Colloidal stability of NanoFibrillated Cellulose: Models, characterization and assembly of fibrils
Lars Wagberg
KTH, Stockholm, Sweden
17:15 - 18:15 Tuesday 21 October 2014 TUG P2 During the last 10-15 years there has been a renewed interest in the use of different types of nanocellulose in different materials and processes. Both Cellulose Nano Fibrils (CNF) and Cellulose Nano Crystals (CNC) have for example been been used in composites, batteries, supercapacitors and as a strength enhancing additives in the production of papers with a high filler content. This rapid development in the use of CNF and CNC has definitely shown that it is absolutely necessary to avoid an uncontrolled aggregation of these materials in order to utilize the excellent inherent properties of the cellulose components. This can be achieved through enhanced control of the colloidal interactions during mixing of the CNF/CNC and other composite components.
We have developed a non-complicated model based on the DLVO-theory to predict the association, i.e. aggregation or gelation, of the CNF and its dependence of salt concentration and pH. To correctly mimic the experimental results, it was necessary to introduce a specific counter-ion binding to theCNF. With this modelling frame, we have used both Dynamic Light Scattering (DLS) and rheology to study the gelation of CNF dispersions and how the CNF particles could be oriented by straining the NFC gel. It was also shown that it is necessary to form the gel in-situ to characterize the interaction between the fibrils in the gel and not the interaction between gel particles. The CNF gels were then used to prepare fibril-reinforced composites by a careful solvent exchange procedure and an in-situ polymerization within the so prepared fibril network.
To fully exploit the excellent stiffness of the CNF /CNC, a considerable effort has been devoted to create oriented structures of these cellulose nanoparticles. As expected, it was difficult to use self-assembly to orient the curved, kinked and slender CNF particles. To orient the shorter CNC, we used a nano-pattern created by strain-induced buckling of a crosslinked PDMS (PolyDiMethylSiloxane) polymer. With this technique, it is possible to create oriented threads and networks of CNC by using micro-contact printing with the nano-patterned surfaces containing the oriented CNC.
The present contribution is a review showing the importance of colloidal forces to the fibril–fibril interactions under dilute conditions and the importance of these interactions on the dry properties of CNF composites.
Figure: Demonstration of the gradual development of gelation of a CNF dispersion, containing acridine orange, following addition of a dilute HCl drop(a-c). The final CNF gel formed from a 1.5 g/l CNF dispersion.
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