A waterborne polyurethane coating, its preparation method and use

Abstract

This invention belongs to the field of polyurethane materials technology, specifically relating to a waterborne polyurethane coating, its preparation method, and its application. This invention uses amino-terminated crosslinking agent polyethyleneimine containing a large number of hydrogen bonds and small molecule amine crosslinking agents as post-crosslinking agents to construct a dense-sparse synergistic double crosslinking network. This network is then emulsified with oligomeric polyols, polyisocyanates, hydrophilic chain extenders, and small molecule chain extenders, while controlling the hard segment content at 41 wt%, to prepare a waterborne polyurethane coating with high transparency, water resistance, alcohol resistance, and shape memory properties. The resulting waterborne polyurethane emulsion not only exhibits excellent transparency (over 89% transmittance in the visible light range), but also combines a loose chemical crosslinking network constructed by short-chain crosslinking agents with a dense physical crosslinking network constructed by multi-branched crosslinking agents. The synthesized waterborne polyurethane emulsion demonstrates excellent water resistance, strong mechanical properties, superior alcohol scratch resistance, and shape memory properties.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane materials technology, specifically relating to a waterborne polyurethane coating, its preparation method, and its application. Background Technology

[0002] Waterborne polyurethane (WPU), as an environmentally friendly polymer material, is widely used in coatings, adhesives, textile finishing, and leather finishing due to its water-based dispersion medium, non-toxicity, non-flammability, low volatile organic compound (VOC) emissions, and excellent film-forming properties and flexibility. With increasingly stringent global environmental regulations and the deepening implementation of "dual-carbon" goals, WPU is gradually replacing traditional solvent-based polyurethanes and becoming the mainstream development direction for surface coating materials. However, although ordinary waterborne polyurethane performs well under normal conditions, it still has significant performance shortcomings when facing extreme chemical environments and high-temperature and humid conditions, severely limiting its in-depth application in high-end medical devices, food packaging, precision electronic protection, and special equipment.

[0003] First, traditional waterborne polyurethane molecular chains typically contain a large number of hydrophilic groups (such as carboxylates, sulfonates, and polyether segments). While these groups impart good stability to the emulsion, they result in a low crosslinking density and a relatively loose linear network structure in the cured coating. This structural defect makes the material highly susceptible to swelling, softening, and even dissolution when exposed to polar organic solvents, especially medical alcohol (75% ethanol). In applications requiring frequent alcohol wiping, such as medical disinfection and electronic screen cleaning, ordinary WPU coatings often exhibit whitening, loss of gloss, peeling, or flaking after only a few dozen wipes, leading to a failure of protective function and inability to meet the demands of high-frequency disinfection.

[0004] Secondly, the resistance of traditional waterborne polyurethane (WPU) to high temperature and humidity environments is also questionable. Water molecules entering the polymer network cause swelling and thermo-water plasticization, weakening hydrogen bonding between hard segments and promoting microphase structure relaxation and rearrangement. This leads to decreased mechanical strength retention, poor dimensional stability, interfacial adhesion failure, and even coating / adhesive film peeling. These shortcomings limit the application of WPU in scenarios such as retort pouch bonding, high-temperature sterilization packaging, boiling water resistant coatings for kitchen and bathroom applications, and high-temperature washing finishing. Therefore, developing a WPU material system that combines environmental friendliness and high-temperature water stability is not only of significant engineering application value but also of clear scientific research significance. Currently, although water resistance can be improved to some extent by introducing isocyanate curing agents or epoxy modification, this often comes at the cost of coating transparency, flexibility, or application period, making it difficult to simultaneously achieve high transparency and excellent boiling water resistance. Summary of the Invention

[0005] In view of the problems and shortcomings of the existing technology, the present invention aims to provide a waterborne polyurethane coating, its preparation method and application.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of this invention provides a method for preparing a waterborne polyurethane coating, comprising the following steps: (1) Add polyisocyanate and catalyst to the dehydrated oligomeric polyol, and react at 85-95℃ for 2-3 hours under closed conditions to obtain prepolymer A; (2) Add a hydrophilic chain extender to prepolymer A and react at 80-90℃ for 2-3 hours to obtain prepolymer B; (3) Add a small molecule chain extender to prepolymer B, react at 70-80℃ for 2-3 hours, then add an organic solvent and mix well to obtain prepolymer C; (4) Add a neutralizing agent to prepolymer C and react at 50-70°C for 15-30 min to obtain prepolymer D; (5) Add the crosslinking agent to water to obtain a crosslinking agent solution, then add the crosslinking agent solution to the prepolymer D, and react at 50-70℃ for 20-40 min to obtain a waterborne polyurethane coating. The crosslinking agent is composed of polyethyleneimine and small molecule amines.

[0007] Preferably, the small molecule amine is any one of diethylenetriamine, 1,4-butanediamine, and triethylenetetramine.

[0008] More preferably, the small molecule amine is diethylenetriamine.

[0009] Preferably, the mass ratio of small molecule amine to polyethyleneimine in the crosslinking agent is 1:20 to 3:4.

[0010] Preferably, the oligomeric polyol is one or more of polycarbonate diol (PCDL), polytetramethylene ether diol (PTMEG), and polycaprolactone diol (PCL).

[0011] More preferably, the oligomeric polyol is polytetramethylene ether diol.

[0012] Preferably, the polyisocyanate is isophorone diisocyanate (IPDI), 4,4-diisocyanate, or 4,4-diisocyanate. ， - One or more of dicyclohexylmethane diisocyanate (HMDI) and diphenylmethane diisocyanate (MDI).

[0013] More preferably, the polyisocyanate is isophorone diisocyanate.

[0014] Preferably, the hydrophilic chain extender is dimethylolpropionic acid (DMPA) and / or dimethylolbutyric acid (DMBA).

[0015] Preferably, the small molecule chain extender is ethylenediamine (EDA) and / or 1,4-butanediol (BDO).

[0016] More preferably, the small molecule chain extender is 1,4-butanediol.

[0017] Preferably, the neutralizing agent is any one of triethylamine (TEA), dimethylethanolamine, ammonia, and sodium hydroxide.

[0018] Preferably, the catalyst is dibutyltin dilaurate (DBTDL).

[0019] Preferably, the organic solvent is one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and acetone.

[0020] Preferably, in step (5), the amount of crosslinking agent used is 2wt% to 3wt% of the mass of the polyisocyanate.

[0021] Preferably, in step (1), the mass ratio of the oligomeric polyol to the polyisocyanate is (1-2):1, and the amount of the catalyst is 0.5wt% to 1wt% of the mass of the polyisocyanate.

[0022] Preferably, the ratio of the total molar number of hydroxyl functional groups in the oligomeric polyol, hydrophilic chain extender, and small molecule chain extender to the molar number of isocyanate functional groups in the polyisocyanate is 1:(1.3~1.7).

[0023] Preferably, the molar ratio of the hydrophilic chain extender to the small molecule chain extender is 1:4 to 3:4; and the molar ratio of the neutralizing agent to the hydrophilic chain extender is 1:1.

[0024] Preferably, the amount of the organic solvent is 5% to 15% of the total mass of the polyisocyanate, oligomeric polyol, hydrophilic chain extender, small molecule chain extender and crosslinking agent.

[0025] A second aspect of the present invention provides an aqueous polyurethane coating prepared by the preparation method described in the first aspect above.

[0026] The third aspect of this invention provides the application of the waterborne polyurethane coating described in the first aspect above in the field of high-temperature cooking protection.

[0027] Preferably, the method of applying the waterborne polyurethane coating to the BOPP film specifically involves: applying the prepared waterborne polyurethane coating onto the BOPP film using a coating rod, thereby imparting the film with properties of resistance to boiling and alcohol scratching, which is used to protect the substrate material and thus extend its service life.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses amino-terminated crosslinking agent polyethyleneimine (PEI) containing a large number of hydrogen bonds and small molecule amine crosslinking agents (DETA, BDA, TETA) as post-crosslinking agents to construct a dense-sparse synergistic double crosslinking network. This network is then emulsified with oligomeric polyols, polyisocyanates, hydrophilic chain extenders, and small molecule chain extenders, with the hard segment content controlled at 41 wt%, to prepare a waterborne polyurethane coating with high transparency, water resistance, alcohol resistance, and shape memory properties. The resulting waterborne polyurethane emulsion not only exhibits excellent transparency (over 89% transmittance in the visible light range), but also combines a loose chemical crosslinking network constructed by short-chain crosslinking agents with a dense physical crosslinking network constructed by multi-branched crosslinking agents. The synthesized waterborne polyurethane emulsion demonstrates excellent water resistance, strong mechanical properties, superior alcohol scratch resistance, and shape memory properties.

[0029] 2. The waterborne polyurethane coating with high transparency, water resistance, alcohol resistance and shape memory properties provided by the present invention can restore transparency in a short time after being boiled in high temperature water, and the mechanical properties of the material are improved after boiling.

[0030] 3. The waterborne polyurethane coating provided by this invention has high transparency, resistance to boiling water, resistance to alcohol wiping, and shape memory properties, with an elastic modulus as high as 65 MPa. During the use of the material, it can effectively reduce the damage caused by harsh external environments. At the same time, through the dual protection of resistance to boiling water and resistance to alcohol scratching, the service life of the material is greatly improved. Attached Figure Description

[0031] Figure 1 Images of waterborne polyurethane coating samples from Examples 1-3, Comparative Example 1, and Comparative Example 2 of this invention; Figure 2 The light transmittance of the waterborne polyurethane coatings in Examples 1-3, Comparative Example 1, and Comparative Example 2 of this invention; Figure 3 The stress-strain curves of the waterborne polyurethane coatings in Examples 1-3, Comparative Examples 1 and 2 of the present invention before and after boiling in water are shown; wherein, (a) is the stress-strain curve before boiling in water, and (b) is the stress-strain curve after boiling in water. Figure 4 These are schematic diagrams illustrating the water boiling changes of the waterborne polyurethane coatings in Examples 1-3, Comparative Example 1, and Comparative Example 2 of the present invention. Figure 5 This is a schematic diagram illustrating the process by which the waterborne polyurethane coatings of Examples 1-3, Comparative Examples 1 and 2 of the present invention regained their transparency after being boiled in water and turning white. Figure 6 This illustrates the application scenario of the waterborne polyurethane coating with alcohol scratch resistance in Embodiment 2 of the present invention. Figure 7 The shape memory properties of the waterborne polyurethane coating in Example 2 of this invention. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Information on the raw materials used in the embodiments of this invention is shown in Table 1.

[0033] Table 1 Raw Material Information Table Example 1

[0034] A method for preparing a waterborne polyurethane coating is as follows: (1) Weigh 8.7202g of PTMEG-1000 and put it into a three-necked flask. Fix the three-necked flask in an oil bath and control the reaction temperature at 120℃. Turn on the stirring machine and use a vacuum pump to remove water until the vacuum reaches -0.1MPa. Remove water for 3.5h. Lower the temperature of the reaction system to 90℃, and then add 7.1852g of IPDI and 48μL of butyltin dilaurate. Seal the system and react for 2.5h.

[0035] (2) Lower the temperature of the reaction system to 80℃, add 0.5133g DMPA, and react for 2.5h.

[0036] (3) Add 0.6898g of BDO into the system and react at 80℃ for 1h. Observe the viscosity of the system. When the system temperature drops to 50℃, add 10mL of acetone to reduce the viscosity of the system.

[0037] (4) Weigh 0.4179g of TEA and slowly add it dropwise into the system. Increase the stirring speed and react at 50℃ for 30min.

[0038] (5) Weigh 50 mL of deionized water, 0.1004 g of PEI and 0.0690 g of DETA, dissolve PEI and DETA in the deionized water, and then slowly add them to the system. React at 50 °C for 40 min to obtain waterborne polyurethane coating.

[0039] Calculate the hard segment content of the waterborne polyurethane coating in this embodiment. Hwt% The calculation formula is as follows:

[0040] In the formula, m 1 、m 2 、m 3 、m 4 The quantities are respectively the mass of oligomeric polyols, polyisocyanates, chain extenders (hydrophilic chain extenders and small molecule chain extenders) and crosslinking agents.

[0041] The hard segment content of the waterborne polyurethane coating in this embodiment is calculated to be 41.0 wt%. This invention relates to the hard segment content of the waterborne polyurethane coating. H wt% The hard segment content is controlled at 30% to 50% because when the hard segment content is controlled at around 41%, the mechanical properties, thermal stability and emulsion stability of the material can be balanced. If the hard segment content is too high, it will destroy the microphase separation structure of the material and lead to a decrease in performance. Example 2

[0042] A waterborne polyurethane coating, the preparation method of which is as follows: (1) Weigh 8.7202g of PTMEG-1000 and put it into a three-necked flask. Fix the three-necked flask in an oil bath and control the reaction temperature at 120℃. Turn on the stirring machine and use a vacuum pump to remove water until the vacuum reaches -0.1MPa. Remove water for 3.5h. Lower the temperature of the reaction system to 90℃, and then add 7.1852g of IPDI and 48μL of butyltin dilaurate. Seal the system and react for 2.5h.

[0043] (2) Lower the temperature of the reaction system to 80℃, add 0.5133g DMPA, and react for 2.5h.

[0044] (3) Add 0.6898g of BDO into the system and react at 80℃ for 1h. Observe the viscosity of the system. When the system temperature drops to 50℃, add 10mL of acetone to reduce the viscosity of the system.

[0045] (4) Weigh 0.4179g of TEA and slowly add it dropwise into the system. Increase the stirring speed and react at 50℃ for 30min.

[0046] (5) Weigh 50 mL of deionized water, 0.1400 g of PEI and 0.0294 g of DETA, dissolve PEI and DETA in the deionized water, and then slowly add them dropwise to the system. React at 50 °C for 40 min to obtain waterborne polyurethane coating. Example 3

[0047] A waterborne polyurethane coating, the preparation method of which is as follows: (1) Weigh 8.7202g of PTMEG-1000 and put it into a three-necked flask. Fix the three-necked flask in an oil bath and control the reaction temperature at 120℃. Turn on the stirring machine and use a vacuum pump to remove water until the vacuum reaches -0.1MPa. Remove water for 3.5h. Lower the temperature of the reaction system to 90℃, and then add 7.1852g of IPDI and 48μL of butyltin dilaurate. Seal the system and react for 2.5h.

[0048] (2) Lower the temperature of the reaction system to 80℃, add 0.5133g DMPA, and react for 2.5h.

[0049] (3) Add 0.6898g of BDO into the system and react at 80℃ for 1h. Observe the viscosity of the system. When the system temperature drops to 50℃, add 10mL of acetone to reduce the viscosity of the system.

[0050] (4) Weigh 0.4179g of TEA and slowly add it dropwise into the system. Increase the stirring speed and react at 50℃ for 30min.

[0051] (5) Weigh 50 mL of deionized water, 0.1624 g of PEI and 0.0070 g of DETA, dissolve PEI and DETA in the deionized water, and then slowly add them to the system. React at 50 °C for 40 min to obtain waterborne polyurethane coating.

[0052] Comparative Example 1: No PEI added A waterborne polyurethane coating, the preparation method of which is as follows: (1) Weigh 8.7202g of PTMEG-1000 and put it into a three-necked flask. Fix the three-necked flask in an oil bath and control the reaction temperature at 120℃. Turn on the stirring machine and use a vacuum pump to remove water until the vacuum reaches -0.1MPa. Remove water for 3.5h. Lower the temperature of the reaction system to 90℃, and then add 7.1852g of IPDI and 48μL of butyltin dilaurate. Seal the system and react for 2.5h.

[0053] (2) Lower the temperature of the reaction system to 80℃, add 0.5133g DMPA, and react for 2.5h.

[0054] (3) Add 0.6898g of BDO into the system and react at 80℃ for 1h. Observe the viscosity of the system. When the system temperature drops to 50℃, add 10mL of acetone to reduce the viscosity of the system.

[0055] (4) Weigh 0.4179g of TEA and slowly add it dropwise into the system. Increase the stirring speed and react at 50℃ for 30min.

[0056] (5) Weigh 50 mL of deionized water and 0.1694 g of DETA, dissolve DETA in the deionized water, and then slowly add it dropwise into the system. React at 50 °C for 40 min to obtain waterborne polyurethane coating.

[0057] Comparative Example 2: No DETA added A waterborne polyurethane coating, the preparation method of which is as follows: (1) Weigh 8.7202g of PTMEG-1000 and put it into a three-necked flask. Fix the three-necked flask in an oil bath and control the reaction temperature at 120℃. Turn on the stirring machine and use a vacuum pump to remove water until the vacuum reaches -0.1MPa. Remove water for 3.5h. Lower the temperature of the reaction system to 90℃, and then add 7.1852g of IPDI and 48μL of butyltin dilaurate. Seal the system and react for 2.5h.

[0058] (2) Lower the temperature of the reaction system to 80℃, add 0.5133g DMPA, and react for 2.5h.

[0059] (3) Add 0.6898g of BDO into the system and react at 80℃ for 1h. Observe the viscosity of the system. When the system temperature drops to 50℃, add 10mL of acetone to reduce the viscosity of the system.

[0060] (4) Weigh 0.4179g of TEA and slowly add it dropwise into the system. Increase the stirring speed and react at 50℃ for 30min.

[0061] (5) Weigh 50 mL of deionized water and 0.1694 g of PEI, dissolve PEI in the deionized water, and then slowly add it dropwise into the system. React at 50 °C for 40 min to obtain waterborne polyurethane coating.

[0062] Performance testing experiments: 1. Particle size, PDI index, and Zeta potential The particle size, PDI index, and Zeta potential of the waterborne polyurethane coatings prepared in Examples 1-3, Comparative Examples 1 and 2 of this invention were tested using a dynamic light scattering instrument. The results are shown in Table 2.

[0063] Table 2. Particle size, PDI index, and Zeta potential of Examples 1-3, Comparative Examples 1 and 2

[0064] Figure 1 Images show waterborne polyurethane coating samples prepared in Examples 1-3, Comparative Example 1, and Comparative Example 2 of this invention. Figure 1As shown in Table 2, all samples exhibit a white emulsion state, and the emulsion is stable. Furthermore, the average particle size of each sample is approximately 270–280 nm, with a relatively uniform particle size distribution. The PDI index is significantly less than 0.7. In addition, the Zeta potential is greater than 35 mV, indicating that the emulsion is stable and can be stored for more than one year without demulsification or precipitation.

[0065] 2. Transparency The waterborne polyurethane coatings prepared in Examples 1-3, Comparative Examples 1 and 2 of this invention were rotary evaporated at 45°C for 1.5 hours using a rotary evaporator. The evaporators were then poured into glass dishes and laid in a vacuum oven to form waterborne polyurethane film samples measuring 2 cm × 2 cm with a thickness of 0.6 mm. The UV transmittance of the waterborne polyurethane film samples was tested using a UV-9000S spectrophotometer (Shanghai Yuanxi Instrument Co., Ltd.), and the results are as follows: Figure 2 As shown. Figure 2 The transmittance refers to the light transmittance of the waterborne polyurethane coatings in Examples 1-3, Comparative Example 1, and Comparative Example 2 of this invention. Figure 2 As can be seen, all samples exhibit excellent transparency, with most having a transmittance of over 80%. This high transparency can broaden their applications in coatings, packaging, and protective materials.

[0066] 3. Mechanical properties The waterborne polyurethane coatings prepared in Examples 1-3, Comparative Example 1, and Comparative Example 2 of this invention were made into tensile specimens and placed in boiling water at 100°C for cooking. The mechanical properties of the tensile specimens before and after cooking were then tested. The results are as follows: Figure 3 As shown in Table 3.

[0067] Table 3. Young's modulus of Examples 1-3, Comparative Examples 1 and 2 before and after boiling.

[0068] Depend on Figure 3 It can be seen that, Figure 3 (a) shows the stress-strain curve before boiling. Figure 3 (b) shows the stress-strain curve after boiling. The mechanical properties of the sample in Example 2 were best before and after boiling. Before boiling, its tensile strength was 30.2 MPa, its elongation at break was 490.4%, and its Young's modulus was as high as 65.0 MPa. After boiling, its mechanical properties improved. After boiling, the Young's modulus of the sample in Example 2 increased from 65 MPa to 85.1 MPa, an increase of 30.9%.

[0069] 4. Water boiling resistance The waterborne polyurethane coatings prepared in Examples 1-3, Comparative Examples 1 and 2 of this invention were used to make tensile specimens, which were then boiled in boiling water at 100°C. The changes in the tensile specimens at different boiling times were recorded. The results are as follows: Figure 4 and Figure 5 As shown. Figure 4 The boiling water reaction results of the waterborne polyurethane coatings in Examples 1-3, Comparative Examples 1 and 2 of the present invention are shown. Figure 4 It can be seen that when the samples were boiled in boiling water, Comparative Example 2 was the first to turn white and soften after 30 minutes. As the boiling time gradually increased, Comparative Example 1, Example 1, and Example 3 all showed whitening after 60 minutes. Then, after boiling Example 2 for another 90 minutes, no whitening occurred. Whitening only occurred after 120 minutes. In summary, Example 2 exhibited the best water resistance, withstanding 120 minutes of boiling. This is because the DETA to PEI ratio was optimal. At this ratio, the "reversible / dynamic dense physical network" and the "more stable / sparser chemical covalent network" achieved the best balance. The chemical covalent cross-linking network was perfectly connected by the dynamic physical cross-linking network, resulting in a highly stable dual-synergistic network of chemical backbone cross-linking and dynamic physical cross-linking. Figure 5 This is a schematic diagram illustrating the process by which the waterborne polyurethane coatings of Examples 1-3, Comparative Examples 1 and 2 of the present invention regain transparency after being boiled in water and turning white. Figure 5 It can be seen that the sample in Example 2 not only has the best water resistance, but also the shortest time to recover its transparency after boiling, taking only 30 minutes to fully recover. This is because its network is "stable enough but not overly hydrophilic / dynamic", minimizing the scattering sources introduced by boiling and eliminating them the fastest.

[0070] 5. Resistance to alcohol wiping The waterborne polyurethane coating of Example 2 of this invention was rotary evaporated at 45°C for 1.5 hours using a rotary evaporator. The evaporator was then poured into a glass dish and laid in a vacuum oven to produce a 2cm × 2cm waterborne polyurethane film sample with a thickness of 0.6mm. The waterborne polyurethane film sample was then rubbed with 75% medical alcohol under a 50g weight using a multi-functional alcohol-rubber abrasion tester. The scratching effect on the sample surface was observed using a super-depth-of-field three-dimensional microscope. The results are as follows: Figure 6 As shown. Among them, Figure 6 (a) is a hyper-depth-of-field photograph taken after the sample has been scratched. Figure 6 (b) A photograph demonstrating the transparency of the sample. Figure 6 (c) is the ultraviolet transmittance diagram. From Figure 6As can be seen, after 200 wiping cycles, the film transparency of the sample in Example 2 only decreased from 88.1% (wavelength 750 nm) to 64.5% (wavelength 750 nm), demonstrating good alcohol scratch resistance. This is because PEI mainly densifies the surface interface and provides a strong interaction network, while DETA mainly provides a more uniform and stable covalent locking network. Therefore, the addition of DETA did not disrupt the density and interaction points of PEI; instead, it maintained the covalent locking by improving covalent locking and reducing swelling and migration. Thus, Example 2 maintained excellent alcohol resistance.

[0071] 6. Shape memory performance The waterborne polyurethane coating prepared in Example 2 of this invention was used to make tensile specimens. The original specimen length was recorded as L1. The specimens were then immersed in 90°C hot water for two minutes, removed, stretched, and then immersed in an ice-water bath for two minutes. The specimen length was recorded as L3. The specimens were then immersed in 90°C hot water for one minute, and the recovered length was recorded as L2. The recovery rate of the specimens was calculated using the following formula:

[0072] The shape memory properties of the waterborne polyurethane coating in Example 2 of this invention are as follows: Figure 7 As shown. By Figure 7 It can be seen that Example 2 has shape memory function with a recovery rate of up to 91.5%. The chemical cross-linking network constructed by DETA / PEI endows the material with "memory", while the glass transition of the sample into a reversible phase realizes the "fixation" and "release" of the shape. The two work together to achieve the shape memory effect.

[0073] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Those skilled in the art can modify or make equivalent substitutions to the technical solutions of the present invention based on the concept of the present invention, without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a waterborne polyurethane coating, characterized in that, Includes the following steps: (1) Add polyisocyanate and catalyst to the dehydrated oligomeric polyol, and react at 85-95℃ for 2-3 hours under closed conditions to obtain prepolymer A; (2) Add a hydrophilic chain extender to prepolymer A and react at 80-90℃ for 2-3 hours to obtain prepolymer B; (3) Add a small molecule chain extender to prepolymer B, react at 70-80°C for 0.5-2 hours, then add an organic solvent and mix well to obtain prepolymer C; (4) Add a neutralizing agent to prepolymer C and react at 50-70°C for 15-30 min to obtain prepolymer D; (5) Add the crosslinking agent to water to obtain a crosslinking agent solution, then add the crosslinking agent solution to the prepolymer D, and react at 50-70℃ for 20-40 min to obtain a waterborne polyurethane coating. The crosslinking agent is composed of polyethyleneimine and small molecule amines.

2. The method for preparing the waterborne polyurethane coating according to claim 1, characterized in that, The small molecule amine is any one of diethylenetriamine, 1,4-butanediamine, and triethylenetetramine.

3. The method for preparing the waterborne polyurethane coating according to claim 1, characterized in that, The mass ratio of small molecule amine to polyethyleneimine in the crosslinking agent is 1:20 to 3:

4.

4. The method for preparing the waterborne polyurethane coating according to claim 1, characterized in that, The oligomeric polyol is one or more of polycarbonate diol, polytetramethylene ether diol, and polycaprolactone diol; the polyisocyanate is isophorone diisocyanate, 4,4-diisocyanate, etc. ， - One or more of dicyclohexylmethane diisocyanate and diphenylmethane diisocyanate; the hydrophilic chain extender is dimethylolpropionic acid and / or dimethylolbutyric acid; the small molecule chain extender is ethylenediamine and / or 1,4-butanediol; the neutralizing agent is any one of triethylamine, dimethylethanolamine, ammonia, and sodium hydroxide.

5. The method for preparing the waterborne polyurethane coating according to claim 1, characterized in that, The catalyst is dibutyltin dilaurate; the organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, and acetone.

6. The method for preparing the waterborne polyurethane coating according to claim 1, characterized in that, In step (5), the amount of crosslinking agent used is 2wt% to 3wt% of the mass of the polyisocyanate.

7. The method for preparing the waterborne polyurethane coating according to claim 1, characterized in that, In step (1), the mass ratio of the oligomeric polyol to the polyisocyanate is (1-2):1, and the amount of catalyst used is 0.5wt% to 1wt% of the mass of the polyisocyanate.

8. The method for preparing the waterborne polyurethane coating according to claim 1, characterized in that, The ratio of the total molar number of hydroxyl functional groups in the oligomeric polyol, hydrophilic chain extender, and small molecule chain extender to the molar number of isocyanate functional groups in the polyisocyanate is 1:(1.3-1.7); the molar ratio of the hydrophilic chain extender to the small molecule chain extender is 1:4 to 3:4; the molar ratio of the neutralizing agent to the hydrophilic chain extender is 1:1; and the amount of the organic solvent is 5% to 15% of the total mass of the polyisocyanate, oligomeric polyol, hydrophilic chain extender, small molecule chain extender, and crosslinking agent.

9. A waterborne polyurethane coating prepared by the preparation method according to any one of claims 1-8.

10. The application of the waterborne polyurethane coating according to claim 9 in the field of high-temperature boiling protection.

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