Electro-active control of the viscous flow and tribological performance of ecolubricants based on phyllosilicate clay minerals and castor oil

Abstract

Lubricating fluids with highly improved tribological performance very often entail the use of environmentally disrespectful formulations. Based on organoclay dispersions in castor oil, the present research explores the development of simple and sustainable lubricants that enable an “active control” of the tribological behavior under the action of electric fields. Storage stable formulations with high electro-rheological (ER) performance were obtained with 2 wt% organo-modified montmorillonites (OMt) dispersions in castor oil. Their strong ER potential was assessed through the dynamic yield stress values obtained from steady flow curves at 25 °C under electric fields up to 4 kV/mm. Broadband dielectric spectroscopy measurements demonstrated a local maximum (or shoulder) in the dielectric loss, ε´´, in the frequency range 5–10 kHz which is related to interfacial polarization. It was found that the OMt clays had a much greater ER potential than other types of clay mineral (e.g. halloysite nanotubes) that did not exhibit any polarization relaxation in ε´´. Moreover, in general, a fairly good correlation of the yield stress values with the drop in the permittivity, Δε´ = ε0´ – ε∞´, was found for the OMt clays. Preliminary electro-tribological tests were carried out using a ball-on-three plates setup, with an optimal formulation prototype consisting of 2 wt% Cloisite 15A dispersion in castor oil. The results demonstrated a reduction of up to 9% of the friction coefficient in the mixed lubrication regime. This outcome, based on organo-modified layered nanosilicates dispersions in castor oil, revealed the feasibility of producing a “new generation” of lubricants which, with no need for hazardous chemicals, may assist in the development of a novel concept of lubrication through electric potentials.