Sustainable development of biocomposites has been limited by the shortage of monomers with an organic base that could combine an organic process and high performance. Recent experimentation on the numerous applications of wood is showing how this material can be transformed through innovative processes unimaginable until recently. For a few years now, experiments on the use of transparent wood instead of glass have been made. Wood can be made completely transparent in order to become way more resistant and with better insulation capacities than glass. However, even though experiments have shown very encouraging results so far, transparent wood is not yet on the market, because of the high costs required to produce it.
A few years ago, researchers at KTH Royal Institute of Technology in Stockholm have been able to transform wood into a transparent material with good insulation properties, which could be used to reduce the consumption of energy. In order to obtain this result lignin is removed, then the material is soaked with polyethylene glycol – a polymer that is very similar to wood and which makes it able to absorb, retain and ultimately also release heat. However, the process to remove lignin implies the use of chemicals and big quantities of energy, with resulting waste materials. This innovative version of wood is resistant to humidity, it is transparent with a low degree of opacity, and it is a thermal insulator.
As an alternative to this, researchers at the University of Maryland have developed a different production process that does not involve the removal of lignin. This process eliminates cromophores, the molecules that give wood its typical brown colour and that impede the filtering of light. Later on hydrogen peroxide has been applied on the surface of one-meter-long, one-millimetre-thick wooden plates then they have been exposed to solar light for one hour. Once the plates have turned white, they have been immersed in ethanol and later made smoother and more resistant thanks to an epoxy resin. This process, which allows 90% of incident light to filter, does not generate any waste materials.
Transparent wood is biodegradable when produced with polyethylene glycol, because both of them have an organic origin. It is not biodegradable when produced with acrylic.
Starting from these considerations, researchers in the Department of Fibre and Polymer Technology at KTH in Stockholm have created yet another completely organic, transparent, biocomposite of wood based on the synthesis of a limonene acrylate monomer obtained from food industry waste, which is impregnated and readily polymerised into a substrate of delignified wood, without using solvents. The resulting nanostructured biocomposite has an excellent optic transmittance (90%) with a width of 1,2 millimetres, a noticeably low opacity (30%) and high mechanic performance.
This demonstrates the high potential for the development of sustainable nanotechnologies using wood for structural applications in which transparency and mechanic performance are combined to create transparent buildings.
The challenge is to find production processes suitable to greater-dimension contexts, offering controlled micro- and nanostructures so that the sustainable nanotechnologies which use wood can compete not only with glass, but also with translucid plastic in applications useful for interior design, lights and energy saving.
Image: Qinqin Xia, University of Maryland, College Park/Science Advances, 2021
