It's a disaster! They've invented windows that can clean themselves! If, like me,
you're one of the minuscule minority who actually enjoy cleaning
your windows, this new technological development will fill you with
horror. For the rest of humanity—the millions of people who loathe
wobbling up ladders, bucket and soapy squeegee in hand—the prospect
of windows that keep themselves sparkling automatically, using
nothing but the sun and the rain, will seem like nothing short of a
miracle. How can a simple piece of glass stay clean all by itself?
Let's take a closer look!
Photo: Say goodbye to this tiresome chore! Self-cleaning glass means you can throw away your window-cleaning kit—or does it? You'll see it sold under brand names such as Pilkington Activ™ and Cardinal Neat® Glass.
Photo: The coating added to self-cleaning glass is extremely thin—like placing a dime on top of the Empire State Building. Photographs in the Carol M. Highsmith Archive,
courtesy of Library of Congress, Prints and Photographs Division.
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.
Photo: Titanium dioxide, an important part of self-cleaning windows, is also the ingredient that puts the "white" into white paint. That's why this tube of artist's paint is labeled "titanium white."
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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)—but let's not get bamboozled
by the jargon. Let's find out more...
Light-activated
Photo: It takes energy to shift dirt from windows, even if they clean themselves. The ultraviolet part of sunlight—the part that gives you sunburn—is the power behind self-cleaning glass.
Titanium dioxide is a photocatalyst: it's 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). All this is illustrated
in the diagram in the box below.
Artwork: Ultraviolet is high-energy, high-frequency (short-wavelength) light just beyond the blue end of the visible light spectrum our eyes can see.
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. Magic!
How does a window clean itself?
In summary, then, here's how a titanium-dioxide-coated window gets itself clean through photocatalysis and hydrophilia:
When UV light (the yellow arrow shown on the left) shines on the titanium dioxide coating, electrons (the tiny, negatively charged particles inside atoms) are released.
The electrons interact with water molecules (H2O) in the air, breaking them up into
hydroxyl radicals
(OH·), which are highly reactive, short-lived, uncharged forms of
hydroxide ions (OH−).
These agile hydroxyl radicals attack the hefty organic
(carbon-based) molecules from which most dirt is made, breaking apart their chemical
bonds and turning them into smaller, harmless substances such as carbon
dioxide and water. This is an example of oxidation.
The hydroxyl radicals also make the glass hydrophilic (water-loving). When it rains, water molecules spread
evenly across it and wipe it clean like a kind of automatic squeegee!
Advantages and disadvantages of self-cleaning windows
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
and sand 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, however, the advantages seem to outweigh the drawbacks, especially
if you really loathe window cleaning or your windows are inaccessible or hard to reach.
Photo: "Ggggrrrr, you mean I still have to clean the insides?"
Not just windows
What works on a window should, theoretically, work on any large flat surface,
so similar photocatalytic technology can be used in many other places.
Self-cleaning solar panels and building facades have already been developed—and for fairly obvious
reasons: they get dirty quickly, are often mounted high up in inaccessible places, and so can be expensive to keep clean.
Although cleaning solar panels doesn't always produce that much gain in efficiency (for small domestic systems, anyway), it can be worthwhile on larger installations; but if you've ever seen a solar farm (with many hectares of solar panels), you'll know
how difficult and impractical that can be. Self-cleaning technology has been developed
for both solar thermal panels (ones that make hot water) and photovoltaic
solar cells (ones that generate electricity).
Artwork: How a typical self-cleaning surface coating works. The surface has a substrate (base material) of glass (1) covered with polyurethane plastic (2) and a titanium dioxide coating (3) made of
nanoparticles less than 100 nanometers in diameter,
on which organic dirt (4) slowly builds up. When ultraviolet light hits the coating (5), photocatalyzed reactions destroy the dirt, releasing harmless carbon dioxide and water (6). Based on an artwork from
Self-cleaning surface coating (photocatalysis) by Rudolf Gensler et al, Siemens AG, February 5, 2013.
Although self-cleaning technology really comes into its own for large surfaces outdoors,
it can also be used more modestly indoors. Since there's obviously no sunlight inside,
indoor surfaces typically need light from an ultraviolet lamp to activate the cleaning process.
(Some self-cleaning surfaces have an inner layer of tiny LEDs concealed under
the outer, working layer, which shine out intermittently to activate the photocatalyst.) Self-cleaning mirrors, tiles, and kitchen surfaces in the home are likely next steps; self-cleaning, antimicrobial surfaces in hospitals are another possible application. And what about self-cleaning eyeglasses and contact lenses?
A Clear Coating, With Green Applications by Anne Eisenberg. The New York Times, January 16, 2010. Slippery plastic coatings provide an alternative approach for self-cleaning surfaces.
Self-cleaning materials by Peter Forbes, Scientific American, August 2008, Vol. 299, No. 2, pp. 88–95. A comparison of several different self-cleaning methods, including the titanium dioxide technology I've described above.
Son of Carwash, the Self-Cleaning House by Patricia Leigh Brown, The New York Times, January 17, 2002. The story of Frances Gabe and her amazing, waterproof, plastic self-cleaning home!
Self-Cleaning Windows to Be Sold in U.S. This Year by Julian Barnes, The New York Times, June 26, 2001. This old article explains why rivals Pilkington and PPG took quite a cautious approach
when they first began marketing self-cleaning windows in the United States.
Books
If you want to know about the science in (much) more detail, these books are worth a look:
Photocatalysis: Fundamentals and Perspectives by
Jenny Schneider et al (eds). Royal Society of Chemistry, 2016. A detailed review of cutting-edge research, with a focus on chemistry and materials science.
Photocatalysis: Science and Technology by Masao Kaneko and Ichiro Okura. Springer, 2010. Covers the basic science of photocatalysis before considering applications in semiconductors and environmental cleaning (including air and water purification) and photo-energy conversion.
Patents
For technical details about how photocatalytic glass is made, it's worth browsing some of the patents in this area. These are just a few examples—and there are many more!
Self-cleaning system and window-glass by Maarten Marinus Johannes Wilhelmus Van Herpen for Koninklijke Philips Electronics N.V., April 3, 2012. This describes self-cleaning surfaces with built-in LEDs for stimulating the photocatalyst.
Method of coating glass by John Andrew Ridealgh et al, for Pilkington Group Limited, August 15, 2013.
And why stop at self-cleaning windows? Frances Gabe invented a complete self-cleaning house, which you can find documented here. It doesn't use photocatalytic technology, however.
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