The solutions for self explosion of tempered glass

as we know, tempered glass has 0.5% self explosion rate, so how to reduce the self explosion rate is a important thing for all the glass processing factory.  

1. Reduce the stress value of tempered glass

The distribution of stress in tempered glass is that the two surfaces of tempered glass are under compressive stress, while the core layer is under tensile stress. The stress distribution in the glass thickness is similar to a parabolic curve. The center of the glass thickness is the vertex of the parabola, which is the point where the tensile stress is maximum; There is compressive stress near the two surfaces of the glass on both sides; The zero stress surface is approximately one-third of the thickness. By analyzing the physical process of tempered quenching, it is known that there is a rough proportional relationship between the surface tension of tempered glass and the maximum internal tensile stress in numerical values, that is, the tensile stress is 1/2 to 1/2 of the compressive stress. Domestic manufacturers generally set the surface tension of tempered glass at around 100MPa, but the actual situation may be higher. The tensile stress of tempered glass itself is about 32MPa~46MPa, and the tensile strength of glass is 59MPa~62MPa. As long as the tension generated by the expansion of nickel sulfide is within 30MPa, it is sufficient to cause self explosion. If the surface stress is reduced, it will correspondingly reduce the inherent tensile stress of tempered glass itself, thereby helping to reduce the occurrence of self explosion.

The surface stress range of tempered glass specified in the American standard ASTMC1048 is greater than 69MPa; The semi tempered (heat strengthened) glass is between 24MPa and 52MPa. The standard BG17841 for curtain wall glass specifies a semi tempered stress range of 24< δ≤ The new national standard GB15763.2-2005 "Safety Glass for Building Purposes - Part 2: Tempered Glass" implemented on March 1st this year in China requires that the surface stress should not be less than 90MPa. This is a decrease of 5MPa from the previous old standard of 95MPa, which is beneficial for reducing self explosion.

2. Make the stress on the glass uniform and consistent

The uneven stress of tempered glass can significantly increase the self explosion rate, which cannot be ignored. The self explosion caused by uneven stress can sometimes be very concentrated, especially in a specific batch of bent tempered glass, where the self explosion rate can reach an alarming level of severity and may occur continuously. The main reason is the uneven local stress and the deviation of the tension layer in the thickness direction, which also has a certain impact on the quality of the original glass sheet itself. Uneven stress significantly reduces the strength of glass, which to some extent increases the internal tensile stress, thereby increasing the self explosion rate. If the stress of tempered glass can be uniformly distributed, it can effectively reduce the self explosion rate.

3. Hot Dip Treatment (HST)

Hot dip interpretation. Hot dip treatment, also known as homogenization treatment, is commonly known as "detonation". Hot dip treatment is the process of heating tempered glass to 290 ℃± 10 ℃ and holding it for a certain period of time to promote the rapid crystal phase transformation of nickel sulfide in tempered glass. This allows tempered glass, which was originally intended to self explode after use, to be artificially broken in advance in the factory's hot dip furnace, thereby reducing the self explosion of tempered glass during installation and use. This method generally uses hot air as the heating medium, and is referred to as "HeatSoakTest" or HST in foreign countries, which is literally translated as hot dip treatment.

Difficulties in hot dipping. From a theoretical perspective, hot dip treatment is neither complex nor difficult. But in reality, achieving this process indicator is very difficult. Research has shown that there are various specific chemical structural formulas for nickel sulfide in glass, such as Ni7S6, NiS, NiS1.01, etc. Not only do the proportions of various components vary, but other elements may also be doped. The speed of its phase transition highly depends on the temperature. Research has shown that the phase transition rate at 280 ℃ is 100 times that at 250 ℃, so it is necessary to ensure that each piece of glass in the furnace undergoes the same temperature regime. Otherwise, on the one hand, low temperature glass cannot completely phase change due to insufficient insulation time, which weakens the effectiveness of hot dip. On the other hand, when the glass temperature is too high, it can even cause the reverse phase transformation of nickel sulfide, causing greater hidden dangers. Both of these situations can lead to ineffective or even counterproductive hot dip treatment. The uniformity of temperature during the operation of hot dip furnaces is so important, and three years ago, the temperature difference inside most domestic hot dip furnaces even reached 60 ℃ during hot dip insulation. It is not uncommon for imported furnaces to have a temperature difference of around 30 ℃. So even though some tempered glass undergoes hot dip treatment, its self explosion rate remains high.

The new standards will be more effective. In fact, the hot dip process and equipment have also been continuously improving. The German standard DIN18516 specifies an insulation time of 8 hours in the 90 year version, while the prEN14179-1:2001 (E) standard reduces the insulation time to 2 hours. The effect of the hot dip process under the new standard is very significant, and there are clear statistical technical indicators: after hot dip, it can be reduced to one self explosion per 400 tons of glass. On the other hand, the hot dip furnace is constantly improving its design and structure, and the heating uniformity has also been significantly improved, which can basically meet the requirements of the hot dip process. For example, the self explosion rate of the hot dip treated glass of China Southern Glass Group has reached the technical indicators of the new European standard, and it has performed extremely satisfactorily in the 120000 square meter Guangzhou New Airport mega project.

Although hot dip treatment cannot guarantee the absolute absence of self explosion, it does reduce the occurrence of self explosion and effectively solve the problem of self explosion that troubles all parties in the project. So hot dip is the most effective method recognized in the world to completely solve the problem of self explosion.