Polyurethane in-situ filled honeycomb composite material, wave-absorbing material prepared from polyurethane in-situ filled honeycomb composite material, preparation method and application of wave-absorbing material

Abstract

The invention belongs to the technical field of advanced functional materials, and particularly relates to a polyurethane in-situ filled honeycomb composite material, a wave-absorbing material prepared from the same, a preparation method and application. The honeycomb material is filled with the polyurethane foaming liquid in an in-situ foaming mode, the mechanical property of the aramid honeycomb core is enhanced, and meanwhile the structure of an aramid honeycomb is reserved; the prepared composite material has the advantages of light weight, easiness in processing, high compression resistance and the like. Meanwhile, the filled open-cell foam provides favorable conditions for loading a wave-absorbing agent, the wave-absorbing material is prepared by dipping the polyurethane in-situ filled honeycomb composite material into a wave-absorbing agent dispersion liquid, and the wave-absorbing material has the advantages of stable and adjustable wave-absorbing performance, wide wave-absorbing frequency band, high reflection loss to electromagnetic waves and the like; the material can be used as an electromagnetic wave stealth material, an electromagnetic shielding material and the like and is applied to the fields of aerospace, precision equipment and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of advanced materials, and particularly relates to a polyurethane in-situ filled honeycomb composite material, an electromagnetic wave absorbing material made therefrom, a preparation method and uses thereof. Background Art

[0002] With the breakthrough development of electromagnetic wave technology in the fields of wireless communication and radar detection, while it is widely used in military defense and civilian facilities, it also causes problems of high-intensity electromagnetic pollution and external detection interference, seriously threatening the stability of precision electronic systems. Electromagnetic wave absorbing materials, based on the function of electromagnetic wave energy attenuation, show remarkable effectiveness in suppressing the reflection of target radar signals and constructing electromagnetic shielding barriers, and have become key functional materials for achieving electromagnetic compatibility optimization in fields such as aerospace and electronic countermeasures, and are of strategic significance for ensuring the safe operation of highly sensitive equipment. In recent years, researchers have discovered or invented various new types of electromagnetic wave absorbing materials, such as conductive carbon materials like graphene, carbon nanotubes, biomass porous carbon, Mxene, etc., and magnetic materials such as ferrites, metal magnetic powders, carbonyl iron, and nano magnetic materials. However, they all have disadvantages such as complex preparation, poor load-bearing capacity, high cost, and uncontrollable effective absorption band (EAB), which greatly limit the application scope of electromagnetic wave absorbing materials in emerging fields.

[0003] Currently, the method for electromagnetic wave absorption modification of honeycomb materials mainly uses an impregnating solution of impregnating resin and an electromagnetic wave absorber. For example, the patent with publication number CN115296039B uses a white honeycomb impregnated with a reflection impregnating solution (a mixture of metal powder and phenolic resin) and an electromagnetic wave absorption impregnating solution (a mixture of an absorber and phenolic resin) to optimize the electromagnetic wave absorption performance of the electromagnetic wave absorbing honeycomb material in the T direction. However, this impregnation method has disadvantages such as unstable performance, complex process, serious weight gain, and the need for high-temperature curing, which seriously affects the mass production and application of the modified honeycomb electromagnetic wave absorbing material.

[0004] In addition to the method of impregnating white honeycomb, there is also a method of endowing the filler with electromagnetic wave absorption performance to endow the material with electromagnetic wave absorption performance. For example, the patent with publication number CN113211883B uses carbon black, carbon nanotubes or magnetic wave absorbers added during filling and foaming for simultaneous foaming filling to achieve the purpose of endowing the honeycomb material with electromagnetic wave absorption performance. However, the electromagnetic wave absorber will greatly affect the foaming process of polyurethane foam, and adding an excessive amount of electromagnetic wave absorber will also cause synthetic defects such as pore structure collapse, sharp increase in filler dropout rate, and viscoelastic distortion of the precursor, which limits the effective loading of the electromagnetic wave absorber and results in poor adjustability of the electromagnetic wave absorption performance of the product.

[0005] The patent with the publication number CN112126114B also discloses a method combining two impregnation methods with the filling of wave-absorbing agents: First, the white honeycomb is impregnated with a wave-absorbing impregnating solution, and then a wave-absorbing rigid foam is foamed in the honeycomb pores to fill the honeycomb. This method improves the disadvantage of poor environmental resistance of traditional wave-absorbing honeycombs due to the introduction of rigid wave-absorbing foam. However, this method also has disadvantages such as complex preparation process, poor repeatability, and high density, making it difficult to be applied on a large scale.

[0006] To solve the problems existing in current wave-absorbing materials, it is urgent to develop a wave-absorbing material that is lightweight, has strong compressive resistance, simple preparation method, low cost, stable and adjustable wave-absorbing performance. Summary of the Invention

[0007] Aiming at the problems of the prior art, the present invention provides a polyurethane in-situ filled honeycomb composite material, the wave-absorbing material made therefrom, a preparation method and uses thereof. The present invention uses a polyurethane foaming solution to foam and fill the honeycomb material in-situ, enhancing the mechanical properties of the aramid honeycomb core and retaining the structure of the aramid honeycomb itself; at the same time, by impregnating the polyurethane in-situ filled honeycomb composite material with a wave-absorbing dispersion liquid, the obtained wave-absorbing material has the advantages of stable and adjustable wave-absorbing performance, wide wave-absorbing frequency band and strong reflection loss of electromagnetic waves, and can be used as an electromagnetic wave stealth material, an electromagnetic shielding material, etc., and is applied to fields such as aerospace and precision equipment.

[0008] The present invention provides a polyurethane in-situ filled honeycomb composite material, which is made by foaming and filling a honeycomb material in a lightweight open-cell polyurethane pre-foaming mixture; the lightweight open-cell polyurethane pre-foaming mixture is prepared by mixing the following raw materials in parts by weight:

[0009] Polyol 10 - 500 parts,

[0010] Isocyanate 10 - 500 parts,

[0011] Chain extender 0.5 - 50 parts,

[0012] Foaming agent 0.5 - 50 parts,

[0013] Catalyst 0.5 - 50 parts,

[0014] Silicone oil 0.5 - 50 parts.

[0015] Preferably, the lightweight open-cell polyurethane pre-foaming mixture is made from the following raw materials in parts by weight:

[0016] Polyol 100 - 160 parts,

[0017] Isocyanate 160 - 200 parts,

[0018] Chain extender 8 - 9 parts,

[0019] 5 - 8 parts of blowing agent,

[0020] 2.5 - 6.5 parts of catalyst,

[0021] 3 - 5 parts of silicone oil.

[0022] Preferably, the honeycomb material is selected from aramid honeycomb, paper honeycomb or aluminum honeycomb.

[0023] Preferably, the molecular weight of the polyol is 2500 - 4000, the hydroxyl value is 40 - 60, and the viscosity is 400 - 700; the polyol is selected from at least one of polyether polyol, polyester polyol, and aromatic polyol;

[0024] And / or, the isocyanate is one or more of polymethylene polyphenyl polyisocyanate, diphenylmethane diisocyanate, and toluene diisocyanate;

[0025] And / or, the chain extender is at least one of ethylenediamine, 1,4 - butanediol, and ethylene glycol;

[0026] And / or, the blowing agent is pure water;

[0027] And / or, the catalyst is at least one of triethanolamine and dimethylethanolamine;

[0028] And / or, the silicone oil is at least one of emulsified polydimethylsiloxane and amorphous polydimethylsiloxane.

[0029] Preferably, the isocyanate is selected from polymethylene polyphenyl polyisocyanate and diphenylmethane diisocyanate; the mass ratio of polymethylene polyphenyl polyisocyanate to diphenylmethane diisocyanate is (2 - 5):1.

[0030] Preferably, the mass ratio of polymethylene polyphenyl polyisocyanate to diphenylmethane diisocyanate is 3:1.

[0031] The present invention also provides a wave - absorbing material, which is obtained by impregnating the above polyurethane in - situ filled honeycomb composite material into a wave - absorbing agent dispersion liquid and drying.

[0032] Preferably, the wave - absorbing agent dispersion liquid is at least one of carbon nanotube dispersion liquid, graphene sheet dispersion liquid, and carbon black dispersion liquid.

[0033] Preferably, the wave - absorbing agent dispersion liquid comprises the following raw materials in parts by weight:

[0034] 1 - 3 parts of dispersant,

[0035] 100 - 300 parts of ethanol,

[0036] 10 - 30 parts of carbon material;

[0037] The carbon material is selected from at least one of carbon nanotubes, graphene sheets or carbon black;

[0038] The dispersant is selected from at least one of polyvinylpyrrolidone k10, polyvinylpyrrolidone k30, and polyvinylpyrrolidone k90.

[0039] The present invention also provides a method for preparing the above-mentioned wave-absorbing material, which includes the following steps:

[0040] (1) Take the honeycomb material and put it into the above-mentioned lightweight open-cell polyurethane pre-foaming mixture for foaming and filling to obtain a polyurethane in-situ filled honeycomb composite material;

[0041] (2) Immerse the polyurethane in-situ filled honeycomb composite material into the wave-absorbing agent dispersion liquid and dry it to obtain the product.

[0042] Preferably, the concentration of the wave-absorbing agent dispersion liquid is 1-3 g / L;

[0043] And / or, the impregnation time is 30-60 min; the drying temperature is 70-90 °C, and the drying time is 40-50 h.

[0044] The present invention also provides the use of the above-mentioned wave-absorbing material in the preparation of electromagnetic wave stealth materials or electromagnetic shielding materials.

[0045] The present invention prepares a composite material by in-situ foaming and filling of aramid honeycomb with a polyurethane foaming solution. During the preparation process, only the non-destructive filling and impregnation processes of the aramid honeycomb are involved. The preparation process is simple and has high repeatability. At the same time, the composite material has advantages such as light weight and strong compression resistance; in terms of wave-absorbing performance, the wave-absorbing performance of the aramid honeycomb wave-absorbing composite material prepared by the present invention is stable and adjustable, with a wide wave-absorbing frequency band and stronger reflection loss of electromagnetic waves, and has good application prospects in the field of electromagnetic wave stealth.

[0046] Obviously, based on the above content of the present invention, according to the common general knowledge and conventional means in the art, without departing from the above basic technical idea of the present invention, various other forms of modifications, substitutions or changes can be made.

[0047] The following is a further detailed description of the above content of the present invention through specific embodiments in the form of examples. However, this should not be construed as limiting the scope of the above subject matter of the present invention to the following examples. All technologies implemented based on the above content of the present invention belong to the scope of the present invention. Description of the Drawings

[0048] Figure 1 It is the reflection loss result of the composite material obtained in Example 1.

[0049] Figure 2 Reflection loss results of the composite material obtained in Example 2.

[0050] Figure 3 Reflection loss results of the composite material obtained in Example 3.

[0051] Figure 4 Reflection loss results of the composite material obtained in Comparative Example 1.

[0052] Figure 5 SEM images of the polyurethane foam after impregnation during the preparation of Comparative Example 2. a Magnification is 50 times; b Magnification is 100 times; c Magnification is 10,000 times.

[0053] Figure 6 Reflection loss results of the composite material obtained in Comparative Example 2.

[0054] Figure 7 Optical images of the broadband microwave absorbing polyurethane in-situ filled aramid honeycomb composite material with a length of 50 cm × width of 50 cm × height of 5 cm obtained in Example 2. a is the top view of the upper bottom surface; b is the front view of the upper bottom surface; c is the front view of the upper bottom surface; d is the top view of the lower bottom surface; e is the front view of the lower bottom surface; f is the front view of the lower bottom surface.

[0055] Figure 8 SEM images during the preparation of the aramid honeycomb composite material in Example 2: a, b, and c are SEM images of the unfilled and unimpregnated polyurethane foam. d, e, f are SEM images of the unfilled and impregnated microwave absorbing polyurethane foam. g, h, i are SEM images of the microwave absorbing polyurethane in-situ filled aramid honeycomb composite material after filling and impregnation. The magnification of a, d, g, h is 50 times; the magnification of b, e is 100 times; the magnification of c, f, i is 10,000 times.

[0056] Figure 9 Comparison diagram of the stress-strain curves of the white honeycomb and the composite material before impregnation. Detailed implementation manners

[0057] In the following examples and experimental examples, the reagents and raw materials not specifically described are all commercially available products.

[0058] The dispersant polyvinylpyrrolidone used has models k10, k30, and k90 (Shanghai Aladdin Biochemical Technology Co., Ltd.). The microwave absorber is carbon nanotubes (purity: >99%, resistivity: 800 - 1200 ohm, length: 8 - 14 μm, Shenzhen Suiheng Graphene Technology Co., Ltd.). The density of the aramid honeycomb is 32 - 160 kg / m 3, the side length of the hole is 1.83 - 5.50 mm (Jiaxing Yagang Composite Materials Co., Ltd.). Polyols (polyether polyols, polyester polyols, aromatic polyols with a molecular weight of 2500 - 4000, a hydroxyl value of 40 - 60, and a viscosity of 400 - 700), silicone oil (emulsified polydimethylsiloxane, amorphous polydimethylsiloxane), and isocyanates (liquid polymethylene polyphenyl polyisocyanate, diphenylmethane diisocyanate) were purchased from Zhonglan Chenguang Chemical Co., Ltd. Chain extenders (ethylenediamine, 1,4 - butanediol, ethylene glycol), blowing agents (deionized water), and absolute ethanol were purchased from Sichuan Haihong Innovation Technology Co., Ltd., and catalysts (triethanolamine, dimethylethanolamine) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All reagents were of analytical grade and were used without purification.

[0059] When preparing the impregnating solution in the present invention, the dispersant concentration is 5 - 10 g / L. First, an original impregnating solution with a solid content of 10% is prepared for dispersion. The original impregnating solution prepared by this method has good dispersibility and stable solution performance. After dilution, the absorber concentration of the impregnating solution is 1 - 4 g / L. The broadband electromagnetic wave absorbing polyurethane in - situ filled aramid honeycomb composite board impregnated within this range has the ability to absorb electromagnetic waves in a broadband. It should be noted that the absorbing materials impregnated with the impregnating solution prepared by the components and methods of the present invention should all be within the scope of patent authorization protection. As a new impregnating method, the same impregnation mode with a concentration lower / higher than this does not break through the authorization scope of the present invention and should all be within the protection scope of the present invention.

[0060] The present invention tests the electromagnetic wave absorbing performance by using the bow - shaped method to measure the reflectivity.

[0061] Example 1: Polyurethane In - Situ Filled Aramid Honeycomb Electromagnetic Wave Absorbing Composite Material 1

[0062] 1. Preparation of Polyurethane In - Situ Filled Aramid Honeycomb Composite Material

[0063] 1) Mix 3.0 g of catalyst (the catalyst is composed of triethanolamine and dimethylethanolamine in a mass ratio of 1:1), 4.6 g of silicone oil (the silicone oil is composed of emulsified polydimethylsiloxane and amorphous polydimethylsiloxane mixed in a mass ratio of 2:1), 100 g of polyol (polyether polyol with a molecular weight of 3500, a hydroxyl value of 55, and a viscosity of 600), 7.2 g of blowing agent (deionized water), and 8.0 g of chain extender (the chain extender is composed of ethylenediamine and ethylene glycol in a mass ratio of 1:5) and stir evenly to form Component A, and stir 160 g of isocyanate (the isocyanate is composed of polymethylene polyphenyl polyisocyanate and diphenylmethane diisocyanate in a mass ratio of 3:1) evenly to form Component B;

[0064] 2) Add component B to component A and mix evenly to obtain 282.8 g of polyurethane foam pre-foaming liquid. Quickly pour the pre-foaming liquid into a self-made rectangular mold (55×55×15 cm). Subsequently, place an aramid honeycomb panel (64 kg / m 3 , 2.75 mm, 50×50×5 cm) into the mold for in-situ filling of polyurethane foam. Wait for the foam to expand and grow until it overflows the top of the aramid honeycomb and stops growing. Let it stand for 2 h until the foam is completely cured. After the foaming is completed, clean the excess part to obtain the filled aramid honeycomb composite board, that is, the aramid honeycomb composite material filled with polyurethane in-situ; after the polyurethane foam is filled, the density of the aramid honeycomb increases by 35 kg / m 3 .

[0065] 2. Preparation of Aramid Honeycomb Absorbing Composite Material Filled with Polyurethane In-situ

[0066] 1) Dissolve 3.0 g of dispersant (polyvinylpyrrolidone k90) in 300 g of absolute ethanol and stir for 60 min. Subsequently, ultrasonically disperse 30 g of wave-absorbing agent (carbon nanotubes) in the above solution for 30 min to prepare a conductive solution with a solid content of 10% for later use.

[0067] 2) Take 180 g of the above conductive solution and dissolve it in 18 L of absolute ethanol solution to prepare a conductive solution with a concentration of 1 g / L. Place the filled aramid honeycomb composite board into the conductive solution for impregnation. After the liquid level of the conductive solution completely covers the composite board, let it stand for 30 min. Take it out and dry it in an oven at 80 °C for 48 h to obtain the aramid honeycomb absorbing composite material filled with polyurethane in-situ.

[0068] 3. Wave Absorbing Performance of Aramid Honeycomb Absorbing Composite Material Filled with Polyurethane In-situ

[0069] The above-obtained aramid honeycomb absorbing composite material filled with polyurethane in-situ has excellent broadband wave absorbing performance (EAB = 12.6 GHz) within 2 - 18 GHz. The minimum reflection loss appears at a frequency of 18.0 GHz and is -24.5 dB( Figure 1 ).

[0070] Example 2. Aramid Honeycomb Absorbing Composite Material Filled with Polyurethane In-situ 2

[0071] 1. Preparation of Aramid Honeycomb Composite Material Filled with Polyurethane In-situ

[0072] 1) Mix 3.0 g of catalyst (mass ratio of triethanolamine to dimethylethanolamine is 1:1), 4.6 g of silicone oil (silicone oil is a mixture of emulsified polydimethylsiloxane and amorphous polydimethylsiloxane in a mass ratio of 2:1), 100 g of polyol (polyether polyol with a molecular weight of 3500, hydroxyl value of 55, and viscosity of 600), 7.2 g of foaming agent (deionized water), and 8.0 g of chain extender (ethylenediamine and ethylene glycol in a mass ratio of 1:5) thoroughly by stirring to form Component A, and stir 160 g of isocyanate (mass ratio of polymethylene polyphenyl polyisocyanate to diphenylmethane diisocyanate is 3:1) evenly to form Component B;

[0073] 2) Add Component B to Component A and mix them evenly to obtain 282.8 g of polyurethane foam pre-foaming liquid. Quickly pour the pre-foaming liquid into a self-made rectangular mold (55×55×15 cm). Subsequently, place an aramid honeycomb panel (64 kg / m 3 , 2.75 mm, 50×50×5 cm) into the mold for in-situ filling of polyurethane foam. Wait until the foam grows and overflows the top of the aramid honeycomb and then stops growing. Let it stand for 2 h until the foam cures completely. After foaming, clean the excess part to obtain the filled aramid honeycomb composite panel, namely the polyurethane in-situ filled aramid honeycomb composite material; after the polyurethane foam is filled, the density of the aramid honeycomb increases by 34 kg / m 3 .

[0074] 2. Preparation of Polyurethane In-Situ Filled Aramid Honeycomb Absorbing Composite Material

[0075] 1) Dissolve 3.0 g of dispersant (polyvinylpyrrolidone k90) in 300 g of absolute ethanol and stir for 60 min. Subsequently, ultrasonically disperse 30 g of wave-absorbing agent (carbon nanotubes) in the above solution for 30 min to prepare a conductive solution with a solid content of 10% for later use.

[0076] 2) Take 360 g of the above conductive solution and dissolve it in 18 L of absolute ethanol solution to prepare a conductive solution with a concentration of 2 g / L. Immerse the filled aramid honeycomb composite panel in the conductive solution. After the liquid level of the conductive solution completely covers the composite panel, let it stand for 30 min. Take it out and dry it in an oven at 80 °C for 48 h to obtain the polyurethane in-situ filled aramid honeycomb absorbing composite material ( Figure 7 ).

[0077] 3. Wave Absorbing Performance of Polyurethane In-Situ Filled Aramid Honeycomb Absorbing Composite Material

[0078] The obtained polyurethane in-situ filled aramid honeycomb absorbing composite material has excellent broadband wave absorbing performance (EAB = 10.3 GHz) within 2 - 18 GHz, and the minimum reflection loss appears at a frequency of 16.5 GHz, which is -32.3 dB ( Figure 2 ).

[0079] Example 3: Polyurethane In-situ Filled Aramid Honeycomb Absorbing Composite Material 3

[0080] 1. Preparation of Polyurethane In-situ Filled Aramid Honeycomb Composite Material

[0081] 1) Mix 3.0 g of catalyst (mass ratio of triethanolamine to dimethylethanolamine is 1:1), 4.6 g of silicone oil (emulsified polydimethylsiloxane and amorphous polydimethylsiloxane are mixed by mass ratio of 2:1), 100 g of polyol (polyether polyol with molecular weight of 3500, hydroxyl value of 55, and viscosity of 600), 7.2 g of foaming agent (deionized water), and 8.0 g of chain extender (mass ratio of ethylenediamine to ethylene glycol is 1:5) and stir evenly to form Component A, and stir 160 g of isocyanate (mass ratio of polymethylene polyphenyl polyisocyanate to diphenylmethane diisocyanate is 3:1) evenly to form Component B;

[0082] 2) Add Component B to Component A and mix evenly to obtain 282.8 g of polyurethane foam pre-foaming liquid. Quickly pour the pre-foaming liquid into a self-made rectangular mold (55×55×15 cm), and then place the aramid honeycomb board (64 kg / m 3 , 2.75 mm, 50×50×5 cm) into the mold for in-situ filling of polyurethane foam. Wait for the foam to expand and grow until it overflows the top of the aramid honeycomb and then stops growing. Place it for 2 h to wait for the foam to cure completely. After the foaming is completed, clean the excess part to obtain the filled aramid honeycomb composite board, that is, the polyurethane in-situ filled aramid honeycomb composite material; after the polyurethane foam is filled, the density of the aramid honeycomb increases by 35 kg / m 3 .

[0083] 2. Preparation of Polyurethane In-situ Filled Aramid Honeycomb Absorbing Composite Material

[0084] 1) Dissolve 3.0 g of dispersant (polyvinylpyrrolidone k90) in 300 g of absolute ethanol and stir for 60 min. Then, ultrasonically disperse 30 g of absorbing agent (carbon nanotubes) in the above solution for 30 min to prepare a conductive solution with a solid content of 10% for later use.

[0085] 2) Take 540 g of the above conductive solution and dissolve it in 18 L of absolute ethanol solution to prepare a conductive solution with a concentration of 3 g / L. Place the filled aramid honeycomb composite board into the conductive solution for impregnation. After the liquid level of the conductive solution completely covers the composite board, let it stand for 30 min. Take it out and dry it in an oven at 80 °C for 48 h to obtain the polyurethane in-situ filled aramid honeycomb absorbing composite material.

[0086] 3. Absorbing Performance of Polyurethane In-situ Filled Aramid Honeycomb Absorbing Composite Material

[0087] The obtained polyurethane in-situ filled aramid honeycomb microwave absorbing composite material has excellent broadband microwave absorbing properties in the range of 2 - 18 GHz (EAB = 15.8 GHz), and the minimum reflection loss appears at a frequency of 2.6 GHz, which is -28.6 dB( Figure 3 ).

[0088] The following is the preparation method of the control sample.

[0089] Comparative Example 1, Polyurethane in-situ filled aramid honeycomb microwave absorbing composite materials with different densities

[0090] 1. Preparation of polyurethane in-situ filled aramid honeycomb composite material

[0091] 1) Mix 3.0 g of catalyst (the mass ratio of triethanolamine to dimethylethanolamine is 1:1), 4.6 g of silicone oil (emulsified polydimethylsiloxane and amorphous polydimethylsiloxane are mixed by mass ratio of 2:1), 100 g of polyol (polyether polyol with a molecular weight of 3500, hydroxyl value of 55, and viscosity of 600), 7.2 g of foaming agent, and 8.0 g of chain extender (the mass ratio of ethylenediamine to ethylene glycol is 1:5) and stir evenly to form Component A. Stir 160 g of isocyanate (polymethylene polyphenyl polyisocyanate) evenly to form Component B;

[0092] 2) Add Component B to Component A and mix evenly to obtain 282.8 g of polyurethane foam pre-foaming liquid. Quickly pour the pre-foaming liquid into a self-made rectangular mold (55×55×15 cm), and then place the aramid honeycomb board (64 kg / m 3 , 2.75 mm, 50×50×5 cm) into the mold for in-situ filling of polyurethane foam. Wait for the foam to expand and grow until it overflows the top of the aramid honeycomb and stops growing. Let it stand for 2 h until the foam cures completely. After foaming, clean up the excess part to obtain the filled aramid honeycomb composite board, that is, the polyurethane in-situ filled aramid honeycomb composite material; after filling with polyurethane foam, the density of the aramid honeycomb increases by 45 kg / m 3 .

[0093] 2. Preparation of polyurethane in-situ filled aramid honeycomb microwave absorbing composite material

[0094] 1) Dissolve 3.0 g of dispersant (polyvinylpyrrolidone k90) in 300 g of absolute ethanol and stir for 60 min. Then, ultrasonically disperse 30 g of microwave absorber (carbon nanotubes) in the above solution for 30 min to prepare a conductive solution with a solid content of 10% for later use.

[0095] 2) Dissolve 540 g of the above conductive solution in 18 L of anhydrous ethanol solution to prepare a conductive solution with a concentration of 3 g / L. Immerse the filled aramid honeycomb composite board in the conductive solution. After the liquid level of the conductive solution completely covers the composite board, let it stand for 30 min. Take it out and dry it in an oven at 80 °C for 48 h to obtain a polyurethane in-situ filled aramid honeycomb microwave absorption composite material.

[0096] 3. Microwave Absorbing Properties of Polyurethane In-situ Filled Aramid Honeycomb Microwave Absorption Composite Material

[0097] The polyurethane in-situ filled aramid honeycomb microwave absorption composite material obtained in Comparative Example 1 has excellent broadband microwave absorption properties (EAB = 10.1 GHz) within 2 - 18 GHz, and the minimum reflection loss appears at a frequency of 16.4 GHz, which is -34.0 dB( Figure 4 ).

[0098] Comparative Example 2, Lightweight Polyurethane In-situ Filled Aramid Honeycomb Microwave Absorption Composite Material

[0099] 1. Preparation of Lightweight Polyurethane In-situ Filled Aramid Honeycomb Composite Material

[0100] According to the preparation method of Example 2, the weight of the polyurethane formulation remains unchanged. The difference is that the ratio of the two isocyanates is changed (polymethylene polyphenyl polyisocyanate: diphenylmethane diisocyanate = 1:1) to prepare a lighter polyurethane foam filled aramid honeycomb composite material; after filling with polyurethane foam, the density of the aramid honeycomb increases by 25 kg / m 3 .

[0101] 2. Preparation of Polyurethane In-situ Filled Aramid Honeycomb Microwave Absorption Composite Material

[0102] 1) Dissolve 3.0 g of a dispersant (polyvinylpyrrolidone k90) in 300 g of anhydrous ethanol and stir for 60 min. Then, ultrasonically disperse 30 g of a microwave absorber (carbon nanotubes) in the above solution for 30 min to prepare a conductive solution with a solid content of 10% for later use.

[0103] 2) Take 540 of the above conductive solution and dissolve it in 18 L of anhydrous ethanol solution to prepare a conductive solution with a concentration of 3 g / L. Immerse the filled aramid honeycomb composite board in the conductive solution. After the liquid level of the conductive solution completely covers the composite board, let it stand for 30 min. Take it out and dry it in an oven at 80 °C for 48 h to obtain a polyurethane in-situ filled aramid honeycomb microwave absorption composite material.

[0104] 3. Microwave Absorbing Properties of Polyurethane In-situ Filled Aramid Honeycomb Microwave Absorption Composite Material

[0105] The lightweight polyurethane in-situ filled aramid honeycomb microwave absorbing composite prepared in Comparative Example 2 has poor microwave absorbing performance: broadband microwave absorbing performance (EAB = 2.7 GHz), and the minimum reflection loss appears at a frequency of 18.0 GHz, which is -12.3 dB( Figure 6 ).

[0106] Table 1

[0107]

[0108]

[0109] The microwave absorbing performances of the microwave absorbing composites prepared in Examples 1-3 and Comparative Examples 1-2 are shown in Table 1. It can be seen that for polyurethanes with the same formulation, the density of the aramid honeycomb is relatively stable after foaming and filling (Examples 1-3). The obtained aramid honeycomb composite is lightweight, and at the same time, its microwave absorbing performance is stable and adjustable, with a wide microwave absorbing frequency band and stronger reflection loss of electromagnetic waves. By changing the formulation of the isocyanate (Comparative Example 1), the density of the aramid honeycomb increases significantly, making the aramid honeycomb composite not have the advantage of being lightweight. By changing the isocyanate ratio (Comparative Example 2), the density of the aramid honeycomb can only increase by 25 kg / m 3 , but the microwave absorbing performance of the composite becomes worse, with a narrow microwave absorbing frequency band and weak reflection loss of electromagnetic waves. The reason is that although increasing the proportion of diphenylmethane diisocyanate can reduce the overall foam density, the overall closed-cell rate will also increase, and more wall films hinder the impregnating solution from impregnating into the interior of the foam material, thereby reducing the microwave absorbing performance of the material( Figure 5 ).

[0110] The above data show that through the optimization of the polyurethane foam formulation in the present invention, after it is filled into the aramid honeycomb material, it can meet the requirements of both low density (lightweight) and excellent microwave absorbing performance (larger absorption bandwidth). In addition, the present invention can achieve the microwave absorbing effect in a specific frequency band by adjusting the concentration of the conductive solution according to the demand for the microwave absorbing frequency band.

[0111] The beneficial effects of the present invention are demonstrated by the following experimental examples.

[0112] Experimental Example 1: Microscopic morphology of the polyurethane in-situ filled aramid honeycomb microwave absorbing composite

[0113] Unfilled and unimpregnated polyurethane foam: Prepared according to the preparation method of Example 2, except that the obtained polyurethane foam is not filled and impregnated.

[0114] Unfilled and impregnated polyurethane foam: Prepared according to the preparation method of Example 2, except that the obtained polyurethane foam is not filled but impregnated with the impregnating solution.

[0115] In-situ filled aramid honeycomb absorbing composite material of polyurethane after filling and impregnation: Prepared according to the preparation method of Example 1.

[0116] I. Experimental method

[0117] The micro-morphologies of polyurethane in free foaming, after impregnation, and after filling and impregnation were observed by scanning electron microscope ( Figure 8 ).

[0118] II. Experimental results

[0119] Figure 8 a, b, and c are SEM images of unfilled and unimpregnated polyurethane foam. It can be seen from Figure 8 a and b that the polyurethane foam has an obvious cell structure with clear skeletons and cracks on the cell walls, which makes it easy for the impregnating liquid to enter the interior of the foam. Figure 8 It can be seen from c that there are some wrinkles on the surface of the polyurethane cell walls and no other substances remain. It can be seen from Figure 8 d and e that the structure of the polyurethane foam changes little after impregnation, and some of the skeletons are slightly broken, which may be caused by movement or impregnation. Figure 8 It can be seen from f that the absorbent is evenly and densely dispersed on the surface of the polyurethane. This distribution of the absorbent can form a conductive path inside and endow the polyurethane foam with absorbing properties. Figure 8 g, h, and i are scanning images of in-situ foamed and filled aramid honeycomb impregnated with polyurethane. It can be clearly seen from Figure 8 g and h that the hexagonal shape of the aramid honeycomb pore diameter is wrapped by the polyurethane foam, which also illustrates the in-situ growth process of the polyurethane and the shape is Figure 8 similar to d and e, indicating good stability after filling and impregnation. Figure 8 i shows that the absorbent adheres to the surface of the filled polyurethane, and the distribution is more aggregated compared with Figure 8 f, which provides good absorbing properties for the aramid honeycomb composite material.

[0120] Experimental Example 2: Compressive resistance of in-situ filled aramid honeycomb composite material of polyurethane

[0121] In-situ filled aramid honeycomb composite material of polyurethane after filling and without impregnation: Prepared according to the preparation method of Example 2, except that the obtained polyurethane foam filled with aramid honeycomb was not impregnated.

[0122] I. Experimental method

[0123] The compressive strength of the samples was measured using a universal testing machine (UTM 4204, SUNS, China) at a loading speed of 2.0 mm / min.

[0124] II. Experimental method

[0125] Figure 9 Through compression loading tests, the compression properties of white honeycomb (aramid honeycomb core) and polyurethane-filled aramid honeycomb core were evaluated. Honeycomb samples with dimensions of approximately 5 cm × 5 cm × 2 cm were used in the experiments, and the relationship curve between stress (unit: MPa) and strain was recorded when the strain reached 16%. The results showed that in the low-strain (2 - 4%) stage, the white honeycomb exhibited a peak stress of 5 MPa while the polyurethane-filled aramid honeycomb core exhibited a peak stress of approximately 7 MPa. For the unfilled honeycomb core, the stress rapidly decreased to 4 - 5 MPa after the peak, while the stress of the filled honeycomb core decreased more gently and remained above 5 MPa. This difference is due to the fact that the polyurethane filling enhanced the structural stability and effectively inhibited crack propagation, enabling the filled honeycomb core to exhibit better energy absorption capacity. The experimental results indicate that the polyurethane-filled aramid honeycomb core is significantly superior to the unfilled honeycomb core in terms of compressive performance and is suitable for applications in scenarios with higher durability requirements.

[0126] In summary, the present invention uses polyurethane foaming liquid to in-situ foam-fill aramid honeycomb, and the prepared composite material has advantages such as light weight, easy processing, and strong compression resistance. In terms of wave absorption performance, the wave absorption performance of the polyurethane in-situ filled aramid honeycomb wave absorption composite material is stable and adjustable, has a wide wave absorption frequency band, and has stronger reflection loss to electromagnetic waves, showing good application prospects in the field of electromagnetic wave stealth.

Claims

1. A polyurethane in-situ filled honeycomb composite material, characterized in that, It is made by putting honeycomb material into a light open-cell polyurethane pre-foaming mixture for foaming and filling; the light open-cell polyurethane pre-foaming mixture is made by mixing raw materials in the following weight parts: Polyol 10 - 500 parts, Isocyanate 10 - 500 parts, Chain extender 0.5 - 50 parts, Foaming agent 0.5 - 50 parts, Catalyst 0.5 - 50 parts, Silicone oil 0.5 - 50 parts.

2. The polyurethane in-situ filled honeycomb composite material according to claim 1, characterized in that The light open-cell polyurethane pre-foaming mixture is made by mixing raw materials in the following weight parts: Polyol 100 - 160 parts, Isocyanate 160 - 200 parts, Chain extender 8 - 9 parts, Foaming agent 5 - 8 parts, Catalyst 2.5 - 6.5 parts, Silicone oil 3 - 5 parts.

3. The polyurethane in-situ filled honeycomb composite material according to claim 1, wherein The honeycomb material is selected from aramid honeycomb, paper honeycomb or aluminum honeycomb.

4. The polyurethane in-situ filled honeycomb composite material according to claim 1 or 2, characterized in that, The polyol has a molecular weight of 2500 - 4000, a hydroxyl value of 40 - 60, and a viscosity of 400 - 700; the polyol is selected from at least one of polyether polyol, polyester polyol, and aromatic polyol; and / or, the isocyanate is one or more of polymethylene polyphenyl polyisocyanate, diphenylmethane diisocyanate, and toluene diisocyanate; and / or, the chain extender is at least one of ethylenediamine, 1,4-butanediol, and ethylene glycol; and / or, the foaming agent is pure water; and / or, the catalyst is at least one of triethanolamine and dimethylethanolamine; and / or, the silicone oil is at least one of emulsified polydimethylsiloxane and amorphous polydimethylsiloxane.

5. The polyurethane in-situ filled honeycomb composite material according to claim 4, wherein The isocyanate is selected from polymethylene polyphenyl polyisocyanate and diphenylmethane diisocyanate; the mass ratio of polymethylene polyphenyl polyisocyanate to diphenylmethane diisocyanate is (2 - 5):

1.

6. An electromagnetic wave absorbing material, characterized in that, It is obtained by impregnating the polyurethane in-situ filled honeycomb composite material according to any one of claims 1 - 5 into an absorbent dispersion liquid and drying.

7. The wave-absorbing material according to claim 6, characterized in that, The absorbent dispersion liquid is at least one of a carbon nanotube dispersion liquid, a graphene sheet dispersion liquid, and a carbon black dispersion liquid.

8. The wave-absorbing material according to claim 7, characterized in that, The absorbent dispersion liquid comprises raw materials in the following weight parts: Dispersant 1 - 3 parts, Ethanol 100 - 300 parts, Carbon material 10 - 30 parts; The carbon material is selected from at least one of carbon nanotubes, graphene sheets, and carbon black; The dispersant is selected from at least one of polyvinylpyrrolidone k10, polyvinylpyrrolidone k30, and polyvinylpyrrolidone k90.

9. The preparation method of the wave-absorbing material according to any one of claims 6-8, characterized in that, Comprises the following steps: (1) Take the honeycomb material and put it into the light open-cell polyurethane pre-foaming mixture according to claim 1 or 2 for foaming and filling to obtain a polyurethane in-situ filled honeycomb composite material; (2) Impregnate the polyurethane in-situ filled honeycomb composite material into the absorbent dispersion liquid and dry to obtain it.

10. Use of the absorbent material according to any one of claims 6 - 8 in preparing an electromagnetic wave stealth material or an electromagnetic shielding material.

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