Water-redispersible nanofibrillated cellulose powder
Drying an aqueous suspension of conventional NFC will lead to the irreversible agglomeration of the fibrils due to the large number of hydrogen bonds formed between their hydroxyl groups. Therefore, NFC is usually shipped and further processed as aqueous suspensions, generating high costs and making the material susceptible to bacterial degradation. Thus, there is great interest in the preparation of water-redispersible NFC powder. Moreover, the effect of these powders on the mechanical performance in nanocomposites has not been investigated so far.
Civil and Mechanical Engineering
ture under successive stirring. The resulting powder formed a stable suspension for at least 20 hours when redispersed in water. Freeze-dried samples of the suspension were analyzed by SEM (Fig.1). The carboxymethylated NFC formed a network of fibrils with diameters below the micrometer level, providing evidence for the prevention of agglomeration during the drying process. Furthermore, as shown by DMA experiments (Fig.2), the storage modulus of a HPC film was similarly increased when compounded with 20% of carboxymethylated NFC powder (red, open circles) or carboxymethylated NFC suspension (red dots). In contrast, HPC composites prepared with untreated RBP suspension (blue dots) revealed a lower reinforcing effect, which was even less pronounced when dried RBP was used (blue open circles). HPC / NFC 20% (aq) HPC / NFC 20% (p)
HPC / RBP 20% (aq) HPC / RBP 20% (p)
Christian Eyholzer, Tanja Zimmermann, in collaboration with Kristiina Oksman, LTU (SE)
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Nanofibrillated Cellulose (NFC) has attracted great interest as a reinforcing component in nanocomposites. Water-redispersible NFC powder was prepared by a combination of carboxymethylation and mechanical disintegration of refined beech pulp. Scanning electron microscopy (SEM) analyses and sedimentation experiments confirmed that carboxymethylation was crucial to prevent irreversible agglomeration of the fibrils during drying. Consistently, dynamic mechanical analysis (DMA) experiments of hydroxypropyl cellulose (HPC) films showed an almost identical increase in storage modulus when compounded with a carboxymethylated NFC powder or a never dried suspension of carboxymethylated NFC. The study shows that carboxymethylated NFC powder can be used as an alternative to conventional NFC in aqueous suspensions.
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The main strategy proposed to overcome irreversible agglomeration during drying is the introduction of electrostatic groups on the surface of the fibrils. Following this approach, a refined beech pulp (RBP) was first carboxymethylated, then mechanically disintegrated and finally dried from an alcohol mix-
Fig. 2: Storage modulus versus temperature of HPC films compounded with untreated RBP and carboxymethylated NFC in powder form (p) or in suspension (aq).
Consequently, water-redispersible NFC powder can be used as a valuable alternative to aqueous NFC suspensions for the reinforcement of polymer matrices.
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Fig.1: Electron micrograph of a freeze-dried suspension of water-redispersed NFC powder.
Contact: christian.eyholzer@empa.ch Reference: Ch. Eyholzer, N. Bordeanu, F. Lopez-Suevos, D. Rentsch, T. Zimmermann, K. Oksman, Cellulose, 17(1), 19 –30 (2010)
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