentration at bending points and causing varnish film cracking. To avoid varnish film cracking, a larger bending radius must be used, limiting the close arrangement of insulated wires in complex slots, resulting in a reduced motor slot fill factor and consequently affecting power density improvement.

[0003] Against this backdrop, modifying the structure and materials of polyamide-imide insulated wires to significantly improve their flexibility without sacrificing heat resistance and insulation performance has become crucial for achieving high slot full rate winding designs. By improving the bending performance of the wire, a more compact coil arrangement can be achieved, thereby reducing the stator core size at the same output power, effectively reducing the size and weight of the motor, and helping new energy vehicles develop towards "high power density, high efficiency, and lightweight".

[0004] Therefore, developing an insulated wire that combines high heat resistance and high flexibility is a technical challenge that urgently needs to be solved. Summary of the Invention

[0005] In view of this, the main objective of the present invention is to provide a method for preparing an insulated wire.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows.

[0007] A first aspect of the present invention includes a method for preparing an insulated wire, comprising the following steps:

[0008] S1: Roll the conductor to the target size at least once to obtain a rolled conductor;

[0009] S2: Anneal the rolled conductor to obtain an annealed conductor;

[0010] S3: Apply insulating varnish to the outer periphery of the annealed conductor to form a first pre-formed core wire. Line the first pre-formed core wire coated with insulating varnish into the enameling machine. The insulating varnish is cured at high temperature to form an insulating layer. Repeat the coating and curing steps multiple times to obtain a second pre-formed core wire with an insulating layer of a predetermined thickness.

[0011] S4: Apply a topcoat to the outside of the insulation layer to form a third prefabricated core wire. The third prefabricated core wire coated with the topcoat is then fed into an enameling machine. After high-temperature curing, a topcoat layer is formed on the outside of the insulation layer. The coating and curing steps are repeated multiple times to obtain an insulated wire.

[0012] In this invention, the predetermined thickness insulation layer in step S3 includes a base layer and optionally an intermediate layer. The varnish forming the base layer is optionally a conventional polyamide-imide varnish, such as commercially available polyamide-imide varnishes, for example, the TONGMID595 series from ELANTAS or the HI406 series from RESONAC. The varnish forming the intermediate layer is optionally a polyimide varnish, preferably a corona-resistant polyimide varnish or a low-dielectric polyimide varnish. The low-dielectric polyimide varnish refers to a polyimide varnish with a dielectric constant less than 3.5, particularly a polyimide varnish with a dielectric constant less than 3. The corona-resistant polyimide varnish is a polyimide varnish with anti-ionization breakdown properties. For example, commercially available TONGTHERM537 series from ELANTAS or SI-750D series from RESONAC. In this method, the second pre-fabricated core wire with an insulation layer of the predetermined thickness is obtained by sequentially coating and curing the above-mentioned insulating varnishes.

[0013] A second aspect of the present invention is to provide a method for preparing a topcoat, wherein the topcoat is a modified polyamide-imide paint.

[0014] The modified polyamide-imide paint is prepared by dissolving a modified polyamide-imide intermediate in solvent B. The modified polyamide-imide intermediate is prepared by an addition reaction of diisocyanate with a mixed acid anhydride, wherein the mixed acid anhydride includes short-chain acid anhydride and long-chain acid anhydride.

[0015] The short-chain anhydride includes at least one of the following: aromatic short-chain anhydride, aliphatic short-chain anhydride, and alicyclic short-chain anhydride. The aromatic short-chain anhydride refers to an aromatic anhydride having a chain of aliphatic substituents with no more than 5 carbon atoms on its aromatic ring, or an unsubstituted aromatic anhydride. The aliphatic short-chain anhydride refers to an aliphatic anhydride whose molecule contains a chain of aliphatic structures with no more than 5 carbon atoms. The alicyclic short-chain anhydride refers to an alicyclic anhydride whose cyclic alicyclic structure has no more than 7 carbon atoms. The molecular structure of the short-chain anhydride should contain at least one anhydride structure and one carboxyl group; or it should have a dianhydride structure.

[0016] In a preferred embodiment of the present invention, the short-chain anhydride is an aromatic short-chain anhydride.

[0017] In a preferred embodiment of the present invention, the aromatic short-chain anhydride includes at least one of the following anhydrides: trimellitic anhydride, methyl-substituted trimellitic anhydride, ethyl-substituted trimellitic anhydride, and pyromellitic dianhydride.

[0018] In this invention, the long-chain anhydride includes at least one of the following anhydrides: alicyclic anhydrides, aromatic anhydrides, and bio-based long-chain anhydrides. The molecular structure of the long-chain anhydride should contain at least one anhydride structure and one carboxyl group. More specifically, the long-chain anhydride is prepared from a long-chain aliphatic diacid or its derivative and a short-chain carboxylic acid or its derivative.

[0019] In this invention, the long-chain aliphatic diacid or its derivative refers to an aliphatic chain structure with not less than 6 carbon atoms or an alicyclic structure with not less than 8 carbon atoms in its molecular structure. The short-chain carboxylic acid or its derivative refers to an aromatic monocarboxylic acid or its derivative with an aliphatic chain structure with not more than 5 carbon atoms in its molecular structure, or with an aromatic monocarboxylic acid or its derivative having aliphatic substituents with not more than 5 carbon atoms.

[0020] It should be noted that the term "not greater than" in this invention includes the case of 0.

[0021] In this invention, the modified polyamide-imide intermediate is prepared by reacting a mixed anhydride comprising long-chain anhydride and short-chain anhydride with a diisocyanate.

[0022] In the mixed anhydrides, the molar ratio of long-chain anhydrides is less than 10%, preferably 3-5%.

[0023] In this invention, the diisocyanate is selected from aromatic diisocyanates, preferably at least one of toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI).

[0024] A third aspect of the present invention relates to a method for preparing a modified polyamide-imide intermediate.

[0025] The method includes the following steps:

[0026] (1) In step S4, long-chain aliphatic dicarboxylic acid and monocarboxylic acid are fed in a 1:1 molar ratio, and solvent A, activator and catalyst are added to react and react to obtain long-chain anhydride.

[0027] (2) Further, in step S4, the long-chain anhydride obtained after the reaction is mixed with the short-chain anhydride and reacted with diisocyanate to obtain the modified polyamide imide intermediate;

[0028] (3) Dissolve the modified polyamide-imide intermediate in solvent B to obtain modified polyamide-imide varnish; solvent B is at least one of NMP, DMF or DMAC.

[0029] In step (1), the activator is DCC (dicyclohexylcarbodiimide), the catalyst is DMAP (4-dimethylaminopyridine), and solvent A includes dichloromethane and ethylbenzene.

[0030] The activator DCC is diluted and added dropwise in a stirred reactor, wherein the concentration of the diluted DCC is not greater than 3 mol / L. During the DCC addition process, the reaction temperature is preferably 0~10℃, more preferably 0~5℃.

[0031] In this invention, preferably, the activator DCC is added in excess, based on the molar amount of the monocarboxylic acid in the mixed carboxylic acids. The molar ratio of DCC to the monocarboxylic acid is 1.05~1.2:1.

[0032] In step (1), DMAP is added at 10% to 20% of the molar amount of the monocarboxylic acid.

[0033] In step (2), the feeding ratio of long-chain anhydrides is no more than 10% of the mixed anhydrides, preferably 3-5%. The feeding ratio is based on molar ratio. In step (2), based on the theoretical amount of diisocyanate added to react with the anhydride, the actual added diisocyanate in this invention is added in excess relative to the anhydride molar ratio, wherein the isocyanate group in the diisocyanate is in excess relative to the sum of the carboxyl group and the anhydride group in the corresponding anhydride by 10-20%. That is, for this invention, when the reactant is a diisocyanate reacting with an anhydride or diacid anhydride containing one anhydride and a carboxyl group, the molar ratio of diisocyanate to anhydride is 1.1-1.2:1.

[0034] A fourth aspect of the present invention is to provide an insulating wire comprising the modified polyamide-imide topcoat layer.

[0035] The obtained modified polyamide-imide intermediate is dissolved in solvent B to obtain modified polyamide-imide varnish, i.e., topcoat. The modified polyamide-imide varnish is applied to the wire and cured by heating to obtain an insulated wire containing a topcoat layer.

[0036] The insulated wire of the present invention includes a conductor and an enameled layer around the conductor. The enameled layer has a multi-layer structure, comprising an insulating layer and a topcoat layer arranged sequentially from the inside out. The insulating layer optionally includes a base layer and / or an intermediate layer.

[0037] In this invention, the bottom layer uses a conventional polyamide-imide varnish, such as a commercially available general-purpose polyamide-imide varnish. The middle layer uses a polyimide varnish, preferably a corona-resistant polyimide varnish or a low-dielectric polyimide varnish. The low-dielectric polyimide varnish refers to a polyimide varnish with a dielectric constant less than 3.5, particularly a polyimide varnish with a dielectric constant less than 3. The corona-resistant polyimide varnish is a polyimide varnish with resistance to ionization breakdown.

[0038] In this invention, the topcoat layer is formed by coating and curing a modified polyamide-imide varnish.

[0039] In a preferred embodiment of the present invention, the thickness of the topcoat layer accounts for 10% to 50% of the total thickness of the entire enameled layer. The total thickness of the enameled layer includes the thickness of the insulating layer disposed on the conductor and the thickness of the topcoat layer formed by curing the modified polyamide-imide varnish. That is, the thickness of the topcoat layer accounts for 10% to 50% of the sum of the thicknesses of the insulating layer in step S3 and the topcoat layer in step S4.

[0040] In this invention, the total thickness of the enamel coating is 50~500μm, preferably 50~200μm.

[0041] Compared with the prior art, the present invention has the following advantages.

[0042] The modified polyamide-imide intermediate of this invention serves as the main component of the topcoat. After coating and curing, it forms a topcoat layer that coats the surface of the insulated wire, enhancing its flexibility. Furthermore, the insulated wire with this invention's insulation layer maintains good electrical insulation properties even at higher operating temperatures, is less prone to cracking, and exhibits good flexibility. This allows for closer arrangement of complex slots, improving motor slot fill factor and reducing stator core volume for the same power output, indirectly reducing motor weight and achieving lightweighting requirements. Detailed Implementation

[0043] Example 1

[0044] Step 1) In a 500 mL three-necked flask, add dodecanoic acid (20.2 g, 0.1 mol), benzoic acid (12.2 g, 0.1 mol), and DMAP (4-dimethylaminopyridine) (1.22 g, 0.01 mol) sequentially. Add 200 mL of anhydrous dichloromethane and stir until the solids are completely dissolved. Place the three-necked flask in an ice bath (0-5 °C) and slowly add a dichloromethane solution of DCC (dicyclohexylcarbodiimide) (24.7 g, 0.12 mol of DCC dissolved in 50 mL of anhydrous dichloromethane), controlling the temperature ≤10 °C during the addition. After the addition is complete, remove the ice bath and stir the reaction at room temperature (25-30 °C) for 12-16 hours. After the reaction stops, filter the solution, wash the filtrate with anhydrous petroleum ether, and rotary evaporate under reduced pressure (0.08 MPa) to obtain a pale yellow oily crude product. Further separation was performed by silica gel column chromatography with ethyl acetate / petroleum ether as eluent. After purification and drying, a white solid terminal carboxyl dodecanoyl-benzoyl anhydride was obtained, as shown in Formula 1.

[0045] Formula 1.

[0046] In a flask equipped with a reflux condenser, 5 mmol of terminal carboxyl-dodecanoyl-benzoic anhydride was added, followed by 95 mmol of trimellitic anhydride (TMA), 500 mL of ethylbenzene, and 0.12 mol of diphenylmethane diisocyanate (MDI). The mixture was refluxed at 190 °C for 2 hours. After the reaction was complete, the product was washed with n-hexane, and the solvent was removed under reduced pressure to obtain the modified polyamide-imide intermediate.

[0047] 2) Dissolve the modified polyamide-imide intermediate obtained in step 1) in NMP (N-methylpyrrolidone) to prepare modified polyamide-imide paint (solid content 30%).

[0048] 3) A pre-formed core wire is formed by coating a base layer of polyamide-imide insulating varnish around the annealed conductor. The pre-formed core wire coated with polyamide-imide insulating varnish is then fed into an enameling machine. The insulating varnish is cured at high temperature. Repeated coating and curing steps are used to obtain a polyamide-imide base layer with a thickness of 12 μm. A corona-resistant polyamide varnish intermediate layer is further coated and cured on the outside of the base layer. Repeated coating and curing steps are used to obtain a corona-resistant polyamide intermediate layer with a thickness of 83 μm. The modified polyamide-imide varnish obtained in step 2) is coated on the outside of the intermediate layer. The modified polyamide-imide varnish is cured at high temperature to form a modified polyamide-imide varnish layer on the outside of the intermediate layer. Repeated coating and curing steps are used to obtain a modified polyamide-imide topcoat layer disposed on the outside of the intermediate layer, thus obtaining an insulated wire covered with a topcoat layer. The thickness of the topcoat layer obtained by curing the modified polyamide-imide varnish is 25 μm.

[0049] Example 2

[0050] Using the method described in Example 1, octadecanoic acid was used to prepare a carboxyl-terminated octadecanoyl-benzoic anhydride with the structure shown in Formula 2.

[0051] Formula 2;

[0052] In a flask equipped with a reflux condenser, 3 mmol of terminal carboxyl octadecyl-benzoyl anhydride was added, followed by 97 mmol of TMA, 500 ml of ethylbenzene, and 0.12 mol of MDI. The mixture was refluxed at 190 °C for 2 hours to prepare a modified polyamide-imide intermediate. A modified polyamide-imide varnish was then prepared and coated onto a pre-coated and cured base layer of polyamide-imide and a mid-layer of low-dielectric polyimide insulation. The base layer was 12 μm thick, and the mid-layer was 48 μm thick. After multiple coatings, baking, and curing processes, a modified polyamide-imide topcoat layer with a thickness of approximately 60 μm was obtained, covering the wire to form an insulated wire.

[0053] Example 3

[0054] Using the method described in Example 1, a carboxyl-terminated hexadecanoic acid anhydride was prepared from hexadecanoic acid, with the structure shown in Formula 3.

[0055] Formula 3;

[0056] In a flask equipped with a reflux condenser, 10 mmol of terminal carboxyl hexadecanoyl-benzoic anhydride was added, followed by 90 mmol of TMA, 500 ml of ethylbenzene, and 0.12 mol of MDI. The mixture was refluxed at 190 °C for 2 hours to prepare a polyamide-imide intermediate. A polyamide-imide varnish was then prepared and coated onto an insulated wire with a pre-coated and cured bottom layer of polyamide-imide and an intermediate layer of polyamide-imide insulation. The bottom layer was 12 μm thick, and the intermediate layer was 96 μm thick. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of approximately 12 μm was obtained, covering the wire and thus yielding the insulated wire.

[0057] Example 4

[0058] Using the same method as in Example 1, a modified polyamide-imide varnish was prepared and coated onto a cured polyimide insulated wire. The polyimide insulating layer was 95 μm thick. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of approximately 25 μm was obtained, covering the wire to form an insulated wire.

[0059] Example 5

[0060] Using the same method as in Example 1, a modified polyamide-imide varnish was prepared and coated onto an insulated wire with a bottom layer of polyamide-imide and a middle layer of corona-resistant polyamide insulation that had already been coated and cured. The bottom layer was 12 μm thick and the middle layer was 83 μm thick. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of about 25 μm was obtained to cover the wire, thus obtaining an insulated wire.

[0061] Comparative Example 1

[0062] The method described in step 1) of Example 1 is adopted, except that: 0.1 mol TMA, 500 ml ethylbenzene, and 0.12 mol MDI are added to a flask equipped with a reflux condenser, and the mixture is refluxed at 190°C for 2 hours to prepare a polyamide-imide intermediate. Further, following the same method as steps 2) and 3) of Example 1, the polyamide-imide intermediate prepared in the above steps is dissolved in NMP to obtain a polyamide-imide varnish. This varnish is then coated onto an insulated wire with a pre-coated and cured bottom layer of polyamide-imide and an intermediate layer of corona-resistant polyimide insulation. The bottom layer has a thickness of 12 μm, and the intermediate layer has a thickness of 83 μm. The polyamide-imide varnish is baked and cured to obtain a polyamide-imide topcoat layer with a thickness of approximately 25 μm covering the wire, thus obtaining an insulated wire.

[0063] Comparative Example 2

[0064] Using the method described in step 1) of Example 1, terminal carboxyl-dodecanoyl-benzoyl anhydride was prepared, with the difference being that: 20 mmol of terminal carboxyl-dodecanoyl-benzoyl anhydride was added to a flask equipped with a reflux condenser, followed by 80 mmol of TMA, 500 ml of ethylbenzene, and 0.12 mol of MDI. The mixture was refluxed at 190°C for 2 hours to prepare a polyamide-imide intermediate. Further, following the same method as steps 2) and 3) of Example 1, a modified polyamide-imide varnish was prepared and coated onto an insulated wire with a pre-coated and cured bottom layer of polyamide-imide and an intermediate layer of corona-resistant polyimide insulation. The bottom layer had a thickness of 12 μm, and the intermediate layer had a thickness of 83 μm. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of approximately 25 μm was obtained, covering the wire, thus obtaining the insulated wire.

[0065] Comparative Example 3

[0066] The method described in step 1) of Example 1 is adopted, except that DCC is added dropwise while the temperature is controlled at 20°C during the dropwise addition process. Modified polyamide-imide varnish is prepared using the same method as in steps 2) and 3) of Example 1, and coated onto the insulated wire with a pre-coated and cured bottom layer of polyamide-imide and an intermediate layer of corona-resistant polyimide insulation. The bottom layer has a thickness of 12 μm, and the intermediate layer has a thickness of 83 μm. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of approximately 25 μm is obtained, covering the wire, thus obtaining the insulated wire.

[0067] Comparative Example 4

[0068] The method described in step 1 of Example 1 was employed, except that: in a 500 mL three-necked flask, malonic acid (10.4 g, 0.1 mol), benzoic acid (12.2 g, 0.1 mol), and DMAP (4-dimethylaminopyridine) (1.22 g, 0.01 mol) were added sequentially, followed by 200 mL of anhydrous dichloromethane. The mixture was stirred until the solid was completely dissolved to prepare benzoylmalonic anhydride (PhCO-O-COCH2COOH). Further addition of 5 mmol of benzoylmalonic anhydride and 95 mmol of trimellitic anhydride prepared a modified polyamide-imide intermediate. The modified polyamide-imide varnish was prepared using the same method as steps 2) and 3) in Example 1. It was then coated onto the insulated wire with a base layer of polyamide-imide and a middle layer of corona-resistant polyimide insulation that had already been coated and cured. The base layer was 12 μm thick and the middle layer was 83 μm thick. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of about 25 μm was obtained to cover the wire, thus obtaining the insulated wire.

[0069] Comparative Example 5

[0070] Using the same method as in Example 1, a modified polyamide-imide varnish was prepared and coated onto an insulated wire with a bottom layer of polyamide-imide and a middle layer of corona-resistant polyimide insulation that had already been coated and cured. The bottom layer was 12 μm thick and the middle layer was 103 μm thick. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of about 5 μm was obtained to cover the wire, thus obtaining an insulated wire.

[0071] Comparative Example 6

[0072] Using the same method as in Example 1, a modified polyamide-imide varnish was prepared and coated onto an insulated wire with a bottom layer of polyamide-imide and a middle layer of corona-resistant polyimide insulation that had already been coated and cured. The bottom layer was 12 μm thick and the middle layer was 38 μm thick. After baking and curing, a modified polyamide-imide topcoat layer with a thickness of about 70 μm was obtained to cover the wire, thus obtaining an insulated wire.

[0073] The insulated wires of the embodiments and comparative examples were tested according to the following methods.

[0074] 1. Flexibility Test. The flexibility test is conducted using the flexibility test method provided in IEC 60851-32019 Test Method 8. The specific steps are as follows.

[0075] Take two 500mm long straight insulated wires. Bend each wire 180±2° around a polished test core. One wire is wound horizontally (core diameter = N times wire thickness, where N is 1.0, 1.5, or 2.0), and the other is wound vertically (core diameter = N times wire width). In this test, after horizontal and vertical winding, a product with a smooth, crack-free surface is recorded as "qualified"; a product with surface cracks is recorded as "unqualified".

[0076] 2. Thermal shock test at 260℃ for 30 minutes.

[0077] The test was conducted using the test method provided in GB / T4074.6-2024 Test Method 9.

[0078] Prepare the test specimens according to the specifications in GB / T4074.3-2024, section 6.2.2. Take a 400mm straightening sample wire and bend it 180±2° around a polished test shaft (shaft diameter = wire thickness N times). Place the specimens in a forced-ventilation oven and heat at 260±5℃ for 30 minutes. Remove the specimens from the oven and cool to room temperature. Then, examine the enameled coating for cracks using a 10x magnifying glass. The presence of cracks indicates a failure; the absence of cracks indicates a pass.

[0079] 3. Softening breakdown test.

[0080] The softening breakdown tester (model HTC600) was used for testing. The sample line was placed in the instrument, and the temperature rise method was used for testing. The temperature at which the breakdown occurred was recorded and evaluated.

[0081] Test conditions:

[0082] Weight of the weight: 1KG is equivalent to 9.8N;

[0083] Heating rate: 2℃ / min;

[0084] Test voltage: 100V;

[0085] Target temperature: 600℃.

[0086] Evaluation criteria:

[0087] A: Above 500℃;

[0088] B: Above 400℃, below 500℃;

[0089] C: less than 400℃.

[0090] The results are shown in the table below.

[0091]

[0092] Note: 2x indicates that the diameter of the core when the insulated wire is bent is twice the wire width or thickness; 1.5x indicates that the diameter of the core when bent is 1.5 times the wire width or thickness; 1.0x indicates that the diameter of the core is one time the wire width or thickness.

[0093] As can be seen from the above tests, the insulated wires prepared in Examples 1-5 of this invention showed no cracking in the 2.0x, 1.5x, and 1.0x winding flexibility tests, with a pass rate of 100%. In Comparative Example 1, due to the use of short-chain anhydrides, the prepared insulated wires lacked flexibility, and the thermal shock resistance was also significantly reduced. In Comparative Example 2, no cracking was observed in the flexibility test, but the wound wires underwent undesirable deformation in the 260℃ 30min thermal shock test. The polyamide-imide coated wire obtained using the reaction of Comparative Example 3 of this invention showed outer melt flow during the 260℃ 30min thermal shock test, and the experiment could not be conducted. The wire coated with the modified polyamide-imide topcoat layer obtained using the reaction of Comparative Example 4 of this invention showed cracking in many samples during the flexibility test at 25℃ and the 260℃ 30min thermal shock test. In Comparative Example 5, due to insufficient coating thickness of the modified polyamide-imide varnish and insufficient topcoat layer thickness, cracking occurred during the flexibility tests of the 1.5x and 1.0x winding groups. Unexpected deformation also occurred during the thermal shock test, resulting in insufficient overall flexibility. In Comparative Example 6, due to excessive coating thickness of the modified polyamide-imide varnish, with the topcoat layer exceeding 50% of the total thickness, the thermal shock performance and heat resistance decreased, the softening breakdown test result dropped to B, the overall heat resistance level decreased, and the mechanical properties decreased.

[0094] In summary, the insulated wire provided by this invention improves the flexibility of the wire and maintains good electrical insulation performance even at higher operating temperatures. It is less prone to cracking, adapts to more compact arrangement of complex slots, thereby improving the slot fill factor of the motor. This reduces the volume of the stator core at the same power, indirectly reducing the weight of the motor and achieving the requirement of lightweighting.

Claims

1. A method for producing an insulated electric wire, characterized by, The method comprises the following steps: S1: conducting at least one time of calendering to the conductor to a target size to obtain a calendered conductor; S2: conducting annealing treatment to the calendered conductor to obtain an annealed conductor; S3: coating insulating paint on the outer periphery of the annealed conductor to form a first precore, and feeding the first precore coated with the insulating paint into a varnishing machine, wherein the insulating paint is cured at high temperature to form an insulating layer, and the coating and curing steps are repeated multiple times to obtain a second precore with a predetermined thickness of the insulating layer; S4: coating finish paint on the outer side of the insulating layer of the second precore to form a third precore, and feeding the third precore coated with the finish paint into the varnishing machine, wherein the finish paint layer is formed on the outer side of the insulating layer after curing at high temperature, and the coating and curing steps are repeated multiple times to obtain an insulated wire; The insulating paint in step S3 comprises at least one of polyimide paint and polyamide-imide paint. The preparation method of the finish paint in step S4 further comprises the following steps: (1) feeding long-chain aliphatic dicarboxylic acid and monocarboxylic acid according to a molar ratio of 1:1, adding solvent A, activator and catalyst to mix and react to obtain long-chain anhydride, wherein the long-chain aliphatic dicarboxylic acid refers to an aliphatic chain structure with not less than 6 carbon atoms or an alicyclic structure with not less than 8 carbon atoms in the molecular structure; the activator is dicyclohexyl carbodiimide, and the catalyst is 4-dimethylaminopyridine; (2) mixing the obtained long-chain anhydride with short-chain anhydride to obtain mixed anhydride, and reacting the mixed anhydride with diisocyanate to obtain modified polyamide-imide intermediate, and further dissolving the modified polyamide-imide intermediate in solvent B to obtain modified polyamide-imide paint.

2. The method of producing an insulated electric wire according to claim 1, characterized by, In step S3, the predetermined thickness insulating layer comprises a bottom layer and / or an intermediate layer.

3. The method of producing an insulated electric wire according to claim 1, characterized by, In step S4, the thickness of the finish paint layer accounts for 10% to 50% of the sum of the thickness of the insulating layer in step S3 and the thickness of the finish paint layer in step S4.

4. The method of producing an insulated electric wire according to claim 1, characterized by, In step (1), the molar ratio of the activator to the monocarboxylic acid is 1.05 to 1.2:

1.

5. The method of producing an insulated electric wire according to claim 1, characterized by, In step (1), the activator is added in a dropwise manner, and the reaction temperature during the dropwise addition process is 0 to 10°C.

6. The method of producing an insulated electric wire according to claim 1, characterized by, In step (1), the catalyst is added in an amount of 10% to 20% based on the molar amount of the monocarboxylic acid.

7. The method of producing an insulated electric wire according to claim 1, characterized by, In step (2), the feeding amount of the long-chain anhydride is not more than 10% of the mixed anhydride.

8. The method of producing an insulated electric wire according to claim 7, characterized by, In step (2), the feeding amount of the long-chain anhydride is 3% to 5% of the mixed anhydride.

9. The method of producing an insulated electric wire according to claim 1, wherein In step (2), diisocyanate is added in excess relative to the molar ratio of the mixed anhydride.

10. The method of producing an insulated electric wire according to claim 9, wherein In step (2), the molar ratio of diisocyanate to mixed anhydride is 1.1 to 1.2:1.

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