Materials scientists at the University of Maryland, College Park, have discovered a method to make wood stronger by making it denser. In this Scientific American article, team member Hu Liangbing claims their product has up to 50 times the strength in compression and 20 times the stiffness (resistance to deformation) than the original wood.
It’s a two-step process. First the wood is boiled and soaked in a chemical solution that removes some of the lignin (a polymer) from the plant cells but retains most of the cellulose (the main source of strength). Then the wood is compressed, crushing the cell walls into the empty spaces, and gently heated to form new molecular bonds. “You have all these nanofibres aligned in the growth direction,” explains Hu. The result is three times as dense as the original wood, moldable into different shapes, and moisture resistant – a quality never before achieved with densified wood.
The chemicals themselves are sodium hydroxide, also known as lye, and sodium sulfite. Neither chemical is difficult or energy-intensive to manufacture. (In fact, lye can be made at home from water and table salt plus a power source.) Both are soluble in water and harmless when diluted or neutralized, though they need to be handled carefully when concentrated as their high alkalinity (pH>9) can cause severe burns. So the process seems OK from an environmental perspective.
Hu’s team is talking about human-scale applications like lightweight furniture and body armor. It would take a huge scale-up in the process to make densified wood an affordable option for homebuilding and other structural uses. In the journal Nature, other materials scientists assert the two-step process isn’t worth the expense compared to techniques like steaming the wood or adding resins, which also improve strength.