Analysis of typical failure modes of insulating glass in building curtain walls.
1. The preface
The development of high-efficiency and energy-saving glass is one of the important ways to reduce building energy consumption. Insulating glass is a glass product made of two or more pieces of glass, separated by a spacer frame with desiccant in the middle, and sealed around the perimeter. It has good thermal insulation properties and has been widely used in building doors, windows, and curtain walls. At present, the most widely used structural glass in new buildings around the world is insulating glass. Insulating glass is also known as "today's glass" with a sense of the times. The production technology and application technology of insulating glass in some developing countries are quite mature and are widely used, bringing significant energy-saving effects to commercial buildings and home buildings.
The edges of insulating glass are made of organic materials (structural sealant, polysulfide sealant, etc.), which have shortcomings such as easy aging, relatively poor weather resistance, air tightness, and water tightness. The energy-saving effect of insulating glass is easy to decline or even fail over time, and its durability is difficult to achieve. After the insulating glass structural adhesive is used for a certain period, its physical aging will also weaken the strength of the bonding interface, causing interface cracking and air leakage under environmental cyclic loads. Due to the huge amount of insulating glass used worldwide and the factors of insulating glass itself, the quality and safety issues of insulating glass in some developing countries are relatively prominent. This article systematically summarizes the various failure modes of insulated glass on building curtain walls and details the reasons for their failure. The analysis of this article is expected to guide technicians to design and use curtain wall insulating glass more rationally, reduce the occurrence of safety and quality accidents, and thereby improve the safety, reliability, and durability of insulating glass.
2. Typical failure modes of insulating glass
2.1 Dew point, condensation, and water accumulation of insulating glass
Dew point and condensation in insulating glass are the most common failure modes. In the production process of insulating glass, if a sealant with a large gas permeability coefficient is selected, it will easily lead to poor sealing of the insulating glass, or the sealing components of the insulating glass may be debonded or broken, or the sealant may fail or age. The watertightness of the sealing components is insufficient; for some oversized insulating glass, the sealing components at the edge of the insulating glass are dislocated and deformed due to temperature differences or during installation, resulting in sealing failure. The above-mentioned factors open the way for water vapor to enter the insulating glass, causing the dew point of the insulating glass and even water accumulation in the cavity (as shown in Figure 1), which directly causes the insulating glass to fail. When the insulating glass is coated glass, due to the action of water vapor, the rainbow phenomenon will appear in the coated insulating glass cavity or the film layer will be corroded, oxidized, etc.
（a）The condensation on external surface
（b）Dew point and water accumulation in insulating glass cavity
Figure 1 Dew point, condensation, and water accumulation caused by insulating glass seal failure
2.2 Insulated glass sealant flows and leaks oil
Silicone sealants are produced with hydrogen-terminated polysiloxane as the base material and dimethyl silicone oil as the plasticizer. The product quality is stable. As market competition becomes increasingly fierce, some companies incorporate low-boiling point substances, such as white oil, into silicone sealant products to replace dimethyl silicone oil to reduce costs, greatly reducing the durability of the product. White oil is a colorless, odorless white oily long-chain alkane produced by high-pressure hydrogenation and refining of petroleum lubricating oil. It is often used as a textile lubricant and coolant. When used in trace amounts, it can improve the surface gloss of rubber products. The basic composition of white oil is a saturated sintered structure. It has a small molecular weight, low boiling point, and is easy to volatilize. Especially when the ambient temperature is high, the white oil will volatilize and ooze out, and the silicone glue will gradually harden over time. , shrink, or even crack, resulting in bond failure (see Figure 2). The main component of butyl glue, the first sealant used in insulating glass, is polyisobutylene. Its molecular chain is dominated by C-C bonds, which are similar to white oil, and the polarity between the two is similar. According to the principle of similar miscibility, when butyl sealant encounters white oil, it will be swollen and dissolved, resulting in the flow of insulating glass sealant (see Figure 3). Once the insulating glass sealant flows and leaks oil, the insulating glass seal will also fail.
Figure 2 The secondary sealant for insulating glass hardens, shrinks, and cracks(The card can be inserted into the gap formed by the debonding)
Figure 3 The state of oil leakage inside the insulating glass sealant
2.3 The outer piece of insulating glass falls off
There are mainly four factors that cause the outer piece of insulating glass to fall off:
(1) The bonding strength between the insulating glass sealant and the glass does not meet the relevant requirements.
The stability of the insulating glass system is achieved by the insulating glass sealant. At present, the sealing structure of insulating glass used on curtain walls is mainly a double sealing structure, which consists of two sealants. Butyl hot melt adhesive is usually used as the first seal, coupled with structural sealant, such as polysulfide sealant or silicone sealant is used as the second seal. Glass processing companies generally use a rotating sealant coating table or a fully automatic butyl extruder coating machine for the second seal. For some glass curtain walls that adopt a hidden frame structure, during the use of insulating glass, since the weight of the outer piece of insulating glass is completely borne by the second sealant, once the second sealant has insufficient bonding performance or causes bonding due to aging, etc.
Performance degradation and other phenomena can cause the outer piece to fall off as a whole, causing serious safety hazards. In addition, some insulating glass produced and processed in developing countries uses polysulfide sealant as the secondary sealant. Due to the poor UV resistance of polysulfide sealant, the sharp decline in bonding strength with the glass after aging, and its insufficient strength, when this type of glass is used in hidden frame glass curtain walls, it can easily cause the outer piece of insulating glass to fall. The international industry standards "Technical Specifications for Glass Curtain Wall Engineering" and "Silicone Structural Sealant for Insulated Glass" both put forward requirements for the compatibility of the secondary sealant of insulated glass and the materials in contact with it.
Before use, silicone structural sealants should be tested for compatibility with glass, metal frames, spacers, positioning blocks, and other sealants. They can only be used after passing the compatibility test. If the silicone structural sealant is incompatible with the materials in contact with it, the bonding strength of the secondary sealant will be reduced or completely lost, and it will not be able to withstand the wind load of the outer glass and the weight of the glass, resulting in the appearance of the insulating glass.
(2) The width of the second sealant injection for insulating glass does not meet the requirements.
The international industry standard "Insulated Glass" stipulates that the width of double-pass sealing outer sealant injection is 5~7mm, and products with special specifications or special requirements shall be agreed upon by both parties. The international industry standard "Technical Specifications for Glass Curtain Wall Engineering" stipulates that silicone structural sealants should be subjected to load-bearing limit state calculations based on different stress conditions. The bonding width and bonding thickness should be determined through calculation respectively, the bonding width should not be less than 7 mm, and the bonding thickness should not be less than 6 mm. The international industry standard "Quality Inspection Standard for Glass Curtain Wall Engineering" stipulates that the width of the secondary silicone structural sealant layer of insulating glass should meet the structural calculation requirements. The international industry standard "Technical Specifications for Glass Curtain Wall Engineering" is the basic basis for the design and calculation of glass curtain walls. It stipulates that the width and thickness of the load-bearing silicone structural sealant in hidden frame and semi-hidden frame glass curtain walls should be determined through calculation. and stipulates minimum width and thickness. For insulated glass used on hidden frame or semi-hidden frame glass curtain walls, the second layer of silicone structural sealant bears wind load, earthquake load, and self-weight load. The injection width and thickness should also be calculated based on the load of the insulating glass. The international industry standard "Insulating Glass Production Regulations" also stipulates that the injection width and thickness of silicone structural adhesive should meet the design requirements, the width will be less than 7 mm, and the thickness shall not be less than 6 mm. The injection width of insulating glass The thickness requirements are consistent with the "Technical Specifications for Glass Curtain Wall Engineering".
(3) There is air leakage in the insulating glass sealing component (the gas in the insulated layer is connected to the outside atmosphere).
When air leakage occurs in the insulating glass sealing component (such as secondary sealant debonding, breakage, etc.), at this time, in addition to the failure of the heat insulation function of the insulating glass, the load-bearing performance of the insulating glass is also changed.
Figure 4 is a schematic diagram of the load-bearing between the insulated layer of insulating glass and the outside atmosphere in the case of sealing and air leakage.
In the sealed state, external load (wind load) fatigue failure. In particular, some curtain wall opening fans use insulating glass assembled with hidden frames. Since the opening fans are subject to constant artificial opening and closing, the second sealant bears the dynamic load of the glass gravity, which can easily cause and accumulate dynamic fatigue damage, accelerating the process. The debonding failure of the second sealant caused the entire outer piece of the insulating glass of the opening fan to fall off.
2.4 Environmental temperature difference and pressure difference cause breakage and deformation of insulating glass
There is often a certain difference between the ambient temperature and air pressure corresponding to the insulated glass during production and service. This will cause the gas sealed in the hollow layer to expand or contract. When the insulating glass is in service, the ambient temperature is higher than the temperature during production, or the atmospheric pressure is lower than the atmospheric pressure during production, the insulating glass will expand outward, otherwise it will be concave. When the recess is severe, the inner and outer pieces of the insulating glass may stick to each other, as shown in Figure 5.
Figure 5 The temperature difference causes the inner and outer sheets of the insulating glass to come into contact.
When the ambient temperature or pressure difference changes sufficiently, it will have a very negative impact on the insulating glass, and even directly cause the insulating glass to break and fail. The author once worked on a curtain wall project where the insulating glass cracked due to air pressure differences. The insulating glass showed signs of expansion after being transported from the glass processing factory to the project implementation site. Insulating glass made of untempered glass had A large number of cracks that occurred during transportation and continued to crack after being installed on the curtain wall. The probability of damage reached more than 30%, and even some tempered insulating glass also cracked. Therefore, the problem of insulating glass rupture caused by environmental pressure differences requires sufficient attention, and sometimes corresponding measures must be taken, such as using a pressure balancing device inside the insulating glass.
2.5 Temperature changes cause deformation and failure of insulating glass sealing components
During the service of insulated glass, due to the cyclic effects of indoor and outdoor temperature differences, environmental temperature changes, and other factors, the sealing unit of the insulated glass product will undergo certain expansion and dislocation deformation. Once the edge sealing component deforms too much, it will cause the first sealant to be extruded, resulting in insufficient width and thickness of the sealant, or even degumming and breakage, accelerating the airtight failure of the insulating glass, causing dew point and condensation of the insulating glass, and affecting the service life of the insulating glass. During the inspection of the existing glass curtain wall, the author found that it is very common for the first sealant (butyl glue) of the insulating glass to be squeezed out, causing the glue to break and the aluminum frame strips to be exposed (see Figure 6). This phenomenon is not caused by the quality of the insulating glass itself, but by changes in ambient temperature.
Butyl extruder is squeezed out to reveal the aluminum frame strips
Figure 6 The phenomenon of butyl extruder being squeezed out of insulating glass
In addition, as the application of insulated glass continues to become larger and larger, manufacturers and designers are also very concerned about the reliability of edge-sealing components. Because the size of insulated glass increases, the ambient temperature will cause greater edge dislocation and deformation, thus causing greater edge dislocation and deformation. It is more likely to cause the insulating glass sealing component to fail.
2.6 Tempered glass explodes
The self-explosion of tempered glass is common during the service of insulating glass. Especially when the outer piece self-explodes, the tempered glass fragments fall from high altitudes, which can easily cause safety accidents. The fundamental factor causing the self-explosion of tempered glass is the impurities inside the glass (mainly NiS impurities). Since the self-explosion of tempered glass is difficult to predict and control, it is considered a "cancer of glass." After the self-explosion of tempered glass, the obvious self-explosion source can be seen in the damage morphology, which is in the shape of a "butterfly spot", as shown in Figure 7. Near the "butterfly spot", through a magnifying glass, a heterogeneous particle can often be seen, as shown in the figure. As shown in 8. The self-explosion rate of tempered glass from various manufacturers is not consistent, ranging from 3% to 0.3%.
Generally speaking, the self-explosion rate is calculated based on the number of pieces, without considering the area size and thickness of the single piece of glass, so it is not accurate enough and cannot make a more scientific comparison. To uniformly calculate the self-destruction rate, unified assumptions must be determined. Unified conditions are stipulated: every 5 to 8 tons of glass contains one nickel sulfide sufficient 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 it is calculated that the self-explosion rate of 6mm thick tempered glass is about 3‰~5‰. This is consistent with the actual detection values of high-level glass deep processing companies around the world.
Figure 7 Self-explosion rupture morphology of tempered glass caused by heterogeneous particles
(An obvious impurity can be seen near the self-explosion point, similar to the eyeballs in a cat’s eye, and the overall appearance is like a butterfly shape)
Figure 8 Magnified view of heterogeneous particles from the self-explosion source of tempered glass
The self-explosion of tempered glass is caused by local stress concentration in the tensile stress layer, and the stress concentration is caused by pressure or microcrack expansion at the interface between impurity particles and glass. The particle interface pressure can be caused by a variety of factors, such as nickel sulfide. It is caused by particle phase change or thermal deformation of various other impurity particles during temperature changes (only impurity particles whose expansion coefficient is the same as that of glass do not generate interface pressure). Therefore, there is only one direct reason for the self-explosion of tempered glass, which is local stress concentration, and there are many indirect reasons. The degree of stress concentration is affected by many factors, and the defects or impurities that cause this stress concentration are also diverse.
Detecting or predicting each defect or impurity is much more difficult and troublesome than detecting stress concentration. Therefore, we only need to find ways to detect stress concentration points to find the source of self-explosion risk. For glass, which is a transparent material, stress concentrations with large stress gradient changes can easily be determined using the photoelastic method. As long as there is stress concentration inside the tempered glass, it can be detected by the photoelastic meter. A large amount of testing experience has shown that by irradiating glass with a photoelastic instrument and viewing it with the naked eye, stress spots caused by heterogeneous particles above 0.1mm inside the tempered glass can be found. Using ordinary industrial cameras for inspection, the stress spots caused by heterogeneous particles above 0.3mm inside the tempered glass can be identified. The morphology of the heterogeneous particles inside the tempered glass and the stress spots nearby are shown in Figure 9. Therefore, the photoelastic method is used to detect the self-explosion source of tempered glass and evaluate its self-explosion risk, which has good accuracy and operability for heterogeneous particles inside tempered glass.
(b) Stress spot near heterogeneous particles
Figure 9 Tempered glass self-explosion source and its stress photoelastic spots
3. The summary
The safe and reliable service of insulating glass is directly related to the performance of building curtain walls. Insulated glass is subject to temperature during service, wind load, vibration load, rain, and direct sunlight will cause various failure modes. LIJIANG Glass comprehensively analyzes typical failure modes and causes of insulating glass sealing failure, outer sheet falling off, and tempered glass self-explosion, to help relevant engineering and technical personnel design and use curtain wall insulating glass more rationally and reduce the occurrence of safety and quality accidents, providing a certain reference basis for improving service life.
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