On the Electro-Active Control of Nanocellulose-Based Functional Biolubricants

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.