Surface wettability of lignin materials from supercritical water hydrolysis of wood

To meet the demands of the evolving circular economy, there is a growing need for renewable resources as base materials for innovative, easily recyclable products. Lignin, the second most abundant biopolymer, has emerged as a promising source of aromatics and reinforcing agent in polymer composites. For the successful manufacturing of homogeneous composite materials, good bonding between the coexisting phases is essential to prevent the formation of voids and agglomerates. Therefore, understanding the surface properties of these materials is crucial for designing optimal composite compounds. In this study, the wettability of lignin-cellulose composites and lignin samples obtained through supercritical water hydrolysis (SCWH) of birch wood is investigated. The contact angle (CA) technique, specifically the sessile drop method, was employed to assess and compare the wettability of SCWH lignin with commercially available lignin and raw birch wood. The results provide insights into their surface energy, adhesion, and hydrophilic or hydrophobic characteristics under processing conditions. All samples exhibited hydrophilicity, with an initial CA approximately 40 °, except for raw birch wood, which had a higher initial CA of ∼ 64°. Notably, when lignin is accompanied by significant amounts of cellulose, different trends in CA changes over time were observed. The influence of pressure on the CA between water and these polymers was also analyzed, but no significant impact was detected. This research advances the development of lignin-based materials with tailored surface properties for various industrial applications.