Method for producing ethylene-vinyl alcohol copolymer

Abstract

This method for producing EVOH comprises: a saponification step (I) in which an EVAc solution containing EVAc and an alcohol is supplied to the tower upper part of a tower reactor, alcohol vapor is supplied to the tower lower part and discharged from the tower upper part, the EVAc is saponified using a base catalyst, and the EVOH solution is taken out from the tower bottom; and an aldehyde reduction step (II) for obtaining an alcohol (b) by recovering the alcohol used in the saponification step (I) and reducing the aldehyde in the recovered alcohol (a) obtained, the aldehyde reduction step (II) including a step for bringing the recovered alcohol (a) into contact with an acetalation catalyst to reduce the aldehyde, and in the saponification step (I), the alcohol vapor supplied to the lower portion of the tower contains the alcohol (b), and the alcohol (b) contains crotonaldehyde. As a result, provided is a method for producing EVOH in which hue deterioration of the resulting EVOH is suppressed and environmental burden is reduced.

Description

TECHNICAL FIELD

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

[0002] Ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as "EVOH") is a useful polymer material excellent in oxygen barrier property, oil resistance, non-charging property, mechanical strength, and the like, and is widely used as various packaging materials such as films, sheets, containers, and the like. As a method for producing EVOH, for example, a method of subjecting an ethylene-vinyl acetate copolymer (hereinafter sometimes abbreviated as "EVAc") obtained by copolymerizing ethylene and vinyl acetate in the presence of a saponification catalyst in an organic solvent containing an alcohol to saponification is generally used.

[0003] From the viewpoint of reducing environmental burden, it is desired to recover and reuse unreacted raw materials and used solvents in the production process of EVOH, and in particular, it is extremely important to reuse alcohol-based solvents such as methyl alcohol.

[0004] As an example of reusing methyl alcohol solvent, in the examples of Patent Literature 1, it is described that, for the copolymerization of ethylene and vinyl acetate in an alcohol-based solvent, the methyl alcohol used in the polymerization is recovered, and the recovered methyl alcohol is treated with a cation exchange resin to acetalize aldehydes contained therein, thereby reducing the aldehyde compound concentration, and thereafter is used again in the polymerization process, whereby the appearance (fish eye number and colored visual evaluation) of a single layer film produced from the obtained EVOH pellets is good.

[0005] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Laid-Open No. 2002-060403 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION The methanol recovered from the polymerization step described in the above prior art method has a small amount of crotonal generated by the polycondensation of acetaldehyde, and thus, it is relatively easy to achieve solvent reuse in the polymerization step. However, since the saponification step is a reaction system in which acetaldehyde polycondensation is relatively likely to occur, the methanol recovered from the saponification step tends to contain a large amount of crotonal. Here, as one of the reasons that the removal of acetaldehyde affects the quality of the final product, polyene formation caused by acetaldehyde polycondensation can be considered, and it can be considered that the reuse of methanol containing crotonal and the like formed by polyene formation adversely affects the manufacturing process and reduces the quality of the final product. It has been known that in the acetaldehyde reduction treatment based on acetalization described in Patent Document 1, although acetaldehyde can be removed, it is difficult to remove crotonal, and in order to remove crotonal, either distillation with a large environmental burden must be performed, or acetalization treatment must be performed for a long time. In particular, it has been known that when methanol containing crotonal and the like is used in the polymerization step as described in Patent Document 1, the quality of EVOH pellets is reduced.

[0007] In view of the above, the present inventors and the like conducted intensive studies, and as a result, surprisingly found that even when the alcohol introduced as alcohol vapor in the saponification step contains a specific amount of crotonal, the hue of the resulting EVOH does not deteriorate. Thus, it was found that by recycling and reusing the alcohol used in the saponification step and using it as a raw material for alcohol vapor in the saponification step, it is possible to suppress quality reduction and effectively utilize resources (reduce environmental burden), and thus the present application was completed.

[0008] That is, the object of the present application is to provide a method for producing EVOH that suppresses deterioration of the hue of the resulting EVOH and reduces environmental burden.

[0009] Means for solving the problem According to the present application, the above object is achieved by the following technical solution.

[0010] [1] 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 portion of a column of a column reactor, alcohol vapor is supplied to a lower portion of the column and is discharged from the upper portion, and the aforementioned EVAc is saponified using a base catalyst, and an EVOH solution containing EVOH having a saponification degree of 80 mol% or more and 100 mol% or less and the alcohol is taken out from a bottom portion; and an acetaldehyde reduction step (II) in which the alcohol used in the saponification step (I) is recovered, and the acetaldehyde in the resulting recovered alcohol (a) is reduced to obtain an alcohol (b), the acetaldehyde reduction step (II) includes a step in which the recovered alcohol (a) is brought into contact with an acetalization catalyst to reduce acetaldehyde, in the saponification step (I), the alcohol vapor supplied to the lower portion of the column contains the alcohol (b), and the alcohol (b) contains crotonal; [2] The production method according to [1], wherein the aldehyde-reducing step (II) further includes a step of reducing the aldehyde concentration in the recovered alcohol (a) to obtain alcohol (b) by mixing the recovered alcohol (a) with an alcohol different from the recovered alcohol (a). [3] The production method according to [2], wherein the alcohol different from the recovered alcohol (a) is contained in a recovered alcohol recovered in a step other than the saponification step (I). [4] The production method according to [2] or [3], wherein the alcohol different from the recovered alcohol (a) is an unused alcohol. [5] The production 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 aforementioned column reactor is 0 to 150 ppm. [6] The production 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 aforementioned column reactor 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 aforementioned column reactor is 20 mol% or more and less than 60 mol%. [8] EVOH obtained by the production method according to any one of [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), the EVOH satisfying the following formula (1).

[0011] 10 ≤ b1 / (b2 + b3) < 150... (1) In the above formula (1), b1 is the content (ppm) of acetaldehyde (B1) with respect to EVOH (A), b2 is the content (ppm) of 2,4-hexadienal (B2) with respect to EVOH (A), and b3 is the content (ppm) of 2,4,6-octatrienal (B3) with respect to EVOH (A).

[0012] Effects of the Invention The production method according to the present application can provide a production method of EVOH that suppresses deterioration of the hue of the obtained EVOH and reduces environmental burden. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 Schematic diagram of the column reactor used in the examples and comparative examples. DETAILED DESCRIPTION

[0014] The present invention is a production method of EVOH, which has a saponification step (I) of supplying an EVAc solution containing EVAc and an alcohol to an upper portion of a column of a column reactor, supplying alcohol vapor to a lower portion of the column and discharging from the upper portion while saponifying the aforementioned EVAc using an alkali catalyst, and taking out an EVOH solution containing EVOH having a saponification degree of 80 mol% or more and 100 mol% or less and an alcohol from a bottom portion, and an aldehyde reduction step (II) of recovering the alcohol used in the saponification step (I) and reducing aldehyde in the obtained recovered alcohol (a) to obtain an alcohol (b), the aldehyde reduction step (II) including a step of reducing aldehyde by bringing the recovered alcohol (a) into contact with an acetalization catalyst, in the saponification step (I), the alcohol vapor supplied to the lower portion of the column contains the alcohol (b), and the alcohol (b) contains crotonaldehyde. According to this production method, even if the recovered alcohol-based solvent is used as the alcohol vapor of the saponification step (I), coloring of the obtained EVOH can be suppressed and environmental burden at the time of production can be reduced. Generally, the alcohol used in the saponification step (I) has crotonaldehyde and the like generated by polyenization of acetaldehyde mixed therein. Therefore, when the alcohol used in the saponification step (I) is recovered, a certain amount of crotonaldehyde is contained. Crotonaldehyde and the like have a conjugated structure, and therefore, it is considered that a compound further generated by polyenization is a coloring cause substance of the final product (EVOH), but the current situation is that crotonaldehyde is difficult to remove by acetalization, and therefore, recovery of the alcohol containing crotonaldehyde has to rely on a means such as distillation which has a large energy consumption (environmental burden). However, it was surprisingly found that even if a certain amount of crotonaldehyde is contained when used as the alcohol vapor in the saponification step (I), coloring of the final product (EVOH) can be suppressed, and it was found that EVOH having high quality (coloring is suppressed) can be provided by a production method having low environmental burden. Hereinafter, the present invention will be described in detail.

[0015] The production method of the present invention has a saponification step (I) of supplying an EVAc solution containing EVAc and an alcohol to an upper portion of a column of a column reactor, supplying alcohol vapor to a lower portion of the column and discharging from the upper portion while saponifying the aforementioned EVAc using an alkali catalyst, and taking out an EVOH solution containing EVOH having a saponification degree of 80 mol% or more and 100 mol% or less and an alcohol from a bottom portion. In the saponification step (I), the EVAc in the EVAc solution is saponified by using an alkali catalyst, and thereby the EVOH solution is obtained.

[0016] Figure 1 is a schematic view of the column reactor used in the examples described later. The saponification step (I) will be described with reference to Figure 1 The EVAc solution containing EVAc and an alcohol is supplied to the upper portion of the column of the column reactor. In the saponification step (I), the alcohol vapor supplied to the lower portion of the column contains the alcohol (b), and the alcohol (b) contains crotonaldehyde. Figure 1In this process, the aforementioned EVAc solution is supplied to the tower reactor from the EVAc solution supply port 2 at the top of the tower. Furthermore, the alkaline catalyst is supplied from a position lower than or at the same location as the EVAc solution supply point at the top of the tower. Figure 1 In this process, an alkaline catalyst is supplied to the tower reactor from an alkaline catalyst supply port 3, which is positioned lower than the EVAc solution supply port 2. Additionally, alcohol vapor is supplied to the lower part of the tower and discharged from the upper part. The location for supplying the alcohol vapor is preferably higher than the location where the EVOH solution is removed (described later). From the viewpoint of more effectively removing residual aldehydes and other impurities from the EVAc solution, the location for discharging the alcohol vapor is preferably higher than the location for supplying the EVAc solution. Similarly, from the same viewpoint, the location for discharging the alcohol vapor is also preferably the top of the tower. Figure 1 In the middle, alcohol vapor is blown in from the alcohol vapor inlet 4 at the bottom of the tower and discharged from the alcohol vapor outlet 1 at the top of the tower.

[0017] The aforementioned EVAc solution supplied to the upper part of the column is transported from the top to the bottom of the column reactor. The EVAc solution supplied to the upper part of the column comes into contact with the aforementioned alcohol vapor, and byproducts such as aldehydes and acetates are discharged from the upper part of the column (alcohol vapor outlet 1) along with the aforementioned alcohol vapor. Furthermore, the EVAc solution transported to the location where the aforementioned alkaline catalyst is supplied (alkaline catalyst supply port 3) undergoes saponification by contacting the aforementioned alkaline catalyst, and then the EVOH solution is removed from the bottom of the column (EVOH solution outlet 5). By supplying the alkaline catalyst to the lower part of the upper part of the column compared to the location where the aforementioned EVAc solution is supplied, impurities such as aldehydes remaining in the aforementioned EVAc solution can be removed in advance by the aforementioned alcohol vapor in the upper part of the column before saponification. Therefore, the coloring of the aforementioned EVAc during saponification is suppressed. It should be noted that the alkaline catalyst can be introduced from the same location as the EVAc solution.

[0018] The EVAc used in the saponification step (I) can be manufactured by copolymerizing ethylene and vinyl acetate according to known methods. The polymerization method and solvent are not limited, and solution polymerization using methanol as a solvent is suitable. As a polymerization catalyst, free radical initiators, such as various azonitrile initiators and organic peroxide initiators, can be used. Furthermore, as long as it does not impair the effects of the present invention, the aforementioned EVAc may contain other monomers besides ethylene and vinyl acetate (e.g., α-olefins such as propylene, unsaturated acids such as acrylic acid, various nitriles, various amides) capable of copolymerizing with ethylene and vinyl acetate. The content of units derived from the aforementioned other monomers in the aforementioned EVAc is typically 10 mol% or less.

[0019] The ethylene unit content of the EVAc used in the saponification step (I) is preferably 20 mol% or more and 60 mol% or less. If the ethylene unit content is 20 mol% or more, the melt-forming properties of the resulting EVOH tend to improve. It is more suitable to be 24 mol% or more. On the other hand, by keeping the ethylene unit content at 60 mol% or less, the gas barrier properties of the resulting EVOH become good. 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) can be a partially saponified EVAc that has been saponified by a known method, or it can be an EVAc that has not been partially saponified.

[0020] The aforementioned EVAc solution contains an alcohol. The saponification reaction of EVAc proceeds via an exchange reaction between the acetate groups of EVAc and an alcohol. Therefore, by using an alcohol as the solvent for the EVAc solution, it is advantageous to minimize the use of a base catalyst and to achieve efficient saponification. Examples of such alcohols include methanol, ethanol, 1-propanol, and 2-propanol, with methanol being preferred.

[0021] The concentration of EVAc in the aforementioned EVAc solution is not particularly limited, but from a productivity point of view, it is preferably 70% by mass or less, more preferably 60% by mass or less. On the other hand, from a productivity point of view, 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.

[0022] Examples of alkaline catalysts used in the saponification step (I) include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide; and alkali metal alkoxides such as sodium methoxide, sodium ethoxide, and potassium tert-butoxide. Sodium hydroxide, potassium hydroxide, sodium methoxide, and sodium ethoxide are preferred, and sodium hydroxide is more preferred. The aforementioned alkaline catalysts can be used directly or in solution form. When used in solution form, the same solvent as the aforementioned EVAc solution can be used.

[0023] Generally, if a saponification reaction is carried out, the solubility in the solvent decreases. Therefore, it is preferable to pressurize the tower reactor in the saponification step (I) and carry out the reaction at a high temperature. The pressure of the tower reactor is preferably 0.1 to 1.0 MPa. More preferably, the aforementioned pressure is 0.8 MPa or less, even more preferably 0.6 MPa or less, and particularly preferably 0.55 MPa or less. The aforementioned pressure can be 0.2 MPa or more.

[0024] The preferred temperature for the tower reactor is 60 to 180°C. From the viewpoint of improving reaction efficiency, this 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 preferred temperature for the tower reactor is 150°C or lower, even more preferably 140°C or lower, and particularly preferably 130°C or lower.

[0025] The amount of alkaline catalyst added in the saponification step (I) is preferably 0.01 to 10 parts by weight relative to 100 parts by weight of EVAc. More preferably, this amount is 0.05 parts by weight or more, even more preferably 0.1 parts by weight or more, and particularly preferably 0.3 parts by weight or more. On the other hand, the aforementioned amount is more preferably 5 parts by weight or less, and even more preferably 3 parts by weight or less.

[0026] In the saponification step (I), the alcohol vapor supplied to the lower part of the tower contains alcohol (b) that has undergone aldehyde reduction treatment, obtained from the aldehyde reduction step (II) described later. Since the aforementioned alcohol vapor contains alcohol (b), the amount of unused alcohol is suppressed, thus reducing the environmental burden. The content of alcohol (b) in the alcohol vapor is not particularly limited, but 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). Other compounds besides alcohol (b) that may be included in the alcohol vapor include, for example, alcohols that have not undergone aldehyde reduction treatment.

[0027] Under alcohol-based solvent conditions, the saponification reaction of EVAc proceeds via transesterification between the acetate groups of EVAc and the alcohol. Therefore, by supplying alcohol vapor to the lower part of the column, the use of the alkaline catalyst can be minimized, enabling efficient saponification. Suitable alcohol-based solvents include methanol, ethanol, 1-propanol, and 2-propanol, with methanol being particularly suitable.

[0028] In the saponification step (I), the amount of alcohol vapor supplied to the tower reactor is preferably 10 parts by mass or more and 1000 parts by mass or less relative to 100 parts by mass of EVAc supplied to the tower reactor. With this high amount of alcohol vapor, the amount of aldehyde contained in the recovered alcohol (a) can be reduced.

[0029] The alcohol used in the saponification step (I) is recovered as recycled alcohol (a) and supplied to the aldehyde reduction treatment in the aldehyde reduction step (II) described later. Here, the alcohol used in the saponification step (I) mainly refers to the alcohol discharged from the alcohol vapor outlet 1. There is no particular limitation on the method of recovering as recycled alcohol (a). One method is to condense the alcohol discharged from the alcohol vapor outlet 1, and then distill to remove acetate esters such as methyl acetate generated by the saponification reaction contained in the aggregated alcohol, thereby recovering the recycled alcohol (a). In this specification, the distillation operation to remove acetate esters such as methyl acetate to obtain recycled alcohol (a) is not considered as the aldehyde reduction treatment in the aldehyde reduction step (II).

[0030] In the saponification step (I), the residual vinyl acetate in the EVAc solution is saponified to generate acetaldehyde, which undergoes self-condensation under saponification conditions. Therefore, the recovered alcohol (a) typically contains acetaldehyde and crotonaldehyde. The amount of acetaldehyde in the recovered alcohol (a) is not particularly limited, for example, it is 50 ppm or more and 1000 ppm or less. Similarly, the amount of crotonaldehyde in the recovered alcohol (a) is not particularly limited, for example, it is 5 ppm or more and 200 ppm or less. As a means of adjusting the content of acetaldehyde and crotonaldehyde in the recovered alcohol (a), adjustments can be made based on, for example, the content of residual vinyl acetate in the EVAc solution, the supply rate of alcohol vapor, and the concentration of the alkaline catalyst.

[0031] The manufacturing method of the present invention includes an aldehyde reduction step (II), in which aldehyde reduction treatment is performed on recovered alcohol (a) to obtain alcohol (b). By including the aldehyde reduction step (II), the manufacturing method of the present invention can suppress the amount of unused alcohol, thereby reducing the environmental burden. The aldehyde reduction step (II) includes a step of contacting the recovered alcohol (a) with an acetalization catalyst to reduce aldehydes. The aldehyde reduction treatment based on acetalization uses less energy than distillation, thus reducing the environmental burden. On the other hand, there is a tendency to be unable to effectively reduce crotonaldehyde, but even if the alcohol (b) contained in the alcohol vapor contains a certain amount of crotonaldehyde, the coloring of the resulting EVOH can be suppressed. Therefore, by this acetalization-based aldehyde reduction treatment, the coloring of the resulting EVOH can be suppressed and the environmental burden reduced. In the aldehyde reduction treatment of 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% or more by mass of the amount of acetaldehyde in the recovered alcohol (a) supplied to the aldehyde reduction step (II), more preferably by 80% or more by mass.

[0032] Acetalization catalysts can be used without particular limitation as long as they are insoluble in alcohol-based solvents and can function as acetalization catalysts with acid points on their surfaces. 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). Zeolite catalysts and silica-alumina catalysts, which are representative solid acid catalysts, can be used. Additionally, minerals such as montmorillonite can also be used. However, for commercial-scale implementation, cation exchange resins are particularly suitable.

[0033] Cation exchange resins function as insoluble solid acids, and various products suitable for large-volume liquid processing and repeated use are available on the market. As cation exchange resins, weakly acidic cation exchange resins can be used, with strongly acidic cation exchange resins being preferred. Sulfonic acid-type strongly acidic ion exchange resins manufactured by DUNPONT, such as AMBERLIST (registered trademark), AMBERLITE (registered trademark), and AmberSep (registered trademark), can be used.

[0034] When using cation exchange resin for acetalization, for example, it is sufficient to simply feed the recovered alcohol (a) into a column filled with bead-shaped cation exchange resin and allow it to pass through. In this case, the temperature inside the column is not particularly limited as long as the recovered alcohol (a) is in liquid temperature range; a temperature near room temperature is generally acceptable. Furthermore, from the viewpoint of solvent processing efficiency, the residence time of the alcohol solvent inside the column is preferably 15 seconds to 30 minutes.

[0035] As part of the aldehyde reduction process (II), other aldehyde reduction processes besides acetalization can be performed. That is, alcohol (b) may be an alcohol that has only undergone acetalization, or an alcohol that has undergone acetalization and other aldehyde reduction processes besides acetalization. Examples of other aldehyde reduction processes besides acetalization include: distillation, mixing with a low-aldehyde-concentration alcohol solvent recovered from processes other than saponification (I), and mixing with an unused alcohol solvent. When performing the aforementioned other aldehyde reduction processes, the process can be performed before or after acetalization. In the case where the aldehyde reduction process is distillation, from the viewpoint of reducing the energy used, it is preferable to perform distillation after acetalization. In the case of mixing with a low-aldehyde-concentration alcohol solvent, the aldehyde reduction process is preferably performed before acetalization. Here, a low-aldehyde-concentration alcohol refers to an alcohol in which at least one of acetaldehyde and crotonaldehyde is present in a lower concentration than the recovered alcohol (a).

[0036] When distillation is performed as an aldehyde reduction process (II) to reduce aldehyde content, the content of crotonaldehyde can be reduced, but it requires a large amount of energy compared to not performing distillation. Therefore, it is preferable not to include distillation-based aldehyde reduction processes.

[0037] The aldehyde reduction process (II) may include a method of reducing the aldehyde concentration in the recovered alcohol (a) by mixing it with an alcohol different from the recovered alcohol (a). Examples of alcohols different from the recovered alcohol (a) include alcohols recovered in processes other than the saponification process (I) and unused alcohols.

[0038] When the aldehyde reduction process (II) is carried out as an aldehyde reduction treatment and is mixed with the alcohol solvent with a low aldehyde concentration recovered in a process other than the saponification process (I), the processes other than the saponification process (I) may include copolymerization process, unreacted vinyl acetate recovery process, concentration process, refining process, etc.

[0039] As an example of a recovery method, the following describes the recovery of alcohol in the concentration process. As a post-treatment method for the EVOH solution after the saponification reaction, in a column-type vessel, a mixture of alcohol solvent and water vapor is supplied from the bottom of the vessel, and the EVOH solution is supplied from a position higher than the supply position of the mixed vapor. This replaces a portion of the solvent in the supplied EVOH solution with water, thus producing a high-concentration EVOH solution. The alcohol vapor and water vapor exiting from the top of the column are condensed in a condenser and can be recovered as an alcohol-water solution. The alcohol obtained by distilling and separating this mixture containing alcohol and water in a distillation column can be used.

[0040] When mixing with unused alcohol as part of the aldehyde reduction process (II), since the unused alcohol does not contain impurities such as aldehydes, the aldehyde concentration in the recovered alcohol (a) can be easily reduced. However, the environmental impact of using large quantities of unused alcohol is significant, so it is preferable to use only small quantities.

[0041] The alcohol supplied to the aldehyde reduction process (II) may contain, in addition to the recovered alcohol (a), an alcohol with a higher content of acetaldehyde and crotonaldehyde compared to the recovered alcohol (a). The content of the recovered alcohol (a) in the alcohol supplied to the aldehyde reduction process (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. The alcohol supplied to the aldehyde reduction process (II) may consist solely of the recovered alcohol (a).

[0042] The acetaldehyde content 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. If the acetaldehyde content in the alcohol (b) is below the above-mentioned upper limit, it tends to suppress the coloring of the resulting EVOH. The alcohol (b) is preferably free of acetaldehyde, but may contain a small amount. That is, it may be 0 ppm or more. In the saponification step (I), the suitable manner for the acetaldehyde content in the alcohol vapor supplied from the bottom of the tower is also the same as the suitable manner for the acetaldehyde content in the alcohol (b).

[0043] The alcohol (b) obtained by the aldehyde reduction process (II) contains crotonaldehyde. The content of crotonaldehyde in the alcohol (b) is preferably 200 ppm or less, more preferably 100 ppm or less, and can 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 in the alcohol (b) can be 5 ppm or more. If the content of crotonaldehyde is below the above-mentioned upper limit, there is a tendency to suppress the coloring of the resulting EVOH. On the other hand, considering that distillation is required when reducing the content of crotonaldehyde, by keeping the content of crotonaldehyde above the above-mentioned lower limit, the energy required to reduce the content of crotonaldehyde can be reduced, that is, the environmental burden can be reduced. In the saponification process (I), the suitable method for the content of crotonaldehyde in the alcohol vapor supplied from the bottom of the tower is also the same as the suitable method for the content of crotonaldehyde in the alcohol (b).

[0044] The alcohol (b) obtained by the aldehyde reduction process (II) is not preferably used in the polymerization process. The alcohol (b) contains crotonaldehyde, so if it is used in the polymerization process, it will reduce the quality of the resulting EVOH pellets and cause poor appearance during molding.

[0045] The degree of saponification of the EVOH in the EVOH solution obtained by the saponification step (I) is preferably 80 mol% or more, preferably 95 mol% or more, and more preferably 99 mol% or more. By fully saponifying, the gas barrier properties of the obtained EVOH are improved.

[0046] Regarding the EVOH solution obtained through the saponification process (I), in this application specification, substances such as pastes that are not completely homogeneous and have undergone phase separation are also considered to be included in the EVOH solution. As a post-treatment method for the EVOH solution after the saponification reaction, in a tower-type vessel, a mixture of solvent and water vapor is supplied from the bottom of the vessel, and the EVOH solution is supplied from a position higher than the supply position of the mixed vapor, thereby replacing a portion of the solvent in the supplied EVOH solution with water, thus producing a high-concentration EVOH solution. The concentration of EVOH in the EVOH solution supplied to the aforementioned tower-type vessel is preferably 15-50% by mass, more preferably 25-40% by mass. Furthermore, the ratio of the EVOH solution supplied to the mixed vapor supply (solution supply / vapor supply) by mass is preferably 100 / 400 to 100 / 8. Moreover, the water content in the mixed vapor is preferably 20-70% by mass. The solvent used in the aforementioned mixed vapor is preferably an alcohol with a boiling point of 130°C or lower. Examples of such alcohols include methanol, ethanol, propanol, and butanol. More preferably, it is an alcohol with a boiling point of 100°C or lower. From the viewpoints of easy availability, low price, low boiling point, and easy processing, methanol is preferred.

[0047] The high-concentration EVOH solution obtained through this operation is preferably granulated using known methods. Examples of granulation methods include: cooling and solidifying the EVOH solution and then cutting it; or melting and mixing the EVOH using an extruder, followed by spraying and cutting. Specific examples of EVOH cutting methods include: extruding the EVOH into strands and then cutting it using a granulator; or cutting the EVOH sprayed from the die using a central thermal cutting method or an underwater cutting method. If the EVOH solution is granulated, aqueous EVOH granules are formed.

[0048] When obtaining aqueous EVOH granules by cooling and solidifying an EVOH solution, the aqueous EVOH granules are preferably washed and dehydrated using known methods. Alternatively, the following chemical treatment is preferred: immersing the EVOH in a solution containing boron compounds, alkali metal salts, alkaline earth metal salts, etc., using known methods, so that the aqueous EVOH contains the compounds. The presence of these compounds improves the mechanical properties and thermal stability of the EVOH molded body. Furthermore, when obtaining aqueous EVOH by melt-blending and granulation of EVOH, the washing, dehydration, and chemical treatment of the EVOH can be performed in an extruder.

[0049] EVOH granules can be obtained by drying the obtained water-containing EVOH granules using a known method. The moisture content of the dried EVOH granules is preferably set to 0.08% by mass or less. The drying method is not particularly limited, and examples include static drying combined with air drying or nitrogen drying, flow drying, or vacuum drying. A multi-stage drying method combining several drying methods is preferred, and a multi-stage drying method incorporating pre-drying and main drying is more preferably used.

[0050] The yellowness index (YI) of the dried EVOH is preferably 13 or less, more preferably 9.5 or less. According to the manufacturing method of the present invention, it is possible to produce EVOH with less coloring and to reduce environmental burden.

[0051] The EVOH obtained by the manufacturing method of the present invention contains an ethylene-vinyl alcohol copolymer (A) and acetaldehyde (B1) with an ethylene unit content of 20 mol% or more and 60 mol% or less, and also contains at least one selected from 2,4-hexadienal (B2) and 2,4,6-octtrienal (B3), and satisfies the following formula (1).

[0052] 10≤b1 / (b2+b3)<150…(1) In the above formula (1), b1 is the content of acetaldehyde (B1) relative to EVOH (A) (ppm), b2 is the content of 2,4-hexadienal (B2) relative to EVOH (A) (ppm), and b3 is the content of 2,4,6-octtrienal (B3) relative to EVOH (A) (ppm).

[0053] That is, by manufacturing EVOH that meets the above conditions, it is possible to provide EVOH that simultaneously inhibits coloring and reduces environmental burden.

[0054] The EVOH obtained by the method of the present invention can be molded into various molded products such as films, sheets, containers, tubes, and fibers. Example

[0055] The present invention will be specifically described below using examples, but the present invention is not limited to the following examples.

[0056] [Evaluation Method] (1) Determination of ethylene unit content and saponification degree of EVOH The dried EVOH granules obtained in the examples and comparative examples were pulverized. 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). The solution was then analyzed using a 500 MHz microscope. 1H-NMR (GX-500 manufactured by NEC Corporation) was performed at 80°C. The content of ethylene units and the degree of saponification were determined based on the peak intensity ratio of ethylene units, ethylene alcohol units, and ethylene ester units.

[0057] (2) Quantitative analysis of acetaldehyde and crotonaldehyde in alcohol solvents In the examples and comparative examples, 10 mL of a mixed solution of acetonitrile / acetic acid (weight ratio: acetonitrile / acetic acid = 9:1) 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). 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 (fractionation system), UV detector) to quantify the aldehyde content. It should be noted that a standard curve prepared using commercially available aldehyde-DNPH compounds (or synthetic compounds) was used for quantification.

[0058] (3) Quantitative analysis of acetaldehyde, crotonaldehyde, 2,4-hexadienal, and 2,4,6-octtrienal. 50.0 mg of the sample obtained by freeze-pulverizing 0.50 g of dried EVOH granules from the Examples and Comparative Examples was weighed into a glass tube of a heated desorption gas chromatography-mass spectrometry (HCGC-MS) apparatus to prepare a sample tube. Using the HCGC-MS apparatus described below, the sample was heated under the following conditions to allow all volatile gases to be adsorbed from the sample into the adsorption tube in one go. The gas released again from the adsorption tube was then separated using a column, and the peak of each component was detected. Standard curves were constructed using the peak areas of standard samples of acetaldehyde, crotonaldehyde, 2,4-hexadienal, and 2,4,6-octtrienal, and quantification was performed using the absolute standard curve method. It should be noted that when determining the standard samples, the standard samples were impregnated into the adsorption tube (Made by Tenax / Carboxen), and the adsorption tube impregnated with the standard samples was used instead of the sample tube. The temperature at which the sample was released after adsorption was changed from 170°C in the sample tube to 260°C in the adsorption tube. Otherwise, the determination was performed using the same method as when determining the sample tube.

[0059] (Heating and desorption section) Device: TurboMatrix-ATD (manufactured by Perkin Elmer Japan) Temperatures at which samples are adsorbed onto the adsorption tube: 170℃ (sample tube), -30℃ (adsorption tube), 250℃ (valve), 260℃ (production line). Adsorption time in the adsorption tube: 10 minutes Temperatures at which samples are released after adsorption: 170℃ (sample tube), 260℃ (adsorption tube), 250℃ (valve), 260℃ (production line) Adsorption tube release time: 35 minutes Carrier gas: Carrier gas flow rate in the helium column: 1.0 ml / min Pressure: 120 kPa (Gas Chromatography-Mass Spectrometry Analysis Department) Devices: 7890B GC System, 7977B MSD (manufactured by Agilent Technologies) Column: DB-WAX UI (Length: 30m, Inner Diameter: 0.25mm, Film Thickness: 0.50μm) Column oven temperature: After maintaining at 40℃ for 5 minutes, increase the temperature to 240℃ at a rate of 10℃ / min and maintain for 10 minutes (total temperature measurement time is 35 minutes). Assembly line (connection section) temperature: 240℃ Ionization conditions: EI+ Detection ion mass range: m / z = 29-600 Detection method: SCAN (4) Hue evaluation The yellow index (YI) values ​​of the dried EVOH granules obtained in the examples and comparative examples were measured and calculated using Hunter's LAB Scan XE according to JIS K7373:2006. A lower value indicates that yellowing is more suppressed and the hue is better; a YI value above 13 indicates that hue deterioration cannot be suppressed.

[0060] (5) Manufacturing quantity ratio When both recycled alcohol and unused alcohol are used in the saponification reaction of saponification step (I), if only unused alcohol is used, the ratio of the amount of EVOH obtained by using an equal amount of unused alcohol to the amount of EVOH obtained through saponification step (I) is calculated, and the resulting value is set as the production ratio. The higher the value, the less unused alcohol is used, and therefore, the less environmental burden.

[0061] (Example 1) (Alcohol recovery) A 48% by mass EVAc solution, prepared by dissolving EVAc with 32 mol% ethylene unit and 68 mol% vinyl acetate unit in methanol, is supplied to a tower reactor (plate tower, 21 plates, 140 mm inner diameter) for saponification. Figure 1This is a schematic diagram of the aforementioned tower reactor. The EVAc solution is supplied from the EVAc solution supply port 2 (at a tower temperature of 115°C) to the 20th plate at a rate of 10 kg / h, and a 5% by weight methanol solution of sodium hydroxide (as an alkaline catalyst) is supplied from the alkali catalyst supply port 3 to the 19th plate at a rate of 1.6 kg / h. Regarding the alcohol vapor, unused methanol vapor is continuously supplied from the alcohol vapor inlet 4 to the lower part of the 1st plate at a rate of 21.6 kg / h. The acetaldehyde and crotonaldehyde content in the methanol solvent used in the unused methanol vapor were analyzed according to the method described in the above evaluation method (2). The acetaldehyde and crotonaldehyde were below the detection limit and were not detected. The byproduct methyl acetate and aldehyde, along with excess methanol, were distilled off from the top of the tower (alcohol vapor outlet 1) as a mixed vapor, and an EVOH methanol solution was obtained from the bottom of the tower (EVOH solution outlet 5). A mixture obtained by condensing and removing the mixed vapor distilled from the top of the column is introduced into another type of column recoverer (plate column) for distillation, thereby recovering methanol (recovered alcohol (a)) from the bottom of the column. The contents of acetaldehyde and crotonaldehyde were determined according to the method described in the above evaluation method (2) for recovering methanol (recovered alcohol (a)), with acetaldehyde 450 ppm and crotonaldehyde 50 ppm. The results are shown in Table 1.

[0062] (Aldehyde reduction process (II)) Next, aldehyde reduction treatment of the recovered methanol (recovered alcohol (a)) was performed using a cation exchange resin. Specifically, the recovered methanol (recovered alcohol (a)) was continuously fed into a 10L tank of an H+ type strong acidic cation exchanger (manufactured by DUPONT: "AMBERLIST 15") to perform acetalization of acetaldehyde, thereby reducing the aldehyde content and obtaining aldehyde-reduced methanol (alcohol (b)) used as alcohol vapor in the saponification process (I). The contents of acetaldehyde and crotonaldehyde in the obtained methanol (alcohol (b)) were determined according to the method described in the evaluation method (2) above, with acetaldehyde content of 0 ppm and crotonaldehyde content of 50 ppm. The results are shown in Table 1.

[0063] (Saponification process (I)) As alcohol vapor, methanol (alcohol (b)) obtained through aldehyde reduction process (II) was used. In addition, saponification was performed using the same method as the alcohol recovery described above, yielding an EVOH methanol solution (EVOH concentration: 25% by mass) from the bottom of the tower (EVOH solution outlet 5) (saponification process (I)). In this saponification process (I), no unused alcohol is used, resulting in a low environmental impact. The results are shown in Table 1.

[0064] (Manufacturing of EVOH granules) Acetic acid, in an equimolar amount to sodium hydroxide supplied to the tower reactor, was added to the EVOH methanol solution obtained through saponification step (I) to neutralize the residual sodium hydroxide. The solution was then concentrated until the copolymer concentration reached 40% by mass. The concentrated solution was extruded from a 3.5 mm nozzle into a methanol-water mixed solvent (methanol / water = 10 / 90 by mass) maintained at 5°C, causing it to solidify into strands. These strands were then cut using a cutting machine to obtain hydrated EVOH granules. The hydrated EVOH granules were then washed in a large amount of 0.1 g / L acetic acid aqueous solution to remove residual methanol and sodium acetate. The granules were then dried at 60°C for 5 hours, and then at 110°C for 10 hours to obtain dried EVOH granules. The dried EVOH granules were evaluated according to the methods described in evaluation methods (1), (3), and (4) above. The results are shown in Table 1.

[0065] (Example 2) The rate at which methanol vapor is supplied to the tower reactor during alcohol recovery and the rate at which methanol vapor is supplied in the saponification step (I) are changed to 43.2 kg / h. Otherwise, dried EVOH granules are prepared using the same method as in Example 1 and evaluated. The results are shown in Table 1.

[0066] (Example 3) In the aldehyde reduction process (II), 25 parts by mass of methanol obtained by acetalization using a cation exchange resin and treated with aldehyde reduction were mixed with 75 parts by mass of methanol recovered from processes other than the saponification process (I) to further reduce aldehyde content. The aldehyde content of the methanol (alcohol (b)) used in the alcohol vapor of the saponification process (I) was adjusted as described in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Example 1 and evaluated. The results are shown in Table 1.

[0067] (Examples 4 and 5) In the aldehyde reduction process (II), the content of methanol recovered from processes other than the saponification process (I) was changed as described in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Example 3 and evaluated. The results are shown in Table 1.

[0068] (Example 6) In the aldehyde reduction process (II), instead of methanol recovered from processes other than the saponification process (I), the unused methanol was changed to the content shown in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Example 3 and evaluated. The results are shown in Table 1.

[0069] (Example 7) In the aldehyde reduction process (II), 10 parts by mass of recovered methanol (recovered alcohol (a)) were mixed with 90 parts by mass of methanol recovered from processes other than saponification (I), and then acetalized using a cation exchange resin to perform aldehyde reduction treatment. Otherwise, dried EVOH granules were prepared using the same method as in Example 1 and evaluated. The results are shown in Table 1.

[0070] (Examples 8 and 9) In the aldehyde reduction process (II), the content of methanol recovered from processes other than the saponification process (I) was changed as described in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Example 7 and evaluated. The results are shown in Table 1.

[0071] (Example 10) In the aldehyde reduction process (II), instead of methanol recovered from processes other than the saponification process (I), the unused methanol was changed to the content shown in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Example 7 and evaluated. The results are shown in Table 1.

[0072] (Example 11) Ethanol was used as the solvent instead of methanol, and dried EVOH granules were prepared and evaluated using the same method as in Example 1. The results are shown in Table 1.

[0073] (Examples 12 and 13) The ethylene unit content of the resulting EVOH was changed in the manner shown in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Example 1, and the results were evaluated. The results are shown in Table 1. It should be noted that the ethylene unit content of EVAc and EVOH is substantially the same.

[0074] (Example 14) The concentration of sodium hydroxide in the saponification step (I) was adjusted to achieve the degree of saponification of the resulting EVOH as described in Table 1. However, during the drying process, the resins stuck together and could not continue operating, even using the same method as in Example 1. Therefore, the rate of supplying the EVAc solution to the saponification step (I) was changed to 5 kg / h, and the supply rate of the recovered methanol vapor was set to 11 kg / h. This adjusted the concentration of sodium hydroxide to achieve the degree of saponification of the EVOH as described in Table 1. Otherwise, the dried EVOH granules were produced using the same method as in Example 1, and the results were evaluated. The results are shown in Table 1.

[0075] (Comparative Example 1) The methanol vapor supplied to the saponification step (I) was replaced with unused methanol. Otherwise, dried EVOH granules were prepared using the same method as in Example 1 and evaluated. The results are shown in Table 1.

[0076] (Comparative Example 2) The methanol vapor supplied to the saponification step (I) was set to recovered methanol (recovered alcohol (a)) without aldehyde reduction treatment. Otherwise, dried EVOH granules were prepared using the same method as in Example 1 and evaluated. The results are shown in Table 1.

[0077] (Comparative Example 3) The aldehyde reduction process (II) was performed using distillation. Otherwise, dried EVOH granules were prepared using the same method as in Example 1 and evaluated. The results are shown in Table 1.

[0078] (Comparative Example 4) The aldehyde reduction process (II) was modified by mixing 25 parts by mass of recovered methanol (recovered alcohol (a)) with 75 parts by mass of methanol recovered from processes other than saponification (I). Acetalization using a cation exchange resin was not performed. Otherwise, dried EVOH granules were prepared using the same method as in Example 1 and evaluated. The results are shown in Table 1.

[0079] (Comparative Example 5) In the aldehyde reduction process (II), the methanol content recovered from processes other than the saponification process (I) was changed as described in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Comparative Example 4, and the results were evaluated. The results are shown in Table 1.

[0080] (Compare Examples 6 and 7) In the aldehyde reduction process (II), instead of methanol recovered from processes other than saponification process (I), the unused methanol content was changed to the level shown in Table 1. Otherwise, dried EVOH granules were prepared using the same method as in Comparative Example 4, and the results were evaluated. The results are shown in Table 1.

[0081] [Table 1] .

[0082] Explanation of reference numerals in the attached figures 1. Alcohol vapor outlet 2 EVAc solution supply port 3 Alkali catalyst supply port 4. Alcohol vapor blown into the inlet 5. EVOH solution outlet.

Claims

1. A method for manufacturing ethylene-vinyl alcohol copolymer, comprising: In the saponification step (I), an ethylene-vinyl acetate copolymer solution containing an ethylene-vinyl acetate copolymer and an alcohol is fed to the upper part of a column reactor, alcohol vapor is fed to the lower part of the column and discharged from the upper part, and simultaneously, the aforementioned ethylene-vinyl acetate copolymer is saponified using an alkaline catalyst. An ethylene-vinyl alcohol copolymer solution containing an ethylene-vinyl alcohol copolymer and an alcohol with a saponification degree of 80 mol% or more and 100 mol% or less is taken out from the bottom of the column; and The aldehyde reduction process (II) recovers the alcohol used in the saponification process (I) and reduces the aldehyde in the recovered alcohol (a) to obtain alcohol (b). The aldehyde reduction process (II) includes a step of contacting the recovered alcohol (a) with an acetalization catalyst to reduce the aldehyde content. In the saponification process (I), the alcohol vapor supplied to the lower part of the tower contains alcohol (b), which contains crotonaldehyde.

2. The manufacturing method according to claim 1, wherein, The aldehyde reduction process (II) also includes a process of obtaining alcohol (b) by mixing the recovered alcohol (a) with an alcohol different from the recovered alcohol (a) to reduce the aldehyde concentration in the recovered alcohol (a).

3. The manufacturing method according to claim 2, wherein, Alcohols that are different from recovered alcohols (a) include recovered alcohols recovered in processes other than saponification process (I).

4. The manufacturing method according to claim 2 or 3, wherein, Alcohols that are different from recycled alcohols (a) contain unused alcohols.

5. The manufacturing method according to any one of claims 1 to 4, wherein, In the saponification process (I), the concentration of acetaldehyde in the alcohol vapor supplied to the aforementioned tower reactor is 0~150ppm.

6. The manufacturing method according to any one of claims 1 to 5, wherein, In the saponification process (I), the concentration of crotonaldehyde in the alcohol vapor supplied to the aforementioned tower reactor is 5 to 200 ppm.

7. The manufacturing method according to any one of claims 1 to 6, wherein, In the saponification step (I), the ethylene-vinyl acetate copolymer supplied to the aforementioned tower reactor has an ethylene unit content of 20 mol% or more and less than 60 mol%.

8. An ethylene-vinyl alcohol copolymer, which is obtained by the manufacturing method according to any one of claims 1 to 7. It contains an ethylene-vinyl alcohol copolymer (A) with an ethylene unit content of more than 20 mol% and less than 60 mol% and acetaldehyde (B1). It also contains at least one selected from 2,4-hexadienal (B2) and 2,4,6-octtrienal (B3). The ethylene-vinyl alcohol copolymer satisfies the following formula (1). 10≤b1 / (b2+b3)<150…(1) In the above formula (1), b1 is the content of acetaldehyde (B1) relative to ethylene-vinyl alcohol copolymer (A) (ppm), b2 is the content of 2,4-hexadienal (B2) relative to ethylene-vinyl alcohol copolymer (A) (ppm), and b3 is the content of 2,4,6-octtrienal (B3) relative to ethylene-vinyl alcohol copolymer (A) (ppm).

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