Compostable silica-tannin mineral-based coating, coated materials and methods for their preparation

Abstract

The present application provides a compostable silica-tannic acid mineral-based coating, coated material and a preparation method thereof, and belongs to the technical field of paper and paperboard surface functionalization. The coating is made of water-based paint, and the main raw materials include porous silica and natural tannic acid, and are combined with polyvinyl alcohol and other substances to form a film. A key point of the present application is that a special network structure is formed by combining specific metal ions (such as aluminum ions) with tannic acid. This method solves the problem of discoloration (yellowing or darkening) of traditional tannic acid coatings. The paper or paperboard treated with the coating of the present application can effectively block water vapor, oxygen and grease at a low coating amount, thereby protecting the contents of the package. At the same time, the coating can maintain the original high whiteness of the paper and has good recyclability and compostability, and is environmentally 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 silicate 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 aid, humectant and preservative 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 an uncoated paper or uncoated paperboard; the top coat can be on top of a mineral barrier layer, the mineral barrier layer has a content of each of ethylene-vinyl alcohol copolymer, nanoclay and polyacrylic polymer ≤0.10wt%. The top coat has a change in whiteness relative to the uncoated base paper ≤1.0, and a pinhole density ≤5 per 100cm2. The total halogen content of the material is ≤50ppm, and the material is free of perfluoroalkyl and polyfluoroalkyl substances. The coated paper or paperboard has a water vapor transmission rate (WVTR) at 38°C, 90% relative humidity ≤20g / m2·d, an oxygen transmission rate (OTR) at 23°C, 0% relative humidity ≤25cm3 / m2·d, and a Cobb 60 ≤12g / m2, an oil resistance rating ≥9. The material is determined as recyclable by PTS-RH 021:2012 Cat II method evaluation. The final dry coat weight can be selected as 2.0g / m2, 2.5g / m2, 3.0g / m2, 3.5g / m2, 4.0g / m2.

[0015] The present application also provides a preparation method of a mineral barrier layer for the aforementioned coated paper or paperboard material, comprising the following steps:

[0016] Step 1. Determine the components and contents of the base coat, which comprises: a mineral phase 60–80wt%, the mineral phase comprises kaolin, calcium carbonate and talc 0–5wt%; an organic binder as the balance to make up to 100wt%; the organic binder comprises 5–15wt% polyvinyl alcohol (PVOH), 5–20wt% styrene butadiene emulsion (SBR) or carboxyl styrene butadiene emulsion (XSBR) and 0–10wt% cationic starch, wherein the percentage contents of the three components are based on the total dry weight of the organic binder; the auxiliary components comprise carboxymethyl cellulose (CMC) 0.1–0.5wt%, sodium hexametaphosphate 0.05–0.30wt%, non-fluorine defoaming agent 0.05–0.20wt% and humectant 0–1.0wt%; wherein the content of each of ethylene-vinyl alcohol copolymer (EVOH), nanoclay and polyacrylic polymer in the base coat is ≤0.10wt%, and the base coat is free of perfluoroalkyl and polyfluoroalkyl substances (PFAS), to obtain a base coat formula;

[0017] Step 2. Disperse the components in water according to the base coat formula obtained in step 1, the dispersion process comprises high-speed dispersion at a shear rate of 1200–1800s -1 for 15–25min to obtain a base coat dispersion;

[0018] Step 3. The base coating dispersion obtained in Step 2 is adjusted to a solid content of 50-64 wt%, the pH is adjusted to 7.5-8.5 using a non-volatile base, and the apparent viscosity is adjusted to 0.40-1.20 Pa·s to obtain a base coating with qualified parameters;

[0019] Step 4. The base coating with qualified parameters obtained in Step 3 is applied to a paper or paperboard substrate by means of a doctor blade or a metering rod, with a dry coating weight of 3-8 g / m², to obtain a paper or paperboard with a wet coating;

[0020] Step 5. The paper or paperboard with a wet coating obtained in Step 4 is dried at 100-120°C for 20-60 s to obtain a base coating paper or paperboard with a solid content of ≥90%;

[0021] Step 6. The dried base coating paper or paperboard obtained in Step 5 is subjected to a calendering treatment with a linear pressure of 60-120 kN / m to obtain a mineral barrier base coating.

[0022] The present application further provides a method for preparing the coated paper or paperboard material as described above, the method comprising:

[0023] Step 1. Porous silica is dispersed in an aqueous phase at pH 5.0-5.5 using a high-speed disperser for 10 min, and a tannic acid solution is added and stirred for 1-5 min to obtain a pre-composite slurry of silica and tannic acid;

[0024] Step 2. The pre-composite slurry of Step 1 is mixed with a polyvinyl alcohol solution, a carboxymethyl cellulose solution, and a microfibrillated cellulose slurry, and is adjusted to a solid content of 25-40 wt% and an apparent viscosity of 0.15-0.60 Pa·s under low-speed stirring to obtain a film-forming composition;

[0025] Step 3. The film-forming composition of Step 2 is applied to the surface of the mineral barrier base coating by means of a doctor blade, a metering rod, or a curtain, with a dry coating weight controlled to be 2-4 g / m², and is dried at 90-110°C for 60-180 s;

[0026] Step 4. An aluminum lactate solution of 0.1-0.5 mol / L is applied to the surface of the coating obtained in Step 3 in a spraying manner at the dry end of Step 3, with a spraying liquid application amount of 0.8-2.5 g / m², a dry end temperature of 70-90°C, an amount of aluminum ions in the coating of 0.05-0.30 mol / kg, and a total iron content of the solution of ≤10 mg / kg to obtain a metal treatment coating;

[0027] Step 5. The metal treatment coating of Step 4 or the coating of Step 3 is subjected to a calendering treatment with a linear pressure of 40-120 kN / m.

[0028] Compared with the prior art, the following significant beneficial effects can be obtained by using the present application:

[0029] Barrier performance: The coating of the present application effectively improves the barrier ability to gas molecules by the synergistic effect of the tortuous physical path of porous silica and the tannic acid crosslinking network. At a low dry coating amount of 2-4 g / m², the coated paper or paperboard has a WVTR of ≤20 g / m²·d at 38℃ and 90% relative humidity, and an OTR of ≤25 cm³ / m²·d at 23℃ and 0% relative humidity, meeting the requirements of medium-high barrier performance for packaging applications.

[0030] Appearance and food contact compliance: The present application effectively avoids the strong complexation and color reaction between tannic acid and iron ions by selecting non-color-causing 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 applied to high brightness substrates, making the coating have a whiteness color difference (ΔE * ab) of ≤1.0 relative to the uncoated base paper, ensuring excellent appearance quality of the product, and the main raw materials used meet the relevant requirements of food contact materials.

[0031] Recyclability: The coating system of the present application does not contain synthetic polymers that are difficult to disperse, and is mainly composed of mineral fillers and water-soluble / water-dispersible components. This design allows the coating to effectively dissociate and separate from the fibers under standard hydro-pulping conditions, without forming large pieces of adhesive material or screen residue, which can be easily removed by conventional screening equipment. According to the PTS-RH021:2012 Cat II method, the evaluation result is recyclable, indicating that it has good recycling performance and meets the requirements of sustainable development of packaging materials.

[0032] Flexibility and durability: The present application forms a network structure with both rigidity and toughness through the synergistic effect of porous silica, tannic acid and the film-forming system. This structure gives the coating the ability to resist mechanical stress and is not prone to cracking or pinholes during the post-processing of packaging materials such as die cutting, folding and molding. After 50 times of bidirectional folding test, the OTR increase of the coating can be controlled within 25%, indicating that its barrier performance remains high integrity after repeated deformation, ensuring the protection function of the packaging in actual use. DETAILED DESCRIPTION

[0033] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with specific examples. It should be understood that the specific examples described herein are only used to explain the present application and not to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application. Unless otherwise specified, the raw materials used in the examples are commercially available industrial products or can be prepared by conventional methods. The performance test methods are in accordance with the standards described in the summary section unless otherwise specified.

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

[0035]

[0036] Note: Zirconium ammonium carbonate and zirconium acetate solution should meet the food contact regulations when used to form a coordination network with TA; iron (III) chloride hexahydrate is only used for color development and performance comparison in the comparative example and is not used for food contact purposes.

[0037] Table 2: Main analytical instrument names, product models and manufacturers:

[0038]

[0039] Main test methods and standards:

[0040] WVTR: ASTM F1249-20, test temperature 38℃, relative humidity 90%; the present application is equivalent to the principle of this method for testing coated paper / board.

[0041] OTR: ASTM D3985-24 (test temperature 23℃, relative humidity 0% on the sample side, coulomb sensor method).

[0042] Cobb 60 Water absorption: ISO 535:2023 "Determination of water absorption of paper and board (Cobb method)".

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

[0044] Re-pulping / recyclability evaluation: PTS-RH 021:2012 Cat II "Identification of re-pulping recyclability of paper and paperboard packaging and graphic products", performed according to Cat II (packaging products / paperboard recycling path), and the judgment is based on:

[0045] Total rejection rate (including dry non-paper components removed during sample preparation) <20%: recyclable; ≥20% is determined as non-recyclable in the present application;

[0046] Dynamic wettability: ASTM D7334-08 (2022) “Standard Test Method for Evaluating Surface Wettability by Advancing Contact Angle”.

[0047] Folding endurance: TAPPI / ANSI T 511 om-25 “Standard Test Method for Folding Endurance of Paper and Paperboard (MIT Method)”.

[0048] Total halogen content: BS EN 14582:2016 “Waste characteristics - Determination of halogen and sulphur content by oxygen combustion in a closed system”.

[0049] Static coefficient of friction (COF): TAPPI / ANSI T 815 om-24 “Standard Test Method for Static Coefficient of Friction of Packaging Materials (Inclined Plane Method)”.

[0050] Whiteness color difference (ΔE * ab): measured by a portable spectrophotometer (illuminant D65, observer 10°, geometry d / 8, SCI mode) relative to the uncoated base paper.

[0051] Non-staining determination: after equilibration at 23 °C, 50% RH, the ΔE * abof the coating treated with metal salt relative to the uncoated base paper ≤ 1.0; and after aging at 38 °C, 90% RH, 72 h, the ΔE * abincrement ≤ 1.0. The metal ion system satisfying the above determination is referred to as non-staining.

[0052] Disintegration degree: the disintegration rate of the sample during the composting process was determined according to ISO 20200:2023, and the mass percentage (%) passing through the 2 mm screen was calculated, ≥ 90% was considered as complete disintegration.

[0053] Biodegradation degree: the amount of CO2 evolution was measured according to ISO 14855-2:2018, and the ratio of the measured cumulative CO2 release amount to the theoretical CO2 yield (ThCO2) of the sample was calculated, ≥ 90% ThCO2 was determined as complete biodegradation.

[0054] Germination index (GI): the germination index of plants was determined according to EN 13432:2000, and the calculation formula was GI = (sample germination rate x root length) / (control germination rate x root length) x 100%, GI ≥ 90% indicated that the compost product was non-phytotoxic.

[0055] Earthworm survival rate: the non-toxicity of the compost product to soil organisms was evaluated according to AS 5810-2010, and the earthworm survival rate ≥ 90% was considered as ecological safety standard.

[0056] Result determination: WVTR or OTR results are recorded as "NA (failure)" when the sample develops continuous cracking or pinhole density greater than 5 per 100 cm2during the test; the blocking is recorded as "pass" with peel force < 0.5 N / 25 mm. Pinhole density is counted by visual inspection and 10X magnification under standard light source. ΔE * ab < 1.0 is recorded as "appearance pass". Dynamic hydrophobicity determination: tilt table roll-off critical angle < 20° and 0° dwell time < 3 s is recorded as "hydrophobic pass". The larger the KIT grade number, the better the oil resistance.

[0057] General preparation procedure:

[0058] Mineral barrier primer preparation:

[0059] Step 1. Determine the components and contents of the primer coating based on dry solids, where the mineral phase is 60-80 wt% including kaolin (D 50 0.3-0.6 pm), calcium carbonate (GCC, D 50 0.7-1.5 pm, or PCC, D 50 0.5-1.2 pm), and optionally talc 0-5 wt%; the organic binder is 20-40 wt% including PVOH 5-15 wt%, SBR / XSBR 5-20 wt%, and optionally cationic starch 0-10 wt%; the additives include CMC 0.1-0.5 wt%, sodium hexametaphosphate 0.05-0.30 wt%, non-fluoro defoamer 0.05-0.20 wt%, and optionally humectant 0-1.0 wt% (propylene glycol / glycerol); the respective contents of EVOH, nanoclay, and polyacrylic polymer in the primer coating are all < 0.10 wt%, and the primer coating is PFAS-free, to obtain a primer coating formulation.

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

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

[0062] Step 4. The qualified primer coating obtained in Step 3 is applied on the paper or paperboard substrate by means of a doctor blade or a metering rod, with the dry coating weight controlled at 3-8 g / m2, to obtain a paper or paperboard with wet coating.

[0063] Step 5. The paper or paperboard with wet coating obtained in Step 4 is dried at 100-120°C for 20-60 s until the solid content of the primer coating is ≥ 90%, to obtain dried primer coated paper or paperboard.

[0064] Step 6. The dried primer coated paper or paperboard obtained in Step 5 is subjected to calendering treatment, with the line pressure controlled at 60-120 kN / m, to obtain a mineral barrier primer.

[0065] Preparation process of example samples:

[0066] Step 1. Porous silica is added into a pH 5.0-5.5 aqueous phase, and dispersed by a high-speed dispersion machine at 1000-2000 rpm for 10 min; a TA solution is slowly added and stirred for 1-5 min, to obtain a pre-composite slurry of silica and TA. -1

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

[0068] Step 3. The film-forming composition of Step 2 is applied on the surface of the mineral barrier primer by means of a doctor blade, a metering rod or a curtain, with the dry coating weight controlled at 2-4 g / m2, and sent into a 90-110°C drying section for drying for 60-180 s.

[0069] Step 4. After the drying end or completion of Step 3, a 0.1-0.5 mol / L solution of non-color-causing multivalent metal ions is applied to the surface of the coating obtained in Step 3 by means of spraying or atomization; when the metal is aluminum ions, the amount is 0.05-0.30 mol / kg, to obtain a metal-treated coating.

[0070] Step 5. The metal-treated coating of Step 4 or the coating of Step 3 is subjected to calendering treatment, with the line pressure controlled at 40-120 kN / m, to obtain a dense top layer.

[0071] Examples:

[0072] Substrate: A coated white paper with a basis weight of 250-300 g / m2 is selected. In the examples of the present application, aluminum lactate is used as a source of coordination ions.

[0073] ​Table 3 Example Formulation Composition (Top and Bottom Layers, parts by mass):

[0074]

[0075] The mole ratio of aluminum to TA in Example 2 is 0.179; the mole ratio of aluminum to TA in Example 4 is 0.196, and the mole ratio of aluminum to TA in Example 5 is 0.089.

[0076] Comparative Example:

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

[0078]

[0079] Application Example:

[0080] Application Example 1: Base Barrier and Appearance Evaluation

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

[0082] Table 5 Example and Comparative Example Base Barrier and Appearance Evaluation Data:

[0083]

[0084] Analysis: Comparative Example 2 uses a formulation and process that is essentially the same as Example 2, with the key difference being the replacement of the coordination metal ion from a non-staining multivalent metal ion (verified with aluminum ion) to a typical staining multivalent metal ion (iron ion). The results show that this replacement causes the whiteness change ΔE * ab of the coating to increase from 0.8 in Example 2 to 5.8, with the coating exhibiting a dark appearance. This result indicates that the selection of a non-staining multivalent metal ion helps to improve barrier performance while maintaining the appearance of the coating (ΔE * ab≤1.0).

[0085] Application Example 2: Paper Cup Outer Wall Condensation Resistance Cycle Test

[0086] Experimental Note: To simulate the performance of a paper cup when used repeatedly in a refrigerated and room temperature environment, the coated sample is placed in a 4°C environment for 30 minutes, then transferred to a 25°C environment for 30 minutes, which is counted as one cycle, and a total of 3 cycles are performed. After each transfer from low temperature to room temperature, the surface is ensured to have obvious condensation water. After all cycles are completed, the sample is equilibrated in a standard environment for 2 hours, and then tested for Cobb 60Water absorption values and dynamic wetting properties.

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

[0088]

[0089] Analysis: This application example verified the durability of the coating by simulating the harsh conditions of repeated condensation water immersion. The results showed that examples 2 and 4 containing non-discoloring metal ions maintained low water absorption values (Cobb 60 ≤11.2 g / m2) and dynamic hydrophobicity after the cycle test. The metal-polyphenol network structure helped to enhance the coating's ability to resist moisture damage. In contrast, comparative examples 1 and 3 showed a decrease in water resistance after the cycle. The experimental results showed that the selection of non-discoloring metal ions for coordination helped to achieve the durability and appearance stability of the coating.

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

[0091] Experimental explanation: This test evaluates the protective ability of the coating when it comes into contact with oil, especially high-temperature oil. The oil resistance grade at room temperature is evaluated by KIT test, and the hot oil penetration resistance is evaluated by measuring the mass gain of the coating surface after 30 minutes of contact with 150°C rapeseed oil.

[0092] Table 7. Results of oil and hot oil test for examples and comparative examples:

[0093]

[0094] Analysis: This application example evaluated the protective ability of the coating in the key food packaging scenario of contact with room temperature and high-temperature oil (150°C). The experimental results clearly showed that examples 2, 4 and 5, which introduced non-discoloring metal ions (aluminum ions), achieved a higher oil resistance grade (KIT = 10 grade), and had a lower mass gain (≤5.2 g / m2) in the hot oil penetration test. The results showed that the metal-polyphenol network formed by aluminum ions and tannic acid helped to prevent the penetration of oil molecules. Combined with the results of application example 1, the selection of non-discoloring metal ions can achieve oil barrier properties while avoiding the degradation of the coating color.

[0095] Application Example 4: Re-sizing performance and flexibility retention rate test.

[0096] Experimental explanation: The re-sizing performance was evaluated according to the PTS-RH 021:2012 standard. The flexibility was characterized by folding the sample sheet 50 times in two directions according to TAPPI T 511 standard, then testing its OTR at 23°C and 0% relative humidity, and calculating the change rate of OTR value relative to the folding before.

[0097] Table 8 Example and Comparative Example Re-pulpability and flexibility test results:

[0098]

[0099] Analysis: This application example simultaneously evaluates the environmental friendliness (re-pulpability) and mechanical durability (flexibility) of the coating, both of which are core indicators of the practicality of paper-based packaging materials. The analysis results show that all examples and comparative examples are determined to be recyclable by the PTS-RH 021:2012 Cat II method, indicating that the coating system has good recyclable basis. However, in the flexibility test, the oxygen barrier performance of all examples is attenuated to less than 25% after 50 repeated folding, and the attenuation rate of examples 2, 4 and 5 is less than 20%. The results show that the structure formed by porous silica, tannic acid and film-forming system helps the coating resist mechanical stress. In contrast, the barrier performance of each comparative example decreases significantly (OTR increases by >40%) after folding. This comparison shows that the formulation of the present application helps to solve the problem of failure of barrier coating due to brittle cracking during processing and use.

[0100] Application Example 5: Processability evaluation.

[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, stacking) and storage and transportation, mainly including its slipperiness and anti-blocking properties. Slipperiness is characterized by the static coefficient of friction (COF), which is measured by the inclined plate method according to the TAPPI / ANSI T 815 om-24 standard at 23°C and 50% relative humidity, and a lower COF value means that the material runs more smoothly on the processing equipment. Anti-blocking (i.e. blocking resistance) is used to simulate the risk of blocking of coated paper when stacked under pressure and heat, and the test method is as follows: two 25mm wide sample coated surfaces are placed opposite each other, and after being pressed together at 40°C and 50kPa pressure for 24 hours, the peeling force required to separate them is measured, and a lower peeling force indicates better anti-blocking effect.

[0102] Table 9 Example and Comparative Example COF and blocking results:

[0103]

[0104] Analysis: The test results of this application example directly reflect the commercial practicability of the coating, especially its performance on the automatic production line. The data show that all the embodiments of the present application exhibit excellent processing suitability, especially the embodiments 2, 4 and 5 coordinated by non-color-causing metal ions, whose static friction coefficient (≤0.34) and blocking peel force (≤0.24 N / 25 mm) have reached the best level. This is mainly due to the synergistic effect of the metal-polyphenol network and porous silica: the network structure makes the coating surface more dense and stiff, reducing the adhesion effect with the contact surface; at the same time, the micro-rough surface formed by the porous silica particles effectively reduces the real contact area, thereby significantly reducing the friction and adhesion. In contrast, the defects of each comparative example are exposed: comparative example 1 (without tannic acid) and comparative example 3 (using non-porous fillers) exhibit higher friction and adhesion due to the relatively flat and soft surface; comparative example 4 has too high a content of silica, resulting in a rough coating surface, easy to fall off, which in turn increases the friction coefficient and causes serious 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, especially to confirm that it does not contain halogen compounds, in order to meet the increasingly stringent environmental regulations and food contact material standards, such as the ban on perfluoroalkyl substances (PFAS). The test is performed in accordance with the BS EN 14582:2016 standard, which measures the halogen content in the combustion products by burning the sample in a closed oxygen system and then determining the halogen content by ion chromatography and other means. In this test, we focus on the total halogen content, and the standard for judging is that the content is ≤50 ppm, which is recorded as qualified, which is usually the industry standard for determining whether a material is "halogen-free".

[0107] Table 10 Test Results of Total Halogen Content of Examples and Comparative Examples:

[0108]

[0109] The test results of this application example have universal guiding significance, and all the samples of the examples and comparative examples have passed the total halogen content test with excellent performance far below the 50 ppm limit. This result clearly proves that the entire coating system constructed by the present application, from porous silica, natural polyphenol to film-forming system and the introduction of non-color-causing metal salt, does not contain halogen elements in its chemical nature.

[0110] Application Example 7: High Humidity Storage Appearance Stability

[0111] Experimental Note: This application example aims to evaluate the color stability of the coating under long-term high temperature and high humidity environment, to simulate the shelf life performance of the product under extreme storage and transportation conditions such as in tropical regions or cold chain interruption. The coated samples are placed in a constant temperature and humidity chamber at 38°C, 90% relative humidity for continuous aging for 72 hours. After aging, the samples are taken out and balanced in standard environment (23°C, 50% RH), and then the whiteness color difference (ΔΕ * ab) relative to the uncoated base paper is re-determined using the same spectrophotometer and test method as in Application Example 1. * * ab value after aging, and the increment of the initial ΔΕ * ab value, to quantify the degree of color change of the coating under high humidity environment, and the increment ≤1.0 is recorded as appearance compliance.

[0112] Table 11: ΔΕ * ab increment after high humidity storage of Examples and Comparative Examples:

[0113]

[0114] The results of this aging test are directly related to the commercial value and consumer acceptance of the product, and are the final test of the long-term reliability of the coating. The data shows that all examples of the present application exhibit excellent color stability, especially noteworthy are examples 2, 4 and 5, which are coordinated with non-chromogenic metal ions (aluminum ions), and their color change increment is minimal (≤0.4), which strongly proves that the metal-polyphenol network structure formed is highly stable, and will not degrade or further develop color under continuous hydrothermal stress. In sharp contrast to this is comparative example 2, which uses chromogenic iron ions, not only the initial color difference, but also the color further deteriorates after aging, with an increment of up to 2.6, which reveals the instability of iron-polyphenol complex under high humidity environment.

[0115] Application Example 8: Iron pollution challenge color boundary.

[0116] Experimental Note: This application example simulates the effect of iron ion pollution in process water commonly seen in industrial production on the appearance of the coating. Iron chloride is uniformly added in the dry end spray solution of each group of formulations (for formulations without a spray step, in the coating), so that the final concentration of Fe 3+ is 5 mg / kg. Test its initial ΔΕ * ab and the ΔΕ * ab increment after 72 hours of aging at 38°C, 90% RH. The judgment standard is that the initial ΔΕ * ab is ≤1.0 and the increment is ≤1.0 is compliance. At the same time, whether the WVTR / OTR is affected is recorded.

[0117] This application example aims to comprehensively evaluate the color stability of the coating in a simulated industrial production environment, and to verify the key importance of the non-color-causing multivalent metal ion selection emphasized in the present invention. To this end, all samples were prepared under simulated process water conditions containing 5 mg / kg Fe 3+ pollution. In addition, in order to make the most direct performance comparison, we specially set up new comparative examples, which are completely corresponding to the examples containing aluminum ions in terms of formulation, the only difference being that the aluminum ions are replaced by an equal molar amount of color-causing iron ions:

[0118] Comparative Example 1: The same formulation as Example 4, but the aluminum ions are replaced by iron ions.

[0119] Comparative Example 2: The same formulation as Example 5, but the aluminum ions are replaced by iron ions.

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

[0121]

[0122] Analysis: The comprehensive results prove that the examples of the present invention using non-color-causing aluminum ions can effectively resist iron pollution and maintain excellent appearance. In contrast, the formulations without metal ion protection and the comparative examples using iron ions directly, the color is severely deteriorated and cannot meet the standards. This clearly shows that the technical route of the present invention is the key to achieving efficient barrier and excellent commercial appearance at the same time, solving the core contradiction in industrial application.

[0123] Application Example 9: Coating storage stability and foaming

[0124] Experimental description: This application example evaluates the storage stability and process adaptability of the coating composition. The coating prepared in Step 2 is stored at 25°C for 7 days in a sealed state, and is subjected to cyclic shearing (1000 s -1 , 30 min), after which the changes in its viscosity, pH, foaming volume (and 0 / 15 / 30 min defoaming conditions) and particle size (D 50 / D 90 ) are determined. The criteria are: viscosity change ≤ 15%, pH change ≤ 0.2, residual foam volume after 30 minutes ≤ 10%, D 90 particle size increase ≤ 0.5 μm.

[0125] Table 13 Coating storage stability and foaming performance of Examples and Comparative Examples:

[0126]

[0127] Analysis: All example formulations exhibited excellent storage and shear stability, with all parameters within the defined range, indicating a well-dispersed and compatible system with potential for industrial application. In contrast, Comparative Example 2 (with iron ions) and Comparative Example 3 (with calcium carbonate instead) both showed some degree of flocculation and pH drift, with poor stability. Comparative Example 4, with too high a silica content, was extremely unstable, with severe viscosity growth and particle agglomeration. This demonstrates that the defined formulation range of the present application is critical to achieving a stable, processable coating.

[0128] Application Example 10: Food contact migration.

[0129] Experimental explanation: To verify the safety of the coating for food packaging, migration tests were performed according to GB 4806 and GB 31604 series standards. Test items included overall migration using food simulants (10% ethanol, 95% ethanol, olive oil), as well as specific migration of key substances such as aluminum elements and the characteristic substance tannic acid (TA). The criteria are: the overall migration results must meet the limit value requirement of ≤10 mg / dm² in GB 4806.8-2016; among the specific migration substances, the migration amount of aluminum is ≤5 mg / kg, and the characteristic substance tannic acid (TA) should not be detected.

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

[0131]

[0132] Analysis: All examples passed the stringent food contact migration test with excellent results, demonstrating their high safety for application in food packaging. It is particularly noteworthy that Example 2, Example 4, and Example 5, which contain aluminum ions, have very low specific migration amounts of aluminum and tannic acid, indicating that the formed metal-polyphenol network structure is very stable and can firmly anchor the components in the coating. Comparative Example 2 did not pass the test because it is not suitable for food contact. Comparative Examples 3 and 4 resulted in overall migration exceeding the limit due to unstable formulation systems, again demonstrating the integrity and scientific nature of the formulation design of the present application.

[0133] Application Example 11: Home and industrial composting compostability evaluation.

[0134] The application example verifies the degradation performance of the samples of Examples 1-5 and Comparative Examples 1-4 under industrial composting and home composting conditions according to the foregoing composting performance test standard. All samples are cut into 100 mm x 100 mm pieces (n = 3), and industrial composting (58 ± 2°C, strong aeration, periodic turning) and home composting (28 ± 2°C, moisture content 55-60%, turning every week) tests are carried out in a controlled composting environment. The industrial composting experiment lasts for 12 weeks to 180 days, and the home composting experiment lasts for 180 days to 365 days. The average value ± standard deviation (n = 3) is calculated for each item according to the standard, and the threshold value of ≥ 90% is used as the disintegration and biodegradation qualification criterion, and the germination index (GI) and earthworm survival rate are confirmed to meet the ecological safety requirements.

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

[0136]

[0137] Analysis: As can be seen from Table 15, Examples 1-5 all exhibit excellent degradability and ecological safety under industrial and home composting conditions: the disintegration rate is more than 91% in 12 weeks, the biodegradation degree is higher than 90% ThCO2 in 180 days or 365 days, the GI and earthworm survival rate are more than 95%, and the compostable material determination requirements are completely met. Among them, the Al 3+ complexed network of tannic acid in Example 2 and Example 4 has the best comprehensive performance, with high degradation rate and ecological stability; Example 5 remains qualified although it is close to the threshold value under the upper limit of the content of silicon dioxide. As for the comparative examples, Comparative Example 1 without tannic acid disintegrates slightly slower in home composting but still barely meets the requirements; Comparative Example 2 using Fe 3+ complexing has a significant decrease in biodegradation and ecological indicators in the home composting system, indicating that color-causing metal ions can hinder degradation and cause biological toxicity; the calcium carbonate replacement system (Comparative Example 3) can be composted, but the overall performance is not as good as the present application; and Comparative Example 4 with too high an inorganic phase content is not qualified due to high residue rate and insufficient biodegradation. The comprehensive results show that the ratio range of 15-45 parts of porous silicon dioxide, appropriate tannic acid, and 0.05-0.30 mol / kg of non-color-causing metal ions (represented by Al 3+ ) is the key to achieving high barrier and high composting performance.

[0138] Analysis of experimental results:

[0139] The comprehensive test results can fully prove that the porous silica-tannic acid mineral-based top layer barrier coating disclosed in the present application, through precise proportioning and synergistic effect of each key component, achieves the combination of barrier property, appearance, durability, and environmental friendliness at a low coating amount.

[0140] 1. Synergistic effect of key components and establishment of technical core: The core of the present application lies in the synergistic effect of three key components. First, the data of application example 1 clearly indicate that the introduction of tannic acid is the key to achieving high-efficiency barrier (comparative example 1 vs. comparative example 1 without tannic acid); at the same time, it is necessary to select porous silica instead of ordinary fillers (comparative example 1 vs. comparative example 3 using calcium carbonate) to build an effective “zigzag path” physical barrier. Second, the introduction of non-color-causing metal ions to form a coordination network helps to improve the overall performance of the coating. The results of comparative example 2 in application example 1 show that the use of color-causing iron ions will cause a large change in the appearance color of the coating (ΔE * ab is 5.8). The results of application example 7 show that the present application can maintain a small color difference change after high-humidity aging test. The results show that the selection of metal ions with “non-color-causing” properties helps to simultaneously obtain barrier performance and appearance stability.

[0141] 2. Trend analysis of performance and component content: By comparing the data of each example horizontally, clear quantitative influence trends can be observed:

[0142] Content of porous silica: When its content increases from 15 parts (example 3) to 45 parts (example 5), the WVTR and OTR of the coating both show a significant downward trend, and the barrier performance continues to improve. However, as shown in comparative example 4 (50 parts), when the content of porous silica increases, the flexibility and processing suitability of the coating decrease, indicating that the range of 15-45 parts defined by the present application is suitable.

[0143] Content of tannic acid: When its content increases from 1 part (example 5) to 6 parts (example 4), the barrier property, oil resistance (application example 3), and condensation resistance (application example 2) of the coating are all further enhanced, which is due to the increase in crosslinking density.

[0144] Concentration of non-color-causing metal ions: When the concentration of aluminum ions increases from 0 mol / kg (example 1) to 0.30 mol / kg (example 4), the ability of the coating to resist moisture and oil erosion reaches a peak, which is particularly evident in dynamic and severe tests such as condensation resistance cycle (application example 2), hot oil permeation resistance (application example 3), and flexibility retention (application example 4), fully demonstrating the decisive role of the metal-polyphenol network in improving the durability of the coating.

[0145] 3. Comprehensive advantage of overall performance: In addition to the core barrier performance, the present application also performs well in terms of re-sizing performance (application example 4), processing suitability (application example 5), and chemical safety (application example 6), which meets the relevant application requirements.

[0146] In summary, the disclosed porous silica-tannin mineral-based top barrier coating, by taking advantage of the synergistic effect of porous minerals and natural polyphenols, and supplemented by mild metal ion coordination enhancement, achieves comprehensive and efficient barrier to moisture, oxygen and grease at a lower coating basis weight. The coating system has excellent appearance whiteness, good re-slurry recovery performance, and complementary compatibility with existing mineral-based barrier bottom or orientation layer.

[0147] Those skilled in the art should understand that the above embodiments are only exemplary and are not intended to limit the scope of the present application. Any modification, equivalent replacement, improvement, etc. made to the technical solutions of the present application within the spirit and principles of the present application shall be included in the protection scope of the present application.

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 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; 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, and the primer coating formulation is finally obtained; 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 with a linear pressure of 40–120 kN / m.

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