Synthesis of <scp>DOPO</scp> ‐Containing Melamine Flame Retardant and Its Effect on the Flame‐Retardant Properties of Waterborne Polyurethane

ABSTRACT To improve the flame retardancy of waterborne polyurethane (WPU), two novel DOPO‐containing melamine‐based flame retardants were synthesized by combining Schiff bases (derived from p‐hydroxybenzaldehyde, terephthalaldehyde, and melamine) with 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO). FTIR and NMR confirmed the chemical structures. The flame retardants were chemically incorporated into WPU to prepare modified films. Comprehensive testing showed significant improvements: water contact angle increased to 138.79° with 6.45% lower absorption; thermal stability improved by 60°C; LOI reached 33.1% with UL‐94 V‐1 rating. Cone calorimetry revealed 52.33% and 60.47% reductions in peak heat release rate (pHRR) and total heat release (THR), with 11 s longer ignition time. Total smoke production (TSP) decreased by 91.81% and average effective heat of combustion (AEHC) by 53.99%. Although CO and CO 2 emissions increased by 144.23% and 86.29%, the overall fire safety performance was markedly enhanced. Mechanical properties improved substantially, with tensile strength reaching 2.12 MPa (2.83× higher than unmodified WPU). The simultaneous enhancement of flame retardancy, thermal stability, water resistance, and mechanical properties demonstrates the effectiveness of this modification approach for WPU applications requiring improved fire safety.