A sustainable approach to recycling of polylactic acid with environmentally friendly reagents

An important aspect before large-scale production and application of bioplastics such as polylactic acid (PLA), is the need to close the life cycle of the material to reduce the need for first-generation biomass and to prevent waste accumulation in the environment. In this work, starting from a high-mass linear commercial PLA, a depolymerization route based on a bulk alcoholysis reaction in the molten state was developed. For this purpose two polyalcohols, pentaerythritol and dipentaerythritol, and an environmentally friendly catalyst, i.e., zinc stearate, were utilized. The formation and specific polyalcohol dependent structure of star-shaped oligomers characterized by a low glass transition temperature was confirmed by spectroscopical and thermal analysis. Indeed, ¹H NMR characterization evidenced that the most effective polyalcohol was pentaerythritol, which at the highest concentration in the reaction mixture, namely 10 wt.-%, allowed most of the hydroxyl groups to react, resulting in a system with a T_g of about 20 °C, which was much lower than that of the starting linear polymer, characterized by a T_g of about 60 °C. Moreover, GPC as well as DSC analysis in particular demonstrated the active role of zinc stearate in the transesterification reaction, as the samples prepared without adding the catalyst to the reaction mixture showed a modest reduction in Tg and molecular weight, which decreased from 92,000 g⋅mol^-1 to 1700 g⋅mol^-1 for the starting linear polymer and the resulting oligomer, respectively, in the case of the sample prepared with the highest amount of PE and with the addition of zinc stearate.The films produced from the star-shaped PLA oligomers were characterized by poor mechanical properties, but the high concentration of alcohol-functionalities could make them applicable in various polymer formulations. The star-shaped polymers were thereby blended with a multifunctional epoxide from renewable sources. The reactivity and compatibility of the two components was proved along with the specific role of zinc stearate remaining from the alcoholysis, in promoting the reaction between the two compounds. Indeed, the produced materials proved to be homogeneous, manageable and also completely enzymatically degradable.