Improvement of flame resistance and mechanical properties of vinyl ester based carbon fiber composites by a reactive phosphonium ionic liquid

In order to simultaneously enhance the flame retardancy and mechanical performance of vinyl ester resin (VER) and its carbon fiber composites (VER/CF), while maintaining the room temperature curing characteristics of VER, a nitrogen-containing phosphonium ionic liquid, P4447N[BMP], was synthetized through an ion-exchange process. P4447N[BMP] demonstrated excellent solubility in VER and did not interfere with its room temperature curing process. With the incorporation of 15 wt% P4447N[BMP], the limiting oxygen index (LOI) of the modified resin (P4447N[BMP]15/VER) and its carbon fiber composite (P4447N[BMP]15/VER/CF) increased to 27.7% and 36.4%, respectively. In addition, P4447N[BMP]15/VER and P4447N[BMP]15/VER/CF exhibited lower heat and smoke release during combustion than the unmodified systems. Mechanistic investigations indicated that the flame-retardant efficacy of P4447N[BMP] originated from its synergistic action in both the gaseous and condensed phases. Benefiting from the ionic bonds in P4447N[BMP] and its reactivity with both the resin matrix and the sizing agent on the carbon fiber surface, the incorporation of P4447N[BMP] markedly enhanced the mechanical properties of the cured resin and its carbon fiber composites. Specifically, the flexural moduli of P4447N[BMP]15/VER and P4447N[BMP]15/VER/CF exhibited increases of 22.2% and 41.0%, respectively. Moreover, P4447N[BMP] contributed to an increase of the glass transition temperature of both the cured resin and the corresponding carbon fiber composites. This work presents a viable route to engineer flame-resistant VER systems and corresponding carbon-fiber composites for advanced industrial applications.