Thermoplastics are polymers consisting of macromolecules which are held together by weak secondary bonds. If the polymeric molecules have few branches the chains can align to form crystalline regions. A thermoplastic however will never reach a fully crystalline state, the material is at most semi-crystalline. Polymers with a low level of crystallinity are called amorphous. Sophisticated thermoplastic polymers, usually semi-crystalline, can withstand high temperatures and show good mechanical properties. The high-performance and ultra polymers, such as polyphenylenesulfide (PPS) or polyetheretherketone (PEEK) are therefore typically used in fibre reinforced parts for aerospace applications. The strength and stiffness in the composite is provided by reinforcing fibers, for instance glass or carbon. The thermoplastic matrix material binds the fibers and transfers loads between the fibers.
The application of thermoplastic polymers as a matrix material in fibre reinforced composites has grown steadily over the last decades. Thermoplastic composites offer clear advantages over their thermoset counterparts in terms of improved fracture toughness, potential for recycling and, most notably, the possibility to reshape or remold the product at elevated temperatures. The latter allows for rapid processing techniques, such as stamp forming or rubber pressing, as well as novel high-tech bonding techniques such as induction welding and ultrasonic welding.