Effects of chitosan thin barrier layers on the oxygen permeability and optical properties of poly(lactic acid) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) multilayers

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(lactic acid) (PLA) are highly promising biobased and biodegradable polymers in food packaging industry due to their mechanical and optical properties, and improved sustainability. Nevertheless, some properties like O₂ permeability need improvement. In this work we have prepared and tested bilayers from PLA and blends of PLA and PHBV, with an additional thin layer of chitosan (CH) to reduce the oxygen transmission rate (OTR) without compromising optical properties. In these materials, the poor compatibility between hydrophilic CH and hydrophobic biopolymers is a serious handicap, as it leads to weak adhesion between the layers. The hydrophobic polymer surfaces were activated with aqueous NaOH at room temperature for different times, which increased hydrophilic character. Considering that biopolyesters can undergo NaOH-catalised hydrolytic degradation, the effect of NaOH treatment on the structure of the polymers was also studied by using FT-IR spectroscopy, scanning electron microscopy (FE-SEM), microhardness and contact angle measurements. Both the use of PHBV and a thin layer of CH lead to significant decreases in O₂ permeability. The decrease is greater in the case of the inner CH layer, which also reduces UV transmittance without significantly affecting the transparency of the film.