What Next? Windows Made From Wood?

Researchers have developed a way to make wood transparent. This new material might one day find use in everything from architecture to packaging. How will window cleaners clean this?

How to make window ‘glass’ from wood - Researchers have figured out how to make wood transparent: Wood is a great building material. Strong and relatively lightweight, it’s also readily available the world over. One thing it isn’t, however, is see-through. So while it makes first-rate walls, it makes really poor window panes. But now, researchers have come up with a nifty way to make wood largely transparent.

This opens up many possible new uses for wood, researchers say. Engineers and architects could use the new material to make large window-like panels that would let lots of natural light into buildings, for example. This might cut down the need for indoor lighting during the day.

Lignin is the brownish substance in wood that makes it opaque. As a natural polymer, it’s made of many small repeating building blocks — chemical bits —  that are linked into a large, chain-like molecule. Lignin, in turn, bonds tightly to the cellulose and other substances in a plant’s cell walls. That’s part of what makes wood so stiff and strong, explains Lars Berglund. He’s a materials scientist at KTH Royal Institute of Technology in Stockholm, Sweden. Materials scientists analyze how the structure of materials at an atomic and molecular level relates to their overall properties. Materials scientists also analyze existing materials and use that knowledge to design new ones.

Removing lignin from wood is part of the process of making paper. In general, the more lignin you remove, the whiter the paper becomes, notes Berglund. But about 10 years ago, Japanese researchers came up with a way to make see-through paper. Their goal was a material that could be used as flexible display screens for electronic devices. Their material let more than 90 percent of the light shining on it to pass through.

Inspired by those results, Berglund’s group set out to make wood that was just as transparent but that didn’t lose its stiffness, as the Japanese material had. And they succeeded. The researchers described their new transparent wood in the April 11 issue of Biomacromolecules.

The chemistry behind clear wood:

The first step was removing that pesky lignin. To do that, Berglund’s team soaked sheets of wood just 3 millimeters (about one-eighth of an inch) thick in an acid bath for six hours. Thicker sheets, including some 2.5 times that thick, were bathed for 12 hours. These baths tested whether the solution would soak throughout the wood. And it did.

Lignin had started out amounting to 30 percent of the wood’s weight. After the acid bath, it made up only 3 percent. The acid did not affect the wood’s overall structure, however. Even the wood's cell walls remained intact. On a microscopic level, the treated wood looked a lot like a kitchen sponge, with many open spaces. With most lignin gone, much of the framework that remained was made of cellulose, another natural polymer in wood.

In a two-step process, Berglund and his team then soaked the leftover wood framework in a chemical known as methyl methacrylate (Meh-THAK-ruh-layt). Also known as MMA, its molecules can link to form a clear, shatterproof material. That plastic is better known by several trade names, including Plexiglas and Lucite.

In step one, the MMA is heated until some of its molecules bond together — but are still liquid. The researchers poured this liquid onto the framework and let it soak in. To speed the process, they put everything in a vacuum chamber. That helped force the solution into the woody framework. Then they baked the material for 4 hours at 70° Celsius (158° Fahrenheit). This bonded the remaining liquid MMA into a clear solid. The new solid was a combination material, or composite (Kum-PAAZ-it).

Making a composite was important for two reasons, says Berglund. First, losing the lignin had left the woody framework relatively weak. What’s more, that material was a cloudy white. That’s because light entering the framework was repeatedly scattered around in many directions. Every time the light passed from the material in a cell wall into an air-filled space inside a cell, or vice versa, the light’s path bent. (The same sort of bending occurs when light passes from air into water, or from water into air. Did you ever notice how a pencil leaning inside a glass of water looks bent at the water’s surface when viewed from most angles?)

Preventing light from bending too much:

That bending of light results from a process called refraction. Every transparent material has something called an index of refraction. . For most materials, that index is a number between 1 and 2, Berglund notes. The higher the difference in index between two materials, the more that light will bend as it moves from one material to the other, he explains.

The framework’s index of refraction, however, is almost the same as solid MMA. That’s a key part of the team’s innovation, says Berglund. That near-match means that light passing through the Plexiglas-wood composite doesn’t get scattered much. So instead of appearing a cloudy white, the composite is largely transparent.

Nearly 85 percent of the light shining onto one side of a hard sheet of the composite will exit out the other side. It’s even possible to read through the material if the writing is held behind it closely enough. Matching the index of refraction for each material in the new composite “is a very smart approach,” says Amit Naskar. He’s a materials scientist at Oak Ridge National Laboratory in Tennessee. “I like their work.”

Berglund and his colleagues think their transparent wood could be used to make big panels that replace windows. These could let lots of daylight into a building. By day, less artificial lighting — and energy — would be needed in such buildings.

But Naskar can envision other uses. Because it’s both clear and strong, the new composite can be used in the packaging industry, he says. And because the composite is about twice as strong as plain Plexiglas, it could either replace that material or help product designers use less of it. For example, something now made of Plexiglas alone could use the same thickness of the new material and end up with a product twice as strong. Or, they might use just half as much — which would weigh only half as much — and have a material as strong as the original.

Finally, Naskar notes, designers  wouldn’t have to keep the composite transparent. They could dye it any color. He envisions engineers might then then use the material to make things like vehicle parts.

Making Reverse Osmosis Membranes

Su Lv shows off a reverse osmosis membrane cartridge.

Making ultra-thin materials with holes the size of water molecules: While visiting GE's China Technology Center, we got to take a look at reverse osmosis membranes. Reverse osmosis is the most energy-efficient means of removing dissolved substances from water. It's what's used commercially for desalination, the process of producing drinking water from seawater.

The term "membrane" is typically used to mean a thin sheet of some material (in fact, the word "sheet" appears in the definition of the term). But for some of the things GE is using it for, the membranes were thin yet robust tubes, each one capable of supporting the weight of a bowling ball. Despite that toughness, features on the tubes are so fine that they can allow water molecules to pass through but reject many things that are roughly the same size, such as the salt ions found in seawater.

This all raises an obvious question: how do you actually produce anything like that? We decided to look into the process of making reverse osmosis membranes. It quickly became clear that the toughness of the membranes is a key feature. Water purification systems need to survive repeated cleaning cycles and go right back to use. We talked to Sijing Wang in Shanghai, who said that some membranes in the systems GE makes can be used for up to eight years.

That toughness, however, is provided by polymers that are microporous, in that they have features a thousand times larger than what is needed for reverse osmosis. These would do little to help remove salts from water, but they provide structural support for membranes that can. The large pores also ensure that water can easily flow through the system once it has passed through the membrane that acts as a filter.

 A test setup that allows Sijing Wang to see how membranes (held in the rectangular cases, lower right) respond to different types of waste material.

The interactions between that membrane and the water it's purifying help dictate the efficiency of the system. Since the membrane doesn't interact well with water, more water will flow through it when it's thinner. For reverse osmosis membranes, this layer is kept to a micrometer or less in thickness. The pores within it have to be kept small so that the ions of the salts in the water can't pass through a pore without interacting with the membrane, which will repel them. (Technically, the pores aren't small enough to physically block the ions from passing through, but the interactions between ions and the membrane keep them from getting too close to the pore opening.)

So you need to both layer a thin membrane across your support membrane and control the size of the pores that form within it, typically limiting them to less than 10 nanometers.

There are two methods of creating thin membranes. One involves forming a polymer but keeping it dissolved in a solvent that also mixes with water (often an alcohol of some sort). As you increase the fraction of water present, the polymer will eventually precipitate out. There are several ways of doing this. The simplest is to just heat the solution so that the solvent evaporates, which increases the fraction of water until the polymer precipitates. Alternatively, you can place the solution in a humid environment until the fraction of water goes up.

The most common method, however, is to create a viscous, 20 percent polymer solution and dunk it directly in water. The solution is so viscous that it won't mix into the water; instead, water infiltrates it and causes the polymer to drop out of solution. This process is often done on a continuous roll of material that's sent through a vat of water.

In all these instances, the action takes place at the interface between the polymer surface and the environment. As a result, the membrane primarily forms at this interface, creating the very thin barrier needed for reverse osmosis. Polymer deeper in the solution tends to form a larger, more open structure, which allows water to flow freely away from the membrane.

An alternative approach that functions in a similar manner is to use a building block for the polymer that dissolves in some solvents and a chemical activator that dissolves in an immiscible one. The two solvents will form two different layers (much like oil and water), and the building block and activator will only meet each other at the interface. As a result, polymerisation only takes place at this interface, resulting in a very thin layer.

How do you put holes in it? To a certain extent, the process takes care of that itself. As water begins to enter a solvent it's not fully compatible with, it will form tiny droplets that are held together by surface tension. The polymer will form around those droplets, leaving small holes behind. The size of these holes is determined by the speed of the process; the quicker it takes place, the smaller the water droplets will be and the smaller the resulting pores. By varying the solutions being used and the speed of the process, it's possible to have fine control over the pore formation process.

There are also additional layers of control possible. It's possible to include molecules that act as "pore generators" in the solutions, which are then removed when the membrane is rinsed later. Wang said most of the polymers GE uses are made of aromatic polyamine—which means a carbon ring that nitrogens are attached to. These chemicals do allow a certain degree of flexibility, in that they can be different sizes (one or more rings) and have slightly different chemical properties. (They're also carcinogenic before they're polymerised, but they're inert afterward. While water purification systems can be said to "contain a carcinogen," they pose absolutely no threat to human health.)

By adjusting the chemistry of the polymer, as well as the process by which it's formed, it's possible to have very fine control over the membrane that ultimately forms. This allows manufacturers to customise membranes for different tasks and to provide the durability that's needed for multiple years of use.

Sidney Loeb (left) with first RO membrane.

First Demonstration Of Reverse Osmosis: In the late 1940s, researchers began examining ways in which pure water could be extracted from salty water. During the Kennedy administration, saline water conversion was a high priority technology goal-"go to the moon and make the desert bloom" was the slogan. Supported by federal and state funding, a number of researchers quickly advanced the science and technology of sea water conversion, but UCLA made a significant breakthrough in 1959 and became the first to demonstrate a practical process known as reverse osmosis (RO).

At that time, Samuel Yuster and two of his students, Sidney Loeb and Srinivasa Sourirajan, produced a functional synthetic RO membrane from cellulose acetate polymer. The new membrane was capable of rejecting salt and passing fresh water at reasonable flow rates and realistic pressures. The membrane was also durable, and could be cast in a variety of geometric configurations. The impact of this discovery has been felt worldwide, ranging from applications in home demineralizers to "rivers of fresh water" in the Middle East and North Africa, where desalination facilities produce trillions of gallons of pure water every day. About 60 percent of the world's desalination capacity is located on the Arabian peninsula.

The process of osmosis through semipermeable membranes was first observed in 1748 by Jean-Antoine Nollet (pictured). For the following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1949, the University of California at Los Angeles first investigated desalination of seawater using semipermeable membranes. Researchers from both University of California at Los Angeles and the University of Florida successfully produced fresh water from seawater in the mid-1950s, but the flux was too low to be commercially viable until the discovery at University of California at Los Angeles by Sidney Loeb and Srinivasa Sourirajan at the National Research Council of Canada, Ottawa, of techniques for making asymmetric membranes characterised by an effectively thin "skin" layer supported atop a highly porous and much thicker substrate region of the membrane. John Cadotte, of FilmTec Corporation, discovered that membranes with particularly high flux and low salt passage could be made by interfacial polymerisation of m-phenylene diamine and trimesoyl chloride. Cadotte's patent on this process[4] was the subject of litigation and has since expired. Almost all commercial reverse osmosis membrane are now made by this method. By the end of 2001, about 15,200 desalination plants were in operation or in the planning stages worldwide.