Factoring in the price of window cleaning for your building may be a thing of the past, thanks to self-cleaning windows.
Self-cleaning glass includes an applied titanium-dioxide (TiO2) metal coating, which helps windows clean themselves in two ways: 1) the breakdown of organic materials deposited on the glass, and 2) the sheeting of rainwater, which washes the glass.
How it Works
"The titanium dioxide is activated by UV light exposure," says Russell Davies, senior project manager at Simpson Gumpertz & Heger Inc. "It takes about 5 or 7 days of exposure before it becomes fully active."
UV light breaks down organic dirt or material deposited on the window’s surface through hydrolysis (the breakdown of water into hydrogen and oxygen). "This hydrogen is the same type of hydrogen used in compost heaps to break down organic materials," Davies says. "The hydrolysis brings about the breakdown of organic soils that might be on the glass."
In self-cleaning glass, the glass surface is also hydrophilic (meaning that the glass doesn’t form droplets of water, but forms almost an even spread, or sheet, of water over the glass surface). The coating "spreads the water out and, as the water sheets and flows off, especially if it has a slope, it will carry the dirt with it. It carries not only the organic dirt that’s been partially decomposed, but also any inorganic mixed with organic soils that are also on the glass," says Davies.
Another advantage of this coating is that it dries quickly because of the way the water sheets on the glass. "It tends not to leave as many streaks, bubbles, or after effects of the soils being deposited," says Davies.
"The glass uses the natural daylight and the rainwater to break down organic materials," says Sharon Urban, marketing communications specialist at Pilkington North America Inc. "As long as you have those two properties, it’s virtually maintenance free." Self-cleaning glass reduces window cleaning, which provides a savings to building owners.
When the Glass Won’t Clean Itself
When a building is exposed to large amounts of inorganic materials, such as salt or sand, those materials will deposit on the glass, but won’t be broken down. As a result, they’ll stay there until water is introduced to the surface to flush them off, says Davies.
If buildings exposed to salt and sand were then exposed to water, they would still have the advantage of the sheeting action – the inorganic materials would be washed off the glass better than they would be on an uncoated glass. But they wouldn’t have the benefit of a UV reaction taking care of any organic materials that inorganic compounds are in contact with or connected to, Davies explains. "The photocatalytic process doesn’t work, and the glass performs the way it would if it didn’t have any self-cleaning coating on it."
In climates where it doesn’t rain very often, the windows can be sprayed to loosen the organic material, says Urban. "That will reduce window cleaning and provide better views."
The self-cleaning property remains with the glass as long as it’s exposed to UV light (even on cloudy days) and water. "In terms of life-cycle, the coating is integral with the glass," says Davies. As long as the glass remains, there should be no change in the photocatalytic performance of the coating. "It’s a catalyst, so it’s not sacrificial; it maintains itself. It’s integral with the glass and shouldn’t degrade over time."
Kylie Wroblaski (firstname.lastname@example.org) is associate editor at Buildings magazine.