Preparation and application of heat-resistant insulation paper composed of polyimide fiber, aramid pulp and plant fiber

Abstract

The invention belongs to the field of high-performance paper-based materials, and discloses a method for preparing a heat-resistant insulating paper base by coating polyimide and hot-pressing and taking polyimide fibers, aramid pulp and plant fibers as basic composition fibers and application of the heat-resistant insulating paper base. The paper base is prepared by taking polyimide chopped fibers, aramid pulp fibers and plant fibers as raw materials and combining a traditional papermaking process with polyimide solution coating and a hot pressing method, and the finally obtained heat-resistant insulating paper base is superior to commercial aramid paper in heat resistance, insulating property at high temperature and mechanical property. In addition, the composite paper also shows excellent flame retardance, hydrophobicity and moisture resistance, and the characteristics enable the composite paper to be more reliable in application in an extreme environment. In an actual application test, the performance of the motor heat-conducting paper manufactured by using the composite paper is superior to that of similar products manufactured by using Nomex T410; in the test of the high-temperature electrical insulation paper for the rocket, the insulation performance of the high-temperature electrical insulation paper at high temperature also exceeds Nomex T410.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of high-performance paper base, and particularly relates to a method for preparing a heat-resistant insulating paper and application of the heat-resistant insulating paper, which is prepared by coating polyimide and hot pressing and takes polyimide fibers, aramid pulp and plant fibers as basic constituent fibers. BACKGROUND

[0002] Heat-resistant paper has unique advantages in harsh high-temperature environments such as motor structures of new energy electric vehicles and aerospace due to its excellent heat resistance and mechanical properties. However, traditional aramid paper, although it is a typical representative of high-strength heat-resistant paper due to its excellent strength and thermal stability, has limitations in the preparation process. Meta-aramid paper is usually made of short-cut fibers and pulp fibers through a papermaking process, similar to the preparation process of cellulose paper. This paper mainly relies on the physical lapping between fibers to form the main structure, and the bonding force of the fiber interface is weak, resulting in a large number of pores and cavities in the paper, which is difficult to meet the strict application requirements in key fields. In addition, with the wide application of paper-based composites in extreme high-temperature fields such as electric vehicle and rocket motors, higher requirements and challenges are put forward for the heat resistance and insulation performance of the new generation of paper-based composites under high temperature. Therefore, it has become an urgent task to develop a heat-resistant paper composite material with high heat resistance, high strength and excellent insulation performance.

[0003] Currently, researchers have mainly proposed four types of solutions to improve the heat resistance of heat-resistant paper.(1) Adding heat-resistant fibers. Among them, polyimide fibers are the best among many heat-resistant fibers. By adding heat-resistant fibers such as polyimide to the pulp, the thermal stability of the paper is significantly improved. The temperature resistance of pure polyimide paper can be increased to 500℃. However, fibers such as high-performance fibers have smooth surfaces and strong chemical inertness, resulting in weak bonding force between fibers, which affects the overall performance of the paper.(2) Heat-resistant coating. Coating a heat-resistant coating (such as silicone, ceramic precursor, etc.) on the surface of the paper is another common strategy. The coating can form a protective layer at high temperatures, blocking heat and oxygen erosion. The coating can specifically enhance the heat resistance of the paper surface without affecting the original performance of the substrate. However, peeling may occur after long-term use.(3) Chemical cross-linking modification. By reacting with the pulp fibers through a chemical cross-linking agent (such as boric acid, epoxy resin), a three-dimensional network structure is formed, thereby improving the heat resistance. The cross-linked structure can inhibit the thermal motion of the fibers at high temperatures and delay decomposition. However, the cross-linking reaction may introduce toxic chemicals, and the process conditions are relatively complex.(4) Introduction of nanocomposites. Nanomaterials (such as nanometer titanium dioxide, graphene) are added to the pulp to improve the heat resistance by utilizing their high thermal conductivity and thermal stability. However, nanometer particles have poor dispersibility and are prone to agglomeration, and the cost is high, making it difficult to industrialize. Although the above methods have their own advantages, there are still challenges in preparing heat-resistant paper bases with excellent comprehensive performance. Traditional aramid paper is usually made from aramid short fibers and aramid pulp fibers, and is made by inclined screen papermaking wet forming process, but the paper produced by this method still needs to be improved in terms of heat resistance and mechanical properties. Therefore, developing a heat-resistant paper base composite material with high heat resistance, high strength, and good insulation is still an important direction of current research. SUMMARY

[0004] In order to overcome the shortcomings and deficiencies of the prior art, the primary purpose of the present application is to provide a preparation method of high-performance heat-resistant insulating paper, which uses polyimide short fibers, aramid pulp fibers and plant fibers as raw materials, and is prepared by traditional papermaking process combined with polyimide solution coating and hot pressing method. By optimizing the ratio of different fibers, the prepared PI / (PI+cellulose) composite paper not only significantly improves the heat resistance, but also maintains excellent insulation performance and mechanical properties at high temperatures, making it flexible for application in various scenarios.

[0005] Another purpose of the present application is to provide a high-performance heat-resistant insulating paper prepared by the above method. The obtained PI / (PI+cellulose) paper exhibits more excellent heat resistance, insulation performance at high temperatures and mechanical properties than aramid paper, and is very suitable for new energy electric vehicle motor heat conduction and rocket motor insulation applications.

[0006] The application further aims to provide the application of the high-performance heat-resistant insulating paper in the heat conduction field of new energy motor and the insulating field of rocket motor.

[0007] The application aims to achieve the above-mentioned purposes by the following solutions.

[0008] The application further aims to provide the application of the high-performance heat-resistant insulating paper in the heat conduction field of new energy motor and the insulating field of rocket motor.

[0009] (1) Preparation of PAA (polyamide acid) solution: under the condition of nitrogen, diamine monomer is added into an organic solvent to be stirred and dissolved to obtain a transparent diamine solution, and then dianhydride monomer is added to be stirred to obtain a transparent and viscous PAA solution;

[0010] (2) Preparation of PAA solution containing catalyst: the PAA solution in step (1) is added with a catalyst to be uniformly mixed to obtain a PAA solution containing catalyst;

[0011] (3) Preparation of ((polyimide) PI + cellulose) paper: PI short-cut fiber, pulp fiber and plant fiber are mixed and defibrated, and then a fiber paper containing polyimide and cellulose is made by papermaking;

[0012] (4) Preparation of polyimide / ((polyimide) PI + cellulose) paper: the fiber paper containing polyimide and cellulose prepared in step (3) is treated by hot pressing process, and then is laid on a glass plate, and then the PAA solution containing catalyst in step (2) is coated on the fiber paper containing polyimide and cellulose, and then is subjected to thermal imidization, and finally is treated by hot pressing process, to obtain the high-performance heat-resistant insulating paper.

[0013] The diamine monomer in step (1) is at least one of 4,4'-oxydianiline (ODA), 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (6FAP), 2,2'-bis(trifluoromethyl)-4,4'-oxydianiline (6FODA), 4,4'-diaminodiphenyl-2,2'-dicarboxylic acid (2,2`-DCB), and preferably is 4,4'-oxydianiline (ODA); the dianhydride monomer is at least one of pyromellitic dianhydride (PMDA), 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3'4,4'-benzophenonetetracarboxylic dianhydride (BTDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and preferably is pyromellitic dianhydride (PMDA); the molar ratio of the diamine monomer and the dianhydride monomer is 1:1-1.02, and preferably is 1:1;

[0014] The stirring and dissolving of the diamine monomer in the organic solvent in step (1) to obtain a transparent diamine solution is performed under a cold water bath, preferably under a 0-12℃ cold water bath, more preferably under an 8℃ cold water bath. The dianhydride monomer in step (1) needs to be dried before use, preferably dried in a vacuum drying oven at 150℃ for 10 hours.

[0015] The organic solvent in step (1) is at least one of N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), preferably N,N-dimethylacetamide (DMAC).

[0016] The amount of the organic solvent in step (1) is sufficient to maintain the solid content of the reaction system at 8wt.%-25wt.%, preferably 10wt.%; wherein the solid content refers to the mass of the added reactants, diamine monomer and dianhydride monomer, and the percentage of the total mass of the system (dianhydride monomer + diamine monomer + organic solvent).

[0017] The addition of the dianhydride monomer in step (1) is preferably in batches to better react at low temperature;

[0018] The stirring reaction temperature in step (1) is 0-25℃, preferably 8℃; the stirring reaction time is 4-24h, preferably 10h.

[0019] The catalyst in step (2) is at least one of 4-hydroxyquinoline (4-Hdql), 4-hydroxyphenylacetic acid (4-Hdpla), pyridine (PD), quinoline (QL), preferably 4-hydroxyquinoline.

[0020] The amount of the catalyst in step (2) is sufficient to maintain the molar ratio of polyamic acid PAA and catalyst at 1:2;

[0021] The mixing in step (2) is preferably stirring and mixing under a nitrogen atmosphere and a 0-25℃ cold water bath.

[0022] The plant fiber in step (3) is preferably a needle-leaf plant fiber;

[0023] The dry mass ratio of the PI short-cut fiber, pulp fiber and plant fiber in step (3) is 6:x:(4-x), wherein x=0-4;

[0024] The defibrating revolution number in step (3) is preferably 10,000-50,000r, preferably 30,000r. PEO can also be added during the defibrating process, wherein the mass of PEO is 0.05-0.2% of the total mass of the dry pulp of the PI short-cut fiber, pulp fiber and plant fiber, preferably 0.1%.

[0025] The papermaking in step (3) is preferably papermaking using a manual sheet former, and the basis weight of the fiber paper containing polyimide and cellulose is 60-120g / m 2 , preferably 80g / m 2 .

[0026] The first hot-pressing process treatment in step (4) refers to hot-pressing treatment using a double-roller hot calender, and the hot-pressing machine parameters are: hot-pressing temperature 200-220℃, preferably 210℃, hot-pressing pressure 110-130N / mm, preferably 120N / mm, hot-pressing speed 0.5-1m / min, preferably 1m / min.

[0027] The relative amount of the fiber paper containing polyimide and cellulose in step (4) and the PAA solution containing catalyst in step (2) satisfies: 30-60g of the PAA solution containing catalyst is used for each hand sheet; the area of the hand sheet is 0.0314m 2 .

[0028] The thermal imidization reaction in step (4) refers to the following process: keeping at 70-90℃ for 0.5h-1h, then increasing the temperature to 110-130℃ for 0.5h-1h, then increasing the temperature to 140-160℃ for 0.5h-1h, then increasing the temperature to 170-190℃ for 0.5h-1h. Preferably, the thermal imidization is carried out at 80℃ / 1h, 120℃ / 1h, 150℃ / 1h, 180℃ / 1h.

[0029] The second hot-pressing process treatment in step (4) refers to hot-pressing treatment using a double-roller hot calender, and the hot-pressing machine parameters are: hot-pressing temperature 180℃, hot-pressing pressure 70-90N / mm, preferably 80N / mm, hot-pressing speed 0.5-1.5m / min, preferably 1m / min.

[0030] The temperatures not specified in steps (1)-(4) are all carried out at room temperature, and the room temperature in the present application refers to 20℃±5℃.

[0031] By using more heat-resistant polyimide fibers as the main network structure and introducing plant fibers to enhance the strength of the paper base, while using highly fibrillated aramid pulp fibers as filling and bonding materials to enhance the strength and heat resistance of the paper base, we can significantly improve the heat resistance of the paper base while ensuring excellent mechanical strength. Polyimide fibers are known for their excellent thermal stability and ability to maintain mechanical and insulating properties in extreme high-temperature environments; plant fibers are widely used due to their abundant source, low cost, and surface rich in hydroxyl groups; aramid pulp uses its fibrillated characteristics to soften under heat during the hot pressing process, forming the overall mechanical structure of the paper through the bonding of short fibers and its own bonding effect. Based on this, if polyimide fibers are combined with plant fibers to form paper and further coated with a polyimide solution to fill the pores and gaps between the fibers in the paper base, a continuous film layer can be formed that can withstand higher temperatures. Subsequently, through the hot pressing process, parameters such as temperature, pressure, and time are controlled to make the microstructure of the coated composite paper more uniform and dense, reducing pores and defects, thereby improving the overall strength and insulating properties of the material. This innovative process not only compensates for the surface defects of the paper base but also significantly improves its overall performance, making it an ideal candidate for heat-resistant paper materials that can play an important role in high-temperature applications.

[0032] A high-strength heat-resistant insulating paper prepared by the above method can be used at temperatures up to 300℃.

[0033] The high-strength heat-resistant insulating paper described above is used in the fields of electric vehicles and aerospace, particularly in the preparation of new energy electric vehicle motor heat-conducting paper and rocket motor insulating paper.

[0034] The mechanism of the present application is as follows:

[0035] We have successfully developed a high-strength heat-resistant insulating high-performance heat-resistant paper. The paper base uses polyimide short fibers, aramid pulp fibers, and plant fibers as raw materials, and is prepared by traditional papermaking process combined with polyimide solution coating and hot pressing method. By optimizing the ratio of different fibers, the prepared PI / (PI+cellulose) composite paper not only significantly improves the heat resistance, but also maintains excellent insulating performance and mechanical properties at high temperatures, making it flexible for use in various scenarios. The final PI / (PI+cellulose) paper has better heat resistance, high-temperature insulating performance, and mechanical properties than commercial aramid paper. In addition, this composite paper also exhibits excellent flame retardancy, hydrophobicity, and moisture resistance, which makes it more reliable in extreme environments. In actual application tests, the motor heat-conducting paper made of this composite paper performs better than the same product made of Nomex T410; in the test of high-temperature electric insulating paper for rockets, its high-temperature insulating performance also exceeds that of Nomex T410.

[0036] The present application has the following advantages and beneficial effects relative to the prior art:

[0037] (1) PI fiber with high heat-resistant insulation performance is conducive to improving the performance of heat-resistant paper.

[0038] (2) Low-temperature imidization catalysts such as 4-hydroxyquinoline are conducive to the synthesis of heat-resistant paper.

[0039] (3) Commonly used polyimide monomers such as PMDA and ODA can be used for the synthesis of heat-resistant paper.

[0040] (4) PI / (PI+cellulose) paper exhibits better tensile strength than Nomex T410 aramid paper at room temperature (85 MPa vs. 80 MPa).

[0041] (5) PI / (PI+cellulose) paper exhibits better heat-resistant performance than Nomex T410 aramid paper under heat treatment (300°C vs. 240°C).

[0042] (6) PI / (PI+cellulose) paper exhibits lower dielectric loss than Nomex T410 at high temperatures.

[0043] (7) In the application of electric vehicle motor heat-conducting paper, PI / (PI+cellulose) paper is superior to Nomex T410 in terms of heat conduction and insulation performance at high temperatures, and its folding resistance is also much better than that of the latter.

[0044] (8) In the application of rocket motor insulation paper, PI / (PI+cellulose) paper exhibits higher breakdown strength retention rate, maintaining 100% for 0.5 hours and >77% for 1 hour, which is significantly better than Nomex T410 (broken down at 300°C for 0.5 hours and 240°C for 1 hour).

[0045] (9) PI / cellulose paper exhibits excellent flame retardancy, hydrophobicity, and moisture resistance. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 (a) and its excellent performance (b) for the preparation process of PAA (4-Hdql) and PI / (PI+cellulose) paper in the examples.

[0047] Figure 2 are the scanning electron microscope (SEM) images of (PI+cellulose) paper before the first heat pressing at different mass ratios, c and d are the scanning electron microscope (SEM) images of (PI+cellulose) paper after the first heat pressing, e and f are the mechanical properties of (PI+cellulose) paper after the first heat pressing.

[0048] Figure 3 The images shown are microscopic images and mechanical properties of PI / (PI+cellulose) paper before and after hot pressing in the examples. Here, a and b are scanning electron microscope (SEM) images of PI / (PI+cellulose) paper with different mass ratios before the second hot pressing, c and d are scanning electron microscope (SEM) images of PI / (PI+cellulose) paper after the second hot pressing, and e and f are the mechanical properties of PI / (PI+cellulose) paper after the second hot pressing.

[0049] Figure 4 The microstructure and fiber analysis of the three fibers in this example are shown. Images a, b, and c show microscopic observations of PI fiber, aramid pulp fiber, and plant fiber; images d, e, and f show scanning electron microscope (SEM) images of PI fiber, aramid pulp fiber, and plant fiber; and images g, h, and i show quantitative analyses of fiber length, thickness, and percentage of bent fibers in PI fiber, aramid pulp fiber, and plant fiber.

[0050] Figure 5 This section compares the heat resistance of PI / (PI+cellulose) paper and Nomex T410 in the examples. In the image, 'a' represents digital images of PI / (PI+cellulose) paper and Nomex T410 with a 6:1:3 ratio (PI fiber: pulp fiber: plant fiber) after being heated to 160℃, 190℃, 210℃, 240℃, 270℃, and 300℃ for 0.5h and 1h, respectively. 'b' represents the stress and modulus values ​​of these samples as line graphs. 'c' and 'd' show the stress and modulus histograms of these samples, respectively. 'e' and 'f' represent the thermogravimetric images of the three fibers, PI / (PI+cellulose) paper, and Nomex T410, respectively.

[0051] Figure 6 This example compares the flame retardant, moisture absorption, and hydrophobic properties of PI / (PI+cellulose) paper and Nomex T410. Figure a shows real-time images of PI / (PI+cellulose) paper and Nomex T410 during flame retardant testing. Figure b shows the moisture absorption data for both. Figure c shows the contact angle data and images for both at 0, 10, 20, and 30 minutes.

[0052] Figure 7 This example compares the dielectric properties of PI / (PI+cellulose) paper and Nomex T410. Figures a and b show the dielectric constant and dielectric loss of PI / (PI+cellulose) paper and Nomex T410 at room temperature (25°C). Figures c and d show the dielectric constant and dielectric loss of PI / (PI+cellulose) paper and Nomex T410 after treatment at 240°C for 1 hour.

[0053] Figure 8 The folding resistance of PI / (PI+cellulose) paper and Nomex T410 in the examples was compared. Wherein a is the folding resistance of PI / (PI+cellulose) paper and Nomex T410 at room temperature (25℃). b is the folding resistance of PI / (PI+cellulose) paper and Nomex T410 after being treated at room temperature (25℃), 210℃ and 300℃ for 1h, respectively. c is a digital image of the folding resistance of PI / (PI+cellulose) paper. d is the folding resistance of PI / (PI+cellulose) paper and Nomex T410 after being treated at 300℃ for 1h, with a ratio of 6:4:0, 6:2:2 and 6:1:3.

[0054] Figure 9 PI / (PI+cellulose) paper was tested for application in motor heat-conducting paper, including heat conduction and high-temperature mechanical properties. Wherein a is a schematic diagram showing the application of motor heat-conducting paper in electric vehicle motors. b-e show digital images and real-time temperature curves of the heat conduction of PI / (PI+cellulose) paper and Nomex T410. f-j show the ring crush strength, bending stiffness, tear strength, burst strength and dimensional stability images of PI / (PI+cellulose) paper and Nomex T410 after being treated at 25℃, 210℃ and 300℃ for 1h.

[0055] Figure 10 PI / (PI+cellulose) paper was tested for application in rocket motor insulation paper, including breakdown strength and volume resistance retention. Wherein a is a schematic diagram showing the application of rocket motor insulation paper in rockets. b-c show the breakdown strength of PI / (PI+cellulose) paper and Nomex T410 after being treated at 25℃, 150℃, 180℃, 210℃, 240℃, 270℃ and 300℃ for 0.5h and 1h, respectively. d-e show the breakdown strength retention of PI / (PI+cellulose) paper and Nomex T410 after being treated at 25℃, 150℃, 180℃, 210℃, 240℃, 270℃ and 300℃ for 0.5h and 1h, respectively. f-g show the volume resistance retention of PI / (PI+cellulose) paper and Nomex T410 after being treated at 25℃, 150℃, 180℃, 210℃, 240℃, 270℃ and 300℃ for 0.5h and 1h, respectively. DETAILED DESCRIPTION

[0056] The application will be further described in conjunction with the examples and drawings, but the embodiments of the application are not limited thereto. In the examples, the specific conditions not specified are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be purchased on the market.

[0057] The reagents used in the examples were commercially available from regular market sources unless otherwise specified. N,N-dimethylacetamide, 4-hydroxyquinoline, pyromellitic dianhydride, 4,4'-oxydianiline, polyethylene oxide (PEO) were purchased from Merck Limited. Aramid paper Nomex T410 was purchased from Bei Zheng Insulating Material Factory. The plant fiber was from a needle-leaf wood fiberboard from Guangzhou Paper Mill, with a fiber length of 0.7-0.8 mm. The pulp fiber was para-aramid pulp, which was purchased through the Taobao application, store: textile supplies factory; item number: 273#, length 0.5-1 mm. The PI short-cut fiber was purchased from Jiangsu Aoshen New Material Co., Ltd., with a length of 3.5-4 mm.

[0058] The thermal properties of PI / (PI+cellulose) paper, Nomex T410 and three kinds of fibers were evaluated using thermogravimetric analysis (TGA) (C308721115, Mettler-Toledo, Switzerland). The tensile properties of PI / (PI+cellulose) paper and Nomex T410 were tested using a universal testing machine (Instron, 5565, America). The microstructure of (PI+cellulose) paper, PI / (PI+cellulose) paper was observed using a scanning electron microscope (Zeiss, EVO 18, Germany). The contact angle of PI / (PI+cellulose) paper and Nomex T410 was measured using a contact angle measuring instrument (Theta Flex, Biolin Scientific, Finland). The dielectric properties of PI / (PI+cellulose) composite paper and Nomex T410 were obtained using a broadband dielectric spectrometer (Novocontrol Gmbh, ALPHa-ANB, Germany). The breakdown strength of PI / (PI+cellulose) paper and Nomex T410 was measured using an alternating current high voltage generator (Huace, HCDJC-1000KV, China) at a rate of 500 V / s. The resistivity of these papers was measured using an electrometer (Keithley, 6517B, America). Fiber analysis was performed using a fiber analyzer (Lorentzen & Wettre, 260, Sweden). The ring crush strength was measured using an L&W crush tester (L&W, 248, Sweden). PI / (PI+cellulose) paper was made by an automatic sheet former (FRANK-PTI, RK3AKWT, Austria) and then hot-pressed by a calender (S-CU5300, Germany). The thermal image of the new energy busbar paper was taken by an infrared thermal imaging camera (Ti400, Fluke, USA). The tear resistance, folding endurance, bending stiffness and burst resistance were measured by a tear tester (009, Lorentzen & Wettre, Sweden), a double-pulley folding endurance tester (S135050000, FRANK-PTI, Austria), a bending stiffness tester (79-25, TMI, USA) and a burst resistance tester (CE180, Lorentzen & Wettre, Sweden), respectively.

[0059] Example 1: Preparation and application of PI / cellulose paper

[0060] (1) Preparation of PAA: The dianhydride PMDA needs to be heat treated before use, i.e. heat treated in a vacuum drying oven at 150°C for 10 hours. Then, the diamine monomer ODA (2.002 g) is added into a 250 mL three-necked flask equipped with mechanical stirring, nitrogen inlet and 8°C cold water bath, and purified DMAC (23.917 g) is added to stir until the ODA is completely dissolved to obtain a transparent diamine solution. The dianhydride monomer PMDA (2.218 g) is added into the flask in three batches (10 min / batch) while adding DMAC to adjust the solid content of the reaction system to 10 wt.%, wherein the solid content refers to the mass of the added diamine monomer and dianhydride monomer and the percentage of the total mass of the system (diamine monomer + dianhydride monomer + DMAC). The molar ratio of diamine to dianhydride is 1:1. The resulting mixture is stirred for 10 hours under a nitrogen atmosphere to obtain a transparent and viscous PAA solution.

[0061] (2) Preparation of PAA (polyamide acid) solution containing 4-Hdql: After obtaining the PAA solution in the above step, as shown in a of Figure 1 , the catalyst 4-Hdql (n pAA : n 催化剂 = 1:2) is added into a triangular flask, and stirring is continued under the conditions of nitrogen and 8°C cold water bath for 2h to obtain a uniformly mixed PAA (polyamide acid) solution + 4-Hdql solution, i.e. a PAA (polyamide acid) solution containing 4-Hdql.

[0062] (3) Preparation of (PI + cellulose) paper

[0063] As shown in a of Figure 1 , PI short-cut fibers, pulp fibers and plant fibers are mixed in different mass ratios shown in Table 1, a high-speed defibrator is used for defibration at 30000r, 0.1% (based on absolute dry pulp) of PEO is added during the defibration process, and a hand sheet former is used to make polyimide fiber paper with a basis weight of 80 g / m 2 and an area of 0.0314 m 2 .

[0064] Table 1 Mass percentage of PI / (PI + cellulose) composite paper

[0065]

[0066] * Wherein the absolute dry of plant fiber is 94%

[0067] (4) Preparation of PI / (PI + cellulose) paper

[0068] As shown in a of Figure 1As shown in a, the (PI+cellulose) paper prepared in step (3) is first subjected to a hot-pressing process. Then, it is laid flat on a glass plate. Next, 40g of the PAA (polyamic acid) solution containing 4-Hdql prepared in step (2) is poured onto one side of the glass plate, tilting it at approximately 60°. A doctor blade or coating tool is used to coat the PAA solution onto the (PI+cellulose) paper, ensuring complete impregnation of the (PI+cellulose) paper with the 4-Hdql PAA (polyamic acid) solution. The coated PAA / (PI+cellulose) paper is then dried at 80°C, 120°C, 150°C, and 180°C for 1 hour each. Finally, a hot-pressing process is performed to produce PI / (PI+cellulose) paper. Figure 1 Figure 'a' shows a schematic diagram of the overall process for preparing PI / (PI+cellulose) paper in the laboratory. Figure 1 Figure b shows that the prepared PI / (PI+cellulose) composite paper has excellent mechanical properties, heat resistance, thermal conductivity, flexibility, and insulation properties at high temperatures, laying the foundation for its application in new energy motors and rocket motors.

[0069] The PI / (PI+cellulose) paper underwent two hot-pressing processes using a twin-roll hot press. The specific parameters of the hot press are as follows:

[0070] First hot pressing treatment: (PI+cellulose) composite paper: hot pressing temperature 210℃, hot pressing pressure 120N / mm, hot pressing speed 1m / min;

[0071] Second hot pressing treatment: PI / (PI+cellulose) composite paper: hot pressing temperature 180℃, hot pressing pressure 80N / mm, hot pressing speed 1m / min.

[0072] Performance testing:

[0073] (1) Mechanical properties of PI / (PI+cellulose) paper (where the mass ratio of PI chopped fiber: pulp fiber: plant fiber is not explicitly defined, it refers to a mass ratio of 6:1:3) and fiber analysis of cellulose paper.

[0074] Figure 2 The microstructure and mechanical properties of paper before coating with a PAA solution containing a catalyst are shown. Figure 2 Images a and b are scanning electron microscope (SEM) images of (PI+cellulose) paper before the first hot pressing, with fiber ratios of 6:4:0 and 6:1:3, respectively. The images show that both types of paper have poor surface smoothness, loose fiber distribution, and a rough overall structure before hot pressing. However, after the first hot pressing treatment... Figure 2(c) and (d) show a significant improvement in paper surface smoothness. This is likely due to the softening of aramid pulp fibers during hot pressing, which bond with PI fibers and plant fibers in the matrix. Simultaneously, the fibers themselves also bond, forming a denser and more uniform mechanical structure, thus enhancing the overall mechanical properties of the paper. Figure 2 The study further revealed the overall mechanical properties of uncoated paper (i.e., (PI+cellulose) paper after the first hot-pressing treatment). Test results showed that uncoated paper had poor mechanical properties, with tensile strength generally below 10 MPa. However, the mechanical properties of (PI+cellulose) paper varied with the fiber ratio, showing a trend of first increasing and then decreasing. The (PI+cellulose) paper with a fiber ratio of 6:1:3 exhibited the highest mechanical properties, reaching 6.5 MPa. This performance improvement may be related to the characteristics of plant fibers during the papermaking process. Plant fibers have good dispersibility in water, allowing them to be evenly distributed in the paper matrix and providing numerous hydrogen bonds. This uniform distribution and hydrogen bonding make the 6:1:3 fiber ratio paper more uniform in microstructure and has higher micro-strength, thus significantly improving the overall mechanical strength of the paper.

[0075] Figure 3 The microstructure and mechanical properties of coated PI / (PI+cellulose) paper are shown. Figure 3 In the diagram, 'a' and 'b' represent the microstructure of PI / (PI+cellulose) paper before the second hot-pressing process. Figure 3 As shown in figures a and b, there is significant PI residue on the surface of the PI-coated paper, especially in the paper pores. However, the surface smoothness and uniformity of the coated paper are still poor, and this unevenness leads to a reduction in the overall mechanical strength of the paper. To further improve the paper's performance, we subjected the coated paper to hot-pressing treatment. Figure 3 In the diagram, c and d represent the microstructure of PI / (PI+cellulose) paper after the second hot-pressing process. Figure 3 As can be seen from figures c and d, after coating and hot-pressing, the surface smoothness and uniformity of the paper are significantly improved, and a uniform layer of PI residue is formed on the paper surface. This treatment improves the paper structure, thereby enhancing the overall mechanical properties of the paper. The final mechanical properties of the finished paper are as follows: Figure 3 As shown in e and f, the mechanical properties of the paper are significantly improved after coating and hot pressing, reaching a maximum of 85 MPa. This result is not only significantly better than the paper before coating, but also exceeds the mechanical strength of 80 MPa of currently commercially available aramid paper.

[0076] The microstructure of paper is the key factor determining its mechanical properties, and the morphology and characteristics of fibers play a crucial role in it. To further analyze the mechanical properties of PI / (PI+cellulose) paper, we conducted micro-morphology observation and fiber analysis on the three types of fibers that make up the paper: PI fiber, aramid pulp fiber, and plant fiber. Figure 4 a, b, c, and Figure 4 d, e, f of FIG. 1 respectively show the microscope observation images and scanning electron microscope (SEM) images of PI fiber, aramid pulp fiber, and plant fiber. As can be seen from the images, PI fiber and plant fiber are longer and straighter than aramid pulp fiber, and their thickness is relatively smaller. In addition, Figure 4 g, h, i of FIG. 1 further confirm the observation results of the microscope images and SEM images through quantitative analysis of fiber length, thickness, and percentage of curved fibers. In an ideal case, fibers in paper should be as long and straight as possible to form an effective network structure inside the paper, providing more binding sites and thus maximizing the inter-fiber bonding force. Therefore, in PI / (PI+cellulose) paper, the increase in the ratio of PI fiber and plant fiber generally improves the mechanical strength of the paper. However, the fibrillation of aramid pulp fiber plays an important role in the paper as well. Fibrillation can significantly increase the actual contact area between fibers, promoting the entanglement and interweaving of fibers. When aramid pulp fiber fills into the network of PI fiber and plant fiber interweaving, it softens under heat during hot pressing and can bond with other fibers in the matrix, forming a more compact and uniform mechanical structure. This structure not only enhances the inter-fiber bonding force but also significantly improves the overall mechanical properties of the paper. Therefore, although PI fiber and plant fiber provide the basic mechanical properties, the fibrillation characteristics of aramid pulp fiber play an indispensable role in the optimization of the microstructure of the paper, enabling the paper to achieve the best balance in mechanical properties.

[0077] (2) Analysis of heat resistance of PI / (PI+cellulose) paper

[0078] In practical applications, it is crucial for the size and mechanical properties of paper materials to remain stable under different environmental conditions, especially in high-temperature environments. For example, Figure 5 As shown in a of FIG. 2, PI / (PI+cellulose) paper does not show significant changes in size at high temperatures up to 300°C, although the color becomes darker, showing good thermal size stability. In contrast, Nomex T410 paper not only becomes darker in color but also shows significant shrinkage in size at high temperatures of 270°C and 300°C, and its size stability decreases significantly as the temperature increases. From the perspective of mechanical properties, the stress and modulus of both materials decrease as the temperature increases (as shown in b and c of FIG. 2). However, the decrease in the stress and modulus of PI / (PI+cellulose) paper is much smaller than that of Nomex T410 paper, indicating that the former has better heat resistance than the latter. Figure 5of b). However, the PI / (PI+cellulose) paper shows a significantly lower rate of mechanical property degradation than Nomex T410 paper. Specifically, the tensile stress of Nomex T410 paper decreases dramatically after 270°C, only maintaining about 50% of the initial value Figure 5 of c). The modulus of PI / (PI+cellulose) paper remains relatively stable at high temperatures Figure 5 of d). In contrast, the modulus of Nomex T410 paper has dropped to less than 300 MPa after treatment at 270°C, far below its initial value of 600 MPa. This indicates that the PI / (PI+cellulose) paper has superior thermal mechanical stability in high temperature environments. Figure 5 of e shows the thermogravimetric curves (TGA) of the three fibers that make up the paper. PI fibers and aramid pulp fibers are difficult to break at high temperatures due to their high rigidity and stable chemical structure of molecular chains, showing thermal stability above 500°C. In contrast, plant fibers are mainly composed of cellulose, hemicellulose and lignin, which have much lower thermal stability than PI fibers and aramid pulp fibers, and generally begin to thermally decompose at around 240°C. In the finished paper product Figure 5 of f), the PI / (PI+cellulose) paper with a ratio of 6:1:3, although it contains plant fibers, resulting in a weight loss step at 240°C, but due to the addition of PI fibers and aramid pulp fibers, its second weight loss step occurs above 500°C, thus ensuring the thermal stability of the overall paper. The thermal stability of Nomex T410 paper is maintained only at around 400°C.

[0079] Figure 5 It is shown that the PI / (PI+cellulose) paper exhibits stable size and mechanical properties in high temperature environments. Its excellent thermal mechanical stability is mainly due to the high thermal stability of PI fibers and aramid pulp fibers, as well as the synergistic effect of plant fibers and PI fibers. This composite structure not only makes up for the lack of thermal stability of plant fibers, but also fully utilizes the advantages of PI fibers and aramid pulp fibers. Figure 5 It is shown that the PI / (PI+cellulose) paper can work stably at temperatures up to 300°C, showing superior high temperature performance than traditional aramid paper (such as Nomex T410).

[0080] (3) Analysis of the flame retardant and hydrophobic properties of PI / (PI+cellulose) paper

[0081] To further evaluate the flame retardant properties of PI / (PI+cellulose) paper and Nomex T410 paper, we conducted a systematic combustion test. Figure 6a demonstrates the real photos of two paper samples before ignition, during combustion, and after combustion. The test results show that the PI / (PI+cellulose) paper exhibits excellent flame retardant performance, with the flame quickly extinguishing after the ignition source is removed, with little evidence of sustained combustion. In contrast, the Nomex T410 paper, while also having some flame retardant properties, has a longer combustion time and more obvious residue under the same conditions. This excellent flame retardant performance is mainly due to the multi-component composite structure of the PI / (PI+cellulose) paper. PI fibers and aramid pulp fibers are inherently flame-resistant materials with extremely high thermal stability and flame-retardant properties. Under high temperature conditions, these two fibers do not melt or drip, but form a stable carbonized layer and a heat insulation layer, effectively isolating oxygen and heat, thereby preventing the spread of fire. Although plant fibers do not have good flame retardant properties, they quickly dehydrate and form a carbonized layer under high temperature, which cooperates with the heat insulation layer formed by PI fibers and aramid pulp fibers to further enhance the heat insulation and flame retardant effect of the paper. In addition, the outer coated PI not only improves the overall thermal stability of the paper, but also forms a continuous carbonized layer under high temperature, effectively preventing the transmission of oxygen and heat. This multi-component synergy allows the PI / (PI+cellulose) paper to effectively suppress the combustion reaction under high temperature, exhibiting excellent flame retardant performance.

[0082] In addition to flame retardant performance, paper also needs to have good waterproof and moisture-proof performance in actual application, especially during production and transportation, paper will inevitably be exposed to humid environment. Therefore, we evaluated the wetting and moisture absorption of the two papers (as shown in Figure 6 b and c). We evaluated the hydrophobicity of both by contact angle; PI / (PI+cellulose) paper and Nomex T410 were placed in an environment of 60°C for 12 hours, dried their moisture, and then tested their moisture absorption rate in an environment of 25°C temperature and 40% humidity. The test results show that the hydrophobicity of PI / (PI+cellulose) paper is significantly better than that of Nomex T410 paper, and the moisture absorption is lower. This property is mainly due to the synergistic effect of the components in the composite paper. PI fibers and aramid pulp fibers themselves have good hydrophobicity and dominate in the composite paper. Although plant fibers have strong hydrophilicity, they are wrapped by the imidized PI layer in the composite paper, and their hydrophilicity is effectively inhibited. In addition, the outer coated PI layer further plays a physical isolation role, blocking the penetration of water. This multi-layer structure not only reduces the moisture absorption of the paper, but also enhances its mechanical strength retention ability in humid conditions, making it more advantageous in actual application.

[0083] (4) Analysis of the dielectric properties of PI / (PI+cellulose) paper

[0084] The rocket motor insulation paper needs to have excellent dielectric properties, which is crucial for the stable operation of the motor. The dielectric constant is a physical quantity that measures the polarization ability of the material in the electric field, reflecting the degree of response of the material to the electric field. Dielectric loss is the energy loss due to polarization hysteresis effect, which is usually dissipated in the form of heat. Low dielectric loss means that the material has less energy loss under the action of the electric field, thereby improving the operating efficiency of the motor. Figure 7 The a and b of the present application show the dielectric properties of PI / (PI+cellulose) paper and Nomex T410 at 25℃, with a frequency range of 1 to 10 6 Hz. The results show that the dielectric constant of PI / (PI+cellulose) paper is 1.55-1.07, and the dielectric loss is 0.309-0.011, both of which are higher than those of Nomex T410 (dielectric constant is 1.22-1.08, dielectric loss is 0.03-0.01). This phenomenon may be related to the presence of hydroxyl (-OH) groups in the cellulose molecular chain. These hydroxyl groups have high polarity, making cellulose more likely to polarize in an electric field, resulting in relatively high dielectric constant and dielectric loss. However, after treatment at 240℃ for 1 hour, the dielectric constant and dielectric loss of PI / (PI+cellulose) paper are significantly reduced, with the dielectric loss (0.022-0.005@1-10 6 Hz) being less than that of Nomex T410 (0.04-0.009@1-10 6 Hz). This change may be due to the partial decomposition and structural rearrangement of cellulose caused by high temperature treatment. Partial decomposition makes the cellulose molecular chain more regular and simple, and the polarization process of its molecular chain segment becomes more stable, reducing hysteresis polarization and thus reducing energy loss and dielectric loss.

[0085] (5) Analysis of the flexibility of PI / (PI+cellulose) paper

[0086] In applications with complex internal structures such as motors, components such as stator slots, high-strength heat-resistant paper-based materials (such as Nomex T410) often need to be cut, folded, wound or wrapped, etc. These processing steps have strict requirements for the flexibility of the material, and good flexibility can ensure that the material does not break or damage during processing. In this invention, we have investigated the flexibility of PI / (PI+cellulose) paper in detail according to the double-clamp folding endurance tester. Figure 8 The a of the present application shows the folding number of PI / (PI+cellulose) paper with a ratio of 6:1:3 and Nomex T410 at 25℃. The results show that both PI / (PI+cellulose) paper and Nomex T410 exhibit excellent folding performance at room temperature, with folding numbers exceeding 3000 times. As the temperature rises, the folding performance of different materials shows significant differences. Figure 8Figure b shows that the folding endurance of Nomex T410 decreases significantly with increasing temperature. After one hour of treatment at 300°C, the folding endurance of Nomex T410 dropped to approximately 100 cycles. Although Nomex T410 retains certain mechanical properties at high temperatures, prolonged exposure to high temperatures leads to thermal aging of its molecular chains and changes in dimensional stability. The molecular chains of aramid fibers degrade at high temperatures, resulting in decreased fiber flexibility and strength, which in turn worsens the folding endurance of the paper. Furthermore, changes in dimensional stability make the paper more prone to stress concentration during folding. This stress concentration accelerates fatigue fracture of the paper, thus significantly reducing the folding endurance. In contrast, PI / (PI+cellulose) paper exhibits excellent folding endurance at high temperatures. This is mainly attributed to its unique composite structure. The PI solution and PI fibers are the same substance with a small difference in modulus, thereby reducing stress concentration during folding. At the same time, PI itself has extremely high heat resistance, allowing it to maintain good flexibility and mechanical properties even at 300°C. Figure 8 c demonstrates the poor folding endurance of PI / cellulose paper. The preparation method of PI / cellulose paper is as follows: plant fiber paper (80g / m³) is... 2 (Purchased from Guangdong Shengtai Archives Supplies) The paper was completely immersed in a PAA (polyamic acid) solution containing 4-Hdql in step (2) for 12 hours, and then dried at 80°C, 120°C, 150°C and 180°C for 1 hour each to produce PI / cellulose paper. This is likely because PI is a polymer with extremely high mechanical strength and rigidity, and its high Young's modulus can significantly improve the tensile strength and modulus of paper. However, this high-rigidity PI coating also limits the bending and folding ability of cellulose paper, thus reducing its flexibility. In contrast, PI / (PI+cellulose) paper optimizes the balance between flexibility and strength through its composite structure. Figure 8 The d-value clearly shows that PI / (PI+cellulose) paper can maintain excellent folding resistance even at temperatures up to 300°C.

[0087] (6) Application Analysis of Thermally Conductive Paper for Tram Motors

[0088] Figure 9Figure 1a shows the application position of the new energy electric vehicle heat-conducting paper. In new energy electric vehicles, the motor generates a large amount of heat when operating at high power, so efficient heat dissipation is needed to ensure that the electrical insulation performance of the motor is not affected by high temperatures. The heat-conducting performance of the insulation material is crucial in this process. The better the heat-conducting performance of the material, the higher the efficiency of heat transfer from the motor winding to the cooling oil, thereby enabling more effective reduction of the operating temperature of the motor. In this invention, we have discussed in detail the heat-conducting performance of PI / (PI+cellulose) paper and its mechanical properties at high temperatures, and compared it with Nomex T410. To evaluate the heat-conducting performance of the two materials, we placed the lower surface of the PI / (PI+cellulose) paper and Nomex T410 on a heating plate at 220°C, respectively, and observed the temperature change on the upper surface of the paper using an infrared thermal imager. Figure 9 Figures 1b-d show the infrared thermal imaging images of the two materials at 0, 30 min and 60 min of heating, while Figure 9 Figure 1e presents their temperature rise curves. The results show that the surface temperature of the paper gradually increases over time, and by 60 min, the surface temperature of the PI / (PI+cellulose) paper has exceeded 215°C, significantly higher than that of Nomex T410. This indicates that the heat-conducting performance of the PI / (PI+cellulose) paper is superior to that of Nomex T410. This may be due to the fact that the PI fiber itself has high heat conductivity, with a regular molecular chain structure and good heat-conducting performance. In the PI / (PI+cellulose) composite paper, the proportion of PI fiber is the highest (60%), forming an effective heat-conducting network structure that can quickly conduct heat, thereby significantly improving the overall heat-conducting performance of the composite paper. In addition, the presence of the PI coating further enhances the heat-conducting performance of the composite paper. The PI coating not only increases the heat-conducting path, but also improves the heat-conducting efficiency through good bonding with the fibers. Figure 9The ring crush strength, bending stiffness, tear strength, burst strength, and thermal dimensional stability of PI / (PI+cellulose) paper and Nomex T410 are shown in f-j of FIG. 1, respectively. As can be seen from the figures, the mechanical properties of PI / (PI+cellulose) paper are significantly better than those of Nomex T410. After treatment at 300℃ for 1 hour, the tear strength and burst strength of Nomex T410 have decreased to less than 10% of the original, which cannot meet the stability requirements in actual applications. In contrast, PI / (PI+cellulose) paper still maintains high mechanical properties after high-temperature treatment, which can effectively ensure the safe operation in actual applications. In addition, the color of both materials becomes darker after high-temperature treatment, but the size of Nomex T410 shrinks significantly, while the size of PI / (PI+cellulose) paper is more stable. This dimensional stability is crucial to ensure the safe operation of motor insulation materials in high-temperature environments. PI / (PI+cellulose) paper, with its excellent thermal conductivity and mechanical stability at high temperatures, exhibits superior comprehensive performance than Nomex T410, which can better meet the heat dissipation and insulation requirements of new energy electric vehicle motors during high-power operation.

[0089] (7) Application analysis of rocket motor insulation paper

[0090] Figure 10 The a of FIG. 1 shows a schematic diagram of the application position of rocket motor insulation paper. The electrical components (such as micro motors) of rocket motors usually need to operate at extremely high temperatures, so their insulation materials must have excellent high-temperature insulation performance to ensure the electrical performance and safety of the motor. In this invention, we compared the electrical insulation performance of PI / (PI+cellulose) paper and Nomex T410. Figure 10 The b-c of FIG. 1 shows the breakdown strength of the two materials after heat treatment at 25℃, 150℃, 180℃, 210℃, 240℃, 270℃, and 300℃ for 0.5 hours and 1 hour. At lower temperatures (<240℃), the breakdown strength of Nomex T410 is higher than that of PI / (PI+cellulose) paper, which may be due to the uneven distribution of fibers in the composite paper, causing local electric field concentration and thus reducing the breakdown strength. However, under high-temperature conditions, PI / (PI+cellulose) paper exhibits significant heat resistance advantages. Figure 10 The d of FIG. 1 shows that after 0.5 hours of high-temperature treatment, PI / (PI+cellulose) paper basically maintains 100% of the breakdown strength, while the breakdown strength retention rate of Nomex T410 is only 92%, and at 300℃, it is directly broken down by voltage due to insufficient heat resistance. When the heat treatment time is extended to 1 hour ( Figure 10The PI / (PI+cellulose) paper has excellent heat resistance, and the breakdown strength retention rate is still as high as 77%, while the Nomex T410 is directly broken down at 240℃. This shows that the PI / (PI+cellulose) paper has a significant insulation performance advantage in a high-temperature environment. Figure 10 Fig. f-g further compares the resistivity retention rates of the two materials. As the temperature rises and the heat treatment time extends, the resistivity retention rate of the PI / (PI+cellulose) paper is always kept above 70%, while the resistivity retention rate of the Nomex T410 is reduced to below 64%. The high resistivity retention rate of the PI / (PI+cellulose) paper effectively reduces the generation of electrons and reduces the possibility of ionization leading to charge carrier multiplication, thereby inhibiting the avalanche breakdown phenomenon of the material. In addition, its excellent heat resistance enables it to maintain excellent insulation performance in a high-temperature environment, providing a strong guarantee for the stability of the use environment. Therefore, the PI / (PI+cellulose) paper, with its excellent heat resistance and insulation performance, shows great potential in high-temperature applications.

[0091] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods and shall be within the scope of protection of the present application.

Claims

1. A method for producing a high performance heat resistant insulation paper, characterized by The method comprises the following steps: (1) Preparation of PAA solution: under nitrogen atmosphere, diamine monomer is added to organic solvent to be dissolved by stirring to obtain transparent diamine solution, and dianhydride monomer is added to be stirred to obtain transparent and viscous PAA solution; (2) Preparation of PAA solution containing catalyst: the PAA solution of step (1) is added with catalyst and mixed uniformly to obtain PAA solution containing catalyst; (3) Preparation of (polyimide + cellulose) paper: PI short-cut fiber, pulp fiber and plant fiber are mixed and defibrated, and then a fiber paper containing polyimide and cellulose is made; (4) Preparation of polyimide / (polyimide + cellulose) paper: the fiber paper containing polyimide and cellulose prepared in step (3) is treated by hot pressing process, then is laid on a glass plate, then the PAA solution containing catalyst in step (2) is coated on the fiber paper containing polyimide and cellulose, then is subjected to thermal imidization, and finally is treated by hot pressing process to obtain high-performance heat-resistant insulating paper.

2. The method according to claim 1, wherein: in step (1), the diamine monomer is at least one of 4,4'-oxydianiline, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 2,2'-bis(trifluoromethyl)-4,4'-oxydianiline, and 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid; the dianhydride monomer is at least one of pyromellitic dianhydride, 4,4'-(hexafluoroisopropylidene) diphthalic anhydride, 3,3'4,4'-benzophenonetetracarboxylic dianhydride, and 3,3',4,4'-biphenyltetracarboxylic dianhydride; the molar ratio of the diamine monomer to the dianhydride monomer is 1:1-1.02; in step (1), the diamine monomer is added to the organic solvent to be dissolved by stirring to obtain transparent diamine solution under cold water bath condition; in step (1), the organic solvent is at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide; in step (1), the amount of the organic solvent satisfies that the solid content of the reaction system is maintained at 8wt.%-25wt.%, wherein the solid content refers to the mass of the added reactants (diamine monomer and dianhydride monomer) and the percentage of the total mass of the system (dianhydride monomer+diamine monomer+organic solvent); in step (1), the stirring reaction temperature is 0-25℃, and the stirring reaction time is 4-24h.

3. The method according to claim 1, wherein: in step (2), the catalyst is at least one of 4-hydroxyquinoline, 4-hydroxyphenylacetic acid, pyridine, and quinoline; in step (2), the amount of the catalyst satisfies that the molar ratio of polyamide acid PAA to the catalyst is 1:2; in step (2), the mixing is stirring mixing under nitrogen atmosphere and cold water bath at 0-25℃.

4. The method according to claim 1, wherein: in step (3), the plant fiber is conifer plant fiber. ​ ​ ​ ​ ​ ​ ​ ​ ​ The dry mass ratio of the PI short-cut fibers, pulp fibers and plant fibers in step (3) is 6: x: (4-x), wherein x = 0-4.

5. The method for preparing high-performance heat-resistant insulation paper according to claim 1, characterized in that: The defibrillation revolution number in step (3) is 10000-50000r; PEO is also added in the defibrillation process, wherein the mass of PEO is 0.05-0.2% of the total mass of the dry pulp of the PI short-cut fibers, pulp fibers and plant fibers; The papermaking in step (3) is performed using a hand sheet maker, and the basis weight of the paper containing the polyimide and the cellulose fibers is 60 to 120 g / m 2 .

6. The method for preparing high-performance heat-resistant insulation paper according to claim 1, characterized in that: The first heat pressing process in step (4) refers to using a double-roller heat calender for heat pressing treatment, and the heat calender parameters are: heat pressing temperature 200-220℃, heat pressing pressure 110-130N / mm, heat pressing speed 0.5-1m / min; The relative amount of the fiber paper containing polyimide and cellulose described in step (4) and the PAA solution containing a catalyst in step (2) satisfies: 30-60 g of the PAA solution containing a catalyst is used per one hand sheet; the area of the hand sheet is 0.0314 m 2 ; The heat imidization reaction in step (4) refers to: first, heat preservation at 70-90℃ for 0.5h-1h, then, heat preservation at 110-130℃ for 0.5h-1h, then, heat preservation at 140-160℃ for 0.5h-1h, and then, heat preservation at 170-190℃ for 0.5h-1h; The second heat pressing process in step (4) refers to using a double-roller heat calender for heat pressing treatment, and the heat calender parameters are: heat pressing temperature 180℃, heat pressing pressure 70-90N / mm, heat pressing speed 0.5-1.5m / min.

7. High-performance heat-resistant insulation paper prepared by the method according to any one of claims 1-6.

8. The high performance heat resistant insulation paper according to claim 7, characterized in that Its service temperature is up to 300℃.

9. Application of the high-performance heat-resistant insulation paper according to claim 7 or 8 in the field of electric vehicles and aerospace.

10. Application of the high-performance heat-resistant insulation paper according to claim 7 or 8 in the preparation of new energy electric vehicle motor heat-conducting paper and rocket motor insulation paper.

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