A high surface-resistance electric strength polyimide based on surface natural micropore structure regulation and preparation method and application thereof

Abstract

The application discloses a kind of high surface natural micropore structure regulation-based high surface withstand voltage polyimide and preparation method and application thereof, belong to high voltage insulating material technical field, after drying diamine is dissolved in solvent A, then add dianhydride, after stirring uniformly, obtain polyamide acid solution;Add pore forming agent again, high-speed stirring is uniformly dispersed, obtain mixed viscous solution C;After coating, drying, imidization high temperature heating, and natural cooling, immerse in liquid B and extract pore forming agent, after drying, obtain high surface natural micropore structure regulation-based high surface withstand voltage polyimide.The polyimide surface has natural pore structure, surface is along the surface of electric field, and the through-hole structure is densely distributed along the surface of electric field, and the electron is easy to drop into the through-hole structure in the jumping multiplication process when flashover, and it is difficult to return to the surface, so that the secondary electron multiplication process is inhibited, the probability of forming secondary electron avalanche is reduced, so as to realize the improvement of surface withstand voltage.

Description

Technical Field

[0001] This invention belongs to the field of high voltage insulating materials technology, specifically relating to a high surface electrical strength polyimide based on the natural microporous structure of its surface, its preparation method, and its application. Background Technology

[0002] Polyimide, a new generation of thermoplastic polymer material with naturally rigid chains, possesses excellent comprehensive properties such as resistance to high and low temperatures, high mechanical strength, wear resistance, radiation resistance, good chemical stability, and dielectric properties, making it highly commercially valuable and widely used in key fields such as electrical insulation, microelectronics, nuclear energy, and aerospace. However, in vacuum insulation systems, the surface dielectric strength of polyimide is far lower than its breakdown strength, making it highly susceptible to surface flashover, a problem that has become one of the key technological bottlenecks restricting the development of spacecraft, pulsed power equipment, high-energy particle accelerators, and semiconductor manufacturing and processing equipment towards high power, miniaturization, and integration. Therefore, developing a polyimide with high surface dielectric strength is of great significance for improving the insulation reliability and safety of industrial systems.

[0003] Vacuum surface flashover is a low-pressure plasma discharge phenomenon along the surface of a solid medium, triggered by secondary electron avalanches. The process can be divided into an initial stage (cathode-induced electron emission), a development stage (secondary electron multiplication), and a breakdown stage (gas desorption ionization and breakdown). Therefore, to improve the surface dielectric strength of polyimide in a vacuum, it is necessary to suppress the development of at least one stage. Based on this theory, researchers both domestically and internationally have proposed numerous suppression methods. Patent CN105405545B achieves the purpose of adjusting the secondary electron emission multiplication process of the insulator by mechanically machining grooves on the insulator surface and embedding electrodes in the grooves. However, this method is complex and expensive, making large-scale industrial production difficult. Patent CN115910497A uses 3D printing technology to obtain insulators with multiple through-holes on the surface and inside, allowing electrons to pass through the through-holes and fall into the cavity during flashover, thereby suppressing secondary electron avalanches and improving the surface dielectric strength of the insulator. However, 3D printing is complex, and the preparation process of polyimide materials, especially polyimide films, is not suitable for 3D printing. Therefore, this approach is not suitable for the preparation of polyimides with high surface electrostatic resistance. Currently, methods such as matrix nanocomposite modification and surface treatment (e.g., fluorination, plasma treatment, composite coating, film deposition, etc.) are also used to suppress surface flashover, but these methods have limited suppression effects and are prone to failure. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the present invention aims to provide a high surface electrical strength polyimide based on the control of the natural microporous structure of the surface, its preparation method and application, so as to solve the technical problem of how to effectively suppress the development of secondary electron avalanche by controlling the natural microporous structure of the polyimide surface, thereby improving its surface electrical strength.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] This invention discloses a method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface, comprising: dissolving a dried diamine in solvent A, adding dianhydride, stirring until homogeneous to obtain a polyamic acid solution; adding a pore-forming agent, stirring at high speed until uniformly dispersed to obtain a mixed viscous solution C; coating the solution, drying it, imidizing it at high temperature, and then naturally cooling it; immersing it in liquid B to extract the pore-forming agent; and drying it to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface.

[0007] Preferably, the molar ratio of dianhydride to diamine is (0.98-1.05):1;

[0008] When adding dianhydride, add it in small amounts multiple times, with each addition spaced 15-30 minutes apart;

[0009] Stir under nitrogen or air atmosphere until a uniform yellow transparent polyamic acid solution is formed;

[0010] The solid content of the polyamic acid solution is 15-20 wt%.

[0011] Preferably, the diamine is one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminobiphenyl, 2,2'-bis(4-aminophenyl)hexafluoropropane, 4,4'-diaminophenyl sulfone, hexamethylenediamine, 2,2'-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 4,4'-(4,4'-isopropylidene diphenyl-1,1'-dimethyldioxy)diphenylamine and related derivatives.

[0012] Preferably, the dianhydride is one or more of the following: pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropene) phthalic anhydride, 4,4'-terephthalodioxydiphthalic anhydride, 4,4'-diphenyl ether dianhydride, and related derivatives.

[0013] Preferably, solvent A is any one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone; liquid B is ethanol or methyl ketone; liquid B can fully dissolve the pore-forming agent without damaging or dissolving the polyimide.

[0014] Preferably, the pore-forming agent is any one of dodecyl dimethyl ammonium bromide, dibutyl phthalate, glycerol, polyethylene glycol, polyethylene oxide, and polyhedral oligomeric silsesquioxane;

[0015] The mass of the pore-forming agent is 5%-40% of the mass of the mixed viscous solution C;

[0016] When the thermal decomposition temperature of the pore-forming agent is not higher than 440℃, imidization is carried out at high temperature and then naturally cooled to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface.

[0017] When the thermal decomposition temperature of the pore-forming agent is higher than 440℃, the imidization is carried out at high temperature and then naturally cooled. The pore-forming agent is then extracted by immersion in liquid B and dried to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface.

[0018] Preferably, the high-speed stirring rate is 800-1400 rpm; the high-speed stirring time is 0.5 h.

[0019] Preferably, the imidization high-temperature heating adopts a gradient heating and holding method, starting from 80°C, with a heating rate of 5°C / min, heat treatment every 60°C for 40 minutes, heating to 300-450°C, heat treatment for 6 hours, and then cooling to room temperature; if the thermal decomposition temperature of the pore-forming agent is not higher than 440°C, then the thermal decomposition temperature of the pore-forming agent plus 10°C is set as the maximum temperature.

[0020] The present invention also discloses a polyimide with high surface electrical strength based on the natural microporous structure of the surface. The polyimide with high surface electrical strength based on the natural microporous structure of the surface is a surface along the electric field, and the surface along the electric field is densely covered with micron-sized pores.

[0021] This invention also discloses the application of the high surface electrical strength polyimide prepared by the above-mentioned method based on the control of the natural microporous structure of the surface in the preparation of insulating components for spacecraft, vacuum circuit breakers, high-energy particle accelerators, pulsed power equipment, and semiconductor manufacturing and processing equipment.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] This invention discloses a method for preparing a high surface electrical strength polyimide based on the natural microporous structure of its surface. The method involves adding a pore-forming agent to a polyamic acid precursor solution, followed by basic chemical methods such as imidization at high temperature or liquid extraction. The resulting polyimide exhibits a porous surface along an electric field. When an electric field is applied to the polyimide surface, the presence of through-holes along the electric field allows primary electrons generated at the cathode and secondary electrons generated during their development to fall into these through-holes as they move towards the anode. Furthermore, the polyimide itself is porous, making it difficult for these electrons to escape and return to the surface, thus suppressing secondary electron avalanche. Structurally, the natural through-holes on the polyimide surface can capture electrons avalanche that propagate towards the anode along the electric field direction during flashover development. Since electrons falling into the through-holes are unlikely to return to the surface, this weakens electron avalanche development and significantly improves the vacuum surface electrical strength of the polyimide. Meanwhile, due to the porous nature of both the surface and interior of polyimide, its dielectric constant is significantly lower than that of ordinary polyimide resins, exhibiting superior application advantages in the fields of communications and microelectronics. Since it directly produces natural polyimide with a microporous surface, the preparation process is simple and convenient, allowing for one-step molding, making it highly suitable for large-scale mass production in industrial production lines. By using pore-forming agent components of different sizes, contents, and concentrations, the pore size and porosity can be controlled. This method allows for the selection of appropriate pore-forming agents and ratios according to industrial needs, controlling the degree of secondary electron avalanche development and regulating the surface dielectric strength of the polyimide.

[0024] This invention also discloses a high surface electrical strength polyimide prepared by the above-described method, based on the regulation of the natural microporous structure of the surface. This high surface electrical strength polyimide, based on the regulation of the natural microporous structure of the surface, has a densely distributed micron-sized pore structure along the surface of the electric field. During flashover, electrons easily fall into the pore structure during their electron multiplication process and are unlikely to return to the polyimide surface. This suppresses the secondary electron multiplication process and reduces the probability of secondary electron avalanche, thereby improving the surface electrical strength of the polyimide. The microporous structure size of the surface of the high surface electrical strength polyimide based on the regulation of the natural microporous structure of the surface is on the order of micrometers. If the aperture size is too small, the probability of electrons falling into the aperture in the electric field decreases, and electrons cannot effectively suppress secondary electron avalanche by bypassing the aperture. If the aperture size is too large, it will damage the mechanical strength of the polyimide and reduce its industrial applicability.

[0025] This invention also discloses the application of the high surface electrical strength polyimide prepared by the above-mentioned method, based on the surface natural microporous structure, in the fabrication of insulating components for spacecraft, vacuum circuit breakers, high-energy particle accelerators, pulsed power equipment, and semiconductor manufacturing and processing equipment. Spacecraft operate in extreme space environments, placing extremely high demands on insulating materials. The high surface electrical strength polyimide of this invention, based on the surface natural microporous structure, effectively suppresses secondary electron avalanches, significantly improving the material's electrical resistance under vacuum and radiation environments, thereby ensuring the stability and reliability of spacecraft electrical systems. Furthermore, its low dielectric constant helps reduce signal transmission losses and improve communication efficiency. Vacuum circuit breakers are critical protection devices in power systems, and their performance directly affects the safe and stable operation of the power grid. Using the high surface electrical strength polyimide of this invention, based on the surface natural microporous structure, as an insulating component can significantly improve the breaking capacity and arc resistance of circuit breakers, extend their service life, and reduce maintenance costs. Simultaneously, the porous structure also aids in heat dissipation, improving the thermal stability of the equipment. In high-energy particle accelerators, insulating components need to withstand extremely high electric field strength and radiation dose. The high surface electrical strength polyimide of this invention, based on a surface natural microporous structure, ensures the stability and safety of accelerators under long-term high-load operation due to its excellent electrical resistance and radiation resistance, providing strong support for scientific research and technological applications. Pulsed power equipment has wide applications in military, scientific research, and other fields, and its insulating components need to withstand extremely short pulses of high voltage and strong current. The high surface electrical strength polyimide of this invention, based on a surface natural microporous structure, effectively suppresses electron avalanche development due to its special microporous structure, improving the electrical strength and reliability of insulating components and ensuring the stable operation of pulsed power equipment. In semiconductor manufacturing, the purity and electrical resistance of insulating components are crucial. The high surface electrical strength polyimide of this invention, based on a surface natural microporous structure, not only has excellent electrical resistance, but its porous structure also helps to adsorb and fix impurities, improving material purity. Furthermore, its low dielectric constant helps reduce signal interference between semiconductor devices, improving integration and performance. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the high surface electrical strength polyimide based on the natural microporous structure of the surface provided by the present invention;

[0027] Figure 2 This is a flowchart of the method for preparing high surface electrical strength polyimide based on the control of natural surface microporous structure provided in Embodiment 1 of the present invention;

[0028] Figure 3This is a SEM microstructure image of a polyimide surface with high surface electrical strength based on the natural microporous structure of the surface, according to Embodiment 1 of the present invention. Detailed Implementation

[0029] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0030] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. All reagents and raw materials used in this invention are available through conventional commercial channels, and unless otherwise specified, they are used in accordance with conventional methods in the art or as per product instructions. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described in this patent are for illustrative purposes only.

[0032] The present invention will now be described in further detail with reference to the accompanying drawings:

[0033] According to one aspect of the present invention, a high surface electrical strength polyimide based on the natural microporous structure of the surface is provided. The polyimide is prepared by adding a pore-forming agent to a precursor polyamic acid solution and then performing imidization, high-temperature heating, or liquid extraction, or other basic chemical methods. The polyimide has a porous structure on its surface, which is a surface along an electric field.

[0034] Specifically, the method for preparing high surface electrical strength polyimide based on the natural microporous structure of the surface provided by this invention is simple. A pore-forming agent is added to a polyamic acid precursor solution, and then the pore-forming agent is thermally decomposed by high-temperature heating or extracted by solvent extraction to prepare a porous polyimide material with a densely packed pore structure on the surface. When an electric field is applied to the surface of this polyimide, due to the pore structure along the electric field, primary electrons generated by the cathode and secondary electrons generated during their development will fall into the pores as they move towards the anode. Furthermore, the interior of the polyimide is also porous, making it difficult for electrons falling into the pores to escape the porous structure and return to the surface, thereby suppressing secondary electron avalanche and ultimately obtaining a high surface electrical strength polyimide based on the natural microporous structure of the surface.

[0035] Optionally, the thickness of the high surface electrical strength polyimide controlled by the natural microporous structure of the surface is not fixed. Preferably, the thickness is in the range of 25-250 μm of commonly used industrial insulating materials.

[0036] Optionally, the size of the surface micropore structure is determined by the size and content of the added solid pore-forming agent or the concentration and content of the added liquid pore-forming agent, thereby achieving controllable adjustment of the surface micropores. The size of the polyimide surface micropore structure is on the nanometer or micrometer scale. Preferably, the size of the high surface electrical conductivity polyimide surface micropore structure controlled by the natural surface micropore structure is on the micrometer scale. If the aperture size is too small, the probability of electrons falling into the aperture due to the electric field decreases, and electrons cannot fall into it while bypassing the aperture, thus failing to effectively suppress secondary electron avalanche. If the aperture size is too large, it will damage the mechanical strength of the polyimide and reduce its industrial applicability.

[0037] Optionally, the polyimide matrix is ​​obtained by polyamic acid solution after diamine and dianhydride undergo condensation acylation reaction in solvent A.

[0038] Optionally, the diamine is selected from one or more of 4,4'-diaminodiphenyl ether (ODA), p-phenylenediamine (PDA), 4,4'-diaminobiphenyl (BPA), 2,2'-bis(4-aminophenyl)hexafluoropropane (6FAP), 4,4'-diaminophenyl sulfone (DDS), hexamethylenediamine (HMDA), 2,2'-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (6F-BAPP), 4,4'-(4,4'-isopropylidene diphenyl-1,1'-dimethyldioxy)diphenylamine (IDDA) and related derivatives;

[0039] The dianhydride is selected from one or more of the following: pyromellitic dianhydride (PMDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-(hexafluoroisopropene) phthalic anhydride (6FDA), 4,4'-terephthalodioxydiphthalic anhydride (HQDA), 4,4'-diphenyl ether dianhydride (ODPA), and related derivatives.

[0040] Solvent A is selected from any one of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP).

[0041] Optionally, the pore-forming agent is a liquid or solid chemical substance that is insoluble in polyamic acid solution and does not react with it. Preferably, the pore-forming agent is selected from bis(dodecyl dimethyl ammonium bromide) (DDAB), dibutyl phthalate (DBP), glycerol (Gly), polyethylene glycol (PEG), polyethylene oxide (PEO), or polyhedral oligomeric silsesquioxane (POSS), etc.

[0042] Optionally, liquid B is ethanol or methyl ketone; liquid B can fully dissolve the pore-forming agent, while liquid B will not damage or dissolve the polyimide portion; the amount of liquid B added ensures that the polyimide can be completely submerged.

[0043] According to another aspect of the present invention, a method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface is provided, comprising the following steps:

[0044] Step 1: Sublimate the diamine and dianhydride separately in a vacuum drying oven before use;

[0045] Step 2: Weigh dianhydride and diamine using an analytical balance, ensuring a molar ratio of dianhydride to diamine of (0.98-1.05):1. Add the diamine to a three-necked flask, weigh solvent A and pour it into the flask. Stir until the diamine is completely dissolved. Then, add the dianhydride to the flask in several batches, with an interval of 15-30 minutes between each addition. Continue stirring under nitrogen or air until a homogeneous, transparent, yellow polyamic acid solution is formed. The solid content of the polyamic acid solution should be 15-20 wt%.

[0046] Step 3: Weigh a certain amount of the pore-forming agent using an analytical balance and add it to the polyamic acid solution obtained in Step 2. Then, stir the solution at 800-1400 rpm under vacuum for 0.5 hours to ensure that the pore-forming agent is uniformly dispersed in the polyamic acid solution, thus obtaining a mixed viscous solution C. The amount of pore-forming agent used is 5%-40% of the sum of the mass of the pore-forming agent and the mass of the polyamic acid solution; that is, the mass of the pore-forming agent is 5%-40% of the mass of the mixed viscous solution C.

[0047] Step 4: Take the mixed viscous solution C obtained in Step 3, spread it evenly on a clean glass plate, coat it with a vacuum coating machine using a scraper, then place it in a vacuum drying oven to dry, and then place it in a high-temperature drying oven to complete the imidization high-temperature heating.

[0048] Step 5: After natural cooling, remove the polyimide material and immerse it in liquid B to extract the dispersed phase—the pore-forming agent. Finally, dry the material to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface. Liquid B can be ethanol or methyl ketone, etc. The amount of liquid B is unlimited, but it is required that the polyimide is completely submerged, and that liquid B can fully dissolve the pore-forming agent without damaging or dissolving the polyimide.

[0049] Specifically, the initial liquid volume in steps three and four can be selected according to actual needs. If a larger membrane area is required, take more solution; otherwise, take less.

[0050] Specifically, in step four, the high-temperature imidization employs a gradient heating and holding method. Starting from 80℃, heat treatment is performed at 60℃ increments (140, 200, 260, ...) for 40 minutes until the required maximum temperature is reached. After heat treatment for 6 hours, the temperature is cooled to room temperature. The heating rate throughout the process is 5℃ / min, and the maximum temperature should not exceed 450℃. If the thermal decomposition temperature of the pore-forming agent component is not higher than 440℃, then the thermal decomposition temperature of the pore-forming agent plus 10℃ is set as the maximum temperature; otherwise, the maximum temperature value should not exceed 450℃ and should not be lower than 300℃.

[0051] Specifically, step five is optional. If the thermal decomposition temperature of the pore-forming agent is not higher than 440°C, then step four is sufficient to obtain polyimide with high surface electrical strength based on the natural microporous structure of the surface; if the thermal decomposition temperature of the pore-forming agent is higher than 440°C, then step five must be completed, i.e., the pore-forming agent is extracted using liquid B.

[0052] Specifically, after coating, the above preparation method involves drying the film in a vacuum oven for a period of time. This serves two purposes: first, to remove gases mixed into the surface and interior of the polyamic acid film; and second, to thermocure the polyamic acid composite film so that it can maintain its shape in subsequent processes, thus laying the foundation for the material to possess good mechanical and insulating properties.

[0053] According to another aspect of the present invention, the present invention also provides the application of the above-described high surface electrical strength polyimide based on the surface natural microporous structure regulation, or the high surface electrical strength polyimide based on the surface natural microporous structure regulation obtained by the above preparation method, in insulating components in high-energy particle accelerators, spacecraft, vacuum circuit breakers, pulsed power equipment, and semiconductor manufacturing and processing equipment.

[0054] Figure 1 This is a schematic diagram of the high surface electrical strength polyimide based on the natural microporous structure of the surface provided by the present invention; as shown. Figure 1 As shown, the high surface electrical strength polyimide based on the natural microporous structure of the surface is an opaque yellow film due to its porous structure.

[0055] Example 1

[0056] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0057] 1) Weigh PMDA and ODA in a molar ratio of 1.02:1, measure DMAc and pour it into a three-necked flask, then add ODA and stir until dissolved. After the ODA is completely dissolved, add PMDA to the three-necked flask in four portions, with an interval of 15 minutes between each addition, stirring until a homogeneous, yellow, transparent polyamic acid (PAA) solution is obtained, with a solid content of 20 wt%.

[0058] 2) Take a certain amount of pore-forming agent dibutyl phthalate (DBP) and add it to the PAA solution obtained in step 1) to make the mass percentage of DBP 25%. Stir at 800 rpm for 0.5 h to ensure that DBP is uniformly dispersed. Coat the film on a clean glass plate with a scraper.

[0059] 3) Place the wet film in a vacuum oven and vacuum it at room temperature for 2 hours. Then place it in a high-temperature forced-air drying oven for imidization. The gradient heating process is as follows: heat to 80℃, 140℃, 200℃, 260℃ and 320℃ at a rate of 5℃ / min. Hold at each temperature for 40 minutes. Finally, heat to 350℃ and hold for 6 hours. Finally, cool to room temperature to remove the film. This will give you a polyimide with high surface electrical strength based on the natural microporous structure of the surface.

[0060] Figure 2This is a flowchart of the preparation method of high surface electrical strength polyimide based on the control of natural surface microporous structure provided in Embodiment 1 of the present invention. As shown in the diagram, a polyamic acid (PAA) solution is first prepared as the basis of the preparation process. A pore-forming agent is added to the PAA solution to control the microporous structure of the polyimide film surface. The mixed solution is uniformly coated onto a substrate to form a film. The coated film is placed in a vacuum environment to remove internal air bubbles and ensure the uniformity and density of the film. The imidization temperature is determined based on whether the thermal decomposition temperature T of the pore-forming agent is greater than 440°C. If T is greater than 440°C, the maximum imidization temperature is set to T + 10°C; if T is not greater than 440°C, thermal imidization is performed at 350°C. After thermal imidization, the film is immersed in solvent B to extract the pore-forming agent and form the desired microporous structure. Finally, the film is peeled off from the substrate and dried to obtain a high surface electrical strength polyimide with a natural surface microporous structure.

[0061] Figure 3 This is a SEM microstructure image of the high surface electrical strength polyimide surface controlled by the natural microporous structure of the surface according to Embodiment 1 of the present invention. The thickness of the polyimide was tested by a professional thickness gauge and found to be 53.86 μm.

[0062] Example 2

[0063] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0064] The preparation method of Example 2 is the same as that of Example 1, except that the amount of dibutyl phthalate (DBP) used in step 2) is different, so that its content is 10%, and the rest of the steps are the same.

[0065] Example 3

[0066] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0067] The preparation method of Example 3 is the same as that of Example 1, except that the amount of dibutyl phthalate (DBP) used in step 2) is different, so that its content is 20%, and the rest of the steps are the same.

[0068] Example 4

[0069] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0070] The preparation method of Example 4 is the same as that of Example 1, except that the amount of dibutyl phthalate (DBP) used in step 2) is different, so that its content is 30%, and the rest of the steps are the same.

[0071] Example 5

[0072] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0073] The preparation method of Example 5 is the same as that of Example 1, except that the maximum heating temperature in step 3) is 320°C and the temperature is maintained for 6 hours. All other steps are the same.

[0074] Example 6

[0075] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0076] The preparation method of Example 6 is the same as that of Example 1, except that the molar ratio of PMDA to ODA in step 1) is 1.05:1, and the other steps are the same.

[0077] Example 7

[0078] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0079] The preparation method of Example 7 is the same as that of Example 1, except that the molar ratio of PMDA to ODA in step 1) is 0.98:1, and the other steps are the same.

[0080] Example 8

[0081] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0082] The preparation method of Example 8 is the same as that of Example 1, except that the pore-forming agent used in step 2) is polyhedral oligomeric silsesquioxane (POSS), and the other steps are the same.

[0083] Example 9

[0084] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0085] The preparation method of Example 9 is the same as that of Example 1, except that the pore-forming agent used in step 2) is polyethylene glycol (PEG), and the rest of the steps are the same.

[0086] Example 10

[0087] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0088] The preparation method of Example 10 is the same as that of Example 1, except that in step 1), BTDA and PDA with a molar ratio of 1.02:1 are weighed, and after step 3), the polyimide film is taken out and immersed in methyl ketone for 12 h, and then dried to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface.

[0089] Example 11

[0090] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0091] The preparation method of Example 11 is the same as that of Example 1, except that in step 1), BPDA and BPA with a molar ratio of 1:1 are weighed, DMAc is measured and poured into a three-necked flask, BPA is added and stirred evenly, and after BPA is completely dissolved, BPDA is added to the three-necked flask in four portions with an interval of 30 minutes each time, and stirred until a uniform yellow transparent polyamic acid (PAA) solution is obtained, so that the solid content of the polyamic acid solution is 15 wt%; in step 3), the maximum heating temperature is 300℃ and the temperature is maintained for 6 h.

[0092] Example 12

[0093] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0094] The preparation method of Example 12 is the same as that of Example 1, except that in step 1), 6FDA and 6FAP with a molar ratio of 0.99:1 are weighed, DMAc is measured and poured into a three-necked flask, 6FAP is added and stirred evenly, and after 6FAP is completely dissolved, 6FDA is added to the three-necked flask in four portions with an interval of 20 minutes each time, and stirred until a uniform yellow transparent polyamic acid (PAA) solution is obtained, so that the solid content of the polyamic acid solution is 18 wt%; in step 3), the maximum heating temperature is 450℃ and the temperature is maintained for 6 h.

[0095] Example 13

[0096] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0097] The preparation method of Example 13 is the same as that of Example 1, except that in step 1), ODPA and DDS with a molar ratio of 1.01:1 are weighed, and after step 3), the polyimide film is taken out and immersed in ethanol for 12 h, and then dried to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface.

[0098] Example 14

[0099] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0100] The preparation method of Example 14 is the same as that of Example 1, except that in step 1), PMDA+BTDA and HMDA with a molar ratio of 0.99:1 are weighed, and solvent A is DMF.

[0101] Example 15

[0102] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0103] The preparation method of Example 15 is the same as that of Example 1, except that in step 1), 6FDA and 6F-BAPP with a molar ratio of 0.99:1 are weighed and solvent A is NMP; in step 2), pore-forming agent is DDAB, and the mass percentage of DDAB is 5%. The mixture is stirred at 1000 rpm for 0.5 h to ensure that DBP is uniformly dispersed, and a film is coated on a clean glass plate with a scraper.

[0104] Example 16

[0105] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0106] The preparation method of Example 15 is the same as that of Example 1, except that in step 1), 6FDA and IDDA with a molar ratio of 0.99:1 are weighed; in step 2), the pore-forming agent is Gly, and the mixture is stirred at 1400 rpm for 0.5 h to ensure uniform dispersion of DBP, and then coated on a clean glass plate with a scraper.

[0107] Example 17

[0108] A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of the surface includes the following steps:

[0109] The preparation method of Example 13 is the same as that of Example 1, except that in step 1), 6FDA and ODA+PDA with a molar ratio of 0.99:1 are weighed; in step 2), the pore-forming agent is PEO, and the mass percentage of PEO is 40%. The mixture is stirred at 1200 rpm for 0.5 h to ensure that DBP is uniformly dispersed, and a film is coated on a clean glass plate with a scraper.

[0110] Comparative Example 1

[0111] Comparative Example 1 is a common polyimide film, which was obtained directly using a coating mechanism with the same amount of yellow transparent PAA solution as in Example 1 and a doctor blade of the same height.

[0112] Experimental Example

[0113] The tensile strength, vacuum surface flashover voltage, maximum secondary electron emission coefficient, and dielectric constant of the high surface dielectric strength polyimides prepared based on the surface natural microporous structure were tested using a universal testing machine, a vacuum surface flashover voltage test device, a secondary electron emission coefficient test system, and a broadband dielectric spectrum test system. The results are shown in Table 1.

[0114] Table 1. Performance test data of the products obtained in Examples 1-9 and Comparative Example 1

[0115]

[0116] As shown in Table 1, compared to ordinary polyimide films, the high surface electrostatic strength polyimide based on the natural microporous structure of the surface provided by the embodiments of the present invention exhibits a significantly increased vacuum surface flashover voltage. Example 1 shows the most significant improvement in vacuum surface flashover performance, increasing by 43%. Examples 2 and 3 vary the content of the pore-forming agent dibutyl phthalate (DBP). When the pore-forming agent content is very low (10%), the polyimide film surface cannot open to form through-holes. When the pore-forming agent content is 20%, although a small number of through-holes are formed, their size is small, thus the suppression of electron avalanche is not optimal. When the pore-forming agent content is high, the formed through-holes are too large, but the flashover voltage does not increase accordingly. Therefore, the flashover voltage is not significantly increased. The voltage does not increase with the increase of the surface micropore size, and blindly creating large-diameter pores will actually reduce the overall performance of polyimide. The highest heating temperature of 320°C in Example 4 cannot reach the boiling point and thermal decomposition temperature of the pore-forming agent dibutyl phthalate, so it is impossible to form a microporous structure on the polyimide surface and thus cannot suppress electron avalanche development. Changing the molar ratio of dianhydride in Examples 6 and 7 will reduce the mechanical properties of the porous film. Therefore, a molar ratio of dianhydride of 1.02:1 is the optimal ratio. In Examples 8 and 9, since the pore-forming agent used is a small-sized pore-forming agent, the pore size formed on the surface is on the order of hundreds of nanometers. This size of pore has a certain inhibitory effect on secondary electron avalanche. Electrons jumping towards the anode will fall into the pores during their movement. However, due to the small size of the pores, the probability of electrons being captured is relatively low. Therefore, the improvement effect on flashover voltage is not as significant as in Example 1. Furthermore, the tensile strength was tested using a universal testing machine. Table 1 shows that the polyimide prepared by the pure coating in Comparative Example 1 exhibits very high tensile strength and excellent mechanical properties. The mechanical properties of the polyimide with a naturally porous surface structure are slightly reduced, which is due to the presence of the porous structure. However, the mechanical properties are not excessively low, and the polyimide with a naturally porous surface structure can still meet the requirements of most industrial applications. Regarding the dielectric constant, the examples show a lower dielectric constant compared to Comparative Example 1. This allows the polyimide with a naturally porous surface structure to achieve better results in applications such as 5G communication, microelectronics, and aerospace, for example, higher signal transmission speed, shorter signal delay time, and lower power consumption.

[0117] This invention discloses a high surface electrical strength polyimide based on the natural microporous structure of its surface, its preparation method, and its application. The high surface electrical strength polyimide based on the natural microporous structure of its surface is prepared by adding a pore-forming agent to a polyamic acid precursor solution, followed by imidization at high temperature or solvent extraction using basic chemical methods. The polyimide surface possesses a natural porous structure, and its surface is along an electric field. The densely distributed through-pore structure on the surface of this polyimide along the electric field allows electrons to easily fall into the through-pore structure during flashover, making it difficult for them to return to the polyimide surface. Therefore, the secondary electron multiplication process is suppressed, reducing the probability of secondary electron avalanche and thus improving the surface electrical strength of the polyimide.

[0118] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0119] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for preparing a polyimide with high surface electrical strength based on the natural microporous structure of its surface, characterized in that, include: The dried diamine was dissolved in solvent A, and then dianhydride was added. After stirring evenly, a polyamic acid solution was obtained. Add the pore-forming agent and stir at high speed until uniformly dispersed to obtain a mixed viscous solution C; After coating, drying, imidization at high temperature and natural cooling, the polyimide with high surface electrical strength based on the natural microporous structure of the surface is obtained by immersion in liquid B to extract the pore-forming agent and drying. The molar ratio of the dianhydride to the diamine is (0.98-1.05):1; the solid content of the polyamic acid solution is 15-20 wt%. Solvent A is any one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone; liquid B is ethanol; liquid B can fully dissolve the pore-forming agent without damaging or dissolving the polyimide; The pore-forming agent is any one of dodecyl dimethyl ammonium bromide, dibutyl phthalate, glycerol, polyethylene glycol, polyethylene oxide, and polyhedral oligomeric silsesquioxane; The mass of the pore-forming agent is 25%-40% of the mass of the mixed viscous solution C; The high-speed stirring rate is 800-1400 rpm; the high-speed stirring time is 0.5 h; The imidization high-temperature heating adopts a gradient heating and holding method, starting from 80°C, with a heating rate of 5°C / min, heat treatment every 60°C for 40 minutes, heating to 350-450°C, heat treatment for 6 hours, and then cooling to room temperature. When the thermal decomposition temperature of the pore-forming agent is not higher than 440℃, imidization is carried out at high temperature and then naturally cooled to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface. When the thermal decomposition temperature of the pore-forming agent is higher than 440℃, the imidization is carried out at high temperature and then naturally cooled. The pore-forming agent is then extracted by immersion in liquid B and dried to obtain a polyimide with high surface electrical strength based on the natural microporous structure of the surface.

2. The method for preparing high surface electrical strength polyimide based on the natural microporous structure of the surface according to claim 1, characterized in that, When adding dianhydride, add it in small amounts multiple times, with each addition spaced 15-30 minutes apart; Stir under nitrogen or air atmosphere until a uniform, yellow, transparent polyamic acid solution is formed.

3. The method for preparing high surface electrical strength polyimide based on the natural microporous structure of the surface according to claim 1, characterized in that, The diamine is one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminobiphenyl, 2,2'-bis(4-aminophenyl)hexafluoropropane, 4,4'-diaminophenyl sulfone, hexamethylenediamine, 2,2'-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 4,4'-(4,4'-isopropylidene diphenyl-1,1'-dimethyldioxy)diphenylamine and related derivatives.

4. The method for preparing high surface electrical strength polyimide based on the natural microporous structure of the surface according to claim 1, characterized in that, The dianhydride is one or more of the following: pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropene) phthalic anhydride, 4,4'-terephthalodioxydiphthalic anhydride, 4,4'-diphenyl ether dianhydride, and their related derivatives.

5. A polyimide with high surface electrical strength based on the natural microporous structure of its surface, characterized in that, The polyimide with high surface electrical strength based on the natural microporous structure of the surface is prepared by any one of the preparation methods described in claims 1-4. The surface of the polyimide is a surface along an electric field, and the surface along the electric field is densely covered with micron-sized pores.

6. The application of the high surface electrical strength polyimide prepared by the preparation method according to any one of claims 1-4, based on the control of the natural microporous structure of the surface, in the preparation of insulating components for spacecraft, vacuum circuit breakers, high-energy particle accelerators, pulsed power equipment, and semiconductor manufacturing and processing equipment.

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