Oxidation of Lignin-Modified Bitumens

The effect of added kraft softwood lignin (20 wt %) on the oxidation rates of two different bitumens was studied using a pressure decay method to measure the oxygen absorption into thin films. Bitumen and lignin-modified binders were oxidized for 40 h at 50 °C under oxygen at near atmospheric pressures. Initial experiments using nitrogen showed little difference between the equilibrium saturation concentrations and diffusion coefficients of the original and lignin-modified materials. The absorption of oxygen by the lignin-modified binders (in mol g –1 of binder) was found to be reduced compared to that of the original bitumen in each case. If the lignin was simply behaving as an inert filler, then a reduction in oxygen absorption of about 20% would be expected. Greater reductions would be expected if the lignin was acting as a bitumen antioxidant. The measured reductions in oxygen absorption were between 7% and 22%, indicating that the lignin was less reactive to oxygen than the bitumens but did not act to reduce the rate of bitumen oxidation. The increase in the shear modulus at 25 °C (and 1.59 Hz) after 40 h of oxidation also showed that the addition of lignin had no marked effect on the oxidation rate.