Corona-resistant polyimide film based on core-shell structure nano SiO2 (at) PS filler and preparation method of corona-resistant polyimide film

Abstract

The invention discloses a corona-resistant polyimide film based on a core-shell structure nano SiO2 (at) PS filler and a preparation method of the corona-resistant polyimide film, and belongs to the technical field of high polymer materials. The preparation method comprises the following steps: dissolving diamine in a solvent under the protection of an inert atmosphere to obtain a diamine solution; preparing a nano SiO2 (at) PS filler dispersion liquid with a core-shell structure; adding the nano SiO2 (at) PS filler dispersion liquid into a diamine solution, fully stirring, and adding dianhydride to obtain polyamide acid resin; and carrying out vacuum negative pressure defoaming on the polyamide acid resin, and then carrying out curtain coating, stretching and imidization to obtain the corona-resistant polyimide film. The preparation method comprises the following steps: carrying out surface amination on a nano SiO2 filler, fixing an ATRP (Atom Transfer Radical Polymerization) initiator, and carrying out surface initiation on ATRP grafted polystyrene to obtain a nano SiO2 (at) PS filler with a core-shell structure; the nanofiller SiO2 (at) PS can effectively avoid the agglomeration phenomenon and is easier to disperse, meanwhile, the nanofiller SiO2 (at) PS has better compatibility in a polyamide acid solution system, and the prepared corona-resistant polyimide film has more uniform corona resistance, insulation and mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler and its preparation method. Background Technology

[0002] Variable frequency speed control motors are among the most critical components of high-speed trains and wind power equipment, and their safe operation is currently a hot research topic both domestically and internationally. Variable frequency motors primarily employ pulse width modulation (PWM) drive technology; however, the partial discharge caused by pulse overvoltage in PWM technology can lead to rapid aging of the inter-turn insulation material in variable frequency motors. Therefore, corona-resistant polyimide film plays a crucial role as the inter-turn insulation material for variable frequency motors.

[0003] Currently, the main approach to improving the corona resistance life of polyimide films is to modify them with nano-inorganic fillers. A specific example is a method for preparing corona-resistant polyimide films proposed in Chinese patent CN111087633B. This method introduces first and second inorganic fillers with different particle sizes into the film, combined with a chemical imidization method, to establish a uniform thermally conductive network within the film, thereby improving the film's corona resistance. Chinese patent CN112574447B discloses a method for preparing a corona-resistant polyimide film. The film includes a pure polyimide layer in the middle and hybrid layers disposed on the upper and lower surfaces of the pure polyimide layer. Each hybrid layer includes, from the surface of the pure polyimide layer outwards, a polyimide hybrid layer doped with aluminum oxide, a polyimide hybrid layer doped with silicon dioxide, and a polyimide hybrid layer doped with titanium dioxide. This invention uses a layer-by-layer roll coating method to prepare the corona-resistant polyimide film. The pure polyimide layer in the middle can maintain the excellent mechanical properties of the composite film. The hybrid layers of the three-layer composite film located on the upper and lower surfaces of the pure polyimide layer achieve a significantly improved corona resistance effect by using different inorganic particles in different layered distributions within the polyimide matrix and by forming a dense protective layer of inorganic particles in the three-dimensional space inside the film.

[0004] While these methods can improve the corona resistance lifetime of polyimide films to some extent, none of them fundamentally solve the problem of the difficulty in dispersing inorganic nanoparticles. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a corona-resistant polyimide film based on a core-shell structured nano-SiO2@PS filler and its preparation method, thereby solving the problems in the prior art.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for preparing a nano-SiO2@PS filler dispersion includes the following steps:

[0008] A coupling agent solution was prepared by mixing silane coupling agent, anhydrous ethanol and deionized water. The solution was then added to a dispersion of nano-SiO2 filler and heated and stirred under a nitrogen atmosphere to carry out a silanization reaction. Afterwards, the surface-aminated SiO2 was collected by centrifugation, washed and dried to obtain SiO2-NH2 nanofiller.

[0009] SiO2-NH2 nanofiller was dispersed in toluene, degassed by bubbling with nitrogen, stirred, and then cooled to 0°C. ATRP initiator was then added dropwise while stirring. After the reaction, the nanoparticles were collected, purified, and dried to obtain SiO2-Br.

[0010] SiO2-Br and styrene monomer were dispersed in a solvent; a catalyst was added, and the mixture was heated and stirred under nitrogen protection; after the reaction was completed, the mixture was cooled; the catalyst was removed by passing it through a neutral alumina column, and then the solid was separated by centrifugation; finally, the solid was dried to obtain SiO2@PS.

[0011] SiO2@PS was dispersed in a polar aprotic solvent to obtain a dispersion of nano-SiO2@PS filler.

[0012] Furthermore, the particle size of the nano-SiO2 filler in the nano-SiO2 filler dispersion is 10–100 nm.

[0013] Furthermore, the silane coupling agent is γ-aminopropyltriethoxysilane, and the mass of the silane coupling agent is 1% to 10% of the mass of the nano-SiO2 filler.

[0014] Furthermore, the ATRP initiator is 2-bromoisobutyryl bromide, with a mass of 50%-150% of the mass of the SiO2-NH2 filler.

[0015] Furthermore, the catalyst is CuBr or CuCl, and its amount is 5%-30% of the molar amount of styrene monomer.

[0016] A nano-SiO2@PS filler dispersion was prepared using the method described above.

[0017] A method for preparing a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler includes the following steps:

[0018] Under an inert atmosphere, the diamine is dissolved in a solvent to obtain a diamine solution;

[0019] The above-mentioned nano-SiO2@PS filler dispersion was added to a diamine solution, and after thorough stirring, dianhydride was added to obtain polyamic acid resin.

[0020] Polyamic acid resin is degassed under vacuum negative pressure, then cast, stretched, and imidized to obtain a corona-resistant polyimide film.

[0021] Furthermore, the mass of the nano-SiO2@PS filler accounts for 10%-40% of the total mass of the diamine and dianhydride.

[0022] Further, the diamine is one or more of the following: 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 1,3-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenyl sulfone, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane; the dianhydride is one or more of the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropyl)bisphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride.

[0023] A corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was prepared using the above-described preparation method.

[0024] The above-mentioned corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler is used in the preparation of inter-turn insulation material for variable frequency motors.

[0025] The above-mentioned nano-SiO2@PS filler dispersion is used in the preparation of polyimide films.

[0026] The beneficial effects of this invention are:

[0027] 1. This invention prepares SiO2@PS nanofillers with a core-shell structure by thoroughly modifying the surface of SiO2 nanoparticles and coating them with a polymer surface layer composed of polystyrene. This reduces the surface energy of inorganic nanoparticles, preventing their aggregation, and at the same time makes them more compatible in polyamic acid solution systems.

[0028] 2. The corona-resistant polyimide film prepared by the present invention based on core-shell structured nano-SiO2@PS filler has significantly improved mechanical properties, insulation properties and corona resistance due to the more uniform dispersion of filler. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1This is a schematic diagram of the process for preparing SiO2@PS according to the present invention. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] A method for preparing a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler includes the following steps:

[0033] S1, under an inert atmosphere, the diamine is dissolved in a solvent to obtain a diamine solution;

[0034] S2, Preparation of core-shell structured nano-SiO2@PS filler dispersion;

[0035] S3, add nano-SiO2@PS filler dispersion to diamine solution, stir thoroughly and then add dianhydride to obtain polyamic acid resin;

[0036] S4. Polyamic acid resin is degassed under vacuum negative pressure, then cast, stretched, and imidized to obtain a corona-resistant polyimide film.

[0037] In S1, the solvent is N,N-dimethylacetamide, the inert gas is nitrogen; the diamine is one or more of the following: 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 1,3-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenyl sulfone, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane.

[0038] like Figure 1 As shown in Figure S2, the specific steps for preparing the core-shell structured nano-SiO2@PS filler dispersion include:

[0039] S21, a coupling agent solution was prepared by mixing silane coupling agent, anhydrous ethanol, and deionized water (DIW) in a volume ratio of 20:72:8. This solution was then added to a nano-SiO2 filler dispersion, and the mixture was heated to 85°C and stirred for 2 hours under a nitrogen atmosphere to carry out a silanization reaction. The surface-aminated SiO2 was collected by centrifugation and washed several times with ethanol. The mixture was then dried in a vacuum oven at 60°C for 24 hours. The resulting amino-modified nano-SiO2 filler product was named SiO2-NH2 nanofiller.

[0040] S22, SiO2-NH2 nanofiller was dispersed in toluene for 10 min, degassed with nitrogen by bubbling, stirred for 30 min, and then cooled to 0 °C in an ice bath. Then, ATRP initiator was added dropwise. The mixture was stirred at 0 °C for 1 h, and then stirred at room temperature for 24 h. After the reaction, the nanoparticles were collected and purified by repeated centrifugation and washing with ethanol. The final product (denoted as SiO2-Br) was dried in a vacuum oven at 50 °C for 24 h.

[0041] S23, add SiO2-Br and styrene monomer to a reaction flask; add solvent and ultrasonically disperse evenly; purge with nitrogen to remove oxygen for more than 30 minutes; add catalyst; heat and stir under nitrogen protection; the reaction time is determined according to the required PS chain length; after the reaction is completed, cool to room temperature and expose to air to terminate the reaction; product purification: remove catalyst by passing the mixture through a neutral alumina column; centrifuge to separate the solid, repeatedly wash the homopolymer with THF to dissolve it, then precipitate with methanol and centrifuge, repeating several times; vacuum dry to obtain PS-grafted SiO2 (SiO2@PS);

[0042] S24, SiO2@PS is dispersed in a polar aprotic solvent (DMAc) to prepare a core-shell structured nano-SiO2@PS filler dispersion.

[0043] In S21, the particle size of the nano-SiO2 filler in the nano-SiO2 filler dispersion is 10-100 nm; the silane coupling agent is γ-aminopropyltriethoxysilane (KH550), and its mass is 1%-10% of the mass of the nano-SiO2 filler.

[0044] In S21, the preparation method of nano-SiO2 dispersion is as follows: the nano-SiO2 aqueous dispersion (30wt%) is ultrasonically diluted in anhydrous ethanol for 10min, and the volume ratio of water to anhydrous ethanol is 1:20 to 1:50.

[0045] In S22, the ATRP initiator 2-bromoisobutyryl bromide (BIBB) has a mass of 50%-150% of the mass of the SiO2-NH2 filler.

[0046] In S23, the solvent is toluene or anisole; the catalyst is CuBr or CuCl, and the amount used is 5%-30% of the molar amount of styrene monomer; the heating temperature of the heating and stirring reaction is 90-110℃, and the reaction time is 5-20h.

[0047] In S3, the dianhydride is: pyromellitic dianhydride, which is one or more combinations of: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropyl)bisphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride; the mass of the nano-SiO2@PS filler accounts for 10%-40% of the total mass of the diamine and dianhydride.

[0048] The technical solution of the present invention will be described in detail below through the following embodiments; wherein the nano-SiO2@PS filler dispersions in Examples 1-5 are prepared by the following methods:

[0049] (1) A coupling agent solution was prepared by mixing silane coupling agent, anhydrous ethanol, and deionized water (DIW) in a volume ratio of 20:72:8. Simultaneously, 25 g of nano-SiO2 aqueous dispersion (30 wt%) was ultrasonically diluted in 1000 mL of anhydrous ethanol for 10 min, and then 1 mL of KH-550 solution was added. The mixture was heated to 85 °C and stirred for 2 h to carry out silanization reaction under a nitrogen atmosphere. The surface-aminated SiO2 was collected by centrifugation and washed several times with ethanol. Then it was dried in a vacuum oven at 60 °C for 24 h. The obtained amino-modified nano-SiO2 filler product was named SiO2-NH2.

[0050] (2) 10 g of SiO2-NH2 nanofiller was dispersed in 50 ml of toluene for 10 min, degassed by bubbling with nitrogen, stirred for 30 min, and then cooled to 0 °C in an ice bath. Then, 10 ml of ATRP initiator was added dropwise. The mixture was stirred at 0 °C for 1 h, and then stirred at room temperature for 24 h. After the reaction, the nanoparticles were collected and purified by repeated centrifugation and washing with ethanol. The final product (denoted as SiO2-Br) was dried in a vacuum oven at 50 °C for 24 h.

[0051] (3) Add 5g SiO2-Br and 10.4g styrene monomer to a reaction flask. Add 100ml toluene and ultrasonically disperse evenly. Purge with nitrogen for more than 30 minutes to remove oxygen. Add 1.43g CuBr. Under nitrogen protection, heat to 90℃ and stir for 5h / 10h / 15h. After the reaction is completed, cool to room temperature and expose to air to terminate the reaction. Product purification: Remove copper catalyst by passing the mixture through a neutral alumina column. Centrifuge to separate the solid, wash repeatedly with THF to dissolve the homopolymer, then precipitate with methanol and centrifuge, repeating several times. Vacuum dry to obtain PS-grafted SiO2 (SiO2@PS-5 / SiO2@PS-10 / SiO2@PS-15, representing PS-grafted SiO2 obtained at reaction times of 5h, 10h and 15h, respectively);

[0052] (4) 5g SiO2@PS was dispersed in 45g DMAc and dispersed for 2h at a speed of 1500r / min using a high-speed disperser to obtain a nano SiO2@PS filler dispersion with a core-shell structure.

[0053] Example 1

[0054] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 70 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 32.7 g of SiO2@PS-5 filler dispersion (of which SiO2@PS accounts for 15% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0055] Example 2

[0056] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 60 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 43.6 g of SiO2@PS-5 filler dispersion (of which SiO2@PS accounts for 20% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0057] Example 3

[0058] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 50 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 54.5 g of SiO2@PS-5 filler dispersion (of which SiO2@PS accounts for 25% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0059] Example 4

[0060] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 50 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 54.5 g of SiO2@PS-10 filler dispersion (of which SiO2@PS accounts for 25% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0061] Example 5

[0062] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 50 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 54.5 g of SiO2@PS-15 filler dispersion (of which SiO2@PS accounts for 25% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0063] Comparative Example 1

[0064] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 70 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 32.7 g of conventional SiO2 filler dispersion (of which SiO2 accounts for 15% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0065] Comparative Example 2

[0066] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 60 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 43.6 g of conventional SiO2 filler dispersion (of which SiO2 accounts for 20% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0067] Comparative Example 3

[0068] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 50 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 54.5 g of conventional SiO2 filler dispersion (of which SiO2 accounts for 25% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0069] Comparative Example 4

[0070] Under a nitrogen atmosphere, 10 g of 4,4'-diaminodiphenyl ether and 50 ml of N,N-dimethylacetamide were added to a round-bottom flask and stirred until completely dissolved. Then, 54.5 g of KH550 modified SiO2 filler dispersion (of which SiO2 accounts for 25% of the total mass of diamine dianhydride) was added, and stirring was continued for 30 min. Then, 21.8 g of pyromellitic dianhydride was added in batches to obtain a polyamic acid solution with suitable viscosity. The resulting solution was then degassed under vacuum and coated on a coating machine. After staged heating at 100℃-150℃-200℃-250℃-300℃-350℃ for 30 min each, a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler was finally obtained.

[0071] Experimental testing;

[0072] The thickness, tensile strength, and insulation strength of the corona-resistant polyimide films prepared in Examples 1-5 and Comparative Examples 1-4 were tested according to GB / T13542-2021 standard.

[0073] The corona resistance of the corona-resistant polyimide films prepared in Examples 1-5 and Comparative Examples 1-4 was tested according to the NB / T31020-2011 standard. The test conditions were: test voltage 2.0KV, pulse frequency 20kHz, test temperature 23℃, pulse rise time 100ns, bipolar pulse square wave, 6mm cylindrical electrode. Nine sets of data were measured for each example, and the median value was taken.

[0074] The test results are shown in Table 1 below:

[0075] Table 1. Performance test data of various corona-resistant polyimide films

[0076]

[0077] As shown in Table 1, the corona resistance of Examples 1, 2, and 3, and Comparative Examples 1, 2, and 3, is significantly improved compared to conventional SiO2 fillers. Simultaneously, the mechanical and insulation properties also show a marked increase, indicating that SiO2@PS-5 has a clear performance advantage over conventional SiO2 fillers. Furthermore, the corona resistance of Examples 1, 2, and 3, and Comparative Examples 1, 2, and 3 initially increases significantly with increasing filler content, but then the improvement slows down. This is because a 15% filler content is insufficient to form a continuous filler layer within the film, resulting in lower corona resistance. However, when the filler content exceeds 20%, a continuous filler layer can be formed within the film, so higher filler content does not significantly improve the performance. Comparing Examples 3, 4, and 5, it is evident that the chain length of the polystyrene grafted onto the SiO2@PS filler surface also greatly affects the filler's dispersion and final performance. Shorter or longer chain segments result in poorer dispersion and performance compared to SiO2@PS-10. As can be seen from the test results of Comparative Example 4 in the table, the dispersibility of SiO2 filler modified by KH550 is better than that of conventional SiO2 filler, but far inferior to that of SiO2@PS. The test data can verify this conclusion.

[0078] The corona-resistant polyimide film of the present invention, through thorough modification of the surface of SiO2 nanoparticles and coating it with a polymer surface layer composed of polystyrene, not only avoids the aggregation of SiO2 nanoparticles, but also makes it more compatible in the polyamic acid solution system, significantly improving the mechanical properties, insulation properties and corona resistance of the film, and has broad application prospects in the fields of rail transit, wind power generation and new energy vehicles.

[0079] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0080] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for preparing a nano-SiO2@PS filler dispersion, characterized in that, Includes the following steps: A coupling agent solution was prepared by mixing silane coupling agent, anhydrous ethanol and deionized water. The solution was then added to a dispersion of nano-SiO2 filler and heated and stirred under a nitrogen atmosphere to carry out a silanization reaction. Afterwards, the surface-aminated SiO2 was collected by centrifugation, washed and dried to obtain SiO2-NH2 nanofiller. SiO2-NH2 nanofiller was dispersed in toluene, degassed by bubbling with nitrogen, stirred, and then cooled to 0°C. ATRP initiator was then added dropwise while stirring. After the reaction, the nanoparticles were collected, purified, and dried to obtain SiO2-Br. SiO2-Br and styrene monomer were dispersed in a solvent; a catalyst was added, and the mixture was heated and stirred under nitrogen protection; after the reaction was completed, the mixture was cooled; the catalyst was removed by passing it through a neutral alumina column, and then the solid was separated by centrifugation; finally, the solid was dried to obtain SiO2@PS. SiO2@PS was dispersed in a polar aprotic solvent to obtain a dispersion of nano-SiO2@PS filler.

2. The method for preparing a nano-SiO2@PS filler dispersion according to claim 1, characterized in that, The nano-SiO2 filler in the nano-SiO2 filler dispersion has a particle size of 10–100 nm.

3. The method for preparing a nano-SiO2@PS filler dispersion according to claim 1, characterized in that, The silane coupling agent is γ-aminopropyltriethoxysilane, and the mass of the silane coupling agent is 1% to 10% of the mass of the nano-SiO2 filler.

4. The method for preparing a nano-SiO2@PS filler dispersion according to claim 1, characterized in that, The ATRP initiator is 2-bromoisobutyryl bromide, and its mass is 50%-150% of the mass of SiO2-NH2 filler.

5. The method for preparing a nano-SiO2@PS filler dispersion according to claim 1, characterized in that, The catalyst is CuBr or CuCl, and its amount is 5%-30% of the molar amount of styrene monomer.

6. A nano-SiO2@PS filler dispersion, characterized in that, It was prepared using the preparation method described in any one of claims 1-5.

7. A method for preparing a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler, characterized in that, Includes the following steps: Under an inert atmosphere, the diamine is dissolved in a solvent to obtain a diamine solution; The nano-SiO2@PS filler dispersion described in claim 6 was added to a diamine solution, and after thorough stirring, dianhydride was added to obtain polyamic acid resin. Polyamic acid resin is degassed under vacuum negative pressure, then cast, stretched, and imidized to obtain a corona-resistant polyimide film.

8. The method for preparing a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler according to claim 7, characterized in that, The mass of the nano-SiO2@PS filler accounts for 10%-40% of the total mass of diamine and dianhydride.

9. The method for preparing a corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler according to claim 7, characterized in that, The diamine is one or more of the following: 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 1,3-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenyl sulfone, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane; the dianhydride is one or more of the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropyl)bisphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride.

10. A corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler, characterized in that, Prepared using the preparation method according to any one of claims 7-9.

11. The application of the corona-resistant polyimide film based on core-shell structured nano-SiO2@PS filler as described in claim 10 in the preparation of inter-turn insulation material for variable frequency motors.

12. The application of the nano-SiO2@PS filler dispersion according to claim 6 in the preparation of polyimide films.

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