Environment

Wood pulp extract stronger than carbon fiber or Kevlar

Wood pulp extract stronger tha...
Underlying structure of the wall of a wood cell, showing the substructure of load-bearing cellulose microfibrils
Underlying structure of the wall of a wood cell, showing the substructure of load-bearing cellulose microfibrils
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Three-dimensional ball and stick model of the cellulose polymer
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Three-dimensional ball and stick model of the cellulose polymer
Micrographs of randomly oriented CNCs
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Micrographs of randomly oriented CNCs
The upper figure shows the structure of the cellulose polymer; the middle figure shows a nanofibril containing both crystalline and amorphous cellulose; the lower figure shows the cellulose nanocrystals after the amorphous cellulose is removed by acid hydrolysis
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The upper figure shows the structure of the cellulose polymer; the middle figure shows a nanofibril containing both crystalline and amorphous cellulose; the lower figure shows the cellulose nanocrystals after the amorphous cellulose is removed by acid hydrolysis
Cross-sectional structure of various types of cellulose nanocrystals showing various crystalline arrangements of the individual cellulose polymer molecules (the rectangular boxes)
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Cross-sectional structure of various types of cellulose nanocrystals showing various crystalline arrangements of the individual cellulose polymer molecules (the rectangular boxes)
Micrographs of cellulose fibers from wood pulp
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Micrographs of cellulose fibers from wood pulp
Cellulose fibers and the smaller structures within them - a) fiber from wood pulp; b) microcrystalline cellulose; c) microfibrils of cellulose; d) nanofibrils of cellulose; e) cellulose nanocrystals from wood pulp; f) CNCs from sea squirts (the only animal source of cellulose microfibrils); and g,h) cellulose nanofibrils from other sources
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Cellulose fibers and the smaller structures within them - a) fiber from wood pulp; b) microcrystalline cellulose; c) microfibrils of cellulose; d) nanofibrils of cellulose; e) cellulose nanocrystals from wood pulp; f) CNCs from sea squirts (the only animal source of cellulose microfibrils); and g,h) cellulose nanofibrils from other sources
Structure of a nanofibril of cellulose down to the locations of individual cellulose chains
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Structure of a nanofibril of cellulose down to the locations of individual cellulose chains
Micrograph of wood pulp derived CNCs
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Micrograph of wood pulp derived CNCs
Underlying structure of the wall of a wood cell, showing the substructure of load-bearing cellulose microfibrils
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Underlying structure of the wall of a wood cell, showing the substructure of load-bearing cellulose microfibrils
The figure shows the first stage of acid hydrolysis, which converts microcrystalline cellulose into cellulose nanofibrils
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The figure shows the first stage of acid hydrolysis, which converts microcrystalline cellulose into cellulose nanofibrils
Cellulose structures in trees from logs to molecules
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Cellulose structures in trees from logs to molecules
Four cellulose molecules held in a crystalline structure by intermolecular hydrogen bonds (dotted lines) (Image: I. Laghi)
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Four cellulose molecules held in a crystalline structure by intermolecular hydrogen bonds (dotted lines) (Image: I. Laghi)

The Forest Products Laboratory of the US Forest Service has opened a US$1.7 million pilot plant for the production of cellulose nanocrystals (CNC) from wood by-products materials such as wood chips and sawdust. Prepared properly, CNCs are stronger and stiffer than Kevlar or carbon fibers, so that putting CNC into composite materials results in high strength, low weight products. In addition, the cost of CNCs is less than ten percent of the cost of Kevlar fiber or carbon fiber. These qualities have attracted the interest of the military for use in lightweight armor and ballistic glass (CNCs are transparent), as well as companies in the automotive, aerospace, electronics, consumer products, and medical industries.

Cellulose is the most abundant biological polymer on the planet and it is found in the cell walls of plant and bacterial cells. Composed of long chains of glucose molecules, cellulose fibers are arranged in an intricate web that provides both structure and support for plant cells. The primary commercial source for cellulose is wood, which is essentially a network of cellulose fibers held together by a matrix of lignin, another natural polymer which is easily degraded and removed.

Cellulose structures in trees from logs to molecules
Cellulose structures in trees from logs to molecules

Wood pulp is produced in a variety of processes, all of which break down and wash away the lignin, leaving behind a suspension of cellulose fibers in water. A typical cellulose wood fiber is only tens of microns wide and about a millimeter long.

Micrographs of cellulose fibers from wood pulp
Micrographs of cellulose fibers from wood pulp

The cellulose in wood pulp, when dry, has the consistency of fluff or lint - a layer of wood pulp cellulose has mechanical properties reminiscent of a wet paper towel. Not what you might expect to be the source of one of the strongest materials known to Man. After all, paper is made from the cellulose in wood pulp, and doesn't show extraordinary strength or stiffness.

Cellulose fibers and the smaller structures within them - a) fiber from wood pulp; b) microcrystalline cellulose; c) microfibrils of cellulose; d) nanofibrils of cellulose; e) cellulose nanocrystals from wood pulp; f) CNCs from sea squirts (the only animal source of cellulose microfibrils); and g,h) cellulose nanofibrils from other sources
Cellulose fibers and the smaller structures within them - a) fiber from wood pulp; b) microcrystalline cellulose; c) microfibrils of cellulose; d) nanofibrils of cellulose; e) cellulose nanocrystals from wood pulp; f) CNCs from sea squirts (the only animal source of cellulose microfibrils); and g,h) cellulose nanofibrils from other sources

Further processing breaks the cellulose fibers down into nanofibrils, which are about a thousand times smaller than the fibers. In the nanofibrils, cellulose takes the form of three-dimensional stacks of unbranched, long strands of glucose molecules, which are held together by hydrogen bonding. While not being "real" chemical bonds, hydrogen bonds between cellulose molecules are rather strong, adding to the strength and stiffness of cellulose nanocrystals.

The upper figure shows the structure of the cellulose polymer; the middle figure shows a nanofibril containing both crystalline and amorphous cellulose; the lower figure shows the cellulose nanocrystals after the amorphous cellulose is removed by acid hydrolysis
The upper figure shows the structure of the cellulose polymer; the middle figure shows a nanofibril containing both crystalline and amorphous cellulose; the lower figure shows the cellulose nanocrystals after the amorphous cellulose is removed by acid hydrolysis

Within these nanofibrils are regions which are very well ordered, in which cellulose chains are closely packed in parallel with one another. Typically, several of these crystalline regions appear along a single nanofibril, and are separated by amorphous regions which do not exhibit a large degree of order. Individual cellulose nanocrystals are then produced by dissolving the amorphous regions using a strong acid.

At present the yield for separating CNCs from wood pulp is about 30 percent. There are prospects for minor improvements, but the limiting factor is the ratio of crystalline to amorphous cellulose in the source material. A near-term goal for the cost of CNCs is $10 per kilogram, but large-scale production should reduce that figure to one or two dollars a kilo.

Cross-sectional structure of various types of cellulose nanocrystals showing various crystalline arrangements of the individual cellulose polymer molecules (the rectangular boxes)
Cross-sectional structure of various types of cellulose nanocrystals showing various crystalline arrangements of the individual cellulose polymer molecules (the rectangular boxes)

CNCs separated from wood pulp are typically a fraction of a micron long and have a square cross-section a few nanometers on a side. Their bulk density is low at 1.6 g/cc, but they exhibit incredible strength. An elastic modulus of nearly 150 GPa, and a tensile strength of nearly 10 GPa. Here's how its strength to compares to some better-known materials:

  • Material...........................Elastic Modulus................Tensile Strength
  • CNC......................................150 GPa.............................7.5 GPa
  • Kevlar 49..............................125 GPa.............................3.5 GPa
  • Carbon fiber.........................150 GPa.............................3.5 GPa
  • Carbon nanotubes..............300 GPa............................20 GPa
  • Stainless steel.....................200 GPa............................0.5 GPa
  • Oak..........................................10 GPa.............................0.1 GPa

The only reinforcing material that is stronger than cellulose nanocrystals is a carbon nanotube, which costs about 100 times as much. Stainless steel is included solely as a comparison to conventional materials. The relatively very low strength and modulus of oak points out how much the structure of a composite material can degrade the mechanical properties of reinforcing materials.
As with most things, cellulose nanocrystals are not a perfect material. Their greatest nemesis is water. Cellulose is not soluble in water, nor does it depolymerize. The ether bonds between the glucose units of the cellulose molecule are not easily broken apart, requiring strong acids to enable cleavage reactions.

The hydrogen bonds between the cellulose molecules are also too strong in aggregate to be broken by encroaching water molecules. Indeed, crystalline cellulose requires treatment by water at 320° C and 250 atmospheres of pressure before enough water intercalates between the cellulose molecules to cause them to become amorphous in structure. The cellulose is still not soluble, just disordered from their near-perfect stacking in the crystalline structure.

But cellulose contains hydroxyl (OH) groups which protrude laterally along the cellulose molecule. These can form hydrogen bonds with water molecules, resulting in cellulose being hydrophilic (a drop of water will tend to spread across the cellulose surface). Given enough water, cellulose will become engorged with water, swelling to nearly double its dry volume.

Swelling introduces a large number of nano-defects in the cellulose structure. Although there is little swelling of a single CNC, water can penetrate into amorphous cellulose with ease, pushing apart the individual cellulose molecules in those regions. In addition, the bonds and interfaces between neighboring CNC will be disrupted, thereby significantly reducing the strength of any material reinforced with CNCs. To make matters worse, water can move easily over the surface/interfaces of the CNCs, thereby allowing water to penetrate far into a composite containing CNCs.

There are several approaches to make CNC composite materials viable choices for real world applications. The simplest, but most limited, is to choose applications in which the composite will not be exposed to water. Another is to alter the surface chemistry of the cellulose so that it becomes hydrophobic, or water-repelling. This is easy enough to do, but will likely substantially degrade the mechanical properties of the altered CNCs. A third approach is to choose a matrix material which is hydrophobic, and preferably that forms a hydrophobic interface with CNCs. While not particularly difficult from a purely chemical viewpoint, there is the practical difficulty that interfaces between hydrophobic and hydrophilic materials are usually severely lacking in strength.

Perhaps the most practical approach will simply be to paint or otherwise coat CNC composite materials in some material that keeps water away. For such a prize - inexpensive strong and rigid materials - we can be sure that innovations will follow to make the theoretical practical.

Source: US Forest Service

38 comments
Justin StGermain
Is it stronger than hemp?
Gene Jordan
Justin beat me to that question. They really need to apply this research to hemp, if they haven't already.
Frank191
CNCs have been studied for many years in Canada and are already produced at the ton scale in a pilot plant. Wonderful product. Its affinity with water is not a problem since it is easy to fluorinate or acetylate the surface. This modification, contrarily to what is pretended in this article, does not alter the properties of the CNCs since the reaction is limited to the surface of the product and the vast majority of the CNC remains unaltered (of course, if it is in a very fine powder form, then it's different)
nutcase
How many trees must be felled to produce enough CNC equivalent to one tree's timber?
Pikeman
Water is easy to protect against but for transparent applications how well does it stand up to UV? .................................................................................. re; nutcase You are making several assumptions without evidence. 1. That the harvested trees are good for lumber. 2. That all the wood from trees harvested for lumber is to make boards out of. 3. That harvesting trees is any worse for the environment than harvesting any other crop. 4. That the harvesting of trees and processing then for CNC even comes close to the environmental cost of producing other composite material.
Randy Noseworthy
I'd like to see something made from this stuff. Who, when, and where is it being used in a production environment?
Harpal Sahota
At 10% of the cost of carbon fibre, and double the tenstile strength, this new material made from the by product of wood, is the best alternative to building cars, planes, ships, trains, glass, speaker materials, ... well anything to be honest! My only concern is its weight??? ... how does it compare to the alternatives available? Its this kind of technologies that will help the states to get out of recession, but, not if the technology is solely owned by, and funded by the military establishment. I have never understood why developers get enticed by initial funding, only later to realise they have no control on the very product they passionately wanted to change the world for the better. Still, that is life, death, what ever one wants to resonate with. Peace!
Fretting Freddy the Ferret pressing the Fret
I like it. It's a high performance renewable material and cheap.
Rob Robinson
I agree with Justin and Gene..... Has anyone thought of Hemp? Seems to me that Hemp would be a more sustainable source of cellulose than trees....and aren't we trying to save the trees?
Joel Joines
I'm also interested in knowing why trees were chosen as the source of the cellulose rathar than a faster growing plant. Kudzu comes to mind. I've seen this stuff take over an entire field in a single summer. If it could provide a reasonable quantity of cellulose per pound it could out perform the slower growing trees.