Integrated Thermophysical and Rheological Characterization of Vegetable Oils and Conventional Emulsion Coolant for Sustainable MQL and Nano-MQL Machining

This study presents a systematic thermophysical and rheological characterization of seven vegetable oils—high oleic soybean oil (HOSO), low oleic soybean oil (LOSO), high oleic canola oil (HOCO), olive oil (OO), coconut oil (CO), palm oil (PO), and a commercial vegetable oil with high tech additives (VO-HTA) and three benchmark fluids: distilled water (DW), conventional emulsion coolant (CEC), and semi-synthetic petroleum oil (SSPO). The objective is to establish a quantitative screening framework for Minimum Quantity Lubrication (MQL) base-fluid selection prior to nanofluid formulation and machining validation. Thermal conductivity was measured using the transient hot-wire (THW-L2) method following ASTM D7896-19 over 25–75 °C. Dynamic viscosity was measured using a cone-plate rotational viscometer across 10–1800 s−1 and 25–75 °C. Thermal conductivity of vegetable oils ranged from 0.155–0.181W·m−1·K−1 and decreased linearly with temperature, significantly lower than CEC (0.62–0.66 W·m−1·K−1) which increases linearly. Dynamic viscosity decreased exponentially with temperature for all vegetable oils. Shear stress–shear rate relationships were linear indicating Newtonian behavior, but dynamic viscosity decreased slightly with share rate at lower temperatures and lower shear rates, indicating near-Newtonian (shear thinning) behavior when fitted with power law model. Palm oil exhibited the highest thermal conductivity and CO the lowest among vegetable oils, but they congealed at 25 °C. OO exhibited highest dynamic viscosity followed by VO–HTA, HOSO and HOCO. HOSO and HOCO demonstrated the most balanced thermophysical behavior, maintaining dynamic viscosity within the desirable 30–60mPa·s range at 45–65 °C. This study provides a screening methodology for vegetable-oil-based fluid selection.