Determination of solvent moisture content in compo

Determination of solvent moisture content in composite printing of plastic flexible packaging (Part 1)

harm of moisture

in the composite and printing of plastic flexible packaging, many solvents are needed, and their own quality has a great impact on products, in which moisture content is one of the important factors related to product quality. Take ethyl acetate solvent as an example, because each mole of water will consume the same mole of curing agent. In other words, in the composite production, 1 part of water will consume 18 parts of curing agent, so the existence of a small amount of water will cause great damage. According to relevant data, the excessive moisture content of ethyl acetate can have the following effects on the composite quality:

(1) moisture consumes the curing agent, making the ratio of main agent and curing agent inaccurate, affecting the curing of the product, and there will be tacky phenomenon

(2) due to the reaction between water and curing agent, carbon dioxide is rapidly generated, which limits the infiltration of adhesive and is easy to make the product bubble

(3) water grabs the curing agent, which can lead to the formation of polyurethane urea (r-nhcohn-r) with relatively high cohesive strength, resulting in the product prone to crystal point and hardening

(4) water dissolves into the solvent, which slows down the volatilization rate of ethyl ester, affects the spreading rate and hardening rate of adhesive, and easily leads to solvent residue

there are two main sources of moisture in ethyl ester. One is the product itself. For example, China's GB industrial ethyl acetate stipulates that the moisture content of superior products is less than 1000ppm, the moisture content of first-class products is less than 2000ppm, and the moisture content of qualified products is less than 4000ppm. These moisture are inevitable in commercial ethyl esters. According to the author's experience, it is best to choose the ethyl ester of superior products when buying. Considering the storage factors, the moisture content should not be greater than 200 before compounding. They will enter thousands of households 0ppm, so as to ensure the compounding quality. Second, the moisture absorbed by ethyl ester during storage, because ethyl ester is a volatile liquid, which needs to absorb heat during the evaporation process. The main heat source of the rapid volatilization of ethyl ester on the surface is the condensation and exothermic heat of water vapor in the air, and the condensed moisture has good radiation resistance, which quickly dissolves into ethyl ester, further increasing its moisture content, which is particularly prominent when the relative humidity of the air is greater than 80% in summer. In view of this situation, it is very important to determine the moisture content in ethyl ester before compounding

comparison of various moisture measuring instruments

there are many moisture measuring instruments on sale, which are divided into the following types according to the test methods:

infrared instruments, small in size, wide in the measurement range, poor in accuracy, are suitable for the determination of wood, paper and other materials with a moisture content of 5% - 90%. Volvo has simple structure and low price

the main principle of Karl Fischer coulometry instrument: using the change of conductivity after chemical reaction to calculate, the structure is complex, the volume is large, and the determination accuracy is the highest, which is suitable for the determination of moisture content below 100ppm. It is generally used for the determination of products in chemical, pharmaceutical and other industries that have very strict requirements on moisture, such as anionic polymerization, or for large-scale color printing plants with multiple frequencies. The price is relatively expensive

Karl Fischer volumetric method, with relatively simple structure, moderate volume and accuracy, is suitable for the determination of moisture content of 10ppm-10%. It is generally used for chemical, pharmaceutical, packaging and other industrial products with strict requirements on moisture for the next reuse determination. The price ranges from thousands of yuan to tens of thousands of yuan

it can be seen that for the general flexible packaging industry, when measuring the moisture content of ethyl acetate and other solvents, the Karl Fischer volumetric moisture meter can fully meet the requirements of 2-10 times a day, and it is relatively economical

determination principle of Karl Fischer volumetric method

the determination of water content by Karl Fischer volumetric method is mainly based on electrochemical reaction:

when I2 and i- exist simultaneously in the solution of the reaction tank, the reaction is carried out at the positive and negative ends of the electrode at the same time, that is, I2 is reduced on one electrode and i- is oxidized on the other electrode, so there is current between the two electrodes. If there is only i- and no I2 in the solution at the same time, there is no current between the two electrodes

Karl Fischer reagent contains substances such as pyridine and iodine, which can undergo the following chemical reactions with the water in the solution to be tested by dosing them into the reaction tank:

the reaction continues, consuming water continuously and generating i-, until the end of reaction titration, and the water consumption is completed. At this time, only when there is a small amount of unreacted Karl Fischer Reagent in the solution, can I2 and i- exist at the same time. The solution between the two platinum electrodes begins to conduct electricity, reaches the end point according to the current indication, and stops titration. Thus, the water content in the solution can be calibrated by measuring the volume (volume) of Karl Fischer reagent that has been consumed

measuring principle of Karl Fischer coulometry

coulometry is based on the fact that after the sample is dissolved in the electrolyte of a special solvent containing a certain amount of iodine, the water consumes iodine, but the required iodine is no longer titrated with the calibrated iodine containing reagent, but through the electrolysis process, the iodine ion in the solution is anodized to iodine:

2i-2e - → I2

the iodine produced reacts with the water in the sample. The end point is indicated by double platinum electrode. When the iodine concentration in the electrolyte returns to the original concentration, stop electrolysis. Then calculate the moisture content of the sample to be tested according to Faraday's Electrolysis Law: