Advanced Structures and Compositions for 3D Forming of Cellulosic Fibers: Dissertation

The objective of this thesis was to systematically investigate strategies to endow fiber-based materials with toughness and formability. Bio-based polymers and green treatments were applied to develop 3D packaging structures. Formability, the material's ability for three-dimensional shaping, was achieved by plastic deformations in paper structures that were defect-free in terms of appearance and functionality. A set of methods to improve paper toughness was explored, including: (a) combined mechanical treatment of fibers in aqueous dispersions of high- and low-solids content, (b) in-plane compression of paper webs followed by unrestrained drying and (c) chemical modification of fiber joints by protein spraying. The mechanical treatment of fiber suspensions at elevated temperature and high solids content induced permanent fiber deformations, including kinks and curls, which are associated with the formation of microcompressions and dislocations. In turn, they increased the extensibility but compromised the axial stiffness of single fibers. Simultaneously, shrinkage of fibers and paper webs were promoted. In contrast, the low-consistency treatment straightened the fibers while their deformations were partly preserved. Fiber bonding was promoted by fibrillation. The application of gelatin affected the strength of fiber joints and improved their deformation ability, making strong fiber webs. The drying shrinkage was also increased. The fiber network was subjected to in-plane compressive treatment and drying shrinkage, which led to fiber buckling and network compression. The role of proteins as compatibilizers and eco-friendly dispersants in composites comprising cellulose nanofibrils (CNF) and thermoformable polylactide (PLA) was also investigated. The combination of mechanical and protein treatment of fibers and their structures improved paper extensibility, from 5% to 29%. Moreover, tray-like shapes were possible with a level of out-of-plane deformation that has not been recorded before for thermoforming with a fixed blank. Overall, this thesis provides fundamental and practical knowledge about the role of several factors contributing to paper toughness and formability. The suggested modification strategies to improve paper toughness are compatible with modern papermaking and conversion processes and can be implemented easily and economically.