Preparation of Eco-Friendly Flame Retardant Modified with Tea Saponin to Enhance Thermal Stability and Fire Safety Performance of Epoxy Composites

Abstract In recent years, intumescent flame retardants (IFRs) have attracted substantial attention as eco-friendly replacements for halogenated counterparts in epoxy resin (EP) applications. In this study, tea saponin (TS)a bio-mass compound derived from agricultural waste tea seed cakewas utilized to prepare a halogen-free flame retardant. First, ethanolamine borate ester (ETBE) was synthesized via the reaction of boric acid with ethanolamine and subsequently reacted with TS to form TS-ETBE. TS-ETBE then underwent an ion-exchange reaction with ammonium polyphosphate (APP) to yield the target flame retardant, TS-ETBE-APP. Thermogravimetric analysis (TGA) demonstrated a significant enhancement in thermal stability; the char yield at 800 °C increased from 7.31% for neat EP to 21.77% for the EP/TS-ETBE-APP (30 wt %) composite. Concurrently, the maximum degradation rate (Rmax) decreased from −27.14 wt %/min to −15.00 wt %/min, while the integral procedural degradation temperature (IPDT) increased from 555 °C to 860 °C. Flammability evaluations revealed that incorporating 30 wt % TS-ETBE-APP elevated the limiting oxygen index (LOI) from 22.2% to 39.4% and achieved a UL-94 V-0 rating. Cone calorimeter tests (CCT) further confirmed superior fire safety and smoke suppression, with the peak heat release rate (pHRR) and total smoke production (TSP) dropping from 806.5 kW/m2 to 464.5 kW/m2 and 9.6 m2/kg to 6.7 m2/kg, respectively. These results demonstrate that the bio-based P−N−B synergistic flame retardant significantly improves both the thermal stability and fire safety performance of epoxy composites.