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Main Types of Glass
Today, flat glass comes in many highly specialised forms intended for different products and applications. Flat glass produced by way of the float process is often further processed (see below) to give it certain qualities or specificities. In this way, the industry can meet the various requirements and needs of the construction, automotive and solar energy industries:
- Annealed glass
- Toughened glass
- Laminated glass
- Coated glass
- Mirrored glass
- Patterned glass
- Extra clear glass
Annealed glass is the basic flat glass product that is the first result of the float process. It is common glass that tends to break into large, jagged shards. It is used in some end products and often in double-glazed windows. It is also the starting material used to produce more advanced products through further processing such as laminating, toughening, coating, etc.
Toughened glass is treated to be far more resistant to breakage than simple annealed glass and to break in a more predictable way when it does break, thus providing a major safety advantage in almost all of its applications.
Toughened glass is made from annealed glass treated with a thermal tempering process. A sheet of annealed glass is heated to above its "annealing point" of 600°C; its surfaces are then rapidly cooled while the inner portion of the glass remains hotter. The different cooling rates between the surface and the inside of the glass produces different physical properties, resulting in compressive stresses in the surface balanced by tensile stresses in the body of the glass.
These counteracting stresses give toughened glass its increased mechanical resistance to breakage, and are also, when it does break, what cause it to produce small, regular, typically square fragments rather than long, dangerous shards that are far more likely to lead to injuries. Toughened glass also has an increased resistance to breakage as a result of stresses caused by different temperatures within a pane.
Toughened glass has extremely broad applications in products for both buildings and, automobiles and transport, as well as in other areas. Car windshields and windows, glass portions of building facades, glass sliding doors and partitions in houses and offices, glass furniture such as table tops, and many other products typically use toughened glass. Products made from toughened glass often also incorporate other technologies, especially in the building and automotive and transport sectors.
Laminated glass is made of two or more layers of glass with one or more "interlayers" of polymeric material bonded between the glass layers.
Laminated glass is produced using one of two methods:
- Poly Vinyl Butyral (PVB) laminated glass is produced using heat and pressure to sandwich a thin layer of PVB between layers of glass. On occasion, other polymers such as Ethyl Vinyl Acetate (EVA) or Polyurethane (PU) are used. This is the most common method.
- For special applications, Cast in Place (CIP) laminated glass is made by pouring a resin into the space between two sheets of glass that are held parallel and very close to each other.
Laminated glass offers many advantages. Safety and security are the best known of these, so rather than shattering on impact, laminated glass is held together by the interlayer. This reduces the safety hazard associated with shattered glass fragments, as well as, to some degree, the security risks associated with easy penetration. But the interlayer also provides a way to apply several other technologies and benefits, such as colouring, sound dampening, resistance to fire, ultraviolet filtering and other technologies that can be embedded in or with the interlayer.
Laminated glass is used extensively in building and housing products and in the automotive and transport industries. Most building facades and most car windscreens, for example, are made with laminated glass, usually with other technologies also incorporated.
Surface coatings can be applied to glass to modify its appearance and give it many of the advanced characteristics and functions available in today's flat glass products, such as low maintenance, special reflection/transmission/absorption properties, scratch resistance, corrosion resistance, etc.
Coatings are usually applied by controlled exposure of the glass surface to vapours, which bind to the glass forming a permanent coating. The coating process can be applied while the glass is still in the float line with the glass still warm, producing what is known as "hard-coated" glass.
Alternatively, in the "off-line" or "vacuum" coating process, the vapour is applied to the cold glass surface in a vacuum vessel.
To produce mirrored glass, a metal coating is applied to one side of the glass. The coating is generally made of silver, aluminium, gold or chrome. For simple mirrored glass, a fully reflective metal coating is applied and then sealed with a protective layer. To produce "one-way" mirrors, a much thinner metal coating is used, with no additional sealing or otherwise opaque layer.
Mirrored glass is gaining a more prominent place in architecture, for important functional reasons as well as for the aesthetic effect.
Patterned glass is flat glass whose surfaces display a regular pattern. The most common method for producing patterned glass is to pass heated glass (usually just after it exits the furnace where it is made) between rollers whose surfaces contain the negative relief of the desired pattern(s).
Patterned glass is mostly used in internal decoration and internal architecture. Today, it is typically used for functional reasons, where light but not transparency is desired, and the patterns are accordingly subtle. However, it has also at times been fashionable as a design feature in itself, in such cases often displaying more prominent patterns.
Extra clear glass is not the result of processing of annealed glass, but instead a specific type of melted glass. Extra clear glass differs from other types of glass by its basic raw material composition. In particular, this glass is made with a very low iron content in order to minimise its sun reflection properties. It therefore lets as much light as possible through the glass. It is most particularly of use for solar energy applications where it is important that the glass cover lets light through to reach the thermal tubes or photovoltaic cells. Anti-reflective properties can be further increased by applying a special coating on the low-iron glass. It can also be used in windows or facades as it offers excellent clarity, which allows occupants to appreciate true colours and to enjoy unimpaired views.