Novel cyclotriphosphazene-based epoxy compound and its application in halogen-free epoxy thermosetting systems: Synthesis, curing behaviors, and flame retardancy

A novel cyclotriphosphazene-based epoxy compound (PN-EPC) as a halogen-free reactive-type flame retardant was synthesized via a two-step synthetic route. The chemical structures and compositions of the cyclotriphosphazene precursor and the final product were characterized by 1H, 13C, and 31P NMR spectroscopy, Fourier transform infrared spectroscopy, mass spectroscopy, and elemental analysis. A series of thermosetting systems based on a conventional epoxy resin and PN-EPC were prepared, and their thermal curing behaviors were investigated. These epoxy thermosets achieved a significant improvement in glass transition temperature and also gained the good thermal stability with a high char yield. The incorporation of PN-EPC could impart an excellent nonflammability to the epoxy thermosets due to a synergistic flame retarding effect as a result of the unique combination of phosphorus and nitrogen from the phosphazene rings, and these epoxy thermosets achieved the high limiting oxygen indexes and the UL-94 V-0 rating when 20 wt.% of PN-EPC was added. The study on flame-retardant mechanism indicates that the pyrolysis products of phosphazene rings acted in both the condensed and gaseous phases to promote the formation of intumescent phosphorus-rich char on the surface of the epoxy thermosets. Such a char layer can supply a much better barrier for underlying thermosets to inhibit gaseous products from diffusing to the flame, to shield the surface of the thermosets from heat and air, and to prevent or slow down oxygen diffusion. As a result, the resulting epoxy thermosets obtained an excellent nonflammability.