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With the rapid development of the real estate industry in recent years, high-rise buildings have sprung up, the insulating glass used for glass curtain walls, residential doors, and windows is increasing day by day, and the architectural glass industry has also achieved unprecedented development. At the same time, the leading companies in the glass deep processing industry are constantly expanding and putting into production in line with market changes.

According to the industry standard insulating glass, the service life of insulating glass is directly related to the quality of edge materials (such as spacers, desiccants, sealants) and the manufacturing process of insulating glass. Spacer strips, desiccant, sealant, and glass form the edge sealing system of insulating glass, which determines the service life of the insulating glass. Qualified insulating glass is expected to have a service life of at least 15 years.

However, there are some unqualified insulating glass products on the market, and the service life is too short, not even more than seven or eight years. Once the insulating glass fails, it needs to spend more money to replace the glass, resulting in a huge waste of national resources. As one of the core materials of the edge sealing system of insulating glass, the desiccant is mainly used to keep the air layer inside the insulating glass dry and avoid fogging inside the insulating glass, which plays an important role in the life of the insulating glass.

Figure 1 How to avoid fogging inside the insulating glass 1

Figure 1 How to avoid fogging inside the insulating glass 1

According to the industry standard "Desiccant for Insulating Glass", according to different materials and production processes, the desiccant for insulating glass is divided into two categories: Type A desiccant 3A molecular sieve (the entry "molecular sieve" is provided by Google), Type B desiccant Desiccant (spherical dry material with attapulgite as the main body).

1. The powerful effect of desiccant on insulating glass

The desiccant in the insulating glass can not only absorb the moisture and residual organic matter in the insulating glass, so that the insulating glass will not be fogged below -60°C, but also balance the internal and external pressure differences due to the great changes in the temperature difference between day and night due to seasons or day and night, and prolong the insulating glass. service life. At present, more than 95% of the failure of insulating glass after being installed on the wall is caused by fog. Once the insulating glass is fogged, it means that the edge sealing system fails, which in turn causes the Low-E film of the Low-E glass to be oxidized and the appearance Functions such as color and energy-saving performance fail.

The desiccant achieves its function by deeply drying the air layer inside the insulating glass:

(1) Absorb the moisture in the air in the sealing cavity of the insulating glass; 

(2) Absorb the moisture that penetrates the insulating glass through the sealant in the later stage; 

(3) Use in combination with other special molecular sieves to absorb the volatilization released by the insulating glass sealant substance.

The fogging of insulating glass is also directly related to sealants and desiccants:

(1) The sealant mainly prevents the moisture in the outside air from diffusing into the insulating glass, and the function of the desiccant is to absorb the moisture diffused into the glass through the sealant; 

(2) If the sealing effect of the sealant fails, the desiccant will be saturated after adsorption. , the insulating glass will soon fog up, and the function will fail; 

(3) At the same time, no matter how good the sealant is, it cannot completely prevent the penetration of moisture. , the insulating glass will also fog up quickly.

2. Insulating glass has very strict requirements on desiccant

In daily life and industrial manufacturing, commonly used desiccants are calcium chloride, silica gel, clay desiccant (montmorillonite), molecular sieve, mineral desiccant (attapulgite), and composite desiccant.

As a desiccant for insulating glass, the requirements are more stringent:

(1) The ultra-low moisture content of the finished product can ensure that the desiccant has strong water absorption performance;

(2) The super deep adsorption capacity can ensure that the moisture in the glass interlayer is fully absorbed;

(3) The ultra-low powder falling degree ensures the beautiful light transmission performance of insulating glass;

(4) The ultra-low nitrogen adsorption capacity ensures that the insulating glass will not be deformed by concave and convex glass under different temperature changes;

(5) Qualified bulk density ensures that the filling amount of desiccant is controlled within the most economical and reasonable range;

(6) Reasonable acidity and alkalinity to ensure that the insulating glass spacer is not easy to be corroded, and the phenomenon of salting out occurs;

(7) Higher crushing strength, reducing the crushing during the filling process due to insufficient strength, forming slag, dust, and contaminating glass;

(8) Low static electricity to avoid the desiccant being adsorbed on the pipe wall and blocking the pipe when the machine is filled.

In the industry standard "Desiccant for Insulating Glass", there are also clear requirements for the technical indicators of the two types of desiccants, as shown in Table 1:

ProjectTechnical Requirement
 Class A Class B
Particle Size mmParticle Size mm
Crush resistance/(N/grain)-≥14.0≥20.0-≥14.0≥20.0
Static water adsorption/%RH=11.3%≥16.5≥11.0
Water absorption rate/%≤0.7≤0.5
Dust amount(NTU)≤30≤20
Bulk density/(g/ml)≥0.70
Static nitrogen adsorption/(mg/g)≤2.0≤1.0
Finished product moisture content/%≤2.0(Baking temperature 550 ℃ )≤2.0(Baking temperature 350 ℃)
Burn vector/%≤2.0(Baking temperature 950 ℃)≤2.0(Baking temperature 550 ℃)

Table 1 Technical requirements for desiccant for insulating glass

In the early days of the development of insulating glass, silica gel, clay minerals, etc. were used as insulating glass desiccants. Since these desiccants could not meet the quality requirements of insulating glass desiccants, they were quickly replaced by 3A molecular sieves with superior performance. However, with the advancement of technology, attapulgite desiccant with a more environmentally friendly and energy-saving production process has been gradually promoted and used in insulating glass.

3. Type 3A molecular sieve desiccant is suitable for insulating glass

A molecular sieve is a kind of crystalline aluminosilicate mineral pellets, which are connected by silicon-oxygen tetrahedron or aluminum-oxygen tetrahedron through oxygen bridges to form a molecular size (usually 0.3 nm to 2 nm) channel and cavity system. Its pore size is comparable to that of general molecules, and only molecules with a diameter smaller than the pore size are allowed to enter. Its pore size can be controlled by different processing techniques. In addition to adsorbing water vapor, it can also adsorb other gases.
According to the proportion and crystal structure of silicon and aluminum, it can be divided into A-type, X-type, Y-type molecular sieves, etc., as shown in Figure 1. According to the pore size of the molecular sieve, it is divided into 3A molecular sieve, 4A molecular sieve, and 5A molecular sieve. Among them, 3A molecular sieve is the most encountered molecular sieve for insulating glass.

Figure 2  A, X, and Y type molecular sieve crystal structure(a) Type A;(b) X type, Y type

Figure 2  A, X, and Y type molecular sieve crystal structure(a) Type A;(b) X type, Y type 1

Among them, the chemical formula of the 3A molecular sieve is XK2O・YNa2O・Al2O3・2SiO2・9/2H2O(X+Y=1), and the effective pore size is about 3Å. The chemical formula of the 4A molecular sieve is Na2O·Al2O3·2SiO2·4.5H2O, and the effective pore size is about 4Å. According to Table 2, the radius of the water molecule is smaller than the pore size of the 3A molecular sieve and can be adsorbed by the 3A molecular sieve. The molecular radius of oxygen and nitrogen is larger than the pore size of a 3A molecular sieve, so they cannot be adsorbed by 3A molecular sieve. However, 4A molecular sieves can adsorb water, oxygen, nitrogen, and even more polar nitrogen.

ProjectWater3A molecular sieveOxygenNitrogen4A molecular sieve

Even more deadly, the nitrogen adsorbed by the 4A molecular sieve is very sensitive to temperature changes. For example, 250ml of 4A molecular sieve can release more than 700ml of gas when it rises from normal temperature to 70℃. The outgassing volume of the 3A molecular sieve will be less than 50ml. For example, as a desiccant for insulating glass, when the outside temperature rises, the 4A molecular sieve will release the adsorbed nitrogen and oxygen, causing the internal pressure of the insulating glass to be stronger than the outside air pressure, and the insulating glass will protrude outward. When the outside air temperature drops, the 4A molecular sieve re-adsorbs nitrogen and oxygen, making the outside pressure stronger than the internal pressure of the insulating glass, and the insulating glass is concave or restored. This kind of "breathing phenomenon" of inhalation and deflation will greatly reduce the service life of the insulating glass. Experience shows that the service life of insulating glass using a 4A molecular sieve is only a quarter of that of a 3A molecular sieve.

Therefore, although there are many types of molecular sieves, theory and practice have proved that only the 3A type molecular sieve desiccant that does not adsorb air is suitable for insulating glass. In the market, since the 3A molecular sieve is obtained by ion exchange through the 4A molecular sieve, and Na+ is replaced by K+, the price of the 3A molecular sieve is higher than that of the 4A molecular sieve, which also leads to the frequent occurrence of 4A molecular sieves in the market often pretending to be 3A. The phenomenon of molecular sieves.

4. Development and limitation of attapulgite insulating glass desiccant

The mineral desiccant mainly uses natural attapulgite as the main desiccant. Attapulgite is a hydrous magnesium-rich aluminosilicate clay mineral with a 2:1 layered chain structure. Its ideal molecular formula is (Mg, Al, Fe)5Si8O20(HO)2(OH2)4 4H2O, in each 2:1 unit structure layer, the tetrahedral wafer corners are reversed at a certain distance, forming a layered chain. . There is a lattice replacement in its structure, and the crystal contains a variable amount of Na+, Ca2+, Fe3+, Al3+, as shown in Figure 2.

Figure 3 The Crystal structure of attapulgite 1

Figure 3 The Crystal structure of attapulgite 1

Due to its unique crystal structure and mineral composition, attapulgite has the characteristics of high specific surface area, excellent adsorption performance, strong selective adsorption capacity, and large cation exchange capacity. It is a very efficient desiccant. Common attapulgite raw ore often contains a large number of impurities, which will seriously affect the performance of attapulgite. Therefore, it needs to be purified and modified to improve and improve the performance of concave and convex to achieve various purposes.

However, as a desiccant for insulating glass, the water absorption performance of attapulgite itself often cannot meet the requirements of insulating glass desiccant. Therefore, in the preparation process, a composite desiccant mixed with attapulgite and calcium chloride is prepared by adding calcium chloride with a low price and remarkable effect. The two materials have complementary properties, that is, calcium chloride makes up for the low water absorption of attapulgite, and attapulgite can also condense the moisture absorbed by calcium chloride to prevent liquid water from appearing. This material is currently used by many desiccant manufacturers. The specific water absorption rate is related to the mixing ratio of the two. The more calcium chloride, the higher the water absorption rate, but the greater the risk of water leakage.

The harm of calcium chloride desiccant to insulating glass must also be explained here:

(1) Calcium chloride desiccant is corrosive, and its corrosiveness increases with the continuous increase of water absorption.

(2) The oxygen in the insulating glass has a certain protective effect on the aluminum strip because the oxygen reacts with the surface of the aluminum strip to form an oxide protective film. The chloride ions released from calcium chloride have a strong corrosive effect on aluminum bars.

(3) Calcium chloride desiccant can easily destroy the butyl rubber, causing the butyl rubber (the entry "butyl" is provided by the industry encyclopedia) to volatilize, and the volatiles forms a rainbow phenomenon on the glass surface, and at the same time destroy the insulating glass edge sealing system.

(4) The clay-supported calcium chloride desiccant also has a certain pulverization effect (the entry "powder" is provided by the industry encyclopedia) and migrates. During the migration process, it is constantly powdered and seeps out from the small holes of the aluminum strip of the insulating glass, which is corrosive to the insulating glass, pollutes the insulating glass, and destroys the aesthetics.

Therefore, concerns and disputes about the calcium chloride-loaded attapulgite desiccant inevitably appeared in the market. The words "calcium chloride desiccant is prohibited" and "calcium chloride and calcium oxide desiccant should not be used" also appear in the engineering technical specifications for building exterior windows or the product promotion catalog in the construction field in the high latitudes of the northern hemisphere.

On the other hand, due to the high viscosity of attapulgite itself, high strength after high-temperature sintering, and no dust, it is very important for insulating glass to keep the interior clean and beautiful. Correspondingly, since the raw powder used in the manufacture of molecular sieves has no viscosity, not only a lot of energy consumption is generated during the sintering process, but also dust is easily generated, which causes certain harm to the human body and the environment.

Under the policies of many countries advocating green energy saving and protecting the environment, combined with the characteristics of attapulgite, environmental protection, and energy saving and consumption reduction, the application of attapulgite in insulating glass desiccant has broad prospects. However, at present, there are few standard requirements for attapulgite desiccant for insulating glass. In particular, the lack of corresponding clear requirements for the calcium chloride content of public concern leads to the uneven quality of attapulgite desiccant in the industry, which makes downstream concerns and concerns in the use process.

In the development process of the use of attapulgite desiccant in insulating glass, on the one hand, it is necessary to improve the water absorption and control the content of calcium chloride by improving the purity of attapulgite, structural modification, and other methods; on the other hand, it is necessary to find more suitable salt water absorption Agents (such as magnesium sulfate, sodium sulfate, etc.) can replace calcium chloride to eliminate the public's concerns about the harm of calcium chloride.

5. The future development trend of insulating glass desiccant

Although there are many types of molecular sieves, the desiccant used for insulating glass in the glass deep processing industry is still dominated by 3A molecular sieve, supplemented by attapulgite desiccant. The problem faced by the 3A molecular sieve is that the 4A molecular sieve is shoddy, and the problem faced by the B-type desiccant with attapulgite as the main material is the use and content of the additive calcium chloride. It is believed that in the future, after the introduction of corresponding policies and standards, the market of desiccants for insulating glass can be standardized and rectified. At the same time, with the development of the industry, the maturity of technology, and the standardization of standards, new desiccants will also be continuously developed and applied to insulating glass producing project.

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