Oxo-biodegradation of polyethylene
Creators: name: Reddy, Murali
Publisher: name: RMIT University
Year: 2024

description: The present work aims to evaluate and improve the oxo-biodegradation process of polyethylene. To achieve this, an attempt was made to fully understand the oxo-biodegradation process by conducting degradability tests on polyethylene and oxo-biodegradable polyethylene (polyethylene loaded with pro-oxidants). Pro-oxidants are transition metal ion complexes which catalyse the oxidation of polyethylene leading to molecular weight reduction facilitating biodegradation. The biodegradation of polyethylene was achieved using the micro-organism Pseudomonas aeruginosa. <br><br>In the degradation experiments, polyethylene film samples were subjected to thermal oxidation for a period of two weeks followed by six weeks of biodegradation. The changes in the molecular weight of polyethylene and the concentration of oxidation products were monitored by size exclusion chromatography and Fourier transform infrared (FTIR) spectroscopy, respectively. After initial thermal oxidation, molecular weight of oxo-biodegradable polyethylene was found to decrease rapidly, accompanied by the formation of low molecular weight carbonyl compounds. These end chain oxidation products are then utilised by microbes in the biodegradation stage. However the influence of microbes is found to occur only on the polymer surface suggesting that the action of pro-oxidant is limited to the oxidation process and is not continued during the microbial degradation stage.<br><br>This work also attempts to improve the overall process of oxo-biodegradation of polyethylene using nanoclay along with a pro-oxidant. Polyethylene nanocomposites were prepared by melt intercalating maleic anhydride grafted polyethylene and montmorillonite clay. It was found that maleic anhydride promotes strong interactions between polyethylene and montmorillonite, leading to a homogeneous dispersion of clay layers. Rheological tests showed that these nanocomposites exhibit shear thinning behaviour, with steady shear viscosities increasing proportionally with clay concentrations. With increase in clay concentration, the tensile strength of nanocomposites increases whereas the elongation at break decreases considerably and barrier properties are found to improve significantly with clay content. <br><br>On comparing the oxo-biodegradation results of polyethylene, oxo-biodegradable polyethylene, polyethylene- and oxo-biodegradable polyethylene nanocomposites, it is found that the molecular weights of both oxo-biodegradable polyethylene and oxo-biodegradable polyethylene nanocomposite decrease substantially as oxidation proceeds. However, the relationship between the carbonyl index (another measure of concentration of carbonyl compounds) and the molecular weight data reveals that the addition of nanoclay does not alter the oxidation mechanism significantly suggesting that the reduction in molecular weight of these two materials is mainly due to the action of pro-oxidant.<br><br>When oxidised (aged) polyethylene- and oxo-biodegradable nanocomposites are subjected to microbial degradation using Pseudomonas aeruginosa, their degradation patterns as indicated by their molecular weight distributions data vary. This could probably be either due to some active role of Pseudomonas aeruginosa leading to further chain cleavage of the polymer or due to the presence of clay that made a difference in the biotic environment. It is clear that montmorillonite has helped in creating a suitable biotic environment conducive to microbial activity and therefore supported the growth of Pseudomonas aeruginosa, which causes further biodegradation of the polymer in nanocomposites.