Degradable high-barrier polylactic acid composite film and preparation method thereof

Abstract

The application discloses a degradable high-barrier polylactic acid composite film and a preparation method thereof, and belongs to the technical field of polylactic acid composite films.The high-barrier polylactic acid composite film comprises a polylactic acid upper surface layer, a high-barrier middle layer and a polylactic acid lower surface layer; the polylactic acid upper surface layer and the polylactic acid lower surface layer are composed of a resin mixture A comprising polylactic acid resin, lactic acid-glycolic acid copolymer, degradable elastomer, compatibility agent, antioxidant and anti-hydrolysis agent; the high-barrier middle layer is composed of a resin mixture B comprising polyglycolic acid, lactic acid-glycolic acid copolymer, compatibility agent and antioxidant; the preparation method is simple, easy to realize industrial production and use, and the prepared polylactic acid composite film is a fully degradable film, which not only has excellent degradability, mechanical properties and processing properties, but also has good flexibility and barrier properties, and can effectively prevent water vapor and oxygen in the air from penetrating.

Description

Technical Field

[0001] This invention belongs to the field of polylactic acid composite membrane technology, specifically relating to a biodegradable high-barrier polylactic acid composite membrane and its preparation method. Background Technology

[0002] Currently, most commercially available high-barrier films are made from petroleum-based polymers such as PET, PP, PE, PA, PE, and PVDC, which pose significant environmental hazards. From an environmental protection and sustainability perspective, developing a biodegradable barrier film can effectively reduce the environmental harm caused by plastic films and alleviate current environmental pressures.

[0003] Polylactic acid (PLA) is a bio-based polymer with high elastic modulus (2-4 GPa), high strength (30-50 MPa), good light transmittance, and compostability. As a novel biodegradable material, PLA is produced from starch extracted from renewable plant resources (such as corn). This starch is saccharified to obtain glucose, which is then fermented with specific microorganisms to produce high-purity lactic acid. Finally, PLA of a certain molecular weight is synthesized through chemical synthesis. It exhibits excellent biodegradability, being completely degraded by microorganisms in nature after use, ultimately producing carbon dioxide and water without polluting the environment. It is a recognized environmentally friendly material and an ideal packaging material. While PLA possesses good safety and biocompatibility, ordinary PLA packaging films have poor barrier properties, making it difficult to use as a standalone barrier film.

[0004] Polyglycolic acid (PGA) is a biodegradable resin with a molecular structure similar to PLA, but it exhibits superior barrier properties. PGA's gas permeability coefficient is approximately 1000 times lower than PLA's, and its water vapor permeability coefficient is approximately 38 times lower. Therefore, PGA can effectively compensate for PLA's shortcomings in barrier properties, improving the barrier performance of films. Furthermore, combining PLA and PGA through copolymerization or blending can further enhance the degradation rate of PLA. Therefore, by combining PGA with polylactic acid (PLA), a biodegradable, high-barrier PLA composite membrane can be designed.

[0005] Patent CN116905231A discloses a polylactic acid composite membrane, its preparation method, and its application. This invention first coats the surface of a polylactic acid fiber membrane with a polydopamine coating, then distributes hydroxyapatite nanowires on the surface of the polydopamine coating through intermolecular forces. However, this polylactic acid fiber membrane focuses on solving the problem in existing technologies where poor dispersion and easy aggregation of bio-electrets in filter materials lead to poor product filtration performance. Patent CN116641228A discloses a polylactic acid composite membrane and its preparation method, including the following steps: in-situ growth of one-dimensional hydroxyapatite crystals: water-soluble calcium salts, water... Soluble phosphates are stirred and dispersed evenly in water, and then placed together with polylactic acid (PLA) fibers in a microwave reactor. After reacting at a preset temperature for a preset time, one-dimensional hydroxyapatite crystals are synthesized in situ on the surface of the PLA fibers to obtain synthetic fibers. The synthetic fibers are then removed and dried for later use. A restricted densification molding process is then performed on the dried synthetic fibers at a preset molding temperature and pressure to obtain a PLA composite membrane modified with hydroxyapatite crystals. This invention achieves simultaneous improvement in the hydrophilicity, biodegradation rate, and mechanical properties of fully degradable PLA composite materials. However, it does not provide relevant technical insights for improving the high-barrier membrane performance of PLA composite membranes. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention discloses a biodegradable high-barrier polylactic acid composite film and its preparation method. The prepared polylactic acid composite film is a fully biodegradable film, which not only has excellent biodegradability, mechanical properties and processing properties, but also exhibits good performance in terms of flexibility and barrier properties.

[0007] The technical solution of the present invention is as follows:

[0008] One objective of this invention is to provide a biodegradable high-barrier polylactic acid (PLA) composite film, wherein the high-barrier PLA composite film comprises a PLA upper surface layer 1, a high-barrier intermediate layer 2, and a PLA lower surface layer 3; wherein,

[0009] The polylactic acid upper surface layer 1 and polylactic acid lower surface layer 3 are resin mixture A, which includes polylactic acid resin, lactic acid-glycolic acid copolymer, biodegradable elastomer, compatibilizer, antioxidant and anti-hydrolysis agent.

[0010] The high-barrier intermediate layer 2 is a resin mixture B, which includes polyglycolic acid, lactic acid-glycolic acid copolymer, compatibilizer and antioxidant.

[0011] Furthermore, based on the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer and biodegradable elastomer, the polylactic acid resin content in resin mixture A is 50-70 wt%, the lactic acid-glycolic acid copolymer content is 15-25 wt%, and the biodegradable elastomer content is 15-25 wt%.

[0012] The compatibilizer content is 1-10 wt% of the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer and biodegradable elastomer;

[0013] The antioxidant content is 0.5-1 wt% of the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer and biodegradable elastomer;

[0014] The content of the anti-hydrolysis agent is 0.1-1 wt% of the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer and biodegradable elastomer.

[0015] Furthermore, based on the total mass of polyglycolic acid and lactic acid-glycolic acid copolymer, the content of polyglycolic acid in resin mixture B is 30-50 wt%, and the content of lactic acid-glycolic acid copolymer is 50-70 wt%.

[0016] The compatibilizer content is 1-10 wt% of the total mass of polyglycolic acid and lactic acid-glycolic acid copolymer;

[0017] The antioxidant content is 0.5-1 wt% of the total mass of polyglycolic acid and lactic acid-glycolic acid copolymer;

[0018] Furthermore, the lactic acid-glycolic acid copolymer is a copolymer with a glycolic acid content greater than 50 mol%.

[0019] Furthermore, the compatibilizer is at least one selected from epoxy compounds, isocyanate compounds, and peroxide compounds.

[0020] Furthermore, the biodegradable elastomer is at least one of polybutylene terephthalate, polybutylene succinate, polybutylene succinate, polycaprolactone, polyhydroxyalkanoate, and carbon dioxide copolymer.

[0021] Furthermore, the antioxidant is at least one of hindered phenols and phosphites.

[0022] Furthermore, the anti-hydrolysis agent is a carbodiimide-based hydrolysis agent.

[0023] The second objective of this invention is to provide a method for preparing a biodegradable high-barrier polylactic acid composite film, specifically including the following steps:

[0024] S1. Weigh the raw materials according to the formula, and mix them by mixing in a mixer to obtain resin mixture A and resin mixture B;

[0025] S2. Resin mixture A and resin mixture B are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion and casting to obtain composite casting sheets. The processing temperature of the main machine is 190-230℃ and the processing temperature of the auxiliary machine is 180-210℃.

[0026] S3. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained.

[0027] Furthermore, in the prepared biodegradable high-barrier polylactic acid composite film, the thickness of the high-barrier intermediate layer 2 is 1-3 μm; the thickness of the polylactic acid upper surface layer 1 and the polylactic acid lower surface layer 3 are both 6-11 μm.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. This invention designs a biodegradable high-barrier polylactic acid (PLA) composite membrane, in which lactic acid-glycolic acid copolymer (PLGA) is added as one of the raw materials in the upper and lower layers and the intermediate layer of PLA. Glycolic acid (GA) has one less side methyl group than lactic acid (LA), making it more hygroscopic and faster in degradation. PLGA degrades faster than PLA. By adjusting the glycolic acid content in the copolymer, the degradation rate and barrier properties of PLGA can be controlled. PLGA also acts as a compatibilizer to some extent, improving the bonding force between the upper and lower PLA layers and the high-barrier intermediate layer, resulting in a tightly bonded high-barrier PLA composite membrane.

[0030] 2. In the method for preparing a biodegradable high-barrier polylactic acid (PLA) composite film according to the present invention, the added biodegradable elastomer, after blending with PLA, can fully utilize its own elasticity to improve the brittleness of PLA, thereby enhancing the toughness of the PLA composite film. The compatibilizer used contains highly reactive groups, which can promote the formation of copolymers between PLA and polyglycolic acid, thereby effectively reducing the size of the dispersed phase and making it uniformly distributed, enhancing interfacial bonding, and improving the mechanical properties of the film. At the same time, the compatibilizer can also promote the formation of covalent bonds between PLA and polyglycolic acids, offsetting the degradation caused by increased processing temperature while retaining the molecular weight of PLA and polyglycolic acid. In addition, the compatibilizer also helps to improve the melt strength of PLA and polyglycolic acid and extend the processing window.

[0031] 3. The preparation method of the biodegradable high-barrier polylactic acid composite film of the present invention is simple. Since lactic acid-glycolic acid copolymer is added between different layers, the different layers have good interfacial compatibility and interfacial bonding force. The composite upper lactic acid layer, high-barrier middle layer and polylactic acid lower layer can be directly cast using a three-layer co-extrusion casting machine to obtain a composite casting with a three-layer structure, which has high practical value and broad application prospects.

[0032] Figure Labels

[0033] Figure 1 This is a schematic diagram of the structure of the biodegradable high-barrier polylactic acid composite membrane described in this invention.

[0034] The reference numerals in the figure are as follows:

[0035] 1. Polylactic acid (PLA) top layer; 2. High-barrier intermediate layer; 3. PLA bottom layer. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0037] In the description of this invention, it should be noted that the terms used in this invention (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and should not be construed as limiting the invention; it should be further understood that the terms used in this invention should be understood to have the same meaning as those in the context of this specification and in the relevant field, and should not be understood in an idealized or overly formal sense, except as expressly defined in this invention.

[0038] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0039] In the quantitative experiments in the following examples, three replicate experiments were set up, and the average value of the results was taken.

[0040] Unless otherwise specified, the experimental methods described in the following examples are conventional methods.

[0041] The following examples illustrate the preparation of resin mixture A-1, which serves as the upper and lower polylactic acid (PLA) surface layer:

[0042] 500g of polylactic acid resin, 250g of lactic acid-glycolic acid copolymer, 250g of polybutylene terephthalate, 10g of ARD, 5.0g of antioxidant 1010, and 1.0g of polycarbodiimide were mixed evenly in a mixing device and then melt-extruded through a twin-screw extruder to obtain resin mixture A-1.

[0043] The following examples illustrate the preparation of resin mixture A-2, which serves as the upper and lower polylactic acid (PLA) surface layer:

[0044] 700g of polylactic acid resin, 150g of lactic acid-glycolic acid copolymer, 150g of polybutylene terephthalate, 50g of ARD, 5.0g of antioxidant 1010, and 1.0g of polycarbodiimide were mixed evenly in a mixing device and then melt-extruded through a twin-screw extruder to obtain resin mixture A-2.

[0045] The following examples illustrate the preparation of resin mixture A-3, which serves as the upper and lower polylactic acid (PLA) surface layer:

[0046] 600g of polylactic acid resin, 200g of lactic acid-glycolic acid copolymer, 200g of polybutylene succinate, 100g of ARD, 10.0g of antioxidant 1010, and 10.0g of polycarbodiimide were mixed evenly in a mixing device and then melt-extruded through a twin-screw extruder to obtain resin mixture A-3.

[0047] Preparation of resin mixture B-1 as a high-barrier intermediate layer in the following examples:

[0048] 300g of polyglycolic acid, 700g of lactic acid-glycolic acid copolymer, 50g of ARD, and 5.0g of antioxidant 1010 were mixed evenly in a mixing device and then melt-extruded through a twin-screw extruder to obtain resin mixture B-1.

[0049] Preparation of resin mixture B-2 as a high-barrier intermediate layer in the following examples:

[0050] 500g of polyglycolic acid, 500g of lactic acid-glycolic acid copolymer, 10g of ARD, and 5.0g of antioxidant 1010 were mixed evenly in a mixing device and then melt-extruded through a twin-screw extruder to obtain resin mixture B-1.

[0051] The following examples illustrate the preparation of resin mixture B-3 as a high-barrier intermediate layer:

[0052] 400g of polyglycolic acid, 600g of lactic acid-glycolic acid copolymer, 100g of ARD, and 10.0g of antioxidant 1010 were mixed evenly in a mixing device and then melt-extruded through a twin-screw extruder to obtain resin mixture B-3.

[0053] In the following examples, the auxiliary machine where resin mixture A is located rotates at 23-28 r / min, and the main machine where resin mixture B is located rotates at 4-6 r / min.

[0054] Example 1

[0055] This embodiment provides a method for preparing a biodegradable high-barrier polylactic acid composite film, including the following steps:

[0056] S1. Resin mixture A-1 and resin mixture B-1 are fed into the auxiliary machine and main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 195-215℃, and the processing temperature of the auxiliary machine is 180-200℃.

[0057] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 3μm, and the thickness of the polylactic acid upper and lower surface layers is 6μm.

[0058] Example 2

[0059] This embodiment provides a method for preparing a biodegradable high-barrier polylactic acid composite film, including the following steps:

[0060] S1. Resin mixture A-1 and resin mixture B-2 are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 200-220℃, and the processing temperature of the auxiliary machine is 180-200℃.

[0061] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 3μm, and the thickness of the polylactic acid upper and lower surface layers is 6μm.

[0062] Example 3

[0063] This embodiment provides a method for preparing a biodegradable high-barrier polylactic acid composite film, including the following steps:

[0064] S1. Resin mixture A-1 and resin mixture B-3 are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 190-210℃, and the processing temperature of the auxiliary machine is 180-200℃.

[0065] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 1 μm, and the thickness of the polylactic acid upper and lower surface layers is 6 μm.

[0066] Example 4

[0067] This embodiment provides a method for preparing a biodegradable high-barrier polylactic acid composite film, including the following steps:

[0068] S1. Resin mixture A-2 and resin mixture B-1 are fed into the auxiliary machine and main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 195-215℃, and the processing temperature of the auxiliary machine is 190-210℃.

[0069] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 3μm, and the thickness of the polylactic acid upper and lower surface layers is 6μm.

[0070] Example 5

[0071] This embodiment provides a method for preparing a biodegradable high-barrier polylactic acid composite film, including the following steps:

[0072] S1. Resin mixture A-2 and resin mixture B-2 are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 200-220℃, and the processing temperature of the auxiliary machine is 190-210℃.

[0073] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 3μm, and the thickness of the polylactic acid upper and lower surface layers is 6μm.

[0074] Example 6

[0075] S1. Resin mixture A-2 and resin mixture B-3 are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 190-210℃, and the processing temperature of the auxiliary machine is 190-210℃.

[0076] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 1 μm, and the thickness of the polylactic acid upper and lower surface layers is 8 μm.

[0077] Example 7

[0078] S1. Resin mixture A-3 and resin mixture B-1 are fed into the auxiliary machine and main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 210-230℃, and the processing temperature of the auxiliary machine is 190-210℃.

[0079] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 2μm, and the thickness of the polylactic acid upper and lower surface layers is 11μm.

[0080] Example 8

[0081] S1. Resin mixture A-3 and resin mixture B-2 are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 210-230℃, and the processing temperature of the auxiliary machine is 180-200℃.

[0082] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 3μm, and the thickness of the polylactic acid upper and lower surface layers is 11μm.

[0083] Example 9

[0084] S1. Resin mixture A-3 and resin mixture B-3 are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion to obtain composite casting sheets. The processing temperature of the main machine is 205-225℃, and the processing temperature of the auxiliary machine is 180-200℃.

[0085] S2. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained, wherein the thickness of the high-barrier intermediate layer is 2μm, and the thickness of the polylactic acid upper and lower surface layers is 8μm.

[0086] Comparative Example 1

[0087] The difference between this comparative example 1 and example 1 is that the upper and lower polylactic acid surfaces are 100% polylactic acid.

[0088] Comparative Example 2

[0089] The difference between this comparative example 2 and example 1 is that the high-barrier intermediate layer is 100% polyglycolic acid.

[0090] Comparative Example 3

[0091] The difference between Comparative Example 3 and Example 1 is that the high-barrier intermediate layer is 100% lactic acid-glycolic acid copolymer.

[0092] Comparative Example 4

[0093] Comparative Example 4 uses a common polylactic acid pure membrane.

[0094] Performance testing

[0095] The polylactic acid composite membranes prepared in Examples 1-9 and Comparative Examples 1-4 were subjected to performance tests. The tensile strength and elongation at break of the polylactic acid composite membranes were tested according to GB / T 1040; the oxygen permeability was determined according to GB / T 1038. The results are shown in Table 1.

[0096] Table 1 Performance test results for Examples 1-9 and Comparative Examples 1-4

[0097]

[0098] The data above shows that both lactic acid-glycolic acid copolymer and polyglycolic acid can effectively improve the barrier properties of the membrane, and the gas barrier properties of the membrane also increase with the increase of glycolic acid content in the membrane. Comparative Example 2 showed the best gas barrier properties; however, polyglycolic acid is more expensive and readily absorbs water, leading to a decline in product performance. In summary, using a blend of lactic acid-glycolic acid copolymer and polyglycolic acid as the high-barrier layer of the membrane allows for better cost control while ensuring membrane barrier properties. Furthermore, the degradation rate of the membrane can be indirectly adjusted by regulating the glycolic acid content in the lactic acid-glycolic acid copolymer.

[0099] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A biodegradable high-barrier polylactic acid composite film, characterized in that, The high-barrier polylactic acid composite film comprises a polylactic acid upper surface layer (1), a high-barrier intermediate layer (2), and a polylactic acid lower surface layer (3); wherein, The polylactic acid upper surface layer (1) and polylactic acid lower surface layer (3) are resin mixture A, which includes polylactic acid resin, lactic acid-glycolic acid copolymer, biodegradable elastomer, compatibilizer, antioxidant and anti-hydrolysis agent; The high-barrier intermediate layer (2) is a resin mixture B, which includes polyglycolic acid, lactic acid-glycolic acid copolymer, compatibilizer and antioxidant; Based on the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer, and biodegradable elastomer, the resin mixture A contains 50-70 wt% polylactic acid resin, 15-25 wt% lactic acid-glycolic acid copolymer, and 15-25 wt% biodegradable elastomer. In the resin mixture A, the compatibilizer content is 1-10 wt% of the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer, and biodegradable elastomer. In the resin mixture A, the antioxidant content is 0.5-1 wt% of the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer, and biodegradable elastomer; In the resin mixture A, the content of the anti-hydrolysis agent is 0.1-1 wt% of the total mass of polylactic acid resin, lactic acid-glycolic acid copolymer, and biodegradable elastomer; Based on the total mass of polyglycolic acid and lactic acid-glycolic acid copolymer, the content of polyglycolic acid in resin mixture B is 30-50 wt%, and the content of lactic acid-glycolic acid copolymer is 50-70 wt%. In the resin mixture B, the compatibilizer content is 1-10 wt% of the total mass of polyglycolic acid and lactic acid-glycolic acid copolymer; In the resin mixture B, the content of antioxidant is 0.5-1 wt% of the total mass of polyglycolic acid and lactic acid-glycolic acid copolymer; The lactic acid-glycolic acid copolymer is a copolymer with a glycolic acid content greater than 50 mol%.

2. The biodegradable high-barrier polylactic acid composite film as described in claim 1, characterized in that, The compatibilizer is at least one of epoxy compounds, isocyanate compounds, and peroxide compounds.

3. The biodegradable high-barrier polylactic acid composite membrane as described in claim 1, characterized in that, The biodegradable elastomer is at least one of polybutylene terephthalate, polybutylene succinate, polycaprolactone, polyhydroxyalkanoate, and carbon dioxide copolymer.

4. The biodegradable high-barrier polylactic acid composite film as described in claim 1, characterized in that, The antioxidant is at least one of hindered phenols and phosphites.

5. The biodegradable high-barrier polylactic acid composite membrane as described in claim 1, characterized in that, The anti-hydrolysis agent is a carbodiimide-based hydrolysis agent.

6. A method for preparing a biodegradable high-barrier polylactic acid composite film as described in any one of claims 1-5, characterized in that, Specifically, the following steps are included: S1. Weigh the raw materials according to the formula, and mix them by mixing in a mixer to obtain resin mixture A and resin mixture B; S2. Resin mixture A and resin mixture B are respectively fed into the auxiliary machine and the main machine of the three-layer co-extrusion casting machine for melt extrusion and casting to obtain composite casting sheets. The processing temperature of the main machine is 190-230℃ and the processing temperature of the auxiliary machine is 180-210℃. S3. After biaxial stretching of the composite casting, a biodegradable high-barrier polylactic acid composite film is obtained.

7. The method for preparing a biodegradable high-barrier polylactic acid composite film as described in claim 6, characterized in that, In the biodegradable high-barrier polylactic acid composite film, the thickness of the high-barrier intermediate layer (2) is 1-3 μm; the thickness of the polylactic acid upper surface layer (1) and the polylactic acid lower surface layer (3) is 6-11 μm.

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