A direct dye composition prepared by low temperature diazotization coupling and a process for its preparation

Abstract

This application relates to the field of dye technology, specifically to a direct dye composition prepared by low-temperature diazotization coupling and its preparation method. The preparation method includes: diazotizing p-aminoazobenzene-4-sulfonic acid with sodium nitrite in a hydrochloric acid medium, and obtaining a diazonium salt solution after digesting excess sodium nitrite; adding the diazonium salt solution to a coupling system containing 2-amino-5-naphthol-7-sulfonic acid and choline, and using choline to adjust the pH for coupling; obtaining a wet filter cake of direct dye red after desalting, filtration, and washing with water; then mixing with lignin sulfonate, an amphoteric solubilizer, polyaspartate, a buffer salt, and an antifoaming agent, and homogenizing to obtain the direct dye composition. This application uses low-temperature diazotization and a choline-organic base coupling system, combined with direct compounding of auxiliaries using the wet filter cake, resulting in a product with good solubility, excellent leveling properties, low inorganic salt content, and high wet fastness, suitable for industrial production.

Description

Technical Field

[0001] This application relates to the field of dye technology, specifically to a direct dye composition prepared by low-temperature diazo coupling and its preparation method. Background Technology

[0002] Direct dyes are a class of water-soluble dyes that can directly dye cellulose fibers in neutral or weakly alkaline dye baths. Their molecular structures typically contain azo, sulfonic acid, hydroxyl, and amino groups, achieving dyeing of fibers such as cotton, viscose, and linen through hydrogen bonds, van der Waals forces, and the affinity resulting from the planarity of the dye molecules. Due to their simple dyeing process, low equipment requirements, comprehensive color spectrum, and low cost, direct dyes have long been valuable in textile dyeing and finishing, paper coloring, and water-based coloring systems.

[0003] The development of direct dyes has generally progressed from monoazo direct dyes to diazo and polyazo direct dyes, and from simply improving dye uptake to considering solubility, levelness, wet fastness, and environmental performance. Early direct dyes typically relied on large amounts of inorganic salts for dyeing, and high proportions of sodium sulfate and sodium chloride were often added as standardized fillers during the commercialization process. This resulted in high electrolyte loads in the dye bath, high salt content in wastewater, and poor dyeing reproducibility. With the increasing demands for water conservation, emission reduction, and cleaner production, low-salt, high-solubility, high-dyeing-uptake, and low-float direct dyes and their commercial compositions have become important areas for improvement in this field.

[0004] The preparation of red direct dyes from p-aminoazobenzenesulfonic acid compounds and naphthalene-based coupling components is a common route in this field. These dyes typically require steps such as diazotization, coupling, salting out, filtration, drying, and commercial compounding. In existing technologies, unstable temperature control during diazotization can easily lead to problems such as diazonium salt decomposition, increased azo byproducts, darker color, and decreased intensity. During coupling, inorganic bases such as sodium carbonate and sodium hydroxide are commonly used to adjust the pH. While convenient, this can create a locally strong alkaline environment, causing oxidation or side reactions of the 2-amino-5-naphthol-7-sulfonic acid coupling components and further increasing the inorganic salt content of the system. Furthermore, if the dye is only standardized through conventional salting out and the addition of inorganic salts after synthesis, the resulting commercial dye is prone to problems such as insufficient cold water solubility, poor hard water stability, excessively rapid initial dyeing, insufficient leveling, and excessive floating color during soaping.

[0005] For example, patent application CN101580651A discloses a pollution-free preparation process for Direct Rose Red FR, which includes preparing an H-acid solution and then performing a first condensation reaction between cyanuric chloride and the aqueous solution at 0-5°C. Although this technical solution involves a low-temperature condensation reaction, its subsequent process route differs from the coupling route in this application, and it still uses a conventional inorganic alkali system for pH adjustment, failing to solve the problems of inorganic salt accumulation and insufficient dye solubility during the coupling process. Patent application CN101161730A discloses a method for synthesizing the direct dye Direct Red 6B, the main process of which involves dissolving aniline in hydrochloric acid, diazotizing it with sodium nitrite, then coupling it with J acid under alkaline conditions to obtain a coupling product, which is then mixed with sodium nitrite and added to dilute hydrochloric acid for repeated diazotization, followed by a coupling reaction with J acid to obtain Direct Red 6B. The technical solution has the following obvious shortcomings: the diazotization process uses conventional temperature control without strictly limiting the diazotization temperature to a low temperature, resulting in insufficient stability of the diazonium salt and easy decomposition and side reactions; the coupling process uses alkaline conditions but does not explicitly use organic bases to adjust the pH, still relying on the traditional inorganic base system, introducing a large amount of inorganic salts; the coupling product needs to undergo repeated diazotization steps, and the process route is different from the simplified coupling route of this application, increasing the risk of side reactions and process complexity; the post-processing still uses the conventional salting out, filtration, and drying process route, without involving low-salinity desalination treatment, resulting in high inorganic salt content in the product, affecting solubility and environmental performance.

[0006] Therefore, existing direct dye red products and their preparation methods still have significant shortcomings in terms of diazotization temperature control, coupling pH adjustment, inorganic salt content control, and commercial compounding processes. There is an urgent need to provide a new direct dye composition and its preparation method to improve the problems of insufficient solubility, uneven dyeing, high inorganic salt content, and poor wet fastness of existing products. Summary of the Invention

[0007] This application aims to overcome at least one of the deficiencies of the prior art and provide a direct dye composition prepared by low-temperature diazo coupling and its preparation method. Through the combination of specific components and preparation methods, the dye achieves high solubility, high dyeing rate, good leveling properties, high wash fastness and low salinity, making it suitable for industrial scale-up production and exhibiting good batch stability.

[0008] In a first aspect, embodiments of this application provide a method for preparing a direct dye composition by low-temperature diazotization coupling, achieved through the following technical solution: A method for preparing a direct dye composition by low-temperature diazotization coupling includes the following steps: S1. Add water, hydrochloric acid and p-aminoazobenzene-4-sulfonic acid to the first reaction vessel, stir to dissolve, cool to 0-5℃, add sodium nitrite solution dropwise, and after the addition is complete, keep the reaction at 0-5℃ for 2-3 hours. Use aminosulfonic acid to digest excess sodium nitrite to obtain diazonium salt solution. S2. Add water, 2-amino-5-naphthol-7-sulfonic acid and choline to the second reaction vessel, stir until dissolved, control the temperature at 10-25℃, add the diazonium salt solution to the second reaction vessel, and adjust the pH of the system to 8.0-9.5 with choline solution during the addition. After the addition is completed, stir the reaction at 15-30℃ for 3-5 hours to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is desalted, filtered, and washed with water to obtain a direct dye red wet filter cake; S4. The wet filter cake of the direct dye red is mixed with lignin sulfonate, amphoteric cosolvent, polyaspartate, buffer salt, and defoamer, and homogenized to obtain a direct dye composition.

[0009] The method for preparing the direct dye composition by low-temperature diazo coupling according to the embodiments of this application has at least the following beneficial effects: This application controls the diazotization temperature at 0–5°C and continues the low-temperature reaction after adding sodium nitrite, so that p-aminoazobenzene-4-sulfonic acid can be fully and stably converted into diazonium salt, reducing the risk of diazonium salt thermal decomposition, azo side reactions, and the formation of dark byproducts. After the reaction, the excess sodium nitrite is digested with aminosulfonic acid, which can reduce the impact of residual nitrite on subsequent coupling reactions and dye color stability, thereby improving the proportion of main products of direct dye red, color intensity, and batch color consistency.

[0010] This application employs a separate reactor method to prepare the diazonium salt solution and the coupling component solution, facilitating independent control of the diazotization acidity, the dissolution state of the coupling component, the reaction temperature, and the pH. Adding the diazonium salt solution to the coupling system containing 2-amino-5-naphthol-7-sulfonic acid and choline reduces side coupling and self-decomposition caused by excessively high local diazonium salt concentrations. Adjusting the pH to 8.0–9.5 with choline solution during coupling avoids a locally hyperalkaline environment caused by strong inorganic bases, reducing oxidation and side reactions of the coupling component, while also minimizing the introduction of inorganic salts.

[0011] This application involves desalting, filtering, and washing the direct dye red reaction solution after synthesis, and then using it as a wet filter cake in the composition compounding step. This process helps remove sodium chloride, sodium sulfate, choline salts, and small molecule impurities, reducing the inorganic electrolyte load of the final composition. Compared to drying the dye into raw powder before secondary compounding, wet filter cake compounding avoids hard agglomeration and coarsening caused by drying shrinkage, reduces the formation of insoluble substances, and improves the wetting, dispersion, and solubility of the product under cold and hard water conditions.

[0012] This application describes a composition prepared by compounding a wet filter cake of direct red dye with lignin sulfonate, amphoteric solubilizer, polyaspartate, buffer salt, and defoamer, followed by homogenization. Lignosulfonate improves dye particle dispersion and anti-aggregation properties; the amphoteric solubilizer enhances hydration and solubilization; polyaspartate helps improve hard water stability and inhibit the influence of metal ions; the buffer salt stabilizes the dye bath pH; and the defoamer improves foam control during processing and use. The components synergistically enhance dye solubility, leveling properties, dyeing stability, and dyeing reproducibility.

[0013] This application integrates dye synthesis, low-salinity treatment, and wet composition compounding in a continuous manner, which can reduce the repetitive processing steps of traditional methods such as "drying wet filter cake into raw powder, crushing, dry mixing or re-pulping, and drying again", thereby reducing energy consumption, dust emissions, and operational contact risks. At the same time, the auxiliaries are more easily and uniformly adsorbed onto the surface of dye particles during the wet homogenization process, resulting in a slurry product with good wettability and a relatively stable particle size distribution, which is suitable for industrial scale-up production and has good batch stability.

[0014] According to some embodiments of this application, in step S4, the raw materials of the direct dye composition include, by weight: 55-85 parts of direct dye red wet filter cake on a dry basis, 5-18 parts of lignin sulfonate, 1-8 parts of amphoteric cosolvent, 0.5-5 parts of polyaspartate, 0.5-4 parts of buffer salt, and 0.05-0.5 parts of defoamer. When raw materials are compounded according to the above dosage, the effective dye content and auxiliary agent dosage can be kept in a suitable balance. 55-85 parts of direct red wet filter cake on a dry basis are beneficial to ensuring the color strength of the composition. 5-18 parts of lignin sulfonate can enhance the dispersion of dye particles and prevent agglomeration. 1-8 parts of amphoteric solubilizer can improve cold water wetting and solubility. 0.5-5 parts of polyaspartate helps to improve hard water stability and reduce metal ion interference. 0.5-4 parts of buffer salt can stabilize the pH of the dye bath and reduce color fluctuations. 0.05-0.5 parts of defoamer can suppress foaming during compounding, homogenization, and dyeing. The synergistic effect of each component in the specified proportions results in a composition with high strength, good solubility, leveling properties, and processing stability.

[0015] According to some embodiments of this application, the molar ratio of p-aminoazobenzene-4-sulfonic acid and sodium nitrite in step S1 and the molar ratio of 2-amino-5-naphthol-7-sulfonic acid and choline in step S2 is 1:1.02~1.12:0.98~1.08:0.8~2.2. By controlling the molar ratio of p-aminoazobenzene-4-sulfonic acid, sodium nitrite, 2-amino-5-naphthol-7-sulfonic acid and choline, sodium nitrite is kept in a moderate excess to ensure the diazotization reaction proceeds fully, while avoiding excessive nitrite from increasing the subsequent digestion burden and affecting the coupling color. Making the coupling component and the diazo component approximately equimolar is beneficial for improving the coupling conversion rate and reducing unreacted aromatic amines or naphthalene components. Within this range, the amount of choline can simultaneously dissolve and activate and adjust the pH, maintaining the weak alkalinity and reaction uniformity of the coupling system, reducing the introduction of inorganic bases and local strong base side reactions, thereby improving the content of the main dye product, color stability, and low salinity level.

[0016] According to some embodiments of this application, the amphoteric co-solvent is selected from at least one of betaine, sodium taurate, N,N-dimethylglycine salt, and cocamidopropyl betaine. The above-mentioned amphoteric co-solvent molecules simultaneously contain hydrophilic ionic groups and organically compatible structures, enabling them to form ionic associations, hydrogen bonds, or hydration with polar groups such as sulfonic acid groups, hydroxyl groups, and amino groups in direct dye red molecules. This weakens the π-π stacking and particle aggregation between dye molecules, improving the wetting, dispersion, and solubility of the dye in water. Simultaneously, its amphoteric characteristics help improve the dye system's adaptability to water hardness and electrolyte fluctuations, slow down the excessively rapid dyeing in the initial dyeing stage, and improve leveling, color uniformity, and dyeing reproducibility.

[0017] According to some embodiments of this application, the buffer salt is selected from at least one of sodium acetate, disodium hydrogen phosphate, sodium bicarbonate, and sodium citrate. Using at least one of sodium acetate, disodium hydrogen phosphate, sodium bicarbonate, and sodium citrate as a buffer salt can maintain the system in a relatively stable weakly acidic to weakly alkaline range during the dissolution and dyeing process of the dye composition, reducing pH fluctuations caused by impurities introduced by water quality, auxiliaries, or fibers, thereby reducing the risk of red light shift and sudden changes in dyeing rate of direct dyes. Simultaneously, the buffer salt, in combination with amphoteric co-solvents and polyaspartic acid salts, can improve the dispersion stability of dyes in hard water and under different dye bath conditions, improving dyeing reproducibility and leveling effect.

[0018] According to some embodiments of this application, the defoamer is a polyether-modified silicone defoamer, such as BYK-024 defoamer or TEGO Foamex 810 defoamer. Using polyether-modified silicone defoamers, especially BYK-024 and TEGO Foamex 810, can rapidly suppress and break up foam during wet filter cake preparation, shear homogenization, and sand milling homogenization, reducing problems such as material transport fluctuations and decreased sand milling efficiency caused by air entrainment in the foam. It combines the water dispersibility of polyether segments with the low surface tension of silicone segments, requiring only a small amount to achieve a stable defoaming effect, and is less likely to affect dye color, solubility, and dyeing performance, thus improving the stability of slurry processing, the continuity of spray drying, and the appearance and batch consistency of the final composition.

[0019] According to some embodiments of this application, the lignin sulfonate is selected from at least one of sodium lignin sulfonate, calcium lignin sulfonate, and modified sodium lignin sulfonate. Lignosulfonates can be used as dispersants and dust-preventing agents to improve the wetting and redispersibility of powders.

[0020] Furthermore, the modified sodium lignin sulfonate is a choline-type modified sodium lignin sulfonate. Choline-type modified sodium lignin sulfonate possesses a lignin aromatic skeleton, sulfonate groups, and a choline quaternary ammonium hydrophilic structure, which enhances its adsorption and hydration on the surface of direct red dye particles, improves the dispersion stability during wet filter cake compounding, reduces particle coarsening, and thus improves the stability and leveling properties of the composition.

[0021] Furthermore, the preparation method of the choline-modified sodium lignosulfonate includes the following steps: adding water and sodium lignosulfonate to a reaction vessel, stirring to dissolve, and then heating to 45-60°C and adjusting the pH to 9.0-10.0; adding epichlorohydrin and reacting for 1-2 hours to activate the phenolic and / or alcoholic hydroxyl groups in the sodium lignosulfonate molecules through epichlorohydrin grafting; subsequently adding choline and continuing the reaction at 55-70°C for 3-5 hours, maintaining the pH at 8.5-9.5 during the reaction; after the reaction is completed, adjusting the pH to 7.0-8.0, and then desalting and concentrating by nanofiltration to obtain the choline-modified sodium lignosulfonate.

[0022] The choline-modified sodium lignosulfonate prepared by the above method, while retaining the aromatic skeleton and hydrophilic groups of sodium lignosulfonate, introduces a choline quaternary ammonium hydrophilic structure, which enhances its adsorption, hydration, and steric stabilization on the surface of direct dye red particles. Epichlorohydrin grafting activation improves the effectiveness of choline binding to sodium lignosulfonate molecules, giving the modified product dispersing, solubilizing, and hard water resistance properties. Nanofiltration desalination and concentration reduce the residue of small molecule salts and unreacted substances, minimizing their impact on dye color and solubility. Its application in wet filter cake compounding systems helps inhibit dye particle aggregation and improves the dispersion stability and dyeing levelness of the composition.

[0023] Furthermore, in the preparation of the choline-modified sodium lignosulfonate, the weight ratio of water, sodium lignosulfonate, epichlorohydrin, and choline is 600:100:4-10:8-20. This ratio allows the sodium lignosulfonate to fully complete epichlorohydrin grafting activation and choline introduction under moderate dilution conditions, ensuring uniform modification reaction and improving the substitution uniformity of the product. Simultaneously, it avoids cross-linking thickening, gelation, and side reactions caused by excessive epichlorohydrin or choline, reducing the content of residual small molecules and impurities, thereby improving the dispersibility, solubility, and storage stability of the obtained modified sodium lignosulfonate.

[0024] According to some embodiments of this application, the desalination in step S3 includes one of nanofiltration desalination, acid precipitation, or salting out. By desalting the direct dye red reaction solution in step S3, sodium chloride, sodium sulfate, choline salts, and other small molecule impurities generated or remaining during diazotization and coupling can be effectively removed, reducing the inorganic salt content of the final direct dye composition and minimizing the impact of high-salt impurities on dye solubility, color stability, and dyeing reproducibility.

[0025] According to some embodiments of this application, the homogenization in step S4 is a combination of shear homogenization and sand milling homogenization. The shear homogenization speed is 1500-4000 r / min, the time is 20-60 min, and the D90 particle size of the slurry after sand milling is 1-8 μm. By combining shear homogenization and sand milling homogenization, and controlling the shear homogenization speed, time, and D90 particle size of the slurry after sand milling within a limited range, the wet filter cake of direct red dye can be fully depolymerized, and auxiliaries such as lignin sulfonate, amphoteric co-solvent, and polyaspartate can be uniformly adsorbed or coated on the surface of dye particles to form a stable dispersion protection system. This can reduce the content of coarse particles and agglomerates in the slurry, improve the wetting speed and hard water dispersion stability of the obtained composition, and at the same time, it is beneficial to improve the diffusion, leveling, and batch reproducibility during dyeing.

[0026] Secondly, embodiments of this application provide direct dye compositions prepared by low-temperature diazo coupling using the above-described method for preparing a direct dye composition by low-temperature diazo coupling.

[0027] The direct dye composition prepared by low-temperature diazo coupling according to the embodiments of this application has at least the following beneficial effects: This direct dye composition is prepared using a low-temperature diazotization, choline-regulated coupling, desalination washing, and wet filter cake compounding process. The composition has low byproduct and inorganic salt content, and the auxiliary agents are evenly distributed on the surface of the dye particles. It exhibits good cold water solubility, wetting and dispersibility, and hard water stability. During the dyeing of cellulose fibers, it provides stable dyeing, bright and stable color, and good leveling properties. It reduces floating dye and color fading during soaping, improves dyeing reproducibility and wet fastness, and lowers the electrolyte load of the dye bath. It is suitable for use on fibers such as cotton, viscose, and linen, as well as paper and water-based coloring systems.

[0028] In this article, the numerical ranges mentioned all include the endpoint values ​​and cover any subranges within that range, such as the ranges obtained by arbitrarily combining the specifically listed numerical values.

[0029] Unless otherwise specified, the weight parts of raw materials refer to the weight ratio between each raw material. For example, 5 to 18 parts of lignin sulfonate indicates the relative amount of lignin sulfonate in the raw materials for preparing the direct dye composition. Based on 1 to 8 parts of amphoteric cosolvent, it can be understood that the weight ratio of lignin sulfonate to amphoteric cosolvent is 5 to 18:1 to 8.

[0030] Direct dye red refers to the dry basis of direct dye red wet filter cake after drying; unless otherwise stated, the amount of direct dye red wet filter cake used in the direct dye compositions of this application is expressed as parts by weight of direct dye red on a dry basis. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a comparison chart showing the solubility and insoluble content of the direct dye compositions prepared in Examples 1-4 and Comparative Examples 1-4 of this application at 25°C. Figure 2 This is a comparison chart showing the dyeing rate and leveling properties of the direct dye compositions prepared in Examples 1-4 and Comparative Examples 1-4 of this application. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this application clearer, a further detailed description will be provided below in conjunction with specific embodiments. The embodiments described herein are merely some examples of this application and should not be construed as limiting the scope of protection of this application. Unless otherwise specified, the raw materials used in the embodiments are all commercially available, and the equipment and testing methods used are conventional equipment and methods in the art.

[0034] The main performance testing methods for this application are as follows: Evaluation of dyeing strength and color: The evaluation was conducted in accordance with GB / T 2374 "General Conditions for Dyeing Determination". The fabric was a pretreated standard cotton fabric. The dye dosage was 2% owf, the liquor ratio was 1:30, 20 g / L of anhydrous sodium sulfate was added, and the dyeing was carried out at 90℃ for 45 min. After washing, soaping, and drying, the K / S value was measured using a colorimeter. The relative strength was taken as 100% with Example 1.

[0035] Color fastness to washing: Tested in accordance with GB / T 3921 "Textiles - Tests for color fastness to washing".

[0036] Color fastness to rubbing: Tested in accordance with GB / T 3920 "Textiles - Color fastness test - Color fastness to rubbing".

[0037] Color fastness to perspiration: Tested in accordance with GB / T 3922 "Textiles - Tests for color fastness to perspiration".

[0038] Water fastness: Tested in accordance with GB / T 5713 "Textiles - Tests for color fastness - Water fastness".

[0039] Color fastness to artificial light: Tested according to GB / T 8427 "Textiles - Tests for color fastness to artificial light: Xenon arc".

[0040] Insoluble matter content: The determination of insoluble matter content shall be carried out in accordance with GB / T 2381 "Determination of insoluble matter content in dyes and dye intermediates".

[0041] Solubility test: A certain amount of dye composition was weighed and added to deionized water at 25℃. After stirring for 10 minutes, the dissolution state was observed. The solution was then filtered through a 0.45μm filter membrane. The endpoint was determined by the absence of obvious filter residue and the filtrate being clear or uniformly transparent. The maximum completely dissolved concentration was recorded. This project was used to evaluate the cold water performance of commercially available direct dye samples.

[0042] Dye uptake test: Measure the absorbance of the dye bath at the maximum absorption wavelength before and after dyeing. Calculate the dye uptake rate based on the change in absorbance using the formula E% = (1 - A1 / A0) × 100%, where A0 is the absorbance of the dye bath before dyeing and A1 is the absorbance of the dye bath after dyeing.

[0043] Evenness test: Take 10 points at different locations on the cotton fabric dyed in the same bath to measure the K / S value and calculate the coefficient of variation (CV%). The lower the CV%, the better the evenness.

[0044] Example 1 Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 2℃. Prepare a sodium nitrite solution by dissolving 1.06 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 3℃. After the addition is complete, keep the reaction at 3℃ for 2.5 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 1.02 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 1.3 kmol of choline to the second reaction vessel, stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves, control the temperature at 15℃, add the diazonium salt solution obtained in step S1 to the second reaction vessel, and adjust the pH of the system to 8.7 with choline aqueous solution during the addition. After the addition is completed, stir the reaction at 22℃ for 4 h to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 70 parts of the direct dye red wet filter cake (dry basis) with 12 parts of choline-modified sodium lignosulfonate, 4 parts of betaine, 3 parts of sodium polyaspartate, 2 parts of sodium acetate, and 0.3 parts of BYK-024 defoamer. Add water to adjust the slurry to a solid content of 28%. Shear homogenize at 2500 r / min for 40 min, and then sand mill until the D90 particle size is 4.5 μm to obtain a slurry-like direct dye composition. The preparation method of the choline-modified sodium lignosulfonate is as follows: 600 kg of water and 100 kg of sodium lignosulfonate are added to a reaction vessel, stirred and dissolved, and then heated to 52°C and the pH is adjusted to 9.5; 7 kg of epichlorohydrin is added and reacted for 1.5 h to activate the phenolic and alcoholic hydroxyl groups in the sodium lignosulfonate molecules through epichlorohydrin grafting; then 14 kg of choline is added and the reaction is continued at 62°C for 4 h, maintaining the pH at 9.0 during the reaction; after the reaction, a 10% hydrochloric acid aqueous solution is added dropwise to adjust the pH to 7.5, and the solution is desalted by nanofiltration and concentrated to obtain choline-modified sodium lignosulfonate.

[0045] Example 2 Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 5°C. Prepare a sodium nitrite solution by dissolving 1.02 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 4°C. After the addition is complete, keep the reaction at 0°C for 3 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 0.98 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 2.2 kmol of choline to the second reaction vessel, and stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves. Control the temperature at 10°C. Add the diazonium salt solution obtained in step S1 to the second reaction vessel. During the addition, adjust the pH of the system to 9.5 using choline aqueous solution. After the addition is completed, stir the reaction at 15°C for 5 hours to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 55 parts of the direct dye red wet filter cake (dry basis) with 18 parts of choline-modified sodium lignosulfonate, 1 part of sodium taurate, 5 parts of sodium polyaspartate, 0.5 parts of sodium citrate, and 0.5 parts of TEGO Foamex 810 defoamer. Add water to adjust the slurry to a solid content of 28%. Shear homogenize at 1500 r / min for 60 min, and then sand mill until the D90 particle size is 1 μm to obtain a slurry-like direct dye composition. The preparation method of the choline-modified sodium lignosulfonate is as follows: 600 kg of water and 100 kg of sodium lignosulfonate are added to a reaction vessel, stirred and dissolved, and then heated to 60°C and the pH is adjusted to 9.0; 10 kg of epichlorohydrin is added and reacted for 1 h to activate the phenolic and alcoholic hydroxyl groups in the sodium lignosulfonate molecules through epichlorohydrin grafting; then 20 kg of choline is added and the reaction is continued at 55°C for 5 h, maintaining the pH at 8.5 during the reaction; after the reaction, 10% hydrochloric acid aqueous solution is added dropwise to adjust the pH to 8.0, and the solution is desalted by nanofiltration and concentrated to obtain choline-modified sodium lignosulfonate.

[0046] Example 3 Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 0℃. Prepare a sodium nitrite solution by dissolving 1.12 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 2℃. After the addition is complete, keep the reaction at 5℃ for 2 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 1.08 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 0.8 kmol of choline to the second reaction vessel, stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves, control the temperature at 25°C, add the diazonium salt solution obtained in step S1 to the second reaction vessel, and adjust the pH of the system to 8.0 with choline aqueous solution during the addition. After the addition is completed, stir the reaction at 30°C for 3 hours to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 85 parts of the wet filter cake of the direct dye red (dry basis) with 5 parts of sodium lignosulfonate, 5 parts of betaine, 3 parts of sodium N,N-dimethylglycine, 0.5 parts of sodium polyaspartate, 4 parts of disodium hydrogen phosphate, and 0.05 parts of BYK-024 defoamer. Add water to adjust the slurry to a solid content of 28%, shear homogenize at 4000 r / min for 20 min, and then sand mill to a D90 particle size of 8 μm to obtain a slurry-like direct dye composition.

[0047] Example 4 Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 3℃. Prepare a sodium nitrite solution by dissolving 1.1 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 3℃. After the addition is complete, keep the reaction at 2℃ for 2.5 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 1.03 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 1.5 kmol of choline to the second reaction vessel, stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves, control the temperature at 20°C, add the diazonium salt solution obtained in step S1 to the second reaction vessel, and adjust the pH of the system to 8.8 with choline aqueous solution during the addition. After the addition is completed, stir the reaction at 23°C for 4 hours to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 75 parts of the direct dye red wet filter cake (dry basis) with 15 parts of choline-modified sodium lignin sulfonate, 3 parts of cocamidopropyl betaine, 2 parts of betaine, 2 parts of sodium polyaspartate, 2 parts of sodium acetate, 1 part of sodium bicarbonate, and 0.2 parts of TEGOFoamex 810 defoamer. Add water to adjust the slurry to a solid content of 28%, shear homogenize at 3000 r / min for 40 min, and then sand mill to a D90 particle size of 5 μm to obtain a slurry-like direct dye composition. The preparation method of the choline-modified sodium lignosulfonate is as follows: 600 kg of water and 100 kg of sodium lignosulfonate are added to a reaction vessel, stirred and dissolved, and then heated to 55°C and the pH is adjusted to 9.6; 8 kg of epichlorohydrin is added and reacted for 1.5 h to activate the phenolic and alcoholic hydroxyl groups in the sodium lignosulfonate molecules through epichlorohydrin grafting; then 15 kg of choline is added and the reaction is continued at 65°C for 4 h, maintaining the pH at 9.2 during the reaction; after the reaction, a 10% hydrochloric acid aqueous solution is added dropwise to adjust the pH to 7.4, and the solution is desalted by nanofiltration and concentrated to obtain choline-modified sodium lignosulfonate.

[0048] Comparative Example 1 (different from Example 1 in that the diazotization temperature in step S1 is 10°C, and sodium carbonate is used instead of choline in step S2 for coupling) Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 2℃. Prepare a sodium nitrite solution by dissolving 1.06 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 10℃. After the addition is complete, keep the reaction at 10℃ for 2.5 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 1.02 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 0.65 kmol of sodium carbonate to the second reaction vessel, and stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves. Control the temperature at 15°C. Add the diazonium salt solution obtained in step S1 to the second reaction vessel. During the addition, adjust the pH of the system to 8.7 using sodium carbonate aqueous solution. After the addition is completed, stir the reaction at 22°C for 4 hours to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 70 parts of the direct dye red wet filter cake (dry basis) with 12 parts of choline-modified sodium lignosulfonate, 4 parts of betaine, 3 parts of sodium polyaspartate, 2 parts of sodium acetate, and 0.3 parts of BYK-024 defoamer. Add water to adjust the slurry to a solid content of 28%. Shear homogenize at 2500 r / min for 40 min, and then sand mill until the D90 particle size is 4.5 μm to obtain a slurry-like direct dye composition. The preparation method of the choline-modified sodium lignosulfonate is as follows: 600 kg of water and 100 kg of sodium lignosulfonate are added to a reaction vessel, stirred and dissolved, and then heated to 52°C and the pH is adjusted to 9.5; 7 kg of epichlorohydrin is added and reacted for 1.5 h to activate the phenolic and alcoholic hydroxyl groups in the sodium lignosulfonate molecules through epichlorohydrin grafting; then 14 kg of choline is added and the reaction is continued at 62°C for 4 h, maintaining the pH at 9.0 during the reaction; after the reaction, a 10% hydrochloric acid aqueous solution is added dropwise to adjust the pH to 7.5, and the solution is desalted by nanofiltration and concentrated to obtain choline-modified sodium lignosulfonate.

[0049] Comparative Example 2 (different from Example 1 in that the diazotization temperature in step S1 is 13°C) Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 2℃. Prepare a sodium nitrite solution by dissolving 1.06 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 13℃. After the addition is complete, keep the reaction at 13℃ for 2.5 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 1.02 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 1.3 kmol of choline to the second reaction vessel, stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves, control the temperature at 15℃, add the diazonium salt solution obtained in step S1 to the second reaction vessel, and adjust the pH of the system to 8.7 with choline aqueous solution during the addition. After the addition is completed, stir the reaction at 22℃ for 4 h to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 70 parts of the direct dye red wet filter cake (dry basis) with 12 parts of choline-modified sodium lignosulfonate, 4 parts of betaine, 3 parts of sodium polyaspartate, 2 parts of sodium acetate, and 0.3 parts of BYK-024 defoamer. Add water to adjust the slurry to a solid content of 28%. Shear homogenize at 2500 r / min for 40 min, and then sand mill until the D90 particle size is 4.5 μm to obtain a slurry-like direct dye composition. The preparation method of the choline-modified sodium lignosulfonate is as follows: 600 kg of water and 100 kg of sodium lignosulfonate are added to a reaction vessel, stirred and dissolved, and then heated to 52°C and the pH is adjusted to 9.5; 7 kg of epichlorohydrin is added and reacted for 1.5 h to activate the phenolic and alcoholic hydroxyl groups in the sodium lignosulfonate molecules through epichlorohydrin grafting; then 14 kg of choline is added and the reaction is continued at 62°C for 4 h, maintaining the pH at 9.0 during the reaction; after the reaction, a 10% hydrochloric acid aqueous solution is added dropwise to adjust the pH to 7.5, and the solution is desalted by nanofiltration and concentrated to obtain choline-modified sodium lignosulfonate.

[0050] Comparative Example 3 (different from Example 1 in that sodium hydroxide is used instead of choline in step S2 for coupling) Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 2℃. Prepare a sodium nitrite solution by dissolving 1.06 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 3℃. After the addition is complete, keep the reaction at 3℃ for 2.5 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 1.02 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 1.3 kmol of sodium hydroxide to the second reaction vessel, stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves, control the temperature at 15℃, add the diazonium salt solution obtained in step S1 to the second reaction vessel, and adjust the pH of the system to 8.7 with sodium hydroxide aqueous solution during the addition. After the addition is completed, stir the reaction at 22℃ for 4 h to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 70 parts of the direct dye red wet filter cake (dry basis) with 12 parts of choline-modified sodium lignosulfonate, 4 parts of betaine, 3 parts of sodium polyaspartate, 2 parts of sodium acetate, and 0.3 parts of BYK-024 defoamer. Add water to adjust the slurry to a solid content of 28%. Shear homogenize at 2500 r / min for 40 min, and then sand mill until the D90 particle size is 4.5 μm to obtain a slurry-like direct dye composition. The preparation method of the choline-modified sodium lignosulfonate is as follows: 600 kg of water and 100 kg of sodium lignosulfonate are added to a reaction vessel, stirred and dissolved, and then heated to 52°C and the pH is adjusted to 9.5; 7 kg of epichlorohydrin is added and reacted for 1.5 h to activate the phenolic and alcoholic hydroxyl groups in the sodium lignosulfonate molecules through epichlorohydrin grafting; then 14 kg of choline is added and the reaction is continued at 62°C for 4 h, maintaining the pH at 9.0 during the reaction; after the reaction, a 10% hydrochloric acid aqueous solution is added dropwise to adjust the pH to 7.5, and the solution is desalted by nanofiltration and concentrated to obtain choline-modified sodium lignosulfonate.

[0051] Comparative Example 4 (different from Example 1 in that choline-modified sodium lignosulfonate, betaine and sodium polyaspartate are not added, and sodium sulfate is used to make up the corresponding mass parts) Preparation of direct dye compositions by low-temperature diazotization coupling: S1. Add 600 kg of water and 120 kg of 31% hydrochloric acid to the first reaction vessel. Add 1 kmol of p-aminoazobenzene-4-sulfonic acid while stirring. After stirring and dissolving, cool to 2℃. Prepare a sodium nitrite solution by dissolving 1.06 kmol of sodium nitrite. Add the sodium nitrite solution dropwise at 0.5 mol / h at 3℃. After the addition is complete, keep the reaction at 3℃ for 2.5 h. Then add aminosulfonic acid solution to digest the excess sodium nitrite to obtain a diazonium salt solution. S2. Add 500 kg of water, 1.02 kmol of 2-amino-5-naphthol-7-sulfonic acid, and 1.3 kmol of choline to the second reaction vessel, stir until the 2-amino-5-naphthol-7-sulfonic acid dissolves, control the temperature at 15℃, add the diazonium salt solution obtained in step S1 to the second reaction vessel, and adjust the pH of the system to 8.7 with choline aqueous solution during the addition. After the addition is completed, stir the reaction at 22℃ for 4 h to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is subjected to nanofiltration desalination. The concentrated solution is adjusted to pH 6.5 and then filtered. The filter cake is washed with deionized water until the conductivity of the filtrate is less than 5 mS / cm to obtain a direct dye red wet filter cake. S4. Mix 70 parts of the wet filter cake of the direct dye red on a dry basis with 19 parts of sodium sulfate, 2 parts of sodium acetate and 0.3 parts of BYK-024 defoamer, add water to adjust the slurry to a solid content of 28%, shear homogenize at 2500 r / min for 40 min, and then sand mill to a D90 particle size of 4.5 μm to obtain a slurry-like direct dye composition.

[0052] Experimental Example The paste-like direct dye compositions prepared in Examples 1-4 and Comparative Examples 1-4 were tested for their main properties. Another portion of the paste-like direct dye compositions was spray-dried at an inlet air temperature of 170°C and an outlet air temperature of 85°C to form a powder. Some properties were tested, and the test data are shown in the table below.

[0053] Table 1. Main properties of direct dye compositions Example 1 106 0.28 105 0.06 88.9 2.8 3.5 Example 2 100 0.32 95 0.08 87.6 3.1 6.4 Example 3 94 0.4 88 0.1 85.8 3.5 7.8 Example 4 103 0.3 100 0.07 88.1 2.9 4.6 Comparative Example 1 91 1.35 55 0.32 79.4 7.8 10.5 Comparative Example 2 93 1.1 78 0.18 82 5.6 6.7 Comparative Example 3 96 0.82 80 1.15 83.1 5.1 8.9 Comparative Example 4 99 0.45 58 0.28 80.6 8.2 18.6 Reference Figures 1-2 As shown in Table 1, the direct dye compositions obtained in Examples 1-4 of this application are superior to the comparative examples in terms of relative strength, solubility, dyeing rate, and leveling properties. Comparative Example 1, using higher temperature diazotization and inorganic base coupling, resulted in poor color stability, low solubility, and high inorganic salt content. Comparative Example 2 demonstrates that, although subsequent compounding remained the same after increasing the diazotization temperature, the amount of dye precursor byproducts increased, leading to a decrease in strength and color stability. Comparative Example 3 shows that when choline was replaced with an inorganic base, the dissolution activation and coupling uniformity of 2-amino-5-naphthol-7-sulfonic acid were insufficient, resulting in decreased dyeing performance. Comparative Example 4 demonstrates that the dye solubility and leveling properties significantly decreased when the compounding auxiliary agent system of this application was absent.

[0054] Table 2 Results of color fastness test Example 1 4-5 4 4-5 4 4 4 4 Example 2 4 4 4-5 3-4 4 4 4 Example 3 4 3-4 4 3-4 4 4 4 Example 4 4-5 4 4-5 4 4 4 4 Comparative Example 1 3 3 4 3 3-4 3-4 3-4 Comparative Example 2 3-4 3 4 3 3-4 3-4 3-4 Comparative Example 3 3-4 3-4 4 3 3-4 3-4 4 Comparative Example 4 3-4 3 4 3 3-4 3-4 4 As can be seen from Table 2, the direct dye compositions obtained in the embodiments of this application have good resistance to soaping, rubbing, perspiration, water, and light. This effect is not solely due to the structure of the dye matrix, but is also related to the reduction of byproducts by low-temperature diazotization, the participation of choline in coupling to increase the proportion of the main product, the reduction of inorganic salts by post-treatment, and the improvement of dyeing and reduction of floating color by compounding auxiliaries.

[0055] Low-salt dyeing was performed on cotton fabrics using the direct dye compositions of Examples 1, 2, Comparative Examples 1, and 4. The dyeing conditions were: dye dosage 2% owf, liquor ratio 1:30, anhydrous sodium sulfate addition of 5 g / L, 10 g / L, and 20 g / L, respectively, dyeing temperature 90℃, and holding time 45 min. The results are shown in the table below.

[0056] Table 3 Dyeing rate under different amounts of dyeing salt Example 1 80.1 85.5 88.9 Example 2 78.5 84.2 87.6 Comparative Example 1 61.7 70.8 79.4 Comparative Example 4 65.2 73.5 80.6 As can be seen from Table 3, the composition of this application still maintains a high dyeing rate under low dyeing salt conditions, indicating that it is suitable for low-salt dyeing processes and is beneficial to reducing the salt content of dyeing wastewater.

[0057] Although embodiments of this application have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions or alterations can be made to these embodiments without departing from the principles and spirit of this application, and the technical solutions resulting from such changes, modifications, substitutions or alterations will all fall within the protection scope of this application.

Claims

1. A method for preparing a direct dye composition by low-temperature diazotization coupling, characterized in that, Includes the following steps: S1. Add water, hydrochloric acid and p-aminoazobenzene-4-sulfonic acid to the first reaction vessel, stir to dissolve, cool to 0-5℃, add sodium nitrite solution dropwise, and after the addition is complete, keep the reaction at 0-5℃ for 2-3 hours. Use aminosulfonic acid to digest excess sodium nitrite to obtain diazonium salt solution. S2. Add water, 2-amino-5-naphthol-7-sulfonic acid and choline to the second reaction vessel, stir until dissolved, control the temperature at 10-25℃, add the diazonium salt solution to the second reaction vessel, and adjust the pH of the system to 8.0-9.5 with choline solution during the addition. After the addition is completed, stir the reaction at 15-30℃ for 3-5 hours to obtain the direct dye red reaction solution. S3. The direct dye red reaction solution is desalted, filtered, and washed with water to obtain a direct dye red wet filter cake; S4. The wet filter cake of the direct dye red is mixed with lignin sulfonate, amphoteric cosolvent, polyaspartate, buffer salt, and defoamer, and homogenized to obtain a direct dye composition.

2. The method for preparing a direct dye composition by low-temperature diazotization coupling according to claim 1, characterized in that, In step S4, the raw materials of the direct dye composition, by weight, include: 55-85 parts of direct dye red wet filter cake on a dry basis, 5-18 parts of lignin sulfonate, 1-8 parts of amphoteric cosolvent, 0.5-5 parts of polyaspartate, 0.5-4 parts of buffer salt, and 0.05-0.5 parts of defoamer.

3. The method for preparing a direct dye composition by low-temperature diazo coupling according to claim 1, characterized in that, The molar ratio of p-aminoazobenzene-4-sulfonic acid and sodium nitrite in step S1 and 2-amino-5-naphthol-7-sulfonic acid and choline in step S2 is 1:1.02~1.12:0.98~1.08:0.8~2.

2.

4. The method for preparing a direct dye composition by low-temperature diazo coupling according to claim 1, characterized in that, The amphoteric co-solvent in step S4 is selected from at least one of betaine, sodium taurate, N,N-dimethylglycinate, and cocamidopropyl betaine; And / or, the buffer salt in step S4 is selected from at least one of sodium acetate, disodium hydrogen phosphate, sodium bicarbonate, and sodium citrate; And / or, the defoamer in step S4 is a polyether-modified silicone defoamer; And / or, the lignin sulfonate in step S4 is selected from at least one of sodium lignin sulfonate, calcium lignin sulfonate, and modified sodium lignin sulfonate.

5. The method for preparing a direct dye composition by low-temperature diazo coupling according to claim 1, characterized in that, The lignin sulfonate is choline-modified sodium lignin sulfonate. The preparation method of the choline-modified sodium lignin sulfonate includes the following steps: adding water and sodium lignin sulfonate to a reaction vessel, stirring to dissolve, and then heating to 45-60°C and adjusting the pH to 9.0-10.0; adding epichlorohydrin and reacting for 1-2 hours to activate the phenolic and / or alcoholic hydroxyl groups in the sodium lignin sulfonate molecules through epichlorohydrin grafting; subsequently adding choline and continuing the reaction at 55-70°C for 3-5 hours, maintaining the pH at 8.5-9.5 during the reaction; after the reaction, adjusting the pH to 7.0-8.0, and then desalting and concentrating by nanofiltration to obtain choline-modified sodium lignin sulfonate.

6. The method for preparing a direct dye composition by low-temperature diazo coupling according to claim 5, characterized in that, The weight ratio of water, sodium lignosulfonate, epichlorohydrin, and choline in the raw materials for preparing the choline-modified sodium lignosulfonate is 600:100:4-10:8-20.

7. The method for preparing a direct dye composition by low-temperature diazotization coupling according to claim 1, characterized in that, The desalination in step S3 includes one of nanofiltration desalination, acid precipitation, or salting out.

8. The method for preparing a direct dye composition by low-temperature diazo coupling according to claim 1, characterized in that, The homogenization in step S4 is a combination of shear homogenization and sand milling homogenization. The shear homogenization speed is 1500-4000 r / min and the time is 20-60 min. The D90 particle size of the slurry after sand milling is 1-8 μm.

9. A direct dye composition prepared by low-temperature diazo coupling, obtained by the method for preparing a direct dye composition by low-temperature diazo coupling as described in any one of claims 1 to 8.

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