Behavioral differences between group I and group II base oils during thermo-oxidative degradation

A simple approach has been adopted to accelerate thermo-oxidative degradation of selected group I and group II base oils by heating each oil in air at specified temperatures. Chromatographic, spectroscopic and thermal techniques are then applied to establish fundamental differences between the oil types by monitoring compositional changes due to their degradation. The oil properties investigated include hydrocarbon type composition, boiling range distribution, the amount of oxygen and types of oxygenated species, molecular weight distribution, and low-temperature fluidity. While most troublesome degradation products from group I oil are insoluble deposits, group II oil yields oil-soluble low as well as high boiling oxygenated species. It is found that the group I oil degradation path reflects preferential reaction of aromatic hydrocarbons to yield polar compounds and insoluble residue. In contrast, the degradation of the group II oil yields high amounts of oxygenated products at the expense of saturates. Since additives are often utilized to protect the base oil from degradation, the above findings are discussed in the context of selection of proper additives for the formulation of engine oil or other lubricants using different oil types.