JPS6357087B2 - - Google Patents

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 has a selectivity that allows it to be suitably used as an ultrafiltration membrane. This invention relates to a permeable membrane and its manufacturing method. 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 membrane's selective removability and water permeability, but in conventional membranes, this dense layer is thick and usually exceeds 1% of the total membrane thickness, so the water permeability is low. Even lower. 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 a selectively permeable membrane made of an ethylene-vinyl alcohol copolymer. An object of the present invention is to provide an ethylene-vinyl alcohol copolymer selective permeable membrane in which at least a portion of the cavities contained in the membrane are in direct contact with the dense layer, and thus have excellent water permeability, and a method for producing the same. 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 mm.
~0.5μ, and has only micropores with a pore size of substantially 500 Å or less, the porous layer extends in the film thickness direction, and has a diameter substantially 1 to 500μ in diameter across the film thickness direction.
100μ and a porous polymeric wall surrounding this cavity and having pores with a pore size substantially in the range of 0.1 to 5μ, with at least some of the cavities directly at the upper end of said compact layer. , and further characterized in that the porous polymer wall is integrally fixed to the water-permeable substrate at a lower portion thereof. The ethylene-vinyl alcohol copolymer used in the present invention preferably has an ethylene content of 5 to 50 mol%, particularly 10 to 50 mol%.
Preferably it is in the range of 40 mol%. The degree of saponification is
It is desirable that the molecular weight is 80% or more, preferably 90% or more, and the molecular weight is about 200,000 or more. If the ethylene content is less than 5 mol%, the water stability of the obtained membrane will be poor, especially because it will become soluble in hot water, while if it exceeds 50 mol%, This is because it becomes difficult to dissolve in organic solvents, 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 with 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. 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 ester, methacrylic acid, methacrylic 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, usually within a range of 10 mol% or less. If the crystallinity of the ethylene-vinyl alcohol copolymer is inhibited, the resulting film will have poor chemical resistance, 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, in the membrane according to the present invention, a dense layer 1 with a thickness of substantially 0.01 to 0.5 μm is formed on the membrane surface. However, it is permissible for the dense layer to partially have a thickness outside the above range. This dense layer has a pore diameter of substantially 500 Å, as shown in Figure 5, which is an electron micrograph of the membrane surface.
It has only the following micropores. As 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. Setting the dense layer to 0.01 μ or less means that
This is not preferred because it lowers the selective removal rate of the membrane. On the other hand, a value of 0.5μ or more is not preferable because it lowers the water permeability of the membrane, although the selective removal rate is excellent. 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, it has a part where no porous polymer layer is interposed between the cavity and the dense layer, and therefore has high water permeability.
Moreover, it has a high selective removal rate due to its thin dense layer. 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 is formed in the membrane thickness to form such a cavity. It extends in the direction of
The polymer wall has pores with a pore size of substantially 0.1 to 5 microns. Note that the form of the cavity is not strictly defined;
Two or more cavities may communicate with each other and include a portion extending 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 1000μ, particularly preferably 100 to 500μ. When it is smaller than 75μ, the membrane is likely to have defects, and when it is larger than 1000μ, it undesirably increases the water permeation resistance. Therefore, the membrane of the present invention has a very high 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, nonwoven fabrics, and other porous sheet materials are preferably used. Specific examples include materials such as polyolefin, polyester, polyamide, polyacrylic, polyvinyl chloride, polyvinyl alcohol, and cellulose. In the present invention, preferably, the water-permeable base material has a large influence on the water permeability of the membrane, and further,
0.05 at 25℃ as it is also related to the durability of the membrane.
It has a water permeability of ml/cm ² ·sec·Kg/cm ² or more, particularly preferably 0.1 ml/cm ² ·sec·Kg/cm ² or more. However, if the water permeability is too large,
The resulting membrane is likely to have defects, and the increased porosity reduces mechanical strength and reduces the durability of the membrane. Furthermore, as described below,
When the film-forming solution is applied to this base material, the film-forming solution easily reaches the surface opposite to the coated surface (hereinafter simply referred to as the back surface), or the film-forming solution is applied to the base material. After that, when the back side is brought into contact with the coagulation solution, the solvent tends to easily reach the side to which the membrane forming solution is applied, making it difficult to obtain an anisotropic membrane having the characteristics of the present invention, which is not preferable. . Therefore, although it depends on the polymer concentration and viscosity in 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, comprises a mixed solution of an aliphatic alcohol having 1 to 4 carbon atoms containing dissolved ethylene-vinyl alcohol copolymer and water on one side of a water-permeable substrate. After applying it to
First, the other side of this water-permeable base material is brought into contact with a coagulation liquid mainly composed of water for a short time, and then the water-permeable base material is immersed in the coagulation liquid to coagulate the polymer. be done. Preferred specific examples of the aliphatic alcohol for preparing the membrane forming solution by dissolving the ethylene-vinyl alcohol copolymer include monohydric alcohols such as methanol, ethanol, propanol, and butanol. Furthermore, in the present invention, polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin are also included in the above alcohol. Monohydric and polyhydric alcohols having 5 or more carbon atoms are generally difficult to dissolve 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 volume mixing ratio in the alcohol-water mixed solvent is usually between 9:1 and 1:9, preferably
It ranges from 3:7 to 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 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 poise 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 film forming solution may drip after being applied to the substrate. This is not preferable because film defects are likely to occur. The water-permeable base material for applying this film-forming solution has already been explained, but in more detail, it is required that it does not dissolve or swell in the film-forming solution, and it is desirable that it has excellent chemical resistance. . According to the method of the present invention, after the film-forming solution is applied to one side of the substrate, the back side is brought into contact with the coagulation 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-vinyl alcohol copolymer. . Further, if necessary, the coagulating liquid may contain a water-soluble salt. The amount is usually 25
% by weight or less. The temperature of the coagulating liquid is not particularly limited, and may generally be room temperature, but is preferably in the range of 0 to 20°C. In order to bring only the back side of the base material coated with the membrane forming solution into contact with the coagulation liquid, specifically, when manufacturing a membrane of a predetermined area in batches, for example, place the back side of the base material facing down. This can be carried out by gently floating the substrate on the surface of the coagulating liquid. In addition, when manufacturing 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, so that the back side of the base material is short. What is necessary is to contact the coagulation liquid for a period of time. As described above, the time period for which the back surface of the base material is brought into contact with the coagulation liquid is usually 1 to 120 seconds, preferably 2 to 60 seconds. If the contact time is too short, it will be difficult to achieve the unique structure of the membrane according to the invention, while if it is too long, the coagulation of the polymer from the back side of the substrate will occur, especially when the film thickness is small. This progress makes it likewise difficult to obtain the unique membrane structure according to the invention. Incidentally, if necessary, the base material may be impregnated in advance with the same solvent as that constituting the film-forming solution, and then the film-forming solution may be applied to one surface thereof. This is to control the replacement rate between the membrane forming solution and the coagulation liquid when the back surface of the base material is brought into contact with the coagulation liquid, and thus to adjust the size of the cavity and the size of the pores of the porous layer. The method of the present invention can be applied to produce films of a predetermined area in batches, and can also be applied to produce continuous films using long substrates. 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 ² ·min or more. The base material, in which only the back surface of the base material has been brought into contact with the coagulating liquid, is then immersed in its entirety in the coagulating liquid.
The immersion time is not particularly limited, and the immersion time may be sufficient to solidify the polymer. Usually 1
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 it in a coagulation solution at a speed of 10 m/min or more. In the present invention, the form of the membrane is not limited at all,
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 membrane-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 the coagulating liquid for a short time, and then The film is formed by immersing the entire base material in a coagulating liquid, 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 membrane surface is very thin, and usually
1 of the thickness of the polymer layer consisting of a dense layer and a porous layer
% or less, it has excellent selective removability, and furthermore, a porous layer having a cavity is formed under this dense layer, and at least a part of this cavity is directly connected to the dense layer at its upper end. Because they are in contact with each other, water permeability is 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. Removal rate (%) = [1-permeate water solute concentration / stock solution solute concentration] × 100 Example 1 Ethylene-vinyl alcohol copolymer with 20 mol% ethylene content (saponification degree 99%, molecular weight approx.
200000) 21.0 parts by weight of isopropyl alcohol
A film-forming solution was prepared by dissolving in 79.0 parts by weight of water (volume ratio 6:4), and this was added to 0.1 ml/ ^cm2・sec・Kg/ ^cm2 (25
After coating one side of a polyester nonwoven fabric substrate with water permeability of
Contact was made for a second. This substrate was then immersed in water to coagulate the polymer. Pure water was passed through the obtained membrane at a temperature of 25° C. and a pressure of 2.0 Kg/cm ² to determine the water permeation rate. 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. An electron micrograph (230x magnification) of the cross section of the membrane obtained above after drying is shown in Figure 1. The electron micrograph in FIG. 3 (2100x) shows the vicinity of the dense layer on the membrane surface enlarged, and FIG. 4 shows the dense layer further enlarged (16000x). Figure 5 shows the surface of the compact layer (38000
times). Comparative Example 1 This comparative example shows the production of a membrane by a conventional conventional dipping method. That is, as in Example 1, the film forming solution was applied to one surface of the base material, and then the entire base material was immersed in water. Example 1 about the membrane thus obtained
Table 1 shows the membrane performance determined under the same conditions. Comparative Example 2 As in Example 1, the membrane forming solution was applied to one side 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. Table 1 shows the membrane performance of the membrane thus obtained, determined under the same conditions as in Example 1.

[Table] Example 2 A membrane was formed in the same manner as in Example 1, except that after applying the membrane forming solution, the time for contacting the back side with water was changed to 3 seconds, 30 seconds, or 90 seconds. I got it. The membrane performances of these membranes determined under the same conditions as in Example 1 are shown in Table 2.

【table】 [Brief explanation of drawings]

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. DESCRIPTION OF SYMBOLS 1... Dense layer, 2... Polymer layer, 3... Porous layer, 4... Cavity, 5... Polymer wall, 6... Water-permeable base material.

Claims (1)

[Claims] 1 The dense layer on the surface of the membrane and the porous layer supporting this dense layer are integrally formed as a polymer layer made of ethylene-vinyl alcohol copolymer, and this porous layer is formed on the water-permeable base material. In the method for producing a fixed selectively permeable membrane, an ethylene-vinyl alcohol copolymer with an ethylene content of 5 to 50 mol% and a degree of saponification of 80% or more is dissolved in a concentration range of 3 to 40% by weight. A mixed solution of aliphatic alcohol having 1 to 4 carbon atoms and water was heated at 0.05 ml/cm ²・sec.
After coating on one side of a water-permeable base material that has a water permeability of Kg/ ^cm2 or more at 25°C, first, the other side of this water-permeable base material is soaked in a coagulating liquid mainly composed of water for 1 to 120 seconds. Then, the water-permeable substrate is immersed in a coagulating liquid to coagulate the polymer, so that the dense layer has a thickness of substantially 0.01 to 0.5 μm, and is 1 μm thicker than the polymer layer. % or less and substantially 500
A cavity having only micropores with a pore diameter of Å or less, the porous layer extending in the film thickness direction, and having a diameter of substantially 1 to 100 μ across the film thickness direction; a porous polymer wall having pores substantially in the range of 0.1 to 5μ, at least some of the cavities are in direct contact with the dense layer at their upper ends, and A method for producing a selectively permeable membrane, characterized in that a selectively permeable membrane is obtained, the wall of which is integrally fixed to the water-permeable substrate at its lower part.