Abstract
ABSTRACT Examination of kinetic data for asphalt oxidative age hardening and the chemistry of the oxidation reactions has led to the proposal of a sequential, dual mechanism for asphalt oxidation. The dual mechanism rationalizes conflicts between results of earlier mechanistic investigations and provides a fundamental explanation for the hyperbolic-like, property-versus-time plots characteristic of asphalt oxidative aging. The oxidation kinetics are in harmony with and provide further confirmation of the microstructural model of asphalt. It is proposed that the rapid oxidation rate at the onset of asphalt oxidation results from reaction of oxygen with a limited amount of highly reactive hydrocarbon components. Final oxidation products of this initial reaction are sulfoxides and most likely aromatized rings. During this initial reaction, a much slower oxidation of benzytic carbons is initiated with both ketones and sulfoxides being formed. The ratio of ketones to sulfoxides formed and the rate of oxidative age hardening were found highly dependent on temperature and oxygen pressure (concentration). Significant differences were found in the oxidation and age hardening kinetics of pressure vessel aged asphalts and asphalts aged at atmospheric pressure. Atmospheric pressure oxidation was significantly more sensitive to variations in temperature and asphalt composition than was pressure vessel oxidation at 100°C.
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