What is Spontaneous Breakage?
Sometimes, after installation of tempered glass, it just breaks by itself without any exact cause. There are instances due to heavy wind load subject onto the glass, the glass will exhibit deflection on its center. Due to this deflection, the glass will move and have contact with the aluminum or the frame. Since the glass edges are the weak point of tempered glass, the continuous abrasion when subjected to heavy wind loads will cause the tempered glass to break spontaneously. There are also instances where the breakage is due to nickel sulfide stones present in the center of tempered glass.
Nickel Sulfide Inclusions
During the production of float glass, there are inadvertently presence of nickel sulfide stones. Although care and checks are being administered to check for nickel sulfide inclusion, it is very difficult to totally eliminate their presence in the glass production process. These nickel sulfide inclusions are also very small in diameter which makes it very hard to be detected by the naked eye.
Since nickel sulfide stones can occur in the production of float glass, they may be present in annealed and heat strengthened glass, as well as in tempered glass. However, spontaneous breakage most of the time occurs in tempered glass only.
Why Spontaneous Breakage Only Occurs in Tempered Glass
Nickel sulfide stone breakage is due to a phase transformation (so called α to β phase change) that results in an increase in the stone size. This volume growth, if the stone is in the center tension zone, can cause stresses potentially resulting in glass breakage.
In the manufacturing process of float glass, the glass cools slowly and steadily in a controlled fashion in order to produce glass with little residual surface and edge compression as possible. During this annealing phase, any stones have time to undergo the α to β phase change and become stable, without causing glass breakage.
When glass is re-heated for heat strengthening or tempering, the nickel sulfide inclusions that are present will shrink back to the smaller high-temperature stable α form. During heat-strengthening process, the cooling process is slow and steady, allowing the nickel sulfide to undergo the α to β phase change safely.
However, during tempering, the glass needs to be rapidly cooled which is inherent in the process. However, the Nickel Sulfide present in the glass gets trapped and may not expand completely and properly during the α to β phase. Then later, due to in-service temperature exposure, the phase change and accompanying volume growth continues and may lead to spontaneous breakage. Hence, spontaneous breakage due to stone inclusions is not an issue with annealed or heat strengthened glass but is most common in tempered glass.