Rheological prediction of the physical stability of concentrated dispersions containing particulates

Abstract

A dynamic rheological technique is presented that can be used for the prediction of physical stability of concentrated dispersions containing particulates, as well as product development and quality control. Typical physical evaluation frequently requires weeks or months before an assessment can be made. By using dynamic rheology, the prediction of physical stability can often be accomplished in less than three (3) hours. The dynamic or oscillatory rheology technique is performed in two parts. First, a strain sweep test (frequency constant, strain variable) defines the linear viscoelastic range (LVR). Second, frequency scans (from 0.01 to 5.0 Hz) are performed at various temperatures, from 5 to 49°C (40–120°F). It is critical that the frequency scans are performed in the LVR. Dynamic rheology measurements yield data on the elastic (G′) and viscous (G″) moduli. Plotting the elastic to viscous moduli ratio as a function of frequency and temperature generates a 3-D surface which is a “fingerprint” of the dispersion’s colloidal stability. The G′/G″ ratios correlate well with observed physical stability properties. A volume index or a simple mean/variance calculation can be used to assign a value to the graphical 3-D representations.