Development of Phosphorus-Free Biobased Epoxy Resin with Further Enhanced Flame Retardancy and Mechanical Properties through Photodimerization

Driven by the imperative to mitigate ecological impacts and enhance material safety, research on developing flame-retardant biobased thermosetting materials has intensified significantly. This work reports a straightforward one-step synthesis of a biobased epoxy monomer (HCA-EP) from renewable p-hydroxycinnamic acid. Subsequent UV irradiation induced photodimerization of HCA-EP, as confirmed through ultraviolet–visible spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and high-resolution mass spectrometry (ESI-MS). The product HCA-EP-2 was obtained after UV irradiation and then was cured with 4,4‘-diaminodiphenylmethane (DDM) to obtain HCA-EP-2/DDM. The HCA-EP-2/DDM exhibited a higher glass transition temperature (197 °C) relative to HCA-EP/DDM (186 °C) and DGEBA/DDM (166 °C). Its tensile strength was measured at 70.4 MPa, surpassing that of both HCA-EP/DDM (64.5 MPa) and DGEBA/DDM (55.2 MPa). HCA-EP-2/DDM achieved a char yield of 33.95%, which is slightly higher than that of the HCA-EP/DDM sample (31.81%) and approximately 2.4 times that of cured DGEBA/DDM (14.21%) at 800 °C under a nitrogen atmosphere. HCA-EP-2/DDM demonstrated a higher LOI than DGEBA/DDM (34.3% vs 23.6%). Also, HCA-EP-2/DDM achieved a V-0 rating in the UL-94 vertical burning test, demonstrating superior flame retardancy compared to the V-1 rating of HCA-EP/DDM. Notably, these results verify that photodimerization of p-Hydroxycinnamic acid significantly enhances material performance, positioning them as viable alternatives to conventional petroleum-based epoxy resins.