Synthesis and Application of Kraft Lignin-Based Polyurethane Coatings for Functional Paper Packaging Materials

The packaging sector presents a significant sustainability challenge, particularly due to the prevalence of plastic packaging. There is a growing interest in sustainable packaging alternatives. The main challenge is to develop packaging with comparable and competitive characteristics. In this context, this manuscript aims to evaluate the performance of lignin-based polyurethane applied as a coating on recycled linerboard. Industrial softwood kraft lignin was fully characterized in terms of purity, functional groups (FTIR and 31P NMR) and molecular weight (GPC). Aiming at coating applications, the lignin sample was solubilized in dimethyl sulfoxide (DMSO) and used as a polyol substitute in the reaction, replacing polyethylene glycol (PEG) at levels of 70%, 80%, and 90%. Subsequently, hexamethylene diisocyanate (HDI) was added to initiate polyurethane formation. After polymerization, the coating was applied in multiple layers onto the linerboard paper. Regarding water resistance, all applications demonstrated effectiveness. The lignin-based polyurethane coating improved the Cobb1800 with reductions in the range of 1147.4 to 1155.8 g/m2 compared to the uncoated paper. Water vapor permeability was reduced by more than 94%. In the evaluation of oil resistance, samples with three layers and 90% lignin replacement performed particularly well, achieving a high value in a kit test for oil and grease (kit test number 12). These results highlight a promising approach to paper-based packaging, with potential applications across a wide range of products.