The Cambridge Multi Pass Rheometer
|The Cambridge Multi Pass Rheometer (MPR) is a novel dual piston capillary rheometer that can be used to investigate the rheological behaviour of melts, liquids and soft solids. It consists of two servo hydraulically controlled pistons enclosed within temperature controlled barrels and a central test section. The pistons can be moved back and forth to drive the material contained within the MPR through the test section (see the schematic diagram to the right). The speed of the pistons can be varied from 0.05mm.s-1 to 200mm.s-1, allowing very high shear rates to be generated.|
Optical and X-ray measurements
A variety of test sections can be used with the MPR from simple capillaries for determining the rheological parameters of the material under test to zig-zag channels used in conjunction with quartz windows to allow flow visualisation studies to be carried out. Any type of channel can be made for the MPR within the limits of manufacturing engineering. An optical module is available that allows the flow of the material within the test section to be observed at the same time as making rheological measurements. This module has been extensively used to carry out flow birefringence studies of polymer melts.
Close up picture of MPR4 set up for optical birefringence measurements.
Another technique that has been successfully used in conjunction with the MPR is small and wide-angle X-ray scattering (SAXS and WAXS). Using a specially designed X-ray cabinet that can be placed around the MPR is possible to measure the change in the degree of crystallinity of a sample as it is sheared.
Close-up picture of MPR3 showing the barrels and mid-section in the middle
The advantages of the MPR
The unique geometry of the MPR gives it several advantages over conventional rheometers.
- Small samples size
The limited volume of the MPR barrels means that only a small amount of sample is required. As little as 10g is needed to fill the barrels and a simple capillary test section. This makes it possible to investigate the behaviour of materials that are either expensive to produce or that are only available in small amounts.
- Pressurisation of sample
The pistons of the MPR can be moved together to pressurise the sample, allowing tests to be conducted above ambient pressure.
- High shear rates
The high piston speeds achievable allow samples to be subjected to very high shear rates. Wall shear rates of upto 160,000s-1 can be applied to samples depending on the geometery selected.
- Displacement and pressure control
In normal operation the displacement of the pistons is varied and the resulting pressure change is measured. However, with the latest control software it is possible to fix the pressure and measure the resulting displacement variation.
- Repeat measurements
Having the sample contained within the barrels of the MPR allows repeated measurements to be carried out. This allows the change of material properties with time to be investigated.
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