The impact of mechanical recycling on the degradation of polyamide

There is currently no agreement on the degradation mechanisms for PA6 as a result of mechanical recycling. In this study, PA6 was extruded five times to simulate mechanical melt recycling process. Both virgin PA6 and the extruded PA6 were characterised to determine the predominant degradation mechanism. Melt flow rate (MFR) increases with increasing number of extrusion passes suggesting a reduction in melt viscosity due to mechanical processing. Gel permeation chromatography (GPC) results confirmed a reduction in molecular weight from 18,600 g/mol to 13,100 g/mol. Chain ends titration showed a decrease in the concentration of both the amine and carboxyl chain ends after the fifth extrusion. There were no changes to the chemical structure of the virgin PA6 and extruded PA6 observed from the Fourier transform infra-red (FTIR) and Nuclear magnetic resonance (NMR) spectra. X-ray photoelectron spectroscopy (XPS) showed a reduction in the C–N peak area supporting the scission of the N-alkylamide bond. The increase in the O=C–N peak area suggests that chain recombination via branching is possible. The shorter chains have higher mobility, and this gives rise to an increase in the crystallisation rate as seen from the differential scanning calorimeter (DSC) thermograms. The appearance of the γ phase after the fifth extrusion was identified in the DSC melting endotherms. The understanding of the degradation process of recycled PA6 will enable targeted modifications involving the incorporation of additives in the recycling process to ensure good mechanical properties in the recycled polymer.