Long Term Degradation of Polymers: Molecular and Macroscopic Effects
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Hydrolytic degradation in water at elevated temperatures is one of the most important reasons for failure in polymeric materials in offshore flexible risers. In this thesis the ageing of polyamide 11 (PA11) in deoxygenated water at 120 °C was studied. Tensile testing and fatigue testing were employed to monitor the influence of ageing on the material mechanical properties. The results show systematic changes in the mechanical properties, where the general trends are increased stiffness and tensile strength with increased ageing times. Measurements of the material weight and density as well as dynamic mechanical thermal analysis were performed to follow changes in the material morphology during ageing. There was observed an increase in crystallinity and glass transition temperature. These results suggest that the changes in mechanical properties can be ascribed to plasticizer depletion and possibly an accumulation of degradation products from the chain scission reaction in the amorphous areas. These assumptions are in accordance with the observed increases in the secant modulus, tensile strength and decreases in fatigue strains, but further analysis of molecular weight and polydispersity are required to confirm the theory.