OXIDATION OF ROADING ASPHALTS

The oxidation of 1.0 mm films of a roading asphalt at 60, 100, and 130-degrees-C in a forced draft oven and at 60-degrees-C under 300 psi of oxygen was studied. In all experiments the increase in viscosity (eta) measured at 45-degrees-C could be fitted to a hyperbolic function of time (t) of the form DELTA(log eta) = t/(a + bt), where a and b are constants. The limiting viscosity DELTA(log eta) --> 1/b was found to be different for each temperature. This was interpreted as being due to differences in the temperature dependence of the rates of competing oxidation reactions and differences in the availability of reactive species due to temperature-dependent structural effects. This contention was supported by infrared spectroscopy which showed that the increase in concentration of carbonyl functionalities as a function of viscosity was the same at each oxidation temperature, whereas the same was not true for sulfoxide functionalities. The rate of increase of sulfoxide concentration was greatest for asphalts oxidized at lower temperatures. Virtually no change in hydrogen type distribution measured by H-1 NMR was observed, except for oxidation at 130-degrees-C where a small decrease in the proportion of aromatic protons was seen. Given the estimated total carbonyl functionality concentration present, the observed constant proportion of benzylic protons suggests that carbonyl formation during oxidation at other than benzylic sites is more significant than supposed in the literature.