Synergistic effect of nanoscale carbon black and ammonium polyphosphate on improving thermal stability and flame retardancy of polypropylene: A reactive network for strengthening carbon layer

Recently it is a hot topic to fabricate high-performance polymer composites via combination strategy. Herein the combination of nanoscale carbon black (CB) and ammonium polyphosphate (APP) into polypropylene (PP) was efficient to remarkably improve its thermal stability and flame retardancy. The optimal ratio corresponding to 7 wt% CB and 18 wt% APP (7CB18APP), presented the increase on T-ma(x) (the temperature at the maximum weight loss rate) by 138.7 degrees C under air. Moreover, its LOI (limited oxygen index) reached to 29.8%, and it passed VO rating at UL-94 (vertical burning test). According to cone calorimeter testing results, it exhibited a decrease on PHRR (peak value of heat release rate) and THR (total heat release) by 75% and 42%, respectively. Based on the analysis for microstructure and mechanical strength of chars as well as rheological behavior, a reactive crosslinking network between CB and APP was confirmed during combustion, which was favorable to the formation of continuous and compact carbon layer in condensed phase. In addition, the trapping radicals effect from CB and APP derivatives might also helpful to the improvement on thermal stability and flame retardancy in gas phase.