A supercritical polyimide foam material and its application

Abstract

This invention provides a supercritical foamed polyimide material and its applications, relating to the field of foamed polyimide technology. The thermoplastic polyimide of this invention contains several ester bonds in its main chain structure. The raw materials for preparing the thermoplastic polyimide include dianhydride compounds, diamine compounds, and di-p-aminophenyl terephthalate. This improves the thermoplasticity of the polyimide while also increasing its affinity for CO2, resulting in better foaming performance under supercritical CO2 foaming conditions. The supercritical foamed material of this invention is suitable for use as a microwave absorbing material.

Description

Technical Field

[0001] This invention belongs to the field of expanded polyimide technology, and relates to a supercritical expanded polyimide foam material and its application. Background Technology

[0002] Polyimide is a high-performance polymer material with excellent high-temperature resistance, outstanding mechanical strength, toughness, creep resistance, and good resistance to chemical solvents. It plays an irreplaceable role in extreme environments such as aerospace and microelectronics. Polymer foam materials are considered one of the ideal carriers for achieving broadband and efficient microwave absorption, especially supercritical foam materials, which are characterized by high cell density and fine pores. However, due to polyimide's insufficient affinity for supercritical fluids (such as supercritical CO2 and supercritical N2), the foaming performance of supercritical polyimide is insufficient, requiring supercritical foaming under relatively harsh conditions. For example, Chinese patent CN102167840A discloses a method for preparing polymer microporous foam materials using supercritical compression molding foaming, where polyimide is foamed using supercritical compression molding foaming at a temperature of 420℃ and a pressure of 25MPa. Such high temperatures make it difficult to achieve industrially.

[0003] Therefore, existing supercritical foaming technology for polyimides urgently needs improvement. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a polyimide supercritical foaming material and its applications.

[0005] The technical solution of the present invention is as follows:

[0006] A supercritical foaming material is obtained by supercritical foaming of composite polyimide using supercritical CO2 fluid at 200-250℃ and 12-22MPa.

[0007] The raw material components of the composite polyimide include thermoplastic polyimide;

[0008] The main chain structure of the thermoplastic polyimide contains several ester bonds;

[0009] The raw materials for preparing the thermoplastic polyimide include dianhydride compounds, diamine compounds, and di-p-aminophenyl terephthalate.

[0010] Preferably, the dianhydride compound is selected from one or a combination of two or more of diphenyl ether tetracarboxylic dianhydride, bisphenol F type diether dianhydride, and bisphenol A type diether dianhydride;

[0011] Preferably, the diamine compound is selected from one or a combination of two or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, and 1,4-cyclohexanediamine.

[0012] Preferably, the ratio of the molar number of the dianhydride compound to the sum of the molar numbers of the diamine compound and the di-p-aminophenyl terephthalate is 0.95-1.1:1.

[0013] Preferably, the molar ratio of the diamine compound to the di-p-aminophenyl terephthalate is 9:1 to 5:5.

[0014] Preferably, the raw material components further include fillers;

[0015] The filler accounts for no more than 15% of the weight of the raw material components.

[0016] More preferably, the filler is selected from microwave absorbing fillers;

[0017] The microwave absorbing filler is selected from carbon nanotubes.

[0018] More preferably, the carbon nanotubes are surface-treated with an epoxy-containing silane coupling agent and then reacted with epoxy groups using a C3-C12 compound containing primary amino and amide structures, wherein the reaction ratio of the epoxy groups is 50-90%.

[0019] Preferably, the composite polyimide is obtained by screw extrusion molding after mixing all the raw material components;

[0020] The holding time for the supercritical foaming is 1-3 hours.

[0021] An application of the polyimide supercritical foam material described in any of the above embodiments, applied to microwave absorbing materials.

[0022] The beneficial effects of this invention are:

[0023] (1) In this invention, an ester bond structure is introduced into the structure of polyimide by bis(p-aminophenyl) terephthalate (BPTP), which not only improves the thermoplasticity of polyimide, but also the ester bond has a good affinity for CO2.

[0024] (2) The thermoplastic polyimide with good affinity for CO2 is used. Supercritical CO2 fluid has good wettability and permeability to thermoplastic polyimide. The thermoplastic polyimide has good supercritical foaming performance and can achieve good supercritical foaming. Attached Figure Description

[0025] Figure 1The thermoplastic polyimide TPI-1 obtained in Example 1 1 H-NMR spectrum.

[0026] Figure 2 The images shown are cross-sectional SEM images of the supercritical foamed materials from Examples 5 and 8.

[0027] Figure 3 The supercritical foaming material of Example 8 exhibits microwave absorption performance in the 2-18 GHz range. Detailed Implementation

[0028] The technical solution of the present invention will be further explained and described below through specific embodiments.

[0029] On the one hand, the present invention proposes a supercritical foaming material, which is obtained by supercritical foaming of composite polyimide using supercritical CO2 fluid at 200-250℃ and 12-22MPa.

[0030] The raw material components of composite polyimide include thermoplastic polyimide (TPI);

[0031] The main chain structure of thermoplastic polyimide (TPI) contains several ester bonds;

[0032] The raw materials for preparing thermoplastic polyimide include dianhydride compounds, diamine compounds, and di-p-aminophenyl terephthalate (BPTP).

[0033] Polyimide has a ring structure, which is highly rigid and conjugated, and has excellent thermal stability and chemical inertness. Hydrogen bonds and π-π stacking exist between molecules, which endow it with high mechanical strength, solvent resistance and low creep. In this invention, ester bonds are introduced into the main chain of polyimide through di-p-aminophenyl terephthalate (BPTP), which has the following effects and purposes: (1) The relatively flexible ester bonds can improve the thermoplasticity of polyimide, reduce the melting temperature and facilitate processing; (2) The structure of ester bonds is similar to that of CO2. Introducing ester bonds can improve the affinity of polyimide for CO2. During supercritical foaming, it is beneficial for CO2 fluid to wet and penetrate TPI, thereby improving the supercritical foaming performance of TPI, such as a higher foaming ratio. Conventional thermoplastic polyimides, such as those using 1,6-hexanediamine to replace part of 4,4'-diaminodiphenyl ether, can also obtain thermoplastic polyimides, but the affinity for CO2 is still not good enough and the foaming performance is not good enough.

[0034] In some embodiments, the dianhydride compound is selected from one or a combination of two or more of diphenyl ether tetracarboxylic dianhydride (α-ODPA), bisphenol F diether dianhydride (BPFDA), and bisphenol A diether dianhydride (BPADA);

[0035] In some embodiments, the diamine compound is selected from one or a combination of two or more of 4,4'-diaminodiphenyl ether (ODA), p-phenylenediamine, m-phenylenediamine, 3,3'-dimethyl-4,4'-diaminodiphenylmethane (MDT), 4,4'-diaminodiphenylmethane (MDA), and 1,4-cyclohexanediamine.

[0036] In some embodiments, the ratio of the molar number of the dianhydride compound to the sum of the molar numbers of the diamine compound and di-p-aminophenyl terephthalate is 0.95-1.1:1. For example, the ratio of the molar number of the dianhydride compound to the sum of the molar numbers of the diamine compound and di-p-aminophenyl terephthalate can be any value or any value between 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, etc., without particular limitation.

[0037] In some embodiments, the molar ratio of the diamine compound to bis(p-aminophenyl) terephthalate (BPTP) is 9:1 to 5:5. For example, the molar ratio of the diamine compound to BPTP can be any value from 9:1, 8:2, 7:3, 6:4, 5:5, etc., without particular limitation. Further, the molar ratio of the diamine compound to BPTP can be 9:1 to 7:3.

[0038] There are no particular limitations on the preparation method of the above-mentioned thermoplastic polyimide. Taking the reaction of BPADA, ODA and BPTP as an example, for example, the molar ratio of BPADA:(ODA+BPTP)=0.98:1 and the molar ratio of ODA:BPTP=8:2, one preparation method is as follows: BPADA, ODA, BPTP and solvent (such as DMF, NMP) are added to a four-necked flask equipped with a mechanical stirrer, a water separator and a nitrogen tube. Under the protection of nitrogen, the reaction is carried out in an ice-water bath (0-4℃) for 2-3 h to generate thermoplastic polyimide acid (TPAA); then the system is changed to an oil bath, heated to 150-180℃ and reacted for 2-3 h with continuous dehydration to obtain a thermoplastic polyimide solution (TPI solution); the TPI solution is poured into deionized water to precipitate, and then pulverized by a high-speed blender, washed several times with hot water and dried to obtain powdered TPI. For the amount of solvent (taking DMF as an example), the weight ratio can be (BPADA+ODA+BPTP):(BPADA+ODA+BPTP+DMF)=0.1-0.3:1.

[0039] In this invention, since the composite polyimide uses TPI with good thermoplasticity and good affinity with CO2 as described above, it can be supercritically foamed under relatively mild conditions. The temperature of the supercritical CO2 fluid can be 200-250℃ and the pressure can be 12-22MPa.

[0040] In some embodiments, the raw material components further include fillers;

[0041] The weight percentage of the filler in the raw material composition shall not exceed 15%. For example, the weight percentage of the filler in the raw material composition may be any value or any value between 1%, 3%, 5%, 6%, 8%, 10%, 12%, 13%, 15%, etc., without any particular restriction.

[0042] In this invention, there are no particular restrictions on the filler material. It can be an inorganic filler material, which can be a non-metallic oxide, a metal oxide, or other types of inorganic filler material. The filler material can also be an organic filler material, such as PTFE micro powder, PE micro powder, etc.

[0043] In some embodiments, the filler is selected from microwave absorbing fillers;

[0044] The microwave absorbing filler is selected from carbon nanotubes.

[0045] Composite polyimide contains microwave absorbing filler, which can be used to obtain microwave absorbing materials. It also combines the characteristics of polyimide materials, such as good heat resistance, good chemical stability, and creep resistance. Moreover, it uses thermoplastic polyimide, which also has good flexibility.

[0046] Common microwave absorbing fillers include resistive, magnetic loss, and dielectric fillers. Resistive fillers can be made of carbon black, carbon fiber, carbon nanotubes (single-walled carbon nanotubes (SWNT) or multi-walled carbon nanotubes (MWNT), graphene, polyaniline, polypyrrole, iron powder, aluminum powder, nickel powder, stainless steel fiber, etc. Magnetic loss fillers can be made of iron(III) oxide, NiZn ferrite, MnZn ferrite, Fe-Ni alloy, Fe-Co alloy, iron(III) oxide / graphene, NiZn ferrite / carbon nanotube, etc. Dielectric fillers can be made of ceramic dielectric fillers such as barium titanate, strontium titanate, lead zirconate titanate, silicon carbide, boron nitride, etc.

[0047] In some embodiments, carbon nanotubes are surface-treated with an epoxy-containing silane coupling agent, followed by reaction with epoxy groups using a C3-C12 compound containing primary amino and amide structures, with the epoxy group reaction ratio being 50-90%. When carbon nanotubes are used as microwave absorbing fillers, the compatibility between carbon nanotubes and polyimide is not good, which may affect the performance of the composite polyimide, including foaming properties and mechanical strength. Therefore, the carbon nanotubes can be surface-treated, for example, using a silane coupling agent (such as an epoxy-containing silane coupling agent, e.g., KH-560). The epoxy groups can react with the polyimide, improving the compatibility between the carbon nanotubes and the polyimide. However, if too many epoxy groups are grafted onto the surface of carbon nanotubes, these epoxy groups will react with polyimide at high temperatures (such as during melt processing), leading to a high cross-linking density of the polyimide. This may also have an adverse effect on supercritical foaming. To address this, compounds containing primary amino and amide structures (such as β-alanine amide, glycine, and p-aminobenzamide) from the C3-C12 range can be used to react with some epoxy groups (such as 50-90% epoxy groups). This reduces the epoxy group density on the surface of the carbon nanotubes and increases the modification of the carbon nanotubes by the amide structure, which is more conducive to improving the compatibility between carbon nanotubes and polyimide, and will not lead to a high cross-linking density of the polyimide.

[0048] In some embodiments, the raw material components are mixed and then extruded using a screw extrusion molding process to obtain a composite polyimide. In this invention, the thermoplastic polyimide has a low melt temperature, allowing for melt extrusion using a screw extruder. It can be directly molded, or extruded and granulated using a screw extruder before injection molding. The product can be sheet, plate, rod, sphere, etc. A twin-screw extruder can be used for screw extrusion. The temperature for both melt extrusion and injection molding can be 330-350°C.

[0049] The holding time for supercritical foaming is 1-3 hours. For example, the holding time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, etc.

[0050] On the other hand, the present invention also proposes the application of the thermoplastic polyimide or the supercritical foaming material described in any of the above embodiments to microwave absorbing materials.

[0051] The technical solution of the present invention will be further described and illustrated below with reference to various embodiments. Unless otherwise specified, the parts mentioned in the following embodiments are parts by weight.

[0052] Examples 1-4 Preparation of thermoplastic polyimide

[0053] Example 1

[0054] BPADA, ODA, and BPTP are the reactants, DMF is the organic solvent, BPADA is 52g (0.1mol), the molar ratio of BPADA:(ODA+BPTP)=0.98:1, the molar ratio of ODA:BPTP=8:2, and the weight ratio of (BPADA+ODA+BPTP):(BPADA+ODA+BPTP+DMF)=0.2:1.

[0055] BPADA, ODA, BPTP, and DMF were added to a four-necked flask equipped with a mechanical stirrer, a water separator, and a nitrogen tube. Under nitrogen protection, the mixture was reacted in an ice-water bath (0-4℃) for 3 h to generate polyimide acid. The mixture was then transferred to an oil bath system, heated to 150-180℃, and reacted for 2.5 h with continuous dehydration to obtain a polyimide solution. The polyimide solution was poured into deionized water to precipitate the polyimide, which was then pulverized using a high-speed blender, washed four times with hot water, and dried overnight in a vacuum oven at 70℃ to obtain powdered thermoplastic polyimide, labeled as TPI-1.

[0056] The thermoplastic polyimide TPI-1 obtained in this embodiment 1 The H-NMR spectrum is attached. Figure 1 As shown.

[0057] Example 2

[0058] The difference between this embodiment and Embodiment 1 is that in Embodiment 1, the molar ratio of BPADA:(ODA+BPTP) was adjusted from 0.98:1 to 1.03:1, and the molar ratio of ODA:BPTP was adjusted from 8:2 to 9:1. The remaining steps remained unchanged. A powdered thermoplastic polyimide was obtained, labeled TPI-2.

[0059] Example 3

[0060] The difference between this embodiment and Embodiment 1 is that in Embodiment 1, the molar ratio of BPADA:(ODA+BPTP) was adjusted from 0.98:1 to 0.96:1, and the molar ratio of ODA:BPTP was adjusted from 8:2 to 7:3. The remaining steps remained unchanged. A powdered thermoplastic polyimide was obtained, labeled TPI-3.

[0061] Example 4

[0062] The difference between this embodiment and Embodiment 1 is that in Embodiment 1, the molar ratio of BPADA:(ODA+BPTP) was adjusted from 0.98:1 to 0.97:1, and the molar ratio of ODA:BPTP was adjusted from 8:2 to 5:5. The remaining steps remained unchanged. A powdered thermoplastic polyimide was obtained, labeled TPI-4.

[0063] Comparative Example 1

[0064] The difference between this comparative example and Example 1 is that in Example 1, BPTP was replaced with an equimolar amount of ODA, meaning that BPTP was not added in this comparative example. All other steps remained unchanged. A powdered thermoplastic polyimide was obtained, labeled TPI-5.

[0065] Comparative Example 2

[0066] The difference between this comparative example and Example 1 is that in Example 1, BPTP was replaced with an equimolar amount of 1,6-hexanediamine. All other steps remained unchanged. A powdered thermoplastic polyimide was obtained, labeled TPI-6.

[0067] Performance testing:

[0068] CO2 adsorption capacity comparison: The polyimide to be tested was placed in CO2 fluid at 50℃ and 10 MPa for different times and then rapidly weighed. The mass before adsorption is m0, and the mass after adsorption is m1. The adsorption rate is calculated as (m1-m0) / m0×100%. The higher the saturated adsorption rate and / or the faster the adsorption, the higher the affinity of the polyimide for CO2. The results are shown in Table 1 below.

[0069] Table 1 Adsorption rate / %

[0070]

[0071] As can be seen from the results in Table 1 above, the thermoplastic polyimide of the present invention has a high adsorption rate for CO2 and a faster adsorption saturation, indicating a high affinity for CO2. Moreover, as the molar ratio of BPTP in the thermoplastic polyimide increases, the adsorption rate for CO2 is even higher and the adsorption saturation is faster.

[0072] Examples 5-12: Preparation of Supercritical Foaming Materials

[0073] Example 5

[0074] The thermoplastic polyimide TPI-1 obtained in Example 1 was extruded and granulated using a twin-screw extruder to obtain TPI granules, which were then injection molded using an injection molding machine to obtain TPI-1 sheets. The processing temperatures of both the extruder and the injection molding machine were between 330-340°C.

[0075] The obtained TPI-1 sheet was placed in a supercritical fluid foaming reactor. The temperature of the CO2 fluid was 220℃ and the pressure was 15MPa. The supercritical foaming was maintained for 2 hours, and then the pressure was quickly released to obtain the supercritical foamed material, which was labeled as TPI-1-F.

[0076] Example 6

[0077] Weigh 6g of SWNT and 94g of thermoplastic polyimide TPI-1 obtained in Example 1, mix them evenly, add them to a twin-screw extruder for extrusion granulation to obtain TPI / CNTs granules, and then injection mold them through an injection molding machine to obtain TPI / CNTs sheets. The processing temperatures of the extruder and injection molding machine are both between 330-340℃.

[0078] The supercritical foaming process of Example 5 was used to foam the material, which was then labeled as TPI-1 / CNTs-F-1.

[0079] Example 7

[0080] The difference between this embodiment and Embodiment 6 is that in Embodiment 6, SWNTs were surface-treated using KH-560 silane coupling agent. The remaining steps remained unchanged. A supercritical foamed material was obtained, labeled TPI-1 / CNTs-F-2.

[0081] The process of surface treatment of SWNT using KH-560 silane coupling agent is as follows: 20g of SWNT is ultrasonically dispersed in a solvent consisting of 1000g of anhydrous ethanol and 100g of deionized water, 1g of KH-560 is added, the mixture is stirred at room temperature for 2h, heated to 60℃, and reacted for another 2h. The mixture is then cooled to room temperature, and the solid and liquid are separated. The solid is washed three times with anhydrous ethanol and dried overnight in a 60℃ oven to obtain the surface-treated SWNT.

[0082] Example 8

[0083] The difference between this embodiment and Embodiment 7 is that in Embodiment 7, SWNTs were surface-treated with KH-560 silane coupling agent and then modified with β-alanine amide. The remaining steps remained unchanged. A supercritical foamed material was obtained, labeled TPI-1 / CNTs-F-3.

[0084] SWNT was surface-treated with KH-560 silane coupling agent and then reacted with β-alanine amide as follows: 10g of the surface-treated SWNT from Example 7 was ultrasonically dispersed in 1000 parts of anhydrous ethanol, and β-alanine amide was added (the molar ratio of β-alanine amide to the epoxy group on the SWNT surface was 7:10). The mixture was stirred at room temperature for 2 hours, then heated to 50°C and reacted for another 4 hours. The mixture was then cooled to room temperature, and the solid and liquid were separated. The solid was washed three times with anhydrous ethanol and dried overnight in a 60°C oven to obtain modified SWNT.

[0085] Example 9

[0086] The difference between this embodiment and Embodiment 6 is that in Embodiment 6, the thermoplastic polyimide TPI-1 obtained in Embodiment 1 is replaced with an equal weight of the thermoplastic polyimide TPI-2 obtained in Embodiment 2. The remaining steps remain unchanged, resulting in a supercritical foamed material, labeled as TPI-2 / CNTs-F-1.

[0087] Example 10

[0088] The difference between this embodiment and Embodiment 6 is that in Embodiment 6, the thermoplastic polyimide TPI-1 obtained in Embodiment 1 is replaced with an equal weight of the thermoplastic polyimide TPI-3 obtained in Embodiment 3. The remaining steps remain unchanged, resulting in a supercritical foamed material, labeled as TPI-3 / CNTs-F-1.

[0089] Comparative Example 3

[0090] The difference between this comparative example and Example 6 is that in Example 6, the thermoplastic polyimide TPI-1 obtained in Example 1 was replaced with the thermoplastic polyimide TPI-5 obtained in Comparative Example 1. The remaining steps remained unchanged. A supercritical foamed material was obtained, labeled TPI-5 / CNTs-F-1.

[0091] Comparative Example 4

[0092] The difference between this comparative example and Example 6 is that in Example 6, the thermoplastic polyimide TPI-1 obtained in Example 1 was replaced with the thermoplastic polyimide TPI-6 obtained in Comparative Example 2. The remaining steps remained unchanged. A supercritical foamed material was obtained, labeled TPI-6 / CNTs-F-1.

[0093] The foaming properties of the polyimide supercritical foam materials of Examples 5-10 and Comparative Examples 3-4 are shown in Table 2 below.

[0094] Table 2 Foaming properties

[0095]

[0096] Therefore, based on the data results in Table 2 above, and comparing Examples 6, 9, 10 and Comparative Examples 3-4, it is evident that the introduction of ester bonds into the main chain structure of polyimide in this invention can significantly improve the supercritical foaming ratio and porosity of the composite polyimide. Moreover, the better the foaming performance, the higher the BPTP molar ratio used. Comparative Examples 5-8 show that adding carbon nanotubes to thermoplastic polyimide, if the carbon nanotubes are not surface-treated, results in slightly poorer foaming performance. Treatment of the carbon nanotubes with KH-560 improves the foaming performance to some extent, and further treatment of the epoxy groups can significantly improve the foaming performance.

[0097] SEM images of the cross-sections of the supercritical foamed materials in Examples 5 and 8 are attached. Figure 2 As shown, the supercritical foaming material TPI-1-F in Example 5 is... Figure 2 In (a), the supercritical foaming material TPI-1 / CNTs-F-3 of Example 8 is Figure 2 As shown in (b), TPI-1 / CNTs-F-3 incorporates SWNTs with good compatibility, resulting in smaller pore size and more rounded morphology compared to TPI-1-F.

[0098] The microwave absorption performance of the supercritical foamed material TPI-1 / CNTs-F-3 in Example 8 was tested using the coaxial method, measuring the absorption performance from 2 to 18 GHz, as shown in the attached figure. Figure 3 As shown, the unique foam structure not only introduces abundant solid-gas interfaces and optimizes the impedance matching of the foam surface, but also significantly enhances dielectric polarization and multiple scattering effects, thereby improving the energy dissipation capability of the absorbing foam. Consequently, the effective absorption bandwidth (EAB) of TPI-1 / CNTs-F-3 exceeds 13.6 GHz. In the low-frequency region, TPI-1 / CNTs-F-3 absorbing foam exhibits typical 1 / 4 wavelength destructive interference characteristics, forming obvious interference peaks and extreme reflection loss values, resulting in outstanding absorption performance in the corresponding frequency band.

[0099] As described above, the basic principles, main features, and advantages of the present invention have been shown and described. Those skilled in the art should understand that the present invention is not limited to the above embodiments, which are merely preferred embodiments and should not be construed as limiting the scope of the invention. All equivalent changes and modifications made in accordance with the scope of the patent and the description should still fall within the scope of the present invention. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A polyimide supercritical foaming material, characterized in that, It is obtained by supercritical foaming of composite polyimide using supercritical CO2 fluid at 200-250℃ and 12-22MPa. The raw material components of the composite polyimide include thermoplastic polyimide; The main chain structure of the thermoplastic polyimide contains several ester bonds; The raw materials for preparing the thermoplastic polyimide include dianhydride compounds, diamine compounds, and di-p-aminophenyl terephthalate.

2. The polyimide supercritical foam material of claim 1, wherein, The dianhydride compounds are selected from one or a combination of two or more of diphenyl ether tetracarboxylic dianhydride, bisphenol F type diether dianhydride, and bisphenol A type diether dianhydride.

3. The polyimide supercritical foam material of claim 1, wherein, The diamine compound is selected from one or a combination of two or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, and 1,4-cyclohexanediamine.

4. The polyimide supercritical foaming material according to claim 1, characterized in that, The ratio of the molar number of the dianhydride compound to the sum of the molar numbers of the diamine compound and the di-p-aminophenyl terephthalate is 0.95-1.1:

1.

5. The polyimide supercritical foam material of claim 1, wherein, The molar ratio of the diamine compound to the di-p-aminophenyl terephthalate is 9:1 to 5:

5.

6. The polyimide supercritical foam material of claim 1, wherein, The raw material components also include fillers; The filler accounts for no more than 15% of the weight of the raw material components.

7. The polyimide supercritical foam material of claim 6, wherein, The filler is selected from microwave absorbing fillers; The microwave absorbing filler is selected from carbon nanotubes.

8. The polyimide supercritical foam material of claim 7, wherein, The carbon nanotubes are surface-treated with an epoxy-containing silane coupling agent, and then reacted with epoxy groups using a C3-C12 compound containing primary amino and amide structures, wherein the reaction ratio of the epoxy groups is 50-90%.

9. The polyimide supercritical foaming material according to claim 1, characterized in that, The composite polyimide is obtained by screw extrusion molding after mixing all the raw material components. The holding time for the supercritical foaming is 1-3 hours.

10. The application of the polyimide supercritical foaming material according to any one of claims 1-9, characterized in that, It is used in microwave absorbing materials.

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