Renewable Cardanol-Based Phosphate as a Flame Retardant Toughening Agent for Epoxy Resins

A biobased flame retardant toughening agent, phosphaphenanthrene groups-containing triscardanyl phosphate (PTCP), was successfully synthesized via debydrochlorination, epoxidation and ring opening reaction from renewable resource cardanol. The chemical structure of PTCP was confirmed by the proton and phosphorus nuclear magnetic resonance. Epoxy resins (EPs) with different contents of PTCP were prepared through a simple mixing method. Thermogravimetric analysis results indicated that the earlier degradation of PTCP catalyzed the char formation of epoxy resins that was beneficial to protecting underlying polymers from further decomposition. The flame retardant properties were enhanced with the increase of the PTCP content. The EP composite containing 30 wt % PTCP showed a limiting oxygen index of 30.5%. Meanwhile, its peak heat release rate, total heat release and average effective heat of combustion values were decreased by 50%, 27% and 32%, respectively, in comparison to those of neat EP. The enhanced flame retardant behavior was attributed to the improved quality of char residue, which effectively inhibited the flammable volatiles, oxygen and heat transfer between degradation zone and flame zone. The impact strength was increased to 19.14 kJ/m2 for EP/PTCP-30% composite from 14.85 kJ/m2 for neat EP, indicating the toughening effect of PTCP on EP. The findings in this study demonstrated that PTCP could be used as a promising flame retardant toughening agent for epoxy resins to overcome their drawbacks of intrinsic brittle and high flammability.