Effect of thermo-mechanical processing on the rheology of oleogels potentially applicable as biodegradable lubricating greases

This work is focused on the development of oleogels potentially suitable as biodegradable lubricating greases by studying the effect that some thermo-mechanical processing variables exert on their rheological properties and microstructure. Oleogels were prepared by dispersing sorbitan monostearate (SMS) in castor oil under different agitation conditions and thermal protocols. The effect of processing variables was evaluated by performing small-amplitude oscillatory shear (SAOS) measurements, AFM observations and some mechanical and tribological tests usually employed to check lubrication performance and stability of lubricating greases. Rheological properties of oleogels can be monitored in situ during processing using a rheo-reactor. Processing conditions such as maximum temperature applied to disperse the gelling agent, heating time, agitation speed and cooling rate significantly influence gel strength. Cooling profile is the most important processing variable affecting the rheological response of oleogels. Differences of more than one decade in the values of SAOS functions were found for oleogels processed by applying different cooling profiles. A moderate agitation during SMS dispersion in oil yields maximum values of oleogel linear viscoelasticity functions. The maximum processing temperature reached during this stage should be only slightly higher than the SMS melting point. In general, oleogel samples exhibited a poor mechanical stability after sample working in a rolling element, but, on the contrary, they recovered completely their initial consistency after a moderate resting time, also exhibiting remarkable lubricant properties. Moreover, oleogel samples tested in a ball-disc tribometer show values of the friction coefficient inferior to those obtained with model lithium lubricating greases.