Analysis of typical failure modes of building curtain wall insulating glass.
1. Preface
The development of high-efficiency energy-saving glass is one of the important ways to reduce building energy consumption. Insulating glass is a glass product composed of two or more pieces of glass, separated by a spacer frame with a desiccant in the middle and sealed around the periphery. It has good thermal insulation performance 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. The production technology and application technology of insulating glass in some developing countries are quite mature and widely used in practice, which has brought significant energy-saving effects to commercial buildings and residential buildings.
The use of organic materials (structural glue, polysulfide glue, etc.) on the edge of insulating glass has defects such as easy aging, weather resistance, air tightness, and relatively poor water tightness. The energy-saving effect of insulating glass is easy to decline or even fail over time, and its durability is difficult to Match the life of the building. After the insulating glass structural adhesive is used for a certain period, its physical aging will also weaken the bonding interface strength, causing interface cracking and air leakage under the action of environmental cyclic loads. Due to the huge usage of insulating glass worldwide and the factors of the insulating glass itself, the quality and safety problems of insulating glass in some developing countries are relatively prominent. This article systematically summarizes various failure modes of insulating glass on the building curtain wall and details the failure reasons. Through the analysis of this article, it is expected to guide technicians to design and use curtain wall insulating glass more reasonably, reduce the occurrence of safety and quality accidents, and improve the safety, reliability, and durability of insulating glass.
2. Typical failure modes of insulating glass
2.1 Insulating glass dew point, condensation, water accumulation
Insulating glass dew point and condensation 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 is easy to cause poor sealing of the insulating glass, degumming and glue breakage of the insulating glass sealing member, sealant failure, and aging. Insufficient water tightness of the sealing member; some super-large insulating glass, due to temperature difference or installation, the sealing member of the insulating glass edge is dislocated and deformed too much, resulting in sealing failure. The above factors open the channel for the water vapor of the insulating glass to enter, resulting in the dew point of the insulating glass, and even the formation of water accumulation in the cavity (as shown in Figure 1), which directly causes the insulation function of the insulating glass to fail. When coated glass is used for insulating glass, due to the action of water vapor, a rainbow phenomenon will appear in the coated insulating glass cavity, or the film layer will be corroded and oxidized.
Figure 1 Dew point, condensation, and water accumulation caused by the failure of the insulating glass seal
2.2 The insulating glass sealant is flowing and seeping oil
The production of silicone sealant is based on hydrogen-terminated polysiloxane and dimethyl silicone oil as a plasticizer, and the product quality is stable. With the increasingly fierce market competition, some companies add low-boiling substances, such as white oil, to the silicone sealant products to reduce the cost to replace dimethyl silicone oil, which greatly reduces the durability of the product. White oil is a colorless, odorless, white oily long-chain alkane obtained by high-pressure hydro refining of petroleum lubricating oil reserves. It is often used in textile lubricants and coolants. A small amount of oil can improve the surface gloss of rubber products. The basic composition of white oil is a saturated sintered structure, with small molecular weight, low boiling point, and volatility. Especially in the case of high ambient temperature, white oil will volatilize and exude, and the silicone glue will gradually harden over time. , shrinkage, and even cracking, resulting in bond failure (see Figure 2). The first sealant used for insulating glass - the main component of butyl rubber is polyisobutylene, and its molecular chain is dominated by C-C bonds, similar to white oil, and the polarities between the two are similar. According to the principle of similar compatibility, when the butyl sealant encounters white oil, it will be swelled and dissolved by it, resulting in the flow phenomenon of the insulating glass sealant (as shown in Figure 3). Once the insulating glass sealant flows and seeps oil, the insulating glass seal will also be declared invalid.
Figure 2 Hardening, shrinking, and cracking of insulating glass secondary sealant(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 sheet of insulating glass falls off
Four main factors cause the outer sheet of insulating glass to fall off:
(1) The bonding strength of insulating glass sealant and glass cannot meet the relevant requirements.
The stability of the insulating glass system is achieved by the insulating glass sealant. At present, the sealing structure of the insulating glass applied to the curtain wall is mainly a double sealing structure, which consists of two sealants, usually butyl hot melt adhesive is used as the first seal, and it is matched with a structural glue such as Polysulfide glue or silicone glue is used as the second seal, and glass deep processing enterprises generally use a rotary glue coating table or an automatic butyl coating machine sealing robot for the second seal. For some glass curtain walls in the form of the hidden frame structure, during the use of insulating glass, since the weight of the outer sheet of insulating glass is completely borne by the secondary sealant, once the secondary sealant has insufficient bonding performance, or due to aging, etc. Performance degradation and other phenomena can cause the outer sheet 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 the polysulfide sealant, the sharp decline in the bonding strength with the glass after aging, and the lack of its strength, when the polysulfide sealant has poor UV resistance, When this type of glass is applied to the hidden frame glass curtain wall, it is very easy to cause the falling accident of the insulating glass outer sheet. International industry standards "Technical Specifications for Glass Curtain Wall Engineering" and "Silicone Structural Sealant for Insulating Glass" all put forward requirements on the compatibility of the secondary sealant for insulating glass and the materials in contact with it.
Before using the silicone structural sealant, the compatibility test with glass, metal frame, spacer, positioning block, and other sealants should be carried out, and the compatibility test can be used only after passing the test. If the silicone structural sealant is incompatible with the materials in contact with it, the bonding strength of the secondary sealant will decrease or be completely lost, and it will not be able to withstand the wind load on the outer glass, and the self-weight of the glass, resulting in the outer surface of the insulating glass. sheet detaches.
(2) The injection width of the insulating glass secondary sealant does not meet the requirements.
The international industry standard "Insulating Glass" stipulates that the glue injection width of the double-sealed outer sealant is 5~7mm, and the products with special specifications or special requirements shall be negotiated by both parties. In the international industry standard "Technical Specifications for Glass Curtain Wall Engineering", it is stipulated that the limit state of the bearing capacity of the silicone structural sealant should be checked according to different stress conditions, and the bonding width and bonding thickness should be determined by calculation respectively, and the structural adhesive 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 insulating glass secondary silicone structural sealant layer 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 silicone structural sealant under load in the hidden frame and semi-hidden frame glass curtain walls should be determined by calculation. And specifies the minimum width and thickness. Applied to the insulating glass on the hidden frame and semi-hidden frame glass curtain wall, the second layer of silicone structural sealant bears wind load, earthquake load, and self-weight load. Similarly, the glue injection width and thickness should be calculated according to the load on the insulating glass. . The international industry standard "Insulating Glass Production Regulations" also stipulates that the injection width and thickness of the silicone structural adhesive should meet the design requirements, and the width should be less than 7 mm and the thickness should not be less than 6 mm. The requirements for thickness are consistent with the "Technical Specifications for Glass Curtain Wall Engineering".
(3) The insulating glass sealing member has permeability leakage (the gas in the hollow layer communicates with the outside atmosphere).
When there is a permeable gas leakage in the insulating glass sealing member (such as degumming of the secondary sealant, glue breaking, etc.), at this time, in addition to the failure of the heat insulation function, the insulating glass also changes the bearing performance of the insulating glass. FIG. 4 is a schematic diagram of the bearing between the hollow layer of the insulating glass and the outside atmosphere under the condition of sealing and permeability leakage. In the sealed state, the external load (wind load) fatigue fails.
Figure 4 Schematic diagram of load-bearing performance of insulating glass in sealed and leaked states
(4) The secondary sealant bears lasting and vibration loads.
When assembling the insulating glass applied to the hidden frame curtain wall, if there is no support block under the hidden frame glass, the secondary sealant has been subjected to the permanent self-weight load of the glass, resulting in creep and fatigue failure. In particular, some curtain wall opening fans use hollow glass assembled with hidden frames. Since the opening fans are constantly artificially started and closed, the secondary sealant is subjected to the dynamic load of the glass gravity, which is extremely prone to dynamic fatigue damage and accumulation, which accelerates The debonding aging of the second sealant and causes the outer sheet of the insulating glass of the opening fan to fall off as a whole.
2.4 The rupture and deformation of insulating glass caused by ambient temperature difference and pressure difference
There is often a certain difference between the ambient temperature and air pressure corresponding to the insulating glass during production and service, which will cause the gas sealed in the hollow layer to expand or shrink, and the ambient temperature during service is higher than the temperature during production, or if the atmospheric pressure is lower than the atmospheric pressure at the time of production, the insulating glass will expand outward, and vice versa. When the inner concave is serious, the inner and outer sheets of the insulating glass can be attached, as shown in Figure 5.
Figure 5. The temperature difference causes touch between the inner and outer sheets of the insulating glass.
When the ambient temperature or pressure difference changes enough, it will bring very adverse effects on the insulating glass, and even directly cause the insulating glass to break and fail. The author has been exposed to a curtain wall project where the insulating glass is broken due to air pressure differences. After the insulating glass is transported to the project implementation site from the glass deep-processing factory, there is an expansion phenomenon. The insulating glass made of untempered glass is in A large number of cracks occurred during transportation and continued to burst after being installed on the curtain wall. The probability of damage is more than 30%, and even some tempered insulating glass is also cracked. Therefore, enough attention should be paid to the rupture of insulating glass caused by environmental pressure differences, and sometimes corresponding measures must be taken, such as the use of a pressure balance device inside the insulating glass.
2.5 Deformation and failure of insulating glass sealing components caused by temperature changes
During the service process of insulating glass, due to the cyclic action of factors such as indoor and outdoor temperature differences, environmental temperature change, etc., the sealing unit of the insulating glass product will have a certain expansion and dislocation deformation. Once the deformation of the edge sealing member is too large, it will cause the first sealant to be extruded, resulting in insufficient width and thickness of a sealant, even degumming, and glue breakage, accelerating the airtight failure of the insulating glass, causing the 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 the first sealant (butyl glue) of the insulating glass was extruded, causing its glue breakage and the exposure of the aluminum frame strip to be very common (as shown in Figure 6). The situation is not caused by the quality problem of the insulating glass itself but caused by the change in ambient temperature.
Butyl glue is extruded to expose aluminum frame strips
Figure 6 The phenomenon of extrusion of insulating glass butyl rubber
In addition, as the application of insulating glass continues to increase in size, manufacturers and designers are also very concerned about the reliability of edge sealing components. Because of the large size of insulating glass, the effect of ambient temperature will make the edge dislocation and deformation greater, so It is more likely to cause the failure of the insulating glass sealing member.
2.6 Self-explosion of tempered glass
The self-explosion of tempered glass generally exists in the service process of insulating glass, especially when the outer sheet is self-exploded, the tempered glass fragments fall from a high altitude, which can easily lead to safety accidents. The fundamental factor that causes the self-explosion of tempered glass is the impurities inside the glass (mainly NiS impurities). Because the self-explosion of tempered glass is difficult to predict and control, it is considered to be "the cancer of glass". After the self-explosion of the tempered glass, an obvious source of self-explosion can be seen, and it 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 Figure 7 8 shown. The self-explosion rate of tempered glass of various manufacturers is not consistent, ranging from 3% to 0.3%. Generally speaking, the self-explosion rate is calculated in units 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. To measure the self-destruction rate uniformly, a uniform assumption must be determined. The uniform conditions are stipulated: 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 it is calculated that the calculated self-explosion rate of 6mm thick tempered glass is about 3‰ to 5‰. This is consistent with the actual detection value of high-level glass deep-processing enterprises around the world.
Figure 7 Self-explosion fracture morphology of tempered glass caused by heterogeneous particles
(An obvious impurity can be seen near the self-explosion point, similar to the eyeball in a cat's eye, and the overall appearance is like a butterfly)
Figure 8 Enlarged view of tempered glass self-explosion source heterogeneous particles
The self-explosion of tempered glass is caused by the local stress concentration in the tensile stress layer, and the stress concentration is caused by the pressure or the expansion of micro-cracks at the interface between the impurity particles and the glass. The particle interface pressure can be caused by various factors, such as nickel sulfide. It is caused by the particle phase transition or the thermal deformation of various other impurity particles in the process of temperature change (only the impurity particles whose expansion coefficient is the same as that of the glass do not generate interfacial pressure). Therefore, there is only one direct reason for the self-explosion of tempered glass, that is, local stress concentration, and there are various indirect reasons. The degree of stress concentration is affected by a variety of factors, and the defects or impurities that cause this stress concentration are also diverse. It is much more difficult and troublesome to detect or predict each defect or impurity than to detect the stress concentration. Therefore, we only need to find a way to detect the stress concentration point, and then we can find the source of the risk of self-explosion. For transparent glass, stress concentrations with large stress gradients are easily determined using photoelastic methods. As long as there is a stress concentration inside the tempered glass, it can be detected by a photoelastic analyzer. A large amount of testing experience has shown that: when the glass is irradiated with a photoelastic instrument and viewed with the naked eye, the stress spot caused by heterogeneous particles above 0.1 mm in the tempered glass can be found. Using ordinary industrial camera detection, it can identify the stress spot caused by heterogeneous particles larger than 0.3mm inside the tempered glass. Figure 9 shows the morphology of the heterogeneous particles inside the tempered glass and the stress spot morphology in its vicinity. 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 of heterogeneous particles inside the tempered glass.
(b) Stress spot near the heterogeneous particle
Figure 9 Tempered glass self-explosion source and its stress photoelastic spot
3. Summary
The safe and reliable service of insulating glass is directly related to the performance of the building curtain wall. During the service process of insulating glass, it is subject to temperature Various failure modes can occur due to the effects of temperature, wind load, vibration load, rain, and direct sunlight. Jinan LIJIANG GLass comprehensively analyzes the typical failure modes and failure causes of insulating glass sealing failure, the outer sheet falling off, tempered glass self-explosion, etc., to design and use curtain wall insulating glass more reasonably for related engineers and technicians, and reduce the risk of safety and quality accidents. occur, and provide a certain reference basis for improving the service life.
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