Reversible cross-linked paper coating and reslurry switching process

Abstract

The present application discloses a reversible cross-linked paper coating and repulpable switching process, belonging to the technical field of paper coating. The coating is composed of an acrylate emulsion, a polyhydroxy fatty acid ester dispersion and a dynamic cross-linking system, the system being one of carbonyl-hydrazine, imine, Diels-Alder, ester exchange, borate or polyvalent metal-carboxylate. Coated at 6-15 g / m2, the Cobb 60 ≤18 g / m2, Kit≥10, 38℃, 90% RH, WVTR≤80 g / (m2·d), 23℃ / 80% RH, OTR≤200 cm3 / (m2·d), seal initiation temperature≤180℃, seal strength≥6.0 N / 15 mm. In the repulping stage, the network is triggered by alkali, acid (containing CO2 injection), chelating agent or ultrasonic assistance, the fiber recovery rate is≥98%, the coarse residue rate is≤1.0%, and the adhesion is≤1 grade according to CEPI V3 / PTS-RH 021:2012. The formula does not contain PFAS, and a high bio-based carbon content can be achieved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of paper coating, and particularly relates to a reversible crosslinking paper coating and a repulping switching process. BACKGROUND

[0002] In order to impart water resistance, oil resistance, heat sealing, and barrier properties against oxygen and water vapor to fiber-based materials such as paper and paperboard, it is common to apply one or more layers of polymeric coating on the surface. In order to ensure that the coating has sufficient mechanical strength, chemical resistance, and adhesion to the substrate during use, a crosslinking agent is often introduced into the coating formulation to form a stable network structure.

[0003] Commonly used crosslinking agents in the prior art, such as polycarbodiimide (PCDI), aziridine, or isocyanate, react with active groups on the polymer chain, such as carboxyl and hydroxyl groups, to form irreversible covalent bonds. This permanent crosslinking network effectively improves the performance of the coating, but also poses a great challenge to the recycling of paper products. In the traditional alkaline repulping process, these highly crosslinked coating fragments are difficult to effectively detach and disperse from the fibers, and are prone to form coarse insoluble adhesives, commonly known as "adhesives". These impurities can adhere to the papermaking equipment, causing production interruptions, or remain in the recycled pulp, causing paper defects, and seriously affecting the quality and value of the recycled pulp. Therefore, the use of irreversible crosslinking technology in coated paper results in low recycling rate, high energy consumption, and large water consumption, which is contrary to the current requirements of sustainable development and circular economy. In addition, the industry is actively seeking environmentally friendly barrier solutions that do not contain perfluoroalkyl substances (PFAS), which poses new requirements for coating technology.

[0004] The technical routes in this field can be mainly classified into the following categories. The first category is “self-crosslinking reinforcement system”. The existing self-crosslinking system of waterborne acrylic emulsion, such as the technology based on diacetone acrylamide (DAAM) / adipic dihydrazide (ADH) or acetoacetoxyethyl methacrylate (AAEM) / hydrazine, has been widely studied and applied in coatings, inks and adhesives. For example, industry publications such as the magazine “Paint & Coatings Industry” have reported the development of crosslinking technology for waterborne coatings, with the core goal being to form stable chemical bonds through room temperature or heated self-crosslinking reactions to improve the water resistance, solvent resistance, scrub resistance and mechanical strength of the coating. Similarly, technical materials 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 the durability of the coating. However, the original intention of these technologies is to form a permanent network as stable as possible, and the focus of their research is on optimizing crosslinking efficiency and final film-forming performance, without considering or teaching how to break or dissociate the network in the subsequent recycling process. Therefore, this type of technology does not solve the core problem of paper-based coatings in the papermaking repulping process, i.e., the contradiction between triggering network dissociation and achieving high yield and low residue rate. Its behavior in a specific industrial repulping window (e.g., pH 10.5-12.0, 45-55°C) is unpredictable and not optimized.

[0005] The second category is “recyclable structure or laminated scheme”. In recent years, technology development has focused more on achieving recyclability through the design of multi-layer composite structures. For example, patent US 11879214 discloses an environmentally friendly repulpable heat-sealable barrier coating, but its repulpable mechanism relies on phase structure and morphology control, and does not teach how to achieve active triggering of network dissociation and a more than 90% decrease in gel fraction in the CEPI standard window through dynamic reversible crosslinking chemical bonds. Patent WO2023 / 049120 discloses a waterborne, heat-sealable, high-bio-based-content barrier coating, with the composition raw materials focusing on the compatibility of ethylene acrylic acid (EAA) and zinc salt, but does not involve the deep matching path of hydrazone, imine or Diels-Alder and other dynamic covalent bonds with the repulping window covered by the present invention. Patent EP 2220153 B1 describes a fast-drying film-forming waterborne barrier coating, but its technical core is to accelerate the physical drying process, and does not teach how to achieve selective deconstruction of the network in a specific repulping window through chemical reverse reactions. Patent US 6066379 proposed as early as 2000 to use ion crosslinking (such as Ca 2+) to achieve repulpable waterproof paperboard, but it does not coordinate the technology with hydrazone, imine, Diels-Alder, etc. multi-trigger window, and it is not verified according to the current CEPI / PTS quantitative recovery standard. These schemes usually emphasize the control of interlayer adhesion or the use of adhesive layers that are easy to disperse in repulping, thereby achieving mechanical separation of the coating or barrier layer from the paper fibers. These technologies do not involve reversible design of the chemical cross-linking network of the single-layer water-based barrier coating itself, and its recyclability depends on physical separation rather than chemical network breaking. Recent academic research, such as the paper on "Reversible imine crosslinking in waterborne self-healing polymer coatings", although it explores the application of dynamic covalent bonds (such as imine bonds) in coatings, its main goal is to achieve the "self-healing" function of the material, i.e. restoring the integrity of the coating after damage through external stimuli such as moisture, which is completely different from the goal and application scenario of this invention, which aims to achieve complete separation of the coating from the fiber through specific chemical triggers (such as strong alkaline or acidic conditions of industrial repulping) for recycling and reuse. Therefore, for single-layer coatings that require high cross-linking density to achieve excellent barrier performance, these schemes cannot provide a solution, nor do they give specific reversible chemical trigger windows and their corresponding processes and quantitative recycling indicators.

[0006] BASF JONCRYL HPB 1631-A is disclosed as a low water vapor transmission rate (WVTR) barrier, food contact compliant; we use it as a "carboxylated barrier emulsion", but the innovation lies in: the reversible hydrazone network of AAEM / ADH or Ca 2+ - Multi-path coupling and network breaking of carboxylate ion clusters and deep matching with CEPI V3 repulping window, not just a simple material replacement. Dow RHOPLEX B-60A is a traditional acrylic emulsion for paper coating, we use it as the matrix, and through the introduction of dynamic cross-linking units AAEM / ADH and polyhydroxyalkanoate (PHA), we build a reversible network to achieve "use state-repulp state" performance switching, which is different from pure ethylene-acrylic or conventional thermal cross-linking systems.

[0007] The present invention is in this technical background, by introducing a multi-path reversible cross-linking network, and coupling its dissociation conditions with the CEPI / PTS repulping standard window of the papermaking industry, for the first time in a single-layer coating system, it realizes the stable performance in use state and the rapid network breaking in trigger state in the recycling stage, and achieves the comprehensive control of WVTR, oxygen transmission rate (OTR), heat sealing performance and repulping indicators. Therefore, it has become a technical problem to be solved in the field to develop an "intelligent" coating system that can provide strong performance in the use stage and easily and efficiently separate from paper fibers in the recycling stage. SUMMARY

[0008] The present invention aims to provide a reversible crosslinking paper coating and repulpability switching process, aiming to solve the problems of paper coating in the prior art, which is difficult to recycle, low repulpability yield and poor quality of recycled pulp due to irreversible crosslinking.

[0009] The present invention provides a reversibly crosslinked paper coating composition comprising, based on polymer dry solids: 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 polyhydroxyaliphatic acid ester 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 a dynamic reversible crosslinking unit, 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 is free of artificially added PFAS, for forming a dry film of 6-15 g / m2, 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, on paper or paperboard, and meets at least one of the following performances when in use: Cobb 60 ≤ 18 g / m2; Kit ≥ 10; WVTR ≤ 80 g / (m2·d) measured at 38 °C, 90% relative humidity (RH); OTR ≤ 200 cm3 / (m2·d) measured at 23 °C, 80% RH; seal initiation temperature ≤ 180 °C; MEK double rubs ≥ 50 times.

[0010] In one embodiment, the dynamic reversible crosslinking unit comprises one or more of: a hydrazone or oxime network formed from a monomer containing acetoacetic acid functionality with a diamine or dihydrazine compound; an imine network formed from a carbonyl compound with a primary or secondary amine; an acetal or ketal network formed from a polyhydroxy with a carbonyl compound; a Diels-Alder reversible network composed of a furfuryl-maleimide pair; a transesterification type dynamic reversible crosslinking unit; a borate type dynamic reversible crosslinking unit; an ionic cluster reversible crosslinking unit formed from a polyvalent metal ion with a carboxylate salt, wherein, in one particular example, the hydrazone network formed from a monomer containing acetoacetic acid functionality with a dihydrazine compound is formed from an acetoacetic acid ethyl methacrylate monomer reacted with adipic acid dihydrazine crosslinking agent.

[0011] In one particular example, the ionic cluster reversible crosslinking unit formed from a polyvalent metal ion with a carboxylate salt is formed from an acrylate polymer containing carboxyl groups with a salt selected from Ca2+ Mg 2+ Al 3+ Zr 4+ or Ti 4+ form.

[0012] In another specific example, the molar ratio of acetoacetic acid functional groups on the monomer comprising acetoacetic acid functional groups to hydrazine groups on the dihydrazide compound is from 2: 1 to 3: 1, such as can be 2: 1, 2.2: 1, 2.5: 1, 2.8: 1, or 3: 1.

[0013] To improve the biobased attribute of the product, in one embodiment, the polyhydroxyalkanoate is selected from one or more of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-4-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

[0014] In a preferred embodiment, the acrylic emulsion is an aqueous dispersion of a copolymer comprising acrylic, methacrylic, vinyl ester or combinations thereof, optionally copolymerized with styrene or other vinyl unsaturated monomers, and the copolymer comprises functional groups capable of participating in or compatible with the formation of the dynamic reversible crosslinking units, such as the acrylic emulsion is one or more of a pure acrylic emulsion, a styrene-acrylic emulsion, a biobased acrylic emulsion, an ethylene-butyl acrylate copolymer emulsion, an ethylene-acrylic acid copolymer emulsion, an ethylene-ethyl acrylate copolymer emulsion, or an ethylene-methyl acrylate copolymer.

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

[0016] The present application also provides a coated paper product comprising a paper or paperboard substrate, and a coating formed from the composition of any of the preceding. To accommodate 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, molded pulp product, or cup paper base paper. In one specific example, the product is a paper cup, and there is no significant leakage after soaking in hot water at 90 °C for 30 minutes.

[0017] The present application also provides a method for preparing the aforementioned coated paper product, which comprises coating the aforementioned composition on a paper or paperboard substrate, followed by annealing treatment. In order to fully form and stabilize the dynamic network in the use stage, the annealing treatment is carried out at a temperature of 100-130℃ for 20-90s. For example, the temperature can be 100℃, 105℃, 110℃, 115℃, 120℃, 125℃ or 130℃, and the time can be 20s, 30s, 45s, 60s, 75s or 90s.

[0018] In order to achieve high-value recycling of the coated paper product, the present application further provides a repulping method for processing the coated paper product prepared according to the aforementioned preparation method. The core of this method is to trigger the dissociation of the dynamic network, and the processing method includes one or more of the following: treatment under alkaline conditions; treatment under acidic conditions; treatment under ultrasonic conditions; or treatment by adding a chelating agent. Specifically, the method comprises: adjusting the pH to 10.5-12.0, for example 10.5, 10.8, 11.0, 11.5 or 12.0, using sodium hydroxide, treating at 45-55℃, for example 45℃, 48℃, 50℃, 52℃ or 55℃, for 10-15min, for example 10min, 12min, 14min or 15min; or adjusting the pH to 4.0-5.0, for example 4.0, 4.2, 4.5, 4.8 or 5.0, by injecting carbon dioxide, treating at 40-50℃, for example 40℃, 42℃, 45℃, 48℃ or 50℃, for 10-20min, for example 10min, 12min, 15min, 18min or 20min; or adding 0.1-0.5wt% of a chelating agent selected from disodium ethylenediaminetetraacetate, citrate or phosphate, for example 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt% or 0.5wt%. An important technical effect of this method is that the method makes the gel fraction of the coating layer decrease by ≥90%. By implementing the repulping method of the present application, excellent recycling results can be achieved, with a fiber recovery rate of ≥98% and a coarse residue rate of ≤1.0%, and an adhesive level of ≤1 level.

[0019] The present application also discloses a method for improving the deinking efficiency of printed coated paper products, which comprises coating the aforementioned composition on the paper product before printing, so that the coated paper product has an EPRC deinking score of greater than 71 when subjected to deinking test according to INGEDE Method 11.

[0020] Based on the above excellent performance, the present application also discloses the use of the aforementioned composition in the preparation of a coated paper product. The coated paper product is particularly suitable for use in a paper machine closed-loop recycling system that requires a high recycling rate to produce high-purity recycled pulp, or for use in food packaging, e-commerce delivery bags and molded pulp products.

[0021] Compared with the prior art, the following remarkable beneficial effects can be obtained by using the present application:

[0022] High efficiency recyclability: by introducing a dynamic reversible crosslinking network, the network can be quickly disintegrated under specific basic, acidic, chelating agent or ultrasound assisted triggering conditions, achieving nearly complete fiber recycling, with a fiber recycling rate of ≥98%, much higher than traditional irreversible crosslinking coatings.

[0023] High-quality regenerated pulp: the coarse residue rate and adhesion generated during the re-pulping process is extremely low, and the regenerated pulp obtained has high purity and excellent physical properties, and can be used to produce high value-added paper products, truly realizing high-quality closed-loop recycling.

[0024] Performance and environmental protection: while ensuring excellent water resistance, oil resistance, water vapor and oxygen barrier performance and other use properties, the formula does not contain PFAS, and high biobased carbon content can be achieved by adjusting the PHA content, meeting the market demand for environmentally friendly and sustainable products.

[0025] Energy saving and consumption reduction: the re-pulping conditions are mild, low temperature and short time, significantly reducing energy and chemical consumption compared to the harsh conditions required for treating traditional difficult-to-treat waste paper. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below in combination with specific examples. It should be understood that the specific examples described herein are only used to explain the present application and are not used to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application. Unless otherwise specified, the raw materials used in the examples are commercially available industrial products or can be prepared by conventional methods. Unless otherwise specified, the performance test methods are carried out according to the standards described in the summary of the invention.

[0027] Main reagents and raw materials:

[0028] Table 1 Name, specification / model and supplier of main reagents and raw materials:

[0029]

[0030] Main analysis and detection instruments:

[0031] Table 2 Name, model / specification and supplier of main analysis and detection instruments:

[0032]

[0033] General process for preparing PHA aqueous dispersion:

[0034] The PHA aqueous dispersion used in the present invention is prepared by the following general procedure, which ensures the safety for food contact applications, avoiding the use of chemicals with off-odors or toxicity.

[0035] Step 1. Pre-mixing: 50 parts by weight of PHA powder is added to 50 parts of deionized water, while adding 0.5 parts of food-grade surfactant polysorbate 80. A high-speed disperser is used to pre-disperse for 30 minutes at 3000 rpm, forming a uniform coarse suspension.

[0036] Step 2. High-pressure homogenization: The above coarse suspension is pumped into a high-pressure homogenizer. The homogenization pressure is set to 1000 bar, and 15 cycles of treatment are performed. During the homogenization process, the material passes through a narrow valve gap under high pressure, subjected to shear, collision, and cavitation effects, effectively breaking and dispersing the PHA particles.

[0037] Step 3. Cooling and characterization: The product after homogenization is rapidly cooled to room temperature by a cooling system, obtaining a uniform, stable, milky white aqueous dispersion. Laser particle size analyzer is used to detect, ensuring that the average particle size (D 50 ) of the final dispersion is not greater than 3 μm, and the solid content is about 45 wt%. The dispersion can be stored stably at room temperature for at least 3 months without obvious sedimentation or stratification.

[0038] Main test standards:

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

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

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

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

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

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

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

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

[0047] Heat seal strength: ASTM F88 / F88M

[0048] Seal 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] Re-pulping performance: CEPI Recyclability Test Method - Part I, Version 3 (Feb. 2025); PTS-RH 021:2012 (Cat. II);

[0052] Printed sheet de-inking suitability: INGEDE Method 11 (packaging version, 2025-03);

[0053] Gel fraction: Soxhlet extraction method;

[0054] Recycled paper sheet performance: 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 regulations: EU No 10 / 2011 (merged to 2025-01-20), Annex V;

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

[0059] Total fluorine / organofluorine (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 method for coating preparation:

[0063] The acrylate emulsion, PHA aqueous dispersion and functional monomer (e.g. AAEM) were mixed well under stirring. In another container, the crosslinker (e.g. ADH) or ion source (e.g. CaCl2) was dissolved in a small amount of deionized water. The crosslinker solution was slowly added to the main emulsion, and stirring was continued for 30 minutes to obtain the coating liquid. The coating liquid was coated on the base paper using a laboratory coater, with a target dry gram weight of 6-15 g / m2. The coated paper was dried in a hot air oven at 105°C for 2 minutes, and then subjected to a short annealing treatment at 115°C for 60 seconds.

[0064] Table 3 Example and Comparative Formulation (all parts are wt% based on dry polymer solids):

[0065]

[0066] Application and Experiment:

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

[0068] Application Example 1: In-use performance testing.

[0069] To comprehensively evaluate the core performance of the coating in use, the coated paper samples of all examples and comparative examples at a standard dry gram weight of 15 g / m2were systematically tested for their key application indicators of water resistance, oil resistance, solvent resistance, barrier, and heat sealing, etc.

[0070] Table 4 Example and Comparative In-use Performance Test Results (15 g / m2):

[0071]

[0072] Analysis: Table 4 data shows that all examples meet the key in-use performance indicators. In comparison, Comparative Example 1, although excellent in some performance, has a too high heat sealing temperature. Comparative Example 2, due to the lack of crosslinking, has very poor water resistance, oil resistance, barrier, and solvent resistance. Comparative Example 3 (DAAM / ADH system) exhibits comparable good performance in use as the examples, which is consistent with the expectation of it as an effective crosslinking system. However, Comparative Example 4 (high bio-based content but no crosslinking) has significantly decreased performance, demonstrating that the crosslinking network is essential to achieve the required barrier and durability.

[0073] Application Example 2: Gram weight window verification.

[0074] To verify the performance stability of the coating of the present application at different coating amounts, the present experiment selected three lower dry gram weight grades of 6 g / m2, 8 g / m2 and 10 g / m2, and tested the performance of all examples and comparative examples to determine the process window of their application.

[0075] Table 5 Performance of the coating at different dry gram weights (6 g / m2):

[0076]

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

[0078]

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

[0080]

[0081] Analysis: The results of Tables 5-7 prove that the coating of the present application has excellent and stable performance in the application gram weight range of 6 to 10 g / m2. Comparative Example 1 and Comparative Example 3 also show stable use performance, but Comparative Example 2 and Comparative Example 4 have a sharp deterioration in performance at low gram weight, and cannot form an effective functional coating.

[0082] Application Example 3: Reslurry experiment (triggered state).

[0083] To verify the reversible disentanglement and recyclability of the coating, the present experiment simulated the industrial reslurry process, treated the coated paper samples under different chemical trigger conditions (alkaline, acidic, complexing and ultrasonic assistance), and quantitatively evaluated the fiber recovery rate and coarse residue rate according to the CEPI V3 standard method.

[0084] Path A (alkali / complexing trigger): adjust to pH 11.0±0.1 with NaOH; 50°C, 15 min. For Example 5, additionally add 0.3wt% EDTA-2Na.

[0085] Path B (acid trigger): adjust to pH 4.0±0.1 with HCl, 45°C, treat for 15 min.

[0086] Path C (CO2-acid / ultrasonic assistance): adjust the pH of the pulp to 4.5±0.2 with CO2 injection, 45°C, 15 min. At the same time, use 40 kHz ultrasonic treatment for 5-10 min.

[0087] Table 8 Results of reslurry performance test of examples and comparative examples (distinguished according to trigger path):

[0088]

[0089] Analysis: The results of Table 8 are the core of the present application. All examples exhibit excellent recyclability under specific triggering conditions. Comparative Example 1 (irreversible crosslinking) and Comparative Example 3 (conventional self-crosslinking) cannot be effectively re-pulped under all test routes, generating a large amount of coarse residue rate, proving the uniqueness and superiority of the reversible network of the present application. Comparative Example 2 and Comparative Example 4 (without effective crosslinking) are easy to re-pulp, but as shown in Table 4, they do not have basic use performance.

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

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

[0092] Table 9 Example and Comparative Example Sealing, Processing and Stability Test Results:

[0093]

[0094] Analysis: All examples exhibit good sealing strength (≥6.0 N / 15 mm) and hot tack. Comparative Example 1 and Comparative Example 3 have higher sealing strength, but this is at the expense of recyclability. Comparative Example 2 and Comparative Example 4 have almost no sealing performance.

[0095] Application Example 5: Multi-regional Food Contact Safety Verification.

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

[0097] Table 10 Example and Comparative Example "Fluorine-Free" Double Path Verification Results:

[0098]

[0099] Table 11 Example and Comparative Example Sensory, MOSH / MOAH and Migration Test Results:

[0100]

[0101] Analysis: All examples and comparative examples meet the main food contact safety regulations, proving the chemical safety of the formula components.

[0102] Application Example 6: Controlled Moisture OTR.

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

[0104] Table 12 Example and Comparative Example Controlled Humidity OTR Results:

[0105]

[0106] * Note: Test was conducted at 23°C, 80% RH.

[0107] Analysis: The OTR values of all examples were < 200 cm3 / (m2·d). The OTR of Comparative Example 1 and Comparative Example 3 were slightly lower, but at the expense of recyclability.

[0108] Application Example 7: Substrate Adaptation Window

[0109] To verify the universality of the coating formulation of the present application on different types of paper substrates, the formulations of all examples and comparative examples were coated on bamboo pulp paper, bagasse paper and cup paper base paper at 10 g / m2, and their key performances were tested. Among them, all samples were subjected to hot water leakage test to simulate the actual use scene.

[0110] Table 13 Example Substrate Adaptation Performance:

[0111]

[0112] Table 14 Comparative Example Substrate Adaptation Performance:

[0113]

[0114] Analysis: The results of Table 13 and Table 14 show that all example formulations can achieve performance indicators on various substrates such as bamboo pulp paper, bagasse paper and cup paper base paper. The results of the cup paper hot water leakage test (Note: This test is usually used for cup paper. To evaluate the extreme liquid resistance of bamboo pulp and bagasse paper, they were temporarily formed for testing) further show that only coatings that have been effectively crosslinked (all examples and Comparative Example 1 and Comparative Example 3) can provide reliable liquid barrier function, while Comparative Example 2 and Comparative Example 4, which are not crosslinked or insufficiently crosslinked, show obvious leakage.

[0115] Application Example 8: Print Deinking Adaptability

[0116] To evaluate the impact of the coating on the recycling value of printed matter, standard water-based ink printing was performed on the coated paper of all examples and comparative examples, followed by a flotation deinking experiment according to INGEDE Method 11, and the brightness increment and ink residue (ERIC) were measured to evaluate the deinking efficiency.

[0117] Table 15 Deinking indicators of examples and comparative examples:

[0118]

[0119] Analysis: The reversible network of examples can effectively release ink particles after the repulping trigger, and the deinking effect is good. The irreversible crosslinking of Comparative Example 1 and Comparative Example 3 seriously inhibits the dissociation and removal of ink particles. Although Comparative Example 2 and Comparative Example 4 have good deinking effect, it is because the coating itself is not firm and has no practical value.

[0120] Application Example 9: Repulping switching and hot water post-recycling of cup paper scenarios

[0121] In order to simulate the recycling scenario of products such as hot drink cups after actual use, all cup paper samples prepared in Application Example 7 were soaked in 90°C hot water for 30 minutes, and then subjected to path A (alkaline / complexing) repulping experiment to test whether the coating still maintains high efficient recyclability after experiencing hot and humid conditions.

[0122] Table 16 Repulping performance after hot water soaking:

[0123]

[0124] Analysis: Hot water soaking did not weaken the trigger state network release efficiency of the coating of examples, and the repulping performance was still excellent, proving the high value recycling feasibility of the technology after actual application scenarios.

[0125] Verification of network reversibility and recycled pulp quality:

[0126] Gel fraction test:

[0127] In order to directly verify the reversibility of the crosslinked network from the mechanism level, the coating was made into an independent film, and the gel fraction (GF) before and after alkaline or acidic treatment was determined by Soxhlet extraction method to quantify the dissociation degree of the crosslinked network.

[0128] Table 17 Gel fraction test results of examples and comparative examples:

[0129]

[0130] Analysis: Table 17 verifies the reversibility of the network from the mechanism level. The gel fraction of all examples of the coating decreases by more than 90% after treatment. On the contrary, Comparative Example 1 and Comparative Example 3 use irreversible crosslinking, and the gel fraction before and after treatment is basically unchanged.

[0131] Evaluation of recycled pulp quality:

[0132] To finally evaluate the value of the recycling scheme of the present application, the good pulp (i.e. recycled fibers) obtained after repulping was used to make handsheets according to standard methods, and the key physical properties (tensile index), optical properties (brightness) and impurities content (dirt, stickies) were evaluated.

[0133] Table 18. Results of the quality evaluation of the recycled pulp (good pulp) from the examples and comparative examples:

[0134]

[0135] Analysis: the good pulp obtained from the coated paper of the examples has maintained a high level of physical and optical properties. In contrast, the recycled pulp from comparative example 1 and comparative example 3 has a significantly lower quality, with high impurities content, and cannot be used for high quality production.

[0136] Results and analysis of the experiments:

[0137] The core of the present application is to construct an intelligent coating system with switchable properties between "use state" and "recycling state". Through systematic verification of a series of application examples, the following conclusions can be drawn:

[0138] 1. Comparable performance with irreversible systems: from the data in Tables 4, 5, 7 and 10, it can be seen that all examples have reached a level comparable to or even better than the conventional irreversible crosslinking agent (comparative example 1) and the conventional self-crosslinking system (comparative example 3) in terms of water resistance (Cobb), oil resistance (Kit), MEK double rubs, barrier (WVTR / OTR) and sealing processing performance. This proves that the dynamic reversible crosslinking network of the present application is stable and efficient under use conditions, and is sufficient to meet the actual application requirements. In contrast, comparative examples 2 and 4 without crosslinking perform poorly in all properties, highlighting the necessity of crosslinking.

[0139] 2. Excellent and controllable recyclability: the repulping experiment results in Table 8 are the key distinguishing point of the present application. All examples have achieved a fiber recovery rate of more than 98% and a coarse residue rate of less than 1.0% under specific acidic or basic trigger conditions, reaching the standard of high-quality recycling. This is in sharp contrast to the very low recovery rate (<90%) and large amount of coarse residue rate (>8%) of comparative examples 1 and 3. The gel fraction test (Table 17) confirms the difference in mechanism: the gel fraction of the examples decreases by more than 90% after triggering, while the network structure of comparative examples 1 and 3 remains essentially unchanged.

[0140] 3. High-quality recycled product value: The advantage of the present application is not only in the high fiber recovery rate, but also in the quality of the recovered fibers. As shown in Table 18, the recycled paper sheet made from the good pulp recovered by the examples maintains a high level of physical strength, whiteness and cleanliness, and can be used to produce high-value-added products. The recycled pulp of Comparative Example 1 and Comparative Example 3 has a large amount of adhesion and impurities, and the performance is severely degraded, so the application value is very low. In addition, the deinking experiment results of Table 15 show that the reversible network of the present application is also beneficial to the removal of ink, further improving 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 application on a variety of different fiber substrates. Application Example 9 (Table 16) verifies that it can still maintain excellent recyclability after experiencing actual use scenarios such as hot water soaking. At the same time, the comprehensive test results of Application Example 5 (Tables 10 and 11) show that the formulation system of the present application does not contain PFAS and other harmful substances, and meets the mainstream global food contact safety regulations.

[0142] In summary, the present application successfully solves the contradiction between high-performance paper coating and efficient recycling and reuse by constructing a dynamic reversible crosslinking network that stably exists in the "use state" and quickly dissociates in the "trigger state". The coating system of the present application not only has excellent performance, but also is environmentally friendly, providing a practical technical solution for realizing high-value closed-loop recycling of paper product packaging.

[0143] Those skilled in the art should understand that the above examples are only exemplary and are not intended to limit the scope of the present application. Any modification, equivalent replacement, improvement, etc. to the technical solutions of the present application within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A reversibly crosslinked paper coating composition comprising, by dry polymer solids: 40-100 wt% of an acrylate-based polymer emulsion; 0-60 wt% of a polyhydroxyaliphatic ester aqueous dispersion; 0.2-7.0 wt% of a dynamic reversible crosslinking unit; The composition is free of artificially added PFAS, is used to form a dry film of 6-15 g / m2 on paper, and meets at least one of the following properties when in use: Cobb 60 ≤ 18 g / m²; Kit > 10; Water vapor transmission rate < 80 g / (m2.d) measured at 38°C, 90% relative humidity; Oxygen transmission rate < 200 cm3 / (m2.d) measured at 23°C, 80% relative humidity; Seal initiation temperature < 180°C; MEK double rubs > 50 times; The dynamic reversible crosslinking unit is a hydrazone network formed from a monomer containing acetoacetic acid functional group and a dihydrazine compound, which is formed from the reaction of acetoacetyl ethyl methacrylate monomer and adipic acid dihydrazine crosslinking agent.

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

1.

3. The composition of claim 1, wherein, The polyhydroxyaliphatic ester is selected from: one or more of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3- hydroxyvalerate), poly(3-hydroxybutyrate-co-4-hydroxybutyrate), poly(3- hydroxybutyrate-co-3-hydroxyhexanoate).

4. The composition of claim 1, wherein, The acrylate-based polymer emulsion is an aqueous dispersion of a copolymer comprising acrylate, methacrylate, vinyl ester or a combination thereof, which is optionally copolymerized with styrene or other vinyl unsaturated monomers.

5. The composition of claim 4, wherein, The acrylate-based 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.

6. A coating formed from the composition of claim 1, wherein, The coating has a seal strength of > 6.0 N / 15 mm.

7. A coated paper product, characterized by A paper substrate, and a coating formed from the composition of any one of claims 1 to 6.

8. The coated paper product according to claim 7, characterized in that The paper substrate is selected from bleached kraft paper, unbleached kraft paper, bamboo pulp paper, bagasse paper, wheat fiber paper or recycled fiber paper.

9. The coated paper product of claim 7, wherein, The paper substrate is selected from molded pulp products or cup paper base paper.

10. The coated paper product of claim 8, wherein, The product is a paper cup, and has no obvious leakage after soaking in hot water at 90°C for 30 minutes.

11. Use of the composition of any one of claims 1 to 6 in the preparation of coated paper products, which are applied to paper machine closed loop recycling systems requiring high recovery rate to produce high purity recycled pulp.

12. Use according to claim 11, characterized in that, The coated paper products are applied to food packaging, e-commerce delivery bags and molded pulp products.

13. A method of making a coated paper product according to claim 7, characterized in that, A method comprising coating the composition of any one of claims 1 to 6 on a paper substrate, followed by annealing treatment.

14. The method of claim 13, wherein, The conditions of the annealing treatment are temperature 100-130°C, time 20-90 s.

15. A method for re-pulping a coated paper product prepared according to the method of claim 13 or 14, characterized by, The method comprises one or more of the following treatment methods: Treatment under alkaline conditions; Treatment under acidic conditions; Treatment under ultrasonic conditions; Treatment with the addition of a chelating agent.

16. The repulping method according to claim 15, characterized in that, The method comprises: adjusting the pH to 10.5-12.0 using sodium hydroxide, treating at 45-55℃ for 10-15 min; or injecting carbon dioxide to adjust the pH to 4.0-5.0, treating at 40-50℃ for 10-20 min; or adding 0.1-0.5wt% 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 makes the gel fraction of the coating decrease by ≥90%.

18. The repulping method as claimed in claim 16, characterized by, The method has a fiber recovery rate of ≥98%, a coarse residue rate of ≤1.0%, and a stickies level of ≤1 grade.

19. A method of improving the deinking efficiency of a printed coated paper product, characterized by, Coating the composition of any one of claims 1-6 on a paper product prior to printing such that the coated paper product has an EPRC deinking score greater than 71 when subjected to a deinking test according to INGEDE Method 11.

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