National Physical Laboratory

Extensional rheometry

Extensional rheometry can be divided into two basic categories: ones in which quantitatively accurate transient extensional viscosity data are determined, and the other in which qualitative data are determined. Stretching rheometers of the sort defined by ISO/DIS 20965 fit into the first category. Fibre spinning and entrance flow measurements typically fit into the second category.

In the first category the measurements are more complex, but are generally easier to interpret. They are suitable for generating transient extensional viscosity data for modelling or for more involved materials characterisation. In comparison, fibre-spinning and entrance flow measurements are more appropriate for quality control type functions.

Both of these qualitative techniques can be carried out using an extrusion device, e.g. an extrusion rheometer. For the fibre spinning method, a haul-off device is required. The test is normally carried out under non-isothermal conditions. For entrance flow measurements, approximate extensional viscosities are derived using analytical models, e.g. the Cogswell model.

NPL Facility: Measurement of extensional viscosity using (a) Cogswell method with capillary extrusion rheometry entrance pressure drop measurement, (b) by a fibre drawing method.

Last Updated: 18 Aug 2016
Created: 1 Sep 2008

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