Transparent Multifunctional Cellulose Nanocrystal Films Prepared Using Trivalent Metal Ion Exchange for Food Packaging

To better preserve food from the interaction with oxygen, moisture, and (light) ultraviolet (UV) radiation, typical polymer film packaging is produced as opaque or metalized sheets. There is increasing consumer demand for multifunctional transparent packaging or a transparent window to allow viewing of food before consumer purchase. Cellulose nanocrystals (CNCs), as a type of sustainable and biodegradable material, show great potential for transparent food packaging by providing good oxygen permeation resistance. However, CNCs are poor in preventing UV light transmission, sensitive to water, and too brittle to make flexible films. This work aimed to produce high-quality CNC films with multifunctional properties using a facile processing method. CNC suspensions were modified with different concentrations of trivalent metal ions (Al3+ and Fe3+). Homogeneous, transparent, and flexible CNC films were prepared by solvent casting. CNC films underwent cross-linking between metal ions and sulfate half-ester groups of the CNCs, leading to high UV absorption properties. In addition, the water vapor and oxygen transmission rates of the cross-linked films were decreased, and the durability of films in water was improved. The CNC films displayed excellent oil/grease resistance and fire self-extinguishment.