Chemical Interactions between Polyamide 66 and Phosphorus Flame Retardants

To elucidate chemistry taking place during thermal degradation of polyamide 66 in the presence of phosphorus flame retardants and understand how commonly used phosphorus flame retardants achieve their flame retardancy, PA66 and a series of model compounds which include N,N'-dibutylhexanediamide, 1-butylazepane-2,7-dione and 1,8-diazacyclotetradecane-2,7‑dione have been subjected to thermal treatments together with melamine polyphosphate, aluminum diethylphosphinate or their combination respectively. Analyses of degradation products indicate that aluminum diethylphosphinate accelerates a cyclization reaction but does not change degradation mechanisms of polyamide 66 and its model compounds while melamine polyphosphate significantly shifts their degradation pathways to dehydration via a modified Hofmann elimination reaction which produces adiponitrile and water. Adiponitrile intumesces strongly when heated with polyphosphoric acid. Water hydrolyzes amides to amines, accompanied by the formation of amino-6-oxohexanoic acid which cyclizes to cyclopentanone. Water and amines together convert aluminum diethylphosphinate to diethylphosphinic acid. On the basis of the identification and transformation of reactive species, modes of flame retardant action of the combination of aluminum diethylphosphinate and melamine polyphosphate for PA66 have been proposed.