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Oxidation induced embrittlement in polypropylene

Bruno Fayolle 
Abstract : The thermooxidative ageing of polypropylene (PP) films, in the 70-140°C temperature range, has been studied by common (FTIR spectrophotometry, tensile testing) and less common (molecular mass measurements, Essential Work of Fracture) methods. It appears that the polymer, initially very ductile, adopts a brittle behaviour in tensile test, whereas oxidation remains undetectable by FTIR measurements. This embrittlement, corresponding to a localization of the plastic deformation, appears to be linked to a random chain scission process: ductile-brittle transition occurs when the weight average molecular weight reaches a critical value M'C=200 kg/mol. This value is consistent with the available data on virgin samples of different molecular weight. This result is difficult to reconcile with the theories, frequently found in the literature : according to them, embrittlement would be due to the formation of defects induced by heterogeneous oxidation at small scale. On the contrary, a detailed analysis suggests that oxidation is rather homogeneous in the amorphous phase and affects the interlamellar connectivity whose crucial role in plastic deformation process is well known. Taking in account this result, a kinetic model is proposed. It predicts structural changes, especially the molecular weight evolution and the thickness distribution of oxidation products through a diffusion-reaction coupling. This model allowed us to give lifetime predictions using embrittlement at M=M'C as endlife criterion. The comparison with experimental results (shape of kinetic curves, lifetime values, thickness profile of degradation products) demonstrates the good predictive value of the model. It is noteworthy that this model is not build from empirical parameter.
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Submitted on : Tuesday, July 15, 2008 - 8:00:00 AM
Last modification on : Tuesday, July 15, 2008 - 8:00:00 AM
Long-term archiving on: : Thursday, September 30, 2010 - 8:31:17 PM


  • HAL Id : pastel-00004023, version 1



Bruno Fayolle. Oxidation induced embrittlement in polypropylene. Engineering Sciences [physics]. Arts et Métiers ParisTech, 2001. English. ⟨NNT : 2001ENAM0016⟩. ⟨pastel-00004023⟩



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