Teng Li and Liangbing Hu, co-leaders of a team at the University of Maryland, have developed a way to make wood more than 10 times times stronger and tougher than before, creating a natural substance that is stronger than many titanium alloys.
“It is both strong and tough, which is a combination not usually found in nature,” said Teng Li, the Samuel P. Langley Professor of mechanical engineering at the university. 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.”
Hu added the material is also comparable to carbon fiber, but much less expensive. “This kind of wood could be used in cars, airplanes, buildings – any application where steel is used,” Hu said.
The team's challenge was that wood in its natural form isn't strong or tough enough for many advanced engineering structures, even when pretreated with steam, heat, ammonia or cold rolling to make the wood's density greater. Without achieving a very high level of densification, the treated wood still is subject expansion due to environmental factors like humidity. Expansion makes the treated wood unstable, creating weakness.
How they do it:
The team’s process begins by removing the wood’s lignin, the part of the wood that makes it both rigid and brown in color, using a boiling process in a solution of sodium hydroxide, sodium sulfite and water.
The wood is then compressed under mild heat, at about 150 F. This causes a total collapse of the wood's cell walls and the complete densification of the natural wood with highly aligned cellulose nanofibres. The cellulose fibers become very tightly packed and any defects like holes or knots are crushed together.
Hu and Li says the wood’s fibers are pressed together so tightly that they can form strong hydrogen bonds, like a crowd of people who can’t budge – who are also holding hands.
The compression makes the wood five times thinner than its original size. "This strategy is shown to be universally effective for various species of wood. Our processed wood has a specific strength higher than that of most structural metals and alloys, making it a low-cost, high-performance, lightweight alternative."
Above: Magnified images of (1) untreated wood and (2) the same wood treated by a new process invented by engineers at the University of Maryland that compresses the natural structures of wood into a new material five times thinner.
The team tested their new wood material and natural wood by shooting bullet-like projectiles at it. The projectile blew straight through the natural wood. The fully treated wood stopped the projectile partway through.
“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 said.
“Their process provides a highly promising route to the design of light weight high performance structural materials, with tremendous potential for a broad range of applications where high strength, large toughness and superior ballistic resistance are desired, “ said Dr. Huajian Gao, a professor at Brown University, who was not involved in the study. “It is particularly exciting to note that the method is versatile for various species of wood and fairly easy to implement.”
Orlando J. Rojas, a professor at Aalto University in Finland said, "Of outstanding interest is the fact that that wood densification leads to both increased strength and toughness, two properties that usually offset each other.”
Hu’s research has explored the capacities of wood’s natural nanotechnology. They previously made a range of emerging technologies out of nanocellulose related materials: (1) super clear paper for replacing plastic; (2) photonic paper for improving solar cell efficiency by 30%; (3) a battery and a supercapacitor out of wood; (4) a battery from a leaf; (5) transparent wood for energy efficient buildings; (6) solar water desalination for drinking and specifically filtering out toxic dyes. These wood-based emerging technologies are being commercialized through a UMD spinoff company, Inventwood LLC.
image: University of Maryland