Method for producing ethylene-vinyl alcohol copolymer

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

Technical Field

[0001] The present invention relates to a method for producing an ethylene-vinyl alcohol copolymer. Background Art

[0002] An ethylene-vinyl alcohol copolymer (hereinafter, sometimes abbreviated as "EVOH") is a polymer material which is excellent in oxygen barrier properties, oil resistance, antistatic property, mechanical strength and so on, and is extensively used as a variety of packaging materials and so on such as films, sheets and vessels.

[0003] EVOH has conventionally been produced by saponifying an ethylenevinyl acetate copolymer (hereinafter sometimes abbreviated as EVAc) with an alkali catalyst or the like, and methods for improving the saponification step have been proposed in order to improve productivity and quality of EVOH.

[0004] For example, Patent Literature No. 1 has described a method for producing a saponified EVAc comprising (1) the step of saponifying EVAc having an ethylene content of 15 to 60 mol% to a saponification degree of 70 to 98 mol% with an alkali catalyst in an alcohol solvent using a tower reactor, to provide a solution of a partially-saponified EVAc; (2) the step of adding water or water / an alcohol to the solution of a partially-saponified EVAc using a tower reactor to form a mixed solution followed by re-saponification in the presence of an alkali catalyst, to provide a high saponification degree solution of a saponified EVAc having a saponification degree of a vinyl acetate component of 99.4 mol% or more; and (3) the step of extruding the high saponification degree solution of a saponified EVAc into a coagulating bath for precipitation followed by acid treatment. It is said that according to such a method, there can be provided pellets of a saponified EVAc which are uniform and have excellent melt moldability.

[0005] Patent Literature No. 2 has described a method for producing a saponified EVAc, comprising adding an alkali catalyst to an EVAc solution; mixing the mixture; then feeding the resulting EVAc solution having a saponification degree of 5 mol% or less to an upper portion of a tower reactor while feeding solvent vapor to a lower portion of the tower and discharging the solvent vapor from an upper portion of the tower, while feeding an alkali catalyst to a position below the position of feeding the EVAc solution, to saponify EVAc. It is described that this production method can suppress coloration of the saponified EVAc by preliminarily removing vinyl esters as impurities and acetaldehyde as a by-product and then subjecting the EVAc to a saponification reaction. Citation List Patent Literature

[0006] Patent Literature 1: JP 9-67411 A Patent Literature 2: JP 2009-242645 A Summary of Invention Technical Problem

[0007] However, in the method described in Patent Literature No. 1, during resaponification, the saponified EVAc product is discharged from the tower reactor together with the solvent, which causes decrease in production efficiency. In addition, in the method described in Patent Literature No. 2, when EVOH is continuously produced for a long period of time, the partially-saponified EVAc may become solidified in the tower reactor for saponification, which causes decrease in production efficiency.

[0008] To solve the above problems, an objective of the present invention is to provide a highly productive method for producing EVOH, which can efficiently saponify EVAc. Solution to Problem

[0009] The above problems can be solved by providing a method for producing an EVOH comprising step (I) of saponifying an EVAc in a solution of the EVAc using an alkali catalyst to provide a solution of a partially-saponified EVAc having a saponification degree of 10 mol% or more and less than 60 mol%; and step (II) of feeding the solution of the partially-saponified EVAc to an upper portion of a tower reactor (A) while feeding solvent vapor to a lower portion of the tower and discharging the solvent vapor from an upper portion of the tower and feeding an alkali catalyst to the tower reactor (A) below the position of feeding the solution of the partially-saponified EVAc, to further saponify the partially-saponified EVAc and then discharging a resulting solution of EVOH having a saponification degree of 99 mol% or more and 100 mol% or less from the tower bottom.

[0010] It is preferable that a saponification degree of the partially-saponified EVAc provided by step (I) is 10 mol% or more and less than 50 mol%. It is also preferable that an ethylene unit content of the EVAc used in step (I) is 10 mol% or more and less than 60 mol%. The solvent in the solution of the EVAc used in step (I) and the solvent vapor used in step (II) are preferably an alcohol, more preferably methanol. It is also preferable that a concentration of the solution of the EVAc used in step (I) is 60% by mass or less. It is also preferable that the amount of the solvent vapor fed to the tower reactor (A) in step (II) is 50 parts by mass or more and less than 500 parts by mass based on 100 parts by mass of the partially-saponified EVAc fed to the tower reactor (A). It is also preferable that in step (I), the solution of the EVAc and the alkali catalyst are mixed in a non-tower blender, to provide a solution of the partially-saponified EVAc having a saponification degree of 10 mol% or more and less than 60 mol%. Advantageous Effects of Invention

[0011] According to the production method of the present invention, even when an EVOH is continuously produced for a long period of time, the ethylene-vinyl acetate copolymer can be efficiently saponified, so that the EVOH can be produced at good productivity. Brief Description of Drawings

[0012] FIG. 1 schematically shows a tower reactor used in Examples 1 to 13 and Comparable Examples 1 to 4. Description of Embodiments

[0013] The present invention provides a method for producing an EVOH comprising step (I) of saponifying an EVAc in a solution of the EVAc using an alkali catalyst to provide a solution of a partially-saponified EVAc having a saponification degree of 10 mol% or more and less than 60 mol%; and step (II) of feeding the solution of the partially-saponified EVAc to an upper portion of a tower reactor (A) while feeding solvent vapor to a lower portion of the tower and discharging the solvent vapor from an upper portion of the tower and feeding an alkali catalyst to the tower reactor (A) below the position of feeding the solution of the partially-saponified EVAc, to further saponify the partially-saponified EVAc and then discharging a resulting solution of EVOH having a saponification degree of 99 mol% or more and 100 mol% or less from the tower bottom. According to the production method of the present invention, even when an EVOH is continuously produced for a long period of time, the ethylene-vinyl acetate copolymer can be efficiently saponified, so that the EVOH can be produced at good productivity.

[0014] In step (I), EVAc in an EVAc solution (a solution in which EVAc is dissolved) is saponified using an alkali catalyst to obtain a solution of a partially-saponified EVAc.

[0015] The EVAc used in step (I) can be produced by copolymerizing ethylene and vinyl acetate according to a general method. There are no restrictions to the polymerization method, solvent and the like, but solution polymerization using methanol as a solvent is preferred. A polymerization catalyst which can be used is a radical initiator such as various azonitrile initiators and organic peroxide initiators. In addition, as long as the effects of the present invention are not impaired, the EVAc can contain monomers other than ethylene and vinyl acetate, which can be copolymerized with ethylene and vinyl acetate (for example, a-olefins such as propylene, unsaturated acids such as acrylic acid, various nitriles, and various amides). A content of units derived from the other monomers in the EVAc is usually 10 mol% or less.

[0016] An 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 plug the holes in the shelf plate, causing increase in pressure, which may shorten a continuous operable time. The ethylene unit content is more preferably 10 mol% or more, further preferably 15 mol% or more, and particularly preferably 20 mol% or more. If the ethylene unit content is 70 mol% or more, gas barrier properties of the resulting EVOH may be reduced. The ethylene unit content is more preferably 65 mol% or less, further preferably 60 mol% or less, and particularly preferably 55 mol% or less.

[0017] The solvent for the EVAc solution is not particularly limited as long as it can dissolve EVAc, but an alcohol is preferred. When an alcohol is used as the solvent, the saponification reaction of EVAc proceeds by transesterification between the acetate group of EVAc and an alcohol compound, so that the amount of the alkali catalyst used can be reduced and the saponification reaction can efficiently proceed. Preferable examples of the alcohol include methanol, ethanol, 1-propanol, and 2-propanol, and methanol is preferred.

[0018] A concentration of EVAc in the EVAc solution is preferably, but not limited to, 70% by mass or less. If the concentration exceeds 70% by mass, it may be difficult to uniformly mix the EVAc solution with the alkali catalyst, or in step (II), the partially-saponified EVAc in the tower reactor (A) may precipitate and plug the holes in the shelf plate, causing increase in pressure, thereby shortening a continuous operable time. The concentration of EVAc is more preferably 60% by mass or less. 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.

[0019] Examples of the alkali catalyst include compounds such as alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide; and alkali metal alkoxides such as sodium methoxide, sodium ethoxide, and potassium t-butoxide. Among these, more preferred are sodium hydroxide, potassium hydroxide, sodium methoxide, and sodium ethoxide, and further preferred is sodium hydroxide. The alkali catalyst can be used as it is, or can be used in the form of a solution. When used in the form of a solution, the solvent can be the same as that of the EVAc solution.

[0020] The EVAc solution is mixed with the alkali catalyst to saponify the EVAc in the mixed solution. A mixing method can be, but not limited to, a method using a mixer for a non-tower reactor or a tower reactor such as a static mixer, a mechanical stirrer and a dynamic mixer. In the light of excellent mixing efficiency, the mixer is preferably that for a non-tower reactor, and among these, more preferably a static mixer.

[0021] A concentration of the alkali catalyst in the mixed solution containing the EVAc and the alkali 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, further 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, a degree of saponification of the resulting partially-saponified EVAc may be excessively high. The concentration is more preferably 0.5 mol / L or less, and further preferably 0.3 mol / L or less.

[0022] A mixing time of the EVAc solution and the alkali catalyst is preferably, but not limited to, 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 further preferably 5 minutes or more. On the other hand, if the mixing time exceeds 100 minutes, a production efficiency may decrease, and a degree of saponification of the partially-saponified EVAc may become excessively high. The mixing time is more preferably 50 minutes or less, and further preferably 30 minutes or less.

[0023] A temperature of the mixture during mixing of the EVAc solution and the alkali catalyst is preferably 10 to 80°C. The temperature is more preferably 20°C or higher, further 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.

[0024] A degree of saponification of the partially-saponified EVAc obtained by mixing the EVAc solution with the alkali catalyst must be 10 mol% or more and less than 60 mol%. By saponifying EVAc to a predetermined degree of saponification in this way, impurities such as aldehydes as by-products are discharged together with the solvent from an upper portion of the tower in step (II) before condensation, so that EVOH with being less colored is obtained. If a saponification degree of the partially-saponified EVAc is less than 10 mol%, viscosity of the solution increases in step (II) as a saponification degree of the partially-saponified EVAc increases, and increase in viscosity leads to poor fluidity. On the other hand, when a saponification degree of the partially-saponified EVAc is 10 mol% or more, viscosity of the solution becomes almost the same as 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 partially-saponified EVAc in the tower reactor (A) is less likely to be solidified. Therefore, pressure increase caused by the hole in the shelf being plugged is suppressed, and EVOH can be produced efficiently. The saponification degree is more preferably 11 mol% or more, further 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%, discharge of the partially-saponified EVAc from the tower reactor (A) together with the solvent vapor in step (II) is suppressed. The saponification degree is more preferably 55 mol% or less, and further preferably 50 mol% or less.

[0025] In step (II), the solution of the partially-saponified EVAc is fed to an upper portion of a tower reactor (A), solvent vapor is fed to a lower portion of the tower and discharged from an upper portion of the tower, and an alkali catalyst is fed to the tower reactor (A) below the position where the solution of the partially-saponified EVAc is fed, to further saponify the partially-saponified EVAc, and the resulting solution of EVOH having a saponification degree of 99 mol% or more and 100 mol% or less is discharged from the bottom of the tower. In this way, in step (II), the partially-saponified EVAc is resaponified to provide EVOH with a high saponification degree.

[0026] FIG. 1 schematically shows a tower reactor (A) used in the examples described later. Step (II) will be described with reference to FIG. 1. The solution of the partially-saponified EVAc obtained in step (I) is fed to an upper portion of the tower reactor (A). In FIG. 1, the solution of the partially-saponified EVAc is fed to the tower reactor (A) from a partially-saponified EVAc solution feeding port 2 at an upper portion of the tower. An alkali catalyst is fed to an upper portion of the tower below the position of feeding the solution of the partially-saponified EVAc. In FIG. 1, an alkali catalyst is fed into the tower reactor (A) from an alkali catalyst feeding port 3 below the partially-saponified EVAc solution feeding port 2. Solvent vapor is fed to a lower portion of the tower and discharged from an upper portion of the tower. The position at which the solvent vapor is fed is preferably above the position at which the solution of the partially-saponified EVAc is discharged, which will be described later. The position from which the solvent vapor is discharged is preferably above the position from which the solution of the partially-saponified EVAc is fed, in the light of more efficiently removing impurities such as aldehyde remaining in the solution of the partially-saponified EVAc. From a similar point of view, 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 through a solvent vapor inlet 4 at a lower portion of the tower, and the solvent vapor is discharged from a solvent vapor outlet 1 at the top of the tower.

[0027] The solution of the partially-saponified EVAc fed to an upper portion of the tower is transported from the top to the bottom of the tower reactor (A). The solution of the partially-saponified EVAc fed to an upper portion of the tower comes into contact with the solvent vapor, and by-products such as an aldehyde and an acetate are discharged from an upper portion of the tower (solvent vapor outlet 1) together with the solvent vapor. The solution of the partially-saponified EVAc transported to the position where the alkaline catalyst is fed (alkaline catalyst feeding port 3) comes into contact with the alkaline catalyst, to resaponify the partially-saponified EVAc, and then the EVOH solution is discharged from the bottom of the tower (EVOH solution outlet 5). By feeding the alkaline catalyst to a position below the position where the partially-saponified EVAc solution is fed at an upper portion of the tower, impurities such as aldehyde remaining in the solution of the partially-saponified EVAc can be removed in advance by the solvent vapor at an upper portion of the tower, and then the partially-saponified EVAc can be resaponified. Thus, coloration of the partially-saponified EVAc during resaponification is suppressed.

[0028] In step (II), the same alkali catalyst as in step (I) is used. The solvent vapor is the same as the solvent of the EVAc solution used in step (I). In terms of the solvent used in step (II), the saponification reaction needs to proceed until a saponification degree of the partially-saponified EVAc is 99 mol% or more, but in a reaction system containing a large amount of water in the solvent, a hydrolysis reaction of the acetate ester becomes dominant, which may make it difficult to efficiently carry out the saponification reaction, and thus the amount of the alkali catalyst used may be increased to increase a saponification degree. Therefore, it is preferable that the solvent used in step (II) does not contain water, but if it contains water, the content is preferably 1.5% by mass or less, more preferably 1% by mass or less, and further preferably 0.8% by mass or less in the light of decreasing the amount of the alkali catalyst. When the water content of the solvent used in step (II) is equal to or less than the upper limit, the amount of the residual catalyst contained in the obtained EVOH is reduced, and as a result, the deterioration of thermal stability can be preferably suppressed. A concentration of the partially-saponified EVAc in the solution of the partially-saponified EVAc subjected to step (II) is preferably, but not limited to, 70% by mass or less. If the concentration is more than 70% by mass, the partially-saponified EVAc may precipitate in the tower reactor (A) and plug the holes in a shelf, causing increase in pressure, which may shorten a period of continuous operation. The concentration of the partially-saponified EVAc is more preferably 60% by mass or less. On the other hand, in the light of productivity, the concentration of the partially-saponified EVAc is preferably 10% by mass or more, more preferably 20% by mass or more, further preferably 30% by mass or more, and particularly preferably 40% by mass or more.

[0029] Generally, as the saponification reaction proceeds, solubility of the partially-saponified EVAc in the solvent decreases, so it is preferable to pressurize the tower reactor (A) in step (II) and to carry out the reaction at a high temperature. A pressure in the column reactor (A) is preferably 0.1 to 1.0 MPa. The pressure is more preferably 0.8 MPa or less, further preferably 0.6 MPa or less, and particularly preferably 0.55 MPa or less. The pressure can be 0.2 MPa or more.

[0030] A temperature of the tower reactor (A) is preferably 60 to 180°C. In the light of improving a reaction efficiency the temperature is more preferably 70°C or higher, further 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, further preferably 140°C or lower, and particularly preferably 130°C or lower.

[0031] In step (II), the amount of solvent vapor fed to the tower reactor (A) is preferably 50 parts by mass or more and less than 500 parts by mass based on 100 parts by mass of the partially-saponified EVAc fed 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 partially-saponified EVAc 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 partially-saponified EVAc may be discharged together with the solvent vapor. The amount of the solvent vapor is more preferably 300 parts by mass or less, further 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. A temperature of the solvent vapor can be, for example, about the boiling point of the solvent at a pressure inside the tower.

[0032] The amount of the alkali catalyst in step (II) is preferably 0.01 to 10 parts by mass based on 100 parts by mass of the partially-saponified EVAc. The amount is more preferably 0.05 parts by mass or more, further 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 further preferably 3 parts by mass or less.

[0033] 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. Thus, EVOH can be produced more efficiently. The number of days for continuously producing EVOH is more preferably 40 days or more, further preferably 45 days or more, even more preferably 50 days or more, and particularly preferably 55 days or more.

[0034] The EVOH solution obtained in step (II) is preferably pelletized by a known method. Examples of the pelletization method include a method comprising cooling and solidifying the EVOH solution and cutting it; and a method comprising melt-kneading the EVOH in an extruder, discharging it, and cutting it. Specific examples of the EVOH cutting method include a method comprising extruding EVOH into a strand shape and cutting it with a pelletizer; and a method comprising cutting EVOH discharged from a die using a center hot cut method or an underwater cut method. When the EVOH solution is pelletized, it becomes water-containing EVOH pellets.

[0035] When the EVOH solution is solidified by cooling to obtain water-containing EVOH pellets, the water-containing EVOH pellets are preferably washed and deliquified by a known method. Also, it is preferable to perform chemical treatment by immersing the water-containing EVOH 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 water-containing EVOH. Incorporation of these compounds can improve the mechanical properties and thermal stability of an EVOH molded article. Also, when the water-containing EVOH is obtained by meltkneading and pelletizing EVOH, the EVOH can be washed, deliquified, and chemically treated in an extruder.

[0036] The obtained water-containing EVOH pellets can be dried by a known method to obtain EVOH pellets. A moisture content of the dried EVOH pellets is preferably 0.5% by mass or less. There are no particular restrictions to a drying method, and examples thereof include a stationary drying method combined with air drying or nitrogen drying, fluidized drying, and vacuum drying. A multi-stage drying method combining several drying methods is preferable, which is more preferably a multi-stage drying method comprising preliminary drying and main drying.

[0037] A 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 production method of the present invention, it is possible to efficiently produce EVOH having such a high saponification degree. The saponification degree is preferably 99.3 mol% or more, more preferably 99.5 mol% or more, and further preferably 99.7 mol% or more.

[0038] An yellowing index (Yl) of the EVOH is preferably 20 or less, more preferably 15 or less, further preferably 13 or less, and particularly preferably 9.5 or less. According to the production method of the present invention, such EVOH with little coloration can be efficiently produced.

[0039] The EVOH obtained by the method of the present invention can be molded into various types of articles such as films, sheets, containers, pipes and fibers. Examples

[0040] The present invention will be described in more detail with reference to Examples.

[0041] [Evaluation method] (1) Saponification degree of a 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 dried partially-saponified EVAc. Twenty mg of the resulting dried partially-saponified EVAc was dissolved in 6 mL of CDCh and subjected to 1H-NMR measurement under the following measurement conditions, and a degree of saponification was calculated from a ratio of the integrated value of the methine hydrogen of the vinyl alcohol unit (saponified site) observed at 3.1 to 4.1 ppm to the integrated value of the methine hydrogen of the vinyl acetate unit (unsaponified site) observed at 4.5 to 5.2 ppm. (Measurement conditions) Device: JEOL superconducting nuclear magnetic resonance spectrometer Lambda 500 Observation frequency: 500 MHz Measurement temperature: 25°C Cumulative number: 1024 times

[0042] (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 1H-NMR measurement was performed under the measurement conditions described below. A saponification degree was calculated from a ratio of the integrated value of the methine hydrogen of the vinyl alcohol unit observed at 3.1 to 4.1 ppm (saponified site) to the integrated value of the methyl group hydrogen of the vinyl acetate unit observed at 1.9 to 2.0 ppm (unsaponified site). (Measurement conditions) Device: JEOL superconducting nuclear magnetic resonance spectrometer Lambda 500 Observation frequency: 500 MHz Measurement temperature: 80°C Cumulative number: 128 times

[0043] (3) Bottom pressure of the tower reactor (A) in the step (II) A pressure at the bottom of the tower was measured with a pressure gauge installed at the bottom (the lower part of the first plate) of the tower reactor (A) [shelf plate tower (saponification tower, total number of plates: 21)] used in step (II) in the Examples and Comparative Examples. Table 1 shows a pressure at the bottom of the tower three days after the initiation of EVOH production.

[0044] (4) Number of days of continuous production 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 initiation of production to the stop of production was determined as the number of days of continuous production.

[0045] (5) Distillate of step (II) Three days after the initiation of the production of EVOH in the Examples and Comparative Examples, the mixed vapor discharged (distilled) from the top of the tower in step (II) was condensed as a distillate, which was visually inspected for the presence of turbidity and a resin, and was evaluated according to the following criteria. Rating D was determined to be poor in production efficiency. Evaluation: Criteria A: Colorless and transparent B: Slightly cloudy C: Severely cloudy D: Presence of resin (6) Yellowing evaluation of EVOH In the examples and comparative examples, the Yl (yellowing index) of the dried EVOH pellets obtained 3 days after the initiation of production was measured in accordance with JIS-K-7103.

[0046] [Example 1] (Step I) A 57% by mass EVAc solution in methanol, 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 solution in methanol at a volume ratio of 95:5 at 60°C for 10 minutes in a static mixer to obtain a solution of a partially-saponified EVAc. For the resulting solution of a partially-saponified EVAc, a saponification degree was determined in accordance with the procedure as described in Evaluation Method (1). The results are shown in Table 1.

[0047] (Step II) Subsequently, the partially-saponified EVAc solution in methanol obtained in step (I) was further saponified using a tower reactor (A) (shelf plate tower (saponification tower, total number of plates: 21), tower inner diameter: 140 mm, height: 4700 mm). FIG. 1 schematically shows the tower reactor (A). The partially-saponified EVAc solution in methanol was continuously fed to the 20th shelf plate of the tower reactor (A) at a rate of 8 kg / h from the partially-saponified EVAc solution feeding port 2 at a tower inner temperature of 118°C. Methanol vapor was continuously fed from the solvent vapor feeding port 4 to the lower part of the first plate in an amount of 75 parts by mass based on 100 parts by mass of the partially-saponified EVAc fed to the tower reactor (A). A methanol solution containing sodium hydroxide in an amount of 1 part by mass based on 100 parts by mass of the partially-saponified EVAc fed to the tower reactor (A) was continuously fed to the 14th shelf plate from the alkali catalyst feeding 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 a solution of EVOH in methanol was obtained from the bottom of the tower (EVOH solution outlet 5). A bottom pressure, the number of days of continuous production, and distillate in step (II) were evaluated in accordance with the methods described in the above evaluation methods (3) to (5). The results are shown in Table 1.

[0048] (Production of EVOH pellets) To the solution of EVOH in methanol obtained in step (II) was added acetic acid equimolar to sodium hydroxide fed to the tower reactor (A) in step (II), to neutralize the remaining sodium hydroxide. Next, the solution was concentrated until the copolymer concentration was 40% by mass. This solution was extruded from a nozzle with a diameter of 3.5 mm into a methanolwater mixed solvent (methanol / water = 10 / 90 by weight) kept at 5°C, to be solidified into a strand shape, which was cut with a cutter to obtain water-containing EVOH pellets. Furthermore, these pellets were washed by putting these into a large amount of 0.1 g / L aqueous acetic acid solution, and after removing the remaining methanol and sodium acetate, these were dried at 60°C for 5 hours and further dried at 110°C for 10 hours to obtain dry EVOH pellets. The obtained dry EVOH pellets were measured for a saponification degree and yellowing according to the methods described in the above evaluation methods (2) and (6). The results are shown in Table 1.

[0049] (Examples 2 and 5, and Comparative Examples 1 to 4) Dry EVOH pellets were prepared and evaluated as described in Example 1, except that a concentration of sodium hydroxide in step (I) was adjusted such that a saponification degree of the resulting partially-saponified EVAc was to be as shown in Table 1. The results are shown in Table 1.

[0050] (Examples 3, 6, and 7) Dry EVOH pellets were prepared and evaluated as described in Example 1, except that an EVAc having an ethylene unit content as described in Table 1 was used and a concentration of sodium hydroxide in step (I) was adjusted such that a saponification degree of the resulting partially-saponified EVAc was to be as shown in Table 1. The results are shown in Table 1.

[0051] (Examples 4, 10, and 11) EVOH pellets were prepared and evaluated as described in Example 1, except that EVOH pellets were produced and evaluated in the same manner as in Example 1, except that the amount of methanol vapor fed to the tower reactor (A) in the step (II) was changed as shown in Table 1. The results are shown in Table 1.

[0052] (Example 8) EVOH pellets were prepared and evaluated as described in Example 1, substituting ethanol for methanol steps (I) and (II). The results are shown in Table 1.

[0053] (Example 9) EVOH pellets were prepared and evaluated as described in Example 1, except that in step (I), a concentration of the EVAc solution in methanol used was changed as shown in Table 1, and a concentration of sodium hydroxide was adjusted such that a saponification degree of the resulting partially-saponified EVAc was to be as shown in Table 1. The results are shown in Table 1.

[0054] (Examples 12 and 13) Dry EVOH pellets were prepared and evaluated as described 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.

[0055] [Table 1] Step (I) Step (II) Evaluation Ethylene unit content EVAc solution Saponification degree Mixing method Solvent vapor Step (II) EVOH Solvent Concentration Solvent Feeding amount *1 Tower bottom pressure Number of days of continuous production Distillate Saponification degree Yl mol% - % by mass mol% - Parts by mass MPa Days - mol% - Example 1 44 Methanol 57 14 Static mixer Methanol 75 0.37 60 A 99.9 9 Example 2 44 Methanol 57 45 Static mixer Methanol 75 0.36 63 A 99.9 8 Example 3 24 Methanol 57 14 Static mixer Methanol 75 0.38 57 A 99.9 9 Example 4 44 Methanol 57 14 Static mixer Methanol 187 0.38 57 A 99.9 9 Example 5 44 Methanol 57 55 Static mixer Methanol 75 0.37 60 C 99.9 8 Example 6 9 Methanol 57 14 Static mixer Methanol 75 0.45 36 A 99.9 9 Example 7 68 Methanol 57 14 Static mixer Methanol 75 0.37 60 A 99.2 9 Example 8 44 Ethanol 57 14 Static mixer Ethanol 75 0.37 60 A 99.4 9 Example 9 44 Methanol 65 14 Static mixer Methanol 75 0.43 42 A 99.9 9 Example 10 44 Methanol 57 14 Static mixer Methanol 38 0.41 48 A 99.9 9 Example 11 44 Methanol 57 14 Static mixer Methanol 223 0.37 60 B 99.9 9 Example 12 44 Methanol 57 14 Mechanical stirrer Methanol 75 0.37 60 A 99.9 14 Example 13 44 Methanol 57 14 Tower reactor (no stirring) Methanol 75 0.37 60 A 99.9 17 Comparative Example 1 44 Methanol 57 1.2 Static mixer Methanol 75 0.48 27 A 99.9 10 Comparative Example 2 44 Methanol 57 95 Static mixer Methanol 75 0.37 60 D 99.9 8 Comparative Example 3 44 Methanol 57 8 Static mixer Methanol 75 0.47 30 A 99.9 10 Comparative Example 4 44 Methanol 57 70 Static mixer Methanol 75 0.37 60 D 99.9 8 *1 the amount of solvent vapor based on 100 parts by mass of EVAc fed to the tower reactor Reference Signs List

[0056] 1: Solvent vapor outlet 2: Partially-saponified EVAc solution feeding port 3: Alkali catalyst feeding port 4: Solvent vapor inlet 5: EVOH solution outlet

Claims

1. A method for producing an ethylene-vinyl alcohol copolymer comprisingstep (I) of saponifying an ethylene-vinyl acetate copolymer in a solution of the ethylene-vinyl acetate copolymer using an alkali catalyst to provide a solution of a partially-saponified ethylene-vinyl acetate copolymer having a saponification degree of 10 mol% or more and less than 60 mol%; andstep (II) offeeding the solution of the partially-saponified ethylene-vinyl acetate copolymer to an upper portion of a tower reactor (A) whilefeeding solvent vapor to a lower portion of the tower and discharging the solvent vapor from an upper portion of the tower andfeeding an alkali catalyst to the tower reactor (A) below the position of feeding the solution of the partially-saponified ethylene-vinyl acetate copolymer, to further saponify the partially-saponified ethylene-vinyl acetate copolymer and then discharging a resulting solution of ethylene-vinyl alcohol copolymer having a saponification degree of 99 mol% or more and 100 mol% or less from the tower bottom.

2. The production method according to Claim 1, wherein a saponification degree of the partially-saponified ethylene-vinyl acetate copolymer provided by step (I) is 10 mol% or more and less than 50 mol%.

3. The production method according to Claim 1 or 2, wherein an ethyleneunit content of the ethylene-vinyl acetate copolymer used in step (I) is 10 mol% or more and less than 60 mol%.

4. The production method according to Claim 1 or 2, wherein the solvent in the solution of the ethylene-vinyl acetate copolymer used in step (I) and the solvent vapor used in step (II) are an alcohol.

5. The production method according to Claim 4, wherein the alcohol is methanol.

6. The production method according to Claim 1 or 2, wherein a concentration of the solution of the ethylene-vinyl acetate copolymer used in step (I) is 60% by mass or less.

7. The production method according to Claim 1 or 2, wherein the amount of the solvent vapor fed to the tower reactor (A) in step (II) is 50 parts by mass or more and less than 500 parts by mass based on 100 parts by mass of the partially-saponified ethylene-vinyl acetate copolymer fed to the tower reactor (A).

8. The production method according to Claim 1 or 2, wherein in step (I), the solution of the ethylene-vinyl acetate copolymer and the alkali catalyst are mixed in a non-tower blender, to provide a solution of the partially-saponified ethylene-vinyl acetate copolymer having a saponification degree of 10 mol% ormore and less than 60 mol%.INTERNATIONAL SEARCH REPORT International application No. PCT / JP2024 / 016633A. CLASSIFICATION OF SUBJECT MATTER C08F210 / 02(2006.01)1; C08F 8 / 12(2006.01)1; C08F 216 / 06(2006.01)1; C08F 218 / 08(2006.01)1 FI: C08F210 / 02; C08F8 / 12; C08F216 / 06; C08F218 / 08 According to International Patent Classification (IPC) or to both national classification and IPC B. FIELDS SEARCHED Minimum documentation searched (classification system followed by classification symbols) C08F210 / 02: C08F8 / 12; C08F216 / 06; C08F218 / 08 Documentation searched other than minimum documentation to the extent that such documents are included in the fields searched Published examined utility model applications of Japan 1922-1996 Published unexamined utility model applications of Japan 1971-2024 Registered utility model specifications of Japan 1996-2024 Published registered utility model applications of Japan 1994-2024 Electronic data base consulted during the international search (name of data base and, where practicable, search terms used) C. DOCUMENTS CONSIDERED TO BE RELEVANT Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. X Y Y A JP 2009-242645 A (KURARAY CO., LTD.) 22 October 2009 (2009-10-22) claims, paragraphs [0008]-[0011], [0017]-[0020], [0031], examples CN 110316690 A (HKC CO., LTD.) 11 October 2019 (2019-10-11) claims, paragraphs [0002]-[0005], [0052]-[0055], examples JP 2010-077352 A (KURARAY CO., LTD.) 08 April 2010 (2010-04-08) claims, paragraphs [0008]-[0011], [0016], [0017], [0037], examples 1-8 1-8 1-8 1-8 A A JP 49-027591 A (THE NIPPON SYNTHETIC CHEMICAL INDUSTRY CO., LTD.) 12 March 1974 (1974-03-12) claims, p. 1, lower right column, line 12 to p. 2, upper left column, line 13, p. 3, lower left column, line 4 to lower right column, line 18, p. 4, lower left column, lines 7-15, examples WO 2023 / 068242 Al (MITSUBISHI CHEMICAL CORPORATION) 27 April 2023 (2023-04-27) entire text 1-8 1-8 | | Further documents are listed in the continuation of Box C. | | See patent family annex. * Special categories of cited documents: “T” later document published after the international filing date or priority “A” document defining the general state of the art which is not considered date and not in conflict with the application but cited to understand the to be of particular relevance principle or theory underlying the invention “D” document cited by the applicant in die international application “X” document of particular relevance; the claimed invention cannot be ■SE” earlier application orpatent but published on or after the international considered novel or cannot be considered to involve an inventive step filing date when the document is taken alone •SL” document which may throw doubts on priority claim(s) or which is “Y” document of particular relevance; the claimed invention cannot be cited to establish the publication date of another citation or other considered to involve an inventive step when the document is special reason (as specified) combined with one or more other such documents, such combination “O” document referring to an oral disclosure, use, exhibition or other being obvious to a person skilled in the art means document member of the same patent family “P” document published prior to the international filing date but later than the priority date claimed Date of the actual completion of the international search 20 June 2024 Date of mailing of the international search report 02 July 2024 Name and mailing address of the ISA / JP Japan Patent Office (ISA / JP) 3-4-3 Kasumigaseki, Chiyoda-ku, Tokyo 100-8915 Japan Authorized officer Telephone No.INTERNATIONAL SEARCH REPORT Information on patent family membersInternational application No.PCT / JP2024 / 016633Patent document cited in search report Publication date (day / month / year) Patent family member)s) Publication date (day / month / year) JP 2009-242645 A 22 October 2009 (Family: none) CN 110316690 A 11 October 2019 (Family: none) JP 2010-077352 A 08 April 2010 (Family: none) JP 49-027591 A 12 March 1974 (Family: none) WO 2023 / 068242 Al 27 April 2023 (Family: none)

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