Operational challenges in the mechanical recycling of polymer waste: the effect of feedstock variability on process stability and regranulate quality

Abstract

Mechanical recycling remains the principal route for material recovery from thermoplastic polymer waste; however, its further development is constrained by the substantial variability of real feedstock streams. Post-consumer materials differ in polymer composition, service and degradation history, and in the contents of additives, pigments, fillers, as well as organic, inorganic and polymeric contaminants. This article reviews international research on how this variability affects the stability of mechanical recycling and the quality of regranulate. Particular attention is paid to PP-HDPE-PET cross-contamination, degradation over successive processing cycles, and changes in temperature, pressure, extruder load, throughput, melt filtration and energy demand. The literature indicates that chain scission dominates in PP during repeated processing and increases the melt flow rate (MFR), whereas branching and crosslinking may be more important in polyethylene. Small changes in PCR composition can also cause measurable shifts in plant operating parameters. A promising direction is the use of in-line monitoring and machine learning to detect changes in feedstock and predict regranulate quality from process data.