RHEOLOGICAL TESTS continued
modes are still possible within certain frequency, angular deflection and torque ranges.
A rheometer equipped with various measuring geometries—ranging from coaxial cylinders over vane-type rotors to parallel plates and cone/plate fixtures—has the flexibility to test a broad range of samples. Medium- to high-viscosity samples can be tested in oscillatory mode (see specifica- tions/measuring range for oscillation tests with the HAAKE Viscotester iQ rheometer in Table 1).
Experiments and results Oscillary experiments were done to test the performance of the mechan- ical-bearing rheometer. A certified mineral oil provided by the German Calibration Service was used as a Newtonian standard fluid, and a poly- isobutylene dissolved in 2,6,10,14-tetramethyl-pentadecane, provided by the National Institute of Standards and Technology (NIST), was used as a non-Newtonian reference material.
One test showed the frequency sweeps performed with the Newtonian standard fluid at varying temperatures (see results in Figure 2). The tests were done using a 35-mm parallel plate geometry with the measuring gap set to 0.5 mm. As temperature decreased, the material became more viscous and the measuring range was extended toward lower frequencies.
Table 1 – Specifications of the HAAKE Viscotester iQ rheometer for oscillatory experiments
When the viscosity data obtained was compared with the certified values for the dynamic viscosity provided by the German Calibration Service (see Table 2), the deviation from the certified viscosity was shown to be <7% for all data measured.
An amplitude sweep was also conducted on the non-Newtonian standard material with the mechanical-bearing rheometer and, for comparison, with an air-bearing rheometer (see Figure 3). The test on the mechanical- bearing rheometer was performed with a 60-mm parallel plate geometry with the measuring gap set to 0.5 mm, while the air-bearing rheometer test was performed with a 35-mm parallel plate geometry and the measur- ing gap set to 0.5 mm. The results were comparable, with the maximum difference between the two being less than 5%. The modulus data clearly allows differentiation between the linear and the nonlinear viscoelastic range of the tested standard sample.
Frequency sweeps were performed within the linear viscoelastic range using the information obtained from the amplitude sweep. A frequency range from 0.1 to 20 Hz (the maximum for the HAAKE Viscotester iQ rhe- ometer) was selected, with a deformation of 10%. Again, the results were comparable for the measured and certified values from the mechanical- bearing rheometer and the data provided by NIST (see results in Figure 4).
Table 2 – Results of frequency sweeps with German Calibration Service Newtonian standard fluid at different temperatures
Figure 2 – Loss modulus G’’ and complex viscosity Iη*I as a function of the frequency for German Calibration Service Newtonian standard fluid at three different temperatures.
Figure 3 – Storage modulus G’ and loss modulus G’’ as a function of the deformation γ for NIST non-Newtonian standard material at 25 °C.
AMERICAN LABORATORY • 24 • AUGUST 2015
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