Research into prepregs began in the 1960s following the successful development of high-performance fibers (such as carbon fibre and aramid fiber). The initial method of preparation involved arranging bundles of fibers in parallel on a glass plate and impregnating them with a resin matrix. In the 1970s, continuous high-performance fibers entered industrial production, and the wet-lay-up method for manufacturing prepregs also progressed to a mechanised production stage. However, this method suffered from drawbacks such as simple equipment, solvent evaporation, and low precision in controlling resin content. Subsequently, the dry-lay-up process was developed. As this process does not require the dissolution of resin in a solvent, there is no issue of solvent evaporation, and the precision of resin content control is higher than that of the wet method; consequently, it gradually replaced the wet-lay-up process. The following sections outline the preparation methods for thermosetting and thermoplastic prepregs. Generally, thermosetting prepregs are prepared using either the wet or dry method, whilst thermoplastic prepregs are typically produced using melt impregnation, powder impregnation, solution impregnation, fibre blending, or film lamination.
Wet-lay-up Process
The wet-lay-up process, also known as the solution process, involves dissolving the resin matrix in a solvent with a low boiling point to form a solution of a specific concentration. Fibre bundles or fabrics are then impregnated in the resin solution at a controlled rate, with the resin content regulated by metering rollers. The pre-impregnated material is then dried in an oven to volatilise the low-boiling-point solvent, after which the impregnated material is wound onto a roll. The wet method is characterised by simple equipment, ease of operation and high versatility. However, a disadvantage of this method is the difficulty in precisely controlling the ratio of reinforcing fibres to resin matrix, making it challenging to achieve a uniform distribution of the resin matrix within the prepreg. Furthermore, the amount of solvent volatilisation is difficult to control and the ash residue causes environmental pollution; consequently, the wet process has gradually been phased out in other countries.
Dry Process
The dry process, also known as the hot-melt process, involves first melting the resin at high temperature and then impregnating the reinforcing fibers using various methods to produce pre-impregnated fabrics. Depending on the processing stage following resin melting, the dry process can be divided into a one-step method and a two-step method. In the one-step method, the fibers are directly impregnated by passing them through a resin bath containing molten resin, followed by drying and winding. In the two-step method, the molten resin is first evenly coated onto impregnation paper on a film coater to form a film, which is then laminated with fibers or fabric and subjected to high-temperature treatment. The advantage of the dry method is that the resin content of the prepreg can be precisely controlled; consequently, the finished products not only have an excellent surface appearance but also result in composite materials with low porosity, thereby avoiding stress concentrations caused by voids. Its disadvantages include relatively complex equipment, a cumbersome preparation process, and specific requirements regarding the resin’s melting point. For thicker prepregs, resin penetration tends to be uneven. In terms of product appearance and the mechanical properties of the composite materials, the hot-melt method is superior to the solution method.
Melt Impregnation Process
Reinforcing fibers and (molten) thermoplastic resin are fed continuously through a pre-impregnation unit, which contains an extruder capable of continuously feeding and heating the resin. The resin content is controlled by metering rollers. The resin comes into direct contact with the fibers, thoroughly impregnating them to produce a composite pre-impregnate. The advantages of this method include easy control of resin content and low ash content in the prepreg (solvent-free), which reduces environmental pollution whilst minimising material loss (particularly of the reinforcing material). Its disadvantages are also evident: firstly, it places high demands on resin viscosity, and the resin must possess good flow properties during processing. Secondly, as the resin remains in a molten state at high temperatures for extended periods, the likelihood of resin decomposition is significantly increased, which consequently affects the performance of the resulting composite material.
Powder Impregnation Method
The powder impregnation method involves depositing electrically charged resin powder onto pre-dispersed fibers, followed by high-temperature treatment to melt the resin and embed it into the fibers. The key advantage of the powder method is its ability to rapidly produce thermoplastic prepregs in a continuous process, with minimal fibre damage, a short processing time, and reduced risk of polymer dispersion, whilst offering cost-effectiveness. A drawback of this method is that the resin powder particles typically have a diameter of 5–10 μm; however, the preparation of resin particles larger than 10 μm is challenging and depends on the time, temperature and pressure required for impregnation.
Solution Impregnation
The solution impregnation method involves completely dissolving the matrix resin in a small amount of solvent to form a solution. A bundle of fibers is then passed through a sizing tank containing the matrix resin solution, allowing the fibers to become coated with resin. After allowing the resin to fully impregnate the fibers, the solvent is removed and the material is dried, ultimately yielding a prepreg. The advantages of this method include the ease with which the resin matrix penetrates the reinforcing material, the ability to produce thick prepregs, and the relatively low cost of equipment. Its disadvantages include specific requirements regarding the boiling point of the resin; for PEEK and PPS (crystalline) resins, there are no suitable low-boiling-point solvents available, so the solution method cannot be used for their prepregs. Furthermore, the use of solvents can cause significant environmental pollution.
Fiber Blending Method
The fibre blending method involves spinning thermoplastic resin into fibers or fibre tapes. The reinforcing fibers and resin fibers are then blended in a specific ratio according to the resin content to form a compact blended yarn. This blended yarn is subsequently woven into a specific product shape, and finally, the resin is melted under high temperature and embedded within the fibers. The advantages of the fibre-blending method include the ease of controlling resin content, the fact that the resin impregnation process occurs simultaneously with the curing of the pre-impregnated material, ensuring thorough fibre impregnation, and the ability to directly wind the material into complex-shaped components. The disadvantages are that uniform resin impregnation is difficult to achieve during the process, and the weaving process can easily cause fiber damage.
Film-layering Method
The film-layering method involves placing reinforcing fibers between layers of polymer film, then heating the film to melt the resin and impregnate the fibers, thereby producing a composite prepreg. The advantage of this process is its simplicity. The disadvantage is that thermoplastic resins—particularly high-performance thermoplastic resins—have very high viscosity in the molten state, which hinders fibre impregnation; consequently, the composites produced by this method do not exhibit high performance.