Thermo-rheological and tribological properties of low- and high-oleic vegetable oils as sustainable bio-based lubricants†

Abstract

Vegetable oil-based lubricants have attracted increased research attention in recent decades as sustainable alternatives to conventional petroleum-based lubricants in metal machining. However, more studies are required to fully elucidate the thermo-rheological and tribological properties. This study presents an investigation of the thermo-rheological and tribological properties of different vegetable oils, including low- and high-oleic soybean oil, high-oleic sunflower, safflower, and canola oils. The lubricity, and evolution of viscosity and thermodynamic properties as a function of temperature were investigated to obtain important parameters including the viscosity index, flow behavior index, flow activation energy, specific heat capacity, thermal conductivity, coefficient of friction, contact angle, and thermal-oxidative decomposition profile. The properties were compared with those obtained with mineral oil, conventional emulsion coolant (CEC), and a commercial bio-based lubricant, Acculube LB-2000, commonly used for metal cutting applications. The vegetable oils displayed comparable properties to the commercial LB-2000 lubricant and pure mineral oil, featuring Newtonian fluid characteristics, high viscosity indices, high flow activation energy, low specific heat capacity and thermal conductivity, and high thermal-oxidative stability. Generally, vegetable oils with high oleic acid content featured higher rheo-thermal stability, higher contact angle, and better performance in reducing the coefficient of friction. On the other hand, CEC displayed non-Newtonian fluid behavior with lower initial viscosity and flow activation energy, and lower thermal-oxidative stability, but comparatively higher specific heat capacity and thermal conductivity compared to the vegetable oils. Compared to pure mineral oil, the vegetable oils show higher oxidative-thermal stability, thermal conductivity and specific heat capacity, and better lubrication performance in the mixed and hydrodynamic lubrication regimes of the Stribeck curve. The results provide important datasets that will contribute to improving the database on the properties of vegetable oils to guide their utilization in designing sustainable vegetable-oil-based biodegradable lubricants.