Waterborne polyurethane composite coating and preparation method thereof

Abstract

The preparation method comprises the following steps: adding a titanate coupling agent into a graphene oxide dispersion liquid A, and carrying out a water bath reaction, washing and drying to obtain functionalized graphene oxide; dispersing the functionalized graphene oxide in an organic solvent to obtain dispersion liquid B; the preparation method comprises the following steps: dehydrating polytetrahydrofuran under reduced pressure, adding isocyanate and polyol, and carrying out a heating reaction under the protection of nitrogen and the action of a catalyst to obtain a first reaction solution; adding a chain extender and the dispersion liquid B into the first reaction liquid, and reacting at 70-80 DEG C for 1.5-2.5 hours to obtain a second reaction liquid; cooling the second reaction liquid, and performing neutralization reaction and emulsification treatment to obtain the waterborne polyurethane composite coating. According to the functionalized graphene oxide, the interface crosslinking density can be improved, a corrosive medium can be effectively blocked, and the prepared coating is high in mechanical property, good in water resistance and excellent in corrosion resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of water-based coating, in particular to a water-based polyurethane composite coating and a preparation method thereof. BACKGROUND

[0002] With the increasingly stringent global environmental regulations and the growing demand for green materials from consumers, water-based polyurethane (WPU) coatings have become a popular choice to replace traditional solvent-based coatings due to their low volatile organic compound (VOC) emissions and excellent film-forming properties. However, the shortcomings of WPU coatings in mechanical strength, chemical corrosion resistance, etc. limit their application in high-end fields such as automobiles, buildings, and electronic devices.

[0003] In recent years, the introduction of nanomaterials has provided a new way to improve the performance of WPU. Among them, graphene oxide (GO) has attracted much attention due to its unique two-dimensional structure and high specific surface area. GO is a carbon-based nanomaterial modified with oxygen-containing functional groups, and its high mechanical strength and chemical stability make it an ideal reinforcing agent for polymer composites. However, the strong van der Waals forces between GO layers easily lead to agglomeration, hindering its uniform dispersion in WPU. Therefore, the dispersion and interfacial compatibility of GO in the polymer matrix are still technical bottlenecks. How to optimize the synergistic effect of GO and WPU through surface modification technology has become a key challenge to promote the development of environmentally friendly coatings.

[0004] At the same time, the improvement of water-based polyurethane in the prior art can only achieve a single performance improvement, such as mechanical strength, water resistance, or corrosion resistance, and cannot achieve a balance of comprehensive performance. SUMMARY

[0005] The present application aims to overcome the above technical deficiencies and provide a water-based polyurethane composite coating and a preparation method thereof, which solves the technical problems of the difficulty of GO to disperse uniformly in the polymer matrix and the inability to simultaneously improve the mechanical strength, water resistance, and corrosion resistance of water-based polyurethane in the prior art.

[0006] To achieve the above technical purposes, the technical solutions provided by the present application are as follows: In a first aspect, the present application provides a preparation method of waterborne polyurethane composite coating, comprising the following steps: S1, adding a titanate coupling agent to a graphene oxide dispersion liquid A, and obtaining functionalized graphene oxide through water bath reaction, washing and drying; dispersing the functionalized graphene oxide in an organic solvent to obtain a dispersion liquid B; S2, adding isocyanate and polyol to polytetrahydrofuran after dehydration under reduced pressure, and performing heating reaction under the protection of nitrogen and the action of a catalyst to obtain a first reaction liquid; adding a chain extender and the dispersion liquid B to the first reaction liquid, and reacting at 70-80℃ for 1.5-2.5h to obtain a second reaction liquid; S3, performing neutralization reaction and emulsification treatment on the second reaction liquid after cooling to obtain the waterborne polyurethane composite coating.

[0007] In a second aspect, the present application provides the waterborne polyurethane composite coating prepared by the above preparation method.

[0008] In a third aspect, the present application provides a waterborne polyurethane composite coating layer, which is prepared by curing the above composite coating into a film.

[0009] Compared with the prior art, the present application has the following beneficial effects: The present application modifies the graphene oxide by a titanate coupling agent, adjusts the surface energy of GO through chemical bonding, and improves the interfacial interaction with the polymer matrix; the obtained functionalized graphene oxide (HGO) is added in the process of waterborne polyurethane polymerization, so that the hydroxyl groups on the HGO react with the isocyanate groups, and the HGO is connected with the waterborne polyurethane through the action of chemical bonds, which improves the interfacial crosslinking density, and combines the electrostatic action and hydrogen bond action to improve the dispersion stability and interfacial strengthening effect in the waterborne polyurethane; in the process of preparing the coating layer, the functionalized graphene oxide can be arranged in the form of a sheet layer parallel to the surface of the metal substrate, so as to effectively block the corrosion medium; the coating layer prepared by the composite coating of the present application has strong mechanical properties (the tensile strength can reach 7.68-31.97 MPa), good water resistance (the water absorption rate is only 8.60-17.11%), and excellent corrosion resistance (the corrosion current density is in the order of 10 -7 ~ 10 -9 ). BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 is a synthesis mechanism diagram of the present application, wherein (a) is a HGO synthesis reaction formula, and (b) is a HGO / CWPU composite structure; Figure 2 is an XRD diagram of GO and HGO in Example 1 of the present application; Figure 3 is a Raman diagram of GO and HGO in Example 1 of the present application; Figure 4 is a contact angle test diagram of the coating layers obtained in Comparative Example 1 and Example 4 of the present application; Figure 5 is an electrochemical corrosion resistance test chart of the present application. DETAILED DESCRIPTION

[0011] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0012] In view of the defects in the prior art that graphene oxide is difficult to be uniformly dispersed in a polymer matrix such as waterborne polyurethane, and the prior art can only improve the performance of waterborne polyurethane in one aspect (such as mechanical strength, water resistance or corrosion resistance), the present application provides a waterborne polyurethane composite coating and a preparation method thereof. The GO is chemically bonded and modified by a titanate coupling agent to realize high dispersibility, strong interfacial bonding, excellent water resistance / corrosion resistance and environmentally friendly preparation of GO in CWPU (cationic waterborne polyurethane), and at the same time realize the balance of high mechanical strength, excellent water resistance and corrosion resistance of the waterborne polyurethane composite coating.

[0013] In a first aspect, the present application provides a preparation method of a functionalized graphene oxide reinforced waterborne polyurethane composite coating, comprising the following steps: S1, adding a titanate coupling agent to the graphene oxide dispersion A, and performing water bath reaction, washing and drying to obtain functionalized graphene oxide; dispersing the functionalized graphene oxide in an organic solvent to obtain dispersion B; S2, adding isocyanate and polyol to the polytetrahydrofuran after dehydration under reduced pressure, and performing heating reaction under the action of nitrogen protection and a catalyst to obtain a first reaction liquid; adding a chain extender and the dispersion B to the first reaction liquid, and reacting at 70-80°C for 1.5-2.5h to obtain a second reaction liquid; S3, after the second reaction liquid is cooled, performing neutralization reaction and emulsification treatment to obtain the functionalized graphene oxide reinforced waterborne polyurethane composite coating.

[0014] The present application greatly utilizes the performance of the material itself. The surface of GO is functionally modified by a titanate coupling agent. The hydroxyl groups on the obtained functionalized graphene oxide (HGO) react with isocyanate groups to realize the connection of HGO and CWPU in a chemical bond manner, improve the interfacial crosslinking density, and solve the dispersion stability and interfacial strengthening problems of GO in CWPU by combining electrostatic action and hydrogen bond action. The functionalized and modified graphene oxide platelets can be arranged in parallel on the surface of a metal substrate to maximize the "labyrinth effect" to block the corrosion medium, so that the functionalized graphene oxide has excellent mechanical strength, water resistance and corrosion resistance.

[0015] Preferably, in step S1, the graphene oxide dispersion A is prepared by adding graphene oxide into deionized water, ultrasonic dispersion, and adjusting the pH value to 9.5-10.5; the ratio between graphene oxide and deionized water is 0.1 g:(100-110) mL. Specifically, the pH value is adjusted to 9.5, 9.8, 10, 10.2, or 10.5, etc.; the ratio between graphene oxide and deionized water includes but is not limited to 0.1 g:100 mL, 0.1 g:102 mL, 0.1 g:105 mL, 0.1 g:106 mL, 0.1 g:108 mL, 0.1 g:110 mL, etc.

[0016] Preferably, in step S1, the titanate coupling agent includes diisopropyl bis(triethanolamine) titanate (HY-1803).

[0017] Preferably, in step S1, the mass ratio of graphene oxide and titanate coupling agent is 1:(6-8), and specifically, the mass ratio includes but is not limited to 1:6, 1:6.5, 1:7, 1:7.5, 1:8, etc.

[0018] Preferably, in step S1, the temperature of water bath reaction is 75-85°C, and the time is 4-6 h. Specifically, the temperature of water bath reaction includes but is not limited to 75°C, 78°C, 80°C, 82°C, 85°C, etc.; the time includes but is not limited to 4 h, 4.5 h, 5 h, 5.5 h, 6 h, etc.

[0019] Preferably, in step S1, the washing liquid includes ethanol and deionized water.

[0020] Preferably, in step S1, the organic solvent includes N-methyl pyrrolidone (NMP).

[0021] Preferably, in step S1, the mass fraction of dispersion B is 0.1-0.9%. Specifically, the mass fraction of dispersion B includes but is not limited to 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, etc.

[0022] Preferably, in step S2, the temperature of polytetrahydrofuran under reduced pressure dehydration is 140-160°C, and the time is 1.5-2.5 h. Specifically, the temperature of reduced pressure dehydration includes but is not limited to 140°C, 145°C, 150°C, 155°C, 160°C, etc.; the time includes but is not limited to 1.5 h, 2 h, 2.5 h, etc.

[0023] Preferably, in step S2, the mass ratio of polytetrahydrofuran, isocyanate and polyol is (10-14):(8-9.5):(0.15-0.25); specifically, the mass ratio includes but is not limited to 10:8:0.15, 10:9:0.2, 12.5:8.75:0.205, 13:8.5:0.18, 14:9.5:0.25, etc.

[0024] Preferably, in step S2, the isocyanate includes isophorone diisocyanate; the polyol includes 1,4-butanediol; and the catalyst includes dibutyltin dilaurate.

[0025] Preferably, in step S2, after the polytetrahydrofuran is dehydrated under reduced pressure, the isocyanate and the polyol are added at 70-80℃; the temperature for heating reaction is 92-98℃, and the time is 2-4h. Specifically, the temperature for adding the isocyanate and the polyol includes but is not limited to 70℃, 72℃, 75℃, 78℃, 80℃, etc.; the temperature for heating reaction includes but is not limited to 92℃, 95℃, 98℃, etc., and the time includes but is not limited to 2h, 2.5h, 3h, 3.5h, 4h, etc.

[0026] Preferably, in step S2, the chain extender and the dispersion liquid B are added after the first reaction liquid is cooled to 70-80℃. Specifically, the temperature for cooling the first reaction liquid includes but is not limited to 70℃, 72℃, 75℃, 78℃, 80℃, etc.

[0027] Preferably, in step S2, the chain extender includes N-methyldiethanolamine.

[0028] Preferably, in step S2, the mass ratio of the isocyanate to the chain extender is (8-9.5):(2-2.5), including but not limited to 8:2, 8:2.2, 8:2.5, 8.75:2, 8.75:2.25, 8.75:2.5, 9.5:2.5, etc.

[0029] Preferably, in step S2, the mass ratio of the isocyanate to the functionalized graphene oxide in the dispersion liquid B is (8-9.5):(0.02-0.22), including but not limited to 8:0.02, 8.75:0.02, 8.75:0.07, 8.75:0.11, 8.75:0.17, 8.75:0.21, 9.5:0.22, etc.

[0030] Preferably, in step S3, after the second reaction liquid is cooled, neutralization reaction and emulsification treatment are performed, specifically including: the second reaction liquid is cooled to 30-40℃, and glacial acetic acid is added for neutralization reaction; then deionized water is added and stirred for emulsification, to obtain the functionalized graphene oxide reinforced waterborne polyurethane composite coating.

[0031] Further preferably, the mass ratio of isocyanate to glacial acetic acid is (8-9.5):(0.85-1); the ratio of isocyanate to deionized water is (8-9.5)g:(40-50)mL. Specifically, the mass ratio of isocyanate to glacial acetic acid includes but is not limited to 8:0.85, 8.75:0.85, 8.75:0.9, 8.75:0.95, 9.5:1, etc.; the ratio of isocyanate to deionized water includes but is not limited to 8g:40mL, 8g:45mL, 8.75g:40mL, 8.75g:45mL, 9.5g:50mL, etc.

[0032] In a second aspect, the present application provides a functionalized graphene oxide reinforced waterborne polyurethane composite coating prepared by the above preparation method.

[0033] In a third aspect, the present application provides a functionalized graphene oxide reinforced waterborne polyurethane composite coating film prepared by curing the above composite coating.

[0034] In the present application, the GO is grafted by a titanate coupling agent. As a high-efficiency surface modifier, the titanate coupling agent effectively reduces the surface energy of GO and enhances the hydrophobicity through chemical bonding. The functionalized graphene oxide (HGO) layers dispersed in the CWPU matrix form a physical barrier, which significantly improves the mechanical strength, water resistance and corrosion resistance of the coating.

[0035] For details, see Figure 1 The -OH groups of HGO react with -isocyanate groups to form chemical bonds with CWPU, thereby enhancing the interfacial crosslinking density and improving the dispersion stability and interfacial strengthening performance of CWPU through electrostatic and hydrogen bonding effects, and endowing the material with excellent mechanical strength, water resistance and corrosion resistance.

[0036] The present application will be further described in detail through specific examples.

[0037] Example 1 A preparation method of a functionalized graphene oxide reinforced waterborne polyurethane composite coating, comprising the following steps: S1, 0.1g of graphene oxide was placed in a conical flask and 105mL of ethanol water was added for ultrasonic treatment to disperse it. After the graphene oxide was completely dispersed, the pH was adjusted to 10 to obtain a graphene oxide dispersion A; 0.7g of bis(triethanolamine) titanium diisopropyl titanate (HY-1803) was added to the graphene oxide dispersion A, and the mixture was heated to reflux at 80℃ in a water bath for 5h; after washing and drying with ethanol and deionized water, functionalized graphene oxide (HGO) powder was obtained; 0.0237g of functionalized graphene oxide was dispersed in NMP and ultrasonically dispersed for 30min to obtain a dispersion B with a mass fraction of 0.1%; S2, 12.5 g of polytetrahydrofuran was placed in a three-necked flask, dehydrated at 150°C under reduced pressure for 2 h, cooled to 75°C, 8.75 g of isophorone diisocyanate and 0.205 g of 1,4-butanediol were added, nitrogen was passed, and the temperature was raised to 95°C. After 0.5 h, one drop of dibutyltin dilaurate was added, and the reaction was carried out for 2.5 h to obtain a first reaction solution; the first reaction solution was cooled to 75°C, 2.25 g of N-methyldiethanolamine and the dispersion B prepared in step S1 were added, and the reaction was carried out for 2 h (during which the viscosity of the reaction system was adjusted with NMP), to obtain a second reaction solution; S3, the second reaction solution was cooled to 35°C, 0.914 g of glacial acetic acid was added, 45 mL of deionized water was added after 5 min, and the stirring speed was adjusted to 1000 rpm, and the stirring was carried out at high speed for 1 h to emulsify and obtain a functionalized graphene oxide reinforced waterborne polyurethane composite coating (hereinafter referred to as a composite coating); S4, 15 g of the composite coating was placed in a polytetrafluoroethylene mold to disperse the composite coating uniformly, and the composite coating was left to stand at room temperature for 24 h and was cured at 60°C for two days to form a film. The tensile properties, water absorption and corrosion resistance were tested, and the results are shown in Table 1.

[0038] Example 2 Compared with Example 1, the only difference is that 0.0711 g of functionalized graphene oxide was dispersed in NMP, and ultrasonic dispersion was carried out for 30 min to obtain a dispersion B with a mass fraction of 0.3%, and the other steps and conditions were the same as those of Example 1.

[0039] Example 3 Compared with Example 1, the only difference is that 0.1185 g of functionalized graphene oxide was dispersed in NMP, and ultrasonic dispersion was carried out for 30 min to obtain a dispersion B with a mass fraction of 0.5%, and the other steps and conditions were the same as those of Example 1.

[0040] Example 4 Compared with Example 1, the only difference is that 0.1659 g of functionalized graphene oxide was dispersed in NMP, and ultrasonic dispersion was carried out for 30 min to obtain a dispersion B with a mass fraction of 0.7%, and the other steps and conditions were the same as those of Example 1.

[0041] Example 5 Compared with Example 1, the only difference is that 0.2133 g of functionalized graphene oxide was dispersed in NMP, and ultrasonic dispersion was carried out for 30 min to obtain a dispersion B with a mass fraction of 0.9%, and the other steps and conditions were the same as those of Example 1.

[0042] Comparative Example 1 Compared with Example 1, the only difference is that S1 was omitted, i.e., no functionalized graphene oxide was added, and the other steps and conditions were the same as those of Example 1.

[0043] Comparative Example 2 Compared with Example 1, the only difference is that HY-1803 added in step S1 is replaced by 0.7g bis(dioctylpyrophosphato)ethyl titanate (LD-311), and other steps and conditions are the same as Example 1.

[0044] Comparative Example 3 Compared with Example 1, the only difference is that HY-1803 added in step S1 is replaced by 0.7g vinyl tri(β-methoxyethoxy)silane (A-172), and other steps and conditions are the same as Example 1.

[0045] Performance test Figure 2 and Figure 3 are respectively the XRD and Raman test results of GO and HGO in Example 1, and Figure 2 It can be seen that the peak position of modified graphene oxide moves to the left, the peak intensity decreases, and the interlayer spacing increases. Figure 3 It can be seen that the intensity ratio (I D / I G ) of D peak and G peak of HGO is greater than that of GO, and the increase of I D / I G value indicates that (a) the number of structural defects increases; (b) the amount of amorphous carbon increases; (c) the crystal size decreases.

[0046] Figure 4 The contact angle test results are shown in the table, and the contact angle of HGO / CWPU composite film increases first and then decreases with the increase of HGO content. When the content of HGO is 0.7%, the contact angle reaches 101.6°, and a better hydrophobic effect is achieved.

[0047] According to Chapter 9 of GB / T 16777-2008, the mechanical properties of the coating samples obtained in the above examples and comparative examples are tested, the water absorption rate is tested according to GB / T 19250-2013; and the corrosion resistance of the test sample is tested by using American AMETEK PARSTAT MC 2000 multi-channel electrochemical test system, with 3.5% NaCl aqueous solution as the corrosion medium, platinum electrode with a surface area of 1cm² as the counter electrode, mercury-mercury electrode as the reference electrode, and the coating sample as the working electrode with an exposed area of 1cm², the three electrodes form a three-electrode circuit. The determination of Tafel curve, the initial voltage is-1.000V, the terminal voltage is 2.000V, the scanning rate is 0.01V / s, and the waiting time is 2s. The collected parameters include corrosion current density and the results are shown in Table 1 and Figure 5 .

[0048] Figure 5For electrochemical corrosion performance (Tafel) test results, by calculation can get corrosion rate CR and corrosion efficiency P EF . By Figure 5 It can be seen that with the introduction of HGO, the polarization curve changes obviously, the corrosion current density moves obviously to the negative direction of y axis, and the corrosion potential moves to the positive direction of x axis, which shows that the introduction of HGO has a certain promoting effect on the corrosion resistance of the coating. And with the increase of HGO content, the corrosion potential shows a trend of first increasing and then decreasing. The corrosion current density of the coating in example 4 (dispersion B is 0.7%) is I corr 3.937 x 10 -9 A / cm 2 , which is 3 orders of magnitude lower than the corrosion current density of pure CWPU (comparative example 1) I corr , and the corrosion efficiency P EF reached 99.98%; This is mainly due to the physical barrier formed by the HGO sheet in the polymer matrix. Graphene oxide (GO) has nanoscale thickness and super large specific surface area. When HGO two-dimensional sheet is uniformly dispersed in cationic waterborne polyurethane (CWPU) matrix, a barrier network structure overlapping with each other will be formed. Corrosion medium (such as water, oxygen, Cl - , H + ) must bypass HGO sheet to reach the metal surface, which leads to the significant extension of penetration path (i.e. labyrinth effect) and the significant reduction of penetration rate. And cationic CWPU enhances the adhesion of coating through electrostatic adsorption, while the interface interaction between HGO and CWPU reduces defects, which together prevent interfacial corrosion.

[0049] Table 1 HGO / CWPU composite coating performance test

[0050] As can be seen from table 1, the tensile strength of HGO / CWPU-0.7% composite coating obtained in example 4 is 31.97 MPa, which is 4.54 times higher than that of original CWPU (7.05 MPa) in comparative example 1. The process reduces the water absorption rate by 53.03% and the corrosion current density by three orders of magnitude.

[0051] In summary, the graphene oxide is modified by titanate coupling agent in the application, the surface energy of GO is adjusted by chemical bonding, and the interface interaction with the polymer matrix is improved; the obtained functionalized graphene oxide (HGO) is added in the process of waterborne polyurethane polymerization, the hydroxyl groups on the HGO react with isocyanate groups, and the waterborne polyurethane is connected through the action of chemical bond, the interface crosslinking density is improved, and the dispersion stability and interface strengthening effect in the CWPU are improved by combining the electrostatic action and hydrogen bond action; the obtained composite coating can be parallelly arranged in the form of sheet layer on the surface of the metal substrate in the process of preparing the coating, so that the corrosion medium is effectively blocked; the coating prepared by the composite coating has strong mechanical properties (the tensile strength can reach 7.68-31.97 MPa), good water resistance (the water absorption rate is only 8.60-17.11%), and excellent corrosion resistance (the corrosion current density is in the order of 10 -7 ~10 -9 ), and can realize the corrosion prevention efficiency of 99.98%.

[0052] The specific embodiments of the application described above do not constitute a limitation on the protection scope of the application. Any various other corresponding changes and modifications made according to the technical concept of the application should be included in the protection scope of the claims of the application.

Claims

1. A method for preparing an aqueous polyurethane composite coating, characterized by, The preparation method comprises the following steps: S1, adding a titanate coupling agent to a graphene oxide dispersion A, and performing water bath reaction, washing and drying to obtain functionalized graphene oxide; the functionalized graphene oxide is dispersed in an organic solvent to obtain a dispersion B; S2, adding isocyanate and polyol to polytetrahydrofuran after vacuum dehydration, and performing heating reaction under nitrogen protection and in the presence of a catalyst to obtain a first reaction liquid; adding a chain extender and the dispersion B to the first reaction liquid, and reacting at 70-80℃ for 1.5-2.5h to obtain a second reaction liquid; S3, performing neutralization reaction and emulsification treatment on the second reaction liquid after cooling to obtain the water-based polyurethane composite coating.

2. The method for preparing an aqueous polyurethane composite coating according to claim 1, characterized in that, In step S1, the graphene oxide dispersion A is prepared by adding graphene oxide to deionized water, performing ultrasonic dispersion, and adjusting the pH value to 9.5-10.5; the ratio between the graphene oxide and the deionized water is 0.1g: (100-110)mL; The titanate coupling agent comprises diisopropyl bis(triethanolamine) titanate; The mass ratio of the graphene oxide and the titanate coupling agent is 1: (6-8).

3. The method for preparing an aqueous polyurethane composite coating according to claim 1, characterized by, In step S1, the temperature of the water bath reaction is 75-85℃, and the time is 4-6h; The organic solvent comprises N-methyl pyrrolidone; The mass fraction of the dispersion B is 0.1-0.9%.

4. The method for preparing the aqueous polyurethane composite coating according to claim 1, characterized in that, In step S2, the mass ratio of the polytetrahydrofuran, isocyanate and polyol is (10-14):(8-9.5):(0.15-0.25); The isocyanate comprises isophorone diisocyanate; The polyol comprises 1,4-butanediol; The catalyst comprises dibutyl tin dilaurate.

5. The method for preparing the aqueous polyurethane composite coating according to claim 1, characterized in that, In step S2, the temperature of the vacuum dehydration of the polytetrahydrofuran is 140-160℃, and the time is 1.5-2.5h; The isocyanate and polyol are added to the polytetrahydrofuran after vacuum dehydration at 70-80℃; The temperature of the heating reaction is 92-98℃, and the time is 2-4h; The first reaction liquid is cooled to 70-80℃, and the chain extender and the dispersion B are added.

6. The method for preparing the aqueous polyurethane composite coating according to claim 1, characterized in that, In step S2, the chain extender comprises N-methyldiethanolamine; The mass ratio of the isocyanate and the chain extender is (8-9.5):(2-2.5); The mass ratio of the isocyanate and the functionalized graphene oxide in the dispersion B is (8-9.5):(0.02-0.22).

7. The method for preparing the aqueous polyurethane composite coating according to claim 1, characterized in that, In step S3, the second reaction liquid after cooling is subjected to neutralization reaction and emulsification treatment, which specifically comprises: cooling the second reaction liquid to 30-40℃, adding glacial acetic acid to perform neutralization reaction, and then adding deionized water and stirring to emulsify, thereby obtaining the water-based polyurethane composite coating.

8. The method for preparing the aqueous polyurethane composite coating according to claim 7, characterized in that, The mass ratio of the isocyanate and the glacial acetic acid is (8-9.5):(0.85-1); and the ratio of the isocyanate and the deionized water is (8-9.5)g:(40-50)mL.

9. The water-based polyurethane composite coating prepared by the preparation method in any one of claims 1-8.

10. An aqueous polyurethane composite coating, characterized in that, The composite coating is prepared by curing the composite coating in claim 9 into a film.

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