Thermal and flame retardant properties of recyclable disulfide based epoxy vitrimers

The 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 vitrimer characteristics of a common epoxy vitrimer based on DGEBA and Butanediol diglycidyl ether cured with 4-Aminophenyldisulfide (4-AFD) with different reactive and additive Phosphorus based flame retardants (FR) with different chemical environments 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) were prepared aiming to improve the flame retardant properties while maintaining the dynamism and reprocessability. The neat EV (Tg ~125 °C) showed no classification and heavy dripping in the UL-94 vertical burning test. With 1.5 wt% of Phosphorous (P) in the EV a V-0 classification with excellent self-extinguishing behavior was achieved. Especially gas phase active FRs with a lower number of oxygen atoms bond to the P showed better flame retardant properties 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.