Influence of oil polarity and material combination on the tribological response of greases formulated with biodegradable oils and bentonite and highly dispersed silica acid

Abstract

Different biodegradable lubricating greases formulated with esters of fatty acids, as base oils, and bentonite and highly dispersed silica acid, as thickener agents, were tribologically investigated in a nanotribometer and compared with polyalpha olefin greases with equal thickeners. Material combinations of steel ball on steel disc and sapphire ball on steel disc were used with different normal loads. Several friction and wear effects were found depending on the thickener and the base oil. The influence of grease components is also different in both material combinations evaluated. On the one hand, the base oil exerts a much higher impact on friction and wear in grease systems thickened with highly dispersed silica acid than in those thickened with bentonite. On the other hand, the latter reacts more sensitively to a change in material combination. Results were discussed and explained on the basis of polarity influences of the base oils and solid surfaces. Copyright © 2012 John Wiley & Sons, Ltd.

Tribological behaviour of novel chemically modified biopolymer-thickened lubricating greases investigated in a steel–steel rotating ball-on-three plates tribology cell

“…the friction coefficient decreases with the normal load. This is not the expected normal force influence in the boundary and mixed lubrication regimes [54] but it is consistent with the higher rheological resistance offered by the grease to the rotational motion at lower normal loads [28].…”

“…The development of fully bio-based lubricating greases is even a more challenging issue that needs not only the replacement of the mineral oil by a suitable vegetable one but also the selection of an appropriate thickening agent obtained from renewable resources. Oil substitution has been satisfactorily addressed [28][29][30], but the replacement of the thickener is a much more complex task to solve due to the high technical efficiency of traditional metallic soaps and urea derivatives, which impart the desired rheological, thermal and tribological properties to the bulk system. Up to now, vegetable oil-based calcium soap-or clay-thickened greases are considered environmentally-friendly, although showing some performance limitations like, for instance, the lower thermal resistance of calcium greases.…”

“…Up to now, vegetable oil-based calcium soap-or clay-thickened greases are considered environmentally-friendly, although showing some performance limitations like, for instance, the lower thermal resistance of calcium greases. Some lubricating greases based on biodegradable oils with traditional thickeners were tested in different tribological contacts [28,31] showing low values of friction coefficient and small wear scar size for some specific formulations, basically depending on the balance between oil polarity and type of thickener.…”

Influence of Base Oil Polarity on the Transient Shear Flow of Biodegradable Lubricating Greases

“…This results in the formation of permanent dipoles [13], high tendencies towards the formation of Keesom forces and consequently high polarities of oleochemicals. In recent studies it has been proven that high base oil polarities, caused by the use of biodegradable oleochemicals in lubricating greases, may heavily influence their tribological characteristics due to surface adhesion processes in tribo-contacts, as shown in [14]. The polarity-induced influence described therein gives reason to presume that base oil polarities not only affect the tribological but also the rheological characteristics of lubricating greases.…”