Improving fire safety and mechanical properties of waterborne polyurethane by montmorillonite-passivated black phosphorus

A novel phospho-silicon co-flame retardant material composed of montmorillonite modified black phosphorus nanosheets (MMT-BP) is synthesized using a liquid phase intercalation method. MMT-BP nanosheets are utilized to improve the thermal stability, mechanical properties, and fire safety of waterborne polyurethane (WPU). After passivation, MMT-BP nanosheets exhibit high stability during degradation even when immersed in 25 °C water for three months. Compared to pure WPU, the tensile strength of the 0.4-MMT-BP/WPU composite is increased by 59.6%, and the elongation at break is maintained. The limiting oxygen index of the 0.4-MMT-BP/WPU composite is increased from 21.7% to 27.2%. Furthermore, the peak heat release rate, total heat release, and the peak value of smoke production rate of 0.4-MMT-BP/WPU are significantly reduced by 33.43%, 13.85% and 32.40%, respectively. The superior flame-retardancy observed in the WPU nanocomposites containing 0.4 wt% MMT-BP is attributed to the formation of an Si-O-P bond between BP and MMT during passivation and the enhanced catalytic carbonization of BP in the condensed phase. In this paper, an effective passivating method of black phosphorus is proposed and a highly efficient flame retardant based on black phosphorus is obtained.