Method for producing ethylene-vinyl alcohol copolymer

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

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

[0002] Ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as "EVOH") is a useful polymer material that has excellent oxygen barrier properties, oil resistance, antistatic properties, mechanical strength, etc., and is widely used as various packaging materials such as films, sheets, containers, etc. A common method for producing EVOH is, for example, to saponify an 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.

[0003] From the viewpoint of reducing the environmental load, it is desirable to recover and reuse unreacted raw materials and used solvents in the EVOH production process, and it is particularly important to reuse alcohol solvents such as methanol.

[0004] As an example of recycling a methanol solvent, Patent Document 1 describes in its working examples that in copolymerization of ethylene and vinyl acetate in an alcoholic solvent, the methanol used during polymerization is recovered, and the recovered methanol is then treated with a cation exchange resin to acetalize the aldehyde contained therein, thereby reducing the concentration of aldehyde compounds. The recovered methanol is then reused in the polymerization step, and the appearance (visual evaluation of the number of fisheyes and coloration) of a monolayer film produced from the resulting EVOH pellets is good.

[0005] Japanese Patent Application Laid-Open No. 2002-060403

[0006] Methanol recovered from the polymerization process described in the above-mentioned conventional method produces a small amount of crotonaldehyde, which is obtained by polycondensation of acetaldehyde, making it 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 thought to be one of the factors that affect the quality of the final product when acetaldehyde is removed. Therefore, it has been thought that reusing methanol containing crotonaldehyde and other compounds produced by polyenation would adversely affect the manufacturing process and reduce the quality of the final product. In the aldehyde reduction process by acetalization described in Patent Document 1, although acetaldehyde can be removed, removal of crotonaldehyde is difficult. It has been found that to remove crotonaldehyde, either distillation, which is environmentally harmful, or a long-term acetalization process is required. In particular, it has been found that when methanol containing crotonaldehyde or the like is used in the polymerization step as described in Patent Document 1, the quality of the EVOH pellets deteriorates.

[0007] In light of the above circumstances, the present inventors conducted extensive research and surprisingly found that even if the alcohol introduced as alcohol vapor in the saponification step contains a specific amount of crotonaldehyde, the color of the resulting EVOH does not deteriorate. Therefore, the present inventors discovered that by recovering and reusing the alcohol used in the saponification step and using it as a raw material for the alcohol vapor in the saponification step, it is possible to effectively utilize resources (reduce environmental impact) while suppressing deterioration in quality, and arrived at the present invention.

[0008] That is, an object of the present invention is to provide a method for producing EVOH that reduces the environmental load while suppressing deterioration in the color of the resulting EVOH.

[0009] According to the present invention, the above-mentioned object is achieved by providing: [1] 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; saponifying the EVAc using an alkali catalyst; and extracting an EVOH solution containing EVOH and the alcohol having a degree of saponification of 80 mol % to 100 mol % 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) comprises a step of bringing the recovered alcohol (a) into contact 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) comprises crotonaldehyde; [2] The production method of [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 of [2], wherein the alcohol different from the recovered alcohol (a) comprises a recovered alcohol recovered in a step other than the saponification step (I); [4] The production method of [2] or [3], wherein the alcohol different from the recovered alcohol (a) comprises a virgin alcohol; [5] The production method of any of [1] to [4], wherein the acetaldehyde concentration in the alcohol vapor supplied to the column reactor in the saponification step (I) is 0 to 150 ppm; [6] The production method of any of [1] to [5], wherein the crotonaldehyde concentration in the alcohol vapor supplied to the column reactor in the saponification step (I) is 5 to 200 ppm; [7] The production 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], the EVOH 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).

[0010] According to the production method of the present invention, it is possible to provide a production method for EVOH that suppresses deterioration in the color of the resulting EVOH and reduces the environmental impact.

[0011] FIG. 1 is a schematic diagram of a column reactor used in Examples and Comparative Examples.

[0012] The present invention provides a method for producing EVOH, comprising: a saponification step (I) in which an EVAc solution containing EVAc and an alcohol is supplied to an upper part of a tower reactor; alcohol vapor is supplied to a lower part of the tower and discharged from the upper part; and the EVAc is saponified using an alkali catalyst; and an EVOH solution containing EVOH and the alcohol, having a degree of saponification of 80 mol % or more and 100 mol % or less, is withdrawn 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 recovered alcohol (a) is reduced in aldehyde to obtain alcohol (b), wherein the aldehyde reduction step (II) comprises a step of bringing the recovered alcohol (a) into contact 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 alcohol (b) comprises crotonaldehyde. According to this production method, even if the recovered alcohol solvent is used as the alcohol vapor in the saponification step (I), discoloration of the resulting EVOH can be suppressed while reducing the environmental impact during production. Typically, the alcohol used in the saponification step (I) contains crotonaldehyde and other compounds produced by polyenation of acetaldehyde. Therefore, when the alcohol used in the saponification step (I) is recovered, a certain amount of crotonaldehyde is contained. Because crotonaldehyde and other compounds have a conjugated structure, compounds that undergo further polyenation are thought to be the cause of discoloration in the final product (EVOH). However, because crotonaldehyde is difficult to remove by acetalization, recovery of alcohol containing crotonaldehyde has traditionally relied on methods such as distillation, which consume a large amount of energy (leading to a large environmental impact). However, surprisingly, it has been found 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 it has been found that high-quality EVOH (with suppressed discoloration) can be provided by a production method with low environmental impact. The present invention will be described in detail below.

[0013] The production method of the present invention includes a saponification step (I) in which an EVAc solution containing EVAc and an 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, the EVAc is saponified using an alkali catalyst, and an EVOH solution containing EVOH and the alcohol having a saponification degree of 80 mol % or more and 100 mol % or less is withdrawn from the bottom of the tower. In the saponification step (I), the EVAc in the EVAc solution is saponified using the alkali catalyst to obtain an EVOH solution.

[0014] FIG. 1 is a schematic diagram of a column reactor used in the Examples described below. The saponification step (I) will be described with reference to FIG. 1 . An EVAc solution containing EVAc and an alcohol is supplied to the top of the column reactor. In FIG. 1 , the EVAc solution is supplied to the column reactor through an EVAc solution supply port 2 at the top of the column. An alkali catalyst is supplied from a position at the same position as or below the position at which the EVAc solution is supplied. In FIG. 1 , an alkali catalyst is supplied to the column reactor through an alkali catalyst supply port 3 located below the EVAc solution supply port 2. Furthermore, alcohol vapor is supplied to the bottom of the column and discharged from the top of the column. The alcohol vapor is preferably supplied above the position at which the EVOH solution described below is discharged. The alcohol vapor is preferably discharged 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 alcohol vapor be discharged from the top of the column. In FIG. 1, alcohol vapor is blown into the column through an alcohol vapor inlet 4 at the bottom, and is discharged from an alcohol vapor outlet 1 at the top of the column.

[0015] The EVAc solution supplied to the top of the column is transported from the top to the bottom of the column reactor. The EVAc solution supplied to the top of the column comes into contact with the alcohol vapor, and by-products such as aldehydes and acetate esters are discharged together with the alcohol vapor from the top of the column (alcohol vapor outlet 1). 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 to saponify the EVAc, and the EVOH solution is then discharged from the bottom of the column (EVOH solution outlet 5). By supplying the alkali catalyst below the position where the EVAc solution is supplied at the top of the column, impurities such as aldehydes remaining in the EVAc solution can be removed in advance by the alcohol vapor at the top of the column before saponification. This suppresses discoloration of the EVAc during saponification. The alkali catalyst may be introduced at the same location as the EVAc solution.

[0016] The EVAc used in the saponification step (I) can be produced by copolymerizing ethylene and vinyl acetate according to a known method. While there are no limitations on the polymerization method or solvent, solution polymerization using methanol as the solvent is preferred. Radical initiators, such as various azonitrile initiators and organic peroxide initiators, can be used as the polymerization catalyst. Furthermore, the EVAc may contain other monomers copolymerizable with ethylene and vinyl acetate (e.g., α-olefins such as propylene, unsaturated acids such as acrylic acid, various nitriles, and various amides) in addition to ethylene and vinyl acetate, as long as the effects of the present invention are not impaired. The content of units derived from the other monomers in the EVAc is typically 10 mol% or less.

[0017] 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. It is more preferably 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 saponified by a known method, or may be non-partially saponified EVAc.

[0018] The EVAc solution contains an alcohol. The saponification reaction of EVAc proceeds through a transesterification reaction between the acetate ester group of EVAc and the alcohol. Therefore, using an alcohol as the solvent for the EVAc solution has the advantage that the amount of alkaline catalyst used can be reduced to a small amount, allowing the saponification reaction to proceed efficiently. Examples of the alcohol include methanol, ethanol, 1-propanol, and 2-propanol, with methanol being preferred.

[0019] The concentration of EVAc in the EVAc solution is not particularly limited, but is preferably 70% by mass or less, more preferably 60% by mass or less, from the viewpoint of productivity, while 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, from the viewpoint of productivity.

[0020] Examples of the alkali catalyst used in the saponification step (I) 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, sodium hydroxide, potassium hydroxide, sodium methoxide, and sodium ethoxide are preferred, and sodium hydroxide is more preferred. The alkali catalyst may be used as is or in the form of a solution. When used as a solution, the solvent may be the same as that used for the EVAc solution.

[0021] Generally, as the saponification reaction progresses, the solubility in the solvent decreases, so it is preferable to pressurize the column reactor in the saponification step (I) and carry out the reaction at a high temperature. The pressure in the column 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.

[0022] 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.

[0023] The amount of alkali catalyst added in the saponification step (I) is preferably 0.01 to 10 parts by mass relative to 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, even more preferably 3 parts by mass or less.

[0024] In the saponification step (I), the alcohol vapor supplied to the bottom of the column contains alcohol (b) that has been subjected to an aldehyde reduction treatment and that has been 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. The alcohol vapor may consist solely 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.

[0025] In an alcoholic solvent, the saponification reaction of EVAc proceeds by a transesterification reaction between the acetate ester group of EVAc and the alcohol, so by supplying alcohol vapor to the bottom of the column, the amount of alkali catalyst used can be reduced, and the saponification reaction can proceed efficiently. Suitable alcoholic solvents include methanol, ethanol, 1-propanol, and 2-propanol, with methanol being particularly suitable.

[0026] In the saponification step (I), the amount of alcohol vapor supplied to the column 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 column reactor. When the amount of alcohol vapor is large, the amount of aldehyde contained in the recovered alcohol (a) can be reduced.

[0027] The alcohol used in the saponification step (I) is recovered as recovered alcohol (a) and subjected to an aldehyde reduction treatment in the aldehyde reduction step (II) described below. Here, the alcohol used in the saponification step (I) mainly refers to the alcohol discharged from the alcohol vapor outlet 1. The method for recovering the recovered alcohol (a) is not particularly limited, 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 and contained in the condensed alcohol are distilled off to recover the recovered alcohol (a). In this specification, the distillation operation for distilling off acetic acid esters such as methyl acetate to obtain the recovered alcohol (a) is not considered to be part of the aldehyde reduction treatment in the aldehyde reduction step (II).

[0028] In the saponification step (I), the residual vinyl acetate contained in the EVAc solution is saponified to produce acetaldehyde, which then self-condenses under saponification conditions. Therefore, the recovered alcohol (a) typically contains acetaldehyde and crotonaldehyde. The amount of acetaldehyde contained in the recovered alcohol (a) is not particularly limited, but is, for example, 50 ppm to 1000 ppm. The content of crotonaldehyde contained in the recovered alcohol (a) is not particularly limited, but is, for example, 5 ppm to 200 ppm. The contents of acetaldehyde and crotonaldehyde in the recovered alcohol (a) can be adjusted by, for example, adjusting the content of residual vinyl acetate in the EVAc solution, the amount of alcohol vapor supplied, the concentration of the alkali catalyst, etc.

[0029] The production method of the present invention includes an aldehyde reduction step (II) in which recovered alcohol (a) is subjected to an aldehyde reduction treatment to obtain alcohol (b). By including the aldehyde reduction step (II), the production method of the present invention can reduce the amount of unused alcohol used, thereby reducing the environmental impact. 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. While aldehyde reduction treatment by acetalization uses less energy than distillation and can reduce the environmental impact, it tends not to efficiently reduce crotonaldehyde. However, even if alcohol (b) contained in the alcohol vapor contains a certain amount of crotonaldehyde, discoloration of the resulting EVOH can be suppressed. Therefore, the aldehyde reduction treatment by acetalization can reduce the environmental impact while suppressing discoloration of the resulting EVOH. In the aldehyde reduction treatment in the aldehyde reduction step (II), the amount of acetaldehyde in the alcohol (b) obtained by the aldehyde reduction step (II) is preferably reduced by 70% by mass or more, more preferably 80% by mass or more, of the amount of acetaldehyde in the recovered alcohol (a) to be subjected to the aldehyde reduction step (II).

[0030] The acetalization catalyst can be used without any particular limitation as long as it is insoluble in an alcohol-based solvent, has acid sites on its surface, and can act as an acetalization catalyst. Examples of solid acids include metal oxides (e.g., Al ２ O ３ , V ２ O ５ ), sulfates (e.g., NiSO ４ , CuSO ４ ), phosphates (e.g., AlPO ４ ), chlorides (e.g., AlCl ３ , CuCl ３ Typical solid acid catalysts such as zeolite catalysts and silica-alumina catalysts may also be used. Minerals such as montmorillonite may also be used. However, for commercial scale implementation, cation exchange resins are particularly suitable.

[0031] Cation exchange resins function as insoluble solid acids, and various products suitable for treating large amounts of liquid and for 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.

[0032] When acetalization is carried out using a cation exchange resin, for example, the recovered alcohol (a) may be introduced into a column packed with bead-shaped cation exchange resin and passed through the column. In this case, the temperature inside the column is not particularly limited as long as the recovered alcohol (a) is in a liquid temperature range, and a temperature around room temperature may usually be used. In addition, from the viewpoint of solvent treatment efficiency, the residence time of the alcohol solvent inside the column is preferably 15 seconds to 30 minutes.

[0033] The aldehyde reduction treatment in the aldehyde reduction step (II) may include an aldehyde reduction treatment other than acetalization. That is, the alcohol (b) may be an alcohol that has been subjected to only acetalization, or an aldehyde reduction treatment other than acetalization. Examples of aldehyde reduction treatment 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 an 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 energy consumption. 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 low aldehyde concentration alcohol refers to an alcohol in which the content of at least one of acetaldehyde and crotonaldehyde is lower than that of the recovered alcohol (a).

[0034] When distillation is performed as the aldehyde reduction treatment in the aldehyde reduction step (II), the crotonaldehyde content can be reduced, but since a larger amount of energy is required compared to when distillation is not performed, it is preferable not to include an aldehyde reduction treatment by distillation.

[0035] The aldehyde reduction treatment 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.

[0036] When the aldehyde reduction treatment in the aldehyde reduction step (II) involves mixing with an alcohol solvent having a low aldehyde concentration recovered in a step other than the saponification step (I), the step other than the saponification step (I) may include a copolymerization step, a step for recovering unreacted vinyl acetate, a concentration step, a purification step, etc.

[0037] As an example of a recovery method, the recovery of alcohol in the concentration step will be described below. As a post-treatment method for the EVOH solution after the saponification reaction, a mixed vapor of an 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. This replaces a portion of the solvent present in the supplied EVOH solution with water, thereby producing 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 this mixed liquid containing alcohol and water through distillation in a distillation tower can be used.

[0038] When mixing with unused alcohol is carried out as an aldehyde reduction treatment in the aldehyde reduction step (II), the aldehyde concentration in the recovered alcohol (a) can be easily reduced because the unused alcohol does not contain impurities such as aldehydes. However, since using a large amount of unused alcohol has a large impact on the environment, it is preferable to use a small amount of unused alcohol.

[0039] The alcohol to be 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 the recovered alcohol (a) in the alcohol to be 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 to be subjected to the aldehyde reduction step (II) may consist solely of the recovered alcohol (a).

[0040] The acetaldehyde content of the alcohol (b) obtained in 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 acetaldehyde content of the alcohol (b) is equal to or less than the above upper limit, coloration of the resulting EVOH tends to be suppressed. Although it is preferable that the alcohol (b) does not contain acetaldehyde, it may contain a small amount of acetaldehyde. That is, it may contain 0 ppm or more. In the saponification step (I), the preferred embodiment of the acetaldehyde content of the alcohol vapor supplied from the bottom of the column is the same as the preferred embodiment of the acetaldehyde content of the alcohol (b).

[0041] The alcohol (b) obtained in the aldehyde reduction step (II) contains crotonaldehyde. The crotonaldehyde content 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 crotonaldehyde content in the alcohol (b) may be 5 ppm or more. When the crotonaldehyde content is equal to or less than the upper limit, discoloration of the resulting EVOH tends to be suppressed. On the other hand, considering that distillation or the like is required to reduce the crotonaldehyde content, when the crotonaldehyde content is equal to or greater than the lower limit, the energy required to reduce the crotonaldehyde content, i.e., the environmental load, can be reduced. In the saponification step (I), the preferred embodiment of the crotonaldehyde content in the alcohol vapor supplied from the bottom of the column is the same as the preferred embodiment of the crotonaldehyde content in the alcohol (b).

[0042] It is not preferable to use the alcohol (b) obtained in the aldehyde reduction step (II) in the polymerization step, because the alcohol (b) contains crotonaldehyde, and using it in the polymerization step reduces the quality of the resulting EVOH pellets and causes poor appearance during molding.

[0043] The degree of saponification of the EVOH contained in the EVOH solution obtained in 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 the saponification sufficiently, the gas barrier property of the obtained EVOH is improved.

[0044] In this specification, the EVOH solution obtained in the saponification step (I) is considered to be a paste-like solution that is not completely homogeneous throughout but has undergone phase separation. A post-treatment method for the EVOH solution after the saponification reaction involves supplying a mixed vapor of solvent and water to a tower vessel from the bottom of the vessel and supplying the EVOH solution from a position above the mixed vapor supply position, thereby replacing a portion of the solvent in the supplied EVOH solution with water and producing a high-concentration EVOH solution. The EVOH concentration in the EVOH solution supplied to the tower vessel is preferably 15 to 50% by mass, more preferably 25 to 40% by mass. It is also preferable that the ratio of the amount of the EVOH solution supplied to the mixed vapor supply (solution supply amount / steam supply amount) be 100 / 400 to 100 / 8 by mass. Furthermore, it is preferable that the water content in the mixed vapor be 20 to 70% by mass. The solvent used for the mixed vapor is preferably an alcohol having a boiling point of 130° C. or less, and examples of such alcohol include alcohols such as methanol, ethanol, propanol, butanol, etc. Alcohols having a boiling point of 100° C. or less are more preferred, and among these, methanol is preferred because it is easily available, inexpensive, has a low boiling point, and is easy to handle.

[0045] The high-concentration EVOH solution thus obtained is preferably pelletized by a known method. Examples of the pelletization method 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 the method 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 discharged from a die is cut by a center hot cut method or an underwater cut method. When the EVOH solution is pelletized, water-containing EVOH pellets are obtained.

[0046] When water-containing EVOH pellets are obtained by cooling and solidifying an EVOH solution, the water-containing EVOH pellets are preferably washed and deliquored by a known method. Alternatively, the water-containing EVOH pellets are preferably immersed 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 compound into the water-containing EVOH. The incorporation of these compounds can improve the mechanical properties, thermal stability, and the like of EVOH molded articles. When water-containing EVOH is obtained by melt-kneading and pelletizing EVOH, the washing, deliquoring, and chemical treatment of the EVOH may be carried out in an extruder.

[0047] The obtained hydrous EVOH pellets can be dried by a known method to obtain EVOH pellets. The moisture content of the dried EVOH pellets is preferably 0.08% by mass or less. The drying method is not particularly limited, and examples thereof include stationary drying combined with air drying or nitrogen drying, fluidized bed drying, and vacuum drying. However, multi-stage drying combining several drying methods is preferred, and multi-stage drying including preliminary drying and main drying is more preferred.

[0048] The yellow index (YI) of the dried EVOH is preferably not more than 13, more preferably not more than 9.5. According to the production method of the present invention, such EVOH with little coloring can be produced, and the environmental load can be reduced.

[0049] 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, 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 is both inhibited from discoloring and reduces the environmental load.

[0050] The EVOH obtained by the method of the present invention can be molded into various types of articles such as films, sheets, containers, pipes, fibers, etc.

[0051] The present invention will be specifically described below using examples, but the present invention is not limited to the following examples.

[0052] [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 resulting 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). １ Measurement was performed at 80°C using H-NMR (GX-500 manufactured by JEOL Ltd.), and the ethylene unit content and degree of saponification were determined from the peak intensity ratio of ethylene units, vinyl alcohol units, and vinyl ester units.

[0053] (2) Quantification 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 using a Shimadzu high-performance liquid chromatograph (column: Shiseido CAPCELL PAK C18 MG type, solvent: acetonitrile / water (gradient system), UV detector) to quantify the aldehyde content. For quantification, a calibration curve prepared using commercially available aldehyde-DNPH products (or synthesized products) was used.

[0054] (3) Quantitation 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, and 50.0 mg of the sample was weighed into 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 conditions described below to adsorb all of the volatile gases from the sample into the adsorption tube, and then the gases re-emitted from the adsorption tube were separated using a column to detect the peaks for each component. Calibration curves were created from the peak areas of standard samples of acetaldehyde, crotonaldehyde, 2,4-hexadienal, and 2,4,6-octatrienal, and each was quantified using 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 temperature of the sample tube, 170°C, to the temperature of the adsorption tube, 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 (manufactured by PerkinElmer Japan) Temperature when adsorbing sample onto adsorption tube: 170°C (sample tube), -30°C (adsorption tube), 250°C (valve), 260°C (transfer line) Adsorption time onto adsorption tube: 10 minutes Temperature when releasing after sample adsorption: 170°C (sample tube), 260°C (adsorption tube), 250°C (valve), 260°C (transfer line) Adsorption tube release 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 (manufactured by Agilent Technologies) Column: DB-WAX UI (Length: 30 m, inner diameter: 0.25 mm, film thickness: 0.50 μm) Column oven temperature: held at 40°C for 5 minutes, then heated at a rate of 10°C / min to 240°C and held for 10 minutes (total measurement temperature: 35 minutes) Transfer line (connection) temperature: 240°C Ionization conditions: EI+ Detected ion mass range: m / z = 29-600 Detection method: SCAN.

[0055] (4) Evaluation of Hue The yellow index (YI) values ​​of the dried EVOH pellets obtained in the Examples and Comparative Examples were measured and calculated using a LAB Scan XE manufactured by Hunter Co. in accordance with JIS K7373: 2006. The smaller the value, the more yellowing was suppressed, and it could be judged that the hue was excellent. A YI value of 13 or more was judged that the deterioration of hue was not suppressed.

[0056] (5) Production volume ratio When both recovered alcohol and virgin alcohol are used in the saponification reaction in the saponification step (I), the production volume ratio is calculated as 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 production volume ratio, the less virgin alcohol is used, and the smaller the environmental impact.

[0057] Example 1 (Alcohol Recovery) EVAc solution with a concentration of 48% by mass, obtained by dissolving EVAc having an ethylene unit content of 32 mol% and a vinyl acetate unit content of 68 mol% in methanol, was supplied to a column reactor (plate column, 21 plates, column inner diameter 140 mm) and saponified. FIG. 1 is a schematic diagram of the column reactor. The column temperature was 115°C. The EVAc solution was supplied from EVAc solution supply port 2 to the 20th plate at a rate of 10 kg / h, and a 5 wt% methanol solution of sodium hydroxide as an alkali catalyst was supplied from alkali catalyst supply port 3 to the 19th plate at a rate of 1.6 kg / h. Unused methanol vapor was continuously supplied to the lower part of the first plate through alcohol vapor inlet 4 at a rate of 21.6 kg / h. The acetaldehyde and crotonaldehyde contents in the methanol solvent used for the unused methanol vapor were analyzed according to the method described in Evaluation Method (2) above. Acetaldehyde and crotonaldehyde were below the detection limit and were not detected. By-product methyl acetate and aldehyde were distilled as a mixed vapor together with excess methanol from the top of the column (alcohol vapor outlet 1), and an EVOH methanol solution was obtained from the bottom of the column (EVOH solution outlet 5). The mixed liquid obtained by condensing the mixed vapor distilled from the top of the column was introduced into another column-type recovery vessel (tray column) and distilled, and methanol (recovered alcohol (a)) was extracted and recovered from the bottom of the column. The acetaldehyde and crotonaldehyde contents of the recovered methanol (recovered alcohol (a)) were measured according to the method described in Evaluation Method (2) above. The acetaldehyde and crotonaldehyde contents were 450 ppm and 50 ppm, respectively. The results are shown in Table 1.

[0058] (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 H+-type strongly acidic cation exchanger (DuPont's "Amberlyst 15") to acetalize acetaldehyde, thereby performing an aldehyde reduction treatment. This produced aldehyde-reduced methanol (alcohol (b)) for use as alcohol vapor in the saponification step (I). The acetaldehyde and crotonaldehyde contents of the obtained methanol (alcohol (b)) were measured according to the method described in the evaluation method (2) above. The results were found to contain 0 ppm acetaldehyde and 50 ppm crotonaldehyde. The results are shown in Table 1.

[0059] (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 column (EVOH solution outlet 5) (saponification step (I)). In this saponification step (I), no virgin alcohol was used, which reduces the environmental impact. The results are shown in Table 1.

[0060] (Production of EVOH Pellets) To the EVOH methanol solution obtained in the saponification step (I), an equimolar amount of acetic acid to the sodium hydroxide supplied to the column reactor was added to neutralize the remaining sodium hydroxide, and the solution was concentrated until the copolymer concentration reached 40% by mass. The concentrated solution was extruded through a 3.5 mm nozzle into a methanol-water mixed solvent (methanol / water = 10 / 90 by mass) maintained at 5°C, solidifying into strands, which were then cut with a cutter to obtain hydrous EVOH pellets. The obtained hydrous EVOH pellets were washed by pouring them into a large amount of 0.1 g / L aqueous acetic acid solution. After removing the remaining methanol and sodium acetate, the pellets were dried at 60°C for 5 hours and then 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.

[0061] (Example 2) Dried EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the feed rate of methanol vapor to the column reactor during alcohol recovery and the feed rate of methanol vapor in the saponification step (I) were changed to 43.2 kg / h. The results are shown in Table 1.

[0062] Example 3 In the aldehyde reduction step (II), 25 parts by mass of methanol that had been subjected to an aldehyde reduction treatment and obtained by acetalization with a cation exchange resin was mixed with 75 parts by mass of methanol recovered from a step other than the saponification step (I) to carry out further aldehyde reduction treatment, and the aldehyde content of the 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.

[0063] (Examples 4 and 5) Dried 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 steps other than the saponification step (I) was changed as shown in Table 1. The results are shown in Table 1.

[0064] (Example 6) Dried 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 methanol recovered from a step other than the saponification step (I) so as to have a content as shown in Table 1. The results are shown in Table 1.

[0065] Example 7 Dried 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 the mixture was subjected to acetalization with a cation exchange resin to perform an aldehyde reduction treatment. The results are shown in Table 1.

[0066] (Examples 8 and 9) Dried 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 steps other than the saponification step (I) was changed as shown in Table 1. The results are shown in Table 1.

[0067] (Example 10) Dried 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 methanol recovered from a step other than the saponification step (I) so as to have a content as shown in Table 1. The results are shown in Table 1.

[0068] Example 11 Dried 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.

[0069] (Examples 12 and 13) Dried 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 the EVAc and EVOH were substantially the same.

[0070] Example 14 An attempt was made to prepare dried EVOH pellets in the same manner as in Example 1, except that the concentration of sodium hydroxide in the saponification step (I) was adjusted so that the degree of saponification of the resulting EVOH would be as shown in Table 1. However, the resin particles stuck together during the drying step, making it impossible to continue the operation. Therefore, 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 changed 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. Dry EVOH pellets were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0071] Comparative Example 1 Dried 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.

[0072] Comparative Example 2 Dried 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.

[0073] Comparative Example 3 Dried EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that distillation was used as the aldehyde reduction treatment method in the aldehyde reduction step (II). The results are shown in Table 1.

[0074] Comparative Example 4 Dried 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 acetalization using a cation exchange resin was not performed. The results are shown in Table 1.

[0075] (Comparative Example 5) Dried 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 steps other than the saponification step (I) was changed as shown in Table 1. The results are shown in Table 1.

[0076] (Comparative Examples 6 and 7) Dried 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 methanol recovered from a step other than the saponification step (I) so as to achieve the contents shown in Table 1. The results are shown in Table 1.

[0077]

[0078] 1 Alcohol vapor outlet 2 EVAc solution supply port 3 Alkaline catalyst supply port 4 Alcohol vapor blowing port 5 EVOH solution outlet

Claims

1. a saponification step (I) in which an ethylene-vinyl acetate copolymer solution containing an ethylene-vinyl acetate copolymer and an alcohol is supplied to an upper part of a column reactor, alcohol vapor is supplied to a lower part of the column and discharged from the upper part of the column, and the ethylene-vinyl acetate copolymer is saponified using an alkali catalyst, and an ethylene-vinyl alcohol copolymer solution containing an ethylene-vinyl alcohol copolymer and an alcohol having a saponification degree of 80 mol % or more and 100 mol % or less is taken out from the bottom of the column; and an aldehyde reduction step (II) for reducing the aldehyde in a recovered alcohol (a) obtained by recovering the alcohol used in the saponification step (I) to obtain an alcohol (b); the aldehyde reduction step (II) includes a step of contacting the recovered alcohol (a) with an acetalization catalyst to reduce the aldehyde; A method for producing an ethylene-vinyl alcohol copolymer, wherein in the saponification step (I), the alcohol vapor supplied to the bottom of the column contains an alcohol (b), and the alcohol (b) contains crotonaldehyde.

2. The production 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) to obtain the alcohol (b).

3. The production 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).

4. The method according to claim 2 or 3, wherein the alcohol different from the recovered alcohol (a) includes unused alcohol.

5. The production method according to any one of claims 1 to 3, 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 claims 1 to 3, 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 production method according to any one of claims 1 to 3, wherein in the saponification step (I), the ethylene-vinyl acetate copolymer supplied to the column reactor has an ethylene unit content of 20 mol% or more and less than 60 mol%.

8. An ethylene-vinyl alcohol copolymer obtained by the production method according to any one of claims 1 to 3, The composition 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 containing 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).