Modified wood found stronger than steel
By Marvin Pirila
Imagine the possibilities…
Engineers at the University of Maryland, College Park, have found a way to make wood more than ten times stronger and tougher than before, creating a natural substance that is stronger than many titanium alloys. See paper at Nature, volume 554, pages 224-228 (08 February 2018).
Professors Liangbing Hu and Teng Li report “This new way to treat wood makes it twelve times stronger than natural wood and ten times tougher,” said Liangbing Hu, the leader of the team that did the research, published in the journal Nature. “This could be a competitor to steel or even titanium alloys, it is so strong and durable. It’s also comparable to carbon fiber, but much less expensive.” Hu is an associate professor of materials science and engineering and a member of the Maryland Energy Innovation Institute.
The process used by Li’s team first removes the wood’s lignin, the part of the wood that makes it both rigid and brown in color. Then it is compressed under mild heat, causing the cellulose fibers to become very tightly packed, crushing together any defects like holes or knots.
The result is a wooden plank one-fifth the thickness, but three times the density of natural wood — and 11.5 times stronger. “It is both strong and tough, which is a combination not usually found in nature,” said Teng Li, the co-leader of the team and the Samuel P. Langley Professor of mechanical engineering at the University of Maryland. His team measured the dense wood’s mechanical properties. “It is as strong as steel, but six times lighter. It takes 10 times more energy to fracture than natural wood. It can even be bent and molded at the beginning of the process.”
Note: Strength measures the resistance of a material to failure, given by the applied stress (or load per unit area). Toughness measures the energy required to crack a material; it is important for things which suffer impact.
This lightweight steel alternative allows for many applications where weight is a factor, particularly for vehicles and airplanes.
Hu says that his study’s main finding is that removing the right amount of lignin is key to maximizing performance. In his team’s experiments, removing too much of the polymer resulted in less-dense, brittle wood, suggesting that some leftover lignin is helpful in binding the cellulose fibers when they are hot-pressed. The wood was strongest when roughly 45% of the lignin was removed.
“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,” Hu. “This kind of wood could be used in cars, airplanes, buildings ─ any application where steel is used.”
This is an exceptional benefit as hardwoods such as maple and oak become harder to find.
Imagine the possibilities of having a viable alternative to steel for many applications, especially one that is renewable and beneficial to everyone in the northland dependent on the wood industry.