Fabrication of a core-shell structure from boron nitride nanosheets and ammonium polyphosphate (BN@APP) for reducing the fire hazards in waterborne polyurethane

Waterborne polyurethane (WPU) has the advantage of good flexibility in the coating film, high and low-temperature resistance, and the advantages of environmental protection. Currently, there is a high demand for flame-retardant WPU. Herein, a flame-retardantcomposites (WPU/BN@APP/DIDOPO) is successfully developed by incorporating ammonium polyphosphate (APP) modified boron nitride (BN@APP) and DOPO derivatives into WPU via simple casting technology.The results indicate that BN@APP formed the intended composite structure and was well dispersed in the WPU matrix. This system significantly enhanced the char residue at 700 ℃ from 0.23% to 5.22% and reducing the peak heat release rate (pHRR) and total heat release (THR) by 58.9% and 36.9%, respectively, compared to pure WPU. Performance analysis shows that during combustion, a dense and stable expanded carbon layer forms, effectively delaying heat and mass transfer. The mechanical properties of the WPU/0.5BN@APP/5DIDOPO composite material are optimal and its breaking strength and elongation at break are superior to those of components. This work demonstrates the potential of the BN@APP/DIDOPO composite system for enhancing flame retardancy in polymeric materials, providing valuable insights for designing advanced flame-retardant systems.