Polypropylene (PP) when polymerized exhibits a high molecular weight (MW) and, consequently, high viscosity, which might be tricky for its processability. To improve this latter, MW can be tailored by different degradation methods. One of the most popular is the degradation of the PP induced by peroxide, because it is a straightforward and cost-effective approach.
Peroxide-induced degradation of PP mechanism is composed of a series of free-radical reactions:
As a result of these molecular changes, the viscoelastic behavior of the PP is deeply changed.
Thus, this study reports two parameters characteristics of PP degradation: MW and complex viscosity (η*), both ploted along the screw.
PP is tumbled-mixed with the peroxide (in powder form) and processed in a laboratory twin screw extruder (TSE). The screw profile (Figure 1) contains a series of conveying elements, separated by three mixing zones, consisting of kneading disks and a left-handed element.
The first block of the kneading disks ensures the melting of PP and the homogenization of the PP/peroxide blend. The left-handed element creates a plug of material upstream of the venting port, at which any eventual residual peroxide is removed by a vacuum pump.
The throughput is kept constant but two values of screw speeds, barrel temperature and peroxide concentration are tested in order to determine the influence of each one of them.
The degradation reaction is simulated with a 1-D simulation software, Ludovic®, which computes the main flow parameters (temperature, pressure, shear rate, viscosity, residence time,…) and the kinetic reaction model. This is a 2-step computation.
First computation without coupling, with rheological and physical properties of the virgin material (i.e. PP without peroxide).
Second computation, taking into account the interactions between the reaction conversion rate, the rheological changes and the flow parameters.
Before checking the validity of the reactive model (second computation), the accuracy of the flow computation without the reaction (first computation) has to be confirmed. Figure 2 provides the comparison of the computed and measured temperature profiles.
All the computed temperatures are within the +/-5% range of the experimental values, thus confirming the ability of Ludovic® to compute the flow conditions in a TSE.
This simulation approach, allows to determine the influence of operating condtions and peroxide concentration on MW and η*. Below, figure 3 shows the evolution of the MW and η* of PP processed under the same conditions, but with two different peroxide concentration (0.05% and 0.10%).
The higher the amount of peroxide is, the more significant the decline is in MW because of the generation of more free radicals, which promoted more chain scission. Also, the increase of peroxide content, involves a decrease in complex viscosity more dramatic but at the same location, and an equivalent plateau is identified.
As a conclusion, in this study Ludovic® has been able to capture the main process features and describe accurately the evolution of PP properties which are molecular weight (MW) and complex viscosity (η*), occuring during its degradation caused by peroxide in a cororating TSE.
*based on: “Evolution of the Peroxide-Induced Degradation of Polypropylene Along a Twin-Screw Extruder: Experimental Data and Theoretical Predictions” by F. Berzin (Centre d’Etudes et de Recherches en Matériaux et Emballages – ESIEC), B. Vergnes (Cemef – Mines ParisTech), S.V. Canevarolo (Department of Materials Engineering – Universidade Federal de São Carlos), A.V Machado et J.A. Covas (Institute of Polymers and Composites – University of Minho) (Journal of Applied Polymer Science – 2006).