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

Abstract

The invention discloses a bio-based PHA oil-resistant and moisture-permeable coating for hamburger paper, coated paper and a preparation method, and belongs to the technical field of bio-based high polymer materials and paper-based food packaging. The coating is a PHA aqueous dispersion, and mainly comprises a semi-crystalline matrix PHA and a low-crystallinity or amorphous toughening PHA. The core of the coating is that after the coating is dried to form a film, a functional gradient structure can be spontaneously formed in the thickness direction of the coating: an interface layer close to a paper base is rich in toughening PHA so as to enhance toughness and adhesive force; and the surface layer of the coating is rich in matrix PHA so as to maximize the oil resistance. By means of the functional gradient structural design, the packaging material integrates oil resistance, moisture permeability and flexibility of the packaging material, has the degradable environment-friendly characteristic and is suitable for packaging high-grease food.

Description

Technical Field

[0001] The present invention belongs to the technical field of bio-based polymer materials and paper-based food packaging, and particularly relates to a bio-based PHA oil-resistant and moisture-permeable coating for hamburger paper, coated paper and a preparation method. Background Art

[0002] For wrapping paper used for freshly made, high-fat hot foods like hamburgers, the industry has long pursued multiple performance objectives, including high oil resistance, moderate moisture permeability, good folding durability, and environmental friendliness. These include repulpability, biodegradability, and the avoidance of per- and polyfluoroalkyl substances (PFAS). Traditional polyethylene (PE)-coated paper offers excellent oil and permeability resistance, but the film and fiber layers are difficult to separate during the repulping process, leading to the formation of a "sticky" substance, which increases recycling costs. Furthermore, its low moisture permeability allows moisture to accumulate within the hot packaging, impacting its quality. Another common approach is to surface-treat grease-proof paper with PFAS-containing substances. While this can achieve a high Kit, due to its environmental persistence and potential health risks, several jurisdictions have tightened restrictions on the use of PFAS in food contact materials. For example, the US Food and Drug Administration has promoted the phase-out of PFAS-containing coatings in food-contact paper products.

[0003] In this context, polyhydroxyalkanoates (PHAs) have become an ideal candidate to replace these traditional solutions due to their renewable sources and biodegradability. However, the application of PHAs in high-performance coatings faces fundamental challenges brought about by their inherent material properties: PHA's barrier properties rely on high crystallinity, while flexibility requires low crystallinity, and these two cannot be achieved in a single homogeneous material.

[0004] Although the existing technologies have confirmed that PHA aqueous dispersions can be used for paper-based barrier coatings, none of them have effectively solved the above-mentioned core contradictions. For example, the international public document WO2020036843 discloses a barrier coating formula based on PHA aqueous dispersions that can achieve Kit ≥ 5, but it teaches a homogeneous system and does not involve the technical idea of ​​constructing a functional gradient structure by blending different PHAs to synergistically improve barrier properties and flexibility, nor does it disclose any technical solutions for the performance retention rate of the coating after folding. Similarly, U.S. Patent US5977250 also proposes the use of PHA emulsions for barrier purposes for paper. These technologies provide the field with a consensus that PHA can be used as a barrier coating for paper, but they are essentially still limited to the scope 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 as a method for improving surface functionality has been studied in traditional coatings, its implementation mechanism is fundamentally different from that of the present invention. For example, the self-stratification mechanism described in U.S. Patent Application Publication No. US20120201965 relies on the huge surface energy difference between the fluorinated component and the non-fluorinated component (such as acrylic or epoxy resin), which is a macroscopic phase separation between heterogeneous materials. This technical teaching does not provide any technical inspiration for how to induce the formation of a quantifiable, functionally synergistic micro-gradient structure by regulating the drying and crystallization kinetics within a fully PHA homologous polymer system with similar chemical structures and weak surface energy differences.

[0006] In summary, the existing technology has failed to provide a solution that can achieve top-level oil resistance, excellent folding resistance and moderate moisture permeability in a single, all-biobased, fluorine-free coating system. The long-standing technical gap in this field is how to break through the limitations of the PHA material's own performance, without introducing heterologous or fluorine-containing functional components, and only utilizing the subtle physical property differences between the same family of PHA polymers. Through a controllable process, a specific physical structure that can synergistically exert barrier and flexibility functions is constructed, and key application indicators such as folding performance retention and hot oil crease penetration time are used as measurement standards. This constitutes a unique technical challenge to be solved by the present invention. Summary of the Invention

[0007] The purpose of the present 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 long-standing inherent contradiction between the barrier properties and flexibility of bio-based polymer coatings within a single coating, 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] Matrix PHA, a semi-crystalline PHA polymer.

[0010] and toughened PHA, a low-crystallinity or amorphous PHA polymer having a glass transition temperature lower than that of the base PHA.

[0011] In the coating, the total mass fraction of PHA is ≥95% based on the total weight of dry solids of the coating; and after drying and film formation, the coating can form a functional gradient structure in the thickness direction of the coating, which satisfies the gradient index GI measured by time-of-flight secondary ion mass spectrometry depth profile ≥1.2, and the relative mass fraction of the toughening PHA in the interface region close to the paper base is higher than its relative mass fraction on the coating surface, and the relative mass fraction of the matrix PHA on the coating surface is higher than its relative mass fraction in the interface region close to the paper base.

[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 toughened PHA is selected from one or more of 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%. In the case of P34HB, the molar fraction of 4-hydroxybutyrate may be 10% to 20%, for example, 10%, 12%, 15%, 18%, or 20%. In the case of PHBHHx, the molar fraction of 3-hydroxyhexanoate may 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 artificially added organic fluorine compounds, and may contain food contact permitted additives in an amount of ≤5% based on the total weight of the coating dry solids, and the additives are selected from one or more of alkyl polysaccharide nonionic surfactants, natural polysaccharide rheology modifiers, low molecular weight polyhydroxyalkanoate oligomers or polyvinyl alcohol.

[0016] Additionally, the coating has a biobased carbon content of ≥85% on dry solids.

[0017] The present invention also provides a high oil resistance hamburger packaging paper, which comprises 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 / m 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 weight of the coating is 3-8g / m 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 has the following properties: an air permeability of 350 to 900 s / 100 mL as tested according to TAPPI T 460; a Kit ≥ 11, and a Kit drop of ≤ 2 after crease preparation according to TAPPI T512 sp-21; a breakthrough time at the crease of ≥ 180 seconds as tested at 60°C using palm kernel oil as a medium according to ISO 16532-1:2008; and a water vapor transmission rate of 300–900 g·m 2 at 37.8°C and 90% relative humidity (RH) according to TAPPI / ANSI T 464 om-12 (R2022). -2 ·24h.

[0018] The present invention also provides a method for preparing the above-mentioned high-oil-resistance hamburger wrapping paper, which comprises: forming the functionally gradient structure on the base paper by two coatings rich in matrix PHA and toughening PHA, respectively, through dual-nozzle synchronous coating, multi-layer curtain co-coating, or multi-layer slot die co-coating; or performing a single coating of the aforementioned coatings and forming a thickness-direction functionally gradient structure with a GI ≥ 1.2 by controlling the temperature and solvent volatilization rate during the drying process.

[0019] Compared with the prior art, the present invention can achieve the following significant beneficial effects:

[0020] Innovative functional gradient structure design and synergistic mechanism: This invention optimizes the technical approach of traditional homogeneous blending. By precisely controlling the drying dynamics, it constructs a spontaneous, 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 toughening PHA with a low glass transition temperature spontaneously accumulates in the interface layer close to the paper base, effectively absorbing and dissipating the stress generated when the paper is folded, acting as a flexible buffer layer to prevent brittle cracking of the coating; while the highly crystalline matrix PHA is enriched 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 in the three key performance dimensions of high oil resistance, moderate moisture permeability and high-speed processing adaptability. 2 With a coating amount of 100%, the top oil resistance of ≥11 after folding can be achieved; at the same time, the water vapor transmission rate (WVTR) is controlled at 300-900g·m -2 Within an ideal 24-hour window, the packaging paper is given a "breathing" function, preventing moisture infiltration from hot food while also preventing excessive water loss that could affect taste. This is superior to PE-coated paper with zero moisture permeability or conventional grease-proof paper with uncontrollable moisture permeability. Furthermore, the crystalline-rich coating surface imparts an ideal static friction coefficient of 0.4–0.5 and excellent anti-blocking properties (no blocking), meeting the requirements of high-speed automated bag-making and packaging production lines while avoiding industrial application challenges such as surface stickiness and paper jams that can arise from excessively high levels of toughening components.

[0022] Environmental friendliness and industrial feasibility of the all-bio-based fluorine-free solution: The present invention provides a high-performance oil-resistant solution without artificially added organic fluorine compounds. Its total fluorine content is less than 50ppm and does not contain PFAS substances that are under key industry control, thereby avoiding the environmental persistence and health risks brought by PFAS. The main body of the coating is composed of PHA with a bio-based carbon content of ≥85%, which has excellent renewability and biodegradability. More importantly, the coating of the present invention does not affect the recycling and reuse of paper, and the repulp fiber yield is as high as more than 98%. Compared with PE coated paper that causes the "viscose" problem, it shows significant circular economy advantages. Its technical route of realizing a gradient structure through a single coating and controlled drying is simple in process and highly compatible with existing paper coating equipment. It does not require the investment of complex multi-layer coating equipment, and provides a practical and feasible technical path for the low-cost and large-scale industrial production of high-performance, environmentally friendly food packaging materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 : 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. Paper base; 2. Central zone of the area where the toughened polyhydroxyalkanoate is higher in the lower part of the coating; 3. Central zone of the area where the base polyhydroxyalkanoate is higher in the upper part of the coating. DETAILED DESCRIPTION

[0025] To make the purpose, technical solutions and advantages of the present invention clearer, the present 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 for the purpose of explaining the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included within the scope of protection of the present invention. Unless otherwise specified, the raw materials used in the present embodiment are all commercially available industrial products or can be prepared by conventional methods. Performance test methods, unless otherwise specified, are carried out in accordance with the standards described in the summary of the invention.

[0026] Main reagents and raw materials:

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

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

[0029]

[0030] Main analytical testing instruments:

[0031] Wire rod coater: K202, RK PrintCoat Instruments Ltd.;

[0032] Blast drying oven: DHG-9140A, Shanghai Yiheng Scientific Instrument Co., Ltd.

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

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

[0035] Density meter: Gurley 4110, Gurley Precision Instruments;

[0036] Crease / anti-fold equipment: T512 crease fixture and pressure roller;

[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 (CIC) system: 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 (ASTM D3359-22 / -23 applicable);

[0043] Grid marker: Elcometer 1542 (1mm / 2mm / 3mm blade 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 test standards:

[0048] All samples were conditioned at 23±1°C and 50±2% RH for at least 24 hours before testing according to TAPPI T402 sp-21 standards.

[0049] Statistical requirements: The test sample size for each indicator should be n ≥ 5; report the mean ± standard deviation; if graded scores are involved, such as Kit or blocking, report the frequency distribution and median.

[0050] Kit: Complies with TAPPI T 559 cm-12 (R2022). While this standard was originally used to evaluate fluorine-containing surface-treated paper, this paper uses it as a comparative grading indicator for non-fluorine coatings. For the post-fold Kit test, a cross fold is first prepared according to TAPPI T512 sp-21, followed by T559 evaluation.

[0051] Grease penetration: Complies with ISO 16532-1:2008.

[0052] Air permeability: Complies with TAPPI T 460 om-21.

[0053] Crease Preparation: Following the principles of TAPPI T512 SP-21, a standardized cross-fold crease was prepared using the following equivalent parameters: Place the sample on a hard, flat surface. A circular folding rod with a width of 3.2 mm was used to create a crease across the sample at a constant speed of approximately 10 mm / s under a load of 1000 ± 50 g. The sample was rotated 90 degrees and the procedure repeated to form a cross-fold crease. The entire procedure was performed under standard temperature and humidity (23 ± 1°C, 50 ± 2% RH). After folding, the sample was allowed to equilibrate in this environment for at least one hour before further testing.

[0054] Water Vapor Transmission Rate: Comply with TAPPI / ANSI T 464 om-12 (R2022), 37.8°C, 90% RH. Refer to TAPPI T 448 om-21 (23°C, 50% RH) if necessary.

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

[0056] Surface water contact angle: Static contact angle measurement was performed using a 5µL water droplet volume, with the contact angle value recorded at 10 seconds. Propulsion angle measurements were also performed, using ASTM D7334-22 as the standard practice.

[0057] Repulpability: Complies with CEPI / 4evergreen Part I (conventional process) v3, 2025 edition.

[0058] Food contact compliance: Sensory testing follows EN 1230-1 / -2:2009, odor / smell, with no fewer than 6 test subjects.

[0059] Verification of Unintentional Addition of Fluorinated Compounds: Cross-verification is performed using the following two methods to confirm that no organofluorinated compounds are intentionally added to the formula:

[0060] 1) Total fluorine content (TF): Tested using combustion-ion chromatography (CIC). The total fluorine content must be less than 50 mg / kg (50 ppm). The limit of quantification (LOQ) of this method is 10 mg / kg.

[0061] 2) Targeted analysis of specific per- and polyfluoroalkyl substances (PFAS): Analysis is performed using liquid chromatography-tandem mass spectrometry (LC-MS / MS). Sample pretreatment is as follows: mince 1g of sample, add 10mL of methanol, and ultrasonically extract at 60°C for 30 minutes. After centrifugation, the supernatant is filtered through a 0.22μm filter for analysis. All target substances on the list of specific PFAS of industry concern (e.g., substances listed in US EPA Method 1633) must be detected. The analytical report must list all target compounds and their respective limits of quantification (LOQs), with the LOQ for each target substance being ≤0.5µg / kg.

[0062] Migration compliance: Refer to GB 4806.8-2022 "Paper and paperboard materials and articles for food contact".

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

[0064] Process Suitability: Blocking / Blocking Test: Conducted using an internal company-controlled method, the test conditions are equivalent to the historical standard ASTM D918-99. The specific steps are as follows: Two 10 cm x 10 cm samples, coated surfaces facing each other, are placed between two clean glass plates, and a uniform pressure of 3.4 kPa is applied. The entire assembly is placed in a constant temperature and humidity chamber at 49°C and 75% RH for 21 hours. After removal, the test is allowed to cool for one hour at a standard temperature and humidity (23 ± 1°C, 50 ± 2% RH). Evaluation criteria: The two samples are manually separated. If they can be separated easily without any damage or adhesion to the coating, the test is scored as "No Blocking"; if there is slight resistance during separation but the coating is intact, the test is scored as "Minor"; if the coating tears or transfers during separation, the test is scored as "Significant". The coefficient of friction is measured according to TAPPI T549, horizontal method, and both the static coefficient of friction (μs) and the dynamic coefficient of friction (μk) are measured. 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 was performed using time-of-flight secondary ion mass spectrometry.

[0067] Sputtering and analysis conditions: using 2keV Cs + The ion beam is used as the sputtering source with a beam current of about 100nA at 300×300µm. 2 The analysis source is Bi 30keV. 3+ Pulsed ion beam, at 100×100µm 2 Data were collected in the center of the sputtering crater to avoid edge effects.

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

[0069] Depth window definition and GI calculation: The total thickness of the coating is determined by monitoring the characteristic signal (such as Si + or Al + The intensity of the secondary ions is determined by a sudden increase in the sputtering signal (spatter) and confirmed by combining a pre-calibrated sputtering rate (approximately 0.5 nm / s) on a pure PHA film of known thickness. The "surface" is defined as the region from the outermost surface to a depth of 10% of the total thickness, and the "interface" is defined as the region extending from the paper-based interface upward to 10% of the total thickness. The average value of the characteristic secondary ion intensity in these two regions is calculated, and the gradient index (GI) is 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 is the intensity of the corresponding characteristic secondary ion normalized by the total PHA signal. When GI ≥ 1.2, an effective functional gradient structure is considered to have been formed.

[0072] Examples and Comparative Examples:

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

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

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

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

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

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

[0079] Step 5. The homogenized emulsion is cooled to room temperature while stirring to obtain a milky white, stable PHA aqueous dispersion. The solid content of each dispersion prepared is 40-45wt%, and the average particle size is D 50 The prepared base 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 ingredients according to the quantities listed in Table 2. First, mix the base PHA aqueous dispersion, toughening PHA1 aqueous dispersion, and toughening PHA2 aqueous dispersion until the total solids content is 100 parts. Then, add the corresponding quantities of APG, natural polysaccharides, and other additives listed in Table 2 to this 100 parts PHA solids weight, and continue stirring until thoroughly mixed. Finally, adjust the solids 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] Use a No. 12 wire rod coater to evenly coat the prepared coating on a 40g / m 2 On the food grade base paper, the dry basis coating amount is controlled to be about 5g / m 2 .

[0084] Table 2 Main materials of the formula of the embodiment and the comparative example:

[0085]

[0086] The present invention adopts the following two main drying processes:

[0087] Gradient Drying Process: After coating, a two-stage gradient drying process is employed. First, slow drying is performed at a chamber temperature of 60°C for 120 seconds, followed by rapid drying at a chamber temperature of 120°C for 30 seconds. Parallel intermittent sampling and weighing, along with infrared temperature measurement of the membrane surface (see instrument list), track the timeline of the solids content increasing from approximately 30wt% to over 95wt%. This confirms the drying trajectory of "slow volatilization followed by rapid densification," thereby inducing the formation of a functionally gradient structure.

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

[0089] Preparation of each embodiment and comparative example:

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

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

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

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

[0094] The basic performance tests were performed on the coated paper samples prepared in the above examples and comparative examples. 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, the coating is considered to form a functional gradient structure in the thickness direction.

[0098] 2 Note: Refers to the Kit level value after the discount compared to the pre-discount value.

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

[0100] Application performance evaluation:

[0101] In order to fully verify the performance of the coated paper of the present invention in actual 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 exposed to high-temperature grease, especially at the weak link after folding, a hot oil penetration test was conducted. The results are shown in Table 4.

[0103] Table 4 Hot oil penetration results of Examples and Comparative Examples (ISO 16532-1, creased samples, 60°C):

[0104]

[0105] Analysis: This test simulated the most demanding use scenario of a folded hot oil burger. All examples achieved breakthrough times far exceeding the target of 180 seconds, demonstrating excellent resistance to hot oil folding. Comparative Example 1, a pure matrix, failed almost instantly due to brittleness; while Comparative Example 3, a homogeneous blend, performed far worse than the gradient structure of Example 2, further demonstrating the necessity of the gradient structure. Comparative Examples 2 and 4, while flexible, exhibited insufficient barrier properties, resulting in significantly shorter oil retention times than the examples of the present invention.

[0106] To evaluate the ability of the packaging paper to regulate the humidity inside the package and prevent the accumulation of water vapor in the food, which would affect the taste, the water vapor transmission rate of the samples was tested. The results are shown in Table 5.

[0107] Table 5 Water vapor transmission rate results of the examples and comparative examples (T 464, 37.8°C, 90% RH, unfolded samples):

[0108]

[0109] Analysis: Water vapor management is crucial to maintaining the texture of the burger. The WVTR values ​​of all examples were between 300 and 900 g·m -2 The ideal 24-hour range achieves "moderate moisture permeability." Comparative Example 1 is virtually impermeable to water vapor, allowing food to become damp. Comparative Examples 2 and 4, however, exhibit excessively high WVTRs, potentially causing food to lose water too quickly and dry out. 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 nature of the coated paper, we conducted a comprehensive evaluation of samples according to the internationally recognized industrial composting standard EN13432 / ASTM D6868. This test simulates industrial composting conditions, requiring the material to biodegrade and disintegrate within a specified timeframe, with the composted product being non-toxic to plant growth. This is a key indicator of whether the material truly achieves organic recycling and returns to nature.

[0111] Table 6 Composting results of Examples and Comparative Examples:

[0112]

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

[0114] In stark contrast, the conventional polyethylene (PE) coated paper, used as a negative control, showed little disintegration under the same conditions. This highlights the fundamental advantages of the present invention's solution over traditional petrochemical-based plastic coated paper in terms of waste disposal and environmental impact. The hamburger paper of this invention can be disposed of in an industrial composting system, ultimately converting it into valuable compost, completing the 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 ensure that it would not adversely affect the flavor of food, a sensory evaluation of odor and taste was conducted, and the results are shown in Table 7.

[0116] Table 7 Summary of sensory results (EN 1230-1 / -2) of 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-flavor or odor. This demonstrates that the material system employed in this invention is inherently safe for food contact and does not affect food flavor.

[0119] To verify that the present 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 samples. The results are shown in Table 8.

[0120] Table 8 PFAS falsification results of Examples and Comparative Examples (CIC-TOF+targeted LC-MS / MS):

[0121]

[0122] ¹Note: The limit of quantification (LOQ) for all target compounds in the targeted PFAS 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. Highly sensitive LC-MS / MS targeted analysis also revealed no specific PFAS substances of industry concern. This dual validation strongly demonstrates that the coating system of this invention is a truly fluorine-free solution, without the artificial addition of organic fluorine compounds.

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

[0125] Table 9 Blockage and friction coefficient results (T 549) of the embodiments and comparative examples:

[0126]

[0127] Analysis: The examples of the present invention exhibit ideal friction coefficients and excellent anti-blocking properties due to their crystalline-rich surface layers, making them ideal for high-speed automated bag making and packaging. Comparative Example 1 exhibits an overly slippery surface, potentially leading to unstable conveying. Comparative Examples 2 and 4, which contain high levels of toughened PHA, exhibit a sticky surface, easily causing blocking and paper jams during processing.

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

[0129] Table 10 Total migration test results of Example and Comparative Example GB 4806.8-2022:

[0130]

[0131] Analysis: Test results show that the total migration levels for all examples and comparative examples are well below the national standard limit of 10 mg / dm². This fully demonstrates that both the functionally gradient coating and the homogeneous or single-component PHA coatings of the present invention possess a high degree of chemical inertness. Under conditions simulating long-term contact with high-temperature, high-fat foods, virtually no substances migrate into the food, ensuring the highest safety standards for food contact materials.

[0132] Comprehensive analysis of application performance:

[0133] The comprehensive results of these application performance tests clearly demonstrate the comprehensive advantages of the present invention over its comparative counterparts. Whether using pure component materials, homogeneous blends, or even formulations outside the specified range, none of these achieve the same optimal balance of hot oil barrier, steam management, process sealing, and food safety as the gradient structure coating of the present invention.

[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 gradient structure created through a gradient drying process. A comparison of Example 2 and Comparative Example 3 clearly reveals the crucial role of this structure. The two samples have identical chemical formulas (base PHA:toughening PHA 1 = 90:10), differing only in the drying process. The former utilizes gradient drying to create a gradient structure (GI = 1.65), while the latter utilizes rapid drying to create a homogeneous structure (GI = 1.03).

[0137] The basic performance data in Table 3 demonstrates that this structural difference directly leads to a significant difference in performance. When unfolded, both samples initially achieved the highest Kit rating of 12, demonstrating that even homogeneous blends achieve good barrier properties under static conditions. However, after a folding operation simulating actual use, the Kit rating of Example 2, with its gradient structure, dropped only one level to 11, demonstrating excellent flexibility and structural stability. In contrast, the Kit rating of Comparative Example 3, with its homogeneous structure, plummeted by five levels to 7 after folding, significantly degrading its oil barrier properties.

[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 and prevents the coating from cracking; while the rigid matrix PHA enriched in the surface layer is able to maintain its structural integrity and continue to exert its excellent barrier function. In addition, the hot oil penetration test results in Table 4 further confirm this point. The breakthrough time of Example 2 at the crease is as long as 260 seconds, which is much higher than the 95 seconds of Comparative Example 3. At the same time, the gradient structure also improves the bonding strength between the coating and the paper base. The adhesion of Example 2 reaches the highest level of 5B, which is better than the 4B level of Comparative Example 3. These data together prove that it is the functional gradient structure of the present invention, rather than a simple blend of chemical components, that achieves the synergistic unity of oil resistance and folding resistance.

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

[0140] To validate the scientific nature of the specified ratio range (base PHA:toughening PHA, 95:5 to 60:40) in this invention, we systematically examined the effects of increasing the toughening agent content of toughening PHA1 from 5 to 40 parts per part on various properties in Examples 1 to 5.

[0141] Trend of barrier properties: With the increase of toughening PHA content (from Example 1 to Example 5), the barrier properties of the coating show a predictable and gradual change. Table 3 shows that the Kit grade before folding slightly decreased from 12 to 11, which still remains at a high level. However, the change in gas permeability index is more sensitive: Gurley permeability decreased from 520s / 100mL to 360s / 100mL, indicating that the gas barrier property has decreased. The water vapor transmission rate (WVTR) data in Table 5 also show the same trend, from 480g·m -2 Increased to 810 g·m in 24 hours -2 24 hours. This is consistent with theoretical expectations, as the low-crystallinity toughening PHA inherently has higher permeability than the high-crystallinity matrix PHA. However, within the 40% content upper limit defined by the present invention, the air permeability and WVTR values ​​of all examples remained within the target window for hamburger paper applications.

[0142] Stability of folding resistance: Throughout the entire formulation range defined in the present invention, the folding resistance remains at an extremely high level. As shown in Table 3, the Kit grade drop of Examples 1-5 stably remains at 1 level, which indicates that even with the addition of only 5 parts of toughening PHA (Example 1), the gradient structure formed is sufficient to effectively protect the coating from catastrophic damage when folded.

[0143] Verification of the boundaries 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 the present invention. Although its gradient structure is still effectively formed (GI = 1.41) and the folding resistance is excellent (Kit grade drops by 1 level), its barrier properties have exceeded the application limit. Its initial Kit grade is only 9 levels, the Gurley air permeability is less than 100 s / 100 mL, and the WVTR is more than 1500 g·m -2 ·24h, which makes it unable to meet the basic requirements of high oil barrier packaging.

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

[0145] 3. Universality of different toughening systems.

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

[0147] First, Comparative Example 2 (matrix:toughening agent 1 = 90:10) and Example 6 (matrix:toughening agent 2 = 90:10) were compared, both of which used a single toughening agent but had different chemical structures. The results showed that the two core performance indicators were highly consistent: both formed effective gradient structures (GI was 1.65 and 1.63, respectively), and the Kit grade drop after folding was 1 level. In terms of adhesion, hot oil permeation, water vapor transmission rate, and processing performance, there was no significant difference between the two.

[0148] Next, we compared single toughening agent systems with blends. For example, in Examples 3 (base:toughening agent 1 = 80:20), 7 (base:toughening agent 2 = 80:20), 9 (base:toughening agent 1:toughening agent 2 = 80:15:5), and 10 (base:toughening agent 1:toughening agent 2 = 80:5:15), the ratio of base PHA to total toughening PHA was 80:20. The data in Tables 3 and 4 demonstrate that, regardless of whether a single toughening agent was used or two toughening agents were blended in varying ratios, effective gradient structures were formed. The post-fold Kit rating reduction remained consistently at level 1, and the hot oil penetration time was consistently above 225 seconds, demonstrating highly consistent and excellent performance.

[0149] These results clearly demonstrate that the core advantage of this invention lies in the physical gradient structure formed through a specific process. This principle has excellent platform versatility and is not limited to a specific toughening PHA chemical structure or a single toughening agent. As long as the toughening component (whether a single polymer or a blend) meets the requirements of low crystallinity or amorphous state and a glass transition temperature lower than that of the matrix PHA, it can be used.

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

[0151] Those skilled in the art will appreciate 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 to the technical solutions of the present invention that fall within the spirit and principles of the present invention shall be included within the scope of protection 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 a water-based dispersion and comprises: Base polyhydroxyalkanoate: a semicrystalline polyhydroxyalkanoate polymer; Toughened polyhydroxyalkanoate: a low-crystallinity or amorphous polyhydroxyalkanoate polymer having a glass transition temperature lower than that of the base polyhydroxyalkanoate; The total mass fraction of polyhydroxyalkanoate is ≥95% based on the total dry solid weight of the coating; After drying and forming a film, the coating can form a functional gradient structure in the thickness direction of the coating and satisfy a gradient index GI of ≥1.2 as measured by time-of-flight secondary ion mass spectrometry depth profile; the relative mass fraction of the toughened polyhydroxyalkanoate in the interface region near the paper base is higher than its relative mass fraction on the coating surface, and the relative mass fraction of the base polyhydroxyalkanoate on the coating surface is higher than its relative mass fraction in the interface region near the paper base; No organic fluorine compound is artificially added to the coating.

2. The coating according to claim 1, characterized in that The total dry weight ratio of the base polyhydroxyalkanoate to the toughened polyhydroxyalkanoate is 95:5 to 60:

40.

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

4. The coating according to claim 1, characterized in that The coating contains food contact approved additives in an amount of ≤5% 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 polyhydroxyalkanoate oligomers or polyvinyl alcohol.

5. The coating according to claim 1, characterized in that The coating has a biobased carbon content of ≥85% on dry solids.

6. A high oil resistance hamburger packaging paper, characterized in that: The structure thereof comprises a base paper layer and a coating layer formed on at least one surface of the base paper layer by the coating material according to any one of claims 1 to 5.

7. The high oil resistance hamburger wrapping paper according to claim 6, characterized in that: The basis weight of the base paper layer is 35-60 g / m 2 The dry coating weight of the coating is 3-8 g / m 2 .

8. The high oil resistance hamburger wrapping paper according to claim 6, characterized in that: The packaging paper has the following properties: an air permeability of 350 to 900 s / 100 mL as tested according to TAPPIT 460; a Kit rating of ≥ 11, with a Kit drop of ≤ 2 after creases prepared according to TAPPI T512 sp-21; a breakthrough time at the crease of ≥ 180 seconds as tested using palm kernel oil at 60°C according to ISO 16532-1:2008; and a water vapor transmission rate of 300–900 g·m·s as tested according to TAPPI / ANSI T 464 om-12 (R2022) at 37.8°C and 90% relative humidity. -2 ·24h.

9. A method for preparing the high oil-resistance hamburger wrapping paper according to any one of claims 6 to 8, characterized in that: The method includes: The functional gradient structure is formed on the base paper by two coatings respectively rich in base polyhydroxyalkanoate and toughened polyhydroxyalkanoate through dual-nozzle synchronous coating, multi-layer curtain co-coating or multi-layer slot die co-coating; Alternatively, the coating according to any one of claims 1 to 6 is applied once, and by controlling the temperature and solvent evaporation rate during the drying process, vertical phase separation of different polyhydroxyalkanoate components is induced to form the functional gradient structure.

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