Synergistic effects of nanofillers on treated natural fiber composites: A comprehensive review of mechanical and moisture resistance enhancement

Natural fiber-reinforced polymer composites have gained increasing attention as eco-friendly alternatives to synthetic composites due to their low cost, renewability, and lightweight properties. Nevertheless, their performance is often hindered by poor interfacial adhesion and high moisture sensitivity. This review provides a comprehensive and integrated analysis of the synergistic effects of chemical surface treatments and nanofiller incorporation. It distinguishes itself from previous studies that treat these approaches in isolation. Chemical surface treatments -such as alkaline, silane, acetylation, and peroxide modifications - improve compatibility between hydrophilic fibers and the hydrophobic matrices by enhancing surface roughness and removing impurities; however, residual hydroxyl groups and microvoids still limit effectiveness. The incorporation of nanofillers such as nanoclay, SiO₂, TiO₂, GO, CNTs, and CNCs has proven highly effective in addressing these deficiencies. Nanofillers fill interfacial voids, bridge cracks, and generate tortuous diffusion paths, leading to superior load transfer and moisture resistance. A critical comparison indicates that high-aspect-ratio nanofillers (e.g., CNTs and GO) are more effective in improving tensile and impact properties, whereas particulate fillers (e.g., TiO₂, SiO₂) enhance barrier properties and compressive performance. Combined treatment and nanofiller systems achieve up to 65% improvements in mechanical properties and 70% reductions in water absorption compared with singly modified composites. This review systematically analyzes the synergistic mechanisms governing these enhancements. It identifies key challenges in nanofiller dispersion, interfacial compatibility, and durability. It outlines strategies for scalable processing, bio-based filler development, and standardized evaluation protocols to advance sustainable, high-performance natural fiber composites. • Synergistic fiber-nanofiller mechanisms explained. • Up to 65% mechanical improvement achieved. • Nanofillers reduce moisture up to 70%. • Dispersion controls composite performance. • Hybrid systems enhance multifunctionality.