Reactive Grafting of a Novel Phosphorus‐Nitrogen Synergistic Flame Retardant Enables V‐0, Low‐Smoke, and Mechanically Reinforced Waterborne Polyurethane Films

ABSTRACT A phosphorus‐nitrogen synergistic flame retardant, pentaerythritol diphosphate‐1‐piperazine ethanol phosphate (PDPE), is incorporated into a waterborne polyurethane (WPU) backbone via reactive grafting to fabricate flame‐retardant WPU (FRWPU) films. The chemical structures are confirmed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The incorporation of PDPE significantly enhances both flame retardancy and mechanical properties. Compared to pristine WPU, the limiting oxygen index (LOI) of FRWPU‐9 increases from 19.0% to 28.6%, achieving a UL‐94 V‐0 rating. Cone calorimeter tests reveal that peak heat release rate (PHRR) and total smoke production (TSP) decrease by 36.6% and 73.4%, respectively. Moreover, char residue at 600°C increases from 3.8% to 11.4%. Notably, the tensile strength reaches 20.21 MPa, a 37.8% improvement over pristine WPU. This superior performance is attributed to a dual‐phase mechanism involving gas‐phase radical capture and dilution, alongside condensed‐phase catalytic carbonization. This work demonstrates an effective strategy for developing high‐performance WPU films through phosphorus‐nitrogen synergy.