Searching for controlled exceedance of biomass degradation phenomena in polyethylene – wood flour and coffee spent grounds melt-blending pre-processing for composites manufacturing by rotational molding

Limitations in the processing of thermoplastic composites containing biomass as fillers via rotational molding (RM) are mainly due to the uncontrolled release of gaseous thermal-decomposition products, which deteriorate the final product's structure and mechanical performance. At the same time, the beneficial stabilizing effects resulting from the antioxidant-rich chemical structure of low-processed biomass create new possibilities in forming composites with plant derivatives in harsh technological environments, increasing their oxidation resistance. This study aims to investigate the effect of pre-processing temperatures (180-240 °C) by extrusion on changes in the structure and stabilizing efficiency of coffee spent ground (CSG) and beech wood flour (WF) introduced into high-density polyethylene (HDPE). Deliberately pre-processing the composites at temperatures above the biomass's degradation onset allowed for reduced gasification during final processing by rotational molding. Furthermore, the introduction of 5 wt% of WF processed at gradually increasing pre-processing temperatures resulted in the release of low-molecular-weight phenolic compounds with high antioxidant activity. In effect, OIT was extended for composites compared to HDPE from 6 and 44 min (180 °C) to 23 and 128 min (240 °C) for the CSG-/WF-series, respectively. Using melt processing beyond the thermal decomposition range of the fillers yielded favorable mechanical properties in RM parts. For the 220 °C-extruded series, the elastic modulus is 1.16 GPa and 1.33 GPa, compared to 1.08 GPa (HDPE), while reducing tensile strength by only 1-3 MPa. The new approach for manufacturing composite powders enabled the production of temperature-stable materials for RM, with increased resistance to oxidation-induced degradation and limited porosity.