Strong Cellulose Filaments
July 2, 2014
Wood has always been a useful structural material; but, as the fable of the Three Little Pigs asserts, its strength is somewhat limited. A house built of sticks proved to be stronger than a house built of straw, but not as strong as a house built of brick. This fable is still an object lesson, when we replace the wolf's huffing and puffing by hurricane-strength winds.
As most people realize, wood has a texture, or grain, along its growth direction. Wood, as harvested, has different mechanical properties in different directions. That's because its component material, cellulose, is a long, straight polymer strand, bonded together by hydrogen, as I discussed in a recent article (Cellulosic Carbon, April 30, 2014). Since the mechanical strength for forces longitudinal to the grain is larger than the strength in the transverse direction, trees sway in the wind but maintain their length.
In the mid-nineteenth century, Immanuel Nobel (1801-1872), who was the father of Alfred Nobel of the eponymous Nobel Prize, realized that a strong laminated board could be made from several thin layers of wood bonded together in a cross-grain arrangement. He invented a lathe that peeled sheets of wood from logs to make such laminates. Plywood manufacturers produce wood sheets with high strength by laminating the wood so that alternate layers have their grain aligned perpendicular to each other.
Such materials engineering of wood on a macroscopic scale level is so successful that its encouraged scientists to attempt improvements at the nanoscopic level. Cellulose nanocrystals are straight, needle-shaped, about 3 nanometers wide by 500 nanometers long, and their mechanical properties are highly anisotropic. They have an estimated Young's modulus along the  (c-axis) of 206 GPa, which is comparable to that of steel, while their Young's modulus along  (a-axis) is estimated to be only 19 GPa.
Recent research in combining nanoscale cellulose strands into filaments has been undertaken by a team of scientists from the KTH Royal Institute of Technology ( KTH, Stockholm, Sweden), the Deutsches Elektronen-Synchrotron elektronsynkrotronen (DESY, Hamburg, Germany), the University of Kiel (Kiel, Germany), the Helmholtz-Zentrum Geesthacht, Institute for Materials Research (Geesthacht, Germany) and Innventia AB (Stockholm, Sweden). They've developed a process that combines a hydrodynamic alignment of the cellulose nanofibrils with a dispersion–gel transition to produce homogeneous, smooth filaments from a low-concentration dispersion of cellulose nanofibrils in water.[3-4]
Cellulose nanofibrils are easily obtained from trees, for which each single cellulose fiber is composed of tens of millions smaller fibers, called "fibrils. There are several ways to separate these from the fibers, and the research team developed a process to rebind these fibrils together into new fibers with controlled mechanical properties. Says study coauthor, Fredrik Lundell, an associate professor of mechanics at KTH,
"We have taken out fibrils from natural cellulose fibers... then we have assembled fibrils again into very strong filament. It is about 10 to 20 microns thick, much like a strand of hair."
The research team was able to produce such fibers with a specific ultimate strength considerably higher than previously reported cellulose filaments. Specific strength is the strength normalized to a per weight basis. On such a per weight basis, these fibers can be stronger than steel or aluminum. Such strength arises from the process used in making the filaments, which achieves an alignment of the nanofibrils on a nanoscale.
Some representative mechanical properties of cellulose filaments for several different processing cases are shown in the following table.
The developed process for producing these filaments is also environmentally friendly. Aside from water and cellulose, there's just table salt (sodium chloride), used as a means of binding the nanofibrils together. Says L. Daniel Söderberg, an adjunct professor of mechanics at KTH and a study coauthors, remarks that "...the material is 100 percent compatible with nature... Cows eat cellulose. Likewise, dead trees and plants are broken down by natural processes."
The material is environmentally friendly in another way. Since the mechanical properties of the filaments can be controlled, they can be made flexible. When the fibrils are perfectly aligned in the thread, there's high strength, but misalignment allows flexibility. Trees do this naturally, letting some parts, such as branches, to bend in the wind rather than breaking. Flexible cellulose filaments could substitute for cotton in clothing.
In a world where we've reached peak oil and possible peaks in the production of various other raw materials, some believe that we're close to reaching peak cultivation of cotton. Additionally, says Lundell,
"Cotton cultivation requires large amounts of water... Take for example the Aral Sea, which more or less disappeared as a result of the cultivation of cotton in Asia. If we are to have a 100 percent sustainable society then we need more materials that have a natural place in the natural cycle."
Considerable variation of the process is imagined, such as adding carbon nanotubes to allow for electrical conductivity. These filaments might also be used to replace glass fibers in fiberglass. The next focus of the team's research is to scale the process to higher volume production. This research was mostly done at the Wallenberg Wood Science Center at KTH, with cooperation by DESY in Hamburg, Germany.
- Fernando L. Dri, Louis G. Hector Jr., Robert J. Moon and Pablo D. Zavattieri, "Anisotropy of the Elastic Properties of Crystalline Cellulose Iβ from First Principles Density Functional Theory with Van der Waals Interactions," Cellulose, vol. 20, no. 6 (December 2013), pp 2703-2718 .
- Cellulose nanocrystals possible 'green' wonder material, Purdue University Press Release, December 16, 2013.
- Karl M. O. Håkansson, Andreas B. Fall, Fredrik Lundell, Shun Yu, Christina Krywka, Stephan V. Roth, Gonzalo Santoro, Mathias Kvick, Lisa Prahl Wittberg, Lars Wågberg and L. Daniel Söderberg, "Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments," Nat. Communications, vol. 5 (June 2, 2014), Article No. 4018, doi: 10.1038/ncomms5018. This is an open access paper with a PDF file available, here.
- Peter Larsson, "Stronger-than-steel fibre spun from wood," KTH Royal Institute of Technology Press Release, June 9, 2014.
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