Technical article

How to use molecular sieves


1.1 Definition of Molecular Sieve
Silicate molecular sieve is a kind of microporous crystalline material with skeleton structure, which has a caged structure, and water molecules will be sucked into the interior of the molecular sieve under the action of adsorption force, so as to achieve the purpose of drying.
1.2 Reasons for choosing molecular sieve as desiccant for insulating glass
The function of insulating glass molecular sieve desiccant is to absorb the moisture in the insulating glass inter layer to reduce the dew point and prevent the glass from fogging. Molecular sieves are selected as insulating glass desiccants, mainly because molecular sieves have the characteristics of strong adsorption performance and stable chemical properties under high temperature and low humidity conditions.
Molecular sieves have good water absorption properties at lower moisture content and higher temperature conditions. This is an important reason for choosing molecular sieve as desiccant for insulating glass.
The lower the water content of the molecular sieve, the lower the corresponding dew point. When filling the desiccant, the faster it is, the better it is to maintain the water content of the molecular sieve, and the better it is to reduce the dew point of the insulating glass.
The chemical properties of molecular sieves are stable, non-toxic and tasteless. Under normal conditions, they will not have chemical reactions with aluminum strips and glass, so as to ensure the service life of insulating glass. This is another reason for choosing molecular sieve as desiccant for insulating glass.
1.3 The difference between 3A and 4A molecular sieves
3A molecular sieve is obtained from 4A molecular sieve by potassium ion exchange, so 3A molecular sieve is more expensive than 4A molecular sieve. It can be seen from the molecular formula that the 3A and 4A molecular sieves have the same composition of other elements except for the potassium and sodium content.
Molecular formula of 3A molecular sieve: 0.67K2O·0.33Na2O·Al2O3·2SiO2·4.5H2O
Molecular formula of 4A molecular sieve: Na2O·Al2O3·2SiO2·4.5H2O
It can be seen that 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 3A molecular sieve, and cannot be adsorbed by 3A molecular sieve. In the same way, 4A molecular sieve can adsorb water, oxygen and nitrogen.
In addition to absorbing moisture in the air, 4A molecular sieve also adsorbs oxygen and nitrogen, which will cause the insulating glass to be convex or concave due to changes in outside air temperature and pressure (when the outside temperature rises, 4A molecular sieve will release the adsorbed nitrogen and oxygen. As a result, the internal pressure of the insulating glass is stronger than the external air pressure, and the insulating glass will be convex. When the external temperature decreases, the 4A molecular sieve will adsorb nitrogen and oxygen, so that the external pressure is stronger than the internal pressure of the insulating glass, and the convex phenomenon of the insulating glass disappears or is concave). Breathing phenomenon will reduce the service life of insulating glass. 3A molecular sieve does not appear to breathe, so insulating glass must use 3A molecular sieve as a desiccant.
3A and 4A molecular sieves cannot be identified with the naked eye, but they can be identified through simple experiments. The experimental steps of the "balloon" method are as follows:
1) Take out a certain amount of molecular sieve from the package and put it in the air for 20 minutes, put it into a dry 250ml conical flask, fill it up, and put the balloon on the cup and seal it.
2) Put the conical flask into the constant temperature box, raise the temperature to 70 °C, and after 4 hours of constant temperature, the volume of the gas in the beaker increases due to the thermal expansion of the gas, and the increase is very small, indicating that the molecular sieve itself does not adsorb Therefore, it is concluded that it is 3A molecular sieve; the gas adsorbed in the molecular sieve is released during the heating process, which greatly increases the volume of the balloon, which proves that the pore size of the molecular sieve is ≥4Å, so it is concluded that it is not a 3A molecular sieve.
2. Correct use of molecular sieves
After purchasing a qualified molecular sieve, it must be used correctly to achieve a good drying effect. The following points should be noted when using molecular sieves.
①When filling the desiccant, it should be carried out in a dry and dust-free room.
② After the desiccant is opened, it is strictly forbidden to expose it to the air for a long time. From filling to gluing, it should be completed continuously within the shortest time (the interval frame filled with molecular sieve should be within 45 minutes).
③ If the molecular sieve package is damaged, it can only be used after passing the test.
④ There shall be no air leakage around the sealed insulating glass.
⑤ After the insulating glass is glued, it is strictly forbidden to expose it to the sun.
The operation method can be summed up in one sentence: dry, dust-free, indoor, fast-loading, regular testing, and preventing air leakage and exposure.
3. The technical index of 3A molecular sieve
①Determination conditions of static water adsorption: 10% relative humidity, 25℃.
②The nitrogen adsorption measurement conditions are: standard atmospheric pressure, 25℃.
For the detailed detection method, I will only introduce the most commonly used detection method: temperature rise method.
Temperature rise detection method:
1) Measure 20ml of water with a 50ml graduated cylinder, and measure the initial temperature T1.
2) Quickly add 20ml of molecular sieve sample into this graduated cylinder filled with water, and measure the highest temperature T2.
3) Calculate the temperature rise △T= T2- T1. (△T≥30℃, qualified)
If the temperature rise index is qualified, the water absorption performance of the molecular sieve is qualified. This method can be used to detect the water absorption performance of molecular sieves before storage or use of molecular sieves.

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Tracy Chen
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