A phosphorus-imidazole-based ionic liquid curing agent with high fire resistance and rapid curing capability, endowing epoxy resins with ultra-high toughness

In this work, the synthesis of simple and convenient flame retardant curing agents for epoxy resin (EP) was investigated to solve the problems of existing flame retardant curing agents due to their complicated preparation process and poor mechanical properties. In this paper, a new type of ionic liquid curing agent (ZIM) was successfully synthesized by introducing a one-step reaction between n-octylphosphoric acid (OPA), which contains a flexible chain, and imidazole (IM) at room temperature. The results showed that the curing agent had a gel time of only 3.5 min at 120 °C, and could achieve UL94 V-0 rating with an ultimate oxygen index of 32 % or more at any additive amount of 15 phr or more. The peak exothermic rate and total exothermic rate are reduced by 55.45 % and 33.45 %, which can meet the requirements of fire safety standards in the industrial field. In addition, the incorporation of flexible chain segments and the absorption of energy dissipation by ionic bonding resulted in a 118.33 % increase in elongation at break and 127.44 % increase in impact strength of EP/ZIM-15, which greatly improved the mechanical properties. This work provides an efficient and simple method for preparing high performing EP curing agents.