JPH08120105A - Method for producing polyvinyl alcohol-based porous sheet - Google Patents

Abstract

PURPOSE: To provide a method for producing a polyvinyl alcohol-based porous sheet of a continuous porous structure, good in hydrophilic properties and wet rubber elasticities, and having a high porosity by extending a reaction fluid consisting of a polyvinyl alcohol, a cross-linking agent and a catalyst and containing no pore forming material in a state of sheet over a heated plate, and reacting for cross-linking thereof. CONSTITUTION: This method for producing a polyvinyl alcohol-based porous sheet comprises feeding a reaction fluid consisting of a polyvinyl alcohol (PVA), a cross- linking agent and a cross-linking catalyst and containing no pore forming material over a hot plate heated at a prescribed temperature, extending the fluid in the state of a sheet and then reacting for cross-linking thereof. A completely saponified PVA having 1500-3800 average degree of polymerization is used. As the cross-linking agent formaldehyde, acetaldehyde, etc., are used and as the cross-linking catalyst sulfuric acid, hydrochloric acid, etc., are used. As the hot plate a resin coated metal plate or a water impermeable woven fabric at 60-100 deg.C is preferable. The reaction fluid is prepared as a 10-20wt.% PVA aqueous solution by dissolving PVA with warm water, and by adding the cross-linking agent and the catalyst resulting to the fluid having approximately 1000-3000cps viscosity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a sheet-like polyvinyl alcohol-based porous body having fine pores.

[0002]

2. Description of the Related Art A polyvinyl alcohol-based porous sheet is a continuous porous structure having a high porosity, and has excellent hydrophilicity and favorable rubber elasticity when wet. Widely used for various applications such as filtration media.

Conventionally, a polyvinyl alcohol type porous sheet is prepared by adding starch as a pore-forming material, aldehydes as a cross-linking agent, and acids as a cross-linking catalyst to an aqueous solution of polyvinyl alcohol (hereinafter abbreviated as "PVA"). After casting this in a predetermined mold, it is heated at a temperature of 50 to 70 ° C. in a hot water bath or an air bath to cause a cross-linking reaction to form a reaction product, which is then washed with water to remove the pore-forming material and block-shaped porosity. It is manufactured by a method of forming a body and slicing it to a desired thickness to form a sheet.

By the way, the above-mentioned method requires 15 minutes for the crosslinking reaction.
It takes up to 25 hours and usually takes 2 to 2 hours to obtain a sheet.
More days were required, and the production efficiency was extremely low. Therefore, if the reaction time is shortened by raising the temperature of the cross-linking reaction in order to improve the production efficiency, the reaction time is certainly shortened, but the obtained porous body has a nonuniform pore diameter distribution, and the There is a problem that a large difference occurs in the size of the pores between the portion and the outer peripheral portion, and in some cases, the reaction product is released from the mold and tends to thermally shrink.

Further, in the conventional method, since a block-shaped porous body is sliced into a sheet, many portions of the block-shaped porous body which are not used for a sheet-shaped product are generated in a peripheral portion, and if the product yield is low, the block-shaped porous body is not used. There was a problem.

Further, since starch is used as the pore-forming material in the conventional method, the range of pore diameters that can be produced is limited to some extent. That is, even when the finest starch is used, the average pore diameter is about 10 to 20 μm,
It is extremely difficult to obtain a porous body having a pore size smaller than this.

When the cross-linking reaction is carried out without adding the starch, which is a pore-forming material, a porous body having a fine pore diameter can be obtained. It shrinks significantly and has a very low porosity.

As a method of preventing shrinkage when starch is not added, for example, a method of adding needles such as asbestos can be mentioned. Certainly, the effect of suppressing the shrinkage phenomenon of the reaction example is exhibited, and a porous body having a high porosity can be obtained. However, the pore size of the porous body produced by this method is about 8 to 10 μm, and it is difficult to obtain a pore size smaller than this.

[0009]

DISCLOSURE OF THE INVENTION As a result of intensive studies conducted by the present inventors in view of the above-mentioned problems, as a result of spreading the reaction solution in a sheet form on a heated hot plate, P having a fine pore diameter is obtained.
It was found that a VA-based porous sheet can be obtained, and the present invention has been completed. An object of the present invention is to provide a method for efficiently producing a PVA-based porous sheet having a porous structure having an extremely fine pore size.

[0010]

An object of the present invention is to continuously react a reaction liquid comprising polyvinyl alcohol, a cross-linking agent and a cross-linking catalyst, which is not mixed with a pore-forming material, on a hot plate heated to a predetermined temperature. It is achieved by a method for producing a polyvinyl alcohol-based porous sheet, which comprises supplying and heating the reaction solution to cause a crosslinking reaction.

According to the production method of the present invention, the reaction liquid to which the above-mentioned pore-forming material is not added and mixed is directly spread on the hot plate and is heated by the heat of the hot plate to carry out the crosslinking reaction. is there.

In the method of the present invention, PVA is a completely saponified product having an average degree of polymerization of 1500 to 3800 (saponification degree of 98%).
It is preferable to use the above), but a mixture of a fully saponified product and a small amount of a partially saponified product may be used. For the purpose of the present invention, fully saponified PVA tends to have shorter crosslinking reaction time than partially saponified PVA.
It is preferable to use A.

The cross-linking agent used in the present invention is an aldehyde, for example, an aliphatic aldehyde such as formaldehyde, acetaldehyde, butyraldehyde and acrylaldehyde, an aliphatic dialdehyde such as glyoxal, and an aromatic aldehyde such as benzaldehyde. Can be mentioned.

The crosslinking catalyst used in the present invention is an acid,
For example, inorganic acids or organic acids such as sulfuric acid, hydrochloric acid, phosphoric acid, and maleic acid can be exemplified. In the present invention, sulfuric acid is preferred.

The temperature of the hot plate used in the present invention is preferably 60 to 100 ° C. The hot plate is preferably a metal plate coated with a heat-resistant and acid-resistant resin, or a heat-resistant woven fabric having no water permeability. By coating with resin, while preventing corrosion due to strongly acidic reaction solution,
The releasability of the reaction product can be improved. Further, in the present invention, the hot plate may be a continuous body capable of continuously supplying the reaction liquid, such as a belt conveyor.

The method of the present invention can be carried out by an apparatus as shown in FIGS. 1 to 3, for example. FIG. 1 is a configuration explanatory view showing an example of an apparatus applied to the method of the present invention. In this apparatus, the hot plate is composed of a belt conveyor. FIG. 2 is an explanatory plan view showing the structure of the belt conveyor section of this apparatus, and FIG. 3 is an enlarged sectional explanatory view of the essential parts showing the belt conveyor section of this apparatus.

This apparatus comprises a rotating belt conveyor 1
A reaction solution discharge unit 2 for flowing a reaction solution into the belt conveyor 1; and rotating drums 3a, 3b, 3c for stretching the belt conveyor 1 and applying a rotational force to the belt conveyor 1.
A plurality of heating rollers 4 for horizontally supporting the belt conveyor 1 and heating the belt conveyor 1; a heating drum 5 for heating the belt conveyor 1;
A water washing device 6 for washing water with water and a water absorbing roller 7 for wiping the surface of the belt conveyor 1 are wiped. Further, the belt conveyor 1 has a heat-resistant woven fabric 1a coated with a fluororesin 1b on the surface thereof, and restricts the flow of the reaction solution so that the reaction solution does not spill near both ends of the surface of the belt conveyor. Protruding portion 8 is provided over the entire circumference of the belt conveyor.

In the method of the present invention using the above apparatus, PV
To manufacture the A-based porous sheet, for example, the following may be performed. First, PVA is dissolved in warm water to prepare an approximately 10 to 20% by weight PVA aqueous solution. After adding the cross-linking agent and the cross-linking catalyst thereto, they are sufficiently stirred and mixed to form a uniform slurry reaction liquid. The PVA concentration of this reaction solution is preferably adjusted to 6 to 10% by weight based on the weight of the reaction solution. The reaction liquid is preferably a viscous liquid having a viscosity of, for example, about 1000 to 3000 cps.

This reaction liquid is continuously supplied from the reaction liquid discharge section 2 to the upper surface of the rotating belt conveyor 1. The supplied reaction solution is spontaneously cast and spreads in a sheet shape.
The thickness of the reaction solution spread in a sheet form may be appropriately set according to the desired thickness of the sheet, but if it is too thick, the reaction time becomes long and the porosity becomes extremely low. There is a tendency.

The reaction liquid 10 on the belt conveyor is heated by the heated belt conveyor 1 and the crosslinking reaction proceeds,
Eventually, the reaction liquid gradually loses fluidity and becomes a sheet-shaped reaction product on the belt conveyor. The belt conveyor 1 is previously heated to 70 to 95 ° C. by a heating drum 5 and additionally heated by a heating roller 4.
The thickness of the obtained reaction product sheet can be changed by adjusting the discharge amount of the reaction liquid, the rotation speed of the belt conveyor, and the like. In addition, you may enable each heating roller 4 to set a temperature individually, respectively.

The belt conveyor moves at a speed of, for example, several tens of centimeters to 1 meter per minute, and the reaction time on the belt conveyor varies depending on the temperature of the belt conveyor, the supply amount of the reaction solution, etc. Usually, about 10 to 20 minutes is sufficient. The sheet-shaped reaction product 31 obtained as described above is peeled from the belt conveyor, and after cooling, is washed with water to remove unreacted raw materials, a crosslinking catalyst, and the like, and is a PVA-based PVA system having rubber elasticity in a wet state. It is a porous sheet.

The belt conveyor 1 from which the sheet-like reaction products have been peeled off is washed by the water washing device 6 to wash away the remaining unreacted raw materials and the attached substances such as the crosslinking catalyst, and the surface is washed cleanly, and then the water-absorbing roller 7 is attached. After the washing water is wiped off, the above crosslinking step is repeated.

[0023]

According to the method of the present invention, it is possible to obtain a PVA-based porous sheet having an extremely fine pore structure which cannot be obtained by the conventional method. Further, in the method of the present invention, the reaction solution is cast on a hot plate and heated in a state of being spread in a sheet to perform a crosslinking reaction, so that the crosslinking reaction time can be dramatically shortened as compared with the conventional method. it can. Furthermore, since the slicing step required in the conventional method can be omitted,
The production efficiency of the PVA-based porous sheet is remarkably improved, and the product yield is remarkably improved.

[0024]

【Example】

Example 1 400 g of completely saponified PVA with an average degree of polymerization of 1500 was added to water to make a total amount of 2500 ml, and this was heated to PV.
A was completely dissolved. Then add water to bring the total amount to 360
Bring to 0 ml and cool with stirring.

When the PVA aqueous solution was cooled to 50 ° C., 500 ml of 37% formaldehyde aqueous solution and 50
% Sulfuric acid (900 ml) was added, and the mixture was uniformly mixed with stirring to obtain a reaction solution.

Next, using the apparatus shown in FIG. 1, the obtained reaction solution was poured onto the belt conveyor 1 heated to 80 ° C. The belt conveyor used here is a fluororesin-coated metal plate. The reaction solution was cast on the belt conveyor 1 in a sheet form. The reaction solution was heated on a belt conveyor for 10 minutes to perform a crosslinking reaction, thereby obtaining a sheet-shaped reaction product. Subsequently, the obtained reaction product was peeled off from the belt conveyor and washed sufficiently with water to remove starch and the like to obtain a PVA-based porous sheet. The PVA-based porous sheet obtained had a thickness of 5 mm and a pore diameter of 1 to 2 μ.
It was a porous structure having an extremely fine and uniform continuous pore structure of m.

[Brief description of drawings]

FIG. 1 is a configuration explanatory view showing an example of an apparatus applied to a method of the present invention.

FIG. 2 is an explanatory plan view showing a belt conveyor section of the apparatus shown in FIG. 1;

FIG. 3 is an enlarged cross-sectional explanatory view of essential parts showing a belt conveyor section of the apparatus of FIG.

[Explanation of symbols]

 1 Belt Conveyor 1a Heat Resistant Woven Cloth 1b Fluororesin 2 Reaction Liquid Discharging Section 3a, 3b, 3c Rotating Drum 4 Heating Roller 5 Heating Drum 6 Water Washing Device 7 Water Absorbing Roller 8 Projection 10 Reaction Liquid 11 Reaction Product

Claims (1)

[Claims] 1. A reaction solution comprising polyvinyl alcohol, a cross-linking agent and a cross-linking catalyst, which is not mixed with a pore-forming material,
A method for producing a polyvinyl alcohol-based porous sheet, which comprises supplying a hot plate heated to a predetermined temperature to spread it into a sheet, and then heating the reaction solution to cause a crosslinking reaction.