Smart biolubricants derived from waste cooking oils: electrorheological and tribological response of cellulose nanofibril dispersions

This work investigates the formulation and performance of smart biolubricants developed by dispersing cellulose nanofibrils (CNF) in waste cooking oils (WCO) of diverse origins: mixed origin (WM-WCO), common restaurant (CR-WCO), traditional fryery (FR-WCO), and fast-food restaurant (FF-WCO). Steady-state viscous flow tests were conducted at 25 °C under external electric fields up to 4 kV/mm to evaluate the electrorheological (ER) activity of 2 wt.% CNF dispersions.

Without an electric field, all formulations exhibited Newtonian behaviour. Upon application of an electric field, the dispersions acquired a highly pseudoplastic character. The ER effect was significantly more pronounced in biolubricants formulated with FF-WCO and FR-WCO, particularly at shear rates <10 s^-1 and electric field strengths between 0.8 and 2.4 kV/mm. This superior performance is primarily attributed to the higher free fatty acid (FFA) content in these specific base oils, which enhances the ER structure-forming capacity. To further isolate the role of oil fractions, molecular distillation was utilized to separate FR-WCO and FF-WCO into heavy (residue, R) and light (distillate, D) fractions. Formulations based on the light fraction demonstrated a weaker ER effect at fields below 1.6 kV/mm while they surpassed both the crude oil and the heavy fraction at fields of 2.4 kV/mm and above. Additionally, FR-D formulation withstood electric fields up to 4 kV/mm without electrical breakdown.

Furthermore, tribological response was assessed under fully flooded conditions in axial bearings (5 N, 500 rpm) using 3.3 wt.% CNF dispersions in WM-WCO. Application of voltage (500 and 1000 V) induced instantaneous and reversible increases in the coefficient of friction (COF) proportional to the field strength, allowing for on-demand electro-active control of the COF. Under 1000 V, CNF dispersions achieved an extraordinary COF increment of 80.9%. These findings highlight the exceptional potential of CNF-based waste cooking oil formulations as tunable, highly responsive smart biolubricants.