Sulfathiazole derivative with phosphaphenanthrene group: Synthesis, characterization and its high flame-retardant activity on epoxy resin

A sulfathiazole derivative with phosphaphenanthrene group (STZ) was successfully synthesized by covalently bonding DOPO and imine intermediate obtained from the condensation of sulfathiazole with p-hydroxybenzaldehyde. Its effect on the thermal, flame-retardant and mechanical properties of epoxy thermosets were comprehensively studied. The presence of STZ decreased the initial degradation temperature and glass transition temperature (T-g) of epoxy thermoset. STZ significantly improved the flame retardancy and fire safety of epoxy thermoset. With the addition of only 4 wt% STZ, the modified epoxy thermoset achieved an LOI value of 29.1% and V-0 rating in UL-94 test. Moreover, the peak heat release rate (PHRR) and total heat release (THR) in cone calorimeter test were decreased by 34.4% and 32.1% in comparison with those of pure epoxy thermoset. STZ showed high flame-retardant activity not only by releasing nonflammable gases and phosphorous radicals in the gas phase, but also by promoting the formation of dense and intumescent char layer with honeycomb-like hollow structure inside in the condensed phase. Because of the strong intermolecular action between STZ and epoxy macromolecular chain, the presence of STZ increased the tensile strength, while decreased the elongation at break of epoxy thermoset. (C) 2020 Elsevier Ltd. All rights reserved.