Antioxidant action mechanisms of hindered amine stabilisers

Kinetic modeling and simulation studies reveal that the antioxidant effectiveness of Hindered Amine Stabilisers (HAS) cannot be accounted for by the currently accepted action mechanisms. Obviously, hindered aminoxyl radicals are capable of reacting with alkylperoxyl radicals. N-alkoxyamines and oxygen are formed in the reaction with tertiary alkylperoxyl radicals, and the free amines, ketones and oxygen in the reaction with sec-alkylperoxyls. In oxidations inhibited by hindered aminoxyls, these reactions are kinetically most pronounced when the chain propagation and termination rates are low as in the AIBN-initiated thermooxidation of cumene inhibited by 2,2,6,6-tetramethyl-pyridinoxyls (TEMPO) where a pronounced induction period of the oxygen uptake is observed which cannot be accounted for by the currently accepted action mechanisms of hindered amine stabilisers (HAS). In the inhibited oxidation of tetralin, however, where the propagation and termination rates are higher, only a retardation of the oxygen uptake but no induction period are observed. But here too, the consumption of the aminoxyl is faster and the oxygen uptake slower than computed without taking into account the reaction of aminoxyl with peroxyl radicals. (C) 2002 Elsevier Science Ltd. All rights reserved.