17.11.2022 by Aileen Sammler

How to Apply the Cox-Merz Rule

A viscometry measurement determines the shear viscosity of a material as a function of the shear rate. In this type of test, a sample is placed between two plates. The upper plate rotates with a defined shear rate. The shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress required to reach this shear rate is determined. Both values are used to calculate the shear viscosity.

With the unique combination of rotational and capillary rheometer measurements offered by NETZSCH, very broad shear rate ranges are achieved. This is important, for example, for the analysis of polymers, because their behavior depends strongly of the shear rate to which they are subjected.

But the shear rate range of such a measurement can be limited. If the centrifugal force (tending to move the material outwards) exceeds the normal force (that pushes the upper geometry upwards), the sample may be ejected out of the measurement gap. In this case, the resulting viscosity values are not valid and representative of the sample.


How to Obtain Shear Viscosity at Higher Shear Rates?

An easy way to obtain results at high shear rates in a rotational rheometer is to use the Cox-Merz rule. This empirical relationship stipulates that for most unfilled polymer melts, the shear viscosity, η, can be predicted by the complex viscosity, η*, which is determined by an oscillation measurement.

The Cox-Merz Rule

An alternative solution for measuring flow behavior at faster processing conditions or higher shear rates can be achieved with the use of a high-pressure capillary rheometer.

Read our Application Note and learn more about the Cox-Merz rule determining shear viscosity values using an oscillation measurement:

Please visit our website for further information about our wide rheology product portfolio: Rheometers - NETZSCH Analyzing & Testing

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