The cause analysis of insulating glass failure and The insulating glass production process control.

1. The preface

The insulating glass has been widely used in buildings as energy-saving transparent structural parts of building exterior walls and has many applications in other industries such as trains, airplanes, refrigeration equipment, greenhouses, and other industries. Due to insufficient attention paid to the sealing of insulating glass in the production process, it is very easy to cause the sealing failure of insulating glass and lose the energy-saving performance of insulating glass. To ensure the long-term effective use of insulating glass, the international industry standard "Insulating Glass" stipulates that there are clear requirements for the life of the insulating glass, and the expected service life of insulating glass should be at least 15 years. The international industry standard "Insulating Glass" gives an exact definition of the service life: the end of the service life of the insulating glass means the failure of the insulating glass, or the generation of visible water vapor in the cavity of the insulating glass is the failure of the insulating glass. The standard clearly explains the cause of failure: the moisture in the environment where the insulating glass is used continuously penetrates from the edge of the insulating glass into the hollow cavity, and the desiccant in the edge sealing system tends to be saturated due to the continuous adsorption of water molecules and eventually loses moisture. The adsorption capacity leads to the increase of water vapor content in the cavity of the insulating glass, and when the dew point is higher than the specified temperature, it is the cause of the failure of the insulating glass.

As a component, insulating glass is widely used in glass curtain wall construction, home door, and window building materials, train windows, refrigerator doors, etc. Once the hollow seal fails after installation, it will be very difficult and expensive to replace the glass. To ensure the long-term effective use of insulating glass, glass deep-processing manufacturers should take corresponding measures to ensure that the insulating glass has a long enough effective use time to meet the needs of various purposes.

2. The main reason for the failure of insulating glass

Insulating glass is a glass product sealed between two or more pieces of glass, and a desiccant is used to absorb the moisture in the air layer of the airtight hollow cavity. The direct cause affecting the effective service time of insulating glass is the gathering speed of moisture in the middle layer. Many factors affect the rate of moisture accumulation in the middle layer, such as material performance, manufacturing process, and control, installation method, environmental aging, etc. The dew point of insulating glass refers to the temperature when the air humidity in the air layer reaches saturation. Below this temperature, the water vapor in the air layer will condense into liquid or solid water. The corresponding relationship between the dew point and the relative humidity of the air and the water content in the air is shown in Table 1.

Table 1 Relationship between dew point and relative humidity and water content

Generally speaking, the international industry standard "Insulating Glass" stipulates that the dew point temperature is <-40°C. Obviously, the higher the water content, the higher the dew point temperature of the air. When the temperature of the inner surface of the glass is lower than the dew point of the air in the air layer, the moisture in the air in the insulating glass cavity will accumulate in the insulating glass cavity, causing the dew point to rising. When the ambient temperature drops and the temperature of the inner surface of the glass are lower than the dew point of the air layer, the water vapor in the air layer will condense or frost on the inner surface of the glass (when the temperature of the inner surface of the glass is higher than 0°C, condensation will be formed when the temperature is lower than 0°C frost).

^{Figure 1 The dew point of inner surface of insulating glass 1}

Condensation or frost on the inner surface of the glass will seriously affect the perspective of the insulating glass and reduce the heat insulation effect of the insulating glass. At the same time, long-term condensation will cause mildew on the inner surface of the glass or return to alkali to produce white spots. , seriously affect the use of insulating glass.

The rise of the dew point of insulating glass is mainly because the moisture from the outside enters the air layer and cannot be absorbed by the desiccant. The following three reasons can cause the dew point of insulating glass to rise:

(1) Impurities in the sealant or capillary pores that exist due to improper extrusion during the injection process 

Under the action of pressure difference or concentration gradient inside and outside the air layer, water in the air enters the air layer through gas circulation or diffusion In the process, the moisture content in the air layer of the insulating glass is increased.

(2) Moisture diffuses into the air layer through the polymer (butyl sealant is generally a high molecular polymer).

Any polymer is not airtight, and the sealants usually used for insulating glass include butyl rubber, polysulfide glue, silicone structural adhesive, etc. For these polymer materials, the existence of fugacity difference (pressure difference or concentration difference) on both sides constitutes the driving force for the isothermal diffusion of the polymer. On the higher fugacity side, polymer molecules absorb gas molecules (air and water) into the solid polymer, move through the polymer chains, and from the other side of the polymer - the lower fugacity side. side released. For the sealant of insulating glass, the main diffusion is the moisture in the air. The diffusion of water follows the relationship (1):

J = P / L*△P   (1)

In the formula: 

J is the diffusion rate, which refers to the diffusion rate of gas passing through a polymer with a certain thickness per unit area per unit time; 

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

L is the thickness of the polymer; 

△P is the gas partial pressure difference across the polymer. 

It can be seen from the above formula that the factors affecting the diffusion of water vapor are mainly 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 layer. The diffusion of water vapor is the main cause of the failure of insulating glass.

(3) The effective adsorption capacity of the desiccant is low.

The effective adsorption capacity of the insulating glass desiccant refers to the adsorption capacity of the desiccant after it is sealed in the air layer. It is a function of factors such as the performance of the desiccant, air humidity, filling capacity, and time in the air. The desiccant sealed in the air layer of the insulating glass has two main functions. One is to absorb the moisture sealed in the air during production so that the insulating glass has a qualified initial dew point; the other is to continuously absorb moisture from the environment through the seal. The glue penetrates the moisture in the hollow layer so that the insulating glass always has a dew point that meets the requirements of use. Therefore, the desiccant is required to have a strong adsorption capacity. If the adsorption capacity of the desiccant is poor, it cannot effectively absorb the moisture that enters the air layer through diffusion, which will cause moisture to accumulate in the insulating glass cavity, the moisture pressure will increase, and the dew point of the insulating glass will rise.

^{Figure 2 The insulating glass molecular sieve desiccant filling 1}

3. The process control of insulating glass production

3.1 Strictly control the humidity of the production environment and the exposure time of molecular sieves in the air

The humidity of the production environment mainly affects the effective adsorption capacity and residual adsorption capacity of the desiccant. The remaining adsorption capacity means that after the insulating glass is sealed, the desiccant absorbs the moisture in the airtight cavity to make the initial dew point meet the requirement. Before the insulating glass is assembled, the molecular sieve that has been poured into the aluminum strip is exposed to the ambient air in the workshop. The molecular sieve in the aluminum strip is constantly absorbing the moisture in the ambient air in the workshop. After the assembly, the molecular sieve in the aluminum strip has a certain adsorption capacity after the moisture is sucked in and sealed, and this part of the adsorption capacity is called the remaining adsorption capacity. The remaining adsorption capacity is equal to the effective adsorption capacity minus the adsorption capacity after desiccant absorbs and consumes water in the air sealed in the air layer.

The function of the remaining adsorption capacity is to continuously absorb the moisture that infiltrates into the air layer of the insulating glass cavity from the periphery of the closed insulating glass cavity. The size of the remaining adsorption capacity determines the moisture adsorption capacity of the insulating glass desiccant to the insulating glass penetrating the air layer of the hollow cavity through the edge sealant during use. The size of the adsorption capacity also determines the speed of moisture accumulation in the air, thus determining the effective use time of the insulating glass. The improvement of residual adsorption capacity is more necessary to meet the national standard accelerated durability test.

When the humidity of the insulating glass production environment is too high, first of all, there is a lot of moisture sealed in the air, and the desiccant consumption will increase the adsorption capacity, and its remaining adsorption capacity will be small. It can be seen from Table 1 that the greater the humidity of the air, the higher the water content. When the relative humidity of the environment increases from 40% to 80%, the moisture content in the air doubles. 

Secondly, ambient humidity has a great influence on the adsorption rate of the desiccant. The higher the humidity, the faster the adsorption rate of the desiccant. In the production process, the general desiccant will be exposed to the air for some time. During this period, the adsorption capacity of the desiccant is proportional to the ambient humidity, and the remaining adsorption capacity of the desiccant decreases with the increase in humidity. 

Therefore, the influence of humidity on the effective use time of insulating glass is very important. To prolong the effective use time of insulating glass, it is necessary to control the humidity of the production environment to be lower.

^{Figure 3 The production environment of insulating glass processing 1}

3.2 Reduce the chance of water vapor penetrating through the edge seal

(1) Choose a sealant with a low permeability coefficient

Selecting an insulating glass sealant with a low water vapor permeability coefficient is one of the effective measures to reduce the water vapor penetration rate. Commonly used sealants for insulating glass production are butyl rubber, polysulfide rubber, and silicone structural adhesive, etc., the water vapor permeability, of butyl rubber: is 1~1.5 g/m*d*cm, polysulfide rubber: is 7~ 8 g/m*d*cm, silicone structural adhesive: 10~15 g/m*d*cm. It can be seen that the water vapor permeability coefficient of butyl rubber is the smallest, so due to the use of butyl rubber in double-sealed glass, its effective service life is better than that of single-sealed insulating glass. Polysulfide glue should be used as much as possible for the double-channel sealing insulating glass sealant of non-hidden frame curtain walls; the double-channel sealing of silicone structural adhesive must be used for the insulating glass of hidden frame curtain wall.

(2) Reasonably determine the thickness of the glue layer

It can be seen from formula (1) that the amount of gas infiltrated through the sealant is inversely proportional to the thickness of the adhesive layer. The thicker the adhesive layer, the lower the permeability. Therefore, the national standard stipulates that when using double-channel sealant, the adhesive thickness of the layer is 5~7 mm. Ensuring the thickness of the adhesive layer is also an important link to reducing water vapor penetration. During production, the thickness and uniformity of the adhesive layer must be guaranteed.

(3) Process control of key parts sealing butyl rubber

For the production of insulating glass with double-channel sealant, it is more important to control the thickness between the spacer frame and the glass, because the metal spacer frame can be considered impermeable to water vapor, and water vapor must pass through the gap between the glass and the frame. The thinner the thickness between the spacer frame and the glass, the lower the water vapor permeability. The water vapor permeability of the above 2.2 (1) butyl rubber is only the lowest of 1~1.5 g/m*d*cm, and the key seal of the insulating glass is this butyl rubber, so the material and production process of the butyl rubber should be well controlled It is the key to determine the sealing performance of insulating glass.

How to control the first sealing of insulating glass?

One is to strictly control the material selection process of butyl rubber. The selection of high-quality butyl rubber starts from the following aspects. The first smell, the second look, and the third experiment are effective methods for selecting high-quality butyl rubber without testing equipment.

One smell: After heating the sol, smell whether the butyl rubber has a pungent smell of waste rubber;

Second look: See if the butyl rubber is shiny (quality) after the glue is applied, and whether the temperature of the sol exceeds 120c (bonding strength);

Third, do more experiments:

(1) Drawing experiment:

When the butyl rubber is pulled to a filament with a diameter of less than 0.5 mm, let go to see if the butyl rubber will rebound.

(2) Raab experiment:

Take a piece of butyl rubber and pull it into a cloth with a thickness of 0.01 mm to see if the butyl rubber is still cloth-like or has ants. The main purpose is to test the fineness and compactness of the butyl rubber.

(3) Glue coating experiment:

Take an aluminum frame, adjust the glue width of the automatic insulating glass butyl rubber coating machine to 3 mm, the thickness of the glue to 0.8 mm, and the temperature of the glue nozzle to be above 120℃. Use the automatic insulating glass plate-pressing production line to carry out the plate pressing. After the plate pressing, check whether the butyl glue can fully press the aluminum strip, whether the total thickness of both sides of the butyl rubber after pressing the plate is within 0.5 mm, and check the ductility of the butyl rubber; it is most suitable to be able to press the aluminum strip and not exceed the aluminum strip, keep the equipment parameters unchanged and then take one-meter-long coating glue the aluminum strips. After gluing, tear off the butyl rubber on both sides of the aluminum strips, including the glue head and tail, and weigh them. Check the specific gravity of the glue and calculate the cost of glue per square meter of glass. Generally, high-quality glue is less than 6 per linear meter. 

In terms of process, we must pay attention to the uniform and continuous glue application of butyl glue, and the gap between the glue nozzle of the butyl glue applicator and the aluminum strip is about 1mm. The glue head and tail must be pinched to the glue-coated surface at the corner of the aluminum strip, and there must be no gaps in the corner where the glue is broken, to ensure a seamless connection between the aluminum strip and the glass. The back of the middle interface of the bent aluminum strip should be sealed with butyl glue, and when the aluminum strip is plugged in, not only the glue-coated surface at the corner will not be glued, but also the interface between the aluminum strip and the plug-in piece should be sealed with butyl glue, and the insulating glass plate that has been laid together The pressure must be uniform up, down, left, and right, and the butyl rubber must be fully pressed on the aluminum strip without showing white. In this way, the airtightness of the insulating glass will be very good, and the service life of the insulating glass will reach the service life required by the national standard.

(4) Reduce the humidity difference between the inside and outside of the insulating glass glue layer

The amount of gas diffusion is proportional to the water vapor partial pressure difference inside and outside the insulating glass, reducing the water vapor partial pressure difference inside and outside the air layer can reduce the amount of water vapor diffusing through the glue layer. As insulating glass, the lower the humidity (water vapor partial pressure) of the air layer, the better. To reduce the water vapor partial pressure difference, only reduce the humidity (or water vapor partial pressure) of the external environment.

During the installation process of insulating glass, drain holes can be opened on the installation frame, so that the accumulated water flowing along the glass surface to the inside of the frame can be quickly discharged, to keep the periphery of the glass dry and prolong the effective use time of the insulating glass.

(5) Select high-quality outer sealant and use mechanical injection to seal

The high-quality outer sealant has less filler, fewer mechanical impurities, and less chance of micropores. Mechanical glue injection (Automatic insulated glass sealing robot or Semi-automatic sealing spreading  table applicator + two-component glue gun for secondary sealing of insulating glass) can avoid extrusion. The capillary pores that are not compacted can prevent water vapor from passing through.

^{Figure 4 The main parts of automatic insulating glass sealant sealing robot machine}

3.3 Shorten the production process time

Shortening the production process time refers to minimizing the time that the desiccant is in contact with the atmosphere and reducing the loss of adsorption capacity so that the desiccant has a higher adsorption capacity. Taking the northern hemisphere as an example, the aluminum spacer bars filled with molecular sieves should not be exposed to the atmosphere for more than 2 hours in high-latitude dry areas, and should not exceed 30 minutes in high-temperature and high-humidity seasons in low-latitude areas or high-latitude areas.

3.4 Reasonably control the air guide gap of the interval frame

The desiccant is generally poured into the interval frame in a sealed condition, and the water in the atmosphere is adsorbed through the air guide slot. The larger the air guide gap, the faster the water absorption rate of the desiccant, and the greater the loss of effective adsorption capacity. Therefore, the air guide gap of the insulating glass spacer is required to be as small as possible. As long as the insulating glass can be guaranteed to have an initial dew point that meets the requirements of the standard, it is more appropriate.

3.5 Choose a desiccant with a large adsorption capacity

The greater the adsorption capacity of the desiccant, the stronger the water vapor adsorption capacity for insulating glass. The adsorption capacity of molecular sieves is much greater than that of silica gel and alumina. The adsorption capacity of molecular sieves is about 4 times that of silica gel or alumina. The deep adsorption capacity is even lower, so try to use a 3A molecular sieve as the insulating glass desiccant.

4. The conclusion

To improve the service life of the insulating glass and prolong the effective use time to meet the standard requirements, it should be controlled from various links such as material selection, processing, manufacturing, structural design, and installation.

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