Intrinsically flame retardant cardanol-based epoxy monomer for high-performance thermosets

Abstract

Petroleum-based diglycidyl ether bisphenol A (DGEBA)-type epoxy is one of the most popular thermosetting materials currently, but it suffers from brittleness, flammability and non-renewability. In this study, an intrinsically flame retardant bio-based epoxy monomer (HECarCP) was synthesized from cardanol and cyclophosphazene. Subsequently, HECarCP was cured by 4,4’-diaminodiphenyl methane (DDM) to obtain a thermosetting material. Compared to the cured DGEBA/DDM thermoset, the cured HECarCP/DDM thermoset showed lower initial decomposition temperature; however, the earlier thermal decomposition caused by cyclophosphazene core contributed to char formation in return. As well, the HECarCP/DDM exhibited slightly lower tensile strength but much higher elongation at break than the DGEBA/DDM, overcoming the drawback of brittleness of the DGEBA/DDM. More importantly, the cyclophosphazene core provided intrinsic flame retardancy to the HECarCP/DDM thermoset, which displayed a relatively high LOI value of 33.0% and UL-94 V-0 classification. In cone calorimeter measurement, the peak heat release rate, total heat release and total smoke production of the HECarCP/DDM thermoset was declined by 63%, 24% and 72%, respectively, compared to those of the DGEBA/DDM, owing to the formation of a thermally stable and insulating char layer during combustion. This study paves the way for broadening applications of cardanol in fabrication of high-performance bio-based thermosetting polymers with excellent comprehensive properties.