P/N/Si‐containing synergistic flame retardants endowing lignin‐based epoxy resin with good flame retardancy, mechanical properties and heat resistance

Biobased epoxy resins are gaining attention as a renewable alternative to petroleum‐based resins. However, their inherent flammability requires flame‐retardant treatment. In this study, hexaphenoxycyclotriphosphazene (HPCP) and various synergists were utilized to improve the flame retardancy of lignin‐based epoxy resin (EP‐L). Results showed that (3‐aminopropyl)triethoxysilane (APTES) had the most effective synergistic effect for HPCP to form a P/N/Si‐containing system, resulting in a compact crosslinked char layer. Upon the incorporation of HPCP and APTES (6∶1, 12 wt% of EP mass) into EP, a V‐0 classification was achieved in the UL‐94 vertical burning assessment, with a limiting oxygen index of 33.6% and a 47.77% decrease in the peak of heat release rate. This system minimally influenced the tensile strength of the composite (a 16.51% reduction), and maximumly increased its initial degradation temperature by 83 °C, compared with EP‐L. © 2023 Society of Industrial Chemistry. Phosphonitrile flame retardants are mainly used to compound with silane coupling agents to make biobased epoxy resins with good flame‐retardant properties without destroying their mechanical properties.