Low-flammability epoxy resins with improved mechanical properties using a Lewis base based on phosphaphenanthrene and 2-aminothiazole

As is known to all, the reduced flammability of modified epoxy resin is always accompanied with damaged mechanical properties. Surprisingly, in this work, we synthesized a novel Lewis base, namely D–AZ sourced from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and 2-aminothiazole to modify epoxy resin, and found that it could not only impart excellent flame retardance to epoxy resin, but also improve the strength of epoxy resin simultaneously. For instance, epoxy resin with 5% loading of D–AZ possessed limited oxygen index as high as 34.7%, and it passed V-0 rating. By comparison with neat epoxy resin, tensile strength of epoxy resin with 10% loading of D–AZ increased by 20%, flexural strength by 38%, and izod unnotched impact strength by 92%. Besides, D–AZ also played an important role in inhibiting the heat release of epoxy resin during combustion, thanks to its gaseous phase and condensed-phase flame-retardant mechanism. Imperfectly, it exhibited that the addition of D–AZ caused some decrease in the thermal stability of epoxy resin, especially in the initial decomposition temperature. Nevertheless, epoxy resin with 10% loading of D–AZ still displayed glass transition at 157 °C, and there was not much difference in the storage and loss modulus of epoxy resins in the low temperature. Briefly, epoxy resins containing D–AZ possessed excellent flame retardance and outstanding mechanical properties with comparison to neat epoxy resin, showing high promise for application.