Material of the Month – ETFE

Materials World magazine
,
2 Mar 2016

This month, Anna Ploszajski explores the material that has formed part one of the most remarkable landmarks in Cornwall.

Our story begins in 1938, in the laboratory of Roy Plunkett, also known as the birthplace of the material behind the non-stick frying pan – Teflon (poly-tetrafluoroethylene, PTFE). Plunkett’s discovery of a high temperature, corrosion resistant, fluorinated polymer opened the doors to a whole new branch of polymer science. DuPont followed Teflon with Teflon FEP (fluorinated ethylene propylene) in 1960, which improved on Teflon’s inability to be melt-extruded and injection moulded. Finally, in 1970, they introduced ethylene tetrafluoroethylene, known as ETFE – a modified co-polymer made from ethylene and tetrafluoroethylene.

Here was a polymer that was transparent to UV light, strong, virtually tear-resistant, didn’t weather and, crucially, could be commercially extruded into thin films. Its first use was in the relatively unglamorous setting of greenhouse poly-tunnels, as an upgrade to polyethylene sheets. But this material caught the eye of German mechanical engineer and sailing enthusiast Stefan Lehnert. In 1981, Lehnert invented a way to weld large sheets of ETFE foil to itself, using a drop bar welding technique. He hoped to make waves with this as an exciting new sail material, but this didn’t quite work out – instead, he co-founded Vector Foiltec with architect Ben Morris, with the idea of making ETFE into a lightweight construction material.

Their first job was in 1982 at Burgers Zoo, the Netherlands, to fit an ETFE roof on the Mangrove Hall. At just 1% of the weight of glass, roofs made from ETFE films significantly reduce the weight of a structure, the amount of supporting architecture and the overall cost. With its fantastic resistance to chemical degradation and UV light, the original ETFE roof is still in place and fully functional to this day, with no sign of visible wear after more than 30 years. Being transparent to visible and UV light was critical for the Zoo’s purpose, since the ETFE allowed plant growth inside while withstanding the elements.

Designing Eden

Other structures using ETFE followed across Europe including roofs for hospitals and schools. It wasn’t until 2001, when Nicholas Grimshaw & Partners produced the unmistakable geodesic domes of the Eden Project in Cornwall from so-called ‘cushions’ of ETFE that this material came into global prominence in the architectural engineering field. These ‘cushions’ are made from two or more ETFE films with an aluminium perimeter, and a pocket of pressurised air inside, maintained by small inflation units. This air provides structural stability and thermal insulation, and is dried prior to use to avoid condensation in the system. Only one 60–100W unit is required every 1,400 square metres of roofing, and the entire system can be automated with in-built sensors. The ETFE foils can also be modified to adjust their optical transparency, by fritting a number of different patterns onto the surface. What’s more, an ETFE surface is non-stick, similar to Teflon, and is therefore self-cleaning because dirt finds it difficult to adhere, and rainwater runs straight off.

The success of the Eden Project catapulted ETFE on a global scale, and it was followed in 2005 by a 20,000m2 window at the Tropical Islands resort in Germany and in 2008 by the National Aquatic Centre, also known as the Watercube, at the Beijing Olympics. This stunning architectural design by PTW Architects involved cladding the entire building in more than 100,000m2 of ETFE panels, fitted with LED lighting, so that the building resembled a freestanding cuboid block of bubbles. It remains the largest ETFE film structure in the world.

The explosion in popularity of this novel material for striking building design has seen it applied across the globe, from the National Stadium in Singapore to an outdoor music venue in Kansas City and a shopping centre in Athens. There are many examples closer to home, too, ETFE architecture features in several British railway stations and the Heathrow Terminal 5 station has parts of its roofing made from ETFE laminate panels, allowing natural daylight into the underground platform tunnel. Other stations include Manchester Victoria concourse, Newport and Barnsley Interchange.

ETFE has the green credentials of being 100% recyclable, as well as having minimal energy requirements for transportation and installation, thanks to its low weight. Furthermore, Vector Foiltec has patented a method of laminating flexible photovoltaic (PV) cells into ETFE, which was first applied to the library of John Wheatley College in Glasgow by ABK Architects. Completed in 2007, the roof comprises a 3mm thick PV membrane made from thin slivers of stainless steel bonded with amorphous silicon to a mesh, encased in a vacuum-formed ETFE wrapper, and then laminated to the outer face of the ETFE cushions.

Not just architecture

For a material with such an advantageous combination of properties, you’d be surprised that the only mainstream application for it is in architecture, although it does have some niche applications. ETFE has better corrosion resistance than Teflon, and a wide working temperature range of -185°C to 150°C. Together with electrical and high energy radiation resistance, these properties make it an excellent material to use as a cover for electrical and fibre optic wiring in critical applications operating in harsh environments such as in aircraft and spacecraft wiring and in the nuclear industry.

These same properties make ETFE films in fibre reinforced polymer laminates useful as a thermoplastic liner in pipes, tanks or other vessels, preventing them from corrosion. They can also be used as a mould-release film in shaping processes – the industrial analogue of the Teflon coating on a toasted sandwich maker.

ETFE has safely secured its place in the Olympic history books, and is breaking into the mainstream of sustainable building materials. Next time you find yourself in a brightly sun-lit indoor space, look up. You might be lucky enough to enjoy this material for yourself.