Peroxide Degradation of Polypropylene during Reactive Extrusion

Abstract

<p>The production of controlled-rheology polypropylene polymers by means of peroxide initiated degradation during reactive extrusion has been studied experimentally and numerically. Experiments were carried out using various amounts of a peroxide to promote chain scission reactions for the purpose of modifying the molecular weight distribution of commercial isotactic polypropylene resins. Experimental runs were carried out in a single screw plasticating extruder and in glass ampoules. The polymers produced were evaluated in terms of their molecular, rheological and mechanical properties. During the extruder runs the residence time distribution in the extruder screw channel was measured by using a radioactive tracer technique in order to study the effects of peroxide concentration, barrel temperature and screw speed.</p> <p>A fully-predictive mathematical model was developed for the peroxide degradation of polypropylene in a single-screw plasticating extruder. The model was developed using conventional plasticating extrusion theory and a simple set of chemical reactions which caused chain scission. The interactions of the flow and reaction phenomena were taken into account via residence time distribution and the chemorheology of the reactive melt. The model includes several submodels for the flow of solids in a feed hopper, flow in the solids conveying zone of the extruder, melting of the polymer, flow and reaction in the melt pumping zone and in the die region. Given the operating conditions and the geometrical configuration of the screw, the model can predict: the mass flow rate of the polymer, the pressure, temperature and molecular weight profiles along the screw channel and in the die, extrudate swell at the die exit and the residence time distribution of the material in the extruder channel. Model predictions are in good agreement with measurements on a 38 mm diameter extruder.</p>