Dewatering of single- and multilayer nanopapers

During the past decade, the paper and board industry has increasingly explored the application of nanomaterials in furnish components. This is due to their potential to create innovative, high-value products that will dominate future markets. This includes cellulosic nanomaterials such as microand nanofibrillated cellulose (MNFC). MNFC consists of cellulose micro- and nanofibrils obtained by deaggregating cellulose macrofibers. These fibrils are considerably smaller and bind more water than the parent pulp fibers. MNFC presents opportunities to innovate natural fiber products with enhanced performance characteristics, including greater mechanical and barrier properties. However, the small size, high swelling, and large surface area of MNFC causes processing issues such as poor water removal properties. This thesis focuses on forming and dewatering sheets containing cellulose nanomaterials and investigates possible solutions to overcome the dewatering challenges. The main hypothesis is to help dewatering by restructuring the fiber network and increasing the permeability of the wet web. The reduced permeability of the wet web is directly related to sheet sealing, which is an established phenomenon in the papermaking process. Different mechanisms of sheet sealing are presented, and we propose approaches to prevent sheet sealing. We show that the enrichment of small fibrils on the exit layer should be avoided. MNFC/fiber flocs and their agglomeration in the suspension are modified through the addition of cationic micro-and nanobubbles. This also changes the z-distribution (localized concentration) of the MNFC fibrils in the web so that there will be more located in the upper layers, which reduces sheet sealing. This method allows for easier dewatering and producing samples with up to 25% MNFC content. Multilayer forming of sheet is suggested as a direct way of structuring. We show that the application of a very thin fibre layer (as thin as 5 gsm) on the screen has a significant effect on the dewatering rate. We also investigate the application of engineered fibers that provide desired functional properties and maintain dewatering properties. This material involves enhanced external fibrillation of fibers without generating excess flake fines and fiber fragments. The composition of this highly swelling material with parent pulp fibers provides low-density, highstrength paperboards. The work summarized in this thesis delivers new insights into dewatering challenges of novel paper/board grades containing nanomaterials and identifies potential routes to overcome these challenges.