A short-cut fiber reinforced flexible polymer thermal protection material modified by graphene oxide and aramid nanofiber hybridization, and a preparation method and use thereof

Abstract

The application provides a kind of chopped fiber reinforced flexible polymer thermal protection material by graphene oxide and aramid nanofiber hybrid modification and its preparation method and use, belongs to the field of advanced materials.Aiming at the problem that the modulus difference between fiber and flexible matrix is huge, the interface compatibility is poor, and the stress concentration is serious, by constructing the hybrid interface phase composed of graphene oxide and aramid nanofiber, the microstructure of the inert surface of the chopped fiber is improved, the multi-scale mechanical interlocking structure between the fiber and the flexible matrix is promoted, the modulus mismatch between the fiber and the rubber matrix is effectively relieved, and the uniform and efficient stress transfer in the interface area is realized.In the process of tensile deformation, dynamic load and high temperature ablation, the hybrid interface phase can keep the close interface bonding between the fiber and the flexible matrix, significantly improve the ablation and erosion resistance of the flexible composite material and the mechanical properties, especially the flexible ablation performance under high dynamic and deformation state, and has good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of advanced materials technology, specifically relating to a flexible polymer thermal protection material reinforced by short-cut fibers modified by hybridization of graphene oxide and aramid nanofibers, its preparation method, and its applications. Background Technology

[0002] Fiber-reinforced polymer matrix composites, due to their low density, high specific strength, excellent corrosion resistance, and designability, have shown broad application prospects in aerospace and defense equipment, especially in the field of ablation-resistant and thermal protection materials. As the service environment of aircraft becomes increasingly harsh, these materials not only need to minimize density while possessing excellent thermal stability and ablation resistance, but also need to maintain the continuity and density of the carbonized protective layer during ablation under complex thermo-mechanical coupling loads, and have the ability to resist high-temperature airflow erosion and mechanical spalling, thereby maintaining the integrity and service reliability of the overall thermal protection structure.

[0003] In the field of thermal protection materials, flexible composite materials with both flexible deformation capabilities and efficient thermal insulation properties have become an important development direction for thermal protection systems of advanced equipment such as hypersonic aircraft. However, interfacial failure between the flexible matrix and high-modulus reinforcing fibers can easily induce structural damage during ablation, becoming a key factor restricting the improvement of their ablation performance. Existing research has shown that the interfacial structure has a decisive influence on the ablation behavior of flexible thermal protection composite materials. For example, Lu et al. (Industrial & Engineering Chemistry Research, 2025, 64: 14487-14499) significantly reduced the ablation rate of the composite material and decreased the back-side temperature rise under tensile deformation conditions by enhancing the interfacial interaction between silicone rubber and carbon fiber.

[0004] High-performance organic fibers such as aramid and polyimide possess high modulus, excellent heat resistance, and good mechanical strength retention, showing significant application potential in flexible thermal protection composites. However, the high rigidity of these fiber molecular chains, strong surface chemical inertness, and limited content of polar functional groups result in poor interfacial compatibility with most polymer matrices, leading to insufficient fiber-matrix interfacial bonding. This interfacial defect not only limits the effective transfer of mechanical loads and weakens the mechanical strength of composites, but also makes them a preferred pathway for oxygen diffusion and thermal erosion during high-temperature ablation, causing interfacial debonding, crack propagation, and char layer peeling. To improve the interfacial bonding between fibers and the matrix, existing technologies mainly focus on surface modification of fibers or the introduction of interfacial phases. For example, surface activation of aramid fibers is achieved through acid-base treatment, plasma treatment, and coupling agent coating; or transition interfaces are constructed by designing multilayer structures and introducing nanoparticles. Ma et al. (Composites Part A: Applied Science and Manufacturing. 2023, 165: 107370) designed a novel lightweight structure by modifying aramid fibers with bilayer silsesquioxane-bonded norbornene and using it to reinforce EPDM rubber, which significantly reduced the linear ablation rate of the composite material. Wu et al. (RSC Advances. 2016, 6:75390-75399) developed a novel alkaline solvent mixture to treat the surface of polyimide fibers, and successfully prepared polyimide fiber-reinforced phenolic resin composite materials with improved interfacial bonding and mechanical properties.

[0005] Despite the progress made by the above methods, the following limitations still exist: First, most modification methods focus on the single regulation of the physical morphology or chemical groups on the fiber surface, and pay insufficient attention to the evolution of the microstructure of the interface region during ablation and its correlation mechanism with macroscopic properties; Second, traditional modification methods often fail to construct multi-scale interface structures on the fiber surface that combine strong bonding, high thermal stability and good energy dissipation capacity; Third, existing research has not systematically revealed the dynamic response law of the fiber / matrix interface under high-temperature ablation environment, and also lacks a hybrid modification strategy that starts from the design of interface microstructure to simultaneously improve the mechanical properties and ablation resistance of composite materials.

[0006] In summary, in composite systems composed of flexible polymer matrices and high-modulus chopped fibers, the significant modulus difference between the two leads to complex and severe microscopic deformations in the interfacial region during loading and ablation, becoming the main cause of interfacial failure and decreased ablation performance. Therefore, from the perspective of interfacial structure design and mechanism, constructing an interfacial construction method that enables coordinated deformation of fibers and matrix under high dynamic conditions, effectively alleviates stress concentration induced by modulus mismatch, and forms a dense and stable residual carbon layer in a high-temperature ablation environment is a key scientific and technological issue for achieving synergistic improvement in the mechanical properties and ablation resistance of chopped fiber-reinforced flexible heat-resistant composite materials. Summary of the Invention

[0007] The purpose of this invention is to provide a polymer matrix composite material reinforced with aramid F-12 fiber by hybrid modification of graphene oxide and aramid nanofibers, its preparation method and uses.

[0008] This invention provides a modified chopped fiber flexible polymer thermal protection material, the raw materials of which include modified chopped fibers and a polymer matrix; the modified chopped fibers are chopped fibers modified by a hybrid of graphene oxide and aramid nanofibers; the mass ratio of graphene oxide to aramid nanofibers is (1~5):(20~100).

[0009] The preferred mass ratio of graphene oxide to aramid nanofibers is (0.3~0.7):(8~12), and more preferably the mass ratio of graphene oxide to aramid nanofibers is 0.5:10.

[0010] Furthermore, the chopped fibers are selected from aramid fibers, polyimide fibers, carbon fibers, and quartz fibers.

[0011] Furthermore, the aramid fiber is preferably aramid F-12 fiber.

[0012] Furthermore, the modified chopped fibers are prepared by the following method:

[0013] (1) Dissolve polyethyleneimine in buffer solution, then stir and add tannic acid to obtain polyethyleneimine-tannic acid solution; then, immerse short-cut fibers in polyethyleneimine-tannic acid solution, stir, then take out short-cut fibers, wash and dry to obtain pre-modified short-cut fibers;

[0014] (2) Dilute the graphene oxide dispersion with water to obtain a graphene oxide suspension. Then add aramid nanofibers to the graphene oxide suspension and stir to obtain a graphene oxide / aramid nanofiber dispersion.

[0015] (3) Immerse the pre-modified short-cut fibers in a dispersion of graphene oxide / aramid nanofibers, and then take them out and dry them to obtain the modified short-cut fibers.

[0016] Furthermore, the raw materials for modified chopped fiber reinforced flexible polymer thermal protection materials also include at least one of the following components: activator, softener, reinforcing agent, accelerator, and curing agent.

[0017] Furthermore, the modified chopped fiber reinforced flexible polymer thermal protection material comprises the following raw materials in parts by weight: 8-12 parts of modified chopped fiber, 90-110 parts of polymer matrix, 3-7 parts of activator, 0.2-0.7 parts of softener, 8-12 parts of reinforcing agent, 0.5-2 parts of accelerator and 1-3 parts of curing agent.

[0018] Preferably, the modified chopped fiber reinforced flexible polymer thermal protection material comprises the following raw materials in parts by weight: 10 parts modified chopped fiber, 100 parts polymer matrix, 5 parts activator, 0.5 parts softener, 10 parts reinforcing agent, 1 part accelerator, and 1.5 parts curing agent.

[0019] Further, the polymer matrix is ​​at least one of EPDM rubber, silicone rubber, thermoplastic vulcanizate, and propylene rubber; the activator is at least one of zinc oxide, magnesium oxide, zinc carbonate, and zinc dodecyl sulfate; the softener is at least one of stearic acid, zinc stearate, calcium stearate, and paraffin oil; the reinforcing agent is at least one of silica, carbon black, and calcium silicate; the accelerator is at least one of diphenylguanidine, tetramethylthiourea, and dibenzothiazole disulfide; and the curing agent is at least one of sulfur, dicumyl peroxide, benzoyl peroxide, and tert-butyl benzoate peroxide.

[0020] Preferably, the curing agent is a mixture of sulfur and dicumyl peroxide, wherein the mass ratio of sulfur to dicumyl peroxide is (0.3~0.7):(1~2).

[0021] Preferably, the mass ratio of sulfur to dicumyl peroxide is 0.5:1.5.

[0022] This invention also provides a method for preparing modified chopped fiber reinforced polymer matrix composites, comprising the following steps:

[0023] (1) According to the weight proportions of each raw material, first add the polymer matrix to the open mill, and after it wraps around the rolls, add the remaining raw materials in sequence;

[0024] (2) After the raw materials are fully mixed, a thin-pass treatment is carried out; then the roller gap is adjusted and the sheet is cut to obtain the compound rubber;

[0025] (3) The compound is vulcanized to obtain the modified short-cut fiber reinforced polymer matrix composite material.

[0026] This invention also provides the application of modified chopped fiber reinforced polymer matrix composites in the aerospace field.

[0027] This invention constructs a hybrid interfacial phase composed of graphene oxide and aramid nanofibers between chopped fibers and rubber. This structure promotes the formation of a denser cross-linked network on the fiber surface and also creates a uniformly dispersed, high-temperature-resistant graphene oxide structure, effectively modifying the short fibers. This modification method is particularly effective for aramid F-12 fibers. When modified aramid F-12 fibers are combined with EPDM rubber to form a flexible polymer-based thermal protection material, the pull-out force and ablation performance are significantly improved. Experiments demonstrate that the aramid F-12 fibers modified by the hybridization of graphene oxide and aramid nanofibers have a rough interfacial surface, overcoming the original numerous surface grooves in aramid F-12 fibers that hinder rubber molecule penetration. This effectively improves the inert interfacial microstructure of aramid F-12 fibers and increases interfacial adhesion. Meanwhile, the hybrid modification layer of graphene oxide and aramid nanofibers can better form a mechanical interlocking structure between aramid F-12 fibers and rubber and achieve better stress transfer in the interface region, improve the interfacial adhesion between aramid F-12 fibers and rubber, form a strong interfacial bonding layer, improve the stress transfer efficiency and tensile strength of the composite material, and significantly improve the ablation and erosion resistance of the composite material. This provides key guidance for the research and development of high-performance rubber-based thermal insulation materials, enabling them to be more widely used in the aerospace field.

[0028] Obviously, based on the above description of the present invention, and according to common technical knowledge and conventional methods in the field, various other modifications, substitutions or alterations can be made without departing from the basic technical concept of the present invention.

[0029] The following detailed embodiments further illustrate the above-described content of the present invention. However, this should not be construed as limiting the scope of the present invention to the following embodiments. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention. Attached Figure Description

[0030] Figure 1 These are surface morphology images of RF-12, F-12 / TP, F-12 / GO, and F-12 / GO / ANF: a~d are SEM images at high magnification; e~h are SEM images at low magnification.

[0031] Figure 2 These are TEM and AFM images of the RF-12, F-12 / TP, F-12 / GO, and F-12 / GO / ANF: a~d are TEM images; e~h are AFM images.

[0032] Figure 3These are the thermogravimetric curves of aramid F-12 fibers before and after modification in a nitrogen atmosphere: (a) Temperature-mass change rate curves of RF-12, F-12 / GO, F-12 / GO / ANF, F-12 / TP, and GO; (b) Temperature-mass change rate curves of RF-12, F-12 / GO, F-12 / GO / ANF, F-12 / TP, and GO.

[0033] Figure 4 The following are elemental analysis diagrams of aramid F-12 fibers before and after modification: (a) Full elemental analysis spectrum of RF-12, F-12 / TP, F-12 / GO, and F-12 / GO / ANF; (b) Fine-fit spectrum of F-12 / GO / ANF-C1s; (c) Fine-fit spectrum of RF-12-C1s.

[0034] Figure 5 These are XRD patterns of RF-12, F-12 / TP, F-12 / GO, F-12 / GO / ANF, and GO.

[0035] Figure 6 These are the H-pull-out force test diagrams for RF-12, F-12 / ANF, F-12 / GO, and F-12 / GO / ANF.

[0036] Figure 7 The relaxation time (a), crosslinking density (b), and crosslinking ratio test graph (b) are for EPDM / RF-12, EPDM / F-12 / GO, and EPDM / F-12 / GO / ANF.

[0037] Figure 8 The stress-strain curves (a), tensile properties (b), and elongation at break test results (b) for EPDM / RF-12, EPDM / F-12 / GO, and EPDM / F-12 / GO / ANF are shown.

[0038] Figure 9 The displacement-load curves (a) and in-situ tensile electron microscope images (b) of EPDM / RF-12 and EPDM / F-12 / GO / ANF are shown.

[0039] Figure 10 a~c are tensile cross-sectional morphology images of RF-12, F-12 / GO, and F-12 / GO / ANF at high magnification; d~f are tensile cross-sectional morphology images of RF-12, F-12 / GO, and F-12 / GO / ANF at low magnification.

[0040] Figure 11The figures are (a) ablation performance test diagrams of EPDM / RF-12, EPDM / F-12 / ANF, EPDM / F-12 / GO, and EPDM / F-12 / GO / ANF, and (b~d) microscopic cross-sectional ablation morphology diagrams and three-dimensional contour diagrams of EPDM / RF-12, EPDM / F-12 / GO, and EPDM / F-12 / GO / ANF. Detailed Implementation

[0041] The raw materials and equipment used in this invention are all known products, obtained by purchasing commercially available products.

[0042] The aramid F-12 fiber was purchased from Jiangsu Xiannuo New Material Technology Co., Ltd., and was a short-cut aramid F-12 fiber with a length of 3mm; the graphene oxide dispersion was purchased from Shandong Jinlite New Material Technology Co., Ltd.; the aramid nanofiber was purchased from Shanghai Xinchuang Houpu New Material Technology Co., Ltd., and was an aramid nanofiber with a length of 0.6-1.8 micrometers; and the EPDM rubber was purchased from Jilin Petrochemical, grade 4045.

[0043] Example: Preparation, characterization, and performance testing of modified aramid F-12 fiber reinforced polymer matrix composites.

[0044] 1. Preparation of modified aramid F-12 fiber

[0045] The material was prepared using the following method:

[0046] (1) Dissolve 2g of polyethyleneimine (PEI) in Tris (tris) buffer solution (adjusted with sodium hydroxide aqueous solution, pH=9.0), then stir and add 0.4g of tannic acid (TA) to obtain a TA-PEI solution with a volume of 1L; then fix 25g of cleaned aramid F-12 (F-12) fiber in a support and put it into the TA-PEI solution. After stirring for 30min, take out the F-12 fiber, rinse it with deionized water, and then dry it at 60℃ for 6h to obtain pre-modified F-12 fiber, denoted as F-12 / TP;

[0047] The cleaning method for F-12 fiber is as follows: add F-12 fiber to 500ml of acetone, sonicate for 3 hours, remove and repeatedly wash in deionized water.

[0048] (2) Take 50 ml of graphene oxide dispersion with a concentration of 10 mg / ml and place it in a 500 ml beaker. Then add 450 ml of deionized water to dilute it and obtain graphene oxide suspension (GO / H2O). Next, add 10 g of aramid nanofiber (ANF) to GO / H2O and ultrasonically stir the mixture for 1 h to obtain a uniform graphene oxide / aramid nanofiber (GO / ANF) aqueous dispersion.

[0049] (3) Immerse F-12 / TP in GO / ANF aqueous dispersion for 2 minutes, then remove and squeeze out excess solution. Repeat the above operation three times, and then dry the sample at 60°C for 2 hours to obtain modified aramid F-12 fiber, denoted as F-12 / GO / ANF.

[0050] Preparation of other aramid F-12 fiber comparative materials:

[0051] (1) Immerse F-12 / TP in a graphene oxide suspension, wherein the graphene oxide concentration is the same as that in the GO / ANF aqueous dispersion. After immersion for 2 minutes, remove the sample and squeeze out the excess solution. Repeat the above operation three times, and then dry the sample at 60°C for 2 hours to obtain graphene oxide modified aramid F-12 fiber, denoted as F-12 / GO.

[0052] (2) Immerse F-12 / TP in an aramid nanofiber suspension, wherein the concentration of aramid nanofiber is the same as that of aramid nanofiber in GO / ANF aqueous dispersion. After immersion for 2 minutes, remove the sample and squeeze out the excess solution. Repeat the above operation three times, and then dry the sample at 60°C for 2 hours to obtain aramid nanofiber modified aramid F-12 fiber, denoted as F-12 / ANF.

[0053] (3) Weigh 15g of F-12 fiber into a 500ml beaker, add 500ml of acetone, sonicate for 3 hours, and then take it out and wash it repeatedly in deionized water to obtain desizing F-12 fiber, which is denoted as RF-12.

[0054] 2. Preparation of modified aramid F-12 fiber reinforced polymer matrix composites

[0055] Modified aramid F-12 fiber-reinforced polymer matrix composites were prepared using the following method:

[0056] 100g of ethylene propylene diene monomer (EPDM) rubber was added to a two-roll mill at 60℃. After the rubber wrapped around the rolls, 5g of zinc oxide, 0.5g of stearic acid, 10g of silica, 1g of accelerator D, 0.5g of sulfur, and 1.5g of dicumyl peroxide were added in sequence, along with 10g of F-12 / GO / ANF. After thorough mixing, the mixture was subjected to six thin-pass treatments. Finally, the roll gap was adjusted to 2mm, and the mixture was sheeted out and left to stand for at least 16 hours for later use. The prepared mixture was then laid flat in a flat mold and vulcanized at 160℃ and 10MPa for 30 minutes to obtain a modified aramid F-12 fiber reinforced polymer matrix composite material, namely the EPDM / F-12 / GO / ANF composite material.

[0057] Preparation of other comparative aramid F-12 fiber reinforced polymer matrix composites:

[0058] Referring to the preparation method of EPDM / F-12 / GO / ANF composite material, F-12 / GO / ANF was replaced with other aramid F-12 fiber comparative materials prepared in step 1, and other comparative aramid F-12 fiber reinforced polymer matrix composite materials were obtained respectively.

[0059] 3. Characterization methods for modified aramid F-12 fibers

[0060] (1) The surface morphology of the fiber was observed using a scanning electron microscope (SEM) from Phenom-world BV (Netherlands); the microstructure of the fiber was observed using a transmission electron microscope (TEM) from Thermo Scientific (USA); and the surface morphology of the fiber was tested using a MultiMode III atomic force microscope (AFM) from Bruker (Germany).

[0061] (2) Thermogravimetric analysis of the fibers was performed using a thermogravimetric analyzer (TGA) from NETZSCH GmbH, Germany;

[0062] (3) Using an X-ray photoelectron spectroscopy (XPS) instrument from Thermo Fisher Scientific, USA, with Al Kα rays as the excitation source, the surface elemental composition and chemical state of the fiber were analyzed.

[0063] (4) The crystallinity of the modified fiber was analyzed using an X-ray diffractometer (XRD) from Rigaku Corporation (Ultima IV) of Japan.

[0064] (5) The crosslinking density and relaxation time of the composite material were tested using a low-field nuclear magnetic resonance analyzer from Suzhou Newmai Analytical Instrument Co., Ltd.

[0065] 4. Performance testing methods for modified aramid F-12 fiber reinforced polymer matrix composites

[0066] (1) According to GB / T 2942-2009 standard, the prepared fiber / rubber H-type pull-out specimen was clamped on a universal tensile testing machine and tested at a tensile rate of 200 mm / min to characterize the interfacial bonding strength between the fiber and the rubber matrix.

[0067] (2) In accordance with GB / T 528-2009 standard, tensile tests were performed on the Type III rubber composite material specimens at a test rate of 200 mm / min, and their tensile strength and elongation at break were determined.

[0068] (3) The microstructural changes of the composite material during the tensile process were observed in real time using the Phenom Xl in-situ scanning electron microscope of Phenom Scientific in the Netherlands, and its mechanical failure mechanism was analyzed.

[0069] (4) The ablation behavior of the composite material was evaluated using the oxy-acetylene ablation tester of Beijing Qinhe Technology Co., Ltd., China. Four samples (30 mm in diameter × 10 mm in thickness) were prepared for each formulation and ablated for 20 s at an ablation heat flux density of 4 MW / m2. The temperature changes on the surface and back of the sample were recorded at the same time.

[0070] (5) Through mass ablation rate (R m ) and carbonization rate (R c The ablation performance of the composite material was quantified, and the calculation formulas are shown in Equation (1) and Equation (2) respectively:

[0071] R m =(m1-m2) / t (1)

[0072] R c =(h1-h2) / t (2)

[0073] In the formula: m1 is the initial mass of the sample before ablation, m2 is the residual mass of the sample after ablation, h1 is the initial height of the sample before ablation, h2 is the minimum height of the carbonized layer peeled off after ablation, and t is the ablation duration.

[0074] 5. Characterization results of modified aramid F-12 fiber

[0075] (1) Micromorphology of the interface of aramid F-12 fiber modified by hybridization of graphene oxide and aramid nanofiber

[0076] like Figure 1 The electron micrograph shows that after F-12 fibers are impregnated with TA / PEI solution, a film-like structure with large particles forms on its surface. This is the TP layer, which can increase surface activity. Figure 1 b, f); After F-12 / TP was coated with GO, large areas of blocky material were observed on the surface of the F-12 fiber under both low and high magnification electron microscopes. These are scattered graphene oxide sheets on the fiber surface, indicating that the coating is not complete. Figure 1 c, g); Furthermore, electron micrographs of F-12 / GO / ANF show numerous filamentous structures between the F-12 fiber surface and GO, entangled GO on the F-12 surface, increasing interfacial bonding and making the coating more complete. Figure 1(d, h). The surface modification in this embodiment increases the roughness of the fiber surface, enhances the permeability of the rubber during the fiber-rubber composite process, increases interfacial friction, and increases the number of locking sites between the fiber and the rubber matrix.

[0077] As can also be seen in atomic force microscopy and transmission electron microscopy, the surface of RF-12 itself is relatively smooth. Figure 2 a, e); After impregnation with TA / PEI solution, an active layer is formed on the surface, causing particles to appear on the fiber surface ( Figure 2 b, f); After being coated with GO, the surface morphology transforms into a large number of sheet-like protrusions that are uneven ( Figure 2 c, g); After further addition of ANF, the surface became rough and the material distribution was uniform with no significantly different protrusions. Figure 2 (d, h); proved that the addition of ANF is beneficial to the bonding between graphene oxide and the F-12 surface, and the roughness is increased on the original basis, effectively improving the inert interface microstructure of F-12.

[0078] (2) Interfacial microstructure analysis of modified aramid F-12 fiber

[0079] The thermogravimetric curves and parameters of F-12 / GO / ANF can intuitively reflect the changes in substances adsorbed on the surface of F-12 fibers. From... Figure 3 As shown in Table 1, at temperatures below 100°C, the evaporation of water adsorbed on the GO surface leads to GO mass loss. Simultaneously, the thermal decomposition of oxygen-containing functional groups on GO (such as carboxyl, hydroxyl, and epoxy groups) produces CO2, CO, and water vapor, resulting in a peak mass loss at 120°C. Similarly, both the F-12 / GO and F-12 / GO / ANF samples exhibited GO thermal degradation at around 200°C, indicating successful GO introduction onto the F-12 fiber surface. Notably, the F-12 / GO sample showed a 5% weight loss at 182.23°C, suggesting a more sparsely distributed GO coating on the fiber surface, indicating relatively weak interfacial adhesion. In contrast, the F-12 / GO / ANF sample exhibited a more stable initial degradation curve, indicating that the ANF / GO mixture formed a stronger coating and that ANF facilitates GO coating on the fiber surface, resulting in stronger bonding between the F-12 fiber and the GO surface.

[0080] Table 1. Thermogravimetric parameters of aramid F-12 fibers before and after modification in nitrogen atmosphere.

[0081]

[0082] (3) Interfacial element analysis of modified aramid F-12 fiber

[0083] XPS spectra and elemental composition fractions of F-12 / GO / ANF are as follows: Figure 4 As shown in Table 2, the nitrogen (N) content increased significantly after depositing a TP layer on the surface of F-12 fibers, due to the abundance of amino groups (-NH2) in the TP layer. After modification with GO and GO / ANF, the carbon (C) content remained relatively unchanged, while the oxygen (O) content and oxygen-carbon ratio (O / C) increased from 0.122 to 0.192, indicating that GO was heavily adsorbed on the surface of F-12 fibers (Table 2). After coating with GO / ANF, the content of CO and C=O bonds in F-12 fibers increased significantly, and a new satellite peak appeared near 290.8 eV, due to the π-electron conjugation system in GO and ANF molecules. Figure 4 c). This demonstrates that GO and ANF are stably coated on the surface of F-12 fibers.

[0084] Table 2. Elemental content of aramid F-12 fibers before and after modification.

[0085]

[0086] (4) Structural characterization of modified aramid F-12 fiber

[0087] XRD was used to characterize the crystallinity of the fiber surface, such as Figure 5 As shown, the F-12 fiber exhibits a broad diffraction peak at 2θ=20.5°, corresponding to the (1,1,0) crystal plane, confirming its semi-crystalline structure. For F-12 / TP, the diffraction peak remains unchanged compared to the F-12 fiber, indicating that the TP layer does not affect the crystallinity of the fiber. The pure GO sample shows a distinct peak at 2θ=11.00°, which is indexed by the (0,0,1) plane of its multilayer structure, with an interlayer spacing of approximately 0.75 nm. For F-12 / GO / ANF, a shifted peak appears at 2θ=11.5°, confirming the success of the GO coating. Since ANF has a semi-crystalline structure and exhibits a bimodal distribution in the 20-30° range, this may be the direct reason for the new peak value of F-12 / GO / ANF near 2θ=26°. In addition, the enhanced peak intensity at 2θ=20.5° in F-12 / GO / ANF indicates that the bonding of GO / ANF stabilizes the crystalline domain of F-12 fibers, further demonstrating that the entangled ANF network enhances the integrity of the coating and the interfacial adhesion.

[0088] The above characterization results demonstrate that graphene oxide and aramid nanofibers are stably coated on the surface of aramid F-12 fibers. The aramid nanofibers facilitate the coating of graphene oxide on the surface of aramid F-12 fibers, making the surface bonding between aramid F-12 fibers and graphene oxide stronger, while forming a rough and strong bonding interface. Through the hybrid modification of graphene oxide and aramid nanofibers, the mechanical locking sites between the fibers and the rubber matrix are increased, effectively improving the inert interfacial microstructure of aramid F-12 fibers and increasing their interfacial adhesion.

[0089] 6. Performance test results of modified aramid F-12 fiber reinforced polymer matrix composites

[0090] (1) Interfacial bonding properties of EPDM / F-12 / GO / ANF composite materials

[0091] H pull-out test results are as follows: Figure 6 As shown, the pull-out force (H) of F-12 fiber is relatively low, only 5.88 N. After GO treatment, the H pull-out force increases slightly to about 7.84 N, indicating that GO-coated fiber improves the interface to some extent by simply increasing roughness, but the effect is limited. After GO / ANF treatment, the H pull-out force of F-12 fiber increases by 75.3%, indicating that the GO / ANF hybrid modification layer can better form a mechanical interlocking structure between F-12 fiber and EPDM rubber and achieve better stress transfer in the interface area. This proves that the GO / ANF coating can further improve the interfacial adhesion between F-12 fiber and EPDM rubber and can make up for the shortcomings of using a single graphene oxide material.

[0092] use 1 Low-field NMR characterizes the crosslinked network structure of the EPDM / F-12 composite system before and after modification, and obtains relevant information such as the transverse relaxation time (T2) distribution, the overall T2 of the modified material, and the crosslinking density. Figure 7 As shown, the relaxation time of the fiber coated with GO is similar to that of the original fiber composite material, but the right-hand peak shifts to the right, indicating that the mobility of the suspended chain in the composite material is increased. In contrast, the EPDM / F-12 composite material prepared by the fiber coated with GO / ANF shows a significant decrease in relaxation time, and the right-hand peak also shows a significant left shift. Furthermore, the two peaks merge, indicating that the degree of cross-linking is the highest in the system, and the movement of free chain segments is more restricted.

[0093] from Figure 7 As shown in Table 3, the crosslinking density gradually increases with interface modification, with a significant increase, indicating that strong hydrogen bonds and greater interfacial roughness effectively bind the rubber molecular chains. After ANF entanglement with GO-coated fibers, the crosslinking density increases through π-π... The interaction firmly adheres to the GO surface to form a whole nano-interface layer. The roughness increases the mechanical interlocking and physical cross-linking effect between the rubber and the interface layer. The hydrogen bonds on the graphene oxide further inhibit the movement of rubber molecules in the interface layer.

[0094] Experimental results show that EPDM / F-12 composite materials prepared by fibers coated with GO / ANF enhance interfacial interactions. The enhanced interface restricts the movement of rubber molecules, making molecular chain separation require higher stress.

[0095] Table 3 Relaxation time, crosslinking density, and crosslinking ratio of aramid F-12 fiber / rubber composite system

[0096]

[0097] (2) Mechanical properties of EPDM / F-12 / GO / ANF composite material

[0098] like Figure 8 As shown in Table 4, due to the inert interface of F-12 fibers, the tensile strength (TS) and elongation at break (Eb) of EPDM / RF-12 are 5.34 MPa and 484.11%, respectively. After GO modification, the tensile strength of EPDM / F-12 / GO decreased to 4.99 MPa, indicating that the easily detachable graphene oxide is unevenly dispersed in the EPDM matrix to form aggregates, resulting in a decrease in tensile strength and elongation at break. The tensile strength of F-12 fibers treated with GO / ANF increased to 6 MPa. The tensile strength of the composite material was 49 MPa, which is 21.5% higher than that of the unmodified fiber / rubber composite. This indicates that the material on the surface of the F-12 fiber is more firmly locked, without affecting the original properties of EPDM, but rather further improving them. On the other hand, the F-12 fiber modified with GO / ANF has a larger specific surface area and roughness in contact with the EPDM matrix, and at the same time, it also strengthens the GO layer on the surface. The rough surface of F-12 / GO / ANF also increases the mechanical interlocking effect with EPDM, thereby improving the tensile strength of the composite material.

[0099] Table 4 Tensile properties of aramid F-12 fiber / ethylene propylene diene monomer (EPDM) rubber composites before and after modification.

[0100]

[0101] Furthermore, the tensile damage evolution process of the composite material was studied using in-situ scanning electron microscopy, such as... Figure 9As shown, the four captured regions correspond to key nodes on the displacement-load curve: upper yield point ①, lower yield point ②, and termination point ④. At the upper yield point, the unmodified composite material exhibits fiber exposure, indicating severe debonding between the fibers and the matrix; while the EPDM / F-12 / GO / ANF composite material only shows slight interfacial cracks, reflecting the load-bearing capacity of the interfacial layer; at the lower yield point, the unmodified composite material undergoes progressive interfacial separation and matrix cracking perpendicular to the tensile direction, leading to a decrease in load-bearing capacity; although the EPDM / F-12 / GO / ANF composite material shows local interfacial separation, the degree of matrix cracking is slight, and the elastomer shrinkage is good.

[0102] At the termination point, the unmodified EPDM / RF-12 composite exhibited extensive fiber pull-out, complete interfacial failure, severe matrix tearing, and significant shrinkage. Although the modified EPDM / F-12 / GO / ANF composite also showed some fiber pull-out, it maintained better matrix integrity and exhibited lower shrinkage, indicating that the EPDM / F-12 / GO / ANF composite has a robust interfacial layer that improves stress transfer efficiency. Throughout the tensile process, the fibers were oriented along the direction of the external force, confirming effective transverse stress transfer.

[0103] Experiments have shown that the EPDM / F-12 / GO / ANF composite material has higher elongation at break and tensile strength.

[0104] The tensile fracture morphology of the composite material before and after modification is as follows: Figure 10 As shown, numerous fiber fracture surfaces and nearly exposed pulled-out fibers were observed in the cross-section of EPDM / RF-12. Figure 10 (a, d) indicates that interfacial failure and fiber breakage are the main failure mechanisms of the composite material. After single GO coating, the number of fiber breaks in EPDM / F-12 / GO further increases, and a large number of fibers remain in the rubber matrix. The pore area between the fiber and the matrix decreases, indicating that the GO attached to the fiber surface improves the interfacial bonding between F-12 fiber and EPDM to some extent. However, the interface becomes disordered with many sheet-like structures, which is caused by the weakly bonded graphene oxide detaching from the fiber surface and dispersing in the matrix. Figure 10 (b, e) After ANF entanglement and GO coating of fibers, the sheet-like material in the cross-section is significantly reduced, indicating that GO is more firmly locked onto the fiber surface. Figure 10c and f show that the fiber and matrix are more tightly bonded, the pores are significantly smaller, and the number of fiber breaks is less, indicating that the fibers are more likely to be completely pulled out. Rubber clusters are clearly attached to the fiber surface, indicating that the EPDM / F-12 / GO / ANF composite material has strong interfacial bonding performance, and the main failure mode is the destruction of the cohesive energy of the rubber matrix.

[0105] (3) Ablation properties of EPDM / F-12 / GO / ANF composite material

[0106] like Figure 11 As shown in Figure 11b-d, after performing three-dimensional profile measurements on the ablated samples, it was found that the unmodified samples exhibited significant surface peeling under thermal erosion. In contrast, the surface of the EPDM / F-12 / GO / ANF composite material was smoother. This phenomenon indicates that under the same thermal erosion conditions, the erosion resistance of the EPDM / F-12 / GO / ANF composite material was significantly enhanced, and its three-dimensional profile was also more intact.

[0107] like Figure 11 As shown in a and Table 5, the mass ablation rate (Ra) m ) and carbonization rate (R c Quantifying the ablation properties of the composite material, it can be seen that the R of the EPDM / F-12 / GO composite material... m Compared to EPDM / RF-12 composite materials, R m The decrease indicates that F-12 fibers coated with GO have a stronger ability to resist high-temperature decomposition, allowing the fiber structure to remain relatively intact after ablation; the R of the EPDM / F-12 / ANF composite material... m Compared to EPDM / RF-12 composite materials, R m The lack of significant reduction indicates that the ANF coating, similar in structure to the F-12 fiber itself, belongs to the aramid family and has similar thermal decomposition properties, thus failing to improve the material's ablation resistance. However, the R of the EPDM / F-12 / GO / ANF composite material... m The ablation resistance was further reduced and improved on the basis of EPDM / F-12 / GO composite material, indicating that after GO / ANF modified F-12 fiber, a strong GO / ANF protective layer was formed, which synergistically exerted the ablation resistance, making EPDM / F-12 / GO / ANF composite material have better ablation resistance than EPDM / F-12 / GO composite material.

[0108] Table 5 Ablation properties of aramid F-12 fiber / ethylene propylene diene monomer (EPDM) rubber composites before and after modification.

[0109]

[0110] Figure 11 The image shows a scanning electron microscope (SEM) image of the cross-section of the carbon layer in the composite material prepared using stepwise modified fibers. At the macroscopic carbon layer scale, the GO-containing composite material shows more exposed fibers and abundant lamellar graphite structures at the pore edges. These graphite domains improve the crystallinity and longitudinal erosion resistance of the carbon layer. However, due to the discontinuity of the lamellar structure and the inherent flammability of the unanchored graphene flakes, the GO carbon layer still has a large number of pores. At the single fiber level, F-12 fibers undergo severe thermal decomposition after oxyacetylene ablation testing, exhibiting a hollow structure with only a single "fiber shell" remaining. This may be because F-12 is a product of structural modification and optimization of poly(p-phenylene terephthalamide), and its incomplete structure leads to easy chain breakage and degradation. However, after ablation, the F-12 fibers coated with GO showed a significant increase in outer shell thickness, indicating that graphene oxide provided some protection and thermal insulation. This protective effect may be due to the crystalline nature of the added graphene, which retained some crystalline structure and properties after ablation, thus significantly improving the degree of graphitization. Further addition of aramid nanofibers with a more complete structure resulted in smaller void diameters, indicating the formation of a dense pyrolytic carbon barrier. The aramid fibers also exhibited high thermal stability, synergistically forming a "nano-ceramic-carbon" biphase protective layer with GO, greatly enhancing the thermal protection performance of the composite material.

[0111] In summary, experiments have demonstrated that graphene oxide and aramid nanofibers stably coat the surface of aramid F-12 fibers. Furthermore, the aramid nanofibers facilitate the coating of graphene oxide onto the aramid F-12 fiber surface, resulting in a stronger bonding between the aramid F-12 fibers and the graphene oxide surface, while simultaneously forming a rough, strong bonding interface. Hybrid modification with graphene oxide and aramid nanofibers can create a rough structure on the smooth surface of aramid F-12 fibers, increasing the number of locking sites with the rubber matrix, effectively improving the inert interfacial microstructure of aramid F-12 fibers, and increasing their interfacial bonding strength with the rubber matrix. The properties of the modified aramid F-12 fiber composite material with EPDM rubber are also demonstrated. Tests have demonstrated that the GO / ANF hybrid modified layer can better form a mechanical interlocking structure between F-12 fiber and EPDM rubber and achieve better stress transfer in the interface region, further improving the interfacial adhesion between F-12 fiber and EPDM rubber. This results in a larger specific surface area and roughness at the contact surface between F-12 fiber and EPDM rubber, increasing the mechanical interlocking effect between F-12 fiber and EPDM, forming a robust interface layer, and improving the stress transfer efficiency and tensile strength of the composite material. At the same time, GO and ANF synergistically enhance the ablation resistance, significantly improving the ablation resistance of the EPDM / F-12 / GO / ANF composite material.

Claims

1. A modified chopped fiber reinforced flexible polymer thermal protection material, characterized in that, Its raw materials include modified chopped fibers and polymer matrix; The modified chopped fibers are prepared by the following method: (1) Dissolve polyethyleneimine in buffer solution, then stir and add tannic acid to obtain polyethyleneimine-tannic acid solution; then, immerse short-cut fibers in polyethyleneimine-tannic acid solution, stir, then take out short-cut fibers, wash and dry to obtain pre-modified short-cut fibers; (2) Dilute the graphene oxide dispersion with water to obtain a graphene oxide suspension. Then add aramid nanofibers to the graphene oxide suspension and stir to obtain a graphene oxide / aramid nanofiber dispersion. (3) Immerse the pre-modified short-cut fibers in a dispersion of graphene oxide / aramid nanofibers, then remove and dry them to obtain the modified short-cut fibers. The mass ratio of graphene oxide to aramid nanofibers is (1~5):(20~100).

2. The modified short-cut fiber reinforced flexible polymer thermal protection material according to claim 1, characterized in that, The chopped fibers are selected from aramid fibers, polyimide fibers, carbon fibers, and quartz fibers.

3. The modified short-cut fiber reinforced flexible polymer thermal protection material according to claim 2, characterized in that, The aramid fiber is aramid F-12 fiber.

4. The modified short-cut fiber reinforced flexible polymer thermal protection material according to claim 1, characterized in that, Its raw materials also include at least one of the following components: activator, softener, reinforcing agent, accelerator, and curing agent.

5. The modified chopped fiber reinforced flexible polymer thermal protection material according to claim 4, characterized in that, It comprises the following raw materials in parts by weight: 10-30 parts modified chopped fibers, 90-150 parts polymer matrix, 0.1-20 parts activator, 0.1-20 parts softener, 0.1-20 parts reinforcing agent, 0.1-20 parts accelerator, and 0.1-20 parts curing agent.

6. The modified chopped fiber reinforced flexible polymer thermal protection material according to claim 5, characterized in that: The polymer matrix is ​​at least one of EPDM rubber, silicone rubber, thermoplastic vulcanizate, and propylene rubber; the activator is at least one of zinc oxide, magnesium oxide, zinc carbonate, and zinc dodecyl sulfate; the softener is at least one of stearic acid, zinc stearate, calcium stearate, and paraffin oil; the reinforcing agent is at least one of silica, carbon black, and calcium silicate; the accelerator is at least one of diphenylguanidine, tetramethylthiourea, and dibenzothiazole disulfide; and the curing agent is at least one of sulfur, dicumyl peroxide, benzoyl peroxide, and tert-butyl benzoate peroxide.

7. A method for preparing the modified chopped fiber reinforced flexible polymer thermal protection material according to any one of claims 1 to 6, characterized in that, Includes the following steps: (1) According to the weight proportions of each raw material, first add the polymer matrix to the open mill, and after it wraps around the rolls, add the remaining raw materials in sequence; (2) After the raw materials are fully mixed, a thin-pass treatment is carried out; then the roller gap is adjusted and the sheet is cut to obtain the compound rubber; (3) The compound is vulcanized to obtain the modified short-cut fiber reinforced polymer matrix composite material.

8. The application of the modified chopped fiber reinforced flexible polymer thermal protection material according to any one of claims 1 to 6 in the aerospace field.

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