The featured image was bought on Istock.com. Copyrights.
Densifying wood improves its mechanical properties and diversifies its uses. Various types of this processed wood are used as fuel, in residential construction, furniture manufacturing, etc. However, in the long term, this material shows a dimensional instability that makes it less interesting for certain applications. A team of researchers has succeeded in producing a much more stable densified wood.
Researchers at the University of Maryland’s Department of Materials Science and Engineering developed a densification process that makes wood more resistant and durable than many metals. They presented it in a study entitled “Processing bulk natural wood into a high-performance structural material”, published on February 8, 2018, in the scientific journal Nature, nicknaming it “super wood”.
The project is co-directed by Teng Li, professor of mechanical engineering, and Hu Liangbing, lecturer and member of the Bing Research Group who focus their research on nanomaterials, energy and flexible electronics. The project started in 2016, after six years of research on wood.
“Super Wood”—An Inexpensive Solution for Structural Materials
According to Li, this wood can be used in car manufacturing. Since it is as strong as and lighter than the metals used in this industry, it could lead to the design of more fuel-efficient vehicles.
“Super wood” is six times lighter than steel and much less expensive than carbon fibre. Like natural wood, it can be transformed by moulding and bending. The purpose of producing densified wood is to replace precious and threatened wood species like teak, with faster-growing wood like pine or balsa. Current densification processes produce wood with inconsistent density that expands when exposed to moisture. The method created by the team makes it possible to produce wood that is resistant to several distortion and spoilage factors.
Steps Involved in the Transformation of Natural Wood into Densified Wood
The first step is to partially remove the hemicellulose and lignin. Lignin is found inside and between the cells, giving the wood its brown colour and rigidity. Hemicellulose binds the cellulose fibres. To break it down, the wood is boiled in a sodium-hydroxide (NaOH) and sodium-sulphite (Na2SO3) solution for seven hours. This treatment does not affect the starchy cellulose.
Then, the wood is compressed for 24 hours under moderate heat, at about 100 °C. This operation compacts the wood by squeezing the cellulose fibres and eliminating cavities and knots.
The two operations brought approximately an 80% reduction in wood thickness. The resulting material is 3 times denser than natural wood, 11.5 times more resistant to mechanical stress and 10 times more durable.
Tests and Conclusions
The team performed a variety of tests on the wood’s mechanical properties, demonstrating the following results:
- Bending strength 18 times higher than natural wood (perpendicular to wood growth)
- Compressive strengths 33 to 52 times higher (perpendicular to wood growth)
Exposed to 95% humidity for 128 hours, the wood expanded by only 8.4%.
Ballistic tests were also conducted to verify strength and rigidity. The researchers fired bullets from a gun used to test the impact resistance of military vehicles. A 3-millimetre thick panel was able to stop a 46-gram steel projectile moving at about 30 metres per second. According to Hu, this is comparable to the speed of an automobile before a collision, which shows how adaptable this material could be for the automotive field.