By-Products from Food Industry as a Promising Alternative for the Conventional Fillers for Wood–Polymer Composites

The present paper describes the application of two types of food-industry by-products, brewers’ spent grain (BSG), and coffee silverskin (ŁK) as promising alternatives for the conventional beech wood flour (WF) for wood–polymer composites. The main goal was to investigate the impact of partial and complete WF substitution by BSG and ŁK on the processing, structure, physicochemical, mechanical, and thermal properties of resulting composites. Such modifications enabled significant enhancement of the melt flowability, which could noticeably increase the processing throughput. Replacement of WF with BSG and ŁK improved the ductility of composites, which affected their strength however. Such an effect was attributed to the differences in chemical composition of fillers, particularly the presence of proteins and lipids, which acted as plasticizers. Composites containing food-industry by-products were also characterized by the lower thermal stability compared to conventional WF. Nevertheless, the onset of decomposition exceeding 215 °C guarantees a safe processing window for polyethylene-based materials.