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Wood pulp extract stronger than carbon fiber or Kevlar

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September 3, 2012

Underlying structure of the wall of a wood cell, showing the substructure of load-bearing ...

Underlying structure of the wall of a wood cell, showing the substructure of load-bearing cellulose microfibrils

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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) micr...

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 n...

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 cryst...

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

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer.   All articles by Brian Dodson
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37 Comments

Is it stronger than hemp?

Justin StGermain
3rd September, 2012 @ 03:19 am PDT

Justin beat me to that question. They really need to apply this research to hemp, if they haven't already.

Gene Jordan
3rd September, 2012 @ 04:59 am PDT

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)

Frank191
3rd September, 2012 @ 07:58 pm PDT

How many trees must be felled to produce enough CNC equivalent to one tree's timber?

nutcase
3rd September, 2012 @ 10:25 pm PDT

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.

Pikeman
4th September, 2012 @ 02:48 am PDT

I'd like to see something made from this stuff. Who, when, and where is it being used in a production environment?

Randy Noseworthy
4th September, 2012 @ 03:53 am PDT

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!

Harpal Sahota
4th September, 2012 @ 04:10 am PDT

I like it. It's a high performance renewable material and cheap.

Fretting Freddy the Ferret pressing the Fret
4th September, 2012 @ 04:45 am PDT

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?

Rob Robinson
4th September, 2012 @ 05:26 am PDT

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.

Joel Joines
4th September, 2012 @ 07:01 am PDT

Trees were chosen because there is a LARGE amount of "wood byproduct" produced by our lumber industry....it's material that would otherwise be burned or otherwise unusable. It's RECYCLING, folks.

Oh, and I want a clear CNC reinforced kayak ASAP, please. :)

Bryan Paschke
4th September, 2012 @ 09:48 am PDT

The figures cited for carbon fibers are way too low. A run of the mill standard modulus carbon fibre like Torayca T700 has an elastic modulus of 230 GPa, and a tensile strength of 4.9 GPa. See the data-sheet: http://www.toraycfa.com/pdfs/T700SDataSheet.pdf

A high-strength carbon fiber like Torayca T1000G has a modulus of 294 GPa and a strength of 6.37 GPa: http://www.toraycfa.com/pdfs/T1000GDataSheet.pdf

Ultra high modulus carbon fibers like Dialead K63A12 can reach a elastic modulus of 790 GPa, albeit at the price of a lower tensile strength of 2.6 GPa: http://www.mpi.co.jp/english/products/industrial_materials/pitch_based_carbon_fiber/pbcf001.html#data3

@Justin: the strength of hemp fibers varies a lot depending on growth conditions and treatment, between 0.8 and 1.5 GPa; http://www.risoe.dtu.dk/rispubl/BIO/biopdf/ris-phd-11.pdf

Roland Smith
4th September, 2012 @ 10:22 am PDT

I fell for this once before and was an early adopter of LP Siding.

The fatal flaw in this material is capenter ants. They eat cellulose like it's candy.

solutions4circuits
4th September, 2012 @ 10:28 am PDT

I would like to see answers posted to the thoughtful questions raised by the Commenters.

Barry Dennis
4th September, 2012 @ 10:40 am PDT

I have been privileged to have seen a similar system in action and we plan to use it in our transformational method of construction that is about to be introduced.

This relatively new system uses any agricultural waste, be it trees, grass, corn/rice husks, etc. to create structural members that are accurate to 0.002 inches and are 2 to 6 times stronger than the best Douglas Fir that we used to be able to buy. One of my clients/investor/board members owns some of the prime Douglas Fir in the nation. He determined that his company will get a greater return from this process than from making plywood and 2x4s from a tree.

The military does not control it but uses it. I have seen the bullet/mortar/grenade resistant 4x8 sheets that look like ordinary chip board that have been used in testing. Without the dents being pointed out, I defy you to find which boards have been used in testing. They line the Army's current tents and that is why the suicide bombers have to get inside the tents to do any damage.

The Coast Guard is using the system successfully as bulkheads in ports with freezing salt water as well as boats pounding them year round.

And we will use the system to create all the structural members, including the foundation in our transformational method of construction that HUD official project will do to construction what Henry Ford did to the automobile.

technotard

technotard
4th September, 2012 @ 11:07 am PDT

When talking about trees most people above are missing the point. The Forest Industry is well and healthy, with millions of acres harvested properly all over. There is tones of by-products full of celulose content (sawmill dust and scraps) that could find good use with this new use. Before promoting Hemp or any other plant into fiber production, use wood by-products and keep concern on how to produce food instead in any other land still suitable.

Max Fdez
4th September, 2012 @ 11:12 am PDT

1. Is this new wood pulp extract curently being produced in mass quantities?

2. If yes, Is it currently being used by manufacturers?

3. How eco friendly is it?

4.While it is stronger, will products also be more durable and light wieght?

5. How soon will the new product be used in afforable automobiles?

Gargamoth
4th September, 2012 @ 12:13 pm PDT

One more reason to encourage people to plant threes, not Brassica napus oleifera

Iosif Eugen Olimpiu
4th September, 2012 @ 12:29 pm PDT

To those concerned about trees - we are fast approaching what I believe will be a huge problem - too many old trees in cities where they must be cut down to protect buildings. Just imagine how many thousands of trees were planted thru the late40's, 50's and 60's. Many of these trees are at the end of their life cycle. Technology such as this would be a great way to make good use of all that pulp. Remember, trees take in carbon, but release it when they decompose (or burn). Using it for mulch might be good, but all the carbon they've been storing is now going back into the atmosphere. Google "carbon cycle".

Old J Hawthorne
4th September, 2012 @ 12:48 pm PDT

OK, so recycle the wood waste products but don't rule out hemp and or kudzu as CNC source material. It seems that many woody or pulpy fibrous vegetative material would possibly work.

Terry Foy
4th September, 2012 @ 01:17 pm PDT

What is the size of this stuff? Is it used as a long continuous fiber like carbon fiber, or is it a tiny short fiber like carbon nanotubes? Is it something that can be wound or braided or is it something that can be injection molded? Where is it available?

also I like nutcase's question that pikeman didn't answer. but, i imagine that the product of this material is not directly comparable to standard lumber.

why are there no pictures at all of the CNC stuff on a macro scale? no products? no full size material that we can make sense of? does the stuff exist? or is this just an article to create hype?

Abe Allen
4th September, 2012 @ 03:00 pm PDT

Do we have to worry about termites?

Raja Dharani Singh
4th September, 2012 @ 03:43 pm PDT

Has work been done it's applicability to 3D printing - either mixed into other materials or as a printed skeleton in which other material can be embedded or maybe even as a stand-alone material. Are there any references to 3D printing with CNC materials.

rf_in_the_bush
4th September, 2012 @ 06:12 pm PDT

I agree that if this material is truly easy to obtain and as wonderful as indicated that we should devote our wood waste to it, but I further believe that we should actively seek the same crystals in other cellulose waste such as grass clippings, other forms of yard waste, farm waste.

I'd love to know if bamboo would work for producing these nano crystals. Bamboo grows quickly and is already a commodity traded around the world. I know it's growth is slower than kudzu but not by much and you can cut bamboo at ground level and a new plant will be there next growing season, making it highly renewable.

Overall I would say that any source material that exists and is cheap to produce, such as waste products from current production of food, lumber, lawn care etc should be viewed as an asset unless they are proven to be too difficult to harvest those wonder crystals from.

rpjacks
5th September, 2012 @ 06:10 am PDT

"The cellulose in wood pulp..."

Was this product design or accident?

donwine
5th September, 2012 @ 01:59 pm PDT

CNC + Liquipel technology = success!

ou812
9th September, 2012 @ 11:10 am PDT

To recycle these new products, will engineers use carpenter ants and termites or will we need to engineer a new animal?

Gargamoth
8th November, 2012 @ 05:17 pm PST

@gene

Damn straight ... cannabis ruderalis = 80% cellulose, lumber = 30% hmmm....

Funny how the cotton and forestry industry played a major role in the prohibition of marijuana isn't it?

bigL
2nd December, 2012 @ 07:27 pm PST

Can someone explain to me how the crystalline silica is extracted from this process? There is mention of amorphous and crystalline 'cellulose' in this article, is this referring to amorphous/crystalline silica or will the cellulose contain it? Is it the crystalline silica in the 'crystalline nano cellulose' that is giving it this strength? It is well documented that natural wood and also wood composite boards contain crystalline silica (silica is not inert by the way), some more than others such as oak and teak to name a couple. Wood dust per se is a group 1 carcinogen classified by the IARC as such in 1995. However, it is well documented that they do not know which element it is of wood dust from either wood or wood composite board (the dust from both are treated equally as carcinogenic) that renders it carcinogenic. So how is the carcinogenic element of the wood they use in this process extracted to ensure it's safe or has this not been considered? Therefore, how can this process where any elements are extracted from wood or wood composite board be safe, as it is not known whether or not the part they are extracting for CNC is the unsafe/carcinogenic element? Crystalline silica is also a Group 1 carcinogen. What concerns me is the dust that will be emitted particularly when machined which will undoubtedly be harmful to health. If the c/silica for example is not extracted then any dust from this will be harmful to health. Those producing CNC will also be at risk surely. There are still many companies who do not provide any PPE or adequate PPE/extraction. There is already a problem in the wood industry and has been for decades resulting in much peer reviewed research whereby the c/silica emitted in the wood dust is proven to be causing tool/saw blade wear of the industrial strength diamond and tungsten carbide tipped blades. If there is sufficient c/silica to blunt these extra strong metal blades then there is more than sufficient to damage the lungs of those in the wood machining industry in particular, who will be inhaling it daily and also will be taking it home, albeit unwittingly, to loved ones and children on their work clothes, hair and skin which will also contaminate homes/cars. When you add old recycled wood extracted from such as construction and demolition waste ( C & D waste) from the demolition of old buildings/properties, which is what most wood composite boards such as MDF/chipboard, etc are made of in the UK, then how are they going to extract the many toxic and carcinogenic chemicals and substances that this old waste wood will have the potential to contain such as asbestos, heavy metals, silica, lead, CCA from treated woods, preservatives, pesticides, fungicides and so on, many of which were banned from use many years ago but will now be finding their way back into the wood chain under the disguise of 'recycling'. Then it will be emitted in the dust and inhaled by woodworkers, particularly those working in the wood machining trade, classified by the IARC as those most at risk from wood dust. Why is it deemed safe for wood workers to inhale and take home wood dust, a group 1 carcinogen on the same level as asbestos? Would it be allowed to happen if it were asbestos? No it would not. Then you have to add into the equation that this is nano sized. It is well documented and concerns have been raised that particles that are nano sized show properties good and bad that are different from the same substance when of a much larger particle size. Nano sized particles can cross the blood brain barrier for example and once inhaled can pass through the alveolar of the lungs into the blood stream and can cross through vein walls so can reach any part of the body. These nano sized particles of CNC will be undetectable/unseen by the human eye so will go undetected. What research has been done to ensure this product is safe for those working with it or with products made from it? As it is bamboo and rice grains are also high in c/silica so using those would be problematic too. Clearly not enough thought is being put into this as regards any harm it may cause to human health for those xposed to any dust from it. They simply will not know what they are unleashing until it is too late for many of it's victims. Clearly no lessons have been learnt from the many thousands of sadly unecessary and totally avoidable deaths cause by asbestos and smoking. How many victims will be needed this time before anyone does anything about it? Clearly the Precautionary Principle totally ignored here too! Clearly no one cares, financial viability and company profits over people's health and lives as always!

woody
28th December, 2012 @ 08:22 pm PST

Wow, make furniture with this stuff, I buy it, over the cardboard crap they are selling in the stores now.

SaysMe
23rd January, 2013 @ 12:35 pm PST

Woody -- You seem to be using very many words to tell us that ... wood is carcinogenic? (As well as rice & bamboo--) You seem obsessed with the threat of silica in wood, to the point that all working with wood is suspect. That's a much larger topic, for what it's worth, than the scope of this article.

Also you are confusing "nano sized" with nanoparticles. Incorporating nanoparticles into consumer products is indeed questionable, for the reasons you mention. But that has nothing to do with the product being discussed. The fact that understanding or producing the product depends on manipulating "nano sized" components has nothing to do with producing nanoparticles or incorporating them into the finished product.

Bill in Seattle
26th January, 2013 @ 05:37 pm PST

Couldn't you just treat the surface of the composite material so that water doesn't come into contact with it?

Once your enameled layers of CNC are at structural spec, enamel protective surface layer. For glass replacements this could be transparent neverwet technology.

Andrew Hoffman
27th July, 2013 @ 02:07 pm PDT

I don't know whether I missed anything but what bonds the filaments together? Is it some sort of resin?

When you come to think of it, probably everything gives off nano particles. If you can smell something, then it is nano particles that are affecting your nasal membranes. It is a dangerous world out there!

windykites1
26th August, 2013 @ 08:48 am PDT

Silica is silicon dioxide, woody huffed abit to much sawdust.

Its made by chemically breaking wood fiber down through acid hydrolysis...

The nanocrystals are held together by a hydrogen bond

If you wany to know more, read more.

David P.
8th September, 2013 @ 10:50 pm PDT

I assume this will be used simalar to carbon fiber in structures, impregnated with epoxy. When hardened. No worry about water or bugs.

Chuckdw
12th December, 2013 @ 03:32 pm PST

I was thinking about using recycled lumber from demolition as a good source of cellulose, but Woody brought up the problem with that, what with asbestos and other carcinogens piggybacking the used lumber.

I love these forums, but it would be nice to be able to comment or subcomment on posts. Notifications via email would be nice when someone comments.

dsiple
20th December, 2013 @ 10:22 am PST

Shoot, I just now saw this, but I only got as far as "...and ballistic glass (CNCs are transparent) ..." and all I could think about was Transparent Aluminum, as I watch "Spock's Brain" on the boob tube.

Mark Lee
5th April, 2014 @ 06:49 pm PDT
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