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Article summary: According to the current standard, the elastic seal of the hollow glass unit (IGU) is a double seal: the inner channel or a sealant mainly uses thermoplastic polyisobutylene (PIB) or butyl rubber to reduce water vapor and gas.The permeability of the sealing edge, in addition, also acts as a processing aid to fix the spacer in a suitable position during the processing of the insulating glass. Some hollow glass units also use double-sided tape as processing aids, but this tape has not been tested for water vapor transmission control-so this type of hollow glass unit cannot be considered as a double seal.  

1. Introduction

According to the current standard, the elastic seal of the hollow glass unit (IGU) is a double seal: the inner channel or a sealant mainly adopts thermoplastic polyisobutylene (PIB) or butyl rubber to reduce moisture and gas on the sealing edge. Permeability, in addition, also acts as a processing aid to fix the spacer in a suitable position during the processing of insulating glass. Some hollow glass units also use double-sided tape as processing aids, but this tape has not been tested for water vapor transmission control-so this type of hollow glass unit cannot be considered as a double seal.

The function of the external sealant or the second sealant is to act as an adhesive to fix the glass units together while restricting the transfer of water into the glass unit and the leakage of gas from the unit. The secondary sealant can be made of different materials, but there are three main ones, namely: polysulfide (PS), polyurethane (PUR), and silicone (SI).

In 2007, more than 450 million square meters of insulating glass produced globally used 65 million liters of polysulfide sealant, 30 million liters of polyurethane sealant, and 15 million liters of silicone sealant. It can be seen from these figures that polysulfide sealant is still the market leader among all kinds of elastic secondary sealants for insulating glass. This prominent position is not only due to the unique processing and comprehensive product performance of polysulfide insulating glass sealant but also due to its nearly 50 years of long-term use experience and its applicability to today's glass windows and buildings.

Generally speaking, the final selection of the second sealing material should consider the following points:

(1) The performance of edge sealing  

(2) Protection provided by the entire glass system   

(3) The environment exposed by the secondary sealant during the application process

In daily practice, the above indicators more or less lead people to choose different secondary sealants for different application ranges: For commercial building glass (glass curtain walls, structural glass), silicone glue is generally preferred, while polyurethane sealants are mainly used For automatic production lines that produce glass with basically the same size and shape, this kind of "normal" glass can have a larger output. Polysulfide sealant can be used for all kinds of insulating glass (for glass curtain walls where the edge of the insulating glass must be protected, polysulfide is restricted to use). This special limitation is due to the development of insulating glass processing and sealant design in the past 10 years. It is also related to the performance of the cured sealant (such as some special advantages) and the processing performance of the sealant.

Insulating glass manufacturers and insulating glass users have different views on the performance of sealants: in addition to cost, the two have the same point of producing high-quality insulating glass units. In addition, the insulating glass manufacturer also cares about handling performance, easy processing, zero defective products, and so on. Processing performance is very important to them. Both sets of performance are discussed in this article. The data used in this article comes from Akzo Nobel's test results and related literature on the glues of the major European sealant manufacturers on the market. Since the processing properties are discussed in detail in many articles and product introductions, this article only briefly describes them. (E.g. [1], [2]). Here we mainly focus on the performance of the cured sealant.

2. Processing performance

The use of automatic production lines puts forward some additional requirements for insulating glass sealants: high output, low wear, easy handling, reduction of waste and reuse, etc. Therefore, basic polymer and sealant manufacturers should now pay more attention to the processability of their products. Modern insulating glass sealant compounds must have [2]:

2.1 Special rheology

Good wettability to the substrate (glass and spacer) is a prerequisite for good adhesion. The sealant must be applied continuously along with the four corners of the glass. Therefore, the sealing material needs to have low viscosity and non-sagging properties. All the tested secondary sealants for insulating glass exhibit the required pseudoelasticity.

2.2 High activity

Sufficiently long service life and fast curing are also required by insulating glass manufacturers. The following table summarizes the curing performance of different insulating glass secondary sealants on the market:

Sealing gluePS14PS23 PS30SI25SI72SI73 PUR28PUR32
Use period minutes  3029204033203520
Surface drying time minutes 504035240150509050
 Shore A hardness 
After curing time1h1320.51673.54.611

Table 1 Curing of the second sealant for insulating glass

A good polysulfide insulating glass two-layer sealant formula, the hardness of the product after curing for 4 hours at room temperature should be able to reach 80% of its final hardness. This unique performance is the result of oxidative curing with active manganese dioxide. The curing speed of other types of products is much slower than that of polysulfide. This has been discussed in [2]. Most insulating glass sealants reach the final Shore A hardness value of approximately 50 after 24 hours.

2.3 Fast generation of adhesive force

The rapid generation of adhesive force is a problem to be paid special attention to. The faster the adhesion is generated, the fewer problems will arise during the transportation of the hollow glass unit. The formulated polysulfide insulating glass sealant can reach its full adhesion within 4 hours after curing at room temperature.

The adhesion test described in EN 1279-6 (the adhesion test is carried out after curing at room temperature for 24 hours, and the sample must be subjected to a certain load for at least 10 minutes). The results clearly show that most of the hollow glass has two channels The sealant performed well.

Sealant PS14PS 23PS 30SI25    SI72SI73PUR28PUR32
Aluminum spacerPassPassPass70% AFPassPass 80% AF60% AF
Time 3.4 minutes later 1.25 minutes later1.5 minutes later

Table 2 Initial adhesion results measured according to EN 1279-6  

AF: Bond failure

Regarding processing performance, polysulfide insulating glass secondary sealant is superior to other types of glue in some aspects, but in the European market, most of the tested sealant products perform well and can meet the requirements.

3. The function and performance of the edge sealing of the insulating glass unit

3.1 Theoretical aspects

Insulating glass units will withstand various loads caused by handling (opening, closing), wind, temperature, and air pressure changes. These loads will cause the deformation of the unit (Figure 1), and the sealant will have to be extended, compressed, and sheared.

Figure 1 Load and deformation of the hollow glass unit

Figure 1 Load and deformation of the hollow glass unit

The ability to adapt to additional humidity, ultraviolet radiation, and heat conditions determines the service life of the sealant. Moisture condensation (condensation) occurs in the hollow glass cavity (for gas-filled glass, it is gas leakage), which means the end of the service life of an insulating glass unit.

For the hollow glass unit, several important aspects about the water vapor and gas transmission rate are what we need to consider:

In contrast to porous materials (such as filter paper), mass transport through polymeric materials takes place in the form of active diffusion.

In principle, there are two possible diffusion paths: through the second and first sealant, or along the surface of the glass and sealant. The possibility of diffusion along the interface is much higher than diffusion through the sealant [4].

Figure 2 The function and performance of the edge sealing of the insulating glass unit

Figure 2 The function and performance of the edge sealing of the insulating glass unit

For a double-sealed glass unit, the resistance to diffusion is the sum of each seal.

The transmittance of the sealant is always proportional to its area. If the balance is established, it is generally inversely proportional to its thickness.

If equilibrium has not been reached, the time required to reach equilibrium is roughly proportional to the square of the thickness (Fick"s and Henry"s law).

Therefore, the thickness of the sealant can increase its barrier performance more during the period before the balance is reached than after the balance is reached.

The looseness of the network structure-for example, the plasticized or swollen structure-will increase the permeability.

3.2 Water vapor penetration (water vapor transmission rate (MVTR))

In the case of perfect bonding between the glass and the sealant, water vapor can only enter the hollow glass cavity through the sealant. In case the bonding between the first sealant and the glass fails, the second sealant has to take on the only task of blocking the penetration of moisture. If the bonding between the second sealant and the glass fails, the insulating glass can no longer be used and needs to be replaced. The early failure of insulating glass is mainly caused by some errors in the production process or the use of inferior sealant, or both.

Table 3 summarizes the water vapor transmission through different types of sealants and double seals (butyl + one of them as the second sealant)

Water vapor transmission rate

Water vapor transmission rate [g/m 2 days]Water vapor transmission rate [%]
DIN 53 122-3 mm sealant test
20 °C60 °C
 23 °C
23 °C
Sealant type
Silicone (two-component, neutral)7-1640-7015-20159.2<1.2

Table 3 Water vapor transmission rate

The data in the table clearly shows that the water vapor transmission rate (MVT rate) depends on the type of polymer, and it increases in proportion to the temperature. The material with the worst gas and water vapor barrier is silicone rubber. What is interesting is that silicone rubber only slightly swells in water. However, the test results show that choosing a material with a good water barrier effect cannot just look at its swelling in water as an indicator as we usually do.

Polyisobutylene has a strong moisture barrier and determines the diffusion resistance of the insulating glass unit. Therefore, we see that the water vapor transmission rate of all double-sealed insulating glass units is relatively close.

3.3 Inert gas penetration

The hollow glass unit is filled with an inert gas such as argon or krypton to improve its thermal and sound insulation capabilities. The diffusion of inert gas depends on the temperature and the pressure difference between the cavity and the environment.

3.4 Design of insulating glass unit-sealant glue size

Although the second sealant of silicone insulating glass exhibits the worst ability to resist the diffusion of gas and moisture, some people still use it to produce air-filled insulating glass. However, people need to consider this when designing insulating glass. Therefore, many European insulating glass manufacturers have established their internal standards to meet the requirements of EN1279 for inflatable insulating glass. The thickness and amount of the first butyl sealant have the strictly limit, which are based on different types of second sealants

Summarized the corresponding data in Figure 3 and Table 5

Figure 3 The insulated glass sealant size

Figure 3 The insulated glass sealant size

Insulating glass secondary sealant typePolysulfide or PolyurethaneSilicone
Butyl rubber dosage g/m2.53.5
   [Part C in Figure 3] (One side)
Secondary sealant thickness mm 2.5-34 -5
 [Part B in Figure 3]

Table 5 Sealant dosage (internal standards of European insulating glass manufacturers)

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Jinan LIJIANG Automatic Sealant Sealing Robot Machine

As mentioned earlier, silicone sealant has a weaker ability to block gas and moisture. It can also be produced the higher quality insulating glass, could by increasing the amount of sealant and carefully sealing the end of the spacer (together with the butyl). 

Inert gas gram diffusion can also be used to estimate the service life of insulating glass: The argon loss rate of polysulfide/butyl sealed insulating glass can be determined by the HOLLER method, and its range is 1-8*10-3/year. It was found that the value of the polyurethane/butyl system is 6-25*10-3 /year.

FELDMEIER and SCHMID [8] predict that if the annual loss rate of argon is about 1%, the service life of insulating glass is about 20 years. Based on this calculation, the polysulfide/butyl-sealed insulating glass has an annual loss rate of less than 1% due to its uniform inert gas, so its service life is estimated to be 30 to 40 years.

3.5 Strength, relaxation, adhesion

A problem that people often discuss is that the function of the second sealant is only for elastic bonding, while the barrier function is provided by the first seal. The content of the previous chapters shows that for the performance of insulating glass, the problem of water vapor and gas passing through the second sealant is also very important. However, strength and adhesion to glass and spacers are closely related to the ability of the sealant to bond the glass sheets together and prevent water vapor (and inert gas) from penetrating the interface between the glass and the sealant.

Strength (and relaxation) and adhesion are different due to different types of polymers. When considering the cost issue, the dependence of these two indicators on the formulation should be emphasized. Take polysulfide sealant as an example, the ratio of polymer/plasticizer/filler determines the performance of the sealant. As mentioned before, the permeability of gas and water vapor will decrease with the increase of the polymer content, and the stress recovery will increase proportionally with the increase of the compound content.

At the same time, the components that can produce plastic deformation in the second sealant will be reduced. The influence of the components of the sealant on the performance of the adhesive has been described in some documents and reports. 

4. Conclusion

For the secondary sealant of insulating glass, it has good processing (technical) performance, transportability under various loads and the ability to resist environmental influences is the most important. Due to the individual advantages of certain types of glues, different sealants suitable for special applications have been developed. Based on long-term practical experience, not only the quality of the sealant, but also the quality of each component of the insulating glass has been greatly improved, and its design has also been optimized. Regarding the second-layer insulating glass sealant, if the field data confirms that various sealants can meet the requirements of high-performance insulating glass, it means that the formulation of these glues is of the highest level.

What needs to be mentioned in this article is the research results of MOGNATO and others. They concluded after conducting the only effective inspection during the production of insulating glass and following strict rules to achieve the lowest defective rate. To achieve this high-quality level, the necessary third-party inspections are also on-site at the same time to ensure excellent product quality.

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

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