JPS5888011A - Asymmetric membrane - Google Patents

Abstract

PURPOSE:To prolong filtering life, in an asymmetric membrane for filtration, by enlarging the pore size of the fine pore of a nonwoven fabric used in a support from the front surface toward the back surface thereof to prevent the clogging thereof. CONSTITUTION:A nonwoven fabric of which the pore size is continuously or discontinuously enlarged from the front surface to the back surface thereof in a thickness direction such as ones integrally formed by laying staple fibers in a multilayer form such that the average length thereof becomes longer (or shorter) in a thickness direction or one integrally formed by laying staple fibers in a multilayer form such that the fiber diameters thereof becomes thicker (or finer) in the thickness direction is impregnated and coated with a solution of a thermoplastic resin and thus treated nonwoven fabric is dried to obtain an asymmetric membrane. About this membrane, because particles generating clogging during filtration are stepwise replenished in the thickness direction, the abrupt reduction in a filtering speed is not generated. Therefore, filtering life is enhanced to a large extent and three times or more of a total filtering amount can be attained as compared to a conventional membrane using a known nonwoven fabric.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an asymmetric membrane used for filtration and the like. A mixed solution prepared by mixing a thermoplastic resin dissolved in a good solvent for the resin with a non-solvent that is compatible with the solvent but does not dissolve the resin is impregnated onto a nonwoven fabric, and then dried to form a fine powder. There is a method to make the pores longer, but the microporous material obtained by this method uses a nonwoven fabric made of short fibers with almost uniform fiber length and fiber diameter, so it cannot be formed into a nonwoven fabric IJx. The diameter of the opening becomes almost uniform in the thickness direction of the microporous body.

When such a microporous body is used to remove impurity particles from water, there is a drawback that impurity particles larger than the diameter of the micropores are trapped only on the surface and easily clog.

The present invention eliminates the above-mentioned conventional internal point, and uses a nonwoven fabric whose pore diameter increases continuously or discontinuously from the front surface to the back surface in the thickness direction, and a solution containing a thermoplastic resin dissolved in a good solvent for the resin. A mixed solution made by mixing a non-solvent that is compatible with the solvent and does not dissolve the resin is impregnated and coated, and then dried to obtain an asymmetric membrane, and the particles that clog during filtration are removed stepwise in the thickness direction. The purpose is to capture and extend filtration life.

The general principle of the asymmetric membrane formation of the present invention is that when the thermoplastic resin solution impregnated into the pores of the nonwoven fabric undergoes phase separation, the portions of the nonwoven fabric close to the pore walls experience shrinkage of the thermoplastic resin that occurs as the solvent evaporates. This prevents the pore diameter from increasing, and conversely, the far part tends to contract, so the pore diameter becomes smaller. This means that when the pore diameter of the nonwoven fabric is large, the diameter of the micropores formed in the pores becomes smaller on average because the thermoplastic resin tends to shrink, and conversely, when the pore diameter of the nonwoven fabric is small, This shows that the micropore diameter becomes larger. Therefore, by changing the pore diameter in the thickness direction of the nonwoven fabric, a microporous membrane having asymmetric pore diameters in the thickness direction can be obtained.

The nonwoven fabric used in the present invention is made of short fibers of inorganic or organic polymers, and the pore diameter increases continuously or discontinuously from the front surface to the back surface in the thickness direction.For example, the average length of the short fibers increases in the thickness direction. There are two methods: one is to layer short fibers in the order of their length (or the smallest) and then form one piece, or the other is to layer short fibers in the thickness direction in order of their diameter (in order of thicker fibers) (or thinner fibers) and form one piece. These can generally be obtained according to the above-mentioned configuration by paper-making techniques using a wet method or a dry method.

These pore diameters in the thickness direction may be a continuous change in pore diameter due to a series of gradient distributions, and do not necessarily have to be a smooth gradient distribution.

Non-woven materials include polyester, polyethylene,
These include thermoplastic resins such as polyethylene, natural polymers such as linters and pulp, and inorganic materials such as potassium titanate, glass, and asbestos.

This will be explained below using examples.

16 parts by weight of chlorinated polyvinyl chloride was dissolved in 87 parts by weight of tetrahydrofuran, and 50 parts by weight of isopropyl alcohol and 0.6 parts by weight of Tong ester were added to 70 parts of this solution, and the mixture was mixed and dissolved uniformly. . This solution was applied to the six types of polyester nonwoven fabrics shown below (each A, B, and C).
) and dried in an atmosphere of 25° C. and 60% humidity to obtain a microporous membrane. Nonwoven fabric A was formed into three layers in which short fibers had diameters of 8 μm, 15 μm, and 25 μm, respectively, each layer had a thickness of 6 μm, and each short fiber had a length of 5 μm. Nonwoven fabric B has a structure in which six layers of I each having short fiber lengths of 5 fins, 30 m, and 60 w- are formed in order,
The layer thickness of each layer was 60 ttm, and the short fiber diameter was 8 μm. Nonwoven fabric C was a comparative example for nonwoven fabrics A and B, and was a nonwoven fabric with a thickness of 180 μm in which short fibers with a fiber length of 5 μm and a fiber diameter of 8 μm occupied the entire layer.

Table 1 shows the retention properties and filtration properties of microporous bodies made from the above six types of nonwoven fabrics.

Table - ■ (Note) 17 The surface is the side with a smaller fiber diameter or shorter fiber length.

(Note) 2. The total filtration amount is the total amount of tap water filtrated after 5 minutes at a p filtration area of 13.8 cIII when constant pressure filtration is performed from the surface at a differential pressure of 0.5 Kg/cJ.

(Note) 6. Pure water was used for water permeation rate.

(Note) 4. The average pore diameter is a value measured by Akimasa Takiko and mirror observation.

As can be seen from Table-■, in the case of microporous membranes using nonwoven fabrics A and B, the average pore diameter is approximately 4 times different between the front and back surfaces.
It is a very good asymmetric membrane, and the total filtration amount of tap water is more than 6 times that of the microporous membrane using nonwoven fabric C. Also, in terms of water permeability, the microporous membranes made of nonwoven fabrics A and B are more than twice as efficient as the microporous membranes made of nonwoven fabric C.
You can see that it is.

As shown above, the asymmetric membrane according to the present invention has a smaller pore diameter from the front surface to the back surface, which effectively utilizes the thickness of the membrane to prevent clogging, prevent a sudden decrease in filtration rate, and extend the filtration life. He made the large cloth head towards the hill.

The asymmetric membrane according to the present invention can be used for water treatment in food, medicine and electronic industries, and is of great industrial value.

Applicant Yuasa Battery Co., Ltd.

Claims (1)

[Scope of Claims] 1) A nonwoven fabric whose pore diameter is increased continuously or discontinuously from the front surface to the back surface in the thickness direction, and a thermoplastic resin is dissolved in a good solvent for the resin, and the solution is mixed with the solvent. An asymmetric membrane obtained by impregnating and coating a mixed solution prepared by mixing a non-solvent that does not dissolve the resin, followed by drying. 2) The asymmetric membrane according to claim 1, which is a nonwoven fabric that is elongated continuously or discontinuously from the front surface to the back surface in the average length or thickness direction of short fibers constituting the nonwoven fabric. b) The average fiber diameter of the short fibers constituting the nonwoven fabric is continuous from the front surface to the back surface in the thickness direction. 2. The asymmetric membrane according to claim 1, which is a nonwoven fabric that is thickened continuously or discontinuously.