Where is toughened glass used?

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Toughened glass can have various uses, mainly for when all the properties of normal glass are required, but also the stronger properties that toughened glass has.

It is used in situations that glass would just not be strong enough for, such as in frameless shower screens or office partitions inside buildings, or on the exterior of buildings where large spans of glass are required, such as office buildings where the exterior is mainly glass. It is also used a great deal in all forms of transport for windows and windscreens, as regular glass could not stand up to the conditions that many vehicles face.

The main use of toughened glass being focussed focused on today is in car windscreens. Toughened glass must be used for these, and in most countries there are regulations in place regarding the thickness and strength to ensure that safety is always at a high standard.

It is necessary to use toughened glass in this situation due to the fact that regular glass could prove very dangerous to the occupants of the car if it were to break. When normal glass breaks in a car accident, it would break into large, jagged pieces with very sharp edges, with many smaller pieces which could fly backwards into the car at a high speed. This could cause very severe injuries to any occupants of the car and can greatly increase the severity of any accident.

Glass, as with other materials, will only break when it cannot withstand the amount of stress that it is under. One cause of this stress could be cracks in the glass, which will then propagate through the glass, increasing the stress as they get deeper.

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The normal stress pattern within glass is evenly distributed so that no one point of the glass is under more stress than another. When a crack appears in the glass, the stress in the area will concentrate around the tip of the crack, causing it to propagate. These stresses on the bonds between the glass molecules become so high that the bonds eventually break. This means that the stress then moves onto the next set of molecules below, breaking those too. This means that the crack extends deeper into the glass until it has eventually broken all the way through all of the bonds in the glass. This means that there will now be a visible line on the glass where there are no longer any bonds between the molecules.

Toughened glass will not break in the same way as regular glass. It also has exceptional strength in comparison. This is a result of the manufacturing process, which is actually largely the same as the manufacturing process involved with regular glass. The only difference is an extra stage at the end which adds the greater strength to the glass. However, it is important that all required processing of the glass (such as cutting to size, polishing the edges or drilling holes in the glass) is carried out before the toughening process starts. Otherwise, if these are attempted on the treated glass, it will just shatter due to the incredibly high tension and compression within the glass. This extra stage is a heat treatment process which, when performed correctly, should make the surface be in compression while the centre is in tension.

Basically, the glass is heated to a very high temperature (approx. 620°C) until it begins to soften. It is then cooled very rapidly using cold jets of water or air. This brings the temperature of the surface of the glass down to room temperature quickly, so quickly that the centre of the glass is still very hot. This means that the surface molecules settle as they are, as they do not have time to move around from being heated to cooled. However, the particles in the center of the glass cool down much more slowly, meaning that they move closer together causing the center of the glass to shrink while the surface is already solid, therefore cannot shrink. This causes the surface of the glass to be forced into compression and the center is put into tension.

This is why toughened glass is much stronger than normal glass, about 5 times stronger. Because normal glass breaks because of applied tension on the surface, when the same amount of stress is applied to toughened glass, the stress has to overcome the compression of the surface before it can put the particles into tension and try to separate them, which is much more difficult. This means that toughened glass is very appropriate for use in car windscreens, as if the car was in an accident, the glass could be put under high stresses, but the windscreen would be more unlikely to break if it were made of toughened glass.

However, this does not mean that toughened glass is unbreakable. It is much harder to break, and when it does break, it breaks in a different way to regular glass. Instead of breaking into large jagged shards, it breaks into many tiny, dull edged and more rounded pieces. This is because the energy which is stored in the compression and tension in the glass is released as it breaks, as the bonds between the molecules breaks, so lots of new surfaces are formed in the tiny pieces of glass. Also, because there is so much energy stored in the bonds as they break, a lot of it is expelled as kinetic and sound energy, so it creates an almost explosive effect as the pieces fly out in all directions very quickly with a loud bang. Although this sounds more dramatic than the breakage of normal glass, it is much less dangerous as none of the pieces are as sharp. This is another reason why toughened glass is more approriate for use in car windscreens. If the windscreen was to break in an accident, the small, dull pieces of glass would cause very little damage to the occupants of the car, as opposed to the large jagged pieces of regular glass.

Other properties

The way that toughened glass breaks is not the only difference from normal glass. Some of its other properties are also different.

For example, the toughening process involved in making toughened glass can slightly reduce the optical quality. Slight distortion of the surface can be seen when light reflects off the glass at certain angles, but not greatly enough to affect the vision of drivers when it is used in car windscreens.

The resistance of the surface of the toughened glass to scratches is not changed by the heating and cooling from that of regular glass. Generally, toughened glass is never any thinner than 6mm with a density of about 2.5kg/m squared.

It also has a high compressive strength value of 1000Mpa. This means that 10 tonnes would be required to shatter 1cm cubed of toughened glass. The Young’s Modulus of toughened glass is about 70Gpa (Giga Pascals). It can withstand temperatures up to 295°C.

All of these properties make it the most suitable choice for car windscreens, as it can withstand much more stress and pressure, and is a stronger and safer option that normal glass.