Toughening and strengthening epoxy resins with a new bi-DOPO biphenyl reactive flame retardant

It is still challenging to simultaneously improve the strength and toughness of flame retardant epoxy resins. In this study, a new reactive flame retardant (DOPO-M) including bi-DOPO and biphenyl structures was successfully synthesized and applied as a co-curing agent. The introduction of DOPO-M accelerated the ring-opening reaction of epoxy resin. E51-DM-P composites exhibited improved transparency and retained good thermal resistance and thermal stability. Notably, the rigid biphenyl structure of DOPO-M and the in situ microphase separation effect created by DOPO-M-rich domains can both improve the tensile strength and toughness of composites by 28.6% and 24.4%, respectively. Meanwhile, the flame retardant property and fire safety of E51-DM-P composites have improved. With only 2.8 wt% DOPO-M (0.25 wt% of phosphorus), the E51-DM-P composites achieved UL-94 V-0 and a high LOI value of 30.2%. Furthermore, the peak heat release rate (PHRR) and total heat release (THR) of E51-DM-P0.5% decreased by 30.4% and 27.3%, respectively. All of these findings indicate that DOPO-M is a multifunctional and highly effective flame retardant that can be applied to manufacture epoxy resins with superior comprehensive properties.