An aramid honeycomb microwave absorbing structure and its interface control method

Abstract

This invention relates to the field of aramid honeycomb structure materials, and more particularly to an aramid honeycomb microwave absorbing structure and its interface control method, comprising: step 1, pretreatment of the molded aramid honeycomb substrate; step 2, composite surface wetting and micro-interface homogenization; step 3, chemical grafting of a reactive interface bridging layer; step 4, construction of a low-modulus stress buffer transition layer; step 5, coating of a high-filling microwave absorbing functional layer; and step 6, interfacial cross-linking and gradient curing. This invention sequentially constructs a composite surface homogenization layer, a reactive interface bridging layer, and a low-modulus stress buffer transition layer between the molded aramid honeycomb and the subsequent microwave absorbing functional layer. Through gradient co-curing of the subsequent microwave absorbing layer and the transition layer, a stable cross-layer bonding structure is formed between the composite surface of the honeycomb cell wall and the high-filling microwave absorbing layer, thereby improving the interfacial bonding strength and interfacial durability under vibration conditions of the microwave absorbing structure.

Description

Technical Field

[0001] This invention relates to the field of aramid honeycomb structure material technology, and in particular to an aramid honeycomb microwave absorbing structure and its interface control method. Background Technology

[0002] Existing aramid honeycomb structures are typically made from aramid paper impregnated with phenolic resin or a phenolic resin / epoxy resin mixture, followed by stretching, shaping, and curing. The resulting honeycomb cell wall surface is not a single aramid surface, but a composite surface composed of aramid fibers and cured resin. This composite surface exhibits inhomogeneities in surface composition, microstructure, and local stiffness in straight areas, edges, and nodal areas of the cell walls. During subsequent impregnation or coating with microwave-absorbing slurries, problems such as insufficient wetting consistency, unstable local interfacial bonding, and stress concentration at the interface after curing can easily occur. Consequently, under service environments such as vibration and humid heat, this can lead to cracking, debonding, or fatigue peeling of the microwave-absorbing layer, affecting the functional retention and service reliability of the structural components.

[0003] To address the aforementioned issues, this application provides an aramid honeycomb absorbing structure and its interface control method. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing an aramid honeycomb microwave absorbing structure and its interface control method. Between the molded aramid honeycomb and the subsequent microwave absorbing functional layer, a composite surface homogenization layer, a reactive interface bridging layer, and a low-modulus stress buffer transition layer are sequentially constructed. Through gradient co-curing of the subsequent microwave absorbing layer and the transition layer, a stable cross-layer bonding structure is formed between the composite surface of the honeycomb wall and the highly filled microwave absorbing layer, thereby improving the interface bonding strength and interface durability of the microwave absorbing structure under vibration conditions.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for interface control of an aramid honeycomb microwave absorbing structure includes the following steps:

[0007] Step 1: Pretreatment of the aramid honeycomb substrate:

[0008] Using aramid honeycomb molded by impregnation and curing with phenolic resin or phenolic / epoxy resin as a substrate, it is immersed in anhydrous ethanol or acetone solvent and ultrasonically cleaned for 5 to 15 minutes to remove residual mold release agent and free oligomers on the surface, and then dried at 60 to 80°C; at this time, the pore walls of the aramid honeycomb are a low surface energy heterogeneous composite surface in which aramid fibers and highly cross-linked cured resin coexist.

[0009] Step 2, Composite Surface Wetting and Micro-interface Homogenization:

[0010] Immerse the aramid honeycomb treated in step 1 in a low-solids interface homogenizing solution for 1–3 minutes; after removing it, blow away excess liquid droplets in the honeycomb pores with compressed air at 0.1–0.2 MPa to prevent pore blockage; then bake at 80–100℃ for 10–20 minutes to form an interface homogenizing layer on the honeycomb pore wall.

[0011] Step 3: Chemical grafting of reactive interfacial bridging layers:

[0012] The aramid honeycomb treated in step 2 is immersed in the hydrolysate of the reactive silane coupling agent for 5 to 15 minutes; after removal, it is blown through with compressed air at 0.2 to 0.3 MPa and placed at 110 to 130°C for 20 to 30 minutes to carry out a condensation reaction, forming an interfacial bridging layer on the surface of the interface homogenization layer; the coupling agent is anchored to the surface of the homogenization layer through covalent bonds or hydrogen bonds.

[0013] Step 4: Construction of a low-modulus stress buffer transition layer:

[0014] The aramid honeycomb treated in step 3 is immersed in a flexible polymer precursor solution for 1 to 3 minutes; after extraction, it is blown through the holes with an airflow of 0.3 to 0.5 MPa so that the flexible polymer precursor solution only adheres to the surface of the honeycomb pore walls to form a thin layer; then, it is semi-cured at 80 to 90°C for 30 to 60 minutes to form a low-modulus transition layer.

[0015] Step 5: Coating of a high-filling microwave absorbing functional layer:

[0016] The semi-cured aramid honeycomb obtained in step 4 is immersed in the microwave absorbing slurry for 3 to 5 minutes; after removal, the channels are purged with a pulsed high-pressure airflow of 0.4 to 0.6 MPa to remove excess slurry, leaving only the microwave absorbing functional layer attached to the outside of the low modulus transition layer.

[0017] Step 6: Interfacial cross-linking and gradient curing:

[0018] The aramid honeycomb structure obtained in step 5 is placed in a programmed temperature rise oven for gradient curing, so that a co-cured bonding interface is formed between the interface homogenization layer, the interface bridging layer, the low modulus transition layer and the microwave absorbing functional layer.

[0019] Preferably, in step 2, the low-solids interface homogenizing liquid is composed of the following components by mass percentage: 0.5% to 2.0% aqueous polyurethane dispersion or aqueous epoxy resin dispersion, 0.05% to 0.2% nonionic surfactant, and the balance being a mixed solvent of deionized water and ethanol; the volume ratio of deionized water to ethanol is 1:1 to 3:1; the nonionic surfactant is one or both of polyether-modified siloxane or octylphenol polyoxyethylene ether.

[0020] By adopting the above technical solution: the surface tension of the liquid is greatly reduced (<30mN / m) by the use of surfactant, which enables it to completely wet the hydrophobic phenolic resin region, while the trace amount of resin dispersion fills the micro-energy difference of the composite surface and provides a consistent primary interface rich in polar groups (such as -OH, -NH-).

[0021] Preferably, in step 3, the mass concentration of the silane coupling agent in the reactive silane coupling agent hydrolysate is 1.0% to 3.0%, and the solvent is an ethanol / water mixture with the pH adjusted to 4.5 to 5.5 by glacial acetic acid, and the volume ratio of ethanol to water is 90:10.

[0022] Preferably, the silane coupling agent is γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, or a combination of the two.

[0023] Preferably, in step 4, the flexible polymer precursor liquid is a two-component flexible polyurethane prepolymer system or a hydroxyl-terminated polybutadiene toughened modified epoxy resin system, diluted with ethyl acetate or methyl ethyl ketone to a solid content of 10% to 20%, and its viscosity is 10 to 30 mPa·s.

[0024] Preferably, in step 4, the thickness of the low-modulus transition layer is 1–3 μm, and its Shore hardness after complete curing is 60–85.

[0025] By adopting the above technical solution, the Shore hardness of the transition layer after complete curing is controlled at 60-85, and its elastic modulus is significantly lower than that of the rigid phenolic resin in step (1) and the microwave absorbing coating in the subsequent step (5), thus acting as a "mechanical damper" in high-frequency vibration environment.

[0026] Preferably, in step 5, the absorbing slurry is composed of a polymer resin matrix, a radar wave absorber uniformly dispersed therein, and a polar mixed solvent, and the impregnation viscosity of the absorbing slurry is 200-500 mPa·s; the polymer resin matrix is ​​a two-component epoxy resin system or a rigid polyurethane resin system.

[0027] The radar wave absorber is a magnetic loss absorber, a dielectric loss absorber, or a mixture of both; the magnetic loss absorber is selected from one or more of carbonyl iron powder, iron-silicon-aluminum alloy powder, iron-cobalt alloy powder, iron-nickel alloy powder, amorphous alloy micro powder, spinel ferrite, or magnetoplumbium ferrite; the dielectric loss absorber is selected from one or more of carbon nanotubes, graphene, conductive carbon black, silicon carbide micro powder, chopped carbon fibers, or MXene two-dimensional materials.

[0028] When the radar wave absorber is mainly composed of magnetically depleting metal micropowder, its mass fraction in the dried and cured radar-absorbing functional layer is 60% to 80%.

[0029] When the radar wave absorber is mainly a dielectric loss type carbon-based powder, its mass fraction in the dried and cured radar absorption functional layer is 3% to 15%.

[0030] Preferably, the gradient curing procedure in step 6 is as follows: first, maintain the temperature at 60-80℃ for 1-2 hours; then, raise the temperature to 110-120℃ and maintain for 2 hours; finally, raise the temperature to 140-150℃ and maintain for 1-2 hours.

[0031] By adopting the above technical solution, a three-dimensional cross-linked interpenetrating network consisting of an interface homogenization layer, a coupling bridging layer, and a flexible buffer layer is finally constructed between the lightweight aramid honeycomb substrate and the heavy microwave absorbing metal layer, thus completely solving the interface delamination problem.

[0032] The present invention also provides an aramid honeycomb microwave absorbing structure, which is prepared by the above-mentioned interface control method; the honeycomb pore wall includes an interface control layer, a low modulus transition layer and a microwave absorbing functional layer from the inside to the outside, wherein the interface control layer is composed of an interface homogenization layer and an interface bridging layer, and the layers are bonded to each other by co-curing with a resin matrix to form an interlayer bonding interface.

[0033] Preferably, the thickness of the interface homogenization layer is less than 0.5 μm, the absorber particles are mainly distributed in the microwave absorption functional layer, and are connected to the honeycomb pore wall through a low modulus transition layer.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. This invention focuses on interface control of the pore wall surface of the heterogeneous aramid / cured resin composite formed after impregnation and curing, which is more consistent with the actual interface state of the formed aramid honeycomb and overcomes the limitation of the prior art that only modifies the surface of bare aramid.

[0036] 2. This invention improves the wetting consistency and interlayer bonding stability of the composite pore wall surface by sequentially constructing an interface homogenization layer, a reactive interface bridging layer and a low-modulus transition layer between the honeycomb pore wall and the microwave absorbing functional layer, and alleviates the abrupt change in stiffness between the high-filling microwave absorbing functional layer and the honeycomb matrix.

[0037] 3. The present invention, combined with the gradient co-curing process, helps to reduce the interface stress concentration under vibration environment, improve the peel resistance and interface durability of the microwave absorbing functional layer, and facilitates the connection with the existing aramid honeycomb microwave absorbing structure manufacturing process. Attached Figure Description

[0038] Figure 1This is a process flow diagram of the present invention;

[0039] Figure 2 This is a schematic diagram of the aramid honeycomb absorbing structure of the present invention. Detailed Implementation

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings, so that those skilled in the art can better understand the advantages and features of the present invention, thereby making a clearer definition of the scope of protection of the present invention. The embodiments described in this invention are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0041] Example 1 (High-filling magnetic loss heavy metal system)

[0042] This embodiment provides an interface control method for an aramid honeycomb microwave absorbing structure, the specific steps of which are as follows:

[0043] Step 1: Take a cured phenolic resin aramid honeycomb with a pore size of 3.2 mm and a density of 48 kg / m³, ultrasonically clean it in anhydrous ethanol for 10 min, and dry it at 70℃ for later use.

[0044] Step 2: Preparation of homogenization solution: 1.0 wt% aqueous polyurethane dispersion + 0.1 wt% polyether-modified siloxane + 98.9 wt% ethanol / water (volume ratio 2:1). Immerse the honeycomb in the solution for 2 min, remove it, purge with 0.15 MPa compressed air, and bake at 90℃ for 15 min.

[0045] Step 3: Preparation of bridging solution: Dissolve 2.0 wt% KH550 (γ-aminopropyltriethoxysilane) in an aqueous ethanol solution at pH 5.0. Immerse the honeycomb structure for 10 min, remove it, purge with 0.25 MPa, and perform a condensation reaction at 120℃ for 25 min.

[0046] Step 4: Preparation of the transition solution: Dilute the two-component flexible polyurethane prepolymer with ethyl acetate to a solid content of 15% (viscosity 20 mPa·s). After impregnation for 2 min, blow holes at 0.4 MPa, and then semi-cur at 85℃ for 45 min. The theoretical thickness of this layer is approximately 2 μm. After complete curing, the Shore A hardness of the same material film is tested to be 75.

[0047] Step 5: Preparation of microwave absorbing slurry: Using a two-component epoxy resin (E-51 and polyamide) as the matrix, add 70% by mass of flake carbonyl iron powder and adjust the viscosity to 350 mPa·s. Impregnate the semi-cured honeycomb for 4 min, then remove and purge with a 0.5 MPa pulsed high-pressure airflow.

[0048] Step 6, Gradient co-curing: Place in an oven and sequentially maintain at 70℃ for 1.5 h, 120℃ for 2 h, and 150℃ for 1.5 h. The final aramid honeycomb microwave absorbing structure is obtained, denoted as S-1.

[0049] Example 2 (Lightweight carbon-based system with low dielectric loss)

[0050] The method in this embodiment is basically the same as that in Embodiment 1, except that:

[0051] In step 3, KH560 (containing epoxy silane) is used instead of KH550;

[0052] In step 4, a hydroxyl-terminated polybutadiene (HTPB) toughened modified epoxy resin system is used as a flexible precursor liquid (the fully cured Shore A hardness is 82).

[0053] In step 5, the absorber in the microwave absorbing slurry is replaced with 8% by mass of multi-walled carbon nanotubes (MWCNTs), and the resin matrix is ​​polyurethane resin. The resulting aramid honeycomb microwave absorbing structure is denoted as S-2.

[0054] Comparative Example 1 (excluding step 2: no micro-interface homogenization layer)

[0055] The process is essentially the same as in Example 1, except step 2 is omitted. After cleaning, the aramid honeycomb directly proceeds to step 3 for silane coupling agent treatment. The remaining steps and parameters are completely identical. This is denoted as C-1.

[0056] Comparative Example 2 (excluding step 3: non-reactive chemical bridging layer)

[0057] The process is essentially the same as in Example 1, except step 3 is omitted. After homogenization in step 2, the aramid honeycomb directly proceeds to step 4, where a low-modulus transition layer is coated. The remaining steps and parameters are completely identical. This is denoted as C-2.

[0058] Comparative Example 3 (excluding step 4: no low-modulus stress buffer layer, abrupt change in stiffness)

[0059] The process is essentially the same as in Example 1, except step 4 is omitted. After the bridging treatment in step 3, the aramid honeycomb directly proceeds to step 5, where a highly filled carbonyl iron absorbing slurry is applied, followed by step 6, which is then cured. This comparative example represents the conventional practice in the prior art of "directly coating a hard, heavy-duty absorbing coating onto a hard honeycomb surface." It is denoted as C-3.

[0060] Comparative Example 4 (Non-gradient co-curing: disruption of interlayer bonding mechanism)

[0061] The process is basically the same as in Example 1, but the curing logic is changed: after step 4, the honeycomb layer is baked at 150°C for 2 hours to fully cure the low-modulus transition layer, and then step 5 is performed to impregnate the microwave absorbing slurry. In step 6, only the microwave absorbing layer is cured at 150°C. This is denoted as C-4.

[0062] Performance Testing Methodology Description

[0063] Interfacial peel strength (plate tensile method): Referring to GB / T 1452 standard, the microwave absorbing honeycomb core material is bonded to the aluminum alloy test block on both sides with high-strength structural adhesive, and its tensile breaking strength (MPa) is determined using a universal testing machine. The main focus is on the bonding force between the coating and the honeycomb wall.

[0064] High-frequency vibration durability test (simulating aircraft engine environment): Following the vibration test standard GJB 150.16A, the coating was subjected to continuous vibration for 10 hours under random vibration conditions with a frequency of 50–2000 Hz and an RMS acceleration of 15g. The coating weight loss rate (%) before and after vibration and the peel strength retention rate (%) after vibration were measured.

[0065] Electromagnetic absorption performance test: The NRL (normative linear lattice) test method was used, with a test frequency range of 2–18 GHz. The minimum reflection loss (RL_min) and the effective absorption bandwidth (EAB) of less than -10 dB were recorded. The test results are shown in Table 1 below.

[0066] Table 1:

[0067]

[0068] In summary, this invention sequentially constructs a composite surface homogenization layer, a reactive interface bridging layer, and a low-modulus stress buffer transition layer between the molded aramid honeycomb and the subsequent microwave absorbing functional layer. Through gradient co-curing of the subsequent microwave absorbing layer and the transition layer, a stable cross-layer bonding structure is formed between the composite surface of the honeycomb cell wall and the highly filled microwave absorbing layer, thereby improving the interfacial bonding strength and interfacial durability of the microwave absorbing structure under vibration environment.

[0069] The descriptions and practices disclosed in this invention are readily apparent and understandable to those skilled in the art, and various modifications and refinements can be made without departing from the principles of this invention. Therefore, any modifications or improvements made without departing from the spirit of this invention should also be considered within the scope of protection of this invention.

Claims

1. A method for interface control of an aramid honeycomb microwave absorbing structure, characterized in that, Includes the following steps: Step 1: Pretreatment of the aramid honeycomb substrate: Using aramid honeycomb molded by impregnation and curing with phenolic resin or phenolic / epoxy resin as the substrate, immerse it in anhydrous ethanol or acetone solvent and ultrasonically clean it for 5 to 15 minutes to remove residual mold release agent and free oligomers on the surface, and then dry it at 60 to 80°C. Step 2, Composite Surface Wetting and Micro-interface Homogenization: The aramid honeycomb treated in step 1 is immersed in a low-solids interface homogenizing solution for 1 to 3 minutes; after removal, excess droplets in the honeycomb pores are blown away with compressed air at 0.1 to 0.2 MPa, and then baked at 80 to 100°C for 10 to 20 minutes to form an interface homogenizing layer on the honeycomb pore wall. Step 3: Chemical grafting of reactive interfacial bridging layers: The aramid honeycomb treated in step 2 is immersed in a reactive silane coupling agent hydrolysate for 5 to 15 minutes; after extraction, it is blown through with compressed air at 0.2 to 0.3 MPa and placed at 110 to 130°C for condensation reaction for 20 to 30 minutes to form an interface bridging layer on the surface of the interface homogenization layer. Step 4: Construction of a low-modulus stress buffer transition layer: The aramid honeycomb treated in step 3 is immersed in a flexible polymer precursor solution for 1–3 minutes; after extraction, it is blown through the holes with an airflow of 0.3–0.5 MPa so that the flexible polymer precursor solution only adheres to the surface of the honeycomb pore walls to form a thin layer; then, it is semi-cured at 80–90°C for 30–60 minutes to form a low-modulus transition layer. Step 5: Coating of a high-filling microwave absorbing functional layer: The semi-cured aramid honeycomb obtained in step 4 is immersed in the microwave absorbing slurry for 3 to 5 minutes; after removal, the channels are purged with a pulsed high-pressure airflow of 0.4 to 0.6 MPa to remove excess slurry, leaving only the microwave absorbing functional layer attached to the outside of the low modulus transition layer. Step 6: Interfacial cross-linking and gradient curing: The aramid honeycomb structure obtained in step 5 is placed in a programmed temperature rise oven for gradient curing, so that a co-cured bonding interface is formed between the interface homogenization layer, the interface bridging layer, the low modulus transition layer and the microwave absorbing functional layer.

2. The interface control method for an aramid honeycomb microwave absorbing structure according to claim 1, characterized in that, In step 2, the low-solids interface homogenization liquid is composed of the following components by mass percentage: 0.5% to 2.0% aqueous polyurethane dispersion or aqueous epoxy resin dispersion, 0.05% to 0.2% nonionic surfactant, and the balance being a mixed solvent of deionized water and ethanol; the volume ratio of deionized water to ethanol is 1:1 to 3:1; the nonionic surfactant is one or both of polyether-modified siloxane or octylphenol polyoxyethylene ether.

3. The interface control method for an aramid honeycomb microwave absorbing structure according to claim 1, characterized in that, In step 3, the mass concentration of the silane coupling agent in the hydrolysate of the reactive silane coupling agent is 1.0% to 3.0%, and the solvent is an ethanol / water mixture with the pH adjusted to 4.5 to 5.5 by glacial acetic acid, and the volume ratio of ethanol to water is 90:

10.

4. The interface control method for an aramid honeycomb microwave absorbing structure according to claim 3, characterized in that, The silane coupling agent is γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, or a combination of the two.

5. The interface control method for an aramid honeycomb microwave absorbing structure according to claim 1, characterized in that, In step 4, the flexible polymer precursor liquid is a two-component flexible polyurethane prepolymer system or a hydroxyl-terminated polybutadiene toughened modified epoxy resin system, diluted with ethyl acetate or methyl ethyl ketone to a solid content of 10% to 20% and a viscosity of 10 to 30 mPa·s.

6. The interface control method for an aramid honeycomb microwave absorbing structure according to claim 5, characterized in that, In step 4, the thickness of the low-modulus transition layer is 1–3 μm, and its Shore hardness after complete curing is 60–85.

7. The interface control method for an aramid honeycomb microwave absorbing structure according to claim 1, characterized in that, In step 5, the absorbing slurry is composed of a polymer resin matrix, a radar wave absorber uniformly dispersed therein, and a polar mixed solvent. The impregnation viscosity of the absorbing slurry is 200-500 mPa·s. The polymer resin matrix is ​​a two-component epoxy resin system or a rigid polyurethane resin system. The radar wave absorber is a magnetic loss absorber, a dielectric loss absorber, or a mixture of both; the magnetic loss absorber is selected from one or more of carbonyl iron powder, iron-silicon-aluminum alloy powder, iron-cobalt alloy powder, iron-nickel alloy powder, amorphous alloy micro powder, spinel ferrite, or magnetoplumbium ferrite; the dielectric loss absorber is selected from one or more of carbon nanotubes, graphene, conductive carbon black, silicon carbide micro powder, chopped carbon fibers, or MXene two-dimensional materials. When the radar wave absorber is a magnetically depleting metal micropowder, its mass fraction in the dried and cured radar-absorbing functional layer is 60% to 80%. When the radar wave absorber is a dielectric loss type carbon-based powder, its mass fraction in the dried and cured radar-absorbing functional layer is 3% to 15%.

8. The interface control method for an aramid honeycomb microwave absorbing structure according to claim 1, characterized in that, The gradient curing procedure in step 6 is as follows: first, maintain the temperature at 60-80℃ for 1-2 hours; then, raise the temperature to 110-120℃ and maintain for 2 hours; finally, raise the temperature to 140-150℃ and maintain for 1-2 hours.

9. An aramid honeycomb microwave absorbing structure, characterized in that, It is prepared by the interface control method according to any one of claims 1 to 8; the honeycomb pore wall includes a microwave absorbing functional layer, a low modulus transition layer and an interface control layer from the inside to the outside, wherein the interface control layer is composed of an interface homogenization layer and an interface bridging layer, and the layers are co-cured with a resin matrix to form an interlayer bonding interface.

10. An aramid honeycomb microwave absorbing structure according to claim 9, characterized in that, The interface homogenization layer is less than 0.5 μm thick, and the absorber particles are distributed in the microwave absorption functional layer and connected to the honeycomb pore wall through a low-modulus transition layer.

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