Continuous fiber reinforced thermoplastic composite material (CFRT for short) refers to a lightweight, high-strength, high-rigidity, high-toughness, and recyclable formed by thermoplastic resin as a matrix and continuous fiber as a reinforcing material through resin melt impregnation and extrusion. New thermoplastic composite materials.
Films and sheets made of CFRTP can be combined in various ways, thus providing designers with more design freedom
Image: Covestro AG
Continuous fiber reinforced thermoplastic composites have excellent properties and have broad application prospects in transportation, aerospace and defense, industrial and sports goods. According to Lucintel's forecast, from 2018 to 2023, the global CFT market is expected to grow at a compound annual growth rate of 8.0%.
Application fields of CFRT Picture from GPM
The production of thermoplastic composite parts generally requires two processes:
(1) Preparation of sheets (pre-impregnated);
(2) Forming of parts.
Prepreg is a resin matrix impregnated with continuous fibers or fabrics under strictly controlled conditions to make a composition of resin matrix and reinforcement. It is an intermediate material for manufacturing composite materials. Certain properties of prepreg are directly brought into composite materials and are the basis of composite materials. The performance of the composite material depends largely on the performance of the prepreg.
Continuous fiber reinforced thermoplastic composite material unidirectional prepreg tape
Compared with thermosetting resin-based composites, continuous fiber reinforced thermoplastic composites are more complicated to impregnate. Due to the high melt viscosity of thermoplastic resins, generally exceeding 100 Pa·s, the melt flow is difficult, making it difficult for reinforced fibers to obtain good impregnation. Therefore, solving the problem of the impregnation of continuous reinforcing fibers with thermoplastic resin has become a key technology for preparing CFRTP sheets.
The impregnation methods of continuous fiber reinforced thermoplastic composite materials mainly include solution impregnation method, melt impregnation method, powder impregnation method, slurry resin deposition method, mixed weaving method, film lamination method and reactive impregnation.
01 Solution immersion method
Solution impregnation is to dissolve the resin in a suitable solvent to reduce its viscosity to a certain level, then use the process used in thermosetting resin impregnation to infiltrate the fiber, and finally remove the solvent by heating.
Advantages of solution immersion method:
Overcoming the shortcomings of high melt viscosity of thermoplastic resin, the fiber is well impregnated;
The preparation process is simple and the equipment is simple.
Disadvantages of solution dipping method:
The solvent must be completely removed, otherwise the solvent resistance of the product will decrease;
There is physical stratification in the process of solvent removal, penetration along the resin fiber interface and the solvent may accumulate in the small pores and voids on the fiber surface, resulting in a poor resin-fiber interface and affecting solvent resistance;
Solvent evaporation and recovery are expensive and pollute the environment.
Nevertheless, some high-performance resin composite materials that are not easy to be impregnated with other preparation techniques are mostly still prepared by solution impregnation.
02 Melt dipping method
Melt impregnation is a preparation technique in which the thermoplastic resin is heated and melted and then the fibers are impregnated. It can be achieved in two ways:
One is melt extrusion dipping, which uses an extruder to feed the melt into a die through which the fibers pass. The main factor affecting the melt extrusion and impregnation process is the speed at which the melt polymer penetrates the fiber layer, which depends on the structure of the reinforcement material.
The other is melt pultrusion and impregnation. A pultrusion die with a special structure is used to allow uniformly dispersed and pre-tensioned continuous fiber bundles to pass through a series of wheel trains with molten matrix resin flowing between the roller system. Subsequent to alternating changes, the fiber and melt are forced to impregnate to achieve the ideal impregnation effect. But this method can only be used to produce long-fiber reinforced particles (typically 6-10mm in length) instead of sheets.
The above two methods put a lot of pressure on the fiber, which will cause fiber damage. The main advantage of the melt dipping method is that it does not require any solvent.
03 Powder dipping method
The powder impregnation method is to absorb the resin fine powder on the surface of the fiber monofilament through electrostatic action in the vulcanized bed, and then heat to fuse the powder, and finally infiltrate the fiber during the molding process. Due to the impregnation in the dry state, the processing process is not limited by the viscosity of the matrix, and the polymer with high molecular weight can be distributed into the fiber.
The diameter of the polymer particles that can be adsorbed on the fiber is in the range of 5~25μm, and the diameter of the resin powder is preferably 5~10μm.
Advantages of powder dipping method:
Less fiber damage, no polymer degradation;
The processing speed is fast and the cost is low.
Disadvantages of powder impregnation:
The infiltration can only be completed during the molding process, and the powder is easy to lose;
The time, temperature and pressure required for infiltration depend on the size and distribution of the powder particle size.
04 Paste resin deposition method
The slurry resin deposition method was developed by the French paper company Arjomari and the British Wiggins Teape company, and its process is similar to the papermaking process. Arjomari disperses glass fibers with a chopped length of 6-25mm, resin powder and emulsifier in water together to form an aqueous suspension, and then adds flocculants to agglomerate on the filter screen of the hydraulic forming machine to make the aggregates and water Separated, hot pressed into felt-like aggregates, and melted into sheets.
The advantages of the slurry resin deposition method are good fiber dispersion, less damage, less heat, and high production efficiency; the disadvantages are high technical difficulty and high equipment cost.
05 Mixed method
The mixing method is to tightly combine the thermoplastic resin spun into fibers or film tapes with reinforcing fibers in a certain proportion to form mixed sand, and then pass through a high-temperature sealing impregnation zone to melt the resin fibers into a matrix.
The mixed fibers can be easily made into fabrics by the general weaving process, and the mixing is about uniform. The lower the pressure required for curing, the ideal state of mixing is that each reinforcing fiber is adjacent to the matrix fiber, but due to the reinforcement The physical properties of the fiber and the matrix fiber are quite different, which 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 infiltrated, and the mixed yarn can be woven into various complex shapes, including three-dimensional structures, or it can be wound directly to produce 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 stacking method
The film lamination method is to superimpose the fiber reinforced material layer and the thermoplastic material sheet, heat and press to make the polymer flow between the reinforced materials, and then solidify.
The pressure applied by the film lamination method should be large enough so that the melt can enter between the fiber layers without flowing between the reinforcing layers. The typical pressure value is less than 2MPa. The composite after cooling should have no holes, and vacuum-assisted pressure can ensure that the sheet is non-porous. This method is widely used for forming sheets with complex surface shapes.
Advantages of film stacking method:
High-quality laminates can be produced, but due to the high viscosity of the melt, higher pressure is required.