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

Abstract

This invention relates to a chromium-free tanning agent based on hydrogen bond reinforcement and its preparation method. The invention first prepares single-terminated and double-terminated -NCO ureapyrimidine ketone compounds, then reacts these compounds with an aqueous solution of bisulfite to prepare a water-soluble reactive ureapyrimidine ketone-based chromium-free non-metallic tanning agent. Compared with conventional organic synthetic tanning agents, the chromium-free tanning agent developed in this invention can both covalently bind with skin collagen and further form a quadruple hydrogen-bonded dimer in the leather fiber network through the hydrogen bond recognition effect of ureapyrimidine ketones (Upy). This strong hydrogen bonding between tanning agent molecules strengthens the molecular cross-linking network structure of the tanned leather, resulting in higher shrinkage temperature and mechanical strength. In particular, the tanning agent developed in this invention contains two types of tannin molecules of different sizes, exhibiting excellent selective filling effect. The tanned leather is full and uniform, with good softness, and can be used in the production of light-colored lightweight leather.

Description

Technical Field

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

[0002] Tanning is the most crucial step in leather manufacturing. 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 its resistance to acids, alkalis, enzymes, and other chemicals.

[0003] Currently, most leather products still use chrome tanning agents. However, if the tanning process is not properly controlled or affected by factors such as the storage environment, chrome-tanned leather products may have excessive levels of hexavalent chromium. These hexavalent chromium compounds can be absorbed by the body through the skin, sweat, respiratory tract, and digestive tract, affecting cell vitality and causing carcinogenic and lesioning effects on the digestive and respiratory tracts. Therefore, stricter standards and regulations have been introduced both domestically and internationally. From the perspective of coordinated development of resources, environment, and economy, developing chrome-free eco-friendly leather tanning technology is of great significance to promoting the sustainable development of the global leather industry. From the perspective of existing technologies, chrome-free tanning methods mainly include vegetable tanning, non-chrome metallic tanning, aldehyde tanning and non-aldehyde organic tanning, as well as combined tanning based on these methods.

[0004] Before the invention of chrome tanning in 1858, vegetable tanning agents were the primary tanning agents used in leather tanning. Vegetable tanning agents, also known as tannins, are mixtures whose active ingredients are vegetable tannins, or tannins, belonging to the polyphenolic compound family. Their structure contains numerous phenolic hydroxyl groups, which can form multi-point hydrogen bonds with the -OH, -NH2, and -COOH groups on the side chains of collagen. Although chrome tanning agents currently dominate the production of light leather (such as clothing and shoe uppers) in the leather industry, vegetable tanning agents have long been considered environmentally friendly tanning materials due to their non-toxicity, biodegradability, and the fact that their raw materials are renewable resources. With the increasing environmental pressure in recent years, there has been a growing interest in using vegetable tannins, a green resource, to replace the more polluting chrome tanning agents.

[0005] The practical problem is that, because the vegetable tanning system is maintained by weak hydrogen bonds, pure vegetable-tanned leather has a low shrinkage temperature and requires a large amount of vegetable tanning agent (≥20%) to achieve hygrothermal stability of 75-80℃. Therefore, vegetable-tanned leather is generally stiff, has poor softness, the tanning agent does not bond firmly to the leather, it is not resistant to washing, and is prone to detanning; moreover, tannin itself has a certain color, resulting in a darker color for the raw leather. To overcome these shortcomings, leather makers mainly focus on the combination tanning of vegetable tanning agents with other tanning agents to achieve a synergistic effect between various tanning methods. For example, vegetable-aluminum tanning and vegetable-aldehyde tanning can produce lightweight leather products with higher hygrothermal stability. However, these combined tanning methods are either too time-consuming and complex, or their use is restricted due to the presence of aluminum salts, or they may produce problems such as free formaldehyde. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes a hydrogen-bonded chromium-free tanning agent and its preparation method. This overcomes the problems of weak hydrogen bond bonding in conventional organic chromium-free tanning systems, large amounts of tanning agent required, and minimal increase in shrinkage temperature of the raw leather. Furthermore, the use of aldehyde-based chromium-free tanning agents easily releases free formaldehyde, while the combined tanning process involving vegetables and aldehydes is complex and time-consuming.

[0007] The chromium-free tanning agent of this invention has carbamoyl sulfonate groups at both ends, and the main chain contains four-fold hydrogen-bonded ureidopyrimidinone (Upy) structural units. The carbamoyl sulfonate groups provide excellent water solubility to the tanning agent and can react with the amino groups of collagen under weakly alkaline conditions to form urea bonds, bridging the collagen molecules through covalent bonds. The Upy units in the molecule can form four-fold hydrogen-bonded dimers through hydrogen bond recognition, further cross-linking the polymer chains of different tanning agent molecules through strong four-fold hydrogen bonds. This creates a dense cross-linked network between the tanning agent and collagen, and between tanning agents themselves, resulting in a significant tanning effect.

[0008] The technical solution of the present invention is as follows:

[0009] A chromium-free tanning agent based on hydrogen bond reinforcement, which consists of compounds having the following two general structural formulas:

[0010] Compound (1): ;

[0011] Compound (2): ;

[0012] Where -R- is , , , , , , , Any one of them.

[0013] The ratio of the two compounds is determined based on the molar ratio of the single-terminal -NCO ureidopyrimidinone compound and the double-terminal -NCO ureidopyrimidinone compound.

[0014] The above-mentioned method for preparing a chromium-free tanning agent based on hydrogen bond reinforcement includes the following steps:

[0015] (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.

[0016] (2) Preparation of di-NCO-terminated ureidopyrimidine ketone compounds: A measured amount of vacuum-dried 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and diisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and a catalyst was added. The reaction was carried out at 90-100℃ for 2-48h. A precipitant with a volume of 7-10 times that of diisocyanate was added. The mixture was stirred, filtered and the precipitate was collected. The precipitate was washed with n-hexane several times and then vacuum-dried at 50-60℃ for 4-6h before being sealed and stored for later use.

[0017] (3) Preparation of chromium-free tanning agent emulsion: The single-terminal-NCO ureidopyrimidinone compounds and the double-terminal-NCO ureidopyrimidinone compounds in steps (1) and (2) are mixed in a certain proportion, a diluent is added, and then a mixed aqueous solution of sodium bisulfite and sodium sulfite is added for dispersion. After stirring and reacting for a period of time, water is added to adjust the solid content to 30% to obtain a light white emulsion. Dilute acid is added to adjust the pH of the emulsion to about 5-6 to obtain chromium-free tanning agent.

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

[0019] In step (2), the molar ratio of 5-(2-hydroxyethyl)-6-methyl-2-aminouracil to diisocyanate is 1:8-15;

[0020] The catalyst in step (2) is any one of dibutyltin dilaurate, stannous octanoate, bismuth isooctanoate, and tetrabutyl titanate, and the amount added is 0.1%-2% of the total mass of 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and diisocyanate;

[0021] The diisocyanate in steps (1) and (2) is any one or more of 1,5-pentane diisocyanate, hexamethylene diisocyanate, isoflurone diisocyanate, dicyclohexyl diisocyanate, phenyldimethyl diisocyanate, cyclohexanedimethyl diisocyanate, and norbornene diisocyanate.

[0022] In step (2), the precipitant is any one or two of petroleum ether, n-pentane, n-hexane, n-heptane, chloroform, and diisopropyl ether.

[0023] Furthermore, in step (3), the molar ratio of the single-terminal -NCO ureidopyrimidinone compound and the double-terminal -NCO ureidopyrimidinone compound is 1:6-10;

[0024] The diluent in step (3) is any one of acetone, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, ethanol, and isopropanol;

[0025] In step (3), the molar amount of sodium bisulfite is 2-3 times the total molar amount of the single- and double-terminal -NCO ureidopyrimidine ketone compounds; the molar ratio of sodium bisulfite to sodium sulfite is 1:0.1-0.4.

[0026] In step (3), the reaction temperature of the mixed aqueous solution with the -NCO ureidopyrimidine ketone compound is 20-40℃, and the dispersion time is 0.5h-6h.

[0027] The beneficial effects of this invention are:

[0028] (1) Traditional vegetable tanning systems based on multi-point hydrogen bond crosslinking often require combined tanning to achieve higher hydrothermal stability due to the weak hydrogen bond forces between natural plant polyphenols and collagen. This involves using tanning agents other than vegetable tanning agents to further enhance the tanning effect, such as vegetable-aldehyde tanning and vegetable-aluminum tanning. These combined tanning processes are complex and time-consuming. The chromium-free tanning agent developed in this invention combines the characteristics of non-aldehyde organic tanning and vegetable tanning, but the tanning reaction is carried out in one step. At the same time, it introduces covalent crosslinking and strong hydrogen bond crosslinking between the skin collagen fiber network (tanning mechanism see...). Figure 1 This avoids the cumbersome step-by-step tanning process, simplifies the tanning operation, shortens the tanning process time, and improves the production efficiency of raw leather.

[0029] (2) The tanning agent of the present invention contains carbamoyl sulfonate active groups, which have high reactivity with the amino groups of collagen and form urea bonds. In particular, the tanning agent molecules containing double-ended active reactive groups will further extend and grow during the tanning process. In this way, multiple UPy units are introduced into a molecular chain of a tanning agent macromolecule polymer of different molecular weights. Compared with general single hydrogen bonds and double hydrogen bonds, this strong hydrogen bond network increases the physical connection points of the urea bond covalently cross-linked collagen network, which further strengthens the three-dimensional fiber cross-linked network inside the tanned leather, and the tanned leather has better mechanical strength.

[0030] (3) The tanning agent of the present invention contains tanning agent molecules with single-terminal active reactive groups and tanning agent molecules with double-terminal active reactive groups, which can effectively cross-link collagen in different fiber weave densities and thicknesses in raw hides; the former molecules are small and have good penetration, easily penetrating into thick areas with tightly woven collagen, such as the neck, back, and buttocks, while the latter can cross-link and fill between loose edge and belly collagen fibers. That is, the tanning agent developed in this invention has excellent penetration and binding ability, good selective filling properties, and the tanned leather not only has a high shrinkage temperature but also small differences in location, resulting in uniform and full leather. Furthermore, this tanning agent does not contain polyphenols, aldehydes, or other active groups that easily cause yellowing, and the tanned wet white leather is resistant to yellowing and has excellent white color, which can be used in the manufacture of leather products with relatively bright and vivid colors, such as light-colored leather. Attached Figure Description

[0031] Figure 1 This is a diagram illustrating the tanning mechanism of the tanning agent of this invention.

[0032] Figure 2 The infrared spectra of intermediate products in steps (1) and (2) of Example 1 and tanning agent in the final step (3) are shown. Detailed Implementation

[0033] 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.

[0034] Example 1

[0035] (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 60°C for 4 h, the mixture was sealed and stored for later use.

[0036] (2) Preparation of di-NCO-terminated ureidopyrimidine ketones: 1 mol of vacuum-dried 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and 8 mol of 1,5-pentanediisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and 15 g of bismuth isooctanoate was added. The reaction was carried out at 90 °C for 5 h. Petroleum ether with a volume of 7 times that of diisocyanate was added. The mixture was stirred, filtered and the precipitate was collected. The precipitate was washed with n-hexane several times and then vacuum-dried at 60 °C for 5 h before being sealed and stored for later use.

[0037] (3) Preparation of chromium-free tanning agent emulsion: 0.1 mol of the single-terminal-NCO ureidopyrimidinone compound obtained in step (1) and 0.6 mol of the double-terminal-NCO ureidopyrimidinone compound obtained in step (2) were mixed, a small amount of isopropanol was added, and then a mixed aqueous solution of 1.4 mol sodium bisulfite and 0.14 mol sodium sulfite was added. The mixture was vigorously stirred and dispersed at 30°C for 2 h. Water was added to adjust the solid content to 30% to obtain a light white emulsion. Dilute hydrochloric acid was added to adjust the pH of the emulsion to 6 to obtain a tanning agent composed of the following two types of compounds:

[0038] Compound (1): ;

[0039] Compound (2):

[0040] .

[0041] Example 2

[0042] (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.

[0043] (2) Preparation of di-NCO-terminated ureidopyrimidine ketones: 1 mol of vacuum-dried 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and 10 mol of isoflurane diisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and 25 g of bismuth isooctanoate was added. The reaction was carried out at 100 °C for 10 h. Then, n-heptane with a volume of 7 times that of diisocyanate was added. The mixture was stirred, filtered and the precipitate was collected. The precipitate was washed with n-hexane several times and then vacuum-dried at 50 °C for 6 h before being sealed and stored for later use.

[0044] (3) Preparation of chromium-free tanning agent emulsion: 0.1 mol of the single-terminal-NCO ureidopyrimidinone compound obtained in step (1) and 0.8 mol of the double-terminal-NCO ureidopyrimidinone compound obtained in step (2) were mixed, a small amount of ethanol was added, and then a mixed aqueous solution of 2 mol sodium bisulfite and 0.25 mol sodium sulfite was added. The mixture was stirred vigorously at 35°C for 4 h. Water was added to adjust the solid content to 30% to obtain a light white emulsion. Dilute hydrochloric acid was added to adjust the pH of the emulsion to 6 to obtain a tanning agent composed of the following two types of compounds:

[0045] Compound (1): ;

[0046] Compound (2): .

[0047] Example 3

[0048] (1) Preparation of single-terminal -NCO ureidopyrimidine ketone compounds: 1 mol of 2-amino-4-hydroxy-6-methylpyrimidine and 10 mol of cyclohexanedimethyl 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 24 h. After reaching the theoretical NCO value, the mixture was cooled to room temperature, n-hexane was added, the precipitate was collected by suction filtration, and the precipitate was washed with acetone several times. After vacuum drying at 50℃ for 4 h, the mixture was sealed and stored for later use.

[0049] (2) Preparation of di-NCO-terminated ureidopyrimidine ketones: 1 mol of vacuum-dried 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and 12 mol of cyclohexanedimethyl diisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and 25 g of bismuth isooctanoate was added. The reaction was carried out at 100 °C for 12 h. Then, 10 times the volume of diisocyanate in n-hexane was added. The mixture was stirred, filtered and the precipitate was collected. The precipitate was washed with n-hexane several times and then vacuum-dried at 50 °C for 5 h before being sealed and stored for later use.

[0050] (3) Preparation of chromium-free tanning agent emulsion: 0.1 mol of the single-terminal-NCO ureidopyrimidinone compound obtained in step (1) and 0.9 mol of the double-terminal-NCO ureidopyrimidinone compound obtained in step (2) were mixed, a small amount of ethanol was added, and then a mixed aqueous solution of 2.2 mol sodium bisulfite and 0.25 mol sodium sulfite was added. The mixture was stirred vigorously at 40°C for 4 h and then water was added to adjust the solid content to 30% to obtain a light white emulsion. Dilute hydrochloric acid was added to adjust the pH of the emulsion to 6 to obtain a tanning agent composed of the following two types of compounds:

[0051] Compound (1): ;

[0052] Compound (2):

[0053] .

[0054] Comparative Example 1

[0055] 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, and n-hexane was added. The precipitate was collected by filtration and washed repeatedly with acetone. After vacuum drying at 60°C for 4 h, the precipitate was removed and cooled to room temperature. A small amount of isopropanol was added to the precipitate, followed by a mixed aqueous solution of 2 mol sodium bisulfite and 0.2 mol sodium sulfite. The mixture was vigorously stirred and dispersed at 30°C for 2 h. Water was added to adjust the solid content to 30%, resulting in a pale white emulsion. Dilute hydrochloric acid was added to adjust the pH of the emulsion to 6, yielding a tanning agent of a single-terminated carbamoylsulfonate ureidopyrimidinone compound.

[0056] Comparative Example 2

[0057] 1 mol of vacuum-dried 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and 8 mol of 1,5-pentanediisocyanate were added to a reactor equipped with a stirrer, thermometer, and condenser. Nitrogen gas was introduced, and 15 g of bismuth isooctanoate was added. The reaction was carried out at 90°C for 5 h. Then, petroleum ether with a volume of 7 times that of the diisocyanate was added. The mixture was stirred, filtered, and the precipitate was collected. The precipitate was washed several times with n-hexane and dried under vacuum at 60°C for 5 h. After cooling to room temperature, a small amount of isopropanol was added to the precipitate, followed by a mixed aqueous solution of 2 mol sodium bisulfite and 0.2 mol sodium sulfite. The mixture was vigorously stirred and dispersed at 30°C for 2 h. Water was added to adjust the solid content to 30%, resulting in a pale white emulsion. Dilute hydrochloric acid was added to adjust the pH of the emulsion to 6, yielding a tanning agent consisting of a dicarboxylic acid sulfonate ureidopyrimidinone compound.

[0058] Comparative Example 3

[0059] The steps (1) and (2) are the same as in Example 1, except that the preparation of the chromium-free tanning emulsion in step (3) is changed to mixing 0.1 mol of the single-terminal-NCO ureidopyrimidinone compound obtained in step (1) and 0.2 mol of the double-terminal-NCO ureidopyrimidinone compound obtained in step (2), adding a small amount of isopropanol, and then adding a mixed aqueous solution of 0.6 mol sodium bisulfite and 0.06 mol sodium sulfite. The mixture is then vigorously stirred and dispersed at 30°C for 2 hours. Water is added to adjust the solid content to 30% to obtain a light white emulsion. Dilute hydrochloric acid is added to adjust the pH of the tanning emulsion to 6.

[0060] Comparative Example 4

[0061] The steps (1) and (2) are the same as in Example 1, except that the preparation of the chromium-free tanning emulsion in step (3) is changed to mixing 0.1 mol of the single-terminal-NCO ureidopyrimidinone compound obtained in step (1) and 1.1 mol of the double-terminal-NCO ureidopyrimidinone compound obtained in step (2), adding a small amount of isopropanol, and then adding a mixed aqueous solution of 2.4 mol sodium bisulfite and 0.24 mol sodium sulfite. The mixture is then vigorously stirred and dispersed at 30°C for 2 hours. Water is added to adjust the solid content to 30% to obtain a light white emulsion. Dilute hydrochloric acid is added to adjust the pH of the tanning emulsion to 6.

[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 products 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 (2) are at 2287 cm⁻¹ -1 The final tanning product (3) has a distinct -NCO peak at 2287 cm⁻¹. -1 The NCO peak disappears. 1700cm -1 1665cm -1 These are the vibrational absorption peaks of the carbonyl group (C=O) on the urea carbonyl group and the ureidopyrimidinone, respectively, at 1258 cm⁻¹. -1 The peak represents the stretching vibration of the CN bond; 1587 cm⁻¹ -1 The characteristic peak of NH on the pyrimidine ring is 1532 cm⁻¹. -1 Characteristic peak of NH on the straight chain; 1130 cm⁻¹ -1 1030cm -1 The infrared results indicate the successful synthesis of the tanning agent, with the peak representing the -S=O deformation vibration absorption peak.

[0069] The shrinkage temperature, tensile strength, tear strength, and sensory properties of the tanned white wet leather are shown in Table 2. Table 2 shows that the chromium-free tanning agent of this invention (Examples 1-3) produces white wet sheepskin with a high shrinkage temperature, high tear strength, and high tensile strength. The leather is full-bodied with a clean, fine grain, which is closely related to the reinforcing and filling effect of quadruple hydrogen bonds. The tanning agent in Comparative Example 1 has a lower shrinkage temperature, possibly because its tanning agent only contains single-terminal carbamoyl sulfonic acid groups, causing side-linking between the tanning agent and collagen molecules, and forming dimer crosslinks solely through quadruple hydrogen bonds. The rough surface and large differences in the location of the raw leather tanned in Comparative Example 2 may be due to the tanning agent containing only double-terminal carbamoyl sulfonate reactive groups, resulting in strong astringency. This causes excessive binding of tanning agent molecules on the leather surface, blocking collagen fiber channels and causing surface over-tanning, making it difficult for subsequent tanning agents to penetrate and further bind with the collagen fibers inside the leather. The leather tanned with the tanning agent in Comparative Example 3 is slightly thinner and flatter, possibly because it contains too much tanning agent with single-terminated carbamoyl sulfonic acid groups and too little tanning agent with double-terminated carbamoyl sulfonic acid groups, resulting in weak filling and insufficient astringency. The leather tanned with the tanning agent in Comparative Example 4 has a slightly rougher surface, which is related to its excessive tanning agent with double-terminated carbamoyl sulfonic acid groups and excessive astringency.

Claims

1. A chromium-free tanning agent based on hydrogen bond reinforcement, characterized in that, The tanning agent is a mixture of compound (1) and compound (2), with the following general structural formula: Compound (1): ; Compound (2): ; Where -R- is , , , , , , , Any one of them; The molar ratio of compound (1) to compound (2) is 1:6-10.

2. A method for preparing a chromium-free tanning agent based on 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 di-NCO-terminated ureidopyrimidine ketones: A measured amount of vacuum-dried 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and diisocyanate were added to a reactor equipped with a stirrer, thermometer and condenser. Nitrogen gas was introduced and a catalyst was added. The reaction was carried out at 90-100℃ for 2-48h. A precipitant with a volume of 7-10 times that of diisocyanate was added. The mixture was stirred, filtered and the precipitate was collected. The precipitate was washed with n-hexane several times and then vacuum-dried at 50-60℃ for 4-6h before being sealed and stored for later use. (3) Preparation of chromium-free tanning agent emulsion: The single-terminal-NCO ureidopyrimidinone compound from step (1) and the double-terminal-NCO ureidopyrimidinone compound from step (2) are mixed at a molar ratio of 1:6-10. A diluent is added, followed by a mixed aqueous solution of sodium bisulfite and sodium sulfite for dispersion. After stirring and reacting for a period of time, water is added to adjust the solid content to 30% to obtain a light white emulsion. Dilute acid is added to adjust the pH of the emulsion to 5-6 to obtain chromium-free tanning agent.

3. The method for preparing a chromium-free tanning agent based on 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; In step (2), the molar ratio of 5-(2-hydroxyethyl)-6-methyl-2-aminouracil to diisocyanate is 1:8-15; The catalyst in step (2) is any one of dibutyltin dilaurate, stannous octanoate, bismuth isooctanoate, and tetrabutyl titanate, and the amount added is 0.1%-2% of the total mass of 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and diisocyanate; The diisocyanate in steps (1) and (2) is any one or more of 1,5-pentane diisocyanate, hexamethylene diisocyanate, isoflurone diisocyanate, dicyclohexyl diisocyanate, phenyldimethyl diisocyanate, cyclohexanedimethyl diisocyanate, and norbornene diisocyanate. In step (2), the precipitant is any one or more of petroleum ether, n-pentane, n-hexane, n-heptane, chloroform, and diisopropyl ether.

4. The method for preparing a chromium-free tanning agent based on hydrogen bond reinforcement according to claim 2, characterized in that, The diluent in step (3) is any one of acetone, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, ethanol, and isopropanol; In step (3), the molar amount of sodium bisulfite is 2-3 times the total molar amount of the single- and double-terminal -NCO ureidopyrimidine ketone compounds; the molar ratio of sodium bisulfite to sodium sulfite is 1:0.1-0.

4. In step (3), the reaction temperature of the mixed aqueous solution with the -NCO ureidopyrimidine ketone compound is 20-40℃, and the dispersion time is 0.5h-6h.

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