Cellulose nanofibrils (CNF) are renewable and biodegradable material with high mechanical strength and specific surface area. CNF has a width below 100 nm and the length is several micrometers. It is necessary to remove water from CNF suspension by dewatering and drying to utilize as a value-added product because CNF suspension has a lot of water. By dewatering and drying, the CNF can be converted to porous products. It is crucial to understand and control the nanostructure of porous CNF material. In this study, porous CNF sheet was prepared and its characteristics depending on the physicochemical properties of fiber and suspension, and drying condition were investigated. In the first, the effects of preparation conditions such as nanofibrillation degree of fibers, sheet forming condition, drying condition on the structural properties of CNF sheet were investigated. In the second, the properties of CNF suspension and CNF sheet were investigated depending on the degree of flocculation or dispersion of CNF by changing the electrical charge or surface tension of suspension. Further, the potential application of the porous CNF sheet as an air filter media was examined.
The CNF sheet with low density and high porosity could be prepared by supercritical drying, solvent exchange drying, and freeze drying due to the inhibition of hydrogen bonds and a low shrinkage during drying. The room temperature drying and hot pressing produced a very dense structure. Porosity of the CNF sheets was affected by the degree of nanofibrillation and dewatering as well as drying condition. The CNF sheet consisted of mainly mesopores in BET analysis. The CNF sheets showed a high tensile strain at break.
The degree of flocculation or dispersion of CNF was varied by controlling the electrical charge of CNF or surface tension of suspension. The electrical charge of CNF was controlled by chemical treatment (carboxymethylation and TEMPO-mediated oxidation) and chemical additive (salt), and their effects on the characteristics of CNF suspension and sheet were investigated. Carboxymethylation was carried out on pulp fibers as a pre-treatment before preparation of CNF. The gel-like and translucent CNF hydrogel was obtained. The viscosity of CNF suspension was increased by carboxymethylation due to the increased electrostatic repulsion, and the drainage rate was decreased significantly. The freeze dried sheet showed higher transparency than untreated CNF sheet. TEMPO-mediated oxidation was carried out as a post-treatment of CNF. The density of sheet made of TEMPO-oxidized CNF was increased with the amount of oxidizing agent. The flocculation of CNF was caused by the addition of salt because the degree of compression of electrical double layer changed by adsorption of cations onto CNF, which depends on the type and concentration of salt. Viscosity, storage modulus, and yield stress of the CNF suspension increased with an increase in ionic strength of suspension, showing more solid-like behavior. Bivalent cations were more effective for increasing network strength and dewatering at a small addition amount of salt compared to monovalent cations. The flocculation degree of nanofibrils affected the porous structure of CNF sheet. The surface tension of suspension was changed by the addition of surfactant. The total drained water content and drainage rate were increased with the decreased in surface tension of water. The highly porous CNF sheet was produced by decreasing the surface tension of water, showing the maximum porosity of 91%.
To investigate the applicability of porous CNF sheet as a filter media, the porous sheet with high mechanical strength was prepared with natural pulp fibers or polyethylene terephthalate (PET) by adding CNF, and its characteristics were evaluated. The apparent density and porosity of porous sheets changed with relation to the morphology of the mixed fibers and drying methods. The tensile properties of the sheets were improved by the addition of CNF. The applicability of porous sheets as a filter media was examined by evaluating the filtration efficiency and pressure drop. The filtration efficiency of CNF-contained sheet greater than 99.97% was achieved by supercritical drying. The high pressure drop during filtration would be solved by surface treatment and controlling the fiber dimension. The porous CNF sheet is expected to be used as substrate for nanocomposite, water membrane filter as well as air filter media depending on its pore characteristics.