# ETFE windows

Modern buildings have a lot of windows. If you look at a recent skyscraper, its facade is likely to be mostly glass. Why use glass for that? Why not use a sheet of plastic instead? Glass sheets are heavy and brittle; plastic film would be much lighter and cheaper.

The main problem with plastic film is UV degradation. Most plastic degrades over time when exposed to sunlight and air. Is it possible to make clear plastic with good resistance to UV and water? Could that be used instead of glass for buildings? Yes, and people have done that.

When ethylene and tetrafluoroethylene are polymerized together, they naturally tend to alternate, producing [ETFE](https://en.wikipedia.org/wiki/ETFE). Each carbon is either fluorinated or next to a fluorinated carbon, which increases UV resistance.

That wikipedia page [lists some buildings](https://en.wikipedia.org/wiki/ETFE#Notable_buildings) using it, notably including the [Eden Project](https://www.youtube.com/watch?v=Ma0o8V3uJJY&t=78s) and the "[water cube](https://en.wikipedia.org/wiki/Beijing_National_Aquatics_Centre)". It also notes:
> ETFE's first large-scale use architecturally came in 2001 at the Eden Project where ETFE was selected as it can be printed and layered to control solar conditions and because it was found to have a low friction coefficient, which saves on maintenance as dust and dirt do not stick.

Current ETFE building facades typically have 3 layers clamped at the edges with aluminum frame, with some air pumped between the layers. So, current designs typically use some electricity. The frames are more expensive than the ETFE film, and my understanding is that they result in ETFE being slightly more expensive than glass. Cost reduction of such frames seems possible.

ETFE has some notable advantages over glass:
- As thermal insulation, ETFE is slightly better than the same number of glass layers: 3 layers are slightly better than triple-glazed windows. That's good.
- The frames are heavier than the ETFE film, but the overall system is still typically <0.5x the weight of windows. That's good. (That lower weight is advantageous when [moving building sections](https://www.youtube.com/watch?v=FGqB-Mexi20) on wheels.)
- ETFE is slippery. Water will run off it at small slopes. Dust adheres poorly to it, so cleaning isn't usually needed. That's good.
- Windows with irregular shapes are easier with ETFE than glass. That's good as long as you trust architects to use this capability wisely.

There are also some ways in which ETFE is different from glass but not exactly better or worse:
- ETFE film is much lighter and less rigid than glass, so it transmits sound much more and reflects sound less.
- ETFE film is mostly transparent to UV down to 200nm. Soda-lime glass blocks UV below ~320nm but mostly transmits UV-A, which means you can get a suntan through a window but are less likely to get a sunburn. (Despite what you may have heard from Feynman, watching a nuclear explosion through a car windshield is not safe for your eyes.) The extra UV transmitted through ETFE also causes slightly more heating inside buildings, might have some disinfectant effects, and might cause more degradation of plastic objects.
- ETFE film generally transmits slightly more visible light than glass, but it's usually curved, which can cause some slight visual distortion.

Overall, I expect ETFE usage for buildings to increase significantly.