Ethylene-vinyl alcohol copolymer and preparation method thereof

Abstract

The invention discloses an ethylene-vinyl alcohol copolymer and a preparation method thereof, and relates to the technical field of resin. According to the technical scheme, the preparation method comprises the following steps: adding ethylene, low-carbon alkane, alkyl acid ethyl ester, alkyl acid vinyl ester and a carbon-carbon skeleton initiator into a reaction kettle, and carrying out a free radical polymerization reaction; adding a polymerization inhibitor to terminate the reaction, removing low-carbon alkane, unreacted ethylene and alkyl acid vinyl ester, and preparing the reaction liquid into an alcoholic solution; adding a catalyst into the alcohol solution for saponification reaction, and then performing acid pickling, water washing and drying to obtain the ethylene-vinyl alcohol copolymer. The low-carbon alkane which is low in boiling point, easy to vaporize and high in latent heat of vaporization and the saturated alkyl acid ethyl ester with good solubility are adopted as the mixed solvent to replace a traditional alcohol solvent, so that removal and control of reaction heat are facilitated, generation of insoluble gel is reduced, a copolymer with narrow molecular weight distribution is easy to obtain, and the method is suitable for industrial production. The cost that azeotrope formed by the alcohol solvent and the monomer is difficult to separate and recover is greatly reduced, and the production efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of resins, and particularly to ethylene-vinyl alcohol copolymers and their preparation methods. Background Art

[0002] Ethylene-vinyl alcohol copolymer (EVOH) combines the properties of polyethylene and polyvinyl alcohol, and is a high-barrier and solvent-resistant resin material with wide applications. Therefore, it is widely used in fields such as food, medical, and cosmetic packaging, as well as automotive fuel tanks and industrial pipelines. In the EVOH polymer chain, ethylene and vinyl alcohol segments are statistically distributed. The ethylene segments endow it with flexibility, making it more processable than polyvinyl alcohol; while the vinyl alcohol segments, through strong intra- and intermolecular hydrogen bond interactions, result in a small free volume between polymer networks, making it difficult for small molecules such as gases and solvents to diffuse through, thus endowing it with excellent gas barrier properties and solvent resistance. The quantity and distribution of the two units directly determine the properties of EVOH. Therefore, controlling the quantity and distribution of the two repeating units in EVOH is crucial for regulating the performance of EVOH.

[0003] Since vinyl alcohol monomers are unstable and easily isomerize to acetaldehyde, EVOH cannot be directly obtained by the polymerization of ethylene and vinyl alcohol. Its synthesis process usually involves two steps. The first step is the polymerization reaction of ethylene and vinyl acetate to obtain ethylene-vinyl acetate copolymer (EVA), and the second step is the alcoholysis of EVA to obtain EVOH and by-product methyl acetate. Among them, the polymerization stage of the first step directly determines the polymer structure and unit distribution, and in the polymerization process, due to chain transfer or branching crosslinking, etc., a small amount of insoluble gel substances are easily generated. The alcoholysis stage of the second step is closely related to the quality and stability of the product. In traditional EVOH preparation methods, a large amount of methanol or tert-butanol is generally used as a solvent in the polymerization stage of the first step, resulting in a large pollution in the preparation process and relatively high energy consumption; moreover, vinyl acetate and alcohol solvents are prone to form azeotropes, with great separation difficulty, high energy consumption and affecting the effect of subsequent solvent reuse. Chinese invention patent CN1179989C discloses a preparation method of ethylene-vinyl acetate copolymer and the saponified product of the copolymer obtained by this method and the molded article containing it, which points out that alcohol solvents are prone to undergo transesterification side reactions with vinyl acetate monomers, generating aldehyde substances, seriously affecting the coloring of the product and the formation of gels.

[0004] Therefore, there is an urgent need to find a class of high-efficiency and low-consumption polymerization solvents. On the one hand, it can conveniently remove the reaction heat, control the reaction process and reduce the generation of side reaction gels in the system, so as to avoid adhesion to the reactor wall and blockage of pipelines; on the other hand, it can quickly separate the solvent from the monomers, improve the recovery efficiency and reduce the operating cost. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide an ethylene-vinyl alcohol copolymer and a preparation method thereof. By using a low-boiling, easily vaporizable lower alkane with a relatively high heat of vaporization and a saturated ethyl alkylate with good solubility as a mixed solvent to replace the traditional alcohol solvent for the polymerization reaction, environmental pollution is reduced and the heat removal efficiency of the reaction and the separation and recovery efficiency of the solvent are improved.

[0006] The technical solution of the present invention is as follows: On the one hand, the present invention provides a preparation method of an ethylene-vinyl alcohol copolymer, comprising the following steps: S1: Add ethylene, a lower alkane, an ethyl alkylate, a vinyl alkylate, and a carbon-carbon backbone initiator to a reaction kettle for free radical polymerization reaction; wherein, the lower alkane is propane or isobutane; the vinyl alkylate is vinyl acetate, vinyl propionate, or vinyl benzoate, and the ethyl alkylate is ethyl acetate, ethyl propionate, or ethyl benzoate; the mass ratio of the lower alkane, the ethyl alkylate to the vinyl alkylate is (0.1-0.3):(0.005-0.02):1; the carbon-carbon backbone initiator is dimethyl 2,3-dicyano-2,3-diphenylsuccinate, diethyl 2,3-dicyano-2,3-diphenylsuccinate, or diisobutyl 2,3-dicyano-2,3-diphenylsuccinate, and the addition amount is 0.01-0.05 wt.% of the total mass of the vinyl alkylate and ethylene; S2: Add an inhibitor to terminate the reaction to obtain a reaction solution of an ethylene-vinyl alkylate copolymer, remove the lower alkane, unreacted ethylene, and vinyl alkylate therein, and then prepare the reaction solution of the ethylene-vinyl alkylate copolymer into an alcohol solution; the inhibitor is hydroquinone, tert-butylcatechol, n-octyl mercaptan, tert-dodecyl mercaptan, or copper acetate; S3: Add a catalyst to the alcohol solution for saponification reaction to obtain a solution containing an ethylene-vinyl alcohol copolymer; the catalyst is sodium hydroxide; S4: Acid wash, water wash, and dry the solution containing the ethylene-vinyl alcohol copolymer to obtain the ethylene-vinyl alcohol copolymer.

[0007] Preferably, in step S1, the free radical polymerization reaction pressure is 2-6 MPaG, the free radical polymerization reaction temperature is 60-70 °C, and the free radical polymerization reaction time is 2-6 h.

[0008] Preferably, in step S2, the mass ratio of the inhibitor to the vinyl alkylate is (0.005-0.03):1.

[0009] Preferably, in step S2, the alcohol in the alcohol solution is methanol, and the concentration of the alcohol solution is 10-30 wt.%.

[0010] Preferably, in step S3, the molar ratio of the catalyst to the vinyl acetate unit in the ethylene-vinyl acetate copolymer is (0.02-0.05):1.

[0011] Preferably, in step S3, the saponification reaction temperature is 60-80°C, and the saponification reaction time is 2-5 h.

[0012] Preferably, in step S4, the acid used for pickling is acetic acid, boric acid or phosphoric acid; the molar ratio of the acid to the catalyst is 1:(0.5-2).

[0013] Preferably, in step S4, the drying temperature is 60-120°C.

[0014] On the other hand, the present invention provides an ethylene-vinyl alcohol copolymer prepared by the above preparation method of the ethylene-vinyl alcohol copolymer. The ethylene molar content in the ethylene-vinyl alcohol copolymer is 27-44%, the content of insoluble gel is <0.1 wt.%, and the saponification degree is ≥99.5%.

[0015] Compared with the prior art, the present invention has the following beneficial effects: In the present invention, a low-boiling, easily vaporizable and high-vaporization latent heat lower alkane and a saturated ethyl alkylate with good solubility are used as a mixed solvent to replace the traditional alcohol solvent for the polymerization reaction. First, since such a mixed solvent is easily vaporizable and has a high vaporization latent heat, it can conveniently remove and control the reaction heat, and can reduce the environmental pollution caused by alcohol solvents and the generation amount of by-product acetaldehyde between the alcohol solvents and vinyl acetate; second, by using the "like dissolves like" principle that the ethyl alkylate is similar to the polymer structure and solubility, the generation of insoluble gel is reduced to avoid adhesion to the reactor wall surface and pipeline valves; third, the carbon-carbon skeleton initiator used in the present invention has high thermal stability, does not contain yellowing groups, and is easy to obtain a copolymer with a narrow molecular weight distribution; finally, after the lower alkane is depressurized and flashed, it can be quickly separated from monomers and by-products, etc., greatly reducing the cost of difficult separation and recovery of the azeotrope formed by alcohol solvents and monomers, and improving production efficiency. Description of the Drawings

[0016] Figure 1 is the H 1 NMR spectrum of the EVOH product prepared in Example 1 of the present invention. Detailed Embodiments

[0017] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention.

[0018] Example 1 The preparation method of the ethylene-vinyl alcohol copolymer in this example includes the following steps: S1: After testing that the airtightness of a 2000 mL stainless steel reactor (equipped with a jacket and an internal coil cooler) is intact, heat it up to 120 °C, use a vacuum pump to draw a negative pressure to -100 kPa, maintain it for 30 min, then displace it with nitrogen three times, and then displace it with ethylene gas twice; after the reactor cools down to below 40 °C, add 780 g of vinyl acetate, and then add a mixed solution of the carbon-carbon skeleton initiator diisobutyl 2,3-dicyano-2,3-diphenylsuccinate and 8 g of ethyl acetate to the reactor. Then, inject 160 g of isobutane into the reactor using a piston pump, and control the addition amount of the carbon-carbon skeleton initiator in the system to be 0.03 wt.% of the total mass of vinyl acetate and ethylene; start stirring and charge 3.5 MPaG of ethylene. After heating up to 65 °C, continuously charge ethylene to keep the pressure of the reaction system stable. The reaction heat of the system is removed by the evaporation and vaporization of isobutane, and react for 3 h; S2: After the reaction is completed, cool it down to room temperature, release the pressure, add hydroquinone at 100 ppm of the mass of vinyl acetate, and then discharge the material to obtain a reaction solution of ethylene-vinyl acetate copolymer. The conversion rate of vinyl acetate is 35%, and the ethylene molar content is 32%; then, remove the residual monomers in the reaction solution of ethylene-vinyl acetate copolymer by vacuum distillation, and prepare the reaction solution of ethylene-vinyl acetate copolymer into a 20 wt.% methanol solution; S3: Add sodium hydroxide to the methanol solution, and the molar ratio of sodium hydroxide to the vinyl acetate unit in the ethylene-vinyl acetate copolymer is 0.04:1. Then, heat it up to 60 °C. During this period, continuously remove methanol by nitrogen bubbling and supplement methanol to maintain its concentration unchanged. After reacting for 5 h, obtain a solution containing ethylene-vinyl alcohol copolymer; S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved with acetic acid for pickling and neutralization, where the molar ratio of acetic acid to sodium hydroxide is 1:1, then wash it with pure water three times, and filter to obtain a precipitate; place the above precipitate in a vacuum oven at 60 °C and dry it for 24 h to obtain a crude EVOH product with a saponification degree of 99.6% and a yellowness index YI of 2.1.

[0019] The H 1 NMR spectrum of the EVOH product prepared in this example is as Figure 1 shown. It can be seen that: the peak with a chemical shift of 0.7 - 0.9 ppm represents the hydrogen on -CH3 of the side chain; the peak with a chemical shift of 1.4 - 1.5 ppm represents the hydrogen on -CH2 of ethylene and vinyl alcohol; the peak with a chemical shift of 3.4 - 3.9 ppm represents the hydrogen on -CH of vinyl alcohol; the peak with a chemical shift of 3.9 - 4.5 ppm represents the hydrogen on -OH of vinyl alcohol; among them, 2.5 ppm and 3.4 ppm are the peak positions of H2O and DMSO respectively. In summary, it is proved that the EVOH product is successfully prepared in this example.

[0020] Example 2 The preparation method of the ethylene-vinyl alcohol copolymer in this example includes the following steps: S1: After testing the airtightness of a 2000 mL stainless steel reactor (equipped with a jacket and an internal coil cooler) and finding it intact, heat it to 120 °C, evacuate to a negative pressure of -100 kPa with a vacuum pump, maintain for 30 min, then displace with nitrogen 3 times, and then displace with ethylene gas 2 times; after the reactor cools to below 40 °C, add 780 g of vinyl acetate, and then add a mixture of the carbon-carbon backbone initiator dimethyl 2,3-dicyano-2,3-diphenylsuccinate and 4 g of ethyl acetate to the reactor. Then inject 80 g of isobutane into the reactor with a plunger pump, and control the addition amount of the carbon-carbon backbone initiator in the system to be 0.05 wt.% of the total mass of vinyl acetate and ethylene; start stirring and charge 2 MPaG of ethylene, heat to 60 °C, and continuously charge ethylene to keep the reaction system pressure stable. The heat of reaction of the system is removed by the evaporation and vaporization of isobutane, and react for 6 h; S2: After the reaction is completed, cool to room temperature, release the pressure, add 300 ppm of tert-butylcatechol based on the mass of vinyl acetate, and then discharge to obtain a reaction solution of ethylene-vinyl acetate copolymer. The conversion rate of vinyl acetate is 42%, and the ethylene molar content is 27%; then, the residual monomers in the reaction solution of ethylene-vinyl acetate copolymer are removed by vacuum distillation, and the reaction solution of ethylene-vinyl acetate copolymer is formulated into a 10 wt.% methanol solution; S3: Add sodium hydroxide to the methanol solution, and the molar ratio of sodium hydroxide to the vinyl acetate unit in the ethylene-vinyl acetate copolymer is 0.02:1. Then heat to 80 °C, and continuously take away methanol by nitrogen bubbling and supplement methanol during this period to keep its concentration unchanged. After reacting for 2 h, obtain a solution containing ethylene-vinyl alcohol copolymer; S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved with phosphoric acid for pickling and neutralization, where the molar ratio of phosphoric acid to sodium hydroxide is 0.5:1, wash with pure water 3 times, and filter to obtain a precipitate; place the above precipitate in a vacuum oven at 80 °C and dry for 16 h to obtain a crude EVOH product with a saponification degree of 99.8% and a yellowness index YI of 2.3.

[0021] Example 3 The preparation method of the ethylene-vinyl alcohol copolymer in this example includes the following steps: S1: After testing and ensuring the airtightness of a 2000 mL stainless steel reactor (equipped with a jacket and an internal coil cooler), heat it up to 120 °C, evacuate to a negative pressure of -100 kPa using a vacuum pump, maintain for 30 min, then displace with nitrogen three times and then displace with ethylene gas twice. After the reactor cools down to below 40 °C, add 780 g of vinyl acetate. Then add a mixture of the carbon-carbon backbone initiator diethyl 2,3-dicyano-2,3-diphenylsuccinate and 12 g of ethyl acetate to the reactor. Then inject 230 g of isobutane into the reactor using a plunger pump, and control the addition amount of the carbon-carbon backbone initiator in the system to be 0.01 wt.% of the total mass of vinyl acetate and ethylene. Start stirring and charge 6 MPaG of ethylene. After heating up to 70 °C, continuously charge ethylene to keep the pressure of the reaction system stable. The heat of reaction of the system is removed by the evaporation and vaporization of isobutane, and react for 2 h; S2: After the reaction is completed, cool to room temperature, release the pressure, add 50 ppm of n-octyl mercaptan based on the mass of vinyl acetate, and then discharge to obtain a reaction solution of ethylene-vinyl acetate copolymer. The conversion rate of vinyl acetate is 32%, and the ethylene molar content is 44%. Then, remove the remaining monomers in the reaction solution of ethylene-vinyl acetate copolymer by vacuum distillation, and prepare the reaction solution of ethylene-vinyl acetate copolymer into a 20 wt.% methanol solution; S3: Add sodium hydroxide to the methanol solution, and the molar ratio of sodium hydroxide to the vinyl acetate unit in the ethylene-vinyl acetate copolymer is 0.03:1. Then heat up to 60 °C. During this period, continuously remove methanol by nitrogen bubbling and supplement methanol to keep its concentration constant. After reacting for 4 h, obtain a solution containing ethylene-vinyl alcohol copolymer; S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved with boric acid for pickling and neutralization, where the molar ratio of boric acid to sodium hydroxide is 2:1. Then wash with pure water three times, filter to obtain a precipitate. Place the above precipitate in a vacuum oven at 100 °C and dry for 12 h to obtain a crude EVOH product with a saponification degree of 99.6% and a yellowness index YI of 2.1.

[0022] Example 4 The preparation method of the ethylene-vinyl alcohol copolymer in this example includes the following steps: S1: After testing and ensuring the airtightness of a 2000 mL stainless steel reactor (equipped with a jacket and an internal coil cooler), heat it up to 120 °C, evacuate to a negative pressure of -100 kPa using a vacuum pump and maintain for 30 min, then displace with nitrogen 3 times and then displace with ethylene gas 2 times; after the reactor cools down to below 40 °C, add 800 g of vinyl propionate, then add a mixture of the carbon-carbon skeleton initiator diisobutyl 2,3-dicyano-2,3-diphenylsuccinate and 16 g of ethyl propionate to the reactor, and then inject 160 g of propane into the reactor using a plunger pump. Control the addition amount of the carbon-carbon skeleton initiator in the system to be 0.04 wt.% of the total mass of vinyl propionate and ethylene; start stirring and charge 4 MPaG of ethylene, after heating up to 65 °C, continuously charge ethylene to keep the reaction system pressure stable, and remove the reaction heat of the system by the evaporation and vaporization of propane, and react for 4 h; S2: After the reaction is completed, cool to room temperature, relieve the pressure, add copper acetate at 100 ppm of the mass of vinyl propionate, and then discharge the material to obtain a reaction solution of ethylene-vinyl propionate copolymer. The conversion rate of vinyl propionate is 38%, and the ethylene molar content is 32%; then, remove the remaining monomers in the reaction solution of ethylene-vinyl propionate copolymer by vacuum distillation, and then prepare the reaction solution of ethylene-vinyl propionate copolymer into a 30 wt.% methanol solution; S3: Add sodium hydroxide to the methanol solution, and the molar ratio of sodium hydroxide to the vinyl propionate unit in the ethylene-vinyl propionate copolymer is 0.05:1, then heat up to 70 °C, and continuously remove methanol by nitrogen bubbling and supplement methanol during this period to maintain its concentration unchanged. After reacting for 3 h, obtain a solution containing ethylene-vinyl alcohol copolymer; S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved with acetic acid for pickling and neutralization, where the molar ratio of acetic acid to sodium hydroxide is 1:1, wash with pure water 3 times, and filter to obtain a precipitate; place the above precipitate in a vacuum oven at 120 °C and dry for 8 h to obtain a crude EVOH product with a saponification degree of 99.7% and a yellowness index YI of 2.6.

[0023] Example 5 The preparation method of the ethylene-vinyl alcohol copolymer in this example includes the following steps: S1: After testing and ensuring the airtightness of a 2000 mL stainless steel reactor (equipped with a jacket and an internal coil cooler), heat it up to 120 °C, evacuate it to a negative pressure of -100 kPa using a vacuum pump, and maintain this for 30 min. Then, displace the air with nitrogen three times and then with ethylene gas twice. After the reactor cools down to below 40 °C, add 780 g of vinyl benzoate. Next, add a mixture of the carbon-carbon backbone initiator diisobutyl 2,3-dicyano-2,3-diphenylsuccinate and 8 g of ethyl benzoate to the reactor. Then, inject 160 g of propane into the reactor using a plunger pump, and control the addition amount of the carbon-carbon backbone initiator in the system to be 0.03 wt.% of the total mass of vinyl benzoate and ethylene. Start stirring and charge 3.5 MPaG of ethylene. After heating up to 65 °C, continuously charge ethylene to keep the pressure of the reaction system stable. The heat of reaction of the system is removed by the evaporation and vaporization of propane, and react for 3 h. S2: After the reaction is completed, cool it down to room temperature, release the pressure, add tert-dodecyl mercaptan at 100 ppm based on the mass of vinyl benzoate, and then discharge the material to obtain a reaction solution of ethylene-vinyl benzoate copolymer. The conversion rate of vinyl benzoate is 35%, and the molar content of ethylene is 32%. Then, remove the remaining monomers in the reaction solution of ethylene-vinyl benzoate copolymer by vacuum distillation, and then prepare the reaction solution of ethylene-vinyl benzoate copolymer into a 20 wt.% methanol solution. S3: Add sodium hydroxide to the methanol solution. The molar ratio of sodium hydroxide to the vinyl benzoate unit in the ethylene-vinyl benzoate copolymer is 0.03:1. Then, heat it up to 60 °C. During this process, continuously remove methanol by nitrogen bubbling and supplement methanol to keep its concentration constant. After reacting for 4 h, obtain a solution containing ethylene-vinyl alcohol copolymer. S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water containing acetic acid for pickling and neutralization. The molar ratio of acetic acid to sodium hydroxide is 1:1. Then, wash it with pure water three times, and filter to obtain a precipitate. Place the above precipitate in a vacuum oven at 60 °C and dry it for 24 h to obtain a crude EVOH product with a saponification degree of 99.6% and a yellowness index YI of 2.1.

[0024] Comparative Example 1 The difference from Example 1 is that in step S1, isobutane is replaced by methanol.

[0025] Comparative Example 2 The difference from Example 1 is that in step S1, the addition amount of isobutane is 320 g.

[0026] Comparative Example 3 The difference from Example 1 is that in step S1, ethyl acetate is not added.

[0027] Comparative Example 4 The difference from Example 1 is that in step S1, isobutane is replaced with n-hexane.

[0028] Comparative Example 5 The difference from Example 1 is that in step S1, isobutane is replaced with n-heptane.

[0029] Comparative Example 6 The difference from Example 1 is that in step S1, the addition amount of ethyl acetate is 20 g.

[0030] Comparative Example 7 The difference from Example 1 is that in step S1, the carbon-carbon skeleton initiator is replaced with azobisisobutyronitrile.

[0031] Perform performance tests on the products of Examples 1-5 and Comparative Examples 1-7. Among them, the test method for the content of insoluble gel is as follows: Filter the reaction solution by suction, and then calculate the mass percentage of the substance remaining on the filter paper in the filtrate; the residual methanol content in the monomer: Test by the LC-MS method; the residual acetaldehyde content in EVA: Refer to "GB 31604.60-2024 National Food Safety Standard Food Contact Materials and Products Solvent Residue Amount", and test by headspace gas chromatography; saponification degree: Refer to "GB / T 41877.2-2022 Plastics Ethylene-vinyl alcohol (EVOH) copolymer molding and extrusion materials Part 2: Specimen preparation and property determination", and test by titration method; yellowness index (YI): Test by a yellowness index instrument. The test results are shown in Table 1: Table 1 Test Results of the Products of Examples 1-5 and Comparative Examples 1-7

[0032] As can be seen from Table 1, in Comparative Example 1, traditional methanol was used as the solvent, which would cause side reactions after polymerization, generating a large amount of acetaldehyde, affecting the yellowness index of the product, and it was difficult to remove the reaction heat. There was a tendency for the insoluble gel in the system to increase, affecting the subsequent saponification process, and a large amount of methanol still remained after monomer separation. In Comparative Example 2, isobutane was added in excess, and in Comparative Example 3, ethyl acetate was not added. Although the effect of removing the reaction heat was good, it would make the solubility of the system worse, increasing the amount of insoluble gel and affecting the molecular structure of the product. In Comparative Examples 4-5, conventional high-boiling alkane solvents were used. In addition to reducing by-products and the residual solvent in the monomer, due to the difficulty of removing the reaction heat, a large number of side reactions still occurred, resulting in corresponding cross-linking and branching, affecting the subsequent alcoholysis and yellowness index. In Comparative Example 6, an excessive amount of ethyl acetate was added as the solvent. Although the polymerization effect was good, it remained in the product in a large amount, affecting the subsequent saponification process and reducing the saponification degree. As can be seen from Comparative Example 7, when conventional azobisisobutyronitrile was used as the initiator, the molecular weight distribution of the prepared EVOH product became wider. This is because the carbon-carbon skeleton initiator simultaneously has the functions of "initiation-transfer-termination" and can play the role of "controlled polymerization", reducing the molecular weight distribution to a certain extent.

[0033] In summary, the present invention uses a mixed solvent of low-boiling and easily vaporizable lower alkanes and highly soluble saturated alkyl acid ethyl esters. On the one hand, it can achieve the efficient separation of monomers and solvents, preventing the large sacrifice of separation energy consumption and efficiency due to azeotropy with methanol during monomer separation and recycling. With the mixed solvent of the present invention, the purity of the monomer is guaranteed. In addition, the phase change of the solvent can be realized by using the heat of vaporization, making the removal of the reaction heat more convenient and efficient. Thirdly, due to the low polarity and small chain transfer constant of the mixed solvent, the chain transfer effect of the monomer during polymerization can be reduced, the molecular weight can be increased and the molecular weight distribution can be reduced, making the branching more uniform. And by using the "like dissolves like" principle that the saturated alkyl acid ethyl ester is similar to the polymer structure and solubility, the content of the generated insoluble gel is reduced, making the saponification reaction more efficient and the saponification degree higher. Finally, it reduces the aldehyde impurities generated by the side reaction of the traditional alcohol solvent and vinyl alkylate, reduces the gel degree of the polymer, and effectively improves the color value.

Claims

1. A method for preparing ethylene-vinyl alcohol copolymer, characterized in that, It includes the following steps: S1: Add ethylene, lower alkanes, ethyl alkylate, vinyl alkylate and a carbon-carbon backbone initiator into a reaction kettle for free radical polymerization reaction; wherein, the lower alkanes are propane or isobutane; the vinyl alkylate is vinyl acetate, vinyl propionate or vinyl benzoate, and the ethyl alkylate is ethyl acetate, ethyl propionate or ethyl benzoate; the mass ratio of the lower alkanes, ethyl alkylate to vinyl alkylate is (0.1 - 0.3):(0.005 - 0.02):1; the carbon-carbon backbone initiator is dimethyl 2,3-dicyano-2,3-diphenylsuccinate, diethyl 2,3-dicyano-2,3-diphenylsuccinate or diisobutyl 2,3-dicyano-2,3-diphenylsuccinate, and the addition amount is 0.01 - 0.05 wt.% of the total mass of vinyl alkylate and ethylene; S2: Add an inhibitor to terminate the reaction to obtain a reaction solution of ethylene-vinyl alkylate copolymer, remove the lower alkanes, unreacted ethylene and vinyl alkylate therein, and then prepare the reaction solution of ethylene-vinyl alkylate copolymer into an alcohol solution; the inhibitor is hydroquinone, tert-butylcatechol, n-octyl mercaptan, tert-dodecyl mercaptan or copper acetate; S3: Add a catalyst to the alcohol solution for saponification reaction to obtain a solution containing ethylene-vinyl alcohol copolymer; the catalyst is sodium hydroxide; S4: Carry out pickling, water washing and drying on the solution containing ethylene-vinyl alcohol copolymer to obtain ethylene-vinyl alcohol copolymer.

2. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, wherein In step S1, the pressure of the free radical polymerization reaction is 2 - 6 MPaG, the temperature of the free radical polymerization reaction is 60 - 70 °C, and the time of the free radical polymerization reaction is 2 - 6 h.

3. The method for preparing ethylene-vinyl alcohol copolymer according to claim 1, characterized in that, In step S2, the mass ratio of the inhibitor to vinyl alkylate is (0.005 - 0.03):

1.

4. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, wherein In step S2, the alcohol in the alcohol solution is methanol, and the concentration of the alcohol solution is 10 - 30 wt.%.

5. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, characterized in that, In step S3, the molar ratio of the catalyst to the vinyl alkylate unit in the ethylene-vinyl alkylate copolymer is (0.02 - 0.05):

1.

6. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, characterized in that, In step S3, the saponification reaction temperature is 60 - 80 °C, and the saponification reaction time is 2 - 5 h.

7. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, wherein, In step S4, the acid used for pickling is acetic acid, boric acid or phosphoric acid; the molar ratio of the acid to the catalyst is 1:(0.5 - 2).

8. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, characterized in that, In step S4, the drying temperature is 60 - 120 °C.

9. Ethylene-vinyl alcohol copolymer, characterized in that, Prepared by the preparation method of ethylene-vinyl alcohol copolymer according to any one of claims 1 - 8, the ethylene molar content in the ethylene-vinyl alcohol copolymer is 27 - 44%, the content of insoluble gel is < 0.1 wt.%, and the saponification degree is ≥ 99.5%.

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