JP7650652B2 - Method for producing ethylene-vinyl alcohol copolymer pellets - Google Patents

Description

The present invention relates to a method for producing ethylene-vinyl alcohol copolymer pellets. In particular, the present invention relates to a method for producing ethylene-vinyl alcohol copolymer pellets that can reduce energy consumption when drying wet pellets to produce dry pellets.

In a typical manufacturing method for 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 alkaline 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.

The hydrous EVOH pellets thus obtained 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 from the pellets.

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 salts thereof, into dry EVOH pellets. For this purpose, a method is adopted in which hydrous EVOH pellets are immersed in an aqueous solution containing such acids or salts to incorporate the acid or salt into the pellets. This type of process for incorporating acids or salts into hydrous pellets is called chemical processing. For example, Patent Document 2 describes a chemical processing method for hydrous EVOH pellets, in which a tower-type container 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.

JP 2002-60413 A Japanese Patent Application Publication No. 11-152307

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 thoroughly dried. However, because 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.

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 of immersing 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 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;
The method for producing ethylene-vinyl alcohol copolymer pellets includes a drying step of drying the intermediate pellets to obtain dried pellets having a moisture content (W ₃ ) of 0.5 mass % or less, and satisfies the following formulas (1) and (2):
T ₂ −T ₁ ≧5 (1)
W ₂ /W ₁ ≦0.98 (2)

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 (W ₁ ) of 40 to 80% by mass and a temperature (T ₁ ) 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 (T ₂ ) of 30 to 85° C. to obtain intermediate pellets having a moisture content (W ₂ ) of 40 to 70% by mass.

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 the cut paste, and have a moisture content (W ₁ ) of 30 to 60% by mass and a temperature (T ₁ ) of 20 to 50° 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 (T ₂ ) of 40 to 85° C. to obtain intermediate pellets having a moisture content (W ₂ ) of 20 to 50% by mass.

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-type vessel, continuously introduce water or an aqueous solution into the vessel, bring the hydrous pellets into contact with the water or the aqueous solution in the vessel, and continuously extract 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.

The method for producing ethylene-vinyl alcohol copolymer pellets of the present invention can reduce the energy required to dry the water-containing EVOH pellets.

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 of immersing 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 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;
The method for producing ethylene-vinyl alcohol copolymer pellets includes a drying step of drying the intermediate pellets to obtain dried pellets having a moisture content (W ₃ ) of 0.5 mass % or less, and satisfies the following formulas (1) and (2):
T ₂ −T ₁ ≧5 (1)
W ₂ /W ₁ ≦0.98 (2)

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 with 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.

First, the synthesis method of EVOH will be explained. EVOH is obtained by saponifying an ethylene-vinyl ester copolymer. As the vinyl ester, fatty acid vinyl esters such as vinyl acetate, vinyl propionate, and vinyl pivalate can be used, with vinyl acetate being preferred. The polymerization of ethylene and the vinyl ester may be by solution polymerization, suspension polymerization, emulsion polymerization, or bulk polymerization, with solution polymerization being preferred. The polymerization may be by either a continuous or batch system.

As the solvent used in the case of solution polymerization, alcohol is preferred, but other organic solvents capable of dissolving ethylene, vinyl ester, and ethylene-vinyl acetate copolymer (such as dimethyl sulfoxide) can also be used. As the alcohol, methanol, ethanol, 1-propanol, 2-propanol, etc. can be used, with methanol being particularly preferred.

Polymerization catalysts that can be used include azonitrile 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), as well as organic peroxide 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.

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 vinyl ester charged. 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 amount of the other monomers are the same as those of EVOH, which will be described later.

After a predetermined time of polymerization, when a predetermined polymerization rate is reached, a polymerization inhibitor is added as necessary, unreacted ethylene gas is evaporated and removed, and then unreacted vinyl ester is expelled. As a method for expelling unreacted vinyl ester from the ethylene-vinyl ester copolymer solution from which ethylene has been evaporated and removed, 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, 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 the unreacted vinyl ester has been removed is taken out from the bottom of the tower.

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 use a tower reactor to carry out the saponification reaction continuously and to extract the EVOH solution from the bottom of the tower. Examples of alkali catalysts that can be used include sodium hydroxide, potassium hydroxide, and alkali metal alcoholates. Furthermore, methanol is preferable as the solvent used for saponification, 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.

The preferred saponification conditions are as follows. The ethylene-vinyl ester copolymer concentration 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, and an EVOH solution is obtained. The EVOH content in the solution is preferably 10 to 50% by mass, more preferably 20 to 45% by mass.

The degree of saponification of the vinyl ester units of the EVOH thus obtained 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, even more preferably 98 mol% or more, and particularly preferably 99 mol% or more. If the degree of saponification is less than 80 mol%, there is a risk that the barrier properties, long-run properties, and moisture resistance will deteriorate.

From the viewpoint of obtaining a molded product having excellent barrier properties and melt moldability, the ethylene unit content of the EVOH of the present invention is preferably 5 to 60 mol%. If the ethylene unit content is less than 5 mol%, the melt moldability may be deteriorated. 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 properties may be insufficient. The ethylene unit content is more preferably 55 mol% or less, and even more preferably 50 mol% or less.

EVOH can also be copolymerized with small amounts 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, 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 suitable EVOH monomers 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, and methallyl sulfonic acid, or 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, and vinylidene chloride. When EVOH contains other monomer units, the copolymerization amount thereof is preferably 10 mol % or less, more preferably 5 mol % or less.

There are no particular limitations on the method for obtaining hydrous EVOH pellets from the EVOH alcohol solution, preferably methanol solution, obtained as described above, but the following two methods are preferred.

One method for obtaining hydrous EVOH pellets is to replace part of the alcohol in an alcohol solution of EVOH after saponification with water, and extrude the resulting water/alcohol solution of EVOH into an aqueous alcohol solution to precipitate strands, which are then cut. Here, the water/alcohol solution of EVOH refers to EVOH dissolved in a mixed solvent of water and alcohol.

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 the solvent (alcohol) and water to the alcohol solution in a container and vaporizing 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 discharged 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 EVOH with a high concentration 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 with a boiling point of 130°C or less, and examples of such alcohol include alcohols such as methanol, ethanol, propanol, and butanol. Alcohols with a boiling point of 100°C or less are more preferable, and among them, methanol is preferable because it is easily available, inexpensive, has a low boiling point, and is easy to handle.

The water/alcohol solution of EVOH obtained by solvent replacement in this manner contains, for every 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 even 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.

The water/alcohol solution of EVOH thus obtained is extruded into a coagulation bath consisting of water or an aqueous alcohol solution, where it is 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. This mass ratio is more preferably 2/98 or more, and even more preferably 5/95 or more. On the other hand, this 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 their moisture content 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.

The lower limit of the temperature at which the EVOH water/alcohol solution 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 EVOH water/alcohol solution is extruded into the coagulation liquid in the form of strands 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 EVOH water/alcohol solution is extruded in the form of strands from the nozzle. At this time, the strand does not necessarily have to be a single 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, it may be washed as necessary as described below. The size of such 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, and 1 mm to 10 mm in diameter in the case of a spherical shape.

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.

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 large-scale processing, it is preferable to contact the EVOH with water in a container, and in particular 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 the water vapor.

There is no particular limitation on the method for contacting the alcohol solution of EVOH introduced into the container with water vapor within the container, and either a continuous method or a batch method may be used. There is also no particular limitation on the shape of the container, but a tower-type container is preferable for the continuous method, and a tank-type container is preferable for the batch method. From the viewpoint of production efficiency, the continuous method is industrially preferable. Examples of tower-type containers include plate towers such as perforated plate towers and bubble cap towers, and packed towers containing ring-type packing.

From the viewpoint of alcohol removal efficiency, it is preferable that the alcohol solution of EVOH introduced into the container and the water vapor are brought into contact with each other in a countercurrent manner. For example, a preferred embodiment is a method in which, in a tower-type container, an alcohol solution of EVOH is introduced from the top of the tower, water vapor is introduced from the bottom of the tower and brought into countercurrent contact with the solution, a water-containing EVOH paste is discharged from the bottom of the tower, and the alcohol is discharged from the top of the tower together with the water vapor. The alcohol vapor and water vapor discharged from the top of the tower 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 container 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 container while maintaining its fluidity and without gelling.

If the amount of water vapor introduced into the container is too small, the alcohol removal efficiency will be poor, and conversely, 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 water vapor introduced relative to the amount of the alcohol solution of EVOH introduced, expressed as a weight ratio. The water vapor that is brought into contact with the alcohol solution of EVOH may contain up to 10% by weight of alcohol, but in order to remove alcohol efficiently, it is preferable that the water vapor does not contain alcohol.

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 will be insufficient, and there is a risk of gelling or blockage inside the container. A temperature of 100°C or higher is more preferable, and 110°C or higher is even more preferable. On the other hand, if the temperature inside the container exceeds 170°C, the EVOH may deteriorate. A temperature of 160°C or lower is more preferable, and 150°C or lower is even more preferable.

The water content of the water-containing EVOH paste discharged from the device is preferably 30 to 60% by weight. If the water content is less than 30% by weight, the fluidity in the container is insufficient. It is preferably 35% by weight or more. On the other hand, if the water content exceeds 60% by weight, the pellets cut when cut in a molten state may fuse together or become non-uniform in shape. It is preferably 50% by weight 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 prevents alcohol from volatilizing in subsequent processes such as precipitation, making it possible to protect the working environment and the surrounding environment.

The water-containing EVOH paste taken out of the device is extruded from the nozzle and cut in a molten state. At this time, the paste may be kneaded in a kneader or other kneading device before being extruded, if necessary. Suitable methods for cutting the water-containing EVOH paste include a hot cut method and an underwater cut method. From the viewpoint of ease of handling the pellets, the nozzle diameter is preferably 2 to 5 mmφ (φ is the diameter; the same applies below). The temperature of the water-containing 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 water-containing 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 water-containing EVOH paste exceeds 170°C, the EVOH may deteriorate. More preferably, it is 150°C or lower.

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. The moisture content is also preferably 50% by mass or less.

The water-containing EVOH pellets are obtained by the method described above. The water-containing EVOH pellets obtained may be subjected to a temperature adjustment process in which the temperature of the pellets is adjusted by immersing them in water or an aqueous alcohol solution at a predetermined temperature. The alcohol content of the water or aqueous alcohol solution is preferably 50% by mass or less, and more preferably 40% by mass or less. The temperature adjustment process may be performed by immersing the water-containing 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 water-containing EVOH pellets are subjected to a dehydration process after undergoing the temperature adjustment process.

The temperature (T ₁ ) of the water-containing EVOH pellets immediately before being subjected to the dehydration step is 0 to 50° C. The temperature (T ₁ ) is preferably 5° C. or higher, and more preferably 10° C. or higher. Meanwhile, the temperature (T ₁ ) is preferably 40° C. or lower, more preferably 35° C. or lower, and may be 30° C. or lower. Also, the water content (W ₁ ) of the water-containing EVOH pellets immediately before being subjected to the dehydration step is 30 to 80% by mass. The water content (W ₁ ) is preferably 35% by mass or higher. Meanwhile, the water content (W ₁ ) is preferably 70% by mass or lower.

In the dehydration step, the hydrous EVOH pellets at a temperature (T ₁ ) are immersed in water or an aqueous solution at a temperature (T ₂ ) of 30 to 85° C. If the temperature (T ₂ ) is too low, the dehydration effect is insufficient. The temperature (T ₂ ) 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 (T ₂ ) is too high, the pellets will stick together during the dehydration treatment. The moisture content (W ₂ ) of the intermediate pellets after the dehydration step is 20 to 70% by mass. The moisture content (W ₂ ) may be 25% by mass or higher, or may be 30% by mass or higher. On the other hand, the moisture content (W ₂ ) is preferably 60% by mass or lower, more preferably 55% by mass or lower, and even more preferably 50% by mass or lower.

At this time, the temperature (T ₁ ), the temperature (T ₂ ), the moisture content (W ₁ ) and the moisture content (W ₂ ) satisfy the following formulas (1) and (2).
T ₂ −T ₁ ≧5 (1)
W ₂ /W ₁ ≦0.98 (2)

That is, in the present invention, by making the temperature difference (T ₂ -T ₁ ) 5°C or more, the moisture content (W ₂ ) after dehydration can be reduced, and the moisture content ratio (W ₂ /W ₁ ) can be made 0.98 or less. In this way, the inventors have now found for the first time that the moisture content of hydrous EVOH pellets can be reduced simply by 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 would cause the pellets to shrink remains and is fixed. Therefore, it is presumed that when solidified pellets with such residual stress are immersed in hot water or aqueous solution, the stress is relieved and the pellets shrink, causing the water in the pellets to be released to the outside, thereby reducing the moisture content of the pellets.

Until now, in order to dry hydrous EVOH pellets, the water had to be removed by vaporizing it using a hot air oven or similar, but because water has a large latent heat of vaporization, this required a lot of energy. On the other hand, if the pellets are simply contracted by coming into contact with high-temperature water, it is only necessary to provide the sensible heat necessary to raise the pellets' temperature, which can greatly reduce the amount of energy required.

The temperature (T ₁ ) and the temperature (T ₂ ) preferably satisfy the formula (1a), more preferably satisfy the formula (1b), further preferably satisfy the formula (1c), and particularly preferably satisfy the formula (1d). _{T 2} -T ₁ is usually 90°C or less.
T ₂ −T ₁ ≧10 (1a)
T ₂ −T ₁ ≧20 (1b)
T ₂ −T ₁ ≧30 (1c)
T ₂ −T ₁ ≧40 (1d)

The moisture content (W ₁ ) and the moisture content (W ₂ ) preferably satisfy the formula (2a), more preferably satisfy the formula (2b), and even more preferably satisfy the formula (2c). _{W 2} /W ₁ is usually 0.5 or more.
W ₂ /W ₁ ≦0.95 (2a)
W ₂ /W ₁ ≦0.92 (2b)
W ₂ /W ₁ ≦0.89 (2c)

The method for carrying out the dehydration step 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 suitable for the continuous method, and a tank-type vessel is suitable for the batch method. In terms of production efficiency, the continuous method is preferable industrially. 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.

In the above 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 these residues remain in the hydrous EVOH pellets obtained by solidifying it. By washing the EVOH in 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 can 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.

In addition, in the above 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, etc. can be obtained by going through a so-called chemical treatment step in which the hydrous EVOH pellets are immersed in an aqueous solution containing an acid or its salt to incorporate the additive into the pellets. 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.

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 in terms of cost and availability. If the carboxylic acid content in the dried pellets is too low, discoloration may occur during melt molding, and if the content is too high, interlayer adhesion may be insufficient. Therefore, the carboxylic acid content is preferably 10 to 5000 ppm. The lower limit of the carboxylic acid content is preferably 30 ppm or more, and more preferably 50 ppm or more. The upper limit of the carboxylic acid content is preferably 1000 ppm or less, and more preferably 500 ppm or less.

Boron compounds include, but are not limited to, boric acids, boric acid esters, borate salts, and boron hydrides. Specifically, boric acids include orthoboric acid, metaboric acid, and tetraboric acid, borate esters include triethyl borate and trimethyl borate, and borates include alkali metal salts, alkaline earth metal salts, and borax of the above-mentioned various borates. 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 of the boron compound in the dried pellets is preferably 10 to 2000 ppm, more preferably 50 to 1000 ppm, in terms of boron.

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 primary phosphate, secondary phosphate, or 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, or dipotassium hydrogen phosphate. The content of the phosphoric acid compound in the dried pellets 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 occurrence 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 the molded product being more likely to have gels and bumps.

Examples of alkali metal salts include aliphatic carboxylates, aromatic carboxylates, phosphates, and metal complexes of monovalent metals. Examples include sodium acetate, potassium acetate, sodium phosphate, lithium phosphate, sodium stearate, potassium stearate, and sodium salts of ethylenediaminetetraacetic acid. Of these, sodium acetate, potassium acetate, and sodium phosphate are preferred. The content of the alkali metal salt in the dried pellets is preferably 5 to 5,000 ppm in terms of the alkali metal element. It is more preferably 20 to 1,000 ppm, and even more preferably 30 to 750 ppm.

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 alkaline earth metal content in the dry pellets is preferably 10 to 1000 ppm, more preferably 20 to 500 ppm, calculated as metal. If the alkaline earth metal content 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.

In the dehydration step, a continuous tower-type vessel is suitable as an apparatus for performing washing and 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, contact may be performed in a countercurrent flow or in a parallel flow.

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 processes, or only one of them may be a dehydration process. Whether or not a process is a dehydration process is determined by whether the above-mentioned temperature and moisture content conditions are met.

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.

The dried EVOH pellets obtained in this way are melt molded into various molded articles such as films, sheets, containers, pipes, and fibers.

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.

(1) Measurement of Moisture Content Using 3 g of the hydrous EVOH pellets, the moisture content of the hydrous EVOH pellets was 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.

(2) Evaluation of Sticking 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.

[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-produced 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.

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 extruded from a die with a 2 mm diameter hole into a coagulation bath consisting of an aqueous methanol solution (methanol/water = 10/90, mass ratio) at 5°C 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.

The obtained hydrous EVOH pellets were brought into contact with an _{aqueous methanol solution (methanol/water=30/70, mass ratio) at 25° C. to adjust the temperature (T 1 )} of the pellets to 25° C. The water content (W 1 ) of the EVOH pellets whose temperature (T ₁ ) reached 25° C. was measured according to the measurement method described in (1) above, and was found to be 57.1 mass %. The results of the temperature adjustment step are shown in Table 1.

The above-mentioned hydrous EVOH pellets (temperature (T ₁ ) 25° C., water content (W ₁ ) 57.1% by mass) were introduced into the top of a tower-type washer having a tower diameter of 0.5 m and a 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 allowed to overflow 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 continuous operation for 24 hours, the pellets did not stagnate or reflux, and always settled toward the bottom of the tower. The internal temperature (T ₂ ) 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 (W ₂ ) according to the measurement method described in (1) above. The results of the above cleaning steps are shown in Table 1.

The hydrous EVOH pellets discharged from the bottom of the tower were introduced into a tower reactor (height 7 m, diameter 1 m) from the top at a rate of 150 kg/hour, while an aqueous solution at 80°C 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 was introduced into the bottom of the reactor at a rate of 200 L/hour. 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 chemical treatment process are shown in Table 1.

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 (W ₃ ) of the obtained dried EVOH pellets was measured according to the measurement method described in (1) above. The results are shown in Table 1.

[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 for washing without adjusting the temperature to 25° C. The experimental method and the results are shown in Table 1.

[ 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 shown in Table 1.

[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 washing device was set to 25° C. The experimental method and the results are shown in Table 1.

[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 ethylene-vinyl acetate copolymer were 24 mol % (Example 5), 27 mol % (Example 6), and 44 mol % (Example 7), respectively. The experimental methods and results are shown in Table 1.

[ 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 having 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 with each other 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 were condensed in a condenser to be 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 having a diameter of 50 mm and a length of 660 mm (L/D=13.2) and having a liquid outlet. The screw rotation speed at this time was 100 rpm. Subsequently, this water-containing EVOH composition was 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 by a hot cutter having four blades and supplied with cooling water at 5° C. The cut water-containing EVOH pellets were cooled with cooling water at 5° C. to obtain solidified water-containing pellets. The rotation speed of the cutter blade was 1200 rpm.

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

[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 shown in Table 1.

[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 results are shown in Table 1.

[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 results are shown in Table 1.

Claims (7)

a dehydration step of immersing ethylene-vinyl alcohol copolymer hydrous pellets having a moisture content (W ₁ ) of 30 to 80% by mass and a temperature (T ₁ ) of 0 to 50° C. (excluding cases where the temperature exceeds 45° C.) in water or an aqueous solution having a temperature (T ₂ ) of 30 to 85° C. (excluding cases where the temperature is less than 40° C.) to obtain intermediate pellets having a moisture content (W ₂ ) 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 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 satisfies the following formulas (1d) and (2):
T ₂ −T ₁ ≧40 (1d)
W ₂ /W ₁ ≦0.98 (2) The method for producing pellets according to claim 1, wherein the water - containing pellets have a water content (W ₁ ) of 40 to 80 mass % and a temperature (T ₁ ) of 0 to 40°C. 3. The method for producing pellets according to claim 2, wherein in the dehydration step, intermediate pellets having a moisture content (W ₂ ) of 40 to 70 mass % are obtained by immersing in water or an aqueous solution having a temperature (T ₂ ) of 30 to 85° C. 4. The method for producing pellets according to claim 1 , wherein in the dehydration step, the hydrated pellets are washed to obtain intermediate pellets from which a saponification catalyst residue has been removed. 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. 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 of the vessel. 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.