Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2011
Publication Date: 5/24/2012
Citation: Bantchev, G.B., Biresaw, G., Cermak, S.C. 2012. Elastohydrodynamic study of blends of bio-based esters with polyalphaolefin in the low film thickness regime. Journal of the American Oil Chemists' Society. 89(6):1091-1099.
Interpretive Summary: This study determined the film thicknesses in the low film thickness regime of lubricating bio-based oils and their blends with synthetic polyalphaolefins. The bio-based oils were soybean oil (SBO), and an ester of an oleic estolide. Polyalphaolefins are oils used for the formulation of high-performance lubricants. Lubricants act by forming a film between the moving parts. The lubricant film reduces the friction and wear of the parts. In the so-called “mixed regime of lubrication,” there is a lubricating film between the parts, but it is so thin that it cannot prevent an occasional contact between them. Currently, there is active research to understand the fundamental laws governing lubrication in the mixed regime, but there is a scarcity of studies with bio-based lubricating oils. In the current study, the film thickness was measured down to a few nanometers (nm). The influence of the lubricating oil, speed of the parts, temperature, and time was investigated. The data was compared with an existing theoretical model that allows predicting the film thickness. It was shown that when estolide is added to low viscosity polyalphaolefin, it immediately forms a film that is a few nm thicker than that predicted by the theory and keeps the parts separated even at low speeds. SBO in polyalphaolefin forms similar films, although at a slower rate – after more than 15 min. Formation of such films is important for the prevention of direct contact since there are studies showing that even a few nm increase of the film thickness can drastically reduce the wear in the mixed regime of lubrication.
Technical Abstract: The film thickness in elastohydrodynamic conditions for soybean oil (SBO), oleic estolide ester (Est) and their binary blends with polyalphaolefins (PAO2 or PAO40) were studied at 30 and 100°C. Changes with time, for up to 200 min, were monitored. SBO and its blends with the lower viscosity PAO2 showed initially good agreement with the Hamrock-Dowson (H-D) equation down to 1-3 nm film thickness. Sixty minutes or more after the start of the measurements, boundary layers with thickness up to 4.7 nm were observed. The blend of SBO with the more viscous PAO40 showed initially a good agreement with the H-D equation at 100°C. Negative deviations in film thickness were observed 15 min after the start of the measurements. At extended periods of time, up to 200 min, they were less pronounced but still detectable. The Est–PAO2 blend showed initial formation of boundary layers with thickness around 2 nm. The boundary layer at 30°C did not change for 200 min, while at 100°C showed a decrease in thickness and/or viscosity with time. The Est and the Est–PAO40 blends showed good agreement with the H-D equation and did not display boundary or fractionation layer within 200 min.