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Desiccant for insulating glass

According to the national standard GB / T 11944-2012 insulating glass, the service life of insulating glass is directly related to the quality of edge materials (such as spacer, desiccant and sealant) and the manufacturing process of insulating glass. The generation of visible water vapor in the inner cavity of insulating glass is the sign of the expiration of the service life of insulating glass. The spacer, desiccant, sealant and glass form the edge sealing system of insulating glass to ensure that the inner cavity of insulating glass is dry without fog. Desiccant is the key to keep the air layer inside insulating glass dry and avoid water mist inside insulating glass, which plays an important role in the service life of insulating glass. This paper introduces the definition, classification and characteristic analysis of desiccant. Through the analysis, it is concluded that 3A molecular sieve is the most ideal choice of desiccant for insulating glass at this stage.


The service life of insulating glass is expected to be at least 15 years. However, at present, the biggest problem in the insulating glass industry is that the service life of more than 80% of insulating glass in the market is too short, and the average life will not exceed 10 years. Most insulating glass will reach its service life in seven or eight years and need to be replaced. With the popularization of insulating glass, China will spend tens of billions of yuan to replace insulating glass every year. This huge waste will seriously hinder the development of national energy-saving strategy. Where is the problem of short service life caused by so many insulating glass with short service life in the market? Two materials are the core of insulating glass: one is desiccant and the other is sealant. The quality of sealant has been greatly improved due to the correct guidance of the standard, but the situation of desiccant is completely different. There are two materials in the industry standard JC / T 2072 of insulating glass desiccant, class a desiccant and class B desiccant.


How much is the performance difference between the two desiccants? Does it affect the service life of insulating glass? What kind of desiccant is the most suitable for insulating glass? Before answering this question, we first popularize the basic knowledge of desiccant.


What is desiccant


Desiccant refers to the substance that can remove moisture from wet substances, which is often divided into two categories: the first is the desiccant with unstable physical state after water absorption, such as calcium sulfate and calcium chloride, which is dried by combining with water to form hydrate; The second type is desiccant with stable physical state after water absorption. This kind of desiccant generally contains micro pores, such as molecular sieve desiccant, clay desiccant, silica gel and activated alumina. It is dried by physically adsorbing water. Among them, the pore of molecular sieve is the smallest, which can reach the size of molecular level in crystal structure.


The purpose of desiccant is to absorb excess water and avoid the failure of insulating glass. Facing so many desiccant products, what method can we use to select the desiccant products we need? Firstly, the desiccant with corresponding characteristics should be selected according to the characteristics of the desiccant for insulating glass. Insulating glass requires long service life, low dew point (i.e. especially dry) and no corrosion. In daily life and industrial manufacturing, commonly used desiccants include calcium chloride, silica gel, clay desiccant (montmorillonite), molecular sieve, mineral desiccant (attapulgite) and composite desiccant. Let's take a look at the characteristics of these commonly used desiccants:


calcium chloride



Calcium chloride desiccant is a kind of high-efficiency adsorption material, which has no pores and is mainly composed of calcium chloride. Water molecules react with calcium chloride to form calcium chloride dihydrate and calcium chloride hexahydrate. Calcium chloride hexahydrate deliquesces into liquid at room temperature. Under the environment of 9% relative humidity, the static water adsorption capacity of calcium chloride desiccant can reach 32.4% and become calcium chloride dihydrate. At this time, the physical state will change and form crystals. Due to the change of environmental humidity conditions, these crystals will dehydrate and crystallize repeatedly. After a long time, they will pulverize and diffuse and pollute the dried objects. If the humidity of the surrounding environment rises again, It will become calcium chloride hexahydrate and deliquesce. At 30 ℃, it will become a strong electrolyte liquid with electrochemical corrosion characteristics.


silica gel



Silica gel is a highly active adsorption material with pore structure; The main component is amorphous silicon dioxide, which belongs to amorphous material. Its shape is transparent and irregular sphere, and its chemical molecular formula is msio2 · nH2O. It is usually prepared by the reaction of sodium silicate and sulfuric acid, aging, acid bubble and a series of post-treatment processes.


The chemical composition and physical structure of silica gel determine that it has many characteristics that are difficult to be replaced by other similar materials: high adsorption performance, good thermal stability, stable chemical properties, high mechanical strength and so on. The interior of silica gel desiccant is a very fine pore network structure. These pores can absorb water and retain water through their physical attraction. As a desiccant, silica gel desiccant is widely used in the drying and moisture-proof of aviation parts, computer devices, electronic products, leather products, medicine, food and other industries. Even if the silica gel desiccant is completely immersed in water, it will not soften or liquefy. It has the characteristics of non-toxic, tasteless, corrosion-free and pollution-free, so it can be in direct contact with any object.


Clay desiccant (montmorillonite)



The appearance of clay desiccant is gray ball, which is most suitable for moisture absorption in the environment below 50 ℃. When the temperature is higher than 50 ℃, the "drainage" degree of clay is greater than that of "water absorption". The significant advantage of clay desiccant is its low price.


Montmorillonite clay desiccant has been found and used in the field of desiccant by German Southern Chemical Company since 1980s. Because of its low price and good adsorption effect, montmorillonite clay desiccant has gradually become one of the international mainstream mineral desiccant raw materials. Montmorillonite desiccant is also known as bentonite desiccant and clay desiccant. Color: purple, gray, purple red. Its characteristics are: ① environmental protection. It is made of pure natural montmorillonite desiccant by drying and activation. It does not contain any additives and soluble substances. It is a non-corrosive, non-toxic and pollution-free green environmental protection product. After use, it can be treated as general waste, will not pollute the environment and can be degraded naturally. ② Adaptability. In the environment of various temperatures, the moisture absorption performance can remain stable. ③ Moisture resistance. The moisture absorption performance is good, and the saturated moisture absorption rate is more than 50% of its own weight, which is 1.5 times that of the traditional desiccant.


Mineral desiccant (attapulgite)



Concave soil desiccant is refined from pure natural mineral raw material attapulgite through activation treatment. It is green, environment-friendly, non-toxic, tasteless and harmless to human body. It has good adsorption activity at room temperature and general humidity, and can be used for static dehumidification, odor removal and decolorization. It is widely used in products that cannot be sealed with oil seal and gas phase, such as optical instruments, electronic products, medical and health care, food packaging and dry air sealing of military products and civil products. Its characteristics are: ① good moisture absorption capacity under high humidity; ② The hygroscopic capacity at high temperature is better than that of montmorillonite.


Molecular sieve desiccant



Molecular sieve desiccant is a synthetic desiccant product with strong adsorption to water molecules. It is a crystalline aluminosilicate compound. Its crystal structure has regular and uniform pores. The pore size is the order of magnitude of the molecular size. It only allows molecules with a diameter smaller than the pore size to enter. Therefore, it can screen the molecules in the mixture according to the size, so it is called molecular sieve. The pore size can be controlled by different processing processes. In addition to adsorbing water and gas, it can also adsorb other gases. At high temperatures above 230 degrees Celsius, it can still hold water molecules well.


Analysis of various desiccants


For the desiccant used for insulating glass, its function is realized through the deep drying of the sealing air layer inside the insulating glass: adsorbing the moisture in the air in the sealing cavity of the insulating glass; Absorb the water continuously penetrating into the hollow glass through the sealant in the later stage; It is used in combination with other special molecular sieves to adsorb the volatile substances released by insulating glass sealant.


Meanwhile, the desiccant for insulating glass must also meet the following requirements (one high, one strong, two in four low):


1. The ultra-low water content of the finished product can ensure that the desiccant has strong water absorption performance.


2. Super deep adsorption capacity can ensure that the water in the glass interlayer is fully absorbed.


3. Ultra low powder dropping degree ensures the beautiful and transparent performance of insulating glass.


4. The ultra-low nitrogen adsorption capacity ensures that the hollow glass will not have concave convex deformation under different temperature changes.


5. Qualified bulk density to ensure 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 salting out.


7. High compressive crushing strength to reduce crushing during filling due to insufficient strength, forming slag and dust and polluting glass.


8. Low static electricity to ensure that the desiccant is adsorbed on the pipe wall and blocks the pipe when the machine is filled.


To sum up, the characteristics of various desiccants and the requirements for insulating glass desiccants have been listed. Below, we analyze whether various desiccants are suitable for insulating glass as follows:


Calcium chloride desiccant calcium chloride desiccant can be used at an ambient temperature of - 5 ° C to 90 ° C. its active components have good stability and are difficult to react with other substances except water. It can be used in almost all environments. Due to its chemical adsorption characteristics, the adsorbed water and calcium chloride generally react as follows: CaCl2 + 2H2O = CaCl2 · 2H2O, but different crystalline water compounds can be formed at different temperatures (1, 2, 3 and 6 crystalline water can be formed). Aqueous calcium chloride can form electrochemical reaction with the aluminum strip of insulating glass, which corrodes the aluminum strip and destroys the sealant, thus affecting the service life of insulating glass. Therefore, it is not suitable for insulating glass desiccant.


Although silica gel desiccant is physically adsorbed, the most suitable moisture absorption environment for silica gel desiccant is room temperature (20 ° C to 32 ° C) and high temperature (60 ° C to 90 ° C). It can not meet the service environment of insulating glass, and the adsorption depth of silica gel desiccant is low, which can not meet the requirements of insulating glass for dew point, Therefore, it is not suitable for insulating glass desiccant.


Montmorillonite clay desiccant is also physical adsorption and has many excellent characteristics. It was used as insulating glass desiccant in the early stage, but it has basically withdrawn from the insulating glass desiccant market because its adsorption depth can not meet the requirements, the insulating glass fogs under harsh environment and low service life.


Attapulgite desiccant has larger adsorption capacity than montmorillonite desiccant under low adsorption conditions and absorbs more water under high humidity conditions. However, in the actual working environment of insulating glass, when the humidity is low, the adsorption capacity will be very low, which will affect the dew point of insulating glass and make it difficult to ensure the service life of insulating glass.


Molecular sieve desiccant has 3a, 4a, 5A and other categories, but not every kind of molecular sieve is suitable for insulating glass desiccant. At present, some insulating glass manufacturers in the market use 4A molecular sieve as desiccant, and the lattice pore of 4A molecular sieve is about 4 angstrom. Although 4A molecular sieve has strong adsorption capacity for water, it will also adsorb some nitrogen and other inert gases in the air, and adsorb and desorb nitrogen with the rise and fall of ambient temperature; When the temperature decreases slightly, 4A molecular sieve will adsorb the air in the insulating glass, resulting in a vacuum state; When the temperature rises, the molecular sieve releases the air into the insulating glass compartment, resulting in the expansion of the air in the insulating glass. With the change of temperature difference between day and night and seasonal change, insulating glass is easy to be twisted and broken due to expansion and contraction, and its service life is shortened; The sudden breaking of insulating glass may lead to major safety accidents. Therefore, 4A molecular sieve can not be used as insulating glass desiccant.


Let's take another look at 3A molecular sieve. Generally, 3A molecular sieve raw powder and clay are formed into spherical particles, which are roasted at high temperature to make 3A special desiccant for insulating glass, that is, class a desiccant in JC / t2072-2011 desiccant for insulating glass. Its lattice pore is about 3 angstrom (1 Å = 0.1nm), and has high affinity with water. It only adsorbs water molecules and does not adsorb other molecules greater than 3 Å. The molecular diameters of some common gases are: he (helium) 0.26 nm, H2 (hydrogen) 0.289 nm, CO2 (carbon dioxide) 0.33 nm, AR (argon) 0.34 nm, O2 (oxygen) 0.346 nm, N2 (nitrogen) 0.364 nm, CO (carbon monoxide) 0.376 nm, CH4 (methane) 0.38 nm, HCHO (formaldehyde) 0.45 nm and water molecule 0.28 nm.


There is also a kind of concave soil desiccant containing calcium chloride desiccant on the market. The manufacturer believes that it belongs to class B desiccant in the industry standard desiccant for insulating glass (JC / T 2072-2011). According to industry standards and different materials and production processes, desiccants for insulating glass are divided into two categories: class a desiccant (3A molecular sieve) and class B desiccant (spherical drying material with attapulgite as the main body). Desiccant for insulating glass shall meet the requirements in Table 1:



Class B desiccant is a spherical desiccant made of attapulgite as the main material. As a desiccant for insulating glass, the water absorption performance of attapulgite itself often can not meet the requirements of insulating glass desiccant. Therefore, in the preparation process, the low-cost and effective calcium chloride is added to make a composite desiccant mixed with attapulgite and calcium chloride, that is, calcium chloride makes up for the low water absorption of attapulgite when the environment is dry, and attapulgite can also condense the liquid during the deliquescence of calcium chloride. It sounds that the properties of the two materials are complementary, However, the adsorption of Attapulgite on deliquescent liquid is physical adsorption, which can not change the essence that these liquids are calcium chloride electrolyte solution. These attapulgite adhered to calcium chloride solution still has electrochemical corrosion, but this material is currently used by many insulating glass manufacturers, which greatly reduces the service life of insulating glass. In order to reduce the corrosion of calcium chloride electrolyte, calcium chloride can be added less, but there is too little calcium chloride, which can not meet the requirements of class B desiccant in the industry standard desiccant for insulating glass, so the addition amount of calcium chloride can only be increased. Although all indexes of this product can meet the standard requirements, the excessive addition of calcium chloride will aggravate the damage of butyl rubber, cause the volatilization of butyl rubber, form a rainbow phenomenon on the glass surface, and damage the edge sealing system of insulating glass. The excessive addition of calcium chloride also has a certain pulverization and migration effect. During the migration process, it is constantly pulverized 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.


Moreover, when the humidity environment is low, the adsorption capacity of class B desiccant will be very low, which will affect the dew point of insulating glass and make insulating glass prone to fogging. Although all indicators of this kind of desiccant meet the requirements of industry standards for class B desiccant, due to the existence of more calcium chloride, the following reactions generally occur when adsorbing water in insulating glass, CaCl З + 2H З o = CaCl З · 2H З o, but it can form different crystalline water compounds (1, 2, 4 and 6 crystalline water can be formed at different temperatures). Aqueous calcium chloride can form electrochemical reaction with the aluminum strip of insulating glass, which corrodes aluminum strip or other spacer strip, destroys sealant and affects the service life of insulating glass. In conclusion, class B desiccant is not a high-quality choice for insulating glass desiccant.


conclusion


3A molecular sieve has the smallest pore among the commonly used molecular sieves. When it comes into contact with air, it can only absorb gaseous water molecules in the air and can not absorb other components in the air (the content of helium and hydrogen in the air accounts for only about 0.007%, which can be ignored). Due to the special technology in the manufacturing process, it has insulating glass desiccant