Surface coating of biomass-modified black phosphorus enhances flame retardancy of rigid polyurethane foam and its synergistic mechanism

A novel surface coating based on chitosan (CS) modified black phosphorus (BP) is obtained through layer-by-layer self-assembly and applied to flame-retardant rigid polyurethane foam (PUF). The CS-BP coating tightly adheres to the PUF surface through hydrogen bonding and electrostatic interaction, improving the PUF's thermal stability, photothermal conversion performance and fire safety performance without compromising its mechanical properties. The 9L-CS-BP/PUF exhibits reduced the peak of CO₂ release, PHRR, and THR by 36.7%, 39.5%, and 28.8%, respectively, while increasing its residual carbon content by 35.2%. Moreover, a systematic comparison is made between the residual carbon inside and on the surface before and after combustion, and the results indicate that a large number of P-O-C structures and phosphoric acid derivatives are generated on the surface. The surface carbon layer formed by CS-BP effectively inhibited the thermo-oxidative degradation process of inner PUF. In the gas phase, modification of CS increases both the temperature range for the release of phosphorus-containing compounds during BP pyrolysis and the release of non-combustible gases during combustion, synergistically enhancing the gas-phase flame retardancy of CS-BP coating. This study provides a new approach to enhance the flame retardancy efficiency and broaden the application of BP-based flame retardants.