A team of scientists at China’s Nanjing University has developed a technique for increasing the strength of wood by removing the hollow tubes that run through its cellulose structure.
Called the “lumen”, the tubes limit the strength of wood.
According to an article published in the Journal of Bioresources and Bioproducts, the Nanjing team has found a chemical method that makes the wood more dense by filling these tubes (see diagram).
They report that wood treated in this way “exhibits ultra-high tensile strength (496.1 MPa), flexural strength (392.7 MPa) and impact toughness (75.2 kJ/sq m), surpassing those of compressed densified wood and traditional metal materials like aluminum alloys”.
This “self-densification” is caused by boiling the wood in a mixture of caustic soda and sodium sulfite to remove some of its lignin.
It is then soaked in a mixture of lithium chloride and a solvent known as dimethylacetamide. This causes the cellulose and lignin to swell and eliminate the lumen tubes, after which it is left to dry – and shrink.
The main attraction of this technique is that it produces superior results to the “hot pressing” method, which uses mechanical force to compress the wood along one axis. By contrast, the chemical technique causes uniform shrinking.
The authors of the paper, led by Dafang Huang, an academic at Nanjing university’s Department of Materials Science and Engineering, say this uniform shrinkage leads to an “order-of-magnitude enhancement in the overall mechanical performance of the wood”, presenting a “significant advantage over compressed densified wood”.
They say super-strong yet lightweight wood has “great potential for application as a sustainable engineering material, replacing traditional structural materials such as metals and alloys”.
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