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

A novel phosphorus-nitrogen synergistic flame retardant, pentaerythritol diphosphate-1-piperazine ethanol phosphate (PDPE), was successfully synthesized via a two-step reaction using phosphorus oxychloride, pentaerythritol, and 1-piperazineethanol as raw materials, and subsequently incorporated into waterborne polyurethane (WPU) backbone through reactive-type grafting to fabricate flame-retardant waterborne polyurethane (FRWPU) films. The chemical structures were confirmed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The incorporation of PDPE significantly enhanced both the flame retardancy and mechanical properties of WPU films. Compared to pristine WPU, the limiting oxygen index (LOI) of FRWPU-9 increased from 19.0% to 28.6%, representing a remarkable enhancement of 50.5%, while successfully achieving UL-94 V-0 rating. Cone calorimeter tests revealed that the peak heat release rate (PHRR), total heat release (THR), peak smoke production rate (SPR), and total smoke production (TSP) of FRWPU-9 were reduced by 36.6%, 42.7%, 66.7%, and 73.4%, respectively. Moreover, the char residue at 600 °C increased by 200%, from 3.8% to 11.4%. Notably, the tensile strength of FRWPU-6 reached 20.21 MPa, representing a 37.8% improvement over pristine WPU (14.67 MPa). The superior flame-retardant performance is attributed to the dual-phase mechanism of PDPE: in the gas phase, the release of non-combustible gases (N2, CO, NH3, CO2) and phosphorus-oxygen radicals (•PO, •PO2) effectively dilutes oxygen concentration and captures combustion-propagating radicals (•OH, •H); in the condensed phase, the formation of polyphosphoric acid catalyzes dehydration and carbonization of the polymer matrix, constructing a dense protective char layer that serves as a physical barrier against heat and oxygen transfer. This work demonstrates an effective strategy for developing high-performance flame-retardant WPU films with simultaneously enhanced fire safety and mechanical properties through phosphorus-nitrogen synergistic effect.