Fabrication of ZrP nanosheet decorated macromolecular charring agent and its efficient synergism with ammonium polyphosphate in flame-retarding polypropylene

Poor efficiency is one of the biggest challenges for halogen-free flame retardant polymer. Catalyzing the carbonization of polymer itself during combustion is proposed to be a promising way to address this issue. In this work, a novel macromolecular charring agent (MCA) decorated by zirconium phosphate nanosheet named ZrP-d-MCA was synthesized and characterized. Subsequently, it was combined with ammonium polyphosphate (APP) to reduce the flammability of polypropylene (PP). When the contents of ZrP-d-MCA and APP were 5 wt% and 15 wt%, respectively, PP/Zr-d-MCA/APP could reach a limiting oxygen index of 32.5% and achieve UL-94 V-0 rating. Moreover, the bench-scale combustion performance determined by the cone calorimeter was significantly improved. The flame-retardant mechanism of ZrP-d-MCA/APP was revealed: during combustion, ZrP nanosheet could efficiently catalyze the charring reactions of MCA to form closed micro-nano char-cages, in which the degradation products of PP would be trapped and catalyzed into thermostable graphitization char.