Method for producing ethylene-vinyl ester copolymer, and saponified ethylene-vinyl ester copolymer

Abstract

Provided is a method for producing an ethylene-vinyl ester copolymer, in which coloration due to thermal degradation can be suppressed in a saponified ethylene-vinyl ester copolymer. The method for producing an ethylene-vinyl ester copolymer by copolymerizing a vinyl ester monomer and ethylene, wherein the polymerization activity of the vinyl ester monomer, as measured using the measurement method described below, is 550 seconds or more.　Measurement method: a glass test tube containing 10 mL of a vinyl ester monomer and 0.130 g of lauroyl peroxide as a polymerization catalyst is immersed in a constant temperature bath at 20°C, the polymerization catalyst is dissolved in the vinyl ester monomer, the number of seconds between a point when the test tube is immersed in a constant temperature bath at 65.0ºC and a point when foam is generated and the height from the surface of a foam layer to the bottom of the foam layer is 3 mm or more is measured, and this number of seconds is taken to be the polymerization activity.

Description

Method for producing ethylene-vinyl ester copolymer, and saponified ethylene-vinyl ester copolymer

[0001] The present invention relates to a method for producing an ethylene-vinyl ester copolymer, and more specifically to a method for producing an ethylene-vinyl ester copolymer, which can suppress coloration due to thermal degradation when the ethylene-vinyl ester copolymer is saponified to form a saponified ethylene-vinyl ester copolymer.

[0002] Saponified ethylene-vinyl ester copolymer (hereinafter sometimes referred to as "EVOH resin") has very strong intermolecular forces due to hydrogen bonds between hydroxyl groups present in the polymer side chains. As a result, it has high crystallinity and, because the intermolecular forces are also strong in the amorphous portion, gas molecules and the like do not easily pass through EVOH resin, demonstrating excellent gas barrier properties.

[0003] EVOH resin is produced by saponifying an ethylene-vinyl ester copolymer obtained by copolymerizing ethylene with a vinyl ester monomer.

[0004] Vinyl ester monomers, such as vinyl acetate, which are raw materials for EVOH resins, are usually produced by reacting ethylene, oxygen, and acetic acid in the presence of a catalyst, and purifying the resulting crude product by distillation or the like.

[0005] Vinyl acetate is highly reactive and easily undergoes radical polymerization in the presence of heat, metal ions, or peroxides. As a result, polymerization products adhere to and accumulate on the walls of production tanks, storage tanks, distillation columns, and transport pipes, which can reduce thermal efficiency and clog transport pipes.

[0006] Therefore, conventionally, vinyl acetate has been subjected to polymerization inhibition using reaction-inhibiting substances such as oxygen and polymerization inhibitors during storage, etc., and when vinyl acetate is subjected to polymerization, the reaction-inhibiting substances have been removed as much as possible before the polymerization is carried out.

[0007] For example, Patent Document 1 discloses an EVOH resin film in which fisheyes are reduced by using a vinyl ester monomer in which the oxygen concentration during storage and / or transportation and during polymerization is set within a specific range, and discloses that it is preferable to reduce the oxygen concentration during polymerization as much as possible from the viewpoint of inhibiting polymerization.

[0008] International Publication No. 2015 / 098885

[0009] However, when a polymerization inhibitor is used to suppress the reaction of a vinyl ester monomer, if an ethylene-vinyl ester copolymer is produced by completely removing the polymerization inhibitor before polymerization and the resulting ethylene-vinyl ester copolymer is used to produce an EVOH resin, it has been difficult to obtain an EVOH resin having a desired color tone.

[0010] Under these circumstances, an object of the present invention is to provide a method for producing an ethylene-vinyl ester copolymer, which can suppress coloration due to thermal degradation when an ethylene-vinyl ester copolymer is saponified to form a saponified ethylene-vinyl ester copolymer.

[0011] However, the present inventors have conducted extensive research in light of the above circumstances and have found that, by increasing the polymerization activity of a vinyl ester monomer to a specific level or higher, coloration due to thermal degradation can be suppressed when an ethylene-vinyl ester copolymer is saponified to form a saponified ethylene-vinyl ester copolymer.

[0012] That is, the present invention has the following aspects. [1] A method for producing an ethylene-vinyl ester copolymer by copolymerizing a vinyl ester monomer with ethylene, wherein the polymerization activity of the vinyl ester monomer measured by the following measurement method is 550 seconds or more. Measurement method: A glass test tube (inner diameter 18 mm, length 200 mm) containing 10 mL of vinyl ester monomer and 0.130 g of lauroyl peroxide as a polymerization catalyst is immersed in a thermostatic bath at 20°C to dissolve the polymerization catalyst in the vinyl ester monomer, and the test tube is then immersed in a thermostatic bath at 65.0°C. The number of seconds from the time when bubbles are generated and the height from the surface of the bubble layer to the bottom of the bubble layer is 3 mm or more is measured, and this number of seconds is defined as the polymerization activity. The heights of the surface and the bottom of the bubble layer are measured at the center of the test tube. [2] A method for producing an ethylene-vinyl ester copolymer according to [1], wherein the polymerization activity of the vinyl ester monomer is 550 to 850 seconds. [3] The ethylene-vinyl ester copolymer according to [1] or [2], comprising a step of adjusting the polymerization activity for 550 seconds or longer by adding a polymerization inhibitor to the vinyl ester monomer. [4] The method for producing an ethylene-vinyl ester copolymer according to [3], wherein the content of the polymerization inhibitor is 5 to 12,000 ppb based on the vinyl ester monomer. [5] The method for producing an ethylene-vinyl ester copolymer according to any one of [1] to [4], wherein the vinyl ester monomer is a vinyl ester monomer after distillation purification. [6] The method for producing an ethylene-vinyl ester copolymer according to any one of [1] to [5], further comprising a chelating agent. [7] The method for producing an ethylene-vinyl ester copolymer according to any one of [1] to [6], wherein the oxygen concentration of the vinyl ester monomer is less than 5 ppm based on the vinyl ester monomer. [8] A saponified ethylene-vinyl ester copolymer obtained by saponifying the ethylene-vinyl ester copolymer obtained by the production method according to any one of [1] to [7]. [9] A pellet comprising the saponified ethylene-vinyl ester copolymer according to [8].

[10] A multilayer structure having a layer containing the saponified ethylene-vinyl ester copolymer according to [8].

[11] A method for producing the multilayer structure according to

[10] , comprising a step of melt-molding the saponified ethylene-vinyl ester copolymer.

[0013] The present invention can suppress coloration due to thermal degradation when an ethylene-vinyl ester copolymer is saponified to form a saponified ethylene-vinyl ester copolymer.

[0014] The present invention will be described below based on examples of embodiments for carrying out the present invention, but the present invention is not limited to the embodiments described below.

[0015] As used herein, "x and / or y (x and y are any configuration)" refers to at least one of x and y, and can mean three things: x only, y only, or x and y. In this specification, when "X to Y" (X and Y are any numbers) is used, unless otherwise specified, it also means "X or more and Y or less," as well as "preferably greater than X" or "preferably smaller than Y." In this specification, when "X or more" (X is any number) or "Y or less" (Y is any number) is used, it also means "preferably more than X" or "preferably less than Y." For numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. Furthermore, in the numerical ranges described herein, the upper or lower limit of the numerical range can also be replaced with the values ​​shown in the examples.

[0016] In this specification, the term "main component" means a component that has a significant effect on the properties of the target object, and the content of the component is usually 50% by mass or more, preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, and may be 100% by mass.

[0017] <<Method for Producing Ethylene-Vinyl Ester Copolymer>> Typically, when producing an ethylene-vinyl ester copolymer, ethylene, a vinyl ester monomer, a polymerization solvent, a polymerization catalyst (polymerization initiator), and the like are charged into a polymerization (reaction) vessel and polymerization is carried out. This embodiment is characterized by using a vinyl ester monomer having a specific polymerization activity, and as long as this polymerization activity is satisfied, known methods can be used for other conditions. For example, either a continuous system or a batch system may be used, and other polymerization conditions may be set appropriately depending on the polymerization system. First, the vinyl ester monomer used in this embodiment will be described.

[0018] <Vinyl Ester Monomer> As the vinyl ester monomer used in this embodiment, vinyl acetate is typically used because of its commercial availability and the efficiency of impurity treatment during production. Other examples include aliphatic vinyl esters such as vinyl formate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, vinyl stearate, and vinyl versatate, and aromatic vinyl esters such as vinyl benzoate. These are aliphatic vinyl esters typically having 3 to 20 carbon atoms, preferably 4 to 10 carbon atoms, and particularly preferably 4 to 7 carbon atoms. These are typically used alone, but multiple types may be used simultaneously as necessary.

[0019] The polymerization activity of the vinyl ester monomer during polymerization is 550 seconds or more, preferably 550 to 900 seconds, more preferably 550 to 850 seconds, even more preferably 555 to 800 seconds, and particularly preferably 560 to 650 seconds. When the polymerization activity of the vinyl ester monomer is equal to or less than the upper limit, the polymerization reactivity tends to be excellent, and when it is equal to or greater than the lower limit, the resin degradation of the EVOH resin tends to be suppressed and the color tone tends to be stable.

[0020] The polymerization activity of such vinyl ester monomers can be measured by the following method. Measurement method: A glass test tube (inner diameter 18 mm, length 200 mm) containing 10 mL of vinyl ester monomer and 0.130 g of lauroyl peroxide as a polymerization catalyst is immersed in a thermostatic bath at 20°C to dissolve the polymerization catalyst in the vinyl ester monomer. The test tube is then immersed in a thermostatic bath at 65.0°C. The number of seconds from the time when bubbles are generated to the time when bubbles form to a height of 3 mm or more from the surface of the bubble layer to the bottom of the bubble layer is measured, and this number of seconds is regarded as the polymerization activity. The heights of the surface and bottom of the bubble layer are measured at the center of the test tube. Note that the "center" in this specification refers to the vertical center portion along the central axis of the cylindrical shape of the test tube.

[0021] The method for producing an ethylene-vinyl ester copolymer according to the present embodiment, in which a vinyl ester monomer and ethylene are copolymerized, preferably includes a step of adjusting the polymerization activity for 550 seconds or more.

[0022] The method for adjusting the polymerization activity of the vinyl ester monomer to the specific range is not particularly limited. For example, it is preferable to adjust the polymerization activity by adding a specific amount of a polymerization inhibitor to a vinyl ester monomer that has been purified by distillation in advance.

[0023] The polymerization activity of the vinyl ester monomer can be adjusted to the specific range by appropriately adjusting the conditions of distillation purification (by adjusting the oxygen and / or polymerization inhibitor contained in advance in the vinyl ester monomer). Alternatively, the polymerization activity of the vinyl ester monomer can be adjusted by appropriately adjusting the conditions of distillation purification and adding a specific amount of a polymerization inhibitor to the vinyl ester monomer.

[0024] (Polymerization Inhibitor) The polymerization inhibitor is not particularly limited, but examples thereof include N,N-dialkylhydroxylamine, styrene derivatives, hydroquinone derivatives, quinone derivatives, piperidine derivatives, conjugated polyenes, etc. Two or more of such polymerization inhibitors can also be used in combination.

[0025] Examples of hydroquinone derivatives include hydroquinone, hydroquinone monomethyl ether, dipentylhydroquinone, p-methoxyphenol, p-butoxyphenol, p-pentoxyphenol, and monobenzone.

[0026] Examples of conjugated polyenes include isoprene, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-t-butyl-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, and 3-methyl-1,3-pentadiene. 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-phenyl-1,3-butadiene, 1,4-diphenyl-1,3-butadiene, 1-methoxy-1,3-butadiene, 2-methoxy-1,3-butadiene, 1-ethoxy-1,3-butadiene, olefins having two carbon-carbon double bonds, such as 2-ethoxy-1,3-butadiene, 2-ethoxy-1,3-butadiene, 2-nitro-1,3-butadiene, chloroprene, 1-chloro-1,3-butadiene, 1-bromo-1,3-butadiene, 2-bromo-1,3-butadiene, fulvene, tropone, ocimene, phellandrene, myrcene, farnesene, cembrene, sorbic acid, sorbic acid esters, sorbates, and abietic acid. Examples of suitable polyenes include conjugated dienes having a conjugated structure; conjugated trienes having a conjugated structure with three carbon-carbon double bonds, such as 1,3,5-hexatriene, 2,4,6-octatriene-1-carboxylic acid, eleostearic acid, tung oil, and cholecalciferol; and conjugated polyenes having a conjugated structure with four or more carbon-carbon double bonds, such as cyclooctatetraene, 2,4,6,8-decatetraene-1-carboxylic acid, retinol, and retinoic acid. In addition, for compounds having multiple stereoisomers, such as 1,3-pentadiene, myrcene, and farnesene, any of these may be used. Two or more of these polyene compounds may also be used in combination.

[0027] Examples of quinone derivatives include o-benzoquinone, p-benzoquinone, diphenoquinone, 1,4-naphthoquinone, and anthraquinone.

[0028] Examples of N,N-dialkylhydroxylamines include N,N-dimethylhydroxylamine, N,N-diethylhydroxylamine, N,N-dipropylhydroxylamine, and N,N-dibutylhydroxylamine.

[0029] Examples of the styrene derivative include at least one selected from cinnamic alcohol, cinnamic acid, and derivatives thereof. In addition to cinnamic alcohol and cinnamic acid, examples include cinnamic acid esters (ethyl cinnamate, methyl cinnamate, etc.), cinnamic acid chlorides (cinnamoyl chloride, etc.), cinnamic acid amide, cinnamic acid nitrile, and cinnamic acid salts (sodium cinnamate, calcium cinnamate, etc.).

[0030] Examples of piperidine derivatives include 4-hydroxyl-2,2,6,6-tetramethylpiperidine-1-oxyl, hexahydroxypicolinic acid, hexahydroquinolinic acid, and hexahydrocinchomeronic acid.

[0031] Among these polymerization inhibitors, hydroquinone derivatives and conjugated polyenes are preferred, and hydroquinone and sorbic acid are more preferred.

[0032] The content of the polymerization inhibitor relative to the vinyl ester monomer is preferably 10 to 15,000 ppb, more preferably 20 to 10,000 ppb, even more preferably 40 to 8,000 ppb, and particularly preferably 50 to 6,000 ppb. When the content is equal to or greater than the lower limit, the color tone improving effect of the present invention tends to be easily obtained, while when the content is equal to or less than the upper limit, sufficient reaction activity tends to be easily obtained. The content of the polymerization inhibitor can be measured using gas chromatography or the like.

[0033] The content of the polymerization inhibitor can be appropriately set within the above range, and may be, for example, 5 to 12,000 ppb, 10 to 5,500 ppb, 20 to 4,000 ppb, 30 to 3,000 ppb, or 40 to 2,000 ppb relative to the vinyl ester monomer.

[0034] The vinyl ester monomer may also contain oxygen as a polymerization inhibitor. When oxygen is contained in the vinyl ester monomer, it is theoretically preferable that oxygen be removed from the vinyl ester monomer during polymerization as much as possible, preferably less than 5 ppm, more preferably less than 3 ppm, and even more preferably less than 2 ppm relative to the vinyl ester monomer. It is believed that if the oxygen concentration is 1 ppm or less, polymerization inhibition by oxygen is not substantially observed. If the oxygen concentration is too high, the reactivity of the vinyl ester monomer decreases, and the oxygen in the polymerization solution acts as a polymerization inhibitor, making it difficult to proceed with polymerization. While there is no particular lower limit for the oxygen concentration, it is usually difficult to continuously maintain an oxygen concentration of less than 1 ppm industrially. Therefore, it is preferable to carry out polymerization of the vinyl ester monomer at an oxygen concentration of about 1 ppm. The oxygen concentration of the vinyl ester monomer can be measured using gas chromatography.

[0035] Methods for reducing the oxygen concentration of the vinyl ester monomer during polymerization include, for example, distillation, bubbling with an inert gas, and creating an inert gas atmosphere in the system to refresh the gas-liquid interface. Examples of such a method for refreshing the interface include stirring, flow, etc. Examples of the inert gas include nitrogen, argon, helium, etc., and generally, nitrogen is preferably used.

[0036] <Ethylene-Vinyl Ester Copolymer> As a method for introducing ethylene into a vinyl ester monomer, ordinary pressurized ethylene polymerization may be carried out, and the amount of ethylene introduced can be controlled by the pressure of ethylene, which is usually selected from the range of 2.5 to 8.0 MPa, although it depends on the target ethylene content and cannot be generally determined.

[0037] Examples of solvents used in such copolymerization include lower alcohols such as methanol, ethanol, propanol, and butanol, and ketones such as acetone and methyl ethyl ketone, with methanol being preferred for industrial use. The amount of solvent used can be appropriately selected in accordance with the desired degree of polymerization of the copolymer, taking into consideration the chain transfer constant of the solvent. For example, when the solvent is methanol, the amount is selected from the range of S (solvent) / M (monomer) = 0.01 to 10 (mass ratio), preferably 0.05 to 7 (mass ratio).

[0038] A polymerization catalyst is used in the copolymerization. Examples of such a polymerization catalyst include known radical polymerization catalysts such as azobisisobutyronitrile, acetyl peroxide, benzoyl peroxide, and lauryl peroxide; peroxyesters such as t-butylperoxyneodecanoate, t-butylperoxypivalate, α,α'-bis(neodecanoylperoxy)diisopropylbenzene, cumylperoxyneodecanoate, 1,1,3,3-tetramethylbutylperoxyneodecanoate, 1-cyclohexyl-1-methylethylperoxyneodecanoate, t-hexylperoxyneodecanoate, and t-hexylperoxypivalate; and di-n-propylperoxymethyl ... Examples of the low-temperature active radical polymerization catalyst include peroxydicarbonates such as dipropyl peroxydicarbonate, di-iso-propyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, bis(4-t-butylcyclohexyl)peroxydicarbonate, di-2-ethoxyethyl peroxydicarbonate, di(2-ethylhexyl)peroxydicarbonate, dimethoxybutyl peroxydicarbonate, and di(3-methyl-3-methoxybutylperoxy)dicarbonate, and diacyl peroxides such as 3,3,5-trimethylhexanoyl peroxide, diisobutyryl peroxide, and lauroyl peroxide. The amount of the polymerization catalyst used varies depending on the type of catalyst and cannot be determined in general, but is selected appropriately depending on the polymerization rate. For example, when azobisisobutyronitrile or acetyl peroxide is used, the amount is preferably 0.0001 to 0.2 parts by mass, more preferably 0.005 to 0.1 parts by mass, per 100 parts by mass of the vinyl ester monomer.

[0039] In this embodiment, the coexistence of a chelating agent such as a hydroxylactone compound or a hydroxycarboxylic acid together with the catalyst is preferred in that it improves the color tone of the resulting resin (approaching colorless). The hydroxylactone compound is not particularly limited as long as it is a compound having a lactone ring and a hydroxyl group in the molecule, and examples thereof include L-ascorbic acid, erythorbic acid, and glucono-delta-lactone, with L-ascorbic acid and erythorbic acid being preferred. Furthermore, examples of hydroxycarboxylic acids include glycolic acid, lactic acid, glyceric acid, malic acid, tartaric acid, citric acid, and salicylic acid, with citric acid being preferred.

[0040] The amount of the chelating agent used, whether in a batchwise or continuous process, is preferably 0.00001 to 0.1 parts by mass, more preferably 0.0005 to 0.05 parts by mass, and particularly preferably 0.001 to 0.03 parts by mass, per 100 parts by mass of the vinyl ester monomer. Having the content within the above range tends to facilitate polymerization of the vinyl ester monomer. In this specification, the term "chelating agent" refers to a compound having a multidentate ligand that bonds with a metal ion to form a chelate compound.

[0041] The content of the chelating agent is preferably 0.1 to 1000 ppm, more preferably 5 to 500 ppm, even more preferably 10 to 300 ppm, particularly preferably 15 to 100 ppm, and most preferably 20 to 60 ppm, based on the ethylene-vinyl ester copolymer.

[0042] The chelating agent is charged into the polymerization system in a manner not particularly limited, but is usually diluted with a solvent such as a lower aliphatic alcohol (methanol, ethanol, propanol, tert-butanol, etc.), an aliphatic ester containing a vinyl ester monomer (methyl acetate, ethyl acetate, etc.), water, or a mixed solvent thereof, and then charged into the polymerization reaction system.

[0043] The reaction temperature of the copolymerization reaction varies depending on the solvent and pressure used, but is usually not more than the boiling point of the solvent, and is usually preferably 40 to 80° C., particularly preferably 55 to 80° C. When the temperature is equal to or higher than the lower limit, the reaction tends to be able to be carried out with a small amount of catalyst in a short time, while when the temperature is equal to or lower than the upper limit, the polymerization tends to be easily controlled.

[0044] Furthermore, in the case of a batch system, the polymerization time is preferably 4 to 10 hours, more preferably 6 to 9 hours. If the polymerization time is equal to or greater than the lower limit, the amount of catalyst tends to be reduced, and if it is equal to or less than the upper limit, productivity tends to be excellent. In the case of a continuous system, the average residence time in the polymerization vessel is preferably 2 to 8 hours, more preferably 2 to 6 hours. If the residence time is equal to or greater than the lower limit, the amount of catalyst tends to be reduced without excessively increasing the polymerization temperature, and if it is equal to or less than the upper limit, productivity tends to be excellent.

[0045] The polymerization rate of the vinyl ester monomer is set as high as possible within the range where polymerization control is possible from the viewpoint of productivity, and is preferably 20 to 95%, more preferably 30 to 90%, and even more preferably 40 to 85%. When the polymerization rate is equal to or higher than the lower limit, productivity is excellent and the amount of unpolymerized vinyl ester monomer remaining tends to be small, while when it is equal to or lower than the upper limit, polymerization control tends to be easy.

[0046] The ethylene content of the ethylene-vinyl ester copolymer is preferably 20 to 95 mol%, more preferably 30 to 90 mol%, and even more preferably 35 to 85 mol%. When the ethylene content is equal to or greater than the lower limit, the gas barrier property and melt moldability at high humidity tend to be excellent, while when the ethylene content is equal to or less than the upper limit, sufficient gas barrier property tends to be easily obtained.

[0047] The above-described production method can provide an ethylene-vinyl ester copolymer useful for producing an EVOH resin that can be inhibited from discoloring due to thermal degradation.

[0048] <EVOH Resin> EVOH resin is obtained by saponifying the ethylene-vinyl ester copolymer obtained by the above-mentioned production method. EVOH resin is a water-insoluble thermoplastic resin that is mainly composed of ethylene structural units and vinyl alcohol structural units, and also contains a small amount of vinyl ester structural units that remain unsaponified.

[0049] For saponification, known methods can be employed, and the saponification is carried out using an alkali or acid catalyst with the copolymer obtained above dissolved in alcohol or aqueous alcohol. Examples of alcohol include methanol, ethanol, propanol, and tert-butanol, with methanol being particularly preferred. The concentration of the copolymer in the alcohol is appropriately selected depending on the viscosity of the system, but is typically selected from the range of 10 to 60% by mass. Examples of catalysts used for saponification include alkali catalysts such as hydroxides or alcoholates of alkali metals, such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and lithium methylate, and acid catalysts such as sulfuric acid, hydrochloric acid, nitric acid, metasulfonic acid, zeolite, and cation exchange resin.

[0050] The amount of such saponification catalyst used is appropriately selected depending on the saponification method, the desired degree of saponification, etc., but when an alkali catalyst is used, it is usually appropriate to use 0.001 to 0.1 equivalents, preferably 0.005 to 0.05 equivalents, based on the total amount of monomers such as vinyl ester monomers. Regarding such saponification methods, any of batch saponification, continuous saponification on a belt, and continuous tower saponification are possible depending on the desired degree of saponification, etc., and tower saponification under a constant pressure is preferably used because it allows for a reduction in the amount of alkali catalyst used during saponification and the saponification reaction tends to proceed with high efficiency.

[0051] The pressure during saponification varies depending on the target ethylene content, but is selected from the range of 0.1 to 0.8 MPa, the saponification temperature is 80 to 150°C, preferably 100 to 130°C, and the saponification time is selected from the range of 0.5 to 3 hours.

[0052] In this manner, an EVOH resin is obtained. In this embodiment, the ethylene content, degree of saponification, and melt flow rate (MFR) (210° C., load 2,160 g) of the obtained EVOH resin are not particularly limited.

[0053] The ethylene content of the EVOH resin is usually 20 to 60 mol%, preferably 21 to 55 mol%, particularly preferably 25 to 50 mol%, and especially 29 to 48 mol%. When the content is equal to or greater than the lower limit, the gas barrier properties and appearance of the resulting molded article, particularly the stretched film, at high humidity tend to be excellent, while when the content is equal to or less than the upper limit, the gas barrier properties of the stretched film tend to be excellent. The ethylene content (the proportion of ethylene structural units) is usually 1 It is measured by H-NMR measurement. For example, 1 The measurement method employs H-NMR measurement, using DMSO-d as the measurement solvent and setting the measurement temperature at 50°C.

[0054] The saponification degree of the vinyl ester component in the EVOH resin is usually 90 mol% or more, preferably 93 to 99.99 mol%, and particularly preferably 98 to 99.99 mol%. When the saponification degree is equal to or higher than the lower limit, the gas barrier property and moisture resistance of the stretched film tend to be excellent. The saponification degree of the vinyl ester component is usually 1 It is measured by H-NMR measurement. For example, 1 The measurement method employs H-NMR measurement, using DMSO-d as the measurement solvent and setting the measurement temperature at 50°C.

[0055] The melt flow rate (MFR) of the EVOH resin (210°C, load 2,160 g) is usually 1 to 100 g / 10 min, preferably 2 to 50 g / 10 min, and particularly preferably 3 to 30 g / 10 min. When the MFR is equal to or lower than the upper limit, the molded article tends to have excellent mechanical strength, while when the MFR is equal to or higher than the lower limit, the extrusion processability during molding tends to be excellent.

[0056] The EVOH resin of this embodiment may further contain a structural unit derived from the following comonomer: α-olefins such as propylene, isobutene, α-octene, α-dodecene, and α-octadecene; hydroxyl group-containing α-olefins such as 3-butene-1-ol, 4-pentene-1-ol, and 3-butene-1,2-diol; hydroxyl group-containing α-olefin derivatives such as esters and acylates thereof; unsaturated carboxylic acids or their salts, partial alkyl esters, complete alkyl esters, nitriles, amides, and anhydrides; unsaturated sulfonic acids or their salts; vinylsilane compounds; vinyl chloride; and styrene.

[0057] Furthermore, EVOH resins that have been "post-modified" by urethanization, acetalization, cyanoethylation, oxyalkylenation, or the like can also be used.

[0058] Among the above-mentioned modified products, EVOH resins in which primary hydroxyl groups have been introduced into the side chains by copolymerization are preferred in that they have good secondary formability in stretching treatments and vacuum / pressure molding, etc., and among these, EVOH resins having a 1,2-diol structure in the side chains are preferred.

[0059] The EVOH resin obtained by such a method can be used as it is, but may further contain additives generally added to EVOH resins, such as heat stabilizers, antioxidants, antistatic agents, colorants, ultraviolet absorbers, lubricants, plasticizers, light stabilizers, surfactants, antibacterial agents, desiccants, antiblocking agents, flame retardants, crosslinking agents, curing agents, foaming agents, crystal nucleating agents, antifogging agents, biodegradable additives, silane coupling agents, and oxygen absorbers, within limits that do not impair the effects of the present invention.

[0060] As the heat stabilizer, additives such as organic acids such as acetic acid, propionic acid, butyric acid, lauric acid, stearic acid, oleic acid, and behenic acid, or salts thereof such as alkali metal salts (sodium, potassium, and the like), alkaline earth metal salts (calcium, magnesium, and the like), and zinc salts, or inorganic acids such as sulfuric acid, sulfurous acid, carbonic acid, phosphoric acid, and boric acid, or salts thereof such as alkali metal salts (sodium, potassium, and the like), alkaline earth metal salts (calcium, magnesium, and the like), and zinc salts, may be added for the purpose of improving various physical properties such as thermal stability during melt molding. Of these, it is particularly preferable to add acetic acid, boric acid, and boron compounds including salts thereof, acetates, and phosphates.

[0061] When acetic acid is added, the amount added is usually 0.001 to 1 part by mass, preferably 0.005 to 0.2 parts by mass, and particularly preferably 0.010 to 0.1 parts by mass, per 100 parts by mass of the EVOH resin. When the amount of acetic acid added is equal to or greater than the lower limit, the effect of containing acetic acid tends to be sufficiently obtained, whereas when the amount is equal to or less than the upper limit, a uniform film tends to be easily obtained.

[0062] When a boron compound is added, the amount added is usually 0.001 to 1 part by mass, preferably 0.002 to 0.2 parts by mass, and particularly preferably 0.005 to 0.1 parts by mass, in terms of boron (analyzed by ICP atomic emission spectrometry after ashing) per 100 parts by mass of EVOH resin. When the amount of the boron compound added is equal to or greater than the lower limit, the effect of adding the boron compound tends to be sufficient, while when it is equal to or less than the upper limit, a uniform film tends to be obtained.

[0063] The amount of acetate or phosphate (including hydrogen phosphate) added is typically 0.0005 to 0.1 parts by mass, preferably 0.001 to 0.05 parts by mass, and particularly preferably 0.002 to 0.03 parts by mass, calculated as metal (analyzed by ICP atomic emission spectrometry after ashing) per 100 parts by mass of EVOH resin. When the amount added is equal to or greater than the lower limit, the effect of inclusion tends to be readily obtained, while when the amount added is equal to or less than the upper limit, a uniform film tends to be readily obtained. When two or more types of salts are added to the EVOH resin, the total amount is preferably within the above-mentioned range.

[0064] The method for adding acetic acid, a boron compound, an acetate, and a phosphate to an EVOH resin is not particularly limited, and examples thereof include: (i) a method in which a porous precipitate of an EVOH resin having a water content of 20 to 80% by mass is brought into contact with an aqueous solution of an additive to incorporate the additive into the porous EVOH resin, and then the resulting porous EVOH resin is dried; (ii) a method in which an additive is incorporated into a homogeneous solution of an EVOH resin (such as a water / alcohol solution), the resulting solution is extruded into a coagulation liquid in the form of a strand, and the resulting strand is cut into pellets and further dried; (iii) a method in which an EVOH resin and an additive are mixed together and then melt-kneaded in an extruder or the like; and (iv) a method in which an alkali (such as sodium hydroxide or potassium hydroxide) used in a saponification step during the production of an EVOH resin is neutralized with an organic acid such as acetic acid, and the amounts of the remaining organic acid such as acetic acid and by-produced salts are adjusted by washing with water. To obtain the effects of the present invention more significantly, methods (i) and (ii) are preferred because they provide excellent dispersibility of additives, and method (iv) is preferred when an organic acid and its salt are to be incorporated.

[0065] The EVOH resin obtained in this manner is preferably in the form of pellets. The pellets may be, for example, spherical, oval, cylindrical, cubic, or rectangular, but are usually oval or cylindrical. From the viewpoint of convenience when the pellets are subsequently used as a molding material, the size of the pellets is such that, in the case of an oval pellet, the minor axis is usually 1 to 10 mm, preferably 2 to 6 mm, and more preferably 2.5 to 5.5 mm, and the major axis is usually 1.5 to 30 mm, preferably 3 to 20 mm, and more preferably 3.5 to 10 mm. In the case of a cylindrical pellet, the diameter of the base is usually 1 to 6 mm, preferably 2 to 5 mm, and the length is usually 1 to 6 mm, preferably 2 to 5 mm.

[0066] The water content of the EVOH resin is usually 0.01 to 0.5% by mass, preferably 0.05 to 0.35% by mass, and more preferably 0.1 to 0.3% by mass.

[0067] The moisture content of the EVOH resin is measured and calculated by the following method: The mass (W1) of the EVOH resin before drying is weighed on an electronic balance, dried in a hot air dryer at 150°C for 5 hours, and then cooled in a desiccator for 30 minutes, after which the mass (W2) is weighed and calculated using the following formula: Moisture content (mass%) = [(W1 - W2) / W1] x 100

[0068] The EVOH resin according to this embodiment has an excellent discoloration suppression effect, and for example, the proportion of color number "1911" in the EVOH resin, as measured by the method described in the Examples below, is usually 45% or less, preferably 40% or less, more preferably 30% or less, even more preferably 20% or less, particularly preferably 15% or less, and most preferably 12% or less. Also, the proportion of color number "1637" in the EVOH resin, as measured by the method described in the Examples below, is usually 2.5% or less, preferably 2.0% or less, more preferably 1.5% or less, even more preferably 1.0% or less, and particularly preferably 0.5% or less.

[0069] When the EVOH resin is in the form of pellets, it is also preferable to adhere a known lubricant to the surface of the pellets in order to stabilize the feedability during melt molding. Examples of types of lubricants include higher fatty acids having 12 or more carbon atoms (e.g., lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, etc.), higher fatty acid esters (methyl esters, isopropyl esters, butyl esters, octyl esters of higher fatty acids, etc.), higher fatty acid amides (e.g., saturated higher fatty acid amides such as lauric acid amide, myristic acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, etc.; unsaturated higher fatty acid amides such as oleic acid amide, erucic acid amide, etc.; bis-higher fatty acid amides such as ethylene bisstearic acid amide, ethylene bisoleic acid amide, ethylene biserucic acid amide, ethylene bislauric acid amide, etc.), low-molecular-weight polyolefins (e.g., low-molecular-weight polyethylene or low-molecular-weight polypropylene with a molecular weight of about 500 to 10,000, or acid-modified products thereof), higher alcohols having 6 or more carbon atoms, ester oligomers, and fluorinated ethylene resins. These compounds may be used alone or in combination of two or more. The content of such lubricants is usually 5% by mass or less, preferably 1% by mass or less, of the EVOH resin. The lower limit is usually 0% by mass.

[0070] The obtained EVOH pellets are preferably washed because it is preferable that the amount of remaining active polymerization catalyst, polymerization inhibitor, and saponification catalyst is small from the viewpoint of improving the appearance of the obtained film and multilayer structure described below. Examples of solvents used for washing include water, alcohols having 1 to 5 carbon atoms such as methanol, ethanol, propanol, and tert-butanol, and esters having 3 to 10 carbon atoms such as methyl acetate and ethyl acetate. The washing method is not particularly limited, and known methods can be used, such as immersing the EVOH resin in such a solvent, spraying the solvent on the resin, and allowing it to flow over the resin.

[0071] The EVOH resin obtained in this manner is prepared into various forms, such as pellets, powder, or liquid, and is provided as a molding material for various molded articles. In particular, in this embodiment, when the EVOH resin is provided as a material for melt molding, the effects of the present invention tend to be more efficiently obtained, which is preferable. Note that the EVOH resin also includes resin compositions obtained by mixing resins other than EVOH resin.

[0072] Examples of the molded article include a single layer film molded from an EVOH resin, and a multilayer structure having at least one layer made of an EVOH resin.

[0073] <Multilayer Structure> A multilayer structure according to one embodiment of the present invention (hereinafter referred to as "the present multilayer structure") includes a layer containing an EVOH resin, and preferably includes a layer consisting of only an EVOH resin. The layer containing an EVOH resin (hereinafter simply referred to as "EVOH resin layer") can be laminated with another substrate containing a thermoplastic resin other than EVOH resin as a main component (hereinafter the resin used in the substrate may be abbreviated as "substrate resin") to impart further strength, protect the EVOH resin layer from the effects of moisture, etc., or impart other functions.

[0074] Examples of the base resin include polyethylene-based resins such as linear low-density polyethylene, low-density polyethylene, very low-density polyethylene, medium-density polyethylene, high-density polyethylene, ethylene-propylene (block and random) copolymers, and ethylene-α-olefin (α-olefin having 4 to 20 carbon atoms) copolymers; polypropylene-based resins such as polypropylene and propylene-α-olefin (α-olefin having 4 to 20 carbon atoms) copolymers; (unmodified) polyolefin-based resins such as polybutene, polypentene, and polycyclic olefin-based resins (polymers having a cyclic olefin structure in the main chain and / or side chain); and polyolefin-based resins obtained by combining these polyolefins with unsaturated carboxylic acids or esters thereof. Examples of the polyolefin resin include polyolefin resins in the broad sense, including modified olefin resins such as unsaturated carboxylic acid-modified polyolefin resins graft-modified with ter, ionomers, ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-acrylic acid ester copolymers, polyester resins, polyamide resins (including copolymerized polyamides), polyvinyl chloride, polyvinylidene chloride, acrylic resins, polystyrene resins, vinyl ester resins, polyester elastomers, polyurethane elastomers, polystyrene elastomers, halogenated polyolefins such as chlorinated polyethylene and chlorinated polypropylene, and aromatic or aliphatic polyketones. These may be used alone or in combination of two or more.

[0075] Among these, hydrophobic resins such as polyamide-based resins, polyolefin-based resins, polyester-based resins, and polystyrene-based resins are preferred, and more preferred are polyolefin-based resins such as polyethylene-based resins, polypropylene-based resins, polycyclic olefin-based resins, and unsaturated carboxylic acid-modified polyolefin-based resins thereof.

[0076] The layer structure of the present multilayer structure can be any combination, such as a / b, b / a / b, a / b / a, a1 / a2 / b, a / b1 / b2, b2 / b1 / a / b1 / b2, or b2 / b1 / a / b1 / a / b1 / b2, where a denotes the EVOH resin layer and b denotes the base resin layer. Furthermore, when R denotes a recycled layer containing a mixture of EVOH resin and a thermoplastic resin other than EVOH resin, obtained by remelting and molding edges or defective products generated during the manufacturing process of the present multilayer structure, the layer structure can also be b / R / a, b / R / a / b, b / R / a / R / b, b / a / R / a / b, b / R / a / R / a / R / b, or b / R / a / R / a / R / b. The total number of layers in the present multilayer structure is usually 2 to 15, and preferably 3 to 10. In the above layer structure, an adhesive resin layer containing an adhesive resin may be interposed between the respective layers, if necessary.

[0077] Known adhesive resins can be used, and may be selected appropriately depending on the type of thermoplastic resin used in the base resin layer "b." Representative examples include modified polyolefin polymers containing carboxy groups, which are obtained by chemically bonding an unsaturated carboxylic acid or its anhydride to a polyolefin resin via an addition reaction, a graft reaction, or the like. Examples of modified polyolefin polymers containing carboxy groups include maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, maleic anhydride-grafted ethylene-propylene (block and random) copolymers, maleic anhydride-grafted ethylene-ethyl acrylate copolymers, maleic anhydride-grafted ethylene-vinyl acetate copolymers, maleic anhydride-grafted polycyclic olefin resins, and maleic anhydride-grafted polyolefin resins. These may be used alone or in combination of two or more.

[0078] In the present multilayer structure, when adhesive resin layers are used between the EVOH resin layer and the base resin layer, it is preferable to use an adhesive resin with excellent hydrophobicity, since the adhesive resin layers are located on both sides of the EVOH resin layer.

[0079] The base resin and adhesive resin may contain conventionally known plasticizers, fillers, clay (montmorillonite, etc.), colorants, antioxidants, antistatic agents, lubricants, core materials, antiblocking agents, waxes, etc., within a range that does not impair the spirit of the present invention (for example, 30% by mass or less, preferably 10% by mass or less, based on the total mass of the resin). These may be used alone or in combination of two or more kinds.

[0080] The lamination of the EVOH resin layer and the base resin layer (including the case where an adhesive resin layer is interposed) can be performed by a known method. Examples include a method of melt-extrusion laminating the base resin onto an EVOH resin film, sheet, or the like, a method of melt-extrusion laminating the EVOH resin onto the base resin layer, a method of co-extruding the EVOH resin and the base resin, a method of dry-laminating the EVOH resin (layer) and the base resin (layer) using a known adhesive such as an organic titanium compound, an isocyanate compound, a polyester compound, or a polyurethane compound, and a method of applying a solution of the EVOH resin onto the base resin and then removing the solvent. Among these, in consideration of cost and environmental considerations, a method including a step of melt-molding a layer including the EVOH resin layer is preferred for production, and specifically, a co-extrusion method is preferred.

[0081] The multilayer structure may be subjected to a (heat) stretching treatment as needed. The stretching treatment may be either uniaxial stretching or biaxial stretching, and in the case of biaxial stretching, it may be simultaneous stretching or sequential stretching. Furthermore, the stretching method may be a roll stretching method, tenter stretching method, tubular stretching method, stretch-blow method, vacuum pressure forming, or the like, whichever provides a higher stretch ratio. The stretching temperature is a temperature near the melting point of the multilayer structure, and is usually selected from the range of 40 to 170°C, preferably about 60 to 160°C. When the stretching temperature is equal to or higher than the lower limit, the stretchability tends to be good, and when it is equal to or lower than the upper limit, a stable stretched state tends to be easily maintained.

[0082] The stretched multilayer structure may be heat-set to provide dimensional stability. Heat-setting can be performed by known means, for example, by subjecting the stretched multilayer structure to heat treatment while maintaining tension, typically at 80 to 180°C, preferably 100 to 165°C, for typically 2 to 600 seconds.

[0083] When the stretched multilayer structure is to be used as a shrink film, the heat-shrinkability can be imparted to the multilayer structure by, for example, cooling and fixing the multilayer structure by applying cold air thereto, without carrying out the heat-setting process described above.

[0084] The thickness of the present multilayer structure (including a stretched structure), and further the thickness of the EVOH resin layer, base resin layer, and adhesive resin layer constituting the multilayer structure, cannot be generally determined depending on the layer configuration, type of base resin, type of adhesive resin, intended use, packaging form, required physical properties, etc., but the thickness of the present multilayer structure (including a stretched structure) is usually 10 to 5,000 μm, preferably 30 to 3,000 μm, and particularly preferably 50 to 2,000 μm. The EVOH resin layer is usually 1 to 500 μm, preferably 3 to 300 μm, and particularly preferably 5 to 200 μm; the base resin layer is usually 5 to 3,000 μm, preferably 10 to 2,000 μm, and particularly preferably 20 to 1,000 μm; and the adhesive resin layer is usually 0.5 to 250 μm, preferably 1 to 150 μm, and particularly preferably 3 to 100 μm.

[0085] Furthermore, the thickness ratio of the EVOH resin layer to the base resin layer in the multilayer structure (EVOH resin layer / base resin layer), expressed as the ratio between the thickest layers when there are multiple layers of each type, is usually 1 / 99 to 50 / 50, preferably 5 / 95 to 45 / 55, and particularly preferably 10 / 90 to 40 / 60. Furthermore, the thickness ratio of the EVOH resin layer to the adhesive resin layer in the multilayer structure (EVOH resin layer / adhesive resin layer), expressed as the ratio between the thickest layers when there are multiple layers of each type, is usually 10 / 90 to 99 / 1, preferably 20 / 80 to 95 / 5, and particularly preferably 50 / 50 to 90 / 10.

[0086] The multilayer structure can also be used to produce cup- or tray-shaped multilayer containers. In this case, a drawing method is typically used, specifically vacuum forming, pressure forming, vacuum-pressure forming, plug-assisted vacuum-pressure forming, etc. Furthermore, blow molding is used to produce tube- or bottle-shaped multilayer containers (laminate structure) from a multilayer parison (a hollow tubular preform before blowing). Specific examples include extrusion blow molding (double-head, mold-moving, parison-shift, rotary, accumulator, horizontal parison, etc.), cold parison blow molding, injection blow molding, and biaxial stretch blow molding (extrusion-type cold parison biaxial stretch blow molding, injection-type cold parison biaxial stretch blow molding, injection-molding in-line biaxial stretch blow molding, etc.). The resulting laminate may be subjected to heat treatment, cooling treatment, rolling treatment, printing treatment, dry lamination treatment, solution or melt coating treatment, bag making, deep drawing, box processing, tube processing, splitting processing, etc. as required.

[0087] Single layer films formed from the EVOH resin, bags made of this multilayer structure, and containers and lids made of cups, trays, tubes, bottles, etc. are useful as packaging materials and containers for various purposes, including general foods, as well as seasonings such as mayonnaise and dressings, fermented foods such as miso, oily foods such as salad oil, beverages, cosmetics, pharmaceuticals, etc.

[0088] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. In the examples, "parts" and "%" are based on mass.

[0089] Example 1 Preparation of Vinyl Ester Monomer Vinyl acetate was purified by distillation in advance, and hydroquinone was added as a polymerization inhibitor to the vinyl acetate at a concentration of 60 ppb. The polymerization activity of the resulting vinyl acetate was 561 seconds, and the oxygen concentration of the vinyl acetate was 1 ppm. The polymerization activity of the vinyl acetate was calculated as follows, and the oxygen concentration of the vinyl acetate was measured using gas chromatography.

[0090] [Measurement of vinyl acetate polymerization activity] 100 mL of the obtained vinyl acetate was weighed into an Erlenmeyer flask and immersed in a 20 ° C. thermostatic bath for 1 hour. 0.130 g of lauroyl peroxide (Kishida Chemical Co., Ltd.) was weighed into a glass test tube (Nichiden Rika Glass Co., Ltd., P-21) with an inner diameter of 18 mm (outer diameter 21 mm) and a length of 200 mm. 10 mL of the thermostatically maintained vinyl acetate was weighed and added to the test tube, and the lauroyl peroxide was completely dissolved in the vinyl acetate in the thermostatic bath at 20 ° C. After dissolution, the test tube was immersed in a 65.0 ° C. thermostatic bath. The number of seconds from the time the test tube was immersed in the 65.0 ° C. thermostatic bath to the time when bubbles appeared and the height from the surface of the bubble layer to the bottom of the bubble layer reached 3 mm or more was measured, and this number of seconds was taken as the polymerization activity. The height of the surface of the bubble layer and the bottom of the bubble layer was measured at the center of the test tube.

[0091] [Preparation of Ethylene-Vinyl Ester Copolymer] Ethylene-vinyl acetate copolymer was polymerized using a polymerization vessel equipped with a stirrer under the following conditions. (Polymerization conditions) - 7,800 parts vinyl acetate - 1,500 parts methanol - 0.79 parts peroxyester (polymerization catalyst) - 0.23 parts citric acid - 3.5 MPa ethylene pressure - 67°C reaction temperature - 6.5 hours The ethylene content of the ethylene-vinyl acetate copolymer obtained was 29 mol %, and the conversion of vinyl acetate to polymerization was 53%. After completion of the reaction, a methanol solution of sorbic acid was added (260 ppm of sorbic acid relative to the charged vinyl acetate).

[0092] [Preparation of EVOH Resin] 300 parts of methanol was added to 100 parts of a methanol solution containing 50% of the ethylene-vinyl acetate copolymer, and 71 parts of a methanol solution containing 0.05 equivalents of sodium hydroxide relative to the remaining vinyl acetate groups in the copolymer was added, followed by a saponification reaction at 80°C. Next, 100 parts of an aqueous methanol solution with a water content of 60% was added to the methanol solution of the EVOH resin under azeotropic conditions, and the methanol was distilled off until the resin concentration in the methanol / aqueous solution of the EVOH resin reached 40%, yielding a homogeneous methanol / water solution of the EVOH resin. The resulting methanol / aqueous solution of the EVOH resin was then solidified into a thin plate and cut with a knife to obtain porous pellets with sides of 3 mm. In the solution washing treatment, 100 parts of the EVOH resin intermediate pellets were added to 250 parts of an aqueous solution containing 150 ppm of acetic acid, 180 ppm of sodium acetate, 150 ppm of calcium dihydrogen phosphate, and 130 ppm of sodium dihydrogen phosphate, and the mixture was stirred at 30 to 35° C. for 1 hour, after which the aqueous solution was replaced and the solution washing treatment was carried out five times in total in the same manner. The pellets were dried at 121° C. for 10 hours in a nitrogen gas stream having an oxygen concentration of 0.5% by volume or less, to obtain EVOH resin pellets.

[0093] Example 2 EVOH resin pellets were obtained by the same procedure as in Example 1, except that hydroquinone was added to vinyl acetate so as to give a concentration of 1,000 ppb relative to vinyl acetate. The polymerization activity of this vinyl acetate was 584 seconds, and the oxygen concentration was 1 ppm.

[0094] Example 3 EVOH resin pellets were obtained by the same procedure as in Example 1, except that hydroquinone was added to vinyl acetate so that the concentration was 5,000 ppb relative to the vinyl acetate. The polymerization activity of this vinyl acetate was 716 seconds, and the oxygen concentration was 1 ppm.

[0095] Example 4 EVOH resin pellets were obtained by the same procedure as in Example 1, except that sorbic acid was used instead of hydroquinone added to vinyl acetate in Example 1. The polymerization activity of this vinyl acetate was 555 seconds, and the oxygen concentration was 1 ppm.

[0096] Example 5 EVOH resin pellets were obtained by the same procedure as in Example 1, except that hydroquinone was added to vinyl acetate so that the concentration was 10,000 ppb relative to the vinyl acetate. The polymerization activity of this vinyl acetate was 888 seconds, and the oxygen concentration was 1 ppm.

[0097] Example 6 EVOH resin pellets were obtained in the same manner as in Example 1, except that hydroquinone was added to vinyl acetate so as to give a concentration of 1,000 ppb relative to the vinyl acetate, and citric acid was not used in preparing the ethylene-vinyl ester copolymer. The polymerization activity of this vinyl acetate was 584 seconds, and the oxygen concentration was 1 ppm.

[0098] Comparative Example 1 EVOH resin pellets were obtained by the same procedure as in Example 1, except that no polymerization inhibitor was added to the distillation-purified vinyl acetate in Example 1. The polymerization activity of this vinyl acetate was 544 seconds, and the oxygen concentration was 1 ppm.

[0099] Comparative Example 2: A mixed gas of air and nitrogen (1:10) was injected into the vinyl acetate purified by distillation in Example 1 to adjust the oxygen concentration of the vinyl acetate to 3 ppm. EVOH resin pellets were obtained by the same procedure as in Example 1, except that no polymerization inhibitor was added to the vinyl acetate. The polymerization activity of the vinyl acetate was 547 seconds, and the oxygen concentration was 3 ppm.

[0100] The following coloration evaluation was carried out using the obtained EVOH resin pellets of Examples 1 to 6 and Comparative Examples 1 and 2. The results are shown in Table 1 below.

[0101] [Coloration Evaluation] The EVOH resin pellets (1 g) obtained in Examples 1 to 6 and Comparative Examples 1 and 2 were used as samples, and coloration was evaluated using a visual analyzer IRIS VA400 (manufactured by Alphamos). Data analysis software: AlphaSoft V14.3; Objective lens: 25 mm (manufactured by Basler); Illumination mode: Top and bottom illumination. Evaluation method: EVOH resin pellets (1 g) were placed on a tray in the chamber of the visual analyzer, and a planar image of the entire sample for color evaluation was taken with a CCD camera. The image was then processed using the data analysis software to evaluate the color pattern of the sample. The resulting color pattern was evaluated for the percentage of color number "1911" (R: 120, G: 120, B: 120) in the EVOH resin and the percentage of color number "1637" (R: 104, G: 104, B: 88) in the EVOH resin. Color numbers "1911" and "1637" are colors with a very deep yellow tint. The smaller this ratio, the more suppressed the coloring and the higher the thermal stability.

[0102]

[0103] As can be seen from Table 1, the EVOH resins of Examples 1 to 6 obtained using vinyl acetate having a specific polymerization activity had a smaller proportion of a very deep yellowish color and were less susceptible to discoloration due to thermal degradation than the EVOH resins of Comparative Examples 1 and 2, which did not use vinyl acetate having no specific polymerization activity. Furthermore, the multilayer structures including a layer containing the EVOH resin of Examples 1 to 6 also exhibited less discoloration due to thermal degradation.

[0104] Although the above examples show specific embodiments of the present invention, the examples are merely illustrative and should not be construed as limiting. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention.

[0105] The EVOH resin obtained from the ethylene-vinyl ester copolymer produced by the production method of the present invention can suppress discoloration due to thermal degradation, and is therefore useful as a packaging material for various foods, as well as for seasonings such as mayonnaise and dressings, fermented foods such as miso, oily foods such as salad oil, beverages, cosmetics, medical products, etc., and as a water-soluble film for packaging materials for agricultural chemicals, detergents, civil engineering additives, disinfectants, dyes, pigments, etc.

Claims

1. A method for producing an ethylene-vinyl ester copolymer by copolymerizing a vinyl ester monomer with ethylene, wherein the polymerization activity of the vinyl ester monomer measured by the following measurement method is 550 seconds or more. Measurement method: A glass test tube (inner diameter 18 mm, length 200 mm) containing 10 mL of vinyl ester monomer and 0.130 g of lauroyl peroxide as a polymerization catalyst was immersed in a thermostatic bath at 20°C to dissolve the polymerization catalyst in the vinyl ester monomer. The test tube was then immersed in a thermostatic bath at 65.0°C. The number of seconds from the time when bubbles were generated until the height from the surface of the bubble layer to the bottom of the bubble layer was 3 mm or more was measured, and this number of seconds was taken as the polymerization activity. The height of the surface of the bubble layer and the height of the bottom of the bubble layer are measured at the center of the test tube.

2. The method for producing an ethylene-vinyl ester copolymer according to claim 1, wherein the polymerization activity of the vinyl ester monomer is 550 to 850 seconds.

3. The method for producing an ethylene-vinyl ester copolymer according to claim 1 or 2, further comprising a step of adjusting the polymerization activity of the vinyl ester monomer to 550 seconds or more by adding a polymerization inhibitor.

4. The method for producing an ethylene-vinyl ester copolymer according to claim 3, wherein the content of the polymerization inhibitor is 5 to 12,000 ppb based on the vinyl ester monomer.

5. The method for producing an ethylene-vinyl ester copolymer according to claim 3, wherein the vinyl ester monomer is a vinyl ester monomer after distillation purification.

6. The method for producing an ethylene-vinyl ester copolymer according to claim 1 or 2, further comprising a chelating agent.

7. The method for producing an ethylene-vinyl ester copolymer according to claim 1 or 2, wherein the oxygen concentration of the vinyl ester monomer is less than 5 ppm relative to the vinyl ester monomer.

8. A saponified ethylene-vinyl ester copolymer obtained by saponifying the ethylene-vinyl ester copolymer obtained by the production method according to claim 1 or 2.

9. Pellets containing the saponified ethylene-vinyl ester copolymer according to claim 8.

10. A multilayer structure having a layer containing the saponified ethylene-vinyl ester copolymer according to claim 8.

11. A method for producing the multilayer structure according to claim 10, comprising the step of melt-molding the saponified ethylene-vinyl ester copolymer.

Images