Transparent wood is sustainable and suitable for structural use

Transparent wood was first introduced in 2016 as a potential structural material for buildings which – in addition to the other useful properties of wood – allows natural light to pass through.

Key to making wood into a transparent composite material is to strip out its lignin, the major-light absorbing component in wood which is essential for forming rigid cell walls. The empty pores left by the removal of lignin, however, need to be filled with a substance which restores the strength of wood and allows light to be transmitted through the material.

In previous iterations of their composite, the KTH researchers had used conventional petroleum-based polymers. Now, they have successfully developed and tested a sustainable alternative which uses limonene acrylate: a monomer made from limonene.

“The new limonene acrylate it is made from renewable citrus, such as peel waste that can be recycled from the orange juice industry,” said Céline Montanari, a PhD candidate and lead author of the study.

Montanari and her colleagues used an extract from orange juice production to create the polymer, which was suitable for restoring the strength of the delignified wood which allowing light to pass through. At a thickness of 1.2mm, the composite material offers 90 per cent optical transmittance and surprisingly low haze. Notably, the material is intended for structural use; it demonstrated heavy-duty mechanical performance with strength and elasticity suitable for structural applications.

According to Professor Lars Berglund, head of the university’s Department of Fibre and Polymer Technology, the group has wanted to create a sustainable version of the wood composite for years.

“Replacing the fossil-based polymers has been one of the challenges we have had in making sustainable transparent wood,” he said. He said that the material is made with no solvents and all chemicals are derived from bio-based raw materials.

The advances could, the researchers say, enable an exciting and unexplored ranged of applications such as in wood nanotechnology, with possibilities including smart windows; wood for heat storage; a wooden laser, and wood with a built-in lighting function. The researchers are working with the KTH photonics group to explore these possibilities further.

“We have looked at where the light goes and what happens when it hits the cellulose,” Berglund added. “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.”