From molecular design to fire-safe epoxies: mechanistic insights into the dual functionality of phosphorus–imidazole flame retardants

In this study, a new rationally engineered phosphorus–imidazole derivative was created to overcome the innate limitations of imidazole-based phosphorus flame retardants. The molecular structure was designed to boost reactivity and to work compatibly with conventional hardeners, aiming to address the typical insufficient curing performance of this class. However, systematic testing showed that despite significant improvements in flame retardancy, the redesigned compound still does not fully resolve curing inefficiencies. At 5 phr loading, the additive promotes effective char formation, reduces heat and smoke release, and achieves a V-0 UL-94 rating, confirming strong fire safety. Yet, both in standalone use and when combined with standard curing agents, it decreases curing efficiency and alters crosslink density, ultimately impacting the quality of the final epoxy network. Comprehensive characterization—including FTIR, NMR, mass spectrometry, DSC, TGA, mechanical testing, and cone calorimetry—reveals that while the molecule participates in crosslinking through amine addition and anionic mechanisms, its structural features also introduce kinetic barriers that limit complete network formation. These findings show that even with rational molecular redesign, curing issues remain inherent to phosphorus–imidazole derivatives, highlighting the need for alternative design strategies to achieve high curing efficiency and fire-safe epoxy resins.