Photodegradation of polylactic acid: Characterisation of glassy and melt behaviour as a function of molecular weight

During the COVID-19 pandemic, UV-C germicidal lamps became widely available, even for household applications. However, their long-term degradation effects on the mechanical and rheological properties of polylactic acid (PLA) are still not well established. The relationship between degradation and its effects on the molecular structure and macroscale properties are hardly known. In this study, we investigated the effects of long-term exposure to UV-C irradiation on the properties of PLA and interpreted the results at the molecular scale. We performed gel permeation chromatography, Fourier-transform infrared spectroscopy and UV-Vis spectroscopy to analyse changes in chemical structure induced by the UV-irradiation. Then, we carried out thermal, rheological and tensile tests to investigate mechanical and melting properties, and we investigated the applicability of these test results to estimate molecular weight loss. We have created a 3D irradiation map that can facilitate the design of disinfection devices. Based on our results, we propose a maximum number of sterilisation cycles (13 cycles) for the tested PLA films that do not result in significant changes in tensile strength and modulus.