Pellets containing ethylene-vinyl alcohol copolymer and method for producing the same

Abstract

To provide EVOH pellets which, when being molded using an extruder, can suppress torque fluctuation of a motor of the extruder and pressure fluctuation in the extruder and which can afford uniform molded products.SOLUTION: Pellets containing an ethylene-vinyl alcohol copolymer, in which a ratio of the number of pellets containing air bubbles of a diameter equal to or larger than 0.1 mm to the number of all pellets is 0.05 / 100 to 3 / 100, are obtained by: extruding an ethylene-vinyl alcohol copolymer hydrous composition, which contains 30-50 pts.mass of water relative to 100 pts.mass of an ethylene-vinyl alcohol copolymer and having a temperature of 90-115°C, through a circular nozzle having a diameter of 2-5 mm at a linear velocity of 8-18 m / min.; then cutting; and thereafter drying it.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to pellets containing an ethylene-vinyl alcohol copolymer and a method for producing the same. [Background technology]

[0002] Ethylene-vinyl alcohol copolymer (hereinafter sometimes referred to as EVOH) is a thermoplastic resin that has excellent gas barrier properties, fuel barrier properties, chemical resistance, stain resistance, anti-static properties, mechanical strength, etc. Taking advantage of these characteristics, EVOH is melt-molded into forms such as films, sheets, bottles, cups, tubes, and pipes, and is used in a variety of applications, including packaging containers.

[0003] EVOH is usually produced as follows: First, ethylene and vinyl acetate are copolymerized in a methanol solution to synthesize an ethylene-vinyl acetate copolymer. Next, the resulting ethylene-vinyl acetate copolymer is saponified in solution to obtain an EVOH solution. The methanol 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.

[0004] Patent Document 1 describes a method for post-processing a methanol solution of EVOH obtained by saponification. The method involves concentrating the methanol solution of EVOH as needed, adding water in an amount sufficient to prevent EVOH precipitation, and producing a methanol-water mixed solution of EVOH containing 15 to 45% by weight. This solution is then extruded into water at 50°C or below or into a methanol-water mixed solution with a lower methanol concentration than the EVOH solution, resulting in strand-like precipitation, which is then cut. It is stated that the methanol-water mixed solution in the coagulation bath preferably contains 10 to 50% by weight of methanol. The resulting pellets are porous, allowing for easy removal of saponification catalyst residue by washing with water and making them easy to handle in the subsequent washing and drying process. However, when EVOH pellets are produced by precipitating a large amount of methanol-containing EVOH solution in a coagulation bath, methanol can volatilize during the precipitation process, and improvements are needed.

[0005] In response to this, a method has been proposed in which an aqueous EVOH composition containing almost no alcohol such as methanol is cut to produce EVOH pellets, which is said to be able to suppress the evaporation of alcohol.

[0006] Patent Document 2 describes a method for producing EVOH resin pellets, which comprises feeding EVOH to an extruder, maintaining the resin melt temperature in the extruder in the range of 70 to 170°C, adjusting the amount of water in the extruder so that the water content of the copolymer immediately after being discharged from the extruder is 5 to 40% by weight, and cutting the water-containing EVOH composition discharged from the extruder.

[0007] Patent Document 3 describes a method for producing EVOH pellets, which comprises introducing a methanol solution of EVOH into a container, bringing the solution into contact with water vapor in the container, and extracting the methanol together with the water vapor to obtain an aqueous EVOH composition containing 0 to 10 parts by weight of an alcohol having a boiling point of 100°C or less and 10 to 500 parts by weight of water per 100 parts by weight of EVOH, and cutting the composition.

[0008] Patent Document 4 describes a method for producing EVOH pellets, in which an EVOH aqueous composition obtained in the same manner as in Patent Document 3 and containing 0 to 10 parts by weight of an alcohol having a boiling point of 100°C or less and 10 to 1,000 parts by weight of water per 100 parts by weight of EVOH is supplied to an extruder, the water content in the extruder is adjusted while melt-kneading, and the pellets are then discharged from the extruder and cut. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Special Publication No. 47-38634 [Patent Document 2] U.S. Patent No. 6,686,405 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-121290 [Patent Document 4] Japanese Patent Application Laid-Open No. 2002-284811 Summary of the Invention [Problem to be solved by the invention]

[0010] Conventional methods such as those described in Patent Documents 2 to 4 can suppress alcohol volatilization during the production of EVOH pellets. However, when the pellets thus obtained are fed to an extruder for melt molding, torque fluctuations in the extruder motor and pressure fluctuations within the extruder are likely to occur, resulting in unevenness in the resulting molded product.

[0011] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide EVOH pellets that can suppress torque fluctuations in the extruder motor and pressure fluctuations inside the extruder when melt-molding using an extruder, thereby enabling the production of homogeneous molded products, and a method for producing such EVOH pellets. [Means for solving the problem]

[0012] The above problem is solved by providing pellets containing an ethylene-vinyl alcohol copolymer, in which the ratio of the number of pellets containing bubbles with a diameter of 0.1 mm or more to the number of all pellets is 0.05 / 100 to 3 / 100.

[0013] In this case, the pellets preferably contain at least one selected from the group consisting of a carboxylic acid and an alkali metal salt, a boron compound, a phosphate compound, and an alkaline earth metal salt. The pellets preferably have a cylindrical or ellipsoidal shape. It is also preferable that the half-width of the particle size distribution of the pellets is 0.7 mm or less.

[0014] The above-mentioned object can also be achieved by providing a method for producing the pellets, which comprises extruding an ethylene-vinyl alcohol copolymer aqueous composition containing 30 to 50 parts by mass of water per 100 parts by mass of ethylene-vinyl alcohol copolymer and having a temperature of 90 to 115°C through a circular nozzle having a diameter of 2 to 5 mm at a linear velocity of 8 to 18 m / min, cutting the extruded composition, and then drying the extruded composition.

[0015] In this case, it is preferable that the ethylene-vinyl alcohol copolymer aqueous composition is extruded through the circular nozzle, cut in a molten state, and then dried. It is also preferable that the aqueous composition contains a carboxylic acid and an alkali metal salt, and further contains at least one selected from the group consisting of a boron compound, a phosphoric acid compound, and an alkaline earth metal salt. [Effects of the Invention]

[0016] The EVOH pellets of the present invention can suppress torque fluctuations in the extruder motor and pressure fluctuations in the extruder when molding using an extruder, thereby producing homogeneous molded products. Furthermore, the production method of the present invention can produce such pellets. [Brief explanation of the drawings]

[0017] [Figure 1]FIG. 1 is a schematic diagram showing the configuration of an extruder used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0018] The pellets of the present invention are pellets containing an ethylene-vinyl alcohol copolymer, and the proportion of pellets containing bubbles with a diameter of 0.1 mm or more is 0.05 / 100 to 3 / 100 of the total number of pellets. By including a certain proportion of pellets containing bubbles in the total pellets, the stability during extrusion molding of EVOH pellets is improved, and fluctuations in the torque of the extruder motor and pressure fluctuations within the extruder can be suppressed, resulting in the production of homogeneous molded products. Conventionally, it has been required that all pellets be homogeneous, so it is surprising that the stability during extrusion molding can be improved by including a certain proportion of pellets containing bubbles.

[0019] The EVOH contained in the pellets of the present invention is preferably one obtained by saponifying an ethylene-vinyl ester copolymer. A typical vinyl ester used in producing the EVOH is vinyl acetate, but other fatty acid vinyl esters (such as vinyl propionate and vinyl pivalate) can also be used. Furthermore, other comonomers can also be copolymerized within a range that does not impede the objectives of the present invention.

[0020] The ethylene unit content of the EVOH contained in the pellets of the present invention is preferably 5 to 70 mol%. If the ethylene unit content is less than 5 mol%, the melt moldability will be insufficient. On the other hand, if the ethylene unit content exceeds 70 mol%, the gas barrier properties will be insufficient. The ethylene unit content is more preferably 10 mol% or more, even more preferably 20 mol% or more, and particularly preferably 22 mol% or more. Furthermore, the ethylene unit content is more preferably 60 mol% or less, even more preferably 50 mol% or less.

[0021] The saponification degree of the vinyl ester component is preferably 80 to 100 mol%. From the viewpoint of obtaining a molded product with excellent gas barrier properties, it is more preferably 95 mol% or more, even more preferably 98 mol% or more, and particularly preferably 99 mol% or more. If the saponification degree is less than 80 mol%, the barrier properties, long-run properties, and moisture resistance may deteriorate. In particular, when producing pellets with excellent melt stability and long-run properties, a high saponification degree of EVOH is preferred.

[0022] The preferred melt flow rate (MFR) of the pellets of the present invention (measured at 190°C under a load of 2160 g; however, for pellets with melting points 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 reciprocal of absolute temperature on the horizontal axis and the melt flow rate (logarithmic) on the vertical axis, and the value 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 may become too high during molding, 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.

[0023] In the pellets of the present invention, the ratio of the number of pellets containing bubbles with a diameter of 0.1 mm or more to the total number of pellets is 0.05 / 100 to 3 / 100. The bubble diameter here is the circle-equivalent diameter obtained from the area of ​​the bubble when the pellet is observed from any direction, and can be obtained by image processing of the image of the pellet. Furthermore, pellets containing bubbles with a diameter of 0.1 mm or more can also be selected by a human eye trained to detect the presence or absence of bubbles with a diameter of 0.1 mm or more.

[0024] When the ratio of pellets containing bubbles to the total number of pellets is less than 0.05 / 100 or more than 3 / 100, molding stability in the extruder decreases. The reason for this is unclear, but it is thought that the buffering effect of the small amount of gas trapped in the pellets is adequately achieved. The ratio of pellets containing bubbles is preferably 0.07 / 100 or more. Furthermore, this ratio is preferably 1.5 / 100 or less, and more preferably 0.8 / 100 or less.

[0025] Furthermore, it is preferable from the viewpoint of thermal stability that the pellets of the present invention contain at least one selected from the group consisting of a boron compound, a phosphoric acid compound, and an alkaline earth metal salt, in addition to the carboxylic acid and the alkali metal salt, and this can suppress deterioration of the EVOH resin, particularly when melt extrusion is performed over a long period of time.

[0026] The carboxylic acid contained in the pellets of the present invention is not particularly limited. Examples include acetic acid, lactic acid, oxalic acid, succinic acid, benzoic acid, and citric acid, with carboxylic acids having four or fewer carbon atoms being preferred. Among these, acetic acid is preferred from the standpoints of cost and availability. The content of carboxylic acid in the dried EVOH resin pellets of the present invention is preferably 10 to 5,000 ppm, since too little may cause discoloration during melt molding, while too much may result in insufficient interlayer adhesion. The carboxylic acid content is preferably 30 ppm or more, more preferably 50 ppm or more. The carboxylic acid content is also preferably 1,000 ppm or less, more preferably 500 ppm or less.

[0027] Examples of alkali metal salts contained in the pellets of the present invention include aliphatic carboxylates, aromatic carboxylates, and phosphates. Examples include sodium acetate, potassium acetate, sodium phosphate, lithium phosphate, sodium stearate, potassium stearate, and the sodium salt of ethylenediaminetetraacetic acid. Among these, sodium acetate, potassium acetate, and sodium phosphate are preferred. The content of alkali metal salts in the EVOH resin pellets of the present invention after drying is preferably 5 to 5,000 ppm, calculated as the alkali metal element, more preferably 20 to 1,000 ppm, and even more preferably 30 to 750 ppm.

[0028] Examples of boron compounds contained in the pellets of the present invention include, but are not limited to, boric acids, boric acid esters, borate salts, and boron hydrides. Specifically, boric acids include orthoboric acid, metaboric acid, and tetraboric acid. Examples of borate esters include triethyl borate and trimethyl borate. Examples of borates include alkali metal salts, alkaline earth metal salts, and borax of the various boric acids listed above. Among these compounds, orthoboric acid (hereinafter simply referred to as boric acid) is preferred. 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, of boron, since too little improves thermal stability and too much can lead to gelation and poor moldability.

[0029] Examples of the phosphate compound contained in the pellets of the present invention include various acids such as phosphoric acid and phosphorous acid, and their salts. The phosphate may be contained in the form of any of monophosphate, diphosphate, and triphosphate, and the cation species is not particularly limited, but alkali metal salts and alkaline earth metal salts are preferred. Among these, it is preferable to add the phosphate compound in the form of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, or dipotassium hydrogen phosphate. The content of the phosphate compound in the dried EVOH resin pellets of the present invention is preferably 1 to 1,000 ppm in terms of phosphate radicals. Addition within this range can suppress discoloration and the occurrence of gels and particles in the molded product. If the content of the phosphate compound is less than 1 ppm, discoloration may occur easily during melt molding. If the content exceeds 1,000 ppm, gels and particles may occur easily in the molded product.

[0030] Examples of alkaline earth metal salts contained in the pellets of the present invention 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 acetate and phosphate are preferred. The content of alkaline earth metal salt in the dried EVOH resin pellets of the present invention is preferably 10 to 1,000 ppm, more preferably 20 to 500 ppm, calculated as metal. If the content of alkaline earth metal salt is less than 10 ppm, the effect of improving long-run properties may be insufficient. Furthermore, if it exceeds 1,000 ppm, the resin may be more likely to discolor when melted.

[0031] The proportion of EVOH in the resin components contained in the pellets 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 essentially composed of EVOH or may consist of only EVOH. Furthermore, the proportion of EVOH in the pellets 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.

[0032] A method for producing pellets of the present invention will be described below. A suitable method for producing pellets of the present invention comprises the steps of adding an alkali catalyst to an alcohol solution of an ethylene-vinyl ester copolymer to saponify the ethylene-vinyl ester copolymer to obtain an alcohol solution of EVOH, adding water to the saponified solution and removing the alcohol to obtain a water-containing composition of EVOH, cutting the obtained water-containing composition into pellets, and drying the obtained pellets.

[0033] First, ethylene and a 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 copolymer. Here, solution polymerization is carried out using an alcohol such as methanol as a solvent. The polymerization method may be either continuous or batchwise. 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 method) 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%.

[0034] Examples of the catalyst 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 peroxyneodecanoate, diisopropyl peroxycarbonate, di-n-propyl peroxydicarbonate, t-butyl peroxyneodecanoate, lauroyl peroxide, benzoyl peroxide, and t-butyl hydroperoxide.

[0035] Monomers copolymerizable 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 ... 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 Copolymerization can also be carried out in the presence of small amounts of alkenes having an ester group such as olefin propane or their saponification products; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, and 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 methallylsulfonic acid or their salts; vinyl silane compounds such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(β-methoxyethoxy)silane, and γ-methacryloxypropylmethoxysilane; alkyl vinyl ethers, vinyl ketones, N-vinylpyrrolidone, vinyl chloride, and vinylidene chloride. In such cases, the amount of copolymerization is typically 5 mol % or less, and it is preferable that these compounds are substantially absent.

[0036] After a predetermined time of polymerization, when a predetermined polymerization rate is reached, a polymerization inhibitor is added as necessary, unreacted ethylene gas is removed by evaporation, 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, for example, a method is employed 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.

[0037] An alkali catalyst is added to an alcohol solution of an ethylene-vinyl ester copolymer from which unreacted vinyl ester has been removed, and the vinyl ester component in the copolymer is saponified to obtain an alcohol solution of an ethylene-vinyl alcohol copolymer. The saponification method can be either continuous or batchwise. Examples of alkali catalysts that can be used include sodium hydroxide, potassium hydroxide, and alkali metal alcoholates. The saponification conditions are as follows: the ethylene-vinyl ester copolymer concentration is preferably 10 to 50% by 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 reaction) is preferably 10 minutes to 6 hours.

[0038] Water is added to the alcohol solution of EVOH obtained in this way, and the alcohol is removed to obtain a hydrous EVOH composition. The hydrous composition is in a fluid state containing EVOH and water, and is cut to produce pellets. When cutting, the hydrous composition in a fluid state may be cut as is, or it may be cut after it has solidified.

[0039] The method for obtaining the above-mentioned aqueous EVOH composition is not particularly limited. For example, a methanol-water mixed solution of EVOH is prepared by the method described in Patent Document 1, and the resulting solution is extruded into a methanol-water mixed solution having a lower methanol concentration than the above-mentioned solution to precipitate in the form of strands, which are then cut into hydrous pellets. The resulting hydrous pellets are supplied to an extruder and melt-kneaded by the method described in Patent Document 2 to obtain an aqueous EVOH composition in a fluid state.

[0040] Alternatively, a water-containing EVOH composition containing 0 to 10 parts by weight of methanol and 10 to 500 parts by weight of water per 100 parts by weight of EVOH can be obtained by introducing a methanol solution of EVOH into a vessel, bringing the solution into contact with water vapor in the vessel, and extracting the methanol together with the water vapor, using a method such as that described in Patent Document 3. Furthermore, a water-containing EVOH composition obtained in the same manner as in Patent Document 3 can be fed into an extruder, and after adjusting the water content in the extruder while melt-kneading, the composition can be discharged from the extruder to obtain a water-containing EVOH composition (Patent Document 4).

[0041] The water content of the resulting aqueous EVOH composition is not particularly limited, but preferably contains 30 to 50 parts by mass of water per 100 parts by mass of EVOH. If the water content is less than 30 parts by mass per 100 parts by mass of EVOH, the proportion of pellets containing bubbles will be too low, which may result in reduced stability during extrusion molding. The water content is more preferably 32 parts by mass or greater. On the other hand, if the water content exceeds 50 parts by mass per 100 parts by mass of EVOH, the proportion of pellets containing bubbles will be too high, which may also result in reduced stability during extrusion molding. The water content is more preferably 45 parts by mass or less, even more preferably 40 parts by mass or less, and particularly preferably 38 parts by mass or less. The aqueous EVOH composition may contain a small amount of organic solvent, such as methanol, in addition to water. However, from an environmental perspective, the organic solvent content is preferably low; typically, it is 10 parts by mass or less, preferably 3 parts by mass or less, more preferably 1 part by mass or less, per 100 parts by mass of EVOH. It is particularly preferred that the organic solvent content be substantially zero.

[0042] When the aqueous EVOH composition contains a carboxylic acid and an alkali metal salt and further contains at least one additive selected from the group consisting of a boron compound, a phosphoric acid compound, and an alkaline earth metal salt, the resulting pellets contain these additives. The method for adding these additives is not particularly limited. For example, the aqueous pellets obtained by precipitating the aqueous EVOH composition in the form of strands and cutting them are immersed in an aqueous solution containing at least one of these additives and then melt-kneaded to obtain the aqueous EVOH composition. Alternatively, an aqueous solution containing at least one of these additives can be injected into the molten aqueous EVOH composition in an extruder.

[0043] The aqueous EVOH composition is extruded through a nozzle and cut into pellets. The extrusion method is not particularly limited, and the aqueous EVOH composition can be extruded using a single-screw or twin-screw extruder or kneader.

[0044] The temperature at which the aqueous EVOH composition is extruded from the nozzle is not particularly limited as long as the aqueous EVOH composition can be maintained in a molten state, but is preferably 90 to 115°C. If the temperature is below 90°C, the proportion of pellets containing bubbles will be too small, resulting in a decrease in stability during extrusion molding. The temperature is more preferably 95°C or higher, and even more preferably 100°C or higher. On the other hand, if the temperature exceeds 115°C, the proportion of pellets containing bubbles will be too large, resulting in a decrease in melt molding stability. The temperature is more preferably 110°C or lower.

[0045] The linear velocity when the aqueous EVOH composition is extruded from the nozzle is not particularly limited, but is preferably 8 to 18 m / min. If the linear velocity is less than 8 m / min, the proportion of pellets containing bubbles becomes too small, resulting in a decrease in stability during extrusion molding. The linear velocity is more preferably 9 m / min or more. On the other hand, if the linear velocity exceeds 18 m / min, the proportion of pellets containing bubbles becomes too large, resulting in a decrease in melt molding stability. The linear velocity is more preferably 15 m / min or less. The linear velocity (cm / min) is calculated by multiplying the mass (g / min) of the aqueous EVOH composition passing through the nozzle per unit time by the cross-sectional area (cm) of the nozzle. 2 ) and specific gravity (g / cm 3 ) and then divide the linear velocity (cm / min) obtained by 100 to obtain the linear velocity (m / min). In the case of a nozzle with multiple holes, the cross-sectional area of ​​the nozzle is calculated by multiplying the cross-sectional area of ​​one hole by the number of holes. The specific gravity of the EVOH water-containing composition varies depending on the composition and water content of the EVOH, but is usually about 1.2 g / cm 3 is.

[0046] The shape of the nozzle through which the aqueous EVOH composition is extruded is not particularly limited, but is preferably circular. The diameter of the nozzle is preferably 2 to 5 mm. If the diameter is less than 2 mm, the proportion of pellets containing bubbles becomes too large, resulting in a decrease in stability during extrusion molding. The nozzle diameter is more preferably 2.5 mm or more. On the other hand, if the nozzle diameter exceeds 5 mm, the proportion of pellets containing bubbles becomes too small, resulting in a decrease in stability during extrusion molding. The nozzle diameter is more preferably 4 mm or less.

[0047] The extruded hydrous composition is extruded through a nozzle and cut to obtain hydrous pellets. Here, the hydrous composition can be cut in a molten state to obtain ellipsoidal pellets, or the hydrous composition can be solidified into strands and then cut with a cutter to obtain cylindrical pellets. Elliptical pellets obtained by cutting in a molten state tend to be irregular in shape and often have poor molding stability. Therefore, it is highly significant to employ the pellets of the present invention, in which the proportion of pellets containing bubbles falls within a predetermined range. Furthermore, cutting in a molten state requires simpler equipment. A hot cutter or an underwater cutter can be used as a cutting device when cutting in a molten state. The moisture content of the pellets obtained by cutting in a molten state immediately after cutting is substantially the same as the moisture content of the hydrous EVOH composition at the time of extrusion.

[0048] The EVOH aqueous composition is cut into pellets and then dried. The drying method is not particularly limited, and a hot air dryer or the like can be used. The dryer may be a fluidized-bed dryer or a static dryer, or a combination of these. Among these, a method in which fluidized-bed drying is first performed, followed by static drying is preferred. The drying temperature is not particularly limited, but a temperature of about 70 to 120°C is typically used, and the temperature can be increased as the drying progresses. The moisture content after drying is typically 1 part by mass or less, and preferably 0.5 parts by mass or less, per 100 parts by mass of EVOH. The pellets may be dehydrated using a centrifugal dehydrator or the like before being placed in the dryer. The dried pellets thus obtained are then subjected to melt molding.

[0049] The shape of the resulting pellets is not particularly limited, but is preferably cylindrical or ellipsoidal. Here, ellipsoidal includes ellipsoids with two orthogonal axes of equal radius and spheres with three orthogonal axes of equal radius. Cylindrical pellets are obtained when the strands are formed and then cut. Furthermore, when the strands are cut with a hot cutter or underwater cutter, ellipsoidal pellets, particularly slightly flattened ellipsoidal pellets, are obtained. The terms "cylinder" and "ellipsoid" are not used in the strict sense, and slight distortion is acceptable. The average particle size of the pellets is typically 2 to 7 mm. Here, the average particle size refers to the circle-equivalent diameter (diameter) calculated from the area of ​​a two-dimensionally observed pellet image. It is preferable that all pellets of the present invention have approximately the same shape, but it is acceptable for pellets of other shapes to be present in small amounts within a range that does not impair the effects of the present invention.

[0050] The half-value width of the particle size distribution of the EVOH pellets of the present invention is preferably 0.7 mm or less. By producing pellets with a small half-value width and uniform particle size, stability during extrusion molding is further improved, and homogeneous molded products can be continuously obtained over a long period of time. The half-value width is preferably 0.6 mm or less, and more preferably 0.5 mm or less. The half-value 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.

[0051] As such, methods for obtaining pellets with a uniform particle size include a method of devising a cutting method and a method of sieving. Among these, the method of sieving is preferred because it can reliably and simply narrow the half-value 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.

[0052] The method for melt-molding the pellets of the present invention thus obtained is not particularly limited, but extrusion molding is preferred. By using the pellets of the present invention, torque fluctuations in the extruder motor and pressure fluctuations within the extruder can be suppressed, allowing for the production of homogeneous molded products. The melting temperature varies depending on the melting point of the EVOH, but is preferably about 150 to 270°C. Various molded products such as films, sheets, pipes, and fibers can be obtained by extrusion molding.

[0053] The resulting molded article may be a molded article consisting of a single layer of EVOH resin, but is preferably a multilayer structure containing at least one layer of EVOH resin. Examples of layer configurations of multilayer structures include, but are not limited to, E / T, T / E / T, E / Ad / T, and T / Ad / E / Ad / T, where E represents the EVOH resin, Ad represents the adhesive resin, and T represents the other thermoplastic resin. Each layer shown here may be a single layer or a multilayer.

[0054] The method for producing the multilayer structure is not particularly limited. Examples include co-extrusion of an EVOH resin with another thermoplastic resin, melt-extrusion of a thermoplastic resin onto a molded product (film, sheet, etc.) made of an EVOH resin, conversely co-extrusion of an EVOH resin with another thermoplastic resin onto a substrate such as a thermoplastic resin, and lamination of a molded product obtained from an EVOH resin with a film or sheet of another substrate using a known adhesive. Among these, the method of co-extrusion with another thermoplastic resin is preferred.

[0055] The multilayer structure thus obtained can be used as a film, sheet, container, etc. Because of its excellent barrier properties and moldability, it is suitable for food packaging containers, fuel containers, chemical containers, etc. [Example]

[0056] The present invention will be described in more detail below with reference to Examples. The pellets obtained in the Examples and Comparative Examples were evaluated according to the evaluation methods shown below.

[0057] (1) Content of air-bubble pellets The pellets obtained in the examples and comparative examples were counted for the number of bubble-containing pellets among 10,000 pellets to measure the bubble-containing pellet content. Pellets having visually visible bubbles of 0.1 mm or more were determined to be bubble-containing pellets.

[0058] (2) Flow irregularities in the film A single layer film formation test was carried out on the pellets obtained in the Examples and Comparative Examples using an extruder and a T-die having the following specifications. Extruder: GT-40-A manufactured by Plastics Technology Research Institute Co., Ltd. Type: Single-screw extruder (non-vent type) Length / Distance: 26 CR:2.8 ·Aperture: 40mmφ Screw: Double flight type Rotation speed: 40 rpm Drive: Sumitomo Heavy Industries DC motor SCR-DC218B Motor capacity: DC 7.5KW (rated 45A) Extruder heater: 3-split type ·C1 / C2 / C3=190℃ / 240℃ / 260℃ Ten resin pressure gauges are installed at equal intervals on the cylinder. Die width: 550mm ·Resin temperature inside die: 240℃ Take-up speed: 10m / min Film thickness: 20μm

[0059] One hour after the start of film production, the number of flow spots in the film was counted and the number of flow spots was measured. 2 The film was held up to a fluorescent lamp, and flow spots of approximately 1 cm or more in width, where changes in refractive index could be seen with the naked eye, were counted.

[0060] 1.0m 2 The appearance of the film was evaluated based on the number of flow spots per film, as follows: A rating of A or B was considered to indicate that flow spots were suppressed. A: Less than 1 B: 1 or more but less than 3 C: 3 or more but less than 10 D: 10 or more

[0061] (3) Thickness unevenness In the single-layer film formation test (2) above, a sample was taken in the MD (longitudinal) direction one hour after the start of film formation, and the thickness was measured over a 2-m length range using a continuous thickness meter. The thickness was measured at 25 mm intervals, and the standard deviation (μm) was calculated. The thickness unevenness was evaluated according to the following criteria. A rating of A or B was considered to have suppressed thickness unevenness. A: 2μm or less B: More than 2μm and less than 4μm C: More than 4μm and less than 6μm D: More than 6μm and less than 8μm E: More than 8μm

[0062] (4) Pressure fluctuation A single layer film production test was carried out in the same manner as in (2) above. During the 6-hour continuous film production from 1 hour to 7 hours after the start of film production, the resin pressure P 10 The maximum pressure P measured at MAX and the minimum pressure P MIN The difference ΔP between the pressure fluctuations was evaluated according to the following criteria. If the evaluation was A or B, it was determined that the pressure fluctuations were suppressed. A: ΔP≦0.3 B: 0.3<ΔP≦0.6 C: 0.6<ΔP≦1.0 D:1.0<ΔP

[0063] (5) Torque fluctuation During the continuous film production from 1 hour to 6 hours after the start of film production (2) above, the fluctuation range of the motor torque of the extruder was evaluated according to the following criteria. If the rating was A or B, it was determined that torque fluctuation was suppressed. A: 5N m or less B: More than 5N·m and less than 10N·m C: More than 10N·m and less than 15N·m D: More than 15N m

[0064] (6) Half-width of pellet particle size distribution Using 500 g of the pellets obtained in the examples and comparative examples, the half-width (mm) was calculated from the particle size distribution obtained using Verder Scientific's "CAMSIZER XT." The particle size was defined as the equivalent circle diameter (diameter) calculated by dynamic image analysis in accordance with ISO 13322-2 (2006).

[0065] Example 1 A saponification reactor was charged with 100 parts by weight of an ethylene-vinyl acetate copolymer with an ethylene unit content of 32 mol% and 400 parts by weight of methanol, followed by 0.16 parts by weight 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 the by-product methyl acetate was removed from the system along with the methanol. The reaction was then terminated by neutralization with acetic acid, yielding a methanol solution of EVOH consisting of 57 parts by weight of EVOH and 75 parts by weight of methanol. The saponification degree of this EVOH was 99.98 mol%.

[0066] The resulting EVOH solution was extruded through a gold plate with a circular opening into an aqueous solution containing a small amount of methanol, solidifying into strands. These strands were then cut into pellets approximately 3 mm in diameter and 5 mm in length. The pellets were dewatered using a centrifuge and washed by repeatedly adding and dewatering large amounts of water. The resulting hydrous EVOH pellets contained 120 parts by weight of water per 100 parts by weight of EVOH. The hydrous EVOH pellets were dried for 1 hour at 80°C in a hot air dryer using air with a dew point temperature of -10°C, yielding hydrous EVOH pellets with a water content of 60 parts by weight per 100 parts by weight of EVOH.

[0067] The resulting hydrous EVOH pellets were fed into the cylindrical barrel 10 of the twin-screw extruder 20 shown in FIG. 1 through the raw material supply section 1. The fluidized hydrous EVOH composition was fed forward through the full-flight screw section 2, kneaded through the reverse-flight screw section 3, and adjusted for moisture content by removing excess moisture through the vent 7. The composition was then fed forward through the full-flight screw section 4, and an aqueous solution of acetic acid, sodium acetate, phosphoric acid, and boric acid was injected through the additive solution injection section 8. The composition was then kneaded through 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 located near the discharge port, was 105°C.

[0068] The amount of EVOH fed per unit time was 23.7 kg / hr (including the mass of water contained therein), and the amount of additive solution fed per unit time was 1.1 L / hr. The additive solution was an aqueous solution containing 10.0 g / L of acetic acid, 7.1 g / L of sodium acetate, 0.11 g / L of phosphoric acid, and 9.8 g / L of boric acid.

[0069] The specifications of the twin-screw extruder 20 are as follows: Length: 45.5 ·Caliber: 44mmφ Screw: Same direction full intermeshing type Rotation speed: 250 rpm Cylinder temperature: 100℃ Die temperature: 105℃

[0070] The aqueous EVOH composition in a fluid state was discharged from the discharge port 11 of the twin-screw extruder 20. The discharge rate per unit time was 20 kg / hr. The aqueous EVOH composition delivered from the discharge port 11 was discharged from a die having a nozzle with six 3 mm diameter holes. A hard chrome-plated die was used. The linear velocity of the aqueous EVOH composition at the nozzle holes was 9.44 m / min. This linear velocity was calculated by dividing the discharge volume of the aqueous EVOH composition per unit time by the total area of ​​the six holes. The discharge volume per unit time was calculated by multiplying the discharge rate per unit time by the specific gravity (1.2 g / cm) of the aqueous EVOH composition. 3 ) The temperature of the aqueous EVOH composition in the die was 105°C. The discharged aqueous EVOH composition was cut in a fluid state by a rotating blade using a center hot cutter immediately after being extruded from the nozzle into the air. The center hot cutter had two blades, and the blade rotation speed was 600 to 2800 rpm. In this way, spheroidal aqueous EVOH pellets were obtained. The pellets contained 35 parts by mass of water per 100 parts by mass of EVOH and were substantially free of methanol.

[0071] The resulting EVOH hydrous composition pellets were dehydrated in a centrifugal dehydrator to yield pellets containing 20 parts by weight of water per 100 parts by weight of EVOH. The dehydrated pellets were then dried in a fluidized bed dryer at 90°C for 15 hours and subsequently at 105°C for 15 hours in a static dryer to yield dried EVOH resin composition pellets. The resulting dried pellets contained 0.2 parts by weight of water per 100 parts by weight of EVOH. The dried EVOH resin composition pellets contained 300 ppm of acetic acid, 270 ppm of boron compounds (calculated as boron), 98 ppm of phosphate compounds (calculated as phosphate radicals), and 130 ppm of sodium alkali metal salts (calculated as metals). The MFR (190°C, 2160 g load) was 1.5 g / 10 min.

[0072] The pellets were then sieved through 6.5 mesh, 7 mesh, and 8 mesh sieves in that order for 10 minutes, and the pellets remaining on the 8 mesh were collected to obtain pellets for melt molding. The resulting pellets were spheroidal, like a sphere crushed from above and below, with a longitudinal length of 3.2 mm and a lateral length of 2.1 mm. The resulting pellets were evaluated according to the above evaluation methods, and the results are shown in Table 1.

[0073] Examples 2 and 3, Comparative Examples 1 and 2 Dry EVOH resin pellets were produced and evaluated in the same manner as in Example 1, except that the moisture content of the hydrous EVOH pellets immediately after cutting, the amount discharged per unit time from the discharge port 11, the temperature of the hydrous EVOH composition in the die, and the linear velocity of the hydrous EVOH composition at the nozzle hole were changed as shown in Table 1. The results are summarized in Table 1.

[0074] [Table 1] [Explanation of symbols]

[0075] 1 Raw material supply section 2,4,6 Full flight screw section 3,5 Reverse flight screw section 7. Vent 8 Additive solution injection section 9. Temperature Sensor 10 cylinder barrel 11 Discharge port 20 Twin-screw extruder

Claims

1. Pellets containing an ethylene-vinyl alcohol copolymer, wherein the ratio of the number of pellets containing bubbles with a diameter of 0.1 mm or more to the number of all pellets is 0.05 / 100 to 3 / 100.

2. 2. The pellet according to claim 1, which contains a carboxylic acid and an alkali metal salt, and further contains at least one member selected from the group consisting of a boron compound, a phosphoric acid compound, and an alkaline earth metal salt.

3. 3. The pellet according to claim 1, wherein the pellet has a cylindrical or ellipsoidal shape.

4. The pellets according to any one of claims 1 to 3, wherein the half width of the particle size distribution of the pellets is 0.7 mm or less.

5. 5. The method for producing pellets according to claim 1, wherein an ethylene-vinyl alcohol copolymer hydrous composition containing 30 to 50 parts by mass of water per 100 parts by mass of the ethylene-vinyl alcohol copolymer and having a temperature of 90 to 115°C is extruded through a circular nozzle having a diameter of 2 to 5 mm at a linear velocity of 8 to 18 m / min, and then cut and dried.

6. 6. The method for producing pellets according to claim 5, wherein the aqueous ethylene-vinyl alcohol copolymer composition is extruded through the circular nozzle, then cut in a molten state, and then dried.

7. 7. The method for producing pellets according to claim 5 or 6, wherein the aqueous composition contains a carboxylic acid and an alkali metal salt, and further contains at least one selected from the group consisting of a boron compound, a phosphoric acid compound, and an alkaline earth metal salt.

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