Evaluation of chemical additives in hydrothermal pre-treatment of wood for the integrated production of monosugars and hydrolysis lignins for PLA-based biocomposites

The use of chemical additives (1- and 2-naphthol, formaldehyde) in hydrothermal pre-treatments of pine, birch, and willow wood was evaluated to minimize lignin condensation reactions and consequently improve the saccharification of the pre-treated materials. The residual hydrolysis lignins were then tested in the preparation of polylactic acid (PLA)-based composites. The results showed that addition of 2-naphthol to the hydrothermal pre-treatment increased the saccharification of pine wood by twofold compared to water-only experiments, but the sugar yield was only 50% of the carbohydrate content in the pre-treated pine. The use of 2-naphthol and formaldehyde also improved somewhat the saccharification of bark-containing willow wood, while the use of 1-naphthol had no effect on the saccharification of any of the pre-treated materials. In birch and willow (without bark), almost complete saccharification could be achieved even without additives. Analyses of hydrolysis lignins from birch revealed structural changes caused by the additives, which were consistent with a reduction of condensation reactions. Selected lignins were successfully used at 20% loading in PLA/lignin composites; however, variation in mechanical properties among composites prepared with different lignins was only minor. In general, the use of lignin decreased the strength and ductility of PLA but increased the stiffness. Based on these results, the use of additives may only be beneficial in the pre-treatment of pine or other softwoods to improve the sugar yields.