Dependence of carbonyl, sulphoxide and hydroxyl functional group formation on oxygen concentration during the oxidation of roading bitumen

Abstract

The aim of this study was to provide insights into the chemical mechanism by which, over the course of its lifetime, bitumen in road surfacings reacts with oxygen in the atmosphere. Oxidation was measured at 50 °C (a realistic in-service temperature) to determine the effect of oxygen concentration on the formation of key oxidation products. Experiments were conducted on thin films of bitumen (<0.03 mm) so that the effect of oxygen diffusion on the rate was negligible. The films were oxidised under air (oxygen partial pressure 22.23 kPa) and pure oxygen (101.33 kPa).The rate of formation of carbonyl, sulphoxide and hydroxyl group containing reaction products was measured by infrared spectroscopy. The reactions were carried out over a time-frame sufficient to cover both the “fast” and “slow” (linear) reaction rate regions and the results discussed in terms of the reaction mechanism. It was observed that the rate of formation of all three functional groups was dependent on oxygen concentration. The apparent reaction order with respect to oxygen concentration was about 0.5 in each case and did not change significantly as the reaction moved from the fast and into the linear rate regions. The non-zero rate dependence observed despite the relatively high oxygen partial pressures used, is significant in that it is inconsistent with a conventional free radical autoxidation chain mechanism and suggests a parallel oxidation process is occurring.