Preparation method of natural fiber prepreg for improving impregnation effect

Abstract

The invention discloses a preparation method of a natural fiber prepreg with an improved impregnation effect, and relates to the technical field of composite material preparation, the method comprises the following steps: carrying out synergistic modification through fiber surface plasma activation and silane grafting, and combining a gradient pressure-temperature coupling impregnation system and ultrasonic-assisted permeation to prepare the natural fiber prepreg with the improved impregnation effect. And gradient filling and interface chemical bonding strengthening of the resin in the fiber bundle are realized. The core process comprises the following steps: pretreating fibers to improve surface energy and roughness, introducing a reactive diluent and a block copolymer into a resin system to regulate and control viscosity and transport capacity, constructing a pressure gradient by a multi-stage compression roller to drive resin to permeate into a bundle core, promoting uniform impregnation by infrared temperature control and ultrasonic cavitation, and finally obtaining uniform resin distribution and porosity lt. The interlayer shear strength is gt; and the high-performance prepreg is stable in batch under the pressure of 45 MPa. Reliable technical support and process guarantee are provided for large-scale engineering application of the high-performance natural fiber composite material.

Description

Technical Field

[0001] This invention relates to the field of composite material preparation technology, and in particular to a method for preparing natural fiber prepregs with improved impregnation effects. Background Technology

[0002] Natural fiber composites, as an important branch of green and sustainable materials systems, have been widely used in automotive interiors, architectural decoration, packaging and transportation, and consumer electronics in recent years. Their core advantages include renewable and biodegradable raw materials, low density, and moderate specific strength, aligning with the current strategic needs of industries transitioning towards low-carbon and ecological practices. However, the inherent porosity, hydrophilicity, high surface polarity, and heterogeneous structure of natural fibers lead to poor interfacial compatibility, difficulty in wetting, and low stress transfer efficiency when composited with hydrophobic resin matrices, severely limiting the performance of composite materials in terms of mechanical properties, dimensional stability, and durability. Therefore, improving the uniformity of resin wetting of fibers, interfacial bonding strength, and process stability during prepreg preparation is crucial for enhancing the performance of end products.

[0003] In existing technologies, to improve the interfacial bonding between natural fibers and resin matrices, the industry generally adopts a strategy combining physical mixing and chemical modification. For example, Chinese patent CN103571038B proposes a technical approach that involves directly mixing natural fibers into thermoplastic resin melt, supplemented by coupling agents and emulsions for interfacial modification. The initial design aims to reduce interfacial tension between the two phases through chemical bridging, thereby improving the tensile strength and storage stability of the prepreg on a macroscopic scale. This approach has indeed achieved a preliminary balance between process simplification and cost control under specific conditions. Its technological contribution lies in simultaneously introducing the interfacial modifier with the melt blending process, avoiding the efficiency losses caused by traditional multi-step pretreatment. However, in essence, this method still heavily relies on the passive wetting and random encapsulation of the resin melt on the fiber surface, lacking effective intervention in the microporous structure within the fiber bundle. Natural fiber bundles are typically composed of tens to hundreds of tightly stacked monofilaments. The capillary network formed within these bundles, lacking directional driving forces, easily creates "dead zones" for resin penetration, leading to insufficient impregnation within the bundle and subsequently causing localized stress concentration and early interfacial debonding. Furthermore, the surface pretreatment methods employed in this approach are mostly limited to simple coating of coupling agents, failing to deeply activate or reconstruct the topological structure of active functional groups such as hydroxyl and carboxyl groups on the fiber surface. This results in insufficient density of chemical bonding sites, with interfacial bonding still primarily relying on physical adsorption, making them prone to performance degradation under dynamic loads or humid and hot environments.

[0004] On the other hand, to improve the resin penetration kinetics in fiber systems, some technologies attempt to introduce mechanically forced impregnation devices. The melt impregnation mold disclosed in patent CN104827686B, combined with a multi-stage roller pressing system, aims to drive the resin melt into the fiber gaps through an external pressure gradient, achieving continuous production. This solution constructs a multi-parameter coupled control framework of pressure, temperature, and speed at the equipment level, theoretically increasing the resin coverage on the fiber bundle surface. However, its technical principle is inherently limited by the attenuation effect of pressure transmission and the nonlinear response of resin rheological behavior. Specifically, when facing high-density or large-diameter fiber bundles, the normal pressure generated by roller pressing decreases exponentially as it penetrates the fiber layer thickness, resulting in the core area remaining in a low-pressure or even pressureless state, making it difficult for the resin to overcome capillary resistance and achieve full filling. Simultaneously, this process is extremely sensitive to resin melt viscosity and processing temperature window; slightly higher viscosity hinders penetration, and temperature fluctuations easily lead to localized degradation or uneven crystallization, resulting in a significant increase in batch-to-batch performance dispersion and difficulty in ensuring process robustness. More importantly, such mechanically forced methods are essentially "external force compression" impregnation, failing to build an active penetration mechanism from the fiber structure or resin molecular movement level, and therefore cannot fundamentally solve the structural defects of residual pores and weak interfacial bonding within the bundle. Summary of the Invention

[0005] To address the above problems, this invention provides a method for preparing natural fiber prepregs that improves the impregnation effect, thereby solving the problems of insufficient resin impregnation inside natural fiber bundles, insufficient interfacial bonding strength, and poor process stability in the prior art.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing natural fiber prepreg with improved impregnation effect, comprising the following steps: Step S1: The natural fiber bundles are physically opened and dusted by an airflow fiber separator, and then introduced into a plasma activation chamber. Step S2: The activated fiber bundles are continuously introduced into a chemical grafting reaction tank and reacted for 8 minutes in a 5% (w / w) aqueous solution of γ-glycidyl etheroxypropyltrimethoxysilane, at a pH of 4.5 and a temperature of 60°C, so that a siloxane network layer is formed on the fiber surface. After the reaction, the fiber bundles are washed with deionized water and then dried in a hot air drying oven at 80°C and a wind speed of 2 m / s for 3 minutes. Step S3, preparing the resin system: using bisphenol A type epoxy resin with an epoxy equivalent of 185 g / eq as the matrix, add 15% by mass of 1,4-butanediol diglycidyl ether reactive diluent, 3% by mass of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer surfactant, and 2% by mass of dicyandiamide microcapsule latent curing agent. Degas in a vacuum stirred tank for 30 minutes, then transfer to a constant temperature storage tank and maintain at 80°C. Step S4: Construct a multi-stage gradient pressure-temperature coupled impregnation system, which sequentially includes a fiber introduction section, a pre-impregnation section, a main impregnation section, a pressure gradient control section, an ultrasonic-assisted penetration section, and a cooling and shaping section; the fiber bundle passes through a tension controller in the introduction section; in the pre-impregnation section, it is initially coated with a resin film by a resin spray head at a pressure of 0.1 MPa and a spray angle of 45°; in the main impregnation tank, it is completely immersed in 90°C resin melt for 120 seconds, with a liquid level height of 150 mm; at the outlet, it is initially compacted by a first-stage roller press device; In step S5, in the pressure gradient control section, the fiber bundle passes sequentially through three sets of hydraulic pressure roller units. The surface of the pressure rollers is covered with a 3mm thick polytetrafluoroethylene elastic layer with a Shore hardness of 70A. The applied pressures are 1.2MPa, 1.8MPa, and 2.5MPa, respectively, and the roller spacing is 80mm, 60mm, and 40mm, respectively. An infrared heating module is set between each set of pressure rollers to maintain the resin viscosity at 800mPa·s ± 50mPa·s. In step S6, in the ultrasonic-assisted permeation section, a 300mm long area is subjected to an array of four 40kHz ultrasonic transducers at a power density of 3W / cm² to promote resin permeation into the core bundle; then it enters the cooling and shaping section; at the outlet, the resin content is monitored in real time by a resin content detector, and the liquid level in the main impregnation section or the pressure of the pressure roller is adjusted accordingly. In step S7, the prepreg after cooling and setting is placed in a hot air setting chamber at 120°C and 1.5 m / s for 60 seconds, and then the tension is controlled at 1.0 N by a tension control roller group. It is then wound up by an automatic winding machine, and the winding roller is covered with a rubber layer with a Shore hardness of 60A. After winding, it is placed in an environment of 25°C and 50% relative humidity for 24 hours to obtain a natural fiber prepreg with uniform resin distribution and strong interfacial bonding.

[0007] A secondary plasma activation section is added after the chemical grafting reaction tank, using nitrogen plasma to increase the density of epoxy groups on the surface of the silane layer and the density of interfacial crosslinking.

[0008] 1% by mass of surface-hydroxylated silica nanoparticles were further added to the resin system. After being dispersed at 5000 rpm for 30 minutes by a high-speed shear disperser, the nanoparticles migrated to the fiber-resin interface to form Si-O-Si covalent bonds, thus constructing a three-phase interface structure of "fiber-nanoparticle-resin".

[0009] A vacuum-assisted degassing section is added after the pressure gradient control section. The pressure inside the chamber is maintained at -0.08MPa, the processing time is 60 seconds, and a pressure balancing valve is installed at the outlet to control the pressure transition gradient to ≤0.02MPa / s.

[0010] A focused ultrasonic transducer is used in the ultrasonic-assisted permeation section, with the sound field focused on the fiber bundle core and a sound pressure level of 160dB. The ultrasonic module and the infrared heating module are linked for control to ensure the viscosity of the resin is stable in the high shear zone.

[0011] A magnetic field-assisted orientation section is added before the cooling and shaping section, and a permanent magnet array is set to generate a 0.5T magnetic field with an effective area length of 200mm; if the resin system contains 0.5wt% iron oxide nanoparticles, they will be induced to align along the fiber axis.

[0012] A surface coating device is added before winding. A 25μm thick polyethylene film is used to simultaneously hot-press the upper and lower surfaces of the prepreg. The hot-pressing temperature is 100℃, the pressure is 0.3MPa, and the linear speed is 1.0m / min. After coating, the prepreg is frozen and stored at -18℃ for ≥6 months. There is no performance degradation after thawing.

[0013] The natural fibers are selected from one or more of flax, hemp, jute, sisal or coconut fiber, with a fiber length of 30mm to 50mm, a single filament diameter of 20μm to 50μm, and a moisture content of ≤8%. The epoxy resin system can be replaced with anhydride-cured epoxy resin, phenolic resin or unsaturated polyester resin, and the reactive diluent and curing agent system can be adjusted accordingly to match the reactivity.

[0014] The multi-stage gradient pressure-temperature coupled impregnation system integrates an online quality monitoring system, including a resin content analyzer, fiber tension sensor, temperature sensor, pressure sensor, and PLC controller. The resin content analyzer has a sampling frequency of 1Hz and an accuracy of ±0.5wt%; the tension sensor has a range of 0-5N and an accuracy of ±0.05N; the temperature sensor is Pt100 with an accuracy of ±0.5℃; and the pressure sensor has a range of 0-5MPa and an accuracy of ±0.02MPa. The PLC controller adjusts the resin pump flow rate, pressure roller hydraulic pressure, heating power, and traction motor speed in real time based on feedback signals.

[0015] The method is applicable to the preparation of chopped fiber felt prepregs with fiber lengths of 25mm to 50mm and areal density of 300g / m². The impregnation stage adopts a double-sided resin spraying + roller pressing penetration process with a spraying pressure of 0.2MPa, a roller pressing pressure of 1.5MPa, an impregnation temperature of 95℃, and a residence time of 90 seconds. The resulting prepreg has a resin content of 36wt%, a porosity of ≤2.0%, and an interlaminar shear strength of ≥42MPa.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention constructs a four-dimensional synergistic mechanism of "fiber surface gradient activation, resin molecule directional transport, bundle pore gradient filling, and interfacial chemical bonding reinforcement," which increases the core pore filling rate from 85% in traditional processes to 98%, the penetration depth to 97% of the fiber bundle radius, and the interlaminar shear strength to over 45 MPa. This completely eliminates the structural defect of "solid outside and hollow inside" in natural fiber prepregs, ensuring the full penetration and uniform distribution of resin within the fiber bundle.

[0017] The prepreg prepared by this invention, after hot pressing, achieves a composite material flexural strength of over 320 MPa, an impact toughness of over 80 kJ / m², a water absorption rate of less than 1.2%, and a strength retention rate of over 85% after wet heat aging. The performance indicators fully meet the engineering requirements of high-end application fields such as automotive structural parts, rail transit interior panels, and building load-bearing components, significantly expanding the application scope of natural fiber composite materials.

[0018] This invention integrates intelligent technology modules such as online quality monitoring system, intelligent pressure roller feedback control, and real-time defect detection to ensure that process parameters are always within the optimal window, the process robustness index Cpk is greater than 1.67, the batch-to-batch performance standard deviation is less than 3%, and the porosity of prepreg is stably controlled below 1.5%. This fundamentally solves the technical problems of poor process stability and large product quality fluctuations in the traditional natural fiber prepreg preparation process. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. The following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a schematic flowchart of a method for preparing a natural fiber prepreg with improved impregnation effect, provided by an embodiment of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but is merely a selection of embodiments of the present invention.

[0022] Please refer to Figure 1 , Figure 1 This is a flowchart of a method for preparing a natural fiber prepreg with improved impregnation effect, provided by an embodiment of the present invention, including the following steps: In step S1, the natural fiber bundles are physically opened and dusted using an airflow fiber separator. The spindle speed is 1200 rpm, the airflow pressure is 0.3 MPa, and the processing time is 15 minutes, so that the fiber bundle dissociation degree is ≥85% and the diameter of a single fiber bundle is controlled within the range of 0.8 mm to 1.2 mm. Then, the fiber bundles are introduced into a plasma activation chamber and treated for 90 seconds under the conditions of argon to oxygen volume ratio of 9:1, gas flow rate of 200 sccm, and discharge power of 300 W, so that the hydroxyl density on the fiber surface increases from 0.8 mmol / g to 2.3 mmol / g and the surface roughness Ra value increases from 0.2 μm to 1.1 μm. First, the primary natural fiber raw material is selected as flax fiber, with a single filament diameter of 35μm, an average length of 40mm, and a moisture content of 7.5%. This flax fiber is then introduced into an airflow fiber separator. In the pretreatment stage of the natural fiber bundles, the raw material undergoes physical opening and dust removal using an airflow fiber separator. The spindle speed is set to 1200rpm, the airflow pressure to 0.3MPa, and the treatment time to 15 minutes, achieving a fiber bundle dissociation degree of over 85%, with the diameter of a single fiber bundle controlled within the range of 0.8mm to 1.2mm. After this physical opening treatment, the fiber bundle dissociation degree reaches 87%, the diameter distribution of single fibers is concentrated in the range of 0.9mm to 1.1mm, and there is no obvious entanglement between fiber bundles, laying a physical foundation for subsequent uniform impregnation.

[0023] Subsequently, the opened fiber bundles were continuously introduced into a plasma activation chamber. This chamber contained parallel plate electrodes with a spacing of 50 mm. The working gas was a mixture of argon and oxygen at a volume ratio of 9:1, a gas flow rate of 200 sccm, a discharge power of 300 W, and a processing time of 90 seconds. The plasma treatment increased the hydroxyl density on the fiber surface from 0.8 mmol / g to 2.3 mmol / g and formed a nanoscale uneven topological structure on the fiber surface, increasing the surface roughness Ra value from 0.2 μm to 1.1 μm. This significantly improved the anchoring ability and wetting and spreading rate of subsequent resin molecules. X-ray photoelectron spectroscopy (XPS) analysis showed that the hydroxyl density on the fiber surface increased from the original 0.8 mmol / g to 2.3 mmol / g; atomic force microscopy (AFM) scanning showed that the surface roughness Ra value increased from 0.2 μm to 1.1 μm, forming a uniformly distributed nanoscale uneven structure with a peak-valley height difference between 50 nm and 150 nm, which significantly enhanced the anchoring ability of resin molecules on the fiber surface.

[0024] Step S2: The activated fiber bundle is continuously introduced into a chemical grafting reaction tank and reacted for 8 minutes in a 5% (w / w) aqueous solution of γ-glycidyl etheroxypropyltrimethoxysilane at pH 4.5 and a temperature of 60°C, with a traction speed of 0.5 m / min. This results in the formation of a siloxane network layer with a thickness of 80 nm to 120 nm on the fiber surface, increasing the surface energy to 68 mN / m and reducing the contact angle to 32°. After the reaction, the fiber bundle is rinsed with deionized water and then dried in a hot air drying oven at 80°C and a wind speed of 2 m / s for 3 minutes. After plasma activation, the fiber bundle immediately enters a chemical grafting reaction tank containing a 5% (w / w) aqueous solution of γ-glycidoxypropyltrimethoxysilane (KH-560). The pH of the solution is adjusted to 4.5, the temperature is maintained at 60°C, the fiber bundle resides in the tank for 8 minutes, and the traction speed is 0.5 m / min. During the reaction, the epoxy groups in the silane molecules undergo ring-opening addition reactions with the hydroxyl groups on the fiber surface, forming a covalently bonded siloxane network layer. The thickness of this network layer, measured by ellipsometry, ranges from 80 nm to 120 nm. Its surface energy increases from 42 mN / m of the original fiber to 68 mN / m, and the contact angle decreases from 85° to 32°, significantly improving the initial wettability between the fiber and the resin matrix.

[0025] Ellipsometry measurements showed that the network layer thickness was 95nm ± 15nm; contact angle testing determined that its static water contact angle decreased from the original 85° to 32°, and the surface energy increased from 42mN / m to 68mN / m. After the reaction, the fiber bundle was washed with deionized water and then dried in a hot air drying oven at 80℃ and 2m / s for 3 minutes to ensure no residual solvent on the fiber surface. The fiber bundle was then cleaned with three sets of deionized water spray heads at a pressure of 0.15MPa for 30 seconds each to ensure complete removal of unreacted silane. At the outlet, the fiber moisture content was reduced to below 0.8%, and no solvent residue remained on the surface.

[0026] Step S3, preparing the resin system: using bisphenol A type epoxy resin with an epoxy equivalent of 185 g / eq as the matrix, add 15% by mass of 1,4-butanediol diglycidyl ether reactive diluent, 3% by mass of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer surfactant, and 2% by mass of dicyandiamide microcapsule latent curing agent. Degas in a vacuum stirred tank at 60°C and -0.09 MPa for 30 minutes, then transfer to a constant temperature storage tank and maintain at 80°C to stabilize the resin viscosity at 800 mPa·s ± 50 mPa·s; In the resin system construction stage, bisphenol A type epoxy resin E-51 with an epoxy equivalent of 185 g / eq was selected as the matrix resin, and its viscosity at 25°C was 12000 mPa·s. To reduce the viscosity of the resin system at the impregnation temperature and improve its reactivity with the functional groups on the fiber surface, a reactive diluent of 15% by mass was added to the epoxy resin. This diluent was 1,4-butanediol diglycidyl ether (BDDGE), with an epoxy equivalent of 145 g / eq and a viscosity of 15 mPa·s at 25°C. Simultaneously, to construct the directional transport capability of resin molecules within the fiber bundle, a block copolymer surfactant, Pluronic F127, of 3% by mass was introduced into the resin system. This surfactant is composed of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock, with a number-average molecular weight of 5800, of which the hydrophilic segment accounts for 40% and the hydrophobic segment accounts for 60%. This surfactant self-assembles into micelle structures in the resin melt, with a critical micelle concentration of 0.8 wt%. The micelle size, determined by dynamic light scattering (DLS), ranges from 25 nm to 40 nm. This micelle structure acts as a carrier for resin molecule transport within the capillary channels of the fiber bundle, reducing the flow resistance of the resin in the micropores. The surfactant also self-assembles into spherical micelles in the resin melt at 80°C. DLS analysis shows an average particle size of 32 nm, a polydispersity index (PDI) of 0.18, and a critical micelle concentration of 0.8 wt%. The micelle structure acts as a carrier for resin molecule transport within the capillary channels, reducing flow resistance and improving penetration efficiency.

[0027] A latent curing agent with a mass fraction of 2% was further added to the resin system. This curing agent was dicyandiamide microcapsules with polymethyl methacrylate as the capsule wall material and dicyandiamide crystals with a purity of 99.5% as the core material. The average particle size of the microcapsules was 5 μm, and the thermal decomposition initiation temperature was 150℃. This curing agent remained inert during the impregnation stage, releasing active amine groups only during the subsequent heat setting stage to undergo a cross-linking reaction with epoxy groups, thereby avoiding premature curing of the resin during impregnation and loss of fluidity. After mixing, the resin system was degassed for 30 minutes in a vacuum stirred tank at 60℃ and -0.09 MPa with a stirring speed of 300 rpm to ensure that no air bubbles remained in the system. Subsequently, it was transferred to a constant temperature storage tank, where the temperature was maintained at 80℃±2℃ and the viscosity was stabilized at 800 mPa·s±30 mPa·s.

[0028] 1% by mass of surface-hydroxylated silica nanoparticles with an average particle size of 20 nm and a specific surface area of ​​200 m² / g were further introduced into the resin system. The nanoparticles were uniformly dispersed in the resin melt, with some migrating to the fiber-resin interface and forming Si-O-Si covalent bonds with the silane layer on the fiber surface, constructing a three-phase interface structure of "fiber-nanoparticle-resin," which improved the interfacial shear strength by more than 15%. Nanoparticle dispersion was achieved using a high-speed shear disperser at 5000 rpm for 30 minutes. After dispersion, the system was analyzed by a laser particle size analyzer, showing a D90 particle size of 85 nm and no agglomerates larger than 100 nm. Simultaneously, to achieve visualized monitoring of the process, 0.1 wt% of coumarin-3-carboxylic acid ethyl ester was added as a fluorescent tracer with an excitation wavelength of 365 nm and an emission wavelength of 450 nm, used for subsequent fluorescence microscopy observation of resin penetration depth and distribution uniformity.

[0029] Step S4: Construct a multi-stage gradient pressure-temperature coupled impregnation system, which sequentially includes a fiber introduction section, a pre-impregnation section, a main impregnation section, a pressure gradient control section, an ultrasonic-assisted penetration section, and a cooling and shaping section. The fiber bundle is tensioned to 0.5N in the introduction section by a tension controller. In the pre-impregnation section, it is initially coated with a resin film by a resin spray head at a pressure of 0.1MPa and a spray angle of 45°. In the main impregnation tank, it is completely immersed in 90°C resin melt for 120 seconds, with a liquid level of 150mm. At the outlet, it is initially compacted by a first-stage roller press device at a pressure of 0.8MPa. During the operation phase of the collaborative impregnation system, a multi-stage gradient pressure-temperature coupled impregnation system was designed and constructed. This system consists of a fiber inlet section, a pre-impregnation section, a main impregnation section, a pressure gradient control section, and a fiber outlet section connected in series, with a total length of approximately 8.5 meters. The fiber inlet section is equipped with a tension controller to maintain the fiber bundle tension at 0.5N, ensuring the fibers remain straight and wrinkle-free in subsequent sections. A closed-loop control system using a magnetic powder brake and tension sensor is employed to stabilize the fiber bundle tension at 0.5N ± 0.05N. The pre-impregnation section is equipped with resin spray heads at a spray pressure of 0.1MPa and a spray angle of 45°, initially coating the fiber bundle surface with a resin film, with a film thickness controlled between 10μm and 20μm. The nozzle orifice diameter is 0.3mm, the spray pressure is 0.1MPa, and the spray angle is 45°, initially coating the fiber bundle surface with a resin film, the thickness of which was measured to be 15μm ± 5μm using a laser thickness gauge. The main impregnation section is equipped with an impregnation tank with a resin level of 150mm, ensuring the fiber bundles are completely submerged in the resin melt. The impregnation temperature is 90℃, and the residence time is 120 seconds. The tank is made of 316L stainless steel with a polished inner wall. The impregnation temperature is maintained at 90℃±1℃ by a bottom heating plate and a side wall insulation layer. At the outlet of the main impregnation section, a first-stage roller pressing device is installed. This device consists of a pair of chrome-plated steel rollers with a diameter of 150mm, a surface hardness of HRC60, an inter-roller pressure of 0.8MPa, and a linear speed synchronized with the fiber traction speed of 1.0m / min. This device is used to initially compact the fiber bundles and extrude excess resin.

[0030] In step S5, in the pressure gradient control section, the fiber bundle passes sequentially through three sets of hydraulic pressure roller units. The surface of the pressure rollers is covered with a 3mm thick polytetrafluoroethylene elastic layer with a Shore hardness of 70A. The applied pressures are 1.2MPa, 1.8MPa, and 2.5MPa, respectively, and the spacing between the pressure rollers is 80mm, 60mm, and 40mm, respectively. An infrared heating module with a wavelength of 3.5μm and a power density of 5W / cm² is installed between each set of pressure rollers to maintain the resin viscosity at 800mPa·s ± 50mPa·s. A pressure gradient control section is set after the main impregnation section. This section consists of three independently controlled hydraulic pressure roller units. Each pressure roller unit includes upper and lower pressure rollers, and the surface of the pressure rollers is covered with a 3mm thick polytetrafluoroethylene elastic layer with a Shore hardness of 70A. The first pressure roller applies a pressure of 1.2MPa, the second 1.8MPa, and the third 2.5MPa, with the spacing between the three pressure rollers decreasing sequentially to 80mm, 60mm, and 40mm, respectively, forming a pressure gradient field along the fiber travel direction. This pressure gradient field forces the resin melt to penetrate from the fiber bundle surface to the core, overcoming capillary resistance and achieving gradient filling of the pores within the bundle. Between each pressure roller unit, an infrared heating module with a wavelength of 3.5μm and a power density of 5W / cm² is installed to locally compensate for the resin temperature drop caused by the pressure, maintaining the resin viscosity within the range of 800mPa·s ± 50mPa·s. A quartz infrared heating tube is used, emitting a wavelength of 3.5μm, with a power density of 5W / cm² and an effective width of 200mm. The total length of the pressure gradient control section is 2.4m, and the fiber bundle residence time in this section is 144 seconds, ensuring sufficient time for resin to complete in-bundle penetration. A miniature pressure sensor and a Pt100 temperature sensor are embedded inside the pressure roller to monitor the pressure and temperature distribution on the roller surface in real time. The standard deviation of the pressure distribution is less than 0.05MPa, and the standard deviation of the temperature distribution is less than 1℃. The hydraulic system and heating power are adjusted via PLC feedback to ensure uniformity of the heating field.

[0031] In step S6, in the ultrasonic-assisted permeation section, a 300mm long area is treated by an array of four 40kHz ultrasonic transducers at a power density of 3W / cm², promoting resin permeation into the core bundle. Subsequently, it enters the cooling and shaping section, where it is cooled to a roller surface temperature of 15℃ by three sets of cooling rollers with 5℃ deionized water, raising the resin viscosity to above 5000mPa·s. At the outlet, the resin content is monitored in real time by a resin content detector, with a target value of 35wt%±1wt%, and feedback is used to adjust the liquid level in the main impregnation section or the pressure of the pressure rollers. Furthermore, at the end of the pressure gradient control section, an ultrasonic-assisted permeation module is installed. This module consists of an array of four ultrasonic transducers with a frequency of 40kHz, a power density of 3W / cm², and an effective area length of 300mm. The ultrasound generates cavitation and microjets in the resin melt, further disrupting the gas-liquid interface within the fiber bundle and promoting resin molecules to enter the microporous structure. This module consists of an array of four 28kHz focused ultrasonic transducers (US-40F model), with the sound field focused on the core region of the fiber bundle, a focal length of 150mm, a sound pressure level of 160dB, and a power density of 3W / cm². After ultrasonic treatment, the fiber bundle enters the cooling and shaping section, which is equipped with three sets of cooling rollers. The cooling medium is 5℃ deionized water, and the surface temperature of the cooling rollers is maintained at 15℃, causing the resin viscosity to rapidly rise to over 5000mPa·s, locking in the impregnation structure. Micro-CT scanning analysis shows that the core pore filling rate increased from 85% to 98%, and the permeation depth reached 97% of the fiber bundle radius. The ultrasonic module and the infrared heating module are linked for control, ensuring that the resin maintains a suitable viscosity in the high-shear region and avoiding localized gelation.

[0032] A vacuum-assisted degassing section is added after the ultrasonic-assisted impregnation section. This section features a vacuum chamber with an internal pressure of -0.08 MPa and a processing time of 60 seconds. This chamber is used to remove trace amounts of air bubbles remaining from the impregnation process, further reducing the porosity of the prepreg. The chamber is made of aluminum alloy with an anodized inner wall, and the internal pressure is maintained at -0.08 MPa by a vacuum pump. A pressure balancing valve is installed at the vacuum chamber outlet to prevent structural damage to the fiber bundles due to sudden pressure changes. The pressure gradient is controlled at 0.02 MPa / s. After vacuum treatment, the prepreg porosity can be reduced to below 1.0%, from 1.5% to 0.9%, and the interlaminar shear strength is increased to over 50 MPa, reaching 51.2 MPa.

[0033] In step S7, the prepreg after cooling and setting is placed in a hot air setting chamber at 120°C and 1.5 m / s for 60 seconds. Then, the tension is controlled at 1.0 N by a tension control roller group, and the prepreg is wound up by an automatic winding machine at a winding tension of 2.0 N and a speed of 1.0 m / min. The winding roller is covered with a rubber layer with a Shore hardness of 60A. After winding, the prepreg is placed in an environment of 25°C and 50% relative humidity for 24 hours to obtain a natural fiber prepreg with uniform resin distribution and strong interfacial bonding.

[0034] A resin content detector is installed at the outlet of the cooling and shaping section. Near-infrared spectroscopy is used to monitor the resin content of the prepreg in real time, with a target value of 35wt% ± 1wt%. If the detected value deviates from the target range, feedback is provided to adjust the resin level in the main impregnation section or the pressure of the pressure roller in the pressure gradient control section, forming a closed-loop control. Near-infrared spectroscopy (NIR) is used, with a light source wavelength of 1200nm to 2200nm, a sampling frequency of 1Hz, and a detection accuracy of ±0.5wt%.

[0035] During the setting and winding stage, the cooled and set prepreg enters a hot air setting chamber at 120°C and 1.5 m / s for 60 seconds. This process partially activates the latent curing agents in the resin system, forming a preliminary cross-linked network and improving the dimensional stability of the prepreg. Subsequently, the prepreg passes through a tension control roller assembly with a tension set at 1.0 N and enters an automatic winding machine at a winding speed of 1.0 m / min and a winding tension of 2.0 N. The winding rollers are coated with a rubber layer with a Shore hardness of 60A to ensure the prepreg roll is wrinkle-free and slip-free. After winding, the prepreg roll is placed in a constant temperature and humidity environment of 25°C and 50% relative humidity for 24 hours to fully complete the interfacial chemical bonding reaction, ultimately obtaining a natural fiber prepreg with uniform resin distribution, no fiber damage, and strong interfacial bonding.

[0036] A surface coating process is added before winding. A 25μm thick polyethylene film is used, simultaneously hot-pressed onto the upper and lower surfaces of the prepreg at 100℃ and 0.3MPa to isolate it from air and moisture, extending the prepreg's shelf life. After coating, the prepreg can be frozen at -18℃ for over 6 months with no performance degradation after thawing. Alternatively, a 25μm thick low-density polyethylene (LDPE) film is used, simultaneously hot-pressed onto the upper and lower surfaces of the prepreg at 100℃, 0.3MPa, and a linear speed of 1.0m / min. After coating, the prepreg can be frozen at -18℃ for up to 6 months with a resin content fluctuation of less than 0.3wt% after thawing and an interlaminar shear strength retention rate of over 98%.

[0037] In terms of quality control, the entire system integrates a PLC controller, connected to a resin content detector, fiber tension sensor, temperature sensor, pressure sensor, and traction motor encoder. The PLC adjusts the resin pump flow rate, pressure roller hydraulic pressure, heating module power, and traction speed based on real-time data to ensure that process parameters are always at their optimal levels. Simultaneously, an online defect detection module is integrated at the end of the impregnation system. This module, composed of a high-resolution linear CCD camera and image processing algorithms, can identify defects such as resin buildup, fiber breakage, and pore aggregation in real time, automatically marking their locations for subsequent slitting and removal.

[0038] Furthermore, the method of this invention can be integrated with an online quality monitoring system, which consists of a resin content analyzer, a fiber tension sensor, a temperature sensor, a pressure sensor, and a PLC controller. The resin content analyzer has a sampling frequency of 1Hz and a detection accuracy of ±0.5wt%; the fiber tension sensor has a range of 0 to 5N and an accuracy of ±0.05N; the temperature sensor is a Pt100 platinum resistance thermometer with an accuracy of ±0.5℃; and the pressure sensor has a range of 0 to 5MPa and an accuracy of ±0.02MPa. Based on the sensor feedback signals, the PLC controller adjusts the resin pump flow rate, the hydraulic pressure of the pressure roller, the power of the heating module, and the speed of the traction motor in real time to ensure that the process parameters are always within the optimal window, achieving fully automated, high-precision, and highly stable continuous production.

[0039] In summary, this invention provides a method for preparing natural fiber prepregs with improved impregnation effects. Its core objective is to address the structural defects in existing technologies, such as insufficient resin impregnation within natural fiber bundles, inadequate interfacial bonding strength, and poor process stability. By constructing a four-dimensional synergistic mechanism of "gradient activation of fiber surface—directional transport of resin molecules—gradient filling of pores within bundle—interfacial chemical bonding strengthening," the invention achieves full-chain impregnation optimization from the microscopic molecular scale to the macroscopic process scale, thereby obtaining natural fiber prepregs with high impregnation uniformity, strong interfacial bonding, excellent mechanical properties, and strong batch stability.

[0040] Examples and Comparative Examples To verify the effectiveness of the present invention, the following embodiments and comparative examples were set up, and performance comparison tests were conducted.

[0041] Example 1: Using the complete process of the present invention, the fiber is flax, the resin system contains 3% Pluronic F127 and 2% dicyandiamide microcapsules, the pressure gradient is 1.2 / 1.8 / 2.5MPa, the ultrasonic power is 3W / cm², and the vacuum degassing is -0.08MPa.

[0042] Example 2: Based on Example 1, 1% nano-silica and 0.1% fluorescent tracer were added to the resin system, and the pressure roller was controlled by intelligent closed loop.

[0043] Example 3: Based on Example 2, a magnetic field-assisted orientation section was added, with a magnetic field strength of 0.5T and 0.5wt% iron oxide nanoparticles in the resin.

[0044] Comparative Example 1: Only conventional melt impregnation process was used, without plasma activation, silane grafting, pressure gradient, or ultrasonic assistance. The resin viscosity was 12000 mPa·s, the impregnation temperature was 120℃, and the pressure was 0.5 MPa.

[0045] Comparative Example 2: The pretreatment and resin system of the present invention were used, but only a single-stage pressure roller (1.5MPa) was used in the impregnation stage, without gradient pressure and ultrasonic assistance.

[0046] Comparative Example 3: The impregnation system of the present invention was used, but the fibers were directly impregnated without plasma and silane treatment.

[0047] Sample number Resin content (wt%) Porosity (%) Interlaminar shear strength (MPa) Bending strength (MPa) Strength retention rate after damp heat aging (%) Example 1 35.2 0.9 51.2 335 88.5 Example 2 34.8 0.7 54.6 342 90.2 Example 3 35.0 0.8 53.1 368 89.7 Comparative Example 1 36.5 3.8 28.4 245 62.3 Comparative Example 2 35.1 2.1 36.7 285 73.8 Comparative Example 3 34.9 2.5 32.1 268 68.9 As can be seen from the table above, the embodiments of the present invention are significantly superior to the comparative examples in terms of porosity control, interlaminar shear strength, flexural strength, and damp heat aging stability. Example 2, due to the introduction of nanoparticles and intelligent pressure rollers, further improves the interface strength and uniformity. Example 3, due to magnetic field-induced orientation, improves the flexural strength by 20%. Comparative example 1, due to the lack of gradient impregnation and surface activation, has high porosity and low strength. Although comparative example 2 has some optimizations, the core impregnation is insufficient due to the lack of pressure gradient and ultrasonic assistance. Comparative example 3 has a low strength retention rate due to the lack of chemical bonding at the interface.

[0048] In addition, the natural fiber prepreg prepared by the method of the present invention has a fiber volume fraction of 55% to 60%, a resin content of 35 wt%, a porosity of less than 1.5% as determined by micro-CT scanning, an interlaminar shear strength of more than 45 MPa as tested by ASTM D2344 standard, a strength retention rate of more than 85% after damp heat aging (85℃ / 85%RH, 1000 hours), and a batch-to-batch performance standard deviation of less than 3%, which meets the engineering application requirements of high-end composite material structural components.

[0049] The natural fibers are selected from one or more of flax, hemp, jute, sisal, or coconut fiber, with a fiber length of 30mm to 50mm, a monofilament diameter of 20μm to 50μm, and a fiber moisture content controlled below 8%. The epoxy resin system can be replaced with anhydride-cured epoxy resin, phenolic resin, or unsaturated polyester resin, but the type of reactive diluent and curing agent system need to be adjusted accordingly to ensure matching reactivity. The block copolymer surfactant can be replaced with polyethylene glycol monomethyl ether-polycaprolactone diblock copolymer, with a number-average molecular weight of 4500 and a hydrophilic segment ratio of 35%, which can also achieve micelle self-assembly and resin transport functions.

[0050] Furthermore, the plasma activation chamber can be replaced with a corona discharge treatment device with a discharge voltage of 15kV and a processing speed of 0.8m / min, which can also achieve the increase of hydroxyl density and topological reconstruction on the fiber surface. The silane coupling agent in the chemical grafting reaction tank can be replaced with isocyanate-based silane, which reacts with the hydroxyl groups on the fiber surface to form urethane bonds, with bond energies higher than ether bonds, further improving the interfacial bonding strength; the latent curing agent can be replaced with organic acid anhydride microcapsules, which have a thermal decomposition temperature of 160℃ and are suitable for high-temperature curing systems.

[0051] The number of pressure roller units in the multi-stage gradient pressure-temperature coupled impregnation system can be expanded to five sets, with pressure gradients set to 1.0MPa, 1.5MPa, 2.0MPa, 2.5MPa, and 3.0MPa, suitable for impregnating coarse fiber bundles with a diameter greater than 1.5mm. The frequency of the ultrasonic-assisted permeation module can be adjusted to 28kHz or 68kHz, suitable for high-viscosity resins or fine fiber bundle systems, respectively. The wavelength of the infrared heating module can be adjusted to 2.8μm or 4.2μm to match the infrared absorption peaks of different resin systems and improve heating efficiency.

[0052] The preparation method of this invention introduces nanofillers into the resin system to further improve interfacial properties. The nanofillers are surface-hydroxylated silica nanoparticles with a particle size of 20 nm and a specific surface area of ​​200 m² / g, added at 1% of the resin mass. The nanoparticles are uniformly dispersed in the resin melt, and some particles migrate to the fiber-resin interface to form Si-O-Si covalent bonds with the silane layer on the fiber surface, constructing a three-phase interfacial structure of "fiber-nanoparticle-resin", which improves the interfacial shear strength by more than 15%. The nanoparticle dispersion is achieved by a high-speed shear disperser at a speed of 5000 rpm for a dispersion time of 30 minutes. After dispersion, the system is detected by a laser particle size analyzer, and no agglomerates larger than 100 nm are found.

[0053] Furthermore, the method of the present invention adds a vacuum-assisted degassing section after the pressure gradient control section. This section is equipped with a vacuum chamber with a pressure of -0.08 MPa and a processing time of 60 seconds, which is used to extract trace amounts of air bubbles remaining during the impregnation process, further reducing the porosity of the prepreg. A pressure balancing valve is installed at the outlet of the vacuum chamber to prevent structural damage to the fiber bundles due to sudden pressure changes. After vacuum treatment, the porosity of the prepreg can be reduced to below 1.0%, and the interlaminar shear strength can be increased to above 50 MPa.

[0054] The preparation method of this invention adds a magnetic field-assisted orientation section before the cooling and shaping section. This section is equipped with a permanent magnet array with a magnetic field strength of 0.5T and an effective area length of 200mm. If magnetic nanoparticles are added to the resin system, the magnetic field can induce the particles to align along the fiber axis, forming a reinforcing phase orientation structure and improving the longitudinal modulus of the composite material. X-ray diffraction analysis confirmed that the particle orientation degree reached more than 85%, and the longitudinal flexural modulus of the composite material was increased by 20%.

[0055] Furthermore, the method of the present invention adds a surface coating process before winding, using a polyethylene film with a thickness of 25μm, which is simultaneously hot-pressed onto the upper and lower surfaces of the prepreg at a temperature of 100℃ and a pressure of 0.3MPa to isolate air and moisture and extend the storage life of the prepreg; after coating, the prepreg is frozen and stored at -18℃, and the storage period can reach more than 6 months, with no performance degradation after thawing.

[0056] The preparation method of this invention is applicable to the preparation of continuous fiber prepregs and also to the preparation of chopped fiber felt prepregs. For chopped fiber felts, the fiber length is 25mm to 50mm and the areal density is 300g / m². In the impregnation stage, a double-sided resin spraying + roller pressing penetration process is adopted, with a spraying pressure of 0.2MPa, a roller pressing pressure of 1.5MPa, and an impregnation temperature of 95℃. This method can also obtain prepregs with uniform resin distribution and a porosity of less than 2.0%. The fiber used is jute, with a length of 30mm and an areal density of 300g / m². In the impregnation stage, a double-sided resin spraying + roller pressing penetration process is adopted, with a spraying pressure of 0.2MPa, a roller pressing pressure of 1.5MPa, an impregnation temperature of 95℃, and a residence time of 90 seconds. The resulting prepreg has a resin content of 36wt%, a porosity of 1.8%, and an interlaminar shear strength of 42MPa, which meets the requirements for non-structural applications.

[0057] Intelligent control and process optimization were employed. All process parameters were determined through experimental design and response surface methodology. Using interlaminar shear strength as the response value and plasma treatment time, silane concentration, resin viscosity, roller pressure gradient, and ultrasonic power as independent variables, a five-factor, three-level Box-Behnken experimental matrix was established. Analysis of variance was used to determine the contribution of each parameter to the impregnation effect. The final optimized parameter combination was: plasma treatment time 90 seconds, silane concentration 5%, resin viscosity 800 mPa·s, roller pressure gradient 1.2 / 1.8 / 2.5 MPa, and ultrasonic power density 3 W / cm². Under this parameter combination, the prepreg achieved a peak interlaminar shear strength of 48.7 MPa, reduced porosity to 1.2%, and a process robustness index (Cpk) greater than 1.67.

[0058] Furthermore, in the implementation of this invention, a complete process database is established to record the raw material parameters, process parameters, test data, and final performance indicators for each batch. A parameter-performance prediction model is established using the Support Vector Regression (SVR) algorithm, with a model determination coefficient R² of 0.93, enabling intelligent recommendation and adaptive adjustment of process parameters. When the input fiber diameter is 1.3 mm, the system automatically recommends a pressure gradient of 1.5 / 2.0 / 2.8 MPa for the pressure roller and adjusts the ultrasonic frequency to 28 kHz to ensure sufficient impregnation.

[0059] Furthermore, in the implementation of the method of the present invention, a complete process database is established to record the raw material parameters, process parameters, test data and final performance indicators of each batch. A parameter-performance prediction model is established through machine learning algorithms to realize intelligent recommendation and adaptive adjustment of process parameters, thereby promoting the development of natural fiber prepreg preparation towards intelligence and digitalization.

[0060] A key technical feature of this invention is the introduction of a fluorescent tracer into the resin system. This tracer is a coumarin derivative with an excitation wavelength of 365 nm and an emission wavelength of 450 nm, added at an amount of 0.1 wt%. By observing the cross-section of the prepreg under a fluorescence microscope, the penetration depth and distribution uniformity of the resin within the fiber bundle can be directly assessed. The penetration depth reaches over 95% of the fiber bundle radius, and the distribution uniformity coefficient is greater than 0.92, providing a visual basis for process optimization.

[0061] The preparation method of this invention employs an intelligent pressure roller in the pressure gradient control section. Pressure and temperature sensors are embedded inside the roller to monitor the pressure and temperature distribution on the roller surface in real time. A closed-loop feedback adjustment hydraulic system ensures the spatial uniformity of the pressure gradient and temperature fields. Testing shows that the standard deviation of the pressure distribution is less than 0.05 MPa, and the standard deviation of the temperature distribution is less than 1 °C, significantly improving impregnation uniformity.

[0062] Furthermore, the method of this invention employs a focused ultrasonic transducer in the ultrasonic-assisted permeation module, focusing the sound field on the core region of the fiber bundle, achieving a sound pressure level of 160 dB and increasing cavitation intensity by 40%, thereby raising the core pore filling rate from 85% to 98% and completely eliminating the "solid outside, hollow inside" defect. The focused ultrasonic module and the infrared heating module are linked for control, ensuring that the resin maintains a suitable viscosity in the high-shear region, avoiding local overheating or gelation.

[0063] The preparation method of this invention adds a plasma secondary activation stage after the chemical grafting reaction tank, using nitrogen plasma at a power of 200W for a processing time of 30 seconds to further activate the epoxy groups on the surface of the silane layer and enhance their reactivity with the epoxy groups of the resin. X-ray photoelectron spectroscopy analysis showed that the surface epoxy group density increased from 1.2 mmol / g to 2.0 mmol / g, the interfacial crosslinking density increased by 35%, and the interlayer shear strength increased by a corresponding 12%.

[0064] Furthermore, the method of this invention introduces reversible dynamic covalent bonds into the resin system. These bonds are disulfide bonds introduced by bis(2-hydroxyethyl) disulfide at an addition amount of 2% of the resin mass. This structure enables the composite material to achieve interface self-repair through heating after damage. After treatment at 160°C for 30 minutes, the crack healing rate reaches 90%, and the mechanical property recovery rate is greater than 85%, significantly improving the service life and reliability of the material.

[0065] The preparation method of the present invention integrates an online defect detection module at the end of the impregnation system. This module consists of a high-resolution linear CCD camera and an image processing algorithm, with a sampling accuracy of 0.1 mm / pixel. It can identify defects such as resin accumulation, fiber breakage, and pore aggregation on the surface of the prepreg in real time, and automatically mark the defect locations for subsequent slitting and removal, ensuring that the finished products are 100% qualified.

[0066] In terms of equipment and system design, all equipment in this invention adopts a modular design, with each functional section capable of independent disassembly and replacement, facilitating process adjustment and equipment maintenance. The control system adopts an industrial Ethernet architecture, supporting remote monitoring and data traceability, and conforms to Industry 4.0 smart manufacturing standards. In terms of energy consumption, the energy consumption per unit output of the entire system is 0.8 kWh / kg, which is 30% lower than the traditional melt impregnation process, meeting green manufacturing requirements. At the equipment level, the entire system adopts a modular design, with each functional section connected by quick-release flanges, facilitating independent maintenance and process adjustment. The control system adopts an industrial Ethernet architecture, supports the OPCUA protocol, and can be connected to the MES system to achieve remote monitoring and data traceability. In terms of energy consumption, the energy consumption per unit output is 0.78 kWh / kg, which is 31% lower than the traditional process, meeting green manufacturing standards.

[0067] Performance characterization and application: The prepreg prepared by this method can be used for hot pressing, compression molding or filament winding processes. The molding temperature is 160℃ to 180℃, the pressure is 5MPa to 10MPa, and the holding time is 10 minutes to 30 minutes. After molding, the flexural strength of the composite material reaches more than 320MPa according to ASTM D790, the impact toughness reaches more than 80kJ / m² according to ASTM D256, and the water absorption rate is less than 1.2% according to ASTM D570, which meets the performance requirements of automotive structural parts, rail transit interior panels and building load-bearing components.

[0068] The prepreg prepared by the method of this invention was tested by thermogravimetric analysis (TGA). The initial decomposition temperature was 320℃, and the char residue at 800℃ was 18%, indicating superior thermal stability compared to unmodified natural fiber composites. Dynamic mechanical analysis showed a glass transition temperature of 145℃ and a storage modulus of 8.5 GPa at room temperature, indicating strong interfacial bonding and high stress transfer efficiency. Scanning electron microscopy revealed complete resin coating on the fiber surface with no exposed areas and no obvious gaps at the fiber-resin interface, confirming thorough impregnation and tight bonding. TGA showed an initial decomposition temperature of 322℃ and a char residue of 18.3% at 800℃. Dynamic mechanical analysis showed a glass transition temperature of 146℃ and a storage modulus of 8.7 GPa at room temperature. Scanning electron microscopy also showed complete resin coating on the fiber surface with no gaps at the interface, confirming thorough impregnation and tight bonding. Fluorescence microscopy showed that the resin penetration depth reached 98% of the fiber bundle radius, with a distribution uniformity coefficient of 0.95.

[0069] In summary, this invention, through the organic integration of five core technology modules—fiber surface gradient activation, precise control of resin molecular structure, multi-level pressure-temperature-ultrasound synergistic impregnation, interfacial chemical bonding strengthening, and intelligent control throughout the entire process—completely solves common industry problems such as uneven impregnation within natural fiber bundles, weak interfacial bonding, and poor process stability. The prepared prepreg exhibits outstanding advantages such as thorough impregnation, strong interfacial bonding, excellent performance, and batch stability, providing reliable technical support and process assurance for the large-scale engineering application of high-performance natural fiber composite materials. It achieves full-scale optimization of natural fiber prepregs from microscopic interfaces to macroscopic structures, resulting in products with high impregnation uniformity, strong interfacial bonding, excellent mechanical properties, and strong batch stability, fully meeting the engineering application requirements of automotive structural components, rail transit interior panels, and building load-bearing components.

[0070] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations will be apparent to those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a natural fiber prepreg with improved impregnation effect, characterized in that, Includes the following steps: Step S1: The natural fiber bundles are physically opened and dusted by an airflow fiber separator, and then introduced into a plasma activation chamber. Step S2: The activated fiber bundles are continuously introduced into a chemical grafting reaction tank and reacted for 8 minutes in a 5% (w / w) aqueous solution of γ-glycidyl etheroxypropyltrimethoxysilane, at a pH of 4.5 and a temperature of 60°C, so that a siloxane network layer is formed on the fiber surface. After the reaction, the fiber bundles are washed with deionized water and then dried in a hot air drying oven at 80°C and a wind speed of 2 m / s for 3 minutes. Step S3, preparing the resin system: using bisphenol A type epoxy resin with an epoxy equivalent of 185 g / eq as the matrix, add 15% by mass of 1,4-butanediol diglycidyl ether reactive diluent, 3% by mass of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer surfactant, and 2% by mass of dicyandiamide microcapsule latent curing agent. Degas in a vacuum stirred tank for 30 minutes, then transfer to a constant temperature storage tank and maintain at 80°C. Step S4: Construct a multi-stage gradient pressure-temperature coupled impregnation system, which sequentially includes a fiber introduction section, a pre-impregnation section, a main impregnation section, a pressure gradient control section, an ultrasonic-assisted penetration section, and a cooling and shaping section; the fiber bundle passes through a tension controller in the introduction section; in the pre-impregnation section, it is initially coated with a resin film by a resin spray head at a pressure of 0.1 MPa and a spray angle of 45°; in the main impregnation tank, it is completely immersed in 90°C resin melt for 120 seconds, with a liquid level height of 150 mm; at the outlet, it is initially compacted by a first-stage roller press device; In step S5, in the pressure gradient control section, the fiber bundle passes sequentially through three sets of hydraulic pressure roller units. The surface of the pressure rollers is covered with a 3mm thick polytetrafluoroethylene elastic layer with a Shore hardness of 70A. The applied pressures are 1.2MPa, 1.8MPa, and 2.5MPa, respectively, and the roller spacing is 80mm, 60mm, and 40mm, respectively. An infrared heating module is set between each set of pressure rollers to maintain the resin viscosity at 800mPa·s ± 50mPa·s. In step S6, in the ultrasonic-assisted permeation section, a 300mm long area is subjected to an array of four 40kHz ultrasonic transducers at a power density of 3W / cm² to promote resin permeation into the core bundle; then it enters the cooling and shaping section; at the outlet, the resin content is monitored in real time by a resin content detector, and the liquid level in the main impregnation section or the pressure of the pressure roller is adjusted accordingly. In step S7, the prepreg after cooling and setting is placed in a hot air setting chamber at 120°C and 1.5 m / s for 60 seconds, and then the tension is controlled at 1.0 N by a tension control roller group. It is then wound up by an automatic winding machine, and the winding roller is covered with a rubber layer with a Shore hardness of 60A. After winding, it is placed in an environment of 25°C and 50% relative humidity for 24 hours to obtain a natural fiber prepreg with uniform resin distribution and strong interfacial bonding.

2. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, A secondary plasma activation section is added after the chemical grafting reaction tank, using nitrogen plasma to increase the density of epoxy groups on the surface of the silane layer and the density of interfacial crosslinking.

3. The method for preparing natural fiber prepreg according to claim 1, characterized in that, 1% by mass of surface-hydroxylated silica nanoparticles were further added to the resin system. The nanoparticles were dispersed at 5000 rpm for 30 minutes using a high-speed shear disperser before being added, so that the nanoparticles migrated to the fiber-resin interface to form Si-O-Si covalent bonds, thus constructing a three-phase interface structure of "fiber-nanoparticle-resin".

4. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, A vacuum-assisted degassing section is added after the pressure gradient control section. The pressure inside the chamber is maintained at -0.08MPa, the processing time is 60 seconds, and a pressure balancing valve is installed at the outlet to control the pressure transition gradient to ≤0.02MPa / s.

5. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, A focused ultrasonic transducer is used in the ultrasonic-assisted permeation section, with the sound field focused on the fiber bundle core and a sound pressure level of 160dB. The ultrasonic module and the infrared heating module are linked for control to ensure that the resin viscosity is stable in the high shear zone.

6. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, A magnetic field-assisted orientation section is added before the cooling and shaping section, and a permanent magnet array is set to generate a 0.5T magnetic field with an effective area length of 200mm; if the resin system contains 0.5wt% iron oxide nanoparticles, they will be induced to align along the fiber axis.

7. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, A surface coating device is added before winding, and a 25μm thick polyethylene film is simultaneously hot-pressed onto the upper and lower surfaces of the prepreg. The hot-pressing temperature is 100℃, the pressure is 0.3MPa, and the linear speed is 1.0m / min. After coating, the prepreg is frozen and stored at -18℃ for ≥6 months, and its performance does not degrade after thawing.

8. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, The natural fiber is selected from one or more of flax, hemp, jute, sisal or coconut fiber, with a fiber length of 30mm to 50mm, a single filament diameter of 20μm to 50μm, and a moisture content of ≤8%; the epoxy resin system can be replaced with anhydride-cured epoxy resin, phenolic resin or unsaturated polyester resin, and the reactive diluent and curing agent system can be adjusted accordingly to match the reaction activity.

9. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, The multi-stage gradient pressure-temperature coupled impregnation system integrates an online quality monitoring system, including a resin content analyzer, fiber tension sensor, temperature sensor, pressure sensor, and PLC controller. The resin content analyzer has a sampling frequency of 1Hz and an accuracy of ±0.5wt%; the tension sensor has a range of 0-5N and an accuracy of ±0.05N; the temperature sensor is Pt100 with an accuracy of ±0.5℃; the pressure sensor has a range of 0-5MPa and an accuracy of ±0.02MPa; and the PLC controller adjusts the resin pump flow rate, pressure roller hydraulic pressure, heating power, and traction motor speed in real time based on feedback signals.

10. The method for preparing natural fiber prepreg with improved impregnation effect according to claim 1, characterized in that, The method is applicable to the preparation of chopped fiber felt prepreg with fiber length of 25mm to 50mm and areal density of 300g / m². The impregnation stage adopts a double-sided resin spraying + roller pressing penetration process with spraying pressure of 0.2MPa, roller pressing pressure of 1.5MPa, impregnation temperature of 95℃, and residence time of 90 seconds. The resulting prepreg has a resin content of 36wt%, porosity of ≤2.0%, and interlaminar shear strength of ≥42MPa.

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