JPS6227004A - Microporous membrane for filtration - Google Patents

Abstract

PURPOSE:To easily obtain the titled water filtration membrane permeable to the air mixed in the water in the filter and having an efficient filtration performance by making a hydrophilic porous membrane with the water permeability improved by hydrophilic inorg. particles partly hydrophobic with alkaline treatment. CONSTITUTION:The hydrophobic resin 1 of a polyolefinic resin such as polyethylene, a fluororesin, etc., and alkali-soluble inorg. particles 2 of silica, etc., having 0.005-5mum mean primary particle diameter are used to form a hydrophilic microporous membrane contg. 10-70wt% hydrophilic inorg. particles 2 in the three-dimensional network structure and having 30-80% porosity, 0.01-10mum mean pore diameter and 10-2,000mum thickness. A part of the membrane is treated with an aq. alkaline soln. to dissolve and remove the hydrophilic inorg. particles 2 and convert the <=20% in area of the porous membrane into a hydrophobic structure. Then a filter is assembled. The formation of the partially hydrophobic membrane by an aq. alkaline soln. may be also performed after the module of the hydrophilic microus membrane is made.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a microporous membrane for filtration, particularly a water filtration membrane, in which a hydrophobic lignophilized membrane with improved water permeability by hydrophilic inorganic particles is used. The present invention relates to a filtration membrane having a structure in which air in water can be easily removed by partially making the porous membrane hydrophobic, and it is efficient but has a filtering performance.

[Prior Art] Various hydrophilic filtration membranes have been proposed in the past. For example, a microporous membrane in which alkali-soluble hydrophilic inorganic particles are retained within a three-dimensional network structure made of a hydrophobic resin has been proposed. has been done. Although this microporous membrane uses ll1li aqueous resin, it is permanently hydrophilized by hydrophilic inorganic particles, and is characterized by a low initial filtration pressure and easy water filtration. . However, although water permeates easily through this microporous membrane, it is difficult for air bubbles mixed in water to pass through the membrane's micropores due to the surface tension of the water, and the smaller the pore size, the higher the pressure. Needless to say, if you don't do this, the air won't be able to pass through.

When a water filter is made using such a microporous wood-treated membrane, it is difficult for air mixed into the water to pass through the porous membrane.
Particularly at low pressures such as household water pressure, air covers the surface of the porous membrane and prevents water from permeating, resulting in an extremely reduced amount of water permeation.

In order to prevent this kind of phenomenon, the conventional methods of removing air are: (1) mechanically attaching an air release valve to the filter; (2)! Methods such as attaching an aqueous porous membrane have been adopted.

However, method 1) requires a special valve to be attached to the filter, requires special consideration in terms of structure, and is not economical. Method (2) takes advantage of the fact that air escapes through a hydrophobic porous membrane, but the hydrophobic porous membrane must be prepared separately and attached to the filter separately from the hydrophilic membrane. The structure is complicated, and additional space is required for installation, which reduces the usable volume of the filter.

[Problems to be Solved by the Invention] In view of the above-mentioned problems, the present invention provides a membrane structure that economically and efficiently reduces the amount of water permeation and provides air removal through a simple filter structure. The purpose of this invention is to provide a water filtration membrane having the following properties.

[Means and effects for solving the problems] According to the present invention, a three-dimensional network structure made of polyolefin or fluororesin has alkali solubility with a uniform primary particle size of 0.005 to 5 san. A porous membrane that retains 10 to 70% by weight of hydrophilic inorganic particles, has a porosity of 30 to 80%, an average pore diameter of 0.01 to 10 µl, and a film thickness of 10 to 2000 µl, is prepared using alkali. The treatment removes hydrophilic inorganic particles from a portion of 20% or less of the porous membrane area, thereby providing a microporous membrane for filtration that has a partially hydrophobic structure.

In the water filtration membrane of the present invention, the resin constituting the microporous membrane is originally hydrophobic, but since the pores contain hydrophilic inorganic particles, the porous membrane is apparently hydrophilic, and water is This makes it easy to do the same thing.

Moreover, the hydrophilic inorganic particles are soluble in an alkaline solution and are partially removed by the alkali, resulting in a microporous membrane in which a hydrophilic part and a hydrophobic part coexist.

The hydrophobic resin used in the present invention is polyethylene, polypropylene, polybutene, a mixture thereof, or a copolymer of two or more of ethylene propylene, butene, and hexene.2. Polyvinylidene fluoride, olefin-tetrafluoroethylene copolymer, and other resins are generally referred to as hydrophobic resins, and they exhibit extremely little deterioration even when in long-term contact with water.

Further, the alkali-soluble hydrophilic inorganic particles used in the present invention are generally insoluble in water, such as silica, magnesium oxide, alumina, barium sulfate, etc., but are dissolved by alkali. The size of the particles is preferably 5 pm or less in diameter, preferably 0.2 to 0.01 p,
A diameter of m is good.

The lignophilized microporous membrane having a three-dimensional network structure used as a filtration membrane in the present invention can be easily obtained, for example, in the following manner. That is, 7 to 42 volume % of hydrophilic inorganic powder and organic liquid plasticizer 3 having a solubility parameter (SP value) of 84 to 99.
0 to 75% by volume, polyolefin 10 to 6 with a weight average molecular weight of less than 300.000 and a number average molecular weight of 15,000 or more
It can be easily obtained by mixing 0% by volume, melt-molding, and extracting the organic liquid plasticizer from the molded product. or,
It can also be obtained by subsequently stretching in at least one direction.

When a crystalline resin is used, the temperature atmosphere during stretching is above the glass transition point and below the melting point, preferably in a temperature range of 100°C or above and 5°C below the melting point, and when a non-grade resin is used. The temperature range is from η°' to the decomposition point, preferably from 100°C to 10°C below the decomposition temperature.

The stretching may be carried out immediately after film formation under the above-mentioned temperature conditions, or the plasticizer may be extracted once the film is formed and then the stretching may be carried out under the above-mentioned temperature conditions. The stretching ratio for orientation is preferably 1.2 times to 3 times.

The parent wood microporous membrane is not limited to a flat membrane, but one made in the form of hollow fibers is also effective.

The water filtration membrane of the present invention can easily dissolve and remove hydrophilic inorganic particles by treating a portion of the lignophilized microporous membrane with an alkaline aqueous solution after forming the lignophilized microporous membrane. 20% or less of the total area of the membrane, preferably 5% or less, is made up of a hydrophobic porous membrane and has a hydrophilic part and a hydrophobic part coexisting, and after the treatment, it can be assembled as a filter to exhibit its effectiveness. It is something.

Partial hydrophobization by alkaline aqueous solution treatment can also be performed after assembly into a filter. In other words, after fabricating the wood-based microporous flat membrane used in the present invention into pleated or spiral-type modules, or after fabricating the wood-based microporous hollow fiber membrane into modules, the upper part of the module, where air in water tends to accumulate, is removed. The water filtration membrane having the structure of the present invention can also be obtained by partially treating the membrane with an alkaline aqueous solution to dissolve and remove the hydrophilic inorganic particles of the lignified microporous membrane.

In the water filtration membrane with the structure of the present invention, since the hydrophobic part does not get wet with water, the air mixed in the water in the filter passes through this part and escapes, and the air covers the part of the lignophilized microporous membrane. This effectively prevents the excess area from decreasing. Of course, the pore size conditions used in the present invention are such that the hydrophilic microporous membrane is impermeable to water and its pores do not get wet under the pressure required for water to permeate through the hydrophilic microporous membrane and perform filtration. It is necessary to design it with a porous membrane.

By using the water filtration membrane of the present invention, the surface of the hydrophilic porous membrane is prevented by air bubbles in the water, which has always been a problem as a hydrophilic porous membrane with a pore size on the order of microns or submicrons. This eliminates the phenomenon of being covered and reducing the water passage area, making it possible to vent air using the same resin that is integrated with the filtration membrane.

Therefore, it is a very simple method in terms of structure, and it has become possible to constantly bleed air without having to install a special air bleed valve and manually operate the air bleed.

In addition, a method has been proposed in which the hydrophobic porous membrane is attached separately to remove air, but it is not integrated with the filtration membrane and requires a part to attach the hydrophobic porous membrane, making the assembly structure complicated. Unavoidably, extra space is required inside the filter.

By using the filtration membrane of the structure of the present invention, the filtration efficiency does not decrease due to the presence of air, and the filtration efficiency is high.The structure is simple, the assembly cost is low, and it is economical, and does not require special operations. The air was removed and a very novel water filtration membrane was obtained.

The functions of the filtration membrane of the present invention will be explained using figures.

A schematic diagram of a filtration device using a wood-treated microporous membrane (without hydrophobic treatment! After the microporous membrane has been wetted with water, air bubbles contained in the raw water are difficult to pass through the membrane and accumulate at the top of the container. Generally, when the pores of a microporous membrane are filled with water and allow air to pass through, if the material is the same, the size of the pore diameter and the pressure applied to the air are inversely proportional, and the smaller the pore diameter, the more air passes through. It is necessary to increase the pressure.

Normally, the relational expression when air passes through the micropores with the micropores filled with liquid is expressed by D=30γP. However, D
: Pore diameter (gm), γ: Liquid surface tension (dyne-'
), P: Pressure (mmHg).

Therefore, as shown in Figure 1, when air is on the outer cylinder side of a filter with a filtration membrane in between, and the filtration membrane is hydrophilic and water fills the pores, the air passes through the filtration membrane. When trying to discharge water, it is estimated using the above formula that when the pore size is 0.2μ, air will not pass through the layer unless a pressure of 15kg/cm2 is applied, and it turns out that extremely high pressure is required. When measured, approximate data is obtained.

However, when the pores are not filled with water, air easily flows through the pores in the direction of lower pressure. Therefore, by making use of this effect and making a part of the hydrophilic porous membrane hydrophobic, we created a part that prevents wood from penetrating into the micropores under low pressure, and this book allows both hydrophilic and hydrophobic properties to coexist. A filtration membrane of the invention was created.

This uses a filter membrane, but the filter device looks like Figure 2.The hydrophobic part created at the top does not contain any water, so the air bubbles that collect at the top permeate through this part due to water pressure and flow out to the outside. . It is most efficient to provide this hydrophobic portion on the upper part of the filtration membrane, and it is effective even if it is not provided all around the filtration membrane but only partially as shown in FIG.

The water will pass through other parts of the parent tree and be filtered. Therefore, if the area of this hydrophobic part is increased, the hydrophilic part decreases and the filtration efficiency decreases, so at least 20% of the total area
It is desirable to keep it below %.

FIG. 4 is an enlarged view of the hydrophobic portion and hydrophilic portion of FIG. 2, and illustrates the mechanism through which air passes through the hydrophobic portion and water through the hydrophilic portion.

The filtration membrane having the structure of the present invention becomes a hydrophobic porous membrane composed only of hydrophobic resin by dissolving alkali-soluble hydrophilic inorganic particles in an alkaline aqueous solution, and becomes a hydrophobic and hydrophilic membrane with a structure as shown in Fig. 5. It becomes a filtration membrane with both properties.

The hydrophobic/hydrophilic portions do not need to be concentrated in one part, and may be dispersed and divided into several locations.

As the hydrophilized microporous membrane used in the present invention, it is also possible to use a microporous membrane stretched uniaxially or biaxially.

[Example] Next, the present invention will be explained in more detail with reference to Examples.

Example 1 A pleated module was fabricated using a porous membrane with a three-dimensional network structure made of 55 parts by weight of polyethylene as a hydrophobic resin and 45 parts by weight of silica with an average particle diameter of 0.1 g as a hydrophilic inorganic particle. 2. A 50% aqueous solution of caustic soda was applied to a part of the porous membrane on the top of one unassembled module, and after washing with water to elute the silica, the module was dried to obtain the filtration membrane of the present invention (Sample 1). The remaining module without caustic soda treatment (Comparative Sample 1) was tested for water permeability. In the test, UFp superhydrant was placed in a water tank, air bubbles were added through an aeration tube, and water was passed through using a pump. The results are shown in Figure 6.Sample 1 shows no change in water permeability over time, indicating that the filtration membrane is functioning normally with little change over time, but comparative sample 1 shows no change over time. It is thought that the water permeation rate decreases and the effective area decreases, and when the filter was stopped and the module was inspected, it was confirmed that it was filled with air.

Example 2 A hollow fiber porous membrane with a three-dimensional network structure was prepared by using 55 parts by weight of polyethylene as a hydrophobic resin and 45 parts by weight of silica with an average particle diameter of 0.1 p as a hydrophilic inorganic particle after spinning and cooling. The filtration membrane used in the present invention was rolled up into a whole shape, a portion was immersed in a 50% caustic soda aqueous solution, the alkali content was immediately washed off with running water, and after drying, it was assembled into a module (sample 2). The membrane was assembled into a module without caustic soda treatment (comparative sample 2) and tested. In the test, OF water was placed in a water tank, carbon particles were suspended in it, and air bubbles were added to the water using an aeration pipe, and the water was permeated through each module using a pump. The results are shown in FIG.

In sample 2, the pressure on the raw water side increases as the water permeability increases, but
The carbon in the raw water clogs the filtration membrane, reducing the amount of water permeation, resulting in overflow characteristics, which is normally expected. However, in Comparative Sample 2, as the water permeation rate increases, the pressure increases abnormally and the permeation characteristics drop extremely. This is thought to be due to the fact that not only the micropores of the filtration membrane are clogged with carbon, reducing the water permeation rate, but also air pockets are created, which reduces the filtration quality.

When we disassembled the module and examined it, we found that in sample 1, the non-hydrophobic parts of the hollow fibers that are the filtration membrane were completely gray, indicating that all the hydrophilic parts were permeable to water, and there were also air pockets. There was none at all. However, in Comparative Sample 2, air was trapped inside the module, and it was clear that almost no carbon was attached to the hollow fibers in the areas where air was trapped, and water did not permeate through the hollow fibers.

[Effects of the Invention] The stretchable microporous membrane of the present invention is a lignophilized porous membrane whose water permeability has been improved by hydrophilic inorganic particles, which is partially made hydrophobic. In addition to ensuring that
Hydrophobic parts can remove air from the water, allowing efficient filtration at low pressure.

[Brief explanation of the drawing]

Figure 1 is a schematic explanatory diagram of a filtration device using a wood-treated microporous membrane (a membrane that has not been subjected to hydrophobization treatment), and Figure 2 is a microporous filtration device with a partially hydrophobic structure according to the present invention. A schematic explanatory diagram of a filtration device using a membrane, FIG. 3 is a sectional view taken along the line xx in FIG. 2, FIG. 4 is an enlarged view of part A in FIG. 2, and FIG.
This is an enlarged view of the hydrophobic and hydrophilic parts of the membrane used in the device shown in the figure, and is a schematic explanatory diagram illustrating the mechanism in which the hydrophobic part allows air to pass through and the hydrophilic part allows water to pass through. Figure 6 is Example 1 FIG. 7 is a graph of the test results in Example 2.

Claims (1)

[Claims] A porous membrane containing 10 to 70% by weight of alkali-soluble hydrophilic inorganic particles with an average primary particle diameter of 0.005 to 5 μm in a three-dimensional network structure made of polyolefin or fluororesin, the porosity of which is A membrane with an average pore size of 0.01 to 10 μm and a thickness of 10 to 2000 μm is treated with an alkali to remove hydrophilic inorganic particles in an area of 20% or less of the porous membrane area, resulting in a partially hydrophobic structure. Microporous membrane for filtration.