Melt flow index versus polymer

Figure 4, Melt Flow Index of polymer versus Hydrogen partial pressure in the gas phase of the reactor - Bulk process —...

Since common processing techniques for polymers involve shear rates of about 100-100 000 s there is no substitute for the comprehensive study of shear-stress versus shear rate over the typical processing windows of shear rate and temperature. Clearly, one-point tests such as melt flow index cannot be used as a guide to processability since the shapes of pseudoplastic curves are not identical (Figure 9.9). 

Fluoropolymer manufacturers and suppliers have developed time-temperature-shear rate data for melt viscosity or melt flow rate (index) to provide an assessment of the thermal stability of these polymers. Figures. 7.26 and 7.27 show the melt viscosity of a few commercial grades of polyvinylidene fluoride as a function of temperature at a fixed shear rate. Relationship between melt viscosity and shear rate and shear stress versus shear rate are presented in Figs. 7.28-7.30. Melt flow index/rate and its relationship with melt viscosity of polyvinylidene fluoride are given in Tables 7.10 and 7.11. 

The General Rheological Model [87] was basically developed for master curves of viscosity versus shear rate for polymer melts using the melt-flow index (MFI) as a normalizing parameter. It can be written in a general form applicable to any viscosity versus shear rate curve of a polymer melt simply by putting MFI as unity to give... 

The method of unification described in the above paragraph is applicable to reprocessed polymers as well [26]. Figure 5.17 shows the normal stress difference versus shear rate data for virgin and reprocessed LDPE [27]. The recycled sample was processed in a Brabender mixer for 60 min at 190 C. The melt flow index of the virgin LDPE was increased from 0.8 to about 1.3, thus indicating that the polymer samples with different processing histories essentially represent polymer grades with different melt flow indices. When plotted in terms of the normal stress difference Ni versus the modified shear rate function ( y/MFI) on... 

Melt index measures how well a molten polymer will flow. This same information can be determined from rheology data. Figure 1 plots the pressure versus piston speed for several of... 

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