Ethylene-vinyl alcohol copolymer, method for producing same, and use thereof

Abstract

This ethylene-vinyl alcohol copolymer contains more than 5 mol% to less than 10 mol% of an ethylene unit and has a saponification degree of 90-99.9 mol%. In the ethylene-vinyl alcohol copolymer, the total contained amount of lactone rings and carboxy groups located at terminals of the molecule chain of the ethylene-vinyl alcohol copolymer is not less than 0.08 mol%. When a heavy water solution containing the ethylene-vinyl alcohol copolymer at a concentration of 10 mass% is prepared and is left still for 14 days at 30°C, and a 1H pulse NMR measurement is performed on the solution by the solid echo method, a crystal component amount A1 determined from a decay curve of the magnetization intensity reflecting spin-spin relaxation obtained from the measurement is not more than 10%. Thus, provided is an ethylene-vinyl alcohol copolymer with which it is possible to prepare an aqueous solution capable of forming a coating film having excellent gas barrier properties even after being stored for a long period of time.

Description

Ethylene-vinyl alcohol copolymer, method for producing the same, and its uses

[0001] This invention relates to an ethylene-vinyl alcohol copolymer, a method for producing the same, and its uses. It also relates to an aqueous solution containing the ethylene-vinyl alcohol copolymer and its uses.

[0002] Polyvinyl alcohol (hereinafter sometimes abbreviated as "PVA") is a water-soluble synthetic polymer and is widely used as a raw material for the synthetic fiber vinylon, as a paper processing agent, a fiber processing agent, an adhesive, a stabilizer for emulsion polymerization and suspension polymerization, an inorganic binder, and a film. When PVA is used as a gas barrier layer, it is favored for its high gas barrier properties, transparency, and the fact that it presents few problems in terms of disposal. However, it is known that while PVA film has high gas barrier properties in a dry atmosphere, its gas barrier properties decrease when the relative humidity exceeds about 70% as the amount of moisture absorbed by the PVA film increases.

[0003] One method used to reduce the hygroscopic properties of PVA is to use an ethylene-vinyl alcohol copolymer in which ethylene is copolymerized at a concentration of 20 mol% or more. However, such ethylene-vinyl alcohol copolymers are insoluble in water, and when handled in solution, organic solvents are used, which has the problem of significantly deteriorating the working environment.

[0004] As a method to solve the above problems, Patent Document 1 describes an ethylene-vinyl alcohol copolymer having an ethylene unit content of 2 to 19 mol%, a degree of polymerization of 200 to 2000, a degree of saponification of 80 to 99.99 mol%, and a total content of carboxyl groups and lactone rings of 0.02 to 0.4 mol%, and states that this ethylene-vinyl alcohol copolymer has excellent gas barrier properties, and that an aqueous solution containing it has excellent low-temperature stability. However, in this ethylene-vinyl alcohol copolymer, carboxyl groups and lactone rings are introduced into the molecular chain by copolymerizing unsaturated carboxylic acids such as maleic anhydride and itaconic acid, and the content of carboxyl groups and lactone rings at the ends of the molecular chain was small.

[0005] Furthermore, Patent Document 2 describes ethylene-vinyl alcohol copolymer particles comprising an ethylene unit content of 1 mol% or more and less than 20 mol%, a viscosity-average degree of polymerization of 200 to 5000, and a degree of saponification of 80 to 99.99 mol%, wherein the degree of crystallinity in water at 30°C Cw(30°C) and the degree of crystallinity in water at 70°C Cw(70°C), as determined by pulsed NMR, satisfy a predetermined relationship, and it is stated that the particles have excellent solubility in water and dissolve in water at a relatively low temperature and in a short time.

[0006] Japanese Patent Publication No. 2000-309607 WO2019 / 078181A1

[0007] However, aqueous solutions of ethylene-vinyl alcohol copolymers described in Patent Documents 1 and 2 sometimes exhibited reduced gas barrier properties when applied to a substrate after prolonged storage. The present invention was made to solve this problem and aims to provide an ethylene-vinyl alcohol copolymer and a method for producing the same that can prepare an aqueous solution capable of forming a coating film with excellent gas barrier properties even after prolonged storage. Furthermore, the present invention aims to provide an aqueous solution containing such an ethylene-vinyl alcohol copolymer, a coating agent consisting of the aqueous solution, a laminate obtained by applying the coating agent to a substrate, and a laminate containing an ethylene-vinyl alcohol copolymer.

[0008] The present invention is as follows: [1] An ethylene-vinyl alcohol copolymer having an ethylene unit content of more than 5 mol% and less than 10 mol%, and a degree of saponification of 90 mol% or more and 99.9 mol% or less; the total content of carboxyl groups and lactone rings located at the ends of the molecular chain of the ethylene-vinyl alcohol copolymer is 0.08 mol% or more; after preparing a 10% by mass heavy aqueous solution of the ethylene-vinyl alcohol copolymer, the solution is left to stand at 30°C for 14 days and then subjected to the Solid Echo method. １ An ethylene-vinyl alcohol copolymer in which the decay curve of magnetization intensity due to spin-spin relaxation obtained by H-pulse NMR measurement is fitted to the following equation (1), and the amount of crystalline component A1 obtained by the following equation (2) is 10% or less.

[0009]

[0010]

[0011] (In formulas (1) and (2), y is the magnetization intensity at time t, y0 is a constant, tau1 is the relaxation time of the crystalline component, tau2 is the relaxation time of the constrained amorphous component, tau3 is the relaxation time of the unconstrained amorphous component, a1 is the relaxation intensity of the crystalline component, a2 is the relaxation intensity of the constrained amorphous component, and a3 is the relaxation intensity of the unconstrained amorphous component.) [2] The ethylene-vinyl alcohol copolymer according to [1], wherein the total content of carboxyl groups and lactone rings located at the terminal or internal of the molecular chain is 0.08 mol% or more and less than 0.2 mol%. [3] The ethylene-vinyl alcohol copolymer according to [1] or [2], wherein the methanol content is 3% by mass or less. [4] An aqueous solution containing the ethylene-vinyl alcohol copolymer according to any one of [1] to [3]. [5] A coating agent comprising the aqueous solution according to [4]. [6] A laminate obtained by coating a substrate with the coating agent according to [5]. [7] A laminate comprising a base material and a layer containing the ethylene-vinyl alcohol copolymer according to any one of [1] to [3]. [8] A method for producing an ethylene-vinyl alcohol copolymer according to any one of [1] to [3], comprising copolymerizing ethylene and vinyl acetate in the presence of a methanol solvent and a radical initiator to obtain an ethylene-vinyl acetate copolymer, and then saponifying it to obtain an ethylene-vinyl alcohol copolymer, wherein the mass ratio of vinyl acetate to methanol is 1.5 or more during copolymerization.

[0012] The ethylene-vinyl alcohol copolymer of the present invention can form a coating film with excellent gas barrier properties even after an aqueous solution containing it has been stored for a long period of time. Furthermore, such an ethylene-vinyl alcohol copolymer can be produced by the manufacturing method of the present invention.

[0013] [Ethylene-Vinyl Alcohol Copolymer] The ethylene-vinyl alcohol copolymer of the present invention is an ethylene-vinyl alcohol copolymer having an ethylene unit content of more than 5 mol% and less than 10 mol%, and a degree of saponification of 90 mol% or more and 99.9 mol% or less; the total content of carboxyl groups and lactone rings located at the ends of the molecular chain of the ethylene-vinyl alcohol copolymer is 0.08 mol% or more; and after preparing a 10% by mass heavy aqueous solution of the ethylene-vinyl alcohol copolymer, the solution was left to stand at 30°C for 14 days and subjected to the Solid Echo method. １ The ethylene-vinyl alcohol copolymer is obtained by fitting the decay curve of magnetization intensity due to spin-spin relaxation, obtained by H-pulse NMR measurement, to the following equation (1), and determining the amount of crystalline component A1 using the following equation (2) to be 10% or less.

[0014]

[0015]

[0016] In equations (1) and (2) above, y is the magnetization intensity at time t, y0 is a constant, tau1 is the relaxation time of the crystalline component, tau2 is the relaxation time of the constrained amorphous component, tau3 is the relaxation time of the unconstrained amorphous component, a1 is the relaxation intensity of the crystalline component, a2 is the relaxation intensity of the constrained amorphous component, and a3 is the relaxation intensity of the unconstrained amorphous component.

[0017] [Crystalline component amount A1] After preparing a 10% by mass heavy aqueous solution of ethylene-vinyl alcohol copolymer, the solution was left to stand at 30°C for 14 days. １ A key feature of the ethylene-vinyl alcohol copolymer of the present invention is that the amount of crystalline component A1, determined from equations (1) and (2) by H-pulse NMR measurement, is 10% or less. This point will be explained below.

[0018] [Pulsed NMR] First, let's explain pulsed NMR. Unlike general-purpose high-resolution NMR used for determining the structure of organic compounds, pulsed NMR is related to the molecular motion within the system. １ This is an analytical method that can measure the relaxation time of the H nucleus. Therefore, it can measure all regardless of chemical shift.１ H nuclei are measured collectively. Furthermore, pulsed NMR, with its high quantitative accuracy, can be used to determine the relative abundance of each moving component within the system.

[0019] A pulsed NMR measuring device has a static magnetic field generated by an electromagnet within the device. In a static magnetic field, the direction of the nuclear spin of a hydrogen nucleus is oriented along the direction of the static magnetic field. When a pulsed magnetic field is applied in this state, the nuclear spin of the hydrogen nucleus becomes tilted 90° from the direction along the static magnetic field (excited state). Subsequently, the direction of the excited nuclear spin returns macroscopically to the direction along the original static magnetic field. The process by which the direction of the nuclear spin returns from the excited state to the original state is called "T2 relaxation," and the time required for this process is called the relaxation time (tau). In the case of single-component relaxation, the magnetization intensity (y) at time (t) is expressed by the following equation (3), using the relaxation intensity (a) in the excited state, the relaxation time (tau), and constants (y0, W).

[0020]

[0021] Here, W is called the Weibull coefficient, and when W = 1, equation (3) becomes of the exponential type, and when W = 2, equation (3) becomes of the Gaussian type. For typical polymers, 1 ≤ W ≤ 2 is satisfied.

[0022] In the case of T2 relaxation, hydrogen nuclei decay from an excited state to their original state while exchanging energy with other hydrogen nuclei. Therefore, if the molecular mobility of the sample is high, the interaction between protons in close proximity is small, making it difficult for the entire system to decay, and thus the relaxation time is longer. Conversely, if the molecular mobility of the sample is low, the relaxation time is shorter. Therefore, in the case of a crystalline polymer, the relaxation time is short in the crystalline component and long in the amorphous component. Furthermore, in the constrained amorphous component, which is the boundary between the crystalline and amorphous components, the relaxation time is intermediate between the two.

[0023] Conventional X-ray measurements can only measure the amounts of crystalline and amorphous components, but pulsed NMR can measure the amounts of constrained amorphous components in addition to crystalline and amorphous components. In particular, pulsed NMR can determine the proportion of each moving component based on the mobility of the crystalline polymer, which cannot be obtained by X-ray measurements. The proportions of these components more accurately reflect the various properties required of water-soluble resins. Therefore, by using pulsed NMR, water-soluble resins can be evaluated more appropriately.

[0024] In actual crystalline polymers, the above-mentioned crystalline components, constrained amorphous components, and unconstrained amorphous components are mixed together. Therefore, when the crystalline polymer is subjected to pulsed NMR measurement, the resulting relaxation curve is observed as the sum of relaxation components originating from crystalline components with short relaxation times, relaxation components originating from unconstrained amorphous components with long relaxation times, and relaxation components originating from constrained amorphous components with relaxation times intermediate between the two.

[0025] In this invention, we attempted to fit the relaxation curve obtained by the linear least squares method to the following equation (4). That is, when the relaxation time of the crystalline component is tau1, the relaxation time of the constrained amorphous component is tau2, and the relaxation time of the unconstrained amorphous component is tau3, the total magnetization intensity (y) of the sample at time (t) is given by the following equation (4), using the constant y0 and the relaxation intensities a1, a2, and a3 in the excited state.

[0026]

[0027] As a result of our diligent verification efforts, we have determined that a fitting function equation (fitting function equation) is available that allows for stable and reproducible fitting between resins produced under various synthesis conditions. This equation fixes the crystalline and constrained amorphous components as Gaussian relaxation (W1=2, W2=2), and the unconstrained amorphous component as Exp-type relaxation (W3=1). In this invention, the following equation (1) obtained in this manner is used in the Solid Echo method. １ We decided to fit the magnetization intensity decay curve due to spin-spin relaxation, obtained by H-pulse NMR measurement.

[0028]

[0029] In the present invention, a1, a2, a3 and y0 derived from the above formula (1) are obtained, and the ratio (%) of each component of a1, a2 and a3 to the total of a1, a2 and a3 (a1+a2+a3) is defined as the amount of crystalline component (A1), the amount of constrained amorphous component (A2), and the amount of unconstrained amorphous component (A3). Accordingly, the value of the amount of crystalline component (A1) (%) is represented by the following formula (2).

[0030]

[0031] In the above formula (1) and formula (2), y is the magnetization intensity at time t, y0 is a constant, tau1 is the relaxation time of the crystalline component, tau2 is the relaxation time of the constrained amorphous component, tau3 is the relaxation time of the unconstrained amorphous component, a1 represents the relaxation intensity of the crystalline component, a2 represents the relaxation intensity of the constrained amorphous component, and a3 represents the relaxation intensity of the unconstrained amorphous component.

[0032] In the pulse NMR measurement according to the present invention, an ethylene-vinyl alcohol copolymer is dissolved in heavy water under 1.1 kgf / cm ２ pressurization conditions at 120°C for 1 hour to prepare a heavy aqueous solution with a concentration of 10% by mass, and after allowing the solution to stand still in a constant temperature chamber at 30°C for 14 days, １ 1H pulse NMR measurement is performed. The amount of each component is calculated from the obtained spin-spin relaxation curve.

[0033] The present inventors have found that when an ethylene-vinyl alcohol copolymer having a crystalline component content (A1) of 10% or less obtained by the above measurement method is used, the stability of the aqueous solution during long-term storage can be controlled.

[0034] In the present invention, it is important to control the amount of crystalline component (A1) to 10% or less. Examples of the method for controlling the amount of crystalline component (A1) within the above range include a method of adjusting the type of ethylene-vinyl alcohol copolymer (degree of saponification, degree of polymerization, content of ethylene units, terminal structures such as carboxy groups and lactone rings, etc.) and adjusting the ratio of vinyl acetate to methanol when copolymerizing ethylene and vinyl acetate. The upper limit of the amount of crystalline component (A1) is preferably 8%, and may be 5% or 3% in some cases.

[0035] [Vinyl Ester] The ethylene-vinyl alcohol copolymer of the present invention is obtained by saponifying an ethylene-vinyl ester copolymer obtained by copolymerizing ethylene and a vinyl ester. Examples of the vinyl ester to be used include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Among these, vinyl acetate is preferred.

[0036] [Ethylene Unit Content] The content of ethylene units in the ethylene-vinyl alcohol copolymer of the present invention is more than 5 mol% and less than 10 mol%. When the content of ethylene units is 5 mol% or less, the gas barrier property under high humidity of a coating film obtained by coating becomes insufficient. The content of ethylene units is preferably 6 mol% or more, which can prevent a decrease in gas barrier property even when an aqueous solution after long-term storage is applied to a substrate. On the other hand, when the content of ethylene units is 10 mol% or more, the water solubility of the ethylene-vinyl alcohol copolymer decreases, and the gas barrier property decreases when the solution is applied to a substrate after long-term storage. The content of ethylene units is preferably 9 mol% or less.

[0037] The content of ethylene units can be determined, for example, from an ethylene-vinyl ester copolymer that is a precursor or reacetylated product of the ethylene-vinyl alcohol copolymer by １ 1H-NMR measurement. After reprecipitation purification of the ethylene-vinyl ester copolymer sample is performed three or more times using a mixed solution of n-hexane and acetone, the sample is dried under reduced pressure at 80°C for 3 days to prepare an ethylene-vinyl ester copolymer for analysis. The ethylene-vinyl ester copolymer for analysis is dissolved in DMSO-d6, and 1H-NMR １ (500 MHz) measurement is performed at 80°C. The content of ethylene units can be calculated using the peak derived from the main chain methine of the vinyl ester (4.7 to 5.2 ppm) and the peak derived from the main chain methylene of ethylene and the vinyl ester (0.8 to 1.6 ppm).

[0038] [Degree of Saponification] The degree of saponification of the ethylene-vinyl alcohol copolymer of the present invention is 90 mol% or more and 99.9 mol% or less. If the degree of saponification is less than 90 mol%, the gas barrier properties of the film obtained by coating with an aqueous solution of the ethylene-vinyl alcohol copolymer will be insufficient. The degree of saponification is preferably 95 mol% or more, and more preferably 97 mol% or more. On the other hand, if the degree of saponification exceeds 99.9 mol%, it is difficult to stably produce the ethylene-vinyl alcohol copolymer. Furthermore, in order to suppress the decrease in gas barrier properties when coated on a substrate after long-term storage, the degree of saponification is preferably 99 mol% or less, and more preferably 98 mol% or less. The degree of saponification of the ethylene-vinyl alcohol copolymer can be measured in accordance with JIS K6726 (1994).

[0039] [Viscosity-Average Degree of Polymerization] The preferred viscosity-average degree of polymerization for the ethylene-vinyl alcohol copolymer of the present invention is 200 to 5000. If the viscosity-average degree of polymerization is less than 200, the gas barrier properties of the resulting coating film may be insufficient. More preferably, the viscosity-average degree of polymerization is 250 or higher, and even more preferably 300 or higher. On the other hand, if the viscosity-average degree of polymerization exceeds 5000, the viscosity of the aqueous solution of the ethylene-vinyl alcohol copolymer becomes too high. More preferably, the viscosity-average degree of polymerization is 4000 or lower, even more preferably 3000 or lower, and particularly preferably 2000 or lower. The viscosity-average degree of polymerization P can be measured in accordance with JIS K6726 (1994). That is, the ethylene-vinyl alcohol copolymer of the present invention can be resaponified to a degree of saponification of 99.5 mol% or higher, purified, and then calculated from the intrinsic viscosity [η] (L / g) measured in water at 30°C using the following formula: P = ([η] × 10000 / 8.29) (1 / 0.62)

[0040] [Carboxylate Group and Lactone Ring Content] The ethylene-vinyl alcohol copolymer of the present invention has a total content of carboxyl groups and lactone rings located at the ends of its molecular chain of 0.08 mol% or more. Here, the total content of carboxyl groups and lactone rings located at the ends of the molecular chain refers to the total content of carboxyl groups and lactone rings located at the ends of the molecular chain relative to the total monomer units of the ethylene-vinyl alcohol copolymer. Having carboxyl groups or lactone rings at the ends of the molecular chain prevents a decrease in gas barrier properties even when an aqueous solution is coated onto a substrate after being stored for a long time. It is more preferable that the total content of terminal carboxyl groups and terminal lactone rings be 0.09 mol% or more. On the other hand, it is preferable that the total content of terminal carboxyl groups and terminal lactone rings be less than 0.2 mol%, and more preferably 0.18 mol% or less. For example, when vinyl acetate is radically polymerized in methanol, terminal carboxyl groups and terminal lactone rings are formed by chain transfer to methanol. Note that terminal carboxyl groups and terminal lactone rings are interconvertible depending on the environment.

[0041] The ethylene-vinyl alcohol copolymer of the present invention may have carboxyl groups or lactone rings located not only at the ends of its molecular chain but also internally. Specifically, by including a monomer capable of generating carboxyl groups or lactone rings as a copolymer component, carboxyl groups or lactone rings are introduced internally rather than at the ends of the molecular chain. In this case, it is preferable that the total content of carboxyl groups and lactone rings located at the ends or internally of the molecular chain is 0.08 mol% or more and less than 0.2 mol%. It is preferable that the ethylene-vinyl alcohol copolymer of the present invention has carboxyl groups or lactone rings only at the ends of its molecular chain.

[0042] Monomers capable of generating a carboxyl group or lactone ring include monomers having a carboxyl group derived from fumaric acid, maleic acid, itaconic acid, maleic anhydride, or itaconic anhydride, acrylic acid esters such as acrylic acid and its salts, methyl acrylate, ethyl acrylate, n-propyl acrylate, and i-propyl acrylate, methacrylic acid esters such as methacrylic acid and its salts, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, and i-propyl methacrylate, acrylamide derivatives such as acrylamide, N-methylacrylamide, and N-ethylacrylamide, and methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, and N-ethylmethacrylamide.

[0043] [Other Monomer Units] The ethylene-vinyl alcohol copolymer of the present invention may contain monomer units other than vinyl alcohol units, ethylene units, vinyl ester units, and monomers capable of generating carboxyl groups or lactone rings, as long as the effects of the present invention are not impaired. Examples of such monomers include α-olefins such as propylene, n-butene, and isobutylene; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinylides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. The content of these monomers varies depending on the purpose and application, but is preferably less than 10 mol%, more preferably less than 5 mol%, even more preferably less than 1 mol%, and particularly preferably less than 0.5 mol%.

[0044] [Method for Producing Ethylene-Vinyl Alcohol Copolymer] A preferred method for producing the ethylene-vinyl alcohol copolymer of the present invention is a method for producing an ethylene-vinyl alcohol copolymer, comprising copolymerizing ethylene and vinyl acetate in the presence of methanol solvent and a radical initiator to obtain an ethylene-vinyl acetate copolymer, and then saponifying it to obtain an ethylene-vinyl alcohol copolymer, wherein the copolymerization is carried out with a mass ratio of vinyl acetate to methanol of 1.5 or more. In this method, when copolymerizing ethylene and vinyl acetate in the presence of methanol solvent and a radical initiator, setting the mass ratio of vinyl acetate to methanol to 1.5 or more facilitates the introduction of carboxyl groups and lactone rings at the ends of the molecular chains. The steps of the production method will be described in detail below.

[0045] [Polymerization Process] The copolymerization method of ethylene and vinyl ester is not particularly limited, but a solution polymerization method in which polymerization is carried out in an alcohol solution is preferred. Examples of the alcohol include lower alcohols such as methanol and ethanol, but methanol is preferred. That is, it is preferable to copolymerize ethylene and vinyl acetate in the presence of methanol solvent and a radical initiator. Here, copolymerizing with a mass ratio of vinyl acetate to methanol of 1.5 or more is preferable because it is possible to efficiently introduce carboxyl groups or lactone rings to the ends of the molecular chains. More preferably, the mass ratio is 2 or more. Also, the mass ratio is usually 10 or less, and preferably 5 or less.

[0046] The initiator used in copolymerization is not particularly limited, but examples include peroxide-based initiators such as n-propyl peroxydicarbonate and benzoyl peroxide, and azo-based initiators such as 2,2'-azobis(isobutyronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 2,2'-azobis(2,4-dimethylvaleronitrile). Among these, peroxide-based initiators are preferred from the viewpoint of efficiently introducing carboxyl groups or lactone rings to the ends of molecular chains, and di-n-propyl peroxydicarbonate is particularly preferred.

[0047] In polymerization operations, batch, semi-batch, and continuous polymerization methods can be employed. Examples of polymerization reactors include batch reactors, tubular reactors, and continuous tank reactors. There are no particular limitations on the polymerization temperature, but 0 to 180°C is preferred, room temperature to 160°C is more preferred, and 30 to 150°C is even more preferred. When polymerization is performed below the boiling point of the solvent used, either vacuum boiling polymerization or atmospheric pressure non-boiling polymerization can be selected. When polymerization is performed above the boiling point of the solvent used, either pressurized non-boiling polymerization or pressurized boiling polymerization can be selected.

[0048] The ethylene pressure in the polymerization reactor during polymerization is preferably 0.1 to 2 MPa, more preferably 0.2 to 1.5 MPa, and even more preferably 0.3 to 1.0 MPa. The polymerization rate at the outlet of the polymerization reactor is not particularly limited, but is preferably 20 to 90%, and more preferably 30 to 80%.

[0049] In the polymerization process, a chain transfer agent may be added to adjust the viscosity-average degree of polymerization of the resulting ethylene-vinyl ester copolymer. Examples of chain transfer agents include aldehydes such as acetaldehyde, propionaldehyde, butyraldehyde, and benzaldehyde; ketones such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; mercaptans such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; and halogenated hydrocarbons such as trichloroethylene and perchloroethylene. Among these, aldehydes and ketones are preferably used. The amount of chain transfer agent added is determined according to the chain transfer constant of the added chain transfer agent and the viscosity-average degree of polymerization of the desired ethylene-vinyl ester copolymer, but is usually 0.1 to 10 parts by mass per 100 parts by mass of vinyl ester used.

[0050] [Saponification Step] The ethylene-vinyl ester copolymer obtained in the polymerization step is saponified in an organic solvent by alcohol decomposition or hydrolysis in the presence of a catalyst. Examples of catalysts used in the saponification step include basic catalysts such as sodium hydroxide, potassium hydroxide, and sodium methoxide; or acidic catalysts such as sulfuric acid, hydrochloric acid, and p-toluenesulfonic acid. The organic solvent used in the saponification step is not particularly limited, but examples include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; and aromatic hydrocarbons such as benzene and toluene. These can be used individually or in combination of two or more. In particular, it is convenient and preferable to use methanol or a mixed solution of methanol and methyl acetate as the solvent and carry out the saponification reaction in the presence of sodium hydroxide, which is a basic catalyst. The amount of saponification catalyst used is preferably 0.01 to 1 in molar ratio to vinyl ester monomer units in the ethylene-vinyl ester copolymer. This molar ratio is more preferably 0.1 or higher, and even more preferably 0.2 or higher. On the other hand, the molar ratio is more preferably 0.7 or less, and even more preferably 0.5 or less.

[0051] A preferred embodiment of the saponification process is as follows. First, a saponification catalyst such as sodium hydroxide is added to the ethylene-vinyl ester copolymer solution obtained in the polymerization process and mixed. The solvent at this time is preferably methanol. Initially, the mixture is a homogeneous liquid, but as the saponification reaction proceeds and the vinyl ester units in the polymer are saponified and converted into vinyl alcohol units, the solubility in the solvent decreases and the polymer precipitates in the solution. At this time, the solution contains methyl acetate produced by alcoholesis with methanol. As the saponification reaction proceeds, the amount of precipitated polymer gradually increases and becomes a slurry, after which a solid block containing the ethylene-vinyl alcohol copolymer and the solvent is obtained.

[0052] The saponification reaction may be carried out in the solid block described above before pulverization, or the saponification reaction may be carried out after pulverization. The saponification temperature is preferably 20 to 80°C. If the saponification temperature is too low, the reaction rate will decrease. The saponification temperature is more preferably 30°C or higher, and even more preferably 40°C or higher. On the other hand, if the saponification temperature is too high, a large amount of solvent will evaporate, reducing the solvent content in the resulting solid block and worsening the solubility of the resulting ethylene-vinyl alcohol copolymer. The saponification temperature is more preferably 70°C or lower, and even more preferably 65°C or lower. The saponification time is preferably 5 minutes to 3 hours. The saponification time is more preferably 10 minutes or more, and even more preferably 15 minutes or more. Furthermore, the saponification time is more preferably 2 hours or less, and even more preferably 90 minutes or less.

[0053] [Neutralization and Washing Process] It is preferable to have a neutralization process after the saponification and grinding processes. Neutralization can be performed using methyl acetate or acetic acid, but the former is preferred from the viewpoint of neutralization efficiency.

[0054] After the saponification and grinding steps, a neutralization step may be provided as needed, followed by a washing step to remove impurities such as sodium acetate, if necessary. Examples of washing solutions include lower alcohols such as methanol and ethanol, water, and mixtures thereof.

[0055] The neutralization step and / or washing step described above can reduce the amount of saponification catalyst residue remaining in the ethylene-vinyl alcohol copolymer. Specifically, it is preferable that the sodium salt content in the ethylene-vinyl alcohol copolymer is 0.6% by mass (metal basis) or less. Reducing the saponification catalyst residue can further suppress the decrease in gas barrier properties. The sodium salt content is more preferably 0.15% by mass or less, and even more preferably 0.1% by mass or less. The sodium salt content can be measured by an ICP emission spectrometer.

[0056] [Drying Process] After the saponification and grinding processes, the material is subjected to a drying process, which may be necessary after a neutralization and / or washing process. The drying method is not particularly limited, but hot air drying is preferred, and the particle temperature during drying is preferably 60 to 120°C. If the temperature is too low, the production efficiency will decrease. A temperature of 70°C or higher is more preferable. On the other hand, if the temperature is too high, some particles will undergo excessive crystallization, and the solubility will deteriorate. A temperature of 100°C or lower is more preferable. The drying time is preferably 2 to 20 hours and can be adjusted as appropriate. In this way, a dried ethylene-vinyl alcohol copolymer can be obtained. From the viewpoint of minimizing the impact on the human body, the methanol content in the dried ethylene-vinyl alcohol copolymer is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1.5% by mass or less. The lower limit of the methanol content is not particularly limited and may be 0% by mass or 0.001% by mass.

[0057] [Ethylene-Vinyl Alcohol Copolymer Aqueous Solution] The ethylene-vinyl alcohol copolymer of the present invention obtained in this way is preferably used as an aqueous solution. The aqueous solution of the present invention can prevent a decrease in gas barrier properties even when the aqueous solution is coated onto a substrate after being stored for a long time. The preferred concentration of the ethylene-vinyl alcohol copolymer of the present invention in the aqueous solution is adjusted according to the application, but is preferably 2 to 50% by mass. The concentration is more preferably 3% by mass or more, and even more preferably 5% by mass or more. On the other hand, the concentration is more preferably 30% by mass or less, and even more preferably 20% by mass or less.

[0058] The aqueous solution described above may or may not contain other vinyl alcohol polymers other than the ethylene-vinyl alcohol copolymer of the present invention. Examples of such other vinyl alcohol polymers include unmodified polyvinyl alcohol that does not contain monomer units other than vinyl alcohol units and vinyl ester units, and modified polyvinyl alcohol that does not contain ethylene units but contains monomer units other than vinyl alcohol units and vinyl ester units. Examples of such other monomer units include those mentioned above as other monomer units that may be contained in the ethylene-vinyl alcohol copolymer of the present invention. It is preferable that the content of such other vinyl alcohol polymers is less than the content of the ethylene-vinyl alcohol copolymer of the present invention.

[0059] [Coating Agent] A suitable use for the aqueous solution of the ethylene-vinyl alcohol copolymer of the present invention is as a coating agent. The coating agent of the present invention is useful as a gas barrier coating agent because it can prevent a decrease in gas barrier properties even when the coating agent is applied to a substrate after being stored for a long time.

[0060] In addition to the ethylene-vinyl alcohol copolymer, the coating agent of the present invention may also contain inorganic layered compounds, crosslinking agents, surfactants, leveling agents, fungicides, preservatives, etc. Examples of inorganic layered compounds include micas, talc, montmorillonite, kaolinite, and vermiculite. Adding inorganic layered compounds improves gas barrier properties, film strength, and handling. Examples of crosslinking agents include epoxy compounds, isocyanate compounds, aldehyde compounds, titanium compounds, silica compounds, aluminum compounds, zirconium compounds, and boron compounds. Among these, silica compounds such as colloidal silica and alkyl silicates are preferred. Adding crosslinking agents can impart water resistance.

[0061] In a preferred embodiment, the coating agent of the present invention contains an antifoaming agent. The inclusion of an antifoaming agent improves the coatability of the coating agent, and the gas barrier properties of the laminate obtained by coating the coating agent onto a substrate tend to be superior. Examples of the antifoaming agent include polyether-based antifoaming agents, mineral oil-based antifoaming agents, natural oil-based antifoaming agents, silicone-based antifoaming agents, and phosphate ester-based antifoaming agents.

[0062] In a preferred embodiment, the coating agent of the present invention contains an alcohol. The inclusion of an alcohol improves the coatability of the coating agent, and the gas barrier properties of the laminate obtained by coating the coating agent onto a substrate tend to be superior. The alcohol is more preferably a C1-C4 alcohol, and even more preferably a C1-C3 alcohol. The alcohol content in the coating agent of the present invention is preferably 1-20% by mass, and more preferably 2-10% by mass.

[0063] [Laminate] A laminate is obtained by coating a substrate with the coating agent of the present invention. The coating agent of the present invention prevents a decrease in gas barrier properties even when the coating agent is applied to the substrate after being stored for a long time, so the resulting laminate has good gas barrier properties. Suitable substrates for coating the coating agent of the present invention include films such as polyolefin films, polyester films, and polyamide films, as well as paper and nonwoven fabrics. The thickness of the substrate (final thickness if stretched) is preferably 5 to 300 μm. The coating method is not particularly limited, and gravure roll coating, reverse gravure coating, reverse roll coating, and Meyer bar coating are suitably used. The thickness of the dried film formed by the coating is preferably 0.1 to 20 μm, and more preferably 0.1 to 9 μm. The film made of the coating agent contains a specific ethylene-vinyl alcohol copolymer, and therefore has excellent gas barrier properties under high humidity. The oxygen permeability of the laminate of the present invention is 5 cc / m ２ It is preferable that the pressure be less than or equal to 2 cc / m³. ２ It is preferable that the daily ATM rate is less than or equal to the daily ATM rate.

[0064] A laminate comprising a substrate and a layer containing the ethylene-vinyl alcohol copolymer of the present invention is also one aspect of the present invention. The method for manufacturing the laminate may be a method of applying the coating of the present invention to the substrate, or it may be by other methods. The laminate may contain layers other than the substrate and the layer containing the ethylene-vinyl alcohol copolymer of the present invention. Preferred embodiments of the substrate are the same as those described above as substrates to which the coating agent of the present invention is applied.

[0065] The applications of the laminate of the present invention are not particularly limited, but it can be used in various applications that take advantage of its good gas barrier properties. It can be used in various applications that require barrier properties, such as food packaging and pharmaceutical packaging.

[0066] A composition containing the ethylene-vinyl alcohol copolymer of the present invention is also one of the present inventions. The composition may be an aqueous solution as described above, or it may be in other forms. The composition may contain an alkali metal salt. The content of the alkali metal salt in the composition is preferably 0.1 parts by mass or less, more preferably 0.01 parts by mass or less, even more preferably less than 0.003 parts by mass, and particularly preferably 0.0025 parts by mass or less, in terms of alkali metal, per 100 parts by mass of ethylene-vinyl alcohol copolymer. The content of the alkali metal salt can be measured by atomic absorption spectrophotometer.

[0067] The present invention will be specifically described below using examples, but the present invention is not limited in any way by these examples. The ethylene-vinyl alcohol copolymer was analyzed according to the following method.

[0068] [Ethylene Unit Content (Ethylene Modification Amount) of Ethylene-Vinyl Alcohol Copolymer] A portion of the methanol solution of the ethylene-vinyl acetate copolymer obtained during the manufacturing process of the ethylene-vinyl alcohol copolymer was taken as a measurement sample. After removing unreacted vinyl acetate monomer from the measurement sample, the sample was purified by precipitation with n-hexane, reprecipitation by dissolution with acetone, and then dried under reduced pressure at 80°C for 3 days to obtain a purified ethylene-vinyl acetate copolymer. The ethylene-vinyl acetate copolymer was dissolved in DMSO-d6 and measured at 80°C using a 500 MHz proton NMR (JEOL-500) to determine the ethylene unit content (mol%). The ethylene unit content of the ethylene-vinyl acetate copolymer is substantially the same as the ethylene unit content of the ethylene-vinyl alcohol copolymer obtained by saponification.

[0069] [Content of terminal carboxyl groups and terminal lactone rings in ethylene-vinyl alcohol copolymers] The content of terminal carboxyl groups and terminal lactone rings in ethylene-vinyl alcohol copolymers was determined by measurement using a 600 MHz proton NMR (JEOL-600) instrument.

[0070] [Viscosity-average degree of polymerization of ethylene-vinyl alcohol copolymer] The viscosity-average degree of polymerization of ethylene-vinyl alcohol copolymer was determined by the method described in JIS K 6726 (1994).

[0071] [Degree of saponification of ethylene-vinyl alcohol copolymer] The degree of saponification of ethylene-vinyl alcohol copolymer was determined by the method described in JIS K 6726 (1994).

[0072] Example 1 (Synthesis of Ethylene-Vinyl Alcohol Copolymer) A polymerization vessel (continuous polymerization tank) equipped with a reflux condenser, raw material supply line, reaction solution removal line, thermometer, nitrogen inlet, ethylene inlet, and stirring blade was used. Vinyl acetate (5500 kg / h), methanol (1380 kg / h), and a 0.4 mass% methanol solution of di-n-propyl peroxydicarbonate (NPP) (40 kg / h) were continuously supplied to the continuous polymerization tank under a nitrogen atmosphere using a metering pump. The ethylene pressure inside the tank was adjusted to 0.67 MPa. The polymerization solution was continuously removed from the polymerization tank so that the liquid level inside the tank remained constant. The polymerization rate at the outlet of the polymerization tank was adjusted to 43%. The residence time (polymerization time) inside the polymerization tank was 5.2 hours. The temperature at the outlet of the polymerization tank was 60°C. The polymerization solution was recovered from the polymerization tank, and unreacted vinyl acetate monomer was removed by introducing methanol vapor into the recovered liquid to obtain a methanol solution of ethylene-vinyl acetate copolymer. Table 1 summarizes the manufacturing conditions for ethylene-vinyl acetate copolymer.

[0073] To a methanol solution (25% by mass) of the ethylene-vinyl acetate copolymer obtained in the polymerization step, a methanol solution (30% by mass) of sodium hydroxide, a saponification catalyst, was added so that the molar ratio of sodium hydroxide to vinyl acetate units in the ethylene-vinyl acetate copolymer was 0.33. The resulting solid block was crushed to obtain particles, which were then left at 60°C for 1 hour to further saponify. Subsequently, the obtained particles were washed with methyl acetate for neutralization, washed with methanol three times, and then dried overnight at 80°C to obtain a purified ethylene-vinyl alcohol copolymer. The obtained ethylene-vinyl alcohol copolymer had a degree of saponification of 97.5 mol%, a viscosity-average degree of polymerization of 1000, an ethylene modification amount of 8.0 mol%, a terminal carboxyl group content of 0.18 mol%, and a methanol content of 0.8% by weight. The manufacturing conditions and analytical results of the ethylene-vinyl alcohol copolymer are summarized in Table 3.

[0074] [Crystal content of ethylene-vinyl alcohol copolymer] 10 g of ethylene-vinyl alcohol copolymer and 90 g of heavy water were placed in a pressure vessel and measured at 1.1 kgf / cm². ２Under pressurized conditions, the ethylene-vinyl alcohol copolymer was dissolved by standing at 120°C for 1 hour to prepare a 10% by mass heavy aqueous solution of the ethylene-vinyl alcohol copolymer. Next, a portion of the heavy aqueous solution of the ethylene-vinyl alcohol copolymer was taken and placed in an NMR tube to prepare a measurement sample. Day 0 was defined as the point when this measurement sample was left to stand at 30°C for 3 hours, and the measurement sample was then stored at 30°C for 14 days. The following conditions were then applied to the measurement sample. １ H-pulse NMR measurements were performed to obtain spin-spin relaxation curves.

[0075] Instrument: Bruker Japan TD-NMR minispec mq20 Pulse program: solid echo Pulse width: 7.22 μs Pulse repetition time (Recycle Delays): 1 sec Dummy Shoot: 4 Pulsed Atten: 0 Gain: Auto Gain Measurement temperature: 30°C (Set value: 294.5 K) Gas flow: 400 l / h Scans: 128 scans Acquisition scale: 8 ms

[0076] The obtained spin-spin relaxation curve was fitted to equation (1) below, and the amount of crystalline component A1 (%) was determined according to equation (2) below, which was found to be 0%. The results are shown in Table 4.

[0077]

[0078]

[0079] In equations (1) and (2) above, y is the magnetization intensity at time t, y0 is a constant, tau1 is the relaxation time of the crystalline component, tau2 is the relaxation time of the constrained amorphous component, tau3 is the relaxation time of the unconstrained amorphous component, a1 is the relaxation intensity of the crystalline component, a2 is the relaxation intensity of the constrained amorphous component, and a3 is the relaxation intensity of the unconstrained amorphous component.

[0080] [Barrier properties immediately after dissolution] (Preparation of aqueous solution) The obtained ethylene-vinyl alcohol copolymer was dissolved in distilled water, and methanol was added to prepare an aqueous solution with a concentration of 10% by mass of ethylene-vinyl alcohol copolymer and a methanol concentration of 4.3% by mass. In this aqueous solution, the ethylene-vinyl alcohol copolymer was completely dissolved and was colorless and transparent.

[0081] (Preparation of coated paper) Using this aqueous solution as the coating solution, a test shim sizer (manufactured by Kumagai Riki Kogyo Co., Ltd.) was used to produce a paper with a basis weight of 70 g / m². ２ The coating was applied to plain paper copier (PPC) paper with an air permeability of 20 sec at a speed of 300 m / min. The coated paper was dried in a 100°C hot air dryer for 5 minutes to form a coating layer with a thickness of 3 μm on the paper, obtaining coated paper (laminated material).

[0082] (Oxygen Permeability Measurement) The obtained coated paper was conditioned at 20°C and 50% RH for 72 hours before measurement. The conditioned coated paper was then subjected to oxygen permeability measurement using a MOCON OX-TRAN2 / 20 model manufactured by Modern Control Co., Ltd., in accordance with the isobaric method described in JIS K 7126 (2006), under conditions of 20°C and 50% RH. Based on the obtained oxygen permeability values, the barrier properties were evaluated according to the following criteria and were rated A. The results are shown in Table 4. A: 2.0 cc / m ２ ・day・atm or less B: 2.0cc / m ２ ・day・atm over 5.0cc / m ２ ・day・atm or less C: 5.0cc / m ２ ・day・atm super

[0083] [Barrier properties after 14 days] The obtained ethylene-vinyl alcohol copolymer was dissolved in distilled water, and methanol was added to prepare an aqueous solution with a concentration of 10% by mass of ethylene-vinyl alcohol copolymer and a methanol concentration of 4.3% by mass. In this aqueous solution, the ethylene-vinyl alcohol copolymer was completely dissolved and was colorless and transparent. This aqueous solution was left to stand at 30°C for 14 days. The aqueous solution after standing was used as a coating solution, and coated paper was prepared and its barrier properties evaluated in the same manner as in the "barrier properties immediately after dissolution" described above, resulting in a rating of A. The results are shown in Table 4.

[0084] Examples 2-4: Ethylene-vinyl alcohol copolymers were synthesized in the same manner as in Example 1, except for the conditions shown in Tables 1 and 3. Table 3 shows the amount of ethylene modification, degree of polymerization, degree of saponification, and total content of terminal carboxyl groups and terminal lactone rings of the obtained ethylene-vinyl alcohol copolymers. The methanol content was 1.2% by weight for Example 2, 0.2% by weight for Example 3, and 0.2% by weight for Example 4. In addition, the amount of crystalline components was measured for each obtained ethylene-vinyl alcohol copolymer, and the barrier properties of the laminate prepared in the same manner as in Example 1 were evaluated immediately after dissolution and after 14 days. However, only in Example 4, an aqueous solution of ethylene-vinyl alcohol copolymer with a concentration of 10% by weight was prepared without adding methanol. The results are summarized in Table 4.

[0085] Comparative Example 1: 76.6 kg of vinyl acetate and 73.3 kg of methanol were charged into a 250 L pressurized reactor equipped with a stirrer, nitrogen inlet, ethylene inlet, initiator addition port, and delay solution addition port. After heating to 60°C, the system was purged with nitrogen. The reactor pressure was then increased to 6.5 kg / cm². ２ Ethylene was introduced to achieve the desired result. As an initiator, a 2.8 g / L solution of 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMV) dissolved in methanol was prepared. After adjusting the temperature inside the polymerization vessel to 60°C, 175 ml of the initiator solution was injected to start polymerization. During polymerization, ethylene was introduced to increase the reaction vessel pressure to 6.5 kg / cm². ２ The polymerization temperature was maintained at 60°C, and polymerization was carried out by continuously adding AMV at a rate of 552 ml / hr using the above initiator solution. After 4 hours, when the polymerization rate reached 20%, the polymerization was stopped by cooling. The reaction vessel was opened to remove ethylene, and then nitrogen gas was bubbled in to completely remove ethylene. Subsequently, unreacted vinyl acetate monomer was removed under reduced pressure to obtain a methanol solution of ethylene-vinyl acetate copolymer. The production conditions for the ethylene-vinyl acetate copolymer are shown in Table 2.

[0086] To the obtained ethylene-vinyl acetate copolymer solution, methanol was added to adjust the concentration to 35%. 286 g of the methanol solution of ethylene-vinyl acetate copolymer (100 g of ethylene-vinyl acetate copolymer in the solution) was then mixed with 46.5 g of an alkaline solution (10% methanol solution of NaOH) (molar ratio of 0.10 to vinyl acetate units in the ethylene-vinyl acetate copolymer) and saponification was carried out. After the system gelled approximately 1 minute after the addition of the alkali, it was pulverized in a pulverizer and left at 60°C for 2 hours to allow saponification to proceed. Then, 1000 g of methyl acetate was added to neutralize the remaining alkali, and the white solid ethylene-vinyl alcohol copolymer obtained by filtration was mixed with 1000 g of water / methanol = 3 / 7 (mass ratio) and left at room temperature for 3 hours for washing. This washing operation was repeated three times. Subsequently, 1000 g of methanol was added to the ethylene-vinyl alcohol copolymer and left at room temperature for 3 hours for washing. This washing operation was repeated two times. Subsequently, the ethylene-vinyl alcohol copolymer obtained by centrifugal deliquidation was left in a dryer at 70°C for two days to obtain dried PVA. The manufacturing conditions and analytical results of the ethylene-vinyl alcohol copolymer are shown in Table 3. In addition, the amount of crystalline component of the obtained ethylene-vinyl alcohol copolymer was measured, and the barrier properties of the laminate prepared in the same manner as in Example 1 immediately after dissolution and after 14 days were evaluated. The results are summarized in Table 4.

[0087] Comparative Examples 2-6: Ethylene-vinyl alcohol copolymers were synthesized in the same manner as in Comparative Example 1, except for the conditions shown in Tables 2 and 3. Table 3 shows the amount of ethylene modification, degree of polymerization, degree of saponification, and total content of terminal carboxyl groups and terminal lactone rings of the obtained ethylene-vinyl alcohol copolymers. In addition, the amount of crystalline components was measured for each obtained ethylene-vinyl alcohol copolymer, and the barrier properties of the laminates prepared in the same manner as in Example 1 were evaluated immediately after dissolution and after 14 days. The results are summarized in Table 4.

[0088] Comparative Example 7 A polymerization vessel (continuous polymerization tank) equipped with a reflux condenser, raw material supply line, reaction solution removal line, thermometer, nitrogen inlet, ethylene inlet, and stirring blades was used. Vinyl acetate (78.3 kg / h), methanol (71.7 kg / h), and 5.3 g of maleic anhydride were charged into the continuous polymerization tank, and after heating to 60°C, nitrogen purging was performed. The reaction tank pressure was then set to 5.7 kg / cm². ２ Ethylene was introduced to achieve the desired result. As an initiator, a 2.8 g / L solution of 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMV) dissolved in methanol was prepared, and 255 mL was injected. A 5% by mass solution of maleic anhydride dissolved in methanol was also prepared as a delay solution. During polymerization, ethylene was introduced to maintain the reactor pressure at 5.7 kg / cm². ２ The polymerization temperature was maintained at 60°C, and polymerization was started by continuously adding AMV at a rate of 804 ml / hr using the above initiator solution, and maleic anhydride using the above delay solution, so that the ratio of vinyl acetate to maleic anhydride in the polymerization system remained constant. After 10 hours, when the polymerization rate reached 60%, the polymerization was stopped by cooling. At this point, the total amount of maleic anhydride delay solution added by delay was 837 ml. After opening the reaction vessel and deethyleneding, nitrogen gas was bubbled to completely remove the ethylene. Then, unreacted vinyl acetate monomer was removed under reduced pressure to obtain a methanol solution of ethylene-vinyl acetate copolymer. The production conditions for the ethylene-vinyl acetate copolymer are summarized in Table 2.

[0089] To the obtained ethylene-vinyl acetate copolymer solution, methanol was added to adjust the concentration to 40%. 333 g of the methanol solution of ethylene-vinyl acetate copolymer (100 g of ethylene-vinyl acetate copolymer in the solution) was then mixed with 46.5 g of an alkaline solution (10% methanol solution of NaOH) (molar ratio of 0.05 to vinyl acetate units in the ethylene-vinyl acetate copolymer) and saponification was carried out. After the system gelled approximately 1 minute after alkali addition, it was pulverized in a pulverizer and left at 40°C for 1 hour to allow saponification to proceed. Then, 1000 g of methyl acetate was added to neutralize the remaining alkali, and the obtained white solid ethylene-vinyl alcohol copolymer was filtered off. 1000 g of methanol was added to the solid and left at room temperature for 3 hours for washing. After repeating the above washing operation three times, the ethylene-vinyl alcohol copolymer obtained by centrifugation was dried in a dryer at 70°C for 2 days to obtain the target product. Table 3 shows the amount of ethylene modification, degree of polymerization, degree of saponification, and total content of carboxyl groups and lactone rings of the obtained ethylene-vinyl alcohol copolymer. At this time, the total content of carboxyl groups and lactone rings was 0.166 mol%, of which the total content of terminal carboxyl groups and terminal lactone rings was 0.03 mol%. The manufacturing conditions and analytical results of the ethylene-vinyl alcohol copolymer are summarized in Table 3.

[0090] Furthermore, the amount of crystalline components in the obtained ethylene-vinyl alcohol copolymer was measured, and the barrier properties of the laminate prepared in the same manner as in Example 1 were evaluated immediately after dissolution and after 14 days. The results are summarized in Table 4.

[0091]

[0092]

[0093]

[0094]

Claims

1. An ethylene-vinyl alcohol copolymer having an ethylene unit content exceeding 5 mol% and less than 10 mol%, and a degree of saponification of 90 mol% or more and 99.9 mol% or less; wherein the total content of carboxyl groups and lactone rings located at the ends of the molecular chain of the ethylene-vinyl alcohol copolymer is 0.08 mol% or more, and after preparing a 10% by mass heavy aqueous solution of the ethylene-vinyl alcohol copolymer, the solution was left to stand at 30°C for 14 days and subjected to the Solid Echo method. １ An ethylene-vinyl alcohol copolymer in which the decay curve of magnetization intensity due to spin-spin relaxation obtained by H-pulse NMR measurement is fitted to the following equation (1), and the amount of crystalline component A1 obtained by the following equation (2) is 10% or less. (In equations (1) and (2), y is the magnetization intensity at time t, y0 is a constant, tau1 is the relaxation time of the crystalline component, tau2 is the relaxation time of the constrained amorphous component, tau3 is the relaxation time of the unconstrained amorphous component, a1 is the relaxation intensity of the crystalline component, a2 is the relaxation intensity of the constrained amorphous component, and a3 is the relaxation intensity of the unconstrained amorphous component.) 2. The ethylene-vinyl alcohol copolymer according to claim 1, wherein the total content of carboxyl groups and lactone rings located at the terminal or internal part of the molecular chain is 0.08 mol% or more and less than 0.2 mol%.

3. The ethylene-vinyl alcohol copolymer according to claim 1, wherein the methanol content is 3% by mass or less.

4. An aqueous solution containing the ethylene-vinyl alcohol copolymer according to any one of claims 1 to 3.

5. A coating agent comprising the aqueous solution described in claim 4.

6. A laminate obtained by coating a substrate with the coating agent described in claim 5.

7. A laminate comprising a base material and a layer containing an ethylene-vinyl alcohol copolymer according to any one of claims 1 to 3.

8. A method for producing an ethylene-vinyl alcohol copolymer, comprising copolymerizing ethylene and vinyl acetate in the presence of methanol solvent and a radical initiator to obtain an ethylene-vinyl acetate copolymer, and then saponifying the copolymer to obtain an ethylene-vinyl alcohol copolymer, wherein the copolymerization is performed with a mass ratio of vinyl acetate to methanol of 1.5 or more, as described in any one of claims 1 to 3.

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