Method for producing ethylene-vinyl alcohol copolymer pellets

Abstract

To reduce the energy required for drying hydrous EVOH pellets.SOLUTION: A method for producing ethylene-vinyl alcohol copolymer pellets includes a dehydration step of immersing an ethylene-vinyl alcohol copolymer hydrous pellet with a moisture content (W1) of 30-80 mass% and a temperature (T1) of 0-50°C in water or an aqueous solution with a temperature (T2) of 30-85°C to prepare an intermediate pellet with a moisture content (W2) of 20-70 mass%; and a drying step for drying the intermediate pellet to prepare a dry pellet with a moisture content (W3) of 0.5 mass% or less, with T2-T1≥5 and W2 / W1≤0.98.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing ethylene-vinyl alcohol copolymer pellets, and more particularly to a method for producing ethylene-vinyl alcohol copolymer pellets that can reduce energy consumption when producing dry pellets by drying wet pellets. [Background technology]

[0002] In a typical method for producing ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as EVOH), first, ethylene and vinyl acetate are copolymerized in a methanol solution to obtain an ethylene-vinyl acetate copolymer. Next, the ethylene-vinyl acetate copolymer is saponified in the presence of an alkali catalyst to obtain a methanol solution of EVOH. At least a portion of the methanol contained in the resulting methanol solution of EVOH is replaced with water, and then the resulting solution is cut to obtain hydrous EVOH pellets.

[0003] The thus obtained hydrous EVOH pellets contain catalyst residues such as sodium acetate, which are usually removed by washing. For example, Patent Document 1 describes a method for washing hydrous EVOH pellets, in which a tower-type vessel is used, hydrous EVOH pellets and an aqueous methanol solution are introduced from the top of the tower, water is introduced from the bottom of the tower, and the hydrous EVOH pellets are allowed to settle from the top to the bottom of the tower while removing the catalyst residues and methanol in the pellets.

[0004] In addition, in order to improve stability during melt molding, it is common to incorporate acids such as acetic acid, boric acid, and phosphoric acid or their salts into the dried EVOH pellets. For this purpose, a method is adopted in which the hydrous EVOH pellets are immersed in an aqueous solution containing the acid or salt to incorporate the acid or salt into the pellets. Such a process of incorporating an acid or salt into the hydrous pellets is called a chemical process. For example, Patent Document 2 describes a chemical process for hydrous EVOH pellets, in which a tower-type vessel is used, hydrous EVOH pellets are introduced from the top of the tower, water is introduced from the middle of the tower, and an aqueous solution containing the acid or salt is introduced from the bottom of the tower, so that the hydrous EVOH pellets and the aqueous solution are brought into countercurrent contact with each other. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2002-60413 A [Patent Document 2] Japanese Patent Application Publication No. 11-152307 Summary of the Invention [Problem to be solved by the invention]

[0006] When EVOH pellets containing water are melt-molded, foaming and resin deterioration occur, so it is important that the EVOH pellets used for melt-molding are sufficiently dried. However, since the water-containing EVOH pellets that have been through the washing and chemical treatment processes contain a large amount of water, a large amount of energy is required to dry such water-containing pellets in a dryer. [Means for solving the problem]

[0007] The present invention relates to a process for producing intermediate pellets having a moisture content (W2) of 20 to 70% by mass, comprising: a dehydration step in which ethylene-vinyl alcohol copolymer hydrous pellets having a moisture content (W1) of 30 to 80% by mass and a temperature (T1) of 0 to 50°C are immersed in water or an aqueous solution having a temperature (T2) of 30 to 85°C to obtain intermediate pellets having a moisture content (W2) of 20 to 70% by mass; A drying step of drying the intermediate pellets to obtain dried pellets having a moisture content (W3) of 0.5% by mass or less; The present invention relates to a method for producing ethylene-vinyl alcohol copolymer pellets that satisfy the following formulae (1) and (2): T2-T1≧5 (1) W2 / W1≦0.98 (2)

[0008] In a preferred embodiment, the hydrous pellets are obtained by extruding a water / alcohol solution of an ethylene-vinyl alcohol copolymer into an aqueous alcohol solution, precipitating the copolymer in the form of strands, and then cutting the extruded pellets, and the hydrous pellets have a moisture content (W1) of 40 to 80% by mass and a temperature (T1) of 0 to 40° C. In this case, it is preferred that in the dehydration step, the pellets are immersed in water or an aqueous solution having a temperature (T2) of 30 to 85° C. to obtain intermediate pellets having a moisture content (W2) of 40 to 70% by mass.

[0009] In another preferred embodiment, the hydrous pellets are obtained by cutting a paste containing an ethylene-vinyl alcohol copolymer and water at 80 to 170° C. and then cooling it, and the hydrous pellets have a hydrous content (W1) of 30 to 60% by mass and a temperature (T1) of 20 to 50° C. In this case, it is preferable that in the dehydration step, the pellets are immersed in water or an aqueous solution having a temperature (T2) of 40 to 85° C. to obtain intermediate pellets having a hydrous content (W2) of 20 to 50% by mass.

[0010] In the dehydration step, it is a preferred embodiment to wash the hydrous pellets to obtain intermediate pellets from which the saponification catalyst residue has been removed. In the dehydration step, it is also a preferred embodiment to impregnate the hydrous pellets with an aqueous solution containing at least one additive selected from carboxylic acids, boron compounds, phosphoric acid compounds, alkali metal salts, and alkaline earth metal salts to obtain intermediate pellets containing the additive. In the dehydration step, it is also preferred to continuously supply the hydrous pellets to the top of a tower-shaped vessel, continuously introduce water or an aqueous solution into the vessel, contact the hydrous pellets with the water or the aqueous solution in the vessel, and continuously discharge the intermediate pellets from the bottom of the vessel. In addition, it is also preferred to dry the intermediate pellets by hot air drying, infrared radiation, or a combination thereof to obtain dried pellets in the drying step. Effect of the Invention

[0011] According to the method for producing ethylene-vinyl alcohol copolymer pellets of the present invention, the energy required for drying the water-containing EVOH pellets can be reduced. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention relates to a process for producing intermediate pellets having a moisture content (W2) of 20 to 70% by mass, comprising: a dehydration step in which ethylene-vinyl alcohol copolymer hydrous pellets having a moisture content (W1) of 30 to 80% by mass and a temperature (T1) of 0 to 50°C are immersed in water or an aqueous solution having a temperature (T2) of 30 to 85°C to obtain intermediate pellets having a moisture content (W2) of 20 to 70% by mass; A drying step of drying the intermediate pellets to obtain dried pellets having a moisture content (W3) of 0.5% by mass or less; The present invention relates to a method for producing ethylene-vinyl alcohol copolymer pellets that satisfy the following formulae (1) and (2): T2-T1≧5 (1) W2 / W1≦0.98 (2)

[0013] The method for producing EVOH pellets of the present invention is characterized in that, when obtaining dry pellets from hydrous pellets, a dehydration step is performed before the drying step. In the dehydration step, intermediate pellets having a reduced moisture content can be obtained by controlling the temperature of the water or aqueous solution in which the hydrous pellets are immersed. This makes it possible to reduce the energy required to obtain dry pellets. The present invention will be described in detail below.

[0014] First, a method for synthesizing EVOH will be described. EVOH is obtained by saponifying an ethylene-vinyl ester copolymer. As the vinyl ester, a fatty acid vinyl ester such as vinyl acetate, vinyl propionate, or vinyl pivalate can be used, with vinyl acetate being preferred. The polymerization of ethylene and the vinyl ester may be any of solution polymerization, suspension polymerization, emulsion polymerization, and bulk polymerization, with solution polymerization being preferred. In addition, either a continuous system or a batch system may be used.

[0015] As the solvent used in the case of solution polymerization, alcohol is preferable, but other organic solvents capable of dissolving ethylene, vinyl ester, and ethylene-vinyl acetate copolymer (e.g., dimethyl sulfoxide) can also be used. As the alcohol, methanol, ethanol, 1-propanol, 2-propanol, etc. can be used, and methanol is particularly preferable.

[0016] As the polymerization catalyst, azonitrile-based initiators such as 2,2-azobisisobutyronitrile, 2,2-azobis-(2,4-dimethylvaleronitrile), 2,2-azobis-(4-methyl-2,4-dimethylvaleronitrile), 2,2-azobis-(4-methoxy-2,4-dimethylvaleronitrile), and 2,2-azobis-(2-cyclopropylpropionitrile), and organic peroxide-based initiators such as isobutyryl peroxide, cumyl peroxy neodecanoate, diisopropyl peroxycarbonate, di-n-propyl peroxy dicarbonate, t-butyl peroxy neodecanoate, lauroyl peroxide, benzoyl peroxide, and t-butyl hydroperoxide can be used.

[0017] The polymerization temperature is preferably 20 to 90°C, more preferably 40 to 70°C. The polymerization time (average residence time in the case of a continuous polymerization) is preferably 2 to 15 hours, more preferably 3 to 11 hours. The polymerization rate is preferably 10 to 90 mol%, more preferably 30 to 80 mol%, based on the charged vinyl ester. The polymer concentration in the solution after polymerization is preferably 5 to 85 mass%, more preferably 20 to 70 mass%. The ethylene unit content of the ethylene-vinyl ester copolymer is the same as the ethylene unit content of EVOH, which will be described later. In addition, the ethylene-vinyl ester copolymer can also be copolymerized with a small amount of other monomers, but the type and copolymerization amount of the other monomers are the same as those of EVOH, which will be described later.

[0018] After a predetermined time of polymerization, when a predetermined polymerization rate is reached, a polymerization inhibitor is added as necessary, unreacted ethylene gas is removed by evaporation, and unreacted vinyl ester is removed. As a method for removing unreacted vinyl ester from the ethylene-vinyl ester copolymer solution from which ethylene has been removed by evaporation, for example, a method is adopted in which the copolymer solution is continuously fed at a constant rate from the top of a tower packed with Raschig rings, an organic solvent vapor such as methanol is blown in from the bottom of the tower, a mixed vapor of the organic solvent such as methanol and unreacted vinyl ester is distilled from the top of the tower, and the copolymer solution from which unreacted vinyl ester has been removed is taken out from the bottom of the tower.

[0019] An alkali catalyst is added to the ethylene-vinyl ester copolymer solution from which 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, but it is preferable to carry out the saponification reaction continuously using a tower reactor and extract the EVOH solution from the bottom of the tower. As the alkali catalyst, sodium hydroxide, potassium hydroxide, alkali metal alcoholate, etc. are used. Furthermore, as the solvent used for saponification, methanol is preferable, and when a tower reactor is used, it is preferable to supply methanol vapor from the bottom of the tower and distill methyl acetate from the top of the tower.

[0020] Suitable saponification conditions are as follows. The concentration of ethylene-vinyl ester copolymer in the reaction solution is preferably 10 to 50% by mass, more preferably 20 to 45% by mass. The reaction temperature is preferably 70 to 150°C, more preferably 80 to 140°C. The amount of catalyst used is preferably 0.005 to 1 equivalent relative to the number of moles of vinyl ester units, more preferably 0.01 to 0.5 equivalents. The reaction time (average residence time in the case of a continuous reaction) is preferably 10 minutes to 6 hours, more preferably 15 minutes to 2 hours. The saponification reaction is allowed to proceed in this manner, thereby obtaining a solution of EVOH. The content of EVOH in the solution is preferably 10 to 50% by mass, more preferably 20 to 45% by mass.

[0021] The saponification degree of the vinyl ester unit of the thus obtained EVOH is preferably 80 to 100 mol%. From the viewpoint of obtaining a molded product with excellent barrier properties, it is more preferably 95 mol% or more, further preferably 98 mol% or more, and particularly preferably 99 mol% or more. If the saponification degree is less than 80 mol%, the barrier properties, long-run properties, and moisture resistance may be deteriorated.

[0022] The ethylene unit content of the EVOH of the present invention is preferably 5 to 60 mol% from the viewpoint of obtaining a molded product having excellent barrier properties and melt moldability. If the ethylene unit content is less than 5 mol%, the melt moldability may deteriorate. The ethylene unit content is more preferably 15 mol% or more, and even more preferably 20 mol% or more. On the other hand, if the ethylene unit content exceeds 60 mol%, the barrier property may become insufficient. The ethylene unit content is more preferably 55 mol% or less, and even more preferably 50 mol% or less.

[0023] EVOH can be copolymerized with a small amount of other monomers as long as the object of the present invention is not hindered. Examples of monomers that can be copolymerized include alkenes such as propylene, butylene, pentene, and hexene; 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, and 4-acyloxy-2-methyl-1-butene. -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-hexene, 6-acyloxy-1-hexene, 5,6-diacyloxy-1-hexene, 1,3-diacetoxy-2-methyl Examples of the EVOH include alkenes having an ester group such as dimethylpropane or saponified products thereof; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, or the like, or the anhydrides, salts, or mono- or dialkyl esters thereof; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefin sulfonic acids such as vinyl sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, or the like, or the salts thereof; vinyl silane compounds such as vinyl trimethoxy silane, vinyl triethoxy silane, vinyl tri(β-methoxy-ethoxy) silane, and γ-methacryloxypropyl methoxy silane; alkyl vinyl ethers, vinyl ketones, N-vinyl pyrrolidone, vinyl chloride, vinylidene chloride, etc. When EVOH has other monomer units, the copolymerization amount thereof is preferably 10 mol % or less, more preferably 5 mol % or less.

[0024] The method for obtaining hydrous EVOH pellets from the thus obtained alcohol solution, preferably methanol solution, of EVOH is not particularly limited, but the following two methods are preferred.

[0025] One method for obtaining hydrous EVOH pellets is to extrude the resulting water / alcohol solution of EVOH into an aqueous alcohol solution, which is obtained by replacing part of the alcohol in the alcohol solution of EVOH after saponification with water, and then cut the extruded solution into strands. Here, the water / alcohol solution of EVOH refers to EVOH dissolved in a mixed solvent of water and alcohol.

[0026] At this time, the method of replacing a part of the alcohol in the alcohol solution of EVOH with water is not particularly limited. A method of adding a mixed vapor of a solvent (alcohol) and water to the alcohol solution in a container to vaporize the solvent (alcohol) from the solution is preferable. The container at this time is preferably a tower-type container such as a plate tower or a packed tower. In this case, the alcohol solution of EVOH is introduced from the top of the tower, and a mixed vapor of the solvent (alcohol) and water is introduced from the bottom of the tower-type container and countercurrently contacted, and the mixed vapor of the solvent (alcohol) and water is drawn out from the top of the tower, and a part of the solvent present in the alcohol solution of EVOH to be supplied is replaced with water, and a water / alcohol solution of high concentration EVOH can be obtained from the bottom of the tower. The mass ratio of alcohol / water in the EVOH solution at this time is preferably 40 / 60 to 90 / 10, more preferably 50 / 50 to 80 / 20. Furthermore, it is preferable that the content of water in the mixed vapor is 20 to 70% by weight. The solvent used for the mixed vapor is preferably an alcohol having a boiling point of 130° C. or less, and examples of such alcohol include alcohols such as methanol, ethanol, propanol, butanol, etc. Alcohols having a boiling point of 100° C. or less are more preferable, and among them, methanol is preferable because it is easily available, inexpensive, has a low boiling point, and is easy to handle.

[0027] The water / alcohol solution of EVOH obtained by the solvent replacement in this manner contains, for 100 parts by mass of EVOH, preferably 50 parts by mass or more of alcohol having a boiling point of 100°C or less. The alcohol content is more preferably 70 parts by mass or more, and even more preferably 80 parts by mass or more. The alcohol content is preferably 1000 parts by mass or less, and more preferably 500 parts by mass or less. By keeping the alcohol content within this range, the fluidity of the EVOH solution is ensured and efficient resin production is possible. The alcohol is preferably methanol.

[0028] The water / alcohol solution of EVOH thus obtained is extruded into a coagulation bath made of water or an aqueous alcohol solution, precipitated in the form of strands, and then cut to obtain hydrous pellets. The aqueous alcohol solution used here is preferably an aqueous methanol solution. EVOH dissolved in a good solvent is precipitated in a poor solvent to form strands. The mass ratio of alcohol / water in the aqueous alcohol solution in the coagulation bath is preferably 0 / 100 to 30 / 70. The mass ratio is more preferably 2 / 98 or more, and even more preferably 5 / 95 or more. On the other hand, the mass ratio is more preferably 20 / 80 or less. The temperature of the coagulation bath is preferably 0 to 30°C, more preferably 20°C or less, and even more preferably 10°C or less. The hydrous pellets after cutting are porous, and the moisture content thereof is preferably 40 to 80% by mass. The moisture content is more preferably 45% by mass or more, and even more preferably 50% by mass or more. The moisture content is more preferably 70% by mass or less, and even more preferably 65% ​​by mass or less.

[0029] The lower limit of the temperature at which the water / alcohol solution of EVOH is brought into contact with the coagulation liquid (bath temperature during pelletization) is preferably -10°C, more preferably 0°C. On the other hand, the upper limit is preferably 40°C, more preferably 20°C, even more preferably 15°C, and even more preferably 10°C. The water / alcohol solution of EVOH is extruded in the form of strands into the coagulation liquid by a nozzle having an arbitrary shape. The shape of such a nozzle is not particularly limited, but a cylindrical shape is preferable. In this way, the water / alcohol solution of EVOH is extruded in the form of strands from the nozzle. At this time, the strand does not necessarily have to be one strand, and any number between several and several hundred strands can be extruded. Next, the EVOH extruded in the form of strands is cut after sufficient coagulation has progressed, and hydrous pellets of EVOH are obtained. Thereafter, washing may be performed as described below, if necessary. The size of such pellets can be, for example, a diameter of 1 mm to 10 mm and a length of 1 mm to 10 mm in the case of a cylindrical shape, and a diameter of 1 mm to 10 mm in the case of a spherical shape.

[0030] Another method for obtaining hydrous EVOH pellets is to cut a paste containing EVOH and water at 80 to 170°C, then cool it to obtain hydrous EVOH pellets. Hydrous EVOH pellets can be obtained by cutting a hot hydrous EVOH paste in a molten state and then cooling it.

[0031] The method for producing the paste containing EVOH and water is not particularly limited. An alcohol solution of EVOH after saponification is introduced into an apparatus, and contacted with water in the apparatus, and the alcohol and water are discharged from the apparatus to obtain a water-containing EVOH paste. The above apparatus is not particularly limited, and the EVOH may be contacted with water in a container or in a kneading device such as an extruder. For mass processing, it is preferable to contact the EVOH with water in a container, and particularly preferable to contact the EVOH with water vapor. That is, it is preferable to contact the EVOH with water vapor in the container and discharge the alcohol together with water vapor.

[0032] The method of contacting the alcohol solution of EVOH introduced into the vessel with water vapor in the vessel is not particularly limited, and may be either a continuous or batch method. The shape of the vessel is also not particularly limited, but a tower-type vessel is preferable for the continuous type, and a tank-type vessel is preferable for the batch type. From the viewpoint of production efficiency, the continuous type is industrially preferable. Examples of the tower-type vessel include a plate tower such as a perforated plate tower or a bubble cap tower, and a packed tower containing ring-type packing.

[0033] From the viewpoint of alcohol removal efficiency, it is preferable that the alcohol solution of EVOH introduced into the vessel is brought into contact with water vapor in a countercurrent manner. For example, in a tower vessel, an alcohol solution of EVOH is introduced from the top of the vessel, water vapor is introduced from the bottom of the vessel and brought into countercurrent contact with the solution, a water-containing EVOH paste is discharged from the bottom of the vessel, and alcohol is discharged together with water vapor from the top of the vessel. The alcohol vapor and water vapor discharged from the top of the vessel are condensed in a condenser and recovered as an alcohol aqueous solution, which can be purified as necessary and reused. The alcohol solution of EVOH comes into direct contact with water vapor in the vessel and the alcohol content gradually decreases, but during that time, the EVOH is in a swollen paste state, and it is possible to discharge the water-containing EVOH paste from the vessel while maintaining its fluidity and without gelling.

[0034] If the amount of steam introduced into the vessel is too small, the alcohol removal efficiency will be poor, whereas if it is too large, it will be disadvantageous in terms of cost, so the amount 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 the alcohol solution of EVOH introduced, expressed as a weight ratio. The steam to be contacted with the alcohol solution of EVOH may contain 10% by weight or less of alcohol, but in order to efficiently remove alcohol, it is preferable that the steam does not contain alcohol.

[0035] The temperature inside the container is preferably 80 to 170°C. If the temperature inside the container is less than 80°C, the fluidity of the water-containing EVOH paste becomes insufficient, and there is a risk of gelation or blockage inside the container. The temperature is more preferably 100°C or higher, and even more preferably 110°C or higher. On the other hand, if the temperature inside the container exceeds 170°C, the EVOH may deteriorate. The temperature is more preferably 160°C or lower, and even more preferably 150°C or lower.

[0036] The water content of the water-containing EVOH paste discharged from the device is preferably 30 to 60% by mass. If the water content is less than 30% by mass, the fluidity in the container is insufficient. The water content is preferably 35% by mass or more. On the other hand, if the water content exceeds 60% by mass, the pellets cut when cut in a molten state may be fused to each other or may have an uneven shape. The water content is preferably 50% by mass or less. In this case, the alcohol content in the water-containing EVOH paste is preferably 0 to 5 parts by weight, more preferably 0 to 1 part by weight, and even more preferably 0 to 0.1 parts by weight. The low alcohol content can prevent the alcohol from volatilizing in a later process such as precipitation, and can protect the working environment and the surrounding environment.

[0037] The hydrous EVOH paste taken out from the device is extruded from a nozzle and cut in a molten state. At this time, the hydrous EVOH paste may be extruded after being kneaded in a kneading device such as a kneader, if necessary. The method of cutting the hydrous EVOH paste is preferably a hot cut method or an underwater cut method. From the viewpoint of easy handling of the pellets, the nozzle diameter is preferably 2 to 5 mmφ (φ is the diameter, the same applies below). The temperature of the hydrous EVOH paste when it is cut is preferably 80 to 170°C. If the temperature inside the container is less than 80°C, the fluidity of the hydrous EVOH paste in the nozzle becomes insufficient. More preferably, it is 100°C or higher, and even more preferably, it is 110°C or higher. On the other hand, if the temperature of the hydrous EVOH paste exceeds 170°C, the EVOH may deteriorate. More preferably, it is 150°C or lower.

[0038] The hydrous EVOH pellets cut in a molten state are cooled with cooling water to solidify. The temperature of the cooling water is preferably 0 to 30°C, more preferably 20°C or less, and even more preferably 10°C or less. The hydrous pellets after cutting are porous, and their moisture content is usually 30 to 60% by mass. The moisture content is preferably 35% by mass or more. Also, the moisture content is preferably 50% by mass or less.

[0039] The hydrous EVOH pellets are obtained by the method described above. The obtained hydrous EVOH pellets may be subjected to a temperature adjustment step in which the pellets are immersed in water or an aqueous alcohol solution at a predetermined temperature to adjust the temperature of the pellets. The alcohol content of the water or aqueous alcohol solution is preferably 50% by mass or less, more preferably 40% by mass or less. The temperature adjustment step may be performed by immersing the hydrous EVOH pellets in water or an aqueous alcohol solution at a constant temperature in a container for a predetermined period of time, or the temperature may be gradually changed while the pellets are in contact with water or an aqueous alcohol solution in a pipe. If necessary, the hydrous EVOH pellets are subjected to a dehydration step after the temperature adjustment step.

[0040] The temperature (T1) of the hydrous EVOH pellets immediately before being subjected to the dehydration step is 0 to 50°C. The temperature (T1) is preferably 5°C or higher, and more preferably 10°C or higher. Meanwhile, the temperature (T1) is preferably 40°C or lower, more preferably 35°C or lower, and may be 30°C or lower. Also, the moisture content (W1) of the hydrous EVOH pellets immediately before being subjected to the dehydration step is 30 to 80% by mass. The moisture content (W1) is preferably 35% by mass or higher. Meanwhile, the moisture content (W1) is preferably 70% by mass or lower.

[0041] In the dehydration step, the hydrous EVOH pellets at a temperature (T1) are immersed in water or an aqueous solution at a temperature (T2) of 30 to 85°C. If the temperature (T2) is too low, the dehydration effect is insufficient. The temperature (T2) is preferably 45°C or higher, more preferably 55°C or higher, even more preferably 65°C or higher, and particularly preferably 72°C or higher. On the other hand, if the temperature (T2) is too high, the pellets stick together during the dehydration treatment. The moisture content (W2) of the intermediate pellets after the dehydration step is 20 to 70% by mass. The moisture content (W2) may be 25% by mass or higher, or may be 30% by mass or higher. On the other hand, the moisture content (W2) is preferably 60% by mass or lower, more preferably 55% by mass or lower, and even more preferably 50% by mass or lower.

[0042] At this time, the temperature (T1), the temperature (T2), the moisture content (W1) and the moisture content (W2) satisfy the following formulas (1) and (2). T2-T1≧5 (1) W2 / W1≦0.98 (2)

[0043] That is, in the present invention, by making the temperature difference (T2-T1) 5°C or more, the moisture content (W2) after the dehydration treatment can be reduced, and the moisture content ratio (W2 / W1) can be made 0.98 or less. Thus, the inventors have found for the first time that the moisture content of hydrous EVOH pellets can be reduced by simply immersing the pellets in hotter water or aqueous solution. Although the mechanism is not entirely clear, it is presumed that when a fluidized EVOH solution or EVOH paste is solidified, the stress that tends to shrink remains and is fixed. Therefore, it is presumed that when a solidified pellet with such residual stress is immersed in hot water or aqueous solution, the stress is relieved and the pellet shrinks, releasing the water in the pellet to the outside, thereby reducing the moisture content of the pellet.

[0044] In the past, in order to dry hydrous EVOH pellets, it was necessary to remove the water by evaporating it using a hot air oven or the like, but because the latent heat of vaporization of water is large, this required a lot of energy.On the other hand, if the pellets are simply contracted by contacting them with high-temperature water, it is only necessary to provide sensible heat sufficient to raise the pellets' temperature, which can greatly reduce the amount of energy required.

[0045] The temperature (T1) and the temperature (T2) preferably satisfy the formula (1a), more preferably satisfy the formula (1b), further preferably satisfy the formula (1c), and particularly preferably satisfy the formula (1d). T2-T1 is usually 90° C. or less. T2-T1≧10 (1a) T2-T1≧20 (1b) T2-T1≧30 (1c) T2-T1≧40 (1d)

[0046] The moisture content (W1) and the moisture content (W2) preferably satisfy the formula (2a), more preferably satisfy the formula (2b), and even more preferably satisfy the formula (2c). W2 / W1 is usually 0.5 or more. W2 / W1≦0.95 (2a) W2 / W1≦0.92 (2b) W2 / W1≦0.89 (2c)

[0047] The method for carrying out the dehydration step is not particularly limited, and may be either a continuous method or a batch method. The shape of the vessel is also not particularly limited, but a tower-type vessel is suitable for the continuous method, and a tank-type vessel is suitable for the batch method. In consideration of production efficiency, the continuous method is industrially preferable. It is preferable to continuously supply the hydrous pellets to the top of a tower-type vessel, continuously introduce water or an aqueous solution into the vessel, contact the hydrous pellets with the water or aqueous solution in the vessel, and continuously extract intermediate pellets from the bottom of the vessel. The water or aqueous solution may be introduced from the bottom of the tower and contacted with the pellets in a countercurrent flow, or may be introduced from the top of the tower and contacted with the pellets in a parallel flow. In the case of countercurrent contact, the water or aqueous solution after contact is taken out from the top of the tower. In the case of parallel contact, the water or aqueous solution after contact is taken out from the bottom of the tower together with the intermediate pellets.

[0048] In the dehydration step, it is preferable to wash the hydrous EVOH pellets to obtain intermediate pellets from which the saponification catalyst residue has been removed. The alcohol solution of EVOH after saponification contains saponification catalyst residues such as sodium acetate, and the residues remain in the hydrous EVOH pellets obtained by solidifying the saponification catalyst residues. By washing the hydrous EVOH pellets with high-temperature water or an aqueous solution, intermediate pellets from which the residues have been removed can be obtained, and the water content of the intermediate pellets can be reduced. The washing liquid used here may be not only water, but also an aqueous solution containing an acid such as acetic acid, or an aqueous solution containing an alcohol such as methanol.

[0049] In the dehydration step, it is also preferable to impregnate the hydrous EVOH pellets with an aqueous solution containing at least one additive selected from carboxylic acids, boron compounds, phosphoric acid compounds, alkali metal salts, and alkaline earth metal salts to obtain intermediate pellets containing the additive. EVOH pellets with improved thermal stability can be obtained by immersing the hydrous EVOH pellets in an aqueous solution containing an acid or its salt to incorporate the additive into the pellets, a so-called chemical treatment step. By carrying out this chemical treatment step in a high-temperature aqueous solution in which the additive is dissolved, intermediate pellets containing the additive can be obtained and the moisture content of the intermediate pellets can be reduced.

[0050] Examples of carboxylic acids used in the chemical treatment include acetic acid, lactic acid, propionic acid, oxalic acid, succinic acid, benzoic acid, and citric acid. Among these, acetic acid is preferred from the standpoint of cost and availability. If the content of the carboxylic acid in the dried pellets is too low, coloring may occur during melt molding, and if the content is too high, interlayer adhesion may be insufficient, so the content is preferably 10 to 5000 ppm. The lower limit of the content of the carboxylic acid is preferably 30 ppm or more, more preferably 50 ppm or more. The upper limit of the content of the carboxylic acid is preferably 1000 ppm or less, more preferably 500 ppm or less.

[0051] Examples of boron compounds include, but are not limited to, boric acids, boric acid esters, borate salts, and boron hydrides. Specifically, examples of boric acids include orthoboric acid, metaboric acid, and tetraboric acid, examples of borate esters include triethyl borate and trimethyl borate, and examples of borates include alkali metal salts, alkaline earth metal salts, and borax of the above-mentioned various boric acids. Among these compounds, orthoboric acid (hereinafter simply referred to as boric acid) is preferred. If the content of the boron compound in the dried pellets is too low, the effect of improving thermal stability is small, and if it is too high, gelation may occur, resulting in poor moldability. Therefore, the content is preferably 10 to 2000 ppm, more preferably 50 to 1000 ppm, in terms of boron.

[0052] Examples of the phosphoric acid compound include various acids such as phosphoric acid and phosphorous acid and their salts. The phosphate may be contained in the form of any of primary phosphate, secondary phosphate, and tertiary phosphate, and the cationic species is not particularly limited, but is preferably an alkali metal salt or an alkaline earth metal salt. Among them, it is preferable to add the phosphoric acid compound in the form of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, and dipotassium hydrogen phosphate. The content of the phosphoric acid compound in the dry pellet is preferably 1 to 1000 ppm in terms of phosphate radical. By adding it in such a range, it is possible to suppress the coloring of the molded product and the generation of gels and bumps. If the content of the phosphoric acid compound is less than 1 ppm, there is a risk of severe coloring during melt molding. Also, if it exceeds 1000 ppm, there is a risk of gels and bumps being easily generated in the molded product.

[0053] Examples of the alkali metal salt include aliphatic carboxylates, aromatic carboxylates, phosphates, and metal complexes of monovalent metals. For example, sodium acetate, potassium acetate, sodium phosphate, lithium phosphate, sodium stearate, potassium stearate, and sodium salt of ethylenediaminetetraacetic acid are listed. Among them, sodium acetate, potassium acetate, and sodium phosphate are preferable. The content of the alkali metal salt in the dry pellets is preferably 5 to 5000 ppm in terms of the alkali metal element. More preferably, it is 20 to 1000 ppm, and further preferably, it is 30 to 750 ppm.

[0054] Examples of alkaline earth metal salts include magnesium salts, calcium salts, barium salts, and beryllium salts, with magnesium salts and calcium salts being particularly preferred. The anion species of the alkaline earth metal salt is not particularly limited, but acetates and phosphates are preferred. The content of alkaline earth metal in the dry pellets is preferably 10 to 1000 ppm, more preferably 20 to 500 ppm, calculated as metal. If the content of alkaline earth metal is less than 10 ppm, the effect of improving long-run properties may be insufficient. If it exceeds 1000 ppm, coloring may become severe when the resin is melted.

[0055] In the dehydration step, a continuous tower-type vessel is suitable as an apparatus for performing washing or chemical treatment. When washing is performed in a tower-type vessel in the dehydration step, it is preferable to contact the washing water and the pellets in a countercurrent flow. On the other hand, when chemical treatment is performed in a tower-type vessel in the dehydration step, the contact may be in a countercurrent flow or in a parallel flow.

[0056] When both washing and chemical treatment are performed, washing is performed first, followed by chemical treatment. In that case, both washing and chemical treatment may be dehydration steps, or only one of them may be a dehydration step. Whether or not a step is a dehydration step is determined based on whether the above-mentioned temperature and moisture content conditions are met.

[0057] In this way, the intermediate pellets whose moisture content has been reduced by the dehydration process can be dried to obtain dried pellets whose moisture content (W3) is 0.5% by mass or less. The drying method is not particularly limited, but the intermediate pellets can be dried by hot air drying, infrared radiation, or a combination thereof to obtain dried pellets. Since the intermediate pellets whose moisture content has been reduced are dried, the required energy can be reduced compared to directly drying the hydrous pellets. By making the moisture content (W3) of the dried pellets 0.5% by mass or less, foaming and deterioration of the resin during melt molding can be prevented.

[0058] The dried EVOH pellets thus obtained are melt-molded into various articles such as films, sheets, containers, pipes, and fibers. EXAMPLES

[0059] The present invention will be specifically described below using examples and comparative examples, but the present invention is not limited to the examples shown below. The measurement and evaluation methods were as follows.

[0060] (1) Moisture content measurement The moisture content of the hydrous EVOH pellets was measured using 3 g of the hydrous EVOH pellets with 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) Stalemate assessment The hydrous EVOH pellets after the dehydration step were visually inspected for the presence or absence of sticking, and were evaluated according to the following criteria. Evaluation: Criteria A: No adhesion between pellets was observed. B: Pellets were observed sticking together.

[0062] [Example 1] From the top of the bubble cap tower with a tower diameter of 0.85 m and the number of stages of 20, a methanol solution of ethylene-vinyl acetate copolymer (ethylene unit content: 32 mol%) (ethylene-vinyl acetate copolymer concentration: 35 mass%) and a methanol solution of sodium hydroxide (sodium hydroxide concentration: 15 mass%) were supplied at 1.3 t / h and 50 kg / h, respectively. In addition, methanol steam at 115°C was blown into the tower from the bottom of the tower at 1.1 t / h, and the by-product methyl acetate was distilled out from the top of the tower together with a part of the methanol. At this time, the temperature in the tower was 110 to 115°C, the pressure was 0.54 MPa, and the residence time of the copolymer in the tower was 30 minutes. In this way, a methanol solution of EVOH with an ethylene unit content of 32 mol% and a saponification degree of 99.5 mol% (EVOH concentration 29 mass%) was obtained from the bottom of the tower.

[0063] A mixed vapor of methanol and water was further blown into the obtained EVOH methanol solution, and the mixed vapor of methanol and water was distilled off to obtain a solution of EVOH in a methanol-water mixed solvent (methanol / water = 65 / 35, mass ratio) (EVOH concentration: 40 mass%). This solution was discharged from a die with a 2 mm diameter hole into a coagulation bath of a 5°C aqueous methanol solution (methanol / water = 10 / 90, mass ratio) and coagulated into strands. The obtained strands were cut with a cutter to obtain cylindrical hydrous EVOH pellets of 2.5 to 3.5 mm.

[0064] The obtained hydrous EVOH pellets were brought into contact with a 25° C. aqueous methanol solution (methanol / water=30 / 70, mass ratio) to adjust the temperature (T1) to 25° C. The water content (W1) of the EVOH pellets whose temperature (T1) reached 25° C. was measured according to the measurement method described in (1) above, and was found to be 57.1% by mass. The results of the temperature adjustment process are shown in Table 1.

[0065] The above-mentioned hydrous EVOH pellets (temperature (T1) 25°C, water content (W1) 57.1% by mass) were introduced into the top of a tower-type washer with a tower diameter of 0.5 m and height of 8 m at a rate of 150 kg / hr together with an aqueous methanol solution (methanol / water = 30 / 70, mass ratio), and water (washing water) at 80°C was introduced into the bottom of the tower at a rate of 200 L / hr. The washing water was overflowed and discharged from the top of the tower, and at the same time, washing water containing the hydrous EVOH pellets and a small amount of methanol was discharged from the bottom of the tower, while continuously performing countercurrent washing of the hydrous EVOH pellets. During the 24-hour continuous operation, the pellets always settled toward the bottom of the tower without being retained or refluxed. The internal temperature (T2) from the middle to the bottom of the tower was 80°C, which was the same as the temperature of the washing water. The hydrous EVOH pellets immediately after being discharged from the bottom of the tower were measured for water content (W2) according to the measurement method described in (1) above. The results of the above washing process are shown in Table 1.

[0066] The hydrous EVOH pellets discharged from the bottom of the tower were introduced into a tower reactor (height 7 m, diameter 1 m) at a rate of 150 kg / hour from the top, while an aqueous solution containing 0.05% by mass of acetic acid, 0.025% by mass of sodium acetate, 0.017% by mass of magnesium acetate, and 0.019% by mass of potassium dihydrogen phosphate at 80°C was introduced from the bottom of the reactor at a rate of 200 L / hour, and the aqueous solution was discharged from the top of the reactor, and at the same time, the treated hydrous EVOH pellets were continuously discharged from the bottom of the reactor. The discharged hydrous EVOH pellets were evaluated for sticking according to the evaluation method described in (2) above. The results of the above chemical treatment process are shown in Table 1.

[0067] The hydrous EVOH pellets after the chemical treatment were dried using a hot air dryer at 80°C for 3 hours, and then at 107°C for 24 hours to obtain dried EVOH pellets. The moisture content (W3) of the obtained dried EVOH pellets was measured according to the measurement method described in (1) above. The results are shown in Table 1.

[0068] [Example 2] Dry EVOH pellets were obtained in the same manner as in Example 1, except that the hydrous EVOH pellets at 5° C. obtained by cutting with a cutter were introduced directly into a tower-type washer and washed without adjusting the temperature to 25° C. The experimental method and the results are summarized in Table 1.

[0069] [ Comparative Example 4 ] Dry EVOH pellets were obtained in the same manner as in Example 1, except that in the washing step, the temperature of the washing water introduced into the tower-type washer was set to 40° C., and in the chemical treatment step, the temperature of the aqueous solution introduced into the tower-type reactor was set to 40° C. The experimental method and the results are summarized in Table 1.

[0070] [Example 4] Dry EVOH pellets were obtained in the same manner as in Example 1, except that in the washing step, the temperature of the washing water introduced into the tower-type washer was set to 25° C. The experimental method and the results are shown in Table 1.

[0071] [Examples 5 to 7] Dry EVOH pellets were obtained in the same manner as in Example 1, except that the ethylene unit contents of the ethylene-vinyl acetate copolymer and the EVOH obtained by saponifying the copolymer were 24 mol % (Example 5), 27 mol % (Example 6), and 44 mol % (Example 7), respectively. The experimental method and the results are shown in Table 1.

[0072] [ Comparative Example 5 ] The EVOH methanol solution obtained in the same manner as in Example 1 was continuously fed at 52 kg / hr from the top of a plate tower with a diameter of 0.3 m and 10 plates, and steam was blown in at 86 kg / hr from the bottom to allow the EVOH solution and steam to come into contact in a countercurrent manner. The temperature in the tower was 130°C, and the pressure in the tower was 0.3 MPa. Methanol steam and steam were distilled off from the top of the tower, and these were condensed in a condenser and recovered as an aqueous methanol solution. A water-containing EVOH composition was extracted from the bottom of the tower. The obtained water-containing EVOH composition (EVOH concentration 60% by mass, water content 40% by mass, methanol content less than 0.1% by mass) was fed at 41 kg / hr to a kneader with a diameter of 50 mm and a length of 660 mm (L / D = 13.2) having a liquid outlet. The screw rotation speed at this time was 100 rpm. The hydrous EVOH composition was then extruded through a die having six holes with a hole diameter of 3 mm, and cut at a distance of 0.05 mm from the die using a hot cutter having four blades and supplied with cooling water at 5° C. The cut hydrous EVOH pellets were cooled with cooling water at 5° C. to obtain solidified hydrous pellets. The rotation speed of the cutter blade was 1200 rpm.

[0073] The obtained hydrous EVOH pellets were subjected to the temperature adjustment step, washing step, chemical treatment step and drying step to obtain dry EVOH pellets in the same manner as in Example 1. The experimental method and the results are shown in Table 1.

[0074] [Comparative Example 1] Dry EVOH pellets were obtained in the same manner as in Example 1, except that in the washing step, the temperature of the washing water introduced into the tower-type washer was set to 30° C., and in the chemical treatment step, the temperature of the aqueous solution introduced into the tower-type reactor was set to 30° C. The experimental method and the results are summarized in Table 1.

[0075] [Comparative Example 2] The hydrous EVOH pellets at 5° C. obtained by cutting with a cutter were introduced directly into a tower washer for washing without adjusting the temperature to 25° C., and dried EVOH pellets were obtained in the same manner as in Example 1, except that in the washing step, the temperature of the washing water introduced into the tower washer was set to 90° C., and in the chemical treatment step, the temperature of the aqueous solution introduced into the tower reactor was set to 90° C. The experimental method and the results are summarized in Table 1.

[0076] [Comparative Example 3] The hydrous EVOH pellets at 5° C. obtained by cutting with a cutter were introduced directly into a tower washer for washing without adjusting the temperature to 25° C., and dried EVOH pellets were obtained in the same manner as in Example 1, except that in the washing step, the temperature of the washing water introduced into the tower washer was set to 20° C., and in the chemical treatment step, the temperature of the aqueous solution introduced into the tower reactor was set to 20° C. The experimental method and the results are summarized in Table 1.

[0077] [Table 1]

Claims

1. Moisture content (W １ ) is 30 to 80 mass%, and the temperature (T １ The ethylene-vinyl alcohol copolymer water-containing pellets having a temperature (T ２ ) is immersed in water or an aqueous solution having a temperature of 30 to 85°C (excluding cases where the temperature is less than 40°C) to measure the water content (W ２ a dehydration step for obtaining intermediate pellets having a content of 20 to 70% by mass; The intermediate pellets are dried to obtain a water content (W ３ A drying step of obtaining dry pellets having a content of 0.5% by mass or less, the hydrous pellets are obtained by extruding a water / alcohol solution of an ethylene-vinyl alcohol copolymer into an aqueous alcohol solution to precipitate the copolymer in the form of strands, and then cutting the strands; The water / alcohol solution of the ethylene-vinyl alcohol copolymer contains 50 parts by mass or more and 1,000 parts by mass or less of alcohol having a boiling point of 100° C. or less per 100 parts by mass of the ethylene-vinyl alcohol copolymer; and A method for producing ethylene-vinyl alcohol copolymer pellets, which satisfies the following formula (1d) and formula (2): T ２ -T １ ≧40 (1d) W ２ / W １ ≦0.98 (2)

2. The moisture content of the hydrous pellet (W １ ) is 40 to 80 mass %, and the temperature (T １ 2. The method for producing pellets according to claim 1, wherein the temperature is 0 to 40° C.

3. In the dehydration step, the temperature (T ２ ) is immersed in water or an aqueous solution having a water content (W ２ 3. The method for producing pellets according to claim 2, wherein the intermediate pellets have a content of 40 to 70 mass %.

4. The method for producing pellets according to any one of claims 1 to 3, wherein in the dehydration step, the hydrous pellets are washed to obtain intermediate pellets from which saponification catalyst residue has been removed.

5. 5. The method for producing pellets according to claim 1, wherein in the dehydration step, the hydrous pellets are impregnated with an aqueous solution containing at least one additive selected from the group consisting of carboxylic acids, boron compounds, phosphoric acid compounds, alkali metal salts, and alkaline earth metal salts to obtain intermediate pellets containing the additive.

6. 6. The method for producing pellets according to claim 1, wherein in the dehydration step, the hydrous pellets are continuously supplied to an upper portion of a tower-shaped vessel, water or an aqueous solution is continuously introduced into the vessel, the hydrous pellets are contacted with the water or the aqueous solution in the vessel, and intermediate pellets are continuously discharged from a bottom portion of the vessel.

7. The method for producing pellets according to any one of claims 1 to 6, wherein in the drying step, the intermediate pellets are dried by hot air drying, infrared radiation, or a combination thereof to obtain dried pellets.

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