Photooxidation of fire retarded polypropylene III. Mechanism of HAS inactivation

This paper, which is the third of a series dealing with the behaviour of fi re retarded polypropylene on exposure to UV light, is devoted to the photos tabilisation aspects. The influence of various Hindered Amine Stabilisers ( HAS) on the stability of fire retarded polypropylene samples, exposed to an irradiation in artificial accelerated media, is reported. The kinetic curv es of oxidation emphasise the poor efficiency of the studied stabilisers in fire retarded media. The antagonisms between fire retardant and stabiliser s are discussed. The FTIR analysis indicate that the structure and the dist ribution of the stabiliser in the irradiated sample are subject to severe m odifications upon irradiation. Particularly, the formation of amine salts c an be observed at the surface of thick samples. This phenomenon is related to the reaction of the amine groups of the stabiliser with hydrobromic acid generated by the halogenated fire retardant. The migration of the HAS, fro m the core of the sample towards the surface, is also observed. The reduced efficiency of the studied HAS to stabilise polypropylene is attributed to the formation of amine salt, that prevents the stabiliser from being conver ted into the active forms (i.e. nitroxy radical and alkoxyamine). An improv ed stability can be obtained by varying the substitution on the nitrogen at om of the HAS molecule, particularly in the case of the N-oxyl substituted HAS. (C) 2000 Elsevier Science Ltd. All rights reserved.