A preparation method of a homogeneous polymethacrylimide wave-absorbing foam

Abstract

The application discloses a preparation method of homogeneous poly-methacrylimide wave-absorbing foam, and belongs to the technical field of functional foam materials. Thermally expandable microspheres are used as carriers, and iron metal framework structures with 5-hydroxy isophthalic acid as monomers are loaded on the surfaces of the carriers to obtain dispersed thermally expandable microspheres, which can be uniformly dispersed in the wave-absorbing foam, realize uniform foaming, and have uniform foaming effect and insignificant layering phenomenon. Meanwhile, core-shell toughening composite powders generated by a molten salt method are used as fillers of the wave-absorbing foam, so that the impact resistance and performance stability of the wave-absorbing foam in the foaming process are effectively ensured. The strong covalence of Ti-B bonds in the shell of the core-shell toughening composite powders makes the core-shell toughening composite powders have high conductivity, can absorb electromagnetic wave energy through a dielectric loss mechanism, and the boron nitride of the core can assist in forming multiple scattering interfaces and prolonging the electromagnetic wave propagation path, so that the wave-absorbing performance of the absorbent is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of functional foam materials, and in particular relates to a method for preparing homogeneous polymethacrylimide wave-absorbing foam. Background Art

[0002] With the development of the electronics industry and the widespread use of electronic devices, electromagnetic wave pollution is becoming increasingly serious, resulting in a growing demand for absorbing and shielding materials. The demand for absorbing materials in practical applications is urgent. In the research of absorbing materials, absorbing foam materials have received considerable attention. Absorbing foam composite materials not only have excellent absorption of electromagnetic waves and infrared rays, but also have low density, high strength, and the advantages of sound absorption and vibration reduction. The application of absorbing foam materials in civilian facilities can effectively reduce the impact of electromagnetic waves on equipment, reduce the load-bearing capacity of the main structure, and provide heat insulation and noise reduction.

[0003] Polymethacrylimide foam is a high-performance foam material with good mechanical properties, excellent flame retardancy, and resistance to open flames. Compared with other types of foam plastics, it does not produce harmful gases during use, has a low density, a wide performance adjustment range, and is easy to install and disassemble. While being resistant to high temperatures, it also has excellent resistance to low-temperature brittleness. As an ideal core material for continuous fiber-reinforced composite materials such as carbon fiber, it has been widely used both at home and abroad.

[0004] Chinese patent publication number CN107857843B discloses a method for preparing a homogeneous polymethacrylimide (PMMA) absorbing foam. This invention employs a two-step process for adding an absorber: a high-density absorber is added to a precursor mixture to produce a foam precursor copolymer sheet. After foaming, the foam precursor copolymer sheet is crushed to form absorbing PMI foam particles. The absorbing PMI foam particles, methacrylic acid or acrylic acid, and methacrylonitrile or acrylonitrile are mixed, and a lightweight absorber is added to achieve uniform dispersion of the absorber in the PMMA foam. The addition of a dispersant allows for high-speed dispersion in the mixture, preventing sedimentation of various additives. This improves the excellent mechanical properties and thermal stability of the PMMA absorbing foam while also achieving excellent absorbing performance. This method is suitable for applications in aerospace, military, and other fields. However, this solution involves mixing multiple additives, and effective dispersion of the additives cannot be achieved using the dispersant alone, resulting in uneven foaming and thus affecting the overall strength of the absorbing foam. Summary of the Invention

[0005] The object of the present invention is to provide a method for preparing a homogeneous polymethacrylimide absorbing foam. Heat-expandable microspheres are used as a carrier, and an iron metal skeleton structure with 5-hydroxyisophthalic acid as a monomer is loaded on the surface of the carrier to obtain dispersed heat-expandable microspheres. The microspheres can be evenly dispersed in the absorbing foam to achieve uniform foaming, a uniform foaming effect, and no significant stratification. At the same time, a molten salt method is used to generate a core-shell toughened composite powder as a filler for the absorbing foam, effectively ensuring the impact resistance and performance stability of the absorbing foam during the foaming process.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A method for preparing homogeneous polymethacrylimide wave-absorbing foam comprises the following steps:

[0008] Methacrylic acid, core-shell toughened composite powder, methacrylonitrile, azobisisobutyronitrile, dispersed heat-expandable microspheres, magnesium oxide as a cross-linking agent, carbon nanotubes, polyamide as an anti-settling agent, and formamide as a nucleating agent are added into a reactor, stirred at 35-40°C and 500-600 r / min for 30-40 minutes, injected into a mold, immersed in deionized water at 50-60°C for polymerization for 80-85 hours, and preheated in an oven at 150-160°C for 2-3 hours, and then foamed at 210-220°C for 2-3 hours to obtain a homogeneous polymethacrylimide wave-absorbing foam.

[0009] Furthermore, the mass ratio of methacrylic acid, core-shell toughened composite powder, methacrylonitrile, azobisisobutyronitrile, dispersed thermally expandable microspheres, magnesium oxide, absorbent, polyamide and formamide is 45-50:10-12:53-55:0.3-0.5:7-9:1-2:2-3:2-3:0.5-0.7.

[0010] Furthermore, the absorbent is any one or more of carbon nanotubes, carbon black or graphene.

[0011] Furthermore, the core-shell toughened composite powder is prepared by the following steps:

[0012] The mixed powder and anhydrous ethanol solution with a mass fraction of 25-30% were added to the reactor in a ratio of 160-180 g: 500-600 mL, stirred at 20-25 ° C and 500-600 r / min for 24-26 h, and the anhydrous ethanol was removed by rotary evaporation. The product was vacuum dried at 60-80 ° C for 1-2 h, and transferred to a muffle furnace. Under argon protection, it was heated to 1400-1440 ° C at a flow rate of 50-55 mL / min and a heating rate of 3-4 ° C / min. It was kept warm for 2-3 h and naturally cooled to room temperature. The product was washed with deionized water 2-3 times and vacuum dried at 60-80 ° C for 1-2 h to obtain a core-shell toughened composite powder.

[0013] Furthermore, the mixed powder is prepared by the following steps:

[0014] The sodium chloride is placed in a muffle furnace and dried at 140-150° C. for 2-3 hours to completely remove moisture from the salt to obtain pretreated sodium chloride; the pretreated sodium chloride, potassium chloride, nano boron nitride and titanium dihydride are evenly mixed to obtain a mixed powder.

[0015] Furthermore, the usage ratio of pretreated sodium chloride, potassium chloride, nano boron nitride and titanium dihydride is 85-95g: 55-60g: 10-15g: 10-12g.

[0016] Furthermore, the dispersed heat-expandable microspheres are prepared by the following steps:

[0017] Heat-expandable microspheres, 5-hydroxyisophthalic acid and deionized water are added to a reactor, sodium hexadecyl sulfate is dissolved in a 70-75 wt% ethanol solution and then added to the reactor, and stirred at 120-130° C. and 400-500 r / min for 1-2 hours. Ferric chloride is added to the reactor, and the stirring reaction is continued for 4-5 hours. The reaction is filtered, and the filter cake is washed with deionized water and anhydrous ethanol for 2-3 times, respectively, and vacuum dried to obtain dispersed heat-expandable microspheres.

[0018] Furthermore, the usage ratio of heat-expandable microspheres, 5-hydroxyisophthalic acid, deionized water, sodium cetyl sulfate, ethanol solution and ferric chloride is 70-80 g: 30-40 g: 700-800 mL: 1-2 g: 120-140 mL: 15-20 g.

[0019] Furthermore, heat-expandable microspheres are prepared by the following steps:

[0020] Sodium chloride, colloidal silica and deionized water are added to a reactor, stirred at 35-40° C. and 500-600 r / min for 30-40 minutes, and the pH value is adjusted to 3.5-4 to obtain an aqueous phase mixture; acrylonitrile, methacrylonitrile, isopentane, isooctane, ethylene glycol dimethacrylate, diallyl carbonate, cyclohexane oil and azobisisobutyronitrile are added to the reactor and stirred uniformly to obtain an oil phase mixture; the aqueous phase mixture and the oil phase mixture are added to the reactor in a mass ratio of 10:1 and stirred uniformly, and high-speed dispersed at 1500-1600 r / min for 15-20 minutes. Under nitrogen protection, the mixture is stirred and polymerized at a temperature of 70-80° C., a rotation speed of 80 r / min and a pressure of 0.5-0.7 MPa for 20-22 hours, filtered, and the filter cake is washed with deionized water and anhydrous ethanol for 2-3 times respectively, and vacuum dried at 60-80° C. for 1-2 hours to obtain heat-expandable microspheres.

[0021] Furthermore, the usage ratio of sodium chloride, colloidal silicon dioxide and deionized water is 80-90 g:35-40 g:800-900 mL.

[0022] Furthermore, the mass ratio of acrylonitrile, methacrylonitrile, isopentane, isooctane, ethylene glycol dimethacrylate, diallyl carbonate, cyclohexane oil and azobisisobutyronitrile is 190-200:80-85:25-30:65-70:1-2:1-2:9-10:1.5-2.

[0023] Beneficial effects of the present invention:

[0024] The present invention prepares a homogeneous polymethacrylimide absorbing foam. Its innovation lies in the use of heat-expandable microspheres as a carrier, on the surface of which is loaded an iron metal skeleton structure composed of 5-hydroxyisophthalic acid as a monomer. This design enables the heat-expandable microspheres to be evenly dispersed throughout the absorbing foam, achieving uniform foaming and a uniform foaming effect with minimal stratification. Furthermore, a core-shell toughened composite powder, generated using a molten salt method, is used as the filler in the absorbing foam, effectively ensuring the foam's impact resistance and performance stability during the foaming process.

[0025] 2. The core-shell toughened composite powder involved in the present invention successfully prepared a core-shell structure with nano-boron nitride as the core and titanium boride as the shell through a molten salt method. This composite powder, used as a filler in homogeneous polymethacrylimide absorbing foam, can significantly improve the compressive strength and impact resistance of the absorbing foam. The strong covalent nature of the Ti-B bond in the titanium boride shell gives it high conductivity, enabling it to absorb electromagnetic wave energy through a dielectric loss mechanism. Boron nitride, with its high thermal stability and layered structure, can assist in forming multiple scattering interfaces, extending the electromagnetic wave propagation path, thereby improving energy dissipation efficiency and enhancing the absorbent's absorbing performance. Furthermore, the layered structure of boron nitride helps to disperse external stress, while the high hardness of the titanium boride shell prevents crack propagation. The synergistic effect of these two factors significantly improves the compressive strength and impact resistance of the absorbing foam, while avoiding the agglomeration problem that often occurs when conventional nano-boron nitride is directly added.

[0026] 3. The dispersed heat-expandable microspheres of the present invention use heat-expandable microspheres as carriers, and an iron metal skeleton structure with 5-hydroxyisophthalic acid as a monomer is loaded on their surface. This structure enhances the crosslinking density of the microsphere shell, improving its thermal stability and compressive resistance. In addition, the hydroxyl groups contained in 5-hydroxyisophthalic acid introduce hydroxyl groups on the surface of the heat-expandable microspheres, reducing particle agglomeration through electrostatic repulsion, allowing the heat-expandable microspheres to be evenly dispersed in the absorbing foam, achieving uniform foaming. DETAILED DESCRIPTION

[0027] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] Example 1: A method for preparing a homogeneous polymethacrylimide wave-absorbing foam, comprising the following steps:

[0029] S1: 80g of sodium chloride, 35g of colloidal silica and 800mL of deionized water were added to a reactor, stirred at 35°C and 500r / min for 30min, and the pH value was adjusted to 3.5 to obtain an aqueous phase mixture; 190g of acrylonitrile, 80g of methacrylonitrile, 25g of isopentane, 65g of isooctane, 1g of ethylene glycol dimethacrylate, 1g of diallyl carbonate, 9g of cyclohexane oil and 1.5g of azobisisobutyronitrile were added to the reactor and stirred evenly to obtain an oil phase mixture; the aqueous phase mixture and the oil phase mixture were added to the reactor in a mass ratio of 10:1 and stirred evenly, and high-speed dispersed at 1500r / min for 15min. Under nitrogen protection, the mixture was stirred and polymerized at a temperature of 70°C, a rotation speed of 80r / min and a pressure of 0.5MPa for 20h, filtered, and the filter cake was washed twice with deionized water and anhydrous ethanol respectively, and vacuum dried at 60°C for 1h to obtain heat-expandable microspheres.

[0030] S2: 70 g of heat-expandable microspheres, 30 g of 5-hydroxyisophthalic acid, and 700 mL of deionized water were added to a reactor. 1 g of sodium hexadecyl sulfate was dissolved in 120 mL of a 70 wt% ethanol solution and added to the reactor. The mixture was stirred at 120°C and 400 r / min for 1 h. 15 g of ferric chloride was added to the reactor. The mixture was stirred and reacted for 4 h. The mixture was filtered and the filter cake was washed twice with deionized water and anhydrous ethanol, respectively, and dried in vacuo to obtain dispersed heat-expandable microspheres.

[0031] The core structure of the heat-expandable microspheres is a liquid foaming agent wrapped in a thermoplastic polymer shell. Sodium hexadecyl sulfate serves as an anionic surfactant and forms micelles after being dissolved in ethanol, thereby reducing the surface tension of the aqueous phase and helping the microspheres to be evenly dispersed in the aqueous phase to prevent agglomeration. The iron ions in ferric chloride react with the carboxylic acid groups of 5-hydroxyisophthalic acid to form an iron metal skeleton structure, which is coated on the surface of the heat-expandable microspheres, enhancing the crosslinking density of the microsphere shell and improving its thermal stability and compressive resistance. 5-hydroxyisophthalic acid contains hydroxyl groups. By introducing hydroxyl groups on the surface of the heat-expandable microspheres, particle agglomeration is reduced through electrostatic repulsion, thereby avoiding the traditional direct addition that causes the heat-expandable microspheres to be unable to be evenly distributed in the absorbing foam, thereby causing uneven foaming.

[0032] The iron metal skeleton and the heat-expandable microspheres coordinate stress through dislocation slip. When ordinary heat-expandable microspheres expand due to the lack of stress buffering mechanism, the internal stress concentration is difficult to release, which can easily lead to microsphere rupture or uneven foaming.

[0033] S3: Place sodium chloride in a muffle furnace and dry it at 150°C for 3 hours to completely remove moisture from the salt to obtain pretreated sodium chloride; mix 85g of pretreated sodium chloride, 55g of potassium chloride, 10g of nano-boron nitride and 10g of titanium dihydride to obtain a mixed powder; add 160g of the mixed powder and 500mL of anhydrous ethanol solution with a mass fraction of 20% into a reactor, stir at 20°C and 500r / min for 24h, remove anhydrous ethanol by rotary evaporation, and vacuum dry at 60°C for 1h. Transfer the product to a muffle furnace and heat it to 1400°C under argon protection at a flow rate of 50mL / min and a heating rate of 3°C / min. Keep warm for 2h and cool naturally to room temperature. Wash the product with deionized water twice and vacuum dry it at 60°C for 1h to obtain a core-shell toughened composite powder.

[0034] Through the molten salt method, nano-boron nitride reacts with TiH2 in a molten salt of sodium chloride and potassium chloride. TiH2 decomposes to generate active titanium and hydrogen. Titanium, as a reaction source, reacts with the boron in the boron nitride micropowder. The titanium dispersed in the molten salt preferentially forms heterogeneous nuclei on the surface of the nano-boron nitride particles, forming nano-scale titanium boride grains and accumulating into a shell layer, generating a core-shell toughened composite powder with nano-boron nitride as the core and titanium boride as the shell.

[0035] S4: 45g of methacrylic acid, 10g of core-shell toughened composite powder, 53g of methacrylonitrile, 0.3g of azobisisobutyronitrile, 7g of dispersed heat-expandable microspheres, 1g of magnesium oxide as a cross-linking agent, 2g of carbon nanotubes, 2g of polyamide as an anti-settling agent, and 0.5g of formamide as a nucleating agent were added to a reactor, stirred at 35°C and 500r / min for 30min, the mixed solution was injected into a mold, and the mold was immersed in 50°C deionized water for polymerization for 80h. The product was placed in a 150°C oven for preheating for 2h, and then foamed at 210°C for 2h to obtain a homogeneous polymethacrylimide absorbing foam.

[0036] Example 2: A method for preparing a homogeneous polymethacrylimide wave-absorbing foam, comprising the following steps:

[0037] S1: 85 g of sodium chloride, 37.5 g of colloidal silica and 850 mL of deionized water were added to a reactor, stirred at 37.5 ° C and 550 r / min for 35 min, and the pH value was adjusted to 3.75 to obtain an aqueous phase mixture; 195 g of acrylonitrile, 82.5 g of methacrylonitrile, 27.5 g of isopentane, 67.5 g of isooctane, 1.5 g of ethylene glycol dimethacrylate, 1.5 g of diallyl carbonate, 9.5 g of cyclohexane oil and 1.75 g of azobisisocyanate were added to the reactor. Butyronitrile was added into the reactor and stirred evenly to obtain an oil phase mixture; the aqueous phase mixture and the oil phase mixture were added into the reactor in a mass ratio of 10:1 and stirred evenly, and dispersed at a high speed of 1550 r / min for 17.5 min. Under nitrogen protection, the mixture was stirred and polymerized at a temperature of 75°C, a rotation speed of 80 r / min and a pressure of 0.6 MPa for 21 h. The mixture was filtered, and the filter cake was washed twice with deionized water and anhydrous ethanol respectively, and dried in vacuum at 70°C for 1.5 h to obtain heat-expandable microspheres.

[0038] S2: 75 g of heat-expandable microspheres, 35 g of 5-hydroxyisophthalic acid, and 750 mL of deionized water were added to a reactor. 1.5 g of sodium hexadecyl sulfate was dissolved in 130 mL of a 72.5 wt% ethanol solution and added to the reactor. The mixture was stirred at 125°C and 450 r / min for 1.5 h. 17.5 g of ferric chloride was added to the reactor, and the reaction was continued with stirring for 4.5 h. The mixture was filtered, and the filter cake was washed twice with deionized water and anhydrous ethanol, respectively, and dried in vacuo to obtain dispersed heat-expandable microspheres.

[0039] S3: Place sodium chloride in a muffle furnace and dry it at 160°C for 3.5 hours to completely remove the moisture in the salt to obtain pretreated sodium chloride; mix 90g of pretreated sodium chloride, 57.5g of potassium chloride, 12.5g of nano-boron nitride and 11g of titanium dihydride to obtain a mixed powder; add 170g of the mixed powder and 550mL of anhydrous ethanol solution with a mass fraction of 25% into a reactor, stir for 25 hours at 22.5°C and 550r / min, remove the anhydrous ethanol by rotary evaporation, and vacuum dry at 70°C for 1.5 hours. Transfer the product to a muffle furnace and heat it to 1420°C under argon protection at a flow rate of 52.5mL / min and a heating rate of 3.5°C / min. Keep warm for 2.5 hours, cool it naturally to room temperature, wash the product with deionized water twice, and vacuum dry it at 70°C for 1.5 hours to obtain a core-shell toughened composite powder.

[0040] S4: 47.5 g of methacrylic acid, 11 g of core-shell toughened composite powder, 54 g of methacrylonitrile, 0.4 g of azobisisobutyronitrile, 8 g of dispersed heat-expandable microspheres, 1.5 g of magnesium oxide as a cross-linking agent, 2.5 g of carbon black, 2.5 g of polyamide as an anti-settling agent, and 0.6 g of formamide as a nucleating agent were added to a reactor, and stirred at 37.5 ° C and 550 r / min for 35 minutes. The mixed solution was injected into a mold, and the mold was immersed in 55 ° C deionized water for polymerization for 82.5 hours. The product was placed in a 155 ° C oven for preheating for 2.5 hours, and then foamed at 215 ° C for 2.5 hours to obtain a homogeneous polymethacrylimide absorbing foam.

[0041] Example 3: A method for preparing a homogeneous polymethacrylimide wave-absorbing foam, comprising the following steps:

[0042] S1: 90g of sodium chloride, 40g of colloidal silica and 900mL of deionized water were added to a reactor, stirred at 40°C and 600r / min for 40min, and the pH value was adjusted to 4 to obtain an aqueous phase mixture; 200g of acrylonitrile, 85g of methacrylonitrile, 30g of isopentane, 70g of isooctane, 2g of ethylene glycol dimethacrylate, 2g of diallyl carbonate, 10g of cyclohexane oil and 2g of azobisisobutyronitrile were added to the reactor and stirred evenly to obtain an oil phase mixture; the aqueous phase mixture and the oil phase mixture were added to the reactor in a mass ratio of 10:1 and stirred evenly, and high-speed dispersed at 1600r / min for 20min. Under nitrogen protection, the mixture was stirred and polymerized at a temperature of 80°C, a rotation speed of 80r / min and a pressure of 0.7MPa for 22h, filtered, and the filter cake was washed with deionized water and anhydrous ethanol three times respectively, and vacuum dried at 80°C for 2h to obtain heat-expandable microspheres.

[0043] S2: 80 g of heat-expandable microspheres, 40 g of 5-hydroxyisophthalic acid, and 800 mL of deionized water were added to a reactor. 2 g of sodium hexadecyl sulfate was dissolved in 140 mL of a 75 wt% ethanol solution and added to the reactor. The mixture was stirred at 130°C and 500 rpm for 2 h. 20 g of ferric chloride was added to the reactor, and the reaction was continued with stirring for 5 h. The mixture was filtered, and the filter cake was washed three times with deionized water and anhydrous ethanol, respectively, and vacuum dried to obtain dispersed heat-expandable microspheres.

[0044] S3: Place sodium chloride in a muffle furnace and dry it at 170°C for 4 hours to completely remove moisture from the salt to obtain pretreated sodium chloride; mix 95g of pretreated sodium chloride, 60g of potassium chloride, 15g of nano-boron nitride and 12g of titanium dihydride to obtain a mixed powder; add 180g of the mixed powder and 600mL of anhydrous ethanol solution with a mass fraction of 30% into a reactor, stir for 26 hours at 25°C and 600r / min, remove the anhydrous ethanol by rotary evaporation, and vacuum dry at 80°C for 2 hours. Transfer the product to a muffle furnace and heat it to 1440°C under argon protection at a flow rate of 55mL / min and a heating rate of 4°C / min. Keep warm for 3 hours and cool naturally to room temperature. Wash the product with deionized water three times and vacuum dry it at 80°C for 2 hours to obtain a core-shell toughened composite powder.

[0045] S4: 50g of methacrylic acid, 12g of core-shell toughened composite powder, 55g of methacrylonitrile, 0.5g of azobisisobutyronitrile, 9g of dispersed heat-expandable microspheres, 2g of magnesium oxide as a cross-linking agent, 3g of graphene, 3g of polyamide as an anti-settling agent, and 0.7g of formamide as a nucleating agent were added to a reactor, stirred at 40°C and 600r / min for 40min, the mixed solution was injected into a mold, and the mold was immersed in deionized water at 60°C for polymerization for 85h. The product was placed in a 160°C oven for preheating for 3h, and then foamed at 220°C for 3h to obtain a homogeneous polymethacrylimide absorbing foam.

[0046] Comparative Example 1: Based on Example 3, without step S2, the dispersed heat-expandable microspheres in step S4 were replaced by the heat-expandable microspheres in step S1, and the rest remained unchanged to prepare a homogeneous polymethacrylimide absorbing foam.

[0047] Comparative Example 2: Based on Example 3, step S3 is not performed, the core-shell toughened composite powder in step S4 is discarded, and the rest remain unchanged to prepare a homogeneous polymethacrylimide absorbing foam.

[0048] Comparative Example 3: Based on Example 3, the core-shell toughened composite powder in step S4 was replaced with nano-boron nitride in step S3, and the rest remained unchanged to prepare a homogeneous polymethacrylimide absorbing foam.

[0049] The homogeneous polymethacrylimide absorbing foams prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were tested for performance, and the results are shown in Tables 1 and 2:

[0050] As can be seen from Table 1, the average density, compressive strength, and heat deformation temperature of the upper, middle, and lower sections of the homogeneous polymethacrylimide absorbing foams prepared in Examples 1 to 3 are significantly higher than those in the comparative example, and the flat plate reflectivity is significantly lower than that in the comparative example. In addition, the fluctuations in the various data of Examples 1 to 3 are small, but the fluctuations in the various data of the comparative example are large. This indicates that the homogeneous polymethacrylimide absorbing foam prepared in the present invention is uniformly foamed, has no obvious delamination, and has stable performance.

[0051] In Comparative Example 1, the dispersed heat-expandable microspheres in step S4 are replaced with the heat-expandable microspheres in step S1, and the iron ions in the ferric chloride react with the carboxylic acid groups of the 5-hydroxyisophthalic acid to form an iron metal skeleton structure, which is coated on the surface of the heat-expandable microspheres, thereby enhancing the crosslinking density of the microsphere shell and improving its thermal stability and compressive resistance. In addition, the 5-hydroxyisophthalic acid contains hydroxyl groups, and the hydroxyl groups are introduced on the surface of the heat-expandable microspheres. The particle agglomeration is reduced by electrostatic repulsion, thereby avoiding the traditional direct addition that causes the heat-expandable microspheres to be not evenly distributed in the absorbing foam, thereby resulting in uneven foaming.

[0052] In Comparative Example 2, the core-shell toughened composite powder in step S4 is discarded, and nano-boron nitride reacts with TiH2 in a molten salt of sodium chloride and potassium chloride through a molten salt method. TiH2 decomposes to generate active titanium and hydrogen. Titanium acts as a reaction source and reacts with the boron in the boron nitride powder. The titanium dispersed in the molten salt preferentially nucleates heterogeneously on the surface of the nano-boron nitride particles to form nano-scale titanium boride grains and accumulate into a shell layer, thereby generating a core-shell toughened composite powder with nano-boron nitride as the core and titanium boride as the shell. The shell layer of titanium boride has high conductivity and can absorb electromagnetic wave energy through dielectric loss. The high thermal stability and layered structure of boron nitride can assist in forming a multiple scattering interface, extend the electromagnetic wave propagation path, and improve energy dissipation efficiency. The layered structure of boron nitride can disperse external stress, and the high hardness of the titanium boride shell can prevent crack propagation. The two work together to significantly improve the compressive strength and impact resistance of the absorbing foam, avoiding the problem of easy agglomeration of traditional direct addition of nano-boron nitride.

[0053] In Comparative Example 3, the core-shell toughened composite powder in step S4 is replaced with nano-boron nitride in step S3. The layered structure of boron nitride can disperse external stress, but nano-boron nitride has certain brittleness and extremely high surface energy, which causes it to tend to reduce the surface area and lower the surface energy by agglomeration, resulting in uneven average density at the top, middle and bottom of the absorbing foam.

[0054] Table 1 Homogeneous polymethacrylimide absorbing foam performance test table

[0055]

[0056]

[0057] While the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations can be made to the embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for preparing a homogeneous polymethacrylimide wave-absorbing foam, characterized in that: The steps include: Sodium chloride, colloidal silica and deionized water are added to a reactor, stirred at 35-40° C. and 500-600 r / min for 30-40 minutes, and the pH value is adjusted to 3.5-4 to obtain an aqueous phase mixture; acrylonitrile, methacrylonitrile, isopentane, isooctane, ethylene glycol dimethacrylate, diallyl carbonate, cyclohexane oil and azobisisobutyronitrile are added to the reactor and stirred uniformly to obtain an oil phase mixture; the aqueous phase mixture and the oil phase mixture are added to the reactor in a mass ratio of 10:1 and stirred uniformly, and high-speed dispersed at 1500-1600 r / min for 15-20 minutes. Under nitrogen protection, the mixture is stirred and polymerized at a temperature of 70-80° C., a rotation speed of 80 r / min and a pressure of 0.5-0.7 MPa for 20-22 hours, filtered, washed and vacuum dried to obtain heat-expandable microspheres; Heat-expandable microspheres, 5-hydroxyisophthalic acid, and deionized water are added to a reactor. Sodium hexadecyl sulfate is dissolved in a 70-75 wt% ethanol solution and then added to the reactor. The mixture is stirred at 120-130° C. and 400-500 r / min for 1-2 hours. Ferric chloride is added to the reactor, and the mixture is stirred and reacted for 4-5 hours. The mixture is filtered, washed, and vacuum-dried to obtain dispersed heat-expandable microspheres. Place the sodium chloride in a muffle furnace and dry it at 140-150° C. for 2-3 hours to completely remove the moisture in the salt to obtain pretreated sodium chloride; The pretreated sodium chloride, potassium chloride, nano boron nitride and titanium dihydride are uniformly mixed to obtain a mixed powder; The mixed powder and 25-30wt% anhydrous ethanol solution were added to a reactor in a ratio of 160-180g:500-600mL, stirred at 20-25°C and 500-600r / min for 24-26h, and the anhydrous ethanol was removed by rotary evaporation. The product was vacuum dried and transferred to a muffle furnace. Under argon protection, the temperature was heated to 1400-1440°C at a flow rate of 50-55mL / min and a heating rate of 3-4°C / min. The mixture was kept warm for 2-3h, cooled naturally, washed, and vacuum dried to obtain a core-shell toughened composite powder. Methacrylic acid, core-shell toughened composite powder, methacrylonitrile, azobisisobutyronitrile, dispersed heat-expandable microspheres, magnesium oxide, an absorbent, polyamide, and formamide are added to a reactor, stirred at 35-40°C and 500-600 r / min for 30-40 minutes, the mixed solution is injected into a mold, the mold is immersed in deionized water at 50-60°C for polymerization for 80-85 hours, the product is preheated in an oven at 150-160°C for 2-3 hours, and then foamed at 210-220°C for 2-3 hours to obtain a homogeneous polymethacrylimide absorbing foam. The mass ratio of the methacrylic acid, core-shell toughening composite powder, methacrylonitrile, azobisisobutyronitrile, dispersed heat-expandable microspheres, magnesium oxide, absorbent, polyamide and formamide is 45-50:10-12:53-55:0.3-0.5:7-9:1-2:2-3:2-3:0.5-0.

7.

2. The method for preparing a homogeneous polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The absorbent is any one or more of carbon nanotubes, carbon black or graphene.

3. The method for preparing a homogeneous polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The usage ratio of the pretreated sodium chloride, potassium chloride, nano boron nitride and titanium dihydride is 85-95g: 55-60g: 10-15g: 10-12g.

4. The method for preparing a homogeneous polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The usage ratio of the heat-expandable microspheres, 5-hydroxyisophthalic acid, deionized water, sodium hexadecyl sulfate, ethanol solution and ferric chloride is 70-80g:30-40g:700-800mL:1-2g:120-140mL:15-20g.

5. The method for preparing a homogeneous polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The usage ratio of the sodium chloride, colloidal silicon dioxide and deionized water is 80-90 g:35-40 g:800-900 mL.

6. The method for preparing a homogeneous polymethacrylimide wave-absorbing foam according to claim 1, characterized in that: The mass ratio of acrylonitrile, methacrylonitrile, isopentane, isooctane, ethylene glycol dimethacrylate, diallyl carbonate, cyclohexane oil and azobisisobutyronitrile is 190-200:80-85:25-30:65-70:1-2:1-2:9-10:1.5-2.

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