How many high-temperature tempered glass self-explodes in the northern hemisphere in summer?

--The causes, identification methods and preventive measures of self-explosion of tempered glass 

Every summer in the northern hemisphere, many regions go into "stove mode". Recently, I learned that many cities in the northern hemisphere have frequently reported the occurrence of high-rise building glass curtain wall tempered glass explosions. 

So is it really the glass that was exploded by the high-temperature barbecue as mentioned in the news? 

What are the identification methods of self-explosion of tempered glass of glass curtain wall? 

And in the daily maintenance of high-rise curtain wall buildings, how should we prevent the self-explosion of tempered glass? 

The following Jinan LIJIANG Glass will take you to understand the reasons for the self-explosion of tempered glass, the building material of the glass curtain wall of high-rise buildings.

In fact, since the birth of tempered glass, especially the tempered glass produced by automatic glass tempering furnace, it has been accompanied by the problem of self-explosion. The self-explosion of tempered glass can be described as the phenomenon that the tempered glass is automatically broken without direct external action. Self-explosion of tempered glass can occur during tempering, storage, transportation, installation, and use.

Tempered glass self-explosion rate

At present, the self-explosion rate of tempered glass produced and processed by production plants in various countries is not consistent, ranging from 3% to 0.3%. Generally, the self-explosion rate is calculated by the number of pieces, without considering the area size and thickness of a single piece of glass, so it is not accurate enough and cannot be compared more scientifically. In order to measure the self-destruction rate uniformly, a uniform assumption must be determined. Set uniform conditions: every 5-8 tons of glass contains a nickel sulfide enough to cause self-explosion; the average area of each piece of tempered glass is 1.8 square meters; the nickel sulfide is evenly distributed. Then the calculated self-explosion rate of 6 mm thick tempered glass is 0.64% to 0.54%, that is, the self-explosion rate of 6 mm tempered glass is about 3‰ to 5‰. This is basically consistent with the actual value of tempered glass produced and processed by high-level glass deep-processing enterprises in the international scope.

Even if it is produced according to the standard, the self-explosion of tempered glass cannot be completely avoided. Large buildings can easily use hundreds of tons of glass, which means that there is a high probability of nickel sulfide and heterophase impurities in the glass. Therefore, although the tempered glass is hot-dipped, self-explosion is still inevitable. 

Self-explosion mechanism of tempered glass 

Self-explosion can be divided into two types according to different causes:

One is the self-explosion caused by visible defects in the tempered glass, such as stones, sand, air bubbles, inclusions, notches, scratches, burst edges, etc.;

The second is the self-explosion of tempered glass caused by nickel sulfide (NiS) impurities and heterogeneous particles in the glass. Nickel sulfide NiS is generally crystalline, and has a thermodynamic tendency to transform from α phase to β phase at room temperature, and is accompanied by a volume expansion of 2% to 3%. The diameter of nickel sulfide that causes self-explosion is between 0.04 and 0.65mm, and the average particle size is 0.2 mm. When the temperature change has corresponding kinetic conditions, the excessive expansion of nickel sulfide (NiS) impurities can cause the self-explosion of tempered glass. There is also a new discovery of the self-explosion of tempered glass caused by heterogeneous phase particles. 

^{Figure 1 The self-explosion of tempered glass caused by nickel sulfide (NIS) impurities 1}

This is the self-explosion factor of two different types of tempered glass. Glass deep-processing enterprises and practitioners should clearly classify them, treat them differently, and use different methods to deal with them. The former is generally visible and relatively easy to detect, so it is controllable in production. The latter is mainly caused by the volume expansion of tiny nickel sulfide particles in the glass, which cannot be visually inspected and therefore uncontrollable. In actual operation and handling, the former can generally be removed before installation, while the latter continues to exist because it cannot be inspected, becoming the main factor for the self-explosion of tempered glass in use. Nickel sulfide is difficult to replace after self-explosion, and the processing cost is high. At the same time, it will be accompanied by large quality complaints and economic losses, resulting in dissatisfaction of the owner and even more serious consequences. 

There are some other internal factors in the self-explosion of tempered glass: unreasonable glass slotting and drilling, poor quality of the original glass, uneven thickness such as patterned glass, uneven stress distribution such as curved tempered glass and regional tempered glass. In addition, external factors such as wind load and glass assembly will also change the stress distribution inside the tempered glass and become a possible cause for the self-explosion of the tempered glass. 

^{Figure 2 Some other internal factors in the self-explosion of tempered glass 1}

How to identify the self-explosion of tempered glass

First of all, look at whether the initiation point (the cracks in the tempered glass are radial and have starting points) is in the middle of the glass, such as at the edge of the glass, generally because the glass has not been chamfered or the edge of the glass is damaged, resulting in stress concentration and the gradual development of cracks If the detonation point is in the middle of the glass, see if the detonation point has a pattern similar to two butterfly wings composed of two small polygons (butterfly spots), if so, carefully observe the common edge of the two small polygons (the torso of the butterfly) If there should be small black particles (nickel sulfide stones) visible to the naked eye, it can be judged to be self-explosion; otherwise, it should be destroyed by external force. The typical feature of glass self-explosion is butterfly spots. The glass fragments are distributed radially, and there are two glass blocks shaped like butterfly wings in the center of the radiation, commonly known as "butterfly spots". Nis stones are located on the interface of two "butterfly spots". 

^{Figure 3 The typical characteristics of glass self-explosion--Butterfly spots 1}

How to reduce the self-explosion rate of tempered glass?

According to the analysis of foreign research statistics, more than 95% of the impurity particles that cause the self-explosion of tempered glass are distributed between 0.04mm and 0.65mm. 

Jinan LIJIANG Glass has performed electron microscope analysis on dozens of impurity particles, and the results show that the diameters of these particles are concentrated between 0.10 mm and 0.30 mm, which also verifies the previous statistical results. 

So what are the requirements for impurity defects in relevant international standards? The international standard "Tempered Glass for Building Doors, Windows and Curtain Walls" puts forward the following requirements for the raw materials of tempered glass for doors, windows and curtain walls. "

4.1.1 The appearance quality of the glass used in the production of tempered glass for building doors and windows shall not be lower than the first-class requirements of the international standard "Flat Glass"."

The requirements for point defects of first-class flat glass in the international standard "Flat Glass" are as follows.

^{Table 1 The appearance quality of flat glass first-class product}

That is, the allowable number of point defects with a diameter of 0.3~0.5 mm in 1 square meter is 2. Obviously, nickel sulfide impurities could not be excluded, which determines that the possibility of self-explosion of tempered glass is inevitable. As a glass deep-processing manufacturer, we can only reduce its self-explosion rate by means of methods. 

How to reduce the self-explosion rate of tempered glass? 

The international standard "Tempered Glass for Building Doors, Windows and Curtain Walls" stipulates raw materials, allowable area, edge processing quality, appearance quality, minimum/maximum allowable number of fragments, and the difference between the maximum and minimum surface stress. 

The raw material requirements have already been mentioned, but here is only a brief introduction to the allowable area, edge processing quality, minimum/maximum allowable number of fragments, and the difference between the maximum and minimum surface stress. 

The allowable area of tempered glass is shown in the table below. 

^{Table 2 Tempered glass allowable area}

This table is basically the same as "Table 7.1.1-1 Maximum allowable area of safety glass" in "7 Anti-human Impact Regulations for Architectural Glass" in the international standard "Technical Regulations for Architectural Glass Application"; The "Regulations" table is considered from the perspective of preventing human impact, while the international standard "Tempered Glass for Building Doors, Windows and Curtain Walls" is from the perspective of reducing the self-explosion rate of tempered glass for doors, windows and curtain walls.

The edge processing quality requirements are as follows. 

"Tempered glass for building doors and windows can be chamfered or finely ground on three sides, and the width of the chamfer should not be less than 1 mm; the tempered glass for glass curtain walls and skylights should be ground on three sides or polished on three sides, and the width of the chamfer should not be less than 1 mm. less than 1 mm.” 

The minimum allowable fragmentation requirements are as follows. 

Take 4 glass samples for testing, the minimum number of fragments of each sample in any 50mm*50mm area should meet the requirements of Table 2, and a small number of long-shaped fragments are allowed, the length of which does not exceed 75mm.

^{Table 3 Minimum allowed number of fragments}

The maximum allowable number of shards is required as follows.

 Take 4 glass samples for testing, the maximum number of fragments of each sample in any 50mm*50mm area should not exceed the requirements of the table 4.

^{Table 4 Maximum number of fragments allowed}

The difference between the maximum and minimum surface stress is required as follows. "The surface stress of tempered glass should not be less than 90MPa, and the difference between the maximum surface stress and the minimum surface stress should not exceed 15MPa." These regulations accurately reflect the "methods to avoid self-explosion of tempered glass" proposed in the preface of the international standard "Tempered Glass for Building Doors, Windows and Curtain Walls". "The main methods to avoid self-explosion of tempered glass are: 

1) Select high-quality flat glass; 

2) Moderate tempering; 

3) Ensure that the surface stress of tempered glass is uniform; 

4) Improve the processing quality of the edge of tempered glass; 

5) Limit the size of tempered glass. So how to defend against such self-destruction? 

1. After the glass is tempered, use lamination to process the glass: The PVB interlayer film is sandwiched between two pieces of tempered glass, processed and produced by a laminated tempered glass production line, and processed into tempered laminated glass by high temperature and high pressure. The toughness of the laminated glass PVB film is very good. When the laminated glass is broken by external force, it can absorb a large amount of impact energy and quickly attenuate it, so the laminated glass is difficult to break down and maintains excellent integrity. When a building using laminated tempered glass is subjected to explosions, wind disasters, earthquakes, etc., even if the glass is broken, it can still remain in the door and window frame, protecting people inside and outside the building from the hazard of flying glass fragments, wind and rain and It is also difficult for other foreign objects to cause damage to the interior. 

The downsides are: 

1) The weight and thickness of the glass increase the bearing and load of the building; 

2) It is not conducive to escape and rescue when a fire occurs; 

2. Apply a film to the glass: Laminate high-performance polyester film on glass. Polyester film is commonly known as safety explosion-proof film. 

When the glass is broken due to various reasons, it can stick to the glass fragments to prevent splashing, protect the people inside and outside the building from the hazard of the splashing glass fragments, and it is difficult for wind, rain and other foreign objects to cause damage to the interior. . 

The safety explosion-proof film can be connected with the frame edge system together with the organic glue to form a glass film protection system to prevent falling.

Advantages: easy to process, without changing the glass specification and layout.

Disadvantages: increase the construction cost. 

3. Installation of enclosure design: Set up guardrails in crowded places to prevent losses caused by broken glass. 

4. Use ultra-white tempered glass: Good thermal stability, high strength, low self-explosion rate (close to 0), safe to use, high light transmittance, good permeability, high-end, high technical content, and wide application range. The above is a summary of Jinan LIJIANG Glass's summary of the self-explosion of tempered glass due to high temperature in the northern hemisphere in summer. .

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