The Chemistry of Radical Scavenging Antioxidants at Elevated Temperatures

In this study, the autoxidation of squalane inhibited by phenolic and aminic antioxidants was analysed between 160 and 220 °C representing piston assembly temperatures of automotive engines. The mechanism of the phenolic antioxidant octadecyl 3-(3,5-di-tert-butyl-4- hydroxyphenyl) (OHPP) in the presence of oxygen at these temperatures was analysed. It was concluded that OHPP autoxidation formed four products; a hydroxyl-substituted phenolic, octadecyl 3-(3,5-ditert-butyl-4-hydroxy-phenyl)-3-hydroxy-propanoate, a hydroxyl-substituted quinone methide (octadecyl 3-(3,5-ditert-butyl-4-oxo-cyclohexa- 2,5-dien-1-ylidene)-2-hydroxy-propanoate), and the previously observed hydroxycinnamate and di-tert-butyl benzoquinone. Quantification of the time development of these products indicates that benzoquinone is formed from hydroxycinnamate and not from the phenoxyl radical as previously thought. Mechanisms are suggested to account for this. In the presence of alkyl hydroperoxide, the aminic antioxidant octylated diphenylamine (ODPA) was found to be less effective at high temperatures with large amounts of ODPA remaining at the end of the induction period where the substrate started to oxidise significantly. This is contrary to most previous studies where all the ODPA was consumed by the end of the induction period. The suggested reason for this is that autoxidation occurs preferentially by abstraction of tertiary hydrogen atoms, forming tertiary alkyl peroxy radicals and hydroperoxides. The difference of the O-H bond strength of tertiary alkyl hydroperoxides and the N-H bond strength of ODPA was relatively small resulting in the abstraction of hydrogen atoms from ODPA by tertiary alkyl peroxy radicals being noticeably reversible at higher temperatures. Aminic antioxidants containing naphthalene rings and heteroatom bridges were found to increase the induction period relative to substituted diphenylamines. The alkylated naphthenic antioxidant N-[4-(1,1,3,3-tetramethylbutyl)phenyl]naphthalen-1-amine could achieve this despite 53% of the reacted antioxidant in the initial stages forming dehydrodimer structures. N-(1-naphthyl)naphthalene-1-amine was investigated and was found to form the product (4E)-4-(1-naphthylimino)-1H-naphthalen-1-ol. Based on the aminic antioxidants used, (1,1,3,3-tetramethylbutyl)-12H-benzo-α-phenothiazine was synthesised and used as an antioxidant in squalane autoxidation.