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1. The sealing life of insulating glass

Needless to say, the energy saving of insulating glass is related to its sealing effect. Whether insulating glass can save energy in the long term depends on the sealing life of insulating glass. When the sealing of insulating glass fails, condensation occurs in the air layer of the insulating glass. There are two results: (1) The liquid state after the condensation of gas-phase water has a greater thermal conductivity than air; (2) The gas in the glass air layer has microcirculation, that is, thermal convection. Therefore, the energy-saving effect of insulating glass with failed sealing is worse than that of sealed insulating glass. Not only that. If we understand that the main function of insulating glass is not only energy saving, but also to ensure the permeability of the building, then the water in the air layer condensed on the surface of the glass due to sealing failure will affect people's vision.

The main reason for the failure of insulating glass sealing is that the sealing sealant of the insulating glass does not play a role in blocking the external interference of water vapor from entering the air layer of the insulating glass, causing the molecular sieve in the spacer to lose its water absorption capacity. When the temperature in the air layer of the insulating glass reaches the dew point temperature, internal condensed water will condense on the surface between the glass.

The following discusses the reasons affecting the sealing life of insulating glass and the expected sealing life of various sealing structures given by experiments.

2. The "pump" phenomenon of insulating glass

During its service life, insulating glass is always affected by natural factors such as temperature difference, air pressure, wind load, and ultraviolet rays, resulting in a "pump" phenomenon. The continuous thermal expansion and contraction of the gas in the air layer of the insulating glass forms stress (tensile and compressive stress) at the sealant of the edge sealing part of the insulating glass, resulting in alternating extension and compression of the colloid. Ultraviolet rays have a destructive effect on the long chain of chemical molecules of organic sealants. Some sealants have poor UV resistance and age under UV irradiation, losing their elasticity and structural strength. Therefore, theoretically, sealed insulating glass will eventually fail to seal. What people can do is find out the various reasons that affect the sealing life, prescribe the right medicine, and extend the sealing life as much as possible.

3. Some reasons affecting the sealing life

At present, insulating glass has the problem of short sealing life, that is, the sealing of insulating glass fails prematurely, which is manifested by condensation in the air layer of the insulating glass, and in severe cases, the dew-point phenomenon occurs.

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 are achieved by insulating glass sealants.

3.1 Sealing structure. 

Insulating glass is always faced with external water vapor penetration, temperature difference, air pressure, and wind load during use. Therefore, the sealant is required to have not only the function of water vapor sealing but also the function of ensuring the structural stability of the insulating glass system. Obviously, the ideal situation is to use a sealant to meet the sealing needs of insulating glass. But in reality, no sealant can have good water and air tightness and structural characteristics at the same time. Therefore, people have to use double-pass sealing to meet the sealing life requirements of insulating glass. Traditional double-pass sealing is sealing first and then structure. Generally speaking, the first seal mainly uses butyl extruder adhesive, which is mainly used to prevent water vapor penetration, prevent inert gas and air from entering and exiting the insulating glass, and play an auxiliary positioning role in the production of insulating glass. Butyl adhesive forms a physical bond with the glass and the spacer frame. The second seal usually uses structural adhesives including polyurethane, polysulfide adhesive, and silicone adhesive. The main function is to bond the glass and the spacer into insulating glass to prevent the molecular sieve in the insulating glass from running to the outside. The two have their own duties and are indispensable. Only in the case of double-pass sealing, when other conditions remain unchanged, can the insulating glass guarantee a longer sealing life. In this regard, we can also use the P1 accelerated aging experiment to prove it. The P1 experimental conditions include 600C (1400F) high temperature, continuous water spraying (r.h.100%), and 2500W/cm2 ultraviolet irradiation. The experimental results simulate the one-year life of insulating glass in nature through each week of life. According to the P1 accelerated aging test of insulating glass sealant, as the temperature increases by 100F, the chemical reaction of the sealant doubles, and the aging process increases by 1 times, see Table 1.

Table 1 one-year life of insulating glass in nature through each week of life

Fahrenheit temperatureWeek
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 sealing structureSealing life achieved by testing
Butyl extruder/aluminum spacer24 hours
Composite strips (Swiggle strips)2 weeks
Silicone sealant/aluminum spacer3 weeks
Polysulfide, polyurethane, or hot melt butyl extruder/aluminum spacer6-8 weeks
Double-channel sealing structure
Butyl, polysulfide/polyurethane, four-side insert aluminum spacer12-18 weeks
Butyl, silicone sealant, four-side insert aluminum spacer15-20 weeks
Silicone sealant, Swiggle strip25 weeks+
Butyl, silicone sealant, continuous elbow aluminum spacer40 weeks+
Super spacer, hot melt butyl extruder100 weeks +

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

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

In summary, whether the sealing structure of insulating glass is reasonable directly affects the sealing life of insulating glass. The use of insulating glass with a double-pass sealing structure should be vigorously promoted to improve the sealing life of insulating glass.

3.2 Sealing materials

As can be seen from Table 6, the sealing materials (spacers and sealants) have a significant impact on the sealing life of insulating glass under the condition of a certain sealing structure, which is worth discussing.

3.2.1 Aluminum spacers. 

Generally, the sealing method of aluminum spacers is that the butyl rubber strips on both sides of the aluminum strips play the main sealing role. During the "pumping" movement of the insulated glass, the butyl extruder strip will be stretched, displaced, sheared, etc., shortening the channel for water vapor penetration. In terms of the grooved aluminum process, whether it is a continuous bend or an angle joint, the sealing of the aluminum strip edge is completed by butyl extruder, but the connection at the back or corner is not completely sealed (only one insufficient seal is enough to cause sealing failure). In addition, the bonding performance of butyl extruder itself is reduced at low temperatures. Any of the above situations can lead to weakened or failed sealing performance.

3.2.2 Types of sealants. 

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

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

Silicone adhesive50
Polysulfide adhesive19
Butyl adhesive2.25

However, especially manufacturers of polysulfide adhesives, intentionally or unintentionally forget when quoting numbers that the experimental conditions for measuring water vapor permeability (MVTR) are carried out at room temperature of 250℃, while the actual situation is that the conditions for the use of insulating glass, especially in summer in most places in my country, are close to or higher than 600C. Under the condition of rising temperature, the water vapor permeability of different adhesives will change differently. In fact, under the condition of 600℃, the MVTR of polysulfide adhesive is very close to the MVTR of silicone adhesive. The results of 20 years of tracking the actual service life of insulating glass in North America also confirm this point, that is, the insulating glass using the double-channel sealing structure of butyl glue/silicone glue has a longer sealing life than the insulating glass using other structures.

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

The method of the immersion test is briefly described as putting a 500-gram sealant block into the water at a temperature of 600℃ for 60 days, and then measuring its volume and weight to observe the changes. The results list (Table 4) is as follows:

Table 4:

TypeVolume change%Weight change%
Silicone adhesive3-102-6
Polysulfide adhesive5030

The above experimental data show that (1) the volume and weight of structural adhesives have increased to varying degrees, and the increase is in the order of polysulfide adhesive>polyurethane>silicone adhesive; (2) the changes in volume and weight before and after water absorption are different from the results expressed by their MVTR (water vapor permeability).

3.2.4 Molecular sieve

Generally speaking, the air layer after the insulating glass is plate-pressed and assembled 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 heat transfer value U value of the insulating glass and reduce the energy-saving effect of the insulating glass. In addition, during the service life of insulating glass, due to uneven coating or breakpoints of butyl extruder of aluminum spacers, pumping phenomenon of insulating glass caused by temperature changes, and water vapor permeability of sealants and many other factors, water vapor enters the air layer of insulating glass, and desiccant is also needed to ensure the low dew point temperature (such as -400℃) in insulating glass. The selection of desiccant for insulating glass can be referred to in Table 5.

Table 5:

Types of desiccantPore diameter/ÅAdsorbableNon-adsorbable
3A molecular sieve3WaterSubstances other than water
4A molecular sieve4Water, air, argon and kryptonSulfur hexafluoride, xenon, solvent
13X molecular sieve8.5

As can be seen from Table 5, if the insulating glass sealant used does not contain solvent, the 3A molecular sieve is most suitable for the desiccant of insulating glass. 3A molecular sieve is hydrophilic and only adsorbs water without adsorbing other substances. , so it can ensure that the sheets of insulating glass are parallel under normal conditions, reduce the stress of the glass at the edge seal, and extend the life of the insulating glass; the use of a 4A molecular sieve will cause the insulating glass to bend inward, the central part of the air layer will shrink, the glass will be visually deformed, the heat transfer will increase, the energy-saving effect will be reduced, and the sealing life of the insulating glass will be reduced; if the insulating glass seal used contains solvents, you should consider using a mixture of two molecular sieves. The usual practice is to use 75% 3A molecular sieve and 25% 13X molecular sieve so that it has the function of adsorbing water and solvents.

4. Super spacer and high-performance insulating glass

At present, high-performance insulating glass has been quite popular in developed countries, such as low-emissivity glass and warm edge technology, with market shares of more than 90% and 80% respectively.

In summary, (1) the warm edge has a better energy-saving effect than the cold edge, but the thermal conductivity coefficients of different warm edges are different; (2) Although the use of a certain warm edge technology can save energy to a certain extent, it is at the expense of the sealing life of the insulating glass; (3) The sealing life of the insulating glass is more important than its local temporary energy-saving effect improvement; (4) The purpose of making insulating glass is to save energy, but the use of aluminum spacers to make insulating glass has become the weak point of insulating glass; (5) Although the life of insulating glass made with improved aluminum spacers (such as continuous bends) has been significantly improved, its disadvantage of high thermal conductivity still exists.

Obviously, in the traditional thinking framework, no matter how hard people try to improve, improve the energy-saving effect of insulating glass, and increase the sealing life, they cannot have both.

In the 1980s, two enterprising and challenging Canadian scientists developed a method for the first time to solve the contradiction between energy saving and durability of insulating glass, namely super spacers, which caused a revolution in the insulating glass industry. Super spacers are continuous elastic spacers made of microporous materials without any metal and containing a 3A molecular sieve.

From the materials used to make super spacers, it is not difficult to understand that compared with other warm edge spacer technologies, the thermal resistance of insulating glass made of super spacers is the largest. Therefore, there is no need to elaborate. In contrast, the focus should be on why super spacers can greatly improve the sealing life of insulating glass. The P1 test results given in Table 6 above show that the longest sealing life of insulating glass made of super spacers can be up to more than 100 years. Given that this product is different from traditional spacer products and processes both in terms of the product itself and in terms of process, it is necessary to focus on introducing it in order to reveal the reasons for the long sealing life. There are two main factors that affect the sealing life of insulating glass with super spacers, namely the elasticity of the material and the reverse double-pass sealing, which are discussed below.

4.1 The elastic spacers

During the use of the middle edge glass, due to the influence of external factors such as temperature difference changes, wind loads, and pressure, it is always in a "pumping" movement of expansion and contraction, forming stress at the edge seal of the glass. When the glass bends inward, stress is formed at the chamfered contact between the inner side of the glass and the upper end of the spacer, the sealant at the upper end is squeezed inward, and the sealant at the lower end, especially the structural sealant is stretched outward; when the glass bends outward, stress is formed at the chamfered contact between the inner side of the glass and the lower end of the spacer, the sealant at the upper end, especially the sealing butyl extruder is stretched outward, shortening the water vapor channel, and the sealant at the lower end, especially the structural sealant, is squeezed outward and inward.

This "pumping" movement of insulating glass is inevitable for insulating glass structures of any structure. However, the degree of performance and the consequences are different. For the grooved aluminum spacer, when the insulating glass expands and contracts, the aluminum metal has rigidity and cannot absorb or buffer the stress generated by the movement of the glass. Therefore, the stress on the contact surface between the inner side of the glass and the aluminum spacer is very large; on the contrary, the use of silicone super spacer made of EPDM is elastic. When the insulating glass expands and contracts, it will be consistent with the movement direction of the glass, so that the stress of the edge cloth is minimized, thereby minimizing the possibility of the insulating glass bursting and increasing the sealing life of the insulating glass. The rigidity of the aluminum spacer not only shortens the sealing life of the insulating glass of this structure but also increases the possibility of the insulating glass bursting, which is most prominent in areas with large temperature differences, especially in cold areas in winter.

4.2 Unique reverse double-pass sealing process

The necessity of double-pass sealing for insulating glass was explained from three different angles. According to Table 7, we know that the MVTR of the insulating glass sealant has a great relationship with the sealing life of the insulating glass. Under certain conditions, the smaller the MVTR, the longer the sealing life of the insulating glass. Generally speaking, the double-pass sealing structure of the insulating glass adopts the method of sealing first and then structuring. From Table 8, the volume and weight of the structural adhesive in the second sealing position in the traditional structure will increase under the condition of long-term contact with water vapor, resulting in the shortening of the water vapor channel of the first sealant butyl glue. In severe cases, it will separate from the bonding surface of the glass, allowing external water vapor to enter the air layer of the insulating glass. When the internal 3A molecular sieve desiccant is saturated, condensation will form inside the insulating glass, resulting in sealing failure.

Different from the traditional sealing structure of sealing first and then structure, the insulating glass made with super spacer strips adopts the opposite approach, that is, structure first and then sealing. Specifically, the super spacer strip and the pre-coated sealant with structural strength on both sides play a structural role, while the outer channel uses hot melt butyl extruder to play the main sealing role. This reverse double-channel sealing structure isolates water vapor from the outside, and it cannot enter the position of the first sealant, extending the water vapor penetration channel; in addition, the back of the super spacer strip has 10 layers of high polyester material, which can not only prevent the bidirectional adsorption of molecular sieves but also play an auxiliary sealing function when the sealing performance of low-temperature butyl extruder is reduced. The elasticity of the material itself and the high-strength pressure-sensitive acrylic adhesive on both sides prevent stretching, displacement, and shearing. Such a structure and sealing method greatly enhance the sealing performance and life of insulating glass.

From the application of super spacers in developed countries, it is in the ascendant and is becoming more and more widely popularized and applied. Due to the excellent performance of super spacers, it has won many national and industrial honors, such as the Canadian Solar Energy Association's Solar Energy Company Award of the Year in 1994, the American Door and Window Association's highest honor Crystal Achievement Award in 2002 and 2004, and the British Door and Window Association's annual G04 Energy Saving Innovation Award in 2004, which is known as the Oscar Award of the industry. In addition, people also figuratively compare insulating glass made with super spacers to Cadillac and Mercedes-Benz cars in insulating glass. In developed countries, when the shelf life of insulating glass made with other spacer sealing structures is only 5-15 years, the shelf life of insulating glass made with super spacers is 20-30 years.

In summary, from the perspective of increasing the sealing life, we can say that the reverse double-pass sealing with super spacers is superior to the ordinary double-pass sealing structure, and the insulating glass made with super spacers is an essential material for making high-performance insulating glass.

5. The conclusion

The energy saving of ordinary insulating glass is very limited. In order to further improve the energy-saving effect of building doors and windows, the configuration of existing insulating glass should be improved, and the application of high-performance insulating glass windows should be vigorously promoted; in the configuration of high-performance insulating glass, Low-E glass, argon gas, and warm edge spacers are the three essential basic elements, and none of them can be missing. Among them, the use of super spacers can basically eliminate the dew-point condensation phenomenon at the edge of the insulating glass.

The sealing life of insulating glass is at least as important as the U value of the heat transfer coefficient, if not more important, and must be given enough attention; if aluminum spacers are used and cannot be changed in a short period of time, continuous elbows should be used instead of four-sided corner spacers; under the condition that conditions are met, super spacers should be used in one step. The use of the latter can also reduce the explosion of insulating glass.

In addition to the above characteristics, we believe that high-performance insulating glass should also have the function of reducing noise. In general, from the perspective of reducing noise, the configuration includes asymmetric thickness glass, laminated glass containing film, increasing the thickness of glass, and using elastic super spacers. The noise reduction effect increased by the configuration can be increased by 7-10 decibels compared with ordinary insulating glass, which is very important for people living near the noise source. High-performance insulating glass is an updated product of ordinary insulating glass, with greener, more environmentally friendly, and healthier functions. Although the current penetration rate of (ordinary) insulating glass in developing country accounts for about 5% of the completed projects that year, it needs to be vigorously developed. But this does not mean that we have to wait until the insulating glass is fully popularized in the whole society before promoting the application of high-performance insulating glass.

6. The suggestions

It should be recognized that the formulation, implementation, and effective role of building energy-saving policies are far from enough to rely solely on the market itself. Relevant departments and government departments should speed up the research, formulation, and implementation of policies and regulations on energy-saving building doors and windows that are suitable for developing country's national conditions and enforce them. In addition, the government should also work with relevant parties to carry out the following work:

Implement popularization activities for the knowledge of insulating glass windows for the whole people, and conduct "literacy" on energy-saving windows for current home buyers.

Establish a detection method for the life of insulating glass. The new detection method should be similar to the P1 detection in the United States, which is different from the existing national standard method.

Compile the detection standard of the U value of the heat transfer value of insulating glass windows as soon as possible, including software simulation methods and laboratory hot box detection methods.

Developers should do some practical energy-saving work, and the government should rate different levels of energy-saving windows and affix labels for consumers to identify.

Insulating glass manufacturers should implement a shelf life of at least 10 years for insulating energy-saving windows. During the warranty period, if the insulating glass fails to seal, the developer will replace it free of charge.

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