Windows that clean themselves

Early in 2001, international glass manufacturers Pilkington launched a revolutionary new product that could mean an end to the traditional 'sponge-and-bucket' method of cleaning external windows. Pilkington Activ™ is the first glass of its kind to possess the ability to clean itself. This may not be a popular breakthrough with window cleaners, but it does offer several benefits. These include the saving of cleaning time for residential and commercial windows, and a reduction in expenditure on cleaning products. Self-cleaning windows will reduce the need for ladders and industrial cleaning equipment that must be operated at extreme heights. On an environmental upside, the glass reduces the run-off of potentially harmful detergents.

The self-cleaning glass uses the ultraviolet rays of the Sun to steadily and continuously break down and dissolve organic dirt in a photocatalytic reaction, while simultaneously reducing the surface tension of water, causing it to spread out and 'sheet' down the surface of the glass and wash away dirt.

The Sun...

A microscopically thin, transparent coating of titanium (IV) oxide (titanium dioxide) gives the glass its special properties. This patented coating is bonded permanently to the surface of the glass while it is in its molten state. This means that the composition becomes part of the surface, rather than simply a coating. The overall thickness of the active coating is around 50nm (50 millionths of a millimetre — the comparative equivalent of the thickness of a penny coin on top of the Empire State Building).

The photocatalytic effect (Figure 1) arises because titanium dioxide can absorb the ultraviolet component of natural sunlight, causing its electrons to become excited (move to a higher energy level). These electrons react with oxygen in the air to produce free radical oxygen. The active oxygen molecules, no longer in their balanced electronic state, are powerful oxidants, capable of attacking organic dirt on the glass. These reactions clean the window by breaking down the organic material to form mainly carbon dioxide and water.

Figure 1. The photocatalytic effect. The coating's photoactivity breaks down organic material, reducing the adherence of dirt to the surface.

And the rain

The hydrophilic properties of titanium dioxide also add to the cleaning effect. On regular glass, rainfall tends to form droplets on the surface, typically 3-4 mm in diameter. These droplets aggregate together and, when they reach a certain mass, flow down the glass in small streams (Figure 2). This concentrates dirt on the window pane, resulting in smears, smudges and droplet drying marks. The titanium dioxide coating has hydroxylated groups at the surface. These alter droplet formations by changing the glass surface from hydrophobic to hydrophilic. The titanium dioxide forms strong hydrogen bonds with the water, overcoming the effects of surface tension. This means that 'droplets' are not formed on the coating because the water spreads out into an almost uniform film on the surface. On Pilkington Activ™ a single droplet of water can cover an area of over 4cm² compared to less than 0.2cm² on regular glass.

Figure 2 Comparison of Pilkington Activ™ and regular float glass in external windows.

COMING TO A WINDOW NEAR YOU

Self-cleaning glass does not clean a surface instantaneously – it operates continuously as the window is exposed to natural sunlight and rainfall. The coating cannot be applied to existing windows because it is deposited during the manufacture of the glass itself. However, once Pilkington Activ™ has been installed, after a short 'running in' period, which allows the coating to absorb enough ultraviolet radiation to start working properly, the durable coating will last for the lifetime of the glass.

Pilkington Activ™ is the latest in a line of innovations from Pilkington, including fire-resistant glass (Pilkington Pyrostop), and Hotscreen-glass embedded with tungsten wires, finer than a human hair, that can defrost an icy windscreen at the touch of a burton. In July 2001, the research scientist who headed the team that pioneered the development of Pilkington Activ™, Dr Kevin Sanderson, won the prestigious Worshipful Company of Glass Sellers’ “Award of Excellence”. In agreement, homeowners everywhere are certain to be celebrating as the new glass appears in window products across Europe (Figure 3) and the window-cleaning chore becomes a thing of the past.

Figure 3. Typical sealed unit of Pilkington Activ™ product for a window.