A highly efficient, colorless phosphorus–nitrogen synergistic flame retardant for durable flame retardancy in wood pulp paper

Wood pulp paper (WPP) is an exemplary substrate material, yet its flammability impedes the widespread application in diverse functional fields. Herein, a highly effective, colorless and transparent phosphorus–nitrogen flame retardant, namely ammonium salt of tris(hydroxymethyl)aminomethane diphosphonic acid (AMPAP-NH4+) without halogen and formaldehyde, has been synthesized for WPP. Chemical grafting was employed to confer highly durable flame retardancy to the WPP, enabling it to maintain excellent fire resistance after functionalization. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analysis demonstrated that AMPAP-NH4+ was directly linked to cellulose fibers via POC bonds, effectively introducing P and N elements into the WPP without altering the surface morphology of the fibers. Results from thermogravimetric analysis, vertical burning test, limiting oxygen index test, cone calorimeter test and thermogravimetric-Fourier infrared spectrometer test demonstrated that the treated WPP exhibited outstanding flame retardancy. The peak heat release rate (pHRR) and total heat release (THR) values decreased by 80.8% and 70.9%, respectively, and the limiting oxygen index (LOI) increased to 41.4%. Furthermore, due to the stability of the POC bond, the treated samples remained unignited after 168 h, with a high LOI value of 35.3%. In summary, AMPAP-NH4+ serves as a phosphorus–nitrogen synergistic transparent flame retardant that can effectively and durably impart paper with flame retardancy without altering its surface morphology.