TWI883007B - Polyvinyl alcohol, method for producing same and use of same - Google Patents

Abstract

A polyvinyl alcohol having a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity average polymerization degree of 400 or more and less than 1800, an aldehyde group of 0.05 mol% or more and less than 0.5 mol% at the end, and an absorbance of 0.1 mass % aqueous solution at 280 nm of 0.17 or more and less than 0.55. By using the polyvinyl alcohol as a dispersion stabilizer for suspension polymerization of vinyl compounds, the formation of coarse particles in the obtained vinyl resin can be suppressed, particles with uniform diameter can be obtained, and the generation of fish eyes can be suppressed. In addition, by using the polyvinyl alcohol, a crosslinked product with excellent water resistance can be easily obtained.

Description

Polyvinyl alcohol, its production method and use

The present invention relates to a polyvinyl alcohol having a saponification degree and a viscosity average polymerization degree within a specific range, further containing a specific amount of aldehyde groups at the terminal, 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 a vinyl compound, and a method for producing a vinyl resin.

Polyvinyl alcohol (hereinafter sometimes referred to as "PVA") having reactive functional groups has been used in adhesives, paper coating agents, polarizing films, and dispersion stabilizers for suspension polymerization of vinyl compounds (such as vinyl chloride) and other products. In particular, the cross-linked polymer synthesized by the cross-linking reaction between the reactive site and the cross-linking agent has better strength, heat resistance, and solvent resistance than the same type of linear polymers because the activity of the molecular chain is restricted to the three-dimensional direction, and is particularly water-resistant, so it is highly useful. In addition, as a wide range of PVA uses, there are various PVAs used as dispersion stabilizers for suspension polymerization of vinyl compounds.

In particular, as a modified PVA having a reactive functional group with high crosslinking performance, there can be cited a modified PVA having a structure represented by an acetylacetyl group in which a methylene hydrogen is capped by two carbonyl groups (Patent Document 1).

Furthermore, in Patent Documents 2 and 3, heat-treated PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds for the purpose of improving the polymerization stability of vinyl compounds and suppressing the coarsening of the obtained ethylene polymer. In addition, polymerization stability in this specification means that since the dispersion of droplets containing vinyl compounds is good during polymerization, as a result, particles of ethylene polymers with coarsening suppressed and uniform diameter can be obtained. [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 2014-205826 [Patent Document 2] Japanese Patent Publication No. 51-45189 [Patent Document 3] Japanese Patent Publication No. 2004-250695

[Problems to be solved by the invention]

However, the benzophenone used in Patent Document 1 is highly toxic to organisms and has the risk of explosion if mixed with air and vapor, and a method for synthesizing modified PVA with reactive functional groups in a safer manner is sought. In addition, 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.

Furthermore, when the modified PVA described in Patent Documents 2 or 3 is used as a dispersion stabilizer for suspension polymerization, although the polymerization stability during polymerization of vinyl compounds is improved, the obtained vinyl polymers are mostly fine powders, which is not sufficient for the requirements pursued in recent years. Furthermore, there are many problems such as fish eyes caused by foreign matter and defects when the vinyl polymers are made into sheets.

The present invention is made to solve the above-mentioned problems, and its purpose is to provide a PVA which, when used as a dispersion stabilizer for suspension polymerization of vinyl compounds, can suppress the formation of coarse particles in the resulting vinyl resin, obtain particles with uniform diameter, and suppress the generation of fish eyes. Another purpose is to provide a PVA that can easily obtain a crosslinked product with excellent water resistance. [Means for solving the problem]

The present invention is based on the discovery of a PVA that solves the above-mentioned problems, and the present invention is completed. The PVA has a saponification degree and a viscosity average polymerization degree within a specific range, further contains a specific amount of aldehyde groups at the end, and has a specific absorbance.

That is, the above-mentioned problem can be solved by providing a polyvinyl alcohol having a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity average polymerization degree of 400 or more and less than 1800, an aldehyde group content of 0.05 mol% or more and less than 0.5 mol% at the end, and an absorbance of 0.1 mass% aqueous solution at 280nm of 0.17 or more and less than 0.55.

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

(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).

Furthermore, X is also preferably an alkylene group having 1 to 6 carbon atoms.

At this time, the preferred method for producing the polyvinyl alcohol is to obtain polyvinyl ester by polymerizing vinyl ester in the presence of dialdehyde or trialdehyde, and then saponifying the polyvinyl ester.

In addition, a preferred embodiment of the present invention is a dispersion stabilizer for suspension polymerization of a vinyl compound containing the above-mentioned polyvinyl alcohol. 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 preferred embodiment of the present invention.

Furthermore, the ideal embodiment of the present invention is a crosslinked product, which is a crosslinked product of the above-mentioned polyvinyl alcohol, wherein the dissolution rate of a film with a thickness of 100 μm containing the above-mentioned crosslinked product is less than 10% when it is immersed in hot water at 80°C for 1 hour. At this time, the ideal method for producing the above-mentioned crosslinked product is to crosslink the above-mentioned polyvinyl alcohol in the presence of an acid catalyst. [Effect of the invention]

If the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, it exhibits high polymerization stability, and can suppress the formation of coarse particles in the resulting vinyl resin, while obtaining particles with uniform diameters, and can suppress the generation of fish eyes. In addition, by using the PVA of the present invention, a crosslinked product with excellent water resistance can be easily obtained.

[Form used to implement the invention]

[Polyvinyl alcohol] The polyvinyl alcohol of the present invention has the following characteristics: a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity average polymerization degree of 400 or more and less than 1800, an aldehyde group content of 0.05 mol% or more and less than 0.5 mol% at the end, and an absorbance of 0.1 mass% aqueous solution at 280nm of 0.17 or more and less than 0.55.

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

Furthermore, when PVA having a saponification degree of less than 70 mol% is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization stability decreases, and while the number of coarse particles in the resulting vinyl resin increases, particles of uniform diameter cannot be obtained. Furthermore, a large number of fish eyes are generated in the resulting vinyl resin. On the other hand, PVA having a saponification degree of more than 99.9 mol% 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 measured in accordance with JIS K 6726:1994.

It is important that the viscosity average degree of polymerization (hereinafter sometimes simply referred to as the degree of polymerization) of PVA is 400 or more and less than 1800. When a PVA with a degree of polymerization of less than 400 is used as a raw material for a crosslinker, the water resistance of the resulting crosslinker decreases. When used as a raw material for a crosslinker, the degree of polymerization of PVA is preferably 550 or more. On the other hand, when the degree of polymerization is greater than 1800, the productivity of PVA decreases. Furthermore, when a PVA with a degree of polymerization of more than 1800 is used as a raw material for a crosslinker, the viscosity of the liquid becomes too high and the operability decreases. The degree of polymerization of PVA is preferably less than 1600.

Furthermore, when PVA having a degree of polymerization of less than 400 is used as a dispersing stabilizer for suspension polymerization of vinyl compounds, the polymerization stability decreases, and while the number of coarse particles in the resulting ethylene resin increases, particles with a uniform diameter 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. Furthermore, when PVA having a degree of polymerization of more than 1800 is used as a dispersing stabilizer for suspension polymerization of vinyl compounds, the polymerization stability decreases, and while the number of coarse particles in the resulting ethylene resin increases, particles with a uniform diameter cannot be obtained. Furthermore, a large number of fish eyes are generated in the resulting ethylene 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 even more preferably less than 1000.

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

It is important that the PVA of the present invention contains 0.05 mol% or more and less than 0.5 mol% of aldehyde groups at the ends. Aldehyde groups are reactive functional groups that can be used for crosslinking reactions due to acetalization with hydroxyl groups, crosslinking due to free radical reactions, etc. In addition, since PVA having aldehyde groups has a high adsorption capacity for vinyl compounds, when used as a dispersing stabilizer for suspension polymerization of vinyl compounds, the polymerization is stable, and the formation of coarse particles and fine powder in the obtained vinyl resin is reduced. Furthermore, when the obtained vinyl resin is made into a sheet, the generation of fish eyes can also be reduced.

When PVA with an aldehyde content of less than 0.05 mol% is used as a raw material for a crosslinker, the water resistance of the resulting crosslinker is reduced. The aldehyde content is preferably 0.08 mol% or more. On the other hand, PVA with an aldehyde content of more than 0.5 mol% has low productivity. In addition, when such PVA is used as a raw material for a crosslinker, the viscosity of the liquid becomes too high and the workability is reduced. The aldehyde content is preferably less than 0.45 mol%, and more preferably less than 0.4 mol%.

Furthermore, when PVA having an aldehyde group content of less than 0.05 mol% is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization stability is reduced, and while the number of coarse particles in the resulting vinyl resin increases, particles with a uniform diameter cannot be obtained. Furthermore, a large number of fish eyes are generated 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 more than 0.5 mol% has low productivity. Furthermore, when such PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization stability is reduced, and while the number of coarse particles in the resulting vinyl resin increases, particles with a uniform diameter cannot be obtained. Furthermore, a large number of fish eyes are generated in the resulting vinyl resin. The content of aldehyde groups is preferably less than 0.45 mol%, and more preferably less than 0.4 mol%. The content of aldehyde groups can be determined from the ¹ H-NMR spectrum of the vinyl ester polymer before saponification in deuterated chloroform solvent, or the ¹ H-NMR spectrum of PVA in heavy DMSO or heavy water solvent.

The PVA of the present invention may also 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 %.

It is important that the absorbance at 280nm of a 0.1 mass% aqueous solution of PVA is not less than 0.17 and not more than 0.55. When the PVA having an absorbance less than 0.17 is used as a raw material for a crosslinker, the water resistance of the resulting crosslinker is reduced. The absorbance is preferably not less than 0.21, more preferably not less than 0.24, and further preferably not less than 0.28. On the other hand, the PVA having an absorbance of not less than 0.55 has low productivity. Furthermore, when such PVA is used as a raw material for a crosslinker, the viscosity of the liquid becomes too high and the operability is reduced. The absorbance is preferably less than 0.52, more preferably less than 0.48, and further preferably less than 0.45.

Furthermore, 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, and while the number of coarse particles in the obtained ethylene resin increases, particles with a uniform diameter cannot be obtained. Furthermore, a large number of fish eyes are generated in the obtained ethylene resin. The absorbance is preferably above 0.21, more preferably above 0.24, and even more preferably above 0.28. On the other hand, the 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 vinyl compounds, the polymerization stability is reduced, and while the number of coarse particles in the obtained ethylene resin increases, particles with a uniform diameter cannot be obtained. Furthermore, a large number of fish eyes are generated in the obtained ethylene resin. The absorbance is preferably less than 0.52, more preferably less than 0.48, and even more preferably less than 0.45.

The above absorbance indicates the amount or chain of ethylene double bonds in the main chain of PVA. When the absorbance is within the above range, the adsorption of PVA to vinyl compounds is improved, and the polymerization stability when used as a dispersion stabilizer for suspension polymerization of vinyl compounds is further improved due to the synergistic effect with the aldehyde group at the end of PVA. When PVA with an absorbance within the above range is used as a raw material for a crosslinked product, the water resistance of the resulting crosslinked product is improved. In addition, the absorption at a wavelength of 280nm originates from the [-CO-(CH=CH) ₂ -] structure in PVA. The introduction of this structure can be achieved by using aldehydes as modifiers, or by using monomers that can introduce ethylene double bonds into the main chain by copolymerization. When the absorbance is outside the above range, the adsorption force between PVA and the vinyl compound is reduced, or PVA and the vinyl compound are dissolved. Therefore, it becomes impossible to play the effect as a dispersion stabilizer for suspension polymerization. In addition, when PVA with an absorbance outside the above range is used as a raw material for a crosslinked product, the water resistance of the obtained crosslinked product is reduced. The conditions such as the measuring device of the absorbance are as described in the examples described below.

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

(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).

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 carbon number of 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 above carbon number, and more preferably an alkylene group having the above carbon number. When X satisfies the above aspects, it has the following tendencies: it is easy to introduce the aldehyde group of PVA, the water solubility of the obtained PVA is good, the polymerization stability is good when PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, or the formation of fish eyes can be suppressed in the obtained vinyl resin.

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, an amino group, and the like.

(Production method of PVA) The production method of PVA of the present invention is not particularly limited, but the ideal production method is to polymerize vinyl ester in the presence of dialdehyde or trialdehyde to obtain polyvinyl ester, and then saponify the polyvinyl ester.

As polymerization methods, there are known methods such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, and dispersion polymerization. From an industrial point of view, solution polymerization, emulsion polymerization, and dispersion polymerization are preferred. Regarding polymerization operations, any of batch, semi-batch, and continuous methods may be used.

Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl octanoate, and vinyl versatate. From an industrial point of view, vinyl acetate is preferred.

The types of dialdehyde or trialdehyde are not particularly limited, and examples thereof include glyoxal, malondialdehyde, succindialdehyde, glutaraldehyde, adipaldehyde, heptanedialdehyde, octanedial, nonanedialdehyde, benzene trimethylenedialdehyde, etc. Among them, dialdehydes or trialdehydes having 2 to 10 carbon atoms are preferably used, and dialdehydes having 2 to 6 carbon atoms are more preferably used, from the viewpoint of easy introduction of aldehyde groups, good water solubility of the resulting PVA, and good polymerization stability when PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds. Glutaraldehyde is further preferably used from the viewpoint of easy availability and suppression of fisheye formation 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, 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. The dialdehyde or trialdehyde may be used alone or in combination of two or more.

It is preferred that a solvent is used in the polymerization step and the mass ratio of vinyl ester to solvent is vinyl ester/solvent = 100/0 to 90/10. If the mass ratio of vinyl ester to solvent is greater than 90/10, when PVA is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the performance of the dispersion stabilizer tends to be reduced.

In the polymerization step, other monomers other than vinyl ester may be copolymerized within the scope of not impairing the gist of the present invention. By copolymerizing other monomers with vinyl ester, the main chain of the obtained polymer has a structure of other monomer units. Examples of the other monomers include α-olefins such as ethylene and propylene; (meth)acrylic acid and its salts; (meth)acrylate esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tertiary butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, octadecyl (meth)acrylate; (meth)acrylamide; N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, diacetone (meth)acrylamide, (meth)acrylamide propanesulfonic acid and its salts. , (meth)acrylamide derivatives such as (meth)acrylamide propyl dimethylamine and its salt or quaternary salt thereof, N-hydroxymethyl (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, tertiary butyl vinyl ether, dodecyl vinyl ether, octadecyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; halogenated vinyls such as vinyl chloride and vinyl fluoride; halogenated vinylides 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; isopropylene acetate, etc. When such other monomers are copolymerized, their content is usually 5 mol% or less.

As the solvent used in the polymerization step, an alcohol solvent is preferably used. Examples of the alcohol solvent include methanol, ethanol, propanol, etc., among which methanol is more preferred. These solvents may be used alone or in combination of two or more.

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

Generally, the productivity of the polymerization step and the subsequent saponification step is reduced by adding water, but in the present invention, it was found that the aldehyde structure can be easily introduced at the terminal by adding water. The reason is not certain, but it is speculated that the side reactions (hemial, acetalization, cyclization) of the dialdehyde or trialdehyde in the polymerization step are suppressed by adding water, and the chemical equilibrium is more inclined to the state of the aldehyde than the state of the product caused by the side reaction, so the reaction between the dialdehyde or trialdehyde and the vinyl ester proceeds efficiently.

The polymerization initiator used in the polymerization step is not particularly limited and can be selected from known polymerization initiators according to the polymerization method. Examples of the polymerization initiator include azo-based polymerization initiators, peroxide-based polymerization initiators, and redox-based polymerization initiators. Examples of the azo-based polymerization initiator include 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(isobutyronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). Peroxide-based polymerization initiators include, for example, percarbonate compounds such as diisopropyl peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, and diethoxyethyl peroxydicarbonate; peroxyester compounds such as tert-butyl peroxyneodecanoate and isopropyl peroxyneodecanoate; acetylcyclohexylsulfonate; 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate, etc. As redox-based polymerization initiators, a combination of an oxidant and a reducing agent may be used. As an oxidant, peroxide is preferred. As a reducing agent, metal ions, reducing compounds, etc. may be cited. As a combination of oxidizing agent and reducing agent, there are: a combination of peroxide and metal ion; a combination of peroxide and reducing compound; a combination of peroxide, metal ion and reducing compound, etc. As peroxide, there are: hydrogen peroxide, cumene hydroperoxide, tertiary butyl hydroperoxide and other hydroperoxides, persulfate (potassium, sodium or ammonium salt), tertiary butyl peracetate, perester (tertiary butyl peroxybenzoate), etc. As metal ions, there are: Fe ²⁺ , Cr ²⁺ , V ²⁺ , Co ²⁺ , Ti ³⁺ , Cu ⁺ and other metal ions that can accept the transfer of 1 electron. As reducing compounds, there can be listed: sodium bisulfite, sodium bicarbonate, tartaric acid, fructose, glucose, sorbitol, bleaching agent, ascorbic acid. Among them, a combination of one or more peroxides selected from the group consisting of hydrogen peroxide, potassium persulfate, sodium persulfate and ammonium persulfate and one or more reducing agents selected from the group consisting of sodium bisulfite, sodium bicarbonate, tartaric acid, bleaching agent and ascorbic acid is more preferred, and a combination of hydrogen peroxide and one or more reducing agents selected from the group consisting of sodium bisulfite, sodium bicarbonate, tartaric acid, bleaching agent and ascorbic acid is more preferred. In addition, a water-soluble polymerization initiator such as potassium persulfate, ammonium persulfate, hydrogen peroxide, cumene hydroperoxide, etc. can be combined with the above-mentioned polymerization initiator to prepare the polymerization initiator. These polymerization initiators can be used alone or in combination of two or more.

When the vinyl ester is polymerized in the presence of dialdehyde or trialdehyde, the polymerization rate of the vinyl ester is not particularly limited, but preferably 10% or more and less than 90%. When the polymerization rate is less than 10%, the productivity of PVA may be reduced. The polymerization rate is more preferably 20% or more. On the other hand, when the polymerization rate is more than 90%, the viscosity of the obtained polyvinyl ester may become too high, the productivity of PVA may be reduced, and the color of the obtained PVA may be deteriorated. The polymerization rate is more preferably less than 70%.

The method for saponifying polyvinyl ester is not particularly limited, and a well-known saponification method can be adopted. For example, alcoholysis reaction or hydrolysis reaction using alkaline catalysts such as sodium hydroxide, potassium hydroxide, sodium methoxide, and acidic catalysts such as p-toluenesulfonic acid can be cited. As solvents that can be used in the reaction, for example, 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 can be cited. 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.

(Application) The PVA of the present invention can be used for various applications. The following are examples but are not limited to them. (1) Application as dispersant: dispersion stabilizer for pigments such as coatings and adhesives, dispersion stabilizer and dispersing aid for suspension polymerization of various vinyl compounds such as vinyl chloride, vinylidene chloride, styrene, (meth)acrylate, vinyl acetate, etc. (2) Application as coating agent: paper coating agent, sizing agent, fiber processing agent, leather finishing agent, coating, antifogging agent, anti-metal corrosion agent, galvanizing gloss agent, anti-static agent Electrolytic agent, pharmaceutical coating agent (3) Adhesive application: Adhesive, adhesive, rewet adhesive, various adhesives, cement and mortar additives (4) Emulsifier application: emulsifier for emulsion polymerization, asphalt and other post-emulsifiers (5) Coagulant application: coagulant for suspended matter and dissolved matter in water, metal coagulant (6) Paper processing application: paper strength enhancer, oil and solvent resistance agent, smoothness enhancer , surface gloss improving agent, coating agent, barrier agent, light fastness agent, water-resistant agent, dye and color developer dispersant, adhesion improver, adhesive (7) Agricultural use: agricultural pesticide adhesive, agricultural pesticide spreading agent, agricultural coating agent, soil improver, anti-corrosion agent, agricultural pesticide dispersant (8) Medical and cosmetic use: granulation adhesive, coating agent, emulsifier, adhesive, binder, film Preparation base material, film forming agent (9) Viscosity modifier application: thickener, rheology modifier (10) Film application: water-soluble film, polarizing film, barrier film, fiber product packaging film, seed care sheet, plant sheet, seed tape, hygroscopic film (11) Molded product application: fiber, pipe, tube, leakproof film, water-soluble lace (chemical lace) water-soluble fiber, sponge (12) Gel use: medical gel, industrial gel (13) Post-reaction use: post-reaction use with low molecular weight organic compounds, high molecular weight organic compounds, inorganic compounds The PVA of the present invention uses an acid catalyst, and the activity of the molecular chain is constrained in the three-dimensional direction, so that it can synthesize a cross-linked product with higher viscosity, higher water resistance, higher strength, heat resistance, and excellent solvent resistance than the same type of linear polymer. Therefore, it can be ideally used in the above-mentioned (2) coating material use, (3) adhesive use, (10) film use, (12) gel use, etc. In addition, the PVA of the present invention is as described below, and can also be ideally used in (1) dispersant use.

(Dispersion stabilizer for suspension polymerization of vinyl compounds) The ideal use of the PVA of the present invention is as a dispersion stabilizer for suspension polymerization of vinyl compounds containing the PVA. If the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization of vinyl compounds, the polymerization reaction is stable and the formation of coarse particles and fine powder can be suppressed. In addition, the formation of fish eyes when the obtained vinyl resin is made into a sheet can be suppressed.

The above-mentioned dispersion stabilizer for suspension polymerization may also contain various additives within the scope that does not impair the purpose of the present invention. Examples of the above-mentioned 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; mold inhibitors; antiblocking agents; defoaming agents; compatibilizers, etc. The content of 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 entire dispersion stabilizer for suspension polymerization.

(Method for producing ethylene resin) Another ideal embodiment of the present invention is a method for producing ethylene resin by suspension polymerization of a vinyl compound in the presence of the PVA of the present invention. In this production method, a particle-shaped ethylene resin can be obtained.

As a method for adding the suspension polymerization dispersion stabilizer of the present invention to the polymerization tank, for example, there are: (i) a method of adding it to the polymerization tank as an aqueous solution, (ii) a method of directly adding it in a powder state, etc. From the viewpoint of uniformity in the polymerization tank, the above method (i) is preferred.

Examples of vinyl compounds include: halogenated vinyls 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 them, it is preferred to use vinyl chloride alone or to use vinyl chloride and a monomer copolymerizable with vinyl chloride in combination. 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, vinyl ether, etc.

In the suspension polymerization of vinyl compounds, oil-soluble or water-soluble polymerization initiators that have been 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; peroxyester compounds such as tert-butyl peroxyneodecanoate, tert-butyl peroxypivalate, tert-hexyl peroxypivalate, and isopropyl peroxyneodecanoate; peroxides such as acetylcyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate, 3,5,5-trimethylhexyl peroxide, and dodecanol 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 water-soluble polymerization initiators include potassium persulfate, ammonium persulfate, hydrogen peroxide, cumene hydroperoxide, etc. These polymerization initiators may be used alone or in combination of two or more.

In the suspension polymerization of the vinyl compound, the polymerization temperature is not particularly limited and can be a low temperature of about 20°C or a high temperature of more than 90°C, preferably about 20 to 60°C. In order to improve the heat removal efficiency of the polymerization reaction system, a polymerization reactor with a reflux condenser can be used.

The obtained ethylene resin can be appropriately mixed with a plasticizer and used for various molded products.

In the suspension polymerization of the vinyl compound, the amount (concentration) of the dispersion stabilizer for suspension polymerization of the present invention is usually 1000 ppm or less and 50 ppm or more 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 above ppm means mass ppm.

In the suspension polymerization of the vinyl compound, in addition to PVA, water-soluble cellulose ethers such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, etc., which are commonly used in the suspension polymerization of the vinyl compound in an aqueous solvent; water-soluble polymers such as gelatin; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, tristearate glyceryl, ethylene oxide propylene oxide block copolymer; water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glyceryl oleate, sodium laurate, etc., can also be used. The amount of addition is not particularly limited, and preferably 0.01 mass part or more and 1.0 mass part or less per 100 mass parts of the vinyl compound.

(Crosslinked product) The ideal use of the PVA of the present invention is a crosslinked product, which is a crosslinked product of the PVA, and the dissolution rate of a film with a thickness of 100 μm containing the crosslinked product is less than 10% when immersed in hot water at 80°C for 1 hour.

Here, the dissolution rate can be measured by the method described in the examples described later. The smaller the dissolution rate, the better the water resistance. In addition, the film to be measured can be produced by a known method such as casting film making method or melt extrusion film making method using a film-making stock solution containing a crosslinked product produced using the PVA of the present invention. In the present invention, it is more preferable that the dissolution rate of the above film is less than 8% when immersed in hot water at 80°C for 1 hour, and it is even more preferable that it is less than 5%.

The method for producing the crosslinked product is not particularly limited, but a method of crosslinking PVA in the presence of an acid catalyst is ideal.

(Acid catalyst) 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.

The method of using the acid catalyst is not particularly limited. The acid catalyst may be used directly or dissolved in a solvent. When mixed with PVA, the acid catalyst may be mixed after the PVA aqueous solution is prepared, or the acid catalyst may be mixed and dissolved while the PVA aqueous solution is prepared. From the viewpoint of suppressing side reactions, the method of mixing the acid catalyst after the PVA aqueous solution is prepared is preferred.

The amount of the acid catalyst used is not particularly limited, but the amount of the acid catalyst used is preferably 0.01 to 5 parts by mass relative to 10 parts by mass of PVA. When the amount used is less than 0.01 parts by mass, there is a risk that a crosslinked product may not be formed smoothly. It is more preferably 0.05 parts by mass or more. On the other hand, when the amount used is greater than 5 parts by mass, there is a risk that a crosslinked product may not be formed smoothly due to a decrease in the relative concentration of PVA. It is more preferably 3 parts by mass or less.

In the production of crosslinked products, in addition to the above-mentioned acid catalyst, a crosslinking agent may also be used. Examples of crosslinking agents include dialdehydes such as glyoxal, malondialdehyde, and glutaraldehyde, glyoxylates such as sodium glyoxylate and calcium glyoxylate, diamines such as ethylenediamine, propylenediamine, and 1,3-bis(aminomethyl)cyclohexane, and dihydrazides such as adipic acid dihydrazide.

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

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

[Saponification degree of PVA] The saponification degree of PVA is measured according to JIS K 6726:1994.

[Terminal aldehyde group content of PVA] The terminal aldehyde group content of PVA is determined as follows: prepare a 10 mass % aqueous solution of PVA, drop 5 g of the aqueous solution into 500 g of a 95/5 solution of methyl acetate/water to precipitate the PVA, recover and dry it, dissolve the separated PVA in DMSO-d ₆ , and measure it using 400 MHz ¹ H-NMR. At this time, the peak derived from the methyl group of the vinyl alcohol unit belongs to 3.2 to 4.0 ppm (integral value P), and the peak derived from the proton of the aldehyde group belongs to around 9.5 to 10 ppm (integral value Q). Then, the content is calculated from each peak by the following formula. Aldehyde group content (molar %) = (Q/P) × 100

[Absorbance of UV absorption spectrum of PVA] A 0.1 mass% aqueous solution of PVA was prepared. Then, the aqueous solution was placed in a tank with a light path length of 1 cm, and the absorbance at 280 nm was measured using an ultraviolet-visible light spectrometer (UV-2450 manufactured by Shimadzu Corporation).

[Production Example 1 Production of PVA1] 1500 parts by mass of vinyl acetate (hereinafter sometimes abbreviated as "VAc") and 10 parts by mass of methanol were added to a polymerization tank. Next, after replacing the polymerization tank with nitrogen, 12 parts by mass of glutaraldehyde and 12 parts by mass of water were added to the polymerization tank, and the polymerization tank was heated to 60°C. In the presence of 2,2'-azobis(isobutyronitrile) as a polymerization initiator, polymerization was carried out until the polymerization rate reached 25%. While adding methanol, the VAc remaining under reduced pressure was ejected from the system together with the methanol, and a methanol solution (concentration 40% by mass) of polyvinyl acetate (hereinafter sometimes abbreviated as "PVAc") was obtained. Next, the concentration of vinyl acetate units in PVAc was diluted to 30% by mass in methanol solvent, and sodium hydroxide was used as a saponification catalyst at a molar ratio of 0.03 relative to PVAc at 40°C for 1 hour. The obtained polyvinyl alcohol was immersed in a cleaning solution of methyl acetate/methanol = 80/20 for cleaning. Next, after centrifugation to remove the solvent, it was dried to obtain PVA1, which has an average degree of polymerization of 1500, a degree of saponification of 99 mol%, a terminal aldehyde content of 0.1 mol%, and an absorbance of 0.1% by mass aqueous solution at 280nm of 0.279.

[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 amount of vinyl acetate, methanol, water, and aldehyde used in the polymerization, the type of aldehyde, the polymerization rate when the polymerization reaction was stopped, 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.

[Production Example 13 (Production of PVA13)] PVA11 was produced by the same method as in Production Example 11, and the obtained PVA11 was heat-treated at 80°C for 1 hour in a hot air dryer to produce PVA13. [Table 1] PVA Aggregation conditions Saponification conditions PVA analysis value Vinyl acetate (mass fraction) Methanol (mass) Water (mass) aldehyde Polymerization rate (%) PVAc concentration (mass %) NaOH/PVAc (mol ratio) Degree of Polymerization Saponification degree (mol%) Aldehyde content (mol%) Absorbance Kind Added amount (weight) Manufacturing example 1 PVA 1 1500 10 12 A 12.0 25 30 0.03 1500 99 0.1 0.279 Manufacturing example 2 PVA 2 1500 10 50 A 50.0 10 30 0.0189 520 88 0.35 0.540 Manufacturing example 3 PVA 3 1500 10 0 A 24.0 50 30 0.013 750 72 0.15 0.377 Manufacturing example 4 PVA 4 1500 10 twenty four A 24.0 50 30 0.0143 720 72 0.25 0.476 Manufacturing example 5 PVA 5 1500 10 27.4 B 27.4 50 30 0.0143 720 72 0.25 0.466 Manufacturing example 6 PVA 6 1500 10 9 A 9.0 50 30 0.03 1500 99 0.04 0.091 Manufacturing example 7 PVA 7 450 1050 0 A 100.0 50 30 0.0189 200 88 0.35 0.532 Manufacturing example 8 PVA 8 1500 10 42 C 42.0 5 30 0.018 720 72 0.54 0.977 Manufacturing example 9 PVA 9 1500 10 twenty four A 24.0 50 30 0.0142 720 65 0.3 0.501 Manufacturing example 10 PVA 10 1500 10 11 A 11.0 1 30 0.0100 1900 72 0.08 0.201 Manufacturing example 11 PVA 11 750 750 0 A 11.0 50 30 0.009 720 72 0.04 0.098 Manufacturing example 12 PVA 12 825 675 0 D 5.0 50 30 0.008 720 72 - 0.18 Manufacturing example 13 PVA 13 750 750 0 A 11.0 50 30 0.009 720 72 0.04 0.172

[Table 2] Kind aldehyde A Glutaraldehyde B Adipaldehyde C Benzene-1,3,5-tricarbaldehyde D Acetaldehyde

Example 1 10 parts by mass of PVA (1) was dissolved in distilled water to prepare 100 parts by mass of a 10% aqueous solution, and 0.5 parts by mass of phosphoric acid was added as an acid catalyst and mixed and stirred to prepare a resin composition aqueous solution. The aqueous solution was cast on a polyethylene terephthalate (PET) film, placed 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. When the water resistance of the obtained film was evaluated using the following method, the dissolution rate was 1.5% by mass.

(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. In addition, the dissolution rate (mass %) was calculated by finding 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, and the dissolution rate (mass %) was calculated using the following formula. The results are shown in Table 3. Dissolution rate (mass %) = [(X1-X2)/X1] × 100

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.

Comparative Example 1 Except for using PVA6 as PVA, a film was prepared in the same manner as in Example 1, and the water resistance was evaluated. As a result, PVA6 showed almost no water resistance due to the low aldehyde content.

Comparative Example 2 Except for using PVA7 as PVA, the water resistance was evaluated in the same manner as in Example 1. As a result, PVA7 showed almost no water resistance due to its low average degree of polymerization.

[Table 3] PVA Acid Catalyst Dissolution rate (mass %) Kind Usage (quantity) Kind Usage (quantity) Embodiment 1 PVA 1 10 Phosphoric acid 0.5 1.5 Embodiment 2 PVA 2 10 Phosphoric acid 0.5 5.3 Comparative example 1 PVA 6 10 Phosphoric acid 0.5 25.6 Comparative example 2 PVA 7 10 Phosphoric acid 0.5 65.5

Example 3 PVA3 was dissolved in deionized water as a dispersing stabilizer for suspension polymerization, and 100 parts by mass of the aqueous solution of PVA3 was added to a sterilizer. The amount of PVA3 added was 850 ppm relative to the amount of vinyl chloride (VCM) added. Next, deionized water was added to make the total amount of deionized water 1200 parts by mass. Next, 0.65 parts by mass of a 70% toluene solution of isopropyl peroxyneodecanoate and 1.05 parts by mass of a 70% toluene solution of tert-butyl peroxyneodecanoate were added to the sterilizer, and nitrogen was introduced into the sterilizer to make the pressure 0.2 MPa. Then, the nitrogen flushing operation was performed a total of 5 times, and after the inside of the autoclave was fully replaced with nitrogen to remove oxygen, 940 parts by mass of vinyl chloride was added. The temperature of the contents in the autoclave was raised to 57°C and the suspension polymerization of vinyl chloride was started under stirring. The pressure in the autoclave at the start of the polymerization was 0.80 MPa. After about 3.5 hours from the start of the polymerization, the polymerization was stopped at the time point when the pressure in the autoclave reached 0.70 MPa, the unreacted vinyl chloride was removed, and the polymerized 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 method shown below.

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

(1) Average Particle Size The particle size distribution was determined using a Tyler mesh sieve by the dry sieving method described in JIS Z 8815: 1994. The results were plotted as a Rosin-Rammler distribution and the average particle size (d _p50 ) was calculated.

(2) Particle size distribution The following evaluation criteria are used to evaluate the content (mass %) of vinyl chloride polymer particles that do not pass through a sieve with a mesh size of 355 μm (JIS standard mesh size is converted to 42 mesh). The above content means the accumulation on the sieve (mass %). In addition, the mesh size of the above sieve is the nominal mesh size W according to JIS Z 8801-1-2006. A: less than 0.5 mass % B: more than 0.5 mass % and less than 1 mass % C: more than 1 mass %

The following evaluation criteria are used to evaluate the content (mass %) of vinyl chloride polymer particles that pass through a sieve with a mesh size of 355 μm and do not pass through a sieve with a mesh size of 250 μm (JIS standard mesh size is converted to 60 mesh). The above content means the accumulation on the sieve (mass %). In addition, the mesh size of the above sieve is the nominal mesh size W according to JIS Z 8801-1-2006. A: less than 5 mass % B: 5 mass % or more and less than 10 mass % C: 10 mass % or more

The following evaluation criteria are used to evaluate the content (mass %) of vinyl chloride polymer particles that pass through a sieve with a mesh size of 75 μm (JIS standard mesh size is converted to 200 mesh). The above content means the accumulation on the sieve (mass %). In addition, the mesh size of the above sieve is the nominal mesh size W according to JIS Z 8801-1-2006. A: less than 1 mass % B: more than 1 mass % and less than 2 mass % C: more than 2 mass %

In addition, the content of vinyl chloride polymer particles that did not pass through the sieve with a mesh size of 355μm and the content of vinyl chloride polymer particles that did not pass through the sieve with a mesh size of 250μm were both smaller, indicating that the coarse particles were less and the particle size distribution was sharper, and the polymerization stability was better. In addition, the content of vinyl chloride polymer particles that passed through the sieve with a mesh size of 75μm was less, indicating that there was less fine powder and the step passability was excellent.

(3) Fisheyes 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, and the number of fisheyes per 1000 ^cm2 was visually measured. The fewer the number of fisheyes, the fewer defects on the sheet.

Examples 4-5 Except that PVA4-5 were used instead of PVA3, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3. 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 dispersing stabilizer for suspension polymerization, the obtained vinyl chloride polymer particles did not become coarse, exhibited good polymerization stability, and had less fine powder and fewer fish eyes.

Comparative Example 3 Except that PVA8 was used as PVA, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since PVA8 has a high content of aldehyde groups, the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine powder is high, and there are also many fish eyes.

Comparative Example 4 Except that PVA9 was used as PVA, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since PVA9 has an excessively low saponification degree, the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine powder is high, and there are also many fish eyes.

Comparative Example 5 Except that PVA10 was used as PVA, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since PVA10 has an excessively high viscosity average polymerization degree, the obtained vinyl chloride polymer particles have a large average particle size, a high proportion of coarse particles and fine powder, and a large number of fish eyes.

Comparative Example 6 Except that PVA11 was used as PVA, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since PVA11 has too little aldehyde content, the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine powder is high, and there are also many fish eyes.

Comparative Example 7 Except that PVA12 was used as PVA, suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3. The evaluation results of the obtained 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 has no aldehyde group at the end of PVA, the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine powder is high, and there are also many fish eyes.

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 PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Although PVA13 has a high absorbance value, the aldehyde content is too low, so the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine powder is high, and there are many fish eyes. [Table 4] PVA Usage (ppm/VCM) Evaluation results of vinyl chloride polymer particles Average particle size (μm) Particle size distribution Fisheye(pcs) Did not pass 42 mesh Not passed 60 mesh Pass 200 mesh Embodiment 3 PVA 3 850 140.0 A A A 8 Embodiment 4 PVA 4 850 130.1 A A A 0 Embodiment 5 PVA 5 850 145.2 A A A 15 Comparative example 3 PVA 8 850 165.5 A B C 55 Comparative example 4 PVA 9 850 199.8 C C C 18000 Comparative example 5 PVA 10 850 222.1 C C C 15000 Comparative example 6 PVA 11 850 203.8 C C B 6000 Comparative example 7 PVA 12 850 210.0 C C B 9000 Comparative example 8 PVA 13 850 185.6 B C B 4000

As shown in the examples, the PVA of the present invention can be easily formed into a crosslinked product using an acid or the like, and the water resistance is excellent at this time. In addition, if the PVA of the present invention is used as a dispersing stabilizer for suspension polymerization of vinyl compounds, the polymerization stability is excellent, and the obtained vinyl chloride polymer particles (ethylene polymer) have a small average particle size, and the generation of coarse particles and fine powder is also small, which can reduce fish eyes. Therefore, the productivity and processability of the ethylene polymer are excellent. Therefore, the industrial usefulness of the present invention is extremely high.

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Claims (6)

A polyvinyl alcohol having a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity average polymerization degree of 400 or more and less than 1800, containing 0.05 mol% or more and less than 0.5 mol% of the structure represented by the following formula (1) at the terminal, and an absorbance of 0.1 mass% aqueous solution at 280 nm of 0.17 or more and less than 0.55,

(In formula (1), X represents an alkylene group having 1 to 6 carbon atoms, and * represents a bond). A method for producing polyvinyl alcohol as claimed in claim 1, which comprises polymerizing vinyl ester in the presence of dialdehyde or trialdehyde to obtain polyvinyl ester, and then saponifying the polyvinyl ester. A dispersion stabilizer for suspension polymerization of vinyl compounds, which contains polyvinyl alcohol as described in claim 1. A method for producing a vinyl resin, which comprises suspension polymerization of a vinyl compound in the presence of polyvinyl alcohol as in claim 1. A crosslinked product, which is a crosslinked product of polyvinyl alcohol as claimed in claim 1, wherein a film with a thickness of 100 μm containing the crosslinked product is immersed in hot water at 80°C for 1 hour, and the dissolution rate is less than 10%. A method for producing a crosslinked product as claimed in claim 5, which comprises crosslinking the polyvinyl alcohol in the presence of an acid catalyst.