Lubrication in vegetable-based lubricating fluids under oxidative conditions

摘要

The use of vegetable-based fluids to replace mineral oil basestocks introduces double bonds of unsaturated fatty acids into the chemical media of the lubricants. Despite some advantages like biodegradability, improved low temperature properties or better additive acceptability, unsaturation can also lead to such problems as low oxidative stability, chemical reactivity, higher Corrosiveness, poor filterability, questionable elastomer compatibility, precipitation, etc. This study addresses one moreissue, important but yet quite unfamiliar in lubricant research. Oxidative gelation is observed in vegetable oils, which are heated in thin film on the metal surface under controlled air flow in the Penn State Micro Oxidation Test. Both appearance andkinetics of the formation of the solids is qualitatively different from fluids, which do not contain unsaturation (mineral oils or synthetic esters usually form a different type of deposit). A number of neat conventional vegetable oils, potentialbiodegradable basestocks for many lubricant applications (canola, high oleic sunflower, soybean, corn and castor) were tested and all of them demonstrated oxidative gelation tendencies.Although experimental evidence that oxidative gelation quantitatively relates to the number of double bonds in the vegetable-based fluids still has to be collected, all observations are consistent with hydroperoxide theory as well as with theory ofgelation. Qualitative theoretical explanations are provided in the study, showing that presence of polyunsaturation, especially in polymerized vegetable-based lubricant.