ETFE, a fluorocarbon-based polymer, is a durable, adaptable plastic that's opening horizons for builders at the Beijing Olympics and beyond
Imagine a swimming arena made out of bubbles. Or a stadium knit from steel girders like a bird's nest. Or even an enormous tent, proudly covering over a million square feet of space. A decade ago, such buildings might have existed only in the imagination. Today, they're being built in Beijing as China's new National Stadium and National Aquatics Center and as the Khan Shatyry Entertainment Center in Astana, Kazakhstan. All thanks to innovative architects, adroit engineers—and the unusual properties of the material called ETFE.
ETFE may be about to get its moment in the architectural spotlight, but in fact, it has been around since the 1970s, when DuPont invented a fluorocarbon-based polymer, Ethylene Tetrafluoroethylene, for use as an insulation material in the aeronautics industry.
The interesting property for architects is that the resin can be spun into a thin, surprisingly durable, film, which manufacturers such as DuPont (also Asahi Glass Company, which calls its version Fluon) pack in rolls—like a sturdier version of plastic cling-wrap. It can be used in sheets or inflated into pillows, and with its incredible versatility, it has become the go-to material for those in search of an alternative to more traditional materials, such as glass.
As it happens, DuPont didn't really care about pushing ETFE to architects at all. That fell to Stefan Lehnert, a German mechanical engineering and business administration student and avid sailor, who stumbled across the material in his search for new sail technologies. Having discounted it as inappropriate for his sailing needs, he nonetheless saw building-material potential in its transparency and its self-cleaning and structural properties.
In 1982, he founded Vector Foiltec—a design and manufacturing company specializing in the use of ETFE—in Bremen, Germany, and began shopping the material around to architectural firms. The company's first project, the roof of a small pavilion at a zoo in Arnheim, Holland.
Since then, ETFE has become increasingly popular, especially in Europe. The 1990s saw it used in office atria, university buildings, medical facilities, exposition halls, and zoos across Britain and Germany. In 2000, the Eden Project, a huge environmental complex in Cornwall, Britain, containing two gigantic geodesic conservatories covered in ETFE, was unveiled. Designed by Grimshaw Architects, the construction was widely acclaimed as an engineering marvel, and created a wave of global interest.
Bubbles and Twigs
With the spotlight on the Beijing Olympics, designers expect ETFE to go mainstream at last. Certainly there's no better illustration of the material's ability to turn architectural fantasy into reality than the Beijing Olympic Green, located at the north end of the central axis of Beijing City. There, less than 500 meters apart, sit the rapidly rising National Stadium and National Aquatic Center.
The two structures could not look more different. The Herzog and de Meuron-designed stadium is crafted out of woven steel and resembles a sturdy but intricate bird's nest. The Aquatics Center, nicknamed the Watercube, is refined and delicately detailed, an iridescent box covered in what appear to be bubbles. When completed later this year, both will showcase innovative uses of ETFE. To protect spectators from rain and wind, the stadium will feature red ETFE cushions inserted in the spaces between the "twigs" of its "nest." The 750,000 square foot Watercube, the largest ETFE project ever, will be clad entirely (roof and four walls) in blue ETFE cushions.
Given the extensive size and expense (an estimated $100 million) of the Watercube project, it's surprising to note that this will be the first time that Sydney, Australia's PTW Architects have actually used the fabric. They're that confident. John Bilmon, managing director at the company, says they chose ETFE over glass and fiberglass because it satisfied the project's engineering needs. Some bubbles in the design span 30 feet without any internal framing—a distance that wouldn't be possible with other materials.
No Sharp Objects
But the ETFE system also cost less (though they wouldn't give specifics) than an equivalent traditional system, freeing up money for higher-quality filtration and water-treatment systems for the center's pools. A more traditional form of cladding would have been not only more expensive and cumbersome, says Bilmon, it would have resulted in a "…less exciting, beautiful, and functional building."
The material's appeal is manifold, and those who work with it praise its unique properties. First, it's extremely light—about 1/100 the weight of glass—and deceptively strong, able to stretch to three times its length without losing its elasticity. (Having said that, a sharp implement like a knife can puncture it—one reason it's used mostly for roofs.)
If the film does tear, it can be patched with other pieces of ETFE. When exposed to fire, it softens and shrinks away from the heat, naturally venting smoke out of a building. And it's naturally nonstick, nonporous surface, which has chemical properties similar to DuPont's other best-selling material, Teflon, is so slick that dirt, snow, and rain simply slide off.
Its light weight reduces corresponding structural costs. Edward Peck, managing director of the North American Division of Foiltec, which now has 12 offices and 250 employees worldwide, estimates that a simple, small roofing project could be 10% cheaper if ETFE were used. For larger, more complicated projects, the overall construction savings could reach 60%.
Then there's the fun factor. ETFE comes in different finishes (transparent, matte) and colors, and can be lit from within using LED lights or decorated with light projections like a giant movie screen. It can be printed with patterns by running it through a special press—something not possible with glass. It can take myriad shapes, too: Strips can be heat-welded together like fabric squares in a quilt. This "sewing" method enables ETFE to be installed in pieces much longer and wider than glass. A large glass panel might measure 10 ft. by 5 ft., whereas a strip of ETFE could be 180 feet long and 12 feet wide, with structural supports.
It also scores well on the environmentally friendly front, particularly crucial given the current call for greener building practices. The film is recyclable (simply melt and reuse), and due to its light weight, doesn't require much energy to transport. The Watercube is designed to gather heat passing through its ETFE walls and roof—energy that can be used to heat the building's water systems or expelled through vents if the building gets too hot.
On an aesthetic level, the cushions reinforce the building's theme. Their pillowy shapes evoke a bubbles's roundness, and their triple-layered construction, which mixes layers of blue film with transparent film, gives the façade a sense of depth and shifting color. Once the games start in August, 2008, officials will be able to transform the walls into a giant TV screen showing simultaneous projections of the swimming activities taking place inside.
For all its wondrous properties, ETFE isn't an entirely perfect material. In its typical usage, two or three layers are welded together and shipped flat to the job site, where they're inflated into panels or "cushions". These cushions require semi-continuous air pressure—to keep them stable and give them thermal properties—so most systems include thin hoses that plug into the cushions' sides.
These air-supply lines connect to a computerized system that monitors the pressure within the cushions. This system can also feed air into, or eject air from, particular chambers or layers to let in more light or create more shade, meaning the ETFE cushions act as a dynamic puffer jacket for buildings. In some installations, this is done automatically using light sensors.
Of course, many projects don't call for such complexity. "You have to evaluate, project by project, what the driving force is for using ETFE," says Foiltec's Peck. "Is it for architectural imagery, for transparency, for structural reasons or thermal performance?" He doesn't advise using it for small-scale or residential projects.
Acoustics can be another drawback. The cushion system, when used on a roof, can amplify the sound of rain because the tension in the cushion acts like a drum. Manufacturers have developed several noise-suppressing techniques, including layering polycarbonate sheets within ETFE cushions, but their use isn't widespread yet.
Interior applications, such as walls within an office, present other sound issues. ETFE transmits more sound than glass or wood, making it ill-suited for meeting rooms or conference centers near airports, to name one ETFE proposal that was quashed by noise concerns. Conversely, ETFE can be beneficial for self-contained, noisy areas like aquatic parks—sound bounces off the walls and floor and escapes through the roof.
Just the Beginning
But plans are already under way to address these concerns, and ETFE is already cropping up in more and more locations. Foiltec has eight projects that will be built in the U.S. next year (and more than 100 projects slated worldwide), and has just completed an atrium roof for a U.S. Federal building. Peck hopes the latter project might prove to be a turning point for ETFE in the U.S., which has been notably slow to catch on to the material's potential. ETFE was named in three of the four submissions for the Beijing National Aquatic Center, and can be seen in several proposals for the 2012 London Olympics.
Another large ETFE project is already on the horizon: The Khan Shatyry Entertainment Center, a 1,076,000-sq.-ft. tent-shaped recreational complex in the capital of Kazakhstan, designed by London firm Foster + Partners is due for completion next year. Other top-tier architecture firms, like Skidmore, Owings & Merrill (SOM) and Gehry Partners, are said to be considering ETFE for upcoming projects.
And other innovations are still being developed. Foiltec is currently testing whether it might be possible to attach photovoltaics to ETFE panels or use an insulating "nanogel" to increase a panel's thermal properties. As Bilmon, the Watercube architect, says: "There's a new realization that the whole of the world is facing sustainability issues—and solutions like ETFE are required for the future."
Click here to view how ETFE is being used in buildings around the world.