An evaluation of the colloidal stability of metal working fluid.

The effect of calcium on the stability of a commercial MWF is characterized through the experimental determination of the stability ratio, W. Three experimental methods of stability ratio evaluation are investigated. (1) The initial slope of the absorbance versus time curve is used to estimate the rate of coagulation. (2) Absorbance measurements are used to estimate N(0)/N with time. The stability ratio is determined from the slope of N(0)/N versus time. (3) Photon correlation spectroscopy (PCS) measurements of the volume distribution with time are used to estimate N(0)/N with time. Electrophoretic mobility was also measured and used to determine the fast coagulation concentration of the MWF. The accuracy of the experimentally determined stability ratios is evaluated using a population balance coagulation model. The model predicts the population distribution of a coagulating dispersion with time based on an initial particle size distribution and stability ratio. The model results were compared with the PCS-measured distributions to determine which stability ratio evaluation method best describes the stability of the MWF emulsion studied. Using the initial slope of the absorbance versus time curve to determine the fast coagulation concentration correlates well with electrophoretic mobility measurements. However, using absorbance measurements to determine the rate of coagulation underestimates the stability ratio of the MWF studied by orders of magnitude. N(0)/N values calculated from absorbance measurements provide a reasonable estimate of the stability ratio but inconsistencies in the method decrease its reliability. The stability ratio derived from PCS measurements appears to provide the most accurate, reliable description of MWF stability.