Concentration-dependent anti-/pro-oxidant activity of natural phenolic compounds in bio-polyesters

In this work, the potential of several naturally occurring phenolic compounds, such as Ferulic Acid (FA), Vanillic Acid (VA), Vitamin E (VE) and Quercetin (Q), as stabilizers against the photo-oxidative degradation of Polylactic acid (PLA) has been assessed. Specifically, PLA-based films containing different amounts of considered stabilizers have been formulated and their photo-stability under UVB exposure has been evaluated. The preliminary characterization of the formulated films shows that all used stabilizers exert plasticizing action, as probed by rheological analysis, due to their low molecular weight. Moreover, no significant modification of the PLA crystallinity has been noticed in presence of used natural compounds. The study of the photo-oxidation behaviour of PLA-based systems suggests that, among all used natural phenolic compounds, FA and Q are suitable anti-oxidant for PLA, if they are added at low content. Surprisingly, all considered natural compounds, exert a pronounced pro-degradant action in PLA matrix, if their content overcomes a certain specific threshold. This finding has been related to the known concentration-dependent anti-/pro-oxidant activity of used phenolic compounds, that results exacerbated in presence of traces of transition metals, such as iron and copper.