Anti-/pro-oxidant concentration-dependent activity of natural phenolic compounds in bio-polyesters

Polylactic acid (PLA) is a biodegradable bio-based polyester of increasing industrial interest because of its unique properties, such as low toxicity, bio- and eco-compatibility. Indeed, PLA is a suitable material for several “green” applications in different fields, among other food packaging, textile fibres and biomedical devices [1]. PLA performances, in terms of durability, are limited by several chemical aging mechanisms, such as thermal decomposition, hydrolysis, photo-oxidation, natural weathering and thermo-oxidation at high temperature [2]. Therefore, to avoid the decay of its physical and mechanical properties, the addition of anti-oxidants and/or light stabilizers is imperative. In this work, the photo-oxidation resistance of PLA-based systems has been investigated, also considering the effect of the content of natural compounds, such as Quercetin (Q), Vitamin E (VE), Ferulic Acid (FA) and Vanillic Acid (VA). PLA-based films containing different amount of natural phenolic compounds have been formulated through melt-mixing and their photo-oxidation behaviour has been investigated with the aim to explore the potential of used molecules as UV light stabilizers in PLA. Accurate spectroscopic analyses indicate that FA, VA, VE and Q show an unexpected pro-oxidant activity if they are present at high content. This finding can be understood considering the intrinsic concentration-dependent pro-oxidant activity of used natural molecules, coupled with the presence in the system of transition metals, since the considered natural compounds show transition metals reducing properties. In all, obtained results open new perspectives in the control of the degradation time of bio-polyesters since the rationale control of the content of natural phenolic compounds in biopolymer-based formulations could allow the obtainment of polymers with tunable lifetim