Method for producing modified poly(vinyl alcohol)

Abstract

This method for producing a modified PVA includes: a step (1) for preparing a modifying agent solution that contains monomers (A) that are an ethylenically unsaturated dicarboxylic acid and a derivative thereof, a basic compound (B) and a solvent; a step (2) for mixing the modifying agent solution with a vinyl ester monomer (C) and copolymerizing the monomers (A) and the vinyl ester monomer (C) to obtain a modified poly(vinyl ester); and a step (3) for saponifying the modified poly(vinyl ester) to obtain a modified PVA. In the modifying agent solution, the molar ratio (B / A) of the basic compound (B) relative to the total amount of monomers (A) is 0.0001-0.3, and the content of an ethylenically unsaturated dicarboxylic acid diester is 10 mol% or less relative to the total amount of monomers (A). Due to this configuration, a modified PVA which contains structural units derived from the ethylenically unsaturated dicarboxylic acid and a derivative thereof and in which the content of components which are insoluble in water is low can be efficiently produced.

Description

Method for producing modified polyvinyl alcohol

[0001] The present invention relates to a method for producing modified polyvinyl alcohol, and more particularly to a method for producing modified polyvinyl alcohol containing structural units derived from an ethylenically unsaturated dicarboxylic acid and its derivatives.

[0002] Polyvinyl alcohol (hereinafter sometimes abbreviated as "PVA") is known as a water-soluble synthetic polymer, and is used as a raw material for the synthetic fiber vinylon, and is also used in a wide range of applications, such as paper processing agents, fiber processing agents, adhesives, stabilizers for emulsion polymerization and suspension polymerization, binders for inorganic materials, and films.

[0003] In particular, modified PVAs containing structural units derived from carboxylic acids and their derivatives are used, taking advantage of the reactivity of the carboxylic acids, as sizing agents for acidic paper containing aluminum sulfate, water-resistant coating films in combination with crosslinking agents, etc. Furthermore, the inclusion of structural units derived from carboxylic acids and their derivatives improves the water solubility of PVAs, making them industrially useful and widely used as packaging films for agricultural chemicals, laundry detergents, and industrial chemicals.

[0004] The introduction of structural units derived from carboxylic acids and their derivatives into PVA can be achieved, for example, by copolymerizing a vinyl ester monomer with a monomer consisting of a carboxylic acid and its derivative to obtain a copolymer, and then saponifying the copolymer. To efficiently introduce structural units derived from carboxylic acids and their derivatives, ethylenically unsaturated dicarboxylic acid derivatives that are highly reactive with vinyl ester monomers are used, and from the viewpoint of industrial availability, the use of maleic acid derivatives, fumaric acid derivatives, and itaconic acid derivatives is known.

[0005] Patent Document 1 discloses a production method in which vinyl acetate and maleic acid or maleic anhydride are copolymerized in an organic solvent in the presence of 0.5 to 2.0 molar equivalents of alkali relative to the maleic acid or maleic anhydride. However, according to the inventors' investigations, the production method disclosed in Patent Document 1 exhibits a slow polymerization rate between vinyl acetate and maleic acid or maleic anhydride, leaving room for improvement. Furthermore, the addition of an excess alkali component to the polymerization system accelerates the hydrolysis of vinyl acetate, resulting in a deterioration in the color of the resulting modified PVA due to the by-product acetaldehyde.

[0006] Furthermore, as described in Non-Patent Document 1, in modified PVA containing structural units derived from derivatives of ethylenically unsaturated dicarboxylic acids (e.g., monoesters, diesters, or anhydrides), a crosslinking reaction between the structural moieties derived from the monoesters, diesters, or anhydrides and the hydroxyl groups of the PVA proceeds at high temperatures, producing water-insoluble components. The water-insoluble components can become the starting point for cracks when molded into films or the like, and can also cause clogging of strainers or the like when the modified PVA is used as an aqueous solution, and therefore must be suppressed.

[0007] Patent Document 2 discloses a technique for suppressing the generation of water-insoluble components in modified PVA containing structural units derived from carboxylic acid and its derivatives by washing with a washing solution having a specific solvent composition. However, since extra energy is required to recover the organic solvent from the washing solution used, this not only increases production costs from an industrial standpoint, but also reduces CO ２ There are still problems remaining from the perspective of environmental burdens such as increased emissions.

[0008] Japanese Patent Application Laid-Open No. 56-43305 International Publication No. 2018 / 061272

[0009] Polymer Vol.38, No.12, pp.2933-2945,1997

[0010] 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 modified PVA containing structural units derived from an ethylenically unsaturated dicarboxylic acid and / or a derivative thereof, which has a high polymerization rate and results in a modified PVA with a small amount of water-insoluble components.

[0011] The present invention is as follows: [1] A method for producing a modified polyvinyl alcohol containing structural units derived from a monomer (A), comprising the steps of: (1) preparing a modifying solution containing the monomer (A), a basic compound (B), and a solvent; (2) mixing the modifying solution with a vinyl ester monomer (C) and copolymerizing the monomer (A) and the vinyl ester monomer (C) to obtain a modified polyvinyl ester; and (3) saponifying the modified polyvinyl ester to obtain a modified polyvinyl alcohol, wherein the monomer (A) is at least one selected from the group consisting of an ethylenically unsaturated dicarboxylic acid and a salt thereof, an ethylenically unsaturated dicarboxylic acid monoester and a salt thereof, an ethylenically unsaturated dicarboxylic acid diester, and an ethylenically unsaturated dicarboxylic acid anhydride; and wherein, in the modifying solution, the molar ratio (B / A) of the basic compound (B) to the total amount of the monomer (A) is 0.0001 or more and 0.3 or less; and The method for producing a vinyl ester monomer (C) in step (2), wherein the content of the ethylenically unsaturated dicarboxylic acid diester in the modifier solution when mixed with the vinyl ester monomer (C) is 10 mol % or less based on the total amount of the monomer (A). [2] The method for producing a vinyl ester monomer (C) in step (2), wherein the content of the ethylenically unsaturated dicarboxylic acid diester in the modifier solution is 10 mol % or less based on the total amount of the monomer (A). [2] The method for producing a vinyl ester monomer (C) in step (2), wherein the content of the ethylenically unsaturated dicarboxylic acid diester in the modifier solution is 10 mol % or less based on the total amount of the monomer (A). [3 ... [4] The method according to any one of [1] to [3], wherein the basic compound (B) is a hydroxide salt. [5] The method according to any one of [1] to [4], wherein the basic compound (B) is at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, sodium bicarbonate, and potassium bicarbonate.[6] The method according to any one of [1] to [5], wherein in step (2), when the modifying agent solution is mixed with the vinyl ester monomer (C), the molar ratio (A / C) of the monomer (A) to the vinyl ester monomer (C) is 0.0005 or more and 0.1 or less. [7] The method according to any one of [1] to [6], wherein the content of units derived from the monomer (A) in the modified polyvinyl alcohol is 0.05 mol % or more and 10 mol % or less, based on the total monomer units. [8] The method according to any one of [1] to [7], wherein the saponification degree of the modified polyvinyl alcohol is 60 mol % or more and 100 mol % or less. [9] The method according to any one of [1] to [8], wherein, when 5 parts by mass of the modified polyvinyl alcohol is dissolved in 95 parts by mass of water at 90°C, the amount of insoluble matter in the modified polyvinyl alcohol is less than 2,000 ppm.

[0012] According to the production method of the present invention, the polymerization rate is high when producing a modified PVA containing structural units derived from an ethylenically unsaturated dicarboxylic acid and / or a derivative thereof, and the resulting modified PVA contains a small amount of water-insoluble components.

[0013] The present invention relates to a method for producing a modified polyvinyl alcohol (hereinafter sometimes abbreviated as "modified PVA") containing structural units derived from an ethylenically unsaturated dicarboxylic acid and / or its derivative. Conventional methods for producing such modified PVAs sometimes result in a high content of water-insoluble components. The inventors investigated the cause of this problem and found that the carboxyl groups contained in the raw material ethylenically unsaturated dicarboxylic acid and / or its derivative are esterified by the alcohol solvent, resulting in an increase in the amount of ethylenically unsaturated dicarboxylic acid diester over time. Furthermore, they found that copolymerization using raw materials with a high content of ethylenically unsaturated dicarboxylic acid diester results in an increase in the content of water-insoluble components in the resulting modified PVA. In response to this problem, they found that adding a basic compound to a solution of an ethylenically unsaturated dicarboxylic acid and / or its derivative can reduce the content of ethylenically unsaturated dicarboxylic acid diester in the solution, thereby reducing the content of water-insoluble components in the resulting modified PVA. The present invention will be described in more detail below.

[0014] [Step (1)] The production method of the present invention includes step (1) of preparing a modifier solution containing a monomer (A), a basic compound (B), and a solvent. The modifier solution obtained in this step is used in the polymerization reaction in step (2).

[0015] [Monomer (A)] The monomer (A) used in the present invention is at least one selected from the group consisting of ethylenically unsaturated dicarboxylic acids and their salts, ethylenically unsaturated dicarboxylic acid monoesters and their salts, ethylenically unsaturated dicarboxylic acid diesters, and ethylenically unsaturated dicarboxylic acid anhydrides. Monomer (A) is preferably at least one selected from the group consisting of maleic acid, salts of maleic acid, monoalkyl maleate esters, salts of monoalkyl maleate esters, dialkyl maleate esters, maleic anhydride, fumaric acid, salts of fumaric acid, monoalkyl fumarate esters, salts of monoalkyl fumarate esters, dialkyl fumarate esters, itaconic acid, salts of itaconic acid, monoalkyl itaconic esters, salts of monoalkyl itaconic esters, dialkyl itaconic esters, and itaconic anhydride.

[0016] Monomers (A) suitable for use in preparing the modifier solution are ethylenically unsaturated dicarboxylic acids, ethylenically unsaturated dicarboxylic acid monoesters, and ethylenically unsaturated dicarboxylic acid anhydrides. In the modifier solution, the ethylenically unsaturated dicarboxylic acids and ethylenically unsaturated dicarboxylic acid monoesters form salts due to the added basic compound (B). It is believed that the formation of salts suppresses the formation of ethylenically unsaturated dicarboxylic acid diesters. Furthermore, ethylenically unsaturated dicarboxylic acid diesters are produced during the preparation of the modifier solution by the reaction of the alcohol solvent with the ethylenically unsaturated dicarboxylic acid monoester. Furthermore, since both ethylenically unsaturated dicarboxylic acids and ethylenically unsaturated dicarboxylic acid anhydrides form ethylenically unsaturated dicarboxylic acid monoesters in alcohol solutions, ethylenically unsaturated dicarboxylic acid diesters are also formed when these compounds are used as raw materials. In the present invention, it is important to suppress the formation of these ethylenically unsaturated dicarboxylic acid diesters in the modifier solution.

[0017] Since the ethylenically unsaturated dicarboxylic acid has low solubility in the vinyl ester monomer (C) such as vinyl acetate, it may precipitate during copolymerization of the monomer (A) and the vinyl ester monomer (C), resulting in a heterogeneous reaction solution and uneven modification. From this viewpoint, the monomer (A) is preferably an ethylenically unsaturated dicarboxylic acid monoester or an ethylenically unsaturated dicarboxylic acid anhydride.

[0018] [Basic Compound (B)] The basic compound (B) is added to the modifying agent solution to suppress the formation of an ethylenically unsaturated dicarboxylic acid diester. The basic compound (B) is not particularly limited as long as it can form a salt with an ethylenically unsaturated dicarboxylic acid and its derivative. The basic compound (B) is preferably a basic inorganic salt, more preferably a sodium salt, potassium salt, magnesium salt, or calcium salt, with sodium salt being even more preferred. It is also preferable that the basic compound (B) is a hydroxide salt. Specific examples of suitable basic compounds (B) include at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, sodium bicarbonate, and potassium bicarbonate. Among these, sodium hydroxide, sodium carbonate, and sodium bicarbonate are more preferred, with sodium hydroxide being even more preferred.

[0019] [Solvent] The solvent used in step (1) is an organic solvent capable of dissolving the monomer (A) and the basic compound (B), and an alcohol is usually used. The alcohol used in step (1) is preferably an alcohol having 1 to 4 carbon atoms, more preferably an alcohol having 1 to 3 carbon atoms, even more preferably methanol or ethanol, and particularly preferably methanol. A mixture of multiple alcohols can also be used. An organic solvent other than alcohol may be contained, and in that case, the content of the organic solvent is less than 50 mass %, preferably less than 10 mass %, of the total solvent.

[0020] [Modifier Solution] In the modifier solution, the molar ratio (B / A) of the basic compound (B) to the total amount of the monomer (A) is 0.0001 or more and 0.3 or less. A molar ratio (B / A) of 0.0001 or more can effectively suppress the formation of an ethylenically unsaturated dicarboxylic acid diester. From the viewpoint of suppressing the formation of an ethylenically unsaturated dicarboxylic acid diester and reducing the content of water-insoluble components in the modified PVA after saponification, the molar ratio (B / A) is preferably 0.001 or more, more preferably 0.005 or more, even more preferably 0.01 or more, and particularly preferably 0.05 or more. On the other hand, if the molar ratio (B / A) exceeds 0.3, the polymerization rate decreases, and a sufficient polymerization rate cannot be obtained unless a large amount of polymerization initiator is added. This increases the content of water-insoluble components in the modified PVA after saponification, and the modified PVA becomes discolored. The molar ratio (B / A) is preferably 0.25 or less, and more preferably 0.2 or less. The molar ratio (B / A) in the denaturant solution does not substantially change with time.

[0021] The content of the monomer (A) in the entire modifying agent solution is preferably 1% by mass or more and 50% by mass or less. The content of the monomer (A) is more preferably 2% by mass or more, and even more preferably 3% by mass or more. On the other hand, from the viewpoint of solubility in methanol and reducing the viscosity of the modifying agent solution, the content of the monomer (A) is more preferably 40% by mass or less, and even more preferably 30% by mass or less.

[0022] The modifying agent solution may or may not contain components other than the monomer (A), the basic compound (B), and the solvent. The total content of the monomer (A), the basic compound (B), and the solvent relative to the entire modifying agent solution is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more.

[0023] The method for preparing the modifier solution is not particularly limited, and the solution can be prepared by stirring the monomer (A), basic compound (B), and solvent in a container. The order in which the monomer (A), basic compound (B), and solvent are mixed is not particularly limited. However, from the viewpoint of minimizing the amount of ethylenically unsaturated dicarboxylic acid diester formed, it is preferable to simultaneously mix the monomer (A), basic compound (B), and solvent, or to add the monomer (A) to a mixture of the basic compound (B) and solvent and then mix them. The time from mixing the monomer (A) with the solvent to initiating the polymerization reaction in step (2) is preferably 1 hour or more, more preferably 2 hours or more, and even more preferably 5 hours or more. If this time is too short, there is a risk that the monomer (A) will not completely dissolve in the modifier solution, and the amount of ethylenically unsaturated dicarboxylic acid diester formed will be small, reducing the need for the present invention. On the other hand, the time until initiating the polymerization reaction in step (2) is preferably 100 hours or less, more preferably 50 hours or less. If the time is too long, the amount of ethylenically unsaturated dicarboxylic acid diester formed may be large. The temperature when preparing the modifying agent solution is usually 0°C or higher and not higher than the boiling point of the solvent. In order to improve solubility, the temperature is preferably 20°C or higher, more preferably 30°C or higher. On the other hand, if the temperature when preparing the solution is too high, the amount of ethylenically unsaturated dicarboxylic acid diester formed may be too large, so the temperature is preferably 60°C or lower, more preferably 50°C or lower.

[0024] [Step (2)] The production method of the present invention includes a step (2) of mixing the modifying agent solution prepared in the step (1) with a vinyl ester monomer (C) to copolymerize the monomer (A) and the vinyl ester monomer (C) to obtain a modified polyvinyl ester. The modified polyvinyl ester (C) obtained in this step is used in the saponification reaction in the step (3).

[0025] In step (2), the content of the ethylenically unsaturated dicarboxylic acid diester in the modifying solution when mixed with the vinyl ester monomer (C) is 10 mol % or less, based on the total amount of the monomer (A). A low content of the ethylenically unsaturated dicarboxylic acid diester in the modifying solution introduced into the polymerization reaction can reduce the content of water-insoluble components in the resulting modified PVA. The content of the ethylenically unsaturated dicarboxylic acid diester is preferably 5 mol % or less, more preferably 3.5 mol % or less, and even more preferably 2.5 mol % or less, based on the total amount of the monomer (A). The lower limit of the content of the ethylenically unsaturated dicarboxylic acid diester is not particularly limited, and may be 0 mol % based on the total amount of the monomer (A).

[0026] Examples of the vinyl ester monomer (C) used in step (2) include vinyl acetate, vinyl formate, vinyl propionate, vinyl caprylate, vinyl versatate, etc. Among these, vinyl acetate is preferred from an industrial viewpoint. These may be used alone or in combination of two or more.

[0027] In step (2), when the modifying agent solution is mixed with the vinyl ester monomer (C), the molar ratio (A / C) of the monomer (A) to the vinyl ester monomer (C) is preferably 0.0005 or more and 0.1 or less. If the molar ratio (A / C) is too small, the effect of introducing the structural unit derived from the monomer (A) is not sufficiently achieved. The molar ratio (A / C) is more preferably 0.001 or more, and even more preferably 0.002 or more. On the other hand, if the molar ratio (A / C) is too large, the modifying agent may precipitate in the reaction solution containing the vinyl ester monomer. Therefore, the molar ratio (A / C) is more preferably 0.06 or less, and even more preferably 0.03 or less.

[0028] In the polymerization, other monomers than the vinyl ester monomer (C) and the monomer (A) may be copolymerized within the scope of the present invention. Examples of 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; p) (meth)acrylamidopropyldimethylamine and its salts or quaternary salts, N-methylol(meth)acrylamide and its derivatives, and other (meth)acrylamide compounds; 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; vinyl halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; 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, and may be 5 mol % or less, or even 2 mol % or less. In this specification, "(meth)acrylic" is a general term for methacrylic and acrylic.

[0029] 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 di(n-propyl)peroxydicarbonate, diisopropylperoxydicarbonate, di(2-ethylhexyl)peroxydicarbonate, and diethoxyethylperoxydicarbonate; perester compounds such as t-butylperoxyneodecanate and α-cumylperoxyneodecanate; acetylcyclohexylsulfonyl peroxide; and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate. Potassium persulfate, ammonium persulfate, hydrogen peroxide, and the like may also be used in combination with the above polymerization initiators. These polymerization initiators may be used alone or in combination of two or more. As the polymerization initiator, an azo polymerization initiator or a peroxide polymerization initiator having a 10-hour half-life temperature of 66°C or less is preferred, as they can accelerate the polymerization rate and achieve a high polymerization rate in a short period of time. Examples of azo-based 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 polymerization initiator used is not particularly limited, but is typically 0.001 to 0.5 parts by mass, more preferably 0.003 to 0.1 parts by mass, and even more preferably 0.005 to 0.05 parts by mass, per 100 parts by mass of vinyl ester monomer. 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. If continuous polymerization is performed, the polymerization initiator is added continuously to the reaction solution.

[0030] The polymerization method used in the polymerization may be batch polymerization, semi-batch polymerization, continuous polymerization, or semi-continuous polymerization. A solution polymerization method is employed as the polymerization method. Examples of solvents used in the solution polymerization method include alcoholic solvents such as methanol, ethanol, and n-propanol, with methanol being particularly preferred. A portion of the solvent is derived from the solvent contained in the modifying agent solution. 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 the polymerization, it is usually preferably 10 parts by mass or more and 150 parts by mass or less, and more preferably 20 parts by mass or more and 100 parts by mass or less, per 100 parts by mass of the vinyl ester monomer.

[0031] 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 too low, a sufficient polymerization rate tends not to be obtained. On the other hand, if the polymerization temperature is too high, 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.

[0032] The polymerization rate of the vinyl ester monomer is not particularly limited, but is preferably 10% or more and 90% or less. If the polymerization rate is less than 10%, the productivity of the modified PVA may decrease. A polymerization rate of 20% or more is more preferable. On the other hand, if the polymerization rate exceeds 90%, the viscosity of the resulting modified polyvinyl ester may become too high, which may decrease the productivity of the modified PVA and may also deteriorate the hue of the resulting modified PVA. A polymerization rate of 70% or less is more preferable.

[0033] [Step (3)] The production method of the present invention includes a step (3) of saponifying the modified polyvinyl ester obtained in the step (2) to obtain a modified PVA.

[0034] In step (3), the modified polyvinyl ester obtained in step (2) is saponified to obtain a modified PVA. The method for saponifying the modified polyvinyl ester is not particularly limited, and known saponification methods can be used. Examples include alcoholysis or hydrolysis using a basic catalyst such as sodium hydroxide, potassium hydroxide, or sodium methoxide, or an acidic catalyst such as p-toluenesulfonic acid. Among these, the method of saponification using sodium hydroxide as a catalyst is simple and preferred. While the solvent used in this reaction is not particularly limited, it is preferred to add a catalyst solution to the modified polyvinyl ester solution obtained in step (2) to allow the saponification reaction to proceed. The amount of saponification catalyst used is preferably 0.001 or more and 0.5 or less, more preferably 0.002 or more and 0.2 or less, in terms of molar ratio relative to the vinyl ester units in the modified polyvinyl ester. The temperature at which saponification is performed is not particularly limited, but is preferably in the range of 20°C to 70°C. As the saponification reaction proceeds, a gel-like product precipitates, which is then pulverized. Thereafter, the mixture is dewatered using a centrifuge, dried in a dryer, and further pulverized to obtain a dried powder of the modified PVA.

[0035] [Modified PVA] In the modified PVA obtained as described above, the content of units derived from the monomer (A) is preferably 0.05 mol % or more and 10 mol % or less relative to the total monomer units. The content of units derived from the monomer (A) is more preferably 0.1 mol % or more, and even more preferably 0.5 mol % or more. On the other hand, the content of units derived from the monomer (A) is more preferably 8 mol % or less, and even more preferably 6 mol % or less. The content of units derived from the monomer (A) is １ It can be determined by H-NMR or the like.

[0036] The degree of saponification of the modified PVA is not particularly limited, but is usually 60 mol% or more and 100 mol% or less. It is more preferably 70 mol% or more, and even more preferably 80 mol% or more. On the other hand, the degree of saponification is usually 99.9 mol% or less. Note that when the degree of saponification is high, the content of water-insoluble components in the modified PVA is originally low, so there is little point in adopting the present invention. Therefore, the degree of saponification is preferably 98 mol% or less, and more preferably 95 mol% or less. The degree of saponification is measured in accordance with JIS K 6726 (1994).

[0037] The viscosity-average degree of polymerization of the modified PVA is not particularly limited, but is usually 100 to 5,000, preferably 200 to 4,000, more preferably 300 to 3,000, and even more preferably 400 to 2,000. 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 intrinsic viscosity [η] (liters / g) measured in water at 30°C for modified PVA saponified to a degree of saponification of 99.5 mol% or more: P = ([η] x 10,000 / 8.29) （１／０．６２）

[0038] When 5 parts by mass of the obtained modified polyvinyl alcohol is dissolved in 95 parts by mass of water at 90°C, the content of insoluble matter (components insoluble in water) in the modified polyvinyl alcohol is preferably less than 2000 ppm. The content of the insoluble components is preferably 500 ppm or less, more preferably 200 ppm or less, and even more preferably 100 ppm or less. The content of the insoluble components can be measured by the method described in the Examples below. In this specification, "ppm" means "ppm by mass."

[0039] The YI value of the resulting modified PVA powder is preferably 40 or less, more preferably 35 or less, and even more preferably 30 or less. The YI can be measured by the method described in the following examples.

[0040] The modified PVA thus obtained can be used in a variety of applications. The modified 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 modified PVA can also be heated and melt-molded. In this case, it is preferable to add a plasticizer such as glycerin before melt-molding. The modified PVA obtained by the production method of the present invention contains a small amount of water-insoluble components and is inhibited from discoloring, making it suitable for applications requiring good appearance and strength.

[0041] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The measurement and evaluation methods used in the following examples and comparative examples are shown below.

[0042] (1) Content of Water-Insoluble Components A 500 mL flask equipped with a stirrer and reflux condenser was placed in a water bath set to 20°C. 285 g of distilled water was added to the flask, and stirring was initiated at 300 rpm. 15 g of the modified PVA powder obtained in the following Examples and Comparative Examples was weighed out and gradually added to the flask. After the entire amount (15 g) of modified PVA powder was added, the temperature of the water bath was immediately raised to 90°C over approximately 30 minutes to dissolve the modified PVA powder, thereby obtaining a modified PVA solution. After the water bath temperature reached 90°C, dissolution was continued for an additional 60 minutes while stirring at 300 rpm. The resulting modified PVA solution was then filtered through a metal filter with a 63 μm mesh. The filter was then thoroughly washed with warm water at 90°C to remove the solution adhering to the filter, leaving only the undissolved particles (insoluble matter) remaining on the filter, and then dried in a heated dryer at 120°C for 1 hour. The weight of the filter after drying was compared with the weight of the filter before use for filtration, and the weight of the undissolved particles was calculated. The weight of the undissolved particles thus obtained was taken as the content of water-insoluble components.

[0043] (2) YI Value of PVA Resin The YI value of the modified PVA powder obtained in the following Examples or Comparative Examples was measured using a "Color meter ZE6000" manufactured by Nippon Denshoku Industries Co., Ltd. Specifically, the modified PVA powder was placed in a round cell, and the container was filled with the powder while gently tapping the side of the container, and the cell was placed on a reflective sample stage to measure the YI value.

[0044] Example 1 A 5% by mass aqueous solution of sodium hydroxide was added to a preparation tank containing methanol, followed by the addition of monomethyl maleate and mixing. The amount of monomethyl maleate added was 20% by mass of the total solution obtained, and the molar ratio of sodium hydroxide to the monomethyl maleate added was 0.0008. The mixture was then mixed at 20°C for 24 hours to prepare a modifier solution. In the modifier solution immediately after preparation, 0.6 mol% of the added monomethyl maleate was converted to dimethyl maleate, and 99.4 mol% remained as monomethyl maleate. Furthermore, after allowing the modifier solution to stand for a further 72 hours at 30°C, 2.9 mol% of the added monomethyl maleate was converted to dimethyl maleate, and 97.1 mol% remained as monomethyl maleate.

[0045] A polymerization reaction was carried out using a polymerization vessel (continuous polymerization apparatus; hereinafter referred to as the polymerization vessel) equipped with a reflux condenser, raw material supply lines, a thermometer, a nitrogen inlet, and an agitator blade. 656 L / h of vinyl acetate (VAM), 171 L / h of methanol (MeOH), 101 L / h of a modifier solution (30°C), and 24 L / h of a 2% methanol solution of 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMV) were continuously fed into the polymerization vessel using a metering pump. The polymerization liquid was continuously withdrawn from the polymerization vessel so that the liquid level in the polymerization vessel remained constant. The residence time in the polymerization vessel was 4 hours, and the conversion of vinyl acetate in the polymerization liquid withdrawn from the polymerization vessel was 40%. The temperature of the polymerization liquid withdrawn from the polymerization vessel was 63°C. The polymerization solution was removed from the polymerization tank, and unreacted vinyl acetate was removed by introducing methanol vapor into the polymerization solution to obtain a methanol solution (concentration: 35%) of modified polyvinyl acetate (hereinafter sometimes abbreviated as "modified PVAc"). The entire amount of the modifier solution prepared at one time was supplied to the polymerization tank over 72 hours. That is, the content of the ethylenically unsaturated dicarboxylic acid diester in the modifier solution when mixed with the vinyl ester monomer (C) was 0.6 mol % or more and 2.9 mol % or less, based on the total amount of the monomer (A).

[0046] Methanol was added to the above-mentioned methanol solution of modified PVAc to prepare a saponification raw material solution of modified PVAc with a concentration of 32 mass %. A saponification catalyst solution, a methanol solution of sodium hydroxide (concentration: 4 mass %), was added so that the molar ratio of sodium hydroxide to vinyl acetate units in the above-mentioned modified PVAc was 0.08. The saponification raw material solution and the saponification catalyst solution were mixed using a static mixer to obtain a mixture. The resulting mixture was placed on a belt and held at 40°C for 18 minutes to allow the saponification reaction to proceed. The gel obtained by the saponification reaction was crushed and dewatered. The resulting modified polyvinyl alcohol (modified PVA) particles were continuously fed at a rate of 600 kg / hr (resin content) into a dryer with a jacket temperature of 105°C. The average residence time of the modified PVA particles in the dryer was 4 hours. Further crushing was then performed to obtain a modified PVA powder. 72 hours after the start of the continuous polymerization, the polymer solution was saponified, and the resulting modified PVA powder was sampled and analyzed. As a result, the viscosity average polymerization degree of the obtained modified PVA was 1,200, the saponification degree was 90.0 mol%, １ The amount of modification by the monomer (A) in the H-NMR analysis was 4.0 mol %. The content of water-insoluble components measured by the above-mentioned method was 50 ppm. The YI was 20.

[0047] Examples 2 to 16, Comparative Examples 1 to 3 A modifying agent solution was prepared in the same manner as in Example 1, except that the type and amount of monomer (A) and the type and molar ratio of basic compound (B) were changed as shown in Table 1. The content of dicarboxylic acid diester in the resulting modifying agent solution was as shown in Table 1. A polymerization reaction was carried out using this modifying agent solution in the same manner as in Example 1 to obtain a methanol solution of modified PVAc. The feed rates of methanol, AMV solution, and modifying agent solution were changed as shown in Table 2. The polymerization rate of vinyl acetate was adjusted to 40% after a 4-hour residence time by mainly adjusting the amount of polymerization initiator (AMV). In Comparative Example 2, when the amount of polymerization initiator was the same as in Example 1 while adding a large amount of basic compound (B), polymerization hardly progressed. Therefore, in Comparative Example 2, operations subsequent to the saponification reaction were not performed.

[0048] Using the resulting methanol solution of modified PVAc, a saponification reaction was carried out in the same manner as in Example 1, except that the amount of methanol supplied and the molar ratio of sodium hydroxide to vinyl acetate units were changed as shown in Table 2. Thereafter, the mixture was crushed, dried, and crushed in the same manner as in Example 1, to obtain a modified PVA powder. The viscosity-average degree of polymerization, degree of saponification, and amount of modification by monomer (A) of the resulting modified PVA are shown in Table 2. The measurement results of the content of water-insoluble components and YI are also shown in Table 2.

[0049]

[0050]

Claims

1. A method for producing modified polyvinyl alcohol containing structural units derived from monomer (A), comprising the steps of: (1) preparing a modifying solution containing monomer (A), a basic compound (B), and a solvent; (2) mixing the modifying solution with a vinyl ester monomer (C) and copolymerizing the monomer (A) and the vinyl ester monomer (C) to obtain a modified polyvinyl ester; and (3) saponifying the modified polyvinyl ester to obtain a modified polyvinyl alcohol, wherein the monomer (A) is at least one selected from the group consisting of ethylenically unsaturated dicarboxylic acids and salts thereof, ethylenically unsaturated dicarboxylic acid monoesters and salts thereof, ethylenically unsaturated dicarboxylic acid diesters, and ethylenically unsaturated dicarboxylic acid anhydrides; and wherein the molar ratio (B / A) of the basic compound (B) to the total amount of monomer (A) in the modifying solution is 0.0001 or more and 0.3 or less; and In the step (2), the content of the ethylenically unsaturated dicarboxylic acid diester in the modifying agent solution when mixed with the vinyl ester monomer (C) is 10 mol % or less based on the total amount of the monomer (A).

2. The method according to claim 1, wherein the monomer (A) is at least one selected from the group consisting of maleic acid, salts of maleic acid, monoalkyl maleate esters, salts of monoalkyl maleate esters, dialkyl maleate esters, maleic anhydride, fumaric acid, salts of fumaric acid, monoalkyl fumarate esters, salts of monoalkyl fumarate esters, dialkyl fumarate esters, itaconic acid, salts of itaconic acid, monoalkyl itaconic esters, salts of monoalkyl itaconic esters, dialkyl itaconic esters, and itaconic anhydride.

3. The production method according to claim 1 or 2, wherein the basic compound (B) is a basic inorganic salt.

4. The method according to claim 1 or 2, wherein the basic compound (B) is a hydroxide salt.

5. The method according to claim 1 or 2, wherein the basic compound (B) is at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, sodium hydrogen carbonate, and potassium hydrogen carbonate.

6. The method according to claim 1 or 2, wherein in step (2), when the modifying agent solution is mixed with the vinyl ester monomer (C), the molar ratio (A / C) of the monomer (A) to the vinyl ester monomer (C) is 0.0005 or more and 0.1 or less.

7. The method according to claim 1 or 2, wherein the content of units derived from the monomer (A) in the modified polyvinyl alcohol is 0.05 mol % or more and 10 mol % or less based on the total monomer units.

8. The method according to claim 1 or 2, wherein the degree of saponification of the modified polyvinyl alcohol is 60 mol % or more and 100 mol % or less.

9. The manufacturing method described in claim 1 or 2, wherein when 5 parts by mass of the modified polyvinyl alcohol is dissolved in 95 parts by mass of water at 90°C, the amount of insoluble matter in the modified polyvinyl alcohol is less than 2000 ppm.

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