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This research work aims to explore the development of functional nanocellulose-based biolubricants, which allow for an electro-active control of the friction behavior. With this purpose, the influence of both nanocellulose concentration and electric field strength on the lubricant's electrorheological behavior was analyzed. Electric field strengths up to 4 kV/mm were imposed and two different kinds of nanocellulose were studied as the polarizable particulate phase: cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs). Nanocellulose particles were added to castor oil at weight fractions ranging from 0 to 6 wt %. All dispersions exhibited a noticeable variation in their dielectric constant, but not in their conductivity, within a wide frequency range between 1 Hz and 200 kHz, and their dielectric behavior was significantly affected by the particle weight fraction. Noteworthily, it was found that the critical value of nanocellulose concentration, 4 wt %, at which the electro-viscous effect displayed by these dispersions was constrained, yielding a limiting electrorheological (ER) behavior. In addition, the dynamic yield stress dependence on the electric field strength showed a critical value within the interval of 0.8−1.2 kV/mm, suggesting a nonlinear conduction model for these nanocellulose-based ER dispersions. Finally, a maximum leak current intensity for 1 wt % CNF or CNC dispersions and an asymptotic decay at higher concentrations were observed. We conclude that both CNC and CNF nanoparticles have demonstrated that they can endow castor oil with significant ER properties, which remarkably reduced the friction coefficient within the boundary and mixed lubrication regions at electric field strengths lower than 40 V.

The newest generation of lubricants needs to adapt to stricter environmental policies. Simple and sustainable formulations with tunable rheological properties under the action of electric potentials may be the key. The present research explored the feasibility of producing electro-sensitive ecolubricants based on nanocellulose (crystalline and fibrillar) or nanoclay (Cloisite 15A montmorillonite and halloysite nanotubes) dispersions in castor oil, at concentrations that ranged from 2 to 6 wt.%. Broadband dielectric spectroscopy (BDS) measurements allowed for a first estimate on the electro-responsive potential of the nanofluids. The nanocelluloses and the montmorillonite suspensions presented a relaxation event in the dielectric loss, ε″, centered at ca. 2–4 kHz, which is related to interfacial polarization. Moreover, their actual electro-rheological (ER) effect under high electric potentials up to 4 kV/mm was assessed by determining the magnitude of the yield stress from steady flow curves at 25 °C. It was found that the nanocelluloses and the montmorillonite showed an enhancement of three orders of magnitude in their yield stress values at 4 kV/m. This enhancement was much greater than in the halloysite nanoclay, which did not exhibit any polarization). This is the starting point for the development of environmentally friendly ER lubricating fluids, based on nanocellulose and montmorillonites (layered nanosilicates), which might assist in reducing the friction and wear through the application of controlled electric fields.

This manuscript describes a rheological and tribological study carried out on eco-friendly lubricants. These ecolubricants were made up of nanoclays as dispersed phase (a layered nanosilicate (montmorillonite Cloisite 15A) and a fiber-like nanoclay (sepiolite Pangel B20)) and vegetable-based oil as continuous phase (castor oil (CO), high oleic sunflower oil (HOSO) and their mixtures). A series of nanoclay-based ecolubricants were prepared by varying both nanoclay concentration and base oil, and thus, its viscosity. Friction and wear behaviors were assessed by using a ball-on-three plates tribometer cell. The results showed that the fiber-like sepiolite Pangel B20 yielded an important reduction in the wear scar diameter, thus revealing its potential as anti-wear and load-carrying additive in ecolubricant formulations, while Cloisite 15A proved to have friction improving properties. These anti-wear and friction reducing properties were found to be influenced by both nanoclay concentration and oil viscosity.

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