Assessment of polyethylene degradation after aging through anaerobic digestion and composting

Impurities in bio-waste enter biogas and compost facilities during bio-waste treatment. The plastic fraction is usually separated and sent for incineration. This study is aimed at evaluating the quality of low density polyethylene after being exposed to anaerobic digestion, composting or both processes sequentially, in order to assess for the optimum post-processing of these plastic fractions, eventually undergoing recycling processes. The alteration in the polymer's quality was first analyzed by non-destructive optical methods, including FTIR, NIR, SEM and EDX. These tests were followed by thermal and mechanical properties' assessment. The surface of the specimens showed signs of degradation, yet the extent of the changes was microscopic. The FTIR and NIR spectra showed higher intensities for C-H bonds in treated samples, which might be an indication of shorter polymer's chain length and/or higher chain scissoring. FTIR showed weak transmission spectral changes at regions representing C-O and O-H bonds for anaerobically digested and sequentially anaerobically digested and composted samples, indicating the existence of oxidized products, which was confirmed by the EDX analysis, where slightly higher oxygen content on the surface of specimens underwent the anaerobic digestion process was observed. However, NIR spectra did not show conforming spectra. It was accordingly concluded that the oxidized products are contaminants (residues on the specimens' surfaces), which was as well confirmed by the DSC analysis. The thermal analysis indicated a decrease in the polymer's crystallinity after being treated, where the decrease effect was the highest by anaerobic digestion, however, the mechanical properties (including tensile strength, elastic modulus and strain at break) pointed out minor quality alteration in treated samples, especially for samples underwent both processes successively. Based on the provided assessment, the general properties of LDPE plastic specimens are acceptable to be recycled, even after undergoing accelerated aging conditions, such as anaerobic digestion and composting. Further analyses are needed to evaluate plastics with additives as well as bags and films. (C) 2018 Elsevier Ltd. All rights reserved.