A corrosion-resistant modified polyurethane coating, and a preparation method and application thereof

Abstract

The application provides a kind of corrosion-resistant modified polyurethane coating and its preparation method and application, the application is by introducing cellulose nanocrystal (CNC) modified polyurethane matrix, first find the source of CNC plant wood fiber and its low eutectic solvent method extraction mode jointly can effectively improve the corrosion resistance of material, mechanical property.The application is with renewable biomass as raw material, all from natural biological materials, to achieve "waste into treasure", adopt green and environment-friendly DES solvent extraction technology, meet the requirements of sustainable development.In addition, the amount of CNC in the application only needs to add 0.1%, can significantly improve the material performance, reduce the production cost.At the same time, for the anticorrosion application in marine environment provides a kind of efficient, green solution, is expected to provide a new research method and way for preparing new type of corrosion-resistant coating, has good application scene in marine anticorrosion coating, ship coating and marine equipment protection etc.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of coatings, and particularly relates to a modified polyurethane coating resistant to corrosion, and a preparation method and application thereof. BACKGROUND

[0002] Under the circumstances of the continuous expansion of the scale of marine engineering and the increase of the complexity of operating environment, marine engineering equipment is facing severe corrosion challenges. The salt, oxygen in seawater and extreme weather conditions pose a long-term corrosion threat to metal equipment, threatening the safety, reliability and life of the equipment, so it is essential to adopt effective corrosion protection means. In order to prolong the service life of marine equipment and reduce maintenance costs, it is essential to develop efficient anti-corrosion coating materials, which provide a reliable protective barrier for marine engineering equipment.

[0003] Polyurethane (PU) coatings, as a kind of functional coatings, have been widely used in different fields such as construction engineering, transportation, military defense, etc. due to their excellent corrosion resistance, low-temperature flexibility, and wear resistance. In terms of corrosion protection, using PU coatings for the corrosion protection of engineering structures is an economical and safe method. In recent years, researchers at home and abroad have developed various methods to modify PU coatings to enhance their corrosion resistance. For example, Liu Jiaqi et al. modified PU coatings with amino silicone oil synthesized from dichloromethylsilane and allylamine (Study on the corrosion resistance of modified polyurethane resin coatings[J]. Coatings and Protection, 2019, 40(12): 38-42.). Jin et al. modified WPU with isophorone diisocyanate, polypropylene glycol, 2,2-dimethylol propionic acid, and two types of epoxy resin (E-20 and E-44) as main raw materials (Study on synthesis and properties of waterborne polyurethane modified by epoxy resin[J]. Applied Mechanics and Materials, 2013, 395-396: 423-426). Invention patent CN114058261A discloses a temperature-resistant and acid-resistant anticorrosive coating for brick desulfurization chimneys and a preparation method thereof, which synthesizes unmodified polyurethane, synthesizes organosilicon oligomers with silane coupling agent, and chemically modifies polyurethane with them. Invention patent CN112759999A discloses a preparation method of an organic silicon modified waterborne acrylic polyurethane anticorrosive coating, which selects polytetrahydrofuran ether polyol, polyhexanedioic acid-1,4-butanediol ester, polyhexanedioic acid hexanediol ester, etc. polyols, isophorone diisocyanate, dicyclohexyl methane diisocyanate, etc. diisocyanate, and introduces organic silicon intermediates and acrylic monomers to copolymerize and modify waterborne polyurethane. Although the above-mentioned researches improve the corrosion resistance of polyurethane to some extent, for example, by introducing organosilicon oligomers, epoxy resins or waterborne acrylic modifiers to improve the temperature resistance and acid resistance of the coating, there are still some limitations. First of all, these methods are insufficient in optimizing the mechanical properties of polyurethane, which limits its applicability in high-strength and high-durability applications. Secondly, the operation process is relatively complex, and the modifiers used are mostly chemically synthesized materials, which are easy to pollute the environment during preparation and are difficult to meet the requirements of sustainable development. In addition, the addition amount of the modifier is high, which directly increases the production cost and reduces the economic efficiency of industrial application. Therefore, it is necessary to develop a coating that can maintain excellent corrosion resistance while improving mechanical properties, and realize a low-cost, low-carbon and environmentally friendly preparation process to meet the demand of modern industry for high-performance corrosion-resistant materials. SUMMARY

[0004] In view of the above problems existing in the prior art, the application provides a corrosion-resistant modified polyurethane coating as well as a preparation method and application thereof. The coating has excellent corrosion resistance, high strength, low cost and environmental protection advantages, can be widely applied to protection requirements in various complex environments, overcomes the limitations of traditional modified polyurethane coatings in practical application, and solves the problems of poor mechanical properties, large amount of added modifier, high cost and non-compliance with sustainable development requirements of the existing modified polyurethane coatings.

[0005] To achieve the above object, the application adopts the following scheme: a preparation method of a corrosion-resistant modified polyurethane coating, comprising the following steps:

[0006] 1) polytetrahydrofuran is dissolved in a solvent and a catalyst is added, and stirring is uniformly carried out to obtain solution A; a diisocyanate compound is dissolved in a solvent to obtain solution B;

[0007] 2) under a nitrogen atmosphere, the obtained solution A and solution B in step 1) are uniformly mixed, and then they are placed at 60-80 DEG C for 2-8 hours to obtain a prepolymer;

[0008] 3) the prepolymer obtained in step 2) is first cooled to room temperature and then transferred to an environment of-20-10 DEG C for continuous stirring, and then a chain extender is slowly added to the prepolymer to obtain a reaction liquid; the reaction liquid further comprises cellulose nanocrystals, the cellulose nanocrystals are added by being added to solution A, solution B, a mixed solution or a reaction liquid, and the cellulose nanocrystals are extracted by using a eutectic solvent method;

[0009] 4) the reaction liquid obtained in step 3) is heated to 60-80 DEG C for continuous reaction for 6-24 hours, after the reaction is completed, heating is stopped, and it is transferred to a mold for vacuum drying to constant weight, and the corrosion-resistant modified polyurethane coating is obtained.

[0010] As preferred, the cellulose nanocrystals are derived from plant wood fibers, preferably but not limited to peanut shell, ramie and / or waste paper fibers.

[0011] As preferred, the eutectic solvent method specifically comprises the following steps:

[0012] S1: choline chloride and guaiacol are heated and stirred, then AlCl3·6H2O is added, and heating and stirring are continuously carried out until a uniform transparent liquid is formed to obtain a eutectic solvent;

[0013] S2: the plant lignocellulose and the deep eutectic solvent are placed in a reduced pressure glass bottle, after being shaken uniformly, they are placed in an oil bath at 100-140 DEG C for 2-4 hours, then deionized water is added to quench the reaction, then the solution is put into a dialysis bag with a molecular weight of 8000-14000 and is placed in deionized water for dialysis until the solution pH becomes neutral;

[0014] S3: the solution obtained in step S2 is placed in an ice bath for ultrasonic treatment until the solution becomes transparent, after the ultrasonic treatment, the supernatant is collected by centrifugation and is freeze-dried, thus obtaining the cellulose nanocrystal.

[0015] Preferably, the molar ratio of the choline chloride, guaiacol and AlCl3.6H2O in step S1 is 1:1:(0.01-0.1); the heating temperature is 70-90 DEG C and the stirring speed is 500-1000 rpm.

[0016] Preferably, the solid-liquid ratio of the plant lignocellulose and the deep eutectic solvent is (0.5-1) g:40 g; the ultrasonic output power during the ultrasonic treatment is 600-1000 W, the on / off time is (1-4 s) / (1-4 s) and the ultrasonic time is 5-15 min.

[0017] Preferably, the diisocyanate compound is one or more of isophorone diisocyanate, 1,6-hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate and dicyclohexylmethane diisocyanate; and the solvent is N,N'-dimethylacetamide, N,N'-dimethylformamide, toluene or acetone.

[0018] Preferably, the mass ratio of the prepolymer and the cellulose nanocrystal in the reaction solution is 100:(0.1-1).

[0019] Preferably, the chain extender is isophorone diamine, dicyclohexylmethane di, ethylenediamine, diaminodiphenylmethane or triethanolamine; and the catalyst is dibutyltin diacetate, dibutyltin diacetate or titanate.

[0020] Another object of the present application is to provide a corrosion-resistant modified polyurethane coating prepared by the above method.

[0021] Another object of the present application is to provide the above-mentioned modified polyurethane coating for corrosion protection of metal substrates in corrosive environments, including but not limited to acidic corrosion environments (such as acid rain or acidic industrial exhaust gas), alkaline corrosion environments (such as alkaline waste liquid or alkaline cleaning agent), salty corrosion environments (such as marine salt spray or deicing salt), and aqueous corrosion environments (such as high humidity or underwater immersion conditions). The coating significantly enhances the adhesion and barrier properties of the metal surface by optimizing the formulation and introducing modified components, effectively preventing the penetration of corrosive media, and prolonging the service life of the metal structure.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] 1、The present application introduces cellulose nanocrystals (CNC) to modify the polyurethane matrix, and for the first time finds that the plant lignocellulose source of CNC and its low eutectic solvent extraction method can effectively improve the corrosion resistance and mechanical properties of the material. On the one hand, CNC has a nanoscale size and a high specific surface area, which can be uniformly dispersed in the polyurethane matrix to form a dense protective layer that effectively blocks the penetration of corrosive media. On the other hand, during the compounding process, CNC introduces more ether bonds and C-H structures into the structure of polyurethane, which is attributed to the physical embedding and interfacial interaction of CNC, resulting in changes in the signal, thereby changing the microstructure and performance characteristics of the material. This change can significantly affect the mechanical, thermal, or interfacial properties of the material, thereby improving the electrochemical stability and corrosion resistance of polyurethane. In addition, the addition of CNC optimizes the interfacial structure of polyurethane, forming a uniform and stable nanometer interfacial distribution, improving the storage modulus (G') and loss modulus (G"), and exhibiting stronger impact resistance and durability. Compared with traditional materials such as polyaniline and epoxy resin, the modified polyurethane of the present application significantly improves the impedance value in a simulated marine environment, exhibiting more excellent corrosion resistance, overcoming the shortcomings of traditional corrosion-resistant materials in mechanical properties, corrosion resistance, and environmental friendliness, and exhibiting excellent comprehensive performance. It provides an efficient and green solution for corrosion protection in marine environments, and is expected to provide a new research method and approach for the preparation of new corrosion-resistant coatings, and provides an innovative idea for the development of a new generation of corrosion-resistant coating materials.

[0024] 2、The application is prepared by taking renewable biomass such as ramie, peanut shell, waste paper fiber as raw materials, which are all from natural biological materials, achieving the purpose of "waste into treasure", and having the advantages of low price, good biocompatibility, and the use of green and environmentally friendly DES solvent extraction technology, which avoids the pollution to the environment in the preparation process of polyaniline and other chemical materials, and meets the requirements of sustainable development. In addition, the amount of CNC in the application only needs to be added by 0.1%, which can significantly improve the material performance, which is far lower than the demand of high proportion of fillers in traditional anticorrosive coating, and reduces the production cost. The modified polyurethane not only improves the corrosion resistance of the material, but also performs excellently in mechanical strength, and can effectively protect the metal substrate in the corrosion environment, and has good application scenarios in marine anticorrosive coating, ship coating and marine equipment protection. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The flow chart for preparation of cellulose nanocrystals.

[0026] Figure 2 The infrared spectrum analysis chart of peanut shell / ramie / waste paper fiber before and after pretreatment in DES solvent.

[0027] Figure 3 The infrared spectrum analysis chart of modified polyurethane coating.

[0028] Figure 4 The DMA test of different modified polyurethane coatings; A is the storage modulus-frequency relationship chart, B is the loss modulus-frequency relationship chart, and C is the loss factor-frequency relationship chart.

[0029] Figure 5 The electrochemical impedance spectrogram of different modified polyurethane coatings. DETAILED DESCRIPTION

[0030] The application will be further described in detail below in combination with specific examples, but the examples are not as a limitation on the application. The experimental methods not specified in the preferred examples are usually carried out according to the conventional conditions, or according to the conditions suggested by the reagent manufacturers.

[0031] I. A preparation method of a corrosion-resistant modified polyurethane coating.

[0032] Example 1

[0033] (1) Preparation of cellulose nanocrystals, the flow chart is shown in Figure 1 , which specifically includes the following steps:

[0034] 1) Choline chloride (ChCl) and guaiacol (GG) were placed in a beaker, and then AlCl3.6H2O was added to make the molar ratio of ChCl, GG and AlCl3.6H2O 1:1:0.07, and then the mixture was heated and stirred on a constant temperature magnetic stirrer at 90°C and a rotation speed of 500 rpm until a homogeneous transparent liquid was formed to obtain a deep eutectic solvent (DES).

[0035] 2) Peanut shell and the deep eutectic solvent were placed in a vacuum glass bottle at a solid-liquid ratio of 1:40 (w / w), and after being shaken uniformly, they were reacted in an oil bath at 120°C for 3 h. After the pretreatment was completed, deionized water was added to quench the reaction, and then the mixture was loaded into a dialysis bag with a molecular weight of 8000-14000 and dialyzed in deionized water until the solution pH became neutral.

[0036] 3) The obtained solution was placed in an ice bath for ultrasonic treatment to keep the temperature below 40°C. The ultrasonic output power was 600 W, the on / off time was 2 / 2 s, and the ultrasonic time was 15 min. After the ultrasonic treatment was completed, the mixture was separated by a centrifuge at a speed of 4000 rpm for 5 min. The turbid supernatant was collected and repeatedly subjected to ultrasonic treatment until the supernatant in the centrifuge tube became transparent. The supernatant was collected by centrifugation and freeze-dried to obtain the cellulose nanocrystals.

[0037] (2) Preparation of a modified polyurethane coating resistant to corrosion

[0038] 1) Polytetrahydrofuran PTMEG was baked at 110°C for 4 h to remove moisture. 14 g of PTMEG was placed in a 100 ml three-necked flask, and then N,N'-dimethylacetamide DMAc was added as a solvent, and 0.01 g of a catalyst dibutyltin dilaurate DBTDL was added dropwise. The mixture was stirred uniformly at a rotation speed of 2000 rpm to obtain solution A. Isophorone diisocyanate IPDI (3.46 g) and 1,6-hexamethylene diisocyanate HDI (2.60 g) were added to a solvent N,N'-dimethylacetamide DMAc, and the mixture was stirred uniformly at a rotation speed of 2000 rpm to obtain solution B. The total amount of the solvent in solution A and the solvent in solution B was 45 ml.

[0039] 2) The obtained solution A and solution B were mixed uniformly under a nitrogen atmosphere, and then the mixture was placed at 80°C for 4 h to obtain a mixed solution.

[0040] 3) The obtained mixture is first cooled to room temperature and then transferred to an environment of 0°C and continues to stir for 10 minutes, then isophorone diamine IPDA (3.80g) is slowly added dropwise within 30min, at the same time, 2mg of cellulose nanocrystals dispersed in DMAC solvent in advance is added to the reaction solution, then heated to 60°C and kept at this temperature for 12h, after the reaction is completed, stop heating and transfer it to a mold, vacuum dried to constant weight, to obtain the corrosion resistant modified polyurethane coating.

[0041] Example 2

[0042] The plant lignocellulose is ramie, and the other steps are the same as those in Example 1.

[0043] Example 3

[0044] The plant lignocellulose is waste paper fiber, and the other steps are the same as those in Example 1.

[0045] Example 4

[0046] The cellulose nanocrystals are directly added to solution A, and the other steps are the same as those in Example 3.

[0047] Example 5

[0048] The cellulose nanocrystals are directly added to solution B, and the other steps are the same as those in Example 3.

[0049] Example 6

[0050] The cellulose nanocrystals are directly added to the prepolymer, and the other steps are the same as those in Example 3.

[0051] Comparative Example 1

[0052] The microcrystalline cellulose is replaced by peanut shell, and the cellulose nanocrystals are extracted by the sulfuric acid method, and the other steps are the same as those in Example 1.

[0053] Comparative Example 2

[0054] The microcrystalline cellulose is replaced by peanut shell, and the other steps are the same as those in Example 1.

[0055] II. Performance detection

[0056] 1. The peanut shell / ramie / waste paper fiber in the present application and the sample before pretreatment with the eutectic solvent are cleaned with 100mL of ethanol solution (ethanol and water are mixed at a ratio of 1:2 (v / v)) and soaked and stirred overnight. The soaked sample is vacuum filtered and washed with ethanol solution until the filtrate is clear and colorless. The solid fraction is dried at 60°C and then placed in a desiccator. Then the sample is analyzed by Fourier transform infrared spectroscopy, and the results are shown in Figure 2 .

[0057] FromFigure 2 As can be seen from the figure, the chemical structure of different biomasses changes after being treated by the eutectic solvent. The O-H stretching vibration peak at 3340 cm-1 is weakened after being treated by the eutectic solvent, indicating that the eutectic solvent treatment reduces the free hydroxyl groups in the sample. This may be due to the partial degradation of hemicellulose and lignin, and the released hydroxyl groups are blocked by the hydrogen bonding in the eutectic solvent. The C-H stretching vibration peak at 3010-2780 cm-1 is slightly enhanced after being treated by the eutectic solvent, especially for the ramie and peanut shell samples. This may be related to the partial dissolution of lignin and the relative increase of cellulose. The C=O stretching vibration peak at 1698-1555 cm-1 is obviously enhanced after being treated by the eutectic solvent, especially for the waste paper sample. This may be due to the degradation of lignin during the eutectic solvent treatment, generating carbonyl or carboxylic acid compounds. The C-O-C stretching vibration peaks at 1246 cm-1 and 1035 cm-1 are slightly reduced after being treated, indicating that part of the C-O-C bonds of hemicellulose are broken or dissolved. The C-H peak at 897 cm-1 is significantly reduced after being treated, especially for the ramie and waste paper samples, indicating that the lignin is partially degraded and the C-H groups are reduced. Therefore, the eutectic solvent of the present application can remove a large amount of lignin and hemicellulose from biomass while retaining the basic skeleton structure of cellulose, and is a mild and efficient treatment method.

[0058] 2. Fourier transform infrared spectrometer was used to analyze the modified polyurethane coatings prepared in Examples 1-3 and Comparative Examples 1-2, and polyurethane (PU) was used as a control, and the results are shown in Figure 3 .

[0059] As can be seen from the figure, the chemical structure of different biomasses changes after being treated by the eutectic solvent. The O-H stretching vibration peak at 3340 cm-1 is weakened after being treated by the eutectic solvent, indicating that the eutectic solvent treatment reduces the free hydroxyl groups in the sample. This may be due to the partial degradation of hemicellulose and lignin, and the released hydroxyl groups are blocked by the hydrogen bonding in the eutectic solvent. The C-H stretching vibration peak at 3010-2780 cm-1 is slightly enhanced after being treated by the eutectic solvent, especially for the ramie and peanut shell samples. This may be related to the partial dissolution of lignin and the relative increase of cellulose. The C=O stretching vibration peak at 1698-1555 cm-1 is obviously enhanced after being treated by the eutectic solvent, especially for the waste paper sample. This may be due to the degradation of lignin during the eutectic solvent treatment, generating carbonyl or carboxylic acid compounds. The C-O-C stretching vibration peaks at 1246 cm-1 and 1035 cm-1 are slightly reduced after being treated, indicating that part of the C-O-C bonds of hemicellulose are broken or dissolved. The C-H peak at 897 cm-1 is significantly reduced after being treated, especially for the ramie and waste paper samples, indicating that the lignin is partially degraded and the C-H groups are reduced. Therefore, the eutectic solvent of the present application can remove a large amount of lignin and hemicellulose from biomass while retaining the basic skeleton structure of cellulose, and is a mild and efficient treatment method. Figure 3 ​

[0060] 3. The modified polyurethane coatings prepared in Examples 1-3 and Comparative Examples 1-2 were tested by dynamic mechanical analysis (DMA). Specifically, the modified polyurethane samples were cut into 30 x 7 x 0.2 mm, and frequency scanning was performed at 0°C with a fixed strain of 1%, and the frequency range was 10 Hz-0.1 Hz, with polyurethane (PU) as a control. The storage modulus, also known as the elastic modulus, refers to the amount of energy stored by a material due to elastic reversible deformation when it is deformed, and reflects the elasticity of the material. The loss modulus, also known as the viscous modulus, refers to the amount of energy lost by a material due to irreversible viscous deformation when it is deformed, and reflects the viscosity of the material. The ratio of the loss modulus to the storage modulus is called the loss factor, which reflects the proportion of viscous elasticity of the material. The results are shown in Figure 4 .

[0061] The storage modulus (G'), loss modulus (G") and loss factor of different modified polyurethane coatings with frequency are shown in Figure 4 A, Figure 4 B and Figure 4 C, respectively. It can be found that the storage modulus of Examples 1-3 is higher than that of Comparative Examples 1-2, and the loss modulus of Examples 1-3 is higher than that of Comparative Examples 1-2, which indicates that the introduction of CNC derived from plant wood fibers has a significant enhancing effect on the dynamic mechanical properties of the polyurethane coating. The dual improvement of the storage modulus and the loss modulus indicates that the material has been balanced and optimized in terms of elastic and viscous behavior. The storage modulus and loss modulus properties of Comparative Example 1 and Comparative Example 2 are similar, but Comparative Example 2 is better, indicating that the CNC extracted by the DES method has more excellent performance in the modified polyurethane. In addition, the loss factor (Tan δ) of Examples 1-3 is less than that of Comparative Examples 1-2, which indicates that the elastic properties of the modified polyurethane coating dominate, and it exhibits higher deformation recovery efficiency. The results show that the type and extraction method of the raw material significantly affect the modification effect of CNC, and the present application significantly improves the resistance to external permanent deformation of the modified polyurethane coating, and also makes the modified polyurethane coating have better viscoelasticity and stiffness, thereby improving its overall mechanical properties.

[0062] 4. The modified polyurethane coatings prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to electrochemical impedance analysis. The electrochemical impedance spectrum (EIS) of the modified polyurethane coating was obtained using an electrochemical workstation in 3.5% NaCl simulated seawater. In a three-electrode system, the contact area of all samples (working electrode) with the corrosive electrolyte was 1 cm 2 . At an alternating voltage of 100 Mv rms, the measurement frequency was from 10 4 Hz to 10 -2Hz. Polyurethane (PU) was used as a control. The electrochemical impedance curves of the modified polyurethane coatings all consisted of a semicircle in the high-frequency region and a sloping line in the low-frequency region, as shown in the following figures. Figure 5 As shown.

[0063] from Figure 5 The electrochemical impedance spectroscopy (EIS) shows that CNC extracted by the DES method significantly improves the corrosion resistance of polyurethane. The modified polyurethane coatings prepared in Examples 1-3 exhibit high impedance values ​​in the low-frequency region (<10Hz), indicating superior shielding effect against corrosive media. Compared with the comparative example and PU, the impedance modulus of the coatings of this invention is increased by nearly four orders of magnitude. Simultaneously, in the high-frequency region, the modified polyurethane coatings prepared in Examples 1-3 also exhibit high impedance values ​​compared with the comparative example and PU, indicating good electrochemical stability. Therefore, this invention significantly improves the corrosion resistance of polyurethane. This suggests that the plant wood fiber source of CNC and its eutectic solvent extraction method work together to improve the electrochemical stability and corrosion resistance of polyurethane. This is likely due to the nanoscale size, high specific surface area, and good dispersibility of CNC derived from plant wood fibers, as well as the mild extraction conditions and surface modification effect of the DES method, which jointly promote the dispersion and interfacial interaction of CNC in the polyurethane matrix. This synergistic effect improves the density and shielding performance of the polyurethane coating, thereby significantly enhancing its electrochemical stability and corrosion resistance.

[0064] It should be noted that the performance of the modified polyurethane coatings obtained in Examples 4 to 6 is similar to that in Example 3, and they are not listed here.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention that do not depart from the spirit and scope of the technical solutions of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a corrosion resistant modified polyurethane coating, characterized by, The method comprises the following steps: 1) dissolving polytetrahydrofuran in a solvent and adding a catalyst, stirring to obtain solution A; dissolving a diisocyanate compound in a solvent to obtain solution B; 2) mixing solution A and solution B obtained in step 1) uniformly under a nitrogen atmosphere, and then reacting at 60-80°C for 2-8h to obtain a prepolymer; 3) cooling the prepolymer obtained in step 2) to room temperature, then transferring it to an environment of-20-10°C for continuous stirring, then slowly adding a chain extender to the prepolymer to obtain a reaction solution; the reaction solution further comprises cellulose nanocrystals, which are added by being added to solution A, solution B, the prepolymer or the reaction solution, and the cellulose nanocrystals are extracted by a deep eutectic solvent method; the cellulose nanocrystals are derived from plant lignocellulose, and the plant lignocellulose is peanut shell, ramie and / or waste paper fiber; the deep eutectic solvent method specifically comprises the following steps: S1: heating and stirring choline chloride and guaiacol, then adding AlCl3·6H2O, continuing to heat and stir until a uniform transparent liquid is formed to obtain a deep eutectic solvent; S2: placing the plant lignocellulose and the deep eutectic solvent in a reduced-pressure glass bottle, shaking uniformly, then placing in an oil bath at 100-140°C for 2-4h, adding deionized water to quench the reaction, then loading into a dialysis bag with a molecular weight of 8000-14000 and placing in deionized water for dialysis until the solution pH becomes neutral; S3: placing the solution obtained in step S2 in an ice bath for ultrasonic treatment until the solution becomes transparent, after ultrasonic treatment, centrifuging to collect the supernatant, and freeze-drying to obtain the cellulose nanocrystals; 4) heating the reaction solution obtained in step 3) to 60-80°C, continuing to react for 6-24h, after the reaction is completed, stopping heating, and transferring it to a mold, vacuum drying to a constant weight to obtain the corrosion-resistant modified polyurethane coating.

2. The method for preparing the corrosion-resistant modified polyurethane coating according to claim 1, characterized in that, In step S1, the molar ratio of choline chloride, guaiacol and AlCl3·6H2O is 1:1:(0.01-0.1); the heating temperature is 70-90°C, and the stirring speed is 500-1000 rpm.

3. The method for preparing the corrosion-resistant modified polyurethane coating according to claim 1, characterized in that, In the deep eutectic solvent method, the solid-liquid ratio of the plant lignocellulose and the deep eutectic solvent is (0.5-1) g:40 g; in the ultrasonic treatment process, the ultrasonic output power is 600-1000 W, the on / off time is (1-4 s) / (1-4 s), and the ultrasonic time is 5-15 min.

4. The method for preparing the corrosion-resistant modified polyurethane coating according to claim 1, characterized in that, The diisocyanate compound is one or more of isophorone diisocyanate, 1,6-hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate and dicyclohexylmethane diisocyanate; the solvent is N,N'-dimethylacetamide, N,N'-dimethylformamide, toluene or acetone.

5. The method for preparing the corrosion-resistant modified polyurethane coating according to claim 1, characterized in that, In the reaction solution, the mass ratio of the prepolymer to the cellulose nanocrystals is 100:(0.1-1).

6. The method for preparing the corrosion-resistant modified polyurethane coating according to claim 1, characterized in that, The chain extender is isophorone diamine, ethylenediamine, diaminodiphenyl methane or triethanolamine; the catalyst is dibutyl tin diacetate or titanate.

7. The corrosion resistant modified polyurethane coating prepared according to the method of any one of claims 1-6.

8. The modified polyurethane coating of claim 7 is applied to a metal substrate for corrosion protection in a corrosive environment, which is an acidic corrosive environment, an alkaline corrosive environment, a salty corrosive environment or an aqueous corrosive environment.

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