Karl Fischer method for determination of moisture in ethyl acetate
1. The harm of moisture
In the dry compound production of plastic flexible packaging, the water content of the ethyl acetate solvent is one of the important factors related to the compound quality. It can cause many adverse effects:
(1) Moisture consumes the curing agent, making the ratio of the main agent and the curing agent inaccurate, which affects the curing of the product and is prone to stickiness.
(2) Due to the reaction of moisture and curing agent, carbon dioxide is quickly generated to limit the infiltration of the adhesive, and the product is also prone to bubbles.
(3) Moisture captures the curing agent, which may cause the adhesive to produce polyurethane urea (R-NHCOHN-R) with a relatively high cohesive strength, and the product is prone to crystal points and hardening.
(4) Moisture dissolves into the solvent, which slows down the evaporation rate of the ethyl ester, which affects the spreading rate and hardening rate of the adhesive, and it is also easy to cause the solvent to remain. There are two main sources of moisture in ethyl acetate. One is the product itself. As specified in China's "GB3728-91 Industrial Ethyl Acetate", the moisture content of the premium product is less than 1000ppm, and the moisture content of the first product is less than 2000ppm. The moisture content of the product is less than 4000ppm. In commercially available ethyl esters, these moistures are inevitable. Second, the ethyl ester absorbs moisture during storage. Because ethyl ester is a volatile liquid, it needs to absorb heat during the volatilization process. The main source of heat for the rapid evaporation of the surface ethyl ester is the condensation and release of water vapor in the air The moisture in the water quickly dissolves in the ethyl ester, further increasing its moisture content, which is especially prominent when the relative humidity of the air in summer is greater than 80%. In view of this situation, it is very important to determine the moisture content of ethyl ester before compounding.
2. Comparison of various moisture analyzers
There are many commercially available moisture analyzers, and there are the following according to the test method:
Infrared method instrument, small size, wide measurement range, poor accuracy, suitable for the measurement of wood, paper and other materials with a moisture content of 5% -90%, simple structure and low price.
The Karl Fischer Coulomb method instrument mainly uses the conductivity change calculation after the chemical reaction. The structure is complex, the volume is large, and the measurement accuracy is the highest. It is suitable for moisture contents below 100PPm, and is generally used for anionic polymerization. The determination of products in the chemical, pharmaceutical and other industries is expensive.
The Karl Fischer volumetric method has a simple structure, moderate volume and accuracy, and is suitable for the determination of moisture content of 10PPm-10%. It is generally used for the determination of products in chemical, pharmaceutical and packaging industries with strict moisture requirements. The price ranges from thousands of yuan To tens of thousands of yuan.
It can be seen that for the flexible packaging industry, when measuring the moisture content of ethyl acetate and other solvents, the use of Karl Fischer volumetric moisture analyzer can fully meet the requirements, and the economy is relatively good.
3. The principle of Karl Fischer volumetric moisture determination
The Karl Fischer volumetric method is used to determine the water content, mainly based on the electrochemical reaction: I2 + 2e
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