Compostable silicon dioxide-tannic acid mineral-based coating, coating material and preparation method thereof

Abstract

The invention provides a compostable silicon dioxide-tannic acid mineral-based coating, a coating material and a preparation method of the compostable silicon dioxide-tannic acid mineral-based coating, and belongs to the technical field of paper and paperboard surface functionalization. The coating is prepared from water-based paint, and main raw materials comprise porous silicon dioxide and natural tannic acid, and are matched with substances such as polyvinyl alcohol to form a film. One key point of the invention is that specific metal ions (such as aluminum ions) are added to be combined with tannic acid to form a special network structure. The method solves the problem that a traditional tannic acid coating is prone to discoloration (yellowing or darkening). Paper or a paperboard treated by the coating can effectively block water vapor, oxygen and grease when the coating amount is low, and packaging contents are protected. Meanwhile, the coating can keep the original high-whiteness appearance of the paper, has good recoverability and compostable performance, and is environment-friendly.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of paper and paperboard surface functionalization, and specifically relates to a compostable silica-tannic acid mineral-based coating, a coated material and a preparation method thereof. The coating is used to improve the moisture resistance, oxygen resistance and grease resistance of paper-based packaging materials, while ensuring their appearance and environmental friendliness. BACKGROUND

[0002] Under the background of promoting sustainable development globally, paper-based packaging materials, as a key solution to replace traditional plastic packaging, are facing growing market demand and technical challenges. The core challenge is how to achieve environmental friendliness of packaging materials without sacrificing food protection performance, especially whether they can be efficiently recycled after consumption.

[0003] To achieve the above goal, the industry has explored various technical routes. One is to use synthetic polymer barrier layers, such as ethylene-vinyl alcohol copolymer (EVOH). EVOH is relatively expensive and extremely sensitive to humidity, and its barrier performance will decrease sharply in a high-humidity environment. Another is to use polyvinyl alcohol (PVOH)-based coatings. PVOH is widely used due to its excellent film-forming property and high gas barrier property in dry conditions, but its inherent hydrophilicity causes its barrier performance to decrease significantly in a high-humidity environment.

[0004] To improve the performance of PVOH, it is often compounded with inorganic fillers. For example, silica (SiO2), especially nano or porous silica, is often used as a filler to improve the physical barrier performance of the coating by creating a "tortuous path" effect. Patent WO2014193572A1 discloses a multilayer coating of PVOH and silica nanoparticles on a substrate. However, simply combining PVOH with porous silica is difficult to achieve ideal high-humidity barrier effect at low coating amount.

[0005] Another modification scheme is to introduce natural polyphenols such as tannic acid (TA). The rich phenolic hydroxyl groups in TA molecules can form a large number of hydrogen bonds with the hydroxyl groups of PVOH, thereby increasing the crosslinking density of the composite film and improving its water resistance and barrier stability in a high-humidity environment. At the same time, the metal-polyphenol network as an emerging surface modification technology has also attracted attention, which uses the coordination of TA and multivalent metal ions to form a stable network structure. Patent US4163679A discloses that various metal ions including aluminum can be added to the TA-containing coating solution to improve the coating performance.

[0006] However, simple combination of the above technologies applied to high- brightness paper has a risk of color development: TA is easy to react with trace Fe 3+Strong complexation occurs and color is caused. For this purpose, the present application proposes to define "non-color-causing multivalent metal ions" as follows: ΔE * ab≤1.0 and stability meets ΔE * ab increment ≤1.0. Under this judgment, aluminum, zirconium, titanium, magnesium, calcium ions are selected and a dry-end spraying process is adopted, which can improve the barrier while maintaining the appearance stability.

[0007] The present application realizes the combination of the barrier property and color stability of the coating by combining specific non-color-causing multivalent metal ions (such as aluminum lactate) with a dry-end spraying process, and achieves the expected technical effect. SUMMARY

[0008] The present application aims to overcome the shortcomings of the prior art, and provides a compostable silica-tannic acid mineral-based coating, a coated material and a preparation method thereof, which aims to solve the problem of color deterioration caused by the reaction with trace metal ions in the industrial application of TA system with high barrier potential, and simultaneously realize the synergistic unification of high barrier property and high whiteness appearance by using a low-cost and process-compatible means.

[0009] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0010] A compostable silica-tannic acid mineral-based coating, the silica-tannic acid mineral-based coating comprises, in terms of dry solids: 15-45 parts of porous silica; 20-40 parts of a film-forming system; 1-6 parts of tannic acid; and 0-1.0 parts of microfibrillated cellulose and 0-2 parts of an auxiliary agent. The film-forming system is selected from one or both of polyvinyl alcohol and carboxymethyl cellulose. The mass ratio of tannic acid to porous silica is 1:5 to 1:45. The compostable silica-tannic acid mineral-based coating is a top coating. The top coating contains non-color-causing multivalent metal ions to form a metal-polyphenol network. The multivalent metal ions are selected from one or more of aluminum ions, zirconium ions, titanium ions, magnesium ions, and calcium ions. The multivalent metal ions are provided in one or more of aluminum lactate, zirconium ammonium carbonate, titanium lactate, magnesium lactate, and calcium lactate, and the addition amount of the multivalent metal ions is 0.05-0.30 mol / kg based on the dry solids of the film-forming phase. When the multivalent metal ions are aluminum ions, the molar ratio of the aluminum ions to tannic acid in the coating is 0.08-0.20, which is calculated by determining the metal element content by ICP-OES or ICP-MS and determining the TA content by TOC or HPLC-UV, both based on the dry weight of the film-forming phase. Furthermore, the total content of ethylene-vinyl alcohol copolymer, nanoclay, and polyacrylic polymer in the top coating is each ≤0.10 wt%.

[0011] The porous silica belongs to the category of silicon oxide fillers selected from one or more of mesoporous silica prepared by precipitation method, fume method, sol-gel method and template method, with volume distribution D 50 0.2-1.5 μm and D 90 ≤3.0 μm. The film-forming system is mainly composed of polyvinyl alcohol and carboxymethyl cellulose with a mass ratio of 5:1 to 1:1; and further contains one or more of water-soluble polysaccharides or cellulose ethers in a total amount of ≤5 wt%. The tannic acid belongs to hydrolyzed gallotannin; the content of total iron in the top layer coating is ≤0.0005 mol / kg, based on the solid content of the film-forming phase. The total amount of the auxiliary agent is 0-2 parts, selected from one or more of defoaming agent, wetting and leveling agent, film-forming auxiliary agent, humectant and antiseptic and mildew-proof agent, and the auxiliary agent does not contain fluorine-containing surfactant or fluorine-containing polymer.

[0012] In the above scheme, the quantity range of each component can be further selected, for example: the number of parts of the porous silica can be selected as 15 parts, 20 parts, 25 parts, 30 parts, 35 parts, 40 parts, 45 parts. The number of parts of the film-forming system can be selected as 20 parts, 23 parts, 25 parts, 28 parts, 30 parts, 35 parts, 40 parts. The number of parts of the tannic acid can be selected as 1 part, 2 parts, 3 parts, 4 parts, 5 parts, 6 parts. The number of parts of the microfibrillated cellulose can be selected as 0 part, 0.2 part, 0.30 part, 0.40 part, 0.50 part, 0.60 part, 0.80 part, 1.0 part. The number of parts of the auxiliary agent can be selected as 0 part, 0.5 part, 1.0 part, 1.5 part, 2.0 part. The mass ratio of polyvinyl alcohol to carboxymethyl cellulose in the film-forming system can be 5:1, 4:1, 3.7:1, 3.6:1, 3:1, 2.75:1, 2:1, 1:1. The mass ratio of tannic acid to porous silica can be selected as 1:5, 1:7.5, 1:8.3, 1:8.75, 1:15, 1:25, 1:40, 1:45. The molar concentration of the added amount of multivalent metal ions (taking aluminum ions as an example) can be selected as 0 mol / kg, 0.05 mol / kg, 0.10 mol / kg, 0.15 mol / kg, 0.20 mol / kg, 0.25 mol / kg, 0.30 mol / kg. When the multivalent metal ions are aluminum ions, the molar ratio of the aluminum ions to tannic acid in the coating can be selected as 0.08, 0.10, 0.15, 0.179, 0.20.

[0013] The present application also provides a coated paper or paperboard material, which comprises: a paper-based substrate and the aforementioned top layer coating formed on at least one surface of the paper-based substrate; wherein the dry coating amount of the top layer coating is 2-4 g / m².

[0014] The substrate is uncoated paper or uncoated paperboard; the top coating may be located on top of a mineral barrier substrate, wherein the content of ethylene-vinyl alcohol copolymer, nano-clay, and polyacrylic acid polymer in the mineral barrier substrate is ≤0.10wt%. The whiteness change of the top coating relative to the uncoated base paper is ≤1.0, and the pinhole density is ≤5 / 100cm². The total halogen content of the material is ≤50ppm, and it does not contain perfluorinated or polyfluoroalkyl substances. The water vapor transmission rate (WVTR) of the coated paper or paperboard at 38℃ and 90% relative humidity is ≤20g / m²·d, and the oxygen transmission rate (OTR) at 23℃ and 0% relative humidity is ≤25cm³ / m²·d, and Cobb 60 ≤12g / m², oil resistance grade ≥9. The material has been evaluated using the PTS-RH 021:2012 Cat II method and determined to be recyclable. The final dry coating weight can be selected as 2.0g / m², 2.5g / m², 3.0g / m², 3.5g / m², or 4.0g / m².

[0015] The present invention also provides a method for preparing a mineral barrier underlayer for the aforementioned coated paper or paperboard material, comprising the following steps:

[0016] Step 1. Determine the composition and content of the primer coating based on dry solids, which includes: 60–80 wt% mineral phase, wherein the mineral phase includes kaolin, calcium carbonate and 0–5 wt% talc; as the balance, an organic binder is added to make up to 100 wt%; the organic binder includes 5–15 wt% polyvinyl alcohol (PVOH), 5–20 wt% styrene-butadiene emulsion (SBR) or carboxylated styrene-butadiene emulsion (XSBR) and 0–10 wt% cationic starch, wherein the percentage content of these three components is based on the total dry weight of the organic binder; additives include 0.1–0.5 wt% carboxymethyl cellulose (CMC), 0.05–0.30 wt% sodium hexametaphosphate, 0.05–0.20 wt% fluorine-free defoamer and 0–1.0 wt% humectant; wherein the content of ethylene-vinyl alcohol copolymer (EVOH), nano-clay and polyacrylic polymer in the primer coating is ≤0.10 wt% and does not contain perfluorinated and polyfluoroalkyl substances (PFAS), thus obtaining the primer coating formulation;

[0017] Step 2. Based on the undercoat formulation obtained in Step 1, disperse the components in water. The dispersion process includes 1200–1800 s. -1 The bottom coating dispersion was obtained by high-speed dispersion at a shear rate of 15–25 min.

[0018] Step 3. Adjust the solid content of the primer dispersion obtained in Step 2 to 50–64 wt%, adjust the pH to 7.5–8.5 using a non-volatile alkali, and make its apparent viscosity 0.40–1.20 Pa·s to obtain a primer with qualified parameters.

[0019] Step 4. Apply the qualified base coating obtained in Step 3 to the paper or paperboard substrate using a scraper or measuring rod. The dry coating amount is 3–8 g / m², resulting in paper or paperboard with a wet coating.

[0020] Step 5. Dry the paper or paperboard with wet coating obtained in Step 4 at 100–120°C for 20–60 seconds to make the bottom solid content ≥90% to obtain dry bottom coated paper or paperboard.

[0021] Step 6. Calender the dried base coated paper or paperboard obtained in Step 5 with a linear pressure of 60–120 kN / m to obtain the mineral barrier base layer.

[0022] The present invention further provides a method for preparing the aforementioned coated paper or paperboard material, the method comprising:

[0023] Step 1. Disperse porous silica in an aqueous phase with pH 5.0–5.5 using a high-speed disperser for 10 min, add tannic acid solution and stir for 1–5 min to obtain a pre-composite slurry of silica and tannic acid;

[0024] Step 2. Add polyvinyl alcohol solution, carboxymethyl cellulose solution and microfibrillated cellulose slurry to the pre-composite slurry of Step 1 and mix. Adjust the mixture under low speed stirring until the solid content is 25-40 wt% and the apparent viscosity is 0.15-0.60 Pa·s to obtain the film-forming composition.

[0025] Step 3. Apply the film-forming composition from Step 2 to the surface of the mineral barrier substrate by means of a doctor blade, metering rod, or curtain, control the dry coating amount to 2–4 g / m², and dry at 90–110°C for 60–180 s.

[0026] Step 4. At the drying end of Step 3, apply a 0.1–0.5 mol / L aluminum lactate solution to the surface of the coating obtained in Step 3 by spraying. The amount of spray solution applied is 0.8–2.5 g / m², the drying end temperature is 70–90℃, the amount of aluminum ions in the coating is 0.05–0.30 mol / kg, and the total iron in the solution is ≤10 mg / kg to obtain a metal-treated coating.

[0027] Step 5. Calender the metal coating from Step 4 or the coating from Step 3 with a linear pressure of 40–120 kN / m.

[0028] Compared with the prior art, the following significant advantages can be obtained by using the present invention:

[0029] Barrier performance: The coating of this invention effectively enhances the barrier ability against gas molecules through the synergistic effect of the tortuous physical path constructed by porous silica and the cross-linked network of tannic acid. At a relatively low dry coating weight of 2–4 g / m², the WVTR of the coated paper or paperboard can be ≤20 g / m²·d at 38°C and 90% relative humidity; and the OTR can be ≤25 cm³ / m²·d at 23°C and 0% relative humidity, meeting the requirements of medium to high barrier performance for packaging applications.

[0030] Appearance and food contact compliance: This invention effectively avoids a strong complexation and color-developing reaction between tannic acid and iron ions by using non-coloring multivalent metal ions such as aluminum ions for coordination and controlling the content of trace iron ions in the system. This method solves the color degradation problem of tannic acid-containing coatings when applied to high-whiteness substrates, resulting in a whiteness difference (ΔE) between the coating and the uncoated backing paper. * The ab) value can be kept below 1.0, ensuring the product's excellent appearance quality, and the main raw materials used meet the relevant requirements for food contact materials.

[0031] Recyclability: The coating system of this invention does not contain difficult-to-disperse synthetic polymers and mainly consists of mineral fillers and water-soluble / water-dispersible components. This design allows the coating to effectively dissociate and separate from the fibers under standard hydraulic pulping conditions, without forming large sheets of adhesive or screen residue, and is easily removed by conventional screening equipment. Evaluation according to PTS-RH021:2012 Cat II method yielded a recyclable result, indicating good recyclability and meeting the requirements of sustainable development for packaging materials.

[0032] Flexibility and Durability: This invention utilizes the synergistic effect of porous silica, tannic acid, and the film-forming system to create a network structure that combines rigidity and toughness. This structure endows the coating with the ability to resist mechanical stress, making it less prone to cracking or pinholes during post-processing of packaging materials, such as die-cutting, folding, and forming. After 50 bidirectional folding tests, the OTR increase of the coating can be controlled within 25%, indicating that its barrier properties maintain high integrity after repeated deformation, ensuring the protective function of the packaging in actual use. Detailed Implementation

[0033] 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. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention. Unless otherwise stated, the raw materials used in this embodiment are commercially available industrial products or can be prepared by conventional methods. Unless otherwise specified, performance testing methods are performed according to the standards described in the invention summary section.

[0034] Table 1. Main reagent and raw material names, product models and manufacturers:

[0035]

[0036] Note: When zirconium carbonate and zirconium acetate solutions are used to form coordination networks with TA, they should comply with food contact regulations; ferric chloride (III) hexahydrate is only used for comparative color development and performance comparison, and is not for food contact applications.

[0037] Table 2 mainly analyzes the names, models, and manufacturers of the testing instruments:

[0038]

[0039] Main testing methods and standards:

[0040] WVTR: ASTM F1249-20, test temperature 38℃, relative humidity 90%; this application applies the same principle to the testing of coated paper / paperboard.

[0041] OTR: ASTM D3985-24 (Test temperature 23°C, relative humidity on the specimen side 0%, coulometric sensor method).

[0042] Cobb 60 Water absorption: ISO 535:2023 Paper and paperboard — Determination of water absorption (Cobb method)

[0043] Oil resistance (KIT test): TAPPI T 559 cm-22 (R2022) "Determination of oil resistance of paper and paperboard (KIT method)".

[0044] Repulping / Recyclability Assessment: PTS-RH 021:2012 Cat II "Identification of Repulping Recyclability of Paper and Paperboard Packaging and Graphic Products", conducted according to Cat II (Packaging Products / Paperboard Recycling Path), based on the following criteria:

[0045] Total rejection rate (including dry non-paper components removed during sample preparation) < 20%: recyclable; ≥ 20% is determined to be non-recyclable in this invention;

[0046] Dynamic wettability: ASTM D7334-08(2022) "Implementation procedure for evaluating surface wettability by means of advancing contact angle".

[0047] Folding endurance: TAPPI / ANSI T 511 om-25 "Determination of folding endurance of paper and paperboard (MIT method)".

[0048] Total halogen content: BS EN 14582:2016 Waste characteristics—Closed system oxygen combustion and determination of halogen and sulfur content.

[0049] Static friction coefficient (COF): TAPPI / ANSI T 815 om-24 "Determination of static friction coefficient of packaging materials (tilted plate method)".

[0050] Whiteness color difference (ΔE) * ab): Calculated relative to uncoated paper using a portable spectrophotometer (light source D65, observer 10°, geometric d / 8, SCI mode).

[0051] Non-coloring determination: After equilibration at 23°C and 50% RH, the ΔE of the metal salt-treated coating relative to the uncoated base paper. * ab≤1.0; and after aging at 38℃, 90% RH for 72h, ΔE * The increment of ab is ≤1.0. Systems that meet the above criteria are called non-coloring metal ion systems.

[0052] Disintegration rate: The physical disintegration rate of the sample during composting is determined according to ISO 20200:2023. The percentage of the mass passing through a 2mm sieve (%) is calculated. ≥90% is considered as complete disintegration.

[0053] Biodegradability: CO2 evolution is measured according to ISO 14855-2:2018. The ratio of the measured cumulative CO2 release to the theoretical CO2 production (ThCO2) of the sample is calculated. ≥90% ThCO2 is considered as completely biodegraded.

[0054] Germination Index (GI): The germination index of plants is determined according to EN 13432:2000. The calculation formula is GI=(sample germination rate×root length) / (control germination rate×root length)×100%. A GI≥90% indicates that the compost product has no plant toxicity.

[0055] Earthworm survival rate: According to AS 5810-2010, the non-toxicity of compost products to soil organisms is evaluated, and an earthworm survival rate of ≥90% is considered to meet the ecological safety standard.

[0056] Result determination: When a sample exhibits continuous cracks or a pinhole density greater than 5 pins / 100cm² during testing, the WVTR or OTR result is recorded as "NA (Failure)"; for adhesion resistance, a peel force ≤ 0.5N / 25mm is recorded as "Pass". Pinhole density is determined based on visual inspection and counting using a 10x magnifying glass under a standard light source. ΔE * An ab ≤ 1.0 rating is considered "appearance compliant". Dynamic hydrophobicity is determined as follows: a critical roll-off angle on the tilting table ≤ 20° and a 0° dwell time ≤ 3s is considered "hydrophobic compliant". A higher KIT rating number indicates better oil resistance.

[0057] General preparation process:

[0058] Preparation of mineral barrier substrate:

[0059] Step 1. Determine the composition and content of the primer coating on a dry solids basis, wherein the mineral phase accounts for 60–80 wt%, including kaolin (D... 50 (0.3–0.6 μm), calcium carbonate (GCC, D) 50 For 0.7–1.5 μm or PCC, D 50 The primer contains 0.5–1.2 μm of EVOH, nano-clay, and optional talc (0–5 wt%); an organic binder comprising 20–40 wt% of PVOH (5–15 wt%), SBR / XSBR (5–20 wt%), and optional cationic starch (0–10 wt%); additives comprising 0.1–0.5 wt% CMC, 0.05–0.30 wt% sodium hexametaphosphate, 0.05–0.20 wt% fluorine-free defoamer, and optional humectant (0–1.0 wt%) (propylene glycol / glycerin); the primer contains ≤0.10 wt% of EVOH, nano-clay, and polyacrylic polymer, and is PFAS-free, thus obtaining the primer formulation.

[0060] Step 2. Based on the underlying coating formulation obtained in Step 1, disperse the mixture in the following order: first add water, then add sodium hexametaphosphate and dissolve it; next, add the mineral phase and disperse it at 1200–1800 s. -1 Disperse at a high shear rate for 15–25 min; then add SBR / XSBR at a reduced rate; finally add PVOH aqueous solution, CMC, cationic starch and other additives in sequence, stir evenly to obtain the bottom coating dispersion.

[0061] Step 3. Adjust the underlying coating dispersion obtained in Step 2 to a solid content of 50–64 wt%. Adjust the pH to 7.5–8.5 using an alkaline buffer system (without ammonia or volatile amine bases) to ensure the apparent viscosity of the coating is 0.40–1.20 Pa·s (25℃, Brookfield RV, Spindle 4, 100 rpm) to obtain an underlying coating with qualified parameters.

[0062] Step 4. Apply the qualified base coating obtained in Step 3 to the paper or paperboard substrate using a doctor blade or measuring rod, controlling the dry coating amount to 3–8 g / m², to obtain paper or paperboard with a wet coating.

[0063] Step 5. Dry the wet-coated paper or paperboard obtained in Step 4 at 100–120°C for 20–60 seconds until the bottom layer solid content is ≥90%, to obtain a dry bottom-coated paper or paperboard.

[0064] Step 6. Calender the dried base coated paper or paperboard obtained in Step 5, controlling the linear pressure to be 60–120 kN / m, to obtain the mineral barrier base layer.

[0065] Example Sample Preparation Process:

[0066] Step 1. Add porous silica to an aqueous phase with a pH of 5.0–5.5 and disperse using a high-speed disperser at 1000–2000 s. -1 Disperse for 10 min; slowly add TA solution and stir for 1–5 min to obtain a pre-composite slurry of silica and TA.

[0067] Step 2. Add PVOH solution (10–15 wt%), CMC solution (1–2 wt%) and MFC slurry sequentially to the pre-composite slurry from Step 1, stir at low speed for 5–10 min and adjust to a solid content of 25–40 wt% and an apparent viscosity of 0.15–0.60 Pa·s to obtain the film-forming composition.

[0068] Step 3. Apply the film-forming composition from Step 2 to the surface of the mineral barrier substrate using a doctor blade, metering rod, or curtain method, controlling the dry coating amount to 2–4 g / m², and then place it in a drying section at 90–110°C for 60–180 s to dry.

[0069] Step 4. At the end of or after the drying in Step 3, apply a 0.1–0.5 mol / L non-coloring polyvalent metal ion solution to the surface of the coating obtained in Step 3 by spraying or atomizing; when the metal is aluminum ions, the amount is 0.05–0.30 mol / kg, to obtain a metal-treated coating.

[0070] Step 5. Calender the metal coating from Step 4 or the coating from Step 3 with a linear pressure of 40–120 kN / m to obtain a dense top layer.

[0071] Example:

[0072] Substrate: Coated white cardboard with a weight of 250–300 g / m². In embodiments of the present invention, aluminum lactate was used as a source of coordinating ions for verification.

[0073] Table 3. Formulation composition of the examples (top and bottom layers, parts by weight):

[0074]

[0075] In Example 2, the molar ratio of aluminum to TA was 0.179; in Example 4, the molar ratio of aluminum to TA was 0.196; and in Example 5, the molar ratio of aluminum to TA was 0.089.

[0076] Comparative example:

[0077] Table 4 Comparative Example Formulation Composition (parts by mass):

[0078]

[0079] Application example:

[0080] Application Example 1: Basic barrier and appearance evaluation.

[0081] Experimental Description: This application example aims to evaluate the basic barrier performance (WVTR, OTR, Cobb) of each embodiment and comparative example. 60 (KIT grade) and appearance (ΔE) * (ab, pinhole density). All samples were prepared and tested according to their formulations.

[0082] Table 5. Basic barrier and appearance evaluation data for the examples and comparative examples:

[0083]

[0084] Analysis: Comparative Example 2 used essentially the same formulation and process as Example 2, with the key difference being the replacement of the non-coloring metal ions (represented by aluminum ions for verification) required by this invention with typical coloring metal ions (iron ions). The results showed that this replacement resulted in a change in the whiteness of the coating, ΔE. * Increasing ab from 0.8 in Example 2 to 5.8 resulted in a darker coating appearance. This result demonstrates that selecting non-coloring multivalent metal ions helps maintain the coating appearance (ΔE) while improving barrier properties. * ab≤1.0).

[0085] Application Example 2: Resistant condensation cycle test of the outer wall of a paper cup.

[0086] Experimental Description: To simulate the performance of paper cups under repeated use in refrigerated and room temperature environments, coated samples were placed in a 4°C environment for 30 minutes, then transferred to a 25°C environment for 30 minutes. This process was counted as one cycle, and a total of 3 cycles were performed. After each transfer from low temperature to room temperature, it was ensured that significant condensation formed on the surface. After all cycles were completed, the samples were equilibrated under standard conditions for 2 hours before testing their Cobb properties. 60Water absorption value and dynamic wetting properties.

[0087] Table 6. Results of condensation cycle resistance tests for the examples and comparative examples:

[0088]

[0089] Analysis: This application example verifies the coating's durability by simulating the harsh conditions of repeated immersion in condensate. The results show that Examples 2 and 4, containing non-coloring metal ions, maintained low water absorption values ​​(Cobb) after cyclic testing. 60 The coating exhibits a viscosity of ≤11.2 g / m² and dynamic hydrophobicity. The metal-polyphenol network structure enhances the coating's resistance to moisture degradation. In contrast, Comparative Examples 1 and 3 showed a decrease in water-blocking performance after cycling. These experimental results demonstrate that selecting non-coloring metal ions for coordination contributes to achieving both coating durability and appearance stability.

[0090] Application Example 3: Oil and heat resistance oil penetration test.

[0091] Experimental Description: This test evaluates the coating's protective ability against greases, especially high-temperature greases. The room-temperature oil resistance rating is assessed using the KIT test, and the heat-resistant oil penetration resistance is evaluated by measuring the weight gain of rapeseed oil at 150°C after 30 minutes of contact with the coating surface.

[0092] Table 7. Results of oil and heat-resistant oil tests for the examples and comparative examples:

[0093]

[0094] Analysis: This application example evaluated the protective capability of the coating in a critical food packaging scenario involving contact with room temperature and high-temperature oils (150°C). The experimental results clearly show that Examples 2, 4, and 5, which introduced non-coloring metal ions (aluminum ions), achieved higher oil resistance ratings (KIT=10) and lower weight gain in the hot oil penetration test (≤5.2 g / m²). The results indicate that the metal-polyphenol network formed by aluminum ions and tannic acid helps prevent oil molecule penetration. Combined with the results of Application Example 1, selecting non-coloring metal ions can achieve oil barrier properties while avoiding coating color degradation.

[0095] Application Example 4: Test of re-sizing performance and flexibility retention.

[0096] Experimental Description: Resizing performance was evaluated according to PTS-RH 021:2012 standard. Flexibility was assessed by subjecting the sample to 50 bidirectional folds according to TAPPI T 511 standard, followed by testing its OTR at 23℃ and 0% relative humidity, and the rate of change of its OTR value relative to the unfolded value was calculated to characterize it.

[0097] Table 8. Results of resizing and flexibility tests for the examples and comparative examples:

[0098]

[0099] Analysis: This application example simultaneously evaluated the environmental friendliness (re-sizing performance) and mechanical durability (flexibility) of the coating, both core indicators for evaluating the practicality of paper-based packaging materials. The analysis results show that all examples and comparative examples, evaluated using the PTS-RH 021:2012 Cat II method, were determined to be recyclable, indicating that the coating system has a good foundation for recyclability. However, in the flexibility test, after 50 repeated folds, the oxygen barrier performance of all examples decreased by less than 25%, with Examples 2, 4, and 5 showing a decrease rate of less than 20%. The results indicate that the structure formed by porous silica, tannic acid, and the film-forming system helps the coating resist mechanical stress. In contrast, the comparative examples showed a significant decrease in barrier performance after folding (OTR increase > 40%). This comparison demonstrates that the formulation of this invention helps solve the problem of barrier coating failure due to brittle cracking during processing and use.

[0100] Application Example 5: Processability Assessment.

[0101] Experimental Description: This application example aims to evaluate the physical surface properties of coated paper during actual post-processing (such as printing, die-cutting, and stacking) and warehousing and transportation, primarily including its slip properties and anti-blocking properties. Slip properties are characterized by the static coefficient of friction (COF), which is determined according to the TAPPI / ANSI T 815 om-24 standard at 23°C and 50% relative humidity using the inclined plate method. A lower COF value indicates smoother material operation on processing equipment. Anti-blocking properties (i.e., anti-sticking performance) are used to simulate the risk of coated paper sticking together when stacked under pressure and heat. The test method involves placing two 25mm wide samples with their coated surfaces facing each other and pressing them together at 40°C and 50kPa for 24 hours. The peel force required to separate them is then measured. A lower peel force indicates better anti-blocking performance.

[0102] Table 9. Results of COF and adhesion resistance in the examples and comparative examples:

[0103]

[0104] Analysis: The test results of this application example directly reflect the commercial applicability of the coating, especially its performance on automated production lines. Data shows that all embodiments of the present invention exhibit excellent processability, particularly Examples 2, 4, and 5, which are coordinated with non-coloring metal ions, achieving optimal levels of static friction coefficient (≤0.34) and anti-adhesion peel force (≤0.24 N / 25 mm). This is mainly attributed to the synergistic effect of the metal-polyphenol network and porous silica: this network structure makes the coating surface denser and harder, reducing the adhesion effect with the contact surface; simultaneously, the micro-rough surface formed by the porous silica particles effectively reduces the actual contact area, thereby significantly reducing friction and adhesion. In contrast, the shortcomings of each comparative example are clearly exposed: Comparative Example 1 (without tannic acid) and Comparative Example 3 (using non-porous fillers) exhibited higher friction and adhesion due to their relatively soft surfaces; Comparative Example 4, due to its excessive silica content, resulted in a rough coating surface that was prone to powdering, which in turn increased the friction coefficient and led to severe adhesion problems.

[0105] Application Example 6: Total Halogen Content Test.

[0106] Experimental Description: This application example aims to verify the chemical safety of the coating material, particularly confirming its absence of halogen compounds to comply with increasingly stringent environmental regulations and food contact material standards, such as the ban on per- and polyfluoroalkyl substances (PFAS). The test was performed according to BS EN 14582:2016, a method that involves burning the sample in a closed oxygen system and subsequently determining the halogen content in the combustion products using methods such as ion chromatography. In this test, we focused on the total halogen content, with a threshold of ≤50 ppm for acceptance. This is generally the industry standard for determining whether a material is "halogen-free."

[0107] Table 10. Total halogen content test results of the examples and comparative examples:

[0108]

[0109] The test results of this application example have general guiding significance. All samples from the examples and comparative examples passed the total halogen content test with excellent performance, far below the 50 ppm limit. This result clearly demonstrates that the entire coating system constructed in this invention, from porous silica and natural polyphenols to the film-forming system and the introduced non-coloring metal salt, is chemically free of halogen elements.

[0110] Application Example 7: Appearance stability during high humidity storage.

[0111] Experimental Description: This application example aims to evaluate the color stability of the coating under prolonged high temperature and humidity conditions, simulating the shelf life performance of the product under extreme storage and transportation conditions such as tropical regions or cold chain disruptions. The coated sample was placed in a constant temperature and humidity chamber at 38°C and 90% relative humidity for 72 hours for continuous aging. After aging, the sample was removed and equilibrated under standard conditions (23°C, 50% RH). Subsequently, using the same spectrophotometer and testing methods as in Application Example 1, its whiteness color difference (ΔE) relative to the uncoated base paper was re-measured. * ab). By calculating ΔE after aging. * ab value and initial ΔE * The increment of the ab value is used to quantify the degree of color change of the coating in a high humidity environment. An increment ≤ 1.0 is considered to meet the appearance standard.

[0112] Table 11 ΔE after high humidity storage in the examples and comparative examples * ab increment:

[0113]

[0114] The results of this aging test directly relate to the product's commercial value and consumer acceptance, serving as the ultimate test of the coating's long-term reliability. Data shows that all embodiments of the present invention exhibit excellent color stability. Particularly noteworthy are Examples 2, 4, and 5, coordinated with non-coloring metal ions (aluminum ions), where the color change increment is negligible (≤0.4), strongly demonstrating the high stability of the formed metal-polyphenol network structure, which does not degrade or undergo further color development even under continuous hydrothermal stress. In stark contrast, Comparative Example 2, which uses coloring iron ions, not only exhibits poor initial color but also further deteriorates after aging, with an increment as high as 2.6, revealing the instability of the iron-polyphenol complex under high humidity conditions.

[0115] Application Example 8: Iron contamination challenges color boundaries.

[0116] Experimental Description: This application example simulates the impact of iron ion contamination in process water, a common phenomenon in industrial production, on the appearance of coatings. Ferric chloride was uniformly added to the dry-end spray solution of each formulation (or, for formulations without a spraying step, to the coating itself) to reduce the Fe... 3+ The final concentration was 5 mg / kg. Its initial ΔE was measured. * ab and ΔE after aging at 38℃ and 90% RH for 72 hours * ab increment. The criterion is the initial ΔE. * The standard is met if ab ≤ 1.0 and increment ≤ 1.0. Also, record whether WVTR / OTR is affected.

[0117] This application example aims to comprehensively evaluate the color stability of the coating under simulated industrial production conditions and to verify the criticality of the non-coloring multivalent metal ion selection emphasized in this invention. For this purpose, all samples were subjected to simulated environments containing 5 mg / kg Fe. 3+ Prepared under contaminated process water conditions. Furthermore, for the most direct performance comparison, we have specifically established new comparative examples, which correspond completely to the examples containing aluminum ions in formulation, the only difference being that the aluminum ions are replaced with an equimolar amount of coloring iron ions:

[0118] Control group 1: The formula is the same as in Example 4, but aluminum ions are replaced with iron ions.

[0119] Control group 2: The formula is the same as in Example 5, but aluminum ions are replaced with iron ions.

[0120] Table 12 Examples and Comparative Examples in Fe 3+ Color stability under pollution:

[0121]

[0122] Analysis: The comprehensive results demonstrate that the embodiments using the non-coloring aluminum ions of this invention effectively resist iron contamination and maintain excellent appearance. Conversely, the formulations without metal ion protection and the comparative examples using iron ions directly exhibited severe color degradation and failed to meet standards. This clearly shows that the technical approach of this invention is key to simultaneously achieving high-efficiency barrier properties and excellent commercial appearance, resolving a core contradiction in industrial applications.

[0123] Application Example 9: Storage stability and foaming of coatings.

[0124] Experimental Description: This application example evaluated the storage stability and process adaptability of the coating composition. The coating prepared in step 2 was stored in a sealed container at 25°C for 7 days and subjected to cyclic shearing (1000 s). -1 After 30 min, its viscosity, pH, foam volume (and defoaming at 0 / 15 / 30 min), and particle size (D) were measured. 50 / D 90 Changes in viscosity (≤15%), pH change (≤0.2), and residual foam volume after 30 minutes (≤10%) were judged. 90 Particle size growth ≤ 0.5 μm.

[0125] Table 13. Storage stability and foaming properties of the coatings in the examples and comparative examples:

[0126]

[0127] Analysis: All formulations in the examples exhibited excellent storage and shear stability, with all indicators remaining within the defined range, indicating uniform dispersion, good component compatibility, and potential for industrial application. In contrast, Comparative Example 2 (containing iron ions) and Comparative Example 3 (using calcium carbonate instead) both showed some degree of flocculation and pH drift, indicating poor stability. Comparative Example 4, due to its excessively high silica content, was extremely unstable, exhibiting severe viscosity increase and particle agglomeration. This demonstrates that the formulation range defined in this invention is crucial for obtaining stable and processable coatings.

[0128] Application Example 10: Food contact migration.

[0129] Experimental Description: To verify the safety of the coating for food packaging, migration tests were conducted according to the GB 4806 and GB 31604 series standards. The tests included overall migration using food simulants (10% ethanol, 95% ethanol, olive oil), as well as specific migration of key substances such as aluminum and the characteristic substance tannic acid (TA). The judgment criteria were: overall migration results must meet the limit requirement of ≤10 mg / dm² in GB 4806.8-2016; among the specific migrating substances, the migration amount of aluminum should be ≤5 mg / kg, and the characteristic substance tannic acid (TA) should be undetectable.

[0130] Table 14. Food contact migration test results of the examples and comparative examples:

[0131]

[0132] Analysis: All examples passed the stringent food contact migration test with excellent results, demonstrating their high safety for food packaging applications. Particularly noteworthy are Examples 2, 4, and 5, which contain aluminum ions, exhibiting extremely low specific migration amounts of aluminum and tannic acid, indicating a highly stable metal-polyphenol network structure that firmly anchors the components within the coating. Comparative Example 2 failed the test due to its unsuitability for food contact. Comparative Examples 3 and 4, however, showed excessive overall migration due to unstable formulation systems, further demonstrating the completeness and scientific validity of the formulation design of this invention.

[0133] Application Example 11: Evaluation of compostability for household and industrial composting.

[0134] Based on the aforementioned compost performance test standards, the degradation performance of the samples of Examples 1–5 and Comparative Examples 1–4 under industrial composting and household composting conditions was verified. For all samples, sheets with dimensions of 100 mm × 100 mm were cut (n = 3), and industrial composting (58 ± 2 °C, strong aeration, periodic turning) and household composting (28 ± 2 °C, moisture content 55–60%, turning once a week) tests were respectively carried out in a controlled composting environment. The industrial composting experiment lasted for 12 weeks to 180 days, and the household composting experiment lasted for 180 days to 365 days. The average value ± standard deviation (n = 3) was calculated for each item according to the standard regulations, and a threshold of ≥ 90% was used as the passing criterion for disintegration and biodegradation. At the same time, the germination index (GI) and earthworm survival rate were confirmed to meet the ecological safety requirements.

[0135] Table 15 Household / Industrial Composting Test Results of Examples and Comparative Examples (n = 3):

[0136]

[0137] Analysis: As can be seen from Table 15, Examples 1–5 showed excellent degradability and ecological safety under both industrial and household composting conditions: the disintegration rate exceeded 91% after 12 weeks, the biodegradation degree after 180 days or 365 days was higher than 90% ThCO2, and both the GI and earthworm survival rate were above 95%, fully meeting the requirements for compostable materials. Among them, Examples 2 and 4 containing the Al 3+ —tannic acid complex network had the best comprehensive performance, with both high degradation rate and ecological stability; Example 5 remained qualified although it was close to the determination threshold under the upper limit condition of silica content. In terms of the comparative examples, Comparative Example 1 without tannic acid disintegrated slightly slower but still勉强 met the standard in household composting; Comparative Example 2 using Fe 3+ 3 complexation showed significant decreases in both biodegradation and ecological indicators in the household composting system, indicating that color-forming metal ions would hinder degradation and cause biological toxicity; the calcium carbonate replacement system (Comparative Example 3) could be composted, but its overall performance was inferior to that of the present invention; while Comparative Example 4 with too high inorganic phase content was unqualified due to a large residue rate and insufficient biodegradation. The comprehensive results show that the proportion range of 15–45 parts of porous silica, appropriate amount of tannic acid, and 0.05–0.30 mol / kg non-color-forming metal ions (represented by Al 3+ is the key to achieving both high barrier and high composting performance.

[0138] Analysis of experimental results: <000??338>Based on the comprehensive test results, it can be fully demonstrated that the porous silica-tannic acid mineral-based top barrier coating disclosed in the present invention realizes the combination of barrier property, appearance, durability and environmental friendliness at a low coating amount through the precise proportioning and synergistic effect of each key component.

[0140] 1. Synergistic Effect of Key Components and Establishment of Core Technology: The core of this invention lies in the synergistic effect of three key components. First, the data from Application Example 1 clearly indicates that the introduction of tannic acid is crucial for achieving efficient barrier properties (Comparative Example 1 vs. Comparative Example 1 without tannic acid); simultaneously, porous silica must be used instead of ordinary fillers (Comparative Example 1 vs. Comparative Example 3 using calcium carbonate) to construct an effective "zigzag path" physical barrier. Second, the introduction of non-coloring metal ions to form a coordination network helps improve the overall performance of the coating. The results of Comparative Example 2 in Application Example 1 show that the use of coloring iron ions leads to a significant change in the coating's appearance color (ΔE). * (ab is 5.8). The results of Application Example 7 demonstrate that the embodiments of the present invention maintain minimal color difference change after high humidity aging tests. The results indicate that selecting metal ions with "non-coloring" properties helps to simultaneously achieve barrier properties and appearance stability.

[0141] 2. Trend Analysis of Performance and Component Content: By comparing the data of various embodiments, a clear trend of quantitative impact can be observed:

[0142] Porous silica content: When its content increased from 15 parts (Example 3) to 45 parts (Example 5), the WVTR and OTR of the coating both showed a significant decreasing trend, and the barrier performance continued to improve. However, as shown in Comparative Example 4 (50 parts), when the porous silica content increased, the coating flexibility and processability decreased, indicating that the 15–45 parts range defined in this invention is a suitable range.

[0143] Tannic acid content: When its content was increased from 1 part (Example 5) to 6 parts (Example 4), the barrier properties, oil resistance (Application Example 3) and condensation resistance (Application Example 2) of the coating were further enhanced, which was attributed to the increase in crosslinking density.

[0144] Concentration of non-coloring metal ions: When the aluminum ion concentration increased from 0 mol / kg (Example 1) to 0.30 mol / kg (Example 4), the coating’s ability to resist moisture and grease erosion reached its peak. This was particularly evident in dynamic and stringent tests such as resistance to condensation cycling (Application Example 2), resistance to hot oil penetration (Application Example 3), and retention of flexibility (Application Example 4), which fully demonstrates the decisive role of the metal-polyphenol network in improving the durability of the coating.

[0145] 3. Comprehensive advantages in overall performance: In addition to the core barrier properties, the present invention also performs well in terms of re-sizing performance (application example 4), processability (application example 5), and chemical safety (application example 6). The coating performs well in terms of re-sizing performance, processability, and chemical safety, meeting the relevant application requirements.

[0146] In summary, the porous silica-tannic acid mineral-based top-layer barrier coating disclosed in this invention achieves comprehensive and efficient barrier properties against moisture, oxygen, and grease at a relatively low coating basis weight by utilizing the synergistic effect of porous minerals and natural polyphenols, supplemented by mild metal ion coordination enhancement. This coating system also possesses excellent whiteness, good resizing and recovery performance, and complementary compatibility with existing mineral-based barrier underlayers or orientation layers.

[0147] Those skilled in the art should understand that the above embodiments are merely exemplary and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, or improvements made to the technical solutions of the present invention within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A compostable silica-tannic acid mineral-based coating, characterized in that, The silica-tannic acid mineral-based coating comprises, on a dry solids basis: 15–45 parts porous silica; 20–40 parts of film-forming system; 1–6 parts tannins; And 0–1.0 parts of microfibrillated cellulose and 0–2 parts of adjuvants; The film-forming system is selected from one or both of polyvinyl alcohol and carboxymethyl cellulose; The mass ratio of tannic acid to porous silica is 1:5 to 1:

45. The compostable silica-tannic acid mineral-based coating is the top coating; The top coating contains non-coloring polyvalent metal ions to form a metal-polyphenol network; The multivalent metal ions are selected from one or more of aluminum ions, zirconium ions, titanium ions, magnesium ions, and calcium ions; The multivalent metal ions are provided as one or more of aluminum lactate, ammonium zirconium carbonate, titanium lactate, magnesium lactate, and calcium lactate, and the amount of multivalent metal ions added is 0.05–0.30 mol / kg based on the dry weight of the film-forming phase solids. When the multivalent metal ion is aluminum ion, its molar ratio with tannic acid in the top coating is 0.08–0.

20. The molar ratio is determined by ICP-OES or ICP-MS to determine the metal element content, and converted by TOC or HPLC-UV to determine the TA content, all based on the dry weight of the film-forming phase solids. Furthermore, the total content of ethylene-vinyl alcohol copolymer, nano clay and polyacrylic polymer in the top coating is ≤0.10wt% each.

2. The coating according to claim 1, characterized in that, The porous silica belongs to a class of silicon oxide fillers. The silicon oxide filler is selected from one or more mesoporous silica prepared by precipitation, vapor phase, sol-gel, and template methods, and its volume distribution D... 50 The size is 0.2–1.5 μm and D 90 ≤3.0μm.

3. The coating according to claim 1 or 2, characterized in that, The film-forming system is mainly composed of polyvinyl alcohol and carboxymethyl cellulose, with a mass ratio of 5:1 to 1:

1.

4. The coating according to claim 3, characterized in that, The film-forming system contains one or more of water-soluble polysaccharides or cellulose ethers in total amount ≤5 wt%.

5. The coating according to claim 1, characterized in that, The tannic acid is hydrolyzed gallic tannin; the total iron content in the top coating is ≤0.0005 mol / kg based on the film-forming phase solids.

6. The coating according to claim 1, characterized in that, The total amount of the additives is 0–2 parts, and they are selected from one or more of the following categories: defoamers, wetting and leveling agents, film-forming aids, humectants, and preservatives and mildew inhibitors. The additives do not contain fluorinated surfactants or fluorinated polymers.

7. A coated paper or paperboard material, characterized in that, It includes: A paper-based substrate and a top coating according to claim 1 formed on at least one surface of the paper-based substrate; The dry coating amount of the top layer is 2–4 g / m².

8. The coated paper or paperboard material according to claim 7, characterized in that, The substrate is uncoated paper or uncoated paperboard; the top coating is located on top of the mineral barrier layer, and the content of ethylene-vinyl alcohol copolymer, nano clay and polyacrylic polymer in the mineral barrier layer is ≤0.10wt%.

9. The coated paper or paperboard material according to claim 7 or 8, characterized in that, The whiteness change of the top coating relative to the uncoated backing paper is ≤1.0, and the pinhole density is ≤5 / 100cm².

10. The coated paper or paperboard material according to claim 9, characterized in that, The material contains a total halogen content of ≤50ppm and is free of perfluorinated and polyfluoroalkyl substances.

11. The coated paper or paperboard material according to claim 7, characterized in that, The coated paper or paperboard has a water vapor transmission rate ≤20 g / m²·d at 38°C and 90% relative humidity, and an oxygen transmission rate ≤25 cm³ / m²·d at 23°C and 0% relative humidity. 60 ≤12g / m², oil resistance grade ≥9.

12. The coated paper or paperboard material according to claim 7, characterized in that, The material was evaluated using the PTS-RH 021:2012 Cat II method and determined to be recyclable.

13. A method for preparing a mineral barrier underlayer for coating paper or paperboard material as described in claim 7 or 8, characterized in that, Includes the following steps: Step 1. Determine the composition and content of the primer coating based on dry solids, which includes: 60–80 wt% mineral phase, wherein the mineral phase includes kaolin, calcium carbonate and 0–5 wt% talc; as the balance, an organic binder is added to make up to 100 wt%; the organic binder includes 5–15 wt% polyvinyl alcohol, 5–20 wt% styrene-butadiene emulsion or carboxylated styrene-butadiene emulsion and 0–10 wt% cationic starch, wherein the percentage content of these three components is based on the total dry weight of the organic binder; additives include 0.1–0.5 wt% carboxymethyl cellulose, 0.05–0.30 wt% sodium hexametaphosphate, 0.05–0.20 wt% fluorine-free defoamer and 0–1.0 wt% humectant; wherein the content of each of the ethylene-vinyl alcohol copolymer, nano-clay and polyacrylic polymer in the primer coating is ≤0.10 wt%, and it does not contain perfluorinated and polyfluoroalkyl substances, thus obtaining the primer coating formulation; Step 2. Based on the undercoat formulation obtained in Step 1, disperse the components in water. The dispersion process includes 1200–1800 s. -1 The bottom coating dispersion was obtained by high-speed dispersion at a shear rate of 15–25 min. Step 3. Adjust the solid content of the primer dispersion obtained in Step 2 to 50–64 wt%, adjust the pH to 7.5–8.5 using a non-volatile alkali, and make its apparent viscosity 0.40–1.20 Pa·s to obtain a primer with qualified parameters. Step 4. Apply the qualified base coating obtained in Step 3 to the paper or paperboard substrate using a scraper or measuring rod. The dry coating amount is 3–8 g / m², resulting in paper or paperboard with a wet coating. Step 5. Dry the paper or paperboard with wet coating obtained in Step 4 at 100–120°C for 20–60 seconds to make the bottom solid content ≥90% to obtain dry bottom coated paper or paperboard. Step 6. Calender the dried base coated paper or paperboard obtained in Step 5 with a linear pressure of 60–120 kN / m to obtain the mineral barrier base layer.

14. A method for preparing coated paper or paperboard material according to claim 8, characterized in that, The preparation method includes the following steps: Step 1. Disperse silica in an aqueous phase at pH 5.0–5.5 using a high-speed disperser for 10 min, add tannic acid solution and stir for 1–5 min to obtain a pre-composite slurry of silica and tannic acid; Step 2. Add polyvinyl alcohol solution, carboxymethyl cellulose solution and microfibrillated cellulose slurry to the pre-composite slurry of Step 1 and mix. Adjust the mixture under low speed stirring until the solid content is 25-40 wt% and the apparent viscosity is 0.15-0.60 Pa·s to obtain the film-forming composition. Step 3. Apply the film-forming composition from Step 2 to the surface of the mineral barrier substrate by means of a doctor blade, metering rod, or curtain, control the dry coating amount to 2–4 g / m², and dry at 90–110°C for 60–180 s. Step 4. At the drying end of Step 3, apply a 0.1–0.5 mol / L aluminum lactate solution to the surface of the coating obtained in Step 3 by spraying. The amount of spray solution applied is 0.8–2.5 g / m², the drying end temperature is 70–90℃, the amount of aluminum ions in the coating is 0.05–0.30 mol / kg, and the total iron in the solution is ≤10 mg / kg to obtain a metal-treated coating. Step 5. Calender the metal coating from Step 4 or the coating from Step 3 with a linear pressure of 40–120 kN / m.

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