Comparison and practice of methods for determining moisture content in water-based coatings

0 Foreword
Due to GB18582-2001 "Indoor decoration material interior wall coating limit of harmful substances" on volatile organic matter (VOC) determination, the method of moisture determination has gas chromatography and Carl Fishu method, the two methods coexist. With the implementation of the new method of VOC test voC in HJ/T 201-2005 Environmental Mark Product Technical Requirements - Water-based Coatings, moisture determination needs to be matched by a more accurate and simpler method of measurement. To this end, we have done a lot of experiments to compare and discuss the two methods
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1 Experimental part
1.1 gas chromatography
1.1.1 principle
using the hydrophobility of waterless methamphetamine, through the separation of gas chromatography technology, the use of internal labeling method to determine moisture in water-based latex paint.
1.1.2 instruments and reagents
instruments: HP-6890 gas chromatography, equipped with thermal conductivity detector.
column: 2 m long, 3.2 mm outside diameter, filled with 177 to 250 μ m polymer porous micro-ball stainless steel column. Micro-μ: 1 x L;
reagents: distilled water (GB/T6682, grade III water);
1.1.3 Test Method
1.1.3.1 Instrument Operating Conditions
the initial temperature of the column temperature is heated by a procedure of 80 degrees C, 5min is maintained, the speed is 30 degrees C/min, the final temperature is 170 degrees C, and the final temperature is 5 min.
detector temperature: 240 degrees C, gas load: purity of 99.99% or more nitrogen, flow rate of 30 mL/min.
1.1.3.2 Determination of water correction factor
in the same plug glass bottle called about 0.2 g distilled water and 0.2 g of isopropyl alcohol, accurate to 0.1 mg, add 2 mL of methamphetamine, mixed. A standard mix sample of 1 syringe was used to record its chromatography. If isopropanol and methamphetamine are not waterless reagents, the same amount of isopropyl alcohol and methamphetamine (mixture) without water as blank, record the blank water peak area. The response factor is calculated as follows, and the results are found in Table 1.
the results of the response factor under the gas chromatography
: R - response factor;
Wi - Isopropyl alcohol quality, g;
WH2 O - Water Quality, g;
ai - the peak area of isopropyl alcohol;
AH2O - peak area of water;
B - blank area of water peak.
1.1.3.3 sample analysis
said to take 0.6 g paint and 0.2 g isopropyl alcohol, accurate to 0.1 mg, added to the plug glass bottle, and then added 2 mL of methamphetamine, cap the plug, force shake the sample bottle 15min, to ensure that the moisture in the coating is completely affected by methamphetamine and release. Place 5 min to precipitate it. It is also suitable to separate it using a low-speed centrifuge, taking its upper liquid 1 sL and injecting it into the gas chromatography.
a chromatography chart, the water mass score H2O(%), calculated according to the following formula:
AH2O - water peak area;
Ai - isopropyl alcohol area;
Wi - isopropyl alcohol weight;
Wc - paint weight;
R - correction factor.
1.2 Carl Fissoufa
1.2.1 Principle
Plus a constant DC voltage on a double platinum electrode, which measures current or voltage during titration, is different from the original Carl Fissou reagent, but they both include the basic components of I2 and SO2. The solvents can be methanol, isopropyl alcohol, chloroethanol, pyridine and various amines. In the water SO2 is oxidized by I2 to H2SO4, reactive:
I 2 plus S O 2 plus 3 R N plus C H 3O H plus H 2 O → 2 R N H 2 plus R N B . . . The acid produced by H S O 4C H 3
is buffered by the background solvent, and when the water reacts completely, a slight excess of I2 is reduced on the electrode, causing changes in current and voltage to show the titration end point.
1.2.2 Instruments and Reagents
(1) Instruments: Vantone's moisture titration meter, Switzerland; In the samples of aldehyde ketones, a special Ka's reagent should be used, the laboratory is equipped with only ordinary Ka's reagents, syringes: 10 μ L, disposable straw: 3 mL.
(2) reagents: distilled water (GB/T 6682, grade III water);
1.2.3 Test Method
1.2.3.1 Calibration of Ka's reagents
adding a new solvent to the titration bottle to cover the electrode is appropriate, with the Ka's reagent titration to the end point. Inhale distilled water with a 10 syringe, inject it into a titration bottle, and use a reduction method to weigh the quality of the water as accurate as 0.1mg. And the quality of water into the titration meter, with the Ka's reagent titration to the end point, record the results displayed by the instrument. Repeat the calibration until the difference between the adjacent calibration results is less than 1%, the average of at least two measurements is calculated, and the average of the calibration results is entered into the titration meter.
1.2.3.2 Sample Analysis
Take the paint sample with a disposable dropper, add 1 to 2 drops to the titration bottle, and add the paint quality by subtracting is 0.2 to 0.4 g, accurate to 0.1 mg. And the quality of the coating into the titration meter, with the Ka's reagent titration sample to the end point, record the results shown by the instrument, see Table 2.
the calibration results
repeatedly determined under the Carl Fisso method, the relative deviation of the two analyses obtained by the same analyst shall not be greater than 3.5%.
2 Results and Discussion
2.1 Deficiencies in the Carl Fishu method
Through the above data, it can be seen that the parallelism of the Carl Fisshu titration method itself is not good, and the relative deviation is large. And even such a result, the operation is more troublesome, the factors affected are as follows:
(1) sample quantity requirements are strict, should be used 0.2 to 0.4 g or so. Because too little sample will bring into the balance of error, when too many samples, added to the solvent will form a large number of flocculation precipitation, in the titration process is easy to wrap electrodes, so that the measurement results produce errors.
(2) environmental humidity has a greater impact. Because of the use of straw sample, it is not able to enter through the sample pad. It is necessary to open the piston of the sample port, easy to bring moisture into the air, the environmental impact is greater.
(3) titration cups change solvents frequently. Due to latex paint encountered polar methanol, will form precipitation, or milky turbidity, so that the complete extraction of moisture caused a certain degree of difficulty, each sample test, must replace the solvent. Otherwise, the moisture content of the next sample will be added with the water that the last sample did not fully dissect, so that the measurement results are too large, resulting in errors.
(4) electrodes are susceptible to contamination. Because latex paint through methanol milk, there are many more complex ingredients into the titration cup, electrodes are easy to be coated, after coating the electrode reaction is not sensitive, so that the measurement results are too large.
(5) titring head is susceptible to contamination, affecting test results. Because the titration cup, whether there is floc-like precipitation or the existence of milky turbidity, will cause pollution to the titration cup, so that the titration head gasket is contaminated with impurities, so that the end control failure, resulting in incorrect results.
(6) strict requirements for sample types. Because when the sample contains aldehyde ketones, a special Kashi reagent must be selected, otherwise, the wrong results will occur. For example, water-based wood coatings. Therefore, when the composition of the sample is not understood, should first be qualitative, choose the appropriate reagents.
2.2 gas chromatography has the advantage that
(1) the requirements for samples apply to many water-based coatings, including water-based wood coatings.
(2) is simple to operate, has good parallelism, and has a relative error ≤ 1%.
(3) test sample time is short.
(4) has lower configuration requirements of its own. All you need is a 2.0 m GD × 102 fill bar. Detector TCD, carrier: N2, the operation process is relatively simple.
results of the two methods are compared to Table 3.
The results of the two methods were compared with
3 conclusions
using gas chromatography and Carl Fishu method to do a lot of experiments and studies on the moisture detection of water-based latex coatings, respectively, describing the characteristics of the two methods and the factors affecting the detection process. Through the control of the test results, it can be seen that the gas chromatography method can be used to detect water-based paint moisture, convenient, fast, less interference factors, high accuracy, can meet the requirements of production monitoring and new product development research, while providing accurate moisture content for VOC detection. Through comparison and practice, the gas chromatography method is more suitable for the implementation and implementation of the newly revised GB18582 national standard and HJ/T201-2005 ring standard.