Paper-based PHA double-sided composite soil biodegradable agricultural mulch film, its preparation method and uses

Abstract

This invention discloses a paper-based PHA double-sided composite biodegradable agricultural mulch film for soil, its preparation method, and its applications, belonging to the technical fields of agricultural covering materials and green polymer materials. The mulch film includes a paper base layer and a polyhydroxyalkanoate (PHA) covering layer on its surface. Through the synergistic effect of a water-based PHA coating on the soil-facing side and a thermoplastic PHA extruded film layer on the windward side, combined with a "wet-to-heat" integrated composite process and trace anhydride grafting, a super-strong bond at the fiber-tear level is achieved at the paper-plastic interface, overcoming the traditional problem of easy delamination in paper-plastic composites. Compared to traditional fluorine-containing products, this invention achieves superior barrier and moisture retention performance with a thinner coating thickness, while maintaining undetectable total organic fluorine levels, utilizing a double barrier. The product has excellent mechanical laying adaptability and long-season weather resistance, and is fully biodegradable, completely solving the problems of microplastic residues and persistent organic pollutants, making it an ideal green agricultural covering material.

Description

Technical Field

[0001] This invention belongs to the technical field of agricultural covering materials and green polymer materials, specifically relating to paper-based-PHA double-sided composite soil biodegradable agricultural mulch film, its preparation method and uses. Background Technology

[0002] While traditional polyethylene (PE) mulch films offer significant yield increases and moisture retention in agricultural production, their slow degradation and the accumulation of residual film in the topsoil have become a serious agricultural non-point source pollution problem. This not only damages soil structure but also hinders crop root growth. Therefore, developing environmentally friendly biodegradable mulch films and paper-based covering materials has become a trend in the industry.

[0003] Paper-based materials, derived from natural plant fibers, possess good biodegradability. However, their hydrophilic and porous structure leads to poor water resistance and wet strength, necessitating modification through coating or lamination techniques to adapt them to agricultural environments. Among numerous modifying materials, polyhydroxyalkanoates (PHAs) have attracted significant attention due to their excellent biocompatibility and complete soil biodegradability. Early studies have disclosed attempts to extrude and laminate paper or paperboard using poly(3-hydroxybutyrate-3-hydroxyvalerate) (PHBV), exploring the impact of paper moisture content on lamination adhesion performance.

[0004] However, existing paper-plastic composite technologies still have shortcomings in controlling the bonding interface of homogeneous materials. On the one hand, although it is known that moisture affects adhesion, there is a lack of systematic quantitative research on the synergistic effects of specific process sequences (such as the "wet-to-heat" interval) and trace chemical modifications (such as anhydride grafting). This makes it difficult for the bonding strength of the composite interface to reach the "fiber tearing" level of failure mode, and the film layer is prone to delamination from the paper substrate during use. On the other hand, commercially available water-based PHA dispersions or coating technologies usually require the addition of a high proportion of surfactants or non-PHA polymers to maintain system stability. This not only reduces the bio-based purity of the coating but may also affect the final mineralization rate of the material in soil.

[0005] Furthermore, the current biodegradable mulch film market is mainly dominated by all-plastic films made of polybutylene terephthalate-butylene adipate (PBAT) or polylactic acid (PLA), and their degradation cycle and synergy with paper-based materials still need optimization. Meanwhile, with increasingly stringent global environmental regulations on perfluorinated and polyfluoroalkyl substances (PFAS), fluorinated additives commonly used in traditional waterproof and oil-resistant paper products face compliance challenges. Therefore, developing a paper-based PHA composite agricultural mulch film with strong interfacial bonding, no fluorides, and controllable soil degradation properties is of great significance for promoting the development of green agriculture. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a paper-based PHA double-sided composite soil biodegradable agricultural mulch film, its preparation method and uses.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a paper-based PHA double-sided composite biodegradable agricultural mulch film for soil, comprising a paper base layer with a basis weight of 25 g / m² to 300 g / m², wherein the basis weight of the paper base layer is, for example, 25 g / m², 30 g / m², 35 g / m², 50 g / m², 70 g / m², 80 g / m², 100 g / m², 150 g / m², 160 g / m², 200 g / m², 250 g / m², or 300 g / m², and PHA covering layers disposed on both sides of the paper base layer. The PHA covering layers include a water-based PHA coating disposed on the soil-contacting surface of the paper base layer and a thermoplastic PHA extruded film layer disposed on the windward and rain-facing surface of the paper base layer.

[0009] The aqueous PHA coating has a dry coating weight of 5 g / m² to 25 g / m², for example, 5 g / m², 6 g / m², 8 g / m², 10 g / m², 12 g / m², 15 g / m², 18 g / m², 20 g / m², 22 g / m² or 25 g / m²; the thermoplastic PHA extruded film layer has a thickness of 20 μm to 80 μm, for example, 20 μm, 25 μm, 30 μm, 32 μm, 35 μm, 40 μm, 50 μm, 60 μm, 75 μm or 80 μm.

[0010] The agricultural mulch film is subjected to a wet-to-heat sequence of 20 to 60 minutes, for example, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, or 60 minutes, and the residual water content of the coating on the soil surface during lamination is 5.0 wt% to 7.0 wt% (on a dry basis), for example, 5.0 wt%, 5.2 wt%, 5.3 wt%, 5.4 wt%, 5.5 wt%, 5.6 wt%, 5.8 wt%, 6.0 wt%, 6.2 wt%, 6.5 wt%, 6.8 wt%, or 7.0 wt%, etc. The windward surface is provided with a PHA-grafted anhydride compatibility layer containing anhydride groups, the mass fraction of which is 0.07wt% to 0.15wt%, for example, 0.07wt%, 0.08wt%, 0.09wt%, 0.10wt%, 0.11wt%, 0.12wt%, 0.13wt%, or 0.15wt%, etc., and the surface tension of the paper is ≥40mN / m before lamination, for example, 40mN / m, 41mN / m, 42mN / m, or 45mN / m, etc.

[0011] The PHA covering layer exhibits a biodegradability of ≥90% in aerobic soil after 24 months according to ISO 17556:2019 standard. The total organic fluorine (TOF) content of both the finished mulch film and the PHA covering layer was undetectable by EN 14582:2016 oxygen bomb combustion-ion chromatography, with a method detection limit ≤5 mg / kg. The PHA is selected from one or more of short-chain PHA, medium- and long-chain PHA, copolymers formed from short-chain PHA monomers and medium- and long-chain PHA monomers, and their grafted modified forms. The short-chain PHA is selected from one or more of poly(3-hydroxybutyrate) (P3HB), poly(3-hydroxybutyrate-4-hydroxybutyrate) (P(3HB-co-4HB)), or poly(3-hydroxybutyrate-3-hydroxyvalerate) (PHBV); the medium- and long-chain PHA is selected from one or more of poly(3-hydroxyhexanoate), poly(3-hydroxyheptanoate), poly(3-hydroxynonanoate), poly(3-hydroxydecanoate), poly(3-hydroxyundecanoate), poly(3-hydroxydodecanate), poly(3-hydroxytridecanate), or poly(3-hydroxytetradecanoate).

[0012] Furthermore, the soil-adhering waterborne PHA coating is a homologous, internally stable system with a PHA solids content ≥97wt%, a total non-PHA polymer content ≤1.0wt%, a total strong surfactant content ≤0.5wt%, and a total wax content ≤3.0wt%, all on a dry basis. The total organic fluorine content in the coating and its preparation formulation is undetectable, and the pH value is between 6.4 and 6.8, for example, 6.4, 6.5, 6.6, 6.7, or 6.8; particle size D... 50 The particle size is 0.18 μm to 0.22 μm, for example, 0.18 μm, 0.19 μm, 0.20 μm, 0.21 μm or 0.22 μm; the solid content is 38 wt% to 42 wt%, for example, 38 wt%, 39 wt%, 40 wt%, 41 wt% or 42 wt%.

[0013] Furthermore, the soil-adhering waterborne PHA coating contains 12wt% to 16wt% of PHA oligomers, such as 12wt%, 13wt%, 14wt%, 15wt%, or 16wt%, based on the coating polymer solids, with a number-average molecular weight of 3kDa to 8kDa, such as 3kDa, 4kDa, 5kDa, 6kDa, 7kDa, or 8kDa, and 2.5wt% to 3.5wt% of PHA-half ester salts, such as 2.5wt%, 2.8wt%, 3.0wt%, 3.2wt%, or 3.5wt%, based on the coating polymer solids. The neutralization degree of the PHA-half ester salts is 40% to 65%, such as 40%, 45%, 50%, 55%, 60%, or 65%. Furthermore, the water-based PHA coating on the soil surface, as the coating body, has a biodegradability of ≥90% after 90 days of testing according to ISO 17556 standard under aerobic soil conditions at 25℃ to 30℃.

[0014] Furthermore, the Cobb of the sample after coating the soil surface. 60 The value decreased by ≥30% compared to uncoated paper, and when the paper basis weight was between 25 g / m² and 160 g / m², the Cobb coating value decreased significantly. 60 Values ​​≤30g / m², such as 26g / m², 27g / m², 28g / m², 29g / m², or 30g / m², etc.; when the paper basis weight is between 160g / m² and 300g / m², the Cobb coating value is... 60 Values ​​≤ 40g / m², such as 33g / m², 34g / m², 35g / m², or 40g / m², etc.

[0015] Furthermore, the thermoplastic PHA extruded film layer is a multilayer co-extruded structure, comprising a PHA-grafted anhydride compatibility layer with a thickness of 2μm to 6μm, such as 2μm, 3μm, 4μm, 5μm or 6μm, and a homologous blended functional layer with a thickness of 18μm to 75μm, such as 18μm, 20μm, 27μm, 30μm, 32μm, 35μm, 37μm, 40μm, 50μm, 60μm or 75μm; and the sum of the thicknesses of the compatibility layer and the functional layer is 20μm to 80μm.

[0016] To illustrate the structure of the present invention more intuitively, it is described in conjunction with the accompanying drawings. The drawings are schematic cross-sectional views of the paper-based PHA composite soil-degradable agricultural mulch film of the present invention. As shown, the mulch film includes a paper base layer and a water-based PHA coating disposed on one side of the base layer. On the other side of the paper base layer, a thermoplastic PHA extruded film layer is disposed on the windward side. This film layer specifically consists of a PHA-grafted anhydride compatible layer adjacent to the paper base layer and a homogeneous blended functional layer located on the outermost side.

[0017] The PHA-grafted anhydride compatibility layer contains 0.07 wt% to 0.15 wt% of anhydride groups, and the source of the anhydride groups, based on the polymer of the compatibility layer, is selected from maleic anhydride or itaconic anhydride. The failure mode of the paper-film interface after lamination is fiber tearing.

[0018] Furthermore, the wind-facing side or the side to be laminated is subjected to corona, plasma, or flame surface activation treatment before lamination, with an equivalent activation energy of 1.0 kJ / m² to 1.4 kJ / m², such as 1.0 kJ / m², 1.1 kJ / m², 1.2 kJ / m², 1.3 kJ / m², or 1.4 kJ / m², etc., and the surface tension after treatment is ≥40 mN / m.

[0019] Furthermore, the PHA coating layer satisfies any of the following optical properties: the transmittance T of the black opaque film at 600 nm. 600≤3%, such as 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 2.0% or 2.5%; or the total reflectance of the white microbubble film in the 400nm to 700nm band under the integrating sphere method is ≥65%, such as 65%, 68%, 70%, 72% or 75%.

[0020] Furthermore, the pulp fiber system of the paper base layer is selected from one or more of bamboo pulp, herbaceous crop straw pulp and wood pulp. The herbaceous crop straw pulp is one or more of rice straw pulp, wheat straw pulp, bagasse pulp or reed pulp. The wood pulp is one or more of softwood pulp or hardwood pulp. The total mass fraction of bamboo pulp or herbaceous crop straw pulp is ≥50wt%, and it contains 0.2wt% to 1.0wt% of cellulose nanofibers (CNF), for example, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.8wt% or 1.0wt%. The paper base layer satisfies a Bekk smoothness of 180s to 500s, for example, 180s, 185s, 190s, 200s, 210s, 260s, 270s, 300s, 340s, 355s, 400s, 420s, or 500s; and a Gurley permeability of 80s / 100mL to 500s / 100mL, for example, 80s / 100mL, 85s / 100mL, 90s / 100mL, 100s / 100mL, 120s / 100mL, 150s / 100mL, 160s / 100mL, 200s / 100mL, 260s / 100mL, 275s / 100mL, 310s / 100mL, 400s / 100mL, or 500s / 100mL.

[0021] Furthermore, the paper base layer is provided with internal penetration reinforcement, staggered micro-cuts and edge pressing lines, and the reinforcement amount is 3g / m² to 5g / m², for example, 3g / m², 3.5g / m², 4g / m², 4.5g / m² or 5g / m², etc. The internal penetration reinforcement is carried out by spraying or impregnation with a homologous internally stable emulsion dilution with a solid content of 10wt% to 15wt%, for example, 10wt%, 11wt%, 12wt%, 13wt%, 14wt% or 15wt%, etc. The length of the staggered micro-cuts is 1mm to 2mm, for example, 1mm, 1.2mm, 1.5mm, 1.8mm or 2mm; the spacing is 10mm to 20mm, for example, 10mm, 12mm, 15mm, 18mm or 20mm; the width of the edge pressing line is 15mm to 25mm, for example, 15mm, 18mm, 20mm, 22mm or 25mm, to balance pressure relief and tear resistance.

[0022] Furthermore, the water vapor transmission rate of the entire film relative to the uncoated paper decreases by 45% to 70%, for example, 45%, 47%, 49%, 50%, 55%, 60%, 62%, 65%, or 70%, etc., tested according to the ASTM E96 / E96M-24a cup method BW procedure, under test conditions of 38°C and 90%RH. In addition, the 180° peel strength at the paper-film interface after lamination is ≥1.5N / 25mm, for example, 1.5N / 25mm, 1.6N / 25mm, 1.7N / 25mm, 1.8N / 25mm, 1.9N / 25mm, 2.0N / 25mm, 2.1N / 25mm, or 2.3N / 25mm, etc. The bio-based carbon content of the PHA coating layer or its constituent polymer solids is ≥90%, for example, 90%, 92%, 95%, or 96%, etc., tested according to ISO 16620-2:2019 standard.

[0023] The present invention also provides a method for preparing the above-mentioned agricultural mulch film, comprising the following steps:

[0024] Step 1. Form a base paper with a weight of 25g / m² to 300g / m² to obtain the paper base layer;

[0025] Step 2. PHA-anhydride is obtained by reactive extrusion grafting, hydrolyzed and partially neutralized to form PHA-half ester salt, and a PHA-half ester salt solution with a pH value of 6.4 to 6.8 is prepared.

[0026] Step 3. Preheat the PHA-half ester salt solution obtained in Step 2 to 85°C to 95°C, for example, 85°C, 88°C, 90°C, 92°C, or 95°C, and mix it with molten PHA in a pressurized emulsifying kettle. Control the mass ratio of PHA-half ester salt to PHA to be 2:100 to 5:100, for example, 2:100, 3:100, 4:100, or 5:100, and perform high-shear emulsification to obtain a solid content of 38wt% to 42wt% and a particle size D. 50 Internally stable emulsions of the same family with a thickness of 0.18 μm to 0.22 μm;

[0027] Step 4. Apply the same family of internally stabilized emulsion to the soil-adhering surface of the paper base layer, apply at a dry coating amount of 5g / m² to 25g / m² and dry in sections at 80℃ to 110℃, for example, at 80℃, 90℃, 100℃ or 110℃, until the exit paper temperature is ≤55℃, to obtain a single-sided coated paper base layer.

[0028] Step 5. Surface activation is performed on the windward side of the single-sided coated paper substrate, and the equivalent activation energy is controlled to be 1.0 kJ / m² to 1.4 kJ / m² to obtain an activated coated paper substrate with a surface tension ≥ 40 mN / m.

[0029] Step 6. Extrude or co-extrude the PHA resin, controlling the melt temperature to 160°C to 185°C, for example, 160°C, 165°C, 170°C, 175°C, 180°C or 185°C, to obtain a thermoplastic PHA melt curtain with a total thickness of 20μm to 80μm and a compatibility layer thickness of 2μm to 6μm.

[0030] Step 7. Following the wet-to-heat integrated lamination process, the thermoplastic PHA melt curtain is laminated with the windward side of the activated coated paper base layer. The time from the completion of Step 4 to entering the lamination roller is controlled to be 20 min to 60 min, and the residual water content of the coating on the base layer is 5.0 wt% to 7.0 wt% during lamination. The residual water content is calculated by cutting a certain area of ​​coated paper sample, drying it to constant weight in an oven at (105±2)℃, and then calculating it as (wet weight - dry weight) / dry weight × 100%. The wet-to-heat integrated lamination process refers to the process of making the melt curtain contact the windward side of the paper base layer at the lamination roller and immediately cooling and shaping it while the coating on the base layer is in the above-mentioned residual water content range, thereby obtaining the paper-plastic composite roll material.

[0031] Step 8. After the paper-plastic composite roll is left to stand at room temperature for ≥24 hours, it is cut to obtain the finished paper-based PHA soil-degradable agricultural mulch film.

[0032] The white microfoamed membrane is prepared by injecting high-pressure carbon dioxide at a pressure of 5.8 MPa to 6.2 MPa, such as 5.8 MPa, 5.9 MPa, 6.0 MPa, 6.1 MPa, or 6.2 MPa; foaming at a temperature of 155°C to 162°C, such as 155°C, 158°C, 160°C, or 162°C; and a draw ratio of 1.55 to 1.70, such as 1.55, 1.60, 1.65, or 1.70.

[0033] The present invention also provides the use of the above-mentioned agricultural mulch film in farmland mulching cultivation, for suppressing weeds, conserving moisture and regulating temperature, and for plowing into the soil after crop harvest. The PHA covering layer has a biodegradability of ≥90% within 24 months according to ISO 17556 standard.

[0034] Compared with the prior art, the following significant advantages can be obtained by using the present invention:

[0035] Low moisture absorption rate and water vapor transmission rate: This invention utilizes a double-sided composite structure of a PHA water-based coating on the soil surface and a PHA thermoplastic film layer on the windward side. By employing a dual mechanism of "coating sealing + film layer barrier", it significantly reduces the moisture absorption rate and water vapor transmission rate of the paper-based material, effectively cuts off the water vapor channel, solves the problem of easy moisture absorption and failure of traditional paper mulch films, and achieves efficient moisture management and moisture retention functions.

[0036] High mechanical peel strength and puncture resistance: This invention innovatively adopts a "wet-to-heat" integrated composite process and a compatibility layer design containing anhydride groups. Under specific moisture residue and activation energy windows, it promotes hydrogen bonding and chemical bridging between paper fibers and PHA molecular chains, realizing a high-strength fiber tearing failure mode at the paper-plastic interface. This significantly improves the mechanical peel strength and puncture resistance of the mulch film, ensuring the integrity of long-season coverage.

[0037] Wide range of applications: This invention achieves precise control of the optical properties of mulch film. The black mulch film has extremely low light transmittance to ensure excellent weed suppression effect, while the white mulch film obtains high reflectivity through micro-foaming structure to reduce soil temperature, thus meeting the agronomic needs of different crops and seasons.

[0038] Green and environmentally friendly: All products of this invention are based on bio-based PHA materials, contain no fluorides, meet the stringent requirements of EN14582 standard regarding the absence of total organic fluorine, and have excellent biodegradability in soil. The degradation products are carbon dioxide and water, which completely eliminates the risks of microplastic residues and persistent organic pollutants, and realizes a green ecological closed loop from source to end. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the cross-sectional structure of the paper-based PHA composite soil-degrading agricultural mulch film of the present invention.

[0040] In the diagram, 1-soil-attached water-based PHA coating; 2-paper base layer; 3-PHA grafted anhydride compatibility layer; 4-homogeneous blended functional layer. Detailed Implementation

[0041] 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.

[0042] like Figure 1As shown, this agricultural mulch film is composed of a multi-layered composite structure. Number 1 in the diagram represents the soil-contacting water-based PHA coating, which is located at the bottom layer of the structure and is used for contact with the soil. Number 2 represents the paper base layer, which serves as the skeleton material and has a relatively large thickness with an internal fibrous interwoven structure, located above coating 1. Immediately above the paper base layer 2 is the extremely thin PHA-grafted anhydride compatibility layer, shown by number 3, which enhances the interfacial bonding force. At the top layer of the structure is the homogeneous blended functional layer, shown by number 4. Together with the compatibility layer 3, it forms a thermoplastic PHA extruded film layer on the windward side through a multi-layer co-extrusion process, providing the mulch film with barrier and mechanical protection functions.

[0043] Main reagents and raw materials:

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

[0045]

[0046] Main analytical and testing instruments:

[0047] Table 2 mainly analyzes the names, models, and manufacturers of the testing instruments:

[0048]

[0049] The "equivalent activation energy" E (kJ / m²) during corona or plasma treatment is calculated using the following formula:

[0050] E = P / (v × w), where P is the processing power (kW), v is the strip linear speed (m / s), and w is the effective strip width (m).

[0051] After treatment, the surface tension is quickly assessed using a dyne pen or surface tension test solution. The highest dyne value that can be evenly spread within 2 seconds and does not shrink within 30 seconds is recorded and converted into surface tension (mN / m) to determine whether it reaches ≥40mN / m.

[0052] Main testing standards:

[0053] Water vapor transmission rate cup method: According to ASTM E96 / E96M-24a cup method BW procedure "Standard Test Method for Water Vapor Transmission Rate of Materials", the test conditions are 38℃ and relative humidity (RH) 90%; water vapor transmission rate (WVTR) is expressed in g / (m²·d). The WVTR decrease (%) is calculated according to [WVTR... 未涂布纸 -WVTR 样品 ] / WVTR 未涂布纸 Calculated as ×100%, where WVTR 未涂布纸 The test values ​​are for the same batch and basis weight of the original paper (uncoated / unlaminated).

[0054] Cobb 60 Water absorption: Determined according to GB / T 1540-2002 "Determination of water absorption of paper and paperboard (Cobb method)". The Cobb method... 60 Decrease (%) by [Cobb] 60未涂布纸 -Cobb 60涂覆后试样 / Cobb 60未涂布纸 Calculated by ×100%, where Cobb 60未涂布纸 The test value is for the same batch and basis weight of the base paper (uncoated / unlaminated) as the coated sample. The acceptance criteria are ≤30g / m² for paper with a weight of 25g / m² to 160g / m² and ≤40g / m² for paper with a weight of 160g / m² to 300g / m².

[0055] 180° Peel Strength: Determined according to ASTM D903-98(2025). Specimen width was (25.0±0.5) mm, peel angle was 180°, and peel rate was 152 mm / min. At least 5 specimens were tested for each group, and the arithmetic mean was taken. The result is expressed as N / 25 mm. The acceptance criterion is a 180° peel strength ≥ 1.5 N / 25 mm after lamination, with fiber tearing as the predominant failure mode.

[0056] Puncture resistance: determined according to ASTM D5748-95 (2019). A pear-shaped probe with a diameter of 19 mm was used, and the test speed was 254 mm / min. Each sample was tested at least 5 times and the arithmetic mean was taken. The maximum load (N) was recorded as the puncture resistance.

[0057] Elmendorf tear strength: determined according to GB / T 455-2002 "Determination of tear strength of paper and paperboard".

[0058] Optical transmittance / haze: Haze and visible light transmittance were determined according to ASTM D1003-21 "Test Method for Transmittance and Haze of Transparent Plastics"; the transmittance T of the black film at 600nm was measured. 600 Measurements were taken using a UV-Vis spectrophotometer in transmission mode.

[0059] White film reflectance: Measured using a UV-Vis spectrophotometer with an integrating sphere attachment in reflectance mode, with a standard white plate (e.g., Spectralon or equivalent diffuse reflectance standard) as the reference. The scanning range was 400 nm to 700 nm, with a step size of, for example, 5 nm. The reflectance R(λ) at each wavelength was recorded. R... 400-700 (%) is defined as the arithmetic mean of R(λ) in the range of 400nm to 700nm:

[0060] R 400-700(%) = [ΣR(λ)] / n (λ represents each sampling point from 400–700 nm, and n is the number of sampling points).

[0061] Each sample was tested at least 5 different locations and the arithmetic mean was taken.

[0062] Bekk smoothness: Determined according to ISO 5627:1995 "Determination of smoothness of paper and paperboard (Bekk method)".

[0063] Gurley permeability: determined according to ISO 5636-5:2013 "Paper and paperboard - Determination of air permeability - Part 5: Gurley method".

[0064] Contact angle: The static drop method was used with 1 μL of deionized water droplet, 25℃, 50%RH, and a reading of 60s. The test sample was a sample with a soil-coated surface.

[0065] Plastic density: Determined according to Procedure A of ASTM D792-20, "Standard Test Methods for Density and Relative Density of Plastics".

[0066] Soil biodegradability: According to ISO 17556:2019 "Determination of final aerobic biodegradability of plastics in soil", the test temperature was 25℃ to 30℃, and aerobic soil conditions were used.

[0067] Bio-based carbon content: determined according to ISO 16620-2:2019 "Plastics - Bio-based content - Part 2: Determination of carbon".

[0068] Total organic fluorine (TOF): determined according to EN 14582:2016 oxygen bomb combustion-ion chromatography (CIC), with a method detection limit ≤5 mg / kg; the pass criterion is that the result is below the detection limit and is recorded as "not detected".

[0069] Black film T-shirt 600 (Transmittance at 600nm): Measured using a UV-Vis spectrophotometer in transmission mode at a wavelength of 600nm, with air as the reference. The sample should be flat and wrinkle-free, and the test area should avoid pressure lines, micro-cuts, and defects. At least five different locations should be tested for each sample, and the arithmetic mean should be taken. 600 Record the instrument readings. The sample preparation and transmittance definition in the test refer to the relevant clauses of ASTM D1003-21.

[0070] General preparation process for self-made intermediates:

[0071] 1. Preparation of PHA-grafted anhydride (PHA-g-anhydride):

[0072] PHA resin and unsaturated anhydride were mixed at a mass ratio of 100:1.0 to 1.5, and 0.2 phr to 0.4 phr of dicumyl peroxide were added, where phr represents the mass parts per 100 parts of PHA resin. The mixture was reactive extruded under a twin-screw extruder with an aspect ratio ≥40, a temperature of 180°C to 185°C, and a speed of 200 rpm to 300 rpm to obtain PHA-grafted anhydride. Samples were taken after reactive extrusion, and the anhydride group content of the grafted product was determined: the sample was dissolved / dispersed in a suitable solvent, and the anhydride groups were completely hydrolyzed to the form of dibasic acids. The acid value was then titrated with a standard alkaline solution, and the anhydride group mass fraction was calculated. Based on the determination results, the anhydride feed ratio, initiator dosage, or screw speed were adjusted to control the final PHA-grafted anhydride anhydride mass fraction within the range of 0.07 wt% to 0.15 wt%.

[0073] 2. Preparation of PHA-half-ester salt solution:

[0074] The PHA-grafted anhydride obtained in the preparation of PHA-grafted anhydride (PHA-g-anhydride) was dispersed in deionized water at 70°C to 80°C with stirring. NaOH, KOH, NH4OH, ethanolamine or diethanolamine solution was added dropwise to adjust the degree of neutralization to 40% to 65% (the acid value was determined by titration with 0.1 mol / L NaOH standard solution and standardized with 0.1 mol / L HCl standard solution; the degree of neutralization was the percentage of the number of anhydride moles neutralized according to the acid value titration results relative to the theoretical total number of anhydride moles) to obtain a PHA-half ester salt solution.

[0075] 3. Preparation of homologous internally stable emulsions:

[0076] Step 1. In the preparation of the PHA-half ester salt solution, the PHA-half ester salt solution obtained in Step 1 is preheated to 85°C to 95°C and mixed with molten PHA (melt temperature 160°C to 180°C; the molten PHA is selected from PHA resin and its blends; if the formulation contains PHA oligomers, poly(3-hydroxybutyrate) P3HB oligomer is selected and premixed with molten PHA before participating in emulsification) in a pressurized emulsification kettle. The mass ratio of PHA-half ester salt to PHA is controlled to be 2:100 to 5:100. High-shear emulsification is performed at a speed of 1000 rpm to 3000 rpm, followed by cooling and discharge to a particle size D. 50 A homologous internally stable emulsion was obtained with a particle size of 0.18 μm to 0.22 μm, a solid content of 38 wt% to 42 wt%, and a pH of 6.4 to 6.8.

[0077] 4. Preparation of PHA oligomers:

[0078] Step 1. Using poly(3-hydroxybutyrate) resin as raw material, pulverize it into powder, making the powder particle size D 50The size ranges from 5 μm to 20 μm to improve the dispersion of the aqueous phase and the controllability of the reaction.

[0079] Step 2. Add the poly(3-hydroxybutyrate) powder obtained in Step 1 to deionized water, control the mass fraction of poly(3-hydroxybutyrate) to be 5 wt% to 15 wt%, and add 0.2 wt% to 1.0 wt% of PHA-half ester salt solution based on the mass of poly(3-hydroxybutyrate) as a homologous wetting and dispersing component; mechanically stir in an atmospheric pressure stirred tank or a pressurized emulsifying tank, and can be combined with high shear dispersion for 10 min to 30 min to obtain a homogeneous aqueous slurry.

[0080] Step 3. Under stirring conditions at 50℃ to 70℃, add NaOH or KOH alkaline solution dropwise to the aqueous slurry obtained in Step 2 to adjust the pH of the system to 11.5 to 12.8. React within this temperature and pH range for 0.5h to 3h for aqueous-phase alkaline catalytic controlled hydrolysis and chain scission. During the reaction, take samples every 10min to 30min. Neutralize the sample solution with HCl to pH 6.5 to 7.0, filter to collect the solid, and dry it. Monitor the number-average molecular weight Mn using gel permeation chromatography (GPC). GPC testing uses chloroform as the mobile phase, a flow rate of 1.0mL / min, and a column temperature of 35℃, and is calibrated using polystyrene standards. Terminate the reaction when Mn falls within the range of 3kDa to 8kDa by rapidly cooling to ≤40℃ and adjusting the pH of the system to 6.4 to 6.8 with HCl.

[0081] Step 4. After terminating the reaction, filter or centrifuge the suspension to collect the solid, wash with deionized water until the pH of the washing solution is 6.5 to 7.0 and the conductivity is stable, and then vacuum dry at 40°C to 50°C to constant weight to obtain poly(3-hydroxybutyrate) oligomer with a number average molecular weight Mn of 3kDa to 8kDa.

[0082] General preparation method of paper-based PHA agricultural mulch film:

[0083] The following general preparation method is used in Examples 1, 4-6 and 7-10 of the present invention (and the comparative examples with corresponding soil-adhering water-based PHA coating B1 and wind-facing thermoplastic PHA extruded film layer B2, unless otherwise stated). Examples 2 and 3 and the comparative examples without one of the layers are based on the following method with corresponding additions, deletions and adjustments to steps 2-7.

[0084] Step 1. Select plant fiber raw materials, and after crushing, grinding and mixing, form them into base paper with a weight of 25g / m² to 300g / m² on a wire or cylinder paper machine to obtain the paper base layer.

[0085] Step 2. Following the methods of "1. Preparation of PHA-grafted anhydride (PHA-g-anhydride)" and "2. Preparation of PHA-half ester salt solution" in the "General Preparation Process of Self-made Intermediates" of this embodiment, PHA-anhydride is obtained by reactive extrusion grafting, hydrolyzed and partially neutralized to obtain a PHA-half ester salt solution with a pH value of 6.4 to 6.8.

[0086] Step 3. Following the method in "3. Preparation of Homologous Internally Stable Emulsions" of the "General Preparation Process of Self-Made Intermediates" in this embodiment, the solution obtained in Step 2 is mixed and emulsified with molten PHA to obtain a solid content of 38wt% to 42wt% and a particle size D. 50 It is a homologous internally stable emulsion with a thickness of 0.18 μm to 0.22 μm.

[0087] Step 4. Apply the same family of internally stabilized emulsion to the soil-adhering surface of the paper base using a coating machine at a dry coating amount of 5 g / m² to 25 g / m², and dry it in a segmented drying oven at 80°C to 110°C until the outlet paper temperature is ≤55°C to obtain a single-sided coated paper base.

[0088] Step 5. Perform surface activation treatment (corona, plasma or flame) on the windward side (i.e. uncoated side) of the single-sided coated paper substrate, and control the equivalent activation energy to 1.0 kJ / m² to 1.4 kJ / m² to obtain an activated coated paper substrate with a surface tension ≥ 40 mN / m.

[0089] Step 6. Add PHA resin (or its masterbatch mixture) to an extrusion coating machine or a multilayer co-extruder, control the melt temperature to 160°C to 185°C, and extrude from the die to obtain a thermoplastic PHA melt curtain with a total thickness of 20μm to 80μm (of which the compatibility layer thickness is 2μm to 6μm).

[0090] Step 7. Using a wet-to-heat integrated lamination process, the thermoplastic PHA melt curtain and the weather-facing surface of the activated coated paper base layer are laminated between a cooling roller and a lamination roller. The time from the completion of Step 4 (out of the oven) to the entry into the lamination roller in this step is strictly controlled to be 20 to 60 minutes, and the residual moisture content of the coating on the surface at the moment of lamination is 5.0 wt% to 7.0 wt%. The residual moisture content is monitored by an online infrared moisture meter or determined by a sampling drying method ((wet weight - dry weight) / dry weight × 100%). Under these conditions, the melt curtain contacts the paper base layer and immediately cools and sets, producing a paper-plastic composite roll.

[0091] Step 8. The paper-plastic composite roll is aged and left to stand at room temperature (20℃ to 25℃) for ≥24 hours to eliminate internal stress and complete crystallization. Then, it is slit using a slitting machine to obtain the finished paper-based PHA soil-degradable agricultural mulch film. This finished product is used for subsequent performance testing and field application.

[0092] Examples and Comparative Examples:

[0093] In the following examples, B1 represents the soil-adhering water-based PHA coating, and B2 represents the wind-facing thermoplastic PHA extruded film layer.

[0094] Example 1 uses a paper base layer with a basis weight of 80 g / m². The pulp fiber system is a mixture of bamboo pulp and wood pulp, with bamboo pulp accounting for 70 wt% and containing 0.3 wt% cellulose nanofibers (CNF) as the paper base layer. In step 4, a homologous internally stabilized emulsion is used as the surface coating B1. The dispersed phase melt is a blend of poly(3-hydroxybutyrate-4-hydroxybutyrate) P (3HB-co-4HB) and poly(3-hydroxybutyrate-3-hydroxyvalerate) PHBV (HV 12 mol%) at a mass ratio of 2:1. 15 wt% of P3HB oligomer and 3 wt% of PHA-hemiester salt are added based on the coating polymer solids. The coating is applied in two coats, with a total dry coating weight of 12 g / m². In step 5, the equivalent activation energy is controlled at 1.2 kJ / m² to achieve a surface tension of 41 mN / m. In step 6, the extruded weatherproof film layer B2 has a multi-layer co-extruded structure, comprising a 3μm poly(3-hydroxybutyrate-3-hydroxyhexanoate) (PHBH) grafted maleic anhydride compatibility layer (0.10wt% anhydride group) and a 32μm PHBH functional black film layer (containing 0.3wt% low PAH carbon black), with a total thickness of 35μm. During the lamination process in step 7, the wet-to-heat interval is controlled at 40 min, and the residual moisture content of the coating on the clay surface is 5.6wt% on the dry basis.

[0095] Example 2 uses a paper base layer with a basis weight of 80 g / m². This example omits the clay-coated surface layer B1 and omits steps 2 to 4. In step 5, the paper surface is activated, with the equivalent activation energy controlled at 1.2 kJ / m² and the surface tension at 40 mN / m. In step 6, the extruded weathering film layer B2 is a PHBH single-layer black film (containing 0.3 wt% low PAH carbon black) with a thickness of 32 μm. In step 7, the melt curtain is directly laminated to the paper base layer, without involving wet-heat synergistic control.

[0096] Example 3 uses a paper base layer with a basis weight of 70 g / m². In step 4, the formulation of the surface coating B1 is the same as in Example 1, with a total dry coating weight of 12 g / m². This example does not include an extruded film layer B2 on the windward side; production ends at step 4, and the finished product is obtained after slitting.

[0097] Example 4 uses a paper base layer with a basis weight of 200 g / m². In step 4, the formulation of the soil-coating layer B1 is the same as in Example 1, with a dry coating weight of 8 g / m². In step 5, the equivalent activation energy is controlled at 1.1 kJ / m², and the surface tension at 41 mN / m. In step 6, the weatherproof film layer B2 has a multi-layer co-extruded structure, comprising a 3 μm thick poly(3-hydroxybutyrate-3-hydroxyhexanoate) grafted maleic anhydride compatibility layer (anhydride mass fraction of 0.10 wt%, based on the polymer of the compatibility layer) and a 37 μm thick PHBH white micro-foamed functional layer; the white micro-foamed functional layer is prepared using a high-pressure carbon dioxide injection foaming process, with an injection pressure of 6.0 MPa, a foaming temperature of 158°C, a draw ratio of 1.60, a closed-cell rate of 30%, and a total thickness of 40 μm for B2. In step 7, during lamination, the wet-to-heat interval is controlled at 35 min, and the residual moisture content of the soil-coating layer on a dry basis is 5.5%.

[0098] Example 5 uses a paper base layer with a basis weight of 35 g / m² and a wrinkling degree of 22%, which is pre-treated with 3 g / m² internal penetration reinforcement (using a 12 wt% solids content homologous internally stabilized emulsion diluent). In step 4, the formulation of the surface coating B1 is the same as in Example 1, with a dry coating weight of 12 g / m². In step 5, the equivalent activation energy is controlled at 1.2 kJ / m², and the surface tension at 40 mN / m. In step 6, the weatherproof film layer B2 has a multi-layer co-extruded structure, comprising a 3 μm P(3HB-co-4HB) grafted maleic anhydride compatible layer (0.10 wt% anhydride group) and a 27 μm P(3HB-co-4HB) functional black film layer, with a total thickness of 30 μm. In step 7, during lamination, the wet-to-heat interval is controlled at 45 min, and the residual water content of the surface coating is 6.0 wt% on the dry basis.

[0099] Example 6 This example mainly verifies the preparation parameters of the homologous internally stable emulsion. In step 3, when preparing the emulsion, the dispersed phase melt is a blend of P(3HB-co-4HB) and PHBV (HV 12mol%) at a mass ratio of 2:1. 15wt% of P3HB oligomer and 3wt% of PHA-hemiester salt (based on the coating polymer solids) are added. The emulsification process is controlled to maintain a particle size D. 50 The film thickness was 0.20 μm, the solid content was 40 wt%, and the pH value was 6.6. Subsequent film preparation was carried out using the same paper base and film layer structure and process parameters as in Example 1.

[0100] Example 7 uses a paper base layer with a basis weight of 80 g / m², containing 70 wt% bamboo pulp and 0.4 wt% CNF. In step 4, the formulation of the clay-coating layer B1 is the same as in Example 1, with a total dry coating weight of 12 g / m². In step 5, the equivalent activation energy is controlled at 1.2 kJ / m², and the surface tension at 41 mN / m. In step 6, the weather-resistant film layer B2 has a multi-layer co-extruded structure, comprising a 3 μm PHBH-grafted maleic anhydride compatible layer (0.10 wt% anhydride) and a 32 μm PHBH functional black film layer, with a total thickness of 35 μm. In step 7, during lamination, the wet-to-heat interval is controlled at 40 min, and the residual moisture content of the clay-coating layer on a dry basis is 5.4 wt%.

[0101] Example 8 uses a paper base layer with a basis weight of 200 g / m². In step 4, the formulation of the soil-coating layer B1 is the same as in Example 1, with a dry coating weight of 10 g / m². In step 5, the equivalent activation energy is controlled at 1.1 kJ / m², and the surface tension at 40 mN / m. In step 6, the weatherproof film layer B2 has a multi-layer co-extruded structure, comprising a 3 μm thick PHBH-grafted maleic anhydride compatibility layer (0.10 wt% anhydride group, based on the polymer of the compatibility layer) and a 37 μm thick PHBH white micro-foamed functional layer; the white micro-foamed functional layer is prepared using a high-pressure carbon dioxide injection foaming process, with an injection pressure of 6.2 MPa, a foaming temperature of 155°C, a draw ratio of 1.60, a closed-cell rate of 35%, and a total thickness of 40 μm for B2. In step 7, during lamination, the wet-to-heat interval is controlled at 30 min, and the residual moisture content of the soil-coating layer on a dry basis is 5.3%.

[0102] Example 9 uses a paper base layer with a basis weight of 25 g / m² and a wrinkling degree of 30%, which is pre-treated with 4 g / m² internal penetration reinforcement (using a 15 wt% solids content homologous internally stabilized emulsion diluent). In step 4, the formulation of the surface coating B1 is the same as in Example 1, with a dry coating weight of 5 g / m². In step 5, the equivalent activation energy is controlled at 1.2 kJ / m², and the surface tension at 40 mN / m. In step 6, the weatherproof film layer B2 has a multi-layer co-extruded structure, comprising a 2 μm P(3HB-co-4HB) grafted itaconic anhydride compatible layer (0.10 wt% anhydride group) and an 18 μm P(3HB-co-4HB) functional black film layer, with a total thickness of 20 μm. In step 7, during lamination, the wet-to-heat interval is controlled at 50 min, and the residual water content of the surface coating is 6.2 wt% on the dry basis.

[0103] Example 10 uses a paper base layer with a basis weight of 300 g / m², which is pre-reinforced with 4 g / m² internal penetration (using a 15 wt% solids content homologous internally stabilized emulsion dilution), and treated with staggered micro-cuts (1.5 mm in length, 15 mm in spacing) and edge pressing (20 mm in width). In step 4, the formulation of the soil-coating layer B1 is the same as in Example 1, with a dry coating weight of 8 g / m². In step 5, the equivalent activation energy is controlled at 1.2 kJ / m², and the surface tension at 41 mN / m. In step 6, the weather-resistant film layer B2 has a multi-layer co-extruded structure, comprising a 5 μm PHBH-grafted maleic anhydride compatible layer (0.10 wt% anhydride group) and a 75 μm PHBH functional black film layer, with a total thickness of 80 μm. In step 7, during lamination, the wet-to-heat interval is controlled at 40 min, and the residual water content of the soil-coating layer on a dry basis is 5.8 wt%.

[0104] Comparative example:

[0105] Comparative Example 1 used a paper base layer with a basis weight of 80 g / m², without a soil-coating layer B1. The wind-facing film layer B2 was a PHBH black film with a thickness of 30 μm. During preparation, the paper surface was activated (1.1 kJ / m², surface tension 39 mN / m), and then laminated using a conventional extrusion coating process without any coating involved, and without the involvement of wet-heat synergistic control.

[0106] Comparative Example 2 used a paper base layer with a basis weight of 200 g / m², without a soil-coating layer B1. The windproof and rainproof film layer B2 was a solid white PHBH film with a thickness of 40 μm. During preparation, the paper surface was activated (1.1 kJ / m², surface tension 39 mN / m) and then laminated using a conventional extrusion coating process.

[0107] Comparative Example 3 used a paper base layer with a basis weight of 80 g / m². The surface to be bonded to the soil was coated with a styrene-acrylic emulsion (a commercially available product), and no extruded film layer B2 was installed on the windward side. The preparation was carried out according to the conventional coated paper process.

[0108] Comparative Example 4 used a paper base layer with a basis weight of 80 g / m², without a soil-coating layer B1. The windproof and rainproof film layer B2 was a 35 μm thick polybutylene adipate (PBAT) black film. During preparation, the paper surface was activated (1.1 kJ / m², surface tension 39 mN / m) and then laminated using a conventional extrusion coating process.

[0109] Comparative Example 5 used a paper base layer with a basis weight of 80 g / m². The surface coating B1 contained polyvinyl alcohol (PVOH), and the windproof and rainproof film layer B2 was a black PBAT film. Conventional coating and lamination processes were used in its preparation, which did not comply with the requirements of homologous internal stability and fluorine-free properties of this invention.

[0110] Comparative Example 6 used a paper base layer with a basis weight of 80 g / m². The weatherproof film layer B2 was a PBAT black film, and the composite interface layer used a PE-g-MAH universal compatibilizer. The activation energy during preparation was 1.2 kJ / m², the surface tension was 39 mN / m, and co-extrusion coating was used for lamination.

[0111] Comparative Example 7 used a paper base layer with a basis weight of 80 g / m². The formulation of the surface coating B1 was the same as in Example 1, with a dry coating weight of 8 g / m². The weatherproof film layer B2 was a black poly(3-hydroxybutyrate) (PHB) film with a thickness of 35 μm. The preparation process employed the same wet-heat process as in Example 1, with a controlled interval of 40 min, a residual water content of 5.2 wt%, an activation energy of 1.2 kJ / m², and a surface tension of 41 mN / m, aiming to verify the influence of the brittleness of the PHB resin itself.

[0112] Comparative Example 8 used a paper base layer with a basis weight of 80 g / m². The formulation of the surface coating B1 was the same as in Example 1, with a dry coating weight of 8 g / m². The windproof and rainproof film layer B2 was a high HV content (20 mol%) PHBV black film with a thickness of 35 μm. The preparation process used was the same wet-heat process as in Example 1, with a controlled interval of 40 min, a residual water content of 5.2 wt%, an activation energy of 1.2 kJ / m², and a surface tension of 41 mN / m, aiming to verify the softening and tensile strength effects of the high HV content resin.

[0113] Application example:

[0114] Application Example 1: Barrier and water absorption performance test.

[0115] This application example primarily evaluates the water vapor barrier performance and liquid water resistance of paper-based agricultural mulch films in actual use environments. The experiment selected examples and comparative samples with different structural designs, and strictly followed the BW procedure (inverted cup method) in ASTM E96 / E96M-24a standard to test the water vapor transmission rate (WVTR) under high temperature and high humidity conditions (38℃, 90% RH) to simulate the osmotic pressure of the mulch film in a humid and hot soil environment. Simultaneously, the water absorption within 60 seconds was determined using the Cobb method according to GB / T 1540-2002 standard. 60 This study aimed to quantitatively evaluate the water-blocking effect of the coating on the soil surface against liquid water. The test verified whether the synergistic effect of the water-based coating and thermoplastic film layer could effectively reduce the moisture absorption rate of the paper substrate and block water vapor channels, thereby ensuring the moisture retention function and physical structural stability of the mulch film during field coverage. The experimental results are shown in Table 3.

[0116] Table 3 Barrier and Water Absorption Data:

[0117]

[0118] Data analysis: Test results show that all embodiments with the B1 coating on the soil surface (Examples 1, 3-5, and 7-10) exhibited Cobb... 60 The values ​​were effectively controlled within the range of 26 g / m² to 35 g / m², meeting the water resistance requirements (≤30 g / m² or ≤40 g / m²) for different paper basis weight ranges. Examples 1 and 5 are representative, corresponding to uncoated Cobb paper. 60 The levels were 41 g / m² and 40 g / m², respectively, which decreased to 28 g / m² and 26 g / m² after B1 coating treatment; the Cobb 60 Price reduction based on (uncoated Cobb paper) 60 - Cobb after application 60 Uncoated Cobb Paper 60 Calculated at 100%, the reduction is greater than 30% in all cases. In particular, Example 5 uses lightweight paper combined with internal permeation reinforcement, Cobb... 60 The concentration reached 26 g / m², indicating that the internally stabilized emulsion has a good covering and filling effect on paper fibers. Regarding water vapor barrier properties, Example 1, with its double-sided composite structure, exhibited excellent barrier performance, with a WVTR reduction of 62%, significantly better than Example 3 (45%) with only a coating and Example 2 (58%) with a single-sided coating, confirming the synergistic effect of the B1 coating and B2 film layer in blocking water vapor pathways. In contrast, Comparative Example 3, although using a non-PHA coating, had insufficient film density, resulting in poor barrier performance (WVTR reduction of only 40%). Comparative Example 5, containing hydrophilic polyvinyl alcohol (PVOH), although initially providing acceptable barrier properties, showed poor barrier performance due to Cobb's presence of hydrophilic polyvinyl alcohol (PVOH). 60 The lower values ​​may be related to the short testing time, raising questions about their long-term water resistance. Although Comparative Examples 7 and 8 used PHA membranes, their overall barrier performance was slightly inferior to the embodiments of this invention due to the lack of a B1 coating or inherent material characteristics (such as differences in the hydrophilicity of PHBV). The data indicate that a dual barrier of "coating + membrane" constructed using PHA materials of the same family can achieve superior moisture management performance.

[0119] Application Example 2: Interface bonding and mechanical performance testing.

[0120] This application example focuses on verifying the effect of the "wet-to-heat" integrated composite process and the anhydride-based compatible layer on improving the bonding strength of the paper-plastic interface, while also examining the overall puncture resistance of the mulch film. The experiment tested the 180° peel strength according to ASTM D903 standard, focusing on observing the interface failure mode (fiber tearing or interface delamination) to determine the strength of the composite. Puncture resistance was tested according to ASTM D5748 standard to simulate the mulch film's ability to resist puncture by plant stems or soil clods during installation and use. Key process parameters during composite testing were recorded, including the wet-to-heat time interval, the residual moisture content of the coating on the soil surface, and the paper surface activation energy, to ensure the process relevance of the data. By comparing the mechanical performance of different embodiments and comparative examples, the aim was to determine the optimal process window and material combination for achieving the interface fiber tearing failure mode. The experimental results are shown in Table 4.

[0121] Table 4 Interface and Puncture Data:

[0122]

[0123] Data Analysis: Experimental results show that within a specific process window of 20-60 min "wet-to-heat" interval, 5.0 wt%-7.0 wt% residual water content in B1, and 1.0 kJ / m²-1.4 kJ / m² activation energy, Examples 1, 4, 5, and 7-10 of this invention all achieved excellent interfacial bonding performance, with peel strength consistently above 1.5 N / 25 mm, and the failure mode was the ideal "fiber tearing". This confirms that moderate moisture retention and the chemical bonding of anhydride groups effectively promoted the fusion of the paper-plastic interface. Although Example 2 did not have a B1 coating on the clay side or employ wet-to-heat synergistic control, it still achieved a fiber tearing failure of 1.8 N / 25 mm under activation energy of 1.2 kJ / m² and surface tension of 40 mN / m, indicating that a single-sided extrusion structure can also meet interfacial bonding requirements to a certain extent, but its overall barrier and durability are inferior to that of a double-sided composite structure. Conversely, Comparative Examples 1, 2, 4, and 6, regardless of whether they used PHBH / PBAT without a compatibility layer or a universal compatibility layer, exhibited low peel strength (1.0-1.2 N / 25 mm), primarily manifesting as interfacial delamination, indicating that physical adhesion or non-specific compatibilizers were insufficient to resist damp heat stress. Comparative Examples 7 and 8, despite using PHA material, suffered from weaker bonding strength and puncture performance (8.2-9.0 N) compared to Example 1 (10.2 N) at the same thickness due to a lack of targeted process control or material toughness matching issues (such as the brittleness of PHB). Furthermore, the puncture strength showed a positive correlation with increasing film thickness (Examples 9-10), verifying the effectiveness of multilayer co-extrusion structures in improving the overall mechanical strength and toughness of the mulch film.

[0124] Application Example 3: Optical performance testing.

[0125] This application example aims to evaluate the optical properties of plastic film to verify its ability to meet the needs of different agricultural planting methods. For black plastic film, the focus is on its shading performance, and the transmittance (T0) at a wavelength of 600 nm is measured using a UV-Vis spectrophotometer in transmission mode. 600 This indicator is directly related to the weed-suppressing effect. For white mulch film, a UV-Vis spectrophotometer with an integrating sphere attachment is used to measure the total reflectance (Ro) in the 400nm to 700nm wavelength band. 400-700 The experiment aimed to evaluate its effectiveness in reflecting sunlight, reducing soil temperature, and increasing crop light scattering. Samples with different carbon black additions and different microfoaming closed-cell ratios were tested. By comparing the optical parameters of the examples and comparative examples, the technical advantages of this invention in functional masterbatch dispersion and foaming structure control were verified, ensuring that the mulch film can perform its expected agronomic functions in field applications. The experimental results are shown in Table 5.

[0126] Table 5 Optical Data:

[0127]

[0128] Data analysis: Test data shows that the black embodiments of the present invention (1, 2, 5, 7, 9, 10), when 0.3 wt% low PAH carbon black is added, have a T 600 The transmittance was controlled below 1.3%, with some exceeding 1.0%, far below the critical weed-suppressing value of 3%, indicating that the carbon black was uniformly dispersed in the PHA matrix, effectively blocking the wavelengths required for photosynthesis and ensuring excellent weed-suppressing performance. In contrast, although comparative examples 4-6 with PBAT substrates also added black masterbatch, their transmittance was generally higher (2.5%-2.8%), possibly related to the decrease in pigment hiding power caused by differences in matrix crystallinity. Regarding white mulch films, Examples 4 and 8 employed high-pressure carbon dioxide micro-foaming technology, achieving reflectances of 70% and 72% respectively, comparable to or slightly better than the pure solid white film of Comparative Example 2. However, due to their microporous structure, they are expected to have a greater advantage in thermal insulation performance. The data confirms that by controlling the dispersion and microstructure of additives in the PHA substrate, the optical properties of the mulch film can be precisely customized to adapt to different scenarios such as long-season weed suppression or summer cooling.

[0129] Application Example 4: Coverage period integrity and mechanical film laying compatibility.

[0130] This application example comprehensively examines the mechanical adaptability and weather resistance of plastic film through actual field laying tests. A standard agricultural film-laying machine was used for continuous operation at a set speed (10-12 m / min) and tension. Each sample group had a continuous laying length of no less than 1.0 km, repeated at least three times. The film breakage rate was calculated as the number of breaks / actual laying length (km), in units of breaks / km; the pass rate was calculated as (actual continuous laying length without breaks / planned laying length) × 100%. Subsequently, field observations were conducted on the laid plastic film, recording the number of days the film maintained its physical structure intact without large-area damage or degradation (integrity days). This test aims to evaluate the material's comprehensive performance under mechanical tension, soil friction, and natural environmental erosion (wind, rain, ultraviolet radiation). The experimental results are shown in Table 6.

[0131] Table 6 Coverage Integrity and Mechanical Film Laying Compatibility:

[0132]

[0133] Data Analysis: Field trial data clearly reflects the correlation between material structure and mechanical properties. The double-sided composite structure samples in Examples 1, 7, and 10 exhibited the best film-laying adaptability, with a breakage rate controlled at 0.5 times / km or less, a pass rate as high as 96%-97%, and a field integrity period generally exceeding 100 days, meeting the needs of long-season crops. This is attributed to the complementary relationship between the skeletal support of the paper base layer and the tough protection of the B2 film layer. In contrast, Examples 3 and Comparative Example 5, with only a coating, lacked the strong protection of the thermoplastic film layer, resulting in poor weather resistance and an integrity period of only 70-90 days. Comparative Example 5 also showed even worse water resistance due to the water sensitivity of PVOH. Comparative Example 7, due to the high crystalline brittleness of the PHB material itself, had a high breakage rate during film laying (1.5 times / km), while the high HV content PHBV in Comparative Example 8 was too soft, easily causing tensile voids to expand under tension, also affecting the laying quality. Data shows that this invention, by using PHA resin and constructing a multi-layered composite structure, successfully solves the shortcomings of single materials in mechanical laying and field durability.

[0134] Application Example 5: Soil biodegradation and total organic fluorine testing.

[0135] This application example assesses the final biodegradation and fluoride risk of mulch film products in soil. Biodegradation testing was conducted according to ISO 17556:2019, using a closed respiration meter to monitor carbon dioxide production or oxygen consumption under aerobic soil conditions and converting the results to mineralization. A blank soil control and a reference material (cellulose) control were set up to correct for soil background respiration. Soil moisture content was maintained between 40% and 60% of its maximum water holding capacity, the test temperature was 25°C to 30°C, and the soil was regularly turned over and watered as required by the standard.

[0136] Total organic fluorine (TOF) was determined by oxygen bomb combustion pretreatment according to EN 14582:2016 and by combustion ion chromatography (CIC). Samples were taken from the finished plastic film and the scraped PHA cover layer (soil-facing dry film and wind-facing thermoplastic film layer), and ground. The sample weight was 0.10 g to 0.50 g, and the absorbent volume was adjusted to 10 mL to 50 mL. The limit of detection (LOD) was calculated using the 3σ principle with a program blank, and the LOD was controlled to be ≤5 mg / kg by the combination of sample weight / volume adjustment. The experimental results are shown in Tables 7 and 8.

[0137] Table 7 Soil biodegradation mineralization:

[0138]

[0139] Table 8. Results of Total Organic Fluorine TOF Tests:

[0140]

[0141] Data Analysis: The biodegradability and environmental safety test results are satisfactory. All PHA-based examples (1-5, 7-10) and comparative examples (1, 2, 7, 8) showed mineralization exceeding 90% and bio-based carbon content as high as 92%-96% during 24-month soil landfill testing, proving that they originate from renewable resources and can completely return to the natural cycle. This contrasts sharply with Comparative Example 3 (non-PHA coating, degradation rate <70%) and Comparative Examples 4-6 (PBAT-based, degradation is slightly slower and bio-based content is low). More importantly, TOF test results showed that no organic fluorine was detected in any of the samples from all examples of this invention (<5 mg / kg), completely eliminating the risk of persistent organic pollutants that may arise from traditional waterproof and oil-resistant coatings. This result confirms that the technical route of this invention, combining physical microstructure design with bio-based chemical modification, successfully achieves both functional and environmental compliance of the mulch film without relying on fluorinated chemicals.

[0142] Application Example 6: Paper Substrate Performance Test.

[0143] This application focuses on monitoring the physical properties of the paper substrate, particularly smoothness (Bekk) and air permeability (Gurley), as these fundamental properties directly affect subsequent coating uniformity, composite interface quality, and the overall air permeability and moisture retention balance of the mulch film. Experiments were conducted according to ISO 5627 and ISO 5636-5 standards, testing base papers with different basis weights and pulp ratios. Quantifying these parameters verifies the stability of the paper substrate's production process and whether its physical structure meets the requirements of the "wet-to-heat" composite process for substrate surface characteristics. This invention uses a Bekk smoothness of 180s to 500s and a Gurley air permeability of 80s / 100mL to 500s / 100mL as acceptance criteria. This test covers a wide range from ultralight paper (25g / m²) to heavyweight paper (300g / m²), ensuring the universality and quality consistency of the invention's technical solution across different product specifications. Experimental results are shown in Table 9.

[0144] Table 9. Data for paper substrates: Bekk and Gurley:

[0145]

[0146] Data analysis: The physical performance test results of the paper base material showed that the Bekk smoothness of the base paper used in each embodiment and comparative example ranged from 185s to 420s, and the Gurley air permeability ranged from 85s / 100mL to 310s / 100mL, all within the acceptable range set by this invention (smoothness 180-500s, air permeability 80-500s / 100mL). With the increase in paper basis weight (from 25g / m² in Example 9 to 300g / m² in Example 10), the smoothness and air permeability resistance showed the expected upward trend, which is consistent with the increase in paper thickness and fiber density. The fluctuations in all samples were within the allowable process error, indicating that the papermaking process is stable and reliable. This suitable smoothness ensures uniform adhesion between the coating and film layers, while the controlled air permeability helps to eliminate interfacial bubbles during the lamination process, while providing the necessary basic micro-permeability for the finished mulch film, preventing anaerobic fermentation in the soil. The data confirms that the selected paper substrate can provide a good physical carrier for the construction of the functional layers.

[0147] Experimental Results and Analysis:

[0148] Based on the above application examples, this invention has achieved significant technological breakthroughs in five dimensions of agricultural mulch film: barrier properties, mechanical strength, optical control, operational adaptability, and environmental degradation, by constructing a paper-based PHA double-sided composite structure.

[0149] 1. Barrier Performance and Hydrophilicity / Hydrophilicity Analysis: Experimental data clearly demonstrate that the synergistic effect of the B1 coating and the B2 film layer is key to achieving efficient moisture management. When the dry coating amount of the B1 coating on the soil surface increased from 5 g / m² (Example 9) to 12 g / m² (Examples 1, 5, and 7), the Cobb... 60 The water absorption value decreased from approximately 30 g / m² to the range of 26 g / m² to 28 g / m², and the contact angle correspondingly increased from 82° to nearly 90°. This confirms the film-forming density and hydrophobic modification effect of the internally stabilized emulsion of the same family on the paper fiber surface. The WVTR reduction of the double-sided composite structure (Example 1) reached 62%, which is significantly better than that of Example 3 (45%) with only coating and Example 2 (58%) with single-sided coating. This indicates that the dual mechanism of "coating sealing + film barrier" effectively cuts off the water vapor channel and solves the natural defect of paper-based materials being easy to absorb moisture and be breathable.

[0150] 2. Interface Bonding and Mechanical Mechanism Analysis: Mechanical performance testing revealed the core value of the "wet-to-heat" integrated composite process. Within the set process window (intervals of 20-60 min, residual water content of B1 5.0-7.0 wt%, activation energy 1.0-1.4 kJ / m²), the 180° peel strength of all examples remained stable above 1.5 N / 25 mm, and the failure mode was fiber tearing. This indicates that appropriate moisture promotes hydrogen bonding between cellulose hydroxyl groups and PHA molecular chains, while the chemical bridging effect of anhydride groups further enhances interfacial adhesion, overcoming the problem of easy delamination in traditional paper-plastic composites. Data analysis of puncture strength showed that as the thickness of the B2 film layer increased from 20 μm (Example 9) to 80 μm (Example 10), the puncture force generally increased with increasing film thickness, verifying the contribution of the multilayer co-extrusion structure to the overall strength and toughness.

[0151] 3. Optical Control Efficiency Analysis: Optical test results confirmed the material formulation's ability to precisely control the photothermal environment. For black mulch film, in this embodiment of the invention, by adding approximately 0.3 wt% low-PAH carbon black, a T0.05 was obtained. 600 The low light transmittance ranges from 0.7% to 1.3%, all well below the 3% weed suppression threshold, ensuring excellent weed control. For white mulch film, the introduction of microfoaming technology significantly improves light reflection performance; when the closed-cell rate varies within the range of 30% to 35%, R... 400-700 The total reflectance remains at a high level of 70% to 72%. This high reflectivity helps to lower ground temperature and increase diffused light, making it particularly suitable for the cultivation of tropical or summer crops.

[0152] 4. Field Adaptability and Degradation Analysis: Field application data shows that the mulch film of this invention has an extremely low breakage rate (≤0.5 times / km) at industrial-scale laying speeds, and its integrity days cover the main crop growth cycle (90-130 days), which is superior to the brittleness of single PHB films and the collapse problem of high HV content PHBV films. This is due to the complementary dimensional stability of the paper base layer and the flexibility of the PHA film layer. More importantly, the soil biodegradability of all embodiments exceeded 90% within 24 months, and total organic fluorine was undetectable, completely solving the microplastic residue and fluorine pollution risks of traditional plastic mulch films, and realizing a green closed loop from source to end.

[0153] In summary, this invention, through innovative material system design and process control, has successfully developed a paper-based PHA agricultural mulch film that combines high performance and fully degradable properties. Its unique "wet-to-heat" composite interface and functionalized coating / film structure not only meet the needs of modern agricultural mechanization and refined management, but also provide an ideal mulch material solution for sustainable agricultural development due to its high bio-based carbon content and fluorine-free environmental friendliness.

[0154] 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 paper-based-PHA double-sided composite soil biodegradable agricultural mulch film, characterized in that, The agricultural mulch film includes a paper base layer with a basis weight of 25 g / m² to 300 g / m², and a polyhydroxyalkanoate covering layer disposed on both sides of the paper base layer; The polyhydroxyalkanoate coating layer includes an aqueous polyhydroxyalkanoate coating disposed on the soil-contacting surface of the paper substrate and a thermoplastic polyhydroxyalkanoate extruded film layer disposed on the windward and rain-facing surface of the paper substrate. The aqueous polyhydroxyalkanoate coating has a dry coating weight of 5 g / m² to 25 g / m²; the thermoplastic polyhydroxyalkanoate extruded film has a thickness of 20 μm to 80 μm. The waterborne polyhydroxyalkanoate coating on the soil surface is a homologous, internally stable system. The polyhydroxyalkanoate solids content is ≥97 wt%, the total non-polyhydroxyalkanoate polymer content is ≤1.0 wt%, the total strong surfactant content is ≤0.5 wt%, and the total wax content is ≤3.0 wt%, all on a dry basis. The total organic fluorine content in the coating and its preparation formulation is undetectable, and it meets the following requirements: pH value 6.4 to 6.8, particle size D... 50 The particle size ranges from 0.18 μm to 0.22 μm, and the solid content ranges from 38 wt% to 42 wt%. The agricultural mulch film is subjected to a wet-to-heat process of 20 to 60 minutes on a dry basis. During the lamination process, the residual water content of the coating on the soil-facing surface is 5.0 wt% to 7.0 wt%. The windward side is provided with a polyhydroxyalkanoate grafted anhydride compatibility layer containing anhydride groups, with an anhydride group mass fraction of 0.07 wt% to 0.15 wt%. Before lamination, the surface tension of the paper surface is ≥40 mN / m. After lamination, the 180° peel strength of the paper-film interface is ≥1.5 N / 25 mm, and the failure mode of the paper-film interface after lamination is fiber tearing. The polyhydroxyalkanoate covering layer showed a biodegradability of ≥90% in aerobic soil after 24 months according to ISO 17556:2019 standard; the total organic fluorine of the finished mulch film and the polyhydroxyalkanoate covering layer was not detected by EN 14582:2016 oxygen bomb combustion-ion chromatography, and the method detection limit was ≤5 mg / kg. The polyhydroxy fatty acid ester is selected from one or more of short-chain polyhydroxy fatty acid esters, medium- and long-chain polyhydroxy fatty acid esters, copolymers formed by short-chain polyhydroxy fatty acid ester monomers and medium- and long-chain polyhydroxy fatty acid ester monomers, and their grafted modified forms. The short-chain polyhydroxy fatty acid ester is selected from one or more of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-4-hydroxybutyrate) or poly(3-hydroxybutyrate-3-hydroxyvalerate). The medium- and long-chain polyhydroxy fatty acid esters are selected from one or more of poly(3-hydroxyhexanoate), poly(3-hydroxyheptanoate), poly(3-hydroxynonanoate), poly(3-hydroxydecanoate), poly(3-hydroxyundecanoate), poly(3-hydroxydodecanate), poly(3-hydroxydodecanate), poly(3-hydroxytridecanate), or poly(3-hydroxytetradecanoate). The thermoplastic polyhydroxyalkanoate extruded film is a multilayer co-extruded structure comprising a polyhydroxyalkanoate-grafted anhydride compatibility layer with a thickness of 2 μm to 6 μm and a homogeneous blended functional layer with a thickness of 18 μm to 75 μm, wherein the sum of the thicknesses of the compatibility layer and the functional layer is 20 μm to 80 μm; the anhydride group content in the polyhydroxyalkanoate-grafted anhydride compatibility layer is 0.07 wt% to 0.15 wt%, and the anhydride group source is selected from maleic anhydride or itaconic anhydride based on the polymer of the compatibility layer.

2. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The waterborne polyhydroxyalkanoate coating on the soil surface contains 12 wt% to 16 wt% of polyhydroxyalkanoate oligomers, with a number average molecular weight of 3 kDa to 8 kDa, based on the coating polymer solids, and 2.5 wt% to 3.5 wt% of polyhydroxyalkanoate-half ester salt, based on the coating polymer solids, wherein the degree of neutralization of the polyhydroxyalkanoate-half ester salt is 40% to 65%.

3. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The water-based polyhydroxyalkanoate coating on the soil surface, as the coating body, has a biodegradability of ≥90% after 90 days of testing according to ISO 17556 standard under aerobic soil conditions at 25℃ to 30℃.

4. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, Cobb of the sample after coating the soil surface 60 The value decreased by ≥30% compared to uncoated paper, and when the paper basis weight was between 25 g / m² and 160 g / m², the Cobb coating value decreased significantly. 60 Value ≤30g / m²; when the paper basis weight is between 160g / m² and 300g / m², the Cobb coating value is ≤30g / m²; 60 Value ≤ 40g / m².

5. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, Before lamination, the wind-facing side or the side to be laminated is subjected to corona, plasma, or flame surface activation treatment with an equivalent activation energy of 1.0 kJ / m² to 1.4 kJ / m², and the surface tension after treatment is ≥40 mN / m.

6. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The polyhydroxyalkanoate coating layer meets any of the following optical properties: the transmittance T of the black opaque film at 600 nm. 600 ≤3%; or the total reflectance of the white microbubble film in the 400nm to 700nm band under the integrating sphere method is ≥65%.

7. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The pulp fiber system of the paper base is selected from one or more of bamboo pulp, herbaceous crop straw pulp and wood pulp. The herbaceous crop straw pulp is one or more of rice straw pulp, wheat straw pulp, sugarcane bagasse pulp or reed pulp. The wood pulp is one or more of softwood pulp or hardwood pulp. The total mass fraction of bamboo pulp or herbaceous crop straw pulp is ≥50wt%, and it contains 0.2wt% to 1.0wt% of cellulose nanofibers.

8. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The paper base layer meets the requirements of Bekk smoothness of 180s to 500s and Gurley air permeability of 80s / 100mL to 500s / 100mL.

9. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The paper base layer is equipped with internal penetration reinforcement, staggered micro-cuts and edge pressing lines, with a reinforcement amount of 3g / m² to 5g / m². The internal penetration reinforcement is carried out by spraying or impregnating with a homologous internally stabilized emulsion dilution with a solid content of 10wt% to 15wt%. The staggered micro-cuts are 1mm to 2mm long and 10mm to 20mm apart. The edge pressing lines are 15mm to 25mm wide to balance pressure relief and tear resistance during laying.

10. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The water vapor transmission rate of the whole film decreased by 45% to 70% compared to the uncoated paper. The test method was based on the ASTM E96 / E96M-24a cup method BW procedure, and the test conditions were 38°C and 90% RH.

11. The paper-based-PHA double-sided composite soil biodegradable agricultural mulch film according to claim 1, characterized in that, The bio-based carbon content of the polyhydroxyalkanoate coating or its constituent polymer solids is ≥90%, and the test method is based on ISO 16620-2:2019.

12. A method for preparing a paper-based-PHA double-sided composite biodegradable agricultural mulch film according to claim 1, characterized in that, Includes the following steps: Step 1. Form a base paper with a weight of 25g / m² to 300g / m² to obtain the paper base layer; Step 2. Obtain polyhydroxy fatty acid ester-anhydride by reactive extrusion grafting, hydrolyze and partially neutralize to form polyhydroxy fatty acid ester-half ester salt, and prepare a polyhydroxy fatty acid ester-half ester salt solution with a pH value of 6.4 to 6.

8. Step 3. Preheat the polyhydroxyalkanoate-hemiester salt solution obtained in Step 2 to 85°C to 95°C, and mix it with the molten polyhydroxyalkanoate in a pressurized emulsifying kettle. Control the mass ratio of polyhydroxyalkanoate-hemiester salt to polyhydroxyalkanoate to be 2:100 to 5:100, and perform high-shear emulsification to obtain a solid content of 38wt% to 42wt% and a particle size D. 50 Internally stable emulsions of the same family with a thickness of 0.18 μm to 0.22 μm; Step 4. Apply the same family of internally stabilized emulsion to the soil-adhering surface of the paper base layer, apply at a dry coating amount of 5g / m² to 25g / m², and dry in sections at 80℃ to 110℃ until the exit paper temperature is ≤55℃ to obtain a single-sided coated paper base layer. Step 5. Surface activation is performed on the windward side of the single-sided coated paper substrate, and the equivalent activation energy is controlled to be 1.0 kJ / m² to 1.4 kJ / m² to obtain an activated coated paper substrate with a surface tension ≥ 40 mN / m. Step 6. Extrude and coat polyhydroxyalkanoate resin or perform multilayer co-extrusion, controlling the melt temperature at 160°C to 185°C, to obtain a thermoplastic polyhydroxyalkanoate melt curtain with a total thickness of 20μm to 80μm and a compatibility layer thickness of 2μm to 6μm; when the melt curtain is a white microfoamed film, it is prepared by high-pressure carbon dioxide injection at 5.8MPa to 6.2MPa, foaming at 155°C to 162°C, and a draw ratio of 1.55 to 1.70; Step 7. Following the wet-to-heat integrated lamination process, the thermoplastic polyhydroxyalkanoate melt curtain is laminated with the windward side of the activated coated paper base layer. The time from the completion of Step 4 to entering the lamination roller is controlled to be 20 min to 60 min. During lamination, the residual moisture content of the coating on the base layer is 5.0 wt% to 7.0 wt% on a dry basis. The residual moisture content is calculated by cutting a certain area of ​​coated paper sample, drying it to constant weight in an oven at (105±2)℃, and then calculating it as (wet weight - dry weight) / dry weight × 100%. The wet-to-heat integrated lamination process refers to the process of contacting the melt curtain with the windward side of the paper base layer at the lamination roller and immediately cooling and shaping it while the coating on the base layer is within the above-mentioned residual moisture content range, thereby producing a paper-plastic composite roll. Step 8. After the paper-plastic composite roll is left to stand at room temperature for ≥24 hours, it is cut to obtain the finished paper-based polyhydroxyalkanoate soil-degradable agricultural mulch film.

13. An application in farmland mulch cultivation, characterized in that, The agricultural mulch film according to claim 1 is used for weed suppression, moisture retention and temperature regulation. After crop harvesting, it is plowed into the soil. The polyhydroxyalkanoate covering layer has a biodegradability of ≥90% within 24 months according to ISO 17556 standard.

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