Why do scientists create transparent wood? | Technology

30 years ago, a German botanist had a simple desire: to see the inner workings of woody plants without taking them apart. Got the Siegfried Fink Create a transparent tree He bleached the pigments of plant cells and published his technique in a journal specializing in wood technology. A 1992 paper was the last word on transparent wood for more than a decade, until a researcher named Lars Berglund stumbled upon it.

Berglund was impressed by Fing's discovery, but not for botanical reasons. The materials scientist, who works at Sweden's KTH Royal Institute of Technology, specializes in polymer composites and was interested in developing a more robust alternative to transparent plastics. And he wasn't the only one interested in the virtues of wood. Across the ocean, researchers at the University of Maryland were busy with a related goal: harnessing the power of wood for nontraditional purposes.

After years of experimentation, these groups' research is now beginning to bear fruit. Transparent wood could also soon be used in structural components such as ultra-durable smartphone screens, soft-light lamps and color-changing windows.

“I really believe this material has a promising future,” says Kiliang Fu, a wood nanotechnologist at Nanjing Forestry University in China who worked in Berglund's lab as a graduate student.

The wood is made up of countless small vertical channels, like a tight bundle or a bundle of hay. These tube-shaped cells transport water and nutrients throughout the tree, and when the wood is cut and moisture evaporates, air pockets remain. To create transparent wood, scientists must first modify or remove the glue called lignin, which binds bundles of cells together and gives trunks and branches their earthy brown tones. After bleaching or removing the color of the lignin, a milky white skeleton of empty cells remains.

This skeleton is opaque because the cell walls bend light differently than the air in the refracting cell cavities, known as the refractive index. By filling the air pockets with a material like epoxy resin that bends light like cell walls, the wood becomes transparent.

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The material the scientists worked with is thin: typically between a millimeter and a centimeter thick. But the cells form a strong honeycomb structure, and smaller wood fibers are stronger than fine carbon fibers, says materials scientist Liangping Hu, who leads the Transparent Wood Research Group at the University of Maryland in College Park. And with added resin, clear wood outperforms plastic and glass: In tests measuring how easily materials break under stress or fracture, clear wood is about three times stronger than clear plastics such as glass. .

“The results are amazing, a piece of wood is as resistant as glass,” says Hu. Wood properties is apparent in Annual Review of Materials Research The year is 2023.

Exposed wood usually retains its grain, giving it a natural aesthetic. The piece, made by scientists at the University of Maryland College Park, looks like frosted glass but is an excellent insulator.Hu Group / University of Maryland College Park

This process works with thick wood, but vision through that material is blurry because it scatters so much light. In their original 2016 study, Hu and Berglund found that millimeter sheets of resin-filled wood skeletons let in 80 to 90 percent of the light. As the thickness approaches a centimeter, the path of light slows down: Berglund's team used 3.7-millimeter-thick wood—roughly the thickness of two five-cent coins—to allow only light to pass through. 40% light.

The slim profile and strength of the material make it a great alternative to thin, breakable plastic or glass cutouts like display screens. French company Voodoo, for example, uses a similar lignin removal process in its wood screens, but leaves some of the lignin behind to create a different color aesthetic. The company adapts its recyclable, touch-sensitive digital displays to products such as car dashboards and billboards.

But much of the research has involved exposed wood as architectural elements, with windows being a particularly promising application, says Biochemical Engineer Prodiut Dhar at the Indian Institute of Technology in Varanasi. Transparent wood is a better insulator than glass, so it can help buildings retain heat or keep it out. Hu and his colleagues found that polyvinyl alcohol, or PVA – a polymer used in adhesives and food packaging – penetrates wood skeletons and transports transparent wood at a faster rate. Five times less than glass The group reported in 2019 Advanced functional materials.

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Researchers are coming up with other modifications to increase wood's ability to retain or release heat, which could be useful for energy-efficient buildings. Celine Montanari, a materials scientist at RISE Research Institute in Sweden, and her colleagues tested phase-change materials that switch from storing to releasing heat when they change from solid to liquid, or vice versa. For example, by adding polyethylene glycol, the scientists discovered that their wood stored heat when it was hot and released it when it cooled, according to their published work. ACS application objects and interfaces In 2019.

Many species of wood have been studied to create expressive woods such as balsa, rubberwood, birch, pine and poplar.knowable

So, transparent wood windows are stronger than traditional glass and can help regulate temperature, but the view through them can be blurry, more like frosted glass than a normal window. However, if users want diffused light, nebulosity can be an advantage: because the wood is thick and strong, it can be a light source that supports part of the weight of a building, such as a roof, providing a soft light. Room, says Berglund.

Hu and Berglund continue to play with ways to impart new properties on transparent wood. About five years ago, Berglund and his colleagues at KTH and the Georgia Institute of Technology discovered they could follow suit. Smart windows, which can change to a transparent color to block visibility or the sun's rays. The researchers sandwiched an electrochromic polymer—a material that changes color with electricity—between layers of transparent wood coated with an electrode polymer to conduct electricity. This is how it was created A dynamic wooden board From transparent to magenta when users pass a small current through it.

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More recently, both groups have focused their attention on improving the sustainability of transparent wood production. For example, the resin used to fill wooden scaffolding is usually a petroleum-derived plastic, so it's best to avoid using it, says Montanari. As an alternative, she and her colleagues discovered a completely biological polymer. Derived from citrus peels. The team first combined acrylic acid and limonene, a chemical found in essential oils extracted from lemon and orange peels. Then they impregnated the beautiful tree with it. Even with fruit filler, bio-based transparent wood maintains its mechanical and optical properties, withstands about 30 MPa of pressure more than conventional wood, and transmits about 90% of light, researchers reported in 2021. Advanced Science.

Hu's lab, for its part, has just published Scientific advances And Green method of lignin bleaching Based on hydrogen peroxide and UV radiation, this further reduces the energy requirement of the product. The team brushed 0.5- to 3.5-millimeter-thick pieces of wood with hydrogen peroxide, then placed them in front of ultraviolet lights to reflect sunlight. UV rays bleached the pigmented parts of the lignin, but left the structural parts intact. , which helped preserve more strength in the wood.

Scientists painted the word tree on a sheet of wood with hydrogen peroxide (top) and then used ultraviolet light to bleach the painted areas (center). The penetration of epoxy into the wood made it transparent (below); A sheet with a transparent pattern distinguishes transparent and opaque sections. K. XIA ET AL / Scientific Advances 2021

These eco-friendly approaches have helped limit the amount of toxic chemicals and fossil polymers used in production, but now glass has a lower environmental impact than transparent wood at the end of its life, according to an analysis by de Tar and his colleagues. The science of the total environment. Adopting greener production methods and increasing production are two necessary steps to introducing transparent wood to mainstream markets, the researchers say, but it will take time. However, they believe it is possible and believe in its potential as a sustainable commodity.

“When you're trying to achieve sustainability, you don't just want to match the properties of fossil-based materials,” says Montanari. “As a scientist, I want to overcome that.”

Article translated Debbie Bonchner.

This article appeared first Spanish is knownA non-profit publication dedicated to making scientific knowledge available to all.

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