A Recycling-Focused Assessment of the Oxidative Thermomechanical Degradation of HDPE Melt Containing Pro-oxidant

Abstract

This study shows the effect of a pro-oxidant (oxo-biodegradable) additive on the oxidative thermomechanical degradation of high-density polyethylene (HDPE). It also allows us to predict the behavior of the material when subjected to mechanical recycling or to biodegradation. When HDPE, one of the most consumed thermoplastics worldwide, is transformed into a product or when subjected to primary and/or secondary recycling it will undergo thermomechanical degradation. According to current standards HDPE is not biodegradable, therefore pro-oxidants are added to many HDPE products, which can compromise the product's life. Knowledge on the influence of pro-oxidants on HDPE in the melt is limited and the objective of this study is to assess the behavior of HDPE containing pro-oxidant manganese stearate (MnSt) in the oxidative thermomechanical degradation process. Thermomechanical degradation tests were conducted in a closed-chamber torque rheometer. FTIR and SEC results agreed with those of torque rheometry and showed that oxidative thermomechanical degradation of HDPE increases with increasing manganese stearate concentration. Degradation resulted in an increase in the number of oxygenated functional groups, mainly ketones, aldehydes and carboxylic acids, and reduction in the weight-average molar mass of HDPE. MnSt may have acted both as a lubricant and pro-oxidant during processing with predominance of one or the other effect at different stages along the oxidative thermomechanical degradation of HDPE.