Efficient polyphosphazene covalent triazine flame retardants for epoxy resin: Double-faced role of benzene ring and flame-ratardant mechanism

Despite significant progress has been made, fabricating organic flame retardants with excellent fire resistance at low level addition remains a challenging task. In this work, four polyphosphazene covalent triazine (PCT) flame retardants which are named as MTP, B-MTP, CTP and B-CTP respectively, are synthesized through simple precipitation polymerization, and introduced into epoxy resin (EP) to investigate the anti-flammability. An addition of 3 wt% B-MTP significantly improves the char residue at 800 °C and enhances the thermal stability of EP. In addition, the peak heat release rate (p-HRR), fire growth index (FGI), total smoke production (TSP), and total heat release (THR) decrease by 40.2 %, 64.9 %, 32.3 %, and 28.8 %, respectively. Subsequently, the flame-ratardant mechanisms of PCT with different structures are studied in detail using thermogravimetric analysis-Fourier transform infrared, X-ray photoelectron, and Raman spectroscopy. Interestingly, for the first time the double-faced role of benzene ring in the flame-ratardant performance of the prepared PCT flame retardants is discovered, probably due to the delayed noncombustible gas generation mechanism. This work provides theoretical support for future construction of efficient PCT based flame retardants.