Methods for manufacturing ethylene-vinyl alcohol copolymers

Abstract

This invention provides an ethylene-vinyl alcohol copolymer with a low content of ethylene structural units, which can reduce the coloring of melt-molded articles. An ethylene-vinyl alcohol copolymer is an ethylene-vinyl alcohol copolymer with an ethylene structural unit content of 1 to 16.5 mol%, wherein the yellowness of the aforementioned ethylene-vinyl alcohol copolymer is 80 or less in the following yellowness test. [Yellowness Test] After heating ethylene-vinyl alcohol copolymer particles in air at 170°C for 5 hours, the yellowness is determined using a reflectance measurement method according to JIS K7373.

Description

Methods for manufacturing ethylene-vinyl alcohol copolymers This invention relates to ethylene-vinyl alcohol copolymers and methods for manufacturing ethylene-vinyl alcohol copolymers. Previously, attempts were made to use polyvinyl alcohol-based resins in melt molding to create food packaging materials that maintain transparency while also providing gas barrier properties, aroma retention, solvent resistance, and oil resistance. Now, there is a need to improve melt moldability by reducing changes in melt viscosity over time, while also minimizing coloring of the melt-molded product, thereby increasing its value as a food packaging material. For example, Patent Document 1 proposes that by making the carbonyl group present in the resin to a certain level and controlling the concentration of at least one of carboxylic acid and alkali metal salt and alkaline earth metal salt, it is possible to achieve the stability of coloring and melt viscosity over time in the melt molding of polyvinyl alcohol resin, and uses the absorbance at 280 nm and pH of a 4% by mass aqueous solution of the resin as its indicators. On the other hand, polyvinyl alcohol-based resins have high melting points and are prone to decomposition in the molten state, so various attempts have been made to lower the melting point. In particular, many attempts have been made to improve quality by copolymerizing ethylene to create ethylene-vinyl alcohol copolymers, which makes melt molding easier and further reduces coloring. For example, Patent Document 2 proposes using an aliphatic alcohol with 4 or fewer carbon atoms as a polymerization solvent in the manufacture of an ethylene-vinyl acetate copolymer with an ethylene content of 5 to 60 moles. The content of acetaldehyde relative to vinyl acetate and the content of saturated acetate are made to be below a specific value, and polymerization is carried out at a specific temperature. This reduces the coloring and gelling of the ethylene-vinyl alcohol copolymer obtained after saponification. Furthermore, Patent Document 3 proposes that in the manufacture of an ethylene-vinyl acetate copolymer with an ethylene content of 5 to 60 mol%, a compound with a molecular weight of less than 1000 and having conjugated double bonds is added after polymerization, resulting in an ethylene-vinyl alcohol copolymer containing 0.1 to 3000 ppm of a compound with a molecular weight of less than 1000 and having conjugated double bonds after saponification. Furthermore, Patent Document 4 proposes a film coating agent comprising a water-soluble polyvinyl alcohol-based polymer containing 3-19 moles of ethylene. [Prior Art Documents] [Patent Documents] Patent Document 1: Japanese Patent Application Publication No. 2011-241234; Patent Document 2: Japanese Patent Application Publication No. 2002-194009; Patent Document 3: Japanese Patent Application Publication No. 2001-206999; Patent Document 4: Japanese Patent Application Publication No. 2000-119585 [The problem that the invention aims to solve] In the aforementioned patent document 1, it is speculated that the substance causing the coloring is due to the formation of -CO-(CH=CH). n The conjugated double bond structure does not record the addition of substances with -CO-(CH=CH) such as sorbic acid at the end of polymerization. n - The compound structure improves the coloring of melt-forming. Furthermore, Patent Document 1 describes the removal of unreacted vinyl acetate monomers from the system after polymerization, but does not describe the control of its value. Moreover, Patent Document 1 does not describe attempts to copolymerize ethylene. In the aforementioned Patent Document 2, it is described that acetaldehyde acts as a chain transfer agent during the polymerization reaction, and if the acetaldehyde content in the raw materials exceeds 200 ppm, the coloring and gelation of the formed product will become more pronounced. Furthermore, Patent Document 2 describes the removal of unreacted vinyl acetate from the solution after polymerization, but it does not describe the control of the amount of vinyl acetate in the solution after polymerization. In the aforementioned Patent Document 3, the following research was conducted: preferably, after polymerization and before the step of removing vinyl acetate, a compound with a molecular weight of less than 1000 containing conjugated double bonds is added. The compound with conjugated double bonds acts as a stabilizer to prevent deterioration. However, there is no record of ethylene-vinyl alcohol copolymers containing compounds with conjugated double bonds of less than 150 ppm. The aforementioned Patent Document 4 only describes a method of coating an aqueous solution onto a substrate to form a barrier layer, but does not describe an ethylene-vinyl alcohol copolymer that can reduce the coloring of melt-molded articles. Previously, many attempts have been made to improve melt formability by reducing the amount of impurities before polymerization and controlling additives after polymerization. However, for ethylene-vinyl alcohol copolymers with an ethylene structural unit content of less than 20 mol%, it remains unknown whether any method can reduce the coloring of melt-formed articles. In view of the above, the present invention provides an ethylene-vinyl alcohol copolymer with an ethylene structural unit content of 1 to 16.5 mol% that can reduce the coloring of melt-formed articles. [Solution to the Problem] However, after repeated and careful review, the inventors of this case found that by setting the content of ethylene structural units in the ethylene-vinyl alcohol copolymer to a specific range, and by heating it in air at 170°C for 5 hours and then using the reflectance measurement method according to JIS K7373, the yellowness was found to be below 80, thereby achieving the above-mentioned objective. That is, the present invention has the following properties. [1] An ethylene-vinyl alcohol copolymer, wherein the content of ethylene structural units is 1 to 16.5 mol%, and the yellowness of the ethylene-vinyl alcohol copolymer is 80 or less in the following yellowness test. [Yellowness test] After heating the ethylene-vinyl alcohol copolymer particles in air at 170°C for 5 hours, the yellowness is determined by the reflectance measurement method according to JIS K7373. [2] The ethylene-vinyl alcohol copolymer as described in [1], wherein the content of sodium acetate as determined according to JIS K6726 is 0.15% by mass or less. [3] The ethylene-vinyl alcohol copolymer as described in [1] or [2], further containing 0.1 ppm to 10 ppm of conjugated polyene. [4] The ethylene-vinyl alcohol copolymer as described in [3], wherein the conjugated polyene is at least one selected from the group consisting of sorbic acid, sorbate, and sorbate salt. [5] An ethylene-vinyl alcohol copolymer as described in any one of [1] to [4], wherein the aforementioned ethylene-vinyl alcohol copolymer comprises an ethylene-vinyl alcohol copolymer having a structural unit having a primary hydroxyl group in the side chain. [6] A method for manufacturing an ethylene-vinyl alcohol copolymer, which is a method for manufacturing an ethylene-vinyl alcohol copolymer as described in any one of [3] to [5], wherein a solution containing copolymerizing components of ethylene and ethylene ester monomers is subjected to a copolymerization reaction, and at the end of the copolymerization reaction, 0.001 to 0.125 parts by mass of a conjugated polyene is added relative to 100 parts by mass of the ethylene ester monomer feed amount, followed by saponification. [7] A method for manufacturing an ethylene-vinyl alcohol copolymer as described in [6], wherein saponification is performed after the amount of unreacted ethylene ester monomers in the solution after the copolymerization reaction is less than 200 ppm. [8] The method for manufacturing ethylene-vinyl alcohol copolymers as described in [6] or [7] further comprises monomers having primary hydroxyl groups on their side chains and / or monomers with primary hydroxyl groups on their side chains protected by esters as the aforementioned copolymerizing components. [Effects of the Invention] The ethylene-vinyl alcohol copolymer of the present invention can reduce coloring when used to make melt-molded articles. Furthermore, due to the reduced coloring, the aforementioned melt-molded articles are suitable for use as food packaging materials requiring high transparency and gas barrier properties, thereby increasing their commercial value. Moreover, the method for manufacturing the ethylene-vinyl alcohol copolymer according to the present invention can efficiently obtain ethylene-vinyl alcohol copolymers with reduced coloring. The present invention will now be described in more detail based on embodiments thereof, but the present invention is not limited to these embodiments. Furthermore, in the present invention, "x and / or y (x and y are any composition or component)" refers to only x, only y, or a combination of x and y. Also, when expressed as "x~y" (x and y are any numbers) in the present invention, unless otherwise specified, it includes the meaning of "x or more and y or less" and "preferably greater than x" or "preferably less than y". Furthermore, when expressed as "x or more" (x is any number) or "y or less" (y is any number), it also includes the meaning of "preferably greater than x" or "preferably less than y". <<Ethylene-Vinyl Alcohol Copolymer>> An example of an embodiment of the present invention is an ethylene-vinyl alcohol copolymer (hereinafter referred to as "EVOH"), wherein the content of ethylene structural units is 1 to 16.5 mol%, and after being heated in air at 170°C for 5 hours, the yellowness determined by reflectance measurement method according to JIS K7373 is 80 or less. Hereinafter, this EVOH will be described. The aforementioned EVOH is usually a thermoplastic resin obtained by saponifying an ethylene-ethylene ester copolymer, which is a copolymer of ethylene and ethylene ester monomers. Examples of vinyl ester monomers used in this EVOH include: vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, trimethylvinyl acetate, vinyl decanoate, vinyl laurate, vinyl stearate, vinyl benzoate, neodecanoate, and vinyl trifluoroacetate. These can be used alone or in combination of two or more. From an economic perspective, vinyl acetate is preferred. Furthermore, as copolymerizing components, in addition to the aforementioned vinyl ester monomers, other copolymerizing monomers such as monomers with primary hydroxyl groups on the side chains, monomers with primary hydroxyl groups on the side chains protected by esters or the like, and vinyl unsaturated monomers can also be used. These can be used alone or in combination with two or more. Regarding the monomers with primary hydroxyl groups in their side chains, examples include: allyl alcohol, 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 6-hepten-1-ol, methyl allyl alcohol, and other monomers containing monohydroxyalkyl groups; and 2-methylene-1,3-propanediol, 3,4-diol-1-butene, 4,5-diol-1-pentene, 4,5-diol-3-methyl-1-pentene, 5,6-diol-1-hexene, glyceryl monoallyl ether, and other monomers containing dihydroxyalkyl groups. Regarding the aforementioned monomers whose primary hydroxyl groups on the side chains are protected by esters or the like, examples include acetates of the aforementioned monomers having primary hydroxyl groups on the side chains. Specifically, examples include monomers containing monoacetoxyalkyl groups such as allyl acetate, 3-butenyl acetate, 4-pentenyl acetate, 5-hexenyl acetate, 6-heptenyl acetate, and methyl allyl acetate; and monomers containing diacetoxyalkyl groups such as 2-methylene-1,3-propanediol diacetate, 3,4-diacetoxy-1-butene, 4,5-diacetoxy-1-pentene, 4,5-diacetoxy-3-methyl-1-pentene, 5,6-diacetoxy-1-hexene, and 3-allyloxy-1,2-propanediol diacetate. Regarding the aforementioned vinyl unsaturated monomers, examples include: olefins such as propylene, 1-butene, and isobutene; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, phthalic acid (anhydride), maleic acid (anhydride), and itconic acid (anhydride), or their salts, or mono- or dialkyl esters with 1 to 18 carbon atoms; acrylamides, N-alkylmethacrylamide, N,N-dimethylacrylamide, 2-acrylamide propanesulfonic acid, or their salts with alkyl groups having 1 to 18 carbon atoms; acrylamides such as acrylamide propyl dimethylamine or its acid salts or its quaternary salts; methacrylamides, alkyl groups with 1 to 18 carbon atoms... Methacrylamides, such as N-alkylmethacrylamide, N,N-dimethylmethacrylamide, 2-methacrylamide propanesulfonic acid or its salts, with 1 to 18 carbon atoms; N-vinylacrylamides, such as N-vinylpyrrolidone, N-vinylmethacrylamide, and N-vinylacetamide; vinyl cyanides, such as acrylonitrile and methacrylonitrile; vinyl ethers, such as alkyl vinyl ethers, hydroxyalkyl vinyl ethers, and alkoxyalkyl vinyl ethers, with 1 to 18 carbon atoms; vinyl halides, such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, and vinyl bromide; and vinyl silanes, etc. Among the aforementioned other copolymerizable monomers, monomers having primary hydroxyl groups on their side chains are preferred, as are monomers whose primary hydroxyl groups on their side chains are protected by esters or the like. For productivity reasons, monomers whose primary hydroxyl groups on their side chains are more preferably protected by esters or the like, and even more preferably 3,4-diacetoxy-1-butene or 2-methylene-1,3-propanediol diacetate, with 3,4-diacetoxy-1-butene being particularly preferred. When monomers having primary hydroxyl groups on their side chains or monomers whose primary hydroxyl groups on their side chains are protected by esters or the like are used as copolymerizable components, EVOH with structural units having primary hydroxyl groups on their side chains can be obtained. This EVOH with structural units having primary hydroxyl groups on its side chains tends to maintain gas barrier properties while improving secondary forming properties. This EVOH is obtained by using the aforementioned ethylene and ethylene ester monomers, as well as other copolymerizable monomers as needed, through a manufacturing process including (I) copolymerization, (II) polymerization termination, (III) saponification, (IV) cleaning, and (V) drying. The following describes each manufacturing step. [(1) Copolymerization Step] In the copolymerization reaction of the copolymerizing components, well-known methods such as bulk polymerization, solution polymerization, suspension polymerization, dispersion polymerization, or emulsion polymerization can be used. Among these, solution polymerization, which is easy to control, is suitable. Hereinafter, solution polymerization, which is a suitable copolymerization reaction, will be described in detail. When carrying out this copolymerization reaction by solution polymerization, examples of solvents used in the solution polymerization include: methanol, ethanol, 1-propanol, 2-propanol, butanol, and other lower alcohols with 1 to 5 carbon atoms, as well as ketones such as acetone and 2-butanone. These can be used alone or in combination of two or more. Among them, methanol is suitable for controlling the polymerization reaction. Furthermore, 2-propanol is suitable for synthesizing copolymers with low degrees of polymerization. The amount of solvent used can be appropriately selected by taking into account the degree of polymerization of the desired EVOH and the chain transfer constant of the solvent. When the solvent is methanol or 2-propanol, the preferred ratio is S (solvent) / M (monomer) = 0.01~10 (mass ratio), more preferably 0.05~7 (mass ratio). Regarding the feeding method of the copolymerizing component in solution polymerization, any method can be adopted, such as initial single feeding, multi-feeding, continuous feeding considering the reactivity ratio of monomers, or the Hanna method. In the above copolymerization reaction, a polymerization initiator is used. Examples of such initiators include: azo-based initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylpentanonitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylpentanonitrile); peroxide-based initiators such as acetyl peroxide, benzoyl peroxide, lauryl peroxide, tributyl peroxyneodecanate, 1,1,3,3-tetramethylbutyl peroxyneodecanate, diisopropyl peroxide dicarbonate, and di-2-ethylhexyl peroxide dicarbonate. These can be used alone or in combination of two or more. The amount of polymerization initiator used varies depending on the type of initiator and therefore cannot be generalized; it should be selected arbitrarily according to the polymerization rate. For example, when using 2,2'-azobisisobutyronitrile or tert-butyl peroxyneodecanate, the amount used relative to 100 parts by weight of ethylene ester monomer is usually 0.001 to 0.2 parts by weight, preferably 0.005 to 0.1 parts by weight. The polymerization temperature of the copolymerization reaction is preferably selected in the range of 40°C to the boiling point, depending on the solvent used and the ethylene pressure. Furthermore, during the copolymerization reaction, the copolymerization reaction can be carried out in the presence of a chain transfer agent, to the extent that it does not impair the effects of the present invention. Examples of chain transfer agents include aldehydes such as acetaldehyde, propionaldehyde, and crotonaldehyde; and thiols such as 2-hydroxyethanethiol. These can be used alone or in combination of two or more. The amount of chain transfer agent added during the copolymerization reaction is determined according to the chain transfer constant of the chain transfer agent and the degree of polymerization of the desired EVOH, but it is generally preferred to be 0.1 to 10 parts by mass relative to 100 parts by mass of the ethylene ester monomer. [(II) Copolymerization Termination Step] In order to reliably terminate the polymerization reaction after it is completed, a conjugated polyene is added as a polymerization inhibitor. The aforementioned conjugated polyenes are structures formed by alternating carbon-carbon double bonds and carbon-carbon single bonds, and are compounds containing two or more carbon-carbon double bonds. These conjugated polyenes can be conjugated dienes with alternating carbon-carbon double bonds and one carbon-carbon single bond, conjugated trienes with alternating carbon-carbon double bonds and two carbon-carbon single bonds, or conjugated polyenes with alternating carbon-carbon double bonds and carbon-carbon single bonds of a number or more. However, if the number of conjugated carbon-carbon double bonds is eight or more, there is a tendency for the formed product to be colored due to the color of the conjugated polyene itself; therefore, the number of conjugated carbon-carbon double bonds is preferably seven or less. Furthermore, the aforementioned conjugated double bonds, consisting of two or more carbon-carbon double bonds, may not be conjugated with each other and may have multiple groups within a molecule. For example, compounds containing three conjugated trienes within the same molecule, such as tung oil, are also included in conjugated polyenes. Specific examples of conjugated polyenes include: isoprene, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-tributyl-1,3-butadiene, 1,3-pentadiene, 2,3-dimethyl-1,3-pentadiene, 2,4-dimethyl-1,3-pentadiene, 3,4-dimethyl-1,3-pentadiene, 3-ethyl-1,3-pentadiene, 2-methyl- 1,3-Pentadiene, 3-Methyl-1,3-Pentadiene, 4-Methyl-1,3-Pentadiene, 1,3-Hexadiene, 2,4-Hexadiene, 2,5-Dimethyl-2,4-Hexadiene, 1,3-Octadiene, 1,3-Cyclopentadiene, 1,3-Cyclohexadiene, 1-Pheny-1,3-Butadiene, 1,4-Diphenyl-1,3-Butadiene, 1-Methoxy-1,3-Butadiene, 2-Methoxy-1,3-Butadiene 1-Butadiene, 1-ethoxy-1,3-butadiene, 2-ethoxy-1,3-butadiene, 2-nitro-1,3-butadiene, chlorobutadiene, 1-chloro-1,3-butadiene, 1-bromo-1,3-butadiene, 2-bromo-1,3-butadiene, fulvin, ketone, aubergine, phellandrene, laurylene, farnesene, cembrene, sorbic acid, sorbate Conjugated dienes such as sorbates and pinoresinic acid, which are composed of a conjugated structure with two carbon-carbon double bonds; conjugated trienes such as 1,3,5-hextriene, 2,4,6-octtrien-1-carboxylic acid, stearic acid, tung oil, and cholecalciferol, which are composed of a conjugated structure with three carbon-carbon double bonds; and conjugated polyenes such as cyclooctatetraene, 2,4,6,8-decatetraene-1-carboxylic acid, retinol, and retinic acid, which are composed of a conjugated structure with four or more carbon-carbon double bonds. Furthermore, for polyenes with multiple stereoisomers, such as 1,3-pentadiene, laurylene, and farnesene, any one of them may be used. These conjugated polyenes may be used alone or in combination of two or more types. Of these, at least one of the group consisting of sorbic acid, sorbate esters and sorbate salts is preferred because it can reliably terminate the copolymerization reaction and is easy to operate, with sorbic acid being particularly preferred. The amount of conjugated polyene added is usually preferably 0.0001~0.3 parts by mass and further preferably 0.001~0.125 parts by mass relative to the feed amount at the beginning of polymerization of 100 parts by mass of vinyl ester-based monomers. The control of the addition amount of conjugated polyenes at the end of polymerization is important. Because the state of the captured radical is stable structure, the conjugated polyene has the time point when addition after the end of the copolymerization will indeed cause the polymerization to terminate, even in the high temperature, inadequate cooling state of the reaction fluid will inhibit the subsequent undesired polymerization reaction in the state of unreacted vinyl ester-based monomers, ethylene, and polymerization starter residues. If the addition of conjugated polyenes at the end of polymerization is too small, the polymerization termination ability is insufficient and thus worse. On the other hand, it is known that conjugated polyenes, especially sorbic acid and its esters, generate new peroxides by reaction with oxygen, even in order to capture the stable structure of free radicals, which give rise to mixtures of acetaldehyde and fumaraldehyde due to heat or photodecomposition. Thereby, if the amount of conjugated polyene is added in excess, there is a tendency for undesirable reactions as described above to occur in the saponification step, the drying step after cleaning, etc. and thus less favorable. The residual amount of the conjugated polyene in the EVOH of the product is also important, and that part is described later. If the amount of conjugated polyene added is too small, the effect of terminating the copolymerization reaction is insufficient, and, as will be described later, there is the disadvantage of decreasing the amount of conjugated polyene in the EVOH of the product. Further, it is better to perform the following (III) saponification step after the amount of unreacted vinyl ester-based monomer remaining in the solution after the termination of the copolymerization reaction becomes less than 200 ppm, more preferably less than 100 ppm, and more preferably less than 80 ppm. When the amount of unreacted vinyl ester-based monomers is higher than 200 ppm, there is a tendency to generate saponifications of unreacted vinyl ester-based monomers, EVOH coloring, and molten formed form due to the saponification catalyst used in the following saponification steps. To make the concentration of the unreacted vinyl ester monomer remaining in the solution below the preceding numerical value, examples may be given: methods of distilling unreacted vinyl ester monomers by adding the solvent used for solution polymerization as described above, heating at a temperature above the boiling point of the vinyl ester monomer. [(III) Saponification step] Saponification of the resulting ethylene-vinyl ester system copolymer was then carried out. The saponification method performed by the saponification step is able to be carried out by using well-known methods, e.g., in dissolving the ethylene-vinyl ester system copolymer obtained above in the state of alcohol or hydro-containing alcohol, by adding a saponification catalyst. In the case of the preceding alcohols, examples may be given: methanol, ethanol, propanol and other lower alcohols with carbon numbers 1~5. These can be used alone or combined with 2 or more types. The concentration of ethylene-vinyl ester-based copolymers in alcohols or aqueous alcohols is chosen appropriately depending on the concentration and is usually 5~60% by mass. Regarding the aforementioned saponification catalysts, examples include: alkaline catalysts such as hydroxides and alcohols of alkali metals such as sodium hydroxide, potassium hydroxide, sodium methoxide, sodium ethoxide, potassium methoxide, and potassium ethoxide; and acid catalysts such as sulfuric acid, hydrochloric acid, nitric acid, and methanesulfonic acid. Preferably, examples include: hydroxides and alcohols of sodium such as sodium hydroxide and sodium methoxide. The saponification temperature is not limited, but a range of 20–140°C is suitable. When the ethylene structural unit content of the ethylene-ethylene ester copolymer is 1–16.5 mol%, particulate matter will be generated as saponification proceeds, indicating that the reaction has completed. At this point, when a gel-like product precipitates, it can be crushed. Furthermore, when using monomers with primary hydroxyl groups protected by side chains, such as esters, as copolymerizing components, the ester is saponified through the aforementioned saponification step, converting it into primary hydroxyl groups. In the saponification step, to improve the degree of saponification, the temporarily generated particulate matter can be washed away and then dispersed again in alcohol or the like, with the addition of a saponification catalyst to further react as a secondary saponification. Furthermore, during the saponification step, this EVOH may not be completely deprotected, leaving a small amount of ester groups remaining. Through this saponification step, a slurry of EVOH in which the ethylene ester units in EVOH are converted into ethylene alcohol units can be obtained. In the aforementioned EVOH slurry, it is preferable to neutralize the saponifying catalyst contained therein. For example, when using sodium hydroxide, sodium methoxide, or other sodium hydroxide or alcohol derivatives as saponifying catalysts, acetic acid is preferable for neutralization, generating sodium acetate in the form of a salt. [(IV) Cleaning Step] The slurry of EVOH after the aforementioned saponification step is separated into solid and liquid components by conventional methods. The EVOH particles obtained in the form of solid components are cleaned, thereby obtaining EVOH particles with reduced residual amounts of conjugated polyenes added in the copolymerization termination step and salts generated during the neutralization of the saponification catalyst. Regarding the solvents used for the aforementioned cleaning, it is preferable to use solvents with low solubility of EVOH and high solubility of conjugated polyenes and sodium acetate, especially solvents with high solubility of conjugated polyenes. Methanol is almost insoluble in EVOH, while sodium acetate can dissolve 13.79g in 100g of saturated methanol solution at 15°C. Sorbic acid, which has a better structure among conjugated polyenes, can dissolve 20% at 30°C, making it a preferred choice. Furthermore, methanol can also dissolve sorbic acid and sorbate esters, making them preferred choices as well. Regarding the liquor ratio in the cleaning step, it is preferably 4 to 8 parts by mass (liquor ratio 4 to 8) relative to 1 part by mass of EVOH generated. Furthermore, the number of cleaning cycles is preferably 2 to 6, the cleaning temperature is preferably 20 to 70°C, and the cleaning time is preferably 0.5 to 3 hours. [(V) Drying Step] The EVOH particles after the washing step are separated into solid and liquid phases using conventional methods and then fed into a drying step to obtain the EVOH particles (hereinafter referred to as "EVOH particles"). Specifically, hot air drying using a cylindrical dryer is preferred, and the temperature of the EVOH particles during drying is preferably 80~120°C, and more preferably 90~110°C. If the temperature is too low, production efficiency tends to decrease. On the other hand, if the temperature is too high, undesirable reactions may occur during drying due to residual sodium acetate, etc., making it easier for the particles to become discolored during melt molding. Furthermore, the drying time is preferably 2~10 hours, and more preferably 3~8 hours. For the EVOH particles, the preferred particle size after drying is 80% or more by mass of particles passing through a 2.36mm mesh JIS Z8801 sieve, and 20% or less by mass of particles passing through a 150μm mesh sieve. If the proportion of particles passing through the 2.36mm mesh sieve is less than 80% by mass, there is a tendency for the drying time to be too long. If the proportion of particles passing through the 150μm mesh sieve exceeds 20% by mass, there will be too much fine powder, which may require new anti-attraction and electrostatic countermeasures when melting and molding. The particle size of this EVOH can be controlled by the content of ethylene structural units in the EVOH, the concentration of ethylene-ethylene ester copolymers in the alcohol or aqueous alcohol during the saponification step, and the saponification temperature. Furthermore, when this EVOH is a copolymer having structural units with primary hydroxyl groups in the side chains of the EVOH, the content of structural units with primary hydroxyl groups in the side chains can also be controlled. The resulting EVOH, obtained in this manner, has a yellowness of 80 or less, preferably 70 or less, and most preferably 60 or less in the following yellowness test. If the yellowness exceeds 80, the coloring of the melt-molded article becomes significant. [Yellowness Test] After heating the ethylene-vinyl alcohol copolymer particles in air at 170°C for 5 hours, the yellowness was determined by the reflectance measurement method according to JIS K7373. Furthermore, the lower limit of yellowness is not specifically defined, but it is generally above 10, preferably above 20, and ideally above 30. To ensure that the yellowness does not reach 20, it is necessary to strictly control the amount of conjugated polyene in the polymerization termination step, the amount of residual ethylene ester monomers in the saponification solution, and the amount of sodium acetate remaining in the dried EVOH, which tends to be uneconomical. Also, the reaction mechanism is not yet fully understood, but when used in melt molding, the viscosity changes significantly over time, making it difficult to control. The content of ethylene structural units in this EVOH is 1~16.5 mol%, preferably 3~15 mol%, more preferably 5~11 mol%, and most preferably 6~10 mol%. If the content of ethylene structural units is too low, the melting point of the EVOH will be high, making it difficult to melt and form. Conversely, if the content of ethylene structural units is too high, the gas barrier properties will decrease. The aforementioned content of ethylene structural units can be controlled by adjusting the ethylene pressure during copolymerization. The saponification degree of this EVOH is typically above 90 mol%, preferably above 95 mol%, and ideally above 99 mol%. If the saponification degree is too low, there is a tendency for the melt-molded product to have reduced gas barrier properties, aroma retention, solvent resistance, and oil resistance. Furthermore, the melt flow rate (MFR) [210°C, 2160g load], which is an indicator of the viscosity of the EVOH when melting to obtain the molded article, is typically 0.1~100g / 10min, preferably 1~50g / 10min, and particularly preferably 3~35g / 10min. If the MFR is too high, it tends to become difficult to control the thickness of the molded product during melt molding; if it is too low, the molding machine tends to bear a high load during melt molding. The MFR is an indicator of the degree of polymerization of the EVOH and can be adjusted by the amount of polymerization catalyst and solvent used during copolymerization of the copolymerizing components. The amount of sodium acetate contained in this EVOH is preferably 0.001% by mass to 0.15% by mass, particularly preferably 0.005% by mass to 0.12% by mass, and even more preferably 0.01% by mass to 0.10% by mass. If the amount of sodium acetate is excessive, although the reaction mechanism is not yet fully understood, it may easily form -CO-(CH=H) that causes coloration when it is melted to form granules or shaped articles. n - The tendency of the structure to cause coloration. Furthermore, if the amount of sodium acetate is too small, although its reaction mechanism is not yet clear, there is a tendency for the viscosity to change significantly over time when used in melt molding, and it tends to become difficult to control. In addition, the aforementioned amount of sodium acetate was determined according to the dissolution titration method for sodium acetate in JIS K6726, the test method for polyvinyl alcohol. If it is not completely dissolved in water, if a solution of water and methanol in a 3:1 (volume ratio) is used according to the same JIS annotation, the EVOH can be dissolved, and sodium acetate can be detected. Furthermore, the branched structure generated by chain transfer in the polymerization reaction is cut off by saponification, and sodium carboxylate generated at the molecular chain ends can also be detected simultaneously. The amount of conjugated polyenes remaining in this EVOH is preferably 0.1 ppm to 10 ppm, and particularly preferably 0.15 ppm to 5 ppm. If the amount of conjugated polyenes is less than 0.1 ppm, the unexpected generation of free radicals in the EVOH will consume them, reducing the effect of preventing degradation. Furthermore, to ensure that the amount is below 0.1 ppm, the EVOH must be excessively cleaned, which tends to be economically disadvantageous. Also, if the amount of conjugated polyenes exceeds 10 ppm, when the EVOH is melted, it may contain -CO-(CH=H) n - The tendency of the structure to increase the amount of coloring substances. EVOH with primary hydroxyl structures in its side chains exhibits better color inhibition after heating. Furthermore, when the EVOH contains structural units with primary hydroxyl structures in its side chains, the content of such primary hydroxyl structures is 2.5 mol% or more, preferably 2.5 to 10 mol%, and particularly preferably 3 to 6 mol%. If the content of primary hydroxyl structures in the side chains is too low, the melting point of the EVOH tends to increase, making melt molding more difficult. Conversely, if the content of structural units with primary hydroxyl structures in the side chains is too high, production costs tend to increase, making it less economical. The content of the aforementioned structural units having primary hydroxyl groups in the side chain can be controlled by the feed amount of monomers having primary hydroxyl groups in the side chain used as comonomers or monomers that protect the aforementioned hydroxyl groups with esters or the like. To obtain this EVOH with the aforementioned characteristics, examples include: (i) a method to make the amount of sodium acetate remaining in the EVOH less than 0.15% by mass; (ii) a method to make the amount of conjugated polyene remaining in the EVOH more than 0.1 ppm and less than 10 ppm; (iii) a method to use at least one selected from the group consisting of sorbic acid, sorbate, and sorbate salt as the conjugated polyene; (iv) a method to add 0.001 to 0.125 parts by mass of the conjugated polyene relative to 100 parts by mass of the ethylene ester monomer feed in the copolymerization termination step (ii) above; and (v) a method to make the amount of unreacted ethylene ester monomer less than 200 ppm in the copolymerization termination step (ii) above, and then proceed to the saponification step (iii). This EVOH can be obtained by using these methods (i) to (v) alone or in combination. Furthermore, this EVOH can be blended with other components to form a resin composition. Examples of such other components, within a range that does not impede the effects of the present invention (e.g., typically 30% by mass or less of the resin composition, preferably 20% by mass or less, more preferably 10% by mass or less, and especially preferably 5% by mass or less), include, for example: other thermoplastic resins besides EVOH, plasticizers, lubricants, stabilizers, surfactants, colorants, ultraviolet absorbers, antistatic agents, desiccants, crosslinking agents, metal salts, fillers, various fibers, etc. These can be used alone or in combination of two or more. This EVOH is suitable for use as a gas barrier material, such as in food packaging. There are no particular limitations on the method of making this EVOH into a gas barrier material. Examples include: (1) coating a solution in which this EVOH is dissolved onto a thin film of a substrate resin and drying it to form a layer composed of EVOH to make a gas barrier material; (2) melting and molding this EVOH to make a gas barrier material, etc. In the method described in (1) above, examples of solvents for dissolving the EVOH include: water, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and other lower alcohols with 1 to 5 carbon atoms. These can be used alone or in combination of two or more. Among these, a mixture of water and 2-propanol is preferred. Furthermore, the concentration of solid components in the solution for dissolving the EVOH is typically 0.5 to 30% by mass, preferably 5 to 20% by mass. Regarding the methods for applying the aforementioned solution that dissolves the EVOH, examples include well-known methods such as bar coating, roller coating, die coating, gravure coating, comma coating, and screen printing. Among these, bar coating is preferred. After coating, it can be dried by heat treatment at 60~105℃ for 0.5~10 minutes to obtain the gas barrier material formed by this EVOH. Furthermore, regarding the melt forming method in the above (2) method, examples include: extrusion molding, injection molding, gas molding, compression molding, blow molding, etc. This process yields a gas barrier material having a layer formed from the EVOH. The aforementioned gas barrier material can be made as a single-layer structure or a multi-layer structure, but a multi-layer structure is preferred. The multi-layer structured gas barrier material preferably has at least one layer composed of the EVOH. Furthermore, the multi-layered gas barrier material can be laminated with layers formed from the EVOH, or it can be laminated with other substrate resins. Examples of the aforementioned base resins include: linear low-density polyethylene, low-density polyethylene, ultra-low-density polyethylene, medium-density polyethylene, high-density polyethylene, polyethylene-propylene (block and random) copolymers, ethylene-α-olefin (α-olefin with 4 to 20 carbon atoms) copolymers, polypropylene-based resins such as polypropylene and propylene-α-olefin (α-olefin with 4 to 20 carbon atoms), polybutene, polypentene, polycyclic olefin resins (polymers in which at least one of the main chain and side chain has a cyclic olefin structure), and other (unmodified) polyolefin resins containing unsaturated carboxylic acids. The term "modified olefin resin" refers to a broad range of polyolefin resins, including unsaturated carboxylic acid-modified polyolefin resins (such as those grafted onto polyolefins), ionomers, ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-acrylate copolymers, polyester resins, polyamide resins (including copolymerized polyamides), polyvinyl chloride, polyvinylidene chloride, acrylic resins, polystyrene, vinyl ester resins, polyester elastomers, polyurethane elastomers, polystyrene elastomers, halogenated polyolefins such as chlorinated polyethylene and chlorinated polypropylene, and aromatic or aliphatic polyketides. These resins can be used alone or in combination of two or more. Furthermore, these base resins can undergo surface treatments such as corona treatment. The thickness of the layer formed by this EVOH is typically 1~200 μm, preferably 1~100 μm, and particularly preferably 1~50 μm. Furthermore, when the above-mentioned gas barrier material has a multilayer structure, the thickness of all layers formed by this EVOH contained in the gas barrier material is included. Furthermore, the oxygen permeability of the layer formed by this EVOH is preferably 5cc. 3μm / m 2 . day. atm ≤ 1cc. 3μm / m 2 . day. atm ≤, ideally 0.1cc. 3μm / m 2 Below 0.003 cc / m. Furthermore, the above oxygen permeability was measured at 23°C and 0%RH; the lower limit of oxygen permeability is typically 0.003 cc / m. 2 • day. atm. Furthermore, the above oxygen permeability can be determined using an oxygen permeability measuring device. [Example] The present invention is further illustrated below with examples, but the invention is not limited to these examples as long as it does not deviate from its spirit. Furthermore, unless otherwise specified, "parts" and "%" in the examples refer to a quality standard. The methods for determining yellowness, the amount of sodium acetate, the amount of conjugated polyene, and the amount of unreacted vinyl ester monomers in the examples are shown below. [Yellowness] 10g of EVOH particles were spread on an aluminum tray and heated in an oven at 170°C for 5 hours. After the particles were removed and cooled to room temperature, the yellowness of the particles was determined using a JIS K7373 method with a Nippon Denshoku Kogyo Co., Ltd. SE6000 spectrophotometer. [Amount of sodium acetate] The amount of sodium acetate remaining in EVOH was determined using the dissolution titration method according to JIS K6726. [Amount of Conjugated Polyene] When the conjugated polyene remaining in EVOH is sorbic acid, it is determined as follows: The sample is cryogenically pulverized, and 200 mg is accurately weighed. It is then moistened with a trace amount of methanol, and 5 mL of water is added. The mixture is heated and dissolved. Further methanol is added to prepare 10 mL. The amount of sorbic acid in this solution is quantified by liquid chromatography. [Amount of Unreacted Vinyl Ester Monomers] The amount of unreacted vinyl ester monomers, when the vinyl ester monomer is vinyl acetate, is quantified by gas chromatography to determine the amount of residual vinyl acetate in the solution after the polymerization reaction. [GC Measurement Conditions] • GC System: Agilent Technologies 7890B [Made by Agilent Technologies] • Detector: FID • Column: Agilent Technologies Catalog 125-7032 (30m × 0.530mm) • Column Temperature: 60℃ <Example 1> [Synthesis of Ethylene-Vinyl Ester Copolymer] In a temperature-controlled autoclave, 460 parts of vinyl acetate, 36 parts of 3,4-diethoxy-1-butene, and 132 parts of methanol were fed. The system was temporarily purged with nitrogen, followed by ethylene, and the temperature was raised to 67°C while stirring. After heating, ethylene was injected at a partial pressure of 1.35 MPa while stirring and maintaining the internal temperature at 67°C for 4 hours. Simultaneously, a 0.2% methanol solution of 0.129 parts of tert-butyl peroxyneodecanate was added as an initiator over 4 hours. After the addition was completed, the mixture was stirred at 67°C for another 3 hours to carry out the copolymerization reaction. Then, a solution of 0.096 parts (0.021 parts relative to 100 parts of the fed vinyl acetate monomer) of sorbic acid dissolved in 100 parts of methanol was added as a copolymerization termination step, and the mixture was cooled to room temperature (23°C). To further reduce unreacted vinyl acetate monomers, the volatile components caused by heating at 75°C were repeatedly removed and methanol was added, thereby obtaining a methanol solution of ethylene-vinyl acetate-3,4-diethoxybutene copolymer. At this point, the amount of unreacted vinyl acetate monomer remaining in the solution was 10 ppm. [Preparation and Properties of EVOH] The above solution was diluted with methanol to a concentration of 10%. While stirring in a flask equipped with a stirrer and cooling tube, the solution temperature was maintained at 45°C. An 8.7% methanol solution of sodium hydroxide, in an amount equal to 10 mmol equivalent of the vinyl acetate units of the copolymer, was added for primary saponification. After approximately 30 minutes, the saponified material precipitated. Stirring continued until a slurry containing particulate matter was obtained. For further saponification, as a secondary saponification, the resulting slurry was temporarily filtered and then dispersed again in 20 times the amount of methanol as the saponified material. While stirring in a flask equipped with a stirrer and cooling tube, an 8.7% methanol solution of sodium hydroxide, in an amount equal to 50 mmol equivalent of the vinyl acetate units of the copolymer before primary saponification, was added. The reaction was carried out at 50°C for 3 hours. After the reaction, the mixture was neutralized with acetic acid, and the slurry was filtered again to obtain a wet cake. The wet cake was washed three times with methanol at 5 times its mass and filtered again. It was then dried in a hot air dryer at 100°C for 8 hours to obtain EVOH particles. The resulting copolymer has a 1,2-butanediol structure with primary hydroxyl groups on the side chains. The saponification degree of the obtained EVOH particles, analyzed using the alkali consumption for the hydrolysis of residual ethylene ester units, was 99.9 mol%. Furthermore, the content of the obtained ethylene structural units, determined by NMR, was 11.4 mol%, and the content of the 1,2-butanediol structural units was 3.2 mol%. The residual sodium acetate content was 0.109 wt%, and the conjugated polyene content was 0.3 ppm. 86% of the obtained EVOH particles passed through a JIS Z8801 sieve with a mesh size of 2.36 mm, and 17% passed through a sieve with a mesh size of 150 μm. After heating the EVOH particles in air at 170°C for 5 hours, the yellowness was determined to be 43.9 using the reflectance measurement method according to JIS K7373. The copolymer composition of EVOH in Example 1 is shown in Table 1 below, and the evaluation results of EVOH are shown in Table 2 below. <Examples 2, 3, 9, 10, Comparative Examples 1, 2, 4, 5, 6> As described in Table 1 below, the amounts of vinyl acetate, 3,4-diethoxy-1-butene, methanol, ethylene partial pressure, tert-butyl peroxydecanoate, and the type and amount of conjugated polyene added in the copolymerization termination step were changed in Example 1, and polymerization was carried out. The unreacted vinyl ester monomers before the saponification step were processed as described in Table 1 below, and then a first saponification was performed. Except for the changes in Table 1 regarding whether there was a second saponification, the alkali equivalent used in the second saponification, and the washing method of the wet cake obtained after saponification, the process was the same as in Example 1, and EVOH particles were obtained. <Examples 4-8, Comparative Example 3> The amounts of vinyl acetate, 3,4-diethoxy-1-butene, methanol, and ethylene partial pressure in the feed of Example 1 were changed as shown in Table 1. The initiator was changed to a methanol solution of 2,2'-azobisisobutyronitrile, and the total amount of this initiator solution was added at the start of polymerization. After addition, the copolymerization reaction was carried out at 67°C for 4 hours. Except for changes to the type and amount of conjugated polyene added in the copolymerization termination step, the amount of unreacted ethylene ester monomer before the primary saponification step, whether secondary saponification was performed, the alkali equivalent used in the secondary saponification, and the washing method of the wet cake obtained after saponification, the process was the same as in Example 1 to obtain EVOH particles. The contents of ethylene structural units, 1,2-butanediol structure, 1,3-propanediol structure, saponification degree, sodium acetate content, conjugated polyene content, the ratio of particles passing through 2.36 mm and 150 μm mesh sieves, and yellowness coefficients of the obtained EVOH particles are described in Table 2 below. [Table 1] ※1: Total amount of methanol solution containing the initiator added at the start of polymerization. ※2: Methanol solution containing the initiator added 4 hours after the start of polymerization, with the reaction continuing for 3 hours after the addition is completed. ※3: Amount added relative to 100 parts of vinyl acetate fed into the polymerization process. ※4: Equivalent weight of vinyl acetate units in the copolymer relative to the amount before the first saponification. ※5: A: Wet cake washed 3 times with 5 times its mass of methanol after saponification. B: Wet cake washed 2 times with 5 times its mass of methanol after saponification. C: Wet cake extracted with methanol using a Soxhlet extractor for 1 day after saponification. [Table 2] ※1: Proportion of particles passing through a 2.36mm mesh sieve ※2: Proportion of particles passing through a 150μm mesh sieve From Examples 1-10 in Tables 1 and 2 above, it is known that copolymerization is performed in a manner that brings the ethylene structural units to a specific range. A specific amount of conjugated polyene is added at the copolymerization termination step, and saponification is performed after the unreacted ethylene ester monomers are brought to a specific amount or less in the saponification step. Further washing is then used to bring the amount of sodium acetate and conjugated polyene in the EVOH particles to a specific range. This reduces the yellowing after heating in air at 170°C for 5 hours. Molded articles obtained from such EVOH particles through melt molding are expected to be suitable for various applications requiring high transparency due to reduced coloration. The above embodiments illustrate specific forms of the present invention, but are merely illustrative and not intended to be limiting. Various modifications that will be obvious to those skilled in the art are included within the scope of this invention. [Industrial Applicability] These EVOH particles can reduce coloring during the molding process, making them suitable for use as food packaging materials requiring high transparency and gas barrier properties.

Claims

1. An ethylene-vinyl alcohol copolymer, wherein the content of ethylene structural units is 1 to 16.5 mol%, the ethylene-vinyl alcohol copolymer comprising structural units having primary hydroxyl groups in the side chains, the ethylene-vinyl alcohol copolymer having a yellowness of 80 or less in the following yellowness test; [Yellowness Test] After heating the ethylene-vinyl alcohol copolymer particles in air at 170°C for 5 hours, the yellowness is determined by reflectance measurement method according to JIS K7373.

2. The ethylene-vinyl alcohol copolymer of claim 1, wherein the content of sodium acetate as determined in accordance with JIS K6726 is less than 0.15% by mass.

3. The ethylene-vinyl alcohol copolymer of claim 1 or 2, further containing 0.1 ppm to 10 ppm of conjugated polyene.

4. The ethylene-vinyl alcohol copolymer of claim 3, wherein the conjugated polyene is at least one selected from the group consisting of sorbic acid, sorbate, and sorbate salt.

5. A method for manufacturing an ethylene-vinyl alcohol copolymer, which is the method for manufacturing an ethylene-vinyl alcohol copolymer as claimed in claim 3 or 4, wherein a solution containing copolymerizing components of ethylene and ethylene ester monomers is subjected to a copolymerization reaction, and at the end of the copolymerization reaction, 0.001 to 0.125 parts by mass of conjugated polyene are added relative to 100 parts by mass of ethylene ester monomers in the feed, followed by saponification.

6. The method for manufacturing the ethylene-vinyl alcohol copolymer as claimed in claim 5, wherein the amount of unreacted ethylene ester monomers in the solution after the copolymerization reaction is less than 200 ppm is followed by saponification.

7. The method for manufacturing an ethylene-vinyl alcohol copolymer as claimed in claim 5 or 6, further comprising a monomer having a primary hydroxyl group on its side chain and / or a monomer having a primary hydroxyl group on its side chain protected by an ester as a copolymerizing component.