Photooxidation of fire retarded polypropylene: III. Mechanism of HAS inactivation

This paper, which is the third of a series dealing with the behaviour of fire retarded polypropylene on exposure to UV light, is devoted to the photostabilisation 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 curves of oxidation emphasise the poor efficiency of the studied stabilisers in fire retarded media. The antagonisms between fire retardant and stabilisers are discussed. The FTIR analysis indicate that the structure and the distribution of the stabiliser in the irradiated sample are subject to severe modifications upon irradiation. Particularly, the formation of amine salts can 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, from 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 converted into the active forms (i.e. nitroxy radical and alkoxyamine). An improved stability can be obtained by varying the substitution on the nitrogen atom of the HAS molecule, particularly in the case of the N-oxyl substituted HAS.