JP7699041B2 - Resin composition containing ethylene-vinyl alcohol copolymer, its method of manufacture and pellets made from same - Google Patents

Description

The present invention relates to a resin composition containing an ethylene-vinyl alcohol copolymer and a method for producing the same. It also relates to pellets made of the resin composition and a method for producing the same. It also relates to a multilayer structure having a layer made of the resin composition.

Ethylene-vinyl alcohol copolymer (hereinafter sometimes referred to as EVOH) is a thermoplastic resin with excellent gas barrier properties, fuel barrier properties, chemical resistance, contamination resistance, antistatic properties, mechanical strength, etc. Taking advantage of these characteristics, EVOH is molded into films, sheets, bottles, cups, tubes, pipes, etc. and is used for a variety of purposes, including packaging containers.

EVOH is usually manufactured as follows. First, ethylene and vinyl acetate are copolymerized in a solution such as methanol to synthesize an ethylene-vinyl acetate copolymer. The resulting ethylene-vinyl acetate copolymer is then saponified in solution to obtain an EVOH solution. The solvent is then removed from the resulting EVOH solution, which is then cut and dried to produce EVOH pellets, which are then subjected to melt molding. At this time, various improvements have been proposed in the EVOH pellet manufacturing process to suppress defects in the molded product after melt molding and to ensure stable melt molding.

Patent Document 1 describes a method for producing a saponified ethylene-vinyl acetate copolymer, which comprises copolymerizing ethylene and vinyl acetate in a methanol solution to produce an ethylene-vinyl acetate copolymer, saponifying the copolymer in a methanol solution, adding water to the methanol solution containing the saponified copolymer to prepare a mixed solution, removing insoluble matter from the mixed solution, and obtaining a saponified ethylene-vinyl acetate copolymer from the mixed solution from which the insoluble matter has been removed. In the examples, it is described that a mixed solution having a weight ratio of methanol/water of 49/51 to 70/30 is filtered through a filter with an opening of 60 μm to remove the insoluble matter, polyvinyl alcohol. It also describes that the filtered solution is solidified into strands, cut, and dried to obtain EVOH pellets, which are then melt-molded at 240°C to produce a film with few fisheyes. However, such a filtration method may not be sufficient to suppress defects in the molded product after melt molding.

Patent Document 2 describes a method for producing saponified ethylene-vinyl acetate copolymer pellets, which is characterized by extruding a solution of saponified ethylene-vinyl acetate copolymer into a solidifying liquid in the form of strands from a nozzle made of one of an aluminum compound, a glass compound, and a thermosetting resin, and then cutting the strands. By selecting the nozzle material, it is said that the strands are less likely to break compared to when a conventionally used stainless steel nozzle is used, EVOH pellets of uniform size are obtained, and feeding fluctuations and load fluctuations of the extruder can be suppressed. In the examples, nozzles made of glass, an aluminum-silicon alloy, and a silicon resin are used, but nozzles made of these materials are insufficient in strength and durability compared to stainless steel nozzles, making them difficult to use industrially.

JP 2002-12618 A Japanese Patent Application Publication No. 11-77672

The present invention has been made to solve the above problems, and aims to provide an EVOH resin composition that can suppress defects in molded articles after melt molding, and a method for producing the same. It also aims to provide pellets made of such an EVOH resin composition that can be stably melt molded, and a method for producing the same.

The above problem is solved by providing a resin composition that contains an ethylene-vinyl alcohol copolymer with an ethylene unit content of 20 to 60 mol% and a degree of saponification of 99 mol% or more; the light transmittance T1 of a solution obtained by dissolving 5 parts by mass of the resin composition in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 60% by mass) is 95% or more, and the light transmittance T2 of a solution obtained by dissolving 5 parts by mass of the resin composition in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 40% by mass) is 95% or more.

In this case, it is preferable that the resin composition contains at least one additive selected from a carboxylic acid, a boron compound, a phosphoric acid compound, an alkali metal salt, and an alkaline earth metal salt.

A preferred embodiment is a pellet made of the resin composition. In this case, it is preferable that the pellet has a cylindrical or ellipsoidal shape and that the half-width of the particle size distribution of the pellet is 0.7 mm or less. A multilayer structure having at least one layer made of the resin composition is also a preferred embodiment.

The above problem can also be solved by providing a method for producing the resin composition, which includes the steps of adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer, adding water to the saponified solution and then filtering it a first time, adding methanol to the solution after the first filtration and then filtering it a second time, and removing the solvent from the solution after the second filtration.

The above problem can also be solved by providing a method for producing the resin composition, which includes the steps of adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer, adding water to the saponified solution and then filtering it a first time, adding more water to the solution after the first filtration and then filtering it a second time, and removing the solvent from the solution after the second filtration.

The above problem can also be solved by providing a method for producing the pellets, which includes a step of adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer, a step of adding water to the saponified solution and then filtering it a first time, a step of adding methanol to the solution after the first filtration and then filtering it a second time, a step of removing the solvent from the solution after the second filtration, a cutting step, and a drying step.

The above problem can also be solved by providing a method for producing the pellets, which includes a step of adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer, a step of adding water to the saponified solution and then filtering it a first time, a step of adding more water to the solution after the first filtration and then filtering it a second time, a step of removing the solvent from the solution after the second filtration, a cutting step, and a drying step.

In the manufacturing method of these pellets, it is preferable to have a screening process after the drying process, in which pellets that are too large and pellets that are too small are removed.

The EVOH resin composition of the present invention can suppress defects in molded articles after melt molding. In addition, the pellets of the present invention can be stably melt molded, thereby making it possible to obtain molded articles with suppressed defects.

FIG. 1 is a schematic diagram of a twin-screw extruder used in the examples and comparative examples.

The resin composition of the present invention is a resin composition containing an ethylene-vinyl alcohol copolymer with an ethylene unit content of 20 to 60 mol% and a degree of saponification of 99 mol% or more; the light transmittance T1 of a solution obtained by dissolving 5 parts by mass of the resin composition in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 60% by mass) is 95% or more, and the light transmittance T2 of a solution obtained by dissolving 5 parts by mass of the resin composition in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 40% by mass) is 95% or more.

A major feature of the present invention is that the light transmittance T1 when dissolved in a 1-propanol/water mixed solvent with a 1-propanol content of 60% by mass, and the light transmittance T2 when dissolved in a 1-propanol/water mixed solvent with a 1-propanol content of 40% by mass, are both 95% or more. This means that the EVOH resin composition of the present invention contains little insoluble matter when dissolved in a 1-propanol/water mixed solvent with a 1-propanol content of 60% by mass and when dissolved in a 1-propanol/water mixed solvent with a 1-propanol content of 40% by mass.

There are few solvents that can dissolve EVOH, and a 1-propanol/water mixed solvent is one of the few good solvents. EVOH does not dissolve in water or 1-propanol, but it will dissolve in a solvent that is a mixture of water and 1-propanol in the appropriate ratio. In this case, EVOH with a high ethylene unit content is more soluble in a mixed solvent with a high 1-propanol content, and EVOH with a high vinyl alcohol unit content is more soluble in a mixed solvent with a high water content.

EVOH is a random copolymer containing ethylene units and vinyl alcohol units, but not all molecular chains contain ethylene units and vinyl alcohol units equally. Some molecular chains contain a large amount of ethylene units, and some contain a large amount of vinyl alcohol units. By having both light transmittances T1 and T2 of 95% or more, an EVOH resin composition can be obtained in which both molecular chains with an excessive amount of ethylene units and molecular chains with an excessive amount of vinyl alcohol units have been removed.

In the past, a method was proposed for removing insoluble matter in the EVOH manufacturing process by filtering a solution in which EVOH was dissolved in a methanol/water mixed solvent (see Patent Document 1), but it was not possible to sufficiently remove both molecular chains with an excessive amount of ethylene units and molecular chains with an excessive amount of vinyl alcohol units. In this study, it was discovered that by performing multiple filtrations using mixed solvents with different compositions, it is possible to effectively remove impurities and suppress defects in molded products after melt molding. In particular, it was found that when melt molding is performed at high temperatures, it is possible to suppress bumps, streaks, and flow irregularities, and to suppress the amount of adhesion inside the die during long-term operation.

The light transmittance T1 of a solution obtained by dissolving 5 parts by mass of the resin composition of the present invention in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 60% by mass) is 95% or more. If the light transmittance T1 is less than 95%, impurities containing molecular chains with too much vinyl alcohol unit content cannot be sufficiently removed, and defects in the molded product after melt molding cannot be suppressed. The light transmittance T1 is preferably 97% or more, and more preferably 98% or more. The light transmittance T1 is the light transmittance at 25°C with a wavelength of 350 nm to 600 nm, and is a relative light transmittance (%) when the light transmittance when a cell containing only a solvent in which EVOH is not dissolved is measured is taken as 100%.

In addition, the light transmittance T2 of a solution in which 5 parts by mass of the resin composition of the present invention is dissolved in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 40% by mass) is also 95% or more. If the light transmittance T2 is less than 95%, impurities containing molecular chains with too high a content of ethylene units cannot be sufficiently removed, and defects in the molded product after melt molding cannot be suppressed. The light transmittance T2 is preferably 97% or more, and more preferably 98% or more. The light transmittance T2 is measured in the same manner as the light transmittance T1.

The resin composition of the present invention contains an ethylene-vinyl alcohol copolymer with an ethylene unit content of 20 to 60 mol% and a degree of saponification of 99 mol% or more.

The EVOH contained in the resin composition of the present invention is obtained by saponifying an ethylene-vinyl ester copolymer, and preferably obtained by saponifying an ethylene-vinyl acetate copolymer. The ethylene unit content is 20 to 60 mol%. If the ethylene unit content is less than 20 mol%, the melt moldability is insufficient. On the other hand, if the ethylene unit content exceeds 60 mol%, the gas barrier properties are insufficient. The ethylene unit content is preferably 25 mol% or more, more preferably 30 mol% or more. The ethylene unit content is preferably 55 mol% or less, more preferably 50 mol% or less.

The saponification degree of EVOH is 99 mol% or more. If the saponification degree is less than 99 mol%, the barrier properties and melt moldability are insufficient. It is more preferable that the saponification degree is 99.3 mol% or more. A high saponification degree is important, especially when producing an EVOH composition with excellent melt stability and good long-run properties.

In addition to EVOH, the resin composition of the present invention preferably further contains at least one additive selected from carboxylic acids, boron compounds, phosphoric acid compounds, alkali metal salts, and alkaline earth metal salts. This can improve qualities such as thermal stability.

Examples of carboxylic acids include oxalic acid, succinic acid, benzoic acid, citric acid, acetic acid, and lactic acid. Among these, acetic acid is preferred from the standpoint of cost, ease of availability, and the like. If the carboxylic acid content in the dried EVOH resin pellets of the present invention is too low, discoloration may occur during melt molding, and if the content is too high, interlayer adhesion may be insufficient. Therefore, the carboxylic acid content is preferably 10 to 5000 ppm. The carboxylic acid content is preferably 30 ppm or more, and more preferably 50 ppm or more. The carboxylic acid content is preferably 1000 ppm or less, and more preferably 500 ppm or less.

Boron compounds include, but are not limited to, boric acids, boric acid esters, borate salts, boron hydrides, etc. Specifically, boric acids include orthoboric acid, metaboric acid, tetraboric acid, etc., borate esters include triethyl borate, trimethyl borate, etc., and borates include alkali metal salts, alkaline earth metal salts, borax, etc. of the above-mentioned various borates. Among these compounds, orthoboric acid (hereinafter simply referred to as boric acid) is preferred. If the content of the boron compound in the dried EVOH resin pellets of the present invention is too low, the effect of improving thermal stability is small, and if it is too high, gelation may occur, resulting in poor moldability. Therefore, the content of the boron compound in the dried EVOH resin pellets of the present invention is preferably 10 to 2000 ppm, more preferably 50 to 1000 ppm, in terms of boron.

Examples of the phosphoric acid compound include various acids such as phosphoric acid and phosphorous acid and their salts. The phosphate may be contained in the form of primary phosphate, secondary phosphate, or tertiary phosphate, and the cationic species is not particularly limited, but is preferably an alkali metal salt or an alkaline earth metal salt. Among them, it is preferable to add the phosphoric acid compound in the form of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, or dipotassium hydrogen phosphate. The content of the phosphoric acid compound in the EVOH resin pellets after drying of the present invention is preferably 1 to 1000 ppm in terms of phosphate radical. By adding it in such a range, it is possible to suppress coloring of the molded product and the occurrence of gels and bumps. If the content of the phosphoric acid compound is less than 1 ppm, there is a risk of severe coloring during melt molding. Also, if it exceeds 1000 ppm, there is a risk of gels and bumps being easily generated in the molded product.

Examples of alkali metal salts include aliphatic carboxylates, aromatic carboxylates, phosphates, and metal complexes of monovalent metals. Examples include sodium acetate, potassium acetate, sodium phosphate, lithium phosphate, sodium stearate, potassium stearate, and sodium salts of ethylenediaminetetraacetic acid. Of these, sodium acetate, potassium acetate, and sodium phosphate are preferred. The content of alkali metal salts in the EVOH resin pellets after drying in the present invention is preferably 5 to 5,000 ppm in terms of alkali metal element. More preferably, it is 20 to 1,000 ppm, and even more preferably, it is 30 to 750 ppm.

Examples of alkaline earth metal salts include magnesium salts, calcium salts, barium salts, and beryllium salts, with magnesium salts and calcium salts being particularly preferred. The anion species of the alkaline earth metal salt is not particularly limited, but acetates and phosphates are preferred. The alkaline earth metal content in the EVOH resin pellets after drying of the present invention is preferably 10 to 1000 ppm, more preferably 20 to 500 ppm, calculated as metal. If the alkaline earth metal content is less than 10 ppm, the effect of improving long-run properties may be insufficient. Furthermore, if it exceeds 1000 ppm, there is a risk of severe coloring when the resin is melted.

The proportion of EVOH in the resin components constituting the EVOH resin composition of the present invention is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 99% by mass or more, or may be substantially composed of EVOH or may be composed of only EVOH. The proportion of EVOH in the EVOH resin composition of the present invention is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 99% by mass or more.

The preferred melt flow rate (MFR) of the EVOH resin composition of the present invention (measured at 190°C under a load of 2160 g; however, for those having a melting point near or exceeding 190°C, measurements are taken under a load of 2160 g at multiple temperatures equal to or higher than the melting point, plotted on a semi-logarithmic graph with the inverse of absolute temperature on the horizontal axis and the melt flow rate on the vertical axis (logarithm), and extrapolated to 190°C) is preferably 0.1 to 200 g/10 min. The MFR is more preferably 0.2 g/10 min or more, even more preferably 0.5 g/10 min or more, and particularly preferably 1 g/10 min or more. The MFR is more preferably 50 g/10 min or less, even more preferably 30 g/10 min or less, and particularly preferably 15 g/10 min or less. If the MFR is too small, the torque of the extruder motor during molding may become too high, making melt extrusion difficult. On the other hand, if the MFR is too high, the mechanical strength of the resulting molded product may be insufficient.

The method for producing the EVOH resin composition of the present invention will be described below. A suitable method for producing the resin composition of the present invention includes the steps of adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer, adding water to the saponified solution and then filtering it a first time, adding methanol or water to the solution after the first filtration and then filtering it a second time, and removing the solvent from the solution after the second filtration.

First, ethylene and vinyl ester are copolymerized using a radical initiator as a catalyst to obtain an ethylene-vinyl ester copolymer. Preferably, vinyl acetate is used as the vinyl ester to obtain an ethylene-vinyl acetate polymer. At this time, solution polymerization is performed using methanol as a solvent. Either a continuous or batch system may be used. The polymerization temperature is 20 to 90°C, preferably 40 to 70°C. The polymerization time (average residence time in the case of a continuous system) is 2 to 15 hours, preferably 3 to 11 hours. The polymerization rate is 10 to 90%, preferably 30 to 80%, based on the vinyl ester charged. The resin content in the solution after polymerization is 5 to 85%, preferably 20 to 70%.

Catalysts used include azonitrile initiators such as 2,2-azobisisobutyronitrile, 2,2-azobis-(2,4-dimethylvaleronitrile), 2,2-azobis-(4-methyl-2,4-dimethylvaleronitrile), 2,2-azobis-(4-methoxy-2,4-dimethylvaleronitrile), and 2,2-azobis-(2-cyclopropylpropionitrile), as well as isobutyryl peroxide, cumyl peroxy neodecanoate, diisopropyl peroxycarbonate, di-n-propyl peroxy dicarbonate, t-butyl peroxy neodecanoate, lauroyl peroxide, benzoyl peroxide, and t-butyl hydroperoxide.

Monomers that can be copolymerized with ethylene and vinyl esters, such as 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 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-hexene, 1,3-diacetoxy-2-methyl Alkenes having an ester group such as olefin propane or saponified products thereof; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, or their anhydrides, salts, or mono- or dialkyl esters; 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 their salts; 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, vinylidene chloride, etc. can also be copolymerized in the presence of small amounts. In this case, the amount of copolymerization is usually 5 mol% or less, and it is preferable that they are substantially not contained.

After a specified time of polymerization, when a specified polymerization rate is reached, a polymerization inhibitor is added as necessary, unreacted ethylene gas is evaporated and removed, and then unreacted vinyl ester is expelled. As a method for expelling unreacted vinyl ester from the ethylene-vinyl ester copolymer solution from which ethylene has been evaporated and removed, for example, a method is adopted in which the copolymer solution is continuously fed at a constant rate from the top of a tower packed with Raschig rings, methanol vapor is blown in from the bottom of the tower, a mixed vapor of methanol and unreacted vinyl ester is distilled from the top of the tower, and the ethylene-vinyl ester copolymer solution from which unreacted vinyl ester has been removed is taken out from the bottom of the tower.

An alkali catalyst is added to a methanol solution of ethylene-vinyl ester copolymer from which unreacted vinyl ester has been removed, and the vinyl ester component in the copolymer is saponified to obtain a methanol solution of ethylene-vinyl alcohol copolymer. The saponification method can be either continuous or batchwise. Sodium hydroxide, potassium hydroxide, alkali metal alcoholate, etc. are used as the alkali catalyst. The saponification conditions are as follows. The ethylene-vinyl ester copolymer concentration is preferably 10 to 50 mass %. The saponification reaction temperature is preferably 30 to 150°C. The amount of alkali catalyst used is preferably 0.005 to 0.6 equivalents (per vinyl ester unit). The saponification time (average residence time in the case of a continuous method) is preferably 10 minutes to 6 hours.

Water is added to the methanol solution of EVOH thus obtained, and then filtration is performed. Suitable methods for filtration include a method in which water is added during the first filtration and methanol is added during the second filtration, and a method in which water is added during both the first and second filtrations.

When water is added during the first filtration and methanol is added during the second filtration, it is preferable to first add water to adjust the methanol content in the methanol/water mixed solvent in the EVOH solution to 50 to 70% by mass. By adjusting the methanol content in the mixed solvent to this range, it is possible to effectively remove only insoluble matter derived from molecular chains containing too many ethylene units in the EVOH. The methanol content is more preferably 52% by mass or more, and even more preferably 54% by mass or more. On the other hand, the methanol content is more preferably 68% by mass or less, even more preferably 66% by mass or less, and particularly preferably 64% by mass or less.

The type of filter used is not particularly limited, but a filter with mesh size of about 20 to 150 μm is preferably used. The differential pressure before and after the filter is preferably 0.1 to 0.7 MPa. If the differential pressure is less than 0.1 MPa, there is a risk of reduced productivity. The differential pressure is more preferably 0.15 MPa or more, and even more preferably 0.2 MPa or more. On the other hand, if the differential pressure exceeds 0.7 MPa, impurities cannot be sufficiently filtered out and may be mixed into the product. The differential pressure is more preferably 0.6 MPa or less, and even more preferably 0.5 MPa or less.

Methanol is added to the solution after the first filtration, and then the second filtration is performed. At this time, it is preferable to add methanol so that the methanol content in the methanol/water mixed solvent in the EVOH solution is 70 to 90% by mass. By setting the methanol content in this range, it is possible to effectively remove only insoluble matter derived from molecular chains containing too much vinyl alcohol units in the EVOH. The methanol content is more preferably 72% by mass or more, even more preferably 74% by mass or more, and particularly preferably 76% by mass or more. On the other hand, the methanol content is more preferably 88% by mass or less, and even more preferably 86% by mass or less.

The type of filter used is not particularly limited, but a filter with mesh sizes of 20 to 100 μm is preferably used. The differential pressure before and after the filter is preferably 0.2 to 1.5 MPa. If the differential pressure is less than 0.2 MPa, there is a risk of reduced productivity. The differential pressure is more preferably 0.3 MPa or more, and even more preferably 0.4 MPa or more. On the other hand, if the differential pressure exceeds 1.5 MPa, impurities cannot be sufficiently filtered out, and there is a risk of them being mixed into the product. The differential pressure is more preferably 1.2 MPa or less, and even more preferably 1.0 MPa or less.

When water is added during each of the first and second filtrations, it is preferable to first add water to adjust the methanol content in the methanol/water mixed solvent in the EVOH solution to 70 to 90% by mass. By adjusting the methanol content to this range, it is possible to effectively remove only insoluble matter derived from molecular chains containing too much vinyl alcohol units in the EVOH. The methanol content is more preferably 72% by mass or more, even more preferably 74% by mass or more, and particularly preferably 76% by mass or more. On the other hand, the methanol content is more preferably 88% by mass or less, even more preferably 86% by mass or less.

The type of filter used is not particularly limited, but a filter with mesh sizes of 20 to 100 μm is preferably used. The differential pressure before and after the filter is preferably 0.2 to 1.5 MPa. If the differential pressure is less than 0.2 MPa, there is a risk of reduced productivity. The differential pressure is more preferably 0.3 MPa or more, and even more preferably 0.4 MPa or more. On the other hand, if the differential pressure exceeds 1.5 MPa, impurities cannot be sufficiently filtered out, and there is a risk of them being mixed into the product. The differential pressure is more preferably 1.2 MPa or less, and even more preferably 1.0 MPa or less.

After the first filtration, water is further added to the solution, and then the second filtration is performed. At this time, it is preferable to add water to make the methanol content in the methanol/water mixed solvent in the EVOH solution 50 to 70 mass%. By making the methanol content in the mixed solvent within this range, it is possible to effectively remove only insoluble matter derived from molecular chains containing too many ethylene units in the EVOH. The methanol content is more preferably 52 mass% or more, and even more preferably 54 mass% or more. On the other hand, the methanol content is more preferably 68 mass% or less, even more preferably 66 mass% or less, and particularly preferably 64 mass% or less.

The type of filter used is not particularly limited, but a filter with mesh size of about 20 to 150 μm is preferably used. The differential pressure before and after the filter is preferably 0.1 to 0.7 MPa. If the differential pressure is less than 0.1 MPa, there is a risk of reduced productivity. The differential pressure is more preferably 0.15 MPa or more, and even more preferably 0.2 MPa or more. On the other hand, if the differential pressure exceeds 0.7 MPa, impurities cannot be sufficiently filtered out and may be mixed into the product. The differential pressure is more preferably 0.6 MPa or less, and even more preferably 0.5 MPa or less.

It is preferable that the difference in methanol content in the EVOH solution between the first filtration and the second filtration is 10 to 30% by mass. By setting an appropriate difference, different impurities can be effectively removed. The difference in methanol content is more preferably 12% by mass or more, and even more preferably 14% by mass or more. On the other hand, the difference in methanol content is more preferably 28% by mass or less, and even more preferably 26% by mass or less.

As described above, in the industrial EVOH production process, the EVOH solution dissolved in a methanol/water mixed solvent is filtered, but in the present invention, the performance of the EVOH resin composition is evaluated using the light transmittance of the EVOH solution dissolved in a 1-propanol/water mixed solvent. This is because EVOH pellets that have been dried and crystallized are difficult to dissolve in a methanol/water mixed solvent, whereas they can be dissolved relatively easily in a 1-propanol/water mixed solvent. If the 1-propanol content in the 1-propanol/water mixed solvent is made less than the methanol content, the solubility can be evaluated in the same way as when a methanol/water mixed solvent is used.

After filtration, the solvent is removed. The method for removing the solvent is not particularly limited as long as it can reduce the solvent content. The EVOH solution can be solidified by extruding it into a poor solvent such as water and solidifying it to reduce the solvent content. Alternatively, water may be mechanically squeezed out of the EVOH solution in an extruder or kneader, or water vapor may be evaporated from a vent. After the solvent is removed in this way, the EVOH solution is cut. There is no particular limit to the method for obtaining pellets by cutting. The solidified strands containing water can be cut with a cutter, or the strands with reduced water content in the extruder or kneader can be cut with a hot cutter or underwater cutter while still in a flowing state.

When the EVOH resin composition pellets contain at least one additive selected from carboxylic acids, boron compounds, phosphoric acid compounds, alkali metal salts, and alkaline earth metal salts, the method of adding the additive is not particularly limited. Methods that can be used include immersing the pellets in a water-containing state in an aqueous solution containing the additive to impregnate them, or injecting an aqueous solution containing the additive into water-containing and fluidized EVOH in an extruder or the like, kneading the mixture, and then cutting the mixture to produce pellets.

After cutting the EVOH resin composition into pellets, it is dried. There are no particular limitations on the drying method, and a hot air dryer or the like can be used. The dryer may be a fluidized or stationary dryer, or a combination of these may be used. Among these, a method in which the material is first dried using the fluidized drying method and then dried using the stationary drying method is preferred. There are no particular limitations on the drying temperature, but a temperature of about 70 to 120°C is usually used, and the temperature can be increased as the drying progresses. The moisture content after drying is usually 1% by weight or less, and preferably 0.5% by weight or less. The dried pellets thus obtained are then subjected to the molding process.

The shape of the pellets obtained is not particularly limited, but is preferably a cylinder or an ellipsoid. Here, the ellipsoid includes a spheroid with two orthogonal axes having equal radii, and a sphere with three orthogonal axes having equal radii. If the strands are formed and then cut, cylindrical pellets are obtained. If the strands are cut with a hot cutter or underwater cutter, ellipsoid pellets, particularly slightly flattened ellipsoid pellets, are obtained. Here, the terms cylinder and ellipsoid are not used in the strict sense, and they may be slightly distorted. The average particle size of the pellets is usually 2 to 7 mm. Here, the average particle size refers to the circle equivalent diameter (diameter) obtained from the area of a pellet image observed two-dimensionally.

The half-width of the particle size distribution of the EVOH resin composition pellets of the present invention is preferably 0.7 mm or less. By producing pellets with a small half-width and uniform particle size, pressure fluctuations in the extruder can be suppressed during melt molding. This makes it possible to continuously obtain homogeneous melt molded products over a long period of time. The half-width of the particle size distribution can be determined from the distribution of diameter values obtained as circle-equivalent diameters from the areas of multiple pellet images observed two-dimensionally. Specifically, it can be measured using a dynamic image analyzer. The half-width is preferably 0.6 mm or less, and more preferably 0.5 mm or less.

Methods for obtaining pellets with a uniform particle size include a cutting method and a sieving method. Among these, the sieving method is preferable because it can reliably and easily reduce the half-width of the particle size distribution. When sieving, it is preferable to use sieves with at least two types of openings to remove pellets that are too large and pellets that are too small.

The EVOH resin composition pellets of the present invention thus obtained can be melt molded to obtain various molded products such as films, sheets, containers, pipes, and fibers. Melt molding methods that can be used include extrusion molding, inflation extrusion, blow molding, melt spinning, and injection molding. The melting temperature varies depending on the melting point of the EVOH, but is preferably around 150 to 270°C.

The obtained molded product may be a molded product consisting of a single layer of the EVOH resin composition of the present invention, but it is preferable to use a multilayer structure containing at least one layer consisting of the EVOH resin composition. Examples of layer configurations of multilayer structures include, but are not limited to, E/T, T/E/T, E/Ad/T, T/Ad/E/Ad/T, etc., where E represents the resin composition of the present invention, Ad represents the adhesive resin, and T represents the other thermoplastic resins. Each layer shown here may be a single layer or a multilayer.

The method for producing the multilayer structure is not particularly limited. Examples include a method of co-extrusion or coinjection of the EVOH resin composition of the present invention with a thermoplastic resin, a method of melt-extruding a thermoplastic resin onto a molded product (film, sheet, etc.) made of the EVOH resin composition, a method of conversely co-extruding the EVOH resin composition with another thermoplastic resin onto a substrate such as a thermoplastic resin, and a method of laminating a molded product obtained from the EVOH resin composition of the present invention with a film or sheet of another substrate using a known adhesive. Among these, the co-extrusion or coinjection method is preferred.

The multilayer structure thus obtained can be used as various packaging materials such as films, sheets, and containers. Because it has excellent barrier properties and moldability, it is suitable for food packaging containers, fuel containers, and chemical containers.

The present invention will be explained in more detail below using examples. The evaluation methods used in the examples are as follows.

(1) Light transmittance A mixed solvent containing 60% by mass of 1-propanol and 40% by mass of water was prepared, and the sample EVOH composition pellets were added thereto and dissolved at 80° C. for 6 hours while stirring to prepare a 5% by mass EVOH solution. The EVOH solution in which the pellets were completely dissolved was cooled and placed in a measurement cell (quartz: optical path length 10 mm), and the light transmittance at 25° C. with wavelengths of 350 nm to 600 nm was measured using a spectrophotometer UV-2450 manufactured by Shimadzu Corporation, and the average value was taken as the light transmittance (T1). In addition, the light transmittance (T2) was measured in the same manner as above, except that a mixed solvent containing 40% by mass of 1-propanol and 60% by mass of water was used. In measuring T1 and T2, the light transmittance when a cell containing only a solvent in which EVOH was not dissolved was measured was taken as 100%, and the relative light transmittance (%) was calculated.

(2) Half-width in particle size distribution of pellets The particle size distribution of the dried pellets obtained in the examples and comparative examples was determined from the circle-equivalent diameter calculated by dynamic image analysis in accordance with ISO 13322-2 (2006) using a Verder Scientific "CAMSIZER XT" for 500 g of pellets as a sample. The circle-equivalent diameter refers to the circle-equivalent diameter calculated from the area of a pellet image observed two-dimensionally. The half-width (mm) was determined from the obtained particle size distribution.

(3) Pimples in the film A single-layer film formation test was carried out using the dried pellets obtained in the examples and comparative examples. A film having a thickness of 20 μm was obtained using an extruder and a T-die having the following specifications.
・ Extruder: "GT-40-A" manufactured by Plastics Engineering Research Institute Co., Ltd.
- Type: Single screw extruder (non-vent type)
・L/D: 26
・CR: 2.8
・Aperture: 40mmφ
・Screw: Double flight type ・Rotation speed: 40 rpm
・Driver: DC motor "SCR-DC218B" manufactured by Sumitomo Heavy Industries, Ltd.
Motor capacity: DC 7.5 kW (rated 45 A)
・Extruder heater: 3-split type ・C1/C2/C3 = 190℃/240℃/260℃
・10 resin pressure gauges are installed at equal intervals on the cylinder ・Die width: 550 mm
・Resin temperature inside die: 260℃
Take-up speed: 10 m/min

The number of bumps (visible to the naked eye with a diameter of about 100 μm or more) in the film one hour after the start of film formation was counted and converted into the number per 1.0 ^m2 . Based on the number, the film was judged as follows.
A: Less than 20 B: 20 to less than 100 C: 100 to less than 500 D: 500 or more

(4) Streaks in the film A single-layer film-forming test was carried out in the same manner as in (3), and the number of streaks (those visible to the naked eye and having a width of about 100 μm or more) in the film one hour after the start of film-forming was counted and converted to per 1.0 ^m2 . The appearance of the film was evaluated according to the number of streaks as follows:
A: Less than 2 B: 2 to 6 C: 6 to 21 D: 21 or more

(5) Flow Spots in the Film A single-layer film-forming test was carried out in the same manner as in (3), and the number of flow spots (those with a width of about 1 cm or more that can be confirmed by the naked eye by holding the film up to a fluorescent lamp and showing a change in refractive index) in the film one hour after the start of film-forming was counted and converted to per 1.0 ^m2 . The appearance of the film was evaluated according to the number of flow spots as follows:
A: Less than 1 place B: 1 place or more but less than 3 places C: 3 places or more but less than 10 places D: 10 places or more

(6) Amount of adhesion inside the die A single-layer film formation test was carried out in the same manner as in (3). Eight hours after the start of film formation, the EVOH resin composition in the extruder was replaced with low-density polyethylene (LDPE) having an MFR (190°C, 2160 g load) of 1 g/10 min over a period of 10 minutes, and the mass of the resin resulting from thermal degradation of EVOH adhering inside the die was measured and evaluated according to the following criteria.
A: Less than 3g B: 3g to less than 5g C: 5g to less than 10g D: 10g or more

Here, the amount of adhesion inside the die is an index showing the long-running properties of melt molding using an EVOH resin composition. Die adhesions continuously accumulate on the metal surface after melt molding has begun, and as molding time progresses, thermal degradation progresses and the material adheres to the inside of the die. As the material has been thermally deteriorated, it becomes discolored and crosslinked, and if it gets mixed into a molded product, it will cause poor appearance. The amount of adhesion inside the die is the quantification of the amount of adhesion to the die after a certain period of time has passed, and is an index for evaluating long-running properties. The amount of adhesion inside the die depends on the shear rate of the molten resin inside the die, and indicates the accumulation of material in the low shear region of 1 to 30 (1/S).

(7) Pressure Fluctuation A single-layer film production test was carried out in the same manner as in (3). During continuous film production for 6 hours from 1 hour to 7 hours after the start of film production, the difference ΔP between the maximum pressure P _MAX and the minimum pressure P _MIN at the resin pressure P10 measured with a pressure gauge at the tip of the extruder (10th) was evaluated according to the following criteria.
A: ΔP≦0.3
B: 0.3<ΔP≦0.6
C: 0.6<ΔP≦1.0
D: 1.0<ΔP

Example 1
A saponification reactor was charged with 100 parts by mass of an ethylene-vinyl acetate copolymer having an ethylene unit content of 32 mol% and 400 parts by mass of methanol, and further charged with 0.16 parts by mass of a methanol solution of sodium hydroxide (80 g/L) (sodium hydroxide/vinyl acetate unit=0.4/1 (molar ratio)). Nitrogen gas was blown into the reactor, and the reaction was carried out at 60° C. for 4 hours while removing the by-product methyl acetate together with methanol from the system. Thereafter, the reaction was stopped by neutralization with acetic acid, and a methanol solution of EVOH consisting of 57 parts by mass of EVOH and 75 parts by mass of methanol was obtained. The saponification degree of this EVOH was 99.95 mol%.

Next, water was added to the methanol solution of EVOH, and the mass ratio of the solvents in the EVOH solution (methanol/water) was set to 60/40. This EVOH solution was extracted from the bottom of the reactor at 60°C using a gear pump and supplied to a bucket-type first filter (first filtration process). The pressure difference between the inlet and outlet pressures of the first filter was set to 0.3 MPa, and a metal element with a mesh size of 100 μm was placed in the filtration section. After filtration, methanol was added to the EVOH solution, and the mass ratio of the solvents in the EVOH solution (methanol/water) was set to 80/20. This EVOH solution was kept at 60°C and supplied to a bucket-type second filter using a gear pump (second filtration process). The pressure difference between the inlet and outlet pressures of the second filter was set to 0.5 MPa, and a metal element with a mesh size of 60 μm was placed in the filtration section.

The EVOH solution that had passed through the first filtration process 1 and the second filtration process 2 was extruded into water through a metal plate with a circular opening to solidify into strands, which were then cut to obtain pellets with a diameter of approximately 3 mm and a length of approximately 5 mm. The pellets were deliquified using a centrifuge, and the deliquification process was repeated by adding a large amount of water.

The EVOH pellets thus obtained (ethylene unit content 32 mol%, saponification degree 99.95 mol%, moisture content 26% by mass) were fed into the cylinder barrel 10 from the raw material supply section 1 of the twin-screw extruder 20 shown in FIG. 1. The water-containing EVOH in a fluidized state was sent forward by the full-flight screw section 2, kneaded by the reverse-flight screw section 3, and the moisture content was adjusted by removing excess moisture at the vent 7. Then, the pellets were sent forward by the full-flight screw section 4, and an aqueous solution of acetic acid/boric acid/sodium acetate/magnesium acetate/potassium dihydrogen phosphate was injected from the additive injection section 8. Then, the pellets were kneaded by the reverse-flight screw section 5, passed through the full-flight screw section 6, and discharged from the discharge port 11. The temperature of the resin composition measured by the temperature sensor 9 installed near the discharge port was 100°C.

The amount of EVOH fed per unit time was 10 kg/hr (including the mass of water contained), the amount of additives fed per unit time was 0.65 L/hr, and the additive composition was an aqueous solution containing 4.3 g/L of acetic acid, 15 g/L of boric acid, 4.6 g/L of sodium acetate, 3.0 g/L of magnesium acetate, and 1.4 g/L of potassium dihydrogen phosphate.

The specifications of the twin-screw extruder 20 are as follows:
L/D: 45.5
・Aperture: 30mmφ
・Screw: Same direction full intermeshing type ・Rotation speed: 300 rpm
Motor capacity: DC 22kW
-Heater: 13-split type

The EVOH resin composition discharged from the discharge port 11 was discharged from a die with six holes of 3 mm diameter. The temperature of the EVOH resin composition in the die at this time was 105°C. The discharged EVOH resin composition was cut with a center hot cutter while still in a fluid state. The rotation speed of the cutter blade was 2700 rpm.

The moisture content of the obtained EVOH resin composition pellets was 17% by mass. The pellets were dried at 100°C for 15 hours using a fluidized bed dryer, and then dried at 100°C for 15 hours using a stationary dryer, resulting in a moisture content of 0.3% by mass. The EVOH resin composition pellets after drying contained 300 ppm of acetic acid, 270 ppm of boron compounds in terms of boron, 100 ppm of phosphate compounds in terms of phosphate radicals, 40 ppm of potassium in terms of metals, 130 ppm of sodium in terms of metals, and 50 ppm of magnesium in terms of metals as alkaline earth metal salts. The MFR (190°C, 2160 g load) was 1.5 g/10 min.

The dried EVOH resin composition pellets were then sieved for 10 minutes through sieves stacked in the order of 6.5 mesh, 7 mesh, and 8 mesh, and the pellets remaining on the 8 mesh were collected to obtain pellets for molding. The resulting pellets were slightly flattened spheroids with a longitudinal length of 3.2 mm and a transverse length of 2.1 mm. The resulting pellets were evaluated according to the evaluation method described above. The evaluation results are shown in Table 1.

Examples 2 , 3, 5, 8, Comparative Examples 1 to 6
Resin composition pellets were prepared and evaluated in the same manner as in Example 1, except that the ethylene unit content, filtration conditions, and sieving conditions of the synthesized EVOH were changed as shown in Table 1. The evaluation results are summarized in Table 1. In Examples 2 , 3, and 5 and Comparative Examples 1 to 6 , EVOH was synthesized in the same manner as in Example 1. The EVOH synthesized in Example 8 had an ethylene unit content of 44 mol%, a saponification degree of 99.7 mol%, and an MFR (190° C., 2160 g load) of 4 g/10 min. In Comparative Example 1, the first filtration step in Example 1 was omitted. In Comparative Example 2, the second filtration step in Example 1 was omitted.

Reference Signs List 1 Raw material supply section 2, 4, 6 Full flight screw section 3, 5 Reverse flight screw section 7 Vent 8 Additive injection section 9 Temperature sensor 10 Cylinder barrel 11 Discharge port 20 Twin screw extruder

Claims (10)

A resin composition comprising an ethylene-vinyl alcohol copolymer having an ethylene unit content of 20 to 60 mol% and a saponification degree of 99 mol% or more;
A resin composition in which a solution obtained by dissolving 5 parts by mass of the resin composition in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 60% by mass) has a light transmittance T1 of 96 % or more, and a solution obtained by dissolving 5 parts by mass of the resin composition in 95 parts by mass of a 1-propanol/water mixed solvent (1-propanol content: 40% by mass) has a light transmittance T2 of 97 % or more. The resin composition according to claim 1, which contains at least one additive selected from carboxylic acids, boron compounds, phosphoric acid compounds, alkali metal salts, and alkaline earth metal salts. Pellets made of the resin composition according to claim 1 or 2. The pellets according to claim 3, wherein the pellets have a cylindrical or ellipsoidal shape and the half-width of the particle size distribution of the pellets is 0.7 mm or less. A multilayer structure having at least one layer made of the resin composition according to claim 1 or 2. 3. A method for producing a resin composition according to claim 1, comprising the steps of: adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer; adding water to the saponified solution to adjust the methanol content to 54 to 66% by mass , and then performing a first filtration; adding methanol to the solution after the first filtration to adjust the methanol content to 74 to 86% by mass, and then performing a second filtration; and removing the solvent from the solution after the second filtration. 3. A method for producing a resin composition according to claim 1, comprising the steps of: adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer; adding water to the saponified solution to adjust the methanol content to 74 to 86% by mass , and then performing a first filtration; further adding water to the solution after the first filtration to adjust the methanol content to 54 to 66% by mass , and then performing a second filtration; and removing the solvent from the solution after the second filtration. 5. The method for producing pellets according to claim 3 or 4, comprising the steps of: adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer; adding water to the saponified solution to adjust the methanol content to 54 to 66% by mass , and then performing a first filtration; adding methanol to the solution after the first filtration to adjust the methanol content to 74 to 86 % by mass, and then performing a second filtration; removing the solvent from the solution after the second filtration; a cutting step; and a drying step. 5. The method for producing pellets according to claim 3 or 4, comprising the steps of: adding an alkali catalyst to a methanol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain a methanol solution of an ethylene-vinyl alcohol copolymer; adding water to the saponified solution to adjust the methanol content to 74 to 86% by mass , and then performing a first filtration; further adding water to the solution after the first filtration to adjust the methanol content to 54 to 66 % by mass , and then performing a second filtration; removing the solvent from the solution after the second filtration; a cutting step; and a drying step. The method for producing pellets according to claim 8 or 9, further comprising a screening step after the drying step, in which pellets that are too large and pellets that are too small are removed.

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