Syringaldehyde‐Derived P/N/S Co‐Curing Agents: Constructing Epoxy Resins With Excellent Flame Retardant and Mechanical Strength

Epoxy resins (EP) are widely used but suffer from high flammability. Conventional flame‐retardant modifications often compromise their transparency or mechanical properties, and most flame retardants are derived from petroleum‐based resources. To overcome these limitations, a new molecular‐designed multifunctional P/N/S‐containing co‐curing agent, designed SED, was synthesized from bio‐based syringaldehyde, sulfaguanidine, and DOPO. Acting as a multifunctional additive, SED accelerated the curing kinetics of EP and improved its mechanical qualities and excellent flame retardancy without sacrificing the high transparency. The EP formulation containing 7 wt% SED received a V‐0 rating in the UL‐94 test and achieved an LOI value of 34.4%. The THR, TSP, and PHRR all significantly decreased using the EP/SED system as compared to EP; the largest reductions were 27.6%, 22.0%, and 38.1%, respectively. Investigation into its flame‐retardant mechanism revealed that SED possessed excellent combined condensed‐phase and gas‐phase flame‐retardant performance, enabling it to achieve high flame‐retardant efficiency to EP at low addition levels. Furthermore, the EP/SED samples maintained high transparency and heat resistance while exhibiting significantly improved mechanical properties in tensile and flexural tests. A novel molecular design strategy is presented in this study for the development of high‐performance EP suitable for applications in rail transportation and high‐end optical devices.