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Abstract
Synthetic oils have come into widespread use due to their inherent stability, consistent physical properties, and reproducible composition. As ever increasing demands are being placed on oil performance in magnetic recording disk drives by increasing rotation rates up to 10,000 rpm, it is important to know the ultimate limitations of these functional fluids due to oxidation; In this study, the authors focus on a pentaerythritol ester oil. Accelerated 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 oxidation was followed by analytical techniques including UV/visible, infrared, and proton nuclear magnetic resonance spectroscopy, and gel permeation chromatography, thin layer chromatography, viscometry, and differential scanning calorimetry. A new technique of laser desorption mass spectrometry with jet cooling, which provides the parent ion mass spectrum, provided the mass distribution of intermediate oxidation products. These techniques enabled determination of the predominant oxidation products. Oxidation proceeds through 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 alkyl chain with the formation of methyl ketone and carboxylic acid end groups. Dissolved iron increased the formation rate of hydroxyl and oxetane without changing the hydroperoxide concentration. Oligomers were absent from the mass spectrum of the oxidized oil. The increase in oil viscosity with accelerated aging is due to increasing intermolecular hydrogen bonding between hydroxyl groups formed on alkyl chains.
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