Furan molecular architecture regulates the pyrolysis behavior and flame retardancy of bio-based DOPO-Derived epoxy thermosets

Bio-based furan-containing phosphorus flame retardants (FRs) are widely considered promising for epoxy resins (EP) because furan structures are generally believed to enhance carbonization. However, how furan molecular architecture regulates the pyrolysis pathway and flame-retardant mechanism remains unclear. Herein, three DOPO-derived FRs containing 2, 4, and 8 furan units were synthesized with an identical phosphorus content to isolate the effect of furan architecture. The results indicate that increasing the number of furan units does not continuously improve the flame retardancy of EP but significantly alters the pyrolysis pathway and phosphorus utilization during thermal degradation. Although higher furan contents enhance the intrinsic self-carbonization ability of the FRs, they simultaneously suppress the release of phosphorus-containing active species, thereby weakening phosphorus-assisted catalytic carbonization of the epoxy matrix. Consequently, EP composites containing higher-furan FRs exhibit increased volatile evolution, less graphitized protective char, and inferior flame-retardant performance despite the higher intrinsic char-forming ability of the FRs. In contrast, DFP-2 exhibits a favorable balance between phosphorus-radical release and catalytic charring, resulting in the best interfacial compatibility, mechanical properties, and fire safety. This work establishes a clear structure–property relationship by revealing how furan molecular architecture regulates pyrolysis behavior, phosphorus release, and catalytic carbonization of epoxy thermosets, providing mechanistic insights for the molecular design of bio-based phosphorus-containing flame retardants.