Development of a CNF- spinning process

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).
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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
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