Determination of thermal oxidation rate constants by an inverse method. Application to polyethylene

A kinetic model for polymer thermal oxidation derived from the standard mechanistic scheme, in which initiation is only due to (uni and bimolecular) hydroperoxide decomposition, has been elaborated. It is composed (for unstabilised, saturated hydrocarbon polymers) of four non-linear differential equations. The usual simplifying hypotheses (steady state, interrelationship between termination rate constants, long kinetic chain and low conversion) are suppressed. We have tried to use the model to solve the inverse problem. i.e. the determination of elementary rate constants from usual experimental data, for instance from oxygen absorption curves and oxidation profiles. A two steps procedure, based first on oxygen absorption curves in oxygen excess, second 2 on oxidation profiles on bulk samples, is proposed. The first step allows us to determine the initiation rate constants and the ratio k(3)(2)/k(6), k(3) and k(6) being the respective rate constants of hydrogen abstraction and bimolecular peroxyl+peroxyl termination. In the second step. it is possible to determine the other termination rate constants k(4) and k(5) (alkyl + alkyl and alkyl + peroxyl terminations, respectively). A numerical application, made on unstabilised HDPE, shows the excellent predictive properties of this model. (C) 2004 Elsevier Ltd. All rights reserved.