Preliminary Insights into Electro-Sensitive Ecolubricants: A Comparative Analysis Based on Nanocelluloses and Nanosilicates in Castor Oil

Abstract

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.

Dielectric Properties of Cellulose and Nanocellulose-Based Materials

“…According to our investigation, similar to many polymer materials, cellulose has a relatively low dielectric constant 𝜀 (3.9 to 7.5) dielectric loss less than ~0.1 (Luca et al 1938;le Bras et al 2015;García-Morales et al 2020), but this range is next only to the ferroelectric polymer-PVDF.…”

“…In this part, we mainly introduce the composite dielectric materials obtained by mixing nanofiber and polymer. Commonly used polymer dielectrics are listed in Table 3, and their permittivity is generally in the range of 2 to 12, while cellulose's permittivity is in the range of 3.9 to 7.5 (Luca et al 1938;le Bras et al 2015;García-Morales et al 2020), which is greater than the permittivity of ordinary polymers except for ferroelectric PVDF. Nanocellulose (including CNCs, CNFs, and CMC) has good thermal stability, chemical stability, and environmental benignancy (Reddy and Yang 2009;Zhou et al 2011;Cherian et al 2011) and is widely used as polymer-based fillers.…”

“…Although low permittivity also has important industrial applications, in this paper, we focus on cellulose-based dielectrics with high permittivity. The dielectric constant of nanocellulose is 3.9 to 7.5 (Luca et al 1938;García-Morales et al 2020), the dielectric loss is less than 0.1 (Bonardd et al 2018a), which are related to the source of cellulose, surface polar groups, crystallinity, hydrogen bond, hygroscopicity, and temperature, etc.…”