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 H 3 PMo 12 O 40 ·nH 2 O to form the protonated salt [EIPN] 3 PMo 12 O 40 . Subsequently, TPU/EIPN and TPU/[EIPN] 3 PMo 12 O 40 composites were prepared. The results showed that both EIPN and [EIPN] 3 PMo 12 O 40 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] 3 PMo 12 O 40 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] 3 PMo 12 O 40 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] 3 PMo 12 O 40 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.