The underlying mechanism related to freezing-induced crosslinking of aldehyde-containing cellulose nanofibrils (CNFs) has been investigated, and the critical parameters behind this process have been identified. The aldehydes introduced by periodate oxidation allows for formation of hemiacetal bonds between the CNFs provided the fibrils are in sufficiently close contact before the water is removed. This is achieved during the freezing process where the cellulose components are initially separated, and the growth of ice crystals forces the CNFs to come into contact in the thin lamellae between the ice crystals. The crosslinked 3-D structure of the CNFs can subsequently be dried under ambient conditions after solvent exchange and still maintain a remarkably low density of 35 kg m⁻³, i.e. a porosity greater than 98%. A lower critical amount of aldehydes, 0.6 mmol g⁻¹, was found necessary in order to generate a crosslinked 3-D CNF structure of sufficient strength not to collapse during the ambient drying. The chemical stability of the 3-D structure can be further enhanced by converting the hemiacetals to acetals by treatment with an alcohol under acidic conditions.

The potential for large-scale applications of cellulose nanofibrils (CNFs) is limited by the high water content of the starting material, which leads to high transportation costs and undesirable environmental impact. However, drying of CNFs results in loss of their nanoscopic dimensions leading to deterioration of their unique inherent mechanical properties. Herein, thorough redispersion studies of both fundamental and applied nature have been conducted in order to evaluate the effect of charge, redispersing agent, and drying method. Freeze-dried CNF dispersions were successfully redispersed by either increasing the charge density or adding redispersing agents. The greatest effect on redispersibility was achieved with fractionated LignoBoost lignin as redispersing agent, and this is attributed to steric repulsion during water removal and reduced CNF adhesion. Furthermore, the results unexpectedly show that redispersion is easier when the CNFs are dried in the form of nanopapers. By using this approach, excellent redispersibility was achieved even without a redispersing agent. Nanopapers formed from the redispersed CNFs was found to have essentially the same mechanical properties as those made from never-dried CNFs. Hence, this work suggests solutions for making CNFs viable for large-scale application while maintaining their nanoscale dimensions and their ability to create nanopapers with excellent mechanical properties.

Article A comparative study of the properties of three nanofibrillated cellulose systems that have been produced at about the same energy consumption levels in the mechanical delamination step was published on August 1, 2016 in the journal Nordic Pulp & Paper Research Journal (volume 31, issue 3).