Phosphorus–Nitrogen Synergistic Flame-Retardant Cellulose Nanofibers from a Reactive Ternary Deep Eutectic Solvent Containing Guanidine Phosphate

Phosphorylation is considered one of the most effective strategies for enhancing the flame retardancy of cellulose materials. Herein, a pretreatment based on a reactive deep eutectic solvent (RDES) containing guanidine phosphate was developed and used for facilitating cellulose nanofibrillation and synthesizing phosphorylated cellulose nanofibers (P-CNFs) with superior phosphorus-nitrogen synergistic flame-retardancy properties and extremely small average diameter (≈3 nm), eliminating external flame retardants. Compared with unmodified softwood pulp, the peak heat release rate (PHRR) and total heat release (THR) of P-CNFs were reduced by 87.2% and 75.3%, respectively. Moreover, the limiting oxygen index (LOI) of P-CNFs increased to 62.7%. The fabricated P-CNF films exhibited remarkable self-extinguishing behavior and demonstrated outstanding mechanical properties, (maximum tensile strength >188 MPa) and exceptional optical transparency (visible light transmittance >90%). This study presents an innovative and efficient strategy for the development of ecofriendly and flame-retardant nanocellulose materials with enhanced mechanical properties, demonstrating significant potential for fire-safe flexible electronics and transparent coatings.