MFI Correlations in Polymer Product Property Evaluation

Despite the fact that MFI is generally considered as a measure of the rheological behavior and processability of the polymers, it has been shown over the years to conelate with final product properties. A general outline of the effect of MFI 

Note Pronounced increase = + +, pronounced decrease increase = +, slight decrease s —, no change - x. 

The simultaneous effect of MFI and crystallinity on the yield stress can be seen from Fig. 9.47 taken from Orespi and Ranalli [SO] who studied the behavior of PP with different isotactic indices. It is seen that, with the same isotactic index, that is the equivalent crystallizable part, lower MFI samples are less crystalline and hence show a lower yield stress, but a small variation of the isotactic index leads to a remarkable increase in the yield stress from samples with the same flowabUity or, in other words, similar MFI. 

The effect of MFI on tenacity of film tapers can be seen from Fig. 9.51 taken from Ref. 53, which shows that increasing the MFI decreases the tenacity of 

Impact strength could be regarded as the determination of flexural stress at a rapid rate of load increase. The impact strength is known to depend on both density as well as MFI [51,57,58]. It is highest for the lowest MFI material, but drops sharply as MFI increases and then irly evens out at higher values as 

Rgure 9.46 Variation of tensile yield strength with temperature for two different melt flow index samples as per ASTM D412-61T, 5 cm/min. (From Ref. 49.) 

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