Oilseed Pomace as a Substitute for Wood Filler in Composites Based on Post-Consumer Polyethylene

The development of composite materials based on post-consumer polymers and agricultural residues is a pragmatic valorization approach that extends the lifetime of materials. This research aimed to analyze the selected physical and mechanical properties of post-consumer-polyethylene-based composites with lignocellulosic fillers. This study explores the ‘ready-to-use’ valorization of untreated oilseed pomaces. The polyethylene ratio was set at 30% and 40%. Wood particles were substituted with oilseed pomace from nigella, rapeseed and evening primrose. The content of the pomace replacing wood particles was 30%, 65% and 100%. The composites made of post-consumer polyethylene and wood particles were used as a reference. The manufacturing process utilized a hybrid approach, combining extrusion with flat pressing. Increasing pomace content generally reduced the modulus of rupture and modulus of elasticity. Surface roughness decreased with higher pomace addition, except for the 30% rapeseed content for the lower polyethylene ratio, i.e., 30%, which showed unusually high roughness. Higher pomace content improved surface wettability, particularly for nigella-based composites. Water absorption and thickness swelling after 2 h and 24 h of soaking were highest at 30% pomace content and decreased with increasing substitution levels. Evening primrose composites consistently exhibited the lowest water uptake and swelling.