Honeycomb enhanced in-situ foaming polyimide wave-absorbing foam composite material and preparation method thereof

Abstract

The invention discloses a honeycomb enhanced in-situ foaming polyimide wave-absorbing foam composite material which is prepared by the following steps: S1, adding aromatic dianhydride, aromatic diamine, triethanolamine, a foaming agent, dibutyltin dilaurate, epoxy resin and a hydroxyl chain extender into an organic solvent, uniformly mixing, adding wave-absorbing functional particles, and uniformly dispersing to obtain a component A; taking polymethylene polyphenyl polyisocyanate as a component B, and uniformly mixing the component A and the component B to obtain a polyimide foaming precursor solution; and S2, pouring the polyimide foaming precursor solution into a mold filled with a honeycomb core material, standing at room temperature for 2-6 hours to enable foam to grow in pores of the honeycomb core material in an in-situ confinement manner, then putting a semi-finished product under a vacuum condition for thermal imidization treatment, and demolding after cooling to obtain a target object. According to the method disclosed by the invention, the precursor solution is subjected to in-situ chemical foaming in the honeycomb pores by a one-step method, and the prepared composite material has excellent wave-absorbing property and mechanical property.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials technology, and in particular to a honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material and its preparation method. Background Technology

[0002] Microwave-absorbing foam, with its excellent electromagnetic wave absorption performance and ideal mechanical properties, is considered a highly promising structural microwave-absorbing material. Current research on microwave-absorbing foam structures mainly focuses on lightweight and flexible applications; its low compressive strength limits its long-term application as a microwave-absorbing foam structure. Although the mechanical properties of foam can be improved through chemical regulation and process optimization, they still fall short of its potential application as a direct load-bearing component in lightweight aerospace equipment. However, using a honeycomb reinforcement method can compensate for the poor mechanical properties of polymer foam materials. Simultaneously, the honeycomb structure can divide the originally monolithic foam into periodic microwave-absorbing units, further enhancing the microwave-absorbing performance of the foam material and achieving a synergistic improvement in both mechanical and microwave-absorbing properties. For example, patent CN202211380907.6 discloses a method for preparing a honeycomb foam composite microwave-absorbing material based on PMI foam. This PMI honeycomb foam composite microwave-absorbing material is foamed, then its surface is functionalized, cut into thin sheets, repeatedly immersed in microwave-absorbing adhesive, and finally pressed layer by layer into the honeycomb. The resulting PMI foam honeycomb microwave-absorbing material exhibits improved mechanical and microwave-absorbing properties. However, this process is relatively cumbersome, and hot pressing makes it difficult to achieve conformal coating, which easily leads to interface problems. Therefore, the current technical challenge is that the weak mechanical properties of microwave absorbing foam and its poor interfacial bonding with honeycomb make it difficult to achieve synergistic enhancement. Summary of the Invention

[0003] To address the technical problem that it is difficult to achieve synergistic enhancement of the mechanical properties and interfacial bonding performance of existing microwave absorbing foams with honeycomb structures, this invention provides a honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material and its preparation method.

[0004] The honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material provided by this invention is prepared by the following two steps: S1. Add aromatic dianhydride, aromatic diamine, triethanolamine, foaming agent, dibutyltin dilaurate, epoxy resin and hydroxyl chain extender to an organic solvent, mix evenly, add microwave absorbing functional particles, and ultrasonically disperse evenly to obtain component A; use polymethylene polyphenyl polyisocyanate (PAPI) as component B, mix component A and component B, stir at 2000-3000 r / min for 20-60 s to obtain polyimide foaming precursor solution.

[0005] In component A, the weight proportions of each component are as follows: 35-60 parts of aromatic dianhydride, 35-60 parts of aromatic diamine, 5-25 parts of epoxy resin, 1-8 parts of hydroxyl chain extender, 1-5 parts of foaming agent, 0.3-3 parts of microwave absorbing functional particles, 0.1-2 parts of triethanolamine, and 0.01-0.3 parts of dibutyltin dilaurate.

[0006] The organic solvent is selected from at least one of N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL), propylene carbonate, ethyl acetate, alcohols and ketones (acetone, MEK), and ethers (THF, dioxane).

[0007] The foaming agent is any one of water, a compound system of bicarbonate and organic acid, azodicarbonamide (AC), ditoluenesulfonyl hydrazine (OBSH), and azobisisobutyronitrile (AIBN).

[0008] The microwave absorbing particles are selected from any one of CNT, graphene, carbonyl iron, polyaniline, and barium ferrite.

[0009] The aromatic dianhydride is preferably pyromellitic dianhydride (PMDA), and the aromatic diamine is preferably 4,4′-diaminodiphenylmethane (MT). The mass ratio of PMDA to MT is (97-102):100.

[0010] The hydroxyl chain extender is polyethylene glycol (PEG), preferably PEG with a number average molecular weight of 400-2000.

[0011] S2. Pour the polyimide foaming precursor solution into a mold containing the honeycomb core material, let it stand at room temperature for 2-6 hours to allow the foam to grow in situ within the pores of the honeycomb core material, and then place the semi-finished product under vacuum conditions for thermal imidization treatment. After cooling, the target honeycomb reinforced in situ foamed polyimide microwave absorbing foam composite material is obtained.

[0012] The honeycomb core material is selected from any one of phenolic honeycomb, aramid honeycomb, polypropylene honeycomb, polycarbonate honeycomb, polyetherimide honeycomb, polyphenylene sulfide honeycomb, metal honeycomb, and ceramic honeycomb. The pore size of the honeycomb core material is 1.5-5.0 mm.

[0013] The thermal imidization treatment specifically involves a three-stage heating process under vacuum conditions: first, heating to 80-120℃ and holding for 1-2 hours; then heating to 150-180℃ and holding for 1-2 hours; and finally heating to 220-260℃ and holding for 1-3 hours.

[0014] Compared with the prior art, the advantages of the present invention are: (1) The method of the present invention introduces a relatively weakly reactive epoxy resin into the precursor of one-step in-situ chemical foaming, so that the in-situ foaming precursor solution has better conformability. Introducing honeycomb during the foaming process, the foam grows in confined space within the honeycomb pores, which increases the foaming pressure and forms a continuous transition interface between the foam and the pore wall, reducing defects introduced by the adhesive layer and multiple interfaces; thereby achieving a comprehensive improvement in the strength of the skeleton and the interfacial bonding strength between the honeycomb pore wall and the foam, and realizing the dual-functional synergistic enhancement of the microwave absorbing foam and the honeycomb, with excellent microwave absorption and load-bearing functions.

[0015] (2) The method of the present invention achieves impedance matching and multiple scattering path construction simultaneously during the preparation process by adjusting the aperture and periodic unit topology of the honeycomb, thus possessing both excellent absorption performance and mechanical properties.

[0016] (3) It omits the long-term polymerization, slicing and multiple impregnation / lamination steps that exist in the existing methods, simplifies the materials and processes, makes the process controllable, reduces the amount of solvent and volatiles, and can be prepared on a large scale.

[0017] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0018] Figure 1 The images show the FTIR spectra of the composite materials prepared in Examples 1-3 and the foam precursor solution PAAE.

[0019] Figure 2 Thermogravimetric curves of the composite materials prepared in Examples 1-3 and Comparative Example 1 in air atmosphere.

[0020] Figure 3 The stress-strain curves are for the composite materials prepared in Examples 1-3 and Comparative Example 1. Detailed Implementation

[0021] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0022] Example 1 A honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material is prepared by the following steps: S1: Preparation of polyimide foaming precursor solution Weigh 41.41 g of PMDA and 41.82 g of MT and dissolve them in 50 ml of DMF, stirring until clear. Then, add 0.42 g of triethanolamine (TEA), 1.67 g of deionized water, 0.04 g of dibutyltin dilaurate (DBTDL), 10.04 g of epoxy resin E-51, and 3.35 g of hydroxyl chain extender PEG-1000 sequentially, stirring until homogeneous. Add 1.25 g of CNT and ultrasonically disperse until homogeneous to obtain component A. Take an equimolar amount of PAPI as component B, add component B to component A, and stir at 3000 r / min for 30 s to obtain a polyimide foaming precursor solution, abbreviated as PAAE.

[0023] S2: Preparation of polyimide microwave absorbing foam composite materials The prepared polyimide foaming precursor solution was immediately poured into a mold containing aramid honeycomb (pore size 1.83 mm), and allowed to stand at room temperature for 6 h to allow the foam to grow in situ within the honeycomb pores. The semi-finished product was then placed in a vacuum oven (vacuum degree 0.05-0.09 MPa) and imidized by segmented heating. The specific heating program was as follows: first, the temperature was raised to 100℃ and held for 2 h, then raised to 170℃ and held for 1.5 h, and then raised to 240℃ and held for 2 h. After cooling and demolding at room temperature, the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material, abbreviated as CPH1, was obtained.

[0024] Example 2 The preparation method of Example 1 is followed, with the only difference being that in step S2, the pore size of the aramid honeycomb is adjusted to 2.75 mm, and the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material, abbreviated as CPH2, is finally obtained.

[0025] Example 3 The preparation method of Example 1 is followed, except that in step S2, the pore size of the aramid honeycomb is adjusted to 4.5 mm, and the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material, abbreviated as CPH3, is finally obtained.

[0026] Comparative Example 1 Step S1: Prepare polyimide foaming precursor solution, as in Step S1 of Example 1.

[0027] Step S2: Immediately pour the polyimide foaming precursor solution into the mold (without using aramid honeycomb), and let it stand at room temperature for 6 hours to allow the foam to expand in the mold; then place the semi-finished product in a vacuum (vacuum degree 0.05-0.09 MPa) oven and heat it in stages to complete imidization. The specific heating program is as follows: first heat to 100℃ and hold for 2 hours, then heat to 170℃ and hold for 1.5 hours, and then heat to 240℃ and hold for 2 hours; cool at room temperature and demold to obtain polyimide microwave absorbing foam composite material, namely CNT / PI / Epoxy composite material, abbreviated as CPE.

[0028] Performance tests are as follows: (1) FTIR spectroscopic tests of CPH1, CPH2, CPH3 and foam precursor solution PAAE prepared in Examples 1-3 are as follows: Figure 1 As shown. Compared to the PAAE foam precursor solution with introduced epoxy groups, the imine rings in the CPH1, CPH2, and CPH3 foam composites have a higher imine ring density at 1780 cm⁻¹. -1 (vas(C=O)), 1720 cm -1 The characteristic peak of (vs(C=O)) corresponds to the asymmetric and symmetric stretching vibration peaks of C=O in the amide bond, at 1366 cm⁻¹. -1 (vs(CN)) and 723 cm -1 At (δ(C=O)), there are characteristic peaks corresponding to the CN stretching vibration and C=O bending vibration in the imide functional group, and at the same time, there is a characteristic peak representing oxazolidinone (1750 cm⁻¹). -1 Polyamic acid-CONH at 1650 cm⁻¹ -1 and 1538 cm -1 Characteristic absorption peaks for C=O and CN were observed only in the foam precursor solution (PAAE). This indicates that the polyamic acid structure was fully converted into a polyimide structure through imidization reaction under high temperature treatment, proving the successful preparation of the PI foam skeleton structure of the matrix material.

[0029] (2) The thermogravimetric curves of the composite materials prepared in Examples 1-3 and Comparative Example 1 in air atmosphere are as follows: Figure 2As shown, compared to the thermal decomposition initiation temperature (T10% = 300 °C) of the non-honeycomb foam structure (CNT / PI / Epoxy) in Comparative Example 1, the thermal decomposition initiation temperatures (T10%) of the aramid honeycomb-containing foam composites CPH1, CPH2, and CPH3 in Examples 1-3 were significantly increased to 320 °C, 330 °C, and 331 °C, respectively. This is mainly attributed to the higher aromatization temperature (400-430 °C) of the aramid honeycomb, which delays the degradation behavior of the soft segments in the foam system. The second significant degradation stage (550-650 °C) represents the skeletal decomposition caused by the breakage of crosslinking points in the chain segments. This indicates that the aramid honeycomb has good thermal stability and, to a certain extent, acts as a physical barrier for the PI foam, thus improving the thermal stability of the foam.

[0030] (3) The mechanical properties and radar absorption properties of the composite materials prepared in Examples 1-3 and Comparative Example 1 were tested. The compressive strength was determined according to GB / T 8813-2008 "Determination of Compressive Properties of Rigid Foamed Plastics". The reflectivity was tested according to GJB 2038A-2011 "Test Method for Reflectivity of Radar Absorbing Materials". The test results are shown in Table 1 and... Figure 3 .

[0031] Table 1. Test results of mechanical properties and microwave absorption properties of the composite materials prepared in Examples 1-3 and Comparative Example 1.

[0032] Because the pore size of the foam cells in the honeycomb wall is relatively uniform, different confined growth spaces are the main factor affecting the impedance matching thickness of the absorbing bandwidth. Aramid honeycomb itself is a wave-transparent material. The original monolithic PI absorbing foam is isolated into periodic absorbing structural units by the confined space of the honeycomb pores. Each unit can attenuate incident electromagnetic waves (e.g., conductivity loss and polarization loss). The addition of the honeycomb introduces more heterogeneous interfaces, further promoting interface polarization. Simultaneously, electromagnetic waves are continuously reflected back and forth on the honeycomb wall surface between the hexagonal absorbing foam units, improving electromagnetic wave dissipation. Therefore, the smaller the pore size of the honeycomb, the more foam units in the same size foam composite material have in the isolation region. The presence of more wave-transparent phases further optimizes the impedance matching characteristics, and the increased interfaces and reflection paths greatly improve the attenuation capability of electromagnetic waves.

[0033] from Figure 3It can be seen that the composite materials in Examples 1-3 all exhibit excellent compressive strength. This is because the high density of rigid segments and crosslinking in the CPE composite foam itself possesses excellent mechanical properties. Simultaneously, the strong interfacial interactions induced by the good hydrogen bonds, π-π covalent bonds, and chemical bonds (-NCO and -OH) between the foam and the honeycomb wall during confined growth result in synergistic enhancement of mechanical properties between the CPE composite foam and the aramid honeycomb. Based on Comparative Example 1, epoxy resin E-51 was removed from the precursor solution in step S1, ultimately yielding pure CNT / PI foam material, abbreviated as CP. Experimental test results show that the CNT / PI foam material exhibits two distinct linear elastic deformation stages: from point contact between the indenter and the foam skeleton during loading to gradual compaction and surface contact. In contrast, the stress of the honeycomb-reinforced foam composite material of this invention significantly increases from the initial loading stage and exhibits linear elastic deformation. This is mainly attributed to the confining effect of the honeycomb pore walls, which provides better support for the foam skeleton, and the surface contact during loading, which enables effective load transfer of the sample. Due to the anisotropic nature of honeycomb structures, the pressure-bearing direction should be parallel to the pore direction during application to maximize the synergistic reinforcement effect of CPE composite foam and aramid honeycomb. As the pore wall size per unit volume increases, the anisotropic reinforcement effect of the honeycomb becomes more significant. The composite foam sample with a pore size of 1.83 mm (CPH1) achieved the highest compressive strength and modulus of 30 MPa and 322.3 MPa, respectively, representing increases of 285% and 201% compared to the 7.8 MPa and 107.1 MPa of pure CNT / PI foam (CP). The foam composites with pore sizes of 2.75 mm (CPH2) and 4.5 mm (CPH3) achieved compressive strengths and moduli of 20 MPa and 223.3 MPa, and 13 MPa and 167.8 MPa, respectively, also showing significant improvements compared to pure CNT / PI foam.

[0034] In summary, the honeycomb-reinforced foam composite material of the present invention can flexibly adapt to assembly requirements by adjusting the thickness while still achieving EAB coverage of the X-band. At the same time, its significant advantage of high compressive strength makes it a more widely used and competitive material, especially with great potential in applications in direct load-bearing structures.

[0035] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material, characterized in that, It includes the following two steps: S1. Add aromatic dianhydride, aromatic diamine, triethanolamine, foaming agent, dibutyltin dilaurate, epoxy resin and hydroxyl chain extender to an organic solvent, mix evenly, add microwave absorbing functional particles, and ultrasonically disperse evenly to obtain component A; use polymethylene polyphenyl polyisocyanate as component B, mix and stir component A and component B for 20-60 s to obtain polyimide foaming precursor solution; S2. Pour the polyimide foaming precursor solution into a mold containing the honeycomb core material, let it stand at room temperature for 2-6 hours to allow the foam to grow in situ within the pores of the honeycomb core material, and then place the semi-finished product under vacuum conditions for thermal imidization treatment. After cooling, the target honeycomb reinforced in situ foamed polyimide microwave absorbing foam composite material is obtained.

2. The preparation method of the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 1, characterized in that, The thermal imidization treatment specifically involves a three-stage heating process under vacuum conditions: first, heating to 80-120℃ and holding for 1-2 hours; then heating to 150-180℃ and holding for 1-2 hours; and finally heating to 220-260℃ and holding for 1-3 hours.

3. The preparation method of the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 1, characterized in that, The honeycomb core material is selected from any one of phenolic honeycomb, aramid honeycomb, polypropylene honeycomb, polycarbonate honeycomb, polyetherimide honeycomb, polyphenylene sulfide honeycomb, metal honeycomb, and ceramic honeycomb.

4. The preparation method of the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 3, characterized in that, The pore size of the honeycomb core material is 1.5-5.0 mm.

5. The preparation method of the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 1, characterized in that, The organic solvent is selected from at least one of DMF, N-methylpyrrolidone, N,N-dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone, propylene carbonate, ethyl acetate, alcohols and ketones, and ethers.

6. The method for preparing the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 1, characterized in that, The aromatic dianhydride is pyromellitic dianhydride, and the aromatic diamine is 4,4′-diaminodiphenylmethane.

7. The preparation method of the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 1, characterized in that, The foaming agent is any one of water, a compound system of bicarbonate and organic acid, azodicarbonamide, ditoluenesulfonyl hydrazine, and azobisisobutyronitrile.

8. The method for preparing the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 1, characterized in that, The microwave absorbing particles are selected from any one of CNT, graphene, carbonyl iron, polyaniline, and barium ferrite.

9. The method for preparing the honeycomb-reinforced in-situ foamed polyimide microwave absorbing foam composite material as described in claim 1, characterized in that, In component A, the weight proportions of each component are as follows: 35-60 parts of aromatic dianhydride, 35-60 parts of aromatic diamine, 5-25 parts of epoxy resin, 1-8 parts of hydroxyl chain extender, 1-5 parts of foaming agent, 0.3-3 parts of microwave absorbing functional particles, 0.1-2 parts of triethanolamine, and 0.01-0.3 parts of dibutyltin dilaurate.

10. A honeycomb-reinforced in-situ foamed polyimide microwave-absorbing foam composite material, characterized in that, It is prepared by the method described in any one of claims 1-9.

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