Carbon fiber reinforced polypropylene for unmanned aerial vehicle and preparation method thereof

Abstract

The invention belongs to the technical field of polymer composite materials, and particularly relates to carbon fiber reinforced polypropylene for an unmanned aerial vehicle and a preparation method of the carbon fiber reinforced polypropylene. The invention aims to solve the technical problems of weak interface bonding between carbon fibers and a polypropylene matrix and single material performance distribution. The composite material is prepared from polypropylene resin, carbon fibers, functionalized foaming microspheres, an antioxidant and a processing aid, the surface of the carbon fiber is pretreated through a multistage gradient interface agent, a coating of a core-shell-arm structure is formed, and the interface bonding force and toughness of the fiber and a matrix are improved; the interior of the composite material is of a bionic sandwich structure, and the composite material comprises a carbon fiber reinforced compact outer layer and a light core layer formed by in-situ expansion of foaming microspheres. Carbon fibers are soaked in a solution containing a multistage gradient interface agent to be dried, so that a nano coating is formed on the surfaces of the fibers; a blank is obtained through double-channel partition co-extrusion; and placing in a mold for hot and cold mold pressing, so that the foaming microspheres are expanded by heating to form a foam core layer structure in the material.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite materials technology, specifically relating to a carbon fiber reinforced polypropylene for unmanned aerial vehicles and its preparation method. Background Technology

[0002] Due to the high requirements for flight load and flight time, unmanned aerial vehicles (UAVs) widely utilize composite materials in their fuselage, wings, and other structures to reduce weight and improve specific strength and specific stiffness. Currently, advanced composite materials, primarily carbon fiber composites, account for 60% to 80% of the structure of UAVs worldwide. In particular, carbon fiber reinforced resin matrix composites possess excellent properties such as low density, high specific strength and modulus, and good fatigue performance, and have been widely used in aerospace structures. The fuselage shell and wing skin of UAVs often employ sandwich structures made of carbon fiber combined with honeycomb or foam core materials to achieve high stiffness and lightweighting. For example, sandwich structures composed of carbon fiber sheets and foam materials are widely used in UAV main beams. However, traditional carbon fiber composites are mostly prepared using thermosetting resin matrices combined with honeycomb core materials, which suffers from high cost, long molding cycles, and difficult maintenance. In contrast, thermoplastic resins such as polypropylene are inexpensive, have short processing cycles, and are recyclable, making them promising lightweight materials for UAVs. However, polypropylene is a non-polar crystalline polymer with low mechanical strength and is prone to creep, often making it difficult to meet requirements when used directly in structural components. Existing patent CN102675734A discloses a method for preparing glass fiber reinforced polypropylene microfoam material. This method involves adding supercritical fluid during injection molding to prepare glass fiber reinforced microporous foamed polypropylene, improving the material's strength and stiffness. Although this glass fiber reinforced microfoam material offers some performance improvement compared to unreinforced materials, it still falls short of fully meeting the requirements of high strength and lightweight for load-bearing structures such as drone fuselages. In existing technologies, most fiber-reinforced polypropylene microfoam materials have uniformly mixed components, resulting in a singular distribution of material properties, making it impossible to simultaneously achieve targeted optimization of both localized high strength and overall weight reduction.

[0003] In summary, the existing technical problems are: the weak interfacial bonding between carbon fiber and polypropylene matrix affects the strength and impact toughness of composite materials; and the lack of a performance gradient structure inside the composite material makes it difficult to simultaneously meet the requirements of high stiffness and lightweight.

[0004] To address the aforementioned issues, it is necessary to develop a novel carbon fiber reinforced polypropylene composite material and process to meet the requirements of high specific strength and lightweight for UAV structural components. Summary of the Invention

[0005] The purpose of this invention is to provide a carbon fiber reinforced polypropylene (CFRP) composite material for unmanned aerial vehicles (UAVs) and its preparation method, overcoming the shortcomings of weak interfacial bonding and uniform material property distribution in existing technologies. In this invention, by constructing a multi-level gradient interface and a biomimetic sandwich structure, the CFRP composite material achieves excellent mechanical properties and toughness while maintaining low density, meeting the requirements of lightweight and high-strength UAV structural components.

[0006] A carbon fiber reinforced polypropylene for unmanned aerial vehicles and its preparation method are described below:

[0007] S1: Under dry nitrogen protection, polypropylene glycol, 2,2-dimethylolpropionic acid, and acetone were mixed and stirred while heating. A mixture of isophorone diisocyanate and catalyst was added dropwise to obtain a prepolymer. The reaction system was cooled to 70°C, and a mixed solution of styrene and maleic anhydride dissolved in acetone was added to the prepolymer. An initiator was added and the mixture was stirred to react. The reaction system was cooled, triethylamine was added, and the mixture was stirred and sheared. Deionized water was added to obtain an aqueous dispersion. The dispersion was distilled under reduced pressure at 45°C to obtain a dispersion with a solid content of 30%. The dispersion was mixed with 20 parts of maleic anhydride-grafted polypropylene emulsion at room temperature and stirred at 300-500 rpm for 1-2 hours to obtain a multi-level gradient interface agent. S2: T700 grade short carbon fiber is immersed in a multi-level gradient interface agent with a solid content of 5%, and after being taken out, excess liquid is removed by extrusion rollers. The sizing rate on the fiber surface is controlled at 1%, and vacuum drying is performed to obtain carbon fiber with an interface agent coating on the surface. S3: Take 70-80 parts of polypropylene resin, 18-22 parts of carbon fiber with an interface agent coating on the surface, 0.4-0.6 parts of antioxidant, and 0.4-0.5 parts of zinc stearate and mix them to obtain reinforcing component A; take 20-30 parts of polypropylene resin and 3-5 parts of functionalized foamed microspheres and mix them to obtain foaming component B; then co-extrude and knead component A and component B in separate areas to form a preform. S4: The above blank is subjected to hot and cold molding foaming to prepare carbon fiber reinforced polypropylene composite board.

[0008] Further, the prepolymer described in step S1 is specifically prepared by mixing and stirring 35-40 parts of polypropylene glycol, 4-5 parts of 2,2-dimethylolpropionic acid and 40-50 parts of acetone under dry nitrogen protection and heating to 60°C, then adding dropwise a mixture of 30-35 parts of isophorone diisocyanate and 0.03-0.05 parts of the catalyst dibutyltin dilaurate, and controlling the temperature at 75-80°C for 2-3 hours to obtain the prepolymer.

[0009] Further, the aqueous dispersion described in step S1 is specifically prepared by adding 4-6 parts of styrene and 4-5 parts of maleic anhydride dissolved in 15-25 parts of acetone to the prepolymer, adding 0.2-0.3 parts of initiator azobisisobutyronitrile and stirring until homogeneous, and reacting at a constant temperature for 4 hours; cooling the reaction system to 40°C, adding 3-4 parts of triethylamine, stirring for 30 minutes, and adding 150 parts of deionized water under high-speed shear at 2000 rpm to obtain the aqueous dispersion.

[0010] Furthermore, the vacuum drying described in step S2 specifically involves drying at 70–80°C in a vacuum oven for 5–6 hours.

[0011] Furthermore, the antioxidant mentioned in step S3 is specifically a mixture of hindered phenolic antioxidant 1010 and phosphite antioxidant 168 in a mass ratio of 1:1.

[0012] Furthermore, the functionalized foamed microspheres described in step S3 are specifically obtained by mixing high-temperature foamable microspheres with a maleic anhydride-grafted polypropylene compatibilizer with a grafting rate of 1% at a weight ratio of 9:1 at 160°C for 10 minutes.

[0013] Furthermore, in step S3, the partitioned co-extrusion mixing specifically involves component A entering channel one, with the temperatures of each section set to 180°C, 180°C, 182°C, and 183°C, and component B entering channel two, with the temperatures of each section set to 170°C, 173°C, 175°C, and 178°C. The two material streams converge within a specially designed co-extrusion die.

[0014] Further, the hot and cold molding foaming process described in step S4 specifically involves first placing the material in the cavity of a mold preheated to 205°C, closing the mold and applying a pressure of 0.8 MPa, holding the pressure for 90 seconds, then using a vacuum suction cup with a heat insulation pad to transfer the hot blank to the closed and pre-cooled mold, immediately closing the mold and applying a pressure of 0.5 MPa, while simultaneously starting the mold cooling circulation system, holding the pressure and cooling for 120 seconds until the mold temperature drops to 60°C, and then releasing the pressure.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention constructs a chemically bonded transition layer on the surface of carbon fiber by using a multi-level gradient interface agent, which greatly improves the bonding strength between the fiber and the matrix, avoids fiber pull-out damage, and improves the interface strength of the composite material; at the same time, the polyurethane shell absorbs impact energy, making the interface area more tough, and the fracture toughness and impact resistance of the material are significantly improved.

[0016] (2) The present invention forms a sandwich structure of dense layer-foam layer inside the material through partitioned co-extrusion and foaming; carbon fiber is mainly distributed in the dense outer layer and bears the main load, while the foam core layer reduces weight and provides a certain thickness to improve bending stiffness; this structural design greatly improves the specific strength and specific stiffness of the material, meeting the application requirements of UAVs and other applications that emphasize both lightweight and high strength.

[0017] (3) The method of the present invention cleverly combines extrusion and molding processes. Compared with the traditional method of first composite fiber and then foaming or bonding sandwich, the process is simplified and the efficiency is high. Dual-channel co-extrusion ensures that the fiber and foaming agent are in their proper positions. The foaming process is completed in the mold, and the product is formed in one step without the need for subsequent assembly of sandwich structure. The whole process is easy to realize continuous production and scale up, and has good industrial application prospects. Attached Figure Description

[0018] Figure 1 This describes a manufacturing process for carbon fiber reinforced polypropylene used in unmanned aerial vehicles (UAVs).

[0019] Figure 2 The TEM image is from Experiment Example 1.

[0020] Figure 3 The image shown is a SEM image from Experiment Example 2. Detailed Implementation

[0021] The following embodiments further explain and illustrate the technical solutions of the present invention. It is particularly noted that each specific embodiment is a specific interpretation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention. Figure 1 The diagram shows a manufacturing process for carbon fiber reinforced polypropylene used in drones. The detailed manufacturing steps are as follows: 1. Synthesis of multi-level gradient interfacial agents In a reactor equipped with a reflux condenser, mechanical stirring, and nitrogen protection, a flexible polyurethane backbone is formed through the chain extension reaction of isophorone diisocyanate and polypropylene glycol. Simultaneously, 2,2-dimethylolpropionic acid is introduced as a hydrophilic chain extender, covalently introducing carboxyl groups into the polyurethane segments. These polyurethane segments exhibit excellent flexibility and adhesion, serving as a basis for future coatings and alleviating interfacial stress in the composite material under stress. The carboxyl groups on 2,2-dimethylolpropionic acid provide active sites for subsequent neutralization, emulsification, and ionic bonding. In an acetone solution of the polyurethane prepolymer, a copolymerization reaction of styrene and maleic anhydride is initiated by a free radical initiator. Free radicals may abstract hydrogen atoms from the polyurethane chains, forming active sites on the polyurethane backbone, thereby grafting SMA side chains. SMA polymerizes around the polyurethane chains, forming a physically entangled interpenetrating network structure. The anhydride ring of maleic anhydride is a highly active functional group, serving as a chemical anchoring point for subsequent chemical reactions with hydroxyl or amino groups on the carbon fiber surface. This forms the basis for chemical bonding, with a strength far exceeding that of physical adsorption. The rigid polystyrene segments combine with the flexible polyurethane segments to form a core-shell or network structure that combines rigidity and flexibility. This structure effectively transfers stress and absorbs energy at the interface layer. Then, triethylamine neutralizes the carboxyl groups on 2,2-dimethylolpropionic acid to generate a carboxylate, which greatly enhances the hydrophilicity of the molecules. Furthermore, the anionic charge stabilizes the emulsion particles due to electrostatic repulsion, preventing precipitation. Finally, the maleic anhydride-grafted polypropylene emulsion is chemically identical to the final polypropylene matrix resin, exhibiting excellent compatibility. The entire process involves solution polymerization followed by phase inversion emulsification to prepare a multi-level gradient interface agent. First, a polyurethane prepolymer with hydrophilic side groups is synthesized in acetone solvent, and styrene-maleic anhydride segments are grafted in situ. After neutralization and salt formation, water is added to undergo a phase inversion and form nanomicelles under shear force. After solvent removal, a core-shell-arm structured dispersion is obtained. This process effectively avoids the side reactions of isocyanate and water and the hydrolysis of anhydride groups, ensuring the functional integrity of the interface agent.

[0022] 2. Carbon fiber pretreatment T700 grade short-cut carbon fibers were mixed and soaked with a multi-gradient interface agent. The multi-gradient interface agent was diluted with deionized water to a solid content of 5%. After being removed, excess liquid was removed by extrusion rollers, and the sizing rate on the fiber surface was controlled to be 1%. The fibers were then dried in a vacuum oven to obtain carbon fibers with an interface agent coating on the surface. Through the interface agent treatment, a nanoscale functional coating is formed on the surface of the carbon fibers, realizing a gradient transition from rigidity to flexibility to thermoplastic compatibility on the fiber surface, effectively solving the problems of poor interfacial adhesion and brittleness between carbon fibers and polypropylene.

[0023] 3. Partitioned co-extrusion and melt composite A biomimetic sandwich structure is formed by melt blending using a dual-channel zoned co-extrusion technology. In the first channel, polypropylene matrix resin is mixed with carbon fibers pretreated with the aforementioned interface agent, along with appropriate amounts of antioxidants and processing aids, and then melt-blended together. The interface-treated carbon fibers are more uniformly dispersed in the polypropylene melt and tightly bonded to the matrix during blending, making them less prone to agglomeration or breakage. The addition of antioxidants prevents degradation and oxidation of polypropylene during high-temperature melting and subsequent foaming. Processing aids reduce melt viscosity and improve the uniformity of carbon fiber dispersion in the melt. In the second channel, polypropylene matrix resin is mixed and melt-blended with functionalized foamed microspheres. These foamed microspheres are modified expandable microsphere particles containing volatile substances as foaming agents, and the outer shell material can be a thermoplastic polymer. By functionalizing the surface of the microspheres, the wettability and dispersibility of the microspheres in the polypropylene melt can be improved, preventing agglomeration and interfacial debinding. Two streams of molten material merge at the extruder die head to form a composite preform. Due to the design of the channels being isolated before merging, and by controlling the temperature of component B channel to be lower than that of component A channel, and by including lubricant in component A, the difference in melt viscosity is appropriately controlled. Therefore, upon merging, the two melt streams adhere but do not completely mix, forming a melt cross-section with a preliminary partitioned layered structure: one side is rich in carbon fibers, and the other side is rich in foamed microspheres. This step achieves a macroscopic layered arrangement of the material: carbon fibers are mainly confined to one layer of the preform, while foamed microspheres are concentrated on the other side, laying the foundation for subsequent compression molding to form a sandwich structure.

[0024] 4. Hot and cold molding foaming The above-mentioned preform is placed in a preheated mold for rapid hot-cold molding foaming. The prepared preform is evenly spread in the cavity of the mold, which has been preheated to 205°C. The mold is then quickly closed, and a pressure of 0.8 MPa is immediately applied and held for 90 seconds. This aims to rapidly transfer heat to the core of the preform, allowing the functionalized foamed microspheres to quickly and synchronously reach the optimal foaming temperature range, inducing uniform expansion. This pressure value is carefully designed; too low a pressure results in low melt viscosity, which may lead to excessive cell coalescence or gas escape. The pressure is slightly higher than atmospheric pressure to limit the excessive expansion of cells in the initial stage. The process involves disordered growth, guiding the formation of uniform cell nuclei while allowing for moderate expansion to create a preliminary cell structure. Immediately after completion, the mold is opened, and a transfer tool is used to quickly transfer the pre-expanded, softened, but largely shape-preserved hot billet to a closed and pre-cooled mold. The mold is then immediately closed, and a pressure of 0.5 MPa is rapidly applied. Simultaneously, the mold cooling circulation system is activated, maintaining pressure and cooling for 120 seconds until the mold temperature decreases. Under this pressure, uniform cells are obtained, resulting in a low-density product that minimizes material weight while ensuring sufficient mechanical properties.

[0025] Example 1 Table 1 Raw Material Information Table

[0026] A carbon fiber reinforced polypropylene for unmanned aerial vehicles and its preparation method are disclosed, and the preparation steps are as follows: S1: Under dry nitrogen protection, 38 parts of polypropylene glycol, 4.5 parts of 2,2-dimethylolpropionic acid, and 45 parts of acetone were mixed and stirred, and the mixture was heated to 60°C. A mixture of 32 parts of isophorone diisocyanate and 0.04 parts of dibutyltin dilaurate catalyst was added dropwise, and the reaction was carried out at 78°C for 2.5 hours to obtain a prepolymer. The reaction system was cooled to 70°C, and then a mixture of 5 parts of styrene and 4.5 parts of maleic anhydride dissolved in 20 parts of acetone was added to the prepolymer. 0. 25 parts of initiator azobisisobutyronitrile were stirred and mixed evenly, and the reaction was kept at the temperature for 4 hours. The reaction system was cooled to 40°C, 3.5 parts of triethylamine were added, and the mixture was stirred for 30 minutes. Under high-speed shear at 2000 rpm, 150 parts of deionized water were added to obtain an aqueous dispersion. The dispersion was distilled under reduced pressure at 45°C to obtain a dispersion with a solid content of 30%. The dispersion was mixed with 20 parts of maleic anhydride-grafted polypropylene emulsion at room temperature and stirred at 400 rpm for 1.5 hours to obtain a multi-gradient interface agent. S2: T700 grade short carbon fiber is immersed in a multi-level gradient interface agent with a solid content of 5%. After being taken out, excess liquid is removed by extrusion rollers. The sizing rate on the fiber surface is controlled at 1%. The fiber is dried in a vacuum oven at 75°C for 5.5 hours to obtain carbon fiber with an interface agent coating on the surface. S3: 75 parts of polypropylene resin, 20 parts of carbon fiber with an interface agent coating, 0.5 parts of antioxidant, and 0.45 parts of zinc stearate are mixed to obtain reinforcing component A. The antioxidant is a compound of hindered phenolic antioxidant 1010 and phosphite antioxidant 168 in a mass ratio of 1:1. 25 parts of polypropylene resin and 4 parts of functionalized foamed microspheres are mixed to obtain foaming component B. The functionalized foamed microspheres are obtained by mixing high-temperature foamed microspheres with a 1% maleic anhydride-grafted polypropylene compatibilizer at a weight ratio of 9:1 at 160°C for 10 minutes. The mixture is then co-extruded in sections. Component A enters channel one, with temperatures set at 180°C, 180°C, 182°C, and 183°C. Component B enters channel two, with temperatures set at 170°C, 173°C, 175°C, and 178°C. The two streams converge in a specially designed co-extrusion die to form a billet. S4: Place the above blank into the cavity of the mold that has been preheated to 205°C, close the mold and apply a pressure of 0.8MPa, hold the pressure for 90 seconds, then use a vacuum suction cup with a heat insulation pad to transfer the hot blank to the mold that has been closed and pre-cooled. Immediately close the mold and apply a pressure of 0.5MPa, while simultaneously starting the mold cooling circulation system, hold the pressure and cool for 120 seconds until the mold temperature drops to 60°C, release the pressure, and obtain the carbon fiber reinforced polypropylene composite sheet.

[0027] Example 2 The preparation method is the same as in Example 1, but with the following differences: In step S1: 35 parts polypropylene glycol, 4 parts 2,2-dimethylolpropionic acid and 40 parts acetone; 30 parts isophorone diisocyanate and 0.03 parts catalyst; the reaction is carried out at 75°C for 3 hours; 4 parts styrene and 4 parts maleic anhydride are dissolved in 25 parts acetone and mixed with 0.2 parts initiator azobisisobutyronitrile; 3 parts triethylamine are added; the mixture is stirred at 300 rpm for 2 hours. In step S2: Dry at 70°C for 6 hours in a vacuum oven; In step S3: 70 parts polypropylene resin, 18 parts carbon fiber with an interface agent coating on the surface, 0.4 parts antioxidant, 0.4 parts zinc stearate; 30 parts polypropylene resin and 3 parts functionalized foamed microspheres; Example 3 The preparation method is the same as in Example 1, but with the following differences: In step S1: 40 parts polypropylene glycol, 5 parts 2,2-dimethylolpropionic acid and 50 parts acetone; 35 parts isophorone diisocyanate and 0.05 parts catalyst; the reaction is carried out at 80℃ for 2 hours; 6 parts styrene and 5 parts maleic anhydride are dissolved in 15 parts acetone and mixed with 0.3 parts initiator azobisisobutyronitrile; 4 parts triethylamine are added; the mixture is stirred at 500 rpm for 1 hour. In step S2: Dry at 80°C for 5 hours in a vacuum oven; In step S3: 80 parts polypropylene resin, 22 parts carbon fiber with an interface agent coating on the surface, 0.6 parts antioxidant, 0.5 parts zinc stearate; 20 parts polypropylene resin and 5 parts functionalized foamed microspheres; Comparative Example 1 The preparation method of Example 1 is followed, but without the multi-level gradient interface treatment, i.e., steps S1 and S2 are omitted. In step S3, 5 parts of maleic anhydride-grafted polypropylene compatibilizer are added to reinforcing component A. The remaining steps are the same.

[0028] Comparative Example 2 Referring to the component dosages of Example 1, but instead of using dual-channel zoned co-extrusion, polypropylene, interface-treated carbon fiber, foamed microspheres, and additives are mixed, extruded, and granulated in a twin-screw extruder in a single step, followed by injection molding. The remaining steps are the same.

[0029] Experimental Example 1 The carbon fibers with an interface agent coating prepared in step S2 of Example 1 were subjected to field emission transmission electron scanning (PES) testing. The samples were cut using focused ion beam microscopy; the slice thickness was <100 nm, and the cutting was perpendicular to the carbon fiber axis; ruthenium tetroxide vapor was used as the staining agent, and the staining time was 20 minutes; the accelerating voltage was set to 200 kV, and the imaging mode was bright field imaging. The test results are as follows: Figure 2 As shown; The core region, composed of T700 grade carbon fiber monofilaments, is a large circular black area located in the center of the image. Due to the high density and graphitization of the carbon fiber itself, it strongly scatters the electron beam, appearing as a uniform dark color in the bright-field image. The shell region, composed of polyurethane main chains and grafted styrene-maleic anhydride, is a dark ring tightly wrapped around the carbon fiber core, with uniform thickness. This is the core coating layer in the core-shell structure. The dye reacted strongly with the benzene rings in the styrene-maleic anhydride and the polar groups in the polyurethane, resulting in a significant increase in electron density in this region. The dark band, second only to carbon fiber, is enhanced by the chemical bonding between maleic anhydride and the carbon fiber surface, thus adhering tightly to the fiber surface. The arm-shaped regions are composed of polypropylene segments from the outer layer of the interface agent. The light-colored areas, resembling whiskers, flocculents, or clouds, extend outward from the dark shell and gradually blend into the background. Since the PP segments are mainly composed of saturated hydrocarbons, they do not react significantly with the dye, resulting in extremely light staining. This plays a role in physical entanglement at the macromolecular level, and the chemical structure is completely consistent with the subsequent composite PP matrix, ensuring excellent compatibility.

[0030] Experiment Example 2 The final product prepared in Example 1 was characterized by SEM using a field emission scanning electron microscope (FESEM) with an accelerating voltage of 5.0 kV-10.0 kV and a secondary electron detector. The product was cut into 10 mm × 10 mm strips and immersed in liquid nitrogen for approximately 15-20 minutes to completely embrittle it. A 5-10 nm thick gold conductive film was sprayed onto the cross-sectional surface using an ion sputtering apparatus. A panoramic image of the cross-section was first taken at 50x, then magnified to 1000x-2000x, focusing on the dense skin layer and the foamed core layer to observe the fiber interface and the microstructure of the foam cells. The test results are as follows: Figure 3 As shown, it consists of three parts: the top is a panoramic cross-sectional view, the bottom left is a microstructure diagram of the core layer, and the bottom right is a diagram of the skin layer and interface. The panoramic cross-sectional image clearly shows a "sandwich" structure, with thin, dense solid layers on the top and bottom, and a thicker porous foamed layer in the middle, confirming the effectiveness of the partitioned co-extrusion process. Component A, rich in carbon fibers, forms a dense skin layer, while component B, rich in microspheres, forms a foamed core layer. The foamed core layer occupies most of the thickness, which not only significantly reduces the density of the board but also increases the bending stiffness by increasing the moment of inertia, meeting the design requirements of lightweight and high-strength UAVs. The microstructure diagram of the core layer shows a dense honeycomb structure, with cell morphology mainly consisting of regular spherical or ellipsoidal shapes and uniform pore size distribution. The cell walls are intact, without obvious through-holes or cracks, representing a typical independent closed-cell structure. This is attributed to the precise control of the rapid hot and cold process during compression molding and foaming. The functionalized microspheres achieved uniform expansion without any collapse, which helps maintain the compressive strength and thermal insulation performance of the material. The skin-interface bonding diagram shows randomly distributed short-cut carbon fibers within the dense PP matrix. The key observation was of the traces of fibers being pulled out of the matrix. The cross-section showed that the carbon fibers were mostly broken rather than pulled out intact over long distances, with relatively short exposed fiber lengths. Moreover, the fiber surface was not smooth and bare, but rather had a small amount of resin matrix debris attached to it. This directly proves that the prepared multi-level gradient interface agent played a crucial role. The polyurethane / styrene-maleic anhydride layer in the interface agent constructed good chemical bonds and physical entanglements between the carbon fibers and the PP matrix, effectively solving the problem of poor interfacial adhesion. The carbon fibers were uniformly dispersed in the skin layer without obvious agglomeration, indicating that the processing aids and interface modification during the co-extrusion process improved the wettability of the fibers in the melt.

[0031] Experimental Example 3 The comprehensive performance of the carbon fiber reinforced polypropylene for UAVs prepared in Examples 1-3 and Comparative Examples 1-2 was tested: Density test: Referencing standard GB / T 1033.1-2008 "Determination of density of non-foamed plastics - Part 1: Immersion method, liquid pyrometer method and titration method", the Archimedes displacement method was adopted and an anti-buoyancy frame was used for the test. Tensile strength test: Referring to standard GB / T 1040.2-2022 "Determination of tensile properties of plastics - Part 2: Test conditions for molded and extruded plastics", a universal testing machine was used to prepare type I dumbbell specimens and test the tensile strength. Bending strength test: The three-point bending method of a universal testing machine was used to test the bending properties of plastics in accordance with the standard GB / T 9341-2008 "Determination of bending properties of plastics". The span-to-thickness ratio was 16:1 and the bending rate was 2 mm / min. Impact strength test: Refer to standard GB / T 1843-2008 "Determination of impact strength of plastic cantilever beams" and use a cantilever beam impact testing machine to test notched specimens. The notch type is type A. The test results are shown in Table 2. Table 2 Comparison of comprehensive performance test results between Examples 1-3 and Comparative Examples 1-2

[0032] The results above show that the tensile strength, flexural strength, and impact strength of Comparative Example 1 are all much lower than those of the Example, proving that the multi-level gradient interface agent of the present invention is significantly better than traditional compatibilizers in improving the bonding of carbon fiber / PP interface. The gradient transition layer formed can more effectively transfer load and prevent the propagation of interfacial cracks, thereby giving the composite material better strength and toughness. The tensile strength, flexural strength, and impact strength of Comparative Example 2 are poor, and the density is high. This is because a carbon fiber-rich surface layer and a dedicated foam core layer were not formed. The stiffness and impact resistance of this homogeneous foamed composite material are poor. This proves the necessity of partitioned co-extrusion to establish a sandwich structure: the carbon fiber mainly bears the outer layer stress, and the foam core layer reduces weight and provides thickness to improve bending performance. The two work together.

Claims

1. A carbon fiber reinforced polypropylene for unmanned aerial vehicles, comprising a polypropylene matrix resin and carbon fibers, characterized in that, The structure consists of a dense outer layer of polypropylene reinforced with carbon fibers and a foam core layer formed by the expansion of functionalized foamed microspheres within the polypropylene matrix resin. The surface of the carbon fibers is coated with a nanoscale coating formed by a multi-level gradient interface agent. The nanoscale coating is composed of polyurethane segments, styrene-maleic anhydride copolymer segments, and polypropylene segments, which are bonded together by chemical bonding or physical entanglement. The functionalized foamed microspheres are high-temperature resistant expandable microspheres and have undergone surface modification treatment.

2. The carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 1, characterized in that, The carbon fiber reinforced polypropylene has the following raw materials and component amounts: 100 parts polypropylene resin, 18-22 parts carbon fiber with an interface agent coating on the surface, 0.4-0.6 parts antioxidant, 0.4-0.5 parts zinc stearate, and 3-5 parts functionalized foamed microspheres.

3. A method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to any one of claims 1-2, characterized in that, S1: Under dry nitrogen protection, polypropylene glycol, 2,2-dimethylolpropionic acid, and acetone were mixed and stirred while heating. A mixture of isophorone diisocyanate and catalyst was added dropwise to obtain a prepolymer. The reaction system was cooled to 70°C, and a mixed solution of styrene and maleic anhydride dissolved in acetone was added to the prepolymer. An initiator was added and the mixture was stirred to react. The reaction system was cooled, triethylamine was added, and the mixture was stirred and sheared. Deionized water was added to obtain an aqueous dispersion. The dispersion was distilled under reduced pressure at 45°C to obtain a dispersion with a solid content of 30%. The dispersion was mixed with 20 parts of maleic anhydride-grafted polypropylene emulsion at room temperature and stirred at 300-500 rpm for 1-2 hours to obtain a multi-level gradient interface agent. S2: T700 grade short carbon fiber is immersed in a multi-level gradient interface agent with a solid content of 5%, and after being taken out, excess liquid is removed by extrusion rollers. The sizing rate on the fiber surface is controlled at 1%, and vacuum drying is performed to obtain carbon fiber with an interface agent coating on the surface. S3: Take 70-80 parts of polypropylene resin, 18-22 parts of carbon fiber with an interface agent coating on the surface, 0.4-0.6 parts of antioxidant, and 0.4-0.5 parts of zinc stearate and mix them to obtain reinforcing component A; take 20-30 parts of polypropylene resin and 3-5 parts of functionalized foamed microspheres and mix them to obtain foaming component B; then co-extrude and knead component A and component B in separate areas to form a preform. S4: The above blank is subjected to hot and cold molding foaming to prepare carbon fiber reinforced polypropylene composite board.

4. The method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 3, characterized in that, The prepolymer described in step S1 is specifically prepared by mixing and stirring 35-40 parts of polypropylene glycol, 4-5 parts of 2,2-dimethylolpropionic acid and 40-50 parts of acetone under dry nitrogen protection and heating to 60°C, then adding dropwise a mixture of 30-35 parts of isophorone diisocyanate and 0.03-0.05 parts of the catalyst dibutyltin dilaurate, and controlling the temperature at 75-80°C for 2-3 hours to obtain the prepolymer.

5. The method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 3, characterized in that, The aqueous dispersion described in step S1 is prepared by adding 4-6 parts of styrene and 4-5 parts of maleic anhydride dissolved in 15-25 parts of acetone to the prepolymer, adding 0.2-0.3 parts of initiator azobisisobutyronitrile and stirring until homogeneous, and reacting at a constant temperature for 4 hours; cooling the reaction system to 40°C, adding 3-4 parts of triethylamine, stirring for 30 minutes, and adding 150 parts of deionized water under high-speed shear at 2000 rpm to obtain the aqueous dispersion.

6. The method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 3, characterized in that, The vacuum drying described in step S2 specifically involves drying at 70–80°C in a vacuum oven for 5–6 hours.

7. The method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 3, characterized in that, The antioxidant mentioned in step S3 is specifically a mixture of hindered phenolic antioxidant 1010 and phosphite antioxidant 168 in a mass ratio of 1:

1.

8. The method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 3, characterized in that, The functionalized foamed microspheres mentioned in step S3 are specifically obtained by mixing high-temperature foamable microspheres with a maleic anhydride-grafted polypropylene compatibilizer with a grafting rate of 1% at a weight ratio of 9:1 at 160°C for 10 minutes.

9. The method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 3, characterized in that, The partitioned co-extrusion mixing described in step S3 specifically involves component A entering channel one, with the temperatures of each section set to 180℃, 180℃, 182℃, and 183℃, and component B entering channel two, with the temperatures of each section set to 170℃, 173℃, 175℃, and 178℃. The two material streams converge in a specially designed co-extrusion die.

10. The method for preparing carbon fiber reinforced polypropylene for unmanned aerial vehicles according to claim 3, characterized in that, The hot and cold molding foaming process described in step S4 is as follows: First, the hot blank is placed in the cavity of a mold that has been preheated to 205°C. The mold is closed and a pressure of 0.8 MPa is applied. The pressure is held for 90 seconds. Then, a vacuum suction cup with a heat insulation pad is used to transfer the hot blank to the mold that has been closed and pre-cooled. The mold is closed immediately and a pressure of 0.5 MPa is applied. At the same time, the mold cooling circulation system is turned on and the pressure is held for 120 seconds until the mold temperature drops to 60°C. Then, the pressure is released.

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