WO2024204499A1

WO2024204499A1 - Method for producing ethylene-vinyl alcohol copolymer

Info

Publication number: WO2024204499A1
Application number: PCT/JP2024/012575
Authority: WO
Inventors: 直人西田; 英里子片平; 瑞子尾下
Original assignee: 株式会社クラレ
Priority date: 2023-03-29
Filing date: 2024-03-28
Publication date: 2024-10-03

Abstract

This method for producing EVOH has: a saponification step (I) for supplying an EVAc solution containing EVAc and an alcohol to the top of a column type reactor, supplying an alcohol vapor to the bottom of the reactor, discharging the alcohol vapor from the top of the reactor, saponifying the EVAc using an alkali catalyst, and discharging an EVOH solution from the bottom of the column; and an aldehyde reduction step (II) for reducing the amount of aldehyde in a recovered alcohol (a) obtained by recovering alcohol used in the saponification step (I), thereby obtaining an alcohol (b). The aldehyde reduction step (II) includes a step for reducing the amount of aldehyde by bringing the recovered alcohol (a) into contact with an acetalation catalyst. In the saponification step (I), the alcohol vapor supplied to the bottom of the reactor contains the alcohol (b), and the alcohol (b) contains crotonaldehyde. Due to this configuration, provided is a method for producing EVOH in which the obtained EVOH has a lower environmental impact while suppressing a deterioration in color hue.

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. A common method for producing EVOH is, for example, to saponify ethylene-vinyl acetate copolymer (hereinafter sometimes abbreviated as "EVAc") obtained by copolymerizing ethylene and vinyl acetate in an organic solvent containing alcohol in the presence of a saponification catalyst.

From the perspective of reducing the environmental impact, it is desirable to recover and reuse unreacted raw materials and used solvents in the EVOH manufacturing process, and it is particularly important to reuse alcohol-based solvents such as methanol.

As an example of reusing the methanol solvent, the working examples in Patent Document 1 state that in the copolymerization of ethylene and vinyl acetate in an alcoholic solvent, the methanol used during polymerization is recovered, and then the recovered methanol is treated with a cation exchange resin to acetalize the aldehyde contained therein, thereby reducing the concentration of aldehyde compounds, and then the methanol is used again in the polymerization process, resulting in a monolayer film formed from the resulting EVOH pellets that has a good appearance (visual evaluation of the number of fisheyes and coloring).

JP 2002-060403 A

Methanol recovered from the polymerization process as described in the above conventional method produces a small amount of crotonaldehyde obtained by polycondensation of acetaldehyde, so it is relatively easy to reuse the solvent in the polymerization process. However, methanol recovered from the saponification process tends to contain a relatively large amount of crotonaldehyde because the saponification process is a reaction system in which polycondensation of acetaldehyde is relatively likely to occur. Here, polyenation by polycondensation of acetaldehyde is considered to be one of the factors that affect the quality of the final product when acetaldehyde is removed, and it was thought that reusing methanol containing crotonaldehyde and the like produced by polyenation would have a negative effect on the manufacturing process and reduce the quality of the final product. In the aldehyde reduction process by acetalization described in Patent Document 1, it is possible to remove acetaldehyde, but it is difficult to remove crotonaldehyde, and it has been found that in order to remove crotonaldehyde, distillation, which has a large environmental impact, or a long-term acetalization process must be performed. In particular, it was found that when methanol containing crotonaldehyde, etc. is used in the polymerization process as described in Patent Document 1, the quality of the EVOH pellets deteriorates.

In light of the above situation, the inventors conducted extensive research and surprisingly discovered that even if the alcohol introduced as alcohol vapor in the saponification process contains a specific amount of crotonaldehyde, the hue of the resulting EVOH does not deteriorate. Therefore, they discovered that by recovering and reusing the alcohol used in the saponification process and using it as a raw material for the alcohol vapor in the saponification process, it is possible to effectively utilize resources (reducing the environmental burden) while suppressing deterioration in quality, which led to the present invention.

In other words, the present invention aims to provide a method for producing EVOH that reduces the environmental impact while suppressing deterioration in the hue of the resulting EVOH.

According to the present invention, the above object is to provide a method for producing EVOH, comprising: a saponification step (I) of supplying an EVAc solution containing EVAc and an alcohol to an upper part of a tower reactor, supplying alcohol vapor to a lower part of the tower and discharging the alcohol vapor from the upper part of the tower, and saponifying the EVAc using an alkali catalyst, and extracting an EVOH solution containing EVOH and an alcohol having a degree of saponification of 80 mol % or more and 100 mol % or less from the bottom of the tower; and an aldehyde reduction step (II) of recovering the alcohol used in the saponification step (I) and reducing aldehyde in recovered alcohol (a) to obtain alcohol (b), wherein the aldehyde reduction step (II) includes a step of contacting the recovered alcohol (a) with an acetalization catalyst to reduce aldehyde, and in the saponification step (I), the alcohol vapor supplied to the lower part of the tower contains alcohol (b), and the alcohol (b) contains crotonaldehyde;
[2] The production method according to [1], further comprising a step of mixing the recovered alcohol (a) with an alcohol different from the recovered alcohol (a) in the aldehyde reduction step (II) to reduce the aldehyde concentration in the recovered alcohol (a) to obtain an alcohol (b);
[3] The production method according to [2], wherein the alcohol different from the recovered alcohol (a) includes a recovered alcohol recovered in a step other than the saponification step (I);
[4] The method according to [2] or [3], wherein the alcohol different from the recovered alcohol (a) contains unused alcohol;
[5] The method according to any one of [1] to [4], wherein in the saponification step (I), the acetaldehyde concentration in the alcohol vapor supplied to the column reactor is 0 to 150 ppm;
[6] The method according to any one of [1] to [5], wherein in the saponification step (I), the crotonaldehyde concentration in the alcohol vapor supplied to the column reactor is 5 to 200 ppm;
[7] The method according to any one of [1] to [6], wherein in the saponification step (I), the ethylene unit content of the EVAc supplied to the column reactor is 20 mol% or more and less than 60 mol%;
[8] An EVOH obtained by any one of the production methods [1] to [7], comprising an ethylene-vinyl alcohol copolymer (A) (hereinafter sometimes abbreviated as "EVOH (A)") having an ethylene unit content of 20 mol% or more and 60 mol% or less, and acetaldehyde (B1), and further comprising at least one selected from the group consisting of 2,4-hexadienal (B2) and 2,4,6-octatrienal (B3), and satisfying the following formula (1):
10≦b1/(b2+b3)<150...(1)
In the above formula (1), b1 is the content (ppm) of acetaldehyde (B1) relative to the EVOH (A), b2 is the content (ppm) of 2,4-hexadienal (B2) relative to the EVOH (A), and b3 is the content (ppm) of 2,4,6-octatrienal (B3) relative to the EVOH (A).
This is achieved by:

The manufacturing method of the present invention can provide a method for producing EVOH that reduces the environmental impact while suppressing deterioration in the hue of the resulting EVOH.

FIG. 1 is a schematic diagram of a column reactor used in the Examples and Comparative Examples.

The present invention is a method for producing EVOH, comprising: a saponification step (I) in which an EVAc solution containing EVAc and alcohol is supplied to the top of a tower reactor, alcohol vapor is supplied to the bottom of the tower and discharged from the top of the tower, the EVAc is saponified using an alkaline catalyst, and an EVOH solution containing EVOH and alcohol having a degree of saponification of 80 mol% or more and 100 mol% or less is extracted from the bottom of the tower; and an aldehyde reduction step (II) in which the alcohol used in the saponification step (I) is recovered and the recovered alcohol (a) is reduced in aldehyde to obtain alcohol (b), wherein the aldehyde reduction step (II) includes a step of contacting the recovered alcohol (a) with an acetalization catalyst to reduce aldehyde, and in the saponification step (I), the alcohol vapor supplied to the bottom of the tower contains alcohol (b), and the alcohol (b) contains crotonaldehyde. According to this production method, even if the recovered alcohol solvent is used as the alcohol vapor in the saponification step (I), the coloring of the obtained EVOH can be suppressed and the environmental load during production can be reduced. Usually, the alcohol used in the saponification step (I) contains crotonaldehyde and the like, which are generated by polyenation of acetaldehyde. Therefore, when the alcohol used in the saponification step (I) is recovered, a certain amount of crotonaldehyde is contained. Since crotonaldehyde and the like have a conjugated structure, compounds that have further progressed in polyenation are considered to be substances that cause coloring of the final product (EVOH). However, since it is difficult to remove crotonaldehyde by acetalization, the recovery of alcohol containing crotonaldehyde has been forced to rely on a method that uses a large amount of energy (environmental load), such as distillation. However, it was surprisingly discovered that when used as alcohol vapor in the saponification step (I), discoloration of the final product (EVOH) can be suppressed even if a certain amount of crotonaldehyde is contained, and that high-quality EVOH (with suppressed discoloration) can be provided using a manufacturing method with low environmental impact. The present invention will be described in detail below.

The manufacturing method of the present invention includes a saponification step (I) in which an EVAc solution containing EVAc and alcohol is supplied to the top of a tower reactor, alcohol vapor is supplied to the bottom of the tower and discharged from the top of the tower, the EVAc is saponified using an alkaline catalyst, and an EVOH solution containing EVOH and alcohol with a saponification degree of 80 mol % to 100 mol % is taken out from the bottom of the tower. In the saponification step (I), an EVOH solution is obtained by saponifying the EVAc in the EVAc solution using an alkaline catalyst.

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

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

EVAc used in the saponification step (I) can be produced by copolymerizing ethylene and vinyl acetate according to a known 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 the saponification step (I) is preferably 20 mol% or more and 60 mol% or less. When the ethylene unit content is 20 mol% or more, the melt moldability of the resulting EVOH tends to be improved. More preferably, it is 24 mol% or more. On the other hand, when the ethylene unit content is 60 mol% or less, the gas barrier properties of the resulting EVOH are improved. The ethylene unit content is preferably 50 mol% or less, and more preferably 45 mol% or less. The EVAc used in the saponification step (I) may be partially saponified EVAc that has been saponified by a known method, or it may be EVAc that has not been partially saponified.

The EVAc solution contains alcohol. The saponification reaction of EVAc proceeds through an ester exchange reaction between the acetate ester group of EVAc and alcohol, so the use of an alkaline catalyst can be reduced to a small amount by using alcohol as the solvent for the EVAc solution, which has the advantage of allowing the saponification reaction to 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 from the viewpoint of productivity, it is preferably 70% by mass or less, and 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 in the saponification step (I) 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.

Generally, as the saponification reaction progresses, the solubility in the solvent decreases, so it is preferable to pressurize the tower reactor in the saponification step (I) and carry out the reaction at a high temperature. The pressure in the tower reactor 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 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 is more preferably 150°C or lower, even more preferably 140°C or lower, and particularly preferably 130°C or lower.

The amount of the alkali catalyst added in the saponification step (I) is preferably 0.01 to 10 parts by mass per 100 parts by mass of EVAc. 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.

In the saponification step (I), the alcohol vapor supplied to the bottom of the tower contains alcohol (b) that has been subjected to an aldehyde reduction treatment, obtained through the aldehyde reduction step (II) described below. The inclusion of alcohol (b) in the alcohol vapor reduces the amount of unused alcohol used, thereby reducing the environmental impact. The content of alcohol (b) contained in the alcohol vapor is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, and the alcohol vapor may consist only of alcohol (b). Examples of compounds other than alcohol (b) that may be contained in the alcohol vapor include alcohol that has not been subjected to an aldehyde reduction treatment.

In an alcohol-based solvent, the saponification reaction of EVAc proceeds through an ester exchange reaction between the acetate ester group of EVAc and alcohol, so by supplying alcohol vapor to the bottom of the tower, the amount of alkaline catalyst used can be reduced, allowing the saponification reaction to proceed efficiently. Suitable alcohol-based solvents include methanol, ethanol, 1-propanol, and 2-propanol, with methanol being particularly suitable.

In the saponification step (I), the amount of alcohol vapor supplied to the tower reactor is preferably 10 parts by mass or more and 1,000 parts by mass or less per 100 parts by mass of EVAc supplied to the tower reactor. If the amount of alcohol vapor is large, the amount of aldehyde contained in the recovered alcohol (a) can be reduced.

The alcohol used in the saponification step (I) is recovered as recovered alcohol (a) and is subjected to aldehyde reduction treatment in the aldehyde reduction step (II) described below. Here, the alcohol used in the saponification step (I) mainly means the alcohol discharged from the alcohol vapor outlet 1. There are no particular limitations on the method of recovering it as recovered alcohol (a), but in one embodiment, the alcohol discharged from the alcohol vapor outlet 1 is condensed, and then acetic acid esters such as methyl acetate produced by the saponification reaction contained in the condensed alcohol are distilled off to recover recovered alcohol (a). In this specification, the distillation operation of distilling off acetic acid esters such as methyl acetate to obtain recovered alcohol (a) is not considered to be an aldehyde reduction treatment in the aldehyde reduction step (II).

In the saponification step (I), the residual vinyl acetate contained in the EVAc solution is saponified to generate acetaldehyde, which then undergoes self-condensation under saponification conditions, so that the recovered alcohol (a) usually contains acetaldehyde and crotonaldehyde. The amount of acetaldehyde contained in the recovered alcohol (a) is not particularly limited, but is, for example, 50 ppm or more and 1000 ppm or less. The content of crotonaldehyde contained in the recovered alcohol (a) is not particularly limited, but is, for example, 5 ppm or more and 200 ppm or less. The content of acetaldehyde and crotonaldehyde contained in the recovered alcohol (a) can be adjusted, for example, by the content of residual vinyl acetate contained in the EVAc solution, the amount of alcohol vapor supplied, the concentration of the alkali catalyst, etc.

The manufacturing method of the present invention has an aldehyde reduction step (II) in which recovered alcohol (a) is subjected to an aldehyde reduction treatment to obtain alcohol (b). By having the aldehyde reduction step (II) in the manufacturing method of the present invention, the amount of unused alcohol used can be reduced, thereby reducing the environmental burden. The aldehyde reduction step (II) includes a step of contacting recovered alcohol (a) with an acetalization catalyst to reduce aldehyde as an aldehyde reduction treatment. Aldehyde reduction treatment by acetalization can reduce the environmental burden because it uses less energy than distillation, etc., but tends not to efficiently reduce crotonaldehyde. However, even if alcohol (b) contained in the alcohol vapor contains a certain amount of crotonaldehyde, it can suppress discoloration of the resulting EVOH, so the aldehyde reduction treatment by acetalization can reduce the environmental burden while suppressing discoloration of the resulting EVOH. In the aldehyde reduction process in the aldehyde reduction step (II), the amount of acetaldehyde in the alcohol (b) obtained in the aldehyde reduction step (II) is preferably reduced by 70% by mass or more, and more preferably by 80% by mass or more, of the amount of acetaldehyde in the recovered alcohol (a) supplied to the aldehyde reduction step (II).

The acetalization catalyst can be used without any particular limitation as long as it is not dissolved in an alcohol-based solvent, has an acid site on the surface, and can act as an acetalization catalyst. Examples of solid acids include metal oxides (e.g., _Al2O3 , _V2O5 ), sulfates (e.g., _NiSO4 , _CuSO4 ), phosphates (e.g., _AlPO4 ) _, _and chlorides (e.g., _AlCl3 , _CuCl3 ). Representative solid acid catalysts such as zeolite catalysts and silica-alumina catalysts may also be used. Minerals such as montmorillonite may also be used. However, cation exchange resins are particularly suitable for commercial scale implementation.

Cation exchange resins function as insoluble solid acids, and various products suitable for treating large amounts of liquid and repeated use are commercially available. Although weakly acidic cation exchange resins may be used, strongly acidic cation exchange resins are preferred, and sulfonic acid-type strongly acidic ion exchange resins manufactured by DuPont, such as Amberlyst (registered trademark), Amberlite (registered trademark), and AmberSep (registered trademark), can be used.

When acetalization is performed using a cation exchange resin, for example, the recovered alcohol (a) may be fed into a tower packed with bead-shaped cation exchange resin and passed through it. In this case, there are no particular limitations on the temperature inside the tower as long as it is within a temperature range in which the recovered alcohol (a) is liquid, and a temperature close to room temperature may usually be used. In addition, from the viewpoint of the efficiency of the solvent treatment, the residence time of the alcohol-based solvent inside the tower is preferably 15 seconds to 30 minutes.

As the aldehyde reduction treatment in the aldehyde reduction step (II), other aldehyde reduction treatments other than acetalization may be performed. That is, the alcohol (b) may be an alcohol that has been subjected to only acetalization, or may be an alcohol that has been subjected to aldehyde reduction treatments other than acetalization and acetalization. Examples of other aldehyde reduction treatments other than acetalization include distillation, mixing with an alcohol solvent having a low aldehyde concentration recovered in a step other than the saponification step (I), and mixing with unused alcohol solvent. When the other aldehyde reduction treatment is performed, such treatment may be performed before or after acetalization. When the aldehyde reduction treatment is distillation, it is preferable to perform the distillation after acetalization from the viewpoint of reducing the energy used. When the aldehyde reduction treatment is mixing with an alcohol solvent having a low aldehyde concentration, it is preferable to perform the distillation before acetalization. Here, the alcohol with a low aldehyde concentration means an alcohol in which the content of at least one of acetaldehyde and crotonaldehyde is less than that of the recovered alcohol (a).

If distillation is performed as the aldehyde reduction treatment in the aldehyde reduction step (II), the crotonaldehyde content can be reduced, but since a large amount of energy is required compared to the case where distillation is not performed, it is preferable not to include an aldehyde reduction treatment by distillation.

The aldehyde reduction process in the aldehyde reduction step (II) may include a method of reducing the aldehyde concentration in the recovered alcohol (a) by mixing an alcohol different from the recovered alcohol (a). Examples of the alcohol different from the recovered alcohol (a) include alcohol recovered in a step other than the saponification step (I), unused alcohol, etc.

When the aldehyde reduction process (II) involves mixing with an alcohol solvent with a low aldehyde concentration recovered in a process other than the saponification process (I), the process other than the saponification process (I) can be a copolymerization process, a process for recovering unreacted vinyl acetate, a concentration process, a purification process, etc.

As an example of a recovery method, the recovery of alcohol in the concentration process is described below. As a post-treatment method for the EVOH solution after the saponification reaction, a mixed vapor of alcohol solvent and water is supplied from the bottom of a tower-type vessel, and the EVOH solution is supplied from a position above the supply position of the mixed vapor, thereby replacing part of the solvent present in the supplied EVOH solution with water to produce a highly concentrated EVOH solution. The alcohol vapor and water vapor drawn out from the top of the tower are condensed in a condenser and can be recovered as an aqueous alcohol solution. The alcohol obtained by separating and distilling this mixed liquid containing alcohol and water in a distillation tower can be used.

When mixing with unused alcohol as an aldehyde reduction treatment in the aldehyde reduction step (II), it is possible to easily reduce the aldehyde concentration in the recovered alcohol (a) since the unused alcohol does not contain impurities such as aldehydes. However, using a large amount of unused alcohol has a large impact on the environment, so it is preferable to use a small amount.

The alcohol subjected to the aldehyde reduction step (II) may contain, in addition to the recovered alcohol (a), an alcohol having a higher content of acetaldehyde and crotonaldehyde than the recovered alcohol (a). The content of recovered alcohol (a) in the alcohol subjected to the aldehyde reduction step (II) may be 5% by mass or more, 20% by mass or more, 50% by mass or more, 70% by mass or more, or 90% by mass or more, and the alcohol subjected to the aldehyde reduction step (II) may consist only of recovered alcohol (a).

The content of acetaldehyde contained in the alcohol (b) obtained by the aldehyde reduction step (II) is preferably 150 ppm or less, more preferably 80 ppm or less, and even more preferably 40 ppm or less. When the content of acetaldehyde contained in the alcohol (b) is equal to or less than the above upper limit, discoloration of the obtained EVOH tends to be suppressed. It is preferable that the alcohol (b) does not contain acetaldehyde, but it may contain some amount. In other words, it may be 0 ppm or more. The preferred embodiment of the content of acetaldehyde contained in the alcohol vapor supplied from the bottom of the tower in the saponification step (I) is the same as the preferred embodiment of the content of acetaldehyde contained in the alcohol (b).

The alcohol (b) obtained by the aldehyde reduction step (II) contains crotonaldehyde. The content of crotonaldehyde contained in the alcohol (b) is preferably 200 ppm or less, more preferably 100 ppm or less, and may be 70 ppm or less, 47 ppm or less, 32 ppm or less, 27 ppm or less, or 23 ppm or less. The content of crotonaldehyde contained in the alcohol (b) may be 5 ppm or more. When the content of crotonaldehyde is equal to or less than the upper limit, discoloration of the obtained EVOH tends to be suppressed. On the other hand, considering that distillation or the like is required to reduce the content of crotonaldehyde, when the content of crotonaldehyde is equal to or more than the lower limit, the energy required to reduce the content of crotonaldehyde, i.e., the environmental load, can be reduced. In the saponification step (I), the preferred embodiment of the crotonaldehyde content contained in the alcohol vapor supplied from the bottom of the tower is the same as the preferred embodiment of the crotonaldehyde content contained in the alcohol (b).

It is not preferable to use the alcohol (b) obtained by the aldehyde reduction process (II) in the polymerization process. Since the alcohol (b) contains crotonaldehyde, using it in the polymerization process will reduce the quality of the EVOH pellets obtained and cause poor appearance during molding.

The degree of saponification of the EVOH contained in the EVOH solution obtained by the saponification step (I) is preferably 80 mol% or more, more preferably 95 mol% or more, and even more preferably 99 mol% or more. By carrying out sufficient saponification, the gas barrier properties of the obtained EVOH are improved.

In the present specification, the EVOH solution obtained by the saponification step (I) includes a paste-like solution that is not completely uniform throughout and has undergone phase separation. As a post-treatment method for the EVOH solution after the saponification reaction, a mixed vapor of a solvent and water is supplied from the bottom of a tower-type vessel, and the EVOH solution is supplied from a position above the supply position of the mixed vapor, thereby replacing a part of the solvent present in the supplied EVOH solution with water to obtain a high-concentration EVOH solution. The concentration of EVOH in the EVOH solution supplied to the tower-type vessel is preferably 15 to 50% by mass, more preferably 25 to 40% by mass. It is also preferable that the ratio of the supply amount of the EVOH solution to the supply amount of the mixed vapor (solution supply amount/vapor supply amount) is 100/400 to 100/8 by mass. It is also preferable that the water content in the mixed vapor is 20 to 70% by mass. The solvent used for the mixed vapor is preferably an alcohol with a boiling point of 130°C or less, and examples of such alcohol include alcohols such as methanol, ethanol, propanol, and butanol. Alcohols with a boiling point of 100°C or less are more preferable, and among them, methanol is preferred because it is easily available, inexpensive, has a low boiling point, and is easy to handle.

The high-concentration EVOH solution thus obtained 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. The moisture content of the dried EVOH pellets is preferably 0.08% by 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 preferred, and multi-stage drying including preliminary drying and main drying is more preferred.

The yellow index (YI) of the dried EVOH is preferably 13 or less, and more preferably 9.5 or less. The manufacturing method of the present invention makes it possible to produce such EVOH with little coloring and reduce the environmental impact.

The EVOH obtained by the production method of the present invention contains an ethylene-vinyl alcohol copolymer (A) having an ethylene unit content of 20 mol % or more and 60 mol % or less, and acetaldehyde (B1), and further contains at least one member selected from the group consisting of 2,4-hexadienal (B2) and 2,4,6-octatrienal (B3), and satisfies the following formula (1):
10≦b1/(b2+b3)<150...(1)
In the above formula (1), b1 is the content (ppm) of acetaldehyde (B1) relative to the EVOH (A), b2 is the content (ppm) of 2,4-hexadienal (B2) relative to the EVOH (A), and b3 is the content (ppm) of 2,4,6-octatrienal (B3) relative to the EVOH (A).
That is, by producing EVOH that satisfies the above conditions, it is possible to provide EVOH that achieves both suppression of coloration and reduction of the environmental load.

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 present invention will be specifically explained below using examples, but the present invention is not limited to the following examples.

[Evaluation method]
(1) Measurement of Ethylene Unit Content and Saponification Degree of EVOH The dried EVOH pellets obtained in the Examples and Comparative Examples 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 measured at 80° C. using a 500 MHz ¹ H-NMR ("GX-500" manufactured by JEOL Ltd.) to determine the ethylene unit content and the degree of saponification from the peak intensity ratios of ethylene units, vinyl alcohol units, and vinyl ester units.

(2) Quantitative determination of acetaldehyde and crotonaldehyde in alcohol solvent In the examples and comparative examples, 10 mL of an acetonitrile/acetic acid mixed solution (weight ratio: acetonitrile/acetic acid = 9:1) (hereinafter referred to as DNPH solution) containing 1000 mg/L of 2,4-dinitrophenylhydrazine was added to 0.5 mL of the recovered alcohol (a) supplied to the aldehyde reduction step (II) and the alcohol vapor (alcohol (b)) supplied to the saponification step (I), and the mixture was heated and stirred at 60 ° C. for 1 hour. The solution was analyzed with a Shimadzu high performance liquid chromatograph (column: Shiseido CAPCELL PAK C18 MG type, solvent: acetonitrile/water (gradient system), UV detector) to quantify the content of aldehydes. In addition, a calibration curve created with a commercially available aldehyde-DNPH product (or a synthesized product) was used for the quantitative determination.

(3) Quantitative determination of acetaldehyde, crotonaldehyde, 2,4-hexadienal, and 2,4,6-octatrienal 0.50 g of the dried EVOH pellets obtained in the Examples and Comparative Examples was freeze-pulverized to obtain a sample, which was weighed out in an amount of 50.0 mg and placed in a glass tube for a thermal desorption gas chromatograph mass spectrometer to prepare a sample tube. Using the thermal desorption gas chromatograph mass spectrometer described below, the sample was heated under the following conditions to adsorb the entire amount of volatile gas from the sample once into the adsorption tube, and the gas re-emitted from the adsorption tube was separated in a column to detect the peaks of each component. A calibration curve was created from the peak areas of standard samples of acetaldehyde, crotonaldehyde, 2,4-hexadienal, and 2,4,6-octatrienal, and each was quantified by the absolute calibration curve method. When measuring the standard sample, the standard sample was impregnated into an adsorption tube (manufactured by Tenax (registered trademark)/Carboxen (registered trademark)), and the adsorption tube impregnated with the standard sample was used instead of the sample tube. The temperature at the time of release after sample adsorption was changed from the sample tube temperature of 170° C. to the adsorption tube temperature of 260° C., but the measurement was performed in the same manner as in the measurement of the sample tube.
(Thermal desorption section)
Apparatus: TurboMatrix-ATD (PerkinElmer Japan)
Temperature when adsorbing the sample to the adsorption tube: 170°C (sample tube), -30°C (adsorption tube), 250°C (valve), 260°C (transfer line)
Adsorption time in the adsorption tube: 10 minutes Temperature at the time of release after sample adsorption: 170°C (sample tube), 260°C (adsorption tube), 250°C (valve), 260°C (transfer line)
Adsorption tube discharge time: 35 minutes Carrier gas: Helium Carrier gas flow rate to column: 1.0 ml/min
Pressure: 120 kPa
(Gas chromatograph mass spectrometry section)
Apparatus: 7890B GC System, 7977B MSD (Agilent Technologies)
Column: DB-WAX UI (length: 30 m, inner diameter: 0.25 mm, film thickness: 0.50 μm)
Column oven temperature: 40° C. for 5 minutes, then heated to 240° C. at a rate of 10° C./min and held for 10 minutes (total measurement temperature: 35 minutes)
Transfer line (connection) temperature: 240°C
Ionization conditions: EI+
Detectable ion mass range: m/z=29-600
Detection method: SCAN

(4) Hue Evaluation The yellow index (YI) value of the dried EVOH pellets obtained in the Examples and Comparative Examples was measured and calculated using a LAB Scan XE manufactured by Hunter Co., Ltd. in accordance with JIS K7373: 2006. The smaller the value, the more yellowing is suppressed and the better the hue is. A YI value of 13 or more was determined to have failed to suppress the deterioration of hue.

(5) Production volume ratio When both recovered alcohol and virgin alcohol are used in the saponification step (I) to carry out the saponification reaction, the production volume ratio is calculated by the ratio of the amount of EVOH obtained in the saponification step (I) when only virgin alcohol is used to the amount of EVOH obtained when the same amount of virgin alcohol is used. The higher the ratio, the less the amount of virgin alcohol used, and therefore the smaller the environmental impact.

Example 1
(Alcohol recovery)
An EVAc solution having a concentration of 48% by mass, in which EVAc having an ethylene unit content of 32 mol% and a vinyl acetate unit content of 68 mol% was dissolved in methanol, was supplied to a tower reactor (plate tower, number of plates: 21, inner diameter of tower: 140 mm) and saponified. FIG. 1 is a schematic diagram of the tower reactor. The EVAc solution was supplied to the 20th plate of the tower reactor, whose temperature inside the tower was 115° C., from an EVAc solution supply port 2 at a rate of 10 kg/h, and a 5 wt % methanol solution of sodium hydroxide as an alkali catalyst was supplied to the 19th plate at a rate of 1.6 kg/h from an alkali catalyst supply port 3. As alcohol vapor, unused methanol vapor was continuously supplied to the lower part of the first plate from an alcohol vapor blowing port 4 at a rate of 21.6 kg/h. The acetaldehyde content and the crotonaldehyde content in the methanol solvent used for the unused methanol vapor were analyzed according to the method described in the evaluation method (2) above, and acetaldehyde and crotonaldehyde were below the detection limit and were not detected. The by-product methyl acetate and aldehyde were distilled as a mixed vapor together with excess methanol from the top of the tower (alcohol vapor outlet 1), and an EVOH methanol solution was obtained from the bottom of the tower (EVOH solution outlet 5). The mixed liquid obtained by condensing the mixed vapor distilled from the top of the tower was introduced into another tower-type recovery vessel (plate tower), and distillation was performed to recover methanol (recovered alcohol (a)) from the bottom of the tower. The recovered methanol (recovered alcohol (a)) was measured for the acetaldehyde and crotonaldehyde contents according to the method described in the evaluation method (2) above, and contained 450 ppm of acetaldehyde and 50 ppm of crotonaldehyde. The results are shown in Table 1.

(Aldehyde Reduction Step (II))
Subsequently, the recovered methanol (recovered alcohol (a)) was subjected to an aldehyde reduction treatment using a cation exchange resin. Specifically, the recovered methanol (recovered alcohol (a)) was continuously charged into a tank (10 L) filled with a strong acid cation exchanger of H+ type (manufactured by DuPont: "Amberlyst 15") to perform acetalization of acetaldehyde, thereby performing an aldehyde reduction treatment, and an aldehyde-reduced methanol (alcohol (b)) for use as alcohol vapor in the saponification step (I) was obtained. The contents of acetaldehyde and crotonaldehyde in the obtained methanol (alcohol (b)) were measured according to the method described in the above evaluation method (2), and it contained 0 ppm of acetaldehyde and 50 ppm of crotonaldehyde. The results are shown in Table 1.

(Saponification step (I))
Saponification was carried out in the same manner as in the alcohol recovery described above, except that the aldehyde-reduced methanol (alcohol (b)) obtained in the aldehyde reduction step (II) was used as the alcohol vapor, and an EVOH methanol solution (EVOH concentration: 25% by mass) was obtained from the bottom of the tower (EVOH solution outlet 5) (saponification step (I)). In this saponification step (I), no unused alcohol was used, and the environmental load is small. The results are shown in Table 1.

(Production of EVOH pellets)
Acetic acid in an amount equivalent to the molar amount of sodium hydroxide fed to the tower reactor was added to the EVOH methanol solution obtained in the saponification step (I), the remaining sodium hydroxide was neutralized, and the solution was concentrated until the copolymer concentration reached 40% by mass. The concentrated solution was extruded from a nozzle with a diameter of 3.5 mm into a methanol-water mixed solvent (methanol/water = 10/90 mass ratio) kept at 5°C, solidified into a strand shape, and cut with a cutter to obtain hydrous EVOH pellets. The obtained hydrous EVOH 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 dried at 60°C for 5 hours and further dried at 110°C for 10 hours to obtain dried EVOH pellets. The obtained dried EVOH pellets were evaluated according to the methods described in the above evaluation methods (1), (3), and (4). The results are shown in Table 1.

Example 2
Dry EVOH pellets were produced and evaluated in the same manner as in Example 1, except that the supply rate of methanol vapor to the column reactor during alcohol recovery and the supply rate of methanol vapor in the saponification step (I) were changed to 43.2 kg/h. The results are shown in Table 1.

Example 3
In the aldehyde reduction step (II), 25 parts by mass of methanol having been subjected to aldehyde reduction treatment obtained by acetalization with a cation exchange resin and 75 parts by mass of methanol recovered from a step other than the saponification step (I) were mixed to carry out further aldehyde reduction treatment, and the aldehyde content of methanol (alcohol (b)) used in the alcohol vapor in the saponification step (I) was adjusted to be as shown in Table 1. Dry EVOH pellets were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Examples 4 and 5)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 3, except that in the aldehyde reduction step (II), the content of methanol recovered from the steps other than the saponification step (I) was changed as shown in Table 1. The results are shown in Table 1.

(Example 6)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 3, except that in the aldehyde reduction step (II), unused methanol was used in place of the methanol recovered from the steps other than the saponification step (I) so as to have the content shown in Table 1. The results are shown in Table 1.

(Example 7)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that in the aldehyde reduction step (II), 10 parts by mass of recovered methanol (recovered alcohol (a)) was mixed with 90 parts by mass of methanol recovered from a step other than the saponification step (I) and then acetalized with a cation exchange resin to carry out an aldehyde reduction treatment. The results are shown in Table 1.

(Examples 8 and 9)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 7, except that in the aldehyde reduction step (II), the content of methanol recovered from the steps other than the saponification step (I) was changed as shown in Table 1. The results are shown in Table 1.

Example 10
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 7, except that in the aldehyde reduction step (II), unused methanol was used in place of the methanol recovered from the steps other than the saponification step (I) so as to have the content shown in Table 1. The results are shown in Table 1.

(Example 11)
Dry 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. 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 ethylene unit content of the EVAc used in the saponification step (I) was changed so that the ethylene unit content of the resulting EVOH would be as shown in Table 1. The results are shown in Table 1. The ethylene unit contents of EVAc and EVOH are substantially the same.

(Example 14)
When an attempt was made to prepare dried EVOH pellets in the same manner as in Example 1 by adjusting the concentration of sodium hydroxide in the saponification step (I) so that the degree of saponification of the resulting EVOH would be as shown in Table 1, the resin particles stuck together in the drying step, making it impossible to continue the operation. Therefore, except that the supply rate of the EVAc solution to the saponification step (I) was changed to 5 kg/h, the supply rate of the recovered methanol vapor was set to 11 kg/h, and the concentration of sodium hydroxide was adjusted so that the degree of saponification of the resulting EVOH would be as shown in Table 1, dried EVOH pellets were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 1)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the methanol vapor supplied to the saponification step (I) was changed to virgin methanol. The results are shown in Table 1.

(Comparative Example 2)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the methanol vapor supplied to the saponification step (I) was recovered methanol (recovered alcohol (a)) that had not been subjected to an aldehyde reduction treatment. The results are shown in Table 1.

(Comparative Example 3)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the aldehyde reduction treatment method in the aldehyde reduction step (II) was changed to distillation. The results are shown in Table 1.

(Comparative Example 4)
Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the aldehyde reduction treatment method in the aldehyde reduction step (II) was changed to a method in which 25 parts by mass of recovered methanol (recovered alcohol (a)) was mixed with 75 parts by mass of methanol recovered from a step other than the saponification step (I) and that acetalization using a cation exchange resin was not performed. The results are shown in Table 1.

(Comparative Example 5)
Dry EVOH pellets were prepared and evaluated in the same manner as in Comparative Example 4, except that in the aldehyde reduction step (II), the content of methanol recovered from the steps other than the saponification step (I) was changed as shown in Table 1. The results are shown in Table 1.

(Comparative Examples 6 and 7)
Dry EVOH pellets were prepared and evaluated in the same manner as in Comparative Example 4, except that in the aldehyde reduction step (II), unused methanol was used in place of the methanol recovered from the steps other than the saponification step (I) so as to have the content shown in Table 1. The results are shown in Table 1.

1 Alcohol vapor exhaust port 2 EVAc solution supply port 3 Alkaline catalyst supply port 4 Alcohol vapor blowing port 5 EVOH solution outlet

Claims

a saponification step (I) of supplying an ethylene-vinyl acetate copolymer solution containing an ethylene-vinyl acetate copolymer and an alcohol to an upper part of a tower reactor, supplying alcohol vapor to a lower part of the tower and discharging it from the upper part of the tower, and saponifying the ethylene-vinyl acetate copolymer using an alkali catalyst to extract an ethylene-vinyl alcohol copolymer solution containing an ethylene-vinyl alcohol copolymer having a saponification degree of 80 mol % or more and 100 mol % or less and an alcohol from the bottom of the tower; and an aldehyde reduction step (II) of reducing aldehyde in recovered alcohol (a) obtained by recovering the alcohol used in the saponification step (I) to obtain alcohol (b),
The aldehyde reduction step (II) comprises a step of contacting the recovered alcohol (a) with an acetalization catalyst to reduce aldehyde;
A method for producing an ethylene-vinyl alcohol copolymer, wherein in the saponification step (I), the alcohol vapor supplied to the lower part of the column contains an alcohol (b), and the alcohol (b) contains crotonaldehyde.
The method according to claim 1, further comprising a step of mixing the recovered alcohol (a) with an alcohol different from the recovered alcohol (a) in the aldehyde reduction step (II) to reduce the aldehyde concentration in the recovered alcohol (a) and obtain the alcohol (b).
The method according to claim 2, wherein the alcohol different from the recovered alcohol (a) includes a recovered alcohol recovered in a step other than the saponification step (I).
The method according to claim 2 or 3, wherein the alcohol different from the recovered alcohol (a) includes unused alcohol.
The method according to any one of claims 1 to 4, wherein in the saponification step (I), the acetaldehyde concentration in the alcohol vapor supplied to the tower reactor is 0 to 150 ppm.
The method according to any one of claims 1 to 5, wherein in the saponification step (I), the crotonaldehyde concentration in the alcohol vapor supplied to the tower reactor is 5 to 200 ppm.
The method according to any one of claims 1 to 6, wherein in the saponification step (I), the ethylene unit content of the ethylene-vinyl acetate copolymer supplied to the tower reactor is 20 mol% or more and less than 60 mol%.
An ethylene-vinyl alcohol copolymer obtained by the production method according to any one of claims 1 to 7,
The present invention comprises an ethylene-vinyl alcohol copolymer (A) having an ethylene unit content of 20 mol% or more and 60 mol% or less, and acetaldehyde (B1),
Further comprising at least one selected from the group consisting of 2,4-hexadienal (B2) and 2,4,6-octatrienal (B3);
An ethylene-vinyl alcohol copolymer satisfying the following formula (1):
10≦b1/(b2+b3)<150...(1)
In the above formula (1), b1 is the content (ppm) of acetaldehyde (B1) relative to the ethylene-vinyl alcohol copolymer (A), b2 is the content (ppm) of 2,4-hexadienal (B2) relative to the ethylene-vinyl alcohol copolymer (A), and b3 is the content (ppm) of 2,4,6-octatrienal (B3) relative to the ethylene-vinyl alcohol copolymer (A).