Extensional indexers

For polymer melts and solutions pen is strongly influenced by the extensional flow occurring at the capillary entrance. Observed values of pen can be much higher than those predicted by eq. 6.2.22. In fact, the use of capillary entrance flow as an extensional indexer is discussed in Section 7.8. 

By far the easiest extensional method is the last listed in Table 7.9.1, entrance pressure drop. Entrance pressure drop data can be readily obtained with a capillary rheometer over a very wide range of rates and viscosities. More work is needed to determine the utility of data from this attractive extensional indexer. Can it give information on extensional response of new materials Can we guess extensional parameters for a constitutive equation and use entrance flow to test them (eg. Schunk and Scriven, 1990)7... 

Figure 24 shows extensional viscometry results for 1 g/10 min melt index LDPE, LLDPE, and mLLDPE. LDPE, with its broader molecular weight... 

The fact that the Trouton ratio eventually approaches a constant value means that the shear and extensional viscosity flow curves are paraUeT in that range, and if the behaviour is power-law, then the power-law index is the same in both cases. (Notice this from the polymer melt examples shown later in the figures 13 - 20.)... 

Such an instrument could measure true viscosity, but one would need to use other dies and weights. With such a short die entrance, losses can consume up to half of ptot (recall Figure 6.2.9). Also the ratio of die to reservoir radius is rather large, R/Rr = 0.219, so reservoir losses are significant. Thus the melt index number is a combined measure of shear and extensional viscosity. 

Comparisons of results from rheometers and indexers are essential in evaluating what material function dominates the indexer s response. Such comparisons can help us to determine when an indexer may give us useful rheological data, as in the case of squeezing flow. Figure 6.4.8. This ability becomes even more important in the next chapter, where we shall see that indexers are the only choice for extensional measurements on low viscosity fluids. 

Table 7.9.1 summarizes the main flow geometries that have been tested as extensional ifaeomemrs. They are listed from top to bottom in the order of this chapter, but also fiom rheometer to indexer, and generally from use with more viscous to less viscous test samples. The key advantages and disadvantages of each method are noted. The types of material function that these rheometers and indexers can measure were summarized in Figure II.3. 

In the low viscosity range, the opposed-nozzle device seems most attractive. Like entrance pressure drop, it is an indexer, but the flow from opposed nozzles is more nearly extensional. 

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