Synthesis of cyclotriphosphazene-containing imidazole as a thermally latent hardener for epoxy resins and its application in carbon fiber reinforced composites

In this study, a latent imidazole hardener (CPI) for epoxy resins was synthesized through the nucleophilic substitution reaction between 2-methyl imidazolium (2MI) and hexachlorocyclotriphosphazene (HCCP). The chemical structure of CPI was characterized by Fourier transform infrared, H-1 NMR, and P-31 NMR. Curing kinetic studies (Kissinger and Ozawa methods) manifest that epoxy systems cured by CPI show higher apparent activation energy and curing temperatures than those cured by 2MI. Compared with 2MI, CPI shows distinctly improved thermal latency toward epoxy groups at both room temperature and 80 degrees C, owing to the steric hindrance and electron-withdrawing effects of the introduced cyclotriphosphazene group. The results of dynamic mechanical analysis (DMA) and three-point bending tests indicate that the epoxy resins cured by CPI exhibit higher glass transition temperatures and comparable flexural properties in comparison to the corresponding epoxy resins cured by 2MI. As expected, the epoxy resins cured by CPI also show improved flame retardancy ascribed to the introduction of phosphorus and nitrogen elements. Finally, the optimized CPI/epoxy system was used as the polymer matrix to fabricate carbon fiber reinforced composites (CFRCs), and the resulting CFRCs show superior mechanical performance compared with the CFRCs based on the 2MI/epoxy system.