Self-cleaning glass
Introduction
In this report self-cleaning glass will be examined. First the properties of self-cleaning glass will be explained. Then the different manufacturing processes of self-cleaning glass and applications are described.
Self-cleaning glass properties
When a material has the ability of self-cleaning the surfaces can repel contaminants such as solid particles, organic liquids and biological contaminants. This can be done by either using a hydrophobic or a hydrophilic surface.
Hydrophobic surface
A hydrophobic surface has a water contact angle of greater than 90°. Typical float glass or silane treated glass surface have a water contact angle of around 30° and 100°. For it to be a self-cleaning surface, which is then super hydrophobic, it has to have a static water contact angle of greater than 160° and a lower roll-off angle. The droplets on the hydrophobic self-cleaning surface will then roll of the surface and during the way it will take away the contamination particles as can be seen in Figure 1. This is also called the lotus effect because of the similar lotus leaf properties.
Hydrophilic surface
The other way a surface can be made self-cleaning is through a hydrophilic coating with photo catalysis properties. These photo catalytic properties were discovered in 1967 by Akira Fujishima at the University of Tokyo and was called the “Honda-Fujishima Effect". And it meant that when the material gets exposed to UV light irradiation and is in the presence of water, oxygen radicals are produced which are able to decompose impurities, such as fats, oils, soot or plant material. It gets chemically broken down and washed away by forming sheets due to the low contact angles. It washes away by the next heavy shower of rain and because of this it can be less often cleaned.
The titanium dioxide is hydrophilic because of the high surface energy. That is why the water does not form drops, but rather a sealed water film. The self-cleaning effect of titanium dioxide is illustrated in Figure 2.
There is currently research being done to modify the properties of the titanium dioxide so that it becomes photo catalytic by irradiation with visible light. This is done by combining it with metal atoms such as, chromium, vanadium, tungsten or carbon. This would mean the glass can also be used indoors and is already been implemented in indoor paints which reduce gaseous pollutants in the air to improve the air quality.
Materials fabrication
To make the self-cleaning glass regular float glass is used with a certain coating or adaption.
Glass
For the manufacturing of float glass the “Pilkington float glass manufacturing process” is mostly used. First the needed raw materials are batched. Silica sand, calcium oxide, soda, magnesium and recycled glasses are mixed into the needed amounts. The materials are melted in a chambered furnace at around 1500°C and then drawn onto a tin bath at around 1000°C. At this moment coatings are applied at high temperature on the material. It is then cooled in an annealing lehr at around 600°C after which the glass is checked on quality, cut and stored or can be treated with a different coating post-production (1). The process also can be seen in Figure 3.
Fabrication of hydrophobic self-cleaning glass
Natural hydrophobic material use microscopic grooves and/or wax for their water resistance. For the fabrication of hydrophobic surfaces one can use different ways: Mechanical stretching, sol-gel process, layer-by-layer-assembly, etching, chemical and electrochemical deposition and chemical vapor deposition (2). However for making a transparent hydrophobic surface usually the sol-gel process is used.
Sol-gel process
The sol-gel process is a process for producing solid materials from small molecules. It is usually used for making metal oxides, in particular the ones of silicon and titanium. They can be modified by adding certain chemical substances to it in order to retain the required properties. (3) Nanoscale particles are produced from the silanes by means of chemical reactions in a solution. The dispersion of particles is called the sol. Depending on the type of sol chosen, the solution evaporates either at room temperature or when it is heated, and the sol becomes a viscous gel because the particles concatenate into a dense web due to their high reactivity. Once it has dried out a compact layer is formed.
The sol can be applied using traditional industrial processes such as dip coating, spraying or spin-coating. To increase the durability it is burned into the surface at high temperatures, during the float bath fase. Because a sol can also dry at room temperature, glazing can also be treated post production. The resulting transparent coatings are only a few nanometers thin.
Fabrication of hydrophilic self-cleaning glass
As hydrophilic self-cleaning glass surfaces are based on titanium dioxide first the manufacturing of that material is explained after which the applicability of it on glass is explained.
Titanium dioxide
Titanium dioxide is being used because it has a high refractive index and a high degree of transparency in the visible region of the spectrum. This makes it highly opaque and bright white and that is why it can be used for transparent coating.
Titanium dioxide, written as the chemical formula TiO2, occurs in nature in three crystalline adaptations; rutile, anatase and brookite and two high pressure forms. Anatase and rutile are the most used adaptions. They are produced through a sulfate or a chloride process. Where the chloride process produces a superior product with less waste products (4). In Figure 4 the two ways of producing titanium dioxide are shown.
In the sulfate process, ore containing titanium is dissolved in sulfuric acid, which has a solution of titanium, iron and other metal sulfates. Then, through chemical reduction, purification, precipitation, washing and calcination, TiO2 is produced. The crystal structure, anatase or rutile, is controlled by nucleation and calcination. In the following formulas the sulfate process reactions are shown:
FeTiO_3+2〖 H〗_2 SO_4→ TiOSO_4+FeSO_4+2 H_2 O
TiOSO_4+H_2 O→TiO_2+H_2 SO_4
The chloride process includes high-temperature, anhydrous vapor phase reactions. The titanium ore is reacted with chlorine gas under reducing conditions to obtain titanium tetrachloride and metallic chloride impurities, which are afterwards removed. Highly purified TiCl4 is then oxidized at high temperature to produce TiO2. Oxidation makes it possible to change the particle size distribution and the crystal type, which has consequences for the hiding power and tinting strength. In the following formulas the reactions of the chloride process are shown:
2 FeTiO_3+7 Cl_2+3C→2 TiCl_4+2 FeCl_3+3 CO_2
TiCl_4+O_2→TiO_2+2 Cl_2
Titanium dioxide coating
Photocatalytic coatings with nano-TiO2 are usually applied using chemical vapor deposition at the same time as the material itself is manufactured. It is important to ensure that silicon seals are not being because their oils seep over the glass, destroying the hydrophilic surface properties and leading to unsightly smears forming (5).
As well as coating by means of CVD, sol-gel processes can also be used to create the surface functionality. This latter method is far more preferable for the manufacturers as it is carried out at lower temperatures, takes less time, and saves energy costs.
Applications
The applications of self-cleaning glass can be quite large and are especially useful for difficult to access places. Such as upper floor windows, conservatory roofs, doors, other sloping roofs, atria (6). But it can also be applied for glass solar panels which means it will have a higher efficiency. The hydrophobic glass, which is not exactly self-cleaning because it is only dirt-resistant, can be applied anywhere where there is water falling on the glass. While the hydrophilic glass can only be applied on places where water and UV light is falling on the glass, which means it can’t be used internally.
Summary
In this report the properties of self-cleaning glass are explained. This can be done using a hydrophobic and a hydrophilic surface. Where it is hydrophobic when the water contact angle is more than 90 degrees and hydrophilic when it is less than 90 degrees. The hydrophobic glazing is self-cleaning because the water rolls of the glass and takes away the dirt while for the hydrophilic glazing an extra coating is needed with photocatalytic properties which decompose organic materials. The material which is usually used for this is a coating of titanium dioxide. Hydrophobic glass is usually made using sol-gel coating which are silicon base atoms that create the hydrophobic surface tension. To make the titanium dioxide two processes are used one is based on sulfate and one on chloride. Where the chloride one is producing less waste materials and a better product. The titanium dioxide can then be placed on the glazing using a coating through chemical vapor deposition at the same time as the material is being produced or also through a sol-gel process. The applications are usually difficult to reach windows, roofs or solar panels. Or when the maintenance costs are high. However the titanium dioxide coated glazing only works when water and UV light is coming on the glazing which means one has to take this into account.
I think that the self-cleaning properties of materials can save a lot of costs. However the fact that it can only be used at certain places remains a problem. This can be solved using additives but then it can still not be used without adding water and thus cannot be internally. It is also important to see whether the glass with titanium dioxide can be recycled, and it seems that there it has no into concrete
References
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