WO2024228377A1

WO2024228377A1 - Method for producing ethylene-vinyl alcohol copolymer

Info

Publication number: WO2024228377A1
Application number: PCT/JP2024/016633
Authority: WO
Inventors: 宏樹大野; 直人西田; 雄大伊藤
Original assignee: 株式会社クラレ
Priority date: 2023-05-01
Filing date: 2024-04-30
Publication date: 2024-11-07

Abstract

The method for producing an ethylene-vinyl alcohol copolymer has a step (I) in which an ethylene-vinyl acetate copolymer in a solution of an ethylene-vinyl acetate copolymer is saponified using an alkali catalyst to obtain a solution of a saponified ethylene-vinyl acetate copolymer having a degree of saponification of from 10 mol% to less than 60 mol% and a step (II) in which the solution of the saponified ethylene-vinyl acetate copolymer is supplied to top of a tower reactor (A), solvent vapor is supplied to the lower part and discharged from the top, and an alkali catalyst is supplied below the location at which the solution of the saponified ethylene-vinyl acetate copolymer is supplied to further saponify the saponified ethylene-vinyl acetate copolymer, and a solution of an ethylene-vinyl alcohol copolymer having a degree of saponification of from 99 mol% to 100 mol% obtained is removed from the tower bottom. According to this production method, even when an ethylene-vinyl alcohol copolymer is produced continuously for a long period of time, the ethylene-vinyl acetate copolymer can be saponified efficiently, so that the ethylene-vinyl alcohol copolymer can be produced at good productivity.

Description

Method for producing ethylene-vinyl alcohol copolymer

The present invention relates to a method for producing an ethylene-vinyl alcohol copolymer.

Ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as "EVOH") is a useful polymeric material with excellent oxygen barrier properties, oil resistance, antistatic properties, mechanical strength, etc., and is widely used as a variety of packaging materials such as films, sheets, and containers.

EVOH has traditionally been produced by saponifying ethylene-vinyl acetate copolymer (hereinafter sometimes abbreviated as EVAc) using an alkaline catalyst, and methods have been proposed to improve the saponification process in order to increase the productivity and quality of EVOH.

For example, Patent Document 1 describes a method for producing saponified EVAc, which includes the steps of: (1) using a tower reactor to saponify EVAc with an ethylene content of 15 to 60 mol% in an alcohol solvent with an alkaline catalyst until the degree of saponification reaches 70 to 98 mol%, thereby obtaining a partially saponified EVAc solution; (2) using a tower reactor to add water or water/alcohol to the partially saponified EVAc solution to form a mixed solution, which is then resaponified in the presence of an alkaline catalyst, thereby obtaining a highly saponified EVAc solution in which the vinyl acetate component has a degree of saponification of 99.4 mol% or more; and (3) extruding the highly saponified EVAc solution into a coagulation bath to precipitate the solution, followed by acid treatment. This method is said to produce pellets of saponified EVAc that are uniform and have excellent melt moldability.

Patent Document 2 describes a method for producing a saponified EVAc product in which an alkali catalyst is added to and mixed with an EVAc solution, the resulting EVAc solution having a degree of saponification of 5 mol% or less is supplied to the top of a tower reactor, solvent vapor is supplied from the bottom of the tower, the solvent vapor is discharged from the top of the tower, and an alkali catalyst is supplied from below the position where the EVAc solution is supplied to saponify the EVAc. It is described that this production method can suppress coloration of the saponified EVAc product by first removing impurities such as vinyl esters and their by-product acetaldehyde before subjecting the EVAc to a saponification reaction.

Japanese Patent Application Publication No. 9-67411 JP 2009-242645 A

However, in the method described in Patent Document 1, during resaponification, the EVAc saponified product is discharged from the tower reactor along with the solvent, which causes a decrease in production efficiency. In addition, in the method described in Patent Document 2, when EVOH is produced continuously over a long period of time, the partially saponified EVAc product can solidify in the tower reactor used for saponification, which causes a decrease in production efficiency.

The present invention has been made to solve the above problems, and aims to provide a highly productive method for producing EVOH that can efficiently saponify EVAc.

The above problem is solved by providing a method for producing EVOH, which includes a step (I) of saponifying EVAc in a solution of EVAc using an alkaline catalyst to obtain a solution of partially saponified EVAc having a degree of saponification of 10 mol% or more and less than 60 mol%, and a step (II) of supplying the solution of partially saponified EVAc to the top of a tower reactor (A), supplying solvent vapor to the bottom of the tower and discharging it from the top of the tower, and supplying an alkaline catalyst to the tower reactor (A) below the position where the solution of partially saponified EVAc is supplied, thereby further saponifying the partially saponified EVAc, and then withdrawing the resulting solution of EVOH having a degree of saponification of 99 mol% or more and 100 mol% or less from the bottom of the tower.

In this case, it is preferable that the degree of saponification of the EVAc partially saponified product obtained in step (I) is 10 mol% or more and less than 50 mol%. It is also preferable that the ethylene unit content of the EVAc used in step (I) is 10 mol% or more and less than 60 mol%. The solvent of the EVAc solution used in step (I) and the solvent vapor used in step (II) are preferably alcohol, more preferably methanol. It is also preferable that the concentration of the EVAc solution used in step (I) is 60 mass% or less. It is also preferable that the amount of solvent vapor supplied to the tower reactor (A) in step (II) is 50 mass parts or more and less than 500 mass parts per 100 mass parts of the EVAc partially saponified product supplied to the tower reactor (A). In step (I), it is also preferable to mix the EVAc solution with the alkali catalyst in a non-tower mixer to obtain a solution of partially saponified EVAc having a degree of saponification of 10 mol% or more and less than 60 mol%.

The manufacturing method of the present invention allows EVAc to be efficiently saponified, even when EVOH is produced continuously over a long period of time, making it possible to produce EVOH with high productivity.

FIG. 1 is a schematic diagram of a tower reactor used in Examples 1 to 13 and Comparative Examples 1 to 4.

The present invention is a method for producing EVOH, comprising the steps of (I) saponifying EVAc in a solution of EVAc using an alkaline catalyst to obtain a solution of EVAc partially saponified product having a degree of saponification of 10 mol% or more and less than 60 mol%, and (II) supplying the solution of the partially saponified EVAc to the top of a tower reactor (A), supplying solvent vapor to the bottom of the tower and discharging it from the top of the tower, and supplying an alkaline catalyst to the tower reactor (A) below the position where the solution of the partially saponified EVAc is supplied, thereby further saponifying the partially saponified EVAc, and then withdrawing the resulting solution of EVOH having a degree of saponification of 99 mol% or more and 100 mol% or less from the bottom of the tower. According to this production method, EVAc can be efficiently saponified even when EVOH is produced continuously for a long period of time, and therefore EVOH can be produced with high productivity.

In step (I), the EVAc in the EVAc solution (a solution in which EVAc is dissolved) is saponified using an alkaline catalyst to obtain a solution of partially saponified EVAc.

EVAc used in step (I) may be produced by copolymerizing ethylene and vinyl acetate according to a general method. There are no limitations on the polymerization method or solvent, but solution polymerization using methanol as the solvent is preferred. As the polymerization catalyst, a radical initiator, for example, various azonitrile initiators and organic peroxide initiators, can be used. In addition, the EVAc may contain other monomers other than ethylene and vinyl acetate that can be copolymerized with ethylene and vinyl acetate (for example, α-olefins such as propylene, unsaturated acids such as acrylic acid, various nitriles, and various amides) within a range that does not impair the effects of the present invention. The content of units derived from the other monomers in the EVAc is usually 10 mol% or less.

The ethylene unit content of the EVAc used in step (I) is preferably 5 mol% or more and less than 70 mol%. If the ethylene unit content is less than 5 mol%, in step (II), the partially saponified EVAc in the tower reactor (A) may precipitate and block the holes in the shelves, causing an increase in pressure, which may shorten the period during which continuous operation is possible. The ethylene unit content is more preferably 10 mol% or more, even more preferably 15 mol% or more, and particularly preferably 20 mol% or more. On the other hand, if the ethylene unit content is 70 mol% or more, the gas barrier properties of the resulting EVOH may be reduced. The ethylene unit content is more preferably 65 mol% or less, even more preferably 60 mol% or less, and particularly preferably 55 mol% or less.

The solvent for the EVAc solution is not particularly limited as long as it can dissolve EVAc, but alcohol is preferred. When alcohol is used as the solvent, the saponification reaction of EVAc proceeds through an ester exchange reaction between the acetate ester group of EVAc and an alcohol-based compound, so the amount of alkaline catalyst used can be kept to a small amount, and the saponification reaction can proceed efficiently. Examples of the alcohol include methanol, ethanol, 1-propanol, and 2-propanol, with methanol being preferred.

The concentration of EVAc in the EVAc solution is not particularly limited, but is preferably 70% by mass or less. If the concentration exceeds 70% by mass, it may be difficult to mix the EVAc solution and the alkaline catalyst uniformly, or in step (II), the pressure may increase due to the precipitation of the partially saponified EVAc in the tower reactor (A) blocking the holes in the shelves, shortening the period during which continuous operation is possible. The concentration of EVAc is more preferably 60% by mass or less. On the other hand, from the viewpoint of productivity, the concentration of EVAc is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 40% by mass or more.

The alkali catalyst may be a compound such as an alkali metal hydroxide, such as sodium hydroxide, potassium hydroxide, or lithium hydroxide; or an alkali metal alkoxide, such as sodium methoxide, sodium ethoxide, or potassium t-butoxide; among these, sodium hydroxide, potassium hydroxide, sodium methoxide, or sodium ethoxide is preferred, and sodium hydroxide is more preferred. The alkali catalyst may be used as it is, or may be used as a solution. When used as a solution, the solvent may be the same as that of the EVAc solution.

The EVAc solution is mixed with the alkali catalyst to saponify the EVAc in the mixed solution. The mixing method is not particularly limited, but examples include a method using a mixer such as a static mixer, a mechanical stirrer, a dynamic mixer, or other non-tower reactor, or a tower reactor. From the viewpoint of excellent mixing efficiency, a non-tower reactor is preferred as the mixer, and among them, a static mixer is more preferred.

The concentration of the alkaline catalyst in the mixed solution containing the EVAc and the alkaline catalyst is preferably 0.005 to 1 mol/L. If the concentration is less than 0.005 mol/L, the EVAc may not be sufficiently saponified. The concentration is more preferably 0.01 mol/L or more, even more preferably 0.05 mol/L or more, and particularly preferably 0.1 mol/L or more. On the other hand, if the concentration exceeds 1 mol/L, the degree of saponification of the resulting partially saponified EVAc may be too high. The concentration is more preferably 0.5 mol/L or less, and even more preferably 0.3 mol/L or less.

The mixing time of the EVAc solution and the alkaline catalyst is not particularly limited, but is preferably 1 to 100 minutes. If the mixing time is less than 1 minute, the EVAc may not be sufficiently saponified. The mixing time is more preferably 3 minutes or more, and even more preferably 5 minutes or more. On the other hand, if the mixing time exceeds 100 minutes, there is a risk that the production efficiency may decrease or the degree of saponification of the partially saponified EVAc may be too high. The mixing time is more preferably 50 minutes or less, and even more preferably 30 minutes or less.

The temperature of the mixture during mixing of the EVAc solution with the alkali catalyst is preferably 10 to 80°C. The temperature is more preferably 20°C or higher, even more preferably 30°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher. On the other hand, the temperature is more preferably 70°C or lower.

The degree of saponification of the partially saponified EVAc obtained by mixing the EVAc solution with the alkaline catalyst must be 10 mol% or more and less than 60 mol%. In this way, by saponifying EVAc to a predetermined degree of saponification, impurities such as aldehydes that are by-produced during this process are discharged together with the solvent from the top of the tower in step (II) before they condense, resulting in EVOH with little coloration. When the degree of saponification of the partially saponified EVAc is less than 10 mol%, an increase in solution viscosity is observed in step (II) as the degree of saponification of the partially saponified EVAc increases, and the high viscosity results in poor flowability. On the other hand, when the saponification degree of the EVAc partially saponified product is 10 mol% or more, the solution viscosity becomes similar to that of the EVAc solution, and surprisingly, as the saponification degree increases in step (II), the viscosity decreases. Therefore, even when EVOH is continuously produced for a long period of time, the EVAc partially saponified product in the tower reactor (A) is less likely to solidify. Therefore, the pressure increase caused by the holes in the shelf being blocked is suppressed, and EVOH can be produced efficiently. The saponification degree is more preferably 11 mol% or more, even more preferably 12 mol% or more, and particularly preferably 13 mol% or more. On the other hand, when the saponification degree is less than 60 mol%, the EVAc partially saponified product is suppressed from being discharged from the tower reactor (A) together with the solvent vapor in step (II). The saponification degree is more preferably 55 mol% or less, and even more preferably 50 mol% or less.

In step (II), the solution of the EVAc partial saponification product is supplied to the top of the tower reactor (A), solvent vapor is supplied to the bottom of the tower and discharged from the top of the tower, and an alkali catalyst is supplied to the tower reactor (A) below the position where the solution of the EVAc partial saponification product is supplied to further saponify the EVAc partial saponification product, and the resulting solution of EVOH having a saponification degree of 99 mol% or more and 100 mol% or less is taken out from the bottom of the tower. In this way, in step (II), the EVAc partial saponification product is resaponified to obtain EVOH with a high degree of saponification.

Figure 1 is a schematic diagram of a tower reactor (A) used in the examples described later. Step (II) will be described with reference to Figure 1. The solution of the EVAc partial saponification product obtained in step (I) is supplied to the top of the tower reactor (A). In Figure 1, the solution of the EVAc partial saponification product is supplied to the tower reactor (A) from an EVAc partial saponification product solution supply port 2 at the top of the tower. An alkaline catalyst is supplied to a position below the position at the top of the tower where the solution of the EVAc partial saponification product is supplied. In Figure 1, an alkaline catalyst is supplied into the tower reactor (A) from an alkaline catalyst supply port 3 installed below the EVAc partial saponification product solution supply port 2. Solvent vapor is also supplied to the bottom of the tower and discharged from the top of the tower. The position at which the solvent vapor is supplied is preferably above the position at which the EVOH solution described later is taken out. The position from which the solvent vapor is discharged is preferably above the position from which the solution of the partially saponified EVAc is supplied, from the viewpoint of more efficiently removing impurities such as aldehyde remaining in the solution of the partially saponified EVAc. From the same viewpoint, it is also preferable that the position from which the solvent vapor is discharged is the top of the tower. In FIG. 1, the solvent vapor is blown in from the solvent vapor inlet 4 at the bottom of the tower, and the solvent vapor is discharged from the solvent vapor outlet 1 at the top of the tower.

The solution of the EVAc partial saponification product supplied to the top of the tower is transported from the top to the bottom of the tower reactor (A). The solution of the EVAc partial saponification product supplied to the top of the tower comes into contact with the solvent vapor, and by-products such as aldehydes and acetate esters are discharged from the top of the tower (solvent vapor outlet 1) together with the solvent vapor. The solution of the EVAc partial saponification product transported to the position where the alkaline catalyst is supplied (alkaline catalyst supply port 3) comes into contact with the alkaline catalyst, and the EVAc partial saponification product is re-saponified, and then the EVOH solution is taken out from the bottom of the tower (EVOH solution outlet 5). By supplying the alkaline catalyst below the position where the solution of the EVAc partial saponification product is supplied at the top of the tower, impurities such as aldehydes remaining in the solution of the EVAc partial saponification product can be removed in advance by the solvent vapor at the top of the tower, and then the EVAc partial saponification product can be re-saponified. This suppresses coloration of the EVAc partial saponification product during re-saponification.

In step (II), the same alkali catalyst as in step (I) is used. The same solvent vapor as that of the EVAc solution used in step (I) is used. Regarding the solvent used in step (II), the saponification reaction must be carried out until the degree of saponification of the partially saponified EVAc reaches 99 mol% or more. In a reaction system containing a large amount of water in the solvent, the hydrolysis reaction of the acetate ester becomes dominant, which may make it difficult to efficiently proceed with the saponification reaction, and the amount of the alkali catalyst used may increase in order to increase the degree of saponification. Therefore, it is preferable that the solvent used in step (II) does not contain water, but if it does contain water, its content is preferably 1.5 mass% or less, more preferably 1 mass% or less, and even more preferably 0.8 mass% or less, from the viewpoint of reducing the amount of the alkali catalyst used. If the water content of the solvent used in step (II) is equal to or less than the upper limit, the amount of catalyst residue contained in the obtained EVOH is reduced, and as a result, deterioration of thermal stability, etc. can be suppressed, which is preferable. The concentration of the EVAc partial saponification product in the solution of the EVAc partial saponification product provided in step (II) is not particularly limited, but is preferably 70% by mass or less. If the concentration exceeds 70% by mass, the EVAc partial saponification product in the tower reactor (A) may precipitate and block the holes in the shelves, causing an increase in pressure, which may shorten the period during which continuous operation is possible. The concentration of the EVAc partial saponification product is more preferably 60% by mass or less. On the other hand, from the viewpoint of productivity, the concentration of the EVAc partial saponification product is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 40% by mass or more.

Generally, as the saponification reaction progresses, the solubility of the partially saponified EVAc in the solvent decreases, so it is preferable to pressurize the tower reactor (A) in step (II) and carry out the reaction at a high temperature. The pressure in the tower reactor (A) is preferably 0.1 to 1.0 MPa. The pressure is more preferably 0.8 MPa or less, even more preferably 0.6 MPa or less, and particularly preferably 0.55 MPa or less. The pressure may be 0.2 MPa or more.

The temperature of the tower reactor (A) is preferably 60 to 180°C. From the viewpoint of increasing the reaction efficiency, the temperature is more preferably 70°C or higher, even more preferably 80°C or higher, and particularly preferably 90°C or higher. On the other hand, the temperature of the tower reactor (A) is more preferably 150°C or lower, even more preferably 140°C or lower, and particularly preferably 130°C or lower.

In step (II), the amount of solvent vapor supplied to the tower reactor (A) is preferably 50 parts by mass or more and less than 500 parts by mass relative to 100 parts by mass of the EVAc partially saponified product supplied to the tower reactor (A). If the amount of the solvent vapor is less than 50 parts by mass, impurities such as aldehydes in the EVAc partially saponified product may not be sufficiently removed, and the resulting EVOH may be colored. The amount of the solvent vapor is more preferably 60 parts by mass or more. On the other hand, if the amount of the solvent vapor is 500 parts by mass or more, the EVAc partially saponified product may be discharged together with the solvent vapor. The amount of the solvent vapor is more preferably 300 parts by mass or less, even more preferably 200 parts by mass or less, even more preferably 150 parts by mass or less, and particularly preferably 100 parts by mass or less. The temperature of the solvent vapor may be, for example, about the boiling point of the solvent at the pressure inside the tower.

The amount of the alkali catalyst added in step (II) is preferably 0.01 to 10 parts by mass per 100 parts by mass of the EVAc partially saponified product. The amount is more preferably 0.05 parts by mass or more, even more preferably 0.1 parts by mass or more, and particularly preferably 0.3 parts by mass or more. On the other hand, the amount is more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less.

According to the above-mentioned production method of the present invention, it is preferable to continuously operate the tower reactor (A) in step (II) for 35 days or more to continuously produce EVOH. This allows EVOH to be produced more efficiently. The number of days for continuous production of EVOH is more preferably 40 days or more, even more preferably 45 days or more, even more preferably 50 days or more, and particularly preferably 55 days or more.

The EVOH solution obtained in step (II) is preferably pelletized by a known method. Examples of pelletization methods include a method in which the EVOH solution is cooled and solidified, and then cut, and a method in which the EVOH is melt-kneaded in an extruder, then discharged, and then cut. Specific examples of methods for cutting EVOH include a method in which EVOH is extruded into strands and then cut with a pelletizer, and a method in which EVOH is discharged from a die and then cut using a center hot cut method or an underwater cut method. When the EVOH solution is pelletized, it becomes hydrous EVOH pellets.

When the EVOH solution is cooled and solidified to obtain hydrous EVOH pellets, it is preferable to wash and deliquor the hydrous EVOH pellets by a known method. It is also preferable to perform a chemical treatment by immersing the pellets in a solution containing a boron compound, an alkali metal salt, an alkaline earth metal salt, or the like by a known method to incorporate the relevant compound into the hydrous EVOH. Incorporating these compounds can improve the mechanical properties and thermal stability of the EVOH molded body. When the hydrous EVOH is obtained by melt-kneading and pelletizing EVOH, the EVOH may be washed, deliquored, and chemically treated in an extruder.

EVOH pellets can be obtained by drying the obtained hydrous EVOH pellets by a known method. It is preferable that the moisture content of the EVOH pellets after drying is 0.5 mass% or less. There are no particular limitations on the drying method, and examples include stationary drying combined with air drying or nitrogen drying, fluidized drying, and vacuum drying, but multi-stage drying combining several drying methods is preferable, and multi-stage drying including preliminary drying and main drying is more preferable.

The saponification degree of the EVOH thus obtained must be in the range of 99 mol% or more and 100 mol% or less. According to the manufacturing method of the present invention, EVOH with such a high saponification degree can be efficiently manufactured. The saponification degree is preferably 99.3 mol% or more, more preferably 99.5 mol% or more, and even more preferably 99.7 mol% or more.

The yellow index (YI) of the EVOH is preferably 20 or less, more preferably 15 or less, even more preferably 13 or less, and particularly preferably 9.5 or less. According to the manufacturing method of the present invention, such EVOH with little coloring can be efficiently manufactured.

The EVOH obtained by the method of the present invention can be molded into various molded products such as films, sheets, containers, pipes, and fibers.

The following provides a more detailed explanation using examples.

[Evaluation method]
(1) Degree of Saponification of Partially Saponified EVAc After Step (I) The partially saponified EVAc solution after step (I) obtained in the Examples and Comparative Examples was concentrated using a rotary evaporator and then dried under reduced pressure at 40° C. for 10 hours to obtain a dry partially saponified EVAc. 20 mg of the resulting dry partially saponified EVAc was dissolved in 6 mL of CDCl ₃ and subjected to ¹ H-NMR measurement under the following measurement conditions, and the degree of saponification was calculated from the ratio of the integral value of the methine hydrogen of the vinyl alcohol unit (saponified site) observed at 3.1 to 4.1 ppm to the integral value of the methine hydrogen of the vinyl acetate unit (unsaponified site) observed at 4.5 to 5.2 ppm.
(Measurement conditions)
Device name: JEOL superconducting nuclear magnetic resonance device Lambda 500
Observation frequency: 500MHz
Measurement temperature: 25°C
Number of times of accumulation: 1024 times

(2) Saponification Degree of EVOH In the Examples and Comparative Examples, dried EVOH pellets obtained 3 days after the start of production were pulverized, and 20 mg of the obtained powder was dissolved in 6 mL of a mixed solution of deuterated dimethyl sulfoxide/deuterated trifluoroacetic acid (mass ratio: deuterated dimethyl sulfoxide/deuterated trifluoroacetic acid=95:5), and then ¹ H-NMR measurement was performed under the measurement conditions described below. The saponification degree was calculated from the ratio of the integral value of the methine hydrogen of the vinyl alcohol unit observed at 3.1 to 4.1 ppm (saponified site) to the integral value of the methyl group hydrogen of the vinyl acetate unit observed at 1.9 to 2.0 ppm (unsaponified site).
(Measurement conditions)
Device name: JEOL superconducting nuclear magnetic resonance device Lambda 500
Observation frequency: 500MHz
Measurement temperature: 80°C
Number of times accumulated: 128

(3) Pressure at the bottom of the tower reactor (A) in step (II) The pressure at the bottom of the tower was measured with a pressure gauge installed at the bottom (lower part of the first plate) of the tower reactor (A) [plate tower (saponification tower, total number of plates: 21)] used in step (II) in the Examples and Comparative Examples. The pressure at the bottom of the tower three days after the start of EVOH production is shown in Table 1.

(4) Continuous Production Days In the Examples and Comparative Examples, when EVOH was continuously produced, the production was stopped when the pressure at the bottom of the tower measured in accordance with the above evaluation method (3) exceeded 0.52 MPa, and the number of days from the start of production to the stop of production was recorded as the continuous production days.

(5) Distillate of step (II) Three days after the start of the production of EVOH described in the Examples and Comparative Examples, the distillate condensed from the mixed vapor discharged (distilled) from the top of the tower in step (II) was visually inspected for the presence or absence of turbidity and resin, and was evaluated according to the following criteria. A grade of D was determined to be poor in production efficiency.
Evaluation: Criteria A: Colorless and transparent B: Slightly cloudy C: Extremely cloudy D: Resin present (6) Evaluation of Yellowness of EVOH In the Examples and Comparative Examples, the YI (yellowness index) of the dried EVOH pellets obtained 3 days after the start of production was measured in accordance with JIS-K-7103.

[Example 1]
(Step I)
A 57% by mass EVAc methanol solution, in which EVAc having an ethylene unit content of 44 mol% and a vinyl acetate unit content of 56 mol% was dissolved in methanol, was mixed with a 120 g/L sodium hydroxide methanol solution in a volume ratio of 95:5 at 60° C. for 10 minutes in a static mixer to obtain a partially saponified EVAc solution. The degree of saponification of the obtained partially saponified EVAc solution was measured according to the method described in the above evaluation method (1). The results are shown in Table 1.

(Step II)
Subsequently, the partially saponified EVAc methanol solution obtained in step (I) was further saponified using a tower reactor (A) (plate tower (saponification tower, total number of stages: 21), tower inner diameter: 140 mm, height: 4700 mm). FIG. 1 is a schematic diagram of the tower reactor (A). The partially saponified EVAc methanol solution was continuously supplied to the 20th plate of the tower reactor (A) at a rate of 8 kg/h from the partially saponified EVAc solution supply port 2 at a tower temperature of 118° C. Methanol vapor was continuously supplied from the solvent vapor supply port 4 to the lower part of the first plate in an amount of 75 parts by mass relative to 100 parts by mass of the partially saponified EVAc supplied to the tower reactor (A). A methanol solution containing sodium hydroxide in an amount of 1 part by mass relative to 100 parts by mass of the partially saponified EVAc supplied to the tower reactor (A) was continuously supplied to the 14th plate from the alkali catalyst supply port 3. Also, by-products such as methyl acetate and aldehyde were distilled as mixed vapor together with excess methanol from the top of the tower (solvent vapor outlet 1), and an EVOH methanol solution was obtained from the bottom of the tower (EVOH solution outlet 5). The bottom pressure, number of continuous production days, and distillate in step (II) were evaluated according to the methods described in the above evaluation methods (3) to (5). The results are shown in Table 1.

(Production of EVOH pellets)
To the EVOH methanol solution obtained in step (II), acetic acid was added in an amount equal to the molar amount of sodium hydroxide supplied to the tower reactor (A) in step (II) to neutralize the remaining sodium hydroxide. Next, the solution was concentrated until the copolymer concentration reached 40% by mass. This solution was extruded from a nozzle with a diameter of 3.5 mm into a methanol-water mixed solvent (methanol/water = 10/90 weight ratio) kept at 5 ° C. to solidify into a strand shape, and cut with a cutter to obtain hydrous EVOH pellets. Furthermore, the pellets were washed by putting them into a large amount of 0.1 g/L acetic acid aqueous solution, and the remaining methanol and sodium acetate were removed, and then the pellets were dried at 60 ° C. for 5 hours, and further dried at 110 ° C. for 10 hours to obtain dried EVOH pellets. The saponification degree and yellowness of the obtained dried EVOH pellets were measured according to the methods described in the above evaluation methods (2) and (6). The results are shown in Table 1.

(Examples 2 and 5, Comparative Examples 1 to 4)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the concentration of sodium hydroxide in step (I) was adjusted so that the degree of saponification of the resulting partially saponified EVAc product would be as shown in Table 1. The results are shown in Table 1.

(Examples 3, 6, and 7)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that EVAc having an ethylene unit content shown in Table 1 was used and the concentration of sodium hydroxide in step (I) was adjusted so that the degree of saponification of the partially saponified EVAc obtained was as shown in Table 1. The results are shown in Table 1.

(Examples 4, 10, and 11)
EVOH pellets were produced and evaluated in the same manner as in Example 1, except that the amount of methanol vapor supplied to the tower reactor (A) in the step (II) was changed as shown in Table 1. The results are shown in Table 1.

(Example 8)
EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that ethanol was used as the solvent instead of methanol in steps (I) and (II). The results are shown in Table 1.

(Example 9)
EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that in step (I), the concentration of the EVAc methanol solution used was changed as shown in Table 1, and the concentration of sodium hydroxide was adjusted so that the degree of saponification of the resulting partially saponified EVAc product was as shown in Table 1. The results are shown in Table 1.

(Examples 12 and 13)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the mixing method in step (I) was changed as shown in Table 1. The results are shown in Table 1.

1 Solvent vapor outlet 2 EVAc partially saponified solution supply port 3 Alkaline catalyst supply port 4 Solvent vapor blowing port 5 EVOH solution outlet

Claims

(I) a step of saponifying the ethylene-vinyl acetate copolymer in the ethylene-vinyl acetate copolymer solution using an alkali catalyst to obtain a solution of a partially saponified ethylene-vinyl acetate copolymer having a saponification degree of 10 mol % or more and less than 60 mol %; and (B) a step of supplying the partially saponified ethylene-vinyl acetate copolymer solution to an upper portion of a tower reactor (A),
Solvent vapor is supplied to the bottom of the column and discharged from the top of the column.
The method for producing an ethylene-vinyl alcohol copolymer comprises a step (II) of supplying an alkali catalyst to a column reactor (A) below a position where the solution of the partially saponified ethylene-vinyl acetate copolymer is supplied, thereby further saponifying the partially saponified ethylene-vinyl acetate copolymer, and then withdrawing the obtained solution of an ethylene-vinyl alcohol copolymer having a saponification degree of 99 mol % or more and 100 mol % or less from the column bottom.
The method of claim 1, wherein the degree of saponification of the partially saponified ethylene-vinyl acetate copolymer obtained in step (I) is 10 mol% or more and less than 50 mol%.
The method according to claim 1 or 2, wherein the ethylene unit content of the ethylene-vinyl acetate copolymer used in step (I) is 10 mol% or more and less than 60 mol%.
The method according to claim 1 or 2, wherein the solvent for the ethylene-vinyl acetate copolymer solution used in step (I) and the solvent vapor used in step (II) are alcohols.
The method of claim 4, wherein the alcohol is methanol.
The method according to claim 1 or 2, wherein the concentration of the ethylene-vinyl acetate copolymer solution used in step (I) is 60% by mass or less.
The method according to claim 1 or 2, wherein the amount of solvent vapor supplied to the tower reactor (A) in step (II) is 50 parts by mass or more and less than 500 parts by mass per 100 parts by mass of the partially saponified ethylene-vinyl acetate copolymer supplied to the tower reactor (A).
The method according to claim 1 or 2, in which in step (I), a solution of ethylene-vinyl acetate copolymer and an alkali catalyst are mixed in a non-tower mixer to obtain a solution of partially saponified ethylene-vinyl acetate copolymer having a degree of saponification of 10 mol% or more and less than 60 mol%.