Transition between regimes in the degradation of thermoplastic polymers

The pyrolysis regimes of thermoplastic polymers (polyethylene) are examined through a mathematical model, including transport phenomena and chemical reactions. Depolymerization and melting are followed by devolatilization. Surface regression, property variation, heat convection and conduction are properly taken into account. Simulations are carried out for external heating conditions corresponding to fixed-bed reactors, hot-plate contact and fire-level radiation exposures. Only very slow external heat transfer rates, associated with low reactivity of the fuel, give rise to a pure kinetic control. For conditions of interest in fixed-bed reactors and radiative heating, a transition from a thermally thin to a thermally thick regime is observed, as the sample size is increased. Two regimes can be established during hot-plate contact, namely an ablative regime or again a thermally thick regime, depending on the relative importance between internal heat transfer and chemical reaction kinetics. (C) 1999 Elsevier Science Ltd. All rights reserved.