Constructing a bio-based transparent epoxy resin with a citral-derived curing agent for enhanced flame retardancy and toughness

Transparent flame-retardant epoxy coatings with high toughness and sustainability are of great interest for advanced protective and electronic applications. In this work, a bio-sourced flame-retardant toughening agent (CTDP) was synthesized using citral and a phosphorus-containing compound. The resulting molecule features dangling aliphatic chains and multiple reactive groups, enabling it to act not only as a flame-retardant toughening agent but also as a co-curing agent within the epoxy resin (EP). At a low loading of 0.5 wt%, the EP/CTDP-0.5 achieved a limiting oxygen index of 27.5 % and achieved the UL-94 V-1 rating, demonstrating significantly enhanced flame retardancy compared to neat EP. Cone calorimeter results further demonstrated a 14.9 % reduction in peak heat release rate and a 13.2 % decrease in total smoke production, highlighting the effective fire-safety performance of CTDP. Remarkably, the incorporation of only 0.5 wt% CTDP led to a significant 209.9 % increase in impact strength, highlighting its excellent toughening effect. This improvement is primarily attributed to the flexible dangling aliphatic chains in CTDP, which promote microphase separation and facilitate the formation of a hyperbranched-like crosslinked structure. Importantly, the EP/CTDP composites maintained excellent optical transparency, with visible-light transmittance remaining high even at increased CTDP content. These findings demonstrate that CTDP effectively integrates flame retardancy, mechanical toughness, and transparency within a single bio-based molecular framework, offering a promising and sustainable strategy toward the design of advanced multifunctional EPs.