Method for producing ethylene-vinyl alcohol copolymer resin pellets

Abstract

For the purpose of providing a method for producing ethylene-vinyl alcohol resin pellets having excellent thermal stability, the present invention provides a method for producing ethylene-vinyl alcohol resin pellets that includes a pelletizing step and a drying step, wherein ethylene-vinyl alcohol copolymer resin intermediate pellets are produced in the pelletizing step, the volatile portion of the intermediate pellets between the pelletizing step and the drying step is 10-90 mass%, and the lower alcohol content of the ethylene-vinyl alcohol resin intermediate pellets carried into the drying step is set to 0.005-15 mass%.

Description

Method for producing ethylene-vinyl alcohol copolymer resin pellets

[0001] The present invention relates to a method for producing ethylene-vinyl alcohol copolymer resin pellets, and more particularly to a method for producing ethylene-vinyl alcohol copolymer resin pellets having excellent thermal stability.

[0002] Conventionally, if solvents such as lower alcohols used in the production of ethylene-vinyl alcohol copolymer (hereinafter sometimes referred to as "EVOH") resin pellets remain, they can cause problems during molding into films and the like, so it is generally required to reduce these as much as possible. Furthermore, from economic and safety perspectives, it is required to recover as much of these as possible. In particular, since release of these solvents into the atmosphere during the drying process poses an environmental problem, efforts are made to reduce their content as much as possible before the drying process (see, for example, Patent Documents 1 and 2).

[0003] Japanese Patent Laid-Open No. 9-316258 Japanese Patent Laid-Open No. 2002-060413

[0004] However, it has been found that there is room for improvement in the thermal stability of EVOH resin pellets obtained by drying EVOH resin intermediate pellets in which the lower alcohol content has been reduced as much as possible before drying.

[0005] In view of the above, the present invention provides a method for producing EVOH resin pellets having excellent thermal stability.

[0006] However, the present inventors have conducted extensive research in light of the above circumstances and have found that the thermal stability of the EVOH resin pellets obtained after drying is improved when the lower alcohol content of the EVOH resin intermediate pellets to be fed to the drying step is set in the range of 0.005 to 15 mass %.

[0007] That is, the present invention has the following aspects. [1] A method for producing ethylene-vinyl alcohol copolymer resin pellets, comprising a pelletizing step and a drying step, wherein ethylene-vinyl alcohol copolymer resin intermediate pellets are produced in the pelletizing step, the volatile content of the intermediate pellets between the pelletizing step and the drying step is 5 to 90 mass %, and the ethylene-vinyl alcohol copolymer resin intermediate pellets carried to the drying step have a lower alcohol content of 0.005 to 15 mass %. [2] A method for producing ethylene-vinyl alcohol copolymer resin pellets, comprising a pelletizing step and a drying step, wherein ethylene-vinyl alcohol copolymer resin intermediate pellets are produced in the pelletizing step, the drying step is performed multiple times, and the ethylene-vinyl alcohol copolymer resin intermediate pellets carried to a first drying step among the multiple drying steps have a lower alcohol content of 0.005 to 15 mass %. [3] The method for producing ethylene-vinyl alcohol copolymer resin pellets according to [1] or [2], wherein the ethylene-vinyl alcohol copolymer resin intermediate pellets to be carried into the drying step have a lower alcohol content of 0.03 to 9% by mass. [4] The method for producing ethylene-vinyl alcohol copolymer resin pellets according to any one of [1] to [3], wherein the volatile content of the ethylene-vinyl alcohol copolymer resin intermediate pellets is 10 to 90% by mass. [5] The method for producing ethylene-vinyl alcohol copolymer resin pellets according to any one of [1] to [4], wherein the drying step is carried out in an enclosed space. [6] The method for producing ethylene-vinyl alcohol copolymer resin pellets according to [2], wherein the first drying step of the plurality of drying steps is carried out in an enclosed space. [7] The method for producing ethylene-vinyl alcohol copolymer resin pellets according to [2] or [6], wherein the first drying step of the plurality of drying steps is carried out using explosion-proof equipment.

[0008] In the present invention, the lower alcohol content of the EVOH resin intermediate pellets to be carried to the drying step is set to 0.0005 to 15% by mass, so that the obtained EVOH resin pellets have excellent thermal stability.

[0009] FIG. 1 is a flow diagram illustrating an embodiment of the present invention.

[0010] 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.

[0011] In the present invention, when "X to Y" (X and Y are any numbers) is expressed, unless otherwise specified, it means "X or more and Y or less", and also means "preferably larger than X" or "preferably smaller than Y". When "X or more" (X is any number) or "Y or less" (Y is any number) is expressed, it also means "preferably larger than X" or "preferably smaller than Y".

[0012] <EVOH Resin> The pellets produced by the present invention are made of EVOH resin. EVOH resin is a resin obtained by saponifying an ethylene-vinyl ester copolymer, which is a copolymer of ethylene and a vinyl ester monomer, and is usually a water-insoluble thermoplastic resin.

[0013] As the vinyl ester monomer, vinyl acetate is typically used because of its commercial availability and the efficiency of impurity removal during production. Examples of vinyl ester monomers other than vinyl acetate 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. Typically, an aliphatic vinyl ester having 3 to 20 carbon atoms, preferably 4 to 10 carbon atoms, and particularly preferably 4 to 7 carbon atoms, is used. These are typically used alone, but multiple types may be used simultaneously as necessary.

[0014] The copolymerization of ethylene and vinyl ester monomers can be carried out using any known polymerization method, such as solution polymerization, suspension polymerization, or emulsion polymerization. Generally, solution polymerization using a lower alcohol, preferably methanol, as the solvent is used. The resulting ethylene-vinyl ester copolymer can also be saponified by a known method. The EVOH resin produced in this manner primarily contains ethylene-derived structural units and vinyl alcohol structural units, with a small amount of vinyl ester structural units remaining unsaponified.

[0015] The content of ethylene structural units in the EVOH resin is usually 20 to 60 mol%, preferably 25 to 50 mol%, and particularly preferably 25 to 35 mol%. The content of the ethylene structural units can be controlled by the ethylene pressure used when copolymerizing the vinyl ester monomer with ethylene. When the content is equal to or greater than the lower limit, the gas barrier properties and melt moldability under high humidity conditions are less likely to deteriorate. Conversely, when the content is equal to or less than the upper limit, the gas barrier properties are less likely to deteriorate. The content of the ethylene structural units can be measured in accordance with ISO 14663.

[0016] The saponification degree of the EVOH resin is usually 90 to 100 mol%, preferably 95 to 100 mol%, and particularly preferably 99 to 100 mol%. The saponification degree can be controlled by the amount, temperature, time, etc. of the saponification catalyst (usually an alkali catalyst) used when saponifying the ethylene-vinyl ester copolymer. If the saponification degree is too low, the gas barrier properties, thermal stability, moisture resistance, etc. tend to decrease. The saponification degree of the EVOH resin can be measured in accordance with JIS K6726 (wherein the EVOH resin is used as a solution uniformly dissolved in a water / methanol solvent).

[0017] The melt flow rate (MFR) (210°C, 2160 g load) of the EVOH resin is usually 0.5 to 100 g / 10 min, preferably 1 to 50 g / 10 min, and particularly preferably 3 to 35 g / 10 min. When the MFR is equal to or less than the upper limit, stability during film formation is less likely to be impaired, while when the MFR is equal to or greater than the lower limit, the viscosity is less likely to become too high, making melt extrusion difficult. The MFR is an index of the degree of polymerization of the EVOH resin, and can be adjusted by the amount of polymerization initiator and the amount of solvent used when copolymerizing ethylene and vinyl ester monomers.

[0018] The EVOH resin may further contain structural units derived from the comonomers shown below within a range that does not impair the effects of the present invention (for example, 10 mol % or less of the EVOH resin).Examples of the comonomer include olefins such as propylene, 1-butene, and isobutene; hydroxyl group-containing α-olefins such as 3-butene-1-ol, 3-butene-1,2-diol, 4-pentene-1-ol, and 5-hexene-1,2-diol, and derivatives thereof such as esters and acylation products; hydroxyalkylvinylidenes such as 2-methylenepropane-1,3-diol and 3-methylenepentane-1,5-diol; 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, and 1,3-dibutyloxy-2-methylenepropane; hydroxyalkylvinylidene diacetates such as hydroxy-2-methylenepropane; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, (anhydrous) phthalic acid, (anhydrous) maleic acid, (anhydrous) itaconic acid, or their salts or mono- or di-alkyl esters in which the alkyl group has 1 to 18 carbon atoms; acrylamide, N-alkylacrylamide in which the alkyl group has 1 to 18 carbon atoms, N,N-dimethylacrylamide, 2-acrylamidopropanesulfonic acid or its salts, acrylamidopropyldimethylamine or its acid salts or quaternary salts thereof acrylamides such as methacrylamide, N-alkylmethacrylamides in which the alkyl group has 1 to 18 carbon atoms, N,N-dimethylmethacrylamide, 2-methacrylamidopropanesulfonic acid or a salt thereof, methacrylamidepropyldimethylamine or an acid salt or a quaternary salt thereof; N-vinylamides such as N-vinylpyrrolidone, N-vinylformamide, and N-vinylacetamide; vinyl cyanides such as acrylonitrile and methacrylonitrile; alkyl vinyl ethers in which the alkyl group has 1 to 18 carbon atoms, hydrochloric acid vinyl ethers such as alkoxyalkyl vinyl ether and alkoxyalkyl vinyl ether; halogenated vinyl compounds such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride and vinyl bromide; vinyl silanes such as trimethoxyvinylsilane; halogenated allyl compounds such as allyl acetate and allyl chloride; allyl alcohols such as allyl alcohol and dimethoxyallyl alcohol; and comonomers such as trimethyl-(3-acrylamido-3-dimethylpropyl)-ammonium chloride and acrylamido-2-methylpropanesulfonic acid.These may be used alone or in combination of two or more.

[0019] Among these, hydroxy group-containing α-olefins are preferred, and 3-butene-1,2-diol and 5-hexene-1,2-diol are particularly preferred. When the hydroxy group-containing α-olefins are copolymerized, the resulting EVOH resin has primary hydroxyl groups in the side chains. Such EVOH resins having primary hydroxyl groups in the side chains, particularly EVOH resins having a 1,2-diol structure in the side chains, are preferred in that they exhibit good secondary moldability while maintaining gas barrier properties.

[0020] When the EVOH resin has a primary hydroxyl group in a side chain, the content of the structural unit derived from the monomer having the primary hydroxyl group is usually 0.1 to 20 mol %, preferably 0.5 to 15 mol %, particularly preferably 1 to 10 mol %, of the EVOH resin.

[0021] Furthermore, as the EVOH resin, EVOH resins that have been "post-modified" by esterification, urethanization, acetalization, cyanoethylation, oxyalkylenation, or the like can also be used.

[0022] When the post-modified EVOH resin is used, the modification rate is usually 10 mol % or less, and preferably 4 mol % or less. When the modification rate of the EVOH resin is equal to or less than the upper limit, the EVOH resin is less susceptible to thermal degradation and the long-run properties are less likely to be reduced.

[0023] Furthermore, the EVOH resin may be a mixture of EVOH resins having different ethylene structural unit contents, saponification degrees, polymerization degrees, copolymerization components, and the like.

[0024] The EVOH resin pellets obtained by the present invention may contain a resin composition obtained by mixing a resin other than the EVOH resin. In this case, the content of the resin composition obtained by mixing a resin other than the EVOH resin is, for example, 20% by weight or less, preferably 10% by weight or less.

[0025] The EVOH resin pellets obtained by the present invention may contain compounding agents generally compounded in EVOH resins, such as heat stabilizers, antioxidants, antistatic agents, colorants, ultraviolet absorbers, 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, oxygen absorbers, etc. These may be used alone or in combination of two or more kinds.

[0026] <Method for producing EVOH resin pellets> The method for producing EVOH resin pellets of the present invention is a method for producing EVOH resin pellets including a drying step, in which the lower alcohol content of the intermediate EVOH resin pellets to be fed to the drying step is set to 0.005 to 15% by mass. The EVOH resin pellets obtained in this manner can be produced, for example, via the following steps as shown in FIG.

[0027] In the present invention, the lower alcohol means an aliphatic alcohol having 1 to 10 carbon atoms, such as methanol, ethanol, propanol, n-butanol, and t-butanol.

[0028] [Pelletization Step] First, as described above, an ethylene copolymer is produced by copolymerizing ethylene and a vinyl ester elastomer, and this copolymer is saponified with an alkali to produce a fluid EVOH resin. The fluid EVOH resin is then pelletized to obtain EVOH resin intermediate pellets (A).

[0029] The fluid EVOH resin can be pelletized, for example, by a method (hot cut method) in which the fluid EVOH resin is extruded from the discharge port of an extruder, cut in a molten state, and then cooled and solidified to produce pellets, or a method (strand cut method) in which the fluid EVOH resin is extruded into a coagulation bath, cooled and solidified to produce EVOH resin strands, and the resulting EVOH resin strands are cut. Of these, the strand cut method is preferably used because it can produce porous pellets.

[0030] In either the hot cut method or the strand cut method, the fluid EVOH resin used as the pellet raw material may be a molten EVOH resin obtained by melting a hydrous EVOH resin composition or EVOH resin pellets (dry EVOH composition).

[0031] The aqueous EVOH resin composition is a composition in which EVOH resin is dissolved in a solvent, and the concentration of EVOH resin in the aqueous EVOH resin composition is usually 20 to 60% by mass.

[0032] As the solvent, a lower alcohol, a water / lower alcohol mixed solvent, etc. can be used, and among these, a water / lower alcohol mixed solvent is preferably used, and the water / lower alcohol mixed mass ratio is preferably 80 / 20 to 5 / 95. Furthermore, as the lower alcohol, methanol is particularly preferably used.

[0033] The aqueous EVOH resin composition preferably contains 0 to 100 parts by mass of a lower alcohol and 10 to 500 parts by mass of water per 100 parts by mass of EVOH resin.

[0034] The method for adjusting the water content of the aqueous EVOH resin composition is not particularly limited, but in order to increase the water content, for example, a method of spraying the solvent onto a solution or slurry of EVOH resin obtained by solution polymerization, a method of mixing the EVOH resin solution or slurry with the solvent, a method of bringing the EVOH resin solution or slurry into contact with vapor of the solvent, etc. In order to decrease the water content, the composition may be appropriately dried, for example, using a fluidized-type hot air dryer or a stationary hot air dryer.

[0035] [Hot-Cut Method] In the hot-cut method, when the hydrous EVOH resin composition is fed into an extruder as a pellet raw material for production by the hot-cut method, the temperature of the hydrous EVOH resin composition in the extruder is preferably 70 to 170°C, more preferably 80 to 170°C, and even more preferably 90 to 170°C. When the temperature of the hydrous EVOH resin composition is equal to or higher than the lower limit, the EVOH resin is less likely to melt completely, and when the temperature is equal to or lower than the upper limit, the EVOH resin is less susceptible to thermal degradation. On the other hand, when a dry EVOH resin composition is fed into the extruder as a pellet raw material, the temperature of the dry EVOH resin composition in the extruder is preferably 150 to 300°C, more preferably 160 to 280°C, and even more preferably 170 to 250°C. The temperatures of the hydrous EVOH resin composition and the dry EVOH resin composition in the extruder are those detected near the discharge port at the tip of the extruder by a temperature sensor installed in the extruder cylinder.

[0036] The hydrous EVOH resin composition or dry EVOH resin composition extruded from the die of the extruder, i.e., the EVOH resin in a molten state, is cut before being cooled and solidified. The cutting method may be either a hot cutting method in which cutting is performed in the atmosphere (air hot cutting method) or an underwater cutting method in which the composition is extruded into a vessel equipped with a cutter filled with cooling water and cut in the cooling water.

[0037] The temperature of the cooling water in the underwater cutting method is such that the EVOH resin extruded in a molten state does not instantly solidify (coagulate), and when a hydrous EVOH resin composition is used as the raw material, the temperature of the cooling water is preferably −20 to 50° C., more preferably −5 to 30° C. Furthermore, when a dry EVOH resin composition is used as the raw material, it is more likely to solidify than when a hydrous EVOH resin composition is used as the raw material, so the temperature of the cooling water in the underwater cutting method is higher than when a hydrous EVOH resin composition is used as the raw material, and is usually 0 to 90° C., preferably 20 to 70° C.

[0038] The cooling water is not limited to water, and other suitable materials include water / alcohol mixed solutions; aromatic hydrocarbons such as benzene; ketones such as acetone and methyl ethyl ketone; ethers such as dipropyl ether; and organic esters such as methyl acetate, ethyl acetate, and methyl propionate. Among these, water or a water / alcohol mixed solution is preferred due to its ease of handling. In a water / alcohol mixed solution, the water / alcohol ratio (mass ratio) is typically 90 / 10 to 99 / 1. The alcohol may be a lower alcohol such as methanol, ethanol, or propanol, with methanol being preferred from an industrial perspective.

[0039] [Strand Cut Method] In the strand cut method, when the aqueous EVOH resin composition is fed into the extruder as a pellet raw material, the temperature of the aqueous EVOH resin composition extruded into the coagulation bath is usually 10 to 100°C, the temperature of the coagulation bath is a temperature at which the extruded EVOH resin can be cooled and solidified, usually -10 to 40°C, and the residence time is usually about 10 to 400 seconds. On the other hand, when a dry EVOH resin composition is fed into the extruder as a pellet raw material, the temperature at which the EVOH resin is extruded into the coagulation bath is usually 150 to 300°C, the temperature of the coagulation bath is usually 0 to 90°C, and the residence time is about 2 to 400 seconds. The coagulation liquid used in the coagulation bath can be the same as the cooling water used in the hot cut method.

[0040] In this manner, EVOH resin intermediate pellets (A) are obtained. From the viewpoint of adjusting the lower alcohol content, which will be described later, it is preferable that the EVOH resin intermediate pellets (A) are porous pellets. The porous pellets can be obtained, for example, by using an alcohol or water / alcohol solution of EVOH resin (aqueous EVOH resin composition) as the pellet raw material in a strand cutting method. When the EVOH resin intermediate pellets (A) are porous, the lower alcohol content can be efficiently adjusted in the "lower alcohol content adjusting step" described later by permeating the pores with a lower alcohol of a predetermined concentration.

[0041] The shape of the EVOH resin intermediate pellets (A) usually depends on the production method, and can be various shapes. Examples of the shape of the EVOH resin intermediate pellets (A) include spherical, oval, cylindrical, cubic, rectangular, and irregular shapes, but they are usually oval or cylindrical. From the viewpoint of convenience when used later as a molding material, the size of the EVOH resin intermediate pellets (A) is such that, in the case of an oval shape, the major axis is usually 1 to 10 mm, preferably 2 to 7 mm, and the minor axis is usually 1 to 6 mm, preferably 2 to 5 mm. In the case of a cylindrical shape, the diameter of the base is usually 1 to 10 mm, preferably 2 to 7 mm, and the length is usually 1 to 10 mm, preferably 3 to 8 mm.

[0042] When the lower alcohol content of the obtained EVOH resin intermediate pellets (A) satisfies the predetermined concentration range, the pellets may be advanced to the "drying step" without passing through the "washing step" and / or the "lower alcohol content adjusting step" described below.

[0043] [Washing Step] The fluid EVOH resin used as the pellet raw material usually contains the alkali catalyst used in the saponification, by-product salts, other impurities, etc. Therefore, the obtained EVOH resin intermediate pellets (A) usually contain these impurities, etc. Therefore, the EVOH resin intermediate pellets (A) are usually subjected to a washing step to remove these impurities, etc.

[0044] The washing liquid used in the washing step is usually an aqueous solution. The washing method may be, for example, to add the EVOH resin intermediate pellets (A) to the washing liquid and stir the mixture. The washing step may be carried out multiple times.

[0045] As mentioned above, in the washing step, it is common technical knowledge among those skilled in the art to wash the EVOH resin intermediate pellets (A) so as to reduce the amount of lower alcohol contained in the pellets as much as possible. In this washing step, washing the EVOH resin intermediate pellets (A) usually makes it possible to obtain EVOH resin intermediate pellets (B) having an extremely low content of lower alcohol (usually 0.005 mass % or less).

[0046] In the washing step, the washing solution is adjusted to have the lower alcohol content specified in the present invention, and a predetermined lower alcohol content can be achieved. However, it is preferable to provide a lower alcohol content adjustment step described below in order to adjust the lower alcohol content with high precision.

[0047] [Lower Alcohol Content Adjustment Step] In order to convert the EVOH resin intermediate pellets (B) into EVOH resin intermediate pellets (C) containing a predetermined amount of lower alcohol, it is preferable to provide a lower alcohol content adjustment step in which the EVOH resin intermediate pellets (B) are brought into contact with a treatment liquid containing a predetermined concentration of lower alcohol.

[0048] That is, in the lower alcohol content adjusting step, EVOH resin intermediate pellets (C) having a predetermined lower alcohol content can be obtained by bringing a treatment liquid containing a predetermined concentration of lower alcohol into contact with the EVOH resin intermediate pellets (B). In this way, EVOH resin intermediate pellets (C) having a predetermined lower alcohol content can be easily produced simply by adjusting the concentration of the lower alcohol contained in the treatment liquid.

[0049] As a method for bringing the EVOH resin intermediate pellets (B) into contact with a treatment liquid containing a predetermined concentration of lower alcohol, for example, a method of immersing the EVOH resin intermediate pellets (B) in the treatment liquid, a method of spraying the treatment liquid onto the EVOH resin intermediate pellets (B), etc. can be used. However, from the viewpoint of being able to uniformly incorporate the lower alcohol, a method of immersing the EVOH resin intermediate pellets (B) in the treatment liquid and stirring the EVOH resin intermediate pellets together with the treatment liquid is preferred.

[0050] The lower alcohol concentration of the treatment liquid used in the lower alcohol content adjusting step varies depending on the lower alcohol content of the EVOH resin intermediate pellets (B) to be contacted, the weight ratio of pellets to the treatment liquid, and the contact time, but is preferably 0.01 to 40% by mass, more preferably 0.02 to 25% by mass, and even more preferably 0.05 to 15% by mass. When the EVOH resin intermediate pellets (B) are immersed in the treatment liquid, the immersion time (time in contact with the treatment liquid) is preferably 1 minute to 6 hours, and more preferably 30 minutes to 3 hours. The temperature of the treatment liquid is preferably 1 to 60°C, and more preferably 10 to 40°C.

[0051] The treatment liquid is usually an aqueous solution. Alternatively, the cleaning liquid used in the cleaning step may contain a predetermined amount of lower alcohol.

[0052] In this way, it is possible to easily obtain EVOH resin intermediate pellets (C) containing a predetermined amount of lower alcohol. The lower alcohol content of the EVOH resin intermediate pellets (C) is 0.005 to 15 mass%, preferably 0.015 to 15 mass%, more preferably 0.025 to 12 mass%, particularly preferably 0.03 to 9 mass%, and especially preferably 0.035 to 7 mass%.

[0053] If a predetermined amount of lower alcohol content has already been achieved in the washing step, this lower alcohol content adjustment step is not necessary. However, since adjusting the lower alcohol content to a predetermined range in the washing step requires complicated operations, it is preferable to provide a separate lower alcohol content adjustment step.

[0054] [Drying Step] The volatile content of the EVOH resin intermediate pellets (C) obtained in this manner is usually 10 to 90 mass%, preferably 20 to 80 mass%, more preferably 30 to 70 mass%, and even more preferably 45 to 65 mass%. Therefore, in the present invention, it is necessary to provide a drying step for drying the EVOH resin intermediate pellets (C). A plurality of such drying steps may be provided.

[0055] The drying step may be carried out in any manner as long as it can dry the EVOH resin intermediate pellets (C). However, for example, a drying step carried out in an enclosed space is preferable from the viewpoint of environmental protection, since it facilitates the recovery of volatile components (water, lower alcohols, etc.). Examples of drying methods that can be carried out in an enclosed space include fluidized drying using a cylindrical / groove-type agitator dryer, cylindrical dryer, rotary dryer, fluidized-bed dryer, vibrating fluidized-bed dryer, conical rotary dryer, etc., and static drying using a batch box dryer, band dryer, tunnel dryer, vertical silo dryer, etc., among which drying using explosion-proof equipment is preferred. Various drying methods may also be used in combination. In the present invention, a "closed space" refers to a space in which the inflow and outflow of air or gas is restricted. That is, in such a space, it is possible to circulate air or gas to limit gas emission to the outside, or to maintain a constant pressure within the space by setting the amount of air or gas emitted (emission amount) to the amount of air or gas introduced (introduction amount) equal to the amount of air or gas introduced (introduction amount). In the present invention, the term "explosion-proof facility" refers to any facility in which explosion-proof measures are taken for electrical equipment that may be an ignition source. In other words, if the drying step is carried out using explosion-proof facility, explosions and fires caused by the leakage of flammable substances can be effectively prevented, which is preferable from the viewpoint of environmental protection.

[0056] The drying temperature in the drying step is preferably set to 40 to 190°C, more preferably 60 to 170°C, and even more preferably 80 to 150°C. Furthermore, efficient drying can be achieved by passing a gas such as air or gas at 80 to 150°C through the dryer. The drying time in the drying step is preferably set to 5 minutes to 100 hours, more preferably 30 minutes to 90 hours, even more preferably 1 to 80 hours, and particularly preferably 10 to 70 hours.

[0057] The volatile content of the EVOH resin pellets thus obtained is usually 0.01 to 0.5% by mass, preferably 0.05 to 0.35% by mass, and particularly preferably 0.1 to 0.3% by mass.

[0058] That is, drying in the present invention means adjusting the volatile content of the resulting EVOH resin pellets to 0.5% by mass or less.

[0059] The volatile content of the EVOH resin intermediate pellets (C) and the EVOH resin pellets is measured and calculated by the following method (Comparative Example 1 is EVOH resin intermediate pellets (B)). That is, in the present invention, the volatile content is calculated by the dry residue method, and specifically, the mass (W1) of the EVOH resin intermediate pellets (B) or (C) or the EVOH resin pellets before drying is weighed on an electronic balance, dried in a hot air dryer at 150°C for 5 hours, and then left to cool in a desiccator for 30 minutes, after which the mass (W2) is weighed and calculated by the following formula: Volatile content (mass%) = [(W1 - W2) / W1] x 100

[0060] As described above, the present invention is contrary to the conventional common technical knowledge that EVOH resin intermediate pellets (B) having a sufficiently low lower alcohol content are introduced into the drying step. Instead, EVOH resin intermediate pellets (C) containing a predetermined amount of lower alcohol are introduced into the drying step. This allows EVOH resin pellets with excellent thermal stability to be obtained. While the reason for this is unclear, it is presumed that the unusual water volatilization state improves the molecular arrangement during drying, resulting in a higher crystallinity, thereby improving the thermal stability of the resulting EVOH resin pellets. Therefore, when multiple drying steps are provided, it is sufficient that the lower alcohol content of the EVOH resin intermediate pellets (C) introduced into the first drying step be set within a predetermined range.

[0061] The EVOH resin pellets thus obtained are used as they are or in various forms such as powder or liquid, and are provided as molding materials for various molded products.

[0062] 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 by mass.

[0063] Example 1: First, an EVOH resin solution (water / methanol mixed solution (water / methanol = 42 / 58 mixed mass ratio, EVOH resin concentration 40%, solution temperature 60°C)) with an ethylene content of 32 mol%, a saponification degree of 99.7 mol%, and an MFR of 12 g / 10 min (210°C, load 2160 g) was prepared. The EVOH resin solution was then extruded into a water tank containing a coagulation liquid (water / methanol = 97 / 3 mixed mass ratio) maintained at 2°C to form strands and coagulate. The solids were then cut with a cutter to obtain cylindrical EVOH resin intermediate pellets (A). In the washing step, 100 parts of the EVOH resin intermediate pellets (A) were added to 200 parts of a water washing solution and stirred at 30 to 35°C for 1 hour. The washing solution was then replaced, and washing was repeated five times in the same manner. The methanol content of the EVOH resin intermediate pellets (B) after the washing treatment was analyzed by gas chromatography and found to be 0.004%. Furthermore, as a lower alcohol content adjustment step, 100 parts of the EVOH resin intermediate pellets (B) were added to 200 parts of a treatment liquid (aqueous solution containing 0.04% methanol) and stirred at 30 to 35°C for 2 hours, thereby adjusting the lower alcohol content once. The methanol content of the EVOH resin intermediate pellets (C) after the lower alcohol content adjustment was analyzed by gas chromatography and found to be 0.02%. The resin concentration was 40.7%, and the volatile content was 59.3%. The EVOH resin intermediate pellets (C) after the lower alcohol content adjustment were dried in a batch vacuum dryer at 120°C for 18 hours, and then dried for 10 hours in a batch ventilated box dryer while blowing in nitrogen gas at 118°C, yielding EVOH resin pellets with a volatile content of 0.2%.

[0064] Example 2 EVOH resin pellets were obtained in the same manner as in Example 1, except that in the lower alcohol content adjusting step, the methanol concentration of the treatment liquid was changed to 0.1%.

[0065] Example 3 EVOH resin pellets were obtained in the same manner as in Example 1, except that in the lower alcohol content adjusting step, the methanol concentration of the treatment liquid was changed to 0.2%.

[0066] Example 4 EVOH resin pellets were obtained in the same manner as in Example 1, except that in the lower alcohol content adjusting step, the methanol concentration of the treatment liquid was changed to 0.4%.

[0067] Example 5 EVOH resin pellets were obtained in the same manner as in Example 1, except that in the lower alcohol content adjusting step, the methanol concentration of the treatment liquid was changed to 11.0%.

[0068] Example 6 EVOH resin pellets were obtained in the same manner as in Example 1, except that in the lower alcohol content adjusting step, the methanol concentration of the treatment liquid was changed to 22.0%.

[0069] [Comparative Example 1] EVOH resin pellets were obtained in the same manner as in Example 1, except that the step of adjusting the lower alcohol content in Example 1 was not carried out. That is, Comparative Example 1 is a conventional example in which the methanol content of the EVOH resin intermediate pellets before drying that are carried to the drying step is extremely low.

[0070] The methanol content (%) and volatile content (%) of these EVOH resin intermediate pellets, and the volatile content (%) of the EVOH resin pellets are shown in Table 1 below.

[0071] The physical properties of the EVOH resin intermediate pellets and EVOH resin pellets were measured as follows. <Methanol content> Measured by gas chromatography (external standard method). That is, 5.0 g of the EVOH resin intermediate pellets or EVOH resin pellets and 20.0 g of distilled water were weighed, and the pellets were stirred in the distilled water for 3 hours to extract the methanol in the pellets. The extract was filtered to prepare a sample, and the methanol concentration was quantified using gas chromatography (GC) under the following conditions. Gas chromatography conditions Vaporizer temperature: 160°C (split ratio 10:1) Carrier gas: He (64.2 kPa) Column: Supel-Q PLOT (length 30 m, inner diameter 0.32 mm) Column temperature: 90°C (equilibration time 3 minutes) Temperature program: 90°C (hold for 10 minutes) → increase temperature at 10.0°C / minute → 150°C (hold for 2 minutes) Detector: FID Detector temperature: 160°C Make-up gas: N2 (30 mL / min) H2 flow rate: 40 mL / min Air flow rate: 400 mL / min

[0072] Then, based on the value quantified by gas chromatography (GC), the methanol content per intermediate pellet or EVOH resin pellet was calculated using the following formula: "Methanol content per pellet (%)" = "Methanol concentration in GC sample (GC analysis value) (%)" × "Weighed mass of distilled water (g) + Weighed mass of volatile matter in pellet (g)" ÷ Weighed mass of pellet (g) = "GC analysis value (%)" × "Weighed mass of distilled water (g) + {Weighed mass of pellet (g) × (Volatile matter in pellet (%) / 100)}" ÷ Weighed mass of pellet (g).

[0073]

[0074] The EVOH resin pellets of Examples 1 to 6 and Comparative Example 1 were subjected to a color evaluation and evaluated for thermal stability as described below. The results are shown in Table 2 below. The lower the color evaluation value, the smaller the proportion of dark color and the higher the thermal stability.

[0075] <Evaluation of Coloring> The EVOH resin pellets were evaluated using a visual analyzer IRIS VA400 (manufactured by Alpha MOS) to evaluate the ratio (2167 / 2440) of color number "2167" (R: 136, G: 120, B: 120) to color number "2440" (R: 152, G: 136, B: 136). Color number "2167" is a dark color, and color number "2440" is a light color. The greater the ratio of dark colors (the greater the coloring evaluation value), the more the sample was deteriorated and colored by heat.

[0076]

[0077] In the coloring evaluation, Examples 1 to 6 all showed a smaller proportion of darker colors than the conventional example (Comparative Example 1), and were found to have excellent thermal stability.

[0078] 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.

[0079] The present invention can produce EVOH resin pellets having excellent thermal stability.

Claims

1. A method for producing ethylene-vinyl alcohol copolymer resin pellets, comprising a pelletizing step and a drying step, wherein ethylene-vinyl alcohol-based copolymer resin intermediate pellets are produced in the pelletizing step, the volatile content of the intermediate pellets from the pelletizing step to the drying step is 5 to 90% by mass, and the ethylene-vinyl alcohol copolymer resin intermediate pellets to be fed to the drying step have a lower alcohol content of 0.005 to 15% by mass.

2. A method for producing ethylene-vinyl alcohol copolymer resin pellets comprising a pelletizing step and a drying step, wherein ethylene-vinyl alcohol copolymer resin intermediate pellets are produced in the pelletizing step, the drying step is performed a plurality of times, and the ethylene-vinyl alcohol copolymer resin intermediate pellets carried to a first drying step among the plurality of drying steps have a lower alcohol content of 0.005 to 15 mass %.

3. The method for producing ethylene-vinyl alcohol copolymer resin pellets according to claim 1 or 2, wherein the ethylene-vinyl alcohol copolymer resin intermediate pellets to be fed to the drying step have a lower alcohol content of 0.03 to 9 mass %.

4. The method for producing ethylene-vinyl alcohol copolymer resin pellets according to claim 2, wherein the volatile content of the ethylene-vinyl alcohol copolymer resin intermediate pellets is 10 to 90% by mass.

5. The method for producing ethylene-vinyl alcohol copolymer resin pellets according to claim 1 or 2, wherein the drying step is carried out in a closed space.

6. The method for producing ethylene-vinyl alcohol copolymer resin pellets according to claim 2, wherein the first drying step among the plurality of drying steps is carried out in an enclosed space.

7. The method for producing ethylene-vinyl alcohol copolymer resin pellets according to claim 2 or 6, wherein the first drying step among the plurality of drying steps is carried out using explosion-proof equipment.

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