Carbon fiber pp composite material, preparation method and application thereof

Abstract

This application provides a carbon fiber PP composite material, its preparation method, and its application. The composite material, through the synergistic effect of a polyamide pre-coating layer, a nano-cellulose crystal-assisted dispersion system, and a pressure-assisted in-situ chemical bonding process, is composed of polyamide-coated carbon fibers, functionalized MXene, carboxylated multi-walled carbon nanotubes, nano-cellulose crystals, compatibilizers, antioxidants, and polypropylene in a specific ratio. Combined with a designated solution impregnation pre-coating and spraying layer-by-layer assembly process, and utilizing the stress buffering effect of the PA flexible transition layer and the network skeleton effect of the nano-cellulose crystals, a multi-level gradient modulus interface layer with chemical bonding is constructed on the carbon fiber surface. This enables the composite material to simultaneously possess high strength, high toughness, and excellent electromagnetic shielding performance, extending the long-term stable service life of UAV cargo compartment structural components. It has the advantages of controllable preparation process, stable performance, and ease of promotion and implementation.

Description

Technical Field

[0001] This application belongs to the field of polymer composite materials technology, specifically relating to a carbon fiber PP composite material, its preparation method and its application. Background Technology

[0002] In existing technologies, the cargo compartment of a drone, as a core structural component ensuring transportation safety and flight stability, directly determines the drone's carrying efficiency, flight safety, and mission execution capabilities through its material properties. To improve the overall performance of a drone, it is necessary to minimize its own structural weight to significantly enhance its endurance, flight speed, and acceleration. Furthermore, the cargo compartment structure must possess excellent mechanical strength and impact resistance to protect the precision instruments or cargo loaded inside. Additionally, with the increasingly complex electromagnetic environment, the drone cargo compartment also needs superior electromagnetic shielding performance to prevent external electromagnetic interference from affecting the electronic equipment and flight control system inside the compartment during flight, thereby ensuring high precision and reliability of the flight trajectory.

[0003] Currently, material solutions for drone cargo compartments are mainly divided into two categories.

[0004] The first category consists of general-purpose plastics, represented by polypropylene (PP), and their chopped fiber reinforced composites. These materials are low in density, easy to process, and inexpensive, meeting the basic requirements for lightweight drones. However, their mechanical strength and modulus are relatively low, failing to provide sufficient support and impact protection for the cabin. While conventional chopped glass fiber or carbon fiber reinforced PP can improve rigidity and strength to some extent, the material's impact toughness decreases sharply with increasing fiber content, making it difficult to achieve a balance between structural strength and toughness. More importantly, these materials lack electromagnetic shielding capabilities and cannot cope with complex electromagnetic interference challenges.

[0005] The second category is continuous carbon fiber reinforced composite materials using thermosetting resins such as epoxy resin or vinyl ester resin as the matrix. These materials have extremely high specific strength and specific modulus, and significant lightweight advantages. However, this approach has unavoidable application shortcomings: firstly, the thermosetting matrix has a long molding cycle and is difficult to recycle and reuse; secondly, the impact damage tolerance of the composite material is not ideal, and internal delamination is easily generated after impact, and the damage is not visible to the naked eye, posing a hidden danger to flight safety; thirdly, the interfacial compatibility and dispersion uniformity between the conductive filler and the thermosetting resin are difficult to control precisely, often resulting in large performance dispersion and low yield.

[0006] Therefore, there is an urgent need to develop a new type of carbon fiber reinforced polypropylene composite material that can precisely control the carbon fiber-polypropylene interface structure, maximize the interface reinforcement efficiency, and at the same time possess the recyclability, high toughness, and high electromagnetic shielding effectiveness of thermoplastic materials, in order to systematically solve the above-mentioned technical bottlenecks and meet the application requirements of next-generation high-performance UAV cargo cabins. Summary of the Invention

[0007] To address the shortcomings of existing technologies, such as insufficient strength, toughness, and functionality of general-purpose polypropylene-based composite materials, non-recyclable thermosetting carbon fiber composite materials with poor impact resistance and delamination resistance, low compatibilization efficiency of existing PP and PA blending methods, and difficulty of PA acting on the carbon fiber interface, this application proposes a carbon fiber polypropylene composite material, its preparation method, and its application.

[0008] The technical solution adopted in this application is as follows: A carbon fiber PP composite material, by weight, is composed of the following components: 12-28 parts of pre-coated carbon fiber, 1-6 parts of functionalized MXene (transition metal carbide), 0.5-2 parts of carboxylated multi-walled carbon nanotubes, 0.5-2 parts of nanocellulose crystals, 3-10 parts of compatibilizer, 0.5-1 part of antioxidant, and 52-75 parts of polypropylene. The pre-coated carbon fiber includes a carbon fiber matrix and a polyamide pre-coating layer covering the outer surface of the carbon fiber matrix.

[0009] In some feasible embodiments, the polypropylene is homopolymer polypropylene granules with a melt flow rate of 10 g / 10 min (230 °C, 2.16 kg); the antioxidant is a mixture of 1010 and 168 in a mass ratio of 1:1.

[0010] In some feasible embodiments, the polyamide pre-coated carbon fiber is made of PA6 or PA66, the pre-coating layer is 50nm-500nm thick, and the polyamide pre-coating layer accounts for 2wt%-10wt% of the total mass of the polyamide pre-coated carbon fiber.

[0011] In some feasible embodiments, the functionalized MXene is Ti3C2T modified by co-crosslinking of polydopamine and polyethyleneimine. x MXene nanosheets.

[0012] In some feasible embodiments, the method for preparing the functionalized MXene includes the following steps: Step 101. Mix Ti3AlC2MAX phase, LiF and 9mol / L hydrochloric acid solution at a mass ratio of 1:(0.8-1.2):(18-25), stir and react at 35℃-45℃ for 40h-56h, centrifuge and wash until pH>6, and then ultrasonically exfoliate in an ice-water bath to obtain monolayer / few-layer MXene nanosheets; Step 102. Disperse the MXene nanosheets obtained in Step 101 in Tris-HCl buffer solution with pH 8.0-9.0, add dopamine and branched polyethyleneimine sequentially, and control the mass ratio of MXene nanosheets, dopamine and polyethyleneimine to be 1:(1.5-2.5):(0.8-1.2). Stir the reaction at 40℃ for 24 h, centrifuge, wash and dry to obtain functionalized MXene.

[0013] In some feasible embodiments, the cellulose nanocrystals have a diameter of 5nm-20nm and a length of 100nm-500nm.

[0014] The nanocellulose crystals are cellulose nanocrystals with sulfonic acid groups and hydroxyl groups on their surface, prepared by sulfuric acid hydrolysis.

[0015] In actual implementation, the preparation method of nanocellulose crystals is as follows: cotton linters or wood pulp are mixed with 64wt% sulfuric acid at a mass ratio of 1:10, stirred and hydrolyzed at 50℃ for 2h, 10 times the volume of deionized water is added to terminate the reaction, centrifuged and washed until pH neutral, and ultrasonically treated in an ice-water bath for 30min to obtain nanocellulose crystals with a diameter of 5nm-20nm and a length of 100nm-500nm.

[0016] In some feasible embodiments, the method for preparing the carboxylated multi-walled carbon nanotubes includes the following steps: Step 201. Disperse multi-walled carbon nanotubes in a mixed acid solution at a mass ratio of 1:(30-50), wherein the mixed acid is a mixture of concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1, and then reflux the reaction for 4-8 hours under the conditions of ultrasonic power of 150W-300W and temperature of 60-80℃. Step 202. After the reaction in step 201 is completed, the product is repeatedly centrifuged and washed with deionized water until the pH of the filtrate is neutral, and then vacuum dried at 80°C for 12 hours to obtain carboxylated multi-walled carbon nanotubes.

[0017] In actual implementation, the carboxylation rate of the carboxylated multi-walled carbon nanotubes was determined to be 1.5wt%-2.1wt% by acid-base titration; the outer diameter of the multi-walled carbon nanotubes was 8nm-15nm, the length was 10μm-50μm, and the purity was 96%-99%. In some feasible embodiments, the compatibilizer is obtained by compounding maleic anhydride-grafted polypropylene and epoxy group-grafted polypropylene in a mass ratio of (1-2):1.

[0018] To address the technical problems raised in this application, this application also provides a method for preparing carbon fiber PP composite material, comprising the following steps: Step 301. The carbon fiber fabric is ultrasonically cleaned in acetone at 60°C for 2 hours, dried, and then heat-treated in an air atmosphere at 400°C in a muffle furnace for 1 hour. Then it is immersed in a 15wt% NaOH solution for 30 minutes, washed with water until neutral, and dried to obtain surface-activated carbon fiber. Step 302. Dissolve PA6 or PA66 in formic acid or a formic acid / dichloromethane mixed solvent to prepare a polyamide solution with a mass fraction of 2wt%-8wt%; then immerse the activated carbon fibers obtained in step 301 into the polyamide solution and impregnate at 25℃-40℃ for 30-120 min. After removal, vacuum dry at 60℃-80℃ to constant weight to obtain polyamide pre-coated carbon fibers. In practice, operators control the thickness of the pre-coating layer to be 50nm-500nm by adjusting the solution concentration and impregnation time, and the mass of the pre-coating layer is 2%-10% of the carbon fiber mass. Step 303. In deionized water, add functionalized MXene, carboxylated multi-walled carbon nanotubes and nanocellulose crystals in sequence according to the preset ratio, and ultrasonically disperse in an ice-water bath at a power of 150W-300W for 15-30 minutes to obtain a uniform multi-component aqueous dispersion. In actual implementation, the concentration of nanocellulose crystals in the total dispersion is 0.5 mg / mL-2 mg / mL; Step 304. The dispersion obtained in step 303 is uniformly sprayed onto the surface of the polyamide pre-coated carbon fiber obtained in step 302 at a flow rate of 0.5-1.0 mL / min using a spraying system, and dried at 50℃-70℃; the spraying-drying cycle is repeated 2-5 times to obtain the carbon fiber precursor loaded with nano-functional fillers. Step 305. The carbon fiber precursor, polypropylene, compatibilizer and antioxidant obtained in step 304 are transferred to a twin-screw extruder according to a preset ratio. After blending and extrusion and granulation under CO2 atmosphere, pressure 5-12MPa and melt temperature 165-185℃, the carbon fiber PP composite material is obtained.

[0019] In some feasible embodiments, in step 305, the CO2 injection amount in the twin-screw extruder is 1wt%-3wt% of the total material mass, and the screw speed is 200rpm-350rpm.

[0020] To address the technical problems raised in this application, this application also provides a drone cargo compartment, wherein the structural components of the drone cargo compartment are made of the aforementioned carbon fiber PP composite material.

[0021] Compared with the prior art, this application has the following beneficial effects: This application provides a carbon fiber-PP composite material, its preparation method, and its application. The composite material is composed of polyamide-pre-coated carbon fibers, functionalized MXene, carboxylated multi-walled carbon nanotubes, nanocellulose crystals, compatibilizers, antioxidants, and polypropylene in a specific ratio. The components simultaneously incorporate a PA6 or PA66 nano-pre-coating layer, a nanocellulose crystal-assisted dispersion system, and a surface-modified MXene and carbon nanotube composite functional filler system. Combined with a pressurized CO2-assisted in-situ chemical bonding process, a multi-level gradient modulus interface layer with chemical bonding is constructed on the carbon fiber surface. Utilizing the flexible transition of the PA pre-coating layer, the network framework of the nanocellulose crystals, and the conductive properties of MXene and carbon nanotubes, the composite material forms a dense, multi-level interface reinforcement and electromagnetic shielding structure. During processing, a covalent bond interface is formed between the PA pre-coating layer, the nanofunctional filler, and the PP matrix, significantly enhancing the composite material's strength, toughness, electromagnetic shielding effectiveness, and interfacial bonding stability. This solves the problems of poor interfacial strength, difficulty in balancing reinforcement and toughness, and inefficient integration of electromagnetic shielding functions in traditional carbon fiber-reinforced polypropylene composites, as well as the challenges in the bonding between PP and PA. The low efficiency of the blending method for increasing capacity can be addressed by extending the long-term stable service life of UAV cargo compartment structural components. It offers advantages such as controllable manufacturing processes, stable performance, and ease of widespread implementation.

[0022] Specifically, this application forms a nanoscale PA pre-coating layer on the surface of carbon fiber by solution impregnation, which significantly improves the compatibilization efficiency of PA.

[0023] More specifically, the amide groups of the PA pre-coating layer form dense hydrogen bond anchors with the oxygen-containing functional groups on the carbon fiber surface. Furthermore, functionalized MXene and carbon nanotube functional networks dispersed with the assistance of nanocellulose crystals are combined on the PA pre-coating layer to construct a multi-level gradient modulus interface structure of carbon fiber-PA flexible layer-nanofunctional filler network-PP matrix. This achieves a gradual transition of stress transfer from rigid carbon fiber to flexible PP matrix and effectively dissipates impact energy.

[0024] During the mixing process, the maleic anhydride / epoxy groups grafted onto the compatibilizer react in situ with the amino / carboxyl groups at the end of the PA pre-coating layer and the hydroxyl groups on the surface of nanocellulose, further enhancing the stability of the interfacial bonding. Attached Figure Description

[0025] Figure 1 These are comparison diagrams of tensile strength of Examples 1-5 and Comparative Examples 1-8 of this application.

[0026] Figure 2 These are comparison diagrams of the notch impact strength of Examples 1-5 and Comparative Examples 1-8 of this application.

[0027] Figure 3 These are comparison diagrams of the electromagnetic shielding effectiveness of Examples 1-5 and Comparative Examples 1-8 of this application.

[0028] Figure 4 These are comparison diagrams of heat distortion temperatures for Examples 1-5 and Comparative Examples 1-8 of this application. Detailed Implementation

[0029] Combination Figures 1-4 Examples 1-5 and Comparative Examples 1-8 further illustrate the technical solutions proposed in this application.

[0030] Example 1 Weigh each component precisely according to the component table shown in Table 1.

[0031] [1] Carbon fiber pretreatment: general-purpose polyacrylonitrile-based carbon fiber twill fabric (tensile strength 3530MPa, modulus 230GPa, areal density 200g / m³) was prepared. 2 After being cut (model T300), it was ultrasonically cleaned in acetone at 60°C for 2 hours, then removed and dried. It was then placed in a muffle furnace and heat-treated at 400°C in air for 1 hour. After cooling, it was immersed in a 15wt% NaOH solution for 30 minutes, repeatedly washed with deionized water until neutral, and dried at 80°C to obtain surface-activated carbon fibers (CF hydroxyl or carboxyl groups).

[0032] [2] Preparation of PA6 pre-coating layer: PA6 (number average molecular weight Mn≈30,000) was dissolved in formic acid to prepare a PA6 / formic acid solution with a mass fraction of 5wt%. The activated carbon fiber fabric obtained in step [1] was completely immersed in the solution and impregnated at 30°C for 60 min. After removal, it was dried in a vacuum oven at 70°C to constant weight (about 12 h) to obtain PA6 pre-coated carbon fiber. The mass of PA6 pre-coating layer was 5.5wt% of the carbon fiber mass, and the average thickness of the pre-coating layer was about 180 nm.

[0033] [3] Preparation of functionalized MXene: Ti3AlC2MAX phase, LiF and 9mol / L HCl solution were mixed in a mass ratio of 1:1.1:22 and stirred at 40℃ for 55h. After washing and exfoliation, MXene (Ti3C2T) was obtained. xThe resulting MXene, PDA (dopamine), and PEI (branched polyethyleneimine, Mw=600Da) were dispersed in Tris-HCl buffer at a mass ratio of 1:2:1. The mixture was stirred at 40°C for 24 h. After centrifugation, washing, and drying, the PDA / PEI co-crosslinked modified functionalized MXene was obtained.

[0034] [4] Preparation of carboxylated multi-walled carbon nanotubes: Multi-walled carbon nanotubes were dispersed in a mixed acid solution at a mass ratio of 1:(30-50). The mixed acid was a mixture of concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1. The mixture was then refluxed at an ultrasonic power of 300W and a temperature of 75℃ for 6 hours. After the reaction was completed, the product was repeatedly centrifuged and washed with deionized water until the pH of the filtrate was neutral. Then, it was vacuum dried at 80℃ for 12 hours to obtain carboxylated multi-walled carbon nanotubes.

[0035] [5] Preparation of multi-component dispersion: Functionalized MXene, carboxylated multi-walled carbon nanotubes and nanocellulose crystals were weighed at a mass ratio of 5:2:3; then deionized water was added at a mass ratio of 1:1000 of total filler to water to obtain a dispersion with a total filler concentration of 0.1wt%; during ultrasonic dispersion, the ultrasonic power input per liter of dispersion was about 225W (ice-water bath), and the ultrasonic time was 40min. After ultrasonic dispersion was completed, a multi-component aqueous dispersion was obtained.

[0036] [6] Construction of a multi-level gradient modulus interface layer on the carbon fiber surface: The PA6 pre-coated carbon fiber fabric obtained in step [2] was fixed on a spraying platform. The dispersion from step [5] was sprayed onto the carbon fiber surface through an ultrasonic nozzle at a flow rate of 0.6 mL / min (50 mL-100 mL of dispersion was sprayed per square meter of carbon fiber fabric), and then dried in the infrared at 60 °C. The spraying-drying cycle was repeated 3 times to obtain a carbon fiber precursor loaded with a uniform functional filler network.

[0037] [7] The carbon fiber precursor, polypropylene granules, maleic anhydride-grafted polypropylene (MAH-g-PP), and epoxy group-grafted polypropylene (GMA-g-PP) obtained in step [6] in a 1:1 weight ratio, along with a compatibilizer and antioxidant, are fed into the main feed port of a twin-screw extruder according to the proportions in Table 1. The temperatures of each section of the extruder from the feed port to the die are set to 155℃, 175℃, 185℃, 185℃, and 180℃. CO2 is injected into the middle section of the melting zone at a pressure of 8MPa, with an injection amount of 2wt% of the total material mass. The screw speed is set to 280rpm. After melt blending, extrusion, water cooling, and pelletizing, the polyamide interface-reinforced carbon fiber polypropylene composite material granules described in this application are obtained.

[0038] 【8】Test specimen preparation: The granules obtained in step 【7】 are dried at 80℃ for 4 hours, and then prepared into dumbbell-shaped tensile specimens, notched impact specimens and electromagnetic shielding test discs conforming to ISO standards by injection molding machine. The barrel temperature of the injection molding machine is 185-205℃, and the mold temperature is 60℃.

[0039] Example 2 According to the component formulation shown in Table 1, the material of the PA6 pre-coating layer was replaced with PA66 (number average molecular weight Mn≈32,000), and the solvent was replaced with a mixture of formic acid and dichloromethane (volume ratio 1:1). The remaining components and processes were the same as in Example 1.

[0040] Example 3 According to the component formulation shown in Table 1, the content of the PA6 pre-coating layer was adjusted to 2.8 wt% of the carbon fiber mass (achieved by reducing the PA6 / formic acid solution concentration to 2.5 wt% and shortening the impregnation time to 40 min), and the polypropylene content was adjusted accordingly. The remaining components and processes were kept consistent with those in Example 1.

[0041] Example 4 According to the component formulation shown in Table 1, the content of the PA6 pre-coating layer was adjusted to 8.8 wt% of the carbon fiber mass (achieved by increasing the PA6 / formic acid solution concentration to 8 wt% and extending the impregnation time to 90 min), and the polypropylene content was adjusted accordingly. The remaining components and processes were kept consistent with those in Example 1.

[0042] Example 5 The difference between this embodiment and embodiment 1 is that CO2 is not injected in step [7], and twin-screw extrusion is performed only under normal atmospheric pressure.

[0043] Comparative Example 1 The difference between this comparative example and Example 1 is that the formulation does not contain any nanocellulose crystals at all, but sodium dodecyl sulfate (SDS), a surfactant of the same mass as the nanocellulose crystals in Example 1, is added to the dispersion in step [5] to disperse functionalized MXene and carbon nanotubes.

[0044] Comparative Example 2 The difference between this comparative example and Example 1 is that the solution impregnation method in step [2] and the spraying method in step [6] are not used. Instead, the PA6 granules are directly melt-blended with all components, including activated carbon fibers, functionalized MXene, carbon nanotubes, nanocellulose crystals, and PP, in step [7]. The amount of PA6 added is consistent with the mass of the PA6 pre-coating layer in Example 1.

[0045] Comparative Example 3 The difference between this comparative example and Example 1 is that there is no PA6 pre-coating layer in the formulation (step [2] is omitted), while the other components and processes are the same as in Example 1.

[0046] Comparative Example 4 The difference between this comparative example and Example 1 is that the formulation does not contain functionalized MXene.

[0047] Comparative Example 5 The difference between this comparative example and Example 1 is that the formulation does not contain carboxylated multi-walled carbon nanotubes.

[0048] Comparative Example 6 The difference between this comparative example and Example 1 is that the nanocellulose crystals are replaced with an equal amount of small molecule polyol dispersant.

[0049] Comparative Example 7 The difference between this comparative example and Example 1 is that the content of the PA6 pre-coating layer was adjusted to 1.2 wt% of the carbon fiber mass (achieved by using a 1 wt% PA6 / formic acid solution and an impregnation time of 25 min).

[0050] Comparative Example 8 Pure polypropylene samples were prepared using the same injection molding process.

[0051] Table 1. Component formulations (parts by weight) for the examples and comparative examples Table 2 Test Content Table 3 Test Results As shown in Table 3, in Examples 1-5, by pre-coating PA6 or PA66 onto the surface of carbon fibers in the form of nano-thin films, and combining this with in-situ chemical bonding assisted by nanocellulose crystal dispersion and CO2 pressurized mixing, a multi-level gradient modulus interface layer structure is introduced into the carbon fiber reinforced polypropylene matrix. During the melt composite process with the polypropylene matrix, the amide groups of the PA pre-coating layer, the surface hydroxyl groups of the nanocellulose crystals, the amino / hydroxyl groups of the functionalized MXene, and the maleic anhydride / epoxy groups of the compatibilizer simultaneously participate in the in-situ chemical bonding reaction, thus completing the construction of the multi-level interface structure. On the one hand, the PA6 or PA66 pre-coating layer, as the first-level modulus transition layer from rigid carbon fiber to flexible PP matrix, forms dense hydrogen bonds between its amide groups and the hydroxyl or carboxyl groups on the carbon fiber surface. Simultaneously, the terminal amino / carboxyl groups react efficiently with the compatibilizer under CO2 pressure, breaking through the bottleneck of the traditional incompatibility between strength and toughness in carbon fiber / polypropylene composites at the interface level. On the other hand, the rigid nanorod morphology of nanocellulose crystals constructs a second-level nanofiber network on the PA pre-coating layer, serving as a dispersion template and interfacial framework for functionalized MXene and carboxylated multi-walled carbon nanotubes. This allows the conductive filler to form a uniform, dense, multi-level overlapping conductive network on the carbon fiber surface, significantly extending the reflection and absorption path of electromagnetic waves at the interface layer. Furthermore, its high-density surface hydroxyl groups can form covalent bonds with the compatibilizer, significantly improving the bonding force between the interface layer and the matrix. When the composite material operates under the complex conditions of UAV flight, it can effectively resist structural stress caused by aerodynamic loads, accidental impact loads, and external electromagnetic interference, significantly improving the mechanical protection stability and electromagnetic protection durability of the cargo hold structure, thereby enhancing the long-term service performance of the UAV cargo hold. During actual flight, drones are less prone to structural deformation, impact cracking, and electromagnetic interference failure, effectively ensuring the safety of equipment and cargo inside the cargo hold.

[0052] Specifically, in the construction of the multi-level gradient modulus interface layer, a PA6 or PA66 pre-coating layer is uniformly coated on the surface of each carbon fiber through a solution impregnation process. Due to the hydrogen bonding between the amide groups of PA6 or PA66 and the oxygen-containing functional groups on the carbon fiber surface, and the similar polarity of the two, the PA layer can form a stable and continuous flexible interface layer on the carbon fiber surface, avoiding the interface defects caused by the low compatibility of PA / PP in the traditional matrix blending method. After reacting with the compatibilizer, a continuous modulus transition is formed from the carbon fiber to the PP matrix. The nanocellulose crystals are prepared by sulfuric acid hydrolysis and are rich in sulfonic acid groups and hydroxyl groups on the surface. They can form a stable colloidal dispersion system in water. Their amphiphilic surface can simultaneously adsorb hydrophobic MXene and carbon nanotubes, and intertwine with functionalized MXene and carbon nanotubes to form a continuous nanofunctional filler network, while providing sufficient active hydroxyl sites for the chemical bonding of the compatibilizer.

[0053] More specifically, the two-dimensional layered structure of functionalized MXene can provide an efficient electromagnetic wave reflection interface, the one-dimensional high aspect ratio structure of carboxylated multi-walled carbon nanotubes can bridge the layers to form long-range conductive pathways, the high-density hydrogen bonds of amide groups in the PA6 or PA66 pre-coating layer can effectively enhance the cohesion of the interface layer, the nanofiber network of nanocellulose crystals can effectively disperse stress and inhibit crack propagation, and a strongly anchored multi-level gradient modulus connection interface is formed between carbon fiber and polypropylene matrix, improving the stress transfer efficiency between fiber and matrix, avoiding interface debonding and fiber pull-out failure, thereby significantly improving the tensile strength, impact toughness, electromagnetic shielding effectiveness of composite materials, and the long-term service stability of UAV cargo compartment structure under complex working conditions.

[0054] Functionalized MXene was modified by co-crosslinking with polydopamine and polyethyleneimine. Firstly, dopamine was used to initially modify MXene nanosheets in a weakly alkaline Tris-HCl buffer system, forming a polydopamine layer on the MXene surface through self-polymerization, increasing the active binding sites. Secondly, branched polyethyleneimine was used for secondary crosslinking modification, effectively improving its interfacial compatibility with nanocellulose crystals and compatibilizers. This facilitates the prevention of excessive agglomeration during subsequent multi-component dispersion preparation and enhances the uniformity of MXene distribution within the nanofiber network framework. (A polyamide pre-coating process was not used.) In Comparative Example 1, no nanocellulose crystals were added, and the small molecule surfactant SDS was used instead, thus failing to introduce a nanofiber network framework structure onto the surface of PA6 pre-coated carbon fibers. Although SDS could temporarily disperse some of the nanofillers, it could not form a physically entangled network on the carbon fiber surface. On the one hand, during the subsequent CO2 pressurized mixing process, SDS was easily thermally decomposed and volatilized, leading to the disordered re-agglomeration of functionalized MXene and carboxylated multi-walled carbon nanotubes. The conductive filler was unevenly distributed at the interface layer, failing to form a continuous conductive path, and the electromagnetic shielding effectiveness dropped to -49.8 dB. On the other hand, the agglomerates became stress concentration points in the matrix, and cracks preferentially initiated and propagated around the agglomerates under stress, reducing the tensile strength and impact strength to 234.8 MPa and 13.2 kJ / m, respectively. 2 On the other hand, the rigid rod-shaped skeleton lacking nanocellulose crystals serves as a transition level for the interfacial modulus, causing a sharp increase in the modulus transition between the PA6 pre-coating layer and the PP matrix. This results in the inability to effectively transfer interfacial stress, significantly reducing the overall mechanical properties of the composite material.

[0055] In Comparative Example 2, excluding the polyamide pre-coating process and the precision spraying process for constructing the interface layer, the traditional matrix blending method was used to melt-blend all components in one step. PA6 was randomly distributed in the PP matrix in an island-like structure, unable to be precisely enriched on the carbon fiber surface. On the one hand, PA6 and PP are thermodynamically incompatible systems; the interfacial defects caused by phase separation make the matrix itself a mechanically weak area, with tensile strength and impact strength of only 217.3 MPa and 10.5 kJ / m, respectively. 2 The performance was even lower than that of Comparative Example 3 without PA, proving that inefficient blending compatibilization is actually detrimental to performance. Furthermore, the nanofunctional fillers are randomly distributed in the matrix and cannot form a continuous and dense conductive network on the carbon fiber surface, resulting in a significant drop in electromagnetic shielding effectiveness to -43.6dB. On the other hand, there is no PA flexible transition layer between the carbon fiber and the PP matrix, and the stress concentration caused by the modulus mismatch cannot be relieved, resulting in obvious brittle fracture characteristics of the composite material.

[0056] In Comparative Example 3, the PA6 or PA66 pre-coating layer was removed, and no flexible transition layer was pre-constructed on the carbon fiber surface. The rigid surface of the carbon fiber (modulus ≈ 230 GPa) was in direct contact with the flexible PP matrix (modulus ≈ 1.5 GPa), resulting in a significant modulus mismatch at the interface. On the one hand, under load, stress could not be effectively transferred from the matrix to the fiber, and the fiber's reinforcing potential was not fully realized, with the tensile strength dropping to 262.4 MPa. On the other hand, there were no chemical bonding anchors at the interface, relying only on the coupling effect of the compatibilizer with a small number of hydroxyl and carboxyl groups on the fiber surface. The interfacial bonding was weak, and the fiber quickly debonded from the matrix upon impact, resulting in limited energy dissipation and a notched impact strength dropping to 16.1 kJ / m. 2 On the other hand, the lack of a hydrogen bond network foundation in the PA pre-coating layer reduces the bonding strength between the functional filler network constructed with nanocellulose crystal assistance and the fiber surface. Some fillers migrate into the matrix during processing, weakening the integrity of the conductive network at the interface.

[0057] In Comparative Example 4, functionalized MXene was removed, and conductivity was provided solely by carboxylated multi-walled carbon nanotubes. Although carbon nanotubes can form a one-dimensional conductive pathway, their one-dimensional structure cannot provide sufficient electromagnetic wave reflection area in the interface layer. Electromagnetic waves are primarily transmitted within the material, resulting in a sharp drop in shielding effectiveness to -28.3 dB. Furthermore, the one-dimensional structure of carbon nanotubes alone cannot form an effective one-dimensional to two-dimensional complementary network with the nanocellulose crystals. The multiple interfacial polarization loss mechanism at the interface is missing, significantly reducing the efficiency of electromagnetic wave energy dissipation as heat. On the other hand, the two-dimensional layered structure of MXene can act as a bridge in the interface layer; its absence reduces the modulus transition level of the interface layer, causing a slight decrease in tensile strength and impact strength to 275.1 MPa and 20.8 kJ / m, respectively. 2 .

[0058] In Comparative Example 5, carboxylated multi-walled carbon nanotubes were removed, and the conductive network was constructed solely using functionalized MXene two-dimensional sheets. Due to the lack of one-dimensional carbon nanotubes acting as conductive bridges between the MXene sheets, the long-range conductive pathway continuity was insufficient, and the electromagnetic shielding effectiveness dropped to -48.1 dB. Furthermore, the high aspect ratio of carbon nanotubes effectively bridges the conductive gaps between adjacent MXene sheets and carbon fibers; their absence resulted in multiple breaks in the interfacial conductive network, reducing electron transport efficiency and causing electromagnetic wave leakage at the gaps. On the other hand, the nanofiber reinforcement effect of carbon nanotubes was absent, leading to a slight decrease in the overall rigidity and load-bearing capacity of the interfacial layer, with tensile strength and impact strength dropping to 268.7 MPa and 19.6 kJ / m, respectively. 2 .

[0059] In Comparative Example 6, the nanocellulose crystals were replaced with an equal amount of small-molecule polyol dispersant, without introducing a rigid nanofiber network framework structure. Although the small-molecule dispersant can temporarily disperse the nanofiller, it lacks the amphiphilic surface and rod-like rigid structure of the nanocellulose crystals. On the one hand, it cannot form a stable porous network on the PA6 pre-coating layer. On the other hand, the functional filler layer after spraying is prone to slippage and detachment during subsequent processing, resulting in poor interfacial integrity and continuity. The tensile strength and impact strength decreased to 241.5 MPa and 14.8 kJ / m, respectively. 2 Furthermore, small molecule dispersants cannot provide a gradient transition function for interfacial modulus, and the problem of interfacial stress concentration is not effectively solved, resulting in weak energy dissipation capacity of composite materials under impact. On the other hand, small molecule dispersants and compatibilizers cannot form chemical bonds, and the interfacial layer is maintained only by physical adsorption, resulting in poor long-term service stability.

[0060] In Comparative Example 7, the PA6 pre-coating content was reduced to 1.2 wt% of the carbon fiber mass. This excessively low PA coating amount failed to form a continuous and uniform flexible transition layer on the carbon fiber surface, resulting in exposed areas on the fiber surface, incomplete modulus transition, and ineffective mitigation of interfacial stress concentration. Consequently, the tensile strength and impact strength decreased to 267.8 MPa and 17.2 kJ / m, respectively. 2 Furthermore, the insufficient density of amide groups on the surface of the PA pre-coating layer reduces the number of hydrogen bond anchors between the nanocellulose crystals and their supported functional filler network and the fiber surface, resulting in decreased interfacial bonding strength. Some fillers also debond during processing, weakening interfacial reinforcement and conductivity.

[0061] In summary, this application provides a carbon fiber PP composite material, its preparation method, and its application. This composite material is composed of polyamide-pre-coated carbon fibers, functionalized MXene, carboxylated multi-walled carbon nanotubes, nanocellulose crystals, compatibilizers, antioxidants, and polypropylene in a specific ratio. The components simultaneously incorporate a PA6 or PA66 nano-pre-coating layer, a nanocellulose crystal-assisted dispersion system, and a surface-modified MXene and carbon nanotube composite functional filler system. Combined with a pressurized CO2-assisted in-situ chemical bonding process, a multi-level gradient modulus interface layer with chemical bonding is constructed on the carbon fiber surface. The flexibility of the PA pre-coating layer is utilized... The transition layer, the network framework of nanocellulose crystals, and the conductive properties of MXene and carbon nanotubes enable the composite material to form a dense, multi-level interface-reinforced and electromagnetic shielding structure. During processing, a covalent bond interface is formed between the PA pre-coating layer, the nanofunctional filler, and the PP matrix, significantly enhancing the composite material's strength, toughness, electromagnetic shielding effectiveness, and interfacial bonding stability. This solves the technical problems of poor interfacial strength, difficulty in balancing reinforcement and toughening, inefficient integration of electromagnetic shielding functions, and low compatibilization efficiency of PP and PA blending methods in traditional carbon fiber reinforced polypropylene composites. It can extend the long-term stable service life of UAV cargo compartment structural components. It has the advantages of controllable manufacturing process, stable performance, and ease of promotion and implementation.

[0062] The embodiments provided by the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A carbon fiber PP composite material, characterized in that, By weight, it consists of the following components: 12-28 parts of pre-coated carbon fiber, 1-6 parts of functionalized MXene, 0.5-2 parts of carboxylated multi-walled carbon nanotubes, 0.5-2 parts of nanocellulose crystals, 3-10 parts of compatibilizer, 0.5-1 part of antioxidant, and 52-75 parts of polypropylene; The pre-coated carbon fiber includes a carbon fiber matrix and a polyamide pre-coating layer covering the outer surface of the carbon fiber matrix.

2. The carbon fiber PP composite material according to claim 1, characterized in that, In the polyamide pre-coated carbon fiber, the polyamide pre-coating layer is made of PA6 or PA66, the pre-coating layer thickness is 50nm-500nm, and the polyamide pre-coating layer accounts for 2wt%-10wt% of the total mass of the polyamide pre-coated carbon fiber.

3. The carbon fiber PP composite material according to claim 1, characterized in that, The functionalized MXene is Ti3C2T modified by co-crosslinking of polydopamine and polyethyleneimine. x MXene nanosheets.

4. The carbon fiber PP composite material according to claim 1, characterized in that, The preparation method of the functionalized MXene includes the following steps: Step 101. Mix Ti3AlC2MAX phase, LiF and 9mol / L hydrochloric acid solution at a mass ratio of 1:(0.8-1.2):(18-25), stir and react at 35℃-45℃ for 40h-56h, centrifuge and wash until pH>6, and then exfoliate by ultrasonication in an ice-water bath to obtain MXene nanosheets; Step 102. Disperse the MXene nanosheets obtained in Step 101 in Tris-HCl buffer solution with pH 8.0-9.0, add dopamine and branched polyethyleneimine sequentially, and control the mass ratio of MXene nanosheets, dopamine and polyethyleneimine to be 1:(1.5-2.5):(0.8-1.2). Stir the reaction at 40℃ for 24 h, centrifuge, wash and dry to obtain functionalized MXene.

5. The carbon fiber PP composite material according to claim 1, characterized in that, The nanocellulose crystals have a diameter of 5nm-20nm and a length of 100nm-500nm.

6. The carbon fiber PP composite material according to claim 1, characterized in that, The preparation method of the carboxylated multi-walled carbon nanotubes includes the following steps: Step 201. Disperse multi-walled carbon nanotubes in a mixed acid solution at a mass ratio of 1:(30-50), wherein the mixed acid is a mixture of concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1, and then reflux the reaction for 4-8 hours under the conditions of ultrasonic power of 150W-300W and temperature of 60-80℃. Step 202. After the reaction in step 201 is completed, the product is repeatedly centrifuged and washed with deionized water until the pH of the filtrate is neutral, and then vacuum dried at 80°C for 12 hours to obtain carboxylated multi-walled carbon nanotubes.

7. The carbon fiber PP composite material according to claim 1, characterized in that, The compatibilizer is obtained by compounding maleic anhydride-grafted polypropylene and epoxy group-grafted polypropylene in a mass ratio of (1-2):

1.

8. A method for preparing a carbon fiber PP composite material according to any one of claims 1-7, characterized in that, Includes the following steps: Step 301. The carbon fiber fabric is ultrasonically cleaned in acetone at 60°C for 2 hours, dried, and then heat-treated in an air atmosphere at 400°C in a muffle furnace for 1 hour. Then it is immersed in a 15wt% NaOH solution for 30 minutes, washed with water until neutral, and dried to obtain surface-activated carbon fiber. Step 302. Dissolve PA6 or PA66 in formic acid or a formic acid / dichloromethane mixed solvent to prepare a polyamide solution with a mass fraction of 2wt%-8wt%; The activated carbon fibers obtained in step 301 are then immersed in the polyamide solution and soaked at 25℃-40℃ for 30-120 minutes. After being removed, they are vacuum dried at 60℃-80℃ to constant weight to obtain polyamide pre-coated carbon fibers. Step 303. In deionized water, add functionalized MXene, carboxylated multi-walled carbon nanotubes and nanocellulose crystals in sequence according to the preset ratio, and ultrasonically disperse in an ice-water bath at a power of 150W-300W for 15-30 minutes to obtain a uniform multi-component aqueous dispersion. Step 304. The dispersion obtained in step 303 is uniformly sprayed onto the surface of the polyamide pre-coated carbon fiber obtained in step 302 through a spraying system at a flow rate of 0.5-1.0 mL / min, and dried at 50℃-70℃ to obtain the carbon fiber precursor loaded with nano-functional fillers. Step 305. The carbon fiber precursor, polypropylene, compatibilizer and antioxidant obtained in step 304 are transferred to a twin-screw extruder according to a preset ratio. After blending and extrusion and granulation under CO2 atmosphere, pressure of 5MPa-12MPa and melt temperature of 165-185℃, the carbon fiber PP composite material is obtained.

9. The method for preparing a carbon fiber PP composite material according to claim 8, characterized in that, In step 305, in the twin-screw extruder, the CO2 injection amount is 1wt%-3wt% of the total material mass, and the screw speed is 200rpm-350rpm.

10. A cargo hold for an unmanned aerial vehicle (UAV), characterized in that, The structural components of the drone cargo compartment are made of a carbon fiber PP composite material as described in any one of claims 1-7.

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