Polymethacrylimide wave-absorbing foam and preparation method thereof

Abstract

The invention relates to the technical field of polymethacrylimide foam, in particular to polymethacrylimide wave-absorbing foam and a preparation method thereof. The high-temperature-resistant acrylic resin is prepared from the following raw materials in parts by weight: 55 to 60 parts of methacrylic acid, 45 to 55 parts of methacrylonitrile, 5 to 8 parts of 1-amino octadecane, 0.3 to 0.6 part of N-acrylamide, 0.4 to 0.7 part of N-hydroxysuccinimide ester, 0.5 to 1.2 parts of carbodiimide, 4 to 7 parts of tridecyl methacrylate, 2 to 8 parts of functional filler, 0.35 to 0.40 part of initiator, 0.6 to 0.8 part of foaming agent, 12 to 15 parts of cross-linking agent and 8 to 12 parts of absorbent. 21-35 parts of modified cellulose, 12-25 parts of a biomass-based additive, and 5-18 parts of a dispersant. The prepared polymethacrylimide wave-absorbing foam is relatively good in mechanical property, and shows excellent chemical stability under the conditions of room temperature and extremely low temperature.

Description

Technical Field

[0001] The present invention relates to the technical field of polymethacrylimide foam, and more specifically, it relates to a polymethacrylimide wave-absorbing foam and a preparation method thereof. Background Art

[0002] Polymethacrylimide (PMI) foam is a thermosetting rigid foam material with a 100% closed-cell structure. Its uniformly cross-linked cell wall structure endows it with outstanding structural stability and excellent mechanical properties. It is an ideal core material for manufacturing lightweight and high-strength composite sandwich structures and has been widely used in large airliners, transport aircraft, fighter jets, helicopters, high-speed trains, wind turbine blades, satellites, launch vehicles, etc.

[0003] In the prior art, the strength of polymethacrylimide foam itself is poor and it is prone to breakage and fracture during use. Generally, nano-fillers such as silica and carbon fiber are added to enhance the mechanical strength of polymethyl methacrylate-based composites. Due to the easy agglomeration characteristics of nano-fillers themselves, they usually need to be organically modified before use, which makes the preparation process of polymethacrylimide foam materials more complex, the preparation process more lengthy, and the addition of modifiers may also affect the performance of polymethacrylimide foam materials.

[0004] Polymethacrylimide foam materials have the following problems: polymethacrylimide itself does not have wave-absorbing function, and the wave-absorbing function needs to be achieved by adding absorbents to the raw materials. However, low-frequency absorbents have a large density and are prone to precipitation, and although nano-scale absorbents have good effects, they are difficult to disperse. Summary of the Invention

[0005] The present invention provides a polymethacrylimide wave-absorbing foam and a preparation method thereof. The wave-absorbing function is achieved by adding absorbents, and the water absorption rate is reduced by adding hydrophobic materials. The prepared polymethacrylimide wave-absorbing foam has good mechanical properties and exhibits excellent chemical stability at room temperature and extremely low temperatures.

[0006] In a first aspect, the present invention provides a polymethacrylimide wave-absorbing foam, which is composed of the following raw materials: 55-60 parts of methacrylic acid, 45-55 parts of methacrylonitrile, 5-8 parts of 1-aminooctadecane, 0.3-0.6 parts of N-acrylamide, 0.4-0.7 parts of N-hydroxysuccinimide ester, 0.5-1.2 parts of carbodiimide, 4-7 parts of tridecyl methacrylate, 2-8 parts of functional filler, 0.35-0.40 parts of initiator, 0.6-0.8 parts of foaming agent, 12-15 parts of cross-linking agent, 8-12 parts of absorbent, 21-35 parts of biomass-based additive, 12-25 parts of environmentally friendly water absorbent, and 5-18 parts of dispersant.

[0007] Preferably, the biomass-based additive is prepared from lignin of biomass origin. Polyols are obtained through liquefaction, and silicon carbide fluoride and ammonium polyphosphate are introduced during the synthesis process. The mass ratio of lignin, silicon carbide fluoride, and ammonium polyphosphate is 2-5:1-3:3-8.

[0008] Preferably, the functional filler is one or more of nano-silica, calcium silicate hydrate, mesoporous material, molecular sieve, porous ceramic, and glass fiber.

[0009] Preferably, the absorbent is one or more of carbon nanotubes, graphene, graphite powder, conductive carbon black, and carbon fiber.

[0010] Preferably, the modified cellulose is prepared by hydrophobically modifying nano-cellulose with azobenzene. The mass ratio of nano-cellulose to azobenzene is 1-2:1-5.

[0011] Preferably, the cross-linking agent is one or more of acrylamide, methacrylamide, magnesium methacrylate, magnesium acrylate, zinc methacrylate, and zinc acrylate.

[0012] Preferably, the initiator is one or more of azobisisobutyronitrile, azobisisoheptonitrile, benzoyl peroxide, or bis(2-ethylhexyl) peroxydicarbonate.

[0013] Preferably, the foaming agent is one or more of tert-butyl methacrylate, isopropanol, tert-butanol, formamide, carbamide, methylurea, and dimethylurea; the dispersant is one or more of hydroxypropyl methylcellulose, hydroxyethyl cellulose, sodium dodecylbenzenesulfonate, and polyvinyl alcohol.

[0014] In a second aspect, the present invention provides a method for preparing a polymethacrylimide microwave-absorbing foam, comprising the following steps: (1) Mix methacrylic acid, methacrylonitrile, 1-aminooctadecane, N-acrylamide, N-hydroxysuccinimide ester, carbodiimide, tridecyl methacrylate, functional filler, initiator, foaming agent, cross-linking agent, absorbent, modified cellulose, biomass-based additive, and dispersant evenly to obtain a mixed solution; (2) Inject the obtained mixed solution into a mold, then perform air extraction treatment, and then seal it completely. Place it in a circulating water bath system. First, polymerize at 35-45°C for 36-72 hours, and then polymerize at 55-65°C for 36-72 hours to obtain a prepolymer; (3) Place the prepolymer in an oven, treat it at 70-120°C for 6-12 hours, then preheat the treated prepolymer at 180°C for 3 hours, then foam at 180-220°C for 2-5 hours, and anneal at 160-180°C for 14-20 hours to obtain a polymethacrylimide microwave-absorbing foam.

[0015] Preferably, the pressure for the air extraction treatment is -0.04 to -0.08 MPa.

[0016] In summary, the present invention has the following beneficial effects: 1. In the present invention, the biomass-based additive uses lignin from biomass as a raw material, and polyols are obtained through liquefaction. During the synthesis process, silicon carbide fluoride and ammonium polyphosphate are introduced to prepare it. As the main component of the plant cell wall, lignin's aromatic structure and rich active groups such as hydroxyl groups and methoxy groups make it an ideal raw material for preparing polyols. The thermal stability of silicon carbide fluoride and ammonium polyphosphate synergistically enhances the effect. The high thermal conductivity of silicon carbide fluoride can accelerate the uniform distribution of the decomposition products of ammonium polyphosphate, promote the formation of a dense carbon layer, and the high-temperature resistance characteristics of SiC-F can delay the thermal degradation of the matrix material and extend the flame retardant protection time. The prepared biomass-based additive has excellent mechanical stability, oil-water separation performance, degradation performance, and flame retardant performance, further improving the mechanical properties of the polymethacrylimide absorbing foam.

[0017] 2. In the present invention, the modified cellulose is prepared by hydrophobically modifying nanocellulose with azobenzene, and it has good thermal stability and biodegradability, being green and environmentally friendly. The Schiff base reaction occurs between the aldehyde group on the nanocellulose and the amino group on the amino azobenzene. Hydrogen bonds are formed between the lignin hydroxyl group and the nanocellulose hydroxyl group to improve the mechanical strength. Nanocellulose is composed of cellulose molecular chains, and its molecular chains are rich in hydroxyl groups (-OH), which endow it with hydrophilicity. Through reactions such as esterification, silanization, and acylation, these hydroxyl groups can be converted into hydrophobic groups such as ester groups, silyl groups, or amide groups. When silicon carbide fluoride binds to nanocellulose, the synergistic effect of fluorine atoms and silyl groups can further reduce the surface energy, and at the same time, the introduction of silicon carbide provides additional mechanical strength and thermal stability. This synergistic effect enables the material to have excellent weather resistance and corrosion resistance while maintaining hydrophobicity. By filling the gaps between SiO2 nanoparticles through the sol-gel method and introducing long-chain alkyl silyl groups, fiber entanglement can be effectively prevented and the hydrophobic performance can be improved.

[0018] 3. Carbon nanotubes, graphene, graphite powder, conductive carbon black, and carbon fibers are added as absorbents to the polymethacrylimide absorbing foam prepared in the present invention, endowing it with electromagnetic wave absorption ability. Through functional modification, the polymethacrylimide absorbing foam combines the mechanical and temperature resistance advantages with the adsorption characteristics of the polymethacrylimide absorbing foam, showing unique value in the high-end industrial and environmental protection fields.

[0019] 4. The poly(methacrylimide) microwave-absorbing foam prepared by the present invention is an adsorption composite material with low moisture absorption rate, good mechanical properties, strong heat resistance, stable chemical properties, low dielectric loss, simple process, easy control of parameters, and easy to realize industrial production. The poly(methacrylimide) microwave-absorbing foam prepared by this method has excellent mechanical properties and low moisture absorption rate.

[0020] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the protection scope of the present invention. Detailed Description of the Invention

[0021] The present invention will be further described in detail below with reference to the embodiments. It should be noted specifically that: for those conditions not specified in the following embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. Except for special instructions, the raw materials used in the following embodiments can all be obtained from ordinary commercial sources. Embodiments Embodiment 1

[0022] A poly(methacrylimide) microwave-absorbing foam is composed of the following raw materials: 55 parts of methacrylic acid, 45 parts of methacrylonitrile, 5 parts of 1-aminooctadecane, 0.3 parts of N-acrylamide, 0.4 parts of N-hydroxysuccinimide ester, 0.5 parts of carbodiimide, 4 parts of tridecyl methacrylate, 2 parts of functional filler, 0.35 parts of initiator, 0.6 parts of foaming agent, 12 parts of crosslinking agent, 8 parts of absorbent, 21 parts of modified cellulose, 12 parts of biomass-based additive, and 5 parts of dispersant.

[0023] The biomass-based additive is prepared by using biomass-derived lignin as a raw material, liquefying it to obtain polyol, and introducing silicon carbide fluoride and ammonium polyphosphate during the synthesis process. The mass ratio of lignin, silicon carbide fluoride, and ammonium polyphosphate is 2:1:3.

[0024] The functional filler is a mixture of nano-silica, hydrated calcium silicate, mesoporous material, and porous ceramic, and the mass ratio is 1:2:1:5.

[0025] The absorbent is a mixture of carbon nanotubes, graphene, and carbon fiber, and the mass ratio is 2:3:5.

[0026] The modified cellulose is prepared by using nano-cellulose as a substrate and performing hydrophobic modification with azobenzene. The mass ratio of nano-cellulose to azobenzene is 1:1.

[0027] The crosslinking agent is acrylamide; the initiator is azobisisobutyronitrile; the foaming agent is tert-butyl methacrylate; the dispersant is hydroxypropyl methylcellulose.

[0028] A preparation method of a poly(methacrylimide) microwave-absorbing foam includes the following steps: (1)Mix methacrylic acid, methacrylonitrile, 1-aminooctadecane, N-acrylamide, N-hydroxysuccinimide ester, carbodiimide, tridecyl methacrylate, functional filler, initiator, foaming agent, crosslinking agent, absorbent, modified cellulose, biomass-based additive and dispersant evenly to obtain a mixed solution; the reaction time of the mixed solution is 25 h; (2)Inject the obtained mixed solution into a mold, then perform air extraction treatment, the pressure of the air extraction treatment is -0.04 MPa, then seal it completely, put it into a circulating water bath system, polymerize at 35 °C for 36 h first, and then polymerize at 55 °C for 36 h to obtain a prepolymer; (3)Place the prepolymer in an oven, treat it at 70 °C for 6 h, then preheat the treated prepolymer at 180 °C for 3 h, then foam at 180 °C for 2 h, and anneal at 160 °C for 14 h to obtain polymethacrylimide microwave absorbing foam. Example 2

[0029] A polymethacrylimide microwave absorbing foam is composed of the following raw materials: 58 parts of methacrylic acid, 52 parts of methacrylonitrile, 7 parts of 1-aminooctadecane, 0.5 part of N-acrylamide, 0.6 part of N-hydroxysuccinimide ester, 0.8 part of carbodiimide, 5 parts of tridecyl methacrylate, 4 parts of functional filler, 0.30 part of initiator, 0.7 part of foaming agent, 14 parts of crosslinking agent, 10 parts of absorbent, 25 parts of modified cellulose, 20 parts of biomass-based additive, and 12 parts of dispersant.

[0030] The biomass-based additive uses biomass-derived lignin as a raw material, obtains polyol through liquefaction, and is prepared by introducing silicon carbide fluoride and ammonium polyphosphate during the synthesis process. The mass ratio of lignin, silicon carbide fluoride and ammonium polyphosphate is 3:2:5.

[0031] The functional filler is a mixture of nano-silica, calcium silicate hydrate, mesoporous material and porous ceramic, and the mass ratio is 1:3:2:4.

[0032] The absorbent is a mixture of carbon nanotubes, graphene and carbon fiber, and the mass ratio is 1:2:5.

[0033] The modified cellulose is prepared by hydrophobic modification of nano-cellulose with azobenzene. The mass ratio of nano-cellulose and azobenzene is 2:1.

[0034] The crosslinking agent is acrylamide; the initiator is azobisisobutyronitrile; the foaming agent is tert-butyl methacrylate; the dispersant is hydroxypropyl methylcellulose.

[0035] A preparation method of a polymethacrylimide microwave absorbing foam includes the following steps: (1) Mix methacrylic acid, methacrylonitrile, 1 - aminooctadecane, N - acrylamide, N - hydroxysuccinimide ester, carbodiimide, tridecyl methacrylate, functional filler, initiator, foaming agent, cross - linker, absorbent, modified cellulose, biomass - based additive, and dispersant evenly to obtain a mixed solution; the reaction time of the mixed solution is 26 h; (2) Inject the obtained mixed solution into a mold, then conduct degassing treatment. The pressure of the degassing treatment is - 0.06 MPa, then seal it completely, and place it in a circulating water bath system. First, polymerize at 40 °C for 48 h, and then polymerize at 60 °C for 48 h to obtain a prepolymer; (3) Place the prepolymer in an oven, treat it at 100 °C for 8 h, then preheat the treated prepolymer at 180 °C for 3 h, then foam at 200 °C for 3 h, and anneal at 170 °C for 16 h to obtain polymethacrylimide microwave - absorbing foam. Example 3

[0036] A polymethacrylimide microwave - absorbing foam is composed of the following raw materials: 60 parts of methacrylic acid, 55 parts of methacrylonitrile, 8 parts of 1 - aminooctadecane, 0.6 part of N - acrylamide, 0.7 part of N - hydroxysuccinimide ester, 1.2 parts of carbodiimide, 7 parts of tridecyl methacrylate, 8 parts of functional filler, 0.40 part of initiator, 0.8 part of foaming agent, 15 parts of cross - linker, 12 parts of absorbent, 35 parts of modified cellulose, 25 parts of biomass - based additive, and 18 parts of dispersant.

[0037] The biomass - based additive is prepared by using biomass - derived lignin as a raw material, liquefying it to obtain polyol, and introducing silicon carbide fluoride and ammonium polyphosphate during the synthesis process. The mass ratio of lignin, silicon carbide fluoride, and ammonium polyphosphate is 5:3:8.

[0038] The functional filler is a mixture of nano - silica, calcium silicate hydrate, mesoporous material, and porous ceramic, and the mass ratio is 1:1:1:3.

[0039] The absorbent is a mixture of carbon nanotubes, graphene, and carbon fiber, and the mass ratio is 1:2:1.

[0040] The modified cellulose is prepared by using nanocellulose as a substrate and performing hydrophobic modification with azobenzene. The mass ratio of nanocellulose and azobenzene is 2:5.

[0041] The cross - linker is acrylamide; the initiator is azobisisobutyronitrile; the foaming agent is tert - butyl methacrylate; the dispersant is hydroxypropyl methylcellulose.

[0042] A preparation method of a polymethacrylimide microwave - absorbing foam includes the following steps: (1)Mix methacrylic acid, methacrylonitrile, 1 - amino octadecane, N - acrylamide, N - hydroxysuccinimide ester, carbodiimide, tridecyl methacrylate, functional filler, initiator, foaming agent, cross - linker, absorbent, modified cellulose, biomass - based additive and dispersant evenly to obtain a mixed solution; the reaction time of the mixed solution is 28 h; (2)Inject the obtained mixed solution into a mold, then conduct air extraction treatment. The pressure of the air extraction treatment is - 0.08 MPa, then seal it completely, and place it in a circulating water bath system. First, polymerize at 45 °C for 72 h, and then polymerize at 55 - 65 °C for 72 h to obtain a prepolymer; (3)Place the prepolymer in an oven, treat it at 120 °C for 12 h, then preheat the treated prepolymer at 180 °C for 3 h, then foam at 220 °C for 5 h, and anneal at 180 °C for 20 h to obtain polymethacrylimide microwave - absorbing foam.

[0043] Comparative Example 1 The difference from Example 1 is that no functional filler is added.

[0044] Comparative Example 2 The difference from Example 1 is that no absorbent is added.

[0045] Comparative Example 3 The difference from Example 1 is that no modified cellulose is added.

[0046] Comparative Example 4 The difference from Example 1 is that no environmentally friendly water absorbent is added.

[0047] Performance Test The polymethacrylimide microwave - absorbing foams prepared in Examples 1 - 3 and Comparative Examples 1 - 4 were tested for mechanical properties in the following manner. As shown in Table 1.

[0048] The standard specimen was obtained according to the standard of QB / T 5491 - 2020 "Polymethacrylimide Foam Sheets". As shown in Table 1.

[0049] Tensile property test: Use a universal mechanical testing machine to conduct tensile tests on the prepared standard tensile mechanical specimens, and the tensile rate is 2 mm / min. Conduct tensile property tests on the products of the examples and comparative examples using a universal testing machine. The test conditions are as follows: Conduct tensile strength and elongation at break tests in accordance with the provisions of GB / T 9641; Calculate the tensile modulus according to GB / T 1040.1. The outer contour dimensions of the specimen are (150 ± 0.5) mm × (40 ± 0.5) mm × (10 ± 0.1) mm, and it is processed according to Figure 1 in the standard of GB / T 9641 to make the neck width (25 ± 0.1) mm; The number of specimens is not less than 5. As shown in Table 1.

[0050] Absorbing performance test: Through the reflectivity test method of radar absorbing materials, the test is carried out according to the GJB 2038A-2011 bow test method. The test frequency band is (2~18) GHz, the sample size is (300±0.5) mm×(300±0.5) mm×(30±0.2) mm, and the number of specimens is 1. As shown in Table 2.

[0051] Table 1 Performance test results Tensile strength (MPa) Young's modulus (GPa) Elongation at break (%) Moisture absorption rate after soaking in water at 30°C for 72 h (%) Example 1 70.25 2.56 4.32 0.25 Example 2 68.56 2.58 4.15 0.28 Example 3 69.13 2.55 4.28 0.31 Comparative Example 1 58.68 1.75 2.16 1.35 Comparative Example 2 60.52 1.88 2.55 1.40 Comparative Example 3 61.35 1.96 2.74 1.38 Comparative Example 4 58.23 1.78 2.72 1.21 Table 2 Absorbing performance test results Thickness (mm) Reflectivity (dB) at frequencies from 2 to 4 GHz Reflectivity (dB) at frequencies from 4 to 8 GHz Reflectivity (dB) at frequencies from 8 to 18 GHz Example 1 30 -6～-7 -12～-16 -13～-17 Example 2 30 -7～-8 -11～-15 -12～-17 Example 3 30 -8～-9 -13～-18 -14～-19 Comparative Example 1 30 -5～-7 -10～-14 -11～-15 Comparative Example 2 30 - - - Comparative Example 3 30 -4～-6 -9～-12 -10～-14 Comparative Example 4 30 -2～-5 -5～-10 -8～-12 As can be seen from Table 1, the polymethacrylimide absorbing foam prepared in Example 1 has good tensile strength, Young's modulus, elongation at break and moisture absorption rate, indicating that the prepared polymethacrylimide absorbing foam is an adsorption composite material, which performs excellently in terms of hydrophobicity, mechanical properties, heat resistance, chemical stability and low dielectric loss.

[0052] As can be seen from Table 2, the polymethacrylimide absorbing foam prepared in Example 1 has good absorbing performance, can absorb electromagnetic waves, prevent electromagnetic wave interference, and can be applied to aerospace equipment, medical devices, electronic products, communication equipment, measuring equipment and other aspects.

[0053] As mentioned above, it is only the exemplary specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A polymethacrylimide wave-absorbing foam, characterized in that: The invention is composed of the following raw materials: 55-60 parts of methacrylic acid, 45-55 parts of methacrylonitrile, 5-8 parts of 1-aminooctadecane, 0.3-0.6 parts of N-acrylamide, 0.4-0.7 parts of N-hydroxysuccinimide ester, 0.5-1.2 parts of carbodiimide, 4-7 parts of tridecyl methacrylate, 2-8 parts of functional filler, 0.35-0.40 parts of initiator, 0.6-0.8 parts of foaming agent, 12-15 parts of cross-linking agent, 8-12 parts of absorbent, 21-35 parts of modified cellulose, 12-25 parts of biomass-based additives and 5-18 parts of dispersant; the modified cellulose is prepared by hydrophobic modification of nanocellulose using azobenzene as a substrate, and the mass ratio of nanocellulose to azobenzene is 1-2:1-5.

2. The polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The biomass-based additive uses biomass-derived lignin as a raw material, obtains polyols through liquefaction, and introduces fluorinated silicon carbide and ammonium polyphosphate during the synthesis process. The mass ratio of lignin, fluorinated silicon carbide and ammonium polyphosphate is 2-5:1-3:3-8.

3. The polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The functional filler is one or more of nano silicon dioxide, hydrated calcium silicate, mesoporous material, molecular sieve, porous ceramic and glass fiber.

4. The polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The absorbent is one or more of carbon nanotubes, graphene, graphite powder, conductive carbon black and carbon fiber.

5. The polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The crosslinking agent is one or more of acrylamide, methacrylamide, magnesium methacrylate, magnesium acrylate, zinc methacrylate and zinc acrylate.

6. The polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The initiator is one or more of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide or di(2-ethyl)hexyl peroxydicarbonate.

7. The polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The foaming agent is one or more of tert-butyl methacrylate, isopropyl alcohol, tert-butyl alcohol, formamide, carbonamide, methyl urea and dimethyl urea; the dispersant is one or more of hydroxypropyl methylcellulose, hydroxyethyl cellulose, sodium dodecylbenzene sulfonate and polyvinyl alcohol.

8. The method for preparing the polymethacrylimide wave-absorbing foam according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) methacrylic acid, methacrylonitrile, 1-aminooctadecane, N-acrylamide, N-hydroxysuccinimide ester, carbodiimide, tridecyl methacrylate, functional filler, initiator, foaming agent, cross-linking agent, absorbent, modified cellulose, biomass-based additive and dispersant are mixed uniformly to obtain a mixed solution; (2) injecting the obtained mixed solution into a mold, evacuating the air, and then completely sealing the mold, placing the mold in a circulating water bath system, polymerizing the mold at 35-45°C for 36-72 hours, and then polymerizing the mold at 55-65°C for 36-72 hours to obtain a prepolymer; (3) placing the prepolymer in an oven and treating it at 70-120° C. for 6-12 hours, then preheating the treated prepolymer at 180° C. for 3 hours, then foaming it at 180-220° C. for 2-5 hours, and annealing it at 160-180° C. for 14-20 hours to obtain polymethacrylimide absorbing foam.

9. The method for preparing the polymethacrylimide wave-absorbing foam according to claim 8, characterized in that: The pressure of the vacuum treatment is -0.04~-0.08MPa.