Toughened safety glass
Tempered safety glass ( ESG ) is tempered glass according to EN 12150-1 in which a strong internal tension is built up through heat treatment . This significantly increases the shock and impact resistance compared to normal flat glass . If it breaks, it breaks into small, compact cubes, which effectively reduces the risk of cuts and impacts.
ESG is used in particular
- where high loads occur, e.g. when stored horizontally and used as a shelf,
- where there is an increased risk of breakage, for example with exposed edges as well as in the interior design of business premises or apartments,
- where there is an increased risk of injury if broken, for example with shower walls or vehicle glazing.
Manufacturing
During processing into toughened safety glass, the glass is heated to temperatures above its transformation temperature and then suddenly cooled again. The near-surface zones of the glass cool faster than the inner core. The result is a tensile stress in the core and a compressive stress in the surface (with metallic materials, the same effect is achieved, for example, by surface hammering and autofrettage ). ESG must therefore be sanded or cut to size prior to heat treatment; subsequent processing is only possible to a very limited extent. Processing that penetrates the tensile stress zone of the glass would lead to breakage of the pane.
properties
When broken, toughened safety glass breaks into small pieces, usually less than 3 cm in length, without the very acute angles that occur with ordinary glass. This significantly reduces the risk of injury compared to normal flat glass . If ESG is provided with large stickers on overhead glazing, the fragments held together by the film can cause more damage than the individual small pieces when they fall down.
The shock and impact resistance is verified by means of the pendulum impact test according to the DIN EN 12600 standard. In addition, the toughened glass is insensitive to large temperature differences. The thermal shock resistance is 200 K.
Partially toughened glass , TVG for short (regulated in DIN EN 1863-1), like fully toughened toughened safety glass (ESG), is subjected to a thermal toughening process. However, the cooling process takes place more slowly. This results in lower tension differences in the glass between the core and the surfaces. The flexural strength of 70 N / mm² lies between that of float glass 45 N / mm² (according to EN 572) and toughened safety glass 120 N / mm² (according to DIN EN 12150). Furthermore, TVG with 100 K has a higher thermal shock resistance than float glass with approx. 40 K (according to EN 572). In the event of breakage, cracks occur that run radially from the center of the break to the edge of the pane, similar to the breakage of float glass. TVG is mainly used for the production of laminated safety glass (VSG). Due to the large-format fragments, LSG made of TVG has an increased residual load-bearing capacity and is mainly used for overhead glazing and fall-proof glazing.
Defects, damage, special features
With conventional ESG, so-called "spontaneous fractures" can occur when heated by solar radiation (e.g. with facade glazing) or high temperature fluctuations, which are caused by corrosive nickel (II) sulfide inclusions . This problem is countered with so-called ESG-H glass: After production, this toughened safety glass is subjected to the so-called heat soak test or hot storage test by storing it for several hours at a temperature of 280 to 300 degrees Celsius. In the process, appropriately predestined panes break. The probability of spontaneous breakage is now low for the remaining panes.
Since ESG and HSG lie on rollers in the furnace during the toughening process, slight surface changes can occur. The ripple (called “roller waves” in specialist circles) cannot always be avoided for physical reasons and leads to an impairment of the reflection image. As a result of this thermal tempering process, chemical and mechanical changes in the surface properties such as the formation of dots (called “roller pick-up”) and roller marks can occur.
See also
literature
- Interpane Glas Industrie AG, "Designing with Glass", 7th revised edition, page 135/136
- "Glass construction practice, construction and measurement", Weller / Nicklisch / Thieme / Weimar, 2nd edition 2010, Verlag Bauwerk