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Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
DESY, Hamburg Germany.
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2014 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, 4018- p.Article in journal (Refereed) Published
Abstract [en]

Cellulose nanofibrils can be obtained from trees and have considerable potential as a building block for biobased materials. In order to achieve good properties of these materials, the nanostructure must be controlled. Here we present a process combining hydrodynamic alignment with a dispersion-gel transition that produces homogeneous and smooth filaments from a low-concentration dispersion of cellulose nanofibrils in water. The preferential fibril orientation along the filament direction can be controlled by the process parameters. The specific ultimate strength is considerably higher than previously reported filaments made of cellulose nanofibrils. The strength is even in line with the strongest cellulose pulp fibres extracted from wood with the same degree of fibril alignment. Successful nanoscale alignment before gelation demands a proper separation of the timescales involved. Somewhat surprisingly, the device must not be too small if this is to be achieved.

Place, publisher, year, edition, pages
2014. Vol. 5, 4018- p.
Keyword [en]
Current International Research, Wood Cell-Walls, Rotational Diffusion, Microfibril Angle, Fibers, Flow, Nanopaper, Nanocomposites, Birefringence, Microchannels
National Category
Chemical Sciences Fluid Mechanics and Acoustics Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-133940DOI: 10.1038/ncomms5018ISI: 000338836700002Scopus ID: 2-s2.0-84901950560OAI: oai:DiVA.org:kth-133940DiVA: diva2:664073
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20140812. Updated from manuscript to article in journal.

Available from: 2013-11-13 Created: 2013-11-13 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Colloidal interactions and orientation of nanocellulose particles
Open this publication in new window or tab >>Colloidal interactions and orientation of nanocellulose particles
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanoparticles are very interesting building blocks. Their large surface-to-bulk ratio gives them different properties from those of larger particles. Controlling their assembly can greatly affect macroscopic material properties. This often happens in nature, resulting in macroscopic materials with properties far better than those of similar human-made materials. However, in this fast-growing research field, we may soon compete with nature in certain areas. This thesis demonstrates that the distribution and orientation of nanocellulose particles can be controlled, which is crucial for many applications.

Nanocellulose is an interesting nanoparticle, for example, because of its high strength, low thermal expansion, and high crystallinity. Nanocellulose particles are called nanofibrillated cellulose (NFC) or cellulose nanocrystals (CNCs). NFC is obtained from wood by mechanically shearing apart fibrils from the fiber wall and to obtain CNCs, parts of the cellulose are broken down by hydrolytic acidic reactions, most commonly, prior to homogenization. NFC particles are longer and less crystalline than are CNCs, but both are similar in width. The particles attract each other in aqueous dispersions and have a high aspect ratio and, thus, a large tendency to aggregate. The rate at which this occurs is typically reduced by charging the particles, generating an electrostatic repulsion between them.

To fully utilize the many interesting properties of nanocellulose, the aggregation and orientation of the particles have to be controlled; examining this delicate task is the objective of this thesis. The limits for particle stability and aggregation are examined in papers 2–3 (as well as in this thesis) and orientation of the particles is investigated in papers 3–5. In addition, the liberation of the nanoparticles from different types of wood fibers is studied in papers 1 and 2.

It was found that the liberation yield improved with increased fiber charge. In addition, the charge of the fibrils is higher than the charge of the original fibers, indicating that the fibrils were liberated from highly charged parts of the fibers and that the low-charge fraction was removed during processing.

Aggregation was both theoretically predicted and experimentally studied. A theoretical model was formulated based on Derjaguin–Landau–Verwey–Overbeek theory, which is intended to predict the influence of salt, pH, and particle charge on the colloidal stability of the NFC. To predict the experimental trends, specific interactions between salt counterions and the particles charges had to be included in the model, which greatly increased the effect of salt on the NFC stability. Below the particle overlap concentration, instability induced by pH or salt created small sedimenting flocs, whereas above the overlap concentration the system gelled. Increasing the particle concentration further also gels the system.

Orientation of nanocellulose was first achieved by shearing, salt- or acid-induced NFC gels. This oriented the fibrils and increased the gel modulus in the direction of shear. The orientation persisted after the shear strain was released and did not cause breakdown of the macroscopic gel. The orientation is probably due to rotation in the interfibril crosslinks, which is possible because the crosslinks are physical, not covalent.

     Second, orientation was also induced by elongational flow. Shear and acceleration forces were combined to align fibrils in the direction of the flow. The orientation was then frozen by gelation (adding salt or reducing the pH). Drying the gel threads created filaments of aligned fibrils with a higher specific strength than that of steel.

     Finally, CNC particles could be aligned on flat surfaces. The particles were first forced to align due to geometrical constraints in grooves on a nanowrinkled surface. The CNCs were then transferred to a flat surface using a contact-printing process. This created surfaces with lines of highly aligned CNCs, where the line–line spacing was controlled with nanometer precision.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. vi, 49 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:47
Keyword
NFC, CNC, CNF, MFC nanocellulose, colloidal stability, orientation
National Category
Chemical Sciences Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-133941 (URN)978-91-7501-912-3 (ISBN)
Public defence
2013-12-06, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20131114

Available from: 2013-11-14 Created: 2013-11-13 Last updated: 2013-11-14Bibliographically approved
2. Orientation of elongated, macro and nano-sized particles in macroscopic flows
Open this publication in new window or tab >>Orientation of elongated, macro and nano-sized particles in macroscopic flows
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Non-spherical particles are present all around us, in biological, industrial and environmental processes. Making predictions of their impact on us and systems in our vicinity can make life better for everyone here on earth. For example, the ash particles from a volcano eruption are non-spherical and their spreading in the atmosphere can hugely impact the air traffic, as was also proven in 2010. Furthermore, the orientation of the wood fibres in a paper sheet influences the final properties of the paper, and the cause of a specific fibre orientation can be traced back to the fluid flows during the manufacturing process of the paper.

In this thesis, experimental and numerical work is presented with the goal to understand and utilize the behavior of elongated particles in fluid flows. Two different experimental setups are used. The first one, a turbulent half channel flow, aims at increasing the understanding of how particles with non-zero inertia behave in turbulence. The second setup is an attempt to design a flow field with the purpose to align nanofibrils and create high performance cellulose filaments.

Experiments were performed in a turbulent half channel flow at different flow set- tings with dilute suspensions of cellulose acetate fibres having three different aspect ratios (length to width ratio). The two main results were firstly that the fibres agglom- erated in streamwise streaks, believed to be due to the turbulent velocity structures in the flow. Secondly, the orientation of the fibres was observed to be determined by the aspect ratio and the mean shear, not the turbulence. Short fibres were oriented in the spanwise direction while long fibres were oriented in the streamwise direction.

In order to utilize the impressive properties (stiffness comparable to Kevlar) of the cellulose nanofibril in a macroscopic material, the alignment of the fibrils must be controlled. Here, a flow focusing device (resulting in an extensional flow), designed to align the fibrils, is used to create a cellulose filament with aligned fibrils. The principle is based on a separation of the alignment and the assembly of the fibrils, i.e. first align the fibrils and then lock the aligned structure. With this process, continuous filaments were created, with properties similar to that of the wood fibre at the same fibril alignment. However, the highest alignment (lowest angle) of the fibrils in a filament created was only 31o from the filament axis, and the next step is to increase the alignment. This thesis includes modeling of the alignment process with the Smoluchowski equation and a rotary diffusion. Finding a model that correctly describes the alignment process should in the end make it possible to create a filament with fully aligned fibrils.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xiii, 81 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014.19
Keyword
Orientation, fibre, fibril, turbulent channel flow, particle streaks, flow focusing, cellulose nanofibrils, extensional flow, Smoluchowski, rotary diffusion
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-150493 (URN)978-91-7595-254-3 (ISBN)
Public defence
2014-09-25, Kollegiesalen, Brinellvägen 8, KTKH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20140908

Available from: 2014-09-08 Created: 2014-09-04 Last updated: 2014-09-08Bibliographically approved

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Prahl Wittberg, LisaWågberg, LarsSöderberg, Daniel

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