Environment-friendly leather retanning agent and preparation method thereof

Abstract

The application discloses an environment-friendly leather re-tanning agent and a preparation method thereof. Firstly, starch is subjected to ultrasonic-assisted degradation to reduce the molecular weight of the starch, then the degraded starch is subjected to dialdehyde modification to obtain dialdehyde starch; then, amino acid containing a double amino group is added into the dialdehyde starch to prepare a dialdehyde starch-amino acid prepolymer, finally, an etherifying agent is added into the dialdehyde starch-amino acid prepolymer to close the excess free aldehyde group, thereby obtaining the environment-friendly leather re-tanning agent, wherein the molar ratio of the primary amino group in the amino acid containing a double amino group to the aldehyde group in the dialdehyde starch is 1:1.2-1:2. The application uses natural starch and amino acid as raw materials to prepare the re-tanning agent, and no formaldehyde is added as a condensing agent in the preparation process, so that the obtained re-tanning agent is safe and environment-friendly, and when the re-tanning agent is used for leather re-tanning, the filling effect is good, the finished leather is full, soft, elastic, has a good hand feeling and high comprehensive performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of tanning leather, especially relates to an environment-friendly leather re-tanning agent and a preparation method thereof. BACKGROUND

[0002] The leather re-tanning process plays an important role in the leather industry, directly affecting the product quality of leather products. After re-tanning with appropriate re-tanning agents, the leather can be endowed with excellent properties, including improved hand feeling and softness, increased fullness, enhanced water resistance, light resistance, and surface charge change, promoting uniform dyeing, etc. Different styles of leather can be formed to better meet the needs of the market for leather products.

[0003] Re-tanning agents are one of the most important factors affecting leather re-tanning quality. There are many types of re-tanning agents, including aromatic synthetic re-tanning agents, resin re-tanning agents, aldehyde re-tanning agents, and oil re-tanning agents. However, the re-tanning agents in the prior art often contain heavy metals or other toxic and harmful chemicals, resulting in a large amount of waste and wastewater. At present, many people have tried to use modified natural products such as starch and crushed leather to prepare re-tanning agents to further meet the needs of society and consumers.

[0004] Starch and amino acids are two inexhaustible, inexhaustible, biodegradable, environmentally friendly, non-polluting, natural renewable resources that are increasingly favored by people. Natural starch has poor solubility and dispersion in cold water, making it difficult to form a stable system. Therefore, various physical and chemical modifications need to be made to the starch during actual application. However, due to uneven modification, the product quality is greatly affected, the use effect is poor, and it cannot meet the actual production needs. Amino acids have small molecular weight and strong water solubility. When applied to leather processing, they have poor filling properties and no obvious re-tanning effect. However, starch and amino acid products have excellent performance and are completely biodegradable, with broad application prospects.

[0005] However, there is no industrialized technology for applying amino acids to modify the process and products of preparing starch leather re-tanning agents in the prior art, and the product quality is difficult to control. SUMMARY

[0006] The present application combines starch and amino acids to propose an environment-friendly leather re-tanning agent and a preparation method thereof. The obtained re-tanning agent is safe and environmentally friendly, and when used for leather re-tanning, it not only has good filling effect, but also has fullness, softness, elasticity, good hand feeling, and high comprehensive performance.

[0007] To achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0008] A preparation method of an environment-friendly leather re-tanning agent, characterized in that it comprises the following steps:

[0009] S1: ultrasonic-assisted degradation of starch to obtain monomers with smaller molecular weight, the smaller the better;

[0010] S2: dialdehyde modification of the degraded starch to obtain dialdehyde starch;

[0011] S3: preparation of dialdehyde starch-amino acid pre-polymer by reacting dialdehyde starch with amino acid containing diamino groups;

[0012] S4: sealing of free aldehyde groups in the dialdehyde starch-amino acid pre-polymer with etherifying agent to obtain the environmentally friendly leather re-tanning agent;

[0013] The molar ratio of primary amino groups in the amino acid to aldehyde groups in the dialdehyde starch is 1:1.2-1:2;

[0014] The proportion of free aldehyde groups in the dialdehyde starch-amino acid pre-polymer is 50%-90%.

[0015] In a preferred embodiment, the ultrasonic-assisted degradation of starch in step S1 specifically includes: adding a degradation agent dropwise to a mixed emulsion of starch and pure water and ethanol and stirring, preferably stirring at 40-50°C for 15-30 min, then performing ultrasonic-assisted degradation of starch, the degradation temperature is 65-95°C, the degradation time is 1-6 h, and the temperature is lowered to below 40°C after the reaction is completed to obtain a degraded starch solution;

[0016] In the formula, the weight ratio of the degradation agent to the starch raw material is 3-15%, and the weight ratio of the pure water to the ethanol is 1:0.5-1:1.

[0017] In a preferred embodiment, the dialdehyde modification in step S2 specifically includes: adding a dialdehyde modifier solution to the degraded starch solution in step S1, adjusting the pH value to 2.5-3.5, and performing dialdehyde modification, the modification conditions are: avoiding light, the modification temperature is 30-50°C, and the modification time is 3-6 h; after the reaction is completed, filtration, washing, and drying are performed to obtain dialdehyde starch;

[0018] In the formula, the weight ratio of the dialdehyde modifier to the starch raw material is 0.1:1-1:1.

[0019] Preferably, after the reaction is completed, filtration is performed, then the dialdehyde starch is washed with alcohol / water mixed solution and filtered multiple times, usually twice, and then dried to obtain the dialdehyde starch.

[0020] Preferably, the alcohol / water mixed solution is an ethanol solution, and the volume ratio of ethanol to water is 1:9-1:1.

[0021] In a preferred embodiment, the preparation of dialdehyde starch-amino acid pre-polymer in step S3 specifically includes:

[0022] S301: Dissolving the amino acid containing double amino group in pure water to obtain an amino acid dispersion; preferably, dissolving at 10-50℃;

[0023] S302: Mixing the dialdehyde starch prepared in step 2 with the amino acid dispersion, adjusting the pH value to 7.5-9.0 with a strong alkali solution, and then reacting at 70-85℃ for 0.5-1.0 hours and continuing to react at 80-95℃ for 10-30 minutes to obtain a dialdehyde starch-amino acid prepolymer.

[0024] In a preferred embodiment, step S4 specifically comprises: adding an etherifying agent containing a hydroxyl group to the dialdehyde starch-amino acid prepolymer prepared in step S3, adjusting the pH value to 3.0-6.0, then incubating at 50-70℃ for 1-3 hours, and then adjusting the pH value to 6.0-8.0.

[0025] The molar ratio of the hydroxyl group in the etherifying agent to the free aldehyde group in the dialdehyde starch-amino acid prepolymer is 2:1.

[0026] Preferably, the amino acid containing double amino group is at least one of lysine, asparagine and glutamine.

[0027] Preferably, the etherifying agent is at least one of methanol, ethanol, isopropanol, diethylene glycol or n-butanol.

[0028] Preferably, the degrading agent is a sulfuric acid or hydrochloric acid solution with a mass concentration of 15-30%.

[0029] Preferably, the dialdehyde agent is a periodic acid or sodium periodate solution with a concentration of 0.3-0.6 mol / L.

[0030] Preferably, in step S1, an ultrasonic generator is used to generate ultrasonic waves, the ultrasonic generator has an ultrasonic wave frequency of 20-60 kHz and an ultrasonic wave intensity of 0.8-1.2 W / cm 2 .

[0031] Preferably, the strong alkali solution is one of sodium hydroxide, potassium hydroxide or a mixture thereof, and the mass concentration of the sodium hydroxide or potassium hydroxide solution is 20-40%.

[0032] Based on the same inventive concept, the present application also provides an environmentally friendly leather refilling agent prepared by the preparation method of all the above embodiments.

[0033] The present application has the following advantages and positive effects compared with the prior art due to the use of the above technical solutions:

[0034] 1. The retanning agent provided by the present application, firstly, the starch is subjected to ultrasonic-assisted degradation, so that the starch is fully and uniformly degraded into a large number of small molecular weight starch units, then the starch is subjected to dialdehyde modification, so that aldehyde groups are uniformly provided on each small molecular starch unit, then amino acid containing double amino groups is added to prepare dialdehyde starch-amino acid prepolymer, and macromolecular dialdehyde starch-amino acid prepolymer is obtained through reaction, so that the retanning agent has good filling property for retanning leather, since the aldehyde groups have the risk of being oxidized during storage and use, the etherifying agent added at last can seal the excess free aldehyde groups, so that the free aldehyde groups are protected, thereby the retanning agent product has good stability, so that the retanning agent is easier to store; in addition, a large number of carboxyl groups are introduced into the retanning agent product after the dialdehyde modified starch reacts with the amino acid, so that the retanning agent product has good water solubility, so that the retanning agent product of the present application will not appear condensation, precipitation and segregation phenomenon during storage and use, and has better leather retanning effect.

[0035] 2. The starch of the present application is degraded and then subjected to aldehyde modification, so that the uniformity of the reaction between the dialdehyde starch and the amino acid can be effectively improved, the active sites are increased, the active groups on the generated dialdehyde starch-amino acid prepolymer are more uniformly distributed, and the product contacts more fully when retanning leather. The starch is degraded into a large number of small molecular weight starch units through the degradation reaction, which is not only beneficial to the subsequent dialdehyde modification and increases the reaction efficiency, but also makes the aldehyde groups on each small molecular weight starch unit more uniformly provided, so that the molar ratio of the primary amino group in the amino acid to the aldehyde group in the dialdehyde starch is controlled during the reaction between the dialdehyde starch and the amino acid, the aldehyde group on each small molecular weight starch unit reacts with the amino group of the amino acid, and a network structure is formed through crosslinking, which on the one hand increases the contact area of the retanning agent when retanning leather, and the retanning reaction is more sufficient, and the retanning effect is better; on the other hand, the molecular weight of the generated prepolymer is also increased, so that the retanning agent has good filling property. Moreover, the amino acid containing double amino groups is used, which on the one hand better reacts with the dialdehyde starch to form a network structure prepolymer, and on the other hand also increases the molecular weight of the prepolymer, so that the retanning agent has good filling property.

[0036] 3. The present application firstly degrades the starch, and then subjects the starch to dialdehyde modification, so that the dialdehyde reaction of the degraded starch can effectively improve the reaction efficiency. In particular, ultrasonic assistance is used in the degradation of the starch, which is beneficial to the degradation agent to easily penetrate into the area of the compact structure of the starch (crystalline region) rather than only the area of the loose structure of the starch (non-crystalline region), which not only can improve the degradation efficiency, but more importantly can more effectively and comprehensively treat the starch, so that there is no dead angle in the microstructure of the starch, thereby obtaining degraded starch with better quality, overcoming the uneven treatment of the existing method, and having better and more uniform degradation performance.

[0037] 4、The preparation method of the present application uses natural degradable starch and amino acid as raw materials to prepare the retanning agent product, which is safe and environmentally friendly; through the adjustment of the starch ultrasonic assisted degradation process and the dialdehyde starch-amino acid prepolymer preparation process, the molecular weight is easily controlled, and the dialdehyde starch-amino acid prepolymer and the retanning agent product have good water solubility and dispersibility. The quality of the leather retanning agent is good, and when used for leather retanning, it not only has good filling effect, but also the finished leather is full, soft, elastic, has good hand feeling, and has high comprehensive performance, which is comparable to traditional amino resin retanning agents. At the same time, the retanning product also has excellent buffing performance, no color fading, and certain improvement effect on the tensile strength and tear strength of the finished leather.

[0038] 5、The retanning agent preparation process of the present application does not use formaldehyde as a condensing agent, and uses natural starch and amino acid as raw materials, so there is no possibility of formaldehyde generation in the production of the retanning agent product, the product itself, and the retanning process of the finished leather. DETAILED DESCRIPTION

[0039] The present application uses natural starch and amino acid in combination, reduces the molecular weight of starch through ultrasonic assisted degradation of starch, then modifies the degraded starch by dialdehyde modification, then reacts the dialdehyde starch with amino acid containing double amino groups to obtain dialdehyde starch-amino acid prepolymer, and finally adds an etherifying agent to block the excess free aldehyde groups to obtain an environmentally friendly leather retanning agent without formaldehyde. The quality of the leather retanning agent is good, and when used for leather retanning, it not only has good filling effect, but also the finished leather is full, soft, elastic, has good hand feeling, and has high comprehensive performance, which is comparable to traditional amino resin retanning agents. At the same time, the retanning product also has excellent buffing performance, no color fading, and certain improvement effect on the tensile strength and tear strength of the finished leather.

[0040] The following will further describe an environmentally friendly leather retanning agent and a preparation method thereof according to the present application in combination with specific embodiments. According to the following description, the advantages and characteristics of the present application will be more apparent.

[0041] In this document, the range represented by "a value to another value" is a summary representation for avoiding listing all the values in the range in the specification. Therefore, the description of a certain value range covers any value in the value range and a smaller value range defined by any value in the value range, as if the any value and the smaller value range are written in the specification.

[0042] As described in the examples herein, "retanning agent", "retanning agent product" and "environmentally friendly leather retanning" have the same meaning.

[0043] It is worth noting that the shrinkage temperature of the leather blank in the following examples and comparative examples is measured by HG shrinkage temperature recorder.

[0044] The method for testing the formaldehyde content is as follows: first, 2 g of the leather sample is put into 50 ml of 0.1% sodium dodecyl sulfate washing solution, and is extracted at 40°C for 1 hour, then the extracted solution is filtered, and is reacted with ethyl acetone to quantitatively determine the formaldehyde content in the leather sample.

[0045] The retanning process used in the following specific examples is as follows:

[0046] The uniform blue hide of the animal skin garment raw material or shoe upper is put into a rotating drum, and is sequentially subjected to softening, neutralization, and washing according to the conventional process, then 8% of the retanning agent and 200% of water are added according to the weight of the uniform blue hide, and the mixture is rotated at a temperature of 35°C and a pH value of 5.0-6.0 for 1 hour, then the retanning process is completed according to the conventional dyeing, fat-liquoring, and acid-fixing processes.

[0047] The application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the application, and are not used to limit the protection scope of the application. Improvements and adjustments made by those skilled in the art according to the application still belong to the protection scope of the application.

[0048] Example 1

[0049] (1) Ultrasonic-assisted degradation of corn starch: 10 Kg of corn starch, 20 liters of pure water, and 10 liters of ethanol are stirred and mixed at room temperature, 7.5 liters of sulfuric acid (20% by mass) is added dropwise into the mixed solution, and the mixture is stirred at 45°C for 20 min, then the degradation of corn starch is carried out under ultrasonic assistance, the degradation temperature is 80°C, the degradation time is 4.5 hours, the ultrasonic frequency of the ultrasonic generator is 40 kHz, and the ultrasonic intensity is 1.0 W / cm 2 After the reaction is completed, the temperature is lowered to below 40°C, and a degraded starch solution is obtained.

[0050] (2) Diallyl modification: 40 liters of periodic acid or sodium periodic acid solution (0.5 mol / L) is added to the degraded starch solution prepared in step (1), and the pH value is adjusted to 3.0, then the diallyl modification is carried out in the dark at a modification temperature of 40°C for 4.5 hours. After the reaction is completed, an appropriate amount of ethanol is added, and the mixture is filtered and washed twice with an alcohol-water mixture (20:80 by volume), and then is dried to obtain dialdehyde starch.

[0051] (3) Preparation of dialdehyde starch-amino acid prepolymer: 4.5 Kg lysine and pure water were dissolved at 40°C, and then added to the dispersion prepared by mixing dialdehyde starch prepared in step (2) and 30 liters of pure water, and stirred uniformly. After adjusting the pH value to 8.0 with sodium hydroxide solution, the reaction was carried out at 80°C for 1.0 hour, and then the temperature was increased to 90°C for 20 minutes to obtain dialdehyde starch-amino acid prepolymer.

[0052] (4) Blocking free aldehyde groups: The content of free aldehyde groups in the dialdehyde starch-amino acid prepolymer obtained in step (3) was determined, 2 times the molar content of free aldehyde groups of methanol was added, the pH value was adjusted to 4.5, and then the temperature was kept at 60°C for 2 hours. Finally, the pH value was adjusted to 7.0 to obtain the retanning agent product.

[0053] The retanning agent prepared in this example was used to retan cowhide upper leather according to the retanning process described herein. The retanned cowhide upper leather was full and compact, the grain surface was fine, and the hand feeling was good.

[0054] Example 2

[0055] (1) Ultrasonic-assisted degradation of corn starch: 10 Kg of corn starch, 20 liters of pure water, and 10 liters of ethanol were mixed at room temperature, and 5 liters of sulfuric acid (20% by mass) was added dropwise to the mixture. The mixture was stirred at 40°C for 25 minutes, and then the ultrasonic-assisted degradation of corn starch was carried out at a temperature of 70°C for 5.5 hours. The ultrasonic frequency of the ultrasonic generator was 30 kHz, and the ultrasonic intensity was 1.1 W / cm 2 After the reaction was completed, the temperature was reduced to below 40°C to obtain a degraded starch solution.

[0056] (2) Dialdehyde modification: 20 liters of periodic acid or sodium periodate solution (0.5 mol / L) was added to the degraded starch solution prepared in step (1), and the pH value was adjusted to 3.0. The dialdehyde modification was carried out in the dark at a temperature of 30°C for 6 hours. After the reaction was completed, an appropriate amount of ethanol was added, filtered, washed twice with an alcohol-water mixture (10:90 by volume), and dried to obtain dialdehyde starch.

[0057] (3) Preparation of dialdehyde starch-amino acid prepolymer: 3.5 Kg of glutamine and pure water were dissolved at 30°C, and then added to the dispersion prepared by mixing dialdehyde starch prepared in step (2) and 30 liters of pure water, and stirred uniformly. After adjusting the pH value to 8.0 with sodium hydroxide solution, the reaction was carried out at 70°C for 1.0 hour, and then the temperature was increased to 80°C for 30 minutes to obtain dialdehyde starch-amino acid prepolymer.

[0058] (4) Blocking free aldehyde groups: the free aldehyde group content in the dialdehyde starch-amino acid prepolymer obtained in step (3) is determined, 2 times the molar amount of diethylene glycol or n-butanol is added, the pH value is adjusted to 4.5, then it is incubated at 50°C for 3 hours, and finally the pH value is adjusted to 7.0 to obtain the retanning agent product.

[0059] The retanning agent prepared in this example is used to retan pigskin garment leather according to the retanning process described herein. The retanned pigskin upper leather is full and compact, the grain is fine, and the hand feeling is good.

[0060] Example 3

[0061] (1) Ultrasonic-assisted degradation of corn starch: 10 kg of corn starch, 20 liters of pure water and 10 liters of ethanol are stirred and mixed at room temperature, 15 liters of sulfuric acid (mass concentration of 20%) is added dropwise into the mixed solution, and the corn starch is degraded under ultrasonic assistance at a degradation temperature of 95°C for 2 hours, with the ultrasonic frequency of the ultrasonic generator being 60 kHz and the ultrasonic intensity being 0.8 W / cm 2 After the reaction is completed, the temperature is lowered to below 40°C to obtain a degraded starch solution.

[0062] (2) Dialdehyde modification: 55 liters of periodic acid or sodium periodate solution (molar concentration of 0.6 mol / L) is added to the degraded starch solution prepared in step (1), and the pH value is adjusted to 3.5 for dialdehyde modification. The modification conditions are: avoiding light, modification temperature of 50°C, and modification time of 3.5 hours. After the reaction is completed, an appropriate amount of ethanol is added, filtered, washed with an alcohol-water mixture (volume ratio of 30:70) twice, and dried to obtain dialdehyde starch.

[0063] (3) Preparation of dialdehyde starch-amino acid prepolymer: 4.5 kg of asparagine is dissolved in pure water at 30°C, and then added to the dispersion liquid prepared by mixing the dialdehyde starch prepared in step (2) with 30 liters of pure water, stirred uniformly, and then the pH value is adjusted to 8.5 with sodium hydroxide solution. After that, it is reacted at a temperature of 85°C for 0.5 hours, and then the temperature is raised to 95°C for continuous reaction for 15 minutes to obtain the dialdehyde starch-amino acid prepolymer.

[0064] (4) Blocking free aldehyde groups: the free aldehyde group content in the dialdehyde starch-amino acid prepolymer obtained in step (3) is determined, 2 times the molar amount of diethylene glycol or n-butanol is added, the pH value is adjusted to 4.5, then it is incubated at 50°C for 3 hours, and finally the pH value is adjusted to 7.0 to obtain the retanning agent product.

[0065] The retanning agent prepared in this example is used to retan pigskin garment leather according to the retanning process described herein. The retanned pigskin upper leather is full and compact, the grain is fine, and the hand feeling is good.

[0066] Comparative Example 1

[0067] Preparation of traditional amino resin retanning agent:

[0068] (1) First, 7.6 Kg of melamine, 34.9 Kg of glutaraldehyde, and 26.2 Kg of pure water were added to a reaction kettle and stirred uniformly. 0.3 Kg of 10% sodium hydroxide solution was used to adjust the pH value of the reaction system to 7.5. Slow heating was performed to 60°C and the temperature was maintained for 1.0 hour to obtain a hydroxylated melamine prepolymer.

[0069] (2) Then, 8.7 Kg of water and 4.9 Kg of sodium bisulfite were added to the hydroxylated melamine prepolymer. The temperature was raised to 85°C and maintained for 2.0 hours.

[0070] (3) Finally, the temperature was lowered to 60°C. 5.8 Kg of hydrolyzed collagen with a molecular weight of about 5000 and 11.6 Kg of pure water were added. The pH value was adjusted to 5.5. After maintaining the temperature for 2.0 hours, the temperature was lowered to room temperature. The pH value was adjusted to neutral. Thus, the retanning agent product of the present comparative example was obtained.

[0071] The retanning agent prepared in the present comparative example was used to retan cow leather for shoe uppers according to the retanning process described herein.

[0072] Application of Comparative Example 1

[0073] The quality of the finished leather retanned with the retanning agent of Example 1 was compared with the quality of the finished leather retanned with the retanning agent of Comparative Example 1. The softness, fullness, firmness, and surface hand feeling of the cow leather for shoe uppers after retanning were further compared. The results are shown in Table 1.

[0074] Table 1

[0075] Project Softness Fullness Compactness Surface feel Example 1 Softer Full Compact Smooth Comparative Example 1 Normal Full Loose Slightly dry

[0076] As can be seen from the above table, the finished leather retanned with the retanning agent of Example 1 has fullness comparable to that of the finished leather retanned with the retanning agent of Comparative Example 1, but is superior in softness, firmness, and surface hand feeling. Therefore, the finished leather retanned with the retanning agent of Example 1 has certain advantages.

[0077] Application of Comparative Example 2

[0078] In the present comparative example, the retanning agent prepared in Comparative Example 1 was used to retan sheepskin according to the retanning process described herein.

[0079] The retanning agents of Examples 1, 2, and 3 above were used to retan sheepskin according to the retanning process described herein. The changes in shrinkage temperature of the finished leather before and after retanning and the free formaldehyde content in the finished leather were further compared. The results are shown in Table 2.

[0080] Table II

[0081]

[0082] From the above table, it can be seen that the amino resin re-tanning agent provided by the examples 1-3 has good product quality and high comprehensive performance, and its re-tanning performance can be comparable to that of the traditional amino resin re-tanning agent, but formaldehyde is not used as raw material in the preparation process, so that the risk of residual formaldehyde in the re-tanning agent in the leather processing can be fundamentally eliminated. Further, the natural corn starch and amino acid are used as main raw materials, and the product is absolutely safe and environmentally friendly.

[0083] The above disclosure is only the preferred embodiments of the present application. The preferred embodiments do not describe all the details and do not limit the application to the specific embodiments described. Obviously, many modifications and variations can be made according to the content of the present application. The present application selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present application, so that those skilled in the art can well utilize the present application. The present application is limited only by the claims and their full scope and equivalents.

[0084] Under the guidance of the present application and the above-mentioned embodiments, it is easy for those skilled in the art to predict that each raw material or its equivalent replacement, each processing method or its equivalent replacement listed or exemplified by the present application can realize the present application, and the upper and lower limits of the parameters of each raw material and processing method, the interval values can realize the present application, and the embodiments are not listed one by one.

Claims

1. A method for preparing an environmentally friendly leather retanning agent, characterized in that, Includes the following steps: S1: The degradation agent is added dropwise to a mixed emulsion of starch, pure water, and ethanol and stirred. Then, the starch is degraded under ultrasonic assistance at a temperature of 65–95°C for 1–6 hours. After the reaction is completed, the temperature is lowered to below 40°C to obtain a degraded starch solution. S2: The degraded starch is modified by dialdehyde to obtain dialdehyde starch; S3: Prepare dialdehyde starch-amino acid prepolymer by reacting dialdehyde starch with amino acids containing diamino groups; S4: The free aldehyde groups in the dialdehyde starch-amino acid prepolymer are blocked by an etherifying agent to obtain the environmentally friendly leather retanning agent. The molar ratio of the primary amino group in the amino acid to the aldehyde group in the dialdehyde starch is 1:1.2 to 1:2, and the proportion of unreacted aldehyde groups in the dialdehyde starch-amino acid prepolymer is 50% to 90%. The amino acid containing two amino groups is at least one of lysine, asparagine, and glutamine.

2. The method for preparing the environmentally friendly leather retanning agent according to claim 1, characterized in that, In step S1, the degradation agent accounts for 3-15% of the weight of the starch raw material, and the weight ratio of pure water to ethanol is 1:0.5-1:

1.

3. The method for preparing the environmentally friendly leather retanning agent according to claim 1, characterized in that, The dialdehyde modification in step S2 specifically includes: adding a dialdehyde agent solution to the degraded starch solution in step S1, adjusting the pH value to 2.5-3.5, and performing dialdehyde modification under the following conditions: protection from light, modification temperature of 30-50℃, and modification time of 3-6 hours; after the reaction is completed, filtering, washing, and drying are performed to obtain dialdehyde starch. The weight ratio of the dual aldehyde oxidizing agent to the starch raw material is 0.1:1 to 1:

1.

4. The preparation method of the environmentally friendly leather retanning agent according to claim 1, characterized in that, The preparation of the dialdehyde starch-amino acid prepolymer in step S3 specifically includes: S301: Dissolve amino acids containing diamino groups in pure water to obtain an amino acid dispersion; S302: The dialdehyde starch obtained in step S2 is mixed and stirred with the amino acid dispersion. The pH value is adjusted to 7.5-9.0 with a strong alkali solution, and the reaction is carried out at a temperature of 70-85℃ for 0.5-1.0 hours. The temperature is then raised to 80-95℃ and the reaction is continued for 10-30 minutes to obtain the dialdehyde starch-amino acid prepolymer.

5. The method for preparing the environmentally friendly leather retanning agent according to claim 1, characterized in that, Step S4 specifically includes: adding an etherifying agent containing hydroxyl groups to the dialdehyde starch-amino acid prepolymer obtained in step S3, adjusting the pH value to 3.0-6.0, then keeping it at 50-70°C for 1-3 hours, and then adjusting the pH value to 6.0-8.

0. The molar ratio of the hydroxyl groups in the etherifying agent to the free aldehyde groups in the dialdehyde starch-amino acid prepolymer is 2:

1.

6. The method for preparing the environmentally friendly leather retanning agent according to claim 1 or 5, characterized in that, The etherifying agent is at least one of methanol, ethanol, isopropanol, diethylene glycol, or n-butanol.

7. The method for preparing the environmentally friendly leather retanning agent according to claim 2, characterized in that, The degradation agent is a sulfuric acid or hydrochloric acid solution with a mass concentration of 15-30%.

8. The method for preparing the environmentally friendly leather retanning agent according to claim 3, characterized in that, The bisaldehyde agent is a periodic acid or sodium periodate solution with a concentration of 0.3–0.6 mol / L.

9. The method for preparing the environmentally friendly leather retanning agent according to claim 2, characterized in that, In step S1, an ultrasonic generator is used to generate ultrasonic waves. The ultrasonic frequency of the ultrasonic generator is 20–60 kHz, and the ultrasonic intensity is 0.8–1.2 W / cm². 2 .

10. The method for preparing the environmentally friendly leather retanning agent according to claim 4, characterized in that, The strong alkaline solution is one of sodium hydroxide and potassium hydroxide or a mixture thereof, and the mass concentration of the sodium hydroxide or potassium hydroxide solution is 20-40%.

11. An environmentally friendly leather retanning agent prepared by the preparation method according to any one of claims 1-10.

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