Most homemakers hate cleaning windows because it’s quite a strenuous exercise that comes with the danger of falling. Imagine painstakingly cleaning the windows of a 100-storey skyscraper with a squeegee and a bucket of soapy water.… No wonder window washing services are pricey. One way to cut corners on the bill is to use self-cleaning glass that doesn’t need a scrub as often. Typically, a self-cleaning coating breaks down organic dirt when catalysed by UV light and the dirt is rinsed off along with any water or rain.
Windows that clean themselves
However, conventional self-cleaning glass curtain walls are expensive because it involves processes in extreme conditions such as Chemical Vapour Deposition (CVD) or sputtering. Many of these thin-film technologies entail high vacuum or intense heat, thereby calling for expensive equipment. Customers have to pay roughly US$10 to 20 per square metre for self-cleaning glass curtain walls whereas non-coated ones can cost as little as US$1 per square metre. The hefty price tag essentially hinders the popularity of self-cleaning glass.
Luckily, Prof. Hong-xing Yang and Dr Lin Lu, Department of Building Services Engineering, and their research team succeeded in bringing down the cost of self-cleaning glass curtain wall substantially by inventing a screen-printable nano composite paste. “Instead of expensive machinery that creates vacuum or high temperature, the paste can be silk-screened onto glass at room temperature and atmospheric pressure. We are talking about low-tech silk-screening machines just like those used for printing on garments. That would cost a lot less than a CVD chamber,” said Prof. Yang. Besides, the paste is also inexpensive, with a material cost of no more than US$1.5 per square metre, far cheaper than conventional self-cleaning thin films. “The screen-printed coating is also thicker than thin films, so that its photocatalytic property in breaking down organic dirt is also more potent than others,” he added.
Clear vision
So far, we have taken care of the self-cleaning aspect of glass curtain walls. But as they function like oversized windows that let in light and the exterior views, they aren’t doing the job if not optically transparent enough. Prof. Yang explains, “It’s about the particle size of the coating. When the particles are roughly in the same order as the wavelength of visible light, the so-called Rayleigh scattering effect occurs. That is to say, light will be scattered by the coating instead of passing through straight. The coating will then appear semi-clear or opaque to our eyes. To make the self-cleaning coating clear, we must reduce the particle size to nano order – about 5 nm – much smaller than the wavelength of visible light. Then Rayleigh effect can be neglected and 98% of the light can pass through without being scattered.”
Better still, the nano composite paste is safe to the environment as it is water based, containing only trace amount of Volatile Organic Compounds (VOCs) and no heavy metals whatsoever. It shows superior cleaning property because of its super-hydrophilic character. It spreads out water on it nicely and brushes away any inorganic dust on the surface or organic dirt broken down by the photocatalytic compound. The coating can withstand higher heat than glass and can be silk-screened over the substrate before tempering. After tempered, the coating forms a robust and durable layer that resists scratching. It lasts for more than 20 years under normal usage.
Apart from curtain walls, mirrors and glass windows, the nano composite paste may also be applied on photovoltaic cells and stainless steel surfaces that benefit most from its self-cleaning property.
