A method for preparing colorless regenerated cellulose from waste blue pure cotton fabric by using metal molten salt

Abstract

The present application belongs to the field of light and textile industry, and discloses a method for preparing colorless regenerated cellulose from waste blue pure cotton fabric by using metal molten salt. The method comprises the following steps: (1) adding the waste blue pure cotton fabric into the aqueous solution of molten salt, and carrying out microwave reaction under the conditions of inert gas protection and stirring to obtain a colorless cotton fabric molten salt solution without solid particle residues; (2) cooling the colorless cotton fabric molten salt solution without solid particle residues, washing with water, and then drying to obtain colorless regenerated cellulose particles. The molten salt used in the present application is a hydrate of lithium halide, which only acts on the chromophore groups of the reactive dyes, so that the conjugated structure in the chromophore groups of the reactive dyes is destroyed, thereby making the cellulose lose the original color. Moreover, the covalent bond between the reactive dyes and the cellulose fibers is not destroyed, so no colored wastewater is generated, which meets the requirements of clean production of the textile industry.

Description

Technical Field

[0001] The invention belongs to the field of light textile industry, and in particular relates to a method for preparing colorless regenerated cellulose from waste blue pure cotton fabric by utilizing metal molten salt. Background Art

[0002] Cellulose is a natural polymer found widely in nature. Regenerating cellulose from cellulose-rich biomass has been a hot topic in both scientific research and industry. Proposed recovery systems include strong alkali (NaOH) systems, N,N-dimethylformamide / lithium chloride systems, N-methylmorpholine-N-oxide (NMMO) systems, ionic liquids, and phosphoric acid systems. These systems essentially utilize highly polar groups to break the hydrogen bonds formed by hydroxyl groups between cellulose fiber macromolecules. Dyed waste textiles, on the other hand, are complexes of polymers and dyes. The paper "Complete separation of colorants from polymeric materials for cost-effective recycling of waste textiles" proposes a method for recovering dyes and fibers separately by leveraging the extraction properties of organic solvents from waste textiles. For example, methods can be used to recover polyester and disperse dyes from disperse dyed polyester fabrics, acid dyes and nylon from acid dyed nylon, and direct dyes and cotton fabric from direct dyed cotton fabrics. However, this method is ineffective for pure cotton fabrics dyed with reactive dyes due to the covalent bonds between the reactive dyes and the cotton fabric. The ether bond formed between the reactive dye and the hydroxyl group is not resistant to acid and oxidants. Therefore, the paper (Recycling of waste cotton fabrics into regenerated cellulose films through three solventsystems: A comparison study) proposed a method of using acid to hydrolyze the ether bond formed between the dye and the hydroxyl group to pre-decolorize the dyed cotton fabric, and then using a strong alkali (NaOH) system to dissolve the cotton fiber. However, after destroying the ether bond between the reactive dye and the hydroxyl group of the cotton fiber, the hydrolyzed dye will form colored wastewater when dissolved in water. In the wastewater treatment, an additional oxidant needs to be added for decolorization to remove the color of the wastewater and substances such as aniline formed during the degradation of the dye. The Chinese invention patent authorization text CN111793223B reports a method for preparing nano-scale regenerated cellulose using a molten salt system, using Li + Mg 2+ , Ca 2+ 、Zn 2+Regenerated cellulose is prepared by dissolving one of microcrystalline cellulose, softwood dissolving pulp, hardwood dissolving pulp, gramineous dissolving pulp, cotton pulp, and commercial bleached pulp in a complex salt hydrate composed of one or more hydrates of chloride, bromide, and perchlorate. However, the macromolecular structure of cotton fabrics dyed with reactive dyes has not only the hydroxyl groups of the cellulose material itself, but also benzene rings, azo groups, sulfonate groups, and other groups. Among them, water-soluble groups such as sulfonate groups will react with Mg. 2+ , Ca 2+ 、Zn 2+ etc. form complexes, making it difficult to dissolve the cotton fabric dyed with reactive dyes; while Li + Chloride, bromide, and perchlorate of reactive dyes will also react with Li due to the water-soluble groups such as sulfonate in the macromolecular structure of cotton fabrics dyed with reactive dyes. + Chlorides, bromides, and perchlorates compete with water molecules, and ultimately conventional methods make it difficult to recover nanoscale colorless regenerated cellulose from cotton fabrics dyed with reactive dyes.

[0003] In summary, the current recovery of cellulose from cotton fabrics dyed with reactive dyes has the following problems:

[0004] (1) Reactive dyes are small organic molecules that form covalent bonds with hydroxyl groups in cotton fabrics. Using strong acids such as sulfuric acid to break the ester bond between the hydroxyl groups and reactive dyes will result in the production of colored wastewater. Using oxidants such as hypochlorite or hydrogen peroxide to bleach or pre-bleach colored cotton fabrics, oxidants have no selectivity for colored dyes and cotton fabrics, which will greatly reduce the strength of cotton fabrics and generate washing wastewater.

[0005] (2) A covalent bond is formed between the reactive dye and the cotton fabric. Traditional organic solvents such as dimethylformamide, dimethyl sulfoxide, and ethanol cannot extract the reactive dye from the colored cotton fabric.

[0006] (3) Conventional metal molten salts, such as Mg 2+ , Ca 2+ 、Zn 2+ 、Fe 2+ 、Fe 3+ etc., will form complexes with strong polar groups on cotton fabrics dyed with reactive dyes, such as sulfonate groups, resulting in the inability of conventional metal molten salts to dissolve cotton fabrics dyed with reactive dyes. At the same time, because the macromolecular structure of cotton fabrics dyed with reactive dyes contains water-soluble groups such as sulfonate groups, they will react with Li + The chlorides, bromides and perchlorates compete with water molecules, which ultimately makes conventional processes unsuitable and difficult to recover colorless regenerated cellulose from dark cotton fabrics dyed with reactive dyes. Summary of the Invention

[0007] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the primary object of the present invention is to provide a method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt.

[0008] The purpose of the present invention is achieved through the following solutions:

[0009] A method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt comprises the following steps:

[0010] (1) Adding waste blue pure cotton fabric to a molten salt aqueous solution, microwave reaction occurs under inert gas protection and stirring conditions to obtain a colorless cotton fabric molten salt solution without solid particles remaining;

[0011] (2) The colorless cotton fabric molten salt solution without any solid particles remaining is cooled, washed with water, and then dried to obtain colorless regenerated cellulose particles.

[0012] The molten salt described in step (1) is a lithium halide (such as lithium chloride, lithium bromide, lithium iodide), which only acts on the chromophore of the active dye, destroying the conjugated structure in the chromophore of the active dye, thereby causing the cellulose to lose its original color; and it does not destroy the covalent bond between the active dye and the cellulose fiber, so no colored wastewater is generated, which meets the requirements of clean production in the textile industry.

[0013] In the aqueous solution of the molten salt described in step (1), the mass fraction of the molten salt is 30-60%.

[0014] The amount of the waste blue pure cotton fabric in step (1) satisfies the requirement that the mass of the waste blue pure cotton fabric is less than 10% of the mass of the aqueous solution of the molten salt.

[0015] The color depth (K / S value) of the waste blue pure cotton fabric described in step (1) is between 5 and 30; the structure of the dye of the waste blue pure cotton fabric described in step (1) is a formazan type, and the dye is preferably a composite dye composed of one or more of Reactive Blue 221 (active group is monochloro-s-triazine + vinyl sulfone), Reactive Blue 220 (active group is vinyl sulfone), Reactive Blue 235 (active group is monofluoro-s-triazine), Reactive Blue 245 (active group is monochloro-s-triazine) and Reactive Blue 160 (active group is monochloro-s-triazine + monochloro-s-triazine).

[0016] The inert atmosphere in step (1) is preferably at least one of nitrogen, carbon dioxide, and argon;

[0017] The stirring speed in step (1) is 50-500 r / min; The microwave reaction in step (1) refers to the reaction at 100-250° C. and a power of 500-1000 W for 10-120 min;

[0018] The cooling described in step (2) refers to cooling to room temperature.

[0019] The washing with water in step (2) refers to washing with water for 3-10 times until the washed solution does not contain molten salt.

[0020] After washing with water in step (2), the washing liquid is collected and the metallic lithium molten salt is recovered by distillation and freeze-drying, thereby realizing the recycling of lithium halide with a recovery rate of more than 99%.

[0021] The drying in step (2) refers to freeze drying, preferably drying at -50 to -80°C for 1 to 72 hours.

[0022] The colorless regenerated cellulose particles obtained in step (2) have a particle size of 1-4 μm.

[0023] The mechanism of the present invention is:

[0024] The hydrogen bonds formed between the numerous hydroxyl groups in the cellulose macromolecular chains will be + Open, so that the cellulose has a certain degree of dispersion in the appropriate solution. In the macromolecular structure of cotton fabric dyed with blue reactive dye, in addition to the hydrogen bonds formed between the numerous hydroxyl groups between the cellulose macromolecular chains, there are also strong polar groups such as sulfonate groups and water-soluble groups. Sulfonate groups will form hydrated ions with water molecules in the molten salt. Therefore, the hydrogen bonds between the hydroxyl groups in the cellulose macromolecular structure modified by the reactive dye are blocked by Li + The difficulty of opening increases and the speed of opening slows down. By introducing microwave, the Li + The violent oscillation generates molecular heat, and at the same time, the halogen anions and Li + The inductive force and dispersion between them are also enhanced in microwaves, thereby accelerating the dissolution of cellulose fibers dyed with reactive dyes.

[0025] The molecular structure of formazan reactive dyes mainly contains complexes of formazan groups (-N=NC=N-NH-) and metal elements such as iron, copper, nickel and chromium, as well as reactive groups and water-soluble groups. Cellulose fibers dyed with formazan reactive dyes are exposed to the presence of highly polar Li + It will attack the coordination bond originally formed by metal elements such as iron, copper, nickel and chromium and formazan, so that the coordinated metal elements of the reactive dyes are replaced. Moreover, the azo structure in the reactive dyes with ortho-hydroxyl groups will produce azo-quinonehydrazone tautomerism and dissociation reactions due to proton transfer. In the presence of halide ions, the dyed cellulose fibers will fade.

[0026] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0027] (1) The molten salt used is a hydrate of lithium halide (such as lithium chloride, lithium bromide, lithium iodide), which only acts on the chromophore of the active dye, destroying the conjugated structure in the chromophore of the active dye, thereby causing the cellulose to lose its original color.

[0028] (2) The molten salt used is a hydrate of lithium halide (such as lithium chloride, lithium bromide, lithium iodide), which does not destroy the covalent bond between the active dye and the cellulose fiber, so it does not produce colored wastewater, which meets the requirements of clean production in the textile industry.

[0029] (3) After the cellulose fibers dyed with the reactive dye are dissolved and decolorized, the lithium halide can be recycled through extraction with distilled water, distillation and freeze-drying, with a recovery rate of more than 99%.

[0030] (4) In the cellulose molecular structure, reactive dyes are most likely to react with the primary hydroxyl groups at C6. However, not all primary hydroxyl groups at C6 will react. In addition, the cellulose molecular structure also contains secondary hydroxyl groups at C2 and C3, which have relatively weaker reactivity. Therefore, after decolorization, cellulose can still be re-dyed with other colors. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a decolorization principle diagram of the present invention.

[0032] Figure 2 3. K / S curve of the discarded blue pure cotton fabric before and after bleaching in Example 1 of the present invention.

[0033] Figure 3 This is the absorption curve of the dye in the washing solution after washing with water after microwave reaction in Example 1.

[0034] Figure 4 The infrared spectra (FT-IR) of waste blue cotton fabric and bleached cellulose.

[0035] Figure 5 These are the Raman spectra of waste blue cotton fabric and bleached cellulose.

[0036] Figure 6 This is the SEM image of the decolorized cellulose particles. DETAILED DESCRIPTION

[0037] The present invention will be described in further detail below with reference to the Examples and accompanying drawings, but the embodiments of the present invention are not limited thereto. Where specific conditions are not specified in the Examples, conventional conditions or conditions recommended by the manufacturer were followed. Reagents or instruments used, where the manufacturer is not specified, are commercially available conventional products.

[0038] Unless otherwise specified, all reagents used in the examples can be purchased from the market.

[0039] Example 1

[0040] A 20g molten salt solution containing 58% LiBr by mass was prepared with distilled water. 0.4g of discarded pure cotton fabric dyed with Reactive Blue 235 to a color depth (K / S) of 22 was accurately weighed. The cotton fabric was pre-cut into 5x5mm pieces with scissors and then added to the molten salt solution. Nitrogen was introduced, mechanical stirring was initiated at 100 rpm, and the temperature was raised to 120°C. A microwave (500 watts output power) was then applied for 90 minutes to produce a colorless cellulose solution. After the solution cooled to room temperature, it was extracted eight times with distilled water. The extracts were collected and the metallic lithium molten salt was recovered by distillation and freeze-drying. The colorless cellulose was transferred to a freeze dryer and dried for 5 hours to obtain decolorized cellulose particles.

[0041] The K / S value of discarded pure cotton fabrics is about 22, while the color depth (K / S value) of the bleached cellulose at the maximum absorption wavelength λmax=600nm is close to 0, indicating that the reactive dyes on the original dyed pure cotton fabrics have lost their color. Figure 2 Take the LiBr extract and test the maximum absorption wavelength in the visible light region (400-800nm). The dye Reactive Blue 235 has a maximum absorbance at 690nm, but the maximum absorption wavelength is basically undetectable in the extract. See the attached Figure 3 , indicating that the extraction solution has no color. Since the chemical bond between the reactive dye and the cellulose fiber is not destroyed, no colored wastewater will be produced. In the infrared spectra of the discarded pure color cotton fabric and the cellulose after decolorization, see the attached Figure 4 , 3333cm -1 The peak at 2899 cm is the peak of hydroxyl (-OH) in the cellulose macromolecular structure. -1 The peak at 1638 cm is the vibration peak between carbon atoms and hydrogen atoms in the cellulose macromolecular structure. -1 The peak at 3350cm is the vibration peak of the carbonyl group (C=O) in the reactive dye 235; in the infrared spectrum of decolorized cellulose, -1 The peak at 1620 cm is the stretching vibration peak of the newly generated amino group. -1 The peak at 2930 cm is the superposition of the stretching vibration peaks of the newly generated carboxylate and the carbonyl group in the quinone hydrazone structure. -1The peak at is the stretching vibration peak of the hydroxyl group in the newly generated carboxylate group. In the Raman spectra of waste blue pure cotton fabric, bleached cellulose, and bleached pure cotton fabric (whiteness 75), see the attached Figure 5 , discarded cotton fabric dyed with reactive blue 235 dye at 1366.49 cm -1 The peak at is the stretching vibration peak of the azo bond (-N=N-). After treatment with LiBr solution, the azo bond in the dye is opened, the conjugated structure of the dye is destroyed, and the dyed cellulose loses its original color. The size of the colorless cellulose particles prepared from waste pure cotton fabric is within 1-4μm. The specific results are shown in the attached Figure 6 .

[0042] Example 2

[0043] The method of Example 2 is basically the same as that of Example 1.

[0044] Example 2 differs from Example 1 in that the color depth (K / S value) of the waste cellulose is only 5, and a colorless cellulose solution is observed after treatment for 30 minutes.

[0045] Example 3

[0046] The method of Example 3 is basically the same as that of Example 1.

[0047] Example 3 differs from Example 1 in that the temperature of the microwave treatment is 150° C., and a colorless cellulose solution is observed after treatment for 30 minutes.

[0048] Example 4

[0049] The method of Example 4 is basically the same as that of Example 1.

[0050] Example 4 differs from Example 1 in that the molten salt is changed from LiBr to LiCl, the mass fraction of the molten salt in the aqueous solution of the molten salt is 41%, the microwave treatment temperature is 130° C., and a colorless cellulose solution is observed after treatment for 45 minutes.

[0051] Example 5

[0052] The method of Example 5 is basically the same as that of Example 1.

[0053] Example 5 differs from Example 1 in that the waste cotton fabric is dyed with Reactive Blue 220, the color depth (K / S value) of the cotton fabric is 18, the microwave treatment temperature is 125°C, and a colorless cellulose solution is observed after treatment for 35 minutes.

[0054] Example 6

[0055] The method of Example 6 is basically the same as that of Example 1.

[0056] Example 6 differs from Example 1 in that the waste cotton fabric is dyed with a mixture of Reactive Blue 245 and 160, the color depth (K / S value) of the cotton fabric is 28, and the microwave treatment temperature is 150°C. A colorless cellulose solution is observed after 120 minutes of treatment.

[0057] Example 7

[0058] The method of Example 7 is substantially the same as that of Example 1.

[0059] Example 7 differs from Example 1 in that the microwave treatment output power is 800 watts, and a colorless cellulose solution is observed after 20 minutes of treatment.

[0060] Comparative Example 1

[0061] The method of Comparative Example 1 is substantially the same as that of Example 1.

[0062] Comparative Example 1 differs from Example 1 in that the treated fabric is changed from waste blue cotton fabric to commercial bleached pure cotton fabric (thread count: 40 x 40, warp and weft density: 120 x 80, weight: 125 g / m 2 ), i.e., a semi-finished fabric before dyeing, and a colorless cellulose solution is observed after 5 minutes of treatment at 120°C without using a microwave.

[0063] Comparative Example 2

[0064] The method of Comparative Example 2 is substantially the same as that of Example 1.

[0065] Comparative Example 2 differs from Example 1 in that a colorless cellulose solution is observed after 5 days of treatment at 150°C without using a microwave.

[0066] Comparative Example 3

[0067] The method of Comparative Example 3 is substantially the same as that of Example 1.

[0068] Comparative Example 3 differs from Example 1 in that a blue cotton fabric molten salt solution without solid particle residues is observed after 72 hours of treatment at 180°C without using a microwave.

[0069] Comparative Example 4

[0070] The method of Comparative Example 4 is substantially the same as that of Example 1.

[0071] Comparative Example 4 differs from Example 1 in that the waste cotton fabric is dyed with Reactive Red 120, the color depth (K / S value) of the cotton fabric is 24, and a red cotton fabric molten salt solution without solid particle residues is observed.

[0072] Comparative Example 5

[0073] The method of Comparative Example 5 is basically the same as that of Example 1.

[0074] Comparative Example 5 differs from Example 1 in that the mass fraction of the molten salt is 25%. Result: No dissolution of the blue cotton fabric was observed.

[0075] Table 1 shows the relationship between the required decolorization time, the yield of colorless cellulose particles (i.e., the mass of the obtained colorless cellulose particles divided by the mass of the waste blue pure cotton fabric) and the recovery rate of the metal molten salt in a method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using a metal molten salt.

[0076] Table 1 Decolorization time required for preparing colorless regenerated cellulose, yield of colorless cellulose particles and recovery rate of metal molten salt in Examples 1-7 and Comparative Examples 1-5

[0077]

[0078]

[0079] Application Examples

[0080] The decolorized cellulose particles prepared in Example 1 were further dyed. The specific dyeing process was as follows: 2 g of decolorized cellulose was placed in a dye solution, incubated at room temperature for 10 minutes, then heated to 80°C at a rate of 2°C / min. After incubation for 30 minutes, the solution was soaped (90°C for 10 minutes, using 1 g / L of AATCC 193 standard detergent) and washed with water (50°C for 10 minutes). The dye was Reactive Blue 220, with an Owf (to cloth weight) of 2%, a dye bath ratio of 20:1, 50 g / L of sodium sulfate, and 20 g / L of soda ash. The dye residue and washing solution were collected, and spectrophotometric analysis revealed a dye fixation rate of 47.3%. This indicates that the cellulose can still be dyed after decolorization, and the dye has a high dye fixation rate.

[0081] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt, characterized in that The following steps are involved: (1) Adding waste blue pure cotton fabric to a molten salt aqueous solution, microwave reaction occurs under inert gas protection and stirring conditions to obtain a colorless cotton fabric molten salt solution without solid particles remaining; (2) cooling the colorless cotton fabric molten salt solution without any solid particles remaining, washing it with water, and then drying it to obtain colorless regenerated cellulose particles; The molten salt in step (1) is a lithium halide; In the aqueous solution of the molten salt described in step (1), the mass fraction of the molten salt is 30-60%; The structure of the dye of the discarded blue pure cotton fabric described in step (1) is formazan.

2. The method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt according to claim 1, characterized in that: The molten salt in step (1) is at least one of lithium chloride, lithium bromide and lithium iodide.

3. The method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt according to claim 1, characterized in that: The amount of the waste blue pure cotton fabric in step (1) satisfies the requirement that the mass of the waste blue pure cotton fabric is less than 10% of the mass of the aqueous solution of the molten salt.

4. The method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt according to claim 1, characterized in that: The color depth K / S value of the waste blue pure cotton fabric described in step (1) is between 5 and 30.

5. The method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt according to claim 1, characterized in that: The dye of the waste blue pure cotton fabric described in step (1) is one or a composite dye consisting of at least two of Reactive Blue 221, Reactive Blue 220, Reactive Blue 235, Reactive Blue 245 and Reactive Blue 160.

6. The method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt according to claim 1, characterized in that: The inert atmosphere in step (1) is at least one of nitrogen, carbon dioxide, and argon.

7. The method for preparing colorless regenerated cellulose from waste blue pure cotton fabric using metal molten salt according to claim 1, characterized in that: The stirring speed in step (1) is 50-500 r / min; the microwave reaction in step (1) refers to the reaction at 100-250° C. and a power of 500-1000 W for 10-120 min.

8. Application of the colorless regenerated cellulose prepared by the method according to any one of claims 1 to 7 in the textile industry.

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