Chrome-free tanning agent and preparation method thereof

Abstract

The invention relates to the technical field of leather tanning, and discloses a chrome-free tanning agent and a preparation method thereof.The preparation method comprises the steps that ZrOCl2. 8H2O and tannic acid are dissolved in a citric acid buffer solution, stirring is conducted at a constant speed to prepare a Zr-TA precursor solution, the precursor solution is transferred into a high-pressure reaction kettle for a reaction, and nanocrystal nucleuses are obtained; kH-550 and deionized water are mixed, hydrochloric acid is added for catalytic hydrolysis, the hydrolysis degree is monitored through dynamic light scattering till the particle size is smaller than 2 nm and the hydrolysis degree is larger than 95%, reaction is stopped, and hydrolysate is obtained; mixing the nanocrystal nucleus with hydrolysate, adding hydrochloric acid for hydrolysis, and preparing an ASP coating layer by adopting a microfluidic device; and the ASP coating layer and Suc-PLA-PEG are treated in an ultrasonic field, a solvent is removed through CO2 supercritical drying, and the chrome-free tanning agent under the powder character is obtained. The chrome-free tanning agent is uniform in particle size distribution and high in stability.

Description

Technical Field

[0001] The present invention relates to the technical field of leather tanning, in particular to a chrome-free tanning agent and a preparation method thereof. Background Art

[0002] With growing environmental awareness and the ongoing green transformation of the leather processing industry, the environmental pollution and health risks associated with traditional chrome tanning agents are becoming increasingly prominent. Hexavalent chromium is potentially carcinogenic and easily remains in the leather tanning process, making it difficult to completely remove. This poses a threat to the human body and the ecological environment during the use and disposal of leather products. Therefore, the development of safe, efficient, and environmentally friendly chrome-free tanning agents has become a key research direction in the leather industry.

[0003] The chrome-free tanning agent systems that are currently being studied mainly include vegetable tanning agents, organic macromolecular tanning agents, and certain metal ion replacement tanning agents, such as titanium, aluminum, and zirconium. However, existing chrome-free tanning agents generally have the following problems during the tanning process: uneven particle size distribution makes it difficult to effectively penetrate leather fibers, resulting in incomplete tanning and low shrinkage temperature; poor stability and insufficient film-forming properties, resulting in the physical properties of tanned leather inferior to those of chrome tanning systems; low metal-organic ligand complexation efficiency affects the tanning agent's yield and tanning effect; traditional preparation methods are complex and lack green processability, and require high-temperature calcination or the use of large amounts of organic solvents, which increases the environmental burden.

[0004] Therefore, there is an urgent need for a new method for preparing a chrome-free tanning agent based on green chemistry, which has efficient complexing ability, controllable particle size, and adaptability to the high performance requirements of leather. Summary of the Invention

[0005] In view of this, the present invention proposes a chrome-free tanning agent and a preparation method thereof, aiming to solve the problems existing in the current technology of uneven particle size distribution, low complexation efficiency, poor stability, insufficient film-forming performance and low green degree of the preparation process of chrome-free tanning agents.

[0006] In one aspect, the present invention provides a method for preparing a chrome-free tanning agent, comprising the following steps: S1: ZrOCl2·8H2O and tannic acid were dissolved in a citric acid buffer solution with a pH of 3.8 to prepare a Zr-TA precursor solution, which was reacted at 60°C with constant stirring to form a complex. The precursor solution was transferred to a high-pressure reactor for reaction to obtain TA-ZrO nanocrystal cores with a particle size of about 5 nm; S2: Mix KH-550 with deionized water, add hydrochloric acid to catalyze hydrolysis, and monitor the degree of hydrolysis by dynamic light scattering. Stop the reaction when the particle size is less than 2 nm and the degree of hydrolysis is greater than 95% to obtain a hydrolyzate. The TA-ZrO nanocrystal cores were mixed with a hydrolysis solution, hydrochloric acid was added for hydrolysis, and an ASP coating layer was prepared using a microfluidic device. S3: The ASP coating layer and Suc-PLA-PEG are treated in an ultrasonic field and the solvent is removed by CO2 supercritical drying to obtain a chrome-free tanning agent in powder form.

[0007] Preferably, the concentration of tannic acid to ZrOCl2·8H2O is 3:1.

[0008] Preferably, the precursor solution is transferred to a high-pressure reactor and reacted at a temperature of 120°C.

[0009] Preferably, after the precursor solution is transferred to a high-pressure reactor, the end point of the reaction is determined by monitoring the disappearance of the phenolic hydroxyl absorption peak by in-situ Fourier transform infrared spectroscopy, wherein the absorption peak is 1580-1600 cm-1.

[0010] Preferably, the volume ratio of KH-550 to deionized water is 1:5, and the concentration of hydrochloric acid used is 0.1 mol / L.

[0011] Preferably, a flow rate ratio of 1:1 is used for liquid inflow in the microfluidic device, the Reynolds number of the reaction fluid is 200, and the thickness of the ASP coating layer is 8-12 nm.

[0012] Preferably, the dosage of the Suc-PLA-PEG is 1.5 times of its CMC value, and the treatment is carried out in an ultrasonic field at a frequency of 40 kHz and a power of 500 W.

[0013] Preferably, the critical temperature in the CO2 supercritical drying is set to 31.1°C and the critical pressure is set to 7.38 MPa.

[0014] Preferably, the specific surface area of ​​the chrome-free tanning agent is ≥200 m2 / g, and the pore size distribution range is 2-50 nm.

[0015] On the other hand, the present invention also protects a chrome-free tanning agent obtained by the above preparation method.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The present invention adopts the complexation reaction of ZrOCl2·8H2O and tannic acid in a buffer system, and controls the temperature of the reaction in an autoclave to prepare TA-ZrO nanocrystalline cores with a particle size of about 5 nm. The particle size distribution is concentrated, and the core has good dispersibility. It can more effectively penetrate the interior of leather collagen fibers, realize a more thorough and uniform tanning process, and significantly improve the physical stability and shrinkage temperature of the leather.

[0017] By hydrolyzing KH-550 to form a silanol solution, combined with TA-ZrO nanocrystal cores, an ASP coating layer with a thickness of 8-12 nm was prepared in a microfluidic system. This achieved surface coating control over the particles, improved the stability of the tanning agent during storage and tanning, prevented aggregation and agglomeration, and ensured the long-term suspension stability and delivery uniformity of the tanning agent.

[0018] Sucinyl-polylactic acid-polyethylene glycol (Suc-PLA-PEG) is used as a polymer synergistic component and compounded with the tanning agent under ultrasonic conditions to improve film-forming properties and the ability to bind to collagen fibers, so that the tanned leather has good softness, fullness and dry and wet friction fastness.

[0019] Compared with the traditional method of using organic solvents or high-temperature drying, the present invention uses CO2 supercritical fluid drying to avoid degradation of heat-sensitive components, significantly improve product purity and activity, and at the same time provide a new green, low-energy, and residue-free drying path for the pharmaceutical industry.

[0020] The present invention optimizes Zr 4+ The complexation ratio and reaction time with tannic acid under acidic conditions (monitored by in situ FTIR at 1580-1600 cm -1 Phenolic hydroxyl absorption peak), ensure complete reaction and high complexation efficiency, improve the yield of tanning agent products and reduce unreacted components from the source, and improve raw material utilization.

[0021] The obtained chrome-free tanning agent has ≥200m 2 / g specific surface area and 2-50nm pore size distribution can be used as a good microporous adsorbent, which not only improves the anchoring ability of tanning agents on collagen fibers, but also facilitates the adsorption and fixation of subsequent active substances such as dyes and auxiliaries, thereby improving the overall performance of leather.

[0022] The core raw materials of tanning agents such as tannic acid, citric acid, polylactic acid, etc. are all derived from natural or renewable resources. The entire process does not involve heavy metal chromium, highly volatile organic solvents or high-temperature calcination processes. It has good ecological compatibility and meets the safety requirements of environmental regulations such as REACH and RoHS for leather products. DETAILED DESCRIPTION

[0023] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention. Example

[0024] Preparation of precursor solution: Accurately weigh 0.30 mol of ZrOCl2·8H2O and 0.10 mol of tannic acid, and dissolve them in a citric acid buffer solution with a pH of 3.8 (buffer preparation method: adjust the pH to 3.8 by adding a certain amount of citric acid and its salt). Mix the two in a molar ratio of 3:1, place in a reactor, control the temperature to 60°C, and stir at a constant speed of 500 rpm for 2 hours to allow ZrOCl2·8H2O to react with tannic acid. 4+ It is fully complexed with tannic acid to form a Zr-TA precursor solution.

[0025] Preparation of nanocrystal cores in high pressure reactor: The precursor solution was transferred to a high-pressure reactor, sealed, and heated to 120°C for 4 hours. During this period, the absorption peak of the phenolic hydroxyl group (1580-1600 cm -1 When the absorption peak significantly weakens or disappears, the complexation reaction is nearly complete. After the reaction is complete, the mixture is cooled to room temperature to obtain a TA-ZrO nanocrystal core solution with a particle size of approximately 5 nm.

[0026] Preparation of hydrolyzate and coating layer: KH-550 was weighed and mixed with deionized water in a volume ratio of 1:5. 0.1 mol / L hydrochloric acid was added as a catalyst. The hydrolysis reaction was monitored online using dynamic light scattering (DLS). The reaction was stopped when the particle size of the hydrolyzed product was less than 2 nm and the degree of hydrolysis exceeded 95%, resulting in a stable hydrolyzate.

[0027] The TA-ZrO nanocrystal core solution was mixed with a hydrolyzate solution, and an appropriate amount of hydrochloric acid was added. The reaction was carried out using a microfluidic device. The flow rate ratio of the microfluidic device was set to 1:1, and the Reynolds number of the reaction fluid was controlled at 200, forming an ASP coating layer with a thickness of 8-12 nm.

[0028] Surface modification and drying treatment: In a 40kHz, 500W ultrasonic field, the ASP coating solution was mixed with a sucrose-based polylactic acid-polyethylene glycol copolymer. The dosage of Suc-PLA-PEG was 1.5 times its critical micelle concentration. Ultrasonic treatment was performed for 30 minutes to promote the full combination of the coating layer and the modifier.

[0029] Finally, the CO2 supercritical drying method was used, with the critical temperature set at 31.1°C and the critical pressure set at 7.38 MPa, to remove the solvent in the system and obtain a powdered chrome-free tanning agent.

[0030] In this example, Suc-PLA-PEG was prepared by the following method: Material preparation Sucrose (purity ≥99%); Polylactic acid monomer (Lactide, purity ≥99%); polyethylene glycol (PEG, adjustable molecular weight, 2000-5000 Da); Catalyst: diethylzinc (ZnEt2) or zinc acetylacetonate (Zn(acac)2); Solvent: anhydrous dichloromethane (DCM) or anhydrous toluene; Other reagents: N,N-diisopropylethylamine (DIEA), azobisisobutyronitrile (AIBN) (if free radical reaction is required).

[0031] Preparation method Step 1: Multifunctional group modification of sucrose; Sucrose activation In an anhydrous environment, sucrose was dissolved in dry toluene, an appropriate amount of N,N-diisopropylethylamine was added as a basic catalyst, and the reaction system was stirred at 80°C for 30 minutes.

[0032] Esterification modification A small amount of active hydroxyl donor for cyclic ring-opening polymerization of lactic acid monomer (Lactide) is slowly added to the system to partially esterify multiple hydroxyl groups on sucrose to form polyhydroxy-modified sucrose (Suc-ol). This process is gentle and can effectively maintain the multi-functional structure of sucrose and provide it with grafting points for polylactic acid.

[0033] Step 2: Sucrose-polylactic acid (Suc-PLA) graft polymerization; Ring-opening polymerization Suc-ol was transferred to a dry reactor, and pre-purified lactic acid monomer (Lactide) was added. A small amount of diethylzinc was added as a catalyst (ZnEt2, 0.1 mol%). Under nitrogen protection, the reaction temperature was set to 120°C, and the ring-opening polymerization reaction was carried out for 6 hours.

[0034] Adjusting the chain length of polylactic acid By adjusting the molar ratio of lactic acid monomer to sucrose, the length of the PLA chain segment was precisely controlled to obtain Suc-PLA grafted products with different molecular weights.

[0035] Step 3: PEG segment copolymerization modification; End-grafted PEG The synthesized Suc-PLA was dissolved in anhydrous dichloromethane, and an equimolar amount of hydroxyl-terminated PEG was added. A free radical reaction initiated by azobisisobutyronitrile (AIBN) or an esterification reaction catalyzed by DCC / DMAP was used to connect the PEG chain segments to the Suc-PLA ends through ester or ether bonds to form a sucrose-polylactic acid-polyethylene glycol triblock copolymer.

[0036] purification After the reaction was completed, dialysis was used to remove unreacted monomers, catalysts and low-molecular impurities, and finally pure Suc-PLA-PEG copolymer powder was obtained by freeze-drying.

[0037] The chrome-free tanning agent in this embodiment was sampled and the performance of the sample was tested. The experimental data record table is shown in Table 1: Systematic performance testing was conducted across multiple dimensions, including thermal stability, water resistance, flexibility, environmental friendliness, tanning uniformity, and long-term stability. The samples used in each test were taken from cowhide treated with the tanning agent described in this invention and then processed through cutting, drying, and shaping before testing.

[0038] The thermal shrinkage temperature (Ts) of tanned leather was measured using a differential scanning calorimeter (DSC) at a heating rate of 10°C / min over a temperature range of 30-200°C. The test results showed that the Ts value of leather tanned with the tanning agent of the present invention was 92.5°C, significantly higher than that of untanned raw hides (approximately 67.3°C) and superior to that obtained with traditional vegetable tanning agents. This demonstrates that the tanning agent imparts excellent thermal stability to the leather, meeting the requirements of subsequent processing steps such as heat drying and hot pressing.

[0039] In terms of water resistance, according to the GB / T 4689.18-2008 standard, leather samples were immersed in distilled water for 24 hours to measure their water absorption rate, and further immersed for 72 hours at pH 3 to measure their tensile strength retention. The results showed that the water absorption rate of leather samples treated with this tanning agent was 37.2%, far lower than the comparative vegetable-tanned leather (approximately 62.5%), and the tensile strength retention rate reached 89.7%, indicating good hydrolytic stability and water resistance.

[0040] In terms of flexibility and hand feel, the samples' softness was measured using a Handle-O-Meter and subjected to ISO 5402 bending fatigue testing. The samples showed no cracking or breaking after 10,000 bends, and their softness approached 93.4% of that of the chrome-tanned control. Ten leather professionals also conducted a blind evaluation and scored the leather's softness, elasticity, and skin-friendliness, with an average score of 4.3 out of 5, demonstrating the excellent wearability and comfort afforded by the tanning agent.

[0041] To verify the environmentally friendly nature of this tanning agent, inductively coupled plasma mass spectrometry (ICP-MS) was used to measure its heavy metal content. Analysis after microwave digestion showed that no toxic heavy metals, such as Cr, Pb, Cd, or Hg, were detected in the tanning agent and its tanned leather samples. This fully complies with the heavy metal limits set forth in REACH regulations and the national ecological leather standard GB / T 19941-2005, demonstrating its high environmental friendliness and biosafety.

[0042] To assess tanning agent penetration and tanning uniformity, Van Gieson dye was used to stain cortical cross-sections to observe the depth of tanning agent penetration. The dye layer coverage exceeded 90%, and the tanning agent's depth of incorporation was evenly distributed. Nitrogen-soluble analysis was also used to determine the degree of cross-linking, which reached 84.3%. This indicates that the tanning agent strongly bonds with collagen fibers and can effectively replace traditional metal tanning agents for deep tanning.

[0043] Finally, to assess its long-term stability and antimicrobial properties, leather samples were subjected to accelerated aging and antibacterial testing. After 500 hours of irradiation in a UV aging chamber, the sample's color difference (ΔE) was 1.2, and its tensile strength decreased by 7.6%, maintaining good mechanical properties. In antibacterial testing, the tanned leather samples exhibited inhibition zones greater than 7 mm in diameter against Staphylococcus aureus and Escherichia coli, demonstrating excellent antibacterial activity and suitability for applications requiring high hygiene standards, such as footwear, furniture, and automotive interiors.

[0044] Table 1

[0045] As can be seen from the table, firstly, in terms of thermal stability, after differential scanning calorimetry (DSC) testing, the thermal shrinkage temperature (Ts) of the tanned leather reached 92.5°C, which is significantly higher than the temperature range required for conventional thermal processing technology. This fully demonstrates that the tanning agent of the present invention imparts excellent thermal stability to the leather, which can meet the requirements of subsequent high-temperature processing technologies such as heat drying, embossing, and lamination, and avoid thermal collapse of the leather structure or performance degradation.

[0046] In the water resistance test, the water absorption rate of the tanned sample was controlled at 37.2%, and the tensile strength retention rate was still 89.7% after immersion in a strong acidic environment of pH=3 for 72 hours, reflecting that the stable cross-linked network formed by the tanning agent can effectively improve the hydrolytic stability of collagen fibers, and is suitable for application in clothing, shoe uppers and outdoor products in high humidity and high acid and alkali environments.

[0047] In terms of flexibility and comfort, after softness instrument testing and 10,000 ISO bending fatigue tests, the sample showed no cracks and its softness was close to that of standard chrome-tanned leather; it received a blind score of 4.3 points (out of 5 points) from professional judges, demonstrating the significant advantages of this tanning agent in maintaining the elasticity, ductility and skin-friendly feel of leather, making it particularly suitable for the development of high-end wearable leather products.

[0048] In terms of environmental protection, the ICP-MS test results show that no toxic heavy metals such as chromium, lead, cadmium, and mercury were detected in the tanning agent and the leather treated therewith, which fully complies with the REACH regulations and the GB / T 19941-2005 national ecological leather standard requirements, highlighting its high green safety and suitability for use in areas with extremely high requirements for eco-toxicity, such as baby products, furniture, and automotive finishes.

[0049] In the tanning uniformity test, the Van Gieson staining results showed that the tanning material penetration depth exceeded 90% and the distribution was uniform and consistent; combined with the nitrogen element solubility analysis method, the cross-linking degree was measured to be 84.3%, indicating that the tanning agent can achieve deep tanning and has a stable and long-lasting bond with collagen fibers, effectively improving the overall structural strength and mechanical consistency of the leather.

[0050] In terms of long-term stability and antibacterial properties, the color difference ΔE value of the sample after 500 hours of UV aging was only 1.2, and the tensile strength decrease rate was as low as 7.6%, demonstrating its excellent anti-aging performance; the antibacterial test results showed that inhibition zones with diameters exceeding 7 mm were formed for Staphylococcus aureus and Escherichia coli, indicating that the leather endowed by the tanning agent of the present invention has good antibacterial activity and has a wide range of sanitary protection value.

[0051] In summary, the chrome-free tanning agent of the present invention not only improves the thermal stability, water resistance, flexibility and comfort of leather, but also has multiple beneficial effects such as high environmental protection, good tanning uniformity, long-lasting structural stability and excellent antibacterial properties. It fully meets the comprehensive performance requirements of modern high-end ecological leather in terms of environmental protection, safety, functionality and comfort, and has broad prospects for industrial application.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a chrome-free tanning agent, characterized in that: The steps include: S1: ZrOCl2·8H2O and tannic acid were dissolved in a citric acid buffer solution with a pH of 3.8 to prepare a Zr-TA precursor solution, which was reacted at 60°C with constant stirring to form a complex. The precursor solution was transferred to a high-pressure reactor for reaction to obtain TA-ZrO nanocrystal cores with a particle size of approximately 5 nm; S2: Mix KH-550 with deionized water, add hydrochloric acid to catalyze hydrolysis, and monitor the degree of hydrolysis by dynamic light scattering. Stop the reaction when the particle size is less than 2 nm and the degree of hydrolysis is greater than 95% to obtain a hydrolyzate. The TA-ZrO nanocrystal cores were mixed with a hydrolysis solution, hydrochloric acid was added for hydrolysis, and an ASP coating layer was prepared using a microfluidic device. S3: The ASP coating layer and Suc-PLA-PEG are treated in an ultrasonic field and the solvent is removed by CO2 supercritical drying to obtain a chrome-free tanning agent in powder form.

2. The method for preparing a chrome-free tanning agent according to claim 1, wherein The concentration of the tannic acid and ZrOCl2·8H2O is 3:

1.

3. The method for preparing a chrome-free tanning agent according to claim 1, wherein The precursor solution was transferred to a high-pressure reactor and reacted at a temperature of 120°C.

4. The method for preparing a chrome-free tanning agent according to claim 1, wherein After the precursor solution was transferred to a high-pressure reactor, the end point of the reaction was determined by monitoring the disappearance of the phenolic hydroxyl absorption peak by in-situ Fourier transform infrared spectroscopy. -1 .

5. The method for preparing a chrome-free tanning agent according to claim 1, wherein The volume ratio of KH-550 to deionized water is 1:5, and the concentration of hydrochloric acid used is 0.1 mol / L.

6. The method for preparing a chrome-free tanning agent according to claim 1, characterized in that: The microfluidic device adopts a flow rate ratio of 1:1 for liquid inflow, the Reynolds number of the reaction fluid is 200, and the thickness of the ASP coating layer is 8-12 nm.

7. The method for preparing a chrome-free tanning agent according to claim 1, characterized in that: The dosage of the Suc-PLA-PEG is 1.5 times of its CMC value, and the treatment is carried out in an ultrasonic field at a frequency of 40 kHz and a power of 500 W.

8. The method for preparing a chrome-free tanning agent according to claim 1, characterized in that: The critical temperature in the CO2 supercritical drying is set to 31.1°C and the critical pressure is set to 7.38 MPa.

9. The method for preparing a chrome-free tanning agent according to claim 1, wherein: The specific surface area of ​​the chrome-free tanning agent is ≥200m 2 / g, and the pore size distribution range is 2-50nm.

10. A chrome-free tanning agent obtained by the preparation method according to any one of claims 1 to 9.

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