The rheological analysis of samples is a fundamental part of developing many types of products. Unlike a viscometer, a rheometer can actually measure sample properties at extremely low shear rates, as in sedimentation, or the high shear rates seen in pumping, mixing and application and making measurements in the correct shear range, we can adequately simulate a flow process and so differentiate good products from poor ones. The rheometer can also determine the effect of adding different quantities of an additive, or process changes and so be used to optimize the formulation and production of a product.
The rheometer not only measures the viscosity of the product at room temperature, but can also be used to evaluate the viscosity during a programmed temperature profile. This can also be used with polymers to evaluate processability and glass transition temperatures. Results are accurate with minimal time spent testing, as a pre-programmed analysis may be started and left to run unattended, or even overnight.
Rotational rheometers can accommodate many different measuring systems, although the most common are the cone and plate, the parallel plates, the coaxial cylinders and the torsional fixtures. In the case of the cone and plate or parallel plates, the sample is loaded onto a temperature controlled flat lower plate and an upper cone or flat plate is lowered onto the sample squeezing it into a defined space. After trimming away excess sample, the upper measuring system is then either sheared in one direction (viscometry) or oscillated rotationally (oscillation, as shown in Figure 1 below).
Viscometry can be used to investigate the yield stress, ie the stress required to initiate sample flow, simulate a shearing process, measure shear stability or analyze how viscosity changes with temperature. Oscillation tests usually investigate the viscoelastic structure of a sample without breaking it down. Initially an amplitude sweep is run to determine how large an oscillation the sample can withstand before the structure breaks down, this is known as the linear viscoelastic region. Once the linear viscoelastic region has been determined, a frequency sweep, time sweep or temperature sweep may be performed to investigate how the viscoelastic structure and viscosity changes under dynamic conditions.