Intrinsically anti-flammable and self-toughened phosphorylated cardanol-derived novolac epoxy thermosets

Abstract To overcome the flammability and brittleness of DGEBA, cardanol was selected to synthesize bio-based epoxy in this work because of its unique structure with both rigid aromatic rings and flexible long fatty chains. A novel intrinsically anti-flammable and self-toughened cardanol-derived novolac epoxy was synthesized through condensation, phosphorylation, and epoxidation processes, with a cardanol-based epoxy monomer without phosphorylation (ECF) as a contrastive sample. TGA results showed that the char yield of ECF-PO(OPh)2/DDM was much higher than those of DGEBA/DDM and ECF/DDM thermosets due to the catalytic charring effect of phosphate species. The DGEBA/DDM and ECF/DDM thermosets exhibited high flammability, whereas the ECF-PO(OPh)2/DDM possessed exceptional anti-flammability. Specifically, the cured ECF-PO(OPh)2/DDM possessed a high LOI value of 33.5 % and UL-94 V-0 rating; the PHRR, THR, and TSP values measured by cone calorimeter test were declined by 69.5 %, 28.6 %, and 16.4 % compared to those of the commercial DGEBA/DDM thermoset. Besides, the cured ECF-PO(OPh)2/DDM thermoset showed good toughness originated from the long fatty chain of cardanol. This as-synthesized phosphorylated cardanol-derived novolac epoxy possesses simultaneously fire-safe and toughening characteristics, which could be a renewable substitute for commercial DGEBA counterpart to overcome its flammability and brittleness.