Thermoplastic starch/poly(lactic acid) sheets coated with cross-linked chitosan

Blends of thermoplastic starch (TPS) and poly(lactic acid) (PLA) (70/30 TPS/PLA wt/wt) were thermopressed to produce biodegradable sheets. These sheets were coated with cross-linked chitosan by two different methods: spraying and immersion. The coated sheets presented a more irregular surface than uncoated samples, which was associated with the chitosan reticulation. The chitosan coating reduced the water solubility and the water vapor permeability of the sheets due to being less hygroscopic. Coated sheets were more rigid and had higher tensile strength than uncoated sheets, because the cross-linking joins the macromolecules covalently, reducing the mobility of the chains. Higher glycerol concentration in the TPS increased the elongation at break due to the glycerol plasticizing effect. Coating by spraying was more effective at changing the sheet properties than coating by immersion, and this technique can be used to reduce the hydrophilic character of biodegradable films, allowing their use as packaging materials.