Process optimization of cellulose fibril production: The effect of process medium composition on energy efficiency and product quality

Cellulosic nanomaterials are a new family of renewable biomaterials that have the potential to widely expand the application range of cellulose fibres. For this reason, they have been under extensive research over the last decade. The purpose of this Master's thesis was to provide an outlook on the process optimization of mechanical processing of cellulose nanofibrils, focusing on the composition of the process medium. More precisely, the effects of the process medium on the energy efficiency and product quality of cellulose fibril production were studied. The effect of the process medium was studied by comparing the fibrillation of never-dried birch kraft pulp dispersed in reverse osmosis water, tap water and a 5 % dose of a green additive. Three additives were tested: glycerol, a Prosoft debonder solution and a choline chloride - glycerol (1:2) deep eutectic solvent (DES). Prior to processing, the pulp was ion-exchanged to the sodium counter-ion form with optimal swelling and fibrillation conditions. A Masuko friction grinder was used for the fibrillation. To assess the quality of the fibrillated materials, their properties were analysed with a combination of characterization methods chosen from literature. The results of the characterization methods were presented as a function of specific net energy consumption to illustrate the relation between energy efficiency and product quality for each sample. The results from the characterization methods consistently demonstrated that reverse osmosis water provided the best fibrillation results, especially at lower energy levels. The effect of glycerol was negligible, while the divalent cations in tap water disrupted the swelling of fibres and their subsequent fibrillation. The deep eutectic solvent behaved similarly as tap water, implying it does not retain its complex form in aqueous solutions but reverts to a halide salt and glycerol, the former of which disrupts the fibrillation similarly as the ions in tap water. The fibres dispersed in the debonder solution showed the weakest results overall, and their behaviour suggests the fibrils form flocks around the cationic polymers of the debonder solution, heavily disrupting their fibrillation and the formation of an interfibrillar network. Finally, the quality parameters of the fibrils peak between net energy levels 2 and 3 kWh/kg for all mediums except the debonder solution. This indicates that prolonged grinding reduces the aspect ratio of the fibrils, resulting in weakening of the fibril network.