Preparation method of polyimide film based on micro-nano filler modification

Abstract

The invention discloses a preparation method of a polyimide film based on micro-nano filler modification, which is characterized in that diamine and dianhydride are used as monomers, and a polyimide film material is prepared by micro-nano filler modification. According to the preparation method disclosed by the invention, the conductivity of the film is effectively regulated and controlled, so that the film is non-linearly changed under different electric field intensities, charges can be quickly and adaptively dissipated, the corona resistance and breakdown resistance of the film are remarkably improved, the micro-nano filler is uniformly and efficiently dispersed in the film, the electrical uniformity and reliability of the material are high, and the preparation method is suitable for industrial production. And the service life of the material in a high-pressure environment is prolonged.

Description

Technical Field

[0001] This invention relates to the field of dielectric composite materials technology, and in particular to a method for preparing polyimide films based on micro / nano filler modification. Background Technology

[0002] Polyimide film, as a high-performance polymer material, possesses excellent mechanical properties, high-temperature resistance, and chemical stability, and is widely used in many fields such as electronics, aerospace, and electrical insulation. However, the conductivity of traditional polyimide film exhibits a linear change under high electric fields. When exposed to strong electric field environments such as corona discharge and partial discharge, it is prone to energy accumulation, leading to problems such as material aging and breakdown, which limits its further application in high-voltage and ultra-high-voltage electrical equipment.

[0003] In recent years, modifying polyimides by introducing nanofillers to regulate their electrical properties has become a research hotspot. Appropriate micro / nanofillers can improve the bulk conductivity of materials and impart certain nonlinear conductivity characteristics to polyimide films, allowing them to exhibit adaptive conductivity changes under different electric field strengths. This effectively dissipates charge, improves the material's corona resistance and breakdown resistance, and meets the growing demands of high-end electrical applications. Furthermore, thermal imidization is a crucial step in polyimide film preparation, achieving dehydration, ring-closing, and cross-linking reactions of the polyamic acid solution through high-temperature treatment. However, the thermal imidization process also presents some technical challenges: inappropriate process conditions cannot ensure uniform distribution of fillers and solvents during thermal imidization, leading to localized defects and reducing the film's electrical breakdown resistance and stability. Summary of the Invention

[0004] To address the above shortcomings, this invention provides a method for preparing polyimide films modified with micro / nano fillers. This method effectively controls the film's conductivity, causing it to exhibit nonlinear changes under different electric field strengths. This allows for rapid and adaptive charge dissipation, significantly improving the film's corona resistance and breakdown resistance. Furthermore, the micro / nano fillers are uniformly and efficiently dispersed in the film, resulting in high electrical uniformity and reliability, and extending the material's lifespan under high-voltage environments. The specific technical solution is as follows: A method for preparing polyimide films based on micro / nano filler modification includes: S1. Place the micro / nano filler in an ultrasonic device, add a dispersant and an organic solvent, and perform ultrasonic dispersion to obtain a filler dispersion. S2. Weigh the dianhydride monomer and diamine monomer, and add them to the organic solvent under a nitrogen atmosphere. Under ultrasonic dispersion, stir continuously at a stirring rate of 80-200 rpm for 1-2 hours to fully dissolve the monomers and allow them to undergo polymerization to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.3-1.5 mL / min. The mixture is stirred continuously at a stirring rate of 150-400 rpm for 6-8 hours at 30-50℃ using a combination of mechanical and magnetic stirring to obtain a nanocomposite solution. S4. Cast the nanocomposite solution obtained in step S3 onto a glass or stainless steel plate to form a thin film. Place it in a vacuum oven and pre-dry it at 80-120℃ for 1-2 hours. Then, under a nitrogen atmosphere, raise the temperature to 150℃, 200℃, and 250℃ in 30-minute intervals, and finally reach 300℃. Hold each target temperature for 1 hour to complete the thermal imidization process. After cooling, peel the film off the glass or stainless steel plate to obtain a polyimide film. The mass fraction of the micro / nano filler in the polyimide film is 15-55%.

[0005] Preferably, the micro / nano filler is one or both of zinc oxide micro / nano particles and silicon carbide micro / nano whiskers, and the particle size distribution range of the micro / nano filler is 50 nm-1 μm.

[0006] More preferably, the micro / nano filler is selected from zinc oxide micro / nano particles (ZnO) and silicon carbide micro / nano whiskers (SiC), wherein the mass fraction of zinc oxide micro / nano particles in the polyimide film is 10%-40%, and the mass fraction of silicon carbide micro / nano whiskers in the polyimide film is 15%.

[0007] Preferably, the dispersant is selected from one of polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS), silane coupling agent, and sodium alkyl diphenyl ether disulfonate.

[0008] Preferably, the organic solvent is selected from one of N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), anhydrous ethanol, chloroform, dichloromethane, and tetrahydrofuran.

[0009] Preferably, the dianhydride monomer is selected from one or more of pyromellitic dianhydride (PMDA), biphenyl dianhydride (BPDA), benzophenone dianhydride, and diphenyl ether dianhydride.

[0010] Preferably, the diamine monomer is selected from one or more of p-phenylenediamine (PDA), 4,4'-diaminodiphenyl ether (ODA), m-phenylenediamine, and 4,4'-diaminodiphenyl sulfone.

[0011] Preferably, the molar ratio of the dianhydride monomer to the diamine monomer is 1.02:1.

[0012] Preferably, the thickness of the polyimide film is 20-30 μm.

[0013] Preferably, the process further includes post-treatment of the prepared polyimide film, including plasma treatment or chemical etching. The plasma treatment power is 80-400W and the treatment time is 3-20min. The chemical etching is specifically the reverse process of hydrolysis reaction, and the etching rate can reach 5-15μm / min in an etchant at 80°C.

[0014] Preferably, the purity of the nitrogen gas is not less than 99.9%.

[0015] Preferably, the ultrasonic dispersion operation time is controlled at 1-2 hours, and the ultrasonic power is set at 100-500W.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention provides a method for preparing polyimide films modified with micro / nano fillers. The polyimide films prepared by this method exhibit nonlinear conductivity. The method incorporates micro / nano fillers, utilizing their quantum confinement effect and interfacial polarization characteristics to effectively control the film's conductivity, causing it to exhibit nonlinear changes under different electric field strengths, enabling rapid and adaptive charge dissipation. The addition of nanofillers significantly improves the film's corona resistance and breakdown resistance. Furthermore, the uniform and efficient dispersion of the micro / nano fillers within the film results in high electrical uniformity and reliability, extending the material's lifespan under high-voltage environments.

[0017] 2. This invention first introduces a dispersant to prepare a filler dispersion and a polyimide precursor solution, respectively, to fully dissolve the filler and monomer, forming a stable dispersion system with suitable reactivity, viscosity, and uniformity. This not only improves the efficiency of the polymerization reaction but also ensures the regularity and stability of the polyimide molecular chains, laying the foundation for the subsequent thermal imidization process. The prepared filler dispersion is then slowly added dropwise to the polyimide precursor solution at a dropping rate of 0.3-1.5 mL / min, and continuously stirred at 150-400 rpm at 30-50°C to ensure that the micro-nano fillers are uniformly dispersed in the polyimide matrix, avoiding agglomeration and uneven dispersion, thus ensuring the stability and consistency of material properties and optimizing the nonlinear conductivity and breakdown voltage performance of the film product.

[0018] 3. By combining pre-baking treatment and gradient temperature curing process, high-quality thermal imidization of polyimide film is achieved, which efficiently promotes the complete conversion of polyamic acid into polyimide, ensures uniform distribution of filler and solvent during thermal imidization, avoids thermal stress that may cause film cracking or deformation, and also avoids local defects caused by excessively high or low temperatures, thus improving the thermal stability of the film and significantly enhancing its corona resistance time.

[0019] 4. Post-treatment of the thin film by plasma treatment or chemical etching further optimizes the surface properties of the thin film. Plasma treatment can remove surface impurities, adjust the surface roughness of the thin film, improve the surface energy of the thin film, and expand its application adaptability. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart of the preparation method of the present invention; Figure 2 This is a scanning electron microscope image of the polyimide film product prepared in Example 1 of the present invention; wherein the micro-nano fillers are uniformly dispersed in the polyimide matrix. Figure 3 The diagram shows the current density-electric field intensity of the polyimide film products obtained in Examples 1-5 of this invention. Detailed Implementation

[0022] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Unless otherwise defined, all technical terms used below have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of the present invention. Unless otherwise specifically stated, all raw materials, reagents, instruments, and equipment used in the present invention are commercially available or can be prepared by existing methods.

[0023] Example 1 This embodiment describes a method for preparing a polyimide film modified with micro / nano fillers, which specifically includes the following steps: S1. Select silicon carbide micron particles with an average particle size of 1.5 μm as micro-nano fillers. Place the micro-nano fillers in an ultrasonic device, add polyvinylpyrrolidone as a dispersant and anhydrous ethanol as an organic solvent. The ratio of silicon carbide, polyvinylpyrrolidone, and anhydrous ethanol is 2 g: 0.5 g: 50 ml. Ultrasonically disperse the filler for 1 h at a power of 300 W to obtain the filler dispersion. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Under ultrasonic dispersion at 300 W, stir continuously at a stirring rate of 120 rpm and a temperature of 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization reaction to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.8 mL / min. The mixture is stirred continuously at 250 rpm for 6 h at 50 °C to obtain a nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 30 μm using a doctor blade. The film is then placed in a vacuum oven and pre-baked at 100°C for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100°C to 150°C, 200°C, and 250°C at 30-minute intervals, finally reaching 300°C. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment at a plasma power of 200 W for 10 min to obtain the finished polyimide film. The mass fraction of silicon carbide microparticles in the polyimide film is 15%; it is named 15%SiC-PI.

[0024] Example 2 This embodiment describes a method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers, which specifically includes the following steps: S1. Zinc oxide nanoparticles with an average particle size of 50 nm and silicon carbide microparticles with an average particle size of 1.5 μm were selected as micro-nano fillers. The micro-nano fillers were placed in an ultrasonic device, and polyvinylpyrrolidone was added as a dispersant and anhydrous ethanol as an organic solvent. The ratio of zinc oxide, silicon carbide, polyvinylpyrrolidone and anhydrous ethanol was 1 g: 1.5 g: 0.8 g: 70 ml. The mixture was ultrasonically dispersed at 300 W for 1 h to obtain a filler dispersion. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Disperse the mixture by ultrasonication at 300 W, and then stir continuously at 120 rpm and 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.8 mL / min. The mixture is stirred continuously at 250 rpm for 6 h at 40 °C to obtain a nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 20 μm using a doctor blade. The film is then placed in a vacuum oven and pre-baked at 100°C for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100°C to 150°C, 200°C, and 250°C at 30-minute intervals, finally reaching 300°C. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment at a plasma power of 200 W for 10 min to obtain the finished polyimide film. The mass fraction of silicon carbide microparticles in the polyimide film is 15%, and the mass fraction of zinc oxide nanoparticles in the polyimide film is 10%; it is named 15%SiC-10%ZnO-PI.

[0025] Example 3 This embodiment describes a method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers, which specifically includes the following steps: S1. Zinc oxide nanoparticles with an average particle size of 50 nm and silicon carbide microparticles with an average particle size of 1.5 μm were selected as micro-nano fillers. The micro-nano fillers were placed in an ultrasonic device, and polyvinylpyrrolidone was added as a dispersant and anhydrous ethanol as an organic solvent. The ratio of zinc oxide, silicon carbide, polyvinylpyrrolidone and anhydrous ethanol was 2 g: 1.5 g: 1.0 g: 100 ml. The mixture was ultrasonically dispersed at 300 W for 1 h to obtain a filler dispersion. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Disperse the mixture by ultrasonication at 300 W, and then stir continuously at 120 rpm and 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.8 mL / min. The mixture is stirred continuously at 250 rpm for 6 h at 30 °C to obtain a nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 25 μm using a doctor blade. The film is then placed in a vacuum oven and pre-baked at 100°C for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100°C to 150°C, 200°C, and 250°C at 30-minute intervals, finally reaching 300°C. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment at a plasma power of 200 W for 10 min to obtain the finished polyimide film. The mass fraction of silicon carbide microparticles in the polyimide film is 15%, and the mass fraction of zinc oxide nanoparticles in the polyimide film is 20%; it is named 15%SiC-20%ZnO-PI.

[0026] Example 4 This embodiment of a method for preparing a polyimide film based on micro / nano filler modification specifically includes the following steps: S1. Zinc oxide nanoparticles with an average particle size of 50 nm and silicon carbide microparticles with an average particle size of 1.5 μm were selected as micro-nano fillers. The micro-nano fillers were placed in an ultrasonic device, and polyvinylpyrrolidone was added as a dispersant and anhydrous ethanol as an organic solvent. The ratio of zinc oxide, silicon carbide, polyvinylpyrrolidone and anhydrous ethanol was 3 g: 1.5 g: 1.2 g: 120 ml. The mixture was ultrasonically dispersed at 300 W for 1 h to obtain a filler dispersion. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Disperse the mixture by ultrasonication at 300 W, and then stir continuously at 120 rpm and 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.3-1.5 mL / min. The mixture is stirred continuously at 250 rpm for 6 h at 40 °C to obtain a nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 30 μm by a doctor blade. The film is then placed in a vacuum oven and pre-baked at 100°C for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100°C to 150°C, 200°C, and 250°C at 30-minute intervals, finally reaching 300°C. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment at a plasma power of 200 W for 10 min to obtain the finished polyimide film. The mass fraction of silicon carbide microparticles in the polyimide film is 15%, and the mass fraction of zinc oxide nanoparticles in the polyimide film is 30%. It is named 15%SiC-30%ZnO-PI.

[0027] Example 5 This embodiment of a method for preparing a polyimide film based on micro / nano filler modification specifically includes the following steps: S1. Zinc oxide nanoparticles with an average particle size of 50 nm and silicon carbide microparticles with an average particle size of 1.5 μm were selected as micro-nano fillers. The micro-nano fillers were placed in an ultrasonic device, and polyvinylpyrrolidone was added as a dispersant and anhydrous ethanol as an organic solvent. The ratio of zinc oxide, silicon carbide, polyvinylpyrrolidone and anhydrous ethanol was 4 g: 1.5 g: 1.5 g: 140 ml. The mixture was ultrasonically dispersed at 300 W for 1 h to obtain a filler dispersion. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Disperse the mixture by ultrasonication at 300 W, and then stir continuously at 120 rpm and 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.8 mL / min. The mixture is stirred continuously at 250 rpm for 6 h at 50 °C to obtain a nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 30 μm using a doctor blade. The film is then placed in a vacuum oven and pre-baked at 100°C for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100°C to 150°C, 200°C, and 250°C at 30-minute intervals, finally reaching 300°C. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment at a plasma power of 200 W for 10 min to obtain the finished polyimide film. The mass fraction of silicon carbide microparticles in the polyimide film is 15%, and the mass fraction of zinc oxide nanoparticles in the polyimide film is 40%. It is named 15%SiC-40%ZnO-PI.

[0028] Comparative Example 1 The thermal imidization process in this comparative example differs from that in Example 5. The specific steps are as follows: S1. Zinc oxide nanoparticles with an average particle size of 50 nm and silicon carbide microparticles with an average particle size of 1.5 μm were selected as micro / nano fillers. The micro / nano fillers were placed in an ultrasonic device, and polyvinylpyrrolidone was added as a dispersant and anhydrous ethanol as an organic solvent. The ratio of zinc oxide, silicon carbide, polyvinylpyrrolidone, and anhydrous ethanol was 4 g:1.5 g:1.5 g:140 ml. The mixture was ultrasonically dispersed at 300 W for 1 h to obtain a filler dispersion. The mass fraction of silicon carbide microparticles in the polyimide film was 15%, and the mass fraction of zinc oxide nanoparticles in the polyimide film was 40%. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Under ultrasonic dispersion conditions, stir continuously at a stirring rate of 120 rpm and a temperature of 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization reaction to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.8 mL / min. The mixture is stirred continuously at 250 rpm for 6 h at 50 °C to obtain a nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 30 μm by a doctor blade. The film is placed in a vacuum oven and heated at 50℃, 100℃, 150℃, 200℃, 250℃ and 300℃ for 1.5 h at 30-minute intervals to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment with a plasma power of 200W for 10 min to obtain the finished polyimide film.

[0029] Comparative Example 2 The difference between this comparative example and Example 5 lies in the preparation method of the nanocomposite solution, which specifically includes the following steps: S1. Four g of zinc oxide nanoparticles with an average particle size of 50 nm and 1.5 g of silicon carbide microparticles with an average particle size of 1.5 μm were selected as micro / nano fillers and added to 240 mL of N-methyl-2-pyrrolidone. The mixture was ultrasonically dispersed at 300 W for 1 h. Then, 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine were added, and the mixture was stirred continuously for 24 h under a nitrogen atmosphere and an ice bath to obtain a nanocomposite solution. The mass fraction of silicon carbide microparticles in the polyimide film was 15%, and the mass fraction of zinc oxide nanoparticles in the polyimide film was 40%. S2. The nanocomposite solution obtained in step S1 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 30 μm by a doctor blade. The film is then placed in a vacuum oven and pre-baked at 100°C for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100°C to 150°C, 200°C, and 250°C at 30-minute intervals, and finally reaches 300°C. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment with a plasma power of 200 W for 10 min to obtain the finished polyimide film.

[0030] Comparative Example 3 The difference between this comparative example and Example 5 is that the polyimide film was not modified. The specific steps are as follows: S1. Select zinc oxide nanoparticles with an average particle size of 50nm as micro-nano fillers. Place the micro-nano fillers in an ultrasonic device, add polyvinylpyrrolidone as a dispersant and anhydrous ethanol as an organic solvent. The ratio of zinc oxide, polyvinylpyrrolidone, and anhydrous ethanol is 5.5g:1.5g:140ml. Perform ultrasonic dispersion at 300W for 1h to obtain the filler dispersion. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Under ultrasonic dispersion conditions, stir continuously at a stirring rate of 120 rpm and a temperature of 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization reaction to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a drop rate of 0.8 mL / min. The mixture is stirred continuously at 250 rpm for 6 hours at 50°C using a combination of mechanical and magnetic stirring to obtain the nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 30 μm by a doctor blade. The film is then placed in a vacuum oven and pre-baked at 80-120℃ for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100℃ to 150℃, 200℃, and 250℃ at 30-minute intervals, finally reaching 300℃. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment at a plasma power of 200W for 10 min to obtain the finished polyimide film. The mass fraction of zinc oxide nanoparticles in the polyimide film is 55%; it is named 55%ZnO-PI.

[0031] Comparative Example 4 The difference between this comparative example and Example 5 is that the polyimide film was not modified. The specific steps are as follows: S1. Select silicon carbide micron particles with an average particle size of 1.5 μm as micro / nano fillers. Place the micro / nano fillers in an ultrasonic device, add 1.5 g of polyvinylpyrrolidone as a dispersant and 140 mL of anhydrous ethanol as an organic solvent. The ratio of silicon carbide, polyvinylpyrrolidone, and anhydrous ethanol is 5.5 g: 1.5 g: 140 mL. Perform ultrasonic dispersion at 300 W for 1 h to obtain the filler dispersion. S2. Weigh 10.2 mmol of phenyltetracarboxylic dianhydride and 10 mmol of p-phenylenediamine, and add them to 100 mL of N-methyl-2-pyrrolidone under a nitrogen atmosphere. Under ultrasonic dispersion conditions, stir continuously at a stirring rate of 120 rpm and a temperature of 30 °C for 1.5 h to allow the monomers to fully dissolve and undergo polymerization reaction to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a drop rate of 0.8 mL / min. The mixture is stirred continuously at 250 rpm for 6 hours at 50°C using a combination of mechanical and magnetic stirring to obtain the nanocomposite solution. S4. The nanocomposite solution obtained in step S3 is cast into a thin film on a glass or stainless steel plate using a casting method. The film thickness is controlled to be 30 μm by a doctor blade. The film is then placed in a vacuum oven and pre-baked at 80-120℃ for 1.5 h. Then, under a nitrogen atmosphere, the temperature is increased sequentially from 100℃ to 150℃, 200℃, and 250℃ at 30-minute intervals, finally reaching 300℃. Each target temperature is held for 1 hour to complete the thermal imidization process. After cooling, the film is peeled off from the glass or stainless steel plate and then subjected to plasma treatment at a plasma power of 200W for 10 min to obtain the finished polyimide film. The mass fraction of silicon carbide nanoparticles in the polyimide film is 55%; it is named 55%SiC-PI.

[0032] 1. Conductivity test The properties of the polyimide film products prepared in Examples 1-5 above were tested. The polyimide film obtained by this invention exhibits nonlinear conductivity and excellent nonlinear coefficient. The conductivity of the film is stable under low electric field, and the conductivity increases rapidly and nonlinearly under high electric field. The test results are shown in the appendix. Figure 3 .

[0033] Current density measures the amount of charge flowing per unit area per unit time and is used to evaluate electrical conductivity. When the nonlinearity coefficient α < 1, the current density increases nonlinearly with the electric field strength. When the nonlinearity coefficient α > 1, the current density increases superlinearly with the electric field strength.

[0034] from Figure 3 It can be seen that the current density-electric field intensity diagram of the binary SiC / PI composite material with 15% filler content consistently maintains a relatively small current density range, exhibiting stable high resistance characteristics. When zinc oxide nanoparticles of 10%-40% and silicon carbide micron-particle fillers of 15% are added, the current density-electric field intensity diagram of the sample is close to that of the binary SiC / PI composite material with 15% filler content at low field strengths, while at high field strengths, the slope of the curve increases significantly, showing significant nonlinear conductivity characteristics; at the same time, the current density of the sample is also much larger than that of the binary SiC / PI composite material with 15% filler content and the sample with low filler content.

[0035] 2. Breakdown voltage and corona resistance test The performance of the polyimide film products prepared in Example 5 and Comparative Examples 1-4 was tested, and the test results are shown in Table 1 below.

[0036] The corona resistance test conditions in Table 1 are as follows: test voltage: 2kV, frequency: 25kHz, pulse rise time: 50ns, temperature: 150±2°C, bipolar pulse square wave, 6mm cylindrical electrode, and the median value of the test results. Breakdown voltage performance testing was conducted according to GB / T13542.2-2009. The polyimide film product prepared in this embodiment of the invention possesses both excellent breakdown voltage and corona resistance performance, with the corona resistance time being improved by more than 50% compared to the comparative polyimide film.

[0037] Table 1 Breakdown voltage and corona resistance time of polyimide films In summary, this invention provides a method for preparing polyimide films modified with micro / nano fillers. The polyimide films prepared by this method have nonlinear conductivity characteristics. The preparation method of this invention introduces micro / nano fillers, and utilizes the quantum confinement effect and interfacial polarization characteristics of the micro / nano fillers to effectively control the conductivity of the film, making it exhibit nonlinear changes under different electric field strengths. This allows for rapid and adaptive charge dissipation, significantly improving the corona resistance and breakdown resistance of the film. Furthermore, the micro / nano fillers are uniformly and efficiently dispersed in the film, resulting in high electrical uniformity and reliability of the material, and extending the service life of the material under high-pressure environments. This invention first introduces a dispersant to prepare a filler dispersion and a polyimide precursor solution, respectively, to fully dissolve the filler and monomer, forming a stable dispersion system with suitable reactivity, viscosity, and uniformity. This not only improves the efficiency of the polymerization reaction but also ensures the regularity and stability of the polyimide molecular chains, laying the foundation for the subsequent thermal imidization process. The prepared filler dispersion is then slowly added dropwise to the polyimide precursor solution at a dropping rate of 0.3-1.5 mL / min, and the mixture is continuously stirred at a stirring rate of 150-400 rpm using a combination of mechanical and magnetic stirring at 30-50°C. This ensures that the micro-nano fillers are uniformly dispersed in the polyimide matrix, avoiding agglomeration and uneven dispersion, thus guaranteeing the stability and consistency of the material properties and optimizing the breakdown voltage performance of the film product. By combining pre-baking and gradient temperature curing processes, high-quality thermal imidization of polyimide films was achieved. This efficiently promotes the complete conversion of polyamic acid into polyimide, ensuring uniform distribution of fillers and solvents during thermal imidization. It avoids thermal stress that could lead to film cracking or deformation, and also prevents localized defects caused by excessively high or low temperatures, thus improving the film's thermal stability and significantly enhancing its corona resistance time. Post-treatment with plasma processing or chemical etching further optimizes the film's surface properties. Plasma processing removes surface impurities, adjusts surface roughness, increases surface energy, and expands its application adaptability.

[0038] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for preparing polyimide films modified with micro / nano fillers, characterized in that, include: S1. Place the micro / nano filler in an ultrasonic device, add a dispersant and an organic solvent for ultrasonic dispersion to obtain a filler dispersion; S2. Weigh the dianhydride monomer and diamine monomer, and add them to the organic solvent under a nitrogen atmosphere. Under ultrasonic dispersion, stir continuously at a stirring rate of 80-200 rpm for 1-2 hours to fully dissolve the monomers and allow them to undergo polymerization to obtain a polyimide precursor solution. S3. The filler dispersion obtained in step S1 is slowly added dropwise to the polyimide precursor solution obtained in step S2 at a dropping rate of 0.3-1.5 mL / min. The mixture is stirred continuously at 150-400 rpm for 6-8 hours at 30-50℃ to obtain a nanocomposite solution. S4. Cast the nanocomposite solution obtained in step S3 onto a glass or stainless steel plate to form a thin film. Place it in a vacuum oven and pre-dry it at 80-120℃ for 1-2 hours. Then, under a nitrogen atmosphere, raise the temperature to 150℃, 200℃, and 250℃ in 30-minute intervals, and finally reach 300℃. Hold each target temperature for 1 hour to complete the thermal imidization process. After cooling, peel the film off the glass or stainless steel plate to obtain a polyimide film. The mass fraction of the micro / nano filler in the polyimide film is 15-55%.

2. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that, The micro / nano filler is one or both of zinc oxide micro / nano particles and silicon carbide micro / nano whiskers, and the particle size distribution range of the micro / nano filler is 50nm-1.5μm.

3. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 2, characterized in that, The micro / nano fillers are zinc oxide micro / nano particles and silicon carbide micro / nano crystals. The mass fraction of zinc oxide micro / nano particles in the polyimide film is 10%-40%, and the mass fraction of silicon carbide micro / nano crystals in the polyimide film is 15%.

4. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that, The dispersant is selected from one of polyvinylpyrrolidone, sodium dodecyl sulfate, silane coupling agent, and sodium alkyl diphenyl ether disulfonate.

5. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that, The organic solvent is selected from one of N,N-dimethylformamide, N-methyl-2-pyrrolidone, anhydrous ethanol, chloroform, dichloromethane, and tetrahydrofuran.

6. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that, The dianhydride monomer is selected from one or more of pyromellitic dianhydride, biphenyl dianhydride, benzophenone dianhydride, and diphenyl ether dianhydride.

7. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that, The diamine monomer is selected from one or more of p-phenylenediamine, 4,4'-diaminodiphenyl ether, m-phenylenediamine, and 4,4'-diaminodiphenyl sulfone.

8. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that, The molar ratio of the dianhydride monomer to the diamine monomer is 1.02:

1.

9. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that, The thickness of the polyimide film is 20-30 μm.

10. The method for preparing a nonlinear conductivity polyimide film modified with micro / nano fillers according to claim 1, characterized in that... The prepared polyimide film is then subjected to post-treatment, which includes plasma treatment or chemical etching. The plasma treatment power is 80-400W and the treatment time is 3-20min. The chemical etching is specifically the reverse process of hydrolysis reaction, and the etching rate can reach 5-15μm / min in an etchant at 80°C.

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