Lightweight and environment-friendly PE film and preparation method thereof

Abstract

The application belongs to the technical field of polyethylene, and discloses a light-weight environment-friendly PE film and a preparation method thereof.A variety of polyethylene materials are compounded to form a base resin system and are subjected to low-temperature solvent-free solid-phase grafting modification, and a nano-composite filler subjected to in-situ hydrophobization is matched to form a reinforcing system, and a supercritical stretching process is combined to form a uniform and dense oriented crystalline structure in the film, so that the key mechanical properties such as the tensile strength and the puncture resistance are ensured while the light weight of the film is realized; meanwhile, the in-situ hydrophobization modification of the filler solves the problems of poor compatibility with the polyethylene matrix and easy agglomeration, and avoids the deterioration of the processing performance and the appearance quality of the film caused by the addition of the filler; the whole production process adopts a solvent-free dry process, and the core links such as the base grafting, the filler modification and the premixing and pre-plasticizing are all completed under low-temperature conditions, and the differential extrusion also adopts a low-temperature plasticizing technology, so that the production energy consumption is reduced and the thermal degradation of the polyethylene molecular chain is reduced.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene technology, specifically a lightweight and environmentally friendly PE film and its preparation method. Background Technology

[0002] PE film is widely used in food packaging, daily necessities packaging, agricultural covering, medical protection, and many other fields. Lightweighting and environmental friendliness of PE film have become core trends in the industry. Lightweighting means reducing resin raw material consumption by decreasing film thickness, thereby reducing plastic waste generation at the source. Environmental friendliness requires the film to achieve low-energy production, low VOC emissions, high recyclability, and controllable degradation throughout its entire life cycle, while consistently meeting performance requirements. However, existing technologies still have the following technical challenges: Simply reducing the thickness will cause a sharp drop in mechanical properties such as tensile strength and puncture resistance of the film, making it unable to meet the actual use requirements. On the other hand, adding inorganic fillers in an attempt to make up for the performance shortcomings will further degrade the film's processing performance and appearance quality due to the poor compatibility between the fillers and the PE matrix and their tendency to agglomerate, while also reducing the film's recyclability.

[0003] The biodegradable components have a large polarity difference with the PE matrix, so solvent-based compatibilizers need to be added to improve compatibility. This not only increases production costs but also leads to excessive VOC emissions during production, which goes against the original intention of environmental modification. Moreover, the existing environmental modification process has high energy consumption and cannot achieve a balance between low energy consumption and environmental protection.

[0004] Existing processes are not suitable for the stable production of ultra-lightweight films. They have poor plasticization uniformity and are prone to problems such as excessive film thickness deviation and surface defects. They are also prone to film breakage and tearing during stretching, resulting in poor production stability. At the same time, the scraps generated during production need to be recycled by offline granulation. The high-temperature treatment during granulation will cause severe degradation of PE molecular chains, resulting in a significant decrease in the performance of recycled materials, which cannot be reused for the production of high-quality films.

[0005] Existing technologies are mostly single-process improvements, lacking multi-technology collaborative design. For example, improving the stretching process alone cannot solve the problem of poor compatibility of fillers, and optimizing the recycling process alone cannot take into account the mechanical properties after lightweighting, making it difficult to achieve the unity of lightweighting, environmental protection and large-scale production. Summary of the Invention

[0006] The purpose of this invention is to provide a lightweight and environmentally friendly PE film and its preparation method, so as to solve one or more problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a lightweight and environmentally friendly PE film, comprising the following specific steps: Preferably, the matrix grafting stage uses a blend of high-density polyethylene, linear low-density polyethylene and metallocene polyethylene as the matrix resin, wherein the mass ratio of high-density polyethylene, linear low-density polyethylene and metallocene polyethylene is 1:(3-5):(0.5-1.5), and the melt index of the matrix resin is 1-4 g / 10 min. The matrix resin, composite graft monomer, composite initiator, and polymerization inhibitor are added to a high-speed solid-phase reactor at a mass ratio of 100:1.5-3.0:0.15-0.4:0.05-0.1. The mixture is heated to 80-100℃ under a nitrogen protective atmosphere and stirred in stages to obtain the grafted modified polyethylene matrix. The composite grafting monomers are maleic anhydride, itaconic acid and isooctyl acrylate in a mass ratio of 2:1:0.5. The composite initiator is dicumyl peroxide and benzoyl peroxide in a mass ratio of 2:1. The polymerization inhibitor is hydroquinone, which realizes low-temperature solvent-free solid-phase grafting modification of the matrix resin.

[0008] Preferably, the filler modification stage is based on the grafted modified polyethylene matrix obtained in the matrix grafting stage, and uses nano-cellulose whiskers, nano-active calcium carbonate and nano-PE wax grafted talc as composite reinforcing fillers, with a mass ratio of 3:(1-5):(0.3-0.8), and the average particle size of the filler is 50-100nm. The composite reinforced filler is added to a high-speed mixer and heated to 70-90℃. The composite modifier is sprayed in under high-speed stirring. The amount of modifier added is 3.0-4.5% of the total mass of the composite reinforced filler. The mixture is kept at the temperature and stirred for 12-22 minutes to obtain the in-situ hydrophobic modified nanocomposite filler. The composite modifier is a compound made of polyethylene wax grafted with maleic anhydride, stearic acid and γ-aminopropyltriethoxysilane in a mass ratio of 3:1:0.5. Through in-situ hydrophobic modification, a double hydrophobic graft layer is formed on the surface of the filler. At the same time, renewable nanocellulose is used as the core reinforcing phase. The in-situ hydrophobic modified nanocomposite filler can enhance the polyethylene matrix at a low addition amount of 2-5 parts by mass.

[0009] Preferably, the premixing and preplasticizing stage is based on the in-situ hydrophobic modified nanocomposite filler obtained in the filler modification stage. According to the mass parts, 85-92 parts of grafted modified polyethylene matrix, 2-5 parts of in-situ hydrophobic modified nanocomposite filler, 3-8 parts of food-grade polycaprolactone, 1-2 parts of composite additives, and 0.3-0.8 parts of PE-compatible anti-dripping agent are added to a low-temperature solid-phase mixer. Under a nitrogen protective atmosphere at 90-110℃, a solid-phase premixing and preplasticizing process was carried out for 12-22 minutes using a high-shear and intermittent exhaust process to obtain a uniformly dispersed preplasticized blend material. The endpoint of solid-phase premixing and preplasticizing is determined by the appearance and dispersibility of the material. Qualified preplasticized blended material is uniform white granules with no obvious agglomeration or color difference. When a small amount of material is placed under a microscope for observation, the in-situ hydrophobic modified nanocomposite filler does not show obvious agglomeration in the matrix, and the dispersed particle size is ≤200nm, which means that the endpoint of premixing and preplasticizing has been reached.

[0010] The composite additive is a composition of a heavy metal-free photosensitive degrader, food-grade antioxidant 1010, antioxidant 168 and ultraviolet absorber UV-531 in a mass ratio of 2:1:1:0.5. The PE-compatible anti-dripping agent is polyethylene grafted polytetrafluoroethylene.

[0011] Preferably, in the differential extrusion stage, the uniformly dispersed pre-plasticized blend material obtained in the premixing and pre-plasticizing stage is added to a twin-screw melt differential extruder for low-temperature plasticizing extrusion. The temperatures of each section of the extruder are set as follows: Zone 1 110-120℃, Zone 2 120-130℃, Zone 3 125-135℃, Zone 4 130-140℃, and the die head temperature 135-140℃. During the extrusion process, the material undergoes multi-channel differential shearing plasticization through the melt differential module. The melt differential module has 40-50 flow channels with a flow channel diameter of 1.2-2.2mm, and the inner wall of the flow channel is provided with spiral grooves. Finally, a melt sheet with a thickness of 0.1-0.3mm is obtained by extrusion through a flat die head.

[0012] Preferably, in the supercritical stretching stage, the melt sheet with a thickness of 0.1-0.3 mm obtained by the differential extrusion stage is rapidly cooled to 40-60°C to obtain a casting with controllable crystallinity. The casting is then introduced into a bidirectional synchronous stretching unit, using a segmented supercritical CO2 introduction process: supercritical CO2 fluid is introduced into the preheating section, and supercritical CO2 fluid diluted with inert gas is introduced into the stretching section. After preheating in a segmented supercritical CO2 atmosphere for 4-9 seconds, the cast film is subjected to bidirectional synchronous stretching at a temperature of 85-95℃, with a longitudinal stretching ratio of 4.5-6.5 times and a transverse stretching ratio of 4.5-6.5 times. After stretching, it is heat-set at 110-120℃ for 5-10 seconds and then rapidly cooled to room temperature to obtain a lightweight PE film.

[0013] Preferably, in the surface modification stage, the lightweight PE film obtained in the supercritical stretching stage is introduced into a low-temperature plasma treatment unit. Under an atmosphere of atmospheric pressure nitrogen and argon mixed at a mass ratio of 8:2, dielectric barrier discharge plasma is used to treat both sides of the film. The discharge power is 800-1200W, the treatment speed is 80-120m / min, and the treatment time is 0.8-2.2s, forming a gradient crosslinked layer with a thickness of 80-220nm on the film surface. The degree of crosslinking of the gradient crosslinking layer gradually decreases from the surface of the film to the interior, with a surface crosslinking degree ≥65% and a crosslinking degree ≤8% at the junction with the grafted modified polyethylene matrix. After the treatment, the film is subjected to electrostatic elimination and surface cleaning to obtain a surface-modified film.

[0014] Preferably, the closed-loop recycling stage is based on the surface-modified film obtained in the surface modification stage and the film scraps and waste film generated during the production process. The collected unqualified finished film and production waste are subjected to low-temperature crushing online, with the crushing temperature controlled at 35-40℃, to obtain recycled powder with a particle size of 20-40 mesh. The recycled powder is then subjected to low-temperature plasma pretreatment under normal pressure nitrogen atmosphere to remove surface impurities and oxide layer. Subsequently, the pretreated recycled powder is added to the premixing and preplasticizing stage at a mass ratio of 15-35% of the total material in the premixing and preplasticizing stage, and premixed and preplasticized simultaneously with the new material. Simultaneously, by linking an online thickness gauge and an online viscosity monitor, the thickness and melt viscosity of the finished film are detected in real time. The detection data is fed back to the extruder and stretching units, and the extrusion amount, stretching ratio and extrusion temperature are controlled in a closed loop. Finally, the finished film is slit and wound to obtain the PE film product.

[0015] The present invention also provides a lightweight and environmentally friendly PE film, which is prepared by the above-mentioned preparation method and is composed of the following components in parts by weight: 85-92 parts of grafted modified polyethylene matrix, 2-5 parts of in-situ hydrophobic modified nanocomposite filler, 3-8 parts of food-grade polycaprolactone, 1-2 parts of composite additives, and 0.3-0.8 parts of PE-compatible anti-dripping agent. The grafted modified polyethylene matrix uses a blend of high-density polyethylene, linear low-density polyethylene, and metallocene polyethylene as the matrix resin, with a mass ratio of high-density polyethylene, linear low-density polyethylene, and metallocene polyethylene of 1:(3-5):(0.5-1.5). The melt index of the matrix resin is 1-4 g / 10 min. The grafted modified polyethylene matrix is ​​prepared by low-temperature solvent-free solid-phase grafting of matrix resin, composite grafting monomer, composite initiator, and polymerization inhibitor in a mass ratio of 100:(1.5-3.0):(0.15-0.4):(0.05-0.1). The composite grafting monomer is a mixture of maleic anhydride, itaconic acid, and isooctyl acrylate in a mass ratio of 2:1:0.5; the composite initiator is a mixture of dicumyl peroxide and benzoyl peroxide in a mass ratio of 2:1; and the polymerization inhibitor is hydroquinone. The in-situ hydrophobic modified nanocomposite filler uses nano-cellulose whiskers, nano-activated calcium carbonate, and nano-PE wax grafted talc powder as composite reinforcing fillers, with a mass ratio of 3:(1-5):(0.3-0.8). The average particle size of the composite reinforcing filler is 50-100 nm. The in-situ hydrophobic modified nanocomposite filler is prepared by in-situ hydrophobic modification of the composite reinforcing filler and the composite modifier, with the amount of the composite modifier added being 3.0-4.5% of the total mass of the composite reinforcing filler. The composite modifier is a mixture of polyethylene wax grafted with maleic anhydride, stearic acid and γ-aminopropyltriethoxysilane in a mass ratio of 3:1:0.5. The composite additive is a mixture of a heavy metal-free photosensitive degrader, food-grade antioxidant 1010, antioxidant 168 and ultraviolet absorber UV-531 in a mass ratio of 2:1:1:0.5. The PE-compatible anti-dripping agent is polyethylene grafted with polytetrafluoroethylene; The lightweight PE film has a thickness of 2-7 μm and a gradient crosslinking layer with a thickness of 80-220 nm on its surface. The surface crosslinking degree of the gradient crosslinking layer is ≥65%, and the crosslinking degree at the junction with the grafted modified polyethylene matrix is ​​≤8%.

[0016] The beneficial effects of this invention are as follows: 1. This invention forms a matrix resin system by compounding multiple polyethylene materials and performing low-temperature solvent-free solid-phase grafting modification. It is then combined with in-situ hydrophobicated nanocomposite fillers to form a reinforcing system. Combined with supercritical stretching technology, a uniform and dense oriented crystalline structure is formed inside the film. This achieves lightweight film while ensuring key mechanical properties such as tensile strength and puncture resistance. At the same time, the in-situ hydrophobic modification of the fillers solves the problems of poor compatibility and easy agglomeration with the polyethylene matrix, avoiding the degradation of film processing performance and appearance quality by the addition of fillers.

[0017] 2. The entire production process of this invention adopts a solvent-free dry process. Core steps such as matrix grafting, filler modification, and premixing and preplasticizing are all completed under low temperature conditions. The differential extrusion also adopts low temperature plasticizing technology, which reduces production energy consumption and reduces the thermal degradation of polyethylene molecular chains. The composite additives are selected without heavy metal components, the supercritical fluid can be recycled, the plasma surface modification has no pollutant emissions, and the compatibility between degradable components and matrix is ​​improved through grafting modification, without the need to add solvent-based compatibilizers.

[0018] 3. This invention improves the uniformity of melt plasticization through the special structure of the melt differential module, and the supercritical stretching process effectively avoids film breakage and tearing problems during film stretching. Combined with online monitoring and closed-loop control of process parameters, it ensures the stability and yield of ultra-lightweight film production. At the same time, the production waste is recycled in situ after online low-temperature crushing and plasma pretreatment. Combined with the system compatibility of the grafted modified matrix, the interfacial compatibility between recycled materials and new materials is improved, avoiding performance degradation of recycled materials and achieving zero discharge of production waste. Attached Figure Description

[0019] Figure 1 This is a complete process flow diagram for the preparation of the lightweight and environmentally friendly PE film of this invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Basic process examples like Figure 1 As shown, a method for preparing a lightweight and environmentally friendly PE film is provided, including the following specific steps: The selected polyethylene resin raw material is vacuum dried at 100-110℃ for 4-6 hours with a vacuum degree of -0.08~-0.09MPa to remove moisture and low molecular weight volatiles from the raw material; the nanocomposite filler raw material is dried with hot air at 80-90℃ for 2-3 hours and passed through a 200-mesh standard sieve to remove large agglomerated particles and surface adsorbed moisture. The pretreated raw material is sealed and stored for later use.

[0022] In the matrix grafting stage, a blend of high-density polyethylene, linear low-density polyethylene and metallocene polyethylene was used as the matrix resin, wherein the mass ratio of high-density polyethylene, linear low-density polyethylene and metallocene polyethylene was 1:(3-5):(0.5-1.5), the melt index of the matrix resin was 1-4 g / 10 min, and the test conditions were 190℃ and 2.16 kg. The matrix resin, composite graft monomer, composite initiator, and polymerization inhibitor are added to a high-speed solid-phase reactor at a mass ratio of 100:(1.5-3.0):(0.15-0.4):(0.05-0.1). The reactor is heated to 80-100℃ under a nitrogen protective atmosphere. The purity of nitrogen and argon used in each process stage of this invention is ≥99.99% and ≥99.99%, respectively. When inert gas is introduced, a slight positive pressure of 0.01-0.03MPa is maintained inside the process equipment to prevent external air from entering and causing oxidation of the material. A segmented stirring method is adopted, with stirring at 500-600 r / min for 5-10 min in the early stage and 300-400 r / min for 10-20 min in the later stage to obtain the grafted modified polyethylene matrix. The high-speed solid-phase reactor used adopts a ribbon agitator with a gap of 1-2 mm between the blade and the inner wall of the reactor. The effective volume loading coefficient of the reactor is controlled at 60%-70%, ensuring that the material is fully mixed without dead corners during the stirring process, thereby improving the uniformity of the grafting reaction.

[0023] The composite grafting monomers are maleic anhydride, itaconic acid and isooctyl acrylate in a mass ratio of 2:1:0.5. The composite initiator is dicumyl peroxide and benzoyl peroxide in a mass ratio of 2:1. The polymerization inhibitor is hydroquinone. No solvent is added throughout the process, which realizes low-temperature solvent-free solid-phase grafting modification of the matrix resin. The controllable grafting rate is 1.2%-2.0%, which improves the polarity of the matrix without destroying the PE molecular chain structure.

[0024] The grafting rate was determined by acid-base titration. The specific procedure was as follows: a certain mass of grafted modified polyethylene matrix sample was weighed, placed in xylene, and heated under reflux until completely dissolved. A sodium hydroxide ethanol solution of known concentration was added and allowed to react fully. The excess sodium hydroxide was then titrated with a standard hydrochloric acid solution, and the grafting rate was calculated based on the titration results. The stirring time was 500-600 r / min for 5-8 min in the initial stage and 300-400 r / min for 15-20 min in the later stage. The stirring time could be adjusted slightly according to the melt index of the matrix resin. If the melt index was too high, the stirring time in the initial stage was shortened and the stirring time in the later stage was extended.

[0025] Both the composite grafting monomer and the composite initiator must be stored in a sealed, dry, and light-protected environment, with the storage temperature controlled between 0-10℃. The shelf life of the composite grafting monomer is 6 months, and the shelf life of the composite initiator is 3 months. Before use, the purity of the raw materials must be tested, and only those with a purity of ≥98% can be put into use.

[0026] The filler modification stage is based on the grafted modified polyethylene matrix obtained in the matrix grafting stage, and uses nano-cellulose whiskers, nano-active calcium carbonate and nano-PE wax grafted talc as composite reinforcing fillers, with a mass ratio of 3:(1-5):(0.3-0.8), and the average particle size of the filler is 50-100nm. The nano-cellulose whiskers have an aspect ratio of 20-50:1 and a crystallinity of ≥85%; the nano-activated calcium carbonate has an activation degree of ≥98% and a whiteness of ≥95%; the nano-PE wax-grafted talc has a grafting rate of ≥3.0% and a melting point of 90-100℃.

[0027] The composite reinforced filler is added to a high-speed mixer, heated to 70-90℃, and the composite modifier is sprayed in under high-speed stirring at 800-1000r / min. The amount of modifier added is 3.0-4.5% of the total mass of the composite reinforced filler. The mixture is kept at the temperature and stirred for 12-22min to obtain the in-situ hydrophobic modified nanocomposite filler. The water contact angle of the in-situ hydrophobically modified nanocomposite filler is ≥110°. The contact angle is measured at room temperature using the seat drop method. The average value is taken from 5 different test points after the filler is pressed into tablets. The dispersed particle size of the modified filler in the polyethylene matrix is ​​≤200nm. The particle size distribution is observed and statistically analyzed using transmission electron microscopy (TEM). The average dispersed particle size is calculated from 10 randomly selected fields of view.

[0028] The effective volumetric loading coefficient of the high-speed mixer is 50%-60%. The composite modifier is added by atomized spraying with a spray flow rate of 5-10 mL / min. The spray head is set to a 360° rotating type to ensure that the modifier and the composite reinforcing filler are in full and uniform contact.

[0029] The composite modifier is a compound of polyethylene wax grafted with maleic anhydride, stearic acid and γ-aminopropyltriethoxysilane (KH-550) in a mass ratio of 3:1:0.5. No solvent is added throughout the process. Through in-situ hydrophobic modification, a double hydrophobic graft layer is formed on the surface of the filler. The inner layer is the hydrophobic group of silane coupling agent and the outer layer is the hydrophobic segment of polyethylene wax grafted with maleic anhydride. This improves the interfacial compatibility between the filler and the PE matrix and avoids filler agglomeration. At the same time, renewable nanocellulose is used as the core reinforcing phase. The in-situ hydrophobic modified nanocomposite filler achieves efficient reinforcement of the polyethylene matrix at a low addition amount of 2-5 parts.

[0030] In the premixing and preplasticizing stage, based on the in-situ hydrophobic modified nanocomposite filler obtained in the filler modification stage, 85-92 parts of grafted modified polyethylene matrix, 2-5 parts of in-situ hydrophobic modified nanocomposite filler, 3-8 parts of food-grade polycaprolactone, 1-2 parts of composite additives, and 0.3-0.8 parts of PE-compatible anti-dripping agent are added to a low-temperature solid-phase mixer according to the mass fractions. Under a nitrogen protective atmosphere at 90-110℃, a high shear rate of 1200-1500 r / min and an intermittent venting process are used to perform solid-phase premixing and preplasticizing treatment for 12-22 min, with venting once every 5 min for 10-15 s each time, to obtain a uniformly dispersed preplasticized blend material. The vacuum degree of intermittent venting is controlled at -0.08~-0.095MPa. During the venting process, the mixer is kept at a low speed of 200-300r / min to avoid material splashing or stratification during the venting process.

[0031] The high-shear blades are double-layered cross-shaped shear blades. The outer layer of the blades has a linear velocity of 8-12 m / s. The blade rotation speed is 300-350 r / min for a shear rate of 1200 r / min and 400-450 r / min for a shear rate of 1500 r / min. During the shearing process, the gap between the blades and the inner wall of the equipment is maintained at 1-2 mm.

[0032] The composite additive is a composition made up of a heavy metal-free photosensitive degrader, food-grade antioxidant 1010, antioxidant 168 and ultraviolet absorber UV-531 in a mass ratio of 2:1:1:0.5. The PE-compatible anti-dripping agent is polyethylene-grafted polytetrafluoroethylene (PE-g-PTFE). The entire process is solvent-free and free of toxic and harmful components. The components are uniformly dispersed through low-temperature solid-phase pre-plasticization at a temperature lower than the melting temperature of PE, thereby reducing VOC emissions and production energy consumption.

[0033] The grafting rate of the polyethylene-grafted polytetrafluoroethylene is 1.5%-3.0%, the average particle size is 1-5μm, and the melt index is 2-8g / 10min (test conditions: 190℃, 2.16kg). It has good compatibility with the polyethylene matrix and no agglomeration.

[0034] In the differential extrusion stage, the uniformly dispersed pre-plasticized blend material obtained in the premixing and pre-plasticizing stage is added to a twin-screw melt differential extruder for low-temperature plasticizing extrusion. The temperatures of each section of the extruder are set as follows: Zone 1 110-120℃, Zone 2 120-130℃, Zone 3 125-135℃, Zone 4 130-140℃, and the die head temperature 135-140℃. During the extrusion process, the material undergoes multi-channel differential shearing plasticization through the melt differential module. The melt differential module has 40-50 channels with a channel diameter of 1.2-2.2mm, and the inner wall of the channels is provided with spiral grooves to achieve uniform plasticization and refinement of the melt. Finally, a melt sheet with a thickness of 0.1-0.3mm is obtained by extrusion through a flat die head. The spiral groove pitch on the inner wall of the flow channel is 3-5mm, the groove depth is 0.2-0.4mm, and the spiral helix angle is 30-45°; the screw length-to-diameter ratio of the twin-screw melt micro-extrusion unit is 36:1~40:1, and the screw speed is 80-150r / min. The speed can be adjusted according to the target thickness of the melt sheet. When the thickness is too thick, the screw speed should be increased appropriately.

[0035] By using melt differential low-temperature plasticizing technology, plasticizing uniformity is improved, production energy consumption is reduced, and thermal degradation of PE molecular chains is reduced, while maintaining the high molecular weight and high melt strength of the matrix.

[0036] In the supercritical stretching stage, the melt sheet with a thickness of 0.1-0.3 mm obtained from the differential extrusion stage is rapidly cooled to 40-60℃ to obtain a casting with controllable crystallinity. The casting is then introduced into a bidirectional synchronous stretching unit, using a segmented supercritical CO2 introduction process: supercritical CO2 fluid is introduced into the preheating section at a pressure of 8-12 MPa and a temperature of 45-55℃; supercritical CO2 fluid diluted with inert gas is introduced into the stretching section, and supercritical CO2 and nitrogen are mixed at a mass ratio of 7:3 at a pressure of 6-10 MPa and a temperature of 50-60℃. The molten sheet is cooled by rollers. The cooling rollers are mirror-plated chrome rollers. The roller temperature is controlled at 40-60℃. The contact time between the molten sheet and the cooling roller is 1-3s. The cooling rate is controlled at 80-120℃ / s. The cooling rate is precisely controlled by adjusting the roller speed to ensure the consistency of the crystallinity of the casting.

[0037] After preheating in a segmented supercritical CO2 atmosphere for 4-9 seconds, the cast film is subjected to bidirectional synchronous stretching. The stretching temperature is controlled at 85-95℃, the longitudinal stretching ratio is 4.5-6.5 times, and the transverse stretching ratio is 4.5-6.5 times. After stretching, it is heat-set at 110-120℃ for 5-10 seconds, and then rapidly cooled to room temperature to obtain a lightweight PE film. The crystallinity of the cast sheet obtained after cooling the molten sheet is controlled at 20%-30%, and the crystallinity is detected by X-ray diffraction (XRD). The supercritical CO2 flow rate in the preheating section is 5-10 L / min, and the supercritical CO2 flow rate in the stretching section is 3-8 L / min. The stretching speed of the bidirectional synchronous stretching is 50-100 mm / s, and the longitudinal and transverse stretching speeds are kept synchronized.

[0038] During the heat setting stage, the ambient pressure is controlled at 0.1-0.3 MPa, and hot air pressurization is used with a hot air velocity of 2-5 m / s to ensure that all parts of the film are heated and pressurized evenly, thus avoiding film shrinkage or deformation.

[0039] Supercritical CO2 can be completely recycled and reused, with no pollutant emissions throughout the process. At the same time, in conjunction with the synergistic heterogeneous nucleation effect of nanocomposite fillers, a uniform and dense oriented crystalline structure is formed inside the film, which avoids the stretching and breaking of the ultrathin film and ensures the mechanical stability of the lightweight film.

[0040] The supercritical CO2 recycling process is as follows: CO2 discharged from the bidirectional synchronous stretching unit is cooled to room temperature, filtered through a precision filter to remove impurities, and then pressurized and heated to a supercritical state by a compressor unit. After its purity is tested to be ≥99.5%, it is recycled back into the preheating section and stretching section for reuse. The CO2 recycling rate is ≥95%.

[0041] In the surface modification stage, the lightweight PE film obtained in the supercritical stretching stage is introduced into a low-temperature plasma treatment unit. Under a mixed atmosphere of nitrogen and argon at atmospheric pressure (mass ratio of nitrogen to argon is 8:2), the film is treated on both sides using dielectric barrier discharge plasma. The discharge power is 800-1200W, the treatment speed is 80-120m / min, and the treatment time is 0.8-2.2s, forming a gradient crosslinked layer with a thickness of 80-220nm on the film surface. The plasma treatment uses a medium-frequency AC power supply with an output frequency of 10-20kHz and an output voltage of 0-5kV. The discharge power and output frequency can be finely adjusted according to the film thickness. When the film thickness is too thin, the discharge power and frequency should be appropriately reduced.

[0042] The degree of crosslinking of the gradient crosslinking layer gradually decreases from the surface of the film to the interior, with a surface crosslinking degree ≥65% and a crosslinking degree ≤8% at the junction with the grafted modified polyethylene matrix. After the treatment, the film is subjected to electrostatic elimination and surface cleaning to obtain a surface-modified film. The discharge gap for low-temperature plasma treatment is 1-3 mm, and the electrode is a stainless steel plate electrode with a uniformity deviation of ≤0.1 mm between electrode spacings. The degree of crosslinking is detected by the gel content method. The specific operation is as follows: a certain mass of surface-modified film sample is weighed, refluxed in boiling xylene for 24 h, filtered, and the insoluble matter is dried to constant weight. The degree of crosslinking is calculated based on the ratio of the mass of insoluble matter to the initial mass of the sample.

[0043] Static electricity elimination is achieved using an ion bar-type static electricity elimination device. The vertical distance between the ion bar and the film surface is 10-15cm, and the air velocity at the outlet of the bar is 3-5m / s. The speed at which the film passes through the ion bar is consistent with the plasma treatment speed. After static electricity elimination, the static voltage on the film surface is ≤±50V, which meets the requirements for subsequent processing.

[0044] Surface cleaning employs a combination of dust-free hot air blowing and dust removal using a sticky roller. The dust-free hot air temperature is at room temperature, and the blowing pressure is 0.05-0.1 MPa. The sticky roller is a high-adhesion polyurethane roller, and its rotation speed is synchronized with the film's running speed. After cleaning, the film surface is free of visible impurities, and the content of particulate matter with a particle size ≥0.01 mm is ≤5 particles / m³. 2 .

[0045] The process employs a dry modification method using atmospheric pressure and low-temperature plasma, which is solvent-free, VOC-free, and energy-efficient. Through a gradient crosslinking structure, it not only improves the surface properties of the film but also maintains the thermoplasticity and heat-sealing properties of the PE matrix. At the same time, it introduces polar groups on the film surface to improve printing adhesion and wettability, thereby reducing production costs.

[0046] The closed-loop recycling stage is based on the surface-modified film obtained in the surface modification stage and the film scraps and waste film generated during the production process. The collected unqualified finished film and production waste are subjected to low-temperature crushing online, with the crushing temperature controlled at 35-40℃, to obtain recycled powder with a particle size of 20-40 mesh. The recycled powder is then subjected to low-temperature plasma pretreatment with a discharge power of 500-700W and a treatment time of 1-2s under normal pressure nitrogen atmosphere to remove surface impurities and oxide layers. Subsequently, the pretreated recycled powder is added to the premixing and preplasticizing stage at a mass ratio of 15-35% of the total material in the premixing and preplasticizing stage, and premixed and preplasticized simultaneously with the new material to achieve closed-loop in-situ recycling of production waste. The cryogenic pulverization uses a liquid nitrogen-assisted high-speed turbine pulverizer. The rotor speed of the pulverizer is 3000-5000 r / min. During the pulverization process, liquid nitrogen is continuously introduced to maintain the temperature inside the equipment at 35-40℃. The pulverized powder is classified by a vibrating screen, and 20-40 mesh powder is selected for subsequent pretreatment. The residue is returned to the pulverization process.

[0047] Simultaneously, by linking an online thickness gauge and an online viscosity monitor, the thickness and melt viscosity of the finished film are detected in real time. The detection data is fed back to the extruder and stretching units, and the extrusion amount, stretching ratio and extrusion temperature are controlled in a closed loop to ensure that the thickness deviation of the finished film is controlled within ±0.15μm. Finally, the finished film is slit and wound up to obtain the PE film product. During online monitoring, when the film thickness deviation exceeds ±0.10μm, the control is activated. If the thickness is too thick, the extrusion amount is reduced by 5%-10% or the stretch ratio is increased by 0.2-0.5 times. If the thickness is too thin, the extrusion amount is increased by 5%-8% or the stretch ratio is reduced by 0.1-0.3 times. When the melt viscosity fluctuation exceeds ±50mPa·s, the temperature of each zone of the extruder is finely adjusted by 1-3℃. The processing distance of plasma pretreatment is 5-10cm, which is the vertical distance between the recovered powder and the plasma electrode.

[0048] The online thickness gauge uses a laser thickness gauge with a detection accuracy of ±0.01μm and a detection frequency of 10 times / second; the online viscosity monitor uses a capillary viscometer with a detection accuracy of ±10mPa·s and a detection frequency of 5 times / second. The detection data is transmitted to the process control system in real time to achieve rapid parameter adjustment.

[0049] By designing a process involving online cryogenic pulverization, plasma pretreatment, and in-situ recycling, and by adapting the grafted modified matrix to the in-situ hydrophobic modified composite filler, the interfacial compatibility between recycled and virgin materials is improved, resource utilization is enhanced, and zero waste discharge is achieved during the production process. At the same time, online closed-loop multi-parameter control ensures the stability and high yield of ultrathin film production, making it suitable for large-scale continuous industrial production.

[0050] The lightweight and environmentally friendly PE film prepared by this invention has a longitudinal and transverse tensile strength of ≥25MPa, an elongation at break of ≥300%, and a puncture resistance of ≥0.5N / μm. Under natural light conditions, the photodegradation rate of the film is ≥30% within 6-12 months after use and ≥60% within 18-24 months. The degradation rate is detected by weighing, that is, the mass loss rate of the film sample is measured periodically.

[0051] This invention also provides a lightweight and environmentally friendly PE film, prepared by the above-described preparation method, and composed of the following components in parts by weight: 85-92 parts of grafted modified polyethylene matrix, 2-5 parts of in-situ hydrophobic modified nanocomposite filler, 3-8 parts of food-grade polycaprolactone, 1-2 parts of composite additives, and 0.3-0.8 parts of PE-compatible anti-dripping agent. The food-grade polycaprolactone used has a number average molecular weight of 10,000-30,000 and a melt index of 5-15 g / 10 min (test conditions: 160℃, 2.16 kg), which meets the relevant national standards for plastic resins for food contact.

[0052] The grafted modified polyethylene matrix uses a blend of high-density polyethylene, linear low-density polyethylene, and metallocene polyethylene as the matrix resin, with a mass ratio of high-density polyethylene, linear low-density polyethylene, and metallocene polyethylene of 1:(3-5):(0.5-1.5). The melt index of the matrix resin is 1-4 g / 10 min (test conditions: 190℃, 2.16 kg). The grafted modified polyethylene matrix is ​​prepared by low-temperature solvent-free solid-phase grafting of matrix resin, composite grafting monomer, composite initiator, and polymerization inhibitor at a mass ratio of 100:(1.5-3.0):(0.15-0.4):(0.05-0.1), with a grafting rate of 1.2%-2.0%. The composite grafting monomer is a mixture of maleic anhydride, itaconic acid, and isooctyl acrylate in a mass ratio of 2:1:0.5; the composite initiator is a mixture of dicumyl peroxide and benzoyl peroxide in a mass ratio of 2:1; and the polymerization inhibitor is hydroquinone. The in-situ hydrophobic modified nanocomposite filler uses nano-cellulose whiskers, nano-activated calcium carbonate, and nano-PE wax grafted talc powder as composite reinforcing fillers, with a mass ratio of 3:(1-5):(0.3-0.8). The average particle size of the composite reinforcing filler is 50-100 nm. The in-situ hydrophobic modified nanocomposite filler is prepared by in-situ hydrophobic modification of the composite reinforcing filler and the composite modifier, with the amount of the composite modifier added being 3.0-4.5% of the total mass of the composite reinforcing filler. The composite modifier is a mixture of polyethylene wax grafted with maleic anhydride, stearic acid and γ-aminopropyltriethoxysilane (KH-550) in a mass ratio of 3:1:0.5. The composite additive is a mixture of a heavy metal-free photosensitive degrader, food-grade antioxidant 1010, antioxidant 168 and ultraviolet absorber UV-531 in a mass ratio of 2:1:1:0.5. The PE-compatible anti-dripping agent is polyethylene-grafted polytetrafluoroethylene (PE-g-PTFE). The lightweight PE film has a thickness of 2-7 μm and a gradient cross-linked layer with a thickness of 80-220 nm on its surface. The surface cross-linking degree of the gradient cross-linked layer is ≥65%, and the cross-linking degree at the junction with the grafted modified polyethylene matrix is ​​≤8%. The film can realize closed-loop in-situ recycling of production waste. The recycled powder is added to the premixing and preplasticizing stage at a mass ratio of 15-35% of the total materials in the premixing and preplasticizing stage. The thickness deviation of the finished film is controlled within ±0.15 μm.

[0053] The lightweight and environmentally friendly PE film prepared by this invention has a heat-sealing temperature of 120-140℃, a heat-sealing pressure of 0.2-0.4MPa, a heat-sealing time of 0.5-1s, and a heat-sealing strength of ≥10N / 15mm, which meets the heat-sealing requirements of food packaging, daily packaging and other fields.

[0054] The finished film is inspected by sampling from roll to roll. 5% of the finished rolls are randomly selected from each batch, and three 1m long samples are cut from each roll for inspection. The core inspection items include thickness deviation, tensile strength, elongation at break, puncture resistance, heat seal strength, surface crosslinking degree, and electrostatic voltage. All inspection items must meet the index range disclosed in this invention. If any one index in a single roll sample fails to meet the standard, the roll is deemed unqualified. If the failure rate of a batch sample is ≥10%, the entire batch is re-inspected. If the re-inspection still fails, the batch is deemed unqualified.

[0055] Example 1 This embodiment uses median parameters / parts to prepare a lightweight and environmentally friendly PE film. The specific steps are as follows: Raw material pretreatment: Polyethylene resin raw material is vacuum dried at 105℃ for 5 hours with a vacuum degree of -0.085MPa; Nanocomposite filler raw material is hot-air dried at 85℃ for 2.5 hours, passed through a 200-mesh standard sieve, and sealed for later use.

[0056] Matrix grafting: The matrix resin is high-density polyethylene: linear low-density polyethylene: metallocene polyethylene = 1:3:0.5 (mass ratio), melt index 2 g / 10 min; matrix resin: composite grafting monomer: composite initiator: polymerization inhibitor = 100:2.0:0.25:0.07 (mass ratio); heated to 90℃ under nitrogen protection (0.02 MPa), stirred at 550 r / min for 7 min, stirred at 350 r / min for 18 min, grafting rate 1.5%.

[0057] Filler modification: The composite reinforcing filler is nano-cellulose whiskers: nano-activated calcium carbonate: nano-PE wax grafted talc powder = 3:3:0.5 (mass ratio), with an average particle size of 80nm; the composite modifier is added at 3.8% (accounting for the total mass of the filler), atomized and sprayed under stirring at 80℃ and 900r / min, and heat-preserved for 18min, with a water contact angle of 115° and a dispersed particle size of 180nm.

[0058] Premixed and pre-plasticized: by mass parts, 88 parts of grafted modified polyethylene matrix, 3 parts of in-situ hydrophobic modified nanocomposite filler, 6 parts of food-grade polycaprolactone, 1.5 parts of composite additives, and 0.5 parts of PE-compatible anti-dripping agent; sheared at 1350 r / min for 18 min at 100℃ under nitrogen protection, with intermittent venting (-0.09 MPa, venting for 12 s every 5 min).

[0059] Differential extrusion: Twin-screw extruder with a length-to-diameter ratio of 38:1, zone temperatures of 115℃ / 125℃ / 130℃ / 135℃, and die head temperature of 138℃; screw speed of 120r / min, melt differential module with 45 flow channels (diameter 1.8mm), and extruded melt sheet thickness of 0.2mm.

[0060] Supercritical stretching: The melt sheet is cooled to 50℃ (crystallization 25%), preheated in the preheating section with supercritical CO2 (50℃, 8MPa, flow rate 8L / min) for 6s, stretched bidirectionally at 90℃ (5.0 times, speed 75mm / s), heat-set at 115℃ and 0.2MPa for 8s, and then rapidly cooled to room temperature.

[0061] Surface modification: Nitrogen:Argon = 8:2 mixed atmosphere, plasma discharge power 1000W, processing speed 100m / min, processing time 1.5s; forming a gradient cross-linked layer with a thickness of 150nm, surface cross-linking degree 70%, and matrix bonding degree 6%; surface voltage ≤±40V after electrostatic elimination.

[0062] Closed-loop recycling: Waste materials are pulverized at 38℃ into 30-mesh powder and pretreated with 600W plasma for 1.5s; the recycled powder is mixed back into the premixing and preplasticizing stage at a ratio of 20%, and the film thickness deviation is ±0.12μm after online control.

[0063] Example 2 This embodiment uses a high-matrix / low-filler addition method to prepare a lightweight and environmentally friendly PE film. The specific steps are as follows: Raw material pretreatment: Polyethylene resin raw material is vacuum dried at 100℃ for 6 hours with a vacuum degree of -0.08MPa; Nanocomposite filler raw material is dried with hot air at 80℃ for 3 hours, passed through a 200-mesh standard sieve, and sealed for later use.

[0064] Matrix grafting: The matrix resin is high-density polyethylene: linear low-density polyethylene: metallocene polyethylene = 1:4:1 (mass ratio), melt index 1g / 10min; matrix resin: composite grafting monomer: composite initiator: polymerization inhibitor = 100:1.5:0.15:0.05 (mass ratio); heated to 85℃ under nitrogen protection (0.01MPa), stirred at 500r / min for 10min, stirred at 300r / min for 20min, grafting rate 1.2%.

[0065] Filler modification: The composite reinforcing filler is composed of nano-cellulose whiskers: nano-activated calcium carbonate: nano-PE wax grafted talc powder = 3:1:0.3 (mass ratio), with an average particle size of 50nm; the composite modifier is added at 3.0% (accounting for the total mass of the filler), atomized and sprayed under stirring at 75℃ and 800r / min, and heat-preserved for 12min, with a water contact angle of 110° and a dispersed particle size of 150nm.

[0066] Premixed and pre-plasticized: by mass parts, 92 parts grafted modified polyethylene matrix, 2 parts in-situ hydrophobic modified nanocomposite filler, 3 parts food-grade polycaprolactone, 1 part composite additive, and 0.3 parts PE-compatible anti-dripping agent; sheared at 1200 r / min for 12 min at 95℃ under nitrogen protection, with intermittent venting (-0.08 MPa, venting for 10 s every 5 min).

[0067] Differential extrusion: Twin-screw extruder with a length-to-diameter ratio of 36:1, zone temperatures of 110℃ / 120℃ / 125℃ / 130℃, and die head temperature of 135℃; screw speed of 80 r / min, melt differential module with 40 flow channels (diameter 1.2 mm), and extruded melt sheet thickness of 0.1 mm.

[0068] Supercritical stretching: The melt sheet is cooled to 40℃ (crystallization 20%), preheated for 4s with supercritical CO2 (45℃, 8MPa, flow rate 5L / min) in the preheating section, stretched bidirectionally at 85℃ (4.5 times the density, speed 50mm / s), heat-set at 110℃ and 0.1MPa for 5s, and then rapidly cooled to room temperature.

[0069] Surface modification: Nitrogen:Argon = 8:2 mixed atmosphere, plasma discharge power 800W, processing speed 80m / min, processing time 0.8s; forming a gradient cross-linked layer with a thickness of 80nm, surface cross-linking degree 65%, and matrix bonding degree 8%; surface voltage ≤±50V after electrostatic elimination.

[0070] Closed-loop recycling: Waste material is pulverized at 35℃ into 20-mesh powder and pretreated with 500W plasma for 1 second; the recycled powder is mixed back into the premixing and preplasticizing stage at a ratio of 15%, and the film thickness deviation is ±0.10μm after online control.

[0071] Table 1. Intermediate Indicator Testing Tables for Examples 1 and 2

[0072] Table 2 Performance test data of lightweight and environmentally friendly PE films in Examples 1 and 2

[0073] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0074] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a lightweight and environmentally friendly PE film, characterized in that, The specific steps include the following: In the matrix grafting stage, a matrix resin system is formed by compounding various polyethylene materials. The matrix resin is mixed with composite grafting monomers, composite initiators and polymerization inhibitors in proportion. The low-temperature solvent-free solid-phase grafting treatment is carried out under inert gas protection and specific low-temperature conditions to prepare grafted modified polyethylene matrix. In the filler modification stage, based on the grafted modified polyethylene matrix, a variety of nanomaterials are selected to form a composite reinforced filler, and a composite modifier is added for in-situ hydrophobic modification treatment to prepare an in-situ hydrophobic modified nanocomposite filler. In the premixing and preplasticizing stage, the grafted modified polyethylene matrix, in-situ hydrophobic modified nanocomposite filler and various auxiliary components are mixed in a preset ratio. Under inert gas protection and at a specific temperature, a low-temperature high-shear intermittent exhaust process is used to carry out solid-phase premixing and preplasticizing to obtain preplasticized blended materials. In the differential extrusion stage, the pre-plasticized blended material is fed into the differential extrusion unit, the extrusion temperature of each section is controlled, and the melt is uniformly plasticized and refined through the differential module to extrude melt sheets of a specific thickness. In the supercritical stretching stage, the melt sheet is cooled to obtain a casting with controllable crystallinity. The casting is then fed into a bidirectional synchronous stretching device. Using a segmented supercritical fluid introduction process, the casting is preheated, bidirectional synchronous stretched, heat-set and cooled sequentially under preset temperature and pressure parameters to prepare a lightweight polyethylene film. In the surface modification stage, the lightweight polyethylene film is fed into a plasma treatment device. Under a specific mixed gas atmosphere, the film is modified on both sides to form a gradient cross-linked layer. After electrostatic elimination and cleaning, the surface-modified film is obtained. In the closed-loop recycling stage, unqualified finished films and waste generated during the production process are collected. After removing impurities and oxide layers through low-temperature crushing and plasma pretreatment, the recycled powder is introduced into the premixing and preplasticizing stage in a preset ratio to premix with the new material. The key process parameters of the extrusion and stretching processes are controlled in a coordinated manner. The final finished product is obtained after slitting and winding.

2. The method for preparing lightweight and environmentally friendly PE film according to claim 1, characterized in that, In the matrix grafting stage, the matrix resin is a blend of high-density polyethylene, linear low-density polyethylene and metallocene polyethylene, with a mass ratio of 1:(3-5):(0.5-1.5) and a melt index of 1-4 g / 10 min. The mass ratio of matrix resin, composite grafting monomer, composite initiator and polymerization inhibitor is 100:1.5-3.0:0.15-0.4:0.05-0.

1.

3. The method for preparing lightweight and environmentally friendly PE film according to claim 2, characterized in that, The composite grafting monomers are maleic anhydride, itaconic acid and isooctyl acrylate in a mass ratio of 2:1:0.5, the composite initiator is dicumyl peroxide and benzoyl peroxide in a mass ratio of 2:1, and the polymerization inhibitor is hydroquinone.

4. The method for preparing lightweight and environmentally friendly PE film according to claim 3, characterized in that, In the filler modification stage, the composite reinforcing filler is a blend of nano-cellulose whiskers, nano-active calcium carbonate and nano-PE wax grafted talc powder, with a mass ratio of 3:(1-5):(0.3-0.8). The average particle size of the composite reinforcing filler is 50-100nm. The amount of composite modifier added is 3.0-4.5% of the total mass of the composite reinforcing filler.

5. The method for preparing lightweight and environmentally friendly PE film according to claim 4, characterized in that, The composite modifier is a compound made of polyethylene wax grafted with maleic anhydride, stearic acid and γ-aminopropyltriethoxysilane in a mass ratio of 3:1:0.

5.

6. The method for preparing lightweight and environmentally friendly PE film according to claim 5, characterized in that, In the premixing and preplasticizing stage, the amounts of each component by mass are as follows: 85-92 parts of grafted modified polyethylene matrix, 2-5 parts of in-situ hydrophobic modified nanocomposite filler, 3-8 parts of food-grade polycaprolactone, 1-2 parts of composite additives, and 0.3-0.8 parts of PE-compatible anti-dripping agent; the composite additives are a mixture of heavy metal-free photosensitive degrader, food-grade antioxidant 1010, antioxidant 168 and ultraviolet absorber UV-531 in a mass ratio of 2:1:1:0.5, and the PE-compatible anti-dripping agent is polyethylene grafted polytetrafluoroethylene.

7. The method for preparing lightweight and environmentally friendly PE film according to claim 6, characterized in that, In the differential extrusion stage, the temperatures of each section of the extruder are: Zone 1 110-120℃, Zone 2 120-130℃, Zone 3 125-135℃, Zone 4 130-140℃, and the die head temperature 135-140℃; the differential module has 40-50 flow channels with a flow channel diameter of 1.2-2.2mm, and extrusion yields melt flakes with a thickness of 0.1-0.3mm.

8. The method for preparing lightweight and environmentally friendly PE film according to claim 7, characterized in that, In the supercritical stretching stage, the molten sheet is cooled to 40-60℃ to obtain a casting. A segmented supercritical CO2 introduction process is adopted, the stretching temperature is 85-95℃, the longitudinal and transverse stretching ratios are both 4.5-6.5 times, and the heat setting temperature is 110-120℃.

9. The method for preparing lightweight and environmentally friendly PE film according to claim 8, characterized in that, In the surface modification stage, the mixed gas is nitrogen and argon in a mass ratio of 8:

2. After treatment, a gradient cross-linked layer with a thickness of 80-220nm is formed on the surface of the film, with a surface cross-linking degree ≥65% and a cross-linking degree ≤8% at the junction with the grafted modified polyethylene matrix. In the closed-loop recycling stage, the recycled powder accounts for 15-35% of the total material in the premixing and preplasticizing stage.

10. A lightweight and environmentally friendly PE film, prepared by the preparation method according to any one of claims 1-9, characterized in that, It is composed of the following components by weight: 85-92 parts of grafted modified polyethylene matrix, 2-5 parts of in-situ hydrophobic modified nanocomposite filler, 3-8 parts of food-grade polycaprolactone, 1-2 parts of composite additives, and 0.3-0.8 parts of PE-compatible anti-dripping agent; the film thickness is 2-7 μm, and the surface is provided with a gradient crosslinking layer, the thickness and crosslinking degree parameters of the gradient crosslinking layer conforming to the limitations of claim 9.

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