Moisture in water-based coatings is measured by gas chromatography

1 With the
increasing attention paid to the quality of indoor environmental protection, the content of volatile organic compounds (VOCs) in water-based coatings, an interior decoration material, has received widespread attention. China has formulated standards on the environmental protection requirements of coatings, GB 18582-2001 "Indoor decoration materials interior wall coatings in the limit of harmful substances", HBC 12-2002 "environmental mark product certification technology requirements water-based coatings" and GB 50325-2001 "civil construction indoor environmental pollution control norms." In these specifications, the VOC or TVOC content determination process involves the determination of the solid content, density and moisture content of the coating three parts, the coating solid content and density determination relative deviation is small, and moisture determination is critical to the overall requirements . There are many methods to determine moisture content, such as gas chromatography, Carl Fishu, extraction, titration and so on. In this article, the moisture content of gas chromatography is studied and discussed.
2 Experimental part
2.1 principle
with the paint prepared after the sample through the solvent dissolution and dispersion, through the separation of gas chromatography technology, qualitative detection of all outflow components, using the internal standard method through the peak area to determine the moisture content.
2.2 Instruments and Reagents
2.2.1 Instruments: HP 5890 Gas Chromatography: HP-1 (Crosslinked Methyl Silonxan) MethylSoxane Capillary Column (Crosslinked Methyl Silonxan) 30m×0.53mm ×2.65mm coating); recorder: HP-3394A integration recorder; micro syringe: 1mL; plugged glass bottle: 10mL.
2.2.2 Reagent N, N-MDMAM: Chromatography Purity; No Water Isopropyl alcohol: Chromatography Purity; Distilled water: GB 6628 Provides purity of at least three levels of water; Sample: Inner Wall Latex Coatings.
2.3 Measurement Method
2.3.1 Instrument operating conditions: sample gasification chamber temperature: 200 degrees C; Initial temperature: 80 degrees C, 1min, heating rate of 40 degrees C/min; final temperature: 200 degrees C; carrier: helium with a purity of 99.995 percent or more; flow rate: 46mL/min.
2.3.2 Measure the correction factor of water
in the same plugged glass bottle called about 0.2g distilled water and 0.2g or so isopropyl alcohol (accurately weighed to 0.1mg), add 2mL of methamphetamine mixed, with trace syringes into the sample, and record its chromatography. If isopropyl alcohol and methamphetamine are non-water reagents, the same amount of isopropyl alcohol and methamphetamine mixture, but without water as a blank, record the blank water peak area.
to calculate the water correction factor R:
: AH 2 O - water peak area;
Ai - isopropyl alcohol peak area;
Wi - isopropyl alcohol weight, g;
Wc - Paint Weight, g;
R - Correction factor.
3 Results and Discussion
3.1 Correction Factor Determination Results (see Table 1)
Results of Correction Factors
The inner subject matter and the parts selected when the internal calibration method determines the Correction Factor must be chromatography pure reagents. Since isopropanol and methamphetamine can be soluble in water or absorb water, it may bring errors to the determination of correction factors and sample analysis, so that the water content in the original blanks is subtracted from the calculation to reduce the error. Before analyzing the sample, the gas chromatography is used in association with the Fuli leaf infrared spectrum to determine that isopropyl alcohol and methamphetamine are free of water, thus reducing errors and convenient calculation. The average measured correction factor is 1.188 and the standard deviation is 0.30%.
3.2 Moisture Content measurement results
Using the above experimental method, different samples were measured in parallel several times (see Table 2), and the relative deviation of the test results was <1.0%. It can be seen that this method has high test accuracy and good confidence (1.0% is the maximum allowable error measured according to this method in the experiment).
Parallel measurement results of different samples and standard error
3.3 selection of experimental conditions
in order to make the sample's various parts, especially isopropyl alcohol, water, dimethyl methamide has a better separation effect, selected methylsiloxane as a fixed phase of the capillary column. Due to the wide boiling point range of the various parts of the sample, the procedure was adopted to heat up during the experiment. Set the initial temperature to 80 degrees C, keep 1min, the boiling point isopropanol and water flow out first, with the increase of column temperature, dihamphetamine outflow. In order to shorten the analysis time, the heating rate was increased by 40 degrees C/min and the air flow rate was 46mL/min, and the phenomenon of no tailing or not sharpness of the peaks was found in each group. Under these chromatography conditions, the separation of the parts is optimal and the ideal chromatography is obtained (see Figure 1).
separate chromatography from each
gas chromatography to determine moisture, using the internal standard method. In determining the correction factor, the internal marker should be equivalent to the amount of water, both 0.2g, and when the sample is measured, the water content of the sample must be estimated according to the solid content to determine the weighing amount. The use of the internal standard method must accurately weigh the various parts, the method will not affect the calculation results because of the change of conditions, convenient and accurate and fast.
3.4 Factors affecting moisture determination
Because of the presence of water in the air itself, the test subject is the water content of water-based coatings, which requires the laboratory working environment is relatively dry; Samples in the process of weighing, must maintain a sealed state, so as to avoid the evaporation of low boiling point organic matter, and after weighing must be fully mixed, so that water in the water-based coating completely mixed with solvents and precipitation, so that the determination of moisture content is more accurate. Some sample preparation mentioned that it can be low-speed centrifugation to make it precipitate, but in the experiment found that this will increase the loss of isopropyl alcohol, so here only natural static precipitation. Before taking the sample, the syringe should be repeatedly extracted to fill it with liquid to be tested and to avoid the presence of bubbles. This eliminates errors in the amount of sample and ensures capacity accuracy.
3.5 Determination of the recovery rate
Add the same amount of pure water to a sample with a known moisture content, determine the total amount of moisture in the mixture according to the above method, and calculate the recovery rate (see table 3).
3 total moisture and recovery rate of the mixture
compared with the analysis, the recovery rate was 95% to 103%. It can be seen that the method is reliable and can ensure the accurate determination of moisture content in water-based coatings. The calculation of VOC or TVOC is related to moisture content and solid content and density. The determination of density and solid content is very accurate and does not produce errors in calculation results, but the accuracy of moisture content has a great impact on it. Gas chromatography does not specify the relative deviation of the measurement results, but for GC analysis, its repetition is generally less than 10% of the relative deviation of the results of the two measurements of the same operator. Such a large error has a huge impact on the calculation results of the final VOC or TVOC, and in carl Fisso's measurement, the national standard stipulates that the relative error of the two analysis results must not be greater than 3.5%. Through repeated experiments, sum up the causes of various errors, as far as possible to eliminate the existence of errors, has basically controlled the relative deviation within 1.0%.4 conclusion
a large number of experiments and studies have been done on the use of gas chromatography to determine moisture content, and this method has been further improved. The method is convenient and fast, the external factors have little influence and high accuracy, which meets the data needed for production monitoring and new product development research, and provides accurate and reliable data for the determination of VOC or TVOC content in water-based coatings.