The impregnation methods for continuous fiber-reinforced thermoplastic composites mainly include solution impregnation, melt impregnation, powder impregnation, slurry resin deposition, commingling, film stacking, and reactive impregnation.

01. Solution Impregnation

Solution impregnation involves dissolving the resin in a suitable solvent to reduce its viscosity to a certain level. Then, the process used for thermosetting resin impregnation is used to wet the fiber, and finally the solvent is removed by heating.

Advantages of Solution Impregnation: Overcomes the drawback of high viscosity of thermoplastic resin melts, enabling good fiber impregnation; simple preparation process and equipment.

Disadvantages of Solution Impregnation: The solvent must be completely removed, otherwise the solvent resistance of the product will decrease; physical delamination occurs during the solvent removal process, penetration along the resin-fiber interface, and the solvent may accumulate in the small holes and voids on the fiber surface, resulting in poor resin-fiber interface and affecting solvent resistance; solvent evaporation and recovery are expensive and pollute the environment.

Despite this, the preparation of many high-performance resin composites that are difficult to impregnate using other preparation techniques still mostly uses solution impregnation.

02. Melt Impregnation

Melt impregnation is a preparation technique in which the thermoplastic resin is heated and melted before impregnating the fibers. This can be achieved in two ways: One is melt extrusion impregnation, which uses an extruder to feed the melt into a mold through which the fibers pass. The main factor affecting the melt extrusion impregnation process is the speed at which the molten polymer penetrates the fiber layer, which depends on the structure of the reinforcing material. The other is melt pultrusion impregnation, which uses a special pultrusion die head. A bundle of uniformly dispersed, pre-tensioned continuous fibers passes through a roller system with molten matrix resin flowing between a series of rollers. Repeated alternating changes force the fibers and melt to impregnate, achieving the desired impregnation effect. However, this method can only be used to produce long fiber-reinforced particles (generally 6-10mm in length) and not sheets.

In both methods, the pressure applied to the fibers is very high, which can cause fiber damage. However, the main advantage of melt impregnation is that it does not require any solvent.

03. Powder Impregnation

In powder impregnation, resin fine powder is adsorbed onto the surface of the fiber filaments by electrostatic action in a fluidized bed, then heated to melt the powder, and finally the fiber is wetted during the molding process. Because impregnation is carried out in a dry state, the processing process is not limited by the viscosity of the matrix, and polymers with relatively high molecular weight can be distributed into the fibers. The diameter of the polymer particles that can be adsorbed on the fibers is in the range of 5-25μm, and the resin powder diameter is preferably 5-10μm.

Advantages of Powder Impregnation: Less fiber damage, no polymer degradation; fast processing speed, low cost.

Disadvantages of Powder Impregnation: Wetting can only be completed during the molding process, and the powder is easily lost; the time, temperature, and pressure required for wetting depend on the size and distribution of the powder particles.

04. Slurry Resin Deposition

The process of slurry resin deposition is similar to that of papermaking. Short glass fibers (6-25mm in length), resin powder, and emulsifier are dispersed in water to form an aqueous suspension. A flocculant is then added to cause coagulation on the filter screen of a hydraulic molding machine, separating the coagulate from the water, and forming a felt-like coagulate by hot pressing and melting it into a sheet.

Advantages of Slurry Resin Deposition: Good fiber dispersion, little damage, less heat, high production efficiency; Disadvantages: High technical difficulty and high equipment cost.

05. Commingling

Commingling involves closely combining thermoplastic resin spun into fibers or films with reinforcing fibers in a certain proportion to form a mixed yarn, and then melting the resin and fiber into a matrix through a high-temperature sealed impregnation zone. Ordinary weaving processes can easily make the mixed fibers into fabrics. The more uniform the mixing, the less pressure is needed for curing. The ideal state of mixing is that each reinforcing fiber is adjacent to the matrix fiber, but because of the large difference in physical properties between the reinforcing fiber and the matrix fiber, this is actually very difficult to achieve. Commingling has good processing performance, easy control of resin content, sufficient fiber wetting, and mixed yarns can be woven into various complex shapes, including three-dimensional structures, and can also be directly wound to produce high-performance composite materials.

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

06. Film Stacking

Film stacking involves stacking layers of fiber-reinforced material and thermoplastic sheets. The polymer is allowed to flow between the reinforcing materials by heating and pressing, and then cured. The pressure applied in film stacking must be sufficient to allow the melt to enter between the fiber layers without flowing between the reinforcing layers. Typical pressure values are less than 2.0 MPa. The composite after cooling should be free of voids; vacuum-assisted pressing can ensure that the sheet is void-free. This method is widely used in molding sheets with complex surface shapes.

Advantages of Film Stacking: High-quality laminated products can be obtained, but due to the high viscosity of the melt, high pressure is required;

Disadvantages of Film Stacking: High resin content leads to high costs; the high viscosity matrix resin is difficult to infiltrate into the fibers.

07. Reaction impregnation method

The reaction impregnation method utilizes the characteristics of monomers or prepolymers, such as their low initial molecular weight, low melt viscosity, and good fluidity, which allows for sufficient fiber infiltration. Continuous fiber-reinforced thermoplastic resin matrix composites are prepared through in-situ polymerization. However, this process has rather demanding conditions and the reaction is difficult to control, and it has not yet been industrialized.

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