Method for producing aramid-like heat-resistant paper by directly synthesizing polyimide through low-temperature imidization in cellulose paper substrate and application of aramid-like heat-resistant paper

Abstract

The invention belongs to the field of high-performance paper bases, and discloses a simple method for producing low-cost and high-performance aramid-like heat-resistant paper by directly synthesizing polyimide in a cellulose paper substrate through low-temperature imidization and application. The heat-resistant paper is directly synthesized and prepared in a paper substrate by using polyimide. According to the method, a traditional fiber pre-synthesis process and a subsequent paper forming process are removed, the heat-resistant paper is manufactured through a one-step method, the production complexity is remarkably reduced, and the product cost is reduced. The resulting PI / cellulose paper exhibits excellent mechanical, thermal and dielectric properties comparable to aramid paper, and is very suitable for lightweight honeycomb structures and electrical insulation applications.

Description

Technical Field

[0001] The present invention belongs to the field of high-performance paper-based materials, and particularly relates to a simple method for directly synthesizing polyimide by low-temperature imidization in a cellulose paper matrix to produce low-cost and high-performance aramid-like heat-resistant paper and its applications. Background Art

[0002] Heat-resistant paper is a special material designed to maintain structural integrity and resist degradation at high temperatures. Aramid paper is a typical representative of such materials, mainly made of aramid fibers and renowned for its excellent strength and thermal stability. These properties endow aramid paper with outstanding mechanical durability and heat resistance, making it crucial for aerospace, automotive, and electrical applications. Traditional aramid paper production involves multiple complex stages, including the pre-synthesis of aramid solutions and subsequent formation of aramid fibers through papermaking. The detailed process includes monomer polymerization, spinning, coagulation, stretching, washing, drying, cutting, fiber dispersion, chemical cross-linking, precise layering, high-temperature drying, and final winding. Despite the great advantages of aramid paper, the complex procedures lead to increased production costs, thus limiting its widespread adoption in various fields. Therefore, there is a need to simplify the manufacturing process of heat-resistant paper and reduce its production cost.

[0003] Current efforts to simplify aramid paper production and reduce costs mainly focus on three main strategies: optimizing current manufacturing processes, exploring alternative manufacturing technologies, and developing alternative heat-resistant fiber products. (1) Optimizing current manufacturing processes: This approach involves partially replacing aramid fibers with more economical fibers such as cellulose, glass, polyester, or recycled aramid fibers, which can improve cost-effectiveness but may affect uniformity, heat resistance, and mechanical integrity. Using alternative binders, such as replacing phenolic resin with polyurethane, may eliminate the need for chemical cross-linking, simplify production but may increase binder costs. Surface modification can improve performance but increases the complexity of pulping and papermaking. Fine-tuning parameters such as temperature, pressure, and residence time can also improve efficiency and reduce costs, but the improvements are limited. (2) Exploring alternative manufacturing technologies: Different from traditional wet papermaking techniques, dry papermaking methods can create non-woven structures but require specialized equipment and may face uniformity issues. Electrospinning technology can control fiber morphology and produce fine fibers with a high surface area, but the production speed is slow and the cost is high. Solvent casting can directly dissolve aramid polymers for casting, but the solvent cost is high. Combining wet spinning with rapid drying can reduce energy requirements but involves significant equipment investment. In addition, advanced 3D printing technology can precisely deposit aramid polymers to achieve complex designs, although the processing speed is slow and the high setup costs persist. (3) Developing alternative heat-resistant fiber products: Options such as glass, basalt, or ceramic fibers offer varying degrees of heat resistance and strength, but suffer from brittleness, low mechanical strength, and reduced flexibility. Collectively, these strategies offer significant advantages for process simplification and cost reduction, but they also present a trade-off between cost and performance. Therefore, developing simple and cost-effective technologies to produce high-performance heat-resistant aramid paper materials remains a significant challenge.

[0004] The new method may hold promise for overcoming these limitations. The production of traditional aramid paper networks requires two key steps: pre-synthesizing heat-resistant fibers and then forming a paper matrix. If we can directly synthesize a heat-resistant polymer network in an existing matrix, we can produce heat-resistant paper in a one-step process. Cellulose paper is a widely used and cost-effective matrix material made from plant cellulose fibers, which offers versatile applications in various fields. It can provide a lightweight, porous structure that adapts to various morphologies, including thin paper, thick boards, and 3D honeycombs. Although cellulose has limited thermal stability (about 240 °C), its low cost and versatility make it an attractive matrix structure. Polyimide (PI) is a high-performance polymer with a rigid aromatic backbone that provides excellent thermal stability (able to withstand temperatures up to 500 °C). Polyimide can be processed into fibers and films through thermal and low-temperature chemical imidization processes. Especially the latter low-temperature process, polyimide can be synthesized at about 180 °C. The unique thermal properties and forming characteristics of polyimide make it a suitable candidate polymer compound to be integrated into the cellulose matrix to create heat-resistant materials. Summary of the Invention

[0005] To overcome the above-mentioned disadvantages and deficiencies of the prior art, the primary object of the present invention is to provide a method for directly synthesizing polyimide in a cellulose paper matrix by low-temperature imidization to produce aramid-like heat-resistant paper. By using polyimide as a substitute raw material and directly synthesizing it in the paper matrix, we bypass the traditional pre-synthesis of aramid fibers and eliminate the subsequent fiber assembly and papermaking processes, significantly simplifying the production complexity and reducing costs. We expect this method to open up a way for large-scale production of high-performance heat-resistant paper materials and apply them to military and civilian fields. Most importantly, this technology removes the traditional fiber pre-synthesis process and the subsequent paper forming process, manufacturing heat-resistant paper in a one-step method, significantly reducing the production complexity and product cost.

[0006] Another object of the present invention is to provide the heat-resistant paper prepared by the above method. The obtained PI / cellulose paper exhibits excellent mechanical, thermal, and dielectric properties comparable to aramid paper, and is very suitable for lightweight honeycomb structures and electrical insulation applications.

[0007] Another object of the present invention is to provide the application of the above heat-resistant paper in the fields of electrical insulation and honeycomb weight reduction.

[0008] The object of the present invention is achieved by the following solutions:

[0009] A method for directly synthesizing polyimide in a cellulose paper matrix by low-temperature imidization to produce aramid-like heat-resistant paper, comprising the following steps:

[0010] (1) Preparation of PAA (polyamic acid) solution: Under nitrogen gas passing condition, diamine monomer is added into an organic solvent and stirred to dissolve to obtain a transparent diamine solution, and then dianhydride monomer is added and stirred for reaction to obtain a transparent and viscous PAA solution;

[0011] (2) Preparation of PAA solution containing catalyst: A catalyst is added to the PAA solution in step (1) and mixed evenly to obtain a PAA solution containing catalyst;

[0012] (3) Preparation of polyimide (PI) / cellulose paper: The plant fiber paper is completely immersed in the PAA solution containing catalyst for impregnation treatment, and then the impregnated plant fiber paper is taken out and subjected to thermal imidization reaction to obtain PI / cellulose paper.

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

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

[0015] The organic solvent described 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 dosage of the organic solvent described in step (1) satisfies that the solid content of the reaction system is maintained at 8 wt.% - 25 wt.%, preferably 15 wt.%; where the solid content refers to the percentage of the mass sum of the added reactant diamine monomer and dianhydride monomer in the total mass of the system (dianhydride monomer + diamine monomer + organic solvent).

[0017] The addition of the dianhydride monomer described in step (1) is preferably carried out in batches to ensure full reaction at low temperature;

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

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

[0020] The dosage of the catalyst described in step (2) satisfies that the molar ratio of polyamic acid to the catalyst is 1:2;

[0021] The mixing described in step (2) is preferably carried out by stirring in a nitrogen atmosphere and a cold water bath at 0 - 25°C.

[0022] The plant fiber paper described in step (3) is a plant fiber paper with a specified amount of 60 - 100 g / m 2 preferably 80 g / m 2 of the plant fiber paper.

[0023] The impregnation time described in step (3) is 8 - 24 h, preferably 12 h. The impregnation refers to impregnation at room temperature.

[0024] The thermal imidization reaction described in step (3) means holding at 70 - 90°C for 0.5 h - 1 h, then heating to 110 - 130°C and holding for 0.5 h - 1 h, then heating to 140 - 160°C and holding for 0.5 h - 1 h, and then heating to 170 - 190°C and holding for 0.5 h - 1 h. It is preferably carried out under the conditions of 80°C / 1 h, 120°C / 1 h, 150°C / 1 h, and 180°C / 1 h for thermal imidization.

[0025] Where the temperature is not specified in steps (1) to (3), it means carried out at room temperature. The room temperature in the present invention refers to 20°C ± 5°C.

[0026] A high - strength heat - resistant paper prepared by the above method.

[0027] The application of the above - mentioned high - strength heat - resistant paper in the fields of electrical insulation, electric vehicles, and aerospace, especially in the preparation of electrical insulation paper and honeycomb structure components in the aerospace field.

[0028] The mechanism of the present invention is:

[0029] We have developed a simple method to produce low-cost and high-performance heat-resistant paper by directly synthesizing polyimide in a cellulose paper matrix. By removing the traditional pre-synthesis of aramid fibers and subsequent paper forming, the production complexity is significantly reduced. The PI / cellulose paper is simply fabricated by a one-step method with the help of a low-temperature imidization catalyst. The obtained paper exhibits unique mechanical, thermal, and dielectric properties compared to the commercial aramid paper Nomex T410. At the same time, this heat-resistant paper also has excellent flame retardancy, hydrophobicity, and moisture absorption. More importantly, this paper is applied to honeycomb structure components and electrical insulation paper. They show better performance than the structures and papers made of aramid paper Nomex T410.

[0030] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0031] (1) The base cellulose matrix with high mechanical strength and porosity is beneficial to improving the performance of the heat-resistant paper.

[0032] (2) Low-temperature imidization catalysts such as 4-hydroxyquinoline are beneficial to the synthesis of heat-resistant paper.

[0033] (3) Commonly used polyimide monomers such as PMDA and ODA can be used for the synthesis of heat-resistant paper.

[0034] (4) The PI / cellulose paper exhibits better tensile strength (89 MPa vs. 80 MPa) than Nomex T410 aramid paper at room temperature.

[0035] (5) The PI / cellulose paper exhibits similar thermo-mechanical properties to Nomex aramid paper under heat treatment at 25 - 240 °C.

[0036] (6) The PI / cellulose paper exhibits a lower dielectric constant and lower dielectric loss than Nomex aramid paper.

[0037] (7) The hand-made honeycomb structure components exhibit better mechanical properties than aramid paper. The mechanical properties of commercial machine-made honeycomb structure components are better than those of hand-made components and far better than aramid paper.

[0038] (8) The breakdown strength of the PI / cellulose paper after 1 hour of heat treatment is higher than that of aramid paper. The PI / cellulose paper exhibits an excellent resistance retention rate compared to aramid paper within 1 to 2 hours under thermal shock at 50 - 140 °C.

[0039] (9) The PI / cellulose paper exhibits excellent flame retardancy, hydrophobicity, and moisture adsorption in humid air. Description of the Drawings

[0040] Figure 1 It is the preparation process of PAA.

[0041] Figure 2 Digital images of six PI films prepared for the examples.

[0042] Figure 3 Preparation process, digital image, and performance comparison chart of PI / cellulose paper and Nomex T410 in the examples.

[0043] Figure 4 Preparation processes of aramid and PI / cellulose paper honeycombs and paper honeycombs in the examples.

[0044] Figure 5 Infrared images and mechanical properties of PI films in the examples.

[0045] Figure 6 TG images of PI films in the examples and comparison of ultimate tensile strength.

[0046] Figure 7 Optimization of the preparation process of PI / cellulose paper in the examples, including optimization of imidization catalyst and cellulose paper matrix.

[0047] Figure 8 Microstructure and fiber analysis of cellulose paper-based fibers in the examples.

[0048] Figure 9 Possible catalytic mechanism of catalyst 4-hydroxyquinoline in the examples.

[0049] Figure 10 Mechanical property comparison between PI / cellulose paper and Nomex T410 in the examples.

[0050] Figure 11 Heat resistance, dielectric property, flame retardancy, wetting, and moisture absorption properties of PI / cellulose paper prepared in the examples.

[0051] Figure 12 Performance tests of PI / cellulose paper honeycomb structure and aramid Nomex T410 prepared in the examples.

[0052] Figure 13 Performance tests of the paper honeycomb structure prepared in the examples.

[0053] Figure 14 Motor application and insulation test of PI / cellulose paper.

[0054] Figure 15 SEM images of PI / cellulose paper and Nomex T410 and insulation mechanism diagram.

[0055] Figure 16 Production cost analysis and potential large-scale manufacturing of PI / cellulose paper. Detailed implementation manners

[0056] The present invention will be further described in detail below in conjunction with embodiments and the accompanying drawings, but the implementation manners of the present invention are not limited thereto. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0057] Unless otherwise specified, the reagents used in the examples can be conventionally purchased from the market. N,N-dimethylacetamide, 4-hydroxyquinoline, 4-hydroxyphenylacetic acid, pyridine, quinoline, pyromellitic dianhydride, and diaminodiphenyl ether were purchased from Macklin. The aramid paper Nomex T410 was purchased from Beizheng Insulating Material Factory. The epoxy resin film was purchased from Jixin Plastic Chemical Auxiliary Business. Cellulose paper 1 (80 g / m 2 ) was purchased from Cytiva. Cellulose paper 2 (100 g / m 2 ) and cellulose paper 3 (80 g / m 2 ) were purchased from Guangdong Shengtai Archival Supplies Factory. The paper honeycomb was purchased from Jingyuan Packaging. The phenolic resin was purchased from Henan Borun New Materials (2310).

[0058] The structural characteristics of polyamic acid (PAA) and polyimide (PI) were characterized using attenuated total reflection (Thermo Scientific Nicolet, IS50, America). The thermal properties of PI, PI / cellulose composite paper, cellulose paper, and Nomex T410 were evaluated using thermogravimetric analysis (TGA). Before the thermogravimetric (TG) test, the PI film and PI / cellulose paper were soaked in alcohol for 12 hours to eliminate the influence of the thermal decomposition of 4-hydroxyquinoline on the TG results. The tensile properties of PI, PI / cellulose composite paper, cellulose paper, and Nomex T410 were tested using a universal testing machine (Instron, 5565, America). In addition, the compression and bending properties of the honeycomb structure were evaluated. The microstructures of the PI / cellulose composite paper and cellulose paper were observed using a scanning electron microscope (Zeiss, EVO 18, Germany). The contact angles of PI, PI / cellulose composite paper, cellulose paper, and Nomex T410 were measured using a contact angle measuring instrument (ThetaFlex, Biolin Scientific, Finland). At room temperature, the dielectric properties of the PI / cellulose composite paper and Nomex T410 were obtained using a broadband dielectric spectrometer (Novocontrol GmbH, ALPHa-ANB, Germany). The breakdown strength of the PI composite paper and Nomex T410 after heat treatment at 50 °C to 140 °C for 1 or 2 hours was measured using an AC high-voltage generator (at a rate of 500 V / s). The resistivity of these papers was measured using an electrometer (Keithley, 6517B, America). The ring crush strength was tested using an L&W crush tester (L&W, 248, Sweden). The impact strength was measured using a pendulum impact tester.

[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, that is, heat-treated in a vacuum drying oven at 150 °C for 10 hours. Then, as Figure 1As shown, diamine monomer ODA (2.002 g) was added to a 250 mL three-necked flask equipped with a mechanical stirrer, a nitrogen inlet, and an 8 °C cold water bath, and purified DMAC (23.917 g) was added and stirred until ODA was completely dissolved to obtain a transparent diamine solution. Dianhydride monomer PMDA (2.218 g) was added to the flask in three batches (10 min / batch), and at the same time, DMAC was added to adjust the solid content of the reaction system to 10 wt.%, where the solid content refers to the percentage of the sum of the masses of the added reactant diamine monomer and dianhydride monomer in the total mass of the system (diamine monomer + dianhydride monomer + DMAC). The molar ratio of diamine to dianhydride was 1:1. The resulting mixture was stirred in a nitrogen atmosphere for 10 hours to obtain a transparent and viscous PAA solution.

[0061] (2) Preparation of PAA (polyamic acid) solution + catalyst (4-Hdql or 4-Hdpla or PD or QL or without catalyst): After obtaining the PAA solution in the above step, catalyst 4-Hdql or 4-Hdpla or PD or QL (n pAA :n 催化剂 = 1:2) or without it was added to a triangular flask, and stirring was continued for 2 h under nitrogen and an 8 °C cold water bath condition to obtain a uniformly mixed PAA (polyamic acid) solution + catalyst (4-Hdql or 4-Hdpla or PD or QL or without catalyst) solution.

[0062] (3) Preparation of PI film

[0063] The solution obtained in step (2) was coated on a glass plate using a scraper, and then, six different golden-yellow PI films and PAA films were prepared under the following conditions, as Figure 2 shown.

[0064] PI (250 °C): Without catalyst, dried at 80 °C, 120 °C, 150 °C, 200 °C, and 250 °C for 1 hour respectively.

[0065] PI (180 °C): Without catalyst, dried at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour respectively.

[0066] PI (4-Hdql): Catalyst 4-Hdql was added, and dried at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour respectively.

[0067] PI (4-Hdpla): Catalyst 4-Hdpla was added, and dried at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour respectively.

[0068] PI(PD): Add catalyst PD and dry at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour respectively.

[0069] PI(QL): Add catalyst QL and dry at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour respectively.

[0070] PAA (without adding catalyst): Without adding catalyst, dry at 80 °C for 4 hours.

[0071] (4) Preparation of PI / cellulose paper

[0072] As Figure 3 shown, the plant fiber paper is completely immersed in the PAA solution containing the catalyst for 12 hours. Then, the impregnated plant fiber paper is dried at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour respectively to produce PI / cellulose paper. As Figure 3 shown in a of Figure 3 is the process schematic diagram of the PI / cellulose paper prepared in the laboratory. Figure 3 b of

[0073] (5) Preparation of honeycomb structure

[0074] Aramid paper honeycomb and PI / cellulose paper honeycomb: As Figure 4 shown in a of

[0075] the theoretical width of the epoxy resin film is expressed as A (5.5 mm). Every two films are placed at a center distance of 4A, and multiple epoxy resin films are evenly pasted on the aramid paper or PI / cellulose paper. Then, the aramid paper or PI / cellulose paper is cut into 1-cm-wide strips perpendicular to the film, and these 1-cm-wide strips are overlapped. The overlapping process is carried out in a staggered manner, aligning the film on one strip with the centers of two films on the subsequent strip. After the overlapping process is completed, the preliminary honeycomb structure is cured at 110 °C for 1 hour. After curing, the preliminary honeycomb structure is stretched and formed into a honeycomb. The aramid paper honeycomb and PI / cellulose paper honeycomb are then immersed in phenolic resin for 5 minutes, taken out and left standing for 1 hour to remove the excess phenolic resin, and then cured at 150 °C for 30 minutes. Figure 4As shown in b of , the paper honeycomb was immersed in the PAA(4-Hdql) solution for 12 hours. Then, the PAA / paper honeycomb was dried at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour to produce PI / paper honeycomb. Subsequently, the PI / paper honeycomb was immersed in phenolic resin for 5 minutes, taken out and left standing for 1 hour to remove the excess phenolic resin, and then cured at 150 °C for 30 minutes.

[0076] Performance test:

[0077] (1) Selection of PI film catalyst

[0078] The infrared images of the six PI films and PAA films prepared in Example 1 are as shown in Figure 5 a of . The characteristic peaks C-N and C=O of PI appeared in all six PI films, and the characteristic peaks C=O(CONH), C-NH, and -OH of PAA disappeared, indicating that all six PI films were successfully prepared. The enlarged infrared images of PI(4-Hdql) and PI(250 °C) are as shown in Figure 5 b of . It can be seen that they have the same characteristic peaks, indicating that the PI film prepared from 4-Hdql has the same structure as the PI film prepared by complete thermal imidization. Figure 5 The six films on the surface of c of have similar thermal decomposition temperatures (550 °C - 600 °C). Figure 5 d of is the tensile curve of the PI film. Among them, the tensile stress of PI(4-Hdql) is 125 MPa, which is closest to the tensile stress of PI(250 °C) at 129 MPa, indicating that the imidization degree of PI(4-Hdql) is better than that of the other three catalysts.

[0079] The thermogravimetric images of the PI films prepared in Example 1 are as shown in Figure 6 a of . Except for the PI films of PI(4-Hdql) and PI(250 °C), obvious weight loss phenomena occurred in the thermogravimetric curves of the other four films after 200 °C, indicating that the imidization degrees of the other two films are not high. We calculated the imidization degrees of the six PI films using the method described in the literature (W. Chen, W. Chen, B. Zhang, S. Yang, C.-Y. Liu, Polymer 2017, 109, 205.) Figure 6 (Figure b of ). The results show that 4-hydroxyquinoline has excellent catalytic ability. Figure 6 c of shows that the mechanical properties of the PI film prepared from 4-hydroxyquinoline are 125 MPa, which is closest to the completely imidized PI film (PI(250 °C)).

[0080] (2) Properties of PI / cellulose paper

[0081] The PI / cellulose paper prepared in Example 1, with the cellulose paper being a component of the PI / cellulose paper, has a significant impact on the microstructure and mechanical properties of the composite paper. Figure 7 a-f in Figure 7 are SEM images of Cellulose Paper 1, Cellulose Paper 2, Cellulose Paper 3, and the PI / cellulose papers prepared from these three cellulose papers. Meanwhile, we carried out imide formation reactions of five PAA solutions (PAA + 4-Hdql, PAA + 4-Hdpla, PAA + PD, PAA + QL, PAA - without catalyst) on three different cellulose papers (dried at 80 °C, 120 °C, 150 °C, and 180 °C for 1 hour respectively), and conducted tensile property tests on the obtained PI / cellulose papers, the raw cellulose papers, and the aramid paper Nomex T410, specifically as Figure 7 shown in g-i of Figure 7 , where PI / cellulose paper (180 °C) represents the PI / cellulose paper prepared without catalyst. The results show that the composite paper containing the catalyst 4-Hdql has the best mechanical properties. Fibers are the basic components of paper, and the structure and basic properties of fibers significantly affect the properties of the finished paper. Figure 8 shows the microstructure and fiber analysis of three kinds of paper fibers (Cellulose Paper 1, Cellulose Paper 2, Cellulose Paper 3). Figure 8 a-c in Figure 8 qualitatively reveal that the fibers of cellulose 1 are finer and shorter than those of cellulose 2 and cellulose 3. Figure 8 d-i in Figure 8 quantitatively show the fiber analysis of the three cellulose papers. The length, width, and roughness of cellulose fiber - 1 (i.e., the fiber of Cellulose Paper 1) are all smaller than those of cellulose fiber - 2 and cellulose fiber - 3 ( Figure 8 shown in d-f of Figure 8 ), resulting in fewer fiber bonding points and areas, and thus lower paper strength. The fibers should be relatively straight and parallel to maximize the bonding force between fibers. Cellulose 1 has more bent and twisted fibers than cellulose 2 and cellulose 3 ( Figure 8 shown in g-h of Figure 8 ), and the bent and twisted fibers may reduce the contact area between fibers, leading to a decrease in paper strength. Although a higher degree of fiber separation increases the fiber bonding points ( Figure 8 shown in i of Figure 8 ), the short fiber length results in lower overall paper strength. 4-Hdql contains the characteristic groups of two kinds of catalysts, namely hydroxy acid and azacyclic, and its potential catalytic mechanism for imide formation is as Figure 9 shown. 4-Hdql has a lone pair of electrons on the nitrogen atom. During the catalytic process, the lone pair of electrons on the nitrogen can participate in the reaction as a nucleophile, endowing the nitrogen with strong nucleophilicity. It can form hydrogen bonds I and II with the PAA molecular chain, resulting in the formation of PI during the imide dehydration process. The -OH group of 4-Hdql may also provide a proton H+, increasing the electropositivity of C=O in the polyamic acid, promoting the nucleophilic attack of nitrogen, thus accelerating the cleavage of C-O and promoting the imide dehydration process to generate PI. Figure 10Figure a shows the comparison of the maximum stress of several PI / cellulose papers 3 (where PI / cellulose paper 3 (180 °C) refers to the PI / cellulose paper obtained by imidization reaction by drying for 1 hour at 80 °C, 120 °C, 150 °C, and 180 °C without adding a catalyst, PI / cellulose paper-3 (4-Hdql) refers to the PI / cellulose paper obtained by imidization reaction by drying for 1 hour at 80 °C, 120 °C, 150 °C, and 180 °C with the addition of catalyst 4-Hdql, PI / cellulose paper-3 (4-Hdpla) refers to the PI / cellulose paper obtained by imidization reaction by drying for 1 hour at 80 °C, 120 °C, 150 °C, and 180 °C with the addition of catalyst 4-Hdpla, PI / cellulose paper-3 (PD) refers to the PI / cellulose paper obtained by imidization reaction by drying for 1 hour at 80 °C, 120 °C, 150 °C, and 180 °C with the addition of catalyst PD, PI / cellulose paper-3 (QL) refers to the PI / cellulose paper obtained by imidization reaction by drying for 1 hour at 80 °C, 120 °C, 150 °C, and 180 °C with the addition of catalyst QL) and Nomex T410. The stress of PI / cellulose paper-3 (4-Hdql) is 89 MPa, which is approximately 12% higher than that of Nomex T410. Figure 10 Figure b visually shows the mechanical properties of PI / cellulose paper-3 (4-Hdql) and Nomex T410; PI / cellulose paper-3 (4-Hdql) can withstand at least 4.7 kg of weight, far exceeding the maximum weight of 3.76 kg that Nomex T410 can withstand.

[0082] The excellent heat resistance of the composite paper is necessary to ensure the normal operation of the motor. Figure 11 Figure a shows that we set six temperature levels between 100 °C and 240 °C for PI / cellulose paper-3 (with catalyst 4-Hdql, thermal imidization conditions: 80 °C, 1 h; 120 °C, 1 h; 150 °C, 1 h; 180 °C, 1 h) and Nomex T410, and each level was heated for 0.5 hours and 1 hour to simulate the heat treatment of the insulating paper in actual applications. The digital images after heat treatment are as Figure 11 shown in Figure b, where the color change of Nomex T410 is very small after high-temperature treatment, while the PI / cellulose paper darkens after high-temperature treatment due to the presence of cellulose paper, which is attributed to the destruction of cellulose at high temperatures. Figure 11 Figure c shows the maximum stress and modulus of rupture of the samples after heat treatment. At room temperature, the stress and modulus of the PI / cellulose paper are higher than those of Nomex T410, which may be attributed to the high surface density of the PI / cellulose paper, while the surface of Nomex T410 is more porous. Figure 11d and e respectively indicate that, compared with Nomex T410, the stress of PI / cellulose paper is comparable, while the modulus of PI / cellulose paper is significantly higher than that of Nomex T410. Figure 11 f shows the TG analysis of PI / cellulose paper, where PI refers to PI(4-hdql). The two distinct steps in the weight loss curve of PI / cellulose paper correspond to the thermal degradation of cellulose and PI respectively. The composite paper with excellent flame retardancy can significantly improve the safety of motor operation and aircraft flight. We conducted flame tests on PI (referring to PI(4-hdql)), PI / cellulose paper, and Nomex T410 ( Figure 11 g). Since the cellulose in PI / cellulose paper is exposed, it will burn slightly when first exposed to the ignition source. However, because its surface and cross-section are wrapped by PI, the flame will be immediately extinguished due to the excellent flame retardancy of PI. Excellent dielectric properties are necessary to ensure the safe operation of insulating materials. Figure 11 h and i show the dielectric parameters of PI / cellulose paper under varying frequency conditions. The dielectric constant and loss of PI / cellulose paper are lower than those of Nomex T410, which helps to maintain the insulation performance and avoid losses during the curing process; the low dielectric loss reduces the energy loss and heating under the influence of the electric field. Figure 11 j shows the wettability of PI (referring to PI(4-hdql)), PI / cellulose paper, and Nomex T410. Wettability is related to the surface energy and pore size of the sample. Nomex T410 has more pores on its surface, allowing water to penetrate faster and having a smaller contact angle. In contrast, PI / cellulose paper has good wettability, providing a guarantee for the normal operation of the equipment. We placed PI / cellulose paper and Nomex T410 in an environment of 60 °C for 12 hours to dry their moisture, and then tested their moisture absorption rates in an environment of 25 °C temperature and 40% humidity. From production to storage and transportation, and then to use, the moisture absorption rate of the composite paper is also a key parameter to ensure a stable use environment. Figure 11 k shows the moisture content curves of PI / cellulose paper and Nomex T410 at different times after being dried and placed at room temperature. It can be seen that the moisture content of PI / cellulose paper and Nomex T410 will rise to a certain extent within one hour after being exposed to room temperature conditions, but both are lower than the moisture content at the time of delivery, which is 6%.

[0083] (3) Performance of honeycomb structure

[0084] Moreover, to evaluate the practicality, we followed the industrial honeycomb core production process, namely "gluing - stacking - drying - stretching - drying - impregnating with phenolic resin - drying", and used PI / cellulose paper (where the cellulose paper is cellulose paper 3, the catalyst is 4 - hdql, and the thermal imidization parameters are 80°C, 1 h; 120°C, 1 h; 150°C, 1 h; 180°C, 1 h) and Nomex T410 to fabricate a robust honeycomb structure and evaluate its mechanical properties. Aramid honeycombs are mainly applied in the aerospace field, such as for aircraft fuselage components, cabin doors, blades, tail fins, etc.( Figure 12 as shown in a) of Figure 12 , which can effectively enhance the structural strength and meet the lightweight requirements. First, to evaluate the advantages of PI / cellulose paper as a component of the honeycomb structure, we conducted a ring compression test on Figure 12 as shown in b) of 2 . The results showed that the ring compression strength of PI / cellulose paper was much higher than that of Nomex T410. Then, we conducted an impact test on the two honeycomb structures Figure 12 as shown in c) of Figure 12 . The results showed that the impact strength of PI / cellulose paper was 1.1 kJ / m Figure 12 , which was better than that of the Nomex T410 honeycomb structure. Then, we conducted compression and bending tests on the two honeycomb structures Figure 12 as shown in d - g) of Figure 12 . The results showed that the compression stress of the PI / cellulose paper honeycomb structure was much better than that of the Nomex T410 honeycomb structure Figure 13 as shown in d) of Figure 13 , and the compression modulus was 67% higher than that of the Nomex T410 honeycomb structure

[0085] (4) Performance of motor insulation paper

[0086] as shown in e) of

[0085] (4) Performance of motor insulation paper

[0086] , which may be attributed to the denser structure of PI / cellulose paper providing better strength for the honeycomb structure. The results of the three - point bending test Figure 12 as shown in f) of Figure 12 showed that compared with the Nomex T410 honeycomb structure, the modulus of the PI / cellulose paper honeycomb structure was 50% higher Figure 13 as shown in g) of Figure 13 , indicating greater bending force, but it was slightly more brittle. Figure 13 shows the large - scale manufacturing process of composite honeycombs in a factory environment, namely pulping - papermaking - automatic paper - forming honeycomb - honeycomb forming. In the laboratory, after the PI in the paper honeycomb forms imide, it is molded into a composite - formed honeycomb. This composite material was then subjected to compression, bending, and impact tests Figure 13 as shown in b and f) of

[0085] (4) Performance of motor insulation paper

[0086] , and its performance exceeded that of the two hand - made honeycombs. This performance improvement may be attributed to the automated forming process of commercial honeycombs, which results in a more uniform stress distribution within the honeycomb, thus bringing more superior performance.

[0085] (4) Performance of motor insulation paper

[0086] To evaluate the stability and reproducibility of PI / cellulose paper, we placed PI / cellulose paper (where the cellulose paper was cellulose paper 3, the catalyst was 4-hdql, and the thermal imidization parameters were 80 °C, 1 h; 120 °C, 1 h; 150 °C, 1 h; 180 °C, 1 h) and Nomex T410 in motor insulating oil for high-temperature heat treatment, that is, at 50 °C, 80 °C, 120 °C, and 140 °C, each holding for 0.5 h or 1 h, to simulate the actual operating environment of the motor and evaluate the main properties of PI / cellulose paper as a motor insulating paper. Figure 14 Figure a shows the position of the insulating paper in an electric vehicle. It is used to isolate the copper wire in the motor slot, reduce electromagnetic interference inside the motor, and improve the stability and efficiency of motor operation. Figure 14 Figures b and c show the breakdown strength and breakdown strength retention rate of PI / cellulose paper and Nomex T410 after being treated at the actual application ambient temperature for 1 hour. The breakdown strength of PI / cellulose paper remained above 40 kV / mm, while the breakdown voltage of Nomex T410 was mostly below 30 kV / mm. The breakdown strength retention rate of PI / cellulose paper was higher than that of Nomex T410. After being treated at the same temperature for 2 hours, the breakdown strength of PI / cellulose paper still remained above 35 kV / mm ( Figure 14 Figure d), and the breakdown strength retention rate was above 80% ( Figure 14 Figure e), showing better performance than Nomex T410. This may be attributed to the more superior mechanical properties and heat resistance of PI / cellulose paper compared to Nomex T410, as well as its denser surface ( Figure 15 Figures a and b). Figure 15 Figures c and d show the high-voltage breakdown diagrams of PI / cellulose paper (where the cellulose paper was cellulose paper 3, the catalyst was 4-hdql, and the thermal imidization parameters were 80 °C, 1 h; 120 °C, 1 h; 150 °C, 1 h; 180 °C, 1 h) and Nomex T410. At high voltages, high mechanical properties help reduce compressive stress breakdown under the electric field; high heat resistance and excellent dielectric properties are beneficial for reducing leakage and dielectric loss when the temperature rises; at the same time, the dense structure generates fewer electrons, reducing the breakdown probability. Figure 14 Figure f shows the resistivity retention rate of PI / cellulose paper and Nomex T410 after being treated at the actual application ambient temperature for 1 hour and 2 hours. The resistivity retention rate of PI / cellulose paper was higher than that of Nomex T410, which helped reduce the conductive path and leakage current, thereby enhancing the insulation effect of the material. Figure 14 Figure g shows the modulus retention rate of PI / cellulose paper and Nomex T410 after high-temperature treatment. The high modulus retention rate ensures the durability of the insulating material under high-temperature applications.

[0087] We have estimated the raw material costs required for manufacturing PI / cellulose paper. Figure 16 The results presented in a of Figure 16 show that the production cost of PI / cellulose paper is significantly lower than that of Nomex T410, with a reduction of 40.5%, indicating the commercial feasibility of industrial-scale production of PI / cellulose paper. In addition, compared with the manufacturing process of traditional aromatic polyamide fiber paper that requires 9 steps, our production process of PI / cellulose paper can be completed in only 4 steps, greatly simplifying the production process and providing economic benefits.

[0088] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for producing aramid-like heat-resistant paper by directly synthesizing polyimide in a cellulose paper matrix at low temperature, characterized in that The following steps are involved: (1) Preparation of PAA solution: under nitrogen flow, diamine monomer is added into an organic solvent and stirred to dissolve to obtain a transparent diamine solution, and then dianhydride monomer is added and stirred to react to obtain a transparent PAA solution; (2) Preparation of a PAA solution containing a catalyst: adding a catalyst to the PAA solution of step (1), and mixing the mixture to obtain a PAA solution containing a catalyst; (3) Preparation of polyimide (PI) / cellulose paper: The plant fiber paper is completely immersed in a PAA solution containing a catalyst for immersion treatment, and then the immersed plant fiber paper is taken out and subjected to a thermal imidization reaction to obtain PI / cellulose paper.

2. The method for producing aramid-like heat-resistant paper by directly synthesizing polyimide by low-temperature imidization in a cellulose paper matrix according to claim 1, characterized in that: The diamine monomer described in step (1) is at least one of 4,4'-diaminodiphenyl ether, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, and 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (2,2'-DCB); the dianhydride monomer is at least one of pyromellitic acid dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 3,3'4,4'-benzophenonetetracarboxylic dianhydride, and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA); the molar ratio of the diamine monomer to the dianhydride monomer is 1:1-1.02, preferably 1:

1.

3. The method for producing aramid-like heat-resistant paper by directly synthesizing polyimide in a cellulose paper matrix through low-temperature imidization according to claim 1, characterized in that: The step (1) of adding the diamine monomer to the organic solvent and stirring to dissolve to obtain a transparent diamine solution is carried out in a cold water bath at 0-12°C; The organic solvent in step (1) is at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone and dimethyl sulfoxide; The amount of the organic solvent in step (1) is such 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; The stirring reaction temperature in step (1) is 0-25° C.; the stirring reaction time is 4-24 h.

4. The method for producing aramid-like heat-resistant paper by directly synthesizing polyimide in a cellulose paper matrix through low-temperature imidization according to claim 1, characterized in that: The catalyst described in step (2) is at least one of 4-hydroxyquinoline, 4-hydroxyphenylacetic acid, pyridine and quinoline, preferably 4-hydroxyquinoline.

5. The method for producing aramid-like heat-resistant paper by directly synthesizing polyimide in a cellulose paper matrix through low-temperature imidization according to claim 1, characterized in that: The amount of the catalyst used in step (2) satisfies the molar ratio of polyamic acid to catalyst of 1:2; The mixing described in step (2) refers to stirring and mixing in a nitrogen atmosphere and a cold water bath at 0-25°C.

6. The method for producing aramid-like heat-resistant paper by directly synthesizing polyimide in a cellulose paper matrix through low-temperature imidization as claimed in claim 1, characterized in that: The plant fiber paper described in step (3) refers to a paper having a basis weight of 60-100 g / m 2 Plant fiber paper.

7. The method for producing aramid-like heat-resistant paper by directly synthesizing polyimide in a cellulose paper matrix through low-temperature imidization as claimed in claim 1, characterized in that: The immersion time in step (3) is 8-24 hours; the immersion refers to immersion at room temperature; The thermal imidization reaction in step (3) refers to keeping the temperature at 70-90°C for 0.5h-1h, then heating to 110-130°C for 0.5h-1h, then heating to 140-160°C for 0.5h-1h, and then heating to 170-190°C for 0.5h-1h.

8. A heat-resistant paper prepared by the method according to any one of claims 1 to 7.

9. Use of the heat-resistant paper according to claim 8 in the fields of electrical insulation, electric vehicles and aerospace.

10. Use of the heat-resistant paper according to claim 8 in the preparation of electrical insulating paper and honeycomb structure components in the aerospace field.

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