Physical ageing of polyethylene terephthalate under natural sunlight: correlation study between crystallinity and mechanical properties

Semi-crystalline polyethylene terephthalate (PET) was aged under the effect of natural UV exposure and outdoor temperature during 670 days. The variation in the mechanical and thermal properties beside to the morphology was tracked by applying different analytical techniques, including scanning electron microscopy, infrared spectroscopy, differential scanning calorimetry and wide angle X-ray diffraction, in addition to tensile strength and hardness measurements. It has been confirmed that the ageing process is the results of physical trend only. The aged PET showed a decrease in both tensile strength and strain with an increase in the degree of crystallinity of aged PET samples during the whole period. These changes in crystallinity were examined by various analysis methods: density, calorimetric and infrared spectroscopy. New peaks in FTIR analysis at 1115 and 1090 cm−1 were characterized and proved that this technique is considered to be an easy tool to track the change in the surface crystallinity of aged PET samples directly. The results of this study showed that an augmentation in the degree of crystallinity of outdoor aged PET samples from 18 to 36 %, accompanied with a decrease in tensile strength from 167.9 to 133.7 MPa. Moreover, a good exponential correlation was found between the degree of crystallinity and the mechanical properties of the aged PET.