Preparation method of polyethylene blown film for fruit and vegetable packaging

Abstract

The invention discloses a preparation method of a polyethylene blown film for fruit and vegetable packaging, which comprises the following steps: respectively introducing a first outer layer material, an inner layer material and a second outer layer material into a hopper of a three-layer co-extrusion blow molding machine for three-layer co-extrusion blow molding, and then performing traction treatment to obtain the polyethylene blown film for fruit and vegetable packaging, the first outer-layer material and the second outer-layer material are linear low-density polyethylene; the material of the inner layer is low-density polyethylene; through structural design and material modification, the heat resistance of the film is effectively improved, and meanwhile, good oxygen isolation performance is obtained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polyethylene film, and particularly relates to a preparation method of a polyethylene blown film for fruit and vegetable packaging. BACKGROUND

[0002] Polyethylene film has excellent chemical stability, heat sealing property and low price advantage, is one of the materials with the largest use amount in agricultural covering planting and industrial production, and can be roughly divided into general film and functional film according to different application scenes, wherein the general film is mainly agricultural polyethylene film and packaging polyethylene film, and the functional film is a film applied to a specific scene, such as a battery separator, a rust-proof film, an ion exchange film and the like.

[0003] Polyethylene is widely used for packaging of various food, meat, vegetables and other agricultural and sideline products due to its non-toxic and odorless properties, as well as good opening, heat sealing and transparency, and can print text and patterns thereon. Although the polyethylene packaging film is convenient to use and simple to operate, it has only certain barrier property to water vapor and insufficient barrier property to oxygen and carbon dioxide, and thus cannot be applied to higher requirement packaging fields. Therefore, a great attention is paid to the development and preparation of high-barrier polyethylene film, and current research methods can be divided into two categories, one of which is to realize high barrier property by adding and fixing an additive to a polymer film, and the other is to prepare a composite film with better barrier property by using a multi-layer composite process.

[0004] A Chinese patent (publication number CN118560132A) discloses a bacteriostatic polyethylene blown film and a preparation method and application thereof. The polyethylene blown film is formed by three-layer co-extrusion blow molding of an inner layer material, a middle layer material and an outer layer material. The preparation raw material of the inner layer material is added with a bacteriostatic master batch, and the bacteriostatic master batch is a silver-zinc ion bacteriostatic master batch or an oyster shell powder antibacterial master batch. The weight of the bacteriostatic master batch in the preparation raw material of the inner layer material is 3-8% of the weight of polyethylene. The invention technology uses three-layer material co-extrusion blow molding to obtain the polyethylene film, but lacks research on the different material layers, and the heat resistance and oxygen barrier property of the polyethylene film also need to be improved.

[0005] Therefore, how to select different polyethylene as the inner and outer layer materials, and perform corresponding modification treatment and traction treatment process to prepare the polyethylene blown film for fruit and vegetable packaging, effectively improve the thermal decomposition temperature of the film, and reduce the oxygen transmission rate, has become a research direction. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a preparation method of polyethylene blown film for fruit and vegetable packaging; the present application uses montmorillonite linear low-density polyethylene composite as outer material, uses plasma-treated low-density polyethylene as inner material, obtains polyethylene blown film through co-extrusion blow molding and traction treatment, effectively improves the heat resistance of the film through structural design and material modification, and at the same time obtains good oxygen barrier performance, so that it can be better applied in the field of fruit and vegetable packaging.

[0007] The technical scheme of the present application is as follows: The present application provides a preparation method of polyethylene blown film for fruit and vegetable packaging, comprising the following steps: The first outer material, the inner material and the second outer material are introduced into the hoppers of the three-layer co-extrusion blow molding machine respectively for three-layer co-extrusion blow molding, and then traction treatment is performed to obtain the polyethylene blown film for fruit and vegetable packaging; The first outer material and the second outer material are montmorillonite modified linear low-density polyethylene; the preparation method of the montmorillonite modified linear low-density polyethylene comprises the following steps: first, long-chain quaternary ammonium salt is used to intercalate and modify sodium-based montmorillonite to obtain quaternary ammonium salt modified montmorillonite; linear low-density polyethylene, a compatibilizer and the quaternary ammonium salt modified montmorillonite are melt blended to obtain a montmorillonite / linear low-density polyethylene composite; The inner material is low-density polyethylene.

[0008] The present application uses sodium-based montmorillonite as raw material, modifies and treats it by using long-chain quaternary ammonium salt, grafts or intercalates organic chain segments to the lamella of sodium-based montmorillonite, thereby obtaining quaternary ammonium salt modified montmorillonite, reducing the surface energy of montmorillonite and facilitating the distribution in the polyethylene matrix, and then adding a compatibilizer polyethylene grafted maleic anhydride to improve the compatibility of montmorillonite and linear low-density polyethylene, and obtaining a montmorillonite / linear low-density polyethylene composite through melt blending.

[0009] As a preferred technical scheme of the present application, the step of intercalation modification is as follows: 4-6 parts of sodium-based montmorillonite are added to 90-100 parts of deionized water to obtain a montmorillonite dispersion liquid by stirring and dispersing; 3-5 parts of long-chain quaternary ammonium salt are added to 30-40 parts of deionized water and mixed uniformly at 70-80 DEG C, and then added to the montmorillonite dispersion liquid and stirred at high speed for 4-6 hours, and then treated to obtain quaternary ammonium salt modified montmorillonite.

[0010] As a preferred technical scheme of the present application, the weight fraction of the sodium-based montmorillonite can be 4 parts, 4.5 parts, 5 parts, 5.5 parts or 6 parts, etc.

[0011] As a preferred technical scheme of the present application, the weight fraction of the long-chain quaternary ammonium salt can be 3 parts, 3.5 parts, 4 parts, 4.5 parts or 5 parts, etc.

[0012] As a preferred technical scheme of the present application, the post-processing step is: reduced pressure filtration, deionized water washing, vacuum drying, and ball milling.

[0013] As a preferred technical scheme of the present application, the long-chain quaternary ammonium salt is selected from cetyltrimethylammonium bromide or octadecyltrimethylammonium chloride.

[0014] As a preferred technical scheme of the present application, the compatibilizer is polyethylene grafted maleic anhydride.

[0015] As a preferred technical scheme of the present application, the step of melt blending is: melt extruding 400-500 parts of linear low-density polyethylene, 15-25 parts of a compatibilizer, and 25-35 parts of the quaternary ammonium salt modified montmorillonite, and then drawing and water-cooling to form granules.

[0016] As a preferred technical scheme of the present application, the linear low-density polyethylene can be 400 parts, 420 parts, 440 parts, 460 parts, 480 parts, or 500 parts, etc.

[0017] As a preferred technical scheme of the present application, the compatibilizer can be 15 parts, 17 parts, 19 parts, 21 parts, 23 parts, or 25 parts, etc.

[0018] As a preferred technical scheme of the present application, the compatibilizer can be 25 parts, 27 parts, 29 parts, 31 parts, 33 parts, or 35 parts, etc.

[0019] In the melt blending process of the montmorillonite / linear low-density polyethylene composite of the present application, the quaternary ammonium salt modified montmorillonite and the linear low-density polyethylene have good compatibility, the quaternary ammonium salt modified montmorillonite can be uniformly dispersed in the resin matrix, the polyethylene molecular chain enters the modified montmorillonite layer and gradually peels off the modified montmorillonite layer, the peeled montmorillonite nanosheet has good gas barrier property for gas molecules such as oxygen, thereby effectively reducing the oxygen transmission rate; at the same time, the montmorillonite nanosheet can form a dense barrier in the polyethylene, effectively hinder the rapid transfer of heat, delay the rise of the matrix temperature, and capture and hinder the small molecular volatile products produced in the polymer thermal decomposition process, thereby significantly improving the thermal decomposition temperature of the material.

[0020] As a preferred technical scheme of the present application, the low-density polyethylene is plasma-treated low-density polyethylene.

[0021] As a preferred technical scheme of the present application, the plasma treatment conditions are: placing the low-density polyethylene in a plasma treatment system, using oxygen as the working gas, the oxygen volume flow rate is 0.008-0.010 L / min, the ionization power is 200-220 W, and the treatment time is 2-3 min.

[0022] The inner layer material of the present application adopts plasma treatment low-density polyethylene, the high-energy particles in the plasma bombard the surface of the low-density polyethylene to break the C-H and C-C bonds, a large number of free radicals are generated on the molecular chain of the low-density polyethylene, and then the oxygen-containing functional groups are introduced in the atmosphere of oxygen, the functional groups contain electronegative atoms or are directly negatively charged, so that the low-density polyethylene as a whole presents negative electricity.

[0023] As a preferred technical solution of the present application, the conditions of the traction treatment are as follows: the horizontal and vertical traction ratio is 3*3, the traction temperature is 118-120 DEG C, the traction speed is 90-100 mm / s, and the cooling time is 10-15 s.

[0024] The outer layer linear low-density polyethylene (LLDPE) of the film material of the present application is short-chain polyethylene, and the inner layer low-density polyethylene (LDPE) is long-chain polyethylene, in the process of traction treatment, the short-chain polyethylene has a fast orientation crystallization speed, the long-chain polyethylene has a slow orientation crystallization speed, so that the interface layer caused by the response difference and the mutual influence appears, the short-chain polyethylene molecular chain segment forms more crystallization precursors in the interface layer, induces the orientation crystallization of the long-chain polyethylene, and the comprehensive performance of the polyethylene blown film is improved through the interface layer with high crystallinity.

[0025] As a preferred technical solution of the present application, the thickness of the first outer layer and the second outer layer is 20-40 mu m, and the thickness of the inner layer is 30-50 mu m.

[0026] Compared with the prior art, the present application has the following beneficial effects: (1) The polyethylene blown film for fruit and vegetable packaging of the present application adopts the montmorillonite linear low-density polyethylene composite as the outer layer material, adopts the plasma treatment low-density polyethylene as the inner layer material, is co-extrusion blow molded, and is combined with traction treatment, so that good heat resistance and oxygen barrier performance are obtained through the structural design and material modification.

[0027] (2) In the process of melt blending, the montmorillonite / linear low-density polyethylene composite of the present application has good compatibility between the quaternary ammonium salt modified montmorillonite and the linear low-density polyethylene, the quaternary ammonium salt modified montmorillonite can be uniformly dispersed in the resin matrix, the polyethylene molecular chain enters the modified montmorillonite sheet layer and gradually peels off the modified montmorillonite sheet layer, the peeled montmorillonite nanosheet layer has good gas barrier property to oxygen and other gas molecules, so that the oxygen transmission rate is effectively reduced; at the same time, the montmorillonite nanosheet layer can form a dense barrier in the polyethylene, effectively hinder the rapid heat transfer, delay the temperature rise of the matrix, capture and hinder the small molecular volatile products generated in the polymer thermal decomposition process, and significantly improve the thermal decomposition temperature of the material.

[0028] (3) The inner layer material of the application is plasma treated low density polyethylene, and the surface of the low density polyethylene is negatively charged. The outer layer material is montmorillonite / linear low density polyethylene composite modified by quaternary ammonium salt, and the outer layer material is cationized. In the material compounding process, the inner layer material and the outer layer material can be electrostatically combined at the interface, so as to form an electrostatic composite network at the interface layer, effectively improve the thermal decomposition temperature, and reduce the oxygen transmission rate.

[0029] (4) The interface layer of the application has a double network structure. The first network structure is formed by physical entanglement between the molecular chains of the outer layer short branched polyethylene and the inner layer long branched polyethylene in the drawing process. The second network structure is an electrostatic composite network formed by electrostatic combination between the inner layer material and the outer layer material at the interface layer. The network structure can change the heat transfer path in the polyethylene film, effectively block the heat conduction, improve the thermal stability and thermal decomposition temperature of the material, and at the same time, the formation of the interface network structure reduces the free volume in the polyethylene film, increases the difficulty of oxygen molecule penetration, and thus reduces the oxygen transmission rate. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the application, the following will briefly introduce the drawings needed to be used in the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0031] Figure 1 It is a schematic diagram of linear low density polyethylene (LLDPE) molecules; Figure 2 It is a schematic diagram of low density polyethylene (LDPE) molecules; Figure 3 It is an XRD diagram of quaternary ammonium salt modified montmorillonite in Example 1; Figure 4 It is a schematic diagram of polyethylene blown film structure in Example 1. DETAILED DESCRIPTION

[0032] In order to facilitate the understanding of the application, the application lists the following embodiments. Those skilled in the art should understand that the embodiments are only to help understand the application, and should not be regarded as a specific limitation on the application.

[0033] Some components in the examples and comparative examples are as follows: Linear low density polyethylene (LLDPE), brand 5110G, short branched polyethylene, density 0.926 g / cm 3 , melt index MI 0.85 g / 10 min (2.16 kg, 190°C), weight average relative molecular weight 140,000, purchased from DuPont Dow Chemical Co., Ltd.; Low density polyethylene (LDPE), brand 2420D, long chain branched polyethylene, density 0.922 g / cm 3 Low density polyethylene (LDPE), brand 2420D, long chain branched polyethylene, density 0.922 g / cm Na-montmorillonite, product number P766867, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Cetyltrimethylammonium bromide, CAS number 57-09-0, purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Cetyltrimethylammonium bromide, CAS number 57-09-0, purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Polyethylene grafted maleic anhydride, product number M87824, purchased from Shanghai Mailei Biochemical Technology Co., Ltd. Example 1

[0034] The present embodiment provides a preparation method of a polyethylene blown film for fruit and vegetable packaging, comprising the following steps: The first outer layer material, the inner layer material and the second outer layer material are introduced into the hoppers of a three-layer co-extrusion blowing machine respectively to perform three-layer co-extrusion blowing, and then traction treatment (the horizontal and vertical traction ratio is 3x3, the traction temperature is 120℃, the traction speed is 100mm / s, and the cooling time is 10s) is performed to obtain the polyethylene blown film for fruit and vegetable packaging (the thickness of the first outer layer and the second outer layer is 40μm, and the thickness of the inner layer is 50μm); The first outer layer material and the second outer layer material are montmorillonite modified linear low density polyethylene; the preparation of the montmorillonite modified linear low density polyethylene is as follows: 6 parts of Na-montmorillonite is added into 100 parts of deionized water to obtain a montmorillonite dispersion liquid by stirring and dispersing; 5 parts of long chain quaternary ammonium salt cetyltrimethylammonium bromide is added into 40 parts of deionized water and mixed uniformly at 80℃, and then added into the montmorillonite dispersion liquid and stirred at high speed for 6h, followed by reduced pressure filtration, deionized water washing, vacuum drying and ball milling to obtain quaternary ammonium salt modified montmorillonite; 500 parts of linear low density polyethylene, 25 parts of compatibilizer polyethylene grafted maleic anhydride and 35 parts of the quaternary ammonium salt modified montmorillonite are subjected to melt extrusion, traction and water cooling to obtain montmorillonite / linear low density polyethylene composite.

[0035] The inner layer material is plasma treated low density polyethylene; the plasma treatment conditions are as follows: the low density polyethylene is placed in a plasma treatment system, oxygen is used as the working gas, the oxygen volume flow rate is 0.010L / min, the ionization power is 220W, and the treatment time is 2min. Example 2

[0036] This embodiment provides a method for preparing polyethylene blown film for fruit and vegetable packaging, including the following steps: The first outer layer material, the inner layer material, and the second outer layer material are respectively fed into the hopper of a three-layer co-extrusion blow molding machine for three-layer co-extrusion blow molding. Then, traction processing is performed (the lateral and longitudinal traction ratio is 3×3, the traction temperature is 118℃, the traction speed is 90mm / s, and the cooling time is 15s) to obtain a polyethylene blown film for fruit and vegetable packaging (the thickness of the first and second outer layers is 20μm, and the thickness of the inner layer is 30μm). The first and second outer layer materials are montmorillonite-modified linear low-density polyethylene. The preparation of the montmorillonite-modified linear low-density polyethylene is as follows: by weight, 4 parts of sodium-based montmorillonite are added to 90 parts of deionized water and stirred to disperse to obtain a montmorillonite dispersion; 3 parts of octadecyltrimethylammonium chloride are added to 30 parts of deionized water at 70°C and mixed evenly, then added to the montmorillonite dispersion and stirred at high speed for 4 hours, filtered under reduced pressure, washed with deionized water, vacuum dried, and ball-milled to obtain quaternary ammonium salt-modified montmorillonite; 400 parts of linear low-density polyethylene, 15 parts of compatibilizer polyethylene grafted with maleic anhydride, and 25 parts of the quaternary ammonium salt-modified montmorillonite are melt-extruded, drawn, and water-cooled granulated to obtain a montmorillonite / linear low-density polyethylene composite.

[0037] The inner layer material is plasma-treated low-density polyethylene; the plasma treatment conditions are as follows: the low-density polyethylene is placed in the plasma treatment system, oxygen is used as the working gas, the oxygen volume flow rate is 0.008 L / min, the ionization power is 200 W, and the treatment time is 3 min. Example 3

[0038] This embodiment provides a method for preparing polyethylene blown film for fruit and vegetable packaging, including the following steps: The first outer layer material, the inner layer material, and the second outer layer material are respectively fed into the hopper of a three-layer co-extrusion blow molding machine for three-layer co-extrusion blow molding. Then, traction processing is performed (the lateral and longitudinal traction ratio is 3×3, the traction temperature is 119℃, the traction speed is 95mm / s, and the cooling time is 12s) to obtain a polyethylene blown film for fruit and vegetable packaging (the thickness of the first and second outer layers is 30μm, and the thickness of the inner layer is 40μm). The first outer layer material and the second outer layer material are montmorillonite modified linear low density polyethylene; the preparation of the montmorillonite modified linear low density polyethylene: 5 parts of sodium-based montmorillonite is added to 95 parts of deionized water to stir and disperse to obtain a montmorillonite dispersion liquid; 4 parts of long-chain quaternary ammonium salt hexadecyl trimethyl ammonium bromide is added to 35 parts of deionized water at 75℃, then added to the montmorillonite dispersion liquid and stirred at high speed for 5h, reduced pressure suction filtration, deionized water washing, vacuum drying, ball milling and pulverizing to obtain quaternary ammonium salt modified montmorillonite; 450 parts of linear low density polyethylene, 20 parts of compatibilizer polyethylene grafted maleic anhydride and 30 parts of the quaternary ammonium salt modified montmorillonite are melt extruded, drawn, water cooled and granulated to obtain a montmorillonite / linear low density polyethylene composite.

[0039] The inner layer material is plasma treated low density polyethylene; the conditions of the plasma treatment are as follows: the low density polyethylene is placed in a plasma treatment system, oxygen is used as the working gas, the oxygen volume flow rate is 0.009L / min, the ionization power is 210W, and the treatment time is 2min.

[0040] Comparative Example 1 The difference between this comparative example and Example 1 is that linear low density polyethylene (brand 5110G) is used to replace the montmorillonite modified linear low density polyethylene as the first outer layer material and the second outer layer material.

[0041] Comparative Example 2 The difference between this comparative example and Example 1 is that low density polyethylene (brand 2420D) is used to replace the plasma treated low density polyethylene.

[0042] Comparative Example 3 The difference between this comparative example and Example 1 is that no drawing treatment is performed after co-extrusion blow molding.

[0043] The performance of the above examples and comparative examples is tested, and the testing method is as follows: Haze test: the test is performed according to the requirements of “ASTM D1003-2013 Standard Test Methods for Haze and Luminous Transmittance of Transparent Plastics”.

[0044] Thermal decomposition temperature test: a thermogravimetric analyzer (model TGA55, TA Instruments, USA) is used to determine the thermal decomposition temperature (5% Td, temperature at 5% mass loss) of the film, and the temperature is raised at a rate of 20℃ / min from room temperature to 600℃.

[0045] Oxygen transmission rate test: the test is performed according to the requirements of “ASTM D3985-17 Standard Test Methods for Measuring the Oxygen Gas Transmission Rate of Plastic Films and Sheeting Using the Coulometric Sensor”.

[0046] The performance test data is shown in Table 1.

[0047] Table 1 Performance test results Haze Thermal decomposition temperature (5% Td) °C Oxygen permeability cm3 / (m2-24h-atm) Example 1 3.5 422 394 Example 2 2.9 419 411 Example 3 3.2 420 403 Comparative Example 1 3.6 383 685 Comparative Example 2 3.1 395 622 Comparative Example 3 2.8 404 574 From the above, the polyethylene blown film is obtained by using the montmorillonite linear low density polyethylene composite as the outer material, using the plasma treated low density polyethylene as the inner material, through co-extrusion blow molding, and cooperating with traction treatment, effectively improving the heat resistance of the film through structural design and material modification, and simultaneously obtaining good oxygen barrier performance (Examples 1-3).

[0048] Compared with Example 1, linear low density polyethylene (brand 5110G) is used to replace the montmorillonite modified linear low density polyethylene as the first outer material and the second outer material, lacking the effect of quaternary ammonium salt modified montmorillonite, so that the film thermal decomposition temperature is reduced and the oxygen barrier performance is poor (Comparative Example 1); compared with Example 1, low density polyethylene (brand 2420D) is used to replace the plasma treated low density polyethylene, lacking the plasma treatment to obtain the electronegative influence interface layer, so that the film thermal decomposition temperature is reduced and the oxygen barrier performance is poor (Comparative Example 2); compared with Example 1, no traction treatment is performed, lacking the interface layer to affect the formation of the double network structure, so that the film thermal decomposition temperature is reduced and the oxygen barrier performance is poor (Comparative Example 3).

Claims

1. A method for preparing a polyethylene blown film for fruit and vegetable packaging, characterized in that, Includes the following steps: The first outer layer material, the inner layer material, and the second outer layer material are respectively fed into the hopper of a three-layer co-extrusion blow molding machine to form a thin sheet through three-layer co-extrusion blow molding. The thin sheet is then subjected to traction processing to obtain a polyethylene blown film for fruit and vegetable packaging. The first outer layer material and the second outer layer material are montmorillonite-modified linear low-density polyethylene; the preparation method of the montmorillonite-modified linear low-density polyethylene includes: firstly, intercalating sodium-based montmorillonite with a long-chain quaternary ammonium salt to obtain quaternary ammonium salt-modified montmorillonite; then, melt-blending linear low-density polyethylene, a compatibilizer, and the quaternary ammonium salt-modified montmorillonite to obtain a montmorillonite / linear low-density polyethylene composite. The inner layer material is low-density polyethylene.

2. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 1, characterized in that, The intercalation modification steps are as follows: by weight, 4-6 parts of sodium-based montmorillonite are added to 90-100 parts of deionized water and stirred to disperse to obtain a montmorillonite dispersion; under conditions of 70-80℃, 3-5 parts of long-chain quaternary ammonium salt are added to 30-40 parts of deionized water and mixed evenly, and then added to the montmorillonite dispersion and stirred at high speed for 4-6 hours, followed by post-treatment to obtain quaternary ammonium salt modified montmorillonite.

3. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 1, characterized in that, The long-chain quaternary ammonium salt is selected from hexadecyltrimethylammonium bromide or octadecyltrimethylammonium chloride.

4. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 1, characterized in that, The compatibilizer is polyethylene grafted with maleic anhydride.

5. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 1, characterized in that, The melt blending step is as follows: 400-500 parts of linear low-density polyethylene, 15-25 parts of compatibilizer and 25-35 parts of the quaternary ammonium salt modified montmorillonite are melt-extruded, drawn and water-cooled granulated.

6. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 1, characterized in that, The low-density polyethylene is plasma-treated low-density polyethylene.

7. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 6, characterized in that, The plasma treatment conditions are as follows: low-density polyethylene is placed in the plasma treatment system, oxygen is used as the working gas, the oxygen volume flow rate is 0.008~0.010L / min, the ionization power is 200~220W, and the treatment time is 2~3min.

8. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 1, characterized in that, The conditions for the traction treatment are: the lateral and longitudinal traction ratio is 3×3, the traction temperature is 118~120℃, the traction speed is 90~100mm / s, and the cooling time is 10~15s.

9. The method for preparing a polyethylene blown film for fruit and vegetable packaging according to claim 1, characterized in that, The thickness of the first and second outer layers is 20~40μm, and the thickness of the inner layer is 30~50μm.

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