Hydrolytic Degradation of Closed Cell Polyethylene Terephthalate Foams. The Role of the Mobile Amorphous Phase in the Ductile-Brittle Transition

Polyethylene terephthalate (PET) foam is receiving increasing interest in recent years for engineering and structural applications. But for its successful and reliable implementation in real life infrastructures, knowledge on its ageing performance is critical for service life predictions. In this study the ageing of commercial closed cell PET under 90ºC and 95% RH is presented and its molecular and mechanical degradation characterized. The aim of the study is to observe the impact of the cellular structure on the hydrolysis kinetics, and provide more insights on the nature of embrittlement in PET. Structural changes are evaluated based on a three-phase model with crystalline, mobile amorphous (MAF) and rigid amorphous (RAF) fractions. The hydrolytical degradation of PET foams does not appear influenced by the cellular structure. Hydrolysys and chemicristalization appear to occur at the expense of the MAF fraction until the embrittlement occurs. The ductile-brittle transition point coincides with the achievement of the minimum MAF content. The link of embrittlement with the structural changes occurring in the mobile chains questions the occurrence of hydrolysis-induced embrittlement below T_g, which would correspond to service conditions. A micromechanical interpretation of embrittlement appears more appropriate than a molecular interpretation.