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

According to statistics, the energy consumption of buildings in developing countries accounts for about 30% of the total national 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. The energy consumption of building heating in the high latitude and cold regions of the northern hemisphere has accounted for more than 20% of the total local energy consumption. For the whole building, the area of doors and windows (the entry "doors and windows" is provided by Google) accounts for more than 20% of the building area, and glass accounts for more than 70% of it. From the perspective of energy saving, the energy consumption of the entire building is Among them, heat dissipation through doors and windows accounts for about 50%, and the proportion is very high. In particular, the heat transfer and airtightness of doors and windows in village buildings are the weakest links in the entire outer protective structure. Therefore, it is an urgent task for us to reduce the energy consumption of building doors and windows and improve the thermal insulation and airtightness of building doors and windows. With the deepening of building energy-saving work, a large number of non-energy-saving village and town buildings will also use insulating glass for exterior doors and windows, and the market potential is huge. Among them, the use of insulating glass is increasing day by day. Improve the thermal insulation and durability of insulating glass and avoid its function Failure is of great significance to the building energy efficiency 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 the failure is that the dew point in the air layer of the insulating glass rises, and the internal dew condensation (see Figure 2), accounts for 63% of the total. %; the second is the rupture of insulating glass, accounting for 26%, these two reasons constitute 89% of the total failure, 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. It must be paid great attention, analyze the reasons, and take certain measures to effectively avoid the condensation of insulating glass. the occurrence of the problem.

^{Figure 1 Condensed water appears on the surface of the insulating glass structure}

1. Analysis of the causes of condensation inside the insulating glass

The definition of dew condensation inside the air layer of insulating glass is the phenomenon that condensation water appears on the surface of the structure when the internal surface temperature of the air layer of the insulating glass is lower than the dew point temperature of the nearby air (the entry "condensation water" is provided by Google). The key to dew condensation is the dew point temperature of the humid air. The dew point of the insulating glass refers to the temperature when the humidity of the air sealed in the air layer reaches a saturated state. When the temperature of the surface layer is lower than this temperature, the water vapor in the air layer will be in the glass. Condensation or frost on the surface (condensation occurs when the inner surface temperature of the glass is higher than 0°C, and frost occurs when the temperature is lower than 0°C). There is a one-to-one correspondence between the dew point and the moisture content in the air and relative humidity (the entry "relative humidity" is provided by Google). The higher the moisture content, the higher the temperature of the dew point. See Table 1.

^{Table 1 The Correspondence diagram of dew point, relative humidity and moisture content in the air}

The relevant industry-standard "Insulating Glass" stipulates that the dew point of insulating glass is -40°C. According to this regulation, there should be no condensation or frost on the inner layer during daily use. The reason for this phenomenon is It can be attributed to the rise in the dew point of the inner air layer. The reason for the rise of the dew point of the air layer of the insulating glass is mainly because the moisture from the outside enters the air layer and 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 properly or contains mechanical impurities

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

1.2 The water vapor enters the spacer air layer through the sealant

The sealant is generally a homogeneous polymer (the entry "polymer" is provided by Google), and the polymer is not airtight. Due to the existence of fugacity difference (pressure difference or concentration difference) on both sides, it becomes The polymer that acts as the driving force for isothermal diffusion. For the sealant of insulating glass, the main diffuser is moisture in the air, and 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 is the gas permeability coefficient, which is an inherent physical property of the material;

Δp—the gas partial pressure difference across the polymer.

It can be seen from formula (1) that the main factors affecting the water vapor diffusion are the gas permeability coefficient (air tightness) of the polymer; the thickness of the adhesive layer and the water vapor partial pressure difference inside and outside 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 sealed in the air by 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 diffused into the air layer through the adhesive layer. Continue to maintain a dew point that meets user requirements. If the adsorption capacity of the desiccant is poor, it cannot effectively absorb the water entering the air layer through diffusion, which will cause the water to accumulate in the air, the water pressure will increase, and the dew point of the insulating glass will rise.

2. Measures to avoid condensation on the inner layer

To prolong the service life of the insulating glass, the rise of the dew point of the insulating glass must be strictly controlled, which needs to be controlled from all aspects.

2.1 Strictly control the humidity of the production environment

The humidity of the production environment is mainly the residual adsorption capacity that affects the effective adsorption capacity of the desiccant. 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 meet the requirements, and the desiccant also has adsorption capacity, which is called residual adsorption capacity or residual adsorption capacity. The role of the remaining adsorption capacity is to continuously adsorb the water diffused into the air layer from the periphery. The size of the remaining adsorption determines the amount of water absorbed into the air layer through diffusion during the use of the insulating glass, which also determines the speed of the water accumulation in the air layer, thus determining the effective use of the insulating glass. length of time. So, what is the appropriate relative humidity control for the insulating glass production workshop? According to the above viewpoints and the preliminary data obtained from foreign production tests, it is more scientific and reasonable to use the humidity balance method. First of all, it is necessary to ensure that enough desiccant is used to remove the moisture that enters the air insulation layer of the insulating glass during production, and the moisture that enters the insulating glass insulation layer during the service life of the insulating glass. According to the analysis and relevant foreign data, the relative humidity is 50% to 55% (20 ± 1°C).

^{Figure 2 The production environment of insulating glass producing} 

2.2 Reduce the diffusion of moisture through the sealant

Choose a sealant with a low permeability coefficient. It can be seen from formula (1) that the diffusion amount of water through the sealant is proportional to the gas permeability coefficient. Therefore, selecting an insulating glass sealant with a low gas permeability coefficient is one of the effective measures to reduce the gas diffusion rate. The commonly used sealants for the production of insulating glass are butyl rubber (the entry "rubber" is provided by Google), polysulfide rubber, and silicone rubber. Their gas permeability coefficients are 1-1.5 g/㎡·d·cm for butyl rubber, 7-8 g/㎡·d·cm for polysulfide rubber, and 10-15 g/㎡·d·cm for silicone rubber. It can be seen that the gas permeability coefficient of butyl rubber is the smallest, so the effective service life of double-channel sealing glass is better than that of single-channel sealing insulating glass due to the use of butyl rubber. The sealant of single-channel sealed insulating glass should use polysulfide rubber instead of silicone rubber.

Reasonably determine the thickness of the adhesive layer. From equation (1), it can be seen that the amount of gas diffusion through the polymer is inversely proportional to the thickness of the adhesive layer. The thicker the adhesive layer, the smaller the amount of diffusion, so insulating glass industry standards stipulates: when using double-layer sealant, the thickness of the adhesive layer is 5-7 mm; when using a single-layer sealant, the thickness of the adhesive layer is 8±2 mm to ensure the adhesive layer Thickness 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 adhesive layer. From formula (1), it can be seen that reducing the water vapor partial pressure difference inside and outside the insulating glass can reduce the diffusion of water vapor through the adhesive layer. As an 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 partial pressure of water vapor) of the external environment, it is possible to open a drainage hole on the installation frame, so that the accumulated water flowing along the glass surface to the inside of the frame can be quickly drained, to keep the periphery of the glass dry and prolong the insulating glass. effective usage time.

2.3 Reduce the contact time between the desiccant and the atmosphere

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

2.4 Air-guiding gaps with reasonable control intervals

The desiccant is generally poured into the spacer frame under the condition of sealing. The adsorption of water in the atmosphere is carried out through the air guide gap. The larger the air guide gap, the faster the water absorption rate of the dry blasting agent, and the loss of effective adsorption capacity. Therefore, the air 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 an appropriate adsorption rate

Appropriate selection of the adsorption rate of the desiccant, reasonable packaging and transportation, and small glass breakage are some very effective measures. In addition, it is important to point out that there is a phenomenon of replacing the insulating glass with double-layer glass in the market at present. When Party A uses it, it is found that there is a phenomenon of dew condensation on the inner layer. The causes of condensation are different. Double-sided glazing generally adopts double-sided stickers or other methods, and two pieces of pre-cut glass are separated by a certain distance, then bonded and fixed, and then sealed with secondary glue to form a double-glazed window. The reason for the condensation on the inner layer of the window is that there is no desiccant in the inner layer of the double-layer glass to absorb moisture so that the relative humidity of the air sealed in the inner layer of the glass is the same as the relative humidity of the production workshop. condensation. If the double glazing is tightly sealed, the dew point temperature of the air in the inner layer of the double glazing corresponds to the temperature and humidity of the production workshop (see Table 2).

^{Table 2 The relationship between the air dew point temperature of the inner layer of the double-glazed glass, the temperature of the production workshop and the relative humidity}

It can be seen from Table 2 that under the same relative humidity, the air dew point temperature of the inner layer of the double glazing increases with the increase of the ambient temperature; at the same temperature, the dew point temperature of the inner layer of the double glazing increases with the relative humidity of the production workshop. The change of relative humidity in the production workshop has a great influence on the dew point of the inner air. Therefore, even in the production of double-layer glass, the temperature and humidity in the production workshop should be strictly controlled. If the dew point temperature of the double glazing is 25℃ and the relative humidity is 60% in the double glazing production workshop, the dew point temperature of the air inside the glass is 16.7℃. During the use of such windows, when the ambient temperature changes, the interlayer air is sealed. And there is no desiccant to absorb moisture, that is to say, when the ambient temperature is lower than 16.7 ℃, condensation will appear on the inner glass of the double glazing. The temperature of 16.7 °C is relatively common in spring and autumn in high latitudes of the northern hemisphere, not to mention winter. Therefore, if such windows are used in high latitudes of the northern hemisphere, there will be a large area of dew condensation on the inner layer.

3. Conclusion

Among the external protective structural components such as doors and windows, exterior walls, roofs, and floors of buildings, doors, and windows have the worst thermal insulation performance and consume the most heat through doors and windows, which is the weakest link in building energy conservation. Therefore, improving the thermal insulation performance of doors and windows is the best way to focus on building energy efficiency. 

As the main product of 50% energy saving in public buildings and 65% energy saving in residential buildings, insulating glass is the top priority of today's building energy-saving technology to improve its quality. Therefore, by controlling various links such as material selection, processing, and manufacturing, process environment, etc., it can prevent the dew condensation failure of insulating glass, prolong its service time, and reduce maintenance costs, which can not only bring economic benefits but also obtain better social benefits...

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