The strength of glass is directly proportional to its cooling rate during its processing
Aug 10, 2021
The strength of glass is directly proportional to its cooling rate during its processing. In this article i’ll limit my scope to types of glass used in construction. There are four main types:
1) Annealed Glass
This type of glass is the base product formed from the cooling stage of the float process. The aim is to relieve internal stress in the glass. This is achieved by a controlled cooling of molten glass until it reaches room temperature (In Lagos, Nigeria that is about 27 0C). Annealed glass is the base type of glass which is the starting point of other more advanced processed glass.
2) Heat Strengthened Glass
Glass can be semi tempered or semi toughened glass. The heat strengthened glass is produced by heating annealed glass to 700 degrees Celsius and then cooling it quickly, although not as fast as toughened glass. The aim is to mechanical and thermal strength of annealed glass, making it twice as tough as annealed glass.
This type of glass is not used in structural applications because of its low tensile or compressive strength compared to tempered or toughened glass.
3) Tempered or Toughened Glass
This is the most sought-after type of glass in Nigeria. Its use ranges from balustrades, shower doors, curtain-walls or similar structural applications. The process involves heating annealed glass to about 700 degrees Celsius by conduction, convection and radiation. The glass is cooled rapidly by a uniform and simultaneous blast of air on both surfaces. The variation in cooling rate between the surface and the inside of the glass improves physical properties, resulting in compressive stresses in the surface balanced by tensile stresses in the body of the glass.
The end product is four to five times stronger and safer than annealed glass
The counteracting stresses or surface compression gives toughened glass its increased mechanical resistance to breakage, and when it does break, causes it to produce small, regular, typically square fragments rather than long, dangerous shards that are far more likely to lead to injuries.
4) Laminated Glass
Laminated glass is made from the combination of more than one panel of glass. The most commonly used finished product is two sheets of toughened glass, laminated together with a 1.52mm thick Polyvinyl Butyral (PVB) inter-layer. This inter-layer is used in applications where strong binding, optical clarity, toughness and flexibility is needed.
Laminated glass offers many advantages. Safety and security are the best known of these, so rather than shattering on impact, laminated glass is held together by the inter-layer. This reduces the safety hazard associated with shattered glass fragments, as well as, to some degree, the security risks associated with easy penetration.
Lead Design Engineer for Wylout Designs Limited