Thermal and flame retardant properties of recyclable disulfide based epoxy vitrimers

he development of flame retardant epoxy vitrimers which are suitable for use as a composite matrix material is essential for the transition to a sustainable circular economy.
In this work the thermal and flame retardant properties as well as the rheological characteristics of a common epoxy vitrimer based on DGEBA and Butanediol diglycidyl ether cured with 4-Aminophenyldisulfide (4-AFD) with different aliphatic and naphthalene-based resins as well as reactive and additive Phosphorus based flame retardants (FR) with different oxidation states were studied. The use of 4-AFD as curing agent imparts vitrimeric properties to the material and allows repairing, reprocessing and recycling. A series of epoxy vitrimers (EV) was prepared aiming to improve the flame retardant properties while maintaining the dynamism and reprocessability. The neat EV (Tg ∼125 °C) showed a limited oxygen index (LOI) of 20.9 ± 0.4 %O2 and no classification and heavy dripping in the UL-94 vertical burning test. The addition of naphthalene-based epoxy resins increased the LOI value up to 23.4 ± 0.4 %O2, but the EV did also not reach a UL-94 classification. With 1.5 wt% of Phosphorous (P) in the EV LOI values up to 38.5 ± 0.4 %O2 and V-0 classification with excellent self-extinguishing behavior was achieved. Especially gas phase active FRs with lower oxidation states showed better flame retardant properties in the LOI and UL-94 vertical burning tests with identical phosphorus content. In cone calorimeter test the EVs with 1.5 wt% P reduced the peak heat release rate (PHRR) up to 43 %. The addition of reactive phosphorous-based FRs did not influence the dynamism and recyclability of the material in dynamic mechanical analysis. After mechanical recycling >73 % of the mechanical strength was recovered and the flame retardant EVs do reach as well a V-0 classification in the UL-94 vertical burning test.