Functionalized ZrP nanosheet with free-radical quenching capability and its synergism in intumescent flame-retardant polypropylene

The combination of catalyzing carbonization and free-radical quenching mechanism is proposed to be a promising strategy for the preparation of high-efficiency flame-retardant polypropylene (PP). Herein, a novel functionalized zirconium phosphate (RQZrP) nanosheet with free-radical quenching capability was fabricated by decorating macromolecular N-alkoxy hindered amine (MNOR) onto the surface of ZrP. It was combined with an intumescent flame retardant (IFR) to flame-retard PP. The results showed that there was a good synergism between RQZrP and IFR, which effectively improved the fire safety of PP. When the content of RQZrP was 2 wt% and IFR was 23 wt%, the limiting oxygen index (LOI) of PP increased from 19.0% to 33.0%, and it achieved a UL-94 V-0 rating. Meanwhile, the peak heat release rate (PHRR), total heat release (THR), carbon monoxide production (COP), and carbon dioxide production (CO2P) were significantly decreased. It revealed that nitroxyl radicals generated by RQZrP could capture alkyl radicals and peroxy radicals that produced during the degradation and combustion of PP. Meanwhile, RQZrP acted as a solid acid that catalyzed PP chains rapidly cross-linking to form char on its surface, and it also played as a supporting skeleton to enhance the strength and compactness of the char layer, thus effectively preventing the transmission of heat, oxygen, and combustible gases.