Effects of water sorption on the physical properties of PET, PBT, and their long fibers composites

The behavior of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) on water aging has been studied above and below the glass transition temperature (Tg). The aging process is caused by: degradation of the matrix and an increase in crystallinity above Tg, and microcavitation at the amorphous/crystalline interface below Tg. Such behavior well explains the deviation of the sorption kinetics from the Fickian model. The apparent water diffusion coefficients and the transport activation energies of PET and PBT have been calculated at temperatures above and below Tg. The mechanical behavior of the two polymers on water aging has been measured by means of fracture mechanics and Izod impact tests at different stress concentration factors. An increase of toughness of PET at short aging times has been shown by mechanical tests and SEM analysis fracture surfaces of differently aged samples. Izod tests of PET and PBT composites reinforced by long glass fibers have shown the contribution of fibers to the total fracture energy.