Recent advances in bast and leaf fiber-based polymer hybrid composites

Natural fiber-reinforced polymer composites (NFRPCs) have emerged as sustainable alternatives to synthetic composites due to their biodegradability, cost-effectiveness, and abundant availability. This review specifically focuses on polymer-based hybrid composites reinforced with bast (hemp, jute, flax) and leaf fibers (banana, sisal, PALF), establishing a clearly bounded fiber-centric scope. This work critically examines bast–leaf hybrid systems as the primary reinforcement framework, while agro-industrial waste-derived microcellulose and synthetic fibers (basalt and glass) are discussed only as secondary hybridization strategies for performance enhancement and benchmarking. The influence of fiber type, content, and chemical treatments on mechanical, thermal, and moisture absorption behaviour is critically examined. A comparison of reported studies shows that bast–leaf hybridization generally improves the balance between strength and toughness, with improvements often in the range of 25–45 % compared to single-fiber composites, particularly when fiber content is maintained around 40–50 wt% and interfacial bonding is improved. Comparative analysis reveals that selective incorporation of glass fibers can reduce equilibrium moisture content by up to 40 %, but this effect becomes less pronounced at higher natural fiber contents (>40 wt%), where increased porosity and interfacial gaps promote moisture uptake. Morphological analysis using SEM confirms that optimal microcellulose filler loading (5–7.5 wt%) enhances interfacial bonding in bast–leaf systems, mainly when the filler is well dispersed and void formation is minimal, whereas excessive loading (≥10 wt%) leads to agglomeration, voids, and a decline in performance. Increasing fiber or filler content improves stiffness and thermal stability, but often reduces ductility and impact resistance due to restricted matrix deformation and stress concentration. Differences in reported mechanical and moisture properties across studies are largely linked to variations in processing methods, fiber quality, dispersion, and testing conditions, rather than fiber type alone. This indicates that performance should be interpreted within specific processing and material conditions rather than as fixed optimal values. The review identifies structure–property–processing relationships and provides insights into fiber synergy and associated trade-offs. The review concludes with focused future directions in surface engineering, bast–leaf hybrid optimization, and data-driven material design, including emerging opportunities in AI-assisted prediction and standardization. These insights establish bast and leaf fiber hybrid composites as a promising pathway toward high-performance, environmentally responsible materials for next-generation engineering applications.