Reversible Photo-Responsive Hydrophobic Coating Synthesized from Lignin-Derivable Molecules on Nanocellulose Films for Packaging Applications

Paper-based packaging can offer a sustainable replacement for plastics. However, paper provides a poor barrier to water, oxygen and moisture. This study presents a novel renewable lignocellulosic composite made from a hydrophobic photo-reversible coating deposited onto a cellulose nanofiber film that has improved barrier properties and can be reprocessed. Diglycerol and lignin-derivable aldehyde were reacted to form a tetra-functional monomer with photo-responsive unsaturated double bonds that can be converted to covalent cyclobutane rings to create reversibly crosslinkable network upon UV-irradiation. The photo-responsive compound was applied as a thin coating of thickness 2.7±0.4 μm over cellulose nanofiber (CNF) films of thickness 80±19 μm. The surface of the coated films became hydrophobic with a contact angle (CA) of 93.1±1.7° and displayed a low water vapour transmission rate (WVTR) of 16±2 g/m²/day vs. 30.7±1.5° CA and 81±11 g/m²/day WVTR for uncoated CNF films. The coated film is also oleophobic, an attractive feature for food packaging applications. The reversible photo-reaction enables the crosslinked covalent network to be broken down to unsaturated double bonds once exposed to a higher-energy UV irradiation, allowing reprocessing and recycling. The novel coating was developed using a sustainable green synthesis method (process simple E factor 0.9).