Kinetic modeling of polyethylene and polypropylene thermal degradation

The actual interest towards thermal degradation of plastics lies in the possibility not only of recovering energy but also of producing useful chemicals. This paper presents a mechanistic kinetic model able to describe the radical chain pyrolysis reactions taking place in the liquid phase. The elementary reaction steps are analyzed and their kinetic parameters are proposed starting from the well known analogous gas phase reactions. On the basis of a very limited number of independent kinetic parameters it is then possible to properly describe this degradation process. The simplifying hypotheses needed to describe this decomposition process in a closed form are carefully discussed. Unfortunately, the enhancing effect of the intermediate formation of unsaturated species forces the use of a numerical approach. As a consequence, the simulation of the pyrolysis process requires the integration of a large system of ordinary differential equations. A few examples of comparisons between model predictions and experimental data confirm the validity of the proposed mechanism for polyethylene and polypropylene degradation. These data refer to very low pressure decomposition experiments as well as to differential thermal analysis at atmospheric pressure. The effect of the molecular weight of the polymer is also discussed.