Research Progress on Preparation Technologies of High-Performance Continuous Fiber-Reinforced Thermoplastic Prepregs and Composites (with Multiple Figures)

Abstract

Thermoplastic composites, with their advantages of excellent fatigue resistance, short molding cycles, and recyclability, exhibit broad application prospects in fields such as aerospace and rail transit. This paper systematically reviews the preparation processes of continuous fiber-reinforced thermoplastic prepregs (including solution impregnation, melt impregnation, suspension hot-melt methods, etc.) and composite molding technologies (such as compression molding, automated placement molding, in-situ consolidation molding, etc.). It analyzes the technical characteristics, engineering application feasibility, and research progress of each process, and provides an outlook on future development directions.

Keywords: Thermoplastic prepreg; Preparation process; Suspension hot-melt method; Molding process; Automated placement

1. Introduction

Resin-based composites occupy an important position in aerospace equipment, and their usage has become a key indicator for measuring the advancement of such equipment. Thermoplastic composites, due to their superior fatigue resistance, short molding cycles, and secondary processability, are ideal choices for lightweight structures. This paper focuses on the preparation of thermoplastic prepregs and composite molding processes, aiming to provide technical references for engineering applications.

2. Preparation Processes of Thermoplastic Prepregs

The core of the preparation process for thermoplastic prepregs lies in achieving uniform impregnation of fibers with resin and structural stabilization.

2.1 Solution Impregnation Method

As a traditional process, the solution impregnation method involves dissolving the resin in a solvent to form a low-viscosity impregnating solution, allowing the fiber bundles to complete impregnation under the extrusion of rollers, followed by removal of the solvent in a high-temperature oven. This process has strict requirements for resin solubility, necessitating that the solvent's boiling point falls between the impregnation temperature and the resin's decomposition temperature, and there is a risk of solvent residue. Currently, it is only used for laboratory research. For example, using PES to modify the surface of T700 carbon fibers and combining it with the solution impregnation method, the tensile strength of the prepared CF/PEEK composite was increased by 13.69% (Shen Weixin et al., 2021).

Figure 1: Process flow chart of the solution impregnation method

2.2 Melt Impregnation Method

The melt impregnation method involves heating the resin to a molten state (e.g., PEEK requires 360-380°C) in a screw extruder and achieving impregnation through dynamic contact between the impregnation die and the fiber bundles. This process offers advantages such as precise control of resin content (±2%) and good fiber alignment, but it has extremely high requirements for resin melt fluidity. Li Xingle's research found that at a pultrusion temperature of 360°C, the tensile strength of PEEK prepreg tows reached 1660 MPa, while excessive temperature (370°C) led to resin degradation. Bai Yanbo successfully prepared prepreg tapes with a fiber volume fraction of 66.5% and a porosity of only 1.33% by optimizing the parameters of the PA6 impregnation die.

Figure 2: Process flow chart of the melt impregnation method

2.3 Film Stacking Method

This method involves first preparing a resin film and then combining it with reinforcing fibers. The process is simple and allows for segmented quality control. However, it is suitable for low-viscosity thermoplastic resins. For high-viscosity resins (such as PEEK), the impregnation effect of the prepreg is poor, requiring subsequent hot-pressing molding to enhance fiber impregnation. However, excessive pressure can easily cause fiber wrinkles, affecting resin impregnation. Therefore, pressure is a critical parameter in the film stacking method.

Figure 3: Process flow chart of the film stacking method

2.4 Powder Impregnation Method

This includes the fluidized bed powder method and the powder sprinkling method. The former involves filling the spaces between fibers with resin powder and then heating for consolidation, which has high requirements for resin particle size and makes it difficult to precisely control resin content. The latter involves sprinkling resin powder onto the fibers and heating for preparation, which is suitable for resins with good fluidity.

Figure 4: Process flow charts of the fluidized bed powder method and the powder sprinkling method

2.5 Suspension Hot-Melt Method

As a mainstream industrial technology, the suspension hot-melt method involves dispersing resin powder (particle size 5-25μm) in a water-based suspension and using ultrasonic cavitation effects to promote the unfolding of fiber bundles. The resin particles are then embedded between the fibers under roller pressure, followed by high-temperature (380-400°C) melting and consolidation. This process is environmentally friendly and suitable for continuous production, and has achieved large-scale preparation of PEEK and PPS prepregs. CF/PEEK prepreg tapes produced by the Barrday company in the United States using this process have a 0° tensile strength of 2669 MPa, and the Cetex TC1220 product from Toray in Japan has an interlayer shear strength exceeding 106 MPa. Domestic research shows that optimizing the dispersant ratio (such as combining Triton-100 and PEG) can extend the suspension stabilization time to 50 minutes and achieve a fiber volume fraction of 60%.

Figure 5: Process flow chart of the suspension hot-melt method

2.6 Fiber Co-Weaving Method

The fiber co-weaving method involves co-weaving resin fibers and reinforcing fibers in a warp and weft pattern, utilizing the melting of resin fibers during hot-pressing to achieve interfacial bonding. This process is suitable for preparing complex curved surface components, but the cost of resin spinning is high, and the fiber volume fraction is limited (usually < 50%). Hasan et al. prepared PA6 composites using recycled carbon fiber co-weaving technology, achieving a tensile strength of 1364 MPa and a flexural modulus of 100 GPa. Zhang Lei et al. prepared laminates with a tensile strength of 825 MPa and a flexural strength of 520 MPa using a carbon fiber/nylon 6 co-weaving process.

Overall, the suspension hot-melt method has become the mainstream preparation technology for high-performance prepregs due to its environmental friendliness and process stability, while the fiber co-weaving method shows unique advantages in the forming of complex structures. Domestically, key technologies in melt impregnation and suspension hot-melt processes have been broken through, but large-scale production equipment still relies on imports. There is a need to strengthen the development of specialized resin systems and the construction of intelligent production lines.

3. Molding Processes of Thermoplastic Composites

The core objective of the molding process for thermoplastic composites is to fully leverage the rapid melt fluidity and reprocessability of thermoplastic resins to achieve efficient manufacturing of structural components.

3.1 Compression Molding

As a representative of traditional processes, compression molding involves placing stacked prepreg layers in a mold for heating and pressurization. It offers advantages such as simple operation and short molding cycles (usually ≤ 10 minutes) and has achieved large-scale applications in flat components such as automotive interior panels and aircraft cabin doors. Research shows that optimizing compression molding parameters (such as a heating temperature of 310-330°C and a molding pressure of 9 MPa) can effectively control the porosity of PPS composites to below 1% and achieve an interlayer shear strength of 53 MPa. However, this process has poor adaptability to complex curved surfaces, limiting its application in primary load-bearing structures in aviation.

3.2 Automated Placement Molding

Automated placement molding technology uses numerical control equipment to precisely place prepreg tapes/tows on the mold surface and combines laser heating (400-420°C) to achieve in-situ local melting, significantly improving placement efficiency (up to 12 m/min) and interlayer bonding strength. Domestic and international research both indicate that the synergistic optimization of placement pressure (10-15 N) and speed (60-100 mm/min) is a critical parameter, enabling the interlayer shear strength of CF/PEEK composites to exceed 68 MPa. It is worth noting that the German DLR Institute successfully prepared an 8-meter-long fuselage skin using this technology, verifying its engineering potential in large aviation structural components.

3.3 In-Situ Consolidation Molding

As an upgrade of automated placement technology, in-situ consolidation molding achieves the integration of placement and curing by precisely controlling the mold temperature field (such as the 130-140°C crystallization window), avoiding the need for a secondary hot-pressing process. Domestic research shows that optimizing laser power (6 kW) and roller pressure (1500 N) can achieve an interlayer bonding degree of 85% for CF/PPS composites, with performance approaching that of autoclave molding. This technology has extremely high requirements for the crystallization kinetics control of prepregs and requires process parameter optimization in combination with molecular chain diffusion models.

3.4 3D Printing Molding

3D printing molding technology achieves the additive manufacturing of complex structures through fused deposition modeling (FDM) or selective laser sintering (SLS), showing unique advantages in fields such as medical implants and customized tooling. Research shows that using a dual-nozzle technology (continuous fibers + short cut fibers reinforcement) can increase the tensile strength of CF/PA6 printed parts to 110 MPa, but the interlayer strength is still lower than that of traditional molding processes (about 60% of that of compression-molded parts). In the future, breakthroughs in interlayer performance bottlenecks need to be achieved through printing path optimization and interface modification.

4. Conclusions and Outlook

4.1 Research Status

Domestically, staged achievements have been made in prepreg preparation and molding processes, but engineering applications still lag behind those in Europe and the United States. The suspension hot-melt method and compression molding technology are relatively mature, while automated processes such as in-situ consolidation and 3D printing are still in the laboratory stage.

In the research field of prepreg preparation and molding processes in China, staged progress has been achieved. However, compared with European and American countries, there is still a gap in engineering applications. Specifically, the suspension hot-melt method and compression molding technology have become relatively mature, but automated processes such as in-situ consolidation and 3D printing are still in the laboratory research stage.

4.2 Development Directions

Looking forward, the development directions in the field of prepreg preparation will mainly focus on the following aspects: First, developing high-stability resin systems and continuous production equipment to address the challenges in the impregnation process of high-performance resins such as PEEK. In terms of molding processes, further research on key technologies such as laser-assisted placement and in-situ consolidation will be conducted to improve interlayer performance and production efficiency. Additionally, efforts will be made to actively promote the demonstration applications of thermoplastic composites in fields such as aviation structural components and rail transit lightweighting, expanding their application scope.

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