Optimization of vegetable oil-based cutting fluid performance in hard water by response surface methodology

To cope with the problems of environmental pollution and resource waste of traditional mineral oil-based cutting fluids, this study optimized the performance of vegetable oil-based cutting fluids under hard water environment (hardness 450 mg L −1 , pH = 9) by response surface methodology (RSM), using soybean oil as the base oil and combining with Span 20 and Tween 80 composite surfactants. The experimental design used hydrophilic-lipophilic balance (HLB) value, surfactant concentration and soybean oil content as variables, as well as emulsion stability (particle size) and friction performance (wear scar diameter, WSD) as response values to construct a prediction model and verify its reliability. The results showed that the optimized cutting fluid formulation (HLB = 11.79, surfactant concentration 0.50 wt%, soybean oil content 10.06 wt%) exhibited excellent emulsion stability (particle size 573.8 nm, microscopically observed average particle size 2.48 μm) and friction performance (WSD = 0.506 mm) in the validation experiments. The emulsion has a lower coefficient of friction (25.7 % lower torque) and higher thermal conductivity than commercial cutting fluids. Microscopic characterization (DSC, rheological analysis) showed that its low-temperature fluidity and wettability were significantly better than that of pure water, and the contact angle was smaller. This research provided the theoretical and technological basis for the development of environmentally friendly and efficient hard water applicable cutting fluids to support the sustainable development goal of green manufacturing.