To make vehicles more recyclable and greener than ever, the idea of biodegradable cars started. Materials of choice were a plastic composite of flax and beet, and a combination of rattan and bamboo.
Scientists and engineers at the University of Maryland, led by Liangbing Hu, have developed a method to densify a wide range of wood species and make them about twelve times stronger and tougher than normal wood, creating a bio-based material that is stronger than many titanium alloys. The study entitled “Processing bulk natural wood into a high-performance structural material” is published in the journal Nature.
“The paper provides a highly promising route to the design of lightweight high-performance structural materials, with tremendous potential for a broad range of applications where high strength, large toughness, and superior ballistic resistance are desired. It is particularly exciting to note that the method is versatile for various species of wood and fairly easy to implement,” said Dr. Huajian Gao, a professor at Brown University, who was not part of the research team.
- Excellent Mechanical Properties, Stronger than Titanium Alloys
The researchers were able to densify wood and make it as strong as steel by compressing its fibers tight enough to form strong hydrogen bonds. The microscopic structures of before and after compression are shown in the images below. This densification allowed the wood to be five times thinner, twelve times stronger, and ten times tougher than its natural state.
“This new way to treat wood makes it twelve times stronger than natural wood and ten times tougher. This could be a competitor to steel or even titanium alloys, it is so strong and durable,” said Liangbing Hu. “It can even be bent and molded at the beginning of the process,” added Samuel P. Langley, a mechanical engineering professor at the University of Maryland.
The toughness of the wood-based material was also tested by shooting bullet-like projectiles at it. While the projectile went through the natural wood, the compressed wood stopped the projectile partway through.
“The most outstanding observation, in my view, is the existence of a limiting concentration of lignin, the glue between wood cells, to maximize the mechanical performance of the densified wood. Too little or too much removal lower the strength compared to a maximum value achieved at intermediate or partial lignin removal. This reveals the subtle balance between hydrogen bonding and the adhesion imparted by such polyphenolic compound. Moreover, of outstanding interest, is the fact that that wood densification leads to both, increased strength and toughness, two properties that usually offset each other,” explains Orlando J. Rojas, a professor at Aalto University in Finland.
“It is as strong as steel, but six times lighter,” said co-leader Teng Li in reference to their compressed wood material. This property makes it a good candidate for vehicles and as previously reported, Japan has also developed a wood-based material for car light-weighting, a trend in making vehicles much faster and more efficient. As the famous Colin Chapman, founder of Lotus Cars said “Simplify, then add lightness. Adding power makes you faster on the straights. Subtracting weight makes you faster everywhere.”
- Low-cost and Simple Processing
“It’s also comparable to carbon fiber, but much less expensive,” said Liangbing Hu. Since wood is a much cheaper material than metal alloys, the compressed wood is a low-cost alternative to structural materials requiring toughness and strength.
In addition, considering that the wood and liquid chemicals used to facilitate its compression are abundantly available, and the treatment process developed by the research team is simple enough proves that the new wood-based material to be a low-cost, high-performing material compared with metals and their alloys.
The simple, two-step process developed by the team “begins by removing 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, at about 150 F. This causes the cellulose fibers to become very tightly packed. Any defects like holes or knots are crushed together. The treatment process was extended a little further with a coat of paint.”
“Given the abundance of wood, as well as other cellulose-rich plants, this paper inspires imagination,” said Dr. Zhigang Suo, a professor of mechanics and materials at Harvard University, who is also not part of the study. Moreover, the densification process developed by the researchers works for a wide variety of woods, including fast-growing softwoods, making them a sustainable source for manufacturing biodegradable cars and building materials. “Softwoods 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.