Single component phosphamide-based intumescent flame retardant with potential reactivity towards low flammability and smoke epoxy resins.

To develop a low flammability and smoke epoxy resin, benzothiazole-based phosphamide (DOP-ABZ) was prepared through the reaction of 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphinane-2-oxide (DOP) and 2-aminobenzothiazole (ABZ). Intumescent flame-retardant (IFR) epoxy thermosets (EP) with different loadings of DOP-ABZ were prepared according to the assigned curing procedure. The thermal stability of IFR-EP decreased as compared to that of EP, but the flame retardancy of IFR-EP were greatly improved. EP/20 wt% DOP-ABZ passed UL-94 V-0 rating and got a high LOI value of 28.3%. Meanwhile, cone calorimeter tests showed that the heat release rate greatly decreased from 1139.7 kW/m2 of EP to 238.9 kW/m2, and the productions of smoke/toxic gases including CO and CO2 were also remarkably reduced. Furthermore, the mechanical strength of EP/17.5 wt% DOP-ABZ was enhanced to some extent, i.e. tensile strength increased from 71 MPa of EP to 81 MPa, flexural strength from 98 to 119 MPa, and impact strength from 22 to 32 kJ/m2. Finally, the flame-retardant mechanism was disclosed that DOP-ABZ produced phosphorus-containing acids so as to dehydrate and carbonize epoxy macromolecules leading to the formation of graphitized chars. Meanwhile, the nitrogen/sulfur-containing intermediates simultaneously released noncombustible gases to expand the as-formed char, and then interrupt the combustion reaction.