Most Important Test Methods

Abstract

A variety of measurement methods and instruments are used for rheological characterization of cosmetic emulsions. At low shear rates [62] for detection of the apparent yield stress, stressed-controlled rheometer are used because with this type of instrument the stress is preset and the onset of flow can be determined more or less accurately depending on the quality of the angle resolution of the instrument. Characterization of processing conditions requires measurements at high shear rates with a torsional rheometer or at extremely high shear gradients with a capillary viscometer. The measurement system used is also important and will depend on the product to be measured. Watery fluids need a cylinder system, viscous samples the cone-plate system and all other solid materials the parallel plate system. Modern rheometers are versatile and allow different types of strains and combinations thereof, permitting simulation of many operations in a production process. One of the first experiments performed with a new, unknown sample is the so-called positive ramp test (Fig. 7.1). Here the applied shear stress is increased continuously with time at a constant temperature. It is also referred to as a stress-time ramp. In this experiment an integral viscosity is obtained as a first estimation of the range in which the actual measurements should be performed. A step test is then performed as the second experiment (Fig. 7.2). In contrast to the first Fig. 7.1. Positive ramp Fig. 7.2. Step test