Synergism between polymer antioxidants—kinetic modelling

Abstract

A classical kinetic model, without the stationary state hypothesis, has been developed to simulate the behaviour of a hydrocarbon polymer of polypropylene type, stabilised by a mixture of a chain breaking antioxidant of hindered phenol type with a hydroperoxide decomposer of the organic sulphide or phosphite type. From a numerical solution of the system of differential equations representing the rate variations of the reactive species, one obtains kinetic curves of hydroperoxide build-up from which induction time can be determined. The variations of the induction time (at 140 °C in O 2 excess) have been studied with the composition of the stabiliser mixture, the nature of the hydroperoxide decomposition process (unimolecular or bimolecular), the reactivity of the chain breaking antioxidant, and the rate of an “extrinsic” initiation process added to the POOH decomposition. Two main results were obtained: (i) The chosen model is able to simulate strong synergistic effects only due to action of both stabilisers at distinct levels of the radical chain (initiation for hydroperoxide decomposer and termination for the chain breaking antioxidant). (ii) The high stabilising efficiency of these systems is strongly linked to the “closed loop” character of oxidation, i.e. to the fact that the radical chain process generates its own initiator (POOH). When an “extrinsic” initiation process is added, this stabilising efficiency and the magnitude of synergistic effects decrease sharply.