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.

Based on the response surface methodology, a rheological and tribological study carried out on eco-friendly lubricants is described. Such ecolubricants consisted of fibrillated or crystalline nanocellulose in vegetable oil (castor oil, high oleic sunflower oil or their mixtures). Cellulose nanoparticles showed noticeable friction-reducing and anti-wear properties within the boundary and mixed lubrication regimes, which were found to be dependent on nanocellulose concentration, base oil composition and applied normal force. In general, both types of nanocellulose performed equally well. An excellent tribological performance, with large wear scar diameter reductions, was achieved with 3.3 wt.% (or higher) nanocellulose dispersions in castor oil-rich mixtures. The observed behavior was explained on the basis of enhanced viscosity of castor oil-rich suspensions and the preferential action of the most polar components, nanocellulose and ricinoleic acid, in the vicinity of the contact surfaces.

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.

Sustainable lubricating oleogels were obtained based on elm (Ulmus minor Mill.) pulp cellulose nanofibers in castor oil. Before their nanofibrillation, two samples of the same bleached pulp were subjected to two different pre-treatments, either mechanical PFI (Paper and Fibre Research Institute) refining or chemical TEMPO-mediated oxidation. Their effect on the oleogels’ viscous flow and oscillatory shear behaviors was determined at different temperatures. Very pronounced differences were observed between these oleogels. The oleogel from the chemically-pretreated nanofibers showed evidences of being much more sensitive to both temperature and large shear deformations. Hence, its shear modulus underwent a very remarkable decay of 95.8% when subjected to a non-linear stress value of 100 Pa for 30 min. In turn, under the same conditions, a decay of only 41.4% was observed for the oleogel based on the nanofiber from the mechanically-pretreated nanofibers. Graphical abstract

A method that enabled the formulation of lubricating oleogels using dried cellulose nanofibers (CNFs) as an eco-friendly thickener in castor oil was studied. In their dehydrated state, strong hydrogen bonding between nanofibers and high hydrophilicity are the main obstacles to their dispersion in oil. Hence, clusters of dried CNFs had to be previously detached by their dispersion in water. The resulting hydrogels were then subjected to methanol washes to displace the water from the nanofibers. After centrifugation, the methanol-wetted precipitate was readily dispersed in castor oil, forming an oleogel once the methanol was removed. Optimization was conducted in terms of the following variables: (a) hydrogel processing method; (b) hydrogel pH; (c) methanol/hydrogel ratio; (d) number of washes; and (e) oleogel CNF concentration. Their effect on the oleogel linear viscoelastic behavior was analyzed. In general, they demonstrated a prevailing elastic behavior denoted by a well-developed plateau region. The CNF concentration was found to have a more remarkable impact on the oleogels’ rheological behavior than any other variable studied. Hence, substantial differences were observed between 1 and 2 wt.%. The CNFs exhibited a very remarkable thickening capacity in castor oil, achieving a plateau modulus of ca. 700 Pa with just 2 wt.%. Moreover, the resulting oleogels maintained a uniform texture even after one year of storage. This indicates that the oleogels were both homogeneous and storage stable, effectively overcoming the stability issues associated with direct dispersion of dried CNFs in castor oil.