Effects of hygrothermal aging on the molar mass and thermal properties of recycled poly(ethylene terephthalate) and its short glass fibre composites

This paper reports on the effects of hygrothermal aging at 70 °C in water, and at 80% relative humidity, on the molar mass and thermal properties of recycled poly(ethylene terephthalate) and its short glass fibre composites. During the initial period of exposure, water uptake increases linearly with the square root of time and apparent diffusivity decreases as fibre content increases and as relative humidity decreases. Samples exposed to 80% RH reach an equilibrium water content, while samples immersed in water do not. Composites absorb more water than expected on the basis of matrix sorption behaviour, thus suggesting that other mechanisms, like capillarity and/or transport by microcracks, are active. The observed molar mass decrease has been successfully modelled as a pseudo first-order reaction whose rate constant depends on the humidity conditions. Due to a chemicrystallization process the crystallinity of rPET and its composites increases during aging. The glass transition temperature, as measured on the second DSC scan, depends on the number-average molar mass reached during hygrothermal aging.