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Transparent wood is durable and suitable for structural use

Researchers at the KTH Royal Institute of Technology in Stockholm have developed a transparent and durable wood. The material allows natural light to pass through, can store thermal energy and is – unlike other transparent woods – intended for structural use.

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

The key to making wood a transparent composite material is to remove lignin, the main light absorbing component in wood that is essential for the formation of rigid cell walls.

However, the empty pores left behind when removing lignin must be filled with a substance that restores wood’s strength and allows light to pass 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 that uses limonene acrylate – a monomer made from limonene.

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

Montanari and her colleagues used an extract from orange juice production to make the polymer, which was suitable for restoring the strength of the delignified wood that transmits light.

With a thickness of 1.2 mm, the composite material offers 90 percent optical transmission and surprisingly low haze. The material is primarily intended for structural use; it exhibited heavy duty mechanical performance with strength and elasticity suitable for structural applications.

According to Professor Lars Berglund, head of the University’s Fiber and Polymer Technology department, the group had wanted to make a sustainable version of the wood composite for years.

Replacing the fossil polymers was one of the challenges we’ve had in making sustainable transparent wood, ”he said.

He said the material is made without solvents and all chemicals are derived from biobased raw materials.

The progress, the researchers say, could enable an exciting and unexplored range of applications, such as in wood nanotechnology, with capabilities such as 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 further explore these possibilities. “We looked at where the light goes and what happens when it hits the cellulose,” Berglund added.

“Part of the light passes straight through the wood and makes the material transparent. Part of the light is refracted and scattered at different angles and gives pleasant effects in lighting applications. “