Functionalizing MXene with PEPA-containing polyphosphazene towards highly fire-safe polypropylene

Phosphorus-nitrogen intumescent flame retardant (IFR) is one of the most promising halogen-free flame retardants in enhancing the fire safety of polypropylene (PP), but its efficiency needs to be further improved. Herein, a novel single-component polyphosphazene IFR containing pentaerythritol caged phosphate (PEPA) functionalized titanium carbide nanosheet (MXene), named PZS-MXene, was prepared. When the amount of PZS-MXene was 25 wt%, PP/PZS-MXene classed a UL-94 V-0 rating with the limiting oxygen index of 32.7%. Compared with PP, its peak of heat release rate, total heat release rate and total smoke production rate were reduced by 74.5%, 33.0%, and 19.8%, respectively. The flame-retardant mechanism revealed that PZS-MXene quenched high-active free radicals to inhibit the degradation of PP during the combustion. MXene rapidly catalyzed the carbonization of PZS to form high-quality intumescent char layer on its surface. Besides, MXene acted as the supporting skeletons and exerted the layered barrier effect to improve the compactness of the intumescent char layer, thus effectively inhibited the degradation and combustion of PP. This work provided new insights for the efficient halogen-free flame-retardant PP.