INLINE MEASUREMENT | PROCESS CONTROL


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 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 and the viscosity was corrected in each case using Bagley (entrance pressure loss) and Rabinowitsch Weissenberg. Elongation viscosity and elongation rate were calculated using the Cogswell model


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Figure 2a and 2b: Comparison of shear and elongation viscosity measurements between online and laboratory measurements of two different LDPE materials. Source: Göttfert


November/December 2023 | PLASTICS RECYCLING WORLD 33


from the entrance pressure loss, which is deter- mined by the Bagley correction, and apparent rheological data. Elongation viscosity function determined in this way provides a simple descrip- tion of the visco-elastic processes. The curves for the two LDPE materials measured


at 190°C show the characteristic curve of the strain hardening for the elongation viscosity due to the polymer branches in the test material (Figure 2a and b). Both materials have the same MFR of 4 g/10 min, so cannot be differentiated using the MFR method. Göttfert says the viscosity slope shows only relatively minor differences, but elongation viscosity differs significantly with the


Table 1: Comparison of the selectivity of melt index, viscosity and elongation viscosity for two LDPE materials


Material


Melt index [g/10 min] Difference [%]


Shear viscosity [Pas] at ý=1/s Difference [%]


Elongation viscosity maximum [Pas] Difference [%]


Source: Göettfert


LDPE1 4 -


5611 -


18600 -


LDPE2 4 0


5006 -10.8


30500 +64%


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