A chemical bath and a hot-press can transform wood into a material that is stronger than steel, researchers report. The process, and others like it, could make the humble material an eco-friendly alternative to using plastics and metals in the manufacture of cars and buildings.
“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 Nature1.
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 four2.
Scanning electron microscopy showed that the latest process crushes the cellulose tubes together until they crumple and interlock. “You have all these nanofibres aligned in the growth direction,” says Hu Liangbing, a materials scientist at the University of Maryland at College Park who was part of the team.
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.
A question of strength
Some researchers say they are underwhelmed by the group’s improvements over previous densification methods. Fred Kamke at Oregon State University in Corvallis says that even without removing lignin, other techniques — such as applying higher temperatures, steaming the wood before treatment, and treating it with resins — can achieve most of the reported increase in performance. “These other methods are probably much less expensive than a 7-hour boil in a caustic solution,” he says. In his own tests, 24 layers of densified wood untreated by chemicals was able to halt a 9-millimetre bullet from a handgun.
Michaela Eder, a plant biomechanics researcher at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, notes that compressing the wood to increase its density should naturally improve its strength — but it was unclear how much the entanglement of the nanofibres contributed. Hu and Li say their team’s simulations suggest that the increase in strength is consistent with the effects of hydrogen bonds forming when the nanofibres tangle. Further evidence, they say, is in previous work4 in which they extracted wooden nanofibres to make paper 40 times stronger and 130 times tougher, but with only a modest increase in density. This suggested the cellulose fibres were bonding to achieve the superior strength, they say.
The latest study also follows work3 published in January in which researchers removed all of the lignin and compressed the material at room temperature — resulting in a threefold increase in strength.
Hu says that his study’s main finding is that removing the right amount of lignin is key to maximizing performance. In his team’s experiments, removing too much of the polymer resulted in less-dense, brittle wood, suggesting that some leftover lignin is helpful in binding the cellulose fibres when they are hot-pressed. The wood was strongest when roughly 45% of the lignin was removed.
“I see a lot of potential in this direction,” says Eder, referring to both papers. “What I like is that they’re trying to make use of the inherent properties of the wood itself. It’s a fantastic material to work on and improve.”
Read the related News & Views article: ‘Wood made denser and stronger’