Bio-based flame retardants containing a biphenyl unit for enhancing mechanical properties and fire safety of epoxy resins

Abstract

A reactive flame retardant containing biphenyl structure (DEU-2DOPO) was prepared from the bio-based raw material eugenol. The incorporation of DEU-2DOPO into the epoxy resin serves to enhance the ring-opening reaction of the epoxy. Furthermore, the inclusion of DEU-2DOPO has been found to have a positive impact on the glass transition temperature and residual carbon rate of the epoxy resin. This additive also significantly improves the mechanical properties of the epoxy resin, leading to a 49.3% increase in tensile strength, an 81.9% increase in breaking elongation, and a 15.8% increase in flexural strength. Notably, when DEU-2DOPO is introduced at a concentration of 5 wt%, it not only achieves a V-0 rating in the UL-94 test but also exhibits an impressive ultimate oxygen index of 34.5%. Additionally, the cone calorimetric testing revealed that the presence of DEU-2DOPO results in lower combustion metrics compared to pure epoxy, as evidenced by a reduction of 15.1% in total heat release (THR) and 10.8% in total smoke release (TSR). An analysis of the flame retardant mechanism elucidates that DEU-2DOPO promotes the formation of a stable and dense residual carbon, thereby impeding gas and heat exchange. Furthermore, the decomposition of DEU-2DOPO generates PO· and PO₂· radicals, which effectively neutralize reactive radicals.