KINETIC MODELING OF POLYETHYLENE AND POLYPROPYLENE THERMAL-DEGRADATION

The actual interest towards thermal degradation of plastics lies in th e possibility not only of recovering energy but also of producing usef ul chemicals. This paper presents a mechanistic kinetic model able to describe the radical chain pyrolysis reactions taking place in the liq uid phase. The elementary reaction steps are analyzed and their kineti c parameters are proposed starting from the well known analogous gas p hase reactions. On the basis of a very limited number of independent k inetic parameters it is then possible to properly describe this degrad ation process. The simplifying hypotheses needed to describe this deco mposition process in a closed form are carefully discussed. Unfortunat ely, 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 lar ge system of ordinary differential equations. A few examples of compar isons between model predictions and experimental data confirm the vali dity of the proposed mechanism for polyethylene and polypropylene degr adation. These data refer to very low pressure decomposition experimen ts as well as to differential thermal analysis at atmospheric pressure . The effect of the molecular weight of the polymer is also discussed. (C) 1997 Elsevier Science B.V.