Photostability of polylactide with respect to blue LED radiation at very high irradiance and ambient temperature

Abstract

For an overall sustainable development of lighting systems, it is crucial to establish alternatives to fossil based optical plastics. One biodegradable plastic alternative, based exclusively on renewable raw materials and having outstanding optical properties, could be the bioplastic polylactide (PLA). In order to evaluate the stability of PLA under the influence of extreme levels of optical LED radiation (λmax = 450 nm, FWHM = 16.3 nm), aging experiments were carried out over a period of 5000 h (~ 7 months) using an innovative self-developed test setup. As a reference the widely used optical plastic polycarbonate (PC) was aged under the same conditions. The novel test setup allowed aging tests at low temperatures of 23.0 and 36.1 °C (below the crystallization temperature of PLA) with irradiances of 7.9 and 16.1 kW/m2, respectively. Photodegradation tests in which temperature can be varied virtually independent of radiant flux were performed. To the best of our knowledge this is a first in degradation experiments. By this, aging can be attributed to more radiation- or temperature-related phenomena. Before, during, and after aging, optical, mechanical and chromatographic methods were used to analyze the samples. PLA was found to be largely resistant to visible blue LED radiation under the selected aging conditions. Only an increase in surface hardness and stiffness, indicating embrittlement, was observed. In contrast, even at the low temperatures used in these experiments, PC shows a significant decrease in transmission in the short-wavelength range of up to 17.0% after prolonged aging. Moreover, known degradation products (by FTIR spectroscopy), a decrease in molar mass (5.1%) and a trend increase in Martens hardness and indentation modulus, were detected for all PC of samples.