PROCESS CONTROL | INLINE MEASUREMENT
Figures 1a and 1b: Comparison and example measurements with Bagley correction for a twin die capillary rheometer (top) and Göttfert’s RTS inline rheometer (bottom) with long and short dies
Source: Göttfert
rheological properties is essential for fast and effective process control of polymer production in polymerisation and pilot plants, as well as in compounding operations.
Continuous MFR Currently, the most common task for online capillary rheometry has been continuous determi- nation of melt flow rate (MFR) and melt volume rate (MVR) in order to adjust production slots for an individual polymer specification more quickly. To characterise the processability of a polymer, other rheological parameters, such as shear and elonga- tion viscosity, are also of particular interest. The Göttfert inline rheometer (RTS) is an online die rheometer with two capillaries that measure simultaneously. The arrangement is similar to the process for making laboratory measurements using a twin-die capillary rheometer. The measurement is carried out with a long and a short capillary, with the pressure loss of each capillary plotted over the length/diameter (L/D) ratio of the die and extrapo- lated to L/D of zero using the Bagley correction to determine entrance pressure loss,then corrected for the shape of the velocity profile by Rabinow- itsch-Weissenberg. Using the Cogswell model, elongation viscosity is calculated from the entrance pressure loss and the apparent shear stress and
46 COMPOUNDING WORLD | November 2023
shear rate of the long capillary (Figure 1a and 1b). Experiments with a high-pressure capillary rheometer are often the only way to determine viscosity at higher, process-relevant shear rates. The measuring principle requires different correc- tions to be made to get true shear rate and shear stress. When these corrections are made, reliable viscosity data can be obtained. Göttfert says that which corrections are actually necessary, and the order in which they are applied, depends on the individual application. The company says the advantage of the use of the long die and zero length die in the RTS is that existing bores on existing devices can be used to retrofit or convert to this test method. This arrange- ment, which is also possible in the company’s MBR-TD bypass system, opens up the possibility of measuring melt index in addition to measuring shear and elongation viscosity. Göttfert has checked the arrangement against
comparative measurements made with its RG75 capillary laboratory rheometer. These were made with a capillary configuration of 40/2 and 0/2 mm L/D for the laboratory system and 60/2 and 0/2 mm on the RTS online device. Two LDPE materials were examined. The company says the investigation was carried out at an average shear rate range of 20-1000 s-1
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