Polyhydroxyalkanoate grafted ethylene-acrylic acid copolymer aqueous dispersions having a self-emulsifying core-shell structure, methods of making, and recyclable paper packaging thereof

Abstract

The application discloses a polyhydroxyaliphatic acid ester grafted ethylene-acrylic acid copolymer aqueous dispersion with a self-emulsifying core-shell structure, a preparation method and recyclable paper packaging, and belongs to the technical field of polymer material reactive modification and green packaging. The application adopts in-situ reactive extrusion technology to construct stable microparticles with hydrophobic polyhydroxyaliphatic acid ester as a core and hydrophilic ethylene-acrylic acid copolymer salt as a shell, and solves the industry problem of poor compatibility of bio-based materials. The aqueous dispersion does not need an additional emulsifier, and can achieve the same excellent barrier and heat sealing effects with a lower coating amount, thereby significantly reducing the use cost. The unique alkali-induced swelling mechanism endows the coating with intelligent peeling characteristics, realizes efficient separation of the coating and fibers in the reslurry process, and greatly improves the fiber recycling purity.

Description

Technical Field

[0001] This invention belongs to the field of reactive modification of polymer materials and green packaging technology, specifically relating to an aqueous dispersion of polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure, its preparation method, and its recyclable paper packaging. Background Technology

[0002] Paper-based packaging holds a significant position in the field of environmentally friendly packaging due to its renewable and recyclable advantages. However, the base paper has insufficient barrier properties against media such as liquid water, water vapor, grease, and oxygen, and its heat-sealing processing window is limited. Therefore, coatings are typically required to impart barrier and heat-sealing functions. Polyhydroxyalkanoates (PHAs) are a type of aliphatic polyester synthesized by microorganisms. PHAs possess good biocompatibility and certain barrier properties, but they generally suffer from high crystallinity, brittle film formation, insufficient folding endurance, and weak interfacial forces with cellulose substrates, easily leading to coating cracking, pinholes, and interfacial delamination. Ethylene-acrylic acid copolymers (EAAs) contain carboxyl groups in their chain segments, which can enhance wetting and adhesion to polar substrates. After neutralization, the carboxyl groups form carboxylates, which can impart charge stability and pH responsiveness to aqueous dispersion systems.

[0003] Regarding the aqueous application of carboxyl-containing ethylene copolymers, existing patent documents disclose technical routes for obtaining aqueous dispersions or emulsions through carboxyl neutralization. For example, US3677989A and US5591806A disclose aqueous dispersions / emulsions of carboxyl-containing EAAs or their salts and their applications. For the aqueous application of hydrophobic polymers, some disclosed methods propose neutralizing carboxyl-containing polymers in a melt mixing or twin-screw extruder and continuously injecting water to achieve phase reversal, thereby obtaining an aqueous dispersion without added emulsifiers for use in coatings. Examples include the process routes disclosed in EP359045A, DE10109992A1, WO2005 / 021638A2, and EP2307132B1.

[0004] Several process routes have been proposed for the aqueous dispersibility of biodegradable polyesters such as PHA: First, the route of preparing PHA latex by melt phase inversion emulsification / melt emulsification, for example, WO2012149407A1 discloses a method for preparing PHA latex by melt emulsification; Second, the route of directly dispersing PHA powder in the aqueous phase using colloidal stabilizers, for example, CN104619748A discloses a method of dispersing PHA powder in an aqueous medium by high shear in the presence of colloidal stabilizers such as polyvinyl alcohol, starch and its derivatives, cellulose and its derivatives; Third, the route of biodegradable polymer dispersions for film-forming and coating applications, for example, WO2017 / 151595A1 discloses aqueous, hydrolysis-resistant biodegradable polymer dispersions for film-forming and coating applications.

[0005] In the field of paper-based packaging coatings, to achieve plastic reduction and greening, published patents have also proposed coating or adhesive solutions based on PHA or its modified systems. For example, CN120759147B discloses a water-based coating with high PHA content for coating paper; CN120944505A discloses a method for preparing pure bio-based PHA water-based dispersions / adhesives by controlling the acid value of PHA through melt end-group activation and combining it with reactive emulsifiers, and mentions re-sizing evaluation; in addition, WO2025 / 250291A1 and WO2025 / 250292A1 disclose biodegradable polymer water-based dispersions and their application in coated paper / paperboard. Furthermore, US20210348338A1 discloses water-based barrier coating systems for paper-based packaging, and combines alkaline conditions to achieve recyclability evaluation. These systems typically achieve water resistance and barrier properties through the compounding of mineral fillers and film-forming components or the introduction of reversible crosslinking / ionic interactions. The above-mentioned route differs from the technical route of this invention in terms of "material composition, film formation mechanism and recycling triggering mechanism". The technical route of this invention uses polyhydroxy fatty acid ester to graft ethylene-acrylic acid copolymer (PHA-g-EAA) to construct self-emulsified core-shell microparticles and relies on the charge response of EAA salt shell to achieve overall swelling and exfoliation.

[0006] Therefore, existing publicly available solutions do not provide a core-shell aqueous dispersion system that constructs a covalent graft structure between PHA and EAA, enriches the shell with EAA carboxylate to achieve self-emulsification and stability, and triggers overall swelling and exfoliation under weakly alkaline conditions.

[0007] Therefore, there is an urgent need for a paper-based coating material system that does not require external emulsifiers or crosslinking agents and can achieve efficient repulping within a weakly alkaline window, in order to solve the problems of poor compatibility, reliance on potentially risky crosslinking systems, and narrow and unstable recycling windows in existing technologies. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide an aqueous dispersion of a polyhydroxy fatty acid ester grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure, a preparation method thereof, and its recyclable paper packaging.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] This invention provides an aqueous dispersion of a polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure, comprising an aqueous phase and polymer particles dispersed in the aqueous phase; the polymer particles have a core-shell structure, wherein the core comprises hydrophobic PHA segments and the shell comprises hydrophilic EAA salt segments; the polymer particles comprise an amphiphilic graft copolymer, wherein the amphiphilic graft copolymer is a graft copolymer obtained by reactive extrusion melt grafting of PHA and EAA under the action of a reactive compatibilizer and an initiator; wherein the EAA is a binary copolymer obtained by copolymerizing ethylene monomer and acrylic acid monomer, and the EAA carboxyl group is neutralized to form an EAA salt; in some embodiments, the aqueous dispersion can be prepared by reactively extruding melt grafting of PHA, EAA, reactive compatibilizer and initiator, followed by neutralization of the EAA carboxyl group and reverse emulsification under high shear conditions with added water.

[0011] Based on a total mass of 100 parts by mass for PHA, EAA, reactive compatibilizer monomers, and initiator active ingredients, PHA comprises 20 to 59.49 parts by mass, for example, 20, 22, 25, 28, 30, 32, 35, 38, 40, 42, 45, 48, 50, 52, 55, 56.5, 58, 59, or 59.49 parts by mass, etc.; EAA comprises 40 to 79.49 parts by mass, for example, 40, 42, 45, 48, 50, 52, 55, 58, 60, 62.3, 64, 65, 65.5, 66. The quantities are 5 parts by weight, 66.7 parts by weight, 68.2 parts by weight, 69.49 parts by weight, 70 parts by weight, 72 parts by weight, 75 parts by weight, 76.5 parts by weight, 78 parts by weight, or 79.49 parts by weight, etc.; the reactive compatibilizer is 0.5 to 5.0 parts by weight, for example, 0.5 parts by weight, 0.8 parts by weight, 1.0 parts by weight, 1.5 parts by weight, 2.0 parts by weight, 2.5 parts by weight, 3.0 parts by weight, 3.5 parts by weight, 4.0 parts by weight, 4.5 parts by weight, 4.8 parts by weight, or 5.0 parts by weight, etc.; the initiator is 0.01 to 1.00 parts by weight, for example, 0.01 parts by weight, 0.05 parts by weight, 0.1 parts by weight, 0.2 parts by weight, 0.3 parts by weight, 0.4 parts by weight, 0.5 parts by weight, 0.6 parts by weight, 0.8 parts by weight, 0.9 parts by weight, or 1.00 parts by weight, etc.

[0012] The PHA is selected from short-chain PHA, medium- and long-chain PHA, or copolymers forming short-chain PHA monomers and medium- and long-chain PHA monomers; the short-chain PHA is selected from one or more of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), and poly(3-hydroxybutyrate-co-4-hydroxybutyrate); the medium- and long-chain PHA is selected from one or more of poly(3-hydroxyhexanoate), poly(3-hydroxyheptanoate), poly(3-hydroxyoctanoate), poly(3-hydroxynonanoate), poly(3-hydroxydecanoate), poly(3-hydroxyundecanoate), poly(3-hydroxydodecanate), poly(3-hydroxytetrate), poly(3-hydroxytetradecanoate), poly(3-hydroxytetradecanoate), poly(3-hydroxypentadecanate), poly(3-hydroxyhexadecanoate), poly(3-hydroxyheptadecanoate), and poly(3-hydroxyheptadecanoate).

[0013] The reactive compatibilizer monomer is selected from one or more of glycidyl methacrylate (GMA), glycidyl acrylate, allyl glycidyl ether, vinyl glycidyl ether, maleic anhydride, itaconic anhydride, and citraconic anhydride; the initiator is selected from one or more of dicumyl peroxide (DCP), di-tert-butyl peroxide, benzoyl peroxide, tert-butyl peroxybenzoate, 2,5-dimethyl-2,5-di-tert-butylperoxyhexane, 2,5-dimethyl-2,5-di-tert-butylperoxyhexyn-3, azobisisobutyronitrile, and azobiscyclohexanenitrile.

[0014] The neutralization degree of the EAA carboxyl group is 85% to 100%, for example, it can be 85%, 86%, 88%, 90%, 92%, 94%, 95%, 96%, 98% or 100%, and the cation in the EAA salt is selected from one or more of sodium ions, potassium ions and lithium ions.

[0015] The aqueous dispersion has a solid content of 40wt% to 45wt%, for example, 40wt%, 41wt%, 42wt%, 43wt%, 44wt%, or 45wt%, and a pH value of 9.0 to 9.5, for example, 9.0, 9.1, 9.2, 9.3, 9.4, or 9.5, wherein the solid content is a mass fraction based on the total mass of the aqueous dispersion; the median particle size of the polymer particles is 120nm to 165nm, for example, 120nm, 125nm, 130nm, 135nm, 138nm, 140nm, or 165nm. The molecular weight of the aqueous dispersion is 42nm, 145nm, 148nm, 150nm, 152nm, 155nm, 160nm, or 165nm, and the polydispersity index is ≤0.22, for example, it can be 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, or 0.22; the absolute value of the zeta potential of the aqueous dispersion is ≥30mV; the volume fraction of the aqueous dispersion after centrifugation at 20000g for 30min is 0%; and after sealed storage at 25℃ for 90 days, the median particle size change rate of the polymer microparticle volume distribution is ≤10%.

[0016] The present invention also provides a method for preparing the above-mentioned aqueous dispersion, comprising the following steps:

[0017] Step 1. Reactive Extrusion: PHA and EAA resins are pre-dried under vacuum at 40°C to 80°C for at least 2 hours until their mass is constant and then cooled for later use. The dried PHA and EAA are continuously fed into the main feed port of a twin-screw extruder. The reactive compatibilizer is injected upstream of the first high-shear kneading zone in liquid or molten form using a metering pump through the liquid injection port. The initiator is added through the main feed port or the side feed port after premixing with the resin. The extruder is equipped with at least two high-shear kneading zones along its length and a vacuum exhaust section is provided to remove residual monomers and volatiles. Small molecules, vacuum degree controlled in vacuum exhaust section is -0.02MPa to -0.095MPa, reactive extrusion is melt-mixed and reacted in the range of 130℃ to 200℃, for example, it can be reacted at 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃ or 200℃, residence time is 30s to 180s, for example, 30s, 45s, 60s, 90s, 120s, 150s or 180s, etc.; the amount of external emulsifier or dispersant added in the preparation method is 0 parts by mass;

[0018] Step 2. Online neutralization: An aqueous solution of alkaline neutralizing agent is introduced into the polymer melt obtained in Step 1 at the alkaline neutralizing agent inlet point of the twin-screw extruder. The mass fraction of the aqueous solution of alkaline neutralizing agent is 5 wt% to 40 wt%, for example, it can be 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, or 40 wt%, and the injection amount is calculated according to the EAA carboxyl content and the target degree of neutralization; mixing is performed to neutralize the EAA carboxyl groups into salts to obtain a neutralized melt;

[0019] Step 3. Reverse Emulsification: Deionized water is continuously injected into the neutralized melt obtained in Step 2 in the high-shear kneading zone of the twin-screw extruder, maintaining an effective shear rate greater than or equal to 500 s⁻¹, for example, 500 s⁻¹, 800 s⁻¹, 1000 s⁻¹, 1100 s⁻¹, 1200 s⁻¹, 1300 s⁻¹, 1400 s⁻¹, 1466 s⁻¹, 1500 s⁻¹, 1600 s⁻¹, 1800 s⁻¹, or 2000 s⁻¹, etc., to reverse the system and obtain an aqueous dispersion; the system outlet temperature is ≤110℃, and the deionized water injection point is located at the alkali injection point in Step 2. The neutralizing agent is introduced downstream of the point of introduction; the total mass of water in the aqueous dispersion, m(water), can be initially set according to the target solid content on a dry basis of the formulation, and the calculation formula is m(water) = m(polymer solids) × (1 / SC-1), where SC is the target solid content mass fraction calculated on a dry basis of the formulation, and m(polymer solids) is the total mass of PHA, EAA, reactive compatibilizer monomers and initiator active ingredients, excluding the mass of non-volatile components corresponding to the cations introduced after the EAA carboxyl groups are neutralized into salts; m(water) is the total mass of water introduced by the alkaline neutralizing agent aqueous solution in step 2 and water injected by the deionized water in step 3. The solid content of the finished aqueous dispersion is determined according to ISO 3251:2019, and the amount of deionized water injected in step 3 can be finely adjusted according to the increase in non-volatile matter introduced by residual water and salt formation after online neutralization. The deionized water can be introduced by single-point continuous injection or multi-point segmented continuous injection. The effective shear rate is the apparent shear rate of the high-shear kneading zone, which is calculated according to γ̇=πDN / δ, where D is the screw outer diameter, N is the screw speed (r / s), and δ is the gap between the screw and the barrel. The screw speed and the kneading block combination are adjusted to make γ̇ greater than or equal to 500s-1.

[0020] Step 4. Rapid cooling and filtration: The aqueous dispersion obtained in step 3 is rapidly cooled to below 50°C using a heat exchanger, and then filtered through a 100-mesh to 300-mesh filter within 10 minutes after cooling. The filtrate is collected to obtain the finished aqueous dispersion.

[0021] The twin-screw extruder has an aspect ratio ≥ 44:1 and is equipped with at least two high-shear kneading zones, and a vacuum exhaust section for removing residual small molecules; the reactive extrusion residence time is 30s to 180s; the system outlet temperature during reverse emulsification is ≤ 110℃; the mass fraction of the alkaline neutralizing agent aqueous solution is 5wt% to 40wt%; the alkaline neutralizing agent is selected from one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, potassium carbonate, and sodium bicarbonate; the alkaline neutralizing agent is introduced in aqueous solution form through the alkaline neutralizing agent introduction point, and the deionized water injection point in step 3 is located downstream of the alkaline neutralizing agent introduction point; the deionized water in step 3 is introduced by single-point continuous injection or multi-point segmented continuous injection.

[0022] This invention also provides a recyclable coated paper, comprising a paper substrate and a coating applied to its surface, wherein the coating is formed by coating and drying the aqueous dispersion into a film; the coating absorbs water and swells under repulping conditions of pH 10 and 45°C and peels off entirely from the paper fiber surface, with a fiber recovery rate ≥99% as measured by the CEPI laboratory test method for recyclability; and the total fluorine content in the free film obtained by drying and peeling the aqueous dispersion is undetectable by oxygen bomb combustion-ion chromatography, with a method detection limit of 5 mg / kg.

[0023] The dry coating amount is 10g / m². 2 The coated paper, under the conditions of an upper end cap temperature of 130℃, a lower end cap temperature of 60℃, a heat sealing pressure of 0.3MPa, and a heat sealing time of 1s, has a heat seal strength ≥12N / 15mm as measured by ASTM F88 / F88M-23 (Technique A), and a leakage rate ≤0.1g after immersion in 90℃ hot water for 30min; after re-sizing, the residue on a 0.15mm slotted sieve, as measured by the CEPI recyclability laboratory test method, is ≤0.5%.

[0024] The present invention also provides a method for recycling paper fibers from the coated paper, comprising the following steps:

[0025] Step 1. Re-sizing: The coated paper is placed in an alkaline aqueous phase with a pH of 10.0 and stirred at 45°C for 15 minutes to allow the coating to swell and peel off due to alkali and separate from the paper fibers, thereby obtaining re-sizing pulp. The pulp concentration of the stirred re-sizing is 10 g / L.

[0026] Step 2. Screening: The re-slurry obtained in Step 1 is screened to obtain recycled fibers. The fiber recovery rate, as measured by the CEPI laboratory test method for recyclability, is ≥99%, and the residue on a 0.15mm slit sieve is ≤0.5%. Furthermore, the macroscopic adhesive area, as measured by the macroscopic adhesive area assessment step in the CEPI laboratory test method for recyclability, is ≤1.0 cm². 2 / m2 The equivalent diameter of the macroscopic adhesive is 0.15 mm to 2.0 mm.

[0027] The present invention also provides the use of the aqueous dispersion in the preparation of barrier coatings or heat-sealing coatings for recyclable paper packaging, the paper packaging including paper cups, paper bowls, lunch boxes, paper lids, paper trays, paper plates, paper-based heat-sealing bags, or paper-based molded tableware.

[0028] The present invention also provides a paper packaging article, comprising the coated paper, and processed into paper cups, paper bowls, lunch boxes, paper lids, paper trays, paper plates, paper-based heat-sealed bags or paper-based molded tableware.

[0029] Compared with the prior art, the following significant advantages can be obtained by using the present invention:

[0030] Excellent self-emulsifying stability and compatibility: This invention utilizes reactive extrusion technology to construct an in-situ chemical graft structure between hydrophobic PHA and hydrophilic EAA, forming stable core-shell microparticles, with PHA as the core and EAA salt as the shell. This structure not only solves the problems of poor compatibility and easy stratification when PHA and EAA are physically blended, but also achieves self-emulsification without the addition of any small molecule emulsifiers by relying on the electrostatic repulsion and steric hindrance provided by the abundant carboxylate groups in the shell. The resulting aqueous dispersion has a fine and uniform particle size, does not stratify upon centrifugation, and exhibits excellent storage stability for more than 90 days with a low particle size change rate.

[0031] Superior barrier and heat-sealing properties: Thanks to its nanoscale micro-dispersion and grafting structure, the dispersion forms a dense and uniform coating after film formation, eliminating the phase separation defects of traditional blended coatings. This coating exhibits extremely low haze and imparts excellent waterproof, oil-proof, oxygen-barrier, and moisture-barrier properties to the paper substrate. Simultaneously, the EAA component enriched on the coating surface provides a wide heat-sealing window and high-strength heat-sealing adhesion, meeting the stringent sealing requirements of paper packaging.

[0032] Intelligent Alkali-Induced Swelling Recovery Mechanism: This invention utilizes the pH-responsive characteristics of the EAA carboxylate shell to design an intelligent mechanism that ensures stability during use and separation during recycling. Under normal operating conditions, the coating remains stable; however, under specific weakly alkaline repulping conditions, the shell rapidly absorbs water and swells, weakening the interfacial bond between the coating and paper fibers. This causes the coating to peel off as a whole in a macroscopic sheet form, rather than breaking down into difficult-to-remove fine adhesive residues. This mechanism ensures extremely high fiber recovery rates and significantly reduces adhesive residue residues, solving the industry problem of difficult repulping and separation in traditional plastic-coated paper.

[0033] Environmental friendliness and safety: The aqueous dispersion of this invention uses water as the entire medium, contains no volatile organic compounds (VOCs), and the coating formulation achieves excellent oil-resistant properties without the addition of any fluorinated compounds, avoiding the introduction of perfluorinated and polyfluoroalkyl substances (PFAS). Simultaneously, the introduction of bio-based PHA components reduces dependence on fossil resources, aligning with the trends of green packaging and sustainable development. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the cross-sectional structure of the recyclable paper packaging described in this invention.

[0035] In the figure, 1-paper substrate; 2-coating; 3-polymer microparticles with core-shell structure; 4-cation; 5-continuous phase matrix. Detailed Implementation

[0036] 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.

[0037] Figure 1 This is a schematic cross-sectional view of the recyclable paper packaging according to the present invention. As shown, the recyclable paper packaging consists of a bottom paper substrate 1 and a specific functional coating 2 covering it. In the microstructure of the coating 2: multiple polymer particles 3 with core-shell structures are uniformly distributed in the continuous phase matrix 5. These particles are formed by the fusion of their outer shells during the drying and curing process to form a stable matrix, and the particles have a dense core at their center; small-sized cations 4 are scattered in the matrix region between the particles; and the continuous phase matrix 5 is formed by the fusion of the shells of the particles and covers the cores of the polymer particles and the cations 4.

[0038] Main reagents and raw materials:

[0039] Table 1. Main reagents and raw materials, product models / specifications and manufacturers:

[0040]

[0041] Table 2 mainly analyzes the names, models, and manufacturers of the testing instruments:

[0042]

[0043] Table 3. Main Test Items and Reference Standards:

[0044]

[0045] Example:

[0046] Example 1 provides a detailed preparation process. Unless otherwise stated, the preparation steps for the aqueous dispersions in Examples 2-15 are the same as those for the aqueous dispersion of the PHA-g-EAA graft copolymer in Example 1, differing only in the formulation composition, the type of PHA or EAA used, the neutralizing agent, the degree of neutralization, and the final solid content adjusted in step 2. The "parts by mass" of each component are based on the dry basis of the polymer solids, and the formulation basis for the polymer solids is "parts by mass of PHA + parts by mass of EAA + parts by mass of reactive compatibilizer + parts by mass of initiator (based on active ingredient)," i.e., the sum of the four is 100 parts. The "parts by mass" of the initiator are based on the active ingredient. When the initiator is a commercial product containing less than 100 wt% active ingredient, the actual weighed amount is calculated based on the active ingredient content. For example, when DCP uses a commercial product with an active ingredient content of 40 wt%, its actual weighed amount = parts by mass of DCP in the formulation / 0.40.

[0047] Example 1: The sources and specifications of the PHA resin raw materials used in this example and Examples 2-15 below are as follows: PHBV is PV3000G from Beijing Microstructure Workshop Biotechnology Co., Ltd.; PHB is PB3000G from Beijing Microstructure Workshop Biotechnology Co., Ltd.; PHBH is Green Planet X131A from Kaneka Chemicals; P34HB is PHACT A1000P from CJ BIO.

[0048] The aqueous dispersion of the PHA-g-EAA graft copolymer in this embodiment was prepared as follows: the formulation consisted of 30 parts by weight of PHBV, 65 parts by weight of high-acid-content EAA, 4.5 parts by weight of glycidyl methacrylate (GMA), and 0.5 parts by weight of dicumyl peroxide (DCP); the neutralizing agent was a 20wt% potassium hydroxide aqueous solution with a neutralization degree of 100%. When the total mass of the active ingredients PHA, EAA, GMA, and DCP was 1000.00g, the amount of PHBV used was 300.00g, the amount of high-acid-content EAA used was 650.00g, the amount of GMA used was 45.00g, and the amount of DCP used was 5.00g; when using Perkadox BC-40B-GR-DD with a DCP content of 40wt%, the actual amount weighed was 12.50g. PHBV resin and high-acid-content EAA resin were pre-dried under vacuum at 60°C for 8 hours until their mass was constant and then cooled for later use. The dried PHBV and high-acid-content EAA were continuously fed into the main feed port of a co-rotating twin-screw extruder. The initiator was premixed with the high-acid-content EAA resin and then added through the main feed port. When the reactive compatibilizer was a liquid monomer, it was injected upstream of the first high-shear kneading zone using a metering pump from the liquid injection port. When the reactive compatibilizer was a solid monomer, it was first heated to complete melting in an 80°C constant-temperature oil bath, and then injected upstream of the first high-shear kneading zone using a metering pump from the liquid injection port. In this embodiment, GMA was a liquid monomer. The extruder used was a TSE-35 co-rotating twin-screw extruder with a length-to-diameter ratio of 44:1. Two high-shear kneading zones were set along the extrusion path, and a vacuum exhaust section was set to remove residual monomers and volatile small molecules. The vacuum degree of the vacuum exhaust section was set to -0.08 MPa. The temperature of the reactive extrusion melt mixing section is set at 160℃, and the average residence time is set at 90s; no additional emulsifiers or dispersants are added during the preparation process.

[0049] Step 1. Online neutralization: Introduce a 20wt% potassium hydroxide aqueous solution into the above polymer melt at the alkaline neutralizer inlet point of the twin-screw extruder. The amount of alkaline neutralizer injected is calculated based on the mass fraction of acrylic monomer w(AA) = 0.205 in the high-acid-content EAA and the target degree of neutralization α = 1.00: n(COOH) = m(EAA) × w(AA) / 72.06, m(KOH) = n(COOH) × 56.11 × α, m(KOH solution) = m(KOH) / 0.20; if calculated according to the amount of 650.00g of high-acid-content EAA used in this embodiment, then n(COOH) = 1.8492mol, m(KOH) = 103.76g, corresponding to a mass of 518.78g of 20wt% potassium hydroxide aqueous solution. The mixing process neutralizes the EAA carboxyl groups to form a salt. A vacuum exhaust section is set downstream of the alkaline neutralizing agent introduction point. The vacuum exhaust section is connected to the condensation recovery device and the vacuum degree is controlled at -0.08MPa to remove residual small molecules and water introduced during online neutralization, resulting in a dehydrated and neutralized melt.

[0050] Step 2. Conversion Emulsification: Deionized water is continuously injected into the dehydrated and neutralized melt obtained in Step 1 in the high-shear kneading zone of the twin-screw extruder. The deionized water injection point is located downstream of the vacuum degassing section, and the deionized water is injected continuously at a single point. The total mass of water in the aqueous dispersion, m(water), is calculated according to the target solids content SC: m(water) = m(polymer solids) × (1 / SC-1); In this embodiment, SC is the target solids content calculated on a dry basis according to the formulation, set to 42.0 wt%. When m(polymer solids) = 1000.00 g, m(water) = 1380.95 g. Since the residual water after vacuum degassing and the non-volatile matter introduced by the neutralization of EAA carboxyl groups will affect the solids content of the finished product measured according to ISO 3251, the amount of deionized water injected in Step 2 is initially set at 1380.95 g, and fine-tuned based on the online sampling results, so that the solids content of the obtained aqueous dispersion is 42.0 wt% as measured by ISO 3251. The apparent shear rate in the high-shear kneading zone was maintained at 1466 s⁻¹ to induce the system to reverse and form an aqueous dispersion. The apparent shear rate was calculated as γ̇ = πDN / δ, where D is 35 mm and δ is 0.50 mm. The screw speed was set to 400 r / min, i.e., N = 6.67 r / s. The system outlet temperature was set to 85 °C.

[0051] Step 3. Rapid cooling and filtration: The aqueous dispersion obtained in step 2 is rapidly cooled to below 40°C using a heat exchanger, and then filtered through a 200-mesh filter within 2 minutes after cooling. The filtrate is collected to obtain the finished aqueous dispersion.

[0052] Example 2: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 40 parts by weight of PHB, 55 parts by weight of high acid content EAA, 4.8 parts by weight of GMA, and 0.2 parts by weight of DCP; the neutralizing agent is a 20wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to make the solid content of the obtained aqueous dispersion 40.0wt%.

[0053] Example 3: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 28 parts by weight of PHBV, 68.2 parts by weight of low-acid content EAA, 3.5 parts by weight of GMA, and 0.3 parts by weight of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 85%; the amount of alkaline neutralizing agent injected in step 1 is calculated based on the mass fraction of acrylic monomer in the low-acid content EAA and the target neutralization degree; the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 43.0 wt% in the resulting aqueous dispersion.

[0054] Example 4: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 35 parts by weight of PHBV, 62.3 parts by weight of high acid content EAA, 2.5 parts by weight of maleic anhydride (MAH), and 0.2 parts by weight of DCP; the reactive compatibilizer used is a solid monomer, added in the same manner as the solid monomer in Example 1; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 90%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 45.0 wt% in the obtained aqueous dispersion.

[0055] Example 5: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 30 parts by weight of PHBV, 66.7 parts by weight of high acid content EAA, 3.0 parts by weight of maleic anhydride (MAH), and 0.3 parts by weight of DCP; the reactive compatibilizer used is a solid monomer, added in the same manner as the solid monomer in Example 1; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 95%; the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 41.0 wt% in the obtained aqueous dispersion.

[0056] Example 6: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the aqueous dispersion of the PHA-g-EAA graft copolymer, except that: the formulation in this example is 30 parts by weight of PHBV, 65.0 parts by weight of high acid content EAA, 4.8 parts by weight of GMA, and 0.2 parts by weight of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 85%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 41.0 wt% in the obtained aqueous dispersion.

[0057] Example 7: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 56.5 parts by weight of PHBV, 40.0 parts by weight of high acid content EAA, 3.0 parts by weight of GMA, and 0.5 parts by weight of DCP; the neutralizing agent is a 20wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 41.0wt% in the obtained aqueous dispersion.

[0058] Example 8: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 20 parts by weight of PHBV, 76.5 parts by weight of high acid content EAA, 3.0 parts by weight of GMA, and 0.5 parts by weight of DCP; the neutralizing agent is a 20wt% potassium hydroxide aqueous solution with a neutralization degree of 90%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 43.0wt% in the obtained aqueous dispersion.

[0059] Example 9: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 30 parts by weight of PHBV, 64.0 parts by weight of high acid content EAA, 5.0 parts by weight of GMA, and 1.00 parts by weight of DCP; the neutralizing agent is a 20wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 42.0wt% in the obtained aqueous dispersion.

[0060] Example 10: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 30 parts by weight of PHBV, 69.49 parts by weight of high acid content EAA, 0.5 parts by weight of GMA, and 0.01 parts by weight of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 42.0 wt% in the obtained aqueous dispersion.

[0061] Example 11: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 30.0 parts by weight of PHBH, 65.0 parts by weight of high acid content EAA, 4.5 parts by weight of GMA, and 0.5 parts by weight of DCP; the neutralizing agent is a 20wt% sodium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 42.0wt% in the obtained aqueous dispersion.

[0062] Example 12: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 30 parts by weight of P34HB, 65 parts by weight of high acid content EAA, 4.5 parts by weight of GMA, and 0.5 parts by weight of DCP; the neutralizing agent is a 20 wt% lithium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 41.0 wt% in the obtained aqueous dispersion.

[0063] Example 13: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 30 parts by weight of PHBV, 66.5 parts by weight of high acid content EAA, 3.0 parts by weight of itaconic anhydride, and 0.5 parts by weight of benzoyl peroxide (BPO); the reactive compatibilizer used is a solid monomer, added in the same manner as the solid monomer in Example 1; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 42.0 wt% in the obtained aqueous dispersion.

[0064] Example 14: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the PHA-g-EAA graft copolymer aqueous dispersion, except that: the formulation in this example is 30 parts by weight of PHBV, 65.5 parts by weight of high acid content EAA, 4.0 parts by weight of allyl glycidyl ether, and 0.5 parts by weight of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 42.0 wt% in the obtained aqueous dispersion.

[0065] Example 15: The preparation steps of the aqueous dispersion in this example are the same as those in Example 1 for the preparation of the aqueous dispersion of the PHA-g-EAA graft copolymer, except that: the formulation in this example is 30 parts by mass of mixed PHA (15 parts by mass of PHB + 15 parts by mass of PHBH), 65 parts by mass of high-acid-content EAA, 4.5 parts by mass of GMA, and 0.5 parts by mass of DCP; the neutralizing agent is a 20wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to achieve a solid content of 42.0wt% in the obtained aqueous dispersion.

[0066] Comparative example:

[0067] Comparative Example 1: This comparative example uses physical blending to prepare an aqueous dispersion. First, a high-acid-content EAA emulsion was prepared: the high-acid-content EAA was added to the melt mixing section of a co-rotating twin-screw extruder and heated to 160°C to form a melt. A 20wt% potassium hydroxide aqueous solution was introduced at the alkaline neutralizer introduction point for online neutralization, achieving a neutralization degree of 100%. The amount of potassium hydroxide aqueous solution injected was calculated based on the mass fraction of acrylic acid monomer w(AA) in the EAA: n(COOH) = m(EAA) × w(AA) / 72.06, m(KOH) = n(COOH) × 56.11, m(KOH solution) = m(KOH) / 0.20. Taking a high-acid-content EAA dosage of 650.00g and w(AA) = 0.205 as an example, n(COOH) = 1.8492mol, m(KOH) = 103.76g, corresponding to a 20wt% potassium hydroxide aqueous solution mass of 518.78g. A vacuum exhaust section is set downstream of the neutralization stage, and the vacuum degree is controlled at -0.08MPa to remove the moisture and volatile small molecules introduced during neutralization, thus obtaining a neutralized melt. The resulting neutralized melt was immediately fed into a jacketed, cooled high-shear dispersion vessel. The jacket circulating cooling water temperature was set to 10°C. Deionized water was injected in stages under stirring conditions, and a high-shear disperser was used to shear the emulsion at 10,000 r / min for 10 min to adjust the solid content to 45.0 wt%. This 45.0 wt% solid content, as determined by ISO 3251:2019, represents the mass fraction of all non-volatile components in the high-acid-content EAA emulsion, including the non-volatile components corresponding to the neutralized EAA salt. During shearing, the system temperature was controlled below 40°C through jacket cooling. After shearing, the emulsion was allowed to stand at 25°C for 30 min to defoam, yielding the high-acid-content EAA emulsion. Then, based on non-volatile components, the PHBV aqueous emulsion and the high-acid-content EAA emulsion were 30.00 parts by mass and 65.00 parts by mass, respectively; the PHBV aqueous emulsion used was BioTen1031 from Dubaicheng, with a solid content of 40.0 wt%; 75.00 g of PHBV aqueous emulsion and 144.44 g of the above-mentioned high-acid-content EAA emulsion were weighed and 12.27 g of deionized water were added, and the mixture was stirred mechanically at 1000 r / min for 15 min at 25°C to obtain the aqueous dispersion of Comparative Example 1; the solid content of the dispersion was measured to be 41.0 wt% according to ISO 3251.

[0068] Comparative Example 2: A basic aqueous dispersion was first prepared according to the method of Comparative Example 1, except that 2.00 parts by weight of adipic acid dihydrazide and 1.00 parts by weight of diacetone acrylamide were added to the total non-volatile components of the basic aqueous dispersion of Comparative Example 1. 231.70 g of the basic aqueous dispersion prepared according to the method of Comparative Example 1 was weighed, and 1.00 g of diacetone acrylamide and 2.00 g of adipic acid dihydrazide were added sequentially. The mixture was stirred mechanically at 1000 rpm for 30 min at 25 °C. Then, 4.32 g of deionized water was added and stirring continued for 5 min, so that the solid content of the resulting Comparative Example 2 dispersion, as measured by ISO 3251, was 41.0 wt%.

[0069] Comparative Example 3: A basic aqueous dispersion was first prepared according to the method of Comparative Example 1, with the difference that zinc acetate dihydrate was added to the total amount of non-volatile components of the basic aqueous dispersion in Comparative Example 1, with 0.30 parts by mass (calculated as zinc ions). The molar mass of zinc acetate dihydrate is 219.50 g / mol, the molar mass of zinc is 65.38 g / mol, and the mass fraction of zinc ions is 65.38 / 219.50 = 0.2978. Therefore, 0.30 parts by mass (calculated as zinc ions) corresponds to 1.01 parts by mass of zinc acetate dihydrate added. Weigh 231.70 g of the basic aqueous dispersion prepared according to the method of Comparative Example 1, add 1.01 g of zinc acetate dihydrate, and mix thoroughly at 25°C with mechanical stirring at 1000 r / min for 10 min; then add 1.46 g of deionized water and continue stirring for 5 min, so that the solid content of the resulting Comparative Example 3 dispersion is 41.0 wt% as measured by ISO 3251.

[0070] Comparative Example 4: A basic aqueous dispersion was first prepared according to the method of Comparative Example 1, except that 0.50 parts by weight of alkylphenol polyoxyethylene ether emulsifier was added to the total non-volatile components of the basic aqueous dispersion of Comparative Example 1. 231.70 g of the basic aqueous dispersion prepared according to the method of Comparative Example 1 was weighed, and 0.50 g of alkylphenol polyoxyethylene ether emulsifier was added. The mixture was stirred mechanically at 1000 r / min for 15 min at 25°C. The system was then placed in a 40°C constant temperature forced-air drying oven for concentration, and weighed every 10 min until the total mass of the system was 227.38 g. The resulting dispersion of Comparative Example 4 had a solid content of 42.0 wt% as determined by ISO 3251. Comparative Example 5: The preparation steps of the aqueous dispersion in this comparative example are the same as those in Example 1 for preparing the aqueous dispersion of the PHA-g-EAA graft copolymer, except that: the formulation of this comparative example is 30 parts by weight of PHBV, 65 parts by weight of high acid content EAA, and 0.5 parts by weight of DCP, without the addition of GMA; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to obtain a solid content of 42.0 wt% in the dispersion.

[0071] Comparative Example 6: The preparation steps of the aqueous dispersion in this comparative example are the same as those in Example 1 for preparing the aqueous dispersion of the PHA-g-EAA graft copolymer, except that: the formulation of this comparative example is 30 parts by mass of PHBV, 65 parts by mass of high acid content EAA, 4.5 parts by mass of GMA, and 0.5 parts by mass of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 50%; and the amount of deionized water injected in step 2 is adjusted to obtain a solid content of 42.0 wt% in the dispersion.

[0072] Comparative Example 7: The preparation steps of the aqueous dispersion in this comparative example are the same as those in Example 1 for preparing the aqueous dispersion of the PHA-g-EAA graft copolymer, except that: the formulation of this comparative example is 70 parts by weight of PHBV, 30 parts by weight of high acid content EAA, 4.5 parts by weight of GMA, and 0.5 parts by weight of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to obtain a solid content of 38.0 wt% in the dispersion.

[0073] Comparative Example 8: The preparation steps of the aqueous dispersion in this comparative example are the same as those in Example 1 for preparing the aqueous dispersion of the PHA-g-EAA graft copolymer, except that: the formulation of this comparative example is 10 parts by mass of PHBV, 90 parts by mass of high acid content EAA, 4.5 parts by mass of GMA, and 0.5 parts by mass of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to obtain a solid content of 43.0 wt% in the dispersion.

[0074] Comparative Example 9: The preparation steps of the aqueous dispersion in this comparative example are the same as those in Example 1 for preparing the aqueous dispersion of the PHA-g-EAA graft copolymer, except that: the formulation of this comparative example is 30 parts by mass of PHBV, 65 parts by mass of high acid content EAA, 7.0 parts by mass of GMA, and 0.5 parts by mass of DCP; the neutralizing agent is a 20 wt% potassium hydroxide aqueous solution with a neutralization degree of 100%; and the amount of deionized water injected in step 2 is adjusted to obtain a solid content of 42.0 wt% in the dispersion.

[0075] Application example:

[0076] Application Example 1: Basic performance and stability testing of aqueous dispersions.

[0077] Experimental Description: This test aims to evaluate the colloidal physicochemical properties, core-shell morphology, and long-term storage stability of the prepared aqueous dispersion. Solid content was determined using the oven drying method. 1.000 g of sample was weighed and placed in a 75 mm diameter glass petri dish, then dried in a 105 °C oven to constant weight (the difference between two weighings was less than 0.002 g). The proportion of non-volatile components was calculated. pH was measured using a pH meter equipped with a composite electrode, directly measuring the stock solution at a constant temperature of 25 °C. The reading stabilization time was set to 30 s. Before particle size analysis, the sample was diluted with 0.22 μm filtered deionized water to a polymer solids mass fraction of 0.050 wt% to avoid multiple scattering effects. The diluted solution was then degassed in an ultrasonic cleaner for 2 min. Particle size was measured using a dynamic light scattering instrument at 25 °C with a scattering angle set to 90°. Each sample was scanned three times consecutively, and the average value was taken. In this manual, D50 is defined as the median particle size of the volume distribution calculated by the instrument software from the scattering intensity distribution according to the Mie scattering model, and PDI is defined as the polydispersity index obtained by the cumulative method. Before the Zeta potential test, the sample was diluted with 1.0 mmol / L potassium chloride solution to a polymer solids mass fraction of 0.050 wt%. The diluent was degassed for 2 min, and the electrophoretic mobility μ was determined by electrophoretic light scattering in a folded capillary sample cell. The Zeta potential was then calculated using the Smoluchowski formula ζ = μη / ε. Storage stability was assessed by placing the sealed sample in a 25°C incubator for 90 days, re-measuring D50, and calculating the particle size change rate as ΔD50 = (D50,90d - D50,0d) / D50,0d × 100%. For the centrifugation stability test, 25 mL of sample was placed in a centrifuge tube and treated under a centrifugal force of 20000 g for 30 min. After standing, the height of the supernatant h (supernatant) and the total height of the sample h (total) were measured. The volume fraction of the stratification was calculated as V (stratification) = h (supernatant) / h (total) × 100%. When there was no visible supernatant stratification or sedimentation layer, it was recorded as 0%.

[0078] Core-shell structure characterization: One part each of Examples 1, 3, 11, 13, 15, and Comparative Example 1 was taken. The dispersion was diluted with deionized water filtered through a 0.22 μm filter to a polymer solids mass fraction of 0.0050 wt%. 5 μL was added to a carbon film copper grid, and after standing for 1 min, excess droplets were absorbed with filter paper. Subsequently, 2.0 wt% phosphotungstic acid solution was added for negative staining for 30 s, and then dried again. After drying at room temperature for 30 min, the samples were observed using a transmission electron microscope. In Examples 1, 3, 11, 13, and 15, concentric core-shell morphologies with significant differences were observed, with core diameters ranging from approximately 90 nm to 120 nm and shell thicknesses from 12 nm to 25 nm. The sample in Comparative Example 1 exhibited heterogeneous particle morphology and obvious phase separation characteristics, lacking a stable core-shell structure. Based on the Zeta potential results and alkali-induced swelling response, it was determined that the core was a PHA segment-enriched phase and the shell was an EAA salt segment-enriched phase.

[0079] Grafting structure verification: 2.000 g each of the lyophilized samples from Example 1 and Comparative Example 5 were subjected to Soxhlet extraction for 8 h using xylene / ethanol (volume ratio 9:1) as solvent to remove free EAA components; subsequently, they were vacuum dried at 50 °C to constant weight. The extraction residues of Example 1 and Comparative Example 5 were 84.6% and 52.4%, respectively. FTIR analysis of the extraction residues was performed; Example 1 showed a FTIR of 1720 cm⁻¹. -1 The absorption peak of the ester group C=O is at 1700 cm⁻¹ -1 Nearby carboxyl-related absorption peaks are also present, and at 910 cm⁻¹ -1 The characteristic absorption peaks related to nearby epoxy groups were significantly weakened; no such changes were observed in Comparative Example 5. Combined with the difference in extraction residual rates, it can be determined that a graft structure was formed between PHA and EAA in Example 1.

[0080] Table 4. Results of basic properties and stability tests of aqueous dispersions:

[0081]

[0082] Explanation of the applicability of the table data: The "90-day particle size change (%)" for Comparative Examples 1, 3, 5, 6, and 7 is marked with "-" because these samples exhibited significant stratification (stratification volume fraction ≥ 12.0%) in the initial centrifugation test, or their particle size distribution was extremely non-uniform (e.g., the bimodal distribution of Comparative Example 1, and the micron-sized particles of Comparative Example 7), indicating that the system was thermodynamically unstable. For these samples whose initial stability was deemed unacceptable, conducting a 90-day storage test is of no practical engineering significance, and their particle size data fluctuates drastically over time, making it unreliable; therefore, this data is not included in the statistics. Furthermore, Comparative Example 1 is not included in the "D" section of Table 4. 50In the "(nm)" column, "85 / 620" indicates that the two peak particle sizes of its volume distribution are 85nm and 620nm, respectively; since the particle size distribution of this sample is a significant bimodal distribution, its median particle size D in the volume distribution is... 50 It lacks representativeness and therefore a single D is not reported separately. 50 Numerical value.

[0083] Analysis: As can be seen from the test data in Table 4, the dispersions prepared in Examples 1-15 of this invention exhibit excellent and highly consistent colloidal stability. The D of these samples... 50 The particle sizes were all controlled between 120 nm and 165 nm, the polydispersity index (PDI) was ≤0.22, and the absolute value of the zeta potential was maintained above 30 mV. This indicates that a stable core-shell structure with EAA salt as the shell was successfully constructed through reactive extrusion grafting and online neutralization processes, providing sufficient electrostatic repulsion. This structure ensured that the dispersion did not stratify under 20,000 g high-gravity centrifugation, and the particle size increase rate was controlled within 10% after 90 days of storage. In contrast, the data from the comparative examples revealed the influence of various key factors. Comparative Example 7, due to the excessively high content of hydrophobic PHA (70 parts), exceeded the self-emulsifying ability of the system, leading to phase inversion failure and the formation of a coarse dispersion with a particle size of 5.2 μm and severe stratification. Comparative Example 9, due to excessive reactive monomers, resulted in microgelation, with the particle size significantly increasing to 350 nm and the PDI deteriorating. The physical blend of Comparative Example 1, lacking chemical bonding, exhibited poor compatibility, a bimodal distribution, and a low Zeta potential (-12 mV), leading to severe stratification during centrifugation. Comparative Example 6, with a neutralization degree of only 50% and insufficient hydrophilicity, also suffered from decreased stability. While Comparative Example 8 showed good stability, this was because its main component was highly dispersible EAA (90 parts), which solved the dispersion problem but sacrificed bio-based content. In conclusion, only under specific component ratios and processing conditions can high-performance aqueous dispersions possessing both nanoscale fine particle size and excellent storage stability be obtained.

[0084] Application Example 2: Coating barrier and heat-sealing performance test.

[0085] Experimental Description: This test is used to evaluate the film-forming quality, water / oil / gas barrier properties, and heat-sealing performance of the coating on a paper substrate. The substrate used has a basis weight of 250 g / m³. 2 Grease-barrier folding carton cardboard. Coating was performed using a laboratory coating machine. The wire rod type was selected based on the measured solid content of each dispersion, and the dry coating amount was controlled to be 10.0 g / m² using a weighing method. 2 The weighing method involves recording the difference in dry mass of the same area of ​​sample before and after coating, dividing by the area to obtain the dry coating weight. The dry coating weight error should be controlled to not exceed 0.5 g / m². 2The coated wet film was first dried in an infrared drying device for 2 minutes, until the sample surface temperature reached and was maintained at 90°C (the sample surface temperature was monitored using an infrared thermometer). Then it was transferred to a 110°C forced-air oven for 1 minute to mature. After that, it was conditioned at 23°C and 50% RH for 24 hours.

[0086] The haze test was conducted using a free film (dry film thickness 30μm) coated on a 30μm thick polyethylene terephthalate (PET) release film, and the haze value was read using a haze meter in accordance with GB / T 2410-2008.

[0087] Cobb value is determined according to the Cobb 60s procedure of ISO 535:2023 for water absorption.

[0088] The KIT oil resistance grade is compared and evaluated with reference to TAPPI T 559 cm-22: KIT standard reagents are tested sequentially from grade 1 to grade 12. A specified volume of reagent is dropped onto the coating surface and the specified contact time is maintained. If the reagent does not penetrate, significantly wet or diffuse, or darken in color within the specified time, it is considered to have passed. The highest grade without failure is recorded as the KIT grade.

[0089] Heat seal strength testing was conducted according to ASTM F88 / F88M-23: Using Technique A (unsupported peel), the coated surfaces were heat-sealed together to form a seal. The heat-sealing conditions were: upper end temperature 130℃, lower end temperature 60℃, pressure 0.3MPa, and time 1s. After cooling to 23℃, the sample was cut into 15mm wide strips and peeled 180° at a speed of 300mm / min using an electronic tensile testing machine. The maximum force was recorded. Failure modes were determined according to the following criteria: if fiber tearing occurred in the paper base after peeling and the torn area accounted for ≥50% of the effective peeled area of ​​the seal, it was classified as "paper tear"; if the peeling interface mainly occurred at the coating-paper fiber interface and the fiber residue area was ≤10%, it was classified as "peeling"; if the peeling mainly occurred inside the coating or at the seal as continuous polymer tearing with a paper fiber residue area between 10% and 50%, it was classified as "coating cohesive failure". After each sample is tested, the failure mode is recorded according to the above standards and marked in parentheses after the heat seal strength data in Table 5; if the failure mode of a sample is not recorded in the test, "not recorded" is marked in parentheses.

[0090] The WVTR (water vapor transmission rate) test was conducted using a W3 / 330 water vapor transmission rate tester, following the GB / T21529-2008 standard, "Determination of Water Vapor Transmission Rate of Plastic Films and Sheets - Electrolytic Sensor Method". The coated paper composite sample was tested for apparent WVTR. The sample was prepared by cutting coated paper into circular pieces and using shielding pads to limit the effective test area. The sample edges were sealed with two-component epoxy resin sealant to prevent edge leakage. The test temperature was set to 38℃, the relative humidity on the wet side was set to 90% RH, and a dry carrier gas was introduced on the dry side. The system was considered to have reached steady state when the relative deviation between two consecutive WVTR readings was ≤5% and the time interval between the two readings was ≥10 min. This result is used to characterize the overall barrier performance of the coating / paper composite system.

[0091] Oxygen permeability (OTR) testing was conducted using an OX-2 / 230 oxygen permeability tester. The apparent OTR of the coated paper composite sample was tested according to GB / T 19789-2021, "Test Method for Oxygen Permeability of Plastic Films and Sheets for Packaging Materials – Coulometric Test". The sample was prepared by cutting coated paper into circular pieces and using shielding pads to limit the effective test area. The edges of the samples were sealed with two-component epoxy resin sealant. The test temperature was set to 23℃, and the test humidity was set to 0% RH. When the relative deviation between two consecutive OTR readings automatically output by the instrument was ≤5% and the time interval between the two readings was ≥10 min, the system was considered to have reached steady state, and the OTR result was read. This result is used to characterize the overall barrier performance of the coating / paper composite system.

[0092] Leakage test: Cut the coated paper into 100mm×150mm samples, heat seal three sides to form a paper-based heat-sealed bag (sealing width 10mm, effective sealing length 100mm), add 100.0g of 90℃ hot water into the paper bag and seal it immediately and record the initial mass m0; immerse the sealed paper bag completely in a constant temperature water bath for 30min, the constant temperature water bath temperature is set to 90℃ and the temperature control accuracy is 1℃; after taking it out, wipe the moisture off the outer surface and weigh the mass m1. The leakage is calculated as m(leakage) = m0 - m1.

[0093] Table 5. Test results of coating barrier and heat-sealing performance:

[0094]

[0095] Explanation of the applicability of the table data: All performance data for Comparative Example 7 are marked with "-". This is because the sample has an excessively large particle size and extremely poor stability (see data from Application Example 1), making it impossible to form a uniform and continuous coating on paper substrates or centrifugal membranes using conventional coating processes. Severe particle accumulation and streaks occurred during the coating process, and the coating flaked off in powder form after drying, making it impossible to prepare a sample that meets the test requirements. Therefore, barrier properties, heat seal strength, and free film haze tests could not be performed.

[0096] Analysis: The experimental data clearly demonstrate the decisive influence of different formulations on the microstructure and macroscopic properties of the coating. The extremely low haze of the free films in Examples 1-15 (2.1%-3.8%) indicates that the graft copolymers achieved nanoscale compatibility between the two phases. This dense microstructure endows the coatings with excellent barrier properties (WVTR≤46g / m). 2 ·d, OTR≤140cm 3 / m 2 • d·atm) and oil resistance (KIT grade ≥8). In particular, Examples 2, 6, and 7 achieved optimal oxygen barrier performance through optimized component ratios. Regarding heat-sealing performance, all examples exhibited strength exceeding 12.0 N / 15 mm, with most showing "paper cracking" failure, demonstrating strong adhesion between the coating and the paper substrate and high cohesiveness. In contrast, Comparative Example 1, due to severe phase separation (haze 18.5%), formed a porous and loose structure, resulting in a significant decrease in barrier performance and easy interface peeling during heat sealing. Although Comparative Example 8 showed good film-forming properties, its barrier performance was extremely poor (WVTR as high as 210 g / m²) due to the lack of highly crystalline PHA component (only 10 parts). 2 •d) This resulted in the loss of the high barrier properties of bio-based materials. While Comparative Example 9 had sufficient functional monomers, the excess led to over-crosslinking, increasing coating brittleness and inhomogeneity (haze 12.5%), and significantly reducing heat-sealing strength. This further confirms that only within the formulation range defined in this invention can compatibility, crystallinity, and processability be balanced to obtain a coating that meets the requirements of high-performance packaging.

[0097] Application Example 3: Repulping and recycling performance test of recyclable coated paper.

[0098] Experimental Description: This test aims to simulate the industrial repulping process and evaluate the fiber recovery efficiency and impurity removal of coated paper under specific alkaline conditions; the coated paper sample used was the coated paper prepared in Application Example 2 (dry coating weight 10.0 g / m²). 2Cut the coated paper to be tested into 20mm×20mm fragments, dry them at 105℃ to constant weight, and weigh out the dry basis mass of the coated paper as 20.80g, which is recorded as m (initial). Prepare an alkaline aqueous solution with pH=10.0 (adjusted with potassium hydroxide), and add 2080mL of deionized water to the pulp disintegrator to make the pulp concentration 10g / L; put the paper fragments into the pulp disintegrator preheated to 45℃, set the rotor speed to 3000r / min, and disintegrate for 15min.

[0099] Evaluation of Coating Peeling Morphology: Immediately after the disintegration process, 200 mL of re-sizing pulp was placed in a white tray and allowed to stand for 30 seconds while observing the separation state of the coating from the paper fibers. "Overall peeling" was defined as: the fiber surface no longer exhibiting a continuous film-like coverage, and sheet-like or curled sheet-like coating fragments were observed in the pulp. Subsequently, the pulp was stirred with a glass rod at 60 r / min for 5 seconds, and the sheet-like coating fragments separated from the fiber clumps and no longer firmly adhered to the fiber surface. Evaluation Results: Overall peeling phenomena conforming to the above definition were observed in Examples 1-15 under the aforementioned re-sizing conditions.

[0100] The screening procedure was performed according to CEPI Paper and Board – Recyclability Laboratory Test Method – Part I (Version 3, February 2025): First, a coarse sieve was performed using a Somerville screening device with a 5mm orifice plate. The screening water flow rate was set to 8.6 L / min, and the coarse sieve time was set to 5 min. The coarse sieve pulp was collected. Then, after homogenizing the coarse sieve pulp, 20.00 g of the oven-dry pulp was taken for fine sieve. A 0.15mm slotted sieve plate was used for fine sieve, with the screening water flow rate set to 8.6 L / min and the fine sieve time set to 20 min. The undersize pulp and the oversize residue were collected. The undersize pulp and the oversize residue were dried to constant weight, and the masses m(fiber) and m(residue) were recorded. The theoretical oven-dry fiber mass of the substrate, m(fiber, theoretical), was calculated as m(fiber, theoretical) = m(initial) × 250 / 260, where 250 g / m 2 For substrate quantitative determination, 260g / m 2 The total basis weight of the coated paper is given; under the test conditions, m(fiber, theoretical) = 20.00 g. The fiber recovery rate is calculated as R(fiber) = m(fiber) / m(fiber, theoretical) × 100%; the residue rate on the 0.15 mm slotted sieve is calculated as R(residue) = m(residue) / m(initial) × 100%.

[0101] The determination of the macroscopic adhesive area was performed according to the CEPI method "Measurement of adhesive particles – macrostickies (optional)" and the image analysis specifications of ISO 15360-2:2015: The residue on the sieve was used to prepare filter sheets on a Rapid-Koethen paper feeder according to ISO 5269-2:2004, and then pressed and dried according to ISO 15360-2:2015. A high-resolution scanner was used to image the filter sheets at an optical resolution of 2000 dpi, and the macroscopic adhesive area was measured using image analysis software conforming to ISO 15360-2:2015. The particle size distribution was set to an equivalent diameter of 0.15 mm to 2.0 mm. The macroscopic adhesive area A (mm²) was obtained. 2 / kg). When it is necessary to express the measurement in terms of coated paper area per unit, the basis weight B (g / m²) of the coated paper being tested shall be used. 2 Unit conversion: A (cm) 2 / m 2 )=A(mm 2 / kg)×B / 100000, where B is the basis weight of the coated paper; the basis weight of the coated paper in this specification is 260g / m³. 2 Therefore, A(cm) 2 / m 2 )=A(mm 2 / kg)×0.0026.

[0102] Table 6. Test results of repulping recovery performance:

[0103]

[0104] Explanation of the applicability of the table data: The recovery performance data of Comparative Example 7 is marked with "-". Since this sample could not form an effective coating (see Application Example 2), there was no physical carrier of "coated paper", and subsequent re-pulping, sieving, and adhesive analysis tests could not be performed.

[0105] Analysis: The test results strongly validate the effectiveness of the "alkali-induced swelling and peeling" mechanism of this invention. In Examples 1-15, under mild resizing conditions (pH 10, 45℃), fiber recovery rates were all above 99.0%, with extremely low residue on the sieve (≤0.5%), and the adhesive area was controlled at 0.9 cm². 2 / m 2 The following (far superior to the industry standard of ≤1.0cm) 2 / m 2This indicates that the high-density carboxylate shell in the coating rapidly absorbs water and swells under alkaline conditions, weakening the bond between the coating and the fibers. This causes the coating to peel off as a whole in large, sheet-like form, rather than breaking down into difficult-to-remove fine adhesive particles. Conversely, Comparative Example 2 (chemical crosslinking) and Comparative Example 3 (ionic crosslinking) introduced additional crosslinking networks, restricting the movement and swelling capacity of polymer chains. This resulted in coatings that were difficult to peel off or broke down into fine particles during fragmentation, significantly increasing the adhesive content (up to 16.0 cm⁻¹). 2 / m 2 This also reduced fiber yield. Comparative Example 9, due to excessive monomer causing excessive internal cross-linking, also exhibited a similar negative effect, with the adhesive area reaching 5.5 cm². 2 / m 2 This method fails to meet the requirements for clean recycling. In Comparative Example 1, due to poor component compatibility, the coating easily fractures into tiny fragments during resizing, which pass through the screen or adhere to the fibers, resulting in poor recycling performance. In summary, this invention achieves intelligent and controllable recycling through precise control of the molecular structure without sacrificing water resistance.

[0106] Application Example 4: Test of total fluorine content in the coating free membrane.

[0107] Experimental Description: This test aims to verify the environmental safety of the coating material and determine the total fluorine content in the free membrane samples. The test subject is the free membrane samples prepared according to each formulation, accurately weighed at 0.5000 g. The test method adopts the oxygen bomb combustion procedure specified in EN 14582:2016, combined with the low-range ion chromatography quantitative conditions specified in this specification. First, the sample is placed in an oxygen bomb filled with 3.0 MPa of high-purity oxygen for complete combustion. The combustion products are absorbed using 50.0 mL of deionized water as the absorbent. After combustion, the inner wall of the oxygen bomb is rinsed with the absorbent and the results are combined in the same absorbent. The absorbent is filtered through a 0.22 μm filter membrane and then injected into the ion chromatograph for analysis. Chromatographic conditions: An anion exchange column is used, with sodium carbonate / sodium bicarbonate buffer solution as the eluent, a flow rate of 1.0 mL / min, and a conductivity detector is used to detect the fluoride ion signal. Working curves were established using a series of fluoride ion standard solutions at concentrations of 0.005 mg / L, 0.010 mg / L, 0.020 mg / L, 0.050 mg / L, and 0.100 mg / L. The correlation coefficient R0 was calculated. 2 ≥0.999. Quantitative analysis was performed using the external standard method, and the total fluoride content was calculated based on the sample mass. The results were reported as 'not detected (<5 mg / kg)' or the measured value. The method detection limit was 5 mg / kg.

[0108] Table 7. Test results of total fluorine content in the free membrane coating:

[0109]

[0110] Explanation of the applicability of the table data: Comparative Example 7 is marked with "-". As mentioned earlier, this formulation cannot form a film and therefore cannot provide a solid film sample that meets the requirements for oxygen bomb combustion testing.

[0111] Analysis: Ion chromatography confirmed that the total fluorine content in all embodiments (1-15) and comparative samples capable of film formation was below the method detection limit of 5 mg / kg, and was therefore determined to be undetectable. This result has significant environmental implications, confirming that the excellent oil-repellent properties (KIT grade 8-10) of this coating material originate entirely from the dense crystalline structure and surface energy characteristics of the polymer matrix itself, rather than relying on any fluorine-containing additives.

[0112] Experimental Results and Analysis:

[0113] To comprehensively verify the performance of the PHA-g-EAA aqueous dispersion with a self-emulsifying core-shell structure described in this invention and its application in recyclable paper packaging, this invention conducted systematic testing and comparative analysis by setting up a series of examples (Examples 1-15) and comparative examples (Comparative Examples 1-9), combined with application examples 1-4. The experimental results are analyzed and summarized in detail below.

[0114] Analysis of self-emulsification mechanism and colloidal stability of dispersions:

[0115] According to the data in Application Example 1 (Table 4), Examples 1-15 prepared using the "reactive extrusion grafting-online neutralization-phase inversion emulsification" process described in this invention all yielded milky-white dispersions with uniform appearance and a distinct blue sheen. Their median particle size distribution (D...) 50 The particles are stably distributed between 120 nm and 165 nm, with a polydispersity index (PDI) ≤ 0.22, and the absolute value of the zeta potential remains above 30 mV (-33 mV to -44 mV). This indicates that, through the action of initiators and reactive compatibilizing monomers (such as GMA, MAH, etc.), the hydrophobic PHA segments and the hydrophilic EAA segments successfully achieve chemical bonding, forming a stable amphiphilic graft copolymer. During neutralization and phase inversion, the hydrophilic EAA salt segments spontaneously migrate to the aqueous phase to form a shell, providing sufficient electrostatic repulsion and steric hindrance, thereby encapsulating the hydrophobic PHA core and achieving excellent self-emulsification. Centrifugal stability tests (0% stratification) and 90-day storage stability tests (particle size change rate ≤ 10%) further confirm the thermodynamic and kinetic stability of this core-shell structure.

[0116] In contrast, Comparative Example 1, using physical blending, exhibited a bimodal particle size distribution (85 / 620 nm) and a low Zeta potential (-12 mV) due to the lack of chemical bonding and high interfacial tension between the two phases. Furthermore, it showed severe centrifugal stratification (25.0%), failing to form a stable system. Comparative Example 5, lacking reactive compatibilizers, could not form graft copolymers, leading to difficulties in phase inversion, large particle size (260 nm), and poor stability. Comparative Example 6 had a neutralization degree of only 50%, with insufficient hydrophilic group density to provide adequate emulsifying capacity, resulting in increased particle size and decreased stability. These results strongly suggest that reactive grafting structures and a high degree of carboxyl neutralization are key to achieving self-emulsification and long-term stability of the system.

[0117] Coating microstructure and barrier / heat-sealing performance analysis:

[0118] Based on the data from Application Example 2 (Table 5), the coating free films of Examples 1-15 exhibited extremely low haze (2.1% to 3.8%), indicating that the PHA and EAA components achieved excellent compatibility at the nanoscale, eliminating the scattering interface caused by macroscopic phase separation. This dense coating structure endowed the coated paper with excellent barrier properties, with a WVTR controlled at 24 to 46 g / m². 2 •d, OTR controlled between 95 and 140 cm 3 / m 2 The coating exhibits a heat-sealing strength greater than 12.0 N / 15 mm and an oil resistance rating (KIT) of 8 to 10. Furthermore, the coating demonstrates excellent heat-sealing performance, with heat-sealing strengths exceeding 12.0 N / 15 mm, and the primary failure mode is "paper cracking," indicating a strong bond and cohesiveness between the coating and the paper substrate.

[0119] In contrast, Comparative Example 1 (physical blend) exhibited a haze as high as 18.5%, indicating severe phase separation and porosity within the coating, resulting in a significant decrease in barrier performance (WVTR = 78 g / m²). 2 ·d, OTR=280cm 3 / m 2 The coating exhibited high cross-linking properties (·d·atm), and was prone to interfacial delamination during heat sealing. Comparative Example 9, due to an excess of reactive monomers (7.0 parts), experienced excessive cross-linking and microgelation, leading to an increase in coating haze (12.5%) and brittleness, as well as a significant decrease in heat-sealing strength (9.0 N / 15 mm). This demonstrates that a reasonable grafting ratio is crucial for maintaining coating integrity and processing performance.

[0120] Intelligent recycling mechanism and repulping performance analysis:

[0121] The results of Application Example 3 (Table 6) strongly validate the "alkali-induced swelling and peeling" recovery mechanism proposed in this invention. In Examples 1-15, under simulated industrial resizing conditions (pH 10, 45°C), fiber recovery rates were all above 99.0%, with residue on a 0.15mm slotted screen ranging from only 0.2% to 0.5%, and the macroscopic adhesive surface area was extremely low (0.3 to 0.9 cm). 2 / m 2 This indicates that the EAA carboxylate shell on the coating surface rapidly absorbs water and swells in alkaline hot water. The resulting swelling stress disrupts the interfacial bond between the coating and the fiber, causing the coating to peel off as a whole in a larger sheet form. This makes it easy to remove through screening equipment without breaking it into difficult-to-handle fine adhesives.

[0122] In stark contrast, Comparative Examples 2 (chemical crosslinking) and 3 (ionic crosslinking) introduced additional crosslinking networks, restricting the movement and swelling capacity of polymer chains. This resulted in the coating failing to effectively peel off or break into fine particles, causing a surge in adhesive content (up to 16.0 cm⁻¹, respectively). 2 / m 2 With 14.0cm 2 / m 2 This reduced fiber recycling rate. Comparative Example 4, although using an added emulsifier, showed decreased water resistance in the coating and was prone to producing fine fragments during resizing, with a high adhesive content (8.0 cm³). 2 / m 2 The yield is still significantly higher than in the examples. This confirms that the non-crosslinked, self-emulsifying core-shell structure of the present invention is the core technical means to achieve high yield and low adhesive recovery.

[0123] Environmental safety analysis:

[0124] Application Example 4 (Table 7) shows that the total fluorine content of the coatings in all examples was not detected (<5 mg / kg), proving that the present invention can achieve excellent oil-repellent properties without adding any fluorine-containing compounds, which meets the strict regulatory requirements and development trend of global PFAS-free food contact materials.

[0125] Trend analysis of the impact of component content changes on performance:

[0126] Based on the data from various embodiments and comparative examples, the influence trend of changes in the content of each component on the dispersion and coating performance can be derived:

[0127] PHA content: In the range of 20 to 59.49 parts by mass (Examples 1-15), the barrier properties (especially oxygen barrier properties) of the coating tend to improve with increasing PHA content, and the bio-based content also increases. However, when the PHA content is too high, reaching 70 parts by mass (Comparative Example 7), the system becomes too hydrophobic, exceeding the emulsifying ability of the EAA salt shell, leading to phase inversion failure and the inability to form a stable aqueous dispersion. When the PHA content is too low, only 10 parts by mass (Comparative Example 8), although the dispersion stability is good, the coating loses the high crystallinity barrier properties provided by PHA, and the WVTR and OTR deteriorate significantly (210 g / m², respectively). 2 ·d and 450cm 3 / m 2 (·d·atm). Therefore, controlling the PHA content between 20 and 60 parts by mass is the key to balancing dispersion stability and barrier properties.

[0128] EAA content: EAA primarily provides the hydrophilic shell, heat-sealing properties, and adhesion to the paper substrate. The system maintains good performance within the range of 40 to 79.49 parts by weight.

[0129] Reactive compatibilizing monomer content: The monomer content directly affects the grafting rate and microstructure. An appropriate amount of monomer (0.5 to 5.0 parts by mass, Examples 1-15) ensures good compatibility and particle size control. When no monomer is added (0 parts by mass, Comparative Example 5), chemical grafting cannot be achieved, resulting in coarse and unstable dispersion particles. Excessive monomer (7.0 parts by mass, Comparative Example 9) leads to cross-linking or gelation of the system, deteriorating the optical properties, heat-sealing properties, and recyclability of the coating (increased adhesive content).

[0130] Initiator content: A trace amount of initiator (0.01 to 1.00 parts by mass) can effectively initiate the grafting reaction and achieve performance transition in conjunction with the monomer.

[0131] In summary, this invention successfully prepared a water-based coating material with high barrier properties, excellent heat-sealing properties, superior colloidal stability, and intelligent recyclability by precisely controlling the ratio of PHA to EAA, introducing appropriate amounts of reactive compatibilizers and initiators, and combining specific reactive extrusion and online neutralization processes. This material achieves both "paper-based plastic replacement" and "fluorination removal," while completely solving the industry pain points of difficult recycling and easy generation of adhesive residues in bio-based coatings.

[0132] Those skilled in the art should understand that the above embodiments are merely exemplary and not intended to limit the scope of the invention. The scope of protection of the present invention is defined by the appended claims. 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 scope of protection of the present invention.

Claims

1. An aqueous dispersion of a polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer having a self-emulsifying core-shell structure, characterized in that: The aqueous dispersion comprises an aqueous phase and polymer particles dispersed in the aqueous phase; the polymer particles have a core-shell structure, the core-shell structure comprising a core and a shell, the core comprising hydrophobic polyhydroxy fatty acid ester segments, and the shell comprising hydrophilic ethylene-acrylic acid copolymer salt segments; The polymer microparticles contain an amphiphilic graft copolymer, which is a graft copolymer obtained by reactive extrusion melt grafting of polyhydroxy fatty acid ester and ethylene-acrylic acid copolymer under the action of reactive compatibilizer and initiator. The reactive compatibilizing monomer is selected from one or more of glycidyl methacrylate, glycidyl acrylate, allyl glycidyl ether, vinyl glycidyl ether, maleic anhydride, itaconic anhydride, and citraconic anhydride. The ethylene-acrylic acid copolymer is a binary copolymer obtained by copolymerizing ethylene monomer and acrylic acid monomer, and the carboxyl groups of the ethylene-acrylic acid copolymer are neutralized to form ethylene-acrylic acid copolymer salt; The degree of neutralization of the carboxyl groups in the ethylene-acrylic acid copolymer is 85% to 100%. Based on a total mass of 100 parts by mass of the polyhydroxyalkanoate, the ethylene-acrylic acid copolymer, the reactive compatibilizer and the active ingredient of the initiator, the polyhydroxyalkanoate comprises 20 to 59.49 parts by mass, the ethylene-acrylic acid copolymer comprises 40 to 79.49 parts by mass, the reactive compatibilizer comprises 0.5 to 5.0 parts by mass, and the initiator comprises 0.01 to 1.00 parts by mass.

2. The aqueous dispersion of polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure according to claim 1, characterized in that: The polyhydroxy fatty acid ester is selected from short-chain polyhydroxy fatty acid esters, medium- and long-chain polyhydroxy fatty acid esters, or copolymers that form short-chain polyhydroxy fatty acid ester monomers and medium- and long-chain polyhydroxy fatty acid ester monomers. The short-chain polyhydroxy fatty acid ester is selected from one or more of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), and poly(3-hydroxybutyrate-co-4-hydroxybutyrate). The medium- and long-chain polyhydroxy fatty acid esters are selected from one or more of poly(3-hydroxyhexanoate), poly(3-hydroxyheptanoate), poly(3-hydroxyoctanoate), poly(3-hydroxynonanoate), poly(3-hydroxydecanoate), poly(3-hydroxyundecanoate), poly(3-hydroxydodecanoate), poly(3-hydroxytetrate), poly(3-hydroxytetradecanoate), poly(3-hydroxytetradecanoate), poly(3-hydroxypentadecanoate), poly(3-hydroxyhexadecanoate), poly(3-hydroxyhexadecanoate), poly(3-hydroxyheptadecanoate), and poly(3-hydroxyheptadecanoate).

3. The aqueous dispersion of polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure according to claim 1, characterized in that: The initiator is selected from one or more of dicumyl peroxide, di-tert-butyl peroxide, benzoyl peroxide, tert-butyl peroxybenzoate, 2,5-dimethyl-2,5-di-tert-butylperoxyhexane, 2,5-dimethyl-2,5-di-tert-butylperoxyhexyn-3, azobisisobutyronitrile, and azobiscyclohexanenitrile.

4. The aqueous dispersion of polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure according to claim 1, characterized in that: The cations in the ethylene-acrylic acid copolymer salt are selected from one or more of sodium ions, potassium ions, and lithium ions.

5. The aqueous dispersion of polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure according to claim 1, characterized in that: The aqueous dispersion has a solid content of 40 wt% to 45 wt% and a pH value of 9.0 to 9.5, wherein the solid content is a mass fraction based on the total mass of the aqueous dispersion. The median particle size of the polymer microparticles is 120 nm to 165 nm and the polydispersity index is ≤0.22; The absolute value of the Zeta potential of the aqueous dispersion is ≥30mV; The aqueous dispersion had a 0% volume fraction after centrifugation at 20000g for 30 min; and after being sealed and stored at 25℃ for 90 days, the median particle size change rate of the polymer microparticle volume distribution was ≤10%.

6. The use of the aqueous dispersion of polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer with a self-emulsifying core-shell structure according to claim 1 in the preparation of barrier coatings or heat-sealing coatings for recyclable paper packaging, characterized in that: The paper packaging includes paper cups, paper bowls, lunch boxes, paper lids, paper trays, paper-based heat-sealed bags, or paper-based molded tableware.

7. A method for preparing an aqueous dispersion of a polyhydroxyalkanoate-grafted ethylene-acrylic acid copolymer having a self-emulsifying core-shell structure according to claim 1, characterized in that, The preparation method includes the following steps: Step 1. Reactive extrusion: Polyhydroxy fatty acid ester resin and ethylene-acrylic acid copolymer resin are fed into a twin-screw extruder for melt mixing. During the melt mixing process, reactive compatibilizers and initiators are introduced to reactively extrude and melt graft the polyhydroxy fatty acid ester and ethylene-acrylic acid copolymer to obtain a polymer melt. Step 2. Online neutralization: An aqueous solution of alkaline neutralizing agent is introduced into the polymer melt obtained in Step 1, and the mixture is kneaded to neutralize the carboxyl groups of the ethylene-acrylic acid copolymer into salts, thereby obtaining a neutralized melt; Step 3. Reverse Emulsification: Deionized water is introduced into the neutralized melt obtained in Step 2 in the high-shear kneading zone of the twin-screw extruder to reverse the system and obtain an aqueous dispersion; Step 4. Rapid cooling and filtration: Cool and filter the aqueous dispersion obtained in step 3 to obtain the finished aqueous dispersion. In the preparation method, the amount of external emulsifier or dispersant added is 0 parts by mass.

8. The preparation method according to claim 7, characterized in that: Before step 1, the polyhydroxy fatty acid ester resin and the ethylene-acrylic acid copolymer resin are pre-dried in vacuum at 40°C to 80°C for more than 2 hours until the mass is constant and then cooled for later use. The reactive extrusion in step 1 involves melt mixing and reaction in the range of 130°C to 200°C, with a residence time of 30s to 180s. In step 1, a vacuum exhaust section is set up to remove residual monomers and volatile small molecules. The vacuum degree of the vacuum exhaust section is controlled to be between -0.02MPa and -0.095MPa. The mass fraction of the alkaline neutralizing agent aqueous solution in step 2 is 5 wt% to 40 wt%. Step 3: Maintain the apparent shear rate of the high-shear kneading zone greater than or equal to 500 s. -1 And the system outlet temperature is ≤110℃; Step 4: Rapidly cool the aqueous dispersion to below 50°C, and filter it using a 100-mesh to 300-mesh filter within 10 minutes after cooling.

9. The preparation method according to claim 7, characterized in that: The twin-screw extruder has an aspect ratio ≥ 44:1 and is provided with at least two high-shear kneading zones. The twin-screw extruder is provided with an online neutralization section and a vacuum exhaust section located downstream of the online neutralization section. The vacuum exhaust section is used to remove residual small molecules and moisture introduced during online neutralization. The alkaline neutralizing agent is selected from one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, potassium carbonate, and sodium bicarbonate. The alkaline neutralizing agent is introduced in the form of an aqueous solution through the alkaline neutralizing agent introduction point, and the deionized water in step 3 is introduced through the deionized water injection point, which is located downstream of the alkaline neutralizing agent introduction point. The deionized water described in step 3 is introduced by either single-point continuous injection or multi-point segmented continuous injection.

10. A recyclable coated paper, characterized in that, The recyclable coated paper includes a paper substrate and a coating applied to its surface; The coating is formed by applying and drying the aqueous dispersion of claim 1 into a film; The coating absorbs water and swells under repulping conditions of pH 10 and temperature 45°C, and peels off entirely from the paper fiber surface, with a fiber recovery rate of ≥99% as measured by the CEPI recyclability laboratory test method; Furthermore, the total fluoride content was not detected in the free membrane obtained by drying and peeling the aqueous dispersion, and the method detection limit was 5 mg / kg.

11. The coated paper according to claim 10, characterized in that: The dry coating amount is 10g / m². 2 ; The coated paper, under the conditions of an upper end cap temperature of 130℃, a lower end cap temperature of 60℃, a heat sealing pressure of 0.3MPa, and a heat sealing time of 1s, has a heat sealing strength ≥12N / 15mm as measured by ASTM F88 / F88M-23, and a leakage rate ≤0.1g after immersion in 90℃ hot water for 30min. After repulping, the residue on a 0.15mm slotted sieve, as measured by the CEPI recyclability laboratory test method, is ≤0.5%.

12. A method for recycling paper fibers of coated paper according to claim 10, characterized in that, The recycling method includes the following steps: Step 1. Re-sizing: The coated paper is placed in an alkaline aqueous phase with a pH of 10.0 and stirred at 45°C for 15 minutes to allow the coating to swell and peel off due to alkali and separate from the paper fibers, thereby obtaining re-sizing pulp. The pulp concentration of the stirred re-sizing is 10 g / L. Step 2. Screening: The re-slurry obtained in Step 1 is screened to obtain recycled fibers. The fiber recovery rate, as measured by the CEPI laboratory test method for recyclability, is ≥99%, and the residue on a 0.15mm slit sieve is ≤0.5%. Furthermore, the macroscopic adhesive area, as measured by the macroscopic adhesive area assessment step in the CEPI laboratory test method for recyclability, is ≤1.0 cm². 2 / m 2 The equivalent diameter of the macroscopic adhesive is 0.15 mm to 2.0 mm.

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