Using the small punch test to analyse the influence of ultraviolet radiation on the mechanical behaviour of recycled polyethylene terephthalate

The aim of this work is to evaluate the feasibility of small punch test as a tool for analysing the mechanical response of small plastic parts or specimens. Here, the small punch test was used to study the possible influence of ultraviolet radiation exposure on the mechanical properties of outdoor furniture made from recycled polyethylene terephthalate in the frame of an industrial project with participants from both industry and academia. The industrial project partners prepared four types of masterbatches in order to stabilize the recycled polyethylene terephthalate and to minimize photodegradation as well as colour change in the final product. The masterbatches contained red pigments, antioxidants and ultraviolet absorbers. Flat sample plates were injection moulded from recycled polyethylene terephthalate and these masterbatches. Some sample plates were used as-moulded, some samples received a thermal treatment in order to post-crystallize the bottle-grade recycled polyethylene terephthalate and some other samples were subjected to ultraviolet radiation for 900 h. In the frame of an industrial project, the ultraviolet ageing was performed superficially only on one side of the plates in order to evaluate the colour change. The three sets of samples were analysed by means of tensile tests, differential scanning calorimetry and intrinsic viscosity. However, the irradiated samples could not be tested with tensile tests due to their small size. Therefore, the small punch test was used to accurately characterize these small samples. The feasibility of the small punch test as a valid mechanical characterization method for these types of materials was demonstrated by comparing the small punch test results from the unradiated samples with the tensile test ones. The mechanical parameters obtained from both types of tests showed the same results. Although the used industrial ultraviolet ageing process produced a colour change in the studied recycled polyethylene terephthalate samples, it was possible to verify via small punch tests that the mechanical properties were not affected.