The impregnation methods of continuous fiber thermoplastic composites mainly include solution impregnation method, melt impregnation method, powder impregnation method, slurry resin deposition method, mixed weaving method, thin film stacking method and reaction impregnation.

01. Solution impregnation method

Solution impregnation is to dissolve the resin in a suitable solvent to reduce its viscosity to a certain level, and then use the process used in the impregnation of thermosetting resin to soak the fiber, and then remove the solvent by heating.

The advantages of the solution impregnation method: it overcomes the shortcomings of the high viscosity of the thermoplastic resin solution, so that the fiber is well impregnated; The preparation process is simple and the equipment is simple.

Deficiencies of the solution impregnation method: the solvent must be completely removed, otherwise it will lead to a decrease in the solvent resistance of the product; In the process of solvent removal, there is physical delamination, penetration along the resin fiber interface, and solvent may accumulate in the small pores and voids on the fiber surface, resulting in poor resin-fiber interface and solvent resistance. Solvent evaporation and recovery are expensive and pollute the environment.

However, most of the high-performance resin composites that are not easy to impregnate by other preparation techniques are still prepared by solution impregnation.

02. Melt impregnation method

Melt impregnation is a preparation technique in which thermoplastic resin is heated and melted and then impregnated with fibers. This can be achieved in two ways: one is melt extrusion impregnation, in which the melt is fed into the mold through which the fibers pass through using an extruder. The factors influencing the melt extrusion impregnation process are mainly the speed at which the melt polymer penetrates the fiber layer, depending on the structure of the material. The other is melt pultrusion impregnation, which adopts a special structure of pultrusion die, so that the continuous fiber bundle of uniform dispersion and pre-tension passes through a roller system with molten matrix resin flowing between a series of wheel trains, and repeatedly undergoes alternating changes to make the fiber and melt forced impregnation to achieve the ideal impregnation effect. However, this method can only be used to produce long fiber particles (generally 6~10mm in length) instead of sheets.

In both of these methods, the pressure exerted on the fibers is very high, which can lead to fiber damage. The main advantage of the melt impregnation method is that it does not require any solvent.

03. Powder impregnation method

The powder impregnation method is to adsorb the resin fine powder on the surface of the fiber monofilament through electrostatic action in the vulcanization bed, and then heat to make the powder melt, and then make the fiber wetted during the molding process. Due to the impregnation in the dry state, the processing is not limited by the viscosity of the matrix, and polymers with relatively high molecular mass can be distributed into the fibers. The diameter of polymer particles that can be adsorbed on the fiber is in the range of 5~25μm, and the diameter of resin powder is 5~10μm.

The advantages of the powder impregnation method: less fiber damage and no degradation of the polymer; The processing speed is fast and the cost is low.

Disadvantages of powder impregnation method: infiltration can only be completed in the molding process, and the powder is easy to be lost; The time, temperature, and pressure required for wetting depend on the size and distribution of the powder particle size.

04. Slurry resin deposition method

The process of slurry resin deposition is similar to that of papermaking. The glass fiber, resin powder and emulsifier with a chopped length of 6~25mm are dispersed together in water to become a water suspension, and then a flocculant is added to make it condense on the filter screen of the hydraulic forming machine, so that the agglomeration is separated from the water, hot-pressed into a felt-like agglomeration, and melted into a sheet.

The advantages of slurry resin deposition method are good fiber dispersion, small damage, less heating, and high production efficiency; The disadvantage is that the technology is difficult and the equipment cost is high.

05. Mixed method

The hybrid method is to tightly combine the thermoplastic resin spun into fibers or film tapes in a certain proportion to make mixed sand, and then melt the resin fibers into a matrix through a high-temperature sealed impregnation zone. The mixed fibers can be easily made into fabrics with the general weaving process, the more uniform the mixing, the less pressure required during curing, the ideal state of mixing is that each fiber is adjacent to the matrix fiber, but due to the large difference in physical properties between the fiber and the matrix fiber, it is actually difficult to achieve. The mixed knitting method has good processing performance, the resin content is easy to control, the fiber can be fully wetted, and the mixed yarn can be woven into various complex shapes, including three-dimensional structure, or can be directly wound to obtain composite materials with excellent performance.

However, this technology is not suitable for the composite of glass fiber materials and the molding of daily necessities or low-temperature thermoplastic engineering materials.

06. Thin film lamination method

In the film lamination method, layers of fibrous material and sheets of thermoplastic material are superimposed, and the polymer flows between the materials under heating and pressurization, and then solidifies. The pressure applied by the film lamination method should be large enough that the melt can enter between the fiber layers without flowing between the layers, and the typical pressure value is less than 2.0MPa. The cooled compound should be free of holes, and vacuum-assisted pressure can ensure that the sheet is non-porous. This method is widely used to form sheets with complex surface shapes.

Advantages of the film lamination method: high-quality laminated products can be prepared, but high pressure is required due to the high viscosity of the melt;

Disadvantages of the film stacking method: high resin content and high cost; High-viscosity matrix resins are difficult to infiltrate into fibers.

07. Reaction impregnation method

The reactive impregnation method is to prepare continuous fiber thermoplastic resin matrix composites by in-situ polymerization by taking advantage of the characteristics of small initial molecular weight, low melt viscosity, good fluidity and full wetting of fibers by monomer or prepolymer. However, this process condition is relatively harsh, the reaction is not easy to control, and industrialization has not yet been realized.

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