13 min read

The application history of organic glue as the secondary seal of insulating glass is relatively early and has been used until now. Therefore, in most people's minds, it has always been considered that organic glue has the best resistance to water vapor permeability as a secondary seal of insulating glass.

1. Comparison of the permeability of insulated glass sealed with silicone and organic glue

The application history of organic glue as a secondary seal for insulating glass has an early history and has been used until now. Therefore, in most people's minds, it has always been believed that organic glue as a secondary seal for insulating glass has the best resistance to water vapor permeability. For single-channel sealed insulating glass, this understanding may be correct, but for double-channel sealed insulating glass, it is not entirely true. Because the actual engineering verification in recent years has proved that the performance of silicone as a secondary sealed insulating glass, especially the performance of resistance to water vapor penetration, is significantly better than that of organic glue-sealed insulating glass.According to the latest foreign research report, different double-sealed insulating glass according to DIN1286-1 temperature/humidity cycle and 3 months of constant high temperature (55 ℃) and constant high humidity (100%) exposure conditions, the water absorption of the molecular sieve is Refer to Figure 1 and Table 1 for the comparison of water vapor permeability. It can be seen from the chart that the water vapor permeability of insulating glass is higher under constant weather conditions than under cycling conditions. The insulating glass that uses silicone as the second seal exhibits lower water vapor permeability than the insulating glass that uses organic glue as the second seal. This result proves that the performance of insulating glass should not and cannot only consider the water vapor permeability of the second sealing single material but the entire system must be considered. Experimental data show that under constant high temperature and high humidity, silicone-sealed insulating glass has significant performance advantages; the water vapor permeability of organic-sealed insulating glass is about 3 times that of silicone. Because the water absorption rate of the desiccant (the term "water absorption rate" provided by Google) is inversely proportional to the average life of the insulating glass, this finding shows that the life of the insulating glass with silicone as the second seal is longer than that of polysulfide or polyurethane. Sealed insulating glass is much better.

Second sealantWater absorption after D IN 1286-1 testWater absorption after 3 months of humidity
Polysulfide 10.55.4
Polysulfide 20.85.6
Polyurethane glue 10.54.2
Polyurethane glue 20.64.5
Silicone glue 10.41.5
Silicone glue 20.41.6

Table 1 Permeability of insulating glass after different accelerated exposure conditions (mass fraction)

The French Insulating Glass Association (CEKAL) earlier analyzed those insulating glass that failed prematurely. They found that all the failed insulating glass had different degrees of secondary seal adhesion damage under ultraviolet radiation, and there was no In one case, silicone was used for the second seal. German doors and windows (the term "doors and windows" is provided by the industry encyclopedia) Institute (IFT) also reported the same trend at about the same time.

Figure 1 Water vapor permeability of insulating glass with different secondary seals under temperature, humidity cycle and constant high temperature and high humidity conditions

Figure 1 Water vapor permeability of insulating glass with different secondary seals under temperature, humidity cycle and constant high temperature and high humidity conditions

2. Usual service life and potential failure factors of insulating glass  

In some developing countries, the service life of insulating glass has not been widely understood.  

In Europe, the service life of insulating glass is required to reach at least 25 years. Gas-filled insulating glass requires a gas leakage rate of less than 1% per year. After accelerated aging (the term "aging" is provided by the industry encyclopedia), within a 25-year service period, the total gas leakage rate is required to be less than 5%. Assuming that the U value of the insulating glass 100% filled with argon gas increases by 0.4W/(m2K), the U value decrease after the gas loss during its service period should be less than 0.04W/(m2K).  

In fact, the service life and thermal conductivity of insulating glass will be affected by the following factors: 

①The anti-diffusion property of the edge seal to water vapor and gas. 

②The effective cross-sectional size of the sealant when water vapor or gas diffusion occurs. 

③Durability of edge seal.  

The water vapor and gas permeability of the first seal is much lower than that of the second seal. Therefore, the usual service life is determined by the diffusion through the cross-section of the first seal, as shown in Figure 2.

Figure 2 Effective cross-sectional area of insulating glass

Figure 2 Effective cross-sectional area of insulating glass

IFT's research report pointed out that more than 95% of potential failure cases are caused by the loss of adhesion of the second seal, such as the poor durability of the second seal.  

3. Changes in the water vapor transmission mechanism caused by the aging of the edge seal   

In order to study the causes of premature failure of insulating glass, Van Santen and Schlensog conducted in-depth research on this. The hollow glass is exposed to the environment or the edge seal is aging due to service degradation factors. The aging can cause changes in the water vapor transmission mechanism and lead to water vapor intrusion or gas leakage. During the use of insulating glass, if the water vapor penetration increases significantly, it indicates that the system is aging, so the increase of water vapor penetration is a good indicator for judging whether the system is aging. Before aging, in a stable state, when the desiccant is far from being saturated, the amount of water vapor intruding into the hollow glass in a unit time should be a constant. When the desiccant in the hollow glass is close to saturation, the intrusion of water vapor should be slowed down because the local pressure difference is balanced. However, when the insulating glass installed in the laboratory and on-site undergoes repeated humidity and temperature cycling tests, as the exposure time increases, the water vapor penetration often shows a non-linear increase (Van Santen1986, Marusch1988), as shown in Figure 3. . Van Santen (1986) attributed this behavior to the movement of the edge seal due to temperature and pressure fluctuations, which caused the physical degradation of the first seal. Due to this repetitive movement, the first seal may fail cohesively or cohesively. These two failure mechanisms can be observed in the actual use of insulating glass, usually accompanied by the first seal of PIB penetrating into the visible cavity.

Figure 3 Non-linear characteristics of water absorption of insulating glass after aging

Figure 3 Non-linear characteristics of water absorption of insulating glass after aging

Schlensog (1986) observed the process of adhesive loss on glass after the second seal of organic glue was irradiated with ultraviolet light through the technique of polarized microscope. We all know that when the sun shines on the conventional insulating glass, a certain amount of incident light reaches the edge sealing part through the internal reflection of the glass. When exposed to such a lethal shortwave spectrum for a certain period of time, the organic glue will lose its glass adhesion. The study also showed that when edge seals can detect adhesion or boundary failure, delamination under microscopic conditions already exists. As the exposure time continues, the delaminations become larger and connected to each other, leading to macroscopic failure. It is possible that before the macroscopic failure occurs, water vapor and gas penetrate into the inner cavity along the damaged area of the interface, and the intrusion of water vapor may accelerate the mechanism of this failure.   

The temperature change of the edge seal will cause periodic shearing and peeling forces between the spacer and the glass due to different thermal expansion coefficients, and generate high stress on the edge seal, which may be superimposed with the aging effect. The shear displacement in the interface will generate high tensile stress near the contact surface of the glue and the substrate, which is almost twice the original shear stress, which is why the sheared sealant is prone to fail on the substrate surface. If the second sealant hardens within the service life, the increase in tensile stress may cause partial or complete loss of adhesion.  

4. Effective diffusion cross-section   

When water vapor or gas diffusion occurs, the control of the effective cross-sectional size through the edge seal often depends on the hollow glass manufacturing process. The correct insulating glass manufacturing process can reduce the effective cross-section through the edge seal when water vapor or gas diffusion occurs. The requirement of the manufacturing process is that in addition to the correct size, the first seal must be free of bubbles and completely wet the contact surface between the spacer and the glass. In the case of rigid spacers, the degree of expansion of the first seal under positive pressure is determined by the tensile strength of the second seal against the force. In fact, a positive pressure difference exists when the atmospheric pressure is low or at a high temperature, and temperature is the cause of most pressure differences. The pressure difference caused by temperature exerts a much higher force on the edge seal than the force caused by wind pressure and atmospheric pressure change (see Figure 4). Therefore, the tensile stress behavior of the second seal at a high temperature (Young's mode Number) must be considered. The higher the tensile resistance of the second seal, the better the diffusion cross-section of the first seal will be maintained.

Figure 4 The effect of stress generated by temperature and pressure fluctuations on the diffusion cross-section

Figure 4 The effect of stress generated by temperature and pressure fluctuations on the diffusion cross-section

In addition, the factor that further affects the service life and U value of the insulating glass is the duration of the first seal expansion. Regardless of the type of secondary seal used, this opening time always exists with the period of the positive pressure difference. As mentioned earlier, the degree of opening depends on the tensile strength of the secondary sealant. However, once the positive pressure difference is weakened and the internal and external pressure difference is balanced, the length of time required for the edge seal to close the first seal opening varies according to the elastic recovery rate of the second sealant used. A sealant with a low elastic recovery rate exhibits viscous fluidity in its stress-strain behavior. Due to this stress relaxation principle, its tensile stress will be reduced during the maintenance extension. The result is that when the external force is eliminated, they no longer have the ability to quickly close the first sealed opening and return to its original size. If the second seal cannot be fully restored, the first seal will be permanently deformed. In addition, because the opening of the diffusion channel mainly exists in the high-temperature period, the second sealant with a poor elastic recovery rate will obviously shorten the service life of the insulating glass during the high-temperature period.  

In addition to fluctuations in temperature and atmospheric pressure, the degree of water vapor standard in the surrounding environment also indirectly affects the opening and closing degree of the first seal. In the presence of high water vapor standards or higher, such as direct contact with liquid water, the absorption of water by the second sealant will increase its volume and reduce its mechanical properties. In short, the lower the crosslink density of the polymer sealant grid (the term "density" is provided by the industry encyclopedia), the higher its water absorption. For most glues, the degree of negative impact caused by water absorption is proportional to the total amount of water absorbed. The water absorption of the second seal will cause the opening of the first seal to expand, so the effective diffusion cross-sectional area is also increased.

5 The impact of environmental factors on insulating glass and the performance requirements of edge sealing

Insulating glass is exposed to different environmental and use factors, which will hurt the average life of the insulating glass. Literature on this aspect often divides these effects into physical effects (such as temperature and pressure fluctuations, etc.) and chemical effects (such as Atmospheric environment, chemicals, etc.) into two categories. Among them, some environmental impacts, such as sunlight and water vapor, will produce physical and chemical effects at the same time.

The high temperature will accelerate most of the physical and chemical reactions, such as aging the insulating glass sealant and accelerating the diffusion of water vapor or gas through the edge seal. Temperature fluctuations cause pressure changes in the insulating glass. These pressures will produce mechanical stress on the edge seal, the strongest effect on the small size, and the greatest stress on the edge seal. In addition, after heating, as mentioned earlier, the different thermal expansion coefficients of the spacers and the glass sheets will cause shear and peel forces to act on the edge seal.

The temperature of the edge seal is affected by the daily and annual changes of the ambient temperature, so that the temperature of the edge seal changes rapidly accordingly. According to statistics, during the service life of insulating glass (in 25 years), 2,000 times of pressure fluctuations are caused by the atmosphere (climate), about 20,000 times are caused by sunlight, and about 400,000 times are caused by wind. Caused by pressure. These pressure fluctuations will repeatedly act on the edge seal, affecting the average life of the insulating glass. Therefore, a good insulating glass edge seal must have the following characteristics:

①Weather resistance, that is, resistance to environmental factors (physical properties and adhesion are considered at the same time).

②It has the structural strength to restrain the movement of the edge seal to minimize the change of the effective diffusion cross-sectional area of the first seal.

③Under the conditions of use, it has low water vapor and gas permeability.

6. The characteristics of insulating glass as the second seal

Silicone as a second sealed insulating glass can maintain excellent performance in harsh natural environments because silicone itself has some chemical properties that organic glues cannot achieve.

(1) Anti-aging properties

The Si-O bond of the silicone polymer itself has high bond energy and a large bond angle, so it is transparent to ultraviolet rays and most invisible light below the wavelength of 280nm, and the Si-O The key will not be easily interrupted. Secondly, the interaction force between the silicone molecules is relatively weak, and the molecular chain is flexible, which makes the silicone polymer have good flexibility and is hardly affected by the environmental temperature, especially the extreme high and low temperature.

(2) Modulus of elasticity

The elastic modulus of organic rubber (the term "modulus" is provided by the industry encyclopedia) gradually decreases with the increase of temperature. From -20 ℃ to 70 ℃, the modulus of polysulfide rubber will decrease by 40%, and polyurethane will decrease About 60%, while silicone is almost unaffected by it, and its modulus remains unchanged.

(3) Elastic recovery rate

Similarly, silicone generally shows a good elastic recovery rate due to its inherent molecular properties, while organic rubber shows its thermoplasticity, that is, as the temperature increases, its elastic recovery rate will gradually decrease(see Figure 5).

Figure 5 The relationship between elastic recovery rate and temperature 1

Figure 5 The relationship between elastic recovery rate and temperature 1

(4) Water absorption

Absorbing water, water, and water vapor can cause the natural stress of the edge seal and the expansion of the insulating glass sealant itself. Organic glues are all hydrophilic (the term "hydrophilicity" is provided by the industry encyclopedia), and generally exhibit a higher water absorption rate, while sealants with special formulations (such as DC3362) exhibit better Ketone sealant has lower water absorption (see Figure 6).

Figure 6 The comparison of water absorption of sealants

Figure 6 The comparison of water absorption of sealants

(5) Water vapor diffusivity at high temperature

Temperature greatly affects the water vapor and gas permeability of the insulating glass sealant. As the temperature increases, the diffusion rate of water vapor and gas will increase. The water vapor permeability (VWP) of the second insulating glass sealant at 60 ℃ is 6 to 8 times that of 20 ℃, and the water vapor permeability of silicone and polysulfide is similar at high temperatures (see Figure 7).

Figure 7 The effect of high temperature on the diffusion of water vapor

Figure 7 The effect of high temperature on the diffusion of water vapor

Statistical results show that insulating glass is generally not prone to failure at low temperatures, and the failure of insulating glass generally occurs during high-temperature periods. Although the edge seal of insulating glass is exposed to more than 30 ℃ each year only accounts for (or less than) 20% of the whole year, in this short high-temperature season, the damage experienced by the insulating glass is more than that of the annual temperature below 30 ℃. The damage experienced during the rest of the time is twice as high. This is because the second seal of the hollow glass sealed with organic glue is aging at a high temperature. After aging, its adhesion, elastic modulus, and the elastic recovery rate are degraded. Water vapor diffusibility will accelerate at high temperatures, so high temperatures accompanied by high humidity will completely reduce the average life of the insulating glass. Silicone is not the case for the second sealed insulating glass. As mentioned above, its water absorption rate under high temperature and humidity is much lower than that of organic sealed insulating glass, which has better durability.

7. Conclusion

The insulating glass with the second seal of silicone has a lower water vapor permeability than the insulating glass with the second seal of the organic glue, because the permeability of the edge seal is almost completely determined by the permeability of the first seal (PIB), and as The silicone of the second seal is better than the organic glue in terms of viscoelasticity, especially the tensile strength and elastic recovery rate under actual use conditions, so it is better than the organic glue in preventing the diffusion of water vapor into the first seal. good. The gas diffusion resistance of the edge seal depends on the permeability of the first and second seals. Silicone as a second-sealed insulating glass has been tested under actual service conditions and proved to have high durability and service life because the physical and bonding properties of the silicone second-sealing are hardly affected by the main environmental aging factors. That is the effects of ultraviolet rays, heat, and humidity.


For more information about Jinan LIJIANG Glass insulating glass processing equipment and insulating glass processing accessories, please click here to learn more. 

Comments
* The email will not be published on the website.