Elastohydrodynamic lubrication of aqueous hydroxyethyl cellulose-glycerol lubricants

Continuous growth in lubricant volume worldwide, environmental hazard of fossil-based lubricants and anthropogenic climate change are reflected in an increasing demand for green, sustainable lubrication. In the presented work, the influence of molecular mass of hydroxyethyl celluloses on rheology, elastohydrodynamic friction and film thickness of aqueous lubricants was investigated experimentally. Polyalphaolefin and polyethylene glycol were used as reference lubricants. All lubricants were formulated to have the same dynamic zero-shear viscosity. The average molar mass of commercial hydroxyethyl cellulose was adjusted by enzymatic hydrolysis. 40 to 70 wt% glycerol were added to improve the film formation capability and high-shear viscosity. The results indicate that commercial hydroxyethyl celluloses with weight average molar masses above 88 kg mol^-1 are not suitable to increase elastohydrodynamic film thickness due to shear-thinning even at moderate shear rates. Hydrolysis increased the critical shear stress by an order of magnitude to 1.13 × 10⁴ Pa. Compared to the reference lubricants, elastohydrodynamic friction was reduced by more than 90 %. Nevertheless, due to the pronounced shear-thinning behavior, a weight average molar mass of 19 kg mol^-1 seems to represent a threshold for relevant lubricant film formation in elastohydrodynamically lubricated contacts. Overall, the results show that the combination of hydroxyethyl cellulose and glycerol is well suited for viscosity modification of aqueous lubricants.