On the autoaccelerated character of the branched oxidation of polyolefins

Photochemical or thermal (at moderate temperatures) oxidation of hydrocarbon polymers is characterised by the existence of a pseudo-induction period, during which oxidation is autoaccelerated, and a steady state, This latter corresponds to the equilibrium between hydroperoxide (POOH) decomposition and formation processes. All the kinetic schemes of thermal ageing,based on the assumption that POOH decomposition is the only source of radicals (whatever its order), have a common feature : the steady-state rate is independent of the rare constant of POOH decomposition, as experimentally observed in samples containing variable concentrations of catalytic residues. Experimental data indicate that the duration of the pseudo induction period is proportional to the reciprocal of the POOH decomposition rate constant and independent of any other rate constant. This is an exclusive property of kinetic schemes in which POOH decomposition is unimolecular (whereas termination is bimolecular). The corresponding set of differential equations has been first resolved from the hypothesis of existence of a stationary state for radical concentration. Although this hypothesis is questionable, the corresponding analytical expression appears as a good approximative solution for the general case (no hypothesis of stationary regime). This expression displays interesting predictive properties. Some eventual kinetic implications of spatial heterogeneity of oxidation are examined, Certain variables, for instance the second derivative of the kinetic curve of carbonyl growth (or oxygen absorption), or simply the kinetic curve of mass variation, vary in a non-monotonous way with exposure time. Consideration of timescale associated to these variations can lead to a quantitative approach of the homogeneity, Schematically, if the characteristic time of non-monotonous variations is of the order of magnitude of the reciprocal of the rate constant of POOH decomposition or lower. the oxidation can be considered almost homogeneous, In the example under study : experimental data on thermal oxidation of poly(propylene) indicate that these systems are not too far from homogeneous ones.