Tag Archives: Transparent

Citrus fruit stands poised to make transparent wood more sustainable, stronger, and more transparent

Transparent wood is getting a citrusy update that’s poised to make it more sustainable, hardier, and even more transparent.

A piece of the new transparent wood. Image credits Céline Montanari.

First developed five years ago by researchers at the KTH Royal Institute of Technology, transparent wood is definitely an interesting material. It has many of the characteristics of regular wood (and, indeed, starts out life as such) but it’s generally stronger, more resilient, transparent, and an ok medium to store thermal energy (heat) in.

Now, new research reports how this material can be further improved with a little help from citrus-derived compounds.

Needs some lemon

“The new limonene acrylate it is made from renewable citrus, such as peel waste that can be recycled from the orange juice industry,” says Céline Montanari, a Ph.D. student at the KTH Royal Institute of Technology and lead author of the study.

The process of making transparent wood involves chemically stripping lignin out of wood. Lignin is a natural polymer that plants such as trees use to give their tissues mechanical strength, but it’s also the main light-absorbing compound in there. The empty spaces left over after all this lignin has been removed are later filled in with another transparent compound to restore the material’s strength while allowing light to pass through.

At first, fossil-based polymers (such as synthetic resins) were used for this role. The new paper reports on an alternative to these polymers: limonene acrylate. This is a monomer (individual building blocks of polymers) produced from limonene, which is, in turn, available in the oils found in citrus fruits.

Transparent wood created using the new approach offers much improved optical properties — a “90% optical transmittance” through a plate 1.2 mm thick and a haze of only 30% — the team explains. Unlike other similar composites developed over the last 5 years, transparent wood produced using limonene acrylate is strong enough (and intended to be used) for structural use such as girders or beams. It’s also more sustainable than previous incarnations of the material.

“Replacing the fossil-based polymers has been one of the challenges we have had in making sustainable transparent wood,” Professor Lars Berglund, the head of the KTH’s Department of Fibre and Polymer Technology and corresponding author of the study.

The material requires no solvents to produce, and all the compounds used in the process are derived from biological raw materials. The novel way this material interacts with light further opens new possibilities in fields such as wood nanotechnology, he adds.

“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 now hard at work exploring some of these potential applications.

The paper “High Performance, Fully Bio‐Based, and Optically Transparent Wood Biocomposites” has been published in the journal Advanced Science.

transparent wood

This transparent wood is stronger than glass

Using a chemical technique, researchers removed the complex organic polymers that give wood its characteristic appearance and, in the process, made the wood transparent. The see-through wood was then imbued with epoxy which made the material stronger than glass.

transparent wood

Credit: Advanced Materials

Earlier this year, ZME Science reported how a group from Sweden made optically transparent wood. The researchers at University of Maryland used a very similar method as their Swedish colleagues, with a couple of notable differences.

First, the lignin is removed from the wood through boiling in a chemical bath for several hours. With the lignin extracted, the woody material became transparent. Epoxy was then poured over to make the wood four to five times stronger, as reported in Advanced Materials

At this point, you might be wondering what’s the point of making wood transparent. Well, wood is a great material because of its mechanical properties, created by its structure and the interactions between cellulose, hemicelluloses and lignin. In electronics,  abundant cellulose nanofibers (CNF) and cellulose nanocrystals extracted from wood are highly sought for due to their desirable optical properties. 

The wood was boiled in water, sodium hydroxide and other chemicals. Credit: Advanced Materials

The wood was boiled in water, sodium hydroxide and other chemicals. Credit: Advanced Materials

What’s really interesting about transparent wood is that the material retains the micro-channels used to shuttle nutrients when it was a tree. This creates a waveguide effect which lets more light in. Traditional glass scatters light.

The applications could be really huge, ranging from really cool see-through furniture, to high-tech optical lab equipment. Before this happens, though, the researchers need to figure a way to scale the process because right now they can’t use the method to make transparent blocks larger than five-by-five inches.


KTH researchers develop transparent wood for use in building and solar panels

Wood, one of the cheapest and most widely used construction materials humanity has ever employed,  just had its range of uses expanded: researchers at Stockholm’s KTH Royal Institute of Technology developed a method that makes wood transparent. The method is suitable for mass production, making it even more attractive.

A close-up look at the transparent wood created at KTH Royal Institute of Technology.
Image credits KTH Royal Institute of Technology.

Optically transparent wood is not a new thing, says Lars Berglund, professor at the Wallenberg Wood Science Center at KTH. But it’s usually only been done in microscopic samples intended for wood anatomy studies. Their new process would allow for transparent wood production and usage on a much larger scale than anything ever before attempted.

“Wood is by far the most used bio-based material in buildings. It’s attractive that the material comes from renewable sources. It also offers excellent mechanical properties, including strength, toughness, low density and low thermal conductivity,” Berglund says.

“Transparent wood is a good material for solar cells, since it’s a low-cost, readily available and renewable resource. This becomes particularly important in covering large surfaces with solar cells.”

These transparent panels can also be employed as windows, or used to create semitransparent facades to allow light in while also maintaining privacy.

Optically transparent wood is actually a type of wood veneer from which lignin, a structurally-important component in the cellular walls of trees, is chemically removed. The resulting porous veneer substrate is saturated with a transparent polymer and the optical properties of the two materials are then matched.

“When the lignin is removed, the wood becomes beautifully white. But because wood isn’t not naturally transparent, we achieve that effect with some nanoscale tailoring,” Berglund adds.

“No one has previously considered the possibility of creating larger transparent structures for use as solar cells and in buildings.”

Wood is a renewable resource, but that doesn’t mean we’re doing it substantially  — we have to grow and harvest it accordingly, not by logging away, chainsaws blazing, at the forests around us. The KTH team is now working on ways to improve the transparency of their material and on scaling-up their production method.

“We also intend to work further with different types of wood,” Berglund concludes.

The full paper, titled “Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance” was published online in the journal Biomacromolecules and can be read here.