Facile Sonochemical Preparation of Air-Stable Black Phosphorus for Enhancing Flame Retardancy of Thermoplastic Polyurethane

The black phosphorus (BP), which receives the increasing attention in polymer flame retardancy, has been greatly restricted due to its extremely easy oxidation in air. Herein, a simple efficient and green sonochemical method was accordingly proposed to realize the exfoliation and preparation of BP with enhanced air stability by utilizing the electrostatic self-assembly strategy in the presence of cation polymer poly(ethylene imine) (PEI) modifier. In addition, the prepared PEI modified BP nanosheets (FLBP@PEI) was further applied as the flame retardant fillers to improve the dispersion, thermal stability and flame retardancy of thermoplastic polyurethane (TPU). The results show that the sonochemical approach proves to be feasible in preparation of the modified BP. The incorporated PEI chains could not only improve the air stability of the BP by forming a coating, but also promote the dispersion of the BP nanosheets in the composites. Incorporation of only 2 wt% loading of FLBP@PEI could impart an excellent flame retardancy to the filled TPU composite, e.g., compared with pure TPU, the peak heat release rate (PHRR) and total heat release (THR) of the composite decrease by 59.2% and 26.2%, respectively. Finally, a possible flame-retarding mechanism of FLBP@PEI for TPU was proposed. During material burning, in the condensed phase, the resultant BP could act as a barrier and catalyze TPU to form the stable char residues, while in the vapor phase, the generated inert small molecules of NH 3 and H 2 O from PEI and TPU and P-containing free radicals could also prevent and delay the combustion propagation.