The use of inverse gas chromatography (IGC) for the examination of base oil oxidation processes

Kinematic viscosity (40degreesC, 100degreesC), average M, coke value, and group composition were established in mineral oils (200SN, brightstock HBS), poly-alpha-olefins (PAO-6, PAO-40), and semisynthetic base oils made from PAO (SN/PAO-6 and HBS/PAO-40). Flory-Huggins interaction (x(12)(infinity) for binaries formed with 9 test compounds) and Hildebrand solubility (delta(2)) parameters, evaluated from IGC data, were found on oxidation: x(12)(infinity),; to rise most in long-chained hydrocarbon oils (HBS/PAO-40, PAO-40, HBS) and moderately or nil (200SN/PAO-6) in others, and delta(2) to fall most in HBS, PAO-40 and HBS/PAO-40 and slightly in others (generally, Deltadelta(2) less than or equal to 2 (MJ/m(3))(0.5)), thus indicating the composition and structure changes caused by oxidation to be large enough to prevent miscibility (e.g. of fresh with used oil).