Flame-retardant mechanism of a novel polymeric intumescent flame retardant containing caged bicyclic phosphate for polypropylene

A novel polymeric intumescent flame retardant, named poly(ethanediamine−1,3,5-triazine-o-bicyclic pentaerythritol phosphate) (PETBP), was synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), solid-state 13C nuclear magnetic resonance (13C NMR), 31P NMR and elemental analysis (EA). The effects of PETBP on the flame retardancy and thermostability of polypropylene (PP) were investigated by limiting oxygen index (LOI), vertical burning test (UL-94V), cone calorimetric test (CCT), thermogravimetric analysis (TGA) and TG-FTIR, respectively. The results showed that PETBP could significantly improve the flame retardancy and thermostability of PP. When the content of PETBP was 25.0 wt%, the PP/PETBP mixture could achieve a LOI value of 29.5% and a UL-94 V-0 rating, and its peak heat release rate (PHRR), total heat release (THR), average mass loss rate (av-MLR), smoke production rate (SPR) and total smoke production (TSP) were considerably decreased. The flame-retardant mechanism of PETBP was investigated by TGA, FTIR, TG-FTIR and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDXS). The results revealed that during the combustion PETBP could quench the free radicals of PP chain scission, and form a continuous and compact intumescent char on the substrate, thus effectively retard the degradation and combustion of PP.