A simple and catalyst-free strategy for enhancing flame retardancy and recyclability of phosphonic/carboxylic mixed-ester epoxy vitrimer

Phosphonic/carboxylic mixed-ester epoxy vitrimers offer an ideal alternative to conventional thermosetting resins, combining the advantages of both thermosetting and thermoplastic materials while exhibiting flame retardancy and recyclability. However, the recycling efficiency of phosphonic acid/carboxylic acid ester-based epoxy vitrimers remains unsatisfactory, as existing enhancement strategies are often inefficient, complex, or detrimental to the material's mechanical properties. In this study, (2-carboxyethyl) phenylphosphonic acid (CEPPA) was used as a curing agent, while phenylphosphonic acid (PPA) and n-octylphosphonic acid (OPA) were partially substituted for CEPPA to prepare dual-dynamic epoxy vitrimers containing both carboxylate and phosphonate ester bonds. Compared to epoxy vitrimers cured solely with CEPPA, the introduction of PPA and OPA simultaneously improved both flame retardancy and recycling efficiency without significantly increasing synthesis complexity or requiring the addition of a catalyst. The recycling efficiency increased from 57.9 % to a maximum of 78.6 % (60.7 MPa), the limiting oxygen index (LOI) improved from 27.5 % to 34.5 %, and the total heat release (THR) in cone calorimetry tests was reduced by 21.6 %. This study presents a simple and effective approach to enhancing the performance of flame-retardant epoxy vitrimers, offering a promising strategy for the development of high-performance, recyclable thermosetting materials.