Mechanical enhancement of sustainable natural fiber composites through filler additives: a comprehensive review

Natural fibre composites (NFCs) are progressively acknowledged as sustainable substitutes for synthetic materials due to their renewability, affordability, and biodegradability. Nonetheless, improving the mechanical performance of NFCs continues to pose a considerable problem. This paper looks closely at how different fillers such as silica, graphene, aluminium, titanium, and montmorillonite-can improve the strength, heat resistance, and moisture resistance of NFCs. These fillers are important for addressing key issues like weak bonding between layers, lower strength against impacts, and the ability to absorb moisture. The review categorises fillers according to their mechanical, thermal, electrical, and chemical properties, and it examines the impact of filler type, concentration, and dispersion on composite performance. The synergistic effects resulting from interactions among fibre, filler, and matrix are emphasised, particularly regarding the use of nanofillers and surface treatments in improving characteristics. We also examine the impact of hybrid fillers and optimal loadings on achieving an ideal equilibrium among strength, stiffness, and durability. Particular emphasis is placed on applications in the automotive, construction, and structural sectors, where there is a significant need for lightweight and sustainable composites. Unlike previous reviews, this work provides a structured comparative framework that maps fillers not only by type but also by aspect ratio, highlighting their synergistic roles in enhancing multiple properties simultaneously. In addition, we synthesise trends across studies to identify optimal filler loadings and practical strategies that can guide experimental design and industrial adoption. The review concludes by pinpointing research deficiencies and suggesting avenues for further investigations, especially in terms of enhancing filler content and processing methods to reduce agglomeration and optimise performance. Furthermore, the emerging role of artificial intelligence in predicting composite behavior and optimizing filler-matrix interactions is discussed as a pathway toward smarter and more sustainable material development.