Processing of Polymeric Materials

Polymeric materials can be readily processed to complex shaped with precise dimensions with a minimum of energy and environmental effect, which to a large extent is due to their thermal and rheological properties. Polymers in general have a low melting temperature, which for semicrystalline polymers is the crystal melting point (Tm) and for amorphous polymers is the glass transition temperature (Tg) (cf. Gedde and Hedenqvist 2019a, b). Thermoplastic processing is carried out at temperatures between the melting temperature (Tm or Tg) and the decomposition temperature, the latter being related to the ceiling temperature (Snow and Frey 1943). In some melt processing methods, the hot polymer melt has the access to oxygen, and in such cases, the polymer needs to be protected by antioxidant to avoid thermal oxidation. Most polymers are processed at 200 ± 50 °C, which is at a much lower temperature than the processing of competing materials, such as inorganic glass (1000–1200 °C), steel (1400–1600 °C), aluminium (>700 °C) and brass (1000 °C). Processing of polymers is more energy and environmentally (CO2) friendly than processing of the alternative materials (cf. Chap. 10), even though the heat capacities of polymeric materials are generally higher than those of the competitive materials. There are three basic steps in polymer processing: heating, shaping (involving shear and extensional flow) and cooling. Heating is usually accomplished externally from electric heaters or infrared radiation, but it occurs also by the heat released from the viscous flow of the polymer melt and the friction between the melt and the surrounding materials. The latter is more prevalent in certain processing methods, such as injection moulding and extrusion. Polymer processing is either carried out as a continuous process (e.g. extrusion) or through repeated cycles (e.g. injection moulding), and in both cases automation is possible.