Synthesis of a novel linear polyphosphazene-based epoxy resin and its application in halogen-free flame-resistant thermosetting systems

The synthesis of a novel linear polyphosphazene-based epoxy resin (LPN-EP) was performed via a six-step reaction pathway, and the chemical structures of the intermediate and target products were characterized by 1H and 31P NMR spectroscopy and Fourier transform infrared spectroscopy. A series of thermosetting systems consisting of diglycidyl ether of bisphenol-A and LPN-EP were prepared, and their mechanical properties, thermal stabilities, and flame retardant properties were investigated. The resulting thermosets exhibited excellent flame resistance with the UL-94 V-0 rating but also achieved a significant improvement in impact toughness as a result of the incorporation of rubbery LPN-EP. The thermosets also showed a good thermal stability highlighted for high char yields. The mechanism study indicates that the synergistic effect from the combination of phosphorus and nitrogen in polyphosphazene segments could effectively enhance the flame retardancy by acting in both condensed and gaseous phases to promote the formation of intumescent phosphorus-rich char on the surface of the thermosets. Such a char layer plays a role of insulating protective shell to prevent the volatiles from transferring to the surface of the thermosets as well as to shield the heat and oxygen diffusion, thus resulting in a self-extinguishing flame rating.