Phosphorus containing group and lignin toward intrinsically flame retardant cellulose nanofibril-based film with enhanced mechanical properties

Cellulose-based film material, due to its remarkable physicochemical properties, environmentally friend and low cost, has been widely applied in many high-end areas. However, the lack of functional properties (e.g., flame resistance) except poor mechanical performance severely hinders their further applications. Herein, bamboo-based phosphorylated cellulose nanofibrils (B-PCNFs) were prepared through a simple two-step method composed of phosphorylation and mechanical grinding process. The resultant film prepared via solvent casting process shows an excellent mechanical strength of 115.9 MPa, remarkable elongation at break of 53.1%, high Young's modulus of 2.5 GPa and impressive work of fracture up to 41.8 MJ m−3, respectively, which should be contributed to the formation of dense and homogeneous polymer networks reinforced by enhanced multiple interactions. When burned against the flame of alcohol lamp (700–800 °C), BHL-PCNF film also delivers appealing structural stability even over 25s, demonstrating a prominent flame resistance. Meanwhile, compared to original one, the peak heat release rate (PHRR) and total heat released (THR) of BHL-PCNF film produce a dramatical reduction, i.e., 87.3% and 86.6% for PHRR and THR, respectively. Based on the evolution of morphological and chemical structure, the highly improved flame resistance is strongly contributed to the synergy of phosphorus containing group and lignin, resulting in a protective layer composed of PxOy compound and carbon layer. The proposed mechanism for the enhanced flame retardant property is also declared and clarified. Thereby, this phosphorylated cellulose-based film that integrates outstanding mechanical performance and excellent intrinsically flame resistance holds promising potential candidate in practical applications, such as flame-retardant packaging materials.