Toughened or tempered glass has another advantage. Besides being much stronger than ordinary glass, toughened or tempered glass is also much safer. Not only does it break much not as fast as ordinary glass, and even if it does, it's not falling apart into shards but in harmless blunted glass beads.
Thermally toughened or tempered glass is also used for the insulation of glazing, laminated safety glass, doors, glass facades, entrances, escalators, elevator shafts, bathroom elements and as glass for furniture applications.
The "Heat Soak" - treatment for thermally toughened glass.
Glass may contain inclusions of nickel sulphide (NiS). The inclusions have a diameter of a few microns (µm) to several millimeters (mm) and another particular property is that their crystal structure is different at a low than at a high temperature. Their volume is bigger at low temperature. If the glass cools down slowly (annealed glass), all NiS particles have the time to reach their structure at low temperature during the cooling of the glass. The volume fluctuations of the inclusions can still be absorbed by the pasty condition of the glass and they do not cause any risk of glass breakage.
On the other hand, when thermally toughened glass is heated till about 650°C, the NiS reaches its stable structure at high temperature at the start of the curing. The following abrupt cooling allows the NiS to reach its stable structure at low temperature before the glass has become completely solid. Therefore the transformation will continue in the operating temperature of the glass and the associated volume increase can cause spontaneous breakage. In order to restrain the risk on glass breakage, they apply the heat-soak treatment. With this treatment the glass is heated in an oven for a certain period of time in order to accelerate the transformation reaction of the NiS. The possible fracture as a result of the presence of critical NiS particles will occur during the treatment. Depending on the use of the thermally toughened glass, it should be mentioned in the specifications whether the Heat Soak treatment should be applied or not. In case of load-bearing structural components (beams, SGG, SVG, ...), all glass elements must be treated.
Semi-toughened, semi-tempered or heat-strengthened glass
Semi-toughened or heat-strengthened glass did have a thermal treatment similar to the thermal curing, but where the obtained voltage is lower than the one of toughened glass, because the cooling was slower.
Semi-toughened or heat-strengthened glass doesn't have the same characteristics compared to the commodities which it was made of:
- Heat strengthened or semi-toughened glass can’t be cut, sawn, drilled or modified after the thermal treatment, Any cutting or drilling of holes must happen before the treatment.
- The load impact is 2 to 3 times bigger compared with standard (float) glass.
- The flexural strength is 2 to 3 times bigger compared with standard (float) glass.
- Heat strengthened or semi-toughened glass has a better resistance to thermal shocks than annealed glass: it resists temperature differences of circa 100°C, while annealed glass breakage can occur with a temperature difference of circa 30°C. However, this value is highly variable and depends on the quality of the edge of the glass.
- Heat strengthened or semi-toughened glass has the same fragmentation as a standard (float) glass. Because this glass has a big risk of injury, heat-strengthened glass is not a safety glass
- Heat strengthened or semi-toughened glass has a better optical quality than toughened or tempered glass by reduction of the optical distortions which are inherent to the temperature and duration of the curing process.
- In case of glass breakage the shards (similar to those of annealed glass) can cause injury. That's why it is not considered a safety glass. The fracture image of semi-toughened glass can easily be recognized because of the star shape.
- The Heat Soak Test, which is applied to reduce the risk of spontaneous breakage due to nickel sulphide inclusions, is not necessary for heat strengthened or semi-toughened glass.
- Heat-strengthened or semi-toughened glass is particularly used to prevent a fracture as a result of large temperature differences in the glass element due to for exemple drop shadows.
- Heat-strengthened or semi-toughened glass is used in glass façades.