The selection of insulating glass sealant

Insulating glass is a kind of deep-processed glass product, and it is also a kind of energy-saving material with a wide range of uses. In recent years, the insulating glass industry has developed rapidly. From special products that were only used in high-end buildings in the past, it quickly spread to ordinary buildings, air-conditioned trains, air-conditioned passenger cars, refrigerators, display cabinets, and other fields. Especially with the promotion and application of plastic steel windows, hollow glass has penetrated ordinary residential buildings. At present, the home air conditioner ownership rate in some key developed cities in developed countries is very high. For example, Los Angeles has reached 80%-85%. According to expert calculations, the energy consumption for every 1°C decrease in the indoor temperature of a building is 6 times that of a 1°C increase... The result is an increase in building energy consumption and increased environmental pollution. Therefore, experts in the building materials industry in some developed countries pointed out that “window glass will be put in place as soon as possible, and hollow glass will be fully adopted.”

^{The selection of insulating glass sealant 1}

Water vapor transmission rate of insulating glass sealant

Insulating glass sealant is the main factor that determines the quality and performance of insulating glass. The performance of insulating glass's heat insulation, sound insulation, and anti-frost and fog performance is achieved through a sealed, dry air layer inside. Therefore, the material's gas barrier performance or air permeability is one of the most important indicators for the selection of insulating glass sealant. It can be seen from Table 1 that among the four common insulating glass sealants, butyl rubber is the lowest and silicone rubber is the highest, but butyl is thermoplastic and is only used for inner sealing, and is generally not used alone. Polysulfide glue has low air permeability and is an ideal material for manufacturing insulating glass.

^{Table 1 　 Water vapor transmission rate of sealant}

Unit: g/m² .24h

Note: PS—polysulfide sealant; 

PU—polyurethane sealant; 

Butrl—butyl hot melt adhesive; 

SR—silicone sealant　　 

1. With the continuous improvement of energy-saving requirements, to improve the insulation performance of insulating glass, inflatable (argon,Xenon, etc.) Insulating glass is the development trend of insulating glass, which inevitably requires the insulating glass sealant to have good gas barrier properties or low air permeability.

2. Air leakage is one of the main factors that cause the natural failure of insulating glass. Compared with polysulfide glue, the air permeability of silicone glue is 8 times that of polysulfide glue, and the service life of insulating glass sealed with silicone glue is longer. 

3. Due to the high air permeability of silicone rubber, to ensure the quality and service life of the insulating glass, the scope of use of silicone rubber for insulating glass manufacturing has been limited, such as relevant international industry standards: "Hollow Elastic sealant for glass" standardIt is stipulated that "silicones are only used for glass structural systems for bonding and assembling silicone structural sealants."

4. At present, some developing countries and some insulating glass manufacturers use single-pass sealing (that is, no hot-melt butyl glue is used for inner sealing). Silicone glue cannot be used in this process, otherwise, the insulating glass cannot be guaranteed.

The performance comparison of insulating glass sealant

^{Table 2 　 Performance comparison of insulating glass sealant}

Table 2 shows the performance comparison of several common insulating glass sealants. It can be seen from Table 2 that the water-resistance of silicone glue is worse than that of polysulfide glue, and the water resistance and chemical solvent performance of the material is also the choice of insulating glass sealant. Important indicators. For example, Low-E hollow glass has the best heat insulation and energy-saving effect, but it has high requirements for the water-resistance of the sealant because the coating of Low-E glass will be destroyed by water contact. The windows of the new air-conditioned trains are all hollow glass. The water leakage and failure of hollow glass is a common problem of current train windows. The reason is that rainwater is easy to accumulate between the glass and the window frame. After the sun is exposed, the water temperature can reach up to about 80 ℃. Under this condition, the adhesive bonding strength will decrease rapidly, coupled with the vibration of the train running, the glue layer and the glass will be debonded, causing the hollow glass to fail. In terms of our accumulated experience, even for polysulfide with good water resistance, the formula must be specially designed and the ingredients must be screened to meet the requirements.

Finally, considering the product price factor, although the specific gravity of silicone rubber is smaller than that of polysulfide glue, that is, the former can hit more extended rice with the same weight, but the price of the former is significantly higher than the latter. In general, using polysulfide glue is still more economical than silicone rubber, and the air permeability and water permeability of silicone rubber are much higher than that of polysulfide. To achieve the effect of polyfluidic rubber, the thickness of the glue layer must be increased, which will inevitably increase the production cost of insulating glass.

In recent years, the hollow glass produced by the rubber strip method has also appeared on the market, and this structure has technical deficiencies. Since the main material of the rubber strip is butyl or polyisobutylene rubber, that is, the rubber strip forms a non-chemical bonding surface layer on the glass under heating and pressurizing conditions. Therefore, it has poor temperature resistance and weather resistance (butyl or polyisobutylene). Polyisobutylene glue tends to creep when exposed to heat, and hardens when exposed to cold). In addition, the rubber strip is a thermoplastic rather than an elastomer, so the resistance to displacement and deformation is very poor. From the actual use effect, many manufacturers using this method report that the hollow glass leaks and leaks seriously.

Polysulfide sealant is the first and most widely used outer layer sealant in the insulating glass industry. In recent years, despite competition from other types of rubber such as silicone and polyurethane, it still occupies a leading position with its excellent comprehensive performance and moderate price. From the perspective of the world market, according to the latest statistics from Japan’s TORAY Co., Ltd.: In 2021, the world’s output of insulating glass was 257 million square meters, of which the European market accounted for 54%. In the European market, polysulfide sealant accounts for 82% of the total amount of insulating glass used (see picture 1). In the Japanese market, polysulfide sealants account for 90% of insulating glass sealants (see Figure 2).

^{In the European market, polysulfide sealant accounts for 82% of the total amount of insulating glass 1}

^{In the Japanese market, polysulfide sealants account for 90% of insulating glass sealants 1}

To sum up, when choosing insulating glass sealant:

1. For building doors and windows, exposed frame glass curtain walls, air-conditioned trains or passenger cars, freezers, and other insulating glass should choose a butyl-polysulfide system (butyl rubber for inner seal and polysulfide rubber for outer seal). 

2. For the hollow glass used for the fully hidden or semi-concealed glass curtain wall, the butyl-silicone structural adhesive system should be selected. Here, the silicone structural adhesive has to bear the structural strength, and the thickness of the adhesive layer should generally be between 10 mm and 20 mm.

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