Bastard, Myriam (2006) Durability thermoplastic pieces: application to polyoxymethylene. PhD thesis Mécanique et Matériaux, ENSAM 2006ENAM0003.

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Abstract

We studied the stabilized and unstabilized polyoxymethylene (POM) thermooxidation at various temperatures, between 90 and 150°C and various oxygen pressures, between the atmospheric pressure and 20 bars. The main characteristics of the ageing process are as follows: slow reduction in the molecular mass followed by an important mass loss, strong dependency on the oxygen pressure, and relative scarcity of the observable oxidative products by infra-red spectrophotometry(principaly formiates in end of chains). This leads us to propose a mechanistic scheme in which radicals peroxyle attack methylenes of the chain to give unstable hydroperoxides whose decomposition leads to the formation of a formiate and leads to a depolymerization mechanism which is characterized by kinetic chain length close to one hundred. A kinetic model was derived from this diagram. It reasonably gives an account of all the experimental results obtained, even if questions remain opened, in particular concerning stabilization. The analysis of embrittlement mechanisms induced by ageing suggests that the most important phenomenon is the random chain scission, the rupture passing of the ductile mode to the brittle mode when the molar mass reaches 80 ± 20 kg.mol-1. However, it is probable that the evolution of morphology (reduction of the amorphous layer between plates), also plays a considerable part. A nonempirical method of lifetime prediction, based on these results, was proposed but the determination of all the parameters characterizing the industrial sample(stabilizers?, shock-proof polyurethane?, . . . ) could not be completely accomplished.

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