Deep eutectic solvents in the production of wood-based nanomaterials

Wood-based nanomaterials (WNMs) are renewable, biodegradable, and inexpensive green alternatives for many current materials. WNMs have especially drawn significant interest due to their potential for numerous high-end applications and replacement of petroleum-based products. The main obstacle in the industrial up-scaling of WNM production is the high energy consumption during the process of mechanical nanofibrillation. Chemical pretreatment methods are seen as beneficial in decreasing energy demand and thus have been extensively investigated. Deep eutectic solvents (DESs) are a novel and versatile class of systems that can be used as solvents, reagents, and catalysts in many fields due to their availability, low toxicity, typically green origin, and relatively low cost. DESs can efficiently interact with biomass and are therefore one of the most promising solvents in sustainable WNM production.

In this thesis work, three DES systems were designed and applied for the production of WNMs. DES pretreatments enhanced WNM production significantly and resulted in high-quality and stable (without sedimentation) WNM suspensions. DESs were used as a non-reactive medium that solely swells the fiber structure without deteriorating its degree of polymerization. In addition, reactive DESs were also developed to produce functionalized WNMs with high anionic or cationic charge densities. The fabricated WNMs had a well-individualized structure and formed homogenous films with high mechanical strengths. Furthermore, WNMs containing lignin or combined with tannin extract resulted in biohybrid films with improved UV-shielding and anti-oxidative properties, while the films obtained from anionic WNM possessed some flame-retardancy. The results indicate that DESs are an up-and-coming pretreatment approach for producing versatile WNMs with highly tailorable characteristics.