Performance and Mechanisms of P–N Synergistic Oligomeric Flame Retardants with Different Phosphorus Oxidation States in Epoxy Resin

Abstract Flame retardancy is essential for epoxy resins (EP) used in safety‐critical applications. Although phosphorus‐based flame retardants have shown effectiveness, most studies have focused on small‐molecule systems, with limited attention on their oligomeric counterparts. In this study, two phosphorus‐nitrogen synergistic oligomeric flame retardants with different phosphorus oxidation states—TEPP (+III) and TEPP (+V)—were synthesized and incorporated into EP. The results indicated that, when TEPP and TEOPP were added at a concentration of 5 wt%, the limiting oxygen index (LOI) values of EP/TEPP and EP/TEOPP were 35.6% and 32.6%, respectively, with the former achieving a UL‐94 V0 rating. Cone calorimeter tests demonstrated that EP/TEOPP outperformed EP/TEPP in reducing heat and smoke release, suggesting distinct flame‐retardant mechanisms: TEPP primarily acts in the gas phase via the release of phosphorus‐containing radicals, while TEOPP promotes char formation in the condensed phase. Both systems also demonstrated good mechanical performance, owing to the hydroxyl groups and branched structures in the flame retardant molecules. This work provides valuable insight into the structure–property relationships of phosphorus–nitrogen oligomers and offers a strategy for designing high‐performance flame‐retardant epoxy systems.