Analysis of causes of condensation on the inner layer of insulating glass and control measures.

According to statistics, building energy consumption in developing countries accounts for about 30% of the country's total energy consumption. At present, the energy consumption per unit area of buildings in some developing countries is still about 2 to 3 times that of developed countries with similar climates. In the cold regions of high latitudes in the northern hemisphere, building heating energy consumption has accounted for more than 20% of the total local energy consumption. At the same time, the use of insulating glass for insulation has become a trend that is gradually heating up across the country. For the entire building, the area of doors and windows accounts for more than 20% of the building area, and glass accounts for more than 70% of it. From an energy-saving perspective, of the energy consumption of the entire building, heat dissipation through doors and windows accounts for about 50%. %, the proportion is very high, especially the heat transfer and air tightness of doors and windows in rural and town buildings are the weakest links in the entire outer envelope structure. Therefore, reducing the energy consumption of building doors and windows and improving the thermal insulation and air tightness of building doors and windows are urgent tasks we face. With the deepening of building energy-saving work, a large number of non-energy-saving building exterior doors and windows will also use insulating glass, which has huge market potential. Among them, the use of insulating glass is increasing day by day, improving the insulation and durability of insulating glass, and avoiding its function Failure is of great significance to the building energy conservation industry.

According to a domestic survey of insulating glass after two years of use, the failure rate of insulating glass is 3% to 5%. The first reason for failure is that the dew point in the air layer of insulating glass rises and internal condensation occurs (see Figure 1), accounting for 63% of the failure rate; The second is the explosion of insulating glass, accounting for 26%. These two reasons constitute 89% of the total failures, and the rest only account for 11%. As can be seen from Figure 1, the problem of condensation inside the air layer of insulating glass is prominent, which not only affects its moisture permeability but also reduces the thermal insulation effect of insulating glass. We must pay great attention to it, analyze the reasons, and take certain measures to effectively avoid condensation in insulating glass. reveal the occurrence of problems.

^{Figure 2 The condensation on the inner layer of plastic steel insulating glass windows}

1. Analysis of causes of condensation inside insulating glass

The definition of condensation inside the air layer of insulating glass is the phenomenon of condensation water appearing on the surface of the structure when the internal surface temperature of the air layer of insulating glass is lower than the dew point temperature of the nearby air. The key to condensation is the dew point temperature of the moist air. The dew point of the insulating glass refers to the temperature when the air humidity sealed in the air layer reaches a saturated state. When the surface temperature is lower than this temperature, the water vapor in the air layer will be inside the glass. Surface condensation or frost (condensation occurs when the inner surface temperature of the glass is higher than 0°C, and frost occurs when it is lower than 0°C). The dew point has a one-to-one correspondence with the moisture content and relative humidity in the air. The higher the moisture content, the higher the dew point temperature. 

Table 1 The correspondence table for dew point, relative humidity and moisture content

The relevant industry standard "Insulated Glass" stipulates that the dew point of insulated glass is -40°C. According to this regulation, plastic steel insulated glass windows for construction should not have inner layer condensation or frost problems in daily use. The reasons for this phenomenon can be attributed to the rising dew point of the inner air layer. The reason why the dew point of the air layer of insulating glass rises is mainly because external moisture enters the air layer but is not absorbed by the desiccant. Specifically, there are the following three reasons that can cause the dew point of insulating glass to rise.

1.1 The sealant is not squeezed tightly or contains mechanical impurities

The insulating glass and the aluminum tube spacer are bonded with two layers of elastic sealant. The first layer is sealed with butyl extruder, and the second layer is sealed with structural silicone extruder. In the actual production process, if there are mechanical impurities in the sealant or the extrusion is not tight during the coating process, capillaries will exist inside the colloid, and under the action of the pressure difference or humidity difference between the inside and outside of the spacer layer, moisture in the air will enter the air layer. Increase the water content in the insulating glass spacer layer.

1.2 Water vapor enters the air layer through the sealant

Sealants are generally homogeneous polymers, and polymers are not airtight. Due to the fugacity difference (pressure difference or concentration difference) on both sides of the sealant, it becomes the driving force for the isothermal diffusion of the polymer. For the sealant of insulating glass, the main diffuser is moisture in the air. The diffusion of moisture follows the following relationship:

J=P/L·Δp 

(1)In the formula, J——diffusion rate, refers to the diffusion amount of gas through a certain thickness of polymer per unit time and unit area;

L——polymer thickness;

P——gas permeability coefficient, which is an inherent physical property of the material;

Δp——The difference in gas partial pressure on both sides of the polymer.

It can be seen from equation (1) that the factors affecting water vapor diffusion are mainly the gas permeability coefficient (air tightness) of the polymer; the thickness of the adhesive layer and the difference in water vapor partial pressure between the inside and outside of the air. Moisture diffusion is the main reason for the failure of insulating glass.

1.3 The effective adsorption capacity of the desiccant is low

The requirement for the desiccant is not only to absorb the moisture in the air that is sealed in the insulating glass sealing unit during the assembly process, so that the insulating glass has a qualified initial dew point, but also to continuously absorb the moisture that diffuses into the air layer through the sealant layer. Continue to maintain a dew point that meets usage requirements. If the desiccant has poor adsorption capacity and cannot effectively absorb moisture that enters the air layer through diffusion, it will cause moisture to accumulate in the air, increase the moisture pressure, and increase the dew point of the insulating glass.

2. Measures to avoid condensation on the inner 

To extend the service life of insulating glass, the dew point rise of insulating glass must be strictly controlled and controlled from all aspects.

2.1 Strictly control the humidity of the production environment

The humidity of the production environment mainly affects the residual adsorption capacity of the desiccant. The residual adsorption capacity means that after the insulating glass is sealed, the desiccant absorbs the moisture in the air layer to make the initial dew point reach the required level. The desiccant also has an adsorption capacity. This part of the adsorption capacity is called the residual adsorption capacity or the remaining adsorption capacity. The function of the remaining adsorption capacity is to continuously adsorb moisture that diffuses from the surroundings into the air layer. The size of the remaining adsorption capacity determines the amount of water adsorption that diffuses into the air layer during the use of the insulating glass. It also determines how quickly water accumulates in the air layer, thus determining the effective use of the insulating glass. The length of time. So, what is the appropriate relative humidity level to control in the insulating glass production workshop? Based on the above point of view and the analysis of preliminary data obtained from foreign production tests, it is more scientific and reasonable to adopt the humidity balance method. First, make sure to use enough desiccant to remove the moisture that enters the air insulation layer of the insulating glass during production, and the moisture that enters the insulation layer of the insulating glass during the service life of the insulating glass. According to analysis and relevant foreign data, the relative humidity is appropriate between 50% and 55% (20±1°C).

2.2 Reduce the diffusion of moisture through the sealant

Choose a sealant with a low permeability coefficient. It can be seen from equation (1) that the diffusion amount of moisture through the sealant is proportional to the gas permeability coefficient. Therefore, choosing an insulating glass sealant with a low gas permeability coefficient is one of the effective measures to reduce the gas diffusion rate. Commonly used sealants in insulating glass production include butyl extruder coating, polysulfide extruder coating, and silicone extruder coating. Their gas permeability coefficients are butyl extruder 1 to 1.5 g/m²·d·cm, polysulfide extruder 7 to 8 g/m²·d·cm, and silicone extruder 10 to 15 g/m²·d·cm. It can be seen that butyl extruder has the smallest gas permeability coefficient, so due to the use of butyl extruder, the effective service life of double-pass sealed glass is better than that of single-pass sealed insulating glass. The sealant for single-pass sealed insulating glass should be polysulfide rather than silicone sealant. 

Reasonably determine the thickness of the glue layer. It can be seen from formula (1) that the amount of gas diffusion through the polymer is inversely proportional to the thickness of the glue layer. The thicker the glue layer, the smaller the diffusion amount. Therefore, international industry standards stipulate that when using double-layer sealant to make insulating glass, the thickness of the glue layer is 5~7mm. When using a single layer of sealant, the thickness of the glue layer is 8±2mm. Ensuring the thickness of the adhesive layer is also an important part of reducing the diffusion of water vapor.

Reduce the humidity difference between the inside and outside of the insulating glass glue layer. It can be seen from formula (1) that reducing the water vapor partial pressure difference inside and outside the insulating glass can reduce the diffusion of water vapor through the buytl extruder layer. As for the insulating glass, the lower the humidity (water vapor partial pressure) of the air layer, the better. To reduce Δp, Only by reducing the humidity (or water vapor partial pressure) of the external environment, you can open drainage holes in the installation frame so that the accumulated water flowing along the glass surface to the inside of the frame can be quickly drained, thereby keeping the periphery of the glass dry and extending the length of the insulating glass. effective usage time.

2.3 Reduce the contact time between desiccant and atmosphere

Shorten the production process time, minimize the contact time between the desiccant and the atmosphere, and improve the remaining adsorption capacity of the desiccant.

2.4 Reasonably controlled air guide gaps

The desiccant is generally poured into the spacer frame under a sealed condition. The adsorption of water in the atmosphere is carried out through the air guide gap. The larger the air guide gap is, the faster the water absorption rate of the dry explosive agent will be, and the effective adsorption capacity will be lost. Therefore, the air guide gap of the insulating glass spacer frame is required to be as small as possible, but it is necessary to ensure that the insulating glass meets the initial dew point required by the standard.

2.5 Select a desiccant with appropriate adsorption rate

Appropriate selection of the adsorption rate of the desiccant, reasonable packaging and transportation, minimizing glass breakage, etc. are some very effective measures. In addition, it is important to point out that there is currently a phenomenon in the market where double-layer glass is used to replace insulating glass. When builders use it, they find condensation failure on the inner layer. The cause of condensation on the inner layer of double-layer glass is related to the above-mentioned problem of the inner layer of insulating glass. The reasons for layer condensation are different. Double-glazed glass generally uses double-sided stickers or other methods. Two pieces of pre-cut glass are spaced at a certain distance, and then bonded and fixed, and then sealed with a second layer of glue to form a double-glazed window. The reason for condensation on the inner layer of this kind of window is that there is no desiccant to absorb moisture in the inner layer of the double-glazed glass, so the relative humidity of the air sealed in the inner layer of the glass is the same as the relative humidity in the production workshop. The humidity is higher and the dew point is higher, which makes it easy to Condensation. If the double-glazed glass is tightly sealed, the dew point temperature of the air inside the double-glazed glass corresponds to the temperature and humidity of the production workshop (see Table 2).

Table 2 Relationship table between air dew point temperature, production workshop temperature and relative humidity in the inner layer of double-glazed glass

It can be seen from Table 2 that under the same relative humidity, the dew point temperature of the air inside the double-glazed glass increases with the increase of the ambient temperature; at the same temperature, the dew point temperature of the air inside the double-glazed glass increases with the relative humidity in the production workshop. The dew point of the inner air is greatly affected by changes in relative humidity in the production workshop. Therefore, even if double-glazed glass is produced, the temperature and humidity in the production workshop should be strictly controlled. If the dew point temperature of double-glazed glass is 25°C and the relative humidity is 60%, the dew point temperature of the air inside the glass is 16.7°C. During the use of such windows, when the ambient temperature changes, the interlayer air is sealed There is no desiccant to absorb moisture, which means that when the ambient temperature is lower than 16.7°C, condensation will appear on the inner glass of the double-glazed glass. Temperatures of 16.7°C are common in spring and autumn in high latitudes in the northern hemisphere, not to mention winter. Therefore, if such windows are used in high latitudes in the northern hemisphere, condensation will occur on the inner layer on a large scale.

3. The conclusion

Among the building's exterior structural components such as doors, windows, exterior walls, roofs, and floors, doors and windows have the worst thermal insulation performance and consume the most heat through doors and windows. They are the weakest link in building energy conservation. Therefore, it is important to improve the thermal insulation performance of doors and windows. The focus of building energy conservation. Insulating glass is the main product that saves 50% of energy in public buildings and 65% in residential buildings. Improving its quality is the top priority of today's building energy-saving technology. Therefore, by controlling various aspects such as material selection, processing and manufacturing, and process environment, we can prevent insulating glass from condensation failure, extend its service life, and reduce maintenance costs. This can not only bring economic benefits but also achieve better social benefits.