Digital coating liquid, preparation method thereof and coating structure

Abstract

The application belongs to the technical field of water-based paint, and discloses a digital coating liquid, a preparation method thereof and a coating structure. The preparation raw material of the digital coating liquid according to the application comprises, in terms of weight parts, 20-35 parts of first water-based polyurethane, 30-40 parts of second water-based polyurethane, 10-30 parts of a dispersion medium, 6-9 parts of a film-forming auxiliary solvent, 4-8 parts of a polar auxiliary solvent, 2-5 parts of a defoaming agent and 2-5 parts of a leveling agent. The dispersion medium comprises pure water. The glass transition temperatures of the first water-based polyurethane and the second water-based polyurethane are Tg1 and Tg2 respectively, wherein Tg1 and Tg2 are different. The application can improve the storage stability and anti-yellowing performance of the digital coating liquid, thereby being beneficial to improving the ink adhesion of the formed coating and the water resistance of the coating.

Description

Technical Field

[0001] This application relates to the field of water-based coating technology, and more specifically, to a digital coating liquid and its preparation method and coating structure. Background Technology

[0002] Traditional offset printing presses rely on pressure and adsorption between rollers to transfer ink, while electronic ink digital printing relies on voltage difference, temperature, and roller pressure. However, whether transferring ink from the ink roller to the printing plate, from the printing plate to the blanket, or from the blanket to the substrate, electronic ink digital printing is used. This means that the quality of the printed product obtained using these methods is affected by the properties of the coating solution.

[0003] Currently, the coating solutions used have poor storage stability, which leads to defects in the printed products prepared using these solutions, such as poor adhesion of electronic inks, poor water resistance of the coating, and poor anti-yellowing performance, which seriously affects the quality of the printed products.

[0004] In view of this, this application provides a digital coating liquid, its preparation method, and the coating structure. Summary of the Invention

[0005] Based on the above problems, this application aims to provide a digital coating liquid, its preparation method, and coating structure.

[0006] In one aspect, this application provides a digital coating liquid.

[0007] In this application, the raw materials for preparing the digital coating liquid, by weight, include: The mixture comprises 20-35 parts of a first waterborne polyurethane, 30-40 parts of a second waterborne polyurethane, 10-30 parts of a dispersion medium, 6-9 parts of a film-forming co-solvent, 4-8 parts of a polar co-solvent, 2-5 parts of a defoamer, and 2-5 parts of a leveling agent; the glass transition temperature of the first waterborne polyurethane is Tg1, and the glass transition temperature of the second waterborne polyurethane is Tg2, wherein Tg1 and Tg2 are different.

[0008] In some embodiments of the first aspect of this application, the digital coating liquid satisfies at least one of the following conditions: (1) Tg1 satisfies 5℃≤Tg1≤45℃; (2) Tg2 satisfies 35℃≤Tg2≤75℃; (3) The difference between Tg2 and Tg1 satisfies 10℃≤(Tg2-Tg1)≤50℃.

[0009] In some embodiments of the first aspect of this application, the digital coating liquid satisfies at least one of the following conditions: (1) Tg1 satisfies 10℃≤Tg1≤30℃; (2) Tg2 satisfies 35℃≤Tg2≤60℃; (3) The difference between Tg2 and Tg1 satisfies 15℃≤(Tg2-Tg1)≤30℃.

[0010] In some embodiments of the first aspect of this application, the first aqueous polyurethane includes an aliphatic aqueous polyurethane dispersion or an aromatic aqueous polyurethane dispersion.

[0011] In some embodiments of the first aspect of this application, the second waterborne polyurethane includes a self-crosslinking waterborne polyurethane dispersion or a highly elastic waterborne polyurethane dispersion.

[0012] In some embodiments of the first aspect of this application, the aliphatic aqueous polyurethane dispersion includes anionic aliphatic aqueous polyurethane dispersion.

[0013] In some embodiments of the first aspect of this application, the aromatic waterborne polyurethane dispersion includes anionic aromatic waterborne polyurethane dispersion.

[0014] In some embodiments of the first aspect of this application, the self-crosslinking aqueous polyurethane dispersion includes anionic self-crosslinking aqueous polyurethane dispersion.

[0015] In some embodiments of the first aspect of this application, the highly elastic waterborne polyurethane dispersion includes anionic highly elastic waterborne polyurethane dispersion.

[0016] In some embodiments of the first aspect of this application, the dispersion medium includes pure water.

[0017] In some embodiments of the first aspect of this application, the dispersion medium comprises pure water with a conductivity ≤10 μS / cm.

[0018] In some embodiments of the first aspect of this application, the film-forming co-solvent and the polar co-solvent are selected from different solvents among alcohol solvents.

[0019] In some embodiments of the first aspect of this application, the alcohol solvent includes at least one selected from methanol, ethanol, n-butanol, ethylene glycol, n-propanol, isopropanol, and n-hexanol.

[0020] In some embodiments of the first aspect of this application, the film-forming co-solvent includes at least one of methanol and ethanol.

[0021] In some embodiments of the first aspect of this application, the polar cosolvent includes at least one of isopropanol and n-propanol.

[0022] In some embodiments of the first aspect of this application, the defoamer includes a defoamer specifically for water-based coatings.

[0023] In some embodiments of the first aspect of this application, the water-based coating-specific defoamer includes at least one of BYK-024, BYK-025, TEGO Airex 901W, and Evonik TEGO Airex 920.

[0024] In some embodiments of the first aspect of this application, the leveling agent includes a leveling agent specifically for water-based systems.

[0025] In some embodiments of the first aspect of this application, the water-based system-specific leveling agent includes at least one of BYK-333, BYK-349, Evonik TEGO Glide 450, and TEGO Wet 270.

[0026] Secondly, this application provides a method for preparing a digital coating liquid.

[0027] It should be understood that the preparation method of this application is based on any of the raw materials for preparing digital coating liquid in the first aspect above, and has all the beneficial effects brought about by its components and their proportions in the first aspect, so this application will not repeat them here.

[0028] In this application, the method for preparing the digital coating liquid includes: Step 1: Weigh the corresponding raw materials according to the following proportions: 20-35 parts of first waterborne polyurethane, 30-40 parts of second waterborne polyurethane, 10-30 parts of dispersion medium, 6-9 parts of film-forming cosolvent, 4-8 parts of polar cosolvent, 2-5 parts of defoamer, and 2-5 parts of leveling agent; Step 2, prepare the mixed solvent: The weighed dispersion medium, film-forming co-solvent, and polar co-solvent are mixed evenly to obtain a mixed solvent; Step 3, prepare the premix: The weighed first and second waterborne polyurethanes are dispersed in the mixed solvent to obtain a premix. Step 4, stirring process: The weighed defoamer and leveling agent are added to the premix and stirred to obtain the digital coating liquid.

[0029] In some embodiments of the second aspect of this application, the stirring rate of the stirring treatment is 300 r / min to 800 r / min, the stirring temperature is 20℃ to 25℃, and the stirring time is 1 h to 6 h.

[0030] Thirdly, this application provides a coating structure.

[0031] In this application, the coating structure includes: Surface layer; A coating liquid layer is disposed on one surface of the face material layer; An electrostatic ink layer is disposed on the surface of the coating liquid layer away from the face material layer; The coating liquid layer includes any digital coating liquid of the first aspect, or a digital coating liquid prepared by any preparation method of the second aspect.

[0032] In some embodiments of the third aspect of this application, the material of the face layer includes at least one of thermal paper, polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polypropylene (PP).

[0033] This application has at least the following beneficial effects: This application uses waterborne polyurethane (including a first waterborne polyurethane and a second waterborne polyurethane) as the main component of the digital coating liquid. The first waterborne polyurethane has a low glass transition temperature, which can endow the digital coating liquid system with excellent flexibility and deformation ability, which is conducive to adapting to various types of substrates. It is mainly used to provide excellent wear resistance, weather resistance and film-forming properties. The second waterborne polyurethane has a high glass transition temperature, which is mainly used to improve the flexibility, impact resistance and chemical resistance of the coating. This application controls the difference between the glass transition temperature Tg1 of the first waterborne polyurethane and the glass transition temperature Tg2 of the second waterborne polyurethane, that is, the existence of a difference, so that the high and low Tg polyurethane segments provided by the first waterborne polyurethane and the second waterborne polyurethane can interpenetrate to form an interpenetrating microstructure. This can suppress the emulsion agglomeration, sedimentation and stratification that occur when a single resin is stored for a long time, and improve the storage stability of the digital coating liquid, thereby improving the ink adhesion and water resistance of the printed product. Furthermore, the dispersion medium in this application includes pure water, which effectively avoids gelation or solid precipitation in the digital coating solution when the solid content is high (≥10%), thereby improving the storage stability of the digital coating solution. This application also incorporates film-forming co-solvents and polar co-solvents, selected from different alcohol solvents, to form a gradient evaporation solvent system. This system sustainably maintains the solvation layer on the resin particle surface, reducing the probability of particle agglomeration. This facilitates the control of the overall coating evaporation rate and synergistically optimizes the final film quality of the digital coating solution, further improving the ink adhesion and water resistance of the printed product. This application also incorporates defoamers and leveling agents, which further improve the bonding effect between the substrate and the coating after it forms a coating, especially reducing the surface tension of the coating and thus improving its water resistance. Furthermore, the first and second waterborne polyurethanes used in this application are combined with a gradient volatile alcohol solvent. During coating and film formation, the solvent slowly precipitates out, reducing micropores in the coating caused by rapid solvent escape and minimizing the penetration of oxygen and moisture into the oxidized resin segments within the coating. Residual initiators and small-molecule additives in the digital coating solution gradually migrate out of the coating along with the alcohol solvent, effectively preventing yellowing during long-term storage and exposure to light, thus improving the coating's anti-yellowing properties. Through the synergistic effect of the various raw material components, the digital coating solution of this application can simultaneously improve the storage stability of the digital coating solution, as well as the electronic ink adhesion, coating water resistance, and anti-yellowing properties of the printed product, thereby improving the quality of the printed product. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the coating structure in a printed product. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description, in conjunction with the accompanying drawings and embodiments, is provided. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0036] In the description of this application, the terms "upper," "lower," "right," etc., refer to the orientation or positional relationship shown in the accompanying drawings, and are used only for ease of description and simplification of operation. They do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. In addition, the terms "first" and "second" are used only for descriptive distinction and have no special meaning.

[0037] The endpoints and any values ​​of the ranges disclosed in this application are not limited to the precise ranges or values, and such ranges or values ​​should be understood to include values ​​close to such ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0038] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions. Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc. Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included. Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0039] Unless otherwise specified, all reagents or pharmaceuticals used in this application are commercially available products.

[0040] It should be emphasized that the preparation methods or operation steps not specifically described in this application all refer to the prior art in this field, which should be known by those skilled in the art, and therefore will not be described again in this application.

[0041] As described in the background section, the quality of printed products obtained using electronic ink digital printing is affected by the performance of the coating solution. For example, the HP Indigo 6K digital printing press uses electronic ink technology. The pattern created on the computer is transferred to the printing press. By charging the printing plate and exposing it with a laser head, images are generated on the printing plate (a single printing plate can be used quickly and multiple times), completing multiple pattern reproductions. The color rendering principle is similar to that of traditional offset printing, using CMYK four-color ink overlay. According to the color rendering principle, the four CMYK inks are superimposed to form a vibrant array of colors. The ink is then transferred from the printing plate to the blanket, and then to the substrate, achieving personalized pattern reproduction. This special personalized printing method is highly dependent on the functionality of the online coating solution; specifically, the coating structure in the resulting printed product is as follows... Figure 1 As shown, it includes a face material layer, a coating liquid layer, and an electrostatic ink layer arranged sequentially.

[0042] However, the coating solutions currently used and the printed products prepared from these solutions have the following problems: (1) Poor storage stability: The synthesis method used in the current domestic popular coating liquids results in gelation of the coating material when preparing coating liquids with high solid content (solid content ≥10%). Therefore, only low solid content (solid content 3%~5%) can be achieved. Moreover, low solid content makes the coating process less adaptable, resulting in the overall product effect not reaching the ideal state. In addition, the current popular coating liquids all use acrylic systems (including McMurphy), which will show solid precipitation after long-term storage, resulting in more solid waste in the material tank and greater difficulty in cleaning.

[0043] (2) Poor ink adhesion: After the electronic ink is successfully transferred, it is very easy to lose ink when pulled with tape, especially on special materials such as PET and PVC. The ink loss is extremely serious and greatly affects the overall yield of finished products.

[0044] (3) Poor water resistance of coating: After the electronic ink transfer is successful, the patterned material is placed in water and soaked for 2 hours. After taking it out and wiping it dry, the ink adhesion is tested again. The ink adhesion is almost completely absent. On substrates such as PET and PVC, the pattern is damaged during the wiping process.

[0045] (4) Poor anti-yellowing effect: During long-term storage, due to the unstable substances in the acrylic resin system, they are gradually reacted by oxygen and ambient temperature and humidity during storage, generating colored chromophores; as the storage time increases, the chromophores accumulate, and the coating gradually shows yellowing, affecting the uniformity of appearance and customer satisfaction.

[0046] In view of this, this application provides a digital coating liquid, its preparation method, and the coating structure.

[0047] In one aspect, this application provides a digital coating liquid.

[0048] The raw materials for preparing the digital coating liquid of this application, by weight, include: The mixture comprises 20-35 parts of a first waterborne polyurethane, 30-40 parts of a second waterborne polyurethane, 10-30 parts of a dispersion medium, 6-9 parts of a film-forming co-solvent, 4-8 parts of a polar co-solvent, 2-5 parts of a defoamer, and 2-5 parts of a leveling agent; the glass transition temperature of the first waterborne polyurethane is Tg1, and the glass transition temperature of the second waterborne polyurethane is Tg2, wherein Tg1 and Tg2 are different.

[0049] It should be noted that this application uses waterborne polyurethane (including a first waterborne polyurethane and a second waterborne polyurethane) as the main component of the digital coating liquid. The first waterborne polyurethane has a low glass transition temperature, which can endow the digital coating liquid system with excellent flexibility and deformation ability, which is conducive to adapting to various types of substrates. It is mainly used to provide excellent wear resistance, weather resistance and film-forming properties. The second waterborne polyurethane has a high glass transition temperature, which is mainly used to improve the flexibility, impact resistance and chemical resistance of the coating. By controlling the difference between the glass transition temperature Tg1 of the first waterborne polyurethane and the glass transition temperature Tg2 of the second waterborne polyurethane, this application can make the high and low Tg polyurethane segments provided by the first waterborne polyurethane and the second waterborne polyurethane interpenetrate to form an interpenetrating microstructure. This can suppress the occurrence of emulsion agglomeration, sedimentation and stratification that occur when a single resin is stored for a long time, and can improve the storage stability of the digital coating liquid, thereby improving the ink adhesion and water resistance of the printed product. Furthermore, the dispersion medium in this application includes pure water, which effectively avoids gelation or solid precipitation in the digital coating solution when the solid content is high (≥10%), thereby improving the storage stability of the digital coating solution. This application also incorporates film-forming co-solvents and polar co-solvents, selected from different alcohol solvents, to form a gradient evaporation solvent system. This system sustainably maintains the solvation layer on the resin particle surface, reducing the probability of particle agglomeration. This facilitates the control of the overall coating evaporation rate and synergistically optimizes the final film quality of the digital coating solution, further improving the ink adhesion and water resistance of the printed product. This application also incorporates defoamers and leveling agents, which further improve the bonding effect between the substrate and the coating after it forms a coating, especially reducing the surface tension of the coating and thus improving its water resistance. Furthermore, the first and second waterborne polyurethanes used in this application, combined with a gradient volatile alcohol solvent, allow for slow solvent release during coating and film formation. This reduces micropores in the coating caused by rapid solvent escape and minimizes the penetration of oxygen and moisture into the oxidized resin segments within the coating. Residual initiators and small-molecule additives in the formulation gradually migrate out of the coating along with the alcohol solvent, effectively preventing yellowing during long-term storage and exposure to light, thus improving the coating's anti-yellowing properties. The digital coating liquid of this application, through the synergistic effect of its various raw material components, can simultaneously improve the storage stability of the digital coating liquid, as well as the electronic ink adhesion, coating water resistance, and anti-yellowing properties of the printed product, thereby enhancing the quality of the printed product.

[0050] In some embodiments of the first aspect of this application, the digital coating liquid satisfies at least one of the following conditions: (1) Tg1 satisfies 5℃≤Tg1≤45℃; (2) Tg2 satisfies 35℃≤Tg2≤75℃; (3) 10℃≤(Tg2-Tg1)≤50℃.

[0051] Based on the above scheme, when Tg1 satisfies 5℃≤Tg1≤45℃, the first waterborne polyurethane can provide excellent wear resistance, weather resistance, and film-forming properties, and can better synergize with the second waterborne polyurethane, further improving the substrate adaptability of the digital coating liquid, thereby improving the adhesion of electronic inks and the water resistance of the coating. When Tg2 satisfies 35℃≤Tg2≤75℃, the second waterborne polyurethane can enhance the flexibility, impact resistance, and chemical resistance of the coating, and can better synergize with the first waterborne polyurethane resin, thereby further improving the substrate adaptability of the digital coating liquid and the adhesion of electronic inks. When the difference between Tg2 and Tg1 satisfies 10℃≤(Tg2-Tg1)≤50℃, it can further promote the interpenetration of high and low Tg polyurethane segments to form an interpenetrating microstructure, better suppressing the emulsion aggregation, sedimentation, and stratification that occur during long-term storage of a single resin, and further improving the storage stability of the digital coating liquid, the ink adhesion of printed products, and the water resistance of the coating.

[0052] In some preferred embodiments of the first aspect of this application, the digital coating liquid satisfies at least one of the following conditions: (1) Tg1 satisfies 10℃≤Tg1≤30℃; (2) Tg2 satisfies 35℃≤Tg2≤60℃; (3) The difference between Tg2 and Tg1 satisfies 15℃≤(Tg2-Tg1)≤30℃.

[0053] Based on the above scheme, when the above range is met, the synergistic relationship between the first waterborne polyurethane and the second waterborne polyurethane can be better realized.

[0054] It should be noted that this application does not limit the specific type of the first waterborne polyurethane, as long as it can achieve the functions of this application. For example, in some embodiments of the first aspect of this application, the first waterborne polyurethane includes an aliphatic waterborne polyurethane dispersion or an aromatic waterborne polyurethane dispersion. Based on the above solutions, the first waterborne polyurethane can better achieve the aforementioned functions.

[0055] In some embodiments of the first aspect of this application, when the first waterborne polyurethane comprises an aliphatic waterborne polyurethane dispersion, the aliphatic waterborne polyurethane dispersion used comprises an anionic aliphatic waterborne polyurethane dispersion.

[0056] Based on the above scheme, an anionic aliphatic waterborne polyurethane dispersion is used as the main film-forming component. The polyurethane molecular backbone does not contain easily oxidized benzene ring structures, reducing the formation of quinone chromophores from the source, thereby better improving the coating's anti-yellowing performance. Furthermore, the combination with a gradient volatile alcohol solvent allows for slow solvent release during coating and film formation, reducing micropores caused by rapid solvent escape and minimizing the penetration of oxygen and moisture into the coating to oxidize resin segments, further improving the coating's water resistance. It also boasts advantages such as low odor and low VOCs.

[0057] It should be noted that this application does not limit the specific type of anionic aliphatic waterborne polyurethane dispersion, as long as it can achieve the above-mentioned effects.

[0058] Exemplary examples, in some embodiments of the first aspect of this application, the anionic aliphatic aqueous polyurethane dispersion used includes at least one of Covestro Bayhydrol® U XP 2750, Wanhua Chemical Archsol 8355, Asahi Kasei Duranate, and Zhejiang Huafeng HF-3108.

[0059] In some embodiments of the first aspect of this application, when the first waterborne polyurethane comprises an aromatic waterborne polyurethane dispersion, the aromatic waterborne polyurethane dispersion used comprises an anionic aromatic waterborne polyurethane dispersion.

[0060] Based on the above scheme, the anionic aromatic waterborne polyurethane dispersion used can, on the basis of achieving the above effects, further improve the water resistance of the formed coating.

[0061] It should be noted that this application does not limit the specific type of anionic aromatic waterborne polyurethane dispersion, as long as it can achieve the above-mentioned effects.

[0062] For example, in some embodiments of the first aspect of this application, the aromatic waterborne polyurethane dispersion used includes at least one of Asahikawa Chemical AX-168, Mitsui Chemicals WPU-228, Stepan SPU-381, and Jiangsu Xindian ND-806.

[0063] This application does not limit the specific type of the second waterborne polyurethane, as long as it can achieve the function of this application. For example, in some embodiments of the first aspect of this application, the second waterborne polyurethane includes a self-crosslinking waterborne polyurethane dispersion or a highly elastic waterborne polyurethane dispersion.

[0064] Based on the above scheme, the second waterborne polyurethane can better improve the flexibility, impact resistance and chemical resistance of the coating.

[0065] In some embodiments of the first aspect of this application, when the second waterborne polyurethane comprises a self-crosslinking waterborne polyurethane dispersion, the self-crosslinking waterborne polyurethane dispersion used includes an anionic self-crosslinking waterborne polyurethane dispersion.

[0066] Based on the above scheme, anionic self-crosslinking waterborne polyurethane dispersions are beneficial for further improving the wear resistance and scratch resistance of digital coating liquids.

[0067] It should be noted that this application does not limit the specific type of anionic self-crosslinking waterborne polyurethane dispersion, as long as it can achieve the above-mentioned effects.

[0068] For example, in some embodiments of the first aspect of this application, the anionic self-crosslinking aqueous polyurethane dispersion used includes at least one of Covestro NeoRez® R-2180 and Wanhua Chemical Archsol 5160.

[0069] In some embodiments of the first aspect of this application, when the second waterborne polyurethane comprises a highly elastic waterborne polyurethane dispersion, the highly elastic waterborne polyurethane dispersion used comprises an anionic highly elastic waterborne polyurethane dispersion.

[0070] Based on the above scheme, anionic high-elasticity waterborne polyurethane dispersions are beneficial for further improving the flexibility of digital coating liquids.

[0071] It should be noted that this application does not limit the specific type of anionic high-elasticity waterborne polyurethane dispersion, as long as it can achieve the above-mentioned effects.

[0072] Exemplary examples, in some embodiments of the first aspect of this application, include at least one of Yantai Wanhua Archsol 8357 and Covestro Impranil® DAH.

[0073] In some embodiments of the first aspect of this application, the dispersion medium includes pure water.

[0074] Based on the above scheme, it is beneficial to ensure that the dispersion medium uniformly disperses each component, which is beneficial to improving the system stability and coating performance of the digital coating liquid. It should be understood that the term "pure water" in this application refers to water that theoretically does not contain other ions or impurities, including but not limited to any form of water such as deionized water, distilled water, or ultrapure water.

[0075] In some embodiments of the first aspect of this application, the dispersion medium comprises pure water with a conductivity ≤10 μS / cm.

[0076] Based on the above scheme, this application uses pure water with a conductivity of ≤10μS / cm as the dispersion medium, which can further avoid impurities in the water, thereby further improving the system stability and coating performance.

[0077] In some embodiments of the first aspect of this application, the film-forming co-solvent and the polar co-solvent are selected from different solvents among alcohol solvents.

[0078] Based on the above scheme, it is beneficial to form a gradient evaporation solvent system with the film-forming co-solvent and polar co-solvent, the first waterborne polyurethane and the second waterborne polyurethane, and to control the overall evaporation rate of the coating. This allows the first waterborne polyurethane and the second waterborne polyurethane to gradually merge together during the slow solvent loss process, significantly optimizing the initial film density.

[0079] This application does not limit the specific type of alcohol solvent, as long as it can achieve the function of this application. For example, in some embodiments of the first aspect of this application, the alcohol solvent includes at least one selected from methanol, ethanol, n-butanol, ethylene glycol, n-propanol, isopropanol, and n-hexanol.

[0080] This application does not limit the specific type of film-forming co-solvent, as long as it can achieve the function of this application. In some embodiments of the first aspect of this application, the film-forming co-solvent includes at least one of methanol and ethanol.

[0081] Exemplarily, in some preferred embodiments of the first aspect of this application, the film-forming co-solvent is more preferably anhydrous ethanol. Based on the above scheme, this application uses anhydrous ethanol not only as a film-forming aid but also as a co-solvent to regulate the evaporation rate of the overall digital coating liquid, thereby improving the workability and initial film-forming effect of the digital coating liquid. In particular, anhydrous ethanol can synergistically form a gradient evaporation solvent system with the polar co-solvent and the first and second waterborne polyurethanes: the evaporation rate of anhydrous ethanol is higher than that of isopropanol, and it preferentially escapes from the surface of the coating in the early stage of coating, reserving the slowly evaporating isopropanol to maintain the solvent content inside the coating, thereby precisely controlling the overall evaporation rate of the coating; at the same time, ethanol can swell polyurethane resin segments, allowing resin particles to fully expand in the solvent system, and optimizing the wet film flow state in conjunction with the emulsifying components in the system, improving the compatibility of application and coating, and ensuring that resin particles can gradually approach and fuse during the slow solvent loss process, significantly optimizing the initial film density. It should be understood that the term "anhydrous ethanol" in this application includes, but is not limited to, industrial-grade anhydrous ethanol.

[0082] This application does not limit the specific type of polar co-solvent, as long as it can achieve the function of this application. For example, in some embodiments of the first aspect of this application, the polar co-solvent includes at least one of isopropanol and n-propanol.

[0083] Exemplarily, in some preferred embodiments of the first aspect of this application, the polar co-solvent includes isopropanol. Based on the above scheme, the use of isopropanol as a polar co-solvent in this application can improve the solubility and system compatibility of the resin, help reduce surface tension, and improve leveling. In particular, isopropanol can work synergistically with the film-forming co-solvent. Specifically, the polarity of isopropanol has a higher compatibility with the ester and hydroxyl groups of the polyurethane resin side chains, which can enhance the solubility of the resin in the mixed solvent system and improve the interfacial compatibility between the resin, anhydrous ethanol, and the aqueous phase. At the same time, isopropanol can reduce the overall surface tension of the system, compensate for the high surface tension of anhydrous ethanol, make the wet film spread more evenly, reduce pinholes and brush marks, and improve leveling. Combined with the fast volatility of ethanol, isopropanol, as a slow volatile component, extends the open time of the coating, providing sufficient time for resin chain migration and stacking. The two work together to achieve solvent gradient evaporation and synergistically optimize the final film quality. It should be understood that the term "isopropanol" in this application includes, but is not limited to, industrial-grade isopropanol.

[0084] This application does not limit the specific type of defoamer, as long as it can achieve the function of this application. For example, in some embodiments of the first aspect of this application, the defoamer includes a defoamer specifically for water-based coatings. It should be understood that the term "defoamer specifically for water-based coatings" in this application refers to a defoamer suitable for water-based coatings. Exemplarily, in some embodiments of the first aspect of this application, the defoamer specifically for water-based coatings includes at least one of BYK-024, BYK-025, TEGOAirex 901W, and Evonik TEGO Airex 920.

[0085] Based on the above solution, the defoamer used in this application can effectively eliminate bubbles during the production and construction process, avoid surface defects such as pinholes and fisheyes, and thus help to further improve the ink adhesion and water resistance of the coating.

[0086] This application does not limit the specific type of leveling agent, as long as it can achieve the function of this application. For example, in some embodiments of the first aspect of this application, the leveling agent includes a leveling agent specifically for water-based systems. It should be understood that the term "leveling agent specifically for water-based systems" in this application refers to a leveling agent suitable for water-based coatings. Exemplarily, in some embodiments of the first aspect of this application, the leveling agent specifically for water-based systems includes at least one of BYK-333, BYK-349, Evonik TEGOGlide 450, and TEGO Wet 270.

[0087] Based on the above scheme, the leveling agent used in this application can reduce the surface tension of the coating, improve leveling and substrate wetting, and enhance the smoothness and gloss of the coating, thereby further improving the ink adhesion and anti-yellowing performance of the coating.

[0088] Secondly, this application provides a method for preparing a digital coating liquid.

[0089] In this application, the method for preparing the digital coating liquid includes the following steps: Step 1: Weigh the corresponding raw materials according to the following proportions: 20-35 parts of first waterborne polyurethane, 30-40 parts of second waterborne polyurethane, 10-30 parts of dispersion medium, 6-9 parts of film-forming co-solvent, 4-8 parts of polar co-solvent, 2-5 parts of defoamer, and 2-5 parts of leveling agent.

[0090] In step 1 of this application, the corresponding mass fractions of each preparation raw material are weighed and set aside according to actual needs. It should be understood that the preparation raw materials used in the preparation method of the second aspect of this application are prepared based on any of the preparation raw materials of the digital coating liquid in the first aspect, and possess all the beneficial effects brought about by its components and their proportions in the first aspect, so this application will not elaborate further here.

[0091] Step 2, prepare the mixed solvent: The weighed dispersion medium, film-forming co-solvent, and polar co-solvent are mixed evenly to obtain a mixed solvent.

[0092] In step 2 of this application, by first mixing the dispersion medium, film-forming co-solvent, and polar co-solvent evenly to form a uniform solvent system, it is possible to promote the uniform dispersion of other components, especially to better and more uniformly disperse the first waterborne polyurethane and the second waterborne polyurethane in the mixed solvent.

[0093] In step 3 of this application, there is no limitation on the specific mixing method, as long as it enables the dispersion medium, film-forming co-solvent, and polar co-solvent to form a homogeneous solvent system. For example, in some embodiments of the second aspect of this application, the dispersion method used includes, but is not limited to, any one of stirring and ultrasonication.

[0094] Step 3, prepare the premix: The weighed first and second waterborne polyurethanes are dispersed in the mixed solvent to obtain a premix.

[0095] In step 3 of this application, there is no limitation on the specific dispersion method, as long as it can uniformly disperse the first waterborne polyurethane and the second waterborne polyurethane in the mixed solvent. For example, in some embodiments of the second aspect of this application, the dispersion method used includes, but is not limited to, any one of stirring and ultrasonication.

[0096] Step 4, stirring process: The weighed defoamer and leveling agent are added to the premix and stirred to obtain the digital coating liquid.

[0097] In some embodiments of the second aspect of this application, the stirring rate of the stirring process is 300 r / min to 800 r / min, the stirring temperature is 20℃ to 25℃, and the stirring time is 1 h to 6 h. Based on the above scheme, it is beneficial to mix the components more uniformly and form a digital coating liquid with uniform components.

[0098] Thirdly, this application provides a coating structure.

[0099] In this application, the coating structure includes: Surface layer; A coating liquid layer is disposed on one surface of the face material layer; An electrostatic ink layer is disposed on the surface of the coating liquid layer away from the face material layer; The coating liquid layer includes any digital coating liquid of the first aspect, or a digital coating liquid prepared by any preparation method of the second aspect.

[0100] It should be understood that the coating structure of the third aspect of this application includes any digital coating liquid of the first aspect, or digital coating liquid prepared by any preparation method of the second aspect. Therefore, it has all the beneficial effects brought about by its components and their proportions in the first aspect, or all the beneficial effects brought about by the preparation method itself in the second aspect. Therefore, this application will not elaborate further.

[0101] In some embodiments of the third aspect of this application, the material of the face layer includes at least one of polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polypropylene (PP).

[0102] The technical solutions in this application will now be described clearly and in detail with reference to the accompanying drawings.

[0103] (a) Visual inspection of the coating liquid: The appearance of the coating liquid to be tested was observed using a visual inspection method.

[0104] (ii) Physical property testing: The coating liquid to be tested was tested according to the test method of GB / T 13217.5-2008: the coating liquid to be tested was baked at 80℃ for 1 hour, then removed and cooled to room temperature, and its physical properties were tested, including the following: 1. Macroscopic appearance inspection: Visually inspect for obvious discoloration (such as yellowing). If there is obvious discoloration, it indicates poor performance stability and poor resistance to yellowing.

[0105] 2. Film-forming property test: Touch the surface of the printed sample after baking to see if it feels sticky. If it feels sticky, it indicates poor film-forming property.

[0106] 3. Bending resistance test: The test shall be conducted in accordance with the test method in GB / T 6742-2007 "Paints and Varnishes - Bending Test (Cylindrical Shaft)" using a standard mandrel with a diameter of 8mm. Observe whether there are cracks or peeling in the coating after bending. If there are no cracks or peeling, the bending resistance is considered to be good.

[0107] (III) Storage stability test: 1) Store the coating solution to be tested at a constant temperature of 50℃ for 30 days, and record the state changes at different storage times. Observe whether there are obvious problems such as stratification, precipitation, or viscosity increase in the coating solution after storage. If there are none, it indicates that the storage stability is good.

[0108] 2) Store the coating solution to be tested in a sealed container at room temperature for 12 months, and record the changes in its state at different storage times. Observe whether there are any obvious problems such as stratification, precipitation, or viscosity increase in the coating solution after storage. If there are none, it indicates that the storage stability is good.

[0109] (iv) Ink adhesion test (100-cross pattern): The test was conducted according to the cross-cut test method in the national standard GB / T 13217.7-2023 "Test Method for Ink Adhesion". Among the test results, level 0 is the best and level 5 is the worst.

[0110] (v) Testing of substrate adaptability: According to the cross-cut adhesion test method in the national standard GB / T 13217.7-2023 "Ink Adhesion Test Method", the ink adhesion of thermal paper, PVC, PET, PP and gray scale substrates were tested. When the test result of ink adhesion was level 0 or level 1, the substrate was deemed to be compatible.

[0111] (vi) Test for the anti-yellowing properties of printed samples: The printed sample to be tested was aged under a xenon lamp for 168 hours to obtain the aged sample. Visual inspection was performed to check for obvious discoloration (such as yellowing) in the aged sample; obvious discoloration indicated poor resistance to yellowing.

[0112] (vii) Ink adhesion performance test of printed samples: The core principle of ink adhesion testing is to evaluate the ability of electronic ink to prevent it from peeling off the substrate after transfer printing. The testing steps include: 1) Sample preparation: Take the printed sample to be tested.

[0113] 2) Adhesion test: Use adhesive tape to adhere to the ink layer, push it with your hand to release air bubbles, stress and gaps between the adhesive tape and the ink layer, let it stand for 1 minute, and then quickly pull it out.

[0114] 3) On the printed surface material: observe the pulling area and the ink peeling.

[0115] 4) Result Evaluation Criteria: Advantages: Ink loss <5%, overall pattern has a bright color, and there is no difference between the pulled and non-pulled areas.

[0116] Good: Ink loss is between 5% and 30%, the pattern integrity is acceptable, and the bottom of the surface material can be seen in the pulled area at a specific angle.

[0117] Poor: Ink loss >30%, pattern is obviously damaged, integrity is poor, and the bottom of the face material can be clearly seen in the pulled area, i.e., severe "through".

[0118] (viii) Water resistance test of printed samples: The core principle of water resistance testing is to evaluate the ability of electronic ink to prevent it from peeling off the substrate after transfer printing. The test steps include: 1) Sample preparation: Take the printed sample to be tested.

[0119] 2) Soaking the sample: Soak the printed sample to be tested in deionized water for 2 hours, then wipe it dry.

[0120] 3) Adhesion test: Use adhesive tape to adhere to the ink layer, push it with your hand to release air bubbles, stress and gaps between the adhesive tape and the ink layer, let it stand for 1 minute, and then quickly pull it out.

[0121] 4) On the printed surface material: observe the pulling area and the ink peeling.

[0122] 5) Result Evaluation Criteria: Advantages: Ink loss <5%, overall pattern has a bright color, and there is no difference between the pulled and non-pulled areas.

[0123] Good: Ink loss is between 5% and 30%, the pattern integrity is acceptable, and the bottom of the surface material can be seen in the pulled area at a specific angle.

[0124] Poor: Ink loss >30%, pattern is obviously damaged, integrity is poor, and the bottom of the face material can be clearly seen in the pulled area, i.e., severe "through".

[0125] Example 1 This embodiment provides a digital coating liquid, and its preparation method includes the following steps: Step 1: Weigh the corresponding raw materials according to the following proportions: 30 parts of first waterborne polyurethane, 35 parts of second waterborne polyurethane, 20 parts of dispersion medium, 8 parts of film-forming cosolvent, 6 parts of polar cosolvent, 3 parts of defoamer, and 3 parts of leveling agent.

[0126] In this embodiment, the first waterborne polyurethane used is Zhejiang Huafeng HF-3108 (Tg1 is 25℃), the second waterborne polyurethane used is Mitsui Chemicals WPU-228 (Tg2 is 50℃), the dispersion medium used is deionized water, the film-forming cosolvent is anhydrous ethanol, the polar additive is isopropanol, the defoamer is BYK-025 from BYK Chemicals, and the leveling agent is BYK-333 from BYK Chemicals.

[0127] Step 2, prepare the mixed solvent: Weigh out the dispersion medium, film-forming co-solvent and polar co-solvent from step 1 above and mix them evenly to obtain a mixed solvent.

[0128] Step 3, prepare the premix: The weighed first and second waterborne polyurethanes from step 1 are dispersed in the mixed solvent obtained in step 2 to obtain a premix.

[0129] Step 4, stirring process: Add the defoamer and leveling agent weighed in step 1 to the premix obtained in step 3, and stir at a stirring rate of 500 r / min at room temperature (25℃) for 2 hours to obtain the digital coating liquid.

[0130] Example 2 The only difference from Example 1 is that Tg1 is 30°C, Tg2 is 45°C, and Tg2-Tg1 is 15°C.

[0131] Example 3 The only difference from Example 1 is that Tg1 is 10°C, Tg2 is 60°C, and Tg2-Tg1 is 50°C.

[0132] Example 4 The only difference from Example 1 is that Tg1 is 5°C, Tg2 is 35°C, and Tg2-Tg1 is 30°C.

[0133] Example 5 The only difference from Example 1 is that Tg1 is 25°C, Tg2 is 35°C, and Tg2-Tg1 is 10°C.

[0134] Example 6 The only difference from Example 1 is that Tg1 is 45°C, Tg2 is 75°C, and Tg2-Tg1 is 30°C.

[0135] Comparative Example 1 The only difference between this comparative example and Example 1 is that: The coating liquid used is F7420 model coating liquid from Hainan Bikai Waterborne New Materials Co., Ltd.

[0136] Comparative Example 2 The only difference from Example 1 is that only the first waterborne polyurethane is used, i.e., Tg1 is 25°C.

[0137] Comparative Example 3 The only difference from Example 1 is that only the second waterborne polyurethane is used, i.e., Tg2 is 50°C.

[0138] Table 1 shows the appearance and physical property test results of the digital coating liquid of Example 1 and the coating liquid of Comparative Example 1.

[0139] Table 1. Results of Appearance and Physical Properties Tests of the Coating Liquid

[0140] According to the test results in Table 1, compared with the existing coating liquid of Comparative Example 1, the coating liquid of this application has better storage stability and anti-yellowing performance, and is suitable for the warehousing and transportation needs after industrial mass production.

[0141] Table 2 shows the test results of ink cross-cut adhesion and substrate compatibility of the coating liquids obtained in Examples 1 to 6, as well as Comparative Examples 2 and 3.

[0142] Table 2. Test results of substrate compatibility and ink cross-cut adhesion.

[0143] According to the test results in Table 2, Example 1 exhibits both excellent ink adhesion and substrate compatibility. Compared to Example 1, in Examples 2 and 5, the decrease in Tg difference leads to a convergence of the properties of the two polyurethane resins, weakening the flexibility and rigidity reinforcement effects, thus resulting in a decline in overall performance. Compared to Example 1, in Examples 3, 4, and 6, the increase in Tg difference leads to a decrease in the compatibility of the two polyurethane resins, slight phase separation at the microscopic level, and poor film uniformity, thus reducing the coating adhesion and substrate compatibility. Compared to Example 1, the test results of Examples 4-6 and Comparative Examples 2 and 3 show that excessively small or large Tg1 and Tg2 also affect the coating adhesion and reduce substrate compatibility.

[0144] In summary, when the difference between the glass transition temperature (Tg2) of the second waterborne polyurethane and the glass transition temperature (Tg1) of the first waterborne polyurethane satisfies 10℃≤(Tg2-Tg1)≤50℃, the high and low Tg polyurethane segments provided by the first and second waterborne polyurethanes can interpenetrate to form an interpenetrating microstructure. This can suppress emulsion aggregation, sedimentation, and stratification that occur during long-term storage of a single resin, improving the storage stability of the digital coating liquid, thereby enhancing the ink adhesion and water resistance of the printed product. In particular, only when Tg1 satisfies 5℃≤Tg1≤45℃, Tg2 satisfies 35℃≤Tg2≤75℃, and the difference between Tg2 and Tg1 satisfies 15℃≤(Tg2-Tg1)≤30℃, can both wide substrate adaptability and excellent ink adhesion be simultaneously achieved.

[0145] Application Examples This application uses the digital coating liquid of Example 1 and the coating liquid of Comparative Example 1 as examples to prepare printed samples with corresponding coating structures according to the following steps: 1) Surface activation treatment: Thermal paper, polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polypropylene (PP) were used as surface layers. The surface layers were corona treated using a corona machine. The process parameters for the corona treatment were: working frequency of 20 kHz, discharge voltage of 20 kV, electrode-grounding roller discharge gap of 1.5 mm, power density of 400 W / m, and target density of 34. This was done to activate the surface of the surface layer to facilitate the subsequent coating of digital coating liquid and electrostatic ink.

[0146] When the substrate is thermal paper and polypropylene (PP), corona treatment is performed only with normal corona power (400W / m). When the substrate is polyethylene terephthalate (PET) and polyvinyl chloride (PVC), corona treatment is performed with normal corona power (400W / m) and increased corona power (800W / m), respectively.

[0147] 2) Formation of the coating liquid layer: The coating process is carried out by rotary coating, in which the digital coating liquid of this embodiment is applied to the surface of the face material layer with a coating thickness of 1.0 μm, and then dried at 55°C to form a coating liquid layer on the surface of the face material layer.

[0148] 3) Forming an electronic ink layer: Using an HP Indigo 6K digital printer, the particle charge of the ink system was controlled as follows: a stable negative charge of 300 μC / g, and a constant temperature of 25℃ in the feed tank; electrostatic field parameters: PIP charging voltage of 900V, developing BID bias voltage of 600V, blanket transfer high voltage of 18kV, blanket heating temperature of 120℃; impression roller substrate temperature of 40℃; and printing speed of 30m / min. Electronic ink was printed onto the surface of the coating liquid layer to form an electronic ink layer on the coating liquid layer surface, resulting in an ink with the following properties: Figure 1 The printed sample with the coating structure shown.

[0149] The electronic ink used in this application is the electronic ink that comes with the HP Indigo 6K digital printer.

[0150] This application also tested the electronic ink adhesion and coating water resistance of printed samples obtained with different substrates in Example 1 and Comparative Example 1, and tested the anti-yellowing performance of printed samples obtained with PVC substrate under normal corona power. The test results are shown in Table 3 below.

[0151] Table 3 Performance test results of printed samples

[0152] In Table 3, "normal corona" indicates a printed sample prepared with a corona power of 400W, "increased corona" indicates a printed sample prepared with a corona power of 800W, and "-" indicates no test.

[0153] According to the test results in Table 3, compared with the conventional coating liquid of Comparative Example 1, the coating liquid of this application has a wider range of substrate adaptability, electronic ink adhesion, and water resistance. In particular, it is more suitable for topcoat layers made of materials such as polyethylene terephthalate, polyvinyl chloride, and polypropylene. That is, compared with conventional coating liquids, the printed products obtained based on the digital coating liquid of this application can improve the electronic ink adhesion, coating water resistance, and anti-yellowing performance of the printed products, thereby improving the quality of the printed products.

[0154] In summary, the digital coating liquid of this application, through the synergistic effect between the various raw material components, can improve the storage stability of the digital coating liquid, as well as the electronic ink adhesion, coating water resistance, and anti-yellowing performance of the printed product, thereby improving the quality of the printed product.

[0155] Obviously, the above embodiments of this application are merely examples for clear illustration and are not intended to limit the implementation of this application. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection claimed by this application.

Claims

1. A digital coating liquid, characterized in that, The raw materials for preparing the digital coating liquid, by weight, include: 20-35 parts of first waterborne polyurethane, 30-40 parts of second waterborne polyurethane, 10-30 parts of dispersion medium, 6-9 parts of film-forming cosolvent, 4-8 parts of polar cosolvent, 2-5 parts of defoamer, and 2-5 parts of leveling agent; The glass transition temperature of the first waterborne polyurethane is Tg1, and the glass transition temperature of the second waterborne polyurethane is Tg2, wherein Tg1 and Tg2 are different.

2. The digital coating liquid according to claim 1, characterized in that, The digital coating liquid meets at least one of the following conditions: (1) Tg1 satisfies 5℃≤Tg1≤45℃; (2) Tg2 satisfies 35℃≤Tg2≤75℃; (3) The difference between Tg2 and Tg1 satisfies 10℃≤(Tg2-Tg1)≤50℃.

3. The digital coating liquid according to claim 1, characterized in that, The digital coating liquid meets at least one of the following conditions: (1) Tg1 satisfies 10℃≤Tg1≤30℃; (2) Tg2 satisfies 35℃≤Tg2≤60℃; (3) The difference between Tg2 and Tg1 satisfies 15℃≤(Tg2-Tg1)≤30℃.

4. The digital coating liquid according to claim 1, characterized in that, The first waterborne polyurethane includes an aliphatic waterborne polyurethane dispersion or an aromatic waterborne polyurethane dispersion; The second waterborne polyurethane includes a self-crosslinking waterborne polyurethane dispersion or a highly elastic waterborne polyurethane dispersion.

5. The digital coating liquid according to claim 4, characterized in that, The aliphatic waterborne polyurethane dispersion includes anionic aliphatic waterborne polyurethane dispersions.

6. The digital coating liquid according to claim 4, characterized in that, The aromatic waterborne polyurethane dispersion includes anionic aromatic waterborne polyurethane dispersions.

7. The digital coating liquid according to claim 4, characterized in that, The self-crosslinking waterborne polyurethane dispersion includes anionic self-crosslinking waterborne polyurethane dispersion.

8. The digital coating liquid according to claim 4, characterized in that, The highly elastic waterborne polyurethane dispersion includes anionic highly elastic waterborne polyurethane dispersion.

9. The digital coating liquid according to claim 1, characterized in that, The dispersion medium includes pure water.

10. The digital coating liquid according to claim 9, characterized in that, The dispersion medium includes pure water with a conductivity ≤10μS / cm.

11. The digital coating liquid according to claim 1, characterized in that, The film-forming co-solvent and the polar co-solvent are selected from different solvents among alcohol solvents.

12. The digital coating liquid according to claim 11, characterized in that, The alcohol solvent includes at least one of methanol, ethanol, n-butanol, ethylene glycol, n-propanol, isopropanol, and n-hexanol.

13. The digital coating liquid according to claim 12, characterized in that, The film-forming co-solvent includes at least one of methanol and ethanol; The polar cosolvent includes at least one of isopropanol and n-propanol.

14. A method for preparing the digital coating liquid according to any one of claims 1 to 13, characterized in that, The preparation method includes: Step 1: Weigh the corresponding raw materials according to the following proportions: 20-35 parts of first waterborne polyurethane, 30-40 parts of second waterborne polyurethane, 10-30 parts of dispersion medium, 6-9 parts of film-forming cosolvent, 4-8 parts of polar cosolvent, 2-5 parts of defoamer, and 2-5 parts of leveling agent; Step 2, prepare the mixed solvent: The weighed dispersion medium, film-forming co-solvent, and polar co-solvent are mixed evenly to obtain a mixed solvent; Step 3, prepare the premix: The weighed first and second waterborne polyurethanes are dispersed in the mixed solvent to obtain a premix. Step 4, stirring process: The weighed defoamer and leveling agent are added to the premix and stirred to obtain the digital coating liquid.

15. The preparation method according to claim 14, characterized in that, The stirring rate of the stirring process is 300 r / min to 800 r / min, the stirring temperature is 20℃ to 25℃, and the stirring time is 1 h to 6 h.

16. A coating structure, characterized in that, The coating structure includes: Surface layer; A coating liquid layer is disposed on one surface of the face material layer; An electrostatic ink layer is disposed on the surface of the coating liquid layer away from the face material layer; The coating liquid layer includes the digital coating liquid according to any one of claims 1 to 13, or the digital coating liquid prepared by the preparation method according to any one of claims 14 to 15.

17. The coating structure according to claim 16, characterized in that, The material of the surface layer includes at least one of polyethylene terephthalate, polyvinyl chloride, and polypropylene.

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