High-gloss coating for coated paper and preparation method of high-gloss coating

Abstract

The invention relates to the technical field of coatings, in particular to a high-gloss coating for coated paper and a preparation method of the high-gloss coating. The coating is prepared by coating, drying and calendaring paint containing modified calcium carbonate slurry, colloidal silicon dioxide, styrene-acrylic emulsion, polyvinyl alcohol and an ammonium zirconium carbonate cross-linking agent. The method is characterized in that a calcium carbonate pigment is subjected to multi-stage modification through silica in-situ compounding, polydopamine adhesion and a dynamic network constructed by firstly adding boric acid and then adding borax, and is subjected to cross-linking cooperation with terminal zirconium, so that the microstructure stability, the surface smoothness and the gloss uniformity of a coating under process fluctuation are remarkably improved; therefore, the coated paper product with high gloss, low roughness, low water absorption and high strength is obtained.

Description

Technical Field

[0001] This invention relates to the field of coating technology, and in particular to a high-gloss coating for coated paper and its preparation method. Background Technology

[0002] In the production of high-grade coated papers such as art paper, high gloss is one of the key quality indicators, directly affecting the color saturation and visual performance of printed materials. Traditional techniques, in pursuit of high gloss, generally use fine-particle-size heavy calcium carbonate or precipitated calcium carbonate as the main pigment, supplemented by a high amount of latex binder. This method relies on the smoothness of the pigment itself, achieving gloss enhancement by applying high pressure during calendering to densely arrange the pigment particles. However, this approach essentially treats the pigment deposition process and the binder film-forming process as two relatively independent stages.

[0003] This separation process is controllable on high-precision, stable production lines, but its limitations become apparent in actual production, especially when faced with the process fluctuations common in small and medium-sized production lines. Firstly, while fine-particle-size pigments are beneficial for forming smooth surfaces, if the interfacial interaction between the pigment and binder is weak during the coating drying and curing process, the pigment particles cannot achieve high orientation and dense packing, easily leading to micropores and amplified surface roughness. These microscopic defects become light scattering points, directly resulting in reduced specular reflection, manifested as decreased or unstable gloss.

[0004] Secondly, process adaptability issues are particularly prominent. Fluctuations in coating line speed, minute changes in the temperature gradient of the drying zone, and disturbances in the gap between calendering rollers are all common variables in mass production. Under traditional formulation systems, these disturbances exacerbate the inhomogeneity of the coating structure. For example, drying temperature gradients may cause binder migration, leading to localized uneven composition in the coating; fluctuations in calendering pressure may cause the already unstable pigment accumulation structure to partially collapse or rearrange. The end result is uneven dark spots and streaks on the coating surface, not only resulting in large fluctuations in gloss values ​​(i.e., poor gloss uniformity) but also severely affecting printing results.

[0005] Therefore, the core contradiction of existing technologies lies in the fact that relying solely on physical fine-particle pigments and high-gloss calendering cannot establish an effective synergistic mechanism between pigment accumulation and binder film consolidation. This makes the stability of the coating microstructure highly dependent on extremely stable process conditions, lacking the ability to resist inherent disturbances in mass production. Consequently, it is difficult to simultaneously achieve high gloss performance with process robustness and batch-to-batch consistency, limiting the application of this technology under a wider range of production line conditions. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a high-gloss coating for coated paper and its preparation method, so as to solve the problem in the existing high-gloss coating technology for coated paper that, due to insufficient synergy between pigment accumulation and binder film formation, the microstructure of the coating is prone to porosity and roughness amplification under the fluctuation of mass production process, which in turn causes uneven gloss and poor stability.

[0007] To achieve the above objectives, the present invention provides a high-gloss coating for coated paper, which is formed by drying and calendering a high-gloss coating for coated paper. By weight, the high-gloss coating for coated paper comprises: 1400 parts of modified calcium carbonate slurry, 140-180 parts of colloidal silica, 180-240 parts of styrene-acrylic emulsion, 60-90 parts of polyvinyl alcohol aqueous solution, and 5-8 parts of ammonium zirconium carbonate crosslinking agent.

[0008] The modified calcium carbonate slurry is prepared by the following steps:

[0009] (1) Disperse heavy calcium carbonate powder in water to obtain calcium carbonate dispersion slurry;

[0010] (2) Add sodium silicate solution to calcium carbonate dispersion slurry and introduce carbon dioxide gas to stabilize the pH of the system at 8.1-8.5. After stopping the gas introduction, age the slurry to obtain silica composite calcium carbonate slurry.

[0011] (3) Add dopamine hydrochloride to silica composite calcium carbonate slurry and form a polydopamine adhesion layer under alkaline conditions to obtain calcium carbonate slurry containing catechol-amine sites;

[0012] (4) Add polyvinyl alcohol aqueous solution, boric acid and sodium tetraborate decahydrate to calcium carbonate slurry containing catechol-amine sites to form a dynamic borate ester network, and then dehydrate under vacuum to obtain modified calcium carbonate slurry with a solid content of 69wt%-71wt%.

[0013] In step (4), the order of adding boric acid is earlier than the order of adding sodium tetraborate decahydrate.

[0014] Preferably, the median particle size of the heavy calcium carbonate powder is 0.5-1 μm.

[0015] Preferably, based on 1000 parts by weight of heavy calcium carbonate powder, the amount of sodium silicate solution added in step (2) is 30-60 parts, and the sodium silicate solution contains 10wt%-12wt% Na2O and 25wt%-28wt% SiO2.

[0016] Preferably, based on 1000 parts by weight of heavy calcium carbonate powder, the amount of dopamine hydrochloride added in step (3) is 0.6-1.2 parts.

[0017] Preferably, based on 1000 parts by weight of heavy calcium carbonate powder, the amount of polyvinyl alcohol aqueous solution added in step (4) is 10-20 parts, the amount of boric acid added is 0.8-1.2 parts, and the amount of sodium tetraborate decahydrate added is 1.6-2.4 parts.

[0018] Furthermore, the high-gloss coating for coated paper also includes: 2 parts dispersant, 3 parts sodium carboxymethyl cellulose, 5 parts calcium stearate, 2 parts defoamer, and 5 parts ammonia.

[0019] Preferably, the concentration of the polyvinyl alcohol aqueous solution is 20 wt%.

[0020] Preferably, the solid content of the colloidal silica is 25wt%-35wt%.

[0021] Preferably, the dispersant is a polyacrylate dispersant.

[0022] Preferably, the solid content of the styrene-acrylic emulsion is 45wt%-55wt%.

[0023] Preferably, the ammonium zirconium carbonate crosslinking agent is of type Bacote 20.

[0024] Preferably, the defoamer is an organosilicone defoamer.

[0025] Furthermore, the present invention also provides a method for preparing a high-gloss coating for coated paper, comprising the following steps: adding modified calcium carbonate slurry to a dispersion vessel, starting stirring and adjusting to 1200 rpm, then adding a dispersant and stirring for 5 min; subsequently adding colloidal silica and continuing to disperse for 10 min; sprinkling sodium carboxymethyl cellulose at 1500 rpm and continuing to disperse for 30 min; then adding calcium stearate and dispersing for 10 min; reducing the speed to 800 rpm and sequentially adding styrene-acrylic emulsion, polyvinyl alcohol aqueous solution, and defoamer and stirring for 10 min; then adding ammonia water with a concentration of 25 wt%; finally adding ammonium zirconium carbonate crosslinking agent and stirring at 500 rpm for 5 min, followed by vacuum degassing at 200 mbar for 5 min to obtain a high-gloss coating for coated paper; then coating at least one side of the base paper with the high-gloss coating for coated paper; drying the coated base paper; and calendering the dried paper to obtain coated paper containing a high-gloss coating.

[0026] Preferably, the base paper has a basis weight of 70 g / m³. 2 Uncoated base paper was equilibrated at 23℃ and 50%RH for 24 hours; coating was performed using a doctor blade, with a wet coating amount of 18-22 g / m². 2 The coating line speed is 80-150m / min.

[0027] Preferably, the drying process employs a three-stage hot air drying method, with the first stage hot air temperature at 105-115℃, the second stage hot air temperature at 115-125℃, and the third stage hot air temperature at 120-130℃, and the total residence time at 25-35 seconds.

[0028] Preferably, the calendering is performed under the conditions of a roller temperature of 55-70℃, a linear pressure of 8-12MPa, and 1-3 calendering passes.

[0029] The beneficial effects of this invention are:

[0030] This invention constructs discrete, dot-like silica-rich sites in situ on the surface of calcium carbonate pigments through sodium silicate adsorption combined with pH-controlled carbon dioxide condensation, endowing the pigment surface with abundant silanol active sites. Building upon this, a polydopamine adhesion layer is further introduced to firmly anchor catechol and amine bifunctional groups at the pigment interface. This composite modification strategy fundamentally alters the physicochemical properties of the pigment surface. During subsequent coating, drying, and calendering processes, these carefully constructed surface sites can generate stronger and more uniform interactions with binder components such as polyvinyl alcohol, significantly promoting the directional alignment and dense packing of pigment particles within the coating. This reduces the formation of micropores and surface roughness in the coating at its source, laying a solid structural foundation for achieving a high and uniform mirror gloss.

[0031] This invention creatively employs a specific sequential addition method, first adding boric acid and then sodium tetraborate decahydrate (borax), to construct a spatially enriched dynamic borate ester crosslinking network on the modified pigment surface. This sequential control ensures that borate activity is effectively controlled during coating preparation and storage, avoiding the viscosity surge and flocculation caused by instantaneous excessive crosslinking in high-solids-content systems, thus guaranteeing good coating flowability and storage stability. In the wet film stage after coating, this dynamic network responds rapidly and strengthens, providing the necessary structural viscosity and self-healing capabilities. This allows the coating to maintain excellent wet film spreading uniformity and thickness consistency across a wide range of coating line speeds and drying temperatures, effectively broadening the process window and combating common disturbances in mass production.

[0032] By introducing an ammonium zirconium carbonate crosslinking agent at the end of the coating formulation, and utilizing the homogenized interface and pre-structured foundation provided by the aforementioned surface modification and dynamic network, this scheme achieves a stronger zirconium ion coordination crosslinking and locking mechanism during the coating drying stage. This mechanism effectively suppresses the migration tendency of the binder under the drying temperature gradient, stabilizes the microstructure formed in the coating during the drying process, and prevents secondary unfavorable rearrangement during subsequent calendering stress. Therefore, the coating ultimately exhibits lower water absorption, higher surface strength, and its high-gloss surface possesses excellent stability, making it less prone to gloss drift or localized defects due to minor changes in process conditions.

[0033] In summary, this invention, through a multi-level synergistic design encompassing pigment surface modification, coating rheological structure regulation, and final drying locking, shifts the core of improving gloss stability from dependence on end-calendering equipment and extremely high process precision to the controllable construction of the material system itself. This approach significantly enhances the inherent stability and adaptability of the coating system, enabling the stable production of coated paper products with high gloss, high uniformity, and excellent printability even on small- to medium-sized production lines or under production conditions with certain process fluctuations. This improves the technology's convertibility and mass production versatility. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0035] The heavy calcium carbonate used in the examples and comparative examples was Hydrocarb 90-FL heavy calcium carbonate powder from Omya, with a median particle size d50 of approximately 0.7 μm and a whiteness of approximately 95% (R457); the dispersant was Dispex N40V EB polyacrylate dispersant from BASF, with an active content of approximately 40 wt%; the sodium silicate solution was sodium silicate solution from Sigma-Aldrich (Merck Group) (Na2O content approximately 10.6 wt%, SiO2 content approximately 26.5 wt%); the polyvinyl alcohol was Kuraray POVAL 26-88LV polyvinyl alcohol resin, with a degree of alcoholysis of approximately 88 mol%; the colloidal silica was Levasil CB30 colloidal silica from Nouryon (solid content approximately 30 wt%); the styrene-acrylic emulsion was Acronal S728na styrene-acrylic emulsion from BASF (solid content approximately 50 wt%); and the defoamer was SILFOAM from Wacker. SE39 is an organosilicon defoamer; sodium carboxymethyl cellulose is Ashland's Blanose 7H3SXF sodium carboxymethyl cellulose; and ammonium zirconium carbonate crosslinking agent is LuxferMEL Technologies' Bacote 20 stabilized ammonium zirconium carbonate aqueous solution.

[0036] Example 1:

[0037] Step S1: Add 240g of deionized water to a container equipped with mechanical stirring and temperature control, heat to 95°C, and slowly add 60g of polyvinyl alcohol resin while stirring at 500rpm. Maintain the temperature at 95°C and continue stirring for 60min. Then cool down to 25°C and seal and let stand for 30min to remove bubbles, to obtain a polyvinyl alcohol aqueous solution.

[0038] Step S2: Add 2300g of deionized water to the mixing tank, start the dispersing paddle and adjust it to 1200rpm, then add 5g of dispersant and stir for 5min to disperse it evenly; then add 1000g of heavy calcium carbonate powder in batches at 1500rpm (control the feeding time to 10min), and continue to disperse for 20min after the feeding is completed to obtain calcium carbonate dispersion slurry;

[0039] Step S3: Take the calcium carbonate dispersion slurry obtained in step S2, heat it to 45℃ and maintain stirring at 1200 rpm, add 40g of sodium silicate solution, and continue stirring for 10 min; then introduce carbon dioxide gas to stabilize the pH of the system at 8.3, stop the gas introduction, and continue stirring and aging at 45℃ for 30 min; after aging, add 6g of 25wt% ammonia water and stir for 10 min to obtain silica composite calcium carbonate slurry;

[0040] Step S4: Take the silica composite calcium carbonate slurry obtained in step S3, cool it to 25°C, and stir at 800 rpm. First, dissolve 800 mg of dopamine hydrochloride in 20 g of deionized water to form a clear solution. Then, add the solution to the slurry all at once, maintain the temperature at 25°C, and stir openly for 20 min. Then, add 2 g of 25 wt% ammonia water and continue stirring for 10 min to obtain calcium carbonate slurry containing catechol-amine sites.

[0041] Step S5: Take the modified calcium carbonate slurry containing catechol-amine sites obtained in step S4, keep it at 25°C, and add 15g of the polyvinyl alcohol aqueous solution prepared in step S1 at 800rpm, and stir for 10min; then add 1g of boric acid and stir for 10min, then add 2g of sodium tetraborate decahydrate and stir for 15min; then dehydrate by vacuum at room temperature to obtain a modified calcium carbonate slurry with a solid content of 70wt%;

[0042] Step S6: Add 1400g of the modified calcium carbonate slurry obtained in step S5 to the dispersion vessel, start stirring and adjust to 1200rpm, then add 2g of dispersant and stir for 5min; then add 160g of colloidal silica and continue dispersing for 10min; sprinkle 3g of sodium carboxymethyl cellulose at 1500rpm and continue dispersing for 30min; then add 5g of calcium stearate and disperse for 10min; reduce the speed to 800rpm and add 200g of styrene-acrylic emulsion, 75g of the polyvinyl alcohol aqueous solution obtained in step S1, and 2g of defoamer in sequence, and stir for 10min; then add 5g of 25wt% ammonia water; finally add 6g of ammonium zirconium carbonate crosslinking agent and stir at 500rpm for 5min, then perform vacuum degassing at 200mbar for 5min to obtain a high-gloss coating for coated paper;

[0043] Step S7: Select a quantitative dose of 70g / m 2Uncoated base paper was used as the base paper and equilibrated at 23°C and 50%RH for 24 hours. The coated paper obtained in step S6 was then coated onto the base paper surface using a doctor blade coating method with a high-gloss coating on one side, with a wet coating weight of 20 g / m². 2 The coating line speed is controlled at 100m / min, and then the paper immediately enters the three-stage hot air drying zone. The hot air temperature is set to 110℃, 120℃, and 120℃ respectively, and the total dwell time is 30s. After the first side is dried, the other side is coated and dried using the same parameters. Finally, the paper is calendered at a roller temperature of 60℃, a line pressure of 10MPa, and two calendering passes to obtain the finished coated paper with a high-gloss coating for coated paper.

[0044] Example 2:

[0045] Compared with Example 1, the differences are as follows: In step S3, the amount of sodium silicate solution used is 30g, carbon dioxide gas is introduced to stabilize the pH of the system at 8.1, and after stopping the gas introduction, the system is stirred and aged at 45°C for 20 minutes; in step S4, the amount of dopamine hydrochloride used is 600mg, and the open stirring time is 15 minutes; in step S5, the amount of polyvinyl alcohol aqueous solution obtained in step S1 is 10g, the amount of boric acid is 0.8g, the amount of sodium tetraborate decahydrate is 1.6g, and the solid content of the modified calcium carbonate slurry obtained by vacuum dehydration is... 69wt%; In step S6, the amount of colloidal silica is 140g, the amount of styrene-acrylic emulsion is 180g, the amount of polyvinyl alcohol aqueous solution obtained in step S1 is 60g, and the amount of ammonium zirconium carbonate crosslinking agent is 5g; In step S7, the wet coating amount is 18g / m2, the coating line speed is 120m / min, the three hot air temperatures are 110℃, 120℃, and 125℃ respectively, the total residence time is 25s, the calender roller temperature is 55℃, the line pressure is 9MPa, and the number of calender passes is 2; the other conditions are the same as in Example 1.

[0046] Example 3:

[0047] Compared with Example 1, the differences are as follows: In step S3, the amount of sodium silicate solution used is 50g, carbon dioxide gas is introduced to stabilize the pH of the system at 8.4, and after stopping the gas introduction, the system is stirred and aged at 45°C for 40 minutes; in step S4, the amount of dopamine hydrochloride used is 1000mg, and the open stirring time is 25 minutes; in step S5, the amount of polyvinyl alcohol aqueous solution obtained in step S1 is 20g, the amount of boric acid is 1.2g, the amount of sodium tetraborate decahydrate is 2.4g, and the solid content of the modified calcium carbonate slurry obtained by vacuum dehydration is... 71 wt%; In step S6, the amount of colloidal silica is 180 g, the amount of styrene-acrylic emulsion is 220 g, the amount of polyvinyl alcohol aqueous solution obtained in step S1 is 90 g, and the amount of ammonium zirconium carbonate crosslinking agent is 7 g; In step S7, the wet coating amount is 22 g / m2, the coating line speed is 80 m / min, the three hot air temperatures are 105℃, 115℃, and 120℃ respectively, the total residence time is 35 s, the calender roller temperature is 70℃, the line pressure is 11 MPa, and the number of calender passes is 3; the other conditions are the same as in Example 1.

[0048] Example 4:

[0049] Compared with Example 1, the differences are as follows: in step S3, the amount of sodium silicate solution used is 60g, and carbon dioxide gas is introduced to stabilize the pH of the system at 8.5; in step S4, the open stirring time is 30min; in step S7, the coating line speed is 150m / min, the three hot air temperatures are 115℃, 125℃, and 125℃ respectively, and the total residence time is 30s; the other conditions are the same as in Example 1.

[0050] Example 5:

[0051] Compared with Example 1, the differences are as follows: in step S3, the amount of sodium silicate solution used is 35g, and carbon dioxide gas is introduced to stabilize the pH of the system at 8.2; in step S4, the amount of dopamine hydrochloride used is 1200mg; in step S5, the amount of boric acid used is 1.1g, and the amount of sodium tetraborate decahydrate used is 2.2g; in step S6, the amount of colloidal silica used is 150g, the amount of styrene-acrylic emulsion used is 240g, and the amount of ammonium zirconium carbonate crosslinking agent used is 8g; in step S7, the temperature of the third stage hot air is 130℃, the temperature of the calender roller is 65℃, and the linear pressure is 12MPa; the remaining conditions are the same as in Example 1.

[0052] Example 6:

[0053] Compared with Example 1, the differences are as follows: In step S3, the amount of sodium silicate solution used is 45g, and after stopping the aeration, it is stirred and aged at 45°C for 25min; in step S4, the amount of dopamine hydrochloride used is 700mg; in step S5, the amount of polyvinyl alcohol aqueous solution obtained in step S1 is 12g, the amount of boric acid is 0.9g, and the amount of sodium tetraborate decahydrate is 1.8g; in step S6, the amount of colloidal silica used is 170g, and the amount of polyvinyl alcohol aqueous solution obtained in step S1 is 70g; in step S7, the wet coating amount is 19g / m2, the coating line speed is 90m / min, the three hot air temperatures are 108°C, 118°C, and 122°C respectively, the total residence time is 28s, the calender roller temperature is 58°C, the line pressure is 8MPa, and the number of calender passes is 1; the other conditions are the same as in Example 1.

[0054] Comparative Example 1:

[0055] The difference from Example 1 is that sodium silicate solution is not added in step S3. All other conditions are the same as in Example 1.

[0056] Comparative Example 2:

[0057] The difference from Example 1 is that in step S3, after adding 40g of sodium silicate solution and stirring for 10 minutes, carbon dioxide gas was not introduced, and the mixture was directly stirred and aged at 45°C for 30 minutes; after aging, the operation of adding 6g of 25wt% ammonia water and stirring for 10 minutes remained unchanged. The remaining conditions were the same as in Example 1.

[0058] Comparative Example 3:

[0059] The difference from Example 1 is that in step S4, dopamine hydrochloride is not added; only 20g of deionized water is added, and the mixture is kept at 25°C and stirred open for 20 minutes. The subsequent addition of 2g of 25wt% ammonia and stirring for another 10 minutes remains unchanged. All other conditions are the same as in Example 1.

[0060] Comparative Example 4:

[0061] The difference from Example 1 is that in step S5, after adding 15g of the polyvinyl alcohol aqueous solution obtained in step S1 and stirring for 10 minutes, 2g of sodium tetraborate decahydrate is added and stirred for 15 minutes, followed by the addition of 1g of boric acid and stirring for 10 minutes. The remaining conditions are the same as in Example 1.

[0062] Comparative Example 5:

[0063] The difference from Example 1 is that in step S5, after adding 15g of the polyvinyl alcohol aqueous solution obtained in step S1 and stirring for 10 minutes, 3g of sodium tetraborate decahydrate is added directly and stirred for 25 minutes, without adding 1g of boric acid. The remaining conditions are the same as in Example 1.

[0064] Comparative Example 6:

[0065] The difference from Example 1 is that in step S5, after adding 15g of the polyvinyl alcohol aqueous solution obtained in step S1 and stirring for 10 minutes, 3g of boric acid is added directly and stirred for 25 minutes, without adding 2g of sodium tetraborate decahydrate. The remaining conditions are the same as in Example 1.

[0066] Comparative Example 7:

[0067] The difference from Example 1 is that no ammonium zirconium carbonate crosslinking agent is added in step S6. The remaining conditions are the same as in Example 1.

[0068] High shear viscosity and viscosity drift (GB / T 9751.1-2008): Immediately after the coated paper obtained in step S6 of each example and comparative example was vacuum degassed at 200 mbar with a high-gloss coating, 50 mL of the sample was taken and placed at a constant temperature of 25℃ for 10 min to eliminate temperature difference; a cone-plate rotational viscometer was used with a cone angle of 1°, a cone diameter of 50 mm, and a shear rate of 10000 s. -1 The dynamic viscosity η1 was measured; the same coating sample was sealed and allowed to stand for 30 minutes, and the dynamic viscosity η was measured again under the same conditions. 30 Calculate the viscosity drift ratio D = (η 30 -η1) / η1×100%; each sample was measured in parallel 3 times and the arithmetic mean was taken.

[0069] 75° Specular Gloss and Gloss Uniformity (GB / T 8941-2013, sample condition according to GB / T 10739-2023): After the finished coated paper samples of each example and comparative example were equilibrated for 24 hours according to GB / T 10739-2023, the specular gloss was measured using a 75° specular gloss meter according to GB / T 8941-2013: 9 measuring points were selected on both the front and back of each sample according to a 3×3 grid, and the values ​​of each point were measured and calculated. The average value of the two sides was taken as the 75° specular gloss G75 of the sample. The standard deviation σG of all 18 measuring points was used to characterize the gloss uniformity. Three sheets of each sample were prepared and the average value of the three sheets was taken.

[0070] Bentsson roughness test (GB / T 22363-2008, sample condition according to GB / T 10739-2023): After each sample of coated paper has been equilibrated for 24 hours according to GB / T 10739-2023, the surface roughness of the paper is determined using a Bentsson roughness tester equipped with a low-range tube according to the Bentsson method in GB / T 22363-2008: 5 measuring points are selected on each of the front and back sides of each sample, the test pressure is 1.47 kPa, and the measuring area is 10 cm². 2 Record the roughness R (mL / min); take the average of the two surfaces and calculate the average of the three samples.

[0071] Cobb 60 water absorption (GB / T 1540-2002, sample condition according to GB / T 10739-2023): After each sample of coated paper has been equilibrated for 24 hours according to GB / T 10739-2023, the Cobb 60 water absorption is tested according to GB / T 1540-2002: a sample of 125mm × 125mm is cut, and the test area is 100cm². 2 Add 100 mL of water, allow 60 seconds of contact time, remove and dewater according to standard specifications, weigh the product, and calculate Cobb60 (g / m³). 2 Each sample was measured 5 times on both the front and back sides, and the average was taken. The average of the two sides was then used as the sample result.

[0072] Printed surface strength (GB / T 22365-2008, sample condition according to GB / T 10739-2023): After each sample of coated paper finished product equilibrates for 24 hours according to GB / T 10739-2023, the printed surface strength is determined using the accelerated method according to GB / T 22365-2008: a uniform ink film is formed using a high-speed ink spreader at a speed of 100 r / min and a spreading time of 120 s. The ink film is then transferred to a printing tray and tested on a printed surface strength tester, with a test area of ​​10.0 cm². 2 The printing pressure was 350N, the fan-shaped radius was 85mm, the fan angle was 155°, the pendulum elevation angle was 165°, the final speed was set to 3.5m / s, and the critical speed Vcrit when paper surface fuzzing occurred was recorded. Each sample was tested 3 times on both sides and the average was taken. The average of the two sides was then taken as the sample result.

[0073] Table 1 Performance test results of the examples and comparative samples

[0074] sample <![CDATA[High shear viscosity η1 / (mPa·s)]]> Viscosity drift rate D / % 75° Specular gloss G75 / % Gloss standard deviation σG / % Bentsen roughness R / (mL / min) <![CDATA[Cobb60 / (g / m 2 )]]> Critical velocity Vcrit / (m / s) Example 1 86 2.8 89.6 0.9 32 20.5 3.12 Example 2 74 2.1 87.4 1.1 38 22.8 3.00 Example 3 98 4.9 92.1 0.8 28 19.6 3.25 Example 4 90 3.6 88.9 1.3 36 21.4 2.92 Example 5 95 5.4 90.8 1.1 30 18.9 3.30 Example 6 82 3.0 89.1 1.0 33 21.0 3.08 Comparative Example 1 79 6.8 81.7 2.6 56 27.4 2.55 Comparative Example 2 81 6.0 83.6 2.3 52 26.3 2.62 Comparative Example 3 84 5.8 85.2 2.0 46 24.9 2.78 Comparative Example 4 97 9.2 82.4 3.1 60 29.1 2.38 Comparative Example 5 100 11.0 80.9 3.4 66 30.4 2.30 Comparative Example 6 70 10.4 79.8 2.9 62 31.2 2.22 Comparative Example 7 78 4.7 86.1 1.8 44 23.8 2.86

[0075] Data Analysis:

[0076] As can be seen from the data in Table 1, the high-gloss coating for coated paper prepared by this invention achieves a balance between high shear viscosity and viscosity drift rate, ensuring that the coating remains flowable under the high shear conditions required for doctor blade coating, while preventing significant thickening and drift during the resting stage, thus guaranteeing stable wet film spreading and coating amount. With the combined effects of the construction of silica-calcium carbonate surface sites, the introduction of catechol-amine sites formed by dopamine hydrochloride, and the stepwise addition of boric acid and sodium tetraborate decahydrate to form a dynamic borate ester network, the coating more easily achieves pigment orientation and dense accumulation during drying and calendering, reducing the inherent roughness of the paper surface, increasing the 75° mirror gloss, and reducing gloss fluctuations. Simultaneously, the ammonium zirconium carbonate crosslinking agent, after coordination during the drying stage, locks in and inhibits binder migration, suppressing the secondary amplification of coating porosity and micro-roughness, thereby reducing Cobb60 water absorption and increasing the critical speed for printing surface strength. It is inferred that this system achieves more stable high gloss and printability without relying on ultra-high pressure light conditions through a three-step synergy of surface sites, dynamic network, and post-locking.

[0077] As can be seen from the data in Table 1 for Example 1 and Comparative Example 1, when sodium silicate solution is not added in step S3, the 75° mirror gloss and gloss uniformity of the paper surface deteriorate simultaneously, the Bentsen roughness and Cobb 60 water absorption increase, and the critical velocity decreases. The main reason is that the calcium carbonate surface lacks dot-like silica sites, making it difficult for the pigment to form a stable orientation and dense accumulation during the drying and calendering stage, and micropores and micro-roughness are more easily amplified; at the same time, the lack of Si-OH-rich structures at the interface makes it difficult for the hydrochloric acid dopamine adhesion layer and the subsequent dynamic borate ester network to be effectively enriched on the particle surface, resulting in the inability to superimpose the gains of multiple mechanisms and weakening the synergistic effect.

[0078] As can be seen from the data in Table 1 for Example 1 and Comparative Example 2, when sodium silicate solution was added but carbon dioxide was introduced to control the pH, although the mirror gloss and gloss uniformity were improved compared to the system without sodium silicate, it still exhibited higher roughness, water absorption, and a lower critical velocity. This is because the formation of silica sites depends on the pH-controlled condensation process. The lack of carbon dioxide regulation leads to insufficient or uneven distribution of surface sites, making it difficult to form a continuous and smooth microscopic reflective interface during calendering. Simultaneously, uneven site distribution weakens the joint regulation of the interface by catechol-amine sites and the dynamic borate ester network, making it difficult to stably establish the pigment accumulation-film formation synergy.

[0079] As can be seen from the data in Table 1 for Example 1 and Comparative Example 3, when only silica-composite calcium carbonate is constructed without the catechol-amine sites formed by dopamine hydrochloride, the critical velocity and water resistance-related indicators are more likely to decrease, and the gloss fluctuations are greater. The main reason is that the catechol-amine sites are important interfacial anchors for the dynamic borate ester network and the ammonium zirconium carbonate crosslinking agent to function. Their absence makes the dynamic network more biased towards bulk phase interaction rather than surface enrichment, resulting in insufficient rapid adhesion and self-healing during the wet film stage, and making it more difficult for the coordination and locking to occur uniformly during the drying stage. Ultimately, this leads to a greater likelihood of secondary rearrangement of the coating microstructure under drying and calendering disturbances, resulting in unpredictable comprehensive deterioration.

[0080] As can be seen from the data in Table 1 for Example 1 and Comparative Example 4, when the order of adding sodium tetraborate decahydrate followed by boric acid in step S5 is adopted, the high shear viscosity and viscosity drift rate increase significantly, while the gloss uniformity deteriorates and the roughness increases. This is because sodium tetraborate decahydrate more easily forms higher borate activity in the system, leading to instantaneous flocculation and thickening of the high-solids pigment slurry. This simultaneously disrupts the high shear flow and static stability during the coating stage, resulting in uneven wet film spreading and localized dark spots and gloss fluctuations after calendering.

[0081] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 5 and 6, both adding sodium tetraborate decahydrate or boric acid in step S5 increases the viscosity drift rate, accompanied by adverse changes in gloss, roughness, water absorption, and critical velocity, but the two exhibit different pathways. Adding only sodium tetraborate decahydrate makes it easier for high viscosity and uneven coating to occur due to excessive bridging in the initial stage; adding only boric acid makes it easier for crosslinking to form slowly and for the coating to thicken continuously during the standing stage, leading to increased drift and decreased wet film consistency. Therefore, the combination of boric acid and sodium tetraborate decahydrate, and the timing of adding boric acid first and then sodium tetraborate decahydrate to achieve a dynamic network enriched on the surface, is the key to obtaining a stable coating window and high gloss, and its effect cannot be replaced by a single boron source.

[0082] As can be seen from the data in Table 1 for Example 1 and Comparative Example 7, when no ammonium zirconium carbonate crosslinking agent is added in step S6, the mirror gloss and gloss uniformity decrease, the water absorption of Cobb60 increases, and the critical velocity decreases. The main reason is that although the dynamic borate ester network can provide adhesion and self-healing in the wet film stage, it lacks the coordination and locking in the drying stage. The migration of binders and the secondary rearrangement of microstructures are more difficult to suppress, and the surface smoothness and density after calendering are more easily affected by the drying temperature gradient and pressure disturbance.

[0083] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A high-gloss coating for coated paper, characterized in that, It is formed by drying and calendering a high-gloss coating for coated paper; by weight, the high-gloss coating for coated paper comprises: 1400 parts modified calcium carbonate slurry, 140-180 parts colloidal silica, 180-240 parts styrene-acrylic emulsion, 60-90 parts polyvinyl alcohol aqueous solution, and 5-8 parts ammonium zirconium carbonate crosslinking agent. The modified calcium carbonate slurry is prepared by the following steps: (1) Disperse heavy calcium carbonate powder in water to obtain calcium carbonate dispersion slurry; (2) Add sodium silicate solution to calcium carbonate dispersion slurry and introduce carbon dioxide gas to stabilize the pH of the system at 8.1-8.

5. After stopping the gas introduction, age the slurry to obtain silica composite calcium carbonate slurry. (3) Add dopamine hydrochloride to silica composite calcium carbonate slurry and form a polydopamine adhesion layer under alkaline conditions to obtain calcium carbonate slurry containing catechol-amine sites; (4) Add polyvinyl alcohol aqueous solution, boric acid and sodium tetraborate decahydrate to calcium carbonate slurry containing catechol-amine sites to form a dynamic borate ester network, and then dehydrate under vacuum to obtain modified calcium carbonate slurry with a solid content of 69wt%-71wt%. In step (4), the order of adding boric acid is earlier than the order of adding sodium tetraborate decahydrate.

2. The high-gloss coating for coated paper according to claim 1, characterized in that, The median particle size of the heavy calcium carbonate powder is 0.5-1 μm.

3. The high-gloss coating for coated paper according to claim 1, characterized in that, Based on 1000 parts by weight of heavy calcium carbonate powder, the amount of sodium silicate solution added in step (2) is 30-60 parts, and the sodium silicate solution contains 10wt%-12wt% Na2O and 25wt%-28wt% SiO2.

4. The high-gloss coating for coated paper according to claim 1, characterized in that, Based on 1000 parts by weight of heavy calcium carbonate powder, the amount of dopamine hydrochloride added in step (3) is 0.6-1.2 parts.

5. The high-gloss coating for coated paper according to claim 1, characterized in that, In step (4), the amount of polyvinyl alcohol aqueous solution added is 10-20 parts, the amount of boric acid added is 0.8-1.2 parts, and the amount of sodium tetraborate decahydrate added is 1.6-2.4 parts.

6. The high-gloss coating for coated paper according to claim 1, characterized in that, The high-gloss coating for coated paper also includes: 2 parts dispersant, 3 parts sodium carboxymethyl cellulose, 5 parts calcium stearate, 2 parts defoamer, and 5 parts ammonia.

7. The high-gloss coating for coated paper according to claim 1, characterized in that, The concentration of the polyvinyl alcohol aqueous solution is 20wt%; the solid content of the colloidal silica is 25wt%-35wt%; the solid content of the styrene-acrylic emulsion is 45wt%-55wt%; and the type of the ammonium zirconium carbonate crosslinking agent is Bacote 20.

8. A method for preparing a high-gloss coating for coated paper according to any one of claims 1-7, characterized in that, The process includes the following steps: Add modified calcium carbonate slurry to a dispersion vessel, start stirring and adjust to 1200 rpm, then add a dispersant and stir for 5 minutes; subsequently add colloidal silica and continue dispersing for 10 minutes; sprinkle sodium carboxymethyl cellulose at 1500 rpm and continue dispersing for 30 minutes; then add calcium stearate and disperse for 10 minutes; reduce the speed to 800 rpm and sequentially add styrene-acrylic emulsion, polyvinyl alcohol aqueous solution, and defoamer, stirring for 10 minutes; then add 25 wt% ammonia water; finally add ammonium zirconium carbonate crosslinking agent and stir at 500 rpm for 5 minutes, followed by vacuum degassing at 200 mbar for 5 minutes to obtain a high-gloss coating for coated paper; then apply the high-gloss coating to at least one side of the base paper; dry the coated base paper; and finally calender the dried paper to obtain coated paper with a high-gloss coating.

9. The method for preparing a high-gloss coating for coated paper according to claim 8, characterized in that, The base paper has a basis weight of 70g / m³. 2 Uncoated base paper was equilibrated at 23℃ and 50%RH for 24 hours; coating was performed using a doctor blade, with a wet coating amount of 18-22 g / m². 2 The coating line speed is 80-150m / min.

10. The method for preparing a high-gloss coating for coated paper according to claim 8, characterized in that, The drying process employs a three-stage hot air drying method, with the first stage hot air temperature at 105-115℃, the second stage hot air temperature at 115-125℃, and the third stage hot air temperature at 120-130℃, and a total residence time of 25-35s. The calendering process is carried out under the conditions of a roller temperature of 55-70℃, a linear pressure of 8-12MPa, and 1-3 calendering passes.