Design and synthesis of a dual-DOPO-based flame retardant: performance evaluation and mechanistic study in waterborne polyurethane systems
Yanbo Ding, Libo Sun, Jiale Qu, Guangxian Yang, Jianing Wang, Xinrui Pan, Quanjie Wang, Lianxiang Feng, Baorong Duan, Yonggen Weng
Abstract
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.