A method for preparing an ink-absorbing coating for photographic paper comprising nanobohmite powder

Abstract

The application provides a preparation method of a photo paper ink absorption coating containing nanoboehmite powder, and comprises the following steps: preparing nanoboehmite powder; dispersing the nanoboehmite powder into a solvent to obtain a filler dispersion liquid; preparing an adhesive solution by using an adhesive; mixing the filler dispersion liquid and the adhesive solution to obtain a mixed slurry; coating the mixed slurry on photo paper and performing solidification to obtain the photo paper ink absorption coating; wherein the nanoboehmite powder is prepared by using weak base, industrial aluminum salt and weak acid as raw materials, performing a low-temperature hydrothermal crystallization reaction at 120-140 DEG C in the presence of a surface modifier after two-step pH adjustment, and the nanoboehmite powder has uniform particle size distribution, high specific surface area and high dispersity; the photo paper ink absorption coating prepared by using the nanoboehmite powder as filler has fast ink absorption speed, uniform ink diffusion, no ink floating phenomenon and high gloss.

Description

TECHNICAL FIELD

[0001] The present application relates to a preparation method of a photographic paper ink absorption coating, and particularly relates to a preparation method of a photographic paper ink absorption coating containing nanometer boehmite powder. BACKGROUND

[0002] The photographic paper ink absorption coating is a key functional layer on the surface of an inkjet printing photographic paper. Its main working principle is to quickly capture and fix the ink sprayed on the surface of the photographic paper in the inkjet printing process through the adsorption of the porous structure and functional fillers in the coating. Specifically, when the ink contacts the surface of the coating, the porous fillers in the coating quickly absorb the solvent components in the ink through capillary action, and the three-dimensional network structure formed by the fillers and the binder limits the migration of the pigment particles, thereby realizing the rapid solidification and accurate positioning of the ink and ensuring the clarity and color stability of the printed image.

[0003] However, the existing photographic paper ink absorption coating still has problems such as insufficient ink adsorption capacity, easy ink floating and bleeding in actual application, which seriously affects the color and durability of the image. SUMMARY

[0004] In view of the above problems of the prior art, the present application provides a preparation method of a photographic paper ink absorption coating containing nanometer boehmite powder, which comprises:

[0005] The present application provides a preparation method of a photographic paper ink absorption coating containing nanometer boehmite powder, which comprises:

[0006] Preparation of nanometer boehmite powder;

[0007] Disperse the nanometer boehmite powder into a solvent to obtain a filler dispersion liquid;

[0008] Prepare an adhesive solution with an adhesive;

[0009] Mix the filler dispersion liquid and the adhesive solution to obtain a mixed slurry;

[0010] Coat the mixed slurry on the photographic paper and perform solidification to obtain a photographic paper ink absorption coating;

[0011] The preparation of nanometer boehmite powder comprises the following steps:

[0012] Step A: Add an aluminum salt solution to a reaction kettle; the concentration of the aluminum salt solution is 10-20wt%;

[0013] Step B: Add an ammonia solution to the aluminum salt solution at a rate of 50-100g / min until the pH value is 9-11 and maintain for 1h; the concentration of the ammonia solution is 5-15wt%;

[0014] Step C: Add a solution containing a weak acid to the solution obtained in step B at a rate of 20-50 g / min until the pH value is 5-6, and maintain for 30 min;

[0015] Step D: Add a surface modifier to the solution obtained in step C;

[0016] Step E: Transfer the liquid reactants obtained in step D into a crystallization kettle, control the crystallization temperature at 120-140℃, and the crystallization time at 2-8 hours to obtain a crystallization solution;

[0017] Step F: The crystallization solution is washed with water to remove impurities and then spray-dried to obtain nano-boehmite powder.

[0018] Furthermore, the aluminum salt is aluminum sulfate; the weak acid is acetic acid.

[0019] Furthermore, the surface modifier is one or more of polyacrylic acid, polyethylene glycol, hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate, and the amount of surface modifier used is 0.1-1 wt% of the mass of the solution obtained in step C.

[0020] Furthermore, the surface modifier is a polar surface modifier, and the adhesive is a polar adhesive.

[0021] Furthermore, the surface modifier is polyacrylic acid, and the adhesive is polyvinyl alcohol.

[0022] Furthermore, the mass ratio of nanoboehmite powder to binder is 10:1 - 15:1.

[0023] Furthermore, the surface modifier is an azophenyl group-modified polyacrylic acid derivative; the method also includes irradiating the photo paper ink-absorbing coating with ultraviolet light before using the photo paper ink-absorbing coating.

[0024] Furthermore, adding a surface modifier to the solution obtained in step C includes: adding a surface modifier to the solution obtained in step C and stirring at 150-250 rpm for 20-40 minutes at a temperature of 30-50°C.

[0025] Further, the liquid reactant obtained in step D is transferred to a crystallization vessel, and the crystallization temperature is controlled at 120-140℃ for 2-8 hours to obtain a crystallization solution. This includes: transferring the liquid reactant obtained in step D into a crystallization vessel, pre-crystallizing at 80-100℃ for 1-2 hours, and then increasing the temperature to 120-140℃ at a rate of 5℃ / min for 4 hours of crystallization.

[0026] Furthermore, mixing the filler dispersion and the binder includes: slowly adding the binder solution to the filler dispersion while stirring at low speed.

[0027] In this embodiment, weak alkali, industrial aluminum salt, and weak acid are used as raw materials. After two-step pH adjustment, a low-temperature hydrothermal crystallization reaction is carried out at 120-140℃ in the presence of a surface modifier to obtain nano-boehmite powder with uniform particle size distribution, high specific surface area, and high dispersibility. The ink-absorbing coating of photographic paper prepared with this nano-boehmite powder as a filler has fast ink absorption speed, uniform ink diffusion, no ink floating phenomenon, and high gloss. Attached Figure Description

[0028] Figure 1 The image shows the XRD pattern of boehmite powder from Example 1, where the horizontal axis represents the diffraction angle and the vertical axis represents the absorbance.

[0029] Figure 2 The image shows the XRD pattern of boehmite powder from Example 2, where the horizontal axis represents the diffraction angle and the vertical axis represents the absorbance.

[0030] Figure 3 The image shows the XRD pattern of boehmite powder from Example 3, where the horizontal axis represents the diffraction angle and the vertical axis represents the absorbance.

[0031] Figure 4 The image shows the XRD pattern of boehmite powder in Comparative Example 1, where the horizontal axis represents the diffraction angle and the vertical axis represents the absorbance.

[0032] Figure 5 The image shows the XRD pattern of boehmite powder in Comparative Example 2, where the horizontal axis represents the diffraction angle and the vertical axis represents the absorbance.

[0033] Figure 6 The image shows the XRD pattern of boehmite powder in Comparative Example 3, where the horizontal axis represents the diffraction angle and the vertical axis represents the absorbance.

[0034] Figure 7 The image shows the XRD pattern of boehmite powder in Comparative Example 4, where the horizontal axis represents the diffraction angle and the vertical axis represents the absorbance.

[0035] Figure 8 The effect of the holding time in step B on the pore volume of boehmite.

[0036] Figure 9 The effect of the feeding rate in step B on the boehmite particle size.

[0037] Figure 10 The effect of the holding time in step C on the boehmite particle size.

[0038] Figure 11 The effect of the final pH value in step C on the pore volume of boehmite.

[0039] Figure 12 The effect of pH at the end of step C on boehmite particle size.

[0040] Figure 13 This is a SEM image of the boehmite powder from Example 1.

[0041] Figure 14 This is a SEM image of the boehmite powder from Example 3.

[0042] Figure 15 The image shows the SEM image of boehmite powder from Comparative Example 1.

[0043] Figure 16 The image shows the SEM image of boehmite powder from Comparative Example 3.

[0044] Figure 17 The image shows the SEM image of boehmite powder from Comparative Example 4. Detailed Implementation

[0045] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0046] This application provides a method for preparing a photographic paper ink-absorbing coating containing nano-boehmite powder, comprising:

[0047] Step S11: Prepare nano-boehmite powder.

[0048] Step S11 includes steps A through F.

[0049] Step A: Add aluminum salt solution to the reaction vessel; the concentration of the aluminum salt solution is 10-20 wt%.

[0050] The aluminum salt solution used in step A can be one or more of aluminum sulfate, aluminum nitrate, and aluminum chloride. After dissolving the aluminum salt to prepare an aqueous solution with a concentration of 10-20 wt%, it is added to the reaction vessel to provide a source of aluminum ions for subsequent reactions.

[0051] Step B: Add ammonia solution to the aluminum salt solution at a rate of 50-100 g / min until the pH value is 9-11, and maintain for 1 h; the concentration of ammonia solution is 5-15 wt%.

[0052] Step B is the neutralization and precipitation stage to obtain the boehmite precursor. The specific operation can be carried out as follows: After adding the aluminum salt solution to the reaction vessel, stir at a speed of 200-350 rpm under the temperature of 20-40℃. At the same time, add an ammonia solution with a concentration of 5-15wt% to the aluminum salt solution at a rate of 50-100 g / min until the pH value of the reaction system reaches 9-11, and then maintain this state for 1 hour.

[0053] Under these conditions, aluminum ions can be completely precipitated and transformed into boehmite precursors, effectively preventing the formation of gibbsite. At the same time, the alkaline environment helps to improve the porosity of boehmite, increasing the specific surface area and pore volume of the final product.

[0054] Through extensive experimentation, the inventors discovered that controlling the final pH value to 9-11 is crucial in step B. Excessively high pH leads to the formation of gibbsite, affecting product purity and reducing its dispersibility. Furthermore, excessively high pH results in high reactant aggregation, making it impossible to completely disperse the reactants when added as a weak acid in subsequent steps, ultimately leading to a larger final product particle size and a wider particle size distribution.

[0055] Through extensive experimentation, the inventors discovered that in step B, using an appropriate concentration of ammonia and controlling the ammonia addition rate at 50-100 g / min are crucial. This is because if the addition rate is too fast, it will cause a sudden increase in the local pH of the reaction system, forming highly alkaline micro-regions. This leads to rapid and large-scale precipitation of aluminum ions locally, easily generating gibbsite impurities. At the same time, the precipitated particles agglomerate severely due to excessively rapid growth, resulting in a wider precursor particle size distribution. Conversely, if the addition rate is too slow, it will prolong the neutralization reaction time, reduce production efficiency, and may lead to incomplete precipitation of aluminum ions due to insufficient local reaction, affecting the yield and purity of the final product. Figure 9 The effect of feeding rate on the average particle size of boehmite is shown.

[0056] The duration of step B also affects the pore volume of boehmite, such as... Figure 8 As shown.

[0057] Step C: Add a solution containing a weak acid to the solution obtained in step B at a rate of 20-50 g / min until the pH value is 5-6, and maintain for 30 min.

[0058] The weak acid used in step C can be one or more of formic acid, acetic acid, and acrylic acid. First, prepare these weak acids into a 2-5 wt% aqueous solution, and then add them to the reaction solution obtained in step B at a rate of 20-50 g / min. Control the pH value at the end of the reaction to be between 5 and 6, and maintain this state for 30 min.

[0059] A weak acid is gradually added to the solution obtained in step B, causing the boehmite precursor seed crystals to gradually hydrolyze, inhibiting excessive seed crystal growth, and resulting in precursor seed crystals with uniform particle size. During this process, the weak acid is added slowly, allowing for a slow and thorough reaction with the precursor seed crystals. This helps reduce the precursor particle size and increase its surface energy, significantly raising the system's free energy. This allows the material to transform into boehmite at a lower temperature, which is a key method for lowering the crystallization temperature.

[0060] In step C, if a strong acid is used, the high concentration and strong reactivity of hydrogen ions released by the strong acid will cause a rapid and violent reaction with the precursor seed crystals, resulting in a sharp drop in the pH value of the system. This will disrupt the slow hydrolysis equilibrium of the seed crystals, not only causing violent agglomeration of the precursor particles, but also leading to incomplete crystal transformation due to the excessively violent reaction, generating non-boehmite phase impurities. At the same time, the excessively acidic environment will severely damage the pore structure, significantly reducing the specific surface area and pore volume of the product.

[0061] Through extensive experimentation, the inventors discovered that the final pH value in step C is crucial. If the pH is too high, the weak acid cannot fully react with the precursor, leading to excessively large particles and a slower phase transition rate. Conversely, a low pH results in excessively high ion concentrations, triggering secondary aggregation. Furthermore, an overly acidic environment affects pore volume growth, reducing the product's oil absorption value and hindering the preparation of ink-absorbing coating layers. The effects of the final pH value in step C on the pore volume and average particle size of boehmite are as follows: Figure 11 and Figure 12 As shown.

[0062] If the weak acid is added too quickly, the local pH value will drop rapidly, causing the precursor seed crystals in that area to react excessively while other areas react insufficiently. This results in uneven particle size and a wider particle size distribution. Furthermore, the rapidly added acid cannot fully contact the precursor seed crystals, making it difficult to achieve uniform particle size control. Ultimately, this affects the dispersibility and ink absorption performance of the boehmite powder.

[0063] Step C, the duration of the treatment, also affects boehmite, such as... Figure 10 As shown.

[0064] Step D: Add a surface modifier to the solution obtained in step C.

[0065] The surface modifiers used in step D include one or more of polyacrylic acid, polyethylene glycol, hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate. When added to the solution obtained in step C, the surface modifiers can bind to the hydroxyl groups on the boehmite surface, coating the powder and preventing agglomeration. This improves the uniformity of the final product's particle size, resulting in a more concentrated particle size distribution. Simultaneously, the improved agglomeration state also enhances the powder's dispersibility.

[0066] Step E: Transfer the liquid reactants obtained in step D into a crystallization kettle, control the crystallization temperature at 120-140℃, and the crystallization time at 2-8h to obtain a crystallized liquid.

[0067] Step E involves transferring the liquid reactants obtained in step D into a crystallization vessel, controlling the crystallization temperature at 120-140℃, and the crystallization time at 2-8 hours, thereby obtaining a crystallized liquid.

[0068] Step F: The crystallization solution is washed with water to remove impurities and then spray-dried to obtain nano-boehmite powder.

[0069] Industrially, boehmite is typically produced using hydrothermal synthesis. Industrial hydrothermal synthesis of boehmite usually involves aluminum salts + sodium hydroxide or nitric acid, sulfuric acid, or hydrochloric acid + sodium aluminate. The hydrothermal synthesis temperature is typically 160℃ or higher, and the synthesis time varies from 6 to 24 hours. Boehmite synthesized in this way has a high number of hydroxyl groups on its surface, making it extremely prone to agglomeration, resulting in a wide particle size distribution and inconsistent particle sizes. Furthermore, while this preparation method is simple to operate, the production process uses large amounts of strong acids and alkalis, which are highly corrosive and pose significant hazards to personnel, equipment, and the environment. Additionally, the high-temperature, long-duration hydrothermal synthesis process consumes a lot of energy, making it difficult to control production costs.

[0070] The inventors of this application have precisely controlled and scientifically designed the reaction process, thus enabling the preparation of nano-boehmite powder with concentrated particle size distribution, high specific surface area, and high dispersibility. The inventors discovered that the combination of rate control, pH endpoint, and maintenance time in the two-step pH adjustment steps plays a decisive role in the product properties. Regarding particle size distribution control, step B, by limiting the ammonia concentration to 5-15 wt% and controlling the addition rate at 50-100 g / min, avoids the formation of gibbsite impurities and particle agglomeration caused by sudden local pH increases. Simultaneously, the endpoint pH value is strictly controlled at 9-11 to ensure uniform precipitation of aluminum ions as boehmite precursors. Step C further adds a 2-5 wt% weak acid solution at a rate of 20-50 g / min to a pH value of 5-6, inhibiting excessive seed crystal growth through slow hydrolysis and forming a precursor with uniform particle size. The combination of these two steps effectively narrows the particle size distribution. Regarding the improvement of high specific surface area and dispersibility, the alkaline environment in step B lays the foundation for the porous structure of boehmite and reduces agglomeration; the weak acid regulation in step C lowers the crystallization temperature and retains more pores; the surface modifier added in step D further prevents agglomeration and improves dispersion performance by binding with the hydroxyl groups on the surface of boehmite, ultimately achieving high specific surface area and good dispersibility.

[0071] The nano-boehmite powder prepared in this application has a concentrated particle size distribution, which ensures a uniform coating structure and avoids localized ink absorption differences. Its high specific surface area and porous structure enable rapid ink adsorption and locking of pigment molecules, reducing smudging and penetration. Furthermore, its high dispersibility ensures uniform powder distribution within the coating, forming a continuous and stable ink-absorbing network, thus improving the printing clarity and drying speed of photographic paper. Therefore, the nano-boehmite powder prepared in this application is highly suitable as a filler for photographic paper ink-absorbing coatings.

[0072] In addition, the slow addition of weak acid in step C causes the precursor seed crystals to hydrolyze slowly, which can reduce the particle size and increase the surface energy, significantly improve the system's free energy, and lower the energy barrier of boehmite phase transformation. This allows the crystallization process, which originally required temperatures above 160°C, to be completed at 120-140°C, while avoiding the violent reactions and high-temperature energy consumption problems of the traditional strong acid and strong base method.

[0073] Step S12: Disperse the nano-boehmite powder in a solvent to obtain a filler dispersion;

[0074] For example, when dispersing nano-boehmite powder in a solvent, the solvent must meet the following requirements: it must not chemically react with the boehmite powder, it must ensure uniform dispersion of the powder, and it must have good compatibility with the subsequent binder solution. Suitable solvents can be water, polar organic solvents, or mixed solvents consisting of polyols and water. It is understood that a pH adjuster can be added after the nano-boehmite powder is dispersed in the solvent to improve the dispersion of the boehmite filler. For example, a pH adjuster can be used to adjust the pH of the slurry to 2-4.

[0075] Step S13: Prepare an adhesive solution using an adhesive.

[0076] In the photographic paper ink-absorbing coating of this application embodiment, the core function of the adhesive is to firmly bond the nano-boehmite powder to the surface of the photographic paper substrate, while avoiding clogging the pore structure of the boehmite, not affecting its ink-absorbing performance, and having good compatibility with the boehmite and other components of the coating. Considering the polar surface characteristics of boehmite (rich in hydroxyl groups) and the functional requirements of the ink-absorbing coating, suitable adhesives can be polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyvinyl butyral (PVB), vinyl acetate-ethylene copolymer (VAE emulsion), etc.

[0077] In the preparation of ink-absorbing coatings for photographic paper, pre-dispersing the binder into a solution before mixing it with the filler ensures that the binder is uniformly dispersed in the solvent at the molecular level, preventing solid agglomeration. When the binder is mixed with the filler in a molecular state, it can more evenly coat the surface of the boehmite powder and the gaps between particles, forming a continuous and uniform adhesive network, preventing defects such as insufficient adhesion or excessive thickness in the coating.

[0078] Taking polyvinyl alcohol as an adhesive as an example, the process of preparing an adhesive solution using adhesive can be as follows: add polyvinyl alcohol powder to deionized water and stir at 70-90℃ for 1-2 hours until completely dissolved to form a transparent and homogeneous liquid.

[0079] Step S14: Mix the filler dispersion and the binder solution to obtain a mixed slurry.

[0080] In addition to fillers and binders, one or more of the following additives may be added to the mixed slurry as needed: dispersants that can further improve the dispersion stability of boehmite powder and prevent agglomeration and sedimentation; defoamers that can eliminate air bubbles generated during mixing and prevent pinholes or pitting after coating drying; leveling agents that can improve the spreading performance of the slurry and ensure a smooth and even coating surface after coating; thickeners that adjust the viscosity of the slurry to meet the needs of different coating processes (such as doctor blade coating and roller coating); and curing agents that crosslink and cure the binder into a network structure.

[0081] Step S15: Apply the mixed slurry onto the photographic paper and cure it to obtain the photographic paper ink-absorbing coating.

[0082] Common coating methods for photo paper ink-absorbing coatings include blade coating, roller coating, spray coating, and dip coating. In actual production, the appropriate coating method can be selected based on factors such as paste properties, coating thickness requirements, and production efficiency to ensure that the coating is uniform and smooth, meeting the requirements for ink absorption performance and appearance quality.

[0083] In this embodiment, weak alkali, industrial aluminum salt, and weak acid are used as raw materials. After two-step pH adjustment, a low-temperature hydrothermal crystallization reaction is carried out at 120-140℃ in the presence of a surface modifier to obtain nano-boehmite powder with uniform particle size distribution, high specific surface area, and high dispersibility. The ink-absorbing coating of photographic paper prepared with this nano-boehmite powder as a filler has fast ink absorption speed, uniform ink diffusion, no ink floating phenomenon, and high gloss.

[0084] In some examples, the aluminum salt is aluminum sulfate; the weak acid is acetic acid.

[0085] In terms of aluminum salt selection, aluminum sulfate has stronger dissociation stability and a milder aluminum ion release rate. When reacting with ammonia, it can avoid the formation of highly alkaline micro-regions caused by a sudden increase in local ion concentration, which greatly reduces the risk of gibbsite impurity formation. Moreover, the reaction byproduct ammonium sulfate is easily and completely removed by water washing, leaving no impurity ions that affect the crystal form, effectively ensuring product purity.

[0086] In terms of weak acid selection, acetic acid, as a typical organic weak acid, has a moderate dissociation constant and slow hydrogen ion release. After being added to the system, it can react mildly and fully with boehmite precursor seeds, precisely maintaining a hydrolysis equilibrium of pH 5-6. This avoids the problems of seed aggregation and incomplete crystal transformation caused by the vigorous reaction of strong acids. Moreover, it is easier to control the reaction rate than weaker acids such as carbonic acid, which can efficiently reduce the precursor particle size and increase the surface energy, laying the foundation for low-temperature crystallization. At the same time, the organic properties of acetic acid have better compatibility with subsequent polyacrylic acid surface modifiers, which can further optimize the powder dispersion performance.

[0087] In other words, by selecting the combination of aluminum sulfate and acetic acid as reactants, we can further improve the process stability and product performance by synergistically controlling the reaction temperature, the amount of residual impurities, and the seed crystal growth state, thus producing nano-boehmite powder with more uniform particle size, higher purity, and better dispersibility.

[0088] In some examples, the surface modifier is one or more of polyacrylic acid, polyethylene glycol, hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate, and the amount of surface modifier used is 0.1-1 wt% of the mass of the solution obtained in step C.

[0089] Through extensive experimentation, the inventors determined the optimal amount of surfactant to use when preparing boehmite powder for ink absorption coatings. Insufficient surfactant fails to completely cover the surface of the boehmite precursor crystals, hindering effective coating and causing powder agglomeration during subsequent crystallization and drying. This results in a wider particle size distribution, a lower dispersion index, and excessive exposure of hydroxyl groups on the powder surface, further exacerbating agglomeration and affecting the dispersion performance of the final product. Conversely, excessive surfactant leads to free movement within the system, potentially clogging the pore structure of the boehmite, reducing the product's specific surface area and pore volume (thus affecting ink absorption and absorption performance). It may also remain in the boehmite powder, potentially leading to decreased coating gloss, reduced printing color density, and even interference with the bonding between the binder and boehmite, impacting the coating's mechanical stability. Only with an appropriate dosage can the surface modifier fully combine with the hydroxyl groups on the boehmite surface to form a uniform coating, effectively preventing agglomeration and ensuring uniform particle size (the dispersion index in the examples all reached 99.9%). At the same time, it does not damage the pore structure, ensuring that the product has both high dispersibility, high specific surface area and good ink absorption performance.

[0090] In some examples, the surface modifier is a polar surface modifier, and the adhesive is a polar adhesive.

[0091] Through extensive experiments, the inventors discovered that after the crystallization reaction, some surfactants bind to the surface of nano-boehmite, exhibiting a modification effect, and thus playing a role in the preparation and application of the ink-absorbing coating. The inventors found that when a polar surfactant is used in the hydrothermal crystallization stage and a polar binder is used in the coating preparation stage, the polar surfactant and polar binder can interact, and further interact with the nano-boehmite powder filler, further improving the performance of the ink-absorbing coating.

[0092] Specifically, polar binders can bind to the hydroxyl groups on the boehmite surface through hydrogen bonds or chemical bonds, keeping the boehmite's pores open for ink adsorption. Furthermore, polar surface modifiers can bind to polar binder molecules through hydrogen bonds, electrostatic attraction, etc., reducing the aggregation or peeling of the binder on the surface of the nano-boehmite filler, further improving the mechanical strength and uniformity of the coating. Simultaneously, because polar surface modifiers can interact more strongly with binder molecules, binder molecules are more inclined to bind to the surface modifier on the outer surface of the boehmite rather than penetrating into the pores of the nano-boehmite filler. This reduces the driving force for binder diffusion into the pores of the nano-boehmite filler, preventing the pores from being occupied by the binder and allowing more pores to be used for ink absorption. Therefore, the use of polar surface modifiers and polar binders can further improve the performance of the ink-absorbing coating.

[0093] In some examples, the surface modifier is polyacrylic acid and the adhesive is polyvinyl alcohol.

[0094] Polyacrylic acid (PAA) is a polar surface modifier, and polyvinyl alcohol (PVA) is a polar binder. As a surface modifier, PAA improves the dispersibility of boehmite and enhances the polar interaction between boehmite and ink through its carboxyl groups, thus aiding in ink capture. As a binder, PVA ensures coating structural stability; its hydroxyl groups can form hydrogen bonds with the hydroxyl groups on the boehmite surface or the carboxyl groups of PAA, constructing a continuous adsorption-fixation network and reducing ink diffusion and floating. Therefore, using PAA as a polar surface modifier and PVA as a polar binder can further improve the performance of the ink-absorbing coating.

[0095] In some examples, the polyvinyl alcohol is polyvinyl alcohol with a degree of polymerization in the range of 3500-4000. Further, the polyvinyl alcohol is polyvinyl alcohol with a degree of polymerization in the range of 3500-3600.

[0096] Polyvinyl alcohol (PVA) with a degree of polymerization within this range is best suited to the preparation process and performance requirements of photo paper ink-absorbing coatings. From the perspective of its impact on ink absorption performance, PVA molecules within this degree of polymerization range have moderate chain lengths, low steric hindrance, and do not easily penetrate the microporous structure of boehmite, thus maximizing the preservation of porosity and ink absorption channels. Simultaneously, its film-forming properties and adhesive strength are sufficient to firmly fix the powder onto the substrate surface, balancing the requirements of "adhesive stability" and "ink absorption capacity." Typically, a 4% solution of PVA with a degree of polymerization of 3500-3600 has a viscosity of 80-110 mPa·s at room temperature. In contrast, PVA molecules with a degree of polymerization higher than 4000 have longer chains, which not only increase solubility but also lead to a significant increase in solution viscosity, making it difficult to obtain a smooth coating during the coating process, which is detrimental to production. While PVA with a degree of polymerization lower than 3000 has excellent water solubility, its excessively short chains may result in insufficient adhesive strength, making the coating prone to peeling off; it is also prone to clogging boehmite pores due to molecular entanglement, thus affecting ink absorption performance. For example, polyvinyl alcohol with a degree of polymerization of 2000 has a theoretical root mean square end-to-end distance of 13.8 nm, which is similar to the pore size of boehmite (8-15 nm). This may allow it to enter the pores of boehmite and thus affect ink absorption performance. On the other hand, polyvinyl alcohol with a degree of polymerization of 3500-4000, especially 3500-3600, can be stably dissolved at 70-90℃ to form a solution with moderate viscosity (meeting the requirements of subsequent coating processes for slurry flowability). This ensures uniform mixing with boehmite powder without causing dispersion difficulties due to excessive viscosity.

[0097] In some examples, the mass ratio of nanoboehmite powder to binder is 10:1 to 15:1.

[0098] Through extensive experimentation, the inventors discovered that when the mass ratio of nano-boehmite powder to binder is 10:1 to 15:1, the relative amounts of surface modifier and binder can balance dispersion stability, bonding strength, and ink absorption performance. The surface modifier needs to prevent agglomeration by coating the powder and work synergistically with the binder, while the binder needs to achieve a strong bond between the powder and the substrate without clogging pores. At this ratio, the surface modifier can fully coat the powder, guiding the binder to distribute evenly in the gaps rather than filling the pores through polarity, while the amount of binder is sufficient to form a continuous bonding network and reduce diffusion into the pores. If there is too much surface modifier, free molecules may weaken the bonding force or clog the pores; if there is too much binder, it is prone to secondary agglomeration of powder and pore clogging, affecting ink absorption performance. Meanwhile, from a process perspective, this formulation keeps the slurry viscosity within the ideal range of 80-150 cP (viscosity test temperature is 55℃; the viscosity of pure water-based slurries is preferably 100-150 cP, and slightly lower for oil-based coatings with added organic solvents such as DMF). It avoids uneven coating due to excessively high viscosity (>300 cP) and powdering after drying due to insufficient adhesion (e.g., when the mass ratio of nano-boehmite powder to binder is >15:1), thus meeting the requirements of processes such as doctor blade coating. Therefore, this dosage ratio achieves a synergistic effect of "dispersion-adhesion," ensuring that the coating possesses high dispersibility, strong mechanical properties, excellent ink absorption capacity, and good processability.

[0099] In some examples, the surface modifier is an azophenyl group-modified polyacrylic acid derivative; the method for preparing the photographic paper ink-absorbing coating further includes irradiating the photographic paper ink-absorbing coating with ultraviolet light before using the photographic paper ink-absorbing coating.

[0100] In the preparation of ink-absorbing coatings for photographic paper, when azophenyl-modified polyacrylic acid derivatives are used as surfactants, their working principle is based on the synergistic effect of molecular structure and function: the derivative retains the carboxyl groups of polyacrylic acid, which can fully bond with the hydroxyl groups on the surface of the boehmite precursor through hydrogen bonds, forming a uniform coating under the stirring conditions in step D. The rigid structure of the azophenyl group enhances the steric hindrance effect, effectively inhibiting powder agglomeration and ensuring the high dispersibility and narrow particle size distribution of nano-boehmite. At the same time, the azophenyl group has photoresponsive cis-trans isomerism. After the coating is cured and before printing, when exposed to ultraviolet light, it will change from the trans configuration to the cis configuration, driving the movement of polymer chain segments, dynamically adjusting the surface polarity of the powder and the porous channel structure of the coating, optimizing the ink affinity properties, reducing ink floating phenomenon and improving ink absorption uniformity. After printing, it can naturally restore ink fixation and prevent ink diffusion.

[0101] The step of irradiating the photo paper ink-absorbing coating with ultraviolet light needs to be performed after the coating is prepared and before inkjet printing. This external stimulus regulates the coating's microstructure and surface properties. Traditional coatings can be directly used for printing after curing, but with the introduction of azobenzene-modified surfactants, ultraviolet light irradiation is required to activate the coating's dynamic ink absorption regulation capability. For example, the parameters used for irradiating the photo paper ink-absorbing coating with ultraviolet light are: ultraviolet wavelength 254-365nm, intensity 5-10mW / cm², and time 5-15min, to ensure the azobenzene groups fully convert from the trans configuration to the cis configuration, thereby optimizing pore openness and surface affinity.

[0102] In some examples, step D includes adding a surface modifier to the solution obtained in step C and stirring at 150-250 rpm for 20-40 minutes at a temperature of 30-50°C. Adding a surfactant and stirring at 150-250 rpm for 20-40 minutes at 30-50°C achieves the following technical effects: First, it improves the uniformity of the surface modifier's coating. Stirring at 30-50°C enhances the molecular mobility of the surface modifier, making it easier to diffuse to the surface of the boehmite precursor seed crystals; the 150-250 rpm speed provides sufficient shear force to break up local agglomerations while avoiding turbulence that could damage the seed crystal structure due to excessive speed. The 20-40 minute stirring time ensures that the functional groups of the modifier molecules fully contact the hydroxyl groups on the seed crystal surface and form stable bonds, reducing uncoated "naked" seed crystals and thus lowering the risk of powder agglomeration in subsequent processes. Secondly, the fully bonded surface modifier can form a uniform steric hindrance layer on the seed crystal surface, hindering the van der Waals attraction between particles, thereby further narrowing the particle size distribution. This makes the powder obtained from subsequent spray drying easier to disperse in the solvent, reducing the agglomeration and precipitation of the dispersion, and laying the foundation for preparing a uniform mixed slurry. Thirdly, it can promote the synergistic effect with the binder. The uniformly coated surface modifier can provide consistent interface sites for the bonding of the binder (such as polyvinyl alcohol) in the subsequent step S14. The surface modifier and the binder form a stable "powder-modifier-binder" network through polar interaction, avoiding local insufficient bonding or pore blockage caused by uneven modification, ensuring that the coating has both good mechanical strength and retains sufficient ink absorption pores.

[0103] In some examples, step E includes: transferring the liquid reactant obtained in step D into a crystallization vessel, pre-crystallizing it at 80-100°C for 1-2 hours, and then increasing the temperature to 120-140°C at a rate of 5°C / min for 4 hours of crystallization.

[0104] Stepwise crystallization and staged heating can suppress the non-directional aggregation of boehmite precursors at low temperatures and promote the uniform development of mesoporous structures. This helps to increase specific surface area and pore volume concentration.

[0105] For example, an inert gas can be introduced for protection during the pre-crystallization stage, with a gas flow rate of 0.5-2 L / min. When the crystallization raw material contains impurities or intermediate products from previous reactions, the temperature conditions (80-100℃) during the pre-crystallization stage may accelerate the reaction with trace amounts of oxygen or carbon dioxide in the system. Introducing an inert gas can isolate oxygen from the air, preventing the oxidation of impurity ions, thereby reducing discolored impurities or defects in the powder and improving the purity and whiteness of the nano-boehmite powder. This is particularly important for photographic paper ink-absorbing coatings, as high-purity powder can prevent yellowing or abnormal color development in the coating, ensuring the color reproduction of the photographic paper. At the same time, the introduction of inert gas can drive away dissolved air and generated bubbles in the system, and at the same time, the airflow disturbance makes the solution heated more evenly, reducing bubble aggregation caused by local overheating, which is conducive to the uniform growth of boehmite grains, forming nanoparticles with narrower particle size distribution and more regular morphology. For example, the inert gas can be nitrogen, argon, etc.

[0106] In some examples, mixing the filler dispersion and the binder solution to obtain a mixed slurry involves: slowly adding the binder solution to the filler dispersion while stirring at low speed.

[0107] In the preparation of the mixed slurry for photographic paper ink-absorbing coating, the practice of first adding the filler dispersion to the mixing tank and then slowly adding the binder solution under low-speed stirring has several advantages: First, this method allows surface modifiers such as polyacrylic acid and binders such as polyvinyl alcohol to synergize more fully through polar interactions. The pre-coated powder of the surface modifier provides uniform binding sites for the binder, reducing the entanglement and aggregation between binder molecules, ultimately forming a uniformly dispersed, stable, and porous mixed slurry, ensuring the ink absorption performance and mechanical strength of the coating after subsequent coating. Second, low-speed stirring avoids the generation of excessive bubbles due to vigorous stirring, reducing the risk of pinholes or pitting after the coating dries, while also providing a gentle environment for the gradual fusion of the two liquids. Third, the order of "adding the filler dispersion first, then slowly adding the binder" allows the binder solution to gradually diffuse around the filler particles, uniformly coating the surface of the boehmite powder and the gaps between particles with the shear force of low-speed stirring, avoiding agglomeration or pore blockage caused by excessive local binder. In addition, this feeding sequence can effectively prevent the "clumping" phenomenon caused by excessive local adhesive concentration, thereby reducing coating thickness deviation and significantly improving the color uniformity of the printed image.

[0108] In some examples, mixing the filler dispersion and the binder solution to obtain a mixed slurry involves: slowly adding the binder solution to the filler dispersion under low-speed stirring, and then adding other additives (leveling agents, curing agents, and other functional additives) to obtain the filler dispersion.

[0109] Example 1:

[0110] A method for preparing a photographic paper ink-absorbing coating containing nano-boehmite powder is implemented according to the following steps:

[0111] Step S11: Prepare nano-boehmite powder, including steps A-F.

[0112] Step A: Add 20 kg of 10 wt% aluminum sulfate solution to a 50 L reactor and heat to 30 °C.

[0113] Step B: With a stirring speed of 250 rpm, add 10% wt ammonia solution to the aluminum sulfate solution in the reactor at a rate of 100 g / min until the pH reaches 10.5, and maintain for 1 h.

[0114] Step C: Add 3wt% acetic acid solution to the solution obtained in step B at a rate of 30g / min until pH 5 is reached, and maintain for 30min.

[0115] Step D: Add 0.15 wt% of the mass of the solution obtained in Step C to the solution obtained in Step C, and stir at 200 rpm for 30 min at 40°C.

[0116] Step E: Transfer the liquid reactants obtained in step D into a crystallization kettle, control the crystallization temperature at 140℃ and the crystallization time at 4h to obtain the crystallization liquid.

[0117] Step F: The obtained crystallization solution is washed with water to remove impurities and spray-dried to obtain ultrafine, easily dispersible nano-boehmite powder.

[0118] Step S12: Disperse the nano-boehmite powder obtained in step F into 1.6L of water, adjust the pH, and disperse at high speed to obtain a filler dispersion.

[0119] Step S13: Dissolve 1 / 12 of the mass of nano-boehmite powder in polyvinyl alcohol (KURARAY POVAL™ 95-88 grade) in 0.65L of water to prepare an adhesive solution.

[0120] Step S14: Under low-speed stirring, slowly add the adhesive solution to the filler dispersion, stir and keep it at 50-60℃, then add other additives (leveling agent, curing agent and other functional additives), stir and keep it at the same temperature, filter and defoam to obtain the mixed slurry.

[0121] Step S15: Apply the mixed slurry onto the photographic paper at a coating amount of 32 g / m² and cure it to obtain the photographic paper ink-absorbing coating.

[0122] Example 2:

[0123] A method for preparing a photographic paper ink-absorbing coating containing nano-boehmite powder is implemented according to the following steps:

[0124] Step S11: Prepare nano-boehmite powder, including steps A-F.

[0125] Step A: Add 20 kg of 15 wt% aluminum sulfate solution to a 50 L reactor and heat to 30 °C.

[0126] Step B: With a stirring speed of 250 rpm, add 15 wt% ammonia solution to the aluminum sulfate solution in the reactor at a rate of 80 g / min until the pH reaches 11, and maintain this for 1 h.

[0127] Step C: Add 3wt% acetic acid solution to the solution obtained in step B at a rate of 20g / min until the pH reaches 6, and maintain for 30min.

[0128] Step D: Add 0.3 wt% of the mass of the solution obtained in step C to the solution obtained in step S120, and stir at 200 rpm for 30 min at 40°C.

[0129] Step E: Transfer the liquid reactants obtained in step D into a crystallization kettle, control the crystallization temperature at 130℃, and the crystallization time at 6h to obtain the crystallization liquid.

[0130] Step F: The obtained crystallization solution is washed with water to remove impurities and spray-dried to obtain ultrafine, easily dispersible nano-boehmite powder.

[0131] Step S12: Disperse the nano-boehmite powder obtained in step F into 1.6L of water, adjust the pH, and disperse at high speed to obtain a filler dispersion.

[0132] Step S13: Dissolve 1 / 12 of the mass of nano-boehmite powder in polyvinyl alcohol (KURARAY POVAL™ 95-88 grade) in 0.65L of water to prepare an adhesive solution.

[0133] Step S14: Under low-speed stirring, slowly add the adhesive solution to the filler dispersion, stir and keep it at 50-60℃, then add other additives (leveling agent, curing agent and other functional additives), stir and keep it at the same temperature, filter and defoam to obtain the mixed slurry.

[0134] Step S15: Apply the mixed slurry onto the photographic paper at a coating amount of 32 g / m² and cure it to obtain the photographic paper ink-absorbing coating.

[0135] Example 3:

[0136] A method for preparing a photographic paper ink-absorbing coating containing nano-boehmite powder is implemented according to the following steps:

[0137] Step S11: Prepare nano-boehmite powder, including steps A-F.

[0138] Step A: Add 20 kg of 20 wt% aluminum sulfate solution to a 50 L reactor and heat to 30 °C.

[0139] Step B: With a stirring speed of 250 rpm, add 13 wt% ammonia solution to the reactor at a rate of 100 g / min until the pH reaches 9.5, and maintain this for 1 h.

[0140] Step C: Add 4wt% acetic acid solution to the solution obtained in step B at a rate of 50 g / min until the pH reaches 5.5, and maintain this for 30 min.

[0141] Step D: Add 0.15 wt% of the mass of the solution obtained in Step C to the solution obtained in Step C, and stir at 200 rpm for 30 min at 40°C.

[0142] Step E: Transfer the liquid obtained in step D into a crystallization kettle, control the crystallization temperature at 120℃, and the crystallization time at 8h to obtain the crystallized liquid.

[0143] Step F: The obtained crystallization solution is washed with water to remove impurities and spray-dried to obtain ultrafine, easily dispersible nano-boehmite powder.

[0144] Step S12: Disperse the nano-boehmite powder obtained in step F into 1.6L of water, adjust the pH, and disperse at high speed to obtain a filler dispersion.

[0145] Step S13: Dissolve 1 / 12 of the mass of nano-boehmite powder in polyvinyl alcohol (KURARAY POVAL™ 95-88 grade) in 0.65L of water to prepare an adhesive solution.

[0146] Step S14: Under low-speed stirring, slowly add the adhesive solution to the filler dispersion, stir and keep it at 50-60℃, then add other additives (leveling agent, curing agent and other functional additives), stir and keep it at the same temperature, filter and defoam to obtain the mixed slurry.

[0147] Step S15: Apply the mixed slurry onto the photographic paper at a coating amount of 32 g / m² and cure it to obtain the photographic paper ink-absorbing coating.

[0148] Example 4: The implementation scheme is the same as that in Example 1, except that the aluminum salt is aluminum chloride and the weak acid is acrylic acid.

[0149] Example 5: The implementation scheme is the same as that of Example 1, except that 2.5 wt% of the mass of the solution obtained in step C is added to the solution obtained in step C.

[0150] Example 6: The implementation scheme is the same as that of Example 1, except that the adhesive used in step 13 is VAE emulsion.

[0151] Example 7: The implementation scheme is the same as that in Example 1, except that the adhesive is polyvinyl alcohol with a degree of polymerization of 2500.

[0152] Example 8: The implementation scheme is the same as that of Example 1, except that the mass ratio of nanoboehmite powder to binder is 16:1.

[0153] Example 9: The implementation scheme is the same as in Example 1, except that the surface modifier is 4-phenylazobenzeneacrylamide-modified polyacrylic acid. Before inkjet testing, the ink-absorbing coating was irradiated with ultraviolet light at a wavelength of 365 nm and an intensity of 5 mW / cm² for 5-15 minutes.

[0154] Example 10: The implementation scheme is the same as that of Example 1, except that in step E, the liquid reactant obtained in step D is transferred to a crystallization vessel, pre-crystallized at 90°C for 1 hour, and then increased to 140°C at a rate of 5°C / min for 4 hours; nitrogen gas is introduced for protection during the pre-crystallization stage, and the gas flow rate is 0.5-2 L / min.

[0155] Comparative Example 1: The implementation scheme is the same as that of Example 1, except that the endpoint pH of step B is controlled at 12, while the other control points are exactly the same as those in Example 1.

[0156] Comparative Example 2: The implementation scheme is the same as that of Example 1, except that the endpoint pH of step C is controlled at 4, while the other control points are exactly the same as those in Example 1.

[0157] Comparative Example 3: The implementation scheme is the same as that of Example 2, except that the method does not include step D, and the remaining control points are exactly the same as those of Example 2.

[0158] Comparative Example 4: The implementation scheme is the same as that of Example 2, except that a weak acid is no longer used to control the pH, i.e. step C is omitted. The remaining control points are exactly the same as those in Example 2.

[0159] Depend on Figures 1-3 As can be seen, Examples 1-3 show obvious and sharp peaks at the characteristic diffraction peak positions of boehmite, and there are no other impurity peaks, indicating that the crystal form of the product is pure boehmite.

[0160] Detection and Results

[0161] The nano-boehmite powders obtained in Examples 1-10 and Comparative Examples 1-4 were subjected to particle size distribution, dispersion index, XRD, BET data, and oil absorption value tests. The ink-absorbing coatings for photographic paper obtained in Examples 1-10 and Comparative Examples 1-4 were subjected to inkjet printing tests.

[0162] The particle size distribution was tested using a zeta potential particle size analyzer. When testing the dispersion index, the nano-boehmite powder was prepared into a 10% aqueous solution, mechanically stirred for 5 minutes, centrifuged at 3000 rpm for 10 minutes, and the bottom precipitate was dried before testing.

[0163] The oil absorption value was determined using the dioctyl phthalate (DOP) test method. 100g of nano-boehmite powder was mixed with 150g of DOP for 30 minutes to allow the nano-boehmite powder to fully adsorb DOP. The mixture was then filtered using a vacuum pump for 30 minutes, and the weight gain of the powder was measured as the oil absorption value.

[0164] The inkjet printing test was conducted using an EPSON 805L high-quality glossy paper in print mode. The main focus was on whether there was ink floating, the gloss of the photo paper (60°∠), and the color density K value (color density was tested using X-Rite i1).

[0165] Table 1

[0166]

[0167] Note: In Tables 1-3, " / " indicates that the actual data cannot be tested due to ink floating on the black color block, and there is no need to test it.

[0168] in, Figures 13-17 The images show SEM images of boehmite powders obtained by spray drying in Examples 1, 3, 1, 3, and 4, respectively. In the spray drying process, the material is atomized into droplets by high-pressure gas, and after high-temperature drying, spherical or near-spherical nanoparticles agglomerate / accumulate. This agglomeration is reversible; the powder disperses upon dissolution in water. To measure the particle size of the nano-boehmite powder, the spray-dried nano-boehmite powder was dissolved in water to form a 0.2%-0.5% aqueous solution, dispersed by mechanical stirring, and then measured.

[0169] As can be seen from the results of Comparative Example 1 in Table 1, it is necessary to control the pH of the system to 9-11 in step B, i.e., the neutralization and precipitation stage to obtain the boehmite precursor. Excessively high pH will lead to the formation of gibbsite, thus affecting product purity and reducing product dispersibility. Simultaneously, excessively high pH results in high reactant aggregation, which cannot be completely dispersed by the addition of weak acid, resulting in a larger final product particle size and wider particle size distribution. Gibbsite has a dense crystal structure and low porosity, significantly different from the porous characteristics of boehmite. This causes the overall specific surface area of ​​the powder to decrease from 260+ m² / g in Examples 1-3 to 202.38 m² / g in Comparative Example 1, and the pore volume to decrease from approximately 0.7 ml / g to 0.585 ml / g. In other words, excessively high pH in step B will cause structural inhomogeneity, leading to a comprehensive deterioration of the ink-absorbing coating in terms of physical adsorption capacity, surface morphology, and color uniformity, resulting in poor ink absorption and low gloss in Comparative Example 1. The poor ink absorption of blue, red, and black blocks is mainly due to the destruction of the porous structure, which directly weakens the coating's physical adsorption capacity for ink, especially reducing its ability to capture small-molecule dye inks such as blue and red. This results in the ink not being quickly locked in the pores, leading to insufficient local penetration or uneven diffusion. Simultaneously, the severe agglomeration caused by high pH leads to a wide particle size distribution, creating "uneven density" defect areas in the coating structure. Dense agglomerates hinder ink penetration, while loose areas cause smudging due to excessive ink diffusion, ultimately manifesting as differences in ink absorption of the color blocks. Gloss depends on the smoothness of the coating surface and the uniformity of light reflection. In Comparative Example 1, the presence of gibbsite and large particle agglomerates makes it easy to form an uneven surface when deposited in the coating, increasing light scattering rather than specular reflection. Furthermore, the uneven gaps between agglomerated particles result in micropores and protrusions on the coating surface after drying, further disrupting the consistency of light reflection, causing the 60° angle gloss to decrease from 58-60 in the example to 49.1. The color density (K) value reflects the color depth and uniformity of black ink, which depends on the uniform adsorption and fixation of carbon black particles by the coating. In Comparative Example 1, the low porosity of the gibbsite impurities makes it difficult to effectively support carbon black particles, resulting in ink accumulation on the surface rather than uniform penetration, leading to "floating ink" or localized insufficient concentration. The large-diameter particles formed by the agglomerates clog the pores inside the coating, preventing carbon black particles from penetrating the coating, resulting in a thin and unevenly distributed color layer. At the same time, the uneven surface causes disordered light reflection, further interfering with the accuracy of color density detection, ultimately making it impossible to obtain effective K value data.

[0170] The results of Comparative Example 2 show that in step C, i.e., the stage of gradually dispersing the precursor using a weak acid, it is necessary to control the pH of the system to 5-6. Too low a pH will lead to excessive H+ in the system. + Increased concentration leads to re-agglomeration of reactants, resulting in larger particle size of the final product. Furthermore, lower pH affects the crystallinity of the final product, reducing its crystallinity. Simultaneously, during actual operation, H... +The excessive introduction of [acid] significantly increases the viscosity of the system, hindering normal production. Furthermore, the excessively low pH in step C causes particle agglomeration, pore destruction, and decreased dispersibility, comprehensively affecting coating performance from ink absorption capacity and surface morphology to color uniformity. This results in large-area ink floating, reduced gloss, and abnormal color density in the photographic paper ink-absorbing coating of Comparative Example 2. Specifically, the excessively low pH in step C disrupts the hydrolysis equilibrium of the boehmite precursor. On one hand, the strong acid environment leads to an excessively high hydrogen ion concentration in the system, causing the precursor seeds to react violently rather than hydrolyze slowly, triggering secondary particle agglomeration, widening the particle size distribution, decreasing surface energy, and deteriorating powder dispersibility. Agglomerated particles clog the pores inside the coating, preventing the porous structure of the boehmite from fully utilizing its ink absorption function. Ink is difficult to be quickly absorbed into the pores, resulting in a large amount of residual "floating ink" on the coating surface. On the other hand, an overly acidic environment damages the pore structure of boehmite, causing the specific surface area to decrease from 260+ m² / g in Examples 1-3 to 216.32 m² / g, the pore volume to decrease from approximately 0.7 ml / g to 0.527 ml / g, and the oil absorption value to decrease from approximately 80 to 72.4. This significantly weakens ink absorption capacity, further exacerbating ink accumulation on the surface and ultimately resulting in large-area ink floating. Gloss depends on the smoothness of the coating surface and the uniformity of light reflection. In Comparative Example 2, the secondary agglomeration of particles caused by excessively low pH resulted in uneven particle size distribution. Larger agglomerates were difficult to disperse evenly during coating application, accumulating to form localized protrusions or depressions. Simultaneously, the damaged pore structure led to uneven moisture evaporation during coating drying, easily causing micro-shrinkage cracks or surface unevenness, increasing the probability of light scattering and weakening the specular reflection effect. Therefore, its 60° angle gloss decreased to 53.3. The color density (K) value reflects the color development stability and uniformity of black ink, which depends on the uniform adsorption and fixation of carbon black particles by the coating. In Comparative Example 2, after the agglomerated particles clogged the pores, the carbon black particles could not penetrate into the coating and only adhered to the surface. Furthermore, due to the reduced ink absorption capacity, the ink distribution was extremely uneven, with excessive accumulation in some areas and insufficient accumulation in others, resulting in disordered color development. In addition, large areas of floating ink caused the ink to form an irregular liquid film on the surface. After drying, the thickness of the color layer varied greatly. Coupled with the light reflection interference caused by the uneven surface, the color density test could not obtain stable data, and ultimately the K value could not be displayed.

[0171] The results of Comparative Example 3 show that the introduction of a surface modifier is necessary. The surface modifier can coat the powder, prevent agglomeration, improve the uniformity of boehmite particle size, making its particle size distribution more concentrated, and simultaneously improve the powder's dispersibility. In this comparative example, due to the lack of a surface modifier, although a boehmite product with a smaller Dav was obtained, its overall particle size distribution was wider, and the dispersion index was only 94.5%, showing a significant decrease. The SEM images show that, due to the absence of a surface modifier, the spherical particles obtained by spray granulation significantly increased, and the surface became rougher. Furthermore, Comparative Example 3, by omitting the polyacrylic acid surface modifier in step D, resulted in deterioration of the dispersibility and interfacial compatibility of the nano-boehmite powder, thus leading to performance defects in the ink-absorbing coating. Specifically, the lack of surface modifiers (such as polyacrylic acid) leads to excessive exposure of hydroxyl groups on the surface of the boehmite precursor, making it prone to severe agglomeration during crystallization and drying (test data show that its D100 reaches 392.4 nm, far greater than the 207.2-267.4 nm of Examples 1-3, with a wide particle size distribution and a dispersion index reduced to 94.5% (lower than 99.9% of Examples 1-3). Agglomerated particles destroy the uniform porous structure of the coating, forming localized dense areas. Since blue and red inks are mostly small-molecule dyes, this can negatively impact the coating's structure. The dependence on porosity is stronger: dense areas hinder penetration, while loose areas cause smudging due to excessive ink diffusion, resulting in uneven ink absorption in blue and red areas. Simultaneously, the interaction between the polarity of the unmodified boehmite surface and pigment molecules is weakened, making it unable to effectively capture dye and further exacerbating the ink absorption differences. Gloss depends on the smoothness of the coating surface and the consistency of light reflection. In Comparative Example 3, the powder agglomeration caused by the lack of surface modifiers resulted in uneven particle size, making it difficult to form a uniform buildup during coating, and leading to an uneven microstructure on the coating surface. The irregular gaps between agglomerates easily lead to micropores or protrusions after drying, increasing light scattering rather than specular reflection. Furthermore, the interfacial compatibility between unmodified boehmite and the binder decreases, preventing the binder from uniformly coating the powder and further compromising surface smoothness, resulting in a decrease in the 60° angle gloss from 58-60 in Examples 1-3 to 52.1. The color density K value reflects the color depth of black ink (carbon black granular type), and its dependence on pore structure is slightly lower than that of dye-based inks, but it is still affected by coating uniformity. Comparative Example 3 Although the carbon black particles in the black ink could be physically adhered to the coating surface without completely failing to develop color, the localized densification of the coating caused by their agglomeration still reduced the effective ink absorption area, resulting in uneven distribution of carbon black particles and a decrease in color depth (K value decreased from 2.28-2.31 in Examples 1-3 to 2.01). Simultaneously, the lack of surface modifiers weakened the bonding force between the binder and boehmite, causing some carbon black particles to detach due to the loose coating structure, further reducing color stability and ultimately resulting in a K value lower than normal.

[0172] The results of Comparative Example 4 show that step C, the introduction of weak acid, is necessary, demonstrating the feasibility and necessity of the two-stage pH control method for producing ultrafine boehmite. After removing the weak acid adjustment stage for boehmite precursor particle size, the average particle size of the obtained boehmite increased significantly, and the overall particle size distribution was wide. SEM images also show that spherical agglomerates with significant particle size differences were obtained during spray granulation. Furthermore, Comparative Example 4, by omitting the weak acid adjustment process in step C, resulted in compromised structural uniformity and performance stability of the nano-boehmite powder, leading to differentiated performance of the ink-absorbing coating. Specifically, although the weak acid adjustment (step C) was not performed, the alkaline precipitation process in step B still generated boehmite precursors, and the crystallized powder still possessed a certain degree of porous structure (test data showed a pore volume of 0.692 ml / g and an oil absorption value of 78.3, close to the levels of Examples 1-3). This porous structure provides a basic physical adsorption space for inks, especially for systems dominated by carbon black particles, such as black inks. It primarily relies on the physical trapping effect of the pores, thus the ink absorption state does not exhibit serious defects and remains at a "good" level. Furthermore, although the powder without weak acid treatment has a larger particle size, it still possesses open pores, enabling basic ink adsorption and fixation. Gloss is highly sensitive to the microscopic smoothness of the coating surface and the uniformity of particle dispersion. In Comparative Example 4, due to the omission of the weak acid adjustment in step C, the boehmite precursor seeds failed to achieve particle size uniformity control through slow hydrolysis, resulting in an extremely wide particle size distribution and irregular particle morphology. Large and unevenly distributed particles are difficult to uniformly accumulate during coating, forming numerous uneven microscopic protrusions and gaps on the coating surface, significantly increasing the probability of light scattering and weakening the specular reflection effect. Meanwhile, the compatibility between the powder and the binder decreased without weak acid treatment, and the binder could not uniformly coat the particle surface. After drying, localized shrinkage differences easily occurred, further damaging surface smoothness. This resulted in a decrease in the 60° angle gloss from 58-60 in Examples 1-3 to 47.2, the lowest level among all tested samples. The color density K value depends on the uniform distribution of black ink in the coating and the depth of color development. In Comparative Example 4, omitting step C caused the powder specific surface area to decrease from 260+㎡ / g in Examples 1-3 to 206.55㎡ / g. The effective adsorption area decreased, and the excessively large particle size led to uneven pore distribution: ink excessively accumulated in densely porous areas, while ink penetration was insufficient in densely packed areas, resulting in decreased color uniformity. Furthermore, the powder without weak acid treatment had lower surface energy, weakening the interaction force with carbon black particles and failing to effectively fix the ink particles. Some carbon black migrated downwards during drying, leading to reduced surface pigment accumulation, decreased blackness, and increased whiteness, resulting in a duller appearance. Although the overall ink absorption was "good," the uneven structure and decreased specific surface area together reduced the color stability and depth of the black ink, ultimately causing the color density K value to drop to 1.86.

[0173] Table 2

[0174]

[0175] The test results of Example 4 show that replacing aluminum sulfate with aluminum chloride and acetic acid with acrylic acid leads to a decrease in reaction control precision and slightly inferior performance compared to using aluminum sulfate and acetic acid, but does not cause defects such as gibbsite impurities or excessive agglomeration. Specifically, aluminum sulfate has strong dissociation stability and mild aluminum ion release, which can avoid the formation of local high-alkalinity micro-regions; while aluminum chloride has a faster aluminum ion release rate, and the local aluminum ion concentration is prone to a sudden increase when reacting with ammonia, resulting in more obvious pH fluctuations in micro-regions and easily causing uneven growth of precursor particles. Therefore, the particle size parameters of Example 4 are slightly larger than those of Example 1, and the particle size distribution is wider. Acetic acid has a moderate dissociation constant and slow hydrogen ion release, which can accurately maintain the hydrolysis equilibrium at a pH of 5-6; while acrylic acid has weaker dissociation and a lower hydrogen ion release rate, resulting in a decrease in the precision of hydrolysis control of precursor crystals, making it difficult to uniformly inhibit excessive seed growth, leading to a decrease in particle size uniformity and a slight decrease in the dispersion index. Acrylic acid is weakly acidic, and the precursor hydrolysis is insufficient, resulting in a smaller increase in seed crystal surface energy compared to the acetic acid system. Pore development is also slightly poorer, leading to a slight decrease in specific surface area and pore volume. Simultaneously, the widened particle size and reduced pore structure cause a slight decrease in oil absorption value, but overall, it remains at a high level. Despite slightly poorer particle size and dispersibility, the powder in Example 4 still possesses a relatively complete porous structure (pore volume 0.682 ml / g, oil absorption value 77.3), and there is no severe agglomeration or impurity interference, basically meeting the ink adsorption requirements. Therefore, the overall ink absorption is good; only slight diffusion is caused by localized particle size unevenness, which is better than the defective state of the comparative example. The widened particle size distribution and reduced dispersibility decrease the microscopic smoothness of the coating surface, weaken the uniformity of light reflection, and reduce the gloss to 56.5. The decrease in specific surface area and uneven pore distribution weakened the adsorption and fixation capacity of black ink (carbon black particles), reduced the color depth, and lowered the color density K value to 2.18; however, because the coating structure was still relatively intact (without floating ink or poor ink absorption problems), the K value was significantly higher than that of the comparative example.

[0176] As can be seen from the test results of Example 5, the performance of Example 5 is slightly inferior to that of Examples 1-3 due to the damage to the pore structure caused by excessive surface modifier. Specifically, the excessive polyacrylic acid can still effectively inhibit agglomeration by coating the hydroxyl groups on the powder surface, thus maintaining a high dispersion index. The particle size parameter is slightly larger than that of Example 1 but still relatively uniform. However, the excess surface modifier molecules are partially free in the system, which may form weak bridges between particles, resulting in a slight increase in D100 and a slight widening of the particle size distribution. Excessive surface modifier can clog the pore structure of boehmite. In Example 5, the excess polyacrylic acid may fill some micropores and mesopores, resulting in a slight decrease in specific surface area and pore volume. Pore clogging directly weakens the powder's ability to adsorb ink, resulting in a slight decrease in the ink absorption value, but it is still higher than the comparative example's ink absorption value. The overall ink absorption is good, but the excessive surface modifier clogs some pores, resulting in insufficient ink adsorption in local areas and slight uneven ink absorption. Due to the residue of free polyacrylic acid, the uniformity of light reflection on the coating surface decreases, and the gloss is slightly reduced. The reduction in specific surface area and pore volume weakens the adsorption and fixation ability of black ink (carbon black particles), resulting in a slight decrease in the color density K value.

[0177] As can be seen from the test results of Example 6, although vinyl acetate-ethylene copolymer is a polar binder, the hydrogen bonding strength between it and the hydroxyl groups on the surface of polyacrylic acid-modified boehmite is weaker than that of polyvinyl alcohol. This slightly reduces the uniformity of the binder's coating on the powder surface, potentially leading to a small amount of ink not being fully fixed and resulting in slight localized ink floating. However, the overall ink absorption capacity remains good. The film-forming properties of vinyl acetate-ethylene copolymer are slightly inferior to those of polyvinyl alcohol, resulting in a slight decrease in the microscopic smoothness of the coating surface, reduced uniformity of light reflection, and a slight decrease in gloss. However, because there is no agglomeration or severe pore damage, the gloss is still higher than that of the comparative example. The decreased interfacial bonding force between the binder and the powder leads to a slight decrease in the fixation stability of the carbon black particles, a slight decrease in color depth, and a slight decrease in the color density K value. However, because the coating structure remains uniform (without severe differences in ink absorption), the K value is significantly higher than that of the comparative example.

[0178] As can be seen from the test results of Example 7, the performance of Example 7 is slightly inferior to that of Examples 1-3 due to the slight pore blockage and decreased overall coating integrity caused by the reduced degree of polymerization of the binder. However, the core indicators are still better than those of Comparative Examples 1-4. Polyvinyl alcohol with a low degree of polymerization has smaller molecular chains, and its theoretical root-mean-square end-to-end distance (representing the size of the random coil formed by the PVA molecules) is close to the boehmite pore size, making it easy to penetrate into the mesoporous structure of the boehmite and block the pores. In Example 7, the polyvinyl alcohol with a degree of polymerization of 2500 has a significantly shorter molecular chain length than that of the polyvinyl alcohol in Example 1. Some molecules easily penetrate into the pores of the boehmite, causing local blockage of the ink absorption channels and preventing uniform ink diffusion, resulting in slight uneven ink absorption. However, since polyvinyl alcohol is still a polar binder, it has hydrogen bonding with the hydroxyl groups on the surface of the polyacrylic acid-modified boehmite, so the ink absorption state is still "good". The reduced degree of polymerization decreases the viscosity of the polyvinyl alcohol solution, leading to a lower crosslinking density during film formation. This results in extremely fine cracks appearing in the coating, reducing its microscopic smoothness and light reflection uniformity, thus causing a certain degree of decrease in gloss. Pore blockage reduces the effective adsorption sites of carbon black particles, resulting in a decrease in color depth and a lower color density (K value).

[0179] The test results from Example 8 show that the mass ratio of boehmite powder to binder needs to balance dispersion stability, bonding strength, and ink absorption performance. A suitable proportion of boehmite powder ensures ink absorption performance while maintaining the integrity of the binder's film formation. In Example 8, the reduced amount of binder prevented the formation of a continuous cross-linked network, weakened the bonding force between some particles, and created tiny voids in the coating structure. This resulted in small protrusions and pores on the coating surface, increasing light scattering and reducing specular reflection, thus slightly decreasing gloss. Insufficient binder led to a loose coating structure, making the ink prone to localized over-expansion within these voids. Carbon black particles migrated downwards during drying, resulting in insufficient surface pigment and a decrease in color density to 2.08.

[0180] As seen in Examples 5-8, in the preparation of photo paper ink-absorbing coatings, the polarity matching and balanced dosage ratio of surface modifiers and binders are crucial. Suitable surface modifiers and binders not only enable them to effectively fulfill their respective functions in both powder preparation and coating preparation stages, but also synergistically regulate the dispersion stability, pore structure retention rate, and coating mechanical strength of nano-boehmite powder by constructing a three-dimensional "powder-modifier-binder" network structure.

[0181] Specifically, when the surface modifier is polyacrylic acid and the binder is the polar polymer polyethylene, the two can achieve synergy through multiple forces, including hydrogen bonding, electrostatic attraction, and complementary steric hindrance. Specifically, the combination of the carboxyl groups of polyacrylic acid and the hydroxyl groups of polyvinyl alcohol enhances interfacial adhesion; the charge interaction of the polar groups makes it easier for the binder to uniformly coat the modified powder surface, avoiding localized agglomeration; and the extensibility of the polar molecular chains reduces the blockage of boehmite pores by binder molecules, preserving ink absorption channels. Polyacrylic acid coats the hydroxyl groups on the boehmite surface with its carboxyl groups, and its exposed polar groups form hydrogen bonds with the hydroxyl groups of polyvinyl alcohol, ensuring that the binder is evenly distributed in the gaps between powder particles rather than inside the pores. This guarantees both bonding strength and high porosity, ultimately achieving a balance between ink absorption speed and coating stability. Conversely, if the polarity of the surface modifier and binder is mismatched (e.g., a combination of a non-polar modifier and a polar binder), it will lead to poor interfacial compatibility, increased risk of pore blockage, and deterioration of ink absorption uniformity.

[0182] When the amount of surface modifier is 0.1-1 wt% of the solution mass in step C, and the mass ratio of nano-boehmite to binder is 10:1-15:1, the 0.1-1 wt% modifier can precisely cover the hydroxyl groups on the boehmite surface, forming a monolayer coating and preventing powder agglomeration. The 10:1-15:1 ratio allows the binder to fill the gaps between powder particles to form a continuous network without causing pore blockage due to excessive amount. The modifier restricts the diffusion of the binder into the pores through steric hindrance, while the appropriate amount of binder further reduces the permeation kinetics, together maintaining high pore volume.

[0183] Table 3

[0184]

[0185] As can be seen from the test results of Example 9, the azobenzene modified surface modifier retains the dispersion and coating capabilities of the original modifier, ensuring the stability of particle size distribution and basic pore structure; it can also dynamically optimize pore openness and ink affinity by taking advantage of the cis-trans isomerism characteristics of UV light response, ultimately achieving a slight improvement in ink absorption performance and printing quality, while not causing negative impacts on other indicators.

[0186] Specifically, azobenzene-modified polyacrylic acid retains the carboxyl groups of the original polyacrylic acid, which can firmly bind to the hydroxyl groups on the surface of the boehmite precursor through hydrogen bonds, achieving uniform coating. The introduced azobenzene group has a rigid aromatic structure, which can enhance the steric hindrance effect and further hinder the agglomeration between powder particles, thus the dispersion index does not decrease. The modified molecular chain length and coating efficiency are close to those of the unmodified polyacrylic acid, so the overall particle size distribution is in the same range as in Examples 1-3, with only slight fluctuations due to the minor influence of the rigid group. The molecular structure of azobenzene-modified polyacrylic acid does not significantly increase steric hindrance and will not excessively block the pores of boehmite, thus the basic porous structure is preserved.

[0187] Under ultraviolet light irradiation, the azophenyl group undergoes a cis-trans isomerization transition (from trans configuration to cis configuration), causing slight movement of polymer chain segments. This opens up pore channels that might otherwise be partially covered by the modifier molecules, increasing the number and openness of effective pores. The open pore structure increases the specific surface area and pore volume, and since the oil absorption value is directly related to the pore adsorption capacity, the oil absorption value increases slightly. After ultraviolet irradiation, the polarity of the cis-configured azophenyl group slightly increases, enhancing the affinity between the coating surface and polar inks. At the same time, the open pore channels accelerate ink penetration, reducing the "floating ink" phenomenon caused by ink retention on the surface. The uniform coating of the azophenyl-modified surface modifier results in more uniform dispersion of boehmite powder in the coating, leading to a higher surface smoothness. Furthermore, ultraviolet irradiation does not damage the coating structure, resulting in a slightly higher gloss level than in Example 1. Uniform ink absorption and open pores can more stably lock in pigment molecules and reduce color density loss caused by ink diffusion; at the same time, the improved surface smoothness of the coating reduces light scattering and makes the color more intense, so the color density K value is slightly higher than that of Example 1.

[0188] As can be seen from the test results of Example 10, the synergistic effect of stepwise crystallization and inert gas protection helps to improve the performance of the ink-absorbing coating. Stepwise heating achieves uniform particle size and orderly pore development by regulating the crystal nucleus growth rate, while nitrogen protection reduces agglomeration and pore blockage by isolating interference and stabilizing the surface. Ultimately, while improving dispersibility and porosity, it significantly optimizes ink absorption and print quality.

[0189] Principles and steps not explicitly described in this invention are all obtainable by those skilled in the art through conventional technical means, and therefore will not be elaborated upon. Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing an inkjet coating of photographic paper comprising a nanoboehmite powder, comprising: preparing a nanoboehmite powder; dispersing the nanoboehmite powder into a solvent to obtain a filler dispersion; preparing a binder solution with a binder; mixing the filler dispersion and the binder solution to obtain a mixed slurry; coating the mixed slurry on the photographic paper and curing to obtain the inkjet coating of the photographic paper; wherein the step of preparing the nanoboehmite powder comprises: step A: adding an aluminum salt solution into a reactor; the concentration of the aluminum salt solution is 10-20 wt%; step B: adding an ammonia solution into the aluminum salt solution at a rate of 50-100 g / min until the pH value is 9-11, and maintaining for 1 h; the concentration of the ammonia solution is 5-15 wt%; step C: adding a solution comprising a weak acid into the solution obtained in step B at a rate of 20-50 g / min until the pH value is 5-6, and maintaining for 30 min; step D: adding a surface modifier into the solution obtained in step C; step E: transferring the liquid reactant obtained in step D into a crystallization reactor, and controlling the crystallization temperature to be 120-140 ℃, and the crystallization time to be 2-8 h to obtain a crystallization liquid; step F: removing impurities from the crystallization liquid by water washing, and spray drying to obtain the nanoboehmite powder; wherein the mass ratio of the nanoboehmite powder to the binder is 10:1-15:

1. the aluminum salt is aluminum sulfate; the weak acid is acetic acid. the surface modifier is one or more of polyacrylic acid, polyethylene glycol, cetyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate; the amount of the surface modifier is 0.1-1 wt% of the mass of the solution obtained in step C. the surface modifier is a polar surface modifier, and the binder is a polar binder. the surface modifier is polyvinyl alcohol, and the binder is polyacrylic acid. the surface modifier is a polyacrylic acid derivative modified by an azobenzene group; the method further comprises: irradiating the inkjet coating of the photographic paper with ultraviolet light before using the inkjet coating of the photographic paper. step D comprises: adding the surface modifier into the solution obtained in step C, and stirring at a speed of 150-250 rpm at a temperature of 30-50 ℃ for 20-40 min. step E comprises: transferring the liquid reactant obtained in step D into a crystallization reactor, pre-crystallizing at 80-100 ℃ for 1-2 h, then increasing the temperature to 120-140 ℃ at a rate of 5 ℃ / min, and crystallizing for 4 h. mixing the filler dispersion and the binder comprises: slowly adding the binder solution into the filler dispersion under low-speed stirring. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2. The production method according to claim 1, wherein ​ 3. The production method according to claim 1, wherein ​ 4. The production method according to claim 3, wherein ​ 5. The production method according to claim 4, wherein ​ 6. The production method according to claim 5, wherein ​ 7. The production method according to claim 1, wherein ​ 8. The production method according to claim 1, wherein ​ ​ 9. The production method according to claim 1, wherein, ​

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