Effect of polymer structure on the functional properties of alginate for film or coating applications

This study aimed to characterize the effect of molecular weight (MW) and mannuronic/guluronic acids ratio (M/G) on the functional properties of four different alginate types, considering both the aqueous film-forming solutions (rheological and interfacial properties) and the self-standing films (mechanical and barrier properties). Regarding the film-forming solutions, a structure-dependent effect was clearly disclosed with respect to the flow properties. The high MW alginates showed greater values of apparent viscosity and a more pronounced shear-thinning behavior than low MW alginates. Surprisingly, the corresponding alginate films did not exhibit any significant difference in terms of Young modulus (≈1.9 GPa), elongation at break (≈8.4 %) and tensile strength (≈42.5 MPa), as well as oxygen and water vapor permeances (PO2 ≈ 7.0 • 10^-15 mol m^-2 s^-1 Pa^-1, and PH2O ≈ 1.0 • 10^-7 mol m^-2 s^-1 Pa^-1 at semi-dry condition and 25 ◦C). In addition, the best oxygen barrier performance was obtained for a film thickness of ~20 μm, above which no significant increase in the oxygen barrier performance was noticeable. The outcome of this study suggests the potential of sodium alginate for specific food packaging applications, such as the manufacturing of edible films or functional coatings for packaging materials (e.g., plastics, paper, etc.), thus possibly meeting the increasing demand for more sustainable products and processes.