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When testing two different products under the same conditions we can expect the following: Samples showing
higher critical stresses in an oscillation stress sweep are expected to possess a higher yield value. Figure 3 shows
the results from a stress sweep performed on two toothpastes. Product A (open symbols ) has a lower critical
stress than product B (filled symbols). As expected, the yield value of toothpaste A is lower. This corresponds to
the observation that product A is easier to press out of the container, product B seems to have a stronger gel
structure. Please note that in a stress sweep the material is tested only at one frequency (e.g. at 1 Hz), so stress
sweep results might be slightly different from static tests.
SUMMARY
Today a general accepted definition of the yield point does not exist. As a consequence many people feel
confused about the different possibilities on how to determine the yield point of a product. In this paper methods
and their application, advantages and disadvantages are discussed. As mentioned before, in all methods the test
results depend on the sample’s history and the test conditions (pre-treatment, sample loading etc.). It is in the
responsibility of the operator to find out the optimum conditions for his special application. Modern instruments,
new techniques and software help to find the best solution for reliable results.
REFERENCES
1.
Gebhard Schramm, Introduction into Rheology and Rheometry
2.
W. Umbach (Ed.), Kosmetik, Thieme -Verlag
3.
D. Weipert, H.-D. Tscheuschner, E. Windhab, Rheologie der Lebensmittel, B. Behrs Verlag, 1993
3.
OICCC, Rev. Int. Choc., 1973, (Sept), 216
4.
Eva-Maria Kutschmann, Viscosity measurements on liquid chocolate using the VT 550, HAAKE-Report V97-
136
E
5.
Hans-Michael Petri, Determining the Yield Point of Food Products, Haake-Report V96-127 E
6.
Hans-Michael Petri, Yield Point Determination of Cosmetic Products with Controlled Stress Rheometers,
Haake-Report V97-137E
The product information in this document was correct at the time of printing, however specifications are subject
to alteration without prior notice due to the Gebruder HAAKE philosophy of continual product innovation and
development and any information contained herein should not be construed as a contractual commitment by
Gebruder HAAKE.
A copy of this paper is available on request by quoting V156E.
RHEO303-SK
Solutions for Chemical & Allied Industries - Application Note: Yield Point Determination - a Critical Discussion of Different Methods
© 2006 Rheology Solutions Pty Ltd
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