Sustainability in Rotational Molding: Influence of Additives

Abstract

Rotational molding is a polymer processing technique uniquely able to produce large hollow parts while maintaining structural integrity. This benefit comes with a significant drawback: large amounts of plastic waste when a part is defective. The present study aims to investigate the feasibility of recycling rotomolded waste generated in manufacturing facilities in an effort to avoid landfilling and increase sustainability. Long heating times in the presence of oxygen is inherent to the rotomolding process which gives rise to the high likelihood of thermo-oxidative degradation and subsequent deterioration in physical and mechanical properties. These effects are exacerbated during recycling when the polymer is processed multiple times. To evaluate the impact of degradation two distinct recycling procedures combined various percentages of recycled material (10-50 wt%) with pre-stabilized virgin resin prior to rotomolding. The first recycling procedure exposed polyethylene to a research standard for recycling evaluation whereby multi-pass extrusion is used to promote significant levels of degradation so that the result of degradation may be clearly seen in a recycled polymer. The second technique was a more novel approach to recycling research, where polyethylene resin was repeatedly rotomolded to more closely replicate industrial experiences. In both recycling procedures, the rheological, physical and mechanical properties of recycled samples were found to be similar to the virgin polymer, which may be attributed to the successful protection against thermo-oxidative degradation mechanisms by the additives in the pre-stabilized virgin resin. The most significant change in properties involved the yellowing of the polyethylene samples, most notably when containing 50% recycled content. The discolouration was likely attributed to by-products formed from phenolic antioxidants during the course of protecting the polymer rather than from degradation products themselves since no mechanical or rheological changes were measured. No reasons were found in this study, due to degradation of properties, for rotomolders to not consider using recycled content when it is present in their facilities; with the caveat that the recycled content must be ground to the same size as the virgin feedstock to maintain comparable sintering performance. At the end of this thesis, a study was introduced on the stabilizers used in rotomolding grades, with respect to their performance in recycling. A standard antioxidant, UV stabilizer and a pre-stabilized rotomolding-grade resin (as an additive) were each blended with a reactor-grade unstabilized polyethylene resin prior to recycling (by the second technique mentioned above) to investigate their individual and combined effects in reducing degradation. The individual contributions of the antioxidant and UV stabilizer did not show significant variations in polymer properties, although their combined action offered minor yet meaningful improvements in sinter-ability as well as impact strength, which may indicate a potential synergy and an unintentional benefit that the UV stabilizer provides in alleviating thermo-oxidative degradation effects during rotomolding. A higher concentration of additives may be required in future studies to verify these findings.