Indigo dyeing method

Abstract

The invention relates to the field of fine chemical engineering, and discloses an indigo dyeing method which comprises the following steps: 1) mixing water, small molecular alcohol, sodium hydroxide, thiourea dioxide and indigo to prepare a dye liquor with the pH value of 8-11; the small molecular alcohol is at least one of methanol, ethanol, isopropanol and tert-butyl alcohol; in the dye liquor, the concentration of the small molecular alcohol is 20-70 vol.%; (2) reducing the dye liquor obtained in the step (1), and cooling to 30 DEG C or below; the reduction treatment comprises the following steps: the treatment temperature is 40-80 DEG C, and the treatment time is 20-90 minutes. 3) dyeing the bleached natural fibers by using the dye liquor obtained in the step 2). According to the method, the reduction of indigo is promoted by the micromolecular alcohol, the alcohol generates reductive alcohol free radicals by capturing oxidizing species in a system, and meanwhile, the alcohol is used as a hydrogen donor, so that the transfer of protons and electrons in the system is promoted, and the reduction of the indigo is enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fine chemicals, in particular to a method for indigo dyeing, more particularly to a method for reducing indigo dye with sulfourea dioxide in an alcohol / water two-component mixed solution system. BACKGROUND

[0002] Indigo dye belongs to the category of vat dyes. Vat dyes are an important class of dyes used for dyeing and printing of cellulose fiber fabrics. They have bright colors, complete color spectrum, and outstanding fastness to light and washing, and are currently the second largest in dye production. However, vat dyes lack hydrophilic groups in their molecular structure, which results in poor water solubility and no affinity to fibers, so they cannot be directly dyed. Vat dyes need to be reduced to leuco form (monosodium or disodium salt) which has water solubility and good affinity to fibers, and then oxidized to convert to the original molecular structure and fixed on the fiber.

[0003] The main reducing agents for indigo used in the industry at present are sodium dithionite (Na2S2O4, sodium hyposulfite) and sulfourea dioxide (CH4N2O2S, TD). Sodium hyposulfite and sulfourea dioxide have strong reducing properties in alkaline conditions and can reduce indigo to leuco form. However, sodium hyposulfite is extremely active in chemical properties and has poor stability during storage and use. It can easily react with oxygen in the air, leading to its own decomposition and failure. This not only requires strict control of its storage conditions, increasing storage costs and risks, but also limits the reducing capacity of sodium hyposulfite. In order to achieve ideal dyeing depth and uniformity, a large amount of sodium hyposulfite is often used, which not only increases production costs but also wastes resources. The many drawbacks of sodium hyposulfite greatly limit its sustainable application in the dyeing field. Sulfourea dioxide has good thermal stability, safe storage and transportation, and no irritating odor, and has become a main substitute for sodium hyposulfite for vat dye reduction in industrial production. However, its reduction process is a high-alkali, high-salt, and high-energy consumption process, and because of the mutual consumption of active species in the system, the actual dosage is much higher than the theoretical dosage.

[0004] In addition, traditional dyeing processes usually use single water as the medium. When reducing indigo in single water medium, there are not only reducing active species but also oxidizing active species in the system, which will consume each other, making the reduction require a large amount of reducing agent and high-temperature high-salt reduction conditions.

[0005] With the continuous enhancement of environmental awareness and the deepening of the concept of sustainable development, it is urgent to develop an environmentally friendly, efficient, and economical indigo dyeing method. Finding a more green and environmentally friendly reduction dyeing system is still an urgent need at present. SUMMARY

[0006] The present application aims at overcoming the problems of high energy consumption, high alkali and high salt, and large amount of reducing agent in the indigo reduction process in the prior art, and provides an indigo dyeing method, which realizes indigo dyeing under the conditions of less salt, less alkali, lower temperature and less reducing agent, and realizes the cleanization and energy saving of the indigo reduction process.

[0007] In order to achieve the above-mentioned purpose, the present application provides an indigo dyeing method, which comprises: 1) mixing water, small molecule alcohol, sodium hydroxide, sulfourea dioxide and indigo and configuring into a dyeing solution with pH of 8-11; The small molecule alcohol is at least one of methanol, ethanol, isopropyl alcohol and tert-butyl alcohol; and the concentration of the small molecule alcohol in the dyeing solution is 20-70 vol.%; 2) reducing the dyeing solution obtained in step 1) and cooling to below 30℃; The reducing treatment comprises: the treatment temperature is 40-80℃, and the treatment time is 20-90 min; 3) using the dyeing solution obtained in step 2) to dye the bleached natural fiber.

[0008] Preferably, in step 1), the molar ratio of the amount of the indigo to the amount of the sulfourea dioxide is 1:2-1:4.

[0009] Preferably, in step 1), the molar ratio of the amount of the indigo to the amount of the sulfourea dioxide is 1:3-1:4.

[0010] Preferably, in step 1), the content of the indigo in the dyeing solution is 6-7 g / L.

[0011] Preferably, in step 1), the concentration of the small molecule alcohol in the dyeing solution is 40-50 vol.%. Preferably, in step 2), the temperature of the reducing treatment is 50-70℃, and the time is 25-40 min.

[0012] Preferably, in step 3), the bath ratio of the natural fiber to the dyeing solution is 1:35-45.

[0013] Preferably, in step 3), the time of the dyeing is 20-40 min.

[0014] Preferably, the method further comprises: 4) oxidizing the natural fiber obtained in step 3) in air, and then sequentially performing soaping and air drying.

[0015] Preferably, the reducing agent is not sodium hydrosulfite in the dyeing solution.

[0016] Compared with the prior art, the present application has the following advantages: 1. The present application introduces a suitable proportion of small molecule alcohol into the existing single water medium to form an alcohol / water two-component mixed solution system, then adds sulfurous anhydride as a reducing agent, adjusts the pH to weak alkaline, and generates indigo leuco sodium salt at low temperature to construct an indigo reduction dyeing reaction system. In the system, the addition of alcohol can solve the problems of high alkali, high salt and high energy consumption in the traditional indigo reduction dyeing process, and realize dyeing under the conditions of less salt, less alkali, lower temperature and less reducing agent, which is a new type of ecological dyeing technology.

[0017] 2. In the alcohol / water mixed solution medium of the present application, small molecule alcohol acts as a hydrogen donor and reacts with oxidizing species in the system to generate alcohol radicals through hydrogen extraction process, which protects the reducing species in the reaction system and reduces the amount of TD. Alcohol radicals can also promote the decomposition of TD, reducing the pH value and reduction reaction temperature of the indigo reduction system.

[0018] 3. The addition of small molecule alcohol in the dyeing system of the present application significantly shortens the reduction reaction time of indigo dyeing solution, and the small molecule alcohol has less harm. In the formed alcohol / water two-component medium, small molecule alcohol is easy to separate and can be recycled, realizing the cleaning and energy saving of the indigo reduction process, which is an efficient and environmentally friendly green dyeing technology.

[0019] 4. Compared with the traditional indigo dyeing system whose pH needs to be maintained above 11, the pH of the dyeing system of the present application only needs to be adjusted to weak alkaline, greatly reducing the consumption of alkali. When the indigo dye of the present application is reduced, the temperature used in the present application is much lower than the existing indigo dyeing temperature, and the reduction treatment time is only 20-90 min, greatly reducing the energy consumption in the indigo dyeing process.

[0020] 5. In the dyeing system of the present application, since alcohol / water two-component medium is used, there is no need to add sodium hydrosulfite, and since the reducing agent used is sulfurous anhydride, which is chemically stable and will not explode and burn to produce high temperature due to impact or water, the reaction is relatively mild.

[0021] 6. The dyeing system of the present application has a wide range of applications, and is suitable for natural fiber materials including cotton, dyed hemp, silk and wool, and has better dyeing effect than traditional indigo dyeing. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a schematic diagram of the time required for complete reduction of indigo and the color change rule in the pure water medium of test example 1 of the present application.

[0023] Figure 2 is a schematic diagram of the time required for complete reduction of indigo and the color change rule in the alcohol / water mixed solution system of test example 2 of the present application. DETAILED DESCRIPTION

[0024] The specific embodiments of the present application are described in detail below. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the application, are given by way of illustration only, and are not by way of limitation.

[0025] The endpoints of the ranges and any values described herein are not limited to the precise values stated. The ranges and values should be construed to be approximations that allow for significant variation. Various ranges of values are stated in terms of being between two specific values. When values are provided as between lower and upper values, this is intended to include the values exactly and also include the values from just above the lower value to the value from just below the upper value. When values are provided as between two specific values, this is intended to include the values exactly and also include the values from just above the lower value to the value from just below the upper value. When values are provided as between lower and upper values, this is intended to include the values exactly and also include the values from just above the lower value to the value from just below the upper value. For ranges of values, the endpoints of the ranges are included in the ranges, and the end points can be combined with the single values to create new ranges within the scope of the present application.

[0026] In the description of the present application, the terms "first", "second", "third", etc., are used only to describe the purpose of the description, and cannot be understood as indicating relative importance, or implying the number of the technical features indicated. Therefore, unless otherwise specified, the features defined with "first", "second" can explicitly or implicitly include one or more of the features; the meaning of "multiple" is two or more. The term "comprise" and any variation thereof means non-exclusive inclusion, and one or more other features, integers, steps, operations, units, components and / or combinations thereof can exist or be added.

[0027] The method for indigo dyeing according to the present application comprises: 1) mixing water, a small molecule alcohol, sodium hydroxide, sulfourea dioxide and indigo to prepare a dyeing solution with a pH of 8-11; The small molecule alcohol is at least one of methanol, ethanol, isopropanol and tert-butanol; the concentration of the small molecule alcohol in the dyeing solution is 20-70 vol.%; 2) reducing the dyeing solution obtained in step 1) and cooling it to below 30℃; The reducing treatment comprises a treatment temperature of 40-80℃ and a treatment time of 20-90 min; 3) using the dyeing solution obtained in step 2) to dye the bleached natural fibers.

[0028] In the preparation of the dyeing solution of step 1) described above, in some specific embodiments, water and a small molecule alcohol can be mixed first, and after the alcohol / water two-component system is obtained, sulfourea dioxide is added, then sodium hydroxide is added to adjust the pH of the solution to 8-11, and finally indigo is added to start the reduction.

[0029] In the dyeing solution of step 1) described above, the alkali agent used to adjust the pH of the dyeing solution is not limited, and those skilled in the art can select it according to the alkali agents commonly used in the art, which can be sodium hydroxide and sodium bicarbonate commonly used, and preferably sodium hydroxide.

[0030] The small molecule alcohol in the application is an alcohol compound with simple molecular structure and small relative molecular mass. In the alcohol compound, the number of carbon atoms is generally 1-4, and the hydroxyl group (-OH) is directly connected to a saturated carbon atom. The small molecule alcohol in the application can be at least one of methanol, ethanol, isopropanol and tert-butanol. In the above step 1), during the preparation of the dye solution, the concentration of the small molecule alcohol in the dye solution can be further preferably 40-50 vol.%. In this range, the solubility of the reducing agent and the alkali agent in the alcohol / water two-component system is better, the reduction speed of indigo can be accelerated, and the dyeing effect can be improved.

[0031] In the application, the alcohol / water two-component system is selected instead of the traditional single water system, which can overcome the need for a large amount of reducing agent and high-temperature and high-alkali reduction conditions due to the influence of active species in the system when using single water as the medium. In the alcohol / water two-component system in the application, the small molecule alcohol promotes the reduction of indigo. The alcohol generates alcohol radicals with reducing properties by capturing oxidizing species in the system. The alcohol as a hydrogen donor promotes the transfer of protons and electrons in the system, and strengthens the reduction speed of indigo. The small molecule alcohol in the dye is easy to separate and can be recycled, realizing green and efficient reduction dyeing technology.

[0032] The sulfur dioxide in the application is a strong reducing agent. The sulfur atom is +4 valence, which is easily oxidized to +6 valence. The reduction potential is higher than that of sodium dithionite (sodium hydrosulfite), and the reducing property is stronger under alkaline conditions. In the above step 1) of the application, during the preparation of the dye solution, the molar ratio of the indigo to the sulfur dioxide can be 1:2-1:4, preferably 1:3-1:4, and further preferably 1:3-3.5. The application selects the sulfur dioxide as the reducing agent because it is chemically stable and will not produce high temperature due to impact or contact with water. In the alcohol / water two-component system constructed in the application, the alcohol radical has stronger nucleophilic ability than OH - which can accelerate the decomposition of sulfur dioxide and reduce the pH and temperature required for decomposition, thereby achieving the purpose of reducing the amount of alkali agent.

[0033] In the above step 1) of the application, the content of the indigo in the dye solution can be 6-7 g / L.

[0034] In the step 2) of the method, the process conditions of the reduction treatment of the dyeing solution can be further preferred as follows: the treatment temperature is 50-70 DEG C, and the treatment time is 25-40 min. Compared with the traditional indigo dyeing solution reduction temperature, the reduction effect is better due to the disappearance of the oxidizing species in the method to generate the reducing active species. The indigo dye is in a uniform dispersed state in the alcohol / water two-component medium, which facilitates the method to use a lower reduction temperature for the reduction treatment of the dyeing solution.

[0035] In the process of indigo dyeing, the indigo is not soluble in water, so the indigo dye is first reduced to water-soluble leuco body sodium salt (i.e. indigo white) under alkaline conditions by a reducing agent. After the leuco body is dyed on the fiber, it is re-oxidized to form insoluble indigo particles, which are mechanically deposited in the fiber pores to form a washable blue color. In this process, the presence of oxygen will oxidize the reducing agent, reduce the reduction efficiency, and result in insufficient leuco body production. In the process of indigo dyeing, the steps 1) and 2) of the method can be carried out in an oxygen-free atmosphere. The oxygen-free atmosphere provided by the method can be an oil layer covering the surface of the dyeing solution to isolate air, or nitrogen gas can be introduced into the dyeing solution to drive oxygen, or the process can be carried out in a sealed container.

[0036] In the step 3) of the method, the dyeing solution after the reduction treatment is used for dyeing natural fibers. The bath ratio of the natural fibers to the dyeing solution can be 1:35-45, preferably 1:38-42. That is, 1 g of natural fibers needs to use 35-45 mL of dyeing solution, preferably 38-42 mL of dyeing solution.

[0037] In the step 3) of the method, the dyeing solution obtained in the step 2) is used for dyeing the bleached natural fibers. The dyeing time can be 20-40 min, preferably 25-30 min.

[0038] In the process of indigo dyeing, after the dyeing of the natural fibers is completed, the fibers are generally contacted with oxygen to oxidize the leuco body adsorbed in the fibers to insoluble indigo to form a firm color deposit. The method generally further includes: 4) the natural fibers obtained in the step 3) are oxidized in air, and then are subjected to soaping and drying. Due to the insufficient oxidation of the leuco body, the dyed fibers have a light color and poor color fastness, especially the rubbing fastness and light fastness. The leuco body that is not completely oxidized can fall off in subsequent washing, resulting in color change and staining. Therefore, the natural fibers need to be fully oxidized during the oxidation in air.

[0039] Unlike the traditional dyeing system, the dyeing solution of the present application does not include hydrosulfite. In the present application, due to the selection of the alcohol / water two-component mixed solution system, the dyeing solution does not need to use hydrosulfite, and only relies on the indigo reductant thiourea dioxide to complete the reduction process of indigo dye under alkaline conditions. Since hydrosulfite is not needed, the dyeing solution of the present application is more stable during storage and use.

[0040] The method for indigo dyeing of the present application will be further illustrated by examples. The examples are implemented on the premise of the technical solutions of the present application, and detailed implementation modes and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0041] In the following examples, the experimental methods are the conventional methods in the art unless otherwise specified. The experimental materials used in the following examples are commercially available unless otherwise specified.

[0042] The following test example 1 and test example 2 are set to compare the speed of reduction of indigo in single water medium and alcohol / water two-component mixed solution system. The specific method is: using the way of controlling variables, under the condition of ensuring other factors unchanged, adding ethanol, using stopwatch and ultraviolet means to monitor the reduction process of indigo dye in two different solutions during the experiment, testing the ultraviolet absorption wave of indigo and recording the time required for the absorbance to become 0 (indigo becomes leuco body after reduction, the ultraviolet out-of-peak position of leuco body and indigo is different, and indigo no longer out-of-peak after complete reduction), so as to judge the speed of reduction.

[0043] Test example 1 Take 0.15g sodium hydroxide and 0.25g thiourea dioxide and add them to 25mL water, stir with a magnetic stirrer until completely dissolved, then add 0.05g indigo, take good anti-oxidation measures, adjust the temperature of the water bath to 80℃ and start the reduction, according to the external absorption wave of leuco body, record the time required for complete reduction as 14min, and the change rule of the color of the dyeing solution with time during the reduction process is as shown in Figure 1 .

[0044] Test example 2 Take 0.15g sodium hydroxide and 0.25g thiourea dioxide and add them to 12.5mL water and 12.5mL ethanol mixed solution, stir with a magnetic stirrer until completely dissolved, then add 0.05g indigo, take good anti-oxidation measures, adjust the temperature of the water bath to 80℃ and start the reduction. According to the external absorption wave of leuco body, record the time required for complete reduction as 4min, and the change rule of the color of the dyeing solution with time during the reduction process is as shown in Figure 2 .

[0045] The reduction time of Test Example 1 and Test Example 2 shows that the two-component mixed solution system of ethanol / water can greatly reduce the time required for reduction compared with the reduction system of single water medium.

[0046] Test Example 3 0.2 g of sodium hydroxide, 0.2 g of thiourea dioxide, 0.01 g of indigo, and different volume concentrations of ethanol solution were weighed and prepared into a solution with a total volume of 500 ml. Under anaerobic conditions, the water bath temperature was adjusted to 80°C to start reduction, and the time required for complete reaction was recorded in Table 1. The volume concentration in Table 1 is the volume concentration of ethanol in the mixed solution.

[0047] Table 1

[0048] As can be seen from Table 1, the reduction time of indigo decreases with the increase of ethanol concentration. When the volume concentration of ethanol reaches 80%, the reduction speed of indigo is affected due to the decrease of solubility of the reducing agent TD and NaOH.

[0049] Example 1 1) 0.24 g of thiourea dioxide and 0.20 g of indigo were weighed and stirred in a mixed solution system of 12 ml of ethanol and 18 ml of water. Then a certain amount of sodium hydroxide was added to adjust the pH of the solution to 9 to obtain a dye liquor.

[0050] 2) The dye liquor obtained in step 1) was placed in a 50°C water bath to start reduction, and the reduction time was 20 min. The above steps 1) and 2) were carried out under anaerobic conditions.

[0051] 3) The dye liquor after completion of the reduction process was cooled to 25°C, and then 0.75 g of pure cotton bleached cloth was put in. Dyeing was carried out for 30 min, and the cloth was stirred every 10 min.

[0052] 4) After dyeing was completed, the cloth was exposed to air for sufficient oxidation, soaping, and drying.

[0053] Example 2 1) 0.29 g of thiourea dioxide and 0.20 g of indigo were weighed and stirred in a mixed solution system of 15 ml of ethanol and 15 ml of water. Then a certain amount of sodium hydroxide was added to adjust the pH of the solution to 9 to obtain a dye liquor.

[0054] 2) The dye liquor obtained in step 1) was placed in a 50°C water bath to start reduction, and the reduction time was 20 min. The above steps 1) and 2) were carried out under anaerobic conditions.

[0055] 3) The dye liquor after completion of the reduction process was cooled to 25°C, and then 0.75 g of pure cotton bleached cloth was put in. Dyeing was carried out for 30 min, and the cloth was stirred every 10 min.

[0056] 4) After dyeing is completed, expose the cloth to air for sufficient oxidation, soaping, and drying.

[0057] Example 3 1) Weigh 0.26 g of sulfourea dioxide and 0.21 g of indigo into a mixed solution system of 21 mL of ethanol and 9 mL of water and stir. Then add a certain amount of sodium hydroxide to adjust the pH of the solution to 9 to obtain a dye liquor.

[0058] 2) Place the dye liquor obtained in step 1) in a 70°C water bath to start reduction, and the reduction time is 40 min. Both the above step 1) and step 2) are carried out under anaerobic conditions.

[0059] 3) After the reduction treatment is completed, cool the dye liquor to 25°C, and then put it into 0.75 g of pure cotton bleached cloth. Dye for 30 min, and stir every 10 min in between.

[0060] 4) After dyeing is completed, expose the cloth to air for sufficient oxidation, soaping, and drying.

[0061] Example 4 Prepare the dye liquor according to the method of Example 1, except that: In step 1), replace the mixed solution of 12 mL of ethanol and 18 mL of water with a mixed solution of 18 mL of ethanol and 12 mL of water.

[0062] Example 5 Prepare the dye liquor according to the method of Example 1, except that: In step 1), replace the mixed solution of 12 mL of ethanol and 18 mL of water with a mixed solution of 21 mL of ethanol and 9 mL of water.

[0063] Example 6 Prepare the dye liquor according to the method of Example 1, except that: In step 1), replace 0.24 g of sulfourea dioxide with 0.20 g of sulfourea dioxide.

[0064] Example 7 Prepare the dye liquor according to the method of Example 1, except that: In step 1), replace 0.24 g of sulfourea dioxide with 0.17 g of sulfourea dioxide.

[0065] Comparative Example 1 Prepare the dye liquor according to the method of Example 1, except that, in step 1), replace the mixed solution of 12 mL of ethanol and 18 mL of water with 30 mL of water.

[0066] Comparative Example 2 The dyeing solution was prepared according to the method of Example 2, except that the 12 mL of ethanol and 18 mL of water mixed solution in Step 1) was replaced by 30 mL of water.

[0067] Comparative Example 3 The dyeing solution was prepared according to the method of Example 6, except that the 12 mL of ethanol and 18 mL of water mixed solution in Step 1) was replaced by 30 mL of water.

[0068] Comparative Example 4 The dyeing solution was prepared according to the method of Example 7, except that the 12 mL of ethanol and 18 mL of water mixed solution in Step 1) was replaced by 30 mL of water.

[0069] Comparative Example 5 The dyeing solution was prepared according to the method of Example 1, except that the 12 mL of ethanol and 18 mL of water mixed solution in Step 1) was replaced by 3 mL of ethanol and 27 mL of water mixed solution.

[0070] The K / S values of the cotton dyed according to Examples 1-10 and Comparative Examples 1-5 were detected and recorded in Table 2, respectively.

[0071] Table 2

[0072] As shown in Table 2, the dyeing solution does not need to use sodium dithionite, but only relies on the indigo reducing agent, thionyl hydride, to complete the reduction process of indigo dye under alkaline conditions.

[0073] The above describes the preferred embodiments of the present application, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as the disclosed content of the present application, and all fall within the protection scope of the present application.

Claims

1. A method of indigo dyeing, characterized in that, The method comprises: 1) mixing water, a small molecule alcohol, sodium hydroxide, sulfourea dioxide and indigo blue to prepare a dyeing solution with a pH of 8-11; The small molecule alcohol is at least one of methanol, ethanol, isopropanol and tert-butyl alcohol; the concentration of the small molecule alcohol in the dyeing solution is 20-70 vol.%; 2) reducing the dyeing solution obtained in step 1) and cooling it to below 30℃; The reducing treatment comprises a treatment temperature of 40-80℃ and a treatment time of 20-90 min; 3) using the dyeing solution obtained in step 2) to dye bleached natural fibers.

2. The method of claim 1, wherein, In step 1), the molar ratio of the indigo blue to the sulfourea dioxide is 1:2-1:

4.

3. The method of claim 2, wherein, In step 1), the molar ratio of the indigo blue to the sulfourea dioxide is 1:3-1:

4.

4. The method according to any one of claims 1 to 3, characterized in that, In step 1), the content of the indigo blue in the dyeing solution is 6-7 g / L.

5. The method according to any one of claims 1 to 4, characterized in that, In step 1), the concentration of the small molecule alcohol in the dyeing solution is 40-50 vol.%.

6. The method according to any one of claims 1 to 5, characterized in that, In step 2), the temperature of the reducing treatment is 50-70℃ and the time is 25-40 min.

7. The method according to any one of claims 1 to 6, characterized in that, In step 3), the bath ratio of the natural fibers to the dyeing solution is 1:35-45.

8. The method according to any one of claims 1 to 7, characterized in that, In step 3), the time of the dyeing is 20-40 min.

9. The method according to any one of claims 1 to 8, characterized in that, The method further comprises: 4) oxidizing the natural fibers obtained in step 3) in air and then sequentially performing soaping and air drying.

10. The method according to any one of claims 1-9, characterized in that, The dyeing solution does not include sodium hydrosulfite.

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