Textiles inevitably involve some finishing agents or auxiliaries in the process of printing and dyeing. These substances are closely related to formaldehyde. Therefore, formaldehyde is widely present in textiles. The determination of formaldehyde content in textiles is mandatory. The test principle can be described as follows: Acetylacetone reacts with formaldehyde in ammonium acetate-acetate buffer solution. The resulting product has maximum absorption at 412 nm, based on the absorbance at this wavelength. The proportional relationship of formaldehyde concentration quantifies formaldehyde. The reaction equation is as follows:
Due to the chemical knowledge involved in this project is more, and the standard GB/T2912.1-2009 "Determination of textile formaldehyde - Part 1: Freely hydrolyzed formaldehyde (water extraction method)" [1] comparison in the details of the detection method Roughly speaking, different testers may have different understandings of the standards. Differences may occur during the actual operation, affecting the reliability and stability of the determination of formaldehyde in textiles. For this reason, this article describes the understanding and precautions of the operational problems in the formaldehyde testing process as follows:
1 sampling principle
No specific guidance is given on how to sample, whether it is a mandatory standard or a reference standard. By consulting relevant data and combining their own experience, this paper suggests that different samples should be sampled according to different methods in order to get the most representative samples as far as possible.
1) Products with fusible interlining
The bonding of the fusible interlining to the fabric mainly depends on the adhesive strength of the adhesive, and the adhesive is usually an amide reacted with formaldehyde to form an N-methylol amide polymer, which is in the cellulose macromolecule or its basic Covalent linkages are formed between the structural units, and the crosslinked covalent bonds are hydrolyzed to generate free formaldehyde under certain temperature and humidity conditions, which may cause excessive formaldehyde content in the fabric. Therefore, samples of such products should be taken from the site containing the fusible interlining, and the ratio of the lining to the fabric should be the same for each sampling. Such as: suits generally take the front chest, collar parts, the same trousers with adhesive lining waistband parts, shirts cuffs, collars and confrontation site.
2) Paint printing products
The color fastness to washing and rubbing of paints and printing products mainly depends on the properties of the adhesives. The currently used adhesives are mainly self-crosslinking adhesives of polyacrylates and their derivatives. The agent releases a large amount of formaldehyde when self-crosslinking or cross-linking with the hydroxyl groups of the fiber. In addition, because the paint-printed fabrics are generally baked without washing, it is easy to have a large amount of residual free formaldehyde in the finished products. Therefore, the formaldehyde content in the printed and non-printed parts is different, and the difference is large. Therefore, paint printing products should be taken hard and have a stamped part. Such as: some cultural shirt front chest, back pattern, some large area of ​​paint printing products as far as possible take the same color, the same depth.
3) Products with lining, fabrics and fillers
Lining, fabrics, and fillers may be treated differently according to the requirements of different functions. For example, some products are treated with a waterproof coating, and the lining is finished with a soft coating. The two different finishes are used for finishing. Different agents, formaldehyde content is also different, so the formaldehyde content of fabrics, linings will be different. For such products, the three should be sampled separately and measured separately.
4) Stitched garments
Should be based on the main fabric or the main fabric specified in the product standards, or indicate the sampling site in the test report. Due to the different fiber composition and content of the spliced ​​products, the rate of adsorption and desorption of the fibers and formaldehyde is also different, and the formaldehyde content of the various parts is also different.
2 Sealing of Samples During the process of selecting a representative sample, the sample should be sealed in a timely manner, otherwise it will affect the accuracy of the test results. In addition, the length of the sampling time will also make the results very different. Due to the volatility of formaldehyde, if the sample is exposed to the air for a long time, the formaldehyde will volatilize and reduce, which will affect the authenticity of the formaldehyde content test results. Therefore, after the sample is received, the sample should be quickly cut and tested to ensure that the test results are true and correct.
The practice of seal preservation is to put the sample in a PE bag, seal it, and then coat the aluminum foil. This will not only prevent formaldehyde from escaping through the pores of the PE film, but also avoid direct contact between the sample and the aluminum foil, causing possible chemical reactions between the remaining catalyst or other unclean chemicals.
3 Select the number of sample test When using spectrophotometry for quantitative analysis, because the concentration of the analyte in the sample is unknown, in order to make the absorbance of the sample fall within the linear range, one method is to dilute the sample in advance according to different dilution ratios to obtain a dilution. In the series, colorimetric analysis is performed again; another method is to roughly estimate the concentration of the sample first, and then decide whether to further dilute or increase the amount of sample added according to the colorimetric situation. In order to reduce the workload, the second method is often used more often.
Using Lambert-Beer's law to quantify formaldehyde, it is very important to choose the measurement range of absorbance. If the absorbance is too large or too small, the quantitative result will be inaccurate. Derived from Lambert-Beer's law, under the premise of guaranteeing the accuracy of the test results, the error of the measured result is the smallest when the amount of sample is selected from the absorbance value of the extract between 0.2 and 0.8. Therefore, under the condition that the volume of the extraction medium is certain, the sample volume becomes the biggest factor that affects the measurement error. If the absorbance of the sample is less than 0.2 due to the small number of samples, the number of samples should be appropriately increased to ensure the accuracy of the test results. This method complies with the provisions of the GB/T2912.1 "Testing of textiles, formaldehyde, Part 1: Freely hydrolyzed formaldehyde (water extraction method)", that is, when the content of formaldehyde in the sample is too low, the number of samples should be increased by 1g. To 2.5g.
4 color reaction control
The effective period and preparation process of acetylacetone have an impact on the formaldehyde test and must be strictly managed to ensure that it is used within the validity period. It should be noted that after the preparation of the acetylacetone reagent has been completed, it must be ensured that it can be used after storage for 12 hours. This is because the Nessler's reagent used for color development is not stable enough after its preparation and it normally takes 12 hours. To achieve stability. The glass bottles used for storage should be brown to prevent the light from reacting with the reagents and affect the color rendering effect.
The standard requires that the mixed solution be 40°C and kept warm for (30±5) minutes so that the reaction is complete. The next step is to protect the solution from light (30 ± 5) minutes at room temperature. This will ensure that the temperature of the solution is cooled to room temperature because the temperature is Absorption and color depth also have an effect, which requires the standard solution and the measured solution to be as consistent as possible during the measurement process. Regarding the preservation of light, on the one hand, because the textile extract is a complex system, some substances may undergo photochromic (enhanced or weakened) phenomenon under light; on the other hand, the unreacted acetylacetone will be colored under illumination. Yellowing affects the test results.
5 Selection of reference solution
In actual absorbance measurements, transmitted light is reduced due to the absorption of reagents such as reflections, solvents, different samples, and different conditions. In order to reduce the light intensity only in relation to the concentration of the measured component in the solution, the aforementioned influence must be corrected. For this purpose, the reference solution can be stored in a cuvette having the same optical properties and the same thickness, the absorbance A can be measured, and then the absorbance A of the solution to be measured can be measured. Based on the absorbance additivity, we can use A total - A reference = A test method, so that the final absorbance of the solution to be measured can really reflect the concentration of the test substance.
In a specific experiment, the influencing factors are different, so the reference solution is also different, which means that the weakening of the light intensity may come from different aspects. When the influence of absorbance is deducted, it should be treated differently. Therefore, the reference solution must be carefully selected to do specific experiments and specific analysis, using targeted methods to solve. Therefore, the reference solution has a "solvent blank" reference solution, a "sample blank" reference solution, and a "developer blank" reference solution. The three reference solutions are explained below.
(1) "Solvent Blank" Reference Solution
When the extract (in GB/T 2912.1-2009 refers to the solution after extracting formaldehyde from textiles with distilled water), the developer (in GB/T2912.1-2009 referred to already prepared acetylacetone) When there is no absorption at the wavelength, a pure solvent (referred to as distilled water in GB/T 2912.1-2009) is used as a reference solution, which is called a "solvent blank" reference solution. This reference solution eliminates some systematic errors. The measured absorbance is equivalent to the calculation formula of GB/T 2912.1-2009 results: A = Ab in Ab-Ab-(A d) (absorbance measured in the standard as blank reagent).
(2) "Sample Blank" Reference Solution
When the wavelength of 412 nm, the developer has no absorption, and the extract is colored (in GB/T 2912.1-2009, the sample is discolored, the color of the textile is present in the test solution after the textile has been extracted, or Is part of the color of the textile), that is, there may be absorption of the extract, the extract should be used as a reference solution, we call the "sample blank" reference solution. The measured absorbance is equivalent to the calculation formula of GB/T 2912.1-2009 results: A = Ab in As = Ab - (A d) (absorbance measured in the standard as a blank sample).
(3) "developer blank" reference solution
If the developer has absorption at the measuring wavelength and the extract does not absorb at the measuring wavelength, in this case, a developer can be used as a reference solution, which we call the "developer blank" reference solution. The "developer blank" reference solution is not used in GB/T 2912.1-2009.
6 Confirmation of false positive results
The measurement of formaldehyde is based on the maximum absorption at 412 nm of the reaction product of formaldehyde and acetylacetone, which is then quantified using the Lambert-Beer law. However, because the textile extract is a complex system, sometimes other substances may absorb at this wavelength, which will result in high measurement results. For this problem, the standard also mentions that if it is suspected that the absorbance value is not caused by formaldehyde, it can be confirmed with dimethiconone because the reaction product of formaldehyde and dimetone does not absorb at 412 nm. The results of such experiments may have the following three conditions:
(1) The absorbance at 412 nm is equal to zero. It means that the absorbance of the test sample is completely generated by formaldehyde, and the test result of the test sample is maintained.
(2) The absorbance at 412 nm is equal to the absorbance of the test sample. This indicates that the absorbance of the test sample was entirely generated by other substances other than formaldehyde, and the formaldehyde content test result of the test sample was not detected.
(3) Absorbance at 412 nm is not equal to zero. And less than the absorbance of the test sample. This indicates that the absorbance of the test sample is partially caused by formaldehyde and partly by other substances. The difference between the absorbance of the test sample and the absorbance of the dimethyone confirmation experiment was calculated, and the actual formaldehyde content of the sample was obtained in combination with the standard curve.
7 Other considerations during testing
The test of formaldehyde content is a test with high operational and technical requirements. The calibration of the formaldehyde stock solution before the test, the determination of the calibration curve, the temperature and time of extract extraction, the selection of the spectrophotometer, etc., all of these factors will have a direct impact on the test results, and therefore must be given the necessary attention to improve Accuracy of test results. In the test of formaldehyde content, the following points must also be noted:
1) There are several cuvettes in general laboratories. Due to slight differences in material uniformity and thickness, the same solution will be tested in different cuvettes and there will be some differences in the measured absorbances. Therefore, during the test process, the test personnel should keep consistent with the cuvette as much as possible. Whether the blank reference or the parallel sample of the extraction solution, the same cuvette should be used as much as possible to minimize the effect of different cuvettes on the test results. influences. In addition, after the cuvette is used, it should be immediately cleaned and periodically washed with acetone (or dilute acid) to avoid coloration of the cuvette.
2) Whether it is the drawing of a formaldehyde standard curve or testing of a sample, a certain volume of related solution needs to be removed, and the accuracy of the formaldehyde concentration test is directly related to the accuracy of the volume of the removed solution. Therefore, in the actual operation, the removal of the relevant solution should be taken seriously and accurately removed. When making the formaldehyde standard curve, when the standard solution needs to be divided, use the same set of pipettes as much as possible to reduce the system error and make the drawn standard curve have higher linear correlation coefficient.
3) The formaldehyde standard solution used in GB/T 2912.1-2009 is prepared and calibrated by the operator. Usually it requires multiple calibrations. The titration end point is difficult to determine. The operation is cumbersome and time-consuming. Man-made uncertainties are more likely to cause accidental errors. And systematic errors, and the standard solution can only be stored for 4 weeks. Therefore, it is recommended that the conditional laboratory can use a certain concentration of standard formaldehyde solution to reduce the experimental error.
8 Conclusion
The test of formaldehyde content in textiles is a technically demanding project. It not only requires the operator to master the standards, but also requires the operator to have certain chemical expertise and skills. Therefore, the project should draw more attention in the implementation process.
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