Flexible, transparent, and fire-proof cellulose nanofibril films with outstanding flame retardancy

Naturally occurring cellulose nanofibril (CNF) is a major component of plant cell walls and possesses numerous advantages, including a high specific surface area, exceptional strength, abundant availability, renewability, and biodegradability. However, the inherent ignitability of CNF hinders its practical application in advanced functional materials. Herein, we report a novel ternary deep eutectic solvent, designed under mild mechanical grinding, capable of converting cheap bamboo cellulose into phosphorylated cellulose nanofibrils (PCNFs) uniform fibril diameters in the range of 3 − 10 nm. The pure PCNF-fabricated film exhibited excellent flame-retardant and self-extinguishing performances (with a high limiting oxygen index of 70.2%) without adding flame-retardants. Compared with unmodified CNF, the peak heat release rate and total heat release of the PCNF films decreased by 96.7% and 96.9%, respectively. Furthermore, defibrillation significantly improved the mechanical and optical properties of the PCNF films, suggesting considerable potential for applying PCNFs in flame-retardant packaging materials.