Reversible cross-linked paper coating and repulp switching process

Abstract

The invention discloses a reversible cross-linked paper coating and a repulping switching process, and belongs to the technical field of paper coatings. The coating is composed of an acrylate emulsion, a polyhydroxyalkanoate dispersion and a dynamic cross-linking system, wherein the system is one of carbonyl-hydrazine, imine, Diels-Alder, ester exchange, borate or polyvalent metal-carboxylate. The coating is coated with 6-15 g / m, the use state is that Cobb60 is smaller than or equal to 18 g / m, Kit is larger than or equal to 10, WVTR is smaller than or equal to 80 g / (m.d) at the temperature of 38 DEG C and the RH of 90%, OTR is smaller than or equal to 200 cm / (m.d) at the RH of 23 DEG C / 80%, the initial sealing temperature is smaller than or equal to 180 DEG C, and the sealing strength is larger than or equal to 6.0 N / 15 mm. In the re-pulping stage, net releasing is triggered through alkali, acid (containing CO2 injection), a chelating agent or ultrasonic assistance, and according to CEPI V3 / PTS-RH 021: 2012, the fiber recovery rate is measured to be larger than or equal to 98%, the coarse slag rate is measured to be smaller than or equal to 1.0%, and the adhesion matter is measured to be smaller than or equal to grade 1. The formula does not contain PFAS, and high bio-based carbon content can be realized.

Description

Technical Field

[0001] This invention belongs to the field of paper coating technology, specifically relating to reversible cross-linked paper coating and repulping switching process. Background Technology

[0002] To impart waterproof, oil-proof, heat-sealing, and oxygen and water vapor barrier properties to fibrous materials such as paper and paperboard, one or more layers of polymer coating are typically applied to their surface. To ensure sufficient mechanical strength, chemical resistance, and adhesion to the substrate during use, crosslinking agents are often introduced into the coating formulation to form a stable network structure.

[0003] Commonly used crosslinking agents in existing technologies, such as polycarbodiimide (PCDI), aziridine, or isocyanates, react with active groups on polymer chains, such as carboxyl and hydroxyl groups, to form irreversible covalent bonds. While this permanent crosslinking network effectively improves coating performance, it also poses significant challenges to the recycling and reuse of paper products. In traditional alkaline repulping processes, these highly crosslinked coating fragments are difficult to effectively detach and disperse from the fibers, easily forming coarse, insoluble adhesives, commonly known as "adhesives." These impurities can adhere to papermaking equipment, causing production interruptions, or remain in recycled pulp, causing paper defects and severely affecting the quality and value of recycled pulp. Therefore, coated paper using irreversible crosslinking technology has low recycling rates, high energy consumption, and high water consumption, which contradicts the current requirements of sustainable development and a circular economy. Furthermore, the industry is actively seeking environmentally friendly barrier solutions free of perfluorinated and polyfluoroalkyl substances (PFAS), which places new demands on coating technology.

[0004] The technical approaches in this field can be mainly divided into the following categories. The first category is "self-crosslinking enhancement systems." Existing self-crosslinking systems for waterborne acrylic emulsions, such as those based on diacetone acrylamide (DAAM) / diazidadiate (ADH) or ethyl acetoacetate (AAEM) / hydrazine, have been widely studied and applied in coatings, inks, and adhesives. For example, industry publications such as *Paint & Coatings Industry* have reported on the development of crosslinking technology for waterborne coatings. The core objective is to form stable chemical bonds through self-crosslinking reactions at room temperature or after heating, thereby improving the coating's water resistance, solvent resistance, scrub resistance, and mechanical strength. Similarly, technical documents from chemical suppliers such as Gantrade also detail the mechanism by which ADH, as a unique crosslinking agent, reacts with DAAM or AAEM to form a permanent network to enhance coating durability. However, the initial design intent of these technologies is to form the most robust permanent network possible. Their research focuses on optimizing crosslinking efficiency and final film-forming properties, without considering or teaching how to break or dissociate this network during subsequent recycling. Therefore, this type of technology has not solved the core problem of paper base coating in paper repulping process, namely the contradiction of triggering wire unwinding-high yield-low coarse residue rate. Its behavior in specific industrial repulping windows (e.g., pH 10.5-12.0, 45-55℃) is unpredictable and unoptimized.

[0005] The second category is "recyclable structures or lamination schemes." Recent technological developments have focused more on achieving recyclability through the design of multilayer composite structures. For example, patent US 11879214 discloses an environmentally friendly, resealable, heat-sealable barrier coating, but its resealability mechanism relies on phase structure and morphology control, and does not teach how to achieve the active triggering of network disassembly and a gel fraction reduction of over 90% within the CEPI standard window through dynamic reversible crosslinking chemical bonds. Patent WO2023 / 049120 discloses a water-based, heat-sealable barrier coating with high bio-based content; its composition focuses on the compatibility of ethylene acrylate (EAA) and zinc salts, but does not address the various dynamic covalent bonds, such as hydrazones, imines, or Diels-Alders, covered in this invention, and their deep matching pathways with the resealability window. Patent EP 2220153 B1 describes a fast-drying, film-forming water-based barrier coating, but its core technology lies in accelerating the physical drying process and does not teach how to achieve selective network disassembly within a specific resealability window through chemical reverse reactions. Patent US 6066379 proposed the use of ionic crosslinking (such as Ca) as early as 2000. 2+While some solutions have achieved resizing of waterproof paperboard, they haven't synergized this technology with multiple trigger windows such as hydrazone, imine, and Diels-Alder, nor have they validated it against current CEPI / PTS quantitative recycling standards. These approaches typically emphasize controlling interlayer adhesion or using adhesive layers that are easily dispersed in resizing, thereby achieving mechanical separation of the coating or barrier layer from the paper fibers. These technologies do not involve reversible design of the chemical crosslinking network of the single-layer waterborne barrier coating itself; their recyclability relies on physical separation rather than chemical de-networking. Recent academic research, such as the paper on "Reversible imine crosslinking in waterborne self-healing polymer coatings," while exploring the application of dynamic covalent bonds (such as imine bonds) in coatings, primarily aims to achieve the "self-healing" function of the material, i.e., restoring the coating integrity after damage through external stimuli such as moisture. This is quite different from the goal and application scenario of this invention, which aims to achieve complete separation of the coating from the fibers for recycling through specific chemical triggers (such as the strong alkaline or acidic conditions of industrial resizing). Therefore, these approaches do not provide a solution for single-layer coatings that require high crosslinking density to achieve excellent barrier performance, nor do they provide specific reversible chemical trigger windows and their corresponding processes and quantitative recovery indicators.

[0006] BASF JONCRYL HPB 1631-A publicly claims low water vapor transmission rate (WVTR) barrier and food contact compliance; we use it as a "carboxylated barrier emulsion," but the innovation lies in: its compatibility with AAEM / ADH reversible hydrazone network or Ca 2+ The multi-path coupling and unwinding of carboxylate ion clusters and their deep matching with the CEPI V3 re-sizing window are not simply material replacements. Dow RHOPLEXB-60A is a traditional acrylic emulsion for paper coating. We used it as a matrix and constructed a reversible network by introducing dynamic crosslinking units AAEM / ADH and polyhydroxyalkanoates (PHA) to achieve performance switching between "use state" and "re-sizing state". This is different from simple ethylene-acrylic acid or conventional thermal crosslinking systems.

[0007] This invention, within this technical context, introduces a multi-path reversible crosslinking network and deeply couples its dissociation conditions with the CEPI / PTS repulping standard window in the paper industry. This achieves, for the first time, robust performance in the service phase and rapid network dissociation in the recycling phase within a single-layer coating system, synergistically enabling comprehensive control of WVTR, Oxygen Transmission Rate (OTR), heat-sealing performance, and repulping indicators. Therefore, developing a "smart" coating system that provides robust performance in the service phase and easily and efficiently separates from paper fibers during recycling has become a pressing technical challenge in this field. Summary of the Invention

[0008] The purpose of this invention is to provide a reversible cross-linked paper coating and repulping switching process, which aims to solve the problems of difficult recycling, low repulping yield and poor quality of recycled pulp caused by irreversible cross-linking of paper coatings in the prior art.

[0009] The present invention provides a reversibly crosslinked paper coating composition comprising, on a polymer dry solids basis: 40–100 wt% of an acrylate polymer emulsion, such as 40 wt%, 45 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%, 95 wt%, 98.5 wt%, or 100 wt%; 0–60 wt% of a polyhydroxyalkanoate aqueous dispersion, such as 0 wt%, 1 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, or 60 wt%; and 0.2–7.0 wt% of dynamically reversibly crosslinked units, such as 0.2 wt%, 0.27 wt%, 0.5 wt%, 1.0 wt%, 2.0 wt%, 3.5 wt%, 5.0 wt%, 6.67 wt%, or 7.0 wt%. The composition contains no artificially added PFAS and is used to form a dry film of 6–15 g / m² on paper or paperboard, such as 6 g / m², 7 g / m², 8 g / m², 9 g / m², 10 g / m², 11 g / m², 13 g / m², or 15 g / m², and satisfies at least one of the following properties when used: Cobb 60 ≤18g / m²; Kit≥10; WVTR≤80g / (m²·d) measured at 38℃ and 90% relative humidity (RH); OTR≤200cm³ / (m²·d) measured at 23℃ and 80% RH; initial sealing temperature≤180℃; MEK double wipe count≥50 times.

[0010] In one embodiment, the dynamic reversible crosslinking unit comprises one or more of the following: an hydrazone or oxime network formed by a monomer containing an acetoacetic acid functional group and a diamine or dihydrazine compound; an imine network formed by a carbonyl compound and a primary or secondary amine; an acetal or ketal network formed by a polyhydroxy compound and a carbonyl compound; a Diels-Alder reversible network composed of furfuryl-maleimide pairs; an ester exchange type dynamic reversible crosslinking unit; a borate ester type dynamic reversible crosslinking unit; and an ionic cluster reversible crosslinking unit formed by a polyvalent metal ion and a carboxylate. In a specific example, the hydrazone network formed by the monomer containing an acetoacetic acid functional group and a dihydrazine compound is formed by reacting ethyl acetoacetate monomer with a dihydrazine adipic acid crosslinking agent.

[0011] In a specific example, the reversible crosslinking unit of the ionic clusters formed by polyvalent metal ions and carboxylates is composed of a carboxyl-containing acrylate polymer and a compound selected from Ca...2+ Mg 2+ Al 3+ Zr 4+ or Ti 4+ The formation of metal ions.

[0012] In another specific example, the molar ratio of the acetoacetic acid functional group on the monomer containing the acetoacetic acid functional group to the hydrazine functional group on the dihydrazine compound is 2:1 to 3:1, for example, it can be 2:1, 2.2:1, 2.5:1, 2.8:1 or 3:1.

[0013] To enhance the bio-based properties of the product, in one embodiment, the polyhydroxy fatty acid ester is selected from one or more of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-4-hydroxybutyrate), and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

[0014] In a preferred embodiment, the acrylate polymer emulsion is an aqueous dispersion comprising a copolymer of acrylates, methacrylates, vinyl esters, or combinations thereof, wherein the copolymer is optionally copolymerized with styrene or other vinyl unsaturated monomers, and the copolymer contains functional groups capable of participating in or compatible with the formation of the dynamic reversible crosslinking unit. For example, the acrylate polymer emulsion is one or more of pure acrylate emulsion, styrene-acrylate emulsion, bio-based acrylate emulsion, ethylene-butyl acrylate copolymer emulsion, ethylene-acrylic acid copolymer emulsion, ethylene-ethyl acrylate copolymer emulsion, or ethylene-methyl acrylate copolymer.

[0015] The coating formed from the above composition has a sealing strength of ≥6.0 N / 15 mm.

[0016] The present invention also provides a coated paper article comprising a paper or paperboard substrate and a coating formed from the composition described in any of the preceding claims. To meet different packaging needs, the paper or paperboard substrate is selected from bleached kraft paper, unbleached kraft paper, bamboo pulp paper, bagasse paper, straw fiber paper, recycled fiber paper, molding pulp products, or cup paper base paper. A specific example is that the article is a paper cup, and shows no significant leakage after being soaked in hot water at 90°C for 30 minutes.

[0017] The present invention also provides a method for preparing the aforementioned coated paper article, the method comprising coating the aforementioned composition onto a paper or paperboard substrate, followed by annealing. To ensure sufficient formation and stability of the dynamic network during use, the annealing conditions are a temperature of 100–130°C and a time of 20–90 s. For example, the temperature can be 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, or 130°C, and the time can be 20 s, 30 s, 45 s, 60 s, 75 s, or 90 s.

[0018] To achieve high-value recycling of coated paper products, this invention further provides a repulping method for treating coated paper products prepared according to the aforementioned preparation method. The core of this method is triggering the dissociation of a dynamic network, and the treatment includes one or more of the following: treatment under alkaline conditions; treatment under acidic conditions; treatment under ultrasonic conditions; or treatment with the addition of a chelating agent. Specifically, the method includes: adjusting the pH to 10.5–12.0 using sodium hydroxide, for example 10.5, 10.8, 11.0, 11.5, or 12.0, and treating at 45–55°C, for example 45°C, 48°C, 50°C, 52°C, or 55°C, for 10–15 min, for example 10 min, 12 min, 14 min, or 15 min; or injecting carbon dioxide to adjust the pH to 4.0–5.0, for example 4.0, 4.2, or 4.5. The solution is 4.8 or 5.0, and treated at 40–50°C, for example, 40°C, 42°C, 45°C, 48°C or 50°C, for 10–20 min, for example, 10 min, 12 min, 15 min, 18 min or 20 min; or 0.1–0.5 wt% of a chelating agent selected from disodium ethylenediaminetetraacetate, citrate or phosphate is added, for example, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt% or 0.5 wt%. A significant technical advantage of this method is that it reduces the gel fraction of the coating by ≥90%. By implementing the repulping method of this invention, excellent recycling results can be achieved, with a fiber recovery rate ≥98%, a coarse slag rate ≤1.0%, and an adhesion grade ≤1.

[0019] The present invention also discloses a method for improving the deinking efficiency of printed coated paper products, wherein the aforementioned composition is coated on the paper product before printing, such that the coated paper product has an EPRC deinking score greater than 71 when the deinking test is performed according to INGEDE Method 11.

[0020] Based on the aforementioned superior properties, this invention also discloses the application of the aforementioned composition in the preparation of coated paper products. The coated paper products are particularly suitable for closed-loop recycling systems in paper machines that require high recycling rates to produce high-purity recycled pulp, or for use in food packaging, e-commerce mailing bags, and molding pulp products.

[0021] Compared with the prior art, the use of this invention can achieve the following significant beneficial effects:

[0022] High efficiency and recyclability: By introducing a dynamic reversible cross-linking network, the network can be quickly unwound under specific alkaline, acidic, chelating agent, or ultrasonic-assisted triggering conditions, achieving near-complete fiber recycling with a fiber recycling rate of ≥98%, which is far higher than that of traditional irreversible cross-linking coatings.

[0023] High-quality recycled pulp: The repulping process produces very little coarse residue and adhering substances, resulting in high-purity recycled pulp with excellent physical properties. It can be used to produce high-value-added paper products, truly achieving high-quality closed-loop recycling.

[0024] Balancing performance and environmental friendliness: While ensuring excellent waterproof, oil-proof, and water vapor and oxygen barrier properties, the formula is PFAS-free and can achieve high bio-based carbon content by adjusting the PHA content, meeting the market demand for environmentally friendly and sustainable products.

[0025] Energy saving and consumption reduction: The repulping conditions are mild, low temperature and short time, which significantly reduces energy and chemical consumption compared to the harsh conditions required to process traditional difficult-to-process waste paper. Detailed Implementation

[0026] 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.

[0027] Main reagents and raw materials:

[0028] Table 1. Names, specifications / models, and suppliers of major reagents and raw materials:

[0029]

[0030] Main analytical and testing instruments:

[0031] Table 2 mainly analyzes the names, models / specifications, and suppliers of the testing instruments:

[0032]

[0033] General process for preparing PHA aqueous dispersions:

[0034] The PHA aqueous dispersion used in this invention is prepared by the following general process, which ensures safety for food contact applications and avoids odorous or toxic chemicals.

[0035] Step 1. Premixing: Add 50 parts (by weight) of PHA powder to 50 parts of deionized water, along with 0.5 parts of food-grade surfactant polysorbate 80. Premix using a high-speed disperser at 3000 rpm for 30 minutes to form a uniform coarse suspension.

[0036] Step 2. High-Pressure Homogenization: Pump the coarse suspension into a high-pressure homogenizer. Set the homogenization pressure to 1000 bar and perform 15 cycles. During homogenization, the material passes through a narrow valve gap under high pressure and is subjected to the combined effects of shearing, collision, and cavitation, which effectively breaks down and disperses the PHA particles.

[0037] Step 3. Cooling and Characterization: The homogenized product was rapidly cooled to room temperature using a cooling system to obtain a uniform and stable milky-white aqueous dispersion. A laser particle size analyzer was used to ensure the average particle size (D) of the final dispersion was measured. 50 The particle size is no larger than 3 μm, and the solid content is approximately 45 wt%. This dispersion can be stored stably at room temperature for at least 3 months without significant sedimentation or stratification.

[0038] Main testing standards:

[0039] Standardized climate control: ISO 187:2022;

[0040] Cobb 60 Water absorption: ISO 535:2023;

[0041] Oil resistance (Kit value): TAPPI T 559cm-12 (R2022);

[0042] Oil permeability (turpentine): TAPPI T 454 om-15;

[0043] WVTR (38℃, 90% RH): ASTM F1249-20;

[0044] OTR (Controlled Humidity): ASTM F1927-20 (e.g., 23°C, 80% RH);

[0045] MEK double-wipe cycles: ASTM D5402-19(2024);

[0046] Heat sealing sample preparation / heat sealing profile: ASTM F2029-16(2021);

[0047] Heat seal strength: ASTM F88 / F88M

[0048] Sealing strength: ASTM F88 / F88M-23;

[0049] Hot tack: ASTM F1921 / F1921M-12(2021);

[0050] Coefficient of friction (COF): ASTM D1894-23 or ISO 8295:2020;

[0051] Recyclability test: CEPI Recyclability Test Method - Part I, Version 3 (Feb. 2025); PTS-RH 021:2012 (Cat. II);

[0052] Printed matter de-inking adaptability: INGEDE Method 11 (Packaging Edition, 2025-03);

[0053] Gel fraction: Soxhlet extraction;

[0054] Recycled paper properties: ISO 5269-1:2005; ISO 1924-2:2008;

[0055] Sensory evaluation: EN 1230-1:2019 (odor); EN 1230-2:2009 (taste);

[0056] Chinese migration regulations: GB 4806.1-2016; GB 4806.8-2022; GB 31604.8-2021;

[0057] EU Migration Regulation: EU No 10 / 2011 (merged to 2025-01-20), Annex V;

[0058] US FDA regulations: 21 CFR 176.170 / 176.180;

[0059] Total fluorine / organic fluorine (TOF / TF): ASTM D7359-23 (combustion-ion chromatography, CIC);

[0060] MOSH / MOAH: EN 16995: 2017 (LC-GC-FID).

[0061] Examples and Comparative Examples:

[0062] General methods for coating preparation:

[0063] The acrylate emulsion, PHA aqueous dispersion, and functional monomer (such as AAEM) are mixed thoroughly under stirring. In a separate container, a crosslinking agent (such as ADH) or ion source (such as CaCl2) is dissolved in a small amount of deionized water. The crosslinking agent solution is slowly added dropwise to the bulk emulsion, and stirring is continued for 30 minutes to obtain the coating solution. The coating solution is applied to the base paper using a laboratory coater, with a target dry basis weight of 6–15 g / m². The coated paper is then dried in a hot air oven at 105°C for 2 minutes, followed by a short-time annealing treatment at 115°C for 60 seconds.

[0064] Table 3. Formulations of Examples and Comparative Examples (all parts are based on polymer dry solids wt%):

[0065]

[0066] Application examples and experiments:

[0067] Unless otherwise specified, all test samples were conditioned for at least 24 hours at 23°C and 50% RH according to ISO 187:2022. All tests had n=3; results are expressed as mean ± standard deviation.

[0068] Application Example 1: Performance testing in use.

[0069] To comprehensively evaluate the core performance of the coating under use, this experiment systematically tested the key application indicators such as water resistance, oil resistance, solvent resistance, barrier properties, and heat sealing of all coated paper samples in the examples and comparative examples at a standard dry weight of 15 g / m².

[0070] Table 4. Performance test results (15g / m²) of the examples and comparative examples in use:

[0071]

[0072] Analysis: Table 4 shows that all examples meet the key performance indicators. In contrast, Comparative Example 1, while exhibiting superior performance in some aspects, suffers from an excessively high heat-sealing temperature. Comparative Example 2, lacking cross-linking, exhibits extremely poor water and oil resistance, barrier properties, and solvent resistance. Comparative Example 3 (DAAM / ADH system) demonstrates comparable good performance to the examples in the service state, consistent with its expected performance as an effective cross-linking system. However, Comparative Example 4 (high bio-based content but no cross-linking) shows a significant performance decline, demonstrating that the cross-linked network is essential for achieving the desired barrier and durability.

[0073] Application Example 2: Weight Window Verification.

[0074] To verify the performance stability of the coating of the present invention under different coating amounts, this experiment selected three lower dry weight levels of 6 g / m², 8 g / m² and 10 g / m² to conduct performance tests on all examples and comparative examples to determine the process window for its application.

[0075] Table 5. Performance of coatings at different dry weights (6 g / m²):

[0076]

[0077] Table 6. Performance of the coatings at different dry weights (8 g / m²):

[0078]

[0079] Table 7 Performance of coatings at different dry weights (10 g / m²):

[0080]

[0081] Analysis: The results in Tables 5-7 demonstrate that the coating of the present invention exhibits excellent and stable performance within the application weight range of 6 to 10 g / m². Comparative Examples 1 and 3 also showed stable performance, but Comparative Examples 2 and 4 showed a sharp deterioration in performance at low weights, failing to form an effective functional coating.

[0082] Application Example 3: Re-particle experiment (triggered state).

[0083] To verify the reversible de-linking and recyclability of the coating, this experiment simulated the industrial repulping process, treating coated paper samples under different chemical triggering conditions (alkaline, acidic, complexing, and ultrasonic-assisted), and quantitatively evaluating their fiber recovery rate and coarse residue rate according to the CEPI V3 standard method.

[0084] Pathway A (alkali / complexation trigger): pH adjusted to 11.0±0.1 with NaOH; 50℃, 15 min. For Example 5, an additional 0.3 wt% of EDTA-2Na was added.

[0085] Pathway B (acid-triggered): Adjust the pH to 4.0±0.1 with HCl, treat at 45℃ for 15 min.

[0086] Pathway C (CO2-acid / ultrasound-assisted): The slurry pH is adjusted to 4.5±0.2 by CO2 injection at 45℃ for 15 min. Simultaneously, ultrasonic treatment at 40 kHz is performed for 5–10 min.

[0087] Table 8. Resizing performance test results of the examples and comparative examples (distinguished by trigger path):

[0088]

[0089] Analysis: The results in Table 8 are the core of this invention. All embodiments exhibited excellent recyclability under specific triggering conditions. Comparative Example 1 (irreversible crosslinking) and Comparative Example 3 (conventional self-crosslinking) failed to reslurry effectively in all test paths, producing a large amount of coarse sludge, demonstrating the uniqueness and superiority of the reversible network of this invention. Comparative Examples 2 and 4 (no effective crosslinking), although easy to reslurry, do not possess basic usability as shown in Table 4.

[0090] Application Example 4: Sealing, Processing and Stability Verification.

[0091] Systematic tests were conducted on sealing strength, thermal adhesion, coefficient of friction, oil penetration resistance, and cold chain stability.

[0092] Table 9. Sealing, processing, and stability test results for the examples and comparative examples:

[0093]

[0094] Analysis: All examples exhibited good sealing strength (≥6.0 N / 15 mm) and thermal tack. Comparative Examples 1 and 3 showed higher sealing strength, but this came at the cost of recyclability. Comparative Examples 2 and 4 showed almost no sealing performance.

[0095] Application Example 5: Multi-domain food contact safety verification.

[0096] To ensure product compliance in the global market, this experiment conducted a detailed chemical safety assessment of the coating in accordance with the major food contact material regulations of China, the European Union, and the United States, including "fluorine-free" verification, sensory evaluation, and migration testing.

[0097] Table 10. Validation results of the "fluorine-free" dual-pathway method for the examples and comparative examples:

[0098]

[0099] Table 11 Sensory, MOSH / MOAH, and migration test results of the examples and comparative examples:

[0100]

[0101] Analysis: All examples and comparative examples comply with major food contact safety regulations, demonstrating the chemical safety of the formulation components.

[0102] Application Example 6: Controlled Humidity OTR.

[0103] To evaluate the oxygen barrier performance of the coating in a humid environment, this experiment used a coulometric OTR tester and tested the OTR of the coating under harsh conditions of 23°C and 80% relative humidity (RH) according to ASTM F1927-20 standard.

[0104] Table 12 Results of Controlled Humidity OTR in Examples and Comparative Examples:

[0105]

[0106] * Note: The test was conducted at 23°C and 80% RH.

[0107] Analysis: The OTR values ​​of all embodiments were ≤200 cm³ / (m²·d). Comparative Examples 1 and 3 had slightly lower OTRs, but at the expense of recyclability.

[0108] Application Example 7: Substrate Adaptation Window.

[0109] To verify the universality of the coating formulation of this invention on different types of paper substrates, this experiment applied the formulations of all examples and comparative examples at 10 g / m² to bamboo pulp paper, bagasse paper, and cup paper base paper, and tested their key properties. All samples underwent hot water leakage testing to simulate real-world usage scenarios.

[0110] Table 13 Substrate compatibility performance of the examples:

[0111]

[0112] Table 14 Comparative Substrate Compatibility Performance:

[0113]

[0114] Analysis: The results in Tables 13 and 14 show that all formulations in the examples achieved the performance indicators on various substrates, including bamboo pulp paper, bagasse paper, and cup paper base paper. The results of the hot water leakage test on cup paper (Note: This test is commonly used for cup paper; to evaluate the ultimate liquid resistance of bamboo pulp and bagasse paper, a temporary molding was used for the test) further indicate that only effectively cross-linked coatings (all examples and Comparative Examples 1 and 3) can provide reliable liquid barrier function, while Comparative Examples 2 and 4, which are not cross-linked or insufficiently cross-linked, showed significant leakage.

[0115] Application Example 8: Printing deinking adaptability.

[0116] To assess the impact of coating on the recycling value of printed matter, standard water-based inks were printed on coated paper in all examples and comparative examples, followed by flotation deinking experiments according to INGEDE Method 11. Deinking efficiency was evaluated by measuring brightness increment and ink residue (ERIC).

[0117] Table 15 Deinking Indicators of Examples and Comparative Examples:

[0118]

[0119] Analysis: The reversible network in the embodiments effectively releases ink particles after re-inking triggering, resulting in good deinking performance. The irreversible crosslinking in Comparative Examples 1 and 3 severely inhibits the dissociation and removal of ink particles. Although Comparative Examples 2 and 4 show good deinking performance, this is because the coating itself is not robust and lacks practical application value.

[0120] Application Example 9: Repulping and Recycling after Hot Water in Paper Cup Scenarios.

[0121] To simulate the recycling scenario of products such as hot beverage cups after actual use, this experiment will soak all the cup paper samples prepared in Example 7 in 90°C hot water for 30 minutes, and then conduct the path A (alkaline / complex) resizing experiment to test whether the coating still maintains high recyclability after experiencing hot and humid conditions.

[0122] Table 16 Performance of re-slurrying after hot water:

[0123]

[0124] Analysis: Hot water immersion did not weaken the triggered-state disintegration efficiency of the coating in the embodiment, and the resizing performance remained excellent, proving the feasibility of high-value recycling of this technology in practical application scenarios.

[0125] Network reversibility and recycled pulp quality verification:

[0126] Gel fraction test:

[0127] To directly verify the reversibility of the cross-linked network at the mechanistic level, this experiment prepared the coating into an independent membrane and determined its gel fraction (GF) before and after alkaline or acidic treatment by Soxhlet extraction to quantify the degree of dissociation of the cross-linked network.

[0128] Table 17. Gel fraction test results for the examples and comparative examples:

[0129]

[0130] Analysis: Table 17 verifies the reversibility of the network at the mechanistic level. The gel fraction of the coatings in all embodiments decreased by more than 90% after treatment. Conversely, Comparative Examples 1 and 3, which used irreversible crosslinking, showed essentially no change in gel fraction before and after treatment.

[0131] Recycled pulp quality assessment:

[0132] To ultimately evaluate the value of the recycling scheme of this invention, this experiment prepared recycled handmade paper samples from the good pulp obtained after repulping (i.e., the recycled fibers) according to standard methods, and comprehensively evaluated its key physical properties (tensile index), optical properties (brightness), and impurity content (dust content, adhering substances).

[0133] Table 18 Quality evaluation results of recycled pulp (good pulp) in the examples and comparative examples:

[0134]

[0135] Analysis: The high-quality pulp obtained from the recycled coated paper in the examples maintained high levels of physical and optical properties. In contrast, the recycled pulp produced in Comparative Examples 1 and 3 showed a significant decline in quality, with high impurity content, making it unsuitable for high-quality production.

[0136] Experimental Results and Analysis:

[0137] The core of this invention lies in constructing a smart coating system with switchable performance between a "use state" and a "recycling state." Through systematic verification using a series of application examples, the following conclusions can be drawn:

[0138] 1. Performance Comparable to Irreversible Systems: Data from Tables 4, 5, 7, and 10 show that all embodiments achieved performance comparable to, or even better than, those using conventional irreversible crosslinking agents (Comparative Example 1) and conventional self-crosslinking systems (Comparative Example 3) in terms of water resistance (Cobb), oil resistance (Kit), MEK double-wipe cycles, barrier properties (WVTR / OTR), and sealing performance. This demonstrates that the dynamic reversible crosslinking network of the present invention is stable and efficient under operating conditions, sufficient to meet practical application requirements. In contrast, the uncrosslinked Comparative Examples 2 and 4 performed poorly in all performance aspects, highlighting the necessity of crosslinking.

[0139] 2. Excellent and Controllable Recyclability: The resizing test results in Table 8 are a key differentiating factor of this invention. All examples, under specific acidic or alkaline triggering conditions, achieved fiber recovery rates exceeding 98% and coarse residue rates below 1.0%, meeting the standards for high-quality recycling. This contrasts sharply with the extremely low recovery rates (<90%) and high coarse residue rates (>8%) of Comparative Examples 1 and 3. The gel fraction test (Table 17) mechanistically confirms that this difference stems from the reversibility of the network: the gel fraction of the examples decreased by more than 90% after triggering, while the network structures of Comparative Examples 1 and 3 remained essentially unchanged.

[0140] 3. High-Quality Value of Recycled Products: The advantage of this invention lies not only in its high fiber recovery rate but also in the quality of the recycled fibers. As shown in Table 18, the recycled paper sheets made from the good pulp recovered in the examples maintain high levels of physical strength, whiteness, and cleanliness, making them suitable for producing high-value-added products. In contrast, the recycled pulps of Comparative Examples 1 and 3 contain a large amount of adhering substances and impurities, resulting in severely degraded performance and extremely low application value. Furthermore, the deinking experiment results in Table 15 demonstrate that the reversible network of this invention also facilitates ink removal, further enhancing the value of the recycled pulp.

[0141] 4. Wide Applicability and Safety: Application Example 7 (Tables 13 and 14) demonstrates the good applicability of the coating of the present invention on a variety of different fiber substrates. Application Example 9 (Table 16) verifies that it still maintains excellent recyclability after experiencing real-world usage scenarios such as hot water immersion. Meanwhile, the comprehensive test results of Application Example 5 (Tables 10 and 11) show that the formulation system of the present invention does not contain PFAS or other harmful substances, and complies with mainstream global food contact safety regulations.

[0142] In summary, this invention successfully resolves the contradiction between high-performance paper coating and efficient recycling by constructing a dynamic, reversible cross-linked network that is stable in the "use state" and can rapidly dissociate in the "trigger state." The coating system of this invention not only boasts excellent performance but is also environmentally friendly, providing a practical technical solution for achieving high-value closed-loop recycling of paper packaging products.

[0143] 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 reversible cross-linked paper coating composition, comprising, based on polymer dry solids: 40–100 wt% acrylate polymer emulsion; 0–60 wt% polyhydroxy fatty acid ester aqueous dispersions; 0.2–7.0 wt% of dynamically reversible crosslinking units; The composition contains no artificially added PFAS and is used to form a dry film of 6–15 g / m² on paper or paperboard, and meets at least one of the following properties when used: Cobb 60 ≤18g / m²; Kit ≥ 10; The water vapor transmission rate was measured to be ≤80g / (m²·d) at 38℃ and 90% relative humidity. The oxygen permeability was measured to be ≤200 cm³ / (m²·d) at 23℃ and 80% relative humidity. Initial sealing temperature ≤180℃; MEK double-wipe count ≥ 50 times.

2. The composition according to claim 1, characterized in that, The dynamic reversible crosslinking unit includes one or more of the following: A hydrazone or oxime network formed by monomers containing acetoacetic acid functional groups and diamines or dihydrazines; An imine network formed by carbonyl compounds and primary or secondary amines; A network of acetals or ketals formed by polyhydroxyl and carbonyl compounds; Diels-Alder reversible network composed of furfural-maleimide pairs; Ester-exchange type dynamic reversible crosslinking unit; Boronate type dynamic reversible crosslinking unit; The reversible crosslinking unit of ionic clusters formed by polyvalent metal ions and carboxylates, wherein the hydrazone network formed by monomers containing acetoacetic acid functional groups and dihydrazine compounds is formed by the reaction of ethyl acetoacetate monomer and dihydrazine adipic acid crosslinking agent.

3. The composition according to claim 2, characterized in that, The reversible crosslinking unit of the ion clusters formed by polyvalent metal ions and carboxylates is composed of carboxyl-containing acrylate polymers and compounds selected from Ca... 2+ Mg 2+ Al 3+ Zr 4+ or Ti 4+ The formation of metal ions.

4. The composition according to claim 2, characterized in that, The molar ratio of the acetoacetic acid functional group on the monomer containing the acetoacetic acid functional group to the hydrazine functional group on the dihydrazine compound is 2:1 to 3:

1.

5. The composition according to claim 1, characterized in that, The polyhydroxy fatty acid ester is selected from: One or more of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-4-hydroxybutyrate), and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

6. The composition according to claim 1, characterized in that, The acrylate polymer emulsion is an aqueous dispersion comprising copolymers of acrylates, methacrylates, vinyl esters, or combinations thereof, wherein the copolymer is optionally copolymerized with styrene or other vinyl unsaturated monomers, and the copolymer contains functional groups capable of participating in or compatible with the formation of the dynamic reversible crosslinking unit.

7. The composition according to claim 6, characterized in that, The acrylate polymer emulsion is one or more of the following: pure acrylate emulsion, styrene-acrylate emulsion, bio-based acrylate emulsion, ethylene-butyl acrylate copolymer emulsion, ethylene-acrylic acid copolymer emulsion, ethylene-ethyl acrylate copolymer emulsion, or ethylene-methyl acrylate copolymer.

8. The coating formed by the composition according to claim 1, characterized in that, The coating has a sealing strength of ≥6.0N / 15mm.

9. A coated paper product, characterized in that, It includes a paper or paperboard substrate and a coating formed by the composition of any one of claims 1 to 8.

10. The use of the composition according to any one of claims 1 to 8 in the preparation of coated paper products, wherein the coated paper products are used in closed-loop recycling systems of paper machines that require high recycling rates to produce high-purity recycled pulp, or in food packaging, e-commerce mailing bags and molding pulp products.

11. The coated paper product according to claim 9, characterized in that, The paper or paperboard substrate is selected from bleached kraft paper, unbleached kraft paper, bamboo pulp paper, bagasse paper, wheat straw fiber paper, recycled fiber paper, molding pulp products, or cup paper base paper.

12. The coated paper product according to claim 11, characterized in that, The product is a paper cup, and there is no obvious leakage after soaking in 90°C hot water for 30 minutes.

13. A method for preparing a coated paper product according to claim 9, characterized in that, This includes applying the composition of any one of claims 1 to 8 onto a paper or paperboard substrate, followed by annealing.

14. The preparation method according to claim 13, characterized in that, The annealing conditions are a temperature of 100–130°C and a time of 20–90 seconds.

15. A repulping method for processing coated paper products prepared according to the method of claim 13 or 14, characterized in that, The method includes one or more of the following processing methods: Treat under alkaline conditions; Treat under acidic conditions; Processed under ultrasonic conditions; Treatment with chelating agents.

16. The repulping method according to claim 15, characterized in that, The method includes: adjusting the pH to 10.5–12.0 using sodium hydroxide and treating at 45–55°C for 10–15 min; or adjusting the pH to 4.0–5.0 by injecting carbon dioxide and treating at 40–50°C for 10–20 min; or adding 0.1–0.5 wt% of a chelating agent selected from disodium ethylenediaminetetraacetate, citrate, or phosphate.

17. The repulping method according to claim 15 or 16, characterized in that, The method reduces the gel fraction of the coating by ≥90%.

18. The repulping method according to claim 16, characterized in that, The fiber recovery rate of the method is ≥98%, the coarse residue rate is ≤1.0%, and the adhesion grade is ≤1.

19. A method for improving the deinking efficiency of printed coated paper products, characterized in that, The composition of any one of claims 1 to 8 is coated onto a paper article prior to printing, such that the coated paper article has an EPRC deinking score greater than 71 when subjected to a deinking test according to INGEDE Method 11.

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