Polyvinyl alcohol, its preparation method and its uses

Abstract

The present invention relates to polyvinyl alcohol, a preparation method thereof, and uses thereof. The polyvinyl alcohol has a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity-average degree of polymerization of 400 or more and less than 1800, contains 0.05 mol% or more and less than 0.5 mol% of aldehyde groups at the ends, and has an absorbance of 0.17 or more and less than 0.55 at 280 nm in a 0.1% by mass aqueous solution. By using the polyvinyl alcohol as a dispersion stabilizer for suspension polymerization of vinyl compounds, the formation of coarse particles in the resulting vinyl resin can be suppressed, particles of uniform diameter can be obtained, and the formation of fisheyes can be suppressed. Furthermore, by using the polyvinyl alcohol, a cross-linked product with excellent water resistance can be easily obtained.

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol having a saponification degree and a viscosity-average degree of polymerization within specific ranges, containing a specific amount of aldehyde groups at the terminals, and exhibiting a specific absorbance. The present invention also relates to a cross-linked product using the polyvinyl alcohol, a dispersion stabilizer for suspension polymerization of a vinyl compound, and a method for producing a vinyl resin. Background Art

[0002] Polyvinyl alcohol (PVA) with reactive functional groups (hereinafter sometimes referred to as "PVA") has long been used in a variety of products, including adhesives, paper coatings, polarizing films, and dispersion stabilizers for suspension polymerization of vinyl compounds (e.g., vinyl chloride). In particular, crosslinked polymers synthesized by crosslinking reactions between reactive sites and crosslinking agents have three-dimensional constraints on the movement of their molecular chains. Therefore, compared to linear polymers of the same type, they generally have superior strength, heat resistance, and solvent resistance, particularly water resistance, resulting in their greater usefulness. Another major use of PVA is as a dispersion stabilizer for suspension polymerization of vinyl compounds, and various PVAs are used.

[0003] In particular, examples of modified PVA having a reactive functional group with high crosslinking performance include modified PVA having a structure in which methylene hydrogen is sandwiched between two carbonyl groups, typified by an acetoacetyl group (Patent Document 1).

[0004] Furthermore, Patent Documents 2 and 3 disclose the use of heat-treated PVA as a dispersion stabilizer for suspension polymerization of vinyl compounds in order to improve polymerization stability during polymerization of the vinyl compound and suppress coarsening of the resulting vinyl polymer. It should be noted that, in this specification, polymerization stability refers to the ability to suppress coarsening and obtain vinyl polymer particles of uniform diameter due to the good dispersibility of droplets containing the vinyl compound during polymerization.

[0005] Prior art literature

[0006] [Patent Document]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-205826

[0008] [Patent Document 2] Japanese Patent Application Laid-Open No. 51-45189

[0009] [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-250695. Summary of the Invention

[0010] Problems to be solved by the invention

[0011] However, the diketene used in Patent Document 1 is highly toxic to living organisms, and there is a risk of explosion when its vapor mixes with air. Therefore, there is a need for a safer method for synthesizing modified PVA having reactive functional groups. Furthermore, Patent Document 1 requires the use of a special crosslinking agent, making it difficult to easily obtain a crosslinked product with excellent water resistance using, for example, an acid.

[0012] Furthermore, when using the modified PVA described in Patent Documents 2 or 3 as a dispersion stabilizer for suspension polymerization, while polymerization stability during polymerization of vinyl compounds is improved, the resulting vinyl polymers contain a high amount of fine powder, which is insufficient for the requirements expected in recent years. Furthermore, problems remain, such as the high number of fish eyes caused by pitting and defects when the vinyl polymers are formed into sheets.

[0013] The present invention is made to solve the above-mentioned problems. Its purpose is to provide a PVA that, when used as a dispersion stabilizer for suspension polymerization of a vinyl compound, can suppress the formation of coarse particles in the resulting vinyl resin, produce particles with uniform diameters, and suppress the formation of fisheyes. Another object of the present invention is to provide a PVA that can easily produce a cross-linked product with excellent water resistance.

[0014] [Solutions to Solve the Problem]

[0015] The present inventors have discovered that PVA having a saponification degree and a viscosity-average polymerization degree within specific ranges, a specific amount of aldehyde groups at the terminals, and a specific absorbance solves the above-mentioned problems, thereby completing the present invention.

[0016] That is, the above-mentioned problems are solved by providing a polyvinyl alcohol having a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity-average degree of polymerization of 400 or more and less than 1800, containing 0.05 mol% or more and less than 0.5 mol% of aldehyde groups at the terminals, and having an absorbance of 0.17 or more and less than 0.55 at 280 nm in a 0.1% by mass aqueous solution.

[0017] In this case, it is preferred that the terminal has a structure represented by the following formula (1).

[0018] [Chemical Formula 1]

[0019]

[0020] (In formula (1), X represents a single bond, an alkylene group which may have a substituent, or an arylene group which may have a substituent, and * represents a connecting bond.)

[0021] Furthermore, X is preferably an alkylene group having 1 to 6 carbon atoms.

[0022] In this case, a suitable method for producing the polyvinyl alcohol is to polymerize a vinyl ester in the presence of a dialdehyde or a trialdehyde to obtain a polyvinyl ester, and then saponify the polyvinyl ester.

[0023] Furthermore, a dispersion stabilizer for suspension polymerization of a vinyl compound containing the above-mentioned polyvinyl alcohol is a suitable embodiment of the present invention. A method for producing a vinyl resin by suspension polymerization of a vinyl compound in the presence of the above-mentioned polyvinyl alcohol is also a suitable embodiment of the present invention.

[0024] Furthermore, a cross-linked product of the polyvinyl alcohol described above, wherein a 100 μm thick film containing the cross-linked product exhibits a dissolution rate of less than 10% when immersed in 80°C hot water for 1 hour, is a suitable embodiment of the present invention. In this case, the polyvinyl alcohol is cross-linked in the presence of an acid catalyst, which is a suitable method for producing the cross-linked product.

[0025] Effects of the Invention

[0026] When the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, it exhibits high polymerization stability, thereby suppressing the formation of coarse particles in the resulting vinyl resin, simultaneously obtaining particles with uniform diameters, and suppressing the formation of fisheyes. Furthermore, by using the PVA of the present invention, a cross-linked product with excellent water resistance can be easily obtained. DETAILED DESCRIPTION

[0027] [Polyvinyl alcohol]

[0028] The polyvinyl alcohol of the present invention is characterized by having a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity-average degree of polymerization of 400 or more and less than 1800, containing 0.05 mol% or more and less than 0.5 mol% of aldehyde groups at the terminals, and an absorbance at 280 nm of a 0.1 mass % aqueous solution of the polyvinyl alcohol of the present invention of 0.17 or more and less than 0.55.

[0029] It is important that PVA have a saponification degree of 70 mol% or more and less than 99.9 mol%. Using PVA with a saponification degree of less than 70 mol% as a raw material for a cross-linked product can reduce the water resistance of the resulting cross-linked product. When used as a raw material for a cross-linked product, the saponification degree of PVA is preferably 80 mol% or more, more preferably 90 mol% or more. On the other hand, PVA with a saponification degree of 99.9 mol% or more is difficult to produce.

[0030] In addition, when PVA with a saponification degree of less than 70 mol% is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, polymerization stability decreases, coarse particles increase in the resulting vinyl resin, and particles with uniform diameters cannot be obtained. In addition, many fish eyes appear in the resulting vinyl resin. On the other hand, PVA with a saponification degree of 99.9 mol% or more is difficult to prepare. When used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the saponification degree of PVA is preferably less than 90 mol%, more preferably less than 85 mol%, and even more preferably less than 80 mol%. The saponification degree is a value measured in accordance with JIS K6726:1994.

[0031] It is important that PVA have a viscosity-average degree of polymerization (hereinafter sometimes referred to as "DP") of 400 or more and less than 1800. Using PVA with a DP of less than 400 as a raw material for a cross-linked product reduces the water resistance of the resulting cross-linked product. When used as a raw material for a cross-linked product, the DP of PVA is preferably 550 or more. On the other hand, a DP of 1800 or more reduces the productivity of the PVA. Furthermore, using PVA with a DP of 1800 or more as a raw material for a cross-linked product results in excessively high liquid viscosity, impairing handling. The DP of PVA is preferably less than 1600.

[0032] In addition, when using PVA with a degree of polymerization of less than 400 as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization stability decreases, coarse particles increase in the resulting vinyl resin, and particles with uniform diameters cannot be obtained. The degree of polymerization of PVA is preferably 550 or more. On the other hand, when the degree of polymerization is 1800 or more, the productivity of PVA decreases. In addition, when using PVA with a degree of polymerization of more than 1800 as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization stability decreases, coarse particles increase in the resulting vinyl resin, and particles with uniform diameters cannot be obtained. In addition, many fish eyes are generated on the resulting vinyl resin. When used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the degree of polymerization of PVA is preferably less than 1500, more preferably less than 1300, and further preferably less than 1000.

[0033] The viscosity-average degree of polymerization is a value measured in accordance with JIS K 6726:1994. Specifically, when the degree of saponification is less than 99.5 mol %, the viscosity-average degree of polymerization (P) is determined by the following formula using the intrinsic viscosity [η] (L / g) measured in water at 30°C for PVA saponified to a degree of saponification of 99.5 mol % or higher.

[0034]

[0035] The PVA of the present invention contains an aldehyde group at the end that is more than 0.05 mol % and less than 0.5 mol %, which is important. The aldehyde group is a reactive functional group that can be used for crosslinking or free radical reaction produced by utilizing a crosslinking reaction or free radical reaction with a hydroxyl group. In addition, the PVA with an aldehyde group has a high adsorption capacity for vinyl compounds, so when used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization is stable, and the formation of coarse particles and fine powder in the resulting vinyl resin decreases. Further, when the resulting vinyl resin is formed into a sheet, the generation of fish eyes can also be reduced.

[0036] When PVA with an aldehyde group content of less than 0.05 mol% is used as a raw material for a cross-linked product, the water resistance of the resulting cross-linked product is reduced. The aldehyde group content is preferably 0.08 mol% or greater. On the other hand, PVA with an aldehyde group content of 0.5 mol% or greater has low productivity. In addition, when such PVA is used as a raw material for a cross-linked product, the viscosity of the liquid becomes excessively high, reducing handleability. The aldehyde group content is preferably less than 0.45 mol%, more preferably less than 0.4 mol%.

[0037] In addition, when a PVA having an aldehyde group content of less than 0.05 mol% is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is reduced, and the number of coarse particles in the resulting vinyl resin increases, while particles with uniform diameters cannot be obtained. In addition, many fish eyes are generated on the resulting vinyl resin. The aldehyde group content is preferably 0.08 mol% or more. On the other hand, the productivity of PVA having an aldehyde group content of more than 0.5 mol% is low. In addition, when such a PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is reduced, and the number of coarse particles in the resulting vinyl resin increases, while particles with uniform diameters cannot be obtained. In addition, many fish eyes are generated on the resulting vinyl resin. The aldehyde group content is preferably less than 0.45 mol%, more preferably less than 0.4 mol%. The aldehyde group content can be determined by the concentration of the vinyl ester polymer in a deuterated chloroform solvent before saponification. 1 H-NMR spectra, PVA in deuterated DMSO or heavy water solvents 1 H-NMR spectrum was obtained.

[0038] The PVA of the present invention may contain functional groups other than aldehyde groups, but the content thereof is preferably less than 5 mol %, more preferably less than 1 mol %, and further preferably less than 0.1 mol %.

[0039] It is important that the absorbance at 280 nm of a 0.1% by mass aqueous solution of PVA is greater than 0.17 and less than 0.55. When a PVA having an absorbance of less than 0.17 is used as a raw material for a cross-linked product, the water resistance of the resulting cross-linked product is reduced. The absorbance is preferably greater than 0.21, more preferably greater than 0.24, and even more preferably greater than 0.28. On the other hand, the productivity of PVA having an absorbance of greater than 0.55 is low. In addition, when such a PVA is used as a raw material for a cross-linked product, the viscosity of the liquid becomes too high, and the handleability is reduced. The absorbance is preferably less than 0.52, more preferably less than 0.48, and even more preferably less than 0.45.

[0040] In addition, when the PVA having an absorbance of less than 0.17 is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization stability is reduced, the amount of coarse particles in the resulting vinyl resin increases, and particles with uniform diameters cannot be obtained. In addition, many fish eyes are generated in the resulting vinyl resin. The absorbance is preferably 0.21 or higher, more preferably 0.24 or higher, and further preferably 0.28 or higher. On the other hand, the productivity of the PVA having an absorbance of 0.55 or higher is low. In addition, when such a PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization stability is reduced, the amount of coarse particles in the resulting vinyl resin increases, and particles with uniform diameters cannot be obtained. In addition, many fish eyes are generated in the resulting vinyl resin. The absorbance is preferably less than 0.52, more preferably less than 0.48, and further preferably less than 0.45.

[0041] The above absorbance indicates the amount or number of olefinic double bonds present in the PVA main chain. Maintaining the absorbance within the above range improves PVA's adsorption to vinyl compounds and, through a synergistic effect with the terminal aldehyde groups of PVA, further enhances polymerization stability when used as a dispersion stabilizer for suspension polymerization of vinyl compounds. Using PVA with an absorbance within the above range as a raw material for crosslinked products improves the water resistance of the resulting crosslinked products. The absorption at a wavelength of 280 nm originates from the [-CO-(CH=CH)2-] structure in PVA. This structure can be introduced by using aldehydes as modifiers or monomers that can introduce olefinic double bonds into the main chain through copolymerization. If the absorbance is outside the above range, PVA's adsorption to vinyl compounds decreases, or PVA and the vinyl compounds become dissolved. Consequently, the effectiveness of the PVA as a dispersion stabilizer for suspension polymerization is lost. Furthermore, using PVA with an absorbance outside the above range as a raw material for crosslinked products reduces the water resistance of the resulting crosslinked products. The conditions for the absorbance measurement apparatus and the like are as described in the following Examples.

[0042] The PVA of the present invention preferably has a structure represented by the following formula (1) at its terminal.

[0043] [Chemical Formula 2]

[0044]

[0045] (In formula (1), X represents a single bond, an alkylene group which may have a substituent, or an arylene group which may have a substituent, and * represents a connecting bond.)

[0046] In formula (1), X is a single bond, an alkylene group which may have a substituent, or an arylene group which may have a substituent. The number of carbon atoms in X is preferably 0 to 8, more preferably 1 to 6, further preferably 1 to 4, particularly preferably 2 to 4, and most preferably 3 to 4. X is preferably an alkylene group or an arylene group having the aforementioned number of carbon atoms, more preferably an alkylene group having the aforementioned number of carbon atoms. When X satisfies the above conditions, there is a tendency that the aldehyde group of PVA is easily introduced, the resulting PVA has good water solubility, the polymerization stability is good when PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, and the formation of fish eyes in the resulting vinyl resin can be suppressed.

[0047] Examples of the substituent that the alkylene group or arylene group may have include an alkyl group, an aryl group, a hydroxyl group, an aldehyde group, a carboxyl group, and an amino group.

[0048] (Preparation method of PVA)

[0049] The method for producing the PVA of the present invention is not particularly limited. A suitable production method is a method of obtaining polyvinyl ester by polymerizing vinyl ester in the presence of dialdehyde or trialdehyde, and then saponifying the polyvinyl ester.

[0050] As the polymerization method, known methods such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, and dispersion polymerization can be enumerated. From an industrial point of view, solution polymerization, emulsion polymerization, and dispersion polymerization are preferred. During the polymerization operation, any of a batch process, a semi-batch process, and a continuous process can also be used.

[0051] Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl octanoate, and vinyl versatate. Among them, vinyl acetate is preferred from an industrial viewpoint.

[0052] The type of dialdehyde or trialdehyde is not particularly limited, and examples thereof include glyoxal, malondialdehyde, succinyldialdehyde, glutaraldehyde, adipic dialdehyde, heptanedial, suberic dialdehyde, nonanedialdehyde, and benzene trimeraldehyde. Among them, from the viewpoint of easy introduction of aldehyde groups, good water solubility of the resulting PVA, and good polymerization stability when using PVA as a dispersion stabilizer for suspension polymerization of vinyl compounds, dialdehydes or trialdehydes with 2 to 10 carbon atoms are suitable, and dialdehydes with 2 to 6 carbon atoms are more suitable. From the viewpoint of ease of availability and suppression of the formation of fisheyes in the resulting vinyl resin, glutaraldehyde is further suitable. The amount of dialdehyde or trialdehyde used is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, relative to 100 parts by mass of vinyl ester. On the other hand, the amount of dialdehyde or trialdehyde used is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less, relative to 100 parts by mass of vinyl ester. One dialdehyde or trialdehyde may be used alone, or two or more may be used in combination.

[0053] It is preferred that a solvent be used in the polymerization step, with the mass ratio of vinyl ester to solvent being in the range of 100 / 0 to 90 / 10. If the mass ratio of vinyl ester to solvent exceeds 90 / 10, the performance of PVA as a dispersion stabilizer for suspension polymerization of vinyl compounds tends to be reduced.

[0054] In the polymerization process, other monomers other than vinyl ester may be copolymerized within the scope that does not impair the gist of the present invention. By copolymerizing other monomers with vinyl ester, the main chain of the obtained polymer may have a structure of other monomer units. Examples of such other monomers include α-olefins such as ethylene and propylene; (meth)acrylic acid and its salts; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide; N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, diacetone (meth)acrylamide, (meth)acrylamidepropanesulfonic acid and its salts, (meth) (Meth)acrylamide derivatives such as acrylamidopropyldimethylamine and its salts or quaternary ammonium salts, N-methylol(meth)acrylamide and its derivatives; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinylidene halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; unsaturated dicarboxylic acids such as maleic acid, itaconic acid, and fumaric acid, and their salts or esters; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. When such other monomers are copolymerized, their content is generally 5 mol% or less.

[0055] As the solvent used in the polymerization step, an alcoholic solvent is preferably used. Examples of the alcoholic solvent include methanol, ethanol, and propanol, with methanol being preferred. These solvents may be used alone or in combination of two or more.

[0056] In the production method of the present invention, a more suitable method is to polymerize a vinyl ester in the presence of a dialdehyde or trialdehyde and water to obtain a polyvinyl ester, and then saponify the polyvinyl ester. The mass of water in this method is preferably at least 0.3 times the mass of the dialdehyde or trialdehyde, and more preferably at least 0.4 times. On the other hand, the mass of water is preferably less than 9 times the mass of the dialdehyde or trialdehyde, more preferably less than 4 times, and even more preferably less than 2 times.

[0057] Typically, the addition of water reduces productivity in the polymerization step and the subsequent saponification step. However, in the present invention, the addition of water has been found to facilitate the introduction of an aldehyde structure at the terminal. The reason for this is uncertain, but it is presumed that the addition of water suppresses side reactions (hemiacetalization, acetalization, and cyclization) of the dialdehyde or trialdehyde during the polymerization step, shifting the chemical equilibrium toward the aldehyde state over the state of products generated by these side reactions, allowing the reaction of the dialdehyde or trialdehyde with the vinyl ester to proceed efficiently.

[0058] The polymerization initiator used in the polymerization step is not particularly limited and can be selected from known polymerization initiators depending on the polymerization method. Examples of polymerization initiators include azo-based polymerization initiators, peroxide-based polymerization initiators, and redox-based polymerization initiators. Examples of azo-based polymerization initiators include 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). Peroxide-based polymerization initiators include percarbonate compounds such as diisopropyl peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, and di(ethoxyethyl) peroxydicarbonate; perester compounds such as tert-butyl peroxyneodecanoate and cumyl peroxyneodecanoate; acetylcyclohexylsulfonyl peroxide; and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate. As a redox polymerization initiator, a polymerization initiator in which an oxidizing agent and a reducing agent are combined can be used. As the oxidizing agent, peroxide is preferred. As the reducing agent, metal ions, reducing compounds, etc. can be listed. As a combination of an oxidizing agent and a reducing agent, a combination of peroxide and metal ions; a combination of peroxide and reducing compounds; a combination of peroxide, metal ions and reducing compounds, etc. can be listed. As peroxides, hydrogen peroxide, cumene hydroperoxide, tert-butyl hydroperoxide and other hydroperoxides, persulfates (potassium, sodium or ammonium salts), tert-butyl peracetate, peresters (tert-butyl perbenzoate), etc. can be listed. As metal ions, Fe 2+ Cr 2+ 、V 2+ 、Co 2+ 、Ti 3+ 、Cu +etc. can accept a metal ion of one electron transfer. As reducing compounds, sodium bisulfite, sodium bicarbonate, tartaric acid, fructose, glucose, sorbitol, inositol, Rongalite, ascorbic acid can be cited. Among them, preferably a combination of one or more peroxides selected from hydrogen peroxide, potassium persulfate, sodium persulfate and ammonium persulfate, and one or more reducing agents selected from sodium bisulfite, sodium bicarbonate, tartaric acid, Rongalite and ascorbic acid, more preferably a combination of hydrogen peroxide, and one or more reducing agents selected from sodium bisulfite, sodium bicarbonate, tartaric acid, Rongalite and ascorbic acid. In addition, water-soluble polymerization initiators such as potassium persulfate, ammonium persulfate, hydrogen peroxide and cumene hydroperoxide can also be combined in the above-mentioned polymerization initiators as polymerization initiators. These polymerization initiators can be used alone or in combination of two or more.

[0059] When polymerizing a vinyl ester in the presence of a dialdehyde or trialdehyde, the polymerization rate of the vinyl ester is not particularly limited, but is preferably 10% or more and less than 90%. A polymerization rate of less than 10% may reduce the productivity of the PVA. A polymerization rate of 20% or more is more preferred. On the other hand, a polymerization rate of 90% or more may lead to problems such as excessively high viscosity of the resulting polyvinyl ester, reduced PVA productivity, and poor hue of the resulting PVA. A polymerization rate of less than 70% is more preferred.

[0060] The method for saponifying polyvinyl ester is not particularly limited, and a known saponification method can be used. Examples include alcoholysis reactions or hydrolysis reactions using alkaline catalysts such as sodium hydroxide, potassium hydroxide, and sodium methoxide, or acidic catalysts such as p-toluenesulfonic acid. Examples of solvents that can be used in this reaction include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; aromatic hydrocarbons such as benzene and toluene. These solvents can be used alone or in combination of two or more. Among them, the method of saponification using methanol or a methanol / methyl acetate mixed solution as a solvent and sodium hydroxide as a catalyst is simple and preferred.

[0061] (use)

[0062] The PVA of the present invention can be used in various applications, and examples thereof are given below, but the applications are not limited thereto.

[0063] (1) Dispersant Application: Dispersion stabilizer for pigments in coatings, adhesives, etc., dispersion stabilizer and dispersing aid for suspension polymerization of various vinyl compounds such as vinyl chloride, vinylidene chloride, styrene, (meth)acrylate, vinyl acetate, etc.

[0064] (2) Application of coating agents: paper coating agents, sizing agents, fiber processing agents, leather brighteners, coatings, antifogging agents, metal anticorrosive agents, galvanizing gloss agents, antistatic agents, and pharmaceutical coating agents

[0065] (3) Adhesive applications: adhesives, adhesives, rewetting adhesives, various adhesives, additives for cement or mortar

[0066] (4) Emulsifier application: emulsifier for emulsion polymerization, post-emulsifier for asphalt, etc.

[0067] (5) Uses of coagulants: coagulants for suspended and dissolved matter in water, metal coagulants

[0068] (6) Paper processing applications: paper strength enhancers, oil and solvent resistance agents, smoothness enhancers, surface gloss improvers, sealants, barrier agents, light resistance enhancers, water resistance agents, dye and color developer dispersants, adhesion improvers, adhesives

[0069] (7) Agricultural uses: agricultural adhesives, agricultural spreaders, agricultural coatings, soil conditioners, erosion inhibitors, agricultural dispersants

[0070] (8) Medical / cosmetic applications: granulation binders, coating agents, emulsifiers, patches, adhesives, film preparation base materials, film forming agents

[0071] (9) Viscosity modifier uses: thickener, rheology modifier

[0072] (10) Film applications: water-soluble films, polarizing films, barrier films, fiber product packaging films, seed care sheets, vegetation sheets, seed tapes, hygroscopic films

[0073] (11) Application of molded products: fibers, pipes, tubes, leak-proof films, water-soluble fibers for lace, sponges

[0074] (12) Gel applications: medical gel, industrial gel

[0075] (13) Post-reaction uses: Post-reaction uses with low molecular weight organic compounds, high molecular weight organic compounds, and inorganic compounds

[0076] The PVA of the present invention uses an acid catalyst to constrain the movement of the molecular chains in three dimensions, allowing the synthesis of cross-linked products with high viscosity, high water resistance, high strength, excellent heat resistance, and excellent solvent resistance compared to similar linear polymers. Therefore, it is suitable for use in the aforementioned (2) coating material applications, (3) adhesive applications, (10) film applications, (12) gel applications, and the like. Furthermore, the PVA of the present invention is also suitable for use in (1) dispersant applications, as described below.

[0077] (Dispersion stabilizer for suspension polymerization of vinyl compounds)

[0078] The PVA of the present invention is preferably used as a dispersion stabilizer for suspension polymerization of vinyl compounds containing the PVA. When the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization reaction is stabilized, and the formation of coarse particles and fine powder can be suppressed. Furthermore, the formation of fish eyes when the resulting vinyl resin is formed into a sheet can be suppressed.

[0079] The above-mentioned dispersion stabilizer for suspension polymerization may contain various additives, as long as they do not impair the scope of the present invention. Examples of such additives include polymerization regulators such as aldehydes, halogenated hydrocarbons, and thiols; polymerization inhibitors such as phenolic compounds, sulfur compounds, and N-oxide compounds; pH adjusters; crosslinking agents; preservatives; antifungal agents, antiblocking agents, defoaming agents, and compatibilizers. The content of these various additives in the dispersion stabilizer for suspension polymerization is preferably 10% by mass or less, and more preferably 5% by mass or less, relative to the total amount of the dispersion stabilizer for suspension polymerization.

[0080] (Preparation Method of Vinyl Resin)

[0081] Another preferred embodiment of the present invention is a method for producing a vinyl resin by suspension polymerization of a vinyl compound in the presence of the PVA of the present invention. In this method, a granular vinyl resin can be obtained.

[0082] Examples of methods for charging the dispersion stabilizer for suspension polymerization of the present invention into a polymerization tank include (i) charging the dispersion stabilizer into the polymerization tank as an aqueous solution and (ii) directly charging the dispersion stabilizer into the polymerization tank as a powder. From the perspective of uniformity in the polymerization tank, method (i) is preferred.

[0083] Examples of vinyl compounds include vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, their esters and salts; maleic acid, fumaric acid, their esters and anhydrides; styrene, acrylonitrile, vinylidene chloride, and vinyl ether. Among these, vinyl chloride is preferably used alone or in combination with a monomer copolymerizable with vinyl chloride. Examples of monomers copolymerizable with vinyl chloride include vinyl esters such as vinyl acetate and vinyl propionate; (meth)acrylates such as methyl (meth)acrylate and ethyl (meth)acrylate; α-olefins such as ethylene and propylene; unsaturated dicarboxylic acids such as maleic anhydride and itaconic acid; acrylonitrile, styrene, vinylidene chloride, and vinyl ether.

[0084] In the suspension polymerization of vinyl compounds, oil-soluble or water-soluble polymerization initiators conventionally used in the polymerization of vinyl chloride can be used. Examples of oil-soluble polymerization initiators include percarbonate compounds such as diisopropyl peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, and di(ethoxyethyl) peroxydicarbonate; perester compounds such as tert-butyl peroxyneodecanoate, tert-butyl peroxypivalate, tert-hexyl peroxypivalate, and isopropylphenyl peroxyneodecanoate; peroxides such as acetylcyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate, 3,5,5-trimethylhexanoyl peroxide, and lauroyl peroxide; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). Examples of the water-soluble polymerization initiator include potassium persulfate, ammonium persulfate, hydrogen peroxide, and cumene hydroperoxide. These polymerization initiators may be used alone or in combination of two or more.

[0085] When the vinyl compound is suspended and polymerized, the polymerization temperature is not particularly limited and can be as low as about 20°C or as high as over 90°C, preferably about 20 to 60°C. Furthermore, a polymerization reactor equipped with a reflux condenser can be used to improve the heat removal efficiency of the polymerization reaction system.

[0086] The obtained vinyl resin can be used for various molded articles by appropriately blending a plasticizer and the like.

[0087] In the suspension polymerization of a vinyl compound, the amount (concentration) of the suspension polymerization dispersion stabilizer of the present invention is generally 1000 ppm or less and 50 ppm or more relative to the vinyl compound. If the amount (concentration) is less than 50 ppm, coarse particles may be easily generated during the suspension polymerization of the vinyl compound. The ppm mentioned above refers to ppm by mass.

[0088] When performing suspension polymerization of a vinyl compound, in addition to PVA, water-soluble cellulose ethers commonly used in suspension polymerization of vinyl compounds in an aqueous medium, such as methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose; water-soluble polymers such as gelatin; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glyceryl tristearate, and ethylene oxide-propylene oxide block copolymers; and water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glyceryl oleate, and sodium laurate may also be used. The amount of these emulsifiers added is not particularly limited, but is preferably 0.01 to 1.0 parts by mass per 100 parts by mass of the vinyl compound.

[0089] (cross-linked product)

[0090] A suitable use of the PVA of the present invention is a cross-linked product of the PVA, wherein a 100 μm thick film comprising the cross-linked product has a dissolution rate of less than 10% when immersed in hot water at 80° C. for 1 hour.

[0091] Here, the dissolution rate can be measured using the method described in the Examples below. A lower dissolution rate indicates superior water resistance. The film used for measurement can be produced using a film-forming stock solution containing a cross-linked product produced using the PVA of the present invention, using known methods such as cast film forming or melt extrusion. In the present invention, the dissolution rate of the film when immersed in hot water at 80°C for one hour is preferably less than 8%, and even more preferably less than 5%.

[0092] The method for producing the cross-linked product is not particularly limited, but a method in which PVA is cross-linked in the presence of an acid catalyst is suitable.

[0093] (Acid Catalyst)

[0094] The type of acid catalyst is not particularly limited, and examples thereof include phosphoric acid, hydrochloric acid, sulfuric acid, etc. Among them, phosphoric acid is preferably used.

[0095] There are no particular limitations on the method for using the acid catalyst. The acid catalyst may be used directly or dissolved in a solvent. Furthermore, when mixing with PVA, the acid catalyst may be mixed after preparing the PVA aqueous solution, or the acid catalyst may be mixed and dissolved while preparing the PVA aqueous solution. Mixing the acid catalyst after preparing the PVA aqueous solution is preferred from the perspective of suppressing side reactions.

[0096] The amount of the acid catalyst used is not particularly limited, but the amount of the acid catalyst used is preferably 0.01 parts by mass or more and 5 parts by mass or less relative to 10 parts by mass of PVA. If the amount used is less than 0.01 parts by mass, a cross-linked product may not be formed smoothly. The amount used is more preferably 0.05 parts by mass or more. On the other hand, if the amount used exceeds 5 parts by mass, the relative concentration of PVA decreases, and therefore a cross-linked product may not be formed smoothly. The amount used is more preferably 3 parts by mass or less.

[0097] In the preparation of the cross-linked product, in addition to the above-mentioned acid catalyst, a cross-linking agent may also be used. Examples of the cross-linking agent include dialdehydes such as glyoxal, malondialdehyde, and glutaraldehyde, glyoxylates such as sodium glyoxylate and calcium glyoxylate, diamines such as ethylenediamine, propylenediamine, and 1,3-diaminomethylcyclohexane, and dihydrazides such as adipic acid dihydrazide.

[0098] The method of using the crosslinking agent is not particularly limited and can be the same as the method of using the acid catalyst. In addition, the amount of the crosslinking agent used is also not particularly limited and can be the same as the amount of the acid catalyst used. Example

[0099] [PVA viscosity average degree of polymerization]

[0100] The viscosity average degree of polymerization of PVA is measured in accordance with JIS K 6726: 1994. Specifically, when the degree of saponification of PVA is less than 99.5 mol %, the viscosity average degree of polymerization (P) is determined by the following formula using the intrinsic viscosity [η] (L / g) measured in water at 30°C for PVA saponified to a degree of saponification of 99.5 mol % or more.

[0101]

[0102] [PVA saponification degree]

[0103] The saponification degree of PVA is measured in accordance with JIS K 6726:1994.

[0104] [Content of terminal aldehyde groups in PVA]

[0105] The content of terminal aldehyde groups of PVA can be obtained by preparing a 10% by mass aqueous solution of PVA, dropping 5 g of the aqueous solution into 500 g of a 95 / 5 solution of methyl acetate / water to precipitate PVA, recovering and drying it, separating it to obtain PVA, dissolving the obtained PVA in DMSO-d6, and using a 400 MHz 1 H-NMR measurements were performed. The peaks derived from the methine groups of the vinyl alcohol units were assigned to peaks at 3.2 to 4.0 ppm (integral value P), and the peaks derived from the protons of the aldehyde groups were assigned to peaks at around 9.5 to 10 ppm (integral value Q). The content of each peak was calculated using the following formula.

[0106]

[0107] [Absorbance obtained using the ultraviolet absorption spectrum of PVA]

[0108] A 0.1 mass % aqueous solution of PVA was prepared and placed in a cuvette with an optical path length of 1 cm, and the absorbance at 280 nm was measured using an ultraviolet-visible spectrophotometer (UV-2450 manufactured by Shimadzu Corporation).

[0109] [Preparation Example 1 Preparation of PVA1]

[0110] A polymerization tank was charged with 1500 parts by mass of vinyl acetate (hereinafter sometimes abbreviated as "VAc") and 10 parts by mass of methanol. The tank was then purged with nitrogen, and then 12 parts by mass of glutaraldehyde and 12 parts by mass of water were added. The tank was heated to 60°C, and polymerization was carried out in the presence of 2,2'-azobisisobutyronitrile as a polymerization initiator until the polymerization rate reached 25%. While adding methanol, the remaining VAc was expelled from the system along with the methanol under reduced pressure, yielding a methanol solution of polyvinyl acetate (hereinafter sometimes abbreviated as "PVAc") (40% concentration). The concentration of vinyl acetate units in the PVAc was then diluted to 30% by mass in a methanol solvent. A saponification reaction was carried out at 40°C using sodium hydroxide as a saponification catalyst at a molar ratio of 0.03 to PVAc, for one hour. The resulting polyvinyl alcohol was then immersed in a washing solution of 80 / 20 methyl acetate / methanol for washing. The solvent was then removed by centrifugation and dried to obtain PVA1 having a viscosity average degree of polymerization of 1500, a saponification degree of 99 mol %, a terminal aldehyde group content of 0.1 mol %, and an absorbance of 0.279 at 280 nm of a 0.1 mass % aqueous solution.

[0111] [Preparation Examples 2 to 12 (Preparation of PVAs 2 to 12)]

[0112] PVA2 to PVA12 were prepared in the same manner as in Preparation Example 1, except that vinyl acetate, methanol used during polymerization, water, the amount of aldehyde used, the type of aldehyde, the polymerization rate at the end of the polymerization reaction, and the saponification conditions were changed as shown in Table 1. The preparation conditions are shown in Table 1, and the types of aldehydes used are shown in Table 2.

[0113] [Preparation Example 13 (Preparation of PVA13)]

[0114] PVA11 was prepared by the same method as in Preparation Example 11, and the obtained PVA11 was heat-treated in a hot air dryer at 80° C. for 1 hour to prepare PVA13.

[0115]

[0116] [Table 2]

[0117] type aldehyde A Glutaraldehyde B adipaldehyde C Benzene-1,3,5-tricarbaldehyde D acetaldehyde

[0118] Example 1

[0119] 10 parts by mass of PVA (1) was dissolved in distilled water to form 100 parts by mass of a 10% aqueous solution. 0.5 parts by mass of phosphoric acid was added as an acid catalyst, and the mixture was stirred to form an aqueous resin composition solution. The aqueous solution was cast onto a polyethylene terephthalate (PET) film, left at 23°C and 50% RH for 48 hours, and then heated at 70°C for 5 minutes to obtain a film with a thickness of 100 μm. The water resistance of the obtained film was evaluated according to the following criteria, and the result was a dissolution rate of 1.5% by mass.

[0120] (Water resistance)

[0121] The resulting film was immersed in 80°C hot water for 1 hour, and the film dissolution rate (mass %) was measured. The dissolution rate (mass %) was calculated by determining the dry mass (X1 (g)) of the film before immersion in hot water and the dry mass (X2 (g)) of the film after immersion in hot water using the following formula. The results are shown in Table 3.

[0122]

[0123] Example 2

[0124] The water resistance was evaluated in the same manner as in Example 1 except that the type of PVA used was changed. The results are shown in Table 3.

[0125] Comparative Example 1

[0126] A film was prepared and water resistance was evaluated in the same manner as in Example 1 except that PVA6 was used as PVA. As a result, since the aldehyde group content of PVA6 was too low, water resistance was hardly exhibited.

[0127] Comparative Example 2

[0128] Evaluation of water resistance was attempted in the same manner as in Example 1, except that PVA7 was used as PVA. As a result, since the viscosity average polymerization degree of PVA7 was too low, water resistance was hardly exhibited.

[0129] [Table 3]

[0130]

[0131] Example 3

[0132] PVA3, a dispersion stabilizer for suspension polymerization, was dissolved in deionized water. 100 parts by mass of the PVA3 aqueous solution was charged to an autoclave. The amount of PVA3 charged was 850 ppm relative to the amount of vinyl chloride (VCM). Deionized water was then added to bring the total to 1200 parts by mass. Next, 0.65 parts by mass of a 70% toluene solution of cumyl peroxyneodecanoate and 1.05 parts by mass of a 70% toluene solution of t-butyl peroxyneodecanoate were added to the autoclave. Nitrogen was introduced into the autoclave to a pressure of 0.2 MPa. Five nitrogen purges were then performed to thoroughly replace the autoclave with nitrogen and remove oxygen. Then, 940 parts by mass of vinyl chloride was added. The contents of the autoclave were heated to 57°C, and suspension polymerization of the vinyl chloride was initiated with stirring. The pressure in the autoclave at the time of polymerization initiation was 0.80 MPa. About 3.5 hours after the initiation of polymerization, the polymerization was terminated when the pressure in the autoclave reached 0.70 MPa. Unreacted vinyl chloride was removed, and the polymerization product was taken out and dried at 65°C for 16 hours to obtain vinyl chloride polymer pellets. The obtained vinyl chloride polymer pellets were evaluated by the following method.

[0133] (Evaluation of vinyl chloride polymer pellets)

[0134] The obtained vinyl chloride polymer particles were evaluated for (1) average particle size, (2) particle size distribution, and (3) fisheye according to the following methods. The evaluation results are shown in Table 4.

[0135] (1) Average particle size

[0136] The particle size distribution was measured using a Tyler sieve according to the dry sieving method described in JIS Z 8815:1994. The results were plotted using the Rosin-Rammler distribution to calculate the average particle size (d p50 ).

[0137] (2) Particle size distribution

[0138] The following evaluation criteria were used to evaluate the content (mass %) of vinyl chloride polymer particles that did not pass through a 355 μm sieve (equivalent to 42 mesh in JIS standard sieve mesh). The above content refers to the cumulative mass % on the sieve. The sieve mesh size conformed to the nominal mesh size W in JIS Z 8801-1-2006.

[0139] A: less than 0.5 mass%

[0140] B: 0.5 mass% or more and less than 1 mass%

[0141] C: 1 mass % or more.

[0142] The following evaluation criteria were used to evaluate the content (mass %) of vinyl chloride polymer particles that passed a 355 μm sieve and did not pass a 250 μm sieve (equivalent to 60 mesh in JIS standard sieve mesh). The above content refers to the cumulative mass % on the sieve. The sieve mesh size conformed to the nominal mesh size W in JIS Z 8801-1-2006.

[0143] A: less than 5% by mass

[0144] B: 5% by mass or more and less than 10% by mass

[0145] C: 10% by mass or more.

[0146] The following evaluation criteria were used to evaluate the content (mass %) of vinyl chloride polymer particles passing through a 75 μm sieve (equivalent to 200 mesh in JIS standard sieve mesh). The above content refers to the cumulative mass % on the sieve. The sieve mesh size conforms to the nominal mesh size W in JIS Z 8801-1-2006.

[0147] A: less than 1% by mass

[0148] B: 1 mass % or more and less than 2 mass %

[0149] C: 2 mass % or more.

[0150] It should be noted that, for both the content of vinyl chloride polymer particles that do not pass through a 355 μm sieve and the content of vinyl chloride polymer particles that do not pass through a 250 μm sieve, lower values indicate fewer coarse particles and a sharper particle size distribution, indicating superior polymerization stability. Furthermore, a lower content of vinyl chloride polymer particles that pass through a 75 μm sieve indicates less fine powder and superior processability.

[0151] (3) Fisheye

[0152] 100 parts by mass of the obtained vinyl chloride polymer pellets, 50 parts by mass of dioctyl phthalate, 5 parts by mass of tribasic lead sulfate and 1 part by mass of zinc stearate were kneaded at 150°C for 7 minutes using a roll to prepare a 0.1 mm thick sheet. The thickness of the sheet per 1000 cm was visually measured. 2 The fewer the number of fisheyes, the fewer defects on the sheet.

[0153] Examples 4-5

[0154] Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA4 to 5 were used instead of PVA3. The evaluation results of the resulting vinyl chloride polymer particles are shown in Table 4. When the PVA of the present invention was used as a dispersion stabilizer for suspension polymerization, the resulting vinyl chloride polymer particles did not become coarse, exhibited good polymerization stability, and contained little fine powder and fish eyes.

[0155] Comparative Example 3

[0156] Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA8 was used as PVA. The evaluation results of the resulting vinyl chloride polymer particles are shown in Table 4. Due to the high aldehyde group content of PVA8, the resulting vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and a large number of fish eyes.

[0157] Comparative Example 4

[0158] Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA9 was used as PVA. The evaluation results of the resulting vinyl chloride polymer particles are shown in Table 4. Due to the low saponification degree of PVA9, the resulting vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and a large number of fish eyes.

[0159] Comparative Example 5

[0160] Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA10 was used as the PVA. The evaluation results of the resulting vinyl chloride polymer particles are shown in Table 4. Due to the high viscosity-average polymerization degree of PVA10, the resulting vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and a large number of fish eyes.

[0161] Comparative Example 6

[0162] Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA11 was used as PVA. The evaluation results of the resulting vinyl chloride polymer particles are shown in Table 4. Since the aldehyde group content of PVA11 was too low, the resulting vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and a large number of fish eyes.

[0163] Comparative Example 7

[0164] Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA12 was used as the PVA. The evaluation results of the resulting vinyl chloride polymer particles are shown in Table 4. Since PVA12 is a PVA derived from polyvinyl acetate obtained by polymerizing vinyl acetate in the presence of a monoaldehyde and does not have an aldehyde group at the end of the PVA, the resulting vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and a high number of fish eyes.

[0165] Comparative Example 8

[0166] Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA13 was used as PVA. The evaluation results of the resulting vinyl chloride polymer particles are shown in Table 4. Although PVA13 had a high absorbance value, the aldehyde group content was too low. As a result, the resulting vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and many fish eyes.

[0167] [Table 4]

[0168]

[0169] As shown in the Examples, the PVA of the present invention can be easily cross-linked using acids and the like, resulting in excellent water resistance. Furthermore, when used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the PVA of the present invention exhibits excellent polymerization stability, resulting in a small average particle size of the resulting vinyl chloride polymer particles (vinyl-based polymer), minimal formation of coarse particles and fine powder, and reduced fisheye formation. Consequently, the productivity and processability of the vinyl-based polymer are excellent. Therefore, the industrial utility of the present invention is extremely high.

Claims

1. Polyvinyl alcohol having a saponification degree of 70 mol% or more and less than 99.9 mol% and a viscosity-average degree of polymerization of 400 or more and less than 1800, Contains 0.08 mol% or more and less than 0.45 mol% of the structure represented by the following formula (1) at the terminal, and The absorbance at 280 nm of a 0.1% by mass aqueous solution is 0.17 or more and less than 0.55, The content (mol %) of the structure represented by the above formula (1) was determined by preparing a 10% by mass aqueous solution of the above polyvinyl alcohol, dropping 5 g of the aqueous solution into 500 g of a 95 / 5 solution of methyl acetate / water to precipitate the polyvinyl alcohol, recovering and drying the solution, and separating the polyvinyl alcohol to obtain the polyvinyl alcohol. The obtained polyvinyl alcohol was dissolved in DMSO-d6 and analyzed using a 400 MHz 1 When the integrated value of the peak derived from the methine group of the vinyl alcohol unit is defined as P and the integrated value of the peak derived from the proton of the aldehyde group is defined as Q by H-NMR, the value is obtained by (Q / P)×100. [Chemical Formula 1] In formula (1), X is an alkylene group having 1 to 6 carbon atoms, and * represents a connecting bond.

2. The method for preparing polyvinyl alcohol according to claim 1, wherein Vinyl ester is polymerized in the presence of a dialdehyde to obtain polyvinyl ester, and then the polyvinyl ester is saponified.

3. A dispersion stabilizer for suspension polymerization of a vinyl compound, comprising the polyvinyl alcohol according to claim 1.

4. A method for preparing a vinyl resin, wherein: The vinyl compound is subjected to suspension polymerization in the presence of the polyvinyl alcohol according to claim 1.

5. A cross-linked product, which is a cross-linked product of polyvinyl alcohol according to claim 1, wherein When a film having a thickness of 100 μm including the cross-linked product was immersed in hot water at 80° C. for 1 hour, the dissolution rate was less than 10% by mass.

6. The method for preparing a cross-linked product according to claim 5, wherein: The polyvinyl alcohol is cross-linked in the presence of an acid catalyst.

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