An Evaluation of the Colloidal Stability of Metalworking Fluid

Metalworking fluids are used for cooling, lubrication, and corrosion control in the metal products industry. As of the mid-2000s, many manufacturers were using filtration or centrifugal systems to remove impurities from metalworking fluids to recycle used fluid back into their production line.

Knowing the quality of the metalworking fluid in the recycling system is important for maintaining the machining tools and eliminating metal chips and shavings. Measuring turbidity to calculate the stability of the metalworking fluid is a fast and easy experiment; however, does that test accurately depict the status of the metalworking fluid system?

ISTC’s Kishore Rajagopalan and colleagues from the University of Illinois and Texas A&M University experimentally tested three different turbidity methods to see if the models accurately described the true stability of the metalworking fluids.

  • Method 1: Estimate the rate of aggregation from the slope of turbidity vs. time curve at a single wavelength in the initial stages of coagulation (most popular method in the literature; Reerink & Overbeek, 1954)
  • Method 2: Estimate the rate of aggregation from the slope of the wavelength exponent and change in turbidity vs. time (Reddy & Fogler, 1981)
  • Method 3: Estimate the rate of aggregation from dynamic light scattering measurements to characterize particle size and dispersion stability changes over time (Tawari et al., 2001; Hanus et al., 2001)

Method 3 was the most accurate for determining colloidal stability. Method 2 was also fairly reliable, but uncertainties in determining the wavelength exponent makes the method less reliable than method 3. Method 1 predicts stability ratios that are orders of magnitude lower than the actual values because of a poor wavelength choice.