Food packaging bag based on polylactic acid and natural plant fiber compounding and preparation method

Abstract

The invention relates to the field of preparation and processing of food packaging bags, in particular to a polylactic acid and natural plant fiber compounded food packaging bag and a preparation method thereof. The preparation method comprises the following steps: grafting modified polylactic acid with maleic anhydride, modifying natural plant fibers with laccase, blending the modified polylactic acid, the modified natural plant fibers and rosin-based natural resin according to a ratio, carrying out twin-screw extrusion grafting compounding, and then preparing composite granules with a nano-microporous structure by adopting a supercritical CO2 foaming process. And carrying out blow molding to obtain the food packaging bag. The natural antibacterial property is achieved through phenolic hydroxyl groups of the rosin-based natural resin, adsorption and slow release of gas are achieved in combination with the nano-microporous structure of the composite material, and the packaging bag is endowed with the inherent modified atmosphere fresh-keeping performance. The prepared packaging bag is free of any chemical additive, simple in preparation process and suitable for industrial production, and has a good application prospect in the field of food packaging.

Description

Technical Field

[0001] This invention relates to the field of food packaging bag preparation and processing, specifically to a food packaging bag based on polylactic acid and natural plant fiber composite and its preparation method. Background Technology

[0002] Biodegradable and additive-free natural food packaging materials have become the core direction of industry development. Among them, polylactic acid (PLA), as a renewable and biodegradable polyester material, has good mechanical properties and food contact safety. Natural plant fibers are widely available, inexpensive, and biodegradable. Packaging materials made from the combination of the two have become the preferred solution to replace traditional petroleum-based plastic packaging. However, there are still many technical problems in the research and application of existing PLA-natural plant fiber composite food packaging materials: PLA is a non-polar polymer material, while natural plant fibers contain a large number of polar groups such as hydroxyl groups. The two have poor interfacial compatibility, and the composite material is prone to delamination and cracking, resulting in a significant decrease in mechanical properties and barrier properties, which cannot meet the actual use requirements of food packaging; existing PLA-based composite packaging with modified atmosphere packaging function mostly achieves its function by adding external gas regulators, antibacterial agents, and humectants, which poses a risk of additives migrating into food and does not meet the food packaging safety standards of no chemical additives; the modified atmosphere function of traditional composite packaging is passive and cannot achieve gas adsorption and slow release according to the food storage needs, resulting in a short preservation period and limited preservation effect for perishable foods such as fruits, vegetables, and fresh meat.

[0003] Therefore, inventing a method for preparing a safe food packaging material that has good material compatibility, can improve mechanical properties and barrier properties, and also has a preservation function is very promising. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a food packaging bag based on polylactic acid (PLA) and natural plant fiber composites, and a method for its preparation. Maleic anhydride is used to graft and modify PLA, and laccase is used to modify natural plant fibers. The modified PLA, modified natural plant fibers, and rosin-based natural resin are blended in a specific ratio, and the composite is grafted and compounded via twin-screw extrusion. Then, a supercritical CO2 foaming process is used to prepare composite granules with a nanoporous structure. Finally, the granules are blow-molded to obtain the food packaging bag. The phenolic hydroxyl groups of the rosin-based natural resin provide natural antibacterial properties, and the nanoporous structure of the composite material enables gas adsorption and slow release, giving the packaging bag inherent modified atmosphere preservation properties.

[0005] This invention discloses a food packaging bag based on polylactic acid and natural plant fiber composite, which is composed of the following components in parts by weight:

[0006] 30-40 parts of modified polylactic acid; 15-20 parts of laccase-modified natural plant fiber; 1-6 parts of rosin-based natural resin; 0.05-0.3 parts of crosslinking agent; 0.001-0.006 parts of antioxidant.

[0007] Preferably, the modified polylactic acid is maleic anhydride-grafted polylactic acid.

[0008] Preferably, the laccase-modified natural plant fiber is obtained by modifying natural plant fiber through laccase catalysis.

[0009] Preferably, the rosin-based natural resin is disproportionated rosin resin.

[0010] Preferably, the crosslinking agent is triglyceride citrate.

[0011] Preferably, the antioxidant is natural vitamin E.

[0012] This invention also discloses a method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite, comprising the following steps: Preparation of S1 modified polylactic acid: Polylactic acid and maleic anhydride are added to a mixer at a mass ratio of 25:1, then an initiator and styrene are added and mixed evenly. The mixture is then reacted at 160~170℃. After the reaction is completed, modified polylactic acid is obtained. Preparation of S2 laccase-modified natural plant fiber: Natural plant fiber was prepared into a suspension with a mass fraction of 13% and a pH value of 6. Laccase was then added and the mixture was stirred at a constant temperature in a 50℃ water bath for 2 hours. The mixture was then filtered, washed, and dried to obtain laccase-modified natural plant fiber. Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, rosin-based natural resin, crosslinking agent, and antioxidant are mixed and added to a twin-screw extruder. The mixture is melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule is added to a high-pressure reactor, and supercritical CO2 is introduced. The reactor is held at 55℃ and 14MPa for 2 hours to obtain composite granules. S4 Food Packaging Bag Forming: The composite granules prepared in step S3 are added to a blow molding machine, melted and plasticized at 180°C, blown into a composite film through a blow molding die with a blow ratio of 2.2:1, and then heat-sealed to form a bag at a heat-sealing temperature of 120°C and a heat-sealing time of 1 second to obtain the food packaging bag.

[0013] Preferably, in step S1, the initiator is benzoyl peroxide, and the mass ratio of the initiator to polylactic acid is 0.001:1.

[0014] Preferably, in step S1, the mass ratio of styrene to polylactic acid is 0.03:1.

[0015] Preferably, in step S3, the composite granules have a nanoporous structure with a pore size of 50~100nm and a porosity of 25~35%.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention provides a food packaging bag based on polylactic acid and natural plant fiber composite and its preparation method. The food packaging bag prepared by the method provided by this invention belongs to biomass conversion materials, and this food packaging bag and its preparation method have the following characteristics: (1) The method provided by the present invention introduces active groups by grafting polylactic acid with maleic anhydride, removes gum and reduces lignin content by modifying natural plant fibers with laccase, and combines the interfacial bridging effect of rosin-based natural resin to improve the interfacial bonding force between polylactic acid and natural plant fibers by more than 70%. The composite material has no delamination or cracking, and the mechanical properties are significantly improved. The tensile strength is increased by 30% compared with the pure polylactic acid-plant fiber composite material, and the elongation at break is increased by 30%.

[0017] (2) The method provided by the present invention utilizes the phenolic hydroxyl groups in the rosin-based natural resin molecules to achieve natural antibacterial activity, with an antibacterial rate of ≥90% against Escherichia coli and Staphylococcus aureus; combined with the nanoporous structure formed by supercritical CO2 foaming, it achieves the adsorption of ethylene released from food and the slow release of CO2 / N2, dynamically adjusts the internal atmosphere of the packaging according to the food storage requirements, has no risk of any additive migration, and meets the food packaging safety standards.

[0018] (3) The dense phase interface and nanoporous structure formed by the composite system significantly reduce the oxygen and water vapor permeability of the material, with oxygen permeability reduced by 39.3% and water vapor permeability reduced by 16.5%. The cold storage preservation period of fruits is extended to 15 days and the cold storage preservation period of fresh meat is extended to 7 days, which is 7 days and 5 days longer than pure polylactic acid packaging, respectively, and can effectively preserve the flavor and nutrition of food.

[0019] (4) All raw materials used in this invention are renewable and biodegradable. The composite material can be completely degraded into CO2 and water under composting conditions, leaving no environmental residue. The preparation process uses conventional extrusion, blow molding, and supercritical foaming equipment in the plastics processing field. The process parameters are controllable, the steps are simple, and it is suitable for large-scale industrial production. The packaging bags prepared by this method have mechanical properties, barrier properties, and modified atmosphere preservation properties, and have good application prospects in the food packaging field. Detailed Implementation

[0020] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0021] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0022] Example 1: A food packaging bag based on polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: 30 parts modified polylactic acid; 15 parts laccase-modified natural plant fiber; 1 part rosin-based natural resin; 0.05 parts crosslinking agent; 0.001 parts antioxidant; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0023] A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite includes the following steps: Preparation of S1 modified polylactic acid: Polylactic acid and maleic anhydride were added to a mixer at a mass ratio of 25:1. Then, an initiator and styrene were added and mixed evenly at 550 rpm. The mixture was then reacted at 160℃ for 30 min. After the reaction was completed, the modified polylactic acid was obtained by water cooling and granulation with a grafting rate of 0.98%. The initiator was benzoyl peroxide, and the mass ratio of initiator to polylactic acid was 0.001:1. The mass ratio of styrene to polylactic acid was 0.03:1.

[0024] Preparation of S2 laccase-modified natural plant fiber: Natural plant fiber was prepared into a suspension with a mass fraction of 13% and a pH value of 6. Then, laccase with a mass of 12% of the plant fiber was added. The mixture was stirred at a constant temperature in a 50℃ water bath for 2 hours. Then, it was filtered, washed, and dried in a 65℃ vacuum drying oven until the moisture content was ≤2% to obtain laccase-modified natural plant fiber.

[0025] Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant are mixed and added to a twin-screw extruder. The mixture is melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule is added to a high-pressure reactor, and supercritical CO2 is introduced. The reactor is held at 55℃ and 14MPa for 2 hours to obtain composite granules. The obtained composite granules have a nanoporous structure with a pore size of 50nm and a porosity of 25%.

[0026] S4 Food Packaging Bag Forming: The composite granules prepared in step S3 are added to a blow molding machine, melted and plasticized at 180°C, blown into a composite film through a blow molding die with a blow ratio of 2.2:1, and then heat-sealed to form a bag at a heat-sealing temperature of 120°C for 1 second to obtain the target product, i.e., a food packaging bag.

[0027] Example 2: A food packaging bag based on polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: 32 parts modified polylactic acid; 16 parts laccase-modified natural plant fiber; 2 parts rosin-based natural resin; 0.1 parts crosslinking agent; 0.002 parts antioxidant; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0028] A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite includes the following steps: Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant were mixed and added to a twin-screw extruder. The mixture was melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule was added to a high-pressure reactor, and supercritical CO2 was introduced. The reactor was held at 55℃ and 14MPa for 1.8 hours to obtain composite granules. The obtained composite granules have a nanoporous structure with a pore size of 60nm and a porosity of 27%.

[0029] The conditions and parameters for steps S1 to S4 are the same as in Example 1.

[0030] Example 3: A food packaging bag based on polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: 34 parts of modified polylactic acid; 17 parts of laccase-modified natural plant fiber; 3 parts of rosin-based natural resin; 0.15 parts of crosslinking agent; 0.003 parts of antioxidant; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0031] A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite includes the following steps: Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant were mixed and added to a twin-screw extruder. The mixture was melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule was added to a high-pressure reactor, and supercritical CO2 was introduced. The reactor was held at 55℃ and 14MPa for 1.6 hours to obtain composite granules. The obtained composite granules have a nanoporous structure with a pore size of 70nm and a porosity of 29%.

[0032] The conditions and parameters for steps S1 to S4 are the same as in Example 1.

[0033] Example 4: A food packaging bag based on polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: 36 parts of modified polylactic acid; 18 parts of laccase-modified natural plant fiber; 4 parts of rosin-based natural resin; 0.2 parts of crosslinking agent; 0.004 parts of antioxidant; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0034] A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite includes the following steps: Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant were mixed and added to a twin-screw extruder. The mixture was melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule was added to a high-pressure reactor, and supercritical CO2 was introduced. The reactor was held at 55℃ and 14MPa for 1.4 hours to obtain composite granules. The obtained composite granules have a nanoporous structure with a pore size of 80nm and a porosity of 31%.

[0035] The conditions and parameters for steps S1 to S4 are the same as in Example 1.

[0036] Example 5: A food packaging bag based on polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: 38 parts of modified polylactic acid; 19 parts of laccase-modified natural plant fiber; 5 parts of rosin-based natural resin; 0.25 parts of crosslinking agent; 0.005 parts of antioxidant; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0037] A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite includes the following steps: Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant were mixed and added to a twin-screw extruder. The mixture was melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule was added to a high-pressure reactor, and supercritical CO2 was introduced. The reactor was held at 55℃ and 14MPa for 1.2 hours to obtain composite granules. The obtained composite granules have a nanoporous structure with a pore size of 90nm and a porosity of 33%.

[0038] The conditions and parameters for steps S1 to S4 are the same as in Example 1.

[0039] Example 6: A food packaging bag based on polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: 40 parts modified polylactic acid; 20 parts laccase-modified natural plant fiber; 6 parts rosin-based natural resin; 0.3 parts crosslinking agent; 0.006 parts antioxidant; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0040] A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite includes the following steps: Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant were mixed and added to a twin-screw extruder. The mixture was melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule was added to a high-pressure reactor, and supercritical CO2 was introduced. The reactor was held at 55℃ and 14MPa for 1.0 h to obtain composite granules. The obtained composite granules have a nanoporous structure with a pore size of 100nm and a porosity of 35%.

[0041] The conditions and parameters for steps S1 to S4 are the same as in Example 1.

[0042] Example 7: The modified polylactic acid in the raw materials was replaced with polylactic acid, and the other raw material components and preparation methods were the same as in Example 5, except that step S1 was omitted.

[0043] Example 8: The laccase-modified natural plant fiber in the raw materials was replaced with natural plant fiber. The other raw material components and preparation methods were the same as in Example 5, except that step S2 was omitted.

[0044] Example 9: The rosin-based natural resin in the raw materials was removed, and the remaining raw material components and preparation method were the same as in Example 5.

[0045] Example 10: A food packaging bag based on polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: 38 parts of modified polylactic acid; 19 parts of laccase-modified natural plant fiber; 5 parts of rosin-based natural resin; 0.25 parts of crosslinking agent; 0.005 parts of antioxidant; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0046] A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite includes the following steps: Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant are mixed and added to a twin-screw extruder, melt-blended at 200℃, and extruded to obtain composite granules.

[0047] The conditions and parameters for steps S1 to S4 are the same as in Example 1.

[0048] Example 11: A conventional food packaging bag made of polylactic acid and natural plant fiber composite, composed of the following components in parts by weight: Polylactic acid 38 parts; natural plant fiber 19 parts; crosslinking agent 0.25 parts; antioxidant 0.005 parts; wherein the rosin-based natural resin is disproportionated rosin resin; the crosslinking agent is triglyceride citrate; and the antioxidant is natural vitamin E.

[0049] A method for preparing a conventional food packaging bag composed of polylactic acid and natural plant fibers includes the following steps: Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, disproportionated rosin resin, crosslinking agent, and antioxidant are mixed and added to a twin-screw extruder, melt-blended at 200℃, and extruded to obtain composite granules.

[0050] The conditions and parameters for steps S1 to S4 are the same as in Example 1.

[0051] The food packaging bags prepared in Examples 1-11 were subjected to performance testing, and the test results are shown in the table below:

[0052] According to the data in the table above: (1) In Examples 1-6, with the optimization of the ratio of modified polylactic acid, laccase-modified plant fiber, and rosin-based resin, the interfacial bonding strength increased from 62% to 72%, and the tensile strength and elongation at break increased by 30% simultaneously. The ratio in Example 5 reached the peak performance. This is because maleic anhydride-grafted polylactic acid introduces active groups, which form chemical bonds with the hydroxyl groups on the surface of laccase-modified plant fiber. Rosin-based natural resin, as an interfacial compatibilizer, interacts with the ester bonds of polylactic acid and the polar groups of plant fiber, constructing a stable three-phase interfacial structure. The ratio in Example 5 has the highest chemical bonding density and the best stress transfer efficiency at the three-phase interface. Comparative Example 7 was prepared by adding unmodified polylactic acid, and Example 8 was prepared by adding unmodified plant fiber. The products obtained in Examples 7 and 8 had a less than 25% improvement in interfacial bonding due to the lack of chemical bonding of active groups, resulting in limited improvement in mechanical properties. In Example 9, after removing rosin-based resin, the interfacial bridging effect disappeared, and the interfacial bonding and mechanical properties deteriorated, further verifying the key role of rosin-based resin.

[0053] (2) In Examples 1-6, the antibacterial rate was ≥90%, and the antibacterial rate of Example 5 was the highest at 93%. The structure of nanopores with a pore size of 50-100 nm and a porosity of 25-35% achieved a dynamic balance between gas adsorption and slow release. Among them, the micropore size of Example 5 was 90 nm and the porosity was 33%, which had the best adsorption capacity for ethylene and CO2 slow release rate. Therefore, the preservation period of fruits reached 15 days and that of fresh meat reached 7 days, which is 7 days and 5 days longer than that of pure polylactic acid packaging, respectively. The reason for the above effects is that the phenolic hydroxyl groups in the rosin-based natural resin molecules destroy the bacterial cell membrane and achieve natural antibacterial effect. Under the ratio of Example 5, the dispersibility of rosin-based resin was the best and the effective contact area of ​​phenolic hydroxyl groups was the largest. The nanopore structure formed by supercritical CO2 foaming can adsorb ethylene released from food and slow release CO2 / N2 to regulate the packaging atmosphere. Compared with Example 9, after removing rosin-based resin, the antibacterial rate dropped to below 12%; Example 10 omitted the supercritical foaming process, had no nanoporous structure, lost the modified atmosphere function, and shortened the shelf life.

[0054] (3) In Examples 1-6, the oxygen permeability reduction rate was the highest at 39.3%, and the water vapor reduction rate was 16.5%. This is because the dense interfacial structure reduced the gas permeation channels, and the pore walls of the nanopores also hindered gas molecules. The shortest degradation time was 160 days. The material of the food packaging bag obtained under the raw material ratio of Example 5 had moderate crystallinity and the highest microbial degradation efficiency. In contrast, the conventional composite system of Example 11 had poor interfacial compatibility, loose structure, and the worst barrier performance, with a degradation time as long as 195 days.

[0055] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A food packaging bag based on polylactic acid and natural plant fiber composite, characterized in that, It consists of the following components in parts by weight: 30-40 parts of modified polylactic acid; 15-20 parts of laccase-modified natural plant fiber; 1-6 parts of rosin-based natural resin; 0.05-0.3 parts of crosslinking agent; 0.001-0.006 parts of antioxidant.

2. A food packaging bag based on polylactic acid and natural plant fiber composite as described in claim 1, characterized in that, The modified polylactic acid is maleic anhydride-grafted polylactic acid.

3. A food packaging bag based on polylactic acid and natural plant fiber composite as described in claim 1, characterized in that, The laccase-modified natural plant fiber is obtained by modifying natural plant fiber through laccase catalysis.

4. A food packaging bag based on polylactic acid and natural plant fiber composite as described in claim 1, characterized in that, The rosin-based natural resin is a disproportionated rosin resin.

5. A food packaging bag based on polylactic acid and natural plant fiber composite as described in claim 1, characterized in that, The crosslinking agent is triglyceride citrate.

6. A food packaging bag based on polylactic acid and natural plant fiber composite as described in claim 1, characterized in that, The antioxidant is natural vitamin E.

7. A method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite, characterized in that, Includes the following steps: Preparation of S1 modified polylactic acid: Polylactic acid and maleic anhydride are added to a mixer at a mass ratio of 25:1, then an initiator and styrene are added and mixed evenly. The mixture is then reacted at 160~170℃. After the reaction is completed, modified polylactic acid is obtained. Preparation of S2 laccase-modified natural plant fiber: Natural plant fiber was prepared into a suspension with a mass fraction of 13% and a pH value of 6. Laccase was then added and the mixture was stirred at a constant temperature in a 50℃ water bath for 2 hours. The mixture was then filtered, washed, and dried to obtain laccase-modified natural plant fiber. Preparation of S3 composite granules: Modified polylactic acid, laccase-modified natural plant fiber, rosin-based natural resin, crosslinking agent, and antioxidant are mixed and added to a twin-screw extruder. The mixture is melt-blended and extruded at 200℃ to obtain a preliminary granule. The preliminary granule is added to a high-pressure reactor, and supercritical CO2 is introduced. The reactor is held at 55℃ and 14MPa for 2 hours to obtain composite granules. S4 Food Packaging Bag Forming: The composite granules prepared in step S3 are added to a blow molding machine, melted and plasticized at 180°C, blown into a composite film through a blow molding die with a blow ratio of 2.2:1, and then heat-sealed to form a bag at a heat-sealing temperature of 120°C and a heat-sealing time of 1 second to obtain the food packaging bag.

8. The method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite according to claim 7, characterized in that, In step S1, the initiator is benzoyl peroxide, and the mass ratio of the initiator to polylactic acid is 0.001:

1.

9. The method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite according to claim 7, characterized in that, In step S1, the mass ratio of styrene to polylactic acid is 0.03:

1.

10. The method for preparing a food packaging bag based on polylactic acid and natural plant fiber composite according to claim 7, characterized in that, In step S3, the composite granules have a nanoporous structure with a pore size of 50~100nm and a porosity of 25~35%.

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