Monday, September 12, 2016

Stronger and Lighter Than Frosted Glass, Translucent Wood Reflects the Future of Construction

This is not frosted glass. It’s translucent wood.

Translucent wood from the lab of Dr. Liangbing Hu.
Image Credit: Eran Moore Rea, American Physical Society

Soon, the frosted windows that let light into your bathrooms and offices while protecting your privacy may be made out of a wood-polymer composite. The innovation comes courtesy of Dr. Liangbing Hu’s University of Maryland Materials Science and Engineering lab.

And that’s not its only purpose. Dr. Tian Li, a post-doc in Hu’s lab, says translucent wood could also be used to improve the efficiency of solar cells by uniformly scattering the photons coming in from the sun across the solar cells.

The first news about translucent wood broke this May. The process hinges on the breakthrough that it is possible to remove the lignin—the “glue” that keeps wood together and gives it its yellow-brown color—without totally destroying the microstructures of wood. Carefully done, it is possible to remove the lignin without destroying the lumens, the tiny tunnels in wood that house the lignin.

Traditional paper-making processes also remove the lignin from wood by crushing the wood into pulp and then bleaching it. But that process destroys the natural microstructures of the wood, including the lumen tunnels.

Now Hu’s group has a new paper to prove and expand on their translucent wood’s applications.

They’ve proven that their substance is as waterproof as glass or plastic. While natural wood is notoriously water-absorbent (as anyone who’s ever tried to light a bonfire with wet sticks knows) the wood-polymer hybrid performed almost as well after three days immersed in water. Many wood materials used in current construction are either actually a mixture of different types of materials, or must be treated to be water-resistant.

“When you’re talking about using this wood for a roof or a window, you don’t want to worry about rainy days,” Li said.

Translucent wood windows are lighter and stronger than glass. Hu’s group dropped the same sharp, pointed object on both glass and translucent wood; the glass shattered but there was barely a dent in the wood. So in earthquakes or other emergencies, it’s more difficult to break translucent wood than glass.

Of course, eventually with enough force the translucent wood will break, but the polymers in the wood lumens allow translucent wood to break cleanly in two, with no wood chip scattering.

If you were using the translucent wood as the roof for an office building, the quality and the brightness of the light in the room would be consistent from sunrise to sunset, because the alignment of translucent wood channels consistently guides light through the material. “If you try to use a glass roof, you’ll always have a really bright area in the room below and a really dim area,” Li said. We are all aware of the problems this can cause, “like when you’re driving during sunset you always have to put up something, because the glare hurts your eyes,” Li said.

And it’s not just the quality of light; translucent wood also has higher transmittance than frosted glass. So even though frosted glass and translucent wood both obscure the details of the inside of your bathroom, translucent wood allows more light to actually make it through the window. Translucent wood makes rooms brighter than they would have been with frosted glass.

Uniform, strong light coming in from a translucent window or roof can help you save money by requiring less electricity to light a room during the day during the day. And transparent wood is a better thermal insulator than glass, allowing you to spend less on air-conditioning and heating.

The American basswood samples Hu’s lab uses look like typical wood slabs—beige-brown with visible wood grains. The first step of Hu’s process, the lignin removal, turns the wood white and makes it Styrofoam-like in texture. The second step, the filling of the lumens, makes the wood stronger and translucent, as shown below.

Image Credit: Eran Moore Rea 
A few centimeters above the page, a translucent slab of wood renders text unreadable,
while still letting a good deal of light through.
Image Credit: Eran Moore Rea
Hu’s lab makes the wood translucent by using a process called index-matching. The index of refraction indicates the speed of light in a material by using Snell’s law to calculate how much a beam of light bends when it travels through a material. A totally transparent material does not absorb light and is made up of components with a similar index of refraction. Normal wood is not transparent or translucent because the lignin absorbs light. Hu’s wood is not translucent after the first step (the lignin removal) because the wood lumens have a very different index of refraction from the air that fills them—much like how a snowflake appears white, even though ice and air are both clear. The index of refraction of the epoxy Hu’s lab uses is almost the same as the wood itself, making the material translucent.

The polymer the group is currently using, epoxy, is not biodegradable, so the translucent wood Hu’s lab is making would not naturally break down like organic wood. Biodegradable epoxies would make the translucent wood process more expensive, but not impossible; Li says it is possible to use a biodegradable polymer to fill the lumens and make biodegradable translucent wood.

As for the cost of the new material, all of the chemicals used in the process are common in the paper industry and readably available. However, the specific process of delignification and then filling the lumen in wood with polymers surprised Hu’s contacts in the paper and lumber industries. “We’re looking at wood on a nanometer (one billionth of a meter) scale,” Hu said. “Traditional wood is a material with a lot of bulk properties, but when we say nano, it’s not just that we have a name for the material, it’s that we’re focusing on the nano-enabled properties of wood, on the properties that become possible at that scale.” Dr. Hu’s lab is currently working towards commercializing translucent wood.

Eran Moore Rea

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