Synthesis of eugenol-based phosphorus-containing epoxy for enhancing the flame-retardancy and mechanical performance of DGEBA epoxy resin

In this work, we designed a eugenol-based intrinsically flame-retardant epoxy monomer bis(2-methoxy-4-(oxirane-2-ylmethyl) phenyl) phenyl phosphonate (BEEP). At first, an intermediate product BEP was com-pounded by eugenol and phenyl phosphonic dichloride, and the bio-based epoxy monomer BEEP was acquired by the epoxidation of BEP with 3-chloroperoxybenzoic acid. Then different ratios of BEEP and commercial bisphenol A-type epoxy resin (DER) were blended and cured with 4,4-diaminodiphenyl methane (DDM). The addition of BEEP enhanced the reactivity of DER/BEEP-DDM systems, whose activation energy fitted by Kis-singer's and Ozawa's theories decreased from 50.66 and 55.12 kJ/mol to 47.49 and 51.63 kJ/mol, respectively. Compared with neat DER-DDM epoxy, the flexural strength and flexural modulus promoted from 69.4 MPa and 1.57 GPa to 74.5 MPa and 3.60 GPa, when adding 20 wt% of BEEP (D8B2), due to the stiffness of benzene rings in BEEP. Importantly, with a low phosphorus content of 1.0% in the D8B2 sample, the cured epoxy achieved 27.5% of the limit oxygen index and UL-94 V0 rating. Overall, this work provides a facile route for preparing a bio-based epoxy monomer to enhance the flame-retardancy and mechanical performance of DGEBA epoxy resin.