Preparation method of ethylene-vinyl alcohol copolymer

Abstract

The present invention relates to a method for preparing an ethylene-vinyl alcohol copolymer, which can obtain an ethylene-vinyl alcohol copolymer with a high saponification degree while minimizing the amount of the used alkali catalyst.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2021-0149099 filed in the Korean Intellectual Property Office on November 2, 2021, the disclosure of which is incorporated herein by reference in its entirety.

[0003] The present invention relates to a method for preparing ethylene-vinyl alcohol copolymer. Background Art

[0004] Since ethylene-vinyl alcohol copolymer (EVOH) has excellent barrier properties to gases such as oxygen, transparency, oil resistance, non-static properties, mechanical strength, and the like, it is widely used as a material for films, sheets, containers, and the like.

[0005] EVOH can be prepared by saponification of ethylene-vinyl acetate copolymer (EVAc) prepared by copolymerization of ethylene and vinyl acetate. As a catalyst for the saponification reaction of EVAc, an alkali catalyst such as sodium hydroxide, potassium hydroxide, alkali metal alcoholate, etc. is mainly used.

[0006] Since EVOH with a higher saponification degree exhibits more excellent gas barrier properties, it is preferred that EVOH used for food packaging, etc. has a high saponification degree of 99% or more. In order to obtain such a high saponification degree, the saponification reaction was previously carried out at a higher temperature, or the amount of the alkali catalyst used was increased.

[0007] However, in the case of a high-temperature saponification reaction, a side reaction may easily occur, and in the case of using a large amount of an alkali catalyst, the amount of catalyst byproducts in the saponification product may increase. Since the catalyst byproducts cause discoloration of EVOH, a cleaning process for removing the catalyst byproducts is involved after the saponification reaction, but if the content of the catalyst byproducts is high, an excessive cleaning process is required, thereby deteriorating the process efficiency and generating an excessive amount of wastewater, which is not preferred in terms of the environment. Summary of the invention

[0008] [Technical issues]

[0009] In order to solve this problem, an object of the present invention is to provide a method for preparing an ethylene-vinyl alcohol copolymer, which method is capable of obtaining an ethylene-vinyl alcohol copolymer having a high saponification degree while minimizing the amount of an alkali catalyst used.

[0010] [Technical solution]

[0011] According to one embodiment of the present invention, there is provided a method for preparing an ethylene-vinyl alcohol copolymer, comprising:

[0012] a first saponification reaction step in which a first alkali catalyst solution is reacted while being added dropwise to an ethylene-vinyl acetate copolymer dispersed in an alcohol solvent; and

[0013] A second saponification reaction step in which the second alkali catalyst solution is reacted while being added dropwise to the first saponification reaction mixture.

[0014] [Effects of the Invention]

[0015] According to the present invention, an ethylene-vinyl alcohol copolymer with a high saponification degree can be obtained while minimizing the amount of the alkali catalyst used. Therefore, according to the present invention, the efficiency of the cleaning process for removing the catalyst by-products contained in the ethylene-vinyl alcohol copolymer after the saponification process can be improved, and the amount of waste water generated during the cleaning process can be minimized. DETAILED DESCRIPTION

[0016] The terms used herein are only for explaining specific embodiments, not for limiting the present invention. Singular expressions include plural expressions thereof, unless explicitly stated or it is obvious from the context that this is not the intention. As used herein, the terms "including", "equipped with" or "having" etc. are intended to indicate the presence of the features, quantities, steps, structural elements or combinations thereof practiced, and they are not intended to exclude the possibility of the presence or addition of one or more other features, quantities, steps, structural elements or combinations thereof.

[0017] Although various modifications can be made to the present invention and the present invention can have various forms, specific examples will be described in detail and explained below. However, it should be understood that these are not intended to limit the present invention to specific disclosures, and the present invention includes all modifications, equivalents or replacements thereof without departing from the spirit and technical scope of the present invention.

[0018] Hereinafter, the present invention will be explained in detail.

[0019] The method for preparing the ethylene-vinyl alcohol copolymer of the present invention comprises:

[0020] a first saponification reaction step in which a first alkali catalyst solution is reacted while being added dropwise to an ethylene-vinyl acetate copolymer dispersed in an alcohol solvent; and

[0021] A second saponification reaction step in which the second alkali catalyst solution is reacted while being added dropwise to the first saponification reaction mixture.

[0022] According to the present invention, the saponification reaction of ethylene-vinyl acetate copolymer (EVAc) is divided into two steps, and the alkali catalyst solution is not introduced at once, but is added dropwise continuously in each step. Here, "dropwise addition" means the dropwise introduction of the solution.

[0023] Therefore, if the alkali catalyst solution is continuously added dropwise, an ethylene-vinyl alcohol copolymer with a high degree of saponification can be obtained even with a small amount of catalyst, compared with the case where the catalyst is introduced all at once, and thus the cleaning method for removing the alkali catalyst by-product contained in the prepared ethylene-vinyl alcohol copolymer after the saponification reaction can be simplified, thereby improving process efficiency and economic feasibility and minimizing the amount of wastewater generated.

[0024] According to the present invention, as the reactant ethylene-vinyl acetate copolymer, a commercially available product may be used, or it may be prepared by copolymerization of ethylene and vinyl acetate monomers.

[0025] Furthermore, the ethylene-vinyl acetate copolymer may be those obtained by copolymerizing ethylene and vinyl acetate, and may also be those obtained by copolymerizing monomers copolymerizable therewith. Examples of such monomers include: α-olefins such as propylene, isobutylene, α-octene, α-dodecene, etc.; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, etc., and their salts, anhydrides, monoalkyl esters or dialkyl esters; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefin sulfonic acids such as ethylene sulfonic acid, methallyl sulfonic acid, etc., or their salts; vinyl monomers such as alkyl vinyl ethers, vinyl ketones, N-vinyl pyrrolidone, vinyl chloride, vinylidene chloride, etc.

[0026] The ethylene content of the ethylene-vinyl acetate copolymer can be appropriately controlled according to the target properties of the ethylene-vinyl alcohol copolymer. According to one embodiment, the ethylene content of the ethylene-vinyl acetate copolymer may be 20 mol% or more, 25 mol% or more, 27 mol% or more, or 30 mol% or more, and 60 mol% or less, 50 mol% or less, 48 ​​mol% or less, or 35 mol% or less. If the ethylene content of the ethylene-vinyl acetate copolymer is too low, the melt forming properties and gas barrier properties of the prepared ethylene-vinyl alcohol copolymer under high humidity may be deteriorated, and if the ethylene content is too high, the gas barrier properties may be deteriorated. Meanwhile, the ethylene content may be adjusted by adjusting the content of the ethylene-vinyl acetate copolymer or the ethylene-vinyl alcohol copolymer. 1 The integrated numerical ratios of the peaks in the H-NMR data were calculated as described in the Examples below.

[0027] The weight average molecular weight (Mw) of the ethylene-vinyl acetate copolymer is not particularly limited, but for example, it may be 180,000 g / mol or more, 200,000 g / mol or more, or 220,000 g / mol or more, and 290,000 g / mol or less, 270,000 g / mol or less, or 260,000 g / mol or less. The weight average molecular weight of the ethylene-vinyl alcohol copolymer obtained by the saponification reaction of the ethylene-vinyl acetate copolymer satisfying such a weight average molecular weight may be 120,000 g / mol or more, 130,000 g / mol or more, or 140,000 g / mol or more, and 180,000 g / mol or less, 170,000 g / mol or less, or 160,000 g / mol or less.

[0028] If the weight average molecular weight of the ethylene-vinyl acetate copolymer is too large, the viscosity of the prepared ethylene-vinyl alcohol copolymer may become too high, making melt extrusion difficult, and if it is too small, the film-forming properties may become unstable when preparing a film. The weight average molecular weight of the ethylene-vinyl acetate copolymer and the ethylene-vinyl alcohol copolymer can be measured by gel permeation chromatography (GPC), as described below.

[0029] As the alcohol solvent, a solvent commonly used in the saponification reaction of ethylene-vinyl acetate copolymer can be used. For example, as the alcohol solvent, a lower alcohol solvent such as methanol, ethanol, propanol, isopropanol or butanol can be used, among which methanol is industrially preferred due to its easy availability and low cost.

[0030] As the first base catalyst and the second base catalyst, one or more selected from sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium methoxide, sodium ethoxide, potassium tert-butoxide, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, sodium acetate, potassium acetate and sodium propionate can be used, and for the preparation of high saponification EVOH, sodium hydroxide, potassium hydroxide, sodium methoxide or sodium ethoxide can be preferably used.

[0031] The first base catalyst and the second base catalyst may be the same as or different from each other, and preferably, as the first base catalyst and the second base catalyst, the same material may be used.

[0032] The total amount of the first base catalyst and the second base catalyst used may be 0.01 mole or more or 0.015 mole or more and less than 0.03 mole or less than 0.02 mole based on 1 mole of the vinyl acetate unit of the ethylene-vinyl acetate copolymer.

[0033] Previously, in order to prepare an ethylene-vinyl alcohol copolymer having a high saponification degree, 0.03 mol or more of alcohol was required based on 1 mol of vinyl acetate unit, but according to the preparation method of the present invention, an ethylene-vinyl alcohol copolymer having a high saponification degree can be prepared while minimizing the amount of alcohol catalyst used. However, if the total amount of the alkaline catalyst used is less than 0.01 mol, the saponification reaction may become too slow, and therefore, it is preferable to satisfy the above catalyst amount.

[0034] Furthermore, within the range satisfying the total amount used, the first base catalyst and the second base catalyst may be used in an amount of 0.0025 mol or more or 0.005 mol or more and 0.02 mol or less or 0.01 mol or less, respectively, based on 1 mol of vinyl acetate units of the ethylene-vinyl acetate copolymer.

[0035] The first alkali catalyst and the second alkali catalyst may be used in a molar ratio of 1: 1 to 1: 4 or 1: 1 to 1: 2. Therefore, by controlling the amount of the second alkali catalyst to be equal to or greater than the amount of the first alkali catalyst, an ethylene-vinyl alcohol copolymer having a high degree of saponification may be prepared.

[0036] The first alkali catalyst and the second alkali catalyst are added dropwise to the ethylene-vinyl acetate copolymer in the form of a solution. Among them, as a solvent, the above-mentioned alcohol solvent can be used, and preferably the same solvent as that used to prepare the ethylene-vinyl acetate copolymer dispersion is used. The concentrations of the alkali catalyst solutions may be the same as or different from each other, and may be controlled according to the concentration of the ethylene-vinyl acetate copolymer dispersion and the target dropwise addition speed of the alkali catalyst solution.

[0037] In the first saponification reaction step, an ethylene-vinyl acetate copolymer is first dispersed in an alcohol solvent to prepare a dispersion, and while a first base catalyst is added dropwise thereto, a saponification reaction is performed.

[0038] The amount of the alcohol solvent used for preparing the ethylene-vinyl acetate copolymer dispersion can be appropriately controlled according to the kind of the alcohol solvent, the kind of the ethylene-vinyl acetate copolymer, the concentration of the alkali catalyst solution, etc. For example, the alcohol solvent can be used in the range of 100 to 1000 parts by weight, or 200 parts by weight or more, or 300 parts by weight or more, and 800 parts by weight or 700 parts by weight or less, or 500 parts by weight, based on 100 parts by weight of the ethylene-vinyl acetate copolymer, but the amount is not limited thereto.

[0039] The dropwise addition speed of the first base catalyst solution can be controlled according to the reaction time, and, for example, the dropwise addition of the first base catalyst can be performed so that the introduction amount of the first base catalyst per minute becomes 1.0×10 -5 mole to 13×10-5 When the dropwise addition rate range is satisfied, the saponification reaction time may not be excessively prolonged, but the reaction efficiency may be improved, thereby preparing an ethylene-vinyl alcohol copolymer having a high saponification degree.

[0040] In this regard, it is preferred to perform the dropwise addition of the first base catalyst solution so that the introduction amount of the first base catalyst becomes 2.0×10 -5 mole or more, or 3.0×10 -5 mole or more, or 4.0×10 -5 mole or more, and 11.0×10 -5 Mole or less, or 10.0×10 -5 Mole or less, or 9.0×10 -5 Below mole.

[0041] The temperature of the first saponification reaction step may be 40° C. or higher, 50° C. or higher, or 60° C. or higher, and 120° C. or lower, 110° C. or lower, or 100° C. or lower. If the reaction temperature is lower than 40° C., the saponification reaction rate may become too slow, and if it is higher than 120° C., a side reaction may easily occur, so it is preferable to satisfy the above range.

[0042] The first saponification reaction may be performed under an inert gas atmosphere, and in order to increase the conversion rate, the methyl acetate by-product may be continuously discharged out of the system while the reaction is being performed.

[0043] At the same time, when the dropwise addition of the first alkali solution is completed, the completion of the first saponification reaction step can be achieved. That is, the dropwise addition of the first alkali solution can be performed continuously from the beginning to the end of the first saponification reaction. Therefore, by adding the first alkali solution dropwise during the reaction time, the conversion rate of EVAc to EVOH can be increased while minimizing side reactions.

[0044] After the first saponification reaction is completed, a second alkali catalyst solution prepared separately is added dropwise to the reaction mixture to perform a second saponification reaction.

[0045] The first saponification reaction step and the second saponification reaction step may be performed continuously. That is, immediately after the first saponification reaction step is completed, the second alkali catalyst solution may be added dropwise to the reaction mixture to perform the second saponification reaction.

[0046] The dropwise addition speed of the second base catalyst solution can be controlled according to the reaction time, and for example, the dropwise addition of the second base catalyst can be performed so that the introduction amount of the second base catalyst per minute becomes 2.0×10 -5 mole to 8.0×10-5 When the dropwise addition rate range is satisfied, the saponification reaction time may not be excessively prolonged, but the reaction efficiency may be improved, thereby preparing an ethylene-vinyl alcohol copolymer having a high saponification degree.

[0047] In this regard, it is preferred to perform the dropwise addition of the second base catalyst solution so that the introduction amount of the second base catalyst per minute becomes 2.0×10 -5 mole or more, or 3.0×10 -5 mole or more, or 4.0×10 -5 mole or more, and 7.0×10 -5 Mole or less or 6.0×10 -5 Below mole.

[0048] The temperature of the second saponification reaction step may be 60° C. or higher, 70° C. or higher, or 80° C. or higher, and 120° C. or lower, 110° C. or lower, or 100° C. or lower. If the reaction temperature is lower than 60° C., the saponification reaction rate may become too slow, and if it is higher than 120° C., a side reaction may easily occur.

[0049] Meanwhile, as long as the above range is satisfied, the temperature of the second saponification reaction step may be equal to or higher than the temperature of the first saponification reaction step. In this case, the reaction efficiency may be improved, and thus an ethylene-vinyl alcohol copolymer having a high degree of saponification may be obtained.

[0050] The second saponification reaction may be performed under an inert gas atmosphere, and in order to increase the conversion rate, the methyl acetate by-product may be continuously discharged out of the system while the reaction is being performed.

[0051] Similar to the first saponification reaction step, the second saponification reaction step can be completed when the dropwise addition of the second alkaline solution is completed. That is, the dropwise addition of the second alkaline solution can be performed continuously from the beginning to the end of the second saponification reaction. Therefore, by adding the second alkaline solution dropwise during the reaction time, the conversion rate of EVAc to EVOH can be increased while minimizing side reactions.

[0052] According to the above preparation method, compared with the existing batch saponification method of ethylene-vinyl acetate copolymer, an ethylene-vinyl alcohol copolymer with a high saponification degree of 99% or more can be obtained by using a small amount of alkali catalyst. Since the ethylene-vinyl alcohol copolymer has a high saponification degree, it has excellent gas barrier properties and can be effectively used for food packaging, etc.

[0053] For example, the ethylene-vinyl alcohol copolymer prepared according to the preparation method may have a saponification degree of 99% or more or 99% to 99.9%; an ethylene content of 20 mol% or more, 25 mol% or more, 27 mol% or more, or 30 mol% or more and 60 mol% or less, 50 mol% or less, 48 ​​mol% or less, or 35 mol% or less; and a weight average molecular weight of 120000 g / mol or more, 130000 g / mol or more, or 140000 g / mol or more and 180000 g / mol or less, 170000 g / mol or less, or 160000 g / mol or less. Such an ethylene-vinyl alcohol copolymer may exhibit excellent moldability and gas barrier properties.

[0054] In the following, in order to better understand the present invention, preferred embodiments will be given, but these embodiments are only given as illustrations of the present invention, and it is obvious to those skilled in the art that various modifications and changes can be made within the scope of the category and technical idea of ​​the present invention, and such modifications and changes fall within the scope of the appended claims.

[0055] <Example>

[0056] Example 1

[0057] 100 parts by weight of ethylene-vinyl acetate copolymer (EVAc, Mw 238×10 3 g / mol) and 400 parts by weight of methanol were placed in a saponification reactor, and 20 parts by weight of a methanol solution of sodium hydroxide (16 g / L) (sodium hydroxide / vinyl acetate unit=0.01 / 1, molar ratio) were continuously added dropwise to the saponification reactor over 2 hours to carry out a first saponification reaction. The dropwise addition rate was controlled so that the molar number of the introduced sodium hydroxide / vinyl acetate unit of EVAc became 8.3×10 -5 While the methanol solution of sodium hydroxide was continuously introduced into the reactor as described above, nitrogen was injected into the reactor and reacted at 60°C for 2 hours while the methyl acetate by-product was removed from the system together with methanol.

[0058] Furthermore, while continuously adding 20 parts by weight of a separately prepared methanol solution of sodium hydroxide (16 g / L) (sodium hydroxide / vinyl acetate unit=0.01 / 1, molar ratio) dropwise to the saponification reactor for 3 hours, a second saponification reaction was performed. The dropwise addition rate was controlled so that the molar number of the introduced sodium hydroxide / vinyl acetate unit of EVAc became 5.6×10 -5While the methanol solution of sodium hydroxide was continuously introduced into the reactor as described above, nitrogen was injected into the reactor and the reaction was carried out at 100° C. for 3 hours while the methyl acetate by-product was removed from the system together with methanol.

[0059] Then, the reaction mixture was neutralized with 120 parts by weight of an acetic acid aqueous solution (9 g / L) (acetic acid / sodium hydroxide=1 / 1, molar ratio) to stop the reaction, and an EVOH methanol / water solution consisting of 60 parts by weight of an ethylene-vinyl alcohol copolymer (EVOH), 120 parts by weight of methanol, and 120 parts by weight of water was obtained. The EVOH solution was cooled to room temperature to condense. Then, it was washed by repeatedly draining water with a centrifuge, adding water, and then draining again. And, it was dried under vacuum at 80°C for 16 hours to prepare an EVOH with a moisture content of 0.01% by mass.

[0060] Example 2

[0061] 100 parts by weight of ethylene-vinyl acetate copolymer (EVAc, Mw 242×10 3 g / mol) and 400 parts by weight of methanol were placed in a saponification reactor, and 10 parts by weight of a methanol solution of sodium hydroxide (16 g / L) (sodium hydroxide / vinyl acetate unit=0.005 / 1, molar ratio) was continuously added dropwise to the saponification reactor over 2 hours to carry out a first saponification reaction. The dropwise addition rate was controlled so that the molar number of the introduced sodium hydroxide / vinyl acetate unit of EVAc became 4.2×10 -5 While the methanol solution of sodium hydroxide was continuously introduced into the reactor as described above, nitrogen was injected into the reactor and reacted at 100° C. for 2 hours while the methyl acetate byproduct was removed from the system together with methanol.

[0062] Furthermore, a second saponification reaction was carried out while continuously adding 20 parts by weight of a sodium hydroxide methanol solution (16 g / L) (sodium hydroxide / vinyl acetate unit=0.01 / 1, molar ratio) dropwise to the saponification reactor over 3 hours. The dropwise addition rate was controlled so that the molar number of the introduced sodium hydroxide / vinyl acetate unit of EVAc became 5.6×10 -5 While the methanol solution of sodium hydroxide was continuously introduced into the reactor as described above, nitrogen was injected into the reactor and reacted at 100° C. for 3 hours while the by-product methyl acetate was removed from the system together with methanol.

[0063] Then, the reaction mixture was neutralized with 120 parts by weight of an acetic acid aqueous solution (9 g / L) (acetic acid / sodium hydroxide=1 / 1, molar ratio) to stop the reaction, thereby obtaining an EVOH methanol / water solution consisting of 60 parts by weight of EVOH, 120 parts by weight of methanol and 120 parts by weight of water. Then, EVOH was obtained from the EVOH solution by the same method as in Example 1.

[0064] Comparative Example 1

[0065] 100 parts by weight of ethylene-vinyl acetate copolymer (EVAc, Mw 241×10 3 g / mol) and 400 parts by weight of methanol are placed in a saponification reactor, and at the same time, 40 parts by weight of a methanol solution of sodium hydroxide (16 g / L) (sodium hydroxide / vinyl acetate unit=0.02 / 1, molar ratio) are introduced into the saponification reactor. Nitrogen is injected into the reactor, and the reaction is carried out at 100° C. for 5 hours, while the methyl acetate by-product is removed from the system together with the methanol. Then, the reaction mixture is neutralized with 120 parts by weight of an acetic acid aqueous solution (9 g / L) (acetic acid / sodium hydroxide=1 / 1, molar ratio) to stop the reaction, thereby obtaining an EVOH methanol / water solution consisting of 60 parts by weight of EVOH, 120 parts by weight of methanol and 120 parts by weight of water. Then, EVOH is obtained from the EVOH solution by the same method as in Example 1.

[0066] Comparative Example 2

[0067] 100 parts by weight of ethylene-vinyl acetate copolymer (Mw 236×10 3 g / mol) and 400 parts by weight of methanol are placed in a saponification reactor, and at the same time, 40 parts by weight of sodium hydroxide methanol solution (16g / L) (sodium hydroxide / vinyl acetate unit=0.02 / 1, molar ratio) are introduced into the saponification reactor. Nitrogen is injected into the reactor, and the reaction is carried out at 100° C. for 8 hours, while the methyl acetate by-product is removed from the system together with the methanol. Then, the reaction mixture is neutralized with 120 parts by weight of acetic acid aqueous solution (9g / L) (acetic acid / sodium hydroxide=1 / 1, molar ratio) to stop the reaction, thereby obtaining an EVOH methanol / water solution composed of 60 parts by weight of EVOH, 120 parts by weight of methanol and 120 parts by weight of water. Then, EVOH is obtained from the EVOH solution by the same method as in Example 1.

[0068] Comparative Example 3

[0069] 100 parts by weight of ethylene-vinyl acetate copolymer (Mw 237×10 3g / mol) and 400 weight parts of methanol are placed in a saponification reactor, and at the same time, 60 weight parts of methanol solution of sodium hydroxide (16g / L) (sodium hydroxide / vinyl acetate unit=0.03 / 1, molar ratio) are introduced into the saponification reactor. Nitrogen is injected into the reactor, and the reaction is carried out at 100° C. for 5 hours, and the methyl acetate by-product is removed from the system together with methanol. Then, the reaction mixture is neutralized with 120 weight parts of acetic acid aqueous solution (9g / L) (acetic acid / sodium hydroxide=1 / 1, molar ratio) to stop the reaction, thereby obtaining an EVOH methanol / water solution composed of 60 weight parts of EVOH, 120 weight parts of methanol and 120 weight parts of water. Then, EVOH is obtained from the EVOH solution by the same method as in Example 1, but due to the high content of catalyst impurities, the washing process is further repeated several times, and therefore, compared with Example 1, the waste water of the washing process increases by 50%.

[0070] Comparative Example 4

[0071] 100 parts by weight of ethylene-vinyl acetate copolymer (Mw 238×10 3 g / mol) and 400 parts by weight of methanol were placed in a saponification reactor, and simultaneously 20 parts by weight of a methanol solution of sodium hydroxide (16 g / L) (sodium hydroxide / vinyl acetate unit=0.01 / 1, molar ratio) were introduced into the saponification reactor. Nitrogen was injected into the reactor, and the reaction was carried out at 60° C. for 2 hours, while the methyl acetate by-product was removed from the system together with the methanol.

[0072] Then, 20 parts by weight of a sodium hydroxide methanol solution (16 g / L) (sodium hydroxide / vinyl acetate unit=0.01 / 1, molar ratio) were introduced into the saponification reactor at once. Nitrogen was injected into the reactor, and the reaction was allowed to proceed at 100° C. for 3 hours, while the methyl acetate by-product was removed from the system together with the methanol. Then, the reaction mixture was neutralized with 120 parts by weight of an acetic acid aqueous solution (9 g / L) (acetic acid / sodium hydroxide=1 / 1, molar ratio) to stop the reaction, thereby obtaining an EVOH methanol / water solution consisting of 60 parts by weight of EVOH, 120 parts by weight of methanol and 120 parts by weight of water. Then, EVOH was obtained from the EVOH solution by the same method as in Example 1.

[0073] The experimental conditions of the Examples and Comparative Examples are summarized in Table 1 below.

[0074] [Table 1]

[0075]

[0076] <Experimental example>

[0077] (1) Ethylene content of ethylene-vinyl acetate copolymer

[0078] Depend on 1 H-NMR data The ethylene content of the ethylene-vinyl acetate copolymers used in Examples 1 to 2 and Comparative Examples 1 to 3 was calculated by integration ratio.

[0079] Specifically, obtain 1 The integrated value of the -COOCH- peak (δ4.78) derived from the vinyl acetate unit in H-NMR, and the integrated value of the -CH2- peak derived from the ethylene unit is obtained from the integrated value of the δ0.74-2.10 peak, and the percentage of the ethylene unit in the total of the vinyl acetate unit and the ethylene unit is calculated to obtain the ethylene content of the ethylene-vinyl acetate copolymer.

[0080] (2) Saponification degree

[0081] Through EVOH 1 The saponification degree of the prepared EVOH was calculated from the integrated ratio of the peaks in the H-NMR data.

[0082] Specifically, obtain 1 The integral value of the -OH peak (δ4.05-4.72) derived from the vinyl alcohol unit data and the integral value of the -CH3COO- peak (δ1.99) derived from the vinyl acetate unit of the H-NMR data were calculated, and the percentage of the vinyl alcohol unit in the sum of the vinyl alcohol unit and the vinyl acetate unit was calculated to obtain the saponification degree.

[0083] (3) Measurement of molecular weight

[0084] The molecular weight was measured after GPC sampling (2.0 mg / mL in DMSO) for each of the EVOHs prepared in Examples 1 to 2 and Comparative Examples 1 to 3. The GPC analysis conditions were as follows.

[0085] <Analysis conditions>

[0086] Column: PLgel Mixed BX 2

[0087] Solvent: DMF (containing 0.05M LiBr)

[0088] Flow rate: 1.0mL / min

[0089] Sample injection volume: 100uL

[0090] Column temperature: 65°C

[0091] Detector: Waters 2414RID

[0092] Data processing: Empower 2

[0093] [Table 2]

[0094]

[0095] As a result of the experiment, it was confirmed that in the case of Examples 1 and 2 in which the sodium hydroxide catalyst was continuously added dropwise and introduced over the entire reaction time in the batch-type saponification method, the saponification degree of EVOH was 99.0 mol% or more, and therefore, Examples 1 and 2 showed higher saponification degrees than Comparative Examples 1 and 2 in which the catalyst was introduced once and Comparative Example 4 in which the catalyst was introduced twice. Also, Examples 1 and 2 also showed improved saponification degrees compared to Comparative Example 3 in which the amount of the catalyst used was high.

[0096] From the results, it was confirmed that according to the production method of the present invention, EVOH having a high saponification degree can be produced while minimizing the amount of the alkali catalyst used.

Claims

1. A method for preparing an ethylene-vinyl alcohol copolymer, the method comprising: a first saponification reaction step in which a first alkali catalyst solution is reacted while being added dropwise to an ethylene-vinyl acetate copolymer dispersed in an alcohol solvent; and a second saponification reaction step in which a second alkali catalyst solution is reacted while being added dropwise to the mixture obtained after the first saponification reaction step, wherein the total amount of the first base catalyst and the second base catalyst used is greater than or equal to 0.01 mole and less than or equal to 0.03 mole based on 1 mole of vinyl acetate units of the ethylene-vinyl acetate copolymer; and The first base catalyst and the second base catalyst are used in a molar ratio of 1:1 to 1:

4. The first alkali catalyst solution is added dropwise so that the first alkali catalyst is introduced in an amount of 1.0×10 -5 mole to 13×10 -5 Moore, and The second alkali catalyst solution is added dropwise so that the amount of the second alkali catalyst introduced per minute is 2.0×10 -5 mole to 8.0×10 -5 Moore.

2. The method according to claim 1, wherein: The first base catalyst and the second base catalyst are respectively used in an amount of 0.0025 mol to 0.02 mol based on 1 mol of the vinyl acetate unit of the ethylene-vinyl acetate copolymer.

3. The method according to claim 1, wherein: The first saponification reaction step is completed when the dropwise addition of the first base catalyst is completed, and / or the second saponification reaction step is completed when the dropwise addition of the second base catalyst is completed.

4. The method according to claim 1, wherein: The first saponification reaction step and the second saponification reaction step are performed continuously.

5. The method according to claim 1, wherein: The temperature of the second saponification reaction step is equal to or higher than the temperature of the first saponification reaction step.

6. The method according to claim 1, wherein: The temperature of the first saponification step is 40°C to 120°C.

7. The method according to claim 1, wherein: The temperature of the second saponification step is 60°C to 120°C.

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