Unmodified technical lignins as sustainable binders in structural biocomposites

Abstract

To understand the impact of lignin structure on composite performance, a broad set of lignins varying in botanical origin (annual plants, hardwood, and softwood), and technical processes (kraft, soda, organosolv, and enzymatic hydrolysis) was evaluated. Through lignin impregnation, the tensile strength and stiffness of biocomposites were enhanced up to seven times compared to the hot-pressed holocellulose, particularly with wood-based organosolv lignins. These lignins exhibited low glass transition temperatures (Tg), low molar masses, and a high proportion of native interunit linkages, promoting better distribution and interfacial bonding during hot-pressing. Furthermore, the surface analysis (XPS) and thermal characterization (DSC, DRIFTS) supported the correlation between lignin distribution, Tg, and composite strength, suggesting that low-Tg lignins distribute more homogeneously, facilitating load transfer. The study highlights the importance of evaluating lignin structural features such as linkage type, hydroxy group content, and polysaccharide impurities in conjunction with Tg to predict the performance of hot-pressed wooden composites. Overall, unmodified technical lignins, particularly of the organosolv type, show strong potential as natural binders in sustainable, high-strength biocomposites for structural applications.