A phosphaphenanthrene-benzimidazole derivative for enhancing fire safety of epoxy resins

In this work, a P/N-derived flame retardant (DOPO-BNM) was synthesized using 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), 2-aminobenzimidazole, and 3-nitrobenzaldehyde as raw materials. The effects of DOPO-BNM on the flame retardancy, and thermal and mechanical performances of epoxy resin (EP) were comprehensively investigated. With the introduction of 8 wt% DOPO-BNM, the EP/8% DOPO-BNM sample passed a V-0 classification in vertical burning (UL-94) test, and its limiting oxygen index (LOI) increased to 32.5%. In comparison to the EP sample, the EP/8% DOPO-BNM sample exhibited 13.0%, 28.8%, 17.5%, 47.9% and 34.5% reductions in the peak heat release rate (PHRR), total heat release (THR), maximum average rate of heat emission (MARHE), total smoke production (TSP) and average effective heat of combustion (AEHC), demonstrating its enhanced fire safety. Furthermore, the char residue of EP/8% DOPO-BNM sample was increased by 41.2% relative to that of EP sample in the cone calorimetry test. During burning, DOPO-BNM mainly reduced the burning degree of volatiles, and also promoted the char formation. Additionally, DOPO-BNM slightly decreased the glass-transition temperature (Tg) of EP thermoset, while it remained the mechanical performances. In summary, the addition of DOPO-BNM not only effectively enhanced the fire safety of epoxy thermoset, but also maintained the thermal and mechanical properties.