Abstract
The rheological characteristics of one mineral oil and two ionic liquid (IL) based lubricating greases were explored as a function of thickener concentration. The ILs used are 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]) and trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P6,6,6,14][TFSI]), with polytetra-fluoroethylene (PTFE) particles used as thickeners. Greases with different base liquid concentrations (60–80 wt %) were investigated using small-amplitude oscillatory shear and viscous flow measurements, and contact angle measurements probed adhesion at base liquid–PTFE interfaces. Rheological properties are influenced by base liquid–PTFE adhesion and the chemical structure of the grease base liquids. With the addition of thickener, the greases generally have higher elasticity, strain resistance, and frequency independent properties. Viscometric rheological tests illustrate non-Newtonian shear-thinning behaviour for all greases. [BMIM][TFSI] based greases show the most elastic properties and strain resistance, as well as the highest initial and lowest final viscosities of the greases tested.
Highlights
Lubricating greases are multiphase systems composed of two main components: base liquids and thickeners
Sessile drop contact angle measurements were performed for the mineral oil and the two ionic liquid (IL) on a PTFE surface
The storage and loss moduli for all [P6,6,6,14 ][TFSI] based greases cross each other at an angular frequency ~50 rad/s (Figure 3c). These results suggest that [BMIM][TFSI] and mineral oil have a stronger microstructure than [P6,6,6,14 ][TFSI], at least when the base liquid is 60 wt % and 70 wt %, which is in line with the strain sweep results
Summary
Lubricating greases are multiphase systems composed of two main components: base liquids and thickeners. The base liquid, generally either a mineral or synthetic oil, acts as the lubricating substance that is bled from the grease structure. Grease thickeners form a three-dimensional gelation network (microstructure) which traps the oil and confers the appropriate rheological and tribological behavior to the grease [1]. Viscoelasticity is the simultaneous presentation of both solid and liquid properties. It is a common characteristic that can be obtained by rheological measurements; this property becomes prominent when assessing colloidal systems, including grease. The two main values used to describe viscoelasticity are the storage (G’) and loss (G”) moduli, which relate to the material’s ability to store and dissipate energy, respectively. The storage modulus is often used as a measure of a material’s elasticity, whilst the loss modulus is used to describe the viscous behavior
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