Ultra-clean copolymerized modified high molecular weight polyethylene as well as preparation method and application thereof

Abstract

The invention provides ultra-clean copolymerized modified high-molecular-weight polyethylene, the viscosity-average molecular weight of which is 550,000-650,000, the melt index of which is 0.5-0.7 g / 10min and the ash content of which is less than or equal to 25ppm. The preparation method comprises the following steps: adding supercritical ethylene into a reactor, keeping the ethylene supercritical condition in the reactor, then adding a polar vinyl monomer, controlling the temperature in the reactor to be 60-120 DEG C and the pressure to be 5.5-15MPa, and reacting for 1-2 hours at the temperature of 60-120 DEG C, so as to obtain the ultra-clean copolymerized modified high-molecular-weight polyethylene. Adding part of an initiator and part of an NMP reagent, and carrying out a prepolymerization reaction; and continuously dropwise adding the rest initiator and NMP reagent after the pre-polymerization is finished, controlling the temperature in the reactor to rise by 20-30 DEG C, continuously reacting until the end, and carrying out post-treatment after the polymerization is finished. According to the invention, supercritical ethylene is taken as a solvent, a small amount of polar vinyl monomer is added as a comonomer, bulk polymerization is carried out under the dual action of an initiator and a free radical trapping agent, and the ultra-low ash content high molecular weight polyethylene is obtained and can be applied to the fields of medical treatment, battery diaphragms, high-end filter materials and the like.

Description

Technical Field

[0001] The invention relates to the technical field of polyethylene preparation, and in particular to an ultra-clean copolymer modified high molecular weight polyethylene, a preparation method and application thereof. Background Art

[0002] Ultra-clean high molecular weight polyethylene is a high-end polyolefin product with high technology content, high application performance and high market value. It is widely used in various high-value fields, including medical field, new energy battery separator materials, high-end filter materials, etc. The key to preparing ultra-clean high molecular weight polyethylene is to control the ash content, where the ash content is the sum of the content of inorganic compounds such as high molecular weight polyethylene and non-combustible metals in the catalyst. A higher ash content means lower product purity and poor cleanliness, which will have an adverse effect on the performance of the prepared terminal product. For example, in lithium battery separator materials, if the ash content in the raw materials is high, it is not conducive to the puncture strength of the battery separator. Therefore, high-performance battery separator products have high requirements for the cleanliness of high molecular weight polyethylene.

[0003] At present, high molecular weight polyethylene is mostly obtained by catalytic polymerization such as Ziegler-Natta catalyst, metallocene catalyst, late transition metal catalyst or chromium catalyst, etc., which all contain metal elements. The inevitable residual incombustible metal and other ash in the obtained product can only be reduced by reducing the amount of catalyst. For example, patent CN115340619A discloses low-ash ultra-high molecular weight polyethylene resin, which adopts Ziegler-Natta catalyst, by giving full play to the catalytic efficiency of the catalyst, improving the polymerization activity of the unit catalyst, achieving the reduction of the amount of catalyst and co-catalyst, and reducing the ash content of the product. However, reducing the amount of catalyst used will affect production efficiency and product polymerization rate, and still cannot fundamentally solve the problem of metal ash residue. In addition, in the polyethylene production process, its physical properties and application scenarios can be improved by adding comonomers. However, the copolymerization difficulty of ethylene and other high-carbon comonomers is high and the reaction conditions are harsh, and it is difficult to prepare ideal products with existing mature processes.

[0004] It is expected that suitable polyethylene products can be obtained through free radical polymerization, but the existing technology only realizes ultra-high pressure free radical reaction to obtain low molecular weight low density polyethylene. For example, patent JP-A-2012-60426 discloses a polyethylene film obtained by high pressure free radical polymerization, but it is difficult to obtain high molecular weight high density polyethylene. When high molecular weight polyethylene is generated by free radicals, the free radicals are prone to isomerization during the reaction process, resulting in difficulty in carbon chain growth. The free radical reaction conditions are harsh and quenching is prone to occur, making it difficult to obtain a high molecular weight polyethylene product. Therefore, how to prepare ultra-clean copolymer modified high molecular weight polyethylene still needs further research. Summary of the invention

[0005] The present invention aims to provide an ultra-clean copolymer-modified high molecular weight polyethylene, a preparation method of the ultra-clean copolymer-modified high molecular weight polyethylene, and its application in the fields of medical treatment, new energy battery diaphragm materials, high-end filter materials, etc., so as to solve the above-mentioned defects existing in the background technology.

[0006] The present invention is achieved by adopting the following technical solutions:

[0007] A method for preparing ultra-clean copolymer-modified high molecular weight polyethylene comprises the following steps:

[0008] S1, adding supercritical ethylene into the reactor, and maintaining the reactor at supercritical ethylene conditions;

[0009] S2, add polar vinyl monomer into the reactor and stir evenly;

[0010] S3, controlling the temperature in the reactor to 60-120° C. and the pressure to 5.5-15 MPa, adding part of the initiator and part of the NMP reagent to carry out a prepolymerization reaction;

[0011] S4. After the prepolymerization reaction is completed, the remaining initiator and NMP reagent are continuously added dropwise, the temperature in the reactor is controlled to rise by 20-30° C., and the reaction is continued until the end. After the polymerization is completed, the ultra-clean copolymer-modified high molecular weight polyethylene is obtained by post-treatment.

[0012] The formation condition of the supercritical ethylene is to keep the ethylene above the supercritical temperature and pressure, that is, the temperature is above 9.6°C and the pressure is above 5.07MPa. At the same time, the temperature in the reactor is kept above 9.6°C and the pressure is kept above 5.07MPa to keep the ethylene in a supercritical state as a solvent for free radical polymerization.

[0013] The polar vinyl monomer is a vinyl ester substance, selected from at least one of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, vinyl hexanoate, 2-methyl butyrate, vinyl 3-ethyl valerate, vinyl 3-ethyl hexanoate, vinyl pivalate, vinyl laurate, and vinyl acrylate.

[0014] The addition amount of the polar vinyl monomer is 1-30 g / 1 liter of supercritical ethylene, preferably 1-20 g / 1 liter of supercritical ethylene, and more preferably 5-15 g / 1 liter of supercritical ethylene.

[0015] The NMP reagent is an alkoxyamine substance selected from 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO), phthalimide N-oxyl free radical (PINO), N-phenyl-N-methylphenyloxy free radical (PBHA), N-isopropyl-N-methylphenyloxy free radical (IBHA), N-tert-butyl-N-methylphenyloxy free radical (TBBHA), 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy free radical (4-Hydroxy-TEMPO), 4-amino-2,2,6,6-tetramethylpiperidinyloxy free radical (4-Amino-TEMPO), etc.

[0016] The initiator may be an initiator commonly used in free radical polymerization, such as peroxide, bifunctional peroxide, azo compound initiator, etc. For example, it may be selected from dibenzoyl peroxide, diisopropylbenzene peroxide, di-tert-butyl peroxide, tert-butyl peroxide isopropylbenzene, tert-butyl peroxy-2-ethylhexanoate, azobisisobutylonitrile, 2,2'-azobisisobutylonitrile, etc.

[0017] Ethylene is a low-activity monomer. Since the growing free radicals in the ethylene polymerization process have no conjugated structure and no other stable groups, the growing free radicals are unstable, which easily leads to poor reactivity of dormant intermediates in the free radical polymerization process. It is difficult to obtain high molecular weight polymers through traditional free radical polymerization. The inventors found that by adding a small amount of polar vinyl monomer as a comonomer in supercritical ethylene, adding initiator and NMP reagent for free radical polymerization, they unexpectedly obtained ultra-clean high molecular weight polyethylene with extremely low ash content and excellent mechanical properties. The specific mechanism is not very clear yet, which may be due to the following reasons:

[0018] 1. Using supercritical ethylene as solvent and utilizing the supercritical special fluid state to carry out bulk polymerization under the action of free radicals helps to improve mass transfer and heat transfer efficiency, increase the activity of ethylene free radical polymerization, accelerate the reaction process, and carry out the reaction under milder conditions.

[0019] 2. By adding a small amount of polar vinyl monomer as a comonomer, under the action of the initiator, the polar vinyl monomer forms a stable free radical with a conjugated structure, which is conducive to promoting chain growth and forming a high molecular weight polymer. In addition, the introduction of polar vinyl monomer increases the polarity of polyethylene, improves the surface energy, makes the polyethylene surface more susceptible to polar substances, and improves the hydrophilicity of polyethylene. In addition, polar vinyl monomer has high thermal stability. When copolymerized with ethylene, it improves the heat resistance of the product and makes it more stable in high temperature environments. The introduction of polar vinyl monomer also improves the mechanical properties of polyethylene, including strength, toughness and wear resistance.

[0020] 3. Using NMP reagent as a free radical scavenger, after the initiator generates primary free radicals, the polymerization reaction generates growth free radicals. The NMP reagent combines with the growth free radicals to make the reaction initiated by the initiator suspended in the middle, thereby controlling the number of growth free radicals and preventing the growth free radicals from being quenched. As the temperature rises, the bonds of the suspended body break, releasing growth free radicals, initiating polymerization again, and growing chain segments, thereby achieving the purpose of increasing the molecular weight of the polymer.

[0021] Based on the above principle, the amount of initiator and NMP reagent added can be reasonably adjusted to control the appropriate number of free radicals, thereby obtaining an ideal polymer molecular weight. The preferred amount of initiator added in the present invention is 0.5-15wt% of the amount of polar vinyl monomer added, more preferably 2-11wt%. The preferred amount of NMP reagent added is 0.5-12wt% of the amount of polar vinyl monomer added, more preferably 1-8wt%.

[0022] As a preferred technical solution, the initiator and NMP reagent are added in the form of ethylene diluent. Specifically, the initiator or NMP reagent is dispersed in supercritical ethylene or liquid ethylene to prepare an initiator diluent or an NMP diluent. The concentration of the initiator diluent is 0.01-5wt%, preferably 0.1-2wt%. The concentration of the NMP diluent is 0.01-5wt%, preferably 0.1-2wt%. Of course, the initiator and NMP reagent can also be dispersed in supercritical ethylene or liquid ethylene according to the above concentrations to prepare an initiator / NMP diluent.

[0023] The initiator added during the prepolymerization reaction in step S3 accounts for 10-40wt% of the total amount of initiator, and more preferably 20-30wt%; the NMP reagent added accounts for 10-40wt% of the total amount of NMP reagent, and preferably 20-30wt%.

[0024] Preferably, ethylene and polar vinyl monomers are first purified before use, specifically, by removing oxygen, carbon monoxide, water and carbon dioxide. Commercially available purified products can be used directly, or conventional methods in the art can be used to purify ethylene and polar vinyl monomers.

[0025] Preferably, in step S3, the prepolymerization temperature is 70-100° C., more preferably 75-85° C.; the pressure is 6-10 MPa, more preferably 7.5-9 MPa; and the prepolymerization reaction time is 1-2 hours.

[0026] Preferably, the polymerization reaction time of step S4 is 3-4 hours.

[0027] Furthermore, the reactor is a stirred reactor with a heat exchange jacket, and the heat exchange jacket is preferably an inner jacket. This design can remove the reaction heat in time, which is conducive to the preparation of polyethylene products.

[0028] Further preferably, a circulating gas outlet is provided at the top of the reactor, and when the pressure in the reactor exceeds a set value, the gas is discharged from the circulating gas outlet.

[0029] Furthermore, the post-treatment described in step S4 includes the following steps: separating the product after the reaction into gas and solid by decompression operation in a flash tank, sending the separated gas phase to a recovery unit, and the separated solid phase polymer entering a degassing and drying unit. The dried polymer powder is sieved and homogenized to obtain the ultra-clean copolymer-modified high molecular weight polyethylene.

[0030] The ultra-clean copolymer modified high molecular weight polyethylene prepared by the present invention has a viscosity average molecular weight of 550,000-650,000, a melt index of 0.5-0.7 g / 10 min, and an ash content of ≤25 ppm.

[0031] The ultra-clean copolymer modified high molecular weight polyethylene of the present invention has extremely low ash content and can be applied to the fields of medical treatment, battery separators, high-end filter materials, etc. For example, it can be applied to the medical field, such as the manufacture of medical devices and implants such as joint replacement and artificial cartilage, with higher safety and reliability. When used in battery separators, it helps to enhance the ionic conductivity of the separator material, improve battery performance, improve the puncture strength of the separator, and improve the thermal stability of the separator material, so that it has better stability and durability in high temperature environments.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention uses supercritical ethylene as a solvent, utilizes a supercritical special fluid state, and performs bulk polymerization under the action of free radicals, which helps to improve mass transfer and heat transfer efficiency, improve ethylene free radical polymerization activity, accelerate the reaction process, and react under milder conditions. The present invention adds a small amount of polar vinyl monomer as a comonomer, and under the action of an initiator, the polar vinyl monomer forms a stable free radical with a conjugated structure, which is conducive to promoting chain growth and forming a high molecular weight polymer. In addition, the introduction of the polar vinyl monomer also enhances the hydrophilicity, thermal stability, mechanical properties, etc. of the product. The present invention uses NMP reagent as a free radical trapping agent, and by utilizing the means of nitrogen oxide stable free radical polymerization, the amount of small molecule free radicals can be better controlled and not terminated by diradicals, thereby continuously allowing the chain segment to be lengthened to obtain a high-performance high molecular weight polyethylene copolymer. DETAILED DESCRIPTION

[0034] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only used as examples, but are not used to limit the scope of the present invention. Those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variations, improvements, equivalent solutions, and other technical solutions that do not deviate from the spirit and scope of the present invention.

[0035] The instruments or reagents in the embodiments of the present invention that do not indicate the manufacturer are all conventional commercial instruments or reagents. A 2L pressure-resistant stirred reactor is used in the embodiment, which is made of stainless steel corrosion-resistant material. The reactor is purged and protected by nitrogen to ensure that the reaction is carried out under anhydrous and oxygen-free conditions. A circulating gas outlet is provided on the top of the stirred reactor. When the pressure in the reactor exceeds the set pressure, the gas will be discharged and collected from the circulating gas outlet. The stirred reactor is provided with a heat exchange jacket, and a cooling medium (such as cooling water) is introduced during polymerization to remove the reaction heat in time.

[0036] Example 1

[0037] (1) Add 1 L of supercritical ethylene into a stirred reactor as the bottom liquid, and keep the reactor above the supercritical condition of ethylene (temperature of 25° C., pressure of 5.4 MPa);

[0038] (2) in a preparation tank, the initiator and the NMP reagent are uniformly mixed with liquid ethylene to form an initiator / NMP reagent dilution solution for standby use, wherein the concentration of the initiator in the ethylene dilution solution is 0.5 wt %, and the concentration of the NMP reagent in the ethylene dilution solution is 0.2 wt %, wherein the initiator is azobisisobutyronitrile and the NMP reagent is TEMPO;

[0039] (3) The reactor is kept in a stirring state (rotation speed 250 rpm), and 11 g of polar vinyl monomer is added into the stirred reactor and stirred evenly, wherein the polar vinyl monomer is vinyl acetate;

[0040] (4) controlling the temperature in the reactor to 78° C. and the pressure to 6.6 MPa, adding 25 ml of the initiator / NMP reagent diluent to carry out a prepolymerization reaction for 1 hour;

[0041] (5) After the prepolymerization reaction is completed, the initiator / NMP reagent dilution solution is continuously added dropwise within 3 hours, the total amount of the initiator / NMP reagent dilution solution added is 75 ml, and the temperature in the reactor is controlled to be 100° C. After the reaction is completed, the reaction slurry is subjected to gas-solid separation by decompression operation in a flash tank, and the separated solid polymer is degassed and dried. The dried polymer powder is sieved and homogenized to obtain a high molecular weight polyethylene sample E1.

[0042] Example 2

[0043] (1) Add 1 L of supercritical ethylene into a stirred reactor as the bottom liquid, and keep the reactor above the supercritical condition of ethylene (temperature of 25° C., pressure of 5.4 MPa);

[0044] (2) In a preparation tank, the initiator and the NMP reagent are uniformly mixed with liquid ethylene to form an initiator / NMP reagent dilution solution for standby use, wherein the concentration of the initiator in the ethylene dilution solution is 1 wt %, and the concentration of the NMP reagent in the ethylene dilution solution is 0.5 wt %, wherein the initiator is benzoyl oxide and the NMP reagent is PI NO;

[0045] (3) The reactor is kept in a stirring state (rotation speed 350 rpm), and 12 g of polar vinyl monomer is added into the stirred reactor and stirred evenly, wherein the polar vinyl monomer is vinyl propionate;

[0046] (4) controlling the temperature in the reactor to 80° C. and the pressure to 6.8 MPa, adding 30 ml of the initiator / NMP reagent dilution solution to carry out a prepolymerization reaction for 2 hours;

[0047] (5) After the prepolymerization reaction, the initiator / NMP reagent dilution solution was continuously added dropwise within 4 hours, the total amount of the initiator / NMP reagent dilution solution added was 70 ml, and the temperature in the reactor was controlled to be 105°C. After the reaction was completed, the reaction slurry was subjected to gas-solid separation by decompression operation in a flash tank, and the separated solid polymer was degassed and dried. The dried polymer powder was sieved and homogenized to obtain the high molecular weight polyethylene sample E2.

[0048] Example 3

[0049] (1) Add 1 L of supercritical ethylene into a stirred reactor as the bottom liquid, and keep the reactor above the supercritical condition of ethylene (temperature of 25° C., pressure of 5.4 MPa);

[0050] (2) in a preparation tank, the initiator and the NMP reagent are uniformly mixed with liquid ethylene to form an initiator / NMP reagent dilution solution for standby use, wherein the concentration of the initiator in the ethylene dilution solution is 0.65 wt %, and the concentration of the NMP reagent in the ethylene dilution solution is 0.36 wt %, wherein the initiator is azobisisobutyronitrile and the NMP reagent is TEMPO;

[0051] (3) The reactor is kept in a stirring state (rotation speed 350 rpm), and 13.6 g of polar vinyl monomer is added into the stirred reactor and stirred evenly, wherein the polar vinyl monomer is vinyl acetate;

[0052] (4) controlling the temperature in the reactor to 81° C. and the pressure to 8.9 MPa, adding 28 ml of the initiator / NMP reagent dilution solution to carry out a prepolymerization reaction for 2 hours;

[0053] (5) After the prepolymerization reaction, the initiator / NMP reagent dilution solution was continuously added dropwise within 4 hours, the total amount of the initiator / NMP reagent dilution solution added was 80 ml, and the temperature in the reactor was controlled to be 112° C. After the reaction was completed, the reaction slurry was subjected to gas-solid separation by decompression operation in a flash tank, and the separated solid polymer was degassed and dried. The dried polymer powder was sieved and homogenized to obtain a high molecular weight polyethylene sample E3.

[0054] Example 4

[0055] (1) Add 1 L of supercritical ethylene into a stirred reactor as the bottom liquid, and keep the reactor above the supercritical condition of ethylene (temperature of 25° C., pressure of 5.4 MPa);

[0056] (2) in a preparation tank, the initiator and the NMP reagent are uniformly mixed with liquid ethylene to form an initiator / NMP reagent dilution solution for standby use, wherein the concentration of the initiator in the ethylene dilution solution is 1.5 wt %, and the concentration of the NMP reagent in the ethylene dilution solution is 1 wt %, wherein the initiator is di-tert-butyl peroxide and the NMP reagent is TBBHA;

[0057] (3) The reactor is kept in a stirring state (rotation speed 350 rpm), and 15 g of polar vinyl monomer is added into the stirred reactor and stirred evenly, wherein the polar vinyl monomer is 3-ethyl valerate;

[0058] (4) controlling the temperature in the reactor to 80° C. and the pressure to 7.9 MPa, adding 26 ml of the initiator / NMP reagent dilution solution to carry out a prepolymerization reaction for 1.5 hours;

[0059] (5) After the prepolymerization reaction, the initiator / NMP reagent dilution solution was continuously added dropwise within 3.5 hours, the total amount of the initiator / NMP reagent dilution solution added was 75 ml, and the temperature in the reactor was controlled to be 108° C. After the reaction was completed, the reaction slurry was subjected to gas-solid separation by decompression operation in a flash tank, and the separated solid polymer was degassed and dried. The dried polymer powder was sieved and homogenized to obtain a high molecular weight polyethylene sample E4.

[0060] Example 5

[0061] (1) Add 1 L of supercritical ethylene into a stirred reactor as the bottom liquid, and keep the reactor above the supercritical condition of ethylene (temperature of 25° C., pressure of 5.4 MPa);

[0062] (2) in a preparation tank, the initiator and the NMP reagent are uniformly mixed with liquid ethylene to form an initiator / NMP reagent dilution solution for standby use, wherein the concentration of the initiator in the ethylene dilution solution is 2 wt %, and the concentration of the NMP reagent in the ethylene dilution solution is 1.5 wt %, wherein the initiator is 2,2'-azobisisobutyronitrile and the NMP reagent is PBHA;

[0063] (3) The reactor is kept in a stirring state (rotation speed 450 rpm), and 10 g of polar vinyl monomer is added into the stirred reactor and stirred evenly, wherein the polar vinyl monomer is vinyl valerate;

[0064] (4) controlling the temperature in the reactor to 75° C. and the pressure to 6.1 MPa, adding 20 ml of the initiator / NMP reagent diluent to carry out a prepolymerization reaction for 2 hours;

[0065] (5) After the prepolymerization reaction, the initiator / NMP reagent dilution solution was continuously added dropwise within 2.5 hours, the total amount of the initiator / NMP reagent dilution solution added was 70 ml, and the temperature in the reactor was controlled to be 98° C. After the reaction was completed, the reaction slurry was subjected to gas-solid separation by decompression operation in a flash tank, and the separated solid polymer was degassed and dried. The dried polymer powder was sieved and homogenized to obtain the high molecular weight polyethylene sample E5.

[0066] Example 6

[0067] (1) Add 1 L of supercritical ethylene into a stirred reactor as the bottom liquid, and keep the reactor above the supercritical condition of ethylene (temperature of 25° C., pressure of 5.4 MPa);

[0068] (2) in a preparation tank, the initiator and the NMP reagent are uniformly mixed with liquid ethylene to form an initiator / NMP reagent dilution solution for standby use, wherein the concentration of the initiator in the ethylene dilution solution is 1.2 wt %, and the concentration of the NMP reagent in the ethylene dilution solution is 1 wt %, wherein the initiator is tert-butyl peroxy-2-ethylhexanoate, and the NMP reagent is PINO;

[0069] (3) The reactor is kept in a stirring state (rotation speed 280 rpm), and 14.4 g of polar vinyl monomer is added into the stirred reactor and stirred evenly, wherein the polar vinyl monomer is vinyl acrylate;

[0070] (4) controlling the temperature in the reactor to 78° C. and the pressure to 8.1 MPa, adding 25 ml of the initiator / NMP reagent diluent to carry out a prepolymerization reaction for 1 hour;

[0071] (5) After the prepolymerization reaction, the initiator / NMP reagent dilution solution was continuously added dropwise within 3 hours, the total amount of the initiator / NMP reagent dilution solution added was 75 ml, and the temperature in the reactor was controlled to be 96°C. After the reaction was completed, the reaction slurry was subjected to gas-solid separation by decompression operation in a flash tank, and the separated solid polymer was degassed and dried. The dried polymer powder was sieved and homogenized to obtain the high molecular weight polyethylene sample E6.

[0072] Comparative Example 1

[0073] Compared with Example 3, the solvent was replaced with hexane, ethylene was fed in gaseous form, and the remaining steps and process conditions were basically the same as those in Example 1. The specific preparation process is as follows:

[0074] (1) Add about 1 L of hexane into a stirred reactor as the bottom liquid;

[0075] (2) in a preparation tank, the initiator and the NMP reagent are mixed evenly with hexane to form an initiator / NMP reagent dilution solution for standby use, wherein the concentration of the initiator in the hexane dilution solution is 0.65 wt %, and the concentration of the NMP reagent in the hexane dilution solution is 0.36 wt %, wherein the initiator is azobisisobutyronitrile and the NMP reagent is TEMPO;

[0076] (3) The reactor was kept in a stirring state (rotation speed 350 rpm), and 266 g of ethylene and 13.6 g of polar vinyl monomer vinyl acetate were added to the bottom liquid in the stirred reactor;

[0077] (4) controlling the temperature in the reactor to 81° C. and the pressure to 8.9 MPa, adding 28 ml of the initiator / NMP reagent dilution solution to carry out a prepolymerization reaction for 2 hours;

[0078] (5) After the prepolymerization reaction, the initiator / NMP reagent dilution solution was continuously added dropwise within 4 hours, the total amount of the initiator / NMP reagent dilution solution added was 80 ml, and the temperature in the reactor was controlled to be 112° C. After the reaction was completed, the reaction slurry was separated by a flash tank decompression operation, and the separated solid polymer was degassed and dried. The dried polymer powder was sieved and homogenized to obtain a high molecular weight polyethylene sample D1.

[0079] Comparative Example 2

[0080] Compared with Example 3, in step (2), no NMP reagent is added, and only a single initiator is used. The remaining steps are the same as those in Example 3, and a high molecular weight polyethylene sample D2 is obtained.

[0081] Comparative Example 3

[0082] Compared with Example 3, no comonomer is added in step (3), and the remaining steps are the same as Example 3 to obtain a high molecular weight polyethylene sample D3.

[0083] Comparative Example 4

[0084] The commercially available product of high molecular weight polyethylene is LPF-8008V, produced by Pu Enjing New Energy Materials (Shanghai) Co., Ltd.

[0085] The molecular weight, melt index and ash content of the above-mentioned high molecular weight polyethylenes E1-E6 and high molecular weight polyethylenes D1-D3 were measured according to the following standards. The specific results are shown in Table 1.

[0086] 1. Molecular weight determination

[0087] The viscosity average molecular weight is measured according to standard GB 1632.3-2010 (α=1.49).

[0088] 2. Melt index determination

[0089] The melt index is tested according to standard GB / T 3682.1-2018 (test conditions 190°C / 21.6kg).

[0090] 3. Ash content determination

[0091] The ash content is tested according to standard GB / T 9345.1 / 2008.

[0092] Table 1 Test results

[0093]

[0094]

[0095] Note: “--” means not tested; “ND” means not detected.

[0096] As shown in Table 1, the viscosity average molecular weight of the products obtained in Examples 1-6 of the present invention is 600,000 on average, which belongs to high molecular weight polyethylene and can be applied to 12-25 μm lithium ion battery separators. Compared with polyethylene products of the same molecular weight specification (Comparative Example 4), the products have a high melt index, good processing performance, and extremely low ash content, and belong to ultra-clean polyethylene products. When used in the production of lithium ion battery separators, the puncture strength of the separator can be improved. However, whether it is a sample using conventional solvents and conventional ethylene feeding methods, without adding NMP reagents, and without adding polar vinyl monomers (Comparative Examples 1-3), high molecular weight (molecular weight of about 600,000) polyethylene products cannot be obtained, and the mechanical properties of low molecular weight polyethylene cannot meet the use requirements.

[0097] It should be understood by those skilled in the art that the above embodiments are only examples and do not limit the present invention. The purpose of the present invention has been fully and effectively achieved. The functions and principles of the present invention have been demonstrated and explained in the embodiments, and the embodiments of the present invention may be modified or altered in any way without departing from the principles.

Claims

1. A method for preparing ultra-clean copolymer-modified high molecular weight polyethylene, characterized in that: The steps include: S1, adding supercritical ethylene into the reactor, and maintaining the reactor at supercritical ethylene conditions; S2, add polar vinyl monomer into the reactor and stir evenly; S3, controlling the temperature in the reactor to 60-120° C. and the pressure to 5.5-15 MPa, adding part of the initiator and part of the NMP reagent to carry out a prepolymerization reaction; S4. After the prepolymerization reaction is completed, the remaining initiator and NMP reagent are continuously added dropwise, the temperature in the reactor is controlled to rise by 20-30° C., and the reaction is continued until the end. After the polymerization is completed, the ultra-clean copolymer-modified high molecular weight polyethylene is obtained by post-treatment.

2. The method according to claim 1, characterized in that The polar vinyl monomer is a vinyl ester substance, selected from at least one of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, vinyl hexanoate, 2-methyl butyrate, vinyl 3-ethyl valerate, vinyl 3-ethyl hexanoate, vinyl pivalate, vinyl laurate, and vinyl acrylate.

3. The method according to claim 1, characterized in that The polar vinyl monomer is added in an amount of 1-30 g / 1 liter of supercritical ethylene.

4. The method according to claim 1, characterized in that The NMP reagent is an alkoxyamine substance, selected from at least one of 2,2,6,6-tetramethyl-1-piperidinyloxy free radical, phthalimide N-oxyl free radical, N-phenyl-N-methylphenyloxy free radical, N-isopropyl-N-methylphenyloxy free radical, N-tert-butyl-N-methylphenyloxy free radical, 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy free radical, and 4-amino-2,2,6,6-tetramethylpiperidinyloxy free radical; the initiator is selected from at least one of dibenzoyl peroxide, diisopropyl peroxide, di-tert-butyl peroxide, tert-butyl peroxide isopropyl benzene, tert-butyl peroxy-2-ethylhexanoate, azobisisobutyronitrile, and 2,2'-azobisisobutyronitrile.

5. The method according to claim 1, characterized in that The amount of the initiator added is 0.5-15wt% of the amount of the polar vinyl monomer added, and the amount of the NMP reagent added is 0.5-12wt% of the amount of the polar vinyl monomer added.

6. The method according to claim 1, characterized in that The initiator and NMP reagent are added in the form of ethylene diluent.

7. The method according to claim 1, characterized in that In the prepolymerization reaction in step S3, the initiator added accounts for 10-40wt% of the total amount of the initiator, and the NMP reagent added accounts for 10-40wt% of the total amount of the NMP reagent.

8. The method according to claim 1, characterized in that The reactor is a stirred reactor with a heat exchange jacket, and a circulating gas outlet is provided on the top of the reactor. When the pressure in the reactor exceeds the set value, the gas is discharged from the circulating gas outlet; the post-treatment described in step S4 includes the following steps: the product after the reaction is subjected to gas-solid separation by decompression operation of a flash tank, the separated gas phase is sent to a recovery unit, the separated solid phase polymer enters a degassing and drying unit, and the dried polymer powder is sieved and homogenized to obtain the ultra-clean copolymer-modified high molecular weight polyethylene.

9. The ultra-clean copolymer-modified high molecular weight polyethylene prepared by the method according to any one of claims 1 to 8, characterized in that: The ultra-clean copolymer modified high molecular weight polyethylene has a viscosity average molecular weight of 550,000-650,000, a melt index of 0.5-0.7 g / 10 min, and an ash content of ≤25 ppm.

10. Application of the ultra-clean copolymer modified high molecular weight polyethylene according to claim 9 in medical devices, battery separators, and high-end filter materials.

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