The 'cleanability' (properties) of glazed areas is one of the most talked-about issues in the application of glass in architecture and in transportation vehicles. The current focus of the scientific and technological efforts of glass companies, laboratories and research institutes is the development of glazing products that remain clean over time and that are also easy and inexpensive to clean when required. In short, glass must retain its original cleanliness and require only minor and infrequent maintenance.
The development of new 'self-cleaning' products is justified by the high costs of cleaning curtain-walls or commercial windows and by the extremely difficult chore of keeping home glazing clean, such as windows, skylights, inside surfaces like shower enclosures, mirrors, glass room dividers, glass table tops, glass sinks.
The approach developed to meet these needs consists in depositing a surface coating that provides "easy-to-clean", "self-cleaning" or photocatalytic properties:
Scientific background: a solid material is held together by the forces at play between its constituent elements (atoms, ions). If we break a solid material in a vacuum, we create two new surfaces, which the bonds that originally took place between close elements are now open and "saturation-prone". The energy required to break theses bonds and create new surfaces is called "surface energy". The surface energy of a newly created and clean surface is very high. If a drop of water is placed onto this surface, the open and saturation-prone bonds tend to attract water molecules. This phenomenon is particularly evident when the so-called 'contact angle' (the angle formed by the material's surface -substrate- and the tangent of the interface between the fluid and the environment at the liquid/solid point of contact (see figures below).
The concept of the contact angle expresses the balance of mechanical strains that occur among different surface tension (expressing the above described surface energy) of the substrate, fluid/solid interface. A clean surface displays a high surface tension that contributes to obtaining a low contact angle; it will therefore be a hydrophilic surface. Glass per se is a polar substrate with hydrophilic properties. The contact angle for a drop of water on a clean glass surface is around 10-15º. The 'wettability' of a surface, an attribute that is directly related to its 'cleanability', is tied not only to the concept of contact angle (substrate surface tension) but also to at least two other parameters:
Polarity to the surface in terms of electrostatic surface charge (chemical characteristics of the surface)
Surface topography: roughness and micro-roughness
The action of various types of contaminants (particulate, aggressive chemical compounds) on the glass surface reduces the surface energy by increasing the contact angle and reducing the wettability and cleanability properties. In this case (contaminated surface), the contact angle increases up to 30-50º, thus making the surface cleaning difficult and time consuming. The foregoing clearly highlights the relationship between the cleanability of a surface and its conditions of use over time (durability). To solve this problem, the first solutions that were worked out consisted of "passive" coatings of the "hydrophobic" and "hydrophilic" type. For the purposes of this article, I will concentrate on the hydrophobic type, as follows:
Hydrophobic coatings: the concept behind hydrophobic coatings consists of depositing a coating with low surface tension onto the glass surface in order to obtain a high contact angle. This resulted in the development of two types of coatings: "easy-to-clean" coatings with contact angles ranging from 85-90º to 100-105º, cleaning of which still requires minimal mechanical action and therefore the possibility that dirt may concentrate in localized areas; and "self-cleaning" coatings, with contact angles in excess of 105º, where the mechanical action required is virtual unnecessary and no accumulation of contaminants occurs.
The most innovative "self-cleaning" coating, for its ease of application (simplicity) and cost-effectiveness (all around efficiency), is the newly developed Diamon Fusion® process, by Diamon Fusion® International, Inc. (DFI), of San Juan Capistrano, California. This patented process creates a unique phenomenon: a 'capping' in the chain of atoms that makes a change in the molecular composition of the glass surface. Thus, the new treated surface becomes, chemically speaking, one 'new surface', with a new bond of substrate. This new development creates additional properties, unknown and unseen ever before, such as: higher scratch resistance and impact resistance. Its unique 'invisible art glass', a process where artwork or lettering is imbedded in clear glass where the art remains invisible until the surface of the glass is fogged, misted or steamed. The most popular application has been on shower doors where logos of hotels can be imbedded.
Diamon Fusion® also provides a more bond-friendly surface to pressure-sensitive adhesives, thus highly increasing the performance of the foam tape (adhesive), in addition to also offering a more aggressive surface for contaminants like dust or dirt that will adhere otherwise.
About the author: Guglielmo Macrelli is a technical and scientific consultant in material science (Ionics Lab, Italy). His specific expertise includes: solar and thermal control glazing; variable transmission glazing; glazing with special mechanical properties: tempering and annealing; techniques for thin film deposition on glass.