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.