A chromium-free tanning agent based on cantilever hydrogen bond reinforcement and its preparation method

Abstract

This invention relates to a chromium-free tanning agent based on cantilever hydrogen bond reinforcement and its preparation method. The invention first prepares a single-terminated -NCO ureidopyrimidinone compound, then utilizes the high reactivity of its -NCO active group with the amino group in an amino alcohol compound to prepare a double-terminated -OH compound containing ureidopyrimidinone in its molecular side chain. Next, it reacts with a polyisocyanate to prepare a double-terminated -NCO prepolymer. Finally, using sulfite-terminated prepolymers, a water-soluble reactive chromium-free non-metallic tanning agent containing cantilever hydrogen bonds in its molecular chain is prepared. Compared with conventional organic synthetic tanning agents, the chromium-free tanning agent developed in this invention can form urea covalent bonds with leather collagen and further form a quadruple hydrogen-bonded dimer in the leather fiber network through the hydrogen bond recognition effect of ureidopyrimidinone (Upy). This strong intermolecular hydrogen bonding strengthens the molecular cross-linking network structure of the tanned leather, resulting in higher shrinkage temperature and mechanical strength. In particular, the chromium-free tanning agent developed in this invention has quadruple hydrogen bonds located on the side chains of the tanning agent molecule. These bonds are easily movable and can readily overcome the adverse effects of steric hindrance in the tanning agent molecule. That is, these UPy bonds, suspended on the main molecular chain, are more likely to form quadruple hydrogen-bonded dimers through hydrogen bond recognition. This strong hydrogen bond network further strengthens the urea bond-based collagen cross-linking network, making the tanning effect of the chromium-free tanning agent developed in this invention more significant.

Description

Technical Field

[0001] This invention relates to the field of leather chemicals, specifically to a chromium-free tanning agent based on cantilever hydrogen bond reinforcement and its preparation method. Background Technology

[0002] The leather industry plays a vital role in human society. From early furs, boots, belts, horse harnesses, and armor to modern bags, sofas, wallets, beds, and upholstered furnishings, it exudes a rich historical feel and youthful vitality. For thousands of years, the leather industry has been closely intertwined with people's lives. With the development of the times and the advancement of leather processing technology, the types and forms of leather products have become more diverse, and the applications of leather have expanded. For the leather industry, the application of tanning agents plays an indispensable role in the transformation of animal hides into leather. The process by which tanning agent molecules penetrate the hide and combine with the active groups of raw hide collagen molecules, thereby altering the properties of the raw hide, is called the tanning process. Tanning causes cross-linking bonds to form between the collagen polypeptide chains, increasing the stability of the collagen structure, improving the shrinkage temperature and resistance to damp heat, and enhancing the hide's resistance to acids, alkalis, enzymes, and other chemicals.

[0003] Currently, trivalent chromium salts are the most widely used tanning agents. However, trivalent chromium, with its complexing ability, carries the risk of transforming into highly toxic hexavalent chromium. Therefore, in recent years, the state has included chromium-containing leather scraps in the list of hazardous waste. Clearly, under this background, pursuing chromium-free tanning is an inevitable choice for the future survival and development of the leather industry. From the perspective of existing technology, chromium-free tanning methods are mainly divided into four categories: vegetable tanning, non-chromium metallic tanning, aldehyde tanning, and non-aldehyde organic tanning. Among these, vegetable tanning agents are relatively non-toxic, less polluting, biodegradable, and renewable resources compared to chromium tanning agents. They have developed into an important tanning method in modern leather production and have become one of the preferred materials for making chromium-free tanned leather. Modern vegetable tanning theory suggests that the phenolic hydroxyl groups of vegetable tanning agents form multi-point hydrogen bonds with the peptide chains, hydroxyl, amino, and carboxyl groups of collagen, creating cross-links between collagen fibers and transforming raw hide into leather. However, natural polyphenols have weak hydrogen bonding and low binding force with collagen. To achieve specific water and heat resistance, a large amount of vegetable tanning agents needs to be added to tan the raw hide during production. This results in a large amount of tannin used in pure vegetable tanning. For example, even using tannin from vitex bark, which has excellent tanning properties, the amount used is at least 20% (by weight of grey hide) to reach a shrinkage temperature of approximately 85°C. Therefore, vegetable tanned leather generally has a large amount of tanning agent filler, resulting in a firm, stiff leather with poor softness and low extensibility, making it difficult to meet the softness requirements of lightweight leathers. Furthermore, this weak hydrogen bonding also makes vegetable tanned leather susceptible to washing and prone to detanning. Moreover, natural vegetable tannins have their own color, resulting in a darker leather color that is difficult to adjust. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a chromium-free tanning agent based on cantilever hydrogen bond reinforcement and its preparation method. This overcomes the issues of conventional chromium-free vegetable tanning, such as high tanning agent dosage, hard leather, low shrinkage temperature, weak bonding, easy detanning, and dark color.

[0005] This invention incorporates a quadruple hydrogen bond, a 2-amino-4-hydroxy-6-methylpyrimidine (UPy) structural unit, into the pendant side chain of the tanning agent molecule. Simultaneously, water-soluble anionic reactive groups (carbamoyl sulfonate) are introduced at both ends of the molecule. The former forms a quadruple hydrogen bond dimer through hydrogen bond recognition, bridging the tanning agent molecules and forming physical cross-linking points. The latter imparts excellent water solubility and high reactivity with collagen amino groups to the tanning agent. Compared to conventional vegetable tanning agents that only bind to collagen through weak hydrogen bonds, the tanning agent developed in this invention, after binding with collagen, anchors itself between collagen fibers in the form of covalent urea bonds, forming effective cross-links. This significantly increases the shrinkage temperature of the hide. Furthermore, due to the strong dimerization effect of the quadruple hydrogen bonds, multiple hydrogen bond structures can be formed between tanning agent molecules, thereby forming a strong hydrogen bond cross-linking network within the collagen fiber network, endowing the hide collagen with better tanning properties.

[0006] The technical solution of the present invention is as follows:

[0007] A chromium-free tanning agent based on cantilever hydrogen bond reinforcement, the general structural formula of which is shown in (1), (2) or (3):

[0008] (1) ;

[0009] (2) ;

[0010] (3) ;

[0011] Where -R1- is , or ;

[0012] -R2- is , , , , , or .

[0013] The above-mentioned method for preparing a chromium-free tanning agent based on cantilever hydrogen bond reinforcement includes the following steps:

[0014] (1) Preparation of single-terminal -NCO ureidopyrimidine ketone compounds: A measured amount of 2-amino-4-hydroxy-6-methylpyrimidine and diisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and the reaction was carried out at 80-100℃ for 8-24h. After reaching the theoretical NCO value, the mixture was cooled to room temperature, n-hexane was added, the precipitate was collected by vacuum filtration, and the precipitate was washed with acetone several times. After vacuum drying at 50-60℃ for 4-6h, the precipitate was sealed and stored for later use.

[0015] (2) Preparation of double-terminal -OH ureidopyrimidinone compounds: The single-terminal -NCO ureidopyrimidinone compounds and amino alcohol compounds prepared in step (1) are added to a reactor equipped with a stirrer, thermometer and condenser. Solvent is added, nitrogen gas is introduced, and the reaction is carried out at 60-100℃ for 2-5 hours. The precipitate is collected by filtration, and the precipitate is washed several times with precipitant. After vacuum drying at 30-40℃ for 2 hours, it is sealed and stored for later use.

[0016] (3) Preparation of double-terminated NCO prepolymer: The double-terminated ureidopyrimidinone compound prepared in step (2) is mixed with a metric amount of polyisocyanate, an organic bismuth catalyst is added, and the mixture is stirred at 60-100℃ for 1.5-5h. After the NCO reaction reaches the theoretical value, the temperature is lowered to 30℃ and a diluent is added to adjust the viscosity.

[0017] (4) Preparation of tanning agent emulsion: Add a measured amount of sodium bisulfite and sodium sulfite mixed aqueous solution to the solution of the double-terminated NCO prepolymer obtained in step (3) at one time, stir rapidly, and strongly disperse to obtain a chromium-free tanning agent emulsion that is slightly blue to white. Add deionized water to adjust the solid content to 30%, add acidic compound, and further adjust the pH of the emulsion to 6 to obtain the tanning agent emulsion.

[0018] Preferably, in step (1), the molar ratio of 2-amino-4-hydroxy-6-methylpyrimidine to diisocyanate is 1:6-20;

[0019] The diisocyanate in step (1) is any one of 1,5-pentanediisocyanate, hexamethylene diisocyanate, or isoflurone diisocyanate;

[0020] In step (2), the amino alcohol compound is any one of 2-amino-2-methyl-1,3-propanediol, 2-amino-1,3-propanediol, and L-threonine.

[0021] In step (2), the molar ratio of the single-terminal -NCO ureidopyrimidinone compound and the amino alcohol compound is 1:1-3.

[0022] The solvent in step (2) is any one of chloroform, N,N-dimethylformamide, and dimethyl sulfoxide.

[0023] In step (2), the precipitant is any one or more of petroleum ether, diethyl ether, ethyl acetate, and acetone.

[0024] Preferably, the polyisocyanate in step (3) is any one or more of 1,5-pentanediisocyanate, hexamethylene diisocyanate, isoflurone diisocyanate, dicyclohexyl diisocyanate, phenyldiisocyanate, 2,2,4-trimethyl-1,6-hexamethylene diisocyanate, and 2,4,4-trimethyl-1,6-hexamethylene diisocyanate.

[0025] In step (3), the molar ratio of the di-OH-terminated ureidopyrimidinone compound to the polyisocyanate is 1:2-3;

[0026] The diluent in step (3) is any one of acetone, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, methanol, ethanol, and isopropanol;

[0027] In step (4), the molar amount of sodium bisulfite is 2-4 times that of the molar amount of the double-ended NCO prepolymer in step (3); the molar ratio of sodium bisulfite to sodium sulfite is 1:0.1-0.2.

[0028] The beneficial effects of this invention are:

[0029] (1) The chromium-free tanning agent developed in this invention is an organic chromium-free and formaldehyde-free tanning agent. It is safe and environmentally friendly in the process of use. The tanned leather does not contain chromium or formaldehyde, thus avoiding the pollution problems of heavy metals and free formaldehyde.

[0030] (2) Vegetable tanning agents bind to collagen through multi-point hydrogen bonds, without forming chemical bonds, resulting in a weak bond. Consequently, the finished leather is not resistant to washing and is prone to detanning. On the other hand, hydrogen bonds have a certain binding strength; however, single and double hydrogen bonds are relatively weak. The same applies to the multi-point hydrogen bonds formed between natural polyphenols and collagen in vegetable tanning agents. Under limited dosage, it is difficult to impart a good cross-linking effect to the leather. It is worth mentioning that multiple hydrogen bonds, represented by quadruple hydrogen bonds, exhibit greater stability and binding energy due to the superposition and synergistic effect between hydrogen bonds. Furthermore, the tanning agent of the present invention has terminal carbamoyl sulfonate groups and quadruple hydrogen bond structural units that can react with the amino groups of collagen, thereby forming covalent urea bonds and strong hydrogen bond crosslinks between the three-dimensional network of collagen fibers. The tanning agent is firmly bound to the collagen and does not detannify. Compared with vegetable tanning agents that only bind to the collagen of the hide through weak hydrogen bonds, the tanning agent of the present invention can obtain white wet leather with a shrinkage temperature higher than 80°C with a smaller dosage (≤8%) due to the covalent bridging between the tanning agent and the collagen and the strong hydrogen bond binding force between the tanning agents.

[0031] (3) The chromium-free tanning agent developed in this invention has four hydrogen bonds located on the side chain of the tanning agent molecule, which are easy to swing and can easily overcome the adverse factors of molecular steric hindrance. That is, the UPy suspended on the molecular backbone is easy to form a four-hydrogen bond dimer through hydrogen bond recognition. Furthermore, due to the reactive characteristics of the carbamoyl sulfonic acid group at the upper end of the tanning agent molecule, the tanning agent molecule will grow further as the tanning process proceeds. In this way, multiple UPy units are introduced into a molecular chain of a tanning agent macromolecular polymer of different molecular weights, which can obtain a supramolecular cross-linked network with UPy dimers as physical cross-linking points. This strong hydrogen bond network further strengthens the collagen cross-linked network based on urea bonds (see Tanning Mechanism). Figure 1 This gives the leather excellent mechanical strength. Attached Figure Description

[0032] Figure 1 A diagram illustrating the tanning mechanism of chromium-free tanning agents based on cantilever hydrogen bond reinforcement;

[0033] Figure 2 The infrared spectra of the intermediate products prepared in steps (1), (2), and (3) of Example 1 and the tanning agent in the final step (4). Detailed Implementation

[0034] The following description is merely a preferred embodiment of the present invention and is only used to describe the present invention. It should not be construed as a limitation on the scope of the present invention.

[0035] Example 1

[0036] (1) Preparation of single-terminal -NCO ureidopyrimidine ketone compounds: 1 mol of 2-amino-4-hydroxy-6-methylpyrimidine and 6 mol of 1,5-pentanediisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and the reaction was carried out at 90°C for 10 h. After reaching the theoretical NCO value, the mixture was cooled to room temperature, n-hexane was added, the precipitate was collected by vacuum filtration, and the precipitate was washed with acetone several times. After vacuum drying at 50°C for 4 h, the mixture was sealed and stored for later use.

[0037] (2) Preparation of double-terminal -OH ureidopyrimidinone compounds: 0.5 mol of the single-terminal -NCO ureidopyrimidinone compound prepared in step (1) and 0.8 mol of 2-amino-1,3-propanediol were added to a reactor equipped with a stirrer, thermometer and condenser. Chloroform was added and nitrogen gas was introduced. The reaction was carried out at 60°C for 5 h. The precipitate was collected by filtration and washed with acetone several times. The precipitate was dried under vacuum at 30°C for 2 h and then sealed for storage.

[0038] (3) Preparation of bi-terminal-NCO prepolymer: 0.2 mol of bi-terminal-OH ureidopyrimidinone compound prepared in step (2) was mixed with 0.4 mol of 1,5-pentanediisocyanate, 2 drops of organic bismuth catalyst were added, and the mixture was stirred at 80 °C for 2 h. The NCO content was measured to the theoretical value using the di-n-butylamine method. The temperature was then lowered to 30 °C, and a small amount of ethanol was added to adjust the viscosity.

[0039] (4) Preparation of tanning agent emulsion: Add a mixed aqueous solution containing 0.4 mol sodium bisulfite and 0.05 mol sodium sulfite to the solution of the double-terminated NCO prepolymer obtained in step (3) at one time, stir rapidly, and disperse strongly to obtain a milky white chromium-free tanning agent emulsion. Add deionized water to adjust the solid content to 30%, add dilute formic acid, and further adjust the pH of the emulsion to 6 to obtain the tanning agent emulsion. The structural schematic of the tanning agent is as follows:

[0040] .

[0041] Example 2

[0042] (1) Preparation of single-terminal -NCO ureidopyrimidine ketone compounds: 1 mol of 2-amino-4-hydroxy-6-methylpyrimidine and 6 mol of 1,5-pentanediisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and the reaction was carried out at 90°C for 10 h. After reaching the theoretical NCO value, the mixture was cooled to room temperature, n-hexane was added, the precipitate was collected by vacuum filtration, and the precipitate was washed with acetone several times. After vacuum drying at 50°C for 4 h, the mixture was sealed and stored for later use.

[0043] (2) Preparation of di-OH ureidopyrimidinone compounds: 0.5 mol of the single-NCO ureidopyrimidinone compound prepared in step (1) and 1 mol of 2-amino-2-methyl-1,3-propanediol were added to a reactor equipped with a stirrer, thermometer and condenser. N,N-dimethylformamide was added, nitrogen gas was introduced and the reaction was carried out at 100°C for 3 h. The precipitate was collected by filtration, washed several times with petroleum ether, dried under vacuum at 40°C for 2 h and then sealed for storage.

[0044] (3) Preparation of bi-terminal-NCO prepolymer: 0.2 mol of bi-terminal-OH ureidopyrimidinone compound prepared in step (2) was mixed with 0.45 mol of hexamethylene diisocyanate, 2 drops of organic bismuth catalyst were added, and the mixture was stirred at 80 °C for 3 h. The NCO was measured to the theoretical value using the di-n-butylamine method. The temperature was then lowered to 30 °C, and a small amount of isopropanol was added to adjust the viscosity.

[0045] (4) Preparation of tanning agent emulsion: Add 0.5 mol of sodium bisulfite and 0.06 mol of sodium sulfite mixed aqueous solution to the solution of the double-terminated NCO prepolymer obtained in step (3) at one time, stir rapidly, and disperse strongly to obtain a milky white chromium-free tanning agent emulsion. Add deionized water to adjust the solid content to 30%, add dilute acetic acid, and further adjust the pH of the emulsion to 6 to obtain the tanning agent emulsion. The structural schematic formula of the tanning agent is as follows:

[0046] .

[0047] Example 3

[0048] (1) Preparation of single-terminal -NCO ureidopyrimidine ketone compounds: 1 mol of 2-amino-4-hydroxy-6-methylpyrimidine and 8 mol of isoflurane diisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and the reaction was carried out at 100℃ for 12 h. After reaching the theoretical NCO value, the mixture was cooled to room temperature, n-hexane was added, the precipitate was collected by filtration, and the precipitate was washed with acetone several times. After drying under vacuum at 50℃ for 6 h, the precipitate was sealed and stored for later use.

[0049] (2) Preparation of double-terminal -OH ureidopyrimidinone compounds: 0.5 mol of the single-terminal -NCO ureidopyrimidinone compound prepared in step (1) and 1 mol of L-threonine were added to a reactor equipped with a stirrer, thermometer and condenser. Dimethyl sulfoxide was added, nitrogen gas was introduced, and the reaction was carried out at 80°C for 5 h. The precipitate was collected by filtration, washed with ethyl acetate several times, dried under vacuum at 40°C for 2 h and then sealed for storage.

[0050] (3) Preparation of bi-terminal-NCO prepolymer: 0.2 mol of bi-terminal-OH ureidopyrimidinone compound prepared in step (2) was mixed with 0.5 mol of 1,5-pentanediisocyanate, 2 drops of organic bismuth catalyst were added, and the mixture was stirred at 90 °C for 2.5 h. The NCO content was measured to the theoretical value using the di-n-butylamine method, the temperature was lowered to 30 °C, and a small amount of acetone was added to adjust the viscosity.

[0051] (4) Preparation of tanning agent emulsion: Add 0.7 mol of sodium bisulfite and 0.08 mol of sodium sulfite aqueous solution to the solution of the double-terminated NCO prepolymer obtained in step (3) at one time, stir rapidly, and disperse strongly to obtain a milky white chromium-free tanning agent emulsion. Add deionized water to adjust the solid content to 30%, add dilute formic acid, and further adjust the pH of the emulsion to 6 to obtain the tanning agent emulsion. The structural schematic formula of the tanning agent is as follows:

[0052] .

[0053] Comparative Example 1

[0054] 0.2 mol of 1,3-propanediol and 0.4 mol of 1,5-pentanediisocyanate were added to a reactor equipped with a stirrer, thermometer, and condenser. Nitrogen gas was introduced, and the reaction was carried out at 50°C with stirring for 2 hours. The NCO content was measured using the di-n-butylamine method to the theoretical value. The temperature was then lowered to 30°C, and a small amount of ethanol was added to adjust the viscosity. A mixed aqueous solution containing 0.4 mol of sodium bisulfite and 0.05 mol of sodium sulfite was added to the reactor in one go. The mixture was stirred rapidly and strongly dispersed to obtain a light blue chromium-free tanning agent emulsion. Deionized water was added to adjust the solid content to 30%, and dilute formic acid was added to further adjust the pH of the emulsion to 6, resulting in a tanning agent emulsion without four-fold hydrogen bonds. The structural diagram of this tanning agent is as follows:

[0055] .

[0056] Comparative Example 2

[0057] 0.2 mol of 2-methyl-1,3-propanediol and 0.4 mol of 1,5-pentanediisocyanate were added to a reactor equipped with a stirrer, thermometer, and condenser. Nitrogen gas was introduced, and the reaction was carried out at 50°C with stirring for 3 hours. The NCO content was measured using the di-n-butylamine method to the theoretical value. The temperature was then lowered to 30°C, and a small amount of ethanol was added to adjust the viscosity. A mixed aqueous solution containing 0.4 mol of sodium bisulfite and 0.05 mol of sodium sulfite was added to the reactor in one go. The mixture was stirred rapidly and strongly dispersed to obtain a light blue chromium-free tanning agent emulsion. Deionized water was added to adjust the solid content to 30%, and dilute formic acid was added to further adjust the pH of the emulsion to 6, resulting in a tanning agent emulsion without four-fold hydrogen bonds. The structural schematic of this tanning agent is shown below.

[0058] .

[0059] Comparative Example 3

[0060] 0.2 mol of 2,2-dimethyl-1,3-propanediol and 0.4 mol of 1,5-pentanediisocyanate were added to a reactor equipped with a stirrer, thermometer, and condenser. Nitrogen gas was introduced, and one drop of organic bismuth catalyst was added. The reaction was stirred at 60°C for 4 hours. The NCO content was measured using the di-n-butylamine method to the theoretical value. The temperature was lowered to 30°C, and a small amount of acetone was added to adjust the viscosity. A mixed aqueous solution containing 0.4 mol of sodium bisulfite and 0.05 mol of sodium sulfite was added to the reactor in one go. The mixture was stirred rapidly and strongly dispersed to obtain a milky white chromium-free tanning agent emulsion. Deionized water was added to adjust the solid content to 30%, and dilute formic acid was added to further adjust the pH of the emulsion to 6, resulting in a tanning agent emulsion without quadruple hydrogen bonds. The structural schematic of this tanning agent is shown below.

[0061] .

[0062] Example Products and Comparative Applications

[0063] Commercially available pickled sheepskin was purchased for tanning experiments. Using the pickled sheepskin as the raw material, the cut pickled hide was weighed before tanning to determine the base weight, and the amount of chemicals needed for deacidification was calculated. After deacidification, the hide underwent two washes. The chemicals were weighed based on 250% of the pickled hide's weight. The specific tanning process is shown in Table 1. The shrinkage temperature, tensile strength, tear strength, and sensory characteristics of the resulting wet white leather are shown in Table 2.

[0064] Table 1 Tanning process parameters

[0065]

[0066] Table 2. Shrinkage temperature, mechanical properties, and appearance results of white wet leather.

[0067]

[0068] To characterize the chemical structure of the tanning agent, the intermediate products and the final tanning agent product (4) prepared in steps (1), (2), and (3) of Example 1 were characterized by infrared spectroscopy. Figure 2 It can be seen that intermediate products (1) and (3) are at 2287 cm⁻¹ -1 The intermediate product (2) and the final tanning product (4) have obvious -NCO peaks. Due to the reaction of -NCO with amino groups and bisulfite to generate urea bonds and carbamoyl sulfonic acid groups, respectively, the peak at 2287 cm⁻¹ is observed. -1 The nearby NCO peak disappears. 1705cm -1 1670cm -1 The peaks at approximately 1260 cm⁻¹ are the vibrational absorption peaks of the carbonyl group (C=O) on the urea carbonyl group and the carbonyl group (C=O) on the urea pyrimidinone, respectively. -1 The peak of the stretching vibration of the CN bond is located nearby; 1584 cm⁻¹ -1 The characteristic NH peak on the pyrimidine ring is located nearby, at 1525 cm⁻¹. -1 The nearby peak is a characteristic NH peak on the straight chain; 1131 cm⁻¹ -1 1029cm -1 The near-S=O deformation vibration absorption peak indicates the successful synthesis of the tanning agent, as shown by infrared results.

[0069] The shrinkage temperature, tensile strength, tear strength, and sensory properties of the tanned wet white leather are shown in Table 2. Table 2 shows that the wet white sheepskin leather tanned in Example 1 had the highest shrinkage temperature of 88°C and a tear strength of 120.03 N·mm. -1The tensile strength is 36.44 MPa, indicating high mechanical strength. The leather is full and has a clean, fine grain. The wet white leather prepared in Examples 2-3 also exhibits high wet heat stability and mechanical strength. This is closely related to the fact that the terminal carbamoyl sulfonic acid groups in the tanning agent molecules can form urea bonds with collagen for cross-linking, as well as the reinforcing effect between the suspended quadruple hydrogen bonds in the tanning agent molecules. The shrinkage temperature of the leather tanned in Comparative Examples 1-3 is between 78-80℃. Compared with the examples, the wet heat stability is obviously lower. This is because although the tanning agents in the comparative examples contain terminal carbamoyl sulfonic acid groups that can cross-link collagen molecules, they lack the reinforcing effect of quadruple hydrogen bonds. They only have the covalent cross-linking effect of a single urea group, without strong hydrogen bond forces. This results in lower thermal stability of the leather collagen fibers compared to the examples, and the raw leather is thin, lacking filling and fullness.

Claims

1. A chromium-free tanning agent based on cantilever hydrogen bond reinforcement, characterized in that, The general structural formulas of chromium-free tanning agents are shown in (1), (2), or (3): （1） ； （2） ； （3） Where -R1- is , or ; -R2- is , , , , , or .

2. A method for preparing a chromium-free tanning agent based on cantilever hydrogen bond reinforcement as described in claim 1, characterized in that, Includes the following steps: (1) Preparation of single-terminal -NCO ureidopyrimidine ketone compounds: Measured amounts of 2-amino-4-hydroxy-6-methylpyrimidine and diisocyanate were added to a reactor equipped with a stirrer, thermometer, and condenser. Nitrogen gas was introduced, and the reaction was carried out at 80-100℃ for 8-24 h. After reaching the theoretical NCO value, the mixture was cooled to room temperature, n-hexane was added, the precipitate was collected by filtration, and the precipitate was washed with acetone several times. After vacuum drying at 50-60℃ for 4-6 h, the precipitate was sealed and stored for later use. (2) Preparation of double-terminal -OH ureidopyrimidine ketone compounds: The single-terminal -NCO ureidopyrimidine ketone compounds and amino alcohol compounds prepared in step (1) are added to a reactor equipped with a stirrer, thermometer and condenser. Solvent is added, nitrogen gas is introduced, and the reaction is carried out at 60-100℃ for 2-5h. The precipitate is collected by filtration, and the precipitate is washed several times with precipitant. After vacuum drying at 30-40℃ for 2h, it is sealed and stored for later use. (3) Preparation of double-terminated NCO prepolymer: The double-terminated ureidopyrimidinone compound prepared in step (2) is mixed with a metric amount of polyisocyanate, an organic bismuth catalyst is added, and the mixture is stirred at 60-100℃ for 1.5-5h. After the NCO reaction reaches the theoretical value, the temperature is lowered to 30℃ and a diluent is added to adjust the viscosity. (4) Preparation of tanning agent emulsion: Add a measured amount of sodium bisulfite and sodium sulfite mixed aqueous solution to the double-ended NCO prepolymer solution obtained in step (3) at one time, stir rapidly, and disperse strongly to obtain a chromium-free tanning agent emulsion that is slightly blue to milky white. Add deionized water to adjust the solid content to 30%, add acidic compound, and further adjust the pH of the emulsion to 6 to obtain the tanning agent emulsion.

3. The method for preparing chromium-free tanning agent based on cantilever hydrogen bond reinforcement according to claim 2, characterized in that, In step (1), the molar ratio of 2-amino-4-hydroxy-6-methylpyrimidine to diisocyanate is 1:6-20; The diisocyanate in step (1) is any one of 1,5-pentanediisocyanate, hexamethylene diisocyanate, or isoflurone diisocyanate; In step (2), the amino alcohol compound is any one of 2-amino-2-methyl-1,3-propanediol, 2-amino-1,3-propanediol, and L-threonine. In step (2), the molar ratio of the single-terminal -NCO ureidopyrimidinone compound and the amino alcohol compound is 1:1-3; The solvent in step (2) is any one of chloroform, N,N-dimethylformamide, and dimethyl sulfoxide. In step (2), the precipitant is any one or more of petroleum ether, diethyl ether, ethyl acetate, and acetone.

4. The method for preparing chromium-free tanning agent based on cantilever hydrogen bond reinforcement according to claim 2, characterized in that, In step (3), the polyisocyanate is any one or more of 1,5-pentanediisocyanate, hexamethylene diisocyanate, isoflurone diisocyanate, dicyclohexyl diisocyanate, phenyldimethyl diisocyanate, 2,2,4-trimethyl-1,6-hexamethylene diisocyanate, and 2,4,4-trimethyl-1,6-hexamethylene diisocyanate. In step (3), the molar ratio of the di-OH-terminated ureidopyrimidinone compound to the polyisocyanate is 1:2-3; The diluent in step (3) is any one of acetone, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, methanol, ethanol, and isopropanol; In step (4), the molar amount of sodium bisulfite is 2-4 times that of the molar amount of the double-ended NCO prepolymer in step (3); the molar ratio of sodium bisulfite to sodium sulfite is 1:0.1-0.2.

Images