Reaction Kinetics Studies on the Autoxidation of Bitumen

Abstract

The oxidation of paving grade bitumens during handling, asphalt mix manufacture and subsequently over many years in the field in road surfacings, leads to hardening and eventual cracking and other forms of surfacing failure. Bitumen is essentially a very complex mixture of hydrocarbons, but there are features of the way in which bitumen oxidizes that are unexpected and don’t align with the conventional free radical chain mechanism of hydrocarbon oxidation- known as autoxidation. Basic information on the rate of oxidation and how this is affected by both the concentration of oxygen present and that of the reactive species in the bitumen, is lacking. Such information is needed to help begin to understand the reaction mechanism and find effective antioxidant compounds. The work presented in this thesis is one of the few studies in this area and the first detailed report of bitumen oxidation rate concentration dependencies over the whole reaction pathway (i.e. in both the fast and linear regions) and apparently the first to report reaction rate dependencies for the formation of hydroxyl and sulphoxide products. In contrast to most studies on the oxidation of bitumen, experiments were conducted at a realistic in-service road temperature of 50 °C, which necessitated very long reaction times but avoided potential artefacts and changes in the reaction mechanism that could occur at unrealistically high temperatures. Specific objectives of the work were: I. To develop and validate methodologies to enable measurement of the reaction rate of bitumen at different concentrations and with different oxygen concentrations. Measurements were made using bitumen solutions in two different solvents and thin solid films. The main assumptions/hypotheses underlying the experimental methods used to measure the bitumen and oxygen concentration rate dependencies were investigated and found to be valid: (a) The carbonyl group concentration is a good measure of the overall extent of reaction. (b) The carbonyl peak area measured by infrared spectroscopy is directly proportional to carbonyl group concentration (Beer’s law). (c) Under the conditions used oxygen diffusion into bitumen solutions and solid films was rapid and diffusion effects could be ignored. (d) Under the conditions used the solvents could be considered inert. (e) For oxygen, Henry’s law was found to apply to both bitumen solutions and solid films. II. To measure how the reaction rate depended on the available concentration of oxygen and bitumen reactive species. The results of the experiments showed that the overall bitumen oxidation rate (based on the carbonyl formation rate) depended on bitumen and oxygen concentrations [B] and [O2] to the power of 1.2 and 0.5 respectively. The formation of sulphoxide and hydroxyl groups (the other major reaction products) also showed oxygen dependence, and this was also approximately half order in oxygen concentration. III. To determine if the measured bitumen and oxygen concentration rate dependencies varied over the course of the reaction (i.e. in both fast and linear rate regions). Notably the dependency of the rate on reactant concentration remained the same in both the fast and linear regions of the reaction. IV: To compare the experimentally determined rate dependencies to those expected from bitumen oxidation mechanisms proposed in the literature. The findings were inconsistent with the currently “standard” reaction mechanism for bitumen autoxidation (the Dual-Sequential mechanism) and show that it is probably not an accurate model for bitumen oxidation. The rate dependency findings, together with the observed lack of an induction period and the evidence for the presence of natural radical inhibitors (confirming previous research), are also inconsistent with a conventional free radical chain mechanism and point to the primary oxygen uptake step being a complex non-radical chain process.