JP2010137196A - Method for manufacturing porous hollow-fiber membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a polyolefin-based hollow-fiber membrane with a highly durable surface. <P>SOLUTION: The method for manufacturing the porous hollow-fiber membrane includes: drawing a polyolefin resin spun to hollow fiber to make it porous; and then heat-treating it under an atmosphere of (Tm-35) to (Tm-5)°C, wherein Tm is the melting point (°C) of the polyolefin resin. In the method, the heat-treatment time is 15 minutes or longer. The polyolefin-family hollow-fiber membrane tends to have an improved crystallinity that is measured by wide angle X-ray diffraction. For example, the obtained polyolefin-based hollow-fiber membrane has an improved crystallinity of 65% or higher. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a porous hollow fiber membrane used for filtration.

In recent years, the demand for high-purity water has been increasing in various applications such as electronics industry, pharmaceutical medical use, household use, boiler use, sewage treatment, and water treatment. Various studies have been made.
For example, a method by microfiltration using a membrane can be mentioned, and water can be purified by removing foreign substances such as bacteria.

As a microfiltration membrane, a hollow fiber-like porous hollow fiber membrane (hereinafter referred to as “hollow fiber membrane”) is known, and has a feature of excellent separation performance per unit area.
Polyolefin resins such as polyethylene and polypropylene are known as raw materials for this hollow fiber membrane, and are characterized by excellent chemical stability and durability.

Patent Documents 1 to 5 describe a method for producing a hollow fiber membrane obtained from polyethylene. A hollow fiber-like porous hollow fiber membrane can be produced by stretching a polyolefin resin melt-spun into a hollow shape under predetermined conditions.
The polyolefin-based hollow fiber membrane does not use a solvent in the production process. In addition, the polyolefin resin as a raw material is excellent in recyclability and extremely excellent in terms of environmental load. Polyolefin-based hollow fiber membranes are used in water purifiers as microfiltration membranes, and in recent years are also used for sewage treatment and water treatment (hereinafter referred to as “water treatment”).

In this water treatment, etc., if the filtration through the membrane is continued, the membrane surface (porous portion) is gradually blocked by various substances contained in the water to be treated, and the filtration efficiency decreases as the filtration differential pressure increases. .
For this reason, when the filtration differential pressure exceeds a certain level, reverse cleaning of the membrane or chemical cleaning of the membrane with sodium hypochlorite or the like is performed.
Further, the membrane surface blockage is also reduced by aeration of the water tank to be treated simultaneously with filtration and bringing bubbles into contact with the outer surface of the membrane.

Therefore, the above-described hollow fiber membrane is required to have various durability for use in such an environment.
For example, chemical cleaning with sodium hypochlorite oxidizes and decomposes organic substances deposited on the membrane surface, but the membrane substrate can also be oxidized and degraded at the same time, so the hollow fiber membrane must have excellent chemical resistance. It is.
Further, during aeration during the filtration operation, a tensile force is continuously applied to the hollow fiber membrane, so that the hollow fiber membrane requires high mechanical strength.

  Furthermore, since the inorganic substance contained in the liquid to be treated may come into strong contact with the outer surface of the hollow fiber membrane or the hollow fiber membranes may come into contact with each other by aeration, However, excellent durability is required.

JP-B 63-35726 JP-A-63-75116 JP-A-62-237910 Patent No. 2896781 Japanese Patent No. 3168036

  An object of this invention is to provide the manufacturing method of the polyolefin-type hollow fiber membrane excellent in durability of the membrane surface.

  In the present invention, the polyolefin resin spun into a hollow shape is stretched and made porous, and when the melting point (° C.) of the polyolefin resin is Tm, the temperature is (Tm−35) ° C. or more and (Tm−5) ° C. or less. The present invention relates to a method for producing a porous hollow fiber membrane, which is heat-treated in an atmosphere.

  According to the present invention, the durability of the hollow fiber membrane surface can be improved, and the filtration characteristics can be maintained over a long period of time.

Hereinafter, preferred embodiments of the present invention will be described.
The polyolefin resin used in the present invention is used as a base material for a hollow fiber membrane. A polyolefin resin is chemically stable, and a hollow fiber membrane that does not contain impurities such as an organic solvent can be produced by melt-spinning the resin into a hollow shape and then making it porous.

  Such a polyolefin resin-based hollow fiber membrane is excellent in productivity because it does not require a step of removing the organic solvent remaining in the hollow fiber membrane, and water to be treated is produced by impurities eluting from the membrane itself. And can be suitably used for applications requiring high-purity filtered water.

The polyolefin resin that can be used in the present invention is not particularly limited as long as it is a crystalline polymer that can be melt-spun into a hollow shape and can be drawn and porous. For example, polyethylene, polypropylene, poly-3-methylbutene-1, Examples thereof include poly-4-methylpentene-1.
These polyolefin resins can be appropriately selected and used as necessary, but are chemically stable, have no fear of elution from the membrane to the water to be treated, and are particularly suitable for producing highly pure water. As the substrate for the hollow fiber membrane, polyethylene is preferable.

  The average molecular weight of these polyolefin resins can be appropriately selected as necessary. From the viewpoint of durability and openness, for example, in the case of polyethylene, the weight average molecular weight is in the range of 100,000 to 1,000,000. Is preferable. The average molecular weight here is obtained from an average value in terms of polystyrene when polyethylene is dissolved in o-dichlorobenzene and the molecular weight distribution is measured by the GPC method.

  In the method for producing a porous hollow fiber membrane of the present invention, the hollow fiber obtained by melt spinning of the above-described polyolefin resin is drawn and porous, and then heat treatment is performed under specific conditions.

  This hollow fiber melt spinning is possible by using a spinning nozzle having a discharge port formed in an annular shape, extruding a polyolefin resin in a heated and melted state from the discharge port of the spinning nozzle, and further cooling and solidifying it. is there.

  The heating temperature of the polyolefin resin at the time of melt spinning must be set to a temperature at which the resin does not solidify during spinning, i.e., the melting point of the resin or more, in order to completely prevent the polyolefin resin from solidifying during spinning. Is preferably set to a temperature 10 to 50 ° C. higher than the melting point of the polyolefin resin. On the other hand, the cooling temperature of the hollow fiber after spinning is preferably set to about 10 to 40 ° C. Moreover, it is preferable that winding of the hollow fiber after cooling is performed at a winding speed of 20 to 600 m / min. However, the present invention is not limited to these production conditions.

  The spun hollow fiber is subjected to an annealing treatment before being drawn and porous. Here, the annealing treatment is performed at a temperature equal to or lower than the melting point of the resin, and is performed in order to grow a stack lamellae of polyolefin resin formed in the hollow fiber in the spinning process and improve the crystal orientation order. .

  The hollow fiber can be made porous by stretching the hollow fiber in the longitudinal direction after the annealing treatment. This stretching is preferably composed of two stages, a cold stretching process in which stretching is performed at a relatively low temperature and a hot stretching process in which stretching is performed at a temperature higher than that in the cold stretching process. By such a two-stage drawing process, the structure of the pores formed in the hollow fiber can be controlled more precisely.

  The cold drawing step is usually performed in the range of 0 to (melting point of polyolefin resin-50) ° C. A preferable cold stretching temperature when polyethylene is used as the polyolefin resin is in the range of 0 to 80 ° C. More preferably, it is the range of 10-50 degreeC.

  By subjecting the above-mentioned hollow fiber to such cold drawing, a part of the crystal structure of the hollow fiber is broken, and uniform and micro crazes (microcracks) can be generated between the stack lamellae. Here, the cold draw ratio (the length of the hollow fiber after cold drawing / the length of the hollow fiber before cold drawing) is preferably in the range of 1.2 to 1.8 times. By making the cold draw ratio 1.2 times or more, microcracks tend to be sufficiently generated, and by making it 1.8 times or less, the deformation of the stack lamella is suppressed, and the high porosity It tends to be easy to obtain a hollow fiber membrane.

  The hot drawing process expands the microcracks formed by the cold drawing described above, forms microfibrils between the stack fibers of the hollow fiber, and has a porous structure as a membrane on the hollow fiber, which is composed of slit-like micropores. Is given.

  It is preferable to perform a heat | fever extending process normally as high as possible in the range which does not exceed the melting point of polyolefin resin. Further, the hot draw ratio (the length of the hollow fiber membrane after the hot drawing / the length of the hollow fiber before the hot drawing) can be appropriately selected depending on the pore diameter of the target pore, but from the viewpoint of process stability From 3 to 10 times the range is preferable. More preferably, it is the range of 4-6 times.

  The hollow fiber membrane obtained through the above stretching process can be heat set for the purpose of improving dimensional stability. In this method, the hollow fiber membrane is heated under a temperature condition that is higher than the thermal stretching temperature and lower than the melting point of the polyolefin in a state where the hollow fiber membrane is constant or relaxed. Since the time required for this heat setting is short and can be sufficiently less than 1 minute, it can be adopted as a part of the continuous production process of the hollow fiber membrane.

The heat treatment in the present invention is performed after the above-mentioned stretching and porosity, and when the melting point (° C.) of the polyolefin resin is Tm, the atmosphere is (Tm−35) ° C. or more and (Tm−5) ° C. or less. Under, preferably for 15 minutes or more. This heat treatment can improve the durability of the hollow fiber membrane surface.
In addition, this Tm is melting | fusing point measured by DSC (temperature increase rate: 10 degree-C / min) of polyolefin resin.

  When the heat treatment temperature is lower than (Tm-35) ° C., the durability of the film surface is not sufficiently improved, or a very long heat treatment tends to be required to obtain a certain improvement. Preferably, it is (Tm-30) ° C. or higher.

  On the other hand, when the heat treatment temperature exceeds (Tm-5) ° C., part of the porous structure formed by stretching and porosification is melted, and accordingly, the durability of the film surface tends to be impaired. Preferably it is (Tm-10) degrees C or less, More preferably, it is (Tm-15) degrees C or less.

  Furthermore, it is preferable that the heat treatment time be 15 minutes or longer because the durability of the film surface tends to be improved more reliably. More preferably, it is 20 minutes or more, More preferably, it is 30 minutes or more.

The upper limit of the heat treatment time is not particularly limited, can be appropriately selected according to the required performance of the hollow fiber membrane, and the heat treatment can be continued for several days.
The heat treatment for a long time can be carried out, for example, by winding a stretched and porous hollow fiber around a bobbin and then allowing it to stand in a heating device such as a dryer. It is also possible to ensure the necessary heat treatment time by reciprocating the stretched and hollowed hollow fiber a plurality of times in a heating furnace. The atmosphere for the heat treatment is not particularly limited and may be nitrogen or air.

  The porous hollow fiber membrane obtained in the present invention can be subjected to a hydrophilic treatment as necessary. This hydrophilization treatment can be performed, for example, by immersing the porous hollow fiber membrane in a hydrophilic polymer solution obtained by dissolving a hydrophilic polymer in a solvent, and then drying the membrane. As the solvent evaporates, a hollow fiber membrane whose membrane surface is coated with a hydrophilic polymer is obtained. Thereby, the surface of the polyolefin-based hollow fiber membrane is hydrophilized, and the water permeability of the membrane can be improved.

In addition, by this hydrophilization treatment, several microfibrils forming the porous hollow fiber membrane are bundled together to form a microfibril bundle, and as a result, the slit-like micropores become elliptical micropores, and the average pore diameter of the membrane Therefore, the permeation flux of the membrane can be improved.
This hydrophilization treatment may be performed before the above heat treatment or after the heat treatment.

  According to the present invention, the resulting polyolefin-based hollow fiber membrane tends to improve the crystallinity measured by wide-angle X-ray diffraction. For example, a polyethylene-based hollow fiber membrane having a crystallinity improved to 65% or more is obtained. be able to. This is considered to contribute to the improvement of the durability of the film surface.

  The detailed reason for improving the durability of the film surface is unknown, but if the crystallinity of the polyolefin resin, which contains both crystalline and amorphous parts, increases near room temperature, the proportion of amorphous parts that are prone to plastic deformation decreases. This is thought to be because the shape retention of the porous structure of the polyolefin-based hollow fiber membrane obtained in the present invention is enhanced.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, it is not limited to these. In addition, the evaluation method and manufacturing apparatus used in the Example are as follows.

<Scanning electron microscope (SEM) observation of hollow fiber membrane surface>
Observation of the surface of the hollow fiber membrane was performed using a scanning electron microscope JSM-6060A manufactured by JEOL.

<Measurement of melting point of hollow fiber membrane>
The melting point of the surface of the hollow fiber membrane was measured by using DSC220 manufactured by SII Nano Technology, Inc., and measuring the temperature rising rate at 10 ° C./min.

<Measurement of crystallinity of hollow fiber membrane>
For the crystallinity of the hollow fiber membrane, RU-200 manufactured by Rigaku Corporation was used. Measurement conditions are output: 40 kV-100 mA, measurement range: 2θ = 10 to 35 °, step: 0.02 °, scan: 1 ° / min, and a hollow fiber membrane of about 5 cm is fixed to a folder for measuring fiber samples. And measured. After the measurement, according to Non-Patent Document 1 and Non-Patent Document 2, the degree of crystallinity was determined using “multiple peak separation method” which is analysis software attached to this apparatus.

Textile Handbook 3rd edition (p81-82), Maruzen Co., Ltd., issued in 2004 Polymer Analysis Handbook (p243), Asakura Shoten Co., Ltd., 1985, first edition

<Model surface durability evaluation of hollow fiber membrane>
For the surface durability of the hollow fiber membrane, a dyeing friction fastness tester manufactured by Daiei Kagaku Seisakusho Co., Ltd. was used. A weight of a predetermined weight is applied to the hollow fiber membrane to be tested, and a load is applied. A hollow fiber membrane of the same type as the hollow fiber membrane to be tested is fixed to the surface of the friction rod. The friction test was performed for a predetermined time by setting so as to be in direct contact. The hollow fiber membranes that have been subjected to the friction test are collected, SEM observation is performed on the surface of the hollow fiber membrane to which friction is applied, and the surface durability of the hollow fiber membrane is modeled by comparing the open state of the hollow fiber membrane surface. Evaluated.

Example 1
Polyethylene-based hollow fiber membrane (EX540T, Mitsubishi Rayon Engineering Co., Ltd.) obtained by drawing a porous polyethylene resin with a melting point (Tm) of 134 ° C. into a hollow shape and drawing it into an air atmosphere to make it hydrophilic. The product was heat-treated at 105 ° C. for 30 minutes while being wound on a bobbin, and the crystallinity of the obtained hollow fiber membrane was measured. The crystallinity of the hollow fiber membrane was 65%. .

(Example 2)
The crystallinity of the hollow fiber membrane was measured in the same manner as in Example 1 except that the heat treatment temperature was 110 ° C. As a result, the crystallinity of the hollow fiber membrane was 67%.

(Example 3)
The crystallinity of the hollow fiber membrane was measured in the same manner as in Example 1 except that the heat treatment temperature was 115 ° C. As a result, the crystallinity of the hollow fiber membrane was 66%.

Example 4
Polyethylene hollow fiber membrane (EX540CB, Mitsubishi Rayon Engineering Co., Ltd.) obtained by drawing and hollowing a polyethylene resin having a melting point (Tm) of 134 ° C. in a hollow shape and making it porous in an air atmosphere. The product was heat-treated at 105 ° C. for 30 minutes and the crystallinity of the obtained hollow fiber membrane was measured. The film crystallinity of the hollow fiber was 65%.

(Example 5)
Except that the heat treatment temperature was 110 ° C., the crystallinity of the hollow fiber membrane was measured in the same manner as in Example 4. As a result, the membrane crystallinity of the hollow fiber was 66%.

(Example 6)
Except that the heat treatment temperature was 115 ° C., the crystallinity of the hollow fiber membrane was measured in the same manner as in Example 4. As a result, the crystallinity of the hollow fiber membrane was 65%.

(Example 7)
FIG. 1 shows the result of SEM observation of the surface of the porous membrane after the friction test was performed for 10 minutes by applying a load of 10 gf / fil using the hollow fiber membrane of Example 5.
Compared with the results of Comparative Example 3 below (FIG. 2), the hollow fiber membrane having a crystallinity improved to 66% by heat treatment has more surface voids even after the friction test, resulting in friction durability. It turns out that it is excellent in.

(Comparative Example 1)
When the crystallinity of the same polyethylene-based hollow fiber membrane as in Example 1 was measured without heat treatment, the crystallinity of the hollow fiber membrane was 64%.

(Comparative Example 2)
When the crystallinity of the same polyethylene hollow fiber membrane as in Example 4 was measured without heat treatment, the crystallinity of the hollow fiber membrane was 63%.

(Comparative Example 3)
FIG. 2 shows the result of SEM observation of the surface of the porous membrane after the friction test was performed for 10 minutes by applying a load of 10 gf / fil using the hollow fiber membrane of Comparative Example 2.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the polyolefin-type hollow fiber membrane excellent in the durability of the film | membrane surface can be provided.

Scanning electron micrograph of Example 7 Scanning electron micrograph of Comparative Example 3

Claims (2)

After the polyolefin resin spun into a hollow shape is drawn and made porous, when the melting point (° C.) of the polyolefin resin is Tm, this is performed in an atmosphere of (Tm−35) ° C. or more and (Tm−5) ° C. or less. A method for producing a porous hollow fiber membrane, in which heat treatment is performed. The manufacturing method of the porous hollow fiber membrane of Claim 1 whose heat processing time is 15 minutes or more.

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