Self-cleaning Windows

What is self-cleaning glass?
The first thing to note about self-cleaning windows is that they’re not, in fact, “simple pieces of glass.” They have a very thin outer coating of titanium dioxide, a white, powdery titanium compound best known for giving that dazzling gleam to paint, toothpaste, and all kinds of other bright white things.

Now if titanium dioxide is, essentially, the white in white paint, it might seem ludicrous to splash it all over a window—something we naturally want to be transparent. But the coating really is ultra-ultra thin. We’re talking about putting a layer 10–25 nanometers deep on glass that might be 4mm thick, which is like sitting a dime on top of the Empire State Building! It reduces the light passing through the glass by no more than about 5 percent.

How does self-cleaning glass work?
The titanium dioxide coating cleans through a double-whammy, two-stage process: it’s photocatalytic (light-activated) and hydrophilic (water-loving).

Titanium dioxide is a photocatalyst, a material that makes chemical reactions happen when the right kind of light shines on it. The right kind of light for titanium dioxide is ultraviolet (UV), the super-blue, high-energy part of sunlight that our eyes can’t see, but that nevertheless can give us sunburn even on a cloudy day. When ultraviolet light hits the titanium dioxide coating of a self-cleaning window, electrons are generated. These turn water molecules from the air into hydroxyl radicals that make chemical oxidation and reduction reactions take place on the coating. In effect, the hydroxyl radicals attack organic (carbon-based) dirt and chop it up into smaller pieces that are much easier for rain to wash away.

Since the reactions happen on the titanium coating, on the very surface of the glass, they attack the lowest layers of the dirt, loosening encrusted muck from the glass very effectively by chipping it away from the inside out (the opposite of normal window cleaning, where you effectively scrub the dirt from the outside in).

Water-loving
Glass is usually hydrophobic or “water hating”: water dropped onto glass tends to “bead” (form droplets), while rain runs down windows in noticeable rivulets, leaving dirty streaks as it goes. The titanium dioxide coating changes all that: the hydroxyl radicals produced by photocatalysis make the glass hydrophilic or “water loving.”

Instead of staying in drops, water molecules spread out evenly across the glass in a very even sheet. So when rain hits a dirty self-cleaning window, it spreads across it like a great big cloth. Since the window is most likely vertical or mounted at an angle, the sheet of water wipes down it neatly and evenly, a bit like a rubber squeegee, and the glass dries without any streaks or smears.

Advantages
Self-cleaning windows look great and the coating is meant to keep working for the lifetime of the window. They save time and money (window cleaning can be expensive if you hire someone to do it) and help to avoid the risk of accidents happening when people wobble up ladders with buckets of water. You can get self-cleaning windows in various different thicknesses (typically 4–10mm), with blue tints (to reduce solar glare in places such as conservatories), and with heat-reflecting inner coatings for improved energy efficiency.

Disadvantages
That said, self-cleaning windows have quite a few drawbacks. The first thing you’ll notice is that they’re about 15–20 percent more costly than conventional glazing. Since the self-cleaning process happens slowly and continually, the end result is not like a sudden visit from the window cleaner: self-cleaning glass is always a work in progress, and never as clean and sparkling as a freshly cleaned pane (but not as dirty as an uncleaned one either).

Another problem is the cleaning process relies on sunlight and rain. Although the coating needs only a small amount of ultraviolet light for the photocatalytic effect to begin (so it works even on cloudy days), the titanium dioxide still takes 12–48 hours to absorb enough energy to work properly. The coating also takes a period of time to activate when it’s first installed (anything from a week in summer to two months in winter, depending on the climate where you live—so longer in Alaska than California). And of course you need water as well as sun, so if it doesn’t rain for a period of time you’ll still need to hose your windows down to remove the dirt. (Pilkington, a leading manufacturer of self-cleaning windows, says “In dry spells… your windows can be washed with a soft cloth and warm soapy water or hosed down gently,” which sounds suspiciously like cleaning the windows yourself!)

Photocatalysis only tackles organic (carbon-based) dirt, so it’s not effective against things like salt deposits (if you live near the coast) and it won’t remove paint splashes if you’re a messy decorator. Although the coating is designed to last as long as the windows, it can still be damaged if you clean your windows with abrasive materials or harsh chemicals. And (though the advertisements conveniently don’t mention this) you’ll still need to clean the insides of the windows yourself.

All told, the advantages seem to outweigh the drawbacks, especially if you really loathe window cleaning or your windows are inaccessible or hard to reach. Another benefit is the likelihood of similar technology being used in many other places. Self-cleaning mirrors and kitchen surfaces in the home are likely next steps; self-cleaning, antimicrobial surfaces in hospitals are another. And what about self-cleaning eyeglasses and contact lenses?

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