A review on lignocellulosic fillers for sustainable thermoplastics

The transition toward renewable, low-carbon footprint materials has intensified interest in lignocellulosic fillers as sustainable reinforcements for thermoplastic composites. These bio-derived fillers mainly include wood flour, agricultural residues, micro- and nanocellulose, and lignin-based particulates. They are especially appealing due to their low density, functional surface chemistry, mechanical reinforcement ability, potential biodegradability, and low environmental footprint. Their practical implementation requires an understanding of the complex interplay among filler morphology, surface chemistry, and polymer–filler interactions. Here, we critically review recent advances in the structure-property-performance relationships in thermoplastic composites reinforced with lignocellulosic fillers. Key considerations include microstructural characteristics, dispersion mechanisms, crystallization behavior, mechanical reinforcement efficiency, viscoelastic/rheological properties, and thermal stability. Special emphasis is placed on compatibilization strategies, interfacial engineering approaches, and derivatives obtained upon carbonization, such as biochar. These carbonaceous fillers improve adhesion, enhance nucleation activity, and provide superior thermo-mechanical robustness. Beyond performance, the review is enriched by updated environmental impact metrics of lignocellulosic fillers and their composite materials, highlighting their potential to advance circular and sustainable bio-based material systems. By considering multiple scales, this work outlines design principles and identifies persistent challenges that must be addressed to enable the use of lignocellulosic fillers in next-generation sustainable thermoplastic composites.