Transparent membranes are becoming increasingly important, particularly in applications where clarity to the entire spectrum of natural light is required and is to be combined with a lightweight structure to create an economical solution. Fluoropolymer membranes such as ETFE (ethyl tetrafluorethylene) foils are being increasingly used for transparent and highly translucent roofs and facades. Though ETFE foils offer the advantages of being transparent, self-cleaning and incredibly versatile, the fluorocarbon-based polymer ETFE is not very well known. Besides the above advantages, ETFE has some interesting properties. It can be manufactured into a thin film, stored in rolls and designed for application in sheets or even inflated into pillow-like units of varying size, shape and color. Though its added benefits include energy efficiency, cost effectiveness and durability, the light weight and delicate appearance of ETFE offers unprecedented design opportunities. A typical three-layer system consists of two layers measuring 250 microns thick and one layer roughly 200 microns thick. It gets really interesting when air is added between both the first and second layers and the second and third layers. Barely measurable with a ruler when deflated, the material expands to create a structural barrier several feet in depth. Weighing at roughly 1% of the weight of glass, it reduces structural costs by 10-60%. The material, which is typically pre-stressed to inflate and take loads optimally, can stretch up to three times its original length. Surprisingly, it's also puncture resistant. In hurricane testing, 2x4s were shot into the material at a high velocity. In most instances, the board didn't blast through but was caught up somewhere in the layers. As is the nature of the plastic, when exposed to fire, ETFE only melts and pulls away in spots where the flame is in direct contact - which reduces the risk of a fire spreading across the material. Only exposed areas would require replacement. For smaller tears, patching is often all that is needed. A sticky-sided tape that is essentially made from the same material can be applied and is virtually invisible. ETFE membranes are flame retardant according to DIN 4102 B1 Tests that have shown that, due to the low mass of the membrane (which is only between 0.08 and 0.2 mm thick), there is minimal danger of any material falling in the event of fire. ETFE is considered recyclable since the material can be melted and used again.
The Beijing Olympics 2008 employed this material in construction of its stadia here natural light considerations are a critical design component, being a prefered choice as compared to traditional options like glass or fiberglass. Nicknamed the Watercube, the 750,000 sq. ft. National Aquatic Center in Beijing, China, is the largest ETFE project to date and showcases the material's ability to bridge the gap between fantasy-like, futuristic design and reality. The project gives the stadium an appearance of a box made of bubbles by utilizing blue-tinted ETFE cushions not only in the roofing structure, but also in all four walls. In stark contrast, the National Stadium is an intricate woven configuration of steel girders laced with red-tinted ETFE cushions. Much like a bird's nest, the structure projects the appearance of a sturdy, safe haven. Underscoring the appeal is that the systematic design is tinged by random, seemingly natural inconsistencies that are possible only because of the light weight and versatility of ETFE.
ETFE membrane is principally used in pneumatic, multi-layered systems. The maximum span width depends on the imposed loads and required geometry of the individual elements. For small span widths single layer ETFE can be used, while multi-layered systems allow projects with varying physical requirements. Owing to the outstanding physical characteristics of this material a long life expectancy is guaranteed, with no reduction in aesthetic appearance.
ETFE membrane is principally used in pneumatic, multi-layered systems. The maximum span width depends on the imposed loads and required geometry of the individual elements. For small span widths single layer ETFE can be used. Multi-layered systems allow projects with varying physical requirements to be accommodated. Owing to the outstanding physical characteristics of this material a long life expectancy is guaranteed, with no reduction in aesthetic appearance.
As ETFE is one of the most stable organic combinations, resistance to all environmental influence is very high. It is self-cleaning due to its chemical composition, and will therefore retain its high translucency throughout its life. Any accumulated dirt is washed off by normal rain if the form and connection details are designed correctly.
Translucency of the ETFE membrane is about 95% within the range of 400�600 Nm, with scattered light at a proportion of 12% and direct light at a proportion of 88%. For a three-layered module (upper layer 200�m, middle layer 100�m, inner layer 200�m), the degree of light transmission for vertical incidence = 0.7. This value gives the translucency important for life (of humans, animals and plants). Dangerous UV-B and UV-C radiation (which causes burning and is carcinogenic) is considerably reduced by filtration.
U-values are as follows:
- For one-layered membranes ca. 5.1 W/m²K
- For two-layered membranes ca. 3.5 W/m²K
- For three-layered membranes ca. 2.0 W/m²K
- For four-layered membranes ca. 1.5 W/m²K
ETFE membranes can be 100% recycled. Additionally, the membrane has an extremely low mass due to the minimal thickness of up to only 200�m. 500 m² of a double-layered ETFE cushion, for example, has a material mass of approximately. 0.15 m³. The ETFE system can be fitted in foil, aluminum clamp-plates and steel bolts, nuts etc. The design of appropriate connection details helps to avoid non-separable glued connections.