People have been using wood for thousands of years: to build houses, as tools or for heating. However, as a natural material it is perishable. However, researchers have developed methods to give wood additional properties that make it more resilient. What can such “high-tech wood” do? And where could it be used?
Wood is a renewable raw material. If only as much wood is felled as can grow back in the forest, it is a sustainable and versatile material. In many areas of everyday life, wood can replace plastic and other less environmentally friendly materials – for example in furniture and construction, as containers or even tableware. However, water, microorganisms and fire can affect the natural substance. However, researchers at the Federal Materials Testing and Research Institute (Empa) led by Ingo Burgert are researching methods that change the properties of wood and thus make it more resistant to environmental influences.
Waterproof wood
Water causes wood to weather and even rot over time. This not only changes the appearance of the wood, but also gradually destroys it. Tropical woods such as teak are considered particularly weatherproof and are therefore often used for garden furniture and other outdoor applications. However, the already threatened tropical forest must be cut down for these types of wood. Researchers are therefore looking for ways to give the wood from other, more common tree species the durability of tropical wood. Teak owes its high resistance to chemical substances such as flavonoids and terpenes. These substances are deposited in the narrow spaces between cells. This leaves little space for water between the cells and protects the teak from weathering.
In order to give other woods these characteristics, the Empa research team looked for substances that have a similar effect to flavonoids. Long-chain, linked molecules such as polymers, which are found in plastics for example, are suitable for this purpose. In order to bring such molecules into the spaces between cells, a carrier fluid is necessary. However, since water would cause the wood to swell, the researchers use chemical solvents such as DMSO and pyridine as a carrier liquid. They are neither healthy nor environmentally friendly, but they show that the idea works in principle.
Natural wood is turned into a water-repellent wood-plastic composite material. “The wood remains water-repellent even if I scratch it, because the plastic is deeply embedded in the material,” explains Etienne Cabane from Empa. However, in order to make a thick piece of wood water-repellent, a different technique is needed: the researchers cut the wood into thin layers, integrate the plastic and then glue the layers back together. This is how veneer wood is created. The research team has already built a washbasin out of it. “Plastic wood” could also be used for window frames, boat parts or other places where wood is regularly exposed to moisture.
Wood that resists fungi and bacteria
Another reason for the weathering of wood is fungi and bacteria, because the natural building material is an ideal breeding ground for microorganisms. This makes wooden facades appear grayish – this prevents many builders and homeowners from using wood as a building material. But even in places where hygiene is particularly important, for example in hospitals or large kitchens, wood has so far been considered taboo because of this microbial colonization. Although varnishes and glazes protect against infestation by microorganisms, they often contain toxic substances.
However, Empa researchers led by Mark Schubert have found a method to make wood naturally resistant to fungi and bacteria. To do this, they use enzymes that are found in some types of fungi. For example, the butterfly tramete, a wood-dwelling fungus species, contains laccases. These enzymes can bind iodine firmly to the wood surface and the resulting iodized wood has an antimicrobial protective layer – without any harmful substances. “The advantage is that the chemically bound iodine is resistant to leaching and is therefore durable,” explains Schubert.
The research team has already patented the process and is working with partners from the furniture, construction and paper industries. Furniture with an antiseptic surface for hospitals, more weather-resistant wooden facades or paper could be produced without toxic binders. Long-term tests with door handles and exterior facades made of wood iodized with laccase are used to test how the material performs in practice.
“Calcified” wood against fire
Despite its ecological advantages, wood has a crucial disadvantage that is particularly noticeable in construction: it burns easily. However, Vivian Merk and her colleagues at Empa have found a way to naturally reduce the combustibility of wood.
To do this, the research team mineralizes the wood by storing lime in its cell structure. To get the mineral deep into the structure of the wood, they soak the wood in an aqueous solution of carbonic acid dimethyl ester and calcium chloride. The researchers then increase the pH value using caustic soda. As a result, the released CO2 reacts with the calcium ions and forms limescale in the cell walls and pores. Fire tests show that the combustibility of the wood could be reduced to around a third.
But the “calcified” wood has other advantages: both the wood itself and the stored lime bind CO2. In addition, the process does not require toxic additives, which are often used in conventional fire protection treatments. This means the wood can be recycled safely.