Foamable thermoplastic polyimide and preparation method of wave-absorbing foam thereof

Abstract

The invention discloses supercritical fluid foaming polyimide wave-absorbing foam and a preparation method thereof.The thermoplastic polyimide is prepared by the steps that aromatic dianhydride, aromatic diamine and aliphatic diamine serve as comonomers, polyimide acid is generated through solution polymerization, and then high-temperature cyclization is conducted; wherein the mole number of the aliphatic diamine accounts for 10-50% of the total mole number of the diamine, and the free volume and glass transition temperature of a molecular chain are effectively regulated and controlled by introduction of the aliphatic diamine; the wave-absorbing foam is prepared by taking polyimide as a matrix, doping 1-20wt% of wave-absorbing filler, and foaming for 1-3 hours in a supercritical carbon dioxide or nitrogen environment at the temperature of 180-220 DEG C and the pressure of 9-25MPa, through molecular structure design, the adsorption capacity and impact resistance of polyimide to carbon dioxide are remarkably improved, and the prepared foam has the advantages of light weight, high porosity, uniform foam structure and excellent heat resistance, can realize effective electromagnetic wave absorption in an X wave band, and is suitable for the fields of aerospace stealth and electromagnetic compatibility protection.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of synthesis and processing of high-performance polymer foams, and in particular, to a kind of foamable thermoplastic polyimide and the preparation method of its wave-absorbing foam. BACKGROUND

[0002] Polymer foams exhibit broad application prospects in the field of electromagnetic wave absorption due to their unique porous structure. The internal rich pore structure introduces a large amount of air phase, which can effectively adjust the equivalent dielectric constant of the material and improve its impedance matching characteristics with free space, thereby promoting the entry of electromagnetic waves into the material interior rather than being reflected by the surface. At the same time, the continuous gas-solid interface inside can induce multiple reflections and scattering of incident electromagnetic waves, significantly prolonging the propagation path of electromagnetic waves, and then enhancing energy dissipation through dielectric loss and magnetic loss mechanisms.

[0003] Polyimide (PI) is a high-performance polymer containing imide rings in its main chain, and has an irreplaceable position in aerospace, electronics and other high-temperature harsh environments due to its excellent thermal stability, excellent mechanical properties and outstanding chemical resistance. However, the molecular chain of traditional polyimide (especially thermosetting polyimide) is rigid, has small free volume, and usually exhibits insoluble and infusible characteristics, resulting in poor solubility and diffusion ability of small molecular gases such as carbon dioxide (CO2), which greatly limits the application of advanced foaming technology in this system.

[0004] Supercritical carbon dioxide (scCO2) fluid foaming technology is an environmentally friendly physical foaming method, which has the advantages of non-toxic, non-residual and clean process of foaming agent. By precisely controlling temperature, pressure and other parameters, this technology can realize the precise preparation of polymer cell morphology, size and distribution, and is an ideal way to prepare high-performance microcellular foam materials. However, when it is applied to polyimide system, there are problems: conventional PI has weak adsorption ability to scCO2, and it is difficult to form uniform and sufficient cell nucleation points; even a few thermoplastic polyimides (TPI) also have high glass transition temperature (Tg) and melt viscosity, which makes it difficult for the cells to grow and stabilize during the foaming process, and it is easy to collapse, merge or rupture.

[0005] In the prior art, in order to improve the foaming performance of the polymer, modification methods such as blending, plasticizing or introducing crystallization zone are usually used, but these means often come at the cost of sacrificing the heat resistance, mechanical strength or chemical stability of the material, which is unacceptable for polyimide wave-absorbing foams requiring extreme environmental reliability. In addition, although there have been attempts to try to combine carbon nanotubes, ferrite and other wave-absorbing fillers with polyimide, the introduction of fillers will further affect the rheological behavior of the matrix and hinder the diffusion of scCO2, increasing the complexity of the control of the cell structure, making it extremely difficult to achieve the synergistic optimization of high-efficiency electromagnetic attenuation performance and fine controllable porous structure.

[0006] Therefore, it has become a key technical problem in the field to develop a new type of thermoplastic polyimide resin which, while retaining the inherent excellent performance of polyimide, has high affinity for scCO2 and a suitable low-temperature thermal processing window, and then to prepare a lightweight and high-efficiency wave-absorbing foam with controllable structure and stable performance through supercritical fluid technology. SUMMARY

[0007] To solve the above problems, the present application provides a foaming thermoplastic polyimide and a preparation method of a wave-absorbing foam thereof, which prepares a lightweight and high-efficiency polyimide wave-absorbing foam through the synergistic innovation of molecular chain structure design, foaming process and functional compounding. Specifically, a new type of thermoplastic polyimide (TPI) is synthesized by introducing aliphatic diamine flexible segments into the traditional aromatic polyimide rigid main chain. On this basis, using the environmentally friendly supercritical carbon dioxide fluid foaming technology, and with the excellent gas affinity of the modified TPI, a lightweight foam skeleton with uniform cells and high porosity is prepared under optimized temperature and pressure parameters.

[0008] In a first aspect, the present application provides a preparation method of a foaming thermoplastic polyimide, characterized in that it comprises the following steps: dissolving aromatic dianhydride, aromatic diamine and aliphatic diamine as comonomers in a solvent, generating polyimide acid through polymerization reaction, and then obtaining thermoplastic polyimide through high-temperature cyclization.

[0009] As a preferred, the aromatic dianhydride is selected from one or two or more of benzophenone tetracarboxylic dianhydride, 4,4'-diphenyl ether dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropyl) bis-o-phthalic anhydride and bisphenol A type diether dianhydride, and the mixture thereof in any proportion.

[0010] As a preferred, the aromatic diamine is selected from one or two or more of 2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone and 1,3-bis(4-aminophenoxy)benzene, and the mixture thereof in any proportion.

[0011] As preferred, the aliphatic diamine is selected from one or two or more of 1,6-hexanediamine, 1,10-decanediamine, 4,4'-diaminodicyclohexylmethane, isophorone diamine, 1,2-diaminocyclohexane, cis-1,4-diaminocyclohexane and trans-1,4-diaminocyclohexane in any proportion.

[0012] As preferred, the aliphatic diamine is 1-50% of the total moles of diamine compounds; the diamine compounds comprise aliphatic diamine and aromatic diamine; the molar ratio of the diamine compounds to aromatic dianhydride is 1:1.0-1.1.

[0013] As preferred, the solvent is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone.

[0014] In a second aspect, the application provides a preparation method of a wave-absorbing foam, characterized in that polyimide prepared by a preparation method of foamable thermoplastic polyimide is used as a matrix, wave-absorbing fillers are doped, and then a supercritical fluid foaming process is used to prepare the foam.

[0015] As preferred, the wave-absorbing fillers are selected from one or two or more of carbon nanotubes, graphene, conductive carbon black, silicon carbide fibers, barium titanate, zinc oxide, boron nitride, dielectric ceramic particles, carbonyl iron powder, ferrite, flaky iron silicon aluminum, nickel-coated carbon fiber, cobalt nanowire, iron-cobalt alloy powder, magnetic iron fiber in any proportion.

[0016] As preferred, the addition amount of the wave-absorbing fillers in the matrix is 1-20 wt%; the wave-absorbing fillers are introduced by in-situ polymerization or physical blending.

[0017] As preferred, the foaming agent of the supercritical fluid foaming is carbon dioxide or nitrogen, the foaming temperature is 180-220℃, the foaming pressure is 9-25 MPa, and the pressure holding time is 1-3 h.

[0018] In summary, the application includes at least one of the following beneficial technical effects: 1. The application precisely designs the molecular structure of polyimide, introduces aliphatic diamine flexible segments, basically retains excellent heat resistance, cooperatively realizes significant increase of free volume and moderate reduction of glass transition temperature, improves the impact toughness of the matrix, improves the dissolution and diffusion behavior of CO2 in the polymer, lays an unprecedented material foundation for subsequent preparation of high-quality polyimide foam with controllable structure by using supercritical fluid technology, and overcomes the technical bottleneck that traditional polyimide is difficult to foam.

[0019] 2. Based on the above modified material, combined with supercritical fluid foaming technology, lightweight foam with uniform cells and high porosity can be prepared under relatively mild process conditions. The cell structure not only directly contributes to the lightweight of the material, but also effectively adjusts the equivalent dielectric constant, improves the impedance matching, and promotes the multiple reflection and scattering of electromagnetic waves inside, thereby synergistically enhancing the loss efficiency of the wave-absorbing filler, so as to achieve wide-band and high-efficiency electromagnetic wave absorption at low filler content.

[0020] 3. The wave-absorbing foam prepared by the present application successfully integrates lightweight, heat resistance, high-efficiency wave absorption and structural reliability. The inherent high-temperature stability of polyimide avoids the performance degradation caused by the addition of plasticizers or the use of chemical foaming agents. At the same time, through molecular modification rather than simple blending, the long-term stability of the foam structure under complex thermal-mechanical environment is ensured. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the description of the embodiments of the present application. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor under the premise of not deviating from the concept of the present application.

[0022] Figure 1 The infrared spectrum of TPI2~TPI5 provided by the embodiments of the present application is shown; Figure 2 The cross-sectional SEM diagram of TPI2~TPI5 provided by the embodiments of the present application is shown, wherein Figure 2 a is TPI2 foam; Figure 2 b is TPI3 foam; Figure 2 c is TPI4 foam; Figure 2 d is TPI5 foam; Figure 3 The reflection loss diagram of TPI6 wave-absorbing foam provided by the embodiments of the present application is shown. DETAILED DESCRIPTION

[0023] 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. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of the present invention.

[0024] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0025] Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of this specification.

[0026] In the description of this invention, it should be understood that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0027] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] To enable those skilled in the art to better understand this application, the following embodiments are provided to illustrate in detail the preparation method of an expandable thermoplastic polyimide and its microwave absorbing foam provided in this application.

[0029] Example Example 1 This embodiment details a method for preparing a foamable thermoplastic polyimide and its foam by copolymerizing 80% aromatic diamine and 20% aliphatic diamine.

[0030] Synthesis of thermoplastic polyimide resin: Under nitrogen protection, the following raw materials were sequentially added into a four-necked flask equipped with a mechanical stirrer, a water separator and a spherical condenser: 4,4'-diphenyl ether dianhydride (ODPA) 46.54 g, 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) 49.26 g, 1,6-hexanediamine (HMDA) 3.48 g, N-methyl pyrrolidone (NMP): 568 g. The molar ratio of the raw materials was HMDA : BAPP = 2 : 8, and the solid content of the system was 15%. The reaction device was placed in an ice water bath at 0°C, and the reaction was continuously stirred for 2 hours to obtain a polyimide acid (TPAA) prepolymer solution.

[0031] Subsequently, the reaction device was transferred to an oil bath at 210°C, and a high-temperature dehydration and cyclization reaction was carried out for 2 hours to convert the TPAA into TPI. After the reaction was completed, the obtained viscous solution was naturally cooled to room temperature, and the TPI was precipitated in the form of fibers or particles when the solution was poured into excess deionized water.

[0032] The precipitated TPI crude product was crushed and washed with hot water at 80°C or higher for 3-4 times, each time for 1.5 hours of stirring and washing, to completely remove the residual solvent. Finally, the product was filtered and dried in a vacuum oven at 120°C for 12 hours to obtain a refined light yellow TPI powder, which was named TPI2.

[0033] The TPI2 powder was melt-extruded and granulated by a twin-screw extruder at a processing temperature of 330-340°C to obtain TPI granules. Subsequently, the granules were injection molded into standard test bars and precursor billets required for foaming using an injection molding machine at the same temperature range.

[0034] The processed TPI2 precursor billets were placed in a supercritical fluid foaming kettle. The foaming temperature was set to 205°C, carbon dioxide gas was introduced, and the pressure in the kettle was increased to 15 MPa. The pressure was maintained for 2 hours to ensure that the TPI2 matrix reached saturation adsorption of CO2.

[0035] After the pressure holding was completed, the pressure was rapidly released to normal pressure within 1 second, and the supersaturated CO2 in the system rapidly nucleated and expanded to finally obtain a TPI2 foam with a uniform closed-cell structure.

[0036] Example 2 The present example provides a method for preparing a foamable thermoplastic polyimide (TPI) and its foam, which is prepared by copolymerization of 70% aromatic diamine and 30% aliphatic diamine.

[0037] Synthesis of thermoplastic polyimide resin: Under nitrogen protection, the following raw materials were sequentially added into a four-necked flask equipped with a mechanical stirrer, a water separator and a spherical condenser: ODPA 46.54 g, BAPP 43.10 g, HMDA 5.23 g, NMP 543 g. The molar ratio of the raw materials was HMDA : BAPP = 3 : 7, and the solid content of the system was 15%. The reaction device was placed in an ice water bath at 0°C, and the reaction was continuously stirred for 2 hours to obtain a TPAA prepolymer solution.

[0038] Subsequently, the reaction device was transferred to an oil bath at 210°C, and a high-temperature dehydration and cyclization reaction was performed for 2 hours to convert TPAA into TPI. After the reaction was completed, the obtained viscous solution was naturally cooled to room temperature, and TPI was precipitated in the form of fibers or particles when the solution was poured into excess deionized water.

[0039] The precipitated TPI crude product was crushed and washed with hot water at 80°C or higher for 3-4 times, each time for 1.5 hours of stirring and washing, to completely remove the residual solvent. Finally, the product was filtered and dried in a vacuum oven at 120°C for 12 hours to obtain a refined light yellow TPI powder, which was named TPI3.

[0040] The TPI3 powder was melt-extruded and granulated by a twin-screw extruder at a processing temperature of 330-340°C to obtain TPI granules. Subsequently, the granules were injection molded into standard test bars and precursor billets required for foaming using an injection molding machine at the same temperature range.

[0041] The processed TPI3 precursor billets were placed in a supercritical fluid foaming kettle. The foaming temperature was set to 205°C, carbon dioxide gas was introduced, and the pressure in the kettle was increased to 15 MPa. The system was kept at this pressure for 2 hours to ensure that the TPI3 matrix reached saturation adsorption of CO2.

[0042] After the pressure holding was completed, the pressure was rapidly released to normal pressure within 1 second, and the supersaturated CO2 in the system rapidly nucleated and expanded to finally obtain a TPI3 foam with a uniform closed-cell structure.

[0043] Example 3 This example provides a method for preparing a foamable thermoplastic polyimide (TPI) and its foam, which is prepared by copolymerization of 60% aromatic diamine and 40% aliphatic diamine.

[0044] Synthesis of thermoplastic polyimide resin: Under nitrogen protection, the following raw materials were sequentially added into a four-necked flask equipped with a mechanical stirrer, a water separator and a spherical condenser: ODPA 46.54 g, BAPP 36.95 g, HMDA 6.97 g, NMP 518 g. The molar ratio of the raw materials was HMDA : BAPP = 4 : 6, and the solid content of the system was 15%. The reaction device was placed in an ice water bath at 0°C, and the reaction was continuously stirred for 2 hours to obtain a TPAA prepolymer solution.

[0045] Subsequently, the reaction device was transferred to an oil bath at 210°C, and a high-temperature dehydration and cyclization reaction was performed for 2 hours to convert TPAA into TPI. After the reaction was completed, the obtained viscous solution was naturally cooled to room temperature, and TPI was precipitated in the form of fibers or particles when the solution was poured into excess deionized water.

[0046] The precipitated TPI crude product was crushed and washed with hot water at 80°C or higher for 3-4 times, each time for 1.5 hours of stirring and washing, to completely remove the residual solvent. Finally, the product was filtered and dried in a vacuum oven at 120°C for 12 hours to obtain a refined light yellow TPI powder, which was named TPI4.

[0047] The TPI4 powder was melt-extruded and granulated by a twin-screw extruder at a processing temperature of 330-340°C to obtain TPI granules. Subsequently, the granules were injection molded into standard test bars and precursor billets required for foaming using an injection molding machine at the same temperature range.

[0048] The processed TPI4 precursor billets were placed in a supercritical fluid foaming kettle. The foaming temperature was set to 205°C, carbon dioxide gas was introduced, and the pressure in the kettle was increased to 15 MPa. The system was kept at this pressure for 2 hours to ensure that the TPI4 matrix reached saturation adsorption of CO2.

[0049] After the pressure holding was completed, the pressure was rapidly released to normal pressure within 1 second, and the supersaturated CO2 in the system rapidly nucleated and expanded to finally obtain a TPI4 foam with a uniform closed-cell structure.

[0050] Example 4 This example provides a method for preparing a foamable thermoplastic polyimide (TPI) and its foam, which is prepared by copolymerization of 50% aromatic diamine and 50% aliphatic diamine.

[0051] Synthesis of thermoplastic polyimide resin: Under nitrogen protection, the following raw materials were sequentially added into a four-necked flask equipped with a mechanical stirrer, a water separator and a spherical condenser: ODPA 46.54 g, BAPP 30.79 g, HMDA 8.72 g, NMP 493 g. The molar ratio of the raw materials was HMDA : BAPP = 5 : 5, and the solid content of the system was 15%. The reaction device was placed in an ice water bath at 0°C, and the reaction was continuously stirred for 2 hours to obtain a TPAA prepolymer solution.

[0052] Subsequently, the reaction device was transferred to an oil bath at 210°C, and a high-temperature dehydration and cyclization reaction was performed for 2 hours to convert the TPAA into TPI. After the reaction was completed, the obtained viscous solution was naturally cooled to room temperature, and the TPI was precipitated in the form of fibers or particles when the solution was poured into excess deionized water.

[0053] The precipitated TPI crude product was crushed and washed with hot water at a temperature above 80°C for 3-4 times, each time for 1.5 hours of stirring and washing, to completely remove the residual solvent. Finally, the product was filtered and dried in a vacuum oven at 120°C for 12 hours to obtain a refined light yellow TPI powder, which was named TPI5.

[0054] The TPI5 powder was melt-extruded and granulated by a twin-screw extruder at a processing temperature of 330-340°C to obtain TPI granules. Subsequently, the granules were injection molded into standard test bars and precursor billets required for foaming using an injection molding machine at the same temperature range.

[0055] The processed TPI5 precursor billets were placed in a supercritical fluid foaming kettle. The foaming temperature was set to 205°C, carbon dioxide gas was introduced, and the pressure in the kettle was increased to 15 MPa. The system was kept at this pressure for 2 hours to ensure that the TPI5 matrix reached saturation adsorption of CO2.

[0056] After the pressure holding was completed, the pressure was rapidly released to normal pressure within 1 second, and the supersaturated CO2 in the system rapidly nucleated and expanded to finally obtain a TPI5 foam with a uniform closed-cell structure.

[0057] Example 5 The TPI5 powder prepared in Example 4 was used as the polymer matrix, and 5% of the total mass of carbon nanotubes (CNTs) was added as the wave-absorbing filler. The same process steps as in Examples 1-4 were used, i.e., melt blending and granulation by a twin-screw extruder at 330-340°C, followed by injection molding, to prepare a TPI5 / CNTs composite sheet.

[0058] The composite sheet is placed in a supercritical fluid foaming kettle, the foaming temperature is set to 208 ℃, carbon dioxide is introduced and the pressure is maintained at 15 MPa, and the system is fully adsorbed CO2 by maintaining the pressure for 2 hours. Subsequently, rapid pressure relief, i.e. TPI6 wave-absorbing foam is obtained.

[0059] To systematically evaluate the structure and performance of the material prepared by the present application, the TPI resins and foams obtained in the above examples were tested and analyzed as follows, and the molecular chain free volume fraction of TPI2 to TPI5 resins was simulated and calculated by using Materials Studio software, and the results are shown in Table 1: Table 1 Free volume table of examples 1-4

[0060] The free volume in the table is according to the following formula:

[0061] Wherein: V f : free volume; V0: occupied volume. As can be seen from Table 1, with the increase of the proportion of aliphatic diamine HMDA, the molecular chain free volume fraction of TPI resin shows a clear upward trend, indicating that the space for accommodating small molecules in the material is increased, which is beneficial to the dissolution of carbon dioxide in the polymer and lays a structural foundation for subsequent efficient foaming.

[0062] The infrared spectrum of example 4 (TPI5) and its intermediate product was tested, and the spectrum is shown in Figure 1 It can be seen that after high-temperature cyclization, the stretching vibration absorption peak of amide carbonyl (-CONH-) in TPAA near 1650 cm -1 disappears, and an asymmetric stretching vibration characteristic peak of C=O on the imide ring of polyimide appears near 1710 cm -1 , proving that the imidization reaction has been successfully realized, and a thermoplastic polyimide is generated.

[0063] The notched impact strength of TPI2 to TPI5 resins was tested according to GB / T 1043.1-2008 standard, and the results are shown in Table 2: Table 2 Impact strength table of example 1-4 samples

[0064] The data show that with the increase of the content of flexible aliphatic chain HMDA, the impact resistance of the material is steadily improved, proving the effectiveness of the molecular structure design in improving the brittleness of the material.

[0065] The foaming ratio and porosity of the foamed materials obtained in each example were tested, and the results are shown in Table 3: Table 3 Foaming ratio and porosity of foams of Examples 1-5

[0066] As can be seen from Table 3, with the increase of HMDA content, the foaming ratio and porosity of pure TPI foams are significantly improved, which is consistent with the trend of the increase of free volume. After the addition of CNTs, the foaming performance slightly decreases, but the porosity remains high, indicating that the composite has good foamability.

[0067] Figure 2 Cross-sectional SEM photos of the foams prepared in Examples 1-4 are shown. It can be observed that, from TPI2 foam to TPI5 foam, under the same foaming process, the average cell size gradually increases with the increase of HMDA content, the cell number increases, and the morphology is more uniform. This directly proves that by adjusting the ratio of aliphatic diamines, the CO2 adsorption amount and foaming behavior can be effectively regulated, and thus the control of the cell structure is realized.

[0068] The electromagnetic wave reflection loss of the TPI6 wave-absorbing foam prepared in Example 5 in the X-band (8-12 GHz) was tested by waveguide method, and the results are shown in Figure 3 . The test shows that the reflection loss valley of the wave-absorbing foam in the X-band is about -17 dB, and the effective absorption bandwidth with reflection loss ≤-10 dB reaches 4 GHz. This shows that the foam material prepared by the present application not only has the characteristics of light weight, but also realizes high-efficiency electromagnetic wave attenuation, which is suitable for stealth technology and electromagnetic compatibility applications in the X-band.

[0069] In summary, the present application systematically demonstrates the complete process of synthesizing a series of thermoplastic polyimides (TPI) by regulating the molar ratio of HMDA in the total diamine feed, and further preparing light foams and wave-absorbing foams by supercritical CO2 fluid foaming technology. The results show that with the increase of the ratio of HMDA, the free volume fraction of TPI molecular chain is significantly increased from 33.59% to 39.60%, which effectively promotes the dissolution of CO2 in the polymer, and then the foaming ratio of the foam material is increased from 2.13 to 4.52, and the porosity is increased from 53.13% to 77.87%. At the same time, the impact strength of the material is increased from 7.3 kJ / m 2 to 9.6 kJ / m 2 , which proves that the introduction of flexible segments not only improves the foaming performance, but also enhances the toughness of the material. Finally, by compounding 5 wt% carbon nanotubes in the TPI5 matrix, a wave-absorbing foam with porous structure and high-efficiency electromagnetic wave attenuation performance is successfully prepared, which has a reflection loss valley of -17 dB in the X-band and an effective bandwidth of 4 GHz, realizing the unification of light weight, heat resistance and high-efficiency wave-absorbing function.

[0070] Various embodiments in the present specification are described in progressive manner, and each embodiment focuses on the difference from other embodiments, and the same or similar parts between various embodiments can be mutually referred to.

[0071] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications to the embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to cover all the preferred embodiments and all the changes and modifications falling within the scope of the present application.

[0072] Finally, it should be noted that, in this document, the terms such as first and second are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or terminal device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or terminal device. Without more limitations, the element defined by the statement "including a" does not exclude the presence of another identical element in the process, method, article or terminal device including the element.

[0073] The above describes in detail the preparation method of the foamable thermoplastic polyimide and the wave-absorbing foam thereof provided by the present application, and the principles and implementation manners of the present application are described by using specific examples; the above embodiment description is only used to help understand the method and core idea of the present application; meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation manner and application range can be changed, and the above description should not be understood as a limitation of the present application.

Claims

1. A process for the preparation of foamable thermoplastic polyimide, characterized in that, The method comprises the following steps: The aromatic dianhydride, the aromatic diamine and the aliphatic diamine are dissolved in a solvent to generate polyimide acid through polymerization, and then the polyimide acid is subjected to high-temperature cyclization to obtain the thermoplastic polyimide.

2. The method for preparing a foamable thermoplastic polyimide according to claim 1, characterized in that, The aromatic dianhydride is selected from one or two or more of benzophenone tetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropyl) bisphthalic anhydride and bisphenol A type diether dianhydride, and the aromatic dianhydride is mixed in an arbitrary ratio.

3. The method for preparing an expandable thermoplastic polyimide according to claim 1, characterized in that, The aromatic diamine is selected from one or two or more of 2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-oxydianiline, 3,4'-oxydianiline, 4,4'-oxydianiline and 1,3-bis(4-aminophenoxy)benzene, and the aromatic diamine is mixed in an arbitrary ratio.

4. The method for preparing a foamable thermoplastic polyimide according to claim 1, characterized in that, The aliphatic diamine is selected from one or two or more of 1,6-hexanediamine, 1,10-decanediamine, 4,4'-diaminodicyclohexylmethane, isophorone diamine, 1,2-diaminocyclohexane, cis-1,4-diaminocyclohexane and trans-1,4-diaminocyclohexane, and the aliphatic diamine is mixed in an arbitrary ratio.

5. The method for preparing a foamable thermoplastic polyimide according to claim 1, characterized in that, The aliphatic diamine accounts for 1-50% of the total number of moles of diamine compounds; the diamine compounds comprise the aliphatic diamine and the aromatic diamine; and the molar ratio of the diamine compounds to the aromatic dianhydride is 1:1.0-1.

1.

6. The method for preparing a foamable thermoplastic polyimide according to claim 1, characterized in that, The solvent is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

7. A method of making a wave-absorbing foam, characterized by, The polyimide prepared by the preparation method of the foamable thermoplastic polyimide according to any one of claims 1-5 is used as a matrix, and then the matrix is doped with a wave-absorbing filler and subjected to a supercritical fluid foaming process to obtain the foamable thermoplastic polyimide.

8. The method of claim 7, wherein the wave-absorbing foam is prepared by the steps of: The wave-absorbing filler is selected from one or two or more of carbon nanotubes, graphene, conductive carbon black, silicon carbide fibers, barium titanate, zinc oxide, boron nitride, dielectric ceramic particles, carbonyl iron powder, ferrite, flaky iron silicon aluminum, nickel-coated carbon fiber, cobalt nanowire, iron-cobalt alloy powder and magnetic iron fiber, and the wave-absorbing filler is mixed in an arbitrary ratio.

9. The method of claim 7, wherein the wave-absorbing foam is prepared by the steps of: The addition amount of the wave-absorbing filler in the matrix is 1-20 wt%; and the wave-absorbing filler is introduced in the form of in-situ polymerization or physical blending.

10. The method of claim 7, wherein the wave-absorbing foam is prepared by the steps of: The foaming agent for the supercritical fluid foaming is carbon dioxide or nitrogen, the foaming temperature is 180-220°C, the foaming pressure is 9-25 MPa, and the pressure holding time is 1-3 h.

Images