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Tempered Glass
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Toughened glass or tempered glass is a type of safety glass that has
increased strength and will usually shatter in small, square pieces
when broken. It is used when strength, thermal resistance and safety
are important considerations. At home you are likely to find toughened glass in shower and sliding glass patio doors. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and any glass where these is a danger of human impact. Using toughened glass can pose a security risk in some situations due to the tendency the glass has to shatter utterly upon hard impact. Toughened glass is typically four to six times the strength of annealed glass. However, this strength comes with a penalty. Due to the balanced stresses in the glass, damage to the glass will eventually result in the glass shattering into thumbnail sized pieces. Although toughened glass is most susceptible to breakage via edge damage, breakage can also occur from impacts in the centre of the glass pane. |
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Shattering may not happen when the damage originally occurs and can
be triggered by a minor stress like heat or small impact that would
not normally affect the toughened glass. If any toughened glass
shows any damage it must be replaced. 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. Also, ironically, the toughened glass surface is not as hard as annealed glass and is slightly more susceptible to scratching. Toughened glass is made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace which heats it to above its annealing point of 600 °C. The glass is then rapidly cooled with forced draughts of air. This rapidly cools the glass surface below its annealing point, causing it to harden and contract, while the inner portion of the glass remains free to flow for a short time. The final contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. This compressive stress on the surface of the glass is typically as high as 50 MPa. It is this compressive stress that gives the toughened glass an increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin. The pattern of cooling during the process can be revealed by observing the glass with polarized light, which shows the strain pattern in the glass. Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In the 1640s, Prince Rupert of Bavaria (1619–1682), who was grandson of James I of England, and nephew of Charles I, brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. These are remarkable 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. These were often used by the King as a practical joke. Chemically strengthened glassChemically strengthened glass is a type of glass that has increased strength. When broken it still shatters in long pointed splinters similar to float (annealed) glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required.Chemically strengthened glass is typically six to eight times the strength of annealed glass. The glass is chemically strengthened by submersing the glass in a bath containing a potassium salt (typically potassium nitrate) at 450 °C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution. These potassium ions are larger than the sodium ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the potassium nitrate solution. This replacement of ions causes the surface of the glass to be in a state of compression and the core in compensating tension. The surface compression of chemically strengthened glass may reach up to 690 MPa. There also exists a more advanced two-stage process for making chemically strengthened glass, in which the glass article is first immersed in a sodium nitrate bath at 450 °C, which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article. Chemical strengthening results in a strengthening similar to toughened glass, however the process does not use extreme variations of temperature and therefore chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. This differs from toughened glass, in which slender pieces can often be significantly bowed. Also unlike toughened glass, chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength. Chemically strengthened glass was used on some fighter aircraft canopies. |
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Masala Chai Tea | Instant tea
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Jul 23, 2010 ... Tea, today, has become an integral part of our life. Right from
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