Charge Transfer Complexing Mechanism of Antioxidants. Fate of Aromatic Amines during Thermal Oxidation of Natural Rubber Vulcanizates

Abstract

Thin vulcanizates containing various amines were oxidized at 100° C under 760 mm Hg oxygen pressure. Oxygen absorption and consumption of the amine were plotted as a function of oxidation time. The fate of amine was traced by extracting the oxidized vulcanizates with acetone and then with dilute hydrochloric acid-ethanol mixture, and by determining free amine or Kjeldahl nitrogen of each of the extracts and of the extracted vulcanizates. We observed that the amine combined with the rubber peroxy radical as an intermediate I, from which nitrogen-containing entity was not extracted with acetone but was extracted with the hydrochloric acid-ethanol mixture. The hydrochloric acid-extracts exhibited absorption spectra of Würster cations of the corresponding amines. Therefore, intermediate I is considered charge transfer complex between the amine and the rubber peroxy radical. We further confirmed that the amine was transformed via the intermediate I into acetone-soluble products II and/or amine-rubber compounds III, from the latter of which the nitrogen-containing entity could not be extracted with hydrochloric acid-ethanol mixture. Antioxidant efficiencies of secondary di- and monoamines and tertiary amine were determined and compared with each other. Furthermore, the efficiencies of I, II, and III (see Results for definitions) from these amines were qualitatively estimated separately by determining oxygen absorptions of specifically prepared vulcanizates. The validity of secondary diamine as antioxidants is interpreted by the results that the complex from secondary diamine reacts further with a second peroxy radical to give two of hydroperoxide and a quinonediimine, while the complex from secondary monoamine and tertiary amine cannot take such a route.