A method for preparing a matte polyethylene substrate film

Abstract

The application belongs to the technical field of chemical industry and base material film, and particularly relates to a preparation method of a matte polyethylene base material film, S1: selecting ethylene as raw material; S2: drying ethylene through a dryer, and then mixing and adding the ethylene and n-hexane diluent into a mixing reaction kettle; S3: adding a catalyst containing manganese monoxide. The application adjusts the molecular weight of high-density polyethylene film, reduces the molecular weight and the molecular chain length, adds impurities in the high-density polyethylene film, and then adjusts the molecular weight of low-density polyethylene film, reduces the molecular weight and the molecular chain length. Through the oxidation reaction of manganese element and the contact between the manganese element in the high-density polyethylene film and the low-density polyethylene in the degradation process of the high-density polyethylene film, the oxidation reaction is improved, the degradation speed is improved, the degradation time of the packaging base material film is greatly shortened, the degradation time of polyethylene in the normal environment is avoided to be too long, and the harm of degradation in different environments to the environment is reduced.

Description

Technical Field

[0001] This invention relates to the fields of chemical engineering and substrate film technology, specifically a method for preparing a matte polyethylene substrate film. Background Technology

[0002] Plastic packaging and plastic packaging products are taking up an increasingly larger share of the market, especially composite plastic flexible packaging, which has been widely used in the food, pharmaceutical, and chemical industries. Among them, food packaging accounts for the largest proportion, such as beverage packaging, frozen food packaging, retortable food packaging, and fast food packaging. These products have brought great convenience to people's lives.

[0003] The majority of substrate films used in food, pharmaceutical, and transportation packaging are for single use, leading to a large amount of discarded polyethylene substrate films. Traditional polyethylene films often take up to a century to degrade. In order to facilitate degradation, various additives are added to the polyethylene substrate in current technology. Although this does improve the degradation efficiency, it causes another kind of damage to the environment. In some cases, the degradation effect is only suitable for a certain degradation method, such as burial in soil, while it can harm the aquatic environment when used in water.

[0004] Therefore, we propose a method for preparing matte polyethylene substrate film, which can significantly shorten the degradation time of packaging substrate film, avoid excessive degradation time of polyethylene in normal environment, reduce the environmental harm caused by degradation in different environments, and is suitable for packaging in many industries while ensuring the strength of the packaging. Summary of the Invention

[0005] In order to overcome the above-mentioned defects of the prior art, the present invention provides a method for preparing a matte polyethylene substrate film to solve the problems existing in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a matte polyethylene substrate film:

[0007] S1: Ethylene is selected as the raw material;

[0008] S2: Dry the ethylene using a dryer, then mix it with n-hexane diluent and add it to the reaction vessel;

[0009] S3: Add a catalyst containing manganese monoxide. The molecular weight of high-density polyethylene is controlled by a hydrogen-containing regulator, thus reducing the molecular weight.

[0010] S4: The mixing reactor raises the temperature to 90°C and the pressure to 1.8MPa, and the reacting ethylene, comonomer and most of the solvent are compressed and cooled through a flash evaporator;

[0011] S5: The dried polymer powder is dried in a fluidized bed and then granulated to form high-density polyethylene;

[0012] S6: After the ethylene is pressurized, it is added to the cooling absorber along with the purified recycled comonomer and cyclohexane. While cooling down, it is fully mixed and melted in the reactor and pressurized to 250-310MPa at a temperature of 100-300℃.

[0013] S7: The molecular weight distribution is reduced by adding hydrogen-containing regulators at different temperatures. Manganese-containing catalysts are added and mixed. A deactivating agent is added inside the reactor to terminate the reaction. The temperature is raised to 300℃ and the catalyst residue is adsorbed by an adsorbent.

[0014] S8: The reacting ethylene, comonomers and most of the solvent are removed by flash evaporation, compressed and cooled, and then low-density polyethylene granules are formed by granulation.

[0015] S9: Low-density polyethylene and high-density polyethylene are laminated by extrusion composite method;

[0016] S10: When making packaging materials, low-density polyethylene is used as the inner layer and high-density polyethylene is used as the outer layer.

[0017] Furthermore: The dryer is a vibrating fluidized bed dryer. The excitation force provided by the vibrating motor causes the material to jump forward on the air distribution plate, while contacting the hot air supplied from below the distribution plate for heat and mass transfer. The lower chamber provides a stable air chamber with a certain pressure for the bed. The induced draft fan maintains a slight negative pressure on the upper part of the bed material in the chamber, maintaining a good drying environment and preventing dust leakage. The air distribution plate supports the material and distributes the hot air.

[0018] Furthermore, hydrogen-containing regulators were added to the reactor at 100°C, 180°C, and 240°C, respectively.

[0019] Furthermore, in the preparation of polyethylene catalysts, deactivators include isopentane, tetrahydrofuran, pentane, and toluene, which are volatile organic compounds.

[0020] Furthermore: the extrusion compounding of low-density polyethylene and high-density polyethylene is carried out through an extrusion unit, which includes: a feeding system, a screw system, a die system, a cooling system, a control system, a cutting device, a traction system, and a winding system.

[0021] Furthermore: the high-density polyethylene film is given a matte finish.

[0022] Compared with the prior art, the technical effects and advantages of the present invention are as follows:

[0023] 1) The method for preparing the matte polyethylene substrate film of the present invention involves adjusting the molecular weight of high-density polyethylene film to reduce its molecular weight and molecular chain length, while adding impurities to the high-density polyethylene film to form a low-density polyethylene film. The method also involves adjusting the molecular weight of the low-density polyethylene film to reduce its molecular weight and molecular chain length. Through the oxidation reaction of manganese and the contact between manganese and low-density polyethylene during the degradation process of the high-density polyethylene film, the oxidation reaction is enhanced, increasing the degradation rate. This significantly shortens the degradation time of the packaging substrate film, avoids excessively long degradation times of polyethylene in normal environments, and reduces the environmental harm caused by degradation under different conditions.

[0024] 2) The method for preparing the matte polyethylene substrate film of the present invention involves setting a high-density polyethylene film as the outer layer and a low-density polyethylene film as the inner layer. In the packaging substrate film, if it is made into a bag, after sealing, the low-density polyethylene film is less likely to come into contact with external oxygen and the external environment, and its aging rate is reduced. The high-density polyethylene of the outer layer improves the structural strength of the overall packaging material and avoids its rapid aging. At the same time, it is easier to add pigments to the outside of the high-density polyethylene, which is beneficial for packaging in many industries and ensures the strength of the packaging. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the high-density polyethylene preparation process of the present invention;

[0026] Figure 2 This is a schematic diagram of the low-density polyethylene preparation process of the present invention. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. In addition, the forms of the various structures described in the following embodiments are merely illustrative. The structures involved in the present invention are not limited to the structures described in the following embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] This invention provides a technical solution: a method for preparing a matte polyethylene substrate film, S1: selecting ethylene as the raw material;

[0029] S2: Dry the ethylene using a dryer, then mix it with n-hexane diluent and add it to the reaction vessel;

[0030] S3: Add a catalyst containing manganese monoxide. The molecular weight of high-density polyethylene is controlled by a hydrogen-containing regulator, thus reducing the molecular weight.

[0031] S4: The mixing reactor raises the temperature to 90°C and the pressure to 1.8MPa, and the reacting ethylene, comonomer and most of the solvent are compressed and cooled through a flash evaporator;

[0032] S5: The dried polymer powder is dried in a fluidized bed and then granulated to form high-density polyethylene;

[0033] S6: After the ethylene is pressurized, it is added to the cooling absorber along with the purified recycled comonomer and cyclohexane. While cooling down, it is fully mixed and melted in the reactor and pressurized to 250-310MPa at a temperature of 100-300℃.

[0034] S7: The molecular weight distribution is reduced by adding hydrogen-containing regulators at different temperatures. Manganese-containing catalysts are added and mixed. A deactivating agent is added inside the reactor to terminate the reaction. The temperature is raised to 300℃ and the catalyst residue is adsorbed by an adsorbent.

[0035] S8: The reacting ethylene, comonomers and most of the solvent are removed by flash evaporation, compressed and cooled, and then low-density polyethylene granules are formed by granulation.

[0036] S9: Low-density polyethylene and high-density polyethylene are laminated by extrusion composite method;

[0037] S10: When making packaging materials, low-density polyethylene is used as the inner layer and high-density polyethylene is used as the outer layer.

[0038] There are three existing methods for preparing high-density polyethylene: slurry polymerization, gas-phase polymerization, and solution polymerization. This application uses two types of slurry valves: stirred tank type and stirred tank type in loop reactor.

[0039] The dual reactors can be used in parallel or in series. Hydrogen, ethylene, catalysts, etc. can be put into the first reactor to cause a polymerization reaction. The polymer exists in hexane in the form of a slurry, hence the name slurry process. Because this process is simple and does not have high requirements for the purity of raw materials, it can be flexibly used in parallel or in a single-peak or double-peak manner to produce high-density polyethylene.

[0040] Another process is the loop reactor process, which uses a chromium-based catalyst. The catalyst must be activated before use. Then, the raw material, ethylene, is mixed with hydrogen and other substances and injected into the reactor, where polyethylene is produced under the catalytic reaction of the catalyst. This process is characterized by lower cost, fewer steps, and easier raw material transportation. However, it requires high purity of the raw materials, which increases production costs for packaging materials.

[0041] Gas-phase polymerization utilizes a low-pressure gas-phase fluidized bed reactor with a chromium-based catalyst to induce polymerization at 85-110 degrees Celsius. The main product density is 0.915-0.970 g / cm³. Its key advantages include low operating pressure, relatively low cost, and easy equipment maintenance. However, it requires high purity raw materials, necessitating the refining of all raw materials, which consequently increases the production cost of packaging materials.

[0042] Solution polymerization involves dissolving ethylene in a solvent and polymerizing it at 160-200°C using a catalyst. The resulting polyethylene dissolves in the solvent. The main catalyst used is a Zn-series catalyst. This process is similar to the slurry polymerization process, but it requires more sophisticated equipment and is less convenient for adding manganese monoxide.

[0043] Therefore, the stirred tank slurry method is used to prepare high-density polyethylene, which has lower requirements for raw materials and facilitates the use of manganese monoxide in the manufacturing process.

[0044] refer to Figure 1 High-purity ethylene is used as the main raw material, while propylene and 1-butene are comonomers, hexane is used as the solvent, and a high-efficiency catalyst containing manganese monoxide is used to carry out a low-pressure polymerization reaction at 70-85℃. The resulting slurry is then separated, dried, mixed, and granulated to obtain high-density polyethylene with various excellent properties.

[0045] In the preparation of low-density polyethylene, all monomers (including olefins and comonomers) entering the polymerization reactor must be deoxidized to remove impurities that are toxic to the catalyst, such as oxygen, carbon dioxide, water, sulfides, methanol, and alkynes. Commonly used deoxidizing and oxide catalyst beds and molecular sieves are used to remove impurities. The polymerization reaction is carried out in a fluidized bed reactor. The lower part of the reactor is cylindrical, and the upper part consists of an inverted cone and a hemisphere. There is a gas distribution plate at the bottom of the reactor, on which a fluidized bed layer formed by powdered resin is placed.

[0046] The manganese-containing catalyst and co-catalyst are directly fed into the reaction bed from the distribution plate. The blower sends in circulating gas to keep the bed in a fluidized state and to make the reactants and catalysts uniformly mixed. At the same time, the heat of reaction is carried away and removed from the system in the circulating gas cooler.

[0047] The gas mass velocity through the bed should be 3-6 times Gmf (minimum gas flow rate required for fluidization). The molecular weight regulator, hydrogen, is also introduced into the system from the bottom of the reactor along with the monomers. The resin properties are adjusted by the amount of catalyst, cocatalyst, comonomer, and hydrogen added. The reaction residence time is about 3 hours. The resin exits the reactor and passes through a special unloading system to remove unreacted monomers. The resin with recovered monomers is recycled back to the reactor and enters the degassing chamber. In this chamber, the hydrocarbons adsorbed in the resin are further removed. A purge gas is introduced from the bottom of the chamber and comes into countercurrent contact with the resin to blow out and carry away the hydrocarbons. At the same time, a small stream of deactivating agent is introduced to kill the residual active centers on the polymer. The degassed resin is then passed through equipment such as a vibrating screen to remove large pieces. Before entering the granulation system, it is mixed with solid and liquid additives. The granulation system is a tightly integrated unit consisting of a mixer, a melt pump, and a granulator. Circulating soft water carries away the granulated chips. After drying and water separation, the chips are sent to the hopper and then sent by air to the blending, storage, transportation, and packaging processes.

[0048] Low-density polyethylene and high-density polyethylene are laminated using a hot-melt co-extrusion method.

[0049] Before lamination, single high-density polyethylene (HDPE) films can undergo corona treatment on the processing line. In simple terms, this involves electrostatically charging the plastic film surface with high-frequency voltage, resulting in improved adhesion for subsequent lamination. The process utilizes high-frequency, high-voltage corona discharge on the treated plastic surface to generate low-temperature plasma, causing free radical reactions that cross-link the polymer. This roughens the surface and increases its wettability to polar solvents. These plasmas penetrate the substrate surface through electrostatic discharge, disrupting its molecular structure and oxidizing and polarizing the surface molecules. The ionic electrostatic erosion of the surface further enhances the adhesion of the substrate. The quality of this treatment is also related to the film's torsion time. If the film is stored for a long time after production, a large amount of precipitated additives may adhere to the film surface, affecting the quality after corona treatment. Furthermore, it is important to consider the potential ignition of the solvents used by the electrostatic discharge.

[0050] Depending on the specific requirements of the film application, corona treatment can be divided into single-sided or double-sided corona treatment. For high-speed casting production lines, some also employ multiple sets of electrodes to meet the surface tension requirements of subsequent plastic film processing.

[0051] Co-extrusion is a molding method that uses two or more different plastics to melt and plasticize each plastic separately through two or more extruders, and then feeds them into a die or combines the plastics supplied by the various extruders into the die through a distributor to produce a composite film. Co-extrusion is low-cost, reducing costs by 20%-30% compared to dry compounding. Furthermore, the co-extrusion process does not use adhesives or anchoring agents (AC agents), so its products are hygienic and do not cause environmental pollution. However, composite films produced by co-extrusion are limited to various thermoplastic plastics.

[0052] The dryer is a vibrating fluidized bed dryer. The excitation force provided by the vibrating motor causes the material to jump forward on the air distribution plate, and at the same time, it comes into contact with the hot air sent in from below the distribution plate to carry out heat and mass transfer. The lower chamber provides a stable air chamber with a certain pressure for the bed. The induced draft fan is adjusted to keep the upper part of the bed material in the chamber under a slight negative pressure, maintain a good drying environment and prevent dust from leaking out. The air distribution plate supports the material and distributes the hot air.

[0053] Before starting work, confirm that the power supply voltage meets the requirements and that the fan is free of foreign objects. Then, turn on the induced draft fan, blower, and steam pipe heating valves to generate hot air and preheat the drying system. Simultaneously, check for air leaks at the joints; if leaks are found, tighten them immediately. After preheating (hot air inlet temperature approximately 130℃), adjust the inlet valves to ensure the drying system is in normal operating condition for airflow and temperature. Then, turn on the vibrating motor and observe whether the lower chamber and bed vibrate smoothly. If instability occurs, immediately turn off the vibrating motor and investigate the cause. When starting feeding, continuously monitor the feeding process. Feeding should be uniform and quantitatively delivered into the vibrating fluidized bed dryer inlet. Then, observe whether the product discharge from the vibrating fluidized bed dryer outlet is uniform. Regularly check the product moisture content and output. Note that the airtight device at the cyclone dust collector outlet should be opened periodically. When the drying system stops working, feeding should be stopped first. When the dryer outlet stops discharging material, the power supply to the vibrating motor can be cut off to stop the vibrating fluidized bed dryer. Then, the steam pipe valves can be closed to stop the generation of hot air and allow the drying system to cool down gradually. Subsequently, the power supply to the blower and other motors can be turned off. At this point, the entire drying system stops working.

[0054] The reactor consists of a steel pressure vessel with a stirring system, a feeding port, an exhaust port, and a temperature and pressure control system. In the ethylene polymerization reactor, ethylene monomer is added to the reactor and polymerizes through the action of a chemical catalyst. During the reaction, it is necessary to control the reaction temperature and pressure.

[0055] The device as a whole also includes a reaction unit, a heating unit, and a control unit; the reaction unit has a fixed lid, a manually raised and lowered body, an electric heater built into the body, and a pre-set stainless steel outer shell insulation layer on the outside of the heater. The lid is equipped with 2 gas phase valves, 1 vacuum valve, 1 mechanical pressure gauge, 1 pressure sensor, 1 safety valve, 1 temperature sensor, 1 vacuum gauge, 1 magnetically coupled stirrer, 1 condenser, 1 reflux valve, 1 liquid phase collection tank, and a pre-installed discharge valve at the bottom of the reaction unit.

[0056] The hydrogen-containing regulator was added to the reactor at 100°C, 180°C and 240°C, respectively.

[0057] In the preparation of polyethylene catalysts, deactivators may be volatile organic compounds such as isopentane, tetrahydrofuran, pentane, and toluene.

[0058] The low-density polyethylene and high-density polyethylene are extruded and compounded through an extrusion unit, which includes: a feeding system, a screw system, a die system, a cooling system, a control system, a cutting device, a traction system, and a winding system.

[0059] The feeding system mainly consists of a hopper and a feeding device. The hopper stores the plastic raw materials, while the feeding device is responsible for feeding the plastic raw materials into the screw of the extruder. The function of the feeding system is to ensure that the extruder can continuously and stably supply material.

[0060] The screw system is the core component of an extruder, mainly consisting of the screw, barrel, and heating system. The screw, through rotation, propels the plastic raw material from the feed inlet to the discharge outlet, heating and melting the plastic during this process. The barrel provides the working space for the screw and, through the heating system, heats both the screw and the plastic to maintain the plastic in a molten state.

[0061] The die head system is used for extruding plastics. It consists of a die head and a mold. The die head determines the shape of the final product by controlling the extrusion speed and shape. The mold provides the plastic with the required shape and size.

[0062] The cooling system mainly consists of a cooling device and a water circulation system. After the plastic is extruded, the cooling device cools the plastic product, causing it to solidify rapidly. The water circulation system is responsible for circulating and supplying cooling water to ensure the cooling effect.

[0063] The control system is the intelligent part of the extruder. It adjusts and controls various parts of the extruder through a control panel. Operators can use the control system to adjust parameters such as extrusion speed and temperature to meet the requirements of different products.

[0064] The traction system is used to pull the product, ensuring a stable extrusion process.

[0065] The cutting device cuts the continuously extruded products into certain lengths according to requirements.

[0066] The traction system neatly winds up the film products cut to a certain length.

[0067] During operation, two sets of material granules are first placed into the hopper of the extruder. The rotating screw feeds the plastic granules into the heating zone of the extruder. In the heating zone, the plastic granules are heated to a certain temperature, making them soft and sticky. After heating, the plastic granules are fed into the extrusion zone of the extruder. Through the rotating screw and high-pressure gas, the plastic granules are extruded into continuous plastic strips or tubes. At the outlet of the extruder, the plastic strips or tubes are cooled, hardening and fixing them. Finally, the plastic products are cut into the required lengths for subsequent processing and use.

[0068] The high-density polyethylene film has a matte finish.

[0069] A matting agent is added during the preparation of high-density polyethylene film.

[0070] Degradation experiment

[0071] The traditional low-density polyethylene and high-density polyethylene composite film, low-density polyethylene film, high-density polyethylene film, and the low-density polyethylene and high-density polyethylene composite film prepared in this way are respectively referred to as A, B, C, and D;

[0072] A, B, C, and D were placed in test chambers with the same environment to ensure high humidity, high temperature, high oxygen, and microbial environment.

[0073]

[0074] Place A, B, C, and D in test chambers with identical environments, ensuring high humidity, high temperature, high oxygen, and high microbial conditions. Also, partially bury A, B, C, and D in the soil.

[0075]

[0076] Place A, B, C, and D in test chambers with identical environments, ensuring high humidity, high temperature, high oxygen, and high microbial conditions within the test chambers. All A, B, C, and D must be completely buried in the soil.

[0077]

[0078] The degradation experiments described above show that, in the laboratory, the degradation time of this application is much shorter than that of other polyethylene materials.

[0079] During the reprocessing of high-density polyethylene (HDPE) polymers, impurities are added and inevitably introduced. Generally, the introduction of impurities accelerates the degradation rate of HDPE polymers. The addition of manganese monoxide, because manganese is easily oxidized in the air and at elevated temperatures, will accelerate the rapid degradation of the outer HDPE layer. Oxidation of the inner low-density polyethylene is one of the main ways it decomposes. Ultraviolet rays under sunlight will cause the CH bonds in polyethylene to gradually break, forming free radicals. These free radicals will continuously react with oxygen, ultimately leading to the decomposition of polyethylene.

[0080] In the traditional degradation process of polyethylene, the generation of hydroperoxides and oxygen-containing groups is key to initiating free radical chain reactions and promoting the abiotic oxidative degradation of polyethylene. The degradation activity of polyethylene mulch film can be characterized by detecting the content and dynamic changes of hydroperoxides or oxygen-containing groups. In addition, by comparing the actual release of CO2 or CH4 generated by polyethylene degradation with the theoretical release of polyethylene molecules after complete decomposition, the rate of complete decomposition of polyethylene materials through microbial assimilation can be estimated. By tracking and monitoring the CO2 release of polyethylene film under soil landfill conditions, the study found that the degradation rate of polyethylene film buried in soil for 10 years was only 0.2%-0.5%.

[0081] The outer layer of high-density polyethylene polymer can easily be exposed to light. By adding light-sensitive substances such as manganese metal complexes, or by introducing carbon groups and aldehydes, ultraviolet light can be used to break the polymer chains, thereby changing the structure of the polymer and causing it to lose its mechanical strength.

[0082] Different structures of high-density polyethylene polymers result in different degradation rates. Those with weak chemical bonds are more prone to breakage under environmental influences, such as polyethylene and polytetrafluoroethylene. Even with the same degree of polymerization, differences in bond energy, bond length, and atomic radius between carbon (CF) and carbon (CH) lead to significant differences in their aging properties. Therefore, in polymer degradation, the presence of elements or groups other than carbon and hydrogen becomes an active site for degradation. Generally, double bonds, hydroxyl groups, carbonyl groups, carboxyl groups, amide groups, and ester groups in polyethylene polymer molecules are prone to initiating degradation.

[0083] Molecular weight has a significant impact on polymer degradation. For most polymers, the larger the molecular weight, the more stable the polymer and the less prone it is to degradation. The degree of branching in polyethylene polymer chains is called the degree of branching; the greater the degree of branching, the easier the polymer is to degrade.

[0084] Density is also closely related to the degradation of polyethylene polymers. After soaking low-density polyethylene and high-density polyethylene in lake water for 6 months, the degradability of polyethylene was compared. The experimental results showed that the maximum weight loss of low-density polyethylene and high-density polyethylene samples was 1.5-2.5% and 0.5-0.8%, respectively. Obviously, the low-density plastic is more easily degraded than the high-density plastic.

[0085] Therefore, when used as packaging material, high-density polyethylene film is used as the outer layer to ensure the packaging strength. When the packaging material is made into bags, the low-density polyethylene is exposed and begins to degrade after the high-density polyethylene degrades.

[0086] During the reprocessing of polyethylene polymers, certain impurities are inevitably introduced, and these impurities generally accelerate polymer degradation. Impurities that are always added during polymer production include plasticizers, dispersants, fillers, emulsifiers, initiators, and pigments.

[0087] The introduction of oxide impurities typically triggers free radical reactions in polymers; pigments usually serve to disperse and prevent aging, such as carbon black. Different impurities have varying degrees of degradability to polymers, making it a significant and undeniable influencing factor.

[0088] When heated, the chemical bonds in polyethylene polymers are easily broken, generating free radicals that promote polymer degradation. However, some free radicals can recombine into molecular chains under the influence of collisions, and temperature can also affect the quantum efficiency of polymer photoreaction by causing changes in the molecular mobility of polymer chains.

[0089] Sunlight is divided into ultraviolet, visible, and infrared light according to different wavelengths. Since the atmosphere can filter out some of the light, different types of light have different energies. Ultraviolet light has a wavelength of 200-400nm and an energy of 299-599kJ, with about 5% reaching the ground. Visible light has a wavelength of 500-800nm ​​and an energy of 299-599kJ, with about 40% reaching the ground. Infrared light has a wavelength of 103-106nm and an energy of 101-102kJ, with about 55% reaching the ground.

[0090] Based on the bond energies of polymer chemical bonds, the OO bond energy is 138.9 kJ / mol, the C-Cl bond energy is 328.4 kJ / mol, the CC bond energy is 347.7 kJ / mol, the CO bond energy is 351.5 kJ / mol, the CH bond energy is 413.4 kJ / mol, the HH bond energy is 436.0 kJ / mol, and the OH bond energy is 462.8 kJ / mol. Therefore, although only 5% of the ultraviolet light reaches the ground, its energy is sufficient to damage the polymer.

[0091] Oxygen, moisture, and microorganisms in the environment can also damage polymers and accelerate their degradation. Oxygen is ubiquitous in the human living environment and can react with many substances. Under the influence of light and heat, polymers inevitably react with oxygen.

[0092] Water-soluble polymers can undergo hydrolysis in water, which promotes the degradation of the polymer by water.

[0093] Under conditions of light, heat, and oxygen, polymers break down from macromolecules into smaller molecules. These smaller molecules are then eroded by microorganisms in the natural environment and eventually completely degraded. Soil, heat, and aerobic environments accelerate the degradation process, while light control, anaerobic environments, and water-saturated environments delay it.

[0094] Biodegradable polyethylene materials are prepared by surface-treating readily biodegradable substances such as starch, fatty acids, and cellulose with polyethylene, followed by blending or grafting. Microorganisms such as Streptomyces and Aspergillus flavus can secrete hydrolytic enzymes to break down starch, and then degrade the polyethylene molecules into smaller molecules, which are then further degraded by fungi or bacteria.

[0095] Microorganisms can secrete polymers such as polysaccharides and proteins on their cell surface to form a mucus layer, which helps them resist adverse environments and gather tiny substances in the air to promote their growth and reproduction.

[0096] During the degradation of polyethylene, this mucus layer can effectively reduce the hydrophobicity of the polyethylene molecular surface, help microorganisms adhere to the material surface, and promote the interaction between the extracellular enzymes secreted by microorganisms and the molecular chain segments or low molecular weight substances on the polyethylene material surface.

[0097] Microorganisms secrete specific enzymes that break down polyethylene molecular chains into low-molecular-weight oligomers, dimers, monomers, and other molecular fragments through multiple reactions such as hydrolysis and oxidation.

[0098] Low molecular weight substances produced by the decomposition of polyethylene pass through the cell membrane into microorganisms. Depending on the type of microorganism and its growth environment, they are metabolized through different pathways such as aerobic respiration, anaerobic respiration, and fermentation to generate electrons, energy, and nutrients that constitute cell components required for microbial growth and reproduction.

[0099] Polyethylene (PE) molecules have long chains, high relative molecular mass, high segment crystallinity, and strong hydrophobicity, making it difficult for them to come into contact with biological or chemical substances or enter microbial cells for decomposition and metabolism. Furthermore, the C-C covalent bonds that make up PE molecules are chemically stable and have high bond energy, requiring significant energy or force to break. These factors determine that PE molecules are difficult to degrade, and the degradation process is slow. Research results show that PE materials buried in soil only exhibit a decrease in surface number-average molecular weight and the appearance of numerous micropores, but the overall structure remains largely intact.

[0100] By reducing the molecular chain length with manganese monoxide and adding more impurities to the polyethylene polymer, the presence of chemical substances inside makes it easier for external microorganisms to enter. At the same time, the molecular weight is adjusted by hydrogen-containing regulators, making the molecular weight smaller. This allows high-density polyethylene to break molecular bonds with lower energy or force without affecting its strength.

[0101] For low-density polyethylene polymers, when the inner low-density polyethylene film begins to come into contact with the external environment, it indicates that the outer high-density polyethylene has begun to degrade or break down. Consequently, the manganese element in the low-density polyethylene film begins to come into contact with the external environment, leading to oxidation and degradation. Furthermore, the low-density polyethylene also begins to come into contact with external microorganisms.

[0102] Soil contains a rich variety of microorganisms, typically 10⁶-10⁹ microorganisms per gram of soil. The types and numbers of these microorganisms vary depending on the soil-forming environment and soil depth. Therefore, this resource-rich habitat was chosen as a screening site for degrading bacteria. By using low-density polyethylene (LDPE) film as a substrate, the degradation capacity of soil microorganisms on LDPE film was investigated. Within two years, the net value of carbon dioxide released, measured by a scintillation counter, accounted for approximately 0.7% of the total radioactivity in the test sample, which was significantly higher than the carbon dioxide produced by non-biological processes in the aged sample. This demonstrates that soil microorganisms catalytically transformed LDPE.

[0103] The ability of soil fungi to degrade PE was tested. Isotope tracing and infrared spectroscopy data showed that PE was effectively degraded by soil fungi. The soil microorganisms were then cultured using the photodegraded PE as the sole carbon and energy source. Changes in molecular weight were confirmed by analyzing the changes in viable cell count, sample weight, and GPC.

[0104] Compared to traditional low-density polyethylene (LDPE) films, the LDPE of this application begins to degrade in soil in 15 months, which is more advantageous than the 25 months of traditional LDPE films.

[0105] Three experimental groups were established using traditional high-density polyethylene (HDPE), low-density polyethylene (LDPE), and the substrate composite film. Two types of aquatic microorganisms were selected, and the materials from the three experimental groups were tested under conditions of pH 7.5 and temperature 30℃ for a one-year degradation experiment. The results showed that under the action of aquatic microorganisms, the weight loss of the heat-pretreated substrate composite film was approximately 19%, while that of the HDPE and LPE samples was 10% and 3.5%, respectively. This indicates that the substrate composite film is more prone to degradation in an aquatic environment.

[0106] The foregoing has shown and described the basic principles and main features of the present invention and its advantages. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description. Therefore, it is intended that all changes falling within the meaning and scope of the equivalents of the claims be included within the present invention, and no reference numerals in the claims should be regarded as limiting the scope of the claims.

[0107] 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 matte polyethylene substrate film, characterized by: S1: selecting ethylene as a raw material; S2: drying ethylene through a dryer, then mixing it with n-hexane diluent and adding it into a reaction kettle; S3: adding a catalyst containing manganese monoxide, controlling the molecular weight of high-density polyethylene through a hydrogen-containing regulator to make the molecular weight smaller; S4: raising the temperature of the mixed reaction kettle to 90℃, and the pressure to 1.8MPa, and compressing and cooling the reacted ethylene, comonomer and most of the solvent through a flash evaporator; S5: drying the polymer powder through a fluidized bed dryer, and granulating the dried polymer to form high-density polyethylene; S6: raising the pressure of ethylene, then adding it into a cooling absorber together with purified recycled comonomer and cyclohexane, fully mixing and dissolving it through the reactor while cooling, and pressurizing it to 250-310MPa and raising the temperature to 100-300℃; S7: adding a hydrogen-containing regulator at different temperatures to complete the reduction of molecular weight distribution, adding a manganese-containing catalyst, adding a deactivator inside the reactor to terminate the reaction, and raising the temperature to 300℃, and adsorbing the catalyst residue through an adsorbent; S8: removing the reacted ethylene, comonomer and most of the solvent through a flash evaporator, and compressing and cooling it, and forming low-density polyethylene particles through a granulator; S9: layering low-density polyethylene and high-density polyethylene through extrusion compounding respectively; S10: when making packaging materials, the low-density polyethylene layer serves as the inner layer, and the high-density polyethylene layer serves as the outer layer.

2. The method for preparing a matte polyethylene substrate film according to claim 1, characterized in that: The dryer is a vibrating fluidized bed dryer, which provides exciting force through a vibrating motor to make the material jump forward on the air distribution plate, and at the same time, it contacts with the hot air sent from below the distribution plate to conduct heat and mass transfer. The lower box provides a stable air chamber with a certain pressure for the bed layer, which maintains a slight negative pressure at the upper part of the bed layer in the box through the adjustment of the air blower to maintain a good drying environment and prevent dust leakage. The air distribution plate supports the material and distributes the hot air.

3. The method for preparing a matte polyethylene substrate film according to claim 1, characterized in that: The reaction kettle is composed of a steel pressure vessel with a stirring system, a feeding port, a waste gas discharge port, a temperature and pressure control system. In the ethylene polymerization reactor, ethylene monomer is added to the reaction kettle and polymerized through the action of a chemical catalyst. During the reaction process, the reaction temperature and pressure need to be controlled.

4. The method for preparing a matte polyethylene substrate film according to claim 1, characterized in that: The hydrogen-containing regulator is added to the reactor at 100℃, 180℃ and 240℃ respectively.

5. The method for preparing a matte polyethylene substrate film according to claim 1, characterized in that: The deactivator is an easily volatile organic substance such as isopentane, tetrahydrofuran, pentane and toluene used in the preparation of polyethylene catalyst.

6. The method for preparing a matte polyethylene substrate film according to claim 1, characterized in that: The extrusion compounding of low-density polyethylene and high-density polyethylene is carried out through an extruder set, which includes a feeding system, a screw system, a die system, a cooling system, a control system, a cutting device, a traction system and a winding system.

7. The method for preparing a matte polyethylene substrate film according to claim 1, characterized in that: The high-density polyethylene film is matte treated.

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