Oxidation chemistry of a pentaerythritol tetraester oil

Synthetic oils have come into widespread use due to their inherent stabilit y, consistent physical properties, and reproducible composition. As ever in creasing demands are being placed on oil performance in magnetic recording disk drives by increasing rotation rates up to 10,000 Ipm, it is important to know the ultimate limitations of these functional fluids due to oxidatio n; In this study: the authors focus on a pentaerythritol ester oil. Acceler ated aging tests were carried out on the oil at elevated temperature. Tests were also done with 50 ppm of dissolved iron in the oil. The progress of o xidation was followed by analytical techniques including UV/visible, infrar ed, and proton nuclear magnetic resonance spectroscopy and gel permeation c hromatography, thin layer chromatography, viscometry, and differential scan ning calorimetry. A new technique of laser desorption mass spectrometry wit h jet cooling, which provides the parent ion mass spectrum, Provided the ma ss distribution of intermediate oxidation products. These techniques enable d determination of the predominant oxidation products. Oxidation proceeds t hrough interchain and intrachain proton abstraction Hydroxyl groups form On alkyl chains. Intrachain proton abstraction leads to formation of oxetane and conjugated ketone on the original alkyl chain and to cleavage of the al kyl chain with the formation of methyl ketone and carboxylic acid end group s. Dissolved iron increased the formation rate of hydroxyl and oxetane with out changing the hydroperoxide concentration. Oligomers were absent from th e mass spectrum of the oxidized oil. The increase in oil viscosity with acc elerated aging is due to increasing intermolecular hydrogen bonding between hydroxyl groups formed on alkyl chains.