Knives on the wrong track: inexpensive and sustainable
Photo: Bo Chen / dpa
Everyone has dozen of kitchen knives at home.
Sometimes they are made of ceramic, but in the vast majority of cases they are made of a steel alloy.
Due to its properties, such as its hardness, this can be processed into sharp blades.
But the production of steel is not only complex and expensive, it also consumes a lot of energy.
It could be easier with wood.
It is biodegradable, grows in nature and the manufacture of wooden knives is also easy.
But due to the lack of hardness, the sharpness wasn't far off - so far.
Researchers have now developed a hardening method that makes wood particularly resistant.
The team around Teng Li from the University of Maryland in College Park demonstrate the potential in the journal »Matter« using nails and a knife that is said to be three times sharper than ordinary table knives made of stainless steel.
The method is inexpensive and allows the knives or other tools to be reused, they say.
Using the example of linden wood, the material researchers demonstrate that the very simple process they have developed makes wood 23 times harder. In general, wood is processed with steam and pressure during manufacture - but this is not sufficient for many purposes. They justify this with the fact that cellulose, the main component of wood, has excellent mechanical properties, but makes up just under half of the weight. It is therefore important to remove other components such as hemicellulose and lignin without destroying the cellulose framework. Lignin are large molecules that are stored in the plant's cell wall and cause lignification. Hemicellulose can also be found in plant-based, especially lignified, cell walls.
"Our current use of wood is hardly exploiting its full potential," Li is quoted in a message from the magazine.
He explains the basic principle of his method: “The process consists of two steps.
In the first step we remove part of the lignin.
Then the actually very stiff wood becomes soft, flexible and a bit spongy.
In the second step, we apply pressure and heat to the processed wood to compact it and remove the water. "
"These knives can often be used if they are sharpened and cared for regularly."
Then the wood is brought into the desired shape and treated with mineral oil to make it water-repellent.
Knives, for example, would retain their sharpness for a long time even after being washed regularly.
Investigations under the microscope show that the method largely makes channels and pores in the hardened wood disappear.
The researchers measured both the hardness of the wood and the sharpness of the knives.
In videos, they cut through a steak with a wooden knife and hammer a wooden nail through three layers of linden wood.
In addition to the ecological advantages, they write, the wooden nails are rustproof in contrast to conventional products.
"In the kitchen we have a lot of wooden utensils that we use for a long time, such as cutting boards, chopsticks or rolling pins," says Li. "Even knives can often be used if they are sharpened and cared for regularly." However, it is not known how often the knives have to be sharpened. Because good quality knives are also characterized by the fact that they keep their sharpness for a long time. With their process, the materials researchers want to enable further applications of hardened wood, for example for particularly resistant floors.
In detail, the hardening process works as follows: After the wood has been brought into the desired block size, a chemical treatment should first remove most of the hemicellulose and lignin. To do this, the blocks are placed in an aqueous solution with a little sodium hydroxide (NaOH; five percent by weight) and sodium sulfite (Na2SO3; 2.5 percent) until the pores are soaked and the wood sinks to the floor.
The solution is then brought to a boil at 100 degrees Celsius - for two to six hours, depending on the desired hardening.
The wood is then rinsed with demineralized water to remove the chemicals and remaining lignin.
The blocks are then pressed at room temperature for several hours to remove most of the water and finally they are heated at 105 degrees until completely dry.
The hardened wood is then immersed in mineral oil - if necessary food-safe - for 48 hours to make it water-repellent.
joe / dpa