Degradation Mechanism
Polymer deterioration under the joint influence of elevated temperatures and atmospheric oxygen results in rapid chemical breakdown during melt processing. The reaction pathway of thermo oxidative degradation involves the formation of free radicals that attack the polyester backbone, causing chain scission and discoloration. This dual attack of heat and oxygen is particularly severe in recycled polymer melt, which may already contain unstable chemical structures.
Chemical Reaction
Autoxidation begins when thermal energy generates radicals that react with oxygen to form hydroperoxides. These intermediate compounds then decompose into reactive alkoxy and alkyl radicals, which propagate the degradation cycle. The result is a rapid reduction in intrinsic viscosity and a distinct yellowing of the polymer melt.
Fibre Performance
Yarn produced from degraded resin suffers from poor dye uptake and reduced tensile strength. Yellowing of the polymer limits its use in white or light-colored fabrics, forcing mills to reserve the material for darker shades. High levels of degradation also increase the frequency of filament breakages during the high-speed spinning process.
Control Variable
Minimizing oxygen exposure within the extruder is critical to preventing this form of degradation. Melt channels are sealed and purged with nitrogen to exclude air from the heating zones. In addition, processing temperatures are kept as low as possible to reduce the rate of radical generation and protect the polymer melt.
Mill technicians routinely measure the yellowness index of the extruded pellets to confirm the absence of oxidative damage. This quality check protects the aesthetic and mechanical performance of the final textile fibers.
