The problems of free formaldehyde determination in water-based coatings are discussed

0 Today,
standards for water-based coatings free formaldehyde limits are becoming more and more stringent, and are mandatory standards that enterprises must implement. Therefore, the accuracy of formaldehyde determination is not only related to the interests of enterprises, but also related to the health of the general public. The method that has been used in national standards and environmental standards is acetyl acetone phosphorescicity method, which is also widely used in food, textile industry, etc., is a classically recognized method, but in different operations, mainly in the control of color temperature: some (40±2) C, heating (30±5) min, placed at room temperature (30±5)
min
; In addition, water or reagents as a ratio, different. The standard stipulates that the heated water bath at 60 degrees C heating 30 min, we found in the test, 60 degrees C heating 30 min formaldehyde color is not complete, heating temperature and heating time must be strictly controlled, otherwise there will be errors.
1 Test Part
1.1 Method Principle
The free formaldehyde in the sample is steamed by distillation, and in the pH-6 ammonium acetic acid buffer solution, formaldehyde and acetyl acetone are heated in a 100-degree C water bath to produce a stable yellow compound, which is cooled to absorb light at a wavelength of 412 nm. The content of free formaldehyde in the sample is calculated according to the standard working curve or linear regression equation.
1.2 reagents
are pure analysis, the water used meets the requirements of three levels of water.
acetyl acetone solution (color-showing agent): by reference to GB 18582-2008 standard appendix C preparation.
formaldehyde standard solution: by reference to Appendix C preparation, and finally equipped with 12.00 μg/mL standard solution, spare.
1.3 instruments
distillation device: 500 mL distillation bottle, serpent condensation tube;
scale color tube: 50 mL;
pipe: 1 mL, 2 mL, 5 mL, 10 mL, 20 mL;
flat-panel electric furnace, electric heating sleeve, 1,000 mL berries;
electronic analysis balance: accuracy 0.1 mg;
ultraviolet visible phosphorescometer: UV-7504PC, Shanghai Xinmao Instrument Co., Ltd.
2 results and discussion
2.1 temperature on the effect of the measurement results
formaldehyde and acetyl acetone reaction, must be carried out under heating conditions, how much temperature color is appropriate, the test believes that can make the color-playing reaction should be the most appropriate temperature. Appendix C gives the condition is not the best condition, the preparation of 2 groups of the same concentration of formaldehyde solution, one group at 60 degrees C heating 30 min, the other group at 100 degrees C heating 10 min, the operation is as follows: in the 50 mL scale tube into 0.0, 1.1. 0, 3.0, 5.0, 7.0, 9.0, 10.0 mL formaldehyde standard solution (1.2), add water to 40 mL scale, add 2.5 mL color agent, shake well. After heating under different conditions, cool to room temperature, add water to the scale and shake well. The absorbent (A) is measured at 412 nm with a 10 mm ratio of cuvettes, with reagents as a ratio, as seen in Table 1.
by the standard curve and linear equation can be seen: 60 degrees C heating 30min formaldehyde does not show color completely, each point of light absorption is lower than 100 degrees C heating 10 min, linear equation slope is small, although also linear relationship, but can only show that in strict control of heating temperature and heating time, the percentage of formaldehyde color is constant, if the control is not strict will produce errors. The heating reaction is complete at 100 degrees C, the linear equation slope is large, the sensitivity is high, the temperature is good to control, but the temperature may be low at high altitude, the heating time can be extended appropriately to make the reaction complete.
2.2 Questions of water or reagents
below, water as a ratio and reagents as a test, as shown in Table 2.
To water as a test or reagent as a test, we must abide by a principle: when making a standard curve with water, testing samples should also be water as a test, making a standard curve with reagents as a test, testing samples should also be used as a reagent. If water is used as a reference, the absorbance of each point on the standard curve includes a reagent blank (i.e. the absorbance at the concentration of 0), and the standard curve does not pass through the coordinate origin and has a intercept. If the reagent is used as a reference, the absorbance of each point does not include the reagent blank, and the standard curve passes through the coordinate origin. In Appendix C, the standard curve is based on water as a ratio, and when testing the sample, it is said that water is the ratio, and then it is said that "at the same time under the same conditions to make a blank sample (water), measured the absorbance of the blank sample." The absorbance of the test is subtracted from the absorbance of the blank image, and the corresponding formaldehyde mass is found on the standard working curve". The result of this operation is equal to the absorption of 2 reagent blanks, so that the measurement results are low.
2.3 Determination of free formaldehyde content
plus 20 mL of water in a distillation bottle, put 4 to 5 glass beads, and then weighed into 0.4 to 0.8 g (called quasi-to 0.1 mg) paint samples, the distillation device (reference to Appendix C). Add 15 mL of water to the 50 mL scale tube, the condenser outlet into the water, and immerse the scale tube in an ice water bath, the upper part of the distillation bottle and the distillation bottle branch should be kept warm, heated to the distillation, disconnect the distillation bottle and condenser, in order to prevent the receiving fluid from being sucked into the distillation bottle. Remove the distillation bottle from the heat sleeve while it is hot and place the mouth side-down to prevent condensate from flowing into the hot distillation bottle causing the bottle to crack. Rinse the condenser with water 2 times, the total volume is controlled at about 45 mL, add color agent 2.5 mL, shake well, boiling water bath heating 10 min, cool to room temperature, add water to 50 mL scale and shake well, dry filtration, the following operation with 2.1.
free formaldehyde content is calculated by type (5):
3 Conclusion
(1) Test proves that the color-showing reaction is complete at 100 degrees C and 10 min, and the sensitivity is high.
(2) In the test of water-based coatings used additives have been free formaldehyde content determination, found that individual manufacturers provided a high level of formaldehyde in the fungicide (13%), if the use of this fungicide will cause the product formaldehyde content exceeds the standard, so the research and production formula eliminated this fungicide.
(3) test found that a small amount of emulsion was taken out when distilling, even strictly controlled distillation speed can not be avoided, so that the receiving liquid belt a certain degree of turbidity, after many measurements, turbidity absorption in about 0.03, so that the determination results are high, the use of dry filtration can eliminate interference.
(4) distillation must be dried, otherwise the results are low.
(5) because of the low temperature of the steaming liquid, large volume, so heating 10 min to ensure full color.