Concept of molecular design towards additive technology for advanced lubricants

A simple model to develop novel friction modifiers and anti-wear agents for synthetic oils was studied. Our attention was focused on the construction of additive molecules to meet the requirement for synthetic oils, which possess a different polarity and solvency from mineral oils. The relation between the molecular structure of additives and their tribological properties were elucidated prior to the preparation of the substance and estimation of its tribological properties. Computer-assisted chemistry was partly applied to predict the properties of the designed molecules. The adsorptive activity of the conventional fatty acids is insufficient to reduce friction when they were dissolved in polar synthetic oils such as polyethers. Introduction of another carboxyl group into the fatty acid, especially at the carbon atom next to the original carboxyl group, makes the molecule polar; resulting in friction modifiers for polyethers. Conversion of the carboxyl groups into trimethylsilyl esters provides soluble friction modifiers for poly(dimethylsilicone)-type synthetic oils. A unique lubrication mechanism based on in situ regeneration of the original molecule was proposed for synthetic oils with poor solvency. Importance of adsorptive activity of additive molecule was also pointed out in phosphate-type anti-wear additives for polar synthetic oils. We found that both the polarity of additives and that of base oils have to be taken into account in lubricant design. Too much adsorption activity of the molecule caused corrosive wear. Hydrolytic stable compounds reduces corrosive wear to a considerable extent. Introduction of a hydroxyl group into the alkyl moiety of the phosphate ester provides polar molecule with low acidity. The additives reduce wear in synthetic esters even at low concentrations of phosphorus. The lubrication mechanism was also discussed; the novel additives provide a thick boundary film composed of iron phosphate on rubbing surfaces. Moreover, an acceleration of the kinetics of the film formation by the hydroxyl group was observed. Copyright © 2007 John Wiley & Sons, Ltd.