CONTROLLED RHEOLOGY POLYPROPYLENES - AN IMPROVED MODEL WITH EXPERIMENTAL VALIDATION FOR THE SINGLE-SCREW EXTRUDER PROCESS

The polypropylene produced in most industrial polymerization processes is a resin which has excellent mechanical properties but broad molecu lar weight distribution and high molecular weight tails which makes th e processability of the material difficult. A process called controlle d 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 distribu tion. The objective of this work is to study experimentally and theore tically the process of controlled rheology of polypropylene. The exper iments were carried out in a single screw extruder at different temper atures, peroxide concentrations and screw speeds. The modifications in duced by the peroxide on the rheological properties of the melt were e valuated measuring the shear viscosity in a Mechanical Spectrometer. H igh temperature size exclusion chromatography was used to establish th e molecular weight distributions of the resulting resins. The model pr esented here includes all the basic reactions that have been proposed to take place during the scission and recombination of the polymer cha ins. This represents an improvement over the simplified models publish ed in the literature. The kinetic constants and the initiation efficie ncy employed in the model were estimated by minimizing the differences between calculated and measured average molecular weights. These para meters were then used in the model at other experimental conditions. G ood agreement was found between the measured and calculated values in all cases.