Bio-based PHA oil-blocking and moisture-permeable coating for hamburger paper, coated paper and preparation method

Abstract

This invention discloses a bio-based PHA oil-blocking and moisture-permeable coating for hamburger paper, the coated paper, and its preparation method, belonging to the fields of bio-based polymer materials and paper-based food packaging technology. The coating is an aqueous dispersion of PHA, mainly comprising semi-crystalline matrix PHA and low-crystallinity or amorphous toughening PHA. Its core feature is that after the coating dries and forms a film, it spontaneously forms a functional gradient structure along the coating thickness direction: the interfacial layer near the paper substrate is rich in toughening PHA to enhance toughness and adhesion; while the surface layer is rich in matrix PHA to maximize oil-blocking performance. Through this functional gradient structure design, this invention balances the oil-blocking, moisture-permeable, and flexible properties of the packaging material, and possesses biodegradable environmental characteristics, making it suitable for high-oil food packaging.

Description

Technical Field

[0001] This invention belongs to the field of bio-based polymer materials and paper-based food packaging technology, specifically relating to bio-based PHA oil-blocking and moisture-permeable coating for hamburger paper, coated paper, and preparation method. Background Technology

[0002] For packaging paper used in freshly made high-fat hot foods such as hamburgers, the industry has long pursued multiple performance objectives simultaneously, including high oil resistance, moderate moisture permeability, good folding durability, and environmental friendliness. These objectives include repulping, biodegradability, and avoiding the use of perfluorinated and polyfluoroalkyl substances (PFAS). Traditional polyethylene (PE) coated paper possesses excellent oil resistance and impermeability, but during repulping, the film layer and fibers are difficult to separate effectively, easily forming "adhesive residue" and increasing recycling costs. Furthermore, zero moisture permeability allows moisture to accumulate inside the hot packaging, affecting food quality. Another common approach is to treat greaseproof paper with PFAS-based substances. While this can achieve high kitting properties, given its persistence in the environment and potential health risks, many jurisdictions have tightened restrictions on the use of PFAS in food contact materials. For example, the U.S. Food and Drug Administration has promoted the gradual withdrawal of PFAS-containing coatings from food contact paper products.

[0003] Against this backdrop, polyhydroxyalkanoates (PHAs) have emerged as an ideal candidate to replace the aforementioned traditional solutions due to their renewable sources and biodegradability. However, applying PHAs to the field of high-performance coatings faces fundamental challenges due to their inherent material properties: the barrier properties of PHAs depend on high crystallinity, while flexibility requires low crystallinity, and these two properties cannot be simultaneously achieved in a single homogeneous material.

[0004] While existing technologies have confirmed the use of PHA aqueous dispersions in paper-based barrier coatings, none have effectively resolved the aforementioned core contradictions. For example, international publication WO2020036843 discloses a barrier coating formulation based on PHA aqueous dispersions, achieving Kit ≥ 5. However, it teaches a homogeneous system and does not address the technical approach of constructing a functionally graded structure through blending different PHAs to synergistically improve barrier properties and flexibility. Furthermore, it does not disclose any technical solutions regarding the coating's performance retention rate after folding. Similarly, US patent US5977250 also proposes the use of PHA emulsions for paper barrier applications. These technologies provide a consensus in the field that PHA can be used as a barrier coating for paper, but they are essentially limited to the category of homogeneous materials, and cannot avoid the technical dilemma of sacrificing flexibility in pursuit of high barrier properties.

[0005] On the other hand, although coating self-stratification or self-gradientization has been studied in traditional coatings as a way to improve surface functionality, its implementation mechanism is fundamentally different from that of this invention. For example, the self-stratification mechanism described in US Patent Application Publication US20120201965 relies on the huge surface energy difference between fluorinated and non-fluorinated components (such as acrylic or epoxy resins), which belongs to macroscopic phase separation between dissimilar materials. This technique teaches no technical inspiration for inducing a quantifiable, functionally synergistic micro-gradient structure in a homogeneous PHA polymer system with similar chemical structures and weak surface energy differences by controlling drying and crystallization kinetics.

[0006] In summary, existing technologies have failed to provide a solution that simultaneously achieves top-tier oil resistance, excellent folding resistance, and moderate moisture permeability in a single, fully bio-based, fluorine-free coating system. A long-standing technological gap in this field lies in how to overcome the limitations of PHA materials' inherent properties, and without introducing heterologous or fluorine-containing functional components, utilize only the subtle differences in physical properties between PHA polymers of the same family to construct a specific physical structure that synergistically performs barrier and flexibility functions through a controllable process, using key application indicators such as post-fold performance retention and hot oil crease penetration time as benchmarks. This constitutes the unique technical challenge that this invention aims to address. Summary of the Invention

[0007] The purpose of this invention is to provide a bio-based PHA oil-blocking and moisture-permeable coating for hamburger paper, coated paper, and preparation method. By precisely designing the coating structure at the microscale, a functional gradient structure is constructed, thereby resolving the inherent contradiction between the barrier properties and flexibility of bio-based polymer coatings in a single coating and achieving a synergistic effect of "1+1>2".

[0008] To achieve the above objectives, the present invention provides a bio-based PHA oil-blocking and moisture-permeable coating for hamburger paper, wherein the coating is an aqueous dispersion and comprises:

[0009] The matrix is ​​PHA, a semi-crystalline PHA polymer.

[0010] And toughened PHA, a low-crystallinity or amorphous PHA polymer with a glass transition temperature lower than that of the matrix PHA.

[0011] In the coating, the total mass fraction of PHA is ≥95% based on the total weight of the dry solids of the coating; and after the coating is dried and formed into a film, it is able to form a functionally graded structure in the coating thickness direction, which satisfies the gradient index GI ≥1.2 measured by time-of-flight secondary ion mass spectrometry depth profile, and the relative mass fraction of the toughening PHA in the interface region near the paper substrate is higher than its relative mass fraction on the coating surface, while the relative mass fraction of the matrix PHA on the coating surface is higher than its relative mass fraction in the interface region near the paper substrate.

[0012] In the technical solution of the present invention, the total dry weight ratio of the matrix PHA to the toughened PHA is 95:5 to 60:40. For example, the ratio can be 95:5, 92:8, 90:10, 85:15, 80:20, 75:25, 70:30, 65:35, or 60:40.

[0013] In the technical solution of the present invention, the matrix PHA is selected from one or more of poly(3-hydroxybutyrate) or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). When it is PHBV, the molar fraction of 3-hydroxyvalerate can be 5% to 20%, for example 5%, 8%, 10%, 15% or 20%.

[0014] The toughening PHA is selected from one or more of the following: poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB), wherein the molar fraction of 4-hydroxybutyrate (4HB) is 10% to 20%; or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), wherein the molar fraction of 3-hydroxyhexanoate (PHH) is 5% to 20%. When it is P34HB, the molar fraction of 4-hydroxybutyrate can be 10% to 20%, for example, 10%, 12%, 15%, 18%, or 20%. When it is PHBHHx, the molar fraction of 3-hydroxyhexanoate can be 5% to 20%, for example, 5%, 8%, 10%, 12%, 15.2%, 18%, or 20%.

[0015] In the technical solution of the present invention, the coating does not contain any artificially added organic fluorine compounds, and may contain food contact permitted additives in a total amount of ≤5% based on the total dry solids weight of the coating. The additives are selected from one or more of alkyl polysaccharide nonionic surfactants, natural polysaccharide rheology modifiers, low molecular weight polyhydroxy fatty acid ester oligomers, or polyvinyl alcohol.

[0016] In addition, the bio-based carbon content of the dry solid coating is ≥85%.

[0017] The present invention also provides a high oil-resistant hamburger wrapping paper, the structure of which includes a base paper layer and a coating formed on at least one surface of the base paper layer by any of the aforementioned coatings. The basis weight of the base paper layer is 35-60 g / m². 2 For example, 35g / m 2 40g / m 2 45g / m 2 50g / m 2 55g / m 2 or 60g / m 2 The dry coating amount is 3-8 g / m². 2 For example, 3g / m 2 4g / m 2 5g / m 2 6g / m 2 7g / m 2 or 8g / m 2 The packaging paper possesses the following properties: air permeability tested according to TAPPI T 460 is 350 to 900 s / 100 mL; Kit grade ≥ 11, and after crease preparation according to TAPPI T 512 sp-21, the Kit decrease is ≤ 2 grades; according to ISO 16532-1:2008, using palm kernel oil as a medium at 60°C, the breakthrough time at the crease is ≥ 180 seconds; and its water vapor transmission rate under TAPPI / ANSI T 464 om-12 (R2022) conditions at 37.8°C and 90% relative humidity (RH) is 300–900 g·m³. -2 ·24h.

[0018] The present invention also provides a method for preparing the above-mentioned high oil-resistant hamburger packaging paper. The method includes: forming the functional gradient structure on the base paper by simultaneously coating with dual nozzles, co-coating with multiple layers of curtains or co-coating with multiple layers of groove molds; or coating the aforementioned coatings in a single application and forming a thickness-direction functional gradient structure with GI≥1.2 by controlling the temperature and solvent evaporation rate during the drying process.

[0019] Compared with the prior art, the following significant advantages can be obtained by using the present invention:

[0020] Innovative Functional Gradient Structure Design and Synergistic Mechanism: This invention optimizes the traditional homogeneous blending technique by precisely controlling drying kinetics to construct a spontaneous and quantifiable functional gradient structure along the coating thickness direction, with a gradient index GI ≥ 1.2. Its core synergistic mechanism lies in the following: the toughened PHA with a low glass transition temperature spontaneously accumulates in the interfacial layer near the paper substrate, effectively absorbing and dissipating the stress generated during paper folding, acting as a flexible buffer layer to prevent brittle cracking of the coating; while the highly crystalline matrix PHA accumulates on the coating surface, maximizing its barrier function. This structural design resolves the inherent contradiction between barrier properties and flexibility in a single material.

[0021] Precise balance of multi-objective performance and high-end application value: This invention achieves a balance across three key performance dimensions: high oil resistance, moderate moisture permeability, and high-speed processing adaptability. (The figure is incomplete and requires further context to translate accurately.) 2 With a coating amount of [amount], top-level oil resistance of ≥11 after folding can be achieved; at the same time, water vapor transmission rate (WVTR) is controlled at 300–900 g·m. -2 Within an ideal 24-hour timeframe, the coating imparts a "breathing" function to the packaging paper, preventing moisture from hot food from seeping in while avoiding excessive water loss that could affect the taste. This is superior to zero-permeability PE coated paper or ordinary greaseproof paper with uncontrollable moisture permeability. Furthermore, the crystalline-rich coating surface gives the material an ideal static friction coefficient of 0.4–0.5 and excellent anti-blocking properties (non-clogging), meeting the requirements of high-speed automated bag making and packaging production lines. This avoids industrial application problems such as surface stickiness and paper jamming caused by excessive toughening component content.

[0022] Environmental friendliness and industrial feasibility of the all-bio-based fluorine-free solution: This invention provides a high-performance oil-blocking solution without the artificial addition of organic fluorine compounds. Its total fluorine content is below 50 ppm and it does not contain PFAS substances, which are subject to key industry controls, thus avoiding the environmental persistence and health risks posed by PFAS. The coating is composed of PHA with a bio-based carbon content of ≥85%, exhibiting excellent renewability and biodegradability. More importantly, the coating of this invention does not affect the recycling and reuse of paper, with a repulping fiber yield of over 98%, demonstrating significant circular economy advantages compared to PE-coated paper that causes "adhesive" problems. Its technology route, which achieves a gradient structure through a single coating and controlled drying, is simple, highly compatible with existing paper coating equipment, and eliminates the need for complex multi-layer coating equipment. This provides a practical and feasible technical path for the low-cost, large-scale industrial production of high-performance, environmentally friendly food packaging materials. Attached Figure Description

[0023] Figure 1 : A schematic diagram of the cross-sectional microstructure of the coating of the present invention on a paper or paperboard substrate.

[0024] In the figure, 1 represents the paper base; 2 represents the central zone of the area with a higher toughening polyhydroxyalkanoate content in the lower part of the coating; and 3 represents the central zone of the area with a higher substrate polyhydroxyalkanoate content in the upper part of the coating. Detailed Implementation

[0025] 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.

[0026] Main reagents and raw materials:

[0027] The main reagents and raw materials used in the embodiments and comparative examples of this invention are shown in Table 1.

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

[0029]

[0030] Main analytical and testing instruments:

[0031] Wire bar coating machine: K202, RK PrintCoat Instruments Ltd.;

[0032] Forced-air drying oven: DHG-9140A, Shanghai Yiheng Scientific Instruments Co., Ltd.;

[0033] Contact angle measuring instrument: DSA100, KRÜSS GmbH;

[0034] Oil resistance test kit: TAPPI T559 Kit Test, Thwing-Albert Instrument Co.;

[0035] Air permeability meter: Gurley 4110, Gurley Precision Instruments;

[0036] Crease / Folding Resistance Equipment: T512 crease clamps and pressure rollers;

[0037] Time-of-flight secondary ion mass spectrometer: IONTOF TOF-SIMS 5;

[0038] Water vapor transmission rate meter: Labthink W3 series;

[0039] Combustion ion chromatography system (CIC): AQF-2100H, Mitsubishi Chemical Analytech, coupled with Dionex ICS-6000 or Integrion HPIC, Thermo Scientific;

[0040] Liquid chromatography-tandem mass spectrometry (LC-MS / MS): InfinityLab Pro iQ, Agilent;

[0041] Fixed infrared thermometer: Optris CSlaser LT (-20~150℃);

[0042] Adhesion test tape: Elcometer 99 (applicable to ASTM D3359-22 / -23);

[0043] Grid cutter: Elcometer 1542 (1mm / 2mm / 3mm cutter spacing);

[0044] Analytical balance: METTLER-TOLEDO XPR205 (0.01 mg);

[0045] High-speed disperser / homogenizer: T 25 digital ULTRA-TURRAX, IKA;

[0046] High-pressure homogenizer: AH-100D, ATS Engineering.

[0047] Main testing standards:

[0048] Before testing, all samples were conditioned for at least 24 hours at 23±1℃ and 50±2% RH, in accordance with the TAPPI T402 sp-21 standard.

[0049] Statistical requirements: Sample size n≥5 for each indicator; report mean ± standard deviation; if rating scales are involved, such as Kit or Block, report frequency distribution and median.

[0050] Kit: Follows TAPPI T 559 cm-12 (R2022). Given that this standard was originally used for evaluating fluorinated surface-treated paper, this invention uses it as a comparative rating indicator for non-fluorinated coatings. For the folded kit test, a cross crease is first prepared according to TAPPI T512 sp-21, followed by T559 evaluation.

[0051] Oil penetration: in accordance with ISO 16532-1:2008.

[0052] Breathability: Follows TAPPI T 460 om-21.

[0053] Crease Preparation: Following the principles of TAPPI T512 sp-21, standardized cross crease preparation was performed using the following equivalent parameters: The sample was placed on a hard, flat surface. Using a circular folding rod with a width of 3.2 mm, a crease was formed by uniformly folding the sample at a speed of approximately 10 mm / s under a load of 1000±50 g. The sample was rotated 90 degrees and the operation was repeated to form a cross crease. The entire process was carried out under standard temperature and humidity (23±1℃, 50±2% RH). After folding, the sample needed to be reequilibrated under this environment for at least 1 hour before subsequent testing.

[0054] Water vapor transmission rate: Follow TAPPI / ANSI T 464 om-12 (R2022), 37.8℃, 90% RH. If necessary, refer to TAPPI T 448 om-21 (23℃, 50% RH).

[0055] Adhesion: Follows ASTM D3359-23 Method B (Cross-Cut).

[0056] Surface water contact angle: The static contact angle measurement method was used, with a water droplet volume of 5µL, and the contact angle value was recorded at the 10th second. Simultaneously, the propulsion angle was measured, referencing ASTM D7334-22 as the practice standard.

[0057] Re-pulping performance: in accordance with CEPI / 4evergreen Part I (conventional process) v3, 2025.

[0058] Food contact compliance: Sensory testing shall comply with EN 1230-1 / -2:2009, odor / taste, with a minimum of 6 subjects.

[0059] Verification of unintentional addition of fluorine compounds: Cross-validation was performed using the following two methods to confirm that no organic fluorine compounds were intentionally added to the formulation:

[0060] 1) Total fluoride content (TF): Tested using combustion-ion chromatography (CIC). The total fluoride content must be below 50 mg / kg (50 ppm), and the limit of quantitation (LOQ) for this method is 10 mg / kg.

[0061] 2) Targeted analysis of specific perfluorinated and polyfluoroalkyl compounds (PFAS): Analysis was performed using liquid chromatography-tandem mass spectrometry (LC-MS / MS). Sample pretreatment was as follows: 1g of sample was chopped, 10mL of methanol was added, and the mixture was ultrasonically extracted at 60℃ for 30 minutes. After centrifugation, the supernatant was filtered through a 0.22μm filter membrane for analysis. All targets listed in the industry-specific PFAS list (e.g., substances listed in US EPA Method 1633) must be undetectable. The analysis report must list all target compounds and their respective limits of quantitation (LOQs), with each target compound having an LOQ ≤ 0.5µg / kg.

[0062] Migration compliance: Refer to GB 4806.8-2022 "Paper and Paperboard Materials and Products for Food Contact".

[0063] Compostability: Complies with EN 13432 or ASTM D6868.

[0064] Processing Compatibility: Blockage / Adhesion Test: Conducted using internal control methods, with test conditions equivalent to the historical standard ASTM D918-99. The specific steps are as follows: Two 10cm × 10cm samples are stacked face-to-face between two clean glass plates, and a uniform pressure of 3.4 kPa is applied. The entire apparatus is placed in a constant temperature and humidity chamber at 49°C and 75% RH for 21 hours. After removal, it is cooled for 1 hour at standard temperature and humidity (23±1°C, 50±2% RH). Evaluation criteria: The two samples are separated by hand. If they can be easily separated without any damage or adhesion to the coating, it is recorded as "non-blockage"; if there is slight resistance during separation but the coating remains intact, it is recorded as "slight"; if the coating tears or transfers during separation, it is recorded as "significant". The coefficient of friction follows TAPPI T549, horizontal method, and the static coefficient of friction (μs) and dynamic coefficient of friction (μk) are tested separately. The static friction coefficient reflects the material's resistance to sliding during startup, while the dynamic friction coefficient reflects the material's resistance during sliding.

[0065] Functional gradient characterization:

[0066] Depth profiling analysis was performed using time-of-flight secondary ion mass spectrometry.

[0067] Sputtering and analysis conditions: Cs using 2keV + An ion beam was used as the sputtering source, with a beam current of approximately 100 nA, at a depth of 300 × 300 µm. 2 Sputtering was performed over an area to form analytical pits. The analytical source was a 30keV BiO2 source. 3+ Pulsed ion beam, at 100 × 100 µm 2 Data is collected in the central area of ​​the sputtering crater to avoid edge effects.

[0068] Characteristic ion selection: Characteristic secondary ions that can distinguish between matrix PHA (e.g., PHBV) and toughening PHA (e.g., P34HB) are selected. For example, m / z = 87.04 (C4H7O2-, representing 3-hydroxybutyrate units) is selected as the total PHA signal for intensity normalization; m / z = 101.06 (C5H9O2-, representing 3-hydroxyvalerate units) is selected as the characteristic signal of matrix PHA; m / z = 115.07 (C6H... 11 O2- (representing 4-hydroxybutyrate unit) serves as a characteristic signal for toughening PHA.

[0069] Depth window definition and GI calculation: The total coating thickness is determined by monitoring characteristic signals (such as Si) sputtered to the paper substrate interface. + Or Al + The signal was identified by a sudden increase and confirmed by a sputtering rate (approximately 0.5 nm / s) pre-calibrated on a pure PHA film of known thickness. The "surface" was defined as the region extending from the outermost surface to a depth of 10% of the total thickness, and the "interface" was defined as the region extending upwards from the paper-based interface to 10% of the total thickness. The average value of the characteristic secondary ion intensity within these two regions was calculated, and the gradient exponent GI was defined as:

[0070] GI=(I 基体PHA特征 / I 增韧PHA特征 ) 表层 / (I 基体PHA特征 / I 增韧PHA特征 ) 界面

[0071] Where I represents the intensity of the corresponding characteristic secondary ion after normalization by the total PHA signal. When GI ≥ 1.2, it is considered that an effective functional gradient structure has been formed.

[0072] Examples and Comparative Examples:

[0073] Bioten TM General preparation steps for PHA aqueous dispersions:

[0074] The PHA aqueous dispersion used in this embodiment of the invention is prepared using an environmentally friendly solvent-free high-energy homogenization method. The specific steps are as follows:

[0075] Step 1. Dissolve 0.5g PVA in 200ml of deionized water and heat to 80℃ to obtain the aqueous phase.

[0076] Step 2. Melt 10g of PHA powder at 175°C.

[0077] Step 3. In a high-shear homogenizer, slowly add the molten PHA obtained in step 2 to the hot water phase in step 1 at a speed of 10,000 rpm, and continue shearing for 10 minutes to form a crude emulsion.

[0078] Step 4. Immediately transfer the hot crude emulsion to a high-pressure homogenizer preheated to 80°C and homogenize it 5-8 times under a pressure of 80-100 MPa.

[0079] Step 5. After homogenizing, the emulsion is cooled to room temperature while stirring to obtain a milky white, stable aqueous dispersion of PHA. The solid content of each dispersion is 40-45 wt%, and the average particle size D is... 50 The micrometer size was controlled within the range of 0.5-2.0 μm. The prepared matrix PHA aqueous dispersion, toughened PHA1 aqueous dispersion, and toughened PHA2 aqueous dispersion were sealed and stored at 4 °C for later use.

[0080] General steps for coating preparation:

[0081] Add the materials according to the proportions listed in Table 2. First, mix the base PHA aqueous dispersion, toughened PHA1 aqueous dispersion, and toughened PHA2 aqueous dispersion thoroughly, ensuring that the total solid content is 100 parts. Then, based on this 100 parts of PHA solid weight, add the corresponding proportions of APG, natural polysaccharides, and other additives listed in Table 2, and continue stirring until thoroughly mixed. Finally, adjust the solid content of the mixture to 30 wt% with deionized water and continue stirring for 30 minutes before use.

[0082] General steps for preparing coated paper:

[0083] The prepared coating was evenly applied to a surface with a g / m² thickness using a No. 12 wire rod coating machine. 2 On food-grade base paper, the dry basis coating amount is controlled to be approximately 5 g / m². 2 .

[0084] Table 2. Main ingredients of the formulations for the examples and comparative examples:

[0085]

[0086] This invention employs the following two main drying processes:

[0087] Gradient drying process: After coating, a two-stage gradient drying process is adopted. First, it is slowly dried at a chamber temperature of 60°C for 120 seconds, and then rapidly dried at a chamber temperature of 120°C for 30 seconds. By intermittently sampling and weighing parallel samples and recording the infrared temperature of the film surface (see instrument list), the time curve of the solid content increasing from about 30 wt% to above 95 wt% is tracked to confirm the drying trajectory of "low-speed volatilization followed by rapid densification", thereby inducing the formation of a functionally graded structure.

[0088] Rapid drying process: After coating, the coating is rapidly dried at 100°C for 60 seconds. This process is designed to form a homogeneous blend coating.

[0089] Preparation of each example and comparative example:

[0090] Examples 1-12: Coatings were prepared using the formulations of Examples 1 to 12 in Table 2, and were dried using a gradient drying process after coating.

[0091] Comparative Examples 1-2: Coatings were prepared using the formulations of Comparative Examples 1 and 2 in Table 2, respectively, and were dried using a rapid drying process after coating.

[0092] Comparative Example 3: The coating was prepared using the formulation of Comparative Example 3 in Table 2 (the same as in Example 2), and dried using a rapid drying process after coating.

[0093] Comparative Example 4: The coating was prepared using the formulation of Comparative Example 4 in Table 2, and then dried using a gradient drying process.

[0094] The basic performance tests were conducted on the coated paper samples prepared in the above embodiments and comparative examples, and the results are shown in Table 3.

[0095] Table 3. Basic performance and structural characterization results of each embodiment and comparative example:

[0096]

[0097] 1 Note: When GI≥1.2, it is considered that the coating forms a functional gradient structure in the thickness direction.

[0098] 2 Note: This refers to the decrease in the Kit's rating after the discount compared to before the discount.

[0099] 3 Note: According to TAPPI T 460 om-21, the recommended range for the liquid column analyzer is 5-1800 s / 100 mL. When the result exceeds the recommended range of T460, report it as >1800 s / 100 mL;

[0100] Application performance evaluation:

[0101] To fully verify the performance of the coated paper of the present invention in practical application scenarios, a series of application tests were conducted on samples of all embodiments and comparative examples, and the results are summarized in Tables 4 to 9.

[0102] To evaluate the resistance of coated paper to penetration when in contact with high-temperature grease, especially in the weak points after folding, a hot oil penetration test was conducted, and the results are shown in Table 4.

[0103] Table 4. Hot oil penetration results for the examples and comparative examples (ISO 16532-1, creased samples, 60℃):

[0104]

[0105] Analysis: This test simulated the most demanding usage scenario of a hot oil-filled hamburger after folding. All embodiments achieved breakthrough times far exceeding the target of 180 seconds, demonstrating excellent heat-resistant oil folding performance. Comparative Example 1, a pure matrix, failed almost instantly due to brittleness; Comparative Example 3, a homogeneous blend, performed significantly worse than Embodiment 2 with its gradient structure, further demonstrating the necessity of the gradient structure. While Comparative Examples 2 and 4 were flexible, their insufficient barrier properties resulted in significantly shorter oil retention times compared to the embodiments of this invention.

[0106] To evaluate the ability of packaging paper to regulate the humidity inside the packaging and prevent food from being affected by moisture buildup, the water vapor transmission rate of the samples was tested, and the results are shown in Table 5.

[0107] Table 5. Water vapor transmission rate results for the examples and comparative examples (T 464, 37.8℃, 90% RH, unbroken sample):

[0108]

[0109] Analysis: Effective steam management is crucial for maintaining the texture of hamburgers. The WVTR values ​​for all embodiments fell between 300 and 900 g·m³. -2 • The ideal 24-hour timeframe achieves "moderate moisture permeability." Comparative Example 1 is almost impermeable to water vapor, causing food to become damp; while Comparative Examples 2 and 4 have excessively high WVTRs, which may cause food to dry out too quickly. This demonstrates that the gradient structure of the present invention can precisely control water vapor permeability while ensuring high oil resistance.

[0110] To verify the environmentally friendly properties of the coated paper of this invention, we conducted a comprehensive evaluation of the samples according to the internationally recognized industrial composting standards EN13432 / ASTM D6868. This test simulates industrial composting environmental conditions, requiring the material to complete biodegradation and disintegration within a specified time, and ensuring that the compost products have no toxic effects on plant growth. This is a key indicator for measuring whether the material truly achieves organic recycling and returns to nature.

[0111] Table 6 Composting results of the examples and comparative examples:

[0112]

[0113] All examples of the PHA coatings based on this invention, as well as all comparative samples, successfully passed the compostability test. After a 12-week testing period, the disintegration rate of the samples all exceeded the standard requirement of 90%, demonstrating excellent biodegradability comparable to the positive control (pure cellulose). This result strongly proves that the PHA material system used in this invention, regardless of whether its internal structure is gradient or homogeneous, is chemically completely biodegradable.

[0114] In stark contrast, conventional polyethylene (PE) coated paper, used as a negative control, showed almost no disintegration under the same conditions. This highlights the fundamental advantages of the present invention's solution over conventional petrochemical-based plastic coated paper in terms of waste management and environmental impact. The hamburger paper of this invention can be incorporated into industrial composting systems after disposal, ultimately transforming into valuable compost, achieving organic recycling of materials rather than becoming persistent plastic waste in the environment.

[0115] To ensure the safety of coated paper as a food contact material and to prevent it from adversely affecting the flavor of food, odor and taste sensory evaluations were conducted, and the results are shown in Table 7.

[0116] Table 7 Summary of sensory results (EN 1230-1 / -2) for the examples and comparative examples:

[0117]

[0118] Analysis: All PHA-based samples, regardless of their formulation and structure, passed rigorous sensory testing without introducing any off-flavors or odors. This indicates that the material system used in this invention possesses excellent food contact safety and will not affect the flavor of food.

[0119] To verify that this invention is a fluorine-free solution, a high-sensitivity analytical method was used to detect the total organic fluorine and specific PFAS substances in the sample. The results are shown in Table 8.

[0120] Table 8. PFAS falsification results of the examples and comparative examples (CIC-TOF + targeted LC-MS / MS):

[0121]

[0122] ¹Note: The limit of quantitation (LOQ) for all target compounds in the PFAS target list is ≤0.5µg / kg.

[0123] Analysis: As shown in Table 8, the total fluorine content (TF) of all samples was below the screening threshold of 50 ppm, and no specific PFAS substances of industry concern were detected by highly sensitive LC-MS / MS targeted analysis. This double verification result strongly proves that the coating system of this invention does not contain any artificially added organic fluorine compounds and is a truly fluorine-free solution.

[0124] To evaluate the processing performance of coated paper in high-speed bag making and packaging processes, its anti-blocking (clogging) properties and surface friction coefficient were tested, and the results are shown in Table 9.

[0125] Table 9. Results of blocking and friction coefficients in the examples and comparative examples (T 549):

[0126]

[0127] Analysis: The embodiments of the present invention exhibit an ideal coefficient of friction and excellent anti-blocking properties due to their crystalline phase-rich surface, making them highly suitable for high-speed automated bag making and packaging. The excessively slippery surface of Comparative Example 1 may lead to unstable conveying, while Comparative Examples 2 and 4, with their high PHA toughening content, have sticky surfaces, easily causing adhesion and paper jams during processing.

[0128] To further confirm the safety of coated paper as a food packaging material, in accordance with the requirements of GB 4806.8-2022 "Paper and Paperboard Materials and Articles for Food Contact", a simulant (50% ethanol) representing high-fat foods was selected and the total migration was tested under harsh conditions (60℃, 2 hours). The results are shown in Table 10.

[0129] Table 10. Total migration test results for the examples and comparative examples according to GB 4806.8-2022:

[0130]

[0131] Analysis: Test results show that the total migration in all examples and comparative examples is far below the national standard limit of 10 mg / dm². This fully demonstrates that whether it is the functionally graded structure coating of this invention or the homogeneous or single-component PHA coating, the material system itself has high chemical inertness. Under simulated long-term contact with high-temperature, high-fat foods, almost no substances migrate into the food, ensuring its highest safety standard as a food contact material.

[0132] Comprehensive application performance analysis:

[0133] The comprehensive results of the above application performance tests clearly reveal the overall advantages of this invention compared to the comparative examples. Neither pure component materials, homogeneous blends, nor out-of-range formulations can achieve the same optimal balance across multiple dimensions as the gradient structure coating of this invention, including hot oil barrier properties, steam management, processing sealing, and food safety.

[0134] Experimental Results and Analysis:

[0135] 1. The decisive role of gradient structure and quantitative evidence.

[0136] The core of this invention lies in the functionally graded structure constructed through a gradient drying process. Comparative Example 2 and Comparative Example 3 clearly demonstrate the decisive role of this structure. The chemical formulations of these two samples are exactly the same (matrix PHA:toughening PHA 1 = 90:10), the only difference being the drying process. The former uses gradient drying to form a gradient structure (GI = 1.65), while the latter uses rapid drying to form a homogeneous structure (GI = 1.03).

[0137] As can be seen from the basic performance data in Table 3, this structural difference directly leads to a significant performance disparity. When folded, both examples initially have the highest Kit level of 12, indicating that homogeneous blending can achieve good barrier properties even under static conditions. However, after simulating actual folding operations, the Kit level of Example 2, with its gradient structure, only drops by 1 level to 11, demonstrating superior flexibility and structural stability. In contrast, the Kit level of Comparative Example 3, with its homogeneous structure, drops sharply by 5 levels to 7 after folding, indicating a severe degradation in oil-blocking performance.

[0138] The fundamental reason for this phenomenon lies in the design of the gradient structure: the flexible toughening PHA enriched in the bottom layer effectively absorbs the stress caused by folding, preventing the coating from cracking; while the rigid matrix PHA enriched in the surface layer maintains its structural integrity and continues to exert its excellent barrier function. Furthermore, the hot oil penetration test results in Table 4 further corroborate this point; Example 2 achieved a breakthrough time of 260 seconds at the crease, far exceeding the 95 seconds of Comparative Example 3. Simultaneously, the gradient structure also improves the adhesion between the coating and the paper substrate; Example 2 achieved the highest adhesion rating of 5B, superior to the 4B rating of Comparative Example 3. These data collectively demonstrate that it is the functional gradient structure of this invention, rather than a simple blend of chemical components, that achieves a synergistic unity of oil resistance and folding endurance.

[0139] 2. Analysis of the influence of key component trends: the ratio of matrix PHA to toughening PHA.

[0140] To establish the scientific validity of the ratio range defined in this invention (base PHA: toughening PHA is 95:5 to 60:40), we systematically examined the influence of toughening PHA1 as a single toughening agent in Examples 1 to 5, with its content gradually increasing from 5 parts to 40 parts, on various properties.

[0141] Trends in Barrier Performance: With the increase of toughening PHA content (from Example 1 to Example 5), the barrier performance of the coating exhibited a predictable, gradual change. Table 3 shows that the pre-fold Kit rating slightly decreased from level 12 to level 11, still remaining at a high level. However, the change in air permeability was more sensitive: Gurley air permeability decreased from 520 s / 100 mL to 360 s / 100 mL, indicating a reduction in gas barrier performance. The water vapor transmission rate (WVTR) data in Table 5 also showed the same trend, decreasing from 480 g·m³. -2 • Increased to 810 g·m in 24 hours -2 • 24h. This is in line with theoretical expectations, as the low-crystallinity toughened PHA itself has higher permeability than the high-crystallinity matrix PHA. However, within the 40% content limit defined in this invention, the air permeability and WVTR values ​​of all embodiments remained within the target window for hamburger paper applications.

[0142] Stability of folding resistance: Throughout the entire formulation range defined by this invention, the folding resistance remains at an extremely high level. As shown in Table 3, the Kit grade decrease after folding in Examples 1 to 5 remains consistently at level 1, indicating that even with only 5 parts of toughening PHA (Example 1), the resulting gradient structure is sufficient to effectively protect the coating from catastrophic damage during folding.

[0143] Boundary verification of the formulation range: The results of Comparative Example 4 (toughening PHA content of 50%) clearly define the upper limit of the formulation range of this invention. Although its gradient structure can still be effectively formed (GI=1.41) and its folding endurance is excellent (Kit grade drops by 1 level), its barrier properties have exceeded the application limits. Its initial Kit grade is only 9, Gurley permeability is less than 100 s / 100mL, and WVTR exceeds 1500 g·m. -2 • 24h, which makes it unable to meet the basic requirements of high-resistance oil packaging.

[0144] Based on the above trend analysis, the mixing ratio range of 95:5 to 60:40 defined in this invention is an optimal working window that can ensure top-notch folding resistance and excellent adhesion, while precisely maintaining barrier performance within the target application range.

[0145] 3. Universality of different toughening systems.

[0146] To verify the universality of the technical solution of this invention, we compared the system using only a single toughening agent and the system using a blend of two toughening agents.

[0147] First, comparing Example 2 (matrix:toughener 1 = 90:10) and Example 6 (matrix:toughener 2 = 90:10), both used a single toughening agent but had different chemical structures. The results showed that the two core performance indicators were highly consistent: both formed an effective gradient structure (GI 1.65 and 1.63, respectively), and both exhibited a Kit grade decrease of 1 level after folding. There were also no significant differences in performance across multiple dimensions, including adhesion, hot oil penetration, water vapor transmission rate, and processing performance.

[0148] Secondly, we compared the single toughening agent system with the blend system. For example, in Examples 3 (matrix:toughening agent 1 = 80:20), 7 (matrix:toughening agent 2 = 80:20), 9 (matrix:toughening agent 1:toughening agent 2 = 80:15:5), and 10 (matrix:toughening agent 1:toughening agent 2 = 80:5:15), the ratio of matrix PHA to total toughening PHA in these four examples was 80:20. As can be seen from the data in Tables 3 and 4, regardless of whether a single toughening agent or two toughening agents were used in different proportions, they all formed an effective gradient structure, and the Kit grade decrease after folding remained at level 1. The hot oil penetration time was also above 225 seconds, demonstrating highly consistent and excellent performance.

[0149] These results fully demonstrate that the core advantage of this invention stems from the physical gradient structure formed through a specific process. This principle has good platform versatility, not limited to a specific toughening PHA chemical structure, nor to a single toughening agent, as long as the toughening component (whether a single polymer or a blend) meets the conditions of low crystallinity or amorphous morphology and a glass transition temperature lower than that of the matrix PHA.

[0150] In summary, this invention successfully solves the long-standing technical challenge of balancing barrier properties and flexibility in the field of bio-based barrier coatings by constructing a novel functionally graded polyhydroxyalkanoate coating structure. This technical solution not only boasts superior performance but also offers high process feasibility and sustainable raw material sourcing, providing the market with a truly environmentally friendly and reliable high-quality food packaging solution.

[0151] Those skilled in the art should understand that the above embodiments are merely exemplary and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the technical solutions of the present invention within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A bio-based polyhydroxyalkanoate oil-blocking and moisture-permeable coating for hamburger paper, characterized in that, The coating is an aqueous dispersion and contains: Matrix polyhydroxy fatty acid ester: a semi-crystalline polyhydroxy fatty acid ester polymer; Toughened polyhydroxy fatty acid ester: a polyhydroxy fatty acid ester polymer with low crystallinity or amorphous form, whose glass transition temperature is lower than that of the matrix polyhydroxy fatty acid ester; The total mass fraction of polyhydroxyalkanoates is ≥95% based on the total dry solids weight of the coating. After the coating is dried into a film using a gradient drying process, it can form a functional gradient structure in the coating thickness direction and satisfy the gradient index GI ≥ 1.2 as measured by time-of-flight secondary ion mass spectrometry depth profile; and the relative mass fraction of the toughened polyhydroxyalkanoate in the interface region near the paper substrate is higher than its relative mass fraction on the coating surface, while the relative mass fraction of the matrix polyhydroxyalkanoate on the coating surface is higher than its relative mass fraction in the interface region near the paper substrate. The coating did not contain any organic fluorine compounds. The total dry weight ratio of the matrix polyhydroxyalkanoate to the toughening polyhydroxyalkanoate in the coating is 95:5 to 60:

40.

2. The coating according to claim 1, characterized in that, The matrix polyhydroxy fatty acid ester is one or a combination of poly(3-hydroxybutyrate) or poly(3-hydroxybutyrate-co-3-hydroxyvalerate); The toughened polyhydroxy fatty acid ester is selected from one or a combination of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

3. The coating according to claim 1, characterized in that, The coating contains ≤5% of food contact permitted additives based on the total weight of the coating dry solids, wherein the additives are selected from one or more of alkyl polysaccharide nonionic surfactants, natural polysaccharide rheology modifiers, low molecular weight polyhydroxy fatty acid ester oligomers, or polyvinyl alcohol.

4. The coating according to claim 1, characterized in that, The bio-based carbon content of the dry solid coating is ≥85%.

5. A high oil-resistant hamburger wrapping paper, characterized in that, Its structure includes a base paper layer and a coating formed on at least one surface of the base paper layer by the coating of any one of claims 1 to 4.

6. The high oil-resistant hamburger wrapping paper according to claim 5, characterized in that, The basis weight of the base paper layer is 35–60 g / m³. 2 The dry coating amount is 3–8 g / m². 2 .

7. The high oil-resistant hamburger wrapping paper according to claim 5, characterized in that, The packaging paper exhibits the following properties: air permeability tested according to TAPPI T 460 is 350 to 900 s / 100 mL; Kit grade ≥ 11, and after crease preparation using TAPPI T 512 sp-21, the Kit grade decreases by ≤ 2; according to ISO 16532-1:2008, using palm kernel oil as a medium at 60°C, the crease breakthrough time is ≥ 180 seconds; and its water vapor transmission rate according to ANSI / TAPPI T 464 om-12 (R2022) at 37.8°C and 90% relative humidity is 300–900 g·m³. -2 ·24h.

8. A method for preparing high oil-resistant hamburger wrapping paper according to any one of claims 5 to 7, characterized in that, The method includes: The functional gradient structure is formed on the base paper by simultaneously coating with dual nozzles, co-coating with multiple layers of curtains, or co-coating with multiple layers of groove molds. The two coatings, which are respectively rich in matrix polyhydroxy fatty acid ester and toughening polyhydroxy fatty acid ester, are applied to the base paper. Alternatively, the coating described in any one of claims 1 to 5 may be applied in a single coat, and by controlling the temperature and solvent evaporation rate during the drying process, vertical phase separation of different polyhydroxy fatty acid ester components may be induced to form the functional gradient structure.

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