What is a Spontaneous Glass Fracture?
Often referred to as ‘glass cancer’, spontaneous glass fractures are tiny glass fractures that occur with the presence of Nickel Sulphide inside of the glass. Although they are quite rare ( only 5 parts in 1012, per tonne of glass) they can have severe consequences. The nickel sulphide inclusions are potentially dangerous when present in toughened glass. This is because there is a delayed phase transformation in nickel sulphide. Nickel sulphide crystals have a high temperature and a low temperature form. The dense crystal form at high temperature swells on cooling to make a less dense crystal form at low temperatures.
In ordinary annealed glass nickel sulphide inclusions do not cause problems because the transformation occurs as the glass is cooled slowly during manufacture. However, the transformation is sluggish and when glass is rapidly cooled as part of the toughening process, the nickel sulphide remains trapped in its high-temperature form until some years later when its transformation breaks the glass.
How Do They Occur?
Flat glass is toughened in an oven that resembles a giant toaster. The glass is transported on rollers and is rolled back and forth inside the oven and heated to a temperature of between 600 and 700 degrees centigrade until it becomes soft. The glass in its softened state is rolled out of the oven into an air shower where both sides of the glass are cooled rapidly. Glass has low thermal conductivity so that the inside of the glass remains hot and soft while the two outer surfaces of the glass cool, solidify and then contract due to thermal contraction (thermal expansion in reverse).
After this, the inside gradually cools, solidifies and contracts. Because the outer surfaces are already cold when the inner region begins to solidify, contraction in the inner region squeezes the outer surfaces. In the finished glass, the regions near the outer surfaces experience high compressive forces which are balanced by high tensile forces generated at the inner region. The toughening process produces a safety glass which is also very strong.
Nickel sulphide is in its high temperature form at above 380 degrees centigrade and should revert to the low temperature form during cooling to room temperature, but in toughened glass does not do so because the transformation is sluggish and because of the rapid cooling rate required by the toughening process. Research shows that the high to low-temperature transformation results in a 4% expansion of nickel sulphide, so that larger inclusions can generate potentially dangerous cracks in the surrounding glass. On the positive side, external and internal stresses impart valuable properties to toughened glass. The compressive stress at the surfaces closes up surface cracks and increases the glass strength by a factor of 3 to 5 times. The internal tensile stress ensures that, if the glass should be broken, then the stress release causes the glass to shatter safely in small dice.
What Will Happen To The Glass If Left?
If a nickel sulphide inclusion is present in the tensile zone there is a downside and the internal tensile stress becomes an “Achilles heel” for toughened glass. The problem is that the swelling of the nickel sulphide inclusions generates cracks in the glass and any small crack in the tensile zone will cause catastrophic failure. The sluggish property of the transformation results in a delay between toughening (which generates the unstable inclusion) and glass failure. The rates of failure are hard to predict since both rate of failure and length of time delay vary from location to location. In some installations all failures occur within 5 years, but in many cases failures continue for 10 years or more after installation.
Toughened glass affords protection in the case of impact by breaking into small dice, but the small dice might still cause injury in the event of spontaneous glass fracture or an explosion.
Protect Windows With Anti Shatter Film
Protecting your windows from potential glass explosions has never been easier with the application of 175 Micron Anti Shatter Film or Ultra S600 Anti Shatter Film both certified to EN12600 1B1 can be applied to hold the glass in place should a spontaneous glass fracture occur. For more information, please get in touch.