Flame-retardant natural fiber-reinforced polymer composites: Material chemistry, fire performance, domestic and industrial applications

Natural fiber reinforced polymer composites (NFRPCs) have garnered significant attention as lightweight alternatives to conventional composites for household, building, automotive, and industrial applications. However, their susceptibility to thermal degradation, ignition, heat release, and smoke formation remains a severe limitation in fire-sensitive areas. This review critically examines the thermal degradation behavior, inherent flame retardant properties, and flame retardant modification strategies of natural fiber composites, with special emphasis on lignocellulosic plant fibers, protein-based animal fibers, and calcium carbonate-rich biological shells. The influence of cellulose, hemicellulose, lignin, keratin, fibroin, and inorganic mineral elements on degradation pathways, volatile evolution, and protective residue formation is examined in depth. Particular attention is paid to the char-generating capability of lignin-rich materials (e.g., wood bark, fruit and seed shells), the specific degradation behavior of protein-based fibers, and the endothermic and barrier-forming properties of mineral-rich biological shells. The review also discusses phosphorus-nitrogen intumescent systems, mineral flame retardants, nanofillers, bio-based additives, surface treatments, and hybrid reinforcement strategies, describing their roles in condensed-phase protection, gas-phase flame inhibition, thermal shielding, and smoke suppression. Experimental results show that reinforcement chemistry, polymer matrix properties, additive interactions, and the structural integrity of protective residues are key factors influencing char production and heat release. However, improved thermal stability or higher residue yield does not necessarily translate to better flame retardancy or mechanical integrity under fire conditions. These techniques offer promising routes to developing fire-safer, mechanically dependable, and resource-efficient natural fiber composites for home and industrial applications.