A new type of wood has the potential to become a high-tech material: it can be deformed in any manner and is three times stronger than natural wood. The very part of the wood that gives it its stability in nature has been removed.
By removing lignin, the wood loses its colour. After compressing, it is three times stronger than the original material.
Empa/ETH
Wood is one of the oldest materials in the world. It is lightweight, has very good mechanical properties, regrows, and absorbs CO2. Against the backdrop of the current climate discussion, the last two properties in particular raise the question of how wood can be used even more and in a better way. Ingo Burgert's research group at Empa and ETH Zurich have been looking into this question for years. Their aim is to improve the natural properties of wood and equip it with new functions in order to increase its application range.
Together with Tanja Zimmerman, the current head of Empa's Functional Materials department, Burgert has developed, for example, door handles made from antimicrobial wood, mineralised wood for improved flame resistance and a noticeboard made from magnetised wood. The latest research work by the Wood Materials Science group at ETH Zurich and Empa has opened up further new possibilities: "We have found a way of significantly improving the mechanical properties of wood and at the same time making it even easier to equip it with new properties", says Burgert.
Flexible when wet, stable when dry
The answer lies in the delignification and compaction of the wood. Wood essentially consists of three chemical components: cellulose, hemicellulose and lignin. The lignin ensures that the long cellulose fibrils are stabilised and do not bend. "We use acid to remove this lignin from the wood and thus remove the natural adhesive", explains Marion Frey, who is currently doing her doctorate in Burgert's team. The result: the wood – or rather the remaining, white cellulose – can easily be shaped into any form when wet. Between the cells where lignin once provided stability, water spreads out, dissolves the cell connections and provides deformability. Drying the delignified wood makes the cells interlock, which in turn leads to stable compounds. The material is additionally compacted by pressing so that it becomes around three times stiffer and more tensile than natural spruce wood. Furthermore, a water-repellent coating ensures that the interior of the wood can no longer become damp and so retains its desired shape.
Simpler functionalisation
In addition to the deformability, the removal of lignin from the wood leads to a higher porosity. "This is a great advantage for the functionalisation of wood. Because there is more space between the cells and in the cell walls, it is easier to introduce other substances into the wood structure that give the modified wood new properties", says Tobias Keplinger of ETH Zurich. For example, if iron oxide is introduced, the wood can be magnetised. In their experiments, the first product ideas have already been implemented. This includes a bicycle helmet, the interior trim of a car and the wind mirror of a vehicle.