Polyvinyl alcohol manufacturing method

Abstract

To provide a method for producing PVA which can control the reaction rate of the saponification reaction while suppressing the generation of impurities.SOLUTION: There is provided a method for producing a polyvinyl alcohol, which comprises a step of obtaining a polyvinyl ester solution by adding water to a solution containing a polyvinyl ester and alcohol and a step of obtaining a polyvinyl alcohol solution containing a polyvinyl alcohol by adding an alkali catalyst to the polyvinyl ester solution to saponify the polyvinyl ester, wherein the addition amount of water is 4.0 mass% or less based on the total mass of the polyvinyl ester solution.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing polyvinyl alcohol. [Background technology]

[0002] Polyvinyl alcohol (hereinafter also referred to as "PVA") is generally obtained by adding a catalyst to an alcohol solution of polyvinyl ester and saponifying the polyvinyl ester. More specifically, PVA is often produced by adding sodium hydroxide, an alkaline catalyst, to a methanol solution of polyvinyl acetate and alkaline saponifying the polyvinyl acetate.

[0003] In the production of PVA, it is desirable to be able to control the reaction rate of alkaline saponification depending on the purpose. For example, Patent Document 1 describes a method for saponifying PVA that does not contain sodium ions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-130328 Summary of the Invention [Problem to be solved by the invention]

[0005] In alkaline saponification reactions, the more alkali added, the faster the saponification rate, but a fast saponification rate poses the problem of making it difficult to control the degree of saponification of PVA. Saponification in a mixed solution of alkali and water improves the ease of controlling the degree of saponification of PVA, but the presence of water in the saponification solution results in the generation of sodium acetate as a by-product. As described in Patent Document 1, depending on the intended use of PVA, the inclusion of sodium ions is undesirable, and PVA with a low sodium acetate content is preferred.

[0006] Therefore, an object of one aspect of the present invention is to provide a method for producing PVA that is capable of controlling the reaction rate of the saponification reaction while suppressing the production of by-products. [Means for solving the problem]

[0007] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that by adding a small amount of water to an alcohol solution of a polyvinyl ester before saponification, it is possible to control the saponification rate in accordance with the amount of water added while suppressing the production of by-products.

[0008] That is, one aspect of the present invention is a method for producing polyvinyl alcohol, comprising the steps of: adding water to a solution containing a polyvinyl ester and an alcohol to obtain a polyvinyl ester solution; and adding an alkali catalyst to the polyvinyl ester solution to saponify the polyvinyl ester to obtain a polyvinyl alcohol solution containing polyvinyl alcohol, wherein the amount of water added is 4.0 mass % or less based on the solids content of the polyvinyl ester solution.

[0009] The amount of water added may be 1.1% by mass or more based on the solid content of the polyvinyl ester solution, which can improve the saponification reaction rate. [Effects of the Invention]

[0010] According to one aspect of the present invention, it is possible to provide a method for producing PVA that is capable of controlling the reaction rate of the saponification reaction while suppressing the generation of impurities. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a graph showing the correlation between the amount of water added and the gelation time of PVA in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes embodiments of the present invention, but the present invention is not limited to the embodiments described below.

[0013] One embodiment of the present invention is a method for producing polyvinyl alcohol, comprising: a step of adding water to a solution containing polyvinyl ester and alcohol to obtain a polyvinyl ester solution (hereinafter also referred to as "first step"); and a step of adding an alkali catalyst to the polyvinyl ester solution to saponify the polyvinyl ester to obtain a polyvinyl alcohol solution containing polyvinyl alcohol (hereinafter also referred to as "second step").

[0014] The polyvinyl ester used in the first step may be a homopolymer of a vinyl ester, or a copolymer of a vinyl ester and a monomer other than the vinyl ester that is copolymerizable with the vinyl ester.

[0015] Examples of vinyl esters include vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, etc. From the viewpoint of ease of polymerization, the vinyl ester is preferably vinyl acetate.

[0016] Examples of monomers other than vinyl esters include α-olefin monomers such as ethylene and propylene; (meth)acrylic acid alkyl ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; unsaturated amide monomers such as (meth)acrylamide and N-methylolacrylamide; unsaturated carboxylic acid monomers such as (meth)acrylic acid, crotonic acid, maleic acid, itaconic acid, and fumaric acid; and alkyl (methyl, ethyl, propyl) monomers of unsaturated carboxylic acids. anhydrides of unsaturated carboxylic acids such as maleic anhydride; salts of unsaturated carboxylic acids with sodium, potassium, ammonium, etc.; glycidyl group-containing monomers such as allyl glycidyl ether and glycidyl (meth)acrylate; sulfonic acid group-containing monomers such as 2-acrylamido-2-methylpropanesulfonic acid or salts thereof; phosphate group-containing monomers such as acid phosphooxyethyl (meth)acrylate and acid phosphooxypropyl (meth)acrylate; alkyl vinyl ether monomers; and the like.

[0017] The polyvinyl ester is preferably a homopolymer of vinyl ester from the viewpoint of the stability of the resulting PVA, and more preferably polyvinyl acetate (homopolymer of vinyl acetate) from the viewpoint of ease of polymerization.

[0018] The polymerization initiator for radical polymerization of a vinyl ester-containing monomer is not particularly limited, and examples thereof include azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile), azobisdimethylvaleronitrile, and azobismethoxyvaleronitrile; peroxides such as acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; and perester compounds such as t-butyl peroxyneodecanate, α-cumyl peroxyneodecanate, and t-butyl peroxyneodecanate, which may be used alone or in combination of two or more.

[0019] The method for polymerizing the vinyl ester is not particularly limited, and may be a known method such as solution polymerization, suspension polymerization, or bulk polymerization. Solution polymerization in alcohol is preferred because it is easy to operate and allows the use of the same solvent as in the saponification reaction, which is the subsequent step.

[0020] The polyvinyl ester is subjected to the first step as an alcohol solution containing alcohol. The alcohol may be, for example, methanol, ethanol, butanol, etc., and is preferably methanol. The concentration of the polyvinyl ester in the alcohol solution may be, for example, 10% by mass or more and 80% by mass or less.

[0021] In the first step, a small amount of water is added to the alcohol solution to obtain a polyvinyl ester solution. The term "adding water" refers to adding pure water alone, and does not include adding water as a solvent in which a solute is dissolved. Pure water can be composed of water alone or water and unavoidable impurities.

[0022] The amount of water added is 4.0% by mass or less, based on the solid content of the polyvinyl ester solution, from the viewpoint of suppressing the production of by-products (alkali salts) accompanying alkali saponification, and from the viewpoint of further suppressing the production of such by-products, is preferably 3.5% by mass or less, more preferably 3.0% by mass or less, even more preferably 2.5% by mass or less, and particularly preferably 2.0% by mass or less. The "solid content" of the polyvinyl ester solution means the components remaining after excluding the solvent (e.g., alcohol) from the polyvinyl ester solution.

[0023] By adjusting the amount of water added within the above range, as described above, it is possible to suppress the generation of by-products (alkali salts) accompanying alkaline saponification, and in addition, it is possible to control the reaction rate of alkaline saponification, making it easier to control the degree of saponification of PVA. Specifically, the greater the amount of water added, the higher the reaction rate. The amount of water added may be, for example, 0.01% by mass or more, 0.1% by mass or more, 0.5% by mass or more, or 1.0% by mass or more, based on the solids content of the polyvinyl ester solution. From the viewpoint of improving the reaction rate of the saponification reaction, it is preferably 1.1% by mass or more, more preferably 1.2% by mass or more, even more preferably 1.5% by mass or more, and particularly preferably 2.0% by mass or more.

[0024] In the second step, an alkali catalyst is added to the polyvinyl ester solution to carry out a saponification reaction, thereby obtaining polyvinyl alcohol. The alkali catalyst may be, for example, an alkali metal hydroxide or alcoholate such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, or potassium methylate, and is preferably sodium hydroxide. The amount of alkali catalyst added is not particularly limited, but may be, for example, 1 mmol equivalent or more and 100 mmol equivalents or less relative to the polyvinyl ester. The temperature at which the saponification reaction is carried out may be, for example, 10°C or more and 70°C or less. The time for the saponification reaction may be, for example, 20 minutes or more and 3 hours or less.

[0025] In the saponification reaction, for example, when polyvinyl acetate is used as the polyvinyl ester and sodium hydroxide is used as the alkaline catalyst, and saponification is carried out in anhydrous methanol, sodium acetate is not produced as a by-product, but the saponification reaction is rapid, making it difficult to control the degree of saponification of the PVA. Because saponification (methanolysis) in methanol is a catalytic reaction, it is difficult to control the reaction rate by the amount of alkaline catalyst added. In this production method, a predetermined amount of water is deliberately added to the alcohol solution of the polyvinyl ester, which controls the saponification reaction rate while suppressing the production of impurities, thereby obtaining PVA with the desired degree of saponification.

[0026] In the production method of the present embodiment, the content of alkali salts by-produced in the saponification reaction can be reduced to 0.3 mass % or less, 0.2 mass % or less, or 0.15 mass % or less, based on the solid content of the polyvinyl alcohol obtained in the second step, depending on the amount of water added in the first step.

[0027] The apparatus for carrying out the saponification reaction may be a conventionally known apparatus. For example, a belt-type reactor equipped with an in-line mixer can be used as the apparatus. In this case, the polyvinyl ester solution and the alkali catalyst are charged into a mixing vessel and mixed using a static mixer (mixer) or the like. The mixture is then placed on a belt, and the saponification reaction can be carried out under predetermined temperature conditions while the belt is moving. In addition to the belt-type reactor, for example, a kneader-type reactor, a column-type reactor, etc. can also be used.

[0028] The polyvinyl alcohol in the polyvinyl alcohol solution obtained by the above-mentioned production method may be in a gel state. From the viewpoint of facilitating crushing of the gel-state polyvinyl alcohol, the hardness of the gel-state polyvinyl alcohol is preferably 20 or more, more preferably 25 or more, even more preferably 30 or more, and preferably 90 or less, more preferably 80 or less. The hardness of the gel-state polyvinyl alcohol is measured using a durometer in accordance with JIS S6050 "6.2 Hardness."

[0029] When polyvinyl alcohol is in a gel state, the reaction rate of the saponification reaction can be estimated from the gelation time from the addition of the alkali catalyst to the completion of gelation of the polyvinyl alcohol. The gelation time is preferably 30 minutes or less, more preferably 25 minutes or less, and even more preferably 20 minutes or less, depending on the amount of water added in the first step. When a belt-type reactor is used in the saponification reaction, gelation can be completed within the limited movement time on the belt by adjusting the gelation time by the amount of water added. Here, gelation refers to a state in which an alkali catalyst is added to an alcohol solution of a polyvinyl ester and the alcohol solution loses its fluidity.

[0030] The method for producing PVA may further include a step of pulverizing the polyvinyl alcohol (particularly gel-like polyvinyl alcohol) after the second step. The pulverization is carried out using a known pulverizer. The method for producing PVA may further include a washing step after the second step to remove impurities (e.g., alkali salts such as sodium acetate) and a step of volatilizing the solvent in the polyvinyl alcohol solution.

[0031] The PVA obtained by the above-described production method may have a degree of saponification of, for example, 70 mol% or more. The degree of saponification of the PVA may be 75 mol% or more, or may be 99 mol% or less, or 90 mol% or less. In this specification, the "saponification degree" refers to the degree of saponification measured and calculated in accordance with JIS K6726 "3.5 Saponification degree."

[0032] The volatile content of the PVA may be 5.0% by mass or less, 4.0% by mass or less, or 3.0% by mass or less, based on the total amount of PVA. The term "volatile content" as used herein refers to the volatile content measured and calculated in accordance with JIS K6726-1994, Section 3.4, "Volatile Content." The volatile content is, for example, a solvent component such as alcohol used in the production of PVA. [Example]

[0033] Hereinafter, the present invention will be described in more detail based on examples. Note that the present invention is not limited to the following examples.

[0034] <Preparation of PVA> [Example 1] Into a polymerization kettle equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts by mass of vinyl acetate, 5.3 parts by mass of methanol, and 0.0005 parts by mass of Peroyl NPP (manufactured by NOF Corporation) as an initiator were charged, and polymerization was carried out at 65 ° C for 5.0 hours while stirring under a nitrogen atmosphere. Next, unreacted vinyl acetate (monomer) was removed outside the polymerization system to obtain a methanol solution of polyvinyl acetate (concentration of polyvinyl acetate: 25 to 26% by mass). Water was added to this methanol solution of polyvinyl acetate so that the amount became 0.02% by mass to obtain a polyvinyl acetate solution.

[0035] Subsequently, 5 mmol equivalent of sodium hydroxide was added to the polyvinyl acetate to the polyvinyl acetate solution. Thereafter, a saponification reaction was carried out at 45 ° C for 50 minutes to obtain PVA with a saponification degree of 79.6 mol%.

[0036] [Examples 2 to 8, Comparative Example 1] The amount of water added before saponification was changed as shown in Table 1, and PVA was obtained in the same procedure as in Example 1 except that the amount of sodium hydroxide added was appropriately adjusted so that the saponification degree of PVA was 80 ± 2 mol%.

[0037] <Measurement of Gelation Time> The time from when sodium hydroxide was added to the polyvinyl acetate solution until the gelation of PVA was completed was measured as the gelation time. In addition, it was judged that the gelation was completed when it was confirmed that the fluidity of the polyvinyl acetate solution after the addition of sodium hydroxide had disappeared.

[0038] <Measurement of Sodium Acetate Content> The sodium acetate content in PVA (based on the solid content in PVA) was measured in accordance with JIS K6726 "3.6 Sodium Acetate".

[0039] <Measurement of Volatile Matter> The volatile content in PVA was measured in accordance with JIS K6726 "3.4 Volatile content".

[0040] <Measurement of Saponification Degree of PVA> The saponification degree of PVA was measured and calculated in accordance with JIS K6726 "3.5 Saponification degree".

[0041] The obtained measurement results are shown in Table 1.

Table 1

[0042] As shown in Table 1, in Examples 1 to 8, the content of sodium acetate by-produced along with the saponification reaction is suppressed. FIG. 1 is a graph showing the correlation between the amount of water added and the gelation time of PVA. As can be seen from FIG. 1, a high correlation was found between the amount of water added and the gelation time. Thus, it was found that the gelation time (i.e., the reaction rate of the saponification reaction) can be controlled by the amount of water added.

Claims

1. a step of adding water to a solution containing a vinyl ester homopolymer and an alcohol to obtain a polyvinyl ester solution; and a step of adding an alkali catalyst to the polyvinyl ester solution to saponify the polyvinyl ester at 70°C or less, thereby obtaining a polyvinyl alcohol solution containing polyvinyl alcohol, A method for producing polyvinyl alcohol (excluding polyvinyl alcohol polymers having 1.7 mol % or more of 1,2-glycol bonds), in which the amount of water added is 1.1 mass % or more and 3.5 mass % or less based on the solids content of the polyvinyl ester solution (excluding a production method comprising a step of producing a vinyl alcohol polymer composition (I) by simultaneously carrying out a saponification reaction of a vinyl carboxylate polymer and a reaction involving at least a portion of functional groups contained in the metal alkoxide (I) and / or oligomer (I) derived from the metal alkoxide (I) in a reaction system obtained by adding a metal alkoxide (I) and / or an oligomer (I) derived from the metal alkoxide (I) to a solution containing the vinyl carboxylate polymer).

2. The method for producing polyvinyl alcohol according to claim 1 , wherein the polyvinyl alcohol is in a gel state.

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