P/N/S synergistic flame retardant holocellulose nanofibrils efficiently pretreated from ternary deep eutectic solvents

•Intrinsically flame retardant PHCNFs were prepared by TDES pretreatment.•The PHCNF suspension had good dispersion and colloidal stability.•The PHCNF films exhibited excellent fire resistance, mechanical and optical properties.•The P/N/S synergistic effect of PHCNF films was investigated. Cellulose nanofibrils (CNFs) have widely attracted significant attention due to the excellent mechanical performance, biodegradability, and high surface functionality. However, the high flammability of CNFs commonly restricts their wider application. In this study, the intrinsically flame-resistant holocellulose nanofibrils (HCNFs, which is rich in both cellulose and hemicellulose) were obtained from bamboo under pretreatment using a highly efficient ternary deep eutectic solvent (TDES) and subsequential mechanical homogenization. The phytates and sulfate-containing HCNFs (PHCNFs) displayed nanofibril diameter of 3–5 nm and high aspect ratio of (∼500), and the aqueous suspension exhibited colloidal stability and high transmittance. The PHCNF films exhibited excellent fire resistance, mechanical and optical properties. Compared with raw HCNF, the limiting oxygen index (LOI) of PHCNF-120 film was up to 52.5 %, and the total heat release (THR) and peak heat release rate (PHRR) of PHCNF-120 films were significantly reduced by 92.2 % and 82.4 %, respectively. The flame retardant mechanism was attributed to the synergistic effect of P/N/S in both condensed phase and gaseous phases during the burning process. This work provides a novel, efficient strategy for developing eco-friendly flame-resistant and fireproof holocellulose-based nanomaterials.