Facile preparation of transparent, structurally robust, fire-resistant NaBH4-modified alkaline lignin/CNF film for active packaging

The intrinsic characteristic of UV resistance, antibacterial properties, and thermal stability enabled its extensive application in the functionalized preparation of biodegradable bioplastics. However, the inherent dark color of lignin restricted its broad applications. In this study, a facile and novel reduction method with sodium borohydride for de-coloring of lignin and constructing more hydroxyl groups was proposed. Not only the physicochemical properties of the modified lignin were detected, but also the comprehensive performance of the lignin and nanocellulose composite films were investigated. Results showed that the maximum hydroxyl group content could achieve 8.74 mmol/g, representing a 314.39% increase compared to crude alkaline lignin, which also led the tensile strength of the light-colored composited films to 152.33 MPa with the lignin addition ratio at 23.1%. Besides, excellent ultra-violet shielding efficiency and visible light transmittance were also exhibited, and the limited oxygen index could achieve about 50%. Antimicrobial tests demonstrated 99.99% inhibition rates against both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), and the biodegradability test also proved its environment compatibility, which was expected to provide a feasible basis for the wide application of transparent lignin composited films.