Analysis of the effect of desiccant on fogging and condensation of insulating glass.

Abstract: There are many reasons for the fogging and condensation of hollow glass, and the desiccant is the most important part. This article comprehensively discusses the reasons for the fogging and condensation of insulating glass from the three aspects of auxiliary material quality, production technology, design and construction, and the influence of the key indicators of desiccant on the fogging and condensation of insulating glass, and points out that in the correct operation Under the conditions of craftsmanship, reasonable design, and qualified auxiliary material selection, the quality problems of fogging and condensation of hollow glass can be avoided.

In order to achieve the goal of carbon peak and carbon neutrality, the requirements for building energy efficiency are becoming more and more stringent. As the first choice for energy-saving glass for doors and windows, insulating glass has a quality and life span related to the effect of building energy-saving and the total consumption of resources. problem. In my country, nearly 500 million square meters of insulating glass are applied to buildings every year, and the most common problem of insulating glass quality is fogging and condensation. After the hollow glass is fogged and condensed, the water vapor in the cavity will repeatedly condense and evaporate, which not only hinders the line of sight but also accelerates heat dissipation, causing it to lose the effect of heat preservation and energy-saving, resulting in a waste of energy. With the widespread use of insulating glass, the country will spend tens of billions of yuan every year to replace the insulating glass. This huge waste of resources and energy will seriously hinder the process of carbon peaking and carbon neutrality. Therefore, in order to avoid greater waste and conform to the new development concept of the country, it is necessary to deeply analyze the reasons for the fogging and condensation of insulating glass.

Influence of auxiliary material quality on fogging and condensation of insulating glass

The hollow glass dew point refers to the temperature required when the air humidity in the insulating glass compartment reaches a saturated state. The higher the moisture content, the higher the glass dew point temperature. When the temperature of the inner surface of the glass is lower than the dew point of the air in the spacer layer, moisture in the air will condense on the inner surface of the glass.

^{Analysis of the effect of desiccant on fogging and condensation of insulating glass 1}

In short, the conditions for condensation are: the air temperature is higher than the temperature of the condensed glass surface, and the temperature of the condensed glass surface is lower than the dew point, and the three are combined into one point. The reason is nothing more than the quality of auxiliary materials, production technology, design, and construction, etc. These aspects are analyzed below:

1. The influence of sealant on fogging and condensation of insulating glass

The main function of the sealant is to prevent moisture in the outside air from penetrating into the hollow glass cavity. That is, the smaller the water vapor penetration of the sealant, the less water vapor permeates into the inner cavity each year, and the desiccant can better exert its adsorption value and delay the fogging and condensation time of the glass. No matter how good the performance of the desiccant is, if the quality of the sealant is not good, after the desiccant is saturated with adsorption, the hollow glass will quickly lose dew condensation. In the same way, no matter how good the sealant is, it cannot completely prevent the penetration of water vapor. If 3A molecular sieve is not used or a poor quality desiccant is used, the hollow glass will quickly become fogged and condensed.

2. The influence of spacers on the fogging and condensation of insulating glass

The spacer is used to carry the desiccant and cooperate with the sealing function. The air permeability of the pores of the spacer will affect the working ability of the desiccant. If the pores are impermeable, the desiccant will not be able to absorb water normally. At the same time, the high-frequency welding on the back of the spacer should ensure that the weld is tight and without gaps, otherwise, it will increase the risk of water vapor penetration. Thirdly, the spacer should be straight, the side arc and wave curvature should not be more than 0.2%, the twist should not be more than 0.5mm, the corners should not be deformed, and the sides should be flat to ensure the sealing width and sealing effect of the butyl rubber.

3. Influence of Desiccant on Fogging and Condensation of Insulating Glass

Desiccant is the most important part. Its direct function is to absorb the water vapor in the inner cavity of the hollow glass to make the initial dew point meet the requirements, and then the remaining adsorption capacity continuously absorbs the water vapor that permeates into the inner cavity every year. It keeps the air Dry to ensure that the insulating glass continues to maintain a qualified dew point during service.

According to the industry-standard "desiccant for insulating glass", the desiccant is divided into A-type and B type. Class A desiccant: 3A molecular sieve, which has a good deep and effective water absorption capacity. Generally speaking, it is the adsorption capacity of a 3A molecular sieve for water vapor in a low humidity environment. Type B desiccant: Attapulgite clay is the main material, but most of the type B desiccants are doped with calcium chloride to achieve the purpose of improving water absorption. Calcium chloride will deliquesce and migrate after absorbing water and will cause chemical or electrochemical corrosion to the insulating glass metal spacers, butyl rubber, etc. After the aluminum spacer is corroded, the back welding will be opened, which will accelerate the penetration of water vapor; the calcium chloride penetrates the pores of the aluminum spacer and spreads to the surface of the sealant or the glass surface, which will accelerate the aging of the sealant and Low-E film And oxidation. Many provinces and local standards and restrictions prohibit the use of such calcium chloride products.
The effective adsorption of desiccant in different stages of insulating glass varies. From production to failure of insulating glass, it can be divided into three stages according to the change of the humidity and dew point of the inner cavity:

In the first stage (production period), the desiccant will consume less than 1% of the adsorption capacity. The air humidity in the inner cavity of the hollow glass is similar to the air humidity in the production environment when the sheets are combined, generally between 40% and 70% RH. At this time, the dew point temperature of the air is relatively high. After the glue is combined, the desiccant begins to effectively dry the air in the inner cavity, and the dew point of the insulating glass begins to drop rapidly. This stage lasts for 24 hours to 48 hours.

In the second stage (lifetime), 99% of the adsorption capacity is consumed. The desiccant continuously absorbs the moisture that penetrates into the cavity through the sealant. The humidity of the cavity is maintained at about 0.5%RH, and the dew point is maintained at about -40~60℃. Generally speaking, the quality of insulating glass auxiliary materials, insulating glass production process, and structure The influence of design and other factors are generally between 15 and 30 years.

In the third stage (expiration period), the desiccant no longer adsorbs moisture. The desiccant adsorption reaches saturation and no longer absorbs the moisture that penetrates into the cavity. The humidity and dew point of the air layer in the hollow glass cavity rise rapidly, and the hollow glass is prone to fog and condensation.

^{Table 1 Saturated water content in the air under different dew points}

It can be seen from Table 1 that when the dew point of the air layer in the inner cavity of the hollow glass is 0℃, the water content in the air is about 4.84g/m3. If the 3A molecular sieve is used to control the dew point to -40℃, it corresponds to the air content. The amount of water is about 0.117g/m3, a decrease of 97.6%; the dew point is -60°C, a further decrease of 90.6%.

For example, 1㎡ of 6+9A+6 insulating glass (1m×1m), according to Table 2 the desiccant filling amount is set to 100g, and the static water adsorption capacity of the national standard GB/T10504-2017 is calculated as 16%, then the 3A molecular sieve is saturated The adsorption capacity is:

16%×100g=16g

The inner cavity volume of 1㎡ 6+9A+6 hollow glass is 0.009m3, assuming an ambient temperature of 25℃ and relative humidity of 50%RH, the moisture content in the cavity after the insulating glass is combined is:

0.009m³ *23g/m³ ×50%=0.1035g

It can be seen that it only needs to consume 0.65% of the adsorption capacity of the 3A molecular sieve to completely absorb the moisture in the air layer of the inner cavity of the hollow glass and ensure the sufficient drying of the hollow glass. Let's assume again that if the moisture permeability of the 1㎡ hollow glass is assumed to be 0.27g/㎡·y per year, (16g-0.1035g) ÷ 0.27g/㎡·y=58 years, then other conditions are perfect, The use of 3A molecular sieve can guarantee the service life of insulating glass for more than 50 years.

Influence of production process on fogging and condensation of insulating glass

The production process of insulating glass consists of cleaning the glass, making the frame, filling the desiccant, applying butyl sealant, combining the sheets, and applying the second sealant. Every link will affect the quality of insulating glass.

The temperature and humidity of the production environment are not strictly controlled; the original glass is not cleaned thoroughly, sweat stains, water stains, and residual water droplets are stuck on the surface, resulting in the original glass not being dry; the bending equipment during frame making causes the back of the spacer to crack and increase water vapor penetration And so on, will cause the hollow glass to fog and condense.
The interval between filling the desiccant and the combined sheet is too long; the desiccant package is damaged; the desiccant filling amount is insufficient, if it is not carried out in accordance with the filling requirements specified in "Insulating Glass Production Regulations", it will also cause The hollow glass is fogged and condensed.

^{Table 2 Filling amount of desiccant}

The butyl sealant should be applied continuously and evenly. Before coating, check the flatness of the tempered glass and the quality of the spacer frame. The warped edges of tempered glass, the burrs at the joints of the spacer frame, and the convex corners of the bent aluminum strips will cause the butyl sealant to break and dew, which will increase the penetration of moisture and cause the hollow glass to fog and condense.

Influence of design and construction on fogging and condensation of insulating glass

Many irregularities in design and construction will also affect the fogging and condensation of insulating glass. For example, the aluminum profiles of doors and windows are not designed with drainage outlets; pre-stress is generated during installation or the sealing structure is damaged, resulting in air leakage of insulating glass; or hollow glass panels Unreasonable surface size and application design cause continuous changes in the internal and external cavity pressure of the glass, resulting in failure of the glass seal, etc.

Sum up

From the above analysis, it can be seen that there are many factors that cause the fogging and condensation of insulating glass. In actual production and application, the cause of fogging and condensation of insulating glass may be one factor, or it may be caused by multiple factors. Therefore, when encountering the problem of fogging and condensation in the production and application of insulating glass, it is necessary to clearly understand the mechanism of fogging and condensation; secondly, it is necessary to clarify which part of the insulating glass the problem occurs and then analyze the materials that cause the problem one by one. And technology, as well as the environmental conditions, design, and construction at the time and other factors; finally, after a comprehensive analysis, the various reasons for the fogging and condensation were confirmed by Jinan LIJIANG Glass, and improvement strategies were formulated to avoid the quality problems of the fogging and condensation of the insulating glass again.

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