Comparison of moisture content measurement methods in VOC content determination of water-based coatings

0 Foreword
emissions of volatile organic volatiles (VOC) are one of the most important causes of photochemical pollution and ozone layer destruction. To this end, the State promulgated relevant laws and regulations and other requirements, such as the national mandatory standard GB 18582-2008, "Indoor decoration materials in the interior wall paint hazardous substances limit", the State Environmental Protection Administration promulgated HJ/T 201-2005 , "environmental mark product certification technical requirements water-based coatings", etc., to strengthen the limit control of volatile organic compounds of water-based coatings, as shown in Table 1.
by Table 2, the VOC content of different coatings should be selected the corresponding testing standards
chinese
. The moisture content of the sample must be determined in the VOC content determination of water-based coatings, and in order to accurately evaluate and control the VOC index in water-based coating products, we conducted a comparative test of the accuracy and accuracy of the two methods commonly used to measure the moisture content, Carl Fishu and gas chromatography.
2 Water-based coating moisture content determination
samples measured in this test are water-based wood coatings provided by a coating company in Jiangmen, solid content of 51.66 percent, theoretical water content of 32.00%, 23 degrees C density of 1.325 2 g/mL.
2.1 Test equipment
2.1.1 Carl Fissofa
instruments: electronic analysis balance (0.000 1 g);
microscopic sampler (10 μL); automatic titration tube (25 mL, fine tip, 0.1 mL in scale, connecting the protective tube filled with desiccant to prevent water vapor from entering the tube in the atmosphere).
reagents: A.R. Ka's test fluid, water drop rate ≥2.0 mg/mL, Guangzhou chemical reagent plant; methanol, content ≥99.5%, pure analysis, Guangzhou chemical reagent factory.
2.1.2 Gas Chromatography
Instruments: Gas Chromatography (with thermal conductivity detector);
reagents: distilled water (GB/T6682, grade III water);
2.2 Test step
2.2.1 Carl Fissofa
2.2.1.1 measurement of the titration of the karst solution
take 10 mL methanol at 1 In the 00 mL iodine bottle, the cassette solution is titred to brown-red, the distilled water is accurately added with a trace sampler, and the volume V1 of the karst solution is immediately dripped to brown with the karst solution. Titration by type (1) calculation:
:
Ww - moisture content in the sample, mass score, %;
Fi - the quality correction factor of water relative to the inner object;
Ws - quality of the inner object, g;
Ai - the peak area of water in the sample;
Wi - the quality of the sample, g;
the peak area of As, the inner marker isopropanol.
2.3 Data Analysis
2.3.1 Carl Fisseufa
The titration of the tested Kaka reagent was 2.44 mg/mL, with a difference of less than 0.01 mg/mL in the adjacent calibration values, in accordance with the standard. In order to obtain the accurate water content of the sample, the sample was measured 3 times, and the results were shown in Table 3. As can be known from Table 3, the relative deviation of the two measurement results after the sample is relatively small, the first measurement results are not accurate, the last two sets of data average as the water content of the sample, and the sample theory value, the relative error of the test is 2.88%.
2.3.2 gas chromatography
This method selects a large diameter polar capillary, the amount of sample is not greater than 1.0 sl, to ensure that the severity ratio is large enough, otherwise the separation effect and peak symmetry is poor. The gasification chamber is equipped with a glass lining tube filled with the appropriate amount of silaneized glass cotton to filter the residue and regularly clean the lining tube. The water content of the sample was measured in parallel 3 times, and the results were shown in Table 4. As can be seen from Table 4, compared with the theoretical value of the sample, the relative error of water content measured by gas chromatography is 0.38%, and the relative deviation is smaller than that of Carl Fisseu method, probably because when measuring water content by Carl Fishu method

3 sample VOC content
sample solid content of 51.66%, theoretical water content of 32.00%, so the sample theory VOC content (mass ratio) of 16.34%. As can be known from Table 2, this product needs to use GB/T23985-2009/ISO11890-1:2007 standard . According to the standard, voC content calculation formula is:
the above two methods to determine the sample moisture content into the above formula, the sample VOC content as shown in Table 5.
As can be seen from Table 5, the accuracy of the sample moisture content determination will lead to the accuracy of the final VOC content determination of the sample, and the relative error of the VOC content obtained by the sample moisture content measured by Carl Fisso method is (in order of Table 5) 5.63 The relative errors of VOC content obtained from the moisture content of gas chromatography samples were 0.73%, 0.73% and 0.46%, respectively.
4 Conclusion
Although Carl Fishu is a more common and simple method for measuring the moisture content of samples, its accuracy and precision are inferior to gas chromatography, in order to better control the VOC content of water-based coating products, we should try to use gas chromatography to determine the moisture content of products, gas chromatography should be the arbitration method.