JPS5959207A - Permselective membrane and preparation thereof - Google Patents

Abstract

PURPOSE:To enhance the water permeability of a membrane comprising an ethylene/vinyl copolymer fixed to a water pervious substrate, by forming voids each directly contacting with a dense layer at the upper end thereof in a porous layer. CONSTITUTION:21.0pts.wt. ethylene/vinyl alcohol copolymer containing 20% ethylene is dissolved in 79.0pts.wt. isopropyl alcohol-water (a volume ratio of 6:4) to prepare a film forming solution. This solution is applied to one surface of a polyester nonwoven fabric substrate and, after the back surface thereof is contacted with a water surface for 10sec at 2 deg.C, the whole is immersed in water to coagulate the copolymer. The obtained membrane consists of a porous layer 3 integrally fixed to a water pervious substrate 6 and a dense layer 1 formed on the surface thereof and voids 4 each extending to a membrane direction are formed in the porous layer 3. When pure water is passed through this membrane at 25 deg.C under a pressure of 2.0kg/cm<2>, a water permeating speed is 14.8m<3>/m<2>. day and the removal ratio of PEG with an M.W. of 20,000 is 30%.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a selectively permeable membrane and a method for producing the same, and more specifically, it is made of an ethylene-vinyl alcohol copolymer, has excellent water permeability and selective removal of solutes, and can be used as an ultrafiltration membrane. The present invention relates to a selectively permeable membrane that can be suitably used and a method for manufacturing the same.

In recent years, ultrafiltration membranes have been widely used in a wide range of fields including industrial wastewater treatment, separation and concentration in food and pharmaceutical manufacturing, and the like. As such ultrafiltration membranes, those made from various polymers such as cellulose acetate, polyacrylonitrile, polysulfone, polyamide, polyvinyl chloride, polyvinyl alcohol, and ethylene-vinyl alcohol copolymer are already known. Among these, ultrafiltration membranes made of ethylene-vinyl alcohol copolymer are hydrophilic and have excellent durability and chemical stability. , an anisotropic selectively permeable membrane has been proposed in which a dense layer on the surface is supported by a porous layer below. However, conventionally known membranes made of ethylene-vinyl alcohol copolymers, including the above-mentioned anisotropic membrane, are generally not fully satisfactory in water permeability. In particular, the dense layer on the membrane surface has a dominant effect on the selective removability and water permeability of the calf.
In conventional membranes, this dense layer is thick and usually exceeds 1% of the total membrane thickness, so the water permeability is still low. Furthermore, even when the porous layer includes cavities, a porous polymer layer is interposed between the cavities and the dense layer, further reducing the water permeability of the membrane. Water permeability greatly influences the cost of membrane separation, and if the dense layer is made thinner, water permeability can be improved, but selective removal performance will be lowered.

The present invention was made to solve the above-mentioned problems in selectively permeable membranes made of ethylene-vinyl alcohol copolymer. To provide a dylene-vinyl alcohol copolymer selective permeable membrane, in which at least a part of the cavities contained in the layer is in direct contact with the dense layer, thereby reducing water permeability, and a method for producing the same. With the goal.

In the present invention, a dense layer on the membrane surface and a porous layer supporting this dense layer are integrally formed from an ethylene-vinyl alcohol copolymer, and this porous layer is immobilized on a water-permeable base material. In the selectively permeable membrane, the dense layer has a thickness of substantially 0.01 to 0.5μ and substantially only micropores with a pore diameter of 500 Å or less, and the porous layer Porous, having a cavity extending in the thickness direction and having a diameter of substantially 1 to 100μ across the film thickness direction, and pores surrounding this cavity and having a pore diameter of substantially 0.1 to 5μ. and at least a portion of the cavity is in direct contact with the dense layer at its upper end, and further, the porous polymer wall is integrally fixed to the water-permeable substrate at its lower part. It is characterized by The ethylene-vinyl alcohol copolymer used in the present invention preferably has an ethylene content of 5 to 50 mol%, particularly preferably 10 to 40 mol%. It is desirable that the degree of saponification is 80% or more, preferably 90% or more, and the molecular weight is about 200,000 or more. When the ethylene content is less than 5 mol%, the water stability of the resulting membrane becomes poor, and in particular it becomes soluble in hot water, whereas when it exceeds 50 mol%,
This is because it becomes difficult to dissolve in solvents as described later, making it difficult to prepare a uniform film-forming solution, and the resistance of the resulting film to common organic solvents decreases.

Furthermore, when the degree of saponification is less than 80%, film forming properties generally deteriorate, making it difficult to obtain a film having good physical properties. A particularly preferred ethylene-vinyl alcohol copolymer is a substantially completely saponified product with a degree of saponification of 99% or more.

On the other hand, in the present invention, the ethylene-vinyl alcohol copolymer may contain a copolymerizable component consisting of other vinyl polymerizable monomers, such as acrylic Acid, acrylonitrile, acrylamide, acrylic acid conistere, methacrylic acid, methacrylic acid ester, styrene, vinyl chloride, butadiene, isoprene, chloroprene, etc. can be mentioned, and the amount of copolymer consisting of these monomer components is ethylene-vinyl The content may be within a range that does not inhibit the crystallinity of the alcohol copolymer and may be contained within a range of 10 mol % or less when energized. If the crystallinity of the ethylene-vinyl alcohol copolymer is inhibited, the obtained film strength will be inferior to the chemical properties, which is not preferable.

The selectively permeable membrane according to the present invention is manufactured by the method described below, and has structural features as described below. FIG. 1 is an electron micrograph showing a cross section in the film thickness direction of a dried film produced in an example described later.

FIG. 2 is a schematic diagram thereof. FIGS. 3 and 4 show enlarged cross-sections of the dense layer on the membrane surface. As is clear from these, the film according to the present invention has a film surface with substantially 0.01 to 0.0
A dense layer 1 with a thickness of 0.5 μm is formed. However, it is permissible for the dense layer to partially have a thickness outside the above range. As shown in FIG. 5, which is an electron micrograph of the film surface, this dense layer has substantially only micropores with a pore diameter of 500 Å or less. As will be explained later, in the membrane of the present invention, the total thickness of the polymer layer 2 is 75 to 1000 microns, but the dense layer has a thickness of 1% or less of this polymer layer. 0 for the dense layer
．． A value of 0.01 μm or less is not preferable because it lowers the selective removal rate of the membrane. On the other hand, setting it to 0.5μ or more improves the selective removal rate, but it is not preferable because it lowers the water permeability of the membrane.

The dense layer is integrally continuous with and supported by the porous layer 3 below. This porous layer consists of a cavity 4 extending in the thickness direction and a porous polymer wall 5 surrounding this cavity. One important feature of the membrane of the present invention is that at least a portion of the cavity is in substantially direct contact with the dense layer at its upper end. That is, there is a part between the cavity and the dense layer where no porous polymer layer exists, and therefore it has high water permeability, and moreover, the thin dense layer has a high selective removal rate. Further, the cavity has a diameter transverse to the membrane thickness, i.e., a maximum length of the minor axis, of substantially 1 to 100 μm, and the porous polymer wall has a diameter across the membrane thickness to form such a cavity. the polymer wall has a pore size of substantially 0.1
It has pores of ~5μ. Note that the form of the cavity is not strictly defined, and two or more cavities may communicate with each other and include a portion extending partially in a direction transverse to the film thickness direction.

Note that it is permissible for the maximum length of the minor axis and the pore diameter to be partially outside the above range.

The thickness of the porous layer is suitably 75 to 1000 microns, particularly preferably 100 to 500 microns.

When it is smaller than 75 μ, the film is likely to have defects and 1
If it is larger than 000μ, the water permeation resistance will be increased unnecessarily, which is not preferable. Therefore, the membrane of the present invention has a non-electrolytically large porosity, and the water permeation resistance due to the porous layer is significantly reduced.

The porous layer as described above is integrally fixed to the water-permeable substrate 6 at the lower part of its polymer wall. That is, the lower portion of the polymer wall penetrates into the structure of the substrate and forms a composite polymer layer with the structure of the substrate. Therefore, the water-permeable base material must also be porous, and for example, woven fabrics, non-woven fabrics, and other porous sheet materials are preferably used. Specific examples include materials such as polyolefin, polyester, polyamide, polyacrylic, polyvinyl chloride, polyhinyl alcohol, and cellulose.

In the present invention, the water-permeable base material preferably has a temperature of 0°05 m1/cJ·sec·kglctA at 25°C, since it has a large effect on the water permeability of the membrane and is also related to the durability of the membrane. or more, particularly preferably,
It has water permeability of 0.1 ml/ctl・sec・kg/ci or more. However, if the water permeability is too high, the membrane obtained will have defects, and the mechanical strength will decrease due to the increase in porosity, which will reduce the durability of the membrane. Furthermore, as will be described later, when the film-forming solution is applied to this base material, the film-forming solution easily spreads over the surface opposite to the coated surface (
Hereinafter, it will simply be referred to as the back side. ) can be easily reached, or
After applying the film-forming solution to the substrate, when the back side of the base material is brought into contact with the coagulation solution, the solvent easily reaches the side to which the film-forming solution is applied, resulting in an anisotropic film having the characteristics according to the present invention. This is not desirable because it tends to make it difficult. Therefore, although it depends on the polymer concentration and viscosity of the membrane-forming solution used, the water-permeable base material must be selected so that its water-permeability is greater than the above-mentioned lower limit and that the above-mentioned disadvantages do not occur.

The selectively permeable membrane described above, in accordance with the present invention, contains a carbon atom containing dissolved ethylene-vinyl alcohol copolymer.
After applying the mixed solution of the aliphatic alcohol and water of ~4 on one side of the water-permeable base material, first, the other side of the water-permeable base material is brought into contact with a coagulation liquid containing water as the main component for a short time, The polymer is then manufactured by immersing the water-permeable substrate in a coagulating liquid to coagulate the polymer.

Preferred specific examples of the aliphatic alcohol for preparing a membrane forming solution by melting an ethylene-vinyl alcohol copolymer include methanol, ethanol, propatool, butanol, etc. Furthermore, in the present invention, polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin are also included in the above alcohol.
Hydrolic and polyhydric alcohols are generally difficult to melt the ethylene-vinyl alcohol copolymer and have poor compatibility with a coagulating liquid mainly composed of water, so they are not suitable for use. The volumetric mixing ratio in the alcohol-water mixed solvent usually ranges between 9:1 and 1:9, preferably between 3nia and 8:2. The mixed solvent may contain a small amount of a water-soluble organic solvent, for example, about 40% by volume or less of acetone, dioxane, dimethyl sulfoxide, N,N-dimethylacetamide, etc. Furthermore, if necessary, a water-soluble inorganic salt may be contained within a range that does not inhibit the dissolution of the polymer.

The concentration of the polymer in the membrane forming solution is suitably 3 to 40% by weight, preferably 5 to 30% by weight.

In terms of viscosity, 1 to 1000 voids is appropriate. If the concentration is too high and/or the viscosity is too high, it will be difficult to apply it uniformly to the substrate, and if the concentration is too small, the solution will drip after being applied to the substrate. This is not preferable because it may cause film defects.

The water-permeable base material for applying this film-forming solution has already been explained, but more specifically, it must not dissolve or swell in the film-forming solution and must have excellent chemical resistance. is desirable.

According to the method of the present invention, after the above-mentioned film-forming agent is applied to one side of the base material, the back side is brought into contact with a coagulating liquid for a short time. As mentioned above, the coagulating liquid is water or a solvent mainly composed of water, and may contain the above-mentioned alcohol or water-soluble organic solvent as long as it does not inhibit the coagulation of the ethylene rubinyl alcohol copolymer. . Further, if necessary, the coagulating liquid may contain a water-soluble salt. The amount is usually 2
It is 5% by weight or less. The temperature of the coagulating liquid is not particularly limited, and may be normal to room temperature, but preferably 0 to 2
It is in the range of 0°C. In order to bring only the back side of the substrate coated with the membrane-forming solution into contact with the coagulation liquid, specifically, when manufacturing a membrane of a predetermined area in a batch process, for example, place the back side of the substrate facing down. This can be carried out by gently floating the substrate on the surface of the coagulating liquid. In addition, when producing a long continuous film, a predetermined amount of coagulation liquid is cast, sprayed, or sprinkled on the back side of the base material while the base material is running at a constant speed. What is necessary is to contact the coagulation liquid for a period of time.

- In this way, the time for which the back side of the base material is brought into contact with the coagulation liquid is usually 1 to 120 seconds, preferably 2 to 6 seconds.
It is 0 seconds. If the contact time is too short, it is difficult to realize the unique structure of the membrane according to the invention;
If the length is too long, especially when the film thickness is small, coagulation of the polymer from the back side of the substrate will progress too much, making it similarly difficult to obtain the unique film structure according to the present invention.

If necessary, the base material may be impregnated in advance with the same solvent as that constituting the membrane forming solution, and then the membrane forming solution may be applied to one side of the membrane forming solution. Controlling the rate of displacement between the membrane forming solution and the coagulation solution during contact, thus
This is to adjust the size of the cavity and the size of the pores in the porous layer.

The method of the present invention can be applied to produce a film of a predetermined area in a hatched manner, and can also be applied to produce a continuous film using a long base material. . In the case of continuous film formation, the speed at which the coagulating liquid is brought into contact with the back surface of the substrate is preferably 0.1//m+·min or more.

In this way, the base material with only the back side of the base material in contact with the coagulation liquid,
Then, the entire body is immersed in a coagulating liquid.

The immersion time is not particularly limited, and the immersion time may be sufficient to solidify the polymer. Usually, it takes about 1 minute to 1 hour. In addition, when forming a film in a continuous manner, it is preferable to guide and immerse the film into a coagulation solution at a speed of 10 m/min or more.

In the present invention, the form of the membrane is not limited in any way; for example,
It may be in any shape such as a flat membrane, a tube, or a spiral.

As described above, the method of the present invention involves applying a film-forming solution containing an ethylene-vinyl alcohol copolymer to one side of a water-permeable substrate, bringing only the back side of the substrate into contact with a coagulating liquid for a short time, and
Next, the entire base material is immersed in a coagulation solution to form a film, and the film obtained by this unique method is different from the conventionally known ethylene-vinyl alcohol copolymer anisotropic film. The dense layer on the surface is very thin, usually
Although it is less than 1% of the thickness of the polymer layer consisting of a dense layer and a porous layer, it is selectively removable, and a porous layer with cavities is formed under the dense layer, and ,
Since at least a portion of this cavity is in direct contact with the dense layer at its upper end, the water permeability is also significantly improved and it can be used as an ultrafiltration membrane with unprecedented high performance.

EXAMPLES The present invention will be specifically explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

In addition, in the following, the removal rate is defined by the following formula.

Example 1 Ethylene-hinyl alcohol copolymer with ethylene content of 20 mol% (degree of saponification 99%, molecular weight approximately 200,000
) 21.0 parts by weight of isopropyl alcohol-water (
A film-forming solution was prepared by dissolving 79.0 parts by weight (volume ratio 6:4), and this was added to 0.11111/cnl・sec・kg/
After coating one side of a polyester nonwoven fabric substrate with water permeability of CJ (25°C), the back side was immersed in water at a temperature of 2°C for 1 hour.
Contact was made for 0 seconds. This substrate was then immersed in water to coagulate the polymer.

The obtained membrane was heated at a temperature of 25°C and a pressure of 2.0 kg/cn.
Pure water was passed through the tube under conditions of 1, and the water permeation rate was determined. Further, an aqueous solution of polyethylene glycol (molecular weight approximately 20,000, hereinafter referred to as PEG) of 5,000 ppm was passed under the same conditions as above, and the removal rate was determined from the water permeation rate.

The results are shown in Table 1.

Electron micrograph of the cross section of the membrane obtained above after drying (23
0x) is shown in Figure 1. Electron micrograph in Figure 3 (21
00 times) is an enlarged view of the vicinity of the dense layer on the membrane surface, and FIG. 4 is a further enlarged view of the dense layer (16,000 times). FIG. 5 shows the surface of the compact layer (38,000 times magnification).

Comparative example 1 This comparative example shows the production of a membrane by a conventional conventional dipping method.

That is, as in Example 1, after applying the film forming/8 solution to one surface of the substrate, the entire substrate was submerged in water. + 'tM. Table 1 shows the membrane performance of the crotch thus obtained under the same conditions as in Example 1.

Comparative Example 2 In the same manner as in Example 1, after applying the M membrane/8 solution to one surface of the substrate, the side to which the membrane forming solution was applied was brought into contact with water for 10 seconds, and then the entire substrate was immersed in water for 94°C. I fxed it. Table 1 shows the membrane performance obtained under the same conditions as in Example 1 for the membrane thus obtained.

Table 1 Example 2 All procedures were the same as in Example 1, except that after applying the film forming/8 solution, the time for contacting the back side with water was changed to 3 seconds, 30 seconds, or 90 seconds. A membrane was obtained. The membrane performances of these membranes determined under the same conditions as in Example 1 are shown in Table 2.

[Brief explanation of the drawing]

FIG. 1 is an electron micrograph showing a cross section of the ethylene-vinyl alcohol copolymer selective permeation membrane according to the present invention, FIG. 2 is a schematic diagram thereof, and FIG. 3 is an electron micrograph showing the vicinity of the dense layer on the membrane surface. FIG. 4 is an electron micrograph showing an enlarged view of the dense layer, and FIG. 5 is an electron micrograph of the membrane surface. 1. Dense layer, 2. Polymer layer, 3. Porous layer, 4.
... Cavity, 5... Polymer wall, 6... Water-permeable base material. -A- Figure 1 Figure 82/1 Figure 3 Figure 4

Claims (1)

[Claims] (11) The dense layer on the membrane surface and the porous layer supporting this dense layer are integrally formed from an ethylene-vinyl alcohol copolymer, and this porous layer has a water-permeable base. In the selectively permeable membrane fixed in the shrine, the dense layer has a thickness of substantially 0.01 to 0.5μ, and substantially has only micropores with a pore diameter of 500 Å or less, and a cavity in which the quality layer extends in the film thickness direction and whose diameter transverse to the film thickness direction is substantially 1 to 100 μ; and pores surrounding this cavity and having a pore diameter substantially in the range of 0.1 to 5 μ a porous M body wall having a structure, at least a part of the cavity is in direct contact with the dense layer at its upper end, and further,
A selectively permeable membrane characterized in that the porous polymer wall is integrally fixed to the water-permeable substrate at a lower portion thereof. (2) The selectively permeable membrane according to claim 1, wherein the ethylene content in the ethylene-vinyl alcohol copolymer is 5 to 50 mol%. (3) Water-permeable base material is 0.05 m1/c at 25°C
The selectively permeable membrane according to claim 1, which has a water permeability of t·sec·kg/ca or more. (4) The selectively permeable membrane according to claim 1, wherein the dense layer is 1% or less of the thickness of the polymer layer consisting of the dense layer and the porous layer. (5) After applying a mixed solution of water and an aliphatic alcohol having 1 to 4 carbon atoms containing a dissolved ethylene-vinyl alcohol copolymer onto one surface of a water-permeable base material, first, the water-permeable base material is coated with water. The other side is brought into contact with a coagulating liquid mainly composed of water,
A method for producing a selectively permeable membrane, which comprises: then soaking the water-permeable substrate in a coagulating solution for 4 to 5 minutes to coagulate the polymer. (6) The method for producing a selectively permeable membrane according to claim 5, wherein the ethylene content in the ethylene-hinyl alcohol copolymer is 5 to 50 mol%. (7) 0.05 ml on water-permeable substrate at 25°C
6. The method for producing a selectively permeable membrane according to claim 5, wherein the selectively permeable membrane has a water permeability of /cl+·sec·kg/c11 or more.