Investigation of the effect of complex flow fields on cellulose nanofibril suspensions by birefringence imaging

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Examensarbete för masterexamen
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2020
Författare
Karlsson, Sara-Louise
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Cellulose is an interesting renewable material with many unique properties. A type of nanocellulose is cellulose nanofibrils (CNF). It has potential to be used as reinforcement in composites, to modify the rheology of complex systems in 3D printing and in fiber production, among other areas. They are relatively long and flexible particles with both crystalline and amorphous parts. This material can be used to make stronger materials by increasing the alignment of the fibrils. In all of these cases, it is crucial to understand the behavior of the material in flow, which is the focus of this thesis. The aim with this project is to study how the alignment in cellulose nanofibril suspensions is affected in different flow types (extensional, shear and two mixed flow types), how it changes with different flow rates (1 - 50 μL/s) and if it is affected by the concentration of the suspensions (0.1, 0.25 and 0.5 wt%). To achieve the different complex flow fields, a fluidic four-roll mill was used. The alignment of the nanofibers were then determined by a birefringence imager which measured the retardance, angle of the fast axis and the intensity. The results showed that the used setup with the fluidic four-roll mill and birefringence imager can successfully measure the alignment in CNF suspensions, and that the experiments can be performed continuously with any one wavelength. Dependence on the concentration of the suspensions, flow rate and flow type was found. The alignment increased in extensional flow with increasing concentration, but not in shear flow. The flow rate dependence showed that the alignment increased with the flow rate for the highest concentration (0.5 wt%), but did not have much effect on the lowest concentration (0.1 wt%). It was found that extensional flow achieved more alignment than shear flow in the two higher concentrations of suspensions, but that the flow type had less effect in the lowest concentration. An unexpected dependency on the age of the sample and the assembly of the device was also observed.
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cellulose nanofibrils, fluidic four-roll mill, birefringence imager, alignment
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