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September 1, 2011

So Beautiful, the Lotus

Nanotechnologies now under development, some nearing market stage, bring with them the promise of smart coatings that may significantly broaden product offerings for the metals supply chain.

The industrial metals supply chain may soon have available to it an array of new products that clean, disinfect and repair themselves, and even improve air quality, through the use of high-tech coatings that make simple paints look like nothing more than pretty colors.

Smart coatings—intricately engineered laminates or polymers that respond to light, heat or other environmental stimuli—have only recently been recognized as a distinct area of research and development, as well as a market for new products. Yet it's a growing market, with its own cohort of university researchers, corporate research and development programs and companies with the latest discoveries. NanoMarkets, a market research firm, estimates that sales of smart coating products in the construction, automotive, textile, military and medical fields should increase 13-fold, to more than $3 billion, by 2018.

In development—and, in a few cases, already being marketed—are products that range from self-healing paint and self-cleaning walls and windows to helicopter rotor blades that re-sheath themselves, ship bottoms that shed barnacles with the touch of a button, and steel panels that can generate electricity when exposed to sunlight. And let's not forget a potential competitor, “smog-eating cement,” with photo-catalytic ingredients that not only effortlessly keep walls clean, but also break down nearby air pollutants.

All of these products are within reach of the market. For example, researchers at Case Western Reserve University, the University of Fribourg and the Army Research Laboratory are developing a polymer that, because of its customized design, can reformulate paint to heal scratches and scrapes when subjected to ultraviolet light. Private technology companies, such as Norcross, Georgia-based nGimat Co., are close to bringing to market their nano-engineered “superhydrophobic” surfaces that clean themselves on windows, solar panels and building exteriors. The U.S. Department of Defense is exploring ways to introduce self-healing polymers for helicopter rotor blades, which are made of complex fiberglass layers that can delaminate after extensive wear and tear. Groton, Connecticut-based PEL Associates has developed a de-fouling system for ship bottoms, using smart coatings of polymer and silicon, which will peel away along with efficiency-reducing barnacles, in response to an electrical signal.

As for the smog-eating cement, Italcementi of Bergamo, Italy, has developed a building material laced with photocatalytic titanium dioxide, which not only releases oxidizing reagents that erase the impurities that attach themselves to cement structures, but also destroy airborne pollutants such as nitrogen oxides and sulfur oxides. The company says it has tested its product on busy streets and buildings around Milan and found reductions in ambient nitrogen oxides of as much as 60%. Italcementi's subsidiary Essroc Cement is now marketing the material in North America, calling it TX Active.

PRACTICAL POLYMERS

Using polymers, with their long, custom-designed molecular chains, and employing chemical processes that emulate plant photosynthesis and the dirt-shedding properties of the lotus flower, researchers are tackling problems as diverse as proliferating greenhouse gases, corrosion on military equipment and keyed paint jobs on cars.

The twin promises of smart coatings are greater longevity and new functionality, says the NanoMarkets report “Smart Coatings Markets 2011,” which was co-authored by William Gathright and Sreekanth Venkataraman. Products can potentially reduce the cost of maintenance, extend the life of structures and equipment (as with smart paints and anti-corrosives) or they can, using “chemical intelligence,” perform tasks that once required many man-hours.

One of the most potentially profitable of the new products engineered for functionality is the organic photovoltaic surface, which can serve both as a structural component of a building and as a power generator. Conventional solar panels are deployed on roofs or in adjoining lots in dress parade rows, tilted to catch the maximum amount of direct sunlight. But the new photovoltaics are coated on building materials—roof panels, walls, vehicle exteriors, even windows—or installed as pre-primed panels, and they work whether there's direct sunlight or not.

Tata Steel and the Australian solar technology company Dyesol have collaborated on a dye-sensitized photovoltaic surface that, they say, will be ready for commercial application within the next three years. At the companies' testing site in Shotton, Wales, the material has been applied in a continuous coating process to rolled steel, producing lengths of 100 meters and more of photovoltaic metal, as opposed to individual cells in linked daisy chains. The treated steel can be used to form building facades or roofs with an integrated photovoltaic function.

“Let's say you want to build a Walmart store,” says Marc Thomas, president of Dyesol North America. “You produce the required amount of material, set it up on site, and connect it to the electrical system. Now you have a building that's a power plant while meeting all the requirements of a standard steel structure.”

The coating is long-lasting. “It does not 'wear' per se,” Thomas says. “Its lifetime is unknown, but testing [so far] demonstrates usable life beyond 30 years.”

The idea of photovoltaic surfaces sprang from the work on artificial photosynthesis of German chemistry professor Michaël Grätzel, a professor at Ã?cole Polytechnique Fédérale De Lausanne, and American chemist Brian O'Regan. In 1991, they developed a stratified coating that mimicked nature, with a porous titanium oxide layer in place of the leaf structure, a photosensitive dye as the “chlorophyll” and an electrolyte as a conductive material. In place of the glucose produced by a leaf, Grätzel and O'Regan's cell produced electricity.

A MAN-MADE “LEAF”

The Dyesol-Tata dye solar cell employs the same principle, Thomas says. A thin but stratified coating is applied to a substratum of steel, glass or polymer. When light strikes the dye, it excites electrons, which are released and absorbed by the titanium dioxide, a photocatalytic white substance that has for years been used more mundanely as toothpaste coloring or the chalk lines on tennis courts. Depending on the location of a fully coated building and the power demands of the activities within, the electricity generated by photovoltaic surfaces should be enough to light it and keep its HVAC services running, Thomas says.

Dyesol and Tata expect their coating to eventually overtake standard solar cell technology. “We're at the same point where solar cells were four or five years ago, before they received a momentous rise in market acceptance,” Thomas says.

The new technology has marked advantages over the old, he says. Conventional solar cells use crystals or purified silicon as semiconductors, both of which are much more expensive than the materials used in photovoltaic coatings. The coatings have the additional advantage of responding to indirect light, as opposed to only direct sunlight.

“They could be in the shade and still work,” Thomas says. “It just requires diffuse light at any level, causing the matrix to wobble and kick off electrons.” That means that the surface can generate small amounts of power even in moonlight, he adds.

The two companies recently announced that their researchers had produced “the world's largest dye-sensitized photovoltaic module,” a 10-foot-long, electron-spewing panel, ready for installation on a roof. The module demonstrates the researchers' ability to translate a laboratory concept to a workable model, says Paul Bates, Tata's operations manager at the Wales site.

Other researchers are in the hunt for low-cost photovoltaic coatings. Somenath Mitra, a professor at the New Jersey Institute of Technology, heads a group that has developed polymers incorporating carbon-based nanostructures that are photoactive. Solar radiation excites the polymer matrix, throwing off electrons. One set of carbon structures traps the electrons, not allowing them to recombine in the polymer matrix, while another set carries the electrical charge.

Mitra's nanotubes—molecular-level carbon formations, each 50,000 times thinner than a human hair—are more conductive than copper. They exhibit “ballistic conductivity,” Mitra says, and serve as “tiny wires.” Because of the infinitesimal scale of the carbon structures, they can be combined with paint and applied to surfaces with a brush.

The major barrier to commercial applications for what Mitra calls his “organic solar cells” is their relative lack of efficiency. Conventional solar cells are able to convert about 10% of the solar radiation that hits them into electrical energy, and the Dyesol-Tata coating is said to be even more efficient than that. So far, the organic cell is stuck somewhere below 3%.

Businesspeople like Thomas tend to see the expansion of the smart coatings market in terms of smart companies pursuing profits or cost savings. The driving force in the burgeoning new market is “positive ROI,” Thomas says. Return on investment. But for scientists, the driving force behind these innovations is the extension of beneficial knowledge by talented researchers.

Many new products, like Mitra's organic cells, are being developed with new microscopy techniques, which allow researchers to visualize materials at a nano scale. Those techniques for seeing molecules permit scientists to reconfigure the long linked molecules of plastics so that they respond to external stimuli in new and useful ways.

HEAL THYSELF

Case Western Reserve chemistry professor Stuart J. Rowan describes how his team of researchers developed a polymer which is “self-healing.” That is, it responds to ultraviolet light by liquefying, then re-bonding, filling in scrapes and scratches. The material, which researchers call a “supramolecular” polymer, differs from other polymers in that it has smaller molecules which are “glued together” with metallic ions. Ultraviolet light melts the molecules' “sticky ends,” Rowan says, allowing the polymer to flow into damaged spots on a surface. Then, with the scratches filled in, the substance re-bonds.

The polymer is designed to take advantage of “weak interactions” between the molecule and the “glue” that holds the molecules together, Rowan says. And no, there's no danger that the paint job on a car's self-healing surface will liquefy in the intense sunlight of, say, Arizona or Florida.

“You need a higher intensity than sunlight provides,” Rowan says. “You need something more akin to the intensity of a dentist's light,” the device that dentists use to cure newly installed fillings with focused ultraviolet light.

Rowan acknowledges that the self-healing paint is “not yet ready for prime time.” Surfaces using current models tend to become less efficient when metal substrates, such as the body of a car, absorb some of the ultraviolet light as heat. “Over a period of time, you might get a slow creep,” Rowan says. Still, such problems are solvable, either with an extra thermal insulator or a thicker coating, Rowan says. The research team is in discussions now with companies that want to use the supramolecular polymer for various purposes, including car, floor and furniture surfaces.

Of all the new smart coatings that are in development, the one that may be closest to market is the “self-cleaning” surface. One of a number of technology development companies that are adapting the research of German botanist Wilhelm Barthlott on the self-cleaning lotus flower is nGimat Co., most of whose principals are Georgia Tech materials science researchers.

In the early 1970s, Barthlott, using a scanning electron microscope, studied the petals of lotus flowers, which are known for staying pristinely clean in muddy environments. He discovered a pattern of microscopic bumps, which created a rough, stick-proof surface that has been compared to a bed of nails. The bumps impede organic material from attaching itself to the petal. They also cause water to bead rather than flatten out. Thus, when dirt falls on the surface of the flower, it balances on the sharp points rather than sticking to the flat areas. When beads of water from rain roll off the petal, they carry the dirt with them. In 1997, Barthlott secured a patent for the “lotus effect” and began to develop self-cleaning products, among them a self-cleaning paint for exterior surfaces, which is now marketed in the United States as Lotusan.

Other researchers are developing polymers and custom-designed organic coatings to mimic the effects of the lotus on glass and metal surfaces. At nGimat, a lotus-like super-hydrophobic coating should be on the market within two years, says Yongdong Jiang, the company's program manager who is a graduate of the Georgia Tech doctorate program in materials science and engineering. The new coating will make its first appearance on self-cleaning automobile windows, he says.

Using federal Small Business Technology Transfer grants, nGimat has developed a coating that will eventually be applicable to stainless steel, copper, aluminum and ceramic surfaces, as well as glass, Jiang says.

Self-healing hydrophilic surfaces (using a layer of that photocatalytic titanium dioxide to break down organic impurities) have been on the market about 10 years, the NanoMarkets report says, mostly as self-cleaning windows. Both Alcoa and ArcelorMittal are launching new panels, which, like Italcementi's innovative cement, offer self-cleaning properties for building exteriors. Alcoa's aluminum panels with Reynobond and ArcelorMittal's steel panels with Granite Forever both employ titanium dioxide as a photocatalytic agent. Some of the new self-cleaning materials already on the market, however, remain “niche products,” NanoMarkets says, with minimal impact on new home or office construction.

NOT EVERYTHING WORKS

Some products don't do well when they're finally offered to potential buyers. What makes NanoMarkets so bullish on smart coatings?

“You have to ask: Do smart coatings solve a real problem for a real paying customer today? Is there a broad applicability across a variety of applications? Do smart coatings enable new functionality? Do smart coatings decrease the overall cost of ownership of the underlying product?” says William Gathright, co-author of the NanoMarkets report.

He believes the answer is a yes to all four. “That is a healthy sign,” says Gathright. “When all four of these factors are in place, there is much less risk of the industry fizzling out, or being a one-hit wonder. If one particular product doesn't work out, there are many other applications to develop.”