Last edited 03 Dec 2016

ETFE

Contents

[edit] Material composition and properties

ETFE (ethylenetetrafluoroethylene) consists of modified copolymers of ethylene and tetrafluoroethylene. It is closely related to PTFE (polytetrafluoroethylene or Teflon), and has many similar properties. It has been widely used in the construction industry in recent years.

ETFE is available as a flexible film. This enables it to be used to create curved transparent facades. It is a super-lightweight material; a double layer cushion weighs only 0.70 kg/m2, whereas a single layer of glass (6mm thickness) weighs 15.0 kg/m2 (Ref 1). As a double layered cushion of ETFE only weighs approximately 4.5% that of conventional glass less structural support is needed; reducing the amount of raw materials used, reducing build time, and reducing building costs. It has been suggested that use of ETFE in construction can reduce build costs by 10% on small projects and up to 60% on large-scale projects (Ref 2). Construction costs are also reduced during the installation process, when sheets of ETFE film can be 'welded' together with a blow torch and spans of up to 180 feet can be achieved with sufficient structural support (Ref 8). This 'thermoforming' has excellent dimensional stability; i.e. the material does not shrink or expand when heated (ref 3).

ETFE is highly transparent to light from the whole visible light spectrum and can allow large amounts of natural light into the building, creating a “bright and open space that can emulate the outdoors” (Ref 4). ETFE can retain this transparency and strength for over 30 years (Ref 5). In addition, ETFE has a high level of heat retention, retaining long wave thermal radiation and creating a 'greenhouse effect', which can reduce energy costs by up to 30% (ref 8).

The structural properties of ETFE can be shown on a stress-strain curve (Ref 6). The long sweeping curve indicates the ductility of the material; it can be stretched at high loads without fracturing. In fact, ETFE is able to stretch up to three times its original length without losing its elasticity (Ref 2). When ETFE does fracture, its strong intermolecular bonds prevent the material from tearing or shattering like glass.

As a fluorocarbon polymer, ETFE has similar non-stick properties to PTFE, making it 'self-cleaning'. With a low co-efficient of friction typically of 0.23 (Ref 7), dust or dirt that lands on ETFE is washed away by rainwater. Maintenance of ETFE is required approximately every 3 years.

Fluorocarbon polymers are relatively inert, and are especially unreactive to the weather and chemical attack.

ETFE can resist temperatures of up to 270°C because of its very stable molecular bonding. It is fire retardant as well as 'self-ventilating', which aids in the removal of smoke and other harmful gases and reduces the need for smoke extraction.

[edit] Applications

ETFE can be made into glass-like sheets or inflated into 'multi-layered' cushions and is being used in some of the most innovative new buildings around the world. ETFE, has greatly increased in popularity as a construction material due to its versatility, light weight, tensile strength and excellent weathering properties.

During the 1990's, ETFE was used in offices, universities, medical facilities, exposition halls, and zoos across Europe. In 2000, the Eden Project in Cornwall used ETFE to cover the two geodesic conservatories. Its application created an environment capable of housing plant species from around the world in tropical rainforest and Mediterranean style climates. The Eden Project's construction was widely acclaimed as an engineering marvel, causing ripples of global interest. EFTE's application in architecture allows the designer to 'turn architectural fantasy into reality' (Ref 8). This is demonstrated for example by the National Stadium and National Aquatic Centre in Beijing. Both of these buildings showcase innovative applications of ETFE. To protect spectators from the weather in the national stadium, red ETFE cushions were installed in the spaces between the 'twigs' of the 'bird's nest'.

The National Aquatic Centre was entirely clad in blue ETFE 'bubbles'. These bubbles allow for covered spaces of up to 30ft to be created without internal structure. The Aquatic Centre used the triple-layer formation which mixes layers of blue film with transparent film thus giving the façade of the building a sense of depth and shifting colour. It also allowed images to be projected onto the wall of the centre similar to the Basel football ground or the Allianz-Arena. Each layer of the cushions can be engineered to transmit, reflect or scatter the projected image, allowing the full facade to be used as a visual device.

Other uses:

  • ETFE is used for covering electrical wiring used in high stress, low fume toxicity situations. A primary example of its application the electrical wiring of aircraft and spacecraft. It is also commonly used in the nuclear industry for tie or cable wraps. This is because ETFE has better mechanical toughness than PTFE.
  • It is also used in applications such as wall coverings and anti-graffiti protection in high-traffic areas.
  • As a dual laminate, ETFE can be bonded with FRP (fibre-reinforced plastic or fibre-reinforced polymer) as a thermoplastic liner. This is then installed in pipes, tanks, and ships for additional corrosion protection.
  • ETFE is also the natural choice in solar panel applications because of its low density elasticity
  • Further research and other innovations are still being developed. The company Foiltec is currently testing the possibilities of attaching photovoltaics to ETFE panels for use as an insulating 'nanogel' to improve a panel's thermal properties.

[edit] Sustainability

The fact that ETFE doesn't degrade under; UV light, sunlight, weather or pollution, means that ETFE could have a life of around more than 50 years (Ref 12). As well as being self-cleaning, ETFE is also very simple to repair, with tears being fixable by welding replacement patches over the affected area (Ref 18) which can all be done from outside the building, in contrast to glass, which requires entire panes to be replaced in the event of damage. ETFE sheets are easily dismantled and are recyclable, with 100% of the material being recycled into new ETFE materials and products (Ref 11).

The production process for ETFE involves the polymerisation of the monomer TFE into ETFE, which is a water based process with no requirement for the use of solvents (Ref 12). It is then extruded to the required thickness, a process which requires very little energy (Ref 12). ETFE is then welded into large sheets, another process with low energy consumption.

Its low weight results in lower C02 emissions (Ref 11) and requires far less structural support than other transparent building systems such as glass. In fact the carbon footprint of ETFE is said to be 80 times lower than that of comparable transparent systems (Ref 13).

ETFE has high level heat retention, which when combined with its ability to allow in more natural light than glass, can reduce energy costs by around 30% compared to glass (Ref 14). ETFE is generally utilised in a cushion system which enhances the insulation properties of the material, whilst still providing good translucency. Typically a double layered ETFE film system will provide a U value of 2.6 W/m2k (Ref 15), which is superior to that of double-glazed glass which is 2.9 W/m2k (Ref 15).

With up to 90% transparency (Ref 15), the use of ETFE can significantly reduce indoor lighting costs and thus contribute to reduced energy consumption. ETFE can also be manipulated to control the light transmission to suit specific requirements.

Overall the ETFE system outperforms any other transparent material systems in terms of insulation, translucency, recyclability, weight and production costs. Edward Peck (architect and lead designer for Foiltech North America) suggested “This product gives you a lot of opportunities as far as day lighting, reduction of steel for support structures, savings on transport. If you reduce the tonnage of steel, reduce the raw building materials, we have a real capacity to lighten up a building.” (Ref 13).

However, ETFE is not without its drawbacks. The many layers of ETFE require inflating to form the cushions, which requires steady air pressure, resulting in a material with a cushion system too complex to use in small residential projects. "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?" (Ref 13)

[edit] Conclusion

Overall, the increasing use of ETFE in the construction industry may well continue because of its inherent properties and versatility. It has become a go-to material for those in search of an alternative to more traditional materials, such as glass.

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