JPH04277020A - Microporous membrane and filter material using the same - Google Patents

Abstract

PURPOSE:To provide a filter material having long filtering life. CONSTITUTION:In a web like microporous membrane, a membrane characterized in that the mean pore size of the membrane is uniform in the lateral direction thereof is rolled into a cylindrical shape in a superposed state so that the end part having a small pore size thereof becomes inside to constitute a cylindrical filter material whose pore size is largest in the outer peripheral part thereof and smallest in the inner peripheral part thereof and a raw solution to be filtered is introduced into the filter material from the outer peripheral part thereof and a filtrate is discharged to the center part thereof to use the part having large pore size of the filter material as a prefilter. By this constitution, filtering life can be extended to a large extent.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a microporous filtration membrane, and is particularly applicable to drugs in the pharmaceutical industry, alcoholic beverages in the food industry, and electronic industries that perform fine processing, including the aforementioned manufacturing industry and semiconductor manufacturing industry. It is used for filtration of purified water, pure water, etc. for the production of ultrapure water used in laboratories of various industries, and other precision filtration. The present invention relates to a microporous membrane for precision filtration that efficiently filters.

0002

[Prior Art] The important points of filtration technology are to improve filtration accuracy, that is, the reliability of capturing fine particles, etc., and to suppress the increase in filtration pressure, which is caused by the lifespan of the filter medium, such as clogging with filtered substances. It has been said that In order to improve the accuracy of filtration, attempts have been made to improve the probability of capturing fine particles. In the case of so-called isotropic membranes in which the pore sizes on both surfaces of the membrane do not substantially change, fine particles to be captured can be captured anywhere throughout the thickness of the membrane, and the probability of capture is high. It exhibits large resistance to fluid flow, can only obtain a small flow rate (i.e., only a small flow rate can be obtained per unit area, per unit time, and per unit differential pressure), and is easily clogged and has a short filtration life. , it had drawbacks such as lack of blocking resistance.

On the other hand, a membrane has a structure called an anisotropic membrane, in which the pore diameter changes continuously or discontinuously in the film thickness direction, and the pore diameter on one surface of the membrane is different from the pore diameter on the other surface. is JP 55-6406, JP 56-1540
As described in No. 51, the membrane has a thin layer with small pores called a dense layer on one surface of the membrane, and relatively large pores or extremely large voids from the inside of the membrane to the other surface. It is something that When this membrane is used with the large pore side upstream of the filtration, large particles will be trapped in the large pores on the upstream side, and essentially the smallest particles that can be filtered out will be trapped in this dense layer. Furthermore, the entire thickness of the membrane can be effectively utilized as a filter medium, thereby extending the filtration life. However, since this type of anisotropic membrane is usually extremely thin in the portion having the minimum average pore diameter, it cannot capture a sufficient amount of filtrate such as fine particles.

0004

[Problems to be Solved by the Invention] When microporous membranes, particularly microfiltration membranes, are used to filter filtrate with a high solids content, it has traditionally been the case that a pre-filter with a large pore size is used before treatment with the microfiltration membrane. The method used was to remove large particles in advance by using In other words, it was common knowledge to use two or more stages of filtration. Therefore, if these filtration steps could be performed using a single filter medium, the amount of filter medium consumed would be reduced, resulting in an extremely large process improvement. For the purpose of these improvements, for example, in pleated cartridges, the primary side of the precision filtration membrane is made of a different material with coarse pores such as nonwoven fabric, or the same material with large pores is layered and folded. 1
Attempts have been made to provide a single cartridge with the dual functions of pre-filter and precision filtration. However, there is a drawback that the cartridge molding process is complicated and the processing cost is high, and there has been a desire to develop a membrane that can efficiently capture fine particles of a wide range of sizes with a single membrane.

[0005]

[Means for Solving the Problems] In order to solve the above problems, we have conducted extensive research and found that the problems can be solved by using the filter material of the present application. In other words, in a web-like microporous membrane, the average pore diameter is uniform in the width direction and changes continuously in the length direction. This method uses a filter medium that is spirally wound around a support body, centering around the part with the smaller pore diameter. The filtration material used in the present invention is not particularly limited, and can be selected depending on the use and purpose of filtration. Examples include polypropylene, polyester, fluororesin, cellulose acetate, nitrocellulose, polysulfone, sulfonated polysulfone, polyethersulfone, polyacrylonitrile, polyamide, polyimide, polyphenylene oxide, and the like. The most suitable method for producing microporous membranes can be selected depending on the material. For example, when polypropylene, polyester, etc. are used as a nonwoven fabric, the basis weight during production can be continuously changed, and the pore diameter can be continuously changed from a dense part to a coarse part in the length direction. In addition, when using the same polypropylene, polyester, fluororesin, etc., the web can be made by continuously changing the stretching ratio in the stretching film forming process as shown in Japanese Patent Publication No. 58-52123, Japanese Patent Application Laid-open No. 57-66114, etc. A membrane is obtained in which the pore size changes continuously in the length direction.

[0006] Polysulfone, sulfonated polysulfone, polyether sulfone, polyacrylonitrile,
In the solution casting process using polyamide, polyimide, polyphenylene oxide, etc., in the method shown in JP-A-63-139929, the wind speed of the humidity-controlled air applied to the surface of the polymer solution cast on the support is continuously changed. By this, the average pore diameter in the length direction can be changed continuously. The rate of change in pore diameter is preferably about 10 to 100 times. The variation in pore diameter in the width direction is preferably 10% or less. In the present invention, the membrane thus obtained is processed and used as a filtration module. There are various forms of filtration modules, and when the membrane of the present invention is folded in order from the side with the smallest pore size to form a stacked type, it is used like a press filter.
The side with larger pore diameter is used as the primary side. Also, when assembled as a cylindrical cartridge type module,
Wrap it around the cylindrical support mesh at the center, starting from the part with the smallest hole diameter, and wrap it in a spiral to form a module. This module is used for filtration with the outer surface as the primary side and the center as the secondary side. Conventionally, even if it is a press type,
Even in the case of a cartridge type module, the only way to form a module with sequentially varying pore diameters was to process and stack membranes with different pore diameters. By using a membrane in which one membrane pore diameter is continuous from a small portion to a large portion, a troublesome membrane processing step is unnecessary and the module can be formed efficiently. Examples of actual module configurations are shown below.

[0007]

[Examples] The present invention will be explained in more detail with reference to specific examples, but the present invention is not limited to the following examples unless it goes beyond the gist of the invention. Example 1 A polypropylene nonwoven fabric with a length of 6 m, a width of 236 mm, and a thickness of 0.4 mm, with a basis weight of 100 g/cm2 at the top.
The fabric weight was continuously changed from 1 to 15 g/cm 2 at the rear end, and was wound around a cylindrical porous base material with an outer diameter of 30 mm from the beginning, and a portion was spot-welded to the outer circumferential portion at the final end. Furthermore, donut-shaped polypropylene disc end caps are welded to both the upper and lower ends of the cylindrically wound filtration material together with the cylindrical porous base material.The filtration material thus formed is installed in municipal water piping to improve filtration performance. was evaluated.

Comparative Example 1 A filter material prepared in the same manner as in Example 1 except that the basis weight was 100 g/cm 2 in the lengthwise direction was used for comparison. Table 1 shows the results of comparing the pressure rise after 10,000 m3 filtration for each filter medium of Example 1 and Comparative Example 1.
Shown below.

[0009]

[Table 1]

Example 2 15 parts of polysulfone (P3500 manufactured by Amoco), N
-70 parts of methylpyrrolidone, 15 parts of polyvinylpyrrolidone
1 part and 3 parts of water are uniformly dissolved to obtain a film-forming stock solution. This solution was continuously cast onto a PET base from a casting coater to a film thickness of 180 μm at a speed of 2 m/min, air adjusted to 25°C and relative humidity of 45% was applied to the surface of the film for 5 seconds, and then immediately water was poured. A microporous membrane was created by immersing it in a coagulation liquid bath filled with . In order to continuously change the pore diameter, the wind speed applied for 5 seconds immediately after casting should be changed from 2 m/sec to 1 m/sec.
The pore size was continuously changed by continuously increasing the speed to 2 m/sec for 2 minutes. The hole diameter at this time is 0.2 at the beginning.
It increased continuously from μm to 10.2 μm at 4 m.

Comparative Example 2 A membrane made by the same membrane forming method as in Example 1 was used, except that the wind speed was fixed at 2 m/sec. The above membrane with a width of 20 cm and a length of 4 m was folded alternately every 20 cm from the end to make a 20-layer membrane (20 cm x 20 cm square), which was set in a filter holder with the side with larger pores as the primary side. . One municipal water supply was used for each filter medium of Example 2 and Comparative Example 2.
Table 2 shows the results of comparing the increase in filtration pressure after filtration of 000 m3.

0012

[Table 2]

[0013]

Effects of the Invention As shown in Examples 1 and 2, the filtration module configured according to the present invention can be used for large-scale processing.
It can be seen that the increase in filtration pressure is small and the filtration life can be significantly extended. Furthermore, module molding is easy, and processing time in the manufacturing process can be made more efficient.

Claims (2)

[Claims] 1. A web-like microporous membrane characterized in that the average pore diameter is uniform in the width direction and the size changes continuously in the length direction. 2. A filter medium characterized in that the membrane according to claim 1 is used in a pleated state or in a state in which it is spirally wound around a cylindrical porous support centering around a portion with a small pore diameter.