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Needless to say, the energy-saving of insulating glass is related to its sealing effect. Whether insulating glass can save energy for a long time depends on the sealing life of the insulating glass. When the insulating glass fails to seal, condensation occurs in the air layer of the insulating glass. 

There are two results: 

(1) The liquid state after condensation of gas-phase water has a larger thermal conductivity than air; 

(2) The gas in the glass-air layer has microcirculation, that is, thermal convection. As a result, the energy-saving effect of insulating glass that fails to seal is inferior to that of sealed insulating glass. If we understand that the main function of insulating glass is not only to save energy but more importantly to ensure the permeability of the building, then the water in the air layer caused by the sealing failure will condense on the surface of the glass, which will affect people's vision.

The main reason for the sealing failure of the insulating glass is that the sealing glue of the insulating glass does not play a role in blocking the interference of external water vapor from entering the air layer of the insulating glass so that the molecular sieve in the spacer loses the water uptake capacity, and the temperature in the air layer of the insulating glass When the dew point temperature is reached, internal condensation will form and condense on the surface between the glasses.

The reasons affecting the sealing life of the insulating glass and the expected sealing life of various sealing structures given by experiments are discussed below.

The "pump" phenomenon of insulating glass

Insulating glass is always affected by natural factors such as temperature difference, air pressure, wind load, ultraviolet light, etc. during its service life, causing a "pump" phenomenon. The continuous thermal expansion and contraction process of the gas in the air layer of the hollow glass creates corresponding stress (tensile and compressive stress) at the sealant at the edge sealing part of the hollow glass, which causes the expansion and compression of the colloid to proceed alternately. Ultraviolet rays have a destructive effect on the long chemical molecular chains of organic sealants. Some sealants have poor resistance to ultraviolet rays, and are aging after ultraviolet radiation, and lose the elasticity and structural strength of the sealants. Therefore, theoretically, the sealed insulating glass will eventually fail to seal. What people can do is to find out the various reasons that affect the life of the seal, prescribe the right medicine, and extend the life of the seal as much as possible.

Reasons that affect the life of the seal

At present, my country's insulating glass has the problem of short sealing life, that is, the sealing failure of the insulating glass prematurely, which means that condensation occurs in the air layer of the insulating glass, and the fish tank phenomenon is serious.

The factors affecting the sealing life of insulating glass mainly include sealing structure and sealing materials. The sealing and structural stability of the insulating glass system is achieved by insulating glass sealant.

(1) Sealed structure. 

Insulating glass is always facing the influence of external water vapor penetration, temperature difference, air pressure, and wind load. Therefore, it is required that the sealant not only has the function of water and air sealing but also must have the function of ensuring the structural stability of the insulating glass system. The ideal situation is to use a sealant to meet the sealing needs of insulating glass. But in reality, no glue can have good water-tightness and structural characteristics at the same time. Therefore, people have to adopt double-channel sealing to meet the sealing life needs of insulating glass. The traditional double-channel seal is structured after sealing. Generally speaking, the first seal mainly uses butyl rubber, which is mainly used to prevent the penetration of water vapor, prevent inert gas and air from entering and exiting the hollow glass, and play an auxiliary positioning role in the production of the hollow glass. The butyl glue forms a physical bond with the glass and the spacer frame. The second seal usually uses structural adhesives including polyurethane, polysulfide adhesives, and silicone adhesives. The main function is to bond the glass and spacers into the hollow glass to prevent the molecular sieve in the hollow glass from escaping to the outside. Both perform their duties and are indispensable. Insulating glass can only guarantee a long sealing life when it is double-sealed and other conditions remain unchanged. In this regard, we can also use the P1 accelerated aging experiment to prove it. P1 experimental conditions include: 600C (1400F) high temperature, continuous water spray (r.h.100%) and 2500W/cm2 ultraviolet radiation. The result of this experiment simulates the one-year life span of insulating glass in nature. It is based on the fact that in the P1 accelerated aging experiment, as the temperature increases by 100F, the chemical reaction of the adhesive doubles and the aging process doubles, see Table 1.

Table 1:

Fahrenheit Temperature ℉Sealing Time
800F1 week
900F2 weeks
1000F4 weeks
1100F8 weeks
1200F16 weeks
1300F32 weeks
1400F64 weeks

P1 experimental results data are shown in Table 2 :

Single-channel seal structureTest the achieved seal life
Butyl rubber/aluminum spacer24 hours
Compound tape (Shiweigao tape) 2 weeks
Polysulfide, polyurethane, or hot melt butyl rubber/aluminum spacer6-8 weeks
Butyl, polysulfide/polyurethane, four-sided gusseted aluminum spacer12-18 weeks
Butyl, silicone rubber, four-sided gusseted aluminum spacer15-20 weeks
Silicone glue, Shiweigao tape 25 weeks+
 Butyl, silicone rubber, continuous elbow aluminum spacer40 weeks+
Super spacer, hot melt butyl rubber
     100 weeks+ 

It can be seen from Table 2 that the sealing life of the insulating glass with the double-channel sealing structure is much longer than that of the single-channel sealing structure. The sealing life of the single-channel sealing structure is between 2-8 years, while the double-channel sealing structure is the shortest 12 years and the longest can reach more than 100 years.

The insulating glass hot melt butyl rubber 1

The insulating glass hot melt butyl rubber 1

The above theoretical statement is not only proved to be correct through experiments but also proved by the North American Insulating Glass Association's statistical survey on the actual use of insulating glass for 20 years. The statistics show that the sealing failure of insulating glass with a single-pass sealing structure accounts for more than 95% of the total number of sealing failures.

To sum up, whether the sealing structure of the insulating glass is reasonable or not directly affects the sealing life of the insulating glass. Should vigorously promote the use of double-sealed insulating glass to improve the sealing life of the insulating glass.

(2) Sealing material  

It can be seen from Table 6 that the sealing material (spacer and sealant) has a significant impact on the sealing life of the insulating glass with a certain sealing structure, and it is worth discussing.

2.1 Aluminum spacer. 

Generally, the sealing method of aluminum spacers is based on the butyl rubber strips on both sides of the aluminum strips for the main sealing function. In the "pump" movement of the inner and outer deflection of the hollow glass, the butyl rubber strip will stretch, shift, shear, etc., which shortens the passage of water vapor penetration. As far as the trough aluminum process is concerned, whether it is a continuous pipe bend or a corner joint key, the sealing of the aluminum strip edge is completed by butyl rubber, but the connection at the back or corner is not completely sealed (it only needs to cause the sealing failure. Insufficient sealing is enough). In addition, the bonding performance of the butyl rubber itself at low temperatures is reduced, and any of the above conditions can lead to weakening or failure of the sealing performance.

2.2 Type of sealant. 

In the double-pass sealing structure, the quality of the butyl rubber itself and the quality of the coating process directly affect the sealing life of the insulating glass. It is generally believed that, compared with structural glue, butyl glue has an 80% influence on the sealing life of the insulating glass. Therefore, the first seal is also called the main seal.

The insulating glass hot melt butyl glue 2

The insulating glass hot melt butyl glue 2

In addition, the second insulating glass sealant, the structural adhesive, also plays a vital role in the sealing effect of the insulating glass. It is generally believed that among polysulfide glue, polyurethane, and silicone glue, polysulfide glue is the most suitable structural glue for hollow glass doors and windows. We think this view is specious. The basis for this view is based on the difference in water vapor permeability (MVTR) of various insulating glass glues. See Table 3.

Type of sealantMVTR
Silicone glue50
Butyl rubber2.25

But people in the insulating glass sealing industry, especially the manufacturers that produce and distribute polysulfide rubber, have intentionally or unintentionally forgotten when quoting the figures. The experimental conditions for measuring water vapor permeability (MVTR) are carried out at room temperature 250C, but the actual situation is hollow. The conditions for the use of glass, especially in the summer in most places in my country, the internal temperature of the hollow glass is close to or higher than 600C. Under the condition of increasing temperature, the water vapor permeability of different glues will change differently. In fact, at 600C, the MVTR of polysulfide rubber is very close to that of silicone rubber. The results of tracking the actual service life of insulating glass for 20 years from North America also confirmed this point. That is, the insulating glass with the / butyl rubber/silicone rubber double sealing structure has a longer sealing life than the insulating glass with other structures. 

In this case, we need to find other methods to replace the MVTR experiment under ideal conditions. Practice shows that the water immersion test with glass glue can more accurately express the sealing performance of the sealant.

The method of the immersion test is briefly described as follows: Put a 500-gram rubber block into the water at a temperature of 600C for 60 days, and then measure its volume and weight to observe the changes. The result list Table 4 is as follows:

Table 4:

Type of sealantVolume change %Weight change%
Silicone glue3-102-6

The above experimental data show that 

(1) the volume and weight of structural adhesives have increased to varying degrees, and the magnitude of the increase is polysulfide adhesive>polyurethane>silicone adhesive; 

(2) the change before and after the water absorption volume and weight and their MVTR (Water vapor permeability) shows different results.

2.3 Molecular Sieve

Generally speaking, the air layer after the hollow glass is made and laminated contains a certain amount of moisture. If it is not dried, internal condensation will form at the dew point temperature, which will not only affect the permeability of the glass but also increase the permeability of the hollow glass. The heat transfer value U value reduces the energy-saving effect of insulating glass. In addition, during the service life of the insulating glass, due to uneven coating or breakpoints of the butyl rubber of the aluminum spacer, the pumping phenomenon of the insulating glass caused by the temperature change, and the water vapor permeability of the sealant, etc., many factors lead to When moisture enters the air layer of the insulating glass, a desiccant is also needed to ensure the low dew point temperature in the insulating glass (such as -400C)

The selection of desiccants for insulating glass can be done with reference to Table 5.
Table 5:

Desiccant typeHole diameter/ÅAdsorbableNon-adsorbable
3A molecular sieve3waterSubstances other than water
4A molecular sieve4Water, air, argon, and kryptonSulfur hexafluoride, xenon, solvent
13X molecular sieve8.5/None
Silica gel20-300/None

It can be seen from Table 5 that if the insulating glass sealant used does not contain solvents, 3A molecular sieve is most suitable for the desiccant of insulating glass. 3A molecular sieve is hydrophilic and only adsorbs water and does not adsorb other substances, so it can be guaranteed under normal conditions The sheets of insulating glass are parallel to each other, reducing the stress of the glass at the edge seal and prolonging the life of the insulating glass; the use of 4A molecular sieve will cause the hollow glass to flex inward, and the central part of the air layer will shrink, which will cause the glass to deform visually. The heat transfer increases, the energy saving effect is reduced, and the sealing life of the insulating glass is reduced. If the insulating glass seal used contains a solvent, a mixture of two molecular sieves should be considered. The usual practice is to use 75% 3A molecular sieves and 25 %13X molecular sieve makes it have the function of adsorbing water and solvent.

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