Submitted to: Thermochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2000
Publication Date: N/A
Citation: Interpretive Summary: Lubrication technology has been a frontline research activity for the last couple of decades. This is primarily due to the advancement in mechanics and machine tools working under severe and stringent conditions. Mineral base oils have been used widely as lubricant base stocks, but of late, are facing increasing environment related problems. This has shifted the interest on vegetable oil (VO) base fluids, which has the potential to deliver similar physical and chemical properties of mineral base oils with added advantage of complete biodegradability. This study explores the feasibility of using various VOs including genetically modified VO for use as lube base oils. Since oxidative degradation is the primary reason resulting in various other problems, i.e. increase in viscosity, sludge, deposit, acidity, corrosion, etc., we have explored the causes of oxidative instability of VO in terms of their varying structural parameters. This includes oil insoluble deposit forming tendency, evaporative loss, and oxidation kinetic studies of VOs explained in terms of their chemical diversity. The direct application of this work highlights rapid thermal and oxidative stability tests, kinetic and thermodynamic measurements. The data indicates that the number and abundance of polyunsaturation in the fatty acid chain, divinyl -CH(2), allylic -CH(2), etc. influence oxidation and deposit formation in varying extent. The kinetic model developed also highlights these phenomena and can be used as rapid evaluation and screening tool for potential VOs for use as future lube base stocks.
Technical Abstract: Evaluation of oxidative stability of a series of vegetable oils and genetically modified vegetable oils were carried out using Pressurized Differential Scanning Calorimetry (PDSC). Kinetic and thermodynamic parameters were computed and variation of results explained in terms of structural data derived from quantitative nuclear magnetic resonance spectroscopy. For a variety of vegetable oil samples used in the study, log b (program rate of heating; i.e. 1, 5, 10, 15, and 20 C/min) was linearly related to the reciprocal of absolute temperature (Peak height temperature) with R(2) = 0.99. The resulting slope made it possible to compute activation energy [E(a)] for oxidation reaction and various other thermodynamic parameters, e.g. rate constant (k), Arrhenius frequency factor (Z), and half-life period (t1/2). The presence of unsaturated C=C in the fatty acid (FA) chain, their nature and relative abundance, affect thermal and oxidative stability of the oil, and subsequently, their kinetic and thermodynamic parameters. Quantitative analysis of the NMR spectra yielded various other structural parameters that were correlated with start (ST) and onset (OT) temperature of vegetable oil oxidation and certain important thermodynamic parameters (Ea and k). This is a novel approach, where statistical models were developed as a predictive tool for quick assessment of oxidative and thermodynamic data. An adjusted R2 of 92.2% and higher was obtained using three or four NMR derived predictor variables. The correlations revealed that apart from nature and abundance of C=C, relative presence and abundance of other structural details (e.g. divinyl -CH2, allylic CH2, alpha-CH2 to C=O, etc.) influence oxidation and kinetic data.