Preparation method of photographic paper ink-absorbing coating containing nano boehmite powder

Abstract

The invention provides a preparation method of a photographic paper ink-absorbing coating containing nano boehmite powder. The preparation method comprises the following steps: preparing the nano boehmite powder; dispersing the nano boehmite powder into a solvent to obtain a filler dispersion liquid; preparing an adhesive solution by using an adhesive; mixing the filler dispersion liquid with the adhesive solution to obtain mixed slurry; coating photographic paper with the mixed slurry, and curing to obtain a photographic paper ink-absorbing coating; wherein weak base, industrial aluminum salt and weak acid are used as raw materials, after two-step pH adjustment, low-temperature hydrothermal crystallization reaction is performed at 120-140 DEG C in the presence of a surface modifier to prepare the nano boehmite powder, and the nano boehmite powder with uniform particle size distribution, high specific surface area and high dispersity can be obtained. A photographic paper ink-absorbing coating prepared by using the nano boehmite powder as a filler has the advantages of high ink-absorbing speed, uniform ink diffusion, no floating ink phenomenon and high glossiness.

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 the 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 the following steps: The present application provides a preparation method of a photographic paper ink absorption coating containing nanometer boehmite powder, which comprises the following steps: Preparation of nanometer boehmite powder; Dispersing the nanometer boehmite powder into a solvent to obtain a filler dispersion liquid; Preparation of an adhesive solution with an adhesive; Mixing the filler dispersion liquid and the adhesive solution to obtain a mixed slurry; Coating the mixed slurry on the photographic paper and performing solidification to obtain the photographic paper ink absorption coating; The step of preparing the nanometer boehmite powder comprises the following steps: Step A: adding an aluminum salt solution into a reaction kettle; the concentration of the aluminum salt solution is 10-20wt%; Step B: adding an ammonia water solution into the aluminum salt solution at a rate of 50-100g / min until the pH value is 9-11, and maintaining for 1h; the concentration of the ammonia water solution is 5-15wt%; Step C: adding a solution containing a weak acid into the solution obtained in step B at a rate of 20-50g / min until the pH value is 5-6, and maintaining for 30min; 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 kettle, controlling the crystallization temperature to be 120-140℃, and controlling the crystallization time to be 2-8h to obtain a crystallization liquid; Step F: the crystallization solution is washed by water to remove impurities, and is spray dried to obtain the nanometer boehmite powder.

[0005] Further, the aluminum salt is aluminum sulfate; the weak acid is acetic acid.

[0006] Further, the surface modifier is one or more of polyacrylic acid, polyethylene glycol, cetyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate, and the surface modifier is used in an amount of 0.1-1wt% of the solution obtained in step C.

[0007] Further, the surface modifier is a polar surface modifier, and the binder is a polar binder.

[0008] Further, the surface modifier is polyacrylic acid, and the binder is polyvinyl alcohol.

[0009] Further, the mass ratio of the nanometer boehmite powder to the binder is 10:1-15:1.

[0010] Further, the surface modifier is an azobenzene group modified polyacrylic acid derivative; and the method further comprises: irradiating the photographic paper ink absorption coating with ultraviolet light before using the photographic paper ink absorption coating.

[0011] Further, adding the surface modifier to the solution obtained in step C comprises: adding the surface modifier to 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.

[0012] Further, the method for obtaining the crystallization solution comprises: transferring the liquid reactant obtained in step D into a crystallization kettle, 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.

[0013] Further, mixing the filler dispersion liquid and the binder comprises: slowly adding the binder solution to the filler dispersion liquid under low-speed stirring.

[0014] The weak base, the industrial aluminum salt, and the weak acid are used as raw materials, and after two-step pH adjustment, the nanometer boehmite powder with uniform particle size distribution, high specific surface area, and high dispersity can be obtained by low-temperature hydrothermal crystallization reaction at 120-140℃ in the presence of the surface modifier. The photographic paper ink absorption coating prepared by using the nanometer boehmite powder as the filler has the advantages of fast ink absorption speed, uniform ink diffusion, no ink floating phenomenon, and high gloss. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1XRD test pattern of boehmite powder of Example 1, in which the abscissa is the diffraction angle and the ordinate is the absorbance.

[0016] Figure 2 XRD test pattern of boehmite powder of Example 2, in which the abscissa is the diffraction angle and the ordinate is the absorbance.

[0017] Figure 3 XRD test pattern of boehmite powder of Example 3, in which the abscissa is the diffraction angle and the ordinate is the absorbance.

[0018] Figure 4 XRD test pattern of boehmite powder of Comparative Example 1, in which the abscissa is the diffraction angle and the ordinate is the absorbance.

[0019] Figure 5 XRD test pattern of boehmite powder of Comparative Example 2, in which the abscissa is the diffraction angle and the ordinate is the absorbance.

[0020] Figure 6 XRD test pattern of boehmite powder of Comparative Example 3, in which the abscissa is the diffraction angle and the ordinate is the absorbance.

[0021] Figure 7 XRD test pattern of boehmite powder of Comparative Example 4, in which the abscissa is the diffraction angle and the ordinate is the absorbance.

[0022] Figure 8 Effect of Step B holding time on boehmite pore volume.

[0023] Figure 9 Effect of Step B feed rate on boehmite particle size.

[0024] Figure 10 Effect of Step C holding time on boehmite particle size.

[0025] Figure 11 Effect of Step C end point pH on boehmite pore volume.

[0026] Figure 12 Effect of Step C end point pH on boehmite particle size.

[0027] Figure 13 SEM of boehmite powder of Example 1.

[0028] Figure 14 SEM of boehmite powder of Example 3.

[0029] Figure 15 SEM of boehmite powder of Comparative Example 1.

[0030] Figure 16SEM image of boehmite powder of Comparative Example 3.

[0031] Figure 17 SEM image of boehmite powder of Comparative Example 4. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present application are described below in conjunction with the accompanying drawings, in which it is understood that the preferred embodiments described below are merely intended to illustrate and explain the present application, and are not intended to limit the present application.

[0033] The present application provides a preparation method of an ink-absorbing coating of photographic paper containing nano boehmite powder, comprising the following steps: Step S11: preparing nano boehmite powder.

[0034] In step S11, steps A-F are included.

[0035] Step A: adding an aluminum salt solution into a reaction kettle; the concentration of the aluminum salt solution is 10-20wt%.

[0036] The aluminum salt solution used in step A can be one or more of aluminum sulfate, aluminum nitrate, and aluminum chloride. After the aluminum salt is dissolved to form an aqueous solution with a concentration of 10-20wt%, the solution is added into the reaction kettle to provide a source of aluminum ions for the subsequent reaction.

[0037] Step B: adding an ammonia solution into the aluminum salt solution at a rate of 50-100g / min until the pH value is 9-11, and maintaining for 1h; the concentration of the ammonia solution is 5-15wt%.

[0038] Step B is a stage of neutralizing and precipitating to obtain boehmite precursor. The specific operation can be performed in the following manner: after the aluminum salt solution is added into the reaction kettle, stirring is performed at a temperature of 20-40℃ and a rotation speed of 200-350rpm, while the ammonia solution with a concentration of 5-15wt% is added into the aluminum salt solution at a rate of 50-100g / min until the pH value of the reaction system reaches 9-11, and then the state is maintained for 1h.

[0039] Under such conditions, the aluminum ions can be completely precipitated and converted into boehmite precursor, effectively avoiding the generation of gibbsite, and the alkaline environment helps to improve the porous performance of boehmite, increase the specific surface area and pore volume of the final product.

[0040] The inventors have found through a large number of experiments that in step B, it is crucial to control the end point pH value to be 9-11. If the pH value is too high, gibbsite will be generated, which will affect the purity of the product and reduce the dispersibility of the product. Meanwhile, the high pH value makes the reaction substance agglomerate to a high degree, which cannot be completely dispersed by the subsequent addition of weak acid, and the final particle size of the product is large and the particle size distribution is wide.

[0041] The inventors have found through a large number of experiments that, in step B, it is crucial to use ammonia water of a suitable concentration and to control the addition rate of ammonia water to be 50-100 g / min. This is because if the addition rate is too fast, the local pH value of the reaction system will rise sharply, forming a high-alkaline microzone, causing the aluminum ions to precipitate rapidly in large quantities in the local area, which is prone to generate boehmite impurities, and the precipitated particles will be severely agglomerated due to rapid growth, resulting in a wide particle size distribution of the precursor. If the addition rate is too slow, the neutralization reaction time will be prolonged, reducing the production efficiency, and the incomplete reaction in the local area may cause incomplete precipitation of aluminum ions, affecting the yield and purity of the final product. Figure 9 The effect of the feeding speed on the average particle size of boehmite is shown.

[0042] The holding time of step B also has an effect on the pore volume of boehmite, as shown in Figure 8

[0043] Step C: A solution containing a weak acid is added to the solution obtained in step B at a rate of 20-50 g / min until the pH value is 5-6, and maintained for 30 min.

[0044] The weak acid used in step C can be one or more of formic acid, acetic acid, and acrylic acid. These weak acids are first configured into a 2-5 wt% aqueous solution, and then added to the reaction solution obtained in step B at a speed of 20-50 g / min, controlling the pH value at the end of the reaction to be between 5-6, and maintaining this state for 30 min.

[0045] Gradual addition of weak acid to the solution obtained in step B allows the resulting boehmite precursor seed to gradually hydrolyze, inhibiting the excessive growth of the seed, and obtaining a precursor seed with uniform particle size. In this process, the slow addition of weak acid allows the weak acid to react slowly and sufficiently with the precursor seed, which is beneficial to reducing the precursor particle size, increasing the surface energy of the precursor, and significantly increasing the free energy of the system, so that the material can be phase-transformed into boehmite at a lower temperature, which is a key means to reduce the crystallization temperature.

[0046] In step C, if a strong acid is used, the hydrogen ion concentration dissociated by the strong acid is high, and the reaction activity is strong, which will quickly and violently react with the precursor seed, causing the pH value of the system to drop sharply, destroying the slow hydrolysis balance of the seed, not only causing the precursor particles to agglomerate violently, but also causing incomplete crystal transformation due to the excessively violent reaction, generating non-boehmite phase impurities, and the excessively strong acidic environment will severely damage the pore structure, greatly reducing the specific surface area and pore volume of the product.

[0047] ​The inventors found through a large number of experiments that the end point pH of step C is crucial. If the pH value is higher, the weak acid cannot fully react with the precursor, which will result in too large particle size and slow phase transition speed, while if the pH value is lower, the high ion concentration of the system will cause secondary agglomeration, and the too acidic environment will affect the pore volume growth and reduce the oil absorption value of the product, which is not conducive to the preparation of ink-absorbing coating layer. The effects of the end point pH of step C on the boehmite pore volume and average particle size are shown in Figure 11 and Figure 12 .

[0048] If the weak acid is added too fast, the local pH value will drop rapidly, causing the precursor crystal seeds in this area to react excessively, while the reaction in other areas is insufficient, resulting in uneven particle size and wide particle size distribution, and the fast-added acid cannot fully contact with the precursor crystal seeds, making it difficult to achieve uniform particle size control, which ultimately affects the dispersibility and ink-absorbing performance of the boehmite powder.

[0049] The holding time of step C also has an effect on the boehmite, as shown in Figure 10 .

[0050] Step D: Adding a surface modifier to the solution obtained in step C.

[0051] The surface modifier used in step D includes one or more of polyacrylic acid, polyethylene glycol, cetyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate. After adding it to the solution obtained in step C, the surface modifier can bind to the hydroxyl groups on the surface of the boehmite, coating the powder and preventing agglomeration, improving the uniformity of the particle size of the final product and making the particle size distribution more concentrated. The improvement in the agglomeration state also helps to improve the dispersibility of the powder.

[0052] Step E: Transferring the liquid reactant obtained in step D into a crystallization kettle, controlling the crystallization temperature at 120-140°C, and the crystallization time at 2-8 hours to obtain a crystallization liquid.

[0053] Step E is to transfer the liquid reactant obtained in step D into a crystallization kettle, control the crystallization temperature at 120-140°C, and the crystallization time at 2-8 hours to obtain a crystallization liquid.

[0054] Step F: Washing the crystallization liquid to remove impurities and spray drying to obtain a nano boehmite powder.

[0055] Boehmite is usually produced by hydrothermal synthesis in industry. Industrial hydrothermal synthesis of boehmite is usually carried out by using aluminum salt + sodium hydroxide or nitric acid, sulfuric acid, hydrochloric acid + sodium metaaluminate. The hydrothermal synthesis temperature is usually above 160℃, and the synthesis time is usually 6-24h. The boehmite synthesized by this method has a large number of surface hydroxyl groups and is prone to agglomeration, resulting in a wide particle size distribution and uneven size. Moreover, although this preparation method is simple, it uses a large amount of strong acid and strong base in the production process, which is highly corrosive and harmful to personnel, equipment, and the environment. In addition, the high-temperature and long-time hydrothermal synthesis has high energy consumption, which is not conducive to controlling production costs.

[0056] The reaction process is precisely controlled and scientifically designed by the inventors, so that a nano boehmite powder with concentrated particle size distribution, high specific surface area and high dispersity can be prepared. The inventors found that the combination of speed control, pH end point and maintenance time of the two-step pH adjustment step plays a decisive role in the properties of the product. In terms of particle size distribution control, step B avoids the generation of gibbsite impurities and particle agglomeration caused by local pH surge by limiting the ammonia concentration to 5-15wt%, controlling the addition rate at 50-100g / min, and strictly controlling the end point pH value at 9-11 to ensure uniform precipitation of aluminum ions into boehmite precursors; step C further adds 2-5wt% weak acid solution at a rate of 20-50g / min to a pH value of 5-6, which inhibits the excessive growth of crystal seeds by slow hydrolysis, forming a precursor with uniform particle size, and the combination of the two effectively narrows the particle size distribution. In terms of improving specific surface area and dispersity, the alkaline environment of step B lays the foundation for the porous structure of boehmite, reducing agglomeration; the weak acid regulation of step C reduces the crystallization temperature, retaining more pores; the surface modifier added in step D further prevents agglomeration and improves dispersibility by combining with the surface hydroxyl groups of boehmite, ultimately achieving high specific surface area and good dispersity.

[0057] The nano boehmite powder prepared in the embodiments of the present application has a concentrated particle size distribution, which can ensure uniform coating structure and avoid local ink absorption differences; high specific surface area and porous structure can quickly adsorb ink and lock pigment molecules, reducing bleeding and penetration; and high dispersity ensures uniform distribution of the powder in the coating, forming a continuous and stable ink absorption network, improving the print clarity and drying speed of the photographic paper. Therefore, the nano boehmite powder prepared in the embodiments of the present application is very suitable as a photographic paper ink absorption coating filler.

[0058] In addition, the slow addition of weak acid in step C allows the precursor crystal seeds to slowly hydrolyze, reducing the particle size and increasing the surface energy, significantly improving the system free energy, reducing the energy barrier of boehmite phase transition, and enabling the crystallization process to be completed at 120-140℃, which avoids the problems of violent reaction and high-temperature energy consumption in traditional strong acid and strong base methods.

[0059] Step S12: dispersing the nanometer boehmite powder into a solvent to obtain a filler dispersion liquid; For example, when dispersing the nanometer boehmite powder into a solvent, the solvent needs to meet the following conditions: no chemical reaction with the boehmite powder, uniform dispersion of the powder, and good compatibility with the subsequent adhesive solution. Suitable solvents can be water, polar organic solvents, and mixed solvents composed of polyhydric alcohols and water. It can be understood that a pH regulator can be added after the nanometer boehmite powder is dispersed in the solvent to better disperse the boehmite filler. For example, the pH of the slurry can be adjusted to 2-4 using a pH regulator.

[0060] Step S13: preparing an adhesive solution with an adhesive.

[0061] In the ink-absorbing coating of the photographic paper in the embodiments of the present application, the core function of the adhesive is to firmly bond the nanometer boehmite powder to the surface of the photographic paper substrate, while avoiding blocking the pore structure of the boehmite and affecting its ink-absorbing performance, and having good compatibility with the boehmite and other components of the coating. Considering the polar surface properties (rich in hydroxyl groups) of the boehmite and the functional requirements of the ink-absorbing coating, suitable adhesives can be polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyvinyl butyral (PVB), and vinyl acetate-ethylene copolymer (VAE emulsion).

[0062] In the preparation of the ink-absorbing coating of the photographic paper, dispersing the adhesive into a solution before mixing with the filler can uniformly disperse the adhesive in the solvent at the molecular level, avoiding solid agglomeration. When the adhesive is mixed with the filler in a molecular state, it can more uniformly wrap the surface of the boehmite powder and the interstitial space between the particles, forming a continuous and uniform bonding network, preventing local bonding deficiencies or excessive thickness defects in the coating.

[0063] Taking polyvinyl alcohol as an example, the process of preparing an adhesive solution with an adhesive can be as follows: adding polyvinyl alcohol powder into deionized water, stirring at a temperature of 70-90°C for 1-2 hours until complete dissolution, forming a transparent and uniform liquid.

[0064] Step S14: mixing the filler dispersion liquid and the adhesive solution to obtain a mixed slurry.

[0065] In addition to the filler and the adhesive, one or more of the following additives can also be added to the mixed slurry according to the requirements: a dispersant that can further improve the dispersion stability of the boehmite powder and prevent agglomeration and sedimentation; a defoaming agent that can eliminate the air bubbles generated during the mixing process, avoiding pinholes or pitting after the coating is dried; a leveling agent that can improve the spreading performance of the slurry, ensuring a smooth and flat surface of the coating after coating; a thickening agent that can adjust the viscosity of the slurry to meet the requirements of different coating processes (such as doctor blade coating and roller coating); and a curing agent that can cross-link and solidify the adhesive into a network structure.

[0066] Step S15: coating the mixed slurry on the photographic paper and curing to obtain an ink-absorbing coating of the photographic paper; The common coating methods of the ink-absorbing coating of the photographic paper mainly include doctor blade coating, roller coating, spraying and dipping, and in actual production, a suitable coating method can be selected according to the performance of the slurry, the thickness requirement of the coating and the production efficiency and the like, so as to ensure that the coating is uniform and flat, and the ink-absorbing performance and appearance quality are met.

[0067] The embodiment of the present application uses weak base, industrial aluminum salt and weak acid as raw materials, and after two-step pH adjustment, the 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 dispersity. The ink-absorbing coating of the photographic paper prepared by using the nano-boehmite powder as filler has the advantages of fast ink-absorbing speed, uniform ink diffusion, no ink floating phenomenon and high gloss.

[0068] In some examples, the aluminum salt is aluminum sulfate; and the weak acid is acetic acid.

[0069] In terms of the selection of the aluminum salt, the dissociation stability of aluminum sulfate is stronger, and the release rate of aluminum ions is moderate. When reacting with ammonia water, the formation of high alkaline micro area caused by the sudden increase of local ion concentration can be avoided, the risk of generation of gibbsite impurities is greatly reduced, the by-product ammonium sulfate can be easily removed by washing with water, and no impurity ions that affect the crystal form are left to affect the purity of the product.

[0070] In terms of the selection of the weak acid, acetic acid, as a typical organic weak acid, has moderate dissociation constant and slow hydrogen ion release. After being added to the system, it can react mildly and fully with boehmite precursor seeds, accurately maintain the hydrolysis balance with pH value of 5-6, avoid the problems of crystal seed agglomeration and incomplete crystal form conversion caused by the violent reaction of strong acid, and control the reaction rate more easily than carbonic acid and other weaker acids, thereby efficiently reducing the precursor particle size and improving the surface energy, laying a foundation for low-temperature crystallization. At the same time, the organic property of acetic acid is more compatible with the subsequent polyacrylic acid surface modifier, which can further optimize the dispersion performance of the powder.

[0071] That is, by selecting aluminum sulfate and acetic acid as the reactants, the nano-boehmite powder with more uniform particle size, higher purity and better dispersity can be prepared by further synergistically regulating the reaction mildness, impurity residue and seed growth state, thereby significantly improving the process stability and product performance.

[0072] In some examples, the surface modifier is one or more of polyacrylic acid, polyethylene glycol, cetyltrimethylammonium bromide, dodecyltrimethylammonium bromide and sodium dodecylbenzenesulfonate, and the amount of the surface modifier is 0.1-1wt% of the mass of the solution obtained in step C.

[0073] The inventors determined the appropriate amount of surfactant for preparing boehmite powder for ink-absorbing coating through a large number of experiments. If the amount of surfactant is too small, the surface of boehmite precursor crystal seeds cannot be completely covered, it is difficult to form effective coating, and the powder is prone to agglomeration during subsequent crystallization and drying, resulting in a wide particle size distribution, a decrease in dispersion index, and an excessive exposure of hydroxyl groups on the surface of the powder, which further aggravates the agglomeration trend and affects the dispersion performance of the final product. If the amount is too large, the excess surface modifier will be free in the system, which not only may block the pore structure of boehmite, reduce the specific surface area and pore volume of the product (and thus affect the oil absorption value and ink absorption performance), but also may not be completely removed by the water washing process, remaining in the boehmite powder, resulting in a decrease in coating gloss and print density, and even interfering with the combination of the binder and boehmite, affecting the mechanical stability of the coating. Only the appropriate amount can make the surface modifier fully combine with the hydroxyl groups on the surface of boehmite, form uniform coating, effectively prevent agglomeration, ensure the uniformity of particle size (the dispersion index of the examples is 99.9%), and at the same time, not destroy the pore structure, ensure that the product has high dispersibility, high specific surface area and good ink absorption performance.

[0074] In some examples, the surface modifier is a polar surface modifier, and the binder is a polar binder.

[0075] The inventors found through a large number of experiments that after the crystallization reaction, part of the surface active agent is combined with the surface of the nano boehmite, showing a modification effect, and then plays a role in the preparation and application of the ink-absorbing coating. The inventors found that when a polar surface modifier is used in the hydrothermal crystallization stage and a polar binder is used in the coating preparation stage, the polar surface modifier and the polar binder can interact with each other and with the nano boehmite powder filler, further improving the performance of the ink-absorbing coating.

[0076] Specifically, the polar binder can combine with the hydroxyl groups on the surface of boehmite through hydrogen bonds or chemical bonds, allowing boehmite to maintain open pores for adsorbing ink. In addition, the polar surface modifier can combine with the polar binder molecules through hydrogen bonds, electrostatic attraction, etc., reducing the agglomeration or peeling of the binder on the surface of the nano boehmite filler, further improving the mechanical strength and uniformity of the coating. At the same time, since the polar surface modifier can interact more strongly with the binder molecules, the binder molecules are more inclined to combine with the surface modifier on the outer surface of boehmite rather than penetrating into the pores of the nano boehmite filler, thus reducing the diffusion of the binder into the pores of the nano boehmite filler, and the pores of the nano boehmite filler are not occupied by the binder, allowing more pores to be used for ink absorption. Therefore, the use of a polar surface modifier and a polar binder can further improve the performance of the ink-absorbing coating.

[0077] In some examples, the surface modifier is polyacrylic acid, and the binder is polyvinyl alcohol.

[0078] Polyacrylic acid is a polar surface modifier, and polyvinyl alcohol is a polar binder. As a surface modifier, polyacrylic acid improves the dispersibility of boehmite and enhances the polar interaction between boehmite and ink through the carboxyl group, thereby assisting boehmite in capturing ink. As a binder, polyvinyl alcohol ensures the stability of the coating structure, and its hydroxyl groups can form hydrogen bonds with the hydroxyl groups on the surface of boehmite or the carboxyl groups of polyacrylic acid, thereby constructing a continuous absorption-fixing network to reduce ink diffusion and ink floating. Therefore, using polyacrylic acid as a polar surface modifier and polyvinyl alcohol as a polar binder can further improve the performance of the ink absorption coating.

[0079] In some examples, the polyvinyl alcohol has a degree of polymerization in the range of 3500-4000. Further, the polyvinyl alcohol has a degree of polymerization in the range of 3500-3600.

[0080] Polyvinyl alcohol with a degree of polymerization in this range can best adapt to the preparation process and performance requirements of the ink absorption coating of photographic paper. In terms of the effect on ink absorption performance, the molecular chain length of polyvinyl alcohol in this degree of polymerization range is moderate, with small steric hindrance, and is not easy to penetrate into the microporous structure of boehmite, thereby maximizing the porosity and ink absorption channels. At the same time, its film-forming performance and bonding strength are sufficient to firmly fix the powder on the surface of the substrate, balancing the needs of "bonding stability" and "ink absorption capacity". Generally, a 4% solution of polyvinyl alcohol with a degree of polymerization of 3500-3600 has a viscosity of 80-110 mpa·s at room temperature. In comparison, polyvinyl alcohol with a degree of polymerization higher than 4000 has a longer molecular chain, which not only increases the solubility but also significantly increases the solution viscosity, making it difficult to obtain a smooth coating in the coating process, which is not conducive to the production process. On the other hand, polyvinyl alcohol with a degree of polymerization lower than 3000 has excellent water solubility, but the short molecular chain may result in insufficient bonding strength, causing the coating to easily fall off. At the same time, it is also easy to block the boehmite pores due to molecular entanglement, thereby affecting the 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), and may enter the boehmite pores, thereby affecting the ink absorption performance. However, polyvinyl alcohol with a degree of polymerization of 3500-4000, especially 3500-3600, can be stably dissolved at 70-90°C to form a solution with moderate viscosity (which meets the subsequent coating process requirements for slurry fluidity), which can ensure uniform mixing with boehmite powder and prevent dispersion difficulties caused by high viscosity.

[0081] In some examples, the mass ratio of nanometer boehmite powder to binder is 10:1-15:1.

[0082] The inventors found through a large number of experiments that when the mass ratio of nano-boehmite powder and binder is 10:1-15:1, the relative amount of surface modifier and binder can balance the dispersion stability, bonding strength and ink absorption performance. The surface modifier needs to be coated on the powder to prevent agglomeration and work together with the binder, and the binder needs to achieve firm bonding of the powder and the substrate and not to block the pores. At this ratio, the surface modifier can fully coat the powder, guide the uniform distribution of the binder in the gap through the polar effect instead of filling the pores, and the amount of binder is sufficient to form a continuous bonding network and reduce diffusion to the pores. If the amount of surface modifier is too much, free molecules may weaken the bonding force or block the pores; if the amount of binder is too much, it is easy to cause secondary agglomeration of the powder and block the pores, affecting the ink absorption performance. At the same time, from the process point of view, this ratio can keep the viscosity of the slurry in the ideal range of 80-150 cP (viscosity test temperature is 55°C, the viscosity of pure water-based slurry is preferably 100-150 cP, and the viscosity of oil-based coating with organic solvents such as DMF is slightly lower), which can neither cause uneven coating due to too high viscosity (>300 cP) nor cause powdering problem after drying due to insufficient bonding force (for example, when the mass ratio of nano-boehmite powder and binder is >15:1), and can meet the requirements of processes such as doctor blade coating. Therefore, this amount ratio can realize the synergistic effect of "dispersion-bonding", ensuring that the coating has high dispersibility, strong mechanical properties, excellent ink absorption capacity and good processability.

[0083] In some examples, the surface modifier is an azobenzene group modified polyacrylic acid derivative; the method for preparing the photographic paper ink absorption coating further comprises: irradiating the photographic paper ink absorption coating with ultraviolet light before using the photographic paper ink absorption coating.

[0084] In the preparation of the photographic paper ink absorption coating, when the azobenzene group modified polyacrylic acid derivative is used as the surface active agent, its working principle is based on the synergistic effect of molecular structure and function: the derivative retains the carboxyl group of polyacrylic acid, which can form a uniform coating under stirring conditions in step D by fully bonding with the surface hydroxyl group of boehmite precursor, and the spatial steric effect is enhanced by the rigid structure of the azobenzene group, effectively inhibiting powder agglomeration and ensuring high dispersibility and narrow particle size distribution of nano-boehmite; at the same time, the azobenzene group has the photoresponsive cis-trans isomerization property, which will change from trans form to cis form when irradiated with ultraviolet light after the coating is cured and before printing, driving the movement of polymer segments, dynamically adjusting the surface polarity of the powder and the porous channel structure of the coating, optimizing the ink affinity, reducing the ink floating phenomenon and improving the ink absorption uniformity. After printing, it can naturally restore the ink fixing property to prevent ink diffusion.

[0085] The step of irradiating the ink-receptive coating of the photographic paper with ultraviolet light is performed before the ink-receptive coating of the photographic paper is prepared and before inkjet printing. The microstructure and surface properties of the coating are regulated by external stimulation. After the traditional coating is cured, it can be directly used for printing. After the azobenzene-modified surfactant is introduced, the dynamic ink-absorption regulating ability of the coating needs to be activated by ultraviolet light irradiation. In an example, the parameters for irradiating the ink-receptive coating of the photographic paper with ultraviolet light are as follows: ultraviolet wavelength 254-365 nm, intensity 5-10 mW / cm2, and time 5-15 min, so as to ensure that the azobenzene groups are fully converted from the trans configuration to the cis configuration, so as to optimize the pore openness and surface affinity.

[0086] In some examples, step D comprises: adding a surface modifier to the solution obtained in step C, and stirring at a temperature of 30-50°C and a speed of 150-250 rpm for 20-40 min. The addition of the surface modifier and the stirring at a temperature of 30-50°C and a speed of 150-250 rpm for 20-40 min can achieve the following technical effects: first, the uniformity of the surface modifier can be improved. The stirring at a temperature of 30-50°C can enhance the molecular mobility of the surface modifier, so that the surface modifier is more easily diffused to the surface of the boehmite precursor seed crystal; the speed of 150-250 rpm can provide sufficient shear force to break local agglomeration, and can also avoid the destruction of the crystal structure caused by excessive speed. The stirring time of 20-40 min ensures that the functional groups of the modifier molecules are in sufficient contact with the hydroxyl groups on the surface of the crystal seed and form stable bonds, reduces the number of uncoated “bare” crystal seeds, and thus reduces the risk of powder agglomeration in subsequent processes. Second, the surface modifier that is sufficiently bonded can form a uniform steric hindrance layer on the surface of the crystal seed, which hinders the Van der Waals force attraction between particles, further narrows the particle size distribution, makes the powder obtained by subsequent spray drying more easily dispersed in the solvent, reduces the agglomeration and precipitation of the dispersion liquid, and lays a foundation for preparing a uniform mixed slurry. Third, the surface modifier can promote the synergistic effect of the binder. The uniformly coated surface modifier can provide consistent interface sites for the binding 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, avoid local bonding deficiency or pore blockage caused by uneven modification, and ensure that the coating has good mechanical strength and retains sufficient ink-absorbing pores.

[0087] In some examples, step E comprises: transferring the liquid reactant obtained in step D into a crystallization kettle, pre-crystallizing at 80-100°C for 1-2 h, then increasing the temperature to 120-140°C at a rate of 5°C / min, and crystallizing for 4 h.

[0088] The step-by-step crystallization and the staged temperature increase can inhibit the non-directional aggregation of the boehmite precursor at a low temperature stage, and promote the uniform development of the mesoporous structure. This is helpful to increase the specific surface area and increase the pore volume concentration.

[0089] For example, inert gas can be introduced during the pre-crystallization stage, with a flow rate of 0.5-2 L / min. When impurities or intermediate products of the previous reaction are present in the crystallization raw material, the warm conditions of the pre-crystallization stage (80-100°C) can accelerate the reaction of trace amounts of oxygen or carbon dioxide in the system. The introduction of inert gas can isolate the oxygen in the air, prevent the oxidation of impurity ions, and thus reduce the color impurities or defects in the powder, thereby improving the purity and whiteness of the boehmite nanopowder. This is particularly important for the ink absorption coating of photographic paper, because high-purity powder can prevent the coating from yellowing or abnormal color development, ensuring the color reproduction of the photographic paper. At the same time, the introduction of inert gas can drive out the dissolved air and generated bubbles in the system, and the airflow disturbance can make the solution more uniformly heated, reducing the local overheating that causes bubble aggregation, which is conducive to the uniform growth of boehmite grains, forming a nanopowder with a narrower particle size distribution and more regular morphology. For example, the inert gas can be nitrogen, argon, or the like.

[0090] In some examples, mixing the filler dispersion and the binder solution to obtain the mixed slurry includes: slowly adding the binder solution to the filler dispersion under low-speed stirring.

[0091] In the preparation of the mixed slurry for the ink absorption coating of photographic paper, the method of first adding the filler dispersion to the stirring tank and slowly adding the binder solution under low-speed stirring has the following advantages: First, this method can make the surface modifier such as polyacrylic acid and the binder such as polyvinyl alcohol more fully cooperate through polar interaction. The powder coated in advance by the surface modifier provides uniform binding sites for the binder, reducing the entanglement and aggregation of binder molecules, and ultimately forming a mixed slurry that is uniformly dispersed, stably bonded, and has open pores, which ensures the ink absorption performance and mechanical strength of the coating after subsequent coating. Second, low-speed stirring can avoid excessive bubbles caused by vigorous stirring, reducing the risk of pinholes or pitting after subsequent coating and drying, and providing a gentle environment for the gradual fusion of the two liquids. Third, the order of "first adding the filler dispersion and then slowly adding the binder" allows the binder solution to gradually spread around the filler particles, and with the help of the shearing force of low-speed stirring, the binder is uniformly wrapped around the boehmite powder surface and the interparticle gaps, avoiding the agglomeration or pore blockage problems caused by excessive local binder. In addition, this order of addition can effectively prevent the "gel" phenomenon caused by excessive local binder concentration, reduce the thickness deviation of the coating, and significantly improve the color uniformity of the printed image.

[0092] In some examples, mixing the filler dispersion and the binder solution to obtain the mixed slurry includes: slowly adding the binder solution to the filler dispersion under low-speed stirring, and then adding other additives (functional additives such as leveling agents and curing agents) to obtain the filler dispersion.

[0093] Example 1: A method for preparing an ink-absorbing coating of photo paper containing nanometer boehmite powder, which is specifically implemented according to the following steps: Step S11: preparing nanometer boehmite powder, including steps A-F.

[0094] Step A: adding 20 kg of 10 wt% aluminum sulfate solution into a 50 L reaction kettle and heating to 30 ℃.

[0095] Step B: adding 10% wt ammonia solution into the aluminum sulfate solution in the reaction kettle at a rate of 100 g / min to pH 10.5 at a stirring rate of 250 rpm for 1 h.

[0096] Step C: adding 3 wt% acetic acid solution into the solution obtained in step B at a rate of 30 g / min to pH 5 and maintaining for 30 min.

[0097] Step D: adding 0.15 wt% polyacrylic acid of the mass of the solution obtained in step C into the solution obtained in step C, stirring at a temperature of 40 ℃ and a stirring rate of 200 rpm for 30 min.

[0098] Step E: transferring the liquid reactant obtained in step D into a crystallization kettle, controlling the crystallization temperature to be 140 ℃ and the crystallization time to be 4 h to obtain a crystallization liquid.

[0099] Step F: obtaining superfine and easily dispersed nanometer boehmite powder by water washing and spray drying the obtained crystallization liquid.

[0100] Step S12: dispersing the nanometer boehmite powder obtained in step F into 1.6 L water, adjusting the pH, and high-speed dispersing to obtain a filler dispersion liquid.

[0101] Step S13: dissolving polyvinyl alcohol (KURARAY POVAL™ 95-88 brand) of 1 / 12 of the mass of the nanometer boehmite powder into 0.65 L water to prepare an adhesive solution.

[0102] Step S14: slowly adding the adhesive solution into the filler dispersion liquid under low-speed stirring, maintaining at 50-60 ℃ after stirring, then adding other functional additives (such as leveling agent, curing agent, etc.), stirring and maintaining, filtering and defoaming to obtain a mixed slurry.

[0103] Step S15: coating the mixed slurry on photo paper at a coating amount of 32 g / ㎡ and curing to obtain an ink-absorbing coating of photo paper.

[0104] Example 2: A method for preparing an ink-absorbing coating of photo paper containing nanometer boehmite powder, which is specifically implemented according to the following steps: Step S11: preparing nanometer boehmite powder, including steps A-F.

[0105] Step A: Add 20 kg of 20 wt% aluminum sulfate solution into a 50 L reactor, and heat to 30℃.

[0106] Step B: Add 15 wt% ammonia solution into the aluminum sulfate solution in the reactor at a rate of 80 g / min, with stirring at 250 rpm, until the pH reaches 11, and maintain for 1 h.

[0107] Step C: Add 3 wt% acetic acid solution into the solution obtained in Step B at a rate of 20 g / min until the pH reaches 6, and maintain for 30 min.

[0108] Step D: Add 0.3 wt% polyacrylic acid based on the mass of the solution obtained in Step C into the solution obtained in Step S120, and stir at 200 rpm at a temperature of 40℃ for 30 min.

[0109] Step E: Transfer the liquid reactant obtained in Step D into a crystallization kettle, and control the crystallization temperature at 130℃ and the crystallization time at 6 h to obtain a crystallization liquid.

[0110] Step F: Remove impurities from the obtained crystallization liquid by water washing, and spray dry to obtain superfine and easily dispersible boehmite nanopowder.

[0111] Step S12: Disperse the boehmite nanopowder obtained in Step F into 1.6 L of water, adjust the pH, and disperse at high speed to obtain a filler dispersion liquid.

[0112] Step S13: Dissolve polyvinyl alcohol (KURARAY POVAL™ 95-88 brand) in an amount of 1 / 12 of the mass of the boehmite nanopowder into 0.65 L of water to prepare an adhesive solution.

[0113] Step S14: Slowly add the adhesive solution to the filler dispersion liquid under low-speed stirring, and after stirring, maintain the temperature at 50-60℃, then add other additives (functional additives such as leveling agents and curing agents), stir and maintain the temperature, filter and defoam to obtain a mixed slurry.

[0114] Step S15: Apply the mixed slurry to photographic paper at a coating amount of 32 g / ㎡ and cure to obtain an ink-absorbing coating for photographic paper.

[0115] Example 3: A method for preparing an ink-absorbing coating for photographic paper containing boehmite nanopowder, which is implemented according to the following steps: Step S11: Prepare boehmite nanopowder, including Steps A-F.

[0116] Step A: Add 20 kg of 20 wt% aluminum sulfate solution into a 50 L reactor, and heat to 30℃.

[0117] Step B: 13 wt% ammonia solution was added to the reactor at a rate of 100 g / min with stirring at 250 rpm until pH 9.5 was reached and maintained for 1 h.

[0118] Step C: 4 wt% acetic acid solution was added to the solution from Step B at a rate of 50 g / min until pH 5.5 was reached and maintained for 30 min.

[0119] Step D: 0.15 wt% polyethylene glycol based on the mass of the solution from Step C was added to the solution from Step C and stirred at 40 °C with stirring at 200 rpm for 30 min.

[0120] Step E: The liquid from Step D was transferred to a crystallization reactor and the crystallization temperature was controlled at 120 °C for 8 h to obtain a crystallized liquid.

[0121] Step F: The crystallized liquid obtained was washed with water to remove impurities and spray dried to obtain superfine, easily dispersible boehmite nanopowder.

[0122] Step S12: The boehmite nanopowder obtained in Step F was dispersed in 1.6 L of water, the pH was adjusted, and high-speed dispersion was performed to obtain a filler dispersion liquid.

[0123] Step S13: Polyvinyl alcohol (KURARAY POVAL™ 95-88 brand) in an amount of 1 / 12 the mass of the boehmite nanopowder was dissolved in 0.65 L of water to prepare an adhesive solution.

[0124] Step S14: The adhesive solution was slowly added to the filler dispersion liquid under low-speed stirring, and after stirring, it was incubated at 50-60 °C, then other auxiliary agents (functional auxiliary agents such as leveling agents, curing agents, etc.) were added, stirred and incubated, filtered, and defoamed to obtain a mixed slurry.

[0125] Step S15: The mixed slurry was coated on photographic paper at a coating amount of 32 g / ㎡ and cured to obtain an ink-absorbing coating for photographic paper.

[0126] Example 4: The implementation scheme is the same as that of Example 1, except that the aluminum salt is aluminum chloride and the weak acid is acrylic acid.

[0127] Example 5: The implementation scheme is the same as that of Example 1, except that 2.5 wt% polyacrylic acid based on the mass of the solution from Step C was added to the solution from Step C.

[0128] Example 6: The implementation scheme is the same as that of Example 1, except that the adhesive used in Step 13 is a VAE emulsion.

[0129] Example 7: The implementation scheme is the same as that of Example 1, except that the adhesive is polyvinyl alcohol with a degree of polymerization of 2500.

[0130] Example 8: The implementation is the same as example 1, except that the mass ratio of nanobohmite powder to binder is 16:1.

[0131] Example 9: The implementation is the same as example 1, except that the surface modifier is 4-phenylazobenzene acrylamide modified polyacrylic acid. Before conducting the inkjet test, the ink-receptive coating is irradiated at a UV wavelength of 365 nm and an intensity of 5 mW / cm2for 5-15 min.

[0132] Example 10: The implementation is the same as example 1, except that in step E, the liquid reactant obtained in step D is transferred to a crystallization kettle, pre-crystallized at 90°C for 1 h, then raised to 140°C at a rate of 5°C / min, and crystallized for 4 h; nitrogen is introduced during the pre-crystallization stage, with a gas flow rate of 0.5-2 L / min.

[0133] Comparative Example 1: The implementation is the same as example 1, except that the end point pH in step B is controlled at 12, and the remaining control points are exactly the same as example 1.

[0134] Comparative Example 2: The implementation is the same as example 1, except that the end point pH in step C is controlled at 4, and the remaining control points are exactly the same as example 1.

[0135] Comparative Example 3: The implementation is the same as example 2, except that the method does not include step D, and the remaining control points are exactly the same as example 2.

[0136] Comparative Example 4: The implementation is the same as example 2, except that the weak acid is no longer used to control the pH, i.e. step C is omitted, and the remaining control points are exactly the same as example 2.

[0137] From the above data, it can be seen that the XRD patterns of the nanobohmite powders obtained in examples 1-3 have clear and sharp peaks at the positions of the characteristic diffraction peaks of boehmite, and no other peaks, indicating that the crystal form of the product is pure boehmite. Figures 1-3 Detection and results

[0138] The nanobohmite 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 testing. The inkjet printing test was conducted on the ink-receptive coating of the photographic paper obtained in examples 1-10 and comparative examples 1-4. The particle size distribution was tested using a zeta potential particle size analyzer; for the dispersion index test, the nanobohmite powder was configured as a 10% aqueous solution, mechanically stirred for 5 min, then centrifuged at 3000 rpm for 10 min, and the bottom sediment was dried and tested.

[0139]

[0140] ​The oil absorption value is measured by the dioctyl phthalate (DOP) test method. 100 g of nanobohmite powder is mixed with 150 g of DOP for 30 min to allow the nanobohmite powder to fully absorb the DOP. The mixture is filtered using a vacuum pump for 30 min. The weight gain of the powder is the oil absorption value.

[0141] The inkjet printing test is performed using the EPSON 805L high-quality glossy paper mode. The main focus is on whether there is ink floating, paper gloss (60°), and color density K value (color density is tested using the X-Rite i1).

[0142] Table 1 Note: In Table 1-Table 3, " / " indicates that the true data cannot be tested due to ink floating in the black block, and it is unnecessary to test it.

[0143] wherein, Figures 13-17 The SEM images of the nanobohmite powder obtained by spray drying in Example 1, Example 3, Comparative Example 1, Comparative Example 3, and Comparative Example 4 are shown in FIGS. 1-4, respectively. In the spray drying process, the material is sprayed into droplets by high-pressure gas, and the spherical or spherical-like nanoparticles are formed by agglomeration / accumulation after high-temperature drying. This agglomeration is reversible, and can be dispersed in water. When measuring the particle size of the nanobohmite powder, the nanobohmite powder obtained by spray drying is dissolved in water to form a 0.2%-0.5% aqueous solution, which is then dispersed by mechanical stirring before measurement.

[0144] From the results of Comparative Example 1 in Table 1, it can be seen that it is necessary to control the pH value of the system to be 9-11 in Step B, i.e., the stage of obtaining boehmite precursor by neutralization and precipitation, and too high pH value will lead to the generation of gibbsite, thereby affecting the purity of the product, reducing the dispersibility of the product, and too high pH value makes the degree of agglomeration of the reactants high, and the weak acid cannot completely disperse, so that the final particle size of the product is large and the particle size distribution is wide; the crystal structure of gibbsite is dense and the porosity is low, which is significantly different from the porous characteristics of boehmite, so that the overall specific surface area of the powder is reduced from 260+ m2 / g of Examples 1-3 to 202.38 m2 / g of Comparative Example 1, and the pore volume is reduced from about 0.7 ml / g to 0.585 ml / g. That is, too high pH value in Step B will cause uneven structure, resulting in the overall deterioration of ink absorption coating from physical adsorption capacity, surface morphology to color development uniformity, leading to the results of poor ink absorption and low gloss of Comparative Example 1. Among them, the poor ink absorption of blue, red and black blocks is mainly because the destruction of the porous structure directly weakens the physical adsorption capacity of the coating to the ink, especially the capture ability of small molecule dye ink such as blue and red is reduced, which leads to the fact that the ink cannot be quickly locked in the pores, resulting in local insufficient penetration or uneven diffusion. At the same time, the serious agglomeration caused by high pH value makes the powder particle size distribution wide, and the "uneven density" defect area is formed in the coating structure, the dense agglomerate area hinders the penetration of the ink, and the loose area causes bleeding due to excessive diffusion of the ink, finally showing the difference in ink absorption of color blocks. Gloss depends on the flatness and uniformity of light reflection of the coating surface. In Comparative Example 1, the existence of gibbsite and large particle agglomerates makes it easy to form uneven surface when stacking in the coating, increasing light scattering rather than mirror reflection. In addition, the gap between the agglomerated particles is uneven, and there are small pores and protrusions on the surface of the coating after drying, further destroying the uniformity of light reflection, resulting in the 60° gloss being reduced from 58-60 of the examples 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 structure of gibbsite impurities cannot effectively support carbon black particles, leading to the fact that the ink is stacked on the surface rather than uniformly penetrated, resulting in "ink floating" or local insufficient concentration; the large particle size of the agglomerates makes the pores in the coating blocked, and the carbon black particles cannot penetrate deep into the coating, resulting in a thin and unevenly distributed color development layer; at the same time, the uneven surface leads to disorderly light reflection, further interfering with the accuracy of color density detection, and finally no effective K value data can be obtained.

[0145] From the results of Comparative Example 2, it can be seen that it is necessary to control the pH value of the system to be 5-6 in Step C, i.e., the stage of gradually dispersing the precursor by using weak acid, and too low pH value will lead to the increase of H + concentration, thereby causing the reactants to agglomerate again, increasing the particle size of the final product, and the low pH value affects the crystallinity of the final product, reducing the crystallinity of the product; at the same time, in the actual operation process, the H +The excessive introduction of the excess amount greatly increases the viscosity of the system, which is not conducive to the normal production. Further, the particle agglomeration, pore damage and dispersion decrease caused by the excessively low pH in step C comprehensively affect the coating performance from ink absorption capacity, surface morphology to color development uniformity, resulting in the results of large-area ink floating, gloss reduction and abnormal color density of the ink-absorbing coating of the photographic paper of Comparative Example 2. Specifically, the excessively low pH in step C can destroy the hydrolysis balance of the boehmite precursor. On the one hand, the strong acid environment makes the hydrogen ion concentration in the system too high, resulting in the violent reaction of the precursor crystal seeds instead of slow hydrolysis, causing secondary agglomeration of the particles, the particle size distribution becoming wide and the surface energy decreasing, and the powder dispersion deteriorating. The agglomerated particles can block the internal pores of the coating, so that the porous structure of the boehmite cannot fully play the role of ink absorption, and the ink is difficult to be quickly absorbed into the pores, and a large amount of ink is left on the surface of the coating to form "ink floating". On the other hand, the over-acid environment can destroy the pore structure of the boehmite, resulting in a decrease of the specific surface area from 260+㎡ / g of Examples 1-3 to 216.32㎡ / g, a decrease of the pore volume from about 0.7ml / g to 0.527ml / g, a decrease of the oil absorption value from about 80 to 72.4, a significant weakening of the ink absorption capacity, and further aggravation of the accumulation of ink on the surface, which finally shows large-area ink floating. The gloss depends on the flatness of the coating surface and the uniformity of light reflection. In Comparative Example 2, the secondary agglomeration of particles caused by the excessively low pH makes the particle size distribution of the powder uneven, and the larger agglomerates are difficult to disperse uniformly during the coating process, and are stacked to form local protrusions or depressions. At the same time, after the pore structure is destroyed, the water evaporation rate is uneven during the drying process of the coating, which is easy to produce small shrinkage cracks or uneven surface, increases the probability of light scattering, and weakens the mirror reflection effect. Therefore, the 60° gloss is reduced to 53.3. The color density K value reflects the color development stability and uniformity of black ink, which needs to rely on the uniform adsorption and fixation of the coating to carbon black particles. In Comparative Example 2, after the agglomerated particles block the pores, the carbon black particles cannot penetrate into the interior of the coating, but are only attached to the surface, and due to the decrease of the ink absorption capacity, the ink distribution is extremely uneven, with excessive accumulation in some areas and insufficient in some areas, resulting in color development disorder. In addition, the large-area ink floating makes the ink form an irregular liquid film on the surface, and after drying, the color development layer has a large thickness difference, and the light reflection interference caused by the uneven surface makes the color density detection unable to obtain stable data, and finally the K value cannot be displayed.

[0146] From the results of Comparative Example 3, it can be seen that the introduction of the surface modifier is necessary, the introduction of the surface modifier can coat the powder, prevent the powder from agglomeration, improve the uniformity of boehmite particle size, make the particle size distribution more concentrated, and improve the dispersion performance of the powder. Due to the absence of the surface modifier, although a boehmite product with a smaller Dav is obtained in the comparative example, the overall particle size distribution is wide, and the dispersion index is only 94.5%, which is significantly reduced; it can be seen from the SEM image that due to the absence of the surface modifier, the spherical particles obtained by spray granulation increase significantly, and the surface is rougher. Further, the absence of the polyacrylic acid surface modifier in Comparative Example 3 leads to the deterioration of the dispersibility and interfacial compatibility of the nano boehmite powder, and further causes the performance defects of the ink absorption coating. Specifically, the absence of the surface modifier (such as polyacrylic acid) exposes too many hydroxyl groups on the surface of the boehmite precursor, which is prone to serious agglomeration during crystallization and drying (test data shows that its D100 is 392.4 nm, which is much larger than 207.2-267.4 nm of Examples 1-3, the particle size distribution is wide and the dispersion index is reduced to 94.5% (lower than 99.9% of Examples 1-3). Agglomerated particles can destroy the uniform and porous structure of the coating, forming local dense areas, while blue and red inks are mostly small molecule dye type, which are more dependent on pores: the dense area hinders the penetration, and the loose area causes bleeding due to excessive diffusion of ink, resulting in uneven absorption of blue and red blocks. At the same time, the interaction between the unmodified boehmite surface polarity and the pigment molecules is weakened, which cannot effectively capture the dye, further exacerbating the ink absorption difference. Gloss depends on the flatness of the coating surface and the uniformity of light reflection. In Comparative Example 3, the powder agglomeration caused by the absence of the surface modifier makes the particle size uneven, it is difficult to form uniform accumulation during coating, and the coating surface appears uneven microstructure. The gap between the agglomerates is irregular, and small pores or protrusions are easily produced after drying, increasing light scattering rather than specular reflection. In addition, the interfacial compatibility between the unmodified boehmite and the binder is reduced, and the binder cannot uniformly wrap the powder, further destroying the surface flatness, resulting in a 60° gloss of 52.1, which is lower than the 58-60 of Examples 1-3. The color density K value reflects the color depth of black ink (carbon black particle type), which is slightly lower than that of dye type ink in terms of dependence on pore structure, but is still affected by coating uniformity. In Comparative Example 3, although the carbon black particles of black ink can be attached to the coating surface by physical filling, there is no complete colorless situation, but the local densification of the coating caused by agglomeration still reduces the effective ink absorption area, making the carbon black particles unevenly distributed, and the color depth decreases (K value from 2.28-2.31 of Examples 1-3 to 2.01). At the same time, the absence of the surface modifier leads to the weakening of the bonding force between the binder and the boehmite, and part of the carbon black particles fall off due to the loose structure of the coating, further reducing the color stability, and finally the K value is lower than the normal level.

[0147] From the results of Comparative Example 4, it can be seen that Step C, i.e. the introduction of weak acid, is necessary, and also indicates the feasibility and necessity of the two-stage pH regulation method for producing ultrafine boehmite; after the removal of the weak acid to adjust the particle size of the boehmite precursor, the average particle size of the boehmite obtained has a very obvious increase, and the overall particle size distribution is very wide. From the SEM image, it can also be seen that during the spray granulation process, spherical agglomerates with obvious particle size difference are obtained. Further, in Comparative Example 4, the weak acid adjustment process of Step C is omitted, resulting in damage to the structural uniformity and performance stability of the nano-boehmite powder, and further causing the differentiation of the ink absorption coating. Specifically, although not subjected to weak acid adjustment (Step C), the alkaline precipitation process of Step B can still generate boehmite precursors, and the powder after crystallization still has a certain porous structure (the detection data shows that the pore volume is 0.692 ml / g, and the oil absorption value is 78.3, close to the level of Examples 1-3). This porous structure provides basic physical adsorption space for ink, especially for black ink and other systems mainly composed of carbon black particles, which mainly rely on the physical capture effect of the pores, so the ink absorption state does not appear serious defects, and can still maintain the "good" level. In addition, although the powder not subjected to weak acid treatment has a larger particle size, there are still open pores in the whole, which can realize the basic ink adsorption and fixation. Gloss is highly sensitive to the micro-flatness and particle dispersion uniformity of the coating surface. In Comparative Example 4, the weak acid adjustment of Step C is omitted, and the boehmite precursor seed cannot be regulated by slow hydrolysis to achieve particle size uniformization, resulting in a very wide particle size distribution of the powder, and the particle morphology is irregular. Large particle size and uneven distribution of particles are difficult to accumulate uniformly during coating, forming a large number of uneven micro-protrusions and gaps on the surface of the coating, significantly increasing the probability of light scattering, and weakening the mirror reflection effect. At the same time, the compatibility of the powder not subjected to weak acid treatment with the binder decreases, and the binder cannot uniformly wrap the surface of the particles, and local shrinkage differences are easy to occur after drying, further damaging the surface flatness, resulting in a 60° gloss of 47.2, which is the lowest level among all test samples. The color density K value depends on the uniform distribution of black ink in the coating and the color development depth. In Comparative Example 4, the omission of Step C results in a decrease in the specific surface area of the powder from 260+㎡ / g in Examples 1-3 to 206.55㎡ / g, a decrease in the effective adsorption area, and a non-uniform pore distribution due to the excessively large particle size: excessive aggregation of ink in local dense pore regions and insufficient penetration of ink in local dense regions, resulting in a decrease in color development uniformity. In addition, the surface energy of the powder not subjected to weak acid adjustment is low, and the interaction force with carbon black particles is weakened, which cannot effectively fix the ink particles, and part of the carbon black migrates downward during the drying process, resulting in a decrease in the accumulation of surface pigments, a decrease in blackness, and an increase in whiteness, thereby making the visual appear dark. Although the ink absorption state is overall "good", the structural non-uniformity and the decrease in specific surface area together result in a decrease in the color development stability and depth of black ink, and finally the color density K value decreases to 1.86.

[0148] Table 2 From the test results of Example 4, it can be seen that replacing aluminum sulfate with aluminum chloride and acetic acid with acrylic acid results in a decrease in reaction regulation accuracy, and the performance is slightly inferior to the case of using aluminum sulfate and acetic acid, but does not cause gibbsite impurities and excessive agglomeration. Specifically, aluminum sulfate has strong dissociation stability, and the release of aluminum ions is gentle, which can avoid the formation of local high alkaline microzones; while the release rate of aluminum ions of aluminum chloride is faster, and the local aluminum ion concentration easily rises when reacting with ammonia water, which causes more obvious pH fluctuations in the micro area, easily leading to 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 widened. The dissociation constant of acetic acid is moderate, and the release of hydrogen ions is slow, which can accurately maintain the hydrolysis equilibrium of pH 5-6; while the dissociation of acrylic acid is weaker, and the release rate of hydrogen ions is lower, which reduces the regulation accuracy of the hydrolysis of precursor seeds, and it is difficult to uniformly inhibit the excessive growth of the seeds, resulting in a decrease in particle size uniformity and a slight decrease in dispersion index. Acrylic acid is less acidic, and the degree of precursor hydrolysis is insufficient, the surface energy of the seed crystal is less than that of the acetic acid system, and the pore development is slightly poor, resulting in a slight decrease in specific surface area and pore volume; at the same time, the widening of the particle size and the decline of the pore structure lead to a slight decrease in oil absorption value, but the overall oil absorption value is still at a high level. Although the particle size and dispersion are slightly poor, the powder of Example 4 still has a relatively complete porous structure (pore volume 0.682 ml / g, oil absorption value 77.3), and there is no serious agglomeration or impurity interference, which basically meets the ink absorption requirements, so the ink absorption state is good overall; only the local particle size is uneven, which leads to slight diffusion, which is better than the defect state of the comparative example. The widening of the particle size distribution and the decrease in dispersion reduce the microsmoothness of the coating surface and weaken the uniformity of light reflection, and the gloss is reduced to 56.5. The decrease in specific surface area and uneven pore distribution lead to a decrease in the adsorption and fixation ability of black ink (carbon black particles), and the color development depth is reduced, and the color density K value is reduced to 2.18; but because the coating structure is still relatively complete (without ink floating or poor ink absorption problems), the K value is significantly higher than that of the comparative example.

[0149] From the test results of Example 5, it can be seen that Example 5 has a slightly inferior performance to Examples 1-3 due to the damaged pore structure caused by the excess surface modifier. Specifically, the excess polyacrylic acid can still effectively inhibit agglomeration by coating the surface hydroxyl groups of the powder, so the dispersion index remains at a high level, and the particle size parameter is slightly larger than that of Example 1 but is still relatively uniform. However, the excess surface modifier molecules are partially free in the system, which can form weak bridges between particles, resulting in a slight increase in D100 and a slight widening of the particle size distribution. Excessive use of surface modifier can block the pore structure of boehmite. In Example 5, the excess polyacrylic acid can fill part of the micropores and mesopores, resulting in a slight decrease in specific surface area and pore volume. Pore blockage directly weakens the adsorption capacity of the powder for ink, and the oil absorption value decreases slightly, but is still higher than the oil absorption value level of the comparative examples. The ink absorption state is overall good, but the excess surface modifier blocks part of the pores, resulting in insufficient ink adsorption in local areas and slight uneven ink absorption. The uniformity of light reflection decreases due to the residual polyacrylic acid on the surface of the coating, and the gloss decreases slightly. The decrease in specific surface area and pore volume weakens the adsorption and fixation ability of black ink (carbon black particles), and the color density K value decreases slightly.

[0150] From the test results of Example 6, it can be seen that although the vinyl acetate-ethylene copolymer is a polar binder, the hydrogen bonding strength between the binder and the surface hydroxyl groups of the polyacrylic acid modified boehmite is weaker than that of polyvinyl alcohol, and the uniformity of the binder coating on the surface of the powder decreases slightly, which may result in a small amount of ink not being fully fixed, causing local slight ink floating, but the overall ink absorption capacity is still good. The film-forming property of the vinyl acetate-ethylene copolymer is slightly inferior to that of polyvinyl alcohol, the micro-flatness of the coating surface decreases slightly, the uniformity of light reflection decreases, and the gloss decreases slightly, but the gloss is still higher than that of the comparative examples because there is no agglomeration or serious damage to the pore structure. The decrease in the interfacial bonding force between the binder and the powder results in a slight decrease in the fixation stability of carbon black particles, a slight decrease in color development depth, and a slight decrease in color density K value, but because the coating structure is still uniform (without serious ink absorption differences), the K value is significantly higher than that of the comparative examples.

[0151] As can be seen from the test results of Example 7, Example 7 has slightly blocked pores and reduced overall coating integrity due to the reduced degree of polymerization of the adhesive, and the performance is slightly inferior to Examples 1-3, but the core indicators are still better than Comparative Examples 1-4. The molecular chain of polyvinyl alcohol with too low degree of polymerization is too small, and the theoretical root mean square end-to-end distance (representing the size of the random coil formed by the curling of PVA molecules) is close to the pore size of boehmite, which is easy to penetrate into the boehmite mesoporous structure to block the pores. The molecular chain length of polyvinyl alcohol with a degree of polymerization of 2500 in Example 7 is significantly lower than that of polyvinyl alcohol in Example 1, and part of the molecules can penetrate into the boehmite pores, causing local ink absorption channels to be blocked, and the ink cannot be evenly spread, resulting in slight uneven ink absorption. However, since the polyvinyl alcohol is still a polar adhesive, there is a hydrogen bond interaction with the hydroxyl groups on the surface of the polyacrylic acid modified boehmite, and the ink absorption state is still "good". The reduction of the degree of polymerization reduces the viscosity of the polyvinyl alcohol solution, and the cross-linking density is reduced during the film forming process, resulting in extremely small cracks in the coating, which reduces the micro-level flatness of the coating and weakens the uniformity of light reflection, thus the gloss decreases to a certain extent. The blockage of the pores reduces the effective adsorption sites of the carbon black particles, resulting in a decrease in color development depth and color density K value.

[0152] As can be seen from the test results of Example 8, the mass ratio of boehmite powder to adhesive needs to balance the dispersion stability, bonding strength and ink absorption performance, and the appropriate proportion of boehmite powder can ensure the ink absorption performance while ensuring the film forming integrity of the adhesive. In Example 8, the amount of adhesive is reduced, which results in the inability to form a continuous cross-linked network, and the bonding force between some particles is weakened, resulting in micro voids in the coating structure, which increases the scattering of light and reduces the specular reflection, thus the gloss slightly decreases. The insufficient amount of adhesive results in a loose coating structure, and the ink is easily over-expanded in these voids, and the carbon black particles migrate downward during the drying process, resulting in insufficient pigment on the surface layer and a color density of 2.08.

[0153] As can be seen from Examples 5-8, in the preparation of the ink absorption coating of photographic paper, the type and polarity matching of the surface modifier and the adhesive and the balanced amount ratio are crucial. The appropriate surface modifier and adhesive not only enable them to function well in the powder preparation and coating preparation stages, respectively, but also can synergistically control the dispersion stability of the nano boehmite powder, the pore structure retention rate and the mechanical strength of the coating by constructing a "powder-modifier-adhesive" three-dimensional network structure.

[0154] Specifically, when the surface modifier is polyacrylic acid and the binder is the polar polymer polyvinyl alcohol, the two can realize synergy through multiple forces such as hydrogen bonding, electrostatic attraction, and steric hindrance complementation. Among them, the combination of the carboxyl group of polyacrylic acid and the hydroxyl group of polyvinyl alcohol can enhance the interfacial bonding force; the charge interaction of the polar groups makes the binder more easily uniformly coated on the surface of the modified powder, avoiding local agglomeration; the flexibility of the polar molecular chain can reduce the blockage of the binder molecules to the boehmite pores, retaining the ink absorption channel. The polyacrylic acid coats the surface of the boehmite through the carboxyl group, and the exposed polar groups form hydrogen bonds with the hydroxyl groups of polyvinyl alcohol, making the binder uniformly distributed in the powder gap rather than the inside of the pores, which not only ensures the bonding strength but also maintains high porosity, ultimately achieving a balance between ink absorption speed and coating stability. Conversely, if the surface modifier and the binder are not polar compatible (such as non-polar modifier combined with polar binder), it will lead to poor interfacial compatibility, increase the risk of pore blockage, and cause deterioration of ink absorption uniformity.

[0155] When the amount of surface modifier is 0.1-1wt% of the mass of the solution in step C, and the mass ratio of nano-boehmite to binder is 10:1-15:1, the amount of 0.1-1wt% of the modifier can exactly cover the surface hydroxyl groups of boehmite, forming a monolayer coating and avoiding powder agglomeration. The ratio of 10:1-15:1 allows the binder to fill the powder gap to form a continuous network, and also prevents excessive binder from causing pore blockage. The modifier limits the diffusion of the binder into the pores through steric hindrance, and the appropriate amount of binder further reduces the penetration driving force, together maintaining high pore volume.

[0156] Table 3 As can be seen from the test results of Example 9, the azobenzene-modified surface modifier retains the dispersion and coating ability of the original modifier, ensuring stable particle size distribution and basic pore structure; it can also dynamically optimize pore openness and ink affinity through the cis-trans isomerization characteristics of the ultraviolet light response, ultimately achieving a small increase in ink absorption performance and printing quality, while not causing negative effects on other indicators.

[0157] Specifically, the azobenzene-modified polyacrylic acid retains the carboxyl groups of the original polyacrylic acid, which can be firmly combined with the hydroxyl groups on the surface of the boehmite precursor through hydrogen bonding, achieving uniform coating. The introduced azobenzene group has a rigid aromatic structure, which can enhance the steric hindrance effect and further hinder the agglomeration of powder particles, so the dispersion index does not decrease. The molecular chain length and coating efficiency of the modified polyacrylic acid are close to those of the unmodified polyacrylic acid, so the overall particle size distribution is in the same interval as Examples 1-3, with only slight fluctuations due to the slight 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, so the basic pore structure is retained.

[0158] The azobenzene group undergoes cis-trans isomerization (from trans form to cis form) under ultraviolet irradiation, which drives the slight movement of polymer segments, opens the pore channels that may be partially covered by the modifier molecules, and increases the effective pore number and openness. The open pore structure improves the specific surface area and pore volume, and the oil absorption value is directly related to the pore adsorption capacity, so the oil absorption value is slightly increased. After ultraviolet irradiation, the polarity of the azobenzene group in the cis form is slightly improved, which enhances the affinity of the coating surface to the polar ink, and the open pore channel accelerates the penetration of the ink, reduces the "ink floating" phenomenon caused by the retention of the ink on the surface. The uniform coating of the azobenzene modified surface modifier makes the boehmite powder more uniformly dispersed in the coating, and the flatness of the coating surface is higher; and the ultraviolet irradiation does not damage the coating structure, so the gloss is slightly higher than that of Example 1. Uniform ink absorption and open pores can more stably lock the pigment molecules, reduce the loss of color density caused by ink diffusion; at the same time, the flatness of the coating surface is improved, which reduces the light scattering, so the color is darker, and therefore the color density K value is slightly higher than that of Example 1.

[0159] 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 absorption coating. Stepwise temperature rise realizes particle size uniformization and pore ordered development by regulating the crystal nucleus growth rate, and nitrogen protection reduces agglomeration and pore blockage by isolating interference and stabilizing the surface, finally improves the dispersion and pore performance, and significantly optimizes the ink absorption effect and printing quality.

[0160] Principles, steps and the like not explicitly described in the present application can be obtained by those skilled in the art through conventional technical means, and therefore will not be described. Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these modifications and variations.

Claims

1. A method for preparing an ink-absorbing 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 photographic paper and curing to obtain the ink-absorbing coating of 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 10-20wt%; step B: adding an ammonia solution into the aluminum salt solution at a rate of 50-100g / min until the pH value is 9-11, and maintaining for 1h; the concentration of the ammonia solution is 5-15wt%; step C: adding a solution comprising a weak acid into the solution obtained in step B at a rate of 20-50g / min until the pH value is 5-6, and maintaining for 30min; 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, controlling the crystallization temperature to be 120-140℃, and the crystallization time to be 2-8h to obtain a crystallization liquid; step F: removing impurities from the crystallization liquid by water washing, and spray drying to obtain the nanoboehmite powder. 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 0.1-1wt% 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 polyacrylic acid, and the binder is polyvinyl alcohol. The mass ratio of the nanoboehmite powder to the binder is 10:1-15:

1. The surface modifier is an azobenzene group modified polyacrylic acid derivative; the method further comprises: irradiating the ink-absorbing coating of photographic paper with ultraviolet light before using the ink-absorbing coating of photographic paper. The step D comprises: adding the surface modifier into the solution obtained in step C, and stirring at a speed of 150-250rpm at a temperature of 30-50℃ for 20-40min. The step E comprises: transferring the liquid reactant obtained in step D into a crystallization reactor, pre-crystallizing at 80-100℃ for 1-2h, then increasing the temperature to 120-140℃ at a rate of 5℃ / min, and crystallizing for 4h. The mixing of 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 4 or 5, wherein, ​ 7. The production method according to claim 5, wherein ​ 8. The production method according to claim 1, wherein ​ 9. The production method according to claim 1, wherein ​ ​ 10. The production method according to claim 1, wherein, ​

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