Determination of volatile organic compounds in water-based coatings

0 Foreword
water-based coatings with its non-toxic environmental protection, odor-free, volatile organic ingredients less, non-combustible high safety, more and more popular with the market. When consumers buy water-based coatings, one of the first factors to consider is whether harmful substances meet the requirements of mandatory standards, which are generally concerned about VOC content. Many coating enterprises are taking various measures to find ways to reduce the VOC content of products, however, to control VOC content, it is necessary to have a set of accurate and reliable measurement methods

. At this time, the old standard (GB18582-2001) has been unable to meet the control requirements, so the new standard "interior decoration materials interior wall coating hazardous substances limit" (GB18582-2008) as a national mandatory standard on October 1, 2008 officially implemented. The biggest difference between the new standard and the old standard is that VOC is defined and measured differently, the old standard is to determine the content of all volatile organic matter by drying, and the new standard is to measure the amount of all organic compounds at the initial boiling point ≤250 degrees C by chromatography. The new standards place more stringent limits on the content of hazardous substances in water-based inner wall coatings, with the following amendments≤: first, the addition of benzene, toluene, ethylbenzene and xylene sum control items, the sum of the content of ≤30
0 mg/kg; In the implementation of the new standard, we have established an easy-to-use, accurate and reliable gas chromatography test method for VOC, a water-based coating. This method can separate VOC in water-based coatings very well, and the method has high precision and good reproducible, which plays a positive role in controlling VOC in water-based coatings.
1 Test Part
1.1 Materials and Reagents
Carrier Gas: High Purity Nitrogen
Gas: High Purity Hydrogen
Gas:Air (supplied by Gas Generator)
Thinner: Acetylene (Chromatography Pure)

Internal markers: ethylene glycol monoclode (analytical pure)
markers: alcohol esters-12 (boiling point 255 degrees C) benzene, toluene, mixed xylene (including ethyl benzene), propylene glycol, are pure analysis.
1.2 Instruments and equipment
GC-7890F gas chromatography (Shanghai Tianmei Instrument Co., Ltd.);
N2000 chromatography workstation (Zhejiang University Zhida Information Engineering Co., Ltd.);
column: PEG20000, 30 cm×0.32 mm×0.25 m;
syringes: 1 sl, 10 sl, 100 sl, 10 mL glass syringe;
sample bottle: 8 mL (Panisilin bottle);
electronic analysis balance: sense 0.1 mg;
desktop centrifuge (Shanghai Anting Scientific Instrument): a maximum speed of 5,000 r/min.
1.3 chromatography
temperature (gasification): 220 degrees C;
detector: FID, temperature 200 degrees C;
column temperature: using second-order procedure heating up:
bar front pressure: 0.04 MPa;
diversion sample: the diversion ratio is adjustable;
sample: 1.0 sl.
1.4 Qualitative Analysis
1.4.1 Determination of the retention time of standard compounds
4 mL acetylene in an 8 mL sample bottle, followed by benzene, toluene, mixed xylene (including ethylbenzene), internal The labels, propylene glycol and alcohol esters-12, each compound added, are injected into the chromatography with a solution of 1 μL, and qualitative analysis is carried out under (1.3) conditions to record the chromatography to determine the relative retention time of each compound, as shown in Figure 1.
1.4.2 coating qualitative
take 1.0 g or so paint in 8 mL sample bottle, and then add several glass beads and 4 mL acetylene, fully shake and put into the centrifuge, with 4,500 r/min centrifugation 10 min, take the upper liquid according to (1.3) conditions for qualitative analysis. It can be seen from the spectral map that the compounds detected are xylene, propylene glycol, unknown organic matter and alcohol-esters-12, as shown in Figure 2.
1.5 Quantitative analysis
1.5.1 Preparation of standard solutions
accurate weighing of the subject matter, propylene glycol, benzene, toluene and xylene in the appropriate amount of acetylene, such as Table 1, shake the spare.
1.5.2 After the determination of the relative mass correction factor Ri
after the chromatography conditions are stable, the above specimen is injected into the chromatography, the Ri value of each organic object is determined, the ri value of each organic object is measured 2 times, and the correction factor is calculated by formula (1), the results are as shown in Tables 2 and 3.
1.5.3 Measurement of the mass fraction of the compound to be tested in the coating
add a few glass beads to the sample bottle, and then accurately weigh the coating with a full shake of about 1 g and 0.004 7 g (5 μL) of the internal object (accurate to 0.1 mg), add 4 mL acetylene, shake well and place in the centrifuge to 4 500 r/min centrifuge 10 min. The chromatography and related data were recorded by the upper liquid 1 sl injection chromatography, with results such as Tables 3 and 2.
the relative quality correction factor of the object to be measured.
mis - mass of the inner object, g;
ms - the quality of the coating, g;
Ais - the peak area of the inner object;
ai - the peak area of the object to be measured.
1.5.4 Mark Recovery Rate Test
In order to verify whether the paint specimen caused interference with the compound test, a mark-up recovery rate test was conducted to verify the accuracy of the method. First of all, the content of each compound in the specimen was measured, and then the reagents equivalent to the content of each compound were added, the total amount of each compound was determined, and compared with the theoretical values, the recovery rate of the mark-up was calculated, and the results were shown in Tables 4 and 4.
2 Results and Discussion
2.1 Conclusion
(1) This test is an easy,possible, accurate and reliable gas chromatography test method for analyzing VOC in water-based coatings. It has been proved by trial and error that this method can separate VOC (including benzene, binary alcohol and film-forming additives) in water-based coatings well and meet the minimum detection limit of 50 mg/kg for benzene in GB18582-2008.
(2) The method used in this test to determine VOC in water-based coatings has good reproducibility and precision, the relative average deviation of the detection benzene is less than 12%, and the relative average deviation of the detection of other organic matter is less than 3%. The method is accurate and reliable, the standard recovery rate of xylene in the test is 80.3% to 117.0%, and the label recovery rate of propylene glycol is 92.1% to 103.7%.
2.2 Discussion
(1) This test uses a second-order procedure to heat up the separation of VOC in water-based coatings, the main purpose of which is to make the baseline more stable, avoid the error caused by baseline instability on the measurement results, and also to make various compounds better separated. This test has used a first-order procedure to heat up (first temperature 70 degrees C, retain 6 min, heating rate of 20 degrees C/min, final temperature of 180 degrees C, retain 8 min), the effect is not very good, Figure 5.
can be seen from Figure 5, the baseline fluctuates greatly at the peak position of propylene glycol at 13.815min, which has a great impact on the measurement results. In the experiment, the precision and accuracy of propylene glycol determination were greatly improved by using second-order procedure heating.
(2) This test requires chromatography to have a high sensitivity and stability, because the content of benzene in water-based coatings is trace (10-6 levels), chromatography ordinate voltage in less than 1 mV to see the peak of benzene, the baseline slightly fluctuates will interfere with the measurement results, so the test requires the baseline to be as smooth as possible.
(3) This test has certain requirements for the purity of the reagent, diluent acetylene with chromatography pure, other reagents are pure analysis, as long as to ensure that the impurities and the tested parts can be completely separated and do not interfere with the determination of benzene.
(4) test of the paint sample only diluted not broken milk, after centrifugal separation of the upper layer clarified, but contains some resin, so the gasification chamber needs to be equipped with quartz glass lining tube, filled with the appropriate amount of quartz cotton, if the quartz cotton brown, the lining tube can be placed in the 600 degrees C Mafu furnace burning 1 to 2 h can be whitened for re-use.
(5) This method can be used to determine VOC in water-based coatings, we also used it for the detection of free monosomes in water-based emulsions, and also obtained satisfactory results.
(6) test can also be used in the internal label diluent addition method, that is, the inner label and diluent to make a certain mass fraction of the inner standard diluent, in the determination of the sample, directly add a certain amount of inner label diluent can be, so that the operation can be easier, more accurate measurement.