Matters Needing Attention In The Determination Of Formaldehyde In Textiles

Because the project involves more chemical knowledge, and the standard GB/T2912.1-2009 "formaldehyde determination of textiles first parts: free hydrolysis of formaldehyde (water extraction method)" [1] in the details of the operation of the detection method is relatively rough, different inspectors because of different understanding of standards, in the actual operation process may be different, affecting the reliability and stability of formaldehyde content determination of textiles.

For this reason, the understanding and notices of operation problems in formaldehyde testing process are described as follows:

1 sampling

No specific guidance is given on how to sample, whether mandatory or referenced.

Through consulting relevant information and combining with our own experience, this paper suggests that different samples should be sampled according to different methods so as to get the most representative samples as far as possible.

1) products containing adhesive lining

The adhesion between adhesive lining and fabric depends mainly on the adhesive force of the adhesive, and the adhesive is usually produced by the reaction of amide with formaldehyde to form N- hydroxymethyl amide polymer, which produces covalent bond between cellulose macromolecules or its basic structural units. The cross-linked covalent bond will hydrolyze to produce free formaldehyde under certain temperature and humidity conditions, which may cause excessive formaldehyde content in the fabric.

Therefore, the samples should be taken with adhesive lining when sampling, and the ratio of lining to fabric should be consistent with each sample.

For example, Western-style clothes are usually taken from the front and collar parts, and the trousers are also taken from the waist of the trousers with adhesive lining, and the shirt is taken from the cuffs, collar and the neck.

2) pigment printing products

The color fastness to washing and rubbing of pigment printing products is mainly determined by the properties of the binder. The binder currently used mainly consists of self crosslinking adhesive of polyacrylate and its derivatives, which release a lot of formaldehyde when crosslinking or crosslinking with the hydroxyl of the fiber.

Because the fabric after printing is directly baked without washing, it is easy to have a large amount of free formaldehyde residue in the finished products. Therefore, the formaldehyde content of the printing part is different from that of the non printing part, and the difference is large.

Therefore, the printing and printing products should be handled with hard handle and printed parts when sampling.

For example, some patterns on the front and back of some cultural shirts, some large areas of pigment printing products try to take the same color and the same depth.

3) products containing fabric, fabric and stuffing.

Linings, fabrics and stuffing may be processed by different processes according to different functional requirements. For example, some products are waterproofing finishing, while the lining is softening and finishing. The finishing agents used in the two kinds of finishing are different in formaldehyde content, so the formaldehyde content in fabrics and linings is different.

For such products, the three should be sampled separately and measured separately.

4) spliced garments

The main fabric should be the main fabric or the main fabric specified in the product standard, or the sample location should be indicated in the inspection report.

Because of the difference in fiber composition and content, the adsorption and desorption rates of fiber and formaldehyde are also different, and the formaldehyde content in different parts is different.

2 storage of samples

In the process of selecting representative samples, the samples should be sealed in a timely manner, otherwise the accuracy of the test results will be affected.

In addition, the length of sampling time will result in a great difference in results.

Because formaldehyde has volatility, if the sample is exposed to air for a long time, formaldehyde will volatilize, which will affect the authenticity of formaldehyde content test results.

Therefore, samples should be quickly clipped and tested after receiving the samples, so as to ensure the authenticity of the test results.

Usually, the seal is kept in a PE bag, sealed, and then wrapped with aluminum foil.

This prevents formaldehyde from passing through the pores of the PE film and avoids direct contact between the sample and the aluminum foil, resulting in chemical reactions that may be left to the residual catalyst or other unwashed chemicals.

3 the number of samples tested.

When quantitative analysis is carried out by spectrophotometric method, because of the unknown concentration of the sample in the sample, in order to make the sample absorbance fall into the linear range, one method is to dilute the sample according to the different dilution ratio first, then obtain a dilution series, then carry out colorimetric analysis; another method is to estimate the sample concentration first, and then decide whether to further dilute or increase the amount of the sample according to the colorimetric condition.

In order to reduce workload, the second methods are commonly used.

It is very important to use Lambert Bill law to quantify formaldehyde and choose the measurement range of absorbance. If the light absorbs too large or too small, it will cause inaccurate quantitative results.

The derivation of Lambert Bill's law shows that the error of the measured results is the smallest when the sample absorbance is between 0.2~0.8, while the accuracy of the test results is guaranteed.

Therefore, under the condition of certain volume of extraction medium, the amount of sample will be the biggest factor affecting the measurement error.

If the sample absorbance is less than 0.2 due to the small number of samples, the number of samples should be increased to ensure the accuracy of the test results.

This method conforms to the first part of GB/T2912.1 "determination of formaldehyde in textiles: the free hydrolysis of formaldehyde (water extraction method)", that is, when the formaldehyde content in samples is too low, the number of samples should be increased from 1g to 2.5G.

Control of 4 color reaction

The period of validity and preparation of acetylacetone affects the formaldehyde test. It must be strictly managed to ensure that it is used within the validity period.

It should be noted that after the completion of the preparation of acetylacetone reagents, it is necessary to ensure that it can be used for 12 hours after storage. This is because the chemical properties of the Nash reagent used for coloration are not stable enough after completion of the preparation, and generally take 12 hours to achieve stability.

Glass bottles used for storage should be brown to prevent chemical reactions between light and reagents, affecting the color rendering effect.

The standard requires a mixture of 40 degrees centigrade and heat preservation (30 + 5) min, so that the reaction is complete. Next, the temperature should be kept at room temperature (30 + 5) min, so that the temperature of the solution can be cooled to room temperature, because the temperature has an effect on the absorption of light and the depth of color. This requires that the temperature of standard solution and the tested solution should be as consistent as possible during the determination process.

On the one hand, the avoidance of light preservation is due to the fact that the textile extract is a complex system. Some substances may be photochromic (enhanced or weakened) under light. On the other hand, acetylacetone does not react with light, and the color will turn yellow, which will affect the detection results.

Selection of 5 reference solutions

In the actual measurement of absorbance, the pmission light is weakened due to the absorption of reagents, such as reflection, solvent, different samples and different conditions.

In order to reduce the intensity of light only related to the concentration of the components in the solution, the above effects must be corrected.

For this reason, a colorimetric solution with the same optical quality and the same thickness can be used to store the reference solution. The absorbance A of the solution is measured and the total absorbance A of the solution to be measured is then measured.

Based on the additivity of absorbance, we can use A total -A parameter = "A" to be tested, so that the final absorbance of the solution to be tested can really reflect the concentration of the substance to be measured.

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In a specific experiment, the influence factors are different, so the reference solution is also different. That is to say, the reduction of light intensity may come from different aspects, and should be treated differently when the influence of absorbance is deducted.

Therefore, the reference solution should be carefully chosen, and specific experiments should be done, and specific methods should be adopted to solve them.

So the reference solution has "solvent blank" reference solution, "sample blank" reference solution and "developer blank" reference solution.

These three reference solutions are explained below.

(1) "solvent blank" reference solution.

When the extraction solution (in GB/T 2912.1-2009 refers to the solution of formaldehyde extracted from the textiles by distilled water) and the chromogenic agent (in GB/T2912.1-2009 refers to the already prepared acetylacetone) is not absorbed at the measuring wavelength, the pure solvent (in GB/T 2912.1-2009 refers to distilled water) is used as the reference solution, and is called the "solvent blank" reference solution.

This reference solution can eliminate some systematic errors.

The measured absorbance is equivalent to the result of GB/T 2912.1-2009 calculation formula: Ab in A= "As-Ab- (A d)" (in the standard is called the absorbance measured in blank reagent).

(2) reference blank for "sample blank"

When the wavelength is 412 nm, the developer is not absorbed, and the extract is colored. (in GB/T2912.1-2009, the sample is discoloured, showing the color of the textile, or part of the color of the fabric in the test solution after the textile is extracted.) when the extract is likely to be absorbed, the extraction solution should be used as the reference solution. We call it the reference blank solution.

The measured absorbance is equivalent to the result of GB/T 2912.1-2009 calculation formula: A D in A= "As-Ab - (A d)" (in the standard is called the absorbance measured in blank sample).

(3) "blank developer" reference solution.

If the chromogenic agent is absorbed at the measuring wavelength, the extraction solution is not absorbed at the wavelength of the measurement. In this case, the developer can be used as the reference solution, which we call the blank developer reference solution.

The blank developer reference solution was not used in GB/T 2912.1-2009.

6 confirmation of false positive results

The measurement of formaldehyde is based on the maximum absorption of formaldehyde and acetyl acetone at the wavelength of 412 nm, and then is quantified by Lambert Bill's law.

However, because the textile extract is a complex system, sometimes other substances may also be absorbed at this wavelength, which will result in a high measurement result.

For this problem, it is also mentioned in the standard that if the suspected absorbance value is not caused by formaldehyde, it can be confirmed by the use of double ketone, because the reaction product of formaldehyde and methyl ketone is not absorbed at 412 nm.

The results of such experiments may be in 3 cases:

(1) the absorbance at 412 nm is equal to zero.

It shows that the absorbance of the test sample is completely produced by formaldehyde, and the test results of the test sample are maintained.

(2) the absorbance at 412 nm is equal to the absorbance of the test sample.

It shows that the absorbance of the test sample is completely produced by other substances other than formaldehyde, and the formaldehyde content of the test sample is not detected.

(3) the absorbance at 412 nm is not equal to zero.

And less than the absorbance of the test sample.

It shows that the absorbance of test sample is produced by formaldehyde and partly by other substances.

The difference between the absorbance of the test sample and the absorbance of the confirmation experiment of the methyl ketone was calculated, and the actual formaldehyde content of the sample was obtained by combining the standard curve.

7 other considerations in the testing process

Formaldehyde content test is a test with high operation and technical requirements.

The calibration of formaldehyde solution, the determination of standard curve, the mastery of temperature and time, and the choice of spectrophotometer before testing will have a direct impact on the test results. Therefore, attention must be paid to the accuracy of the test results.

In the formaldehyde content test process, we should pay attention to the following points:

1) there are several colorimetric dishes in the general laboratory. Due to the uniformity of material and the slight difference of thickness, the same solution is tested with different cuvette. There are some differences in absorbance.

Therefore, during the testing process, the test personnel should keep the consistency of the cuvette as far as possible. No matter the blank reference or the parallel sample of the extraction solution, the same colorimeter should be guaranteed as far as possible, so as to minimize the effect of different colorimetric dishes on the test results.

In addition, after the use of cuvette, it should be immediately cleaned and regularly washed with acetone (or diluted acid) to avoid coloring of cuvette.

2) no matter the drawing of the formaldehyde standard curve or the test of the sample, it is necessary to move a certain volume of related solution. The accuracy of formaldehyde concentration test is directly related to the accuracy of the volume of the solution.

Therefore, in practical operation, the migration of related solutions should be taken seriously and moved accurately.

When the formaldehyde standard curve is produced and the standard solution needs to be moved at a time, the same group of pipettes should be used as far as possible to reduce the system error and make the standard curve have a high linear correlation coefficient.

3) the formaldehyde standard solution used in GB/T 2912.1-2009 is formulated and calibrated by the operator itself. It needs to be calibrated several times. The end-point is difficult to determine. It is cumbersome and time-consuming. There are many man-made uncertainties. It is prone to produce accidental errors and systematic errors, and the standard solution can only be kept for 4 weeks.

Therefore, it is suggested that the conditioned laboratory can be converted to a certain concentration of standard formaldehyde solution to reduce the test error.

8 conclusion

The testing of formaldehyde content in textiles is a highly technical project. It not only requires operators to master standards, but also requires operators to have certain professional knowledge and skills in chemistry.

Therefore, more attention should be paid to the implementation of the project.