Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to shatter into small granular chunks instead of splintering into large jagged shards as ordinary annealed glass does. These smaller, granular chunks are less likely to cause deep penetration when forced into the surface of an object (e.g. by gravity, by wind, by falling onto them, etc.) compared to larger, jagged shards because the reduction in both the mass and the maximum dimension of a glass fragment corresponds with a reduction in both the momentum and the penetration depth of the glass fragment. Tempered glass is used for its safety and strength in a variety of applications, including passenger vehicle windows (apart from windshield), shower doors, aquariums, architectural glass doors and tables, refrigerator trays, mobile phone screen protectors, bulletproof glass components, diving masks, and plates and cookware.
Properties
Tempered glass is about four times stronger than annealed glass. The more rapid contraction of the outer layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. Fully tempered 6-mm thick glass must have either a minimum surface compression of 69 MPa (10 000 psi) or an edge compression of not less than 67 MPa (9 700 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi). As a result of the increased surface stress, when broken the glass breaks into small rounded chunks as opposed to sharp jagged shards. Compressive surface stresses give tempered glass increased strength. Annealed glass has almost no internal stress and usually forms microscopic cracks on its surface. Tension applied to the glass can drive crack propagation which, once begun, concentrates tension at the tip of the crack driving crack propagation at very high speeds. Consequently, annealed glass is fragile and breaks into irregular and sharp pieces. The compressive stresses on the surface of tempered glass contain flaws, preventing their propagation or expansion. Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture. The strain pattern resulting from tempering can be observed by viewing through an optical polarizer, such as a pair of polarizing sunglasses.
Uses Tempered glass is used when strength, thermal resistance, and safety are important considerations. Passenger vehicles, for example, have all three requirements. Since they are stored outdoors, they are subject to constant heating and cooling as well as dramatic temperature changes throughout the year. Moreover, they must withstand small impacts from road debris such as stones as well as road accidents. Because large, sharp glass shards would present additional and unacceptable danger to passengers, tempered glass is used so that if broken, the pieces are blunt and mostly harmless. The windscreen or windshield is instead made of laminated glass, which will not shatter into pieces when broken while side windows and the rear windshield have historically been made of tempered glass. Some newer luxury vehicles have laminated side windows to meet occupant retention regulations, anti-theft purposes, or sound-deadening purposes. Other typical applications of tempered glass include:
Balcony doors Athletic facilities Swimming pools Façades Shower doors and bathroom areas Exhibition areas and displays Computer towers or computer cases (see: Enthusiast computing#Computer cases and Case modding#Window mods). Mobile phone screen protectors
Buildings and structures Tempered glass is also used in buildings for unframed assemblies (such as frameless glass doors), structurally loaded applications, and any other application that would become dangerous in the event of human impact. Building codes in the United States require tempered or laminated glass in several situations including some skylights, glass installed near doorways and stairways, large windows, windows which extend close to floor level, sliding doors, elevators, fire department access panels, and glass installed near swimming pools.
Household uses Tempered glass is also used in the home. Some common household furniture and appliances that use tempered glass are frameless shower doors, glass table tops, glass shelves, cabinet glass and glass for fireplaces.
Food service
"Rim-tempered" indicates that a limited area, such as the rim of the glass or plate, is tempered, and is popular in food service. There are also fully tempered variants for strength and thermal shock resistance. Some countries specify requirements regarding this. Tempered glass has also seen increased usage in bars and pubs, particularly in the United Kingdom and Australia, to prevent broken glass being used as a weapon.
Cooking and baking Some forms of tempered glass are used for cooking and baking. Manufacturers and brands include Glasslock, Pyrex, Corelle, and Arc International. This is also the type of glass used for oven doors.
Touchscreen devices Most touchscreen mobile devices use some form of toughened glass (such as Corning's Gorilla Glass), but there are also separate tempered screen protectors for touchscreen devices sold as an accessory.
Manufacturing
Tempered glass can be made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it well above its glass transition temperature of 564 °C (1,047 °F) to around 620 °C (1,148 °F). The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time. An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1 mm thick into compression by ion exchange of the sodium ions in the glass surface with potassium ions (which are 30% larger), by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal tempering and can be applied to glass objects of complex shapes.
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