Tempered glass

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 crumble into small granular chunks instead of splintering into jagged shards as plate glass (a.k.a. annealed glass) does. The granular chunks are less likely to cause injury.

A vandalized telephone booth with tempered glass

As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors and tables, refrigerator trays, mobile phone screen protectors, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.

Properties
Tempered glass of car rear window. Variations in glass stress are clearly seen when photographed through a polarizing filter (bottom picture).

Tempered glass is about four times stronger than annealed ("regular") glass.[1][2] The greater contraction of the inner 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).[3] 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, if the glass is ever broken it only breaks into small circular pieces as opposed to sharp jagged shards. This characteristic makes tempered glass safe for high-pressure and explosion proof applications.

It is this compressive surface stress that gives the tempered glass increased strength. This is because annealed glass, which has almost no internal stress, usually forms microscopic surface cracks, and in the absence of surface compression, any applied tension to the glass causes tension at the surface, which can drive crack propagation. Once a crack starts propagating, tension is further concentrated at the tip of the crack, causing it to propagate at the speed of sound in the material. Consequently, annealed glass is fragile and breaks into irregular and sharp pieces.[4] On the other hand, the compressive stresses on a tempered glass contain the flaw and prevent its 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
Safety approval markings on an automobile vent window made for a Chrysler car by PPG.
Police van with screen protector

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 are typically tempered glass.

Other typical applications of tempered glass include:

  • Balcony doors
  • Athletic facilities
  • Swimming pools
  • Facades
  • Shower doors and bathroom areas
  • Exhibition areas and displays
  • Computer towers or cases

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, near doorways and stairways, large windows, windows which extend close to floor level, sliding doors, elevators, fire department access panels, and near swimming pools.[5]

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. However, there are also specialist manufacturers that offer a fully tempered/toughened drinkware solution that can bring increased benefits in the form of strength and thermal shock resistance. In some countries these products are specified in venues that require increased performance levels or have a requirement for a safer glass due to intense usage.

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. Tempered glass products can be found in hotels, bars, and restaurants to reduce breakages and increase safety standards.

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.

Mobile devices

Most touchscreen mobile devices use some form of toughened glass (such as Corning's Gorilla Glass), as do some aftermarket screen protectors for these devices.[6]

Manufacturing
Tempered safety glass which has been laminated often does not fall out of its frame when it breaks – usually due to the anti-splinter film applied on the glass, as seen in this grocery store meat case.

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 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.[7]

Disadvantages

Tempered glass must be cut to size or pressed to shape before tempering, and cannot be re-worked once tempered. Polishing the edges or drilling holes in the glass is carried out before the tempering process starts. Because of the balanced stresses in the glass, damage to any portion will eventually result in the glass shattering into thumbnail-sized pieces. The glass is most susceptible to breakage due to damage to the edge of the glass, where the tensile stress is the greatest, but shattering can also occur in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (for example, striking the glass with a hardened point).

Using tempered glass can pose a security risk in some situations because of the tendency of the glass to shatter completely upon hard impact rather than leaving shards in the window frame.[8]

The surface of tempered glass does exhibit surface waves caused by contact with flattening rollers, if it has been formed using this process. This waviness is a significant problem in manufacturing of thin film solar cells.[9] The float glass process can be used to provide low-distortion sheets with very flat and parallel surfaces as an alternative for different glazing applications.[10]

Nickel sulfide defects can cause spontaneous breakage of tempered glass years after its manufacturing.[11]

History

Francois Barthelemy Alfred Royer de la Bastie (1830–1901) of Paris, France is credited with first developing a method of tempering glass[12] by quenching almost molten glass in a heated bath of oil or grease in 1874, the method patented in England on August 12, 1874, patent number 2783. Tempered glass is sometimes known as Bastie glass after de la Bastie. In 1877 the German Friedrich Siemens developed a different process, sometimes called compressed glass or Siemens glass, producing a tempered glass stronger than the Bastie process by pressing the glass in cool molds.[13] The first patent on a whole process to make tempered glass was held by chemist Rudolph A. Seiden who was born in 1900 in Austria and emigrated to the United States in 1935.[14]

Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In about 1660, Prince Rupert of the Rhine brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of King Charles II. These are teardrop-shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. They can withstand a blow from a hammer on the bulbous end without breaking, but the drops will disintegrate explosively into powder if the tail end is even slightly damaged.

See also

References
  1. Ford, Mark (22 January 2001). "How is tempered glass made?". Scientific American. Springer Nature America, LLC. Retrieved 12 June 2020.
  2. Block, Valerie, ed. (2002). The use of glass in buildings. West Conshohocken, PA: ASTM International. ISBN 978-0-8031-3458-4. Retrieved 12 June 2020.
  3. "ASTM C1048-18, Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass". ASTM International. 2018. doi:10.1520/C1048-18. Retrieved 12 June 2020. Cite journal requires |journal= (help)
  4. "Tempered vs. Annealed Glass | Hunker". Hunker.com. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  5. Hageman, J.M.; Beeston, B.E.P.; Hageman, K. Contractor's Guide to the Building Code (6th. ed.). Craftsman Book Co. ISBN 9781572182028.
  6. "PET, TPU, or Tempered Glass – all you need to know to choose a screen protector". phonearena.com. Archived from the original on 2015-08-20.
  7. Pfaender, Heinz G. (1996). Schott Guide to Glass (2 ed.). London: Chapman & Hall. ISBN 9780412620607.
  8. O'Block, Robert L.; Donnermeyer, Joseph F.; Doeren, Stephen E. (1991). Security and Crime Prevention. Butterworth–Heinemann. p. 118. The security value of tempered glass, however, is questionable. Although it will resist a brick or rock, it is susceptible to sharp instruments such as ice picks or screwdrivers. When attacked in this manner, tempered glass tends to crumple easily and quietly, leaving no sharp edges.
  9. Walecki, Wojtek J.; Szondy, Fanny (2008). Dhere, Neelkanth G. (ed.). "Integrated quantum efficiency, topography, and stress metrology for solar cell manufacturing: real space approach". Reliability of Photovoltaic Cells, Modules, Components, and Systems. Proceedings of SPIE. Bellingham, WA: SPIE. doi:10.1117/12.792934. ISSN 0277-786X. Retrieved 12 June 2020.
  10. "FLOAT GLASS TECHNOLOGY". ajzonca.tripod.com. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  11. Barry, John (12 January 2006). "The Achille Heel of a Wonderful Material: Toughened Glass". Glass on Web. Retrieved 16 August 2019.
  12. "Glass." The Encyclopædia Britannica : A Dictionary of Arts, Sciences and General Literature. 9th ed. (American reprint). Vol. 10. Philadelphia: Sherman & co., 1894. 595. Print.
  13. Uhlmann, D.R. and Kreidl, N. J. eds.. Glass. Science and Technology: Elasticity and Strength in Glasses. Vol. 5. New York, N.Y.: Academic, 1980. 197. Print.
  14. Barr, Johathan. "The Glass Tempering Handbook: Understanding the Glass Tempering Process". Self published. "Archived copy" (PDF). Archived (PDF) from the original on 2015-04-02. Retrieved 2015-02-28.CS1 maint: archived copy as title (link) accessed February 28, 2015
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