Patent Description:
In the related art, PVA-based resins are used as various dispersing agents, and are also used as dispersing agents (for example, a dispersing agent for emulsion polymerization and a dispersing agent for suspension polymerization) during polymerization of monomers.

In addition, as a method for industrially producing a vinyl chloride resin, a method of subjecting a vinyl chloride monomer or a mixture of a vinyl chloride monomer and a monomer copolymerizable with the vinyl chloride monomer to suspension polymerization is known. During the polymerization, a dispersing agent (also referred to as a dispersion stabilizer) such as a PVA-based resin, methylcellulose, a vinyl acetate-maleic anhydride copolymer, gelatin or the like is used. Among these, various PVA-based dispersion stabilizers have been studied in order to improve physical properties of the obtained vinyl chloride polymer (resin) particles, such as bulk density, particle diameter distribution, porosity, plasticizer absorbability, and residual monomer. Among the PVA-based dispersion stabilizers, from the viewpoint of improving the surface activity of the PVA-based dispersion stabilizers, a dispersion stabilizer of the PVA-based resin that focuses on the carbonyl group and the vinylene group adjacent thereto in the PVA molecule has been proposed.

The PVA-based resin is subjected to a heat treatment to cause a dehydration or deacetylation reaction to generate a vinylene group in the main chain, and is used for applications such as dispersion stabilizers for suspension and water retention materials during the production of polyvinyl chloride. It is also known to improve the strength by subjecting a film-like or fibrous PVA-based resin to a heat treatment.

The vinylene group in the PVA-based resin can be measured particularly by using an ultraviolet absorption spectrum of a <NUM> wt% aqueous solution. Those having a peak near <NUM> belong to the structure [-CO-CH=CH-], those having a peak near <NUM> belong to the structure [-CO-(CH=CH)<NUM>-], and those having a peak near <NUM> belong to the structure [-CO-(CH=CH)<NUM>-].

As a stabilizer for suspension polymerization, various heat-treated PVA-based resins have been studied.

For example, a dispersion stabilizer for suspension polymerization, in which a divalent or trivalent metal is further added to a PVA-based resin having a carbonyl group, is disclosed (see, for example, Patent Literature <NUM>). In addition, a PVA of a specific block character has been proposed (see, for example, Patent Literature <NUM>). Further, a PVA satisfying all conditions of carbonyl group, block character, absorbance, and the like has also been proposed (see, for example, Patent Literature <NUM>) in recent years. Moreover, Patent Literature <NUM> discloses a further dispersion stabilizer for suspension polymerization of vinyl compounds according to prior art.

However, in Patent Literature <NUM>, since a solution containing a metal compound is added to a saponified PVA-based resin in order that the resin contains a metal salt or hydroxide, then a solvent is shaken off, and the PVA-based resin after shaking (usually, after shaking, the solvent during saponification is contained in <NUM> wt% or more) is subjected to a heat treatment at a high temperature of <NUM>, there is a possibility that the solvent present near the surface of the resin is volatilized rapidly and the inside of the resin is not dry. In addition, since at the initial period of the heat treatment, heat is used to volatilize the solvent, and the heat is applied to the PVA-based resin from a part where the solvent is volatilized, there is a possibility that before the solvent in the resin is completely volatilized, the heat is applied from the vicinity of the surface, unevenness can be generated at a position where the heat is applied, and a distribution in degree of the heat treatment between the particles and within the particles can occur.

In addition, in Patent Literature <NUM>, the PVA-based resin obtained by drying is subjected to a heat treatment at <NUM>; however, since about <NUM> wt% to <NUM> wt% of the solvent is contained during the drying of the PVA-based resin usually, there are possibilities that the drying is not enough, a distribution between the particles and within the particles where the heat is applied can occur, similar to Patent Literature <NUM>, and a distribution can similarly occur between the particles and within the particles, but the degree is lower than that in Patent Literature <NUM>.

In Patent Literature <NUM>, a melting heat treatment using an extruder does not cause a problem with respect to the distribution of the degree of heat treatment between the particles and within the particles in the heat treatment; however, the molten resin is usually cooled in a water bath, while the PVA-based resin is water-soluble, and it is difficult to cool the molten resin treated at a high temperature, resulting in poor productivity.

The present inventors conducted intensive studies in view of the above problems, and found that a PVA-based resin having a small distribution of a degree of heat treatment in which the degree of heat treatment does not depend on a particle diameter can be obtained by bringing a ratio of absorbance at a specific wavelength of a PVA-based resin having a large particle diameter to absorbance at a specific wavelength of a PVA-based resin having a small particle diameter close to <NUM>. Thus, the present invention has been completed.

Namely, the gist of the present invention is in the following <<NUM>> to <<NUM>>.

According to the method of the present invention, a PVA-based resin having a small distribution in degree of heat treatment is obtained. Therefore, by using such a PVA-based resin, the following effects are obtained: the amount of the PVA-based resin effectively acting during suspension polymerization of vinyl chloride is increased, the number of adsorption points for vinyl chloride particles is increased, and the reaction is uniform.

The description of the constituent requirements described below is an example (representative example) of an embodiment of the present invention, and the present invention is not limited to these contents.

In the present invention, the term "(meth)allyl" means allyl or methallyl, the term "(meth)acryl" means acryl or methacryl, and the term "(meth)acrylate" means acrylate or methacrylate.

The PVA-based resin obtained by the method of the present invention contains at least a PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less and a PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less, in which an absorbance at <NUM> of a <NUM> wt% aqueous solution thereof is <NUM> or more and the PVA-based resin obtained by the method of the present invention satisfies the following Formula (<NUM>): <MAT> (in Formula (<NUM>), X<NUM> represents an absorbance at <NUM> of a <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less, and Y<NUM> represents an absorbance at <NUM> of a <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less).

There is a variation in the particle diameter of the obtained PVA-based resin depending on stirring conditions in a saponification step or pulverization conditions in a drying step. In the present invention, the particle diameter of the PVA-based resin is <NUM> to <NUM>, and at least a PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less and a PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less are contained.

The PVA-based resin obtained by the method of the present invention has an absorbance at <NUM> in an ultraviolet absorption spectrum of <NUM> or more when being made to a <NUM> wt% aqueous solution. When the absorbance at <NUM> of the <NUM> wt% aqueous solution is <NUM> or more, the adsorptivity to vinyl chloride particles is improved. The absorbance at <NUM> of the <NUM> wt% aqueous solution is preferably <NUM> or more, and more preferably <NUM> or more. An upper limit thereof is not particularly limited, and is preferably <NUM> or less, and more preferably <NUM> or less.

In order to make the absorbance at <NUM> <NUM> or more, a method of subjecting a PVA-based resin having a carbonyl group in the molecule to a heat treatment to cause a dehydration or deacetylation reaction can be mentioned, for example. With the above method, since a conjugated double bond is introduced into the PVA-based resin, the absorbance at <NUM> can be <NUM> or more.

The vinylene group in the PVA-based resin can be measured particularly by measuring an ultraviolet absorption spectrum of the <NUM> wt% aqueous solution of the PVA-based resin. Those having a peak near <NUM> belong to the structure [-CO-CH=CH-], those having a peak near <NUM> belong to the structure [-CO-(CH=CH)<NUM>-], and those having a peak near <NUM> belong to the structure [-CO-(CH=CH)<NUM>-].

As a method of introducing a vinylene group into the PVA-based resin, for example, a vinylene group is introduced into the main chain of the PVA-based resin by subjecting the PVA-based resin having a carbonyl group in the molecule to a heat treatment to cause a dehydration or deacetylation reaction.

The ultraviolet absorption spectrum of the PVA-based resin can be obtained by measuring the absorbance of the <NUM> wt% aqueous solution of the PVA-based resin at wavelengths of <NUM>, <NUM>, and <NUM> using an ultraviolet visible near infrared spectrophotometer (for example, "V-<NUM>" manufactured by JASCO Corporation). The measurement is performed using a sample container (cell) having a thickness of <NUM>.

In addition, the PVA-type resin obtained by the method of the present invention satisfies following Formula (<NUM>).

In Formula (<NUM>), X<NUM> represents an absorbance at <NUM> of a <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less, and Y<NUM> represents an absorbance at <NUM> of a <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less. When X<NUM>/Y<NUM> is too small or too large, the distribution in the degree of heat treatment is large.

In Formula (<NUM>), it is preferable that <NUM> ≤ X<NUM>/Y<NUM> ≤<NUM>, and it is particularly preferable that <NUM> ≤ X<NUM>/Y<NUM> ≤ <NUM>. The closer the value of X<NUM>/Y<NUM> is to <NUM>, the smaller the distribution in the degree of heat treatment, and the most preferable value is <NUM>.

The absorbance (X<NUM>) at <NUM> of the <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less, and the absorbance (Yi) at <NUM> of the <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less are determined by sieving the PVA-based resin in accordance with JIS Z8801-<NUM>:<NUM> "standard sieve" and measuring the absorbance by the method described above.

In addition, the PVA-based resin obtained by the method of the present invention preferably satisfies the following Formula (<NUM>): <MAT> (in Formula (<NUM>), X<NUM> represents an absorbance at <NUM> of the <NUM> wt% aqueous solution of the polyvinyl alcohol-based resin having a particle diameter larger than <NUM> and <NUM> or less, and Y<NUM> represents an absorbance at <NUM> of the <NUM> wt% aqueous solution of the polyvinyl alcohol-based resin having a particle diameter larger than <NUM> and <NUM> or less).

The absorbance can be determined by the method described above.

When X<NUM>/Y<NUM> is too small or too large, the distribution in the degree of heat treatment is large. In Formula (<NUM>), it is preferable that <NUM> ≤ X<NUM>/Y<NUM> ≤ <NUM>, and it is particularly preferable that <NUM> ≤ X<NUM>/Y<NUM> ≤ <NUM>. The closer the value of X<NUM>/Y<NUM> is to <NUM>, the smaller the distribution in the degree of heat treatment, and the most preferable value is <NUM>.

In addition, the ratio of the absorbance at <NUM> to the absorbance at <NUM> (<NUM> / <NUM>) of the PVA-based resin obtained by the method of the present invention when being made to a <NUM> wt% aqueous solution is preferably <NUM> or more, more preferably <NUM> or more, and still more preferably <NUM> or more. When the absorbance ratio is too small, the surface activity tends to decrease and the suspension polymerization stability tends to decrease when the PVA-based resin obtained by the method of the present invention is used as a dispersing agent for suspension polymerization. The upper limit of the absorbance ratio is not particularly limited, and is usually <NUM> from the viewpoint of productivity.

In the present invention, examples of a method of producing the PVA-based resin includes a method of subjecting a PVA-based resin having a carbonyl group in the molecule to a heat treatment to cause a dehydration or deacetylation reaction, as described above.

First, a method of introducing a carbonyl group will be described. Examples of such a method include the following methods.

The method (ii) above is preferred industrially. Particularly, a method of polymerizing a vinyl acetate monomer in the presence of a chain transfer agent such as aldehydes and ketones and saponifying the obtained polymer to obtain a PVA-based resin containing a carbonyl group is particularly advantageous. Hereinafter, this method will be described in more detail.

Examples of the vinyl ester-based monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate, and other linear or branched saturated fatty acid vinyl esters. From the viewpoint of practice, vinyl acetate is preferred, and usually vinyl acetate is used alone or vinyl acetate is used in combination with a fatty acid vinyl ester compound other than vinyl acetate.

As the chain transfer agent used in the method, examples of the aldehydes include acetaldehyde, propionaldehyde, butyraldehyde, and benzaldehyde, and examples of the ketones include acetone, methyl ethyl ketone, hexanone, and cyclohexanone. Among these, aldehydes are preferred, and acetaldehyde is particularly preferred from the viewpoint of productivity such as solvent recovery. The amount of the chain transfer agent added is slightly different depending on a chain transfer constant of the chain transfer agent to be added and the degree of polymerization of a target PVA-based resin, and is <NUM> wt% to <NUM> wt% and more preferably <NUM> wt% to <NUM> wt%, with respect to the vinyl ester-based monomer. As a method of charging the chain transfer agent, the chain transfer agent may be charged in batch at an initial period or may be charged during the polymerization reaction, and the molecular weight distribution of the PVA-based resin can be controlled by charging using any method.

The method of polymerizing the vinyl ester-based monomer, especially vinyl acetate, is not particularly limited, and any known polymerization method can be used. Usually, solution polymerization using alcohol such as methanol, ethanol or isopropyl alcohol as a solvent is performed. Of course, bulk polymerization, emulsion polymerization, and suspension polymerization can also be used. In the solution polymerization, the vinyl ester-based monomer can be charged by any means such as split charging or batch charging. The polymerization reaction is performed using a known radical polymerization catalyst such as azobisisobutyronitrile, acetyl peroxide, benzoyl peroxide, lauroyl peroxide, azobisdimethylvaleronitrile, and azobismethoxyvaleronitrile. The reaction temperature is selected from the range of <NUM> to the boiling point.

At this time, if necessary, a modified PVA-based resin obtained by copolymerizing a vinyl ester-based monomer and a polymerizable monomer can be used. Examples of such a monomer include: olefins such as ethylene, propylene, isobutylene, α-octene, α-dodecene, and α-octadecene; unsaturated acids such as an acrylic acid, a methacrylic acid, a crotonic acid, a maleic acid, a maleic anhydride, an itaconic acid or a salt or a mono- or dialkyl ester thereof; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefin sulfonic acids such as an ethylene sulfonic acid, an allyl sulfonic acid, a methallyl sulfonic acid or a salt thereof; alkyl vinyl ethers; N-acrylamidomethyltrimethylammonium chloride; allyltrimethylammonium chloride; dimethylallyl vinyl ketone; N-vinylpyrrolidone; vinyl chloride; vinylidene chloride; polyoxyalkylene (meth)allyl ethers such as polyoxyethylene (meth)allyl ether and polyoxypropylene (meth)allyl ether; polyoxyalkylene (meth)acrylates such as polyoxyethylene (meth)acrylate and polyoxypropylene (meth)acrylate; polyoxyalkylene (meth)acrylamides such as polyoxyethylene (meth)acrylamide and polyoxypropylene (meth)acrylamide; polyoxyethylene (<NUM>-(meth)acrylamide-<NUM>,<NUM>-dimethylpropyl) ester; polyoxyethylene vinyl ether; polyoxypropylene vinyl ether; polyoxyethylene allylamine; polyoxypropylene allylamine; polyoxyethylene vinylamine; polyoxypropylene vinylamine; and hydroxy group-containing α-olefins such as <NUM>-buten-<NUM>-ol, <NUM>-penten-<NUM>-ol and <NUM>-hexen-<NUM>-ol, and derivatives such as acylated products thereof. The above monomer can be copolymerized in an amount of <NUM> mol% to <NUM> mol%.

In addition, examples of such a monomer include diol-containing compounds such as <NUM>,<NUM>-dihydroxy-<NUM>-butene, <NUM>,<NUM>-diacyloxy-<NUM>-butene, <NUM>-acyloxy-<NUM>-hydroxy-<NUM>-butene, <NUM>-acyloxy-<NUM>-hydroxy-<NUM>-butene, <NUM>,<NUM>-diacyloxy-<NUM>-methyl-<NUM>-butene, <NUM>,<NUM>-dihydroxy-<NUM>-pentene, <NUM>,<NUM>-diacyloxy-<NUM>-pentene, <NUM>,<NUM>-dihydroxy-<NUM>-methyl-<NUM>-pentene, <NUM>,<NUM>-diasiloxy-<NUM>-methyl-<NUM>-pentene, <NUM>,<NUM>-dihydroxy-<NUM>-hexene, <NUM>,<NUM>-diasiloxy-<NUM>-hexene, glycerin monoallyl ether, <NUM>,<NUM>-diacetoxy-<NUM>-allyloxypropane, <NUM>-acetoxy-<NUM>-allyloxy-<NUM>-hydroxypropane, <NUM>-acetoxy-<NUM>-allyloxy-<NUM>-hydroxypropane, glycerin monovinyl ether, glycerin monoisopropenyl ether, vinyl ethylene carbonate, and <NUM>,<NUM>-dimethyl-<NUM>-vinyl-<NUM>,<NUM>-dioxolane. The above monomer may also be copolymerized in an amount of <NUM> mol% to <NUM> mol%.

In the saponification, the vinyl ester polymer obtained above is dissolved in an alcohol and the saponification is performed in the presence of an alkali catalyst or an acid catalyst. Examples of the alcohol include methanol, ethanol, and butanol. The concentration of the polymer in the alcohol is selected from the range of <NUM> wt% to <NUM> wt%. As the alkali catalyst, for example, alkali catalysts such as hydroxides or alcoholates of alkali metals such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate and potassium methylate can be used. As the acid catalyst, for example, an aqueous solution of inorganic acids such as hydrochloric acid and sulfuric acid, or organic acids such as p-toluenesulfonic acid can be used.

The amount of such a catalyst used is necessary to be <NUM> mmol to <NUM> mmol equivalents with respect to the vinyl ester-based monomer. In such a case, the saponification temperature is not particularly limited, and is usually selected from the range of <NUM> to <NUM>, preferably <NUM> to <NUM>. The reaction is performed over <NUM> hours to <NUM> hours.

The PVA-based resin thus obtained contains a carbonyl group in the molecule, and the content is preferably <NUM> mol% or more, and more preferably <NUM> mol% or more. When the content is too small, the amount of the vinylene group produced tends to be insufficient. The upper limit is usually <NUM> mol%.

In addition, the saponification degree (measured in accordance with JIS K6726) of the PVA-based resin is preferably <NUM> mol% to <NUM> mol%, more preferably <NUM> mol% to <NUM> mol%, still more preferably <NUM> mol% to <NUM> mol%, and particularly preferably <NUM> mol% to <NUM> mol%. When the saponification degree is too low, the solubility of the PVA-based resin in water tends to decrease, the melting point thereof tends to be lowered, and the resin tends to be aggregated and blocked during heat treatment; when the saponification degree is too high, the surface active ability tends to be lowered, the dispersibility of the vinyl chloride monomer tends to be lowered, and a block tends to be formed during the suspension polymerization.

The average degree of polymerization (measured in accordance with JIS K6726) of the PVA-based resin is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and particularly preferably <NUM> to <NUM>. When the average degree of polymerization is too small, protective colloid properties tend to be too low and tend to cause aggregation during the suspension polymerization; when the average degree of polymerization is too large, the amount of the vinylene group at the end of the PVA-based resin tends to decrease and the surface activity tends to decrease.

The PVA-based resin preferably contains a divalent or trivalent metal salt or hydroxide from the viewpoint of promoting the deacetylation reaction. Examples of the divalent or trivalent metal include magnesium, calcium, zinc, and aluminum. Specific examples of the metal salt or hydroxide include magnesium acetate tetrahydrate, calcium acetate, calcium propionate, magnesium butyrate, magnesium carbonate, magnesium hydroxide, zinc acetate, and aluminum hydroxide. Among these, magnesium acetate tetrahydrate and calcium acetate are preferably used since they are dissolved in water and/or methanol and are industrially easy to handle. A method of adding these compounds is not particularly limited as long as they are contained in the above PVA-based resin. The above compounds may be added directly to the paste before saponification or the slurry after saponification. Preferred is a method of dissolving the above compound preferably in an alcohol such as methanol, ethanol and propanol, or water, adding the obtained mixture to a PVA-based resin slurry after saponification in the form of a solution having a concentration of <NUM> wt% to <NUM> wt%, and distributing the solution to the PVA-based resin. The content of the compound in the PVA-based resin is preferably <NUM>µmol/g to <NUM>µmol/g, and more preferably <NUM>µmol/g to <NUM>µmol/g, with respect to the PVA-based resin. When the content is too small, the amount of the vinylene group produced tends to decrease; when the content is too large, coloring or decomposition of the PVA-based resin tends to be severe.

In the present invention, it is preferable to contain the divalent or trivalent metal salt or hydroxide as described above. However, in addition to these compounds, for example, a monovalent metal compound such as sodium acetate can be used in combination in a range (<NUM> wt% or less with respect to the divalent or trivalent metal salt or hydroxide) not impairing the effects of the present invention.

In the present invention, it is preferable from the viewpoint of introduction efficiency of the vinylene group that the above divalent or trivalent metal salt or hydroxide is contained in the PVA-based resin containing a carbonyl group in advance as described above, and it is also possible to contain a carbonyl group by heat treatment after a PVA-based resin not containing a carbonyl group is made to contain the above divalent or trivalent metal salt or hydroxide.

The PVA-based resin obtained as described above is dried after saponification to be a powdery PVA-based resin. In the present invention, when the PVA-based resin is being dried, first preliminary drying is performed. Examples of the drying method for preliminary drying include reduced pressure drying, normal pressure drying, and hot air drying. The drying time is usually <NUM> minutes to <NUM> hours, and preferably <NUM> hour to <NUM> hours. The drying temperature is <NUM> or more and less than <NUM>.

After the drying, the PVA-based resin usually contains <NUM> wt% to <NUM> wt% of a solvent (for example, methanol or ethanol) used in saponification.

A double bond is generated in the molecule by subjecting the PVA-based resin obtained by the preliminary drying to a heat treatment. In the present invention, after the preliminary drying, it is preferable to perform further drying before the heat treatment, that is, to perform drying before heat treatment.

Examples of the method for the drying before heat treatment include the usual drying methods described above, and vacuum drying is particularly preferred from the viewpoint of drying efficiency.

In addition, after the drying before heat treatment, the PVA-based resin is preferably dried until the above solvent is less than <NUM> wt%.

The pressure for the reduced pressure drying is usually <NUM> kPa or less, preferably <NUM> kPa or less, and particularly preferably <NUM> kPa or less. When the pressure is too high, it takes time to perform the drying before heat treatment, which is not preferred in a production step. The lower limit of the pressure is better as it is closer to <NUM> kPa.

As drying conditions in the drying step, the temperature is <NUM> to <NUM>.

In addition, the time of the drying before heat treatment is appropriately selected in consideration of the above temperature and pressure conditions, the weight of the object to be treated, and is preferably set within a range of usually <NUM> minutes to <NUM> minutes.

The PVA-based resin obtained by the method of the present invention can be obtained by performing a heat treatment after the above drying before heat treatment to cause a dehydration or deacetylation reaction to form a double bond. The heat treatment method is not particularly limited, and examples thereof include usually a method of subjecting a PVA-based resin to a specific heat treatment. The temperature condition for the heat treatment is preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>. When the temperature condition is too low, the desired amount of the vinylene group cannot be obtained; when the temperature condition is too high, degradation due to the heat treatment tends to be severe, and the resin tends to melt and adhesion in a tank tends to increase.

The heat treatment time is preferably <NUM> hour to <NUM> hours, and more preferably <NUM> hours to <NUM> hours. When the heat treatment time is too short, the amount of the vinylene group produced tends to decrease; when the heat treatment time is too long, the PVA-based resin tends to be colored or insoluble matters tends to be generated in water.

The above heat treatment is preferably performed in an oxygen atmosphere having an oxygen concentration of <NUM> vol% or less, and more preferably in an oxygen atmosphere having an oxygen concentration of <NUM> vol% to <NUM> vol%. When the oxygen concentration is too high, the coloring of the PVA-based resin tends to be severe, or insolubilization tends to occur. In the heat treatment, those obtained by containing the metal salt mentioned above in a PVA-based resin obtained by a known method can be used. In order to generate a sufficient amount of the vinylene group to obtain good surface activity, the content of the carbonyl group in the PVA-based resin before the heat treatment is preferably <NUM> mol% to <NUM> mol%.

Any device may be used in the above heat treatment, and examples of the above heat treatment include: (<NUM>) a method of performing treatment with a heatable mixing device such as a Nauter mixer or a conical dryer; (<NUM>) a method of performing treatment with a general stationary dryer; and (<NUM>) a method using a flask heated with a heat medium, for example, a method using an evaporator. Among these, a heatable mixing device is preferred from the viewpoint of reducing the distribution of heat treatment in the present invention.

The PVA-based resin thus obtained has an absorbance at <NUM> [belong to the structure of -CO-CH=CH-] of <NUM> or more and preferably <NUM> or more in the ultraviolet absorption spectrum of the <NUM> wt% aqueous solution thereof; has an absorbance at <NUM> [belong the structure of -CO-(CH=CH)<NUM>-] of <NUM> or more and preferably <NUM> or more in the ultraviolet absorption spectrum of the <NUM> wt% aqueous solution thereof; and has an absorbance at <NUM> [belong the structure of -CO-(CH=CH)<NUM>-] of <NUM> or more and preferably <NUM> or more in the ultraviolet absorption spectrum of the <NUM> wt% aqueous solution thereof. The absorbance ratio represented by <NUM> / <NUM> is <NUM> or more, and preferably <NUM> or more. When it is too low, the foaming prevention effect during suspension polymerization of vinyl-based compounds such as vinyl chloride tends to be low.

The saponification degree (measured in accordance with JIS K6726) of the PVA-based resin obtained by the method of the present invention is preferably <NUM> mol% to <NUM> mol%, more preferably <NUM> mol% to <NUM> mol%, still more preferably <NUM> mol% to <NUM> mol%, and particularly preferably <NUM> mol% to <NUM> mol%. When the saponification degree is too low, the dispersibility of the PVA-based resin in water tends to decrease; when the saponification degree is too high, the surface active ability tends to be lowered, the dispersibility of the vinyl chloride monomer tends to be lowered, and a block tends to be formed during the suspension polymerization.

In addition, the average degree of polymerization (measured in accordance with JIS K6726) of the PVA-based resin obtained by the method of the present invention is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and particularly preferably <NUM> to <NUM>. When the average degree of polymerization is too small, protective colloid properties tend to be too low and tend to cause aggregation during the suspension polymerization; when the average degree of polymerization is too large, the amount of the vinylene group at the end of the PVA-based resin tends to decrease and the surface activity tends to decrease.

The PVA-based resin obtained by the method of the present invention is useful as a dispersing agent for stably dispersing solid fine particles in a liquid, and particularly useful as a dispersing agent for suspension polymerization.

Next, a suspension polymerization method for a vinyl-based compound (vinyl chloride) using the PVA-based resin obtained by the method of the present invention as a dispersing agent will be described.

During the suspension polymerization, the PVA-based resin obtained by the method of the present invention is usually added as a dispersing agent to water or a heated aqueous medium to disperse a vinyl chloride monomer, and the polymerization is performed in the presence of an oil-soluble catalyst. The PVA-based resin (dispersing agent) can be added in the form of a powder or a solution. When the PVA-based resin has a low saponification degree and is an aqueous dispersion, the PVA-based resin can be added as an aqueous dispersion liquid. Particularly in the form of a solution, when the PVA-based resin is water soluble, the PVA-based resin can be added as an aqueous solution or as a solution after being dissolved in an organic solvent such as an alcohol, a ketone and an ester or a mixed solvent of the organic solvent and water even when the water solubility is low. Even when the saponification degree is low in the aqueous dispersion liquid, the PVA-based resin can be added to the aqueous dispersion liquid as it is in a case of having self-dispersibility in water.

The dispersing agent may be charged in batch at an initial period of the polymerization, or may be split and charged in the middle of the polymerization. The catalyst used may be any oil-soluble catalyst. For example, benzoyl peroxide, lauroyl peroxide, diisopropyl peroxydicarbonate, α,α'-azobisisobutyronitrile, α,α'-azobis-<NUM>,<NUM>-dimethyl-valeronitrile, acetylcyclohexylsulfonyl peroxide, and a mixture thereof can be used. The polymerization temperature is optionally selected from a range well known to those skilled in the art.

In addition, a known stabilizer other than the PVA-based resin obtained by the method of the present invention, for example, a polymer substance can be used in combination. Examples of the polymer substance include a PVA having an average degree of polymerization of <NUM> to <NUM>,<NUM> and a saponification degree of <NUM> mol% to <NUM> mol% or a derivative thereof. Examples of the derivative of the PVA include a formalized product, an acetalized product, a butyralized product, and an urethanized product of the PVA, and an esterified product of the PVA with a sulfonic acid or a carboxylic acid. Further examples include the above modified PVA-based resin. However, the present invention is not necessarily limited thereto.

In addition, examples of the polymer substance other than the PVA-based resin include: cellulose derivatives such as methylcellulose, ethylcellulose, hydroxymethylcellulose, hydroxypropylmethylcellulose, hydroxybutylmethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, aminomethylhydroxypropylcellulose, and aminoethylhydroxypropylcellulose; starch, tragacanth, pectin, glue, alginic acid or a salt thereof; gelatin, polyvinylpyrrolidone, polyacrylic acid or a salt thereof; polymethacrylic acid or a salt thereof; a copolymer of polyacrylamide, polymethacrylamide or vinyl acetate with an unsaturated acid such as a maleic acid, a maleic anhydride, an acrylic acid, a methacrylic acid, an itaconic acid, a fumaric acid, and a crotonic acid; a copolymer of styrene with the above unsaturated acid; a copolymer of vinyl ether with the above unsaturated acid; and a salt or ester of the above copolymers. Various surfactants or inorganic dispersing agents can be used in combination as an auxiliary during the polymerization, and the PVA-based resin obtained by the method of the present invention can also be used as an auxiliary.

Further, not only homopolymerization of vinyl chloride but also copolymerization of vinyl chloride with a monomer copolymerizable therewith is performed. Examples of the copolymerizable monomer include vinylidene halide, vinyl ether, vinyl acetate, vinyl benzoate, acrylic acid or methacrylic acid and an ester thereof, maleic acid or an anhydride thereof, ethylene, propylene, and styrene. In addition, during the polymerization of vinyl chloride, it is also optional to add a polymerization regulator, a chain transfer agent, a gelation improver, an antistatic agent, a pH adjuster that are to be used as appropriate. The polymerization of vinyl chloride has been mainly described above, but the dispersing agent obtained by the method of the present invention is not necessarily limited to vinyl chloride, and is also used for suspension polymerization of optional vinyl-based compounds such as styrene, methacrylate and vinyl acetate.

Hereinafter, although the present invention is demonstrated further more concretely by ways of Examples, the present invention is not limited to following Examples, unless the gist of the present invention is exceeded. Hereinafter, "%" and "part" refer to a weight basis unless otherwise specified.

<NUM> parts of vinyl acetate, <NUM> parts of acetaldehyde, <NUM> parts of methanol, and <NUM>% acetyl peroxide (APO) with respect to vinyl acetate were charged into a polymerization tank and the air in the polymerization tank was replaced with nitrogen. Thereafter, polymerization was started at the boiling point by heating, and the polymerization was stopped when a polymerization rate reached <NUM>% after <NUM> hours. Next, the unpolymerized vinyl acetate was removed, and the obtained polymer was saponified with sodium hydroxide by a related method, so as to prepare a saponified slurry (solvent: methyl acetate/methanol = <NUM>/<NUM> (weight ratio)) of the PVA-based resin (polymerization degree: <NUM>, saponification degree: <NUM> mol%, the amount of carbonyl group: <NUM> mol%) having a resin content of <NUM>%.

Next, a <NUM>% methanol solution of magnesium acetate tetrahydrate as a metal compound was added to the PVA-based resin prepared above at a rate of <NUM> with respect to <NUM> of the PVA-based resin, and the mixture was stirred at <NUM> for <NUM> hour. Thereafter, the mixture was shaken off with Nutsche and dried at <NUM> for <NUM> hours with a blow dryer (preliminary drying), to obtain a PVA-based resin containing <NUM>µmol/g of magnesium acetate.

Next, the obtained PVA-based resin was dried in a heat treatment tank at <NUM> for <NUM> hours under a reduced pressure of <NUM> kPa (drying before heat treatment), then a gas of nitrogen: air = <NUM>:<NUM> (volume ratio) was poured into the heat treatment tank at a rate of <NUM>/hr, and the heat treatment was performed at <NUM> for <NUM> hours while maintaining an oxygen concentration at <NUM>%. The characteristics of the PVA-based resin after the heat treatment were as follows. Polymerization degree: <NUM> (measured in accordance with JIS K6726); saponification degree: <NUM> mol% (measured in accordance with JIS K6726); magnesium acetate: <NUM>µmol/g (calculated in terms of the amount of magnesium).

The obtained PVA-based resin was sieved with sieves having nominal apertures of <NUM>, <NUM>, <NUM>,<NUM>, and <NUM>,<NUM> (JIS Z8801-<NUM>:<NUM> "standard sieve").

For the measurement of the absorbance in each particle diameter range of the PVA-based resin powder sieved above, the absorbance of a <NUM> wt% aqueous solution of the PVA-based resin was measured at wavelengths of <NUM>, <NUM>, and <NUM> using an ultraviolet visible near infrared spectrophotometer ("V-<NUM>" manufactured by JASCO Corporation). The measurement was performed using a sample container (cell) having a thickness of <NUM>. An absorbance ratio (X<NUM>/Y<NUM>) of an absorbance X<NUM> at <NUM> of a <NUM> wt% aqueous solution of a PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less to an absorbance Y<NUM> at <NUM> of a <NUM> wt% aqueous solution of a PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less and an absorbance ratio (X<NUM>/Y<NUM>) of an absorbance X<NUM> at <NUM> of the <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less to an absorbance Y<NUM> at <NUM> of the <NUM> wt% aqueous solution of the PVA-based resin having a particle diameter larger than <NUM> and <NUM> or less were calculated. The results are shown in Table <NUM>.

The treatment was performed in the same manner as in Example <NUM> except that a PVA-based resin having a saponification degree after heat treatment of <NUM> mol% (measured in accordance with JIS <NUM>) was used, unlike in Example <NUM>. The absorbance was measured by sieving in the same manner, and the absorbance ratios X<NUM>/Y<NUM> and X<NUM>/Y<NUM> were calculated in the same manner as in Example <NUM>. The results are shown in Table <NUM>.

The treatment was performed in the same manner as in Example <NUM> except that the drying after shaking (preliminary drying) was not performed, unlike in Example <NUM>. The absorbance was measured by sieving in the same manner, and the absorbance ratios X<NUM>/Y<NUM> and X<NUM>/Y<NUM> were calculated in the same manner as in Example <NUM>. The results are shown in Table <NUM>.

In Examples <NUM> and <NUM> in Table <NUM> above, the value of the absorbance ratio (<NUM> to <NUM> / <NUM> to <NUM>) between particles at <NUM> and <NUM> is close to <NUM>, indicating that the absorbance at <NUM> for coarse particles and fine particles is at the same level, which means that the difference in absorbance due to the particle diameter is small. On the other hand, it can be seen that, in Comparative Example <NUM>, the absorbance ratio between particles is smaller than that in Examples <NUM> and <NUM>, which means that the difference in absorbance due to the particle diameter is large, and a distribution occurs in the degree of heat treatment between particle diameters.

Therefore, after performing saponification and shaking of the PVA-based resin, first the drying is performed slowly with preliminary drying, then the temperature is raised, the drying is performed again, and then the PVA-based resin is subjected to the heat treatment. Thus, the degree of heat treatment between the PVA-based resin particles can be made uniform, and accordingly it is estimated that following the effects are obtained: the amount of the PVA-based resin effectively acting during the suspension polymerization of vinyl chloride is increased, the number of adsorption points for polyvinyl chloride particles is increased, and the reaction is uniform.

Claim 1:
A method for producing a polyvinyl alcohol-based resin, the polyvinyl alcohol-based resin comprising at least:
a polyvinyl alcohol-based resin having a particle diameter larger than <NUM> and <NUM> or less; and
a polyvinyl alcohol-based resin having a particle diameter larger than <NUM> and <NUM> or less,
wherein an absorbance at <NUM> of a <NUM> wt% aqueous solution of the polyvinyl alcohol-based resin is <NUM> or more; and
the polyvinyl alcohol-based resin satisfies the following Formula (<NUM>): <MAT>
wherein in Formula (<NUM>), X<NUM> represents an absorbance at <NUM> of a <NUM> wt% aqueous solution of the polyvinyl alcohol-based resin having a particle diameter larger than <NUM> and <NUM> or less, and Y<NUM> represents an absorbance at <NUM> of a <NUM> wt% aqueous solution of the polyvinyl alcohol-based resin having a particle diameter larger than <NUM> and <NUM> or less; and
wherein the absorbance was measured according to the method as set out in the description;
the method comprising the following steps <NUM>) to <NUM>) in this order:
<NUM>) drying the polyvinyl alcohol-based resin at a temperature between <NUM> and less than <NUM>,
<NUM>) drying the polyvinyl alcohol-based resin at a temperature between <NUM> and <NUM>,
<NUM>) subjecting the polyvinyl alcohol-based resin having a carbonyl group in a molecule to a heat treatment; and
<NUM>) causing a dehydration or deacetylation reaction to take place.