Natural deep eutectic solvents as green plasticizers for chitosan thermoplastic production with controlled/desired mechanical and barrier properties

There is a worldwide interest in replacing oil-based synthetic plastics by biodegradable and nontoxic materials in food packaging. In this context, chitosan (Chit), and Natural Deep Eutectic Solvents (NADESs) prepared from cheap raw materials seem to be excellent candidates since they are natural products with excellent biological properties. The aim of this work was to investigate the potential use of four choline chloride (ChCl)-based NADESs, prepared with Malic Acid (MA), Lactic Acid (LA), Citric Acid (CA) and Glycerol (Gly) as hydrogen bond donors, as plasticizers on thermoplastic chitosan (DD = 76 and 81) films. Translucent chitosan films were fabricated by thermo-compression molding. The morphology of the films, assessed by FESEM measurements, revealed that NADESs interact differently with each chitosan matrix; the most homogeneous surface and compact structure were observed for Chit76-ChCl/CA films. Chemical changes in the processed films were detected on FTIR analysis. In general, films prepared with Chit76 show lower thickness than films prepared with Chit81. Regarding mechanical properties, Chit76-ChCl/MA film presents the highest elasticity, whereas Chit76-ChCl/CA film exhibits higher values of tensile strength. In respect to water resistance properties, in general, films prepared with Chit76 were less permeable to water vapor (lower values of WVP) than films with Chit81. Additionally, Chit76-ChCl/CA films were the least permeable and present lower amount of water retained at the primary sorption sites of the polymeric film.