Manufacturing method for hydrated ethylene-vinyl alcohol copolymer pellet

Abstract

Provided is a manufacturing method for hydrated EVOH pellets, the method including a first step for introducing a hydrated EVOH, in which the moisture content W1 is 10-90 mass%, into an extruder and performing melt kneading thereof, and a second step for obtaining hydrated EVOH pellets by cutting the hydrated EVOH ejected from the extruder, wherein: the extruder has a back slit; water is discharged from the back slit while the back slit is decompressed by means of a decompression device; the temperature of the hydrated EVOH ejected from the extruder is 80-120°C; a moisture content W2 is 5-50 mass%; and the ratio (W2 / W1) of the moisture content W2 with respect to the moisture content W1 is 0.2 or greater and less than 1. According to such a manufacturing method, because it is possible to efficiently reduce the moisture in a hydrated EVOH while suppressing elution of the EVOH, it is possible to manufacture hydrated EVOH pellets with good productivity.

Description

Method for producing ethylene-vinyl alcohol copolymer hydrous pellets

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

[0002] Ethylene-vinyl alcohol copolymer (hereinafter sometimes referred to as EVOH) is a polymeric material that has excellent oxygen barrier properties, aroma retention, oil resistance, antistatic properties, mechanical strength, etc., and is widely used after being molded into films, sheets, containers, etc. A common method for producing EVOH is to saponify an ethylene-vinyl ester copolymer obtained by copolymerizing ethylene with a vinyl ester such as vinyl acetate in an organic solvent containing alcohol in the presence of a saponification catalyst.

[0003] One known method for post-treating an alcohol solution of EVOH obtained by saponification is a method in which the alcohol solution of EVOH is introduced into an apparatus, and brought into contact with water in the apparatus to replace the alcohol in the solution with water, and the water in the hydrous EVOH discharged from the apparatus is reduced, and then the hydrous EVOH is cut to obtain hydrous EVOH pellets (Patent Document 1).

[0004] Patent Document 1 describes a method for obtaining hydrous EVOH pellets by using a kneader having a liquid outlet to reduce the amount of water in a hydrous EVOH composition obtained by substituting alcohol with water, and then extruding the composition through a die and cutting it. Patent Document 2 describes a method for obtaining hydrous EVOH with a reduced water content by kneading a hydrous EVOH obtained by contacting a water / methanol solution of EVOH with water vapor in a twin-screw extruder (front stage) equipped with a vent downstream of the inlet (on the die side).

[0005] JP 2002-284811 A JP 2010-132881 A

[0006] However, because EVOH is hydrophilic, these methods cannot sufficiently reduce the water content of the hydrous EVOH. Therefore, when cutting the hydrous EVOH discharged from the extruder, poor cutting can occur, resulting in poorly shaped hydrous EVOH pellets. Furthermore, when reducing the water content of the hydrous EVOH, EVOH can leach into the discharged water and be discharged together with the water from the liquid outlet of the kneader or the vent of the twin-screw extruder. Continuous operation can leave traces of EVOH leaching on the liquid outlet or vent, which, if left unattended for a long period of time, can clog the liquid outlet or vent, potentially affecting production. Therefore, to prevent poor cutting and EVOH leaching, operation at a low speed is required. For these reasons, a method for obtaining good pellet shapes while increasing production speed has been desired.

[0007] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for producing hydrous EVOH pellets having a good shape, which can efficiently reduce the water content in hydrous EVOH while suppressing elution of EVOH.

[0008] [1] A method for producing hydrous EVOH pellets, comprising a first step of introducing hydrous EVOH having a moisture content W1 of 10 to 90% by mass into an extruder and melt-kneading the same, and a second step of cutting the hydrous EVOH discharged from the extruder to obtain hydrous EVOH pellets, wherein the extruder has a back slit, and water is discharged from the back slit while reducing the pressure of the back slit with a pressure reducing device, the temperature of the hydrous EVOH discharged from the extruder is 80 to 120°C, the moisture content W2 is 5 to 50% by mass, and the ratio of the moisture content W2 to the moisture content W1 (W2 / W1) is 0.2 or more and less than 1; [2] The method according to [1], wherein the degree of reduction in pressure by the pressure reducing device is -0.07 MPa to -0.005 MPa; [3] The method according to [1] or [2], wherein the temperature of the hydrous EVOH introduced into the extruder is 80 to 130°C; [4] the manufacturing method of any of [1] to [3], wherein the extruder does not have a liquid discharge port downstream of the inlet for the water-containing EVOH; [5] the manufacturing method of any of [1] to [4], wherein the screw rotation speed of the extruder is 10 to 2000 rpm; [6] the manufacturing method of any of [1] to [5], further comprising the steps of introducing an EVOH solution containing 50 parts by mass or more of an alcohol having a boiling point of 100°C or less per 100 parts by mass of EVOH into a container, contacting the EVOH with steam in the container to discharge the alcohol together with the steam, and discharging the water-containing EVOH from the container, wherein the water-containing EVOH discharged from the container is introduced into the extruder; [7] the manufacturing method of any of [1] to [6], wherein the EVOH has an ethylene unit content of 20 to 40 mol%.

[0009] According to the production method of the present invention, the amount of water in the hydrous EVOH can be efficiently reduced while suppressing the elution of EVOH, and therefore hydrous EVOH pellets can be produced with high productivity.

[0010] FIG. 1 is a diagram showing the cylinder configuration, screw configuration, and installation position of a pressure reducing device of a twin-screw extruder in Examples 1 to 11. FIG. 2 is a diagram showing the cylinder configuration, screw configuration, and installation position of a pressure reducing device of a twin-screw extruder in Example 12. FIG. 3 is a diagram showing the cylinder configuration and screw configuration of a twin-screw extruder in Comparative Example 1. FIG. 4 is a diagram showing the cylinder configuration, screw configuration, and installation position of a pressure reducing device of a twin-screw extruder in Comparative Example 2. FIG. 5 is a diagram showing the cylinder configuration and screw configuration of a twin-screw extruder in Comparative Example 3. FIG. 6 is a diagram showing the configuration of a hot cutter in Examples 1 to 12 and Comparative Examples 1 to 3.

[0011] The present invention provides a method for producing hydrous EVOH pellets, comprising a first step of introducing hydrous EVOH having a moisture content W1 of 10 to 90% by mass into an extruder and melt-kneading the extruded hydrous EVOH, and a second step of cutting the extruded hydrous EVOH to obtain hydrous EVOH pellets, the extruder having a back slit, and water being discharged from the back slit while reducing the pressure of the back slit with a pressure reducing device, the temperature of the hydrous EVOH discharged from the extruder being 80 to 120°C, the moisture content W2 being 5 to 50% by mass, and the ratio of the moisture content W2 to the moisture content W1 (W2 / W1) being 0.2 or more and less than 1. This production method efficiently reduces the water in the hydrous EVOH while suppressing EVOH elution, thereby enabling the efficient production of well-shaped hydrous EVOH pellets. In particular, the extruder having a back slit tends to efficiently reduce the water in the hydrous EVOH while suppressing EVOH elution. Furthermore, by reducing the pressure in the back slit using a pressure reducing device, poor dehydration, which can occur particularly when producing EVOH with a low ethylene unit content (e.g., an ethylene unit content of 20 to 40 mol%), tends to be suppressed, and hydrous EVOH pellets with a good shape are likely to be obtained.

[0012] In this specification, the water-containing EVOH introduced into the extruder may be referred to as the water-containing EVOH used in the present invention, and the water-containing EVOH obtained by being discharged from the extruder may be referred to as the water-containing EVOH obtained in the present invention.

[0013] The water-containing EVOH used in the present invention is preferably in the form of a paste containing EVOH and water. The water content W1 of the water-containing EVOH is 10 to 90 mass %. As will be described later, the water-containing EVOH may also contain other components such as alcohols and alkali metal salts.

[0014] The EVOH constituting the water-containing EVOH used in the present invention (hereinafter sometimes referred to as "EVOH used in the present invention" or "the EVOH") is typically obtained by saponifying an ethylene-vinyl ester copolymer. The ethylene content in the EVOH is preferably 20 to 60 mol%. When the EVOH has an ethylene content of 20 mol% or more, the water content of the water-containing EVOH supplied to the extruder is reduced, making it easier to prevent the melt viscosity of the water-containing EVOH in the extruder from becoming too low, thereby further suppressing elution of EVOH. The ethylene content is more preferably 23 mol% or more, and even more preferably 25 mol% or more. On the other hand, when the EVOH has an ethylene content of 60 mol% or less, the gas barrier properties are improved. The ethylene content is more preferably 50 mol% or less, even more preferably 45 mol% or less, and particularly preferably 40 mol% or less. Furthermore, from the viewpoint of efficiently reducing the water content in the EVOH using a pressure reducing device, the ethylene unit content of the EVOH is preferably 20 mol% to 40 mol%, more preferably 20 mol% to 35 mol%, and even more preferably 20 mol% to 33 mol%. If the ethylene unit content of the EVOH exceeds 40 mol%, the water reduction effect in the EVOH becomes sufficient through dehydration through the back slit, and the improvement effect of the pressure reducing device tends to be small. The ethylene content and degree of saponification of EVOH do not substantially change between the end of the saponification step and the end of the subsequent steps described below, and are usually measured after all steps have been completed.

[0015] The method for producing the EVOH will be specifically described below. As mentioned above, the EVOH is usually obtained by saponifying an ethylene-vinyl ester copolymer. The copolymerization of ethylene and vinyl ester may be any of solution polymerization, suspension polymerization, emulsion polymerization, and bulk polymerization. The copolymerization may be either continuous or batchwise. An example of the polymerization conditions for solution polymerization is shown below.

[0016] The solvent used is preferably an alcohol having a boiling point of 100°C or less, from the viewpoints of solubility of the ethylene-vinyl ester copolymer and EVOH, ease of handling, and ability to efficiently replace alcohol with water, etc. The boiling point is more preferably 80°C or less, and even more preferably 70°C or less.

[0017] Examples of alcohols having a boiling point of 100° C. or less include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, and t-butyl alcohol, with methanol being particularly preferred.

[0018] Examples of initiators that can be used in the polymerization include azonitrile initiators such as 2,2-azobisisobutyronitrile, 2,2-azobis-(2,4-dimethylvaleronitrile), 2,2-azobis-(4-methoxy-2,4-dimethylvaleronitrile), and 2,2-azobis-(2-cyclopropylpropionitrile), and organic peroxide initiators such as isobutyryl peroxide, cumyl peroxy neodecanoate, diisopropyl peroxycarbonate, di-n-propyl peroxydicarbonate, t-butyl peroxy neodecanoate, lauroyl peroxide, benzoyl peroxide, and t-butyl hydroperoxide.

[0019] Examples of vinyl esters include fatty acid vinyl esters such as vinyl acetate, vinyl propionate, and vinyl pivalate, with vinyl acetate being preferred. EVOH can also contain 0.0002 to 0.2 mol % of a vinylsilane compound as a copolymerization component. Examples of vinylsilane compounds include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(β-methoxyethoxy)silane, and γ-methacryloxypropylmethoxysilane. Of these, vinyltrimethoxysilane and vinyltriethoxysilane are preferred.

[0020] The polymerization conditions are preferably as follows: (1) Temperature: preferably 20 to 90°C, more preferably 40 to 70°C. (2) Time (average residence time in the case of a continuous polymerization): preferably 2 to 15 hours, more preferably 3 to 11 hours. (3) Conversion rate: preferably 10 to 90%, more preferably 30 to 80%, based on the vinyl ester charged. (4) Resin content in the solution after polymerization: preferably 5 to 85% by mass, more preferably 20 to 70% by mass.

[0021] In addition to ethylene and vinyl esters, monomers copolymerizable therewith, for example, α-olefins such as propylene, butylene, isobutylene, pentene, hexene, α-octene, and α-dodecene; 3-acyloxy-1-propene, 3-acyloxy-1-butene, 4-acyloxy-1-butene, 3,4-diacyloxy-1-butene, 3-acyloxy-4-methyl-1-butene, 4-acyloxy-2-methyl-1-butene, 4-acyloxy-3-methyl-1-butene, 3,4-diacyloxy-2-methyl-1-butene, 4-acyloxy-1-pentene, 5-acyloxy-1-pentene, 4,5-diacyloxy-1-pentene, 4-acyloxy-1-hexene, 5-acyloxy-1- It is also possible to carry out polymerization in the presence of small amounts of alkenes having an ester group such as hexene, 6-acyloxy-1-hexene, 5,6-diacyloxy-1-hexene, 1,3-diacetoxy-2-methylenepropane, etc.; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, etc., anhydrides, salts, mono- or dialkyl esters thereof; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefin sulfonic acids such as ethylene sulfonic acid, allyl sulfonic acid, methallylsulfonic acid, etc., or salts thereof; alkyl vinyl ethers, vinyl ketone, N-vinylpyrrolidone, vinyl chloride, vinylidene chloride, etc. The content of other monomer units in the EVOH other than ethylene, vinyl ester, and vinyl alcohol is preferably 20 mol% or less, and in some cases, 10 mol% or less, 5 mol% or less, 3 mol% or less, 1 mol% or less, or 0.1 mol% or less is preferred. The EVOH may not contain the other monomer units.

[0022] After polymerization for a predetermined time has reached a predetermined polymerization rate, a polymerization inhibitor is added as necessary, unreacted ethylene gas is evaporated and removed, and then unreacted vinyl ester is purged. For example, a method for purging the unreacted vinyl ester may be employed in which the polymerization solution from which ethylene has been removed is continuously fed at a constant rate from the top of a column packed with Raschig rings, vapor of an organic solvent, preferably an alcohol having a boiling point of 100°C or less, and most preferably methanol, is blown into the bottom of the column, a mixed vapor of the organic solvent and unreacted vinyl ester is distilled from the top of the column, and the copolymer solution from which unreacted vinyl ester has been removed is taken out from the bottom of the column.

[0023] An alkali catalyst is added to the copolymer solution from which the unreacted vinyl ester has been removed, and the vinyl ester component in the copolymer is saponified. The saponification method can be either continuous or batchwise. As the alkali catalyst, sodium hydroxide, potassium hydroxide, alkali metal alcoholate, etc. are used. Furthermore, methanol is preferred as the solvent used for saponification. For example, the saponification conditions are as follows: (1) Concentration of ethylene-vinyl ester copolymer in the solution: 10 to 50 mass % (2) Reaction temperature: 30 to 150°C (3) Amount of catalyst used: 0.005 to 0.6 equivalents (per vinyl ester component) (4) Time (average residence time in the case of a continuous method): 10 minutes to 6 hours

[0024] In general, continuous saponification allows for more efficient removal of methyl acetate produced by saponification, resulting in a resin with a higher degree of saponification with a smaller amount of catalyst than batch saponification. Furthermore, continuous saponification requires higher temperatures to prevent EVOH from precipitating. Therefore, continuous saponification preferably uses the following reaction temperature and catalyst amount: Reaction temperature: 70 to 150°C. Catalyst amount used: 0.005 to 0.1 equivalents (per vinyl ester component).

[0025] The degree of saponification of the resulting EVOH varies depending on the purpose, but is preferably 80 mol% or more of the vinyl ester component, and from the viewpoint of further improving the gas barrier property, is more preferably 95 mol% or more, even more preferably 98 mol% or more, and particularly preferably 99 mol% or more. The degree of saponification can be adjusted as desired by changing the conditions.

[0026] As described above, when producing EVOH pellets having particularly excellent melt stability and good long-run properties, the degree of saponification of the EVOH is preferably 99.7 mol % or more, more preferably 99.8 mol % or more, even more preferably 99.9 mol % or more, and particularly preferably 99.95 mol % or more. In order to obtain such an EVOH, it is preferable to further adjust the saponification conditions as follows.

[0027] A continuous system is preferred as a method for obtaining EVOH with a high saponification degree of 99.9 mol% or more. Examples of methods for obtaining a high saponification degree in a continuous system include adding a catalyst from multiple locations in a saponification reaction tower, increasing the amount of catalyst used, and increasing the amount of methanol blown into the bottom of the saponification reaction tower. Examples of methods for obtaining EVOH with a high saponification degree of 99.9 mol% or more in a batch system include adding a catalyst in multiple batches, increasing the amount of catalyst used, and increasing the amount of methanol vapor or nitrogen gas blown into a saponification reaction tank.

[0028] The saponification step yields a solution or paste containing EVOH. The saponification reaction EVOH contains an alkali catalyst, by-product salts such as sodium acetate and potassium acetate, and other impurities, which may be removed by neutralization and washing as necessary. When the saponification reaction EVOH is washed with ion-exchanged water or the like containing almost no metal ions, chloride ions, or the like, some catalyst residues such as sodium acetate and potassium acetate may remain in the EVOH.

[0029] A suitable post-treatment method for the EVOH solution or paste after the saponification reaction involves supplying a mixed vapor of solvent and water from the bottom of a tower-type vessel and supplying the EVOH solution or paste from a position above the supply position of the mixed vapor, thereby replacing a portion of the solvent present in the supplied EVOH solution or paste with water and producing a high-concentration EVOH solution. The EVOH concentration in the EVOH solution supplied to the tower-type vessel is preferably 15 to 50 mass%, more preferably 25 to 40 mass%. It is also preferable that the ratio of the supply rate of the EVOH solution to the supply rate of the mixed vapor (solution supply rate / steam supply rate) is 100 / 400 to 100 / 8 by mass. Furthermore, it is preferable that the water content in the mixed vapor is 20 to 70 mass%. The solvent used in the 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. Alcohols with a boiling point of 100° C. or less are more preferred, and among these, methanol is preferred because it is readily available, inexpensive, has a low boiling point, and is easy to handle.

[0030] The EVOH solution or paste thus obtained typically contains 50 parts by mass or more of an alcohol having a boiling point of 100°C or less per 100 parts by mass of EVOH. The alcohol content is preferably 1,000 parts by mass or less, more preferably 500 parts by mass or less. By setting the alcohol content within this range, the fluidity of the EVOH solution is ensured and efficient resin production becomes possible. The alcohol is preferably methanol. The EVOH solution after saponification may not only be an alcohol solution, but also a mixed solvent solution to which, if necessary, other solvents such as water are added to such an extent that EVOH does not precipitate.

[0031]

[0013] Preferably, the production method for obtaining the water-containing EVOH used in the present invention further comprises the steps of introducing the EVOH solution obtained as described above, which contains 50 parts by mass or more of an alcohol having a boiling point of 100°C or less per 100 parts by mass of EVOH, contacting the EVOH with steam in the vessel to discharge the alcohol together with the steam, and discharging the water-containing EVOH from the vessel, and introducing the water-containing EVOH discharged from the vessel into an extruder described below. Such a method not only enables efficient substitution of the alcohol in the EVOH solution with water, but also makes it easy to adjust the water content and temperature of the EVOH.

[0032] The method for contacting the EVOH solution introduced into the vessel with steam within the vessel is not particularly limited, and may be either a continuous or batch method. The vessel shape is also not particularly limited, but a tower-type vessel is preferred for the continuous method, and a tank-type vessel is preferred for the batch method. Considering production efficiency, the continuous method is industrially preferable. Examples of tower-type vessels include plate towers such as perforated plate towers and bubble cap towers, and packed towers containing ring-type packings.

[0033] In a tower-type vessel, steam is supplied from the bottom of the vessel, and an EVOH solution or paste is supplied from a position above the steam supply position, thereby removing the solvent (alcohol) present in the supplied EVOH solution or paste together with the steam, and discharging a water-containing EVOH having a moisture content of 10 to 90% by mass from the vessel. If the amount of steam introduced is too small, the solvent (alcohol) removal efficiency will be poor, while if it is too large, it will be cost-inefficient. Therefore, the amount of steam introduced is preferably 0.3 to 30 times, more preferably 0.5 to 10 times, and even more preferably 0.7 to 5 times, the amount of EVOH solution or paste introduced, expressed by mass ratio. The steam contacted with the EVOH solution or paste may contain up to 10 parts by mass of the solvent (alcohol) per 100 parts by mass of steam. However, for efficient solvent (alcohol) removal, it is preferable that the steam does not contain the solvent (alcohol). The water-containing EVOH may contain 0 to 10 parts by mass of the solvent (alcohol) per 100 parts by mass of EVOH. The water-containing EVOH may further contain saponification catalyst residues and the like.

[0034] The alcohol vapor and water vapor discharged from the top of the column are condensed in a condenser and recovered as an aqueous alcohol solution, which can be purified and reused as necessary. The EVOH solution or paste comes into direct contact with water vapor in the vessel, gradually reducing the solvent (alcohol) content. During this time, the EVOH remains in a swollen paste state, allowing it to be discharged from the vessel while maintaining its fluidity and without gelling. EVOH dissolves in a methanol / water mixed solvent at atmospheric pressure, for example, at temperatures of approximately 60 to 70°C, but does not dissolve in water alone. However, in the presence of pressurized water vapor at temperatures of, for example, 90°C or higher, EVOH can maintain its fluidity even when it contains essentially only water.

[0035] The temperature inside the container is preferably 100 to 150°C. If the temperature inside the container is less than 100°C, the fluidity of the water-containing EVOH will be insufficient, and gelation or blockage may occur inside the container. A temperature of 110°C or higher is more preferable, and 120°C or higher is even more preferable. On the other hand, if the temperature inside the container exceeds 150°C, the EVOH may deteriorate. A temperature of 140°C or lower is more preferable.

[0036] Furthermore, if the pressure inside the vessel is too low, the efficiency of alcohol removal may decrease. The pressure inside the vessel is preferably 0.1 MPa or more, more preferably 0.15 MPa or more, and even more preferably 0.2 MPa or more. On the other hand, if the pressure inside the vessel is too high, the water content of the water-containing EVOH discharged from the vessel may become too high, which may result in the melt viscosity of the water-containing EVOH introduced into the extruder described below becoming too low. Therefore, the pressure inside the vessel is preferably 0.6 MPa or less, more preferably 0.5 MPa or less, and even more preferably 0.4 MPa or less.

[0037] After the EVOH solution is brought into contact with water vapor as described above, the water-containing EVOH is discharged from the vessel. This water-containing EVOH is preferably used as the water-containing EVOH to be supplied to the extruder in the first step described below.

[0038] In the first step, water-containing EVOH having a water content W1 of 10 to 90% by mass is introduced into an extruder and melt-kneaded. The form of the water-containing EVOH introduced into the extruder is not particularly limited, but examples include a paste-like form. As described above, by directly contacting the EVOH solution with water vapor in a container, a paste-like water-containing EVOH is obtained. The EVOH is discharged from the container while maintaining its fluidity and without gelling.

[0039] The water content W1 of the water-containing EVOH introduced into the extruder is 10 to 90% by mass. If the water content W1 is less than 10% by mass, the melt viscosity of the water-containing EVOH becomes too high, making it impossible to extrude the water-containing EVOH from the tip of the extruder or increasing the amount of EVOH eluted. Furthermore, in order to ensure fluidity, the melt temperature of the water-containing EVOH becomes high, which deteriorates the EVOH and worsens the color of the resulting EVOH. The water content W1 is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 45% by mass or more. On the other hand, if the water content W1 exceeds 90% by mass, the melt viscosity of the water-containing EVOH becomes too low, increasing the amount of EVOH eluted. The water content W1 is preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, and particularly preferably 55% by mass or less. The water content W1 of the water-containing EVOH introduced into the extruder is measured by the method described in the Examples below. The water content W1 can be adjusted, for example, by the conditions for post-treating the EVOH solution or paste, or the conditions for obtaining water-containing EVOH from the EVOH solution or paste.

[0040] The temperature of the water-containing EVOH supplied to the extruder is preferably 80 to 130°C. When the temperature is within the above range, the melt viscosity of the water-containing EVOH falls within an appropriate range, which tends to enable the water-containing EVOH to be stably discharged from the tip of the extruder and tends to suppress elution of the EVOH. The temperature is more preferably 90°C or higher, and even more preferably 95°C or higher. The temperature is more preferably 125°C or lower, and even more preferably 115°C or lower. The temperature of the water-containing EVOH can be adjusted, for example, by the conditions when obtaining the water-containing EVOH from an EVOH solution or paste.

[0041] From the viewpoint of preserving the working environment and the surrounding environment and from the viewpoint of preventing the resulting hydrous EVOH pellets from sticking together, the content of alcohol having a boiling point of 100°C or less in the hydrous EVOH introduced into the extruder is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less.

[0042] The water-containing EVOH introduced into the extruder may contain, for example, about 0.1 to 5% by mass, calculated as the metal, of an alkali metal salt, which corresponds to the residue of the catalyst used in the saponification step, and may also contain by-product salts, other impurities, etc. The content of components other than EVOH, water, and alcohol having a boiling point of 100°C or less in the water-containing EVOH supplied to the extruder is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, and particularly preferably 1% by mass or less.

[0043] Fig. 1 is a diagram showing the cylinder configuration (upper side of Fig. 1) and screw configuration (lower side of Fig. 1) of the extruder used in Example 1 etc., which will be described later. The first step will be further described with reference to Fig. 1.

[0044] The extruder used in the first step may be either a single-screw extruder or a multi-screw extruder, but a twin-screw extruder is preferred. The L / D ratio of the extruder is preferably 8 to 30, more preferably 9 to 25, and even more preferably 10 to 20. A block cylinder or the like is used as the cylinder 1. An inlet 2 for the water-containing EVOH is provided in the cylinder 1 of the extruder. After the water-containing EVOH is introduced into the inlet 2, the water-containing EVOH flows along the axis of the cylinder 1 toward the tip side (downstream side 3) of the cylinder 1 as the screw disposed within the cylinder 1 rotates. At this time, the water content of the water-containing EVOH is reduced by discharging water from a liquid outlet (back slit 4) provided in the cylinder 1 while reducing the pressure therethrough using a pressure reducing device 5. Thus, a major feature of the present invention is that the extruder used in the first step has a back slit 4, and the water in the water-containing EVOH is discharged from the back slit 4 while reducing the pressure therethrough using a pressure reducing device 5. The back slit 4 is a drainage slit (liquid discharge port) located upstream 8 of the water-containing EVOH flow 6 relative to the water-containing EVOH inlet 2. Conventionally, drainage slits (liquid discharge port 10) in extruders have been located below the inlet 2 ( FIG. 5 ) or downstream 3 of the water-containing EVOH flow 6 relative to the inlet 2. However, continuous operation leaves traces of EVOH elution at the liquid discharge port 10, which, if left unattended for a long period of time, can clog the liquid discharge port 10 and affect production, requiring periodic cleaning. The present inventors conducted extensive research to produce water-containing EVOH pellets while suppressing the generation of traces of EVOH elution, and as a result, they surprisingly found that providing the above-mentioned back slit 4 in the cylinder 1 and draining water through it suppresses the elution of EVOH into the discharged water. Furthermore, since EVOH is hydrophilic, when a hydrous EVOH having a low ethylene unit content (for example, an ethylene unit content of 20 mol % to 40 mol %) is used, the water content of the hydrous EVOH cannot be sufficiently reduced, and poor cutting may occur when the hydrous EVOH discharged from the extruder and containing a large amount of water is cut."Cutting defects" refers to the inability to cut the water-containing EVOH due to its low melt viscosity, resulting in the production of hydrous EVOH pellets that are strings of pellets or several centimeters in length. Continuous operation with a large amount of these cut-defective water-containing EVOH pellets could clog the transfer pump or piping during the transfer of the water-containing EVOH pellets, affecting production, and therefore required a reduction in production speed. The present inventors conducted extensive research to produce hydrous EVOH pellets with a good shape while suppressing cut defects and the occurrence of EVOH elution marks. As a result, they found that by attaching a pressure-reducing device 5 to the back slit 4 and reducing the pressure, it is possible to produce hydrous EVOH pellets with a good shape while suppressing the elution of EVOH into the discharged water.

[0045] When the extruder is viewed from above in a direction perpendicular to the axis of the cylinder 1, the back slit 4 is preferably located upstream 8 of the water-containing EVOH flow 6 relative to the water-containing EVOH inlet 2, and the inlet 2 and the back slit 4 do not overlap; that is, the most downstream portion 3 of the back slit 4 is preferably located upstream of the most upstream portion 8 of the inlet 2. It is also preferable that the extruder does not have a liquid discharge port 10 downstream of the water-containing EVOH inlet 2, and it is more preferable that the extruder does not have any liquid discharge port 10 other than the back slit 4. When the extruder does not have any liquid discharge port 10 other than the back slit 4, elution of EVOH tends to be further suppressed. The type of back slit 4 used in the present invention is not particularly limited, and a general dewatering slit is used, specifically a wedge wire dewatering slit, a screen mesh dewatering slit, or the like.

[0046] A pressure reducing device 5 is provided adjacent to the back slit 4 of the extruder, reducing the pressure at the back slit 4. By reducing the pressure at the back slit 4 using the pressure reducing device 5, the water content of the water-containing EVOH can be efficiently reduced while suppressing EVOH elution. This tends to reduce cutting defects, particularly when cutting water-containing EVOH with a low ethylene unit content (e.g., an ethylene unit content of 20 mol% to 40 mol%). The degree of pressure reduction at the back slit 4 is preferably −0.07 MPa to −0.005 MPa. Here, the degree of pressure reduction refers to a gauge pressure that indicates the difference from atmospheric pressure. The degree of pressure reduction is expressed as a negative value relative to atmospheric pressure, with atmospheric pressure being set to zero. Therefore, a “large degree of pressure reduction” means a large absolute value of this negative value. If the degree of pressure reduction is too small, the amount of water in the water-containing EVOH reduced is small, and cutting defects are likely to occur. Therefore, the degree of pressure reduction at the back slit 4 is preferably −0.005 MPa or less, and may be −0.015 MPa or less. On the other hand, if the degree of pressure reduction is too large, the amount of EVOH elution increases. Therefore, the degree of reduced pressure in the back slit 4 is preferably −0.07 MPa or more, more preferably −0.055 MPa or more, even more preferably −0.04 MPa or more, and may be −0.025 MPa or more. The type of the pressure reducing device used in the present invention is not particularly limited, and examples thereof include a vacuum pump, an aspirator, and an ejector.

[0047] The cylinder temperature of the extruder is preferably 70 to 110°C. By keeping the cylinder temperature in this range, elution of EVOH is further suppressed and well-shaped hydrous EVOH pellets can be obtained. The temperature is preferably 105°C or lower, more preferably 100°C or lower, and even more preferably 95°C or lower. Here, the cylinder temperature refers to the maximum temperature at the portion downstream of the hydrous EVOH inlet 2.

[0048] The extruder may be of a segmented type. The screw rotation speed is preferably 10 to 2000 rpm. By setting the screw rotation speed within this range, EVOH elution is further suppressed and well-shaped hydrous EVOH pellets can be obtained. The screw rotation speed is preferably 50 rpm or higher, more preferably 120 rpm or higher, even more preferably 150 rpm or higher, and particularly preferably 200 rpm or higher. Meanwhile, the screw rotation speed is preferably 1500 rpm or lower, even more preferably 1000 rpm or lower, even more preferably 800 rpm or lower, and particularly preferably 500 rpm or lower. When the backslit 4 is further depressurized by the decompression device 5, increasing the screw rotation speed and improving the production rate tends to suppress EVOH elution. Conversely, when the backslit 4 is not sufficiently depressurized, decreasing the screw rotation speed and reducing the production rate tends to improve cuttability.

[0049] The water content W2 of the water-containing EVOH discharged from the extruder needs to be 5 to 50% by mass. If the water content W2 is less than 5% by mass, the melt viscosity of the water-containing EVOH becomes too high, making it impossible to discharge the water-containing EVOH from the tip of the extruder or increasing the amount of EVOH that dissolves. The water content W2 is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more. On the other hand, if the water content W2 exceeds 50% by mass, the melt viscosity of the water-containing EVOH becomes too low, increasing the amount of EVOH that dissolves and causing cutting defects. The water content W2 is preferably 45% by mass or less, more preferably 40% by mass or less.

[0050] In the present invention, the ratio (W2 / W1) of the water content W2 of the water-containing EVOH discharged from the extruder to the water content W1 of the water-containing EVOH introduced into the extruder must be 0.2 or more and less than 1. When the ratio (W2 / W1) is within this range, elution of EVOH is further suppressed, and well-shaped water-containing EVOH pellets can be obtained. The ratio (W2 / W1) is preferably 0.3 or more, more preferably 0.4 or more, even more preferably 0.5 or more, even more preferably 0.55 or more, and particularly preferably 0.6 or more. On the other hand, the ratio (W2 / W1) is preferably 0.9 or less, more preferably 0.85 or less, even more preferably 0.8 or less, even more preferably 0.75 or less, and particularly preferably 0.7 or less. The water content W2 can be adjusted, for example, by the water content W1, cylinder temperature, screw rotation speed, etc. As described above, the water content W2 can be adjusted by providing a back slit and discharging water while reducing the pressure using a pressure reducing device.

[0051] The temperature of the water-containing EVOH discharged from the extruder needs to be 80 to 120°C. If the temperature is below 80°C, the melt viscosity of the water-containing EVOH becomes too high, making it impossible to discharge the water-containing EVOH from the tip of the extruder or increasing the amount of EVOH that dissolves. The temperature is preferably 85°C or higher. On the other hand, if the temperature exceeds 120°C, the melt viscosity of the water-containing EVOH becomes too low, increasing the amount of EVOH that dissolves and causing cutting defects. The temperature is preferably 115°C or lower, more preferably 110°C or lower, even more preferably 100°C or lower, and particularly preferably 93°C or lower. The temperature and water content W2 of the water-containing EVOH discharged from the extruder are measured by the methods described in the Examples.

[0052] From the viewpoint of further suppressing elution of EVOH and obtaining well-shaped hydrous EVOH pellets, it is preferable that the temperature of the hydrous EVOH introduced into the extruder is higher than the temperature of the hydrous EVOH discharged from the extruder. The temperature of the hydrous EVOH introduced into the extruder is more preferably 1°C or more higher, further preferably 5°C or more higher, and particularly preferably 10°C or more higher than the temperature of the hydrous EVOH discharged from the extruder.

[0053] In the second step, the hydrous EVOH discharged from the extruder is cut to obtain hydrous EVOH pellets. The method for this is not particularly limited, and examples include a method in which the hydrous EVOH (molten state) discharged from the extruder is directly cut, or a method in which the hydrous EVOH discharged from the extruder is extruded into a coagulation liquid in the form of strands, solidified, and then cut. Of these, a method in which the hydrous EVOH is directly cut is preferred. Methods for directly cutting the hydrous EVOH discharged from the extruder include hot cutting and underwater cutting. The cutting method for the hydrous EVOH composition obtained in step 1, as described in Patent Document 1, is preferably used. From the viewpoint of ease of handling of the hydrous EVOH pellets, a nozzle diameter of 2 to 5 mmφ (φ is the diameter; the same applies hereinafter) is preferred. The size of the produced hydrous EVOH pellets can be, for example, 1 mm to 10 mm in diameter if they are spherical (or nearly spherical), or 1 mm to 10 mm in diameter and 1 mm to 10 mm in length if they are cylindrical. When the EVOH is extruded into a strand shape, solidified, and then cut, cylindrical pellets are obtained, whereas when the EVOH is directly cut in a molten state, spherical (or nearly spherical) pellets are obtained. As described above, the method of cutting the hydrous EVOH in a molten state is superior to the method of extruding the EVOH into a coagulating liquid, extruding it into a strand shape, solidifying it, and then cutting it, in that it is not necessary to consider the take-up speed at which strands can be stably formed, and therefore is more productive. Even in cases where it is not easy to form strands, such as with EVOH with a low ethylene content, hydrous EVOH pellets can be easily produced by cutting the EVOH in a molten state.

[0054] The method of extruding the water-containing EVOH into a strand shape, solidifying it, and then cutting it includes a method of extruding the water-containing EVOH into a coagulation liquid. Water is used as the coagulation liquid, but it may contain a small amount of alcohol. The temperature of the coagulation liquid is preferably 0 to 50°C, and the temperature of the water-containing EVOH during extrusion is 80 to 120°C. This temperature difference allows the water-containing EVOH to be solidified in a short period of time. The temperature of the coagulation liquid is preferably 0 to 30°C. The solidified strand is cut into pellets with a cutter. A strand cutter is preferably used as the cutter.

[0055] The hydrous EVOH is extruded into the coagulation liquid in the form of strands through a nozzle of any shape. The shape of the nozzle is not particularly limited, but a cylindrical shape is preferred. The hydrous EVOH is extruded from the nozzle in this manner in the form of strands. The strand does not necessarily have to be a single strand; any number between several and several hundred strands can be extruded. The extruded EVOH strands are then cut after sufficient coagulation has progressed, yielding hydrous EVOH pellets. These pellets may then be washed, as described below, if necessary. The size of the pellets can be, for example, 1 mm to 10 mm in diameter and 1 mm to 10 mm in length in the case of a cylindrical shape, or 1 mm to 10 mm in diameter in the case of a spherical shape.

[0056] Before drying the thus obtained hydrous EVOH pellets, washing and addition of additives are carried out as necessary. Methods for washing and addition of additives, such as those described in Patent Document 1, which are applied to the pellets of the hydrous EVOH composition obtained in Step 1, can be used. Furthermore, before drying, the hydrous EVOH pellets may be melt-kneaded again in an extruder and then cut into pellets. The melt-kneading method, cutting method, and drying method for the obtained pellets, such as the melt-kneading method in Step 2, the method for pelletizing the EVOH resin obtained in Step 2, and the method for drying the obtained pellets, which are described in Patent Document 1, can be used.

[0057] The melt flow rate (MFR) (210°C, 2160 g load) of the EVOH resin composition pellets obtained using the hydrous EVOH pellets is preferably 0.1 to 200 g / 10 min, more preferably 0.5 to 50 g / 10 min, and even more preferably 1 to 30 g / 10 min. When the MFR is 0.1 g / 10 min or more, moldability tends to be good, and when the MFR is 200 g / 10 min or less, the mechanical properties of the resulting molded article tend to be good. The MFR is measured by the method described in the Examples below.

[0058] The EVOH thus obtained can be melt-molded into various molded articles such as films, sheets, containers, pipes, fibers, etc., and can also be used for a variety of other applications. The production method of the present invention can improve the productivity of such EVOH.

[0059] The present invention will be explained in more detail below using examples.

[0060] [Evaluation Method] (1) Measurement of Moisture Content Using 3 g of each of the hydrous EVOH and hydrous EVOH pellets obtained in the Examples and Comparative Examples, the moisture contents of the hydrous EVOH and hydrous EVOH pellets were measured using a halogen moisture analyzer "HR73" manufactured by METTLER under conditions of a drying temperature of 180°C and a drying time of 15 minutes.

[0061] (2) Evaluation of Cutting Efficiency Using 100 g of the EVOH hydrous pellets obtained in the Examples and Comparative Examples, the mass of the EVOH hydrous pellets exceeding 2 cm in length was measured, and the proportion thereof was calculated as the cutting unevenness rate. Evaluation was made according to the following evaluation criteria. Operability refers to the transportability after cutting. If the evaluation was A to C, it was determined that there was no problem with the cutting ability. A: Cutting unevenness was less than 5%, and operability was good. B: Cutting unevenness was 5% or more and less than 30%, and operability was possible. C: Cutting unevenness was 30% or more and less than 50%, and operability was possible. D: Cutting unevenness was 50% or more, and operability was unstable.

[0062] (3) Evaluation of EVOH elution Water-containing EVOH pellets were continuously produced by the methods described in the Examples and Comparative Examples, and the presence of white traces of EVOH elution at the liquid outlet of the twin-screw extruder was visually confirmed and evaluated according to the following evaluation criteria. It is believed that the EVOH elution traces at the liquid outlet are caused by EVOH eluted in the discharged water. If the evaluation was A to C, it was determined that EVOH elution was suppressed. A: No traces of EVOH elution were observed at the liquid outlet even after continuous operation for 10 days or more. B: Traces of EVOH elution were observed at the liquid outlet after continuous operation for 7 days or more but less than 10 days. C: Traces of EVOH elution were observed at the liquid outlet after continuous operation for 5 days or more but less than 7 days. D: Traces of EVOH elution were observed at the liquid outlet after continuous operation for less than 5 days.

[0063] (4) Melt Flow Rate (MFR) The EVOH hydrous pellets obtained in the Examples and Comparative Examples were placed in ion-exchanged water (bath ratio 20), stirred and washed for 2 hours, and dewatered three times. The surface water was removed from the drained pellets using a centrifuge. The hydrous pellets, which had a moisture content of 33% by mass after centrifugation, were placed in a twin-screw extruder described below, and melt-kneaded under the following conditions while the resin temperature at the discharge port was set to 100°C and a treatment solution consisting of an acetic acid / sodium acetate / phosphoric acid aqueous solution was added through the trace component addition section at the tip of the discharge port. The EVOH was fed at a rate of 10 kg / hr (including the mass of water contained) and the treatment solution was fed at a rate of 0.67 L / hr. The treatment solution was an aqueous solution containing 6.7 g / L of acetic acid, 11.3 g / L of sodium acetate, and 1 g / L of phosphoric acid. (Twin-screw extruder conditions) Device: 30 mm diameter twin-screw extruder L / D: 45.5 Screw: unidirectional fully intermeshing type Screw rotation speed: 300 rpm Cylinder temperature: 100°C Die temperature: 105°C Number of die holes: 5 holes (3 mm diameter)

[0064] The molten EVOH resin discharged from the twin-screw extruder was then cut with a hot cutter to obtain roughly spherical pellets. The roughly spherical pellets had a moisture content of 20% by mass. The obtained pellets were dried under a nitrogen stream at 90°C for 15 hours and then at 105°C for 15 hours to obtain roughly spherical EVOH pellets (moisture content 0.3% by mass) with a minor axis of 2.7 mm and a major axis of 3.7 mm.

[0065] The MFR of the EVOH pellets obtained above was measured according to the method described in JIS K 7210:2014. Specifically, the EVOH pellets were filled into a cylinder of a melt indexer L244 (manufactured by Takara Kogyo Co., Ltd.) with an inner diameter of 9.55 mm and a length of 162 mm, and melted at 210°C. A load was then uniformly applied to the molten resin composition using a plunger with a mass of 2,160 g and a diameter of 9.48 mm. The amount of resin composition extruded per unit time (g / 10 min) through an orifice with a diameter of 2.1 mm provided in the center of the cylinder was measured.

[0066] (5) Measurement of Alcohol Content Five grams of the hydrous EVOH used in the Examples and Comparative Examples was freeze-pulverized in liquid nitrogen, and immediately after pulverization, a sample (approximately 500 mg) was placed in an HSS vial, and methanol was analyzed under the following conditions. Two measurements were performed for each sample, and the alcohol content was calculated from the average. Headspace (HSS)-GC / MS Oven temperature: 120°C (MHE method: [120°C x 30 minutes] x 5 times) Loop temperature: 200°C Transfer temperature: 200°C, with shaking GC / MS Column: DB-WAXetr (30 m - 0.25 mm - 0.5 μm) Oven: 50°C (hold for 5 minutes) → 10°C / min → 200°C (hold for 10 minutes) Inlet: 230°C (split ratio 20:1) Measurement mode: SIM (m / z = 31)

[0067] Example 1 An EVOH solution containing 100 parts by mass of EVOH having an ethylene unit content of 27 mol% and a degree of saponification of 99.98 mol%, 55 parts by mass of methanol, and 45 parts by mass of water was continuously fed into a 10-plate tower with a diameter of 0.3 m from the top tray. Steam was blown into the bottom tray to countercurrently contact the EVOH solution with the steam. The temperature inside the tower was 130°C, and the pressure inside the tower was 0.3 MPa. The water-containing EVOH obtained by countercurrently contacting the steam was withdrawn from the bottom of the tower. The temperature of the obtained water-containing EVOH was 120°C, and the water content (W1) measured according to the above-mentioned evaluation method was 54.5% by mass. The alcohol content was 0.05% by mass. The results are shown in Table 1.

[0068] The resulting hydrous EVOH with a water content (W1) of 54.5% by mass was heated to a resin temperature of 110°C and supplied at 42 kg / hr to a twin-screw extruder shown in FIG. 1 , which had a liquid outlet (back slit 4) located upstream of the hydrous EVOH inlet 2 at 8. A vacuum pump (decompressor 5) was connected to the back slit 4 via piping, and the pressure in the back slit 4 was reduced to -0.02 MPa. The extruded resin was extruded under the following conditions through a die with eight holes and a hole diameter of 30 mm attached to the tip of the extruder. The resulting melt was cut 0.05 mm from the die with a two-blade hot cutter (FIG. 6) to obtain flattened spherical hydrous EVOH pellets. The resin temperature at this time was 95°C, and the water content (W2) measured according to the above-described evaluation method was 36% by mass. The resin temperature was measured using a temperature sensor in contact with the melt, installed near the discharge outlet at the tip of the cylinder. Hydrous EVOH pellets were continuously produced under the above conditions, and the elution of EVOH into the water discharged from the twin-screw extruder was evaluated according to the above-mentioned evaluation method. Furthermore, the cuttability and MFR of a portion of the obtained hydrous EVOH pellets were measured according to the above-mentioned evaluation method. The results are shown in Table 1. The flow rate of the cutter circulating water was 300 L / min, and the cutter blade rotation speed was 3000 rpm. The obtained hydrous EVOH pellets were then placed in ion-exchanged water (bath ratio 20), stirred and washed for 2 hours, and dewatered three times. Surface water was removed from the dewatered pellets using a centrifuge, and the pellets were then dried at 95°C for 3 hours under a nitrogen stream with an oxygen concentration of 1% by volume or less, yielding dried resin composition pellets with a moisture content of 0.5% by mass. <Conditions of twin-screw extruder> L / D: 14 ​​Diameter: 30 mm Screw: full flight Rotation speed: 200 rpm Cylinder temperature: 90°C (maximum temperature at the part downstream from the water-containing EVOH inlet 2) Die temperature: 120°C Number of die holes: 8

[0069] Examples 2 to 12, Comparative Examples 1 to 3 Water-containing EVOH pellets were produced and evaluated in the same manner as in Example 1, except that the conditions were adjusted so that the ethylene unit content, the temperature and moisture content W1 of the resin supplied to the extruder, the degree of pressure reduction in the pressure reduction device, the location of the liquid outlet and the location of the pressure reduction device, the screw rotation speed, and the temperature and moisture content W2 of the resin extruded from the extruder were as shown in Table 1. The results are shown in Table 1. FIG. 2 shows the cylinder configuration of the twin-screw extruder in Example 12. The twin-screw extruder in Example 12 has a liquid outlet (back slit 4) located 8 upstream of the inlet 2 for the water-containing EVOH and a liquid outlet 10 located below the inlet 2 for the water-containing EVOH, and the pressure reduction device 5 is arranged to reduce the pressure in the back slit 4. FIG. 3 shows the cylinder configuration of the twin-screw extruder in Comparative Example 1. The twin-screw extruder in Comparative Example 1 has a liquid outlet (back slit 4) located 8 upstream of the inlet 2 for the water-containing EVOH. Fig. 4 shows the cylinder configuration of the twin-screw extruder in Comparative Example 2, which has a liquid outlet 10 located below the inlet 2 for water-containing EVOH, and a pressure reducing device 5 is arranged so as to reduce the pressure at the liquid outlet 10 located below the inlet 2 for water-containing EVOH. Fig. 5 shows the cylinder configuration of the twin-screw extruder in Comparative Example 3, which has a liquid outlet 10 located below the inlet 2 for water-containing EVOH.

[0070]

[0071] A comparison of Examples 1 to 9 with Comparative Examples 1 and 2 reveals that when the back slit of the extruder was depressurized with a vacuum pump, EVOH elution into the water discharged from the extruder was suppressed, the cuttability of the hydrous EVOH was good, and hydrous EVOH pellets with a good shape were obtained. More specifically, a comparison of Examples 1 to 3, 9, and 11 reveals that a relatively high degree of vacuum applied by the pressure reducing device results in excellent cuttability and suppression of EVOH elution, while a low degree of vacuum affects cuttability and a high degree of vacuum affects EVOH elution. A comparison of Examples 1 to 3, 9, and 11 reveals that when the ratio (W2 / W1) is within a predetermined range, excellent cuttability and suppression of EVOH elution are achieved, while a low ratio (W2 / W1) affects EVOH elution and a high ratio (W2 / W1) affects cuttability. Furthermore, a comparison of Example 2 with Example 5 reveals that when the degree of vacuum is low, reducing the production speed improves cuttability. On the other hand, a comparison between Example 3 and Example 6 reveals that, when the degree of vacuum is high, increasing the production speed can suppress EVOH elution. A comparison between Examples 1, 7, and 8 reveals that the effects of cuttability and EVOH elution suppression are exhibited even when the ethylene unit content changes. A comparison between Examples 1, 2, and 10 reveals that when the water content W2 is within a predetermined range, cuttability and EVOH elution suppression are excellent, while a high water content W2 affects cuttability. A comparison between Example 1 and Example 12 reveals that the presence of a liquid outlet below the water-containing EVOH inlet in addition to the back slit affects EVOH elution. A comparison between Example 1 and Comparative Example 1 reveals that when the back slit is not reduced in pressure, cuttability deteriorates and the effect of suppressing EVOH elution tends to decrease. A comparison between Example 1 and Comparative Example 2 reveals that when a liquid outlet below the water-containing EVOH inlet other than the back slit is reduced in pressure, EVOH elution cannot be suppressed and cuttability also deteriorates. A comparison between Comparative Example 1 and Comparative Example 3 reveals that the use of an extruder having a back slit can suppress the elution of EVOH compared to the use of an extruder having a liquid outlet other than a back slit.

[0072] REFERENCE SIGNS LIST 1 Cylinder 2 Inlet 3 Downstream side 4 Back slit 5 Pressure reducing device 6 Flow of water-containing EVOH 7 Temperature sensor 8 Upstream side 9 Full-flight screw 10 Liquid outlet 30 Hot cutter 31 Melt supply port 32 Die 33 Rotary blade 34 Rotary shaft 35 Cutter box 36 Cooling water supply port 37 Cooling water 38 Water film 39 Pellet outlet 40 Cooling water and pellets

Claims

1. A first step of introducing a water-containing ethylene-vinyl alcohol copolymer having a water content W1 of 10 to 90% by mass into an extruder and melt-kneading the copolymer; and A second step of cutting the hydrous ethylene-vinyl alcohol copolymer discharged from the extruder to obtain hydrous ethylene-vinyl alcohol copolymer pellets, The extruder has a back slit, and water is discharged from the back slit while reducing the pressure of the back slit by a pressure reducing device; The temperature of the water-containing ethylene-vinyl alcohol copolymer discharged from the extruder is 80 to 120° C., and the water content W2 is 5 to 50% by mass, A method for producing hydrous ethylene-vinyl alcohol copolymer pellets, in which the ratio of the moisture content W2 to the moisture content W1 (W2 / W1) is 0.2 or more and less than 1.

2. The method according to claim 1, wherein the degree of vacuum applied by the pressure reducing device is from -0.07 MPa to -0.005 MPa.

3. 3. The process according to claim 1, wherein the temperature of the water-containing ethylene-vinyl alcohol copolymer introduced into the extruder is 80 to 130°C.

4. The method according to claim 1 or 2, wherein the extruder does not have a liquid discharge port downstream of an inlet for the water-containing ethylene-vinyl alcohol copolymer.

5. The method according to claim 1 or 2, wherein the screw rotation speed of the extruder is 10 to 2000 rpm.

6. The method further comprises the steps of: introducing an ethylene-vinyl alcohol copolymer solution containing 50 parts by mass or more of an alcohol having a boiling point of 100° C. or less per 100 parts by mass of the ethylene-vinyl alcohol copolymer into a vessel; contacting the ethylene-vinyl alcohol copolymer with water vapor in the vessel to extract the alcohol together with the water vapor; and extracting a water-containing ethylene-vinyl alcohol copolymer from the vessel; The method according to claim 1 or 2, wherein the water-containing ethylene-vinyl alcohol copolymer discharged from the vessel is introduced into the extruder.

7. The method according to claim 1 or 2, wherein the ethylene-vinyl alcohol copolymer has an ethylene unit content of 20 to 40 mol %.