Cross-linked chitosan/tannin extract as a biodegradable and repulpable coating for paper with excellent oil-resistance, gas barrier and UV-shielding

A series of cross-linking copolymers of chitosan and tannin extract-based epoxy (CS/xTE, x = 0.125, 0.25, 0.5 and 1) were designed and synthesized using different contents of low-cost tannin extract-based epoxy (derived from bark). The resulting CS/xTE were used as coatings for paper, and they endowed the paper with robust barrier properties to oil, moisture and oxygen because of the dense and continuous film formed by CS/xTE on the surface of the paper. Compared with uncoated paper, the Kit rating of CS/1TE (x = 1)-coated paper increased from 0/12 to 12/12, the water vapor transmittance rate decreased from 1252.9 to 298.6 g/m2·d, and the oxygen permeability decreased from 597.1 to 136.2 cm3/m2·d·0.1 MPa. Furthermore, due to the UV-shielding of tannin extract-based epoxy, the CS/xTE-coated paper exhibited excellent UV-aging resistance. The mechanical properties and water resistance of CS/1TE-coated paper were also increased compared with those of uncoated paper due to the dense cross-linked coating. Additionally, the results suggest that CS/1TE is repulpable and biodegradable. This work not only provides an efficient strategy for high-value applications of waste bark extract but also shows a considerable application advantage in the paper packaging industry.