Mechanisms of sulphur – nitrogen PIN FRs

Six similar sulphur – nitrogen PIN FRs were synthesised and tested in polypropylene (PP) and polyamide (PA6). The S-N compounds were one sulfenamide, one sulfinamide and three sulfonamides (*) plus one phosphorus-containing sulfenamide. They were tested (1) in polypropylene at 1% or 2% loading in combination with 4 – 8 % of a phosphorus PIN FR (cyclic dimethyl spirophosphonate, from pinfa member Thor) and (2) in polyamide-6 at 2 – 10 % loading in combination with 0 – 10 % of a phosphorus PIN FR (aluminium diethyl phosphinate AlPi from pinfa member Clariant). The sulfenamide and the sulfinamide showed generation of highly reactive aminyl and sulfenyl radicals which reduce flammability by triggering breakdown of polymer chains in the solid phase and quenching fire radicals in the gas phase. The sulfonamides did not generate radicals and acting by increasing charring. The S-N compounds showed significant synergy with the cyclic P PIN FR in PP. Fire safety effects were also related to the S-N compound decomposition temperature, which was lower for the sulfinamide. Certain of the S-N compounds tested achieved UL 94 V-2 (1.6 mm), at 1% with 4% cyclic P Pin FR in PP and at 2% (without P PIN FR) in polyamide.

(*) sulfenamide: simple bonds between N – S – carbon ring ;
sulfinamide: N – S(=O) – carbon ring;
sulfonamide: N – S(2x=O) – carbon ring.

“Sulfenamides, sulfinamides and sulfonamides as flame retardants - similarities and differences in the mechanism of action”, T. Aaritalo et al., Polymer Degradation and Stability 239 (2025) 111370, DOI: https://doi.org/10.1016/j.polymdegradstab.2025.111370