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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 .
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 (also known as tempered glass), laminated glass , and wire mesh glass (also known as wired glass).
Thermal fracturing in glass occurs when a sufficient temperature differential is created within glass. [1] As a warmed area expands or a cooled area contracts, stress forces develop, potentially leading to fracture. A temperature differential may be created in many ways, including solar heating, space heating devices, fire, or hot and cold liquids.
In toughened glass, compressive stresses are induced on the surface of the glass, balanced by tensile stresses in the body of the glass. Due to the residual compressive stress on the surface, toughened glass is more resistant to cracks, but shatter into small shards when the outer surface is broken.
The first principle is used in laminated glass where two sheets of glass are separated by an interlayer of polyvinyl butyral. The polyvinyl butyral, as a viscoelastic polymer, absorbs the growing crack. The second method is used in toughened glass and pre-stressed concrete. A demonstration of glass toughening is provided by Prince Rupert's Drop.
Also unlike toughened glass, chemically strengthened glass may be cut after strengthening, but loses its added strength within approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength. Another negative of chemically strengthened glass is the added cost.
Imperfections of the glass, such as bubbles, and in particular surface flaws, such as scratches, have a great effect on the strength of glass and decrease it even more than for other brittle materials. [2] [1] The chemical composition of the glass also impacts its tensile strength. [3]
Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10 −6 K −1 at 20 °C), making them more resistant to thermal shock than any other common glass.