Synthesis of a silicon-containing cyclotriphosphazene derivative for flame-retardant modification of epoxy resin

Although epoxy resin (EP) has been widely used in many fields, it is still urgent to effectively improve its flame retardancy by halogen-free strategies. Herein, a silicon-containing cyclotriphosphazene derivative (HEP-Si) was synthesized by nucleophilic substitution and silicon–hydrogen addition reaction using eugenol, hexachlorophosphazene and triethylsilane as raw materials. Then it served as a halogen-free flame retardant for an EP/4,4′-diaminodiphenylmethane system, and its performance was compared with that of commercial flame retardant hexaphenoxycyclotriphosphazene (HPP). The results showed that although the initial thermal decomposition temperature (T_5%) of the cured products of EP/HEP-Si was slightly lower than that of pure EP, T_5% did not decrease further as the amount of HEP-Si increased, indicating that good thermal stability was maintained. In addition, the char yield of the cured products of EP/HEP-Si was significantly increased (more than 20 wt%) compared to that of pure EP. The cured EP with the addition of HPP and HEP-Si both achieved a V-0 rating in UL-94 vertical burning test at 0.5 wt% phosphorus content, while the limiting oxygen index of the EP/HEP-Si system was higher, reaching 33.2%. From cone calorimetric test results, the peak heat release rate, total heat release and total smoke production of the cured products of EP/HEP-Si decreased by 61.5%, 37.1% and 26.2%, respectively, compared with that of pure EP. The cured EP/HEP-Si materials also exhibited good thermomechanical and mechanical properties. Therefore, HEP-Si is suggested to be a promising P/N/Si-containing halogen-free flame retardant for EP. © 2024 Society of Industrial Chemistry.