Ethylene-vinyl alcohol copolymer and method for producing the same

Abstract

The application discloses ethylene-vinyl alcohol copolymer and a preparation method thereof, and relates to the technical field of resins.The technical scheme is as follows: ethylene, low-carbon alkane, alkyl acid ethyl ester, alkyl acid vinyl ester and carbon-carbon skeleton initiator are added into a reaction kettle to perform free radical polymerization; a polymerization inhibitor is added to terminate the reaction, and low-carbon alkane, unreacted ethylene and alkyl acid vinyl ester are removed; then the reaction liquid is prepared into an alcohol solution; a catalyst is added into the alcohol solution to perform saponification; and then the alcohol solution is subjected to acid washing, water washing and drying to obtain ethylene-vinyl alcohol copolymer.The low-carbon alkane and the saturated alkyl acid ethyl ester with good solubility are used as mixed solvents, the low-carbon alkane has low boiling point, is easy to vaporize and has high vaporization latent heat, the traditional alcohol solvent is replaced, the reaction heat is easy to remove and control, the generation of insoluble gel substances is reduced, the copolymer with narrow molecular weight distribution is easy to obtain, the cost of separating and recycling the azeotrope formed by the alcohol solvent and monomers is greatly reduced, and the production efficiency is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of resin, in particular to ethylene-vinyl alcohol copolymer and its preparation method. BACKGROUND

[0002] Ethylene-vinyl alcohol copolymer (EVOH) has the properties of polyethylene and polyvinyl alcohol, and is a widely used high-barrier and solvent-resistant resin material. Therefore, it is widely used in food, medical and cosmetic packaging, automobile fuel tank, industrial pipeline and other fields. The ethylene and vinyl alcohol segments in the EVOH polymer chain are statistically distributed. The ethylene segments give it flexibility, making it more processable than polyvinyl alcohol. The vinyl alcohol segments, through strong intramolecular and intermolecular hydrogen bonding, make the free volume between the polymer network small, so that small molecules such as gas and solvent are difficult to diffuse through, giving it excellent gas barrier and solvent resistance. The number and distribution of the two units directly determine the properties of EVOH. Therefore, controlling the number and distribution of the two repeating units in EVOH is crucial for the performance regulation of EVOH.

[0003] Because the vinyl alcohol monomer is unstable and easily isomerizes into acetaldehyde, EVOH cannot be directly obtained by polymerization of ethylene and vinyl alcohol. Its synthesis process usually includes two steps. The first step is the polymerization of ethylene and alkyl acid vinyl ester to obtain ethylene-alkyl acid vinyl ester copolymer (EVA), and the second step is the alcoholysis of EVA to obtain EVOH and by-product methyl acetate. In the first step of polymerization, the polymer structure and unit distribution are directly determined, and a small amount of insoluble gel is easily produced in the polymerization process due to chain transfer or branching crosslinking, etc. The alcoholysis stage in the second step is closely related to the quality and stability of the product. In the traditional EVOH preparation method, a large amount of methanol or tert-butyl alcohol is generally used as a solvent in the first step of polymerization, resulting in relatively large pollution and relatively high energy consumption. In addition, alkyl acid vinyl ester and alcohol solvent are easy to form azeotrope, which is difficult to separate, has high energy consumption and affects the effect of subsequent solvent reuse. Chinese invention patent CN1179989C discloses a preparation method of ethylene-vinyl acetate copolymer and a saponified product and a shaped product containing the same obtained by the method. It points out that alcohol solvents are easy to have ester exchange side reactions with alkyl acid vinyl ester monomers, producing aldehyde substances, which seriously affect the coloring and gel formation of the product.

[0004] Therefore, it is urgent to find a kind of efficient and low-consumption polymerization solvent, which can on the one hand facilitate the removal of reaction heat, control the reaction process and reduce the generation of side reaction gel in the system, so as to avoid adhesion to the reaction kettle wall and blockage of the pipeline; on the other hand, it can quickly realize the separation between the solvent and the monomer, improve the recovery efficiency and reduce the operating cost. SUMMARY

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide an ethylene-vinyl alcohol copolymer and a preparation method thereof. By using low-boiling-point, easily vaporized, and high-vaporization latent heat low-carbon alkanes and well-soluble saturated alkyl acid ethyl esters as mixed solvents to replace traditional alcohol solvents for polymerization reactions, environmental pollution is reduced and the efficiency of reaction heat removal, solvent separation, and recovery is improved.

[0006] The technical solution of the present invention is:

[0007] In one aspect, the present invention provides a method for preparing an ethylene-vinyl alcohol copolymer, comprising the following steps:

[0008] S1: Ethylene, low-carbon alkane, alkyl ethyl ester, alkyl vinyl ester and carbon-carbon skeleton initiator are added to the reactor to carry out free radical polymerization reaction; wherein the low-carbon alkane is propane or isobutane; the alkyl vinyl ester is vinyl acetate, vinyl propionate or vinyl benzoate, and the alkyl ethyl ester is ethyl acetate, ethyl propionate or ethyl benzoate; the mass ratio of low-carbon alkane, alkyl ethyl ester and alkyl vinyl ester is (0.1-0.3):(0.005-0.02):1; the carbon-carbon skeleton 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 added amount is 0.01-0.05wt.% based on the total mass of the alkyl vinyl ester and ethylene;

[0009] S2: adding a polymerization inhibitor to terminate the reaction to obtain a reaction solution of ethylene-alkyl vinyl ester copolymer, removing lower alkanes, unreacted ethylene and alkyl vinyl ester therefrom, and then preparing the reaction solution of ethylene-alkyl vinyl ester copolymer into an alcohol solution; the polymerization inhibitor is hydroquinone, tert-butylcatechol, n-octyl mercaptan, tert-dodecyl mercaptan or copper acetate;

[0010] S3: adding a catalyst to the alcohol solution to carry out a saponification reaction to obtain a solution containing ethylene-vinyl alcohol copolymer; the catalyst is sodium hydroxide;

[0011] S4: acid-washing, water-washing and drying the solution containing the ethylene-vinyl alcohol copolymer to obtain the ethylene-vinyl alcohol copolymer.

[0012] 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.

[0013] Preferably, in step S2, the mass ratio of the polymerization inhibitor to the alkyl vinyl ester is (0.005-0.03):1.

[0014] Preferably, in step S2, the alcohol in the alcohol solution is methanol, and the concentration of the alcohol solution is 10-30 wt.%.

[0015] Preferably, in step S3, the molar ratio of the catalyst to the alkylate vinyl ester unit in the ethylene-alkylate vinyl ester copolymer is (0.02-0.05):1.

[0016] Preferably, in step S3, the saponification reaction temperature is 60-80° C., and the saponification reaction time is 2-5 h.

[0017] Preferably, in step S4, the acid used for pickling is acetic acid, boric acid or phosphoric acid; and the molar ratio of the acid to the catalyst is 1:(0.5-2).

[0018] Preferably, in step S4, the drying temperature is 60-120°C.

[0019] On the other hand, the present invention provides an ethylene-vinyl alcohol copolymer prepared by the above-mentioned preparation method of the ethylene-vinyl alcohol copolymer, wherein the ethylene molar content in the ethylene-vinyl alcohol copolymer is 27-44%, the insoluble gel content is less than 0.1wt.%, and the saponification degree is ≥99.5%.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention uses low-boiling-point, easily vaporized, and high-vaporization latent heat of low-carbon alkanes and highly soluble saturated alkyl ethyl esters as mixed solvents to replace traditional alcohol solvents for polymerization reactions. Firstly, because such mixed solvents are easily vaporized and have a high vaporization latent heat, reaction heat can be conveniently removed and controlled, and environmental pollution caused by alcohol solvents and the amount of acetaldehyde produced as a by-product by the alcohol solvents and the alkyl ethyl esters can be reduced. Secondly, the "like dissolves like" principle, in which the alkyl ethyl esters and polymers have similar structures and solubilities, is utilized to reduce the formation of insoluble gels to prevent them from adhering to the reactor wall and pipeline valves. Furthermore, the carbon-carbon skeleton initiator used in the present invention has high thermal stability, does not contain yellowing groups, and is easy to obtain copolymers with a narrow molecular weight distribution. Finally, after flash evaporation at reduced pressure, the low-carbon alkanes can be quickly separated from monomers and by-products, significantly reducing the cost of separating and recovering the alcohol solvents and monomers that form azeotropes, thereby improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the H of the EVOH product prepared in Example 1 of the present invention 1 NMR spectrum. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0024] Example 1

[0025] The preparation method of the ethylene-vinyl alcohol copolymer of the present embodiment comprises the following steps:

[0026] S1: After testing the airtightness of a 2000 mL stainless steel reaction kettle (with jacket and inner coil cooler), it was heated to 120°C, vacuumed to -100 kPa with a vacuum pump, maintained for 30 min, then replaced with nitrogen for 3 times, and then replaced with ethylene gas for 2 times; after the reaction kettle was cooled to below 40°C, 780 g of vinyl acetate was added, then a mixture of carbon-carbon skeleton initiator 2,3-dicyano-2,3-diphenyl butanedioic acid diisobutyl ester and 8 g of ethyl acetate was added into the reaction kettle, then 160 g of isobutane was injected into the reaction kettle with a plunger pump, and the amount of carbon-carbon skeleton initiator added in the system was controlled to be 0.03 wt.% of the total mass of vinyl acetate and ethylene; the stirring was started and 3.5 MPaG of ethylene was filled, then the temperature was raised to 65°C, and the reaction system pressure was kept stable by continuously filling ethylene, and the reaction heat of the system was removed by evaporation vaporization of isobutane, and the reaction was carried out for 3 h;

[0027] S2: After the reaction was completed, it was cooled to room temperature, the pressure was released, 100 ppm of hydroquinone based on the mass of vinyl acetate was added, then the material was discharged, and the reaction liquid of ethylene-vinyl acetate copolymer was obtained, the conversion rate of vinyl acetate was 35%, and the ethylene molar content was 32%; then the residual monomers in the reaction liquid of ethylene-vinyl acetate copolymer were removed by vacuum distillation, and the reaction liquid of ethylene-vinyl acetate copolymer was prepared into a 20 wt.% methanol solution;

[0028] S3: Sodium hydroxide was added to the methanol solution, the molar ratio of sodium hydroxide to vinyl acetate units in the ethylene-vinyl acetate copolymer was 0.04:1, then the temperature was raised to 60°C, during which methanol was continuously removed by nitrogen bubbling and supplemented to maintain its concentration unchanged, and after 5 h of reaction, a solution containing ethylene-vinyl alcohol copolymer was obtained;

[0029] S4: The solution containing ethylene-vinyl alcohol copolymer was poured into pure water containing acetic acid for acid washing and neutralization, the molar ratio of acetic acid to sodium hydroxide was 1:1, then it was washed with pure water for 3 times, and the precipitate was obtained after filtration; the above precipitate was dried in a vacuum oven at 60°C for 24 h to obtain EVOH crude product, the saponification degree was 99.6%, and the yellowness index YI was 2.1.

[0030] The H 1 The NMR spectrum is as follows Figure 1As shown, it can be seen that the peak with a chemical shift of 0.7-0.9 ppm represents the hydrogen on the -CH3 side chain; the peak with a chemical shift of 1.4-1.5 ppm represents the hydrogen on the -CH2 side of ethylene and vinyl alcohol; the peak with a chemical shift of 3.4-3.9 ppm represents the hydrogen on the -CH side of vinyl alcohol; and the peak with a chemical shift of 3.9-4.5 ppm represents the hydrogen on the -OH side of vinyl alcohol. Among them, 2.5 ppm and 3.4 ppm are the peak positions of H2O and DMSO, respectively. In summary, this example proves that EVOH product was successfully prepared.

[0031] Example 2

[0032] The preparation method of the ethylene-vinyl alcohol copolymer of this embodiment comprises the following steps:

[0033] S1: After testing the air tightness of a 2000mL stainless steel reactor (with a jacket and an inner coil cooler), heat it to 120°C, use a vacuum pump to evacuate the negative pressure to -100kPa, maintain for 30 minutes, then replace it with nitrogen three times, and then replace it with ethylene gas twice; after the reactor is cooled to below 40°C, add 780g of vinyl acetate, and then add a mixture of carbon-carbon skeleton initiator 2,3-dicyano-2,3-diphenylsuccinate and 4g of ethyl acetate to the reactor, and then use a plunger pump to inject 80g of isobutane into the reactor, and control the amount of carbon-carbon skeleton initiator added to the system to be 0.05wt.% of the total mass of vinyl acetate and ethylene; start stirring and charge 2MPaG ethylene, after heating to 60°C, continuously charge ethylene to keep the pressure of the reaction system stable, and remove the reaction heat of the system by evaporation of isobutane, and react for 6h;

[0034] S2: After the reaction is completed, the mixture is cooled to room temperature, the pressure is released, 300 ppm by weight of tert-butylcatechol based on the weight of vinyl acetate is added, and the mixture is discharged to obtain a reaction solution of ethylene-vinyl acetate copolymer. The conversion of vinyl acetate is 42% and the molar ethylene content is 27%. Subsequently, the residual monomers in the reaction solution of ethylene-vinyl acetate copolymer are removed by distillation under reduced pressure, and the reaction solution of ethylene-vinyl acetate copolymer is prepared into a 10 wt.% methanol solution.

[0035] S3: adding sodium hydroxide to the methanol solution, wherein the molar ratio of sodium hydroxide to the vinyl acetate unit in the ethylene-vinyl acetate copolymer is 0.02:1, and then heating to 80° C., during which methanol is continuously removed by nitrogen bubbling and replenished to maintain a constant concentration. After reacting for 2 hours, a solution containing the ethylene-vinyl alcohol copolymer is obtained;

[0036] S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved in phosphoric acid for acid washing and neutralization, wherein the molar ratio of phosphoric acid to sodium hydroxide is 0.5:1, and then wash with pure water three times. After filtering, a precipitate is obtained; the above precipitate is placed in a vacuum oven at 80°C and dried for 16 hours to obtain crude EVOH with a saponification degree of 99.8% and a yellowness index YI of 2.3.

[0037] Example 3

[0038] The preparation method of the ethylene-vinyl alcohol copolymer of this embodiment comprises the following steps:

[0039] S1: After testing the air tightness of a 2000mL stainless steel reactor (with a jacket and an inner coil cooler), heat it to 120°C, use a vacuum pump to evacuate the negative pressure to -100kPa, maintain for 30 minutes, then replace it with nitrogen three times, and then replace it with ethylene gas twice; after the reactor is cooled to below 40°C, add 780g of vinyl acetate, and then add a mixture of carbon-carbon skeleton initiator 2,3-dicyano-2,3-diphenylsuccinate and 12g of ethyl acetate to the reactor, and then use a plunger pump to inject 230g of isobutane into the reactor, and control the amount of carbon-carbon skeleton initiator added to the system to be 0.01wt.% of the total mass of vinyl acetate and ethylene; start stirring and charge 6MPaG ethylene, after heating to 70°C, continuously charge ethylene to keep the pressure of the reaction system stable, and remove the reaction heat of the system by evaporation of isobutane, and react for 2h;

[0040] S2: After the reaction is completed, the mixture is cooled to room temperature, the pressure is released, and 50 ppm by weight of n-octyl mercaptan based on the weight of vinyl acetate is added. The mixture is then discharged to obtain a reaction solution of ethylene-vinyl acetate copolymer, with a vinyl acetate conversion of 32% and an ethylene molar content of 44%. Subsequently, the residual monomers in the reaction solution of the ethylene-vinyl acetate copolymer are removed by distillation under reduced pressure, and the reaction solution of the ethylene-vinyl acetate copolymer is prepared into a 20 wt.% methanol solution.

[0041] S3: Sodium hydroxide was added to the methanol solution, with the molar ratio of sodium hydroxide to vinyl acetate units in the ethylene-vinyl acetate copolymer being 0.03:1. The temperature was then raised to 60° C., during which methanol was continuously removed by nitrogen bubbling and replenished to maintain a constant concentration. After reacting for 4 hours, a solution containing ethylene-vinyl alcohol copolymer was obtained.

[0042] S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved with boric acid for pickling and neutralization, wherein the molar ratio of boric acid to sodium hydroxide is 2:1, and then wash with pure water three times. After filtering, a precipitate is obtained; the above precipitate is placed in a vacuum oven at 100°C and dried for 12 hours to obtain crude EVOH with a saponification degree of 99.6% and a yellowness index YI of 2.1.

[0043] Example 4

[0044] The preparation method of the ethylene-vinyl alcohol copolymer of this embodiment comprises the following steps:

[0045] S1: After testing the air tightness of a 2000mL stainless steel reactor (with a jacket and an inner coil cooler), heat it to 120°C, use a vacuum pump to reduce the negative pressure to -100kPa, maintain for 30 minutes, then replace it with nitrogen three times, and then replace it with ethylene gas twice; after the reactor is cooled to below 40°C, add 800g of vinyl propionate, and then add a mixture of carbon-carbon skeleton initiator 2,3-dicyano-2,3-diphenylsuccinate and 16g of ethyl propionate to the reactor, and then use a plunger pump to inject 160g of propane into the reactor, controlling the amount of carbon-carbon skeleton initiator added to the system to be 0.04wt.% of the total mass of vinyl propionate and ethylene; start stirring and charge 4MPaG ethylene, after heating to 65°C, continuously charge ethylene to keep the pressure of the reaction system stable, and remove the reaction heat of the system by evaporation of propane, and react for 4h;

[0046] S2: After the reaction is completed, the mixture is cooled to room temperature, the pressure is released, and 100 ppm by weight of copper acetate based on the weight of the vinyl propionate is added. The mixture is then discharged to obtain a reaction solution of ethylene-vinyl propionate copolymer, with a vinyl propionate conversion of 38% and an ethylene molar content of 32%. Subsequently, the residual monomers in the reaction solution of the ethylene-vinyl propionate copolymer are removed by vacuum distillation, and the reaction solution of the ethylene-vinyl propionate copolymer is then prepared into a 30 wt.% methanol solution.

[0047] S3: adding sodium hydroxide to the methanol solution, wherein the molar ratio of sodium hydroxide to the vinyl propionate unit in the ethylene-vinyl propionate copolymer is 0.05:1, and then heating to 70° C., during which methanol is continuously removed by nitrogen bubbling and replenished to maintain its concentration. After reacting for 3 hours, a solution containing the ethylene-vinyl alcohol copolymer is obtained;

[0048] S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved with acetic acid for acid washing and neutralization, wherein the molar ratio of acetic acid to sodium hydroxide is 1:1, and then wash with pure water three times. After filtering, a precipitate is obtained; the above precipitate is placed in a vacuum oven at 120°C and dried for 8 hours to obtain crude EVOH with a saponification degree of 99.7% and a yellowness index YI of 2.6.

[0049] Example 5

[0050] The preparation method of the ethylene-vinyl alcohol copolymer of this embodiment comprises the following steps:

[0051] S1: After testing the air tightness of a 2000mL stainless steel reactor (with a jacket and an inner coil cooler), heat it to 120°C, use a vacuum pump to evacuate the negative pressure to -100kPa, maintain for 30 minutes, then replace it with nitrogen three times, and then replace it with ethylene gas twice; after the reactor is cooled to below 40°C, add 780g of vinyl benzoate, and then add a mixture of carbon-carbon skeleton initiator 2,3-dicyano-2,3-diphenylsuccinate and 8g of ethyl benzoate to the reactor, and then use a plunger pump to inject 160g of propane into the reactor, controlling the amount of carbon-carbon skeleton initiator added to the system to be 0.03wt.% of the total mass of vinyl benzoate and ethylene; start stirring and charge 3.5MPaG ethylene, after heating to 65°C, continuously charge ethylene to keep the pressure of the reaction system stable, and remove the reaction heat of the system by evaporation of propane, and react for 3h;

[0052] S2: After the reaction is completed, the mixture is cooled to room temperature, the pressure is released, 100 ppm by weight of vinyl benzoate based on tert-dodecyl mercaptan is added, and the mixture is discharged to obtain a reaction solution of ethylene-vinyl benzoate copolymer, with a vinyl benzoate conversion of 35% and an ethylene molar content of 32%. Subsequently, the residual monomers in the reaction solution of the ethylene-vinyl benzoate copolymer are removed by reduced pressure distillation, and the reaction solution of the ethylene-vinyl benzoate copolymer is prepared into a 20 wt.% methanol solution.

[0053] S3: adding sodium hydroxide to the methanol solution, wherein the molar ratio of sodium hydroxide to the vinyl benzoate unit in the ethylene-vinyl benzoate copolymer is 0.03:1, and then heating to 60° C., during which methanol is continuously removed by nitrogen bubbling and replenished to maintain a constant concentration. After reacting for 4 hours, a solution containing the ethylene-vinyl alcohol copolymer is obtained;

[0054] S4: Pour the solution containing ethylene-vinyl alcohol copolymer into pure water dissolved with acetic acid for acid washing and neutralization, wherein the molar ratio of acetic acid to sodium hydroxide is 1:1, and then wash with pure water three times. After filtering, a precipitate is obtained; the above precipitate is placed in a vacuum oven at 60°C and dried for 24 hours to obtain crude EVOH with a saponification degree of 99.6% and a yellowness index YI of 2.1.

[0055] Comparative Example 1

[0056] The difference from Example 1 is that in step S1, isobutane is replaced by methanol.

[0057] Comparative Example 2

[0058] The difference from Example 1 is that in step S1, the amount of isobutane added is 320 g.

[0059] Comparative Example 3

[0060] The difference from Example 1 is that in step S1, ethyl acetate is not added.

[0061] Comparative Example 4

[0062] The difference from Example 1 is that in step S1, isobutane is replaced by n-hexane.

[0063] Comparative Example 5

[0064] The difference from Example 1 is that in step S1, isobutane is replaced by n-heptane.

[0065] Comparative Example 6

[0066] The difference from Example 1 is that in step S1, the amount of ethyl acetate added is 20 g.

[0067] Comparative Example 7

[0068] The difference from Example 1 is that in step S1, the carbon-carbon skeleton initiator is replaced by azobisisobutyronitrile.

[0069] Performance tests were conducted on the products of Examples 1-5 and Comparative Examples 1-7. The insoluble gel content was determined by filtering the reaction solution and calculating the mass percentage of the material remaining on the filter paper. The residual methanol content in the monomer was determined using LC-MS. The residual acetaldehyde content in EVA was determined using headspace gas chromatography, in accordance with GB 31604.60-2024 National Food Safety Standard for Residual Solvents in Food Contact Materials and Products. The saponification degree was determined using titration, in accordance with GB / T 41877.2-2022 Plastics: Ethylene-vinyl alcohol (EVOH) copolymers, molding and extrusion materials - Part 2: Preparation of specimens and determination of properties. The yellowness index (YI) was determined using a yellowness index meter. The test results are shown in Table 1.

[0070] Table 1 Test results of products of Examples 1-5 and Comparative Examples 1-7

[0071]

[0072] As can be seen from Table 1, the comparative example 1 uses traditional methanol as the solvent, and a large amount of acetaldehyde is generated after polymerization, which affects the yellowness index of the product, and the reaction heat is not easy to remove, the insoluble gel of the system tends to increase, which affects the subsequent saponification process, and a large amount of methanol still remains after the monomer is separated. In the comparative example 2, isobutane is added in excess, and in the comparative example 3, ethyl acetate is not added, although the removal effect of the reaction heat is good, but it will make the solubility of the system worse, so that the amount of insoluble gel increases, which affects the molecular structure of the product. In the comparative examples 4-5, conventional high-boiling point alkane solvents are used, in addition to reducing the by-products and residual solvents in the monomer, due to the difficulty in removing the reaction heat, a large amount of side reactions will also occur, which will produce corresponding crosslinking and branching, affecting the subsequent alcoholysis and yellowness index. In the comparative example 6, excess ethyl acetate is added as the solvent, although the polymerization effect is good, but it remains in the product, which affects the subsequent saponification process and reduces the saponification degree. As can be seen from the comparative example 7, when the conventional azobisisobutyronitrile is used as the initiator, the molecular weight distribution of the prepared EVOH product will be widened. This is because the carbon-carbon skeleton initiator has the functions of "initiation- transfer-termination" at the same time, which can play the role of "controlled polymerization", and to a certain extent, the molecular weight distribution is reduced.

[0073] In summary, the present application uses low-boiling-point, easy-vaporization low-carbon alkanes and high-solubility saturated alkyl acid ethyl esters as mixed solvents, which can realize efficient separation of monomers and solvents, prevent the separation and reuse process from causing a substantial sacrifice of energy consumption and efficiency due to the azeotropy with methanol, and ensure the purity of the monomers by using the mixed solvents of the present application. In addition, the phase change of the solvent can be realized by using the evaporation vaporization heat, so that the removal of the reaction heat is 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 in the polymerization process can be reduced, the molecular weight can be improved, the molecular weight distribution can be reduced, the branching can be more uniform, and the content of the insoluble gel generated can be reduced by using the "similar phase dissolving" principle of the saturated alkyl acid ethyl ester and the polymer structure and solubility, so that the saponification reaction is more efficient and the saponification degree is higher. Finally, the aldehyde impurities generated by the side reaction of traditional alcohol solvents and alkyl acid ethyl esters are reduced, the gel degree of the polymer is reduced, and the color value is effectively improved.

Claims

1. A method for preparing an ethylene-vinyl alcohol copolymer, characterized in that: The following steps are involved: S1: Ethylene, low-carbon alkane, alkyl ethyl ester, alkyl vinyl ester and carbon-carbon skeleton initiator are added to the reactor to carry out free radical polymerization reaction; wherein the low-carbon alkane is propane or isobutane; the alkyl vinyl ester is vinyl acetate, vinyl propionate or vinyl benzoate, and the alkyl ethyl ester is ethyl acetate, ethyl propionate or ethyl benzoate; the mass ratio of low-carbon alkane, alkyl ethyl ester and alkyl vinyl ester is (0.1-0.3):(0.005-0.02):1; the carbon-carbon skeleton 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 added amount is 0.01-0.05wt.% based on the total mass of the alkyl vinyl ester and ethylene; S2: adding a polymerization inhibitor to terminate the reaction to obtain a reaction solution of ethylene-alkyl vinyl ester copolymer, removing lower alkanes, unreacted ethylene and alkyl vinyl ester therefrom, and then preparing the reaction solution of ethylene-alkyl vinyl ester copolymer into an alcohol solution; the polymerization inhibitor is hydroquinone, tert-butylcatechol, n-octyl mercaptan, tert-dodecyl mercaptan or copper acetate; S3: adding a catalyst to the alcohol solution to carry out a saponification reaction to obtain a solution containing ethylene-vinyl alcohol copolymer; the catalyst is sodium hydroxide; S4: acid-washing, water-washing and drying the solution containing the ethylene-vinyl alcohol copolymer to obtain the ethylene-vinyl alcohol copolymer.

2. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, wherein 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.

3. The method for preparing an ethylene-vinyl alcohol copolymer according to claim 1, wherein In step S2, the mass ratio of the polymerization inhibitor to the alkyl vinyl ester 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, wherein In step S3, the molar ratio of the catalyst to the alkylate vinyl ester unit in the ethylene-alkylate vinyl ester copolymer is (0.02-0.05):

1.

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

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, wherein In step S4, the drying temperature is 60-120°C.

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

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