Enhancing flame retardancy of waterborne polyurethane via ethylenediamine-intercalated black phosphorus

Ethylenediamine-intercalated black phosphorus nanosheets (eda-BP NSs) were synthesized via a solvothermal process on a 50-g scale. In contrast to the bare black phosphorus nanosheets (BP NSs) which typically involves high-cost gas-phase synthesis and subsequent complex exfoliation process, eda-BP NSs exhibit superior environmental stability and offer greater potential for industrial application in flame retardancy. A series of eda-BP NSs/waterborne polyurethane (WPU) composites with various loadings were prepared, and their flame-retardant properties were evaluated using thermogravimetric analysis, cone calorimetry, and limiting oxygen index (LOI) measurements. With the incorporation of only 0.2 wt.% eda-BP NSs, the composite showed 40.79% reduction in peak of heat release rate, 55.44% decrease in peak of smoke production rate, and 22.5% reduction in total smoke production, along with 2.5% increase in LOI and 2.17 wt.% rise in residual char compared to pure WPU. At the same filler loading, the flame-retardant performance of eda-BP NSs/WPU composites was comprehensively superior to that of BP NSs/WPU. The enhanced flame retardancy is attributed to the unique quasi-monolayer intercalation structure of eda-BP NSs and a phosphorus-nitrogen synergistic mechanism facilitated by the amine groups. The outstanding flame-retardant behavior observed in WPU underscores the promise of this organic–inorganic intercalated phosphorus material for broadening application prospects. Furthermore, such a flame-retardant mechanism arising from this distinctive intercalation structure may provide valuable insights for the development of next-generation flame retardants.