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New Super Wood Beats Metals in Feats of Strength

A new method combining chemical, pressure and heat treatments can create ultra-dense material that is stronger than steel

Material scientist Liangbing Hu (left) holds wood stronger than titanium and tougher than steel after a two-step process. Mechanical engineer Teng Li (right) holds an untreated block of the same wood. (University of Maryland)
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By soaking wood in a mix of chemicals and then compressing and heating it, researchers have created a material that is stronger than steel, writes Sid Perkins for Scientific American.

Material scientists have been working for decades — squeezing wood, cooling and passing it through rollers , treating it with ammonia or steam — all in the name of creating a strong, tough material made from a relatively renewable resource. The new wood is three times as dense as natural wood and 12 times stronger, the researchers report in a new study published in the journal Nature.

“Soft woods like pine or balsa, which grow fast and are more environmentally friendly, could replace slower-growing but denser woods like teak, in furniture or buildings,” says Liangbing Hu, a materials scientist at the University of Maryland, College Park and one of the study authors, according to a press release.

Other methods of "densifying" wood produce material that tends to expand back toward its original size and shape when humidity strikes, tells Perkins. The new approach enhances wood in a way that is moisture resistant by creating a more uniformly dense end result.

“This kind of wood could be used in cars, airplanes, buildings – any application where steel is used,” Hu tells David Grossman, reporting for Popular Mechanics. Skyscrapers of the future could even be built from wood.​

The process starts by soaking wood in a bath of sodium hydroxide and sodium sulfate. These two chemicals work together to partially remove lignin and hemicellulose, two polymers that make the cell walls of plants rigid. Critically, the chemicals leave much of the cellulose, a third polymer, intact. The wood cell walls become less rigid and more porous. Next, the researcher pressed the wood while heating it to about 100 degrees Celsius. Wood has many natural channels called lumina that run along the direction of growth. These channels and cell walls collapse during the heat-and-squeeze part of the researchers' process and new hydrogen bonds form between cellulose molecules, toughening the material.

The end result, as seen under a microscope, are layers of wood packed closely together, the researchers report in their paper. The final material is about one-fifth the thickness of its original size but three times the density.

The researchers then subjected their densified wood to a battery of tests to measure its mechanical propertiesThey tested its capacity to bear a load. They tested its stiffness, it's tensile strength (how well it holds up to pulling forces), and how well it could flex. Overall, the result was one rough and tumble piece of wood. In a humidity test, the wood only swelled about 8.4 percent, while still remaining tough.

They also fired a steel projectile at the wood with a ballistic air gun. Five layers of the material together halted a slug moving at the speed that a car might be before a collision, reports Mark Zastrow for Nature.

The results were impressive, but not enough to convince some experts. Zastrow spoke to Fred Kamke at Oregon State University who was not involved in the new research. Kamke pointed out that 24 layers of densified poplar wood were able to halt a speeding 9mm bullet fired at 25 feet without requiring chemical treatment. “These other methods are probably much less expensive than a seven-hour boil in a caustic solution,” he says. 

But Hu and his colleagues are moving forward with their "super wood," reports Stu Borman for Chemical & Engineering News. A spin-off company called Inventwood hopes to market the technology and see a future of vehicles, airplanes, shipping containers, flooring, armor and more built with boiled, treated and compressed wood that is stronger than steel.

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