Thermally toughened safety glass
Thermally toughened safety glass consists of a monolithic float glass pane, which is tempered by a thermal treatment. This increases the flexural strength and creates a characteristic fracture pattern. Thermally toughened safety glass is produced and marked in accordance with the European Construction Products Regulation. The product properties and specification requirements are defined in DIN EN ISO 12150-1. The factory production control and appropriate product labelling must be carried out in accordance with
DIN EN ISO 12150-2. For heat-soaked thermally toughened safety glass, DIN EN 14179 applies.
At the beginning of the production of thermally toughened safety glass, also called tempered safety glass, the previously cut glass pane is heated above the transformation temperature (softening temperature). To avoid possible marks and deformations of the softening glass, the glass pane is continuously moved back and forth over rollers in the oven. The homogeneously heated glass is then rapidly cooled by a blower. The glass surface cools immediately and solidifies, while the glass core slowly loses temperature and the molecules have more time to structure. The result is a compressive stress on the glass surfaces and a tensile stress in the glass core.
The pre-stressing introduced, significantly changes the properties of the original float glass. The compressive stress developed on the surfaces increases the flexural strength of the glass pane. This allows thermally toughened safety glass to withstand up to 5 times higher loads.
If the glass surface is damaged or the bending tensile strength is exceeded, the glass pane breaks up immediately. The frozen pre-stressing is released suddenly. This leads to a high crack propagation speed, which manifests itself in a small-crumbed fracture pattern. These “break crumbs” are described as blunt edged. Together with the low mass of the individual crumb, this results in a certain safety property.
Due to the frozen pre-stressing, toughened safety glass can not be subsequently processed.
If the pre-stressing introduced is uneven, a point shaped or stripe pattern can be detected under certain lighting conditions. To detect this, a high proportion of polarised radiation must be present in the light. Polarised light can be of natural origin or is created by reflection from adjacent surfaces. These so-called anisotropies, which can be recognised in the glass, do not constitute a product defect according to the current standard (DIN EN ISO 12150). With modern production know-how these appearances can be minimised so that they are barely perceptible.
Another material property of thermally toughened safety glass is the increased risk of spontaneous breakage. In the process, contamination (nickel sulphide, NiS) of the molten glass leads to microscopic inclusions. Under certain circumstances, these inclusions can grow and cause the glass to break unexpectedly. To reduce the risk of spontaneous breakage, the glass can be subjected to a hot storage test. In this the glass panes are heated to over 280° Celsius for four hours. This reduces the risk of spontaneous breakage to a minimum.
As the glass panes are heated above the transformation temperature (softening temperature) during production, they lose their planarity. The “wavy glass surface” creates visual distortions in the viewer and reflection. According to the current standard, the wave depth of 0.30 mm must not be exceeded over a distance of 300 mm. These geometric limits are not sufficient to describe the actual optical quality of the glass. Only when the actual wavelength is directly related to the wave depth can a statement about the optical quality be made.
Millidiopters allow a meaningful description of the optical quality. The wave length and wave depth are thereby evaluated together. The table at the end of this Isolas compass shows the resulting millidiopters depending on the wavelength and wave depth.
Thermally toughened safety glass is always used when there are increased safety features or increased structural requirements.
The increased safety properties of thermally toughened safety glass are based on the assumption that the small crumbly fracture pattern with “blunt” breaklines does not present a great risk of injury. Therefore, thermally toughened safety glass can be used as a partition wall for example.
Although the glass breaks up into small crumbs, the fragments still cling together in clod-shaped islands. The risk of injury is increased if the glass can fall onto traffic areas.
The compressive stress applied on the glass surfaces increases the flexural strength of the glass. Very often you need thermally toughened safety glass for large glass elements, which are exposed to high wind loads. Even with structural load bearing elements, such as glass walls, glass beams or glass columns, the increased strength is an advantage. This also applies to point-fixed glazing, where a concentration of stress can be expected in the hole area.
Often, thermally toughened safety glass is also used when the expected temperature differences in a glass element lead to an increased risk of breakage.