Abstract

The kinetic modeling of the thermal degradation of the main vinyl polymers, which include polystyrene, polypropylene and polyethylene, was studied at a mechanistic level. The aim of this project was to demonstrate that a unifying approach only permits the description of the thermal degradation of these three polymers on the basis of a small set of radical reactions. Bond fissions, hydrogen abstractions, β-decompositions, intra-molecular abstractions and terminations are the controlling reactions in the decomposition process. The independent kinetic rate parameters of these reactions in the liquid phase are similar to the ones already adopted in gas phase hydrocarbon pyrolysis. The kinetic parameters of gas phase reactions are corrected to account for the transposition in the liquid phase. These corrections mostly only become significant for reactions with high activation energies, such as initiation reactions, and are also applied to termination reactions to account for the diffusive limitations. The second goal of this project is a description of the possible numerical solutions of the huge amount of kinetic equations (up to several hundred thousand) using both discrete approaches and the method of moments. Three different numerical methods are analyzed and compared with regard to this problem: (1) the large system of balance equations for all the species with a quasi steady state approximation (QSSA) for a global propagating radical; (2) the moment method which describes only the statistical distribution and evolution of polymer and dead species always with QSSA; (3) the moment method which describes the statistical distribution and evolution of all the species without QSSA. The proposed models are validated by comparison with thermogravimetric analyses (TGA) literature, both dynamic and isothermal. These experimental measurements cover a wide range of operating conditions and also include some detailed information on product distributions.

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