In recent years, fiber-reinforced thermoplastic composites based on thermoplastic resins have developed rapidly, setting off a worldwide boom in the research and development of such high-performance composite materials. Thermoplastic composites are composite materials made of thermoplastic polymers (such as polyethylene (PE), polyamide (PA), polyphenylene sulfide (PPS), polyetherimide (PEI), polyetheretherketone ketone (PEKK), and polyetheretherketone (PEEK), etc.) as the matrix and various continuous/discontinuous fibers (such as carbon fiber, glass fiber, aramid fiber, etc.) as the reinforcing materials.

Thermoplastic resin-based composites mainly include long fiber reinforced granules (LFT), continuous fiber reinforced prepreg MT, and glass fiber reinforced thermoplastic composites (CMT). Depending on the requirements, the resin matrix includes thermoplastic engineering plastics such as PPE, PAPRT, PELPCPES, PEEKPI, and PA. Fiber types include glass fiber, aramid fiber, boron fiber, and all other possible fiber varieties. With the continuous maturity of thermoplastic resin-based composite material technology and the development of recyclability, this type of composite material has developed rapidly. In developed countries in Europe and America, thermoplastic composites already account for more than 30% of the total amount of resin-based composite materials.

Thermoplastic Matrix

A thermoplastic matrix is a thermoplastic material with good mechanical properties and heat resistance, suitable for manufacturing various industrial products. Thermoplastic matrices are characterized by high strength, high heat resistance, and good corrosion resistance. Currently, the thermoplastic resins used in aerospace applications are mainly high-temperature, high-performance resin matrices, including PEEK, PPS, and PEI. Among them, amorphous PEI, with its lower processing temperature and processing cost, is used more in aircraft structures than semi-crystalline PPS and high-forming-temperature PEEK.

Thermoplastic resins have better mechanical properties and chemical corrosion resistance, higher operating temperatures, high specific strength and hardness, excellent fracture toughness and damage tolerance, excellent fatigue resistance, the ability to be molded into complex shapes and structures, adjustable thermal conductivity, recyclability, good stability in harsh environments, repeatability, weldability, and repairability.

Composite materials composed of thermoplastic resins and reinforcing materials have durability, high toughness, high impact resistance, and damage tolerance; fiber prepregs do not need to be stored at low temperatures, with an unlimited prepreg storage period; short molding cycle, weldable, high production efficiency, easy to repair; waste can be recycled and reused; large product design freedom, can be made into complex shapes, wide molding adaptability, and many other advantages.

Reinforcing Materials

The performance of thermoplastic composites depends not only on the properties of the resin and reinforcing fibers but also is closely related to the fiber reinforcement method. The fiber reinforcement methods for thermoplastic composites include three basic forms: short fiber reinforcement, long fiber reinforcement, and continuous fiber reinforcement.

Generally, the length of short fibers is 0.2-0.6 mm. Since the diameter of most fibers is less than 70 μm, short fibers appear more like powder. Short fiber-reinforced thermoplastics generally mix the fibers into molten thermoplastic to manufacture. The fiber length and random orientation in the matrix make good wetting relatively easy. Compared with long fiber and continuous fiber reinforced materials, short fiber composites are the easiest to manufacture but have the least improvement in mechanical properties. Short fiber composites tend to form final parts by molding or extrusion methods because short fibers have less impact on fluidity. The fiber length of long fiber-reinforced composites is generally about 20 mm, usually prepared by cutting continuous fibers impregnated with resin into a certain length. The commonly used process is pultrusion, which produces a continuous roving of fibers and thermoplastic resin mixture by stretching through a special molding die. Currently, the structural performance of long-fiber reinforced PEEK thermoplastic composites formed by FDM printing can reach more than 200 MPa, and the modulus can reach more than 20 GPa. The performance will be better through injection molding.

In continuous fiber-reinforced composites, the fibers are "continuous," ranging in length from a few meters to several kilometers. Continuous fiber composites generally mainly provide laminates, prepregs, or woven fabrics, formed by impregnating continuous fibers with the required thermoplastic matrix.     

What are the characteristics of fiber-reinforced composites?

Fiber-reinforced composites are composite materials formed by reinforcing fiber materials, such as glass fiber, carbon fiber, and aramid fiber, with matrix materials through winding, molding, or pultrusion processes. According to the different reinforcing materials, common fiber-reinforced composites are divided into glass fiber-reinforced composites (GFRP), carbon fiber-reinforced composites (CFRP), and aramid fiber-reinforced composites (AFRP).

Because fiber-reinforced composites have the following characteristics: high specific strength, high specific modulus; designable material properties; good corrosion resistance and durability; thermal expansion coefficient similar to concrete. These characteristics enable FRP materials to meet the needs of modern structures for large spans, high rise, heavy loads, lightweight and high strength, and operation under harsh conditions, while also meeting the requirements of the development of modern building construction industrialization. Therefore, they are increasingly widely used in various civil buildings, bridges, highways, marine, hydraulic structures, and underground structures.

Great Prospects for the Development of Thermoplastic Composites

According to reports, the global thermoplastic composites market size is projected to reach $62.62 billion by 2030, with a compound annual growth rate of 7.8% during the forecast period. This growth can be attributed to the growing product demand from the aerospace and automotive industries and the exponential growth of the construction industry. Thermoplastic composites are used in the construction of residential buildings, infrastructure, and water supply facilities. Properties such as excellent strength, toughness, and the ability to be recycled and remolded make thermoplastic composites an ideal choice for manufacturing in building applications.

Thermoplastic composites will also be used to produce storage tanks, lightweight structures, window frames, utility poles, railings, pipes, panels, and doors. The automotive industry is one of the key application areas. Manufacturers are focusing on improving fuel efficiency, and to this end, they are replacing metals and steel with lightweight thermoplastic composites. For example, carbon fiber weighs one-fifth that of steel, so it helps reduce the overall weight of the vehicle.

The prospects for thermoplastic composites are huge, and domestic manufacturers are also investing heavily in R&D, hoping that with the joint efforts of everyone in the future, domestic composite material technology can achieve a leading position internationally.

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