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Process makes wood stronger than steel

Alkaline boiling and hot pressing collapse wood’s pores, maximizing its density

by Stu Borman
February 8, 2018 | APPEARED IN VOLUME 96, ISSUE 7

Credit: Adapted from Nature
A chemical and mechanical method yields wood that has a specific strength (strength per unit density) much higher than that of most metals and alloys.

Making strong materials like steel for construction, motor vehicles, airplanes, and thousands of other uses is an expensive and environmentally fraught process, requiring mining, huge factories, and lots of energy and producing pollution. Wood is a more readily available, less expensive, and more environmentally benign resource, and it is widely used to build homes and construct furniture. If wood also had the strength, impact resistance, and water resistance of steel, its applications could expand considerably.

Researchers have tried to enhance wood’s properties in the past by treating it with steam, heat, or ammonia and then squashing—or densifying—it. But these treatments condense wood to about 40% of its original thickness at best, leaving some of wood’s internal pores and channels intact. And the resulting material still swells considerably when exposed to high humidity.

Teng Li and Liangbing Hu at the University of Maryland, College Park, have now devised a chemical and mechanical treatment for wood that eliminates its cavities and densifies it about twice as much (Nature 2018, DOI: 10.1038/nature25476).

This slow-motion video compares how three materials of equal thickness respond to a bullet: (from top) natural wood, a monolayer of densified wood, and densified wood laminate, which contains five thin layers of the treated wood at 90° rotations to one another.
Credit: Liangbing Hu

The processed wood has higher specific strength (strength per unit density) than that of most structural metals and alloys and is a low-cost, high-performance, lightweight alternative to such materials, the researchers say. Li notes that “specific strength is a key design parameter in weight-sensitive applications, such as vehicles and aerospace applications.”

The researchers first boil natural wood in a solution of NaOH/Na2SO3 to make it more porous and less rigid. They then compress the treated wood perpendicular to its growth direction at 100 °C.

Wood has a preponderance of the glucose polysaccharide cellulose and lesser amounts of the mostly five-sugar polysaccharide hemicellulose and the phenolic polymer lignin. These components have different stabilities in the chemical bath, so the treatment removes nearly 75% of hemicellulose and 50% of lignin but leaves most cellulose intact. Partial lignin removal is key to compressing wood completely, the researchers found.

Hydrogen-bonding interactions between closely spaced cellulose nanofibers enhance the densified product’s strength. It gets slightly weaker and swells modestly in 95% relative humidity but becomes immune to moisture when given an oil-based coating. In a ballistics test, a 3-mm densified-wood laminate stops an air-gun projectile that glides easily through normal wood of the same thickness.

The University of Maryland spin-off company Inventwood will commercialize the technology, which has possible applications that include vehicle and boat bodies, shipping containers, armor, and flooring.

Li and Hu’s work shows that “enormous enhancement of mechanical properties of a natural material can be achieved by a relatively simple chemical-mechanical process,” says material mechanics expert Zhigang Suo of Harvard University.

Wood materials specialist Lauri Rautkari of Aalto University says he would like to know more about the densified wood’s response to direct water exposure and its load-carrying capacity, but he believes the study may represent a significant advance, saying he’s “super happy that this is being published.”



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Maigua Wachira (February 13, 2018 3:38 AM)
Great works in deed, and an advancement in the field of wood technology. Proud to be associated with the Society of Wood Science & Technology(Kenya). I hold a bachelors degree in Wood Science & Technology, a course that's barely known and rarely appreciated in my country but am always very positive about my choice of the course and profession.Am sure a day is coming when the society will be able to appreciate the contribution this profession has made and will continue to make in the engineering industry as well as the environment.
Rogerson Anokye, PhD. (February 15, 2018 5:40 AM)
Maigua, your worries are not peculiar to only Kenya but the whole of Africa. However, your recognition and appreciation will much be felt from what you can do for your people and the entire world with the knowledge acquired.
Mark H (February 13, 2018 7:13 AM)
The Germans, Japanese, and English made aircraft wings, fuselage, structure and parts from wood during WWII when metals were scarce. This article reminded me that titanium is a scarce, and terribly difficult metal to machine. Yet it was chosen as the only material strong enough for high stress components in swing-wing aircraft. The F-14’s “backbone” I beleive.
'Dare Badejo (February 13, 2018 2:26 PM)
This is a great advancement in wood technology. AS rightly said, it is expedient to know the woods response to load and direct water. Also of importance is the cost of production and the by products of the chemical treatment process;are they environmentally friendly?
Great work once again
RVS (February 15, 2018 10:49 AM)
Very interesting! It seems that the video illustrates not the "specific strength", but the "strength" property. What happens if one compares ballistics not of the same thickness samples, but the same weight?
ed c (February 16, 2018 7:58 AM)
@Mark H:

The British more successfully than the Germans, due to different adhesive technology. The US largely forbade major wooden structure in large commercial aircraft due to structural failures; it was easier to test and verify steel or aluminum structures. Wood has never left aircraft production; it is used by homebuilders today and several quite high performance aircraft, like the Falco, have major wood components.

Titanium is neither particularly rare nor particularly hard to work; it just requires a modicum of care, in the same way as does welding magnesium. It's certainly easier to machine with than beryllium.

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