Self Cleaning Glass, What is self cleaning glass
Self cleaning glass speaks to what is self cleaning glass and why use self cleaning glass.
It is is glass which is coated with titanium oxide on its surface which acts as a catalyst that enables sunlight to convert organic dirt to carbon dioxide and water. It causes rainwater to run down the surface in sheets rather than to bead up.
Ordinary glass tends to attract dirt and must be washed periodically both inside an out to maintain its transparency.
Nonorganic dirt, such as sand, is unaffected by the catalyst but the sheets of water are more effective at removing such matter than beaded water. The coating is applied only to the outside of the glass, therefore the interior surface of the glass must be washed manually.
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, 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. 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 film 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 degradation in the hydrophilic/catalytic effectunless 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 superhydrpphilic (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.
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