Tribological Studies of Potential Vegetable Oil-based Lubricants Containing Environmentally Friendly Viscosity Modifiers

The amphiphilic properties that result from the fatty acid composition of vegetable oils contribute to a better lubricity and effectiveness as anti-wear compounds than mineral or synthetic lubricant oils. Despite these advantages, vegetable oils show only a limited range of viscosities and this constrains their use as suitable biolubricants in many industrial applications. For the reason, ethylene–vinyl acetate copolymer (EVA) and ethyl cellulose (EC) have been added to the vegetable oil-based lubricants studied. To address this issue, the frictional and lubricant film-forming properties of improved vegetable oil-based lubricants (high oleic sunflower (HOSO), soybean (SYO) and castor (CO) oils), blended with 4% (w/w) of EVA and 1% (w/w) of EC, have been studied. It has been found that castor oil shows the best lubricant properties, when compared to high oleic sunflower and soybean oil, with very good film-forming properties and excellent friction and wear behaviour. This can be attributed to its hydroxyl functional group that increases both the viscosity and polarity of this vegetable oil. Regarding the effect of the viscosity modifiers studied, ethylene–vinyl acetate copolymer exerts a slight effect on lubricant film-forming properties and, thus, helps to reduce friction and wear mainly in the mixed lubrication region. Ethyl cellulose, on the other hand, was much more effective, mainly with castor oil, in improving both mixed and boundary lubrication.