Overview

Owing to the nature of polymer processing (mainly casting, injection, extrusion or moulding), deteriorative reactions occur at this early stage when polymers are subjected to heat and mechanical stress, leading to further degradation during the useful life of the materials, when heat and oxygen are the most important degradative factors. In more specialised applications, degradation may be induced by high-energy radiation, ozone, biological action, hydrolysis and many other influences. However, there are also new developments in polymer technology in which degradation processes find positive applications, for instance, recycling of polymeric products, photodegradable plastics and fabrication of fibres such as carbon fibres from polyacronitrile or cellulose. In fact, the microelectronics industry is dependent upon polymer degradation in the manufacture of its circuitry. Besides, degradation and combustion studies are getting more and more involved in the definition of the fire hazards that are associated with polymeric materials. Polymer properties may also be improved by processes like curing and grafting, which may be induced intentionally during processing. The mechanisms and kinetics of all these reactions taking place during processing must be understood if the technology and application of polymers are to continue to advance.

Polymer extrusion is one of the most widely used polymer processing methods. The melting zone during co-rotating twin-screw extrusion has been identified as the most significant determinant in the process in attaining the best properties of the polymeric materials. The axial location in the feed stream of the conversion of solid polymer resin to molten mass...