Novel approach to determining shear viscosity for wall-sliding rubber compounds
To record wall slip effects in the rubber extrusion process, continuous measurement of pressure or wall shear stress is required. In the ‘Extrusion Rheometer’ research project, a combination of innovative extrusion rheometers was developed for this purpose, which can be used to measure both the local pressure at every point of the rheometer as well as the wall shear stress within the flow.
© IKV
Recording the complete pressure curve along a rectangular capillary rheometer makes it possible to detect the transition between wall adhesion and wall sliding. To this end, one side of an in-line rheometer is replaced by a membrane, which undergoes deformation under pressure, thus enabling continuous location-dependent recording of pressure changes.
The expected sharp drop in pressure in the rheometer, however, results in a levelling out of the membrane deformation due to the membrane’s mechanical coupling with itself. This leads to a significant error propagation, which makes it difficult to identify the pressure curve precisely and thus the wall slip. To decouple the membrane, it is divided into individual segments that can deform independently of each other under pressure. With the segmented membrane rheometer, it is possible in principle to map the pressure curve within a capillary. The curvature of the segments provides an indication of the prevailing pressure level underneath.
In some cases, the transition between wall adhesion and wall sliding cannot be detected in the segmented membrane rheometer – either because it does not occur or because the resolution of the rheometer is not high enough. In such cases, the wall shear stress can be measured indirectly. For this purpose, a metal strip is inserted into a conventional rectangular channel, which is clamped only at the inlet of the rheometer and decoupled from the wall by a PTFE layer. It can thus be stretched by the flowing rubber and the wall shear stress then leads to an elongation that accumulates up to the clamping point. The elongation is recorded by a glass fibre strain sensor as shown in the measurement setup in Figure 1.
The wall shear stress can be read from the strain gradient, because a change in the gradient means a change in the flow conditions because the underlying shear stress level changes. In the project, roughening the surface of the measuring strip led to an increase in the strain gradient, which is only possible when wall sliding is present. The wall shear stress can be deduced from the strain gradient, as a change in the gradient indicates a change in the flow conditions because the underlying shear stress level changes. In the project, roughening the surface of the measuring strip led to an increase in the strain gradient, which is only possible when wall sliding occurs.
Project data and funding
We would like to thank the BMWE for funding the IGF project (funding code 23114 N) and the project partners for their cooperation.
Project duration: 01.11.2023 – 31.10.2025
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