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TYPES OF GLASS

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FLOAT GLASS (ALSO CALLED PLATE OR ANNEALED GLASS)

The float process, the method that produces 90 per cent of glass for buildings, is a continuous flow process. The raw ingredients - silica, soda ash, lime and various oxides - are heated to 1100degreesC. This molten material then flows on to a shallow bath of molten tin, where it floats, spreading out to form an even layer, its thickness controlled by the speed of flow. As it flows across the bath, the temperature drops to 600degreesC, and then to 250degreesC when the glass solidifies to a sheet. It is then hard enough to pass on rollers into the annealing chamber which ensures a 'fire- polished' product with virtually parallel surfaces. It is readily available, cheap and easily cut, but fragile, breaks easily into dagger-like pieces.

TOUGHENED GLASS (CALLED TEMPERED GLASS IN THE US)

To produce toughened glass, sheets of float glass are heated to 625-650degreesC , then chilled rapidly by having cold air blown on to both surfaces. The interior core of the sheet stays hot and plastic while the surfaces cool and solidify; then as the core cools and shrinks, compressive stresses are induced in the more solid surfaces of between 1.74 and 2.9N/m2.

(Toughening by heat is the most common process; glass can also be toughened by chemical treatment.)

Toughened glass is about five times stronger than float glass if hit with a sharp pointed object, especially at the edges, will disintegrate into thousands of pieces with dulled edges which are safe to handle. These may cause injury if glass falls from a height.

cannot be cut, edge-polished, or drilled after toughening

has a risk of spontaneous failure due to nickel-sulphide inclusion. To reduce the risk, heat soaking should be specified for all toughened glass. The only recognised standard, DIN18516 Part 3, specifies an eight-hour soak at 290degreesC.

HEAT-STRENGTHENED GLASS

Heat-strengthened glass, not as strong as toughened glass, is used instead of float glass where there is a risk of thermal fracture. It is not a safety glass as it breaks into shards on failure. The manufacturing process is similar to toughened glass - sheets of float glass are heated to 650- 700degreesC - but the cooling process is less severe, inducing compressive stresses in the surfaces.

LAMINATED GLASS

Two or more sheets of float glass are bonded together with one or more interlayers of polyvinyl butyral (pvb).

Laminated glass is produced by a semi-automated process. The sheets of glass are washed to remove any trace of contamination, the pvb interlayer is fitted between them and the edges are trimmed. Any air trapped between the pvb and glass 'sandwich' is then eliminated by heating, which softens the pvb layer, and compressing by heavy rollers. The edges are sealed.

Next the sheets enter an autoclave where glass and pvb are sealed together under pressures of 12-14kg/cm2 and by temperatures of 135-145degreesC. The bonding process of heat and pressure converts pvb from a translucent material to one which is clear and a very strong adhesive.

Laminated glass

combines safety and security. If the glass is hit by any object it will crack, but the crack is limited to the point of impact and the rest of the glass remains transparent. The broken shards are held together by the interlayer, reducing the risk of injury. In most cases the residual energy in the object is absorbed by the interlayer, preventing it from penetrating the glass. The correct specification of glass must be used to meet the type of impact anticipated can be worked after laminating, so is available from stock compares in cost to toughened glass has solar control and sound absorbency properties.

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