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Molecular sieve (also known as synthetic zeolite) is a kind of aluminosilicate microporous mesh crystal material, composed of SiO2 and Al2O3 tetrahedrons, with uniform pore size and large specific surface area. There are metal cations (such as Na+, K+, Ca2+, etc.) in the molecular sieve lattice to balance the excess negative charges in the tetrahedron. Different types of molecular sieves have different pore sizes or shapes and can be used to separate a variety of different molecules. Water is a very polar molecule, 3A molecular sieve has a very high affinity with water, and is often used as an excellent adsorbent.
The lattice pore channel of the 3A type special molecular sieve for insulating glass is about 0.3nm, which is the smallest among the common molecular sieves. When it is in contact with air, it can only absorb gaseous water molecules in the air (The content of helium and hydrogen is only about 0.007%, which can be ignored). Any other molecular sieve with an adsorption pore size larger than 3A will adsorb oxygen, nitrogen and other gases in the air, and this adsorption capacity will be very sensitive due to small changes in temperature.
At present, some insulating glass manufacturers on the market do not use 3A molecular sieve, while some use 4A molecular sieve and type B desiccants. The definition of type B desiccant in the industry standard is a spherical desiccant made of attapulgite as the main material.
Genuine 3A type molecular sieve for insulating glass has a pH value of 8 to 9, which is weakly alkaline. It will not corrode auxiliary materials for insulating glass such as aluminum bars and glue.
The type B desiccant is mainly processed from attapulgite and calcium chloride. Because calcium chloride is corrosive, it will be strongly alkaline when it meets water, with a pH of 11 to 13. When it meets water molecules, after a period of time of absorption, the skin will fall off and corrosive substances will flow out, which will corrode the aluminum strip and the sealant.
Another important difference is hydrolytic adsorption. Although type B desiccant has a low adsorption capacity, its pore size is large, and the temperature requirement for hydrolytic absorption is also low. Generally, when there is sun and the temperature is 30 to 40 degrees Celsius in summer, the type B desiccant will discharge the water molecules in it. At this time, the water molecules will adhere to the inner surface of the insulating glass, causing the insulating glass to fog.
For 3A molecular sieves, the temperature conditions for hydrolytic absorption generally need to be above 150°C, at which water molecules adsorbed in the molecular sieve cavity will be discharged, but this temperature has exceeded the limit of human survival.
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