A process for the preparation of a chromium-free keratin-based tanning agent

Abstract

A kind of chrome-free tanning agent based on keratin and its preparation method, the method is extracted from waste wool or feather keratin as raw material, in the presence of alkaline catalyst, with ethylene glycol glycidyl ether carries out ring-opening grafting reaction, and the modified keratin tanning agent with terminal remaining epoxy group is prepared;Using the rich functional groups on the keratin molecular chain, high active epoxy groups are introduced by chemical modification, so that it can form stable covalent crosslinking with amino groups and other groups on the skin collagen fiber;The prepared tanning agent is applied to the leather tanning process, which can significantly improve the shrinkage temperature of the crude leather to more than 80 DEG C, and the finished leather is full and has excellent mechanical properties;The present application realizes the high-value utilization of waste protein resources, and the obtained product completely eliminates chromium pollution, and provides an efficient and clean chrome-free tanning solution for the leather industry.

Description

Technical Field

[0001] This invention belongs to the field of leather tanning technology, specifically relating to a method for preparing a chromium-free tanning agent based on keratin. Background Technology

[0002] Tanning is the core process for transforming raw hides into leather. Tanning agents impart softness, elasticity, mechanical strength, and water and heat resistance to leather by cross-linking collagen fibers. Currently, chrome tanning agents account for 80% to 85% of global leather production due to their superior tanning performance. However, the chrome tanning process poses serious environmental problems. Approximately 2% to 5% of chromium remains in the waste liquid, and trivalent chromium (Cr(III)) can be oxidized to hexavalent chromium (Cr(VI)) under certain conditions, posing a serious threat to water bodies, soil, and the ecological environment. With the EU REACH regulation imposing strict restrictions on hexavalent chromium, developing environmentally friendly chrome-free tanning agents to replace chromium salts has become a core issue for the green transformation of the leather industry.

[0003] Currently, there are four main types of chromium-free tanning agents, but each has its own limitations:

[0004] 1) Non-chromium metal tanning agents: Represented by zirconium tanning agents, they exist in aqueous solution as tetrameric complexes, but their binding with collagen requires strong acidic conditions at pH 1.5~2.0. The coordinate bonds formed at this time are relatively weak, and water washing easily leads to detanning.

[0005] 2) Aldehyde tanning agents: such as glutaraldehyde. Tanned leather has good water resistance, but it has problems such as low shrinkage temperature and easy yellowing of finished leather. In addition, free aldehydes pose potential hazards to human health and the environment.

[0006] 3) Synthetic tanning agents: These are usually aromatic sulfonic acid compounds, formed by the condensation of sulfonated phenols and formaldehyde. Although they have excellent tanning properties, they pose environmental and health risks due to formaldehyde and phenol residues.

[0007] 4) Vegetable tanning agents: widely available and biodegradable, they produce firm, full-bodied leather with good breathability. However, when used alone, they have a slow penetration rate and a relatively low shrinkage temperature.

[0008] Keratin is a fibrous structural protein extracted from waste wool and poultry feathers used in leather tanning. It contains 7%–11% cysteine ​​residues and forms a stable three-dimensional network structure through numerous disulfide bonds. More importantly, the side chains of keratin molecules are rich in active functional groups such as amino, thiol, hydroxyl, and carboxyl groups. This provides the possibility for chemical modification (such as introducing tanning groups like epoxy and aldehyde groups), enabling it to form stable multi-point covalent cross-links and hydrogen bonds with leather collagen fibers. This makes it a promising high-performance, renewable, chromium-free tanning agent raw material. Summary of the Invention

[0009] To overcome the shortcomings of the prior art, the present invention aims to provide a method for preparing a chromium-free tanning agent based on keratin. This method utilizes renewable biomass resources to replace traditional chromium tanning agents. Through the reaction of keratin with ethylene glycol glycidyl ether, a stable covalent cross-linking network is constructed between molecular chains, thereby giving the resulting tanning agent good resistance to damp heat. This yields an active tanning agent with terminally retained epoxy groups, enabling a strong bond with leather collagen fibers and promoting the cleanliness and sustainable development of the leather tanning process.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is: a method for preparing a chromium-free tanning agent based on keratin, comprising the following steps:

[0011] Step 1: Dissolve keratin in water, stir, add tetramethylethylenediamine as a catalyst, adjust the pH to alkaline, and then transfer to a constant pressure dropping funnel to obtain a keratin solution;

[0012] Step 2: Add ethylene glycol glycidyl ether to a three-necked flask, and slowly drop the keratin solution obtained in Step 1 into the three-necked flask, and carry out a constant temperature stirring reaction.

[0013] Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze-dry it, and obtain a chromium-free tanning agent based on keratin.

[0014] The stirring described in step 1 takes 8-10 minutes.

[0015] In step 1, the keratin solution is added over a period of 45 to 55 minutes.

[0016] In step 1, the mass ratio of keratin to water is 1:5 to 10.

[0017] In step 1, the mass ratio of keratin to tetramethylethylenediamine is 100-200:1.

[0018] In step 1, the pH is 8.5 to 9.0.

[0019] In step 2, the constant temperature stirring time is 6.5 hours.

[0020] In step 2, the molar ratio of keratin to ethylene glycol glycidyl ether is 1:2.0 to 3.5.

[0021] Compared with existing technologies, the keratin-based chromium-free tanning agent, its preparation method, and its application provided by this invention have the following significant advantages:

[0022] 1) Excellent Tanning Performance and Environmental Friendliness: This invention uses keratin extracted from waste hair in leather tanning as the main raw material. Utilizing the abundant active amino, thiol, and hydroxyl functional groups on the side chains of keratin, a nucleophilic ring-opening addition reaction occurs with the epoxy groups at both ends of the ethylene glycol diglycidyl ether molecule. This constructs a stable covalent cross-linked network between keratin molecular chains and between keratin and skin collagen fibers, resulting in a highly stable, chromium-free keratin-based chromium-free tanning agent. This invention uses keratin extracted from waste wool and feathers as raw material, achieving high-value utilization of solid waste resources. The prepared chromium-free tanning agent completely avoids the pollution risk of chromium ions, fundamentally solving the heavy metal pollution and hexavalent chromium carcinogenic hazards caused by traditional chromium tanning processes. This invention, using keratin extracted from waste hair in leather tanning as raw material, is green, environmentally friendly, and non-toxic, meeting the requirements of clean and sustainable development in the leather industry.

[0023] 2) Superior thermal stability enhancement: By epoxidizing keratin, active epoxy groups are successfully introduced into its molecular chain. The epoxy groups are retained at the ends of the keratin-based tanning agent molecules, which can undergo nucleophilic ring-opening addition reactions with the ε-amino groups of lysine and hydroxylysine in the collagen side chains to form stable β-hydroxy secondary amine covalent cross-linking bonds. This facilitates the chemical anchoring of the tanning agent between the collagen fibers in the leather, improving the binding strength and absorption efficiency of the tanning agent. Application examples show that the shrinkage temperature (Ts) of the raw leather tanned using the tanning agent of this invention can reach up to 80.8 ℃, which is significantly higher than the tanning effect of conventional chromium-free tanning agents (such as TWS) and close to the level of traditional chromium tanning. This proves that it forms a stable covalent cross-linking network with the collagen fibers in the leather, giving the leather good resistance to damp heat.

[0024] 3) Ideal mechanical properties and fullness: Experimental data confirms that leather treated with the tanning agent of this invention has a tensile strength of 7.97 MPa and a tear strength of 63.57 N / mm, and the raw leather is full and soft. This is due to the structural characteristics of the keratin macromolecule itself and its multi-site binding with collagen fibers (covalent bonds, hydrogen bonds, etc.), which effectively improves the defect of thin leather that may be caused by some synthetic tanning agents.

[0025] 4) The preparation process is simple and controllable, and easy to industrialize: The preparation method described in this invention has mild reaction conditions (pH 8.5, room temperature reaction), simple steps, and only involves conventional unit operations such as dissolution, catalysis, grafting reaction, precipitation purification and drying. The reagents used (such as ethylene glycol glycidyl ether and tetramethylethylenediamine) are common and readily available, no special equipment is required, the production process is stable, and it is easy to scale up production and promote.

[0026] This invention uses keratin extracted from waste hair in leather tanning as the main raw material. It utilizes the abundant amino, thiol, and hydroxyl groups on the side chains of keratin to undergo nucleophilic ring-opening addition reactions with the epoxy groups at both ends of ethylene glycol glycidyl ether molecules. This results in the construction of a stable covalent cross-linking network between keratin molecular chains and between keratin and skin collagen fibers. This leads to the development of a chromium-free tanning agent with good hygrothermal stability and terminal epoxy activity, which is beneficial to the cleanliness and sustainable development of leather tanning. Attached Figure Description

[0027] Figure 1 This is a comparison chart of the shrinkage temperature of leather tanned with different tanning agents according to the present invention.

[0028] Figure 2 This is a comparison chart of the tensile strength of leather tanned with different tanning agents according to the present invention.

[0029] Figure 3 This is a comparison chart of the tear strength of leather tanned with different tanning agents according to the present invention.

[0030] Figure 4 This is a comparison chart of the absorption rates of waste liquid from leather tanned with different tanning agents according to the present invention.

[0031] Figure 5 This is a comparison chart showing the shrinkage temperature of leather tanned with tanning agents synthesized using different molar ratios of keratin and ethylene glycol glycidyl ether in this invention.

[0032] Figure 6 This is a comparison chart of the epoxy values ​​of tanning agents synthesized in different molar ratios of keratin and ethylene glycol glycidyl ether in this invention. Detailed Implementation

[0033] The structural and working principles of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0034] Example 1

[0035] A method for preparing a chromium-free tanning agent based on keratin includes the following steps:

[0036] Step 1: Weigh 15g of keratin and dissolve it in 75g of water. Stir, add 1% tetramethylethylenediamine (TMD) as a catalyst, adjust the pH to 8.5 with NaOH solution, stir for 8 minutes until evenly dispersed, and then transfer to a constant pressure dropping funnel.

[0037] Step 2: Add 2.61g of ethylene glycol glycidyl ether to a three-necked flask, with a dropwise addition time of 50min, and stir at a constant temperature for 6.5h.

[0038] Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze dry it, and obtain a chromium-free tanning agent based on keratin.

[0039] The mass ratio of keratin to water is 1:5; the molar ratio of keratin to ethylene glycol glycidyl ether is 1:2.0; and the mass ratio of keratin to tetramethylethylenediamine is 100:1.

[0040] Example 2

[0041] A method for preparing a chromium-free tanning agent based on keratin includes the following steps:

[0042] Step 1: Weigh 20g of keratin and dissolve it in 100g of water. Add 1% tetramethylethylenediamine (TMD) as a catalyst and adjust the pH to 8.5 with NaOH solution. Stir for 9 minutes until the mixture is evenly dispersed and then transfer it to a constant pressure dropping funnel.

[0043] Step 2: Add 4.35 g of ethylene glycol glycidyl ether to a three-necked flask, adding it dropwise over 50 minutes. Maintain a constant temperature and stir the mixture for 6.5 hours.

[0044] Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze-dry it, and obtain a chromium-free tanning agent based on keratin.

[0045] The mass ratio of keratin to water is 1:5; the molar ratio of keratin to ethylene glycol glycidyl ether is 1:2.5; and the mass ratio of keratin to tetramethylethylenediamine is 100:1.

[0046] Example 3

[0047] A method for preparing a chromium-free tanning agent based on keratin includes the following steps:

[0048] Step 1: Weigh 25g of keratin and dissolve it in 125g of water. Add 1% tetramethylethylenediamine as a catalyst and adjust the pH to 8.5 with NaOH solution. Stir for 10 minutes until evenly dispersed and then transfer to a constant pressure dropping funnel.

[0049] Step 2: Add 6.53g of ethylene glycol glycidyl ether to a three-necked flask, with a dropwise addition time of 50min, and stir at a constant temperature for 6.5h.

[0050] Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze-dry it, and obtain a chromium-free tanning agent based on keratin.

[0051] The mass ratio of keratin to water is 1:5; the molar ratio of keratin to ethylene glycol glycidyl ether is 1:3.0; and the mass ratio of keratin to tetramethylethylenediamine is 100:1.

[0052] Example 4

[0053] A method for preparing a chromium-free tanning agent based on keratin includes the following steps:

[0054] Step 1: Weigh 30g of keratin and dissolve it in 150g of water. Add 1% tetramethylethylenediamine (TMD) as a catalyst and adjust the pH to 8.5 with NaOH solution. Stir until the mixture is evenly dispersed and then transfer it to a constant pressure dropping funnel.

[0055] Step 2: Add 9.135g of ethylene glycol glycidyl ether to a three-necked flask, with a dropwise addition time of 50min, and stir at a constant temperature for 6.5h.

[0056] Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze-dry it, and obtain a chromium-free tanning agent based on keratin.

[0057] The mass ratio of keratin to water is 1:5; the molar ratio of keratin to ethylene glycol glycidyl ether is 1:3.5; and the mass ratio of keratin to tetramethylethylenediamine is 100:1.

[0058] Example 5

[0059] A method for preparing a chromium-free tanning agent based on keratin includes the following steps:

[0060] Step 1: Weigh 25g of keratin and dissolve it in 200g of water. Add 1.5% tetramethylethylenediamine as a catalyst and adjust the pH to 8 with NaOH solution. Stir until evenly dispersed and then transfer to a constant pressure dropping funnel.

[0061] Step 2: Add 6.53g of ethylene glycol glycidyl ether to a three-necked flask, with a dropwise addition time of 50min, and stir at a constant temperature for 6.5h.

[0062] Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze-dry it, and obtain a chromium-free tanning agent based on keratin.

[0063] The mass ratio of keratin to water is 1:8; the molar ratio of keratin to ethylene glycol glycidyl ether is 1:3.0; and the mass ratio of keratin to tetramethylethylenediamine is 150:1.

[0064] Example 6

[0065] A method for preparing a chromium-free tanning agent based on keratin includes the following steps:

[0066] Step 1: Weigh 25g of keratin and dissolve it in 250g of water. Add 2% tetramethylethylenediamine as a catalyst and adjust the pH to 9 with NaOH solution. Stir until evenly dispersed and then transfer to a constant pressure dropping funnel.

[0067] Step 2: Add 6.53g of ethylene glycol glycidyl ether to a three-necked flask, with a dropwise addition time of 50min, and stir at a constant temperature for 6.5h.

[0068] Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze-dry it, and obtain a chromium-free tanning agent based on keratin.

[0069] The mass ratio of keratin to water was 1:10; the molar ratio of keratin to ethylene glycol glycidyl ether was 1:3.0; and the mass ratio of keratin to tetramethylethylenediamine was 200:1. The keratin-based chromium-free tanning agent prepared in Example 3 above was used in the tanning process of sheepskin.

[0070]

[0071] After the keratin-based chromium-free tanning agent prepared in Example 3 was tanned using the tanning process shown in Table 1, the shrinkage temperature, elongation at break, and tensile strength of the raw leather were tested. The keratin-based chromium-free tanning agent was also characterized by infrared spectroscopy and nuclear magnetic resonance testing, which proved the tanning effect of the chromium-free tanning agent.

[0072] See Figure 1-6 ,See Figure 1 The shrinkage rate of the tanned leather samples was measured using a digital leather shrinkage meter (model: MSW-YD4). According to ISO 3380-2002, a small number of sample strips were suspended on the instrument and immersed in water, heated at a rate of 2 degrees Celsius per minute, while the temperature at which the samples suddenly shrank was observed and recorded. The results show that the shrinkage temperature of leather tanned with the tanning agent of this invention is higher than that of other tanned leathers. See [link to relevant documentation]. Figure 2-4 According to ISO 3376-1976 standard, the tear strength, tensile strength, and elongation at break of tanned leather were measured using the following equipment: a servo-controlled computer tensile testing machine (TH-8203S), a servo-controlled benchtop tensile testing machine (TH-8203S), and a servo-controlled temperature difference tensile testing machine (AI-7000-NGD). The results show that the tensile strength of leather tanned with the tanning agent of this invention is lower than that of other tanned leathers, while the tear strength is higher than that of other tanned leathers and acid leathers. Furthermore, the waste liquor absorption rate after fatliquoring with the tanning agent of this invention is significantly higher than that of other tanning agents. See [link to relevant documentation]. Figure 5 The shrinkage rate of the tanned leather samples was measured using a digital leather shrinkage meter (model: MSW-YD4). According to ISO 3380-2002, a small amount of sample strips were suspended on the instrument and immersed in water, heated at a rate of 2 degrees Celsius per minute, while the temperature at which the sample suddenly shrank was observed and recorded. The results showed that the shrinkage temperature was approximately the same when the molar ratio of keratin to glycidyl ether was 1:3.0 as it was when it was 1:3.5; therefore, a molar ratio of 1:3.0 offered better cost-effectiveness. See [link to relevant documentation]. Figure 6First, 10 mL of concentrated hydrochloric acid was mixed with 40 mL of deionized water to prepare the hydrochloric acid solution for use. Then, 0.2 g of KH-EGDE was weighed and added to an Erlenmeyer flask along with 3 mL of the prepared hydrochloric acid solution. After the KH-EGDE was dissolved, the flask was placed in a shaker and reacted in the dark at 40°C and 60 rpm for 4 hours. After the reaction, the solution was titrated with 0.35 M NaOH solution until the pH of the solution reached neutral. The volume of NaOH consumed was recorded as v1 mL. Next, 3 mL of hydrochloric acid solution was titrated using the same method, and the volume of sodium hydroxide consumed was recorded as v2 mL. The formula for calculating the epoxy value is as follows:

[0073] (1)

[0074] Experimental results show that the epoxy value is highest and the tanning effect is better when the molar ratio of keratin to ethylene glycol glycidyl ether is 1:3.0.

Claims

1. A method for preparing a chromium-free tanning agent based on keratin, characterized in that, Includes the following steps: Step 1: Dissolve keratin in water, stir, add tetramethylethylenediamine as a catalyst, adjust the pH to alkaline, and then transfer to a constant pressure dropping funnel to obtain a keratin solution; Step 2: Add ethylene glycol glycidyl ether to a three-necked flask, and slowly drop the keratin solution obtained in Step 1 into the three-necked flask, and carry out a constant temperature stirring reaction. Step 3: After the reaction is complete, cool the product, precipitate and purify it with anhydrous ethanol, freeze-dry it, and obtain a chromium-free tanning agent based on keratin.

2. The method for preparing a chromium-free tanning agent based on keratin according to claim 1, characterized in that, The stirring described in step 1 takes 8-10 minutes.

3. The method for preparing a chromium-free tanning agent based on keratin according to claim 1, characterized in that, In step 1, the keratin solution is added over a period of 45 to 55 minutes.

4. The method for preparing a chromium-free tanning agent based on keratin according to claim 1, characterized in that, In step 1, the mass ratio of keratin to water is 1:5 to 10.

5. The method for preparing a chromium-free tanning agent based on keratin according to claim 1, characterized in that, In step 1, the mass ratio of keratin to tetramethylethylenediamine is 100-200:

1.

6. The method for preparing a chromium-free tanning agent based on keratin according to claim 1, characterized in that, In step 1, the pH is 8.5 to 9.

0.

7. The method for preparing a chromium-free tanning agent based on keratin according to claim 1, characterized in that, In step 2, the constant temperature stirring time is 6.5 hours.

8. The method for preparing a chromium-free tanning agent based on keratin according to claim 1, characterized in that, In step 2, the molar ratio of keratin to ethylene glycol glycidyl ether is 1:2.0 to 3.5.

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