Process for the preparation of ethylene-vinyl alcohol copolymers

Abstract

The present invention relates to a method for preparing an ethylene-vinyl alcohol copolymer, which can obtain an ethylene-vinyl alcohol copolymer of high purity without an excessive washing process. According to the present invention, the efficiency of a washing method for obtaining an ethylene-vinyl alcohol copolymer after saponification of ethylene-vinyl acetate can be improved, thereby improving productivity and minimizing the amount of waste water generated.

Description

Technical Field

[0001] Cross-reference of related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2021-0185017 filed with the Korean Intellectual Property Office on December 22, 2021 and Korean Patent Application No. 10-2022-0121584 filed on September 26, 2022, the disclosures of which are incorporated herein by reference in their entirety.

[0003] This disclosure relates to a method for preparing ethylene-vinyl alcohol copolymers. Background Technology

[0004] Ethylene-vinyl alcohol copolymer (EVOH) is widely used as a material for membranes, sheets, containers and other applications due to its excellent gas barrier properties such as oxygen, transparency, oil resistance, antistatic properties and mechanical strength.

[0005] EVOH can be prepared by the saponification reaction of ethylene-vinyl acetate copolymer (EVAc), which is prepared by copolymerization of ethylene and vinyl acetate. Specifically, the commonly known preparation method includes the following steps: saponifying EVAc in the presence of an alkaline catalyst, extruding the polymer component, molding it into granules, washing with water to remove residual catalyst and byproducts from the granules, and then drying.

[0006] However, the above method has the disadvantage of not easily removing impurities present in the granules. When an excessive amount of alkaline catalyst is used to increase the saponification degree of EVOH, the content of catalyst impurities in the granules also increases. To fully remove them, an excessive washing process is required, resulting in a large amount of wastewater and reducing overall productivity. Furthermore, if the granule size is reduced to improve the efficiency of the washing process, the productivity of extrusion and granulation decreases, making it undesirable in terms of production efficiency. Summary of the Invention

[0007] [Technical Issues]

[0008] This disclosure provides a method for preparing an ethylene-vinyl alcohol copolymer that can improve the efficiency of the washing process and minimize the amount of wastewater generated.

[0009] [Technical Solution]

[0010] According to one embodiment of the present invention, a method for preparing an ethylene-vinyl alcohol copolymer is provided, comprising the following steps:

[0011] In alkaline catalyst and C 1-4 The ethylene-vinyl acetate copolymer was saponified in the presence of an alcohol solvent to obtain a reaction mixture containing an ethylene-vinyl alcohol copolymer.

[0012] The reaction mixture was concentrated, and then water was added to prepare an ethylene-vinyl alcohol copolymer solution having a solvent C0 ratio of 1:9 to 9:1. 1-4 The weight ratio of alcohol to water and the solid content of 10% to 25% by weight;

[0013] Cooling the ethylene-vinyl alcohol copolymer solution to prepare an ethylene-vinyl alcohol copolymer cake from which the ethylene-vinyl alcohol copolymer solidifies.

[0014] Crushing the ethylene-vinyl alcohol copolymer cake to obtain ethylene-vinyl alcohol copolymer pellets; and

[0015] The particles were washed with water.

[0016] [Beneficial Effects]

[0017] According to this disclosure, high-purity ethylene-vinyl alcohol copolymers can be prepared through a simple washing process compared to conventional preparation methods. Therefore, this invention can improve the productivity of ethylene-vinyl alcohol copolymers and minimize the amount of wastewater generated during the washing process. Attached Figure Description

[0018] Figure 1 An apparatus capable of carrying out the method for preparing the ethylene-vinyl alcohol copolymer of this disclosure is shown. Detailed Implementation

[0019] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. Singular forms also include plural forms unless the context clearly indicates otherwise. It should also be understood that, when used in this specification, the terms “comprising,” “having,” or “possessing” designate the presence of the stated features, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, steps, components, or combinations thereof.

[0020] Since the present invention can be modified in various ways and has various forms, specific embodiments of the invention are shown by way of example and will be described in detail. However, this does not mean that the invention is limited to the specific forms disclosed, and it should be understood that the invention includes all modifications, equivalents, and substitutions within the spirit and scope of the invention.

[0021] Unless otherwise stated in this disclosure, the pressure conditions are atmospheric pressure (760 ± 50 Torr).

[0022] The method for preparing the ethylene-vinyl alcohol copolymer disclosed herein includes the following steps:

[0023] In alkaline catalyst and C 1-4The ethylene-vinyl acetate copolymer was saponified in the presence of an alcohol solvent to obtain a reaction mixture containing the ethylene-vinyl alcohol copolymer (step 1);

[0024] The reaction mixture was concentrated, and then water was added to prepare an ethylene-vinyl alcohol copolymer solution having a solvent C0 ratio of 1:9 to 9:1. 1-4 The weight ratio of alcohol to water and the solid content of 10% to 25% by weight (step 2);

[0025] Cool the ethylene-vinyl alcohol copolymer solution to prepare an ethylene-vinyl alcohol copolymer cake in which the ethylene-vinyl alcohol copolymer is condensed (step 3);

[0026] Crushing the ethylene-vinyl alcohol copolymer cake to obtain ethylene-vinyl alcohol copolymer fragments (step 4); and

[0027] Wash the fragments with water (step 5).

[0028] Conventional methods for preparing ethylene-vinyl alcohol copolymers include the following steps: saponifying and extruding the ethylene-vinyl acetate copolymer, followed by granulation, washing, and drying. However, due to the low washing efficiency of granulated ethylene-vinyl alcohol copolymers, several lengthy washing processes are required to adequately remove impurities such as catalysts and byproducts.

[0029] Therefore, the inventors investigated a method for obtaining high-purity ethylene-vinyl alcohol copolymers from the reaction mixture without excessive washing after the saponification reaction of the ethylene-vinyl acetate copolymer. As a result, it was confirmed that high-purity ethylene-vinyl alcohol copolymers can be obtained efficiently through the above steps, thus completing this invention.

[0030] This disclosure will be described in detail below.

[0031] In this disclosure, firstly, in the presence of an alkaline catalyst and C 1-4 The ethylene-vinyl acetate copolymer is saponified in the presence of an alcohol solvent to obtain a reaction mixture containing the ethylene-vinyl alcohol copolymer (step 1).

[0032] In this disclosure, the ethylene-vinyl acetate copolymer, which is used as a reaction material for preparing the ethylene-vinyl alcohol copolymer, can be prepared by using a commercially available product or by copolymerizing ethylene and vinyl acetate monomers.

[0033] Ethylene-vinyl acetate copolymers can be copolymerized by including monomers in addition to ethylene and vinyl acetate that can be copolymerized with them. Examples of such monomers include α-olefins such as propylene, isobutylene, α-octene, and α-dodecene; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, and itaconic acid or their salts, anhydrides, or alkyl or dialkyl esters; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefin sulfonic acids such as vinyl sulfonic acid, aryl sulfonic acid, and m-aryl sulfonic acid or their salts; vinyl monomers such as alkyl vinyl ethers, vinyl ketones, N-vinylpyrrolidone, vinyl chloride, and vinylidene chloride; and so on.

[0034] The ethylene content of the ethylene-vinyl acetate copolymer can be appropriately adjusted according to the desired physical properties of the ethylene-vinyl alcohol copolymer. For example, in order to achieve gas barrier properties and melt formability of the ethylene-vinyl alcohol copolymer, the ethylene content of the ethylene-vinyl acetate copolymer can be more than 20 mol%, more than 25 mol%, or more than 30 mol%, and less than 60 mol%, less than 50 mol%, or less than 40 mol%, but this disclosure is not limited thereto.

[0035] Meanwhile, the ethylene content can be determined by ethylene-vinyl acetate copolymer or ethylene-vinyl alcohol copolymer. 1 Calculation of peak integral ratio of H-NMR data.

[0036] The weight-average molecular weight (Mw) of the ethylene-vinyl acetate copolymer is not particularly limited, but may be, for example, 150,000 g / mol or more, 170,000 g / mol or more, or 180,000 g / mol or more, and may be 290,000 g / mol or less, 270,000 g / mol or less, or 250,000 g / mol or less. The weight-average molecular weight of the ethylene-vinyl acetate copolymer obtained by saponifying the ethylene-vinyl acetate copolymer that meets the above-mentioned weight-average molecular weight may be 110,000 g / mol or more, 120,000 g / mol or more, or 130,000 g / mol or more, and may be 220,000 g / mol or less, 200,000 g / mol or less, or 190,000 g / mol or less.

[0037] The weight-average molecular weight of ethylene-vinyl acetate copolymers and ethylene-vinyl alcohol copolymers can be measured by gel permeation chromatography (GPC).

[0038] Alcohol solvents are commonly used in the saponification reaction of ethylene-vinyl acetate copolymers. In this disclosure, lower alcohols having 1 to 4 carbon atoms are used as solvents. 1-4 Examples of alcohols may include at least one selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol and tert-butanol, with methanol being preferred.

[0039] There are no particular restrictions on the amount of alcohol used, and it can be determined based on the amount of reactants and reaction conditions.

[0040] The alkaline catalyst may be at least one 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. Preferably, at least one selected from sodium hydroxide, potassium hydroxide, sodium methoxide, and sodium ethoxide may be used.

[0041] To ensure the smooth progress of the saponification reaction of the ethylene-vinyl acetate copolymer, an alkaline catalyst can be used in an amount of 0.01 mol or more, or 0.015 mol or more, but less than 0.05 mol or less than 0.04 mol, based on 1 mol of vinyl acetate unit of the ethylene-vinyl acetate copolymer.

[0042] The reaction temperature during saponification can be above 40°C, 50°C, or 60°C, and below 120°C, 110°C, or 100°C. When the reaction temperature is below 40°C, the saponification rate may be too slow, while when it exceeds 120°C, side reactions are likely to occur. Therefore, it is preferable to meet the above range.

[0043] Furthermore, the reaction pressure during the saponification reaction can be above 0.01 bar or above 0.05 bar, and below 0.5 bar or below 0.4 bar, but this disclosure is not limited thereto.

[0044] The saponification reaction can be carried out in an inert gas atmosphere such as nitrogen or argon, and the reaction can be carried out while the byproduct methyl acetate is continuously discharged to the outside of the system to improve the conversion rate.

[0045] When the saponification reaction proceeds and reaches the desired conversion rate, the reaction is stopped by adding an acid such as acetic acid to neutralize the reaction mixture. Afterwards, a method for purifying the ethylene-vinyl alcohol copolymer from the neutralized reaction mixture is performed (steps 2 to 5).

[0046] The neutralization and purification of the reaction mixture can be achieved, for example, using methods such as... Figure 1 The apparatus shown is used for this purpose.

[0047] The amount of acid added to neutralize the reaction mixture can be adjusted according to the amount of base catalyst used. For example, based on 1 equivalent of base catalyst, more than 0.9 equivalents or more than 1.0 equivalents and less than 1.5 equivalents or less than 1.2 equivalents of acid can be added.

[0048] Subsequently, water was added to concentrate the reaction mixture to prepare C in the solvent. 1-4 An ethylene-vinyl alcohol copolymer solution with an alcohol-to-water weight ratio of 1:9 to 9:1 and a solid content of 10% to 25% by weight (step 2).

[0049] Concentration of the reaction mixture is used to evaporate all or part of the solvent C. 1-4 The method involves alcohols, and is not particularly limited thereto. For example, C can be evaporated by heating the neutralized reaction mixture in neutralization vessel 1 while blowing in an inert gas such as nitrogen or argon. 1-4 Alcohol is used to concentrate the reaction mixture. The heating temperature can be adjusted according to the C used. 1-4 The composition of the alcohol and reaction mixture is appropriately adjusted. For example, at atmospheric pressure (760 Torr), it can be above 35°C or 45°C, and below 65°C or 55°C. Alternatively, C can be evaporated by blowing a lower-temperature inert gas into a neutralization tank 1 under reduced pressure. 1-4 Alcohol. C that remains in the reaction mixture after concentration. 1-4 Alcohols can be used as co-solvents in the preparation of ethylene-vinyl alcohol copolymer solutions.

[0050] Water is added to the concentrated reaction mixture as described above to provide an ethylene-vinyl alcohol copolymer solution having a C1 ratio of 1:9 to 9:1, 7:3 to 3:7, or 4:6 to 6:4. 1-4 The weight ratio of alcohol to water, and the solid content of 10% or more by weight, or 15% or more by weight and less than 25% or less by weight, or less than 20% by weight.

[0051] The water can be distilled or deionized water with a conductivity of 10 μS / cm or less, preferably 5 μS / cm or less. Since lower conductivity is better, there is no limit to the lower limit. However, it can be, for example, 0.1 μS / cm or more, or 0.3 μS / cm or more.

[0052] C 1-4 The alcohol can be retained after the reaction mixture is concentrated. Alternatively, when water is added to achieve the above solvent ratio, it can be achieved by further adding C. 1-4 Alcohols are used to prepare ethylene-vinyl alcohol copolymer solutions.

[0053] Ethylene-vinyl alcohol copolymers are insoluble in C 1-4 It is soluble in alcohol or water as a single solvent, but can be dissolved in C. 1-4 The alcohol and water are mixed in a solvent at a weight ratio of 1:9 to 9:1 as described above. Therefore, using C 1-4 An ethylene-vinyl alcohol copolymer solution is prepared using a mixed solvent of alcohol and water, and then cooled and solidified to prepare a cake-like ethylene-vinyl alcohol copolymer. Alternatively, when the above solvent composition is satisfied, the ethylene-vinyl alcohol copolymer solidified in cake form can be pulverized to a suitable particle size in subsequent processes.

[0054] When the solvent of the ethylene-vinyl alcohol copolymer solution contains C 1-4When the alcohol content is too low or the solids content is too high (more than 25% by weight), it is not easy to crush the ethylene-vinyl alcohol copolymer cake in subsequent processes, which may reduce the efficiency of subsequent washing processes.

[0055] Conversely, when the water content in the solvent is too low or the solids content is less than 10% by weight, the ethylene-vinyl alcohol copolymer cake lacks hardness, preventing it from being crushed into particles and becoming a slurry. Therefore, it is not easy to wash, and the ethylene-vinyl alcohol copolymer appears as fine particles during washing, which is undesirable because losses may increase.

[0056] Next, the ethylene-vinyl alcohol copolymer solution is cooled to prepare an ethylene-vinyl alcohol copolymer cake, wherein the ethylene-vinyl alcohol copolymer is coagulated (step 3), and the ethylene-vinyl alcohol copolymer cake is crushed to obtain granules (step 4).

[0057] Because the fragments have a larger surface area than conventional ethylene-vinyl alcohol copolymer pellets, impurities can be removed more effectively during water washing, thus enabling the preparation of high-purity ethylene-vinyl alcohol copolymers through a simple washing method.

[0058] The cooling process for obtaining the ethylene-vinyl alcohol copolymer cake can be carried out at -15°C to -1°C or at -10°C to -3°C. For example, the ethylene-vinyl alcohol copolymer solution is placed in a cooling and curing mold 3 and placed at -15°C to -1°C to obtain the ethylene-vinyl alcohol copolymer cake.

[0059] The pulverizer 4 used for pulverizing the cake is not particularly limited, and devices such as low-speed crushers, industrial mixers, or industrial shredders can be used as said pulverizer. The average particle size (D50) of the pulverized particles is preferably 0.1 mm or more or 0.5 mm or more, and 2.5 mm or less or 2 mm or less, to prevent loss of the ethylene-vinyl alcohol copolymer while improving washing efficiency. Since the pulverization of the cake is carried out in a solution, the particle size can be adjusted according to the pulverization conditions and the solvent composition and solid content of the solution. By satisfying C as described above... 1-4 The appropriate average particle size can be obtained by adjusting the weight ratio of alcohol to water and the solid content.

[0060] The average particle size (D50) of fragments can be measured using a particle size analyzer, such as a laser diffraction particle size analyzer (e.g., Microtrac S3500). Specifically, the copolymer powder to be measured is dispersed in a dispersion medium and introduced into the laser diffraction particle size analyzer. Then, as the particles pass through the laser beam, the particle size distribution is obtained by measuring the differences in the diffraction pattern according to the particle diameter. In the measuring device, the average particle size (D50) can be obtained by calculating the particle size at the point that reaches 50% of the cumulative distribution of particle volume according to the particle size.

[0061] Subsequently, the fragments were washed with water to obtain purified ethylene-vinyl alcohol copolymer (step 5).

[0062] Since the pulverization step takes place in a solution, the solution containing the pulverized particles can be dehydrated using a centrifugal dehydrator 5 or similar device before washing with water. Afterward, the dehydrated particles are placed in a washing tank 6, water is added and stirred, and then dehydrated using a centrifugal dehydrator 7 to proceed with the washing process with water. The washing process with water can be performed once, or repeated two or more times as needed.

[0063] The water used for washing can be distilled or deionized water with a conductivity of 10 μS / cm or less, preferably 5 μS / cm or less. Since lower conductivity is better, there is no limit to the lower limit. However, it can be, for example, 0.1 μS / cm or more, or 0.3 μS / cm or more.

[0064] The amount of water added during the washing process is preferably 100 parts by weight or more, 200 parts by weight or more, or 300 parts by weight or more, and is less than 1000 parts by weight, 800 parts by weight or less, or 600 parts by weight or less. When the amount of water added is too small, impurities cannot be fully dissolved from the particles, while when the amount of water added is too large, it only increases the consumption of washing water without increasing the washing efficiency. Therefore, it is preferable to meet the above range.

[0065] During washing with water, the water temperature is preferably between 10°C and 40°C, or between 20°C and 30°C, because water within these ranges can effectively remove impurities while preventing damage to the ethylene-vinyl alcohol copolymer.

[0066] Compared to existing particulate copolymers, ethylene-vinyl alcohol copolymer particles have a large surface area and excellent washing efficiency, so even under the above conditions, a single wash can remove more than 90% or more than 93.5% of impurities. However, to obtain ethylene-vinyl alcohol copolymers with even higher purity, washing with water can be repeated two or more times if necessary.

[0067] There is no particular limit to the number of times the washing process can be repeated with water. For example, washing with water can be repeated more than 2 or 3 times, but no more than 7 or 5 times. The number of times the washing process can be adjusted according to the amount of particles to be processed at one time. Typically, under the above conditions, washing with water 3 times removes approximately 99.5% of the impurities, and washing with water 5 times removes approximately 99.9% of the impurities. Considering the economic feasibility and efficiency of this method, the number of times the washing process can be 7 or 5 times or less.

[0068] Alternatively, washing with water can be repeated several times until the conductivity of the dehydrated washing solution is below 10 μS / cm or below 8 μS / cm, preferably below 7 μS / cm. The conductivity of the washing solution is a measure of the amount of residual alkaline catalyst and impurity content in the fragments. A lower conductivity of the washing solution can be used to evaluate the higher purity of the ethylene-vinyl alcohol copolymer.

[0069] Washing with water is performed for more than 20 minutes or 30 minutes in each cycle, but for less than 2 hours or less than 1 hour. As will be demonstrated in the embodiments described below, washing efficiency is significantly improved in this disclosure, ensuring excellent washing results even after washing for up to 2 hours.

[0070] The residual sodium content of the ethylene-vinyl alcohol copolymer obtained after washing with water is below 30 ppm, below 20 ppm, or below 10 ppm, indicating very low impurity content. Lower residual sodium content is considered better; theoretically, residual sodium content can be 0 ppm.

[0071] According to the above preparation method, compared with existing methods for preparing ethylene-vinyl alcohol copolymers, the efficiency of the washing process is significantly improved, thereby producing high-purity ethylene-vinyl alcohol copolymers through a relatively simple washing process. Therefore, according to this disclosure, the production efficiency of ethylene-vinyl alcohol copolymers can be improved, and the amount of wastewater generated in the washing process can be minimized, thereby reducing environmental pollution.

[0072] The invention will be described in more detail below with reference to embodiments. However, these embodiments are for illustrative purposes only, and the invention is not intended to be limited to these embodiments.

[0073] <Example>

[0074] Example 1

[0075] use Figure 1 The apparatus is used to prepare ethylene-vinyl alcohol copolymers by the following method.

[0076] 100 parts by weight of ethylene-vinyl acetate copolymer (EVAc, Mw 230000 g / mol) with an ethylene content of 32 mol% and 400 parts by weight of methanol were placed in a saponification reactor, and 60 parts by weight of a methanol solution of sodium hydroxide (16 g / L) were added. Then, nitrogen gas was blown into the reactor, and the saponification reaction was carried out at 60°C for 6 hours, while the byproduct methyl acetate was removed to the outside along with the methanol, thereby obtaining a reaction mixture containing ethylene-vinyl alcohol (EVOH) copolymer.

[0077] 42 g of acetic acid (AcOH, acetic acid / saponification catalyst = 1 / 1 molar ratio) and 0.24 kg of water were added to 15 kg of the reaction mixture for neutralization. Argon gas was blown into neutralization tank 1 to concentrate the reaction mixture, while 10.8 kg of methanol was removed from the outside of neutralization tank 1 at an internal temperature of 64 °C.

[0078] Then, 2.05 kg of water (distilled water, conductivity 4 μS / cm) was introduced into neutralization tank 1 to prepare an ethylene-vinyl alcohol (EVOH) solution with a methanol to water weight ratio of 6:4 and a total solids content (TSC) of 15%.

[0079] The EVOH solution was placed in the cooling and curing mold 3 and cured at -8°C to obtain an EVOH cake.

[0080] Then, the EVOH cake is crushed into small pieces with an average particle size (D50) of 1 mm to 1.5 mm using a crusher 4.

[0081] EVOH particles were placed in centrifuge 5 for initial dehydration. The initially dehydrated EVOH particles were then placed in washing tank 6, and 500 parts by weight of water (distilled water, conductivity 4 μS / cm) at 25°C were added based on 100 parts by weight of EVOH particles. The mixture was then stirred for 2 hours for washing with water, followed by dehydration in centrifuge 7. The washing process was repeated at least four times, measuring the conductivity of the dehydrated washing solution and the sodium content of the EVOH particles each time. The washing solution and EVOH particles in washing tank 6 were then aliquoted at 30-minute intervals during the first three washes and at 1-hour intervals during the fourth and fifth washes, and the conductivity of the washing solution and the sodium content of the EVOH particles were analyzed.

[0082] After a total of 5 washes with water, a granular EVOH copolymer with a moisture content of 70% by weight was finally obtained.

[0083] Example 2

[0084] The ethylene-vinyl acetate copolymer with an ethylene content of 32 mol% was saponified in the same manner as in Example 1 to obtain a reaction mixture containing an ethylene-vinyl alcohol (EVOH) copolymer.

[0085] 42 g of acetic acid (AcOH, acetic acid / saponification catalyst = 1 / 1 molar ratio) and 0.4 kg of water were added to 15 kg of the reaction mixture for neutralization. Argon gas was blown into neutralization tank 1 to concentrate the reaction mixture, while 12.6 kg of methanol was removed to the outside of neutralization tank 1 at an internal temperature of 100 °C.

[0086] Then, 2 kg of water (distilled water, conductivity 4 μS / cm) was introduced into neutralization tank 1 to prepare an ethylene-vinyl alcohol (EVOH) solution with a methanol to water weight ratio of 4:6 and a total solids content (TSC) of 20%.

[0087] Subsequently, EVOH cakes were prepared and crushed in the same manner as in Example 1, and then washed with water five times to finally obtain a granular EVOH copolymer with a moisture content of 72% by weight.

[0088] Comparative Example 1

[0089] The ethylene-vinyl acetate copolymer with an ethylene content of 32 mol% was saponified in the same manner as in Example 1 to obtain a reaction mixture containing an ethylene-vinyl alcohol (EVOH) copolymer.

[0090] 42 g of acetic acid (AcOH, acetic acid / saponification catalyst = 1 / 1 molar ratio) and 0.1 kg of water were added to 15 kg of the reaction mixture for neutralization. Argon gas was blown into the neutralization vessel to concentrate the reaction mixture, while 13.2 kg of methanol was removed from the outside of the neutralization vessel at an internal temperature of 64 °C.

[0091] Then, 0.57 kg of water (distilled water, conductivity 4 μS / cm) was introduced into the neutralization tank to prepare an EVOH solution with a methanol to water weight ratio of 6:4 and a total solids content (TSC) of 35%.

[0092] The EVOH solution is discharged into a cooling tank filled with water at 2°C using an extruder with an orifice of 1.8 mm diameter, and solidified in the form of wire. The solid wire is then cut into EVOH pellets with a length of 1.5 mm to 2.5 mm using a granulator.

[0093] The EVOH granules obtained above were placed in a washing tank, and water (distilled water, conductivity 4 μS / cm) at 25°C was added. The mixture was then stirred for 2 hours for washing with water, followed by dehydration in a centrifuge. The washing process was repeated at least four times, with the conductivity of the dehydrated washing solution and the sodium content of the EVOH granules measured each time. In the first three washes, the washing liquid and the EVOH pulverized product in washing tank 6 were aliquoted at 30-minute intervals, and in the fourth and fifth washes at 1-hour intervals. The conductivity of the washing liquid and the sodium content of the EVOH pulverized product were analyzed.

[0094] After a total of 5 washes with water, the EVOH copolymer in granular form with a moisture content of 50% by weight was finally obtained.

[0095] Comparative Example 2

[0096] The ethylene-vinyl acetate copolymer with an ethylene content of 32 mol% was saponified in the same manner as in Example 1 to obtain a reaction mixture containing an ethylene-vinyl alcohol (EVOH) copolymer.

[0097] 42 g of acetic acid (AcOH, acetic acid / saponification catalyst = 1 / 1 molar ratio) and 0.1 kg of water were added to 15 kg of the reaction mixture for neutralization. Argon gas was blown into neutralization tank 1 to concentrate the reaction mixture, while 12.9 kg of methanol was removed from the outside of neutralization tank 1 at an internal temperature of 100 °C.

[0098] Then, 0.76 kg of water (distilled water, conductivity 4 μS / cm) was introduced into neutralization tank 1 to prepare an ethylene-vinyl alcohol (EVOH) solution with a methanol to water weight ratio of 6:4 and a total solids content (TSC) of 30%.

[0099] Subsequently, EVOH cakes were prepared and crushed in the same manner as in Example 1, and then washed with water five times to finally obtain a granular EVOH copolymer with a moisture content of 68% by weight.

[0100] Comparative Example 3

[0101] The ethylene-vinyl acetate copolymer with an ethylene content of 32 mol% was saponified in the same manner as in Example 1 to obtain a reaction mixture containing an ethylene-vinyl alcohol (EVOH) copolymer.

[0102] 42 g of acetic acid (AcOH, acetic acid / saponification catalyst = 1 / 1 molar ratio) and 0.4 kg of water were added to 15 kg of reaction mixture for neutralization. Argon gas was blown into neutralization tank 1 to concentrate the reaction mixture, while 2.5 kg of methanol was removed to the outside of neutralization tank 1 at an internal temperature of 100 °C.

[0103] Then, 7.4 kg of water (distilled water, conductivity 4 μS / cm) was introduced into neutralization tank 1 to prepare an ethylene-vinyl alcohol (EVOH) solution with a methanol to water weight ratio of 6:4 and a total solids content (TSC) of 5%.

[0104] The EVOH solution was placed in the cooling and curing mold 3 and cured at -8°C to obtain an EVOH cake.

[0105] Then, an attempt was made to crush the EVOH cake in crusher 4, but due to insufficient hardness, the filter cake was not crushed into particles, and the crushed product was in the form of slurry.

[0106] The slurry is placed in washing tank 6, and water at 25°C (distilled water, conductivity 4 μS / cm) is added in an amount of 500 parts by weight based on 100 parts by weight of the pulverized EVOH product. Thereafter, the mixture is stirred for 2 hours for water washing, and then dehydrated in a centrifugal dehydrator 7. However, the pulverized product (slurry) dissolves in the washing water and is discharged. In addition, EVOH in the form of fine particles passes through the filter, resulting in a significant loss of the obtained product.

[0107] Therefore, it is confirmed that the desired effect of the present invention cannot be obtained under the above conditions.

[0108] <Experimental Example>

[0109] (1) Total Solid Content (TSC) of EVOH Solution

[0110] The total solid content of the EVOH solution prepared in one of the examples and comparative examples was measured by the following method.

[0111] A part (W1) of the EVOH solution is placed in an aluminum pan, dried in a vacuum oven at 75°C for 24 hours, then the weight of the dried product (W2) is measured, and TSC is calculated by the following equation.

[0112] TSC (wt%) = weight of sample after drying (W2) / weight of sample before drying (W1)*100

[0113] (2) Analysis of Residual Sodium Content in EVOH Copolymer

[0114] The EVOH copolymer is dried in a vacuum oven at 75°C for 24 hours. After aliquoting 0.2 g of the dried sample into a container for microwave digestion (MDS), 3 mL of nitric acid and 0.5 mL of hydrogen peroxide are added for MDS (90 bar, heating to 250°C for 30 minutes and maintaining for 15 minutes). After the reaction is completed, 0.1 mL of internal standard solution (Sc 1000 ppm aqueous solution) is added, and ultrapure water is added so that the total volume of the sample is 10 mL, thereby preparing a sample for inductively coupled plasma optical emission spectrometry (ICP-OES).

[0115] After stabilizing the ICP-OES instrument (Optima 8300DV), the sample is injected and analyzed under the following conditions.

[0116] <ICP-OES Analysis Conditions>

[0117] RF power: 1300 W

[0118] Plasma gas flow rate: 15 L / min

[0119] Auxiliary gas flow rate: 0.80 L / min

[0120] Internal standard: Sc

[0121] Plasma gas, auxiliary gas: Ar

[0122] (3) Measurement of the conductivity of the washing liquid

[0123] The conductivity meter (Thermo Scientific, EUTECH COND 6+) was calibrated using an 84 μS / cm standard solution, and then the conductivity of the washing solution at room temperature (25°C) was measured. The washing solution was divided into three equal portions, and the conductivity was measured for each portion. The resulting average values ​​are shown in Tables 1 and 2 below.

[0124] [Table 1]

[0125]

[0126] [Table 2]

[0127]

[0128] Referring to Table 1, it was demonstrated in Examples 1 and 2 that, with only one wash, the sodium content in the ethylene-vinyl alcohol copolymer was reduced by more than 93.5%, and the conductivity of the washing solution decreased sharply, with only a very small amount of sodium remaining after three washes.

[0129] However, in Comparative Example 1, prepared according to the existing method, it was confirmed that even after 3 washes, the residual sodium content exceeded 100 ppm, and even after 5 washes, the conductivity of the washing solution exceeded 10 μS / cm, indicating that the washing efficiency was significantly reduced compared to Examples 1 and 2.

[0130] Furthermore, Comparative Example 2, in which the EVOH solution had a solid content greater than 25% by weight, was superior to Comparative Example 1, but residual sodium was not sufficiently removed compared to Examples 1 and 2. In addition, conductivity results confirmed that Comparative Example 2 had a lower washing efficiency than Examples 1 and 2.

[0131] Furthermore, comparing the conductivity and residual sodium content at each washing time in each washing cycle of Examples 1 and 2, the conductivity of the washing solution and the sodium content of the copolymer were similar within the error range after washing for 0.5 hours and after washing for 2 hours. Therefore, according to the present invention, even if the washing time is reduced to 0.5 hours, a washing efficiency similar to that described above can be expected. On the other hand, since the sodium content of the copolymer gradually decreased with increasing washing time in Comparative Example 1, it was determined that a washing time of more than 2 hours was required in each cycle for sufficient washing.

[0132] Therefore, according to the preparation method of this disclosure, most impurities can be removed even with a single wash, and high-purity ethylene-vinyl alcohol copolymers can be obtained with three or more washes, while significantly reducing washing time. Thus, experimental results confirm that the washing process is significantly more efficient than conventional preparation methods.

[0133] [Symbol Explanation]

[0134] 1: Intermittent neutralization tank

[0135] 2: Condenser

[0136] 3: Ethylene-vinyl alcohol copolymer solution cooling bath

[0137] 4: Crusher

[0138] 5,7: Centrifugal dehydrator

[0139] 6: Washing tub

[0140] 8: Distillate tank

[0141] 9,10: Washing liquid tank

Claims

1. A method for preparing an ethylene-vinyl alcohol copolymer, comprising the following steps: In alkaline catalyst and C 1-4 The ethylene-vinyl acetate copolymer was saponified in the presence of an alcohol solvent to obtain a reaction mixture containing an ethylene-vinyl alcohol copolymer. The reaction mixture was concentrated, and then water was added to prepare an ethylene-vinyl alcohol copolymer solution having a solvent C0 ratio of 1:9 to 9:

1. 1-4 The weight ratio of alcohol to water and the solid content of 15% to 25% by weight; The ethylene-vinyl alcohol copolymer solution was cooled at -15°C to -1°C to prepare an ethylene-vinyl alcohol copolymer cake in which the ethylene-vinyl alcohol copolymer solidified. Crushing the ethylene-vinyl alcohol copolymer cake to obtain ethylene-vinyl alcohol copolymer pellets; and Wash the fragments with water. Wherein, the conductivity of the washing solution obtained after washing with water and centrifugation is less than 10 μS / cm, and The residual sodium content of the ethylene-vinyl alcohol copolymer obtained after washing with water is less than 30 ppm.

2. The method for preparing the ethylene-vinyl alcohol copolymer according to claim 1, wherein, The ethylene-vinyl alcohol copolymer solution has a solvent ratio of 3:7 to 7:3 with C0. 1-4 The weight ratio of alcohol to water.

3. The method for preparing the ethylene-vinyl alcohol copolymer according to claim 1, wherein, The ethylene-vinyl alcohol copolymer solution has a solids content of 15% to 20% by weight.

4. The method for preparing the ethylene-vinyl alcohol copolymer according to claim 1, wherein, The C 1-4 The alcohol is selected from at least one of methanol, ethanol, n-propanol, isopropanol, n-butanol, and tert-butanol.

5. The method for preparing the ethylene-vinyl alcohol copolymer according to claim 1, wherein, The average particle size D50 of the ethylene-vinyl alcohol copolymer fragments is 0.1 mm to 2.5 mm.

6. The method for preparing the ethylene-vinyl alcohol copolymer according to claim 1, wherein, The washing process with water is repeated 2 to 5 times.

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