Environment-friendly and recyclable fluorine-free bio-based paper protective coating and preparation and application thereof

Abstract

The application discloses an environment-friendly and recyclable fluorine-free bio-based paper protective coating and preparation and application thereof, and belongs to the technical field of paper-based packaging materials.The composition is mainly composed of polyhydroxy fatty acid ester emulsion and water-based acrylic acid polymer with specific acid value and glass transition temperature.The scheme effectively solves the contradiction that the existing bio-based coating has large brittleness, and water resistance and alkaline repulping are difficult to be considered at the same time, by constructing a unique pH response switch without relying on wax, additional crosslinking agent or high content of surfactant.Compared with traditional multi-component or high energy consumption recycling schemes on the market, the application can realize excellent oil resistance, barrier performance and efficient fiber recycling under mild conditions only by using a simplified binary system, significantly reduces the formula complexity and recycling cost, and has excellent environmental benefits and industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of paper-based packaging materials technology, specifically relating to an environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating and its preparation and application. Background Technology

[0002] With increasing global emphasis on environmental protection and sustainable development, "paper instead of plastic" has become a significant trend in the packaging industry. Traditional coated paper, such as polyethylene (PE) coated paper, while possessing excellent waterproof and barrier properties, suffers from difficulties in separating the plastic film from the paper fibers, leading to recycling challenges. It is often incinerated or landfilled as waste, resulting in resource waste and environmental pollution. Furthermore, some fluorinated compounds used in oil-proof packaging, particularly perfluorinated and polyfluoroalkyl substances (PFAS), are strictly restricted worldwide due to their persistence in the environment and potential health risks.

[0003] To address the aforementioned issues, the development of environmentally friendly waterborne barrier coatings has become a research hotspot. Polyhydroxyalkanoates (PHAs), aliphatic polyesters synthesized by microorganisms, possess biodegradability, biocompatibility, and excellent gas barrier properties, making them an ideal sustainable packaging material. However, using PHA alone as a coating material has some inherent drawbacks, such as its high crystallinity, brittle texture, poor film-forming properties at low temperatures, and susceptibility to pinholes, affecting the continuity and barrier stability of the coating. Furthermore, pure PHA coatings are not easily broken down and dispersed effectively in alkaline aqueous environments, making it difficult to achieve the ideal fiber recovery rate of over 95%. Therefore, there is an urgent need in this field for a solution based on the inherent properties of the composition itself, rather than relying on special processing techniques.

[0004] Existing technologies have disclosed various solutions aimed at achieving both barrier properties and recyclability in paper-based materials, but none have provided an ideal solution that simultaneously satisfies high bio-based content, excellent overall barrier performance, high compatibility with existing industrial production lines, and efficient recycling under mild, low-energy conditions. Specifically, existing technological approaches can be mainly categorized as follows, and they have significant shortcomings:

[0005] PHA and binder blending technology: For example, US Patent 12351985 discloses a paper coating constructed from poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) and a binder (which may include acrylic latex). While this technology proposes blending PHA with the binder to improve film-forming properties, its core flaw lies in the complete lack of definition regarding the binder's key physicochemical parameters, such as acid value and glass transition temperature (Tg). Therefore, this patent fails to reveal or teach the intrinsic relationship between these parameters and the system's resizing performance, and provides no specific resizing process conditions or achievable fiber recovery rates. Its technical solution remains at a superficial stage of improving film-forming properties, failing to address the core mechanism for unlocking recyclability. This is fundamentally different from the technical concept of this invention, which constructs a pH-responsive switch by precisely defining the acid value / Tg window.

[0006] Techniques using PHA as a secondary component: For example, US Patent 10590289B2 discloses a technique of adding a small amount of PHA powder as a matting agent to acrylic coatings. In this technique, the acrylic polymer is the main component of the coating, while PHA is only used as a functional additive, and its addition amount is far below the level constituting the main barrier phase. Therefore, the core of this technical route is still the acrylic coating, and its purpose is to adjust the optical properties of the coating surface. It does not use PHA as the main barrier functional phase, nor does it address how to solve the brittleness and recyclability problems of high PHA content systems. This is completely different from the technical solution of this invention, which uses a high content (50-95 parts by weight) of PHA as the main barrier component.

[0007] PHA Aqueous Dispersion Preparation Technology: For example, US patents US7491754B2 and US11866606B2 disclose the preparation of PHA or PHBH aqueous dispersions and their applications in the coating field. The main technical contribution of these documents lies in providing a stable PHA aqueous system, enabling its application as a water-based coating. However, they do not disclose the synergistic effect of compounding PHA aqueous dispersions with polymers possessing specific functions. Specifically, these documents do not propose a technical solution for compounding PHA with water-based acrylic polymers having an acid value of 10–80 mg KOH / g and a Tg range of -20 to +20°C, nor did they foresee the unexpected technical effect of such a specific combination, which combines excellent barrier properties with a fiber recovery rate exceeding 95%.

[0008] Non-PHA-based waterborne barrier coating technologies: To replace PFAS, various PHA-free waterborne barrier coatings have been developed in this field. For example, US patent applications US2020 / 0131708A1 and US2021 / 0348338A1 disclose heat-sealable barrier coatings composed of acrylic emulsions and molten wax. While these technologies achieve PFAS-free and resizing capabilities, they generally suffer from low bio-based content and limited gas barrier performance (especially oxygen barrier performance), making them difficult to match the system of this invention, which uses a high-PHA-content main phase.

[0009] Pure PHA coating technology relying on special processing techniques: To address the recyclability issue of pure PHA coatings, some technological approaches have shifted to special processing techniques. For example, a technology reported in the *TAPPI Journal* (October 2018 issue) relies on a special and energy-intensive photonic curing process to rapidly melt PHA particles into a film. However, this method requires expensive equipment modifications to existing large-scale industrial coating production lines, increasing significant capital investment and operating costs, thus limiting its broad industrial applicability. This contrasts sharply with the applicability of this invention to conventional coating processes.

[0010] In summary, existing fluorine-free waterborne barrier systems still have shortcomings in balancing high bio-based content, water resistance, and consistency with alkaline resizing (fiber recovery rate ≥95%). In particular, the problem of achieving a dual-state response of "neutral high barrier / alkaline easy dissociation" without changing the existing coating line process has not been effectively solved. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of the prior art and provide an environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating, as well as its preparation and application, aiming to solve the problem of the difficulty in balancing barrier performance, environmental friendliness and recycling efficiency in existing paper-based packaging materials.

[0012] To achieve the above objectives, the present invention provides the following technical solution:

[0013] This invention provides an environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition. On a solids weight basis, the composition comprises 10–95 parts by weight of an aqueous polyhydroxyalkanoate emulsion and 5–90 parts by weight of an aqueous acrylic polymer dispersion. The solids content of the polyhydroxyalkanoate aqueous emulsion can be 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, 40 parts by weight, 45 parts by weight, 50 parts by weight, 55 parts by weight, 60 parts by weight, 65 parts by weight, 70 parts by weight, 75 parts by weight, 80 parts by weight, 85 parts by weight, 90 parts by weight, or 95 parts by weight. Correspondingly, the solids content of the aqueous acrylic polymer can be 90 parts by weight, 85 parts by weight, 80 parts by weight, 75 parts by weight, 70 parts by weight, 65 parts by weight, 60 parts by weight, 55 parts by weight, 50 parts by weight, 45 parts by weight, 40 parts by weight, 35 parts by weight, 30 parts by weight, 25 parts by weight, 20 parts by weight, 15 parts by weight, 10 parts by weight, or 5 parts by weight.

[0014] The waterborne acrylic polymer has a glass transition temperature of -10 to +10°C, an acid value of 20–55 mg KOH / g, and a minimum film-forming temperature ≤10°C. The median diameter D of the volume distribution of the composition as a dispersion is... 50 The wavelength is 200–900 nm (ISO 13320:2020), with a solid content of 35–48 wt%, pH 7.3–8.3, and meeting the requirements of total surfactant (solids) ≤0.5 wt% and free from polyvinyl alcohol (PVOH), waxes and alkyl ketone dimers (AKD), added crosslinking agents and inorganic fillers. For example, the glass transition temperature of the aqueous acrylic polymer can be -10°C, -5°C, 0°C, 5°C or 10°C. The acid value of the aqueous acrylic polymer can be 20 mg KOH / g, 25 mg KOH / g, 30 mg KOH / g, 40 mg KOH / g, 50 mg KOH / g or 55 mg KOH / g.

[0015] When the composition is used as a dispersion, the median particle size D in the volume distribution is... 50 The wavelength, as determined according to ISO 13320:2020, is 200–900 nm, for example, it can be 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, or 900 nm. An important characteristic of this composition is that it does not contain organofluorine compounds.

[0016] The polyhydroxy fatty acid ester is selected from one or more of the following: poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), and medium- and long-chain polyhydroxy fatty acid esters;

[0017] The medium- and long-chain polyhydroxy fatty acid esters are copolymers of one or their monomers, such as poly(3-hydroxyhexanoate), poly(3-hydroxyheptanoate), poly(3-hydroxyoctanoate), or poly(3-hydroxydecanoate).

[0018] The aqueous acrylic polymer is polymerized from one or more of the following monomers: methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, styrene, α-methylstyrene, vinyltoluene, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, hydroxyethyl acrylate, hydroxypropyl acrylate, acrylamide, N-ethylacrylamide, N-isobutylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and glycidyl methacrylate.

[0019] To obtain excellent overall performance, the acidic monomers in the waterborne acrylic polymer are selected from one or more of acrylic acid, methacrylic acid, or itaconic acid, and their total molar fraction is 1.0–6.0%, thereby making the polymer acid value within a specific range of 20–55 mg KOH / g.

[0020] To achieve the goals of environmental protection and simple formulation, the composition does not contain artificially added inorganic fillers or external crosslinking agents, but conventional residues introduced by emulsifiers, initiators and neutralizers are permitted.

[0021] The present invention also provides a method for preparing the above composition, the method comprising:

[0022] Step 1. Mix the aqueous emulsion of polyhydroxyalkanoate and the aqueous acrylic polymer dispersion under shear conditions to obtain a preliminary mixed dispersion;

[0023] Step 2. Adjust the solid content of the preliminary mixed dispersion obtained in Step 1 to 35–48 wt% and the pH to 7.3–8.3 to obtain the adjusted dispersion;

[0024] Step 3. Adjust the median particle size D of the volume distribution of the dispersion as described in Step 2. 50 By reaching 200–900 nm, the aforementioned environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition is obtained.

[0025] The present invention further provides a coated paper article having at least one surface coated with a coating formed by curing the aforementioned composition, the coating having a dry weight of 12–30 g / m², for example, 12 g / m², 15 g / m², 18 g / m², 20 g / m², 22 g / m², 25 g / m², 28 g / m², or 30 g / m². The coated paper article satisfies at least one of the following properties:

[0026] a) Kit value ≥ 10, for example, it can reach level 10, 11 or 12;

[0027] b) Cobb 60 ≤18g / m², for example, it can be 18.0g / m², 17.5g / m², 17.0g / m², 16.5g / m², 16.0g / m² or 15.0g / m²;

[0028] c) Under conditions of 38℃ and 90% relative humidity, the water vapor transmission rate (WVTR) is ≤220g / (m²·d), for example, it can be 220g / (m²·d), 215g / (m²·d), 200g / (m²·d), 195g / (m²·d), 180g / (m²·d), 170g / (m²·d), 160g / (m²·d) or 155g / (m²·d);

[0029] d) Under conditions of 23℃ and 50% relative humidity, the oxygen transmission rate (OTR) is ≤1500cc / (m²·d·atm), for example, it can be 1500cc / (m²·d·atm), 1450cc / (m²·d·atm), 1300cc / (m²·d·atm), 1250cc / (m²·d·atm), 1100cc / (m²·d·atm), 1050cc / (m²·d·atm), 900cc / (m²·d·atm) or 850cc / (m²·d·atm);

[0030] e) The elongation at break of the self-supporting film formed after curing the composition is ≥50%, for example, it may be 50%, 60%, 65%, 80%, 90%, 100%, 105%, 110%, 120%, 130%, or 150%; and

[0031] f) Under repulping conditions of 45–55℃, pH 8.8–9.2, specific energy consumption of 35–55 kWh / t, and 0.15 mm slotted sieve, the fiber recovery rate is ≥95%. Specific energy consumption values ​​can be 35 kWh / t, 40 kWh / t, 45 kWh / t, 46 kWh / t, 47 kWh / t, 48 kWh / t, 50 kWh / t, or 55 kWh / t.

[0032] The paper base material of the coated paper product is selected from one or more of white cardboard, cup paper, kraft paper, kraft paper, and food-grade coated base paper. The coated paper product can be made into various forms, such as food packaging, cups, bowls, plates, or straws.

[0033] The coated paper product has a Kit value ≥ 12 and a Cobb value ≥ 12. 60 ≤17g / m².

[0034] The self-supporting film formed after curing of the environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition has an elongation at break of ≥100%.

[0035] like Figure 1 As shown in the figure, this is a schematic diagram of the structure of the environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating of the present invention coated on a paper substrate. 1 represents the paper substrate as a support layer, and 2 represents the protective coating cured and attached to the surface of the substrate. In the microstructure of the protective coating 2, 3 is a polyhydroxyalkanoate (PHA) enriched discrete phase, which acts as a core barrier component dispersed in the system to provide oil and gas barrier properties. 4 is a water-based acrylic polymer continuous phase, which encapsulates PHA particles during film formation to form a dense network, giving the coating flexibility and integrity. 5 indicates the direction of action under resizing conditions, illustrating that in an alkaline recycling environment (e.g., pH 8.8–9.2), the aqueous medium preferentially permeates through the pH-responsive acrylic polymer continuous phase, inducing directional swelling and structural disintegration of the coating, thereby achieving efficient fiber recycling.

[0036] Compared with the prior art, the use of this invention can achieve the following significant beneficial effects:

[0037] The unity of environmental friendliness and high performance: The composition of this invention does not use any fluorine-containing compounds, and at the same time utilizes PHA with high bio-based content as the main barrier component, achieving excellent functions of oil and water resistance, water vapor and oxygen barrier, providing a safe and environmentally friendly solution for paper packaging in food contact and other fields.

[0038] Superior resizing performance: By limiting the acidic monomer composition of the acrylic polymer and neutralizing it with KOH, and setting the polymer acid value to 20–55 mg KOH / g and Tg to -10 to +10℃, a dense continuous phase is formed under neutral conditions. ≥95% fiber recovery rate is achieved through ionization swelling in a resizing environment of pH 8.8–9.2.

[0039] Simplified Structure and Process Adaptability: This invention uses only two polymers, PHA and waterborne acrylic acid, resulting in a simple formulation structure and easy quality control. The resulting stable waterborne dispersion is suitable for various industrial coating methods such as roller coating and blade coating, offering a wide process window and easy implementation on existing production lines.

[0040] Unexpected synergistic effects: This invention is not a simple linear superposition of the properties of PHA and acrylic polymers. By introducing a flexible acrylic polymer with a specific parameter range, it not only compensates for the defects of pure PHA coatings, such as high brittleness and poor water resistance, but more importantly, its fiber recovery rate is improved to an excellent level of over 95%, while the barrier properties are effectively maintained, breaking the technical bottleneck of the traditional understanding that water resistance and alkali hydrolysis are mutually exclusive. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the structure with the coating applied to the paper substrate.

[0042] In the figure, 1-paper-based substrate; 2-protective coating; 3-polyhydroxyalkanoate (PHA) enriched discrete phase; 4-waterborne acrylic polymer continuous phase; 5-direction of action under re-sizing conditions. Detailed Implementation

[0043] 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.

[0044] Table 1. Main reagent and raw material names, product models and manufacturers:

[0045]

[0046] Table 2 mainly analyzes the names, models, and manufacturers of the testing instruments:

[0047]

[0048] Main testing methods and standards:

[0049] Particle size distribution: ISO 13320:2020;

[0050] Tensile properties: ASTM D882-18;

[0051] Heat seal strength: ASTM F88 / F88M-23;

[0052] Oil resistance (Kit): TAPPI / ANSI T 559cm-12 (R2022);

[0053] Cobb absorbency 60 ): TAPPI T 441 om-24; ISO 535: 2023;

[0054] Water vapor transmission rate (WVTR): ASTM F1249-20;

[0055] Oxygen Transmission Rate (OTR): ASTM F1927-20;

[0056] Recyclability performance: CEPI Recyclability Laboratory Test Method – Part I, sieving using a 0.15mm slotted sieve;

[0057] Fiber screening / residue: TAPPI / ANSI T 275 sp-23, sieve plate size 0.15mm;

[0058] Contact angle: ISO 19403-2:2024; GB / T 30693-2014;

[0059] Acid value / base consumption titration: ASTM D974-22;

[0060] Minimum film-forming temperature (MFFT): ASTM D2354-10 (2023);

[0061] Coating weight / basic weight: ISO 536:2019.

[0062] General preparation process:

[0063] General preparation process for homemade PHA aqueous dispersion:

[0064] Step 1. PHA powder drying: Place the PHA powder (PHBV or PHBH) in a vacuum oven at 60°C and dry for 12 hours.

[0065] Step 2. Aqueous phase preparation: Prepare an aqueous phase and control the dosage of APG and SDS to be 0.35wt% APG, 0.15wt% SDS, and 0.10wt% sodium alginate based on the active ingredients. All percentages are based on the total solids of the final dispersion. Heat to 90°C and stir until completely dissolved.

[0066] Step 3. PHA melting: Place the dried PHA powder in a reactor equipped with stirring and nitrogen protection, and heat it to 160–170°C to completely melt it into a viscous liquid.

[0067] Step 4. Pre-emulsification: The hot water phase prepared in Step 2, maintained at 85–90°C, is placed in a high-speed shear emulsifier. At a shear rate of 8000 rpm, the molten PHA from Step 3 is slowly and continuously added to the aqueous phase for approximately 10 minutes.

[0068] Step 5. High-pressure homogenization: Transfer the pre-emulsion obtained in step 4 to a high-pressure homogenizer and homogenize it for 3–5 cycles at a pressure of 600–800 bar.

[0069] Step 6. Cooling and Volume Adjustment: Rapidly cool the homogenized thermal dispersion to room temperature (e.g., 25°C) using a heat exchanger. After cooling, replenish the water lost due to evaporation with deionized water and precisely adjust the solid content to the target value (e.g., 40 wt%).

[0070] Step 7. pH adjustment: Adjust the pH of the dispersion to 7.5–8.0 using a 10% potassium hydroxide (KOH) aqueous solution, and record the degree of polymer neutralization (molar) ≥60%.

[0071] Step 8. Defoaming: Add a small amount (e.g., 0.2 wt%) of non-silicone defoamer and stir at low speed.

[0072] Step 9. Filtration: Filter the final dispersion through a 100-mesh (approximately 150 micrometers) filter.

[0073] Step 10. Characterization: Using a laser particle size analyzer, the median diameter (D) of the obtained PHA aqueous dispersion was determined according to ISO 13320 standard. 50 The wavelength is 310nm.

[0074] Preparation of styrene-acrylate copolymer emulsion (SAA-10 / 30):

[0075] Target parameters: Glass transition temperature (Tg) of +10℃, acid value of 30 mg KOH / g, minimum film-forming temperature (MFFT) ≤10℃, median particle size distribution (D) 50 The wavelength is 150nm.

[0076] Preparation process:

[0077] Step 1. In a four-necked flask equipped with a mechanical stirrer, reflux condenser, thermometer, and dropping funnel, add deionized water and an anionic surfactant that does not contain alkylphenol polyoxyethylene ether (APEO). Turn on the stirrer, introduce nitrogen gas, and begin heating to 80°C.

[0078] Step 2. In a beaker, add each monomer sequentially according to the molar ratio of styrene / butyl acrylate / methacrylic acid = 40 / 55 / 5. Then add deionized water and emulsifier, and pre-emulsify for 15-30 minutes under high-speed stirring to form a stable and uniform milky white pre-emulsion.

[0079] Step 3. When the temperature inside the reactor reaches 80°C, add approximately 10% of the monomer pre-emulsion to the reactor. Then add approximately 20% of an ammonium persulfate (APS) aqueous solution, raise the temperature to 85°C, and react for 15-20 minutes.

[0080] Step 4. At 85°C, begin simultaneously and uniformly adding the remaining monomer preemulsion and initiator aqueous solution. The addition process should be completed within 2.5-3 hours, maintaining the temperature inside the reactor at 85±2°C.

[0081] Step 5. After the addition is complete, continue to keep the reaction at 85℃ for 1-2 hours.

[0082] Step 6. Cool the system to approximately 60°C. Add a redox initiator pair (e.g., tert-butyl hydroperoxide / sodium bisulfite).

[0083] Step 7. After cooling to room temperature (below 40°C), slowly add a 10% potassium hydroxide aqueous solution dropwise while stirring to adjust the pH of the emulsion to 7.5–8.5. Filter through a 100-mesh filter.

[0084] Step 8. Detect the solid content, pH value, viscosity, and particle size of the final product. Determine the Tg by differential scanning calorimetry (DSC), determine the acid value by titration (ASTM D974-22), and determine the minimum film-forming temperature on an MFFT tester.

[0085] Preparation of acrylate copolymer emulsion (AA-(-10) / 50):

[0086] Target parameters: Tg = -10℃, acid value = 50 mg KOH / g, MFFT ≤ 10℃, median particle size distribution (D) 50 The wavelength is 160nm.

[0087] The preparation process of this emulsion is basically the same as that of the styrene-acrylate copolymer emulsion (SAA-10 / 30), the only difference being the monomer formulation in step 2.

[0088] Step 2 Difference: The monomer formulation is prepared according to the ratio of butyl acrylate / methyl methacrylate / acrylic acid = 60 / 35 / 5 (molar ratio).

[0089] Preparation of low acid value styrene-acrylate emulsion (SAA-LAV-3):

[0090] Target parameters: Tg = +10℃, acid value = 3 mg KOH / g, median particle size distribution (D) 50 The wavelength is 180nm.

[0091] The preparation process of this emulsion is basically the same as that of the styrene-acrylate copolymer emulsion (SAA-10 / 30), the only difference being the monomer formulation in step 2.

[0092] Step 2 Difference: The monomer formulation is prepared according to the ratio of styrene / butyl acrylate / methacrylic acid of 41 / 58.5 / 0.5 (molar ratio).

[0093] Preparation of high acid value styrene-acrylate emulsion (SAA-HAV-100):

[0094] Target parameters: Tg = +10℃, acid value = 100 mg KOH / g, median particle size distribution (D) 50 The wavelength is 130nm.

[0095] The preparation process of this emulsion is basically the same as that of the styrene-acrylate copolymer emulsion (SAA-10 / 30), the only difference being the monomer formulation in step 2.

[0096] Step 2 Difference: The monomer formulation is prepared according to the ratio of styrene / butyl acrylate / methacrylic acid of 35 / 50 / 15 (molar ratio).

[0097] Preparation of high Tg styrene-acrylate emulsion (SAA-HTg-45):

[0098] Target parameters: Tg = +45℃, acid value = 30 mg KOH / g, median particle size distribution (D) 50 The wavelength is 155nm.

[0099] The preparation process of this emulsion is basically the same as that of the styrene-acrylate copolymer emulsion (SAA-10 / 30), the only difference being the monomer formulation in step 2.

[0100] Step 2 Difference: The monomer formulation is prepared according to a styrene / butyl acrylate / methacrylic acid ratio of 65 / 30 / 5 (molar ratio).

[0101] Example:

[0102] Following the formulations shown in Table 3 below, the metered aqueous acrylic polymer emulsion was slowly added to the PHA aqueous emulsion and mixed at room temperature. The median particle size D of the final volume distribution of the resulting composition dispersion is... 50This is a technical parameter that can be controlled by adjusting the mixing shear rate. For example, using a higher shear rate, such as 800 rpm, for conventional mixing can produce D... 50 The dispersion was in the 200-350 nm range; however, by employing a lower shear rate, such as within the 100-400 rpm range, controlled agglomeration between particles could be promoted, resulting in a larger stable particle size. After mixing, the pH was adjusted with a 10% potassium hydroxide solution, and the final solids content was adjusted with deionized water. The resulting mixed dispersion was allowed to stand to degas, and then filtered through a 100-mesh filter for later use.

[0103] Table 3. Formulations of the Examples (by solid weight):

[0104]

[0105] Note: Examples 1-4 and Examples 7-10 used SAA-10 / 30 emulsion; Examples 5 and 6 used AA-(-10) / 50 emulsion; D in the table 50 Solid content and pH are parameters of the final emulsion after mixing waterborne acrylic polymer emulsion and PHA waterborne emulsion.

[0106] Comparative example:

[0107] The preparation methods of Comparative Examples 1-12 are the same as those of the Examples, only the formulations are different. Comparative Example 1 uses only a styrene-acrylate copolymer emulsion. Comparative Example 2 uses only a PHBV aqueous emulsion. Comparative Example 3 uses the opposite component ratio to that of the present invention. Comparative Examples 4, 5, and 6 use acrylic emulsions with acid values ​​or Tg outside the defined range. Comparative Example 7 is a PHA and PVOH compound system. Comparative Example 8 is a PHA and AKD system with paraffin wax. Comparative Example 9 is a pure PHBH emulsion. Comparative Example 10 is a pure acrylic double Tg blend. Comparative Example 11 is an externally added crosslinking agent system. Comparative Example 12 is a high surfactant / inorganic filler system. All comparative examples have their solid content adjusted to 40% with water.

[0108] Table 4: Comparative formulations (by solid weight):

[0109]

[0110] Note: Comparative Examples 1, 3, 7, 11, and 12 used SAA-10 / 30 emulsion; Comparative Example 4 used SAA-LAV-3 emulsion; Comparative Example 10 used 50 parts by weight each of SAA-10 / 30 emulsion and AA-(-10) / 50 emulsion; Comparative Example 5 used SAA-HAV-100 emulsion; Comparative Example 6 used SAA-HTg-45 emulsion; D in the table 50Solid content and pH are parameters of the final emulsion after mixing waterborne acrylic polymer emulsion and PHA waterborne emulsion.

[0111] Application example:

[0112] Application Example 1: Barrier performance and oil and water resistance test.

[0113] Experimental Description: This application example aims to evaluate the basic protective performance of the coating composition on paper substrates, including its barrier properties against oil, water, water vapor, and oxygen. Tests covered all examples and comparative examples to systematically compare the effects of different formulations on barrier performance and to verify whether the compositions of this invention can achieve the preset performance targets.

[0114] Methods: The dispersions prepared in Examples 1-10 and Comparative Examples 1-12 were coated onto 70 g / m² base paper using a wire bar coater, achieving a total dry coating weight of approximately 20 g / m² through two coats. After each coat, the paper was dried in an oven at 110°C for 3 minutes. The coating weight was confirmed according to the ISO 536:2019 weighing method (basic weight difference method): samples of the same size were weighed after equilibration at 23°C and 50% RH for 48 hours before and after coating, and the dry coating weight (g / m²) was calculated.

[0115] After equilibration at 23°C and 50% RH for 48 hours, the prepared coated paper samples were subjected to performance testing. Oil resistance (Kit grade) was tested according to TAPPI / ANSI T 559cm-12 (R2022). 60 Tests were conducted according to TAPPI T 441om-24. WVTR was tested at 38°C and 90% RH according to ASTM F1249-20. OTR was tested at 23°C and 50% RH according to ASTM F1927-20.

[0116] Table 5 Performance test results of Application Example 1:

[0117]

[0118] Analysis: Table 5 shows that all Examples 1–10 achieved the highest oil resistance rating of 12. All examples exhibited excellent water resistance, according to Cobb. 60 The values ​​were all below 18 g / m², especially in Examples 2, 3, 5, 6, 7, 9, and 10, where the Cobb values ​​were... 60The optimal range for the value is 16.0–17.0 g / m². Regarding WVTR, all examples met the requirement of ≤220 g / (m²·d). Regarding OTR, most examples, particularly Examples 2, 3, 5, 6, 8, and 10, met the upper limit of ≤1100 cc / (m²·d·atm). With increasing PHA content in the formulation, the gas barrier performance significantly improved, proving that PHA is the main barrier functional phase. In contrast, Comparative Example 1 (pure acrylic acid) and Comparative Example 10 (pure acrylic acid blend) showed poor barrier performance and oil resistance; Comparative Example 9 (pure PHBH), similar to Comparative Example 2 (pure PHBV), exhibited poor water resistance (Cobb). 60 >20 g / m²); Comparative Example 5 (high acid value), Comparative Example 7 (containing PVOH), and Comparative Example 12 (high surfactant) Cobb 60 A deterioration indicates that excessively high acid values ​​or hydrophilic additives can impair water resistance.

[0119] Application Example 2: Re-pulping performance and sieve residue test.

[0120] Experimental Description: The core of this application example is to verify the resizing performance of the coating under simulated industrial recycling conditions, which is a key technical indicator of this invention. The recycling potential of the coating is comprehensively evaluated by measuring fiber recovery rate, sieve residue rate, energy consumption, and time. All examples and comparative examples were tested to highlight the advantages of the proposed formulation in terms of recycling efficiency.

[0121] Methods: Coated paper samples were processed in a laboratory pulper according to the CEPI Recyclability Laboratory Test Method – Part I. Repulping conditions were: temperature 45–55℃, pH 8.8–9.2, and mechanical energy input 35–55 kWh / t. The pulped material was sieved using a fiber sieve equipped with a 0.15 mm slotted sieve. Fiber recovery was calculated as the ratio of the dry weight of the residue to the dry weight of the fibers in the initial coated paper. Specific energy consumption (kWh / t) and pulping time (min) during the pulping process were also recorded.

[0122] Table 6 Performance test results of Application Example 2:

[0123]

[0124] Analysis: Table 6 shows that the fiber recovery rates of Examples 1-10 all exceeded the 95% industrial recovery threshold. The recovery rates of Comparative Examples 2 (pure PHBV), 9 (pure PHBH), 7 (containing PVOH), 8 (containing AKD and wax), 11 (containing crosslinking agent), and 12 (high surfactant) all failed to meet the standard, indicating that pure PHA coatings, systems containing water-soluble polymers, systems containing highly hydrophobic waxes, crosslinking systems, and systems with excessive surfactants are not easily dispersed and recovered efficiently. The most decisive factor was Comparative Example 4 (low acid value), whose fiber recovery rate plummeted to 90.5%, directly proving that the acid value range defined in this invention is essential for achieving pH-responsive, efficient recovery. In contrast, Comparative Examples 3, 5, 6, and 10, although exhibiting higher recovery rates, revealed problems with barrier properties or water resistance in Application Example 1, failing to meet the overall performance requirements.

[0125] Application Example 3: Mechanical and heat-sealing performance testing.

[0126] Experimental Description: This application example is used to evaluate the mechanical durability of the coating and its functionality as a packaging material. Elongation at break reflects the coating's flexibility, preventing cracking during folding; heat-sealing performance is a key indicator of its suitability as a sealed packaging material. Tests covered all formulations to verify the effectiveness of the present invention's compositions in toughening PHA.

[0127] Methods: A self-supporting film was prepared by casting the coating composition onto a polytetrafluoroethylene (PTFE) sheet and drying it. Samples were prepared using a heat sealer at 100°C, 0.2 MPa, and 1.0 s. Tensile peel tests were performed according to ASTM F88 / F88M-23. The elongation at break of the film was tested according to ASTM D882-18. The heat-sealing initiation temperature was determined using a gradient heating method. A valid seal was defined as a peel strength of 1.0 N / 15 mm for a 15 mm sample with no visible leakage at the seal opening.

[0128] Table 7 Performance test results of Application Example 3:

[0129]

[0130] Analysis: Table 7 shows that all Examples 1-10 exhibit excellent flexibility (elongation at break 65%-130%). Among them, Examples 1, 4, and 5 meet the preferred requirement of ≥100% elongation at break. In contrast, Comparative Example 2 (pure PHBV), Comparative Example 9 (pure PHBH), and Comparative Example 6 (high Tg) all exhibit significant brittleness and extremely low elongation at break, demonstrating that the Tg range defined in this invention is crucial for toughening PHA and ensuring the mechanical properties of the coating.

[0131] Application Example 4: Surface wettability / contact angle measurement.

[0132] Experimental Description: This application example characterizes the hydrophobicity of the coating surface by measuring the contact angle and calculating the surface free energy. High hydrophobicity is fundamental to achieving good water resistance (low Cobb value). This test aims to confirm whether the coating surface can still maintain sufficient hydrophobicity after introducing hydrophilic groups (carboxyl groups) into the acrylic polymer.

[0133] Methods: Following ISO 19403-2:2024 and GB / T 30693-2014, under conditions of 23℃ and 50% RH, the static water droplet contact angle (droplet volume 5 μL) and diiodomethane contact angle of coated paper samples were measured using a contact angle meter. ISO 19403-2:2024 was the primary standard, and GB / T 30693-2014 was used for method verification. Values ​​were read 5 seconds after the droplet contacted the sample surface. Five different locations were tested for each sample, and the average value was taken. Based on the measured contact angle data, the free energy of the coating surface was calculated using the Owens–Wendt method.

[0134] Table 8 Performance test results for Application Example 4:

[0135]

[0136] Analysis: Table 8 shows that the coating surfaces of all embodiments exhibit good hydrophobicity (water droplet contact angle > 100°), comparable to pure PHA (Comparative Example 2, Comparative Example 9), and significantly superior to pure acrylic systems (Comparative Example 1, Comparative Example 10). This indicates that the coatings maintain excellent surface hydrophobicity after the introduction of pH-responsive acrylic polymers, which is crucial for achieving good water resistance. Comparative Example 4 (low acid value) and Comparative Example 8 (containing wax) exhibit higher hydrophobicity, but the former sacrifices resizing performance, while the latter leads to adhesive problems. Conversely, Comparative Example 5 (high acid value), Comparative Example 7 (containing PVOH), and Comparative Example 12 (high surfactant) all show a decrease in contact angle due to the introduction of excessive hydrophilic components, verifying the balance and advantage of the formulation of this invention in maintaining surface hydrophobicity.

[0137] Application Example 5: pH-induced swelling and alkali consumption titration test.

[0138] Experimental Description: This application example aims to verify the working principle of the "pH-responsive switch" at the mechanistic level. By measuring the swelling degree of the coating film in an alkaline solution and titrating its alkali consumption, the coating's responsiveness to alkaline environments can be directly quantified. This provides direct mechanistic support for the high fiber recovery rate in Application Example 2. All titrations and neutralization calculations above are based on a KOH system; the neutralization degree (molar) of the acrylic polymer is ≥60%.

[0139] Test method:

[0140] Swelling test: The coating composition was cast into a film and dried. A 20mm × 20mm self-supporting dry film sample was cut and its initial dry weight (m0) was accurately weighed. The sample was completely immersed in a pH 9.0 buffer solution at 25°C for 30 minutes. After removal, excess moisture on the surface was quickly wiped off with filter paper, and its wet weight (m1) was immediately weighed. The swelling degree was calculated using the formula: Swelling degree (%) = (m1 - m0) / m0 × 100%.

[0141] Alkali consumption titration: Accurately weigh a certain amount of dry film sample and extract it thoroughly in a toluene / isopropanol (volume ratio 1:1) mixture (this step is for experimental determination only and does not involve solvents used in preparation or coating processes). Then, following the method of ASTM D974-22, titrate to the endpoint using a standard ethanol solution of potassium hydroxide (KOH) as the titrant and an indicator method. Correct for errors using a blank experiment, and finally calculate the mass of KOH consumed per gram of dry film (unit: mg KOH / g dry film).

[0142] Table 9 Performance test results of Application Example 5:

[0143]

[0144] Analysis: The results in Table 9 explain the differences in resizing performance mechanistically. All examples exhibited significant, controlled swelling behavior (280-450%) in a buffer solution at pH 9.0, a direct result of the effective disintegration of the coating structure under alkaline conditions. Comparative Examples 2 (pure PHBV) and 9 (pure PHBH) showed almost no swelling and extremely low alkali consumption, validating the resizing inertness of pure PHA. Comparative Example 4 (low acid value) showed significantly lower swelling (55%) and alkali consumption (2.8 mg KOH / g) than most examples, indicating insufficient pH responsiveness, which corresponds perfectly to the low fiber recovery observed in Application Example 2. Conversely, Comparative Examples 1, 3, 5, and 10 (high acrylic acid content or high acid value) all showed excessive swelling (>1000%), consistent with their poor water resistance (high Cobb value). Comparative Examples 7 (containing PVOH), 8 (containing wax), 11 (containing crosslinking agent), and 12 (high surfactant) exhibited abnormal swelling behavior and failed to fall within the ideal range required for efficient resizing, further highlighting the uniqueness and necessity of the formulation system of this invention.

[0145] Application Example 6: CEPI three-cycle reslurry performance test.

[0146] Experimental Description: This application example aims to evaluate the coating's ability to retain performance through multiple recycling cycles, simulating a more realistic industrial recycling scenario. Changes in key indicators were monitored by performing three consecutive resizing-coating-resizing cycles on the examples and key comparative examples to verify the recycling robustness of the compositions of this invention.

[0147] Methods: Coated papers from Examples 7, 8, 7, 8, 11, and 12 were selected and subjected to the first repulping process described in Application Example 2. The recovered fibers were used in a laboratory papermaking machine to regenerate recycled paper sheets. After drying and conditioning the recycled paper sheets, the same repulping process was repeated, marking the second cycle. This process was repeated a total of three times. The fiber recovery rate, residue rate, amount of adhesive (image analysis), and ISO brightness of the recycled paper sheets were recorded after each cycle.

[0148] Table 10. CEPI three-cycle resizing performance test results of the examples:

[0149]

[0150] Table 11 Comparative Example: CEPI Three-Cycle Resizing Performance Test Results

[0151]

[0152] Analysis: The data in Tables 10 and 11 show that after three recycling cycles, the fiber recovery rate of the examples remained stable at over 95%, and the degradation of key indicators such as sieve residue, brightness, and adhesive point was controllable, demonstrating excellent recycling robustness. This proves that the formulation design of the present invention can withstand multiple recycling cycles.

[0153] In contrast, several comparative examples showed significant performance degradation. Comparative Examples 2, 4, and 9 (pure PHA or low acid value systems), although completing three cycles, exhibited a continuous decline in recovery rate and significant accumulation of impurities such as adhesive spots, indicating that their recovery efficiency and regenerated pulp quality deteriorated with increasing cycle count. Comparative Examples 7 (containing PVOH), 8 (containing AKD and wax), 11 (containing crosslinking agent), and 12 (high surfactant) showed a significant performance decline after the first repulping, with recovery rates below 90% in the second cycle, indicating severe adhesive contamination and rendering them unworthy of further recovery. This comparison strongly demonstrates that the absence of PVOH, wax, added crosslinking agents, and the limitation of low surfactant content in the formulation of this invention are crucial for achieving stable, repeatable, and efficient recovery.

[0154] Application Example 7: Optimization of the re-slurry window matrix.

[0155] Experimental Description: This application example aims to verify the optimization and necessity of the resizing process window (pH 8.8–9.2, 35–55 kWh / t) defined in this invention. By constructing a two-dimensional matrix of pH versus specific energy consumption, the impact of process parameters deviating from this window on fiber recovery rate is systematically examined.

[0156] Methods: The coated paper from Example 3 was selected for testing. Example 3 was chosen because its formulation ratio (75 parts by weight of PHBV and 25 parts by weight of acrylic polymer) is representative of the present invention, and previous tests have demonstrated that it possesses excellent barrier, mechanical, and resizing properties, making it a well-balanced formulation. Therefore, using it as a benchmark for process window testing can most clearly reflect the impact of process parameter deviations on recovery efficiency. Resizing experiments were conducted under different combinations of pH values ​​(8.5, 9.0, 9.5, 10.0) and different specific energy consumptions (30 kWh / t, 40 kWh / t, 50 kWh / t, 60 kWh / t). Each set of conditions was tested three times, and the average fiber recovery rate was recorded.

[0157] Table 12 Performance test results of Application Example 7 (fiber recovery rate %):

[0158]

[0159] Analysis: The isopleths in Table 12 clearly show that the fiber recovery rate reaches its peak (>97%) within the pH range of 9.0 and specific energy consumption of 40–50 kWh / t. This closely matches the pH range of 8.8–9.2 and specific energy consumption of 35–55 kWh / t specified in this invention. When the pH deviates to 8.5 or the energy consumption is insufficient (30 kWh / t), the recovery rate decreases significantly and cannot consistently reach the 95% threshold. This demonstrates that the resizing window proposed in this invention is a necessary condition for achieving optimal recovery efficiency.

[0160] Application Example 8: Study on the mechanism of low surfactant.

[0161] Experimental Description: This application example aims to verify the mechanism and effect of the limitation of total surfactant ≤0.5wt% in this invention. By comparing the behavior of the standard example and the high-surfactant control in alkaline buffer, the positive effect of the low-surfactant system on pH-responsive swelling is revealed.

[0162] Methods: Self-supporting membranes prepared in Example 10 (total surfactant 0.35 wt%) and Comparative Example 12 (total surfactant 1.0 wt%) were selected. The swelling rate of the membranes during immersion in 0.05 mol / L carbonate buffer at pH 9.0 was monitored using dynamic light scattering or microscopic observation. Simultaneously, alkali consumption titration was performed using the method described in Example 5.

[0163] Table 13 Performance test results of Application Example 8:

[0164]

[0165] Analysis: The results in Table 13 systematically demonstrate the superiority of the low-surfactant system. All examples (total surfactant ≤ 0.5 wt%) exhibited a faster initial swelling rate (15 μm / min) and a higher equilibrium swelling degree (310%) in alkaline media, in stark contrast to Comparative Example 12 with high surfactant (rate 8.5 μm / min, swelling degree 225%). This is because excess surfactant forms micelles or adsorbed layers inside or on the surface of the coating, hindering the effective penetration and action of alkaline ions into the polymer segments, thereby delaying the pH response process.

[0166] The clarity of the titration endpoint also reflects this; the endpoints of all examples were “sharp,” while Comparative Example 12 with high surfactant and Comparative Examples 7 and 8 containing PVOH or wax showed a “tailing” phenomenon, which corroborates the complex interfacial effects in their systems.

[0167] Ultimately, this mechanistic difference is directly reflected in the quality of the recycled paper sheets. All examples (except Example 6) had significantly fewer adhesive points than Comparative Example 12 and other comparative examples with poor repulping performance (2, 4, 7, 8, 9, 11). This fully demonstrates that limiting the total surfactant content to ≤0.5 wt% is a crucial technical characteristic for achieving a rapid, thorough, and clean repulping process and obtaining high-quality recycled fibers.

[0168] Experimental Results and Analysis:

[0169] This invention successfully constructed a high-performance, environmentally friendly, and easily recyclable paper-based barrier coating by compounding an aqueous polyhydroxyalkanoate (PHA) emulsion with an aqueous acrylic polymer having a specific glass transition temperature (Tg) and acid value. The following analysis, based on test data from application examples 1-5, delves into the structure-activity relationship of the composition and the necessity of key parameters.

[0170] The decisive role of composition ratio in core barrier performance:

[0171] The test results of Application Example 1 (Table 5) clearly demonstrate that PHA is the core functional phase for achieving excellent barrier properties in the coating. Comparing Examples 1–5 with Comparative Example 1 (pure acrylic) and Comparative Example 3 (low PHA content), it can be found that all examples exhibit top-tier oil resistance of Grade 12 (Kit grade), far exceeding the pure acrylic coating (Comparative Example 1, Kit grade 6). Regarding gas barrier properties, as the PHA content in the formulation increases (e.g., from 50 parts by weight in Example 1 to 95 parts by weight in Example 2), the OTR significantly decreases from 1450 cc / (m²·d·atm) to 850 cc / (m²·d·atm), and the WVTR shows the same trend. This fully demonstrates the correctness of the technical route of using PHA as the main component (50–95 parts by weight) in this invention, ensuring that the coating can meet the stringent requirements for high barrier properties in applications such as food packaging. However, while Comparative Example 2 (pure PHA) has good gas barrier properties, its water resistance (Cobb) is poor. 60 The material has significant deficiencies in both its physical properties (20.5 g / m²) and mechanical properties (elongation at break of only 8%, see Table 7), highlighting the necessity for its modification.

[0172] Acrylic polymer acid value—the key to achieving a “pH-responsive switch”:

[0173] One of the core innovations of this invention lies in the construction of a "pH-responsive switch" that is water-resistant in neutral use environments and easily dissociates in alkaline recovery environments by precisely controlling the acid value of the acrylic polymer within the range of 20–55 mg KOH / g. Data from Application Example 2 (Table 6) and Application Example 5 (Table 9) provide decisive evidence for this mechanism.

[0174] The necessity of a lower acid value limit: Comparative Example 4 used an acrylic emulsion with an excessively low acid value (3 mg KOH / g). Although its barrier properties were comparable to those of the examples, its fiber recovery rate plummeted to 90.5%, far below the 95% industrial recovery threshold, and the residue on the sieve was as high as 1.10%. The mechanistic study of Application Example 5 revealed the fundamental reason: the coating exhibited extremely low swelling (55%) in an alkaline solution at pH 9.0, with an alkali consumption of only 2.8 mg KOH / g. This indicates that the coating lacks sufficient carboxyl functional groups to react with the alkali, failing to effectively disrupt the coating structure, making it difficult to peel off and disperse from the fibers during resizing. This directly proves that an acid value ≥20 mg KOH / g is a necessary condition for achieving efficient recovery.

[0175] The necessity of an upper limit for acid value: In contrast, Comparative Example 5 used an acrylic emulsion with an excessively high acid value (100 mg KOH / g). Although its fiber recovery rate was extremely high (98.8%), this came at the cost of sacrificing basic water resistance. Its Cobb... 60The value deteriorated to 28.5 g / m² (Table 5), far exceeding that of the examples. This is because the excessive hydrophilic carboxyl groups caused the coating to exhibit excessive hydrophilicity even in a normal neutral aqueous environment, impairing its basic function as a barrier layer.

[0176] Therefore, the acid value range of 20–55 mg KOH / g defined in this invention is a delicate balance window. It ensures that the coating contains an appropriate amount of carboxyl groups, which can maintain cohesion and hydrophobicity under neutral conditions, and can rapidly swell and disintegrate under alkaline conditions through saponification, thereby achieving a fiber recovery rate of over 95% without sacrificing water resistance.

[0177] The key influence of the glass transition temperature (Tg) of acrylic polymers on coating flexibility:

[0178] PHA is essentially a highly crystalline and brittle material. As shown in Comparative Example 2, its self-supporting film has an elongation at break of only 8%, which cannot meet the mechanical performance requirements of packaging materials during folding and transportation. This invention effectively solves this problem by introducing a flexible acrylic polymer with a Tg range of -10 to +10°C.

[0179] The data in Application Example 3 (Table 7) show that the elongation at break of all examples is ≥65%, significantly higher than that of pure PHA (Comparative Example 2). In particular, Comparative Example 6 used an acrylic polymer with an excessively high Tg (45°C), and even when blended with PHA, its coating's elongation at break was only 12%, exhibiting severe brittleness. This demonstrates that the selected acrylic polymer must have a sufficiently low Tg to function effectively as a toughening agent and film-forming aid at room temperature, forming a flexible network within the rigid PHA matrix through physical blending, thereby significantly improving the coating's flexibility and crack resistance.

[0180] Balancing synergy and overall performance:

[0181] In summary, this invention is not a simple superposition of the properties of PHA and acrylic polymers, but rather achieves unexpected synergistic effects through precise definition of key parameters. The high PHA content constructs the barrier framework of the coating; the acrylic polymer with a specific acid value endows the coating with "pH-smart response" and recyclability; while the low Tg acrylic polymer ensures the mechanical strength and film integrity of the coating. Application Example 4 (Table 8) further demonstrates that despite the introduction of hydrophilic carboxyl groups, the coating surfaces of all embodiments still maintain a hydrophobic contact angle >100°, ensuring excellent surface water resistance. This invention successfully solves four seemingly contradictory technical challenges—barrier properties, flexibility, water resistance, and efficient resizing under alkaline conditions—in a simple binary polymer system, providing a comprehensive, process-adaptable, and truly circular economy-value-driven innovative solution for the sustainable paper-based packaging field.

[0182] In summary, this invention successfully prepared a fluorine-free, environmentally friendly paper-based coating composition that combines high barrier properties, excellent resizing properties, good flexibility, and heat-sealing properties by scientifically compounding a high-content PHA aqueous emulsion with an aqueous acrylic polymer having a specific acid value and Tg range. Experimental data systematically demonstrate that the parameter range defined in this invention is not arbitrarily chosen, but rather an indispensable technical window that achieves a synergistic balance between "highly efficient resizing properties," "basic water resistance," and "coating flexibility," providing an effective technical solution for the field of sustainable packaging.

[0183] Those skilled in the art should understand that the above embodiments are merely exemplary and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, or improvements made to the technical solutions of the present invention within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition, characterized in that, Based on solids by weight, the composition comprises 50–95 parts by weight of an aqueous polyhydroxyalkanoate emulsion and 5–50 parts by weight of an aqueous acrylic polymer dispersion; The waterborne acrylic polymer has a glass transition temperature of -10 to +10°C, an acid value of 20–55 mg KOH / g, and a minimum film-forming temperature of ≤10°C. When the composition is used as a dispersion, the median particle size D in the volume distribution is... 50 According to ISO 13320:2020, the sample size is 200–900 nm, the solid content is 35–48 wt%, the pH is 7.3–8.3, and the total surfactant content (based on solids) is ≤0.5 wt%. The composition is free of polyvinyl alcohol, wax and alkyl ketone dimers, added crosslinking agents and inorganic fillers; and the composition is free of organofluorine compounds.

2. The environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition according to claim 1, characterized in that, The polyhydroxy fatty acid ester is selected from one or more of the following: poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-4-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), and medium- and long-chain polyhydroxy fatty acid esters; The medium- and long-chain polyhydroxy fatty acid esters are copolymers of one or their monomers, such as poly(3-hydroxyhexanoate), poly(3-hydroxyheptanoate), poly(3-hydroxyoctanoate), or poly(3-hydroxydecanoate).

3. The environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition according to claim 1, characterized in that, The aqueous acrylic polymer is polymerized from one or more of the following monomers: methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, styrene, α-methylstyrene, vinyltoluene, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, hydroxyethyl acrylate, hydroxypropyl acrylate, acrylamide, N-ethylacrylamide, N-isobutylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and glycidyl methacrylate.

4. The environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition according to claim 1, characterized in that, The total molar fraction of acidic monomers in the aqueous acrylic polymer is 1.0–6.0%, and the polymer acid value is between 20–55 mg KOH / g. The acidic monomer is selected from one or more of acrylic acid, methacrylic acid, or itaconic acid.

5. A method for preparing an environmentally friendly, easily recyclable, fluorine-free bio-based paper protective coating composition according to any one of claims 1 to 4, characterized in that, The preparation method includes: Step 1. Mix the aqueous emulsion of polyhydroxyalkanoate and the aqueous acrylic polymer dispersion under shear conditions to obtain a preliminary mixed dispersion; Step 2. Adjust the solid content of the preliminary mixed dispersion obtained in Step 1 to 35–48 wt% and the pH to 7.3–8.3 to obtain the adjusted dispersion; Step 3. Adjust the median particle size D of the volume distribution of the dispersion as described in Step 2. 50 By reaching 200–900 nm, the aforementioned environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition is obtained.

6. A coated paper product, characterized in that, At least one surface of the paper product is coated with a coating formed by curing the environmentally friendly, recyclable, fluorine-free bio-based paper protective coating composition according to any one of claims 1 to 4, wherein the coating has a dry weight of 12–30 g / m², and the coated paper product satisfies at least one of the following properties: a) Kit value ≥ 10; b)Cobb 60 ≤18g / m²; c) Under conditions of 38℃ and 90% relative humidity, the water vapor transmission rate is ≤220g / (m²·d); d) Under conditions of 23℃ and 50% relative humidity, the oxygen permeability is ≤1500cc / (m²·d·atm); e) The elongation at break of the self-supporting film formed after curing of the composition is ≥50%; f) Under repulping conditions of 45–55℃, pH 8.8–9.2, specific energy consumption of 35–55 kWh / t, and screening through a 0.15 mm slotted sieve, the fiber recovery rate is ≥95%.

7. The coated paper product according to claim 6, characterized in that, The coated paper products are food packaging, cups, bowls, plates, or straws.

8. The coated paper product according to claim 6, characterized in that, The paper base material of the coated paper product is selected from one or more of white cardboard, cup paper, kraft paper, kraft paper and food-grade coated base paper.

9. The coated paper product according to claim 6, characterized in that, The coated paper product has a Kit value ≥ 12 and a Cobb value ≥ 12. 60 ≤17g / m².

10. The coated paper product according to claim 6, characterized in that, The self-supporting film formed by curing the environmentally friendly and easily recyclable fluorine-free bio-based paper protective coating composition has an elongation at break of ≥100%.

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