Polyvinyl alcohol polymer and method for producing same

Abstract

Provided are a polyvinyl alcohol polymer and the like.　The polyvinyl alcohol polymer has an ionic skeleton and satisfies at least one requirement selected from the following requirements 1, 2, and 3. Requirement 1: the water-insoluble fraction is 30 mass% or less. Requirement 2: the transparency (430 nm) of a 4 mass% aqueous solution is 1% or more. Requirement 3: the YI value of a 4 mass% dimethyl sulfoxide solution is 30 or less.

Description

Polyvinyl alcohol polymer and method for producing the same

[0001] The present invention relates to a polyvinyl alcohol polymer having an ionic group and a method for producing the same.

[0002] Polyvinyl alcohol-based polymers having ionic groups (hereinafter, polyvinyl alcohol-based polymers or vinyl alcohol-based polymers may be abbreviated as "PVA," "PVA-based polymer," etc.) often differ in physical properties from PVAs that do not have ionic groups, such as being able to significantly improve hydrophilicity due to the presence of the ionic groups. Such PVAs are suitable for applications such as water-soluble films that require a high dissolution rate in water, and can also impart water-dispersibility or water solubility to PVAs with a low degree of saponification (e.g., a degree of saponification of 70 mol % or less), which are not inherently water-soluble. By imparting water solubility or water dispersibility to PVAs with a low degree of saponification, the PVAs can be used in a variety of applications, such as water-soluble hot-melt adhesives.

[0003] For example, Patent Document 1 discloses a modified vinyl alcohol polymer having a sulfonic acid group or a salt thereof in a side chain, in which the modification amount of the sulfonic acid group or the salt thereof is 0.01 mol % or more and 10 mol % or less, and in which the block character of the residual vinyl ester unit is 0.55 or more and 1 or less, and a method for producing the same.

[0004] WO2019 / 159757

[0005] An object of the present invention is to provide a polyvinyl alcohol polymer (PVA) and the like.

[0006] As described above, PVA having an ionic group (modified PVA) is known, such as that described in Patent Document 1. However, according to the investigations of the present inventors, it has been found that PVA having an ionic group is prone to coloration, prone to the production of water-insoluble matter (by-product), and in some cases may even be insoluble in water, rather than being soluble in water.

[0007] However, it has been extremely difficult to find a PVA that has ionic groups but is less colored or has fewer water-insoluble components (excellent water dispersibility or solubility).

[0008] For example, in Patent Document 1, PVA and an aldehyde having a sulfonic acid group or a salt thereof are mixed and reacted under heating to introduce an ionic group. However, according to the investigations of the present inventors, depending on the reaction conditions (and further the properties of the PVA used (degree of polymerization, degree of saponification, particle size)), etc.), the mixing may become uneven or insufficient, resulting in large reaction spots (uneven reaction), and possibly an increase in the amount of water-insoluble components. On the other hand, when the PVA is heated under conditions that allow it to be completely melted in order to achieve uniform mixing, the PVA may be prone to coloration or insolubilization.

[0009] Under these circumstances, the present inventors have found that, by selecting the conditions for introducing an ionic skeleton (ionic group, skeleton having an ionic group) (and further selecting the method for introducing the ionic skeleton), it is possible to efficiently reduce coloration and water-insoluble components, and suppress or prevent insolubilization, even in PVA having an ionic skeleton. In particular, it is possible to efficiently reduce coloration and water-insoluble components, and suppress or prevent insolubilization, while maintaining high water solubility or water dispersibility, even in PVAs that do not have a high degree of saponification, such as a fully saponified type. Based on this finding, the present inventors have completed the present invention after further research.

[0010] That is, the present invention relates to the following inventions, etc. [1] A polyvinyl alcohol-based polymer (A) having an ionic skeleton (a skeleton having an ionic group) and satisfying the following requirement 1. Requirement 1: A water-insoluble fraction of 30% by mass or less. [2] A polyvinyl alcohol-based polymer (A) having an ionic skeleton and satisfying the following requirement 2. Requirement 2: A transparency [at 430 nm (e.g., transparency at 20°C)] of a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer (A) is 1% or more. [3] A polyvinyl alcohol-based polymer (A) having an ionic skeleton and satisfying the following requirement 3. Requirement 3: A YI value [for example, a YI value at 20°C (e.g., a YI value calculated and measured in a wavelength range of 360 to 830 nm)] of a 4% by mass dimethyl sulfoxide solution of the polyvinyl alcohol-based polymer (A) is 30 or less. [4] A polyvinyl alcohol-based polymer (A) having an ionic skeleton and satisfying the following requirements 1 and 2. Requirement 1: The water-insoluble fraction is 30% by mass or less. Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more. [5] A polyvinyl alcohol-based polymer (A) having an ionic skeleton and satisfying the following requirements 1 and / or 2, and requirement 3. Requirement 1: The water-insoluble fraction is 30% by mass or less. Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more. Requirement 3: The YI value of a 4% by mass dimethyl sulfoxide solution is 30 or less. [6] A polyvinyl alcohol-based polymer (A) having an ionic skeleton and satisfying the following requirements 1, 2, and 3. Requirement 1: The water-insoluble fraction is 30% by mass or less. Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more. Requirement 3: The YI value of a 4% by mass dimethyl sulfoxide solution is 30 or less. [7] The polyvinyl alcohol-based polymer (A) according to any one of [1] and [4] to [6], in requirement 1, has a water-insoluble fraction of 10% by mass or less. [8] The polyvinyl alcohol-based polymer (A) according to any one of [2] and [4] to [6], wherein the transparency (430 nm) of a 4% by mass aqueous solution is 2% or more in Requirement 2. [9] The polyvinyl alcohol-based polymer (A) according to any one of [3] to [6], wherein the YI value of a 4% by mass dimethyl sulfoxide solution is 10 or less in Requirement 3.

[10] The polyvinyl alcohol-based polymer (A) according to any one of [1] to [9], wherein, in Requirement 1, the water-insoluble content is 5% by mass or less; in Requirement 2, the transparency (430 nm) of a 4% by mass aqueous solution is 10% or more; and in Requirement 3, the YI value of a 4% by mass dimethyl sulfoxide solution is 5 or less.

[11] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[10] , wherein the ionic skeleton has at least one ionic group selected from a carboxyl group and a base thereof (a carboxylic acid base, a group in which a carboxyl group forms a salt).

[12] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[11] , wherein the ionic skeleton comprises a skeleton derived from a polycarboxylic acid component (a skeleton derived from a polycarboxylic acid component and having at least one ionic group selected from a carboxyl group and a base thereof).

[13] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[12] , wherein the ionic skeleton comprises a skeleton derived from at least one polycarboxylic acid component selected from a dicarboxylic acid component and a tricarboxylic acid component.

[14] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[13] , wherein the ionic skeleton comprises a skeleton introduced via a hydroxyl group of a vinyl alcohol unit (a vinyl alcohol unit contained in the polyvinyl alcohol-based polymer).

[15] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[14] , wherein the ionic skeleton comprises a skeleton derived from an ester bond between a hydroxyl group of the vinyl alcohol unit and a carboxy group (for example, one carboxyl group) of a polycarboxylic acid component [a skeleton formed by an ester bond with a carboxyl group (a skeleton formed by an ester bond and having at least one ionic group selected from a carboxyl group and a base thereof)].

[16] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[15] , wherein the ionic skeleton comprises a skeleton derived from an ester bond between a hydroxyl group of a vinyl alcohol unit and a carboxy group of a polycarboxylic acid component, and the polycarboxylic acid component comprises at least one selected from aliphatic dicarboxylic acids (e.g., succinic acid, maleic acid, citraconic acid, itaconic acid, fumaric acid, etc.), aromatic dicarboxylic acids (e.g., phthalic acid), aliphatic tricarboxylic acids (e.g., citric acid), aromatic tricarboxylic acids, acid anhydrides thereof (e.g., succinic anhydride, maleic anhydride, citraconic anhydride, itaconic anhydride, phthalic anhydride), esters thereof (particularly partial esters, e.g., monomethyl succinate, monomethyl maleate, monomethyl citraconic acid, monomethyl itaconate, monomethyl phthalate, monomethyl citrate), and salts thereof (e.g., alkali metal salts such as sodium).

[17] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[16] , wherein the content of the ionic skeleton per monomer unit is 0.01 to 10 mol %.

[18] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[17] , wherein the block character is 0.35 to 0.7.

[19] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[18] , wherein the degree of saponification is 20 to 90 mol %.

[20] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[19] , wherein the degree of saponification is 70 mol % or less.

[21] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[20] , wherein the degree of polymerization is 120 to 3,000.

[22] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[21] , wherein a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer (A) has a pH of 5 to 9.

[23] The polyvinyl alcohol-based polymer (A) according to any one of [1] to

[22] , wherein the pH of a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer (A) is 5.5 to 8.5 (e.g., 5.8 to 8, 5.5 to 7.5, 6 to 7.5, 6 to 7, etc.).

[24] A method for producing a polyvinyl alcohol-based polymer (A) having an ionic skeleton, comprising a heating step of heating a composition containing a polyvinyl alcohol-based polymer (B) and a component corresponding to the ionic skeleton.

[25] The production method according to

[24] , wherein the component corresponding to the ionic skeleton contains a polycarboxylic acid component (e.g., at least one selected from polycarboxylic acids, polycarboxylic acid anhydrides, partial esters of polycarboxylic acids, and salts thereof).

[26] The production method according to

[24] or

[25] , wherein the component corresponding to the ionic skeleton contains at least one selected from dicarboxylic acids, tricarboxylic acids, anhydrides thereof, partial esters thereof, and salts thereof.

[27] The production method according to any one of

[24] to

[26] , wherein the composition contains a pH adjuster (heating is performed in the presence of a pH adjuster).

[28] The production method according to any one of

[24] to

[27] , wherein the composition contains sodium acetate (sodium acetate as a pH adjuster).

[29] The production method according to any one of

[24] to

[28] , wherein the composition contains a pH adjuster in a ratio of 0.5 to 1.9 moles per mole of the component corresponding to the ionic skeleton.

[30] The production method according to any one of

[24] to

[29] , wherein the heating temperature in the heating step is 80 to 240°C.

[31] The production method according to any one of

[24] to

[30] , wherein the heating time in the heating step is 5 minutes to 5 hours.

[32] The production method according to any one of

[24] to

[31] , wherein the production method further comprises a solvent removal step of removing the solvent from a composition containing the polyvinyl alcohol-based polymer (B), the component corresponding to the ionic skeleton, and a solvent, and wherein the heating step is performed after the solvent removal step or in parallel with or consecutively to the solvent removal step.

[33] The production method according to

[32] , wherein the solvent comprises at least one selected from water, an alcohol, and an ester.

[34] The production method according to

[32] or

[33] , wherein the solvent comprises methanol.

[35] The production method according to any one of

[24] to

[34] , wherein the polyvinyl alcohol polymer (A) satisfies at least one requirement selected from the following requirements 1, 2, and 3:Requirement 1: A water-insoluble fraction is 30% by mass or less. Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more. Requirement 3: The YI value of a 4% by mass dimethyl sulfoxide solution is 30 or less.

[36] A hot melt adhesive comprising the polyvinyl alcohol-based polymer (A) according to any one of [1] to

[23] .

[37] A dispersant for suspension polymerization comprising the polyvinyl alcohol-based polymer (A) according to any one of [1] to

[23] .

[0011] According to the present invention, it is possible to provide PVA (a new or specific PVA) and its uses (hot melt adhesives (for example, water-soluble hot melt adhesives), dispersants, etc.).

[0012] In one embodiment, the PVA of the present invention has ionic groups, but has a small amount of water-insoluble components and / or high transparency in aqueous solution. Such physical properties are considered to be indicators of the uniformity of the PVA (particularly, the uniform introduction of ionic groups) (the lack of reaction spots), and can efficiently exhibit the functions of the PVA (e.g., water solubility or dispersibility).

[0013] In one embodiment, the PVA of the present invention has an ionic group and yet has a specific hue (coloring property).

[0014] In one embodiment of the PVA of the present invention, while having ionic groups, it is possible to simultaneously achieve a small amount of water-insoluble components and / or high transparency of the aqueous solution, and a specific hue (coloring property).

[0015] Therefore, the PVA of the present invention can efficiently exhibit the properties (particularly water solubility (dispersibility)) attributable to the ionic group and also suppress coloration.

[0016] In particular, in one embodiment of the PVA of the present invention, even if the PVA does not have a high degree of saponification such as a fully saponified type (for example, a degree of saponification of 90 mol % or less, or even if the PVA has a degree of saponification that does not inherently exhibit or is difficult to exhibit water solubility (dispersibility) in water (for example, a degree of saponification of 70 mol % or less, 60 mol % or less, 50 mol % or less)), the above-mentioned properties and coloration inhibition can be efficiently exhibited.

[0017] Another aspect of the present invention provides a method for producing a PVA, which makes it easy and efficient to obtain the above-described PVA (e.g., PVA with reduced coloration, PVA that efficiently exhibits water solubility or dispersibility) while having an ionic group.

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the preferred embodiments described below.

[0019] The present invention can provide a specific polyvinyl alcohol-based polymer, i.e., polyvinyl alcohol-based polymer (A) (hereinafter, may be referred to as PVA-based polymer (A), PVA (A), etc.). The present invention will be described in detail below.

[0020] [Polyvinyl Alcohol Polymer (A)] The PVA polymer (A) usually contains vinyl alcohol units (units) [as monomer units], and particularly contains at least vinyl alcohol units and vinyl ester units (units derived from vinyl esters, for example, vinyl acetate units).

[0021] The PVA polymer (A) of the present invention usually has (or further has) an ionic skeleton (a skeleton having an ionic group).

[0022] The ionic group includes an anionic group {for example, an acid group [for example, a carboxyl group, a sulfonic acid group (—SO ) ３ H), phosphate group, etc.}, cationic groups [for example, amino group, ammonium (ammonium cation)], salts thereof (groups in which these form salts), etc.

[0023] The salt may be anionic, cationic, or the like, and examples thereof include metal salts [e.g., alkali or alkaline earth metal salts (e.g., lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, etc.)], halides (e.g., chlorides, bromides, iodides, etc.), etc. When the ionic group is a polybasic acid or the like, the salt may be a single salt (of the same kind) or a combination of two or more kinds.

[0024] The ionic skeleton may have one or more ionic groups, and may have two or more ionic groups. When the ionic skeleton has two or more ionic groups, the ionic groups may be the same or different.

[0025] Among these ionic groups, acid groups (particularly, carboxyl groups and sulfonic acid groups) and salts thereof {salts of acid groups, for example, carboxylates [for example, -COOM (M is an alkali metal such as sodium (or its cation)], sulfonates [for example, -SO ３ M (M is an alkali metal such as sodium (or its cation)) is preferred, and a carboxyl group and its base (a carboxylic acid base, a group in which a carboxyl group forms a salt) are particularly preferred.

[0026] Therefore, the ionic group (ionic skeleton) may contain at least one type of ionic group selected from an acid group (particularly, a carboxyl group) and its base (particularly, a carboxylate base).

[0027] The PVA polymer (A) may have one or a combination of two or more ionic skeletons.

[0028] The form of the ionic skeleton is not limited as long as it has an ionic group, and may be selected depending on the type and number of the ionic group, etc., but typically, the ionic skeleton may be present via a vinyl alcohol unit (a hydroxy group of the vinyl alcohol unit) (or may be present on a side chain via the ionic group).

[0029] Such an ionic skeleton may be, for example, a group (skeleton) represented by the following formula (X).

[0030] -CH ２ —CH(OX)—(X) (wherein X represents a group having at least one ionic group.)

[0031] In the above formula (X), examples of the ionic group include the ionic groups exemplified above (carboxyl group, carboxylate salt group, etc.).

[0032] X may have two or more (for example, 2 to 6, 2 to 4, etc.) ionic groups, and when it has two or more, the ionic groups may be the same or different.

[0033] X may be an ionic group alone depending on the type of ionic group, but may generally be a group having an ionic group (a group substituted with an ionic group).

[0034] Examples of such X include -X1-Y (wherein X1 represents a linking group and Y represents an ionic group-containing group).

[0035] Examples of the linking group X1 include a direct bond, -CO- (carbonyl group), -CS- (thiocarbonyl group), -CO-NH-, etc. In particular, -CO- (carbonyl group) etc.

[0036] Examples of the ionic group-containing group Y include ionic groups (such as carboxy groups and their bases) and groups having an ionic group (such as hydrocarbon groups).

[0037] The group having an ionic group (such as a hydrocarbon group) may have a substituent other than the ionic group (for example, a hydroxy group, an oxo group, an acyl group, etc.).

[0038] Examples of groups having such an ionic group (groups substituted with an ionic group, skeletons having an ionic group) include hydrocarbons (groups in which an ionic group is substituted on a hydrocarbon) such as aliphatic hydrocarbons (for example, saturated aliphatic hydrocarbons (for example, saturated aliphatic hydrocarbons having about 1 to 20 carbon atoms, such as methane, ethane, propane, butane, pentane, hexane, octane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, norbornane, and adamantane), unsaturated aliphatic hydrocarbons (for example, unsaturated aliphatic hydrocarbons having about 2 to 10 carbon atoms, such as ethylene, propylene, butene, pentene, hexene, octene, hexadiene, cyclohexene, norbornene, and 7-oxabicyclo[2.2.1]hept-5-ene)), and aromatic hydrocarbons (for example, aromatic hydrocarbons having about 6 to 20 carbon atoms, such as benzene, toluene, xylene, and naphthalene). Note that the substitution position of the ionic group in such groups having an ionic group is not particularly limited.

[0039] In formula (X), a representative example of X may be a group corresponding to (or derived from) a polycarboxylic acid (for example, a dicarboxylic acid or tricarboxylic acid) component, as described below.

[0040] By incorporating a vinyl alcohol unit, coupled with the selection of the introduction conditions, etc., as described below, it is easy to efficiently satisfy the properties described below (amount of water-insoluble components, transparency of an aqueous solution, YI value, etc.) Furthermore, by incorporating a vinyl alcohol unit, it is easy to efficiently introduce an ionic group and also it appears that it is easy to introduce the ionic group relatively uniformly, which can also be a factor in easily efficiently satisfying the properties described below (amount of water-insoluble components, transparency of an aqueous solution, YI value, etc.).

[0041] As described above, the ionic skeleton is preferably a skeleton having at least one selected from a carboxyl group and its base, and such an ionic skeleton may be derived from a polycarboxylic acid component.

[0042] The polycarboxylic acid component may generally be any component capable of introducing a carboxyl group or a salt thereof into the PVA polymer (A), and examples thereof include polycarboxylic acids (e.g., dicarboxylic acids, tricarboxylic acids, tetracarboxylic acids, etc.), derivatives of polycarboxylic acids (e.g., polycarboxylic acid anhydrides (e.g., dicarboxylic acid anhydrides), polycarboxylic acid esters (particularly, partial esters of polycarboxylic acids such as dicarboxylic acid monoesters and tricarboxylic acid mono- or diesters), and salts thereof (e.g., the aforementioned salts such as sodium salts), etc.

[0043] In the polycarboxylic acid ester, examples of the ester include alkyl esters [for example, alkyl esters such as methyl esters and ethyl esters (for example, lower alkyl esters having about 1 to 4 carbon atoms)].

[0044] In the polycarboxylic acid component, examples of the polycarboxylic acid include dicarboxylic acids, tricarboxylic acids, and polycarboxylic acids having four or more carboxyl groups (for example, carboxylic acids having 2 to 6 carboxyl groups, such as tetracarboxylic acids), and among these, dicarboxylic acids and tricarboxylic acids are preferred.

[0045] Examples of polycarboxylic acids include aliphatic carboxylic acids and aromatic carboxylic acids.

[0046] The polycarboxylic acid may have an unsaturated bond (polymerizable unsaturated bond) or a cyclic structure.

[0047] Specific examples of polycarboxylic acids (di- or tricarboxylic acids) include dicarboxylic acids {for example, aliphatic dicarboxylic acids [for example, saturated or unsaturated aliphatic dicarboxylic acids (for example, dicarboxylic acids having about 2 to 30 carbon atoms (for example, 2 to 20 carbon atoms) such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, malic acid, oxaloacetic acid, maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, cis-1,2-cyclohexanedicarboxylic acid, cis-4-cyclohexene-1,2-dicarboxylic acid, 5-norbornene-2,3-dicarboxylic acid, exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic acid, 1,3-adamantanedicarboxylic acid, etc.)], aromatic dicarboxylic acids [for example, arene dicarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, etc. ( Examples of the dicarboxylic acid include dicarboxylic acids having about 8 to 30 carbon atoms (e.g., about 8 to 20 carbon atoms), tricarboxylic acids (e.g., aliphatic tricarboxylic acids [e.g., saturated or unsaturated aliphatic tricarboxylic acids such as citric acid and aconitic acid (e.g., tricarboxylic acids having about 6 to 30 carbon atoms (e.g., about 2 to 20 carbon atoms)], aromatic tricarboxylic acids [e.g., arene tricarboxylic acids such as trimellitic acid (e.g., tricarboxylic acids having about 9 to 30 carbon atoms (e.g., about 2 to 20 carbon atoms)], and polycarboxylic acids having four or more carboxyl groups (e.g., tetracarboxylic acids such as butanetetracarboxylic acid, benzenetetracarboxylic acid, naphthalenetetracarboxylic acid, biphenyltetracarboxylic acid, and benzophenonetetracarboxylic acid; pentacarboxylic acids such as benzenepentacarboxylic acid; hexacarboxylic acids such as mellitic acid). In addition, when isomers (e.g., cis-trans isomers) exist in the polycarboxylic acid, both isomers (e.g., both cis and trans isomers) are included.

[0048] The polycarboxylic acid components may be used alone or in combination of two or more.

[0049] The polycarboxylic acid component may also be such that a carboxyl group or a salt group thereof is introduced into the PVA polymer (A) preferably via a vinyl alcohol unit (a hydroxy group of the vinyl alcohol unit).

[0050] Therefore, the ionic skeleton may include at least such an ionic skeleton.

[0051] Representative ionic skeletons include skeletons derived from an ester bond between a hydroxyl group of a vinyl alcohol unit and a carboxyl group (particularly one carboxyl group) of a polycarboxylic acid component [a skeleton formed by an ester bond with a carboxyl group (a skeleton formed by an ester bond and having at least one ionic group selected from a carboxyl group and its base)]. In other words, the ionic skeleton may include a skeleton obtained by an esterification reaction between a hydroxyl group (a hydroxyl group of a vinyl alcohol unit) and a polycarboxylic acid component.

[0052] As described above, the polycarboxylic acid component is usually a component into which a carboxyl group or a salt thereof can be introduced, and in the PVA polymer (A), it is the origin of the carboxyl group or the salt thereof.

[0053] For example, a part of the carboxyl groups of a polycarboxylic acid forms an ester bond with a hydroxyl group of a vinyl alcohol unit, thereby introducing the carboxyl groups (remaining carboxyl groups) into the PVA-based polymer (A). In a more specific embodiment, for example, when a di(tri)carboxylic acid is used, a PVA-based polymer (A) having an ionic group derived from the di(tri)carboxylic acid can be obtained by esterifying the hydroxyl groups of the PVA-based polymer with the carboxyl groups (COOH groups) of the di(tri)carboxylic acid.

[0054]

[0033] Depending on whether the polycarboxylic acid ester is a partial ester of polycarboxylic acid (e.g., alkyl ester, etc.), for example, if it is a partial ester, the ester moiety and the hydroxyl group of the vinyl alcohol unit undergo an ester exchange reaction to form an ester bond, thereby introducing (introducing in a residual form) the carboxyl group constituting the partial ester into the PVA-based polymer. More specifically, when a dicarboxylic acid monoalkyl ester is used, a PVA-based polymer (A) having an ionic group derived from the dicarboxylic acid can be obtained by subjecting a hydroxy group in the PVA-based polymer to an ester exchange reaction with the dicarboxylic acid monoalkyl ester.

[0055] Alternatively, polycarboxylic acid anhydrides may be converted into polycarboxylic acids or their partial esters by reacting them with vinyl alcohol units (PVA-based polymer (B) described below) or by dissolving them in an appropriate solvent (e.g., methanol) and then introducing carboxyl groups into the polycarboxylic acids or their partial esters in the same manner as described above. More specifically, when a dicarboxylic acid anhydride is used, a PVA-based polymer (A) having ionic groups derived from dicarboxylic acids can be obtained by esterifying hydroxy groups in the PVA-based polymer with carboxyl groups or ester moieties derived from the dicarboxylic acid anhydride.

[0056] The carboxylate group may be formed as a salt of a polycarboxylic acid using a salt of a polycarboxylic acid as the polycarboxylic acid component, or may be formed as a salt with a component corresponding to the salt (e.g., sodium hydroxide, sodium acetate, etc. in the case of sodium) during the reaction process as described above, or may be formed as a salt with a component corresponding to the salt (e.g., sodium hydroxide, sodium acetate, etc. in the case of sodium) after the reaction (introduction of a carboxyl group). The component corresponding to the salt, such as sodium acetate, may be contained in (or derived from) the PVA polymer (B) used as a raw material.

[0057] Such an ionic skeleton [a skeleton derived from an ester bond between a hydroxyl group of a vinyl alcohol unit and a carboxyl group (particularly one carboxyl group) of a polycarboxylic acid component] may be one corresponding to the formula (X), for example, the formula (X) in which X is -CO-Y1 (wherein Y1 represents at least one ionic group-containing group selected from a carboxyl group and a base thereof).

[0058] Specific examples of the ionic skeleton [for example, an ionic skeleton (skeleton having an ionic group) derived from a polycarboxylic acid component such as a dicarboxylic acid or a tricarboxylic acid] include a skeleton (structural unit) represented by the following formula (1).

[0059] [In the formula, R represents a direct bond or a linking group, and M represents a hydrogen atom or a chemical species corresponding to a salt (e.g., an alkali metal such as sodium).]

[0060] Examples of the linking group include hydrocarbon groups (hydrocarbons and groups corresponding to hydrocarbons). Examples of the hydrocarbon group include those exemplified above (e.g., saturated aliphatic hydrocarbon groups, unsaturated aliphatic hydrocarbon groups, and aromatic hydrocarbon groups).

[0061] Such a linking group (hydrocarbon group) may have one or more carboxyl groups or their salt groups, resulting in a skeleton having two or more carboxyl groups or their salt groups.

[0062] Examples of the aliphatic saturated hydrocarbon group include a group (skeleton) represented by the following formula (2).

[0063] [In the formula, R1 to R4 are hydrogen atoms or substituents (hydrocarbon groups, carboxyl groups, bases thereof, etc.). R1 ​​or R2 and R3 or R4 may be connected via another linking group (hydrocarbon group, etc.) (for example, a cycloalkane structure may be formed).]

[0064] Examples of the aliphatic unsaturated hydrocarbon group include a group (skeleton) represented by the following formula (3).

[0065] [In the formula, R1 and R2 are hydrogen atoms or substituents (hydrocarbon groups, carboxyl groups, bases thereof, etc.). R1 ​​and R2 may be connected via another linking group (hydrocarbon group, etc.) (for example, a cycloalkene structure may be formed).]

[0066] Examples of the aromatic hydrocarbon group include a group (skeleton) represented by the following formula (4).

[0067] [In the formula, R1 to R4 are hydrogen atoms or substituents (hydrocarbon groups, carboxyl groups, bases thereof, etc.). Two or more of R1 to R4 may be connected via another linking group (hydrocarbon group, etc.) (for example, a polycyclic aromatic hydrocarbon structure may be formed).]

[0068] The PVA polymer (A) may have one or more different ionic skeletons.

[0069] In the PVA-based polymer (A), the content (proportion, content ratio) of the ionic skeleton (or ionic group, for example, a skeleton represented by formula (X) or (1)) per monomer unit (based on the total of vinyl alcohol units and other monomer units such as vinyl ester units) may be selected from a range of about 0.001 mol % or more (e.g., 0.005 mol % or more), for example, 0.01 mol % or more, preferably 0.05 mol % or more, more preferably 0.1 mol % or more, and may be 30 mol % or less (e.g., 20 mol % or less, 15 mol % or less), for example, 10 mol % or less, preferably 5 mol % or less, more preferably 2 mol % or less.

[0070] These ranges (upper and lower limits) may be combined appropriately to select a range (for example, 0.05 to 2 mol %, 0.1 to 5 mol %, etc.; the same applies to the ranges hereinafter).

[0071] Specifically, the content of the ionic skeleton may be about 0.01 to 10 mol %, preferably about 0.05 to 5 mol %, and more preferably about 0.1 to 2 mol %, per monomer unit.

[0072] The content of 1 mol% per monomer unit refers to the case where one ionic group derived from di(tri)carboxylic acid (for example, a skeleton represented by formula (X) or (1)) is present per 100 monomer units (for example, the total of monomer units such as vinyl alcohol units and vinyl ester units).

[0073] In the PVA-based polymer (A), the content (proportion) of the ionic skeleton relative to the total amount (total) of vinyl alcohol units (e.g., when an ionic skeleton is introduced via a vinyl alcohol unit, the remaining vinyl alcohol units without being introduced) and ionic skeletons may be selected from a range of, for example, about 0.005 mol% or more (e.g., 0.01 mol% or more), for example, 0.02 mol% or more, preferably 0.05 mol% or more (e.g., 0.1 mol% or more), more preferably 0.2 mol% or more (e.g., 0.3 mol% or more, 0.5 mol% or more, 1 mol% or more, 1.5 mol% or more, 2 mol% or more, 2.5 mol% or more, 3 mol% or more), or may be 80 mol% or less (e.g., 50 mol% or less), for example, 30 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less (e.g., 8 mol% or less, 5 mol% or less). Specifically, the content of the ionic skeleton relative to the total amount (total) of the vinyl alcohol units and the ionic skeleton may be, for example, about 0.01 to 50 mol %, preferably about 0.1 to 30 mol %, and more preferably about 0.3 to 10 mol %.

[0074] The above-mentioned content can efficiently improve the hydrophilicity of the PVA polymer (A), and can efficiently impart water solubility (dispersibility) even to a PVA having a saponification degree that does not inherently exhibit water solubility (dispersibility) (e.g., a saponification degree of 70 mol % or less, 60 mol % or less, 50 mol % or less, or 40 mol % or less).

[0075] In addition, by not setting the upper limit too high, it is easy to suppress the formation of water-insoluble matters and coloration.

[0076] The method for measuring the content of the ionic group (for example, the skeleton represented by formula (X) or (1)) is not particularly limited, and can be measured, for example, by using NMR, titration, UV absorbance, etc. As a specific example, the content of the skeleton represented by formula (X) or (1) can be measured by dissolving the PVA polymer (A) in an appropriate solvent (e.g., d6-DMSO) and measuring the content of the ionic group (for example, the skeleton represented by formula (X) or (1)). １ The amount of carboxyl groups can be measured by H-NMR and analyzed for signals derived from specific substituents (e.g., hydrogen). Alternatively, if necessary, a sample after Soxhlet extraction (e.g., after sodium acetate has been removed) can be dissolved in water and subjected to conductometric titration with dilute hydrochloric acid, thereby determining the amount of carboxyl groups from the amount of hydrochloric acid titrated. Furthermore, when the ionic groups have UV (ultraviolet) absorption, the content of the ionic groups can be determined by measuring the UV absorbance of an aqueous solution containing the PVA polymer (A).

[0077] The method for incorporating (introducing) an ionic group into the PVA polymer (A) is not particularly limited, and any conventional method can be used.

[0078] A typical example of such a method is to react (e.g., esterify) the PVA polymer (B) with a component corresponding to the ionic skeleton (ionic group) [e.g., a polycarboxylic acid component such as di(tri)carboxylic acid or a derivative thereof (e.g., carboxylic acid anhydride, di(tri)carboxylate salt, monoalkyl di(tri)carboxylate or a salt thereof, dialkyl di(tri)carboxylate, etc.)].

[0079] The PVA-based polymer (A) contains at least a vinyl alcohol unit, but may also contain a vinyl alcohol unit and a unit that has not been hydrolyzed (saponified) [for example, a vinyl ester unit (or a unit derived from a vinyl ester-based monomer, such as a vinyl acetate unit)]. The PVA-based polymer (A) may also contain units derived from other monomers, as exemplified in the section on the PVA-based polymer (B) described below. When the PVA-based polymer (A) contains other units, the content of the other units may be appropriately selected depending on the monomers used, and may be, for example, 20% by mass or less (e.g., 0.1 to 20% by mass, 15% by mass or less, 10% by mass or less) based on the PVA-based polymer (A) (or the total amount of its polymerization components).

[0080] The block character of the PVA polymer (A) (block character of the residual vinyl ester units) is not particularly limited and may be selected from the range of 0 to 2. Depending on the application, etc., from the viewpoint of efficient performance of the PVA polymer (A), the block character may be, for example, 0.2 or more (e.g., 0.25 or more), preferably 0.3 or more (e.g., 0.32 or more), more preferably 0.35 or more (e.g., 0.38 or more, 0.4 or more), etc., or 1.5 or less (e.g., 1.2 or less), preferably 1 or less (e.g., 0.9 or less), more preferably 0.8 or less (e.g., 0.75 or less, 0.7 or less), etc. Specific block character ranges include, for example, about 0.3 to 1, 0.35 to 0.85, 0.38 to 0.8, 0.4 to 0.75, and 0.35 to 0.7.

[0081] Here, the block character (block character of the residual vinyl ester unit) (η) is the block character of the residual vinyl ester unit (-CH ２ is an index showing the distribution of —CH(OCOR)—, １３ It is determined by analyzing three peaks that appear in the methylene region of the C-NMR spectrum. The three peaks correspond to three two-unit chain structures corresponding to (OH, OH), (OH, OCOR), and (OCOR, OCOR), and the absorption intensities are proportional to the three structures. The block character (η) is expressed by the following formula (1):

[0082] η=(OH, OCOR) / [2(OH)(OCOR)] Formula (1) [In the formula, (OH, OCOR) represents the proportion of two-unit chain structures in which an OH group and an OCOR group are adjacent, (OH) represents the proportion of vinyl alcohol units, and (OCOR) represents the proportion of remaining vinyl ester units, all of which are expressed as mole fractions.]

[0083] This block character takes a value of 0 to 2, with the closer to 0 the higher the blockiness of the vinyl ester group distribution, the closer to 1 the higher the randomness, and the closer to 2 the higher the alternation. The measurement method for this block character is described in detail in Macromolecules, 10, 532 (1977).

[0084] The block character of the PVA polymer (A) can be adjusted, for example, by the block character of the raw material PVA polymer (B) or the production conditions of the PVA polymer (A). The change (increase) in block character is believed to be due to a transesterification reaction between hydroxyl groups and ester groups in the PVA polymer. This transesterification reaction can be influenced by production conditions that can affect the hue, transparency, and water-insoluble content, as described below. Therefore, the change (increase) in block character often reflects the hue (coloring), transparency, and water-insoluble content, as described below. (For example, when the change in block character is relatively suppressed, the hue, high transparency, and low water-insoluble content (and even low YI), as described below, are likely to be achieved.)

[0085] The lower limit of the saponification degree of the PVA polymer (A) may be, for example, 10 mol% or more (e.g., 15 mol% or more), preferably 20 mol% or more (e.g., 22 mol% or more), more preferably about 25 mol% or more (e.g., 27 mol% or more), or may be 30 mol% or more (e.g., 32 mol% or more).

[0086] The upper limit of the saponification degree of the PVA-based polymer (A) may be, for example, 100 mol% or less (e.g., 99.9 mol% or less, 99 mol% or less, 98 mol% or less, 97 mol% or less, or 95 mol% or less). Alternatively, the saponification degree may be not too high (e.g., 90 mol% or less (e.g., 85 mol% or less), preferably 80 mol% or less (e.g., 75 mol% or less), or further 70 mol% or less (e.g., less than 70 mol%, 65 mol% or less, 60 mol% or less, less than 60 mol%, 55 mol% or less, 50 mol% or less, less than 50 mol%, 45 mol% or less, 40 mol% or less, less than 40 mol%, or 38 mol% or less).

[0087] Specifically, the saponification degree of the PVA polymer (A) may be, for example, about 20 to 90 mol%, preferably about 25 to 80 mol%, and more preferably about 30 to 70 mol%, and particularly may be 70 mol% or less (e.g., less than 70 mol%, 65 mol% or less, 60 mol% or less, less than 60 mol%, 55 mol% or less, 50 mol% or less, less than 50 mol%, 45 mol% or less, 40 mol% or less, less than 40 mol%, or 38 mol% or less).

[0088] In particular, the present invention can provide a PVA polymer (A) having excellent water solubility or dispersibility, which satisfies the requirements of high transparency, low water-insoluble content and low YI, as described below, even if the PVA polymer (A) has a low saponification degree (a degree of saponification that does not or hardly exhibits water solubility in itself).

[0089] When the ionic skeleton is introduced via a vinyl alcohol unit, it is preferable that the degree of saponification is not too low, since the introduction (reaction) efficiency is increased, whereas it is preferable that the degree of saponification is not too high, since the PVA polymer (A) has excellent industrial productivity.

[0090] The saponification degree can be determined, for example, by the method for measuring the saponification degree of PVA specified in JIS K 6726.

[0091] The saponification degree of the PVA polymer (A) can be adjusted, for example, by the saponification degree of the raw material PVA polymer (B). The saponification degree may change when the PVA polymer (B) is converted into the PVA polymer (A). In such a case, the saponification degree of the PVA polymer (B) may be adjusted in anticipation of such a change.

[0092] The viscosity (20°C) of a 4% by mass aqueous solution of the PVA polymer (A) is not particularly limited, and may be selected from the range of, for example, about 1 mPa s or more (e.g., 1.1 mPa s or more), and may be 1.2 mPa s or more (e.g., 1.3 mPa s or more), preferably 1.4 mPa s or more (e.g., 1.5 mPa s or more), and more preferably 1.6 mPa s or more (e.g., 1.7 mPa s or more, 1.8 mPa s or more, 1.9 mPa s or more).

[0093] The upper limit of the viscosity (20°C) of a 4% by mass aqueous solution of the PVA polymer (A) is not particularly limited, and may be selected from a range of about 2000 mPa s or less (e.g., 1500 mPa s or less, 1000 mPa s or less, 500 mPa s or less), typically about 300 mPa s or less (e.g., 250 mPa s or less, 200 mPa s or less, 150 mPa s or less, 120 mPa s or less, 100 mPa s or less, 80 mPa s or less, 50 mPa s or less), or may be 30 mPa s or less (e.g., 20 mPa s or less), preferably 15 mPa s or less (e.g., 10 mPa s or less), more preferably 9 mPa s or less (e.g., 8 mPa s or less).

[0094] Specifically, the viscosity (20°C) of a 4% by mass aqueous solution of the PVA polymer (A) may be, for example, about 1 to 500 mPa s (e.g., 1.2 to 300 mPa s, 1.3 to 100 mPa s, 1.4 to 100 mPa s, 1.5 to 30 mPa s), or about 20 mPa or less (e.g., 1.6 to 15 mPa s, 1.7 to 10 mPa s, 1.8 to 9 mPa s, 1.9 to 8 mPa s).

[0095] The (average) degree of polymerization of the PVA-based polymer (A) is not particularly limited, and may be, for example, 100 or more (e.g., 120 or more), preferably 140 or more (e.g., 160 or more), and more preferably 180 or more (e.g., 200 or more, 220 or more, 250 or more).

[0096] The upper limit of the (average) degree of polymerization of the PVA-based polymer (A) is not particularly limited, and may be selected within a range of, for example, about 10,000 or less (e.g., 8,000 or less, 5,000 or less), and may be 3,000 or less (e.g., 2,500 or less), preferably 2,000 or less (e.g., 1,500 or less), and more preferably 1,000 or less (e.g., 800 or less).

[0097] Specifically, the (average) degree of polymerization of the PVA polymer (A) may be, for example, about 120 to 3,000 (eg, 140 to 2,000), preferably about 160 to 1,500, and more preferably about 180 to 1,000.

[0098] It is preferable that the viscosity of a 4% by mass aqueous solution of the PVA polymer (A) and the degree of polymerization are not too small, because this leads to excellent productivity of the PVA polymer (B) that is the raw material for the PVA polymer (A).On the other hand, it is preferable that the viscosity of a 4% by mass aqueous solution of the PVA polymer (A) and the degree of polymerization are not too large, because this leads to excellent industrial productivity of the PVA polymer (A) and also leads to easy realization of a low water-insoluble content of the PVA polymer (A).

[0099] The viscosity of a 4% by mass aqueous solution (20°C) can be determined, for example, by the method specified in JIS K 6726. The degree of polymerization can be determined, for example, by the method specified in JIS K 6726, or can be a calculated (converted) value based on another analytical method [for example, a calculated (converted) value based on the viscosity of a 4% by mass aqueous solution].

[0100] The viscosity of a 4% by mass aqueous solution and the (average) degree of polymerization of the PVA-based polymer (A) can be adjusted, for example, by changing the viscosity of a 4% by mass aqueous solution and the degree of polymerization of the PVA-based polymer (B) used as a raw material. Usually, the viscosity of a 4% by mass aqueous solution and the degree of polymerization of the PVA-based polymer (B) can be reflected in the viscosity of a 4% by mass aqueous solution and the degree of polymerization of the PVA-based polymer (B).

[0101] The pH (e.g., pH at 20°C) of a 4% by mass aqueous solution of the PVA polymer (A) may vary depending on the type of ionic skeleton, the introduction ratio thereof, etc., but may usually be about more than 3 [e.g., 3.2 or more, 3.5 or more, 4 or more, 4.5 or more, preferably 5 or more (e.g., 5.5 or more, 6 or more)] or less than 10 [e.g., 9.5 or less, 9 or less, 8.5 or less, 8 or less, 7.5 or less, 7.2 or less], preferably about 7 or less (e.g., less than 7, 6.9 or less, 6.8 or less, 6.7 or less, 6.6 or less, 6.5 or less)]. Specific examples of the pH (e.g., pH at 20°C) of a 4% by mass aqueous solution of the PVA polymer (A) include 3.5 to 8.5, 5 to 7, and 5.5 to 6.5. In particular, from the viewpoint of the desired functions and stability of the PVA polymer (A) (and furthermore, achieving them in a well-balanced manner), the pH is preferably not too low and not too high (for example, 5 to 9, 5.5 to 8.5, 5.5 to 8, 6 to 9, etc.).

[0102] The PVA-based polymer (A) may or may not contain sodium acetate. Since sodium acetate may affect the hue (coloring characteristics) and the efficient introduction of an ionic skeleton depending on the content, it is sometimes preferable that the content be small. Therefore, the PVA-based polymer (A) may contain no sodium acetate (substantially no sodium acetate) or may contain only a small amount of sodium acetate. For example, when the PVA-based polymer (A) contains sodium acetate, the proportion of sodium acetate per monomer unit of the PVA-based polymer (A) (based on the total of vinyl alcohol units and other monomer units such as vinyl ester units) may be selected from a range of about 2 mol % or less (e.g., 1.5 mol %) or less, and may be about 1.2 mol % or less (e.g., 1 mol % or less), preferably about 0.8 mol % or less (e.g., 0.7 mol % or less), more preferably about 0.5 mol % or less (e.g., 0.4 mol % or less), or may be 0.3 mol % or less (e.g., 0.2 mol % or less, 0.1 mol % or less, 0.05 mol % or less, 0.01 mol % or less), etc. Similarly, when the PVA-based polymer (A) contains sodium acetate, the content of sodium acetate may be 3% by mass or less (e.g., 2.5% by mass or less), preferably 2% by mass or less (e.g., 1.5% by mass or less), and more preferably 1.2% by mass or less (e.g., 1% by mass or less, 0.8% by mass or less, 0.5% by mass or less, 0.3% by mass or less, 0.2% by mass or less, 0.1% by mass or less, 0.05% by mass or less), based on the PVA-based polymer (A) (PVA-based polymer (A) containing sodium acetate).

[0103] When the PVA-based polymer (A) contains sodium acetate, the lower limit of the content thereof per monomer unit of the PVA-based polymer (A) (based on the total of vinyl alcohol units and other monomer units such as vinyl ester units) is, for example, 0.0001 mol % or more (e.g., 0.001 mol % or more, 0.01 mol % or more, 0.03 mol % or more, 0.05 mol % or more, 0.1 mol % or more), or 0.0001 mass % or more of the PVA-based polymer (A) (the PVA-based polymer (A) containing sodium acetate) (e.g., 0.001 mass % or more, 0.05 mass % or more, 0.1 mass % or more).

[0104] When the PVA-based polymer (A) contains sodium acetate, the specific proportion of sodium acetate may be about 0.01 to 1 mol %, preferably about 0.03 to 0.7 mol %, and more preferably about 0.05 to 0.5 mol %, per monomer unit of the PVA-based polymer (A) (based on the total of vinyl alcohol units and other monomer units such as vinyl ester units).

[0105] The proportion of sodium acetate can be determined, for example, by NMR [for example, １ H-NMR measurement, quantifying the signal derived from sodium acetate, and performing calculations.

[0106] The YI (yellowness index) of a 4 mass% dimethyl sulfoxide (DMSO) solution of the PVA-based polymer (A) may be selected from a range of about 30 or less (e.g., 25 or less, 20 or less, 18 or less), and may be, for example, 15 or less (e.g., 12 or less), preferably 10 or less (e.g., 9 or less), more preferably 8 or less (e.g., 6 or less), particularly 5 or less (e.g., 4 or less, 3.5 or less, 3 or less, 2.5 or less, 2 or less, 1.5 or less, 1.2 or less, 1 or less, less than 1, 0.8 or less, 0.5 or less, 0.3 or less, 0.1 or less).

[0107] The YI of a 4 mass % DMSO solution of the PVA polymer (A) can be calculated from data obtained by measuring the UV-Vis spectrum (for example, in a quartz cell with an optical path length of 10 mm) of the 4 mass % DMSO solution at 20°C using an ultraviolet-visible spectrophotometer (for example, in a wavelength range of 360 to 830 nm).

[0108] Such a YI reflects the coloration, and such a YI results in excellent hue (i.e., coloration is suppressed). Note that, as will be described later, the YI can be easily adjusted to fall within the above range efficiently by selecting the production conditions for the PVA polymer (A).

[0109] The PVA polymer (A) is preferably dissolved or dispersed in water.

[0110] The transparency (430 nm) of a 4% by mass aqueous solution of the PVA-based polymer (A) may be selected from a range of about 0.5% or more, for example, 1% or more, preferably 2% or more (e.g., 3% or more, 4% or more), more preferably 5% or more (e.g., 7% or more), or may be 10% or more (e.g., more than 10%, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, 45% or more, 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 95% or more), etc. In particular, the transparency (20°C) of a 4% by mass aqueous solution of the PVA polymer (A) may be 40% or more, 50% or more, 60% or more, or 68% or more (e.g., 70% or more, 72% or more, 75% or more, 77% or more, 80% or more, 82% or more, or 85% or more).

[0111] The transparency (430 nm) of a 4 mass% aqueous solution can be determined, for example, by measuring the transmittance at 430 nm (for example, in a quartz cell with an optical path length of 20 mm) at 20°C using a UV-visible spectrophotometer, using water as a control.

[0112] The water-insoluble content of the PVA polymer (A) may be selected from a range of about less than 50% by mass (e.g., 45% by mass or less), and may be, for example, 40% by mass or less (e.g., 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 18% by mass or less), preferably 15% by mass or less (e.g., 12% by mass or less, 10% by mass or less, 8% by mass or less). In particular, the water-insoluble content of the PVA polymer (A) may be 20% by mass or less (e.g., 15% by mass or less), preferably 10% by mass or less (e.g., 8% by mass or less), and more preferably 5% by mass or less (e.g., less than 5% by mass, 4% by mass or less, 3% by mass or less, 2% by mass or less, 1.5% by mass or less, 1% by mass or less, 0.8% by mass or less, 0.6% by mass or less, 0.5% by mass or less, 0.4% by mass or less, 0.3% by mass or less, 0.2% by mass or less, 0.1% by mass or less, less than 0.1% by mass, 0.08% by mass or less, 0.06% by mass or less, 0.04% by mass, 0.02% by mass or less, 0.01% by mass or less, etc.).

[0113] The water-insoluble content can be determined by, for example, dissolving a polymer in water at 50°C (e.g., dissolving in 50 times the mass of water) and cooling the resulting solution (mixture) to room temperature (e.g., 15 to 30°C), passing the resulting solution through a filter (wire mesh) with a predetermined mesh size (e.g., 100 mesh), measuring the amount of the polymer that did not pass through the filter (wire mesh) (amount of water-insoluble content), and calculating the water-insoluble content / total amount of PVA polymer (A) × 100 (%). Specifically, the water-insoluble content may be determined by the method described in Examples below.

[0114] The transparency and water-insoluble content of a 4% by mass aqueous solution can be indicators of solubility or dispersibility in water, and a high transparency and / or a low water-insoluble content tend to result in excellent solubility or dispersibility in water. Possible factors that reduce transparency or result in water insolubility include, for example, a low degree of saponification, the presence of a PVA-based polymer in which an ionic skeleton is not sufficiently introduced, and the formation of a crosslinked product (by-product) due to some factor (for example, the presence of a PVA-based polymer with a high degree of polymerization, or conditions for introducing an ionic skeleton, such as reaction temperature, the amount of a component corresponding to the ionic skeleton (ionic group) or a pH adjuster, and the balance between these).

[0115] The transparency and the water-insoluble content are considered to be related to the degree of reaction spotting [e.g., the uniformity of the reaction (esterification reaction) for introducing a skeleton having an ionic group derived from a polycarboxylic acid component]. Therefore, in the production of the PVA-based polymer (A), suppressing such reaction spotting (enhancing the uniformity of the reaction) can efficiently achieve the desired transparency and water-insoluble content. The uniformity of the reactivity can be improved by selecting the production conditions for the PVA-based polymer (A), as described below.

[0116] The PVA-based polymer (A) may satisfy at least one requirement selected from the water-insoluble content (requirement 1), the transparency (20°C, 430 nm) of a 4 mass% aqueous solution (requirement 2), and the YI of a 4 mass% dimethyl sulfoxide (DMSO) solution (requirement 3). Preferably, the PVA-based polymer (A) satisfies two or more of these requirements (e.g., requirements 1 and 2, or requirements 1 and / or 2 and 3), and particularly may satisfy all of requirements 1 to 3.

[0117] [Production Method] The method for producing the PVA polymer (A) is not particularly limited, and a conventional method may be used. In the present invention, the PVA polymer (A) may be produced, for example, by reacting the polyvinyl alcohol polymer (B) with a component (e.g., a polycarboxylic acid component) corresponding to the ionic skeleton (ionic group). Such a reaction / production method will be described below with reference to typical or specific examples (PVA polymer (B), esterification, etc.).

[0118] [Polyvinyl Alcohol Polymer (B)] The polyvinyl alcohol polymer (which may be referred to as PVA polymer (B), PVA (B), etc.) is not particularly limited, but for example, a PVA polymer obtained by saponifying (reacting) a vinyl ester polymer [a saponified product of a vinyl ester polymer (a polymer having a vinyl ester monomer as a polymerization component)] can be used.

[0119] The vinyl ester polymer can be obtained by polymerizing at least a vinyl ester monomer (polymerizing it as a polymerization component). The polymerization method is not particularly limited, and may be any conventionally known method, such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. Considering the control of the polymerization degree and the saponification reaction performed after polymerization, solution polymerization using methanol as a solvent or suspension polymerization using water or water / methanol as a dispersion medium is preferred, but is not limited thereto.

[0120] The vinyl ester monomer usable in the polymerization is not particularly limited, but examples thereof include fatty acid vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl caprylate, and vinyl versatate, and these vinyl ester monomers can be used alone or in combination. Among these, vinyl acetate is preferred from an industrial viewpoint.

[0121] When polymerizing a vinyl ester-based monomer, the vinyl ester-based monomer may be copolymerized with other monomers as long as the effects of the present invention are achieved. In other words, the polymerization components of the vinyl ester-based polymer may contain a vinyl ester-based monomer and other monomers. Examples of other monomers that can be used include, but are not limited to, α-olefins (e.g., ethylene, propylene, n-butene, isobutylene, etc.), (meth)acrylic acid esters [e.g., (meth)acrylic acid alkyl esters (e.g., 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, octadecyl (meth)acrylate, etc.], and the like. １－２０ alkyl, etc.)], (meth)acrylamide, (meth)acrylamide derivatives (for example, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, diacetone(meth)acrylamide, (meth)acrylamidopropanesulfonic acid and salts thereof, (meth)acrylamidopropyldimethylamine and salts thereof or quaternary salts thereof, N-methylol(meth)acrylamide, etc.), vinyl ethers (for example, 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, stearyl vinyl ether, etc. １－２０ alkyl vinyl ethers, etc.), nitriles (e.g., acrylonitrile, methacrylonitrile, etc.), vinyl halides (e.g., vinyl chloride, vinyl fluoride, etc.), vinylidene halides (e.g., vinylidene chloride, vinylidene fluoride, etc.), allyl compounds (e.g., allyl acetate, allyl chloride, etc.), vinylsilyl compounds (e.g., vinyltrimethoxysilane, etc.), fatty acid alkenyl esters (e.g., isopropenyl acetate, etc.), etc. These other monomers can be used alone or in combination.

[0122] When other monomers are used, the content of the other monomers is, for example, 0.1 to 20% by mass based on the total amount of the monomers.

[0123] Furthermore, during polymerization of vinyl ester monomers, a chain transfer agent may be present for the purpose of adjusting the degree of polymerization of the resulting vinyl ester polymer. The chain transfer agent is not particularly limited, but examples include aldehydes such as acetaldehyde, propionaldehyde, butylaldehyde, and benzaldehyde; ketones such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; mercaptans such as 2-hydroxyethanethiol, dodecyl mercaptan, 3-mercaptopropionic acid, mercaptosuccinic acid, and sodium 3-mercapto-1-propanesulfonate; and organic halogens such as carbon tetrachloride, trichloroethylene, and perchloroethylene. Among these, aldehydes and ketones are preferred. The amount of chain transfer agent added is determined depending on the chain transfer constant of the chain transfer agent and the desired degree of polymerization of the vinyl ester polymer, but is generally preferably 0.1 to 10% by mass based on the total amount of monomers.

[0124] The vinyl ester polymer obtained as described above can be subjected to a saponification reaction to produce a PVA polymer (B). The method for saponifying the vinyl ester polymer is not particularly limited, and may be a conventionally known method. For example, alcoholysis or hydrolysis using a basic catalyst (e.g., alkali metal hydroxides such as sodium hydroxide and potassium hydroxide) or an acidic catalyst [e.g., inorganic acids (e.g., hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid), organic acids (e.g., carboxylic acids (e.g., fatty acids such as formic acid, acetic acid, and oxalic acid), sulfonic acids (e.g., p-toluenesulfonic acid)] can be used. The catalysts may be used alone or in combination of two or more.

[0125] Examples of the solvent used in the saponification reaction include, in addition to water, 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 can be used alone or in combination of two or more.

[0126] After the saponification reaction, a neutralizing agent may be used (or added) depending on the type of catalyst remaining. When a base catalyst is used, the neutralizing agent is an acidic substance [for example, an organic acid (e.g., a carboxylic acid such as formic acid, acetic acid, or propionic acid), or an inorganic acid (e.g., phosphoric acid, hydrogen phosphate, sulfuric acid, hydrochloric acid, or nitric acid)], or when an acid catalyst is used, a basic substance (or an alkaline substance, for example, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide) is used. The neutralizing agents may be used alone or in combination of two or more.

[0127] In particular, in the present invention, from the viewpoint of efficient saponification (saponification rate, etc.), it is preferable to use a basic catalyst in the saponification reaction, and it is preferable to use at least one selected from a carboxylic acid (a fatty acid such as acetic acid), phosphoric acid, and a hydrogen phosphate as the neutralizing agent.

[0128] The block character and the degree of saponification of the PVA polymer (B) are not particularly limited. However, the block character and the degree of saponification of the PVA polymer (A) may change from those of the raw material PVA polymer (B) during the subsequent esterification reaction or the like. In such a case, it is preferable to adjust the block character and the degree of saponification of the PVA polymer (B) in anticipation of such change.

[0129] Although there are no particular limitations on the viscosity of a 4% by mass aqueous solution or the degree of polymerization of the PVA polymer (B), it is preferable to adjust the viscosity of a 4% by mass aqueous solution or the degree of polymerization of the PVA polymer (A) to the target viscosity or degree of polymerization of the PVA polymer (A) because these values ​​can reflect the PVA polymer (B) used as a raw material during the subsequent esterification reaction, etc. Depending on the degree of saponification of the PVA polymer (B) (when the degree of saponification is low), the PVA polymer (B) may not dissolve in water, and in such cases, the degree of polymerization may be used as a guide.

[0130] The PVA-based polymer (B) may or may not contain sodium acetate. When sodium acetate is contained, the proportion of sodium acetate in the PVA-based polymer (B) may be, for example, about 0.001 mol % or more (e.g., 0.005 mol % or more), preferably about 0.03 mol % or more (e.g., 0.05 mol % or more), more preferably about 0.07 mol % or more (e.g., 0.08 mol % or more), or about 0.1 mol % or more (e.g., 0.12 mol % or more, 0.15 mol % or more, 0.18 mol % or more, 0.2 mol % or more, 0.22 mol % or more, or 0.25 mol % or more) per monomer unit (based on the total of vinyl alcohol units and other monomer units such as vinyl ester units).

[0131] The upper limit of the proportion of sodium acetate in the PVA-based polymer (B) per monomer unit (based on the total of vinyl alcohol units and other monomer units such as vinyl ester units) may be, for example, 10 mol % or less, 8 mol % or less, 6 mol % or less, 5 mol % or less, 4 mol % or less, 3 mol % or less, 2.5 mol % or less, 2.2 mol % or less, 2 mol % or less, 1.8 mol % or less, 1.5 mol % or less, 1.2 mol % or less, 1 mol % or less, 0.9 mol % or less, 0.8 mol % or less, 0.7 mol % or less, 0.6 mol % or less, 0.5 mol % or less, 0.4 mol % or less, or 0.3 mol % or less.

[0132] When sodium acetate is contained, the proportion of sodium acetate in the PVA-based polymer (B) may be, for example, about 0.001% by mass or more (e.g., 0.005% by mass or more), preferably about 0.03% by mass or more (e.g., 0.05% by mass or more), more preferably about 0.07% by mass or more (e.g., 0.08% by mass or more), or about 0.1% by mass or more (e.g., 0.12% by mass or more, 0.15% by mass or more, 0.18% by mass or more, 0.2% by mass or more, 0.22% by mass or more, 0.25% by mass or more).

[0133] The upper limit of the proportion of sodium acetate in the PVA-based polymer (B) may be, for example, 10% by mass or less, 8% by mass or less, 6% by mass or less, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2.5% by mass or less, 2.2% by mass or less, 2% by mass or less, 1.8% by mass or less, 1.5% by mass or less, 1.2% by mass or less, 1% by mass or less, 0.9% by mass or less, 0.8% by mass or less, 0.7% by mass or less, 0.6% by mass or less, 0.5% by mass or less, 0.4% by mass or less, or 0.3% by mass or less.

[0134] Such sodium acetate may be added or blended into the PVA-based polymer (B) in advance, or may be a by-product or residual product during the production process of the PVA-based polymer (B) (e.g., a saponification step), or a combination of these may be contained in the PVA-based polymer (B). As described below, sodium acetate can also function as a pH adjuster [basic pH adjuster (alkali component) in the reaction (esterification)]. When sodium acetate is used for such a purpose, it is sufficient that sodium acetate is present in the reaction; it is not necessary for sodium acetate to be contained in the PVA-based polymer (B). Depending on the amount of sodium acetate contained in the PVA-based polymer (B), the amount of sodium acetate in the reaction may be adjusted by, for example, adding or blending more sodium acetate in the reaction.

[0135] [Reaction (Esterification, etc.)] As described above, the PVA polymer (A) can be produced by reacting the PVA polymer (B) with a component corresponding to the ionic skeleton (ionic group).

[0136] In such a method, an ionic skeleton can usually be introduced via a vinyl alcohol unit (vinyl alcohol unit of the PVA-based polymer (B)). In particular, when a polycarboxylic acid component is used as a component corresponding to the ionic skeleton (ionic group), a PVA-based polymer (A) having an ionic skeleton (an ionic skeleton derived from a polycarboxylic acid component such as a carboxy group or a salt group thereof) can be produced by a reaction (esterification reaction) between the vinyl alcohol unit and the polycarboxylic acid component.

[0137] Such a reaction (a method for introducing an ionic group into the PVA polymer (B), an esterification reaction, an esterification method) is not particularly limited, but for example, the following methods can be used.

[0138] A specific example of the esterification method (esterification step) is a method that involves a heating step of heating (in a solid state or a molten state) a composition containing the PVA-based polymer (B) and a component corresponding to the ionic skeleton [for example, a polycarboxylic acid component such as di(tri)carboxylic acid and / or a derivative thereof] {a heating step of heating the polyvinyl alcohol-based polymer (B) in the presence of the component corresponding to the ionic skeleton [for example, a polycarboxylic acid component such as di(tri)carboxylic acid and / or a derivative thereof] (a heating step of heating the polyvinyl alcohol-based polymer (B) and the component corresponding to the ionic skeleton)}.

[0139] Representative examples of the method include: (a) a method in which a solution (homogeneous solution, solution-like composition, solvent composition) containing the PVA-based polymer (B), a component corresponding to the ionic skeleton [e.g., a polycarboxylic acid component such as di(tri)carboxylic acid and / or its derivatives], and optionally a pH adjuster is obtained, followed by removing (evaporating) the solvent from the solution and heating the composition from which the solvent has been removed to cause a reaction {esterification [esterification in a solid state (or solid composition) or a molten state (or a molten composition)] to obtain the PVA-based polymer (A)}; and (b) a method in which the PVA-based polymer (B), a component corresponding to the ionic skeleton [e.g., a polycarboxylic acid component such as di(tri)carboxylic acid and / or its derivatives], and optionally a pH adjuster are heated in a solid state (while at least the PVA-based polymer (B) is still in a solid state) without being converted into a solution, to cause a reaction {esterification [esterification in a solid state (or solid composition) or a molten state (or a molten composition), particularly in a molten state)]}.

[0140] Among these, from the viewpoint of efficiently satisfying the aforementioned requirements for hue, transparency, and water-insoluble content, it is preferable to select method (a) (and further select and adjust conditions such as heating temperature, heating time, type of solvent, use of a pH adjuster and its type, which will be described later). Such method (a) may include, for example, a solvent removal step of removing the solvent from a composition (solvent composition, solution-like composition, solution) containing the PVA polymer (B), a component corresponding to the ionic skeleton [for example, a polycarboxylic acid component such as di(tri)carboxylic acid and / or a derivative thereof], and a solvent, and the heating step may be performed after the solvent removal step (for example, after recovering the composition obtained in the solvent removal step, a separate heating step is performed on the composition) or in parallel or successively with the solvent removal step (for example, the composition obtained in the solvent removal step is heated directly (in parallel or successively) in the system in which the solvent removal step was performed).

[0141] In the method (a), the solution of the PVA polymer (B) may be obtained by (i) dissolving the PVA polymer (B) in a solvent, or by (ii) saponifying a vinyl ester polymer in a solvent such as methanol with a basic catalyst such as sodium hydroxide.

[0142] Representative methods (methods for carrying out esterification) include (i) a method in which the solvent is removed (e.g., evaporated) from a solution (homogeneous composition, solvent composition) to obtain a solid (solid-state) composition, and then this solid composition is heated under predetermined conditions; and (ii) a method in which the evaporation of the solvent and the heating of the composition are carried out in parallel or succession while the solution (homogeneous composition, solvent composition) is heated.

[0143] During heating, the composition (from which the solvent has been removed) becomes solid or molten depending on the heating conditions and the like. For example, the composition becomes solid when the heating temperature is lower than the melting point of the PVA-based polymer (B) (and further the resulting PVA-based polymer (A)), and becomes molten when the heating temperature is higher than the melting point.

[0144] The solvent is not particularly limited as long as it can dissolve the PVA polymer (B). However, since it is necessary to evaporate the solvent during the reaction (esterification reaction, etc.), the boiling point of the solvent under normal pressure is preferably 150° C. or lower, more preferably 130° C. or lower, and even more preferably 110° C. or lower, from the viewpoint of efficient evaporation (e.g., easy evaporation without excessive heating), since the solvent needs to be evaporated during the reaction (e.g., esterification reaction).

[0145] Specific examples of the solvent include, in addition to water, alcohols such as methanol and ethanol, esters such as methyl acetate and ethyl acetate, ethers such as diethyl ether and tetrahydrofuran, ketones such as acetone and methyl ethyl ketone, and aromatic compounds (e.g., aromatic hydrocarbons) such as benzene and toluene. However, it is preferable to use a single solvent or a mixed solvent of one or more selected from water, alcohols (e.g., methanol), and esters (e.g., methyl acetate), and in particular, a solvent containing methanol (at least methanol) may be preferably used.

[0146] The type of solvent and the ratio of the mixed solvent may be appropriately changed so that the PVA polymer (B) can be completely dissolved.

[0147] The concentration of the PVA polymer (B) in the solution is not particularly limited, and may be selected depending on the type or physical properties (such as the degree of saponification) of the PVA polymer (B). For example, the concentration of the PVA polymer (B) may usually be about 5 to 70 mass %, and when the degree of polymerization is about 500 or less, the concentration may be relatively high (for example, about 40 to 70 mass %). A higher concentration of the PVA polymer (B) is advantageous because it allows for a smaller amount of solvent to be evaporated in the reaction (such as esterification) step.

[0148] By carrying out a reaction (esterification, etc.) (a heating step) via such a solution (solvent composition), the reaction (esterification, etc.) can be carried out efficiently while suppressing reaction spots, and as a result, the above-mentioned YI value, transparency, water-insoluble content, etc. can be efficiently satisfied.

[0149] The component corresponding to the ionic skeleton is not particularly limited as long as it has a corresponding ionic group and can be introduced into the ionic group. Examples of the component corresponding to the ionic skeleton include polycarboxylic acid components (di(tri)carboxylic acid, derivatives of di(tri)carboxylic acid, etc.) as described above. Examples of di(tri)carboxylic acids include the components exemplified above, for example, aliphatic dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, malic acid, oxaloacetic acid, cis-1,2-cyclohexanedicarboxylic acid, cis-4-cyclohexene-1,2-dicarboxylic acid, 5-norbornene-2,3-dicarboxylic acid, and exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic acid; aliphatic tricarboxylic acids such as citric acid and aconitic acid; aromatic dicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid; and aromatic tricarboxylic acids such as trimellitic acid. Among these, succinic acid, maleic acid, citraconic acid, itaconic acid, phthalic acid, and citric acid are preferred. Examples of derivatives of di(tri)carboxylic acids include the components exemplified above, but for example, monoalkyl di(tri)carboxylates or salts thereof are preferred, and monomethyl di(tri)carboxylate or salts thereof are particularly preferred. Specifically, monomethyl succinate, monomethyl maleate, monomethyl citraconic acid, monomethyl itaconate, monomethyl phthalate, monomethyl citrate, or salts thereof (alkali metal salts) are preferred. Furthermore, as described above, carboxylic acid anhydrides such as succinic anhydride, maleic anhydride, citraconic anhydride, itaconic anhydride, phthalic anhydride, cis-4-cyclohexene-1,2-dicarboxylic anhydride, cis-1,2-cyclohexanedicarboxylic anhydride, 5-norbornene-2,3-dicarboxylic anhydride, and trimellitic anhydride can also be used as derivatives.

[0150] A component corresponding to the ionic skeleton [e.g., a polycarboxylic acid component such as di(tri)carboxylic acid and / or its derivative] can be added to the solution of the PVA polymer (B) either as is or in the form of a solution dissolved in a suitable solvent (e.g., water, methanol). Generally, when a carboxylic acid anhydride is dissolved in water, it is partially or completely converted into the corresponding polycarboxylic acid (e.g., dicarboxylic acid). Furthermore, when a carboxylic acid anhydride is dissolved in methanol, it is partially or completely converted into the corresponding polymonomethyl carboxylate (e.g., monomethyl dicarboxylate). In the present invention, a polycarboxylic acid (e.g., dicarboxylic acid) produced by the reaction of a carboxylic acid anhydride with water or a polycarboxylic acid ester (e.g., monomethyl dicarboxylate) produced by the reaction of a carboxylic acid anhydride with an alcohol (e.g., methanol) can also be used.

[0151] The amount of the component corresponding to the ionic skeleton [e.g., polycarboxylic acid component such as di(tri)carboxylic acid and / or its derivative] to be added depends on the amount of the component corresponding to the ionic skeleton [e.g., ionic group derived from polycarboxylic acid component such as di(tri)carboxylic acid and / or its derivative] introduced, but is, for example, preferably 0.1 part by mass or more, more preferably 0.3 part by mass or more, and more preferably 0.5 part by mass or more, relative to 100 parts by weight of the PVA-based polymer (B), and is preferably 50 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 10 parts by mass or less.

[0152] Although a pH adjuster (e.g., at least one selected from an acid component and an alkali component) is not essential, its presence in the reaction system, if necessary, can easily realize (1) improvement in reaction efficiency, (2) suppression of coloration of the PVA-based polymer (A), (3) improvement in the water solubility (dispersibility) of the PVA-based polymer (A), and (4) suppression of insolubilization of the PVA-based polymer (A), and can easily efficiently satisfy the above-mentioned YI value, transparency, water-insoluble content, etc.

[0153] For example, when the acidity of the reaction system is high (e.g., when the pH of a 4% by mass aqueous solution of the resulting PVA-based polymer (A) is low (e.g., 4.5 or less)), the water-insoluble content may increase, so a basic pH adjuster may be added. When the acidity of the reaction system is low (e.g., when the pH of a 4% by mass aqueous solution of the resulting PVA-based polymer (A) is relatively high (e.g., 6.5 or more, 7 or more, or more than 7)), the reaction efficiency may decrease and the color (YI) may deteriorate, so an acidic pH adjuster may be added. The pH adjuster may be selected depending on the component corresponding to the ionic skeleton. For example, a basic pH adjuster (alkali component) may be preferably used for a polycarboxylic acid component.

[0154] Examples of acidic pH adjusters (acid components) include organic acids (e.g., carboxylic acids such as formic acid, acetic acid, and propionic acid), inorganic acids (e.g., phosphoric acid, hydrogen phosphate, sulfuric acid, hydrochloric acid, and nitric acid). These pH adjusters may be used alone or in combination of two or more. Examples of basic pH adjusters (base components, alkaline components) include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, carbonates such as sodium carbonate and potassium carbonate, bicarbonates such as sodium bicarbonate and potassium bicarbonate, and organic acid salts such as sodium acetate. These pH adjusters may be used alone or in combination of two or more.

[0155] The amount of pH adjuster to be present in the reaction system may be selected, for example, depending on the type and amount of the component corresponding to the ionic skeleton [e.g., a polycarboxylic acid component such as a di(tri)carboxylic acid and / or a derivative thereof], and is particularly preferably selected depending on the amount of the component corresponding to the ionic skeleton [e.g., a polycarboxylic acid component such as a di(tri)carboxylic acid and / or a derivative thereof] [e.g., an amount equal to or less than the amount of the component (or ionic group) corresponding to the ionic skeleton]. For example, the pH adjuster [e.g., a basic pH adjuster (alkali component)] may be present in the reaction system in an amount of 0.3 to 3 moles (equivalent), preferably 0.5 to 1.9 moles (e.g., 0.55 to 1.8 moles, 0.8 to 1.5 moles), more preferably 0.9 to 1.3 moles (e.g., 0.95 to 1.2 moles), per mole of the component corresponding to the ionic skeleton (e.g., a polycarboxylic acid component).

[0156] As described above, when the PVA-based polymer (B) contains sodium acetate, the type and amount of a pH adjuster to be newly added may be adjusted depending on the amount of sodium acetate and the desired pH, taking the amount of sodium acetate into consideration.

[0157] As described above, method (a) includes a step (solvent removal step) of removing (by evaporation, etc.) the solvent from a composition (solvent composition, solution, homogeneous composition) containing a PVA-based polymer (B), a component corresponding to the ionic skeleton [e.g., a polycarboxylic acid component such as di(tri)carboxylic acid and / or a derivative thereof], and, if necessary, a pH adjuster. In such a solvent removal step, the method for removing the solvent is not particularly limited, but a method of evaporating the solvent is convenient. Examples of methods for evaporating the solvent (drying methods) include natural drying, heat drying, reduced pressure, and combinations thereof. When heat drying is performed, the drying temperature can be selected depending on the type of solvent, etc. As described above, the composition that has undergone the solvent removal step is subjected to a heating step after the solvent removal step or in parallel or successively to the solvent removal step.

[0158] The heating temperature (heating temperature in the heating step) is preferably 80° C. or higher, more preferably 100° C. or higher, more preferably 120° C. or higher, and may be 130° C. or higher. The heating temperature is preferably 240° C. or lower, more preferably 220° C. or lower, more preferably 200° C. or lower, and may be 190° C. or lower.

[0159] If the heating temperature is too low, the reaction (esterification, etc.) may not proceed sufficiently, whereas if the heating temperature is too high, it may be difficult to efficiently achieve the above-mentioned hue, transparency, and water-insoluble content (and further, the PVA polymer (A) may become insolubilized or become severely colored).

[0160] In the method (a), the PVA-based polymer (B) and a component corresponding to the ionic skeleton [for example, a polycarboxylic acid component such as di(tri)carboxylic acid and / or its derivative] are uniformly mixed in advance in a solution (solution state). Therefore, it is not necessary to raise the heating temperature to a temperature equal to or higher than the melting point of the PVA-based polymer (B). Even at a temperature equal to or lower than the melting point, the reaction (esterification, etc.) can proceed relatively uniformly. Consequently, it is easy to efficiently satisfy the above-mentioned YI value, transparency, water-insoluble content, etc.

[0161] The heating time is not particularly limited, but is preferably about 5 minutes or more, more preferably 7 minutes or more, and may be 10 minutes or more. It is also preferably 5 hours or less, more preferably 3 hours or less, more preferably 2 hours or less, and may be 1 hour or less. In the case of a method in which the evaporation of the solvent and the heating of the composition are performed in parallel or continuously, the product temperature is not equal to the heating temperature while the solvent is evaporating from the composition, and therefore this period (the drying time in the solvent removal step described above) is not included in the heating time.

[0162] If the heating time is too short, the reaction (esterification, etc.) may not proceed sufficiently, whereas if the heating time is too long, it may be difficult to efficiently achieve the above-mentioned hue, transparency, and water-insoluble content (and further, the PVA polymer (A) may become insolubilized or become severely colored).

[0163] As the heating device, a blower dryer or vacuum dryer with a temperature control function can be used in the case of a batch system, and a thin-film evaporation concentration device or the like can be used in the case of a continuous system.

[0164] [Uses, etc.] The PVA polymer (A) can be used for various uses. As described above, it can be suitably used particularly as a hot melt adhesive (e.g., a water-soluble hot melt adhesive) or a dispersant [or a dispersing aid, for example, a dispersant (dispersing aid) for polymerization (e.g., suspension polymerization)].

[0165] In these various applications, the PVA polymer (A) may be used alone or in combination of two or more thereof. For example, a PVA polymer (A) having a relatively high 4% by mass aqueous solution viscosity (degree of polymerization) or a relatively high degree of saponification may be used in combination with a PVA polymer (A) having a lower 4% by mass aqueous solution viscosity (degree of polymerization) or a lower degree of saponification.

[0166] As an example of such an application, the use of a dispersant (or PVA polymer (A), hereinafter the same) or a method for producing a vinyl polymer by polymerization (particularly suspension polymerization) of a vinyl monomer using the dispersant will be described below.

[0167] Suspension polymerization is a polymerization method in which an insoluble vinyl monomer and an oil-soluble polymerization initiator are added to an aqueous medium, and the mixture is stirred to form minute droplets containing the vinyl monomer, and polymerization is carried out in these droplets. The aqueous medium that can be used here is not particularly limited, and examples thereof include water, aqueous solutions containing various additives, and mixed solvents of water and an organic solvent that is compatible with water.

[0168] The PVA polymer (A) can be used as a dispersant in suspension polymerization of a vinyl monomer. The vinyl monomer is not particularly limited, but is preferably, for example, a vinyl monomer generally used in suspension polymerization, such as vinyl chloride, vinylidene chloride, styrene, acrylic acid ester, methacrylic acid ester, vinyl acetate, acrylonitrile, etc., and particularly preferably, a vinyl chloride monomer.

[0169] Examples of vinyl chloride monomers include vinyl chloride monomer (vinyl chloride) and mixtures of vinyl chloride monomer with other monomers copolymerizable therewith. Examples of other monomers copolymerizable with vinyl chloride monomer include vinylidene chloride, vinyl acetate, ethylene, propylene, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, styrene, vinylalkoxysilane, maleic acid, hydroxyalkyl acrylate, allyl sulfonic acid, vinyl sulfonic acid, and the like.

[0170] Therefore, the dispersant (PVA polymer (A)) is suitable for suspension polymerization of vinyl monomers including vinyl chloride monomers (particularly vinyl chloride), and can be particularly suitably used for homopolymerization of vinyl chloride by suspension polymerization. It can also be used for binary or multi-component copolymerization of vinyl chloride with one or more known monomers copolymerizable with vinyl chloride by suspension polymerization, and can be particularly suitably used as a dispersing aid in copolymerization of vinyl chloride with vinyl acetate by suspension polymerization.

[0171] A vinyl chloride resin can be obtained by suspension polymerization of vinyl monomers including vinyl chloride. In producing the vinyl chloride resin, it is preferable that vinyl chloride accounts for 50 to 100 mol % (or 50 to 100 mass %) of the total amount of vinyl monomers used.

[0172] The polymerization initiator in the suspension polymerization of vinyl monomers may also be a known one, and examples thereof include percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as benzoyl peroxide, t-butyl peroxyneodecanoate, α-cumyl peroxyneodecanoate, and t-butyl peroxydecanoate; peroxides such as acetylcyclohexylsulfonyl peroxide and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; azo compounds such as 2,2′-azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, and azobis(4-methoxy-2,4-dimethylvaleronitrile); benzoyl peroxide; and lauroyl peroxide. These may also be used in combination with potassium persulfate, ammonium persulfate, hydrogen peroxide, and the like.

[0173] The amount of the PVA polymer (A) (or dispersing aid) used is not particularly limited, but may be, for example, about 1 part by mass or less (e.g., 0.002 to 0.5 parts by mass, 0.005 to 0.2 parts by mass) relative to 100 parts by mass of the vinyl monomer.

[0174] The polymerization (suspension polymerization) may be carried out in the presence of a dispersant (another dispersant, a dispersion stabilizer). In other words, the PVA polymer (A) may be used in combination with a dispersant (another dispersant, a dispersion stabilizer) [or may be a dispersant for use (such as a dispersant for suspension polymerization)].

[0175] Examples of other dispersants include water-soluble polymers [e.g., cellulose derivatives (e.g., water-soluble cellulose ethers such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carboxymethyl cellulose), gelatin, polyvinyl alcohol polymers, polyvinylpyrrolidone, etc.], emulsifiers or surfactants [e.g., sorbitan monolaurate, sorbitan trioleate, glycerin tristearate, ethylene oxide propylene oxide block polymers, polyoxyethylene glycerin oleate, sodium laurate, etc.], etc. These other dispersants may be used alone or in combination of two or more.

[0176] Among these, PVA polymers (PVA polymers not belonging to the category of the PVA polymer (A)) may be preferably used.

[0177] The PVA-based polymer (e.g., polyvinyl alcohol) may be any PVA-based polymer that does not belong to the category of the PVA-based polymer (A). In general, the PVA-based polymer may be different (e.g., larger) from the PVA-based polymer (A) in terms of the degree of saponification, the viscosity of a 4% by mass aqueous solution (degree of polymerization), etc.

[0178] For example, the saponification degree of the PVA-based polymer may be, for example, 60 mol% or more, 70 mol% or more, 75 mol% or more, 65 to 90 mol%, etc. The polymerization degree of the PVA-based polymer may be 300 or more (e.g., 500 or more, 1000 or more, 2000 or more) or 10000 or less (e.g., 8000 or less, 5000 or less, 3000 or less).

[0179] The other dispersants may exhibit specific functions depending on the type of PVA polymer (A) to be combined, the type of monomer to be polymerized, etc. For example, the other dispersants (water-soluble polymers, etc.) function as dispersants (primary dispersants, other dispersing aids) for stabilizing the dispersibility of vinyl chloride monomers and adjusting the particle size of the vinyl chloride resin to be produced, and the PVA polymer (A) functions as a dispersant (secondary dispersant, dispersing aid) for increasing the porosity in the vinyl chloride resin.

[0180] In the suspension polymerization of a vinyl-based monomer, the amount of the other dispersant used is not particularly limited, but may be, for example, about 5 parts by mass or less (e.g., 0.005 to 1 part by mass, 0.01 to 0.2 part by mass) relative to 100 parts by mass of the vinyl-based monomer.

[0181] In addition to the polymerization initiator and other dispersants, various additives known in the suspension polymerization of vinyl compounds, such as chain transfer agents, polymerization inhibitors, pH adjusters, scale inhibitors, and crosslinking agents, may be used in combination.

[0182] The polymerization temperature in suspension polymerization is not limited and can be arbitrarily selected depending on the type of vinyl monomer used, the target polymerization degree of the polymer, the polymerization yield, etc., but is usually preferably 40 to 70° C. The polymerization time is also not particularly limited and may be appropriately set depending on the target polymerization yield, etc.

[0183] The present invention will be described in more detail and specifically below with reference to examples, but the present invention is not limited to these examples. In the following examples and comparative examples, "%" and "parts" mean "mass %" (or "wt %") and "parts by mass" (or "parts by weight") unless otherwise specified.

[0184] First, the methods for measuring the degree of polymerization, degree of saponification, YI, block character of residual vinyl ester units of PVA (A), transparency of a 4% aqueous solution, and water-insoluble content in the present examples are described below.

[0185] (Method for Measuring Degree of Polymerization) The measurement was carried out in accordance with the method specified in JIS K 6726.

[0186] (Method for measuring the degree of saponification) The measurement was carried out in accordance with the method specified in JIS K 6726.

[0187] (Method for Measuring YI) A 4% DMSO (dimethyl sulfoxide) solution of PVA (A) was placed in a quartz cell with a light path length of 10 mm, and its UV-Vis spectrum was measured at 20°C using an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation; V-730) (wavelength range of 360 to 830 nm, DMSO as blank). The YI of the 4% DMSO solution was calculated from the obtained data. For the calculation, the color system "Lab," light source "D65," color matching function "JIS Z8701:1999," viewing angle "2 degrees," wavelength range "360 to 830 nm," and data interval "5 nm" were selected.

[0188] (Method of measuring block character) First, PVA / D ２ A mixed solution of 2000kJ / MeOH (methanol) (each composition is changed depending on the degree of polymerization and saponification of PVA) was analyzed using a 400 MHz nuclear magnetic resonance spectrometer manufactured by Bruker. １３ C-NMR measurement (measurement temperature = 50°C) was carried out, and the integral values ​​A to C of the following signals were determined. - Integrated value A = signals from 37.2 to 40.4 ppm; assigned to (OCOR, OCOR) - Integrated value B = signals from 40.4 to 41.8 ppm; assigned to (OH, OCOR) - Integrated value C = signals from 41.8 to 44.9 ppm; assigned to (OH, OH) Next, using the integral values ​​obtained above, the block character (η) was calculated according to the following formula: - Block character (η) = (OH, OCOR) / [2(OH)(OCOR)] where (OH, OCOR), (OH), and (OCOR) are calculated as follows. (OH, OCOR) = B (OH) = (OH, OH) + (OH, OCOR) / 2 (OCOR) = (OCOR, OCOR) + (OH, OCOR) / 2 Note that (OH, OH) and (OCOR, OCOR) are as follows: (OH, OH) = C (OCOR, OCOR) = A

[0189] (Method for measuring the transparency of a 4% aqueous solution of PVA) A 4% aqueous solution of PVA (A) was placed in a quartz cell with an optical path length of 20 mm, and its transmittance at 430 nm was measured using an ultraviolet-visible spectrophotometer (V-730, manufactured by JASCO Corporation) at 20° C. Pure water was used as a blank.

[0190] (Method for measuring the water-insoluble fraction of PVA) 10 g of PVA (A) was dissolved in 500 g of water at 50°C with stirring for 1 hour to prepare an aqueous solution of PVA (A). The entire amount of the aqueous solution of PVA (A) was passed through a 100-mesh wire mesh whose weight had been measured in advance, and the water-insoluble components were captured on the wire mesh. The wire mesh capturing the water-insoluble components was dried at 110°C, and the weight of the dried wire mesh was measured. The water-insoluble fraction [%] was calculated as follows: = (B - A) / 10 x 100. Here, A is the weight [g] of the wire mesh before filtration, and B is the weight [g] of the wire mesh after filtration.

[0191] (Sodium Acetate Content) Sodium acetate was dissolved in d6-DMSO and analyzed using a 400 MHz nuclear magnetic resonance spectrometer manufactured by Bruker. １ H-NMR measurement was performed, and the content of sodium acetate was calculated using the signal derived from sodium acetate.

[0192] (pH) pH (20°C) was measured according to the method specified in JIS K 6726.

[0193] Example 1 Synthesis of PVA Polymer (B) 2.3 parts of a 3% methanol solution of sodium hydroxide (0.3 mol per 100 mol of polyvinyl acetate monomer units) and 1 part of water were added to 100 parts of a 50% methanol solution of polyvinyl acetate having a degree of polymerization of 250, and the mixture was thoroughly mixed. A saponification reaction was carried out at 40°C for 20 minutes. After saponification, 0.7 parts of a 5% aqueous acetic acid solution (0.1 mol per 100 mol of polyvinyl acetate monomer units) was added to terminate the saponification reaction. The resulting reaction solution was homogeneous. The resulting solution was dried at 60°C for 1 hour under reduced pressure of 100 Pa to obtain PVA (B). Analysis revealed a saponification degree of 35 mol%, an average degree of polymerization of 250, and a block character of 0.44. PVA (B) was completely insoluble in water. The PVA (B) contained 0.3 moles of sodium acetate (0.43 mass % in the PVB (B)) relative to 100 moles of polyvinyl acetate monomer units.

[0194] (Synthesis of PVA-based polymer (A)) 100 parts of the PVA (B) obtained above was dissolved in 100 parts of methanol to prepare a homogeneous solution. To this solution, 7.6 parts of a 20% methanol solution of monomethyl phthalate (0.60 mol per 100 mol of monomer units of the PVA-based polymer (B)) and 3.5 parts of a 10% methanol solution of sodium acetate (0.30 mol per 100 mol of monomer units of the PVA-based polymer (B)) as a pH adjuster were added and thoroughly mixed. This homogeneous solution was dried at 60°C for 1 hour under reduced pressure of 100 Pa (to evaporate the solvent) to obtain a solid. The amount of sodium acetate (total amount, charged amount) per mole of monomethyl phthalate used (charged amount) was 1 mole.

[0195] This solid was heated at 150°C for 30 minutes under a reduced pressure of 100 Pa to obtain PVA (A). Analysis of PVA (A) showed that the saponification degree was 35 mol%, the average polymerization degree was 250, and the block character was 0.46. The amount of phthalic acid units introduced was １ H-NMR analysis revealed that the content of sodium acetate per monomer unit of PVA (A) was 0.5 mol % (corresponding to 1.4 mol % of vinyl alcohol units). PVA (A) exhibited water solubility, with a 4% transparency of 83%. The proportion of water-insoluble components was less than 0.1%, and the YI of a 4% by mass DMSO solution was less than 1. Furthermore, the pH (20°C) of a 4% by mass aqueous solution of PVA (A) was 6.1, and it was substantially free of sodium acetate.

[0196] Examples 2 to 23, Reference Example 1 PVA polymers (A) shown in Table 1 were synthesized in the same manner as in Example 1, except that the degree of polymerization and degree of saponification of PVA (B), the content of sodium acetate, the concentration and solvent composition of the PVA (B) solution, the type and amount of di(tri)carboxylic acid or its derivative, the amount of sodium acetate added, the heating conditions during synthesis of PVA (A), etc. were appropriately changed. Note that the PVA (A) obtained in Reference Example 1 had very low transparency in aqueous solution, and its DMSO solution was also quite colored. It was clear that the transparency of a 4% by mass aqueous solution was quite low and the YI of the 4% by mass DMSO solution was quite large, so these values ​​were not measured.

[0197] [Examples 24 and 25] In Example 22, PVA (B) was used without being dissolved (as a solid) to obtain PVA (A). Specifically, a powder mixture of PVA (B), phthalic acid, and sodium acetate (no sodium acetate was added in Comparative Example 3) shown in Table 1 was supplied to a polymer extrusion molding device (Mini Lab III manufactured by Thermo SCIENTIFIC) and heated and mixed under the following conditions to obtain PVA (A). Screw rotation speed: 100 rpm Heating (mixing) temperature: 150°C Heating (mixing) time: 30 minutes

[0198] The results are summarized in Table 1.

[0199]

[0200] Polymerization Example 1 (Suspension Polymerization of Vinyl Chloride) Using the PVA polymer (A) obtained in Example 1 above as a dispersing aid, suspension polymerization of vinyl chloride was carried out under the following conditions. A 100-liter polymerization reactor (pressure-resistant autoclave) was charged with 0.05 parts by mass of partially saponified polyvinyl alcohol (saponification degree 80 mol %, polymerization degree 2500) and 0.02 parts by mass of partially saponified polyvinyl alcohol (saponification degree 72 mol %, polymerization degree 800) dissolved in 100 parts by mass of deionized water. Furthermore, 0.75 parts by mass (0.03 parts by mass of PVA polymer (A)) of the aqueous liquid obtained in Example 1 above (PVA polymer (A) content: 4% by weight) was charged, and 0.05 parts by mass of t-butyl peroxyneodecaate was also charged. Next, the polymerization reactor was degassed to 40 mmHg, and then 100 parts by mass of vinyl chloride monomer was charged to initiate polymerization. The polymerization temperature was set to 57°C and maintained at this temperature until the end of the polymerization. The reaction was terminated when the polymerization conversion rate reached 80%, and the unreacted monomers in the polymerizer were recovered. The polymer slurry was then removed from the system and dehydrated and dried to obtain a vinyl chloride resin. The vinyl chloride resin was evaluated as follows:

[0201] (Evaluation of Vinyl Chloride Resin) The obtained vinyl chloride resin was evaluated for average particle size, plasticizer absorption, and sheet colorability as follows.

[0202] <Average particle size> The particle size distribution was measured using a low tap type vibrating sieve (using a JIS sieve) to determine the average particle size.

[0203] <Plasticizer Absorption> The obtained resin was placed in a cylindrical container with glass fiber packed at the bottom, and excess dioctyl phthalate (hereinafter abbreviated as DOP) was added. The resin was left for 30 minutes to allow the DOP to penetrate the resin. The resin was then centrifuged at 3,000 rpm to remove excess DOP, and the weight of the resin was measured to calculate the DOP absorption per 100 parts of polymer. A larger DOP absorption indicates better plasticizer absorption and superior molding processability.

[0204] <Evaluation of Sheet Colorability of Vinyl Chloride Polymer> 100 parts by weight of a vinyl chloride polymer, 50 parts by weight of di-2-ethylhexyl phthalate, 0.8 parts by weight of a dioctyltin mercapto stabilizer, 0.1 parts by weight of a polyethylene lubricant, and 0.8 parts by weight of a calcium zinc one-pack stabilizer were blended, and the mixture was kneaded with a test roll at 160°C for 5 minutes to form a sheet with a thickness of 0.45 mm. A plurality of the above sheets were stacked and pressed at 160°C for 5 minutes to prepare a test piece measuring 40 mm x 40 mm x 15 mm (thickness). The YI of this test piece was measured using a color difference meter.

[0205] Polymerization Examples 2 to 7 Suspension polymerization of vinyl chloride was carried out in the same manner as in Polymerization Example 1, except that the PVA (A) obtained in Examples 2 to 5, 22, and 23 was used, respectively, instead of the PVA (A) obtained in Example 1. The evaluation results of the obtained vinyl chloride resins are shown in Table 2.

[0206]

[0207] [Application to Hot Melt Adhesive] The PVA (A) obtained in Example 1 was used as a hot melt adhesive to evaluate adhesive strength and water solubility as follows. ２ The coating amount of PVA (A) on the kraft paper is 20 g / m ２ After coating at a coating temperature of 150°C, another piece of kraft paper was placed on top and the pressure was increased to 2 kg / cm ２The pieces were thermocompressed by applying a load of 135°C for 5 seconds. After leaving the pieces at 20°C and 65% RH for 24 hours, the adhesion state was evaluated by 180° peeling. As a result, no interfacial or cohesive failure was observed, and it was confirmed that material failure had occurred, indicating that the adhesive joint had sufficient strength. Furthermore, it was confirmed that the adhesive joint peeled off when the bonded paper pieces were placed in water at 30°C and gently stirred. Since the adhesive can be easily removed when recycling waste paper, it is thought that this can increase the efficiency of recycling.

[0208] The present invention can provide a specific polyvinyl alcohol-based polymer, which can be suitably used as a hot melt adhesive, a dispersant (dispersion aid), etc.

Claims

1. A polyvinyl alcohol polymer (A) having an ionic skeleton and satisfying the following requirement 1. Requirement 1: Water-insoluble content is 30% by mass or less 2. A polyvinyl alcohol polymer (A) having an ionic skeleton and satisfying the following requirement 2: Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more.

3. A polyvinyl alcohol polymer (A) having an ionic skeleton and satisfying the following requirement 3. Requirement 3: YI value of 4% by mass dimethyl sulfoxide solution is 30 or less 4. A polyvinyl alcohol polymer (A) having an ionic skeleton and satisfying the following requirements 1 and 2: Requirement 1: The water-insoluble content is 30% by mass or less. Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more.

5. A polyvinyl alcohol polymer (A) having an ionic skeleton and satisfying the following requirements 1 and / or 2 and 3: Requirement 1: The water-insoluble content is 30% by mass or less. Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more. Requirement 3: The YI value of a 4% by mass dimethyl sulfoxide solution is 30 or less.

6. A polyvinyl alcohol polymer (A) having an ionic skeleton and satisfying the following requirements 1, 2 and 3: Requirement 1: The water-insoluble content is 30% by mass or less. Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more. Requirement 3: The YI value of a 4% by mass dimethyl sulfoxide solution is 30 or less.

7. The polyvinyl alcohol polymer (A) according to any one of claims 1 and 4 to 6, wherein the water-insoluble content is 10% by mass or less in requirement 1.

8. The polyvinyl alcohol polymer (A) according to any one of claims 2 and 4 to 6, wherein in requirement 2, the transparency (430 nm) of a 4% by mass aqueous solution is 2% or more.

9. A polyvinyl alcohol polymer (A) according to any one of claims 3, 5 and 6, wherein, in requirement 3, the YI value of a 4% by mass dimethyl sulfoxide solution is 10 or less.

10. The polyvinyl alcohol polymer (A) according to claim 5 or 6, wherein, in Requirement 1, the water-insoluble content is 5% by mass or less; in Requirement 2, the transparency (430 nm) of a 4% by mass aqueous solution is 10% or more; and in Requirement 3, the YI value of a 4% by mass dimethyl sulfoxide solution is 5 or less.

11. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, wherein the ionic skeleton has at least one ionic group selected from a carboxyl group and a salt thereof.

12. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, wherein the ionic skeleton contains a skeleton derived from a polycarboxylic acid component.

13. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, wherein the ionic skeleton comprises a skeleton derived from at least one polycarboxylic acid component selected from a dicarboxylic acid component and a tricarboxylic acid component.

14. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, wherein the ionic skeleton comprises a skeleton introduced via a hydroxyl group of a vinyl alcohol unit.

15. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, wherein the ionic skeleton comprises a skeleton derived from an ester bond between a hydroxyl group of a vinyl alcohol unit and a carboxyl group of a polycarboxylic acid component.

16. The polyvinyl alcohol-based polymer (A) according to any one of claims 1 to 6, wherein the ionic skeleton comprises a skeleton derived from an ester bond between a hydroxyl group of a vinyl alcohol unit and a carboxy group of a polycarboxylic acid component, and the polycarboxylic acid component comprises at least one selected from the group consisting of an aliphatic dicarboxylic acid, an aromatic dicarboxylic acid, an aliphatic tricarboxylic acid, an aromatic tricarboxylic acid, an acid anhydride thereof, an ester thereof, and a salt thereof.

17. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, wherein the content of the ionic skeleton is 0.01 to 10 mol% per monomer unit.

18. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, having a block character of 0.35 to 0.

70.

19. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, having a degree of saponification of 20 to 90 mol %.

20. A polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, having a degree of saponification of 70 mol % or less.

21. The polyvinyl alcohol polymer (A) according to any one of claims 1 to 6, having a degree of polymerization of 120 to 3,000.

22. The polyvinyl alcohol-based polymer (A) according to any one of claims 1 to 6, wherein a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer (A) has a pH of 5 to 9.

23. The polyvinyl alcohol-based polymer (A) according to any one of claims 1 to 6, wherein a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer (A) has a pH of 5.5 to 8.

5.

24. A method for producing a polyvinyl alcohol-based polymer (A) having an ionic skeleton, comprising a heating step of heating a composition containing a polyvinyl alcohol-based polymer (B) and a component corresponding to the ionic skeleton.

25. The method of claim 24, wherein the component corresponding to the ionic backbone comprises a polycarboxylic acid component.

26. The method according to claim 24, wherein the component corresponding to the ionic skeleton comprises at least one selected from the group consisting of dicarboxylic acids, tricarboxylic acids, anhydrides thereof, partial esters thereof, and salts thereof.

27. The method of claim 24, wherein the composition includes a pH adjuster.

28. The method of claim 24, wherein the composition comprises sodium acetate.

29. The method of claim 24, wherein the composition contains a pH adjuster in a ratio of 0.5 to 1.9 moles per mole of the component corresponding to the ionic framework.

30. The method of claim 24, wherein the heating temperature in the heating step is 80 to 240°C.

31. The method of claim 24, wherein the heating time in the heating step is 5 minutes to 5 hours.

32. The method of claim 24, further comprising a solvent removal step of removing the solvent from a composition containing a polyvinyl alcohol polymer (B), a component corresponding to an ionic skeleton, and a solvent, and a heating step is carried out after the solvent removal step or in parallel with or consecutive to the solvent removal step.

33. The process of claim 32, wherein the solvent comprises at least one selected from water, alcohols, and esters.

34. The process of claim 32, wherein the solvent comprises methanol.

35. The method of claim 24, wherein the polyvinyl alcohol polymer (A) satisfies at least one requirement selected from the following requirements 1, 2, and 3: Requirement 1: The water-insoluble content is 30% by mass or less; Requirement 2: The transparency (430 nm) of a 4% by mass aqueous solution is 1% or more; and Requirement 3: The YI value of a 4% by mass dimethyl sulfoxide solution is 30 or less.

36. A hot melt adhesive comprising the polyvinyl alcohol polymer (A) according to any one of claims 1 to 6.

37. A dispersant for suspension polymerization, comprising the polyvinyl alcohol polymer (A) according to any one of claims 1 to 6.

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