JPH0376969B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Background) The present invention relates to a semipermeable membrane for asymmetric separation having a novel structure. In recent years, reverse osmosis and ultrafiltration methods, which are separation technologies using semipermeable membranes, have been put into practical use in various fields, and semipermeable membranes made from materials suitable for each of these various uses have been put on the market. There is. In particular, well-known materials for semipermeable membranes used in ultrafiltration methods include polyacrylonitrile, cellulose, and polysulfone.

Among these membrane materials, aromatic polysulfone-based membranes are attracting attention as they have high mechanical strength and excellent heat resistance and chemical resistance.

(Prior art and its drawbacks) However, aromatic polysulfone resin has low hydrophilicity and is a material that is difficult to wet with water. The water permeability rate is significantly lower than that of
Overefficiency is bad.

Therefore, various attempts have been made to improve the water permeability of aromatic polysulfone separation membranes.

For example, JP-A-58-104940 describes a polysulfone separation membrane containing polyvinylpyrrolidone with a molecular weight of 100,000 or more and a method for producing the same. However 10
In this method, a hydrophilic polymer with a molecular weight of 10,000 or more is added to a membrane-forming solution (called a dope), which increases the polymer density of the dope. The added hydrophilic polymer remains in the entire membrane and is not removed even during subsequent use, resulting in an extremely dense structure and a reduction in water permeation rate.

On the other hand, JP-A-54-26283 discloses a method in which polyethylene glycol having a molecular weight comparable to that of an oligomer is added to a polysulfone solution and used as a dope. However, this method uses water as a coagulation bath for membrane formation, and oligomer-level polyethylene glycol does not remain in the membrane and is completely eluted into the water, so the hydrophilicity of the polysulfone membrane cannot be substantially increased. No significant improvement in water permeation rate can be expected.

Also, Journal of Applied Polymer Science Vol. 20 (1976) 2377-2394 has a molecular weight of 1.
There have been reports of attempts to improve the spinnability of hollow fiber membranes by incorporating a large amount of polyvinylpyrrolidone as a hydrophilic polymer into a membrane-forming dope. A dense layer is not formed, and an asymmetric structure as in the present invention is not obtained.

(Structure of the present invention) The present inventors have earnestly studied a separation semi-membrane having an asymmetric structure mainly composed of a hydrophobic polymer substance that solves the above-mentioned problems and has good overefficiency and a high water permeation rate. As a result, in the membrane thickness direction of the semipermeable separation membrane, a hydrophilic polymer exists within 5 μm from the membrane surface on the dense layer side, and it does not exist from 5 μm to the back surface, resulting in an asymmetric separation with a new structure. Invented membrane.

In other words, the present invention is a semipermeable membrane for separation mainly composed of a hydrophobic polymer substance having an asymmetric structure with a dense layer on the surface and a porous layer on the back side, in which only the dense layer side is composed of the hydrophobic polymer substance. A semipermeable membrane for separation characterized by being made of a mixture with a hydrophilic polymeric substance.
It is.

As the hydrophobic polymer in the present invention, aromatic polysulfone polymers having the structures of the following formulas () to () are representative.

Polyvinylpyrrolidone is a typical water-soluble polymer used in the present invention, and is a water-soluble polymer obtained by polymerizing vinylpyrrolidone.
It is optimal that the average molecular weight is 70,000 or less and 10,000 or more. If the average molecular weight exceeds 70,000, the polymer density of the membrane becomes too large as described above, and the water permeation rate is rather reduced. Furthermore, even if the average molecular weight is 70,000 or less, molecular weights on the order of oligomers, that is, molecular weights on the order of several thousand, are too water-soluble and are almost completely extracted when immersed in water during the membrane forming process, resulting in poor membrane hydrophilicity and water permeability. Performance is not improved.

Next, as one embodiment of the present invention, a case in which the hydrophobic polymer is aromatic polysulfone and the hydrophilic polymer is polyvinylpyrrolidone will be described in detail.

The dope composition for producing a semipermeable membrane for separation preferably contains 3% by weight or less of the above-mentioned polyvinylpyrrolidone based on the total weight of the dope. If the amount exceeds 3% by weight, the membrane structure will be destroyed, which is disadvantageous, as shown in FIG. 1, which is a photograph of a cross section of the membrane taken with a scanning electron microscope.

Further, it is desirable that the total polymer weight of the hydrophilic polymer and hydrophobic polymer in the dope used in the present invention is 10 to 30% by weight based on the total weight of the dope, and is dissolved in the polar organic solvent. Examples of the polar organic solvent include N,N-dimethylacetamide, N,N-dimethylformamide, N-
Examples include methyl-2-pyrrolidone, dimethylsulfoxide, sulfolane, 2-pyrrolidone, hexamethylphosphoramide, etc., but are not particularly limited to these. Furthermore, a non-solvent of either a hydrophobic polymer or a hydrophilic polymer, or an electrolyte or the like may be added to such a polar organic solvent.

In producing the semipermeable membrane for asymmetric separation of the present invention from the dope described above, a conventional method for producing a semipermeable membrane for asymmetric separation can be employed.

To form a separation membrane in the form of a sheet or tube, a dope is applied to a thickness of several tens of microns to several hundred microns on a suitable support (for example, a glass plate or tube, nonwoven fabric, cloth, etc.) in the form of a sheet or tube. The membrane is cast by an appropriate method and then immersed in a coagulant bath to produce a semipermeable membrane for asymmetric separation by sol-gel phase conversion. In addition, by spinning the dope through a hollow shaped nozzle using a known method, it is possible to produce an asymmetric semipermeable hollow membrane for separation.

The coagulant used in film formation is a non-solvent for aromatic polysulfone, which easily mixes with polar organic solvents, such as water, aqueous solutions of electrolytes such as salt and surfactants, dilute aqueous solutions of polar organic solvents, or aromatic polysulfone. Examples include non-solvents and aqueous solutions thereof, and water is particularly commonly used.

The characteristics of the semipermeable membrane for asymmetric separation of the present invention are manifested by the manufacturing method described above. In other words, the characteristic is that the film surface has Fourier transform infrared absorption.
Although the presence of hydrophilic polymers is confirmed by ATR spectroscopy, their presence is not observed on the back surface of the membrane, indicating an asymmetric structure. Figure 2 shows the Fourier transform infrared absorption of the membrane surface of the asymmetric membrane of the present invention.
In the ATR spectrum (hereinafter referred to as FT-IR ATR spectrum), the vertical axis is transmittance (%) and the horizontal axis is wave number (cm ^-1 ). In this figure, the amide I absorption band 1660 cm ^-1 (arrow in Figure 2) shows the absorption of the carbonyl group of the amide of the hydrophilic polymer polyvinylpyrrolidone.
is clearly recognized. On the other hand, FIG. 3 shows the spectrum of the back surface of the same film, but no similar absorption was observed at all.

(Effects of the present invention) It is clear that by making the membrane surface side extremely hydrophilic, the aromatic polysulfone-based semipermeable membrane for separation, which is originally hydrophobic, has become extremely hydrophilic. Performance is improved. Moreover, the excellent heat resistance and chemical resistance that are characteristic of aromatic polysulfone are maintained, so that conventional semipermeable membranes for separation based on hydrophilic polymers, such as semipermeable membranes for separation made of cellulose acetate, can withstand them. It can be effectively used for membrane separation operations under harsh conditions such as

Next, the present invention will be specifically explained with reference to Examples.
and ovalbumin rejection rate (Ro) ^{, respectively .} The Lp value after 7 hours is L ^o _p , and the Lp value after 7 hours is L〓 _p .) β = L ⁰ / _P - L / _P / L ⁰ / _P × 100, and Ro = (1 - The ovalbumin concentration in the stock solution/the ovalbumin concentration in the stock solution) x 100.

Example 1 Aromatic polysulfone of formula () (trade name
Victrex 300p manufactured by ICI) 18 parts by weight and polyvinylpyrrolidone (hereinafter abbreviated as PVP) with an average molecular weight of 40,000.
(manufactured by Aldvich) was dissolved in 80 parts by weight of a mixed solvent mainly containing 2-pyrrolidone. The viscosity of this dope was 14,000 centipoise at 25°C. This was cast onto a polyester nonwoven fabric to a thickness of 150 μm,
After being left in a room temperature atmosphere for 20 seconds, it was immersed in water at 10°C to obtain a semipermeable membrane for asymmetric separation. The semipermeable membrane for asymmetric separation reinforced with the obtained nonwoven fabric was further immersed in hot water at 90°C for 15 minutes to completely remove the solvent. of this membrane
L ^o _p is 4.5m ³ /m ²・day・Kg/cm ² (25℃), its decreasing rate β
was 4.2%. Moreover, Ro was 100%. Furthermore, the PVP content of the film determined from elemental analysis was 4.5%.

Example 2 When the asymmetric separation membrane of Example 1 was immersed in hot water at 90°C for 50 hours, L ^o _p was 4.88 m ³ /m ² ·day · Kg/cm and β
was 3.7%, and Ro was 100%, and no deterioration in membrane performance was observed. Furthermore, the PVP content was 4.8%, and virtually no PVP elution was observed.

Comparative Example 1 A film was formed in the same manner as in Example 1 except that PVP (manufactured by Wako Pure Chemical Industries, Ltd.) having an average molecular weight of 700,000 was used. The Ro of the obtained film was 100°C, but the L ^o _p was 1.01m ³ /m ² .
It decreased to 1 day・Kg/cm ² .

Comparative Example 2 A film was formed in the same manner as in Example 1, except that 4.2 parts by weight of PVP (manufactured by Aldrich) having an average molecular weight of 40,000 and 77.8 parts by weight of a mixed solvent were used. The Ro of the obtained film is
It decreased to 96%, and L ^o _p also decreased to 2.72m ³ /m ² ·day · Kg/cm ² .

Comparative Example 3 A film was formed in the same manner as in Example 1 except that PVP was not added and the mixed solvent was changed to 82 parts by weight. The L ^op of the obtained membrane was 4.77 m ³ /m ² ·day · Kg/cm ² , and the rejection rate of Ro _was 100.
%, but the decrease rate β was 26.6%, and the water permeation rate decreased significantly over time.

Example 3 A film was formed in the same manner as in Example 1 except that a glass plate with a smooth surface was used as the support. After thoroughly drying this asymmetric separation membrane without reinforcing material, the ATR-
When we measured the IR spectrum, we obtained Figure 2. The absorption band of PVP is clear. However, when we measured the ATR-IR spectrum of the back side of the same film, no PVP absorption band was observed, as shown in Figure 3. Fig. 4 is an electron micrograph showing the cross-sectional structure of the semipermeable membrane obtained here;
No destruction of the membrane structure as shown in the figure was observed. In addition, in FIG. 1, the upper side is the surface dense layer of the membrane, and in FIG. 4, the lower side is the surface dense layer of the membrane.

Example 4 A film was formed in the same manner as in Example 1, except that poly(2-vinylpyridine) (manufactured by Aldrich) was used as the hydrophilic polymer. The L ^o _p of the obtained membrane is 7.56
It shows a large value of m ³ /m ²・day・Kg/cm ² , and
Ro was 100%. The polyvinylpyridine content of the film determined from elemental analysis was 2.9%.

Example 5 The membrane of Example 4 was prepared using a glass plate with a smooth surface as a support. After thoroughly washing with water and drying, the ATR-IR spectrum of the membrane surface was measured, and as shown in FIG. 5, an absorption band of pyridine rings was observed at 1440 cm ^-1 (arrow in FIG. 5). However, when the ATR-IR spectrum of the back surface of the same film was measured, no absorption band of the pyridine ring was observed at 1440 cm ^-1 as shown in Figure 6.

[Brief explanation of drawings]

FIG. 1 is a scanning electron micrograph of a cross section of a membrane prepared in Comparative Example 2 using a glass plate with a smooth surface as a support. FIG. 2 is an FT-IR ATR spectrum of the surface of the film produced in Example 3. Figure 3 is an FT-IR ATR spectrum of the back side of the same film. FIG. 4 is a scanning electron micrograph of a cross section of the membrane obtained in Example 3. FIG. 5 shows the FT-IR ATR spectrum of the front surface of the film obtained in Example 5, and FIG. 6 shows the FT-IR ATR spectrum of the back surface of the same film.

Claims (1)

[Scope of Claims] 1. A semipermeable separation membrane mainly composed of a hydrophobic polymer substance having an asymmetric structure with a dense layer on the front side and a porous layer on the back side, wherein only the dense layer side contains the hydrophobic polymer substance. A semipermeable membrane for separation comprising a mixture of a substance and a hydrophilic polymer substance. 2. The semipermeable membrane for separation according to claim 1, wherein the hydrophobic polymer is an aromatic polysulfone having a repeating unit of any one of structural formulas (), (), or (). 3. The semipermeable membrane for separation according to claim 1, wherein the hydrophilic polymer is polyvinylpyrrolidone with an average molecular weight of 70,000 or less. 4. The semipermeable membrane for separation according to claim 1, wherein the hydrophilic polymer is polyvinylpyridine or vinylpyridine copolymer. 5 As a membrane forming solution, 0.5% of the total weight of the solution
The semipermeable membrane for separation according to claim 1, which uses a polymer solution containing a hydrophilic polymer in an amount of at least 3% by weight.