Windows that wash themselves have long been the dream of building owners, and thanks to clever folks at PPG Industries and Pilkington North America, self-cleaning glass is now reality.

The idea to create a special coating for glass, with the help of which it will not be necessary to wash windows, appeared was as early as in the 1960th. At that time the idea was impracticable because the technology did not let to cover the glass with a coating which would have self-cleaning properties, good adhesion to glass and would be strong and durable. However the science does not stand still and several years ago self-cleaning glass appeared on sale.

How Does It Work? This area divided into two categories, two distinctively

different approaches to the same problem: 1) Glass coated or applied with hydrophilic layer that

uses photocatalytic decomposition; 2) Glass coated or applied with a hydrophobic

protective coating

Hydrophilic glass cleans itself in two Hydrophilic glass cleans itself in two stagesstages

Hydrophilic means that the glass attracts water and make it spread out like a thin film on the surface. Hydrophobic means that the glass will repel water and any water that comes in contact with the glass will attempt to have as little contact as possible with the glass and will create spherical droplets on the surface.

Glass being a very porous and microscopically unflat surface is a magnet to dirt, contamination, bacteria and various other deposits. Over time the dirt, grime and contamination are so deep and so rooted into the surface of the glass that it cannot be removed.

The first stage - photo-catalysis (light-activated)

Self-cleaning glass has a coating of titanium dioxide (TiO2) on its outer surface. The action of sunlight on the titanium dioxide generates a species known as electrons and holes. These electrons and holes, along with a specific property of titanium dioxide migrate to the surface and start a process known as oxidation of any organic material which is present. Effectively, the titanium dioxide

absorbs the ultraviolet (UV) component of sunlight and causes the degradation and break-down of any organic material, dust or debris which are on the surface of the glass. It converts them into carbon dioxide and water.

The second stage - hydrophilicity (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 rivulet, leaving dirty streaks as it goes. The titanium dioxide coating changes all that: the hydroxyl radicals produced by photo-catalysis 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.

Table 1Performance

summary

Normal glass SGG BIOCLEAN SGG BIOCLEAN NATURA

% change compared to normal glass

Light transmission 82% 64% 37% Up to 45%

Solar factor 0.78 0.47 0.29 -0.49

U-Value 2.7 W/m^2*K 1.0W/ m^2*K 1.0W/ m^2*K 1.7W/ m^2*K

When Will the Self-cleaning Function Start to Work?

It is installed and cleaned for the first time, the coating will become activated progressively during the following week as it is exposed to the UV rays present in daylight.

The covering works even on cloudy days, as it requires a small amount of UV-radiation. Therefore glass will successfully function both on southern and on northern facades of the building. Moreover hydrophilic activity keeps even at night.

First of all the coating can cope with organic pollution, and what is especially actual for the cities, products of incomplete combustion of motor fuel, which forms a thin oily film, that is difficult to remove from the glass surface. Besides, the coating helps to

wash away inorganic pollution, such as road dust, cement dust, chalk dust and other types of dust.

What Types of Pollutions Can the Coating Cope With?

Taking into consideration that for the normal functioning of such glass daylight and rainwater are necessary, it is suitable only for external glazing, such as facades, windows, roofs, winter gardens.

Where Can the Self-cleaning Glass Be Used?

The ideal place is the glazing of those parts of the building, where the cost of windows cleaning is high or it is difficult physically to organize regular windows and facades cleaning, for example, the glazing of skyscrapers, glass roofs, greenhouses, clerestories.

The self-cleaning glass should not be set under an awning or a sun visor, where the sun and rain do not have an opportunity to get to glass.

REFERENCES•http://www.balconette.co.uk/self-cleaning-glass/Types.aspx

•http://www.columbuswindowcleaning.com/self_cleaning_glass.htm•http://www.selfcleaningglass.com/technical-support.asp

•http://www.explainthatstuff.com/how-self-cleaning-windows-work.html•http://www.newglasssingapore.com/self-cleaning-glass.html