JPH0239300B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to gas separation membranes. It is already known that porous membranes and dense membranes made of various polymers are effective in separating mixed gases, but in general, gas separation membranes with these properties are not able to achieve both permeability and selective separation. It's difficult. That is, gas separation membranes that are highly permeable to different gases generally have poor selectivity, and conversely, gas separation membranes that are excellent in selectively separating different gases have poor permeability. These are obstacles to practical gas separation using gas separation membranes. For this reason, methods for increasing the permeability and selective separation properties of gas separation membranes have been proposed, but a practical gas separation membrane has not yet been obtained from the viewpoint of production and membrane properties. For example, JP-A-47-16574 discloses a method of improving selective separation between gas mixtures by irradiating a gas separation membrane made of a dense and homogeneous polymer film with a low-energy electron beam. However, with electron beam irradiation, the polymer film undergoes physical and chemical changes throughout its thickness, so although selective separation is improved, gas permeability tends to decrease in many cases. Moreover, since the dense polymer film inherently has low gas permeability, it is difficult to obtain a highly practical gas separation membrane with excellent permeability and selective separation properties. . On the other hand, so-called asymmetric membranes, which have a thin layer on the surface that selectively separates gases and this thin layer is supported by a porous polymer, generally have better gas permeability than dense polymer films. However, even in the case of asymmetric films, electron beam irradiation causes physical and chemical changes throughout the entire film thickness.
Gas permeability tends to decrease. The present invention has been made in order to solve the above-mentioned various problems in gas separation membranes, and an object of the present invention is to substantially improve only the thin layer having gas selective separation properties in an asymmetric membrane made of polysulfone. It is an object of the present invention to provide a gas separation membrane having improved selective separation properties while maintaining high gas permeability. The method for manufacturing a gas separation membrane made of polysulfone according to the present invention comprises a dense thin layer on the surface having different permeation fluxes between at least one gas and other gases, and a porosity of 1 to 90%. The above surface thin layer of porous asymmetric polysulfone membrane is 0.0005 ~
Sputter etching is performed under an atmospheric pressure of 1 Torr so that the processing amount, which is the product of processing power density and processing time, is 1 to 1 × 10 ³ W・sec/cm ² , and hydrogen and helium are substantially eliminated. It is characterized by reducing the permeation flux of gases having a larger molecular weight or higher polarity than these without changing the permeation flux. The asymmetric membrane made of polysulfone used in the present invention has a dense thin layer on the surface having different permeation fluxes between at least one type of gas and other gases;
It has a so-called skin layer and has a porosity of 1 to 90%, preferably 40 to 80%. In general, when the pores of a porous membrane are relatively large, the permeation rate is almost a straight line when plotted against the molecular weight of the gas, as shown by line A in Figure 1. As the pores become smaller, oxygen with a large molecular weight, carbon monoxide, carbon dioxide, etc. with a highly polar molecular structure deviate from the above straight line. The porous asymmetric polysulfone membrane that is particularly preferably used in the present invention is a porous or dense thin film having a pore diameter smaller than that at which deviation from the above linear relationship occurs for gases having a large molecular weight or gases having a highly polar structure. It has layers. Although it is difficult to quantify the diameter of micropores in a thin layer, for example, the separation coefficient of helium to nitrogen can be used as one index for selecting the pore diameter, and in the present invention, the separation coefficient is 3 or more. Good results are obtained using porous asymmetric polysulfone membranes. According to the present invention, the separation coefficient is controlled by controlling the thickness of the dense thin layer on the surface of the membrane, the diameter of the micropores, the density, etc. These include, for example, the concentration and viscosity of the film-forming liquid when forming a film by a wet method, the thickness when casting onto a base material, the casting conditions such as temperature and humidity, the amount of solvent evaporated after casting, the coagulation solvent, etc. It is known that this can be controlled by post-processing conditions such as coagulation temperature. According to the method of the present invention, a porous asymmetric polysulfone membrane having a thin layer as described above is exposed to a cathode dark region in which the ion energy in the discharge region is extremely high, resulting in sputter etching, that is, as a result of discharge. By accelerating cations and causing them to collide with the thin surface layer, only the thin surface layer is physically and chemically changed, and the surface layer is cross-linked into an ultra-thin film. . The sputter etching technique itself is already known, as described in, for example, Japanese Patent Publication No. 53-31827. In the present invention, the sputter etching process is usually carried out at room temperature and under an atmospheric pressure of 0.0005 to 1 Torr, preferably 0.001 to 0.1 Torr. When the atmospheric pressure is less than 0.0005 Torr, the discharge is not sustained, and when it is greater than 1 Torr, the sputter etching speed decreases markedly and the discharge becomes unstable, making it particularly difficult to continuously perform the sputter etching process. This is because it is not possible to obtain a homogeneous treated surface layer when carrying out such treatment. A thin layer processed by sputter etching is expressed as the product of processing power density and processing time, and the optimum processing amount is determined appropriately depending on the pore diameter in the thin layer, the thickness of the thin layer, etc. , 1~1×10 ³ W・
sec/ ^cm2 . This is because when the discharge treatment amount is smaller than 1 W・seo/cm ² , the treatment effect is small and the separation coefficient cannot be sufficiently increased, whereas on the other hand, the discharge treatment amount is larger than 1×10 ³ W・sec/cm ² This is because, although the separation coefficient generally becomes high when using a gas separation membrane, there is a risk that the gas separation membrane may be deformed or contracted, and the permeation rate of each gas decreases, which is undesirable.
Obviously, the lower the processing power density, the longer the processing time needs to be, but it is practically desirable to increase the processing power density and shorten the processing time. As a power source, a high frequency power source of several hundred KHz to several tens of MHz can be used, but in practice 13.56MHz is used.
It is convenient to use the industrially allocated frequencies of The required distance between electrodes is 1/√ when the atmospheric pressure is P
For example, when P is 0.005 Torr, the distance between the electrodes must be 30 mm or more, and normally
Adjusted to about 40mm. In addition, a gap is provided between the cathode electrode and the shield electrode to prevent discharge between the two electrodes.
In the case of 0.005 Torr, the gap is usually about 4 mm. In practice, the atmospheric gas used during sputter etching treatment is inert gas such as argon or nitrogen, air,
Carbon dioxide gas and water vapor are used. Porous fabricated asymmetric polysulfone membranes treated in this way unexpectedly show virtually no change in permeation rate for low molecular weight gases such as helium and hydrogen, whereas nitrogen, oxygen, carbon dioxide, etc. The permeation rate of gases with large molecular weights or gases with highly polar structures, such as, decreases. As a result, the gas separation membrane obtained according to the present invention maintains a high permeation rate for gases with a low molecular weight, while increasing the separation coefficient for gases with a high molecular weight or gases with a highly polar structure. Furthermore, the permeation rate changes between gases with large molecular weights and gases with polar structures, and the separation coefficient is increased. FIG. 2 shows a model diagram of an asymmetric membrane in which a thin layer 1 is integrally supported by a rough porous layer 2, in which a portion of the thin layer has been subjected to sputter etching. When the sputter-etched surface layer is observed under an electron microscope, as a result of surface etching by cations, unevenness and woven hair-like substances are observed on the surface, and the crosslinked layer 3 is formed in the form of an ultra-thin film. It is believed that the formation of this crosslinked layer contributes to the decrease in the permeation rate of gases with large molecular weights and gases with a polar structure. As described above, in the gas separation membrane of the present invention, only the thin surface layer that has gas selective separation properties is etched by the sputter etching process, and the crosslinked layer is formed into an ultra-thin film through a chemical reaction. Therefore, the selectivity between different gases is enhanced while maintaining high permeability to small molecular weight gases such as helium before and after the sputter etching process. Examples of the present invention are listed below, but the present invention is not limited thereto. Example Polysulfone (Union Carbide P-
1700) in N-methyl-2-pyrrolidone to prepare a membrane-forming solution, which was cast onto a glass plate, immersed in a coagulation bath, and air-dried to obtain an asymmetric membrane with a porosity of 56%. The skin layer side of this film was heated at room temperature, under an argon atmosphere pressure of 0.1 Torr, and at a discharge power density of 0.5 W/cm ² for various times.
Sputter etching treatment. FIG. 3 shows changes in transmittance with respect to throughput, and FIG. 4 shows changes in separation coefficient. Incidentally, the permeation rate of each gas (unit: cc (STP)/cm ^{2 )} before treatment and when the processing amount is 600 W sec/cm ²
sec・cmHg) and separation factors are shown in the table below.

[Table] Next, it was confirmed that the thin surface layer of the polysulfone film was crosslinked by the sputter etching treatment, and its weight was reduced. FIG. 5 shows the relationship between the amount of treatment and the weight reduction rate and the relationship between the amount of treatment and the estimated thickness of the crosslinked layer. The weight loss rate was determined by weighing the membrane before and after treatment, and the crosslinked layer thickness was determined by immersing the membrane after treatment in dimethylformamide.
The insoluble matter was scooped up with a wire mesh, and the thickness was calculated from the dry weight. Incidentally, the processing amount is 600W・sec/
cm ² , the weight loss rate was 2%, and the crosslinked layer thickness was estimated to be about 1000 Å. In addition, when the chemical structure of the surface of the treated film was examined over time using an X-ray photoelectron analyzer (ESCA650-B, manufactured by Dupont), as shown in Figure 6, S ₂ p /
Since the O _{1 s} /C 1 s ratio increases as the C ₁ s ratio decreases, it seems that the sulfone groups in the surface layer of the membrane are decomposed while oxygen is provided, and crosslinking progresses through them.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between gas molecular weight and permeation rate for porous membranes having micropores of various pore sizes, with the pore sizes increasing in the order of A, B, and C. Figure 2 is a model diagram showing the cross sections of an untreated gas separation membrane and a gas separation membrane according to the present invention, and Figure 3 shows the sputter etching throughput and gas permeability for various gases for a porous asymmetric membrane made of polysulfone. 4 is a graph showing the relationship between the throughput and the separation coefficient of various gases for nitrogen. FIG. 5 is a graph showing the relationship between the throughput and the thickness of the crosslinked layer. It is a graph showing the processing amount and the atomic ratio in the surface layer.

Claims (1)

[Claims] 1. A dense thin layer having different permeation fluxes between at least one gas and other gases on the surface,
A thin layer on the above surface of a porous asymmetric polysulfone membrane with a porosity of 1 to 90% is processed under an atmospheric pressure of 0.0005 to 1 Torr.The processing amount, which is the product of power density and processing time, is 1 to 1 × 10 ³ W. sec/cm ² for hydrogen and helium without substantially changing their permeation fluxes, and for gases with larger molecular weights or more polarity than these. A method for producing a gas separation membrane made of asymmetric polysulfone, characterized by reducing permeation flux.