Phosphorus–Nitrogen Synergistic Flame-Retardant Cellulose Nanofibers from a Reactive Ternary Deep Eutectic Solvent Containing Guanidine Phosphate
Yutong Zhang, Lebin Zhao, Yun Liu, Xujuan Huang, Kaitao Zhang, Henrikki Liimatainen
Abstract
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.