Development of a CNFspinning process Håkansson K.M.O., Kvick M., Lundell F., Fall B.A., Prahl Wittberg, L., Wågberg, L. and Söderberg D. Wallenberg Wood Science Center KTH Royal Institute of Technology [email protected] 1 INTRODUCTION In present society, wood fibres find widespread use as pulp that is the commodity for paper, board and related products, and the strength of individual wood fibres is obvious from Figure 1, see [1]. Unfortunately, wood as such has several disadvantages: it takes carpenter skills to make products that utilize the potential of the material, and the resulting products demand a considerable amount of maintenance. This has caused mankind to disintegrate wood and use the constituents in different manners (particle boards, Masonite, paper and paperboard, viscose). All these materials are used extensively for various purposes. However, none of them reproduce the properties of the original wood fibre. We have developed a process that seem to be able to produce continuous and homogeneous cellulose fibres with a strength matching the fibres in trees, our present fibre is shown as shown in Figure 1. The produced fibre demonstrates bio-mimetic properties in terms of stress-at-break, elastic modulus and fibril alignment [2]. Figure 1. Specific ultimate strength versus specific Young’s modulus for a number of materials, respectively. Solid red dots represent measurements of cellulose pulp fibres extracted from wood while the open markers are fibre and films made of CNF. Filled stars represent the properties of the fibres prepared using the new process, where the four different colours belong to four different cases. The angle displayed next to some markers represent the CNF alignment, where available (where zero is in the direction of the fibre/filament axis). 2 RESULTS The development of the spinning process has been performed within the Wallenberg Wood Science Centre, which is a joint centre between KTH and Chalmers in Gothenburg. The process that has been implemented is shown in Figure 3, and is based on that a suspension of CNF in water is injected in the centre flow and focused by sheath flows from the sides. As a result, the central flow is accelerated and this acceleration causes the fibrils to align in the flow direction. This acceleration is achieved with minimum cross-stream shear, a very important aspect since it means that we achieve an ordered fibril structure without fibril rotation. This means that the aligned fibrils might be susceptible to self-assembly processes as previously observed for spider-mimetic silk [3]. It should be noted that in a nozzle, i.e. solid walls, the shear at the wall makes the particles rotate even if they are aligned in the mean and this rotation will hinder any self-assembly. The resulting gel-thread is ejected and dried to provide the final fibre. Figure 2. Illustration of continuity-controlled alignment followed by the phase transition from liquid dispersion to gel, induced by electrolytes or acid. The nanofibrils in the focused flow are illustrated as rods. The diffusion of Na+, from addition of NaCl in the focusing liquid, is illustrated with a blue tint. The rows of small images above and below the central image illustrate the hydrodynamic, molecular and electro-chemical processes involved. (I) Brownian diffusion (dashed arrows) affects the orientation of a single fibril, (II) hydrodynamically induced alignment (gray arrows) occurs during the acceleration/stretching, (III) Brownian diffusion continues to act after the stretching has ceased, (IV) Brownian diffusion is prevented by the transition to a gel. The lower row of small images illustrate how the electrostatic repulsion (the red area representing the Debye-length), decreases from (i) to (iv) as the Debye length is decreased with increasing Na+ concentration. Figure 3. The spinning process: Description of experimental setup. (a) and (b) Schematic drawing and photo of the flow cell, respectively. (c) Image of the focusing region of the channel, where the flow is directed downwards. Water is focusing an ink- water mix. (d) A schematic drawing of the flow focusing part. (e)-(g) Images of the ejected jet, where water is focusing CNF in e and a NaCl solution is used to focus CNF in f and g. A higher acceleration is used in g compared to f. The experiments show that he properties of the final fibre is controlled by the degree of orientation and it is obvious that the fibres would be even stronger if the alignment could be further improved, cf. Figure 1. However, it has been observed that it is difficult to increase alignment much further that seen in this, i.e. <35°. The main cause of this is believed to be entanglement/flocculation of the fibrils prior to acceleration, which should be controlled by the Brownian motion, the shear and the number nl3, where n is the number density and l is the fibril length. Furthermore, by modifying the flow inside and at the exit from the nozzle it is possible to alter the characteristics of the spun filament. An example can be found in Figure 4, where the filament has been forced to twist. This modification to the macro-scale structure of the filament alters the mechanical propertied as can be seen in Figure 5. On-going efforts are focused on making the spinning process more robust as well as exploring the possibilities for scaling-up of the process, which will allow for higher production rates. Figure 4. Image of a twisted filament. Figure 5. Properties of the twisted filament in comparison to the non-twisted filament. REFERENCES [1] D.H. Page, F. el Hosseiny, and K. Winkler. Behaviour of single wood fibres under axial tensile strain, Nature 229, 252– 253 (1971). [2] K.M.O. Håkansson, A.B. Fall, F. Lundell, S. Yu, C. Krywka, S. V. Roth, G. Santoro, M. Kvick, L. Prahl Wittberg, L. Wågberg, L. D. Söderberg, Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments, Nature Communications, vol. 5, article number: 4018 (2014). [3] S. Arcidiacono, C. M. Mello, M. Butler, E. Welsh, J. W. Soares, A. Allen, D. Ziegler, T. Laue and S. Chase, 2002 Aqueous processing and fiber spinning of recombinant spider silks. Mac-romolecules 35 (4), 1262–1266, (2002).