Mineral-based readily recyclable compostable water barrier coating and method of making the same

Abstract

The application discloses a mineral-based easily-recyclable compostable water-barrier coating and a preparation method thereof, and belongs to the technical field of paper-based packaging materials. The coating is formed by a biodegradable film-forming system and a hierarchical lamellar mineral system containing three different scales and morphologies. The mineral system contains specific morphology and size of lamellar kaolin, non-asbestos grade lamellar talc and fully exfoliated sodium montmorillonite. The mineral lamella forms a dense parallel spread barrier path in the coating through a specific coating and drying solidification process, and combines a pH value responsive reversible cross-linking system to achieve excellent water vapor and oxygen barrier properties, water resistance and oil resistance at a low dry weight of 8-15 g / m2. The coating system does not contain polyvinylidene chloride and perfluoro and polyfluoro alkyl substances. The paper-based material coated with the coating is easily defibered and recycled under standard alkaline warm water conditions, and the fiber recovery rate is high, and meanwhile, the whole material meets the industrial composting requirements.

Description

Technical Field

[0001] This invention belongs to the technical field of paper-based packaging materials, specifically relating to a mineral-based, easily recyclable, compostable, water-based barrier coating and its preparation method. Background Technology

[0002] With increasing global emphasis on environmental protection and sustainable development, "replacing plastic with paper" has become an irreversible and important trend in the packaging industry. However, cellulose-based materials such as paper and paperboard are inherently porous and hydrophilic, which makes them lack sufficient barrier properties, such as water resistance, oil resistance, and oxygen barrier, making it difficult to meet the needs of many packaging applications. Therefore, coating treatment is usually required to endow paper-based materials with the necessary protective functions.

[0003] In existing technologies, such as US patent application US 2011 / 0293957 A1, a waterborne barrier coating comprising biopolymers and mineral fillers is disclosed. The drawback of this technology is that it uses conventional mineral fillers, such as ordinary kaolin, which have not been optimized for specific morphology and size. This simple physical filling method cannot efficiently construct a dense physical barrier in the coating to prevent the penetration of molecules such as water and oxygen; that is, the "maze effect" is not significant. Therefore, to achieve a moderate barrier effect, a high coating weight is often required, typically much greater than 20 g / m², which not only increases costs but also affects the physical properties of the paper-based material and subsequent processing. This invention differs from this approach in that it precisely designs a "three-scale hierarchical lamellar mineral system" composed of micron-sized kaolin, micron-sized talc, and nano-sized montmorillonite with specific morphology and size, and combines this with a high-shear coating process to induce its high orientation, thereby achieving excellent barrier performance at a much lower coating dry weight of 8-15 g / m².

[0004] Furthermore, to address the insufficient water resistance of water-based coatings, some technologies employ strong, irreversible crosslinking systems. For example, European patent application EP 4122988 A1 may involve the use of aldehyde, epoxy, or isocyanate crosslinking agents to improve coating durability. The fundamental flaw of this method is that the resulting stable covalent crosslinked network cannot be effectively broken under standard neutral or alkaline hydraulic pulping conditions. This leads to the coating being difficult to separate effectively from the fibers, instead peeling off in sheets, forming large, sticky substances that severely contaminate the pulp, significantly reduce fiber recycling rates, and fundamentally contradict the core advantage of easy recycling of paper-based packaging. This invention differs by creatively introducing a "pH-responsive reversible crosslinking system," utilizing ester bonds formed between polycarboxylic acids and the hydroxyl groups of biopolymers. These ester bonds remain stable during coating use, imparting sufficient water resistance; however, under standard alkaline conditions during recycling, they rapidly hydrolyze and break down, "unlocking" the crosslinked network, thereby allowing the coating structure to disintegrate and easily separate from the fibers, cleverly resolving the long-standing contradiction between water resistance and easy recycling.

[0005] In summary, existing technologies have failed to provide an ideal solution that can simultaneously satisfy high barrier performance, environmental friendliness, and excellent recyclability at low basis weight. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a mineral-based, easily recyclable, compostable water-based barrier coating and its preparation method, aiming to solve the problem of difficulty in balancing high performance, environmental friendliness and recyclability in existing paper-based barrier coating technologies.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a mineral-based, easily recyclable, compostable, water-based barrier coating, formed from a composition comprising 100 parts by weight of solids:

[0009] A graded lamellar mineral system of 35–65 parts, comprising: platy kaolin, asbestos-free platy talc, and sodium-type montmorillonite. In one specific embodiment, the amount of the graded lamellar mineral system may be 35, 38, 40, 42, 45, 48, 50, 52, 55, 58, 60, 62, or 65 parts.

[0010] Among them, the D of the flaky kaolin 50 The micrometer size is 0.6–1.2 µm, for example, it can be 0.6 µm, 0.7 µm, 0.8 µm, 0.9 µm, 1.0 µm, 1.1 µm or 1.2 µm; the asbestos-free grade flaky talc has a D... 50The value is 0.7–3µm, for example, it can be 0.7µm, 0.9µm, 1.0µm, 1.2µm, 1.5µm, 1.8µm, 2.0µm, 2.2µm, 2.5µm, 2.8µm or 3.0µm.

[0011] The sodium-type montmorillonite has a cation exchange capacity of not less than 90 cmol(+) / kg as determined by ISO 11260:2018, for example, 90 cmol(+) / kg, 92 cmol(+) / kg, 95 cmol(+) / kg, 98 cmol(+) / kg, 100 cmol(+) / kg, 105 cmol(+) / kg or 110 cmol(+) / kg; and a swelling index of not less than 24 mL / 2g in the ASTM D5890 test, for example, 24 mL / 2g, 25 mL / 2g, 26 mL / 2g, 28 mL / 2g, 30 mL / 2g or 32 mL / 2g.

[0012] The total mass ratio of the flaky kaolin and asbestos-free flaky talc to sodium-type montmorillonite is (2–8):1, for example, it can be 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, or 8:1. The asbestos-free flaky talc accounts for 5–35 wt% of the total mass of the mineral system, for example, it can be 5 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt%, 20 wt%, 22 wt%, 25 wt%, 28 wt%, 30 wt%, 32 wt%, or 35 wt%.

[0013] A biodegradable film-forming system comprising 25–55 parts, wherein the film-forming system comprises two or more of partially saponified polyvinyl alcohol, thermoplastic starch and its derivatives, alginate and cellulose derivatives. In one specific embodiment, the amount of the biodegradable film-forming system may be 25, 28, 30, 32, 35, 38, 40, 42, 45, 48, 50, 52 or 55 parts.

[0014] 0.5–8 parts of a reversible crosslinking system, wherein the reversible crosslinking system is a polycarboxylic acid selected from one or more of citric acid, tartaric acid, malic acid, itaconic acid, fumaric acid, maleic acid, succinic acid, glutaric acid, adipic acid, and aconitic acid; and the reversible crosslinking system can be further crosslinked with divalent or higher valence metal cations, wherein the metal cations are selected from one or more of calcium ions, magnesium ions, and strontium ions. In a specific embodiment, the amount of the reversible crosslinking system may be 0.5 parts, 0.8 parts, 1.0 parts, 1.5 parts, 2.0 parts, 2.5 parts, 3.0 parts, 3.5 parts, 4.0 parts, 4.5 parts, 5.0 parts, 6.0 parts, 7.0 parts, or 8.0 parts.

[0015] 1–6 parts of a plant wax emulsion, wherein the plant wax is selected from one or more of rice bran wax, carnauba wax, candelilla wax, sunflower wax, soybean wax, and palm wax. In one specific embodiment, the amount of plant wax emulsion may be 1 part, 1.5 parts, 2.0 parts, 2.5 parts, 3.0 parts, 3.5 parts, 4.0 parts, 4.5 parts, 5.0 parts, 5.5 parts, or 6.0 parts.

[0016] 2–12 parts of bio-based plasticizer. In one specific embodiment, the amount of bio-based plasticizer may be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 parts.

[0017] Furthermore, the water-based barrier coating may also include 0.01–3 parts of a dispersant and / or surfactant and / or defoamer, for example, 0.01 parts, 0.05 parts, 0.1 parts, 0.2 parts, 0.5 parts, 0.8 parts, 1.0 parts, 1.2 parts, 1.5 parts, 1.8 parts, 2.0 parts, 2.5 parts, or 3.0 parts. The dispersant or surfactant is selected from one or more of alkyl polyglycosides, sucrose fatty acid esters, polyoxyethylene alkyl ethers, polysorbates, polyaspartate, and polycarboxylic acid dispersants.

[0018] In one specific embodiment, in the composition forming the coating, the mass ratio of platy kaolinite, platy talc, and sodium montmorillonite in the graded lamellar mineral system is (2–6):(0.5–3):(1). For example, the proportion of platy kaolinite can be 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, or 6; the proportion of platy talc can be 0.5, 0.8, 1.0, 1.2, 1.5, 1.8, 2.0, 2.2, 2.5, 2.8, or 3.0.

[0019] Furthermore, in the composition forming the coating, the particle size distribution width of the flaky kaolin was determined according to ISO 13320:2020. 90 / D 10It can be no greater than 5, for example, 5, 4.8, 4.5, 4.2, 4.0, 3.8, 3.5, 3.2 or 3.0.

[0020] In one embodiment, in the composition forming the coating, the cation exchange capacity of the sodium-type montmorillonite, as determined according to ISO 11260:2018, is not less than 90 cmol(+) / kg, for example, it can be 90 cmol(+) / kg, 91 cmol(+) / kg, 93 cmol(+) / kg, 95 cmol(+) / kg, 97 cmol(+) / kg, 100 cmol(+) / kg, 102 cmol(+) / kg or 105 cmol(+) / kg.

[0021] In one embodiment, in the composition forming the coating, the mass ratio of partially saponified polyvinyl alcohol to thermoplastic starch in the biodegradable film-forming system is (2:1)–(1:1), for example, it can be 2:1, 1.8:1, 1.6:1, 1.5:1, 1.4:1, 1.2:1 or 1:1.

[0022] In addition, the biodegradable film-forming system that forms the coating also includes one or more of the following: polylactic acid, polybutylene succinate, polybutylene adipate, polybutylene terephthalate, polycaprolactone, polypropylene carbonate, poly-β-hydroxybutyrate, poly(3-hydroxybutyrate-3-hydroxyhexanoate), poly(3-hydroxybutyrate-4-hydroxybutyrate), and poly(3-hydroxybutyrate-3-hydroxyvalerate).

[0023] In one embodiment, in the composition forming the coating, the bio-based plasticizer is selected from one or more of glycerol, sorbitol, xylitol, erythritol, propylene glycol, polyethylene glycol, glyceryl triacetate, triethyl citrate, and tributyl citrate.

[0024] To further improve the performance of the coating, the composition forming the coating also includes 0.2–3 parts of microfibrillated cellulose, for example, 0.2 parts, 0.3 parts, 0.5 parts, 0.7 parts, 1.0 parts, 1.2 parts, 1.5 parts, 1.8 parts, 2.0 parts, 2.2 parts, 2.5 parts, 2.8 parts, or 3.0 parts.

[0025] In the final product form, the composition forming the coating is water-based and has a solids content of 20–50 wt%, for example, 20 wt%, 22 wt%, 25 wt%, 28 wt%, 30 wt%, 33 wt%, 35 wt%, 38 wt%, 40 wt%, 42 wt%, 45 wt%, 48 wt%, or 50 wt%. Simultaneously, the total halogen content of the composition, as determined according to BS EN 14582:2016, is not greater than 50 ppm, for example, 50 ppm, 40 ppm, 30 ppm, 20 ppm, 10 ppm, 1 ppm, or lower, and it is free of polyvinylidene chloride (PVDC) and per- and polyfluoroalkyl substances (PFAS), ensuring the environmental friendliness of the product.

[0026] The present invention also provides a method for preparing an aqueous barrier coating, the method using a composition corresponding to the coating described in any of the foregoing claims, and comprising the following steps:

[0027] Step 1. Exfoliate sodium montmorillonite in an aqueous medium with a pH of 8–10, such as 8.0, 8.5, 9.0, 9.5, or 10.0, for 15–30 minutes, such as 15 minutes, 20 minutes, 25 minutes, or 30 minutes, to obtain sodium montmorillonite exfoliation slurry. Filter the exfoliation slurry through a 0.45µm membrane. The dry solids ratio of the filtrate to the original dry solids is ≥60%, such as 60%, 65%, 70%, 75%, or 80%.

[0028] Step 2. Dissolve a portion of the saponified polyvinyl alcohol at 90–95°C, for example, 90°C, 92°C or 95°C, and gelatinize the thermoplastic starch at 90–95°C, for example, 90°C, 92°C or 95°C, and then mix it with the partially saponified polyvinyl alcohol solution to obtain a film-forming system mixture.

[0029] Step 3. Add the sodium montmorillonite stripping slurry and flaky kaolin obtained in Step 1 to the film-forming system mixture obtained in Step 2, and perform rotor-stator homogenization treatment for 3–5 minutes, for example, 3 minutes, 4 minutes or 5 minutes, to obtain a preliminary mineral-film-forming agent mixture.

[0030] Step 4. Pre-disperse the flake talc with the alkyl polysaccharide for 5–10 minutes, for example, 5 minutes, 7 minutes or 10 minutes, to obtain a talc pre-dispersion. Add the talc pre-dispersion to the mineral-film-forming agent preliminary mixture obtained in Step 3 to obtain a mineral-based mixture.

[0031] Step 5. Cool the mineral-based mixture obtained in Step 4 to no higher than 50°C, for example, 50°C, 45°C, 40°C or 35°C, add bio-based plasticizer, reversible crosslinking system, plant wax emulsion and other additives, adjust the pH value to 4.5–5.5 and filter to obtain the composition to be coated.

[0032] Step 6. With 10 3 –5×10 3 s -1 Apparent shear rate, for example, 1000 s -1 2000s -1 3000s -1 4000s -1 Or 5000s -1 The composition to be coated obtained in step 5 is applied to the substrate to form a wet film.

[0033] Step 7. Dry the wet film formed in step 6 to remove moisture, and then perform a heat curing treatment at 150°C for 90–120 seconds, for example, 90 seconds, 100 seconds, 110 seconds or 120 seconds, to obtain the coating.

[0034] In an optional embodiment, the method further includes, after step 7, applying a calcium salt solution to form surface calcium ion crosslinks to further enhance the barrier properties of the coating.

[0035] A coated paper-based material can be prepared by the above method. The coated paper-based material includes a paper or paperboard substrate, and at least one surface of the substrate has a coating formed by the aforementioned method. The dry weight of the coating is 8–15 g / m², for example, 8 g / m², 9 g / m², 10 g / m², 11 g / m², 12 g / m², 13 g / m², 14 g / m² or 15 g / m².

[0036] The coated paper-based material exhibits excellent barrier properties. For example, the water vapor transmission rate (WVTR) of the coating of the coated paper-based material at 38°C and 90% relative humidity is no greater than 15 g / m²·d, such as 15 g / m²·d, 12 g / m²·d, 10 g / m²·d, 8 g / m²·d, or 5 g / m²·d.

[0037] The oxygen permeability (OTR) of the coating on the paper-based material at 23°C and 50% relative humidity is no greater than 40 cc / m²·d·atm, for example, it can be 40 cc / m²·d·atm, 35 cc / m²·d·atm, 30 cc / m²·d·atm, 25 cc / m²·d·atm or 20 cc / m²·d·atm.

[0038] The Cobb water absorption of the coating on the paper-based material within 60 seconds is no greater than 14 g / m², for example, it can be 14 g / m², 12 g / m², 10 g / m² or 8 g / m², and the KIT is no less than 8, for example, it can be 8, 9, 10, 11 or 12.

[0039] The coated paper-based material of this invention has a wide range of applications, especially suitable for food contact packaging, such as paper cups, cup lids, takeaway food boxes, dry goods packaging, or outer packaging for daily chemical products; the material meets the industrial compostability requirements defined by EN 13432:2000 or ASTM D6400-23 standards.

[0040] The present invention also provides a method for recycling the coated paper-based material, comprising mechanically defibrating the coated paper-based material for 10–15 minutes, for example, 10 minutes, 12 minutes or 15 minutes, under the conditions of a temperature of 45–60°C, such as 45°C, 50°C, 55°C or 60°C, a pH value of 9.5–10.5, such as 9.5, 9.8, 10.0, 10.2 or 10.5, and a pulp concentration of 2%.

[0041] After treatment using the aforementioned recycling method, the fiber recovery rate is not less than 95%, for example, it can be 95%, 96%, 97%, 98% or 99%, and the macroscopic adhesive area is not greater than 5 mm² / m², for example, it can be 5 mm² / m², 4 mm² / m², 3 mm² / m², 2 mm² / m² or 1 mm² / m².

[0042] Compared with the prior art, the following significant advantages can be obtained by using the present invention:

[0043] Highly efficient multi-barrier performance: By precisely controlling the morphology, size, and ratio of three platy minerals with different scales and functions—kaolin, talc, and montmorillonite—and inducing their highly parallel orientation through a high-shear coating process, a "three-scale labyrinth structure" was constructed. This structure greatly extends the penetration path of small molecules such as water and oxygen, enabling the coating to achieve excellent WVTR and OTR at a low basis weight of 8–15 g / m², while maintaining good barrier stability even in high humidity environments.

[0044] Excellent recyclability: The polycarboxylic acid esterification and optional calcium ion crosslinking employed in this invention constitute a smart crosslinking system that is "stable during use and reversible during recycling." During coating use and storage, the crosslinked network provides the necessary water resistance and mechanical strength. During the recycling stage, by adjusting the pH to alkaline and appropriately raising the temperature, the ester bonds hydrolyze, and the calcium ions react with the complexing agent, effectively destroying the crosslinked network. This allows the coating to be rapidly peeled off and dispersed from the fibers, achieving a fiber recovery rate of over 95% and extremely low macroscopic adhesive content.

[0045] Comprehensive environmental friendliness: The coating components are all water-based, with extremely low volatile organic compound (VOC) content. The core raw materials are all biodegradable polymers and natural minerals, free of strictly regulated PVDC and PFAS, and the total halogen content can be controlled at extremely low levels. The coated paper-based material as a whole meets industrial composting standards and can effectively degrade under composting conditions, reducing the persistent environmental impact of waste and conforming to the requirements of circular economy and sustainable development.

[0046] Excellent process applicability and economy: The raw materials used in the coating formulation are widely available and cost-controllable. Its preparation process is highly compatible with existing papermaking and coating industrial equipment, and can be applied online or offline using various conventional methods such as blade coating, rod coating, and curtain coating, without requiring large-scale production line modifications. The introduction of talc optimizes the surface energy and oil resistance of the coating, enabling the reduction of expensive plant wax emulsions while achieving a high kit grade, further enhancing the product's economic competitiveness. Detailed Implementation

[0047] 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.

[0048] In this article, "parts by mass" refers to solids; "%" refers to mass percentage unless otherwise specified.

[0049] Main reagents and raw materials:

[0050] Table 1. Names, Models, and Manufacturers of Main Reagents and Raw Materials:

[0051]

[0052] Main analytical and testing instruments:

[0053] Table 2 lists the main analysis and testing equipment names, models, and manufacturers:

[0054]

[0055] Main testing methods and standards:

[0056] WVTR: ASTM F1249-20, Test conditions: 38℃, 90% RH;

[0057] OTR: ASTM F1927-20, Test conditions: 23℃, 50% RH;

[0058] Cobb absorbency: ISO 535:2023, test time: 60 seconds;

[0059] KIT: TAPPI T 559 cm-22;

[0060] Recyclability (re-pulping): PTS-RH 021:2012 (Cat II), dissociation conditions: temperature 45–60℃, pH 9.5–10.5, pulp concentration 2%, time 10–15 min. Screening was performed using a 0.15 mm slit screen, and fiber recovery rate, macroscopic adhesion area, and filtrate turbidity were calculated.

[0061] Static water contact angle: TAPPI T 458 cm-24 or ISO / TS 14778:2021;

[0062] Total halogen content: BS EN 14582:2016, determined by oxygen bomb combustion combined with ion chromatography;

[0063] Industrial compostability: assessed in accordance with EN 13432:2000 and / or ASTM D6400-23 standards;

[0064] Particle size: determined by laser diffraction according to ISO 13320:2020.

[0065] General preparation process:

[0066] General homemade process for rice bran wax emulsion (plant wax emulsion):

[0067] Pre-emulsified solution: Dissolve 1–2 wt% polyvinyl alcohol (PVOH) and 0.3–0.5 wt% glycerol in deionized water at 70–75°C, add 0.5–1.0 wt% alkyl polysaccharide relative to the total emulsion volume, and stir to form a pre-emulsified solution.

[0068] Wax melting and injection: Melt the plant wax (such as rice bran wax) at 90–95℃ and slowly add it into the system of step 1 in a thin stream, while emulsifying at high shear 8000–10000 rpm for 10–15 min.

[0069] Homogenization and solids content: The system was cooled to 70°C and subjected to a second shear for approximately 5 minutes. The emulsion density (D) was measured according to ISO 13320:2020. 50 The wavelength is 300–400 nm; water is added to bring the target solid content to approximately 30 wt%.

[0070] Cooling and post-treatment: Cool to room temperature, adjust pH to 6.5–7.5, defoam, and filter through approximately 100µm to obtain a plant wax emulsion for subsequent use.

[0071] Examples 1-4 and Comparative Examples 1-4:

[0072] The coating compositions of Examples 1 to 4 and Comparative Examples 1 to 4 were prepared according to the general preparation process described above, and their specific formulations (by solids) are shown in Table 3 below.

[0073] General steps for preparing coated paper-based materials:

[0074] The prepared coating composition was applied to 250 g / m² white cardstock using a laboratory coating machine. The coating dry weight was controlled within the target range by adjusting the coating parameters. The coated samples were first dried in an oven at 105°C for 2 minutes to remove most of the moisture, and then heat-cured in an oven at 150°C for 120 seconds. In some embodiments, after heat curing, a 0.3 g / m² 10% calcium chloride solution was applied via a spray system, followed by light calendering. All prepared samples were equilibrated at 23°C and 50% RH for at least 24 hours before performance testing. For samples with a dry weight of 10–14 g / m², the WVTR was ≤15 g / m²·d according to ASTM F1249-20; the OTR was ≤40 cc / m²·d·atm according to ASTM F1927-20; and the Cobb strength was ≤15 g / m²·atm according to ISO 535:2023. 60 ≤13g / m²; KIT ≥9 was measured according to TAPPI T 559 cm⁻².

[0075] Table 3. Formulation composition of the examples and comparative examples (by solids):

[0076]

[0077] Note:

[0078] Comparative Example 1: Talc was not used; its content was replaced by kaolin to verify the contribution of talc to oil resistance.

[0079] Comparative Example 2: Montmorillonite was added directly to the system without a pre-exfoliation step, and its exfoliation degree was <60%, which was used to verify the necessity of sufficient exfoliation.

[0080] Comparative Example 3: Using D 50 Large-particle-size talc (5µm) was used to verify the effect of talc particle size on coating quality.

[0081] Comparative Example 4: Glutaraldehyde, an irreversible crosslinking agent, was used instead of citric acid to verify the impact of the reversible crosslinking system on recycling performance.

[0082] Application example:

[0083] Application Example 1: Takeout food container application.

[0084] Experimental Description: This application example aims to evaluate the oil-repellent performance of the coating under simulated real-world usage scenarios. The coatings prepared in Examples 1-4 and Comparative Examples 1-4 were applied to the inside of 300 g / m² kraft paper, with the dry weight uniformly controlled at 14 g / m². The cardboard was then fabricated into standard-sized oil-repellent food containers. Each container was filled with 150 g of fried rice containing 15 g of cooking oil and left at 25°C for 4 hours. This test simulates a typical scenario of takeout food being prepared, delivered to the consumer, and left for a period of time. After the test, the food was removed, and the oil penetration on the inner wall of the container and the presence of oil spots on the outer wall were visually observed to comprehensively evaluate its oil-repellent performance.

[0085] Application Results: The coatings of Examples 1, 3, 2, 3, and 4 all exhibited excellent oil-resistant properties, with no obvious oil penetration on the inner wall of the lunchbox and a dry, oil-free outer wall. Examples 2 and 4 performed well, with slight oil stains on the inner wall but no penetration to the outer wall. Comparative Example 1 showed significant oil penetration on the inner wall and small visible oil spots on the outer wall.

[0086] Application Example 2: Static water contact angle.

[0087] Experimental Description: This application example evaluates the hydrophobicity of a coating surface by measuring the static water contact angle. The contact angle is a key indicator of the wettability of a liquid on a solid surface; a larger initial contact angle indicates a more hydrophobic surface. The change in contact angle over time (i.e., contact angle hysteresis) reflects the spread and absorption of the liquid on the surface. The experiment was conducted according to the TAPPI T 458 cm-24 standard, testing all coating samples from the examples and comparative examples (equilibrated for 24 hours at 23°C and 50% RH). Using a contact angle meter, a 5µL droplet of deionized water was precisely added to the sample surface. The initial contact angle at the moment of addition and the contact angle 30 seconds later were recorded, and the difference between the two was calculated as the contact angle hysteresis value. Each sample was tested five times, and the average value was taken.

[0088] Table 4. Static water contact angle results for the examples and comparative examples (unit: °):

[0089]

[0090] Analysis: Table 4 shows that the initial contact angle of all examples is higher than 102°, exhibiting good surface hydrophobicity, with Example 3 showing the best performance. Comparative Example 2 (montmorillonite not peeled) has the lowest initial contact angle and the largest contact angle hysteresis, indicating strong surface hydrophilicity and poor water resistance. This is attributed to the unpeeled montmorillonite aggregates disrupting the coating's density and forming hydrophilic channels. Comparative Example 1 (without talc) has a lower contact angle than the examples, indicating that the addition of talc helps improve surface hydrophobicity. The contact angle of Comparative Example 4 (irreversible crosslinking) is comparable to that of the examples, indicating that the crosslinking method has little effect on initial hydrophobicity, but its recyclability varies significantly.

[0091] Application Example 3: Hot water seepage resistance and cup body stiffness maintenance.

[0092] Experimental Description: This application example aims to evaluate the protective performance of the coating under high-temperature liquid contact conditions and its impact on the structural support of paper cups, which are key indicators for hot beverage cup applications. The coatings from each embodiment and comparative example were prepared into 250 ml standard paper cups for testing. Each paper cup was filled with 95°C hot water and left to stand for 30 minutes. During and after this period, any leakage was observed and recorded, and the leakage rate was calculated. After 30 minutes, the hot water was poured out, and the flexural stiffness of the cup wall was immediately measured and compared with the initial stiffness of paper cups from the same batch that had not been soaked in hot water to calculate the stiffness retention rate. Simultaneously, the appearance of the inner wall of the cup was rated.

[0093] Table 5. Results of hot water permeability resistance and stiffness retention in the examples and comparative examples:

[0094]

[0095] Analysis: Table 5 shows that all examples exhibit excellent hot water permeability resistance, extremely low leakage rate, and stiffness retention of close to or exceeding 90%, with intact inner walls. Comparative Example 2 (unpeeled montmorillonite) is completely unsuitable for this application, exhibiting severe leakage and coating damage during testing due to its poor water resistance (see high Cobb values ​​in Table 10). Comparative Example 3 (large-particle talc) showed localized micro-permeability and a significant decrease in stiffness, indicating that large particles disrupted coating uniformity. Comparative Example 1 (without talc) performed slightly worse than the examples. Comparative Example 4 (irreversible crosslinking) performed well in this test, but at the cost of sacrificing recyclability.

[0096] Application Example 4: Oil resistance rating and hot oil penetration test.

[0097] Experimental Description: This application example aims to quantitatively evaluate the oil-repellent performance of the coating, employing two methods for comprehensive evaluation. First, the KIT rating of the coating was determined according to the TAPPI T 559 cm-22 standard. This method assesses the oil-repellent level of the material using a mixture of castor oil, toluene, and heptane with varying permeability; a higher rating indicates better oil repellency. Second, to simulate a more stringent hot oil contact scenario after cooking, a "hot oil mass gain / penetration time" test was conducted: rapeseed oil at 150°C was quantitatively added to the sample surface and kept in contact for 30 minutes. The amount of oil absorbed was assessed by measuring the mass gain per unit area, while the time it took for the oil to first penetrate to the back of the paper substrate was recorded.

[0098] Table 6 Oil resistance results of the examples and comparative examples:

[0099]

[0100] Analysis: Table 6 clearly reveals the crucial role of talc in oil-repellent properties. Comparative Example 1 (without talc) only achieved a KIT rating of 7, exhibiting rapid hot oil penetration and high absorption. In contrast, all examples containing talc (Examples 1-4) achieved a KIT rating of 8 or higher, with Example 3, possessing the highest talc content, reaching a KIT rating of 10, demonstrating the best oil repellency. This proves that hydrophobic talc with a high aspect ratio can effectively prevent oil penetration and significantly reduce the amount of expensive plant waxes required.

[0101] Application Example 5: Recyclability Assessment.

[0102] Experimental Description: This application example is a key test to verify one of the core advantages of this invention, aiming to evaluate the fiber recovery efficiency and pulp cleanliness of the coated paper-based material in a simulated standard paper recycling process. The experiment employed the PTS-RH 021:2012 (Cat II) method, with all samples subjected to hydratolizing for 10–15 minutes under set alkaline (pH 9.5–10.5) and warm water (45–60°C) conditions. After hydratolizing, the pulp was screened using a 0.15 mm sieve to collect fibers and rejected impurities (mainly undispersed coating fragments). The recyclability was comprehensively evaluated by calculating the fiber recovery rate, measuring the area of ​​rejected macroscopic adhesions, and analyzing the turbidity and total solids content of the recycled white water (filtrate).

[0103] Table 7 Results of resizing and white water in the examples and comparative examples:

[0104]

[0105] Analysis: The data in Table 7 present a stark contrast, powerfully demonstrating the superiority of the reversible crosslinking system of this invention. All examples employing citric acid reversible crosslinking (Examples 1-4) and Comparative Examples 1-3 exhibited a high fiber recovery rate exceeding 95%, along with extremely low macroscopic adhesion and white water contamination levels. However, Comparative Example 4, employing glutaraldehyde irreversible crosslinking, saw its fiber recovery rate plummet to 65.2%, with both macroscopic adhesion area and white water contamination levels severely exceeding standards. This indicates that its coating cannot effectively decompose under alkaline conditions, forming a large amount of sticky material that contaminates the pulp. This proves that the pH-responsive reversible crosslinking system of this invention successfully resolves the contradiction between water resistance and easy recyclability.

[0106] Application Example 6: Total Halogen Content Test.

[0107] Experimental Description: This application example aims to verify the chemical safety of the coating material, particularly confirming that it does not contain halogenated substances that are strictly regulated, such as PVDC or PFAS. The experiment was conducted according to BS EN 14582:2016, using oxygen bomb combustion-ion chromatography to test all dried coating samples from the examples and comparative examples. The samples were completely burned in a sealed oxygen bomb, and the combustion products were absorbed by a specific absorbent. The contents of four halide ions—fluorine, chloride, bromine, and iodine—in the absorbent were then precisely determined by ion chromatography, and the total halogen content was calculated.

[0108] Table 8. Total halogen detection results of the examples and comparative examples (unit: ppm):

[0109]

[0110] Analysis: The results in Table 8 consistently show that the total halogen content of all samples, including all examples and comparative examples, was below the method detection limit of 50 ppm. This result strongly confirms that no halogen-containing chemicals were intentionally added to the formulation system of this invention, complying with increasingly stringent environmental and safety regulations for food packaging materials worldwide.

[0111] Application Example 7: Industrial compostability assessment.

[0112] This application example assesses three indicators—disintegration, final aerobic biodegradation, and ecotoxicity—in parallel according to EN 13432:2000:

[0113] Disintegration test: Refer to EN 13432 and adopt ISO 20200 or ISO 16929, incubate in a test composting environment of 58±2℃ for 12 weeks, dry and pass through a 2mm sieve, and calculate the percentage of dry weight ≥90% passing through.

[0114] Final aerobic biodegradation: CO2 accumulation was continuously monitored under controlled composting conditions according to ISO 14855-1:2012. The relative biodegradation rate was calculated based on the theoretical CO2 (ThCO2) corresponding to the organic carbon (TOC) of the sample. Criterion: ≥90% after 180 days.

[0115] Ecotoxicity (plant growth): According to Annex E of EN 13432 (revised by OECD 208), barley and water celery were selected as two plants. The compost product and the blank compost were mixed at a mass fraction of 50% and used as seedling substrate. Germination rate and aboveground biomass were compared. Judgment: The sample compost was ≥90% of the blank.

[0116] Note: EN 13432 also requires compliance with heavy metal limits and compost physicochemical properties; relevant data can be provided together with other patent application examples / test results.

[0117] Table 9. Results of industrial compostability tests for the examples and comparative examples:

[0118] Sample number Disintegration rate (%) after 12 weeks Biodegradation rate (%) within 180 days Plant growth test Example 1 97.2 94.5 pass Example 2 96.8 93.8 pass Example 3 97.5 95.1 pass Example 4 98.1 95.6 pass Comparative Example 1 97.0 94.2 pass Comparative Example 2 96.5 93.3 pass Comparative Example 3 96.9 93.9 pass Comparative Example 4 45.3 86.2 No biotoxicity testing was conducted (disintegration did not meet standards, and the product did not undergo ecotoxicity testing).

[0119] Table 9 shows that Examples 1–4 and Comparative Examples 1–3 using a reversible crosslinking system all meet the EN 13432 thresholds—disintegration (≥90% in 12 weeks) and final aerobic biodegradation (≥90% in 180 days) are both compliant, and the germination rate and aboveground biomass in the plant growth test are both ≥90% of the blank, thus they are deemed industrially compostable. In contrast, Comparative Example 4 has a disintegration rate of only 45.3% and a final biodegradation rate of 86.2% (both failing to meet the standards), thus failing to comply with EN 13432; the visible fragments it exhibits fall under the category of disintegration failure, which is not a direct criterion for ecotoxicity non-compliance. This comparison demonstrates that the pH-responsive reversible crosslinking system used in this invention, without sacrificing water resistance during the service life, does not hinder the samples from undergoing sufficient physical disintegration and biodegradation under industrial composting conditions; simultaneously, combined with the reslurry conditions described in the patent, the system can also achieve efficient recovery.

[0120] Experimental Results and Analysis:

[0121] Table 10 Core performance test results of the embodiments and comparative examples:

[0122]

[0123] Results analysis:

[0124] Synergistic effect analysis of graded mineral system: Comparing Examples 1-4 and Comparative Examples 1-3, it can be seen that the graded lamellar mineral system designed in this invention is crucial for achieving efficient barrier.

[0125] The role of montmorillonite: In Comparative Example 2, montmorillonite, without high-shear pre-exfoliation treatment, exists in the coating as micron-sized aggregates, making it difficult to fill nanoscale defects in the polymer matrix and form dense stacks with other platy minerals. Its exfoliation degree is below 60%, leading to a decrease in barrier network efficiency, manifested as significant degradation in WVTR and OTR, at 38.2 g / m²·d and 85.3 cc / m²·d·atm, respectively, and a significant increase in Cobb60. This indicates that improving the exfoliation degree is key to constructing an efficient "maze barrier."

[0126] The role of talc: Comparative Example 1, which does not contain talc, has a WVTR and OTR comparable to Example 1, but its KIT rating is only 7. Furthermore, to achieve this rating, the amount of plant wax emulsion had to be increased to 4 parts, significantly increasing costs. Conversely, Example 1 achieved a KIT rating of 9 using only 8 parts talc and 2 parts wax emulsion. In Example 3, increasing the talc content to 12 parts further improved the KIT rating to 10, while the amount of plant wax could be further reduced to 1.5 parts. This clearly demonstrates that high aspect ratio flake talc with a hydrophobic surface significantly improves the oil resistance of the coating and has a synergistic effect with plant wax, optimizing formulation costs while improving performance.

[0127] Influence of mineral ratio: In Example 2, the proportion of nano-sized montmorillonite was increased, and the proportion of PVOH, which has better compatibility with it, was correspondingly increased, resulting in the best OTR performance among all examples (14.5 cc / m²·d·atm). This indicates that montmorillonite nanosheets are particularly effective in blocking small molecule gases such as oxygen, and this formulation is especially suitable for applications with high oxygen barrier requirements. Example 4 reduced the content of montmorillonite and talc and increased the content of kaolinite. Although the barrier performance decreased, it still met the requirements of most applications, and its recovery performance indicators (adhesive and turbidity) were the best. This demonstrates the superiority of the formulation of this invention, which can be flexibly adjusted according to the performance and cost requirements of specific application scenarios.

[0128] Analysis of the impact of process and structure on performance:

[0129] Process and structure correlation: High-shear coating and thermosetting can promote the formation of dense, parallel, spreading barrier paths in the coating. This structural effect does not need to be characterized by spectral parameters and has been verified by direct quantitative results such as WVTR / OTR / Cobb / KIT; Comparative Example 2 shows significant deterioration of WVTR and OTR due to insufficient peeling degree and disordered stacking of the layers.

[0130] Particle size matching: Comparative Example 3 used D 50The talc particles were large, with a diameter of 5 µm. Although their orientation was acceptable, the coarse particles disrupted the microstructure of the coating, easily creating stress concentration points and micro-defects at the particle edges. This provided a "shortcut" for water vapor molecules to penetrate, resulting in a significant increase in WVTR to 16.5 g / m²·d. This demonstrates that the strict limitation of the particle size of each component in the graded mineral system is absolutely necessary for constructing a defect-free, highly efficient barrier layer.

[0131] Analysis of the impact of reversible crosslinking systems on recyclability:

[0132] Comparative Example 4 used glutaraldehyde, a conventional crosslinking agent that forms irreversible covalent bonds. The results show that its barrier and liquid resistance properties were comparable to Example 1, but its performance in the recycling test was extremely poor: the fiber recovery rate plummeted to 65.2%, meaning that more than one-third of the valuable fiber resources were discarded with the coating waste. Simultaneously, its adhesive area and filtrate turbidity both exceeded 500, indicating that the formed strong crosslinked network could not be effectively broken under alkaline warm water conditions; the coating could only break into large, sticky fragments, severely contaminating the pulp and rendering it unusable for reprocessing. In contrast, Examples 1-4, which used citric acid as a crosslinking agent, showed that under recycling conditions, the ester bonds hydrolyzed, the crosslinking points broke, and the coating easily dispersed into small, non-sticky particles, thus achieving excellent recycling performance. The fiber recovery rate exceeded 95%, and the adhesive area and turbidity were at extremely low levels. This comparison demonstrates the effectiveness of the reversible crosslinking system of the present invention.

[0133] Content change trend analysis:

[0134] Based on the analysis of Examples 1-4, it can be found that within the approximate range of total mineral content (50-55 parts) and total film-forming system content (33.5-37.5 parts), the oxygen barrier performance of the coating shows a significant upward trend as the montmorillonite content increases from 5 parts to 14 parts. However, excessively high content may pose challenges to the rheological properties of the coating. Increasing the talc content from 5 parts to 12 parts significantly improves the KIT grade. When the talc content reaches 8 parts or more, the requirement for plant wax can be effectively reduced. Kaolin, as the main barrier pathway builder, achieves stable basic barrier performance when its content is between 30-40 parts. When the kaolin content exceeds 40 parts, as shown in Example 4, the system becomes more sensitive to the ratio of plasticizer and film-forming agent, requiring appropriate adjustment to ensure the flexibility of the coating.

[0135] In summary, this invention, through the meticulous design of a graded lamellar mineral system, combined with pH-responsive reversible crosslinking technology and oriented coating process, has successfully developed a water-based coating that integrates high-efficiency barrier properties, easy recycling, and biodegradability, providing an innovative and practical solution for sustainable paper-based packaging.

[0136] 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, improvements, etc., 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 mineral-based, recyclable, compostable, waterborne barrier coating, characterized in that, The coating is formed from a composition comprising, per 100 parts by solids: 35-65 parts of a hierarchical lamellar mineral system comprising: lamellar kaolin, non-asbestos grade lamellar talc, and sodium montmorillonite; D90of the platelet-shaped kaolin is 0.6 - 1.2 pm, the D90of the non-asbestos grade platelet-shaped talc is 0.7 - 3 pm; 50 D90of the platelet-shaped kaolin is 0.6 - 1.2 pm, the D90of the non-asbestos grade platelet-shaped talc is 0.7 - 3 pm; 50 D90of the platelet-shaped kaolin is 0 The sodium montmorillonite has a cation exchange capacity of not less than 90 cmol(+) / kg as determined by ISO 11260:2018 and a swelling index in the ASTM D5890 test of not less than 24 mL / 2g; The mass ratio of the sum of the mass of the lamellar kaolin and non-asbestos grade lamellar talc to the mass of the sodium montmorillonite is (2-8):1, wherein the mass of the non-asbestos grade lamellar talc is 5-35 wt% of the total mass of the mineral system; 25-55 parts of a biodegradable film-forming system comprising two or more of partially saponified polyvinyl alcohol, thermoplastic starch and its derivatives, alginate, and cellulose derivatives; 0.5-8 parts of a reversible cross-linking system, the reversible cross-linking system being a polycarboxylic acid selected from one or more of citric acid, tartaric acid, malic acid, itaconic acid, fumaric acid, maleic acid, succinic acid, glutaric acid, adipic acid, aconitic acid; and the reversible cross-linking system is further cross-linkable with divalent or higher valent metal cations; The metal cations are selected from one or more of calcium ions, magnesium ions, strontium ions; 1-6 parts of a plant wax emulsion, the plant wax being selected from one or more of rice bran wax, carnauba wax, candelilla wax, sunflower wax, soy wax, palm wax; 2-12 parts of a bio-based plasticizer; Further, the aqueous barrier coating can further comprise 0.01-3 parts of a dispersant and / or a surfactant and / or an antifoam agent; The dispersant or surfactant is selected from one or more of alkyl polyglycoside, sucrose fatty acid ester, polyoxyethylene alkyl ether, polysorbate, polyaspartate, polycarboxylic dispersant; The method of preparing the aqueous barrier coating comprises: The sodium montmorillonite is subjected to high shear exfoliation in an aqueous medium having a pH of 8-10 for 15-30 minutes to obtain an exfoliated sodium montmorillonite slurry, and the exfoliated slurry is filtered through a 0.45 µm membrane, and the mass ratio of the dry solid of the obtained filtrate to the dry solid of the original sample is ≥60%.

2. The coating of claim 1, wherein, In the composition forming the coating, the mass ratio of the lamellar kaolin, lamellar talc, and sodium montmorillonite in the hierarchical lamellar mineral system is (2-6):(0.5-3):(1).

3. The coating according to claim 1 or 2, characterized in that The particle size distribution width of the sheet-like kaolin in the composition forming the coating is determined D according to ISO 13320:2020 90 / D 10 not more than 5.

4. The coating of claim 1, wherein, In the composition forming the coating, the mass ratio of the partially saponified polyvinyl alcohol to the thermoplastic starch in the biodegradable film-forming system is (2:1)-(1:1).

5. The coating of claim 1, wherein, The biodegradable film-forming system forming the coating further comprises one or more of polylactic acid, polybutylene succinate, polybutylene succinate-adipate, polybutylene terephthalate-adipate, polycaprolactone, polypropylene carbonate, poly-β-hydroxybutyric acid, poly(3-hydroxybutyric acid-3-hydroxyhexanoic acid) ester, poly(3-hydroxybutyric acid-4-hydroxybutyric acid) ester, poly(3-hydroxybutyric acid-3-hydroxyvaleric acid) ester.

6. The coating of claim 1, wherein, The bio-based plasticizer in the composition forming the coating is selected from one or more of glycerol, sorbitol, xylitol, erythritol, propylene glycol, polyethylene glycol, glycerol triacetate, triethyl citrate, tributyl citrate.

7. The coating of claim 1, wherein, The composition forming the coating further comprises 0.2-3 parts of microfibrillated cellulose.

8. The coating of claim 1, wherein, The composition forming the coating is carried by water, has a solid content of 20-50 wt%, a total halogen content of no more than 50 ppm as determined by BS EN 14582:2016, and is free of polyvinylidene chloride and perfluoroalkyl and polyfluoroalkyl substances.

9. A method of preparing the coating according to any one of claims 1 to 8, characterized in that, The method comprises the following steps: Step 1. Dissolve part of the saponified polyvinyl alcohol at 90-95℃, and gelatinize the thermoplastic starch at 90-95℃, then mix the saponified polyvinyl alcohol solution with the gelatinized thermoplastic starch to obtain a film-forming system mixture; Step 2. Add the sodium-type montmorillonite exfoliated slurry and the sheeted kaolin according to claim 1 to the film-forming system mixture obtained in step 1, and perform rotor-stator homogenization treatment for 3-5 minutes to obtain a mineral-film-forming agent preliminary mixture; Step 3. Pre-disperse the sheeted talc and the alkyl polyglycoside for 5-10 minutes to obtain a talc pre-dispersion, and add the talc pre-dispersion to the mineral-film-forming agent preliminary mixture obtained in step 2 to obtain a mineral-based mixture; Step 4. Cool the mineral-based mixture obtained in step 3 to no more than 50℃, add the bio-based plasticizer, the reversible crosslinking system, the vegetable wax emulsion and other auxiliaries, adjust the pH value to 4.5-5.5, and then filter to obtain a composition to be coated; Step 5. The resulting composition to be coated from Step 4 is applied to a substrate at an apparent shear rate of 10 3 -5 x 10 3 s -1 to form a wet film. Step 6. Dry the wet film formed in step 5 to remove water, and then perform heat curing treatment at 150℃ for 90-120 seconds to obtain a coating.

10. The method of claim 9, wherein, In step 2 of the method, the microfibrillated cellulose is added at the same time as the sheeted kaolin.

11. The method of claim 9, wherein, The method further comprises, after step 6, a step of applying a calcium salt solution to form a surface layer of calcium ion crosslinking.

12. A coated paper base material characterized by, The coated paper-based material comprises a paper or paperboard substrate, at least one surface of the substrate having a coating formed by the method of any one of claims 9-11, the coating having a dry weight of 8-15 g / m².

13. The coated paper base material according to claim 12, characterized in that, The coating of the coated paper-based material has a water vapor transmission rate of no more than 15 g / m²·d under conditions of 38℃ and 90% relative humidity.

14. The coated paper base material according to claim 12, characterized in that, The coating of the coated paper-based material has an oxygen transmission rate of no more than 40 cc / m²·d·atm under conditions of 23℃ and 50% relative humidity.

15. The coated paper base material according to claim 12, wherein The coating of the coated paper-based material has a Cobb water absorption of no more than 14 g / m² within 60 seconds, and an oil resistance rating of no less than 8 levels.

16. Use of a paper base material coated according to any one of claims 12 to 15, characterized in that, The coated paper-based material is used for food contact packaging, the packaging being a paper cup, a cup cover, a take-out meal box, a dry goods packaging or a daily chemical product outer packaging; the material meets the industrial compostable requirements defined by EN13432:2000 or ASTM D6400-23 standards.

17. A method for recycling a paper-based material coated according to any one of claims 12 to 15, characterized in that, The coated paper-based material is subjected to mechanical defibration under conditions of a temperature of 45-60℃, a pH value of 9.5-10.5, and a pulp concentration of 2% for 10-15 minutes.

18. The recycling method of claim 17, wherein, After the recycling method is used for treatment, the fiber recycling rate is not less than 95%, and the macroscopic adhesive area is not greater than 5 mm² / m².

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