What is tempered glass?
Also called toughened glass, it is processed by heating up Annealed glass (normal / float ) and quickly(rapidly) cooling it to room temperature. This process is called tempering. It’s a type of safety glass and is about 4 times stronger than annealed (normal/float) glass. When broken, it will crumble into chunks instead of big and sharp edges like annealed glass. It is less likely to cause injury, therefore, safety glass.
Reasons for Spontaneous Breakage
There are many reasons attributing to spontaneous breaking. A combination of minor and often unseen damages below could deteriorate the lifespan of glass
- minor damage in the edge of the tempered glass
- Small nick or chip of the glass surface, caused during the glass transportation process or its installation
- glass units’ movements caused by winds and temperature changes due inaccurate frame planning
- thermal stresses causing the center of the glass to be hotter than the edge of the glass. The expansion of the heated glass center results in tensile stress at the edge of the glass. If the thermally induced stress exceeds the edge strength of the glass, breakage occurs
Another possible reason is the presence of Nickel-sulfide (NiS) in tempered glass.
Nickel-sulfide is formed in the production of annealed glass. Despite quality control and procedures to reduce Nickel-sulfide stones to form. There’s no technology to completely eliminate them. Nickel-sulfide stones particularly cause problems to tempered glass as the process of tempering occurs rapidly, giving time NiS insufficient time to expand to its final size. This leads to NiS expanding at a later stage under prolonged heat and cause spontaneous breakage.
How to identify?
Breakage often have two distinctive glass fragments at the fracture origin with a unique shape (roughly hexagonal or pentagonal) and are generally larger than the other fragments, resembling a “butterfly,” or “figure of 8” pattern.
Options available to overcome spontaneous breakage of tempered glass
Using laminated glass.
Is where 2 or more pieces of glass held together by an inter-layer, usually polyvinyl (PVB). This is because when one side of the glass breaks, glass with not shatter everywhere, instead the PVB will hold the broken glass in place before the replacement of new glass.
Using Heat to strengthen glass.
It is 2 times Stronger than normal glass but weaker then tempered glass. It doesn’t shatter into many pieces. Spontaneous breakage will not happen.
Using heat soak test.
It is a process that may expose NiS on tempered glass. By placing the tempered glass into a chamber with a temperature of an estimated 550 degrees Fahrenheit / 287 degrees Celsius. This is to accelerate the NiS expansion and glass that contains NiS inclusions will break in the chamber and reduce the risk of the glass breaking in the potential field.