by Fabian Meyer
Materials Science
Time-Effi cient Execution of Rheological Tests in Oscillation With an Easy-to-Use Quality Control Rheometer
Q
uality control managers are under constant pressure to conduct analy- ses as efficiently and cost-effectively as possible. In certain industries, such as paints and coatings and food, rheological testing in oscillation mode is critical, yet it can be a particularly costly part of the quality control process.
Oscillatory tests can determine the viscous and elastic properties of a material and are consid- ered the ideal method with which to examine storage behavior and shelf-life, primarily be- cause they are deemed to be nondestructive to complex materials—during an experiment, the microstructure of a sample is not disturbed by the forces applied. Rheological testing in oscillation mode can also be used to investi- gate phase transitions and crystallization and curing processes.
Eff ective oscillatory testing, however, requires a rheometer with a low-friction bearing sys- tem, minimal instrument inertia and a highly dynamic motor. Using a robust rotational rheometer with a mechanical-bearing—rather than an air-bearing rheometer, which is stan- dard for oscillatory testing—quality control personnel can improve effi ciency while achiev- ing optimal results.
Stress mode versus
deformation mode When performing a rheological test in oscil- lation, a sample can be approached in either controlled-stress (CS) or controlled deformation (CD) mode. Depending on the type of excitation, the sample responds with a deformation (in CS mode) or a stress (in CD mode). A sine curve appears when the amplitude values of the ap- plied deformation or stress signal are low. Tests that are performed in this way, i.e., in the linear– viscoelastic range, are considered nondestructive,
with the applied forces being too low to alter a sample’s microstructure.
The applied sinusoidal and response signals show a phase shift between 0° and 90°, depend- ing on the type of sample. A phase shift of 90°, which is exhibited in samples such as water or mineral oils, indicates that a sample is purely viscous, with no elastic response. Alternatively, a phase shift of 0° signifi es that a sample is purely elastic, with no viscous response; this is the case with steel or thermoset polymer materials, for example. Most complex materi- als, however, exhibit both viscous and elastic behavior—known as viscoelastic behavior.
Oscillatory measurement techniques are ideal for studying a material’s viscosity and elastic- ity. The shelf-life of a material can be examined using oscillatory testing, as can various phase transitions such as melting, curing and crys- tallization that take place under diff erent conditions. Measurements like amplitude sweep, frequency sweep and time sweep can be made when an oscillatory excitation force is applied to a sample using a Thermo Scientifi c HAAKE Viscotester iQ mechanical-bearing ro- tational rheometer
(Thermo Fisher Scientifi c, Karlsruhe, Germany) (Figure 1).
An oscillatory amplitude sweep is used to determine the linear–viscoelastic range of a material and to derive a yield stress. To do so, the frequency of the exciting sinusoidal signal is kept constant while the amplitude is increased gradually. The increase continues until the microstructure breaks down and the rheological material functions are no longer independent of the set parameter.
Conversely, with an oscillatory frequency sweep, the amplitude of the exciting sinusoidal signal is kept constant while the frequency is increased or decreased gradually. Frequency sweeps can ascertain whether a sample behaves
AMERICAN LABORATORY • 23 • AUGUST 2015
Figure 1 – Thermo Scientifi c HAAKE Viscotester iQ rheometer.
like a viscous or viscoelastic fl uid, gel-like paste or fully cross-linked material.
Finally, an oscillatory time sweep requires that the amplitude and frequency of the exciting sinusoidal signal is kept constant and that the rheological properties of the material are monitored over time. Time sweeps are used to examine structural changes that can occur during curing and gelifi cation reactions as well as drying and relaxation processes.
Benefi ts of a
mechanical-bearing rheometer A compact, rotational rheometer equipped
with a highly dynamic, electronically commu- tated motor allows for rotational rheological experiments in both CS and CR modes. While the bearing friction and total system inertia are much higher with a mechanical-bearing instrument, oscillatory tests in CS and CD
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