Quantitative rheology is impossible without the use of rheometers. Cousin to the old viscometer, rheometers are becoming essential for measuring viscosity and shear moduli of soft materials due to the shear rate and environment-dependent properties of nearly everything soft or fluidic. Often articles and textbooks list procedures and data collected from rheometers, but it is of utmost importance to understand which type of instrument the source is referring to and also to select the appropriate instrument for your characterization needs.
What samples can be measured?
Shear rheometers are available with a variety of measuring systems that allow for measuring water-like solvents and solutions as well as rubbery composites.
Capillary rheometers typically use ground hard plastic fed into a hopper that heats them into a polymer melt. After the polymer melt travels through the capillary tube and past the die, it solidifies. Common polymer processing aspects that can be studied with capillary rheometry include polymer extrusion, degradation, and die swell. (1)
Temperature control is a requirement for any rheometry procedure. Shear rheometers have been expertly designed to maintain a constant temperature or ramped temperature covering a wide range, typically as low as -150°C and even as high as 1,000°C.
Capillary rheometers operate at temperatures between 100 and 400 °C. A high temperature is required to maintain polymers in the melt state, thus limiting the lower temperature.
How does each one work?
Shear rheometers use a motor to rotate a measuring system (aka “fixture”, “geometry”, or “sample holder”) which imparts a shear rate (angular velocity of the rotor with accounting for the thickness of the sample, or “gap”) or apply a sinusoidal shear strain to the sample. Regardless of measuring system, the torque needed to shear the sample is the raw value used to calculate viscosity and modulus. Among shear rheometer models there are strain-controlled, stress-controlled, and both stress and strain controlled models. All of these either apply a set torque and change the displacement angle or vice versa.
Capillary rheometers can maintain shear rates orders of magnitude greater than shear rheometers, as high as 500,000 1/s. (1) This is due to the die of the capillary rheometer, which allows for shear rates at the wall to essentially be as large as required, provided that the sample can be pulled through at the speed necessary for the desired shear rate with applied pressure.
Can you compare data?
In general, as long as data collected from shear rheometers is taken from the same test method, including the same applied values, data point time, and temperature, data can be compared if an absolute measuring system is used. Absolute measuring systems include plates, cones, concentric cylinders, and double-gap cylinders, which all have precise flow fields that allow for exact computation of viscosity and moduli. Spindles and stirrers are not absolute measuring systems, and therefore data collected can only be quantitatively compared with data collected from the same spindle.
Capillary rheometer data should not be quantitatively compared to shear rheometer data. Capillary rheometers impart extensional forces along with shear to the sample, (1) and the design of the instrument is very different from a shear rheometer. In this manner, capillary rheometer data can be compared to other capillary rheometer data taken under the same capillary conditions (pressure, speed, temperature), but not to shear rheometer data.
I want to measure polymer melts but I can’t afford both a shear rheometer and a capillary rheometer. What should I do?
Shear rheometers offer a wider range of capabilities than capillary rheometers, which makes them the go-to instrument for polymer melts. Although shear rates are typically not able to approach the high magnitude required for polymer extrusion studies, there are tricks to obtaining data for high shear through time-temperature superposition and the Cox-Mertz Rule. Shear rheometers also provide information on the behavior of the melt at low shear rates, which most capillary rheometers cannot test. Furthermore, polymer solutions and polymer melt composites can also be examined with a shear rheometer, which makes it highly useful even though it cannot directly measure extremely high shear rates.
For more information on comparing capillary rheometers and shear rheometers, check out this post from Plastics Technology.
For more information on the theory and use of capillary rheometers, check out Adhesives Toolkit.
Click here to access a free inforgraphic on the capillary and shear rheometers.
To learn more about the capabilities and handling of a shear rheometer, contact us.
(1) Cogwell, F., Polymer Melt Rheology: A Guide for Industrial Practice, Elsevier, 1981