Optically transparent wood to replace glazing

Researchers at the University of Maryland (USA) developed a way to transform wood into a transparent material with less chemicals and the power of the sun.

Transparent wood is a relatively new phenomenon. We first reported on it back in 2016, when researchers at KTH Royal Institute in Sweden developed a method that involved stripping the lignin from the material, which is the substance in the cell wall that normally blocks light.

However, while this is an effective method, the lignin in wood is also what makes it strong. In addition, removing the lignin uses a lot of chemicals, and it cannot be controlled where the lignin is removed. The new process developed by the Maryland researchers reduces the amount of needed chemicals and leaves most lignin intact.

The process is relatively simple. The researchers apply hydrogen peroxide (H2O2), generally used to bleach hair, to 1 mm thick wood. They then expose it to UV light, which can be either a UV lamp or just leaving it outside in the sun. This process removes the chromophores in lignin, causing the colour of the wood to change from brown to white. Because the chemical is applied with a brush, the process allows for the making of various patterns.

The treated wood is immersed in ethanol for 5 hours to remove any remaining chemicals and then transferred to toluene so as to exchange the ethanol in the wood. Finally, the material is impregnated with epoxy by vacuum infiltration for 1.5 hours before the epoxy is cured. The white material has now turned optically translucent and looks like glass, but with the strength hand flexibility of wood. The resulting material allows 90 per cent of visible light to pass through.

The researchers used mostly balsa wood, but the study implies other types could be used as well. The transparent wood can be made from both transversely and longitudinally cut natural wood.

So why go through all this trouble? Transparent wood could offer a good alternative to window glass. Transparent wood is lighter, less prone to break, and has better insulating properties. Especially the latter is important because windows are a major source of heat loss in buildings. In addition, the production of transparent wood takes less energy than the production of glass, because no high temperatures are involved.

With transparent wood, we might be heading for buildings made entirely out of wood, windows and all! For more on wooden buildings, check out our book Tomorrow’s Timber.

Images: Qinqin Xia, University of Maryland / USDA Forest Service