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Self-cleaning Walls and Windows

Photocatalysts Used on Buildings and Trains

Researchers in Japan have developed a method for keeping walls and windows clean without human intervention. The key to these self-cleaning surfaces are photocatalysts, substances that mediate chemical reactions and are activated by light energy. When organic matter comes into contact with a photocatalyst, it is oxidized at an increased rate and decomposes into water and carbon dioxide. This property enables photocatalyst-coated walls and windows to break down any organic dirt that sticks to them.

How Does It Work?

TOTO Ltd., a major manufacturer of toilets, baths, and other sanitary ceramics, was the first company to commercialize photocatalytic paint for outside walls when it released Hydrotect Color Coat ECO-700 on to the market in 2002. Then in 2007 it put on sale ECO-EX, which boasts even greater cleaning power.


©TOTO Ltd.

The standard procedure when painting the outside walls of a building is to apply an undercoat and paint the pigment over it. ECO-700 achieves its photocatalytic effect by mixing titanium oxide, a common photocatalyst, into this pigment, thus preventing dirt from building up on outside walls. With this method, however, the photocatalytic effect is limited because the titanium oxide deep inside the pigment layer does not come into contact with organic substances outside. Furthermore, as the paint is degraded by the presence of the titanium oxide, the amount of titanium oxide that can be added to a given amount of paint is limited.

ECO-EX employs a three-layer painting method in which the undercoat and pigment are applied as normal, but then a third layer of transparent titanium oxide is applied and fixed. This enables titanium oxide to be fixed intricately on the surface of the paint, dramatically enhancing the dirt repellence, cleaning power, durability, and design potential of the paint.

Continuing Innovation

According to TOTO, coating the outside walls of an average two-story house with ECO-EX has the same cleaning effect as 15 poplar trees, which are known for their air-purifying properties, making it six times more powerful than ECO-700. The dirt-removing effect of these products greatly reduces the time and cost expended in keeping the outside walls of a building clean. Toto estimates that more than 13 million square meters of building walls have been coated with products in the Hydrotect Color Coat series nationwide, producing a total cleaning effect equivalent to that of 710,000 poplar trees.


©TOTO Ltd.

Central Japan Railway Co., meanwhile, is planning to use photocatalysis to prevent dirt from building up on the windows of its trains. The company, which operates the Tokaido Shinkansen bullet train line linking Tokyo to Nagoya, Kyoto, and Osaka needs to keep the windows of its trains clear so that passengers can enjoy the scenery outside. Photocatalysis is an effective means of doing this, but Shinkansen trains travel at speeds of nearly 270 kilometers per hour, and the cars are washed by brush every other day. This routine means that any ordinary photocatalyst applied to the glass surface would soon peel off. What is more, titanium oxide reacts with the sodium in glass, causing the glass to deteriorate.

Researchers at Central Japan Railway have solved these problems by using niobium as a photocatalyst and fusing a very thin membrane of this metal onto the glass. The company has begun using this technology on an experimental basis. (January 2009)