Novel DOPO-based epoxy curing agents Synthesis and the structure-property relationships of the curing agents on the fire safety of epoxy resins

A series of DOPO-based curing agents with similar structures were prepared through Mannich-type reaction and characterized. The curing agents with weak electron-donating methylene groups, strong electron-withdrawing sulfonyl groups and strong electron-donating ether groups were then incorporated into epoxy resins, and the structure-property relationships of the curing agents on the fire safety of epoxy resins were investigated. Thermal and flame-retardant properties of the resins were investigated by thermogravimetric analysis, cone calorimeter, micro-scale combustion colorimeter, UL-94 and limiting oxygen index. The results showed that the curing agents with proper structure could impart epoxy resins with high thermal stabilities and low flammability. The curing agents containing ether groups exhibited better flame-retardant efficiency compared with other curing agents. In order to further investigate the mechanism, the char residues of the resins were studied by scanning electron microscopy, Fourier transform infrared and Raman spectroscopy. Char residues of the resins containing ether groups exhibited a condense char morphology, associated with a high degree of graphitization. This work not only provides novel epoxy resins with good thermal stability, but also will trigger more scientific interest in the development and investigation of the structure-property relationships of the DOPObased flame retardants.