Rheological Quality Control
The geometry of the melt index apparatus results in the material experiencing a combination of shear and elongational flow as it is extruded from the reservoir through the capillary. Extensional flow dominates in the transition region between the reservoir and the capillary while shear flow dominates in the capillary itself. Polymers behave differently in these types of flows because of their chain-like structure. Various combinations of response to shear and elongational flow could also result in rheologically different polymers exhibiting the same melt index. More complete and accurate rheological characterization can be provided by measurements designed to investigate polymer melts at different temperatures and deformation rates in well controlled flow fields. Cone and plate, parallel plate, capillary, and extensional rheometers are designed to do exactly this. The first improvement in quality control over the melt index is usually measurement of the polymer viscosity as a function of shear rate at a representative temperature. Despite the problems with the melt index outlined above, it is a useful measurement because it is simple, straightforward, and a reasonably effective process simulator for typical polymer processing operations. Most processing operations involve a combination of shear and elongational flows. Many operations involve the extrusion of melt through a narrow channel such as a capillary. Thus, this measurement has widespread use and applicability; however, its limitations must be kept in mind.
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