Method for producing ethylene-vinyl alcohol copolymer

Abstract

To provide a method for producing EVOH that shortens drying time and concurrently suppresses color degradation of EVOH.SOLUTION: A method for producing an ethylene-vinyl alcohol copolymer includes a drying step for drying a hydrous ethylene-vinyl alcohol copolymer by microwave irradiation under reduced pressure of -96.3 kPaG or more and -0.3 kPaG or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as EVOH) is a material with excellent oxygen barrier properties, oil resistance, anti-static properties, mechanical strength, etc., and is widely used after being molded into films, sheets, containers, etc. The most common method for producing EVOH is to polymerize ethylene and a vinyl ester such as vinyl acetate to obtain an ethylene-vinyl ester copolymer, which is then saponified in an organic solvent containing alcohol in the presence of a saponification catalyst.

[0003] As a method for post-treating an alcohol solution of EVOH obtained by saponification, for example, a method is widely used in which the alcohol solution of EVOH is extruded into a coagulating liquid such as water or a water / methanol solution in the form of strands, the strands are cut into pellets, and then dried to produce EVOH pellets (Patent Document 1).

[0004] Hot air drying is commonly used as a method for drying hydrous EVOH pellets, and a suitable combination of fluidized bed drying, static drying, etc. is widely used (Patent Document 2). Furthermore, Patent Document 3 describes that drying with microwaves can suppress whitening of EVOH and the generation of voids. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-216725 [Patent Document 2] International Publication No. 2019 / 039458 [Patent Document 3] Japanese Patent Application Publication No. 11-291245 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the above-mentioned conventional method for drying hydrous EVOH, it was found that if the drying temperature is increased or the microwave irradiation intensity is strengthened in order to increase production efficiency by shortening the drying time required to reduce the moisture content of the dried EVOH pellets to 0.3 mass% or less, the EVOH tends to become discolored.

[0007] The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for producing EVOH that achieves both a shortened drying time and suppression of deterioration in the hue of EVOH. [Means for solving the problem]

[0008] According to the present invention, the above object is to [1] A method for producing an ethylene-vinyl alcohol copolymer, comprising a drying step of drying a hydrous ethylene-vinyl alcohol copolymer by irradiating it with microwaves under a reduced pressure of -96.3 kPaG or more and -0.3 kPaG or less; [2] The manufacturing method according to [1], wherein the resin temperature of the ethylene-vinyl alcohol copolymer in the drying step is 50°C or higher and 160°C or lower; [3] The method for producing [1] or [2], further comprising a saponification step of saponifying an ethylene-vinyl ester copolymer to obtain an ethylene-vinyl alcohol copolymer; [4] The production method according to [3], further comprising a pelletizing step of obtaining hydrous ethylene-vinyl alcohol copolymer pellets from the solution or paste containing the ethylene-vinyl alcohol copolymer obtained by the saponification step, and comprising the drying step simultaneously with or after the pelletizing step; This is achieved by providing [Effects of the Invention]

[0009] According to the production method of the present invention, it is possible to provide a production method that achieves both a shortened drying time and suppression of deterioration in the hue of EVOH. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention relates to a method for producing EVOH, which includes a drying step in which hydrous EVOH is dried by irradiating it with microwaves under a reduced pressure of -96.3 kPaG or more and -0.3 kPaG or less. This method can shorten the drying time compared to hot air drying or microwave drying, and produces EVOH with reduced deterioration in hue. Therefore, this method is an extremely useful production method that can improve quality and production efficiency. It is particularly noteworthy that while EVOH simply dried under reduced pressure or EVOH dried by microwave irradiation shows almost no reduction in hue deterioration, the combination of these methods significantly reduces hue deterioration. In this specification, kPaG refers to gauge pressure, and a gauge pressure of 1 atmosphere is 0 kPaG.

[0011] In this specification, the hydrous EVOH subjected to the drying step in the production method of the present invention may be referred to as the hydrous EVOH used in the present invention, and the EVOH after the drying step may be referred to as the dried EVOH.

[0012] The hydrous EVOH used in the present invention is preferably in the form of pellets containing EVOH and water. The water content of the hydrous EVOH is preferably 5% by mass or more and 60% by mass or less from the viewpoint of processability into pellets. The water content of the hydrous EVOH can be appropriately adjusted by the conditions for producing the hydrous EVOH. The water content of the EVOH after drying is usually 0.3% by mass or less. When the water content of the EVOH after drying is 0.1% by mass to 0.3% by mass, there is no significant difference in the drying time required to achieve that water content.

[0013] The EVOH constituting the hydrous EVOH used in the present invention (hereinafter sometimes referred to as "EVOH used in the present invention" or "the EVOH") is usually obtained by saponifying an ethylene-vinyl ester copolymer. The ethylene content in the EVOH is preferably 20 to 60 mol%. The ethylene content is more preferably 23 mol% or more, even more preferably 25 mol% or more, and particularly preferably 30 mol% or more. The ethylene content is more preferably 50 mol% or less, even more preferably 45 mol% or less, and particularly preferably 40 mol% or less. The ethylene content and degree of saponification of the EVOH do not substantially change between the end of the saponification step and the end of the subsequent steps described below, and are usually measured after all steps have been completed.

[0014] The method for producing the EVOH will be specifically described below. As mentioned above, the EVOH is usually obtained by saponifying an ethylene-vinyl ester copolymer. The copolymerization of ethylene and vinyl ester may be any of solution polymerization, suspension polymerization, emulsion polymerization, and bulk polymerization. The copolymerization may be either continuous or batchwise, and the polymerization conditions for solution polymerization are as follows:

[0015] The solvent used is preferably an alcohol having a boiling point of 100°C or less, from the viewpoints of solubility of the ethylene-vinyl ester copolymer and EVOH, ease of handling, and ability to efficiently replace alcohol with water, etc. The boiling point is more preferably 80°C or less, and even more preferably 70°C or less.

[0016] Examples of alcohols having a boiling point of 100° C. or less include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, and t-butyl alcohol, with methanol being particularly preferred.

[0017] Examples of initiators that can be used in the polymerization include azonitrile initiators such as 2,2-azobisisobutyronitrile, 2,2-azobis-(2,4-dimethylvaleronitrile), 2,2-azobis-(4-methoxy-2,4-dimethylvaleronitrile), and 2,2-azobis-(2-cyclopropylpropionitrile), and organic peroxide initiators such as isobutyryl peroxide, cumyl peroxy neodecanoate, diisopropyl peroxycarbonate, di-n-propyl peroxydicarbonate, t-butyl peroxy neodecanoate, lauroyl peroxide, benzoyl peroxide, and t-butyl hydroperoxide.

[0018] Examples of vinyl esters include fatty acid vinyl esters such as vinyl acetate, vinyl propionate, and vinyl pivalate, with vinyl acetate being preferred. EVOH can also contain 0.0002 to 0.2 mol % of a vinylsilane compound as a copolymerization component. Examples of vinylsilane compounds include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(β-methoxyethoxy)silane, and γ-methacryloxypropylmethoxysilane. Of these, vinyltrimethoxysilane and vinyltriethoxysilane are preferred.

[0019] The polymerization conditions are preferably as follows: (1) Temperature: preferably 20 to 90°C, more preferably 40 to 780°C. (2) Time (average residence time in the case of a continuous system): preferably 2 to 15 hours, more preferably 3 to 11 hours. (3) Polymerization rate: preferably 10 to 90%, more preferably 30 to 80%, based on the charged vinyl ester. (4) Resin content in the solution after polymerization: preferably 5 to 85% by mass, more preferably 20 to 70% by mass.

[0020] In addition to ethylene and vinyl esters, monomers copolymerizable therewith, for example, α-olefins such as propylene, butylene, isobutylene, pentene, hexene, α-octene, and α-dodecene; 3-acyloxy-1-propene, 3-acyloxy-1-butene, 4-acyloxy-1-butene, 3,4-diacyloxy-1-butene, 3-acyloxy-4-methyl-1-butene, 4-acyloxy-2-methyl-1-butene, 4-acyloxy-3-methyl-1-butene, 3,4-diacyloxy-2-methyl-1-butene, 4-acyloxy-1-pentene, 5-acyloxy-1-pentene, 4,5-diacyloxy-1-pentene, 4-acyloxy-1-hexene, 5-acyloxy-1-pentene, 5-acyloxy-1-hex ... Polymerization can also be performed in the presence of small amounts of alkenes having an ester group, such as 1-hexene, 6-acyloxy-1-hexene, 5,6-diacyloxy-1-hexene, and 1,3-diacetoxy-2-methylenepropane; unsaturated acids, such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, and itaconic acid, as well as their anhydrides, salts, and mono- or dialkyl esters; nitriles, such as acrylonitrile and methacrylonitrile; amides, such as acrylamide and methacrylamide; olefin sulfonic acids, such as ethylene sulfonic acid, allyl sulfonic acid, and methallylsulfonic acid, or their salts; alkyl vinyl ethers, vinyl ketone, N-vinylpyrrolidone, vinyl chloride, and vinylidene chloride. The content of other monomer units in the EVOH other than ethylene, vinyl ester, and vinyl alcohol is preferably 20 mol% or less, and in some cases, 10 mol% or less, 5 mol% or less, 3 mol% or less, 1 mol% or less, or 0.1 mol% or less is preferred. The EVOH may not contain the other monomer units.

[0021] After polymerization for a predetermined time has reached a predetermined polymerization rate, a polymerization inhibitor is added as necessary, unreacted ethylene gas is evaporated and removed, and then unreacted vinyl ester is purged. For example, a method for purging the unreacted vinyl ester may be employed in which the polymerization solution from which ethylene has been removed is continuously fed at a constant rate from the top of a column packed with Raschig rings, vapor of an organic solvent, preferably an alcohol having a boiling point of 100°C or less, and most preferably methanol, is blown into the bottom of the column, a mixed vapor of the organic solvent and unreacted vinyl ester is distilled from the top of the column, and the copolymer solution from which unreacted vinyl ester has been removed is taken out from the bottom of the column.

[0022] An alkali catalyst is added to the copolymer solution from which the unreacted vinyl ester has been removed, and the vinyl ester component in the copolymer is saponified. The saponification method can be either continuous or batchwise. Examples of alkali catalysts that can be used include sodium hydroxide, potassium hydroxide, and alkali metal alcoholates. Methanol is preferred as the solvent for saponification. For example, the saponification conditions are as follows: (1) Concentration of ethylene-vinyl ester copolymer in the solution: 10 to 50% by mass (2) Reaction temperature: 30 to 150°C (3) Amount of catalyst used: 0.005 to 0.6 equivalents (per vinyl ester component) (4) Time (average residence time in the case of continuous type): 10 minutes to 6 hours

[0023] Generally, when saponification is carried out in a continuous system, methyl acetate produced by saponification can be removed more efficiently, and therefore a resin with a higher degree of saponification can be obtained with a smaller amount of catalyst than in a batch system. Furthermore, in the case of a continuous system, saponification must be carried out at a higher temperature to prevent the precipitation of EVOH produced by saponification. Therefore, in a continuous system, it is preferable to use a reaction temperature and catalyst amount within the following ranges: Reaction temperature: 70 to 150°C. Amount of catalyst used: 0.005 to 0.1 equivalents (per vinyl ester component).

[0024] The degree of saponification of the resulting EVOH varies depending on the purpose, but is preferably 80 mol % or more of the vinyl ester component, more preferably 95 mol % or more, even more preferably 98 mol % or more, and particularly preferably 99 mol % or more. The degree of saponification can be adjusted as desired depending on the conditions.

[0025] As described above, when producing EVOH pellets that have particularly excellent melt stability and good long-run properties, the saponification degree of the EVOH is preferably 99.7 mol % or more, more preferably 99.8 mol % or more, even more preferably 99.9 mol % or more, and particularly preferably 99.95 mol % or more. To obtain such an EVOH, it is preferable to further adjust the saponification conditions as follows.

[0026] A continuous process is preferred as a method for obtaining EVOH with a high saponification degree of 99.9 mol% or more. Examples of continuous processes for obtaining a high saponification degree include adding a catalyst from multiple locations in the saponification reaction tower, increasing the amount of catalyst used, and increasing the amount of methanol blown into the bottom of the saponification reaction tower. Examples of batch processes for obtaining EVOH with a high saponification degree of 99.9 mol% or more include adding the catalyst in multiple batches, increasing the amount of catalyst used, and increasing the amount of methanol vapor or nitrogen gas blown into the saponification reaction tank.

[0027] The saponification step yields a solution or paste containing EVOH. The saponification reaction EVOH contains an alkali catalyst, by-product salts such as sodium acetate and potassium acetate, and other impurities, which may be removed by neutralization and washing as necessary. When the saponification reaction EVOH is washed with ion-exchanged water or the like containing almost no metal ions or chloride ions, some catalyst residues such as sodium acetate and potassium acetate may remain in the EVOH.

[0028] The EVOH solution or paste thus obtained typically contains 50 parts by mass or more of an alcohol having a boiling point of 100°C or less per 100 parts by mass of EVOH. The alcohol content is preferably 70 parts by mass or more, more preferably 80 parts by mass or more. The alcohol content is preferably 1000 parts by mass or less, 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.

[0029] The resulting EVOH solution or paste is then hydropelletized. The hydropelletization method is not particularly limited, and examples include a method in which the EVOH solution or paste is cooled and solidified, followed by cutting, or a method in which the EVOH solution or paste is melt-kneaded in an extruder, then discharged, and then cut. Another example is a method in which a mixed vapor of solvent and water is supplied from the bottom of a tower vessel, and the EVOH solution is supplied from a position above the supply position of the mixed vapor, thereby replacing part of the solvent in the supplied EVOH solution with water, thereby obtaining a highly concentrated EVOH solution, which is then melt-kneaded in an extruder, then discharged, and then cut. Specific examples of EVOH cutting methods include a method in which EVOH is extruded into strands and then cut with a pelletizer, and a method in which EVOH is extruded from a die and then cut using a hot cut method or an underwater cut method.

[0030] The hydrous EVOH pellets obtained after the saponification reaction may contain alkali metal salts, which are saponification catalyst residues. However, if the content is large, quality problems such as discoloration may occur. Therefore, a step of washing and removing the saponification catalyst residue may be performed. The washing method is not particularly limited, but examples include a method of washing by immersion in water or an aqueous solution of an acid such as acetic acid, or a method of washing by supplying water or an aqueous solution of an acid into an extruder. In this washing method of immersion in water or an aqueous solution, either a batch-type treatment vessel or a continuous treatment vessel can be used. Among these, a method of continuously supplying pellets to a tower-type vessel and treating them is preferred from the standpoint of productivity. The washing temperature is usually in the range of 10 to 80°C. A higher washing temperature is preferable from the standpoint of improving washing efficiency, but an excessively high temperature is undesirable because it causes fusion between hydrous pellets. The lower limit of the washing temperature is preferably 20°C or higher. The upper limit of the washing temperature is preferably 70°C or lower.

[0031] The content of alkali metal salts in the washed hydrous EVOH pellets is preferably 5% by mass or less, more preferably 1% by mass or less, even more preferably 0.1% by mass or less, even more preferably 0.05% by mass or less, and particularly preferably 0.03% by mass or less, calculated as metal.

[0032] The obtained hydrous EVOH pellets may be subjected to a dehydration step etc. to adjust the moisture content. The dehydration method is not particularly limited, but examples thereof include centrifugal dehydration.

[0033] The obtained hydrous EVOH pellets may be subjected to chemical treatment as needed. Examples of such treatment include impregnating the hydrous EVOH pellets with an aqueous solution containing any additives, or melt-kneading the hydrous EVOH with additives in an extruder. Examples of such additives include carboxylic acids, boron compounds, phosphoric acid compounds, alkali metal salts, and alkaline earth metal salts.

[0034] The hydrous EVOH pellets obtained above can be subjected to the drying step as the hydrous EVOH used in the present invention, or the hydrous EVOH can be subjected to the drying step in the production method of the present invention when pelletized, i.e., pelletization can be performed during the drying step. The drying step in the production method of the present invention is a step in which the hydrous EVOH is dried by irradiating it with microwaves under a reduced pressure of -96.3 kPaG to -0.3 kPaG. The degree of reduced pressure in the drying step is preferably -96.3 kPaG to -15 kPaG, more preferably -96.3 kPaG to -30 kPaG, and even more preferably -96.3 kPaG to -45 kPaG. The method for reducing pressure in the drying step is not particularly limited and known methods can be used, such as a vacuum pump, steam ejector, or water aspirator, with a vacuum pump being preferred in terms of its reducing capacity. The type of vacuum pump is not particularly limited and known methods can be used, with an oil pump or diaphragm pump being preferred in terms of preventing backflow of oil.

[0035] The microwaves irradiated in the drying step of the production method of the present invention may be, for example, a microwave generator having a commercial frequency of 2.45 GHz and an output of 0 to 2000 W, preferably an output of 0 to 1000 W, more preferably an output of 0 to 600 W, and even more preferably an output of 0 to 300 W. A lower maximum output during microwave irradiation tends to suppress deterioration of the EVOH after drying, but if the maximum value is too low, the drying efficiency decreases. Therefore, the maximum output during microwave irradiation is preferably 100 W or more. The microwave irradiation time is preferably such that the resin temperature reaches 50°C or higher and 160°C or lower. If the resin temperature is lower than 50°C, the irradiation effect cannot be expected, while if the resin temperature exceeds 150°C, the EVOH may be overheated, causing fusion and deterioration.

[0036] From the viewpoint of drying efficiency, the temperature of the EVOH in the drying step is preferably 50°C or higher and 160°C or lower. The temperature of the EVOH is more preferably 80°C or higher and 150°C or lower, and even more preferably 100°C or higher and 140°C or lower. The temperature of the EVOH in the drying step can be adjusted by intermittently irradiating it with microwaves in the intensity range of 0 to 1000W.

[0037] The melt flow rate (MFR) (190°C, load 2160 g) of the obtained EVOH is preferably 0.5 to 100 g / 10 min, more preferably 1 to 50 g / 10 min, and even more preferably 1.5 to 20 g / 10 min. When the MFR is 0.5 g / 10 min or more, moldability tends to be good, and when the MFR is 100 g / 10 min or less, the mechanical properties of the obtained molded article tend to be good. The MFR is measured by the method described in the examples below.

[0038] The resulting EVOH can be melt-molded into various shapes such as films, sheets, containers, pipes, and fibers. [Example]

[0039] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these.

[0040] [Evaluation method] (1) Moisture content of EVOH 2.00 g of each of the hydrous EVOH pellets used in the Examples and Comparative Examples and the dried EVOH pellets obtained in the Examples and Comparative Examples were weighed, and the moisture content was measured at 220°C using a moisture measuring device CA-310 manufactured by Nitto Seiko Analytech Co., Ltd. in accordance with the method described in JIS K 0068:2001.

[0041] (2) Yellowness index (YI) after melt molding 8.9 g of each dried EVOH pellet obtained in the Examples and Comparative Examples was weighed and loaded into a melt indexer (L227-E300, manufactured by Tateyama Chemical Co., Ltd.) with the furnace temperature set to 220°C. After compressing with a piston, a 2010 g weight was placed on the melt indexer and held for 6 minutes. The resin was then completely discharged from the bottom of the melt indexer onto a stainless steel plate covered with aluminum foil and pressed down between another stainless steel plate to a thickness of 3 mm. After cooling, the YI was measured using a colorimeter (LAB Scan XE, manufactured by Hunter). Under the conditions shown in Table 1, where the resin temperature was 110°C and the ethylene unit content was 27 mol%, a YI of 22 or less after melt molding was considered to be balanced between the drying rate and YI. Furthermore, under the conditions shown in Table 2, where the resin temperature was 130°C and the ethylene unit content was 27 mol%, a YI of 30 or less after melt molding was considered to be balanced between the drying rate and YI. Furthermore, under the conditions in Table 3 where the resin temperature is 130°C and the ethylene unit content is 32 mol%, if the YI after melt molding is 10 or less, it was determined that the drying rate and YI are well balanced.

[0042] (3) Calculation of drying rate The drying rate was calculated from the difference in moisture content before and after drying and the drying time obtained in the examples and comparative examples.

[0043] (4) MFR The MFR of the dried EVOH pellets obtained in the Examples and Comparative Examples was measured according to the method described in JIS K7210:2014. Specifically, the resin composition was filled into a 9.55 mm inner diameter, 162 mm long cylinder of a melt indexer L244 (manufactured by Takara Kogyo Co., Ltd.) and melted at a specific temperature. A load of 2,160 g was applied uniformly to the molten resin composition using a 9.48 mm diameter plunger. The amount of resin composition extruded per unit time (g / 10 min) was measured through a 2.1 mm diameter orifice located in the center of the cylinder. For Examples 1 and 2 and Comparative Examples 1 to 5, which had an ethylene unit content of 27 mol%, the melting temperature was set to 210°C. For Examples 3, 4, and Comparative Example 6, which had an ethylene unit content of 32 mol%, the melting temperature was set to 190°C.

[0044] [Example 1] 40 g of hydrous EVOH pellets (11.27% by mass) with an ethylene unit content of 27 mol% and a degree of saponification of 99.9% were introduced into a 150 mL drying apparatus equipped with a magnetic stirrer, gas inlet, and pressure regulator. The hydrous EVOH pellets were dried by microwave irradiation using a μReactor EX microwave irradiation system (manufactured by Shikoku Keisoku Kogyo Co., Ltd.) under stirring at 500 rpm, a pressure inside the drying apparatus of -94.6 kPaG, and an air (sweep gas) inflow rate of 400 mL / min from the gas inlet. Dried EVOH pellets were obtained. Microwave irradiation was performed for 16.5 hours, with the output adjusted between 0 and 300 W to maintain the EVOH pellet temperature at 110°C. The resulting dried EVOH pellets were evaluated for moisture content, YI after melt molding, drying rate, and MFR according to the evaluation methods (1) to (4) described above. The results are shown in Table 1.

[0045] [Comparative Example 1] Dried EVOH was produced and evaluated in the same manner as in Example 1, except that the pressure in the drying apparatus and the drying time were changed as shown in Table 1. The results are shown in Table 1.

[0046] Comparative Example 2 40 g of the water-containing EVOH pellets (water content 11.27% by mass) with an ethylene unit content of 27 mol% used in Example 1 were placed in a 500 ml beaker and left to stand in a hot air dryer at 110°C for 24 hours to obtain dried EVOH pellets. The obtained dried EVOH pellets were evaluated for water content, YI after melt molding, drying rate, and MFR according to the above evaluation methods (1) to (4). The results are shown in Table 1. Note that since air is circulated during hot air drying, no air was introduced as a sweep gas, and therefore the air flow rate in the table is indicated as "-".

[0047] Comparative Example 3 Dried EVOH pellets were prepared and evaluated in the same manner as in Comparative Example 1, except that the pressure in the dryer and the drying time were changed as shown in Table 1. The results are shown in Table 1.

[0048] [Table 1]

[0049] A comparison between Example 1 and Comparative Example 1 shows that microwave drying under reduced pressure conditions improves the YI after melt-forming and accelerates the drying rate. On the other hand, a comparison between Comparative Example 1 and Comparative Example 2 shows that microwave heating under normal pressure accelerates the drying rate, but the difference in YI after melt-forming is small, and the benefit of microwave heating is not that great. Furthermore, a comparison between Comparative Example 2 and Comparative Example 3 shows that reduced-pressure drying accelerates the drying rate, but the difference in YI after melt-forming is small, and the benefit of reduced-pressure drying alone is not that great.

[0050] [Example 2] Dried EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the microwave irradiation conditions were adjusted so that the resin temperature was 130°C and the drying time was set to 7.5 hours. The results are shown in Table 2.

[0051] Comparative Example 4 Dried EVOH pellets were prepared and evaluated in the same manner as in Example 2, except that the pressure in the dryer and the drying time were changed as shown in Table 2. The results are shown in Table 2.

[0052] Comparative Example 5 Dried EVOH pellets were prepared and evaluated in the same manner as in Comparative Example 2, except that the resin temperature and drying time were changed as shown in Table 2. The results are shown in Table 2.

[0053] [Table 2]

[0054] A comparison between Example 2 and Comparative Example 4 shows that microwave drying under reduced pressure conditions improves the YI after melt-forming and increases the drying rate. Furthermore, a comparison between Example 2 and Comparative Example 5 shows that reduced-pressure microwave drying provides significantly better YI after melt-forming and a faster drying rate than commonly used normal-pressure hot-air drying.

[0055] [Examples 3 and 4, Comparative Example 6] Dried EVOH pellets were prepared and evaluated in the same manner as in Example 1, except that the moisture contents before and after drying and the drying conditions were changed as shown in Table 3. The results are shown in Table 3.

[0056] [Table 3]

Claims

1. a drying step of drying the hydrous ethylene-vinyl alcohol copolymer by irradiating it with microwaves under a reduced pressure of -96.3 kPaG or more and -0.3 kPaG or less, The resin temperature of the ethylene-vinyl alcohol copolymer in the drying step is 50°C or higher and 150°C or lower, and A method for producing an ethylene-vinyl alcohol copolymer, wherein the water content of the ethylene-vinyl alcohol copolymer after drying is 0.3 mass% or less.

2. A manufacturing method as described in claim 1, wherein the microwaves are irradiated intermittently during the drying process.

3. A manufacturing method described in claim 1 or 2, wherein the microwaves are irradiated for 7 hours or more and 16.5 hours or less in the drying process.

4. 3. The method according to claim 1, further comprising a saponification step of saponifying the ethylene-vinyl ester copolymer to obtain an ethylene-vinyl alcohol copolymer.

5. The method according to claim 4, further comprising a pelletizing step of obtaining hydrous ethylene-vinyl alcohol copolymer pellets from the solution or paste containing the ethylene-vinyl alcohol copolymer obtained in the saponification step, and the drying step is carried out simultaneously with or after the pelletizing step.

6. A method for drying a hydrous ethylene-vinyl alcohol copolymer, comprising irradiating a hydrous ethylene-vinyl alcohol copolymer with microwaves under a reduced pressure of -96.3 kPaG or more and -0.3 kPaG or less at a resin temperature of the ethylene-vinyl alcohol copolymer of 50°C or more and 150°C or less, to dry the copolymer so that the water content of the ethylene-vinyl alcohol copolymer after drying is 0.3 mass% or less.

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