Flame-retardant, recyclable, and hydrothermally degradable epoxy resins and their degradation products for high-strength adhesives

To date, sustainable thermosetting polymers and their composites have emerged to address recyclability issues. However, achieving mild degradation of these polymers compromises their comprehensive properties such as flame retardancy and glass transition temperature (T_g). Moreover, the reuse of degradation products after recycling for upcycling remains a significant challenge. This study introduces phosphorus-containing anhydride into tetraglycidyl methylene diphenylamine via a facile anhydride-epoxy curing equilibrium with triethanolamine as a transesterification modifier to successfully prepare flame-retardant, malleable, reprocessable, and easily hydrothermally degradable epoxy vitrimers and recyclable carbon fiber-reinforced epoxy composites (CFRECs). The composite exhibited excellent flame retardancy and a high T_g of 192 °C, while the presence of stoichiometric primary hydroxyl groups along the ester-bonding crosslinks enabled environmentally friendly degradation (in H₂O) at 200 °C without any external catalyst. Under mild degradation conditions, the fibers of the composite material were successfully recycled without being damaged, and the degradation products were reused to create a recyclable adhesive with a peel strength of 3.5 MPa. This work presents a method to produce flame retardants and sustainable CFRECs for maximizing the value of degradation products, offering a new upcycling method for high-end applications.