Polyimide paint as well as preparation method and application thereof

Abstract

The invention discloses polyimide paint as well as a preparation method and application thereof, the polyimide paint is prepared from the following raw materials: aluminum isopropoxide and tetraethoxysilane, the solid content is 20wt%-30wt%, and the viscosity is greater than or equal to 5Pa. S and less than or equal to 50Pa. S. The preparation raw materials further comprise orthosilicic acid or a silane coupling agent. The polyimide paint disclosed by the invention has high solid content, low viscosity and excellent corona resistance at the same time. According to the invention, tetraethoxysilane is used as a main inorganic particle source, and aluminum isopropoxide and orthosilicic acid can play roles of a coupling agent and the inorganic particle source at the same time and are combined with carboxyl and amino in polyamide acid, so that the compatibility of tetraethoxysilane and a molecular chain is improved; the problem of particle agglomeration when inorganic particles or nano dispersion liquid of the inorganic particles is directly blended with polyamide acid is effectively solved.

Description

Technical Field

[0001] The invention relates to the technical field of high-performance materials, and in particular to a polyimide paint and a preparation method and application thereof. Background Art

[0002] Polyimide (PI) insulating varnish (i.e., polyimide varnish) is a type of coating with polyimide as the main film-forming substance. It has excellent heat resistance, electrical insulation, mechanical properties and chemical stability, and can be used as wire enamel, etc. The so-called wire enamel is a coating that can produce a good insulating layer between the wires in the winding. It is mainly used for the outer layer of bare copper wires, alloy wires and glass fiber covered wires of various wire diameters to improve and stabilize the performance of the enameled wire.

[0003] At present, polyimide paint has problems such as high solid content and low viscosity, as well as poor corona resistance. Specifically, in the polyimide paint solution, polyimide exists in the form of its precursor, polyamic acid (PAA). In order to achieve a balance between high solid content and low viscosity, Chinese patent document CN114181392A adopts a partial chemical imidization method to partially imidize polyamic acid to form a PAA-PI structure, thereby reducing the solution viscosity and increasing the solid content, but does not solve the corona resistance of the resulting polyimide product. Chinese patent document CN103788651A reduces the apparent viscosity of the polyamic acid solution by introducing trimethylchlorosilane, with a reduction of up to 90%, but it will affect the mechanical properties and thermal decomposition temperature of the polyimide product. Chinese patent document CN104292459A adjusts the viscosity of polyamic acid solution by introducing adhesion promoter and end-capping agent, and the viscosity range is between 500-10000cp, but does not analyze the influence of adhesion promoter and end-capping agent on the performance of polyimide products.

[0004] Regarding the corona resistance of polyimide insulating varnish, the existing technology usually uses in-situ blending, sol-gel and surface deposition methods to compound ceramic particles with polyimide to improve its corona resistance. For example: "Preparation of polyimide / alumina composite film by ion exchange method and performance research, Functional Materials, 2013, 44(05): 740-743.", "Deposition of alumina film on polyimide surface by sol-gel method and its properties, Proceedings of the CSEE, 2014, 34(12): 1965-1971.", "Preparation and performance research of polyimide / alumina nanocomposite film, Harbin University of Science and Technology, 2009.", "Preparation and performance research of nano-alumina / polyimide (PI) hybrid film, Ceramics, 2020(01): 23-29", "Study on the influence of Al2O3 on the performance of polyimide nanocomposite film, Insulating Materials, 2014, 47(6): 50-53" and other studies have shown that the corona resistance of the film obtained by combining ceramic particles with polyimide can reach more than 10 times that of pure polyimide film. Chinese patent document CN116162407A discloses a high-voltage and corona-resistant polyimide insulating varnish material and its preparation method, which is made of a mixture of resin and nano inorganic particles, and a catalyst needs to be added to convert part of PAA into PI to form a PAA-PI structure. However, the above reaction process needs to be heated to 80°C, which increases energy consumption and reaction complexity.

[0005] As mentioned above, there are three technical routes in the prior art for the research on high solid content and low viscosity polyimide paint. Technical route one is to introduce a catalyst to partially imide PAA to obtain a PAA-PI structure, which can adjust the interaction between the molecular chain and the solvent, thereby achieving the effect of high solid content and low viscosity, but the problem with this route is that the structure of the introduced catalyst is complex, and the complexity of the composition of the solution system is increased, which affects the recovery and reuse of the solvent. Technical route two is to add trimethylchlorosilane, but the problem with this route is that it affects the mechanical properties and thermal properties of the final product. Technical route three is to add an adhesion promoter and a capping agent, but the problem with this route is that the mechanical properties and thermal properties of the resulting film are relatively low.

[0006] In addition, in order to improve the corona resistance of polyimide insulating varnish, the existing technology adopts the technical route of inorganic particle blending, but this technical route is difficult to solve the problem of uniform dispersion of particles. The conventional mechanical mixing process is difficult to avoid the agglomeration of inorganic particles, resulting in larger particle size, which affects the toughness and electrical insulation performance of the paint film.

[0007] In view of this, there is an urgent need to provide a polyimide paint that can overcome the problems existing in the prior art, such as the difficulty in taking both high solid content and low viscosity into consideration, as well as poor corona resistance. Summary of the invention

[0008] In view of the deficiencies in the prior art, the object of the present invention is to provide a polyimide varnish which can solve the problems of high solid content, low viscosity, and difficulty in achieving corona resistance in the prior art polyimide insulating varnish, thereby overcoming the deficiencies in the prior art.

[0009] Another object of the present invention is to provide a method for preparing the polyimide paint.

[0010] Another object of the present invention is to provide application of the polyimide paint.

[0011] According to the purpose of the present invention, the present invention provides a polyimide paint, the preparation raw materials of which include aluminum isopropoxide, tetraethoxysilane, and orthosilicic acid or silane coupling agent, the solid content is 20wt% to 30wt%, and the viscosity is 5Pa·s≤≤50Pa·s.

[0012] The polyimide paint of the invention can simultaneously have high solid content and low viscosity and has excellent corona resistance.

[0013] Preferably, the film is obtained by curing the polyimide paint by heating, and the corona resistance time of the polyimide film is ≥20 min.

[0014] Preferably, the preparation raw material further includes orthosilicic acid or a silane coupling agent (eg, KH550), preferably includes orthosilicic acid.

[0015] According to another object of the present invention, the present invention provides a method for preparing the above-mentioned polyimide paint, which comprises the following steps:

[0016] (1) preparing a polyamic acid solution, wherein the initial solid content of the polyamic acid solution is 20 wt % to 30 wt %;

[0017] (2) mixing aluminum isopropoxide and tetraethoxysilane at a mass ratio of 0.1:100 to 1:100 for 1 to 10 hours to obtain a mixed particle source solution;

[0018] (3) adding the mixed particle source solution obtained in step (2) to the polyamic acid solution obtained in step (1), stirring and reacting at room temperature for 1 to 5 hours, adding orthosilicic acid, stirring and reacting at room temperature for more than 24 hours, and further standing and homogenizing to obtain a polyimide paint, wherein the mass ratio of the mixed particle source solution obtained in step (2) to the polyamic acid solution obtained in step (1) is 5:100 to 30:100, and the mass ratio of the orthosilicic acid to the mixed particle source solution obtained in step (2) is 5:100 to 20:100.

[0019] Preferably, the step (1) is: adding pyromellitic dianhydride (PMDA) or biphenyltetracarboxylic dianhydride (BPDA) to N,N-dimethylacetamide (DMAc) solvent, and then adding 4,4'-diaminodiphenyl ether (ODA) in an amount 1 to 1.2 times the molar number of PMDA, and continuously stirring the reaction for 10 to 72 hours to obtain a polyamic acid solution with an initial solid content of, for example, 25 wt%.

[0020] In the present invention, tetraethoxysilane is the main inorganic particle source, aluminum isopropoxide and orthosilicic acid can simultaneously play the role of coupling agent and inorganic particle source, and combine with carboxyl group and amino group in polyamic acid to improve the compatibility of tetraethoxysilane and molecular chain, and effectively solve the particle agglomeration problem when inorganic particles or their nano-dispersions are directly blended with polyamic acid.

[0021] According to another object of the present invention, the present invention provides application of the polyimide paint in the electrical field.

[0022] In one aspect, the present invention provides a polyimide film, which is obtained by curing the polyimide varnish by heating, and has a thickness of 10 μm to 100 μm.

[0023] Preferably, the corona resistance time of the polyimide film is ≥20 min, preferably ≥200 min, more preferably ≥300 min, further preferably ≥400 min, and further preferably ≥500 min.

[0024] On the other hand, the present invention provides an enameled wire, which includes a bare conductor and an outer layer of the coated wire, wherein the outer layer of the coated wire is the above-mentioned polyimide film.

[0025] Preferably, the bare wire is one or more selected from bare copper wire, alloy wire and glass wire.

[0026] In yet another aspect, the present invention provides a product comprising the above-mentioned enameled wire.

[0027] The enameled wire or polyimide varnish of the present invention is mainly used in the following products, for example: (1) Power tools: enameled wire is widely used in power tools, such as electric drills, electric saws, electric screwdrivers, etc., for coil winding, and bears the electrical load of high-load mechanical moving parts; (2) Auto parts: enameled wire is used in automobiles for ignition coils, engine control units, sensors, generators and other electrical equipment, carrying a large amount of electrical load to ensure efficient operation of the circuit; (3) Motors and transformers: enameled wire plays a conductive and insulating role in the windings of motors and transformers, protecting and insulating the coils to ensure the normal operation of the equipment; (4) Electrical equipment: in various electrical equipment, enameled wire is used to connect and transmit electrical energy, such as household appliances, electronic equipment, etc.; (5) Electronic equipment: in electronic equipment, enameled wire is used for circuit board connection and signal transmission to ensure stable operation of the circuit; (6) Electromagnetic coils: enameled wire is also used to manufacture electromagnetic coils such as electromagnets and solenoid valves; (7) Inductors: in the manufacture of inductors, enameled wire is an indispensable material.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The method of back-feeding and excess diamine monomer is beneficial to reducing the viscosity of the initial polyamic acid solution, and at the same time, a polyamic acid stock solution with amino end-capping can be obtained.

[0030] (2) The dilution effect of tetraethoxysilane can effectively reduce the viscosity of the solution. It is combined with orthosilicic acid and aluminum isopropoxide as coupling agents to combine with the carboxyl and amino groups in polyamic acid, thereby improving the compatibility of tetraethoxysilane with the molecular chain, and obtaining a high solid content, low viscosity solution containing an inorganic precursor, which effectively solves the particle agglomeration problem existing in the heterogeneous solution when inorganic particles or their nano-dispersions are directly blended with polyamic acid.

[0031] (3) Inorganic precursors such as tetraethoxysilane, aluminum isopropoxide, orthosilicic acid, etc. in the polyimide paint liquid can be converted into corresponding inorganic oxide particles after the paint film is formed, thereby playing a role in improving the corona resistance of the paint film.

[0032] (4) The above-mentioned polyimide insulating varnish also has high toughness after film formation, thereby being able to better meet the needs of actual production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a photograph of the paint solution prepared in Example 1;

[0034] Figure 2 This is a photo of the paint solution prepared in Comparative Example 2;

[0035] Figure 3 This is a photograph of the paint liquid prepared in Comparative Example 3. DETAILED DESCRIPTION

[0036] The present invention is further described below in conjunction with specific examples, but the present invention is not limited to the following examples. If no specific conditions are specified in the examples, the conditions are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0037] In the present invention, "parts" refer to parts by weight unless otherwise specified. In addition, the endpoints and any values ​​of the range disclosed in the present invention are not limited to the precise range or value, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered as specifically disclosed in the present invention.

[0038] In the following examples and comparative examples:

[0039] Solid content determination method: add paint liquid on the slide, then heat treat at 180℃ for 60min, and record the mass of the slide (A 0 ), the mass of the slide and the paint solution (A 1 ), and the quality of the slide and paint film after heat treatment (A 2 ).

[0040]

[0041] Viscosity determination method: A rotational viscometer is used to test the solution viscosity with a measurement accuracy of ±2% and a test temperature of room temperature.

[0042] The method for preparing a film sample for testing corona performance and mechanical properties is as follows: a polyimide paint liquid is cured by programmed temperature rise to obtain a polyimide film with a film thickness of about 50 μm.

[0043] The heating program was as follows: room temperature was heated to 135°C over 1 h, kept at 135°C for 0.5 h, then heated to 350°C over 2 h, kept at 350°C for 1 h, and then naturally cooled to room temperature.

[0044] Film corona performance test: Corona aging test was performed using a WPT corona aging instrument. Test conditions: bipolar square wave, pulse amplitude 3kV, rising edge 100ns, frequency 20kHz, temperature 30℃.

[0045] Film mechanical properties test: A universal material testing machine is used to test the tensile strength and modulus of the film. The specific method is: clamp the two ends of the film sample vertically on the tensile fixture, set the stretching rate to 10mm / min, and stretch until the sample breaks. The tensile strength can be directly obtained, and the tensile modulus is calculated through the elastic deformation stage of the stress-strain curve.

[0046] Embodiment 1:

[0047] (1) adding 10.8 g of pyromellitic dianhydride (PMDA) to 55 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 24 hours to obtain a polyamic acid solution with an initial solid content of 25 wt%;

[0048] (2) stirring 0.225 g of aluminum isopropoxide and 22.5 g of tetraethoxysilane at room temperature for 10 h to obtain a mixed particle source solution;

[0049] (3) The mixed particle source solution obtained in step (2) was added to the polyamic acid solution obtained in step (1), and the mixture was stirred at room temperature for 5 h, and then 3.5 g of orthosilicic acid (H 4 SiO 4 ), stirred at room temperature for 48 hours, and then allowed to stand for 10 hours to homogenize to obtain a polyimide paint solution with a solid content of 27.1 wt% and a viscosity of 11.2 Pa·s. Figure 1 As shown, there is no delamination phenomenon.

[0050] Embodiment 2:

[0051] (1) adding 10.8 g of pyromellitic dianhydride (PMDA) to 55 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 24 hours to obtain a polyamic acid solution with an initial solid content of 25 wt%;

[0052] (2) stirring 0.0225 g of aluminum isopropoxide and 22.5 g of tetraethoxysilane at room temperature for 6 h to obtain a mixed particle source solution;

[0053] (3) The mixed particle source solution obtained in step (2) was added to the polyamic acid solution obtained in step (1), and the mixture was stirred at room temperature for 4 h, and then 4.5 g of orthosilicic acid (H 4 SiO 4 ), stirred at room temperature for 72 hours, and then allowed to stand for 10 hours to homogenize to obtain a polyimide paint solution with a solid content of 27.5 wt% and a viscosity of 10.3 Pa·s. Figure 1 Similar, no stratification.

[0054] Embodiment 3:

[0055] (1) adding 10.8 g of pyromellitic dianhydride (PMDA) to 58 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 24 hours to obtain a polyamic acid solution with an initial solid content of 24.1 wt%;

[0056] (2) stirring 0.01 g of aluminum isopropoxide and 4 g of tetraethoxysilane at room temperature for 1 h to obtain a mixed particle source solution;

[0057] (3) Add the mixed particle source solution obtained in step (2) to the polyamic acid solution obtained in step (1), stir at room temperature for 1 h, and then add 0.2 g of orthosilicic acid (H 4 SiO 4 ), stirred at room temperature for 10 hours, and then allowed to stand for 10 hours to homogenize to obtain a polyimide paint solution with a solid content of 24.4 wt% and a viscosity of 45.6 Pa·s. Figure 1 Similar, no stratification.

[0058] Embodiment 4:

[0059] (1) adding 10.8 g of pyromellitic anhydride (PMDA) to 43 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 36 hours to obtain a polyamic acid solution with an initial solid content of 29.8 wt%;

[0060] (2) stirring 0.12 g of aluminum isopropoxide and 15 g of tetraethoxysilane at room temperature for 10 h to obtain a mixed particle source solution;

[0061] (3) The mixed particle source solution obtained in step (2) was added to the polyamic acid solution obtained in step (1), and the mixture was stirred at room temperature for 5 h, and then 2.4 g of orthosilicic acid (H 4 SiO 4 ), stirred at room temperature for 60 hours, and then allowed to stand for 10 hours to homogenize to obtain a polyimide paint solution with a solid content of 30.5 wt% and a viscosity of 26.1 Pa·s. Figure 1 Similar, no stratification.

[0062] Embodiment 5:

[0063] (1) adding 10.8 g of pyromellitic dianhydride (PMDA) to 74 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 24 hours to obtain a polyamic acid solution with an initial solid content of 20 wt%;

[0064] (2) stirring 0.225 g of aluminum isopropoxide and 22.5 g of tetraethoxysilane at room temperature for 10 h to obtain a mixed particle source solution;

[0065] (3) The mixed particle source solution obtained in step (2) was added to the polyamic acid solution obtained in step (1), and the mixture was stirred at room temperature for 5 h, and then 3.5 g of orthosilicic acid (H 4 SiO 4 ), stirred at room temperature for 48 hours, and then allowed to stand for 10 hours to homogenize to obtain a polyimide paint solution with a solid content of 22.9 wt% and a viscosity of 6.8 Pa·s. Figure 1 Similar, no stratification.

[0066] Embodiment 6:

[0067] (1) adding 14.55 g of biphenyltetracarboxylic dianhydride (BPDA) to 55 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 36 hours to obtain a polyamic acid solution with an initial solid content of 28.2 wt%;

[0068] (2) stirring 0.225 g of aluminum isopropoxide and 22.5 g of tetraethoxysilane at room temperature for 10 h to obtain a mixed particle source solution;

[0069] (3) The mixed particle source solution obtained in step (2) was added to the polyamic acid solution obtained in step (1), and the mixture was stirred at room temperature for 5 h, and then 3.5 g of orthosilicic acid (H 4 SiO 4 ), stirred at room temperature for 48 hours, and then allowed to stand for 10 hours to homogenize to obtain a polyimide paint solution with a solid content of 29.4 wt% and a viscosity of 16.7 Pa·s. Figure 1 Similar, no stratification.

[0070] Comparative Example 1:

[0071] The polyamic acid solution obtained in step (1) of Example 1 has a viscosity of 120 Pa·s, and the polyamic acid solution is directly used as a polyimide paint solution.

[0072] Comparative Example 2:

[0073] (1) adding 10.8 g of pyromellitic dianhydride (PMDA) to 55 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 24 hours to obtain a polyamic acid solution with an initial solid content of 25 wt%;

[0074] (2) 22.5 g of tetraethoxysilane was added to the polyamic acid solution obtained in step (1), and the mixture was stirred at room temperature for 48 h to obtain a polyimide paint solution with a solid content of 25.9 wt % and a viscosity of 8.7 Pa·s. However, the solution was immiscible. After further standing for 10 h, the solution separated into layers and did not meet the requirements for stable storage. The paint solution sample was as follows: Figure 2 shown.

[0075] Comparative Example 3:

[0076] (1) adding 10.8 g of pyromellitic dianhydride (PMDA) to 55 g of N,N-dimethylacetamide (DMAc) solvent, and then adding 10 g of 4,4'-diaminodiphenyl ether (ODA), and stirring and reacting for 24 hours to obtain a polyamic acid solution with an initial solid content of 25 wt%;

[0077] (2) stirring 0.225 g of aluminum isopropoxide and 22.5 g of tetraethoxysilane at room temperature for 10 h to obtain a mixed particle source solution;

[0078] (3) The mixed particle source solution obtained in step (2) was added to the polyamic acid solution obtained in step (1), and the mixture was stirred at room temperature for 48 hours to obtain a polyimide paint solution with a solid content of 25.9 wt % and a viscosity of 8.2 Pa·s. However, the solutions were not miscible. After further standing for 10 hours, the solution separated into layers and did not meet the requirements for stable storage. The paint sample was as follows: Figure 3 shown.

[0079] Comparative Example 4:

[0080] The polyamic acid solution obtained in step (1) of Example 6 has a viscosity of 180 Pa·s, and the polyamic acid solution is directly used as a polyimide paint solution.

[0081] The polyimide paint solutions of the above examples and comparative examples were cured by programmed temperature rise to obtain polyimide films, and the mechanical properties and corona resistance of the paint films were measured. The measurement results of the above examples and comparative examples are shown in Table 1 below.

[0082] Table 1 Mechanical properties and corona resistance test of paint film

[0083] Tensile strength(MPa) Tensile modulus(GPa) Elongation at break (%) Corona resistance time (min) Example 1 128 2.9 15 491 Example 2 122 2.8 14 502 Example 3 150 2.4 30 20 Example 4 131 2.6 18 120 Example 5 125 2.9 14 482 Example 6 163 3.6 11 378 Comparative Example 1 116 2.2 31 2 Comparative Example 2 —— —— —— —— Comparative Example 3 —— —— —— —— Comparative Example 4 165 3.1 13 3

[0084] Referring to Table 1, regarding Examples 1 to 6, not only are the mechanical properties excellent, but also the corona resistance is good, and the corona resistance time is ≥ 20 min. For example, the paint liquid prepared in Example 2 and the polyimide film prepared by programmed temperature have a corona resistance time of more than 500 min. Regarding Comparative Examples 1 to 4, the corona resistance is poor or cannot be measured. For example, the paint liquid prepared in Comparative Example 1 and the polyimide film prepared by programmed temperature have a corona resistance time of 2 min. In addition, Comparative Examples 2 and 3 are difficult to form a homogeneous film due to phase separation, and no film forming test is performed.

[0085] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A polyimide paint, wherein the raw materials for preparing the polyimide paint include aluminum isopropoxide, tetraethoxysilane, and orthosilicic acid or a silane coupling agent, the solid content is 20wt% to 30wt%, and the viscosity is 5Pa·s≤≤50Pa·s.

2. The polyimide paint according to claim 1, wherein A film is obtained by heating and curing the polyimide paint, and the corona resistance time of the polyimide film is ≥20 minutes.

3. A method for preparing the polyimide paint according to claim 1 or 2, comprising the following steps: (1) preparing a polyamic acid solution, wherein the initial solid content of the polyamic acid solution is 20 wt % to 30 wt %; (2) mixing aluminum isopropoxide and tetraethoxysilane at a mass ratio of 0.1:100 to 1:100 for 1 to 10 hours to obtain a mixed particle source solution; (3) adding the mixed particle source solution obtained in step (2) to the polyamic acid solution obtained in step (1), stirring and reacting at room temperature for 5 hours, adding orthosilicic acid, stirring and reacting at room temperature for more than 24 hours, and further standing and homogenizing to obtain a polyimide paint; Wherein, in step (3), the mass ratio of the mixed particle source solution obtained in step (2) to the polyamic acid solution obtained in step (1) is 5:100 to 30:100, and the mass ratio of the orthosilicic acid to the mixed particle source solution obtained in step (2) is 5:100 to 20:

100.

4. The preparation method according to claim 3, wherein The step (1) is: adding pyromellitic dianhydride (PMDA) to N,N-dimethylacetamide (DMAc) solvent, then adding 4,4'-diaminodiphenyl ether (ODA) in a molar amount of 1 to 1.2 times that of PMDA, and continuously stirring the reaction for 10 to 72 hours to obtain a polyamic acid solution.

5. A polyimide film obtained by curing the polyimide varnish according to claim 1 or 2 by heating, and having a thickness of 10 μm to 100 μm.

6. The polyimide film according to claim 5, wherein The corona resistance time of the polyimide film is ≥20 min or ≥200 min or ≥300 min or ≥400 min or ≥500 min.

7. An enameled wire, comprising a bare conductor and an outer layer of the covered wire, wherein the outer layer of the covered wire is the polyimide film according to claim 5 or 6.

8. The enameled wire according to claim 7, wherein the bare conductor is one or more selected from bare copper wire, alloy wire and glass wire.

9. A product comprising the enameled wire according to claim 7 or 8.

10. Use of the polyimide paint according to claim 1 or 2 in the electrical field.

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