A novel aromatic imine-containing DOPO-based reactive flame retardant towards enhanced flame-retardant and mechanical properties of epoxy resin

For improving the flame-retardant performance of epoxy thermosets without sacrificing other properties, a novel aromatic imine-containing DOPO-based bisphenol, DOPO-AI, was synthesized. Diglycidyl ether bisphenol F epoxy resin (DGEBF) reacted with DOPO-AI first, followed by curing with 4, 4′-diaminodiphenylmethane (DDM). When the mass of DOPO-AI was 25% of DGEBF, the resulting epoxy thermoset EP/20DOPO-AI exhibited the V-0 rating with the limiting oxygen index of 35.4%. In the cone calorimetry test, the peak heat release rate and total heat release of EP/20DOPO-AI showed the reduction of 26% and 27%, respectively, compared to those of DGEBF/DDM (EP). The mechanism of DOPO-AI in reducing heat release and increasing char-form ability of EP/DOPO-AI was studied. Interestingly, the mechanical and thermal mechanical properties of EP/20DOPO-AI were improved, although the crosslinking density was much lower than that of EP. The tensile strength and storage modulus of EP/20DOPO-AI were increased by 13% and 5%, while the glass transition temperature and the elongation at break were kept, compared with those of EP. The high rigidity of DOPO and aromatic imine was the reason for the improvement of the mechanical properties of EP/20DOPO-AI. Furthermore, EP/20DOPO-AI showed a small reduction in thermal stability, compared with that of EP.