Issue | Apr 2015

Low-cost self-cleaning nano-coating for curtain walls

News Bite on PolyU's Innovation

Conventionally, self-cleaning glass curtain wall is produced under extreme condition

and is too costly to be widely used. So, are a squeegee and a bucket of soapy water

the only sensible way to keep skyscrapers spotless? Prof. Hong-xing Yang and Dr Lin

Lu, Department of Building Services Engineering, led a research team to develop a

self-cleaning nano-coating that can be silk-screened onto glass under atmospheric

pressure and room temperature. The technology not only substantially reduces the

cost of self-cleaning glass curtain wall, but also benefits other industries such as

photovoltaic cells in major ways.

ost 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

M

A small demonstration on a curtain wall at The Hong Kong Polytechnic University

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.

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

Issue | Apr 2015

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.

A comparison between PV modules with and without self-cleaning coatings after one month’s outdoor exposure

Tel: (852) 3400-2929 Fax: (852) 2333-2410 Email: pdadmin@polyu.edu.hk Website: www.ife.polyu.edu.hk

Prof. Hongxing Yang (front row, 5th from right), Dr Lin Lu (front row, 4th from right) and their research team