Developing a new technique, scientists from the United States have created thin sheets of wood that are more resistant to many metal alloys. Over the last few decades several research teams have been experimenting to try to reinforce wood to create new materials that could replace metals or offer a “natural” solution to the scourge of plastics.
The new research work offers by far the best – and most impressive – results.
Researchers at the University of Maryland at Colege Park headed by Teng Li and Liangbing Hu have adopted a new approach, seeking to change the porous structure of natural wood. They first boiled various types of wood, including oak wood, in a solution of sodium hydroxide and sodium sulphite for seven hours. In this way the wood cellulose has remained intact but some other components – such as e.g. lignin, which retains cellulose – largely dissipated, creating voids.
Scientists then put the wood in a hot press at a temperature of 100 degrees Celsius for a day. The resulting compressed wood sheet was as thick as one fifth of the original, but its density was three times greater than natural wood, making it 11.5 times more durable.
As described in the relevant article in the “Nature” Inspection, in tests conducted with a special ballistic aero aircraft such as those used in military vehicle tests, the new material proved to be extremely “hard-core”. Five sheets bonded to each other on a laminated slab of only 3 mm thick could easily repel 46 grams of steel bullets which moved at a speed of about 30 km / h.
This velocity is far less than that of the real spheres, but the impact force of the steel missiles on the stratified plate is similar to that created when the two cars collide. For this reason, researchers from Maryland think that compressed wood can be an ideal material for the automotive industry.
Some scientists who did not participate in the study pointed out that possibly other existing techniques could achieve similar results by removing lignin from wood at a lower cost. Researchers from Maryland do not disagree: they argue that the important finding in their research is the discovery that the amount of lignin removed is decisive for the performance of the material.
In their experiments they saw that when they removed more lignin the wood gained less density and became brittle, which suggests that some lignin is needed to hold the cellulose under compression.