Study on the flame retardant properties of ethoxyimidazole and polyoxometalates autocatalytic phthalonitrile derivatives for thermoplastic polyurethane

Cross-linking phthalonitrile (PN) monomers readily forms substances with high thermal stability, and POM-catalyzed carbonization is a promising method to enhance polymer flame retardancy. In this study, ethoxyimidazole-modified phthalonitrile (EIPN) was synthesized and combined with H₃PMo₁₂O₄₀·nH₂O to form the protonated salt [EIPN]₃PMo₁₂O₄₀. Subsequently, TPU/EIPN and TPU/[EIPN]₃PMo₁₂O₄₀ composites were prepared. The results showed that both EIPN and [EIPN]₃PMo₁₂O₄₀ facilitated self-accelerating thermal crosslinking, leading to the formation of triazine and phthalocyanine rings with high thermal stability. TPU composites containing 3 wt% EIPN and [EIPN]₃PMo₁₂O₄₀ exhibited excellent flame retardant properties, with limiting oxygen index (LOI) of 22.2% and 21.6%, respectively. The peak heat release rate (pHRR) of TPU/3%[EIPN]₃PMo₁₂O₄₀ was reduced by 28.3% compared to TPU, while the total smoke release (TSR) of TPU/3%EIPN reduced by 22.2% in comparison to TPU. Notably, [EIPN]₃PMo₁₂O₄₀ demonstrated dynamic mechanical properties characterized by low energy dissipation and an extended fatigue life. This study offers new insights into using phthalonitrile cross-linking in flame-retardant TPU.