A low-temperature curing polyimide foam material and its preparation method

Abstract

This invention discloses a low-temperature curing polyimide foam material and its preparation method, belonging to the field of polymer materials technology. The low-temperature curing polyimide foam material is prepared from aromatic dianhydride, alcohol solvent, and isocyanate as the main raw materials. The isocyanate group to aromatic dianhydride molar ratio is 160:100, and the alcohol solvent to aromatic dianhydride molar ratio is 0.5. This invention solves the technical problems of high curing temperature, high energy consumption, and low mechanical strength in existing polyimide foams, while simultaneously achieving synergistic improvements in low-temperature curing, mechanical properties, thermal stability, and flame retardancy.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a low-temperature curing polyimide foam material and its preparation method. Background Technology

[0002] Polyimide foam, known as "plastic gold" due to its excellent high and low temperature resistance, inherent flame retardancy, thermal insulation, corrosion resistance, and lightweight properties, can withstand long-term operating temperatures of 200–300℃, short-term temperatures of 400–500℃, and remains brittle even at extremely low temperatures of -269℃. It is one of the best-performing organic polymer foam materials currently available. This material has irreplaceable application value in high-precision fields such as aerospace, shipbuilding and submarines, rail transportation, and electronic new energy, and is also a core material for addressing the weight reduction, thermal insulation, and fire protection needs of high-end equipment.

[0003] Isocyanate-based polyimide foam is prepared through a one-step free foaming process using acid anhydrides and polyisocyanates as the main reactants. This method is characterized by its simple preparation process and high production efficiency, leading to its widespread research and application. However, the poor solubility of aromatic dianhydrides in common organic solvents, which leads to their precipitation in subsequent processes, limits their industrial application. Therefore, the current industrial practice typically involves first esterifying the aromatic dianhydride into a diacid diester structure using an excess of alcohol solvent to prepare foaming material A, which is then mixed with isocyanate for foaming. Chinese patent applications CN202010535963.7, CN202511583650.8, and US patent applications US3314923, US3562189, US3644234, and US3772216 all disclose a method of mixing aromatic dianhydride with a polar solvent, using a low-molecular-weight alcohol to undergo an esterification reaction to obtain an esterification precursor solution; then adding a foaming agent, surfactant, and isocyanate and rapidly mixing, followed by heating and curing to obtain a polyimide foam material.

[0004] Although the diacid diester structure can react with isocyanates to produce polyimide foam while ensuring solubility, its curing temperature is too high and the degree of imidization is low, which greatly limits its application in shipbuilding, aerospace and other fields. At the same time, in the existing technology, simply improving solubility will sacrifice reactivity, and improving reactivity will lead to a decrease in solubility. It is difficult to balance the solubility, reactivity and final performance of the material, and it cannot meet the stringent requirements of high-end military and civilian fields for the comprehensive performance of materials.

[0005] Therefore, there is an urgent need to develop a new preparation method that can effectively solve the problem of dissolving material A, while taking into account the mechanical strength, thermal stability and flame retardancy of the foam, and is simple and easy to industrialize, so as to meet the application needs of high-end military and civilian fields. Summary of the Invention

[0006] In view of this, the main objective of the present invention is to provide a low-temperature curing polyimide foam material and its preparation method, which solves the technical problems of high curing temperature, high energy consumption and low mechanical strength of existing polyimide foams, and at the same time achieves synergistic improvement in low-temperature curing, mechanical properties, thermal stability and flame retardancy of foam materials.

[0007] To achieve the above objectives, as a first aspect of the present invention, a low-temperature curing polyimide foam material is proposed. The polyimide foam material is prepared from the main raw materials aromatic dianhydride, alcohol solvent and isocyanate, wherein the molar ratio of isocyanate groups to aromatic dianhydride in the isocyanate is 160:100; and the molar ratio of alcohol solvent to aromatic dianhydride is 0.5.

[0008] Furthermore, the aromatic dianhydride is selected from monomers having diacid anhydride groups, wherein the monomer is one or a combination of 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), 3,3',4,4'-triphenyl diether tetracarboxylic dianhydride, and 3,3',4,4'-tetracarboxylic diphenyl ether dianhydride.

[0009] Furthermore, the alcohol solvent is selected from one or more combinations of methanol, ethanol, propanol, and furfuryl alcohol.

[0010] Furthermore, the isocyanate may be selected from one or both of PM-200 and PM-400.

[0011] In a second aspect of the present invention, a method for preparing the above-mentioned low-temperature curing polyimide foam material is provided, comprising: (1) preparing foaming material one: aromatic dianhydride, alcohol solvent, tetrahydrofuran and ring-opening catalyst are added sequentially to reaction vessel one, the temperature is raised to 60±3℃ and stirred until the mixture is clear and transparent, and a foam stabilizer is added and mixed evenly to obtain foaming material one; (2) preparing composite foaming material: aromatic dianhydride and N,N-dimethylformamide are added sequentially to reaction vessel two, the temperature is raised to 100±3℃ and stirred until the mixture is dissolved and in a homogeneous state; after cooling to 60±3℃, an anti-precipitation agent is added. Mix the agent and foam stabilizer evenly to obtain foam material 2; mix foam material 1 and foam material 2 evenly to prepare composite foam material; (3) Prepare foam material: at room temperature, add isocyanate, curing agent and deionized water to the composite foam material in sequence, stir the mixture at 1000~2000r / min for 10-15s until the solution turns white, transfer the mixture to the mold, and allow it to foam freely at room temperature and pressure, and allow it to stand still to complete the initial curing; (4) Low temperature curing imidization: send the initially cured foam material into the oven and use step-type low temperature heating to cure it to obtain polyimide foam material.

[0012] Furthermore, step (4) involves stepwise low-temperature heating curing, which includes curing at 60±5℃ for 2 hours, followed by heating at a rate of 3℃ / min to 150±5℃ for 4 hours.

[0013] Furthermore, in step (1), the mass ratio of tetrahydrofuran to aromatic dianhydride is 122:100; the mass ratio of ring-opening catalyst to aromatic dianhydride is 0.2~1:100; the mass ratio of foam stabilizer to aromatic dianhydride is 5~10:100; and the esterification index of foaming material one is 1.

[0014] Furthermore, in step (2), the mass ratio of N,N-dimethylformamide to aromatic dianhydride is 122:100; the mass ratio of anti-precipitation agent to aromatic dianhydride is 5~10:100; the mass ratio of foam stabilizer to aromatic dianhydride is 5~10:100; the esterification index of foaming material II is 0; and the esterification index of composite foaming material is 0.5.

[0015] Furthermore, in step (3), the molar ratio of isocyanate groups to all aromatic dianhydrides in the curing agent is 40:100; the mass ratio of the amount of deionized water added to the total amount of all aromatic dianhydrides is 6:100.

[0016] Furthermore, the ring-opening catalyst is selected from one or more of dimethylimidazolium, isoquinoline, triethanolamine, and dibutyltin dilaurate; the anti-precipitation agent is selected from polyvinylpyrrolidone; the foam stabilizer is selected from one or more of AK-8805, DC-193, DC-198, AK-8803, L550, and octadecyl dimethylamine oxide; and the curing agent is selected from one or more of N-3390 and polymethylene polyphenyl polyisocyanate.

[0017] Based on the above technical solution, it can be seen that the technical solution of the present invention achieves synergistic optimization of solubility, reactivity and material properties by precisely controlling the esterification index of the foaming material. Its chemical principle and technical effect are as follows: This invention involves preparing two foaming materials with esterification indices R=0 and R=1 respectively, and mixing the two materials in a certain proportion to obtain a composite foaming material with an esterification index R=0.5. The composite foaming material is then mixed with isocyanate, subjected to free foaming, and cured by step-low temperature heating to obtain a low-temperature cured polyimide foam material. Specifically, the foaming material with an esterification index R=1 forms a diacid diester structure through the esterification reaction of aromatic dianhydride with an alcohol solvent. This structure can form hydrogen bonds with the solvent and exhibits good solubility. The foaming material with an esterification index R=0 is an aromatic dianhydride solution that has not undergone esterification, retaining complete anhydride groups and exhibiting high reactivity.

[0018] By utilizing the characteristic of the esterification structure of foaming material with an esterification index R=1 to form hydrogen bonds with the solvent, and supplemented by foam stabilizers, small molecule impurities in the system, and anti-precipitation additives, the purity of aromatic dianhydride in foaming material with an esterification index R=0 is reduced, its crystallinity is destroyed, and the precipitation of aromatic dianhydride before foaming is avoided. This completely solves the problem of poor solubility of aromatic dianhydride in existing technologies and provides a guarantee for industrial production.

[0019] The composite foam material combines the high reactivity of an esterification index R=0 with the good solubility of an esterification index R=1, enabling complete imidization and curing at low temperatures. Compared to the high-temperature curing process in existing technologies, this significantly reduces production energy consumption and avoids the problem of molecular chain thermal degradation caused by high temperatures. Furthermore, the composite foam material with an esterification index R=0.5 retains more anhydride groups in its molecular chain structure, increasing the degree of imidization of the foam, reducing the formation of byproducts such as urea groups, leading to increased polymerization degree and reduced internal defects. This, in turn, significantly improves the mechanical properties, thermal stability, and flame retardant properties of the foam material.

[0020] The polyimide foam material prepared by this invention, after testing, can meet the requirements of high-end applications in terms of apparent density, tensile strength, compression set, and flame retardancy. Among them, the limiting oxygen index is ≥39%, which meets the flame retardancy requirements and is fully suitable for the harsh working conditions of aerospace, ships, submarines, etc.

[0021] It should be understood that the description in the Summary of the Invention is not intended to limit the key or essential features of the embodiments of the present invention, nor is it intended to restrict the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below.

[0023] Figure 1 These are the FTIR spectra of polyimide foam materials with different esterification indices according to the present invention; Figure 2 This is a DTG curve of polyimide foam materials with different esterification indices according to the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] The terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of the invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0026] As one of the objectives of this invention, a low-temperature curing polyimide foam material is proposed. This polyimide foam material is prepared from aromatic dianhydride, an alcohol solvent, and isocyanate as the main raw materials. The molar ratio of isocyanate groups to aromatic dianhydride in the isocyanate is 160:100; the molar ratio of alcohol solvent to aromatic dianhydride is 0.5. In preparing the low-temperature curing polyimide foam material, excessive alcohol solvent should not be used to avoid excessive esterification and the formation of diacid esters, thus achieving the requirement of low-temperature curing. The molar ratio of alcohol solvent to aromatic dianhydride is also called the esterification index R. The esterification index R of the composite foaming material of the low-temperature curing polyimide foam material of this invention is 0.5.

[0027] By limiting the molar ratio of isocyanate groups to aromatic dianhydrides in isocyanate, both excess and appropriate amounts of isocyanate groups are ensured. This allows for sufficient reaction with aromatic dianhydrides (and subsequent esterification products) to form polyimide molecular chains, while avoiding internal defects in the foam caused by excessive isocyanate residue. By limiting the molar ratio of alcohol solvents to aromatic dianhydrides, the degree of esterification of aromatic dianhydrides can be precisely controlled. This ensures that some aromatic dianhydrides are esterified to form structures that improve solubility, while retaining enough unesterified anhydride groups to maintain high reactivity. This achieves a preliminary balance between solubility and reactivity, providing a reasonable raw material basis for the subsequent preparation of composite foam materials.

[0028] In a preferred embodiment, the aromatic dianhydride is selected from monomers having diacid anhydride groups, wherein the monomer is one or a combination of 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), 3,3',4,4'-triphenyl diether tetracarboxylic dianhydride, and 3,3',4,4'-tetracarboxylic diphenyl ether dianhydride. The diacid anhydride groups can react fully with the isocyanate groups to form structurally stable polyimide molecular chains, giving the foam excellent high and low temperature resistance and flame retardancy.

[0029] In a preferred embodiment, the alcohol solvent is selected from one or more combinations of methanol, ethanol, propanol, and furfuryl alcohol. These solvents exhibit good polarity and solubility, and can undergo esterification with aromatic dianhydrides, converting some of the aromatic dianhydrides into diacid diester structures. These structures can form hydrogen bonds with the solvent in the system, significantly improving the solubility of the aromatic dianhydrides and preventing their precipitation. These alcohols also have moderate reactivity, do not undergo side reactions with isocyanates, and do not leave behind difficult-to-remove impurities, thus not affecting subsequent imidization reactions and foam properties. Furthermore, these alcohols are low in toxicity and highly volatile, allowing for easy removal during subsequent curing processes without adversely affecting product performance or the environment.

[0030] In a preferred embodiment, the isocyanate is selected from one or both of PM-200 and PM-400. The content of NCO groups is 30.5% to 32.0%. This type of isocyanate has moderate reactivity, allowing it to react with aromatic dianhydrides and their derivatives at low temperatures. It is suitable for step-by-step low-temperature curing processes, avoiding uneven foaming due to excessively high reactivity or insufficient imidization due to excessively low reactivity. Simultaneously, its molecular structure is stable and will not decompose during low-temperature curing, ensuring the foam's temperature resistance and flame retardancy.

[0031] Commercially available polyimide foams typically have a thermal imidization temperature as high as 220°C. During oven heating, the system experiences competing reactions including thermal imidization, thermal decomposition, and other side reactions. In contrast, the foam material of this invention exhibits an increased anhydride group ratio as the esterification index R decreases. This allows the composite foaming material to react more quickly and fully with isocyanates to form a stable imidized structure, improving temperature resistance and preventing other side reactions during further heating.

[0032] Foam samples with esterification indices R of 0, 0.5, 1.0, 1.5, and 2.0, cured at 120℃, were selected and subjected to infrared spectroscopy to obtain the following results: Figure 1 The infrared FTIR spectra shown are obtained by performing thermal decomposition tests. Figure 2 The DTG curve shown.

[0033] Depend on Figure 1 The infrared spectra clearly show that at wavelengths of 1726, 1658, 1539, 1380, and 717, the peak values ​​of samples with esterification index R values ​​of 0 and 0.5 are significantly reduced compared to other samples. This indicates that at a curing temperature of 120°C, samples with esterification index R values ​​of 0 and 0.5 can undergo partial imidization, while no obvious imidization structural features were found in other samples. Therefore, the polyimide foam material of this invention undergoes a certain degree of imidization during low-temperature curing.

[0034] Depend on Figure 2The DTG curve clearly shows that in the 230–300℃ range, the weight loss peak of the sample weakens significantly as the R value decreases. When R drops to 0.5, the weight loss peak almost disappears, indicating that the sample has completed sufficient thermal imidization and can effectively prevent further decomposition at 230–300℃. Therefore, the polyimide foam material of this invention has a higher degree of imidization than other products, which also demonstrates its stable performance.

[0035] The DTG curve of the sample with an esterification index R=0 shows that thermal decomposition mainly occurred in the first stage, from 150 to 280°C, almost covering the two stages of 150–210°C and 230–300°C. This curve, combined with the infrared spectrum of the sample with an esterification index R=0, indicates that although the imidization reaction was the fastest, the DTG curve showed a large weight loss peak, indicating that the sample contained a significant amount of aromatic dianhydride and isocyanate that did not participate in the reaction and did not form an imidized structure, thus undergoing thermal decomposition at high temperatures. Furthermore, the incomplete dissolution of the aromatic dianhydride resulted in a high isocyanate index in the foaming system. The high reactivity of the -NCO functional groups in the isocyanate easily led to the generation of byproducts such as urea groups in the system, resulting in continued high-temperature decomposition. Therefore, compared to R=0.5, the polyimide foam material of this invention exhibits better performance stability.

[0036] As a second objective of this invention, this invention proposes a method for preparing the aforementioned low-temperature curing polyimide foam material, comprising: S1: Preparation of foaming material one: Aromatic dianhydride, alcohol solvent, tetrahydrofuran and ring-opening catalyst are added sequentially into reaction vessel one. The temperature is raised to 60±3℃ and stirred for 3-5 hours until the mixture is clear and transparent. Foam stabilizer is added and mixed evenly to obtain foaming material one. S2: Preparation of composite foaming material: Aromatic dianhydride and N,N-dimethylformamide are added sequentially to reaction vessel two, the temperature is raised to 100±3℃ and stirred for 20±5 min until the mixture dissolves and becomes homogeneous; after cooling to 60±3℃, anti-precipitation additive and foam stabilizer are added and mixed evenly to obtain foaming material two; foaming material one and foaming material two are mixed evenly to prepare composite foaming material; S3: Preparation of foam material: At room temperature, add isocyanate, curing agent and deionized water to the composite foam material in sequence. Stir the mixture at 1000~2000r / min for 10s-15s until the solution turns white. Transfer the mixture to the mold and allow it to foam freely at room temperature and pressure. Let it stand for 20±5min to complete the initial curing. S4: Low-temperature curing imidization: The pre-cured foam material is sent into an oven and cured by step-by-step low-temperature heating to obtain polyimide foam material.

[0037] The preparation method of this invention provides a simple and easily industrialized low-temperature curing preparation method. By precisely controlling the esterification index, the problem of easy precipitation of aromatic dianhydrides is completely solved, realizing low-temperature curing of foam, reducing production energy consumption, and ensuring that the comprehensive performance of foam (mechanical properties, temperature resistance, flame retardancy, etc.) meets the standards, thus satisfying the application needs of high-end fields.

[0038] In a preferred embodiment, the stepped low-temperature curing includes curing at 60±5℃ for 2-3 hours, followed by curing at a rate of 3℃ / min to 150±5℃ for 3-5 hours. The first step of low-temperature curing allows the foam to initially take shape after foaming, preventing deformation and collapse during subsequent heating. The slow heating (3℃ / min) allows solvents and small molecule impurities inside the foam to gradually evaporate, reducing defects such as pores and cracks inside the foam. The second step of medium-temperature curing promotes full cross-linking of molecular chains, achieving complete imidization and ensuring that the foam's temperature resistance, mechanical properties, and flame retardant properties meet the standards. Compared with the high-temperature curing process of the prior art, this stepped low-temperature curing can significantly reduce energy consumption while avoiding thermal degradation of molecular chains caused by high temperatures, thus improving the product qualification rate.

[0039] In a preferred embodiment, in the preparation of foaming material one, the mass ratio of tetrahydrofuran to aromatic dianhydride is 122:100; the mass ratio of ring-opening catalyst to aromatic dianhydride is 0.2~1:100; the mass ratio of foam stabilizer to aromatic dianhydride is 5~10:100; and the esterification index of foaming material one is 1. The aromatic dianhydride used in this process is the first addition. The various proportions of raw materials and additives ensure that the esterification index of foaming material one is consistently 1, exhibiting good solubility, while also guaranteeing the system stability and reactivity of foaming material one. This provides a reliable foundation for the preparation of composite foaming materials and avoids problems such as poor solubility and uneven foaming caused by imbalanced proportions.

[0040] In a preferred embodiment, in the preparation of the composite foaming material, the mass ratio of N,N-dimethylformamide to aromatic dianhydride is 122:100; the mass ratio of the anti-precipitation agent to aromatic dianhydride is 5-10:100; the mass ratio of the foam stabilizer to aromatic dianhydride is 5-10:100; the esterification index of the second foaming material is 0; and the esterification index of the composite foaming material is 0.5. The aromatic dianhydride used in this process is the second addition. These various ratios of raw materials and additives ensure that the esterification index of the second foaming material remains stable at 0, retaining high reactivity, while simultaneously ensuring that the esterification index of the composite foaming material remains stable at 0.5, achieving an optimal balance between solubility and reactivity, and completely solving the problem of aromatic dianhydride precipitation.

[0041] In a preferred embodiment, in the preparation of the foam material, the molar ratio of isocyanate groups to all aromatic dianhydrides in the curing agent is 40:100; the mass ratio of deionized water to the total amount of all aromatic dianhydrides is 6:100. All aromatic dianhydrides are the sum of the aromatic dianhydrides added in the first and second additions. This ratio ensures sufficient foaming and complete curing, reduces defects such as internal pores and cracks in the foam, and improves the overall performance of the foam.

[0042] In the above embodiments, the esterification index is the molar ratio of alcohol solvent to aromatic dianhydride. Precise control of the alcohol solvent in each step is to avoid excessive esterification by excessive alcohol solvent to form diacid diester structure, thereby increasing the curing temperature and reducing the degree of imidization.

[0043] In a preferred embodiment, the ring-opening catalyst is selected from one or more of dimethylimidazolium, isoquinoline, triethanolamine, and dibutyltin dilaurate; the anti-precipitation agent is selected from polyvinylpyrrolidone; the foam stabilizer is selected from one or more of AK-8805, DC-193, DC-198, AK-8803, L550, and octadecyl dimethylamine oxide; and the curing agent is selected from one or more of N-3390 and polymethylene polyphenyl polyisocyanate. The NCO group content in the polymethylene polyphenyl polyisocyanate is 19.6%. The selected ring-opening catalysts all possess excellent catalytic activity, allowing for precise control of the esterification reaction rate to ensure the esterification index of foaming material one meets the standard, without causing side reactions with other components in the system. The anti-precipitation agent is polyvinylpyrrolidone, which has good dispersibility and compatibility, effectively disrupting the crystallinity of aromatic dianhydrides and inhibiting their precipitation without affecting foam performance. The selected foam stabilizer improves the surface tension of the foaming system, resulting in a uniform foam pore structure, avoiding problems such as foam breakage and collapse, and enhancing the structural stability of the foam. The selected curing agent has moderate reactivity, is suitable for low-temperature curing processes, and can promote full cross-linking of molecular chains, improving the degree of polymerization and overall performance of the foam. Multiple additives can be flexibly combined and adjusted according to the type of raw materials and production conditions, reducing production costs and adapting to large-scale industrial production.

[0044] Example 1 This embodiment provides a method for preparing a low-temperature curing polyimide foam material, including the following steps: 1) Add 21.81g of pyromellitic dianhydride, 4.60g of ethanol, 26.65g of tetrahydrofuran, and 0.10g of ring-opening catalyst to a three-necked flask in sequence. Heat to 60℃ and reflux with stirring for 4h until the mixture is clear and transparent. Add 1.63g of DC-193 and mix evenly to obtain foaming material one with esterification index R=1. 2) Take another three-necked flask, add 21.81g of pyromellitic dianhydride and 26.65g of N,N-dimethylformamide in sequence, heat to 100℃ and reflux with stirring until the mixture is homogeneous; let stand and cool to 60℃, then add 1.63g of DC-193 and 1.63g of polyvinylpyrrolidone to obtain foaming material II with esterification index R=0. Mix foaming material II and foaming material I evenly to obtain composite foaming material with esterification index R=0.5. 3) Add 44.80g PM-400, 17.14g N-3390 and 2.61g deionized water to the composite foam material in sequence, stir at 1500r / min for 10s~15s until the solution turns white, transfer the mixture to the mold, allow it to foam freely at room temperature and pressure, and let it stand for 20min to complete the initial curing; then use a stepped heating curing method of 60℃×2h and 150℃×4h to obtain low-temperature curing polyimide foam material.

[0045] Example 2 This embodiment provides a method for preparing a low-temperature curing polyimide foam material, including the following steps: 1) Add 32.22g of 4,4'-benzophenone tetracarboxylic dianhydride, 4.60g of ethanol, 39.30g of tetrahydrofuran, and 0.14g of ring-opening catalyst to a three-necked flask in sequence. Heat to 60℃ and reflux with stirring for 4h until the mixture is clear and transparent. Add 2.40g of DC-193 and mix evenly to obtain foaming material one with esterification index R=1. 2) Take another three-necked flask, add 32.22g of 4,4'-benzophenone tetracarboxylic dianhydride and 39.30g of N,N-dimethylformamide in sequence, heat to 100℃ and reflux with stirring until the mixture is homogeneous; let stand and cool to 60℃, then add 2.40g of DC-193 and 1.63g of polyvinylpyrrolidone to obtain foaming material II with esterification index R=0. Mix foaming material II and foaming material I evenly to obtain composite foaming material with esterification index R=0.5. 3) Add 44.80g PM-400, 17.14g N-3390 and 3.85g deionized water to the composite foam material in sequence, stir at 1500r / min for 10s~15s until the solution turns white, transfer the mixture to the mold, allow it to foam freely at room temperature and pressure, and let it stand for 20min to complete the initial curing; then use a stepped heating curing method of 65℃×2h and 145℃×4h to obtain low-temperature curing polyimide foam material.

[0046] Example 3 This embodiment provides a method for preparing a low-temperature curing polyimide foam material, including the following steps: 1) Add 29.42g of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 4.60g of ethanol, 35.94g of tetrahydrofuran, and 0.13g of ring-opening catalyst to a three-necked flask in sequence. Heat to 60℃ and reflux with stirring for 4h until the mixture is clear and transparent. Add 2.19g of AK8805 and mix evenly to obtain foaming material A1 with esterification index R=1. 2) Take another three-necked flask, add 29.42g of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and 35.94g of N,N-dimethylformamide in sequence, heat to 100℃ and reflux with stirring until the mixture is homogeneous; let stand and cool to 60℃, then add 2.19g of AK8805 and 1.63g of polyvinylpyrrolidone to obtain foaming material II with esterification index R=0. Mix foaming material II and foaming material I evenly to obtain composite foaming material with esterification index R=0.5; 3) Add 44.80g PM-400, 17.14g N-3390 and 3.52g deionized water to the composite foam material in sequence, stir at 1500r / min for 10s~15s until the solution turns white, transfer the mixture to the mold, allow it to foam freely at room temperature and pressure, and let it stand for 20min to complete the initial curing; then use a stepped heating curing method of 55℃×2h and 155℃×4h to obtain low-temperature curing polyimide foam material.

[0047] Comparative Example 1 This comparative example provides a method for preparing a polyimide foam material, comprising the following steps: 1) Take a three-necked flask, add 43.62g of pyromellitic dianhydride and 53.30g of N,N-dimethylformamide in sequence, heat to 100℃ and reflux with stirring until the mixture is homogeneous; let stand and cool to 60℃, then add 1.63g of DC-193 and mix evenly to obtain a foaming material with an esterification index R=0; 2) Add 44.80g PM-400, 17.14g N-3390 and 2.61g deionized water to the foaming material in sequence, stir at 1500r / min for 10s~15s until the solution turns white, transfer the mixture to the mold, allow it to foam freely at room temperature and pressure, and let it stand for 20min to complete the initial curing; then use a stepped heating curing method of 60℃×2h and 150℃×4h to obtain low-temperature curing polyimide foam material.

[0048] Comparative Example 2 This comparative example provides a method for preparing a polyimide foam material, comprising the following steps: 1) Add 43.62g of pyromellitic dianhydride, 18.40g of ethanol, 53.30g of tetrahydrofuran, and 0.20g of ring-opening catalyst to a three-necked flask in sequence. Heat to 60℃ and reflux with stirring for 4h until the mixture is clear and transparent. Add 3.26g of DC-193 and mix evenly to obtain a foaming material with an esterification index R=2. 2) Add 44.80g PM-400, 17.14g N-3390 and 2.61g deionized water to the foaming material in sequence, stir at 1500r / min for 10s~15s until the solution turns white, transfer the mixture to the mold, allow it to foam freely at room temperature and pressure, and let it stand for 20min to complete the initial curing; then use a stepped heating curing method of 60℃×2h and 150℃×4h to obtain low-temperature curing polyimide foam material.

[0049] Comparative Example 3 This comparative example provides a method for preparing a polyimide foam material, comprising the following steps: 1) Add 43.62g of pyromellitic dianhydride, 18.40g of ethanol, 53.30g of tetrahydrofuran, and 0.20g of ring-opening catalyst to a three-necked flask in sequence. Heat to 60℃ and reflux with stirring for 4h until the mixture is clear and transparent. Add 3.26g of DC-193 and mix evenly to obtain a foaming material with an esterification index R=2. 2) Add 44.80g PM-400, 17.14g N-3390 and 2.61g deionized water to the foaming material in sequence, stir at 1500r / min for 10s~15s until the solution turns white, transfer the mixture to the mold, allow it to foam freely at room temperature and pressure, and let it stand for 20min to complete the initial curing; then use a stepped heating curing method of 60℃×2h, 150℃×4h and 220℃×4h to obtain polyimide foam material.

[0050] Comparative Example 4 This comparative example provides a method for preparing a polyimide foam material, comprising the following steps: 1) Add 21.81g of pyromellitic dianhydride, 4.60g of ethanol, 26.65g of tetrahydrofuran, and 0.10g of ring-opening catalyst to a three-necked flask in sequence. Heat to 60℃ and reflux with stirring for 4h until the mixture is clear and transparent. Add 1.63g of DC-193 and mix evenly to obtain foaming material one with esterification index R=1. 2) Take another three-necked flask, add 21.81g of pyromellitic dianhydride and 26.65g of N,N-dimethylformamide in sequence, heat to 100℃ and reflux with stirring until the mixture is homogeneous; let stand and cool to 60℃, then add 1.63g of DC-193 and 1.63g of polyvinylpyrrolidone to obtain foaming material II with esterification index R=0. Mix foaming material II and foaming material I evenly to obtain composite foaming material with esterification index R=0.5. 3) Add 44.80g PM-400, 17.14g N-3390 and 2.61g deionized water to the composite foam material in sequence, stir at 1500r / min for 10s~15s until the solution turns white, transfer the mixture to the mold, allow it to foam freely at room temperature and pressure, and let it stand for 20min to complete the initial curing; then use a stepped heating curing method of 60℃×2h, 150℃×4h and 220℃×4h to obtain polyimide foam material.

[0051] Performance Tests and Results Compared to Example 1, Example 2 uses 4,4'-benzophenone tetracarboxylic dianhydride and has a different curing temperature. Example 3 uses 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and AK8805 as foam stabilizers, and also has a different curing temperature. Comparative Example 1 did not use alcohol solvents (R=0) or polyvinylpyrrolidone in its foaming material. Comparative Example 2 used multiple times more alcohol solvents (R=2). Comparative Example 3 used multiple times more alcohol solvents (R=2) and underwent a stepped high-temperature curing process at 60℃×2h, 150℃×4h, and 220℃×4h. Comparative Example 4 was based on Example 1 with an additional 220℃×4h curing. The test results for the examples and comparative examples are shown in Table 1.

[0052] Table 1 As shown in Table 1, the tensile strength, compression set, and flame retardant properties (oxygen index) of Examples 1-3 with R=0.5 are significantly better than those of Comparative Examples 1-3, which is directly related to the degree of imidization of the material. When R is too low (Comparative Example 1, R=0), some aromatic dianhydrides in the system precipitate, resulting in excess isocyanate, which easily reacts with amino groups to form urea groups. The mechanical properties and heat resistance of urea groups are weaker than those of imide structures, and they may even undergo thermal decomposition during high-temperature imidization. At the same time, unreacted aromatic dianhydrides form defects inside the material as impurities, leading to a decrease in overall performance. When the R value increases (e.g., Comparative Example 2, R=2), the reactivity of the system decreases, so imidization cannot be carried out at 150°C, and it does not have a flame retardant effect. At the same time, due to the influence of competing side reactions, some reactions remain in the intermediate stage, resulting in a decrease in the degree of polymerization of the material and an increase in internal defects. Even after baking at 220°C (Comparative Example 3), complete imidization is still not possible, thus deteriorating the mechanical and flame retardant properties. Therefore, under the condition that the foaming material is completely dissolved, reducing the R value can significantly improve the tensile properties and flame retardant properties of polyimide foam.

[0053] As can be seen from Example 1 and Comparative Example 4, the performance of the sample with R=0.5 did not decrease after the step curing process was followed by a high-temperature curing process. This shows that the composite foaming material is already fully imidized during low-temperature curing and does not require high-temperature baking.

[0054] This invention, while ensuring complete dissolution of the foaming material, further increases the proportion of anhydride groups by introducing hydrogen bonds, small molecule impurities, polyvinylpyrrolidone, and preparing composite foaming materials, thereby ultimately improving the imidization degree of the foam and producing high-performance polyimide foam, promoting its further application in high-precision and cutting-edge fields.

[0055] The foregoing has described specific embodiments of the present invention. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps described in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired results. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-temperature curing polyimide foam material, characterized in that, The polyimide foam material is prepared from aromatic dianhydride, alcohol solvent, and isocyanate as the main raw materials. The isocyanate group in the isocyanate has a molar ratio of 160:100 to the aromatic dianhydride; the alcohol solvent has a molar ratio of 0.5 to the aromatic dianhydride.

2. The polyimide foam material according to claim 1, characterized in that, The aromatic dianhydride is selected from monomers having diacid anhydride groups, wherein, The monomer is one or more combinations of 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), 3,3',4,4'-triphenyl diether tetracarboxylic dianhydride, and 3,3',4,4'-tetracarboxylic diphenyl ether dianhydride.

3. The polyimide foam material according to claim 1, characterized in that, The alcohol solvent is selected from one or a combination of methanol, ethanol, propanol, and furfuryl alcohol.

4. The polyimide foam material according to claim 1, characterized in that, The isocyanate is selected from one or both of PM-200 and PM-400.

5. A method for preparing a low-temperature curing polyimide foam material according to any one of claims 1-4, characterized in that, include: (1) Preparation of foaming material one: Aromatic dianhydride, alcohol solvent, tetrahydrofuran and ring-opening catalyst are added sequentially to reaction vessel one, the temperature is raised to 60±3℃ and stirred until the mixture is clear and transparent, foam stabilizer is added and mixed evenly to obtain foaming material one; (2) Preparation of composite foaming material: Aromatic dianhydride and N,N-dimethylformamide are added sequentially to reaction vessel two, the temperature is raised to 100±3℃ and stirred until the mixture is dissolved and in a homogeneous state; after cooling to 60±3℃, anti-precipitation agent and foam stabilizer are added and mixed evenly to obtain foaming material two; foaming material one and foaming material two are mixed evenly to prepare composite foaming material; (3) Preparation of foam material: At room temperature, add isocyanate, curing agent and deionized water to the composite foam material in sequence, stir the mixture at 1000~2000r / min for 10-15s until the solution turns white, transfer the mixture to the mold, and allow it to foam freely at room temperature and pressure, and allow it to stand to complete the initial curing. (4) Low-temperature curing imidization: The pre-cured foam material is sent into an oven and cured by step-by-step low-temperature heating to obtain polyimide foam material.

6. The polyimide foam material according to claim 5, characterized in that, The stepped low-temperature heating curing described in step (4) includes curing at 60±5℃ for 2 hours, followed by heating at a rate of 3℃ / min to 150±5℃ for 4 hours.

7. The polyimide foam material according to claim 5, characterized in that, In step (1), the mass ratio of tetrahydrofuran to aromatic dianhydride is 122:100; the mass ratio of ring-opening catalyst to aromatic dianhydride is 0.2~1:100; the mass ratio of foam stabilizer to aromatic dianhydride is 5~10:100; and the esterification index of foaming material one is 1.

8. The preparation method according to claim 5, characterized in that, In step (2), the mass ratio of N,N-dimethylformamide to aromatic dianhydride is 122:100; the mass ratio of the anti-precipitation agent to aromatic dianhydride is 5~10:100; the mass ratio of the foam stabilizer to aromatic dianhydride is 5~10:100; the esterification index of the second foaming material is 0; and the esterification index of the composite foaming material is 0.

5.

9. The preparation method according to claim 5, characterized in that, In step (3), the molar ratio of isocyanate groups to all aromatic dianhydrides in the curing agent is 40:100; the mass ratio of the amount of deionized water added to the total amount of all aromatic dianhydrides is 6:

100.

10. The preparation method according to claim 5, characterized in that, The ring-opening catalyst is selected from one or more of dimethylimidazolium, isoquinoline, triethanolamine, and dibutyltin dilaurate; The anti-precipitation agent is polyvinylpyrrolidone; The foam stabilizer is selected from one or more of AK-8805, DC-193, DC-198, AK-8803, L550, and octadecyl dimethylamine oxide; The curing agent is selected from one or more combinations of N-3390 and polymethylene polyphenyl polyisocyanate.

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