Design and synthesis of a dual-DOPO-based flame retardant: performance evaluation and mechanistic study in waterborne polyurethane systems

Waterborne polyurethane (WPU) has gained extensive applications in eco-friendly coatings, adhesives, and related fields owing to its inherent safety, environmental compatibility, and superior performance. Nevertheless, its high flammability severely restricts practical implementation. In the present research, a dual-DOPO-based nitrogen‑phosphorus flame retardant (PHED) was designed and subsequently incorporated into WPU to develop flame-retardant PHED-WPU. Remarkably, the incorporation of 12 wt% PHED elevated the limiting oxygen index of WPU from 21.2 % to 28.6 %. Additionally, PHED-WPU demonstrated exceptional flame inhibition characteristics, reaching a V-0 grade in the vertical burning tests (UL-94, compared to no rating for pristine WPU), along with 43.6 % and 30.5 % reductions in total smoke production and total heat release, respectively. It was found that the char formation of 12 % PHED-WPU at 600 °C could reach 7.37 %, 47.99 % higher than WPU (4.98 %). The enhanced flame retardancy is attributed to a synergistic mechanism involving: 1) gas-phase dilution by non-combustible volatiles, 2) quenching of free radicals via phosphorus-containing species at the condensed phase, and 3) formation of a protective phosphorus-rich characeous barrier. Meanwhile, while the flame retardancy performance was significantly improved, the inherent mechanical properties of WPU were maintained. These findings offer valuable insights for developing environmentally friendly flame-retardant WPU systems with superior comprehensive performance.