Current situation of engineering application of thermoplastic composites

In recent years, the fiber reinforced thermoplastic composites based on thermoplastic resin have developed rapidly, and the research and development of this kind of high-performance composite materials is setting off a high tide in the world. Thermoplastic composite material refers to thermoplastic polymer (such as polyethylene (PE), polyamide (PA), polyphenylene sulfide (PPS), polyetherimide (PEI), polyether ketone ketone (PEKK) and polyether ether ketone (PEEK) as the matrix, Composite materials made of various continuous/discontinuous fibers (such as carbon fiber, glass fiber, aramidon fiber, etc.) as reinforcement materials.

1 Thermoplastic resin matrix and reinforcing material

1.1 Thermoplastic matrix

Thermoplastic resins can be classified according to Figure 1 according to their mechanical properties and temperature resistance levels. At present, the thermoplastic resins applied to the aviation field are mainly high-temperature resistant and high-performance resin substrates, including PEEK, PPS and PEI. Among them, amorphous PEI has more applications in aircraft structures than semi-crystalline PPS and high molding temperature PEEK because of its lower processing temperature and processing cost. The main performance parameters of commonly used thermoplastic matrix are shown in Table 1.

Compared to thermosetting resins, Thermoplastic resin has better mechanical properties and chemical corrosion resistance, higher service temperature, high specific strength and hardness, excellent fracture toughness and damage tolerance, excellent fatigue resistance, can mold complex geometric shapes and structures, adjustable thermal conductivity, recyclability, good stability in harsh environments, repeatable molding, welding and repair characteristics. The composite material composed of thermoplastic resin and reinforcement material has durability, high toughness, high impact resistance and damage tolerance. Fiber prepreg no longer need to be stored at low temperature, unlimited prepreg storage period; Short molding cycle, weldable, high production efficiency, easy repair; Waste can be recycled; The product design freedom is large, can be made into complex shapes, forming adaptability and many other advantages.

1.2 Reinforcement Materials

The properties of thermoplastic composites not only depend on the properties of resins and reinforced fibers, but also closely related to the fiber reinforcement methods, which have three basic forms: short fiber reinforcement, long fiber reinforcement and continuous fiber reinforcement

1.2.1 Short fiber reinforcement

In general, the length of staple fiber reinforcement is 0.2 to 0.6mm, and since most fibers are less than 70μm in diameter, the staple fiber looks more like a powder. Short fiber reinforced thermoplastics are generally manufactured by mixing fibers into a molten thermoplastic. The fiber length and random orientation in the matrix make it relatively easy to achieve good wetting, and short fiber composites are the easiest to manufacture with minimal improvement in mechanical properties compared to long fiber and continuous fiber reinforced materials. Short fiber composites tend to be molded or extruded to form final parts because short fibers have less impact on fluidity.

1.2.2 Long fiber reinforcement

The fiber length of long fiber reinforced composite materials is generally about 20mm, which is usually prepared by continuous fiber wetting resin and cutting into a certain length. The process commonly used is pultrusion molding, which is produced by drawing continuous roving mixed with fiber and thermoplastic resin through a special molding die. In the process, the mold is heated and pressurized to melt the resin material and make it flow around the fiber, ensuring proper wetting, and when completed, the fully mixed fiber is cooled and then cut to the desired length. In order to make the final part, long fiber reinforced composite short cuts are often used in compression molding or extrusion molding processes, where the short cuts are placed in a mold, which is heated and pressurized. Because longer fibers inhibit flow and make it difficult to fill all parts of the mold, the manufacturing process is relatively difficult, but the improvement in mechanical properties is more obvious than that of short fiber reinforcement. At present, the structural properties of long fiber reinforced PEEK thermoplastic composite materials through FDM printing can reach more than 200MPa, the modulus can reach more than 20GPa, and the performance will be better through injection molding.CFRTP

1.2.3 Continuous fiber reinforcement

The fibers in continuous fiber reinforced composites are "continuous", ranging in length from a few meters to several thousand meters, and continuous fiber composites generally provide laminates, prepregs or braided fabrics, etc., by impregnating continuous fibers with the desired thermoplastic matrix. Among them, the thickness of the continuous fiber composite prepreg belt is 0.127~0.762mm, the width is 1.6mm to hundreds of mm, and the wider prepreg belt is usually cut into the required width for final processing. Fiber length has a great impact on the properties of composite materials. Basically, the longer the fiber length, the more conducive to the improvement of material properties. The contribution of fiber length to the strength of composite materials can be understood from two aspects:

(1) When the fiber length is less than the critical length, the interface area between the fiber and the resin increases with the increase of the fiber length. When the composite material breaks, the resistance of the fiber drawn from the resin increases, thus improving the ability to bear the load;

(2) When part of the fiber length reaches the critical length, the fracture of the composite material is accompanied by the fracture of more fibers, which also improves the ability to bear the load.

Thermoplastic composites have begun to be used on a large scale in foreign countries. Material providers represented by TenCate, Victrex, etc., automation equipment providers represented by Automated Dynamics, manufacturing research units represented by KVE, TPRC, FOKKER, etc. Aviation application enterprises represented by Airbus and Boeing have developed systematically and their technologies have become increasingly perfect. At present, the main challenges and research directions for the development of thermoplastic composites are as follows:

(1) Improve the application proportion of the main bearing structure. At present, most foreign thermoplastic composites are used in secondary load bearing structures, and the main load bearing structures are relatively few applications. Under the Thermoplastic Economically Affordable Aviation Main Structure (TAPAS) project launched by the European Union, the proportion of thermoplastic composites used in current and future aircraft will be further increased, especially in main bearing structures such as next-generation aircraft fuselages.

(2) Reduce the raw material cost of high-performance thermoplastic composite materials represented by PEEK. At present, the high melting point temperature, large viscosity and complex prepreg production process of high-performance resins such as PEEK are the main reasons for the high cost of raw materials. Reducing costs by optimizing the manufacturing process and working with customers to increase production, as well as developing new high-quality and affordable materials (such as PAEK).

(3) The challenge of large-scale applications (such as the automotive industry). How to produce products with reliable quality and no defects on a large scale, that is, the reliability and robustness of the production process; The construction of a mature supply chain system; Cost control of the entire production process, such as workshop transformation, automatic production line establishment, raw material storage, etc. The design level is improved based on the performance characteristics of different thermoplastic composites.

(4) Improve the molding process. For the automatic laying in situ curing process, especially for high-performance thermoplastic resin composite materials, the laying speed is improved mainly by optimizing the process and improving the heating method (such as the laying temperature of PEEK matrix composite materials needs to reach 400℃). For cladding molding, continue to improve the interface bonding strength between injection molding and stamping parts. For the welding process (resistance welding and induction welding), it is mainly to improve the welding components and processes, reduce or even eliminate the interface residue generated by the process method, improve the welding performance, and continue to develop welding units suitable for high-performance products such as PEEK, and improve the new welding technology.

(5) Strengthen the development of recycling technology. Through the recycling of thermoplastic composite waste, secondary processing and forming, and economic evaluation.

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