All-in-one bis-phosphonate ammonium salt hardeners: latent curing agents promoting fire protection and recyclability of epoxy-based materials

Polymeric materials are integral to modern life, offering unparalleled versatility and affordability. Among them, thermosets stand out for their remarkable durability, chemical resistance, and mechanical strength. However, these materials face two critical challenges: achieving fire safety and enabling recyclability. This study presents a groundbreaking approach to address both issues simultaneously. By developing a novel family of bis-phosphonate ammonium salt (PS), we have created advanced epoxy-based vitrimers (EV). These innovative hardeners not only enhance fire resistance and promote recyclability through a trans-phosphonate exchange mechanism but also exhibit latent curing behavior, desired in various industrial applications. The chemical, mechanical, thermal, and flame-retardant properties of the newly developed EV were assessed. To assess the durability of the newly developed material, it was subjected to five thermomechanical recycling cycles. Throughout these cycles, no signs of degradation or alterations in its physical, thermal, or flame-retardant properties were observed, confirming its robustness. Incorporating 2 wt% phosphorus, the thermoset demonstrated both self-extinguishing and intumescent characteristics, earning a V-0 classification in flammability testing. Compared to the blank reference material, the formulation with the novel bis-phosphonate ammonium salt achieved a substantial decrease in total heat release by 43 % and peak heat release rate by 73 %. To demonstrate the potential applications and sustainability of our new material, high-performance carbon-fiber-reinforced composites (CFRCs) were prepared, and their mechanical properties were investigated. • A phosphonate salt-based latent curing agent was synthesized and used to manufacture a dynamic covalent network. • The dynamic covalent network achieves V-0 classification at 2 wt% phosphorus content. • Trans -phosphonate exchange allows full thermomechanical recycling. • Compatible with carbon fiber reinforced composites, offering high mechanical performance.