Kinetic modeling of polypropylene thermal oxidation during its processing by rotational molding

Abstract

The main drawback of rotational molding is a long stay (several dozens of minutes) of polymer in melt state at high temperature in atmospheric air. To prevent any significant polymer thermal degradation, it is necessary to define, preliminary, a processing window in a temperature-molar mass map. The objective of this article is to elaborate and check the validity of a general thermal degradation model devoted to determine, in a near future, some important boundaries of this processing window.This model is composed of two distinct levels: (i) The first level is derived from the thermal transfer mechanisms occurring during a processing operation, polymer phase changes (i.e., melting and crystallization) being simulated by the enthalpy method; and (ii) The second level is derived from the oxidation mechanistic scheme of free additive polymer in melt state established in a previous study, but completed, here, by adding the main stabilization reactions of a common synergistic blend of antioxidants, widely used for rotational molding polymer grades. By juxtaposing such "thermal" and "chemical" levels, it is possible to predict the polymer thermal degradation during a whole processing operation. The validity of both levels is successfully checked in real rotational molding conditions for polypropylene