Toughened glass

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A vandalized telephone booth with toughened glass Broken phone box.jpg
A vandalized telephone booth with toughened glass

Toughened or tempered 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.

Safety glass glass with additional safety features that make it less likely to break

Safety glass is glass with additional safety features that make it less likely to break, or less likely to pose a threat when broken. Common designs include toughened glass, laminated glass, wire mesh glass and engraved glass. Wire mesh glass was invented by Frank Shuman. Laminated glass was invented in 1903 by the French chemist Édouard Bénédictus (1878–1930).

In mechanics, compression is the application of balanced inward ("pushing") forces to different points on a material or structure, that is, forces with no net sum or torque directed so as to reduce its size in one or more directions. It is contrasted with tension or traction, the application of balanced outward ("pulling") forces; and with shearing forces, directed so as to displace layers of the material parallel to each other. The compressive strength of materials and structures is an important engineering consideration.

Tension (physics) pulling force transmitted axially by means of a string, cable, chain, or similar 1D continuous object, or by each end of a rod, truss member, or 3D object; action-reaction pair of forces acting at each end of said elements; opposite of compression

In physics, tension may be described as the pulling force transmitted axially by the means of a string, cable, chain, or similar one-dimensional continuous object, or by each end of a rod, truss member, or similar three-dimensional object; tension might also be described as the action-reaction pair of forces acting at each end of said elements. Tension could be the opposite of compression.

Contents

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

Bulletproof glass type of strong but optically transparent material that is particularly resistant to being penetrated when struck

Bulletproof glass is a strong and optically transparent material that is particularly resistant to penetration by projectiles. Like any material, it is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft. The softer layer makes the glass more elastic, so it can flex instead of shatter. The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear, undistorted view through the glass. Bulletproof glass varies in thickness from 3⁄4 to 3 1⁄2 inches.

Diving mask Watertight air-filled face cover with view-ports for improving underwater vision

A diving mask is an item of diving equipment that allows underwater divers, including scuba divers, free-divers, and snorkelers, to see clearly underwater. Surface supplied divers usually use a full face mask or diving helmet, but in some systems the half mask may be used. When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable to focus the light on the retina. By providing an air space in front of the eyes, the eye is able to focus nearly normally. The shape of the air space in the mask slightly affects the ability to focus. Corrective lenses can be fitted to the inside surface of the viewport or contact lenses may be worn inside the mask to allow normal vision for people with focusing defects.

Properties

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

Toughened glass is physically and thermally stronger than normal glass. [1] 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. For glass to be considered toughened, this compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000 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, 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. [2]

It is this compressive stress that gives the toughened glass increased strength. This is because annealed glass, which has almost no internal stress, usually forms microscopic surface cracks, and any applied tension gets magnified at the surface, reducing the applied tension needed to propagate the crack. Once it starts propagating, tension gets magnified even more easily, causing it to propagate at the speed of sound in the material. Consequently, annealed glass is fragile and breaks into irregular and sharp pieces. [3] Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.

Annealing of glass is a process of slowly cooling hot glass objects after they have been formed, to relieve residual internal stresses introduced during manufacture. Especially for smaller, simpler objects, annealing may be incidental to the process of manufacture, but in larger or more complex products it commonly demands a special process of annealing in a temperature-controlled kiln known as a lehr. Annealing of glass is critical to its durability. Glass that has not been properly annealed retains thermal stresses caused by quenching, which indefinitely decrease the strength and reliability of the product. Inadequately annealed glass is likely to crack or shatter when subjected to relatively small temperature changes or to mechanical shock or stress. It even may fail spontaneously.

The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.

Polarizer

A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well-defined polarization, that is polarized light. The common types of polarizers are linear polarizers and circular polarizers. Polarizers are used in many optical techniques and instruments, and polarizing filters find applications in photography and LCD technology. Polarizers can also be made for other types of electromagnetic waves besides light, such as radio waves, microwaves, and X-rays.

Uses

Safety approval markings on an automobile vent window made for a Chrysler car by PPG. Safeguard Glass Markings 1.jpg
Safety approval markings on an automobile vent window made for a Chrysler car by PPG.
Police van with screen protector HKPF Airport Police Patrol Car AM8599.JPG
Police van with screen protector

Toughened 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 such as from road debris such as stones as well as automobile accidents. Because large, sharp glass shards would present additional and unacceptable danger to passengers, toughened 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 is typically toughened glass.

Windshield front window of vehicle

The windshield or windscreen of an aircraft, car, bus, motorbike or tram is the front window. Modern windshields are generally made of laminated safety glass, a type of treated glass, which consists of two (typically) curved sheets of glass with a plastic layer laminated between them for safety, and are bonded into the window frame. Motorbike windshields are often made of high-impact polycarbonate or acrylic plastic.

Laminated glass

Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral (PVB) or ethylene-vinyl acetate (EVA), between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic "spider web" cracking pattern when the impact is not enough to completely pierce the glass. In the case of the EVA, the thermoset EVA, offers a complete bounding (cross-linking) with the material whether it is glass, polycarbonate, PET, or other types of products.

Other typical applications of toughened glass include:

Buildings and structures

Toughened 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. Tempered and heat strengthened glass can be three to seven times stronger than annealed glass. [4] Building codes in the United States require tempered or laminated glass in several situations including some skylights, [5] near doorways and stairways, large windows, windows which extend close to floor level, sliding doors, elevators, fire department access panels, and near swimming pools. [6]

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, replacement glass, glass shelves, cabinet glass and glass for fireplaces. [7]

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. [8]

Cooking and baking

Some forms of tempered glass are used for cooking and baking. Manufacturers and brands include Glasslock, Pyrex, Corelle, and Arc International.

Mobile devices

Most touchscreen mobile devices use some form of toughened glass (such as Corning's Gorilla Glass), [9] as do some aftermarket screen protectors for these devices. [10]

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. Broken Meat Case Window.JPG
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.

Toughened 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 toughening and can be applied to glass objects of complex shapes. [11]

Disadvantages

Toughened glass must be cut to size or pressed to shape before toughening, and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening 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 toughened 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. [12]

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. [13] 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. [14]

History

Francois Barthelemy Alfred Royer de la Bastie (1830–1901) of Paris, France is credited with first developing a method of tempering glass [15] 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. [16] 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. [17]

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

Related Research Articles

Poly(methyl methacrylate) polymer

Poly(methyl methacrylate) (PMMA), also known as acrylic, acrylic glass, or plexiglass as well as by the trade names Crylux, Plexiglas, Acrylite, Lucite, and Perspex among several others, is a transparent thermoplastic often used in sheet form as a lightweight or shatter-resistant alternative to glass. The same material can be used as a casting resin, in inks and coatings, and has many other uses.

Heat treating process of heating something to alter it

Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Carbon steel steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content up to 2.1% by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Prince Ruperts drop glass object created by dripping molten glass into cold water

Prince Rupert's drops are toughened glass beads created by dripping molten glass into cold water, which causes it to solidify into a tadpole-shaped droplet with a long, thin tail. These droplets are characterized internally by very high residual stresses, which give rise to counter-intuitive properties, such as the ability to withstand a blow from a hammer or a bullet on the bulbous end without breaking, while exhibiting explosive disintegration if the tail end is even slightly damaged. In nature, similar structures are produced under certain conditions in volcanic lava and are known as Pele's tears.

Tempering (metallurgy) metallurgy

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.

Shot peening cold metal working process

Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals and composites. It entails impacting a surface with shot with force sufficient to create plastic deformation.

Residual stress

Residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, laser peening imparts deep beneficial compressive residual stresses into metal components such as turbine engine fan blades, and it is used in toughened glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones. However, unintended residual stress in a designed structure may cause it to fail prematurely.

Ninja rocks are broken shards of spark plugs that are formed by smashing the ceramic portion of the spark plug with a hammer or other large object. Since they can quickly and quietly fracture the glass side windows on most cars, ninja rocks have been used in "smash-and-grab" auto burglaries since at least 1995. They have no traditional association with the ninja or ninjutsu, only being named such due to their "silent but deadly" function in burglaries.

Architectural glass

Architectural glass is glass that is used as a building material. It is most typically used as transparent glazing material in the building envelope, including windows in the external walls. Glass is also used for internal partitions and as an architectural feature. When used in buildings, glass is often of a safety type, which include reinforced, toughened and laminated glasses.

Spontaneous glass breakage is a phenomenon by which toughened glass may spontaneously break without any apparent reason. The most common causes are:

Safety and security window films are polyester, or PET films that are applied to glass and glazing in order to hold them together if the glass is shattered. The main difference between film and laminated glass is that these Shatter Safe films can be applied to the glass or glazing after manufacture or installation, i.e., it is a retrofit product. These films are used widely all over the world; they can be found on trains, buses, cars, and buildings.

A Bologna bottle, also known as a Bologna phial or philosophical vial, is a glass bottle which has great external strength, often used in physics demonstrations and magic tricks. The exterior is generally strong enough that one could pound a nail into a block of wood using the bottle as a hammer; however, even a small scratch on the interior would cause it to crumble.

Chemically strengthened glass is a type of glass that has increased strength as a result of a post-production chemical process. When broken, it still shatters in long pointed splinters similar to float glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required. However, chemically strengthened glass is typically six to eight times the strength of float glass.

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

Glazing (window) part of a wall or window, made of glass

Glazing, which derives from the Middle English for 'glass', is a part of a wall or window, made of glass. Glazing also describes the work done by a professional "glazier". Glazing is also less commonly used to describe the insertion of ophthalmic lenses into an eyeglass frame.

Insulated glazing

Insulating glass (IG), more commonly known as double glazing, consists of two or three glass window panes separated by a vacuum or gas filled space to reduce heat transfer across a part of the building envelope.

Glass typically has a tensile strength of 7 megapascals (1,000 psi), however theoretically it can have a strength of 17 gigapascals (2,500,000 psi) which is due to glass's strong chemical bonds. Imperfections on glass such as scratches and bubbles decrease the strength of glass. The imperfections on a piece of glass have a great effect on the strength of glass. The chemical composition of the glass also impacts the tensile strength of glass. The processes of thermal and chemical toughening can increase the tensile strength of glass.

References

  1. ↑ Khatib, Jamal (2016-08-12). Sustainability of Construction Materials. Woodhead Publishing. ISBN   9780081003916.
  2. ↑ "The Benefits of Glass Tempering and Strengthening". Swift Glass. 2015-07-16. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  3. ↑ "Tempered vs. Annealed Glass | Hunker". Hunker.com. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  4. ↑ Redner, A. S., "PC-Based Stress-Measuring System for On-Line Quality Control of Tempered and Heat-Strengthened Glass," Use of Glass in Buildings, ASTM STP 1434, V. Block, ed. ASTM International, West Conshohocken, PA 2002
  5. ↑ Hageman, Jack M., and Brian E. P. Beeston. Contractor's Guide to the Building Code. 6th ed. Carlsbad: Craftsman Book, 2008. 360. Print.
  6. ↑ "Tempered Glass | The Glass Guru". www.theglassguru.com. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  7. ↑ "Tempered Glass - Custom Cut Safety Glass Delivered to Your Home - Dulles Glass and Mirror". www.dullesglassandmirror.com. Archived from the original on 2018-05-12.
  8. ↑ "Arcoroc : materials - tempered glassware". hotel-restaurant.arcoroc.com. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  9. ↑ "What the difference between sapphire glass and gorilla glass?". Gazelle The Horn. 2014-04-24. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  10. ↑ "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.
  11. ↑ H.G. Pfaender (1996) Schott guide to glass. Chapman and Hall ISBN   0-412-62060-X
  12. ↑ O'Block, Robert L.; Donnermeyer, Joseph F.; Doeren, Stephen E. (1991). Security and Crime Prevention. Butterworth–Heinemann. 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.
  13. ↑ Walecki, W.; Szondy, F. (2008). "Integrated quantum efficiency, topography, and stress metrology for solar cell manufacturing: real space approach". Proc. SPIE. 7048: 704804. doi:10.1117/12.792934.
  14. ↑ "FLOAT GLASS TECHNOLOGY". ajzonca.tripod.com. Archived from the original on 2017-12-14. Retrieved 2017-12-13.
  15. ↑ "Glass." The Encyclopædia Britannica : A Dictionary of Arts, Sciences and General Literature. 9th ed. (American reprint). Vol. 10. Philadelphia: Sherman & co., 1894. 595. Print.
  16. ↑ 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.
  17. ↑ 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. accessed February 28, 2015
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