Sulfenamides, sulfinamides and sulfonamides as flame retardants - similarities and differences in the mechanism of action

A series of sulfenamides, sulfinamides and sulfonamides have been synthesized and their thermal degradation pathways have been determined by TGA, GC/MS and 1H NMR and their flame retardant properties in polypropylene and polyamide 6 have been investigated. The results reveal, that sulfenamides and sulfinamides have the specific feature of generating highly reactive aminyl and sulfenyl radicals during pyrolysis that provide flame retardancy for polymeric materials by triggering rapid degradation of polymer chains in the solid phase and by quenching of active fire propagating radicals in the flame zone. In contrast, the sulfonamides thermally decomposed via a non-radical mechanism and their main flame retardancy effect was instead attributed to increased charring of polymeric materials. The thermal stabilities for sulfenamides and sulfonamides were high ranging from 266 °C to 290 °C, Whereas the mass loss of 5 wt% of the corresponding sulfinamide was recorded already at 212 °C. Investigation of synergistic effects with two conventional phosphorous flame retardants, i.e. cyclic dimethyl spirophosphonate and diethyl phosphinic acid aluminium salt revealed that sulfenamides, sulfinamides and sulfonamides display a significantly stronger synergistic effect in combination with dimethyl spirophosphonate compared to diethyl phosphonic acid aluminium salt. The flame retardant efficacies as stand-alone flame retardants and as adjuvants in polypropylene films increased in the following order sulfenamides > sulfinamide > sulfonamides. On the contrary, only sulfonamides in this series were able to provide UL 94 V2 rating in polyamide 6 with very short burning times at the loading of 2.5 wt%.