Amazingly sharp knives or robust nails – made of wood: The rather soft natural material can be transformed into a material with expanded application potential through an uncomplicated treatment process, researchers report. The method increases the hardness of the raw material by a factor of 23 and so the treated wood could represent an environmentally friendly alternative to metal, plastic and ceramics in the manufacture of products.
What gives trunk and branch stability is one of the oldest human materials. The considerable hardness of some types of wood also plays an important role. Nevertheless, as is well known, it is not sufficient for many applications and so metals, plastics and ceramic materials are used instead. However, the manufacture of these materials is often complex: They have to be manufactured at high temperatures, for example, and also have a rather poor ecological balance due to other aspects. “If you look at the hard materials we use in everyday life, you can see that many of them have to be laboriously manufactured because the natural materials don’t necessarily meet our needs,” says Teng Li of the University of Maryland at College Park .
He and his colleagues have therefore now devoted themselves to the development of processes to make wood more usable. Methods to harden it have been around for a long time. For example, to prepare the material for the manufacture of furniture or building materials, wood is processed using steam and pressure in order to provide improved characteristics. But the increase in hardness still leaves something to be desired, say the researchers. “Our current use of wood is barely realizing its potential,” Li is convinced.
Made hard with little effort
As he and his colleagues explain, the hard potential of wood lies in the component cellulose. It consists of 40 to 50 percent of this biomolecule, which is located in the plant cell walls of the natural substance and forms its skeletal structure. The rest of the substance, along with hemicellulose, is made up of the biopolymer lignin, which acts as a kind of binding agent. The new hardening process by Li and his colleagues is based on the removal of this component without destroying the cellulose skeleton and the subsequent compression of the structure.
In the first step, wood is boiled from relatively harmless chemical substances at 100 degrees Celsius in a bath. This broth can then be reused again, emphasize the researchers. The substances are drawn into the material and loosen the lignin from the bandage. This initially leads to a surprising effect: the starting material becomes soft. “In the first step of delignification, the wood becomes flexible and almost a bit spongy. In the second step, however, we then carry out hot pressing by applying pressure and heat to the chemically treated wood. It is compressed and the aqueous solutions are removed, ”explains Li. The hardened wood is then treated with oils to optimize it for use in contact with moisture – because cellulose tends to absorb water. The hardened wood can then be carved or sanded into the desired shape.
Microscopic examinations of the resulting material showed that the process gave wood promising microstructures: “The strength of a material depends heavily on the size and density of defects such as cavities, channels or depressions,” explains Li We process the natural wood, reduce or remove the defects in the natural wood considerably, so that the original channels for the transport of water or other substances in the tree have almost disappeared. “
Sharp wooden knives and stainless nails
Tests then showed how this is reflected in the characteristics of the material: The researchers’ results show that the method can make wood 23 times harder. To demonstrate the potential, the researchers made table knives with sharpened blades from pressed wood. Their sharpness can evidently even surpass that of standard knives. “The knife cuts through a steak effortlessly,” said Li. Afterwards, the hardened wooden knife can be washed and reused, making it a promising alternative to steel, ceramic and disposable plastic knives, say the researchers. “These knives can also be used many times if you re-sharpen them and maintain them regularly,” says Li.
The team also made it clear that the material can be used to manufacture wooden nails, the stability of which can compete with that of conventional steel nails: the scientists hammered three boards together with the wooden nails without damaging the wooden structures. In contrast to metal nails, they have a potentially interesting advantage, the researchers emphasize: wooden nails do not rust.
“The remarkable properties of the wood hardened with our process, combined with its advantages such as sustainability and low costs, make it a possible alternative for steel and plastics in a wide range of applications,” sum up the scientists. Li and his colleagues now want to refine the process and explore other possible applications. Accordingly, the hardened wood material could also be used, for example, for the production of floor coverings that are particularly resistant to scratches and wear.
Source: Cell Press, technical article: Matter, doi: 10.1016 / j.matt.2021.09.020