In recent years, the limit of formaldehyde in leather has become more and more common, especially the tanneries that provide raw materials for brands are aware of this. These limited demands mainly come from the following aspects:
Laws and regulations formulated by the government;
EU eco-label or other similar ecological planning;
The limit requirements of individual companies for formaldehyde.
Because these requirements come from different institutions, different types of standards have been issued in the past few years, so the form is becoming more and more severe for tanneries, and many formaldehyde determination methods have also appeared.
Due to the potential carcinogenicity of formaldehyde to human body, it is urgent to produce leather with low free formaldehyde, which also promotes the development of the determination method of low concentration formaldehyde.
The leather industry (including tanneries, chemical suppliers and leather manufacturers) has adapted to this limited requirement, and the leather produced has met the requirements of authoritative organizations and customers. As a reliable leather supplier in the leather industry, Corrine has been at the forefront of the industry in developing retanning materials with low free formaldehyde. In this process, we also tried a variety of formaldehyde determination methods, and we hope to share our experience with more people to achieve the following purposes:
Exchange experience with researchers in the same industry;
Help tanners or other manufacturers to choose suitable determination methods;
Help the makers of laws and regulations to stipulate appropriate determination methods;
Formaldehyde is one of the most productive chemicals in industry. Generally, the commercially available formaldehyde is a solution with a concentration of 35-50% (mass fraction). However, in order to protect the environment, the high concentration formaldehyde in chemical plants needs to be handled carefully. This paper only discusses the determination methods of trace formaldehyde in chemicals, leather and leather products.
The determination of formaldehyde itself is not difficult to operate, but there are still the following problems in the determination process:
Extraction and separation of formaldehyde;
The formaldehyde content in the extract was determined without the interference of formaldehyde compounds.
According to the extraction methods, we can divide the determination methods of formaldehyde into the following four categories:
(1) Complete extraction method: completely crush the product and collect all formaldehyde added in the production process. Typical is the method described in the first part of BS6806.
(2) Water extraction method: soak the sample in water with or without surfactant, and extract water-soluble substances (including formaldehyde) from the substrate. This method has been widely used to determine the formaldehyde content in shoe upper leather, leather parts and leather toys, but there are also some problems, mainly because the extract contains a lot of formaldehyde compounds, which affects the determination results. When determining the formaldehyde content in the synthetic tanning agent, almost 100% of the synthetic tanning agent is dissolved, and these formaldehyde compounds may decompose to produce formaldehyde during the determination, so the experimental result obtained by this method is "the total amount of free formaldehyde and hydrolyzed formaldehyde". The characteristics of this determination method determine that the determination results will change with the change of determination conditions. This method has been adopted by Japanese regulations 1 12, JIS104, part II of BS6806, IUC 19, ISO/TS 17226: 2003.
When using this extraction method, it is worth noting that the extract can be determined by the following two methods:
Colorimetry: Acetylacetone can react with formaldehyde under buffer condition (pH is about 7) to generate yellow substance, and then the absorbance is measured by ultraviolet-visible spectrophotometer.
HPLC: 2,4-dinitrophenylhydrazine (DNPH) can react with aldehydes (including formaldehyde) and ketones to form complexes, which are also determined by UV-Vis spectrophotometer. After separation by HPLC, the content of aldehydes and ketones can be quantitatively determined. This kind of reaction can be carried out in organic medium (solvent) containing phosphoric acid or hydrochloric acid, and the apparent concentration of formaldehyde measured under different reaction conditions is different.
(3) Saturated steam extraction method: suspend the sample in a sealed bottle filled with distilled water, raise the temperature, and bring the volatile substances into the water with saturated steam. This method is very popular in the textile and automobile industries. Typical methods include JIS/KOOC-0/04/KOOC-0/,AATCC-/KOOC-0//KOOC-0/2, the third part of BS 6806, ISO/KOOC-0/4/KOOC-0/84-2 and individual enterprise standards (such as Volkswagen).
Steam extraction has obvious advantages, that is, only volatile substances can be extracted, and the interference of formaldehyde compounds and substrate color is minimized. However, the determination conditions are harsh, and some chemicals are easy to decompose under these conditions, which leads to high experimental apparent results.
(4) Gas extraction: This method is the development trend in some fields.
For easy handling, the product is made into powder.
For the domestic construction industry and automobile industry, it is impossible to completely soak the leather in the determination process.
Usually, nitrogen flow or air flow is used to extract substrates (leather or chemicals) at high temperature. Typical examples are TEGEWA method for determination of formaldehyde content in leather auxiliaries, JAMA method for determination of automobile volatiles recently adopted, and methods commonly used by some enterprises (such as Toyota Motor Corporation).
The diversity of determination methods will inevitably lead to confusion. Among these methods, the two most critical ones are extraction and detection. No matter which detection method is adopted, we can get quite accurate and repeatable results. In recent years, people have done a lot of research work on detection methods, among which acetylacetone colorimetry and DNPH-HPLC are favored, but the research on extraction methods is quite limited, so the research on extraction methods is particularly important.
In this paper, the application effects of several extraction methods were compared with the actual products and simulated compounds as substrates. Because all the experimental results are selected from our daily work, the results are not systematic.
The following is an overview of the experimental process of the three determination methods:
Water extraction method: soak the ground leather sample in water containing surfactant, keep it at 40℃ for 65438 0 h, and then measure the formaldehyde content in the filtrate by acetylacetone method after filtration.
Steam extraction method: suspend the sample in a sealed bottle filled with water, keep it at a certain temperature for a certain time, and then determine the formaldehyde content in the liquid by acetylacetone colorimetry.
Gas extraction method: the sample was extracted with nitrogen flow at high temperature, and the extracted formaldehyde was collected in an extraction column containing DNPH. Finally, it was analyzed by high performance liquid separation and ultraviolet detection.
2 experiment
2. 1 commercial leather samples
In order to test the applicability of water extraction method and steam extraction method, seven kinds of leather samples (1 leather sample with low, medium and high formaldehyde content and four kinds of leather samples with particularly high formaldehyde content) were selected for comparative study. These leather samples are simply neutralized, retanned and fatliquored, and are not dyed. In previous experiments, we noticed the difference between water extraction and steam extraction, so in this experiment, we chose more samples with higher formaldehyde concentration for comparative study.
The formaldehyde content obtained by different extraction methods (steam extraction under two conditions) is shown in the following table. Each experiment was repeated twice, and the average of the two results was taken.
As can be seen from the table, long-term extraction under saturated steam will decompose the synthetic tanning agent into formaldehyde, which makes the determination result high. This experiment mainly discusses the experimental results obtained under two most commonly used extraction conditions: extracting 65438 0 h in water at 40°C and suspending 3h in saturated steam at 60 C.
Under the condition of low formaldehyde content, the experimental results obtained by these two extraction methods are consistent. But when the formaldehyde content is high, the results obtained by the two extraction methods are quite different. As can be seen from the table, when steam extraction is used, the determination results of sample No.4 are higher, while when water extraction is used, the determination results of samples No.3, No.6 and No.7 are higher, so we can conclude that the determination results will be higher when water extraction is used.
Table 1 Formaldehyde content of leather obtained by different extraction methods (ppm, based on the quality of standard leather samples)
Leather sample
Water extraction with surfactant, 40 C1hr.
Steam extraction method
60 degrees Celsius for 3 hours
Steam extraction method
49 degrees Celsius for 20 hours
1
3.7
8.5
8.6
2
37
36
13 1
three
90
6 1
174
four
204
265
660
five
387
379
1 100
six
143
7 1
409
seven
302
160
475
2.2 Extraction rate
The reason why the experimental results of water extraction method are higher may be that its extraction rate is higher than that of steam extraction method. In order to verify this possibility, we have arranged the following experiments. Firstly, the rawhide was treated with formaldehyde-free chemicals, and then the formaldehyde content of blue wet leather and rawhide was determined by these two extraction methods respectively. The experimental results confirmed that the formaldehyde content was less than 5ppm. The amount of formaldehyde in the sample is known, and then two extraction methods are used to determine the formaldehyde content and calculate the extraction rate. The results are shown in the following table.
Table 2 Formaldehyde Extraction Rate (Leather)
figure
Amount of formaldehyde added
milligram (mg)
Formaldehyde extraction amount (water extraction method)/mg
Extraction speed
(%)
Amount of formaldehyde extracted (steam method)/mg
Extraction speed
(%)
1
0.06
0.07
1 17
0.07
1 17
2
0.09
0.086
96
0.095
106
three
0. 15
0.09
60
0. 13
87
four
0.30
0. 18
60
0.27
90
average value
83
100
The above experimental results clearly show that the steam extraction method has higher extraction efficiency for free formaldehyde and reversibly bound formaldehyde in leather samples, while the water extraction method has lower efficiency, which may be related to the sample preparation process. Steam extraction method does not need special treatment for leather samples, while water extraction method needs grinding and other treatments, which may cause the volatilization of free formaldehyde or reversibly bound formaldehyde and cause losses.
2.3 Hydrolyzed urea-formaldehyde resin
As we all know, urea-formaldehyde resin is easy to decompose at low pH value, so this kind of resin (industrial product) is selected as the comparison sample of several standard determination methods.
Table 3 Formaldehyde content in urea-formaldehyde resin samples (ppm, based on sample quality)
assay method
Formaldehyde content
Water extraction method
35000
Steam extraction method
12000
Gas extraction method
450
It can be clearly seen from the table that the results of water extraction and steam extraction are obviously higher than those of gas extraction, because the products are decomposed in water.
2.4 Chemical research
In order to discuss the difference between these two methods (water extraction and steam extraction), we have done further research. Firstly, the effects of two extraction methods on simulated molecules were compared. In the process of reducing the formaldehyde content in synthetic tanning agents, we also developed a large number of active molecules (also known as "formaldehyde scavengers") that can react with formaldehyde. In this experiment, our preferred formaldehyde scavenger is a small molecule compound with three or more active hydrogen atoms, but we hope that the following experiment can be applied to any kind of formaldehyde scavenger. For commercial reasons, we will call this small molecular substance PLX for short. In addition, the dosage of formaldehyde scavenger is different because of its different reactivity, but it will not affect the experimental results in theory.
Synthesis of mimic molecules: Firstly, 5 mol 5mol PLX, 1mol formaldehyde and 10mol water were mixed, and then the reaction was carried out at 40℃ for 4 hours. After the reaction, the obtained suspension was left at room temperature for at least 48 hours, then filtered to obtain filtrate, and dried to obtain pale yellow crystalline substance, which is the reaction product of PLX and formaldehyde, that is, the simulated molecule used in the experiment.
Formaldehyde analysis: Dissolve the above products in water and prepare solutions with concentrations of 2%, 1%, 0.5% and 0. 1% respectively, and then detect the formaldehyde content in the solutions by the following two methods. The first method is to filter the solution with a concentration of 0.5%, and then extract the formaldehyde from the filter paper by steam extraction. The second method is to directly react the solution with acetylacetone and then calculate the formaldehyde content. The experimental results are shown in the following table, and the theoretical removal rate of formaldehyde is calculated.
Table 4 Apparent formaldehyde concentration of reaction products of PLX and formaldehyde (calculated by dry weight of leather samples)
solution concentration
Direct determination (ppm)
Steam extraction (ppm)
2%
303
44 1
1%
1075
432
0.5%
3 1 12
280
0. 1%
15367
125
As can be seen from the table, with the change of solution concentration, the apparent concentration of formaldehyde obtained by steam extraction method has little change, indicating that the reaction product of PLX and formaldehyde is quite stable under this condition (saturated steam) for a long time (3h).
On the contrary, with the decrease of solution concentration, the apparent concentration of formaldehyde obtained by water extraction increases continuously. When the initial concentration is relatively high (2%), the results obtained by these two methods are not much different. When the initial concentration is low (0. 1%), the apparent concentration of formaldehyde obtained by water extraction is obviously higher than that by steam extraction (about 50 times).
We use the following methods to study the source of error. In general, the reaction product of PLX and formaldehyde is very stable, but it will inevitably release a small amount of free formaldehyde, as shown in equation 1. The reaction product decomposes to produce free formaldehyde, so the water in the determination medium absorbs formaldehyde at a certain rate, and the reaction proceeds to the left when it reaches equilibrium, so it can be said that the reaction product is stable. However, the steam extraction method will not appear this phenomenon, and it has no effect on the reaction regardless of the initial concentration.
For water extraction (equivalent), the situation is more complicated. At the same time, the reaction product of PLX with formaldehyde and acetylacetone is added into water, and the free formaldehyde decomposed from the reaction product reacts with acetylacetone, as shown in Equation 2. Because of the existence of another reactive molecule, the equilibrium of decomposition reaction shifts to the right.
PLX form? PLX+formaldehyde (1)
PLX- form+communication-communication? PLX+form+communication-communication? PLX+communication-communication-form (2)
It can be seen from the equation that the amount of formaldehyde reacting with acetylacetone is related to the ratio of acetylacetone to PLX products. In the determination process, the concentration of acetylacetone is fixed, so the apparent concentration of formaldehyde measured is inversely proportional to the concentration of PLX- formaldehyde used in the determination process. The following chart can prove this argument. As can be seen from the figure below, the relationship between them is not linear, which also shows that the reaction is not a simple first-order reaction. This experiment does not involve specific reaction mechanism research, but it is worth noting that when the concentration of PLX- formaldehyde is low, the measured apparent concentration of formaldehyde increases rapidly.
3 discussion
As we all know, different extraction methods for the determination of formaldehyde content in leather have different experimental results. The experimental results of extraction rate also show that the efficiency of steam extraction method is higher, while the efficiency of water extraction method is lower, which is mainly caused by the following two reasons. Firstly, the water extraction method needs to grind the sample into fine particles, which may cause the loss of free formaldehyde in the grinding process, thus making the apparent concentration of formaldehyde low; Secondly, because leather can effectively absorb formaldehyde, it is impossible to completely extract formaldehyde in this equilibrium reaction. Therefore, theoretically speaking, the efficiency of water extraction method is lower than that of steam extraction method when extracting free formaldehyde or reversibly bonded formaldehyde.
Steam extraction can only extract small molecular substances with high vapor pressure, such as formaldehyde, while water extraction can extract any substance that is easily soluble in water. In the standard water extraction experiment, not only formaldehyde can be extracted, but also unstable synthetic tanning agents, reaction products of formaldehyde and formaldehyde scavenger can be extracted. Therefore, reagents, acetylacetone or DNPH can not only react with formaldehyde, but also capture formaldehyde molecules from some oligomers (synthetic tanning agents). Obviously, the latter is not formaldehyde, but a substance that is stable under normal or even more severe conditions, so the formaldehyde content measured under this condition is untrue and has no practical application value.
In practical application, the release of formaldehyde in leather is usually related to water vapor. For upper leather, the warm and humid environment inside the shoe may lead to the release of formaldehyde. Although this effect is not saturated steam (mild condition), it is an open system, but it is similar to the condition of steam extraction in theory. The same is true of the watchband. In garment leather, decorative leather and other kinds of leather, the release of formaldehyde is very similar to gas phase extraction.
On the other hand, steam extraction plays a strong role, especially when the temperature is high or the treatment time is long, saturated steam is easy to decompose relatively stable molecules, so we need to determine the appropriate conditions according to the application purpose of the finished product.
Acetylacetone chromogenic method is used in this experiment. Some people hope to improve this situation by DNPH/HPLC, but the results show that DNPH/HPLC can only reduce the experimental error caused by color or other similar factors. DNPH is highly reactive with formaldehyde, and it can also extract formaldehyde from formaldehyde compounds (as shown in Formula 2), resulting in the same error as acetylacetone. Therefore, the reaction products cannot be separated by HPLC.
4 conclusion
When the formaldehyde content in leather is about 100ppm, both water extraction and steam extraction are effective. When the concentration is high, the efficiency of steam extraction method remains unchanged, while that of water extraction method decreases, which may be related to the sample preparation process. Gas extraction can only extract free formaldehyde, which has potential application prospects. Water extraction can not only extract free formaldehyde and reversibly bound formaldehyde, but also extract water-soluble synthetic tanning agents, resins or other formaldehyde compounds (a stable substance generated by reaction with other substances, which can be decomposed under certain conditions, that is, formaldehyde compounds under such conditions). The simulated molecular experiment shows that the formaldehyde detection reagent used in formaldehyde detection method can decompose stable formaldehyde compounds, resulting in higher apparent concentration of formaldehyde, which can also be used to explain why the same leather sample is treated by two different extraction methods, but the experimental results are so different. In addition, in order to provide more reference for the makers of laws and regulations and testing experts, we still have a lot of specific work to do.
To annotate ...
There are no official references listed in this article, but the following information should be helpful to relevant personnel, and some standards are listed below. Because this article only discusses some determination methods, it does not pay much attention to the effectiveness of these determination methods at present.
Limit standard and industry requirements of formaldehyde content in leather;
Please refer to Japanese law112;
European Union eco-label on footwear, SG trademark and Oko Tex.
Requirements of individual companies (Volkswagen, Toyota, Nike, etc. )
Carcinogenicity of formaldehyde:
IARC's monograph on formaldehyde was published by the International Cancer Organization in Lyon, France.
Some typical determination methods:
Domestic standards/international standards: BS 6806, JIS/KOOC-0/04/KOOC-0/,ISO/TS/KOOC-0/7226: 2003, ISO/KOOC-0/4/KOOC-0/84;
Associations: IUC 19, AATCC- 1 12, JAMA press releaseNo. 14, February 2005;
Trade organizations: PV3925 of Volkswagen, TSM0508G-a of Toyota, and the determination method proposed by TEGEWA.