Method for producing polyvinyl alcohol resin

Abstract

This method for producing a polyvinyl alcohol resin comprises a washing step for continuously washing polyvinyl alcohol particles. The polyvinyl alcohol particles subjected to a first washing step are obtained by saponifying a polyvinyl ester using an alkali catalyst. In the first washing step, the polyvinyl alcohol particles and a first washing solution containing 50 mass% or more of methanol and 0.1-10 mass% of an acid are mixed to form a slurry, and the alkali catalyst residue content in the polyvinyl alcohol particles is reduced. In a second washing step, the slurry and a second washing solution containing 50 mass% or more of methanol and having an acid concentration of less than 0.1 mass% are brought into contact with each other to reduce the acid content in the polyvinyl alcohol particles. As a result, a method for producing a polyvinyl alcohol resin is provided, whereby an alkali catalyst residue contained in the saponified PVA particles can be continuously and efficiently removed.

Description

Method for producing polyvinyl alcohol resin

[0001] The present invention relates to a method for producing a polyvinyl alcohol resin, which includes a washing step in which polyvinyl alcohol particles are continuously washed.

[0002] Polyvinyl alcohol (PVA) is a water-soluble synthetic resin widely used in various fields, including fibers, films, dispersants, and adhesives. PVA is typically produced by saponifying polyvinyl acetate, obtained by polymerizing vinyl acetate monomer, with an alkaline catalyst such as sodium hydroxide. Therefore, the saponified PVA contains sodium acetate, a residue of the saponification catalyst. If a large amount of sodium acetate is present, the PVA is prone to thermal decomposition and discoloration when heated, and there are applications where the presence of such impurities is undesirable. Therefore, methods for removing sodium acetate from saponified PVA have been investigated.

[0003] For example, Patent Document 1 describes a method for reducing the sodium acetate content of polyvinyl alcohol obtained by saponifying polyvinyl ester by washing with a washing solution containing methyl acetate, methanol, and water in specific ratios. The examples describe a batch production method in which a slurry containing PVA and the washing solution is formed, filtered, a PVA cake is obtained, and then dried. Thus, if the PVA particles are so fine that they form a cake after filtration, the washing efficiency is improved, but the need to handle the cake makes the process less convenient, and thus production by a batch process is unavoidable.

[0004] Patent Document 2 describes a method for washing PVA resin using an apparatus comprising a vertical moving bed apparatus and an inclined screw conveyor, the lower part of which is connected to the vicinity of the lower part of the inclined screw conveyor. A slurry containing solid PVA resin is supplied from the upper part of the vertical moving bed apparatus and conveyed upward through the lower part of the inclined screw conveyor. A washing solution is poured into the vicinity of the upper part of the inclined screw conveyor and discharged from the vicinity of the upper part of the vertical moving bed apparatus, thereby removing impurities from the resin. While this method enables continuous washing of PVA resin, the apparatus is large and complex. Furthermore, the PVA washed in the examples contained 0.1 to 0.3% (1000 to 3000 ppm) of sodium acetate, making it difficult to thoroughly wash the resin.

[0005] JP 2011-178964 A JP 2007-245432 A

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for producing a polyvinyl alcohol resin, which can continuously and efficiently remove alkali catalyst residue contained in PVA particles after saponification.

[0007] The present inventors have conducted extensive research to solve the above problems and have completed the present invention, which relates to the following [1] to

[11] .

[0008] [1] A method for producing a polyvinyl alcohol resin, comprising a washing step of continuously washing polyvinyl alcohol particles, the washing step comprising at least a first washing step and a second washing step, wherein the polyvinyl alcohol particles subjected to the first washing step are obtained by saponifying a polyvinyl ester using an alkali catalyst, the first washing step comprising mixing the polyvinyl alcohol particles with a first washing liquid containing 50% by mass or more of methanol and 0.1 to 10% by mass of an acid to form a slurry, thereby reducing the content of alkali catalyst residue in the polyvinyl alcohol particles, the slurry having been subjected to the first washing step being continuously supplied to a second washing step, and the second washing step comprising contacting the slurry with a second washing liquid containing 50% by mass or more of methanol and having an acid concentration of less than 0.1% by mass to reduce the content of acid in the polyvinyl alcohol particles.

[0009] [2] The method for producing a polyvinyl alcohol resin according to [1], wherein the second washing step is carried out in a tower-type vessel, the slurry having undergone the first washing step being continuously introduced into the upper part of the tower-type vessel, and a second washing liquid being continuously introduced from the lower part of the tower-type vessel. [3] The method for producing a polyvinyl alcohol resin according to [1] or [2], wherein the washing liquid having undergone the second washing step is recovered and reused as the first washing liquid. [4] The method for producing a polyvinyl alcohol resin according to any one of [1] to [3], wherein the acid contained in the first washing liquid is a carboxylic acid. [5] The method for producing a polyvinyl alcohol resin according to [4], wherein the carboxylic acid is acetic acid. [6] The method for producing a polyvinyl alcohol resin according to any one of [1] to [5], wherein the first washing liquid further contains methyl acetate, and the mass ratio of methyl acetate to methanol is 0.05 to 0.8. [7] The method for producing a polyvinyl alcohol resin according to any one of [1] to [6], wherein the first washing liquid further contains water, and the mass ratio of water to methanol is 0.005 to 0.2. [8] The method for producing a polyvinyl alcohol resin according to any one of [1] to [7], wherein the polyvinyl alcohol particles have an average equivalent sphere diameter of 1 to 10 mm. [9] The method for producing a polyvinyl alcohol resin according to any one of [1] to [8], wherein the alkaline catalyst is sodium hydroxide.

[10] The method for producing a polyvinyl alcohol resin according to any one of [1] to [9], further comprising a dewatering step and a drying step after the washing step.

[11] The method for producing a polyvinyl alcohol resin according to any one of [1] to

[10] , wherein the obtained polyvinyl alcohol resin has a sodium acetate content of 500 ppm or less.

[0010] According to the method for producing a polyvinyl alcohol resin of the present invention, it is possible to continuously and efficiently remove the alkali catalyst residue contained in the PVA particles after saponification, and it is also possible to remove the acid that has been incorporated into the PVA particles in the first washing step in the second washing step.

[0011] FIG. 1 is a schematic diagram of the apparatus used to produce dried PVA chips in the examples.

[0012] The production method of the present invention includes a washing step of continuously washing polyvinyl alcohol particles containing alkali catalyst residue, and the washing step further includes at least a first washing step and a second washing step. By including these two washing steps, sodium acetate contained in the PVA particles after saponification can be continuously and efficiently removed. The production method of the present invention will be specifically described below.

[0013] [Polymerization step] Polyvinyl ester is produced by polymerizing vinyl ester using a polymerization initiator. Examples of vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl caprylate, vinyl versatate, etc., and among them, vinyl acetate is preferred from an industrial viewpoint. These may be used alone or in combination of two or more.

[0014] During polymerization, other monomers besides vinyl esters may be copolymerized within the scope of the present invention. Examples of such other monomers include α-olefins such as ethylene, propylene, n-butene, and isobutylene; (meth)acrylic acid and salts thereof; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, diacetone(meth)acrylamide, (meth)acrylamidopropanesulfonic acid and salts thereof, and (meth)acrylamidopropyldimethylamine. and its salts or quaternary salts, (meth)acrylamide compounds such as N-methylol(meth)acrylamide and its derivatives; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinylidene halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; unsaturated dicarboxylic acids and their salts or esters such as maleic acid, itaconic acid, and fumaric acid; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. These may be used alone or in combination of two or more. The amount of copolymerization of other monomers is usually 10 mol % or less, or 5 mol % or less. In this specification, "(meth)acryl" is a general term for methacrylic and acrylic.

[0015] The polymerization initiator used in the polymerization is not particularly limited and may be selected from known polymerization initiators, such as azo polymerization initiators, peroxide polymerization initiators, and redox polymerization initiators, depending on the polymerization method. Examples of azo polymerization initiators include 2,2'-azobis(isobutyronitrile) (AIBN), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMV), and 2,2'-azobis(2,4-dimethylvaleronitrile). Examples of peroxide polymerization initiators include percarbonate compounds such as diisopropyl peroxydicarbonate, di(2-ethylhexyl)peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanate and α-cumyl peroxyneodecanate; acetylcyclohexylsulfonyl peroxide; and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate. Potassium persulfate, ammonium persulfate, hydrogen peroxide, or the like may be combined with the above polymerization initiator to form a polymerization initiator. These polymerization initiators may be used alone or in combination of two or more. The polymerization initiator is preferably an azo-based polymerization initiator or a peroxide-based polymerization initiator having a 10-hour half-life temperature of 66°C or less, in order to increase the polymerization rate and achieve a high polymerization rate in a short period of time. Examples of azo-based polymerization initiators or peroxide-based polymerization initiators having a 10-hour half-life temperature of 66°C or less include AIBN, AMV, 2,2'-azobis(2,4-dimethylvaleronitrile), and di-2-ethylhexyl peroxydicarbonate. The amount of the polymerization initiator used is not particularly limited, but is typically preferably 0.1 to 20 parts by mass, more preferably 0.2 to 10 parts by mass, and even more preferably 0.3 to 5.0 parts by mass, per 100 parts by mass of the vinyl ester. The polymerization initiator may be added all at once at the start of polymerization, or may be added at the start of polymerization and then further added during polymerization.

[0016] The polymerization method may be batch polymerization, semi-batch polymerization, continuous polymerization, or semi-continuous polymerization. Any of known polymerization methods, such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization, can be used. Among these, bulk polymerization, in which polymerization is carried out in the absence of a solvent, and solution polymerization, in which polymerization is carried out in the presence of a solvent, are preferred. Examples of solvents used in solution polymerization include alcoholic solvents such as methanol, ethanol, and n-propanol, with methanol being particularly preferred. One solvent may be used alone, or two or more solvents may be used in combination. The amount of solvent used is not particularly limited, but from the viewpoint of increasing the efficiency of polymerization, it is usually preferably 10 to 150 parts by mass, more preferably 20 to 120 parts by mass, even more preferably 30 to 90 parts by mass, and particularly preferably 30 to 80 parts by mass, per 100 parts by mass of the vinyl ester.

[0017] The polymerization temperature is not particularly limited, but is usually preferably 0°C or higher and 200°C or lower, more preferably 30°C or higher and 140°C or lower, even more preferably 30°C or higher and 100°C or lower, and particularly preferably 30°C or higher and 90°C or lower. If the polymerization temperature is lower than 0°C, a sufficient polymerization rate tends not to be obtained. On the other hand, if the polymerization temperature is higher than 200°C, it tends to be difficult to obtain the target polymer. The pressure during polymerization is not particularly limited, and may be increased as needed, or may be atmospheric pressure.

[0018] The polymerization rate of the vinyl ester is not particularly limited, but is preferably 10 to 90%. If the polymerization rate is less than 10%, the productivity of PVA may decrease. The polymerization rate is more preferably 20% or more, and even more preferably 50% or more. On the other hand, if the polymerization rate exceeds 90%, the viscosity of the resulting polyvinyl ester may become too high, which may decrease the productivity of PVA and may also deteriorate the hue of the resulting PVA. The polymerization rate is more preferably 85% or less, and even more preferably 80% or less.

[0019] [Saponification Step] The polyvinyl ester thus obtained is saponified using an alkali catalyst. For example, saponification can be performed by alcoholysis or hydrolysis using an alkali catalyst such as sodium hydroxide, potassium hydroxide, or sodium methoxide. Among these, sodium hydroxide is preferred. When sodium hydroxide is added to the reaction solution as a saponification catalyst, some or all of it is converted to sodium acetate, which remains in the PVA particles as catalyst residue. Examples of solvents used in the saponification reaction include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; and aromatic hydrocarbons such as benzene and toluene. These solvents may be used alone or in combination. Among these, a method using methanol or a methanol / methyl acetate mixed solution as the solvent and sodium hydroxide as the catalyst is preferred. The amount of alkali catalyst used, expressed as a molar ratio relative to the vinyl ester units in the polyvinyl ester, is preferably 0.001 to 0.5, more preferably 0.002 to 0.2, and even more preferably 0.003 to 0.1. The temperature for saponification is not particularly limited, but is preferably in the range of 20 to 70°C. As the saponification reaction progresses, a gel-like product precipitates, which is then pulverized. This yields polyvinyl alcohol (PVA) particles containing the catalyst residue, sodium acetate, and the solvent. The PVA particles thus obtained can be introduced into the first washing step.

[0020] Alternatively, the saponification reaction may be further carried out while the PVA particles are brought into contact with methanol or a methanol / methyl acetate mixed solution at 30 to 70°C. In this case, a slurry in which the PVA particles are dispersed in the mixed solvent is obtained, and the filtrate is removed from the slurry using a solid-liquid separator to obtain the PVA particles. The solid-liquid separator is not particularly limited as long as it can separate the PVA particles from the solvent, but a continuous sieving device such as an Ultrascreen is preferably used. By contacting the PVA particles with the solvent, a portion of the alkali catalyst residue contained in the PVA particles is removed, but the remainder remains in the PVA particles.

[0021] [PVA Particles] The PVA particles obtained through the saponification process contain a solvent consisting of methanol or a methanol / methyl acetate mixed solution and alkali catalyst residue. The content of the solvent in the PVA particles is typically 20 to 400 parts by mass, preferably 30 to 300 parts by mass, and more preferably 50 to 200 parts by mass, per 100 parts by mass of PVA. The content of methyl acetate in the mixed solvent is typically 50 parts by mass or less, preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of methanol. The content of alkali catalyst residue in the PVA particles is typically 5,000 ppm or more per 100 parts by mass of PVA. The shape of the PVA particles is not particularly limited and may be chips, pellets, granules, or the like. The shape may be rectangular, spherical, cylindrical, or irregular. The average equivalent-sphere diameter of the PVA particles is preferably 1 to 10 mm. If the average equivalent sphere diameter is too small, it becomes difficult to continuously separate the particles from the liquid, which may result in a decrease in productivity. The average equivalent sphere diameter is preferably 2 mm or more, more preferably 3 mm or more. On the other hand, if the average equivalent sphere diameter is too large, the washing speed decreases. The average equivalent sphere diameter is preferably 7 mm or less, more preferably 5 mm or less. Here, the equivalent sphere diameter refers to the diameter of a sphere having the same volume as the PVA particles. When calculating the average value, fine powder with an equivalent sphere diameter of less than 0.5 mm is excluded.

[0022] [First Washing Step] In the production method of the present invention, the PVA particles containing a large amount of alkali catalyst residue obtained as described above are washed in two washing steps: a first washing step and a second washing step. In the first washing step, the PVA particles are mixed with a first washing solution containing 50% by mass or more of methanol and 0.1 to 10% by mass of an acid to form a slurry, thereby reducing the content of alkali catalyst residue in the PVA particles. Furthermore, the acid contained in the first washing solution extracts and removes the alkali catalyst residue from the PVA particles that come into contact with the washing solution, thereby terminating the saponification reaction.

[0023] The first cleaning solution contains 50% by mass or more of methanol and 0.1 to 10% by mass of an acid. By including 0.1% by mass or more of an acid, the alkali catalyst residue in the PVA particles can be extracted into the cleaning solution, thereby effectively reducing the content of alkali catalyst residue in the PVA particles. The type of acid is not particularly limited, but a pKa at 25°C of 3 to 5.5 is preferred, and a pKa of 4 to 5 is even more preferred. The acid is preferably a carboxylic acid, and acetic acid is particularly preferred. The acid content in the first cleaning solution is preferably 0.16% by mass or more. On the other hand, if the acid content is too high, the PVA particles will contain a large amount of acid, making it difficult to remove the acid in the second cleaning step. The acid content in the first cleaning solution is preferably 5% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less. The acid contained in the first cleaning solution may be the same as or different from the acid contained in the second cleaning solution described below.

[0024] It is preferable that the first cleaning solution further contains methyl acetate. Methyl acetate is a by-product of alcoholysis during the saponification reaction and is contained in the reaction solvent after saponification. The ratio of methyl acetate to methanol increases as the solvent is repeatedly recovered and reused. Therefore, it is preferable to use a cleaning solution containing methyl acetate at a predetermined ratio to methanol in terms of recyclability and operating costs. The mass ratio of methyl acetate to methanol in the first cleaning solution (methyl acetate / methanol) is preferably 0.05 to 0.8. This mass ratio is more preferably 0.1 to 0.5, and even more preferably 0.15 to 0.4.

[0025] It is also preferable that the first cleaning solution further contains water. By including water in the first cleaning solution, sodium acetate can be more easily extracted from the PVA pellets. If the water content is too low, the efficiency of removing catalyst residues may decrease. On the other hand, if the water content is too high, extra energy may be required to separate and remove water from the recovered solvent. The mass ratio of water to methanol (water / methanol) in the first cleaning solution is preferably 0.005 to 0.2. This mass ratio is more preferably 0.008 to 0.1, and even more preferably 0.01 to 0.05.

[0026] The washing apparatus for the first washing step is not particularly limited. It is sufficient that the PVA particles containing a large amount of alkali catalyst residue and the first washing solution can be continuously supplied and the washed PVA particles can be continuously removed. A tower-type washing apparatus is preferred because it is simple, requires little energy, and enables uniform washing. In this case, it is preferred that the PVA particles containing a large amount of alkali catalyst residue are continuously introduced into the upper part of the tower-type vessel, and the first washing solution is continuously introduced into the lower part of the tower-type vessel. This allows countercurrent contact between the PVA particles gradually descending due to gravity and the ascending first washing solution, thereby efficiently removing methyl acetate from the PVA particles. The first washing solution introduced into the lower part of the tower-type vessel may be introduced into the tower-type vessel from only one pipe or from multiple pipes. When the first washing solution is introduced into the tower-type vessel from multiple pipes, the composition of the first washing solution is determined by combining the contents of the first washing solution. The PVA particles descending within the column are continuously discharged as a slurry together with the washing liquid from the bottom of the column and are continuously supplied to the second washing step. The washing liquid containing sodium acetate after washing is discharged from the top of the column to the outside of the system, recovered, and purified as necessary for reuse.

[0027] [Second Washing Step] The slurry after the first washing step is continuously supplied to the second washing step, where the PVA particles contained in the slurry are brought into contact with a second washing solution containing 50% by mass or more of methanol and having an acid concentration of less than 0.1% by mass, thereby reducing the acid content in the PVA particles.

[0028] The second cleaning solution contains 50% by mass or more of methanol and an acid concentration of less than 0.1% by mass. By having an acid concentration of less than 0.1% by mass, the acid in the PVA particles can be extracted into the cleaning solution, thereby effectively reducing the acid content in the PVA particles. The acid concentration of the second cleaning solution is preferably 0.05% by mass or less. The lower limit of the acid concentration of the second cleaning solution is not particularly limited and may be 0.0001% by mass. In one embodiment, the acid concentration of the second cleaning solution is preferably 0% by mass, i.e., the second cleaning solution is more preferably substantially acid-free. The amount of methanol contained in the second cleaning solution is preferably 70% by mass or more, more preferably 90% by mass or more. The second cleaning solution may contain methyl acetate or water, but the total content of these is preferably 50% by mass or less, preferably 30% by mass or less, and even more preferably 10% by mass or less.

[0029] The washing apparatus for the second washing step is not particularly limited. It is sufficient that it can continuously supply a slurry containing PVA particles and a washing solution containing a relatively large amount of acid, and the second washing solution, and continuously remove the washed PVA particles. As with the first washing step, a tower-type washing apparatus is preferred because it is simple, requires little energy, and enables uniform washing. In this case, it is preferred that the slurry containing PVA particles and a washing solution is continuously introduced into the upper part of the tower-type vessel, and the second washing solution is continuously introduced into the lower part of the tower-type vessel. This allows countercurrent contact between the PVA particles gradually descending due to gravity and the rising second washing solution, thereby efficiently removing the acid from the PVA particles. The second washing solution introduced into the lower part of the tower-type vessel may be introduced into the tower-type vessel from only one pipe or from multiple pipes. When the second washing solution is introduced into the tower-type vessel from multiple pipes, the combined composition of the second washing solution is the composition of the second washing solution. It is also preferable to use the washing liquid recovered in the solid-liquid separator after the second washing step as part of the second washing liquid. In this case, it is also preferable to add methanol to the recovered washing liquid to form the second washing liquid. The PVA particles that have descended within the tower vessel are continuously discharged from the bottom of the tower vessel together with the washing liquid as a slurry, and are continuously subjected to the deliquoring step. Furthermore, the washing liquid that has completed the second washing step is discharged from the top of the tower vessel. It is also preferable to recover the washing liquid that has completed the second washing step and reuse it as the first washing liquid.

[0030] [Deliquipping Step] The second washing step is preferably followed by a deliquipping step in which the washing liquid is mechanically removed from the PVA particles. When the second washing step is performed in a tower vessel, the slurry discharged from the bottom of the tower vessel is continuously sent to a solid-liquid separator, where it is separated into PVA particles and washing liquid. The solid-liquid separator is not particularly limited as long as it can separate the PVA particles from the washing liquid. It is preferable to use a centrifugal deliquifying machine as the solid-liquid separator, as this can effectively remove the washing liquid adhering to the PVA particle surface and reduce the energy required in the subsequent drying step. The washing liquid separated in the deliquipping step is preferably recovered and used as part of the second washing liquid.

[0031] [Drying Step] After the draining step, the PVA particles are dried in the drying step. The drying device is not particularly limited, but drying with hot air is preferred. The drying temperature is preferably 40 to 130°C, more preferably 80 to 120°C. The drying time is preferably 1 to 20 hours, more preferably 1.5 to 6 hours. The drying step usually reduces the solvent content in the PVA particles to 8% by mass or less.

[0032] The dried PVA particles thus obtained have a low sodium acetate content and a low acid content. The sodium acetate content in the dried PVA particles is preferably 500 ppm or less, more preferably 100 ppm or less, and even more preferably 50 ppm or less. On the other hand, since thorough washing increases cleaning costs, the dried PVA particles usually contain 1 ppm or more of sodium acetate. The acid content in the dried PVA particles is preferably 1000 ppm or less, more preferably 700 ppm or less, and even more preferably 500 ppm or less. On the other hand, since thorough washing increases cleaning costs, the dried PVA particles usually contain 10 ppm or more of acid.

[0033] The degree of saponification of the PVA contained in the dried PVA particles is not particularly limited, but is usually 70 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more, and even more preferably 95 mol% or more. On the other hand, the degree of saponification is usually 99.9 mol% or less. The degree of saponification is measured in accordance with JIS K 6726 (1994). The degree of saponification of the PVA after the saponification step and the degree of saponification of the PVA contained in the dried PVA particles are substantially the same.

[0034] The viscosity-average degree of polymerization of the PVA contained in the dried PVA particles is not particularly limited, but is usually 100 to 5,000, preferably 200 to 4,000, and more preferably 300 to 3,000. The viscosity-average degree of polymerization is a value obtained by measurement in accordance with JIS K 6726 (1994). Specifically, when the degree of saponification is less than 99.5 mol%, the viscosity-average degree of polymerization (P) is calculated by the following formula using the limiting viscosity [η] (liters / g) measured in water at 30°C for polyvinyl alcohol saponified to a degree of saponification of 99.5 mol% or more. Note that the degree of saponification of the PVA after the saponification step and the viscosity-average degree of polymerization of the PVA contained in the dried PVA particles are substantially the same. P = ([η] x 10,000 / 8.29) （１／０．６２）

[0035] The PVA particles thus obtained can be used in a variety of applications. The PVA particles can be dissolved in water to prepare an aqueous solution, which can be used in a wide range of applications, including films, fibers, coating agents, dispersants, emulsifiers, and adhesives. Instead of preparing an aqueous solution, the particles can also be melt-molded. In this case, it is preferable to add a plasticizer such as glycerin before melt-molding. The dried PVA particles of the present invention have a low sodium acetate content, which can suppress discoloration and changes in melt viscosity when heated to melt. Furthermore, the low acid content can also prevent the acid from volatilizing and generating odors.

[0036] Example 1 Dried polyvinyl alcohol (PVA) chips 18 were produced using the apparatus shown in Figure 1. First, 2.0 parts by mass of a methanol solution of sodium hydroxide (concentration: 120 g / L) was added to a mixture of 48 parts by mass of polyvinyl acetate and 52 parts by mass of methanol, and a saponification reaction was allowed to proceed at 36°C. After 15 minutes, the resulting solid was pulverized to obtain polyvinyl alcohol (PVA) chips containing methanol.

[0037] The PVA chips obtained by pulverization were introduced into a column reactor (not shown) containing methanol, and the saponification reaction was further carried out at 64°C for 1.5 hours. Subsequently, a slurry 1 containing the PVA chips and methanol was filtered through an ultrascreen 2 to remove a filtrate 3, yielding PVA chips 4 containing alkali catalyst residue. The resulting PVA chips 4 were rectangular parallelepipeds with average dimensions of 5.0 to 6.0 mm in length and width, approximately 1.0 mm in thickness, and an average spherical equivalent diameter of 3.6 to 4.1 mm. The degree of saponification of the PVA was 99 mol%, and the viscosity-average degree of polymerization was 370.

[0038] The PVA chips 4 thus obtained were continuously introduced into a first washing tower 5 for washing (first washing step). The first washing tower 5 is a tower-shaped washer equipped with a flow path for introducing a washing solution 6 containing water, acetic acid, methanol, and methyl acetate, and a flow path for introducing a washing solution 8 recovered from a second washing tower 7 (described later). The composition of the washing solution (first washing solution) supplied to the first washing tower 5 can be adjusted by adjusting the flow rates of the respective washing solutions. In this example, continuous washing was performed while adjusting the composition of the first washing solution to the range of 0.23 parts by mass of acetic acid, 75 to 80 parts by mass of methanol, 18 to 23 parts by mass of methyl acetate, and 1.5 to 2.0 parts by mass of water. The pKa of acetic acid at 25°C is 4.56. The first washing tower 5 is equipped with a mechanism for discharging the washing solution outside the system. The used washing solution containing alkali catalyst residue and the like is discharged outside the system 9, purified in a solvent recovery step, and reused. The PVA chips 4 containing the alkali catalyst residue were continuously introduced into the upper part of the first washing tower 5 and descended within the tower while contacting the washing liquid, and a slurry 10 containing the PVA chips and the washing liquid was discharged from the lower part of the tower. In the first washing tower 5, the alkali catalyst residue contained in the PVA chips that had come into contact with the washing liquid was extracted and removed by the acetic acid contained in the washing liquid, thereby terminating the saponification reaction.

[0039] Subsequently, the slurry 10 of the PVA chips and washing liquid discharged from the first washing tower 5 was continuously introduced into the second washing tower 7 for washing (second washing step). The second washing tower 7 is a tower-shaped washer, and is provided at its upper part with an inlet for introducing the PVA slurry 10 sent from the first washing tower 5 and an outlet for sending the washing liquid 8 in the second washing tower 7 to the first washing tower 5. The second washing tower 7 is further provided at its lower part with an outlet for sending the slurry 11 containing the PVA chips that have come into contact with the washing liquid in the second washing tower 7 to a solid-liquid separator 12, and with an inlet for introducing methanol 13 and the washing liquid 14 recovered from the solid-liquid separator 12 into the second washing tower 7.

[0040] The washing liquid (second washing liquid 15) introduced from the bottom of the second washing tower 7 was a mixed liquid of the washing liquid 14 recovered from the solid-liquid separator 12 and methanol 13, and was 90% by mass or more of which was methanol, contained small amounts of methyl acetate and water, and had an acid content of less than 0.1% by mass. The washing liquid 15 flowed from the bottom to the top of the second washing tower 7, was discharged from an outlet at the top of the second washing tower 7 together with the washing liquid in the slurry introduced from the top, and was sent to the first washing tower 5 through a flow path and reused as the washing liquid 8. The PVA chips contained in the slurry 10 continuously introduced from the top of the second washing tower 7 descended to the bottom within the second washing tower 7 while coming into contact with the washing liquid mainly composed of methanol. The cleaning solution inside the second cleaning tower 7 had a higher methanol concentration and a lower acetic acid concentration toward the bottom, and the PVA chips passing through the second cleaning tower 7 passed through a solution containing methanol as a main component. Therefore, the acetic acid contained in the PVA chips at the end of the first cleaning step was washed away in the second cleaning step.

[0041] The slurry 11 discharged from the bottom of the second washing tower 7 was continuously sent to the solid-liquid separator 12 and separated into PVA chips 16 and washing liquid 14 by a centrifugal dewatering machine. The resulting PVA chips 16 were dried at 100°C in a dryer 17 to obtain dried PVA chips 18. The solvent content of the dried PVA chips 18 was 5% by mass or less. The washing liquid 14 recovered in the solid-liquid separator 12 was sent to the second washing tower 7 and reused as washing liquid. The dried PVA chips 18 were sampled and divided approximately every four hours. The sodium acetate content in the chips was measured using an atomic absorption spectrophotometer and found to be 5 to 48 ppm. The acetic acid content in the dried PVA chips 18 was measured using gas chromatography and found to be 50 to 400 ppm. These results are summarized in Table 1.

[0042] [Examples 2 to 5] PVA chips were washed, dewatered, and dried to obtain dried chips, which were then analyzed in the same manner as in Example 1, except that the ratios of methanol, methyl acetate, water, and acetic acid contained in the first washing solution were changed as shown in Table 1. The results are summarized in Table 1.

[0043] [Comparative Examples 1 and 2] PVA chips were washed, dewatered, and dried to obtain dried chips, which were then analyzed in the same manner as in Example 1, except that the first cleaning solution did not contain acetic acid and the proportions of methanol, methyl acetate, and water contained in the first cleaning solution were changed as shown in Table 1. The results are summarized in Table 1.

[0044] [Comparative Example 3] Chips that had been subjected to the first washing step in the same manner as in Example 4 were collected, and then dewatered and dried under conditions similar to those in Example 1 to obtain dried PVA chips, which were analyzed in the same manner as in Example 1. The results are summarized in Table 1.

[0045]

[0046] 1, 10, 11 Slurry 2 Ultrascreen 3 Filtrate 4, 16 PVA chips 5 First washing tower 6, 8, 14, 15 Washing liquid 7 Second washing tower 9 Outside the system 12 Solid-liquid separator 13 Methanol 17 Dryer 18 Dried PVA chips

Claims

1. A method for producing a polyvinyl alcohol resin, comprising a washing step of continuously washing polyvinyl alcohol particles; the washing step comprises at least a first washing step and a second washing step, the polyvinyl alcohol particles being obtained by saponifying a polyvinyl ester using an alkali catalyst, the first washing step comprising mixing the polyvinyl alcohol particles with a first washing liquid containing 50% by mass or more of methanol and 0.1 to 10% by mass of an acid to form a slurry, thereby reducing the content of alkali catalyst residue in the polyvinyl alcohol particles, the slurry having been subjected to the first washing step being continuously supplied to a second washing step, and the second washing step comprising contacting the slurry with a second washing liquid containing 50% by mass or more of methanol and having an acid concentration of less than 0.1% by mass to reduce the content of acid in the polyvinyl alcohol particles.

2. The method for producing a polyvinyl alcohol resin according to claim 1, wherein the second washing step is carried out in a tower-type vessel, the slurry having completed the first washing step is continuously introduced into the upper part of the tower-type vessel, and a second washing liquid is continuously introduced into the lower part of the tower-type vessel.

3. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, wherein the cleaning liquid after the second cleaning step is recovered and reused as the first cleaning liquid.

4. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, wherein the acid contained in the first cleaning solution is a carboxylic acid.

5. The method for producing a polyvinyl alcohol resin according to claim 4, wherein the carboxylic acid is acetic acid.

6. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, wherein the first cleaning solution further contains methyl acetate, and the mass ratio of methyl acetate to methanol is 0.05 to 0.

8.

7. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, wherein the first cleaning solution further contains water, and the mass ratio of water to methanol is 0.005 to 0.

2.

8. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, wherein the polyvinyl alcohol particles have an average equivalent spherical diameter of 1 to 10 mm.

9. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, wherein the alkaline catalyst is sodium hydroxide.

10. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, further comprising a dewatering step and a drying step after the washing step.

11. The method for producing a polyvinyl alcohol resin according to claim 1 or 2, wherein the sodium acetate content of the obtained polyvinyl alcohol resin is 500 ppm or less.

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