JPH11279210A - Production of polyvinyl alcohol excellent in stability of viscosity at low temperature - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing polyvinyl alcohol, capable of simply carrying out control, or the like, of aqueous solution, treating it in similar method as conventional polyvinyl alcohol and obtaining the subject polymer excellent in stability of viscosity at low temperature by polymerizing a fatty acid vinyl ester under specific condition and saponifying the resultant polymer. SOLUTION: A fatty acid vinyl ester is polymerized under a pressure in a polymerizer kept in a pressure higher than atmospheric pressure by vapor of unreacted fatty acid vinyl ester in a polymerizer or by a non-condensable gas (e.g. nitrogen) at a temperature higher by 2-80 deg.C, preferably 5-50 deg.C than boiling point temperature of reaction solution under atmospheric pressure, and then, the resultant polymer is saponified. Vinyl acetate is industrially preferable as the fatty acid vinyl ester. Solution polymerization is industrially preferably used as the fatty acid. Methanol is industrially preferable in the polymerization method. The polymerization degree of the objective polymer is usually 50-4,000 and the saponification degree is usually >=60 mol.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for producing polyvinyl alcohol having excellent low-temperature viscosity stability in an aqueous solution state.

[0002]

2. Description of the Related Art Polyvinyl alcohol (hereinafter abbreviated as PVA) is industrially produced by polymerizing an aliphatic vinyl ester in an alcohol solvent at atmospheric pressure and saponifying the resulting aliphatic polyvinyl ester. . PVA is used in many applications such as fiber processing, paper processing, films, adhesives, binders for various inorganic materials, and emulsion stabilizers, but is usually used as an aqueous solution except for special cases. I have.

When PVA having a high degree of saponification is dissolved in water to form an aqueous solution, the viscosity increases with time after preparation of the aqueous solution in winter when the water temperature is low, and the fluidity deteriorates. In such cases, the aqueous solution may gel and lose its fluidity at all. Generally, PVA having a high degree of saponification is often used in the field where water resistance is required. However, when a high concentration aqueous solution of PVA having a high degree of saponification is handled, PVA having a high degree of saponification is used. As for the emulsion polymer and the like used as a protective colloid, deterioration of the fluidity of the PVA aqueous solution during winter or the like is a serious problem.

[0004] In order to improve the low-temperature viscosity stability of an aqueous PVA solution, a method of lowering the degree of saponification, introducing a hydrophobic group, or introducing an ionic group is generally used. Also, a method of copolymerizing an aliphatic vinyl ester and an α-olefin has been proposed in Japanese Patent Application Laid-Open No. 8-81666.

[0005]

However, PV having a reduced degree of saponification or having an ionic group or a hydrophobic group introduced thereinto.
A has a disadvantage that the water resistance of the dried film is low. The low-temperature viscosity stability of PVA obtained by copolymerizing an aliphatic vinyl ester and an α-olefin is better than that of conventional PVA, but must be dissolved at a considerably high temperature when preparing an aqueous solution. PVA has the drawbacks of being difficult to perform, and has a drawback such as a low drying rate of the emulsion polymer when used as a protective colloid agent during emulsion polymerization, and can be handled in the same manner as ordinary PVA, and also has good low-temperature viscosity stability. There is no present.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a PVA which is easy to prepare an aqueous solution, can be handled in the same manner as ordinary PVA, and has excellent low-temperature viscosity stability without impairing water resistance. It is.

[0007]

The process of the present invention achieves the above-mentioned object, and comprises the steps of: during the polymerization of an aliphatic vinyl ester, unreacted aliphatic vinyl ester or vapor of a polymerization medium in a polymerization machine; The pressure inside the polymerization machine is increased to a pressure higher than the atmospheric pressure by a non-condensable gas introduced from the outside of the polymerization machine, and the temperature of the reaction solution is set at a temperature 2 to 80 ° C. higher than the boiling point of the reaction solution under the atmospheric pressure. It is characterized by polymerizing and saponifying an aromatic vinyl ester.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically.

The aliphatic vinyl esters used in the present invention include vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, vinyl stearate and the like, and vinyl acetate is industrially desirable.

Further, an unsaturated monomer copolymerizable with the aliphatic vinyl ester and an aliphatic vinyl ester may be copolymerized within a range that does not impair the effects of the present invention. As the unsaturated monomer copolymerizable with the aliphatic vinyl ester, for example, unsaturated monobasic acids such as acrylic acid or salts thereof, maleic acid, itaconic acid, unsaturated dibasic acids such as fumaric acid or salts thereof Or amide groups such as unsaturated dibasic acid monoalkyl esters such as monomethyl maleate and monomethyl itaconate, (meth) acrylates, acrylamide, dimethylacrylamide, N-methylolacrylamide, and N-vinyl-2-pyrrolidone Containing monomers, lauryl vinyl ether, alkyl vinyl ethers such as stearyl vinyl ether, allyl alcohol, dimethyl allyl alcohol, hydroxyl group containing monomers such as isopropenyl allyl alcohol, allyl acetate,
Acetyl group-containing monomers such as dimethyl allyl acetate and isopropenyl allyl acetate; vinyl sulfonic acid group-containing monomers such as sodium vinyl sulfonate and sodium acrylamide-2-methylpropane sulfonate; and halogens such as vinyl chloride and vinylidene chloride. Examples of the monomer include aromatic monomers such as styrene, but are not limited thereto.

Regarding the polymerization machine for polymerizing the aliphatic vinyl ester used in the present invention, as long as the pressure inside the polymerization machine can be maintained at a pressure higher than the atmospheric pressure, any type of stirring device may be used. The polymerization method is batch polymerization, semi-continuous polymerization,
Any of continuous polymerization may be used. The polymerization method may be a known method such as bulk, solution, suspension, or emulsion polymerization, but industrially, solution polymerization is preferred. Alcohols, water and the like are usually used as the polymerization medium, but methanol is preferred from an industrial viewpoint. Various polymerization initiators are used, and azo compounds and peroxides are usually used.

In the present invention, as a method for maintaining the pressure in the polymerization machine at a pressure higher than the atmospheric pressure, for example, a non-condensable gas whose flow rate is controlled is introduced into the reactor, and the pressure is adjusted to a predetermined pressure. (Pressure control valve, etc.) or pressurize to a predetermined pressure by unreacted aliphatic vinyl ester in the polymerization vessel or vapor of the polymerization medium generated by polymerization heat or external heating to raise the pressure in the polymerization machine. A method of condensing only steam other than that required for increasing the pressure to a predetermined pressure by an internal heat removal coil or a condenser, but is not limited thereto. Examples of the non-condensable gas introduced into the reactor include nitrogen, helium, and argon.

In the present invention, the polymerization reaction temperature needs to be 2 to 80 ° C. higher than the boiling point of the reaction solution under atmospheric pressure. The polymerization reaction temperature is more preferably 5 to 50 ° C.
High temperature. When the temperature of the polymerization reaction solution is lower than the boiling point of the reaction solution at atmospheric pressure + 2 ° C., the effect of improving the low-temperature viscosity stability of the aqueous solution is small. On the other hand, when the temperature of the polymerization reaction solution exceeds the boiling point temperature of the reaction solution at atmospheric pressure + 80 ° C., it is difficult to control the polymerization reaction rate and coloring of the polymer is not preferred.

The aliphatic polyvinyl ester obtained by the polymerization is treated with an alkali metal hydroxide such as sodium hydroxide in a mixed solvent of an alcohol such as methanol or an ester such as methyl acetate or ethyl acetate and an alcohol. Polyvinyl alcohol can be obtained by saponifying a product or an alcoholate such as sodium methylate by a known method using a saponification catalyst. The saponified product can be dried and pulverized by various known methods. The degree of polymerization and the degree of saponification of PVA are not particularly limited, but those having a degree of polymerization of 50 to 4000 and a degree of saponification of 60 mol% or more are usually used. For applications requiring water resistance, those having a saponification degree of 96 mol% or more are preferred.

The reason why the low-temperature viscosity stability of the PVA according to the present invention when it is converted to an aqueous solution despite its high degree of saponification is not fully understood, but is presumed as follows. You. That is, in the saponified aliphatic polyvinyl ester polymerized by the polymerization method of the present invention, the amount of 1,2-glycol bonds and the amount of molecular chain branching present in the polymer chain are appropriately larger than those of ordinary PVA. Therefore, it is intended to improve the low-temperature viscosity stability in an aqueous solution state without impairing the water resistance.

[0016]

The present invention will be described more specifically with reference to the following examples. In addition, evaluation of the obtained PVA was performed by the following method. (1). 1,2 glycol bond amount and carboxylic acid terminal amount The 1,2 glycol bond amount is determined by titrating the amount of periodic acid required when the 1,2 glycol portion is cleaved with periodic acid with sodium thiosulfate. Calculated. The carboxylic acid terminal amount was measured by a potentiometric titration method.

(2) Water resistance The obtained aqueous solution of PVA is cast on a PET film,
Dry at 60 ° C for 3 hours to obtain a film thickness of 100μ
m was prepared. The obtained film was immersed in warm water at 40 ° C. for 24 hours, and the insoluble matter fraction was measured from the dry weight of the film before and after immersion. The water resistance was evaluated according to the following evaluation criteria based on the measured insoluble matter fraction. Evaluation criteria for water resistance…: Insoluble matter fraction is 70% or more. X: Insoluble matter fraction is less than 70%.

(3). Low Temperature Viscosity Stability The obtained 8% aqueous solution of PVA was placed in a glass container, and the temperature of the aqueous solution was set at 20 ° C. Next, the glass container was left in a constant temperature water bath at 5 ° C., and the viscosity after being left for 1 hour and 24 hours was measured. Thickening ratio = viscosity after standing at 5 ° C. for 24 hours / viscosity after standing at 5 ° C. for 1 hour Evaluation of low-temperature viscosity stability was performed based on the measured thickening ratio according to the following evaluation criteria. Evaluation criteria for low-temperature viscosity stability…: Thickening ratio is less than 5 times. ×: The thickening ratio is 5 times or more.

Example 1 A reaction solution (86 parts by weight of vinyl acetate, 14 parts by weight of methanol) and 0.004 parts by weight of azobisisobutyronitrile (boiling point at atmospheric pressure: about 60 ° C.) was heated to a pressure higher than atmospheric pressure. The mixture was introduced into a batch polymerization apparatus capable of being held, and the internal pressure was increased to 2.0 kg / cm ² by introducing nitrogen (while all unreacted vinyl acetate and methanol vapors generated during the reaction were aggregated).
And the temperature of the reaction solution was kept at 80 ° C. to carry out polymerization. The polymerization yield was 60.1% at a polymerization time of 240 minutes. The obtained polyvinyl acetate was alkali-saponified to obtain PVA having a saponification degree of 98 mol%. Table 1 shows the physical properties of this PVA.
As is clear from Table 1, PVA having good low-temperature viscosity stability
was gotten.

Example 2 A reaction solution (80 parts by weight of vinyl acetate, 20 parts by weight of methanol, and 0.004 parts by weight of azobisisobutyronitrile) (boiling point at atmospheric pressure: about 60 ° C.) was pressurized to an internal pressure by introducing nitrogen. It was continuously introduced into a one-stage continuous polymerization apparatus maintained at 1.5 kg / cm ^2, and the temperature of the reaction solution was set to 70 ° C. to continuously perform polymerization. The yield at the reactor outlet when the average residence time was 5 hours was 85%. The obtained polyvinyl acetate was alkali-saponified to obtain PVA having a saponification degree of 98 mol%. Table 1 shows the physical properties of this PVA. As is evident from Table 1, PVA having good low-temperature viscosity was obtained.

Example 3 A two-stage continuous polymerization apparatus was used to prepare a reaction solution (boiling point at atmospheric pressure: about 60 ° C.) comprising 85 parts by weight of vinyl acetate, 15 parts by weight of methanol and 0.002 parts by weight of azobisisobutyronitrile. Was introduced continuously into the first reactor which was opened to the atmosphere, and polymerized at atmospheric pressure at a polymerization temperature of 60 ° C. The polymerization solution was introduced into a second reactor in which the internal pressure was maintained at 3.0 kg / cm ² by introducing nitrogen, and the temperature of the reaction solution was set to 95 ° C. to perform polymerization. First
2nd time when the total average residence time of the 2nd reactor is 6 hours
The yield at the reactor outlet was 75%.

The obtained polyvinyl acetate was saponified with an alkali to obtain PVA having a saponification degree of 98 mol%. Table 1 shows the physical properties of this PVA. As is evident from Table 1, PVA having good low-temperature viscosity was obtained.

EXAMPLE 4 A reaction solution (75.degree. C. at atmospheric pressure, boiling point: about 60.degree. C.) comprising 75 parts by weight of vinyl acetate, 25 parts by weight of methanol, and 0.005 parts by weight of azobisdimethylvaleronitrile was charged to a three-stage continuous polymerization apparatus. It was continuously introduced into the first reactor opened to the atmosphere, and polymerized at a polymerization temperature of 60 ° C. under atmospheric pressure. The polymerization liquid was continuously introduced into a second reactor opened to the atmosphere, and further polymerized at a polymerization temperature of 60 ° C. under atmospheric pressure. The polymerization liquid was introduced into a third reactor in which the internal pressure was maintained at 4.0 kg / cm ² by introducing nitrogen, and the temperature of the reaction liquid was set at 105 ° C. to perform polymerization. The yield at the outlet of the third reactor when the average residence time of the total of the first, second and third reactors was 8 hours was 83%.

The obtained polyvinyl acetate was saponified with an alkali to obtain PVA having a saponification degree of 98 mol%. Table 1 shows the physical properties of this PVA. As is clear from Table 1, PVA having good low-temperature viscosity stability was obtained.

Comparative Example 1 Polymerization was carried out in the same manner as in Example 1 except that 0.015 parts by weight of azobisisobutyronitrile was added using a reactor opened to the atmosphere. Polymerization yield of polyvinyl acetate 7
3.3%. The obtained polyvinyl acetate was alkali-saponified to obtain PVA having a saponification degree of 98 mol%. This P
Table 1 shows the physical properties of VA. As is clear from Table 1, the low-temperature viscosity stability was poor.

Comparative Example 2 The pressure in the reactor was slightly increased by introducing nitrogen to 1.1 k.
The polymerization was carried out in the same manner as in Example 2 except that the pressure was g / cm ² , the polymerization temperature was 62 ° C., and 0.15 parts by weight of azobisisobutyronitrile was added. The polymerization yield of polyvinyl acetate was 62.5%. The polyvinyl acetate thus obtained is alkali-saponified to give a PVA having a saponification degree of 98 mol%.
I got Table 1 shows the physical properties of this PVA. As is clear from Table 1, the low-temperature viscosity stability was poor.

Comparative Example 3 Polymerization was carried out in the same manner as in Example 2 except that the pressure in the reactor was adjusted to 12.0 kg / cm ² by introducing nitrogen.
The polymerization temperature was 140 ° C and the polymerization yield of polyvinyl acetate was 8
8.5%. The obtained polyvinyl acetate was alkali-saponified to obtain PVA having a saponification degree of 98 mol%. This P
Table 1 shows the physical properties of VA. As is clear from Table 1, the obtained PVA had good low-temperature viscosity stability, but had poor water resistance, and the hue of PVA was pale yellow.

[0028]

[Table 1]

[0029]

As is apparent from the above description, according to the present invention, a PVA aqueous solution can be easily prepared, can be handled in the same manner as ordinary PVA, and is excellent in water resistance and low-temperature viscosity stability. Can be provided.

Claims (1)

[Claims] (1) During the polymerization of an aliphatic vinyl ester, the pressure in the polymerization machine is increased from atmospheric pressure by unreacted aliphatic vinyl ester in the polymerization machine, vapor of a polymerization medium, or non-condensable gas introduced from outside the polymerization machine. At a high pressure, the temperature of the reaction solution is 2 to 8 times higher than the boiling point of the reaction solution at atmospheric pressure.
A method for producing polyvinyl alcohol having excellent low-temperature viscosity stability, comprising polymerizing an aliphatic vinyl ester at a temperature higher by 0 ° C. and saponifying it.