Making wood that’s stronger than steel

Nature has a new process for turning trees into a building material that’s tougher than girders or automobile frames:

“It’s a new class of materials with great potential,” says Li Teng, a mechanics specialist at the University of Maryland in College Park and a co-author of the study published on 7 February in Nature.

Attempts to strengthen wood go back decades. Some efforts have focused on synthesizing new materials by extracting the nanofibres in cellulose — the hard natural polymer in the tubular cells that funnel water through plant tissue.

Li’s team took a different approach: the researchers focused on modifying the porous structure of natural wood. First, they boiled different wood types, including oak, in a solution of sodium hydroxide and sodium sulfite for seven hours. That treatment left the starchy cellulose mostly intact, but created more hollow space in the wood structure by removing some of the surrounding compounds. These included lignin, a polymer that binds the cellulose.

Then the team pressed the block — like a panini sandwich — at 100 ºC for a day. The result: a wooden plank one-fifth the thickness, but three times the density of natural wood — and 11.5 times stronger. Previous attempts to densify wood have improved the strength by a factor of about three to four.

To test the toughness of the material, the team fired pellets at it from a ballistic air gun normally used to test the impact resistance of military vehicles. Five layers of the material laminated together — just 3 millimetres thick in total — was able to halt a 46-gram steel projectile travelling at roughly 30 metres per second.

That’s much slower than the several hundred metres per second at which a bullet travels, says Hu, but it is comparable to the speed at which a car might be moving before a collision, making the material possibly suitable for use in vehicles.

Video of strength tests at the link.