JP2019031581A - Low polymerization degree vinyl alcohol-based polymer powder having excellent color tone and production method thereof - Google Patents

Abstract

To provide low polymerization degree vinyl alcohol-based polymer powder having excellent color tone with excellent productivity.SOLUTION: A low polymerization degree vinyl alcohol-based polymer powder that has the viscosity average polymerization degree of 100 to 500, and the saponification degree of 80 to 99.9 mole%, and the yellow index of the powder having a particle size in the range of 100 to 1000 μm of less than 10.0.SELECTED DRAWING: None

Description

  The present invention relates to a low-polymerization degree vinyl alcohol polymer powder excellent in hue and a method for producing the same.

  Polyvinyl alcohol has been conventionally used as an adhesive, a paper coating agent, a polarizing film, a water-soluble film, a pharmaceutical, a composition for cosmetics, a dispersion stabilizer for suspension polymerization of vinyl compounds (for example, vinyl chloride), and the like. It is used for applications, and it is often the case that the added value of the product is increased when the polyvinyl alcohol is less colored and white.

  Causes of coloring include heat history during drying, origin of initiator used during vinyl ester polymer production, initiator residue, amount of alkali used in the saponification process, etc. Especially, polymerization of vinyl alcohol polymer When the degree is low, the influence of the above-mentioned origin on the coloration becomes large, and it has been difficult to obtain a low-polymerization degree vinyl alcohol polymer having an excellent hue.

  For example, in Patent Document 1, by using a specific peroxyester compound as a polymerization initiator, there is little coloring, no adverse effects on production equipment and quality, excellent thermal stability, and coloring by heating. It is said that difficult PVA can be obtained. However, although a production example of a medium to high polymerization degree vinyl alcohol polymer having a polymerization degree of 600 or more is disclosed, a production example of a low polymerization degree vinyl alcohol polymer having a polymerization degree of less than 600 is not disclosed. . Generally, when producing a low-polymerization degree vinyl alcohol polymer, it is necessary to increase the polymerization rate of the vinyl ester monomer. To increase the polymerization rate, methods such as increasing the initiator amount or increasing the reaction time can be considered, but increasing the initiator amount will deteriorate the hue, and increasing the reaction time will decrease the production efficiency. Will do. In the method shown in Patent Document 1, it is considered difficult to produce a low-polymerization degree vinyl alcohol polymer while achieving both suppression of coloring and production efficiency.

JP-A-5-320219

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a low-polymerization degree vinyl alcohol polymer powder excellent in hue with high productivity.

  The above-mentioned problems are that the viscosity average polymerization degree is 100 to 500, the saponification degree is 80 to 99.9 mol%, and the yellow index of the powder having a particle diameter in the range of 100 to 1000 μm is less than 10.0. This is solved by providing a low-polymerization degree vinyl alcohol polymer powder characterized in that.

  At this time, it is preferable that the yellow index when a 5% aqueous solution is used is less than 2.0.

The above-mentioned problem is a method for producing a low-polymerization degree vinyl alcohol polymer powder,
By making the pressure in the polymerization tank higher than atmospheric pressure, the temperature in the polymerization tank is set to a temperature 2 to 40 ° C. higher than the boiling point of the reaction solution under atmospheric pressure,
The amount of polymerization initiator added to the vinyl ester monomer is 0.001 to 0.05 mol%,
The low dissociation is characterized in that a polyvinyl ester polymer is produced by adding an acid and / or salt thereof having a first dissociation index of 2 to 5 to 0.1 to 500 ppm based on the weight of the vinyl ester monomer, and then saponified. The problem can also be solved by providing a method for producing a vinyl alcohol polymer powder having a degree of polymerization.

  At this time, it is a preferred embodiment that the pressure in the polymerization tank is made higher than the atmospheric pressure by the vapor of the vinyl ester monomer or the polymerization solvent or the non-condensable gas introduced from the outside of the polymerization tank.

  According to the present invention, a low-polymerization degree vinyl alcohol polymer powder excellent in hue can be provided with high productivity. Since the hue is excellent, the hue can be improved and the added value can be increased in the low polymerization degree PVA application.

  Hereinafter, the present invention will be described in more detail. The vinyl alcohol polymer (hereinafter sometimes abbreviated as PVA) powder of the present invention has a hue by suppressing an increase in the amount of initiator required at the time of producing a low polymerization degree brand by a specific method in the polymerization step. The cause of deterioration is suppressed, and the obtained vinyl ester polymer is saponified to obtain a low polymerization degree PVA powder excellent in hue.

  It is important that the viscosity average polymerization degree of PVA in the present invention is 100 to 500. When the viscosity average degree of polymerization exceeds 500, the polymerization rate of the vinyl ester polymer in the polymerization process may be low. As a result, the amount of initiator used during the production is small, and the hue deterioration is regarded as a problem. Disappear. Here, when the viscosity average degree of polymerization is 500 or less, it is difficult to produce unless the polymerization rate of the vinyl ester polymer in the polymerization process is high. Increases and the hue deteriorates. The present invention is excellent in hue even with a low polymerization degree PVA having a viscosity average polymerization degree of 500 or less. On the other hand, PVA having a viscosity average degree of polymerization of less than 100 has problems in gel strength, processability, etc. in the saponification process, and production itself is difficult.

  The viscosity average degree of polymerization of PVA was determined by measuring the intrinsic viscosity in an acetone solution (Akio Nakajima) after saponifying the PVA polymer substantially completely and then acetylating it into a vinyl ester polymer. : Polymer chemistry 6 (1949)). Hereinafter, the viscosity average degree of polymerization may be abbreviated as degree of polymerization.

  The degree of saponification of PVA in the present invention is important from the viewpoint of production efficiency, and is preferably from 80 to 99.9 mol%, and preferably from 84 to 99.9 mol%. When the degree of saponification is less than 80 mol%, it becomes easy to pulverize during production, and production becomes difficult. When the degree of saponification exceeds 99.9 mol%, the amount of alkali used becomes enormous in the saponification step during production, and thus it is substantially impossible to produce. The saponification degree of PVA is a value that can be measured according to JIS-K6726.

  The PVA powder of the present invention is characterized in that the yellow index of the powder having a particle diameter in the range of 100 to 1000 μm is less than 10.0. Moreover, it is preferable that the yellow index when it is set as 5% aqueous solution is less than 2.0. A lower value of the yellow index indicates a whiter color and can increase the added value of the PVA. The yellow index of the powder having a particle diameter in the range of 100 to 1000 μm is preferably 9.0 or less, more preferably 8.0 or less, and even more preferably 7.5 or less. Further, the yellow index when a 5% aqueous solution is prepared is more preferably 1.9 or less, and further preferably 1.8 or less. The yellow index of the powder and the yellow index when it is made into a 5% aqueous solution are usually 0 or more. The yellow index of PVA is a value measured and calculated according to JIS-Z8722 and JIS-K7373. When the particle size is too fine and when the particle size is too coarse, the error of the measured value is large, so the value of the particle size portion of 100 μm or more and 1000 μm or less was measured.

  The value of the polymerization rate of the vinyl ester monomer in the polymerization step when synthesizing PVA is not particularly limited, but in view of production efficiency, it is usually 50% or more, preferably 60% or more, more preferably 70% or more, 75% or more is particularly preferable.

  The value of the weight average molecular weight / number average molecular weight (Mw / Mn) of PVA is not particularly limited, but is usually 5 or less, and preferably 4 or less from the viewpoint of production efficiency.

  In the present invention, when producing PVA, it is preferable to carry out saponification after polymerizing a vinyl ester monomer by radical polymerization. Examples of vinyl ester units constituting PVA include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, Examples include vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, and vinyl benzoate. Of these, vinyl acetate is most preferred.

  In the process of polymerizing the vinyl ester monomer in the production of PVA in the present invention, the temperature in the polymerization tank is set to 2 to 40 ° C. higher than the boiling point of the reaction solution under atmospheric pressure by raising the pressure in the polymerization tank to higher than atmospheric pressure. A high temperature is a preferred embodiment. Thus, the hue can be improved by reducing the amount of unreacted initiator due to the effect of reducing the amount of initiator used and the improvement of the initiator reaction rate. When the said temperature is less than 2 degreeC, the hue improvement effect may become inadequate and it is preferable that the said temperature shall be 4 degreeC or more. On the other hand, when the temperature exceeds 40 ° C., the energy required for heating and heat removal becomes enormous, which causes a problem in terms of economy and safety, or the vinyl ester monomer itself decomposes at a high temperature, and coloring It may cause. The temperature is preferably 35 ° C. or lower.

  In the present invention, the pressure at which the pressure in the polymerization tank is made higher than atmospheric pressure is not particularly limited, but is preferably 0.005 MPa higher than atmospheric pressure, more preferably 0.01 MPa higher. The pressure is usually 2 MPa or less.

  In the above, there is no particular limitation on the type of gas when non-condensable gas is introduced from the outside of the polymerization tank, but inert gas for vinyl ester monomers such as methane, ethane, propane, nitrogen, carbon dioxide, helium, argon, etc. Of noble gases. Nitrogen, carbon dioxide, and argon are preferable from the viewpoint of productivity, economy, and safety, and nitrogen is more preferable.

  As the polymerization initiator used for the polymerization, conventionally known azo initiators, peroxide initiators and the like are appropriately selected according to the polymerization method. As the azo initiator, 2,2′-azobisisobutyronitrile, 2,2′-azobis (2,4-dimethylvaleronitrile), 2,2′-azobis (4-methoxy-2,4- Dimethyl valeronitrile) and the like, and peroxide initiators include dinormal propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, diethoxyethyl peroxydicarbonate, etc. Perester compounds such as t-butyl peroxyneodecanoate, t-hexylperoxyneodecanoate, α-cumylperoxyneodecanoate, t-butylperoxydecanoate, and the like; acetyl Cyclohexylsulfonyl peroxide; 2,4,4-trimethyl Such as pentyl-2-peroxy phenoxy acetate. Furthermore, the initiator can be combined with potassium persulfate, ammonium persulfate, hydrogen peroxide, or the like to form an initiator. Among these, a peroxide-based initiator is preferable for improving the hue.

  The addition amount of the polymerization initiator with respect to the vinyl ester monomer is preferably in the range of 0.001 to 0.05 mol% from the viewpoint of suppressing coloring. If the addition amount of the polymerization initiator is less than 0.001 mol%, there may be a problem in productivity, more preferably 0.002 mol% or more, and more preferably 0.003 mol% or more. preferable. On the other hand, when the addition amount of the polymerization initiator exceeds 0.05 mol%, a problem may occur in the hue of PVA, and it is more preferably 0.04 mol% or less.

  In order to suppress coloring due to decomposition of the vinyl ester monomer, it is preferable to add an acid having a first dissociation index of 2 to 5 and / or a salt thereof in the polymerization solution. Examples of acids having a first dissociation index of 2 to 5 and / or salts thereof include acetic acid, propionic acid, glycolic acid, lactic acid, glyceric acid, malic acid, tartaric acid, citric acid, salicylic acid, malonic acid, succinic acid, and the like. An acid, a hydroxycarboxylic acid compound or a salt thereof is used. Among these, hydroxycarboxylic acid and / or a salt thereof are preferable from the viewpoint of coloring prevention effect, lactic acid or tartaric acid is more preferable, and tartaric acid is more preferable. The addition method is not particularly limited, and may be charged in an initial batch according to the polymerization method, or may be added continuously. The addition form may be added as a powder or may be added after dissolving in an arbitrary solvent.

  The first dissociation index of the acid and / or salt thereof added to the polymerization solution is preferably 2 to 5, and if it is less than 2, it acts as an acid and may saponify and decompose the vinyl ester monomer, If it exceeds 5, the effect of suppressing coloring may be lowered.

  The addition amount of the acid and / or salt thereof having a first dissociation index of 2 to 5 with respect to the vinyl ester monomer is preferably 0.1 to 500 ppm based on the weight of the vinyl ester monomer. When the addition amount is less than 0.1 ppm, the effect of suppressing coloring may not be sufficiently exhibited, and when it exceeds 500 ppm, the polymerization reaction may be inhibited.

  Other monomers may be copolymerized in synthesizing PVA within the range not impairing the gist of the present invention. Examples of monomers that can be used include α-olefins such as ethylene, propylene, n-butene, and isobutylene; acrylic acid and its salts; acrylamide; N-methylacrylamide, N-ethylacrylamide, N, N-dimethylacrylamide, Acrylamide derivatives such as diacetone acrylamide, acrylamide propane sulfonic acid and salts thereof, acrylamide propyl dimethylamine and salts thereof or quaternary salts thereof, N-methylol acrylamide and derivatives thereof; methacrylamide; N-methyl methacrylamide, N-ethyl methacryl Amides, methacrylamide propane sulfonic acid and salts thereof, methacrylamide propyldimethylamine and salts thereof or quaternary salts thereof, N-methylol methacrylamide and derivatives thereof Methacrylamide derivatives of; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, 2,3-diacetoxy-1- Vinyl ethers such as vinyloxypropane; Nitriles such as acrylonitrile and methacrylonitrile; Vinyl halides such as vinyl chloride and vinyl fluoride; Vinylidene halides such as vinylidene chloride and vinylidene fluoride; Allyl acetate, 2, 3 -Allyl compounds such as diacetoxy-1-allyloxypropane and allyl chloride; unsaturated dicarboxylic acids such as maleic acid, itaconic acid and fumaric acid and salts or esters thereof; And vinyl silyl compounds such as methoxysilane; isopropenyl acetate and the like.

  As a polymerization method adopted for carrying out the polymerization, any of batch polymerization, semi-batch polymerization, continuous polymerization, and semi-continuous polymerization may be adopted. As the polymerization method, an arbitrary method can be adopted from known methods such as a bulk polymerization method, a solution polymerization method, a suspension polymerization method, and an emulsion polymerization method. Among these, a bulk polymerization method or a solution polymerization method in which polymerization is performed in the absence of a solvent or in the presence of an alcohol solvent is preferably employed. As the alcohol solvent used in the bulk polymerization method or the solution polymerization method, methanol, ethanol, n-propanol or the like can be used, but is not limited thereto. These solvents can be used in combination of two or more.

  Further, for the purpose of adjusting the degree of polymerization of the vinyl ester polymer obtained in the polymerization, the polymerization may be carried out in the presence of a chain transfer agent as long as the gist of the present invention is not impaired. Chain transfer agents include aldehydes such as acetaldehyde and propionaldehyde; ketones such as acetone and methyl ethyl ketone; mercaptans such as 2-hydroxyethanethiol; halogenated hydrocarbons such as trichloroethylene and perchloroethylene; sodium phosphinate 1 And phosphinic acid salts such as hydrates. Of these, aldehydes and ketones are preferably used. What is necessary is just to determine the addition amount of a chain transfer agent according to the chain transfer constant of the chain transfer agent to add, and the polymerization degree of the target vinyl ester polymer. In general, the content is preferably 0.1% by mass or more and 10% by mass or less based on the vinyl ester monomer.

  The stirring blade used for the polymerization is not particularly limited, and any stirring blade such as an anchor blade, a paddle blade, a bister, a max blend blade, or the like can be used.

  In the saponification reaction of the vinyl ester polymer, a conventionally known basic catalyst such as sodium hydroxide, potassium hydroxide or sodium methoxide or an acidic catalyst such as p-toluenesulfonic acid, hydrochloric acid, sulfuric acid or nitric acid is used. Alcohol decomposition reaction or hydrolysis reaction can be applied. Examples of the solvent that can be used in this reaction include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; aromatic hydrocarbons such as benzene and toluene. These can be used alone or in combination of two or more. Among them, it is convenient and preferable to perform the saponification reaction using methanol or a methanol / methyl acetate mixed solution as a solvent and sodium hydroxide as a catalyst. The concentration of the vinyl ester polymer in the alcohol is not particularly limited, but is selected from a range of 10 to 80% by weight. The amount of alkali or acid to be used is adjusted according to the degree of saponification, but the amount of 1 to 100 mmol equivalent to the vinyl ester polymer is used to prevent coloring of the PVA polymer and to reduce the amount of sodium acetate. This is preferable. When saponifying with an alkali, if there is a functional group introduced at the end of the vinyl ester polymer that consumes an alkali such as an acid, the amount of alkali is more than the above range. In addition, saponification may be performed. The saponification temperature is not particularly limited, but is in the range of 10 ° C to 70 ° C, preferably 30 ° C to 40 ° C. The reaction time is not particularly limited, but is about 30 minutes to 5 hours.

  The PVA of the present invention may contain other various additives as long as the gist of the present invention is not impaired. Examples of the additive include polymerization regulators such as aldehydes, halogenated hydrocarbons, and mercaptans; polymerization inhibitors such as phenol compounds, sulfur compounds, and N-oxide compounds; pH adjusters; cross-linking agents; An antifungal agent, an antiblocking agent, an antifoaming agent and the like.

  The PVA obtained in the present invention may be appropriately pulverized after performing a saponification reaction, whereby a PVA powder can be obtained. Although it does not specifically limit about the particle diameter of PVA powder, PVA powder which has a particle diameter in the range of 100-1000 micrometers can be obtained by sieving using the sieve (mesh size: 100 micrometers, 1000 micrometers) from which a mesh size differs. it can.

The PVA of the present invention is used for various applications. Although the example is given to the following, it is not limited to this.
(1) Dispersant use: Dispersion stabilizers for organic and inorganic pigments such as paints and adhesives, and dispersion stabilizers for suspension polymerization of various vinyl compounds such as vinyl chloride, vinylidene chloride, styrene, (meth) acrylate, and vinyl acetate. And dispersion aids.
(2) Coating application: Paper coating agent, sizing agent, textile finishing agent, leather finish agent, paint, antifogging agent, metal corrosion inhibitor, galvanizing brightener, antistatic agent, pharmaceutical coating agent.
(3) Emulsifier use: Emulsifier for emulsion polymerization, post-emulsifier such as bitumen.

  Hereinafter, the present invention will be described in more detail with reference to examples. In the following examples and comparative examples, unless otherwise specified, “part” and “%” represent parts by mass and mass%, respectively.

  The vinyl alcohol polymer (A) obtained by the following production example was evaluated according to the following method.

[Viscosity average polymerization degree of PVA]
The viscosity average degree of polymerization of PVA was determined by measuring the intrinsic viscosity in an acetone solution (Akio Nakajima) after saponifying the PVA polymer substantially completely and then acetylating it into a vinyl ester polymer. : Polymer chemistry 6 (1949)).

[Saponification degree of PVA]
The degree of saponification of PVA was determined by the method described in JIS-K6726.

[Yellow index of PVA powder]
The yellow index of the PVA powder is obtained by pulverizing the obtained PVA in advance and using a sieve (mesh size: 100 μm, 1000 μm) to remove particles less than 100 μm and more than 1000 μm, and then a color meter (SM-T-Suga Test Instruments). Measured using H1). The yellow index is a value measured and calculated according to JIS-Z8722 and JIS-K7373. The yellow index of a 5% aqueous solution was measured using a cell having an optical path length of 1 cm.

[productivity]
Productivity when producing PVA, (A), polymerization rate of vinyl ester monomer in polymerization process, weight ratio of vinyl ester monomer / (weight ratio of vinyl ester monomer + weight ratio of reaction solvent) (B) The average residence time was expressed as (C) (unit h) and quantified. The formula is shown below.
((A) / 100) × (B) × (24 / (C))
((A) / 100) indicates the conversion rate of the vinyl ester monomer, and the productivity per unit residence time is calculated by applying (B), and per day by applying (24 / (C)). Productivity. The larger the value, the higher the productivity, and 1.0 or more was A, 0.8 or more and less than 1.0 was B, and less than 0.8 was C. The average residence time is a value used for continuous polymerization, and in the case of batch polymerization, the average residence time may be replaced with the polymerization time.

[Production Example 1: Production of PVA1]
In this production example, the polymerization is continuously performed while continuously adding the polymerization solution to the next step, but the present invention is not limited to this. After substituting the polymerization tank with nitrogen, as an initiator, dinormalpropyl peroxydicarbonate 0.00626 mol% / vinyl acetate (hereinafter abbreviated as VAc), an acid having a first dissociation index of 2 to 5 and / or a salt thereof A mixed solution of VAc / methanol = 73/27 containing 50 ppm / VAc of tartaric acid was continuously added to the polymerization tank, and polymerization was carried out while maintaining the internal temperature at 70 ° C. Since the boiling point of the reaction solution at atmospheric pressure is about 60 ° C., during the polymerization, the pressure in the polymerization tank is increased by introducing steam generated by the reaction and, if necessary, nitrogen, so that the boiling point is 70 ° C., which is the set internal temperature. Adjust so that it does not fall below. In this production example, polymerization was performed by applying a pressure 0.025 MPa higher than the atmospheric pressure. Polymerization is carried out by adjusting the amount added to the polymerization tank and the amount fed to the next step so that the polymerization rate of vinyl acetate in the mixed liquid added to the polymerization tank averages 88% in 12.8 hours. Then, an operation of driving out unreacted VAc out of the system together with methanol by blowing methanol vapor into the polymerization solution fed to the next step was performed to obtain a methanol solution of polyvinyl acetate (concentration 58%). Subsequently, saponification was performed with sodium hydroxide by a conventional method to obtain PVA1 having a viscosity average polymerization degree of 300, a saponification degree of 98.5 mol%, a powder yellow index of 5.83, and a 5% aqueous solution yellow index of 1.44. . The productivity of this production example was 1.20.

[Production Examples 2 to 11, 15 to 16: Production of PVA 2 to 11 and 15 to 16]
VAc and methanol addition ratio, polymerization rate, polymerization temperature, average residence time, type and amount of initiator used during polymerization, type of acid and / or salt thereof having a first dissociation index of 2 to 5, and PVA 2 to 11 and 15 to 16 were produced in the same manner as in Production Example 1 except that the addition amount was changed. Table 1 shows the manufacturing conditions. Table 3 shows the types of initiators used, and Table 4 shows the types of acids and / or salts thereof having a first dissociation index of 2 to 5.

[Production Examples 12-14: Production of PVA 12-14]
VAc and methanol addition ratio, polymerization rate, average residence time, type of initiator used at the time of polymerization and addition amount thereof, polymerization under atmospheric pressure, first dissociation index of 2-5 PVA 12-14 were produced in the same manner as in Production Example 1 except that a certain acid and / or salt thereof was not used. Table 1 shows the manufacturing conditions. Table 3 shows the types of initiators used.

[Examples 1 to 11]
PVA 1 to 11 shown in Examples 1 to 11 were low polymerization degree PVA excellent in hue with a polymerization degree of less than 500, a powder yellow index of less than 10.0, and a 5% aqueous solution having a yellow index of less than 2.0. .

[Comparative Example 1]
PVA12 shown in Comparative Example 1 is obtained by performing polymerization at the boiling point of the polymerization solution under atmospheric pressure according to a conventional method. An acid having a first dissociation index of 2 to 5 and / or a salt thereof was not used, but the yellow index of the powder was less than 10.0 and the yellow index of the 5% aqueous solution was less than 2.0, and the hue was excellent. . However, the degree of polymerization of PVA12 is 630, and it can be seen that when the degree of polymerization is relatively high, it is possible to produce PVA excellent in hue by a conventional method.

[Comparative Example 2]
PVA13 shown in Comparative Example 2 is synthesized without using an acid having a first dissociation index of 2 to 5 and / or a salt thereof by performing polymerization at a boiling point of a polymerization solution under atmospheric pressure according to a conventional method. The degree of polymerization of PVA13 was 300. Then, the yellow index of the powder exceeded 10.0 and the 5% aqueous solution of yellow was because the polymerization temperature was low and the acid having the first dissociation index of 2 to 5 and / or its salt was not used. The index exceeded 2.0, and a low polymerization degree PVA excellent in hue could not be obtained.

[Comparative Example 3]
PVA14 shown in Comparative Example 3 is synthesized without using an acid having a first dissociation index of 2 to 5 and / or a salt thereof by performing polymerization at a boiling point of a polymerization solution under atmospheric pressure according to a conventional method. The degree of polymerization of PVA14 was 300. From Comparative Example 2, attempts were made to lower the polymerization rate, reduce the proportion of VAc, etc., and to reduce the amount of initiator used by sacrificing productivity. Although a slight hue improvement effect was seen in comparison with Comparative Example 2, the yellow index of the powder exceeded 10.0 and the yellow index of the 5% aqueous solution also exceeded 2.0, indicating a low degree of polymerization excellent in hue. PVA could not be obtained. In addition, productivity was as low as 0.78.

[Comparative Example 4]
PVA15 shown in Comparative Example 4 is synthesized using an acid having a first dissociation index of 2 to 5 and / or a salt thereof, which is polymerized at a temperature higher than the boiling point of the polymerization solution under atmospheric pressure. The degree of polymerization of PVA15 was 300. The yellow index of the powder exceeded 10.0 and the yellow index of the 5% aqueous solution also exceeded 2.0 because the initiator amount was too much, and it was not possible to obtain a low polymerization degree PVA excellent in hue. .

[Comparative Example 5]
PVA16 shown in Comparative Example 5 is synthesized using an acid having a first dissociation index of 2 to 5 and / or a salt thereof, polymerized at 160 ° C. The degree of polymerization of PVA16 was 300. Although the initiator amount was small, the polymerization temperature was too high and the decomposition of vinyl acetate progressed, which was the cause of coloring. The yellow index of the powder exceeded 10.0% and 5% The yellow index of the aqueous solution also exceeded 2.0, and a low polymerization degree PVA excellent in hue could not be obtained.

  As shown in the Examples, it is possible to provide a low-degree-of-polymerization vinyl alcohol polymer powder excellent in hue, which has been difficult to provide, with high productivity. Since the hue is excellent, the hue can be improved and the added value can be increased in the low polymerization degree PVA application. Therefore, the industrial usefulness of the present invention is extremely high.

Claims (4)

  The viscosity average polymerization degree is 100 to 500, the saponification degree is 80 to 99.9 mol%, and the yellow index of the powder having a particle diameter in the range of 100 to 1000 μm is less than 10.0 Low polymerization degree vinyl alcohol polymer powder.   The low-degree-of-polymerization vinyl alcohol polymer powder according to claim 1, having a yellow index of less than 2.0 when a 5% aqueous solution is used. A method for producing a low-polymerization degree vinyl alcohol polymer powder according to claim 1 or 2,
By making the pressure in the polymerization tank higher than atmospheric pressure, the temperature in the polymerization tank is set to a temperature 2 to 40 ° C. higher than the boiling point of the reaction solution under atmospheric pressure,
The amount of polymerization initiator added to the vinyl ester monomer is 0.001 to 0.05 mol%,
The low dissociation is characterized in that a polyvinyl ester polymer is produced by adding an acid and / or salt thereof having a first dissociation index of 2 to 5 to 0.1 to 500 ppm based on the weight of the vinyl ester monomer, and then saponified. A method for producing a vinyl alcohol polymer powder having a degree of polymerization.   The low-polymerization degree vinyl alcohol polymer powder according to claim 3, wherein the pressure in the polymerization tank is made higher than the atmospheric pressure by the vapor of the vinyl ester monomer or the polymerization solvent or the non-condensable gas introduced from the outside of the polymerization tank. Method.