A coating grade low density polyethylene and its preparation and use

Abstract

The present application relates to a kind of coating grade low-density polyethylene.It mainly solves the poor adhesion problem of existing low-density polyethylene for coating product in coating process.It is characterized by: including the following mass ratio of raw materials: prepolymerization raw materials: alpha-olefin: metallocene catalyst is 1000-10000:1;Polymerization raw materials: prepolymerization product and ethylene is 1:10-1:500;Chain transfer agent: ethylene is 1:200-1:2000;Polymerization initiator: ethylene is 1:500-1:5000;Its preparation method: S1, first in prepolymerization kettle, alpha-olefin homopolymer is prepared using metallocene catalyst;S2, the alpha-olefin homopolymer produced is added with ethylene, polymerization initiator and chain transfer agent simultaneously into the reactor running under high-pressure polymerization conditions, and the final product is obtained after polymerization reaction.The coating grade low-density polyethylene, by adding alpha-olefin homopolymer to participate in the reaction, improves the adhesion and peel strength of coating product, fully meets the demand of paper-plastic product.

Description

Technical fields:

[0001] This invention relates to the field of coated products with high adhesion performance, and more particularly to a coating-grade low-density polyethylene, its preparation method, and its application. Background technology:

[0002] Coated low-density polyethylene (LDPE) features good toughness, strong adhesion, uniform film formation, and good hygiene, and is widely used in the outer packaging of food, chemical, and textile products. With the development of the domestic packaging industry, the application scope and quantity of specialized materials will gradually expand, and market demand will increase daily.

[0003] Currently, coating-grade low-density polyethylene is prepared using a single high-pressure reactor. Since EVA or EMA is added during the coating process, uneven mixing of resins during high-speed coating can affect the coating effect. In addition, physical mixing has a poor effect on enhancing the adhesion of the coating. Summary of the Invention:

[0004] This invention aims to overcome the problem of poor adhesion of low-density polyethylene (LDPE) used in coated products during the coating process in the prior art, and provides a coating-grade LDPE. This coating-grade LDPE, by incorporating α-olefin homopolymers to participate in the reaction, improves the adhesion and peel strength of the coated products, fully meeting the requirements of paper-plastic products. This invention also provides a method for preparing coating-grade LDPE and its applications.

[0005] The present invention solves its problem through the following technical solution: a coating-grade low-density polyethylene, wherein the raw material components and their proportions by mass are as follows:

[0006] Prepolymerization reaction feedstock: α-olefin, metallocene catalyst; α-olefin:metallocene catalyst mass ratio is 1000-10000:1;

[0007] Polymerization reaction raw materials: prepolymerization reaction products, ethylene, polymerization initiator, chain transfer agent;

[0008] The mass ratio of the prepolymer product to ethylene is 1:10-1:500, preferably 1:50-1:200;

[0009] The mass ratio of the chain transfer agent to ethylene is 1:200-1:2000, preferably 1:200-1:1000;

[0010] The mass ratio of polymerization initiator to ethylene is 1:500-1:5000.

[0011] Preferably, the metallocene catalyst is a homogeneous metallocene catalyst; the homogeneous metallocene catalyst used is the catalyst disclosed in the invention application filed on September 25, 2019, entitled "Restricted Geometric Configuration Metallocene Catalyst and its Preparation Method and Application" (patent application number 201910909798.4, publication date March 26, 2021).

[0012] Preferably, the α-olefin is one of 1-propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and 1-decene.

[0013] Preferably, the polymerization initiator is one or more combinations of peroxy esters (RCOOOR'); the chain transfer agent is one of olefins, aldehydes, ketones, alcohols, saturated hydrocarbons, and ethers.

[0014] This invention also provides a method for preparing coating-grade low-density polyethylene, comprising the following steps:

[0015] S1. First, α-olefin homopolymers are prepared in a prepolymer reactor using a metallocene catalyst;

[0016] S2. The produced flexible monomer oligomer, along with ethylene, a polymerization initiator, and a chain transfer agent, is simultaneously added to a reactor operating under high-pressure polymerization conditions. After polymerization, the final product is obtained. Then, the produced α-olefin homopolymer, along with ethylene, an initiator, and a chain transfer agent, is simultaneously added to a reactor operating under high-pressure polymerization conditions. After polymerization, the final product is obtained.

[0017] Preferably, the polymerization temperature in each region of the reactor is 100-400℃, more preferably 300-400℃; and the polymerization pressure in each region of the reactor is 100-400MPa, more preferably 150-300MPa.

[0018] Preferably, the polymerization pressure in each region of the reactor is 100-400 MPa, more preferably 150-300 MPa; the temperature in the prepolymer reactor is 100-200℃; the pressure in the prepolymer reactor is 0.5-50 MPa; and the aluminum-titanium ratio is 200-2000.

[0019] Preferably, the reactor operating under high-pressure polymerization conditions is a tubular reactor or a batch reactor, or a combination of both, with a single tubular process being preferred.

[0020] Preferably, the reaction product prepared is an ethylene-based polymer.

[0021] Preferably, the final product prepared using a 2.16 kg weight has a melt flow rate of 1-10 g / 10 min and a density of 0.915-0.925 g / cm³. 3 .

[0022] The present invention also provides an application of coating-grade low-density polyethylene final product in the coating field.

[0023] In the method for preparing coating-grade low-density polyethylene of this invention, α-olefin homopolymers are first prepared in a prepolymerization reactor using a metallocene catalyst. Then, the homopolymers, along with ethylene, an initiator, and a chain transfer agent, are simultaneously introduced into a reactor operating under high-pressure polymerization conditions to prepare the reaction product. The α-olefin homopolymer ethylene-octene copolymer prepared by reactive prepolymerization can insert ethylene polymerization segments to form amorphous elastic segments, thereby improving the adhesion of the coated product.

[0024] For coated low-density polyethylene, industrially, product performance is controlled by controlling melt mass flow rate (MFR) and density.

[0025] MFR decreases with decreasing temperature and increasing pressure. Under most industrial operating conditions, MFR can be controlled with a small amount of chain transfer agent, allowing for increased reactor pressure within industrial limits while maintaining a certain relative molecular mass. At this point, the branching degree of both long and short chains decreases, while the density increases. In free radical polymerization, higher reaction temperatures result in higher product conversion rates. Therefore, it is crucial to control the temperature to prevent it from becoming too high. Excessive temperatures increase chain transfer reactions, leading to increased relative molecular mass and a decrease in MFR; simultaneously, high temperatures can generate macromolecular gels, deteriorating processability.

[0026] Density is related to the degree of branching; density increases with increasing pressure and decreases with increasing temperature. The degree of branching of long-chain products increases with increasing temperature and conversion rate, and decreases with increasing pressure. By changing different types of reactors and operating conditions, products with different degrees of branching of long-chain products can be obtained.

[0027] For coating-grade LDPE resin, MFR, density, and swelling ratio (a comprehensive characterization of branching properties and molecular weight) reflect the polymer's molecular structure. MFR reflects the material's processing performance and also characterizes the average relative molecular mass. For coating processing, good MFR stability is required. For coatings on films, woven fabrics, and paper, an MFR of 7–8 g / 10 min is suitable. Density reflects molecular branching; lower density results in better toughness, lower heat-sealing temperature, and less product deformation. SR reflects the material's molecular weight dispersion (Mw / Mn) and the degree of long-chain branching. A higher SR indicates a wider Mw / Mn ratio and a greater degree of long-chain branching. For resins, higher crystallinity leads to higher mechanical properties (such as rigidity and hardness) and thermal properties (such as melting point, crystallization temperature, and Vicat softening temperature).

[0028] However, peel strength, as one of the important parameters of coating-grade LDPE resin, is only slightly affected by changes in MFR and density. Generally speaking, the higher the melt index (MI) of the resin, the better its flowability, the lower the viscosity of the molten film, and the greater the adhesion. A lower melt index (MI) indicates a larger molecular weight, resulting in poorer fusion and less adhesion to the coated substrate, leading to a decrease in peel strength. Secondly, density is also important; lower resin density indicates higher branch content, making the surface more easily activated and increasing adhesion, which is beneficial for improving peel strength. However, changes in these two factors have very limited impact on improving peel strength.

[0029] For paper-plastic composite products, the strength between the coating and the base paper is crucial; this is the significance of "composite." The concept of "peeling" is figuratively like skinning a cow or sheep during slaughter—removing two bonded layers from each other. Peel force differs from tensile force and shear force; it is not applied simultaneously across the entire bonded surface, but only along a single line. Therefore, peel strength is not defined as the maximum stress that can be withstood per unit area, but rather as the maximum peel force that can be withstood per unit width before the bond fails. Its unit is Newton-meter (N / m).

[0030] Compared with the above-mentioned background technology, the present invention has the following beneficial effects:

[0031] The final product of this invention is extruded through a T-die and then laminated with base paper to form a smooth, sealed film on its surface, creating a coated product. The peel strength between the paper and the film is then tested. The coated-grade low-density polyethylene prepared by this method utilizes an α-olefin homopolymer prepared through reactive prepolymerization, which can insert ethylene polymerization segments to form amorphous elastic segments, improving the adhesion of the coated product. Compared to LDPE resin prepared by existing methods, the peel strength is more than twice higher, as demonstrated in peel performance tests. With improved adhesion, the film is less prone to peeling, resulting in superior performance of the paper-plastic composite product. Detailed implementation method:

[0032] The present invention will be further described below with reference to embodiments:

[0033] The final product of this invention is extruded through a T-die and then laminated with the base paper to form a smooth, sealed film on its surface, thus forming a coated product. The peel strength between the paper and the film is then tested.

[0034] The test conditions for paper-plastic composite processing are shown in Table 1:

[0035] Table 1

[0036]

[0037] The paper-plastic products prepared under the above processing conditions were tested for peel strength according to Method A of GB / T 8808 "Peel Test Method for Flexible Composite Plastic Materials". The specific operation method is as follows:

[0038] (1) Sample preparation: Take two samples each in the warp and weft directions, each 15.0±0.1mm wide and 200mm long. Use your hands or adhesive tape to peel the film and paper apart by 50mm beforehand, ensuring that the peeled part is not obviously damaged.

[0039] (2) After sampling, the sample needs to be stabilized at 23±2℃ for 4 hours at room temperature.

[0040] (3) Test: Clamp the two ends of the peeled part of the sample onto the upper and lower clamps of the electronic tensile testing machine (material testing machine), so that the longitudinal axis of the peeled part of the sample coincides with the line connecting the centers of the upper and lower clamps, and the tightness is appropriate.

[0041] (4) During the test, the unpeeled part is T-shaped with the stretching direction.

[0042] (5) Set the test parameters, where the speed is 300 mm / min.

[0043] (6) Record the peel strength parameters.

[0044] After testing, the peel strength fully meets the requirements for paper-plastic products.

[0045] The metallocene catalyst used in the prepolymerization reaction

[0046] Example 1:

[0047] 1-Octene homopolymer was prepared in a prepolymer reactor using a homogeneous metallocene catalyst. The polymerization conditions were: a mass ratio of 1-octene to catalyst of 5000, a temperature of 120℃, a pressure of 10MPa, and an aluminum-titanium ratio of 2000.

[0048] The produced 1-octene homopolymer and the polymerization feedstock containing ethylene, tert-butyl peroxypentanoate, and propionaldehyde were simultaneously fed into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0049] The mass ratio of 1-octene homopolymer to ethylene is 1:150;

[0050] The mass ratio of propionaldehyde to ethylene is 1:200;

[0051] The mass ratio of tert-butyl peroxypentanoate to ethylene is 1:1000;

[0052] The polymerization temperature in all zones of the reactor is 300℃;

[0053] The polymerization pressure in all zones of the reactor is 200 MPa;

[0054] The final product of the reaction, measured with a 2.16 kg weight, had a melt flow rate of 1.1 g / 10 min and a density of 0.919 g / cm³. 3 .

[0055] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength was tested according to Method A of GB / T 8808 "Peel Strength Test Method for Flexible Composite Plastic Materials". The test result was 5.1 N / 15 mm.

[0056] Example 2:

[0057] 1-Heptene homopolymer was prepared in a prepolymer reactor using a homogeneous metallocene catalyst. The polymerization conditions were: a mass ratio of 1-heptene to catalyst of 3000, a temperature of 170℃, a pressure of 1MPa, and an aluminum-zirconium ratio of 1000.

[0058] The produced 1-heptene homopolymer, along with a polymerization feedstock containing ethylene, 2-ethylhexanoate peroxide, and butene, was simultaneously fed into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0059] The mass ratio of 1-hepten homopolymer to ethylene is 1:50.

[0060] The mass ratio of butene to ethylene is 1:1000;

[0061] The mass ratio of 2-ethylhexanoate peroxide to ethylene is 1:500.

[0062] The polymerization temperature in all zones of the reactor is 350℃;

[0063] The polymerization pressure in all zones of the reactor is 280 MPa;

[0064] The final product of the reaction, measured with a 2.16 kg weight, had a melt flow rate of 9.1 g / 10 min and a density of 0.924 g / cm³. 3 .

[0065] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength was tested according to Method A of GB / T 8808 "Peel Strength Test Method for Flexible Composite Plastic Materials". The test result was 6.3 N / 15 mm.

[0066] Example 3:

[0067] 1-propylene homopolymer was prepared in a prepolymer reactor using a homogeneous metallocene catalyst. The polymerization conditions were: a mass ratio of 1-propylene to catalyst of 10000, a temperature of 150℃, a pressure of 15MPa, and an aluminum-titanium ratio of 700.

[0068] The produced 1-propylene homopolymer, along with polymerization feedstock containing ethylene, tert-butyl peroxide-3,5,5-trimethylhexanoate, and diethyl ether, was simultaneously fed into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0069] The mass ratio of 1-propylene homopolymer to ethylene is 1:200;

[0070] The mass ratio of diethyl ether to ethylene is 1:700;

[0071] The mass ratio of tert-butyl peroxide-3,5,5-trimethylhexanoate to ethylene is 1:2000.

[0072] The polymerization temperature in all zones of the reactor is 310℃;

[0073] The polymerization pressure in each zone of the reactor is 150 MPa;

[0074] The final product of the reaction, measured with a 2.16 kg weight, had a melt flow rate of 6.8 g / 10 min and a density of 0.917 g / cm³. 3 .

[0075] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength of the product was tested according to Method A of GB / T 8808 "Peel Strength Test Method for Flexible Composite Plastic Materials". The test result was 5.3 N / 15 mm.

[0076] Example 4:

[0077] 1-Hexene homopolymer was prepared in a prepolymer reactor using a homogeneous metallocene catalyst. The polymerization conditions were: a mass ratio of 1-hexene to catalyst of 7000, a temperature of 100℃, a pressure of 45MPa, and an aluminum-titanium ratio of 1500.

[0078] The produced 1-hexene homopolymer, along with polymerization feedstock containing ethylene, tert-butyl peroxide (3,5,5-trimethylhexanoate), and propane, was simultaneously fed into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0079] The mass ratio of 1-hexene homopolymer to ethylene is 1:90;

[0080] The mass ratio of propane to ethylene is 1:900;

[0081] The mass ratio of tert-butyl peroxide-3,5,5-trimethylhexanoate to ethylene is 1:3000.

[0082] The polymerization temperature in all zones of the reactor is 330℃;

[0083] The polymerization pressure in all zones of the reactor is 240 MPa;

[0084] The final product of the reaction, measured with a 2.16 kg weight, had a melt flow rate of 2.6 g / 10 min and a density of 0.921 g / cm³. 3 .

[0085] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength was tested according to Method A of GB / T 8808 "Peel Strength Test Method for Flexible Composite Plastic Materials". The test result was 5.5 N / 15 mm.

[0086] Example 5:

[0087] 1-Hexene homopolymer was prepared in a prepolymer reactor using a homogeneous metallocene catalyst. The polymerization conditions were: a mass ratio of 1-decene to catalyst of 1000, a temperature of 200℃, a pressure of 15MPa, and an aluminum-titanium ratio of 300.

[0088] The produced 1-decene homopolymer, along with polymerization feedstock containing ethylene, the initiator tert-butyl peroxypentanoate, and ethanol, was simultaneously fed into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0089] The mass ratio of 1-decene homopolymer to ethylene is 1:75;

[0090] The mass ratio of ethanol to ethylene is 1:900;

[0091] The mass ratio of tert-butyl peroxypentanoate to ethylene is 1:1500.

[0092] The polymerization temperature in all zones of the reactor is 400℃;

[0093] The polymerization pressure in all zones of the reactor is 300 MPa;

[0094] The final product of the reaction, measured by a 2.16 kg weight, had a melt flow rate of 6.1 g / 10 min and a density of 0.925 g / cm³. 3 .

[0095] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength was tested according to Method A of GB / T 8808 "Peel Strength Test Method for Flexible Composite Plastic Materials". The test result was 5.2 N / 15 mm.

[0096] Comparative Example 1:

[0097] The difference from Example 1 is that the coating-grade low-density polyethylene resin is directly produced by ethylene, initiator and chain transfer agent entering a reactor operating under high-pressure polymerization conditions, without the participation of α-olefin homopolymer in the reaction.

[0098] Ethylene, tert-butyl peroxypentanoate, and propionaldehyde are simultaneously introduced into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0099] The mass ratio of propionaldehyde to ethylene is 1:200;

[0100] The mass ratio of tert-butyl peroxypentanoate to ethylene is 1:1000;

[0101] The polymerization temperature in all zones of the reactor is 300℃;

[0102] The polymerization pressure in all zones of the reactor is 200 MPa;

[0103] The final product of the reaction, measured by a 2.16 kg weight, had a melt flow rate of 1.0 g / 10 min and a density of 0.918 g / cm³. 3 .

[0104] The above product was processed into paper-plastic composite with coated paper to obtain the product. The peel strength was tested according to Method A of GB / T 8808 Flexible Composite Plastic Materials Peel Strength Test Method. The test result was 1.3 N / 15 mm.

[0105] Comparative Example 2:

[0106] The difference from Example 2 is that the low-density polyethylene resin of the coated product is directly obtained by ethylene, initiator and chain transfer agent entering a reactor operating under high pressure polymerization conditions, without the participation of α-olefin homopolymer in the reaction.

[0107] Ethylene, 2-ethylhexanoate peroxide, and butene are simultaneously fed into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0108] The mass ratio of butene to ethylene is 1:500;

[0109] The mass ratio of 2-ethylhexanoate peroxide to ethylene is 1:500.

[0110] The polymerization temperature in all zones of the reactor is 350℃;

[0111] The polymerization pressure in all zones of the reactor is 280 MPa;

[0112] The final product of the reaction, measured with a 2.16 kg weight, had a melt flow rate of 8.9 g / 10 min and a density of 0.923 g / cm³. 3 .

[0113] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength of the product was tested according to Method A of GB / T 8808 "Peel Strength Test Method for Flexible Composite Plastic Materials". The test result was 1.9 N / 15 mm.

[0114] Comparative Example 3:

[0115] The difference from Example 3 is that the low-density polyethylene resin of the coated product is directly obtained by ethylene, initiator and chain transfer agent entering a reactor operating under high pressure polymerization conditions, without the participation of α-olefin homopolymer in the reaction.

[0116] Ethylene, tert-butyl peroxide-3,5,5-trimethylhexanoate, and diethyl ether were simultaneously introduced into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0117] The mass ratio of diethyl ether to ethylene is 1:700;

[0118] The mass ratio of tert-butyl peroxide-3,5,5-trimethylhexanoate to ethylene is 1:2000.

[0119] The polymerization temperature in all zones of the reactor is 310℃;

[0120] The polymerization pressure in each zone of the reactor is 150 MPa;

[0121] The final product of the reaction, measured by a 2.16 kg weight, had a melt flow rate of 6.4 g / 10 min and a density of 0.915 g / cm³. 3 .

[0122] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength was tested according to Method A of GB / T 8808 "Peel Test Method for Flexible Composite Plastic Materials". The test result was 1.6 N / 15 mm.

[0123] Comparative Example 4:

[0124] The difference from Example 4 is that the low-density polyethylene resin of the coated product is directly obtained by ethylene, initiator and chain transfer agent entering a reactor operating under high pressure polymerization conditions, without the participation of α-olefin homopolymer in the reaction.

[0125] Ethylene, tert-butyl peroxide (3,5,5-trimethylhexanoate), and propane are simultaneously introduced into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0126] The mass ratio of propane to ethylene is 1:900;

[0127] The mass ratio of tert-butyl peroxide-3,5,5-trimethylhexanoate to ethylene is 1:3000.

[0128] The polymerization temperature in all zones of the reactor is 330℃;

[0129] The polymerization pressure in all zones of the reactor is 240 MPa;

[0130] The final product of the reaction, measured with a 2.16 kg weight, had a melt flow rate of 2.1 g / 10 min and a density of 0.919 g / cm³.3 .

[0131] The above product was then laminated with coated paper to obtain a paper-plastic composite product. The peel strength was tested according to Method A of GB / T 8808 "Peel Strength Test Method for Flexible Composite Plastic Materials". The test result was 1.7 N / 15 mm.

[0132] Comparative Example 5:

[0133] The difference from Example 5 is that the low-density polyethylene resin of the coated product is directly obtained by ethylene, initiator and chain transfer agent entering a reactor operating under high pressure polymerization conditions, without the participation of α-olefin homopolymer in the reaction.

[0134] Ethylene, the initiator tert-butyl peroxypentanoate, and ethanol are simultaneously introduced into a tubular reactor operating under high-pressure polymerization conditions, as follows:

[0135] The mass ratio of ethanol to ethylene is 1:900;

[0136] The mass ratio of tert-butyl peroxypentanoate to ethylene is 1:1500.

[0137] The polymerization temperature in all zones of the reactor is 400℃;

[0138] The polymerization pressure in all zones of the reactor is 300 MPa;

[0139] The final product of the reaction, measured by a 2.16 kg weight, had a melt flow rate of 3.8 g / 10 min and a density of 0.924 g / cm³. 3 .

[0140] The above product was processed into paper-plastic composite with coated paper to obtain the product. The peel strength was tested according to Method A of GB / T 8808 Flexible Composite Plastic Materials Peel Strength Test Method. The test result was 1.4 N / 15 mm.

[0141] The homogeneous metallocene catalyst used in the examples is the catalyst complex 1 prepared in Example 1 of the invention application filed on September 25, 2019, entitled "Restricted Geometric Configuration Metallocene Catalyst and Its Preparation Method and Application" (patent application number 201910909798.4, publication date March 26, 2021).

[0142] The final reaction products prepared in Examples 1-5 above were used to perform paper-plastic composite processing with coated paper to obtain finished products. After testing, the peel strength fully met the requirements for paper-plastic products. In contrast, the peel strength of the products in Comparative Examples 1-5 was only 25.5%-30.9% of that of the products of this invention after testing, which affected the bonding effect and performance of the finished products.

Claims

1. A coating-grade low-density polyethylene, wherein the raw material components and their proportions are as follows by mass: Prepolymerization reaction feedstock: α-olefin, metallocene catalyst; α-olefin: metallocene catalyst ratio is 1000-10000:1; Polymerization reaction raw materials: prepolymerization reaction products, ethylene, polymerization initiator, chain transfer agent; The ratio of prepolymerization product to ethylene is 1:10 to 1:500; Chain transfer agent: Ethylene at a ratio of 1:200 to 1:2000; Polymerization initiator: Ethylene ratio 1:500-1:5000; The metallocene catalyst is a homogeneous metallocene catalyst; The α-olefin is one of 1-propene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and 1-decene; The polymerization initiator is one or more of the peroxide ester RCOOOR'; the chain transfer agent is one of the following: olefin, aldehyde, ketone, alcohol, saturated hydrocarbon, ether; The final product, measured using a 2.16 kg weight, exhibited a melt flow rate of 1-10 g / 10 min and a density of 0.915-0.925 g / cm³. 3 ; The method for preparing the coating-grade low-density polyethylene includes the following steps: S1. First, α-olefin homopolymers are prepared in a prepolymer reactor using a metallocene catalyst; S2. Then, the produced α-olefin homopolymer, ethylene, initiator and chain transfer agent are simultaneously added to a reactor operating under high pressure polymerization conditions, and the final product is obtained after polymerization reaction. The temperature in the prepolymer reactor is 100-200℃; the pressure in the prepolymer reactor is 0.5-50MPa; the aluminum-titanium ratio is 200-2000. The polymerization temperature in each zone of the reactor is 300-400℃; the polymerization pressure in each zone of the reactor is 150-300MPa.

2. A method for preparing coating-grade low-density polyethylene according to claim 1, comprising the following steps: S1. First, α-olefin homopolymers are prepared in a prepolymer reactor using a metallocene catalyst; S2. Then, the produced α-olefin homopolymer, along with ethylene, initiator, and chain transfer agent, is simultaneously added to a reactor operating under high-pressure polymerization conditions. After polymerization, the final product is obtained.

3. The method for preparing coating-grade low-density polyethylene according to claim 2, characterized in that: The polymerization temperature in each zone of the reactor is 300-400℃; the polymerization pressure in each zone of the reactor is 150-300MPa.

4. The method for preparing coating-grade low-density polyethylene according to claim 2, characterized in that: The temperature in the prepolymer reactor is 100-200℃; the pressure in the prepolymer reactor is 0.5-50MPa; and the aluminum-titanium ratio is 200-2000.

5. The method for preparing coating-grade low-density polyethylene according to claim 2, characterized in that: The reactor operating under high-pressure polymerization conditions is a tubular reactor or a batch reactor, or a combination of both.

6. The method for preparing coating-grade low-density polyethylene according to claim 2, characterized in that: The prepared reaction product was an ethylene-based polymer; the melt flow rate of the final product, measured with a 2.16 kg weight, was 1-10 g / 10 min; the density of the final product was 0.915-0.925 g / cm³. 3 .

7. The application of coating-grade low-density polyethylene obtained by the preparation method according to claim 1 or any one of claims 2-6 in the field of coating.

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