Submitted to: Lubrication Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2006
Publication Date: 1/1/2007
Citation: Biresaw, G., Kenar, J.A., Kurth, T.L., Felker, F.C., Erhan, S. 2007. Investigation of the mechanism of lubrication by starch-oil composite dry film lubricants. Lubrication Science. 19(1):41-55.
Interpretive Summary: One way of countering the current oversupply of farm products relative to world-wide demand is by developing new consumer and industrial products from farm-based raw materials. Another method is by using farm-based raw materials to replace petroleum-based components in existing products. Lubricants used in various applications constitute a large new market for farm products. Currently, lubricants are formulated almost exclusively using petroleum-based ingredients. Successful replacement of petroleum-based components in lubricants requires a thorough understanding of the lubrication properties of farm-based raw materials and their derivatives. In this work, the friction properties of starch-oil composites or FanteskTM were investigated as a function of starch-to-oil ratio, starch chemistry, and oil chemistry. The result showed that the friction properties of FanteskTM composites can be manipulated by a proper selection of these variables. This means that FanteskTM lubricants can be developed for metalworking applications that may have different friction requirements.
Technical Abstract: The boundary coefficient of friction (COF) of FanteskTM starch-oil composite dry film lubricants was investigated as a function of starch chemistry (waxy vs. normal purified food grade corn starch), oil chemistry (hexadecane vs. oleic acid and various vegetable oils), and starch-to-oil ratio. Based on the results, a mechanism of starch-oil interaction in these composites was proposed. According to the proposed mechanism: a) the oil in the composite is distributed between the bulk and the surface of the starch, and b) the fraction of the oil trapped in the bulk and that adsorbed on the surface are related to each other by an equilibrium constant, and are functions of the total oil concentration in the composite. In line with the proposed mechanism, an adsorption model was used to quantify the free energy of adsorption ('Gads) of the polar oils onto the starch surface. The analysis gave 'Gads values that were higher than those reported for the adsorption of the same polar oils on steel surfaces. This result is consistent with the effect of the relative surface energies of steel and starch on the adsorption of polar oils. The adsorption property of the non-polar hexadecane, relative to the polar oils, was estimated by comparing their interfacial tensions with starch. The result showed a higher interfacial tension for hexadecane-starch than that for the polar oil-starch composites. This result predicts a relatively poorer compatibility, and hence, poorer adsorption of hexadecane on starch leading to higher COF, as was observed in the friction measurements.