Environment-friendly dyeing process for leather

Abstract

The application discloses an environment-friendly leather dyeing process, which comprises the following steps: placing tanned and evenly cut leather in a rotating drum for pretreatment, adjusting pH value, neutralizing and washing to obtain pretreated leather; and placing the pretreated leather in a dyeing solution, and adopting gradient dyeing process of low-temperature holding and temperature rising to dye the leather, wherein the dyeing solution comprises the following raw materials in percentage: 2-5 wt% of plant-based dye, 0.3-0.8 wt% of dispersing agent, 0.2-0.5 wt% of penetrating agent, 0.1-0.3 wt% of color fixing agent, and the rest of water; wherein the plant-based dye is modified from plant raw materials by using quaternized gum arabic and modified pullulan; after dyeing, water is added, heated, and color fixed, and then washed, dried and dyed. The application solves the technical problems of poor color fixing and non-environment-friendly process in leather dyeing.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of leather production, and particularly relates to an environmentally-friendly leather dyeing process. BACKGROUND

[0002] Leather dyeing is a key process in leather processing, and directly determines the appearance color, color fastness and product grade of the leather product. With the increasingly strict national environmental protection policy, the leather industry is transforming towards low emission, low energy consumption and clean production. The traditional leather dyeing process generally uses high-concentration dyes, a large amount of dyeing and fixing aids, and is combined with high-temperature and long-time dyeing process. Although the leather can be colored, the overall production mode has poor environmental protection.

[0003] The traditional process has low dye utilization rate, and a large amount of unbound dyes and aids are left in the dyeing waste liquid, resulting in high colority and high organic matter content of the wastewater, high treatment difficulty, greatly increased environmental protection treatment cost, high energy consumption of high-temperature dyeing, and easy generation of volatile pollutants. The existing environmentally-friendly improved process reduces the dye, reduces the aid, and reduces the temperature to achieve emission reduction, but causes problems such as insufficient dye penetration, light color, obvious color difference, poor color fastness, and easy to cause uneven stress on the leather fiber, strength reduction, and stiff hand feeling, which cannot meet the requirements of environmental protection production, leather mechanical strength and high dyeing quality. Therefore, it is urgent to develop a new dyeing process that takes into account environmental protection, leather strength and high dyeing effect. SUMMARY

[0004] The application provides an environmentally-friendly leather dyeing process.

[0005] Therefore, the application provides an environmentally-friendly leather dyeing process, which comprises the following steps:

[0006] S1: placing the tanned and evenly cut leather in a rotating drum for pretreatment, adjusting the pH value, and washing to neutral to obtain pretreated leather;

[0007] S2: placing the pretreated leather in a dyeing liquid, and using a gradient dyeing process of low-temperature and temperature holding for dyeing, wherein the dyeing liquid comprises the following percentage of raw materials: 2-5wt% of plant-based dyes, 0.3-0.8wt% of dispersing agents, 0.2-0.5wt% of penetrating agents, 0.1-0.3wt% of fixing agents, and the balance of water; wherein the plant-based dyes are modified from plant raw materials by quaternary ammonium modified arabic gum and modified pullulan;

[0008] S3: after dyeing, adding water, heating, temperature holding, fixing, and then washing and drying to complete the dyeing.

[0009] Further, an environmentally-friendly leather dyeing process comprises the following steps:

[0010] S1: Place the tanned and evenly shaved leather in a drum and wash it with water at 40-45℃ for 15-25 minutes to remove surface grease, ash and impurities. Add sodium bicarbonate to adjust the pH to 5-5.5 and treat for 40-60 minutes to complete the neutralization of the leather. After neutralization, take it out and wash the leather with water until it is neutral to obtain the pre-treated leather.

[0011] S2: The pretreated leather is placed in the dyeing solution and dyed using a gradient dyeing process of low temperature holding and high temperature holding. The dyeing solution comprises the following raw materials in percentage: 2-5 wt% plant-based dye, 0.3-0.8 wt% dispersant, 0.2-0.5 wt% penetrant, 0.1-0.3 wt% fixing agent, and the balance being water; wherein, the plant-based dye is obtained by modifying plant raw materials with quaternized gum arabic and modified pullulan polysaccharide.

[0012] S3: After dyeing, add water at 30-50% of the leather weight from step S1, heat to 55-60℃, keep warm and fix the color for 40-60 minutes, then wash and dry to complete the dyeing process.

[0013] Optionally, the plant-based dye is prepared using the following method:

[0014] A1: Mix and crush the branches of sappanwood, the fruit of annatto, and the fruit of gardenia, add water, ultrasonically extract, flocculate, take the upper liquid, concentrate under reduced pressure to obtain concentrated pigment liquid.

[0015] A2: Quaternized gum arabic and modified pullulan polysaccharide are added to the concentrated pigment solution, the pH value is adjusted, a cross-linking agent is added, the mixture is mixed evenly, and after homogenization and sterilization, plant-based dyes are obtained.

[0016] Furthermore, the plant-based dye is prepared using the following method:

[0017] A1: Mix and crush the branches of Sappanwood, the fruit of annatto, and the fruit of gardenia, pass them through a 40-60 mesh sieve, add them to water, and ultrasonically extract them for 30-40 minutes at a power of 200-300W and a frequency of 30-40kHz. Then, treat them with chitosan for flocculation, let them stand and settle for 20-30 minutes, take the upper layer of liquid, and concentrate it under reduced pressure to obtain concentrated pigment liquid.

[0018] A2: Add quaternized gum arabic and modified pullulan polysaccharide to the concentrated pigment solution, adjust the pH value to 5.5-6.5 with citric acid-sodium citrate buffer solution, add crosslinking agent, mix well, react for 2.5-3.5h, homogenize and sterilize to obtain plant-based dye;

[0019] In step A1, the total weight ratio of chitosan to sappanwood branches, annatto fruit, and gardenia fruit is (0.08-0.12):1. The cross-linking agent is genipin, and the amount of cross-linking agent added to the concentrated pigment solution is 0.4-0.8 wt% of the concentrated pigment solution.

[0020] Optionally, the weight ratio of the sappanwood branches, annatto fruit, and gardenia fruit is (2-4):(1-3):1.

[0021] Optionally, the weight ratio of the concentrated pigment solution, quaternized gum arabic, and modified pullulan is 1:(1.1-1.3):(1.7-1.9).

[0022] Optionally, the quaternized gum arabic is prepared by the following method: gum arabic is placed in water, glycidyltrimethylammonium chloride is added, mixed evenly, heated to a higher temperature, and reacted. After the reaction is completed, the mixture is dialyzed to remove unreacted glycidyltrimethylammonium chloride and byproducts. The dialysate is collected, freeze-dried, and then the quaternized gum arabic is obtained.

[0023] Furthermore, the quaternized gum arabic is prepared by the following method: gum arabic is added to water to prepare a gum arabic aqueous solution with a mass fraction of 10-15 wt%, and stirred until completely dissolved; glycidyltrimethylammonium chloride is added at 25-35% of the mass of gum arabic, mixed evenly, and then heated to 55-65℃ and stirred at a constant temperature for 3-4 hours; after the reaction is completed, heating is stopped and the mixture is allowed to cool naturally to room temperature; the resulting reaction solution is transferred to a dialysis bag with a molecular weight cutoff of 8000-14000 Da, and dialyzed with deionized water for 24-30 hours, changing the water every 4-6 hours during the process to remove unreacted glycidyltrimethylammonium chloride monomer and small molecule salt byproducts; the dialysis purified solution in the dialysis bag is collected, freeze-dried, and powdered quaternized gum arabic is obtained.

[0024] Optionally, the modified pullulan is prepared using the following method:

[0025] Add citric acid to water to adjust the pH value, then add blueberry anthocyanins and chondroitin sulfate, mix well to obtain a mixture;

[0026] Put pullulan into water and mix well to obtain a pullulan aqueous solution;

[0027] The mixture was added dropwise to pullulan aqueous solution, mixed evenly, allowed to stand, matured, and dialyzed to remove unreacted blueberry anthocyanins and chondroitin sulfate. Finally, it was freeze-dried to obtain modified pullulan.

[0028] Furthermore, the modified pullulan polysaccharide is prepared using the following method:

[0029] Add citric acid to water to adjust the pH to 4.0-5.0, control the system temperature at 25-35℃, add blueberry anthocyanins and chondroitin sulfate, mix well to obtain a mixture;

[0030] Add pullulan to water, mix well, and prepare a pullulan aqueous solution with a mass fraction of 2.5-3.5 wt%.

[0031] The mixture was slowly added dropwise to pullulan polysaccharide aqueous solution at a rate of 1-2 mL / min. After the addition was complete, the mixture was stirred for 40-60 min and allowed to stand for 50-70 min to mature. Then, it was placed into a dialysis bag with a molecular weight cutoff of 8000-14000 Da and dialyzed with water for 18-24 h, with the water changed every 4-6 h to remove unreacted blueberry anthocyanins and chondroitin sulfate. Finally, it was freeze-dried to obtain modified pullulan polysaccharide.

[0032] The amount of blueberry anthocyanins added is 8-12 wt% of pullulan, and the amount of chondroitin sulfate added is 16-24 wt% of pullulan.

[0033] Optionally, the penetrant is one or more of bio-based sodium lignin sulfonate and tea saponin, the penetrant is one or more of penetrant T, and the color-fixing agent is one or more of color-fixing agent K-301 and color-fixing agent JYB-215.

[0034] Furthermore, the penetrant is tea saponin, the penetrant is penetrant T, and the color-fixing agent is color-fixing agent K-301.

[0035] Optionally, the gradient staining process involves first holding the dye at 30-32°C for 15-20 minutes, then raising the temperature to 45-50°C and holding it for 40-60 minutes.

[0036] Furthermore, the gradient staining process involves first holding the dye at 30°C for 18 minutes, then raising the temperature to 47°C and holding it for 50 minutes.

[0037] As can be seen from the above technical solutions, the embodiments of the present invention have the following advantages:

[0038] 1. This invention discloses an environmentally friendly dyeing process for leather. Compared with traditional chemical synthetic dye dyeing processes, the overall process of this invention is green and clean. It constructs the dyeing system with natural plant-derived components, does not use formaldehyde or heavy metal dyes throughout the process, and the wastewater is easy to treat. It meets the requirements for clean leather production and can improve the dyeing uniformity and color fastness of leather while ensuring that the original feel and mechanical properties of the leather are not damaged. It also reduces the color and organic pollutant emissions of wastewater during the dyeing process, thus achieving low-pollution dyeing processing of leather.

[0039] 2. This invention employs a synergistic modification of plant pigments using a combination of quaternized gum arabic and modified pullulan polysaccharide. The two polysaccharides are functionally complementary, structurally nested, and have clearly defined roles, forming a dual-effect system of charge adsorption + steady-state encapsulation. The specific synergistic mechanism is as follows:

[0040] First, quaternized gum arabic functions as an adsorbent and facilitates rapid dyeing. The molecular chain of quaternized gum arabic carries a high density of permanent positive charges, enabling it to strongly electrostatically bind with anionic plant pigment molecules. This anchors free and easily aggregated plant pigment molecules uniformly onto the polymer chain, effectively improving the water solubility and dispersibility of the pigments. Simultaneously, the overall dye particles in the dyeing system carry a positive charge, allowing them to actively target and adsorb the negative potential points on the surface of leather collagen fibers. This significantly increases the adsorption rate and initial dyeing amount on the leather surface, solving the problems of weak adsorption, easy ionization, and low dyeing rate of plant dyes.

[0041] Secondly, modified pullulan polysaccharides provide structural protection, resistance to environmental interference, and long-lasting color stabilization. Modified pullulan polysaccharides possess a stable structure encapsulated by electrostatic hydrogen bonds of blueberry anthocyanins and chondroitin sulfate, exhibiting a dense polymeric network cavity structure. This structure can encapsulate and spatially lock plant pigment molecules, shielding them from damage caused by external high temperatures, acid-base fluctuations, water washing shear forces, and oxidative environments. It inhibits pigment degradation and fading, significantly improving the dye's thermal stability, acid-base stability, and storage stability, thus compensating for the poor stability and insufficient color fastness of natural plant pigments.

[0042] Finally, the two work synergistically: quaternized gum arabic firmly holds the pigment in the inner layer through electrostatic bonds, enabling rapid dye adsorption, uniform color application, and improved dyeing rate; modified pullulan forms a high-molecular hydrogen bond protective layer on the outer layer, comprehensively encapsulating and locking the inner pigment-polysaccharide complex. The inner layer's electrostatic bonding fixes the color-producing body, while the outer layer's spatial structure resists external environmental damage, forming a stable double-layer structure of "internal anchoring and external protection."

[0043] The two polysaccharides do not interfere with each other and have nested functions, which not only solves the shortcomings of poor dispersion, weak adsorption and low dyeing uptake of plant dyes, but also solves the defects of plant dyes being not heat-resistant, not acid and alkali-resistant, easy to fade and poor color fastness. Ultimately, it achieves an environmentally friendly dyeing effect with high dyeing uniformity, high dyeing uptake, excellent color fastness and strong storage stability.

[0044] 3. This invention limits the amount of sappanwood branches, annatto fruit, and gardenia fruit added. The pigment from sappanwood branches is mainly red, the pigment from annatto fruit provides an orange-yellow tone, and the pigment from gardenia fruit provides a bright yellow tone. The combination of the three can achieve complementary color tones and create the composite color required for leather dyeing.

[0045] When the proportion of sappanwood branches is low, the red component is insufficient, resulting in low red saturation on the leather surface after dyeing and an overall yellowish tone. When the proportion of sappanwood branches is high, the red component is excessive, resulting in an overly dark tone, which can easily lead to a dark and dull leather surface. It can also cause pigment accumulation and lead to color spots.

[0046] When the proportion of annatto fruit is low, its contribution to the orange tone is insufficient, resulting in a monotonous color range. If the proportion is too high, the orange pigment becomes excessive, leading to an overall orange hue that makes it difficult to achieve the desired composite color. Gardenia fruit, as a base yellow component, serves as a reference in the formulation, stabilizing the system's base color. Within this ratio range, the three plant pigments synergistically produce a balanced and harmonious color, resulting in a strong color-developing ability in the extracted composite pigment solution. This produces a full and even color on the dyed leather, effectively avoiding the problems of thin color tone and poor color controllability associated with single-plant pigment dyeing.

[0047] 4. This invention limits the amount of quaternized gum arabic and modified pullulan added. Quaternized gum arabic mainly provides cationic electrostatic binding sites to achieve anchoring and adsorption of plant pigments, thereby improving the dyeing ability of dyes on leather fibers. Modified pullulan mainly plays an outer encapsulation and protection role, improving the thermal and acid-base stability of dyes and enhancing color fastness.

[0048] If the amount of quaternized gum arabic added is low, the system has insufficient cationic binding sites, which cannot fully complex plant pigment molecules, and the pigments are prone to agglomeration and precipitation, resulting in a decrease in dyeing rate. If the amount of quaternized gum arabic added is high, the excessive cationic polymer will cause the dye system to be too viscous and the dye molecules to be too large, making it difficult to penetrate into the leather fibers and only remaining on the leather surface, resulting in poor dyeing effect and increasing the organic load of dyeing wastewater.

[0049] With a low amount of modified pullulan polysaccharide, the outer encapsulation component is insufficient, and a complete and effective protective layer cannot be formed. The improvement in the dye's resistance to high temperature and acid and alkali interference is limited, and the color fastness after dyeing is not significantly improved. With a high amount of modified pullulan polysaccharide, the polysaccharide component is excessive, the viscosity of the system increases significantly, the dye particles are too large, the penetration is blocked, and more floating color is easily produced. The color fastness after washing decreases, and the raw material cost is increased at the same time.

[0050] Within this formulation range, the matching amounts of quaternized gum arabic and modified pullulan polysaccharide can form a double-layer composite structure of "internal anchoring and external protection," which not only ensures the full complexation and dispersion of plant pigments but also constructs a complete stable protective layer, taking into account the dye's dispersibility, penetration ability, dyeing rate, and color fastness, thus achieving optimal overall performance. Detailed Implementation

[0051] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in the present invention can be purchased on the market or prepared by existing methods.

[0052] Preparation Example

[0053] Preparation Example 1

[0054] A quaternized gum arabic, prepared by the following method:

[0055] Gum arabic was added to water to prepare a 12 wt% gum arabic aqueous solution, which was stirred until completely dissolved. Glycidyltrimethylammonium chloride was added at 30% of the gum arabic mass, mixed evenly, and heated to 60°C. The mixture was stirred at a constant temperature for 3.5 h. After the reaction was completed, heating was stopped, and the mixture was allowed to cool naturally to room temperature. The resulting reaction solution was transferred to a dialysis bag with a molecular weight cutoff of 11000 Da and dialyzed with deionized water for 26 h, with the deionized water being replaced every 5 h to remove unreacted glycidyltrimethylammonium chloride monomer and small molecule salt byproducts. The purified dialysis solution in the dialysis bag was collected, freeze-dried, and powdered quaternized gum arabic was obtained.

[0056] Preparation Example 2

[0057] A modified pullulan polysaccharide, prepared by the following method:

[0058] Add citric acid to water to adjust the pH to 4.5, control the system temperature at 30℃, add blueberry anthocyanins and chondroitin sulfate, mix well to obtain a mixture;

[0059] Add pullulan to water, mix well, and prepare a 3wt% pullulan aqueous solution;

[0060] The mixture was slowly added dropwise to pullulan aqueous solution at a rate of 1.5 mL / min. After the addition was complete, the mixture was stirred for 50 min and allowed to stand for 60 min to mature. Then, it was placed in a dialysis bag with a molecular weight cutoff of 11000 Da and dialyzed with water for 21 h, with the water being changed every 5 h to remove unreacted blueberry anthocyanins and chondroitin sulfate. Finally, it was freeze-dried to obtain modified pullulan.

[0061] The amount of blueberry anthocyanins added is 10 wt% of pullulan polysaccharide, and the amount of chondroitin sulfate added is 20 wt% of pullulan polysaccharide.

[0062] Preparation Example 3

[0063] A plant-based dye, prepared by the following method:

[0064] A1: Mix and crush the branches of Sappanwood, the fruit of annatto, and the fruit of gardenia, pass them through a 50-mesh sieve, add them to water, and ultrasonically extract them for 35 minutes at a power of 250W and a frequency of 35kHz. Then, treat them with chitosan for flocculation, let them stand and settle for 25 minutes, take the upper layer of liquid, and concentrate them under reduced pressure to obtain concentrated pigment liquid.

[0065] A2: Quaternized gum arabic prepared in Preparation Example 1 and modified pullulan polysaccharide prepared in Preparation Example 2 were added to the concentrated pigment solution. The pH was adjusted to 6 with citric acid-sodium citrate buffer solution, a crosslinking agent was added, the mixture was stirred evenly, and the reaction was carried out for 3 hours. After homogenization and sterilization, the plant-based dye was obtained.

[0066] In step A1, the total weight ratio of chitosan to sappanwood branches, annatto fruit, and gardenia fruit is 0.1:1. The cross-linking agent is genipin. The amount of cross-linking agent added to the concentrated pigment solution is 0.6 wt% of the concentrated pigment solution. The weight ratio of sappanwood branches, annatto fruit, and gardenia fruit in step A1 is 2:1:1. In step A2, the weight ratio of concentrated pigment solution, quaternized gum arabic, and modified pullulan polysaccharide is 1:1.1:1.7.

[0067] Preparation Example 4

[0068] A plant-based dye, which differs from Preparation Example 3 in that the amount of sappanwood branches added is different. In Preparation Example 4, the weight ratio of sappanwood branches, annatto fruit, and gardenia fruit is 3:1:1.

[0069] Preparation Example 5

[0070] A plant-based dye, which differs from Preparation Example 3 in that the amount of sappanwood branches added is different. In Preparation Example 5, the weight ratio of sappanwood branches, annatto fruit, and gardenia fruit is 4:1:1.

[0071] Preparation Example 6

[0072] A plant-based dye, which differs from Preparation Example 4 in the amount of annatto fruit added. In Preparation Example 6, the weight ratio of sappanwood branches, annatto fruit, and gardenia fruit is 3:2:1.

[0073] Preparation Example 7

[0074] A plant-based dye, which differs from Preparation Example 4 in the amount of annatto fruit added. In Preparation Example 7, the weight ratio of sappanwood branches, annatto fruit, and gardenia fruit is 3:3:1.

[0075] Preparation Example 8

[0076] A plant-based dye, which differs from Preparation Example 6 in the amount of quaternized gum arabic added. In Preparation Example 8, the weight ratio of concentrated pigment solution, quaternized gum arabic, and modified pullulan polysaccharide is 1:1.2:1.7.

[0077] Preparation Example 9

[0078] A plant-based dye, which differs from Preparation Example 6 in the amount of quaternized gum arabic added. In Preparation Example 9, the weight ratio of concentrated pigment solution, quaternized gum arabic, and modified pullulan polysaccharide is 1:1.3:1.7.

[0079] Preparation Example 10

[0080] A plant-based dye, which differs from Preparation Example 8 in the amount of modified pullulan added. In Preparation Example 10, the weight ratio of concentrated pigment solution, quaternized gum arabic, and modified pullulan is 1:1.2:1.8.

[0081] Preparation Example 11

[0082] A plant-based dye, which differs from Preparation Example 8 in that the amount of modified pullulan added is different. In Preparation Example 11, the weight ratio of concentrated pigment solution, quaternized gum arabic, and modified pullulan is 1:1.2:1.9.

[0083] Example

[0084] Example 1

[0085] An environmentally friendly leather dyeing process includes the following steps:

[0086] S1: Place the tanned and evenly shaved leather in a drum and wash it with 42°C water for 20 minutes to remove surface grease, ash and impurities. Add sodium bicarbonate to adjust the pH to 5.2 and treat for 50 minutes to complete the neutralization of the leather. After neutralization, take it out and wash the leather with water until it is neutral to obtain the pre-treated leather.

[0087] S2: The pretreated leather is placed in the dyeing solution and kept at 30°C for 18 minutes, then heated to 47°C and kept at 47°C for 50 minutes. The raw material ratio of the dyeing solution is shown in Table 1. The plant-based dye is prepared using Preparation Example 3.

[0088] S3: After dyeing, add water equal to 40% of the leather weight from step S1, heat to 58℃, keep warm and fix the color for 50 minutes, then wash and dry to complete the dyeing process.

[0089] Examples 2-5

[0090] An environmentally friendly leather dyeing process differs from Example 1 in that the raw material ratio of the plant-based dye is different, as shown in Table 1.

[0091] Table 1. Weight percentage of each raw material in plant-based dyes (%)

[0092]

[0093] Examples 6-14

[0094] An environmentally friendly leather dyeing process, which differs from Example 4 in that the plant-based dyes in Examples 6-14 are from different sources. The plant-based dyes in Examples 6-14 were prepared using Preparation Examples 4-11, respectively.

[0095] Comparative Example

[0096] Comparative Example 1

[0097] An environmentally friendly leather dyeing process, which differs from Example 1 in that the plant-based dyes in the dyeing solution are not processed in step A2.

[0098] Comparative Example 2

[0099] An environmentally friendly leather dyeing process, which differs from Example 1 in that the quaternized gum arabic in the plant-based dye is replaced in equal amounts with modified pullulan.

[0100] Comparative Example 3

[0101] An environmentally friendly leather dyeing process, which differs from Example 1 in that the modified pullulan polysaccharide in the plant-based dye is replaced in equal amounts with quaternized gum arabic.

[0102] Comparative Example 4

[0103] An environmentally friendly leather dyeing process, which differs from Example 1 in that the weight ratio of concentrated pigment solution, quaternized gum arabic, and modified pullulan polysaccharide is adjusted to 1:1.0:2.0.

[0104] Comparative Example 5

[0105] An environmentally friendly leather dyeing process is a traditional chemical dyeing process, which uses commercially available industrial azo acid synthetic dyes as the main material, combined with fatty alcohol polyoxyethylene ether leveling agent, penetrant JFC, and dicyandiamide formaldehyde condensate formaldehyde-type fixing agent. The pH of the dye bath is adjusted with formic acid and acetic acid. No plant pigments or polysaccharide modifying auxiliaries are added throughout the process. The pretreatment process is consistent with the present invention. After tanning and leveling, the leather is washed and neutralized, and then directly put into the dye bath. It is dyed for 90 minutes at a constant temperature of 65°C, without a low-temperature gradient pre-dyeing process. Afterwards, the temperature is directly raised to 72°C without adding water, and the formaldehyde fixing agent is added and kept at this temperature for 70 minutes for color fixation. After a simple cold water wash, the leather is squeezed and dried.

[0106] Performance testing

[0107] The following performance tests were performed on the dyed leathers from Examples 1-14 and Comparative Examples 1-5:

[0108] Tensile strength: The tensile strength of the leather was determined according to GB / T46469-2025 "Physical and mechanical tests of leather - Determination of tensile strength and elongation". The test results are shown in Table 2.

[0109] Tear strength: The tear strength of the leather was determined according to QB / T2711 "Leather Tear Strength Test", and the test results are shown in Table 2.

[0110] Color fastness to rubbing: The color fastness to rubbing of leather was determined according to GB / T 40920-2021 "Leather Color Fastness Test - Reciprocating Rubbing Fastness". The test results are shown in Table 2.

[0111] Formaldehyde content: The formaldehyde content of the leather was determined according to GB / T1994.1-2019 "Determination of formaldehyde content in leather and fur - Part 1: High performance liquid chromatography". The test results are shown in Table 2.

[0112] Color: The color of the wastewater after dyeing was measured according to GB / T11903-1989 "Determination of color of water". The test results are shown in Table 2.

[0113] COD: The wastewater after dyeing was measured according to GB / T11914-89 "Determination of Chemical Oxygen Demand in Water - Dichromate Method". The test results are shown in Table 2.

[0114] Table 2 Test Results

[0115]

[0116] From the overall performance of the leather's mechanical properties, the tensile strength and tear strength of all embodiments of this invention are consistently higher than the minimum requirements of the national industry standard, and the overall mechanical properties are significantly better than all comparative sample samples. The performance and formulation ratio show a clear correlation; the sample with the optimal polysaccharide ratio exhibits the best fiber strength. As the ratio of polysaccharides and plant pigments gradually deviates from the appropriate range, the mechanical strength of the leather gradually decreases. All comparative samples exhibit varying degrees of mechanical shortcomings. Lack of polysaccharide modification, single polysaccharide modification, imbalanced polysaccharide ratios, and traditional high-temperature chemical dyeing processes all damage the integrity of the leather collagen fibers or cause uneven dyeing, leading to localized weakness in the fibers and ultimately reducing the durability of the finished leather. This also proves that the low-temperature gradient dyeing process of this invention can effectively protect the leather fibers and will not negatively affect the mechanical properties of the leather itself.

[0117] As a core indicator for evaluating the color fastness to rubbing, the data shows a significant synergistic effect between the two polysaccharides. Samples using a suitable ratio of quaternized gum arabic and modified pullulan polysaccharide combined with modified plant pigments can form a stable double-layer structure with an inner layer of electrostatically anchored pigment and an outer layer of polymer protection, achieving colorfastness to high-grade leather products. Other examples with ratios slightly deviating from the optimal range also meet the color fastness requirements for conventional leather products. Conversely, the performance of the comparative groups shows a significant decline. Plant pigments lacking polysaccharide modification have weak bonding with leather fibers, making them prone to fading under rubbing. Using only a single polysaccharide can only achieve a single function of adsorption or protection, resulting in a significant decrease in color fastness. An imbalanced polysaccharide ratio cannot construct a complete double-layer coating structure, leading to a simultaneous decrease in color fastness. Even with formaldehyde auxiliaries for color fastness, traditional chemical dyeing still cannot match the optimal formulation of this invention in terms of rubbing resistance. This fully demonstrates that the simultaneous use of two polysaccharides in a reasonable ratio is a key condition for ensuring high color fastness.

[0118] The formaldehyde test results of the finished product directly reflect the environmental safety of the process. All embodiments of this invention, as well as the comparative example using only a plant-based dye modification system, did not add any formaldehyde-containing fixing agents throughout the entire process. No formaldehyde was detected in the finished leather products, meeting the safety standards for eco-friendly leather and intimate leather products. In contrast, the comparative example of traditional chemical dyeing relies on formaldehyde condensates for color fixing, resulting in significant formaldehyde residue in the finished product. Although this residue does not exceed national standards, it poses a risk of skin irritation and sensitization, making it unsuitable for high-end, export-oriented environmentally friendly leather products. This invention completely eliminates the formaldehyde residue hazard from the raw material stage, resulting in a significant advantage in the safety of the finished product.

[0119] The color and COD of the dyeing wastewater reflect the process's emission reduction capacity. The overall wastewater pollution load of all embodiments was significantly lower than that of the comparative examples. The sample with the optimal polysaccharide ratio showed sufficient dye uptake, minimal residual pigment and organic impurities in the wastewater, and the lowest pollution level. Samples with formulations deviating from the suitable range showed only a slight increase in wastewater pollutant indicators, maintaining an overall low pollution level. The wastewater pollution levels of the comparative examples increased significantly. Lack of polysaccharide modification, mono-polysaccharide modification, and imbalanced polysaccharide ratios all reduced dye uptake, leaving a large amount of free pigment and organic matter in the wastewater. Traditional high-temperature chemical dyeing processes use high-concentration synthetic dyes and various chemical auxiliaries, resulting in low dye utilization and wastewater color and organic pollutant loads far exceeding those of the process of this invention. This significantly increases wastewater treatment costs and environmental pressure, highlighting the outstanding clean production advantages of the plant-based modified dyeing system of this invention.

[0120] A comprehensive analysis of all testing dimensions leads to the conclusion that this invention, through the combination of compound plant pigments, precise proportions of dual polysaccharides, and a low-temperature gradient dyeing process, simultaneously considers the mechanical properties of leather, color fastness, and environmental friendliness in production. Quaternized gum arabic and modified pullulan are both indispensable; only when their proportions fall within a defined range can a complete and stable double-layer coating structure be formed, achieving optimal overall performance. Several comparative examples with different designs respectively verified the inherent defects of processes without polysaccharide modification, with single polysaccharides, with unbalanced proportions, and traditional high-temperature chemical dyeing processes. This invention not only solves the industry pain points of traditional dyeing—high-temperature damage to leather, formaldehyde residue, and severe wastewater pollution—but also overcomes the inherent shortcomings of natural plant dyes, such as low dye uptake and poor color fastness.

[0121] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An environmentally friendly leather dyeing process, characterized in that: Includes the following steps: S1: The tanned and evenly shaved leather is placed in a drum for pretreatment. The pH value is adjusted and the leather is washed with water until neutral to obtain the pretreated leather. S2: The pretreated leather is placed in the dyeing solution and dyed using a gradient dyeing process of low temperature holding and high temperature holding. The dyeing solution comprises the following raw materials in percentage: 2-5 wt% plant-based dye, 0.3-0.8 wt% dispersant, 0.2-0.5 wt% penetrant, 0.1-0.3 wt% fixing agent, and the balance being water; wherein, the plant-based dye is obtained by modifying plant raw materials with quaternized gum arabic and modified pullulan polysaccharide. S3: After dyeing, add water, heat to raise the temperature, keep warm to fix the color, and then wash and dry to complete the dyeing process.

2. The environmentally friendly leather dyeing process according to claim 1, characterized in that: The plant-based dyes are prepared using the following method: A1: Mix and crush the branches of sappanwood, the fruit of annatto, and the fruit of gardenia, add water, ultrasonically extract, flocculate, take the upper liquid, concentrate under reduced pressure to obtain concentrated pigment liquid. A2: Quaternized gum arabic and modified pullulan polysaccharide are added to the concentrated pigment solution, the pH value is adjusted, a cross-linking agent is added, the mixture is mixed evenly, and after homogenization and sterilization, plant-based dyes are obtained.

3. The environmentally friendly leather dyeing process according to claim 2, characterized in that: The weight ratio of the sappanwood branches, annatto fruit, and gardenia fruit is (2-4):(1-3):

1.

4. The environmentally friendly leather dyeing process according to claim 2, characterized in that: The weight ratio of the concentrated pigment solution, quaternized gum arabic, and modified pullulan is 1:(1.1-1.3):(1.7-1.9).

5. The environmentally friendly leather dyeing process according to claim 2, characterized in that: The quaternized gum arabic is prepared by the following method: gum arabic is placed in water, glycidyltrimethylammonium chloride is added, mixed evenly, heated to a higher temperature, and reacted. After the reaction is completed, it is dialyzed to remove unreacted glycidyltrimethylammonium chloride and byproducts. The dialysate is collected, freeze-dried, and then quaternized gum arabic is obtained.

6. The environmentally friendly leather dyeing process according to claim 2, characterized in that: The modified pullulan polysaccharide was prepared using the following method: Add citric acid to water to adjust the pH value, then add blueberry anthocyanins and chondroitin sulfate, mix well to obtain a mixture; Put pullulan into water and mix well to obtain a pullulan aqueous solution; The mixture was added dropwise to pullulan aqueous solution, mixed evenly, allowed to stand, matured, and dialyzed to remove unreacted blueberry anthocyanins and chondroitin sulfate. Finally, it was freeze-dried to obtain modified pullulan.

7. The environmentally friendly leather dyeing process according to claim 1, characterized in that: The penetrant is one or more of bio-based sodium lignin sulfonate and tea saponin; the penetrant is one or more of phosphorus-free penetrant JFC-S and penetrant T; and the fixing agent is one or more of fixing agent K-301 and fixing agent JYB-215.

8. The environmentally friendly leather dyeing process according to claim 1, characterized in that: The gradient staining process involves first holding the dye at 30-32℃ for 15-20 minutes, then raising the temperature to 45-50℃ and holding it for 40-60 minutes.

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