Toughened or tempered glass is glass that has undergone processes of controlled thermal treatment to increase its strength.
Toughened glass is made from annealed glass that has been heated to approximately 650⁰C and then rapidly cooled. Due to the increased heat treatment and rapid cooling of the glass, especially between the surface and the inside of the glass, the treatment produces different physical properties. This results in compressive stress on the surface and improved bending strength of glass.
Before toughening, the glass must be cut to size or pressed to shape. This is because once it is toughened, it cannot be re-worked on. Toughened glass is widely used in a number of applications.
Characteristics of Toughened Glass
Toughened glass is four to five times stronger than annealed glass of the same size and thickness against impact.Toughened glass has higher thermal strength and can withstand a high temperature differential upto 250°C.Toughened glass is considered as safety glass. It is difficult to break and even in the event of a breakage, disintegrates into small globules, which are relatively harmless.
Toughening does not alter the basic characteristics of glass such as light transmission and solar radiant heat properties.
After heat treatment, the surface of toughened glass has the same resistance to surface damage as annealed glass.Toughened glass cannot be cut, drilled or altered.
Heat soak test is recommended to be carried out on Toughened glass that is used for overhead and horizontal application, to prevent the spontaneous breakage caused due to nickel sulphide inclusion.
Toughening increases tensile strength, thermal shock resistance and safety of normal glass. It also increases its heat withstanding capacity. Because of these properties, toughened glass is preferred in applications where strength, safety and thermal resistance are significant.
Strength & Safety Considerations
Resistance to Thermal Breakage
Uneven heating due to direct sunlight makes glass expand and contract at different rates. This difference in the rates, creates stress in glass which may lead to breakage. When temperature differential is above 50°C, the risk of thermal breakage is high for annealed glass and when the difference is upto 250°C, the risk of thermal breakage is prevented by using toughened glass.
Thermal Breakage Pattern
Thermal breakage starts at the edge and is perpendicular to the glass edge. Breakage may be single or multiple depending on the built up stress. Several cracks indicate high thermal stress across the glass pane. A fully toughened glass has significantly higher edge strength to withstand chances of thermal breakage.
Applications of Toughened Glass
Toughened glass is specifically designed for use in areas where there is a high risk of contact and breakage. It can be used for exteriors and interiors, such as facades, showroom shelves, dividers, shower cabins, doors, etc.
In the automobile industry, toughened glass is commonly used for side lights and back lights.