Catalyst Effects on Recyclability and Flame Retardancy of Epoxy Vitrimers with Mixed Carboxylate/Phosphonate Transesterification

Epoxy vitrimer, as a class of recyclable thermosetting polymers, offers a promising combination of the thermal stability of traditional thermosets and the recyclability of thermoplastics. However, epoxy vitrimers still exhibit insufficient flame retardancy, limiting their application in fire-sensitive environments. Although various strategies have been proposed to improve their flame resistance, the effect of catalysts on the flame retardancy of epoxy vitrimers remains unexplored. This study investigates the impact of trace amounts of catalysts on the properties of epoxy vitrimers. Epoxy vitrimers were synthesized using (2-carboxyethyl)phenylphosphinic acid (CEPPA) as the epoxy curing agent, with 0.34 wt % of different catalysts (1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), zinc acetylacetonate (Zn(acac)2), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)) incorporated. Compared to the catalyst-free epoxy vitrimer, Zn(acac)2 significantly improved recyclability, increasing the recovery efficiency from 57.9% to 103%, while DBU provided the most notable enhancement in flame retardancy, raising the limiting oxygen index from 27.5% to 34%, reducing the average CO yield by 33.9% and total heat release (THR) by 42.9%. This study demonstrates that trace catalysts can simultaneously enhance the flame retardancy and recyclability.