Enhanced physical, mechanical and barrier properties of chitosan films <i>via</i> tannic acid cross-linking

Growing environmental concerns over the extensive use of petroleum-based polymer packaging have spurred interest in the development of bio-based alternatives. In this work, the incorporation of tannic acid as a cross-linker into chitosan at concentrations of 0-60 wt% was explored. The resulting cross-linking between chitosan chains induced by tannic acid through hydrogen and Schiff-base covalent bonding was confirmed by X-ray photoelectron spectroscopy and gel content measurements. This significantly enhanced the films' thermal stability, water uptake, mechanical properties, and barrier properties. The cross-linking minimized the interaction between chitosan functional groups and water molecules, improving water resistance. The chitosan films with 30 wt% tannic acid displayed significant improvements in tensile stress and Young's modulus by 74% and 110%, respectively, compared with the neat chitosan films, which were ascribed to the strong interaction between chitosan and tannic acid. In addition, the cross-linked films effectively blocked UV light transmission while maintaining transparency levels greater than 85%, offering potential protection against photo-oxidation and photo-discoloration of food produce caused by sunlight exposure. However, increasing tannic acid loading negatively affected the antibacterial properties, wettability, and appearance (increased yellowness) of the cross-linked chitosan films. Furthermore, packaging developed from these cross-linked chitosan films successfully extended the shelf life of chilies, demonstrating their application in food packaging. Compared with petroleum-based polymers and biopolymer packaging films, these cross-linked chitosan films offer promising mechanical and barrier properties and UV-shielding capability, making them a sustainable alternative for packaging applications.