Method for producing ethylene-vinyl acetate copolymer saponified resin composition

Abstract

To provide a method for producing an EVOH resin composition that ensures fast adsorption of boron compounds such as a boric acid and suppresses whitening of the resultant resin composition.SOLUTION: A method for producing an EVOH resin composition includes step (I) for bringing a hydrous EVOH with a moisture content of 15% or more and 60% or less into contact with a boron compound-containing solution, where the boron compound-containing solution contains 10 ppm or more and 150 ppm or less of metal ions (A) and 20 ppm or more and 250 ppm or less of acid ions (B) relative to a content of the boron compound in the solution, where the metal ions (A) are at least one selected from a group consisting of alkali metal ions and alkaline-earth metal ions.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a saponified ethylene-vinyl acetate copolymer resin composition. [Background technology]

[0002] Saponified ethylene-vinyl acetate copolymer (hereinafter sometimes abbreviated as "EVOH") is a polymeric material with excellent gas barrier properties, oil resistance, antistatic properties, mechanical strength, etc., and because it is a melt-processable resin, it is widely used as a variety of packaging materials such as films, sheets, and containers.

[0003] A technique for blending a boron compound with EVOH to improve the melt moldability of EVOH is known. For example, Patent Document 1 describes that in producing a resin composition containing EVOH and a specific amount of a boron compound, EVOH having a water content of 20 to 80% by mass is brought into contact with an aqueous solution of a boron compound, and the content of the boron compound in the aqueous solution of the boron compound is set to 0.001 to 0.5 parts by mass per 100 parts by mass of the total amount of the water contained in the EVOH and the water contained in the aqueous solution of the boron compound, thereby obtaining an EVOH resin composition that has excellent melt moldability, is capable of suppressing the occurrence of fish eyes and the like, particularly during the production of a multilayer laminate, and also has good long-run moldability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] 1678057842192_0 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above-mentioned conventional technology, when trying to increase the production rate of EVOH, the adsorption rate of boric acid to EVOH pellets becomes the rate limiting factor, and the production rate may not be increased. In addition, when the present inventors investigated how to increase the adsorption rate of boric acid, they found that whitening of the resin may occur.

[0006] The present invention has been made to solve the above problems, and has an object to provide a method for producing an EVOH resin composition which has a high adsorption rate for boron compounds such as boric acid and which suppresses whitening of the resulting resin composition. [Means for solving the problem]

[0007] According to the present invention, the above object is to [1] A method for producing an EVOH resin composition, comprising: a step (I) of contacting a hydrous ethylene-vinyl acetate copolymer saponification product having a water content of 15% to 60% (hereinafter sometimes abbreviated as "hydrous EVOH") with a boron compound-containing solution, the boron compound-containing solution containing metal ions (A) in an amount of 10 ppm to 150 ppm and acid ions (B) in an amount of 20 ppm to 250 ppm relative to the content of the boron compound in the solution, the metal ions (A) being at least one selected from the group consisting of alkali metal ions and alkaline earth metal ions; [2] the production method according to [1], in which the EVOH resin composition has a yellowness index (YI1) of 8.0 or more after melt molding; [3] The production method according to [1] or [2], wherein in step (I), the time required for the boron compound to be adsorbed into the aqueous EVOH at 800 ppm relative to the EVOH is 9 hours or less; [4] The hydrous EVOH in step (I) has pores, and the pore specific surface area at 0.005 to 100 μm is 25 to 60 m 2 / g; [5] The method according to any one of [1] to [4], wherein the hydrous EVOH in the step (I) is in the form of pellets, and the hydrous EVOH pellets have an average particle size of 2.5 mm or more and 8 mm or less; This is achieved by providing Effect of the Invention

[0008] According to the present invention, there can be provided a method for producing an EVOH resin composition which has a high adsorption rate of a boron compound and in which whitening of the resulting resin composition is suppressed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present invention will be described. In the following description, specific materials (compounds, etc.) may be exemplified as materials that exhibit specific functions, but the present invention is not limited to the embodiment using such materials. Furthermore, the exemplified materials may be used alone or in combination of two or more types, unless otherwise specified.

[0010] The present invention is a method for producing an EVOH resin composition, comprising a step (I) of contacting a water-containing EVOH having a water content of 15% to 60% with a boron compound-containing solution, the boron compound-containing solution containing 10 ppm to 150 ppm of metal ions (A) and 20 ppm to 250 ppm of acid ions (B) relative to the content of the boron compound in the solution, the metal ions (A) being at least one selected from the group consisting of alkali metal ions and alkaline earth metal ions. By containing specific amounts of metal ions (A) and acid ions (B) in the boron compound-containing solution to be contacted with the water-containing EVOH, the adsorption rate of the boron compound is improved and the whitening of the resulting EVOH resin composition tends to be suppressed. The present invention will be described in detail below.

[0011] The water-containing EVOH used in the present invention is not particularly limited, but the degree of saponification of the vinyl acetate unit of the water-containing EVOH is preferably 80 to 100 mol%. From the viewpoint of obtaining a molded product having excellent barrier properties, the degree of saponification is more preferably 95 mol% or more, further preferably 98 mol% or more, and particularly preferably 99 mol% or more. From the viewpoint of obtaining a molded product having excellent barrier properties and melt moldability, the ethylene unit content of the water-containing EVOH is preferably 20 to 60 mol%, more preferably 23 to 50 mol%. When the ethylene unit content is 20 mol% or more, the melt moldability is improved, and when it is 60 mol% or less, the gas barrier property is improved. The water-containing EVOH may have other monomer units other than ethylene, vinyl acetate, and vinyl alcohol. Examples of the monomers include alkenes such as propylene, butylene, pentene, and hexene; 3-acyloxy-1-propene, 3-acyloxy-1-butene, 4-acyloxy-1-butene, 3,4-diacyloxy-1-butene, 3-acyloxy-4-methyl-1-butene, 4- Acyloxy-2-methyl-1-butene, 4-acyloxy-3-methyl-1-butene, 3,4-diacyloxy-2-methyl-1-butene, 4-acyloxy-1-pentene, 5-acyloxy-1-pentene, 4,5-diacyloxy-1-pentene, 4-acyloxy-1-hexene, 5-acyloxy-1-hexene, 6-acyloxy-1-hexene, 5,6-diacyloxy-1- Examples of the EVOH include alkenes having an ester group such as hexene and 1,3-diacetoxy-2-methylenepropane, or saponified products thereof; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, or anhydrides, salts, or mono- or dialkyl esters thereof; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefin sulfonic acids such as vinyl sulfonic acid, allyl sulfonic acid, and methallyl sulfonic acid, or salts thereof; vinyl silane compounds such as vinyl trimethoxy silane, vinyl triethoxy silane, vinyl tri(β-methoxy-ethoxy) silane, and γ-methacryloxypropyl methoxy silane; alkyl vinyl ethers, vinyl ketones, N-vinyl pyrrolidone, vinyl chloride, and vinylidene chloride. When EVOH has other monomer units, the copolymerization amount thereof is preferably 10 mol% or less, more preferably 5 mol% or less.

[0012] The hydrous EVOH used in the present invention can be produced by a known method, for example, an ethylene-vinyl acetate copolymer obtained by polymerizing ethylene and vinyl acetate is saponified in an organic solvent containing alcohol in the presence of a saponification catalyst to produce an EVOH alcohol solution, and the resulting alcohol solution can be post-treated by extruding the EVOH alcohol solution into a coagulation liquid such as water or a water / methanol solution in the form of a strand, cutting the strand to produce hydrous EVOH pellets, or by introducing the EVOH alcohol solution into an apparatus, contacting it with water in the apparatus to replace the alcohol in the solution with water, reducing the water in the hydrous EVOH discharged from the apparatus, and then cutting the strand to obtain hydrous EVOH pellets. There are no particular limitations on the means for making the water content of the obtained hydrous EVOH 15% to 60%, and the water content can be adjusted by, for example, the concentration of the EVOH alcohol solution, the water / methanol ratio of the coagulation liquid, the bath ratio of the coagulation liquid to the EVOH solution, the amount of water to be contacted in the apparatus, and treatment such as dehydration after obtaining the hydrous EVOH.

[0013] The hydrous EVOH used in the present invention preferably has pores, and the pore specific surface area (specific surface area) of the pores in the range of 0.005 to 100 μm is 25 m. 2 / g~60m 2 When the pore specific surface area is within the above range, the adsorption rate of the boron compound tends to increase. 2 / g or more. The specific pore surface area is preferably 55 m 2 / g or less is more preferable, and 50m 2 / g or less is particularly preferred. The pore specific surface area can be calculated from the pore distribution, and specifically, can be measured according to the method described in the Examples. The pore specific surface area of ​​the hydrous EVOH used in the present invention is set to 25 to 60 m 2 The means for achieving this is not particularly limited, and can be adjusted by, for example, adjusting the concentration of the EVOH alcohol solution, the water / methanol ratio of the coagulation liquid, the bath ratio of the coagulation liquid to the EVOH solution, the amount of water brought into contact in the apparatus, and treatment such as dehydration after obtaining the hydrous EVOH.

[0014] The shape of the hydrous EVOH pellets used in the present invention is preferably spherical (or nearly spherical) or cylindrical. The average particle size of the hydrous EVOH pellets is preferably 2.5 mm or more and 8 mm or less, more preferably 2.8 mm or more and 5 mm or less. When the hydrous EVOH pellets have a particle size in the above range and a specific pore specific surface area, the adsorption rate of the boron compound tends to be improved. There is no particular limitation on the means for making the particle size of the hydrous EVOH pellets of the present invention within the above range, but it can be adjusted, for example, by the nozzle diameter or discharge hole diameter when obtaining the hydrous EVOH pellets, or the interval and rotation speed of the cutter blade when cutting the hydrous EVOH. The average particle size of the hydrous EVOH pellets can be measured by the method described in the Examples.

[0015] The production method of the present invention includes a step (I) of contacting a water-containing EVOH having a water content of 15% or more and 60% or less with a solution containing a boron compound. Step (I) will be described in detail below.

[0016] The boron compounds contained in the boron compound-containing solution include boric acid or its metal salts, such as calcium borate, cobalt borate, zinc borate (zinc tetraborate, zinc metaborate, etc.), aluminum potassium borate, ammonium borate (ammonium metaborate, ammonium tetraborate, ammonium pentaborate, ammonium octaborate, etc.), cadmium borate (cadmium orthoborate, cadmium tetraborate, etc.), potassium borate ( Potassium metaborate, potassium tetraborate, potassium pentaborate, potassium hexaborate, potassium octaborate, etc.), silver borate (silver metaborate, silver tetraborate, etc.), copper borate (cupric borate, copper metaborate, copper tetraborate, etc.), sodium borate (sodium metaborate, sodium diborate, sodium tetraborate, sodium pentaborate, sodium hexaborate, sodium octaborate, etc.), lead borate (lead metaborate, lead hexaborate, etc.), nickel borate ( Nickel orthoborate, nickel diborate, nickel tetraborate, nickel octaborate, etc.), barium borate (barium orthoborate, barium metaborate, barium diborate, barium tetraborate, etc.), bismuth borate, magnesium borate (magnesium orthoborate, magnesium diborate, magnesium metaborate, trimagnesium tetraborate, pentamagnesium tetraborate, etc.), manganese borate (manganese borate, manganese metaborate, etc.), Examples of suitable borate minerals include boron nitride, manganese tetraborate, lithium borate (lithium metaborate, lithium tetraborate, lithium pentaborate, etc.), borax, kernite, inyoite, cottite, suianite, and seibeiite, and preferred are borax, boric acid, and sodium borate (sodium metaborate, sodium diborate, sodium tetraborate, sodium pentaborate, sodium hexaborate, sodium octaborate, etc.). Among these, boric acid is preferred from the viewpoint of availability.

[0017] The boron compound-containing solution contains 10 ppm to 150 ppm of metal ions (A) and 20 ppm to 250 ppm of acid ions (B) relative to the content of the boron compound. If the content of metal ions (A) relative to the content of the boron compound is less than 10 ppm, the adsorption rate of the boron compound is delayed, and if it exceeds 150 ppm, the resulting EVOH resin composition tends to whiten. If the content of acid ions (B) relative to the content of the boron compound is less than 20 ppm, the adsorption rate of the boron compound is delayed, and if it exceeds 250 ppm, the resulting EVOH resin composition tends to whiten.

[0018] The metal ion (A) is not particularly limited as long as it is at least one selected from the group consisting of alkali metal ions and alkaline earth metal ions. From the viewpoint of the hue and thermal stability of the resulting EVOH resin composition, it is preferably at least one selected from the group consisting of sodium ions, potassium ions, calcium ions, and magnesium ions, more preferably at least one selected from the group consisting of sodium ions and magnesium ions, and even more preferably sodium ions.

[0019] As the acid ion (B), from the viewpoints of the hue and thermal stability of the resulting EVOH resin composition, at least one selected from the group consisting of acetate ion, phosphate ion, sulfate ion, nitrate ion and halide ion is preferred, at least one selected from the group consisting of nitrate ion and sulfate ion is more preferred, and sulfate ion is even more preferred.

[0020] The boron compound-containing solution may contain metal ions other than the metal ion (A). Examples of metal ions other than the metal ion (A) include iron ions, cobalt ions, nickel ions, and copper ions. When the solution contains metal ions other than the metal ion (A), the content of the metal ions other than the metal ion (A) relative to the boron compound is preferably 5 ppm or less, more preferably 1 ppm or less, and may be 0.5 ppm or less, 0.1 ppm or less, or may be absent, from the viewpoint of suppressing whitening of the resulting EVOH resin composition.

[0021] The concentration of the boron compound-containing solution is preferably 0.001% to 1%, more preferably 0.003 to 0.5%. By adjusting the concentration to the above range, a predetermined amount of the boron compound can be contained in the resin composition while maintaining the appearance of the resulting EVOH resin composition.

[0022] Examples of the solvent for the boron compound-containing solution include alcohol, water, and mixed solvents thereof, with water being preferred.

[0023] In the production method of the present invention, in step (I), it is preferable that the time required for 800 ppm of the boron compound to be adsorbed in the hydrous EVOH relative to EVOH is within 9 hours. In order to achieve such an adsorption rate within 9 hours, it is important that the boron compound-containing solution contains metal ions (A) and acid ions (B) relative to the boron compound within the above-mentioned ranges. Such an adsorption rate is more preferably 8 hours or less, and even more preferably 7 hours or less. Such an adsorption rate may be 3 hours or more.

[0024] In the production method of the present invention, the water content of the EVOH used in step (I) is adjusted to 50% by mass, and 2000 g of the water-containing EVOH with a water content of 50% by mass is added to an aqueous solution containing 3.8 g of the boron compound used in step (I) in terms of boric acid and 19000 g of water, and the mixture is stirred. It is preferable that the time until 800 ppm of the boron compound is adsorbed in the EVOH is within 9 hours. In order to make the adsorption speed within 9 hours, it is important to contain the metal ion (A) and the acid ion (B) relative to the boron compound in the boron compound-containing solution within the above-mentioned range. The adsorption speed is more preferably 8 hours or less, and even more preferably 7 hours or less. The adsorption speed may be 3 hours or more.

[0025] Methods for contacting the hydrous EVOH pellets with the boron compound-containing solution include spraying the boron compound-containing solution onto the hydrous EVOH pellets, immersing the hydrous EVOH pellets in the boron compound-containing solution, adding the hydrous EVOH pellets while stirring the boron compound-containing solution, kneading the hydrous EVOH and the boron compound-containing solution with an extruder, etc. Among these, the method of adding the hydrous EVOH pellets while stirring the boron compound-containing solution is preferred, since it allows the boron compound to be efficiently incorporated into the pellets.

[0026] In the method of adding hydrous EVOH pellets while stirring the boron compound-containing solution, the bath ratio (kg / L) of the boron compound-containing solution to EVOH is not particularly limited, but is preferably 10 or more and 30 or less.

[0027] In the case of the method in which hydrous EVOH pellets are added to the solution containing the boron compound while stirring the solution, the temperature is preferably 10°C or higher and 50°C or lower.

[0028] The amount of the boron compound contained in the EVOH resin composition obtained by contacting with the boron compound-containing solution is preferably 10 to 10,000 ppm, more preferably 20 to 5,000 ppm, even more preferably 30 to 3,000 ppm, and particularly preferably 40 to 1,500 ppm, calculated as boron atoms. If the content is too small, the effect of adding the boron compound tends to be insufficient, and if the content is too large, the appearance of the molded product finally obtained tends to deteriorate. In the contact treatment with the aqueous solution of the boron compound, the content of the boron compound can be controlled by the concentration of the boron compound, the contact treatment time, the contact treatment temperature, the stirring speed during the contact treatment, the water content of the EVOH resin pellets to be treated, and the like. The content of the boron compound can be measured by ICP atomic emission spectrometry (ICP-AES) using a test solution obtained by treating the EVOH resin composition together with concentrated nitric acid by microwave decomposition, adding pure water to the solution obtained, and then measuring the volume of the test solution. Specifically, it can be measured by the method described in the Examples.

[0029] If necessary, a washing step may be included before step (I). Examples of the washing solution used in the washing step include water, an aqueous solution containing an acid such as acetic acid, and an aqueous solution containing an alcohol such as methanol.

[0030] In addition, the hydrous EVOH obtained before or after step (I) may be subjected to a chemical treatment, if necessary. The treatment may be a method of impregnating the hydrous EVOH pellets with an aqueous solution containing any additive. The additive may be, in addition to the boron compound used in the present invention, a carboxylic acid, a phosphoric acid compound, an alkali metal salt, an alkaline earth metal salt, or the like.

[0031] The hydrous EVOH after step (I) is preferably subjected to a drying step, which can be carried out by a known method, such as hot air drying or infrared irradiation.

[0032] The melt flow rate (MFR) (190°C, load 2160g) of the EVOH resin composition after drying obtained by the production method of the present invention is preferably 0.1 to 200g / 10min, more preferably 0.5 to 50g / 10min, and even more preferably 1 to 30g / 10min. When the MFR is 0.1g / 10min or more, the moldability tends to be good, and when the MFR is 200g / 10min or less, the mechanical properties of the obtained molded product tend to be good. The MFR is measured by the method described in the examples below.

[0033] The yellowness index (YI1) of the EVOH resin composition obtained by the production method of the present invention after melt molding is preferably 8.0 or more. YI1 can be used mainly as an index showing the degree of whitening of the EVOH resin composition, and when it is 8.0 or more, it can be said that whitening is suppressed. YI1 can be adjusted by the concentration of alkali metal and / or alkaline earth metal ions and the concentration of acid ions in the boron compound-containing solution. The yellowness index after melt molding can be measured by the method described in the Examples.

[0034] The ratio (YI2 / YI1) of the yellowness (YI2) of the EVOH resin composition obtained without adding a boron compound after melt molding to the yellowness (YI1) of the EVOH resin composition obtained by the production method of the present invention after melt molding is preferably 1.5 or less. The ratio (YI2 / YI1) can be used mainly as an index showing the degree of whitening of the EVOH resin composition, and when the ratio (YI2 / YI1) is 1.5 or less, it can be said that the whitening of the resin composition is suppressed. The ratio (YI2 / YI1) can be adjusted by the concentration of the metal ion (A) and the concentration of the acid ion (B) in the boron compound-containing solution. The yellowness after melt molding can be measured by the method described in the Examples.

[0035] The EVOH resin composition thus obtained can be molded by melt molding into various molded articles such as films, sheets, containers, pipes, and fibers. EXAMPLES

[0036] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these.

[0037] [Evaluation method] (1) Metal ion (A) content in boron compounds 0.5 g of the boron compound used in the examples and comparative examples and 5 ml of 35% hydrochloric acid were transferred to a 50 mL measuring flask (TPX (registered trademark)) and made up to 100 mL with pure water. The metal ion (A) in the obtained solution was analyzed using an ICP emission spectrometer (OPTIMA4300DV manufactured by PerkinElmer) to calculate the content of the metal ion (A). In addition, a calibration curve prepared using a commercially available standard solution was used for this quantification.

[0038] (2) Acid ion (B) content in boron compounds 0.5 g of the boron compound used in the examples and comparative examples was placed in a 50 ml measuring flask and diluted with pure water to obtain a solution. The resulting solution was analyzed using an ion chromatograph (ICS-1500 manufactured by Nippon Dionex Co., Ltd.) to calculate the content of acid ions (B). An anion analysis column (IC-Pac A25S, guard column A25G) was used as the column. The content was calculated using a calibration curve created using sulfuric acid.

[0039] (3) Boron compound content in dry EVOH 0.5 g of dried EVOH obtained in the examples and comparative examples was placed in a Teflon (registered trademark) pressure vessel, and 5 mL of concentrated nitric acid was added thereto and decomposed at room temperature for 30 minutes. After 30 minutes, the vessel was closed and heated at 150°C for 10 minutes and then at 180°C for 5 minutes using a wet decomposition apparatus (MWS-2, manufactured by Actac Co., Ltd.) to decompose the EVOH, and then cooled to room temperature. This treatment liquid was transferred to a 50 mL measuring flask (manufactured by TPX (registered trademark)) and made up to 10 mL with pure water. The borate ion was analyzed for the obtained solution using an ICP emission spectrometer (OPTIMA4300DV, manufactured by PerkinElmer Co., Ltd.) to calculate the content of borate ion. In addition, a calibration curve created using a commercially available standard solution was used for this quantification.

[0040] (4) Boron compound adsorption time 2000g of hydrous EVOH pellets (moisture content 50% by mass) used in the examples and comparative examples were immersed in a 50L poly container (Φ500mmx415mmH) containing 3.8g of boric acid as a boron compound and 19000g of pure water, and stirred with a mechanical stirrer. Approximately 100g of hydrous EVOH pellets were sampled every hour, dried at 80°C for 3 hours using a hot air dryer, and then dried at 110°C for 18 hours to obtain dry EVOH pellets with a moisture content of 0.3% by mass or less. The boron compound content of the obtained dry EVOH pellets was measured by the method described in the above evaluation method (3), and the time until dry EVOH pellets containing 800ppm or more of boron compounds were obtained (the adsorption time of the boron compound) was measured and evaluated. It was determined that the adsorption time of the boron compound was preferably 8 hours or less from the viewpoint of production speed.

[0041] (5) Change in yellowness index (YI) after melt molding with or without boron compound The hydrous EVOH pellets used in the examples and comparative examples were dried at 80°C for 3 hours using a hot air dryer, and then dried at 110°C for 18 hours to produce dried EVOH pellets containing no boron compounds and having a moisture content of 0.3% by mass or less. In addition, the ratio of change in yellowness after melt molding due to the presence or absence of a boron compound was calculated using the dried EVOH pellets containing a boron compound obtained in the examples and comparative examples. The yellowness after melt molding was measured by the following method. 8.9 g of the dried EVOH pellets were weighed and filled into a melt indexer (L227-E300 manufactured by Tateyama Chemical Industry Co., Ltd.) with the furnace temperature set to 220°C, compressed with a piston, a 2010 g weight was placed on it, and the pellets were held for 6 minutes. After that, all the resin was discharged from the bottom of the melt indexer onto a stainless steel plate covered with aluminum foil, and the pellets were sandwiched and pressed with another stainless steel plate so that the thickness was 3 mm. After cooling, the YI of the 3 mm thick sheet was measured using a colorimeter (Hunter LAB Scan XE). From the obtained YI value, the ratio of the YI of the EVOH pellets after melt molding without a boron compound to the YI of the EVOH pellets after melt molding with a boron compound (YI (without boron compound) / YI (with boron compound)) was calculated. When the YI ratio was 1.5 or less, it was determined that the whitening of EVOH due to the boron compound was suppressed.

[0042] (6) Melt flow rate (MFR) The MFR of the dried EVOH pellets obtained in the examples and comparative examples was measured according to the method described in JIS K7210:2014. Specifically, the resin composition was filled into a cylinder of Melt Indexer L244 (manufactured by Takara Kogyo Co., Ltd.) with an inner diameter of 9.55 mm and a length of 162 mm, and melted at a specific temperature. A load was evenly applied to the molten resin composition using a plunger with a mass of 2,160 g and a diameter of 9.48 mm. The amount of the resin composition extruded per unit time (g / 10 min) was measured from an orifice with a diameter of 2.1 mm provided in the center of the cylinder. The melting temperature was set to 190°C for measurement.

[0043] (7) Specific pore surface area The hydrous EVOH pellets used in the examples and comparative examples were frozen at -80°C, then freeze-dried and returned to room temperature to obtain a pore measurement sample. Approximately 0.5 g of the measurement sample was placed in a standard 5 cc powder cell (stem volume 0.4 cc), and the pore distribution was measured using a Micromeritics pore distribution measurement device (Shimadzu Corporation, Autopore V9620) under an initial pressure of 2.6 kPa. The mercury parameters were set to a mercury contact angle of 130 degrees and a mercury surface tension of 485 dynes / cm. The pore specific surface area was calculated to be a value for a pore diameter of 0.005 to 100 μm.

[0044] (8) Average particle diameter The average particle size of the measurement material (freeze-dried EVOH porous pellets) was measured using the "CAMSIZER XT" from Verder Scientific for 100 g of the pore measurement sample prepared by the above evaluation method (7). The particle size (Q3 50.0%) at which the cumulative particle size distribution from the small particle side of the circle equivalent particle size calculated by the dynamic image analysis method in accordance with ISO 13322-2 (2021) is 50% (volume basis) was defined as the average particle size.

[0045] [Example] Example 1 High-purity boric acid manufactured by Yoneyama Chemical Industry Co., Ltd. was purified three times using a recrystallization method. Sodium hydroxide and sulfuric acid were dissolved in pure water to add to the purified boric acid. The concentrations of sodium hydroxide and sulfuric acid in the aqueous solution were adjusted so that the sodium ion was 114 ppm and the sulfate ion was 211 ppm based on the mass of boric acid. The obtained aqueous boric acid solution was subjected to a reprecipitation operation to obtain boric acid containing 114 ppm of sodium ion and 211 ppm of sulfate ion. The contents of sodium ion and sulfate ion in the boron compound were measured for the obtained boric acid according to the methods described in the above evaluation methods 1 and 2. The results are shown in Table 1.

[0046] The pore specific surface area and average particle size of the hydrous EVOH pellets (water content: 50% by mass) with an ethylene unit content of 32 mol% were measured according to the methods described in the above evaluation methods (7) and (8). The results are shown in Table 1.

[0047] Pure water was added to 3.8 g of the boric acid obtained above so that the amount of pure water was 19000 g, to prepare an aqueous solution for immersing EVOH pellets. 2000 g of the hydrous EVOH pellets (water content 50 mass%) with an ethylene unit content of 32 mol% were immersed in the aqueous solution for immersing EVOH pellets obtained above and stirred with a mechanical stirrer for 5 hours, then dried at 80°C for 3 hours using a hot air dryer, and then dried at 110°C for 18 hours to prepare dried EVOH pellets with a water content of 0.3 mass% or less. The hydrous EVOH pellets were immersed while measuring the adsorption time of the boric acid compound according to the method described in the evaluation method (4) above, and the immersion time was the time until 800 ppm or more of the boron compound was adsorbed. The obtained dried EVOH pellets were measured for the boron compound content, the change ratio of yellowness after melt molding, and the MFR according to the methods described in the evaluation methods (4) to (6) above. The results are shown in Table 1.

[0048] (Examples 2 to 6, Comparative Examples 1 to 8) The boron compound was prepared and dried EVOH pellets were produced and evaluated in the same manner as in Example 1, except that the type and content of the metal ion (A) contained in the boric acid and the type and content of the acid ion (B) contained in the boric acid were adjusted to be as shown in Table 1. The results are shown in Table 1.

[0049] Example 7 As the hydrous EVOH pellets, hydrous EVOH pellets (moisture content 20% by mass, specific pore surface area 28.2 m 2 Except for using 1,000 ethanol (1.0 g / g), the boron compound was prepared and dried EVOH pellets were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0050] [Table 1]

[0051] From Examples 1 to 6, it is found that in the case of a boron compound containing 10 ppm or more and 150 ppm or less of metal ion (A) and 20 ppm or more and 250 ppm or less of acid ion (B), the adsorption rate to EVOH is fast, and the whitening of the resulting dried EVOH pellets after melt molding can be suppressed. From the comparison of Example 1 with Comparative Examples 1 to 3, it is found that if the content of metal ion (A) is more than 150 ppm, the whitening after melt molding cannot be suppressed. From the comparison of Example 1 with Comparative Examples 4 to 6, it is found that if the content of metal ion (A) is less than 10 ppm, the adsorption rate of the boron compound is significantly reduced. From the comparison of Example 2 with Comparative Example 7, it is found that if the content of acid ion (B) is more than 250 ppm, the whitening after melt molding cannot be suppressed. From the comparison of Example 2 with Comparative Example 8, it is found that if the content of acid ion (B) is less than 20 ppm, the adsorption rate of the boron compound is significantly reduced. Comparing Example 1 with Example 7, it is clear that when the pore specific surface area is large, the adsorption rate of the boron compound increases.

Claims

1. A method for producing a saponified ethylene-vinyl acetate copolymer resin composition, comprising: a step (I) of contacting a hydrous saponified ethylene-vinyl acetate copolymer having a moisture content of 15% or more and 60% or less with a boron compound-containing solution, the boron compound-containing solution containing metal ions (A) in an amount of 10 ppm or more and 150 ppm or less and acid ions (B) in an amount of 20 ppm or more and 250 ppm or less relative to the content of the boron compound in the solution, the metal ions (A) being at least one selected from the group consisting of alkali metal ions and alkaline earth metal ions.

2. 2. The method according to claim 1, wherein the yellowness index (YI1) of the saponified ethylene-vinyl acetate copolymer resin composition after melt molding is 8.0 or more.

3. 3. The method according to claim 1, wherein in the step (I), the time required for 800 ppm of the boron compound to be adsorbed into the hydrous saponified ethylene-vinyl acetate copolymer is within 9 hours.

4. The hydrous ethylene-vinyl acetate copolymer saponified product in step (I) has pores, and the pore specific surface area at 0.005 to 100 μm is 25 to 60 m ２ The method according to claim 1 or 2, wherein the molecular weight of the polymer is 1000 or more.

5. 3. The method according to claim 1, wherein the hydrous ethylene-vinyl acetate copolymer saponified product in the step (I) is in the form of pellets, and the average particle size of the hydrous ethylene-vinyl acetate copolymer saponified product pellets is 2.5 mm or more and 8 mm or less.