Answer to the November 15, 2000 Quiz Question

Rheological Quality Control

Perspective Meter:

The apparatus used for determination of the melt index is illustrated in the figure below. Polymer in pellet or powder form is loaded into the reservoir and heated to a specified temperature. A weight is used to extrude the molten polymer through the capillary. The results are reported in terms of the number of grams polymer extruded in ten minutes. Appropriate melt temperatures and extrusion weights are specified in the ASTM standard. As a result of the test one might report, for example, that a polystyrene was found to have a melt index of 4.2 grams/10 min under condition G (200oC and 5kg load).

The major shortcoming of the melt index is that it is a single point measurement of a mixed shear and elongational flow. The rheological behavior of polymers is strongly dependent on both temperature and shear rate and a single point measurement cannot capture this level of detail. For example, consider the viscosity as a function of shear rate for the two polymers illustrated at right. Rheologically these are very different materials; however, if the effective shear rate for the melt index measurement happens to be at the point where the two curves cross, they may exhibit the same melt index. This is one possible reason why two different lots of the same polymer may have the same melt index but exhibit different rheological behavior during processing.

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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