Effect of Thickener Type on Change the Tribological and Rheological Characteristics of Vegetable Lubricants

Abstract

This paper presents the results of a study on the effect of the dispersed phase on the lubricating and rheological properties of selected lubricant compositions. A vegetable oil base (rapeseed oil) was used to prepare vegetable lubricants, which were then thickened with lithium stearate, calcium stearate, aluminum stearate, amorphous silica, and montmorillonite. Based on the results of the tribological tests of selected lubricating compositions, it was found that calcium stearate and montmorillonite have the most beneficial effect on the anti-wear properties of the tested lubricating greases, while silica thickeners (amorphous silica and montmorillonite) provide the effective anti-wear protection in compared to the lubricants produced on lithium and aluminum stearate. The lowest structural viscosity was found for grease thickened with montmorillonite. Much higher values of this parameter were observed for composition, where aluminum stearate was the dispersed phase, while the highest value of structural viscosity was observed for composition, where aerosol–amorphous silica was the thickener. The composition thickened with amorphous silica had the highest yield point value, while the composition in which montmorillonite was the dispersed phase had the lowest value. Dynamic viscosity decreases with temperature, which is characteristic of lubricants. No significant differences in dynamic viscosity were found for the lubricating compositions tested at temperatures above 50 [°C]. The most favorable rheological properties were observed for composition, which was produced using calcium stearate, as it allows the lowest dynamic viscosity at −20 [°C]. Lubricants produced with lithium stearate or aluminum stearate were characterized by higher viscosity at low temperatures. For grease, in which the lithium stearate was used as a thickener, the value of the elasticity index determines the weak viscoelastic properties of tested grease and a greater tendency to change structure under the influence of applied forces. For vegetable grease thickened with aluminum stearate, more than 15 times lower values of the MSD function were observed, and the calculated elasticity index value proves the stronger viscoelastic properties of the aluminum stearate grease in relation to grease thickened with the lithium stearate. The elasticity index value for grease thickened with amorphous silica was lower than for greases thickened with lithium and aluminum stearate, indicating its stronger viscoelastic properties in relation to these two greases. For grease composition prepared on the vegetable oil base and thickened with montmorillonite. The value of the elasticity index was lower than most of the tested grease compositions, without the composition, in which the calcium stearate was used as a thickener. Such results testify to moderately strong viscoelastic properties, which leads to the conclusion that the produced lubricant was a stable substance on changes in chemical structure under the influence of variable conditions prevailing during work in tribological joints.