Molecular Engineering of Strong, Flame-Retardant and Closed-Loop Recyclable Crosslinked Polyamide for All-Aramid Composites

Aramid fiber reinforced composites (AFRCs) are widely used in military, aerospace, and transportation applications. However, the non-recyclability and flammability of traditional epoxy-AFRCs remain significant challenges that limit their future developments. Herein, a flame-retardant and closed-loop recyclable crosslinked network based on poly-m-phenylene isophthalamide (PMIA) is reported for novel AFRCs. The deliberately incorporated phosphorus (P) element in thermally stable PMIA molecular chains, and the selected P and nitrogen riched aldehyde-cyclophosphazene crosslinkers endows the crosslinked P-containing PMIA (CCPMIA) with exceptional flame-retardant properties. The tailor-made P containing PMIA segments and optimized crosslinking ratio with crosslinkers also enable CCPMIA to exhibit excellent mechanical strength. The good interfacial compatibility between CCPMIA and para -aramid fibers confers comparable interfacial shear strength and mechanical properties of CCPMIA-AFRCs to those of traditional epoxy-AFRCs, while the highly flame retarding CCPMIA matrix significantly enhances the flame retardancy of CCPMIA-AFRCs with high limited oxygen index of 39.8% and the highest UL-94 rating of V-0. Additionally, the recycling of the CCPMIA matrix and para -aramid fibers without deterioration on intrinsic properties is achieved by exploiting the catalytic effect of water and heat on the reversible reaction of imine bonds, offering a facile way for the recycling of imine-based materials.