JPS6144525B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for increasing the salt permeability of a hydrophobic porous membrane, and more specifically, to a method for increasing salt permeability of a hydrophobic porous membrane.
The present invention relates to a method of increasing the salt permeability of a porous membrane (abbreviated as PTFE) by subjecting the membrane to surface treatment using a plasma polymerization method.

PTFE is chemically stable and has excellent acid resistance, alkali resistance, organic solvent resistance, and heat resistance, so its porous membranes are used for membrane separation under various conditions. In order to carry out membrane separation efficiently, it is generally desirable to suppress the membrane permeability of the substance to be blocked as much as possible, and to allow the substance to permeate through the membrane as quickly as possible. Therefore, a technique is needed to increase permeability without changing the pore size, membrane thickness, etc.

By the way, there is a plasma polymerization method in which the gas molecules or their decomposition products are chemically activated and polymerized by causing glow discharge in a reduced pressure gas. In this gas, a suitable substrate such as
If a porous PTFE membrane is included, the polymer can be deposited on the substrate. Plasma polymerization is a method for treating and modifying substrates with simple operations and under mild conditions, and its advantages are well known.

The present inventors have demonstrated that the salt permeability of a PTFE porous membrane surface-treated by slowly precipitating a plasma polymerized product using pyridine as a gas under a lower pressure than is generally used is increased compared to that of an untreated porous PTFE membrane. Based on this knowledge, we have completed this invention. Conventionally, a method of making the surface of a PTFE membrane hydrophilic by plasma polymerizing pyridine was known (for example, as described in Japanese Patent Application Laid-Open No.
No. 55-131026) is intended merely to improve adhesion, and it is not known that ion permeability can be increased by appropriate treatment.

That is, this invention subjects a polytetrafluoroethylene porous membrane to a glow discharge treatment in a pyridine atmosphere of 0.01 to 0.02 Torr within the time when the porosity reaches the first 10%, thereby forming a plasma polymerized layer on its surface. This is a method for increasing salt permeability of a polytetrafluoroethylene porous membrane, which is characterized by:

The salts to be permeated may be metal halides, metal sulfates, nitrates, or any other salts.

In order to carry out the method of this invention, it is desirable that the pressure inside the reaction tube during treatment is 0.01 to 0.02 Torr. When the pressure of pyridine is 0.02 Torr or higher, no significant increase in salt permeability occurs, probably as a result of the reaction between the active species proceeding too quickly. A pressure of 0.01 Torr or less is not preferable because it destabilizes the plasma state. To achieve pressures in the above range during processing, the pressure before generating the plasma can be adjusted to e.g.
It is recommended to set it to 0.02 to 0.05 Torr. These are considerably lower settings than the pressures normally used for plasma processing.

The salt permeation rate increases rapidly as the PTFE membrane surface is modified by plasma treatment. However, as the treatment progresses and the plasma polymerized layer becomes too thick, the porosity decreases and the salt permeation rate gradually decreases. Therefore, for the purpose of the present invention, it is desirable that the initial porosity is maintained at 10% or more.

Hereinafter, the configuration of the present invention will be explained in more detail based on the accompanying drawings. FIG. 1 is a sectional view and a perspective view showing an example of an apparatus suitable for carrying out the present invention, in which a glass reaction tube 1 is connected to an exhaust system (not shown), It can exhaust to around ^-3 Torr. In this example, reaction tube 1 has an inner diameter of 45
It is composed of mm cylindrical glass. The reaction tube 1 is provided with a tube 2 for supplying monomer gas, which is connected via a flow regulator to a gas source, namely a container containing purified pyridine (not shown). Connecting. Pyridine vapor is supplied from the gas supply pipe 2 to the reaction tube. A coil 3 is wound around the outside of a part of the reaction tube.

To perform the plasma polymerization process, the coil is connected to a radio frequency (13, 54 MHz) power source to supply power. The gas pressure in the reaction tube is previously adjusted to a desired range, 0.02 to 0.05 Torr in this example, using the flow rate regulator. When a glow discharge occurs in a gas, the gas ionizes and becomes a low-temperature plasma. Pyridine decomposes due to collisions between molecules, producing a large number of chemically active radicals of various types, which form brown polymers. Precipitates as a substance. In addition to the method using the coil shown in the figure, there is a method for causing glow discharge, in which two electrodes are provided inside the reaction tube and an alternating current voltage is applied between them.

A substrate support 4 made of an aluminum plate is installed inside the reaction tube. As shown in Figure 1b, the cross section of the support 4 is semicircular, the center of the circle coincides with the center of the cross section of the reaction tube wall, and the radius of the circle is
It is 19mm. Here, add PTFE to follow this shape.
When the plasma polymerization treatment is performed after fixing the porous membrane, a thin polymer film is gradually deposited on the surface of the PTFE membrane.

The reason for creating such a concentric support is that the plasma density inside the cylindrical tube is distributed symmetrically around the central axis, so processing on this type of support can form a uniform thin film. This is because I thought it was advantageous. The support stand 4 is used by being placed on a slide glass 5.

The present invention is a method as described above, and by using the method, only the ion permeability of the porous PTFE membrane can be increased without damaging the structure of the porous PTFE membrane.

Example 1 The following conditions were adopted: pyridine vapor flow rate 0.041STP-cm ³ /min, pyridine vapor pressure 0.03Torr, and power supply 10W. Fluoropore FP manufactured by Sumitomo Electric was used as the substrate.
A -010 membrane (nominal pore size 0.1 μm) was used and only one side thereof was treated with the method of the invention. FIG. 2 shows the results of investigating the relationship between polymerization treatment time and increase in substrate weight (per 1 cm ² ). The substrate weight increased linearly with time, indicating that a polymer film was gradually formed.

As the polymer accumulates, the white surface of the substrate turns brown, and the strongly hydrophobic surface becomes hydrophilic and does not repel water. The deposits are solid
It adhered to the surface of the PTFE membrane, and did not peel off even after being immersed in water at 30°C for 10 days, and even after repeated so-called peeling tests using adhesive tape several times. When the treatment time exceeded 180 minutes, the releasability did not change, but the substrate began to curl, and when it was forcibly spread, there was a tendency for the thin film to crack.

When the treated membrane was observed using a scanning electron microscope, the large number of pores in the untreated PTFE membrane were gradually filled with plasma polymerization products, and after about 100 minutes, the surface porosity decreased to about 10% of the initial level. It was found that the pores almost disappeared after 190 minutes of treatment. Examples of micrographs are shown in FIGS. 3 and 4. FIG. 3 is a photograph of the membrane surface, in which a shows an untreated membrane, b shows a membrane treated for 20 minutes, c shows a membrane treated for 100 minutes, and d shows a membrane treated for 190 minutes.
Figure 4 is a photograph of the cross section of the membrane, where a is the membrane treated for 20 minutes and b is the membrane treated for 20 minutes.
The membrane was treated for 100 minutes, and c was the membrane treated for 190 minutes. From the cross-sectional photograph of the membrane treated for 190 minutes (Fig. 4d), it was clear that a dense thin film of the product was uniformly formed with a thickness of about 1.8 μm. Furthermore, no change was observed in the pore structure or film thickness of the PTFE substrate itself, regardless of the treatment time. Even when measured using a micrometer, the film thicknesses of the untreated film and the treated film were the same within a range of ±1 μm.

Example 2 After various membranes with different treatment times were prepared under the conditions of Example 1, they were installed in a dialysis cell, and 100 ml of the membrane was placed on one side of the membrane.
Figure 5 shows the membrane permeability obtained by placing a 0.5 mol/Kg aqueous sodium chloride solution and an equal amount of distilled water on the other side and measuring the rate of increase in electrical conductivity on the distilled water side. The measurements were performed in constant temperature water at 30°C, and the liquids on both sides were stirred at approximately 500 rpm using a magnetic stirrer. The membrane permeability was determined by dividing the amount of sodium chloride permeated per 1 s of time per 1 cm ² of cross-sectional area by the concentration difference on both sides of the membrane.

According to Figure 5, as the plasma polymerization treatment time increases, the ion permeability of the untreated membrane increases rapidly, reaches a maximum value at 30-70 minutes of treatment, then decreases again, and after 190 minutes of treatment, the ion permeability of the untreated membrane increases rapidly. It is no different from the untreated membrane.

As mentioned above, the thickness of the PTFE membrane itself does not change during the plasma polymerization process, and its weight only increases monotonically as shown in Figure 2, so the increase in ion permeability is due to the influence of the plasma. Therefore, this is not due to a decrease in the thickness of the base film. Nor is it because the structure of the PTFE film has changed or been destroyed, as is known from the electron micrograph. The increase in ion permeability is thought to be due to the hydrophobic PTFE membrane surface becoming hydrophilic through plasma polymerization treatment. However, there is an optimum treatment time for the treatment to have its effect, and treatment beyond this time will actually reduce the ion permeability and also cause the substrate membrane to curl. Moderate processing makes hydrophobic PTFE
It contributes to increasing the rate of ion uptake into the membrane, but if the treatment is too long, a dense thin film will be formed.
It probably inhibits ion permeation.

Example 3 The same plasma treatment conditions as in Example 1 were adopted, and Fluoropore EP-100 membrane (nominal pore size 1.0μ) was used as the substrate.
m) for 40 minutes. of sodium chloride, copper sulfate or nickel chloride using treated membranes.
A 0.3 mol/Kg aqueous solution was permeated at 30°C.

The salt permeation rate of treated membranes is different from that of untreated membranes.
They all increased significantly: 2.7 times, 1.3 times, and 1.1 times.

[Brief explanation of the drawing]

FIG. 1 shows an example of an apparatus suitable for carrying out the present invention, in which a is a sectional view thereof and b is a partial perspective view thereof. In the figure, 1 is a reaction tube, 2 is a gas supply tube, 3 is a coil, 4 is a substrate support, 5 is a slide glass, 6 is a direction in which an exhaust system is connected, and 7 is a gas supply direction. FIG. 2 is a graph showing the relationship between polymerization treatment time and increase in substrate weight. FIG. 3 is a scanning electron micrograph of the membrane surface, in which a is an untreated membrane, b is a membrane treated for 20 minutes, c is a membrane treated for 100 minutes, and d is a membrane treated for 190 minutes. Figure 4 is a scanning electron micrograph of a cross section of the membrane.
a indicates a membrane treated for 20 minutes, b indicates a membrane treated for 100 minutes, and c indicates a membrane treated for 190 minutes. FIG. 5 is a graph showing the relationship between polymerization treatment time and sodium chloride permeability.

Claims (1)

[Claims] 1 Polytetrafluoroethylene porous membrane, 0.01~
Salt permeability of polytetrafluoroethylene porous membrane characterized by glow discharge treatment in a pyridine atmosphere of 0.02 Torr within the time when the porosity reaches the initial 10%, and a plasma polymerized layer is formed on the surface of the membrane. How to increase.