Characterization of different types of lignin and their potential use in green adhesives

Lignin is the second most abundant component of lignocellulosic biomass after cellulose. This aromatic polymer has an immense potential for applications in composite materials, organic synthesis and pharmaceutical industries. In this study, different types of lignin (Kraft and three organosolv lignins from different processes) were characterized and incorporated into a pressure sensitive adhesive (PSA) aqueous formulation based on polycarboxylate polyether (PCE), which was selected for its low ecotoxicity. All lignin samples were evaluated and compared using various characterization methods, such as GPC, 13C NMR and 31P NMR spectroscopy. Thermal properties were investigated using TGA and DSC techniques. Based on these analyses, organosolv lignin samples presented lower molecular weight (Mw), lower polydispersity and higher amounts of phenolic OH groups. Optimizing molecular weight and hydroxyl contents were found to be the main factors affecting PSA properties. One of our formulations (5-OL-B2), which incorporated the organosolv lignin of lowest molecular weight (Mw ≈ 1270 g/mol) and highest OH content (~7.08 mmolOH/glignin), leads to an adhesive performance similar to that the commercial PSA, such as low peel-off losses and lower distance traveled by the ball (higher tackiness).