Preparation method of EVOH

Abstract

This invention discloses a method for preparing EVOH, relating to the field of EVOH technology. This invention innovatively employs a low-temperature (0-5℃) methanol-n-hexane mixed solvent system, combined with a penetration enhancer to achieve synergistic extraction, ultimately realizing the simultaneous and deep removal of polar and non-polar small molecule impurities from EVA. This significantly reduces the impurity content in EVA and effectively inhibits the self-polymerization reaction of residual monomers, thus providing a crucial guarantee for the subsequent preparation of high-quality EVOH products. Films processed from EVOH prepared by this invention are characterized by a low number of crystal points and high product quality, enabling applications in the field of high-barrier packaging film materials.

Description

Technical Field

[0001] This invention relates to the field of EVOH technology, and more specifically to a method for preparing EVOH. Background Technology

[0002] Ethylene-vinyl alcohol copolymer (EVOH) is an important material in modern polymer industry. It is mainly produced by the copolymerization reaction of ethylene and vinyl acetate to obtain ethylene-vinyl acetate copolymer (EVA), and EVOH can be obtained from EVA through alcoholysis. Due to its excellent gas barrier properties, oil resistance, and transparency, EVOH is widely used in high-barrier packaging materials, automotive fuel tanks, and medical materials.

[0003] In traditional depressurized flash evaporation processes, due to drastic temperature fluctuations, residual vinyl acetate (VAc) is prone to self-polymerization. Simultaneously, this process results in a higher concentration of small-molecule impurities in the EVA, with the generated polyvinyl acetate mixing into the EVA. This polyvinyl acetate then undergoes alcoholysis to convert to polyvinyl alcohol (PVA), leading to numerous crystal points in the final EVOH film. Crystal points refer to locally visible optical inhomogeneities on the film surface, a type of defect. To reduce the PVA content and small-molecule impurities in EVOH, multiple washing and purification processes are typically required, but this not only increases production costs but also significantly reduces overall production efficiency.

[0004] Chinese invention patent CN115991801B discloses a high-performance EVOH and its preparation method. It employs azeotropic treatment to remove vinyl acetate, and filters the polymerization and alcoholysis solutions separately using a polymerization filter and an alcoholysis filter, significantly reducing the number of defects in the EVOH membrane and improving product quality. However, this patent suffers from drawbacks such as high equipment cost, complex filtration system, and high maintenance cost. Chinese invention patent CN112694639A discloses a method and apparatus for purifying the polymerization solution during EVOH generation. It achieves efficient removal of ethylene and vinyl acetate by pressurizing the polymerization solution with a flash tank and pressurizing it with a pump, then contacting the polymer solution with gaseous methanol in a demonolysis tower, and collecting the dilute vinyl acetate from the methanol at the top of the tower. However, this method suffers from drawbacks such as a complex separation system and high separation temperature.

[0005] Therefore, developing a method for preparing EVOH that can efficiently reduce the impurity content in EVA, inhibit the self-polymerization of residual monomers, and is cost-effective and simple has become a key issue that the industry urgently needs to address. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for preparing EVOH. By washing and separating the residual vinyl acetate and non-polar low molecular weight polymer dissolved in EVA under low temperature conditions, the polar and non-polar small molecule impurities inside EVA are removed, which realizes the separation of impurity content in EVA, significantly reduces the impurity content in EVA, effectively inhibits the self-polymerization reaction of residual monomers, and improves the quality of the final EVOH product.

[0007] The technical solution of this invention is as follows:

[0008] The preparation method of EVOH includes the following steps:

[0009] S1: Methanol, n-hexane and initiator are purified, and vinyl acetate is depolymerized and then used for later use;

[0010] S2: A mixture of methanol and n-hexane is placed in a cryogenic reactor for later use. A penetration enhancer is added to the methanol and n-hexane mixture. The temperature of the cryogenic reactor is 0-5℃. The mass ratio of methanol to n-hexane is 5:(1-2). The penetration enhancer is polyoxyethylene lauryl ether, lauryl ether phosphate, or isomeric decayl alcohol polyoxyethylene ether. The amount of penetration enhancer added is 0.05-0.1 wt.% of the methanol and n-hexane mixture.

[0011] S3: Under a nitrogen atmosphere, add solvent, initiator and vinyl acetate to the reactor, introduce ethylene, and heat to 65-70℃ to carry out the polymerization reaction;

[0012] S4: After the reaction is complete, add a terminator to stop the reaction. After purging the ethylene with nitrogen, transfer the resulting polymer solution to the low-temperature reactor of step S2 and stir continuously to obtain a mixture. The terminator is tert-butylcatechol or copper acetate.

[0013] S5: After centrifugation, the EVA is separated from the methanol-n-hexane mixed solvent containing impurities such as vinyl acetate monomer and low molecular weight polymers to obtain crude ethylene-vinyl acetate copolymer.

[0014] S6: The crude ethylene-vinyl acetate copolymer is vacuum dried at 30-45℃ to further reduce solvent and impurity residues. The resulting ethylene-vinyl acetate copolymer is then alcoholyzed to obtain EVOH.

[0015] Preferably, in step S1, the purification treatment of methanol and n-hexane is as follows: methanol and n-hexane are respectively subjected to vacuum distillation to remove residual impurities; the purification treatment of initiator is as follows: the initiator is crystallized to remove impurities; the method for removing polymerization inhibitor from vinyl acetate is as follows: vinyl acetate is subjected to vacuum distillation to remove polymerization inhibitor.

[0016] Preferably, in step S3, the ethylene pressure is 1-3 MPa.

[0017] Preferably, in step S3, the solvent is methanol; the initiator is azobisisobutyronitrile, dimethyl azobisisobutyrate, or tert-butyl peroxide.

[0018] Preferably, in step S3, the mass-volume ratio of solvent, initiator and vinyl acetate is (2-5) mL:(0.002-0.005) g:10 g, and the mass ratio of vinyl acetate to the methanol and n-hexane mixed solvent in step S2 is 1:(3-5).

[0019] Preferably, in step S4, the amount of the terminator added is 300-800 ppm of the mass of the ethylene-vinyl acetate copolymer in the system.

[0020] Preferably, in step S4, the stirring time is 1-2 hours.

[0021] Preferably, in step S6, the vacuum degree of the vacuum drying process is 0.09-0.1 MPa, and the time is 3-5 hours.

[0022] Preferably, in step S6, the obtained EVOH is washed with deionized water and vacuum dried to obtain EVOH powder, which is then fed into an extrusion device, pelletized, and washed to obtain the final EVOH.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. Efficient removal of impurities from EVA and improved EVOH purity: This invention innovatively employs a low-temperature (0-5℃) methanol-n-hexane mixed solvent system, combined with a penetration enhancer to achieve synergistic extraction. The mixed solvent possesses complementary polar and non-polar properties. The hydrophilic and lipophilic structure of the penetration enhancer reduces interfacial tension, penetrates deep into the EVA phase, and significantly increases the contact area between the two phases. Hexane can penetrate into the amorphous region of EVA, interacting with ethylene segments, increasing the gaps between polymer chains and maintaining an "open" structural state. Methanol efficiently removes residual vinyl acetate and non-polar low-molecular-weight polymers dissolved in EVA, ultimately achieving simultaneous and deep removal of polar and non-polar small-molecule impurities from within EVA, significantly improving the purity of the subsequent EVOH product.

[0025] 2. Inhibiting residual monomer self-polymerization and reducing crystal point formation: The low-temperature environment of the cryogenic reactor (0-5℃) can precisely inhibit the self-polymerization reaction of residual vinyl acetate during the transfer of the polymerization solution, avoiding the formation of additional polyvinyl acetate. Based on the data from the examples, the PVA content in the EVOH product prepared by this invention can be controlled at 50-65 ppm, per 1m³. 2The EVOH membrane sample has only 6-7 crystal points, which effectively solves the core defect of excessive crystal points in EVOH membranes in traditional processes.

[0026] 3. Simplified process flow, balancing cost and efficiency: This invention uses a combination of "low-temperature mixed solvent extraction + centrifugal separation" to achieve efficient purification of EVA without the need for multiple washing and purification processes in traditional processes. This not only simplifies the production process but also reduces energy consumption and production costs. At the same time, it shortens the production cycle, significantly improves production efficiency, and is more suitable for the needs of industrial-scale production. Attached Figure Description

[0027] Figure 1 It is the EVOH prepared in Example 1 of this invention. 1 H-NMR spectrum. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention.

[0029] Example 1

[0030] The preparation method of EVOH in this embodiment includes the following steps:

[0031] S1: Methanol and n-hexane were separately subjected to vacuum distillation to remove residual impurities. The impurity content of methanol after treatment was 50 ppm, and the impurity content of n-hexane was 39 ppm. Azobisisobutyronitrile was dissolved in ethanol at 65°C, filtered while hot, and the filtrate was cooled and filtered under vacuum to obtain purified azobisisobutyronitrile with a purity of 99.9%. Vinyl acetate was subjected to vacuum distillation to remove polymerization inhibitors.

[0032] S2: Add 1000g methanol and 300g n-hexane to a low-temperature reactor (3℃), and add 0.8g polyoxyethylene lauryl ether.

[0033] S3: Check the airtightness of the reactor, purge the air with nitrogen (flow rate of 50 mL / min) for 30 min to ensure that the oxygen content in the reactor and pipeline is <10 ppm; add 100 mL of methanol, 400 g of vinyl acetate and 0.15 g of azobisisobutyronitrile to the reactor, purge with ethylene, maintain the system pressure at 3 MPa during the reaction, start stirring at 300 rpm, raise the temperature to 70℃ and maintain the temperature for polymerization reaction.

[0034] S4: After the reaction is complete, add 500 ppm (based on EVA mass) of tert-butylcatechol to the reactor to stop the reaction. Purge the reactor with nitrogen gas (flow rate of 50 mL / min) and open the vent valve (flow rate of 60 mL / min). After 5 min, transfer the obtained polymer solution to a cryogenic reactor and stir for 2 h to obtain a mixture.

[0035] S5: The mixture is then centrifuged to separate EVA from the methanol-n-hexane mixed solvent containing impurities such as vinyl acetate monomer and low molecular weight polymers, to obtain crude EVA.

[0036] S6: Place the crude EVA in a vacuum drying oven at 40℃ with a vacuum degree of 0.09MPa and dry for 5 hours. The obtained EVA is then subjected to alcoholysis to obtain crude EVOH.

[0037] S7: Pour crude EVOH into 1000mL of deionized water and stir until EVOH is completely precipitated. Filter and collect the white precipitate, wash it 5 times with deionized water, and dry it under vacuum at 60℃ for 12h to obtain EVOH powder. Enter the extrusion equipment, and then obtain EVOH after pelleting and washing.

[0038] The EVOH prepared in this embodiment 1 H-NMR spectrum as shown Figure 1 As shown, the peaks with chemical shifts of 0.8-0.9 ppm represent hydrogen atoms at the terminal -CH3; the peaks with chemical shifts of 1.2-1.7 ppm represent hydrogen atoms at the -CH2 of ethylene and vinyl alcohol; the peaks with chemical shifts of 3.3-4.1 ppm represent hydrogen atoms at the -CH of vinyl alcohol; and the peaks with chemical shifts of 4.1-4.6 ppm represent hydrogen atoms at the -OH of vinyl alcohol. The peaks at 2.5 ppm and 3.2 ppm are the elution positions of DMSO and H2O, respectively. In summary, this example demonstrates the successful preparation of EVOH.

[0039] Example 2

[0040] The preparation method of EVOH in this embodiment includes the following steps:

[0041] S1: Methanol and n-hexane were separately subjected to vacuum distillation to remove residual impurities. The impurity content of methanol after treatment was 46 ppm, and the impurity content of n-hexane was 35 ppm. Dimethyl azobisisobutyrate was dissolved in methanol at 30°C, and the temperature was gradually reduced to 0°C during stirring. After centrifugation, purified dimethyl azobisisobutyrate with a purity of 99.9% was obtained. Vinyl acetate was subjected to vacuum distillation to remove polymerization inhibitors.

[0042] S2: Add 1000g methanol and 200g n-hexane to a low-temperature reactor (0℃), and add 0.6g lauryl ether phosphate.

[0043] S3: Check the airtightness of the reactor, purge the air with nitrogen (flow rate of 50 mL / min) for 30 min to ensure that the oxygen content in the reactor and pipeline is <10 ppm; add 80 mL of methanol, 400 g of vinyl acetate and 0.08 g of dimethyl azobisisobutyrate to the reactor, purge with ethylene, maintain the system pressure at 2 MPa during the reaction, start stirring at 300 rpm, raise the temperature to 65℃ and maintain the temperature for polymerization reaction.

[0044] S4: After the reaction is complete, add 300 ppm (based on EVA mass) of copper acetate to stop the reaction. Purge nitrogen gas into the reactor (flow rate 50 mL / min) and open the vent valve (flow rate 60 mL / min). After 5 min, transfer the obtained polymer solution to a low-temperature reactor and stir for 1 h to obtain a mixture.

[0045] S5: The mixture is then centrifuged to separate EVA from the methanol-n-hexane mixed solvent containing impurities such as vinyl acetate monomer and low molecular weight polymers, to obtain crude EVA.

[0046] S6: Place the crude EVA in a vacuum drying oven at 45℃ with a vacuum degree of 0.1MPa and dry it for 4 hours. The resulting EVA is then subjected to alcoholysis to obtain crude EVOH.

[0047] S7: Pour crude EVOH into 1000mL of deionized water and stir until EVOH is completely precipitated. Filter and collect the white precipitate, wash it 5 times with deionized water, and dry it under vacuum at 60℃ for 12h to obtain EVOH powder. Enter the extrusion equipment, and then obtain EVOH after pelleting and washing.

[0048] Example 3

[0049] The preparation method of EVOH in this embodiment includes the following steps:

[0050] S1: Methanol and n-hexane were separately subjected to vacuum distillation to remove residual impurities. The impurity content of methanol after treatment was 52 ppm, and the impurity content of n-hexane was 45 ppm. Tert-butyl peroxide was dissolved in ethanol at 50°C, filtered while hot, and the filtrate was cooled and filtered under vacuum to obtain purified tert-butyl peroxide with a purity of 99.9%. Vinyl acetate was subjected to vacuum distillation to remove polymerization inhibitors.

[0051] S2: Add 1000g methanol and 400g n-hexane to a low-temperature reactor (5℃), and add 1.4g isomeric deca-ol polyoxyethylene ether.

[0052] S3: Check the airtightness of the reactor, purge the air with nitrogen (flow rate of 50 mL / min) for 30 min to ensure that the oxygen content in the reactor and pipeline is <10 ppm; add 140 mL of methanol, 280 g of vinyl acetate and 0.14 g of tert-butyl peroxide to the reactor, purge with ethylene, maintain the system pressure at 1 MPa during the reaction, start stirring at 300 rpm, raise the temperature to 68℃ and maintain the temperature for polymerization reaction.

[0053] S4: After the reaction is complete, add 800 ppm (based on EVA mass) of tert-butylcatechol to the reactor to stop the reaction. Purge the reactor with nitrogen gas (flow rate of 50 mL / min) and open the vent valve (flow rate of 60 mL / min). After 5 min, transfer the obtained polymer solution to a cryogenic reactor and stir for 1.5 h to obtain a mixture.

[0054] S5: The mixture is then centrifuged to separate EVA from the methanol-n-hexane mixed solvent containing impurities such as vinyl acetate monomer and low molecular weight polymers, to obtain crude EVA.

[0055] S6: Place the crude EVA in a vacuum drying oven at 30°C with a vacuum degree of 0.09 MPa and dry for 3 hours. The resulting EVA is then alcoholyzed to obtain crude EVOH.

[0056] S7: Pour crude EVOH into 1000mL of deionized water and stir until EVOH is completely precipitated. Filter and collect the white precipitate, wash it 5 times with deionized water, and dry it under vacuum at 60℃ for 12h to obtain EVOH powder. Enter the extrusion equipment, and then obtain EVOH after pelleting and washing.

[0057] Comparative Example 1

[0058] The difference between Comparative Example 1 and Example 1 is that in step S2, the temperature of the cryogenic vessel is 40°C.

[0059] Comparative Example 2

[0060] The difference between Comparative Example 2 and Example 1 is that in step S2, an equal amount of methanol is used instead of n-hexane in the low-temperature reactor.

[0061] Comparative Example 3

[0062] The difference between Comparative Example 3 and Example 1 is that in step S2, an equal amount of n-hexane is used instead of methanol in the low-temperature reactor.

[0063] Comparative Example 4

[0064] The difference between Comparative Example 4 and Example 1 is that in step S2, 800g of methanol and 500g of n-hexane, and 0.8g of polyoxyethylene lauryl ether are added to the low-temperature reactor.

[0065] Comparative Example 5

[0066] The difference between Comparative Example 5 and Example 1 is that polyoxyethylene lauryl ether is not added to the low-temperature reactor in step S2.

[0067] Comparative Example 6

[0068] The difference between Comparative Example 6 and Example 1 is that the polymerization liquid is not transferred to the cryogenic reactor in step S4.

[0069] Comparative Example 7

[0070] The difference between Comparative Example 7 and Example 1 is that in step S2, the amount of polyoxyethylene lauryl ether added is 2.1g.

[0071] Comparative Example 8

[0072] The difference between Comparative Example 8 and Example 1 is that step S6 is not performed.

[0073] The EVOH prepared in Examples 1-3 and Comparative Examples 1-8 were processed into EVOH thin film samples with a thickness of 45±1 μm, and the number of crystal points and PVA content in the samples were detected by the following methods:

[0074] (1) Number of crystal points: According to Method 1 in Section 6.10.2 of GB / T 11115-2009 Polyethylene (PE) Resin, the number of crystal points per 1m was measured. 2 The number of fisheyes in the membrane sample with a major diameter < 0.2 mm, a major diameter between 0.2 mm and 0.4 mm, a major diameter between 0.4 mm and 0.8 mm, and a major diameter > 0.8 mm;

[0075] (2) PVA content: Place the membrane sample in hot water at 85-95℃, stir to dissolve it completely, slowly cool the resulting hot solution, centrifuge after the solid precipitates, and test the PVA content in the aqueous solution.

[0076] The test results are shown in Table 1:

[0077] Table 1. Detection results of crystal point number and PVA content in EVOH membrane samples of Examples 1-3 and Comparative Examples 1-8

[0078]

[0079] As shown in Table 1, Comparative Example 1, without low-temperature treatment and with a cryogenic reactor temperature of 40°C, resulted in 35 crystal points in the EVOH film sample, significantly higher than the 6 in Example 1. The PVA content also reached 650 ppm, far exceeding the 50 ppm in Example 1, indicating a significant deterioration in both indicators. This is because one of the core advantages of this invention is the suppression of residual vinyl acetate self-polymerization through a low-temperature environment of 0-5°C. Comparative Example 1, however, used a higher temperature of 40°C, which is a suitable condition for vinyl acetate self-polymerization, leading to a large amount of unreacted vinyl acetate in the polymerization solution self-polymerizing to form polyvinyl acetate (PVAc). During subsequent alcoholysis, PVAc further converts to PVA, and PVA easily aggregates to form crystal points during EVOH film formation, ultimately resulting in a significant increase in both PVA content and the number of crystal points.

[0080] Comparative Example 2 used only methanol as a solvent, resulting in 43 crystal spots and a PVA content of 201 ppm in the EVOH film sample, both significantly higher than in Example 1. This is because methanol is only a polar solvent. Although it can dissolve polar impurities in EVA (such as a small amount of residual vinyl acetate), it cannot penetrate into the amorphous region of EVA, which is mainly composed of non-polar low-molecular-weight polymer impurities. Since non-polar impurities cannot be removed by methanol washing, the residual non-polar impurities will form locally optically inhomogeneous crystal spots during the EVOH film processing. At the same time, some of the unremoved vinyl acetate will still undergo a small amount of self-polymerization, leading to an increase in PVA content.

[0081] Similarly, Comparative Example 3 used only hexane as a single solvent, resulting in 45 crystal points and a PVA content of 445 ppm in the EVOH membrane sample, which was inferior to Example 1. This is because hexane is a non-polar solvent, which can only penetrate into the amorphous region of EVA and remove non-polar impurities, but cannot dissolve polar impurities (such as residual vinyl acetate and a small amount of polar oligomers). After a large amount of polar impurities (especially vinyl acetate) remain, they will undergo self-polymerization to form PVAc in subsequent processes, and then be converted into PVA; moreover, polar impurities have poor compatibility with EVOH and easily form crystal points in the film, ultimately leading to a significant increase in PVA content and the number of crystal points.

[0082] In Comparative Example 4, the excessive proportion of hexane in the mixed solvent resulted in 40 crystal spots in the EVOH membrane sample and a PVA content of 350 ppm, failing to achieve the effect of Example 1. This is because, in this invention, the mass ratio of methanol to hexane 5:(1-2) is the optimal ratio for achieving simultaneous removal of polar and non-polar impurities: methanol is responsible for dissolving polar impurities, while hexane is responsible for permeating and removing non-polar impurities. In Comparative Example 4, the excessive proportion of hexane led to a relatively insufficient methanol content, which could not fully dissolve the polar impurities (such as vinyl acetate) in the polymerization solution; undissolved vinyl acetate would undergo self-polymerization to form PVA, and excessive hexane might reduce the overall extraction efficiency of the solvent system for impurities, with residual impurities further forming crystal spots.

[0083] Comparative Example 5, without the addition of a penetration enhancer, resulted in 66 crystal spots in the EVOH membrane sample, with a PVA content of 543 ppm, several times higher than that of Example 1. This is because the core function of the penetration enhancer is to reduce the interfacial tension between EVA and the mixed solvent through its hydrophilic and lipophilic structure, increasing the contact area between the two phases, while simultaneously assisting hexane penetration into the amorphous region of EVA and methanol dissolving polar impurities. Without the addition of a penetration enhancer, the interfacial resistance between EVA and the mixed solvent is high, resulting in insufficient contact, and neither polar nor non-polar impurities can be deeply removed; a large number of residual impurities (including vinyl acetate and low molecular weight polymers) will cause vinyl acetate to self-polymerize to generate more PVA, and the impurities will aggregate to form a large number of crystal spots.

[0084] Comparative Example 6, which did not add methanol-n-hexane or a permeation enhancer and did not undergo low-temperature transfer, resulted in an EVOH membrane sample with 174 crystal spots and a PVA content as high as 786 ppm, the worst among all comparative examples. This is because this comparative example completely lacked the core purification step of this invention—low-temperature mixed solvent extraction. Residual vinyl acetate and polar / non-polar low-molecular-weight impurities in the polymerization solution were not removed in any way. In subsequent processes, vinyl acetate will extensively self-polymerize to form PVAc and then convert to PVA. Simultaneously, many impurities have extremely poor compatibility with EVOH, leading to the dense formation of crystal spots during film formation, ultimately resulting in peak PVA content and crystal spot count.

[0085] In Comparative Example 7, the excessive amount of permeation enhancer resulted in 68 crystal spots and a PVA content of 178 ppm in the EVOH membrane sample, leading to poor performance. This is because the present invention controls the amount of permeation enhancer added within the range of 0.05-0.1 wt.% of the mixed solvent, which assists extraction without introducing new problems. In Comparative Example 7, the excessive amount of permeation enhancer formed an emulsion system or micelle structure in the mixed solvent, hindering the dissolution of polar impurities by methanol and the permeation of non-polar impurities by n-hexane. Simultaneously, the permeation enhancer that was not fully involved in the extraction itself became a new impurity, forming crystal spots in the EVOH membrane, leading to an increase in the number of crystal spots and the PVA content.

[0086] Comparative Example 8, which did not undergo the low-temperature vacuum drying process of step S6, resulted in an EVOH film sample with 19 crystal spots and a PVA content of 87 ppm. While this was better than the other comparative examples, it was still inferior to Example 1. This is because the low-temperature vacuum drying process can further remove residual mixed solvents (methanol, n-hexane) and trace impurities from the crude EVA. Without this treatment, a small amount of solvent and impurities remain in the crude EVA: the residual solvent may promote a small amount of self-polymerization of vinyl acetate in the subsequent alcoholysis process, leading to a slight increase in PVA content; at the same time, the mixture of residual impurities and solvents will form a small number of crystal spots in the EVOH film, ultimately making the performance slightly worse than that of Example 1.

Claims

1. A method for preparing EVOH, characterized in that, Includes the following steps: S1: Methanol, n-hexane and initiator are purified, and vinyl acetate is depolymerized and then used for later use; S2: A mixture of methanol and n-hexane is placed in a cryogenic reactor for later use. A penetration enhancer is added to the methanol and n-hexane mixture. The temperature of the cryogenic reactor is 0-5℃. The mass ratio of methanol to n-hexane is 5:(1-2). The penetration enhancer is polyoxyethylene lauryl ether, lauryl ether phosphate, or isomeric decayl alcohol polyoxyethylene ether. The amount of penetration enhancer added is 0.05-0.1 wt.% of the methanol and n-hexane mixture. S3: Under a nitrogen atmosphere, add solvent, initiator and vinyl acetate to the reactor, introduce ethylene, and heat to 65-70℃ to carry out the polymerization reaction; S4: After the reaction is complete, add a terminator to stop the reaction. After purging the ethylene with nitrogen, transfer the resulting polymer solution to the low-temperature reactor of step S2 and stir continuously to obtain a mixture. The terminator is tert-butylcatechol or copper acetate. S5: The mixture is centrifuged to obtain crude ethylene-vinyl acetate copolymer; S6: The crude ethylene-vinyl acetate copolymer is vacuum dried at 30-45℃, and the resulting ethylene-vinyl acetate copolymer is alcoholyzed to obtain EVOH.

2. The method for preparing EVOH as described in claim 1, characterized in that, In step S1, the purification treatment of methanol and n-hexane is as follows: methanol and n-hexane are respectively subjected to vacuum distillation to remove residual impurities; the purification treatment of initiator is as follows: the initiator is crystallized to remove impurities; the method of removing polymerization inhibitor from vinyl acetate is as follows: vinyl acetate is subjected to vacuum distillation to remove polymerization inhibitor.

3. The method for preparing EVOH as described in claim 1, characterized in that, In step S3, the ethylene pressure is 1-3 MPa.

4. The method for preparing EVOH as described in claim 1, characterized in that, In step S3, the solvent is methanol; the initiator is azobisisobutyronitrile, dimethyl azobisisobutyrate, or tert-butyl peroxide.

5. The method for preparing EVOH as described in claim 1, characterized in that, In step S3, the mass-volume ratio of solvent, initiator and vinyl acetate is (2-5) mL:(0.002-0.005) g:10 g, and the mass ratio of vinyl acetate to the mixed solvent of methanol and n-hexane in step S2 is 1:(3-5).

6. The method for preparing EVOH as described in claim 1, characterized in that, In step S4, the amount of the terminator added is 300-800 ppm of the mass of the ethylene-vinyl acetate copolymer in the system.

7. The method for preparing EVOH as described in claim 1, characterized in that, In step S4, the stirring time is 1-2 hours.

8. The method for preparing EVOH as described in claim 1, characterized in that, In step S6, the vacuum degree of the vacuum drying process is 0.09-0.1 MPa, and the time is 3-5 h.

9. The method for preparing EVOH as described in claim 1, characterized in that, In step S6, the obtained EVOH is washed with deionized water and vacuum dried to obtain EVOH powder, which is then fed into an extrusion device, pelletized, and washed to obtain the final EVOH.

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