TRADITIONAL METHODS
Traditionally, in many industries the yield value was
extrapolated from controlled rate flow curves or calculated
using regression models such as Casson or Hershel-Bulkley.
Unfortunately, these results depend strongly on the
experimental conditions and the shear rate range used in
the regression model. Comparable results are obtained if a
well defined and general accepted procedure exists. One
of these worldwide accepted procedures is used in the
chocolate industry when testing liquid chocolates at 40°C
according to the OICCC recommendations
3,4
.
Figure 1: Flow and Viscosity Curve of chocolate at 40°C
CONTROLLED DEFORMATION
A simple but accurate method for yield point determination is the CD-test (controlled deformation) which can be
carried out using the HAAKE Viscotester VT 550. By using special star shaped rotors the sample’s structure is not
destroyed before the test. In the controlled deformation mode the deformation is stepwise increased until the
structure breaks down. The yield stress value is shown on the display. When the instrument is connected to a
computer the measured stress is plotted as a function of time. The VT550 application software allows a
calculation of the maximum stress that corresponds to the yield stress. This method is a quick but accurate
possibility for determination of yield values in quality control.
STRESS RAMPS
As the CD test is not sensitive enough for low yield stresses
(
e.g. below 10 Pa), the preferred method is a stress ramp
performed on a controlled stress rheometer. There are
different opinions on how to evaluate the test results.
Some people plot the viscosity as a function of stress and
calculate the yield value from the maximum viscosity.
Another possibility is plotting deformation
γ
versus stress
τ
in a double logarithmic plot. The yield stress can be clearly
detected at a pronounced breakpoint in the slope of two
power law regressions. The application software allows the
exact calculation of the intercept of the two curve fits. This
method gives good reproducibility and accurate results
even for low yield values.
Figure 2: Yield Point determination from a stress ramp
OSCILLATION STRESS SWEEP
Very often dynamic tests are run in order to characterize
the products viscoelastic behaviour. From oscillation stress
sweeps we derive in general the linear viscoelastic range of
the product. But these tests give us some idea on the yield
stress, too. At stresses below the critical stress, the sample
material behaves like a viscoelastic solid. At higher stresses,
the material starts to flow as its yield value is exceeded.
The structure of the sample is then irreversibly destroyed.
Figure 3: Oscillation Stress Sweep on two different
toothpastes
(2)
© 2006 Rheology Solutions Pty Ltd
Solutions for Chemical & Allied Industries - Application Note: Yield Point Determination - a Critical Discussion of Different Methods
