Moisture uptake and effects of hygrothermal exposure on closed-cell semicrystalline polyethylene terephthalate foam

Abstract

Polyethylene terephthalate (PET) foam is receiving increasing interest in recent years for its use in structural applications. This is favoured by its balance between mechanical properties and cost, as well as recyclability and wide availability of recycled material. Knowledge on its ageing behaviour upon hygrothermal exposure is needed to ensure the fulfilment of the required service life and foster market uptake. A six-month ageing programme under 40ºC and 80% RH and a one-year water immersion ageing programme were undertaken, in order to understand the moisture uptake mechanism, and observe in-situ as well as irreversible degradation caused by moisture. These conditions were selected in order to ensure being below the Tg of PET, and so in the same structural state as in the use phase. No degradation of the PET foam was found after the completion of both trials. It was demonstrated that the hygrothermal degradation of the polymeric foams is not diffusion-controlled. Moisture uptake by the PET matrix was analogous to that previously reported in films. Liquid water passage into the cell cavities was progressive and much slower, but did not produce any significant additional effect on the mechanical behaviour of the PET foam.