Effect of mechanical recycling on molecular structure and rheological properties of high-density polyethylene (HDPE)

This study investigates the degradation of high-density polyethylene (HDPE) during successive closed-loop mechanical recycling via multiple extrusion. The main objective is to perform a comprehensive analysis of the rheological property changes associated with molecular characteristics. High temperature size exclusion chromatography (SEC) was performed first showing that more than 95% mass recovery was obtained after every mechanical recycling generation, thus excluding the possibility of extensive crosslinking in the rHDPE investigated. Then, small amplitude oscillatory shear (SAOS) measurements revealed significant increases of the zero-shear viscosity (ηo) by up to six-times, especially at a later stage between the 4th and 8th recycling cycles. Additionally, the van Gurp-Palmen (v-GP) plots suggest long chain branching due to the generation of free radicals during mechanical recycling, as radicals are created by chain scission. Extensional rheological measurements showed no detectable strain hardening effect, which is in contradiction with the hypothesis of an ill-defined long chain branched structure. This assumption is further corroborated through nuclear magnetic resonance (NMR) analyses, which detect branching sites in both 13C and 1H spectra. Moreover, the 'branch-on-branch' (BoB) constitutive model yields insights into the molecular topologies present within the recyclates, including different structures such as star-shaped and comb-type configuration. Overall, this study provides in-depth insights into topological changes during the mechanical recycling of HDPE, most likely from linear to a randomly branched, star-like structure, which is of fundamental interest for polyolefin polymer reprocessing; i.e. HDPE recycling.