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Abstract
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General purpose isotactic polypropylene (iPP) grades are not very suitable for processing operations subjected to sever elongation flow field due to their tension thinning behavior and low melt strength of their essentially linear nature. Yet, by some modification in their structure, or even changing the shaping process design, they may be readily be used in processes such as blow molding, fiber spinning, BOPP processing, etc. In this work, suitability and molecular structure requirement of two commercial PP homo-polymer film grades for slit-die extrusion were investigated. Molecular weight distribution (MWD), as the main parameter to describe the molecular structure of a linear polymer, of these resins were evaluated through measurements of GPC and rheology. From their dynamic shear data the relaxation spectra, h(?), were calculated from which MWD was estimated using molecular viscoelastic theories and then compared with the GPC results. Generally good agreement, but with narrower MWD rheology curves was found. Exception was observed for the grade for which rheological data predicted bimodal distribution curve comprising a small shoulder of high MW, not seen in the corresponding GPC curve. A higher value of the generalized mixing parameter than that of the double reptation model which was found for this bimodal grade was attributed to an increase in the number of entanglements and better network connectivity.
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