Synthesis of a novel charring agent containing pentaerythritol and triazine structure and its intumescent flame retardant performance for polypropylene

A novel charring agent (PEPAPC) containing pentaerythritol and triazine structure, was successfully prepared via the nucleophilic substitution reaction of cyanuric chloride, 2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol (PEPA), and piperazine. Then, different proportions of PEPAPC and ammonium polyphosphate (APP) were added into polypropylene (PP) at a 20% total loading via melt blending. The results of limiting oxygen index (LOI), vertical burning test (UL-94), thermogravimetric analysis (TGA) and cone calorimeter test revealed that PEPAPC/APP system could effectively enhance the thermal stability and flame retardance of PP. When the weight ratio of APP to PEPAPC was 3:1 with a 20% loading (PP6), the PP composite achieved the highest LOI value of 28% and passed the vertical burning test (UL-94) V-0 rating. Meanwhile, PP6 showed the highest amount of char residues via TGA test and the lowest PHRR (253 kW/m2) via cone calorimeter test. Moreover, Scanning electron microscopy (SEM) also indicated that PEPAPC/APP system benefited to the formation of more compact char layer which hindered the transfer of volatiles and heat during burning. The investigation of their water resistance demonstrated that the PP6 still obtained a UL-94 V-0 rating after 72 h dipping in hot water.