A piece of the transparent wood and a slice of orange, which can help make the wood more sustainable. Photo: Céline Montanari.
A piece of the transparent wood and a slice of orange, which can help make the wood more sustainable. Photo: Céline Montanari.

Since it was first introduced in 2016, transparent wood has continued to be developed by researchers at KTH Royal Institute of Technology in Stockholm, Sweden, as an innovative structural material for building construction. It lets natural light through and can even store thermal energy.

The key to making wood into a transparent composite material is to strip out its lignin, the major light-absorbing component in wood. But the empty pores left behind by the absence of lignin need to be filled with something that restores the wood's strength and allows light to permeate.

In earlier versions of the composite, researchers at KTH's Wallenberg Wood Science Centre used fossil-based polymers. Now, the researchers have successfully tested an eco-friendly alternative – limonene acrylate, a monomer made from limonene. They report this work in a paper in Advanced Science.

"The new limonene acrylate is made from renewable citrus, such as peel waste that can be recycled from the orange juice industry," says Céline Montanari, a PhD student and lead author of the paper.

The new composite material offers optical transmittance of 90% at 1.2mm thickness and a remarkably low haze of 30%, the researchers report. Unlike other transparent wood composites developed over the past five years, this material is intended for structural use, as it shows heavy-duty mechanical performance: with a strength of 174MPa (25.2ksi) and an elasticity of 17GPa (or about 2.5Mpsi).

Yet all along, sustainability has been a priority for the research group, says Lars Berglund, head of KTH's Department of Fibre and Polymer Technology. "Replacing the fossil-based polymers has been one of the challenges we have had in making sustainable transparent wood."

According to Berglund, environmental considerations and so-called green chemistry permeate the entire work. The transparent wood is made with no solvents, and all chemicals are derived from bio-based raw materials.

The new advances could lead to a yet unexplored range of applications, such as in wood nanotechnology, Berglund says. Possibilities include smart windows, wood for heat-storage and wood with a built-in lighting function – even a wooden laser.

"We have looked at where the light goes, and what happens when it hits the cellulose," Berglund says. "Some of the light goes straight through the wood, and makes the material transparent. Some of the light is refracted and scattered at different angles and gives pleasant effects in lighting applications."

The team is also working with Sergei Popov's photonics group at KTH to explore the nanotechnology possibilities even further.

This story is adapted from material from KTH Royal Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.