Polyvinyl alcohol, its production method and uses

Abstract

To provide PVA which can suppress formation of coarse particles in a vinyl-based resin obtained when used as a dispersion stabilizer for suspension polymerization of a vinyl compound and can obtain particles having uniform diameters, and can suppress occurrence of fish eyes, and to provide PVA capable of simply obtaining a crosslinked body having excellent water resistance.SOLUTION: Polyvinyl alcohol has a degree of saponification of 70 mol% or more and less than 99.9 mol%, has a degree of viscosity average polymerization of 400 or more and less than 1,800, contains 0.05 mol% or more and less than 0.5 mol% of an aldehyde group at its terminal, and has an absorbance at 280 nm of 0.1 mass% aqueous solution of 0.17 or more and less than 0.55.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyvinyl alcohol having a degree of saponification and a viscosity-average degree of polymerization within a specific range, containing a specific amount of terminal aldehyde groups, and having a specific absorbance.The present invention also relates to a crosslinked product using the polyvinyl alcohol, a dispersion stabilizer for suspension polymerization of vinyl compounds, and a method for producing a vinyl resin. [Background technology]

[0002] Polyvinyl alcohol (hereafter, polyvinyl alcohol may be abbreviated as "PVA"), which has reactive functional groups, has 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). Crosslinked polymers, synthesized by crosslinking reactions between reactive sites and crosslinkers, have molecular chains that are constrained in three dimensions. Therefore, they typically have superior strength, heat resistance, and solvent resistance, especially water resistance, compared to similar linear polymers, making them highly useful. Another major application of PVA is as a dispersion stabilizer for suspension polymerization of vinyl compounds, and various PVAs are used.

[0003] An example of a modified PVA having reactive functional groups with particularly high crosslinking performance is a modified PVA having a structure in which a methylene hydrogen is sandwiched between two carbonyl groups, such as an acetoacetyl group (Patent Document 1).

[0004] In Patent Documents 2 and 3, heat-treated PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds in order to improve the polymerization stability during polymerization of the vinyl compounds and suppress coarsening of the resulting vinyl polymer. In this specification, polymerization stability means that the dispersibility of droplets of the vinyl compound is good during polymerization, and as a result, coarsening is suppressed and vinyl polymer particles with a uniform size are obtained. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-205826 [Patent Document 2] Japanese Patent Application Publication No. 51-45189 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-250695 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the diketene used in Patent Document 1 is highly toxic to living organisms and poses a risk of explosion when mixed with air and steam, so there has been a demand for the development of a safer method for synthesizing modified PVA having reactive functional groups. Furthermore, Patent Document 1 requires the use of a special crosslinking agent, and it is difficult to easily obtain a crosslinked product with excellent water resistance using an acid or the like.

[0007] Furthermore, when the modified PVA described in Patent Documents 2 or 3 is used as a dispersion stabilizer for suspension polymerization, although an improvement in polymerization stability during vinyl compound polymerization is observed, the vinyl polymer obtained contains a large amount of fine powder, which is insufficient for meeting recent demands. Furthermore, when the vinyl polymer is formed into a sheet, there are many problems, such as fish eyes due to lumps or defects.

[0008] The present invention has been made to solve the above-mentioned problems, and aims to provide a PVA that, when used as a dispersion stabilizer for suspension polymerization of vinyl compounds, can suppress the formation of coarse particles in the resulting vinyl resin, can produce particles with uniform diameters, and can also suppress the occurrence of fisheyes. Another aim is to provide a PVA that can easily produce a crosslinked product with excellent water resistance. [Means for solving the problem]

[0009] The present inventors have found that the above-mentioned problems can be solved by a PVA having a degree of saponification and a viscosity-average degree of polymerization within a specific range, containing a specific amount of aldehyde groups at its terminals, and having a specific absorbance, and have completed the present invention based on this finding.

[0010] That is, the above-mentioned problems can be solved by providing a polyvinyl alcohol having a degree of saponification 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, a terminal aldehyde group content of 0.05 mol % or more and less than 0.5 mol %, and an absorbance at 280 nm of a 0.1 mass % aqueous solution of polyvinyl alcohol of 0.17 or more and less than 0.55.

[0011] In this case, it is preferable that the terminal has a structure represented by the following formula (1).

[0012] [ka] (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 bond.)

[0013] It is also preferred that X is an alkylene group having 1 to 6 carbon atoms.

[0014] In this case, a suitable method for producing the polyvinyl alcohol is to polymerize a vinyl ester in the presence of a dialdehyde or trialdehyde to obtain a polyvinyl ester, and then saponify the polyvinyl ester.

[0015] A preferred embodiment of the present invention is a dispersion stabilizer for suspension polymerization of vinyl compounds containing the polyvinyl alcohol.A preferred embodiment of the present invention is also a method for producing a vinyl resin by suspension polymerizing a vinyl compound in the presence of the polyvinyl alcohol.

[0016] Furthermore, a preferred embodiment of the present invention is a crosslinked product obtained by crosslinking the polyvinyl alcohol, wherein a 100 μm-thick film made of the crosslinked product has an elution rate of less than 10% when immersed in hot water at 80° C. for 1 hour. In this case, a preferred method for producing the crosslinked product is to crosslink the polyvinyl alcohol in the presence of an acid catalyst. [Effects of the Invention]

[0017] 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, enabling the production of particles with uniform diameters, and also suppressing the occurrence of fisheyes.Furthermore, by using the PVA of the present invention, a crosslinked product with excellent water resistance can be easily obtained. DETAILED DESCRIPTION OF THE INVENTION

[0018] [Polyvinyl alcohol] The polyvinyl alcohol of the present invention is characterized by having a degree of saponification of 70 mol % or more and less than 99.9 mol %, a viscosity-average degree of polymerization of 400 or more and less than 1,800, containing aldehyde groups at its terminals in an amount of 0.05 mol % or more and less than 0.5 mol %, and having an absorbance at 280 nm of 0.17 or more and less than 0.55 in a 0.1% by mass aqueous solution.

[0019] It is important that the degree of saponification of PVA is 70 mol% or more and less than 99.9 mol%. When PVA with a degree of saponification of less than 70 mol% is used as a raw material for a crosslinked body, the water resistance of the resulting crosslinked body decreases. When used as a raw material for a crosslinked body, the degree of saponification of PVA is preferably 80 mol% or more, more preferably 90 mol% or more. On the other hand, PVA with a degree of saponification of 99.9 mol% or more is difficult to produce.

[0020] Furthermore, when PVA with a degree of saponification of less than 70 mol% is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, polymerization stability decreases, resulting in an increase in coarse particles in the resulting vinyl resin, and particles with a uniform diameter cannot be obtained. Furthermore, numerous fish eyes occur in the resulting vinyl resin. On the other hand, PVA with a degree of saponification of 99.9 mol% or more is difficult to produce. 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 obtained by measurement in accordance with JIS K 6726:1994.

[0021] It is important that the viscosity-average degree of polymerization (hereinafter sometimes abbreviated as degree of polymerization) of PVA is 400 or more and less than 1800. When PVA with a degree of polymerization less than 400 is used as a raw material for a crosslinked body, the water resistance of the resulting crosslinked body decreases. When used as a raw material for a crosslinked body, 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. Furthermore, when PVA with a degree of polymerization of 1800 or more is used as a raw material for a crosslinked body, the viscosity of the liquid becomes too high, resulting in poor handleability. The degree of polymerization of PVA is preferably less than 1600.

[0022] Furthermore, when PVA having a degree of polymerization of less than 400 is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, polymerization stability decreases, resulting in an increase in coarse particles in the resulting vinyl resin, and particles with a uniform diameter cannot be obtained. The polymerization degree of PVA is preferably 550 or higher. On the other hand, when the polymerization degree is 1800 or higher, PVA productivity decreases. Furthermore, when PVA having a degree of polymerization of 1800 or higher is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, polymerization stability decreases, resulting in an increase in coarse particles in the resulting vinyl resin, and particles with a uniform diameter cannot be obtained. Furthermore, numerous fisheyes occur in the resulting vinyl resin. When used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization degree of PVA is preferably less than 1500, more preferably less than 1300, and even more preferably less than 1000.

[0023] The viscosity-average degree of polymerization is a value obtained by measurement in accordance with JIS K 6726: 1994. Specifically, when the degree of saponification is less than 99.5 mol%, the viscosity-average degree of polymerization (P) is calculated by the following formula using the intrinsic viscosity [η] (L / g) measured in water at 30°C for PVA saponified to a degree of saponification of 99.5 mol% or more. P = ([η] × 10 4 / 8.29) (1 / 0.62)

[0024] It is important that the PVA of the present invention contains 0.05 mol% or more but less than 0.5 mol% of aldehyde groups at its terminals. Aldehyde groups are reactive functional groups and can be used for crosslinking reactions such as acetalization with hydroxyl groups and radical crosslinking. Furthermore, because PVA containing aldehyde groups has a high adsorption capacity for vinyl compounds, when used as a dispersion stabilizer for the suspension polymerization of vinyl compounds, the polymerization is stabilized, resulting in reduced formation of coarse particles and fine powder in the resulting vinyl resin. Furthermore, the occurrence of fisheyes can be reduced when the resulting vinyl resin is formed into a sheet.

[0025] When PVA having an aldehyde group content of less than 0.05 mol% is used as a raw material for the crosslinked body, the water resistance of the resulting crosslinked body decreases. The aldehyde group content is preferably 0.08 mol% or more. On the other hand, PVA having an aldehyde group content of 0.5 mol% or more has low productivity. Furthermore, when such PVA is used as a raw material for the crosslinked body, the viscosity of the liquid becomes too high, resulting in poor handleability. The aldehyde group content is preferably less than 0.45 mol%, more preferably less than 0.4 mol%.

[0026] Furthermore, 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, polymerization stability decreases, resulting in an increase in coarse particles in the resulting vinyl resin, and particles with a uniform diameter cannot be obtained. Furthermore, numerous fish eyes occur in the resulting vinyl resin. The aldehyde group content is preferably 0.08 mol% or more. On the other hand, PVA having an aldehyde group content of 0.5 mol% or more has low productivity. Furthermore, when such a PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, polymerization stability decreases, resulting in an increase in coarse particles in the resulting vinyl resin, and particles with a uniform diameter cannot be obtained. Furthermore, numerous fish eyes occur in 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 is determined by the aldehyde group content of the vinyl ester polymer in a deuterated chloroform solvent before saponification. 1 H-NMR spectrum and PVA in deuterated DMSO or deuterated water solvents 1 It can be determined from the H-NMR spectrum.

[0027] 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 even more preferably less than 0.1 mol %.

[0028] It is important that the absorbance at 280 nm of a 0.1% by mass aqueous solution of PVA is 0.17 or more but less than 0.55. If PVA with an absorbance of less than 0.17 is used as a raw material for a crosslinked body, the water resistance of the resulting crosslinked body will be reduced. The absorbance is preferably 0.21 or more, more preferably 0.24 or more, and even more preferably 0.28 or more. On the other hand, PVA with an absorbance of 0.55 or more has low productivity. Furthermore, if such PVA is used as a raw material for a crosslinked body, the viscosity of the liquid will be too high, resulting in reduced handleability. The absorbance is preferably less than 0.52, more preferably less than 0.48, and even more preferably less than 0.45.

[0029] Furthermore, when a PVA having an absorbance of less than 0.17 is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, polymerization stability is reduced, resulting in an increase in coarse particles in the resulting vinyl resin, and particles with a uniform diameter cannot be obtained. Furthermore, numerous fisheyes occur in the resulting vinyl resin. The absorbance is preferably 0.21 or more, more preferably 0.24 or more, and even more preferably 0.28 or more. On the other hand, PVA having an absorbance of 0.55 or more has low productivity. Furthermore, when such a PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, polymerization stability is reduced, resulting in an increase in coarse particles in the resulting vinyl resin, and particles with a uniform diameter cannot be obtained. Furthermore, numerous fisheyes occur in the resulting vinyl resin. The absorbance is preferably less than 0.52, more preferably less than 0.48, and even more preferably less than 0.45.

[0030] The absorbance indicates the number or chain length of ethylenic double bonds present in the main chain of the PVA. Having an absorbance within the above range improves the adsorption of PVA to vinyl compounds, and the synergistic effect with the terminal aldehyde groups of the PVA further improves polymerization stability when used as a dispersion stabilizer for suspension polymerization of vinyl compounds. When PVA with an absorbance within the above range is used as a raw material for crosslinked products, the water resistance of the resulting crosslinked products is improved. The absorption at a wavelength of 280 nm is derived from the [—CO—(CH═CH)2—] structure in PVA. This structure can be introduced by using aldehydes as modifiers or by using monomers capable of introducing ethylenic double bonds into the main chain by copolymerization. If the absorbance is outside the above range, the adsorption between PVA and vinyl compounds decreases, or the PVA and vinyl compounds enter a dissolved state. As a result, the PVA will not function as a dispersion stabilizer for suspension polymerization. Furthermore, when PVA with an absorbance outside the above range is used as a raw material for crosslinked products, the water resistance of the resulting crosslinked products is reduced. The conditions for the absorbance measurement device and the like are as described in the Examples below.

[0031] The PVA of the present invention preferably has a structure represented by the following formula (1) at its terminal.

[0032] [ka] (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 bond.)

[0033] 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. X preferably has 0 to 8 carbon atoms, more preferably 1 to 6, even more preferably 1 to 4, particularly preferably 2 to 4, and most preferably 3 to 4. X is preferably an alkylene group or arylene group having the above number of carbon atoms, and more preferably an alkylene group having the above number of carbon atoms. When X satisfies the above aspects, there is a tendency that the introduction of aldehyde groups in PVA is easy, the water solubility of the resulting PVA is good, the polymerization stability when the PVA is used as a dispersion stabilizer for the suspension polymerization of vinyl compounds is good, or the occurrence of fisheyes in the resulting vinyl resin can be suppressed.

[0034] Examples of the substituent that the alkylene group or arylene group may have include an alkyl group, an aryl group, a hydroxy group, an aldehyde group, a carboxy group, and an amino group.

[0035] (PVA manufacturing method) The method for producing the PVA of the present invention is not particularly limited, but a suitable production method is a method in which a vinyl ester is polymerized in the presence of a dialdehyde or trialdehyde to obtain a polyvinyl ester, and then the polyvinyl ester is saponified.

[0036] Examples of the polymerization method include known methods such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, and dispersion polymerization, and from an industrial viewpoint, solution polymerization, emulsion polymerization, and dispersion polymerization are preferred. In the polymerization operation, any of a batch method, a semi-batch method, and a continuous method can be used.

[0037] Examples of vinyl esters include vinyl acetate, vinyl formate, vinyl propionate, vinyl caprylate, and vinyl versatate, with vinyl acetate being preferred from an industrial viewpoint.

[0038] The type of dialdehyde or trialdehyde is not particularly limited, and examples include glyoxal, malondialdehyde, succinaldehyde, glutaraldehyde, adipaldehyde, heptanedial, octanedial, nonanedial, and benzenetricarbaldehyde. Among these, dialdehydes or trialdehydes having 2 to 10 carbon atoms are preferred, and dialdehydes having 2 to 6 carbon atoms are more preferred, from the viewpoints of ease of introduction of aldehyde groups, good water solubility of the resulting PVA, and good polymerization stability when the PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds. Glutaraldehyde is even more preferred, from the viewpoints of ease of availability and ability to suppress the occurrence of fisheyes in the resulting vinyl resin. 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, per 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 the vinyl ester. One type of dialdehyde or trialdehyde may be used alone, or two or more types may be used in combination.

[0039] It is preferable that a solvent is used in the polymerization step, and the mass ratio of the vinyl ester to the solvent is vinyl ester / solvent = 100 / 0 to 90 / 10. If the mass ratio of the vinyl ester to the solvent exceeds 90 / 10, when PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, its performance as a dispersion stabilizer tends to decrease.

[0040] In the polymerization step, a monomer other than the vinyl ester may be copolymerized within the scope of the present invention. By copolymerizing the other monomer with the vinyl ester, the main chain of the resulting polymer can have a structure of the other monomer unit. 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, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide; N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, diacetone(meth)acrylamide, (meth)acrylamidopropanesulfonic acid and its salts, and (meth)acrylamidopropyldimethylamine and its salts. or its quaternary salts, (meth)acrylamide derivatives such as N-methylol (meth)acrylamide and its derivatives; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinyl halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; unsaturated dicarboxylic acids and their salts or esters such as maleic acid, itaconic acid, and fumaric acid; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. When such other monomers are copolymerized, the content thereof is usually 5 mol% or less.

[0041] The solvent used in the polymerization step is preferably an alcohol-based solvent. Examples of alcohol-based solvents include methanol, ethanol, and propanol, with methanol being preferred. These can be used alone or in combination of two or more.

[0042] In the production method of the present invention, a more preferred 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 used in this method is preferably 0.3 times or more, more preferably 0.4 times or more, the amount of the dialdehyde or trialdehyde. On the other hand, the mass of water is preferably less than 9 times, more preferably less than 4 times, and even more preferably less than 2 times the amount of the dialdehyde or trialdehyde.

[0043] Although the addition of water usually reduces productivity in the polymerization step and the subsequent saponification step, it has been found in the present invention that the addition of water makes it possible to easily introduce an aldehyde structure at the terminal. Although the reason for this is unclear, it is presumed that the addition of water suppresses side reactions (hemiacetalization, acetalization, cyclization) of dialdehydes or trialdehydes in the polymerization step, shifting the chemical equilibrium toward the aldehyde state rather than the state of the product resulting from the side reaction, thereby allowing the reaction between the dialdehyde or trialdehyde and the vinyl ester to proceed efficiently.

[0044] 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 polymerization initiators, peroxide polymerization initiators, and redox polymerization initiators. Examples of azo polymerization initiators include 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(isobutyronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). Examples of peroxide polymerization initiators include percarbonate compounds such as diisopropyl peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanoate and cumyl peroxyneodecanoate; acetylcyclohexylsulfonyl peroxide; and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate. As the redox polymerization initiator, a combination of an oxidizing agent and a reducing agent can be used. As the oxidizing agent, a peroxide is preferred. As the reducing agent, a metal ion, a reducing compound, etc. can be mentioned. As the combination of an oxidizing agent and a reducing agent, a combination of a peroxide and a metal ion, a combination of a peroxide and a reducing compound, a combination of a peroxide, a metal ion, and a reducing compound can be mentioned. As the peroxide, a hydroxyperoxide such as hydrogen peroxide, cumene hydroxyperoxide, t-butyl hydroxyperoxide, persulfates (potassium, sodium, or ammonium salts), t-butyl peracetate, peresters (t-butyl perbenzoate), etc. can be mentioned. As the metal ion, Fe 2+ , Cr 2+ , V 2+ , Co 2+ , Ti 3+ , Cu +Examples of suitable reducing compounds include metal ions capable of undergoing one-electron transfer, such as sodium bisulfite, sodium bicarbonate, tartaric acid, fructose, dextrose, sorbose, inositol, rongalite, and ascorbic acid. Among these, a combination of one or more peroxides selected from the group consisting of hydrogen peroxide, potassium persulfate, sodium persulfate, and ammonium persulfate with one or more reducing agents selected from the group consisting of sodium bisulfite, sodium bicarbonate, tartaric acid, rongalite, and ascorbic acid is preferred, and a combination of hydrogen peroxide with one or more reducing agents selected from the group consisting of sodium bisulfite, sodium bicarbonate, tartaric acid, rongalite, and ascorbic acid is more preferred. Furthermore, the polymerization initiator may be combined with a water-soluble polymerization initiator such as potassium persulfate, ammonium persulfate, hydrogen peroxide, or cumene hydroperoxide to form a polymerization initiator. These polymerization initiators can be used alone or in combination.

[0045] When a vinyl ester is polymerized 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%. If the polymerization rate is less than 10%, the productivity of PVA may decrease. A polymerization rate of 20% or more is more preferable. On the other hand, if the polymerization rate is 90% or more, the viscosity of the resulting polyvinyl ester may become too high, resulting in problems such as a decrease in the productivity of PVA and a deterioration in the hue of the resulting PVA. A polymerization rate of less than 70% is more preferable.

[0046] The method for saponifying polyvinyl ester is not particularly limited, and known saponification methods can be used. Examples include alcoholysis or hydrolysis using a basic catalyst such as sodium hydroxide, potassium hydroxide, or sodium methoxide, or an acidic catalyst such as p-toluenesulfonic acid. 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; and aromatic hydrocarbons such as benzene and toluene. These solvents can be used alone or in combination. Among these, a method of saponification using methanol or a mixed solution of methanol and methyl acetate as the solvent and sodium hydroxide as the catalyst is simple and preferred.

[0047] (Application) The PVA of the present invention can be used in a variety of applications, including but not limited to the following examples: (1) Dispersant Uses: Dispersion stabilizer for pigments in paints, adhesives, etc.; Dispersion stabilizer and dispersion aid for suspension polymerization of various vinyl compounds such as vinyl chloride, vinylidene chloride, styrene, (meth)acrylate, and vinyl acetate. (2) Coating agent applications: paper coating agents, sizing agents, textile processing agents, leather finishing agents, paints, anti-fogging agents, metal corrosion inhibitors, zinc plating brighteners, antistatic agents, pharmaceutical coating agents (3) Adhesive applications: adhesives, pressure sensitive adhesives, re-moistening adhesives, various binders, additives for cement and mortar (4) Emulsifier Use: Emulsifier for emulsion polymerization, post-emulsifier for bitumen, etc. (5) Flocculant Use: Flocculant for suspended and dissolved matter in water, metal flocculant (6) Paper processing applications: paper strength enhancers, oil and solvent resistance agents, smoothness improvers, surface gloss improvers, sealing agents, barrier agents, light resistance agents, water resistance agents, dispersants for dyes and color developers, adhesive strength improvers, binders (7) Agricultural uses: pesticide binders, pesticide spreaders, agricultural coating agents, soil conditioners, erosion inhibitors, pesticide dispersants (8) Medical and cosmetic applications: granulation binders, coating agents, emulsifiers, patches, binders, film preparation base materials, film-forming agents (9) Viscosity modifier Uses: Thickener, rheology modifier (10) Film applications: water-soluble film, polarizing film, barrier film, textile packaging film, seed protection sheet, vegetation sheet, seed tape, moisture-absorbing film (11) Molded product applications: fibers, pipes, tubes, leak-proof membranes, water-soluble fibers for chemical lace, sponges (12) Gel applications: medical gels, industrial gels (13) Post-reaction applications: Post-reaction applications with low molecular weight organic compounds, high molecular weight organic compounds, and inorganic compounds The PVA of the present invention, when used with an acid catalyst, constrains the movement of molecular chains in three dimensions, enabling the synthesis of crosslinked products that have higher viscosity, water resistance, strength, heat resistance, and solvent resistance than linear polymers of the same type. Therefore, the PVA of the present invention is suitable for use in the above-mentioned (2) coating materials, (3) adhesives, (10) films, and (12) gels. Furthermore, as described below, the PVA of the present invention is also suitable for use in (1) dispersants.

[0048] (Dispersion stabilizer for suspension polymerization of vinyl compounds) 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 can be stabilized and the formation of coarse particles and fine powder can be suppressed. Furthermore, the occurrence of fisheyes can be suppressed when the resulting vinyl resin is formed into a sheet.

[0049] The dispersion stabilizer for suspension polymerization may contain various additives within the scope of the present invention. Examples of the additives include polymerization regulators such as aldehydes, halogenated hydrocarbons, and mercaptans; polymerization inhibitors such as phenol compounds, sulfur compounds, and N-oxide compounds; pH adjusters; crosslinking agents; preservatives; antifungal agents; antiblocking agents; antifoaming agents; and compatibilizers. The content of the various additives in the dispersion stabilizer for suspension polymerization is preferably 10% by mass or less, and more preferably 5% by mass or less, based on the total amount of the dispersion stabilizer for suspension polymerization.

[0050] (Method of manufacturing vinyl resin) 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, which produces a particulate vinyl resin.

[0051] The dispersion stabilizer for suspension polymerization of the present invention can be charged into a polymerization tank by, for example, (i) preparing an aqueous solution and charging it into the polymerization tank, or (ii) charging it in the form of a powder, etc. From the viewpoint of uniformity in the polymerization tank, the above method (i) is preferred.

[0052] Examples of vinyl compounds include vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, esters and salts thereof; maleic acid, fumaric acid, esters and anhydrides thereof; styrene, acrylonitrile, vinylidene chloride, vinyl ether, etc. Among these, it is preferable to use vinyl chloride 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)acrylic acid esters 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, vinyl ether, etc.

[0053] For 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 the oil-soluble polymerization initiator include percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanoate, t-butyl peroxypivalate, t-hexyl peroxypivalate, and cumyl 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'-azobis(isobutyronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). Examples of the water-soluble polymerization initiator include potassium persulfate, ammonium persulfate, hydrogen peroxide, cumene hydroperoxide, etc. These polymerization initiators can be used alone or in combination of two or more.

[0054] In the suspension polymerization of a vinyl compound, the polymerization temperature is not particularly limited, and may be a low temperature of about 20° C. or a high temperature exceeding 90° C., with a temperature of about 20 to 60° C. being preferred. In addition, a polymerization vessel equipped with a reflux condenser may be used to increase the efficiency of heat removal from the polymerization reaction system.

[0055] The vinyl resin thus obtained can be used for various molded products by appropriately blending a plasticizer or the like.

[0056] In the suspension polymerization of a vinyl compound, the amount (concentration) of the dispersion stabilizer for suspension polymerization of the present invention used is usually 50 ppm or more and 1000 ppm or less relative to the vinyl compound. If it is less than 50 ppm, coarse particles may be easily generated during the suspension polymerization of the vinyl compound. The ppm means ppm by mass.

[0057] In the suspension polymerization of a vinyl compound, in addition to PVA, water-soluble cellulose ethers typically used in the suspension polymerization of a vinyl compound in an aqueous medium, such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and hydroxypropylmethyl cellulose; water-soluble polymers such as gelatin; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glycerin tristearate, and ethylene oxide propylene oxide block copolymers; and water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate, and sodium laurate, may be used in combination. The amount of these emulsifiers added is not particularly limited, and is preferably 0.01 to 1.0 parts by mass per 100 parts by mass of the vinyl compound.

[0058] (Crosslinked body) A suitable use of the PVA of the present invention is a crosslinked product obtained by crosslinking the PVA, in which the elution rate when a 100 μm-thick film made of the crosslinked product is immersed in hot water at 80°C for 1 hour is less than 10%.

[0059] The elution rate can be measured by the method described in the Examples below, and it can be said that the smaller the elution rate, the better the water resistance. The film to be measured can be produced by a known method, such as a casting film production method or a melt extrusion film production method, using a film-forming solution containing a crosslinked product produced using the PVA of the present invention. In the present invention, the elution rate when the film is immersed in hot water at 80°C for 1 hour is preferably less than 8%, and even more preferably less than 5%.

[0060] The method for producing the crosslinked product is not particularly limited, but a production method in which PVA is crosslinked in the presence of an acid catalyst is preferred.

[0061] (acid catalyst) The type of acid catalyst is not particularly limited, and examples thereof include phosphoric acid, hydrochloric acid, sulfuric acid, etc. Among these, phosphoric acid is preferably used.

[0062] The method of using the acid catalyst is not particularly limited. The acid catalyst may be used as is or may be dissolved in a solvent. When mixing with PVA, the acid catalyst may be mixed after preparing an aqueous PVA solution, or the acid catalyst may be mixed and dissolved simultaneously with the preparation of the aqueous PVA solution. The method of mixing the acid catalyst after preparing the aqueous PVA solution is preferred from the viewpoint of suppressing side reactions.

[0063] The amount of acid catalyst used is not particularly limited, but is preferably 0.01 to 5 parts by mass per 10 parts by mass of PVA. If the amount used is less than 0.01 part by mass, a crosslinked product may not be formed properly. More preferably, the amount used is 0.05 parts by mass or more. On the other hand, if the amount used is more than 5 parts by mass, the relative concentration of PVA decreases, and a crosslinked product may not be formed properly. More preferably, the amount used is 3 parts by mass or less.

[0064] In producing the crosslinked product, a crosslinking agent can be used in addition to the acid catalyst. Examples of the crosslinking agent include dialdehydes such as glyoxal, malondialdehyde, and glutaraldehyde, glyoxylates such as sodium glyoxylate and calcium glyoxylate, diamines such as ethanediamine, propanediamine, and 1,3-bisaminomethylcyclohexane, and dihydrazides such as adipic acid dihydrazide.

[0065] The method of using the crosslinking agent is not particularly limited and may be the same as the method of using the acid catalyst. The amount of the crosslinking agent used is also not particularly limited and may be the same as the amount of the acid catalyst used. [Example]

[0066] [Viscosity average degree of polymerization of PVA] The viscosity-average degree of polymerization of PVA was measured in accordance with JIS K 6726: 1994. Specifically, when the degree of saponification of PVA was less than 99.5 mol%, the viscosity-average degree of polymerization (P) was calculated 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. P = ([η] × 10 4 / 8.29) (1 / 0.62)

[0067] [Saponification degree of PVA] The degree of saponification of PVA was measured in accordance with JIS K 6726:1994.

[0068] [PVA terminal aldehyde group content] The content of terminal aldehyde groups in PVA was determined by preparing a 10% by mass aqueous solution of PVA, dropping 5 g of this solution into 500 g of a 95:5 mixture of methyl acetate and water to precipitate PVA, recovering it and drying it. The isolated PVA was dissolved in DMSO-d6 and subjected to a 400 MHz 1 It can be determined by measuring using H-NMR. In this case, the peak derived from the methine of the vinyl alcohol unit is at 3.2 to 4.0 ppm (integral value P), and the peak derived from the proton of the aldehyde group is at around 9.5 to 10 ppm (integral value Q). Then, the content was calculated from each peak using the following formula. Aldehyde group content (mol%) = (Q / P) × 100

[0069] [UV absorption spectrum of PVA] A 0.1% by mass aqueous solution of PVA was prepared, and then the aqueous solution was placed in a cell with an optical path length of 1 cm, and the absorbance at 280 nm was measured using a UV-visible spectrophotometer (UV-2450, manufactured by Shimadzu Corporation).

[0070] [Production Example 1: Production of PVA1] A polymerization vessel was charged with 1,500 parts by mass of vinyl acetate (hereinafter sometimes abbreviated as "VAc") and 10 parts by mass of methanol. Next, after replacing the atmosphere inside the polymerization vessel with nitrogen, 12 parts by mass of glutaraldehyde and 12 parts by mass of water were added to the polymerization vessel, and the polymerization vessel was heated to 60°C. Polymerization was carried out in the presence of 2,2'-azobis(isobutyronitrile) as a polymerization initiator until a polymerization rate of 25% was reached. The remaining VAc was expelled from the system together with methanol under reduced pressure while adding methanol, and a methanol solution of polyvinyl acetate (hereinafter sometimes abbreviated as "PVAc") (concentration 40% by mass) was obtained. Next, a methanol solvent in , PVAc The concentration of the polyvinyl alcohol was diluted to 30% by mass, and saponification reaction was carried out for 1 hour at 40°C using sodium hydroxide as a saponification catalyst in a molar ratio of 0.03 relative to PVAc. The resulting polyvinyl alcohol was immersed in a cleaning solution of methyl acetate / methanol = 80 / 20 and washed. The solvent was then removed by centrifugation and the resulting solution was dried to obtain PVA1, which had a viscosity-average degree of polymerization of 1500, a degree of saponification of 99 mol%, a terminal aldehyde group content of 0.1 mol%, and an absorbance at 280 nm of 0.279 in a 0.1% by mass aqueous solution.

[0071] [Production Examples 2 to 12 (Production of PVA2 to 12)] PVA2 to PVA12 were produced in the same manner as in Production Example 1, except that the amounts of vinyl acetate, methanol, water, and aldehyde used during polymerization, the type of aldehyde, the polymerization rate at the time of terminating the polymerization reaction, and the saponification conditions were changed as shown in Table 1. The production conditions are shown in Table 1, and the types of aldehydes used are shown in Table 2.

[0072] [Production Example 13 (Production of PVA13)] PVA11 was produced in the same manner as in Production Example 11, and the resulting PVA11 was heat-treated in a hot air dryer at 80°C for 1 hour to produce PVA13. [Table 1]

[0073] [Table 2]

[0074] Example 1 10 parts by weight of PVA (1) was dissolved in distilled water to prepare 100 parts by weight of a 10% by weight aqueous solution, to which 0.5 parts by weight of phosphoric acid was added as an acid catalyst, and the mixture was stirred to prepare an aqueous resin composition solution. This aqueous solution was cast onto a polyethylene terephthalate (PET) film, left at 23°C and 50% RH for 48 hours, and then heat-treated at 70°C for 5 minutes to obtain a 100 μm thick film. The water resistance of the obtained film was evaluated as follows, and the elution rate was 1.5% by weight.

[0075] (water resistance) The obtained film was immersed in hot water at 80°C for 1 hour, and the dissolution rate (mass%) of the film was measured. 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 were determined, and the dissolution rate (mass%) was calculated using the following formula. The results are shown in Table 3. Elution rate (mass%)=[(X1-X2) / X1]×100

[0076] Example 2 Except for changing the type of PVA used, the water resistance was evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0077] Comparative Example 1 A film was prepared in the same manner as in Example 1, except that PVA6 was used instead of PVA, and an attempt was made to evaluate water resistance. As a result, PVA6 exhibited almost no water resistance because the content of aldehyde groups was too low.

[0078] Comparative Example 2 Except for using PVA7 as the PVA, evaluation of water resistance was attempted in the same manner as in Example 1. As a result, PVA7 exhibited almost no water resistance because its viscosity average degree of polymerization was too low.

[0079] [Table 3]

[0080] Example 3 PVA3 was dissolved in deionized water as a suspension polymerization dispersion stabilizer, and 100 parts by mass of the PVA3 aqueous solution was charged into an autoclave. The amount of PVA3 charged was 850 ppm relative to the amount of vinyl chloride (VCM). Deionized water was then added so that the total amount of deionized water was 1200 parts by mass. Next, 0.65 parts by mass of a 70% by mass toluene solution of cumyl peroxyneodecanoate and 1.05 parts by mass of a 70% by mass toluene solution of t-butyl peroxyneodecanoate were added to the autoclave, and nitrogen was introduced into the autoclave to a pressure of 0.2 MPa. This nitrogen purging operation was then performed a total of five times to thoroughly replace the atmosphere inside the autoclave with nitrogen and remove oxygen, after which 940 parts by mass of vinyl chloride was added. The contents of the autoclave were heated to 57°C, and suspension polymerization of vinyl chloride was initiated with stirring. The pressure inside the autoclave at the start of polymerization was 0.80 MPa. Approximately 3.5 hours after the start of polymerization, when the pressure inside the autoclave reached 0.70 MPa, the polymerization was stopped, unreacted vinyl chloride was removed, and the polymerization reaction product was taken out and dried at 65°C for 16 hours to obtain vinyl chloride polymer particles. The obtained vinyl chloride polymer particles were evaluated by the following methods.

[0081] (Evaluation of vinyl chloride polymer particles) The vinyl chloride polymer particles thus obtained were evaluated for (1) average particle size, (2) particle size distribution, and (3) fisheyes according to the following methods. The evaluation results are shown in Table 4.

[0082] (1) Average particle size The particle size distribution was measured by the dry sieving method described in JIS Z 8815:1994 using a Tyler mesh sieve. The results were plotted using the Rosin-Rammler distribution equation to obtain the average particle diameter (d p50 ) was calculated.

[0083] (2) Particle size distribution The content (% by mass) of vinyl chloride polymer particles that did not pass through a sieve with 355 μm mesh (equivalent to 42 mesh in JIS standard sieve mesh conversion) was evaluated according to the following evaluation criteria. The content means the cumulative amount (% by mass) on the sieve. The mesh size of the sieve conforms to the nominal mesh size W of JIS Z 8801-1-2006. A: Less than 0.5% by mass B: 0.5% by mass or more and less than 1% by mass C: 1% by mass or more

[0084] The content (% by mass) of vinyl chloride polymer particles that passed through a sieve with a mesh size of 355 μm but did not pass through a sieve with a mesh size of 250 μm (equivalent to 60 mesh in JIS standard mesh conversion) was evaluated according to the following evaluation criteria. The content means the cumulative amount (% by mass) on the sieve. The mesh size of the sieve conforms to the nominal mesh size W of JIS Z 8801-1-2006. A: Less than 5% by mass B: 5% by mass or more and less than 10% by mass C: 10% by mass or more

[0085] The content (% by mass) of vinyl chloride polymer particles that passed through a sieve with 75 μm mesh (200 mesh in JIS standard mesh conversion) was evaluated according to the following evaluation criteria. The content means the cumulative amount (% by mass) on the sieve. The mesh size of the sieve conforms to the nominal mesh size W of JIS Z 8801-1-2006. A: Less than 1% by mass B: 1% by mass or more and less than 2% by mass C: 2% by mass or more

[0086] For both the content of vinyl chloride polymer particles that did not pass through a 355 μm sieve and the content of vinyl chloride polymer particles that did not pass through a 250 μm sieve, the smaller the value, the fewer coarse particles there are, the sharper the particle size distribution, and the better the polymerization stability.Furthermore, a lower content of vinyl chloride polymer particles that passed through a 75 μm sieve indicates the absence of fine powder and excellent processability.

[0087] (3) Fisheye 100 parts by mass of the obtained vinyl chloride polymer particles, 50 parts by mass of dioctyl phthalate, 5 parts by mass of tribasic lead sulfate, and 1 part by mass of zinc stearate were roll-kneaded at 150°C for 7 minutes to prepare a 0.1 mm thick sheet. 2 The number of fisheyes per sheet was determined visually, with fewer fisheyes indicating fewer defects on the sheet.

[0088] Examples 4-5 Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA4 to PVA5 were used instead of PVA3. The evaluation results of the obtained 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 obtained vinyl chloride polymer particles did not become coarse, exhibited good polymerization stability, contained little fine powder, and had a small number of fisheyes.

[0089] Comparative Example 3 Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA8 was used as the PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Because PVA8 has a high aldehyde group content, the obtained vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and many fisheyes.

[0090] Comparative Example 4 Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA9 was used as the PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Because PVA9 had an excessively low degree of saponification, the obtained vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and many fisheyes.

[0091] Comparative Example 5 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 obtained vinyl chloride polymer particles are shown in Table 4. Because PVA10 had an excessively high viscosity-average degree of polymerization, the obtained vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and many fisheyes.

[0092] Comparative Example 6 Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA11 was used as the PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Because PVA11 had an excessively low aldehyde group content, the average particle size of the obtained vinyl chloride polymer particles was large, with a high proportion of coarse particles and fine powder, and many fisheyes.

[0093] Comparative Example 7 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. PVA12 is a PVA derived from polyvinyl acetate obtained by polymerizing vinyl acetate in the presence of a monoaldehyde, and since PVA does not have an aldehyde group at its terminal, the resulting vinyl chloride polymer particles had a large average particle size, a high proportion of coarse particles and fine powder, and many fisheyes.

[0094] Comparative Example 8 Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3, except that PVA13 was used as the PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Although PVA13 had a high absorbance value, the aldehyde group content was too low, resulting in a large average particle size of the obtained vinyl chloride polymer particles, a high proportion of coarse particles and fine powder, and many fisheyes. [Table 4]

[0095] As shown in the examples, the PVA of the present invention can be easily crosslinked using an acid or the like, and the crosslinked product exhibits excellent water resistance. Furthermore, when the PVA of the present invention is used as a dispersion stabilizer for the suspension polymerization of vinyl compounds, the polymerization stability is excellent, and the resulting vinyl chloride polymer particles (vinyl polymers) have a small average particle size, with minimal generation of coarse particles and fine powder, resulting in reduced fisheyes. Therefore, the productivity and processability of vinyl polymers are excellent. Therefore, the present invention is extremely useful industrially.

Claims

1. a degree of saponification 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 0.05 mol % or more and less than 0.5 mol % of a structure represented by the following formula (1) at its terminal, and A polyvinyl alcohol having an absorbance at 280 nm of a 0.1% by mass aqueous solution of 0.17 or more but less than 0.55, The content (mol %) of the structure represented by the formula (1) is determined by preparing a 10% by mass aqueous solution of the polyvinyl alcohol, dropping 5 g of this aqueous solution into 500 g of a 95 / 5 solution of methyl acetate and water to precipitate polyvinyl alcohol, recovering and drying the precipitated polyvinyl alcohol, dissolving the isolated polyvinyl alcohol in DMSO-d 6 , and measuring the polyvinyl alcohol using 400 MHz 1 H-NMR. P is the integral of the peak derived from the methine of the vinyl alcohol unit, and Q is the integral of the peak derived from the aldehyde group. The content (mol %) of the polyvinyl alcohol is determined by (Q / P)×100. 【Chemical 1】 (In formula (1), X represents an alkylene group having 1 to 6 carbon atoms, and * represents a bond.)

2. A method for producing polyvinyl alcohol as described in claim 1, which comprises polymerizing a vinyl ester in the presence of a dialdehyde to obtain a polyvinyl ester, and then saponifying the polyvinyl ester.

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

4. A method for producing a vinyl resin, which comprises suspension polymerizing a vinyl compound in the presence of the polyvinyl alcohol according to claim 1.

5. A crosslinked product obtained by crosslinking the polyvinyl alcohol described in claim 1, wherein the elution rate (mass%) of a 100 μm thick film made of the crosslinked product when immersed in hot water at 80°C for 1 hour is less than 10%.

6. The method for producing a crosslinked product according to claim 5, wherein the polyvinyl alcohol is crosslinked in the presence of an acid catalyst.

Images