A Light Scattering Study of the Aggregation Kinetics of Carbon Black Suspensions in Group1 and Group 2 Base Oils. Influence of Dispersant Additives

The stabilization of non-dispersed particles against aggregation is a major challenge for diesel lubricant formulators. New engine technologies, especially exhaust gas recirculation (EGR), and the increase in drain intervals have given rise to increasingly soot-laden oils. To help formulators understand these complicated systems, the additive chemists might study aggregation of the colloidal particles such as Carbon Black suspensions by Light Scattering techniques. The connection of such experiments is discussed taking into consideration of aggregation kinetics of colloidal particles, in particular it is shown that the time evolution of fractal aggregates of Carbon Black particles leads to the stability parameter W. From the Light Scattering experiments it is found that Group I and Group II base oils behave differently as regards Carbon Black stability. In the case of a Group I base oil, no aggregation is observed, whereas in the presence of a Group II base oil, a large increase of the aggregates is measured. This result is poorly understood and at odds with some other experiments. The effect of dispersants in a Group I base oil is to reduce slightly the size of the aggregates. In a Group II base oil, depending on the type of dispersant, the aggregation kinetics is either slowed down or stopped. These results are confirmed by sedimentation and confocal microscopy techniques. Finally, using a relation which links W to the Van der Waals' steric and electrostatic potentials and numerical values from the literature, the relative influence of attractive and repulsive forces is considered in the case of the Group II base oil.