Mototada Fukuhara

Surface nature of UV deterioration in properties of solid poly(ethylene terephthalate)

We investigated the changes in the molecular weight and also in the mechanical properties with the distance to the exposed surface of the irradiated stacked poly(ethylene terephthalate) (PET) film samples. A relation between the molecular weight and the mechanical properties of the irradiated PET was established. The relation demonstrates that the decrease in molecular weight is one of the main origins causing the deterioration in the mechanical properties. The photodegradation process developing in PET was quantitatively studied by investigating the degradation kinetics of stacked PET film samples. Our results show that the strongest degradation takes place at the exposed surface, and the degradation rate decreases with increasing the distance. This further implies that the capability to bear a tensile stress in the area near the exposed surface is much lower than that in bulk. Therefore, irradiated PET may be fractured in a lower stress. These results indicate the surface nature of ultraviolet deterioration in the physical properties of PET.

Two-step photodegradation process of poly(ethylene terephthalate)

After investigating kinetics of the UV photodegradation of PET film samples having a thickness of 4.4 μm, we found that the photodegradation process takes place in two steps: a very rapid initial step followed by a normal step. This phenomenon is explained by using a concept of weak links. We have obtained the rate constants of degradation of the normal links k N = 9.0 × 10 -7 h -1 and weak links k W = 0.46 h - 1 and the number of scissions of weak links per molecules P W = 0.22. For the samples treated by a UV stabilizer, we found k N = 2.0 × 10 -7 h -1 , k W = 0.11 h -1 , and P W = 0.27. The ratios of the rate constants of the untreated to treated samples are 4.2 for k W and 4.5 for k N . These results indicate that the UV stabilizer slows down the photodegradation rate of each step to the same extent, but hardly affects the number of scissions of weak links. Importantly, it is an implication that the lifetime of the PET thin film can be prolonged by a factor of 4.2 to 4.5 in the irradiation conditions used after being treated by the UV stabilizer.