Influence of wood-based reinforcements on the mechanical and physical properties of postconsumer recycled polypropylene biocomposites

Abstract Polypropylene (PP) is widely used as a polymer matrix in cellulose- and lignocellulose-fibre biocomposites, but commercial PP-based composites often rely on carefully selected virgin PP grades to ensure stable processing and performance. A more sustainable alternative is recycled polypropylene (rPP); however, its suitability for wood-based biocomposites, especially at high filler loadings, remains less established because recycled PP may originate from mixed grades and show variable melt flow behaviour. In this study, circular-grade recycled polypropylene (PP-Circo) was reinforced with two wood-based fillers, bleached softwood kraft pulp (BSKP) fibres and sawdust side-stream, at 20, 30, and 50 wt% filler loadings. The aim was to evaluate how filler type and content affect the processability and performance of injection-moulded rPP-based biocomposites. The composites were characterised in terms of density, melt flow index (MFI), thermal behaviour, mechanical properties, dimensional stability, and water absorption. Increasing filler content increased stiffness, improved heat deflection temperature (HDT), and reduced thermal expansion, while impact strength and melt flowability decreased. Compared with sawdust, BSKP fibres provided stronger reinforcement, resulting in higher flexural strength and modulus and a greater reduction in the coefficient of thermal expansion (CTE). Differential scanning calorimetry showed that PP melting temperature was largely unaffected by the fillers, while BSKP slightly increased crystallinity, suggesting a possible nucleating effect. Water absorption increased with filler loading and was highest for sawdust-filled materials, especially at 50 wt%.