Novel Cyclolinear Cyclotriphosphazene-Linked Epoxy Resin for Halogen-Free Fire Resistance: Synthesis, Characterization, and Flammability Characteristics

A novel halogen-free flame-retardant epoxy resin was designed by introducing phosphazene rings into the backbone in a cyclolinear-linked mode, and then it was synthesized through a three-step synthetic route. The chemical structures and compositions of all the cyclotriphosphazene precursors and the final product were confirmed by 1H and 31P NMR spectroscopy, elemental analysis, and Fourier transform infrared spectroscopy. The thermal curing behaviors of the synthesized epoxy resin with dicyandiamide, 4,4′-diaminodiphenylmethane, and novolak as hardeners were investigated by differential scanning calorimetry (DSC), and the thermal properties were also evaluated by DSC and thermogravimetric analysis. These thermosets achieved high glass transition temperatures over 150 °C and simultaneously displayed good thermal stability with high char yields. Moreover, these thermosets have higher tensile and flexural strength but lower impact toughness in comparison with the conventional epoxy thermosets. The flammability characteristics of the thermosets obtained by curing this epoxy with three hardeners were studied on the basis of the limiting oxygen index (LOI) and UL–94 vertical burning experiments as well as the analysis of residual chars of the tested bars after vertical burring. The high LOI values and the V–0 classification for these epoxy thermosets indicate that the incorporation of phosphazene rings into the molecular backbone imparts flame retardancy on the epoxy resin. This may result from a unique combination of phosphorus and nitrogen in the phosphazene ring. The epoxy resin synthesized in this study is a green functional polymer and may become a potential candidate for fire- and heat-resistant applications in electronic and microelectronic fields.