Controlled Rheology Polypropylenes. An Improved Model with Experimental Validation for the Single Screw Extruder Process

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Abstract
The polypropylene produced in most industrial polymerization processes is a resin which has excellent mechanical properties but broad molecular weight distribution and high molecular weight tails which makes the processability of the material difficult. A process called controlled rheology is being employed to solve this problem. It consists of the addition of organic peroxides in a post-reaction extrusion process to partially degrade the polymer modifying its molecular weight distribution. The objective of this work is to study experimentally and theoretically the process of controlled rheology of polypropylene. The experiments were carried out in a single screw extruder at different temperatures, peroxide concentrations and screw speeds. The modifications induced by the peroxide on the rheological properties of the melt were evaluated measuring the shear viscosity in a Mechanical Spectrometer. High temperature size exclusion chromatography was used to establish the molecular weight distributions of the resulting resins. The model presented here includes all the basic reactions that have been proposed to take place during the scission and recombination of the polymer chains. This represents an improvement over the simplified models published in the literature. The kinetic constants and the initiation efficiency employed in the model were estimated by minimizing the differences between calculated and measured average molecular weights. These parameters were then used in the model at other experimental conditions. Good agreement was found between the measured and calculated values in all cases.