A Practical Approach to Rheology and Rheometry

Rheology
calculation on the slit viscosity data and the extrapolation of the determined
regression equation. The two zero-shear viscosity values η 01 and η 02 differ by
a full decade of magnitude and most likely both are wrong!
As long as the viscosity curve is defined by measured data points in the shear
thinning phase – the viscosity curve is characterized by an angle � 0 – the
Carreau equation takes the last data points of the lowest and highest shear
rates and bends the regression curves for lower and higher shear rates to
parallel the abscissa → η 0 and η ∞. Adding more data points – as in this example
with the slit capillary rheometer – changes the turning points of the
regression curve and leads to new values of η 0. To state the truth, the Carreau
regression calculation also requires measured data points in the part of the
viscosity curve in which even non-Newtonian liquids show a Newtonian flow
behavior, e.g. their viscosity has become independent of shear rate.
viscosity [Pas]
η02
η01
Carreau regression
on slit data
Carreau regression
on rod data
Slit capillary Rod capillary
← →←
→
shear rate [1/s]
Fig. 134 Combining the rod die– and the slit die data points provides two
different Carreau regression curves and two values η01 and η02. One might conclude from this example of an extrapolated evaluation that one
should better do without. If one needs rheological information covering a very
wide shear rate range then one will have to measure original data within that
range. That will most likely require several tests with different rheometer dies
or even the usage of types of rheometers such as CR- or CS-rotational viscometers
(Fig. 85).
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