Self-cleaning glass
Encyclopedia
Self-cleaning glass is a specific type of glass
with a surface which keeps itself free of dirt and grime.
These two types of coating both clean themselves through the action of water, the former by rolling droplets and the latter by sheeting water that carries away dirt.
Hydrophilic coatings based on titania
, however, have an additional property: they can chemically break down adsorbed dirt in sunlight.
The requirements for a self-cleaning hydrophobic surface are a very high static water contact angle θ, the condition often quoted is θ>160°, and a very low roll-off angle, i.e. the minimum inclination angle necessary for a droplet to roll off the surface.
The second class of self-cleaning surfaces are hydrophilic surfaces which do not rely solely on the flow of water to wash away dirt. These coatings chemically break down dirt when exposed to light, a process known as photocatalysis
. Despite the commercialization of a hydrophilic self-cleaning coating in a number of products, the field is far from mature; investigations into the fundamental mechanisms of self-cleaning and characterizations of new coatings are regularly published in the primary literature.
titania
coating. The film can be applied by spin coating of organo-titanate chelated precursor (for example titanium iso-tetrapropoxide chelated by acetylacetone), followed by heat treatment at elevated temperatures to burn the organic residues and to form the anatase phase. In that case, sodium might diffuse from the glass into the nascent titanium dioxide, causing a deleterious effect unless preventive measures are taken. The glass cleans itself in two stages. The "photocatalytic" stage of the process breaks down the organic dirt on the glass using ultraviolet
light and makes the glass superhydrophilic (normally glass is hydrophilic). During the following "superhydrophilic" stage rain
washes away the dirt, leaving almost no streaks, because water spreads evenly on superhydrophilic surfaces.
Glass announced the development of the first self-cleaning windows, Pilkington Activ™, and in the following months several other major glass companies released similar products. As a result, glazing is perhaps the largest commercialization of self-cleaning coatings to date. All of these windows are coated with a thin transparent layer of titanium dioxide a coating which acts to clean the window in sunlight through two distinct properties: photocatalysis causes the coating to chemically break down organic dirt adsorbed onto the window, while hydrophilicity causes water to form “sheets” rather than droplets, contact angles are reduced to very low values in sunlight and dirt is washed away.
(titanium dioxide) has become the material of choice for self-cleaning windows, and hydrophilic self-cleaning surfaces in general, because of its favorable physical and chemical properties. Not only is titania highly efficient at photocatalysing dirt in sunlight and reaching the superhydrophilic state, it is also non-toxic, chemically inert in the absence of light, inexpensive, relatively easy to handle and deposit into thin films and is an established household chemical that is used as a pigment in cosmetics and paint and as a food additive.
phase is the most photocatalytic among its polimorphic structures. Moreover, ultraviolet irradiation creates surface oxygen vacancies at bridging sites, resulting in the conversion of relevant Ti4+ sites to Ti3+ sites which are favourable for dissociative water adsorption. These defects presumably influence the affinity to chemisorbed water of their surrounding sites, forming hydrophilic domains, whereas the rest of the surface remains oleophilic. Hydrophilic domains are areas where dissociative water is adsorbed, associated with oxygen vacancies that are preferentially photogenerated along the [001] direction of the (110) plane; the same direction in which oxygen bridging sites align.
screens, where fingerprint
s are undesirable.
Titanium dioxide–based glass cannot decompose thick non-transparent deposits, such as paint or silicone
, waterstop
fingerprints or bleeding after weathering, or stucco
dust produced during construction.
Since 2001 the TC24 "Coatings on Glass" committee International Commission on Glass has been trying to set up test methods for evaluation of photocatalytic self-cleaning coatings on glass.
Pilkington Activ™ consists of a 20–30 nm layer of nanocrystalline anatase titanium dioxide deposited by an atmospheric pressure chemical vapor deposition
technique onto soda-lime silicate float glass. The result is a product with extremely favorable visible transmission and reflectance properties; Activ™ has a visible reflectance of around 7% and a visible haze of less than 1%, but absorbs 20% of incident solar UV light which is used in the self-cleaning process. The coating is also robust and cannot be damaged by application of Scotch tape or moderate mechanical abrasion; Pilkington claim that the coating will last the lifetime of the window frame.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
with a surface which keeps itself free of dirt and grime.
Introduction
The field of self-cleaning coatings on glass is divided into two categories: hydrophobic and hydrophilic.These two types of coating both clean themselves through the action of water, the former by rolling droplets and the latter by sheeting water that carries away dirt.
Hydrophilic coatings based on titania
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
, however, have an additional property: they can chemically break down adsorbed dirt in sunlight.
The requirements for a self-cleaning hydrophobic surface are a very high static water contact angle θ, the condition often quoted is θ>160°, and a very low roll-off angle, i.e. the minimum inclination angle necessary for a droplet to roll off the surface.
Self-cleaning surfaces
Several techniques are known for the patterning of hydrophobic surfaces through the use of moulded polymers and waxes, by physical processing methods such as ion etching and compression of polymer beads, and by chemical methods such as plasma-chemical roughening, which can all result in ultra-hydrophobic coatings. While these surfaces are effective self-cleaners, they suffer from a number of drawbacks which have so far prevented widespread application. Batch processing a hydrophobic material is a costly and time consuming technique, and the coatings produced are usually hazy, precluding applications on lenses and windows, and fragile materials.The second class of self-cleaning surfaces are hydrophilic surfaces which do not rely solely on the flow of water to wash away dirt. These coatings chemically break down dirt when exposed to light, a process known as photocatalysis
Photocatalysis
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the ability of the catalyst to create electron–hole pairs,...
. Despite the commercialization of a hydrophilic self-cleaning coating in a number of products, the field is far from mature; investigations into the fundamental mechanisms of self-cleaning and characterizations of new coatings are regularly published in the primary literature.
The discovery of self-cleaning behavior
The first self-cleaning glass was based on a thin filmThin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
titania
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
coating. The film can be applied by spin coating of organo-titanate chelated precursor (for example titanium iso-tetrapropoxide chelated by acetylacetone), followed by heat treatment at elevated temperatures to burn the organic residues and to form the anatase phase. In that case, sodium might diffuse from the glass into the nascent titanium dioxide, causing a deleterious effect unless preventive measures are taken. The glass cleans itself in two stages. The "photocatalytic" stage of the process breaks down the organic dirt on the glass using ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light and makes the glass superhydrophilic (normally glass is hydrophilic). During the following "superhydrophilic" stage rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
washes away the dirt, leaving almost no streaks, because water spreads evenly on superhydrophilic surfaces.
The first commercial product
In 2001 PilkingtonPilkington
Pilkington Group Limited is a multinational glass manufacturing company headquartered in St Helens, United Kingdom. It is a subsidiary of the Japan-based NSG Group...
Glass announced the development of the first self-cleaning windows, Pilkington Activ™, and in the following months several other major glass companies released similar products. As a result, glazing is perhaps the largest commercialization of self-cleaning coatings to date. All of these windows are coated with a thin transparent layer of titanium dioxide a coating which acts to clean the window in sunlight through two distinct properties: photocatalysis causes the coating to chemically break down organic dirt adsorbed onto the window, while hydrophilicity causes water to form “sheets” rather than droplets, contact angles are reduced to very low values in sunlight and dirt is washed away.
The use of titania in self-cleaning applications
TitaniaTitanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
(titanium dioxide) has become the material of choice for self-cleaning windows, and hydrophilic self-cleaning surfaces in general, because of its favorable physical and chemical properties. Not only is titania highly efficient at photocatalysing dirt in sunlight and reaching the superhydrophilic state, it is also non-toxic, chemically inert in the absence of light, inexpensive, relatively easy to handle and deposit into thin films and is an established household chemical that is used as a pigment in cosmetics and paint and as a food additive.
The mechanism
The anataseAnatase
Anatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but,...
phase is the most photocatalytic among its polimorphic structures. Moreover, ultraviolet irradiation creates surface oxygen vacancies at bridging sites, resulting in the conversion of relevant Ti4+ sites to Ti3+ sites which are favourable for dissociative water adsorption. These defects presumably influence the affinity to chemisorbed water of their surrounding sites, forming hydrophilic domains, whereas the rest of the surface remains oleophilic. Hydrophilic domains are areas where dissociative water is adsorbed, associated with oxygen vacancies that are preferentially photogenerated along the [001] direction of the (110) plane; the same direction in which oxygen bridging sites align.
Other applications
It is speculated that other possible application areas are computer monitors and PDAPersonal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...
screens, where fingerprint
Fingerprint
A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...
s are undesirable.
Titanium dioxide–based glass cannot decompose thick non-transparent deposits, such as paint or silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
, waterstop
Waterstop
A waterstop is an element of a concrete structure, intended to prevent the passages of fluids when embedded in and running continuously through concrete joints...
fingerprints or bleeding after weathering, or stucco
Stucco
Stucco or render is a material made of an aggregate, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as decorative coating for walls and ceilings and as a sculptural and artistic material in architecture...
dust produced during construction.
Since 2001 the TC24 "Coatings on Glass" committee International Commission on Glass has been trying to set up test methods for evaluation of photocatalytic self-cleaning coatings on glass.
Brands
- The Pilkington Activ brand by PilkingtonPilkingtonPilkington Group Limited is a multinational glass manufacturing company headquartered in St Helens, United Kingdom. It is a subsidiary of the Japan-based NSG Group...
is claimed by the company to be the first self-cleaning glass.
Pilkington Activ™ consists of a 20–30 nm layer of nanocrystalline anatase titanium dioxide deposited by an atmospheric pressure chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
technique onto soda-lime silicate float glass. The result is a product with extremely favorable visible transmission and reflectance properties; Activ™ has a visible reflectance of around 7% and a visible haze of less than 1%, but absorbs 20% of incident solar UV light which is used in the self-cleaning process. The coating is also robust and cannot be damaged by application of Scotch tape or moderate mechanical abrasion; Pilkington claim that the coating will last the lifetime of the window frame.
- The SunClean brand by PPG IndustriesPPG IndustriesPPG Industries is a global supplier of paints, coatings, optical products, specialty materials, chemicals, glass and fiber glass. With headquarters in Pittsburgh, Pennsylvania, PPG operates in more than 60 countries around the globe. Sales in 2010 were $13.4 billion...
also uses a coating of titanium dioxide, applied by a patented process. - Neat Glass by Cardinal Glass Industries has a titanium dioxide layer less than 10 nm thick applied by magnetron sputtering
- SGG Aquaclean (1st generation, hydrophilic only, 2002) and Bioclean (2nd generation, both photoactive and hydrophilic, 2003) by Saint-GobainSaint-GobainSaint-Gobain S.A. is a French multinational corporation, founded in 1665 in Paris and headquartered on the outskirts of Paris at La Défense and in Courbevoie. Originally a mirror manufacturer, it now also produces a variety of construction and high-performance materials.The company has its head...
. The Bioclean coating is applied by chemical vapor deposition. - REIBORG HIKARI by Nippon Sheet GlassNippon Sheet Glassis a Japanese glass manufacturer. In 2006 it purchased Pilkington of the United Kingdom. This makes NSG/Pilkington one of the four largest glass companies in the world alongside with another Japanese company Asahi Glass, Saint-Gobain, and Guardian Industries Corp.....
. Hyoumenkagaku Vol. 26, No. 11, pp. 700―703, 2005, T.Anzaki , movie