Capillary Rheometry

With the Rosand capillary rheometer, a sample is loaded into a cylindrical barrel that is pre-set at the required test temperature. A servo-drive controlled piston is then used to extrude the sample material through a cylindrical or rectangular slot die in place at the end of the barrel at a very controlled series of speeds (volumetric flow rate). The pressure drop across the die is continuously monitored and measured with a pressure transducer placed just above the die. A cut away is shown in Figure 1.  The Rosand capillary rheometers can accommodate a wide range of pressure transducers and dies, making them versatile for measuring a broad spectrum of sample types.  Typical sample viscosities can range from ink jet ink to highly-filled, high modulus rubber samples. The standard instrument’s temperature range is generally from ambient to 400°C (with cryogenic cooling and 500°C max temperature as option s). 

Capillary rheometers can be used to generate shear viscosity, extensional viscosity and elasticity measurements.  Also, there are modules for thermal degradation tests, flow – no flow testing, pressure volume temperature (PVT) tests, haul-off (fibre spinning), stress relaxation, wall slip analysis and others. 

Figure 1: The heart of the Rosand capillary rheometerFigure 1: The heart of the Rosand capillary rheometer

Generating a Viscosity Flow Curve

The constant shear test is designed to investigate the relationship between shear stress and shear rate for a material, with shear viscosity being the ratio of the two parameters. The test routine involves pre-setting the test temperature, then loading the sample with periodic tamping to ensure a uniform fill to reduce voids and air entrapment. Loading the sample is followed by further pre-test compressions to ensure the sample is de-aerated as much as possible and fully compacted. A series of discrete piston speeds (shear rates) are selected across the shear rate range of interest and the sampled extruded until pressure equilibrium is detected at each speed. Pressure is monitored during the test and shear stress calculated at each data pointed collected. To ensure accurate results of the true flow properties of the sample, the user has the option to apply up to two corrections associated with errors of entrance pressure and non-Newtonian flow.